Doc. no. | D????
Date: | 2026-06-15
Project: | Programming Language C++
Reply to: | Jonathan Wakely <lwgchair@gmail.com>

# C++ Standard Library Defect Reports and Accepted Issues (Revision R126)

Revised 2026-06-15 at 14:46:45 UTC

Reference ISO/IEC IS 14882:2024(E)

Also see:

- Table of Contents for all library issues.
- Index by Section for all library issues.
- Index by Status for all library issues.
- Library Active Issues List
- Library Closed Issues List

This document contains only library issues which have been closed by the Library Working Group (LWG) after being found to be defects in the standard. That is, issues which have a status of DR, TC1, C++11, C++14, C++17, or Resolved. See the Library Closed Issues List for issues closed as non-defects. See the Library Active Issues List for active issues and more information. The introductory material in that document also applies to this document.

## Revision History

- R126: 2026-06-15 Post-Brno 2026
  - Summary:
    - 561 open issues, down by 167.
    - 3506 closed issues, up by 334.
    - 51 reassigned issues, down by 6.
    - 4118 issues total, up by 161.
  - Details:
    - Added the following 5 Ready issues: 4547, 4562, 4577, 4587, 4588.
    - Added the following 3 Tentatively NAD issues: 4475, 4494, 4542.
    - Added the following 39 New issues: 4458, 4473, 4479, 4488, 4489, 4502, 4505, 4508, 4516, 4524, 4538, 4539, 4543, 4545, 4546, 4551, 4559, 4561, 4564, 4565, 4566, 4569, 4571, 4573, 4574, 4576, 4580, 4581, 4582, 4584, 4585, 4586, 4590, 4591, 4592, 4593, 4594, 4595, 4596.
    - Added the following 8 LEWG issues: 4503, 4509, 4530, 4558, 4560, 4563, 4572, 4589.
    - Added the following 2 Core issues: 4507, 4578.
    - Added the following 3 SG1 issues: 4519, 4570, 4575.
    - Added the following SG16 issue: 4541.
    - Added the following 8 WP issues: 4471, 4487, 4490, 4521, 4531, 4567, 4568, 4579.
    - Added the following 76 C++26 issues: 4438, 4439, 4440, 4441, 4442, 4443, 4444, 4445, 4446, 4447, 4448, 4449, 4450, 4451, 4452, 4453, 4454, 4455, 4456, 4457, 4459, 4460, 4461, 4462, 4463, 4464, 4465, 4467, 4468, 4469, 4472, 4474, 4476, 4477, 4478, 4480, 4481, 4483, 4485, 4486, 4491, 4492, 4493, 4496, 4497, 4499, 4500, 4504, 4506, 4510, 4511, 4512, 4514, 4517, 4522, 4523, 4525, 4527, 4528, 4529, 4532, 4533, 4535, 4536, 4537, 4540, 4544, 4548, 4549, 4550, 4552, 4553, 4554, 4555, 4556, 4557.
    - Added the following 10 Resolved issues: 4436, 4437, 4466, 4470, 4501, 4513, 4518, 4520, 4526, 4534.
    - Added the following 6 NAD issues: 4482, 4484, 4495, 4498, 4515, 4583.
    - Changed the following 20 issues to Ready (from New): 2592, 2595, 2713, 2714, 2811, 2819, 2983, 3053, 3126, 3172, 3353, 3584, 3647, 3696, 4022, 4039, 4055, 4367, 4397, 4406.
    - Changed the following 3 issues to Ready (from Open): 2136, 2137, 2220.
    - Changed the following issue to Tentatively NAD (from New): 4417.
    - Changed the following issue to Open (from New): 4356.
    - Changed the following issue to Core (from New): 2237.
    - Changed the following issue to SG6 (from New): 3402.
    - Changed the following 9 issues to WP (from New): 4121, 4125, 4158, 4182, 4218, 4249, 4267, 4359, 4385.
    - Changed the following issue to WP (from SG1): 3417.
    - Changed the following 13 issues to WP (from C++23): 2697, 2779, 2960, 3002, 3010, 3020, 3071, 3136, 3411, 3413, 3414, 3443, 3771.
    - Changed the following 4 issues to C++26 (from Ready): 4137, 4243, 4315, 4423.
    - Changed the following 57 issues to C++26 (from Tentatively Ready): 2991, 3090, 3627, 4020, 4136, 4166, 4253, 4255, 4256, 4257, 4265, 4266, 4269, 4274, 4275, 4276, 4280, 4286, 4291, 4292, 4293, 4294, 4297, 4299, 4300, 4301, 4305, 4312, 4317, 4318, 4328, 4340, 4341, 4342, 4343, 4345, 4346, 4349, 4351, 4366, 4370, 4372, 4375, 4377, 4382, 4384, 4398, 4399, 4403, 4407, 4412, 4413, 4415, 4416, 4422, 4425, 4426.
    - Changed the following 46 issues to C++26 (from New): 2746, 3504, 3599, 3662, 3797, 3831, 3891, 4026, 4122, 4133, 4143, 4151, 4223, 4230, 4251, 4259, 4260, 4272, 4290, 4304, 4308, 4314, 4316, 4324, 4325, 4339, 4347, 4358, 4360, 4369, 4376, 4378, 4379, 4383, 4388, 4420, 4424, 4427, 4428, 4429, 4430, 4431, 4432, 4433, 4434, 4435.
    - Changed the following 7 issues to C++26 (from Open): 2414, 3343, 3454, 3777, 4015, 4090, 4130.
    - Changed the following 3 issues to C++26 (from LEWG): 4302, 4361, 4396.
    - Changed the following issue to C++26 (from SG1): 4354.
    - Changed the following 140 issues to C++26 (from WP): 2392, 2994, 3203, 3216, 3305, 3431, 3436, 3578, 3749, 3767, 3809, 3884, 3885, 3886, 3887, 3892, 3893, 3894, 3897, 3899, 3900, 3903, 3904, 3905, 3912, 3914, 3915, 3918, 3919, 3925, 3927, 3935, 3938, 3940, 3944, 3946, 3947, 3948, 3949, 3950, 3951, 3953, 3956, 3957, 3965, 3970, 3973, 3974, 3975, 3984, 3987, 3990, 4001, 4011, 4012, 4013, 4014, 4016, 4023, 4024, 4025, 4027, 4030, 4031, 4035, 4036, 4037, 4038, 4043, 4044, 4045, 4053, 4054, 4060, 4061, 4064, 4071, 4072, 4074, 4076, 4079, 4082, 4083, 4084, 4085, 4088, 4096, 4098, 4105, 4106, 4112, 4113, 4119, 4124, 4126, 4134, 4135, 4140, 4141, 4142, 4144, 4147, 4148, 4153, 4154, 4157, 4164, 4169, 4170, 4172, 4175, 4179, 4186, 4188, 4189, 4191, 4196, 4198, 4200, 4201, 4202, 4203, 4204, 4205, 4208, 4209, 4217, 4222, 4224, 4227, 4231, 4232, 4233, 4234, 4235, 4236, 4239, 4242, 4245, 4247.
    - Changed the following 35 issues to Resolved (from New): 2478, 2479, 2480, 2481, 2507, 3095, 3109, 3192, 3210, 3217, 3537, 3613, 3624, 3638, 3653, 4000, 4109, 4131, 4146, 4187, 4190, 4213, 4215, 4238, 4248, 4264, 4278, 4282, 4283, 4344, 4368, 4381, 4393, 4414, 4419.
    - Changed the following 2 issues to Resolved (from Open): 3625, 4387.
    - Changed the following 11 issues to Resolved (from LEWG): 936, 961, 4329, 4330, 4331, 4332, 4333, 4334, 4335, 4336, 4390.
    - Changed the following 2 issues to Resolved (from Pending NAD): 2302, 2382.
    - Changed the following issue to NAD Editorial (from Pending NAD Editorial): 2125.
    - Changed the following 30 issues to NAD (from Tentatively NAD): 3908, 3909, 3958, 3980, 3981, 3982, 3992, 4003, 4006, 4009, 4050, 4095, 4163, 4171, 4184, 4194, 4219, 4228, 4229, 4244, 4246, 4271, 4309, 4321, 4337, 4362, 4363, 4371, 4391, 4404.
    - Changed the following 18 issues to NAD (from New): 2248, 2472, 2737, 2858, 3162, 3220, 3268, 3484, 3615, 3641, 3686, 3722, 3845, 3952, 3979, 4220, 4221, 4402.
    - Changed the following 2 issues to NAD (from Open): 3161, 3840.
    - Changed the following 4 issues to NAD (from LEWG): 1484, 1488, 1493, 2884.
    - Changed the following 2 issues to NAD (from SG1): 2236, 4075.
    - Changed the following issue to NAD (from Deferred): 2202.
    - Changed the following issue to Dup (from New): 4195.
- R125: 2025-10-30 Pre-Kona 2025
  - Summary:
    - 728 open issues, up by 298.
    - 3172 closed issues, up by 172.
    - 57 reassigned issues, up by 21.
    - 3957 issues total, up by 491.
  - Details:
    - Added the following 4 Ready issues: 4137, 4243, 4315, 4423.
    - Added the following 54 Tentatively Ready issues: 4020, 4136, 4166, 4253, 4255, 4256, 4257, 4265, 4266, 4269, 4274, 4275, 4276, 4280, 4286, 4291, 4292, 4293, 4294, 4297, 4299, 4300, 4301, 4305, 4312, 4317, 4318, 4328, 4340, 4341, 4342, 4343, 4345, 4346, 4349, 4351, 4366, 4370, 4372, 4375, 4377, 4382, 4384, 4398, 4399, 4403, 4407, 4412, 4413, 4415, 4416, 4422, 4425, 4426.
    - Added the following 28 Tentatively NAD issues: 3958, 3980, 3981, 3982, 3992, 4003, 4006, 4009, 4050, 4095, 4163, 4171, 4184, 4194, 4219, 4228, 4229, 4244, 4246, 4271, 4309, 4321, 4337, 4362, 4363, 4371, 4391, 4404.
    - Added the following 249 New issues: 3945, 3952, 3954, 3955, 3959, 3960, 3961, 3962, 3963, 3964, 3966, 3967, 3968, 3969, 3972, 3976, 3977, 3979, 3983, 3985, 3986, 3989, 3991, 3993, 3994, 3995, 3997, 3998, 3999, 4000, 4002, 4005, 4007, 4008, 4010, 4017, 4018, 4021, 4022, 4026, 4028, 4029, 4032, 4033, 4034, 4039, 4040, 4041, 4046, 4047, 4048, 4049, 4051, 4052, 4055, 4057, 4059, 4062, 4063, 4065, 4066, 4067, 4068, 4073, 4077, 4078, 4080, 4081, 4089, 4091, 4092, 4093, 4094, 4099, 4100, 4101, 4102, 4103, 4104, 4107, 4109, 4110, 4111, 4114, 4115, 4116, 4117, 4118, 4120, 4121, 4122, 4123, 4125, 4127, 4128, 4129, 4131, 4132, 4133, 4138, 4139, 4143, 4145, 4146, 4151, 4152, 4155, 4158, 4159, 4160, 4161, 4162, 4165, 4167, 4168, 4173, 4176, 4180, 4181, 4182, 4183, 4185, 4187, 4190, 4192, 4193, 4195, 4197, 4199, 4206, 4207, 4210, 4211, 4212, 4213, 4214, 4215, 4216, 4218, 4220, 4221, 4223, 4225, 4226, 4230, 4237, 4238, 4240, 4248, 4249, 4251, 4252, 4254, 4258, 4259, 4260, 4261, 4262, 4263, 4264, 4267, 4268, 4270, 4272, 4273, 4277, 4278, 4279, 4281, 4282, 4283, 4285, 4287, 4288, 4290, 4295, 4296, 4298, 4303, 4304, 4306, 4307, 4308, 4311, 4313, 4314, 4316, 4319, 4320, 4322, 4323, 4324, 4325, 4326, 4327, 4339, 4344, 4347, 4352, 4353, 4355, 4356, 4357, 4358, 4359, 4360, 4364, 4367, 4368, 4369, 4373, 4374, 4376, 4378, 4379, 4380, 4381, 4383, 4385, 4386, 4388, 4392, 4393, 4394, 4395, 4397, 4400, 4402, 4405, 4406, 4409, 4410, 4411, 4414, 4417, 4418, 4419, 4420, 4421, 4424, 4427, 4428, 4429, 4430, 4431, 4432, 4433, 4434, 4435.
    - Added the following 6 Open issues: 3988, 4015, 4070, 4090, 4130, 4387.
    - Added the following 24 LEWG issues: 4058, 4097, 4241, 4250, 4284, 4289, 4302, 4329, 4330, 4331, 4332, 4333, 4334, 4335, 4336, 4338, 4348, 4350, 4361, 4365, 4390, 4396, 4401, 4408.
    - Added the following Core issue: 4310.
    - Added the following 5 SG1 issues: 4004, 4075, 4174, 4177, 4354.
    - Added the following 3 SG9 issues: 4019, 4108, 4389.
    - Added the following 2 SG16 issues: 4087, 4156.
    - Added the following 105 WP issues: 3946, 3947, 3948, 3949, 3950, 3951, 3953, 3956, 3957, 3965, 3970, 3973, 3974, 3975, 3984, 3987, 3990, 4001, 4011, 4012, 4013, 4014, 4016, 4023, 4024, 4025, 4027, 4030, 4031, 4035, 4036, 4037, 4038, 4043, 4044, 4045, 4053, 4054, 4060, 4061, 4064, 4071, 4072, 4074, 4076, 4079, 4082, 4083, 4084, 4085, 4088, 4096, 4098, 4105, 4106, 4112, 4113, 4119, 4124, 4126, 4134, 4135, 4140, 4141, 4142, 4144, 4147, 4148, 4153, 4154, 4157, 4164, 4169, 4170, 4172, 4175, 4179, 4186, 4188, 4189, 4191, 4196, 4198, 4200, 4201, 4202, 4203, 4204, 4205, 4208, 4209, 4217, 4222, 4224, 4227, 4231, 4232, 4233, 4234, 4235, 4236, 4239, 4242, 4245, 4247.
    - Added the following 5 Resolved issues: 3978, 4042, 4069, 4149, 4150.
    - Added the following NAD Editorial issue: 4178.
    - Added the following 4 NAD issues: 3971, 3996, 4056, 4086.
    - Changed the following 2 issues to Tentatively Ready (from New): 3090, 3627.
    - Changed the following issue to Tentatively Ready (from LEWG): 2991.
    - Changed the following 4 issues to Open (from New): 2939, 3099, 3343, 3625.
    - Changed the following issue to Open (from LEWG): 3454.
    - Changed the following 2 issues to Open (from SG1): 2533, 3941.
    - Changed the following 17 issues to WP (from Tentatively Ready): 2994, 3884, 3885, 3887, 3893, 3894, 3903, 3904, 3905, 3912, 3914, 3915, 3925, 3927, 3935, 3938, 3940.
    - Changed the following 14 issues to WP (from New): 2392, 3203, 3216, 3431, 3436, 3578, 3749, 3809, 3886, 3892, 3897, 3899, 3900, 3919.
    - Changed the following 2 issues to WP (from Open): 3305, 3767.
    - Changed the following issue to WP (from LEWG): 3918.
    - Changed the following issue to WP (from SG16): 3944.
    - Changed the following 313 issues to C++23 (from WP): 2191, 2195, 2295, 2381, 2697, 2731, 2743, 2762, 2774, 2779, 2818, 2820, 2839, 2960, 2997, 3002, 3010, 3020, 3028, 3032, 3036, 3071, 3085, 3088, 3117, 3118, 3120, 3121, 3123, 3136, 3143, 3146, 3152, 3170, 3171, 3177, 3195, 3204, 3211, 3236, 3249, 3265, 3293, 3306, 3361, 3391, 3392, 3403, 3404, 3405, 3406, 3407, 3410, 3411, 3413, 3414, 3419, 3420, 3421, 3422, 3425, 3426, 3427, 3428, 3430, 3432, 3433, 3434, 3435, 3437, 3441, 3443, 3446, 3447, 3448, 3449, 3450, 3453, 3455, 3460, 3461, 3462, 3464, 3465, 3466, 3467, 3470, 3471, 3472, 3473, 3474, 3476, 3477, 3480, 3481, 3482, 3483, 3490, 3492, 3494, 3495, 3498, 3500, 3502, 3505, 3506, 3515, 3517, 3518, 3519, 3520, 3521, 3522, 3523, 3525, 3526, 3527, 3528, 3529, 3530, 3532, 3533, 3535, 3536, 3539, 3540, 3541, 3542, 3543, 3544, 3545, 3546, 3548, 3549, 3551, 3552, 3553, 3554, 3555, 3557, 3559, 3560, 3561, 3563, 3564, 3566, 3567, 3568, 3569, 3570, 3571, 3572, 3573, 3574, 3580, 3581, 3585, 3589, 3590, 3591, 3592, 3593, 3594, 3595, 3597, 3598, 3600, 3601, 3607, 3610, 3612, 3616, 3617, 3618, 3619, 3621, 3622, 3629, 3631, 3632, 3636, 3643, 3645, 3646, 3648, 3650, 3654, 3655, 3656, 3657, 3659, 3660, 3661, 3664, 3670, 3671, 3672, 3677, 3683, 3687, 3692, 3701, 3702, 3703, 3704, 3705, 3707, 3708, 3709, 3710, 3711, 3712, 3713, 3715, 3717, 3719, 3720, 3721, 3723, 3724, 3732, 3733, 3734, 3736, 3737, 3738, 3742, 3743, 3745, 3746, 3747, 3750, 3751, 3753, 3754, 3755, 3756, 3757, 3759, 3760, 3761, 3762, 3764, 3765, 3766, 3769, 3770, 3771, 3772, 3773, 3774, 3775, 3778, 3781, 3782, 3784, 3785, 3786, 3788, 3790, 3792, 3795, 3796, 3798, 3801, 3803, 3807, 3810, 3811, 3814, 3816, 3817, 3818, 3819, 3820, 3821, 3822, 3823, 3824, 3825, 3826, 3827, 3828, 3833, 3834, 3836, 3839, 3841, 3842, 3843, 3847, 3848, 3849, 3850, 3851, 3853, 3857, 3860, 3862, 3865, 3866, 3867, 3869, 3870, 3871, 3872, 3875, 3876, 3877, 3878, 3879, 3880, 3881.
    - Changed the following 4 issues to Resolved (from New): 2933, 3064, 3508, 3861.
    - Changed the following 3 issues to Resolved (from Open): 2088, 2524, 3003.
    - Changed the following 3 issues to Resolved (from LEWG): 2095, 2922, 2985.
    - Changed the following 6 issues to NAD (from Tentatively NAD): 2457, 3635, 3714, 3901, 3930, 3936.
    - Changed the following issue to NAD (from New): 2413.
    - Changed the following 3 issues to NAD (from LEWG): 532, 2885, 3776.
    - Changed the following issue to NAD (from SG1): 3485.
    - Changed the following issue to NAD (from SG9): 3913.
- R124: 2023-06-09 Pre-Varna
  - Summary:
    - 430 open issues, up by 25.
    - 3000 closed issues, up by 92.
    - 36 reassigned issues, down by 4.
    - 3466 issues total, up by 113.
  - Details:
    - Added the following 16 Tentatively Ready issues: 3884, 3885, 3887, 3893, 3894, 3903, 3904, 3905, 3912, 3914, 3915, 3925, 3927, 3935, 3938, 3940.
    - Added the following 5 Tentatively NAD issues: 3901, 3908, 3909, 3930, 3936.
    - Added the following 52 New issues: 3832, 3835, 3837, 3838, 3845, 3846, 3852, 3854, 3855, 3856, 3861, 3863, 3864, 3873, 3882, 3883, 3886, 3888, 3889, 3890, 3891, 3892, 3895, 3896, 3897, 3898, 3899, 3900, 3902, 3906, 3907, 3910, 3911, 3916, 3917, 3919, 3920, 3921, 3922, 3923, 3924, 3926, 3928, 3929, 3931, 3932, 3933, 3934, 3937, 3939, 3942, 3943.
    - Added the following 2 Open issues: 3840, 3844.
    - Added the following 2 LEWG issues: 3868, 3918.
    - Added the following SG1 issue: 3941.
    - Added the following SG9 issue: 3913.
    - Added the following SG16 issue: 3944.
    - Added the following 30 WP issues: 3833, 3834, 3836, 3839, 3841, 3842, 3843, 3847, 3848, 3849, 3850, 3851, 3853, 3857, 3860, 3862, 3865, 3866, 3867, 3869, 3870, 3871, 3872, 3875, 3876, 3877, 3878, 3879, 3880, 3881.
    - Added the following Resolved issue: 3859.
    - Added the following 2 NAD issues: 3858, 3874.
    - Changed the following issue to Tentatively Ready (from Open): 2994.
    - Changed the following 2 issues to Tentatively NAD (from New): 2457, 3635.
    - Changed the following issue to Tentatively NAD (from LEWG): 3714.
    - Changed the following 2 issues to Open (from Tentatively NAD): 2116, 3357.
    - Changed the following issue to Open (from New): 3507.
    - Changed the following issue to Open (from SG16): 3767.
    - Changed the following 12 issues to WP (from Ready): 2195, 2295, 3032, 3085, 3664, 3720, 3756, 3769, 3807, 3811, 3820, 3825.
    - Changed the following issue to WP (from Tentatively Ready): 3819.
    - Changed the following 10 issues to WP (from New): 3204, 3622, 3631, 3645, 3655, 3723, 3734, 3772, 3790, 3803.
    - Changed the following 5 issues to WP (from Open): 3441, 3733, 3742, 3786, 3821.
    - Changed the following 3 issues to WP (from LEWG): 3810, 3827, 3828.
    - Changed the following 9 issues to Resolved (from New): 3223, 3234, 3412, 3673, 3676, 3700, 3718, 3787, 3791.
    - Changed the following 2 issues to Resolved (from Open): 3442, 3698.
    - Changed the following 4 issues to Resolved (from SG16): 3565, 3576, 3639, 3780.
    - Changed the following 12 issues to NAD (from Tentatively NAD): 2432, 3726, 3727, 3735, 3739, 3740, 3741, 3752, 3768, 3779, 3789, 3800.
    - Changed the following issue to NAD (from New): 3806.
- R123: 2022-11-24 Post-Kona
  - Summary:
    - 405 open issues, up by 50.
    - 2908 closed issues, up by 100.
    - 40 reassigned issues, up by 1.
    - 3353 issues total, up by 151.
  - Details:
    - Added the following 7 Ready issues: 3720, 3756, 3769, 3807, 3811, 3820, 3825.
    - Added the following Tentatively Ready issue: 3819.
    - Added the following 11 Tentatively NAD issues: 3726, 3727, 3735, 3739, 3740, 3741, 3752, 3768, 3779, 3789, 3800.
    - Added the following 51 New issues: 3681, 3682, 3684, 3685, 3686, 3688, 3689, 3690, 3691, 3693, 3694, 3696, 3697, 3699, 3700, 3706, 3716, 3718, 3722, 3723, 3725, 3728, 3729, 3730, 3731, 3734, 3744, 3748, 3749, 3758, 3763, 3772, 3783, 3787, 3790, 3791, 3793, 3794, 3797, 3799, 3802, 3803, 3804, 3805, 3806, 3809, 3812, 3813, 3829, 3830, 3831.
    - Added the following 6 Open issues: 3698, 3733, 3742, 3777, 3786, 3821.
    - Added the following 5 LEWG issues: 3714, 3776, 3810, 3827, 3828.
    - Added the following 2 SG16 issues: 3767, 3780.
    - Added the following 65 WP issues: 3683, 3687, 3692, 3701, 3702, 3703, 3704, 3705, 3707, 3708, 3709, 3710, 3711, 3712, 3713, 3715, 3717, 3719, 3721, 3724, 3732, 3736, 3737, 3738, 3743, 3745, 3746, 3747, 3750, 3751, 3753, 3754, 3755, 3757, 3759, 3760, 3761, 3762, 3764, 3765, 3766, 3770, 3771, 3773, 3774, 3775, 3778, 3781, 3782, 3784, 3785, 3788, 3792, 3795, 3796, 3798, 3801, 3814, 3816, 3817, 3818, 3822, 3823, 3824, 3826.
    - Added the following Resolved issue: 3815.
    - Added the following 2 NAD issues: 3695, 3808.
    - Changed the following 3 issues to Ready (from New): 2295, 3032, 3664.
    - Changed the following 2 issues to Ready (from Open): 2195, 3085.
    - Changed the following 2 issues to Tentatively NAD (from Open): 2116, 3357.
    - Changed the following issue to Tentatively NAD (from EWG): 2432.
    - Changed the following issue to Open (from New): 3167.
    - Changed the following issue to Open (from NAD): 617.
    - Changed the following issue to LEWG (from New): 3679.
    - Changed the following 3 issues to WP (from Tentatively Ready): 3670, 3671, 3672.
    - Changed the following 13 issues to WP (from New): 3028, 3118, 3136, 3177, 3411, 3569, 3594, 3597, 3600, 3617, 3629, 3636, 3677.
    - Changed the following 2 issues to WP (from Open): 3545, 3656.
    - Changed the following 2 issues to WP (from LEWG): 3515, 3646.
    - Changed the following issue to WP (from SG1): 3659.
    - Changed the following issue to WP (from SG9): 3564.
    - Changed the following 2 issues to Resolved (from New): 2561, 2564.
    - Changed the following 2 issues to Resolved (from Open): 2114, 2349.
    - Changed the following issue to Resolved (from EWG): 2813.
    - Changed the following issue to TS (from Tentatively Ready): 3649.
    - Changed the following 2 issues to NAD (from Tentatively NAD): 3579, 3667.
    - Changed the following issue to NAD (from New): 2352.
    - Changed the following issue to NAD (from Open): 3068.
    - Changed the following issue to NAD (from SG1): 3611.
- R122: 2022-03-04 March 2022
  - Summary:
    - 355 open issues, up by 41.
    - 2808 closed issues, up by 222.
    - 39 reassigned issues, up by 4.
    - 3202 issues total, up by 267.
  - Details:
    - Added the following 4 Tentatively Ready issues: 3649, 3670, 3671, 3672.
    - Added the following 2 Tentatively NAD issues: 3579, 3667.
    - Added the following 103 New issues: 3416, 3418, 3423, 3424, 3429, 3431, 3436, 3438, 3439, 3444, 3451, 3456, 3457, 3459, 3463, 3475, 3484, 3487, 3489, 3491, 3493, 3496, 3497, 3499, 3501, 3503, 3504, 3507, 3508, 3511, 3512, 3513, 3516, 3531, 3537, 3538, 3550, 3556, 3569, 3577, 3578, 3582, 3583, 3584, 3586, 3587, 3594, 3597, 3599, 3600, 3602, 3603, 3604, 3605, 3606, 3608, 3609, 3613, 3614, 3615, 3617, 3620, 3622, 3623, 3624, 3625, 3626, 3627, 3628, 3629, 3630, 3631, 3633, 3634, 3635, 3636, 3637, 3638, 3640, 3641, 3642, 3644, 3645, 3647, 3651, 3653, 3655, 3658, 3662, 3663, 3664, 3665, 3666, 3668, 3669, 3673, 3674, 3675, 3676, 3677, 3678, 3679, 3680.
    - Added the following 5 Open issues: 3441, 3442, 3488, 3545, 3656.
    - Added the following 6 LEWG issues: 3445, 3454, 3486, 3515, 3534, 3646.
    - Added the following 4 SG1 issues: 3417, 3485, 3611, 3659.
    - Added the following SG9 issue: 3564.
    - Added the following 3 SG16 issues: 3565, 3576, 3639.
    - Added the following 120 WP issues: 3414, 3419, 3420, 3421, 3422, 3425, 3426, 3427, 3428, 3430, 3432, 3433, 3434, 3435, 3437, 3443, 3446, 3447, 3448, 3449, 3450, 3453, 3455, 3460, 3461, 3462, 3464, 3465, 3466, 3467, 3470, 3471, 3472, 3473, 3474, 3476, 3477, 3480, 3481, 3482, 3483, 3490, 3492, 3494, 3495, 3498, 3500, 3502, 3505, 3506, 3517, 3518, 3519, 3520, 3521, 3522, 3523, 3525, 3526, 3527, 3528, 3529, 3530, 3532, 3533, 3535, 3536, 3539, 3540, 3541, 3542, 3543, 3544, 3546, 3548, 3549, 3551, 3552, 3553, 3554, 3555, 3557, 3559, 3560, 3561, 3563, 3566, 3567, 3568, 3570, 3571, 3572, 3573, 3574, 3580, 3581, 3585, 3589, 3590, 3591, 3592, 3593, 3595, 3598, 3601, 3607, 3610, 3612, 3616, 3618, 3619, 3621, 3632, 3643, 3648, 3650, 3654, 3657, 3660, 3661.
    - Added the following 11 Resolved issues: 3452, 3458, 3469, 3478, 3479, 3509, 3510, 3514, 3524, 3547, 3575.
    - Added the following NAD Editorial issue: 3558.
    - Added the following 7 NAD issues: 3415, 3440, 3468, 3562, 3588, 3596, 3652.
    - Changed the following 2 issues to Open (from LEWG): 3068, 3357.
    - Changed the following issue to Open (from NAD): 1102.
    - Changed the following issue to NAD Future (from LEWG): 1025.
    - Changed the following 4 issues to WP (from Ready): 3117, 3211, 3236, 3249.
    - Changed the following issue to WP (from Tentatively Ready): 2820.
    - Changed the following 22 issues to WP (from New): 2191, 2743, 2774, 2997, 3088, 3123, 3143, 3146, 3152, 3195, 3265, 3293, 3306, 3361, 3391, 3403, 3404, 3405, 3406, 3407, 3410, 3413.
    - Changed the following 10 issues to WP (from Open): 2381, 2731, 2762, 2818, 2839, 3036, 3120, 3121, 3170, 3171.
    - Changed the following issue to WP (from LEWG): 3392.
    - Changed the following 2 issues to C++20 (from New): 3290, 3386.
    - Changed the following 280 issues to C++20 (from WP): 1203, 2139, 2164, 2183, 2184, 2243, 2412, 2444, 2593, 2597, 2682, 2783, 2816, 2843, 2849, 2851, 2859, 2870, 2899, 2932, 2935, 2936, 2937, 2940, 2941, 2942, 2943, 2944, 2945, 2946, 2948, 2950, 2952, 2953, 2954, 2958, 2961, 2964, 2965, 2966, 2969, 2970, 2972, 2974, 2975, 2976, 2977, 2978, 2979, 2980, 2981, 2982, 2988, 2989, 2993, 2995, 2996, 2998, 3000, 3001, 3004, 3005, 3007, 3008, 3009, 3012, 3013, 3014, 3015, 3017, 3018, 3024, 3025, 3026, 3030, 3031, 3034, 3035, 3037, 3038, 3039, 3040, 3041, 3042, 3043, 3045, 3048, 3050, 3051, 3054, 3055, 3058, 3062, 3065, 3067, 3070, 3074, 3075, 3076, 3077, 3079, 3080, 3083, 3087, 3094, 3096, 3100, 3101, 3102, 3103, 3104, 3112, 3116, 3119, 3122, 3127, 3128, 3129, 3130, 3131, 3132, 3133, 3137, 3140, 3141, 3144, 3145, 3147, 3148, 3149, 3150, 3153, 3154, 3158, 3160, 3169, 3173, 3175, 3179, 3180, 3182, 3183, 3184, 3185, 3186, 3187, 3190, 3191, 3194, 3196, 3198, 3199, 3200, 3201, 3202, 3206, 3208, 3209, 3218, 3221, 3222, 3224, 3225, 3226, 3230, 3231, 3232, 3233, 3235, 3237, 3238, 3241, 3242, 3243, 3244, 3245, 3246, 3247, 3248, 3250, 3251, 3252, 3253, 3254, 3255, 3256, 3257, 3259, 3260, 3262, 3264, 3266, 3269, 3270, 3272, 3273, 3274, 3276, 3277, 3280, 3281, 3282, 3284, 3285, 3286, 3291, 3292, 3294, 3296, 3299, 3300, 3301, 3302, 3303, 3304, 3307, 3310, 3313, 3314, 3315, 3316, 3317, 3318, 3319, 3320, 3321, 3323, 3324, 3325, 3326, 3327, 3328, 3329, 3330, 3331, 3332, 3334, 3335, 3338, 3340, 3346, 3347, 3348, 3349, 3350, 3351, 3352, 3354, 3355, 3356, 3358, 3359, 3360, 3362, 3363, 3364, 3367, 3369, 3371, 3372, 3373, 3374, 3375, 3377, 3379, 3380, 3381, 3382, 3383, 3384, 3385, 3387, 3388, 3389, 3390, 3393, 3395, 3396, 3397, 3398.
    - Changed the following issue to Resolved (from Tentatively Ready): 3368.
    - Changed the following 16 issues to Resolved (from New): 2154, 2375, 3006, 3110, 3135, 3156, 3212, 3213, 3258, 3278, 3283, 3289, 3345, 3366, 3376, 3408.
    - Changed the following 4 issues to Resolved (from Open): 2363, 2741, 3155, 3168.
    - Changed the following 5 issues to Resolved (from LEWG): 2690, 2814, 2825, 3052, 3228.
    - Changed the following 10 issues to Resolved (from Tentatively Resolved): 2155, 2292, 2498, 2808, 2832, 2999, 3125, 3151, 3178, 3322.
    - Changed the following 5 issues to NAD (from Tentatively NAD): 2335, 3207, 3365, 3394, 3399.
    - Changed the following issue to NAD (from Open): 1396.
    - Changed the following issue to NAD (from LEWG): 3165.
- R121: 2020-03-02 post-Prague mailing
  - Summary:
    - 314 open issues, down by 66.
    - 2586 closed issues, up by 114.
    - 35 reassigned issues, up by 0.
    - 2935 issues total, up by 48.
  - Details:
    - Added the following Tentatively Ready issue: 3368.
    - Added the following 2 Tentatively NAD issues: 3394, 3399.
    - Added the following 20 New issues: 3366, 3370, 3376, 3378, 3386, 3391, 3400, 3401, 3402, 3403, 3404, 3405, 3406, 3407, 3408, 3409, 3410, 3411, 3412, 3413.
    - Added the following LEWG issue: 3392.
    - Added the following 24 WP issues: 3367, 3369, 3371, 3372, 3373, 3374, 3375, 3377, 3379, 3380, 3381, 3382, 3383, 3384, 3385, 3387, 3388, 3389, 3390, 3393, 3395, 3396, 3397, 3398.
    - Changed the following 4 issues to Ready (from New): 3117, 3211, 3236, 3249.
    - Changed the following issue to Tentatively Ready (from Open): 2820.
    - Changed the following 2 issues to Tentatively NAD (from New): 2335, 3365.
    - Changed the following 2 issues to Open (from New): 2844, 3305.
    - Changed the following 2 issues to Tentatively Resolved (from New): 2498, 3151.
    - Changed the following issue to Tentatively Resolved (from Open): 3125.
    - Changed the following 25 issues to WP (from Ready): 3264, 3280, 3281, 3292, 3302, 3303, 3304, 3307, 3310, 3313, 3315, 3316, 3317, 3318, 3319, 3320, 3321, 3323, 3324, 3325, 3326, 3327, 3329, 3330, 3331.
    - Changed the following 19 issues to WP (from Tentatively Ready): 3194, 3233, 3254, 3284, 3285, 3286, 3291, 3294, 3296, 3299, 3300, 3332, 3338, 3346, 3349, 3350, 3351, 3356, 3360.
    - Changed the following 36 issues to WP (from New): 3018, 3141, 3150, 3175, 3200, 3201, 3226, 3237, 3238, 3242, 3243, 3247, 3248, 3250, 3251, 3252, 3255, 3260, 3262, 3269, 3270, 3301, 3314, 3328, 3334, 3335, 3340, 3347, 3348, 3352, 3355, 3358, 3359, 3362, 3363, 3364.
    - Changed the following 4 issues to WP (from Open): 1203, 2859, 3050, 3282.
    - Changed the following issue to WP (from LEWG): 3354.
    - Changed the following issue to Resolved (from New): 3336.
    - Changed the following 3 issues to NAD (from New): 3271, 3298, 3333.
    - Changed the following issue to NAD (from Open): 2860.
- R120: 2020-01-13 pre-Prague mailing
  - Summary:
    - 380 open issues, up by 4.
    - 2472 closed issues, up by 7.
    - 35 reassigned issues, up by 0.
    - 2887 issues total, up by 11.
  - Details:
    - Added the following 2 Tentatively Ready issues: 3356, 3360.
    - Added the following 8 New issues: 3355, 3358, 3359, 3361, 3362, 3363, 3364, 3365.
    - Added the following LEWG issue: 3357.
    - Changed the following 2 issues to Ready (from New): 3280, 3292.
    - Changed the following issue to Ready (from LEWG): 3304.
    - Changed the following 5 issues to Tentatively Ready (from New): 3194, 3254, 3349, 3350, 3351.
    - Changed the following issue to LEWG (from New): 3354.
    - Changed the following issue to Tentatively Resolved (from Ready): 3322.
    - Changed the following 3 issues to Resolved (from New): 2957, 3023, 3176.
    - Changed the following 3 issues to Resolved (from Open): 2894, 3061, 3295.
    - Changed the following issue to Resolved (from EWG): 2089.
- R119: 2019-12-09 post-Belfast mailing
  - Summary:
    - 376 open issues, up by 25.
    - 2465 closed issues, up by 32.
    - 35 reassigned issues, down by 1.
    - 2876 issues total, up by 56.
  - Details:
    - Added the following 21 Ready issues: 3302, 3303, 3307, 3310, 3313, 3315, 3316, 3317, 3318, 3319, 3320, 3321, 3322, 3323, 3324, 3325, 3326, 3327, 3329, 3330, 3331.
    - Added the following 5 Tentatively Ready issues: 3299, 3300, 3332, 3338, 3346.
    - Added the following 27 New issues: 3301, 3305, 3306, 3308, 3309, 3314, 3328, 3333, 3334, 3335, 3336, 3337, 3339, 3340, 3341, 3342, 3343, 3344, 3345, 3347, 3348, 3349, 3350, 3351, 3352, 3353, 3354.
    - Added the following LEWG issue: 3304.
    - Added the following 2 Dup issues: 3311, 3312.
    - Changed the following issue to Ready (from New): 3264.
    - Changed the following issue to Ready (from LEWG): 3281.
    - Changed the following 7 issues to Tentatively Ready (from New): 3233, 3284, 3285, 3286, 3291, 3294, 3296.
    - Changed the following issue to Open (from New): 3295.
    - Changed the following issue to Open (from LEWG): 3282.
    - Changed the following issue to LEWG (from New): 3228.
    - Changed the following 4 issues to WP (from Ready): 3070, 3103, 3149, 3190.
    - Changed the following 23 issues to WP (from Tentatively Ready): 3218, 3221, 3222, 3224, 3225, 3230, 3231, 3232, 3235, 3241, 3244, 3245, 3246, 3253, 3256, 3257, 3259, 3266, 3272, 3273, 3274, 3276, 3277.
    - Changed the following 2 issues to Resolved (from New): 3239, 3279.
    - Changed the following issue to Resolved (from SG1): 2334.
- R118: 2019-10-06 pre-Belfast mailing
  - Summary:
    - 351 open issues, up by 48.
    - 2433 closed issues, up by 0.
    - 36 reassigned issues, up by 2.
    - 2820 issues total, up by 50.
  - Details:
    - Added the following 10 Tentatively Ready issues: 3253, 3256, 3257, 3259, 3266, 3272, 3273, 3274, 3276, 3277.
    - Added the following 38 New issues: 3249, 3250, 3251, 3252, 3254, 3255, 3258, 3260, 3261, 3262, 3263, 3264, 3265, 3267, 3268, 3269, 3270, 3271, 3275, 3278, 3279, 3280, 3283, 3284, 3285, 3286, 3287, 3288, 3289, 3290, 3291, 3292, 3293, 3294, 3295, 3296, 3297, 3298.
    - Added the following 2 LEWG issues: 3281, 3282.
    - Changed the following 5 issues to Tentatively Ready (from New): 3235, 3241, 3244, 3245, 3246.
- R117: 2019-08-05 post-Cologne mailing
  - Summary:
    - 303 open issues, up by 8.
    - 2433 closed issues, up by 20.
    - 34 reassigned issues, down by 1.
    - 2770 issues total, up by 27.
  - Details:
    - Added the following 6 Tentatively Ready issues: 3222, 3224, 3225, 3230, 3231, 3232.
    - Added the following 21 New issues: 3223, 3226, 3227, 3228, 3229, 3233, 3234, 3235, 3236, 3237, 3238, 3239, 3240, 3241, 3242, 3243, 3244, 3245, 3246, 3247, 3248.
    - Changed the following 2 issues to Ready (from New): 3070, 3190.
    - Changed the following 2 issues to Ready (from Open): 3103, 3149.
    - Changed the following 2 issues to Tentatively Ready (from New): 3218, 3221.
    - Changed the following 2 issues to Open (from New): 2818, 3170.
    - Changed the following 6 issues to WP (from Ready): 2899, 3055, 3158, 3169, 3186, 3187.
    - Changed the following 11 issues to WP (from Tentatively Ready): 3183, 3184, 3185, 3191, 3196, 3198, 3199, 3202, 3206, 3208, 3209.
    - Changed the following issue to Resolved (from New): 3091.
    - Changed the following issue to NAD (from New): 3139.
    - Changed the following issue to NAD (from LEWG): 3138.
- R116: 2019-06-17 pre-Cologne mailing
  - Summary:
    - 295 open issues, up by 25.
    - 2413 closed issues, up by 0.
    - 35 reassigned issues, up by 0.
    - 2743 issues total, up by 25.
  - Details:
    - Added the following 6 Tentatively Ready issues: 3198, 3199, 3202, 3206, 3208, 3209.
    - Added the following Tentatively NAD issue: 3207.
    - Added the following 18 New issues: 3197, 3200, 3201, 3203, 3204, 3205, 3210, 3211, 3212, 3213, 3214, 3215, 3216, 3217, 3218, 3219, 3220, 3221.
    - Changed the following 2 issues to Tentatively Ready (from New): 3191, 3196.
- R115: 2019-04-02 post-Kona mailing
  - Summary:
    - 270 open issues, down by 19.
    - 2413 closed issues, up by 31.
    - 35 reassigned issues, up by 0.
    - 2718 issues total, up by 12.
  - Details:
    - Added the following 2 Ready issues: 3186, 3187.
    - Added the following Tentatively Ready issue: 3185.
    - Added the following 9 New issues: 3188, 3189, 3190, 3191, 3192, 3193, 3194, 3195, 3196.
    - Changed the following 3 issues to Ready (from New): 3055, 3158, 3169.
    - Changed the following issue to Ready (from Open): 2899.
    - Changed the following 2 issues to Tentatively Ready (from New): 3183, 3184.
    - Changed the following 4 issues to Open (from New): 3036, 3161, 3168, 3171.
    - Changed the following 5 issues to Tentatively Resolved (from New): 2292, 2808, 2832, 2999, 3178.
    - Changed the following issue to Tentatively Resolved (from Open): 2155.
    - Changed the following 2 issues to WP (from Ready): 3012, 3119.
    - Changed the following 11 issues to WP (from Tentatively Ready): 3040, 3077, 3087, 3101, 3112, 3133, 3144, 3173, 3179, 3180, 3182.
    - Changed the following 2 issues to Resolved (from New): 2951, 3113.
    - Changed the following 16 issues to NAD (from Tentatively NAD): 708, 935, 1121, 1154, 1188, 1217, 1235, 1282, 1289, 1317, 1406, 1499, 2226, 2417, 2600, 3106.
- R114: 2019-01-21 pre-Kona mailing
  - Summary:
    - 289 open issues, up by 14.
    - 2382 closed issues, up by 1.
    - 35 reassigned issues, up by 0.
    - 2706 issues total, up by 15.
  - Details:
    - Added the following 4 Tentatively Ready issues: 3173, 3179, 3180, 3182.
    - Added the following 10 New issues: 3170, 3171, 3172, 3174, 3175, 3176, 3177, 3178, 3183, 3184.
    - Added the following NAD issue: 3181.
    - Changed the following 4 issues to Tentatively Ready (from New): 3077, 3112, 3133, 3144.
- R113: 2018-11-26 post-San Diego mailing
  - Summary:
    - 275 open issues, down by 45.
    - 2381 closed issues, up by 52.
    - 35 reassigned issues, up by 0.
    - 2691 issues total, up by 7.
  - Details:
    - Added the following 4 New issues: 3166, 3167, 3168, 3169.
    - Added the following LEWG issue: 3165.
    - Added the following 2 NAD issues: 3163, 3164.
    - Changed the following issue to Ready (from New): 3119.
    - Changed the following issue to Ready (from Open): 3012.
    - Changed the following issue to Tentatively Ready (from New): 3087.
    - Changed the following issue to Tentatively Ready (from Open): 3040.
    - Changed the following issue to Tentatively Ready (from LEWG): 3101.
    - Changed the following 7 issues to Open (from New): 2962, 2994, 3057, 3061, 3103, 3121, 3149.
    - Changed the following 2 issues to Open (from Core): 2859, 2860.
    - Changed the following issue to LEWG (from New): 3138.
    - Changed the following issue to LEWG (from Open): 2307.
    - Changed the following 10 issues to WP (from Ready): 2183, 2184, 2412, 2682, 2697, 2936, 2995, 2996, 3054, 3116.
    - Changed the following 24 issues to WP (from Tentatively Ready): 2943, 2960, 3008, 3025, 3031, 3037, 3038, 3065, 3096, 3122, 3127, 3128, 3129, 3130, 3131, 3132, 3137, 3140, 3145, 3147, 3148, 3153, 3154, 3160.
    - Changed the following issue to Resolved (from Tentatively Ready): 3134.
    - Changed the following 6 issues to Resolved (from New): 2318, 2836, 2841, 2929, 3022, 3115.
    - Changed the following 6 issues to Resolved (from Open): 1052, 2151, 2499, 2780, 2797, 3111.
    - Changed the following 2 issues to NAD (from New): 2563, 3016.
    - Changed the following issue to NAD (from Open): 1242.
- R112: 2018-10-08 pre-San Diego mailing
  - Summary:
    - 320 open issues, up by 36.
    - 2329 closed issues, up by 3.
    - 35 reassigned issues, up by 0.
    - 2684 issues total, up by 39.
  - Details:
    - Added the following 15 Tentatively Ready issues: 3127, 3128, 3129, 3130, 3131, 3132, 3134, 3137, 3140, 3145, 3147, 3148, 3153, 3154, 3160.
    - Added the following 22 New issues: 3124, 3126, 3133, 3135, 3136, 3138, 3139, 3141, 3142, 3143, 3144, 3146, 3149, 3150, 3151, 3152, 3156, 3157, 3158, 3159, 3161, 3162.
    - Added the following 2 Open issues: 3125, 3155.
    - Changed the following 6 issues to Tentatively Ready (from New): 2960, 3025, 3031, 3037, 3065, 3096.
    - Changed the following 3 issues to Tentatively Ready (from Open): 2943, 3008, 3038.
    - Changed the following issue to Tentatively NAD (from New): 3106.
    - Changed the following 5 issues to Open (from New): 2220, 3081, 3085, 3092, 3120.
    - Changed the following issue to EWG (from Open): 2813.
    - Changed the following issue to Resolved (from Tentatively NAD): 1150.
    - Changed the following issue to Resolved (from Open): 2840.
    - Changed the following issue to Resolved (from LEWG): 1201.
- R111: 2018-06-25 post-Rapperswil mailing
  - Summary:
    - 284 open issues, up by 15.
    - 2326 closed issues, up by 25.
    - 35 reassigned issues, down by 26.
    - 2645 issues total, up by 14.
  - Details:
    - Added the following Ready issue: 3116.
    - Added the following Tentatively Ready issue: 3122.
    - Added the following 10 New issues: 3110, 3112, 3113, 3115, 3117, 3118, 3119, 3120, 3121, 3123.
    - Added the following Open issue: 3111.
    - Added the following LEWG issue: 3114.
    - Changed the following 4 issues to Ready (from Review): 2412, 2682, 2697, 2936.
    - Changed the following 4 issues to Ready (from New): 2183, 2184, 2995, 3054.
    - Changed the following issue to Ready (from LEWG): 2996.
    - Changed the following 16 issues to Tentatively NAD (from LEWG): 708, 935, 1121, 1150, 1154, 1188, 1217, 1235, 1282, 1289, 1317, 1406, 1499, 2226, 2417, 2600.
    - Changed the following issue to Open (from Review): 2708.
    - Changed the following 7 issues to Open (from New): 2173, 2286, 2829, 3008, 3038, 3050, 3082.
    - Changed the following 11 issues to Open (from LEWG): 423, 523, 1052, 1203, 1238, 1242, 1396, 1521, 2471, 2762, 2780.
    - Changed the following 3 issues to LEWG (from New): 3052, 3068, 3101.
    - Changed the following 3 issues to WP (from Ready): 2139, 3058, 3076.
    - Changed the following 12 issues to WP (from Tentatively Ready): 2970, 3062, 3067, 3071, 3074, 3079, 3080, 3083, 3094, 3100, 3102, 3104.
    - Changed the following 4 issues to Resolved (from New): 2511, 2693, 2776, 2938.
    - Changed the following 2 issues to Resolved (from Open): 2800, 2897.
    - Changed the following 2 issues to Resolved (from LEWG): 2040, 2419.
    - Changed the following 2 issues to NAD (from New): 2121, 2892.
- R110: 2018-05-07 pre-Rapperswil mailing
  - Summary:
    - 269 open issues, up by 17.
    - 2301 closed issues, up by 0.
    - 61 reassigned issues, up by 0.
    - 2631 issues total, up by 17.
  - Details:
    - Added the following 4 Tentatively Ready issues: 3094, 3100, 3102, 3104.
    - Added the following 13 New issues: 3093, 3095, 3096, 3097, 3098, 3099, 3101, 3103, 3105, 3106, 3107, 3108, 3109.
    - No issues changed.
- R109: 2018-04-02 post-Jacksonville mailing
  - Summary:
    - 252 open issues, down by 5.
    - 2301 closed issues, up by 36.
    - 61 reassigned issues, down by 1.
    - 2614 issues total, up by 30.
  - Details:
    - Added the following Ready issue: 3076.
    - Added the following 6 Tentatively Ready issues: 3067, 3071, 3074, 3079, 3080, 3083.
    - Added the following 22 New issues: 3063, 3064, 3065, 3066, 3068, 3069, 3070, 3072, 3073, 3077, 3078, 3081, 3082, 3084, 3085, 3086, 3087, 3088, 3089, 3090, 3091, 3092.
    - Added the following WP issue: 3075.
    - Changed the following issue to Ready (from New): 3058.
    - Changed the following issue to Ready (from Open): 2139.
    - Changed the following 2 issues to Tentatively Ready (from New): 2970, 3062.
    - Changed the following 2 issues to Open (from New): 2823, 3049.
    - Changed the following issue to SG1 (from New): 2506.
    - Changed the following 5 issues to WP (from Ready): 2843, 2969, 2975, 3014, 3030.
    - Changed the following 27 issues to WP (from Tentatively Ready): 2164, 2243, 2816, 2849, 2851, 2989, 3000, 3002, 3004, 3005, 3007, 3009, 3010, 3013, 3015, 3017, 3020, 3026, 3034, 3035, 3039, 3041, 3042, 3043, 3045, 3048, 3051.
    - Changed the following issue to WP (from New): 2946.
    - Changed the following 2 issues to Resolved (from LEWG): 2831, 2968.
- R108: 2018-02-12 pre-Jacksonville mailing
  - Summary:
    - 257 open issues, up by 16.
    - 2265 closed issues, up by 7.
    - 62 reassigned issues, up by 0.
    - 2584 issues total, up by 23.
  - Details:
    - Added the following 6 Tentatively Ready issues: 3041, 3042, 3043, 3045, 3048, 3051.
    - Added the following 16 New issues: 3044, 3046, 3047, 3049, 3050, 3052, 3053, 3054, 3055, 3056, 3057, 3058, 3059, 3060, 3061, 3062.
    - Added the following Open issue: 3040.
    - Changed the following 6 issues to Tentatively Ready (from New): 2243, 2816, 2849, 2851, 2989, 3015.
    - Changed the following issue to Tentatively Ready (from Open): 2164.
    - Changed the following 3 issues to Review (from Open): 2682, 2708, 2936.
    - Changed the following 4 issues to Open (from New): 2839, 2840, 2990, 3003.
    - Changed the following 2 issues to C++17 (from Open): 2569, 2587.
    - Changed the following 3 issues to Resolved (from New): 2734, 2821, 2856.
    - Changed the following issue to Resolved (from Open): 2055.
    - Changed the following issue to NAD (from New): 2772.
- R107: 2017-11-28 2017 post-Albuquerque mailing
  - Summary:
    - 241 open issues, down by 16.
    - 2258 closed issues, up by 29.
    - 62 reassigned issues, up by 0.
    - 2561 issues total, up by 13.
  - Details:
    - Added the following Ready issue: 3030.
    - Added the following 3 Tentatively Ready issues: 3034, 3035, 3039.
    - Added the following 7 New issues: 3027, 3028, 3031, 3032, 3036, 3037, 3038.
    - Added the following Open issue: 3029.
    - Added the following NAD Editorial issue: 3033.
    - Changed the following 4 issues to Ready (from New): 2843, 2969, 2975, 3014.
    - Changed the following 11 issues to Tentatively Ready (from New): 3000, 3002, 3004, 3005, 3007, 3009, 3010, 3013, 3017, 3020, 3026.
    - Changed the following 6 issues to Open (from New): 2800, 2833, 2897, 2943, 3011, 3012.
    - Changed the following 23 issues to WP (from Ready): 2779, 2870, 2935, 2941, 2944, 2945, 2948, 2950, 2952, 2953, 2964, 2965, 2972, 2976, 2977, 2978, 2979, 2980, 2981, 2982, 2988, 2993, 2998.
    - Changed the following 2 issues to WP (from Tentatively Ready): 3001, 3024.
    - Changed the following issue to WP (from New): 2958.
    - Changed the following 2 issues to Resolved (from New): 2541, 2799.
- R106: 2017-10-16 2017 pre-Albuquerque mailing
  - Summary:
    - 257 open issues, up by 22.
    - 2229 closed issues, up by 0.
    - 62 reassigned issues, up by 0.
    - 2548 issues total, up by 22.
  - Details:
    - Added the following Tentatively Ready issue: 3024.
    - Added the following 21 New issues: 3005, 3006, 3007, 3008, 3009, 3010, 3011, 3012, 3013, 3014, 3015, 3016, 3017, 3018, 3019, 3020, 3021, 3022, 3023, 3025, 3026.
    - No issues changed.
- R105: 2017-07-30 2017 post-Toronto mailing
  - Summary:
    - 235 open issues, down by 22.
    - 2229 closed issues, up by 36.
    - 62 reassigned issues, up by 8.
    - 2526 issues total, up by 22.
  - Details:
    - Added the following 3 Ready issues: 2988, 2993, 2998.
    - Added the following Tentatively Ready issue: 3001.
    - Added the following 14 New issues: 2983, 2984, 2986, 2987, 2989, 2990, 2994, 2995, 2997, 2999, 3000, 3002, 3003, 3004.
    - Added the following 3 LEWG issues: 2985, 2991, 2996.
    - Added the following NAD issue: 2992.
    - Changed the following 19 issues to Ready (from New): 2870, 2935, 2941, 2944, 2945, 2948, 2950, 2952, 2953, 2964, 2965, 2972, 2976, 2977, 2978, 2979, 2980, 2981, 2982.
    - Changed the following issue to Ready (from LEWG): 2779.
    - Changed the following 6 issues to Open (from New): 2731, 2741, 2813, 2820, 2899, 2936.
    - Changed the following 6 issues to LEWG (from New): 2883, 2884, 2885, 2922, 2968, 2973.
    - Changed the following issue to WP (from Ready): 2932.
    - Changed the following 11 issues to WP (from Tentatively Ready): 2444, 2593, 2597, 2783, 2937, 2940, 2942, 2954, 2961, 2966, 2974.
    - Changed the following 2 issues to C++17 (from New): 2901, 2956.
    - Changed the following 279 issues to C++17 (from WP): 1169, 2016, 2059, 2062, 2063, 2072, 2076, 2101, 2106, 2111, 2119, 2127, 2129, 2133, 2156, 2160, 2166, 2168, 2170, 2181, 2192, 2212, 2217, 2218, 2219, 2221, 2223, 2224, 2230, 2233, 2234, 2239, 2244, 2250, 2259, 2260, 2261, 2266, 2273, 2276, 2296, 2309, 2310, 2312, 2325, 2328, 2336, 2340, 2353, 2354, 2361, 2364, 2365, 2367, 2369, 2376, 2377, 2378, 2380, 2384, 2385, 2387, 2393, 2394, 2396, 2399, 2400, 2401, 2403, 2404, 2406, 2407, 2408, 2411, 2415, 2420, 2422, 2425, 2426, 2427, 2428, 2433, 2434, 2435, 2436, 2437, 2438, 2439, 2440, 2441, 2442, 2447, 2448, 2450, 2454, 2455, 2458, 2459, 2460, 2462, 2464, 2466, 2467, 2468, 2469, 2470, 2473, 2475, 2476, 2477, 2482, 2483, 2484, 2485, 2486, 2487, 2488, 2489, 2492, 2495, 2503, 2510, 2514, 2519, 2520, 2523, 2531, 2534, 2536, 2537, 2540, 2542, 2544, 2545, 2549, 2550, 2556, 2557, 2559, 2560, 2562, 2565, 2566, 2567, 2571, 2572, 2576, 2577, 2578, 2579, 2581, 2582, 2583, 2584, 2585, 2586, 2589, 2590, 2591, 2596, 2598, 2664, 2667, 2669, 2670, 2671, 2672, 2673, 2674, 2676, 2678, 2679, 2680, 2681, 2683, 2684, 2685, 2686, 2687, 2688, 2689, 2694, 2696, 2698, 2699, 2704, 2706, 2707, 2709, 2710, 2711, 2712, 2716, 2718, 2719, 2720, 2721, 2722, 2723, 2724, 2725, 2726, 2727, 2728, 2729, 2732, 2735, 2736, 2738, 2739, 2740, 2742, 2744, 2747, 2748, 2749, 2752, 2755, 2756, 2758, 2759, 2760, 2765, 2767, 2768, 2769, 2770, 2771, 2773, 2777, 2778, 2781, 2782, 2784, 2785, 2786, 2787, 2788, 2789, 2790, 2793, 2794, 2795, 2796, 2801, 2802, 2804, 2806, 2807, 2812, 2824, 2826, 2834, 2835, 2837, 2838, 2842, 2850, 2853, 2855, 2857, 2861, 2866, 2868, 2872, 2873, 2874, 2875, 2876, 2878, 2890, 2900, 2903, 2904, 2905, 2908, 2911, 2921, 2934.
    - Changed the following 2 issues to Resolved (from New): 2880, 2955.
    - Changed the following 3 issues to Resolved (from Open): 2070, 2368, 2665.
    - Changed the following issue to Resolved (from Pending Resolved): 2798.
    - Changed the following 4 issues to NAD (from Tentatively NAD): 760, 2337, 2692, 2717.
    - Changed the following 11 issues to NAD (from New): 2852, 2871, 2886, 2891, 2893, 2902, 2907, 2909, 2916, 2967, 2971.
    - Changed the following issue to Dup (from New): 2910.
    - Changed the following 77 issues to TS (from WP): 2371, 2374, 2389, 2390, 2395, 2409, 2410, 2418, 2451, 2463, 2494, 2500, 2509, 2515, 2516, 2517, 2518, 2521, 2522, 2525, 2526, 2527, 2539, 2551, 2555, 2558, 2568, 2570, 2573, 2574, 2575, 2588, 2601, 2602, 2603, 2605, 2606, 2607, 2608, 2609, 2614, 2615, 2616, 2618, 2619, 2621, 2622, 2624, 2625, 2627, 2629, 2632, 2633, 2634, 2635, 2636, 2637, 2640, 2641, 2644, 2645, 2647, 2648, 2649, 2650, 2652, 2653, 2655, 2656, 2657, 2658, 2660, 2662, 2733, 2745, 2750, 2792.
- R104: 2017-06-18 2017 pre-Toronto mailing
  - Summary:
    - 257 open issues, up by 38.
    - 2193 closed issues, up by 17.
    - 54 reassigned issues, down by 19.
    - 2504 issues total, up by 36.
  - Details:
    - Added the following 4 Tentatively Ready issues: 2954, 2961, 2966, 2974.
    - Added the following 32 New issues: 2947, 2948, 2949, 2950, 2951, 2952, 2953, 2955, 2956, 2957, 2958, 2959, 2960, 2962, 2963, 2964, 2965, 2967, 2968, 2969, 2970, 2971, 2972, 2973, 2975, 2976, 2977, 2978, 2979, 2980, 2981, 2982.
    - Changed the following 2 issues to Tentatively Ready (from New): 2940, 2942.
    - Changed the following issue to Tentatively Ready (from LEWG): 2593.
    - Changed the following issue to Open (from New): 2797.
    - Changed the following 3 issues to Open (from LEWG): 484, 1422, 2055.
    - Changed the following issue to Pending Resolved (from Open): 2798.
    - Changed the following issue to Resolved (from LEWG): 2232.
    - Changed the following issue to NAD (from New): 2898.
    - Changed the following 14 issues to NAD (from LEWG): 255, 851, 877, 933, 1031, 1053, 1112, 1120, 1184, 1320, 2242, 2430, 2446, 2854.
- R103: 2017-03-20 2017 post-Kona mailing
  - Summary:
    - 219 open issues, down by 86.
    - 2176 closed issues, up by 104.
    - 73 reassigned issues, up by 0.
    - 2468 issues total, up by 18.
  - Details:
    - Added the following Ready issue: 2932.
    - Added the following Tentatively Ready issue: 2937.
    - Added the following 13 New issues: 2929, 2933, 2935, 2936, 2938, 2939, 2940, 2941, 2942, 2943, 2944, 2945, 2946.
    - Added the following Open issue: 2931.
    - Added the following WP issue: 2934.
    - Added the following NAD issue: 2930.
    - Changed the following issue to Tentatively Ready (from New): 2783.
    - Changed the following 2 issues to Tentatively Ready (from Open): 2444, 2597.
    - Changed the following issue to Review (from SG1): 2412.
    - Changed the following issue to Open (from New): 2894.
    - Changed the following 2 issues to Core (from New): 2859, 2860.
    - Changed the following 6 issues to WP (from Ready): 2781, 2782, 2784, 2785, 2786, 2787.
    - Changed the following 18 issues to WP (from Tentatively Ready): 2260, 2768, 2769, 2789, 2794, 2795, 2804, 2812, 2824, 2826, 2834, 2835, 2837, 2838, 2842, 2850, 2853, 2855.
    - Changed the following 3 issues to WP (from Review): 2676, 2790, 2796.
    - Changed the following 22 issues to WP (from New): 2788, 2801, 2802, 2806, 2807, 2861, 2866, 2868, 2872, 2873, 2874, 2875, 2876, 2878, 2890, 2900, 2903, 2904, 2905, 2908, 2911, 2921.
    - Changed the following issue to WP (from LEWG): 2857.
    - Changed the following issue to Resolved (from Review): 2245.
    - Changed the following 27 issues to Resolved (from New): 2715, 2803, 2862, 2863, 2864, 2867, 2869, 2877, 2879, 2882, 2887, 2888, 2889, 2895, 2912, 2913, 2914, 2915, 2917, 2918, 2919, 2920, 2924, 2925, 2926, 2927, 2928.
    - Changed the following 19 issues to Resolved (from Tentatively Resolved): 839, 1041, 2179, 2208, 2241, 2294, 2343, 2370, 2391, 2424, 2443, 2501, 2502, 2505, 2529, 2548, 2663, 2677, 2757.
    - Changed the following 3 issues to NAD (from New): 2256, 2449, 2865.
    - Changed the following 2 issues to Dup (from New): 2764, 2896.
- R102: 2017-02-06 2017 pre-Kona mailing
  - Summary:
    - 305 open issues, up by 89.
    - 2072 closed issues, up by 9.
    - 73 reassigned issues, up by 5.
    - 2450 issues total, up by 103.
  - Details:
    - Added the following 9 Tentatively Ready issues: 2826, 2834, 2835, 2837, 2838, 2842, 2850, 2853, 2855.
    - Added the following 89 New issues: 2827, 2829, 2832, 2833, 2836, 2839, 2840, 2841, 2843, 2844, 2845, 2846, 2847, 2848, 2849, 2851, 2852, 2856, 2858, 2859, 2860, 2861, 2862, 2863, 2864, 2865, 2866, 2867, 2868, 2869, 2870, 2871, 2872, 2873, 2874, 2875, 2876, 2877, 2878, 2879, 2880, 2881, 2882, 2883, 2884, 2885, 2886, 2887, 2888, 2889, 2890, 2891, 2892, 2893, 2894, 2895, 2896, 2897, 2898, 2899, 2900, 2901, 2902, 2903, 2904, 2905, 2906, 2907, 2908, 2909, 2910, 2911, 2912, 2913, 2914, 2915, 2916, 2917, 2918, 2919, 2920, 2921, 2922, 2923, 2924, 2925, 2926, 2927, 2928.
    - Added the following 3 LEWG issues: 2831, 2854, 2857.
    - Added the following Resolved issue: 2830.
    - Added the following NAD Editorial issue: 2828.
    - Changed the following 5 issues to Tentatively Ready (from New): 2789, 2795, 2804, 2812, 2824.
    - Changed the following 2 issues to Tentatively Ready (from Open): 2768, 2769.
    - Changed the following 2 issues to Review (from New): 2790, 2796.
    - Changed the following 2 issues to LEWG (from New): 2814, 2825.
    - Changed the following 2 issues to WP (from New): 2792, 2793.
    - Changed the following 3 issues to Resolved (from New): 2805, 2809, 2810.
    - Changed the following issue to Resolved (from Open): 2754.
    - Changed the following issue to NAD (from New): 2822.
- R101: 2016-11-28 2016 post-Issaquah mailing
  - Summary:
    - 216 open issues, down by 44.
    - 2063 closed issues, up by 85.
    - 68 reassigned issues, up by 0.
    - 2347 issues total, up by 41.
  - Details:
    - Added the following 3 Ready issues: 2785, 2786, 2787.
    - Added the following Tentatively Ready issue: 2794.
    - Added the following 34 New issues: 2788, 2789, 2790, 2792, 2793, 2795, 2796, 2797, 2799, 2800, 2801, 2802, 2803, 2804, 2805, 2806, 2807, 2808, 2809, 2810, 2811, 2812, 2813, 2814, 2815, 2816, 2818, 2819, 2820, 2821, 2822, 2823, 2824, 2825.
    - Added the following Open issue: 2798.
    - Added the following 2 Resolved issues: 2791, 2817.
    - Changed the following 3 issues to Ready (from New): 2781, 2782, 2784.
    - Changed the following issue to Tentatively Ready (from Open): 2260.
    - Changed the following issue to Review (from New): 2697.
    - Changed the following 3 issues to Open (from Tentatively Ready): 2569, 2754, 2768.
    - Changed the following 2 issues to Open (from New): 2597, 2730.
    - Changed the following issue to LEWG (from New): 2780.
    - Changed the following issue to Pending NAD Editorial (from Open): 2178.
    - Changed the following 66 issues to WP (from Tentatively Ready): 2062, 2166, 2221, 2223, 2261, 2394, 2460, 2468, 2475, 2503, 2510, 2514, 2519, 2531, 2534, 2536, 2540, 2544, 2556, 2562, 2567, 2570, 2578, 2584, 2589, 2591, 2598, 2664, 2672, 2678, 2679, 2680, 2681, 2686, 2694, 2696, 2699, 2712, 2722, 2729, 2732, 2733, 2735, 2736, 2738, 2739, 2740, 2742, 2744, 2745, 2747, 2748, 2749, 2750, 2752, 2755, 2756, 2758, 2759, 2760, 2765, 2767, 2771, 2773, 2777, 2778.
    - Changed the following 4 issues to WP (from New): 2518, 2521, 2525, 2527.
    - Changed the following 3 issues to WP (from Open): 2568, 2588, 2770.
    - Changed the following 2 issues to Resolved (from Tentatively Ready): 2543, 2753.
    - Changed the following issue to Resolved (from New): 2763.
    - Changed the following issue to Resolved (from Open): 2465.
    - Changed the following issue to Resolved (from SG1): 2445.
    - Changed the following issue to NAD Editorial (from New): 2201.
    - Changed the following issue to NAD (from New): 2668.
    - Changed the following 2 issues to NAD (from Open): 1173, 2199.
- R100: 2016-10-17 2016 pre-Issaquah mailing
  - Summary:
    - 260 open issues, up by 31.
    - 1978 closed issues, up by 5.
    - 68 reassigned issues, up by 5.
    - 2306 issues total, up by 41.
  - Details:
    - Added the following 21 Tentatively Ready issues: 2744, 2745, 2747, 2748, 2749, 2750, 2752, 2753, 2754, 2755, 2756, 2758, 2759, 2760, 2765, 2767, 2768, 2771, 2773, 2777, 2778.
    - Added the following 13 New issues: 2746, 2751, 2763, 2764, 2766, 2772, 2774, 2776, 2780, 2781, 2782, 2783, 2784.
    - Added the following 2 Open issues: 2769, 2770.
    - Added the following 2 LEWG issues: 2762, 2779.
    - Added the following Tentatively Resolved issue: 2757.
    - Added the following NAD issue: 2761.
    - Added the following Dup issue: 2775.
    - Changed the following 45 issues to Tentatively Ready (from New): 2166, 2221, 2261, 2394, 2460, 2475, 2503, 2514, 2519, 2531, 2534, 2536, 2540, 2544, 2556, 2562, 2567, 2569, 2570, 2578, 2584, 2589, 2591, 2598, 2664, 2672, 2678, 2679, 2680, 2681, 2686, 2694, 2696, 2699, 2712, 2722, 2729, 2732, 2733, 2735, 2736, 2738, 2739, 2740, 2742.
    - Changed the following 5 issues to Tentatively Ready (from Open): 2062, 2223, 2468, 2510, 2543.
    - Changed the following issue to Review (from New): 2676.
    - Changed the following issue to Review (from Open): 2245.
    - Changed the following 13 issues to Open (from New): 2158, 2358, 2381, 2465, 2512, 2530, 2532, 2568, 2587, 2588, 2665, 2682, 2708.
    - Changed the following 3 issues to LEWG (from New): 2242, 2471, 2593.
    - Changed the following 3 issues to LEWG (from Open): 2095, 2152, 2153.
    - Changed the following 6 issues to Tentatively Resolved (from New): 2343, 2501, 2502, 2548, 2663, 2677.
    - Changed the following 4 issues to Tentatively Resolved (from Open): 2294, 2370, 2424, 2505.
    - Changed the following 3 issues to Tentatively Resolved (from LEWG): 839, 1041, 2443.
    - Changed the following issue to NAD Editorial (from New): 2701.
    - Changed the following 2 issues to NAD (from New): 2161, 2535.
- R99: 2016-07-11 2016 post-Oulu mailing
  - Summary:
    - 292 open issues, down by 26.
    - 1973 closed issues, up by 51.
    - 2265 issues total, up by 25.
  - Details:
    - Added the following 16 New issues: 2722, 2729, 2730, 2731, 2732, 2733, 2734, 2735, 2736, 2737, 2738, 2739, 2740, 2741, 2742, 2743.
    - Added the following 9 WP issues: 2719, 2720, 2721, 2723, 2724, 2725, 2726, 2727, 2728.
    - Changed the following issue to Tentatively NAD (from New): 2717.
    - Changed the following issue to Tentatively Resolved (from Open): 2241.
    - Changed the following 21 issues to WP (from Ready): 2181, 2309, 2310, 2328, 2393, 2426, 2436, 2441, 2451, 2516, 2542, 2549, 2550, 2551, 2555, 2573, 2667, 2669, 2670, 2671, 2673.
    - Changed the following 12 issues to WP (from Tentatively Ready): 2509, 2596, 2674, 2683, 2684, 2685, 2688, 2689, 2698, 2706, 2707, 2710.
    - Changed the following 6 issues to WP (from New): 2687, 2704, 2709, 2711, 2716, 2718.
    - Changed the following 2 issues to WP (from Open): 2312, 2422.
    - Changed the following issue to NAD (from New): 2700.
- R98: 2016-05-29 2016 pre-Oulu mailing
  - Summary:
    - 318 open issues, up by 50.
    - 1922 closed issues, up by 0.
    - 2240 issues total, up by 50.
  - Details:
    - Added the following 10 Tentatively Ready issues: 2596, 2683, 2684, 2685, 2688, 2689, 2698, 2706, 2707, 2710.
    - Added the following Tentatively NAD issue: 2692.
    - Added the following 37 New issues: 2595, 2597, 2598, 2599, 2675, 2676, 2677, 2678, 2679, 2680, 2681, 2682, 2686, 2687, 2691, 2693, 2694, 2695, 2696, 2697, 2699, 2700, 2701, 2702, 2703, 2704, 2705, 2708, 2709, 2711, 2712, 2713, 2714, 2715, 2716, 2717, 2718.
    - Added the following 2 LEWG issues: 2600, 2690.
    - Changed the following issue to Tentatively Ready (from New): 2674.
    - Changed the following issue to Tentatively Ready (from Open): 2509.
    - Changed the following 2 issues to Tentatively Resolved (from New): 2208, 2529.
    - Changed the following issue to Tentatively Resolved (from Open): 2179.
    - Changed the following issue to Tentatively Resolved (from LEWG): 2391.
- R97: 2016-03-22 2016 post-Jacksonville mailing
  - Summary:
    - 268 open issues, down by 37.
    - 1922 closed issues, up by 40.
    - 2190 issues total, up by 3.
  - Details:
    - Added the following 3 New issues: 2592, 2593, 2594.
    - Changed the following 7 issues to Ready (from Review): 2181, 2309, 2310, 2328, 2393, 2441, 2516.
    - Changed the following 11 issues to Ready (from New): 2542, 2549, 2550, 2551, 2555, 2573, 2667, 2669, 2670, 2671, 2673.
    - Changed the following issue to Ready (from Open): 2426.
    - Changed the following 2 issues to Ready (from LEWG): 2436, 2451.
    - Changed the following issue to Open (from Review): 2424.
    - Changed the following issue to Open (from New): 2368.
    - Changed the following 3 issues to WP (from Ready): 2276, 2523, 2537.
    - Changed the following 25 issues to WP (from Tentatively Ready): 2192, 2450, 2520, 2522, 2539, 2545, 2557, 2558, 2559, 2560, 2565, 2566, 2571, 2572, 2574, 2575, 2576, 2577, 2579, 2581, 2582, 2583, 2585, 2586, 2590.
    - Changed the following issue to WP (from Review): 2296.
    - Changed the following issue to Resolved (from New): 2554.
    - Changed the following issue to Resolved (from Open): 2456.
    - Changed the following issue to NAD Editorial (from New): 2666.
    - Changed the following issue to NAD (from Review): 2402.
    - Changed the following issue to NAD (from New): 2553.
    - Changed the following issue to NAD (from LEWG): 2372.
    - Changed the following 2 issues to NAD Arrays (from Ready): 2253, 2255.
    - Changed the following 3 issues to NAD Arrays (from Open): 2254, 2264, 2277.
- R96: 2016-02-12 2016 pre-Jacksonville mailing (includes the FS TS bugs for the first time)
  - Summary:
    - 305 open issues, up by 48.
    - 1882 closed issues, up by 63.
    - 2187 issues total, up by 111.
  - Details:
    - Added the following 19 Tentatively Ready issues: 2557, 2558, 2559, 2560, 2565, 2566, 2571, 2572, 2574, 2575, 2576, 2577, 2579, 2581, 2582, 2583, 2585, 2586, 2590.
    - Added the following 30 New issues: 2554, 2555, 2556, 2561, 2562, 2563, 2564, 2567, 2568, 2569, 2570, 2573, 2578, 2584, 2587, 2588, 2589, 2591, 2663, 2664, 2665, 2666, 2667, 2668, 2669, 2670, 2671, 2672, 2673, 2674.
    - Added the following 3 NAD Future issues: 2611, 2612, 2654.
    - Added the following 41 WP issues: 2601, 2602, 2603, 2605, 2606, 2607, 2608, 2609, 2614, 2615, 2616, 2618, 2619, 2621, 2622, 2624, 2625, 2627, 2629, 2632, 2633, 2634, 2635, 2636, 2637, 2640, 2641, 2644, 2645, 2647, 2648, 2649, 2650, 2652, 2653, 2655, 2656, 2657, 2658, 2660, 2662.
    - Added the following 2 NAD Editorial issues: 2639, 2659.
    - Added the following 14 NAD issues: 2580, 2604, 2610, 2613, 2617, 2623, 2626, 2628, 2630, 2631, 2638, 2642, 2646, 2661.
    - Added the following 2 Dup issues: 2643, 2651.
    - Changed the following issue to Tentatively Ready (from New): 2545.
    - Changed the following 2 issues to Review (from Open): 2310, 2516.
    - Changed the following issue to NAD (from New): 2552.
- R95: 2015-11-15 2015 post-Kona mailing
  - Summary:
    - 257 open issues, down by 37.
    - 1819 closed issues, up by 47.
    - 2076 issues total, up by 10.
  - Details:
    - Added the following 10 New issues: 2544, 2545, 2546, 2547, 2548, 2549, 2550, 2551, 2552, 2553.
    - Changed the following 2 issues to Ready (from New): 2523, 2537.
    - Changed the following issue to Ready (from Open): 2276.
    - Changed the following 3 issues to Tentatively Ready (from New): 2520, 2522, 2539.
    - Changed the following 2 issues to Tentatively Ready (from Open): 2192, 2450.
    - Changed the following issue to Review (from Ready): 2181.
    - Changed the following 4 issues to Review (from Open): 2309, 2393, 2402, 2441.
    - Changed the following issue to Open (from Tentatively Ready): 2510.
    - Changed the following 10 issues to Open (from New): 2117, 2164, 2290, 2468, 2499, 2505, 2509, 2516, 2524, 2543.
    - Changed the following issue to SG1 (from New): 2533.
    - Changed the following 35 issues to WP (from Ready): 1169, 2072, 2101, 2111, 2119, 2127, 2133, 2156, 2218, 2219, 2244, 2250, 2259, 2336, 2353, 2367, 2380, 2384, 2385, 2435, 2447, 2462, 2466, 2469, 2473, 2476, 2477, 2483, 2484, 2485, 2486, 2487, 2489, 2492, 2494.
    - Changed the following 8 issues to WP (from Tentatively Ready): 2224, 2234, 2273, 2495, 2500, 2515, 2517, 2526.
    - Changed the following issue to Resolved (from Core): 2165.
    - Changed the following 2 issues to NAD (from New): 2474, 2538.
    - Changed the following issue to NAD (from Open): 2379.
    - Changed the following issue to NAD (from Resolved): 2319.
- R94: 2015-09-25 2015 pre-Kona mailing
  - Summary:
    - 294 open issues, up by 38.
    - 1772 closed issues, up by 2.
    - 2066 issues total, up by 40.
  - Details:
    - Added the following 4 Tentatively Ready issues: 2510, 2515, 2517, 2526.
    - Added the following 36 New issues: 2504, 2505, 2506, 2507, 2508, 2509, 2511, 2512, 2513, 2514, 2516, 2518, 2519, 2520, 2521, 2522, 2523, 2524, 2525, 2527, 2528, 2529, 2530, 2531, 2532, 2533, 2534, 2535, 2536, 2537, 2538, 2539, 2540, 2541, 2542, 2543.
    - Changed the following 2 issues to Tentatively Ready (from New): 2495, 2500.
    - Changed the following 2 issues to Tentatively Ready (from Open): 2234, 2273.
    - Changed the following issue to Resolved (from Open): 2051.
    - Changed the following issue to NAD (from New): 2326.
- R93: 2014-05-22 2015 post-Lenexa mailing
  - Summary:
    - 256 open issues, down by 36.
    - 1770 closed issues, up by 48.
    - 2026 issues total, up by 12.
  - Details:
    - Added the following 2 Ready issues: 2492, 2494.
    - Added the following 10 New issues: 2493, 2495, 2496, 2497, 2498, 2499, 2500, 2501, 2502, 2503.
    - Changed the following 2 issues to Ready (from Review): 2111, 2380.
    - Changed the following 20 issues to Ready (from New): 2244, 2250, 2259, 2336, 2353, 2367, 2384, 2385, 2435, 2462, 2466, 2473, 2476, 2477, 2483, 2484, 2485, 2486, 2487, 2489.
    - Changed the following 12 issues to Ready (from Open): 1169, 2072, 2101, 2119, 2127, 2133, 2156, 2181, 2218, 2219, 2447, 2469.
    - Changed the following issue to Tentatively Ready (from Open): 2224.
    - Changed the following issue to Review (from New): 2296.
    - Changed the following issue to Review (from Open): 2328.
    - Changed the following 11 issues to Open (from New): 2262, 2289, 2338, 2348, 2349, 2370, 2398, 2402, 2422, 2450, 2456.
    - Changed the following 8 issues to Open (from SG1): 2245, 2265, 2276, 2309, 2363, 2379, 2426, 2441.
    - Changed the following issue to LEWG (from New): 2372.
    - Changed the following issue to EWG (from New): 2432.
    - Changed the following issue to Deferred (from Open): 2202.
    - Changed the following 14 issues to WP (from Ready): 2160, 2168, 2364, 2403, 2406, 2411, 2425, 2427, 2428, 2433, 2434, 2438, 2439, 2440.
    - Changed the following 18 issues to WP (from Tentatively Ready): 2059, 2076, 2239, 2369, 2378, 2410, 2415, 2418, 2437, 2448, 2454, 2455, 2458, 2459, 2463, 2467, 2470, 2482.
    - Changed the following 3 issues to WP (from New): 2420, 2464, 2488.
    - Changed the following issue to WP (from Open): 2063.
    - Changed the following 2 issues to WP (from SG1): 2407, 2442.
    - Changed the following issue to Resolved (from Review): 2228.
    - Changed the following 3 issues to Resolved (from Open): 1526, 2274, 2397.
    - Changed the following 5 issues to NAD (from New): 2079, 2251, 2351, 2373, 2386.
    - Changed the following issue to NAD (from Open): 2388.
- R92: 2015-04-09 pre-Lenexa mailing
  - Summary:
    - 292 open issues, up by 33.
    - 1722 closed issues, up by 0.
    - 2014 issues total, up by 33.
  - Details:
    - Added the following 5 Tentatively Ready issues: 2459, 2463, 2467, 2470, 2482.
    - Added the following 27 New issues: 2460, 2461, 2462, 2464, 2465, 2466, 2468, 2471, 2472, 2473, 2474, 2475, 2476, 2477, 2478, 2479, 2480, 2481, 2483, 2484, 2485, 2486, 2487, 2488, 2489, 2490, 2491.
    - Added the following Open issue: 2469.
    - Changed the following issue to Tentatively Ready (from Review): 2378.
    - Changed the following 11 issues to Tentatively Ready (from New): 2076, 2239, 2369, 2410, 2415, 2418, 2437, 2448, 2454, 2455, 2458.
    - Changed the following issue to Tentatively Ready (from Open): 2059.
    - Changed the following issue to Tentatively NAD (from New): 2337.
    - Changed the following issue to Tentatively NAD (from Open): 760.
    - Changed the following 5 issues to Open (from New): 2312, 2388, 2393, 2444, 2447.
    - Changed the following 4 issues to LEWG (from New): 2391, 2417, 2436, 2451.
    - Changed the following issue to EWG (from Open): 2089.
    - Changed the following issue to Core (from New): 2452.
    - Changed the following 13 issues to SG1 (from New): 2236, 2245, 2265, 2276, 2309, 2334, 2363, 2379, 2407, 2412, 2426, 2442, 2445.
    - Changed the following issue to SG1 (from Open): 2441.
- R91: 2014-11-23 post-Urbana mailing
  - Summary:
    - 259 open issues, up by 32.
    - 1722 closed issues, down by 20.
    - 1981 issues total, up by 12.
  - Details:
    - Added the following 12 New issues: 2447, 2448, 2449, 2450, 2451, 2452, 2453, 2454, 2455, 2456, 2457, 2458.
    - Changed the following 2 issues to Ready (from Review): 2160, 2364.
    - Changed the following 11 issues to Ready (from New): 2403, 2406, 2411, 2425, 2427, 2428, 2433, 2434, 2438, 2439, 2440.
    - Changed the following issue to Ready (from Open): 2168.
    - Changed the following issue to Review (from New): 2424.
    - Changed the following 5 issues to Open (from New): 2307, 2310, 2383, 2414, 2441.
    - Changed the following 2 issues to Open (from NAD Future): 760, 1173.
    - Changed the following 4 issues to LEWG (from New): 2419, 2430, 2443, 2446.
    - Changed the following 48 issues to LEWG (from NAD Future): 255, 423, 484, 523, 532, 708, 839, 851, 877, 933, 935, 936, 961, 1025, 1031, 1041, 1052, 1053, 1112, 1120, 1121, 1150, 1154, 1184, 1188, 1201, 1203, 1217, 1235, 1238, 1242, 1282, 1289, 1317, 1320, 1396, 1406, 1422, 1459, 1484, 1488, 1493, 1499, 1521, 2040, 2055, 2226, 2232.
    - Changed the following 2 issues to Pending NAD (from Tentatively NAD): 2302, 2382.
    - Changed the following 11 issues to WP (from Ready): 2016, 2170, 2340, 2354, 2377, 2396, 2399, 2400, 2401, 2404, 2408.
    - Changed the following 12 issues to WP (from Tentatively Ready): 2106, 2129, 2212, 2217, 2230, 2233, 2266, 2325, 2361, 2365, 2376, 2387.
    - Changed the following issue to Resolved (from Ready): 2319.
    - Changed the following issue to Resolved (from Review): 2118.
    - Changed the following issue to Resolved (from New): 2416.
    - Changed the following issue to Resolved (from Open): 2108.
    - Changed the following issue to NAD (from New): 2429.
- R90: 2014-10-13 pre-Urbana mailing
  - Summary:
    - 227 open issues, up by 31.
    - 1742 closed issues, up by 0.
    - 1969 issues total, up by 31.
  - Details:
    - Added the following 31 New issues: 2416, 2417, 2418, 2419, 2420, 2421, 2422, 2423, 2424, 2425, 2426, 2427, 2428, 2429, 2430, 2431, 2432, 2433, 2434, 2435, 2436, 2437, 2438, 2439, 2440, 2441, 2442, 2443, 2444, 2445, 2446.
    - No issues changed.
- R89: 2014-07-08 post-Rapperswil mailing
  - Summary:
    - 196 open issues, up by 14.
    - 1742 closed issues, up by 12.
    - 1938 issues total, up by 26.
  - Details:
    - Added the following 6 Ready issues: 2396, 2399, 2400, 2401, 2404, 2408.
    - Added the following 15 New issues: 2391, 2392, 2393, 2394, 2398, 2402, 2403, 2406, 2407, 2410, 2411, 2412, 2413, 2414, 2415.
    - Added the following Open issue: 2397.
    - Added the following 3 WP issues: 2390, 2395, 2409.
    - Added the following NAD issue: 2405.
    - Changed the following issue to Ready (from New): 2377.
    - Changed the following 2 issues to Ready (from Deferred): 2253, 2255.
    - Changed the following 2 issues to Tentatively Ready (from New): 2325, 2387.
    - Changed the following issue to Tentatively NAD (from New): 2382.
    - Changed the following 3 issues to Review (from New): 2364, 2378, 2380.
    - Changed the following 2 issues to Review (from Open): 2118, 2160.
    - Changed the following 3 issues to Open (from New): 2168, 2238, 2273.
    - Changed the following 3 issues to Open (from Deferred): 2254, 2264, 2277.
    - Changed the following 3 issues to WP (from New): 2371, 2374, 2389.
    - Changed the following 4 issues to Resolved (from Deferred): 2282, 2283, 2287, 2333.
    - Changed the following issue to NAD (from Deferred): 2305.
- R88: 2014-05-24 pre-Rapperswil mailing
  - Summary:
    - 182 open issues, up by 29.
    - 1730 closed issues, up by 0.
    - 1912 issues total, up by 29.
  - Details:
    - Added the following 3 Tentatively Ready issues: 2361, 2365, 2376.
    - Added the following 26 New issues: 2362, 2363, 2364, 2366, 2367, 2368, 2369, 2370, 2371, 2372, 2373, 2374, 2375, 2377, 2378, 2379, 2380, 2381, 2382, 2383, 2384, 2385, 2386, 2387, 2388, 2389.
    - Changed the following 5 issues to Tentatively Ready (from Open): 2106, 2129, 2212, 2230, 2233.

## Accepted Issues

### 1(i). C library linkage editing oversight

Section: 16.4.3.3 [using.linkage] Status: TC1 Submitter: Beman Dawes Opened: 1997-11-16 Last modified: 2016-08-09

Priority: Not Prioritized

View all other issues in [using.linkage].

View all issues with TC1 status.

Discussion:

The change specified in the proposed resolution below did not make it into the Standard. This change was accepted in principle at the London meeting, and the exact wording below was accepted at the Morristown meeting.

Proposed resolution:

Change 16.4.3.3 [using.linkage] paragraph 2 from:

> It is unspecified whether a name from the Standard C library declared with external linkage has either extern "C" or extern "C++" linkage.

to:

> Whether a name from the Standard C library declared with external linkage has extern "C" or extern "C++" linkage is implementation defined. It is recommended that an implementation use extern "C++" linkage for this purpose.

### 3(i). Atexit registration during atexit() call is not described

Section: 17.5 [support.start.term] Status: TC1 Submitter: Steve Clamage Opened: 1997-12-12 Last modified: 2016-08-09

Priority: Not Prioritized

View other active issues in [support.start.term].

View all other issues in [support.start.term].

View all issues with TC1 status.

Discussion:

We appear not to have covered all the possibilities of exit processing with respect to atexit registration.
Example 1: (C and C++)

        #include <stdlib.h>
        void f1() { }
        void f2() { atexit(f1); }

        int main()
        {
            atexit(f2); // the only use of f2
            return 0; // for C compatibility
        }

At program exit, f2 gets called due to its registration in main. Running f2 causes f1 to be newly registered during the exit processing. Is this a valid program? If so, what are its semantics?

Interestingly, neither the C standard, nor the C++ draft standard nor the forthcoming C9X Committee Draft says directly whether you can register a function with atexit during exit processing.

All 3 standards say that functions are run in reverse order of their registration. Since f1 is registered last, it ought to be run first, but by the time it is registered, it is too late to be first.

If the program is valid, the standards are self-contradictory about its semantics.

Example 2: (C++ only)

        void F() { static T t; } // type T has a destructor

        int main()
        {
            atexit(F); // the only use of F
        }

Function F registered with atexit has a local static variable t, and F is called for the first time during exit processing. A local static object is initialized the first time control flow passes through its definition, and all static objects are destroyed during exit processing. Is the code valid? If so, what are its semantics?

Section 18.3 "Start and termination" says that if a function F is registered with atexit before a static object t is initialized, F will not be called until after t's destructor completes.

In example 2, function F is registered with atexit before its local static object O could possibly be initialized. On that basis, it must not be called by exit processing until after O's destructor completes. But the destructor cannot be run until after F is called, since otherwise the object could not be constructed in the first place.

If the program is valid, the standard is self-contradictory about its semantics.

I plan to submit Example 1 as a public comment on the C9X CD, with a recommendation that the results be undefined. (Alternative: make it unspecified. I don't think it is worthwhile to specify the case where f1 itself registers additional functions, each of which registers still more functions.)

I think we should resolve the situation in the whatever way the C committee decides.

For Example 2, I recommend we declare the results undefined.

[See reflector message lib-6500 for further discussion.]

Proposed resolution:

Change section 18.3/8 from:

> First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit are called in the reverse order of their registration. A function registered with atexit before an object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after an object obj2 of static storage duration is initialized will be called before obj2's destruction starts.

to:

> First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Non-local objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit are called in the reverse order of their registration, except that a function is called after any previously registered functions that had already been called at the time it was registered. A function registered with atexit before a non-local object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after a non-local object obj2 of static storage duration is initialized will be called before obj2's destruction starts. A local static object obj3 is destroyed at the same time it would be if a function calling the obj3 destructor were registered with atexit at the completion of the obj3 constructor.

Rationale:

See 99-0039/N1215, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.

### 5(i). String::compare specification questionable

Section: 27.4.3.7.8 [string.swap] Status: TC1 Submitter: Jack Reeves Opened: 1997-12-11 Last modified: 2016-11-12

Priority: Not Prioritized

View all other issues in [string.swap].

View all issues with TC1 status.

Duplicate of: 87

Discussion:

At the very end of the basic_string class definition is the signature: int compare(size_type pos1, size_type n1, const charT* s, size_type n2 = npos) const; In the following text this is defined as: returns basic_string<charT,traits,Allocator>(*this,pos1,n1).compare( basic_string<charT,traits,Allocator>(s,n2);

Since the constructor basic_string(const charT* s, size_type n, const Allocator& a = Allocator()) clearly requires that s != NULL and n < npos and further states that it throws length_error if n == npos, it appears the compare() signature above should always throw length error if invoked like so: str.compare(1, str.size()-1, s); where 's' is some null terminated character array.

This appears to be a typo since the obvious intent is to allow either the call above or something like: str.compare(1, str.size()-1, s, strlen(s)-1);

This would imply that what was really intended was two signatures int compare(size_type pos1, size_type n1, const charT* s) const int compare(size_type pos1, size_type n1, const charT* s, size_type n2) const; each defined in terms of the corresponding constructor.

Proposed resolution:

Replace the compare signature in 27.4.3 [basic.string] (at the very end of the basic_string synopsis) which reads:

> `int compare(size_type pos1, size_type n1,
> const charT* s, size_type n2 = npos) const;`

with:

> `int compare(size_type pos1, size_type n1,
> const charT* s) const;
> int compare(size_type pos1, size_type n1,
> const charT* s, size_type n2) const;`

Replace the portion of 27.4.3.7.8 [string.swap] paragraphs 5 and 6 which read:

> `int compare(size_type pos, size_type n1,
> charT * s, size_type n2 = npos) const;
> ` Returns:`
> basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
> basic_string<charT,traits,Allocator>( s, n2))`

with:

> `int compare(size_type pos, size_type n1,
> const charT * s) const;
> ` Returns:`
> basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
> basic_string<charT,traits,Allocator>( s ))
> int compare(size_type pos, size_type n1,
> const charT * s, size_type n2) const;
> ` Returns:`
> basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
> basic_string<charT,traits,Allocator>( s, n2))`

Editors please note that in addition to splitting the signature, the third argument becomes const, matching the existing synopsis.

Rationale:

While the LWG dislikes adding signatures, this is a clear defect in the Standard which must be fixed. The same problem was also identified in issues 7 (item 5) and 87.

### 7(i). String clause minor problems

Section: 27 [strings] Status: TC1 Submitter: Matt Austern Opened: 1997-12-15 Last modified: 2016-11-12

Priority: Not Prioritized

View all other issues in [strings].

View all issues with TC1 status.

Discussion:

(1) In 27.4.3.7.4 [string.insert], the description of template <class InputIterator> insert(iterator, InputIterator, InputIterator) makes no sense. It refers to a member function that doesn't exist. It also talks about the return value of a void function.

(2) Several versions of basic_string::replace don't appear in the class synopsis.

(3) basic_string::push_back appears in the synopsis, but is never described elsewhere. In the synopsis its argument is const charT, which doesn't makes much sense; it should probably be charT, or possible const charT&.

(4) basic_string::pop_back is missing.

(5) int compare(size_type pos, size_type n1, charT* s, size_type n2 = npos) make no sense. First, it's const charT* in the synopsis and charT* in the description. Second, given what it says in RETURNS, leaving out the final argument will always result in an exception getting thrown. This is paragraphs 5 and 6 of 27.4.3.7.8 [string.swap]

(6) In table 37, in section 27.2.2 [char.traits.require], there's a note for X::move(s, p, n). It says "Copies correctly even where p is in [s, s+n)". This is correct as far as it goes, but it doesn't go far enough; it should also guarantee that the copy is correct even where s in in [p, p+n). These are two orthogonal guarantees, and neither one follows from the other. Both guarantees are necessary if X::move is supposed to have the same sort of semantics as memmove (which was clearly the intent), and both guarantees are necessary if X::move is actually supposed to be useful.

Proposed resolution:

ITEM 1: In 21.3.5.4 [lib.string::insert], change paragraph 16 to
EFFECTS: Equivalent to insert(p - begin(), basic_string(first, last)).
ITEM 2: Not a defect; the Standard is clear.. There are ten versions of replace() in the synopsis, and ten versions in 21.3.5.6 [lib.string::replace].
ITEM 3: Change the declaration of push_back in the string synopsis (21.3, [lib.basic.string]) from:

void push_back(const charT)
to
void push_back(charT)
Add the following text immediately after 21.3.5.2 [lib.string::append], paragraph 10.
void basic_string::push_back(charT c);
EFFECTS: Equivalent to append(static_cast<size_type>(1), c);
ITEM 4: Not a defect. The omission appears to have been deliberate.
ITEM 5: Duplicate; see issue 5 (and 87).
ITEM 6: In table 37, Replace:
"Copies correctly even where p is in [s, s+n)."
with:
"Copies correctly even where the ranges [p, p+n) and [s, s+n) overlap."

### 8(i). Locale::global lacks guarantee

Section: 28.3.3.1.6 [locale.statics] Status: TC1 Submitter: Matt Austern Opened: 1997-12-24 Last modified: 2016-08-09

Priority: Not Prioritized

View all issues with TC1 status.

Discussion:

It appears there's an important guarantee missing from clause 22. We're told that invoking locale::global(L) sets the C locale if L has a name. However, we're not told whether or not invoking setlocale(s) sets the global C++ locale.

The intent, I think, is that it should not, but I can't find any such words anywhere.

Proposed resolution:

Add a sentence at the end of 28.3.3.1.6 [locale.statics], paragraph 2:

> No library function other than `locale::global()` shall affect the value returned by `locale()`.

### 9(i). Operator new(0) calls should not yield the same pointer

Section: 17.6.3 [new.delete] Status: TC1 Submitter: Steve Clamage Opened: 1998-01-04 Last modified: 2016-08-09

Priority: Not Prioritized

View all other issues in [new.delete].

View all issues with TC1 status.

Discussion:

Scott Meyers, in a comp.std.c++ posting: I just noticed that section 3.7.3.1 of CD2 seems to allow for the possibility that all calls to operator new(0) yield the same pointer, an implementation technique specifically prohibited by ARM 5.3.3.Was this prohibition really lifted? Does the FDIS agree with CD2 in the regard? [Issues list maintainer's note: the IS is the same.]

Proposed resolution:

Change the last paragraph of 3.7.3 from:

> Any allocation and/or deallocation functions defined in a C++ program shall conform to the semantics specified in 3.7.3.1 and 3.7.3.2.

to:

> Any allocation and/or deallocation functions defined in a C++ program, including the default versions in the library, shall conform to the semantics specified in 3.7.3.1 and 3.7.3.2.

Change 3.7.3.1/2, next-to-last sentence, from :

> If the size of the space requested is zero, the value returned shall not be a null pointer value (4.10).

to:

> Even if the size of the space requested is zero, the request can fail. If the request succeeds, the value returned shall be a non-null pointer value (4.10) p0 different from any previously returned value p1, unless that value p1 was since passed to an operator delete.

5.3.4/7 currently reads:

> When the value of the expression in a direct-new-declarator is zero, the allocation function is called to allocate an array with no elements. The pointer returned by the new-expression is non-null. [Note: If the library allocation function is called, the pointer returned is distinct from the pointer to any other object.]

Retain the first sentence, and delete the remainder.

18.5.1 currently has no text. Add the following:

> Except where otherwise specified, the provisions of 3.7.3 apply to the library versions of operator new and operator delete.

To 18.5.1.3, add the following text:

> The provisions of 3.7.3 do not apply to these reserved placement forms of operator new and operator delete.

Rationale:

See 99-0040/N1216, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.

### 11(i). Bitset minor problems

Section: 22.9.2 [template.bitset] Status: TC1 Submitter: Matt Austern Opened: 1998-01-22 Last modified: 2016-08-09

Priority: Not Prioritized

View all other issues in [template.bitset].

View all issues with TC1 status.

Discussion:

(1) bitset<>::operator[] is mentioned in the class synopsis (23.3.5), but it is not documented in 23.3.5.2.

(2) The class synopsis only gives a single signature for bitset<>::operator[], reference operator[](size_t pos). This doesn't make much sense. It ought to be overloaded on const. reference operator[](size_t pos); bool operator[](size_t pos) const.

(3) Bitset's stream input function (23.3.5.3) ought to skip all whitespace before trying to extract 0s and 1s. The standard doesn't explicitly say that, though. This should go in the Effects clause.

Proposed resolution:

ITEMS 1 AND 2:
In the bitset synopsis (22.9.2 [template.bitset]), replace the member function
`reference operator[](size_t pos);
`
with the two member functions
`bool operator[](size_t pos) const;
reference operator[](size_t pos);
`
Add the following text at the end of 22.9.2.3 [bitset.members], immediately after paragraph 45:

> `bool operator[](size_t pos) const;`
> Requires: pos is valid
> Throws: nothing
> Returns: `test(pos)`
> `bitset<N>::reference operator[](size_t pos);`
> Requires: pos is valid
> Throws: nothing
> Returns: An object of type `bitset<N>::reference` such that `(*this)[pos] == this->test(pos)`, and such that `(*this)[pos] = val` is equivalent to `this->set(pos, val);`

Rationale:

The LWG believes Item 3 is not a defect. "Formatted input" implies the desired semantics. See 31.7.5.3 [istream.formatted].

### 13(i). Eos refuses to die

Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: William M. Miller Opened: 1998-03-03 Last modified: 2017-04-22

Priority: Not Prioritized

View all other issues in [istream.extractors].

View all issues with TC1 status.

Discussion:

In 27.6.1.2.3, there is a reference to "eos", which is the only one in the whole draft (at least using Acrobat search), so it's undefined.

Proposed resolution:

In [istream::extractors], replace "eos" with "charT()"

### 14(i). Locale::combine should be const

Section: 28.3.3.1.4 [locale.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09

Priority: Not Prioritized

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Discussion:

locale::combine is the only member function of locale (other than constructors and destructor) that is not const. There is no reason for it not to be const, and good reasons why it should have been const. Furthermore, leaving it non-const conflicts with 22.1.1 paragraph 6: "An instance of a locale is immutable."

History: this member function originally was a constructor. it happened that the interface it specified had no corresponding language syntax, so it was changed to a member function. As constructors are never const, there was no "const" in the interface which was transformed into member "combine". It should have been added at that time, but the omission was not noticed.

Proposed resolution:

In 28.3.3.1 [locale] and also in 28.3.3.1.4 [locale.members], add "const" to the declaration of member combine:

>     template <class Facet> locale combine(const locale& other) const;

### 15(i). Locale::name requirement inconsistent

Section: 28.3.3.1.4 [locale.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09

Priority: Not Prioritized

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Discussion:

locale::name() is described as returning a string that can be passed to a locale constructor, but there is no matching constructor.

Proposed resolution:

In 28.3.3.1.4 [locale.members], paragraph 5, replace "`locale(name())`" with "`locale(name().c_str())`".

### 16(i). Bad ctype_byname<char> decl

Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09

Priority: Not Prioritized

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Discussion:

The new virtual members ctype_byname<char>::do_widen and do_narrow did not get edited in properly. Instead, the member do_widen appears four times, with wrong argument lists.

Proposed resolution:

The correct declarations for the overloaded members `do_narrow` and `do_widen` should be copied from 28.3.4.2.4 [facet.ctype.special].

### 17(i). Bad bool parsing

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09

Priority: Not Prioritized

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Discussion:

This section describes the process of parsing a text boolean value from the input stream. It does not say it recognizes either of the sequences "true" or "false" and returns the corresponding bool value; instead, it says it recognizes only one of those sequences, and chooses which according to the received value of a reference argument intended for returning the result, and reports an error if the other sequence is found. (!) Furthermore, it claims to get the names from the ctype<> facet rather than the numpunct<> facet, and it examines the "boolalpha" flag wrongly; it doesn't define the value "loc"; and finally, it computes wrongly whether to use numeric or "alpha" parsing.
I believe the correct algorithm is "as if":

      // in, err, val, and str are arguments.
      err = 0;
      const numpunct<charT>& np = use_facet<numpunct<charT> >(str.getloc());
      const string_type t = np.truename(), f = np.falsename();
      bool tm = true, fm = true;
      size_t pos = 0;
      while (tm && pos < t.size() || fm && pos < f.size()) {
        if (in == end) { err = str.eofbit; }
        bool matched = false;
        if (tm && pos < t.size()) {
          if (!err && t[pos] == *in) matched = true;
          else tm = false;
        }
        if (fm && pos < f.size()) {
          if (!err && f[pos] == *in) matched = true;
          else fm = false;
        }
        if (matched) { ++in; ++pos; }
        if (pos > t.size()) tm = false;
        if (pos > f.size()) fm = false;
      }
      if (tm == fm || pos == 0) { err |= str.failbit; }
      else                      { val = tm; }
      return in;

Notice this works reasonably when the candidate strings are both empty, or equal, or when one is a substring of the other. The proposed text below captures the logic of the code above.

Proposed resolution:

In 28.3.4.3.2.3 [facet.num.get.virtuals], in the first line of paragraph 14, change "&&" to "&".

Then, replace paragraphs 15 and 16 as follows:

> Otherwise target sequences are determined "as if" by calling the members `falsename()` and `truename()` of the facet obtained by `use_facet<numpunct<charT> >(str.getloc())`. Successive characters in the range `[in,end)` (see [lib.sequence.reqmts]) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iterator `in` is compared to `end` only when necessary to obtain a character. If and only if a target sequence is uniquely matched, `val` is set to the corresponding value.

> The `in` iterator is always left pointing one position beyond the last character successfully matched. If `val` is set, then err is set to `str.goodbit`; or to `str.eofbit` if, when seeking another character to match, it is found that `(in==end)`. If `val` is not set, then err is set to `str.failbit`; or to `(str.failbit|str.eofbit)`if the reason for the failure was that `(in==end)`. [Example: for targets `true`:"a" and `false`:"abb", the input sequence "a" yields `val==true` and `err==str.eofbit`; the input sequence "abc" yields `err=str.failbit`, with `in` ending at the 'c' element. For targets `true`:"1" and `false`:"0", the input sequence "1" yields `val==true` and `err=str.goodbit`. For empty targets (""), any input sequence yields `err==str.failbit`. --end example]

### 18(i). Get(...bool&) omitted

Section: 28.3.4.3.2.2 [facet.num.get.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09

Priority: Not Prioritized

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Discussion:

In the list of num_get<> non-virtual members on page 22-23, the member that parses bool values was omitted from the list of definitions of non-virtual members, though it is listed in the class definition and the corresponding virtual is listed everywhere appropriate.

Proposed resolution:

Add at the beginning of 28.3.4.3.2.2 [facet.num.get.members] another get member for bool&, copied from the entry in 28.3.4.3.2 [locale.num.get].

### 19(i). "Noconv" definition too vague

Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09

Priority: Not Prioritized

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Duplicate of: 10

Discussion:

In the definitions of codecvt<>::do_out and do_in, they are specified to return noconv if "no conversion is needed". This definition is too vague, and does not say normatively what is done with the buffers.

Proposed resolution:

Change the entry for noconv in the table under paragraph 4 in section 28.3.4.2.5.3 [locale.codecvt.virtuals] to read:

> `noconv`: `internT` and `externT` are the same type, and input sequence is identical to converted sequence.

Change the Note in paragraph 2 to normative text as follows:

> If returns `noconv`, `internT` and `externT` are the same type and the converted sequence is identical to the input sequence `[from,from_next)`. `to_next` is set equal to `to`, the value of `state` is unchanged, and there are no changes to the values in `[to, to_limit)`.

### 20(i). Thousands_sep returns wrong type

Section: 28.3.4.4.1.3 [facet.numpunct.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09

Priority: Not Prioritized

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Discussion:

The synopsis for numpunct<>::do_thousands_sep, and the definition of numpunct<>::thousands_sep which calls it, specify that it returns a value of type char_type. Here it is erroneously described as returning a "string_type".

Proposed resolution:

In 28.3.4.4.1.3 [facet.numpunct.virtuals], above paragraph 2, change "string_type" to "char_type".

### 21(i). Codecvt_byname<> instantiations

Section: 28.3.3.1.2.1 [locale.category] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the second table in the section, captioned "Required instantiations", the instantiations for codecvt_byname<> have been omitted. These are necessary to allow users to construct a locale by name from facets.

Proposed resolution:

Add in 28.3.3.1.2.1 [locale.category] to the table captioned "Required instantiations", in the category "ctype" the lines

>     codecvt_byname<char,char,mbstate_t>,
>     codecvt_byname<wchar_t,char,mbstate_t>

### 22(i). Member open vs. flags

Section: 31.10.4.4 [ifstream.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The description of basic_istream<>::open leaves unanswered questions about how it responds to or changes flags in the error status for the stream. A strict reading indicates that it ignores the bits and does not change them, which confuses users who do not expect eofbit and failbit to remain set after a successful open. There are three reasonable resolutions: 1) status quo 2) fail if fail(), ignore eofbit 3) clear failbit and eofbit on call to open().

Proposed resolution:

In 31.10.4.4 [ifstream.members] paragraph 3, and in 31.10.5.4 [ofstream.members] paragraph 3, under open() effects, add a footnote:

> A successful open does not change the error state.

Rationale:

This may seem surprising to some users, but it's just an instance of a general rule: error flags are never cleared by the implementation. The only way error flags are are ever cleared is if the user explicitly clears them by hand.

The LWG believed that preserving this general rule was important enough so that an exception shouldn't be made just for this one case. The resolution of this issue clarifies what the LWG believes to have been the original intent.

### 23(i). Num_get overflow result

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current description of numeric input does not account for the possibility of overflow. This is an implicit result of changing the description to rely on the definition of scanf() (which fails to report overflow), and conflicts with the documented behavior of traditional and current implementations.

Users expect, when reading a character sequence that results in a value unrepresentable in the specified type, to have an error reported. The standard as written does not permit this.

Further comments from Dietmar:

I don't feel comfortable with the proposed resolution to issue 23: It kind of simplifies the issue to much. Here is what is going on:

Currently, the behavior of numeric overflow is rather counter intuitive and hard to trace, so I will describe it briefly:

- According to 28.3.4.3.2.3 [facet.num.get.virtuals] paragraph 11 `failbit` is set if `scanf()` would return an input error; otherwise a value is converted to the rules of `scanf`.
- `scanf()` is defined in terms of `fscanf()`.
- `fscanf()` returns an input failure if during conversion no character matching the conversion specification could be extracted before reaching EOF. This is the only reason for `fscanf()` to fail due to an input error and clearly does not apply to the case of overflow.
- Thus, the conversion is performed according to the rules of `fscanf()` which basically says that `strtod`, `strtol()`, etc. are to be used for the conversion.
- The `strtod()`, `strtol()`, etc. functions consume as many matching characters as there are and on overflow continue to consume matching characters but also return a value identical to the maximum (or minimum for signed types if there was a leading minus) value of the corresponding type and set `errno` to `ERANGE`.
- Thus, according to the current wording in the standard, overflows can be detected! All what is to be done is to check `errno` after reading an element and, of course, clearing `errno` before trying a conversion. With the current wording, it can be detected whether the overflow was due to a positive or negative number for signed types.

Further discussion from Redmond:

The basic problem is that we've defined our behavior, including our error-reporting behavior, in terms of C90. However, C90's method of reporting overflow in scanf is not technically an "input error". The `strto_*` functions are more precise.

There was general consensus that `failbit` should be set upon overflow. We considered three options based on this:

1. Set failbit upon conversion error (including overflow), and don't store any value.
2. Set failbit upon conversion error, and also set `errno` to indicated the precise nature of the error.
3. Set failbit upon conversion error. If the error was due to overflow, store +-numeric_limits<T>::max() as an overflow indication.

Straw poll: (1) 5; (2) 0; (3) 8.

Discussed at Lillehammer. General outline of what we want the solution to look like: we want to say that overflow is an error, and provide a way to distinguish overflow from other kinds of errors. Choose candidate field the same way scanf does, but don't describe the rest of the process in terms of format. If a finite input field is too large (positive or negative) to be represented as a finite value, then set failbit and assign the nearest representable value. Bill will provide wording.

Discussed at Toronto: N2327 is in alignment with the direction we wanted to go with in Lillehammer. Bill to work on.

Proposed resolution:

Change 28.3.4.3.2.3 [facet.num.get.virtuals], end of p3:

> Stage 3: The result of stage 2 processing can be one of The sequence of `char`s accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header `<cstdlib>`:

> - A sequence of `char`s has been accumulated in stage 2 that is converted (according to the rules of `scanf`) to a value of the type of val. This value is stored in val and `ios_base::goodbit` is stored in err. For a signed integer value, the function `strtoll`.
> - The sequence of `char`s accumulated in stage 2 would have caused `scanf` to report an input failure. `ios_base::failbit` is assigned to err. For an unsigned integer value, the function `strtoull`.
> - For a floating-point value, the function `strtold`.

> The numeric value to be stored can be one of:

> - zero, if the conversion function fails to convert the entire field. `ios_base::failbit` is assigned to err.
> - the most positive representable value, if the field represents a value too large positive to be represented in val. `ios_base::failbit` is assigned to err.
> - the most negative representable value (zero for unsigned integer), if the field represents a value too large negative to be represented in val. `ios_base::failbit` is assigned to err.
> - the converted value, otherwise.

> The resultant numeric value is stored in val.

Change 28.3.4.3.2.3 [facet.num.get.virtuals], p6-p7:

>     iter_type do_get(iter_type in, iter_type end, ios_base& str,
>                      ios_base::iostate& err, bool& val) const;

> > -6- Effects: If `(str.flags()&ios_base::boolalpha)==0` then input proceeds as it would for a `long` except that if a value is being stored into val, the value is determined according to the following: If the value to be stored is 0 then `false` is stored. If the value is 1 then `true` is stored. Otherwise `err|=ios_base::failbit` is performed and no value `true` is stored. and `ios_base::failbit` is assigned to err.

> > -7- Otherwise target sequences are determined "as if" by calling the members `falsename()` and `truename()` of the facet obtained by `use_facet<numpunct<charT> >(str.getloc())`. Successive characters in the range `[in,end)` (see 23.1.1) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iterator in is compared to end only when necessary to obtain a character. If and only if a target sequence is uniquely matched, val is set to the corresponding value. Otherwise `false` is stored and `ios_base::failbit` is assigned to err.

### 24(i). "do_convert" doesn't exist

Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 72

Discussion:

The description of codecvt<>::do_out and do_in mentions a symbol "do_convert" which is not defined in the standard. This is a leftover from an edit, and should be "do_in and do_out".

Proposed resolution:

In 28.3.4.2.5 [locale.codecvt], paragraph 3, change "do_convert" to "do_in or do_out". Also, in 28.3.4.2.5.3 [locale.codecvt.virtuals], change "do_convert()" to "do_in or do_out".

### 25(i). String operator<< uses width() value wrong

Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 67

Discussion:

In the description of operator<< applied to strings, the standard says that uses the smaller of os.width() and str.size(), to pad "as described in stage 3" elsewhere; but this is inconsistent, as this allows no possibility of space for padding.

Proposed resolution:

Change 27.4.4.4 [string.io] paragraph 4 from:
"... where `n` is the smaller of `os.width()` and `str.size()`; ..."
to:
"... where `n` is the larger of `os.width()` and `str.size()`; ..."

### 26(i). Bad sentry example

Section: 31.7.5.2.4 [istream.sentry] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

In paragraph 6, the code in the example:

      template <class charT, class traits = char_traits<charT> >
      basic_istream<charT,traits>::sentry(
               basic_istream<charT,traits>& is, bool noskipws = false) {
          ...
          int_type c;
          typedef ctype<charT> ctype_type;
          const ctype_type& ctype = use_facet<ctype_type>(is.getloc());
          while ((c = is.rdbuf()->snextc()) != traits::eof()) {
            if (ctype.is(ctype.space,c)==0) {
              is.rdbuf()->sputbackc (c);
              break;
            }
          }
          ...
       }

fails to demonstrate correct use of the facilities described. In particular, it fails to use traits operators, and specifies incorrect semantics. (E.g. it specifies skipping over the first character in the sequence without examining it.)

Proposed resolution:

Remove the example above from [istream::sentry] paragraph 6.

Rationale:

The originally proposed replacement code for the example was not correct. The LWG tried in Kona and again in Tokyo to correct it without success. In Tokyo, an implementor reported that actual working code ran over one page in length and was quite complicated. The LWG decided that it would be counter-productive to include such a lengthy example, which might well still contain errors.

### 27(i). String::erase(range) yields wrong iterator

Section: 27.4.3.7.5 [string.erase] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-11-12

Priority: Not Prioritized

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Discussion:

The string::erase(iterator first, iterator last) is specified to return an element one place beyond the next element after the last one erased. E.g. for the string "abcde", erasing the range ['b'..'d') would yield an iterator for element 'e', while 'd' has not been erased.

Proposed resolution:

In 27.4.3.7.5 [string.erase], paragraph 10, change:

> Returns: an iterator which points to the element immediately following _last_ prior to the element being erased.

to read

> Returns: an iterator which points to the element pointed to by _last_ prior to the other elements being erased.

### 28(i). Ctype<char>is ambiguous

Section: 28.3.4.2.4.3 [facet.ctype.char.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 236

Discussion:

The description of the vector form of ctype<char>::is can be interpreted to mean something very different from what was intended. Paragraph 4 says

> Effects: The second form, for all *p in the range [low, high), assigns vec[p-low] to table()[(unsigned char)*p].

This is intended to copy the value indexed from table()[] into the place identified in vec[].

Proposed resolution:

Change 28.3.4.2.4.3 [facet.ctype.char.members], paragraph 4, to read

> Effects: The second form, for all *p in the range [low, high), assigns into vec[p-low] the value table()[(unsigned char)*p].

### 29(i). Ios_base::init doesn't exist

Section: 31.4.3 [narrow.stream.objects] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Sections 31.4.3 [narrow.stream.objects] and 31.4.4 [wide.stream.objects] mention a function ios_base::init, which is not defined. Probably they mean basic_ios<>::init, defined in 31.5.4.2 [basic.ios.cons], paragraph 3.

Proposed resolution:

[R12: modified to include paragraph 5.]

In 31.4.3 [narrow.stream.objects] paragraph 2 and 5, change

> ios_base::init

to

> basic_ios<char>::init

Also, make a similar change in 31.4.4 [wide.stream.objects] except it should read

> basic_ios<wchar_t>::init

### 30(i). Wrong header for LC_*

Section: 28.3.3.1.2.1 [locale.category] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 2 implies that the C macros LC_CTYPE etc. are defined in <cctype>, where they are in fact defined elsewhere to appear in <clocale>.

Proposed resolution:

In 28.3.3.1.2.1 [locale.category], paragraph 2, change "<cctype>" to read "<clocale>".

### 31(i). Immutable locale values

Section: 28.3.3.1 [locale] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 378

Discussion:

Paragraph 6, says "An instance of `locale` is immutable; once a facet reference is obtained from it, ...". This has caused some confusion, because locale variables are manifestly assignable.

Proposed resolution:

In 28.3.3.1 [locale] replace paragraph 6

> An instance of `locale` is immutable; once a facet reference is obtained from it, that reference remains usable as long as the locale value itself exists.

with

> Once a facet reference is obtained from a locale object by calling use_facet<>, that reference remains usable, and the results from member functions of it may be cached and re-used, as long as some locale object refers to that facet.

### 32(i). Pbackfail description inconsistent

Section: 31.6.3.5.4 [streambuf.virt.pback] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The description of the required state before calling virtual member basic_streambuf<>::pbackfail requirements is inconsistent with the conditions described in 27.5.2.2.4 [lib.streambuf.pub.pback] where member sputbackc calls it. Specifically, the latter says it calls pbackfail if:

traits::eq(c,gptr()[-1]) is false

where pbackfail claims to require:

traits::eq(*gptr(),traits::to_char_type(c)) returns false

It appears that the pbackfail description is wrong.

Proposed resolution:

In 31.6.3.5.4 [streambuf.virt.pback], paragraph 1, change:

> "`traits::eq(*gptr(),traits::to_char_type( c))`"

to

> "`traits::eq(traits::to_char_type(c),gptr()[-1])`"

Rationale:

Note deliberate reordering of arguments for clarity in addition to the correction of the argument value.

### 33(i). Codecvt<> mentions from_type

Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

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Duplicate of: 43

Discussion:

In the table defining the results from do_out and do_in, the specification for the result error says

> encountered a from_type character it could not convert

but from_type is not defined. This clearly is intended to be an externT for do_in, or an internT for do_out.

Proposed resolution:

In 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 4, replace the definition in the table for the case of _error_ with

> encountered a character in `[from,from_end)` that it could not convert.

### 34(i). True/falsename() not in ctype<>

Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In paragraph 19, Effects:, members truename() and falsename are used from facet ctype<charT>, but it has no such members. Note that this is also a problem in 22.2.2.1.2, addressed in (4).

Proposed resolution:

In 28.3.4.3.3.3 [facet.num.put.virtuals], paragraph 19, in the Effects: clause for member put(...., bool), replace the initialization of the string_type value s as follows:

>     const numpunct& np = use_facet<numpunct<charT> >(loc);
>     string_type s = val ? np.truename() : np.falsename();

### 35(i). No manipulator unitbuf in synopsis

Section: 31.5 [iostreams.base] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 31.5.5.1 [fmtflags.manip], we have a definition for a manipulator named "unitbuf". Unlike other manipulators, it's not listed in synopsis. Similarly for "nounitbuf".

Proposed resolution:

Add to the synopsis for <ios> in 31.5 [iostreams.base], after the entry for "nouppercase", the prototypes:

>     ios_base& unitbuf(ios_base& str);
>     ios_base& nounitbuf(ios_base& str);

### 36(i). Iword & pword storage lifetime omitted

Section: 31.5.2.6 [ios.base.storage] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the definitions for ios_base::iword and pword, the lifetime of the storage is specified badly, so that an implementation which only keeps the last value stored appears to conform. In particular, it says:

The reference returned may become invalid after another call to the object's iword member with a different index ...

This is not idle speculation; at least one implementation was done this way.

Proposed resolution:

Add in 31.5.2.6 [ios.base.storage], in both paragraph 2 and also in paragraph 4, replace the sentence:

> The reference returned may become invalid after another call to the object's iword [pword] member with a different index, after a call to its copyfmt member, or when the object is destroyed.

with:

> The reference returned is invalid after any other operations on the object. However, the value of the storage referred to is retained, so that until the next call to copyfmt, calling iword [pword] with the same index yields another reference to the same value.

substituting "iword" or "pword" as appropriate.

### 37(i). Leftover "global" reference

Section: 28.3.3.1 [locale] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the overview of locale semantics, paragraph 4, is the sentence

> If Facet is not present in a locale (or, failing that, in the global locale), it throws the standard exception bad_cast.

This is not supported by the definition of use_facet<>, and represents semantics from an old draft.

Proposed resolution:

In 28.3.3.1 [locale], paragraph 4, delete the parenthesized expression

> (or, failing that, in the global locale)

### 38(i). Facet definition incomplete

Section: 28.3.3.2 [locale.global.templates] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It has been noticed by Esa Pulkkinen that the definition of "facet" is incomplete. In particular, a class derived from another facet, but which does not define a member id, cannot safely serve as the argument F to use_facet<F>(loc), because there is no guarantee that a reference to the facet instance stored in loc is safely convertible to F.

Proposed resolution:

In the definition of std::use_facet<>(), replace the text in paragraph 1 which reads:

> Get a reference to a facet of a locale.

with:

> Requires: `Facet` is a facet class whose definition contains the public static member `id` as defined in 28.3.3.1.2.2 [locale.facet].

[ Kona: strike as overspecification the text "(not inherits)" from the original resolution, which read "... whose definition contains (not inherits) the public static member `id`..." ]

### 39(i). istreambuf_iterator<>::operator++(int) definition garbled

Section: 24.6.4.4 [istreambuf.iterator.ops] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2017-11-29

Priority: Not Prioritized

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Discussion:

Following the definition of istreambuf_iterator<>::operator++(int) in paragraph 3, the standard contains three lines of garbage text left over from a previous edit.

>     istreambuf_iterator<charT,traits> tmp = *this;
>     sbuf_->sbumpc();
>     return(tmp);

Proposed resolution:

In [istreambuf.iterator::op++], delete the three lines of code at the end of paragraph 3.

### 40(i). Meaningless normative paragraph in examples

Section: 99 [facets.examples] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 3 of the locale examples is a description of part of an implementation technique that has lost its referent, and doesn't mean anything.

Proposed resolution:

Delete 99 [facets.examples] paragraph 3 which begins "This initialization/identification system depends...", or (at the editor's option) replace it with a place-holder to keep the paragraph numbering the same.

### 41(i). Ios_base needs clear(), exceptions()

Section: 31.5.2 [ios.base] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 157

Discussion:

The description of ios_base::iword() and pword() in 31.5.2.5 [ios.members.static], say that if they fail, they "set badbit, which may throw an exception". However, ios_base offers no interface to set or to test badbit; those interfaces are defined in basic_ios<>.

Proposed resolution:

Change the description in 31.5.2.6 [ios.base.storage] in paragraph 2, and also in paragraph 4, as follows. Replace

> If the function fails it sets badbit, which may throw an exception.

with

> If the function fails, and `*this` is a base sub-object of a `basic_ios<>` object or sub-object, the effect is equivalent to calling `basic_ios<>::setstate(badbit)` on the derived object (which may throw `failure`).

[Kona: LWG reviewed wording; setstate(failbit) changed to setstate(badbit).]

### 42(i). String ctors specify wrong default allocator

Section: 27.4.3 [basic.string] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The basic_string<> copy constructor:

    basic_string(const basic_string& str, size_type pos = 0,
                 size_type n = npos, const Allocator& a = Allocator());

specifies an Allocator argument default value that is counter-intuitive. The natural choice for a the allocator to copy from is str.get_allocator(). Though this cannot be expressed in default-argument notation, overloading suffices.

Alternatively, the other containers in Clause 23 (deque, list, vector) do not have this form of constructor, so it is inconsistent, and an evident source of confusion, for basic_string<> to have it, so it might better be removed.

Proposed resolution:

In 27.4.3 [basic.string], replace the declaration of the copy constructor as follows:

>     basic_string(const basic_string& str);
>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>                  const Allocator& a = Allocator());

In 27.4.3.2 [string.require], replace the copy constructor declaration as above. Add to paragraph 5, Effects:

> In the first form, the Allocator value used is copied from `str.get_allocator()`.

Rationale:

The LWG believes the constructor is actually broken, rather than just an unfortunate design choice.

The LWG considered two other possible resolutions:

A. In 27.4.3 [basic.string], replace the declaration of the copy constructor as follows:

>     basic_string(const basic_string& str, size_type pos = 0,
>                  size_type n = npos);
>     basic_string(const basic_string& str, size_type pos,
>                  size_type n, const Allocator& a);

In 27.4.3.2 [string.require], replace the copy constructor declaration as above. Add to paragraph 5, Effects:

> When no `Allocator` argument is provided, the string is constructed using the value `str.get_allocator()`.

B. In 27.4.3 [basic.string], and also in 27.4.3.2 [string.require], replace the declaration of the copy constructor as follows:

>     basic_string(const basic_string& str, size_type pos = 0,
>                  size_type n = npos);

The proposed resolution reflects the original intent of the LWG. It was also noted by Pete Becker that this fix "will cause a small amount of existing code to now work correctly."

[ Kona: issue editing snafu fixed - the proposed resolution now correctly reflects the LWG consensus. ]

### 44(i). Iostreams use operator== on int_type values

Section: 31 [input.output] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Many of the specifications for iostreams specify that character values or their int_type equivalents are compared using operators == or !=, though in other places traits::eq() or traits::eq_int_type is specified to be used throughout. This is an inconsistency; we should change uses of == and != to use the traits members instead.

Proposed resolution:

[Pre-Kona: Dietmar supplied wording]

List of changes to clause 27:

1. In lib.basic.ios.members paragraph 13 (postcondition clause for 'fill(cT)') change

   >          fillch == fill()

   to

   >          traits::eq(fillch, fill())

2. In lib.istream.unformatted paragraph 7 (effects clause for 'get(cT,streamsize,cT)'), third bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq(c, delim) for the next available input character c

3. In lib.istream.unformatted paragraph 12 (effects clause for 'get(basic_streambuf<cT,Tr>&,cT)'), third bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq(c, delim) for the next available input character c

4. In lib.istream.unformatted paragraph 17 (effects clause for 'getline(cT,streamsize,cT)'), second bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq(c, delim) for the next available input character c

5. In lib.istream.unformatted paragraph 24 (effects clause for 'ignore(int,int_type)'), second bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq_int_type(c, delim) for the next available input
   >          character c

6. In lib.istream.unformatted paragraph 25 (notes clause for 'ignore(int,int_type)'), second bullet, change

   >          The last condition will never occur if delim == traits::eof()

   to

   >          The last condition will never occur if
   >          traits::eq_int_type(delim, traits::eof()).

7. In lib.istream.sentry paragraph 6 (example implementation for the sentry constructor) change

   >          while ((c = is.rdbuf()->snextc()) != traits::eof()) {

   to

   >          while (!traits::eq_int_type(c = is.rdbuf()->snextc(), traits::eof())) {

List of changes to Chapter 21:

1. In lib.string::find paragraph 1 (effects clause for find()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

2. In lib.string::rfind paragraph 1 (effects clause for rfind()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

3. In lib.string::find.first.of paragraph 1 (effects clause for find_first_of()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

4. In lib.string::find.last.of paragraph 1 (effects clause for find_last_of()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

5. In lib.string::find.first.not.of paragraph 1 (effects clause for find_first_not_of()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

6. In lib.string::find.last.not.of paragraph 1 (effects clause for find_last_not_of()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

7. In lib.string.ios paragraph 5 (effects clause for getline()), second bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq(c, delim) for the next available input character c

Notes:

- Fixing this issue highlights another sloppyness in lib.istream.unformatted paragraph 24: this clause mentions a "character" which is then compared to an 'int_type' (see item 5. in the list below). It is not clear whether this requires explicit words and if so what these words are supposed to be. A similar issue exists, BTW, for operator*() of istreambuf_iterator which returns the result of sgetc() as a character type (see lib.istreambuf.iterator::op* paragraph 1), and for operator++() of istreambuf_iterator which passes the result of sbumpc() to a constructor taking a char_type (see lib.istreambuf.iterator::operator++ paragraph 3). Similarily, the assignment operator ostreambuf_iterator passes a char_type to a function taking an int_type (see lib.ostreambuf.iter.ops paragraph 1).
- It is inconsistent to use comparisons using the traits functions in Chapter 27 while not using them in Chapter 21, especially as some of the inconsistent uses actually involve streams (eg. getline() on streams). To avoid leaving this issue open still longer due to this inconsistency (it is open since 1998), a list of changes to Chapter 21 is below.
- In Chapter 24 there are several places with statements like "the end of stream is reached (streambuf_type::sgetc() returns traits::eof())" (lib.istreambuf.iterator paragraph 1, lib.ostreambuf.iter.ops paragraph 5). It is unclear whether these should be clarified to use traits::eq_int_type() for detecting traits::eof().

### 46(i). Minor Annex D errors

Section: 99 [depr.str.strstreams] Status: TC1 Submitter: Brendan Kehoe Opened: 1998-06-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

See lib-6522 and edit-814.

Proposed resolution:

Change 99 [depr.strstreambuf] (since streambuf is a typedef of basic_streambuf<char>) from:

             virtual streambuf<char>* setbuf(char* s, streamsize n);

to:

             virtual streambuf* setbuf(char* s, streamsize n);

In [depr.strstream] insert the semicolon now missing after int_type:

         namespace std {
           class strstream
             : public basic_iostream<char> {
           public:
             // Types
             typedef char                                char_type;
             typedef typename char_traits<char>::int_type int_type
             typedef typename char_traits<char>::pos_type pos_type;

### 47(i). Imbue() and getloc() Returns clauses swapped

Section: 31.5.2.4 [ios.base.locales] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 27.4.2.3 specifies how imbue() and getloc() work. That section has two RETURNS clauses, and they make no sense as stated. They make perfect sense, though, if you swap them. Am I correct in thinking that paragraphs 2 and 4 just got mixed up by accident?

Proposed resolution:

In 31.5.2.4 [ios.base.locales] swap paragraphs 2 and 4.

### 48(i). Use of non-existent exception constructor

Section: 31.5.2.2.1 [ios.failure] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

27.4.2.1.1, paragraph 2, says that class failure initializes the base class, exception, with exception(msg). Class exception (see 18.6.1) has no such constructor.

Proposed resolution:

Replace [ios::failure], paragraph 2, with

> EFFECTS: Constructs an object of class `failure`.

### 49(i). Underspecification of ios_base::sync_with_stdio

Section: 31.5.2.5 [ios.members.static] Status: CD1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Two problems

(1) 27.4.2.4 doesn't say what ios_base::sync_with_stdio(f) returns. Does it return f, or does it return the previous synchronization state? My guess is the latter, but the standard doesn't say so.

(2) 27.4.2.4 doesn't say what it means for streams to be synchronized with stdio. Again, of course, I can make some guesses. (And I'm unhappy about the performance implications of those guesses, but that's another matter.)

Proposed resolution:

Change the following sentence in 31.5.2.5 [ios.members.static] returns clause from:

> `true` if the standard iostream objects (27.3) are synchronized and otherwise returns `false`.

to:

> `true` if the previous state of the standard iostream objects (27.3) was synchronized and otherwise returns `false`.

Add the following immediately after 31.5.2.5 [ios.members.static], paragraph 2:

> When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by

>       fputc(f, c);

> is the same as the effect of

>       str.rdbuf()->sputc(c);

> for any sequence of characters; the effect of extracting a character c by

>       c = fgetc(f);

> is the same as the effect of:

>       c = str.rdbuf()->sbumpc(c);

> for any sequences of characters; and the effect of pushing back a character c by

>       ungetc(c, f);

> is the same as the effect of

>       str.rdbuf()->sputbackc(c);

> for any sequence of characters. [Footnote: This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream. In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer. --End Footnote]

[pre-Copenhagen: PJP and Matt contributed the definition of "synchronization"]

[post-Copenhagen: proposed resolution was revised slightly: text was added in the non-normative footnote to say that operations on the two streams can be mixed arbitrarily.]

### 50(i). Copy constructor and assignment operator of ios_base

Section: 31.5.2 [ios.base] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

As written, ios_base has a copy constructor and an assignment operator. (Nothing in the standard says it doesn't have one, and all classes have copy constructors and assignment operators unless you take specific steps to avoid them.) However, nothing in 27.4.2 says what the copy constructor and assignment operator do.

My guess is that this was an oversight, that ios_base is, like basic_ios, not supposed to have a copy constructor or an assignment operator.

Jerry Schwarz comments: Yes, its an oversight, but in the opposite sense to what you're suggesting. At one point there was a definite intention that you could copy ios_base. It's an easy way to save the entire state of a stream for future use. As you note, to carry out that intention would have required a explicit description of the semantics (e.g. what happens to the iarray and parray stuff).

Proposed resolution:

In 31.5.2 [ios.base], class ios_base, specify the copy constructor and operator= members as being private.

Rationale:

The LWG believes the difficulty of specifying correct semantics outweighs any benefit of allowing ios_base objects to be copyable.

### 51(i). Requirement to not invalidate iterators missing

Section: 23.2 [container.requirements] Status: TC1 Submitter: David Vandevoorde Opened: 1998-06-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The std::sort algorithm can in general only sort a given sequence by moving around values. The list<>::sort() member on the other hand could move around values or just update internal pointers. Either method can leave iterators into the list<> dereferencable, but they would point to different things.

Does the FDIS mandate anywhere which method should be used for list<>::sort()?

Matt Austern comments:

I think you've found an omission in the standard.

The library working group discussed this point, and there was supposed to be a general requirement saying that list, set, map, multiset, and multimap may not invalidate iterators, or change the values that iterators point to, except when an operation does it explicitly. So, for example, insert() doesn't invalidate any iterators and erase() and remove() only invalidate iterators pointing to the elements that are being erased.

I looked for that general requirement in the FDIS, and, while I found a limited form of it for the sorted associative containers, I didn't find it for list. It looks like it just got omitted.

The intention, though, is that list<>::sort does not invalidate any iterators and does not change the values that any iterator points to. There would be no reason to have the member function otherwise.

Proposed resolution:

Add a new paragraph at the end of 23.1:

> Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.

Rationale:

This was US issue CD2-23-011; it was accepted in London but the change was not made due to an editing oversight. The wording in the proposed resolution below is somewhat updated from CD2-23-011, particularly the addition of the phrase "or change the values of"

### 52(i). Small I/O problems

Section: 31.5.3.3 [fpos.operations] Status: TC1 Submitter: Matt Austern Opened: 1998-06-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

First, 31.5.4.2 [basic.ios.cons], table 89. This is pretty obvious: it should be titled "basic_ios<>() effects", not "ios_base() effects".

[The second item is a duplicate; see issue 6(i) for resolution.]

Second, 31.5.3.3 [fpos.operations] table 88 . There are a couple different things wrong with it, some of which I've already discussed with Jerry, but the most obvious mechanical sort of error is that it uses expressions like P(i) and p(i), without ever defining what sort of thing "i" is.

(The other problem is that it requires support for streampos arithmetic. This is impossible on some systems, i.e. ones where file position is a complicated structure rather than just a number. Jerry tells me that the intention was to require syntactic support for streampos arithmetic, but that it wasn't actually supposed to do anything meaningful except on platforms, like Unix, where genuine arithmetic is possible.)

Proposed resolution:

Change 31.5.4.2 [basic.ios.cons] table 89 title from "ios_base() effects" to "basic_ios<>() effects".

### 53(i). Basic_ios destructor unspecified

Section: 31.5.4.2 [basic.ios.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-06-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There's nothing in 27.4.4 saying what basic_ios's destructor does. The important question is whether basic_ios::~basic_ios() destroys rdbuf().

Proposed resolution:

Add after 31.5.4.2 [basic.ios.cons] paragraph 2:

> `virtual ~basic_ios();`

> Notes: The destructor does not destroy `rdbuf()`.

Rationale:

The LWG reviewed the additional question of whether or not `rdbuf(0)` may set `badbit`. The answer is clearly yes; it may be set via `clear()`. See 31.5.4.3 [basic.ios.members], paragraph 6. This issue was reviewed at length by the LWG, which removed from the original proposed resolution a footnote which incorrectly said "`rdbuf(0)` does not set `badbit`".

### 54(i). Basic_streambuf's destructor

Section: 31.6.3.2 [streambuf.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-06-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The class synopsis for basic_streambuf shows a (virtual) destructor, but the standard doesn't say what that destructor does. My assumption is that it does nothing, but the standard should say so explicitly.

Proposed resolution:

Add after 31.6.3.2 [streambuf.cons] paragraph 2:

> `virtual ~basic_streambuf();`

> Effects: None.

### 55(i). Invalid stream position is undefined

Section: 31 [input.output] Status: TC1 Submitter: Matt Austern Opened: 1998-06-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Several member functions in clause 27 are defined in certain circumstances to return an "invalid stream position", a term that is defined nowhere in the standard. Two places (27.5.2.4.2, paragraph 4, and 27.8.1.4, paragraph 15) contain a cross-reference to a definition in _lib.iostreams.definitions_, a nonexistent section.

I suspect that the invalid stream position is just supposed to be pos_type(-1). Probably best to say explicitly in (for example) 27.5.2.4.2 that the return value is pos_type(-1), rather than to use the term "invalid stream position", define that term somewhere, and then put in a cross-reference.

The phrase "invalid stream position" appears ten times in the C++ Standard. In seven places it refers to a return value, and it should be changed. In three places it refers to an argument, and it should not be changed. Here are the three places where "invalid stream position" should not be changed:

> 31.8.2.5 [stringbuf.virtuals], paragraph 14
> 31.10.3.5 [filebuf.virtuals], paragraph 14
> 99 [depr.strstreambuf.virtuals], paragraph 17

Proposed resolution:

In 31.6.3.5.2 [streambuf.virt.buffer], paragraph 4, change "Returns an object of class pos_type that stores an invalid stream position (_lib.iostreams.definitions_)" to "Returns `pos_type(off_type(-1))`".

In 31.6.3.5.2 [streambuf.virt.buffer], paragraph 6, change "Returns an object of class pos_type that stores an invalid stream position" to "Returns `pos_type(off_type(-1))`".

In 31.8.2.5 [stringbuf.virtuals], paragraph 13, change "the object stores an invalid stream position" to "the return value is `pos_type(off_type(-1))`".

In 31.10.3.5 [filebuf.virtuals], paragraph 13, change "returns an invalid stream position (27.4.3)" to "returns `pos_type(off_type(-1))`"

In 31.10.3.5 [filebuf.virtuals], paragraph 15, change "Otherwise returns an invalid stream position (_lib.iostreams.definitions_)" to "Otherwise returns `pos_type(off_type(-1))`"

In 99 [depr.strstreambuf.virtuals], paragraph 15, change "the object stores an invalid stream position" to "the return value is `pos_type(off_type(-1))`"

In 99 [depr.strstreambuf.virtuals], paragraph 18, change "the object stores an invalid stream position" to "the return value is `pos_type(off_type(-1))`"

### 56(i). Showmanyc's return type

Section: 31.6.3 [streambuf] Status: TC1 Submitter: Matt Austern Opened: 1998-06-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The class summary for basic_streambuf<>, in 27.5.2, says that showmanyc has return type int. However, 27.5.2.4.3 says that its return type is streamsize.

Proposed resolution:

Change `showmanyc`'s return type in the 31.6.3 [streambuf] class summary to `streamsize`.

### 57(i). Mistake in char_traits

Section: 27.2.4.6 [char.traits.specializations.wchar.t] Status: TC1 Submitter: Matt Austern Opened: 1998-07-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

21.1.3.2, paragraph 3, says "The types streampos and wstreampos may be different if the implementation supports no shift encoding in narrow-oriented iostreams but supports one or more shift encodings in wide-oriented streams".

That's wrong: the two are the same type. The <iosfwd> summary in 27.2 says that streampos and wstreampos are, respectively, synonyms for fpos<char_traits<char>::state_type> and fpos<char_traits<wchar_t>::state_type>, and, flipping back to clause 21, we see in 21.1.3.1 and 21.1.3.2 that char_traits<char>::state_type and char_traits<wchar_t>::state_type must both be mbstate_t.

Proposed resolution:

Remove the sentence in 27.2.4.6 [char.traits.specializations.wchar.t] paragraph 3 which begins "The types streampos and wstreampos may be different..." .

### 59(i). Ambiguity in specification of gbump

Section: 31.6.3.4.2 [streambuf.get.area] Status: TC1 Submitter: Matt Austern Opened: 1998-07-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

27.5.2.3.1 says that basic_streambuf::gbump() "Advances the next pointer for the input sequence by n."

The straightforward interpretation is that it is just gptr() += n. An alternative interpretation, though, is that it behaves as if it calls sbumpc n times. (The issue, of course, is whether it might ever call underflow.) There is a similar ambiguity in the case of pbump.

(The "classic" AT&T implementation used the former interpretation.)

Proposed resolution:

Change 31.6.3.4.2 [streambuf.get.area] paragraph 4 gbump effects from:

> Effects: Advances the next pointer for the input sequence by n.

to:

> Effects: Adds `n` to the next pointer for the input sequence.

Make the same change to 31.6.3.4.3 [streambuf.put.area] paragraph 4 pbump effects.

### 60(i). What is a formatted input function?

Section: 31.7.5.3.1 [istream.formatted.reqmts] Status: TC1 Submitter: Matt Austern Opened: 1998-08-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 162, 163, 166

Discussion:

Paragraph 1 of 27.6.1.2.1 contains general requirements for all formatted input functions. Some of the functions defined in section 27.6.1.2 explicitly say that those requirements apply ("Behaves like a formatted input member (as described in 27.6.1.2.1)"), but others don't. The question: is 27.6.1.2.1 supposed to apply to everything in 27.6.1.2, or only to those member functions that explicitly say "behaves like a formatted input member"? Or to put it differently: are we to assume that everything that appears in a section called "Formatted input functions" really is a formatted input function? I assume that 27.6.1.2.1 is intended to apply to the arithmetic extractors (27.6.1.2.2), but I assume that it is not intended to apply to extractors like

        basic_istream& operator>>(basic_istream& (*pf)(basic_istream&));

and

        basic_istream& operator>>(basic_streammbuf*);

There is a similar ambiguity for unformatted input, formatted output, and unformatted output.

Comments from Judy Ward: It seems like the problem is that the basic_istream and basic_ostream operator <<()'s that are used for the manipulators and streambuf* are in the wrong section and should have their own separate section or be modified to make it clear that the "Common requirements" listed in section 27.6.1.2.1 (for basic_istream) and section 27.6.2.5.1 (for basic_ostream) do not apply to them.

Additional comments from Dietmar Kühl: It appears to be somewhat nonsensical to consider the functions defined in [istream::extractors] paragraphs 1 to 5 to be "Formatted input function" but since these functions are defined in a section labeled "Formatted input functions" it is unclear to me whether these operators are considered formatted input functions which have to conform to the "common requirements" from 31.7.5.3.1 [istream.formatted.reqmts]: If this is the case, all manipulators, not just `ws`, would skip whitespace unless `noskipws` is set (... but setting `noskipws` using the manipulator syntax would also skip whitespace :-)

It is not clear which functions are to be considered unformatted input functions. As written, it seems that all functions in 31.7.5.4 [istream.unformatted] are unformatted input functions. However, it does not really make much sense to construct a sentry object for `gcount()`, `sync()`, ... Also it is unclear what happens to the `gcount()` if eg. `gcount()`, `putback()`, `unget()`, or `sync()` is called: These functions don't extract characters, some of them even "unextract" a character. Should this still be reflected in `gcount()`? Of course, it could be read as if after a call to `gcount()` `gcount()` return `0` (the last unformatted input function, `gcount()`, didn't extract any character) and after a call to `putback()` `gcount()` returns `-1` (the last unformatted input function `putback()` did "extract" back into the stream). Correspondingly for `unget()`. Is this what is intended? If so, this should be clarified. Otherwise, a corresponding clarification should be used.

Proposed resolution:

In 27.6.1.2.2 [lib.istream.formatted.arithmetic], paragraph 1. Change the beginning of the second sentence from "The conversion occurs" to "These extractors behave as formatted input functions (as described in 27.6.1.2.1). After a sentry object is constructed, the conversion occurs"

In 27.6.1.2.3, [lib.istream::extractors], before paragraph 1. Add an effects clause. "Effects: None. This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 2. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 4. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 12. Change the first two sentences from "If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). Extracts characters from *this..." to "Behaves as a formatted input function (as described in 27.6.1.2.1). If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). After a sentry object is constructed, extracts characters from *this...".

In 27.6.1.3, [lib.istream.unformatted], before paragraph 2. Add an effects clause. "Effects: none. This member function does not behave as an unformatted input function (as described in 27.6.1.3, paragraph 1)."

In 27.6.1.3, [lib.istream.unformatted], paragraph 3. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"

In 27.6.1.3, [lib.istream.unformatted], paragraph 5. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"

In 27.6.1.3, [lib.istream.unformatted], paragraph 7. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 10, because it refers to paragraph 7.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 12. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 15.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 17. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 23.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 24. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

In 27.6.1.3, [lib.istream.unformatted], before paragraph 27. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, reads but does not extract the current input character."

In 27.6.1.3, [lib.istream.unformatted], paragraph 28. Change the first sentence of the Effects clause from "If !good() calls" to Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"

In 27.6.1.3, [lib.istream.unformatted], paragraph 30. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"

In 27.6.1.3, [lib.istream.unformatted], paragraph 32. Change the first sentence of the Effects clause from "If !good() calls..." to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls..." Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"

In 27.6.1.3, [lib.istream.unformatted], paragraph 34. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls". Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"

In 27.6.1.3, [lib.istream.unformatted], paragraph 36. Change the first sentence of the Effects clause from "If !rdbuf() is" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if rdbuf() is"

In 27.6.1.3, [lib.istream.unformatted], before paragraph 37. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount()." Change the first sentence of paragraph 37 from "if fail()" to "after constructing a sentry object, if fail()".

In 27.6.1.3, [lib.istream.unformatted], paragraph 38. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()

In 27.6.1.3, [lib.istream.unformatted], paragraph 40. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()

In 27.6.2.5.2 [lib.ostream.inserters.arithmetic], paragraph 1. Change the beginning of the third sentence from "The formatting conversion" to "These extractors behave as formatted output functions (as described in 27.6.2.5.1). After the sentry object is constructed, the conversion occurs".

In 27.6.2.5.3 [lib.ostream.inserters], before paragraph 1. Add an effects clause: "Effects: None. Does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 2. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 4. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 6. Change the first sentence from "If sb" to "Behaves as a formatted output function (as described in 27.6.2.5.1). After the sentry object is constructed, if sb".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 2. Change the first sentence from "Inserts the character" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, inserts the character".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 5. Change the first sentence from "Obtains characters" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, obtains characters".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 7. Add a new sentence at the end of the paragraph: "Does not behave as an unformatted output function (as described in 27.6.2.6, paragraph 1)."

Rationale:

See J16/99-0043==WG21/N1219, Proposed Resolution to Library Issue 60, by Judy Ward and Matt Austern. This proposed resolution is section VI of that paper.

### 61(i). Ambiguity in iostreams exception policy

Section: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The introduction to the section on unformatted input (27.6.1.3) says that every unformatted input function catches all exceptions that were thrown during input, sets badbit, and then conditionally rethrows the exception. That seems clear enough. Several of the specific functions, however, such as get() and read(), are documented in some circumstances as setting eofbit and/or failbit. (The standard notes, correctly, that setting eofbit or failbit can sometimes result in an exception being thrown.) The question: if one of these functions throws an exception triggered by setting failbit, is this an exception "thrown during input" and hence covered by 27.6.1.3, or does 27.6.1.3 only refer to a limited class of exceptions? Just to make this concrete, suppose you have the following snippet.

      char buffer[N];
      istream is;
      ...
      is.exceptions(istream::failbit); // Throw on failbit but not on badbit.
      is.read(buffer, N);

Now suppose we reach EOF before we've read N characters. What iostate bits can we expect to be set, and what exception (if any) will be thrown?

Proposed resolution:

In 27.6.1.3, paragraph 1, after the sentence that begins "If an exception is thrown...", add the following parenthetical comment: "(Exceptions thrown from `basic_ios<>::clear()` are not caught or rethrown.)"

Rationale:

The LWG looked to two alternative wordings, and choose the proposed resolution as better standardese.

### 62(i). `Sync`'s return value

Section: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The Effects clause for sync() (27.6.1.3, paragraph 36) says that it "calls rdbuf()->pubsync() and, if that function returns -1 ... returns traits::eof()."

That looks suspicious, because traits::eof() is of type traits::int_type while the return type of sync() is int.

Proposed resolution:

In 31.7.5.4 [istream.unformatted], paragraph 36, change "returns `traits::eof()`" to "returns `-1`".

### 63(i). Exception-handling policy for unformatted output

Section: 31.7.6.4 [ostream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Clause 27 details an exception-handling policy for formatted input, unformatted input, and formatted output. It says nothing for unformatted output (27.6.2.6). 27.6.2.6 should either include the same kind of exception-handling policy as in the other three places, or else it should have a footnote saying that the omission is deliberate.

Proposed resolution:

In 27.6.2.6, paragraph 1, replace the last sentence ("In any case, the unformatted output function ends by destroying the sentry object, then returning the value specified for the formatted output function.") with the following text:

> If an exception is thrown during output, then `ios::badbit` is turned on [Footnote: without causing an `ios::failure` to be thrown.] in `*this`'s error state. If `(exceptions() & badbit) != 0` then the exception is rethrown. In any case, the unformatted output function ends by destroying the sentry object, then, if no exception was thrown, returning the value specified for the formatted output function.

Rationale:

This exception-handling policy is consistent with that of formatted input, unformatted input, and formatted output.

### 64(i). Exception handling in `basic_istream::operator>>(basic_streambuf*)`

Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: Matt Austern Opened: 1998-08-11 Last modified: 2017-04-22

Priority: Not Prioritized

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Discussion:

27.6.1.2.3, paragraph 13, is ambiguous. It can be interpreted two different ways, depending on whether the second sentence is read as an elaboration of the first.

Proposed resolution:

Replace [istream::extractors], paragraph 13, which begins "If the function inserts no characters ..." with:

> If the function inserts no characters, it calls `setstate(failbit)`, which may throw `ios_base::failure` (27.4.4.3). If it inserted no characters because it caught an exception thrown while extracting characters from `sb` and `failbit` is on in `exceptions()` (27.4.4.3), then the caught exception is rethrown.

### 66(i). Strstreambuf::setbuf

Section: 99 [depr.strstreambuf.virtuals] Status: TC1 Submitter: Matt Austern Opened: 1998-08-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

D.7.1.3, paragraph 19, says that strstreambuf::setbuf "Performs an operation that is defined separately for each class derived from strstreambuf". This is obviously an incorrect cut-and-paste from basic_streambuf. There are no classes derived from strstreambuf.

Proposed resolution:

99 [depr.strstreambuf.virtuals], paragraph 19, replace the setbuf effects clause which currently says "Performs an operation that is defined separately for each class derived from strstreambuf" with:

> Effects: implementation defined, except that `setbuf(0,0)` has no effect.

### 68(i). Extractors for char* should store null at end

Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: Angelika Langer Opened: 1998-07-14 Last modified: 2017-04-22

Priority: Not Prioritized

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Discussion:

Extractors for char* (27.6.1.2.3) do not store a null character after the extracted character sequence whereas the unformatted functions like get() do. Why is this?

Comment from Jerry Schwarz: There is apparently an editing glitch. You'll notice that the last item of the list of what stops extraction doesn't make any sense. It was supposed to be the line that said a null is stored.

Proposed resolution:

[istream::extractors], paragraph 7, change the last list item from:

> A null byte (`charT()`) in the next position, which may be the first position if no characters were extracted.

to become a new paragraph which reads:

> Operator>> then stores a null byte (`charT()`) in the next position, which may be the first position if no characters were extracted.

### 69(i). Must elements of a vector be contiguous?

Section: 23.3.13 [vector] Status: TC1 Submitter: Andrew Koenig Opened: 1998-07-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The issue is this: Must the elements of a vector be in contiguous memory?

(Please note that this is entirely separate from the question of whether a vector iterator is required to be a pointer; the answer to that question is clearly "no," as it would rule out debugging implementations)

Proposed resolution:

Add the following text to the end of 23.3.13 [vector], paragraph 1.

> The elements of a vector are stored contiguously, meaning that if v is a `vector<T, Allocator>` where T is some type other than `bool`, then it obeys the identity `&v[n] == &v[0] + n` for all `0 <= n < v.size()`.

Rationale:

The LWG feels that as a practical matter the answer is clearly "yes". There was considerable discussion as to the best way to express the concept of "contiguous", which is not directly defined in the standard. Discussion included:

- An operational definition similar to the above proposed resolution is already used for valarray (29.6.2.4 [valarray.access]).
- There is no need to explicitly consider a user-defined operator& because elements must be copyconstructible (23.2 [container.requirements] para 3) and copyconstructible (16.4.4.2 [utility.arg.requirements]) specifies requirements for operator&.
- There is no issue of one-past-the-end because of language rules.

### 70(i). Uncaught_exception() missing throw() specification

Section: 17.9 [support.exception], 99 [uncaught] Status: TC1 Submitter: Steve Clamage Opened: 1998-08-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In article 3E04@pratique.fr, Valentin Bonnard writes:

uncaught_exception() doesn't have a throw specification.

It is intentional ? Does it means that one should be prepared to handle exceptions thrown from uncaught_exception() ?

uncaught_exception() is called in exception handling contexts where exception safety is very important.

Proposed resolution:

In [except.uncaught], paragraph 1, 17.9 [support.exception], and 99 [uncaught], add "throw()" to uncaught_exception().

### 71(i). Do_get_monthname synopsis missing argument

Section: 28.3.4.6.2 [locale.time.get] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The locale facet member `time_get<>::do_get_monthname` is described in 28.3.4.6.2.3 [locale.time.get.virtuals] with five arguments, consistent with do_get_weekday and with its specified use by member get_monthname. However, in the synopsis, it is specified instead with four arguments. The missing argument is the "end" iterator value.

Proposed resolution:

In 28.3.4.6.2 [locale.time.get], add an "end" argument to the declaration of member do_monthname as follows:

      virtual iter_type do_get_monthname(iter_type s, iter_type end, ios_base&,
                                         ios_base::iostate& err, tm* t) const;

### 74(i). Garbled text for `codecvt::do_max_length`

Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Matt Austern Opened: 1998-09-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The text of `codecvt::do_max_length`'s "Returns" clause (22.2.1.5.2, paragraph 11) is garbled. It has unbalanced parentheses and a spurious n.

Proposed resolution:

Replace 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 11 with the following:

> Returns: The maximum value that `do_length(state, from, from_end, 1)` can return for any valid range `[from, from_end)` and `stateT` value `state`. The specialization `codecvt<char, char, mbstate_t>::do_max_length()` returns 1.

### 75(i). Contradiction in `codecvt::length`'s argument types

Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Matt Austern Opened: 1998-09-18 Last modified: 2023-03-29

Priority: Not Prioritized

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Discussion:

The class synopses for classes `codecvt<>` (22.2.1.5) and `codecvt_byname<>` (22.2.1.6) say that the first parameter of the member functions `length` and `do_length` is of type `const stateT&`. The member function descriptions, however (22.2.1.5.1, paragraph 6; 22.2.1.5.2, paragraph 9) say that the type is `stateT&`. Either the synopsis or the summary must be changed.

If (as I believe) the member function descriptions are correct, then we must also add text saying how `do_length` changes its `stateT` argument.

Proposed resolution:

In 28.3.4.2.5 [locale.codecvt], and also in 28.3.4.2.6 [locale.codecvt.byname], change the `stateT` argument type on both member `length()` and member `do_length()` from

> `const stateT&`

to

> `stateT&`

In 28.3.4.2.5.3 [locale.codecvt.virtuals], add to the definition for member `do_length` a paragraph:

> Effects: The effect on the `state` argument is ``as if'' it called `do_in(state, from, from_end, from, to, to+max, to)` for `to` pointing to a buffer of at least `max` elements.

### 76(i). Can a `codecvt` facet always convert one internal character at a time?

Section: 28.3.4.2.5 [locale.codecvt] Status: CD1 Submitter: Matt Austern Opened: 1998-09-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This issue concerns the requirements on classes derived from `codecvt`, including user-defined classes. What are the restrictions on the conversion from external characters (e.g. `char`) to internal characters (e.g. `wchar_t`)? Or, alternatively, what assumptions about `codecvt` facets can the I/O library make?

The question is whether it's possible to convert from internal characters to external characters one internal character at a time, and whether, given a valid sequence of external characters, it's possible to pick off internal characters one at a time. Or, to put it differently: given a sequence of external characters and the corresponding sequence of internal characters, does a position in the internal sequence correspond to some position in the external sequence?

To make this concrete, suppose that `[first, last)` is a sequence of M external characters and that `[ifirst, ilast)` is the corresponding sequence of N internal characters, where N > 1. That is, `my_encoding.in()`, applied to `[first, last)`, yields `[ifirst, ilast)`. Now the question: does there necessarily exist a subsequence of external characters, `[first, last_1)`, such that the corresponding sequence of internal characters is the single character `*ifirst`?

(What a "no" answer would mean is that `my_encoding` translates sequences only as blocks. There's a sequence of M external characters that maps to a sequence of N internal characters, but that external sequence has no subsequence that maps to N-1 internal characters.)

Some of the wording in the standard, such as the description of `codecvt::do_max_length` (28.3.4.2.5.3 [locale.codecvt.virtuals], paragraph 11) and `basic_filebuf::underflow` (31.10.3.5 [filebuf.virtuals], paragraph 3) suggests that it must always be possible to pick off internal characters one at a time from a sequence of external characters. However, this is never explicitly stated one way or the other.

This issue seems (and is) quite technical, but it is important if we expect users to provide their own encoding facets. This is an area where the standard library calls user-supplied code, so a well-defined set of requirements for the user-supplied code is crucial. Users must be aware of the assumptions that the library makes. This issue affects positioning operations on `basic_filebuf`, unbuffered input, and several of `codecvt`'s member functions.

Proposed resolution:

Add the following text as a new paragraph, following 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 2:

> A `codecvt` facet that is used by `basic_filebuf` (31.10 [file.streams]) must have the property that if

>         do_out(state, from, from_end, from_next, to, to_lim, to_next)

> would return `ok`, where `from != from_end`, then

>         do_out(state, from, from + 1, from_next, to, to_end, to_next)

> must also return `ok`, and that if

>         do_in(state, from, from_end, from_next, to, to_lim, to_next)

> would return `ok`, where `to != to_lim`, then

>         do_in(state, from, from_end, from_next, to, to + 1, to_next)

> must also return `ok`. [Footnote: Informally, this means that `basic_filebuf` assumes that the mapping from internal to external characters is 1 to N: a `codecvt` that is used by `basic_filebuf` must be able to translate characters one internal character at a time. --End Footnote]

[Redmond: Minor change in proposed resolution. Original proposed resolution talked about "success", with a parenthetical comment that success meant returning `ok`. New wording removes all talk about "success", and just talks about the return value.]

Rationale:

The proposed resoluion says that conversions can be performed one internal character at a time. This rules out some encodings that would otherwise be legal. The alternative answer would mean there would be some internal positions that do not correspond to any external file position.

An example of an encoding that this rules out is one where the `internT` and `externT` are of the same type, and where the internal sequence `c1 c2` corresponds to the external sequence `c2 c1`.

It was generally agreed that `basic_filebuf` relies on this property: it was designed under the assumption that the external-to-internal mapping is N-to-1, and it is not clear that `basic_filebuf` is implementable without that restriction.

The proposed resolution is expressed as a restriction on `codecvt` when used by `basic_filebuf`, rather than a blanket restriction on all `codecvt` facets, because `basic_filebuf` is the only other part of the library that uses `codecvt`. If a user wants to define a `codecvt` facet that implements a more general N-to-M mapping, there is no reason to prohibit it, so long as the user does not expect `basic_filebuf` to be able to use it.

### 78(i). Typo: event_call_back

Section: 31.5.2 [ios.base] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

typo: event_call_back should be event_callback

Proposed resolution:

In the 31.5.2 [ios.base] synopsis change "event_call_back" to "event_callback".

### 79(i). Inconsistent declaration of polar()

Section: 29.4.2 [complex.syn], 29.4.7 [complex.value.ops] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 29.4.2 [complex.syn] polar is declared as follows:

       template<class T> complex<T> polar(const T&, const T&);

In 29.4.7 [complex.value.ops] it is declared as follows:

       template<class T> complex<T> polar(const T& rho, const T& theta = 0);

Thus whether the second parameter is optional is not clear.

Proposed resolution:

In 29.4.2 [complex.syn] change:

       template<class T> complex<T> polar(const T&, const T&);

to:

       template<class T> complex<T> polar(const T& rho, const T& theta = 0);

### 80(i). Global Operators of complex declared twice

Section: 29.4.2 [complex.syn], 29.4.3 [complex] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Both 26.2.1 and 26.2.2 contain declarations of global operators for class complex. This redundancy should be removed.

Proposed resolution:

Reduce redundancy according to the general style of the standard.

### 83(i). String::npos vs. string::max_size()

Section: 27.4.3 [basic.string] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 89

Discussion:

Many string member functions throw if size is getting or exceeding npos. However, I wonder why they don't throw if size is getting or exceeding max_size() instead of npos. May be npos is known at compile time, while max_size() is known at runtime. However, what happens if size exceeds max_size() but not npos, then? It seems the standard lacks some clarifications here.

Proposed resolution:

After 27.4.3 [basic.string] paragraph 4 ("The functions described in this clause...") add a new paragraph:

> For any string operation, if as a result of the operation, `size()` would exceed `max_size()` then the operation throws `length_error`.

Rationale:

The LWG believes length_error is the correct exception to throw.

### 86(i). String constructors don't describe exceptions

Section: 27.4.3.2 [string.require] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The constructor from a range:

    template<class InputIterator>
             basic_string(InputIterator begin, InputIterator end,
                          const Allocator& a = Allocator());

lacks a throws clause. However, I would expect that it throws according to the other constructors if the numbers of characters in the range equals npos (or exceeds max_size(), see above).

Proposed resolution:

In 27.4.3.2 [string.require], Strike throws paragraphs for constructors which say "Throws: length_error if n == npos."

Rationale:

Throws clauses for length_error if n == npos are no longer needed because they are subsumed by the general wording added by the resolution for issue 83(i).

### 90(i). Incorrect description of operator >> for strings

Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The effect of operator >> for strings contain the following item:

`isspace(c,getloc())` is true for the next available input character c.

Here `getloc()` has to be replaced by `is.getloc()`.

Proposed resolution:

In 27.4.4.4 [string.io] paragraph 1 Effects clause replace:

> `isspace(c,getloc())` is true for the next available input character c.

with:

> `isspace(c,is.getloc())` is true for the next available input character c.

### 91(i). Description of operator>> and getline() for string<> might cause endless loop

Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Operator >> and getline() for strings read until eof() in the input stream is true. However, this might never happen, if the stream can't read anymore without reaching EOF. So shouldn't it be changed into that it reads until !good() ?

Proposed resolution:

In 27.4.4.4 [string.io], paragraph 1, replace:

> Effects: Begins by constructing a sentry object k as if k were constructed by typename basic_istream<charT,traits>::sentry k( is). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:

with:

> Effects: Behaves as a formatted input function (31.7.5.3.1 [istream.formatted.reqmts]). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:

In 27.4.4.4 [string.io], paragraph 6, replace

> Effects: Begins by constructing a sentry object k as if by typename basic_istream<charT,traits>::sentry k( is, true). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c) until any of the following occurs:

with:

> Effects: Behaves as an unformatted input function (31.7.5.4 [istream.unformatted]), except that it does not affect the value returned by subsequent calls to basic_istream<>::gcount(). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c) until any of the following occurs:

[Redmond: Made changes in proposed resolution. `operator>>` should be a formatted input function, not an unformatted input function. `getline` should not be required to set `gcount`, since there is no mechanism for `gcount` to be set except by one of `basic_istream`'s member functions.]

[Curaçao: Nico agrees with proposed resolution.]

Rationale:

The real issue here is whether or not these string input functions get their characters from a streambuf, rather than by calling an istream's member functions, a streambuf signals failure either by returning eof or by throwing an exception; there are no other possibilities. The proposed resolution makes it clear that these two functions do get characters from a streambuf.

### 92(i). Incomplete Algorithm Requirements

Section: 26 [algorithms] Status: CD1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The standard does not state, how often a function object is copied, called, or the order of calls inside an algorithm. This may lead to surprising/buggy behavior. Consider the following example:

    class Nth {    // function object that returns true for the nth element
      private:
        int nth;     // element to return true for
        int count;   // element counter
      public:
        Nth (int n) : nth(n), count(0) {
        }
        bool operator() (int) {
            return ++count == nth;
        }
    };
    ....
    // remove third element
        list<int>::iterator pos;
        pos = remove_if(coll.begin(),coll.end(),  // range
                        Nth(3)),                  // remove criterion
        coll.erase(pos,coll.end());

This call, in fact removes the 3rd AND the 6th element. This happens because the usual implementation of the algorithm copies the function object internally:

    template <class ForwIter, class Predicate>
    ForwIter std::remove_if(ForwIter beg, ForwIter end, Predicate op)
    {
        beg = find_if(beg, end, op);
        if (beg == end) {
            return beg;
        }
        else {
            ForwIter next = beg;
            return remove_copy_if(++next, end, beg, op);
        }
    }

The algorithm uses find_if() to find the first element that should be removed. However, it then uses a copy of the passed function object to process the resulting elements (if any). Here, Nth is used again and removes also the sixth element. This behavior compromises the advantage of function objects being able to have a state. Without any cost it could be avoided (just implement it directly instead of calling find_if()).

Proposed resolution:

Add a new paragraph following 26 [algorithms] paragraph 8:

> [Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely. Programmers for whom object identity is important should consider using a wrapper class that points to a noncopied implementation object, or some equivalent solution.]

[Dublin: Pete Becker felt that this may not be a defect, but rather something that programmers need to be educated about. There was discussion of adding wording to the effect that the number and order of calls to function objects, including predicates, not affect the behavior of the function object.]

[Pre-Kona: Nico comments: It seems the problem is that we don't have a clear statement of "predicate" in the standard. People including me seemed to think "a function returning a Boolean value and being able to be called by an STL algorithm or be used as sorting criterion or ... is a predicate". But a predicate has more requirements: It should never change its behavior due to a call or being copied. IMHO we have to state this in the standard. If you like, see section 8.1.4 of my library book for a detailed discussion.]

[Kona: Nico will provide wording to the effect that "unless otherwise specified, the number of copies of and calls to function objects by algorithms is unspecified". Consider placing in 26 [algorithms] after paragraph 9.]

[Santa Cruz: The standard doesn't currently guarantee that functions object won't be copied, and what isn't forbidden is allowed. It is believed (especially since implementations that were written in concert with the standard do make copies of function objects) that this was intentional. Thus, no normative change is needed. What we should put in is a non-normative note suggesting to programmers that if they want to guarantee the lack of copying they should use something like the `ref` wrapper.]

[Oxford: Matt provided wording.]

### 98(i). Input iterator requirements are badly written

Section: 24.3.5.3 [input.iterators] Status: CD1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 72 in 24.3.5.3 [input.iterators] specifies semantics for `*r++` of:

`{ T tmp = *r; ++r; return tmp; }`

There are two problems with this. First, the return type is specified to be "T", as opposed to something like "convertible to T". This is too specific: we want to allow *r++ to return an lvalue.

Second, writing the semantics in terms of code misleadingly suggests that the effects *r++ should precisely replicate the behavior of this code, including side effects. (Does this mean that *r++ should invoke the copy constructor exactly as many times as the sample code above would?) See issue 334(i) for a similar problem.

Proposed resolution:

In Table 72 in 24.3.5.3 [input.iterators], change the return type for `*r++` from `T` to "convertible to T".

Rationale:

This issue has two parts: the return type, and the number of times the copy constructor is invoked.

The LWG believes the the first part is a real issue. It's inappropriate for the return type to be specified so much more precisely for *r++ than it is for *r. In particular, if r is of (say) type `int*`, then *r++ isn't `int`, but `int&`.

The LWG does not believe that the number of times the copy constructor is invoked is a real issue. This can vary in any case, because of language rules on copy constructor elision. That's too much to read into these semantics clauses.

Additionally, as Dave Abrahams pointed out (c++std-lib-13703): since we're told (24.1/3) that forward iterators satisfy all the requirements of input iterators, we can't impose any requirements in the Input Iterator requirements table that forward iterators don't satisfy.

### 103(i). set::iterator is required to be modifiable, but this allows modification of keys

Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Set::iterator is described as implementation-defined with a reference to the container requirement; the container requirement says that const_iterator is an iterator pointing to const T and iterator an iterator pointing to T.

23.1.2 paragraph 2 implies that the keys should not be modified to break the ordering of elements. But that is not clearly specified. Especially considering that the current standard requires that iterator for associative containers be different from const_iterator. Set, for example, has the following:

`typedef implementation defined iterator;
// See _lib.container.requirements_`

23.2 [container.requirements] actually requires that iterator type pointing to T (table 65). Disallowing user modification of keys by changing the standard to require an iterator for associative container to be the same as const_iterator would be overkill since that will unnecessarily significantly restrict the usage of associative container. A class to be used as elements of set, for example, can no longer be modified easily without either redesigning the class (using mutable on fields that have nothing to do with ordering), or using const_cast, which defeats requiring iterator to be const_iterator. The proposed solution goes in line with trusting user knows what he is doing.

Other Options Evaluated:

Option A. In 23.2.7 [associative.reqmts], paragraph 2, after first sentence, and before "In addition,...", add one line:

> Modification of keys shall not change their strict weak ordering.

Option B. Add three new sentences to 23.2.7 [associative.reqmts]:

> At the end of paragraph 5: "Keys in an associative container are immutable." At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both `iterator` and `const_iterator` are constant iterators. It is unspecified whether or not `iterator` and `const_iterator` are the same type."

Option C. To 23.2.7 [associative.reqmts], paragraph 3, which currently reads:

> The phrase ``equivalence of keys'' means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 in the same container are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false.

add the following:

> For any two keys k1 and k2 in the same container, comp(k1, k2) shall return the same value whenever it is evaluated. [Note: If k2 is removed from the container and later reinserted, comp(k1, k2) must still return a consistent value but this value may be different than it was the first time k1 and k2 were in the same container. This is intended to allow usage like a string key that contains a filename, where comp compares file contents; if k2 is removed, the file is changed, and the same k2 (filename) is reinserted, comp(k1, k2) must again return a consistent value but this value may be different than it was the previous time k2 was in the container.]

Proposed resolution:

Add the following to 23.2.7 [associative.reqmts] at the indicated location:

> At the end of paragraph 3: "For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value."

> At the end of paragraph 5: "Keys in an associative container are immutable."

> At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both `iterator` and `const_iterator` are constant iterators. It is unspecified whether or not `iterator` and `const_iterator` are the same type."

Rationale:

Several arguments were advanced for and against allowing set elements to be mutable as long as the ordering was not effected. The argument which swayed the LWG was one of safety; if elements were mutable, there would be no compile-time way to detect of a simple user oversight which caused ordering to be modified. There was a report that this had actually happened in practice, and had been painful to diagnose. If users need to modify elements, it is possible to use mutable members or const_cast.

Simply requiring that keys be immutable is not sufficient, because the comparison object may indirectly (via pointers) operate on values outside of the keys.

The types `iterator` and `const_iterator` are permitted to be different types to allow for potential future work in which some member functions might be overloaded between the two types. No such member functions exist now, and the LWG believes that user functionality will not be impaired by permitting the two types to be the same. A function that operates on both iterator types can be defined for `const_iterator` alone, and can rely on the automatic conversion from `iterator` to `const_iterator`.

[Tokyo: The LWG crafted the proposed resolution and rationale.]

### 106(i). Numeric library private members are implementation defined

Section: 29.6.5 [template.slice.array] Status: TC1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This is the only place in the whole standard where the implementation has to document something private.

Proposed resolution:

Remove the comment which says "// remainder implementation defined" from:

- 29.6.5 [template.slice.array]
- 29.6.7 [template.gslice.array]
- 29.6.8 [template.mask.array]
- 29.6.9 [template.indirect.array]

### 108(i). Lifetime of exception::what() return unspecified

Section: 17.7.3 [type.info] Status: TC1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 18.6.1, paragraphs 8-9, the lifetime of the return value of exception::what() is left unspecified. This issue has implications with exception safety of exception handling: some exceptions should not throw bad_alloc.

Proposed resolution:

Add to 17.7.3 [type.info] paragraph 9 (exception::what notes clause) the sentence:

> The return value remains valid until the exception object from which it is obtained is destroyed or a non-const member function of the exception object is called.

Rationale:

If an exception object has non-const members, they may be used to set internal state that should affect the contents of the string returned by `what()`.

### 109(i). Missing binders for non-const sequence elements

Section: 99 [depr.lib.binders] Status: CD1 Submitter: Bjarne Stroustrup Opened: 1998-10-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There are no versions of binders that apply to non-const elements of a sequence. This makes examples like for_each() using bind2nd() on page 521 of "The C++ Programming Language (3rd)" non-conforming. Suitable versions of the binders need to be added.

Further discussion from Nico:

What is probably meant here is shown in the following example:

    class Elem {
      public:
        void print (int i) const { }
        void modify (int i) { }
    };

    int main()
    {
        vector<Elem> coll(2);
        for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::print),42));    // OK
        for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::modify),42));   // ERROR
    }

The error results from the fact that bind2nd() passes its first argument (the argument of the sequence) as constant reference. See the following typical implementation:

>     template <class Operation>
>     class binder2nd
>       : public unary_function<typename Operation::first_argument_type,
>                               typename Operation::result_type> {
>     protected:
>       Operation op;
>       typename Operation::second_argument_type value;
>     public:
>       binder2nd(const Operation& o,
>                 const typename Operation::second_argument_type& v)
>           : op(o), value(v) {}

>      typename Operation::result_type
>       operator()(const typename Operation::first_argument_type& x) const {
>         return op(x, value);
>       }
>     };

The solution is to overload operator () of bind2nd for non-constant arguments:

>     template <class Operation>
>     class binder2nd
>       : public unary_function<typename Operation::first_argument_type,
>                               typename Operation::result_type> {
>     protected:
>       Operation op;
>       typename Operation::second_argument_type value;
>     public:
>       binder2nd(const Operation& o,
>                 const typename Operation::second_argument_type& v)
>           : op(o), value(v) {}

>      typename Operation::result_type
>       operator()(const typename Operation::first_argument_type& x) const {
>         return op(x, value);
>       }
>       typename Operation::result_type
>       operator()(typename Operation::first_argument_type& x) const {
>         return op(x, value);
>       }
>     };

Proposed resolution:

Howard believes there is a flaw in this resolution. See c++std-lib-9127. We may need to reopen this issue.

In 99 [depr.lib.binders] in the declaration of binder1st after:

> `typename Operation::result_type
> operator()(const typename Operation::second_argument_type& x) const;`

insert:

> `typename Operation::result_type
> operator()(typename Operation::second_argument_type& x) const;`

In 99 [depr.lib.binders] in the declaration of binder2nd after:

> `typename Operation::result_type
> operator()(const typename Operation::first_argument_type& x) const;`

insert:

> `typename Operation::result_type
> operator()(typename Operation::first_argument_type& x) const;`

[Kona: The LWG discussed this at some length.It was agreed that this is a mistake in the design, but there was no consensus on whether it was a defect in the Standard. Straw vote: NAD - 5. Accept proposed resolution - 3. Leave open - 6.]

[Copenhagen: It was generally agreed that this was a defect. Strap poll: NAD - 0. Accept proposed resolution - 10. Leave open - 1.]

### 110(i). istreambuf_iterator::equal not const

Section: 24.6.4 [istreambuf.iterator], 24.6.4.4 [istreambuf.iterator.ops] Status: TC1 Submitter: Nathan Myers Opened: 1998-10-15 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

Member istreambuf_iterator<>::equal is not declared "const", yet [istreambuf.iterator::op==] says that operator==, which is const, calls it. This is contradictory.

Proposed resolution:

In 24.6.4 [istreambuf.iterator] and also in [istreambuf.iterator::equal], replace:

>     bool equal(istreambuf_iterator& b);

with:

>     bool equal(const istreambuf_iterator& b) const;

### 112(i). Minor typo in `ostreambuf_iterator` constructor

Section: 24.6.5.2 [ostreambuf.iter.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-10-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The requires clause for `ostreambuf_iterator`'s constructor from an `ostream_type` (24.5.4.1, paragraph 1) reads "s is not null". However, s is a reference, and references can't be null.

Proposed resolution:

In 24.6.5.2 [ostreambuf.iter.cons]:

Move the current paragraph 1, which reads "Requires: s is not null.", from the first constructor to the second constructor.

Insert a new paragraph 1 Requires clause for the first constructor reading:

> Requires: `s.rdbuf()` is not null.

### 114(i). Placement forms example in error twice

Section: 17.6.3.4 [new.delete.placement] Status: TC1 Submitter: Steve Clamage Opened: 1998-10-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 196

Discussion:

Section 18.5.1.3 contains the following example:

    [Example: This can be useful for constructing an object at a known address:
            char place[sizeof(Something)];
            Something* p = new (place) Something();
     -end example]

First code line: "place" need not have any special alignment, and the following constructor could fail due to misaligned data.

Second code line: Aren't the parens on Something() incorrect? [Dublin: the LWG believes the () are correct.]

Examples are not normative, but nevertheless should not show code that is invalid or likely to fail.

Proposed resolution:

Replace the first line of code in the example in 17.6.3.4 [new.delete.placement] with:

>     void* place = operator new(sizeof(Something));

### 115(i). Typo in strstream constructors

Section: 99 [depr.strstream.cons] Status: TC1 Submitter: Steve Clamage Opened: 1998-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

D.7.4.1 strstream constructors paragraph 2 says:

> Effects: Constructs an object of class strstream, initializing the base class with iostream(& sb) and initializing sb with one of the two constructors:

> - If mode&app==0, then s shall designate the first element of an array of n elements. The constructor is strstreambuf(s, n, s).

> - If mode&app==0, then s shall designate the first element of an array of n elements that contains an NTBS whose first element is designated by s. The constructor is strstreambuf(s, n, s+std::strlen(s)).

Notice the second condition is the same as the first. I think the second condition should be "If mode&app==app", or "mode&app!=0", meaning that the append bit is set.

Proposed resolution:

In [depr.ostrstream.cons] paragraph 2 and 99 [depr.strstream.cons] paragraph 2, change the first condition to `(mode&app)==0` and the second condition to `(mode&app)!=0`.

### 117(i). `basic_ostream` uses nonexistent `num_put` member functions

Section: 31.7.6.3.2 [ostream.inserters.arithmetic] Status: CD1 Submitter: Matt Austern Opened: 1998-11-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The effects clause for numeric inserters says that insertion of a value `x`, whose type is either `bool`, `short`, `unsigned short`, `int`, `unsigned int`, `long`, `unsigned long`, `float`, `double`, `long double`, or `const void*`, is delegated to `num_put`, and that insertion is performed as if through the following code fragment:

    bool failed = use_facet<
       num_put<charT,ostreambuf_iterator<charT,traits> >
       >(getloc()).put(*this, *this, fill(), val). failed();

This doesn't work, because `num_put<>`::put is only overloaded for the types `bool`, `long`, `unsigned long`, `double`, `long double`, and `const void*`. That is, the code fragment in the standard is incorrect (it is diagnosed as ambiguous at compile time) for the types `short`, `unsigned short`, `int`, `unsigned int`, and `float`.

We must either add new member functions to `num_put`, or else change the description in `ostream` so that it only calls functions that are actually there. I prefer the latter.

Proposed resolution:

Replace 27.6.2.5.2, paragraph 1 with the following:

> The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting. When val is of type bool, long, unsigned long, double, long double, or const void*, the formatting conversion occurs as if it performed the following code fragment:

>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(), val). failed();

> When val is of type short the formatting conversion occurs as if it performed the following code fragment:

>     ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield;
>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(),
>           baseflags == ios_base::oct || baseflags == ios_base::hex
>              ? static_cast<long>(static_cast<unsigned short>(val))
>              : static_cast<long>(val)). failed();

> When val is of type int the formatting conversion occurs as if it performed the following code fragment:

>     ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield;
>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(),
>           baseflags == ios_base::oct || baseflags == ios_base::hex
>              ? static_cast<long>(static_cast<unsigned int>(val))
>              : static_cast<long>(val)). failed();

> When val is of type unsigned short or unsigned int the formatting conversion occurs as if it performed the following code fragment:

>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(), static_cast<unsigned long>(val)).
>     failed();

> When val is of type float the formatting conversion occurs as if it performed the following code fragment:

>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(), static_cast<double>(val)).
>     failed();

[post-Toronto: This differs from the previous proposed resolution; PJP provided the new wording. The differences are in signed short and int output.]

Rationale:

The original proposed resolution was to cast int and short to long, unsigned int and unsigned short to unsigned long, and float to double, thus ensuring that we don't try to use nonexistent num_put<> member functions. The current proposed resolution is more complicated, but gives more expected results for hex and octal output of signed short and signed int. (On a system with 16-bit short, for example, printing short(-1) in hex format should yield 0xffff.)

### 118(i). `basic_istream` uses nonexistent `num_get` member functions

Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: CD1 Submitter: Matt Austern Opened: 1998-11-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.formatted.arithmetic].

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Discussion:

Formatted input is defined for the types `short`, `unsigned short`, `int`, `unsigned int`, `long`, `unsigned long`, `float`, `double`, `long double`, `bool`, and `void*`. According to section 27.6.1.2.2, formatted input of a value `x` is done as if by the following code fragment:

    typedef num_get< charT,istreambuf_iterator<charT,traits> > numget;
    iostate err = 0;
    use_facet< numget >(loc).get(*this, 0, *this, err, val);
    setstate(err);

According to section 28.3.4.3.2.2 [facet.num.get.members], however, `num_get<>::get()` is only overloaded for the types `bool`, `long`, `unsigned short`, `unsigned int`, `unsigned long`, `unsigned long`, `float`, `double`, `long double`, and `void*`. Comparing the lists from the two sections, we find that 27.6.1.2.2 is using a nonexistent function for types `short` and `int`.

Proposed resolution:

In 31.7.5.3.2 [istream.formatted.arithmetic] Arithmetic Extractors, remove the two lines (1st and 3rd) which read:

>     operator>>(short& val);
>     ...
>     operator>>(int& val);

And add the following at the end of that section (27.6.1.2.2) :

>     operator>>(short& val);

> The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

>       typedef num_get< charT,istreambuf_iterator<charT,traits> > numget;
>       iostate err = 0;
>       long lval;
>       use_facet< numget >(loc).get(*this, 0, *this, err, lval);
>             if (err == 0
>                     && (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval))
>                     err = ios_base::failbit;
>       setstate(err);

>     operator>>(int& val);

> The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

>       typedef num_get< charT,istreambuf_iterator<charT,traits> > numget;
>       iostate err = 0;
>       long lval;
>       use_facet< numget >(loc).get(*this, 0, *this, err, lval);
>             if (err == 0
>                     && (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval))
>                     err = ios_base::failbit;
>       setstate(err);

[Post-Tokyo: PJP provided the above wording.]

### 119(i). Should virtual functions be allowed to strengthen the exception specification?

Section: 16.4.6.14 [res.on.exception.handling] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [res.on.exception.handling].

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Discussion:

Section 16.4.6.14 [res.on.exception.handling] states:

"An implementation may strengthen the exception-specification for a function by removing listed exceptions."

The problem is that if an implementation is allowed to do this for virtual functions, then a library user cannot write a class that portably derives from that class.

For example, this would not compile if ios_base::failure::~failure had an empty exception specification:

    #include <ios>
    #include <string>

    class D : public std::ios_base::failure {
    public:
            D(const std::string&);
            ~D(); // error - exception specification must be compatible with
                  // overridden virtual function ios_base::failure::~failure()
    };

Proposed resolution:

Change Section 16.4.6.14 [res.on.exception.handling] from:

"may strengthen the exception-specification for a function"

to:

"may strengthen the exception-specification for a non-virtual function".

### 120(i). Can an implementor add specializations?

Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The original issue asked whether a library implementor could specialize standard library templates for built-in types. (This was an issue because users are permitted to explicitly instantiate standard library templates.)

Specializations are no longer a problem, because of the resolution to core issue 259. Under the proposed resolution, it will be legal for a translation unit to contain both a specialization and an explicit instantiation of the same template, provided that the specialization comes first. In such a case, the explicit instantiation will be ignored. Further discussion of library issue 120 assumes that the core 259 resolution will be adopted.

However, as noted in lib-7047, one piece of this issue still remains: what happens if a standard library implementor explicitly instantiates a standard library templates? It's illegal for a program to contain two different explicit instantiations of the same template for the same type in two different translation units (ODR violation), and the core working group doesn't believe it is practical to relax that restriction.

The issue, then, is: are users allowed to explicitly instantiate standard library templates for non-user defined types? The status quo answer is 'yes'. Changing it to 'no' would give library implementors more freedom.

This is an issue because, for performance reasons, library implementors often need to explicitly instantiate standard library templates. (for example, std::basic_string<char>) Does giving users freedom to explicitly instantiate standard library templates for non-user defined types make it impossible or painfully difficult for library implementors to do this?

John Spicer suggests, in lib-8957, that library implementors have a mechanism they can use for explicit instantiations that doesn't prevent users from performing their own explicit instantiations: put each explicit instantiation in its own object file. (Different solutions might be necessary for Unix DSOs or MS-Windows DLLs.) On some platforms, library implementors might not need to do anything special: the "undefined behavior" that results from having two different explicit instantiations might be harmless.

Proposed resolution:

Append to 16.4.5.3 [reserved.names] paragraph 1:

> A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.

[Kona: changed the wording from "a user-defined name" to "the name of a user-defined type"]

Rationale:

The LWG considered another possible resolution:

> In light of the resolution to core issue 259, no normative changes in the library clauses are necessary. Add the following non-normative note to the end of 16.4.5.3 [reserved.names] paragraph 1:

> > [Note: A program may explicitly instantiate standard library templates, even when an explicit instantiation does not depend on a user-defined name. --end note]

The LWG rejected this because it was believed that it would make it unnecessarily difficult for library implementors to write high-quality implementations. A program may not include an explicit instantiation of the same template, for the same template arguments, in two different translation units. If users are allowed to provide explicit instantiations of Standard Library templates for built-in types, then library implementors aren't, at least not without nonportable tricks.

The most serious problem is a class template that has writeable static member variables. Unfortunately, such class templates are important and, in existing Standard Library implementations, are often explicitly specialized by library implementors: locale facets, which have a writeable static member variable `id`. If a user's explicit instantiation collided with the implementations explicit instantiation, iostream initialization could cause locales to be constructed in an inconsistent state.

One proposed implementation technique was for Standard Library implementors to provide explicit instantiations in separate object files, so that they would not be picked up by the linker when the user also provides an explicit instantiation. However, this technique only applies for Standard Library implementations that are packaged as static archives. Most Standard Library implementations nowadays are packaged as dynamic libraries, so this technique would not apply.

The Committee is now considering standardization of dynamic linking. If there are such changes in the future, it may be appropriate to revisit this issue later.

### 122(i). streambuf/wstreambuf description should not say they are specializations

Section: 31.6.3 [streambuf] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 27.5.2 describes the streambuf classes this way:

> The class streambuf is a specialization of the template class basic_streambuf specialized for the type char.

> The class wstreambuf is a specialization of the template class basic_streambuf specialized for the type wchar_t.

This implies that these classes must be template specializations, not typedefs.

It doesn't seem this was intended, since Section 27.5 has them declared as typedefs.

Proposed resolution:

Remove 31.6.3 [streambuf] paragraphs 2 and 3 (the above two sentences).

Rationale:

The `streambuf` synopsis already has a declaration for the typedefs and that is sufficient.

### 123(i). Should valarray helper arrays fill functions be const?

Section: 29.6.5.4 [slice.arr.fill], 29.6.7.4 [gslice.array.fill], 29.6.8.4 [mask.array.fill], 29.6.9.4 [indirect.array.fill] Status: CD1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

One of the operator= in the valarray helper arrays is const and one is not. For example, look at slice_array. This operator= in Section 29.6.5.2 [slice.arr.assign] is const:

`void operator=(const valarray<T>&) const;`

but this one in Section 29.6.5.4 [slice.arr.fill] is not:

`void operator=(const T&);`

The description of the semantics for these two functions is similar.

Proposed resolution:

29.6.5 [template.slice.array] Template class slice_array

> In the class template definition for slice_array, replace the member function declaration

>           void operator=(const T&);

> with

>           void operator=(const T&) const;

29.6.5.4 [slice.arr.fill] slice_array fill function

> Change the function declaration

>           void operator=(const T&);

> to

>           void operator=(const T&) const;

29.6.7 [template.gslice.array] Template class gslice_array

> In the class template definition for gslice_array, replace the member function declaration

>           void operator=(const T&);

> with

>           void operator=(const T&) const;

29.6.7.4 [gslice.array.fill] gslice_array fill function

> Change the function declaration

>           void operator=(const T&);

> to

>           void operator=(const T&) const;

29.6.8 [template.mask.array] Template class mask_array

> In the class template definition for mask_array, replace the member function declaration

>           void operator=(const T&);

> with

>           void operator=(const T&) const;

29.6.8.4 [mask.array.fill] mask_array fill function

> Change the function declaration

>           void operator=(const T&);

> to

>           void operator=(const T&) const;

29.6.9 [template.indirect.array] Template class indirect_array

> In the class template definition for indirect_array, replace the member function declaration

>           void operator=(const T&);

> with

>           void operator=(const T&) const;

29.6.9.4 [indirect.array.fill] indirect_array fill function

> Change the function declaration

>           void operator=(const T&);

> to

>           void operator=(const T&) const;

[Redmond: Robert provided wording.]

Rationale:

There's no good reason for one version of operator= being const and another one not. Because of issue 253(i), this now matters: these functions are now callable in more circumstances. In many existing implementations, both versions are already const.

### 124(i). ctype_byname<charT>::do_scan_is & do_scan_not return type should be const charT*

Section: 28.3.4.2.3 [locale.ctype.byname] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [locale.ctype.byname].

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Discussion:

In Section 28.3.4.2.3 [locale.ctype.byname] ctype_byname<charT>::do_scan_is() and do_scan_not() are declared to return a const char* not a const charT*.

Proposed resolution:

Change Section 28.3.4.2.3 [locale.ctype.byname] `do_scan_is()` and `do_scan_not()` to return a `const charT*`.

### 125(i). valarray<T>::operator!() return type is inconsistent

Section: 29.6.2 [template.valarray] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In Section 29.6.2 [template.valarray] valarray<T>::operator!() is declared to return a valarray<T>, but in Section 29.6.2.6 [valarray.unary] it is declared to return a valarray<bool>. The latter appears to be correct.

Proposed resolution:

Change in Section 29.6.2 [template.valarray] the declaration of `operator!()` so that the return type is `valarray<bool>`.

### 126(i). typos in Effects clause of ctype::do_narrow()

Section: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Typos in 22.2.1.1.2 need to be fixed.

Proposed resolution:

In Section 28.3.4.2.2.3 [locale.ctype.virtuals] change:

       do_widen(do_narrow(c),0) == c

to:

       do_widen(do_narrow(c,0)) == c

and change:

       (is(M,c) || !ctc.is(M, do_narrow(c),dfault) )

to:

       (is(M,c) || !ctc.is(M, do_narrow(c,dfault)) )

### 127(i). auto_ptr<> conversion issues

Section: 99 [auto.ptr] Status: TC1 Submitter: Greg Colvin Opened: 1999-02-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There are two problems with the current `auto_ptr` wording in the standard:

First, the `auto_ptr_ref` definition cannot be nested because `auto_ptr<Derived>::auto_ptr_ref` is unrelated to `auto_ptr<Base>::auto_ptr_ref`. Also submitted by Nathan Myers, with the same proposed resolution.

Second, there is no `auto_ptr` assignment operator taking an `auto_ptr_ref` argument.

I have discussed these problems with my proposal coauthor, Bill Gibbons, and with some compiler and library implementors, and we believe that these problems are not desired or desirable implications of the standard.

25 Aug 1999: The proposed resolution now reflects changes suggested by Dave Abrahams, with Greg Colvin's concurrence; 1) changed "assignment operator" to "public assignment operator", 2) changed effects to specify use of release(), 3) made the conversion to auto_ptr_ref const.

2 Feb 2000: Lisa Lippincott comments: [The resolution of] this issue states that the conversion from auto_ptr to auto_ptr_ref should be const. This is not acceptable, because it would allow initialization and assignment from _any_ const auto_ptr! It also introduces an implementation difficulty in writing this conversion function -- namely, somewhere along the line, a const_cast will be necessary to remove that const so that release() may be called. This may result in undefined behavior [7.1.5.1/4]. The conversion operator does not have to be const, because a non-const implicit object parameter may be bound to an rvalue [13.3.3.1.4/3] [13.3.1/5].

Tokyo: The LWG removed the following from the proposed resolution:

In 21.3.5 [meta.unary], paragraph 2, and 21.3.5.4 [meta.unary.prop], paragraph 2, make the conversion to auto_ptr_ref const:

>     template<class Y> operator auto_ptr_ref<Y>() const throw();

Proposed resolution:

In 21.3.5 [meta.unary], paragraph 2, move the `auto_ptr_ref` definition to namespace scope.

In 21.3.5 [meta.unary], paragraph 2, add a public assignment operator to the `auto_ptr` definition:

>     auto_ptr& operator=(auto_ptr_ref<X> r) throw();

Also add the assignment operator to 21.3.5.4 [meta.unary.prop]:

>     auto_ptr& operator=(auto_ptr_ref<X> r) throw()

> Effects: Calls `reset(p.release())` for the `auto_ptr p` that `r` holds a reference to.
> Returns: `*this`.

### 129(i). Need error indication from seekp() and seekg()

Section: 31.7.5.4 [istream.unformatted], 31.7.6.2.5 [ostream.seeks] Status: TC1 Submitter: Angelika Langer Opened: 1999-02-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.unformatted].

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Discussion:

Currently, the standard does not specify how seekg() and seekp() indicate failure. They are not required to set failbit, and they can't return an error indication because they must return *this, i.e. the stream. Hence, it is undefined what happens if they fail. And they can fail, for instance, when a file stream is disconnected from the underlying file (is_open()==false) or when a wide character file stream must perform a state-dependent code conversion, etc.

The stream functions seekg() and seekp() should set failbit in the stream state in case of failure.

Proposed resolution:

Add to the Effects: clause of seekg() in 31.7.5.4 [istream.unformatted] and to the Effects: clause of seekp() in 31.7.6.2.5 [ostream.seeks]:

> In case of failure, the function calls `setstate(failbit)` (which may throw `ios_base::failure`).

Rationale:

Setting failbit is the usual error reporting mechanism for streams

### 130(i). Return type of container::erase(iterator) differs for associative containers

Section: 23.2.7 [associative.reqmts], 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 1999-03-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 451

Discussion:

Table 67 (23.1.1) says that container::erase(iterator) returns an iterator. Table 69 (23.1.2) says that in addition to this requirement, associative containers also say that container::erase(iterator) returns void. That's not an addition; it's a change to the requirements, which has the effect of making associative containers fail to meet the requirements for containers.

Proposed resolution:

In 23.2.7 [associative.reqmts], in Table 69 Associative container requirements, change the return type of `a.erase(q)` from `void` to `iterator`. Change the assertion/not/pre/post-condition from "erases the element pointed to by `q`" to "erases the element pointed to by `q`. Returns an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, a.end() is returned."

In 23.2.7 [associative.reqmts], in Table 69 Associative container requirements, change the return type of `a.erase(q1, q2)` from `void` to `iterator`. Change the assertion/not/pre/post-condition from "erases the elements in the range `[q1, q2)`" to "erases the elements in the range `[q1, q2)`. Returns q2."

In 23.4.3 [map], in the `map` class synopsis; and in 23.4.4 [multimap], in the `multimap` class synopsis; and in 23.4.6 [set], in the `set` class synopsis; and in 23.4.7 [multiset], in the `multiset` class synopsis: change the signature of the first `erase` overload to

       iterator erase(iterator position);

and change the signature of the third `erase` overload to

      iterator erase(iterator first, iterator last);

[Pre-Kona: reopened at the request of Howard Hinnant]

[Post-Kona: the LWG agrees the return type should be `iterator`, not `void`. (Alex Stepanov agrees too.) Matt provided wording.]

[ Sydney: the proposed wording went in the right direction, but it wasn't good enough. We want to return an iterator from the range form of erase as well as the single-iterator form. Also, the wording is slightly different from the wording we have for sequences; there's no good reason for having a difference. Matt provided new wording, (reflected above) which we will review at the next meeting. ]

[ Redmond: formally voted into WP. ]

### 132(i). list::resize description uses random access iterators

Section: 23.3.11.3 [list.capacity] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The description reads:

-1- Effects:

             if (sz > size())
               insert(end(), sz-size(), c);
             else if (sz < size())
               erase(begin()+sz, end());
             else
               ;                           //  do nothing

Obviously list::resize should not be specified in terms of random access iterators.

Proposed resolution:

Change 23.3.11.3 [list.capacity] paragraph 1 to:

Effects:

             if (sz > size())
               insert(end(), sz-size(), c);
             else if (sz < size())
             {
               iterator i = begin();
               advance(i, sz);
               erase(i, end());
             }

[Dublin: The LWG asked Howard to discuss exception safety offline with David Abrahams. They had a discussion and believe there is no issue of exception safety with the proposed resolution.]

### 133(i). map missing get_allocator()

Section: 23.4.3 [map] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The title says it all.

Proposed resolution:

Insert in 23.4.3 [map], paragraph 2, after operator= in the map declaration:

        allocator_type get_allocator() const;

### 134(i). vector constructors over specified

Section: 23.3.13.2 [vector.cons] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The complexity description says: "It does at most 2N calls to the copy constructor of T and logN reallocations if they are just input iterators ...".

This appears to be overly restrictive, dictating the precise memory/performance tradeoff for the implementor.

Proposed resolution:

Change 23.3.13.2 [vector.cons], paragraph 1 to:

-1- Complexity: The constructor template <class InputIterator> vector(InputIterator first, InputIterator last) makes only N calls to the copy constructor of T (where N is the distance between first and last) and no reallocations if iterators first and last are of forward, bidirectional, or random access categories. It makes order N calls to the copy constructor of T and order logN reallocations if they are just input iterators.

Rationale:

"at most 2N calls" is correct only if the growth factor is greater than or equal to 2.

### 136(i). seekp, seekg setting wrong streams?

Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I may be misunderstanding the intent, but should not seekg set only the input stream and seekp set only the output stream? The description seems to say that each should set both input and output streams. If that's really the intent, I withdraw this proposal.

Proposed resolution:

In section 27.6.1.3 change:

    basic_istream<charT,traits>& seekg(pos_type pos);
    Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).

To:

    basic_istream<charT,traits>& seekg(pos_type pos);
    Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::in).

In section 27.6.1.3 change:

    basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir);
    Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).

To:

    basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir);
    Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::in).

In section 27.6.2.4, paragraph 2 change:

    -2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).

To:

    -2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::out).

In section 27.6.2.4, paragraph 4 change:

    -4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).

To:

    -4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::out).

[Dublin: Dietmar Kühl thinks this is probably correct, but would like the opinion of more iostream experts before taking action.]

[Tokyo: Reviewed by the LWG. PJP noted that although his docs are incorrect, his implementation already implements the Proposed Resolution.]

[Post-Tokyo: Matt Austern comments:
Is it a problem with basic_istream and basic_ostream, or is it a problem with basic_stringbuf? We could resolve the issue either by changing basic_istream and basic_ostream, or by changing basic_stringbuf. I prefer the latter change (or maybe both changes): I don't see any reason for the standard to require that std::stringbuf s(std::string("foo"), std::ios_base::in); s.pubseekoff(0, std::ios_base::beg); must fail.
This requirement is a bit weird. There's no similar requirement for basic_streambuf<>::seekpos, or for basic_filebuf<>::seekoff or basic_filebuf<>::seekpos.]

### 137(i). Do use_facet and has_facet look in the global locale?

Section: 28.3.3.1 [locale] Status: TC1 Submitter: Angelika Langer Opened: 1999-03-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 28.3.3.1 [locale] says:

-4- In the call to use_facet<Facet>(loc), the type argument chooses a facet, making available all members of the named type. If Facet is not present in a locale (or, failing that, in the global locale), it throws the standard exception bad_cast. A C++ program can check if a locale implements a particular facet with the template function has_facet<Facet>().

This contradicts the specification given in section 28.3.3.2 [locale.global.templates]:
template <class Facet> const Facet& use_facet(const locale& loc);
-1- Get a reference to a facet of a locale.
-2- Returns: a reference to the corresponding facet of loc, if present.
-3- Throws: bad_cast if has_facet<Facet>(loc) is false.
-4- Notes: The reference returned remains valid at least as long as any copy of loc exists

Proposed resolution:

Remove the phrase "(or, failing that, in the global locale)" from section 22.1.1.

Rationale:

Needed for consistency with the way locales are handled elsewhere in the standard.

### 139(i). Optional sequence operation table description unclear

Section: 23.2.4 [sequence.reqmts] Status: TC1 Submitter: Andrew Koenig Opened: 1999-03-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The sentence introducing the Optional sequence operation table (23.1.1 paragraph 12) has two problems:

A. It says ``The operations in table 68 are provided only for the containers for which they take constant time.''
That could be interpreted in two ways, one of them being ``Even though table 68 shows particular operations as being provided, implementations are free to omit them if they cannot implement them in constant time.''
B. That paragraph says nothing about amortized constant time, and it should.

Proposed resolution:

Replace the wording in 23.1.1 paragraph 12 which begins ``The operations in table 68 are provided only..." with:

> Table 68 lists sequence operations that are provided for some types of sequential containers but not others. An implementation shall provide these operations for all container types shown in the ``container'' column, and shall implement them so as to take amortized constant time.

### 141(i). basic_string::find_last_of, find_last_not_of say pos instead of xpos

Section: 27.4.3.7.4 [string.insert], 27.4.3.7.6 [string.replace] Status: TC1 Submitter: Arch Robison Opened: 1999-04-28 Last modified: 2016-11-12

Priority: Not Prioritized

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Discussion:

Sections 21.3.6.4 paragraph 1 and 21.3.6.6 paragraph 1 surely have misprints where they say:
— `xpos <= pos` and `pos < size();`

Surely the document meant to say ```xpos < size()`'' in both places.

[Judy Ward also sent in this issue for 21.3.6.4 with the same proposed resolution.]

Proposed resolution:

Change Sections 21.3.6.4 paragraph 1 and 21.3.6.6 paragraph 1, the line which says:
— `xpos <= pos` and `pos < size();
` to:
`
` — `xpos <= pos` and `xpos < size();`

### 142(i). lexicographical_compare complexity wrong

Section: 26.8.11 [alg.lex.comparison] Status: TC1 Submitter: Howard Hinnant Opened: 1999-06-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The lexicographical_compare complexity is specified as:
"At most min((last1 - first1), (last2 - first2)) applications of the corresponding comparison."
The best I can do is twice that expensive.

Nicolai Josuttis comments in lib-6862: You mean, to check for equality you have to check both < and >? Yes, IMO you are right! (and Matt states this complexity in his book)

Proposed resolution:

Change 26.8.11 [alg.lex.comparison] complexity to:

> At most `2*min((last1 - first1), (last2 - first2))` applications of the corresponding comparison.

Change the example at the end of paragraph 3 to read:

> [Example:
> for ( ; first1 != last1 && first2 != last2 ; ++first1, ++first2) {
> if (*first1 < *first2) return true;
> if (*first2 < *first1) return false;
> }
> return first1 == last1 && first2 != last2;
> --end example]

### 144(i). Deque constructor complexity wrong

Section: 23.3.5.2 [deque.cons] Status: TC1 Submitter: Herb Sutter Opened: 1999-05-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 23.3.5.2 [deque.cons] paragraph 6, the deque ctor that takes an iterator range appears to have complexity requirements which are incorrect, and which contradict the complexity requirements for insert(). I suspect that the text in question, below, was taken from vector:

> Complexity: If the iterators first and last are forward iterators, bidirectional iterators, or random access iterators the constructor makes only N calls to the copy constructor, and performs no reallocations, where N is last - first.

The word "reallocations" does not really apply to deque. Further, all of the following appears to be spurious:

> It makes at most 2N calls to the copy constructor of T and log N reallocations if they are input iterators.1)

> 1) The complexity is greater in the case of input iterators because each element must be added individually: it is impossible to determine the distance between first abd last before doing the copying.

This makes perfect sense for vector, but not for deque. Why should deque gain an efficiency advantage from knowing in advance the number of elements to insert?

Proposed resolution:

In 23.3.5.2 [deque.cons] paragraph 6, replace the Complexity description, including the footnote, with the following text (which also corrects the "abd" typo):

> Complexity: Makes last - first calls to the copy constructor of T.

### 146(i). complex<T> Inserter and Extractor need sentries

Section: 29.4.6 [complex.ops] Status: TC1 Submitter: Angelika Langer Opened: 1999-05-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The extractor for complex numbers is specified as:

> template<class T, class charT, class traits>
> basic_istream<charT, traits>&
> operator>>(basic_istream<charT, traits>& is, complex<T>& x);
> Effects: Extracts a complex number x of the form: u, (u), or (u,v), where u is the real part and v is the imaginary part (lib.istream.formatted).
> Requires: The input values be convertible to T. If bad input is encountered, calls is.setstate(ios::failbit) (which may throw ios::failure (lib.iostate.flags).
> Returns: is .

Is it intended that the extractor for complex numbers does not skip whitespace, unlike all other extractors in the standard library do? Shouldn't a sentry be used?
The inserter for complex numbers is specified as:

> template<class T, class charT, class traits>
> basic_ostream<charT, traits>&
> operator<<(basic_ostream<charT, traits>& o, const complex<T>& x);
> Effects: inserts the complex number x onto the stream o as if it were implemented as follows:
> template<class T, class charT, class traits>
> basic_ostream<charT, traits>&
> operator<<(basic_ostream<charT, traits>& o, const complex<T>& x)
> {
> basic_ostringstream<charT, traits> s;
> s.flags(o.flags());
> s.imbue(o.getloc());
> s.precision(o.precision());
> s << '(' << x.real() << "," << x.imag() << ')';
> return o << s.str();
> }

Is it intended that the inserter for complex numbers ignores the field width and does not do any padding? If, with the suggested implementation above, the field width were set in the stream then the opening parentheses would be adjusted, but the rest not, because the field width is reset to zero after each insertion.

I think that both operations should use sentries, for sake of consistency with the other inserters and extractors in the library. Regarding the issue of padding in the inserter, I don't know what the intent was.

Proposed resolution:

After 29.4.6 [complex.ops] paragraph 14 (operator>>), add a Notes clause:

> Notes: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions.

Rationale:

For extractors, the note is added to make it clear that skipping whitespace follows an "all-or-none" rule.

For inserters, the LWG believes there is no defect; the standard is correct as written.

### 147(i). Library Intro refers to global functions that aren't global

Section: 16.4.6.4 [global.functions] Status: TC1 Submitter: Lois Goldthwaite Opened: 1999-06-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The library had many global functions until 17.4.1.1 [lib.contents] paragraph 2 was added:

> All library entities except macros, operator new and operator delete are defined within the namespace std or namespaces nested within namespace std.

It appears "global function" was never updated in the following:

> 17.4.4.3 - Global functions [lib.global.functions]
> -1- It is unspecified whether any global functions in the C++ Standard Library are defined as inline (dcl.fct.spec).
> -2- A call to a global function signature described in Clauses lib.language.support through lib.input.output behaves the same as if the implementation declares no additional global function signatures.*
> [Footnote: A valid C++ program always calls the expected library global function. An implementation may also define additional global functions that would otherwise not be called by a valid C++ program. --- end footnote]
> -3- A global function cannot be declared by the implementation as taking additional default arguments.
> 17.4.4.4 - Member functions [lib.member.functions]
> -2- An implementation can declare additional non-virtual member function signatures within a class:

> > -- by adding arguments with default values to a member function signature; The same latitude does not extend to the implementation of virtual or global functions, however.

Proposed resolution:

Change "global" to "global or non-member" in:

> 17.4.4.3 [lib.global.functions] section title,
> 17.4.4.3 [lib.global.functions] para 1,
> 17.4.4.3 [lib.global.functions] para 2 in 2 places plus 2 places in the footnote,
> 17.4.4.3 [lib.global.functions] para 3,
> 17.4.4.4 [lib.member.functions] para 2

Rationale:

Because operator new and delete are global, the proposed resolution was changed from "non-member" to "global or non-member.

### 148(i). Functions in the example facet BoolNames should be const

Section: 99 [facets.examples] Status: TC1 Submitter: Jeremy Siek Opened: 1999-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 99 [facets.examples] paragraph 13, the do_truename() and do_falsename() functions in the example facet BoolNames should be const. The functions they are overriding in numpunct_byname<char> are const.

Proposed resolution:

In 99 [facets.examples] paragraph 13, insert "const" in two places:

> `string do_truename() const { return "Oui Oui!"; }
> string do_falsename() const { return "Mais Non!"; }`

### 149(i). Insert should return iterator to first element inserted

Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Andrew Koenig Opened: 1999-06-28 Last modified: 2016-11-12

Priority: Not Prioritized

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Discussion:

Suppose that c and c1 are sequential containers and i is an iterator that refers to an element of c. Then I can insert a copy of c1's elements into c ahead of element i by executing

>     c.insert(i, c1.begin(), c1.end());

If c is a vector, it is fairly easy for me to find out where the newly inserted elements are, even though i is now invalid:

>     size_t i_loc = i - c.begin();
>     c.insert(i, c1.begin(), c1.end());

and now the first inserted element is at c.begin()+i_loc and one past the last is at c.begin()+i_loc+c1.size().
But what if c is a list? I can still find the location of one past the last inserted element, because i is still valid. To find the location of the first inserted element, though, I must execute something like

>     for (size_t n = c1.size(); n; --n)
>        --i;

because i is now no longer a random-access iterator.
Alternatively, I might write something like

>     bool first = i == c.begin();
>     list<T>::iterator j = i;
>     if (!first) --j;
>     c.insert(i, c1.begin(), c1.end());
>     if (first)
>        j = c.begin();
>     else
>        ++j;

which, although wretched, requires less overhead.
But I think the right solution is to change the definition of insert so that instead of returning void, it returns an iterator that refers to the first element inserted, if any, and otherwise is a copy of its first argument.

[ Summit: ]

> Reopened by Alisdair.

[ Post Summit Alisdair adds: ]

> In addition to the original rationale for C++03, this change also gives a consistent interface for all container insert operations i.e. they all return an iterator to the (first) inserted item.

> Proposed wording provided.

[ 2009-07 Frankfurt ]

> Q: why isn't this change also proposed for associative containers?

> A: The returned iterator wouldn't necessarily point to a contiguous range.

> Moved to Ready.

Proposed resolution:

23.2.4 [sequence.reqmts] Table 83 change return type from `void` to `iterator` for the following rows:

> Table 83 — Sequence container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition
> --- | --- | ---
> `a.insert(p,n,t)` | `void iterator` | Inserts `n` copies of `t` before `p`.
> `a.insert(p,i,j)` | `void iterator` | Each iterator in the range `[i,j)` shall be dereferenced exactly once. pre: `i` and `j` are not iterators into `a`. Inserts copies of elements in `[i, j)` before `p`
> `a.insert(p,il)` | `void iterator` | `a.insert(p, il.begin(), il.end())`.

Add after p6 23.2.4 [sequence.reqmts]:

> -6- ...

> The iterator returned from `a.insert(p,n,t)` points to the copy of the first element inserted into `a`, or `p` if `n == 0`.

> The iterator returned from `a.insert(p,i,j)` points to the copy of the first element inserted into `a`, or `p` if `i == j`.

> The iterator returned from `a.insert(p,il)` points to the copy of the first element inserted into `a`, or `p` if `il` is empty.

p2 23.3.5 [deque] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert(const_iterator position, size_type n, const T& x);
>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);
>       void iterator insert(const_iterator position, initializer_list<T>);

23.3.5.4 [deque.modifiers] change return type from `void` to `iterator` on following declarations:

>       void iterator insert(const_iterator position, size_type n, const T& x);
>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

Add the following (missing) declaration

>     iterator insert(const_iterator position, initializer_list<T>);

[forwardlist] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert_after(const_iterator position, initializer_list<T> il);
>     void iterator insert_after(const_iterator position, size_type n, const T& x);
>     template <class InputIterator>
>       void iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

p8 [forwardlist.modifiers] change return type from `void` to `iterator`:

>     void iterator insert_after(const_iterator position, size_type n, const T& x);

Add paragraph:

> Returns: position.

p10 [forwardlist.modifiers] change return type from `void` to `iterator`:

>     template <class InputIterator>
>       void iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

Add paragraph:

> Returns: position.

p12 [forwardlist.modifiers] change return type from `void` to `iterator` on following declarations:

>     void iterator insert_after(const_iterator position, initializer_list<T> il);

change return type from `void` to `iterator` on following declarations:

p2 23.3.11 [list] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert(const_iterator position, size_type n, const T& x);

>     template <class InputIterator>
>     void iterator insert(const_iterator position, InputIterator first, InputIterator last);

>     void iterator insert(const_iterator position, initializer_list<T>);

23.3.11.4 [list.modifiers] change return type from `void` to `iterator` on following declarations:

>     void iterator insert(const_iterator position, size_type n, const T& x);

>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

Add the following (missing) declaration

>     iterator insert(const_iterator position, initializer_list<T>);

p2 23.3.13 [vector]

Update class definition, change return type from `void` to `iterator`:

>     void iterator insert(const_iterator position, T&& x);

>     void iterator insert(const_iterator position, size_type n, const T& x);

>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

>     void iterator insert(const_iterator position, initializer_list<T>);

23.3.13.5 [vector.modifiers] change return type from `void` to `iterator` on following declarations:

>     void iterator insert(const_iterator position, size_type n, const T& x);

>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

Add the following (missing) declaration

>     iterator insert(const_iterator position, initializer_list<T>);

p1 23.3.14 [vector.bool] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert (const_iterator position, size_type n, const bool& x);

>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

>       void iterator insert(const_iterator position, initializer_list<bool> il);

p5 27.4.3 [basic.string] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert(const_iterator p, size_type n, charT c);

>     template<class InputIterator>
>       void iterator insert(const_iterator p, InputIterator first, InputIterator last);

>     void iterator insert(const_iterator p, initializer_list<charT>);

p13 27.4.3.7.4 [string.insert] change return type from `void` to `iterator`:

>     void iterator insert(const_iterator p, size_type n, charT c);

Add paragraph:

> Returns: an iterator which refers to the copy of the first inserted character, or `p` if `n == 0`.

p15 27.4.3.7.4 [string.insert] change return type from `void` to `iterator`:

>     template<class InputIterator>
>       void iterator insert(const_iterator p, InputIterator first, InputIterator last);

Add paragraph:

> Returns: an iterator which refers to the copy of the first inserted character, or `p` if `first == last`.

p17 27.4.3.7.4 [string.insert] change return type from `void` to `iterator`:

>     void iterator insert(const_iterator p, initializer_list<charT> il);

Add paragraph:

> Returns: an iterator which refers to the copy of the first inserted character, or `p` if `il` is empty.

Rationale:

[ The following was the C++98/03 rationale and does not necessarily apply to the proposed resolution in the C++0X time frame: ]

> The LWG believes this was an intentional design decision and so is not a defect. It may be worth revisiting for the next standard.

### 150(i). Find_first_of says integer instead of iterator

Section: 26.6.9 [alg.find.first.of] Status: TC1 Submitter: Matt McClure Opened: 1999-06-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Proposed resolution:

Change 26.6.9 [alg.find.first.of] paragraph 2 from:

> Returns: The first iterator i in the range [first1, last1) such that for some integer j in the range [first2, last2) ...

to:

> Returns: The first iterator i in the range [first1, last1) such that for some iterator j in the range [first2, last2) ...

### 151(i). Can't currently clear() empty container

Section: 23.2.4 [sequence.reqmts] Status: TC1 Submitter: Ed Brey Opened: 1999-06-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

For both sequences and associative containers, a.clear() has the semantics of erase(a.begin(),a.end()), which is undefined for an empty container since erase(q1,q2) requires that q1 be dereferenceable (23.1.1,3 and 23.1.2,7). When the container is empty, a.begin() is not dereferenceable.
The requirement that q1 be unconditionally dereferenceable causes many operations to be intuitively undefined, of which clearing an empty container is probably the most dire.
Since q1 and q2 are only referenced in the range [q1, q2), and [q1, q2) is required to be a valid range, stating that q1 and q2 must be iterators or certain kinds of iterators is unnecessary.

Proposed resolution:

In 23.1.1, paragraph 3, change:

> p and q2 denote valid iterators to a, q and q1 denote valid dereferenceable iterators to a, [q1, q2) denotes a valid range

to:

> p denotes a valid iterator to a, q denotes a valid dereferenceable iterator to a, [q1, q2) denotes a valid range in a

In 23.1.2, paragraph 7, change:

> p and q2 are valid iterators to a, q and q1 are valid dereferenceable iterators to a, [q1, q2) is a valid range

to

> p is a valid iterator to a, q is a valid dereferenceable iterator to a, [q1, q2) is a valid range into a

### 152(i). Typo in `scan_is()` semantics

Section: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The semantics of `scan_is()` (paragraphs 4 and 6) is not exactly described because there is no function `is()` which only takes a character as argument. Also, in the effects clause (paragraph 3), the semantic is also kept vague.

Proposed resolution:

In 28.3.4.2.2.3 [locale.ctype.virtuals] paragraphs 4 and 6, change the returns clause from:

> "... such that `is(*p)` would..."

to: "... such that `is(m, *p)` would...."

### 153(i). Typo in `narrow()` semantics

Section: 28.3.4.2.4.3 [facet.ctype.char.members] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 207

Discussion:

The description of the array version of `narrow()` (in paragraph 11) is flawed: There is no member `do_narrow()` which takes only three arguments because in addition to the range a default character is needed.

Additionally, for both `widen` and `narrow` we have two signatures followed by a Returns clause that only addresses one of them.

Proposed resolution:

Change the returns clause in 28.3.4.2.4.3 [facet.ctype.char.members] paragraph 10 from:

Returns: do_widen(low, high, to).

to:

Returns: do_widen(c) or do_widen(low, high, to), respectively.

Change 28.3.4.2.4.3 [facet.ctype.char.members] paragraph 10 and 11 from:

            char        narrow(char c, char /*dfault*/) const;
            const char* narrow(const char* low, const char* high,
                               char /*dfault*/, char* to) const;

            Returns: do_narrow(low, high, to).

to:

            char        narrow(char c, char dfault) const;
            const char* narrow(const char* low, const char* high,
                               char dfault, char* to) const;

            Returns: do_narrow(c, dfault) or
                     do_narrow(low, high, dfault, to), respectively.

[Kona: 1) the problem occurs in additional places, 2) a user defined version could be different.]

[Post-Tokyo: Dietmar provided the above wording at the request of the LWG. He could find no other places the problem occurred. He asks for clarification of the Kona "a user defined version..." comment above. Perhaps it was a circuitous way of saying "dfault" needed to be uncommented?]

[Post-Toronto: the issues list maintainer has merged in the proposed resolution from issue 207(i), which addresses the same paragraphs.]

### 154(i). Missing `double` specifier for `do_get()`

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The table in paragraph 7 for the length modifier does not list the length modifier `l` to be applied if the type is `double`. Thus, the standard asks the implementation to do undefined things when using `scanf()` (the missing length modifier for `scanf()` when scanning `double`s is actually a problem I found quite often in production code, too).

Proposed resolution:

In 28.3.4.3.2.3 [facet.num.get.virtuals], paragraph 7, add a row in the Length Modifier table to say that for `double` a length modifier `l` is to be used.

Rationale:

The standard makes an embarrassing beginner's mistake.

### 155(i). Typo in naming the class defining the class `Init`

Section: 31.4 [iostream.objects] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There are conflicting statements about where the class `Init` is defined. According to 31.4 [iostream.objects] paragraph 2 it is defined as `basic_ios::Init`, according to 31.5.2 [ios.base] it is defined as `ios_base::Init`.

Proposed resolution:

Change 31.4 [iostream.objects] paragraph 2 from "`basic_ios::Init"` to "`ios_base::Init"`.

Rationale:

Although not strictly wrong, the standard was misleading enough to warrant the change.

### 156(i). Typo in `imbue()` description

Section: 31.5.2.4 [ios.base.locales] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There is a small discrepancy between the declarations of `imbue()`: in 31.5.2 [ios.base] the argument is passed as `locale const&` (correct), in 31.5.2.4 [ios.base.locales] it is passed as `locale const` (wrong).

Proposed resolution:

In 31.5.2.4 [ios.base.locales] change the `imbue` argument from "`locale const" to "locale const&".`

### 158(i). Underspecified semantics for `setbuf()`

Section: 31.6.3.5.2 [streambuf.virt.buffer] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The default behavior of `setbuf()` is described only for the situation that `gptr() != 0 && gptr() != egptr()`: namely to do nothing. What has to be done in other situations is not described although there is actually only one reasonable approach, namely to do nothing, too.

Since changing the buffer would almost certainly mess up most buffer management of derived classes unless these classes do it themselves, the default behavior of `setbuf()` should always be to do nothing.

Proposed resolution:

Change 31.6.3.5.2 [streambuf.virt.buffer], paragraph 3, Default behavior, to: "Default behavior: Does nothing. Returns this."

### 159(i). Strange use of `underflow()`

Section: 31.6.3.5.3 [streambuf.virt.get] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The description of the meaning of the result of `showmanyc()` seems to be rather strange: It uses calls to `underflow()`. Using `underflow()` is strange because this function only reads the current character but does not extract it, `uflow()` would extract the current character. This should be fixed to use `sbumpc()` instead.

Proposed resolution:

Change 31.6.3.5.3 [streambuf.virt.get] paragraph 1, `showmanyc()`returns clause, by replacing the word "supplied" with the words "extracted from the stream".

### 160(i). Typo: Use of non-existing function `exception()`

Section: 31.7.5.2 [istream] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The paragraph 4 refers to the function `exception()` which is not defined. Probably, the referred function is `basic_ios<>::exceptions()`.

Proposed resolution:

In 31.7.5.2 [istream], 31.7.5.4 [istream.unformatted], paragraph 1, 31.7.6.2 [ostream], paragraph 3, and 31.7.6.3.1 [ostream.formatted.reqmts], paragraph 1, change "`exception()" to "exceptions()"`.

[Note to Editor: "exceptions" with an "s" is the correct spelling.]

### 161(i). Typo: `istream_iterator` vs. `istreambuf_iterator`

Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The note in the second paragraph pretends that the first argument is an object of type `istream_iterator`. This is wrong: It is an object of type `istreambuf_iterator`.

Proposed resolution:

Change 31.7.5.3.2 [istream.formatted.arithmetic] from:

> The first argument provides an object of the istream_iterator class...

to

> The first argument provides an object of the istreambuf_iterator class...

### 164(i). do_put() has apparently unused fill argument

Section: 28.3.4.6.4.3 [locale.time.put.virtuals] Status: TC1 Submitter: Angelika Langer Opened: 1999-07-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 28.3.4.6.4.3 [locale.time.put.virtuals] the do_put() function is specified as taking a fill character as an argument, but the description of the function does not say whether the character is used at all and, if so, in which way. The same holds for any format control parameters that are accessible through the ios_base& argument, such as the adjustment or the field width. Is strftime() supposed to use the fill character in any way? In any case, the specification of time_put.do_put() looks inconsistent to me.
Is the signature of do_put() wrong, or is the effects clause incomplete?

Proposed resolution:

Add the following note after 28.3.4.6.4.3 [locale.time.put.virtuals] paragraph 2:

> [Note: the `fill` argument may be used in the implementation-defined formats, or by derivations. A space character is a reasonable default for this argument. --end Note]

Rationale:

The LWG felt that while the normative text was correct, users need some guidance on what to pass for the `fill` argument since the standard doesn't say how it's used.

### 165(i). `xsputn()`, `pubsync()` never called by `basic_ostream` members?

Section: 31.7.6.2 [ostream] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 2 explicitly states that none of the `basic_ostream` functions falling into one of the groups "formatted output functions" and "unformatted output functions" calls any stream buffer function which might call a virtual function other than `overflow()`. Basically this is fine but this implies that `sputn()` (this function would call the virtual function `xsputn()`) is never called by any of the standard output functions. Is this really intended? At minimum it would be convenient to call `xsputn()` for strings... Also, the statement that `overflow()` is the only virtual member of `basic_streambuf` called is in conflict with the definition of `flush()` which calls `rdbuf()->pubsync()` and thereby the virtual function `sync()` (`flush()` is listed under "unformatted output functions").

In addition, I guess that the sentence starting with "They may use other public members of `basic_ostream` ..." probably was intended to start with "They may use other public members of `basic_streamuf`..." although the problem with the virtual members exists in both cases.

I see two obvious resolutions:

1. state in a footnote that this means that `xsputn()` will never be called by any ostream member and that this is intended.
2. relax the restriction and allow calling `overflow()` and `xsputn()`. Of course, the problem with `flush()` has to be resolved in some way.

Proposed resolution:

Change the last sentence of 27.6.2.1 (lib.ostream) paragraph 2 from:

> They may use other public members of basic_ostream except that they do not invoke any virtual members of rdbuf() except overflow().

to:

> They may use other public members of basic_ostream except that they shall not invoke any virtual members of rdbuf() except overflow(), xsputn(), and sync().

[Kona: the LWG believes this is a problem. Wish to ask Jerry or PJP why the standard is written this way.]

[Post-Tokyo: Dietmar supplied wording at the request of the LWG. He comments: The rules can be made a little bit more specific if necessary be explicitly spelling out what virtuals are allowed to be called from what functions and eg to state specifically that flush() is allowed to call sync() while other functions are not.]

### 167(i). Improper use of `traits_type::length()`

Section: 31.7.6.3.4 [ostream.inserters.character] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 4 states that the length is determined using `traits::length(s)`. Unfortunately, this function is not defined for example if the character type is `wchar_t` and the type of `s` is `char const*`. Similar problems exist if the character type is `char` and the type of `s` is either `signed char const*` or `unsigned char const*`.

Proposed resolution:

Change 31.7.6.3.4 [ostream.inserters.character] paragraph 4 from:

> Effects: Behaves like an formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts characters. The number of characters starting at s to be inserted is traits::length(s). Padding is determined as described in lib.facet.num.put.virtuals. The traits::length(s) characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).

to:

> Effects: Behaves like a formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts n characters starting at s, where n is the number that would be computed as if by:

> - traits::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*, and also for the overload where the first argument is of type basic_ostream<char, traits>& and the second is of type const char*.
> - std::char_traits<char>::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*.
> - traits::length(reinterpret_cast<const char*>(s)) for the other two overloads.

> Padding is determined as described in lib.facet.num.put.virtuals. The n characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).

[Santa Cruz: Matt supplied new wording]

[Kona: changed "where n is" to " where n is the number that would be computed as if by"]

Rationale:

We have five separate cases. In two of them we can use the user-supplied traits class without any fuss. In the other three we try to use something as close to that user-supplied class as possible. In two cases we've got a traits class that's appropriate for char and what we've got is a const signed char* or a const unsigned char*; that's close enough so we can just use a reinterpret cast, and continue to use the user-supplied traits class. Finally, there's one case where we just have to give up: where we've got a traits class for some arbitrary charT type, and we somehow have to deal with a const char*. There's nothing better to do but fall back to char_traits<char>

### 168(i). Typo: formatted vs. unformatted

Section: 31.7.6.4 [ostream.unformatted] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The first paragraph begins with a descriptions what has to be done in formatted output functions. Probably this is a typo and the paragraph really want to describe unformatted output functions...

Proposed resolution:

In 31.7.6.4 [ostream.unformatted] paragraph 1, the first and last sentences, change the word "formatted" to "unformatted":

> "Each unformatted output function begins ..."
> "... value specified for the unformatted output function."

### 169(i). Bad efficiency of `overflow()` mandated

Section: 31.8.2.5 [stringbuf.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 8, Notes, of this section seems to mandate an extremely inefficient way of buffer handling for `basic_stringbuf`, especially in view of the restriction that `basic_ostream` member functions are not allowed to use `xsputn()` (see 31.7.6.2 [ostream]): For each character to be inserted, a new buffer is to be created.

Of course, the resolution below requires some handling of simultaneous input and output since it is no longer possible to update `egptr()` whenever `epptr()` is changed. A possible solution is to handle this in `underflow()`.

Proposed resolution:

In 31.8.2.5 [stringbuf.virtuals] paragraph 8, Notes, insert the words "at least" as in the following:

> To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus at least one additional write position.

### 170(i). Inconsistent definition of `traits_type`

Section: 31.8.5 [stringstream] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The classes `basic_stringstream` (31.8.5 [stringstream]), `basic_istringstream` (31.8.3 [istringstream]), and `basic_ostringstream` (31.8.4 [ostringstream]) are inconsistent in their definition of the type `traits_type`: For `istringstream`, this type is defined, for the other two it is not. This should be consistent.

Proposed resolution:

Proposed resolution:

To the declarations of `basic_ostringstream` (31.8.4 [ostringstream]) and `basic_stringstream` (31.8.5 [stringstream]) add:

>     typedef traits traits_type;

### 171(i). Strange `seekpos()` semantics due to joint position

Section: 31.10.3.5 [filebuf.virtuals] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Overridden virtual functions, seekpos()

In 31.10.3 [filebuf] paragraph 3, it is stated that a joint input and output position is maintained by `basic_filebuf`. Still, the description of `seekpos()` seems to talk about different file positions. In particular, it is unclear (at least to me) what is supposed to happen to the output buffer (if there is one) if only the input position is changed. The standard seems to mandate that the output buffer is kept and processed as if there was no positioning of the output position (by changing the input position). Of course, this can be exactly what you want if the flag `ios_base::ate` is set. However, I think, the standard should say something like this:

- If `(which & mode) == 0` neither read nor write position is changed and the call fails. Otherwise, the joint read and write position is altered to correspond to `sp`.
- If there is an output buffer, the output sequences is updated and any unshift sequence is written before the position is altered.
- If there is an input buffer, the input sequence is updated after the position is altered.

Plus the appropriate error handling, that is...

Proposed resolution:

Change the unnumbered paragraph in 27.8.1.4 (lib.filebuf.virtuals) before paragraph 14 from:

> pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);

> Alters the file position, if possible, to correspond to the position stored in sp (as described below).

> - if (which&ios_base::in)!=0, set the file position to sp, then update the input sequence

> - if (which&ios_base::out)!=0, then update the output sequence, write any unshift sequence, and set the file position to sp.

to:

> pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);

> Alters the file position, if possible, to correspond to the position stored in sp (as described below). Altering the file position performs as follows:

> 1. if (om & ios_base::out)!=0, then update the output sequence and write any unshift sequence;

> 2. set the file position to sp;

> 3. if (om & ios_base::in)!=0, then update the input sequence;

> where om is the open mode passed to the last call to open(). The operation fails if is_open() returns false.

[Kona: Dietmar is working on a proposed resolution.]

[Post-Tokyo: Dietmar supplied the above wording.]

### 172(i). Inconsistent types for `basic_istream::ignore()`

Section: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Greg Comeau, Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 31.7.5.2 [istream] the function `ignore()` gets an object of type `streamsize` as first argument. However, in 31.7.5.4 [istream.unformatted] paragraph 23 the first argument is of type `int.`

As far as I can see this is not really a contradiction because everything is consistent if `streamsize` is typedef to be `int`. However, this is almost certainly not what was intended. The same thing happened to `basic_filebuf::setbuf()`, as described in issue 173(i).

Darin Adler also submitted this issue, commenting: Either 27.6.1.1 should be modified to show a first parameter of type int, or 27.6.1.3 should be modified to show a first parameter of type streamsize and use numeric_limits<streamsize>::max.

Proposed resolution:

In 31.7.5.4 [istream.unformatted] paragraph 23 and 24, change both uses of `int` in the description of `ignore()` to `streamsize`.

### 173(i). Inconsistent types for `basic_filebuf::setbuf()`

Section: 31.10.3.5 [filebuf.virtuals] Status: TC1 Submitter: Greg Comeau, Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 31.10.3 [filebuf] the function `setbuf()` gets an object of type `streamsize` as second argument. However, in 31.10.3.5 [filebuf.virtuals] paragraph 9 the second argument is of type `int`.

As far as I can see this is not really a contradiction because everything is consistent if `streamsize` is typedef to be `int`. However, this is almost certainly not what was intended. The same thing happened to `basic_istream::ignore()`, as described in issue 172(i).

Proposed resolution:

In 31.10.3.5 [filebuf.virtuals] paragraph 9, change all uses of `int` in the description of `setbuf()` to `streamsize`.

### 174(i). Typo: `OFF_T` vs. `POS_T`

Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

According to paragraph 1 of this section, `streampos` is the type `OFF_T`, the same type as `streamoff`. However, in paragraph 6 the `streampos` gets the type `POS_T`

Proposed resolution:

Change 99 [depr.ios.members] paragraph 1 from "`typedef OFF_T streampos;`" to "`typedef POS_T streampos;`"

### 175(i). Ambiguity for `basic_streambuf::pubseekpos()` and a few other functions.

Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

According to paragraph 8 of this section, the methods `basic_streambuf::pubseekpos()`, `basic_ifstream::open()`, and `basic_ofstream::open` "may" be overloaded by a version of this function taking the type `ios_base::open_mode` as last argument argument instead of `ios_base::openmode` (`ios_base::open_mode` is defined in this section to be an alias for one of the integral types). The clause specifies, that the last argument has a default argument in three cases. However, this generates an ambiguity with the overloaded version because now the arguments are absolutely identical if the last argument is not specified.

Proposed resolution:

In 99 [depr.ios.members] paragraph 8, remove the default arguments for `basic_streambuf::pubseekpos()`, `basic_ifstream::open()`, and `basic_ofstream::open().`

### 176(i). `exceptions()` in `ios_base`...?

Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The "overload" for the function `exceptions()` in paragraph 8 gives the impression that there is another function of this function defined in class `ios_base`. However, this is not the case. Thus, it is hard to tell how the semantics (paragraph 9) can be implemented: "Call the corresponding member function specified in clause 31 [input.output]."

Proposed resolution:

In 99 [depr.ios.members] paragraph 8, move the declaration of the function `exceptions()`into class `basic_ios`.

### 179(i). Comparison of const_iterators to iterators doesn't work

Section: 23.2 [container.requirements] Status: CD1 Submitter: Judy Ward Opened: 1998-07-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Currently the following will not compile on two well-known standard library implementations:

>     #include <set>
>     using namespace std;

>     void f(const set<int> &s)
>     {
>       set<int>::iterator i;
>       if (i==s.end()); // s.end() returns a const_iterator
>     }

The reason this doesn't compile is because operator== was implemented as a member function of the nested classes set:iterator and set::const_iterator, and there is no conversion from const_iterator to iterator. Surprisingly, (s.end() == i) does work, though, because of the conversion from iterator to const_iterator.

I don't see a requirement anywhere in the standard that this must work. Should there be one? If so, I think the requirement would need to be added to the tables in section 24.1.1. I'm not sure about the wording. If this requirement existed in the standard, I would think that implementors would have to make the comparison operators non-member functions.

This issues was also raised on comp.std.c++ by Darin Adler. The example given was:

>     bool check_equal(std::deque<int>::iterator i,
>     std::deque<int>::const_iterator ci)
>     {
>     return i == ci;
>     }

Comment from John Potter:

> In case nobody has noticed, accepting it will break reverse_iterator.

> The fix is to make the comparison operators templated on two types.

>         template <class Iterator1, class Iterator2>
>         bool operator== (reverse_iterator<Iterator1> const& x,
>                          reverse_iterator<Iterator2> const& y);

> Obviously: return x.base() == y.base();

> Currently, no reverse_iterator to const_reverse_iterator compares are valid.

> BTW, I think the issue is in support of bad code. Compares should be between two iterators of the same type. All std::algorithms require the begin and end iterators to be of the same type.

Proposed resolution:

Insert this paragraph after 23.2 [container.requirements] paragraph 7:

> In the expressions

>         i == j
>         i != j
>         i < j
>         i <= j
>         i >= j
>         i > j
>         i - j

> Where i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.

[post-Toronto: Judy supplied a proposed resolution saying that `iterator` and `const_iterator` could be freely mixed in iterator comparison and difference operations.]

[Redmond: Dave and Howard supplied a new proposed resolution which explicitly listed expressions; there was concern that the previous proposed resolution was too informal.]

Rationale:

The LWG believes it is clear that the above wording applies only to the nested types `X::iterator` and `X::const_iterator`, where `X` is a container. There is no requirement that `X::reverse_iterator` and `X::const_reverse_iterator` can be mixed. If mixing them is considered important, that's a separate issue. (Issue 280(i).)

### 180(i). Container member iterator arguments constness has unintended consequences

Section: 27.4.3 [basic.string] Status: CD1 Submitter: Dave Abrahams Opened: 1999-07-01 Last modified: 2016-11-12

Priority: Not Prioritized

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Discussion:

It is the constness of the container which should control whether it can be modified through a member function such as erase(), not the constness of the iterators. The iterators only serve to give positioning information.

Here's a simple and typical example problem which is currently very difficult or impossible to solve without the change proposed below.

Wrap a standard container C in a class W which allows clients to find and read (but not modify) a subrange of (C.begin(), C.end()]. The only modification clients are allowed to make to elements in this subrange is to erase them from C through the use of a member function of W.

[ post Bellevue, Alisdair adds: ]

> This issue was implemented by N2350 for everything but `basic_string`.

> Note that the specific example in this issue (`basic_string`) is the one place we forgot to amend in N2350, so we might open this issue for that single container?

[ Sophia Antipolis: ]

> This was a fix that was intended for all standard library containers, and has been done for other containers, but string was missed.

> The wording updated.

> We did not make the change in `replace`, because this change would affect the implementation because the string may be written into. This is an issue that should be taken up by concepts.

> We note that the supplied wording addresses the initializer list provided in N2679.

Proposed resolution:

Update the following signature in the `basic_string` class template definition in 27.4.3 [basic.string], p5:

>     namespace std {
>       template<class charT, class traits = char_traits<charT>,
>         class Allocator = allocator<charT> >
>       class basic_string {

>         ...

>         iterator insert(const_iterator p, charT c);
>         void insert(const_iterator p, size_type n, charT c);
>         template<class InputIterator>
>           void insert(const_iterator p, InputIterator first, InputIterator last);
>         void insert(const_iterator p, initializer_list<charT>);

>         ...

>         iterator erase(const_iterator const_position);
>         iterator erase(const_iterator first, const_iterator last);

>         ...

>       };
>     }

Update the following signatures in 27.4.3.7.4 [string.insert]:

>     iterator insert(const_iterator p, charT c);
>     void insert(const_iterator p, size_type n, charT c);
>     template<class InputIterator>
>       void insert(const_iterator p, InputIterator first, InputIterator last);
>     void insert(const_iterator p, initializer_list<charT>);

Update the following signatures in 27.4.3.7.5 [string.erase]:

>     iterator erase(const_iterator const_position);
>     iterator erase(const_iterator first, const_iterator last);

Rationale:

The issue was discussed at length. It was generally agreed that 1) There is no major technical argument against the change (although there is a minor argument that some obscure programs may break), and 2) Such a change would not break const correctness. The concerns about making the change were 1) it is user detectable (although only in boundary cases), 2) it changes a large number of signatures, and 3) it seems more of a design issue that an out-and-out defect.

The LWG believes that this issue should be considered as part of a general review of const issues for the next revision of the standard. Also see issue 200(i).

### 181(i). make_pair() unintended behavior

Section: 22.3 [pairs] Status: TC1 Submitter: Andrew Koenig Opened: 1999-08-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The claim has surfaced in Usenet that expressions such as
`make_pair("abc", 3)`
are illegal, notwithstanding their use in examples, because template instantiation tries to bind the first template parameter to `const char (&)[4]`, which type is uncopyable.
I doubt anyone intended that behavior...

Proposed resolution:

In 22.2 [utility], paragraph 1 change the following declaration of make_pair():

>     template <class T1, class T2> pair<T1,T2> make_pair(const T1&, const T2&);

to:

>     template <class T1, class T2> pair<T1,T2> make_pair(T1, T2);

In 22.3 [pairs] paragraph 7 and the line before, change:

>     template <class T1, class T2>
>     pair<T1, T2> make_pair(const T1& x, const T2& y);

to:

>     template <class T1, class T2>
>     pair<T1, T2> make_pair(T1 x, T2 y);

and add the following footnote to the effects clause:

> According to 12.8 [class.copy], an implementation is permitted to not perform a copy of an argument, thus avoiding unnecessary copies.

Rationale:

Two potential fixes were suggested by Matt Austern and Dietmar Kühl, respectively, 1) overloading with array arguments, and 2) use of a reference_traits class with a specialization for arrays. Andy Koenig suggested changing to pass by value. In discussion, it appeared that this was a much smaller change to the standard that the other two suggestions, and any efficiency concerns were more than offset by the advantages of the solution. Two implementors reported that the proposed resolution passed their test suites.

### 182(i). Ambiguous references to size_t

Section: 16 [library] Status: CD1 Submitter: Al Stevens Opened: 1999-08-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Many references to `size_t` throughout the document omit the `std::` namespace qualification.

For example, 16.4.5.6 [replacement.functions] paragraph 2:

>      operator new(size_t)
>      operator new(size_t, const std::nothrow_t&)
>      operator new[](size_t)
>      operator new[](size_t, const std::nothrow_t&)

Proposed resolution:

In 16.4.5.6 [replacement.functions] paragraph 2: replace:

> `- operator new(size_t)
> - operator new(size_t, const std::nothrow_t&)
> - operator new[](size_t)
> - operator new[](size_t, const std::nothrow_t&)`

by:

>     - operator new(std::size_t)
>     - operator new(std::size_t, const std::nothrow_t&)
>     - operator new[](std::size_t)
>     - operator new[](std::size_t, const std::nothrow_t&)

In [lib.allocator.requirements] 20.1.5, paragraph 4: replace:

> The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*, T const*, size_t, and ptrdiff_t, respectively.

by:

> The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*, T const*, std::size_t, and std::ptrdiff_t, respectively.

In [lib.allocator.members] 20.4.1.1, paragraphs 3 and 6: replace:

> 3 Notes: Uses ::operator new(size_t) (18.4.1).

> 6 Note: the storage is obtained by calling ::operator new(size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires.

by:

> 3 Notes: Uses ::operator new(std::size_t) (18.4.1).

> 6 Note: the storage is obtained by calling ::operator new(std::size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires.

In [lib.char.traits.require] 21.1.1, paragraph 1: replace:

> In Table 37, X denotes a Traits class defining types and functions for the character container type CharT; c and d denote values of type CharT; p and q denote values of type const CharT*; s denotes a value of type CharT*; n, i and j denote values of type size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and state denotes a value of type X::state_type.

by:

> In Table 37, X denotes a Traits class defining types and functions for the character container type CharT; c and d denote values of type CharT; p and q denote values of type const CharT*; s denotes a value of type CharT*; n, i and j denote values of type std::size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and state denotes a value of type X::state_type.

In [lib.char.traits.require] 21.1.1, table 37: replace the return type of X::length(p): "size_t" by "std::size_t".

In [lib.std.iterator.tags] 24.3.3, paragraph 2: replace:
typedef ptrdiff_t difference_type;
by:
typedef std::ptrdiff_t difference_type;

In [lib.locale.ctype] 22.2.1.1 put namespace std { ...} around the declaration of template <class charT> class ctype.
In [lib.iterator.traits] 24.3.1, paragraph 2 put namespace std { ...} around the declaration of:
template<class Iterator> struct iterator_traits
template<class T> struct iterator_traits<T*>
template<class T> struct iterator_traits<const T*>

Rationale:

The LWG believes correcting names like `size_t` and `ptrdiff_t` to `std::size_t` and `std::ptrdiff_t` to be essentially editorial. There there can't be another size_t or ptrdiff_t meant anyway because, according to 16.4.5.3.5 [extern.types],

> For each type T from the Standard C library, the types ::T and std::T are reserved to the implementation and, when defined, ::T shall be identical to std::T.

The issue is treated as a Defect Report to make explicit the Project Editor's authority to make this change.

[Post-Tokyo: Nico Josuttis provided the above wording at the request of the LWG.]

[Toronto: This is tangentially related to issue 229(i), but only tangentially: the intent of this issue is to address use of the name `size_t` in contexts outside of namespace std, such as in the description of `::operator new`. The proposed changes should be reviewed to make sure they are correct.]

[pre-Copenhagen: Nico has reviewed the changes and believes them to be correct.]

### 183(i). I/O stream manipulators don't work for wide character streams

Section: 31.7.7 [std.manip] Status: CD1 Submitter: Andy Sawyer Opened: 1999-07-07 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [std.manip].

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Discussion:

31.7.7 [std.manip] paragraph 3 says (clause numbering added for exposition):

> Returns: An object s of unspecified type such that if [1] out is an (instance of) basic_ostream then the expression out<<s behaves as if f(s) were called, and if [2] in is an (instance of) basic_istream then the expression in>>s behaves as if f(s) were called. Where f can be defined as: ios_base& f(ios_base& str, ios_base::fmtflags mask) { // reset specified flags str.setf(ios_base::fmtflags(0), mask); return str; } [3] The expression out<<s has type ostream& and value out. [4] The expression in>>s has type istream& and value in.

Given the definitions [1] and [2] for out and in, surely [3] should read: "The expression out << s has type basic_ostream& ..." and [4] should read: "The expression in >> s has type basic_istream& ..."

If the wording in the standard is correct, I can see no way of implementing any of the manipulators so that they will work with wide character streams.

e.g. wcout << setbase( 16 );

Must have value 'wcout' (which makes sense) and type 'ostream&' (which doesn't).

The same "cut'n'paste" type also seems to occur in Paras 4,5,7 and 8. In addition, Para 6 [setfill] has a similar error, but relates only to ostreams.

I'd be happier if there was a better way of saying this, to make it clear that the value of the expression is "the same specialization of basic_ostream as out"&

Proposed resolution:

Replace section 31.7.7 [std.manip] except paragraph 1 with the following:

> 2- The type designated smanip in each of the following function descriptions is implementation-specified and may be different for each function.
> `smanip resetiosflags(ios_base::fmtflags mask);`
> -3- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:*
> [Footnote: The expression cin >> resetiosflags(ios_base::skipws) clears ios_base::skipws in the format flags stored in the basic_istream<charT,traits> object cin (the same as cin >> noskipws), and the expression cout << resetiosflags(ios_base::showbase) clears ios_base::showbase in the format flags stored in the basic_ostream<charT,traits> object cout (the same as cout << noshowbase). --- end footnote]
> `ios_base& f(ios_base& str, ios_base::fmtflags mask)
> {
> // reset specified flags
> str.setf(ios_base::fmtflags(0), mask);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
> `smanip setiosflags(ios_base::fmtflags mask);`
> -4- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:
> `ios_base& f(ios_base& str, ios_base::fmtflags mask)
> {
> // set specified flags
> str.setf(mask);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
> `smanip setbase(int base);`
> -5- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, base) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, base) were called. The function f can be defined as:
> `ios_base& f(ios_base& str, int base)
> {
> // set basefield
> str.setf(base == 8 ? ios_base::oct :
> base == 10 ? ios_base::dec :
> base == 16 ? ios_base::hex :
> ios_base::fmtflags(0), ios_base::basefield);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
> `smanip setfill(char_type c);
> `
> -6- Returns: An object s of unspecified type such that if out is (or is derived from) basic_ostream<charT,traits> and c has type charT then the expression out<<s behaves as if f(s, c) were called. The function f can be defined as:
> `template<class charT, class traits>
> basic_ios<charT,traits>& f(basic_ios<charT,traits>& str, charT c)
> {
> // set fill character
> str.fill(c);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out.
> `smanip setprecision(int n);`
> -7- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
> `ios_base& f(ios_base& str, int n)
> {
> // set precision
> str.precision(n);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in
> .
> `smanip setw(int n);
> `
> -8- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
> `ios_base& f(ios_base& str, int n)
> {
> // set width
> str.width(n);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.

[Kona: Andy Sawyer and Beman Dawes will work to improve the wording of the proposed resolution.]

[Tokyo - The LWG noted that issue 216(i) involves the same paragraphs.]

[Post-Tokyo: The issues list maintainer combined the proposed resolution of this issue with the proposed resolution for issue 216(i) as they both involved the same paragraphs, and were so intertwined that dealing with them separately appear fraught with error. The full text was supplied by Bill Plauger; it was cross checked against changes supplied by Andy Sawyer. It should be further checked by the LWG.]

### 184(i). numeric_limits<bool> wording problems

Section: 17.3.5.3 [numeric.special] Status: CD1 Submitter: Gabriel Dos Reis Opened: 1999-07-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

bools are defined by the standard to be of integer types, as per 6.9.2 [basic.fundamental] paragraph 7. However "integer types" seems to have a special meaning for the author of 18.2. The net effect is an unclear and confusing specification for numeric_limits<bool> as evidenced below.

18.2.1.2/7 says numeric_limits<>::digits is, for built-in integer types, the number of non-sign bits in the representation.

4.5/4 states that a bool promotes to int ; whereas 4.12/1 says any non zero arithmetical value converts to true.

I don't think it makes sense at all to require numeric_limits<bool>::digits and numeric_limits<bool>::digits10 to be meaningful.

The standard defines what constitutes a signed (resp. unsigned) integer types. It doesn't categorize bool as being signed or unsigned. And the set of values of bool type has only two elements.

I don't think it makes sense to require numeric_limits<bool>::is_signed to be meaningful.

18.2.1.2/18 for numeric_limits<integer_type>::radix says:

> For integer types, specifies the base of the representation.186)

This disposition is at best misleading and confusing for the standard requires a "pure binary numeration system" for integer types as per 3.9.1/7

The footnote 186) says: "Distinguishes types with base other than 2 (e.g BCD)." This also erroneous as the standard never defines any integer types with base representation other than 2.

Furthermore, numeric_limits<bool>::is_modulo and numeric_limits<bool>::is_signed have similar problems.

Proposed resolution:

Append to the end of 17.3.5.3 [numeric.special]:

> The specialization for bool shall be provided as follows:

>         namespace std {
>            template<> class numeric_limits<bool> {
>            public:
>              static const bool is_specialized = true;
>              static bool min() throw() { return false; }
>              static bool max() throw() { return true; }

>              static const int  digits = 1;
>              static const int  digits10 = 0;
>              static const bool is_signed = false;
>              static const bool is_integer = true;
>              static const bool is_exact = true;
>              static const int  radix = 2;
>              static bool epsilon() throw() { return 0; }
>              static bool round_error() throw() { return 0; }

>              static const int  min_exponent = 0;
>              static const int  min_exponent10 = 0;
>              static const int  max_exponent = 0;
>              static const int  max_exponent10 = 0;

>              static const bool has_infinity = false;
>              static const bool has_quiet_NaN = false;
>              static const bool has_signaling_NaN = false;
>              static const float_denorm_style has_denorm = denorm_absent;
>              static const bool has_denorm_loss = false;
>              static bool infinity() throw() { return 0; }
>              static bool quiet_NaN() throw() { return 0; }
>              static bool signaling_NaN() throw() { return 0; }
>              static bool denorm_min() throw() { return 0; }

>              static const bool is_iec559 = false;
>              static const bool is_bounded = true;
>              static const bool is_modulo = false;

>              static const bool traps = false;
>              static const bool tinyness_before = false;
>              static const float_round_style round_style = round_toward_zero;
>            };
>          }

[Tokyo: The LWG desires wording that specifies exact values rather than more general wording in the original proposed resolution.]

[Post-Tokyo: At the request of the LWG in Tokyo, Nico Josuttis provided the above wording.]

### 185(i). Questionable use of term "inline"

Section: 22.10 [function.objects] Status: CD1 Submitter: UK Panel Opened: 1999-07-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 4 of 22.10 [function.objects] says:

> [Example: To negate every element of a: transform(a.begin(), a.end(), a.begin(), negate<double>()); The corresponding functions will inline the addition and the negation. end example]

(Note: The "addition" referred to in the above is in para 3) we can find no other wording, except this (non-normative) example which suggests that any "inlining" will take place in this case.

Indeed both:

> 17.4.4.3 Global Functions [lib.global.functions] 1 It is unspecified whether any global functions in the C++ Standard Library are defined as inline (7.1.2).

and

> 17.4.4.4 Member Functions [lib.member.functions] 1 It is unspecified whether any member functions in the C++ Standard Library are defined as inline (7.1.2).

take care to state that this may indeed NOT be the case.

Thus the example "mandates" behavior that is explicitly not required elsewhere.

Proposed resolution:

In 22.10 [function.objects] paragraph 1, remove the sentence:

> They are important for the effective use of the library.

Remove 22.10 [function.objects] paragraph 2, which reads:

> Using function objects together with function templates increases the expressive power of the library as well as making the resulting code much more efficient.

In 22.10 [function.objects] paragraph 4, remove the sentence:

> The corresponding functions will inline the addition and the negation.

[Kona: The LWG agreed there was a defect.]

[Tokyo: The LWG crafted the proposed resolution.]

### 186(i). bitset::set() second parameter should be bool

Section: 22.9.2.3 [bitset.members] Status: CD1 Submitter: Darin Adler Opened: 1999-08-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In section 22.9.2.3 [bitset.members], paragraph 13 defines the bitset::set operation to take a second parameter of type int. The function tests whether this value is non-zero to determine whether to set the bit to true or false. The type of this second parameter should be bool. For one thing, the intent is to specify a Boolean value. For another, the result type from test() is bool. In addition, it's possible to slice an integer that's larger than an int. This can't happen with bool, since conversion to bool has the semantic of translating 0 to false and any non-zero value to true.

Proposed resolution:

In 22.9.2 [template.bitset] Para 1 Replace:

>     bitset<N>& set(size_t pos, int val = true );

With:

>     bitset<N>& set(size_t pos, bool val = true );

In 22.9.2.3 [bitset.members] Para 12(.5) Replace:

>     bitset<N>& set(size_t pos, int val = 1 );

With:

>     bitset<N>& set(size_t pos, bool val = true );

[Kona: The LWG agrees with the description. Andy Sawyer will work on better P/R wording.]

[Post-Tokyo: Andy provided the above wording.]

Rationale:

`bool` is a better choice. It is believed that binary compatibility is not an issue, because this member function is usually implemented as `inline`, and because it is already the case that users cannot rely on the type of a pointer to a nonvirtual member of a standard library class.

### 187(i). iter_swap underspecified

Section: 26.7.3 [alg.swap] Status: CD1 Submitter: Andrew Koenig Opened: 1999-08-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The description of iter_swap in 25.2.2 paragraph 7,says that it ``exchanges the values'' of the objects to which two iterators refer.
What it doesn't say is whether it does so using swap or using the assignment operator and copy constructor.
This question is an important one to answer, because swap is specialized to work efficiently for standard containers.
For example:

>     vector<int> v1, v2;
>     iter_swap(&v1, &v2);

Is this call to iter_swap equivalent to calling swap(v1, v2)? Or is it equivalent to

>     {
>     vector<int> temp = v1;
>     v1 = v2;
>     v2 = temp;
>     }

The first alternative is O(1); the second is O(n).

A LWG member, Dave Abrahams, comments:

> Not an objection necessarily, but I want to point out the cost of that requirement:

> > `iter_swap(list<T>::iterator, list<T>::iterator)`

> can currently be specialized to be more efficient than iter_swap(T*,T*) for many T (by using splicing). Your proposal would make that optimization illegal.

[Kona: The LWG notes the original need for iter_swap was proxy iterators which are no longer permitted.]

Proposed resolution:

Change the effect clause of iter_swap in 25.2.2 paragraph 7 from:

> Exchanges the values pointed to by the two iterators a and b.

to

> `swap(*a, *b)`.

Rationale:

It's useful to say just what `iter_swap` does. There may be some iterators for which we want to specialize `iter_swap`, but the fully general version should have a general specification.

Note that in the specific case of `list<T>::iterator`, iter_swap should not be specialized as suggested above. That would do much more than exchanging the two iterators' values: it would change predecessor/successor relationships, possibly moving the iterator from one list to another. That would surely be inappropriate.

### 189(i). setprecision() not specified correctly

Section: 31.5.2.3 [fmtflags.state] Status: TC1 Submitter: Andrew Koenig Opened: 1999-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

27.4.2.2 paragraph 9 claims that setprecision() sets the precision, and includes a parenthetical note saying that it is the number of digits after the decimal point.
This claim is not strictly correct. For example, in the default floating-point output format, setprecision sets the number of significant digits printed, not the number of digits after the decimal point.
I would like the committee to look at the definition carefully and correct the statement in 27.4.2.2

Proposed resolution:

Remove from 31.5.2.3 [fmtflags.state], paragraph 9, the text "(number of digits after the decimal point)".

### 193(i). Heap operations description incorrect

Section: 26.8.8 [alg.heap.operations] Status: TC1 Submitter: Markus Mauhart Opened: 1999-09-24 Last modified: 2024-01-25

Priority: Not Prioritized

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Duplicate of: 216

Discussion:

25.3.6 [lib.alg.heap.operations] states two key properties of a heap [a,b), the first of them is
"(1) *a is the largest element"
I think this is incorrect and should be changed to the wording in the proposed resolution.

Actually there are two independent changes:

> A-"part of largest equivalence class" instead of "largest", cause 25.3 [lib.alg.sorting] asserts "strict weak ordering" for all its sub clauses.

> B-Take 'an oldest' from that equivalence class, otherwise the heap functions could not be used for a priority queue as explained in 23.2.3.2.2 [lib.priqueue.members] (where I assume that a "priority queue" respects priority AND time).

Proposed resolution:

Change 26.8.8 [alg.heap.operations] property (1) from:

> (1) *a is the largest element

to:

> (1) There is no element greater than `*a`

### 195(i). Should `basic_istream::sentry`'s constructor ever set eofbit?

Section: 31.7.5.2.4 [istream.sentry] Status: TC1 Submitter: Matt Austern Opened: 1999-10-13 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Suppose that `is.flags() & ios_base::skipws` is nonzero. What should `basic_istream<>::sentry`'s constructor do if it reaches eof while skipping whitespace? 27.6.1.1.2/5 suggests it should set failbit. Should it set eofbit as well? The standard doesn't seem to answer that question.

On the one hand, nothing in [istream::sentry] says that `basic_istream<>::sentry` should ever set eofbit. On the other hand, 31.7.5.2 [istream] paragraph 4 says that if extraction from a `streambuf` "returns `traits::eof()`, then the input function, except as explicitly noted otherwise, completes its actions and does `setstate(eofbit)"`. So the question comes down to whether `basic_istream<>::sentry`'s constructor is an input function.

Comments from Jerry Schwarz:

> It was always my intention that eofbit should be set any time that a virtual returned something to indicate eof, no matter what reason iostream code had for calling the virtual.

> The motivation for this is that I did not want to require streambufs to behave consistently if their virtuals are called after they have signaled eof.

> The classic case is a streambuf reading from a UNIX file. EOF isn't really a state for UNIX file descriptors. The convention is that a read on UNIX returns 0 bytes to indicate "EOF", but the file descriptor isn't shut down in any way and future reads do not necessarily also return 0 bytes. In particular, you can read from tty's on UNIX even after they have signaled "EOF". (It isn't always understood that a ^D on UNIX is not an EOF indicator, but an EOL indicator. By typing a "line" consisting solely of ^D you cause a read to return 0 bytes, and by convention this is interpreted as end of file.)

Proposed resolution:

Add a sentence to the end of 27.6.1.1.2 paragraph 2:

> If `is.rdbuf()->sbumpc()` or `is.rdbuf()->sgetc()` returns `traits::eof()`, the function calls `setstate(failbit | eofbit)` (which may throw `ios_base::failure`).

### 198(i). Validity of pointers and references unspecified after iterator destruction

Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Beman Dawes Opened: 1999-11-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Is a pointer or reference obtained from an iterator still valid after destruction of the iterator?

Is a pointer or reference obtained from an iterator still valid after the value of the iterator changes?

>     #include <iostream>
>     #include <vector>
>     #include <iterator>

>     int main()
>     {
>         typedef std::vector<int> vec_t;
>         vec_t v;
>         v.push_back( 1 );

>         // Is a pointer or reference obtained from an iterator still
>         // valid after destruction of the iterator?
>         int * p = &*v.begin();
>         std::cout << *p << '\n';  // OK?

>         // Is a pointer or reference obtained from an iterator still
>         // valid after the value of the iterator changes?
>         vec_t::iterator iter( v.begin() );
>         p = &*iter++;
>         std::cout << *p << '\n';  // OK?

>         return 0;
>     }

The standard doesn't appear to directly address these questions. The standard needs to be clarified. At least two real-world cases have been reported where library implementors wasted considerable effort because of the lack of clarity in the standard. The question is important because requiring pointers and references to remain valid has the effect for practical purposes of prohibiting iterators from pointing to cached rather than actual elements of containers.

The standard itself assumes that pointers and references obtained from an iterator are still valid after iterator destruction or change. The definition of reverse_iterator::operator*(), 24.5.1.5 [reverse.iter.conv], which returns a reference, defines effects:

>     Iterator tmp = current;
>     return *--tmp;

The definition of reverse_iterator::operator->(), [reverse.iter.op.star], which returns a pointer, defines effects:

>     return &(operator*());

Because the standard itself assumes pointers and references remain valid after iterator destruction or change, the standard should say so explicitly. This will also reduce the chance of user code breaking unexpectedly when porting to a different standard library implementation.

Proposed resolution:

Add a new paragraph to 24.3.4 [iterator.concepts]:

> Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.

Replace paragraph 1 of 24.5.1.5 [reverse.iter.conv] with:

> Effects:

>       this->tmp = current;
>       --this->tmp;
>       return *this->tmp;

> [Note: This operation must use an auxiliary member variable, rather than a temporary variable, to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.3.4 [iterator.concepts].) The name of this member variable is shown for exposition only. --end note]

[Post-Tokyo: The issue has been reformulated purely in terms of iterators.]

[Pre-Toronto: Steve Cleary pointed out the no-invalidation assumption by reverse_iterator. The issue and proposed resolution was reformulated yet again to reflect this reality.]

[Copenhagen: Steve Cleary pointed out that reverse_iterator assumes its underlying iterator has persistent pointers and references. Andy Koenig pointed out that it is possible to rewrite reverse_iterator so that it no longer makes such an assupmption. However, this issue is related to issue 299(i). If we decide it is intentional that `p[n]` may return by value instead of reference when `p` is a Random Access Iterator, other changes in reverse_iterator will be necessary.]

Rationale:

This issue has been discussed extensively. Note that it is not an issue about the behavior of predefined iterators. It is asking whether or not user-defined iterators are permitted to have transient pointers and references. Several people presented examples of useful user-defined iterators that have such a property; examples include a B-tree iterator, and an "iota iterator" that doesn't point to memory. Library implementors already seem to be able to cope with such iterators: they take pains to avoid forming references to memory that gets iterated past. The only place where this is a problem is `reverse_iterator`, so this issue changes `reverse_iterator` to make it work.

This resolution does not weaken any guarantees provided by predefined iterators like `list<int>::iterator`. Clause 23 should be reviewed to make sure that guarantees for predefined iterators are as strong as users expect.

### 199(i). What does `allocate(0)` return?

Section: 16.4.4.6 [allocator.requirements] Status: TC1 Submitter: Matt Austern Opened: 1999-11-19 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [allocator.requirements].

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Discussion:

Suppose that `A` is a class that conforms to the Allocator requirements of Table 32, and `a` is an object of class `A` What should be the return value of `a.allocate(0)`? Three reasonable possibilities: forbid the argument `0`, return a null pointer, or require that the return value be a unique non-null pointer.

Proposed resolution:

Add a note to the `allocate` row of Table 32: "[Note: If `n == 0`, the return value is unspecified. --end note]"

Rationale:

A key to understanding this issue is that the ultimate use of allocate() is to construct an iterator, and that iterator for zero length sequences must be the container's past-the-end representation. Since this already implies special case code, it would be over-specification to mandate the return value.

### 200(i). Forward iterator requirements don't allow constant iterators

Section: 24.3.5.5 [forward.iterators] Status: CD1 Submitter: Matt Austern Opened: 1999-11-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In table 74, the return type of the expression `*a` is given as `T&`, where `T` is the iterator's value type. For constant iterators, however, this is wrong. ("Value type" is never defined very precisely, but it is clear that the value type of, say, `std::list<int>::const_iterator` is supposed to be `int`, not `const int`.)

Proposed resolution:

In table 74, in the `*a` and `*r++` rows, change the return type from "`T&`" to "`T&` if `X` is mutable, otherwise `const T&`". In the `a->m` row, change the return type from "`U&`" to "`U&` if `X` is mutable, otherwise `const U&`".

[Tokyo: The LWG believes this is the tip of a larger iceberg; there are multiple const problems with the STL portion of the library and that these should be addressed as a single package. Note that issue 180(i) has already been declared NAD Future for that very reason.]

[Redmond: the LWG thinks this is separable from other constness issues. This issue is just cleanup; it clarifies language that was written before we had iterator_traits. Proposed resolution was modified: the original version only discussed *a. It was pointed out that we also need to worry about *r++ and a->m.]

### 201(i). Numeric limits terminology wrong

Section: 17.3 [support.limits] Status: CD1 Submitter: Stephen Cleary Opened: 1999-12-21 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

In some places in this section, the terms "fundamental types" and "scalar types" are used when the term "arithmetic types" is intended. The current usage is incorrect because void is a fundamental type and pointers are scalar types, neither of which should have specializations of numeric_limits.

[Lillehammer: it remains true that numeric_limits is using imprecise language. However, none of the proposals for changed wording are clearer. A redesign of numeric_limits is needed, but this is more a task than an open issue.]

Proposed resolution:

Change 17.3 [support.limits] to:

> -1- The headers `<limits>`, `<climits>`, `<cfloat>`, and `<cinttypes>` supply characteristics of implementation-dependent fundamental arithmetic types (3.9.1).

Change [limits] to:

> -1- The `numeric_limits` component provides a C++ program with information about various properties of the implementation's representation of the fundamental arithmetic types.

> -2- Specializations shall be provided for each fundamental arithmetic type, both floating point and integer, including `bool`. The member `is_specialized` shall be `true` for all such specializations of `numeric_limits`.

> -4- Non-fundamentalarithmetic standard types, such as `complex<T>` (26.3.2), shall not have specializations.

Change 17.3.5 [numeric.limits] to:

> -1- The member `is_specialized` makes it possible to distinguish between fundamental types, which have specializations, and non-scalar types, which do not.

### 202(i). unique() effects unclear when predicate not an equivalence relation

Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Andrew Koenig Opened: 2000-01-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

What should unique() do if you give it a predicate that is not an equivalence relation? There are at least two plausible answers:

> 1. You can't, because 25.2.8 says that it it "eliminates all but the first element from every consecutive group of equal elements..." and it wouldn't make sense to interpret "equal" as meaning anything but an equivalence relation. [It also doesn't make sense to interpret "equal" as meaning ==, because then there would never be any sense in giving a predicate as an argument at all.]

> 2. The word "equal" should be interpreted to mean whatever the predicate says, even if it is not an equivalence relation (and in particular, even if it is not transitive).

The example that raised this question is from Usenet:

>     int f[] = { 1, 3, 7, 1, 2 };
>     int* z = unique(f, f+5, greater<int>());

If one blindly applies the definition using the predicate greater<int>, and ignore the word "equal", you get:

> Eliminates all but the first element from every consecutive group of elements referred to by the iterator i in the range [first, last) for which *i > *(i - 1).

The first surprise is the order of the comparison. If we wanted to allow for the predicate not being an equivalence relation, then we should surely compare elements the other way: pred(*(i - 1), *i). If we do that, then the description would seem to say: "Break the sequence into subsequences whose elements are in strictly increasing order, and keep only the first element of each subsequence". So the result would be 1, 1, 2. If we take the description at its word, it would seem to call for strictly DEcreasing order, in which case the result should be 1, 3, 7, 2.
In fact, the SGI implementation of unique() does neither: It yields 1, 3, 7.

Proposed resolution:

Change 26.7.9 [alg.unique] paragraph 1 to:

> For a nonempty range, eliminates all but the first element from every consecutive group of equivalent elements referred to by the iterator `i` in the range [first+1, last) for which the following conditions hold: `*(i-1) == *i` or `pred(*(i-1), *i) != false`.

Also insert a new paragraph, paragraph 2a, that reads: "Requires: The comparison function must be an equivalence relation."

[Redmond: discussed arguments for and against requiring the comparison function to be an equivalence relation. Straw poll: 14-2-5. First number is to require that it be an equivalence relation, second number is to explicitly not require that it be an equivalence relation, third number is people who believe they need more time to consider the issue. A separate issue: Andy Sawyer pointed out that "i-1" is incorrect, since "i" can refer to the first iterator in the range. Matt provided wording to address this problem.]

[Curaçao: The LWG changed "... the range (first, last)..." to "... the range [first+1, last)..." for clarity. They considered this change close enough to editorial to not require another round of review.]

Rationale:

The LWG also considered an alternative resolution: change 26.7.9 [alg.unique] paragraph 1 to:

> For a nonempty range, eliminates all but the first element from every consecutive group of elements referred to by the iterator `i` in the range (first, last) for which the following conditions hold: `*(i-1) == *i` or `pred(*(i-1), *i) != false`.

Also insert a new paragraph, paragraph 1a, that reads: "Notes: The comparison function need not be an equivalence relation."

Informally: the proposed resolution imposes an explicit requirement that the comparison function be an equivalence relation. The alternative resolution does not, and it gives enough information so that the behavior of unique() for a non-equivalence relation is specified. Both resolutions are consistent with the behavior of existing implementations.

### 206(i). operator new(size_t, nothrow) may become unlinked to ordinary operator new if ordinary version replaced

Section: 17.6.3.2 [new.delete.single] Status: CD1 Submitter: Howard Hinnant Opened: 1999-08-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

As specified, the implementation of the nothrow version of operator new does not necessarily call the ordinary operator new, but may instead simply call the same underlying allocator and return a null pointer instead of throwing an exception in case of failure.

Such an implementation breaks code that replaces the ordinary version of new, but not the nothrow version. If the ordinary version of new/delete is replaced, and if the replaced delete is not compatible with pointers returned from the library versions of new, then when the replaced delete receives a pointer allocated by the library new(nothrow), crash follows.

The fix appears to be that the lib version of new(nothrow) must call the ordinary new. Thus when the ordinary new gets replaced, the lib version will call the replaced ordinary new and things will continue to work.

An alternative would be to have the ordinary new call new(nothrow). This seems sub-optimal to me as the ordinary version of new is the version most commonly replaced in practice. So one would still need to replace both ordinary and nothrow versions if one wanted to replace the ordinary version.

Another alternative is to put in clear text that if one version is replaced, then the other must also be replaced to maintain compatibility. Then the proposed resolution below would just be a quality of implementation issue. There is already such text in paragraph 7 (under the new(nothrow) version). But this nuance is easily missed if one reads only the paragraphs relating to the ordinary new.

N2158 has been written explaining the rationale for the proposed resolution below.

Proposed resolution:

Change 18.5.1.1 [new.delete.single]:

>     void* operator new(std::size_t size, const std::nothrow_t&) throw();

> > -5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a `bad_alloc` exception.

> > -6- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -7- Required behavior: Return a non-null pointer to suitably aligned storage (3.7.4), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the (possibly replaced) ordinary version. This requirement is binding on a replacement version of this function.

> > -8- Default behavior:

> > - Calls `operator new(size)`.
> > - If the call to `operator new(size)` returns normally, returns the result of that call, else
> > - if the call to `operator new(size)` throws an exception, returns a null pointer.
> > - Executes a loop: Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to the Standard C library function `malloc` is unspecified.
> > - Returns a pointer to the allocated storage if the attempt is successful. Otherwise, if the last argument to `set_new_handler()` was a null pointer, return a null pointer.
> > - Otherwise, the function calls the current new_handler (18.5.2.2). If the called function returns, the loop repeats.
> > - The loop terminates when an attempt to allocate the requested storage is successful or when a called new_handler function does not return. If the called new_handler function terminates by throwing a `bad_alloc exception`, the function returns a null pointer.

> > -9- [Example:

> > >     T* p1 = new T;                 // throws bad_alloc if it fails
> > >     T* p2 = new(nothrow) T;        // returns 0 if it fails

> > --end example]

>     void operator delete(void* ptr) throw();
>     void operator delete(void* ptr, const std::nothrow_t&) throw();

> > -10- Effects: The deallocation function (3.7.4.2) called by a delete-expression to render the value of `ptr` invalid.

> > -11- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -12- Requires: the value of `ptr` is null or the value returned by an earlier call to the default (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t, const std::nothrow_t&)`.

> > -13- Default behavior:

> > - For a null value of `ptr`, do nothing.
> > - Any other value of `ptr` shall be a value returned earlier by a call to the default `operator new`, which was not invalidated by an intervening call to `operator delete(void*)` (17.4.3.7). For such a non-null value of `ptr`, reclaims storage allocated by the earlier call to the default `operator new`.

> > -14- Remarks: It is unspecified under what conditions part or all of such reclaimed storage is allocated by a subsequent call to `operator new` or any of `calloc`, `malloc`, or `realloc`, declared in `<cstdlib>`.

>     void operator delete(void* ptr, const std::nothrow_t&) throw();

> > -15- Effects: Same as above, except that it is called by the implementation when an exception propagates from a nothrow placement version of the new-expression (i.e. when the constructor throws an exception).

> > -16- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -17- Requires: the value of `ptr` is null or the value returned by an earlier call to the (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t, const std::nothrow_t&)`.

> > -18- Default behavior: Calls `operator delete(ptr)`.

Change 18.5.1.2 [new.delete.array]

>     void* operator new[](std::size_t size, const std::nothrow_t&) throw();

> > -5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a `bad_alloc` exception.

> > -6- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -7- Required behavior: Same as for operator `new(std::size_t, const std::nothrow_t&)`. This nothrow version of operator `new[]` returns a pointer obtained as if acquired from the ordinary version. Return a non-null pointer to suitably aligned storage (3.7.4), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the (possibly replaced) `operator new[](std::size_t size)`. This requirement is binding on a replacement version of this function.

> > -8- Default behavior: Returns `operator new(size, nothrow)`.

> > - Calls `operator new[](size)`.
> > - If the call to `operator new[](size)` returns normally, returns the result of that call, else
> > - if the call to `operator new[](size)` throws an exception, returns a null pointer.

>     void operator delete[](void* ptr) throw();
>     void operator delete[](void* ptr, const std::nothrow_t&) throw();

> > -9- Effects: The deallocation function (3.7.4.2) called by the array form of a delete-expression to render the value of `ptr` invalid.

> > -10- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -11- Requires: the value of `ptr` is null or the value returned by an earlier call to `operator new[](std::size_t)` or `operator new[](std::size_t, const std::nothrow_t&)`.

> > -12- Default behavior: Calls `operator delete(ptr)` or `operator delete[](ptr, std::nothrow)` respectively.

Rationale:

Yes, they may become unlinked, and that is by design. If a user replaces one, the user should also replace the other.

[ Reopened due to a gcc conversation between Howard, Martin and Gaby. Forwarding or not is visible behavior to the client and it would be useful for the client to know which behavior it could depend on. ]

[ Batavia: Robert voiced serious reservations about backwards compatibility for his customers. ]

### 208(i). Unnecessary restriction on past-the-end iterators

Section: 24.3.4 [iterator.concepts] Status: TC1 Submitter: Stephen Cleary Opened: 2000-02-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 24.1 paragraph 5, it is stated ". . . Dereferenceable and past-the-end values are always non-singular."

This places an unnecessary restriction on past-the-end iterators for containers with forward iterators (for example, a singly-linked list). If the past-the-end value on such a container was a well-known singular value, it would still satisfy all forward iterator requirements.

Removing this restriction would allow, for example, a singly-linked list without a "footer" node.

This would have an impact on existing code that expects past-the-end iterators obtained from different (generic) containers being not equal.

Proposed resolution:

Change 24.3.4 [iterator.concepts] paragraph 5, the last sentence, from:

> Dereferenceable and past-the-end values are always non-singular.

to:

> Dereferenceable values are always non-singular.

Rationale:

For some kinds of containers, including singly linked lists and zero-length vectors, null pointers are perfectly reasonable past-the-end iterators. Null pointers are singular.

### 209(i). basic_string declarations inconsistent

Section: 27.4.3 [basic.string] Status: TC1 Submitter: Igor Stauder Opened: 2000-02-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In Section 27.4.3 [basic.string] the basic_string member function declarations use a consistent style except for the following functions:

>     void push_back(const charT);
>     basic_string& assign(const basic_string&);
>     void swap(basic_string<charT,traits,Allocator>&);

- push_back, assign, swap: missing argument name
- push_back: use of const with charT (i.e. POD type passed by value not by reference - should be charT or const charT& )
- swap: redundant use of template parameters in argument basic_string<charT,traits,Allocator>&

Proposed resolution:

In Section 27.4.3 [basic.string] change the basic_string member function declarations push_back, assign, and swap to:

>     void push_back(charT c);

>     basic_string& assign(const basic_string& str);
>     void swap(basic_string& str);

Rationale:

Although the standard is in general not consistent in declaration style, the basic_string declarations are consistent other than the above. The LWG felt that this was sufficient reason to merit the change.

### 210(i). distance first and last confused

Section: 26 [algorithms] Status: TC1 Submitter: Lisa Lippincott Opened: 2000-02-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In paragraph 9 of section 26 [algorithms], it is written:

> In the description of the algorithms operators + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of [...] a-b is the same as of
> `return distance(a, b);`

Proposed resolution:

On the last line of paragraph 9 of section 26 [algorithms] change `"a-b"` to `"b-a".`

Rationale:

There are two ways to fix the defect; change the description to b-a or change the return to distance(b,a). The LWG preferred the former for consistency.

### 211(i). operator>>(istream&, string&) doesn't set failbit

Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Scott Snyder Opened: 2000-02-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The description of the stream extraction operator for std::string (section 21.3.7.9 [lib.string.io]) does not contain a requirement that failbit be set in the case that the operator fails to extract any characters from the input stream.

This implies that the typical construction

>     std::istream is;
>     std::string str;
>     ...
>     while (is >> str) ... ;

(which tests failbit) is not required to terminate at EOF.

Furthermore, this is inconsistent with other extraction operators, which do include this requirement. (See sections 31.7.5.3 [istream.formatted] and 31.7.5.4 [istream.unformatted]), where this requirement is present, either explicitly or implicitly, for the extraction operators. It is also present explicitly in the description of getline (istream&, string&, charT) in section 27.4.4.4 [string.io] paragraph 8.)

Proposed resolution:

Insert new paragraph after paragraph 2 in section 27.4.4.4 [string.io]:

> If the function extracts no characters, it calls is.setstate(ios::failbit) which may throw ios_base::failure (27.4.4.3).

### 212(i). Empty range behavior unclear for several algorithms

Section: 26.8.9 [alg.min.max] Status: TC1 Submitter: Nico Josuttis Opened: 2000-02-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The standard doesn't specify what min_element() and max_element() shall return if the range is empty (first equals last). The usual implementations return last. This problem seems also apply to partition(), stable_partition(), next_permutation(), and prev_permutation().

Proposed resolution:

In 26.8.9 [alg.min.max] - Minimum and maximum, paragraphs 7 and 9, append: Returns last if first==last.

Rationale:

The LWG looked in some detail at all of the above mentioned algorithms, but believes that except for min_element() and max_element() it is already clear that last is returned if first == last.

### 214(i). set::find() missing const overload

Section: 23.4.6 [set], 23.4.7 [multiset] Status: CD1 Submitter: Judy Ward Opened: 2000-02-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 450

Discussion:

The specification for the associative container requirements in Table 69 state that the find member function should "return iterator; const_iterator for constant a". The map and multimap container descriptions have two overloaded versions of find, but set and multiset do not, all they have is:

>     iterator find(const key_type & x) const;

Proposed resolution:

Change the prototypes for find(), lower_bound(), upper_bound(), and equal_range() in section 23.4.6 [set] and section 23.4.7 [multiset] to each have two overloads:

>     iterator find(const key_type & x);
>     const_iterator find(const key_type & x) const;

>     iterator lower_bound(const key_type & x);
>     const_iterator lower_bound(const key_type & x) const;

>     iterator upper_bound(const key_type & x);
>     const_iterator upper_bound(const key_type & x) const;

>     pair<iterator, iterator> equal_range(const key_type & x);
>     pair<const_iterator, const_iterator> equal_range(const key_type & x) const;

[Tokyo: At the request of the LWG, Judy Ward provided wording extending the proposed resolution to lower_bound, upper_bound, and equal_range.]

### 217(i). Facets example (Classifying Japanese characters) contains errors

Section: 99 [facets.examples] Status: TC1 Submitter: Martin Sebor Opened: 2000-02-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The example in 22.2.8, paragraph 11 contains the following errors:

1) The member function `My::JCtype::is_kanji()' is non-const; the function must be const in order for it to be callable on a const object (a reference to which which is what std::use_facet<>() returns).

2) In file filt.C, the definition of `JCtype::id' must be qualified with the name of the namespace `My'.

3) In the definition of `loc' and subsequently in the call to use_facet<>() in main(), the name of the facet is misspelled: it should read `My::JCtype' rather than `My::JCType'.

Proposed resolution:

Replace the "Classifying Japanese characters" example in 22.2.8, paragraph 11 with the following:

    #include <locale>

    namespace My {
        using namespace std;
        class JCtype : public locale::facet {
        public:
            static locale::id id;     //  required for use as a new locale facet
            bool is_kanji (wchar_t c) const;
            JCtype() {}
        protected:
            ~JCtype() {}
        };
    }

    //  file:  filt.C
    #include <iostream>
    #include <locale>
    #include "jctype"                 //  above
    std::locale::id My::JCtype::id;   //  the static  JCtype  member
    declared above.

    int main()
    {
        using namespace std;
        typedef ctype<wchar_t> wctype;
        locale loc(locale(""),              //  the user's preferred locale...
                   new My::JCtype);         //  and a new feature ...
        wchar_t c = use_facet<wctype>(loc).widen('!');
        if (!use_facet<My::JCtype>(loc).is_kanji(c))
            cout << "no it isn't!" << endl;
        return 0;
    }

### 220(i). ~ios_base() usage valid?

Section: 31.5.2.8 [ios.base.cons] Status: TC1 Submitter: Jonathan Schilling, Howard Hinnant Opened: 2000-03-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The pre-conditions for the ios_base destructor are described in 27.4.2.7 paragraph 2:

> Effects: Destroys an object of class ios_base. Calls each registered callback pair (fn,index) (27.4.2.6) as (*fn)(erase_event,*this,index) at such time that any ios_base member function called from within fn has well defined results.

But what is not clear is: If no callback functions were ever registered, does it matter whether the ios_base members were ever initialized?

For instance, does this program have defined behavior:

>     #include <ios>

>     class D : public std::ios_base { };

>     int main() { D d; }

It seems that registration of a callback function would surely affect the state of an ios_base. That is, when you register a callback function with an ios_base, the ios_base must record that fact somehow.

But if after construction the ios_base is in an indeterminate state, and that state is not made determinate before the destructor is called, then how would the destructor know if any callbacks had indeed been registered? And if the number of callbacks that had been registered is indeterminate, then is not the behavior of the destructor undefined?

By comparison, the basic_ios class description in 27.4.4.1 paragraph 2 makes it explicit that destruction before initialization results in undefined behavior.

Proposed resolution:

Modify 27.4.2.7 paragraph 1 from

> Effects: Each ios_base member has an indeterminate value after construction.

to

> Effects: Each ios_base member has an indeterminate value after construction. These members must be initialized by calling basic_ios::init. If an ios_base object is destroyed before these initializations have taken place, the behavior is undefined.

### 221(i). num_get<>::do_get stage 2 processing broken

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Matt Austern Opened: 2000-03-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Stage 2 processing of numeric conversion is broken.

Table 55 in 22.2.2.1.2 says that when basefield is 0 the integral conversion specifier is %i. A %i specifier determines a number's base by its prefix (0 for octal, 0x for hex), so the intention is clearly that a 0x prefix is allowed. Paragraph 8 in the same section, however, describes very precisely how characters are processed. (It must be done "as if" by a specified code fragment.) That description does not allow a 0x prefix to be recognized.

Very roughly, stage 2 processing reads a char_type ct. It converts ct to a char, not by using narrow but by looking it up in a translation table that was created by widening the string literal "0123456789abcdefABCDEF+-". The character "x" is not found in that table, so it can't be recognized by stage 2 processing.

Proposed resolution:

In 22.2.2.1.2 paragraph 8, replace the line:

>     static const char src[] = "0123456789abcdefABCDEF+-";

with the line:

>     static const char src[] = "0123456789abcdefxABCDEFX+-";

Rationale:

If we're using the technique of widening a string literal, the string literal must contain every character we wish to recognize. This technique has the consequence that alternate representations of digits will not be recognized. This design decision was made deliberately, with full knowledge of that limitation.

### 222(i). Are throw clauses necessary if a throw is already implied by the effects clause?

Section: 16.3.2.4 [structure.specifications] Status: TC1 Submitter: Judy Ward Opened: 2000-03-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 21.3.6.8 describes the basic_string::compare function this way:

>     21.3.6.8 - basic_string::compare [lib.string::compare]

>     int compare(size_type pos1, size_type n1,
>                     const basic_string<charT,traits,Allocator>&  str ,
>                     size_type  pos2 , size_type  n2 ) const;

>     -4- Returns:

>         basic_string<charT,traits,Allocator>(*this,pos1,n1).compare(
>                      basic_string<charT,traits,Allocator>(str,pos2,n2)) .

and the constructor that's implicitly called by the above is defined to throw an out-of-range exception if pos > str.size(). See section 27.4.3.2 [string.require] paragraph 4.

On the other hand, the compare function descriptions themselves don't have "Throws: " clauses and according to 17.3.1.3, paragraph 3, elements that do not apply to a function are omitted.

So it seems there is an inconsistency in the standard -- are the "Effects" clauses correct, or are the "Throws" clauses missing?

Proposed resolution:

In 16.3.2.4 [structure.specifications] paragraph 3, the footnote 148 attached to the sentence "Descriptions of function semantics contain the following elements (as appropriate):", insert the word "further" so that the foot note reads:

> To save space, items that do not apply to a function are omitted. For example, if a function does not specify any further preconditions, there will be no "Requires" paragraph.

Rationale:

The standard is somewhat inconsistent, but a failure to note a throw condition in a throws clause does not grant permission not to throw. The inconsistent wording is in a footnote, and thus non-normative. The proposed resolution from the LWG clarifies the footnote.

### 223(i). reverse algorithm should use iter_swap rather than swap

Section: 26.7.10 [alg.reverse] Status: TC1 Submitter: Dave Abrahams Opened: 2000-03-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Shouldn't the effects say "applies iter_swap to all pairs..."?

Proposed resolution:

In 26.7.10 [alg.reverse], replace:

> Effects: For each non-negative integer i <= (last - first)/2, applies swap to all pairs of iterators first + i, (last - i) - 1.

with:

> Effects: For each non-negative integer i <= (last - first)/2, applies iter_swap to all pairs of iterators first + i, (last - i) - 1.

### 224(i). clear() complexity for associative containers refers to undefined N

Section: 23.2.7 [associative.reqmts] Status: TC1 Submitter: Ed Brey Opened: 2000-03-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the associative container requirements table in 23.1.2 paragraph 7, a.clear() has complexity "log(size()) + N". However, the meaning of N is not defined.

Proposed resolution:

In the associative container requirements table in 23.1.2 paragraph 7, the complexity of a.clear(), change "log(size()) + N" to "linear in `size()`".

Rationale:

It's the "log(size())", not the "N", that is in error: there's no difference between O(N) and O(N + log(N)). The text in the standard is probably an incorrect cut-and-paste from the range version of `erase`.

### 225(i). std:: algorithms use of other unqualified algorithms

Section: 16.4.6.4 [global.functions] Status: CD1 Submitter: Dave Abrahams Opened: 2000-04-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Are algorithms in std:: allowed to use other algorithms without qualification, so functions in user namespaces might be found through Koenig lookup?

For example, a popular standard library implementation includes this implementation of std::unique:

>     namespace std {
>         template <class _ForwardIter>
>         _ForwardIter unique(_ForwardIter __first, _ForwardIter __last) {
>           __first = adjacent_find(__first, __last);
>           return unique_copy(__first, __last, __first);
>         }
>         }

Imagine two users on opposite sides of town, each using unique on his own sequences bounded by my_iterators . User1 looks at his standard library implementation and says, "I know how to implement a more efficient unique_copy for my_iterators", and writes:

>     namespace user1 {
>         class my_iterator;
>         // faster version for my_iterator
>         my_iterator unique_copy(my_iterator, my_iterator, my_iterator);
>         }

user1::unique_copy() is selected by Koenig lookup, as he intended.

User2 has other needs, and writes:

>     namespace user2 {
>         class my_iterator;
>         // Returns true iff *c is a unique copy of *a and *b.
>         bool unique_copy(my_iterator a, my_iterator b, my_iterator c);
>         }

User2 is shocked to find later that his fully-qualified use of std::unique(user2::my_iterator, user2::my_iterator, user2::my_iterator) fails to compile (if he's lucky). Looking in the standard, he sees the following Effects clause for unique():

> Effects: Eliminates all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first, last) for which the following corresponding conditions hold: *i == *(i - 1) or pred(*i, *(i - 1)) != false

The standard gives user2 absolutely no reason to think he can interfere with std::unique by defining names in namespace user2. His standard library has been built with the template export feature, so he is unable to inspect the implementation. User1 eventually compiles his code with another compiler, and his version of unique_copy silently stops being called. Eventually, he realizes that he was depending on an implementation detail of his library and had no right to expect his unique_copy() to be called portably.

On the face of it, and given above scenario, it may seem obvious that the implementation of unique() shown is non-conforming because it uses unique_copy() rather than ::std::unique_copy(). Most standard library implementations, however, seem to disagree with this notion.

[Tokyo: Steve Adamczyk from the core working group indicates that "std::" is sufficient; leading "::" qualification is not required because any namespace qualification is sufficient to suppress Koenig lookup.]

Proposed resolution:

Add a paragraph and a note at the end of 16.4.6.4 [global.functions]:

> Unless otherwise specified, no global or non-member function in the standard library shall use a function from another namespace which is found through argument-dependent name lookup (6.5.4 [basic.lookup.argdep]).

> [Note: the phrase "unless otherwise specified" is intended to allow Koenig lookup in cases like that of ostream_iterators:
> Effects:

> > *out_stream << value;
> > if(delim != 0) *out_stream << delim;
> > return (*this);

> > --end note]

[Tokyo: The LWG agrees that this is a defect in the standard, but is as yet unsure if the proposed resolution is the best solution. Furthermore, the LWG believes that the same problem of unqualified library names applies to wording in the standard itself, and has opened issue 229(i) accordingly. Any resolution of issue 225(i) should be coordinated with the resolution of issue 229(i).]

[Toronto: The LWG is not sure if this is a defect in the standard. Most LWG members believe that an implementation of `std::unique` like the one quoted in this issue is already illegal, since, under certain circumstances, its semantics are not those specified in the standard. The standard's description of `unique` does not say that overloading `adjacent_find` should have any effect.]

[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution for this issue is in accordance with Howard's paper.]

Rationale:

It could be argued that this proposed isn't strictly necessary, that the Standard doesn't grant implementors license to write a standard function that behaves differently than specified in the Standard just because of an unrelated user-defined name in some other namespace. However, this is at worst a clarification. It is surely right that algorithsm shouldn't pick up random names, that user-defined names should have no effect unless otherwise specified. Issue 226(i) deals with the question of when it is appropriate for the standard to explicitly specify otherwise.

### 226(i). User supplied specializations or overloads of namespace std function templates

Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Dave Abrahams Opened: 2000-04-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The issues are:

1. How can a 3rd party library implementor (lib1) write a version of a standard algorithm which is specialized to work with his own class template?

2. How can another library implementor (lib2) write a generic algorithm which will take advantage of the specialized algorithm in lib1?

This appears to be the only viable answer under current language rules:

>     namespace lib1
>     {
>         // arbitrary-precision numbers using T as a basic unit
>         template <class T>
>         class big_num { //...
>         };

>         // defining this in namespace std is illegal (it would be an
>         // overload), so we hope users will rely on Koenig lookup
>         template <class T>
>         void swap(big_int<T>&, big_int<T>&);
>     }

>     #include <algorithm>
>     namespace lib2
>     {
>         template <class T>
>         void generic_sort(T* start, T* end)
>         {
>                 ...
>             // using-declaration required so we can work on built-in types
>             using std::swap;
>             // use Koenig lookup to find specialized algorithm if available
>             swap(*x, *y);
>         }
>     }

This answer has some drawbacks. First of all, it makes writing lib2 difficult and somewhat slippery. The implementor needs to remember to write the using-declaration, or generic_sort will fail to compile when T is a built-in type. The second drawback is that the use of this style in lib2 effectively "reserves" names in any namespace which defines types which may eventually be used with lib2. This may seem innocuous at first when applied to names like swap, but consider more ambiguous names like unique_copy() instead. It is easy to imagine the user wanting to define these names differently in his own namespace. A definition with semantics incompatible with the standard library could cause serious problems (see issue 225(i)).

Why, you may ask, can't we just partially specialize std::swap()? It's because the language doesn't allow for partial specialization of function templates. If you write:

>     namespace std
>     {
>         template <class T>
>         void swap(lib1::big_int<T>&, lib1::big_int<T>&);
>     }

You have just overloaded std::swap, which is illegal under the current language rules. On the other hand, the following full specialization is legal:

>     namespace std
>     {
>         template <>
>         void swap(lib1::other_type&, lib1::other_type&);
>     }

This issue reflects concerns raised by the "Namespace issue with specialized swap" thread on comp.lang.c++.moderated. A similar set of concerns was earlier raised on the boost.org mailing list and the ACCU-general mailing list. Also see library reflector message c++std-lib-7354.

J. C. van Winkel points out (in c++std-lib-9565) another unexpected fact: it's impossible to output a container of std::pair's using copy and an ostream_iterator, as long as both pair-members are built-in or std:: types. That's because a user-defined operator<< for (for example) std::pair<const std::string, int> will not be found: lookup for operator<< will be performed only in namespace std. Opinions differed on whether or not this was a defect, and, if so, whether the defect is that something is wrong with user-defined functionality and std, or whether it's that the standard library does not provide an operator<< for std::pair<>.

Proposed resolution:

Adopt the wording proposed in Howard Hinnant's paper N1523=03-0106, "Proposed Resolution To LWG issues 225, 226, 229".

[Tokyo: Summary, "There is no conforming way to extend std::swap for user defined templates." The LWG agrees that there is a problem. Would like more information before proceeding. This may be a core issue. Core issue 229 has been opened to discuss the core aspects of this problem. It was also noted that submissions regarding this issue have been received from several sources, but too late to be integrated into the issues list. ]

[Post-Tokyo: A paper with several proposed resolutions, J16/00-0029==WG21/N1252, "Shades of namespace std functions " by Alan Griffiths, is in the Post-Tokyo mailing. It should be considered a part of this issue.]

[Toronto: Dave Abrahams and Peter Dimov have proposed a resolution that involves core changes: it would add partial specialization of function template. The Core Working Group is reluctant to add partial specialization of function templates. It is viewed as a large change, CWG believes that proposal presented leaves some syntactic issues unanswered; if the CWG does add partial specialization of function templates, it wishes to develop its own proposal. The LWG continues to believe that there is a serious problem: there is no good way for users to force the library to use user specializations of generic standard library functions, and in certain cases (e.g. transcendental functions called by `valarray` and `complex`) this is important. Koenig lookup isn't adequate, since names within the library must be qualified with `std` (see issue 225), specialization doesn't work (we don't have partial specialization of function templates), and users aren't permitted to add overloads within namespace std. ]

[Copenhagen: Discussed at length, with no consensus. Relevant papers in the pre-Copenhagen mailing: N1289, N1295, N1296. Discussion focused on four options. (1) Relax restrictions on overloads within namespace std. (2) Mandate that the standard library use unqualified calls for `swap` and possibly other functions. (3) Introduce helper class templates for `swap` and possibly other functions. (4) Introduce partial specialization of function templates. Every option had both support and opposition. Straw poll (first number is support, second is strongly opposed): (1) 6, 4; (2) 6, 7; (3) 3, 8; (4) 4, 4.]

[Redmond: Discussed, again no consensus. Herb presented an argument that a user who is defining a type `T` with an associated `swap` should not be expected to put that `swap` in namespace std, either by overloading or by partial specialization. The argument is that `swap` is part of `T`'s interface, and thus should to in the same namespace as `T` and only in that namespace. If we accept this argument, the consequence is that standard library functions should use unqualified call of `swap`. (And which other functions? Any?) A small group (Nathan, Howard, Jeremy, Dave, Matt, Walter, Marc) will try to put together a proposal before the next meeting.]

[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution is the one proposed by Howard.]

[Santa Cruz: the LWG agreed with the general direction of Howard's paper, N1387. (Roughly: Koenig lookup is disabled unless we say otherwise; this issue is about when we do say otherwise.) However, there were concerns about wording. Howard will provide new wording. Bill and Jeremy will review it.]

[Kona: Howard proposed the new wording. The LWG accepted his proposed resolution.]

Rationale:

Informally: introduce a Swappable concept, and specify that the value types of the iterators passed to certain standard algorithms (such as iter_swap, swap_ranges, reverse, rotate, and sort) conform to that concept. The Swappable concept will make it clear that these algorithms use unqualified lookup for the calls to `swap`. Also, in 29.6.3.3 [valarray.transcend] paragraph 1, state that the valarray transcendentals use unqualified lookup.

### 227(i). std::swap() should require CopyConstructible or DefaultConstructible arguments

Section: 26.7.3 [alg.swap] Status: TC1 Submitter: Dave Abrahams Opened: 2000-04-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

25.2.2 reads:

> `template<class T> void swap(T& a, T& b);`
> Requires: Type T is Assignable (_lib.container.requirements_).
> Effects: Exchanges values stored in two locations.

The only reasonable** generic implementation of swap requires construction of a new temporary copy of one of its arguments:

>     template<class T> void swap(T& a, T& b);
>       {
>           T tmp(a);
>           a = b;
>           b = tmp;
>       }

But a type which is only Assignable cannot be swapped by this implementation.

**Yes, there's also an unreasonable implementation which would require T to be DefaultConstructible instead of CopyConstructible. I don't think this is worthy of consideration:

>     template<class T> void swap(T& a, T& b);
>     {
>         T tmp;
>         tmp = a;
>         a = b;
>         b = tmp;
>     }

Proposed resolution:

Change 25.2.2 paragraph 1 from:

> Requires: Type T is Assignable (23.1).

to:

> Requires: Type T is CopyConstructible (20.1.3) and Assignable (23.1)

### 228(i). Incorrect specification of "..._byname" facets

Section: 28.3.4 [locale.categories] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-20 Last modified: 2018-08-10

Priority: Not Prioritized

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Discussion:

The sections 28.3.4.2.3 [locale.ctype.byname], 28.3.4.2.6 [locale.codecvt.byname], 28.3.4.4.2 [locale.numpunct.byname], 28.3.4.5.2 [locale.collate.byname], 28.3.4.6.5 [locale.time.put.byname], 28.3.4.7.5 [locale.moneypunct.byname], and 28.3.4.8.3 [locale.messages.byname] overspecify the definitions of the "..._byname" classes by listing a bunch of virtual functions. At the same time, no semantics of these functions are defined. Real implementations do not define these functions because the functional part of the facets is actually implemented in the corresponding base classes and the constructor of the "..._byname" version just provides suitable date used by these implementations. For example, the 'numpunct' methods just return values from a struct. The base class uses a statically initialized struct while the derived version reads the contents of this struct from a table. However, no virtual function is defined in 'numpunct_byname'.

For most classes this does not impose a problem but specifically for 'ctype' it does: The specialization for 'ctype_byname<char>' is required because otherwise the semantics would change due to the virtual functions defined in the general version for 'ctype_byname': In 'ctype<char>' the method 'do_is()' is not virtual but it is made virtual in both 'ctype<cT>' and 'ctype_byname<cT>'. Thus, a class derived from 'ctype_byname<char>' can tell whether this class is specialized or not under the current specification: Without the specialization, 'do_is()' is virtual while with specialization it is not virtual.

Proposed resolution:

Change section 22.2.1.2 (lib.locale.ctype.byname) to become:

         namespace std {
           template <class charT>
           class ctype_byname : public ctype<charT> {
           public:
             typedef ctype<charT>::mask mask;
             explicit ctype_byname(const char*, size_t refs = 0);
           protected:
            ~ctype_byname();             //  virtual
           };
         }

Change section 22.2.1.6 (lib.locale.codecvt.byname) to become:

        namespace std {
          template <class internT, class externT, class stateT>
          class codecvt_byname : public codecvt<internT, externT, stateT> {
          public:
           explicit codecvt_byname(const char*, size_t refs = 0);
          protected:
          ~codecvt_byname();             //  virtual
           };
         }

Change section 22.2.3.2 (lib.locale.numpunct.byname) to become:

         namespace std {
           template <class charT>
           class numpunct_byname : public numpunct<charT> {
         //  this class is specialized for  char  and  wchar_t.
           public:
             typedef charT                char_type;
             typedef basic_string<charT>  string_type;
             explicit numpunct_byname(const char*, size_t refs = 0);
           protected:
            ~numpunct_byname();          //  virtual
           };
         }

Change section 22.2.4.2 (lib.locale.collate.byname) to become:

         namespace std {
           template <class charT>
           class collate_byname : public collate<charT> {
           public:
             typedef basic_string<charT> string_type;
             explicit collate_byname(const char*, size_t refs = 0);
           protected:
            ~collate_byname();           //  virtual
           };
         }

Change section 22.2.5.2 (lib.locale.time.get.byname) to become:

         namespace std {
           template <class charT, class InputIterator = istreambuf_iterator<charT> >
           class time_get_byname : public time_get<charT, InputIterator> {
           public:
             typedef time_base::dateorder dateorder;
             typedef InputIterator        iter_type

             explicit time_get_byname(const char*, size_t refs = 0);
           protected:
            ~time_get_byname();          //  virtual
           };
         }

Change section 22.2.5.4 (lib.locale.time.put.byname) to become:

         namespace std {
           template <class charT, class OutputIterator = ostreambuf_iterator<charT> >
           class time_put_byname : public time_put<charT, OutputIterator>
           {
           public:
             typedef charT          char_type;
             typedef OutputIterator iter_type;

             explicit time_put_byname(const char*, size_t refs = 0);
           protected:
            ~time_put_byname();          //  virtual
           };
         }"

Change section 22.2.6.4 (lib.locale.moneypunct.byname) to become:

         namespace std {
           template <class charT, bool Intl = false>
           class moneypunct_byname : public moneypunct<charT, Intl> {
           public:
             typedef money_base::pattern pattern;
             typedef basic_string<charT> string_type;

             explicit moneypunct_byname(const char*, size_t refs = 0);
           protected:
            ~moneypunct_byname();        //  virtual
           };
         }

Change section 22.2.7.2 (lib.locale.messages.byname) to become:

         namespace std {
           template <class charT>
           class messages_byname : public messages<charT> {
           public:
             typedef messages_base::catalog catalog;
             typedef basic_string<charT>    string_type;

             explicit messages_byname(const char*, size_t refs = 0);
           protected:
            ~messages_byname();          //  virtual
           };
         }

Remove section 28.3.4.2.5 [locale.codecvt] completely (because in this case only those members are defined to be virtual which are defined to be virtual in 'ctype<cT>'.)

[Post-Tokyo: Dietmar Kühl submitted this issue at the request of the LWG to solve the underlying problems raised by issue 138(i).]

[Copenhagen: proposed resolution was revised slightly, to remove three last virtual functions from `messages_byname`.]

### 229(i). Unqualified references of other library entities

Section: 16.4.2.2 [contents] Status: CD1 Submitter: Steve Clamage Opened: 2000-04-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Throughout the library chapters, the descriptions of library entities refer to other library entities without necessarily qualifying the names.

For example, section 25.2.2 "Swap" describes the effect of swap_ranges in terms of the unqualified name "swap". This section could reasonably be interpreted to mean that the library must be implemented so as to do a lookup of the unqualified name "swap", allowing users to override any ::std::swap function when Koenig lookup applies.

Although it would have been best to use explicit qualification with "::std::" throughout, too many lines in the standard would have to be adjusted to make that change in a Technical Corrigendum.

Issue 182(i), which addresses qualification of `size_t`, is a special case of this.

Proposed resolution:

To section 17.4.1.1 "Library contents" Add the following paragraph:

> Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects section for library function F is described as calling library function G, the function ::std::G is meant.

[Post-Tokyo: Steve Clamage submitted this issue at the request of the LWG to solve a problem in the standard itself similar to the problem within implementations of library identified by issue 225(i). Any resolution of issue 225(i) should be coordinated with the resolution of this issue.]

[post-Toronto: Howard is undecided about whether it is appropriate for all standard library function names referred to in other standard library functions to be explicitly qualified by `std`: it is common advice that users should define global functions that operate on their class in the same namespace as the class, and this requires argument-dependent lookup if those functions are intended to be called by library code. Several LWG members are concerned that valarray appears to require argument-dependent lookup, but that the wording may not be clear enough to fall under "unless explicitly described otherwise".]

[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. This paper resolves issues 225 and 226. In light of that resolution, the proposed resolution for the current issue makes sense.]

### 230(i). Assignable specified without also specifying CopyConstructible

Section: 16 [library] Status: CD1 Submitter: Beman Dawes Opened: 2000-04-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Issue 227(i) identified an instance (std::swap) where Assignable was specified without also specifying CopyConstructible. The LWG asked that the standard be searched to determine if the same defect existed elsewhere.

There are a number of places (see proposed resolution below) where Assignable is specified without also specifying CopyConstructible. There are also several cases where both are specified. For example, 29.5.3 [rand.req].

Proposed resolution:

In 23.2 [container.requirements] table 65 for value_type: change "T is Assignable" to "T is CopyConstructible and Assignable"

In 23.2.7 [associative.reqmts] table 69 X::key_type; change "Key is Assignable" to "Key is CopyConstructible and Assignable"

In 24.3.5.4 [output.iterators] paragraph 1, change:

> A class or a built-in type X satisfies the requirements of an output iterator if X is an Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:

to:

> A class or a built-in type X satisfies the requirements of an output iterator if X is a CopyConstructible (20.1.3) and Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:

[Post-Tokyo: Beman Dawes submitted this issue at the request of the LWG. He asks that the 26.7.5 [alg.replace] and 26.7.6 [alg.fill] changes be studied carefully, as it is not clear that CopyConstructible is really a requirement and may be overspecification.]

[Portions of the resolution for issue 230 have been superceded by the resolution of issue 276(i).]

Rationale:

The original proposed resolution also included changes to input iterator, fill, and replace. The LWG believes that those changes are not necessary. The LWG considered some blanket statement, where an Assignable type was also required to be Copy Constructible, but decided against this because fill and replace really don't require the Copy Constructible property.

### 231(i). Precision in iostream?

Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: James Kanze, Stephen Clamage Opened: 2000-04-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

What is the following program supposed to output?

    #include <iostream>

        int
        main()
        {
            std::cout.setf( std::ios::scientific , std::ios::floatfield ) ;
            std::cout.precision( 0 ) ;
            std::cout << 1.00 << '\n' ;
            return 0 ;
        }

From my C experience, I would expect "1e+00"; this is what `printf("%.0e" , 1.00 );` does. G++ outputs "1.000000e+00".

The only indication I can find in the standard is 22.2.2.2.2/11, where it says "For conversion from a floating-point type, if (flags & fixed) != 0 or if str.precision() > 0, then str.precision() is specified in the conversion specification." This is an obvious error, however, fixed is not a mask for a field, but a value that a multi-bit field may take -- the results of and'ing fmtflags with ios::fixed are not defined, at least not if ios::scientific has been set. G++'s behavior corresponds to what might happen if you do use (flags & fixed) != 0 with a typical implementation (floatfield == 3 << something, fixed == 1 << something, and scientific == 2 << something).

Presumably, the intent is either (flags & floatfield) != 0, or (flags & floatfield) == fixed; the first gives something more or less like the effect of precision in a printf floating point conversion. Only more or less, of course. In order to implement printf formatting correctly, you must know whether the precision was explicitly set or not. Say by initializing it to -1, instead of 6, and stating that for floating point conversions, if precision < -1, 6 will be used, for fixed point, if precision < -1, 1 will be used, etc. Plus, of course, if precision == 0 and flags & floatfield == 0, 1 should be = used. But it probably isn't necessary to emulate all of the anomalies of printf:-).

Proposed resolution:

Replace 28.3.4.3.3.3 [facet.num.put.virtuals], paragraph 11, with the following sentence:

> For conversion from a floating-point type, `str.precision()` is specified in the conversion specification.

Rationale:

The floatfield determines whether numbers are formatted as if with %f, %e, or %g. If the `fixed` bit is set, it's %f, if `scientific` it's %e, and if both bits are set, or neither, it's %g.

Turning to the C standard, a precision of 0 is meaningful for %f and %e. For %g, precision 0 is taken to be the same as precision 1.

The proposed resolution has the effect that if neither `fixed` nor `scientific` is set we'll be specifying a precision of 0, which will be internally turned into 1. There's no need to call it out as a special case.

The output of the above program will be "1e+00".

[Post-Curaçao: Howard provided improved wording covering the case where precision is 0 and mode is %g.]

### 232(i). "depends" poorly defined in 17.4.3.1

Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Peter Dimov Opened: 2000-04-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

17.4.3.1/1 uses the term "depends" to limit the set of allowed specializations of standard templates to those that "depend on a user-defined name of external linkage."

This term, however, is not adequately defined, making it possible to construct a specialization that is, I believe, technically legal according to 17.4.3.1/1, but that specializes a standard template for a built-in type such as 'int'.

The following code demonstrates the problem:

>     #include <algorithm>

>     template<class T> struct X
>     {
>      typedef T type;
>     };

>     namespace std
>     {
>      template<> void swap(::X<int>::type& i, ::X<int>::type& j);
>     }

Proposed resolution:

Change "user-defined name" to "user-defined type".

Rationale:

This terminology is used in section 2.5.2 and 4.1.1 of The C++ Programming Language. It disallows the example in the issue, since the underlying type itself is not user-defined. The only possible problem I can see is for non-type templates, but there's no possible way for a user to come up with a specialization for bitset, for example, that might not have already been specialized by the implementor?

[Toronto: this may be related to issue 120(i).]

[post-Toronto: Judy provided the above proposed resolution and rationale.]

### 233(i). Insertion hints in associative containers

Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 2000-04-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 192, 246

Discussion:

If `mm` is a multimap and `p` is an iterator into the multimap, then `mm.insert(p, x)` inserts `x` into `mm` with `p` as a hint as to where it should go. Table 69 claims that the execution time is amortized constant if the insert winds up taking place adjacent to `p`, but does not say when, if ever, this is guaranteed to happen. All it says it that `p` is a hint as to where to insert.

The question is whether there is any guarantee about the relationship between `p` and the insertion point, and, if so, what it is.

I believe the present state is that there is no guarantee: The user can supply `p`, and the implementation is allowed to disregard it entirely.

Additional comments from Nathan:
The vote [in Redmond] was on whether to elaborately specify the use of the hint, or to require behavior only if the value could be inserted adjacent to the hint. I would like to ensure that we have a chance to vote for a deterministic treatment: "before, if possible, otherwise after, otherwise anywhere appropriate", as an alternative to the proposed "before or after, if possible, otherwise [...]".

[Toronto: there was general agreement that this is a real defect: when inserting an element x into a multiset that already contains several copies of x, there is no way to know whether the hint will be used. The proposed resolution was that the new element should always be inserted as close to the hint as possible. So, for example, if there is a subsequence of equivalent values, then providing a.begin() as the hint means that the new element should be inserted before the subsequence even if a.begin() is far away. JC van Winkel supplied precise wording for this proposed resolution, and also for an alternative resolution in which hints are only used when they are adjacent to the insertion point.]

[Copenhagen: the LWG agreed to the original proposed resolution, in which an insertion hint would be used even when it is far from the insertion point. This was contingent on seeing a example implementation showing that it is possible to implement this requirement without loss of efficiency. John Potter provided such a example implementation.]

[Redmond: The LWG was reluctant to adopt the proposal that emerged from Copenhagen: it seemed excessively complicated, and went beyond fixing the defect that we identified in Toronto. PJP provided the new wording described in this issue. Nathan agrees that we shouldn't adopt the more detailed semantics, and notes: "we know that you can do it efficiently enough with a red-black tree, but there are other (perhaps better) balanced tree techniques that might differ enough to make the detailed semantics hard to satisfy."]

[Curaçao: Nathan should give us the alternative wording he suggests so the LWG can decide between the two options.]

[Lillehammer: The LWG previously rejected the more detailed semantics, because it seemed more loike a new feature than like defect fixing. We're now more sympathetic to it, but we (especially Bill) are still worried about performance. N1780 describes a naive algorithm, but it's not clear whether there is a non-naive implementation. Is it possible to implement this as efficently as the current version of insert?]

[Post Lillehammer: N1780 updated in post meeting mailing with feedback from Lillehammer with more information regarding performance. ]

[ Batavia: 1780 accepted with minor wording changes in the proposed wording (reflected in the proposed resolution below). Concerns about the performance of the algorithm were satisfactorily met by 1780. 371(i) already handles the stability of equal ranges and so that part of the resolution from 1780 is no longer needed (or reflected in the proposed wording below). ]

Proposed resolution:

Change the indicated rows of the "Associative container requirements" Table in 23.2.7 [associative.reqmts] to:

Associative container requirements
expression | return type | assertion/note pre/post-condition | complexity
--- | --- | --- | ---
`a_eq.insert(t)` | `iterator` | inserts `t` and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to `t` exists in `a_eq`, `t` is inserted at the end of that range. | logarithmic
`a.insert(p,t)` | `iterator` | inserts `t` if and only if there is no element with key equivalent to the key of `t` in containers with unique keys; always inserts `t` in containers with equivalent keys. always returns the iterator pointing to the element with key equivalent to the key of `t`. iterator `p` is a hint pointing to where the insert should start to search. `t` is inserted as close as possible to the position just prior to `p`. | logarithmic in general, but amortized constant if `t` is inserted right after before `p`.

### 234(i). Typos in allocator definition

Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In paragraphs 12 and 13 the effects of `construct()` and `destruct()` are described as returns but the functions actually return `void`.

Proposed resolution:

Substitute "Returns" by "Effect".

### 235(i). No specification of default ctor for reverse_iterator

Section: 24.5.1.2 [reverse.iterator] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The declaration of `reverse_iterator` lists a default constructor. However, no specification is given what this constructor should do.

Proposed resolution:

In section 24.5.1.4 [reverse.iter.cons] add the following paragraph:

> `reverse_iterator()`

> Default initializes `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a default constructed iterator of type `Iterator`.

[pre-Copenhagen: Dietmar provide wording for proposed resolution.]

### 237(i). Undefined expression in complexity specification

Section: 23.3.5.2 [deque.cons] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The complexity specification in paragraph 6 says that the complexity is linear in `first - last`. Even if `operator-()` is defined on iterators this term is in general undefined because it would have to be `last - first`.

Proposed resolution:

Change paragraph 6 from

> Linear in first - last.

to become

> Linear in distance(first, last).

### 238(i). Contradictory results of stringbuf initialization.

Section: 31.8.2.2 [stringbuf.cons] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-05-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 27.7.1.1 paragraph 4 the results of calling the constructor of 'basic_stringbuf' are said to be `str() == str`. This is fine that far but consider this code:

      std::basic_stringbuf<char> sbuf("hello, world", std::ios_base::openmode(0));
      std::cout << "'" << sbuf.str() << "'\n";

Paragraph 3 of 27.7.1.1 basically says that in this case neither the output sequence nor the input sequence is initialized and paragraph 2 of 27.7.1.2 basically says that `str()` either returns the input or the output sequence. None of them is initialized, ie. both are empty, in which case the return from `str()` is defined to be `basic_string<cT>()`.

However, probably only test cases in some testsuites will detect this "problem"...

Proposed resolution:

Remove 27.7.1.1 paragraph 4.

Rationale:

We could fix 27.7.1.1 paragraph 4, but there would be no point. If we fixed it, it would say just the same thing as text that's already in the standard.

### 239(i). Complexity of unique() and/or unique_copy incorrect

Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The complexity of unique and unique_copy are inconsistent with each other and inconsistent with the implementations. The standard specifies:

for unique():

> -3- Complexity: If the range (last - first) is not empty, exactly (last - first) - 1 applications of the corresponding predicate, otherwise no applications of the predicate.

for unique_copy():

> -7- Complexity: Exactly last - first applications of the corresponding predicate.

The implementations do it the other way round: unique() applies the predicate last-first times and unique_copy() applies it last-first-1 times.

As both algorithms use the predicate for pair-wise comparison of sequence elements I don't see a justification for unique_copy() applying the predicate last-first times, especially since it is not specified to which pair in the sequence the predicate is applied twice.

Proposed resolution:

Change both complexity sections in 26.7.9 [alg.unique] to:

> Complexity: For nonempty ranges, exactly last - first - 1 applications of the corresponding predicate.

### 240(i). Complexity of adjacent_find() is meaningless

Section: 26.6.10 [alg.adjacent.find] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The complexity section of adjacent_find is defective:

>     template <class ForwardIterator>
>     ForwardIterator adjacent_find(ForwardIterator first, ForwardIterator last
>                                   BinaryPredicate pred);

> -1- Returns: The first iterator i such that both i and i + 1 are in the range [first, last) for which the following corresponding conditions hold: *i == *(i + 1), pred(*i, *(i + 1)) != false. Returns last if no such iterator is found.

> -2- Complexity: Exactly find(first, last, value) - first applications of the corresponding predicate.

In the Complexity section, it is not defined what "value" is supposed to mean. My best guess is that "value" means an object for which one of the conditions pred(*i,value) or pred(value,*i) is true, where i is the iterator defined in the Returns section. However, the value type of the input sequence need not be equality-comparable and for this reason the term find(first, last, value) - first is meaningless.

A term such as find_if(first, last, bind2nd(pred,*i)) - first or find_if(first, last, bind1st(pred,*i)) - first might come closer to the intended specification. Binders can only be applied to function objects that have the function call operator declared const, which is not required of predicates because they can have non-const data members. For this reason, a specification using a binder could only be an "as-if" specification.

Proposed resolution:

Change the complexity section in 26.6.10 [alg.adjacent.find] to:

> For a nonempty range, exactly `min((i - first) + 1, (last - first) - 1)` applications of the corresponding predicate, where i is `adjacent_find`'s return value.

[Copenhagen: the original resolution specified an upper bound. The LWG preferred an exact count.]

### 241(i). Does unique_copy() require CopyConstructible and Assignable?

Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Some popular implementations of unique_copy() create temporary copies of values in the input sequence, at least if the input iterator is a pointer. Such an implementation is built on the assumption that the value type is CopyConstructible and Assignable.

It is common practice in the standard that algorithms explicitly specify any additional requirements that they impose on any of the types used by the algorithm. An example of an algorithm that creates temporary copies and correctly specifies the additional requirements is accumulate(), 29.5.3 [rand.req].

Since the specifications of unique() and unique_copy() do not require CopyConstructible and Assignable of the InputIterator's value type the above mentioned implementations are not standard-compliant. I cannot judge whether this is a defect in the standard or a defect in the implementations.

Proposed resolution:

In 25.2.8 change:

> -4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap.

to:

> -4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap. The expression *result = *first must be valid. If neither InputIterator nor OutputIterator meets the requirements of forward iterator then the value type of InputIterator must be copy constructible. Otherwise copy constructible is not required.

[Redmond: the original proposed resolution didn't impose an explicit requirement that the iterator's value type must be copy constructible, on the grounds that an input iterator's value type must always be copy constructible. Not everyone in the LWG thought that this requirement was clear from table 72. It has been suggested that it might be possible to implement `unique_copy` without requiring assignability, although current implementations do impose that requirement. Howard provided new wording.]

[ Curaçao: The LWG changed the PR editorially to specify "neither...nor...meet..." as clearer than "both...and...do not meet...". Change believed to be so minor as not to require re-review. ]

### 242(i). Side effects of function objects

Section: 26.7.4 [alg.transform], 29.5 [rand] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The algorithms transform(), accumulate(), inner_product(), partial_sum(), and adjacent_difference() require that the function object supplied to them shall not have any side effects.

The standard defines a side effect in 6.10.1 [intro.execution] as:

> -7- Accessing an object designated by a volatile lvalue (basic.lval), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.

As a consequence, the function call operator of a function object supplied to any of the algorithms listed above cannot modify data members, cannot invoke any function that has a side effect, and cannot even create and modify temporary objects. It is difficult to imagine a function object that is still useful under these severe limitations. For instance, any non-trivial transformator supplied to transform() might involve creation and modification of temporaries, which is prohibited according to the current wording of the standard.

On the other hand, popular implementations of these algorithms exhibit uniform and predictable behavior when invoked with a side-effect-producing function objects. It looks like the strong requirement is not needed for efficient implementation of these algorithms.

The requirement of side-effect-free function objects could be replaced by a more relaxed basic requirement (which would hold for all function objects supplied to any algorithm in the standard library):

> A function objects supplied to an algorithm shall not invalidate any iterator or sequence that is used by the algorithm. Invalidation of the sequence includes destruction of the sorting order if the algorithm relies on the sorting order (see section 25.3 - Sorting and related operations [lib.alg.sorting]).

I can't judge whether it is intended that the function objects supplied to transform(), accumulate(), inner_product(), partial_sum(), or adjacent_difference() shall not modify sequence elements through dereferenced iterators.

It is debatable whether this issue is a defect or a change request. Since the consequences for user-supplied function objects are drastic and limit the usefulness of the algorithms significantly I would consider it a defect.

Proposed resolution:

Things to notice about these changes:

1. The fully-closed ("[]" as opposed to half-closed "[)" ranges are intentional. we want to prevent side-effects from invalidating the end iterators.
2. That has the unintentional side-effect of prohibiting modification of the end element as a side-effect. This could conceivably be significant in some cases.
3. The wording also prevents side-effects from modifying elements of the output sequence. I can't imagine why anyone would want to do this, but it is arguably a restriction that implementors don't need to place on users.
4. Lifting the restrictions imposed in #2 and #3 above is possible and simple, but would require more verbiage.

Change 25.2.3/2 from:

> -2- Requires: op and binary_op shall not have any side effects.

to:

> -2- Requires: in the ranges [first1, last1], [first2, first2 + (last1 - first1)] and [result, result + (last1- first1)], op and binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]

Change 25.2.3/2 from:

> -2- Requires: op and binary_op shall not have any side effects.

to:

> -2- Requires: op and binary_op shall not invalidate iterators or subranges, or modify elements in the ranges [first1, last1], [first2, first2 + (last1 - first1)], and [result, result + (last1 - first1)]. [Footnote: The use of fully closed ranges is intentional --end footnote]

Change 26.4.1/2 from:

> -2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op shall not cause side effects.

to:

> -2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the range [first, last], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of a fully closed range is intentional --end footnote]

Change 26.4.2/2 from:

> -2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op1 and binary_op2 shall not cause side effects.

to:

> -2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the ranges [first, last] and [first2, first2 + (last - first)], binary_op1 and binary_op2 shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]

Change 26.4.3/4 from:

> -4- Requires: binary_op is expected not to have any side effects.

to:

> -4- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]

Change 26.4.4/2 from:

> -2- Requires: binary_op shall not have any side effects.

to:

> -2- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]

[Toronto: Dave Abrahams supplied wording.]

[Copenhagen: Proposed resolution was modified slightly. Matt added footnotes pointing out that the use of closed ranges was intentional.]

### 243(i). `get` and `getline` when sentry reports failure

Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2000-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

basic_istream<>::get(), and basic_istream<>::getline(), are unclear with respect to the behavior and side-effects of the named functions in case of an error.

27.6.1.3, p1 states that "... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input..." It is not clear from this (or the rest of the paragraph) what precisely the behavior should be when the sentry ctor exits by throwing an exception or when the sentry object returns false. In particular, what is the number of characters extracted that gcount() returns supposed to be?

27.6.1.3 p8 and p19 say about the effects of get() and getline(): "... In any case, it then stores a null character (using charT()) into the next successive location of the array." Is not clear whether this sentence applies if either of the conditions above holds (i.e., when sentry fails).

Proposed resolution:

Add to 27.6.1.3, p1 after the sentence

> "... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input."

the following

> "Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array."

Rationale:

Although the general philosophy of the input functions is that the argument should not be modified upon failure, `getline` historically added a terminating null unconditionally. Most implementations still do that. Earlier versions of the draft standard had language that made this an unambiguous requirement; those words were moved to a place where their context made them less clear. See Jerry Schwarz's message c++std-lib-7618.

### 247(i). `vector`, `deque::insert` complexity

Section: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Lisa Lippincott Opened: 2000-06-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 2 of 23.3.13.5 [vector.modifiers] describes the complexity of `vector::insert`:

> Complexity: If first and last are forward iterators, bidirectional iterators, or random access iterators, the complexity is linear in the number of elements in the range [first, last) plus the distance to the end of the vector. If they are input iterators, the complexity is proportional to the number of elements in the range [first, last) times the distance to the end of the vector.

First, this fails to address the non-iterator forms of `insert`.

Second, the complexity for input iterators misses an edge case -- it requires that an arbitrary number of elements can be added at the end of a `vector` in constant time.

I looked to see if `deque` had a similar problem, and was surprised to find that `deque` places no requirement on the complexity of inserting multiple elements (23.3.5.4 [deque.modifiers], paragraph 3):

> Complexity: In the worst case, inserting a single element into a deque takes time linear in the minimum of the distance from the insertion point to the beginning of the deque and the distance from the insertion point to the end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to the copy constructor of T.

Proposed resolution:

Change Paragraph 2 of 23.3.13.5 [vector.modifiers] to

> Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.

[For input iterators, one may achieve this complexity by first inserting at the end of the `vector`, and then using `rotate`.]

Change 23.3.5.4 [deque.modifiers], paragraph 3, to:

> Complexity: The complexity is linear in the number of elements inserted plus the shorter of the distances to the beginning and end of the deque. Inserting a single element at either the beginning or the end of a deque causes a single call to the copy constructor of T.

Rationale:

This is a real defect, and proposed resolution fixes it: some complexities aren't specified that should be. This proposed resolution does constrain deque implementations (it rules out the most naive possible implementations), but the LWG doesn't see a reason to permit that implementation.

### 248(i). time_get fails to set eofbit

Section: 28.3.4.6 [category.time] Status: CD1 Submitter: Martin Sebor Opened: 2000-06-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There is no requirement that any of time_get member functions set ios::eofbit when they reach the end iterator while parsing their input. Since members of both the num_get and money_get facets are required to do so (22.2.2.1.2, and 22.2.6.1.2, respectively), time_get members should follow the same requirement for consistency.

Proposed resolution:

Add paragraph 2 to section 22.2.5.1 with the following text:

> If the end iterator is reached during parsing by any of the get() member functions, the member sets ios_base::eofbit in err.

Rationale:

Two alternative resolutions were proposed. The LWG chose this one because it was more consistent with the way eof is described for other input facets.

### 250(i). splicing invalidates iterators

Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: Brian Parker Opened: 2000-07-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 23.3.11.5 [list.ops] states that

      void splice(iterator position, list<T, Allocator>& x);

invalidates all iterators and references to list `x`.

This is unnecessary and defeats an important feature of splice. In fact, the SGI STL guarantees that iterators to `x` remain valid after `splice`.

Proposed resolution:

Add a footnote to 23.3.11.5 [list.ops], paragraph 1:

> [Footnote: As specified in [default.con.req], paragraphs 4-5, the semantics described in this clause applies only to the case where allocators compare equal. --end footnote]

In 23.3.11.5 [list.ops], replace paragraph 4 with:

> Effects: Inserts the contents of x before position and x becomes empty. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

In 23.3.11.5 [list.ops], replace paragraph 7 with:

> Effects: Inserts an element pointed to by i from list x before position and removes the element from x. The result is unchanged if position == i or position == ++i. Pointers and references to *i continue to refer to this same element but as a member of *this. Iterators to *i (including i itself) continue to refer to the same element, but now behave as iterators into *this, not into x.

In 23.3.11.5 [list.ops], replace paragraph 12 with:

> Requires: [first, last) is a valid range in x. The result is undefined if position is an iterator in the range [first, last). Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

[pre-Copenhagen: Howard provided wording.]

Rationale:

The original proposed resolution said that iterators and references would remain "valid". The new proposed resolution clarifies what that means. Note that this only applies to the case of equal allocators. From [default.con.req] paragraph 4, the behavior of list when allocators compare nonequal is outside the scope of the standard.

### 251(i). basic_stringbuf missing allocator_type

Section: 31.8.2 [stringbuf] Status: CD1 Submitter: Martin Sebor Opened: 2000-07-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopsis for the template class `basic_stringbuf` doesn't list a typedef for the template parameter `Allocator`. This makes it impossible to determine the type of the allocator at compile time. It's also inconsistent with all other template classes in the library that do provide a typedef for the `Allocator` parameter.

Proposed resolution:

Add to the synopses of the class templates basic_stringbuf (27.7.1), basic_istringstream (27.7.2), basic_ostringstream (27.7.3), and basic_stringstream (27.7.4) the typedef:

      typedef Allocator allocator_type;

### 252(i). missing casts/C-style casts used in iostreams

Section: 31.8 [string.streams] Status: CD1 Submitter: Martin Sebor Opened: 2000-07-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

27.7.2.2, p1 uses a C-style cast rather than the more appropriate const_cast<> in the Returns clause for basic_istringstream<>::rdbuf(). The same C-style cast is being used in 27.7.3.2, p1, D.7.2.2, p1, and D.7.3.2, p1, and perhaps elsewhere. 27.7.6, p1 and D.7.2.2, p1 are missing the cast altogether.

C-style casts have not been deprecated, so the first part of this issue is stylistic rather than a matter of correctness.

Proposed resolution:

In 27.7.2.2, p1 replace

      -1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.

with

      -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In 27.7.3.2, p1 replace

      -1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.

with

      -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In 27.7.6, p1, replace

      -1- Returns: &sb

with

      -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In D.7.2.2, p1 replace

      -2- Returns: &sb.

with

      -2- Returns: const_cast<strstreambuf*>(&sb).

### 253(i). valarray helper functions are almost entirely useless

Section: 29.6.2.2 [valarray.cons], 29.6.2.3 [valarray.assign] Status: CD1 Submitter: Robert Klarer Opened: 2000-07-31 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This discussion is adapted from message c++std-lib-7056 posted November 11, 1999. I don't think that anyone can reasonably claim that the problem described below is NAD.

These valarray constructors can never be called:

       template <class T>
             valarray<T>::valarray(const slice_array<T> &);
       template <class T>
             valarray<T>::valarray(const gslice_array<T> &);
       template <class T>
             valarray<T>::valarray(const mask_array<T> &);
       template <class T>
             valarray<T>::valarray(const indirect_array<T> &);

Similarly, these valarray assignment operators cannot be called:

         template <class T>
         valarray<T> valarray<T>::operator=(const slice_array<T> &);
         template <class T>
         valarray<T> valarray<T>::operator=(const gslice_array<T> &);
         template <class T>
         valarray<T> valarray<T>::operator=(const mask_array<T> &);
         template <class T>
         valarray<T> valarray<T>::operator=(const indirect_array<T> &);

Please consider the following example:

       #include <valarray>
       using namespace std;

       int main()
       {
           valarray<double> va1(12);
           valarray<double> va2(va1[slice(1,4,3)]); // line 1
       }

Since the valarray va1 is non-const, the result of the sub-expression va1[slice(1,4,3)] at line 1 is an rvalue of type const std::slice_array<double>. This slice_array rvalue is then used to construct va2. The constructor that is used to construct va2 is declared like this:

         template <class T>
         valarray<T>::valarray(const slice_array<T> &);

Notice the constructor's const reference parameter. When the constructor is called, a slice_array must be bound to this reference. The rules for binding an rvalue to a const reference are in 8.5.3, paragraph 5 (see also 13.3.3.1.4). Specifically, paragraph 5 indicates that a second slice_array rvalue is constructed (in this case copy-constructed) from the first one; it is this second rvalue that is bound to the reference parameter. Paragraph 5 also requires that the constructor that is used for this purpose be callable, regardless of whether the second rvalue is elided. The copy-constructor in this case is not callable, however, because it is private. Therefore, the compiler should report an error.

Since slice_arrays are always rvalues, the valarray constructor that has a parameter of type const slice_array<T> & can never be called. The same reasoning applies to the three other constructors and the four assignment operators that are listed at the beginning of this post. Furthermore, since these functions cannot be called, the valarray helper classes are almost entirely useless.

Proposed resolution:

slice_array:

- Make the copy constructor and copy-assignment operator declarations public in the slice_array class template definition in 29.6.5 [template.slice.array]
- remove paragraph 3 of 29.6.5 [template.slice.array]
- remove the copy constructor declaration from [cons.slice.arr]
- change paragraph 1 of [cons.slice.arr] to read "This constructor is declared to be private. This constructor need not be defined."
- remove the first sentence of paragraph 1 of 29.6.5.2 [slice.arr.assign]
- Change the first three words of the second sentence of paragraph 1 of 29.6.5.2 [slice.arr.assign] to "These assignment operators have"

gslice_array:

- Make the copy constructor and copy-assignment operator declarations public in the gslice_array class template definition in 29.6.7 [template.gslice.array]
- remove the note in paragraph 3 of 29.6.7 [template.gslice.array]
- remove the copy constructor declaration from [gslice.array.cons]
- change paragraph 1 of [gslice.array.cons] to read "This constructor is declared to be private. This constructor need not be defined."
- remove the first sentence of paragraph 1 of 29.6.7.2 [gslice.array.assign]
- Change the first three words of the second sentence of paragraph 1 of 29.6.7.2 [gslice.array.assign] to "These assignment operators have"

mask_array:

- Make the copy constructor and copy-assignment operator declarations public in the mask_array class template definition in 29.6.8 [template.mask.array]
- remove the note in paragraph 2 of 29.6.8 [template.mask.array]
- remove the copy constructor declaration from [mask.array.cons]
- change paragraph 1 of [mask.array.cons] to read "This constructor is declared to be private. This constructor need not be defined."
- remove the first sentence of paragraph 1 of 29.6.8.2 [mask.array.assign]
- Change the first three words of the second sentence of paragraph 1 of 29.6.8.2 [mask.array.assign] to "These assignment operators have"

indirect_array:

- Make the copy constructor and copy-assignment operator declarations public in the indirect_array class definition in 29.6.9 [template.indirect.array]
- remove the note in paragraph 2 of 29.6.9 [template.indirect.array]
- remove the copy constructor declaration from [indirect.array.cons]
- change the descriptive text in [indirect.array.cons] to read "This constructor is declared to be private. This constructor need not be defined."
- remove the first sentence of paragraph 1 of 29.6.9.2 [indirect.array.assign]
- Change the first three words of the second sentence of paragraph 1 of 29.6.9.2 [indirect.array.assign] to "These assignment operators have"

[Proposed resolution was modified in Santa Cruz: explicitly make copy constructor and copy assignment operators public, instead of removing them.]

Rationale:

Keeping the valarray constructors private is untenable. Merely making valarray a friend of the helper classes isn't good enough, because access to the copy constructor is checked in the user's environment.

Making the assignment operator public is not strictly necessary to solve this problem. A majority of the LWG (straw poll: 13-4) believed we should make the assignment operators public, in addition to the copy constructors, for reasons of symmetry and user expectation.

### 254(i). Exception types in clause 19 are constructed from `std::string`

Section: 19.2 [std.exceptions], 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: Dave Abrahams Opened: 2000-08-01 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [std.exceptions].

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Discussion:

Many of the standard exception types which implementations are required to throw are constructed with a const std::string& parameter. For example:

         19.1.5  Class out_of_range                          [lib.out.of.range]
         namespace std {
           class out_of_range : public logic_error {
           public:
             explicit out_of_range(const string& what_arg);
           };
         }

       1 The class out_of_range defines the type of objects  thrown  as  excep-
         tions to report an argument value not in its expected range.

         out_of_range(const string& what_arg);

         Effects:
           Constructs an object of class out_of_range.
         Postcondition:
           strcmp(what(), what_arg.c_str()) == 0.

There are at least two problems with this:

1. A program which is low on memory may end up throwing std::bad_alloc instead of out_of_range because memory runs out while constructing the exception object.
2. An obvious implementation which stores a std::string data member may end up invoking terminate() during exception unwinding because the exception object allocates memory (or rather fails to) as it is being copied.

There may be no cure for (1) other than changing the interface to out_of_range, though one could reasonably argue that (1) is not a defect. Personally I don't care that much if out-of-memory is reported when I only have 20 bytes left, in the case when out_of_range would have been reported. People who use exception-specifications might care a lot, though.

There is a cure for (2), but it isn't completely obvious. I think a note for implementors should be made in the standard. Avoiding possible termination in this case shouldn't be left up to chance. The cure is to use a reference-counted "string" implementation in the exception object. I am not necessarily referring to a std::string here; any simple reference-counting scheme for a NTBS would do.

Further discussion, in email:

...I'm not so concerned about (1). After all, a library implementation can add const char* constructors as an extension, and users don't need to avail themselves of the standard exceptions, though this is a lame position to be forced into. FWIW, std::exception and std::bad_alloc don't require a temporary basic_string.

...I don't think the fixed-size buffer is a solution to the problem, strictly speaking, because you can't satisfy the postcondition
`strcmp(what(), what_arg.c_str()) == 0`
For all values of what_arg (i.e. very long values). That means that the only truly conforming solution requires a dynamic allocation.

Further discussion, from Redmond:

The most important progress we made at the Redmond meeting was realizing that there are two separable issues here: the const string& constructor, and the copy constructor. If a user writes something like `throw std::out_of_range("foo")`, the const string& constructor is invoked before anything gets thrown. The copy constructor is potentially invoked during stack unwinding.

The copy constructor is a more serious problem, becuase failure during stack unwinding invokes `terminate`. The copy constructor must be nothrow. Curaçao: Howard thinks this requirement may already be present.

The fundamental problem is that it's difficult to get the nothrow requirement to work well with the requirement that the exception objects store a string of unbounded size, particularly if you also try to make the const string& constructor nothrow. Options discussed include:

- Limit the size of a string that exception objects are required to throw: change the postconditions of 19.2.4 [domain.error] paragraph 3 and 19.2.8 [runtime.error] paragraph 3 to something like this: "strncmp(what(), what_arg._str(), N) == 0, where N is an implementation defined constant no smaller than 256".
- Allow the const string& constructor to throw, but not the copy constructor. It's the implementor's responsibility to get it right. (An implementor might use a simple refcount class.)
- Compromise between the two: an implementation is not allowed to throw if the string's length is less than some N, but, if it doesn't throw, the string must compare equal to the argument.
- Add a new constructor that takes a const char*

(Not all of these options are mutually exclusive.)

Proposed resolution:

Change 19.2.3 [logic.error]

>     namespace std {
>       class logic_error : public exception {
>       public:
>         explicit logic_error(const string& what_arg);
>         explicit logic_error(const char* what_arg);
>       };
>     }

> ...

> `logic_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `logic_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.4 [domain.error]

>     namespace std {
>       class domain_error : public logic_error {
>       public:
>         explicit domain_error(const string& what_arg);
>         explicit domain_error(const char* what_arg);
>       };
>     }

> ...

> `domain_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `domain_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.5 [invalid.argument]

>     namespace std {
>       class invalid_argument : public logic_error {
>       public:
>         explicit invalid_argument(const string& what_arg);
>         explicit invalid_argument(const char* what_arg);
>       };
>     }

> ...

> `invalid_argument(const char* what_arg);`

> > -4- Effects: Constructs an object of class `invalid_argument`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.6 [length.error]

>     namespace std {
>       class length_error : public logic_error {
>       public:
>         explicit length_error(const string& what_arg);
>         explicit length_error(const char* what_arg);
>       };
>     }

> ...

> `length_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `length_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.7 [out.of.range]

>     namespace std {
>       class out_of_range : public logic_error {
>       public:
>         explicit out_of_range(const string& what_arg);
>         explicit out_of_range(const char* what_arg);
>       };
>     }

> ...

> `out_of_range(const char* what_arg);`

> > -4- Effects: Constructs an object of class `out_of_range`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.8 [runtime.error]

>     namespace std {
>       class runtime_error : public exception {
>       public:
>         explicit runtime_error(const string& what_arg);
>         explicit runtime_error(const char* what_arg);
>       };
>     }

> ...

> `runtime_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `runtime_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.9 [range.error]

>     namespace std {
>       class range_error : public runtime_error {
>       public:
>         explicit range_error(const string& what_arg);
>         explicit range_error(const char* what_arg);
>       };
>     }

> ...

> `range_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `range_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.10 [overflow.error]

>     namespace std {
>       class overflow_error : public runtime_error {
>       public:
>         explicit overflow_error(const string& what_arg);
>         explicit overflow_error(const char* what_arg);
>       };
>     }

> ...

> `overflow_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `overflow_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.11 [underflow.error]

>     namespace std {
>       class underflow_error : public runtime_error {
>       public:
>         explicit underflow_error(const string& what_arg);
>         explicit underflow_error(const char* what_arg);
>       };
>     }

> ...

> `underflow_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `underflow_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change [ios::failure]

>     namespace std {
>       class ios_base::failure : public exception {
>       public:
>         explicit failure(const string& msg);
>         explicit failure(const char* msg);
>         virtual const char* what() const throw();
>     };
>     }

> ...

> `failure(const char* msg);`

> > -4- Effects: Constructs an object of class `failure`.

> > -5- Postcondition: `strcmp(what(), msg) == 0`.

Rationale:

Throwing a bad_alloc while trying to construct a message for another exception-derived class is not necessarily a bad thing. And the bad_alloc constructor already has a no throw spec on it (18.4.2.1).

Future:

All involved would like to see const char* constructors added, but this should probably be done for C++0X as opposed to a DR.

I believe the no throw specs currently decorating these functions could be improved by some kind of static no throw spec checking mechanism (in a future C++ language). As they stand, the copy constructors might fail via a call to unexpected. I think what is intended here is that the copy constructors can't fail.

[Pre-Sydney: reopened at the request of Howard Hinnant. Post-Redmond: James Kanze noticed that the copy constructors of exception-derived classes do not have nothrow clauses. Those classes have no copy constructors declared, meaning the compiler-generated implicit copy constructors are used, and those compiler-generated constructors might in principle throw anything.]

[ Batavia: Merged copy constructor and assignment operator spec into `exception` and added `ios::failure` into the proposed resolution. ]

[ Oxford: The proposed resolution simply addresses the issue of constructing the exception objects with `const char*` and string literals without the need to explicit include or construct a `std::string`. ]

### 256(i). typo in 27.4.4.2, p17: copy_event does not exist

Section: 31.5.4.3 [basic.ios.members] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

27.4.4.2, p17 says

> -17- Before copying any parts of rhs, calls each registered callback pair (fn,index) as (*fn)(erase_event,*this,index). After all parts but exceptions() have been replaced, calls each callback pair that was copied from rhs as (*fn)(copy_event,*this,index).

The name copy_event isn't defined anywhere. The intended name was copyfmt_event.

Proposed resolution:

Replace copy_event with copyfmt_event in the named paragraph.

### 258(i). Missing allocator requirement

Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Matt Austern Opened: 2000-08-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

From lib-7752:

I've been assuming (and probably everyone else has been assuming) that allocator instances have a particular property, and I don't think that property can be deduced from anything in Table 32.

I think we have to assume that allocator type conversion is a homomorphism. That is, if x1 and x2 are of type X, where X::value_type is T, and if type Y is X::template rebind<U>::other, then Y(x1) == Y(x2) if and only if x1 == x2.

Further discussion: Howard Hinnant writes, in lib-7757:

I think I can prove that this is not provable by Table 32. And I agree it needs to be true except for the "and only if". If x1 != x2, I see no reason why it can't be true that Y(x1) == Y(x2). Admittedly I can't think of a practical instance where this would happen, or be valuable. But I also don't see a need to add that extra restriction. I think we only need:

> if (x1 == x2) then Y(x1) == Y(x2)

If we decide that == on allocators is transitive, then I think I can prove the above. But I don't think == is necessarily transitive on allocators. That is:

Given x1 == x2 and x2 == x3, this does not mean x1 == x3.

Example:

> x1 can deallocate pointers from: x1, x2, x3
> x2 can deallocate pointers from: x1, x2, x4
> x3 can deallocate pointers from: x1, x3
> x4 can deallocate pointers from: x2, x4

> x1 == x2, and x2 == x4, but x1 != x4

[Toronto: LWG members offered multiple opinions. One opinion is that it should not be required that `x1 == x2` implies `Y(x1) == Y(x2)`, and that it should not even be required that `X(x1) == x1`. Another opinion is that the second line from the bottom in table 32 already implies the desired property. This issue should be considered in light of other issues related to allocator instances.]

Proposed resolution:

Accept proposed wording from N2436 part 3.

[Lillehammer: Same conclusion as before: this should be considered as part of an allocator redesign, not solved on its own.]

[ Batavia: An allocator redesign is not forthcoming and thus we fixed this one issue. ]

[ Toronto: Reopened at the request of the project editor (Pete) because the proposed wording did not fit within the indicated table. The intent of the resolution remains unchanged. Pablo to work with Pete on improved wording. ]

[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 259(i). `basic_string::operator[]` and const correctness

Section: 27.4.3.5 [string.capacity] Status: CD1 Submitter: Chris Newton Opened: 2000-08-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paraphrased from a message that Chris Newton posted to comp.std.c++:

The standard's description of `basic_string<>::operator[]` seems to violate const correctness.

The standard (21.3.4/1) says that "If `pos < size()`, returns `data()[pos]`." The types don't work. The return value of `data()` is `const charT*`, but `operator[]` has a non-const version whose return type is `reference`.

Proposed resolution:

In section 21.3.4, paragraph 1, change "`data()[pos]`" to "`*(begin() + pos)`".

### 260(i). Inconsistent return type of `istream_iterator::operator++(int)`

Section: 24.6.2.3 [istream.iterator.ops] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopsis of istream_iterator::operator++(int) in 24.5.1 shows it as returning the iterator by value. 24.5.1.2, p5 shows the same operator as returning the iterator by reference. That's incorrect given the Effects clause below (since a temporary is returned). The `&' is probably just a typo.

Proposed resolution:

Change the declaration in 24.5.1.2, p5 from

     istream_iterator<T,charT,traits,Distance>& operator++(int);

to

     istream_iterator<T,charT,traits,Distance> operator++(int);

(that is, remove the `&').

### 261(i). Missing description of `istream_iterator::operator!=`

Section: 24.6.2.3 [istream.iterator.ops] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

24.5.1, p3 lists the synopsis for

       template <class T, class charT, class traits, class Distance>
            bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
                            const istream_iterator<T,charT,traits,Distance>& y);

but there is no description of what the operator does (i.e., no Effects or Returns clause) in 24.5.1.2.

Proposed resolution:

Add paragraph 7 to the end of section 24.5.1.2 with the following text:

       template <class T, class charT, class traits, class Distance>
            bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
                            const istream_iterator<T,charT,traits,Distance>& y);

-7- Returns: !(x == y).

### 262(i). Bitmask operator ~ specified incorrectly

Section: 16.3.3.3.3 [bitmask.types] Status: CD1 Submitter: Beman Dawes Opened: 2000-09-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The ~ operation should be applied after the cast to int_type.

Proposed resolution:

Change 17.3.2.1.2 [lib.bitmask.types] operator~ from:

       bitmask operator~ ( bitmask X )
         { return static_cast< bitmask>(static_cast<int_type>(~ X)); }

to:

       bitmask operator~ ( bitmask X )
         { return static_cast< bitmask>(~static_cast<int_type>(X)); }

### 263(i). Severe restriction on `basic_string` reference counting

Section: 27.4.3 [basic.string] Status: CD1 Submitter: Kevlin Henney Opened: 2000-09-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The note in paragraph 6 suggests that the invalidation rules for references, pointers, and iterators in paragraph 5 permit a reference- counted implementation (actually, according to paragraph 6, they permit a "reference counted implementation", but this is a minor editorial fix).

However, the last sub-bullet is so worded as to make a reference-counted implementation unviable. In the following example none of the conditions for iterator invalidation are satisfied:

        // first example: "*******************" should be printed twice
        string original = "some arbitrary text", copy = original;
        const string & alias = original;

        string::const_iterator i = alias.begin(), e = alias.end();
        for(string::iterator j = original.begin(); j != original.end(); ++j)
            *j = '*';
        while(i != e)
            cout << *i++;
        cout << endl;
        cout << original << endl;

Similarly, in the following example:

        // second example: "some arbitrary text" should be printed out
        string original = "some arbitrary text", copy = original;
        const string & alias = original;

        string::const_iterator i = alias.begin();
        original.begin();
        while(i != alias.end())
            cout << *i++;

I have tested this on three string implementations, two of which were reference counted. The reference-counted implementations gave "surprising behavior" because they invalidated iterators on the first call to non-const begin since construction. The current wording does not permit such invalidation because it does not take into account the first call since construction, only the first call since various member and non-member function calls.

Proposed resolution:

Change the following sentence in 21.3 paragraph 5 from

> Subsequent to any of the above uses except the forms of insert() and erase() which return iterators, the first call to non-const member functions operator[](), at(), begin(), rbegin(), end(), or rend().

to

> Following construction or any of the above uses, except the forms of insert() and erase() that return iterators, the first call to non- const member functions operator[](), at(), begin(), rbegin(), end(), or rend().

### 264(i). Associative container `insert(i, j)` complexity requirements are not feasible.

Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: John Potter Opened: 2000-09-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 102

Discussion:

Table 69 requires linear time if [i, j) is sorted. Sorted is necessary but not sufficient. Consider inserting a sorted range of even integers into a set<int> containing the odd integers in the same range.

Related issue: 102(i)

Proposed resolution:

In Table 69, in section 23.1.2, change the complexity clause for insertion of a range from "N log(size() + N) (N is the distance from i to j) in general; linear if [i, j) is sorted according to value_comp()" to "N log(size() + N), where N is the distance from i to j".

[Copenhagen: Minor fix in proposed resolution: fixed unbalanced parens in the revised wording.]

Rationale:

Testing for valid insertions could be less efficient than simply inserting the elements when the range is not both sorted and between two adjacent existing elements; this could be a QOI issue.

The LWG considered two other options: (a) specifying that the complexity was linear if [i, j) is sorted according to value_comp() and between two adjacent existing elements; or (b) changing to Klog(size() + N) + (N - K) (N is the distance from i to j and K is the number of elements which do not insert immediately after the previous element from [i, j) including the first). The LWG felt that, since we can't guarantee linear time complexity whenever the range to be inserted is sorted, it's more trouble than it's worth to say that it's linear in some special cases.

### 265(i). std::pair::pair() effects overly restrictive

Section: 22.3 [pairs] Status: CD1 Submitter: Martin Sebor Opened: 2000-09-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I don't see any requirements on the types of the elements of the std::pair container in 20.2.2. From the descriptions of the member functions it appears that they must at least satisfy the requirements of 20.1.3 [lib.copyconstructible] and 20.1.4 [lib.default.con.req], and in the case of the [in]equality operators also the requirements of 20.1.1 [lib.equalitycomparable] and 20.1.2 [lib.lessthancomparable].

I believe that the the CopyConstructible requirement is unnecessary in the case of 20.2.2, p2.

Proposed resolution:

Change the Effects clause in 20.2.2, p2 from

> -2- Effects: Initializes its members as if implemented: `pair() : first(T1()), second(T2()) {}`

to

> -2- Effects: Initializes its members as if implemented: `pair() : first(), second() {}`

Rationale:

The existing specification of pair's constructor appears to be a historical artifact: there was concern that pair's members be properly zero-initialized when they are built-in types. At one time there was uncertainty about whether they would be zero-initialized if the default constructor was written the obvious way. This has been clarified by core issue 178, and there is no longer any doubt that the straightforward implementation is correct.

### 266(i). bad_exception::~bad_exception() missing Effects clause

Section: 17.9.4 [bad.exception] Status: CD1 Submitter: Martin Sebor Opened: 2000-09-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopsis for std::bad_exception lists the function ~bad_exception() but there is no description of what the function does (the Effects clause is missing).

Proposed resolution:

Remove the destructor from the class synopses of `bad_alloc` (17.6.4.1 [bad.alloc]), `bad_cast` (17.7.4 [bad.cast]), `bad_typeid` (17.7.5 [bad.typeid]), and `bad_exception` (17.9.4 [bad.exception]).

Rationale:

This is a general problem with the exception classes in clause 18. The proposed resolution is to remove the destructors from the class synopses, rather than to document the destructors' behavior, because removing them is more consistent with how exception classes are described in clause 19.

### 268(i). Typo in locale synopsis

Section: 28.3.3.1 [locale] Status: CD1 Submitter: Martin Sebor Opened: 2000-10-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopsis of the class std::locale in 22.1.1 contains two typos: the semicolons after the declarations of the default ctor locale::locale() and the copy ctor locale::locale(const locale&) are missing.

Proposed resolution:

Add the missing semicolons, i.e., change

        //  construct/copy/destroy:
            locale() throw()
            locale(const locale& other) throw()

in the synopsis in 22.1.1 to

        //  construct/copy/destroy:
            locale() throw();
            locale(const locale& other) throw();

### 270(i). Binary search requirements overly strict

Section: 26.8.4 [alg.binary.search] Status: CD1 Submitter: Matt Austern Opened: 2000-10-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.binary.search].

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Duplicate of: 472

Discussion:

Each of the four binary search algorithms (lower_bound, upper_bound, equal_range, binary_search) has a form that allows the user to pass a comparison function object. According to 25.3, paragraph 2, that comparison function object has to be a strict weak ordering.

This requirement is slightly too strict. Suppose we are searching through a sequence containing objects of type X, where X is some large record with an integer key. We might reasonably want to look up a record by key, in which case we would want to write something like this:

        struct key_comp {
          bool operator()(const X& x, int n) const {
            return x.key() < n;
          }
        }

        std::lower_bound(first, last, 47, key_comp());

key_comp is not a strict weak ordering, but there is no reason to prohibit its use in lower_bound.

There's no difficulty in implementing lower_bound so that it allows the use of something like key_comp. (It will probably work unless an implementor takes special pains to forbid it.) What's difficult is formulating language in the standard to specify what kind of comparison function is acceptable. We need a notion that's slightly more general than that of a strict weak ordering, one that can encompass a comparison function that involves different types. Expressing that notion may be complicated.

Additional questions raised at the Toronto meeting:

- Do we really want to specify what ordering the implementor must use when calling the function object? The standard gives specific expressions when describing these algorithms, but it also says that other expressions (with different argument order) are equivalent.
- If we are specifying ordering, note that the standard uses both orderings when describing `equal_range`.
- Are we talking about requiring these algorithms to work properly when passed a binary function object whose two argument types are not the same, or are we talking about requirements when they are passed a binary function object with several overloaded versions of `operator()`?
- The definition of a strict weak ordering does not appear to give any guidance on issues of overloading; it only discusses expressions, and all of the values in these expressions are of the same type. Some clarification would seem to be in order.

Additional discussion from Copenhagen:

- It was generally agreed that there is a real defect here: if the predicate is merely required to be a Strict Weak Ordering, then it's possible to pass in a function object with an overloaded operator(), where the version that's actually called does something completely inappropriate. (Such as returning a random value.)
- An alternative formulation was presented in a paper distributed by David Abrahams at the meeting, "Binary Search with Heterogeneous Comparison", J16-01/0027 = WG21 N1313: Instead of viewing the predicate as a Strict Weak Ordering acting on a sorted sequence, view the predicate/value pair as something that partitions a sequence. This is almost equivalent to saying that we should view binary search as if we are given a unary predicate and a sequence, such that f(*p) is true for all p below a specific point and false for all p above it. The proposed resolution is based on that alternative formulation.

Proposed resolution:

Change 25.3 [lib.alg.sorting] paragraph 3 from:

> 3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For the algorithms to work correctly, comp has to induce a strict weak ordering on the values.

to:

> 3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For algorithms other than those described in lib.alg.binary.search (25.3.3) to work correctly, comp has to induce a strict weak ordering on the values.

Add the following paragraph after 25.3 [lib.alg.sorting] paragraph 5:

> -6- A sequence [start, finish) is partitioned with respect to an expression f(e) if there exists an integer n such that for all 0 <= i < distance(start, finish), f(*(begin+i)) is true if and only if i < n.

Change 25.3.3 [lib.alg.binary.search] paragraph 1 from:

> -1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is in order according to the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.

to:

> -1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is partitioned with respect to an expression formed by binding the search key to an argument of the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.

Change 25.3.3.1 [lib.lower.bound] paragraph 1 from:

> -1- Requires: Type T is LessThanComparable (lib.lessthancomparable).

to:

> -1- Requires: The elements e of [first, last) are partitioned with respect to the expression e < value or comp(e, value)

Remove 25.3.3.1 [lib.lower.bound] paragraph 2:

> -2- Effects: Finds the first position into which value can be inserted without violating the ordering.

Change 25.3.3.2 [lib.upper.bound] paragraph 1 from:

> -1- Requires: Type T is LessThanComparable (lib.lessthancomparable).

to:

> -1- Requires: The elements e of [first, last) are partitioned with respect to the expression !(value < e) or !comp(value, e)

Remove 25.3.3.2 [lib.upper.bound] paragraph 2:

> -2- Effects: Finds the furthermost position into which value can be inserted without violating the ordering.

Change 25.3.3.3 [lib.equal.range] paragraph 1 from:

> -1- Requires: Type T is LessThanComparable (lib.lessthancomparable).

to:

> -1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)

Change 25.3.3.3 [lib.equal.range] paragraph 2 from:

> -2- Effects: Finds the largest subrange [i, j) such that the value can be inserted at any iterator k in it without violating the ordering. k satisfies the corresponding conditions: !(*k < value) && !(value < *k) or comp(*k, value) == false && comp(value, *k) == false.

to:

       -2- Returns:
             make_pair(lower_bound(first, last, value),
                       upper_bound(first, last, value))
           or
             make_pair(lower_bound(first, last, value, comp),
                       upper_bound(first, last, value, comp))

Change 25.3.3.3 [lib.binary.search] paragraph 1 from:

> -1- Requires: Type T is LessThanComparable (lib.lessthancomparable).

to:

> -1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)

[Copenhagen: Dave Abrahams provided this wording]

[Redmond: Minor changes in wording. (Removed "non-negative", and changed the "other than those described in" wording.) Also, the LWG decided to accept the "optional" part.]

Rationale:

The proposed resolution reinterprets binary search. Instead of thinking about searching for a value in a sorted range, we view that as an important special case of a more general algorithm: searching for the partition point in a partitioned range.

We also add a guarantee that the old wording did not: we ensure that the upper bound is no earlier than the lower bound, that the pair returned by equal_range is a valid range, and that the first part of that pair is the lower bound.

### 271(i). basic_iostream missing typedefs

Section: 31.7.5.7 [iostreamclass] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Class template basic_iostream has no typedefs. The typedefs it inherits from its base classes can't be used, since (for example) basic_iostream<T>::traits_type is ambiguous.

Proposed resolution:

Add the following to basic_iostream's class synopsis in 31.7.5.7 [iostreamclass], immediately after `public`:

      // types:
      typedef charT                     char_type;
      typedef typename traits::int_type int_type;
      typedef typename traits::pos_type pos_type;
      typedef typename traits::off_type off_type;
      typedef traits                    traits_type;

### 272(i). Missing parentheses around subexpression

Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 569

Discussion:

27.4.4.3, p4 says about the postcondition of the function: If rdbuf()!=0 then state == rdstate(); otherwise rdstate()==state|ios_base::badbit.

The expression on the right-hand-side of the operator==() needs to be parenthesized in order for the whole expression to ever evaluate to anything but non-zero.

Proposed resolution:

Add parentheses like so: rdstate()==(state|ios_base::badbit).

### 273(i). Missing ios_base qualification on members of a dependent class

Section: 31 [input.output] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

27.5.2.4.2, p4, and 27.8.1.6, p2, 27.8.1.7, p3, 27.8.1.9, p2, 27.8.1.10, p3 refer to in and/or out w/o ios_base:: qualification. That's incorrect since the names are members of a dependent base class (14.6.2 [temp.dep]) and thus not visible.

Proposed resolution:

Qualify the names with the name of the class of which they are members, i.e., ios_base.

### 274(i). a missing/impossible allocator requirement

Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I see that table 31 in 20.1.5, p3 allows T in std::allocator<T> to be of any type. But the synopsis in 20.4.1 calls for allocator<>::address() to be overloaded on reference and const_reference, which is ill-formed for all T = const U. In other words, this won't work:

template class std::allocator<const int>;

The obvious solution is to disallow specializations of allocators on const types. However, while containers' elements are required to be assignable (which rules out specializations on const T's), I think that allocators might perhaps be potentially useful for const values in other contexts. So if allocators are to allow const types a partial specialization of std::allocator<const T> would probably have to be provided.

Proposed resolution:

Change the text in row 1, column 2 of table 32 in 20.1.5, p3 from

> any type

to

> any non-const, non-reference type

[Redmond: previous proposed resolution was "any non-const, non-volatile, non-reference type". Got rid of the "non-volatile".]

Rationale:

Two resolutions were originally proposed: one that partially specialized std::allocator for const types, and one that said an allocator's value type may not be const. The LWG chose the second. The first wouldn't be appropriate, because allocators are intended for use by containers, and const value types don't work in containers. Encouraging the use of allocators with const value types would only lead to unsafe code.

The original text for proposed resolution 2 was modified so that it also forbids volatile types and reference types.

[Curaçao: LWG double checked and believes volatile is correctly excluded from the PR.]

### 275(i). Wrong type in num_get::get() overloads

Section: 28.3.4.3.2.2 [facet.num.get.members] Status: CD1 Submitter: Matt Austern Opened: 2000-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 22.2.2.1.1, we have a list of overloads for num_get<>::get(). There are eight overloads, all of which are identical except for the last parameter. The overloads are:

- long&
- unsigned short&
- unsigned int&
- unsigned long&
- short&
- double&
- long double&
- void*&

There is a similar list, in 22.2.2.1.2, of overloads for num_get<>::do_get(). In this list, the last parameter has the types:

- long&
- unsigned short&
- unsigned int&
- unsigned long&
- float&
- double&
- long double&
- void*&

These two lists are not identical. They should be, since `get` is supposed to call `do_get` with exactly the arguments it was given.

Proposed resolution:

In 28.3.4.3.2.2 [facet.num.get.members], change

      iter_type get(iter_type in, iter_type end, ios_base& str,
                    ios_base::iostate& err, short& val) const;

to

      iter_type get(iter_type in, iter_type end, ios_base& str,
                    ios_base::iostate& err, float& val) const;

### 276(i). Assignable requirement for container value type overly strict

Section: 23.2 [container.requirements] Status: CD1 Submitter: Peter Dimov Opened: 2000-11-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

23.1/3 states that the objects stored in a container must be Assignable. 23.4.3 [map], paragraph 2, states that map satisfies all requirements for a container, while in the same time defining value_type as pair<const Key, T> - a type that is not Assignable.

It should be noted that there exists a valid and non-contradictory interpretation of the current text. The wording in 23.1/3 avoids mentioning value_type, referring instead to "objects stored in a container." One might argue that map does not store objects of type map::value_type, but of map::mapped_type instead, and that the Assignable requirement applies to map::mapped_type, not map::value_type.

However, this makes map a special case (other containers store objects of type value_type) and the Assignable requirement is needlessly restrictive in general.

For example, the proposed resolution of active library issue 103(i) is to make set::iterator a constant iterator; this means that no set operations can exploit the fact that the stored objects are Assignable.

This is related to, but slightly broader than, closed issue 140(i).

Proposed resolution:

23.1/3: Strike the trailing part of the sentence:

> , and the additional requirements of Assignable types from 23.1/3

so that it reads:

> -3- The type of objects stored in these components must meet the requirements of CopyConstructible types (lib.copyconstructible).

23.1/4: Modify to make clear that this requirement is not for all containers. Change to:

> -4- Table 64 defines the Assignable requirement. Some containers require this property of the types to be stored in the container. T is the type used to instantiate the container. t is a value of T, and u is a value of (possibly const) T.

23.1, Table 65: in the first row, change "T is Assignable" to "T is CopyConstructible".

23.2.1/2: Add sentence for Assignable requirement. Change to:

> -2- A deque satisfies all of the requirements of a container and of a reversible container (given in tables in lib.container.requirements) and of a sequence, including the optional sequence requirements (lib.sequence.reqmts). In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on deque that are not described in one of these tables or for operations where there is additional semantic information.

23.2.2/2: Add Assignable requirement to specific methods of list. Change to:

> -2- A list satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the the optional sequence requirements (lib.sequence.reqmts). The exceptions are the operator[] and at member functions, which are not provided. [Footnote: These member functions are only provided by containers whose iterators are random access iterators. --- end foonote]

> list does not require the stored type T to be Assignable unless the following methods are instantiated: [Footnote: Implementors are permitted but not required to take advantage of T's Assignable properties for these methods. -- end foonote]

>          list<T,Allocator>& operator=(const list<T,Allocator>&  x );
>          template <class InputIterator>
>            void assign(InputIterator first, InputIterator last);
>          void assign(size_type n, const T& t);

> Descriptions are provided here only for operations on list that are not described in one of these tables or for operations where there is additional semantic information.

23.2.4/2: Add sentence for Assignable requirement. Change to:

> -2- A vector satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the optional sequence requirements (lib.sequence.reqmts). The exceptions are the push_front and pop_front member functions, which are not provided. In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on vector that are not described in one of these tables or for operations where there is additional semantic information.

Rationale:

list, set, multiset, map, multimap are able to store non-Assignables. However, there is some concern about `list<T>`: although in general there's no reason for T to be Assignable, some implementations of the member functions `operator=` and `assign` do rely on that requirement. The LWG does not want to forbid such implementations.

Note that the type stored in a standard container must still satisfy the requirements of the container's allocator; this rules out, for example, such types as "const int". See issue 274(i) for more details.

In principle we could also relax the "Assignable" requirement for individual `vector` member functions, such as `push_back`. However, the LWG did not see great value in such selective relaxation. Doing so would remove implementors' freedom to implement `vector::push_back` in terms of `vector::insert`.

### 278(i). What does iterator validity mean?

Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: P.J. Plauger Opened: 2000-11-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 23.3.11.5 [list.ops] states that

      void splice(iterator position, list<T, Allocator>& x);

invalidates all iterators and references to list `x`.

But what does the C++ Standard mean by "invalidate"? You can still dereference the iterator to a spliced list element, but you'd better not use it to delimit a range within the original list. For the latter operation, it has definitely lost some of its validity.

If we accept the proposed resolution to issue 250(i), then we'd better clarify that a "valid" iterator need no longer designate an element within the same container as it once did. We then have to clarify what we mean by invalidating a past-the-end iterator, as when a vector or string grows by reallocation. Clearly, such an iterator has a different kind of validity. Perhaps we should introduce separate terms for the two kinds of "validity."

Proposed resolution:

Add the following text to the end of section 24.3.4 [iterator.concepts], after paragraph 5:

> An invalid iterator is an iterator that may be singular. [Footnote: This definition applies to pointers, since pointers are iterators. The effect of dereferencing an iterator that has been invalidated is undefined.]

[post-Copenhagen: Matt provided wording.]

[Redmond: General agreement with the intent, some objections to the wording. Dave provided new wording.]

Rationale:

This resolution simply defines a term that the Standard uses but never defines, "invalid", in terms of a term that is defined, "singular".

Why do we say "may be singular", instead of "is singular"? That's becuase a valid iterator is one that is known to be nonsingular. Invalidating an iterator means changing it in such a way that it's no longer known to be nonsingular. An example: inserting an element into the middle of a vector is correctly said to invalidate all iterators pointing into the vector. That doesn't necessarily mean they all become singular.

### 280(i). Comparison of reverse_iterator to const reverse_iterator

Section: 24.5.1 [reverse.iterators] Status: CD1 Submitter: Steve Cleary Opened: 2000-11-27 Last modified: 2020-03-29

Priority: Not Prioritized

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Discussion:

This came from an email from Steve Cleary to Fergus in reference to issue 179(i). The library working group briefly discussed this in Toronto and believed it should be a separate issue. There was also some reservations about whether this was a worthwhile problem to fix.

Steve said: "Fixing `reverse_iterator`. `std::reverse_iterator` can (and should) be changed to preserve these additional requirements." He also said in email that it can be done without breaking user's code: "If you take a look at my suggested solution, reverse_iterator doesn't have to take two parameters; there is no danger of breaking existing code, except someone taking the address of one of the reverse_iterator global operator functions, and I have to doubt if anyone has ever done that. . . But, just in case they have, you can leave the old global functions in as well -- they won't interfere with the two-template-argument functions. With that, I don't see how any user code could break."

Proposed resolution:

Section: 24.5.1.2 [reverse.iterator] add/change the following declarations:

      A) Add a templated assignment operator, after the same manner
            as the templated copy constructor, i.e.:

      template < class U >
      reverse_iterator < Iterator >& operator=(const reverse_iterator< U >& u);

      B) Make all global functions (except the operator+) have
      two template parameters instead of one, that is, for
      operator ==, !=, <, >, <=, >=, - replace:

           template < class Iterator >
           typename reverse_iterator< Iterator >::difference_type operator-(
                     const reverse_iterator< Iterator >& x,
                     const reverse_iterator< Iterator >& y);

      with:

          template < class Iterator1, class Iterator2 >
          typename reverse_iterator < Iterator1 >::difference_type operator-(
                     const reverse_iterator < Iterator1 > & x,
                     const reverse_iterator < Iterator2 > & y);

Also make the addition/changes for these signatures in [reverse.iter.ops].

[ Copenhagen: The LWG is concerned that the proposed resolution introduces new overloads. Experience shows that introducing overloads is always risky, and that it would be inappropriate to make this change without implementation experience. It may be desirable to provide this feature in a different way. ]

[ Lillehammer: We now have implementation experience, and agree that this solution is safe and correct. ]

[2020-03-29; Jonathan Wakely comments]

The issue title is misleading, it is not about comparing to `const`-qualified `reverse_iterator`s, but comparing to `reverse_iterator<const-iterator>`.

### 281(i). std::min() and max() requirements overly restrictive

Section: 26.8.9 [alg.min.max] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 486

Discussion:

The requirements in 25.3.7, p1 and 4 call for T to satisfy the requirements of `LessThanComparable` ( [lessthancomparable]) and `CopyConstructible` (16.4.4.2 [utility.arg.requirements]). Since the functions take and return their arguments and result by const reference, I believe the `CopyConstructible` requirement is unnecessary.

Proposed resolution:

Remove the `CopyConstructible` requirement. Specifically, replace 25.3.7, p1 with

-1- Requires: Type T is `LessThanComparable` ( [lessthancomparable]).

and replace 25.3.7, p4 with

-4- Requires: Type T is `LessThanComparable` ( [lessthancomparable]).

### 282(i). What types does numpunct grouping refer to?

Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: Howard Hinnant Opened: 2000-12-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 16 mistakenly singles out integral types for inserting thousands_sep() characters. This conflicts with the syntax for floating point numbers described under 22.2.3.1/2.

Proposed resolution:

Change paragraph 16 from:

> For integral types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 28.3.4.4.1.3 [facet.numpunct.virtuals].

To:

> For arithmetic types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 28.3.4.4.1.3 [facet.numpunct.virtuals].

[ Copenhagen: Opinions were divided about whether this is actually an inconsistency, but at best it seems to have been unintentional. This is only an issue for floating-point output: The standard is unambiguous that implementations must parse thousands_sep characters when performing floating-point. The standard is also unambiguous that this requirement does not apply to the "C" locale. ]

[ A survey of existing practice is needed; it is believed that some implementations do insert thousands_sep characters for floating-point output and others fail to insert thousands_sep characters for floating-point input even though this is unambiguously required by the standard. ]

[Post-Curaçao: the above proposed resolution is the consensus of Howard, Bill, Pete, Benjamin, Nathan, Dietmar, Boris, and Martin.]

### 283(i). std::replace() requirement incorrect/insufficient

Section: 26.7.5 [alg.replace] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 483

Discussion:

(revision of the further discussion) There are a number of problems with the requires clauses for the algorithms in 25.1 and 25.2. The requires clause of each algorithm should describe the necessary and sufficient requirements on the inputs to the algorithm such that the algorithm compiles and runs properly. Many of the requires clauses fail to do this. Here is a summary of the kinds of mistakes:

1. Use of EqualityComparable, which only puts requirements on a single type, when in fact an equality operator is required between two different types, typically either T and the iterator's value type or between the value types of two different iterators.
2. Use of Assignable for T when in fact what was needed is Assignable for the value_type of the iterator, and convertability from T to the value_type of the iterator. Or for output iterators, the requirement should be that T is writable to the iterator (output iterators do not have value types).

Here is the list of algorithms that contain mistakes:

- 25.1.2 std::find
- 25.1.6 std::count
- 25.1.8 std::equal
- 25.1.9 std::search, std::search_n
- 25.2.4 std::replace, std::replace_copy
- 25.2.5 std::fill
- 25.2.7 std::remove, std::remove_copy

Also, in the requirements for EqualityComparable, the requirement that the operator be defined for const objects is lacking.

Proposed resolution:

20.1.1 Change p1 from

In Table 28, `T` is a type to be supplied by a C++ program instantiating a template, `a`, `b`, and `c` are values of type `T`.

to

In Table 28, `T` is a type to be supplied by a C++ program instantiating a template, `a`, `b`, and `c` are values of type `const T`.

25 Between p8 and p9

Add the following sentence:

When the description of an algorithm gives an expression such as `*first == value` for a condition, it is required that the expression evaluate to either true or false in boolean contexts.

25.1.2 Change p1 by deleting the requires clause.

25.1.6 Change p1 by deleting the requires clause.

25.1.9

Change p4 from

-4- Requires: Type `T` is `EqualityComparable` (20.1.1), type Size is convertible to integral type (4.7.12.3).

to

-4- Requires: The type `Size` is convertible to integral type (4.7.12.3).

25.2.4 Change p1 from

-1- Requires: Type `T` is `Assignable` (23.1 ) (and, for `replace()`, `EqualityComparable` (20.1.1 )).

to

-1- Requires: The expression `*first = new_value` must be valid.

and change p4 from

-4- Requires: Type `T` is `Assignable` (23.1) (and, for `replace_copy()`, `EqualityComparable` (20.1.1)). The ranges `[first, last)` and `[result, result + (last - first))` shall not overlap.

to

-4- Requires: The results of the expressions `*first` and `new_value` must be writable to the result output iterator. The ranges `[first, last)` and `[result, result + (last - first))` shall not overlap.

25.2.5 Change p1 from

-1- Requires: Type `T` is `Assignable` (23.1). The type `Size` is convertible to an integral type (4.7.12.3).

to

-1- Requires: The expression `value` must be is writable to the output iterator. The type `Size` is convertible to an integral type (4.7.12.3).

25.2.7 Change p1 from

-1- Requires: Type `T` is `EqualityComparable` (20.1.1).

to

-1- Requires: The value type of the iterator must be `Assignable` (23.1).

Rationale:

The general idea of the proposed solution is to remove the faulty requires clauses and let the returns and effects clauses speak for themselves. That is, the returns clauses contain expressions that must be valid, and therefore already imply the correct requirements. In addition, a sentence is added at the beginning of chapter 25 saying that expressions given as conditions must evaluate to true or false in a boolean context. An alternative would be to say that the type of these condition expressions must be literally bool, but that would be imposing a greater restriction that what the standard currently says (which is convertible to bool).

### 284(i). unportable example in 20.3.7, p6

Section: 22.10.8 [comparisons] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The example in 22.10.8 [comparisons], p6 shows how to use the C library function `strcmp()` with the function pointer adapter `ptr_fun()`. But since it's unspecified whether the C library functions have `extern "C"` or `extern "C++"` linkage [16.4.3.3 [using.linkage]], and since function pointers with different the language linkage specifications (9.12 [dcl.link]) are incompatible, whether this example is well-formed is unspecified.

Proposed resolution:

Change 22.10.8 [comparisons] paragraph 6 from:

> [Example:

>         replace_if(v.begin(), v.end(), not1(bind2nd(ptr_fun(strcmp), "C")), "C++");

> replaces each `C` with `C++` in sequence `v`.

to:

> [Example:

>         int compare(const char*, const char*);
>         replace_if(v.begin(), v.end(),
>                    not1(bind2nd(ptr_fun(compare), "abc")), "def");

> replaces each `abc` with `def` in sequence `v`.

Also, remove footnote 215 in that same paragraph.

[Copenhagen: Minor change in the proposed resolution. Since this issue deals in part with C and C++ linkage, it was believed to be too confusing for the strings in the example to be "C" and "C++". ]

[Redmond: More minor changes. Got rid of the footnote (which seems to make a sweeping normative requirement, even though footnotes aren't normative), and changed the sentence after the footnote so that it corresponds to the new code fragment.]

### 285(i). minor editorial errors in fstream ctors

Section: 31.10.4.2 [ifstream.cons] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-31 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

31.10.4.2 [ifstream.cons], p2, 31.10.5.2 [ofstream.cons], p2, and 31.10.6.2 [fstream.cons], p2 say about the effects of each constructor:

... If that function returns a null pointer, calls `setstate(failbit)` (which may throw `ios_base::failure`).

The parenthetical note doesn't apply since the ctors cannot throw an exception due to the requirement in 31.5.4.2 [basic.ios.cons], p3 that `exceptions()` be initialized to `ios_base::goodbit`.

Proposed resolution:

Strike the parenthetical note from the Effects clause in each of the paragraphs mentioned above.

### 286(i). <cstdlib> requirements missing size_t typedef

Section: 26.13 [alg.c.library] Status: CD1 Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The <cstdlib> header file contains prototypes for bsearch and qsort (C++ Standard section 25.4 paragraphs 3 and 4) and other prototypes (C++ Standard section 21.4 paragraph 1 table 49) that require the typedef size_t. Yet size_t is not listed in the <cstdlib> synopsis table 78 in section 25.4.

Proposed resolution:

Add the type size_t to Table 78 (section 25.4) and add the type size_t <cstdlib> to Table 97 (section C.2).

Rationale:

Since size_t is in <stdlib.h>, it must also be in <cstdlib>.

### 288(i). <cerrno> requirements missing macro EILSEQ

Section: 19.4 [errno] Status: CD1 Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

ISO/IEC 9899:1990/Amendment1:1994 Section 4.3 States: "The list of macros defined in <errno.h> is adjusted to include a new macro, EILSEQ"

ISO/IEC 14882:1998(E) section 19.3 does not refer to the above amendment.

Proposed resolution:

Update Table 26 (section 19.3) "Header <cerrno> synopsis" and Table 95 (section C.2) "Standard Macros" to include EILSEQ.

### 291(i). Underspecification of set algorithms

Section: 26.8.7 [alg.set.operations] Status: CD1 Submitter: Matt Austern Opened: 2001-01-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The standard library contains four algorithms that compute set operations on sorted ranges: `set_union`, `set_intersection`, `set_difference`, and `set_symmetric_difference`. Each of these algorithms takes two sorted ranges as inputs, and writes the output of the appropriate set operation to an output range. The elements in the output range are sorted.

The ordinary mathematical definitions are generalized so that they apply to ranges containing multiple copies of a given element. Two elements are considered to be "the same" if, according to an ordering relation provided by the user, neither one is less than the other. So, for example, if one input range contains five copies of an element and another contains three, the output range of `set_union` will contain five copies, the output range of `set_intersection` will contain three, the output range of `set_difference` will contain two, and the output range of `set_symmetric_difference` will contain two.

Because two elements can be "the same" for the purposes of these set algorithms, without being identical in other respects (consider, for example, strings under case-insensitive comparison), this raises a number of unanswered questions:

- If we're copying an element that's present in both of the input ranges, which one do we copy it from?
- If there are n copies of an element in the relevant input range, and the output range will contain fewer copies (say m) which ones do we choose? The first m, or the last m, or something else?
- Are these operations stable? That is, does a run of equivalent elements appear in the output range in the same order as as it appeared in the input range(s)?

The standard should either answer these questions, or explicitly say that the answers are unspecified. I prefer the former option, since, as far as I know, all existing implementations behave the same way.

Proposed resolution:

Add the following to the end of 26.8.7.3 [set.union] paragraph 5:

> If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then max(m, n) of these elements will be copied to the output range: all m of these elements from [first1, last1), and the last max(n-m, 0) of them from [first2, last2), in that order.

Add the following to the end of 26.8.7.4 [set.intersection] paragraph 5:

> If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the first min(m, n) of those elements from [first1, last1) are copied to the output range.

Add a new paragraph, Notes, after 26.8.7.5 [set.difference] paragraph 4:

> If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the last max(m-n, 0) elements from [first1, last1) are copied to the output range.

Add a new paragraph, Notes, after 26.8.7.6 [set.symmetric.difference] paragraph 4:

> If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then |m - n| of those elements will be copied to the output range: the last m - n of these elements from [first1, last1) if m > n, and the last n - m of these elements from [first2, last2) if m < n.

[Santa Cruz: it's believed that this language is clearer than what's in the Standard. However, it's also believed that the Standard may already make these guarantees (although not quite in these words). Bill and Howard will check and see whether they think that some or all of these changes may be redundant. If so, we may close this issue as NAD.]

Rationale:

For simple cases, these descriptions are equivalent to what's already in the Standard. For more complicated cases, they describe the behavior of existing implementations.

### 292(i). effects of a.copyfmt (a)

Section: 31.5.4.3 [basic.ios.members] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The Effects clause of the member function `copyfmt()` in 27.4.4.2, p15 doesn't consider the case where the left-hand side argument is identical to the argument on the right-hand side, that is `(this == &rhs)`. If the two arguments are identical there is no need to copy any of the data members or call any callbacks registered with `register_callback()`. Also, as Howard Hinnant points out in message c++std-lib-8149 it appears to be incorrect to allow the object to fire `erase_event` followed by `copyfmt_event` since the callback handling the latter event may inadvertently attempt to access memory freed by the former.

Proposed resolution:

Change the Effects clause in 27.4.4.2, p15 from

> -15- Effects:Assigns to the member objects of `*this` the corresponding member objects of `rhs`, except that...

to

> -15- Effects:If `(this == &rhs)` does nothing. Otherwise assigns to the member objects of `*this` the corresponding member objects of `rhs`, except that...

### 294(i). User defined macros and standard headers

Section: 16.4.5.3.3 [macro.names] Status: CD1 Submitter: James Kanze Opened: 2001-01-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 2 of 16.4.5.3.3 [macro.names] reads: "A translation unit that includes a header shall not contain any macros that define names declared in that header." As I read this, it would mean that the following program is legal:

      #define npos 3.14
      #include <sstream>

since npos is not defined in <sstream>. It is, however, defined in <string>, and it is hard to imagine an implementation in which <sstream> didn't include <string>.

I think that this phrase was probably formulated before it was decided that a standard header may freely include other standard headers. The phrase would be perfectly appropriate for C, for example. In light of 16.4.6.2 [res.on.headers] paragraph 1, however, it isn't stringent enough.

Proposed resolution:

For 16.4.5.3.3 [macro.names], replace the current wording, which reads:

> Each name defined as a macro in a header is reserved to the implementation for any use if the translation unit includes the header.168)

> A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.

> 168) It is not permissible to remove a library macro definition by using the #undef directive.

with the wording:

> A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.

> A translation unit shall not #define or #undef names lexically identical to keywords.

[Lillehammer: Beman provided new wording]

### 295(i). Is abs defined in <cmath>?

Section: 29.7 [c.math] Status: CD1 Submitter: Jens Maurer Opened: 2001-01-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 80 lists the contents of the <cmath> header. It does not list `abs()`. However, 26.5, paragraph 6, which lists added signatures present in <cmath>, does say that several overloads of `abs()` should be defined in <cmath>.

Proposed resolution:

Add `abs` to Table 80. Also, remove the parenthetical list of functions "(abs(), div(), rand(), srand())" from 29.6 [numarray], paragraph 1.

[Copenhagen: Modified proposed resolution so that it also gets rid of that vestigial list of functions in paragraph 1.]

Rationale:

All this DR does is fix a typo; it's uncontroversial. A separate question is whether we're doing the right thing in putting some overloads in <cmath> that we aren't also putting in <cstdlib>. That's issue 323(i).

### 296(i). Missing descriptions and requirements of pair operators

Section: 22.3 [pairs] Status: C++11 Submitter: Martin Sebor Opened: 2001-01-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopsis of the header `<utility>` in 22.2 [utility] lists the complete set of equality and relational operators for `pair` but the section describing the template and the operators only describes `operator==()` and `operator<()`, and it fails to mention any requirements on the template arguments. The remaining operators are not mentioned at all.

[ 2009-09-27 Alisdair reopens. ]

> The issue is a lack of wording specifying the semantics of `std::pair` relational operators. The rationale is that this is covered by catch-all wording in the relops component, and that as relops directly precedes `pair` in the document this is an easy connection to make.

> Reading the current working paper I make two observations:

> 1. relops no longer immediately precedes `pair` in the order of specification. However, even if it did, there is a lot of `pair` specification itself between the (apparently) unrelated relops and the relational operators for `pair`. (The catch-all still requires `operator==` and `operator<` to be specified explicitly)
> 2. No other library component relies on the catch-all clause. The following all explicitly document all six relational operators, usually in a manner that could have deferred to the relops clause.

> >     tuple
> >     unique_ptr
> >     duration
> >     time_point
> >     basic_string
> >     queue
> >     stack
> >     move_iterator
> >     reverse_iterator
> >     regex submatch
> >     thread::id

> The container components provide their own (equivalent) definition in 23.2.2 [container.requirements.general] Table 90 -- Container requirements and do so do not defer to relops.

> `Shared_ptr` explicitly documents `operator!=` and does not supply the other 3 missing operators (`>`,`>=`,`<=`) so does not meet the reqirements of the relops clause.

> `Weak_ptr` only supports `operator<` so would not be covered by relops.

> At the very least I would request a note pointing to the relops clause we rely on to provide this definition. If this route is taken, I would recommend reducing many of the above listed clauses to a similar note rather than providing redundant specification.

> My preference would be to supply the 4 missing specifications consistent with the rest of the library.

[ 2009-10-11 Daniel opens 1233(i) which deals with the same issue as it pertains to `unique_ptr`. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready

Proposed resolution:

After p20 22.3 [pairs] add:

>     template <class T1, class T2>
>     bool operator!=(const pair<T1,T2>& x, const pair<T1,T2>& y);

> > Returns: `!(x==y)`

>     template <class T1, class T2>
>     bool operator> (const pair<T1,T2>& x, const pair<T1,T2>& y);

> > Returns: `y < x`

>     template <class T1, class T2>
>     bool operator>=(const pair<T1,T2>& x, const pair<T1,T2>& y);

> > Returns: `!(x < y)`

>     template <class T1, class T2>
>     bool operator<=(const pair<T1,T2>& x, const pair<T1,T2>& y);

> > Returns: `!(y < x)`

Rationale:

[operators] paragraph 10 already specifies the semantics. That paragraph says that, if declarations of operator!=, operator>, operator<=, and operator>= appear without definitions, they are defined as specified in [operators]. There should be no user confusion, since that paragraph happens to immediately precede the specification of `pair`.

### 297(i). const_mem_fun_t<>::argument_type should be const T*

Section: 22.10.10 [logical.operations] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The class templates `const_mem_fun_t` in 20.5.8, p8 and `const_mem_fun1_t` in 20.5.8, p9 derive from `unary_function<T*, S>`, and `binary_function<T*, A, S>`, respectively. Consequently, their `argument_type`, and `first_argument_type` members, respectively, are both defined to be `T*` (non-const). However, their function call member operator takes a `const T*` argument. It is my opinion that `argument_type` should be `const T*` instead, so that one can easily refer to it in generic code. The example below derived from existing code fails to compile due to the discrepancy:

`template <class T>`
`void foo (typename T::argument_type arg) // #1`
`{`
`typename T::result_type (T::*pf) (typename T::argument_type) const = // #2`
`&T::operator();`
`}`

`struct X { /* ... */ };`

`int main ()`
`{`
`const X x;`
`foo<std::const_mem_fun_t<void, X> >(&x); // #3`
`}`

#1 `foo()` takes a plain unqualified `X*` as an argument
#2 the type of the pointer is incompatible with the type of the member function
#3 the address of a constant being passed to a function taking a non-const `X*`

Proposed resolution:

Replace the top portion of the definition of the class template const_mem_fun_t in 20.5.8, p8

`template <class S, class T> class const_mem_fun_t`
`: public unary_function<T*, S> {`

with

`template <class S, class T> class const_mem_fun_t`
`: public unary_function<const T*, S> {`

Also replace the top portion of the definition of the class template const_mem_fun1_t in 20.5.8, p9

`template <class S, class T, class A> class const_mem_fun1_t`
`: public binary_function<T*, A, S> {`

with

`template <class S, class T, class A> class const_mem_fun1_t`
`: public binary_function<const T*, A, S> {`

Rationale:

This is simply a contradiction: the `argument_type` typedef, and the argument type itself, are not the same.

### 298(i). ::operator delete[] requirement incorrect/insufficient

Section: 17.6.3.3 [new.delete.array] Status: CD1 Submitter: John A. Pedretti Opened: 2001-01-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The default behavior of `operator delete[]` described in 18.5.1.2, p12 - namely that for non-null value of ptr, the operator reclaims storage allocated by the earlier call to the default `operator new[]` - is not correct in all cases. Since the specified `operator new[]` default behavior is to call `operator new` (18.5.1.2, p4, p8), which can be replaced, along with `operator delete`, by the user, to implement their own memory management, the specified default behavior of`operator delete[]` must be to call `operator delete`.

Proposed resolution:

Change 18.5.1.2, p12 from

> -12- Default behavior:

> - For a null value of `ptr` , does nothing.
> - Any other value of `ptr` shall be a value returned earlier by a call to the default `operator new[](std::size_t)`. [Footnote: The value must not have been invalidated by an intervening call to `operator delete[](void*)` (16.4.5.9 [res.on.arguments]). --- end footnote] For such a non-null value of `ptr` , reclaims storage allocated by the earlier call to the default `operator new[]`.

to

> -12- Default behavior: Calls `operator delete(`ptr) or `operator delete(ptr, std::nothrow)` respectively.

and expunge paragraph 13.

### 300(i). list::merge() specification incomplete

Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: John Pedretti Opened: 2001-01-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The "Effects" clause for list::merge() (23.3.11.5 [list.ops], p23) appears to be incomplete: it doesn't cover the case where the argument list is identical to *this (i.e., this == &x). The requirement in the note in p24 (below) is that x be empty after the merge which is surely unintended in this case.

Proposed resolution:

In 23.3.11.5 [list.ops], replace paragraps 23-25 with:

> 23 Effects: if (&x == this) does nothing; otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()). The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i in the range other than the first, the condition comp(*i, *(i - 1)) will be false.

> 24 Notes: Stable: if (&x != this), then for equivalent elements in the two original ranges, the elements from the original range [begin(), end()) always precede the elements from the original range [x.begin(), x.end()). If (&x != this) the range [x.begin(), x.end()) is empty after the merge.

> 25 Complexity: At most size() + x.size() - 1 applications of comp if (&x ! = this); otherwise, no applications of comp are performed. If an exception is thrown other than by a comparison there are no effects.

[Copenhagen: The original proposed resolution did not fix all of the problems in 23.3.11.5 [list.ops], p22-25. Three different paragraphs (23, 24, 25) describe the effects of `merge`. Changing p23, without changing the other two, appears to introduce contradictions. Additionally, "merges the argument list into the list" is excessively vague.]

[Post-Curaçao: Robert Klarer provided new wording.]

### 301(i). basic_string template ctor effects clause omits allocator argument

Section: 27.4.3.2 [string.require] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The effects clause for the basic_string template ctor in 21.3.1, p15 leaves out the third argument of type Allocator. I believe this to be a mistake.

Proposed resolution:

Replace

> -15- Effects: If `InputIterator` is an integral type, equivalent to

> > `basic_string(static_cast<size_type>(begin), static_cast<value_type>(end))`

with

> -15- Effects: If `InputIterator` is an integral type, equivalent to

> > `basic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)`

### 303(i). Bitset input operator underspecified

Section: 22.9.4 [bitset.operators] Status: CD1 Submitter: Matt Austern Opened: 2001-02-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [bitset.operators].

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Discussion:

In 23.3.5.3, we are told that `bitset`'s input operator "Extracts up to N (single-byte) characters from is.", where is is a stream of type `basic_istream<charT, traits>`.

The standard does not say what it means to extract single byte characters from a stream whose character type, `charT`, is in general not a single-byte character type. Existing implementations differ.

A reasonable solution will probably involve `widen()` and/or `narrow()`, since they are the supplied mechanism for converting a single character between `char` and arbitrary `charT`.

Narrowing the input characters is not the same as widening the literals `'0'` and `'1'`, because there may be some locales in which more than one wide character maps to the narrow character `'0'`. Narrowing means that alternate representations may be used for bitset input, widening means that they may not be.

Note that for numeric input, `num_get<>` (22.2.2.1.2/8) compares input characters to widened version of narrow character literals.

From Pete Becker, in c++std-lib-8224:

> Different writing systems can have different representations for the digits that represent 0 and 1. For example, in the Unicode representation of the Devanagari script (used in many of the Indic languages) the digit 0 is 0x0966, and the digit 1 is 0x0967. Calling narrow would translate those into '0' and '1'. But Unicode also provides the ASCII values 0x0030 and 0x0031 for for the Latin representations of '0' and '1', as well as code points for the same numeric values in several other scripts (Tamil has no character for 0, but does have the digits 1-9), and any of these values would also be narrowed to '0' and '1'.

> ...

> It's fairly common to intermix both native and Latin representations of numbers in a document. So I think the rule has to be that if a wide character represents a digit whose value is 0 then the bit should be cleared; if it represents a digit whose value is 1 then the bit should be set; otherwise throw an exception. So in a Devanagari locale, both 0x0966 and 0x0030 would clear the bit, and both 0x0967 and 0x0031 would set it. Widen can't do that. It would pick one of those two values, and exclude the other one.

From Jens Maurer, in c++std-lib-8233:

> Whatever we decide, I would find it most surprising if bitset conversion worked differently from int conversion with regard to alternate local representations of numbers.

> Thus, I think the options are:

> - Have a new defect issue for 22.2.2.1.2/8 so that it will require the use of narrow().
> - Have a defect issue for bitset() which describes clearly that widen() is to be used.

Proposed resolution:

Replace the first two sentences of paragraph 5 with:

> Extracts up to N characters from is. Stores these characters in a temporary object str of type `basic_string<charT, traits>`, then evaluates the expression `x = bitset<N>(str)`.

Replace the third bullet item in paragraph 5 with:

- the next input character is neither `is.widen(0)` nor `is.widen(1)` (in which case the input character is not extracted).

Rationale:

Input for `bitset` should work the same way as numeric input. Using `widen` does mean that alternative digit representations will not be recognized, but this was a known consequence of the design choice.

### 305(i). Default behavior of codecvt<wchar_t, char, mbstate_t>::length()

Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Howard Hinnant Opened: 2001-01-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [locale.codecvt.byname].

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Discussion:

22.2.1.5/3 introduces codecvt in part with:

> codecvt<wchar_t,char,mbstate_t> converts between the native character sets for tiny and wide characters. Instantiations on mbstate_t perform conversion between encodings known to the library implementor.

But 22.2.1.5.2/10 describes do_length in part with:

> ... codecvt<wchar_t, char, mbstate_t> ... return(s) the lesser of max and (from_end-from).

The semantics of do_in and do_length are linked. What one does must be consistent with what the other does. 22.2.1.5/3 leads me to believe that the vendor is allowed to choose the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in performs so that it makes his customers happy on a given platform. But 22.2.1.5.2/10 explicitly says what codecvt<wchar_t,char,mbstate_t>::do_length must return. And thus indirectly specifies the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in must perform. I believe that this is not what was intended and is a defect.

Discussion from the -lib reflector:
This proposal would have the effect of making the semantics of all of the virtual functions in `codecvt<wchar_t, char, mbstate_t>` implementation specified. Is that what we want, or do we want to mandate specific behavior for the base class virtuals and leave the implementation specified behavior for the codecvt_byname derived class? The tradeoff is that former allows implementors to write a base class that actually does something useful, while the latter gives users a way to get known and specified---albeit useless---behavior, and is consistent with the way the standard handles other facets. It is not clear what the original intention was.

Nathan has suggest a compromise: a character that is a widened version of the characters in the basic execution character set must be converted to a one-byte sequence, but there is no such requirement for characters that are not part of the basic execution character set.

Proposed resolution:

Change 22.2.1.5.2/5 from:

The instantiations required in Table 51 (lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and codecvt<char,char,mbstate_t>, store no characters. Stores no more than (to_limit-to) destination elements. It always leaves the to_next pointer pointing one beyond the last element successfully stored.

to:

Stores no more than (to_limit-to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. codecvt<char,char,mbstate_t> stores no characters.

Change 22.2.1.5.2/10 from:

> -10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiations required in Table 51 (21.1.1.1.1), namely codecvt<wchar_t, char, mbstate_t> and codecvt<char, char, mbstate_t>, return the lesser of max and (from_end-from).

to:

> -10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiation codecvt<char, char, mbstate_t> returns the lesser of max and (from_end-from).

[Redmond: Nathan suggested an alternative resolution: same as above, but require that, in the default encoding, a character from the basic execution character set would map to a single external character. The straw poll was 8-1 in favor of the proposed resolution.]

Rationale:

The default encoding should be whatever users of a given platform would expect to be the most natural. This varies from platform to platform. In many cases there is a preexisting C library, and users would expect the default encoding to be whatever C uses in the default "C" locale. We could impose a guarantee like the one Nathan suggested (a character from the basic execution character set must map to a single external character), but this would rule out important encodings that are in common use: it would rule out JIS, for example, and it would rule out a fixed-width encoding of UCS-4.

[Curaçao: fixed rationale typo at the request of Ichiro Koshida; "shift-JIS" changed to "JIS".]

### 306(i). offsetof macro and non-POD types

Section: 17.2 [support.types] Status: CD1 Submitter: Steve Clamage Opened: 2001-02-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Spliced together from reflector messages c++std-lib-8294 and -8295:

18.1, paragraph 5, reads: "The macro `offsetof` accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9). The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."

For the POD requirement, it doesn't say "no diagnostic required" or "undefined behavior". I read 4.1 [intro.compliance], paragraph 1, to mean that a diagnostic is required. It's not clear whether this requirement was intended. While it's possible to provide such a diagnostic, the extra complication doesn't seem to add any value.

Proposed resolution:

Change 18.1, paragraph 5, to "If type is not a POD structure or a POD union the results are undefined."

[Copenhagen: straw poll was 7-4 in favor. It was generally agreed that requiring a diagnostic was inadvertent, but some LWG members thought that diagnostics should be required whenever possible.]

### 307(i). Lack of reference typedefs in container adaptors

Section: 23.3.11 [list] Status: CD1 Submitter: Howard Hinnant Opened: 2001-03-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

From reflector message c++std-lib-8330. See also lib-8317.

The standard is currently inconsistent in 23.3.11.3 [list.capacity] paragraph 1 and 23.3.11.4 [list.modifiers] paragraph 1. 23.2.3.3/1, for example, says:

> -1- Any sequence supporting operations back(), push_back() and pop_back() can be used to instantiate stack. In particular, vector (lib.vector), list (lib.list) and deque (lib.deque) can be used.

But this is false: vector<bool> can not be used, because the container adaptors return a T& rather than using the underlying container's reference type.

This is a contradiction that can be fixed by:

1. Modifying these paragraphs to say that vector<bool> is an exception.
2. Removing the vector<bool> specialization.
3. Changing the return types of stack and priority_queue to use reference typedef's.

I propose 3. This does not preclude option 2 if we choose to do it later (see issue 96(i)); the issues are independent. Option 3 offers a small step towards support for proxied containers. This small step fixes a current contradiction, is easy for vendors to implement, is already implemented in at least one popular lib, and does not break any code.

Proposed resolution:

Summary: Add reference and const_reference typedefs to queue, priority_queue and stack. Change return types of "value_type&" to "reference". Change return types of "const value_type&" to "const_reference". Details:

Change 23.2.3.1/1 from:

      namespace std {
        template <class T, class Container = deque<T> >
        class queue {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;

        public:
          explicit queue(const Container& = Container());

          bool      empty() const             { return c.empty(); }
          size_type size()  const             { return c.size(); }
          value_type&       front()           { return c.front(); }
          const value_type& front() const     { return c.front(); }
          value_type&       back()            { return c.back(); }
          const value_type& back() const      { return c.back(); }
          void push(const value_type& x)      { c.push_back(x); }
          void pop()                          { c.pop_front(); }
        };

to:

      namespace std {
        template <class T, class Container = deque<T> >
        class queue {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::reference             reference;
          typedef typename Container::const_reference       const_reference;
          typedef typename Container::value_type            value_type;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;

        public:
          explicit queue(const Container& = Container());

          bool      empty() const             { return c.empty(); }
          size_type size()  const             { return c.size(); }
          reference         front()           { return c.front(); }
          const_reference   front() const     { return c.front(); }
          reference         back()            { return c.back(); }
          const_reference   back() const      { return c.back(); }
          void push(const value_type& x)      { c.push_back(x); }
          void pop()                          { c.pop_front(); }
        };

Change 23.2.3.2/1 from:

      namespace std {
        template <class T, class Container = vector<T>,
                  class Compare = less<typename Container::value_type> >
        class priority_queue {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;
          Compare comp;

        public:
          explicit priority_queue(const Compare& x = Compare(),
                                  const Container& = Container());
          template <class InputIterator>
            priority_queue(InputIterator first, InputIterator last,
                           const Compare& x = Compare(),
                           const Container& = Container());

          bool      empty() const       { return c.empty(); }
          size_type size()  const       { return c.size(); }
          const value_type& top() const { return c.front(); }
          void push(const value_type& x);
          void pop();
        };
                                      //  no equality is provided
      }

to:

      namespace std {
        template <class T, class Container = vector<T>,
                  class Compare = less<typename Container::value_type> >
        class priority_queue {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::reference             reference;
          typedef typename Container::const_reference       const_reference;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;
          Compare comp;

        public:
          explicit priority_queue(const Compare& x = Compare(),
                                  const Container& = Container());
          template <class InputIterator>
            priority_queue(InputIterator first, InputIterator last,
                           const Compare& x = Compare(),
                           const Container& = Container());

          bool      empty() const       { return c.empty(); }
          size_type size()  const       { return c.size(); }
          const_reference   top() const { return c.front(); }
          void push(const value_type& x);
          void pop();
        };
                                      //  no equality is provided
      }

And change 23.2.3.3/1 from:

      namespace std {
        template <class T, class Container = deque<T> >
        class stack {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;

        public:
          explicit stack(const Container& = Container());

          bool      empty() const             { return c.empty(); }
          size_type size()  const             { return c.size(); }
          value_type&       top()             { return c.back(); }
          const value_type& top() const       { return c.back(); }
          void push(const value_type& x)      { c.push_back(x); }
          void pop()                          { c.pop_back(); }
        };

        template <class T, class Container>
          bool operator==(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator< (const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator!=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator> (const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator>=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator<=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
      }

to:

      namespace std {
        template <class T, class Container = deque<T> >
        class stack {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::reference             reference;
          typedef typename Container::const_reference       const_reference;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;

        public:
          explicit stack(const Container& = Container());

          bool      empty() const             { return c.empty(); }
          size_type size()  const             { return c.size(); }
          reference         top()             { return c.back(); }
          const_reference   top() const       { return c.back(); }
          void push(const value_type& x)      { c.push_back(x); }
          void pop()                          { c.pop_back(); }
        };

        template <class T, class Container>
          bool operator==(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator< (const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator!=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator> (const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator>=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator<=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
      }

[Copenhagen: This change was discussed before the IS was released and it was deliberately not adopted. Nevertheless, the LWG believes (straw poll: 10-2) that it is a genuine defect.]

### 308(i). Table 82 mentions unrelated headers

Section: 31 [input.output] Status: CD1 Submitter: Martin Sebor Opened: 2001-03-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 82 in section 27 mentions the header <cstdlib> for String streams (31.8 [string.streams]) and the headers <cstdio> and <cwchar> for File streams (31.10 [file.streams]). It's not clear why these headers are mentioned in this context since they do not define any of the library entities described by the subclauses. According to 16.4.2.2 [contents], only such headers are to be listed in the summary.

Proposed resolution:

Remove <cstdlib> and <cwchar> from Table 82.

[Copenhagen: changed the proposed resolution slightly. The original proposed resolution also said to remove <cstdio> from Table 82. However, <cstdio> is mentioned several times within section 31.10 [file.streams], including 31.13 [c.files].]

### 310(i). Is errno a macro?

Section: 16.4.2.3 [headers], 19.4 [errno] Status: CD1 Submitter: Steve Clamage Opened: 2001-03-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Exactly how should errno be declared in a conforming C++ header?

The C standard says in 7.1.4 that it is unspecified whether errno is a macro or an identifier with external linkage. In some implementations it can be either, depending on compile-time options. (E.g., on Solaris in multi-threading mode, errno is a macro that expands to a function call, but is an extern int otherwise. "Unspecified" allows such variability.)

The C++ standard:

- 17.4.1.2 says in a note that errno must be macro in C. (false)
- 17.4.3.1.3 footnote 166 says errno is reserved as an external name (true), and implies that it is an identifier.
- 19.3 simply lists errno as a macro (by what reasoning?) and goes on to say that the contents of of C++ <errno.h> are the same as in C, begging the question.
- C.2, table 95 lists errno as a macro, without comment.

I find no other references to errno.

We should either explicitly say that errno must be a macro, even though it need not be a macro in C, or else explicitly leave it unspecified. We also need to say something about namespace std. A user who includes <cerrno> needs to know whether to write `errno`, or `::errno`, or `std::errno`, or else <cerrno> is useless.

Two acceptable fixes:

- errno must be a macro. This is trivially satisfied by adding
  #define errno (::std::errno)
  to the headers if errno is not already a macro. You then always write errno without any scope qualification, and it always expands to a correct reference. Since it is always a macro, you know to avoid using errno as a local identifer.

- errno is in the global namespace. This fix is inferior, because ::errno is not guaranteed to be well-formed.

[ This issue was first raised in 1999, but it slipped through the cracks. ]

Proposed resolution:

Change the Note in section 17.4.1.2p5 from

> Note: the names defined as macros in C include the following: assert, errno, offsetof, setjmp, va_arg, va_end, and va_start.

to

> Note: the names defined as macros in C include the following: assert, offsetof, setjmp, va_arg, va_end, and va_start.

In section 19.3, change paragraph 2 from

> The contents are the same as the Standard C library header <errno.h>.

to

> The contents are the same as the Standard C library header <errno.h>, except that errno shall be defined as a macro.

Rationale:

C++ must not leave it up to the implementation to decide whether or not a name is a macro; it must explicitly specify exactly which names are required to be macros. The only one that really works is for it to be a macro.

[Curaçao: additional rationale added.]

### 311(i). Incorrect wording in basic_ostream class synopsis

Section: 31.7.6.2 [ostream] Status: CD1 Submitter: Andy Sawyer Opened: 2001-03-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 31.7.6.2 [ostream], the synopsis of class basic_ostream says:

      // partial specializationss
      template<class traits>
        basic_ostream<char,traits>& operator<<( basic_ostream<char,traits>&,
                                                const char * );

Problems:

- Too many 's's at the end of "specializationss"
- This is an overload, not a partial specialization

Proposed resolution:

In the synopsis in 31.7.6.2 [ostream], remove the // partial specializationss comment. Also remove the same comment (correctly spelled, but still incorrect) from the synopsis in 31.7.6.3.4 [ostream.inserters.character].

[ Pre-Redmond: added 31.7.6.3.4 [ostream.inserters.character] because of Martin's comment in c++std-lib-8939. ]

### 312(i). Table 27 is missing headers

Section: 22 [utilities] Status: CD1 Submitter: Martin Sebor Opened: 2001-03-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 27 in section 20 lists the header <memory> (only) for Memory (lib.memory) but neglects to mention the headers <cstdlib> and <cstring> that are discussed in 21.3.7 [meta.rel].

Proposed resolution:

Add <cstdlib> and <cstring> to Table 27, in the same row as <memory>.

### 315(i). Bad "range" in list::unique complexity

Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: Andy Sawyer Opened: 2001-05-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

23.3.11.5 [list.ops], Para 21 describes the complexity of list::unique as: "If the range (last - first) is not empty, exactly (last - first) -1 applications of the corresponding predicate, otherwise no applications of the predicate)".

"(last - first)" is not a range.

Proposed resolution:

Change the "range" from (last - first) to [first, last).

### 316(i). Vague text in Table 69

Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 69 says this about a_uniq.insert(t):

> inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair indicates whether the insertion takes place and the iterator component of the pair points to the element with key equivalent to the key of t.

The description should be more specific about exactly how the bool component indicates whether the insertion takes place.

Proposed resolution:

Change the text in question to

> ...The bool component of the returned pair is true if and only if the insertion takes place...

### 317(i). Instantiation vs. specialization of facets

Section: 28.3 [localization] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The localization section of the standard refers to specializations of the facet templates as instantiations even though the required facets are typically specialized rather than explicitly (or implicitly) instantiated. In the case of ctype<char> and ctype_byname<char> (and the wchar_t versions), these facets are actually required to be specialized. The terminology should be corrected to make it clear that the standard doesn't mandate explicit instantiation (the term specialization encompasses both explicit instantiations and specializations).

Proposed resolution:

In the following paragraphs, replace all occurrences of the word instantiation or instantiations with specialization or specializations, respectively:

> 22.1.1.1.1, p4, Table 52, 22.2.1.1, p2, 22.2.1.5, p3, 22.2.1.5.1, p5, 22.2.1.5.2, p10, 22.2.2, p2, 22.2.3.1, p1, 22.2.3.1.2, p1, p2 and p3, 22.2.4.1, p1, 22.2.4.1.2, p1, 22,2,5, p1, 22,2,6, p2, 22.2.6.3.2, p7, and Footnote 242.

And change the text in 22.1.1.1.1, p4 from

> An implementation is required to provide those instantiations for facet templates identified as members of a category, and for those shown in Table 52:

to

> An implementation is required to provide those specializations...

[Nathan will review these changes, and will look for places where explicit specialization is necessary.]

Rationale:

This is a simple matter of outdated language. The language to describe templates was clarified during the standardization process, but the wording in clause 22 was never updated to reflect that change.

### 318(i). Misleading comment in definition of numpunct_byname

Section: 28.3.4.4.2 [locale.numpunct.byname] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The definition of the numpunct_byname template contains the following comment:

        namespace std {
            template <class charT>
            class numpunct_byname : public numpunct<charT> {
        // this class is specialized for char and wchar_t.
            ...

There is no documentation of the specializations and it seems conceivable that an implementation will not explicitly specialize the template at all, but simply provide the primary template.

Proposed resolution:

Remove the comment from the text in 22.2.3.2 and from the proposed resolution of library issue 228(i).

### 319(i). Storage allocation wording confuses "Required behavior", "Requires"

Section: 17.6.3.2 [new.delete.single], 17.6.3.3 [new.delete.array] Status: CD1 Submitter: Beman Dawes Opened: 2001-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The standard specifies 16.3.2.4 [structure.specifications] that "Required behavior" elements describe "the semantics of a function definition provided by either the implementation or a C++ program."

The standard specifies 16.3.2.4 [structure.specifications] that "Requires" elements describe "the preconditions for calling the function."

In the sections noted below, the current wording specifies "Required Behavior" for what are actually preconditions, and thus should be specified as "Requires".

Proposed resolution:

In 17.6.3.2 [new.delete.single] Para 12 Change:

> Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...

to:

> Requires: the value of ptr is null or the value returned by an earlier call ...

In 17.6.3.3 [new.delete.array] Para 11 Change:

> Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...

to:

> Requires: the value of ptr is null or the value returned by an earlier call ...

### 320(i). list::assign overspecified

Section: 23.3.11.2 [list.cons] Status: CD1 Submitter: Howard Hinnant Opened: 2001-05-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 23.3.11.2 [list.cons], paragraphs 6-8 specify that list assign (both forms) have the "effects" of a call to erase followed by a call to insert.

I would like to document that implementers have the freedom to implement assign by other methods, as long as the end result is the same and the exception guarantee is as good or better than the basic guarantee.

The motivation for this is to use T's assignment operator to recycle existing nodes in the list instead of erasing them and reallocating them with new values. It is also worth noting that, with careful coding, most common cases of assign (everything but assignment with true input iterators) can elevate the exception safety to strong if T's assignment has a nothrow guarantee (with no extra memory cost). Metrowerks does this. However I do not propose that this subtlety be standardized. It is a QoI issue.

Existing practise: Metrowerks and SGI recycle nodes, Dinkumware and Rogue Wave don't.

Proposed resolution:

Change 23.3.11.2 [list.cons]/7 from:

> Effects:

>        erase(begin(), end());
>        insert(begin(), first, last);

to:

> Effects: Replaces the contents of the list with the range [first, last).

In 23.2.4 [sequence.reqmts], in Table 67 (sequence requirements), add two new rows:

          a.assign(i,j)     void      pre: i,j are not iterators into a.
                                      Replaces elements in a with a copy
                                      of [i, j).

          a.assign(n,t)     void      pre: t is not a reference into a.
                                      Replaces elements in a with n copies
                                      of t.

Change 23.3.11.2 [list.cons]/8 from:

> Effects:

>        erase(begin(), end());
>        insert(begin(), n, t);

to:

> Effects: Replaces the contents of the list with n copies of t.

[Redmond: Proposed resolution was changed slightly. Previous version made explicit statement about exception safety, which wasn't consistent with the way exception safety is expressed elsewhere. Also, the change in the sequence requirements is new. Without that change, the proposed resolution would have required that assignment of a subrange would have to work. That too would have been overspecification; it would effectively mandate that assignment use a temporary. Howard provided wording. ]

[Curaçao: Made editorial improvement in wording; changed "Replaces elements in a with copies of elements in [i, j)." with "Replaces the elements of a with a copy of [i, j)." Changes not deemed serious enough to requre rereview.]

### 321(i). Typo in num_get

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Kevin Djang Opened: 2001-05-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 22.2.2.1.2 at p7 states that "A length specifier is added to the conversion function, if needed, as indicated in Table 56." However, Table 56 uses the term "length modifier", not "length specifier".

Proposed resolution:

In 22.2.2.1.2 at p7, change the text "A length specifier is added ..." to be "A length modifier is added ..."

Rationale:

C uses the term "length modifier". We should be consistent.

### 322(i). iterator and const_iterator should have the same value type

Section: 23.2 [container.requirements] Status: CD1 Submitter: Matt Austern Opened: 2001-05-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It's widely assumed that, if X is a container, iterator_traits<X::iterator>::value_type and iterator_traits<X::const_iterator>::value_type should both be X::value_type. However, this is nowhere stated. The language in Table 65 is not precise about the iterators' value types (it predates iterator_traits), and could even be interpreted as saying that iterator_traits<X::const_iterator>::value_type should be "const X::value_type".

Related issue: 279(i).

Proposed resolution:

In Table 65 ("Container Requirements"), change the return type for X::iterator to "iterator type whose value type is T". Change the return type for X::const_iterator to "constant iterator type whose value type is T".

Rationale:

This belongs as a container requirement, rather than an iterator requirement, because the whole notion of iterator/const_iterator pairs is specific to containers' iterator.

It is existing practice that (for example) iterator_traits<list<int>::const_iterator>::value_type is "int", rather than "const int". This is consistent with the way that const pointers are handled: the standard already requires that iterator_traits<const int*>::value_type is int.

### 324(i). Do output iterators have value types?

Section: 24.3.5.4 [output.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2001-06-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 73 suggests that output iterators have value types. It requires the expression "*a = t". Additionally, although Table 73 never lists "a = t" or "X(a) = t" in the "expressions" column, it contains a note saying that "a = t" and "X(a) = t" have equivalent (but nowhere specified!) semantics.

According to 24.1/9, t is supposed to be "a value of value type T":

> In the following sections, a and b denote values of X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T.

Two other parts of the standard that are relevant to whether output iterators have value types:

- 24.1/1 says "All iterators i support the expression *i, resulting in a value of some class, enumeration, or built-in type T, called the value type of the iterator".
- 24.3.1/1, which says "In the case of an output iterator, the types iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_type are both defined as void."

The first of these passages suggests that "*i" is supposed to return a useful value, which contradicts the note in 24.1.2/2 saying that the only valid use of "*i" for output iterators is in an expression of the form "*i = t". The second of these passages appears to contradict Table 73, because it suggests that "*i"'s return value should be void. The second passage is also broken in the case of a an iterator type, like non-const pointers, that satisfies both the output iterator requirements and the forward iterator requirements.

What should the standard say about `*i`'s return value when i is an output iterator, and what should it say about that t is in the expression "*i = t"? Finally, should the standard say anything about output iterators' pointer and reference types?

Proposed resolution:

24.1 p1, change

> All iterators `i` support the expression `*i`, resulting in a value of some class, enumeration, or built-in type `T`, called the value type of the iterator.

to

> All input iterators `i` support the expression `*i`, resulting in a value of some class, enumeration, or built-in type `T`, called the value type of the iterator. All output iterators support the expression `*i = o` where `o` is a value of some type that is in the set of types that are writable to the particular iterator type of `i`.

24.1 p9, add

> `o` denotes a value of some type that is writable to the output iterator.

Table 73, change

>     *a = t

to

>     *r = o

and change

>     *r++ = t

to

>     *r++ = o

[post-Redmond: Jeremy provided wording]

Rationale:

The LWG considered two options: change all of the language that seems to imply that output iterators have value types, thus making it clear that output iterators have no value types, or else define value types for output iterator consistently. The LWG chose the former option, because it seems clear that output iterators were never intended to have value types. This was a deliberate design decision, and any language suggesting otherwise is simply a mistake.

A future revision of the standard may wish to revisit this design decision.

### 325(i). Misleading text in moneypunct<>::do_grouping

Section: 28.3.4.7.4.3 [locale.moneypunct.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2001-07-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The Returns clause in 22.2.6.3.2, p3 says about moneypunct<charT>::do_grouping()

> Returns: A pattern defined identically as the result of numpunct<charT>::do_grouping().241)

Footnote 241 then reads

> This is most commonly the value "\003" (not "3").

The returns clause seems to imply that the two member functions must return an identical value which in reality may or may not be true, since the facets are usually implemented in terms of struct std::lconv and return the value of the grouping and mon_grouping, respectively. The footnote also implies that the member function of the moneypunct facet (rather than the overridden virtual functions in moneypunct_byname) most commonly return "\003", which contradicts the C standard which specifies the value of "" for the (most common) C locale.

Proposed resolution:

Replace the text in Returns clause in 22.2.6.3.2, p3 with the following:

> Returns: A pattern defined identically as, but not necessarily equal to, the result of numpunct<charT>::do_grouping().241)

and replace the text in Footnote 241 with the following:

> To specify grouping by 3s the value is "\003", not "3".

Rationale:

The fundamental problem is that the description of the locale facet virtuals serves two purposes: describing the behavior of the base class, and describing the meaning of and constraints on the behavior in arbitrary derived classes. The new wording makes that separation a little bit clearer. The footnote (which is nonnormative) is not supposed to say what the grouping is in the "C" locale or in any other locale. It is just a reminder that the values are interpreted as small integers, not ASCII characters.

### 327(i). Typo in time_get facet in table 52

Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: Tiki Wan Opened: 2001-07-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 447

Discussion:

The `wchar_t` versions of `time_get` and `time_get_byname` are listed incorrectly in table 52, required instantiations. In both cases the second template parameter is given as OutputIterator. It should instead be InputIterator, since these are input facets.

Proposed resolution:

In table 52, required instantiations, in 28.3.3.1.2.1 [locale.category], change

        time_get<wchar_t, OutputIterator>
        time_get_byname<wchar_t, OutputIterator>

to

        time_get<wchar_t, InputIterator>
        time_get_byname<wchar_t, InputIterator>

[Redmond: Very minor change in proposed resolution. Original had a typo, wchart instead of wchar_t.]

### 328(i). Bad sprintf format modifier in money_put<>::do_put()

Section: 28.3.4.7.3.3 [locale.money.put.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2001-07-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The sprintf format string , "%.01f" (that's the digit one), in the description of the do_put() member functions of the money_put facet in 22.2.6.2.2, p1 is incorrect. First, the f format specifier is wrong for values of type long double, and second, the precision of 01 doesn't seem to make sense. What was most likely intended was "%.0Lf"., that is a precision of zero followed by the L length modifier.

Proposed resolution:

Change the format string to "%.0Lf".

Rationale:

Fixes an obvious typo

### 329(i). vector capacity, reserve and reallocation

Section: 23.3.13.3 [vector.capacity], 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Anthony Williams Opened: 2001-07-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There is an apparent contradiction about which circumstances can cause a reallocation of a vector in Section 23.3.13.3 [vector.capacity] and section 23.3.13.5 [vector.modifiers].

23.3.13.3 [vector.capacity],p5 says:

> Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the size specified in the most recent call to reserve().

Which implies if I do

      std::vector<int> vec;
      vec.reserve(23);
      vec.reserve(0);
      vec.insert(vec.end(),1);

then the implementation may reallocate the vector for the insert, as the size specified in the previous call to reserve was zero.

However, the previous paragraphs (23.3.13.3 [vector.capacity], p1-2) state:

> (capacity) Returns: The total number of elements the vector can hold without requiring reallocation

> ...After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise...

This implies that vec.capacity() is still 23, and so the insert() should not require a reallocation, as vec.size() is 0. This is backed up by 23.3.13.5 [vector.modifiers], p1:

> (insert) Notes: Causes reallocation if the new size is greater than the old capacity.

Though this doesn't rule out reallocation if the new size is less than the old capacity, I think the intent is clear.

Proposed resolution:

Change the wording of 23.3.13.3 [vector.capacity] paragraph 5 to:

> Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity().

[Redmond: original proposed resolution was modified slightly. In the original, the guarantee was that there would be no reallocation until the size would be greater than the value of capacity() after the most recent call to reserve(). The LWG did not believe that the "after the most recent call to reserve()" added any useful information.]

Rationale:

There was general agreement that, when reserve() is called twice in succession and the argument to the second invocation is smaller than the argument to the first, the intent was for the second invocation to have no effect. Wording implying that such cases have an effect on reallocation guarantees was inadvertant.

### 331(i). bad declaration of destructor for ios_base::failure

Section: 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: PremAnand M. Rao Opened: 2001-08-23 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

With the change in 16.4.6.14 [res.on.exception.handling] to state "An implementation may strengthen the exception-specification for a non-virtual function by removing listed exceptions." (issue 119(i)) and the following declaration of ~failure() in ios_base::failure

        namespace std {
           class ios_base::failure : public exception {
           public:
               ...
               virtual ~failure();
               ...
           };
         }

the class failure cannot be implemented since in 17.7.3 [type.info] the destructor of class exception has an empty exception specification:

        namespace std {
           class exception {
           public:
             ...
             virtual ~exception() throw();
             ...
           };
         }

Proposed resolution:

Remove the declaration of ~failure().

Rationale:

The proposed resolution is consistent with the way that destructors of other classes derived from `exception` are handled.

### 333(i). does endl imply synchronization with the device?

Section: 31.7.6.5 [ostream.manip] Status: CD1 Submitter: PremAnand M. Rao Opened: 2001-08-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

A footnote in 31.7.6.5 [ostream.manip] states:

> [Footnote: The effect of executing cout << endl is to insert a newline character in the output sequence controlled by cout, then synchronize it with any external file with which it might be associated. --- end foonote]

Does the term "file" here refer to the external device? This leads to some implementation ambiguity on systems with fully buffered files where a newline does not cause a flush to the device.

Choosing to sync with the device leads to significant performance penalties for each call to endl, while not sync-ing leads to errors under special circumstances.

I could not find any other statement that explicitly defined the behavior one way or the other.

Proposed resolution:

Remove footnote 300 from section 31.7.6.5 [ostream.manip].

Rationale:

We already have normative text saying what `endl` does: it inserts a newline character and calls `flush`. This footnote is at best redundant, at worst (as this issue says) misleading, because it appears to make promises about what `flush` does.

### 334(i). map::operator[] specification forces inefficient implementation

Section: 23.4.3.3 [map.access] Status: CD1 Submitter: Andrea Griffini Opened: 2001-09-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current standard describes map::operator[] using a code example. That code example is however quite inefficient because it requires several useless copies of both the passed key_type value and of default constructed mapped_type instances. My opinion is that was not meant by the comitee to require all those temporary copies.

Currently map::operator[] behaviour is specified as:

      Returns:
        (*((insert(make_pair(x, T()))).first)).second.

This specification however uses make_pair that is a template function of which parameters in this case will be deduced being of type const key_type& and const T&. This will create a pair<key_type,T> that isn't the correct type expected by map::insert so another copy will be required using the template conversion constructor available in pair to build the required pair<const key_type,T> instance.

If we consider calling of key_type copy constructor and mapped_type default constructor and copy constructor as observable behaviour (as I think we should) then the standard is in this place requiring two copies of a key_type element plus a default construction and two copy construction of a mapped_type (supposing the addressed element is already present in the map; otherwise at least another copy construction for each type).

A simple (half) solution would be replacing the description with:

      Returns:
        (*((insert(value_type(x, T()))).first)).second.

This will remove the wrong typed pair construction that requires one extra copy of both key and value.

However still the using of map::insert requires temporary objects while the operation, from a logical point of view, doesn't require any.

I think that a better solution would be leaving free an implementer to use a different approach than map::insert that, because of its interface, forces default constructed temporaries and copies in this case. The best solution in my opinion would be just requiring map::operator[] to return a reference to the mapped_type part of the contained element creating a default element with the specified key if no such an element is already present in the container. Also a logarithmic complexity requirement should be specified for the operation.

This would allow library implementers to write alternative implementations not using map::insert and reaching optimal performance in both cases of the addressed element being present or absent from the map (no temporaries at all and just the creation of a new pair inside the container if the element isn't present). Some implementer has already taken this option but I think that the current wording of the standard rules that as non-conforming.

Proposed resolution:

Replace 23.4.3.3 [map.access] paragraph 1 with

> -1- Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.

> -2- Returns: A reference to the mapped_type corresponding to x in *this.

> -3- Complexity: logarithmic.

[This is the second option mentioned above. Howard provided wording. We may also wish to have a blanket statement somewhere in clause 17 saying that we do not intend the semantics of sample code fragments to be interpreted as specifing exactly how many copies are made. See issue 98(i) for a similar problem.]

Rationale:

This is the second solution described above; as noted, it is consistent with existing practice.

Note that we now need to specify the complexity explicitly, because we are no longer defining `operator[]` in terms of `insert`.

### 335(i). minor issue with char_traits, table 37

Section: 27.2.2 [char.traits.require] Status: CD1 Submitter: Andy Sawyer Opened: 2001-09-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 37, in 27.2.2 [char.traits.require], descibes char_traits::assign as:

      X::assign(c,d)   assigns c = d.

And para 1 says:

> [...] c and d denote values of type CharT [...]

Naturally, if c and d are values, then the assignment is (effectively) meaningless. It's clearly intended that (in the case of assign, at least), 'c' is intended to be a reference type.

I did a quick survey of the four implementations I happened to have lying around, and sure enough they all have signatures:

        assign( charT&, const charT& );

(or the equivalent). It's also described this way in Nico's book. (Not to mention the synopses of char_traits<char> in 21.1.3.1 and char_traits<wchar_t> in 21.1.3.2...)

Proposed resolution:

Add the following to 21.1.1 para 1:

> r denotes an lvalue of CharT

and change the description of assign in the table to:

      X::assign(r,d)   assigns r = d

### 336(i). Clause 17 lack of references to deprecated headers

Section: 16 [library] Status: CD1 Submitter: Detlef Vollmann Opened: 2001-09-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

From c++std-edit-873:

16.4.2.3 [headers], Table 11. In this table, the header <strstream> is missing.

This shows a general problem: The whole clause 17 refers quite often to clauses 18 through 27, but D.7 is also a part of the standard library (though a deprecated one).

Proposed resolution:

To 16.4.2.3 [headers] Table 11, C++ Library Headers, add "<strstream>".

In the following places, change "clauses 17 through 27" to "clauses 17 through 27 and Annex D":

- 2 [intro.refs] Normative references/1/footnote 1
- 3 [intro.defs] Definitions/1
- [dcl.dcl] Library introduction/9
- 16.3 [description] Method of description (Informative)/1
- 16.3.3.3.4 [character.seq] Character sequences/1/bullet 2
- 16.3.3.5 [functions.within.classes] Functions within classes/1
- 16.3.3.6 [objects.within.classes] Private members/1/(2 places)
- 16.4 [requirements] Library-wide requirements/1
- 16.4.2.3 [headers] Headers/4
- 16.4.5.6 [replacement.functions] Replacement functions/1
- 16.4.6.4 [global.functions] Global or non-member functions/2
- 16.4.6.12 [protection.within.classes] Protection within classes/1

### 337(i). replace_copy_if's template parameter should be InputIterator

Section: 26.7.5 [alg.replace] Status: CD1 Submitter: Detlef Vollmann Opened: 2001-09-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

From c++std-edit-876:

In section 26.7.5 [alg.replace] before p4: The name of the first parameter of template replace_copy_if should be "InputIterator" instead of "Iterator". According to 16.3.3.3 [type.descriptions] p1 the parameter name conveys real normative meaning.

Proposed resolution:

Change `Iterator` to `InputIterator`.

### 338(i). is whitespace allowed between `-' and a digit?

Section: 28.3.4 [locale.categories] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

From Stage 2 processing in 28.3.4.3.2.3 [facet.num.get.virtuals], p8 and 9 (the original text or the text corrected by the proposed resolution of issue 221(i)) it seems clear that no whitespace is allowed within a number, but 28.3.4.4.1 [locale.numpunct], p2, which gives the format for integer and floating point values, says that whitespace is optional between a plusminus and a sign.

The text needs to be clarified to either consistently allow or disallow whitespace between a plusminus and a sign. It might be worthwhile to consider the fact that the C library stdio facility does not permit whitespace embedded in numbers and neither does the C or C++ core language (the syntax of integer-literals is given in 5.13.2 [lex.icon], that of floating-point-literals in 5.13.4 [lex.fcon] of the C++ standard).

Proposed resolution:

Change the first part of 28.3.4.4.1 [locale.numpunct] paragraph 2 from:

> The syntax for number formats is as follows, where `digit` represents the radix set specified by the `fmtflags` argument value, `whitespace` is as determined by the facet `ctype<charT>` (22.2.1.1), and `thousands-sep` and `decimal-point` are the results of corresponding `numpunct<charT>` members. Integer values have the format:

>       integer   ::= [sign] units
>       sign      ::= plusminus [whitespace]
>       plusminus ::= '+' | '-'
>       units     ::= digits [thousands-sep units]
>       digits    ::= digit [digits]

to:

> The syntax for number formats is as follows, where `digit` represents the radix set specified by the `fmtflags` argument value, and `thousands-sep` and `decimal-point` are the results of corresponding `numpunct<charT>` members. Integer values have the format:

>       integer   ::= [sign] units
>       sign      ::= plusminus
>       plusminus ::= '+' | '-'
>       units     ::= digits [thousands-sep units]
>       digits    ::= digit [digits]

Rationale:

It's not clear whether the format described in 28.3.4.4.1 [locale.numpunct] paragraph 2 has any normative weight: nothing in the standard says how, or whether, it's used. However, there's no reason for it to differ gratuitously from the very specific description of numeric processing in 28.3.4.3.2.3 [facet.num.get.virtuals]. The proposed resolution removes all mention of "whitespace" from that format.

### 339(i). definition of bitmask type restricted to clause 27

Section: 28.3.4.2 [category.ctype], 16.3.3.3.3 [bitmask.types] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The ctype_category::mask type is declared to be an enum in 28.3.4.2 [category.ctype] with p1 then stating that it is a bitmask type, most likely referring to the definition of bitmask type in 16.3.3.3.3 [bitmask.types], p1. However, the said definition only applies to clause 27, making the reference in 22.2.1 somewhat dubious.

Proposed resolution:

Clarify 17.3.2.1.2, p1 by changing the current text from

> Several types defined in clause 27 are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (22.9.2 [template.bitset]).

to read

> Several types defined in clauses lib.language.support through lib.input.output and Annex D are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (lib.template.bitset).

Additionally, change the definition in 22.2.1 to adopt the same convention as in clause 27 by replacing the existing text with the following (note, in particluar, the cross-reference to 17.3.2.1.2 in 22.2.1, p1):

> 22.2.1 The ctype category [lib.category.ctype]

>     namespace std {
>         class ctype_base {
>         public:
>             typedef T mask;

>             // numeric values are for exposition only.
>             static const mask space = 1 << 0;
>             static const mask print = 1 << 1;
>             static const mask cntrl = 1 << 2;
>             static const mask upper = 1 << 3;
>             static const mask lower = 1 << 4;
>             static const mask alpha = 1 << 5;
>             static const mask digit = 1 << 6;
>             static const mask punct = 1 << 7;
>             static const mask xdigit = 1 << 8;
>             static const mask alnum = alpha | digit;
>             static const mask graph = alnum | punct;
>         };
>     }

> The type `mask` is a bitmask type (16.3.3.3.3 [bitmask.types]).

[Curaçao: The LWG notes that T above should be bold-italics to be consistent with the rest of the standard.]

### 340(i). interpretation of `has_facet<Facet>(loc)`

Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It's unclear whether 22.1.1.1.1, p3 says that `has_facet<Facet>(loc)` returns true for any `Facet` from Table 51 or whether it includes Table 52 as well:

> For any locale `loc` either constructed, or returned by locale::classic(), and any facet `Facet` that is a member of a standard category, `has_facet<Facet>(loc)` is true. Each locale member function which takes a `locale::category` argument operates on the corresponding set of facets.

It seems that it comes down to which facets are considered to be members of a standard category. Intuitively, I would classify all the facets in Table 52 as members of their respective standard categories, but there are an unbounded set of them...

The paragraph implies that, for instance, `has_facet<num_put<C, OutputIterator> >(loc)` must always return true. I don't think that's possible. If it were, then `use_facet<num_put<C, OutputIterator> >(loc)` would have to return a reference to a distinct object for each valid specialization of `num_put<C, OutputIteratory>`, which is clearly impossible.

On the other hand, if none of the facets in Table 52 is a member of a standard category then none of the locale member functions that operate on entire categories of facets will work properly.

It seems that what p3 should mention that it's required (permitted?) to hold only for specializations of `Facet` from Table 52 on `C` from the set { `char`, `wchar_t` }, and `InputIterator` and `OutputIterator` from the set of { {i,o}`streambuf_iterator`<{`char`,`wchar_t`}`>` }.

Proposed resolution:

In 28.3.3.1.2.1 [locale.category], paragraph 3, change "that is a member of a standard category" to "shown in Table 51".

Rationale:

The facets in Table 52 are an unbounded set. Locales should not be required to contain an infinite number of facets.

It's not necessary to talk about which values of InputIterator and OutputIterator must be supported. Table 51 already contains a complete list of the ones we need.

### 341(i). Vector reallocation and swap

Section: 23.3.13.3 [vector.capacity] Status: CD1 Submitter: Anthony Williams Opened: 2001-09-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It is a common idiom to reduce the capacity of a vector by swapping it with an empty one:

      std::vector<SomeType> vec;
      // fill vec with data
      std::vector<SomeType>().swap(vec);
      // vec is now empty, with minimal capacity

However, the wording of 23.3.13.3 [vector.capacity]paragraph 5 prevents the capacity of a vector being reduced, following a call to reserve(). This invalidates the idiom, as swap() is thus prevented from reducing the capacity. The proposed wording for issue 329(i) does not affect this. Consequently, the example above requires the temporary to be expanded to cater for the contents of vec, and the contents be copied across. This is a linear-time operation.

However, the container requirements state that swap must have constant complexity (23.2 [container.requirements] note to table 65).

This is an important issue, as reallocation affects the validity of references and iterators.

If the wording of 23.2.4.2p5 is taken to be the desired intent, then references and iterators remain valid after a call to swap, if they refer to an element before the new end() of the vector into which they originally pointed, in which case they refer to the element at the same index position. Iterators and references that referred to an element whose index position was beyond the new end of the vector are invalidated.

If the note to table 65 is taken as the desired intent, then there are two possibilities with regard to iterators and references:

1. All Iterators and references into both vectors are invalidated.
2. Iterators and references into either vector remain valid, and remain pointing to the same element. Consequently iterators and references that referred to one vector now refer to the other, and vice-versa.

Proposed resolution:

Add a new paragraph after 23.3.13.3 [vector.capacity] paragraph 5:

>       void swap(vector<T,Allocator>& x);

> Effects: Exchanges the contents and capacity() of `*this` with that of `x`.

> Complexity: Constant time.

[This solves the problem reported for this issue. We may also have a problem with a circular definition of swap() for other containers.]

Rationale:

swap should be constant time. The clear intent is that it should just do pointer twiddling, and that it should exchange all properties of the two vectors, including their reallocation guarantees.

### 343(i). Unspecified library header dependencies

Section: 16 [library] Status: Resolved Submitter: Martin Sebor Opened: 2001-10-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopses of the C++ library headers clearly show which names are required to be defined in each header. Since in order to implement the classes and templates defined in these headers declarations of other templates (but not necessarily their definitions) are typically necessary the standard in 17.4.4, p1 permits library implementers to include any headers needed to implement the definitions in each header.

For instance, although it is not explicitly specified in the synopsis of `<string>`, at the point of definition of the `std::basic_string` template the declaration of the `std::allocator` template must be in scope. All current implementations simply include `<memory>` from within `<string>`, either directly or indirectly, to bring the declaration of `std::allocator` into scope.

Additionally, however, some implementation also include `<istream>` and `<ostream>` at the top of `<string>` to bring the declarations of `std::basic_istream` and `std::basic_ostream` into scope (which are needed in order to implement the string inserter and extractor operators (21.3.7.9 [lib.string.io])). Other implementations only include `<iosfwd>`, since strictly speaking, only the declarations and not the full definitions are necessary.

Obviously, it is possible to implement `<string>` without actually providing the full definitions of all the templates `std::basic_string` uses (`std::allocator`, `std::basic_istream`, and `std::basic_ostream`). Furthermore, not only is it possible, doing so is likely to have a positive effect on compile-time efficiency.

But while it may seem perfectly reasonable to expect a program that uses the `std::basic_string` insertion and extraction operators to also explicitly include `<istream>` or `<ostream>`, respectively, it doesn't seem reasonable to also expect it to explicitly include `<memory>`. Since what's reasonable and what isn't is highly subjective one would expect the standard to specify what can and what cannot be assumed. Unfortunately, that isn't the case.

The examples below demonstrate the issue.

Example 1:

It is not clear whether the following program is complete:

    #include <string>

    extern std::basic_ostream<char> &strm;

    int main () {
        strm << std::string ("Hello, World!\n");
    }

or whether one must explicitly include `<memory>` or `<ostream>` (or both) in addition to `<string>` in order for the program to compile.

Example 2:

Similarly, it is unclear whether the following program is complete:

    #include <istream>

    extern std::basic_iostream<char> &strm;

    int main () {
        strm << "Hello, World!\n";
    }

or whether one needs to explicitly include `<ostream>`, and perhaps even other headers containing the definitions of other required templates:

    #include <ios>
    #include <istream>
    #include <ostream>
    #include <streambuf>

    extern std::basic_iostream<char> &strm;

    int main () {
        strm << "Hello, World!\n";
    }

Example 3:

Likewise, it seems unclear whether the program below is complete:

    #include <iterator>

    bool foo (std::istream_iterator<int> a, std::istream_iterator<int> b)
    {
        return a == b;
    }

    int main () { }

or whether one should be required to include `<istream>`.

There are many more examples that demonstrate this lack of a requirement. I believe that in a good number of cases it would be unreasonable to require that a program explicitly include all the headers necessary for a particular template to be specialized, but I think that there are cases such as some of those above where it would be desirable to allow implementations to include only as much as necessary and not more.

[ post Bellevue: ]

> Position taken in prior reviews is that the idea of a table of header dependencies is a good one. Our view is that a full paper is needed to do justice to this, and we've made that recommendation to the issue author.

[ 2009-07 Frankfurt ]

> Resolved. Handled by LWG 1178(i).

Proposed resolution:

For every C++ library header, supply a minimum set of other C++ library headers that are required to be included by that header. The proposed list is below (C++ headers for C Library Facilities, table 12 in 17.4.1.2, p3, are omitted):

    +------------+--------------------+
    | C++ header |required to include |
    +============+====================+
    |<algorithm> |                    |
    +------------+--------------------+
    |<bitset>    |                    |
    +------------+--------------------+
    |<complex>   |                    |
    +------------+--------------------+
    |<deque>     |<memory>            |
    +------------+--------------------+
    |<exception> |                    |
    +------------+--------------------+
    |<fstream>   |<ios>               |
    +------------+--------------------+
    |<functional>|                    |
    +------------+--------------------+
    |<iomanip>   |<ios>               |
    +------------+--------------------+
    |<ios>       |<streambuf>         |
    +------------+--------------------+
    |<iosfwd>    |                    |
    +------------+--------------------+
    |<iostream>  |<istream>, <ostream>|
    +------------+--------------------+
    |<istream>   |<ios>               |
    +------------+--------------------+
    |<iterator>  |                    |
    +------------+--------------------+
    |<limits>    |                    |
    +------------+--------------------+
    |<list>      |<memory>            |
    +------------+--------------------+
    |<locale>    |                    |
    +------------+--------------------+
    |<map>       |<memory>            |
    +------------+--------------------+
    |<memory>    |                    |
    +------------+--------------------+
    |<new>       |<exception>         |
    +------------+--------------------+
    |<numeric>   |                    |
    +------------+--------------------+
    |<ostream>   |<ios>               |
    +------------+--------------------+
    |<queue>     |<deque>             |
    +------------+--------------------+
    |<set>       |<memory>            |
    +------------+--------------------+
    |<sstream>   |<ios>, <string>     |
    +------------+--------------------+
    |<stack>     |<deque>             |
    +------------+--------------------+
    |<stdexcept> |                    |
    +------------+--------------------+
    |<streambuf> |<ios>               |
    +------------+--------------------+
    |<string>    |<memory>            |
    +------------+--------------------+
    |<strstream> |                    |
    +------------+--------------------+
    |<typeinfo>  |<exception>         |
    +------------+--------------------+
    |<utility>   |                    |
    +------------+--------------------+
    |<valarray>  |                    |
    +------------+--------------------+
    |<vector>    |<memory>            |
    +------------+--------------------+

Rationale:

The portability problem is real. A program that works correctly on one implementation might fail on another, because of different header dependencies. This problem was understood before the standard was completed, and it was a conscious design choice.

One possible way to deal with this, as a library extension, would be an `<all>` header.

Hinnant: It's time we dealt with this issue for C++0X. Reopened.

### 345(i). type tm in <cwchar>

Section: 27.5 [c.strings] Status: CD1 Submitter: Clark Nelson Opened: 2001-10-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

C99, and presumably amendment 1 to C90, specify that <wchar.h> declares struct tm as an incomplete type. However, table 48 in 27.5 [c.strings] does not mention the type tm as being declared in <cwchar>. Is this omission intentional or accidental?

Proposed resolution:

In section 27.5 [c.strings], add "tm" to table 48.

### 346(i). Some iterator member functions should be const

Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Jeremy Siek Opened: 2001-10-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Iterator member functions and operators that do not change the state of the iterator should be defined as const member functions or as functions that take iterators either by const reference or by value. The standard does not explicitly state which functions should be const. Since this a fairly common mistake, the following changes are suggested to make this explicit.

The tables almost indicate constness properly through naming: r for non-const and a,b for const iterators. The following changes make this more explicit and also fix a couple problems.

Proposed resolution:

In 24.3.4 [iterator.concepts] Change the first section of p9 from "In the following sections, a and b denote values of X..." to "In the following sections, a and b denote values of type const X...".

In Table 73, change

        a->m   U&         ...

to

        a->m   const U&   ...
        r->m   U&         ...

In Table 73 expression column, change

        *a = t

to

        *r = t

[Redmond: The container requirements should be reviewed to see if the same problem appears there.]

### 347(i). locale::category and bitmask requirements

Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: P.J. Plauger, Nathan Myers Opened: 2001-10-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 28.3.3.1.2.1 [locale.category] paragraph 1, the category members are described as bitmask elements. In fact, the bitmask requirements in 16.3.3.3.3 [bitmask.types] don't seem quite right: `none` and `all` are bitmask constants, not bitmask elements.

In particular, the requirements for `none` interact poorly with the requirement that the LC_* constants from the C library must be recognizable as C++ locale category constants. LC_* values should not be mixed with these values to make category values.

We have two options for the proposed resolution. Informally: option 1 removes the requirement that LC_* values be recognized as category arguments. Option 2 changes the category type so that this requirement is implementable, by allowing `none` to be some value such as 0x1000 instead of 0.

Nathan writes: "I believe my proposed resolution [Option 2] merely re-expresses the status quo more clearly, without introducing any changes beyond resolving the DR.

Proposed resolution:

Replace the first two paragraphs of 28.3.3.1.2 [locale.types] with:

>         typedef int category;

> Valid category values include the `locale` member bitmask elements `collate`, `ctype`, `monetary`, `numeric`, `time`, and `messages`, each of which represents a single locale category. In addition, `locale` member bitmask constant `none` is defined as zero and represents no category. And locale member bitmask constant `all` is defined such that the expression

>         (collate | ctype | monetary | numeric | time | messages | all) == all

> is `true`, and represents the union of all categories. Further the expression `(X | Y)`, where `X` and `Y` each represent a single category, represents the union of the two categories.

> `locale` member functions expecting a `category` argument require one of the `category` values defined above, or the union of two or more such values. Such a `category` argument identifies a set of locale categories. Each locale category, in turn, identifies a set of locale facets, including at least those shown in Table 51:

[Curaçao: need input from locale experts.]

Rationale:

The LWG considered, and rejected, an alternate proposal (described as "Option 2" in the discussion). The main reason for rejecting it was that library implementors were concerened about implementation difficult, given that getting a C++ library to work smoothly with a separately written C library is already a delicate business. Some library implementers were also concerned about the issue of adding extra locale categories.

> Option 2:
> Replace the first paragraph of 28.3.3.1.2 [locale.types] with:

> > Valid category values include the enumerated values. In addition, the result of applying commutative operators | and & to any two valid values is valid, and results in the setwise union and intersection, respectively, of the argument categories. The values `all` and `none` are defined such that for any valid value `cat`, the expressions `(cat | all == all)`, `(cat & all == cat)`, `(cat | none == cat)` and `(cat & none == none)` are true. For non-equal values `cat1` and `cat2` of the remaining enumerated values, `(cat1 & cat2 == none)` is true. For any valid categories `cat1` and `cat2`, the result of `(cat1 & ~cat2)` is valid, and equals the setwise union of those categories found in `cat1` but not found in `cat2`. [Footnote: it is not required that `all` equal the setwise union of the other enumerated values; implementations may add extra categories.]

### 349(i). Minor typographical error in ostream_iterator

Section: 24.6.3 [ostream.iterator] Status: CD1 Submitter: Andy Sawyer Opened: 2001-10-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

24.5.2 [lib.ostream.iterator] states:

        [...]

        private:
        // basic_ostream<charT,traits>* out_stream; exposition only
        // const char* delim; exposition only

Whilst it's clearly marked "exposition only", I suspect 'delim' should be of type 'const charT*'.

Proposed resolution:

In 24.6.3 [ostream.iterator], replace `const char* delim` with `const charT* delim`.

### 352(i). missing fpos requirements

Section: 27.2.3 [char.traits.typedefs] Status: CD1 Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

(1) There are no requirements on the `stateT` template parameter of `fpos` listed in 27.4.3. The interface appears to require that the type be at least Assignable and CopyConstructible (27.4.3.1, p1), and I think also DefaultConstructible (to implement the operations in Table 88).

21.1.2, p3, however, only requires that `char_traits<charT>::state_type` meet the requirements of CopyConstructible types.

(2) Additionally, the `stateT` template argument has no corresponding typedef in fpos which might make it difficult to use in generic code.

Proposed resolution:

Modify 21.1.2, p4 from

Requires: `state_type` shall meet the requirements of CopyConstructible types (20.1.3).

Requires: state_type shall meet the requirements of Assignable (23.1, p4), CopyConstructible (20.1.3), and DefaultConstructible (20.1.4) types.

Rationale:

The LWG feels this is two issues, as indicated above. The first is a defect---std::basic_fstream is unimplementable without these additional requirements---and the proposed resolution fixes it. The second is questionable; who would use that typedef? The class template fpos is used only in a very few places, all of which know the state type already. Unless motivation is provided, the second should be considered NAD.

### 353(i). `std::pair` missing template assignment

Section: 22.3 [pairs] Status: Resolved Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The class template `std::pair` defines a template ctor (20.2.2, p4) but no template assignment operator. This may lead to inefficient code since assigning an object of `pair<C, D>` to `pair<A, B>` where the types `C` and `D` are distinct from but convertible to `A` and `B`, respectively, results in a call to the template copy ctor to construct an unnamed temporary of type `pair<A, B>` followed by an ordinary (perhaps implicitly defined) assignment operator, instead of just a straight assignment.

Proposed resolution:

Add the following declaration to the definition of `std::pair`:

        template<class U, class V>
        pair& operator=(const pair<U, V> &p);

And also add a paragraph describing the effects of the function template to the end of 20.2.2:

        template<class U, class V>
        pair& operator=(const pair<U, V> &p);

Effects: `first = p.first;` `second = p.second;` Returns: `*this`

[Curaçao: There is no indication this is was anything other than a design decision, and thus NAD. May be appropriate for a future standard.]

[ Pre Bellevue: It was recognized that this was taken care of by N1856, and thus moved from NAD Future to NAD EditorialResolved. ]

### 354(i). Associative container lower/upper bound requirements

Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Hans Aberg Opened: 2001-12-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Discussions in the thread "Associative container lower/upper bound requirements" on comp.std.c++ suggests that there is a defect in the C++ standard, Table 69 of section 23.1.2, "Associative containers", [lib.associative.reqmts]. It currently says:

> a.find(k): returns an iterator pointing to an element with the key equivalent to k, or a.end() if such an element is not found.

> a.lower_bound(k): returns an iterator pointing to the first element with key not less than k.

> a.upper_bound(k): returns an iterator pointing to the first element with key greater than k.

We have "or a.end() if such an element is not found" for `find`, but not for `upper_bound` or `lower_bound`. As the text stands, one would be forced to insert a new element into the container and return an iterator to that in case the sought iterator does not exist, which does not seem to be the intention (and not possible with the "const" versions).

Proposed resolution:

Change Table 69 of section 23.2.7 [associative.reqmts] indicated entries to:

> a.lower_bound(k): returns an iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.

> a.upper_bound(k): returns an iterator pointing to the first element with key greater than k, or a.end() if such an element is not found.

[Curaçao: LWG reviewed PR.]

### 355(i). Operational semantics for a.back()

Section: 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Yaroslav Mironov Opened: 2002-01-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 68 "Optional Sequence Operations" in 23.1.1/12 specifies operational semantics for "a.back()" as "*--a.end()", which may be ill-formed [because calling operator-- on a temporary (the return) of a built-in type is ill-formed], provided a.end() returns a simple pointer rvalue (this is almost always the case for std::vector::end(), for example). Thus, the specification is not only incorrect, it demonstrates a dangerous construct: "--a.end()" may successfully compile and run as intended, but after changing the type of the container or the mode of compilation it may produce compile-time error.

Proposed resolution:

Change the specification in table 68 "Optional Sequence Operations" in 23.1.1/12 for "a.back()" from

>     *--a.end()

to

>       { iterator tmp = a.end(); --tmp; return *tmp; }

and the specification for "a.pop_back()" from

>     a.erase(--a.end())

to

>       { iterator tmp = a.end(); --tmp; a.erase(tmp); }

[Curaçao: LWG changed PR from "{ X::iterator tmp = a.end(); return *--tmp; }" to "*a.rbegin()", and from "{ X::iterator tmp = a.end(); a.erase(--tmp); }" to "a.erase(rbegin())".]

[There is a second possible defect; table 68 "Optional Sequence Operations" in the "Operational Semantics" column uses operations present only in the "Reversible Container" requirements, yet there is no stated dependency between these separate requirements tables. Ask in Santa Cruz if the LWG would like a new issue opened.]

[Santa Cruz: the proposed resolution is even worse than what's in the current standard: erase is undefined for reverse iterator. If we're going to make the change, we need to define a temporary and use operator--. Additionally, we don't know how prevalent this is: do we need to make this change in more than one place? Martin has volunteered to review the standard and see if this problem occurs elsewhere.]

[Oxford: Matt provided new wording to address the concerns raised in Santa Cruz. It does not appear that this problem appears anywhere else in clauses 23 or 24.]

[Kona: In definition of operational semantics of back(), change "*tmp" to "return *tmp;"]

### 358(i). interpreting `thousands_sep` after a `decimal_point`

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I don't think `thousands_sep` is being treated correctly after decimal_point has been seen. Since grouping applies only to the integral part of the number, the first such occurrence should, IMO, terminate Stage 2. (If it does not terminate it, then 22.2.2.1.2, p12 and 22.2.3.1.2, p3 need to explain how `thousands_sep` is to be interpreted in the fractional part of a number.)

The easiest change I can think of that resolves this issue would be something like below.

Proposed resolution:

Change the first sentence of 22.2.2.1.2, p9 from

> If discard is true then the position of the character is remembered, but the character is otherwise ignored. If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.

to

> If `discard` is true, then if `'.'` has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if `'.'` has already been accumulated, the character is discarded and Stage 2 terminates. ...

Rationale:

We believe this reflects the intent of the Standard. Thousands sep characters after the decimal point are not useful in any locale. Some formatting conventions do group digits that follow the decimal point, but they usually introduce a different grouping character instead of reusing the thousand sep character. If we want to add support for such conventions, we need to do so explicitly.

### 359(i). num_put<>::do_put (..., bool) undocumented

Section: 28.3.4.3.3.2 [facet.num.put.members] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

22.2.2.2.1, p1:

        iter_type put (iter_type out, ios_base& str, char_type fill,
                       bool val) const;
        ...

        1   Returns: do_put (out, str, fill, val).

AFAICS, the behavior of do_put (..., bool) is not documented anywhere, however, 22.2.2.2.2, p23:

>     iter_type put (iter_type out, ios_base& str, char_type fill,
>                    bool val) const;

> Effects: If (str.flags() & ios_base::boolalpha) == 0 then do out = do_put(out, str, fill, (int)val) Otherwise do

>                  string_type s =
>                      val ? use_facet<ctype<charT> >(loc).truename()
>                          : use_facet<ctype<charT> >(loc).falsename();

> and then insert the characters of s into out. out.

This means that the bool overload of `do_put()` will never be called, which contradicts the first paragraph. Perhaps the declaration should read `do_put()`, and not `put()`?

Note also that there is no Returns clause for this function, which should probably be corrected, just as should the second occurrence of "out." in the text.

I think the least invasive change to fix it would be something like the following:

Proposed resolution:

In 28.3.4.3.3.3 [facet.num.put.virtuals], just above paragraph 1, remove the `bool` overload.

In 28.3.4.3.3.3 [facet.num.put.virtuals], p23, make the following changes

> Replace `put()` with `do_put()` in the declaration of the member function.

> Change the Effects clause to a Returns clause (to avoid the requirement to call `do_put(..., int)` from `do_put (..., bool))` like so:

> 23 Returns: If `(str.flags() & ios_base::boolalpha) == 0` then `do_put (out, str, fill, (long)val)` Otherwise the function obtains a string `s` as if by

>                  string_type s =
>                     val ? use_facet<ctype<charT> >(loc).truename()
>                         : use_facet<ctype<charT> >(loc).falsename();

> and then inserts each character `c` of s into out via `*out++ = c` and returns `out`.

Rationale:

This fixes a couple of obvious typos, and also fixes what appears to be a requirement of gratuitous inefficiency.

### 360(i). locale mandates inefficient implementation

Section: 28.3.3.1 [locale] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

22.1.1, p7 (copied below) allows iostream formatters and extractors to make assumptions about the values returned from facet members. However, such assumptions are apparently not guaranteed to hold in other cases (e.g., when the facet members are being called directly rather than as a result of iostream calls, or between successive calls to the same iostream functions with no interevening calls to `imbue()`, or even when the facet member functions are called from other member functions of other facets). This restriction prevents locale from being implemented efficiently.

Proposed resolution:

Change the first sentence in 22.1.1, p7 from

> In successive calls to a locale facet member function during a call to an iostream inserter or extractor or a streambuf member function, the returned result shall be identical. [Note: This implies that such results may safely be reused without calling the locale facet member function again, and that member functions of iostream classes cannot safely call `imbue()` themselves, except as specified elsewhere. --end note]

to

> In successive calls to a locale facet member function on a facet object installed in the same locale, the returned result shall be identical. ...

Rationale:

This change is reasonable becuase it clarifies the intent of this part of the standard.

### 362(i). bind1st/bind2nd type safety

Section: 99 [depr.lib.binders] Status: CD1 Submitter: Andrew Demkin Opened: 2002-04-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The definition of bind1st() (99 [depr.lib.binders]) can result in the construction of an unsafe binding between incompatible pointer types. For example, given a function whose first parameter type is 'pointer to T', it's possible without error to bind an argument of type 'pointer to U' when U does not derive from T:

       foo(T*, int);

       struct T {};
       struct U {};

       U u;

       int* p;
       int* q;

       for_each(p, q, bind1st(ptr_fun(foo), &u));    // unsafe binding

The definition of bind1st() includes a functional-style conversion to map its argument to the expected argument type of the bound function (see below):

      typename Operation::first_argument_type(x)

A functional-style conversion (99 [depr.lib.binders]) is defined to be semantically equivalent to an explicit cast expression (99 [depr.lib.binders]), which may (according to 5.4, paragraph 5) be interpreted as a reinterpret_cast, thus masking the error.

The problem and proposed change also apply to 99 [depr.lib.binders].

Proposed resolution:

Add this sentence to the end of 99 [depr.lib.binders]/1: "Binders `bind1st` and `bind2nd` are deprecated in favor of `std::tr1::bind`."

(Notes to editor: (1) when and if tr1::bind is incorporated into the standard, "std::tr1::bind" should be changed to "std::bind". (2) 20.5.6 should probably be moved to Annex D.

Rationale:

There is no point in fixing bind1st and bind2nd. tr1::bind is a superior solution. It solves this problem and others.

### 363(i). Missing exception specification in 27.4.2.1.1

Section: 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: Walter Brown and Marc Paterno Opened: 2002-05-20 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

The destructor of ios_base::failure should have an empty throw specification, because the destructor of its base class, exception, is declared in this way.

Proposed resolution:

Change the destructor to

      virtual ~failure() throw();

Rationale:

Fixes an obvious glitch. This is almost editorial.

### 364(i). Inconsistent wording in 27.5.2.4.2

Section: 31.6.3.5.2 [streambuf.virt.buffer] Status: CD1 Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

31.6.3.5.2 [streambuf.virt.buffer] paragraph 1 is inconsistent with the Effects clause for seekoff.

Proposed resolution:

Make this paragraph, the Effects clause for setbuf, consistent in wording with the Effects clause for seekoff in paragraph 3 by amending paragraph 1 to indicate the purpose of setbuf:

Original text:

> 1 Effects: Performs an operation that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).

Proposed text:

> 1 Effects: Influences stream buffering in a way that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).

Rationale:

The LWG doesn't believe there is any normative difference between the existing wording and what's in the proposed resolution, but the change may make the intent clearer.

### 365(i). Lack of const-qualification in clause 27

Section: 31 [input.output] Status: CD1 Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Some stream and streambuf member functions are declared non-const, even thought they appear only to report information rather than to change an object's logical state. They should be declared const. See document N1360 for details and rationale.

The list of member functions under discussion: `in_avail`, `showmanyc`, `tellg`, `tellp`, `is_open`.

Related issue: 73(i)

Proposed resolution:

In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13

Replace

      bool is_open();

with

      bool is_open() const;

Rationale:

Of the changes proposed in N1360, the only one that is safe is changing the filestreams' is_open to const. The LWG believed that this was NAD the first time it considered this issue (issue 73(i)), but now thinks otherwise. The corresponding streambuf member function, after all,is already const.

The other proposed changes are less safe, because some streambuf functions that appear merely to report a value do actually perform mutating operations. It's not even clear that they should be considered "logically const", because streambuf has two interfaces, a public one and a protected one. These functions may, and often do, change the state as exposed by the protected interface, even if the state exposed by the public interface is unchanged.

Note that implementers can make this change in a binary compatible way by providing both overloads; this would be a conforming extension.

### 369(i). io stream objects and static ctors

Section: 31.4 [iostream.objects] Status: CD1 Submitter: Ruslan Abdikeev Opened: 2002-07-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Is it safe to use standard iostream objects from constructors of static objects? Are standard iostream objects constructed and are their associations established at that time?

Surpisingly enough, Standard does NOT require that.

27.3/2 [lib.iostream.objects] guarantees that standard iostream objects are constructed and their associations are established before the body of main() begins execution. It also refers to ios_base::Init class as the panacea for constructors of static objects.

However, there's nothing in 27.3 [lib.iostream.objects], in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init], that would require implementations to allow access to standard iostream objects from constructors of static objects.

Details:

Core text refers to some magic object ios_base::Init, which will be discussed below:

> "The [standard iostream] objects are constructed, and their associations are established at some time prior to or during first time an object of class basic_ios<charT,traits>::Init is constructed, and in any case before the body of main begins execution." (27.3/2 [lib.iostream.objects])

The first non-normative footnote encourages implementations to initialize standard iostream objects earlier than required.

However, the second non-normative footnote makes an explicit and unsupported claim:

> "Constructors and destructors for static objects can access these [standard iostream] objects to read input from stdin or write output to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])

The only bit of magic is related to that ios_base::Init class. AFAIK, the rationale behind ios_base::Init was to bring an instance of this class to each translation unit which #included <iostream> or related header. Such an inclusion would support the claim of footnote quoted above, because in order to use some standard iostream object it is necessary to #include <iostream>.

However, while Standard explicitly describes ios_base::Init as an appropriate class for doing the trick, I failed to found a mention of an _instance_ of ios_base::Init in Standard.

Proposed resolution:

Add to 31.4 [iostream.objects], p2, immediately before the last sentence of the paragraph, the following two sentences:

> If a translation unit includes <iostream>, or explicitly constructs an ios_base::Init object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.

[Lillehammer: Matt provided wording.]

[Mont Tremblant: Matt provided revised wording.]

Rationale:

The original proposed resolution unconditionally required implementations to define an ios_base::Init object of some implementation-defined name in the header <iostream>. That's an overspecification. First, defining the object may be unnecessary and even detrimental to performance if an implementation can guarantee that the 8 standard iostream objects will be initialized before any other user-defined object in a program. Second, there is no need to require implementations to document the name of the object.

The new proposed resolution gives users guidance on what they need to do to ensure that stream objects are constructed during startup.

### 370(i). Minor error in basic_istream::get

Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Ray Lischner Opened: 2002-07-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Defect report for description of basic_istream::get (section 31.7.5.4 [istream.unformatted]), paragraph 15. The description for the get function with the following signature:

      basic_istream<charT,traits>& get(basic_streambuf<char_type,traits>&
      sb);

is incorrect. It reads

> Effects: Calls get(s,n,widen('\n'))

which I believe should be:

> Effects: Calls get(sb,widen('\n'))

Proposed resolution:

Change the Effects paragraph to:

> Effects: Calls get(sb,this->widen('\n'))

[Pre-Oxford: Minor correction from Howard: replaced 'widen' with 'this->widen'.]

Rationale:

Fixes an obvious typo.

### 371(i). Stability of multiset and multimap member functions

Section: 23.2 [container.requirements] Status: CD1 Submitter: Frank Compagner Opened: 2002-07-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The requirements for multiset and multimap containers (23.1 [lib.containers.requirements], 23.1.2 [lib.associative.reqmnts], 23.3.2 [lib.multimap] and 23.3.4 [lib.multiset]) make no mention of the stability of the required (mutating) member functions. It appears the standard allows these functions to reorder equivalent elements of the container at will, yet the pervasive red-black tree implementation appears to provide stable behaviour.

This is of most concern when considering the behaviour of erase(). A stability requirement would guarantee the correct working of the following 'idiom' that removes elements based on a certain predicate function.

      multimap<int, int> m;
      multimap<int, int>::iterator i = m.begin();
      while (i != m.end()) {
          if (pred(i))
              m.erase (i++);
          else
              ++i;
      }

Although clause 23.1.2/8 guarantees that i remains a valid iterator througout this loop, absence of the stability requirement could potentially result in elements being skipped. This would make this code incorrect, and, furthermore, means that there is no way of erasing these elements without iterating first over the entire container, and second over the elements to be erased. This would be unfortunate, and have a negative impact on both performance and code simplicity.

If the stability requirement is intended, it should be made explicit (probably through an extra paragraph in clause 23.1.2).

If it turns out stability cannot be guaranteed, i'd argue that a remark or footnote is called for (also somewhere in clause 23.1.2) to warn against relying on stable behaviour (as demonstrated by the code above). If most implementations will display stable behaviour, any problems emerging on an implementation without stable behaviour will be hard to track down by users. This would also make the need for an erase_if() member function that much greater.

This issue is somewhat related to LWG issue 130(i).

Proposed resolution:

Add the following to the end of 23.2.7 [associative.reqmts] paragraph 4: "For `multiset` and `multimap`, `insert`and `erase` are stable: they preserve the relative ordering of equivalent elements.

[Lillehammer: Matt provided wording]

[Joe Gottman points out that the provided wording does not address multimap and multiset. N1780 also addresses this issue and suggests wording.]

[Mont Tremblant: Changed set and map to multiset and multimap.]

Rationale:

The LWG agrees that this guarantee is necessary for common user idioms to work, and that all existing implementations provide this property. Note that this resolution guarantees stability for multimap and multiset, not for all associative containers in general.

### 373(i). Are basic_istream and basic_ostream to use (exceptions()&badbit) != 0 ?

Section: 31.7.5.3.1 [istream.formatted.reqmts], 31.7.6.3.1 [ostream.formatted.reqmts] Status: CD1 Submitter: Keith Baker Opened: 2002-07-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 31.7.5.3.1 [istream.formatted.reqmts] and 31.7.6.3.1 [ostream.formatted.reqmts] (exception()&badbit) != 0 is used in testing for rethrow, yet exception() is the constructor to class std::exception in 17.7.3 [type.info] that has no return type. Should member function exceptions() found in 31.5.4 [ios] be used instead?

Proposed resolution:

In 31.7.5.3.1 [istream.formatted.reqmts] and 31.7.6.3.1 [ostream.formatted.reqmts], change "(exception()&badbit) != 0" to "(exceptions()&badbit) != 0".

Rationale:

Fixes an obvious typo.

### 375(i). basic_ios should be ios_base in 27.7.1.3

Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Ray Lischner Opened: 2002-08-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In Section 31.8.2.5 [stringbuf.virtuals]: Table 90, Table 91, and paragraph 14 all contain references to "basic_ios::" which should be "ios_base::".

Proposed resolution:

Change all references to "basic_ios" in Table 90, Table 91, and paragraph 14 to "ios_base".

Rationale:

Fixes an obvious typo.

### 376(i). basic_streambuf semantics

Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Ray Lischner Opened: 2002-08-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In Section 31.8.2.5 [stringbuf.virtuals], Table 90, the implication is that the four conditions should be mutually exclusive, but they are not. The first two cases, as written, are subcases of the third.

As written, it is unclear what should be the result if cases 1 and 2 are both true, but case 3 is false.

Proposed resolution:

Rewrite these conditions as:

> (which & (ios_base::in|ios_base::out)) == ios_base::in

> (which & (ios_base::in|ios_base::out)) == ios_base::out

> (which & (ios_base::in|ios_base::out)) == (ios_base::in|ios_base::out) and way == either ios_base::beg or ios_base::end

> Otherwise

Rationale:

It's clear what we wanted to say, we just failed to say it. This fixes it.

### 379(i). nonsensical ctype::do_widen() requirement

Section: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The last sentence in 22.2.1.1.2, p11 below doesn't seem to make sense.

      charT do_widen (char c) const;

      -11- Effects: Applies the simplest reasonable transformation from
           a char value or sequence of char values to the corresponding
           charT value or values. The only characters for which unique
           transformations are required are those in the basic source
           character set (2.2). For any named ctype category with a
           ctype<charT> facet ctw and valid ctype_base::mask value
           M (is(M, c) || !ctw.is(M, do_widen(c))) is true.

Shouldn't the last sentence instead read

           For any named ctype category with a ctype<char> facet ctc
           and valid ctype_base::mask value M
           (ctc.is(M, c) || !is(M, do_widen(c))) is true.

I.e., if the narrow character c is not a member of a class of characters then neither is the widened form of c. (To paraphrase footnote 224.)

Proposed resolution:

Replace the last sentence of 28.3.4.2.2.3 [locale.ctype.virtuals], p11 with the following text:

           For any named ctype category with a ctype<char> facet ctc
           and valid ctype_base::mask value M,
           (ctc.is(M, c) || !is(M, do_widen(c))) is true.

[Kona: Minor edit. Added a comma after the M for clarity.]

Rationale:

The LWG believes this is just a typo, and that this is the correct fix.

### 380(i). typos in codecvt tables 53 and 54

Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Tables 53 and 54 in 28.3.4.2.6 [locale.codecvt.byname] are both titled "convert result values," when surely "do_in/do_out result values" must have been intended for Table 53 and "do_unshift result values" for Table 54.

Table 54, row 3 says that the meaning of partial is "more characters needed to be supplied to complete termination." The function is not supplied any characters, it is given a buffer which it fills with characters or, more precisely, destination elements (i.e., an escape sequence). So partial means that space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state.

Proposed resolution:

Change the title of Table 53 to "do_in/do_out result values" and the title of Table 54 to "do_unshift result values."

Change the text in Table 54, row 3 (the partial row), under the heading Meaning, to "space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state."

### 381(i). detection of invalid mbstate_t in codecvt

Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

All but one codecvt member functions that take a state_type argument list as one of their preconditions that the state_type argument have a valid value. However, according to 22.2.1.5.2, p6, codecvt::do_unshift() is the only codecvt member that is supposed to return error if the state_type object is invalid.

It seems to me that the treatment of state_type by all codecvt member functions should be the same and the current requirements should be changed. Since the detection of invalid state_type values may be difficult in general or computationally expensive in some specific cases, I propose the following:

Proposed resolution:

Add a new paragraph before 22.2.1.5.2, p5, and after the function declaration below

        result do_unshift(stateT& state,
        externT* to, externT* to_limit, externT*& to_next) const;

as follows:

        Requires: (to <= to_end) well defined and true; state initialized,
        if at the beginning of a sequence, or else equal to the result of
        converting the preceding characters in the sequence.

and change the text in Table 54, row 4, the error row, under the heading Meaning, from

        state has invalid value

to

        an unspecified error has occurred

Rationale:

The intent is that implementations should not be required to detect invalid state values; such a requirement appears nowhere else. An invalid state value is a precondition violation, i.e. undefined behavior. Implementations that do choose to detect invalid state values, or that choose to detect any other kind of error, may return error as an indication.

### 383(i). Bidirectional iterator assertion typo

Section: 24.3.5.6 [bidirectional.iterators] Status: CD1 Submitter: ysapir (submitted via comp.std.c++) Opened: 2002-10-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Following a discussion on the boost list regarding end iterators and the possibility of performing operator--() on them, it seems to me that there is a typo in the standard. This typo has nothing to do with that discussion.

I have checked this newsgroup, as well as attempted a search of the Active/Defect/Closed Issues List on the site for the words "s is derefer" so I believe this has not been proposed before. Furthermore, the "Lists by Index" mentions only DR 299(i) on section 24.1.4, and DR 299(i) is not related to this issue.

The standard makes the following assertion on bidirectional iterators, in section 24.1.4 [lib.bidirectional.iterators], Table 75:

                             operational  assertion/note
    expression  return type   semantics    pre/post-condition

    --r          X&                        pre: there exists s such
                                           that r == ++s.
                                           post: s is dereferenceable.
                                           --(++r) == r.
                                           --r == --s implies r == s.
                                           &r == &--r.

(See http://lists.boost.org/Archives/boost/2002/10/37636.php.)

In particular, "s is dereferenceable" seems to be in error. It seems that the intention was to say "r is dereferenceable".

If it were to say "r is dereferenceable" it would make perfect sense. Since s must be dereferenceable prior to operator++, then the natural result of operator-- (to undo operator++) would be to make r dereferenceable. Furthermore, without other assertions, and basing only on precondition and postconditions, we could not otherwise know this. So it is also interesting information.

Proposed resolution:

Change the guarantee to "postcondition: r is dereferenceable."

Rationale:

Fixes an obvious typo

### 384(i). equal_range has unimplementable runtime complexity

Section: 26.8.4.4 [equal.range] Status: CD1 Submitter: Hans Bos Opened: 2002-10-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 26.8.4.4 [equal.range] states that at most 2 * log(last - first) + 1 comparisons are allowed for equal_range.

It is not possible to implement equal_range with these constraints.

In a range of one element as in:

        int x = 1;
        equal_range(&x, &x + 1, 1)

it is easy to see that at least 2 comparison operations are needed.

For this case at most 2 * log(1) + 1 = 1 comparison is allowed.

I have checked a few libraries and they all use the same (nonconforming) algorithm for equal_range that has a complexity of

         2* log(distance(first, last)) + 2.

I guess this is the algorithm that the standard assumes for equal_range.

It is easy to see that 2 * log(distance) + 2 comparisons are enough since equal range can be implemented with lower_bound and upper_bound (both log(distance) + 1).

I think it is better to require something like 2log(distance) + O(1) (or even logarithmic as multiset::equal_range). Then an implementation has more room to optimize for certain cases (e.g. have log(distance) characteristics when at most match is found in the range but 2log(distance) + 4 for the worst case).

Proposed resolution:

In 26.8.4.2 [lower.bound]/4, change `log(last - first) + 1` to `log2(last - first) + O(1)`.

In 26.8.4.3 [upper.bound]/4, change `log(last - first) + 1` to `log2(last - first) + O(1)`.

In 26.8.4.4 [equal.range]/4, change `2*log(last - first) + 1` to `2*log2(last - first) + O(1)`.

[Matt provided wording]

Rationale:

The LWG considered just saying O(log n) for all three, but decided that threw away too much valuable information. The fact that lower_bound is twice as fast as equal_range is important. However, it's better to allow an arbitrary additive constant than to specify an exact count. An exact count would have to involve `floor` or `ceil`. It would be too easy to get this wrong, and don't provide any substantial value for users.

### 386(i). Reverse iterator's operator[] has impossible return type

Section: 24.5.1.6 [reverse.iter.elem] Status: CD1 Submitter: Matt Austern Opened: 2002-10-23 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

In [reverse.iter.op-=], `reverse_iterator<>::operator[]` is specified as having a return type of `reverse_iterator::reference`, which is the same as `iterator_traits<Iterator>::reference`. (Where `Iterator` is the underlying iterator type.)

The trouble is that `Iterator`'s own operator[] doesn't necessarily have a return type of `iterator_traits<Iterator>::reference`. Its return type is merely required to be convertible to `Iterator`'s value type. The return type specified for reverse_iterator's operator[] would thus appear to be impossible.

With the resolution of issue 299(i), the type of `a[n]` will continue to be required (for random access iterators) to be convertible to the value type, and also `a[n] = t` will be a valid expression. Implementations of `reverse_iterator` will likely need to return a proxy from `operator[]` to meet these requirements. As mentioned in the comment from Dave Abrahams, the simplest way to specify that `reverse_iterator` meet this requirement to just mandate it and leave the return type of `operator[]` unspecified.

Proposed resolution:

In 24.5.1.3 [reverse.iter.requirements] change:

>     reference operator[](difference_type n) const;

to:

>     unspecified operator[](difference_type n) const; // see 24.3.5.7 [random.access.iterators]

[ Comments from Dave Abrahams: IMO we should resolve 386 by just saying that the return type of reverse_iterator's operator[] is unspecified, allowing the random access iterator requirements to impose an appropriate return type. If we accept 299's proposed resolution (and I think we should), the return type will be readable and writable, which is about as good as we can do. ]

### 387(i). `std::complex` over-encapsulated

Section: 29.4 [complex.numbers] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The absence of explicit description of `std::complex<T>` layout makes it imposible to reuse existing software developed in traditional languages like Fortran or C with unambigous and commonly accepted layout assumptions. There ought to be a way for practitioners to predict with confidence the layout of `std::complex<T>` whenever `T` is a numerical datatype. The absence of ways to access individual parts of a `std::complex<T>` object as lvalues unduly promotes severe pessimizations. For example, the only way to change, independently, the real and imaginary parts is to write something like

    complex<T> z;
    // ...
    // set the real part to r
    z = complex<T>(r, z.imag());
    // ...
    // set the imaginary part to i
    z = complex<T>(z.real(), i);

At this point, it seems appropriate to recall that a complex number is, in effect, just a pair of numbers with no particular invariant to maintain. Existing practice in numerical computations has it that a complex number datatype is usually represented by Cartesian coordinates. Therefore the over-encapsulation put in the specification of `std::complex<>` is not justified.

Proposed resolution:

Add the following requirements to 29.4 [complex.numbers] as 26.3/4:

> If `z` is an lvalue expression of type cv `std::complex<T>` then

> - the expression `reinterpret_cast<cv T(&)[2]>(z)` is well-formed; and
> - `reinterpret_cast<cv T(&)[2]>(z)[0]` designates the real part of `z`; and
> - `reinterpret_cast<cv T(&)[2]>(z)[1]` designates the imaginary part of `z`.

> Moreover, if `a` is an expression of pointer type cv `complex<T>*` and the expression `a[i]` is well-defined for an integer expression `i` then:

> - `reinterpret_cast<cv T*>(a)[2*i]` designates the real part of `a[i]`; and
> - `reinterpret_cast<cv T*>(a)[2*i+1]` designates the imaginary part of `a[i]`.

In 29.4.3 [complex] and [complex.special] add the following member functions (changing `T` to concrete types as appropriate for the specializations).

>     void real(T);
>     void imag(T);

Add to 29.4.4 [complex.members]

>     T real() const;

> > Returns: the value of the real component

>     void real(T val);

> > Assigns `val` to the real component.

>     T imag() const;

> > Returns: the value of the imaginary component

>     void imag(T val);

> > Assigns `val` to the imaginary component.

[Kona: The layout guarantee is absolutely necessary for C compatibility. However, there was disagreement about the other part of this proposal: retrieving elements of the complex number as lvalues. An alternative: continue to have `real()` and `imag()` return rvalues, but add `set_real()` and `set_imag()`. Straw poll: return lvalues - 2, add setter functions - 5. Related issue: do we want reinterpret_cast as the interface for converting a complex to an array of two reals, or do we want to provide a more explicit way of doing it? Howard will try to resolve this issue for the next meeting.]

[pre-Sydney: Howard summarized the options in n1589.]

[ Bellevue: ]

> Second half of proposed wording replaced and moved to Ready.

[ Pre-Sophia Antipolis, Howard adds: ]

> Added the members to [complex.special] and changed from Ready to Review.

[ Post-Sophia Antipolis: ]

> Moved from WP back to Ready so that the "and [complex.special]" in the proposed resolution can be officially applied.

Rationale:

The LWG believes that C99 compatibility would be enough justification for this change even without other considerations. All existing implementations already have the layout proposed here.

### 389(i). Const overload of valarray::operator[] returns by value

Section: 29.6.2.4 [valarray.access] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 77

Discussion:

Consider the following program:

        #include <iostream>
        #include <ostream>
        #include <vector>
        #include <valarray>
        #include <algorithm>
        #include <iterator>
        template<typename Array>
        void print(const Array& a)
        {
        using namespace std;
        typedef typename Array::value_type T;
        copy(&a[0], &a[0] + a.size(),
        ostream_iterator<T>(std::cout, " "));
        }
        template<typename T, unsigned N>
        unsigned size(T(&)[N]) { return N; }
        int main()
        {
        double array[] = { 0.89, 9.3, 7, 6.23 };
        std::vector<double> v(array, array + size(array));
        std::valarray<double> w(array, size(array));
        print(v); // #1
        std::cout << std::endl;
        print(w); // #2
        std::cout << std::endl;
        }

While the call numbered #1 succeeds, the call numbered #2 fails because the const version of the member function valarray<T>::operator[](size_t) returns a value instead of a const-reference. That seems to be so for no apparent reason, no benefit. Not only does that defeats users' expectation but it also does hinder existing software (written either in C or Fortran) integration within programs written in C++. There is no reason why subscripting an expression of type valarray<T> that is const-qualified should not return a const T&.

Proposed resolution:

In the class synopsis in 29.6.2 [template.valarray], and in 29.6.2.4 [valarray.access] just above paragraph 1, change

      T operator[](size_t const);

to

      const T& operator[](size_t const);

[Kona: fixed a minor typo: put semicolon at the end of the line wehre it belongs.]

Rationale:

Return by value seems to serve no purpose. Valaray was explicitly designed to have a specified layout so that it could easily be integrated with libraries in other languages, and return by value defeats that purpose. It is believed that this change will have no impact on allowable optimizations.

### 391(i). non-member functions specified as const

Section: 28.3.3.3.2 [conversions.character] Status: CD1 Submitter: James Kanze Opened: 2002-12-10 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

The specifications of toupper and tolower both specify the functions as const, althought they are not member functions, and are not specified as const in the header file synopsis in section 28.3.3 [locales].

Proposed resolution:

In [conversions], remove `const` from the function declarations of std::toupper and std::tolower

Rationale:

Fixes an obvious typo

### 395(i). inconsistencies in the definitions of rand() and random_shuffle()

Section: 29.7 [c.math] Status: CD1 Submitter: James Kanze Opened: 2003-01-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 29.7 [c.math], the C++ standard refers to the C standard for the definition of rand(); in the C standard, it is written that "The implementation shall behave as if no library function calls the rand function."

In 26.7.13 [alg.random.shuffle], there is no specification as to how the two parameter version of the function generates its random value. I believe that all current implementations in fact call rand() (in contradiction with the requirement avove); if an implementation does not call rand(), there is the question of how whatever random generator it does use is seeded. Something is missing.

Proposed resolution:

In [lib.c.math], add a paragraph specifying that the C definition of rand shal be modified to say that "Unless otherwise specified, the implementation shall behave as if no library function calls the rand function."

In [lib.alg.random.shuffle], add a sentence to the effect that "In the two argument form of the function, the underlying source of random numbers is implementation defined. [Note: in particular, an implementation is permitted to use `rand`.]

Rationale:

The original proposed resolution proposed requiring the two-argument from of `random_shuffle` to use `rand`. We don't want to do that, because some existing implementations already use something else: gcc uses `lrand48`, for example. Using `rand` presents a problem if the number of elements in the sequence is greater than RAND_MAX.

### 396(i). what are characters zero and one

Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

23.3.5.1, p6 [lib.bitset.cons] talks about a generic character having the value of 0 or 1 but there is no definition of what that means for charT other than char and wchar_t. And even for those two types, the values 0 and 1 are not actually what is intended -- the values '0' and '1' are. This, along with the converse problem in the description of to_string() in 23.3.5.2, p33, looks like a defect remotely related to DR 303(i).

    23.3.5.1:
      -6-  An element of the constructed string has value zero if the
           corresponding character in str, beginning at position pos,
           is 0. Otherwise, the element has the value one.

    23.3.5.2:
      -33-  Effects: Constructs a string object of the appropriate
            type and initializes it to a string of length N characters.
            Each character is determined by the value of its
            corresponding bit position in *this. Character position N
            ?- 1 corresponds to bit position zero. Subsequent decreasing
            character positions correspond to increasing bit positions.
            Bit value zero becomes the character 0, bit value one becomes
            the character 1.

Also note the typo in 23.3.5.1, p6: the object under construction is a bitset, not a string.

[ Sophia Antipolis: ]

> We note that `bitset` has been moved from section 23 to section 20, by another issue (842(i)) previously resolved at this meeting.

> Disposition: move to ready.

> We request that Howard submit a separate issue regarding the three `to_string` overloads.

[ post Bellevue: ]

> We are happy with the resolution as proposed, and we move this to Ready.

[ Howard adds: ]

> The proposed wording neglects the 3 newer `to_string` overloads.

Proposed resolution:

Change the constructor's function declaration immediately before 22.9.2.2 [bitset.cons] p3 to:

        template <class charT, class traits, class Allocator>
        explicit
        bitset(const basic_string<charT, traits, Allocator>& str,
               typename basic_string<charT, traits, Allocator>::size_type pos = 0,
               typename basic_string<charT, traits, Allocator>::size_type n =
                 basic_string<charT, traits, Allocator>::npos,
               charT zero = charT('0'), charT one = charT('1'))

Change the first two sentences of 22.9.2.2 [bitset.cons] p6 to: "An element of the constructed string has value 0 if the corresponding character in str, beginning at position pos, is zero. Otherwise, the element has the value 1.

Change the text of the second sentence in 23.3.5.1, p5 to read: "The function then throws invalid_argument if any of the rlen characters in str beginning at position pos is other than zero or one. The function uses traits::eq() to compare the character values."

Change the declaration of the `to_string` member function immediately before 22.9.2.3 [bitset.members] p33 to:

        template <class charT, class traits, class Allocator>
        basic_string<charT, traits, Allocator>
        to_string(charT zero = charT('0'), charT one = charT('1')) const;

Change the last sentence of 22.9.2.3 [bitset.members] p33 to: "Bit value 0 becomes the character `zero`, bit value 1 becomes the character `one`.

Change 22.9.4 [bitset.operators] p8 to:

Returns:

      os << x.template to_string<charT,traits,allocator<charT> >(
          use_facet<ctype<charT> >(os.getloc()).widen('0'),
          use_facet<ctype<charT> >(os.getloc()).widen('1'));

Rationale:

There is a real problem here: we need the character values of '0' and '1', and we have no way to get them since strings don't have imbued locales. In principle the "right" solution would be to provide an extra object, either a ctype facet or a full locale, which would be used to widen '0' and '1'. However, there was some discomfort about using such a heavyweight mechanism. The proposed resolution allows those users who care about this issue to get it right.

We fix the inserter to use the new arguments. Note that we already fixed the analogous problem with the extractor in issue 303(i).

### 400(i). redundant type cast in lib.allocator.members

Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

20.2.10.2 [allocator.members] allocator members, contains the following 3 lines:

      12 Returns: new((void *) p) T( val)
         void destroy(pointer p);
      13 Returns: ((T*) p)->~T()

The type cast "(T*) p" in the last line is redundant cause we know that std::allocator<T>::pointer is a typedef for T*.

Proposed resolution:

Replace "((T*) p)" with "p".

Rationale:

Just a typo, this is really editorial.

### 401(i). incorrect type casts in table 32 in lib.allocator.requirements

Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I think that in par2 of [default.con.req] the last two lines of table 32 contain two incorrect type casts. The lines are ...

      a.construct(p,t)   Effect: new((void*)p) T(t)
      a.destroy(p)       Effect: ((T*)p)?->~T()

.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:

      alloc<T>             a     ;// an allocator for T
      alloc<T>::pointer    p     ;// random access iterator
                                  // (may be different from T*)
      alloc<T>::reference  r = *p;// T&
      T const&             t     ;

For that two type casts ("(void*)p" and "(T*)p") to be well-formed this would require then conversions to T* and void* for all alloc<T>::pointer, so it would implicitely introduce extra requirements for alloc<T>::pointer, additionally to the only current requirement (being a random access iterator).

Proposed resolution:

Accept proposed wording from N2436 part 1.

Note: Actually I would prefer to replace "((T*)p)?->dtor_name" with "p?->dtor_name", but AFAICS this is not possible cause of an omission in 12.4.6 [over.ref] (for which I have filed another DR on 29.11.2002).

[Kona: The LWG thinks this is somewhere on the border between Open and NAD. The intend is clear: `construct` constructs an object at the location p. It's reading too much into the description to think that literally calling `new` is required. Tweaking this description is low priority until we can do a thorough review of allocators, and, in particular, allocators with non-default pointer types.]

[ Batavia: Proposed resolution changed to less code and more description. ]

[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]

[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 402(i). wrong new expression in [some_]allocator::construct

Section: 16.4.4.6 [allocator.requirements], 20.2.10.2 [allocator.members] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This applies to the new expression that is contained in both par12 of 20.2.10.2 [allocator.members] and in par2 (table 32) of [default.con.req]. I think this new expression is wrong, involving unintended side effects.

20.2.10.2 [allocator.members] contains the following 3 lines:

      11 Returns: the largest value N for which the call allocate(N,0) might succeed.
         void construct(pointer p, const_reference val);
      12 Returns: new((void *) p) T( val)

[default.con.req] in table 32 has the following line:

      a.construct(p,t)   Effect: new((void*)p) T(t)

.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:

      alloc<T>             a     ;// an allocator for T
      alloc<T>::pointer    p     ;// random access iterator
                                  // (may be different from T*)
      alloc<T>::reference  r = *p;// T&
      T const&             t     ;

Cause of using "new" but not "::new", any existing "T::operator new" function will hide the global placement new function. When there is no "T::operator new" with adequate signature, every_alloc<T>::construct(..) is ill-formed, and most std::container<T,every_alloc<T>> use it; a workaround would be adding placement new and delete functions with adequate signature and semantic to class T, but class T might come from another party. Maybe even worse is the case when T has placement new and delete functions with adequate signature but with "unknown" semantic: I dont like to speculate about it, but whoever implements any_container<T,any_alloc> and wants to use construct(..) probably must think about it.

Proposed resolution:

Replace "new" with "::new" in both cases.

### 403(i). basic_string::swap should not throw exceptions

Section: 27.4.3.7.8 [string.swap] Status: CD1 Submitter: Beman Dawes Opened: 2003-03-25 Last modified: 2016-11-12

Priority: Not Prioritized

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Discussion:

std::basic_string, 27.4.3 [basic.string] paragraph 2 says that basic_string "conforms to the requirements of a Sequence, as specified in (23.1.1)." The sequence requirements specified in (23.1.1) to not include any prohibition on swap members throwing exceptions.

Section 23.2 [container.requirements] paragraph 10 does limit conditions under which exceptions may be thrown, but applies only to "all container types defined in this clause" and so excludes basic_string::swap because it is defined elsewhere.

Eric Niebler points out that 27.4.3 [basic.string] paragraph 5 explicitly permits basic_string::swap to invalidates iterators, which is disallowed by 23.2 [container.requirements] paragraph 10. Thus the standard would be contradictory if it were read or extended to read as having basic_string meet 23.2 [container.requirements] paragraph 10 requirements.

Yet several LWG members have expressed the belief that the original intent was that basic_string::swap should not throw exceptions as specified by 23.2 [container.requirements] paragraph 10, and that the standard is unclear on this issue. The complexity of basic_string::swap is specified as "constant time", indicating the intent was to avoid copying (which could cause a bad_alloc or other exception). An important use of swap is to ensure that exceptions are not thrown in exception-safe code.

Note: There remains long standing concern over whether or not it is possible to reasonably meet the 23.2 [container.requirements] paragraph 10 swap requirements when allocators are unequal. The specification of basic_string::swap exception requirements is in no way intended to address, prejudice, or otherwise impact that concern.

Proposed resolution:

In 27.4.3.7.8 [string.swap], add a throws clause:

Throws: Shall not throw exceptions.

### 404(i). May a replacement allocation function be declared inline?

Section: 16.4.5.6 [replacement.functions], 17.6.3 [new.delete] Status: CD1 Submitter: Matt Austern Opened: 2003-04-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The eight basic dynamic memory allocation functions (single-object and array versions of ::operator new and ::operator delete, in the ordinary and nothrow forms) are replaceable. A C++ program may provide an alternative definition for any of them, which will be used in preference to the implementation's definition.

Three different parts of the standard mention requirements on replacement functions: 16.4.5.6 [replacement.functions], 17.6.3.2 [new.delete.single] and 17.6.3.3 [new.delete.array], and 6.8.6.4 [basic.stc.auto].

None of these three places say whether a replacement function may be declared inline. 17.6.3.2 [new.delete.single] paragraph 2 specifies a signature for the replacement function, but that's not enough: the `inline` specifier is not part of a function's signature. One might also reason from 9.2.3 [dcl.fct.spec] paragraph 2, which requires that "an inline function shall be defined in every translation unit in which it is used," but this may not be quite specific enough either. We should either explicitly allow or explicitly forbid inline replacement memory allocation functions.

Proposed resolution:

Add a new sentence to the end of 16.4.5.6 [replacement.functions] paragraph 3: "The program's definitions shall not be specified as `inline`. No diagnostic is required."

[Kona: added "no diagnostic is required"]

Rationale:

The fact that `inline` isn't mentioned appears to have been nothing more than an oversight. Existing implementations do not permit inline functions as replacement memory allocation functions. Providing this functionality would be difficult in some cases, and is believed to be of limited value.

### 405(i). qsort and POD

Section: 26.13 [alg.c.library] Status: CD1 Submitter: Ray Lischner Opened: 2003-04-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 26.13 [alg.c.library] describes bsearch and qsort, from the C standard library. Paragraph 4 does not list any restrictions on qsort, but it should limit the base parameter to point to POD. Presumably, qsort sorts the array by copying bytes, which requires POD.

Proposed resolution:

In 26.13 [alg.c.library] paragraph 4, just after the declarations and before the nonnormative note, add these words: "both of which have the same behavior as the original declaration. The behavior is undefined unless the objects in the array pointed to by base are of POD type."

[Something along these lines is clearly necessary. Matt provided wording.]

### 406(i). vector::insert(s) exception safety

Section: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Dave Abrahams Opened: 2003-04-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There is a possible defect in the standard: the standard text was never intended to prevent arbitrary ForwardIterators, whose operations may throw exceptions, from being passed, and it also wasn't intended to require a temporary buffer in the case where ForwardIterators were passed (and I think most implementations don't use one). As is, the standard appears to impose requirements that aren't met by any existing implementation.

Proposed resolution:

Replace 23.3.13.5 [vector.modifiers] paragraph 1 with:

> 1- Notes: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor or assignment operator of T or by any InputIterator operation there are no effects.

[We probably need to say something similar for deque.]

### 407(i). Can singular iterators be destroyed?

Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Clause 24.3.4 [iterator.concepts], paragraph 5, says that the only expression that is defined for a singular iterator is "an assignment of a non-singular value to an iterator that holds a singular value". This means that destroying a singular iterator (e.g. letting an automatic variable go out of scope) is technically undefined behavior. This seems overly strict, and probably unintentional.

Proposed resolution:

Change the sentence in question to "... the only exceptions are destroying an iterator that holds a singular value, or the assignment of a non-singular value to an iterator that holds a singular value."

### 409(i). Closing an fstream should clear error state

Section: 31.10.4.4 [ifstream.members], 31.10.5.4 [ofstream.members] Status: CD1 Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

A strict reading of [fstreams] shows that opening or closing a basic_[io]fstream does not affect the error bits. This means, for example, that if you read through a file up to EOF, and then close the stream and reopen it at the beginning of the file, the EOF bit in the stream's error state is still set. This is counterintuitive.

The LWG considered this issue once before, as issue 22(i), and put in a footnote to clarify that the strict reading was indeed correct. We did that because we believed the standard was unambiguous and consistent, and that we should not make architectural changes in a TC. Now that we're working on a new revision of the language, those considerations no longer apply.

Proposed resolution:

Change 31.10.4.4 [ifstream.members], para. 3 from:

> Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)].

to:

> Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().

Change 31.10.5.4 [ofstream.members], para. 3 from:

> Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)).

to:

> Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().

Change 31.10.6.4 [fstream.members], para. 3 from:

> Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) )

to:

> Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) ), else calls clear().

[Kona: the LWG agrees this is a good idea. Post-Kona: Bill provided wording. He suggests having open, not close, clear the error flags.]

[Post-Sydney: Howard provided a new proposed resolution. The old one didn't make sense because it proposed to fix this at the level of basic_filebuf, which doesn't have access to the stream's error state. Howard's proposed resolution fixes this at the level of the three fstream class template instead.]

### 410(i). Missing semantics for stack and queue comparison operators

Section: 23.3.11.2 [list.cons], 23.3.11.4 [list.modifiers] Status: CD1 Submitter: Hans Bos Opened: 2003-06-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Sections 23.3.11.2 [list.cons] and 23.3.11.4 [list.modifiers] list comparison operators (==, !=, <, <=, >, =>) for queue and stack. Only the semantics for queue::operator== (23.3.11.2 [list.cons] par2) and queue::operator< (23.3.11.2 [list.cons] par3) are defined.

Proposed resolution:

Add the following new paragraphs after 23.3.11.2 [list.cons] paragraph 3:

>       operator!=

> Returns: `x.c != y.c`

>       operator>

> Returns: `x.c > y.c`

>       operator<=

> Returns: `x.c <= y.c`

>       operator>=

> Returns: `x.c >= y.c`

Add the following paragraphs at the end of 23.3.11.4 [list.modifiers]:

>       operator==

> Returns: `x.c == y.c`

>       operator<

> Returns: `x.c < y.c`

>       operator!=

> Returns: `x.c != y.c`

>       operator>

> Returns: `x.c > y.c`

>       operator<=

> Returns: `x.c <= y.c`

>       operator>=

> Returns: `x.c >= y.c`

[Kona: Matt provided wording.]

Rationale:

There isn't any real doubt about what these operators are supposed to do, but we ought to spell it out.

### 411(i). Wrong names of set member functions

Section: 26.8.7 [alg.set.operations] Status: CD1 Submitter: Daniel Frey Opened: 2003-07-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

26.8.7 [alg.set.operations] paragraph 1 reads: "The semantics of the set operations are generalized to multisets in a standard way by defining union() to contain the maximum number of occurrences of every element, intersection() to contain the minimum, and so on."

This is wrong. The name of the functions are set_union() and set_intersection(), not union() and intersection().

Proposed resolution:

Change that sentence to use the correct names.

### 412(i). Typo in 27.4.4.3

Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2003-07-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 429

Discussion:

The Effects clause in 31.5.4.4 [iostate.flags] paragraph 5 says that the function only throws if the respective bits are already set prior to the function call. That's obviously not the intent. The typo ought to be corrected and the text reworded as: "If (state & exceptions()) == 0, returns. ..."

Proposed resolution:

In 31.5.4.4 [iostate.flags] paragraph 5, replace "If (rdstate() & exceptions()) == 0" with "If ((state | (rdbuf() ? goodbit : badbit)) & exceptions()) == 0".

[Kona: the original proposed resolution wasn't quite right. We really do mean rdstate(); the ambiguity is that the wording in the standard doesn't make it clear whether we mean rdstate() before setting the new state, or rdsate() after setting it. We intend the latter, of course. Post-Kona: Martin provided wording.]

### 413(i). Proposed resolution to LDR#64 still wrong

Section: 31.7.5.3.3 [istream.extractors] Status: CD1 Submitter: Bo Persson Opened: 2003-07-13 Last modified: 2017-04-22

Priority: Not Prioritized

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Discussion:

The second sentence of the proposed resolution says:

"If it inserted no characters because it caught an exception thrown while extracting characters from sb and ..."

However, we are not extracting from sb, but extracting from the basic_istream (*this) and inserting into sb. I can't really tell if "extracting" or "sb" is a typo.

[ Sydney: Definitely a real issue. We are, indeed, extracting characters from an istream and not from sb. The problem was there in the FDIS and wasn't fixed by issue 64(i). Probably what was intended was to have *this instead of sb. We're talking about the exception flag state of a basic_istream object, and there's only one basic_istream object in this discussion, so that would be a consistent interpretation. (But we need to be careful: the exception policy of this member function must be consistent with that of other extractors.) PJP will provide wording. ]

Proposed resolution:

Change the sentence from:

> If it inserted no characters because it caught an exception thrown while extracting characters from sb and failbit is on in exceptions(), then the caught exception is rethrown.

to:

> If it inserted no characters because it caught an exception thrown while extracting characters from *this and failbit is on in exceptions(), then the caught exception is rethrown.

### 414(i). Which iterators are invalidated by v.erase()?

Section: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Matt Austern Opened: 2003-08-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Consider the following code fragment:

>     int A[8] = { 1,3,5,7,9,8,4,2 };
>     std::vector<int> v(A, A+8);

>     std::vector<int>::iterator i1 = v.begin() + 3;
>     std::vector<int>::iterator i2 = v.begin() + 4;
>     v.erase(i1);

Which iterators are invalidated by `v.erase(i1)`: i1, i2, both, or neither?

On all existing implementations that I know of, the status of i1 and i2 is the same: both of them will be iterators that point to some elements of the vector (albeit not the same elements they did before). You won't get a crash if you use them. Depending on exactly what you mean by "invalidate", you might say that neither one has been invalidated because they still point to something, or you might say that both have been invalidated because in both cases the elements they point to have been changed out from under the iterator.

The standard doesn't say either of those things. It says that erase invalidates all iterators and references "after the point of the erase". This doesn't include i1, since it's at the point of the erase instead of after it. I can't think of any sensible definition of invalidation by which one can say that i2 is invalidated but i1 isn't.

(This issue is important if you try to reason about iterator validity based only on the guarantees in the standard, rather than reasoning from typical implementation techniques. Strict debugging modes, which some programmers find useful, do not use typical implementation techniques.)

Proposed resolution:

In 23.3.13.5 [vector.modifiers] paragraph 3, change "Invalidates all the iterators and references after the point of the erase" to "Invalidates iterators and references at or after the point of the erase".

Rationale:

I believe this was essentially a typographical error, and that it was taken for granted that erasing an element invalidates iterators that point to it. The effects clause in question treats iterators and references in parallel, and it would seem counterintuitive to say that a reference to an erased value remains valid.

### 415(i). behavior of std::ws

Section: 31.7.5.5 [istream.manip] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

According to 27.6.1.4, the ws() manipulator is not required to construct the sentry object. The manipulator is also not a member function so the text in 27.6.1, p1 through 4 that describes the exception policy for istream member functions does not apply. That seems inconsistent with the rest of extractors and all the other input functions (i.e., ws will not cause a tied stream to be flushed before extraction, it doesn't check the stream's exceptions or catch exceptions thrown during input, and it doesn't affect the stream's gcount).

Proposed resolution:

Add to 31.7.5.5 [istream.manip], immediately before the first sentence of paragraph 1, the following text:

> Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to is.gcount(). After constructing a sentry object...

[Post-Kona: Martin provided wording]

### 416(i). definitions of XXX_MIN and XXX_MAX macros in climits

Section: 17.3.6 [climits.syn] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

Given two overloads of the function foo(), one taking an argument of type int and the other taking a long, which one will the call foo(LONG_MAX) resolve to? The expected answer should be foo(long), but whether that is true depends on the #defintion of the LONG_MAX macro, specifically its type. This issue is about the fact that the type of these macros is not actually required to be the same as the the type each respective limit.
Section 18.2.2 of the C++ Standard does not specify the exact types of the XXX_MIN and XXX_MAX macros #defined in the <climits> and <limits.h> headers such as INT_MAX and LONG_MAX and instead defers to the C standard.
Section 5.2.4.2.1, p1 of the C standard specifies that "The values [of these constants] shall be replaced by constant expressions suitable for use in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the following shall be replaced by expressions that have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions."
The "corresponding type converted according to the integer promotions" for LONG_MAX is, according to 6.4.4.1, p5 of the C standard, the type of long converted to the first of the following set of types that can represent it: int, long int, long long int. So on an implementation where (sizeof(long) == sizeof(int)) this type is actually int, while on an implementation where (sizeof(long) > sizeof(int)) holds this type will be long.
This is not an issue in C since the type of the macro cannot be detected by any conforming C program, but it presents a portability problem in C++ where the actual type is easily detectable by overload resolution.

[Kona: the LWG does not believe this is a defect. The C macro definitions are what they are; we've got a better mechanism, `std::numeric_limits`, that is specified more precisely than the C limit macros. At most we should add a nonnormative note recommending that users who care about the exact types of limit quantities should use <limits> instead of <climits>.]

Proposed resolution:

Change [c.limits], paragraph 2:

> -2- The contents are the same as the Standard C library header `<limits.h>`. [Note: The types of the macros in `<climits>` are not guaranteed to match the type to which they refer.--end note]

### 419(i). istream extractors not setting `failbit` if `eofbit` is already set

Section: 31.7.5.2.4 [istream.sentry] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

[istream::sentry], p2 says that `istream::sentry` ctor prepares for input if `is.good()` is true. p4 then goes on to say that the ctor sets the `sentry::ok_` member to true if the stream state is good after any preparation. 31.7.5.3.1 [istream.formatted.reqmts], p1 then says that a formatted input function endeavors to obtain the requested input if the sentry's `operator bool()` returns true. Given these requirements, no formatted extractor should ever set `failbit` if the initial stream `rdstate() == eofbit`. That is contrary to the behavior of all implementations I tested. The program below prints out

>     eof = 1, fail = 0
>     eof = 1, fail = 1

on all of them.

    #include <sstream>
    #include <cstdio>

    int main()
    {
        std::istringstream strm ("1");

        int i = 0;

        strm >> i;

        std::printf ("eof = %d, fail = %d\n",
                     !!strm.eof (), !!strm.fail ());

        strm >> i;

        std::printf ("eof = %d, fail = %d\n",
                     !!strm.eof (), !!strm.fail ());
    }

Comments from Jerry Schwarz (c++std-lib-11373):
Jerry Schwarz wrote:
I don't know where (if anywhere) it says it in the standard, but the formatted extractors are supposed to set `failbit` if they don't extract any characters. If they didn't then simple loops like
while (cin >> x);
would loop forever.
Further comments from Martin Sebor:
The question is which part of the extraction should prevent this from happening by setting `failbit` when `eofbit` is already set. It could either be the sentry object or the extractor. It seems that most implementations have chosen to set `failbit` in the sentry [...] so that's the text that will need to be corrected.

Pre Berlin: This issue is related to 342(i). If the sentry sets `failbit` when it finds `eofbit` already set, then you can never seek away from the end of stream.

Kona: Possibly NAD. If `eofbit` is set then `good()` will return false. We then set ok to false. We believe that the sentry's constructor should always set `failbit` when ok is false, and we also think the standard already says that. Possibly it could be clearer.

[ 2009-07 Frankfurt ]

> Moved to Ready.

Proposed resolution:

Change [istream::sentry], p2 to:

>     explicit sentry(basic_istream<charT,traits>& is , bool noskipws = false);

> -2- Effects: If `is.good()` is `true` `false`, calls `is.setstate(failbit)`. Otherwise prepares for formatted or unformatted input. ...

### 420(i). is std::FILE a complete type?

Section: 31.10 [file.streams] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

7.19.1, p2, of C99 requires that the FILE type only be declared in <stdio.h>. None of the (implementation-defined) members of the struct is mentioned anywhere for obvious reasons.

C++ says in 27.8.1, p2 that FILE is a type that's defined in <cstdio>. Is it really the intent that FILE be a complete type or is an implementation allowed to just declare it without providing a full definition?

Proposed resolution:

In the first sentence of [fstreams] paragraph 2, change "defined" to "declared".

Rationale:

We don't want to impose any restrictions beyond what the C standard already says. We don't want to make anything implementation defined, because that imposes new requirements in implementations.

### 422(i). explicit specializations of member functions of class templates

Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It has been suggested that 17.4.3.1, p1 may or may not allow programs to explicitly specialize members of standard templates on user-defined types. The answer to the question might have an impact where library requirements are given using the "as if" rule. I.e., if programs are allowed to specialize member functions they will be able to detect an implementation's strict conformance to Effects clauses that describe the behavior of the function in terms of the other member function (the one explicitly specialized by the program) by relying on the "as if" rule.

Proposed resolution:

Add the following sentence to 16.4.5.3 [reserved.names], p1:

> It is undefined for a C++ program to add declarations or definitions to namespace std or namespaces within namespace `std` unless otherwise specified. A program may add template specializations for any standard library template to namespace `std`. Such a specialization (complete or partial) of a standard library template results in undefined behavior unless the declaration depends on a user-defined type of external linkage and unless the specialization meets the standard library requirements for the original template.168) A program has undefined behavior if it declares

> - an explicit specialization of any member function of a standard library class template, or
> - an explicit specialization of any member function template of a standard library class or class template, or
> - an explicit or partial specialization of any member class template of a standard library class or class template.

> A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.

[Kona: straw poll was 6-1 that user programs should not be allowed to specialize individual member functions of standard library class templates, and that doing so invokes undefined behavior. Post-Kona: Martin provided wording.]

[Sydney: The LWG agrees that the standard shouldn't permit users to specialize individual member functions unless they specialize the whole class, but we're not sure these words say what we want them to; they could be read as prohibiting the specialization of any standard library class templates. We need to consult with CWG to make sure we use the right wording.]

### 425(i). return value of std::get_temporary_buffer

Section: 99 [depr.temporary.buffer] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

The standard is not clear about the requirements on the value returned from a call to get_temporary_buffer(0). In particular, it fails to specify whether the call should return a distinct pointer each time it is called (like operator new), or whether the value is unspecified (as if returned by malloc). The standard also fails to mention what the required behavior is when the argument is less than 0.

Proposed resolution:

Change 21.3.4 [meta.help] paragraph 2 from "...or a pair of 0 values if no storage can be obtained" to "...or a pair of 0 values if no storage can be obtained or if n <= 0."

[Kona: Matt provided wording]

### 426(i). search_n(), fill_n(), and generate_n() with negative n

Section: 26.6.15 [alg.search], 26.7.6 [alg.fill], 26.7.7 [alg.generate] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The complexity requirements for these function templates are incorrect (or don't even make sense) for negative n:

25.1.9, p7 (search_n):
Complexity: At most (last1 - first1) * count applications of the corresponding predicate.

25.2.5, p3 (fill_n):
Complexity: Exactly last - first (or n) assignments.

25.2.6, p3 (generate_n):
Complexity: Exactly last - first (or n) assignments.

In addition, the Requirements or the Effects clauses for the latter two templates don't say anything about the behavior when n is negative.

Proposed resolution:

Change 25.1.9, p7 to

> Complexity: At most (last1 - first1) * count applications of the corresponding predicate if count is positive, or 0 otherwise.

Change 25.2.5, p2 to

> Effects: Assigns value through all the iterators in the range [first, last), or [first, first + n) if n is positive, none otherwise.

Change 25.2.5, p3 to:

> Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.

Change 25.2.6, p1 to (notice the correction for the misspelled "through"):

> Effects: Invokes the function object genand assigns the return value of gen through all the iterators in the range [first, last), or [first, first + n) if n is positive, or [first, first) otherwise.

Change 25.2.6, p3 to:

> Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.

Rationale:

Informally, we want to say that whenever we see a negative number we treat it the same as if it were zero. We believe the above changes do that (although they may not be the minimal way of saying so). The LWG considered and rejected the alternative of saying that negative numbers are undefined behavior.

### 427(i). Stage 2 and rationale of DR 221

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The requirements specified in Stage 2 and reiterated in the rationale of DR 221 (and echoed again in DR 303) specify that num_get<charT>:: do_get() compares characters on the stream against the widened elements of "012...abc...ABCX+-"

An implementation is required to allow programs to instantiate the num_get template on any charT that satisfies the requirements on a user-defined character type. These requirements do not include the ability of the character type to be equality comparable (the char_traits template must be used to perform tests for equality). Hence, the num_get template cannot be implemented to support any arbitrary character type. The num_get template must either make the assumption that the character type is equality-comparable (as some popular implementations do), or it may use char_traits<charT> to do the comparisons (some other popular implementations do that). This diversity of approaches makes it difficult to write portable programs that attempt to instantiate the num_get template on user-defined types.

[Kona: the heart of the problem is that we're theoretically supposed to use traits classes for all fundamental character operations like assignment and comparison, but facets don't have traits parameters. This is a fundamental design flaw and it appears all over the place, not just in this one place. It's not clear what the correct solution is, but a thorough review of facets and traits is in order. The LWG considered and rejected the possibility of changing numeric facets to use narrowing instead of widening. This may be a good idea for other reasons (see issue 459(i)), but it doesn't solve the problem raised by this issue. Whether we use widen or narrow the `num_get` facet still has no idea which traits class the user wants to use for the comparison, because only streams, not facets, are passed traits classes. The standard does not require that two different traits classes with the same `char_type` must necessarily have the same behavior.]

Informally, one possibility: require that some of the basic character operations, such as `eq`, `lt`, and `assign`, must behave the same way for all traits classes with the same `char_type`. If we accept that limitation on traits classes, then the facet could reasonably be required to use `char_traits<charT>`.

[ 2009-07 Frankfurt ]

> There was general agreement that the standard only needs to specify the behavior when the character type is char or wchar_t.

> Beman: we don't need to worry about C++1x because there is a non-zero possibility that we would have a replacement facility for iostreams that would solve these problems.

> We need to change the following sentence in [locale.category], paragraph 6 to specify that C is char and wchar_t:

> "A template formal parameter with name C represents the set of all possible specializations on a parameter that satisfies the requirements for a character on which any member of the iostream components can be instantiated."

> We also need to specify in 27 that the basic character operations, such as eq, lt, and assign use std::char_traits.

> Daniel volunteered to provide wording.

[ 2009-09-19 Daniel provided wording. ]

[ 2009-10 Santa Cruz: ]

> Leave as Open. Alisdair and/or Tom will provide wording based on discussions. We want to clearly state that streams and locales work just on `char` and `wchar_t` (except where otherwise specified).

[ 2010-02-06 Tom updated the proposed wording. ]

> [ The original proposed wording is preserved here: ]

> > 1. Change 28.3.3.1.2.1 [locale.category]/6:

> >    > [..] A template formal parameter with name `C` represents the set of all possible specializations on a `char` or `wchar_t` parameter that satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]

> > 2. Add the following sentence to the end of 28.3.4.3 [category.numeric]/2:

> >    > [..] These specializations refer to [..], and also for the `ctype<>` facet to perform character classification. Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations.

> > 3. Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3:

> >    > Stage 2: If `in==end` then stage 2 terminates. Otherwise a `charT` is taken from `in` and local variables are initialized as if by

> >    > >     char_type ct = *in;
> >    > >     using tr = char_traits<char_type>;
> >    > >     const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct);
> >    > >     char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms
> >    > >                  pos ? pos - atoms : sizeof(src) - 1];
> >    > >     if (tr::eq(ct, ct == use_facet<numpunct<charT>(loc).decimal_point()))
> >    > >         c = '.';
> >    > >     bool discard =
> >    > >         tr::eq(ct, ct == use_facet<numpunct<charT>(loc).thousands_sep())
> >    > >         && use_facet<numpunct<charT> >(loc).grouping().length() != 0;

> >    > where the values `src` and `atoms` are defined as if by: [..]

> >    [Remark of the author: I considered to replace the initialization "`char_type ct = *in;`" by the sequence "`char_type ct; tr::assign(ct, *in);`", but decided against it, because it is a copy-initialization context, not an assignment]

> > 4. Add the following sentence to the end of 28.3.4.6 [category.time]/1:

> >    > [..] Their members use [..] , to determine formatting details. Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations.

> > 5. Change 28.3.4.6.2.2 [locale.time.get.members]/8 bullet 4:

> >    - The next element of `fmt` is equal to `'%'` For the next element `c` of `fmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true`, [..]

> > 6. Add the following sentence to the end of 28.3.4.7 [category.monetary]/2:

> >    > Their members use [..] to determine formatting details. Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations.

> > 7. Change 28.3.4.7.2.3 [locale.money.get.virtuals]/4:

> >    > [..] The value `units` is produced as if by:

> >    > >     for (int i = 0; i < n; ++i)
> >    > >       buf2[i] = src[char_traits<charT>::find(atoms, atoms+sizeof(src), buf1[i]) - atoms];
> >    > >     buf2[n] = 0;
> >    > >     sscanf(buf2, "%Lf", &units);

> > 8. Change 28.3.4.7.3.3 [locale.money.put.virtuals]/1:

> >    > [..] for character buffers `buf1` and `buf2`. If for the first character `c` in `digits` or `buf2` is equal to `ct.widen('-')``char_traits<charT>::eq(c, ct.widen('-')) == true`, [..]

> > 9. Add a footnote to the first sentence of 31.7.5.3.2 [istream.formatted.arithmetic]/1:

> >    > As in the case of the inserters, these extractors depend on the locale's `num_get<>` (22.4.2.1) object to perform parsing the input stream data.(footnote) [..]

> >    > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 10. Add a footnote to the second sentence of 31.7.6.3.2 [ostream.inserters.arithmetic]/1:

> >     > Effects: The classes `num_get<>` and `num_put<>` handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 11. Add a footnote after the first sentence of 31.7.8 [ext.manip]/4:

> >     > Returns: An object of unspecified type such that if in is an object of type `basic_istream<charT, traits>` then the expression `in >> get_money(mon, intl)` behaves as if it called `f(in, mon, intl)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 12. Add a footnote after the first sentence of 31.7.8 [ext.manip]/5:

> >     > Returns: An object of unspecified type such that if `out` is an object of type `basic_ostream<charT, traits>` then the expression `out << put_money(mon, intl)` behaves as a formatted input function that calls `f(out, mon, intl)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 13. 13) Add a footnote after the first sentence of 31.7.8 [ext.manip]/8:

> >     > Returns: An object of unspecified type such that if `in` is an object of type b`asic_istream<charT, traits>` then the expression `in >>get_time(tmb, fmt)` behaves as if it called `f(in, tmb, fmt)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 14. Add a footnote after the first sentence of 31.7.8 [ext.manip]/10:

> >     > Returns: An object of unspecified type such that if `out` is an object of type `basic_ostream<charT, traits>` then the expression `out <<put_time(tmb, fmt)` behaves as if it called `f(out, tmb, fmt)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

[ 2010 Pittsburgh: ]

> Moved to Ready with only two of the bullets. The original wording is preserved here:

> > 1. Change 28.3.3.1.2.1 [locale.category]/6:

> >    > [..] A template formal parameter with name `C` represents the set of all possible specializations on a of types containing `char`, `wchar_t`, and any other implementation-defined character type parameter that satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]

> > 2. Add the following sentence to the end of 28.3.4.3 [category.numeric]/2:

> >    > [..] These specializations refer to [..], and also for the `ctype<>` facet to perform character classification. [Note: Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations - end note].

> > 3. Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3:

> >    > Stage 2: If `in==end` then stage 2 terminates. Otherwise a `charT` is taken from `in` and local variables are initialized as if by

> >    > >     char_type ct = *in;
> >    > >     using tr = char_traits<char_type>;
> >    > >     const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct);
> >    > >     char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms
> >    > >                  pos ? pos - atoms : sizeof(src) - 1];
> >    > >     if (tr::eq(ct, ct == use_facet<numpunct<charT>(loc).decimal_point()))
> >    > >         c = '.';
> >    > >     bool discard =
> >    > >         tr::eq(ct, ct == use_facet<numpunct<charT>(loc).thousands_sep())
> >    > >         && use_facet<numpunct<charT> >(loc).grouping().length() != 0;

> >    > where the values `src` and `atoms` are defined as if by: [..]

> >    [Remark of the author: I considered to replace the initialization "`char_type ct = *in;`" by the sequence "`char_type ct; tr::assign(ct, *in);`", but decided against it, because it is a copy-initialization context, not an assignment]

> > 4. Add the following sentence to the end of 28.3.4.6 [category.time]/1:

> >    > [..] Their members use [..] , to determine formatting details. [Note: Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations - end note].

> > 5. Change 28.3.4.6.2.2 [locale.time.get.members]/8 bullet 4:

> >    - The next element of `fmt` is equal to `'%'` For the next element `c` of `fmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true`, [..]

> > 6. Add the following sentence to the end of 28.3.4.7 [category.monetary]/2:

> >    > Their members use [..] to determine formatting details. [Note: Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations - end note].

> > 7. Change 28.3.4.7.2.3 [locale.money.get.virtuals]/4:

> >    > [..] The value `units` is produced as if by:

> >    > >     for (int i = 0; i < n; ++i)
> >    > >       buf2[i] = src[char_traits<charT>::find(atoms, atoms+sizeof(src), buf1[i]) - atoms];
> >    > >     buf2[n] = 0;
> >    > >     sscanf(buf2, "%Lf", &units);

> > 8. Change 28.3.4.7.3.3 [locale.money.put.virtuals]/1:

> >    > [..] for character buffers `buf1` and `buf2`. If for the first character `c` in `digits` or `buf2` is equal to `ct.widen('-')``char_traits<charT>::eq(c, ct.widen('-')) == true`, [..]

> > 9. Add a new paragraph after the first paragraph of 31.2.3 [iostreams.limits.pos]/1:

> >    > In the classes of clause 27, a template formal parameter with name `charT` represents one of the set of types containing `char`, `wchar_t`, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.

> > 10. Add a footnote to the first sentence of 31.7.5.3.2 [istream.formatted.arithmetic]/1:

> >     > As in the case of the inserters, these extractors depend on the locale's `num_get<>` (22.4.2.1) object to perform parsing the input stream data.(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 11. Add a footnote to the second sentence of 31.7.6.3.2 [ostream.inserters.arithmetic]/1:

> >     > Effects: The classes `num_get<>` and `num_put<>` handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 12. Add a footnote after the first sentence of 31.7.8 [ext.manip]/4:

> >     > Returns: An object of unspecified type such that if in is an object of type `basic_istream<charT, traits>` then the expression `in >> get_money(mon, intl)` behaves as if it called `f(in, mon, intl)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 13. Add a footnote after the first sentence of 31.7.8 [ext.manip]/5:

> >     > Returns: An object of unspecified type such that if `out` is an object of type `basic_ostream<charT, traits>` then the expression `out << put_money(mon, intl)` behaves as a formatted input function that calls `f(out, mon, intl)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 14. Add a footnote after the first sentence of 31.7.8 [ext.manip]/8:

> >     > Returns: An object of unspecified type such that if `in` is an object of type b`asic_istream<charT, traits>` then the expression `in >>get_time(tmb, fmt)` behaves as if it called `f(in, tmb, fmt)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 15. Add a footnote after the first sentence of 31.7.8 [ext.manip]/10:

> >     > Returns: An object of unspecified type such that if `out` is an object of type `basic_ostream<charT, traits>` then the expression `out <<put_time(tmb, fmt)` behaves as if it called `f(out, tmb, fmt)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

Proposed resolution:

1. Change 28.3.3.1.2.1 [locale.category]/6:

   > [..] A template formal parameter with name `C` represents the set of all possible specializations on a of types containing `char`, `wchar_t`, and any other implementation-defined character type parameter that satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]

2. Add a new paragraph after the first paragraph of 31.2.3 [iostreams.limits.pos]/1:

   > In the classes of clause 27, a template formal parameter with name `charT` represents one of the set of types containing `char`, `wchar_t`, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.

### 428(i). string::erase(iterator) validity

Section: 27.4.3.7.5 [string.erase] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-11-12

Priority: Not Prioritized

View all other issues in [string.erase].

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Discussion:

23.1.1, p3 along with Table 67 specify as a prerequisite for a.erase(q) that q must be a valid dereferenceable iterator into the sequence a.

However, 21.3.5.5, p5 describing string::erase(p) only requires that p be a valid iterator.

This may be interepreted as a relaxation of the general requirement, which is most likely not the intent.

Proposed resolution:

Remove 27.4.3.7.5 [string.erase] paragraph 5.

Rationale:

The LWG considered two options: changing the string requirements to match the general container requirements, or just removing the erroneous string requirements altogether. The LWG chose the latter option, on the grounds that duplicating text always risks the possibility that it might be duplicated incorrectly.

### 430(i). `valarray` subset operations

Section: 29.6.2.5 [valarray.sub] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

The standard fails to specify the behavior of valarray::operator[](slice) and other valarray subset operations when they are passed an "invalid" slice object, i.e., either a slice that doesn't make sense at all (e.g., slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray object (e.g., slice (2, 1, 1) for a valarray of size 1).

[Kona: the LWG believes that invalid slices should invoke undefined behavior. Valarrays are supposed to be designed for high performance, so we don't want to require specific checking. We need wording to express this decision.]

[ Bellevue: ]

> Please note that the standard also fails to specify the behavior of slice_array and gslice_array in the valid case. Bill Plauger will endeavor to provide revised wording for `slice_array` and `gslice_array`.

[ post-Bellevue: Bill provided wording. ]

[ 2009-07 Frankfurt ]

> Move to Ready.

[ 2009-11-04 Pete opens: ]

> The resolution to LWG issue 430(i) has not been applied — there have been changes to the underlying text, and the resolution needs to be reworked.

[ 2010-03-09 Matt updated wording. ]

[ 2010 Pittsburgh: Moved to Ready for Pittsburgh. ]

Proposed resolution:

Replace 29.6.2.5 [valarray.sub], with the following:

> The member operator is overloaded to provide several ways to select sequences of elements from among those controlled by `*this`. Each of these operations returns a subset of the array. The const-qualified versions return this subset as a new `valarray`. The non-const versions return a class template object which has reference semantics to the original array, working in conjunction with various overloads of `operator=` (and other assigning operators) to allow selective replacement (slicing) of the controlled sequence. In each case the selected element(s) must exist.

>     valarray<T> operator[](slice slicearr) const;

> > This function returns an object of class `valarray<T>` containing those elements of the controlled sequence designated by `slicearr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABCDE", 5);
> > >     v0[slice(2, 5, 3)] = v1;
> > >     // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)

> > end example]

>     valarray<T> operator[](slice slicearr);

> > This function selects those elements of the controlled sequence designated by `slicearr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABCDE", 5);
> > >     v0[slice(2, 5, 3)] = v1;
> > >     // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)

> > end example]

>     valarray<T> operator[](const gslice& gslicearr) const;

> > This function returns an object of class `valarray<T>` containing those elements of the controlled sequence designated by `gslicearr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     const size_t lv[] = {2, 3};
> > >     const size_t dv[] = {7, 2};
> > >     const valarray<size_t> len(lv, 2), str(dv, 2);
> > >     // v0[gslice(3, len, str)] returns
> > >     // valarray<char>("dfhkmo", 6)

> > end example]

>     gslice_array<T> operator[](const gslice& gslicearr);

> > This function selects those elements of the controlled sequence designated by `gslicearr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABCDEF", 6);
> > >     const size_t lv[] = {2, 3};
> > >     const size_t dv[] = {7, 2};
> > >     const valarray<size_t> len(lv, 2), str(dv, 2);
> > >     v0[gslice(3, len, str)] = v1;
> > >     // v0 == valarray<char>("abcAeBgCijDlEnFp", 16)

> > end example]

>     valarray<T> operator[](const valarray<bool>& boolarr) const;

> > This function returns an object of class `valarray<T>` containing those elements of the controlled sequence designated by `boolarr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     const bool vb[] = {false, false, true, true, false, true};
> > >     // v0[valarray<bool>(vb, 6)] returns
> > >     // valarray<char>("cdf", 3)

> > end example]

>     mask_array<T> operator[](const valarray<bool>& boolarr);

> > This function selects those elements of the controlled sequence designated by `boolarr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABC", 3);
> > >     const bool vb[] = {false, false, true, true, false, true};
> > >     v0[valarray<bool>(vb, 6)] = v1;
> > >     // v0 == valarray<char>("abABeCghijklmnop", 16)

> > end example]

>     valarray<T> operator[](const valarray<size_t>& indarr) const;

> > This function returns an object of class `valarray<T>` containing those elements of the controlled sequence designated by `indarr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     const size_t vi[] = {7, 5, 2, 3, 8};
> > >     // v0[valarray<size_t>(vi, 5)] returns
> > >     // valarray<char>("hfcdi", 5)

> > end example]

>     indirect_array<T> operator[](const valarray<size_t>& indarr);

> > This function selects those elements of the controlled sequence designated by `indarr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABCDE", 5);
> > >     const size_t vi[] = {7, 5, 2, 3, 8};
> > >     v0[valarray<size_t>(vi, 5)] = v1;
> > >     // v0 == valarray<char>("abCDeBgAEjklmnop", 16)

> > end example]

### 431(i). Swapping containers with unequal allocators

Section: 16.4.4.6 [allocator.requirements], 26 [algorithms] Status: Resolved Submitter: Matt Austern Opened: 2003-09-20 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [allocator.requirements].

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Discussion:

Clause 16.4.4.6 [allocator.requirements] paragraph 4 says that implementations are permitted to supply containers that are unable to cope with allocator instances and that container implementations may assume that all instances of an allocator type compare equal. We gave implementers this latitude as a temporary hack, and eventually we want to get rid of it. What happens when we're dealing with allocators that don't compare equal?

In particular: suppose that `v1` and `v2` are both objects of type `vector<int, my_alloc>` and that `v1.get_allocator() != v2.get_allocator()`. What happens if we write `v1.swap(v2)`? Informally, three possibilities:

1. This operation is illegal. Perhaps we could say that an implementation is required to check and to throw an exception, or perhaps we could say it's undefined behavior.

2. The operation performs a slow swap (i.e. using three invocations of `operator=`, leaving each allocator with its original container. This would be an O(N) operation.

3. The operation swaps both the vectors' contents and their allocators. This would be an O(1) operation. That is:

>         my_alloc a1(...);
>         my_alloc a2(...);
>         assert(a1 != a2);

>         vector<int, my_alloc> v1(a1);
>         vector<int, my_alloc> v2(a2);
>         assert(a1 == v1.get_allocator());
>         assert(a2 == v2.get_allocator());

>         v1.swap(v2);
>         assert(a1 == v2.get_allocator());
>         assert(a2 == v1.get_allocator());

[Kona: This is part of a general problem. We need a paper saying how to deal with unequal allocators in general.]

[pre-Sydney: Howard argues for option 3 in N1599. ]

[ 2007-01-12, Howard: This issue will now tend to come up more often with move constructors and move assignment operators. For containers, these members transfer resources (i.e. the allocated memory) just like swap. ]

[ Batavia: There is agreement to overload the container `swap` on the allocator's Swappable requirement using concepts. If the allocator supports Swappable, then container's swap will swap allocators, else it will perform a "slow swap" using copy construction and copy assignment. ]

[ 2009-04-28 Pablo adds: ]

> Fixed in N2525. I argued for marking this Tentatively-Ready right after Bellevue, but there was a concern that N2525 would break in the presence of the RVO. (That breakage had nothing to do with swap, but never-the-less). I addressed that breakage in in N2840 (Summit) by means of a non-normative reference:

> > [Note: in situations where the copy constructor for a container is elided, this function is not called. The behavior in these cases is as if `select_on_container_copy_construction` returned `x` — end note]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2982.

Proposed resolution:

### 432(i). stringbuf::overflow() makes only one write position available

Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Christian W Brock Opened: 2003-09-24 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [stringbuf.virtuals].

View all other issues in [stringbuf.virtuals].

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Discussion:

27.7.1.3 par 8 says:

> Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).

The sentences "plus one additional write position." and especially "(as does the write end pointer epptr())" COULD by interpreted (and is interpreted by at least my library vendor) as:

> post-condition: epptr() == pptr()+1

This WOULD force sputc() to call the virtual overflow() each time.

The proposed change also affects Defect Report 169.

Proposed resolution:

27.7.1.1/2 Change:

> 2- Notes: The function allocates no array object.

to:

> 2- Postcondition: str() == "".

27.7.1.1/3 Change:

> -3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which . Then copies the content of str into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to which. If which & ios_base::out is true, initializes the output sequence with the underlying sequence. If which & ios_base::in is true, initializes the input sequence with the underlying sequence.

to:

> -3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which. Then copies the content of str into the basic_stringbuf underlying character sequence. If which & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (which & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If which & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.

27.7.1.2/1 Change:

> -1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the buffer is only created in input mode, the underlying character sequence is equal to the input sequence; otherwise, it is equal to the output sequence. In case of an empty underlying character sequence, the function returns basic_string<charT,traits,Allocator>().

to:

> -1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the basic_stringbuf was created only in input mode, the resultant basic_string contains the character sequence in the range [eback(), egptr()). If the basic_stringbuf was created with (which & ios_base::out) being true then the resultant basic_string contains the character sequence in the range [pbase(), high_mark) where high_mark represents the position one past the highest initialized character in the buffer. Characters can be initialized either through writing to the stream, or by constructing the basic_stringbuf with a basic_string, or by calling the str(basic_string) member function. In the case of calling the str(basic_string) member function, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_string). Otherwise the basic_stringbuf has been created in neither input nor output mode and a zero length basic_string is returned.

27.7.1.2/2 Change:

> -2- Effects: If the basic_stringbuf's underlying character sequence is not empty, deallocates it. Then copies the content of s into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to the mode stored when creating the basic_stringbuf object. If (mode&ios_base::out) is true, then initializes the output sequence with the underlying sequence. If (mode&ios_base::in) is true, then initializes the input sequence with the underlying sequence.

to:

> -2- Effects: Copies the content of s into the basic_stringbuf underlying character sequence. If mode & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (mode & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If mode & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.

Remove 27.2.1.2/3. (Same rationale as issue 238: incorrect and unnecessary.)

27.7.1.3/1 Change:

> 1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof().

to:

> 1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof(). Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.

27.7.1.3/9 Change:

> -9- Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).

to:

> -9- The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position.

27.7.1.3/12 Change:

> -12- _ If (newoff + off) < 0, or (xend - xbeg) < (newoff + off), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .

to:

> -12- _ If (newoff + off) < 0, or if (newoff + off) refers to an uninitialized character (as defined in 31.8.2.4 [stringbuf.members] paragraph 1), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .

[post-Kona: Howard provided wording. At Kona the LWG agreed that something along these lines was a good idea, but the original proposed resolution didn't say enough about the effect of various member functions on the underlying character sequences.]

Rationale:

The current basic_stringbuf description is over-constrained in such a way as to prohibit vendors from making this the high-performance in-memory stream it was meant to be. The fundamental problem is that the pointers: eback(), gptr(), egptr(), pbase(), pptr(), epptr() are observable from a derived client, and the current description restricts the range [pbase(), epptr()) from being grown geometrically. This change allows, but does not require, geometric growth of this range.

Backwards compatibility issues: These changes will break code that derives from basic_stringbuf, observes epptr(), and depends upon [pbase(), epptr()) growing by one character on each call to overflow() (i.e. test suites). Otherwise there are no backwards compatibility issues.

27.7.1.1/2: The non-normative note is non-binding, and if it were binding, would be over specification. The recommended change focuses on the important observable fact.

27.7.1.1/3: This change does two things: 1. It describes exactly what must happen in terms of the sequences. The terms "input sequence" and "output sequence" are not well defined. 2. It introduces a common extension: open with app or ate mode. I concur with issue 238 that paragraph 4 is both wrong and unnecessary.

27.7.1.2/1: This change is the crux of the efficiency issue. The resultant basic_string is not dependent upon epptr(), and thus implementors are free to grow the underlying buffer geometrically during overflow() *and* place epptr() at the end of that buffer.

27.7.1.2/2: Made consistent with the proposed 27.7.1.1/3.

27.7.1.3/1: Clarifies that characters written to the stream beyond the initially specified string are available for reading in an i/o basic_streambuf.

27.7.1.3/9: Made normative by removing "Notes:", and removed the trailing parenthetical comment concerning epptr().

27.7.1.3/12: Restricting the positioning to [xbeg, xend) is no longer allowable since [pbase(), epptr()) may now contain uninitialized characters. Positioning is only allowable over the initialized range.

### 434(i). bitset::to_string() hard to use

Section: 22.9.2.3 [bitset.members] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [bitset.members].

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Discussion:

It has been pointed out a number of times that the bitset to_string() member function template is tedious to use since callers must explicitly specify the entire template argument list (3 arguments). At least two implementations provide a number of overloads of this template to make it easier to use.

Proposed resolution:

In order to allow callers to specify no template arguments at all, just the first one (charT), or the first 2 (charT and traits), in addition to all three template arguments, add the following three overloads to both the interface (declarations only) of the class template bitset as well as to section 23.3.5.2, immediately after p34, the Returns clause of the existing to_string() member function template:

        template <class charT, class traits>
        basic_string<charT, traits, allocator<charT> >
        to_string () const;

        -34.1- Returns: to_string<charT, traits, allocator<charT> >().

        template <class charT>
        basic_string<charT, char_traits<charT>, allocator<charT> >
        to_string () const;

        -34.2- Returns: to_string<charT, char_traits<charT>, allocator<charT> >().

        basic_string<char, char_traits<char>, allocator<char> >
        to_string () const;

        -34.3- Returns: to_string<char, char_traits<char>, allocator<char> >().

[Kona: the LWG agrees that this is an improvement over the status quo. Dietmar thought about an alternative using a proxy object but now believes that the proposed resolution above is the right choice. ]

### 435(i). bug in DR 25

Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [string.io].

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Discussion:

It has been pointed out that the proposed resolution in DR 25(i) may not be quite up to snuff:
http://gcc.gnu.org/ml/libstdc++/2003-09/msg00147.html

It looks like Petur is right. The complete corrected text is copied below. I think we may have have been confused by the reference to 22.2.2.2.2 and the subsequent description of `n' which actually talks about the second argument to sputn(), not about the number of fill characters to pad with.

So the question is: was the original text correct? If the intent was to follow classic iostreams then it most likely wasn't, since setting width() to less than the length of the string doesn't truncate it on output. This is also the behavior of most implementations (except for SGI's standard iostreams where the operator does truncate).

Proposed resolution:

Change the text in 21.3.7.9, p4 from

> If bool(k) is true, inserts characters as if by calling os.rdbuf()->sputn(str.data(), n), padding as described in stage 3 of lib.facet.num.put.virtuals, where n is the larger of os.width() and str.size();

to

> If bool(k) is true, determines padding as described in lib.facet.num.put.virtuals, and then inserts the resulting sequence of characters `seq` as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size()`;

[Kona: it appears that neither the original wording, DR25, nor the proposed resolution, is quite what we want. We want to say that the string will be output, padded to os.width() if necessary. We don't want to duplicate the padding rules in clause 22, because they're complicated, but we need to be careful because they weren't quite written with quite this case in mind. We need to say what the character sequence is, and then defer to clause 22. Post-Kona: Benjamin provided wording.]

### 436(i). are cv-qualified facet types valid facets?

Section: 28.3.3.1.2.2 [locale.facet] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [locale.facet].

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Discussion:

Is "const std::ctype<char>" a valid template argument to has_facet, use_facet, and the locale template ctor? And if so, does it designate the same Facet as the non-const "std::ctype<char>?" What about "volatile std::ctype<char>?" Different implementations behave differently: some fail to compile, others accept such types but behave inconsistently.

Proposed resolution:

Change 22.1.1.1.2, p1 to read:

Template parameters in this clause which are required to be facets are those named Facet in declarations. A program that passes a type that is not a facet, or a type that refers to volatile-qualified facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed. A const-qualified facet is a valid template argument to any locale function that expects a Facet template parameter.

[Kona: changed the last sentence from a footnote to normative text.]

### 438(i). Ambiguity in the "do the right thing" clause

Section: 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Howard Hinnant Opened: 2003-10-20 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [sequence.reqmts].

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Discussion:

Section 23.2.4 [sequence.reqmts], paragraphs 9-11, fixed up the problem noticed with statements like:

    vector<int> v(10, 1);

The intent of the above statement was to construct with:

    vector(size_type, const value_type&);

but early implementations failed to compile as they bound to:

    template <class InputIterator>
    vector(InputIterator f, InputIterator l);

instead.

Paragraphs 9-11 say that if InputIterator is an integral type, then the member template constructor will have the same effect as:

    vector<static_cast<size_type>(f), static_cast<value_type>(l));

(and similarly for the other member template functions of sequences).

There is also a note that describes one implementation technique:

> One way that sequence implementors can satisfy this requirement is to specialize the member template for every integral type.

This might look something like:

>     template <class T>
>     struct vector
>     {
>          typedef unsigned size_type;

>          explicit vector(size_type) {}
>          vector(size_type, const T&) {}

>          template <class I>
>          vector(I, I);

>          // ...
>     };

>     template <class T>
>     template <class I>
>     vector<T>::vector(I, I) { ... }

>     template <>
>     template <>
>     vector<int>::vector(int, int) { ... }

>     template <>
>     template <>
>     vector<int>::vector(unsigned, unsigned) { ... }

>     //  ...

Label this solution 'A'.

The standard also says:

> Less cumbersome implementation techniques also exist.

A popular technique is to not specialize as above, but instead catch every call with the member template, detect the type of InputIterator, and then redirect to the correct logic. Something like:

>     template <class T>
>     template <class I>
>     vector<T>::vector(I f, I l)
>     {
>          choose_init(f, l, int2type<is_integral<I>::value>());
>     }

>     template <class T>
>     template <class I>
>     vector<T>::choose_init(I f, I l, int2type<false>)
>     {
>         // construct with iterators
>     }

>     template <class T>
>     template <class I>
>     vector<T>::choose_init(I f, I l, int2type<true>)
>     {
>         size_type sz = static_cast<size_type>(f);
>         value_type v = static_cast<value_type>(l);
>         // construct with sz,v
>     }

Label this solution 'B'.

Both of these solutions solve the case the standard specifically mentions:

    vector<int> v(10, 1);  // ok, vector size 10, initialized to 1

However, (and here is the problem), the two solutions have different behavior in some cases where the value_type of the sequence is not an integral type. For example consider:

>          pair<char, char>                     p('a', 'b');
>          vector<vector<pair<char, char> > >   d('a', 'b');

The second line of this snippet is likely an error. Solution A catches the error and refuses to compile. The reason is that there is no specialization of the member template constructor that looks like:

    template <>
    template <>
    vector<vector<pair<char, char> > >::vector(char, char) { ... }

So the expression binds to the unspecialized member template constructor, and then fails (compile time) because char is not an InputIterator.

Solution B compiles the above example though. 'a' is casted to an unsigned integral type and used to size the outer vector. 'b' is static casted to the inner vector using it's explicit constructor:

    explicit vector(size_type n);

and so you end up with a static_cast<size_type>('a') by static_cast<size_type>('b') matrix.

It is certainly possible that this is what the coder intended. But the explicit qualifier on the inner vector has been thwarted at any rate.

The standard is not clear whether the expression:

         vector<vector<pair<char, char> > >   d('a', 'b');

(and similar expressions) are:

1. undefined behavior.
2. illegal and must be rejected.
3. legal and must be accepted.

My preference is listed in the order presented.

There are still other techniques for implementing the requirements of paragraphs 9-11, namely the "restricted template technique" (e.g. enable_if). This technique is the most compact and easy way of coding the requirements, and has the behavior of #2 (rejects the above expression).

Choosing 1 would allow all implementation techniques I'm aware of. Choosing 2 would allow only solution 'A' and the enable_if technique. Choosing 3 would allow only solution 'B'.

Possible wording for a future standard if we wanted to actively reject the expression above would be to change "static_cast" in paragraphs 9-11 to "implicit_cast" where that is defined by:

>     template <class T, class U>
>     inline
>     T implicit_cast(const U& u)
>     {
>          return u;
>     }

Proposed resolution:

Replace 23.2.4 [sequence.reqmts] paragraphs 9 - 11 with:

For every sequence defined in this clause and in clause lib.strings:

- If the constructor

             template <class InputIterator>
             X(InputIterator f, InputIterator l,
               const allocator_type& a = allocator_type())

  is called with a type InputIterator that does not qualify as an input iterator, then the constructor will behave as if the overloaded constructor:

             X(size_type, const value_type& = value_type(),
               const allocator_type& = allocator_type())

  were called instead, with the arguments static_cast<size_type>(f), l and a, respectively.

- If the member functions of the forms:

             template <class InputIterator>          //  such as  insert()
             rt fx1(iterator p, InputIterator f, InputIterator l);

             template <class InputIterator>          //  such as  append(), assign()
             rt fx2(InputIterator f, InputIterator l);

             template <class InputIterator>          //  such as  replace()
             rt fx3(iterator i1, iterator i2, InputIterator f, InputIterator l);

  are called with a type InputIterator that does not qualify as an input iterator, then these functions will behave as if the overloaded member functions:

             rt fx1(iterator, size_type, const value_type&);

             rt fx2(size_type, const value_type&);

             rt fx3(iterator, iterator, size_type, const value_type&);

  were called instead, with the same arguments.

In the previous paragraph the alternative binding will fail if f is not implicitly convertible to X::size_type or if l is not implicitly convertible to X::value_type.

The extent to which an implementation determines that a type cannot be an input iterator is unspecified, except that as a minimum integral types shall not qualify as input iterators.

[ Kona: agreed that the current standard requires `v('a', 'b')` to be accepted, and also agreed that this is surprising behavior. The LWG considered several options, including something like implicit_cast, which doesn't appear to be quite what we want. We considered Howards three options: allow acceptance or rejection, require rejection as a compile time error, and require acceptance. By straw poll (1-6-1), we chose to require a compile time error. Post-Kona: Howard provided wording. ]

[ Sydney: The LWG agreed with this general direction, but there was some discomfort with the wording in the original proposed resolution. Howard submitted new wording, and we will review this again in Redmond. ]

[Redmond: one very small change in wording: the first argument is cast to size_t. This fixes the problem of something like `vector<vector<int> >(5, 5)`, where int is not implicitly convertible to the value type.]

Rationale:

The proposed resolution fixes:

      vector<int> v(10, 1);

since as integral types 10 and 1 must be disqualified as input iterators and therefore the (size,value) constructor is called (as if).

The proposed resolution breaks:

      vector<vector<T> > v(10, 1);

because the integral type 1 is not *implicitly* convertible to vector<T>. The wording above requires a diagnostic.

The proposed resolution leaves the behavior of the following code unspecified.

      struct A
      {
        operator int () const {return 10;}
      };

      struct B
      {
        B(A) {}
      };

      vector<B> v(A(), A());

The implementation may or may not detect that A is not an input iterator and employee the (size,value) constructor. Note though that in the above example if the B(A) constructor is qualified explicit, then the implementation must reject the constructor as A is no longer implicitly convertible to B.

### 441(i). Is fpos::state const?

Section: 31.5.3 [fpos] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In section 31.5.3.2 [fpos.members] fpos<stateT>::state() is declared non const, but in section 31.5.3 [fpos] it is declared const.

Proposed resolution:

In section 31.5.3.2 [fpos.members], change the declaration of `fpos<stateT>::state()` to const.

### 442(i). sentry::operator bool() inconsistent signature

Section: 31.7.6.2.4 [ostream.sentry] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-18 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

In section [ostream::sentry] paragraph 4, in description part basic_ostream<charT, traits>::sentry::operator bool() is declared as non const, but in section 27.6.2.3, in synopsis it is declared const.

Proposed resolution:

In section [ostream::sentry] paragraph 4, change the declaration of `sentry::operator bool()` to const.

### 443(i). filebuf::close() inconsistent use of EOF

Section: 31.10.3.4 [filebuf.members] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In section 31.10.3.4 [filebuf.members] par6, in effects description of basic_filebuf<charT, traits>::close(), overflow(EOF) is used twice; should be overflow(traits::eof()).

Proposed resolution:

Change overflow(EOF) to overflow(traits::eof()).

### 444(i). Bad use of casts in fstream

Section: 31.10 [file.streams] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-20 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

31.10.4.4 [ifstream.members] p1, 31.10.5.4 [ofstream.members] p1, 31.10.6.4 [fstream.members] p1 seems have same problem as exposed in LWG issue 252(i).

Proposed resolution:

[Sydney: Genuine defect. 27.8.1.13 needs a cast to cast away constness. The other two places are stylistic: we could change the C-style casts to const_cast. Post-Sydney: Howard provided wording. ]

Change 27.8.1.7/1 from:

> Returns: (basic_filebuf<charT,traits>*)&sb.

to:

> Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).

Change 27.8.1.10/1 from:

> Returns: (basic_filebuf<charT,traits>*)&sb.

to:

> Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).

Change 27.8.1.13/1 from:

> Returns: &sb.

to:

> Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).

### 445(i). iterator_traits::reference unspecified for some iterator categories

Section: 24.3.2.3 [iterator.traits] Status: CD1 Submitter: Dave Abrahams Opened: 2003-12-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The standard places no restrictions at all on the reference type of input, output, or forward iterators (for forward iterators it only specifies that *x must be value_type& and doesn't mention the reference type). Bidirectional iterators' reference type is restricted only by implication, since the base iterator's reference type is used as the return type of reverse_iterator's operator*, which must be T& in order to be a conforming forward iterator.

Here's what I think we ought to be able to expect from an input or forward iterator's reference type R, where a is an iterator and V is its value_type

- *a is convertible to R
- R is convertible to V
- static_cast<V>(static_cast<R>(*a)) is equivalent to static_cast<V>(*a)

A mutable forward iterator ought to satisfy, for x of type V:

          { R r = *a; r = x; } is equivalent to *a = x;

I think these requirements capture existing container iterators (including vector<bool>'s), but render istream_iterator invalid; its reference type would have to be changed to a constant reference.

(Jeremy Siek) During the discussion in Sydney, it was felt that a simpler long term solution for this was needed. The solution proposed was to require `reference` to be the same type as `*a` and `pointer` to be the same type as `a->`. Most iterators in the Standard Library already meet this requirement. Some iterators are output iterators, and do not need to meet the requirement, and others are only specified through the general iterator requirements (which will change with this resolution). The sole case where there is an explicit definition of the reference type that will need to change is `istreambuf_iterator` which returns `charT` from `operator*` but has a reference type of `charT&`. We propose changing the reference type of `istreambuf_iterator` to `charT`.

The other option for resolving the issue with `pointer`, mentioned in the note below, is to remove `pointer` altogether. I prefer placing requirements on `pointer` to removing it for two reasons. First, `pointer` will become useful for implementing iterator adaptors and in particular, `reverse_iterator` will become more well defined. Second, removing `pointer` is a rather drastic and publicly-visible action to take.

The proposed resolution technically enlarges the requirements for iterators, which means there are existing iterators (such as `istreambuf_iterator`, and potentially some programmer-defined iterators) that will no longer meet the requirements. Will this break existing code? The scenario in which it would is if an algorithm implementation (say in the Standard Library) is changed to rely on `iterator_traits::reference`, and then is used with one of the iterators that do not have an appropriately defined `iterator_traits::reference`.

The proposed resolution makes one other subtle change. Previously, it was required that output iterators have a `difference_type` and `value_type` of `void`, which means that a forward iterator could not be an output iterator. This is clearly a mistake, so I've changed the wording to say that those types may be `void`.

Proposed resolution:

In 24.3.2.3 [iterator.traits], after:

> be defined as the iterator's difference type, value type and iterator category, respectively.

add

> In addition, the types

>     iterator_traits<Iterator>::reference
>     iterator_traits<Iterator>::pointer

> must be defined as the iterator's reference and pointer types, that is, the same type as the type of `*a` and `a->`, respectively.

In 24.3.2.3 [iterator.traits], change:

> In the case of an output iterator, the types

>     iterator_traits<Iterator>::difference_type
>     iterator_traits<Iterator>::value_type

> are both defined as `void`.

to:

> In the case of an output iterator, the types

>     iterator_traits<Iterator>::difference_type
>     iterator_traits<Iterator>::value_type
>     iterator_traits<Iterator>::reference
>     iterator_traits<Iterator>::pointer

> may be defined as `void`.

In 24.6.4 [istreambuf.iterator], change:

>     typename traits::off_type, charT*, charT&>

to:

>     typename traits::off_type, charT*, charT>

[ Redmond: there was concern in Sydney that this might not be the only place where things were underspecified and needed to be changed. Jeremy reviewed iterators in the standard and confirmed that nothing else needed to be changed. ]

### 448(i). Random Access Iterators over abstract classes

Section: 24.3.5.7 [random.access.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2004-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 76, the random access iterator requirement table, says that the return type of a[n] must be "convertible to T". When an iterator's value_type T is an abstract class, nothing is convertible to T. Surely this isn't an intended restriction?

Proposed resolution:

Change the return type to "convertible to T const&".

### 449(i). Library Issue 306 Goes Too Far

Section: 17.2 [support.types] Status: CD1 Submitter: Pete Becker Opened: 2004-01-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Original text:

> The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9). The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."

Revised text:

> "If type is not a POD structure or a POD union the results are undefined."

Looks to me like the revised text should have replaced only the second sentence. It doesn't make sense standing alone.

Proposed resolution:

Change 18.1, paragraph 5, to:

> The macro offsetof accepts a restricted set of type arguments in this International Standard. If type is not a POD structure or a POD union the results are undefined. The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."

### 453(i). basic_stringbuf::seekoff need not always fail for an empty stream

Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

      pos_type basic_stringbuf::seekoff(off_type, ios_base::seekdir,
                                        ios_base::openmode);

is obliged to fail if nothing has been inserted into the stream. This is unnecessary and undesirable. It should be permissible to seek to an effective offset of zero.

[ Sydney: Agreed that this is an annoying problem: seeking to zero should be legal. Bill will provide wording. ]

Proposed resolution:

Change the sentence from:

> For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer, the positioning operation fails.

to:

> For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails.

### 455(i). cerr::tie() and wcerr::tie() are overspecified

Section: 31.4 [iostream.objects] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Both cerr::tie() and wcerr::tie() are obliged to be null at program startup. This is overspecification and overkill. It is both traditional and useful to tie cerr to cout, to ensure that standard output is drained whenever an error message is written. This behavior should at least be permitted if not required. Same for wcerr::tie().

Proposed resolution:

Add to the description of cerr:

> After the object cerr is initialized, cerr.tie() returns &cout. Its state is otherwise the same as required for basic_ios<char>::init (lib.basic.ios.cons).

Add to the description of wcerr:

> After the object wcerr is initialized, wcerr.tie() returns &wcout. Its state is otherwise the same as required for basic_ios<wchar_t>::init (lib.basic.ios.cons).

[Sydney: straw poll (3-1): we should require, not just permit, cout and cerr to be tied on startup. Pre-Redmond: Bill will provide wording.]

### 456(i). Traditional C header files are overspecified

Section: 16.4.2.3 [headers] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The C++ Standard effectively requires that the traditional C headers (of the form <xxx.h>) be defined in terms of the newer C++ headers (of the form <cxxx>). Clauses 17.4.1.2/4 and D.5 combine to require that:

- Including the header <cxxx> declares a C name in namespace std.
- Including the header <xxx.h> declares a C name in namespace std (effectively by including <cxxx>), then imports it into the global namespace with an individual using declaration.

The rules were left in this form despited repeated and heated objections from several compiler vendors. The C headers are often beyond the direct control of C++ implementors. In some organizations, it's all they can do to get a few #ifdef __cplusplus tests added. Third-party library vendors can perhaps wrap the C headers. But neither of these approaches supports the drastic restructuring required by the C++ Standard. As a result, it is still widespread practice to ignore this conformance requirement, nearly seven years after the committee last debated this topic. Instead, what is often implemented is:

- Including the header <xxx.h> declares a C name in the global namespace.
- Including the header <cxxx> declares a C name in the global namespace (effectively by including <xxx.h>), then imports it into namespace std with an individual using declaration.

The practical benefit for implementors with the second approach is that they can use existing C library headers, as they are pretty much obliged to do. The practical cost for programmers facing a mix of implementations is that they have to assume weaker rules:

- If you want to assuredly declare a C name in the global namespace, include <xxx.h>. You may or may not also get the declaration in namespace std.
- If you want to assuredly declare a C name in namespace std, include <cxxx>. You may or may not also get the declaration in the global namespace.

There also exists the possibility of subtle differences due to Koenig lookup, but there are so few non-builtin types defined in the C headers that I've yet to see an example of any real problems in this area.

It is worth observing that the rate at which programmers fall afoul of these differences has remained small, at least as measured by newsgroup postings and our own bug reports. (By an overwhelming margin, the commonest problem is still that programmers include <string> and can't understand why the typename string isn't defined -- this a decade after the committee invented namespace std, nominally for the benefit of all programmers.)

We should accept the fact that we made a serious mistake and rectify it, however belatedly, by explicitly allowing either of the two schemes for declaring C names in headers.

[Sydney: This issue has been debated many times, and will certainly have to be discussed in full committee before any action can be taken. However, the preliminary sentiment of the LWG was in favor of the change. (6 yes, 0 no, 2 abstain) Robert Klarer suggests that we might also want to undeprecate the C-style `.h` headers.]

Proposed resolution:

Add to 16.4.2.3 [headers], para. 4:

> Except as noted in clauses 18 through 27 and Annex D, the contents of each header cname shall be the same as that of the corresponding header name.h, as specified in ISO/IEC 9899:1990 Programming Languages C (Clause 7), or ISO/IEC:1990 Programming Languages-C AMENDMENT 1: C Integrity, (Clause 7), as appropriate, as if by inclusion. In the C++ Standard Library, however, the declarations and definitions (except for names which are defined as macros in C) are within namespace scope (3.3.5) of the namespace std. It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations (9.10 [namespace.udecl]).

Change [depr.c.headers], para. 2-3:

> -2- Every C header, each of which has a name of the form name.h, behaves as if each name placed in the Standard library namespace by the corresponding cname header is also placed within the global namespace scope. of the namespace `std` and is followed by an explicit using-declaration (9.10 [namespace.udecl]). It is unspecified whether these names are first declared or defined within namespace scope (6.4.6 [basic.scope.namespace]) of the namespace `std` and are then injected into the global namespace scope by explicit using-declarations (9.10 [namespace.udecl]).

> -3- [Example: The header `<cstdlib>` assuredly provides its declarations and definitions within the namespace `std`. It may also provide these names within the global namespace. The header `<stdlib.h>` makes these available also in assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespace `std`. -- end example]

### 457(i). bitset constructor: incorrect number of initialized bits

Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Dag Henriksson Opened: 2004-01-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The constructor from unsigned long says it initializes "the first M bit positions to the corresponding bit values in val. M is the smaller of N and the value CHAR_BIT * sizeof(unsigned long)."

Object-representation vs. value-representation strikes again. CHAR_BIT * sizeof (unsigned long) does not give us the number of bits an unsigned long uses to hold the value. Thus, the first M bit position above is not guaranteed to have any corresponding bit values in val.

Proposed resolution:

In 22.9.2.2 [bitset.cons] paragraph 2, change "M is the smaller of N and the value CHAR_BIT * sizeof (unsigned long). (249)" to "`M` is the smaller of `N` and the number of bits in the value representation (section 6.9 [basic.types]) of `unsigned long`."

### 460(i). Default modes missing from basic_fstream member specifications

Section: 31.10 [file.streams] Status: CD1 Submitter: Ben Hutchings Opened: 2004-04-01 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

The second parameters of the non-default constructor and of the open member function for basic_fstream, named "mode", are optional according to the class declaration in 27.8.1.11 [lib.fstream]. The specifications of these members in 27.8.1.12 [lib.fstream.cons] and 27.8.1.13 lib.fstream.members] disagree with this, though the constructor declaration has the "explicit" function-specifier implying that it is intended to be callable with one argument.

Proposed resolution:

In 31.10.6.2 [fstream.cons], change

      explicit basic_fstream(const char* s, ios_base::openmode mode);

to

      explicit basic_fstream(const char* s,
                             ios_base::openmode mode = ios_base::in|ios_base::out);

In 31.10.6.4 [fstream.members], change

      void open(const char*s, ios_base::openmode mode);

to

      void open(const char*s,
                ios_base::openmode mode = ios_base::in|ios_base::out);

### 461(i). time_get hard or impossible to implement

Section: 28.3.4.6.2.3 [locale.time.get.virtuals] Status: CD1 Submitter: Bill Plauger Opened: 2004-03-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Template time_get currently contains difficult, if not impossible, requirements for do_date_order, do_get_time, and do_get_date. All require the implementation to scan a field generated by the %x or %X conversion specifier in strftime. Yes, do_date_order can always return no_order, but that doesn't help the other functions. The problem is that %x can be nearly anything, and it can vary widely with locales. It's horribly onerous to have to parse "third sunday after Michaelmas in the year of our Lord two thousand and three," but that's what we currently ask of do_get_date. More practically, it leads some people to think that if %x produces 10.2.04, we should know to look for dots as separators. Still not easy.

Note that this is the opposite effect from the intent stated in the footnote earlier in this subclause:

> "In other words, user confirmation is required for reliable parsing of user-entered dates and times, but machine-generated formats can be parsed reliably. This allows parsers to be aggressive about interpreting user variations on standard formats."

We should give both implementers and users an easier and more reliable alternative: provide a (short) list of alternative delimiters and say what the default date order is for no_order. For backward compatibility, and maximum latitude, we can permit an implementation to parse whatever %x or %X generates, but we shouldn't require it.

Proposed resolution:

In the description:

    iter_type do_get_time(iter_type s, iter_type end, ios_base& str,
            ios_base::iostate& err, tm* t) const;

2 Effects: Reads characters starting at suntil it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce the format specified by 'X', or until it encounters an error or end of sequence.

change: 'X'

to: "%H:%M:%S"

Change

    iter_type do_get_date(iter_type s, iter_type end, ios_base& str,
            ios_base::iostate& err, tm* t) const;

    4 Effects: Reads characters starting at s until it has extracted those
    struct tm members, and remaining format characters, used by
    time_put<>::put to produce the format specified by 'x', or until it
    encounters an error.

to

    iter_type do_get_date(iter_type s, iter_type end, ios_base& str,
            ios_base::iostate& err, tm* t) const;

4 Effects: Reads characters starting at s until it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce one of the following formats, or until it encounters an error. The format depends on the value returned by date_order() as follows:

            date_order()  format

            no_order      "%m/%d/%y"
            dmy           "%d/%m/%y"
            mdy           "%m/%d/%y"
            ymd           "%y/%m/%d"
            ydm           "%y/%d/%m"

An implementation may also accept additional implementation-defined formats.

[Redmond: agreed that this is a real problem. The solution is probably to match C99's parsing rules. Bill provided wording. ]

### 464(i). Suggestion for new member functions in standard containers

Section: 23.3.13 [vector], 23.4.3 [map] Status: CD1 Submitter: Thorsten Ottosen Opened: 2004-05-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [vector].

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Discussion:

To add slightly more convenience to vector<T> and map<Key,T> we should consider to add

1. add vector<T>::data() member (const and non-const version) semantics: if( empty() ) return 0; else return buffer_;
2. add map<Key,T>::at( const Key& k ) member (const and non-const version) semantics: iterator i = find( k ); if( i != end() ) return *i; else throw range_error();

Rationale:

- To obtain a pointer to the vector's buffer, one must use either operator[]() (which can give undefined behavior for empty vectors) or at() (which will then throw if the vector is empty).
- tr1::array<T,sz> already has a data() member
- e cannot use operator[]() when T is not DefaultDonstructible
- Neither when the map is const.
- when we want to make sure we don't add an element accidently
- when it should be considered an error if a key is not in the map

Proposed resolution:

In 23.3.13 [vector], add the following to the `vector` synopsis after "element access" and before "modifiers":

      // [lib.vector.data] data access
      pointer       data();
      const_pointer data() const;

Add a new subsection of 23.3.13 [vector]:

> 23.2.4.x `vector` data access

>        pointer       data();
>        const_pointer data() const;

> Returns: A pointer such that [data(), data() + size()) is a valid range. For a non-empty vector, data() == &front().

> Complexity: Constant time.

> Throws: Nothing.

In 23.4.3 [map], add the following to the `map` synopsis immediately after the line for operator[]:

      T&       at(const key_type& x);
      const T& at(const key_type& x) const;

Add the following to 23.4.3.3 [map.access]:

>       T&       at(const key_type& x);
>       const T& at(const key_type& x) const;

> Returns: A reference to the element whose key is equivalent to x, if such an element is present in the map.

> Throws: `out_of_range` if no such element is present.

Rationale:

Neither of these additions provides any new functionality but the LWG agreed that they are convenient, especially for novices. The exception type chosen for `at`, `std::out_of_range`, was chosen to match `vector::at`.

### 465(i). Contents of <ciso646>

Section: 16.4.2.3 [headers] Status: CD1 Submitter: Steve Clamage Opened: 2004-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [headers].

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Discussion:

C header <iso646.h> defines macros for some operators, such as not_eq for !=.

Section 16.4.2.3 [headers] "Headers" says that except as noted in clauses 18 through 27, the <cname> C++ header contents are the same as the C header <name.h>. In particular, table 12 lists <ciso646> as a C++ header.

I don't find any other mention of <ciso646>, or any mention of <iso646.h>, in clauses 17 thorough 27. That implies that the contents of <ciso646> are the same as C header <iso646.h>.

Annex C (informative, not normative) in [diff.header.iso646.h] C.2.2.2 "Header <iso646.h>" says that the alternative tokens are not defined as macros in <ciso646>, but does not mention the contents of <iso646.h>.

I don't find any normative text to support C.2.2.2.

Proposed resolution:

Add to section 17.4.1.2 Headers [lib.headers] a new paragraph after paragraph 6 (the one about functions must be functions):

> Identifiers that are keywords or operators in C++ shall not be defined as macros in C++ standard library headers. [Footnote:In particular, including the standard header <iso646.h> or <ciso646> has no effect.

[post-Redmond: Steve provided wording.]

### 467(i). char_traits::lt(), compare(), and memcmp()

Section: 27.2.4.2 [char.traits.specializations.char] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [char.traits.specializations.char].

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Discussion:

Table 37 describes the requirements on Traits::compare() in terms of those on Traits::lt(). 21.1.3.1, p6 requires char_traits<char>::lt() to yield the same result as operator<(char, char).

Most, if not all, implementations of char_traits<char>::compare() call memcmp() for efficiency. However, the C standard requires both memcmp() and strcmp() to interpret characters under comparison as unsigned, regardless of the signedness of char. As a result, all these char_traits implementations fail to meet the requirement imposed by Table 37 on compare() when char is signed.

Read email thread starting with c++std-lib-13499 for more.

Proposed resolution:

Change 21.1.3.1, p6 from

> The two-argument members assign, eq, and lt are defined identically to the built-in operators =, ==, and < respectively.

to

> The two-argument member assign is defined identically to the built-in operator =. The two argument members eq and lt are defined identically to the built-in operators == and < for type unsigned char.

[Redmond: The LWG agreed with this general direction, but we also need to change `eq` to be consistent with this change. Post-Redmond: Martin provided wording.]

### 468(i). unexpected consequences of ios_base::operator void*()

Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iostate.flags].

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Discussion:

The program below is required to compile but when run it typically produces unexpected results due to the user-defined conversion from std::cout or any object derived from basic_ios to void*.

        #include <cassert>
        #include <iostream>

        int main ()
        {
            assert (std::cin.tie () == std::cout);
            // calls std::cout.ios::operator void*()
        }

Proposed resolution:

Replace std::basic_ios<charT, traits>::operator void*() with another conversion operator to some unspecified type that is guaranteed not to be convertible to any other type except for bool (a pointer-to-member might be one such suitable type). In addition, make it clear that the pointer type need not be a pointer to a complete type and when non-null, the value need not be valid.

Specifically, change in [lib.ios] the signature of

        operator void*() const;

to

        operator unspecified-bool-type() const;

and change [lib.iostate.flags], p1 from

        operator void*() const;

to

    operator unspecified-bool-type() const;

         -1- Returns: if fail() then a value that will evaluate false in a
          boolean context; otherwise a value that will evaluate true in a
          boolean context. The value type returned shall not be
          convertible to int.

         -2- [Note: This conversion can be used in contexts where a bool
          is expected (e.g., an if condition); however, implicit
          conversions (e.g., to int) that can occur with bool are not
          allowed, eliminating some sources of user error. One possible
          implementation choice for this type is pointer-to-member.  - end
          note]

[Redmond: 5-4 straw poll in favor of doing this.]

[Lillehammer: Doug provided revised wording for "unspecified-bool-type".]

### 469(i). vector<bool> ill-formed relational operators

Section: 23.3.13 [vector] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [vector].

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Discussion:

The overloads of relational operators for vector<bool> specified in [lib.vector.bool] are redundant (they are semantically identical to those provided for the vector primary template) and may even be diagnosed as ill-formed (refer to Daveed Vandevoorde's explanation in c++std-lib-13647).

Proposed resolution:

Remove all overloads of overloads of relational operators for vector<bool> from [lib.vector.bool].

### 471(i). result of `what()` implementation-defined

Section: 17.9.3 [exception] Status: C++11 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [exception].

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Discussion:

[lib.exception] specifies the following:

        exception (const exception&) throw();
        exception& operator= (const exception&) throw();

        -4- Effects: Copies an exception object.
        -5- Notes: The effects of calling what() after assignment
            are implementation-defined.

First, does the Note only apply to the assignment operator? If so, what are the effects of calling what() on a copy of an object? Is the returned pointer supposed to point to an identical copy of the NTBS returned by what() called on the original object or not?

Second, is this Note intended to extend to all the derived classes in section 19? I.e., does the standard provide any guarantee for the effects of what() called on a copy of any of the derived class described in section 19?

Finally, if the answer to the first question is no, I believe it constitutes a defect since throwing an exception object typically implies invoking the copy ctor on the object. If the answer is yes, then I believe the standard ought to be clarified to spell out exactly what the effects are on the copy (i.e., after the copy ctor was called).

[Redmond: Yes, this is fuzzy. The issue of derived classes is fuzzy too.]

[ Batavia: Howard provided wording. ]

[ Bellevue: ]

> Eric concerned this is unimplementable, due to nothrow guarantees. Suggested implementation would involve reference counting.

> Is the implied reference counting subtle enough to call out a note on implementation? Probably not.

> If reference counting required, could we tighten specification further to require same pointer value? Probably an overspecification, especially if exception classes defer evalutation of final string to calls to what().

> Remember issue moved open and not resolved at Batavia, but cannot remember who objected to canvas a disenting opinion - please speak up if you disagree while reading these minutes!

> Move to Ready as we are accepting words unmodified.

[ Sophia Antipolis: ]

> The issue was pulled from Ready. It needs to make clear that only homogenous copying is intended to be supported, not coping from a derived to a base.

[ Batavia (2009-05): ]

> Howard supplied the following replacement wording for paragraph 7 of the proposed resolution:

> > -7- Postcondition: `what()` shall return the same NTBS as would be obtained by using `static_cast` to cast the rhs to the same types as the lhs and then calling `what()` on that possibly sliced object.

> Pete asks what "the same NTBS" means.

[ 2009-07-30 Niels adds: ]

> Further discussion in the thread starting with c++std-lib-24512.

[ 2009-09-24 Niels provided updated wording: ]

> I think the resolution should at least guarantee that the result of `what()` is independent of whether the compiler does copy-elision. And for any class derived from `std::excepion` that has a constructor that allows specifying a `what_arg`, it should make sure that the text of a user-provided `what_arg` is preserved, when the object is copied. Note that all the implementations I've tested already appear to satisfy the proposed resolution, including MSVC 2008 SP1, Apache stdcxx-4.2.1, GCC 4.1.2, GCC 4.3.2, and CodeGear C++ 6.13.

> The proposed resolution was updated with help from Daniel Krügler; the update aims to clarify that the proposed postcondition only applies to homogeneous copying.

[ 2009-10 Santa Cruz: ]

> Moved to Ready after inserting "publicly accessible" in two places.

Proposed resolution:

Change 17.9.3 [exception] to:

> -1- The class `exception` defines the base class for the types of objects thrown as exceptions by C++ standard library components, and certain expressions, to report errors detected during program execution.

> Each standard library class `T` that derives from class `exception` shall have a publicly accessible copy constructor and a publicly accessible copy assignment operator that do not exit with an exception. These member functions shall preserve the following postcondition: If two objects lhs and rhs both have dynamic type `T`, and lhs is a copy of rhs, then `strcmp(lhs.what(), rhs.what()) == 0`.

> ...

>     exception(const exception& rhs) throw();
>     exception& operator=(const exception& rhs) throw();

> > -4- Effects: Copies an exception object.

> > -5- Remarks: The effects of calling `what()` after assignment are implementation-defined.

> > -5- Postcondition: If `*this` and rhs both have dynamic type `exception` then `strcmp(what(), rhs.what()) == 0`.

### 473(i). underspecified `ctype` calls

Section: 28.3.4.2.2 [locale.ctype] Status: C++11 Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Most `ctype` member functions come in two forms: one that operates on a single character at a time and another form that operates on a range of characters. Both forms are typically described by a single Effects and/or Returns clause.

The Returns clause of each of the single-character non-virtual forms suggests that the function calls the corresponding single character virtual function, and that the array form calls the corresponding virtual array form. Neither of the two forms of each virtual member function is required to be implemented in terms of the other.

There are three problems:

1. One is that while the standard does suggest that each non-virtual member function calls the corresponding form of the virtual function, it doesn't actually explicitly require it.

Implementations that cache results from some of the virtual member functions for some or all values of their arguments might want to call the array form from the non-array form the first time to fill the cache and avoid any or most subsequent virtual calls. Programs that rely on each form of the virtual function being called from the corresponding non-virtual function will see unexpected behavior when using such implementations.

2. The second problem is that either form of each of the virtual functions can be overridden by a user-defined function in a derived class to return a value that is different from the one produced by the virtual function of the alternate form that has not been overriden.

Thus, it might be possible for, say, `ctype::widen(c)` to return one value, while for `ctype::widen(&c, &c + 1, &wc)` to set `wc` to another value. This is almost certainly not intended. Both forms of every function should be required to return the same result for the same character, otherwise the same program using an implementation that calls one form of the functions will behave differently than when using another implementation that calls the other form of the function "under the hood."

3. The last problem is that the standard text fails to specify whether one form of any of the virtual functions is permitted to be implemented in terms of the other form or not, and if so, whether it is required or permitted to call the overridden virtual function or not.

Thus, a program that overrides one of the virtual functions so that it calls the other form which then calls the base member might end up in an infinite loop if the called form of the base implementation of the function in turn calls the other form.

Lillehammer: Part of this isn't a real problem. We already talk about caching. 22.1.1/6 But part is a real problem. `ctype` virtuals may call each other, so users don't know which ones to override to avoid avoid infinite loops.

This is a problem for all facet virtuals, not just `ctype` virtuals, so we probably want a blanket statement in clause 22 for all facets. The LWG is leaning toward a blanket prohibition, that a facet's virtuals may never call each other. We might want to do that in clause 27 too, for that matter. A review is necessary. Bill will provide wording.

[ 2009-07 Frankfurt, Howard provided wording directed by consensus. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add paragraph 3 to 28.3.4 [locale.categories]:

> -3- Within this clause it is unspecified if one virtual function calls another virtual function.

Rationale:

We are explicitly not addressing bullet item #2, thus giving implementors more latitude. Users will have to override both virtual functions, not just one.

### 474(i). confusing Footnote 297

Section: 31.7.6.3.4 [ostream.inserters.character] Status: CD1 Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ostream.inserters.character].

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Discussion:

I think Footnote 297 is confused. The paragraph it applies to seems quite clear in that widen() is only called if the object is not a char stream (i.e., not basic_ostream<char>), so it's irrelevant what the value of widen(c) is otherwise.

Proposed resolution:

I propose to strike the Footnote.

### 475(i). May the function object passed to for_each modify the elements of the iterated sequence?

Section: 26.6.5 [alg.foreach] Status: CD1 Submitter: Stephan T. Lavavej, Jaakko Jarvi Opened: 2004-07-09 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [alg.foreach].

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Discussion:

It is not clear whether the function object passed to for_each is allowed to modify the elements of the sequence being iterated over.

for_each is classified without explanation in [lib.alg.nonmodifying], "25.1 Non-modifying sequence operations". 'Non-modifying sequence operation' is never defined.

25(5) says: "If an algorithm's Effects section says that a value pointed to by any iterator passed as an argument is modified, then that algorithm has an additional type requirement: The type of that argument shall satisfy the requirements of a mutable iterator (24.1)."

for_each's Effects section does not mention whether arguments can be modified:

> "Effects: Applies f to the result of dereferencing every iterator in the range [first, last), starting from first and proceeding to last - 1."

Every other algorithm in [lib.alg.nonmodifying] is "really" non-modifying in the sense that neither the algorithms themselves nor the function objects passed to the algorithms may modify the sequences or elements in any way. This DR affects only for_each.

We suspect that for_each's classification in "non-modifying sequence operations" means that the algorithm itself does not inherently modify the sequence or the elements in the sequence, but that the function object passed to it may modify the elements it operates on.

The original STL document by Stepanov and Lee explicitly prohibited the function object from modifying its argument. The "obvious" implementation of for_each found in several standard library implementations, however, does not impose this restriction. As a result, we suspect that the use of for_each with function objects that modify their arguments is wide-spread. If the restriction was reinstated, all such code would become non-conforming. Further, none of the other algorithms in the Standard could serve the purpose of for_each (transform does not guarantee the order in which its function object is called).

We suggest that the standard be clarified to explicitly allow the function object passed to for_each modify its argument.

Proposed resolution:

Add a nonnormative note to the Effects in 26.6.5 [alg.foreach]: If the type of 'first' satisfies the requirements of a mutable iterator, 'f' may apply nonconstant functions through the dereferenced iterators passed to it.

Rationale:

The LWG believes that nothing in the standard prohibits function objects that modify the sequence elements. The problem is that for_each is in a secion entitled "nonmutating algorithms", and the title may be confusing. A nonnormative note should clarify that.

### 478(i). Should forward iterator requirements table have a line for r->m?

Section: 24.3.5.5 [forward.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2004-07-11 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [forward.iterators].

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Duplicate of: 477

Discussion:

The Forward Iterator requirements table contains the following:

     expression  return type         operational  precondition
                                      semantics
      ==========  ==================  ===========  ==========================
      a->m        U& if X is mutable, (*a).m       pre: (*a).m is well-defined.
                  otherwise const U&

      r->m        U&                  (*r).m       pre: (*r).m is well-defined.

The second line may be unnecessary. Paragraph 11 of [lib.iterator.requirements] says:

> In the following sections, a and b denote values of type const X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T, o denotes a value of some type that is writable to the output iterator.

Because operators can be overloaded on an iterator's const-ness, the current requirements allow iterators to make many of the operations specified using the identifiers a and b invalid for non-const iterators.

Related issue: 477(i)

Proposed resolution:

Remove the "r->m" line from the Forward Iterator requirements table. Change

> "const X"

to

> "X or const X"

in paragraph 11 of [lib.iterator.requirements].

Rationale:

This is a defect because it constrains an lvalue to returning a modifiable lvalue.

### 482(i). Swapping pairs

Section: 22.3 [pairs], 22.4 [tuple] Status: Resolved Submitter: Andrew Koenig Opened: 2004-09-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [pairs].

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Discussion:

(Based on recent comp.std.c++ discussion)

Pair (and tuple) should specialize std::swap to work in terms of std::swap on their components. For example, there's no obvious reason why swapping two objects of type pair<vector<int>, list<double> > should not take O(1).

[Lillehammer: We agree it should be swappable. Howard will provide wording.]

[ Post Oxford: We got `swap` for `pair` but accidently missed `tuple`. `tuple::swap` is being tracked by 522(i). ]

Proposed resolution:

Wording provided in N1856.

Rationale:

Recommend NADResolved, fixed by N1856.

### 485(i). output iterator insufficiently constrained

Section: 24.3.5.4 [output.iterators] Status: Resolved Submitter: Chris Jefferson Opened: 2004-10-13 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [output.iterators].

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Discussion:

The note on 24.1.2 Output iterators insufficiently limits what can be performed on output iterators. While it requires that each iterator is progressed through only once and that each iterator is written to only once, it does not require the following things:

Note: Here it is assumed that `x` is an output iterator of type `X` which has not yet been assigned to.

a) That each value of the output iterator is written to: The standard allows: `++x; ++x; ++x;`

b) That assignments to the output iterator are made in order `X a(x); ++a; *a=1; *x=2;` is allowed

c) Chains of output iterators cannot be constructed: `X a(x); ++a; X b(a); ++b; X c(b); ++c;` is allowed, and under the current wording (I believe) `x,a,b,c` could be written to in any order.

I do not believe this was the intension of the standard?

[Lillehammer: Real issue. There are lots of constraints we intended but didn't specify. Should be solved as part of iterator redesign.]

[ 2009-07 Frankfurt ]

> Bill provided wording according to consensus.

[ 2009-07-21 Alisdair requests change from Review to Open. See thread starting with c++std-lib-24459 for discussion. ]

[ 2009-10 Santa Cruz: ]

> Modified wording. Set to Review.

[ 2009-10 Santa Cruz: ]

> Move to Ready after looking at again in a larger group in Santa Cruz.

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added below.

Rationale:

Solved by N3066.

Proposed resolution:

Change Table 101 — Output iterator requirements in 24.3.5.4 [output.iterators]:

> Table 101 — Output iterator requirements
> Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
> --- | --- | --- | ---
> `X(a)` |  |  | `a = t` is equivalent to `X(a) = t`. note: a destructor is assumed.
> `X u(a);` `X u = a;` |  |  |
> `*r = o` | result is not used |  | Post: `r` is not required to be dereferenceable. `r` is incrementable.
> `++r` | `X&` |  | `&r == &++r` Post: `r` is dereferenceable, unless otherwise specified. `r` is not required to be incrementable.
> `r++` | convertible to `const X&` | `{X tmp = r; ++r; return tmp;}` | Post: `r` is dereferenceable, unless otherwise specified. `r` is not required to be incrementable.
> `*r++ = o;` | result is not used |  |

### 488(i). rotate throws away useful information

Section: 26.7.11 [alg.rotate] Status: CD1 Submitter: Howard Hinnant Opened: 2004-11-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.rotate].

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Discussion:

rotate takes 3 iterators: first, middle and last which point into a sequence, and rearranges the sequence such that the subrange [middle, last) is now at the beginning of the sequence and the subrange [first, middle) follows. The return type is void.

In many use cases of rotate, the client needs to know where the subrange [first, middle) starts after the rotate is performed. This might look like:

      rotate(first, middle, last);
      Iterator i = advance(first, distance(middle, last));

Unless the iterators are random access, the computation to find the start of the subrange [first, middle) has linear complexity. However, it is not difficult for rotate to return this information with negligible additional computation expense. So the client could code:

      Iterator i = rotate(first, middle, last);

and the resulting program becomes significantly more efficient.

While the backwards compatibility hit with this change is not zero, it is very small (similar to that of lwg 130(i)), and there is a significant benefit to the change.

Proposed resolution:

In 26 [algorithms] p2, change:

>       template<class ForwardIterator>
>         void ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
>                     ForwardIterator last);

In 26.7.11 [alg.rotate], change:

>       template<class ForwardIterator>
>         void ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
>                     ForwardIterator last);

In 26.7.11 [alg.rotate] insert a new paragraph after p1:

> Returns: `first + (last - middle)`.

[ The LWG agrees with this idea, but has one quibble: we want to make sure not to give the impression that the function "advance" is actually called, just that the nth iterator is returned. (Calling advance is observable behavior, since users can specialize it for their own iterators.) Howard will provide wording. ]

[Howard provided wording for mid-meeting-mailing Jun. 2005.]

[ Toronto: moved to Ready. ]

### 495(i). Clause 22 template parameter requirements

Section: 28.3 [localization] Status: CD1 Submitter: Beman Dawes Opened: 2005-01-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [localization].

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Discussion:

It appears that there are no requirements specified for many of the template parameters in clause 22. It looks like this issue has never come up, except perhaps for Facet.

Clause 22 isn't even listed in 17.3.2.1 [lib.type.descriptions], either, which is the wording that allows requirements on template parameters to be identified by name.

So one issue is that 17.3.2.1 [lib.type.descriptions] Should be changed to cover clause 22. A better change, which will cover us in the future, would be to say that it applies to all the library clauses. Then if a template gets added to any library clause we are covered.

charT, InputIterator, and other names with requirements defined elsewhere are fine, assuming the 17.3.2.1 [lib.type.descriptions] fix. But there are a few template arguments names which I don't think have requirements given elsewhere:

- internT and externT. The fix is to add wording saying that internT and externT must meet the same requirements as template arguments named charT.
- stateT. I'm not sure about this one. There already is some wording, but it seems a bit vague.
- Intl. [lib.locale.moneypunct.byname] The fix for this one is to rename "Intl" to "International". The name is important because other text identifies the requirements for the name International but not for Intl.

Proposed resolution:

Change 16.3.3.3 [type.descriptions], paragraph 1, from:

> The Requirements subclauses may describe names that are used to specify constraints on template arguments.153) These names are used in clauses 20, 23, 25, and 26 to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.

to:

> The Requirements subclauses may describe names that are used to specify constraints on template arguments.153) These names are used in library clauses to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.

In the front matter of class 22, locales, add:

> Template parameter types internT and externT shall meet the requirements of charT (described in 27 [strings]).

Rationale:

Again, a blanket clause isn't blanket enough. Also, we've got a couple of names that we don't have blanket requirement statements for. The only issue is what to do about stateT. This wording is thin, but probably adequate.

### 496(i). Illegal use of "T" in vector<bool>

Section: 23.3.13 [vector] Status: CD1 Submitter: richard@ex-parrot.com Opened: 2005-02-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the synopsis of the std::vector<bool> specialisation in 23.3.13 [vector], the non-template assign() function has the signature

      void assign( size_type n, const T& t );

The type, T, is not defined in this context.

Proposed resolution:

Replace "T" with "value_type".

### 497(i). meaning of numeric_limits::traps for floating point types

Section: 17.3.5.2 [numeric.limits.members] Status: CD1 Submitter: Martin Sebor Opened: 2005-03-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

18.2.1.2, p59 says this much about the traps member of numeric_limits:

> static const bool traps;
> -59- true if trapping is implemented for the type.204)
> Footnote 204: Required by LIA-1.

It's not clear what is meant by "is implemented" here.

In the context of floating point numbers it seems reasonable to expect to be able to use traps to determine whether a program can "safely" use infinity(), quiet_NaN(), etc., in arithmetic expressions, that is without causing a trap (i.e., on UNIX without having to worry about getting a signal). When traps is true, I would expect any of the operations in section 7 of IEEE 754 to cause a trap (and my program to get a SIGFPE). So, for example, on Alpha, I would expect traps to be true by default (unless I compiled my program with the -ieee option), false by default on most other popular architectures, including IA64, MIPS, PA-RISC, PPC, SPARC, and x86 which require traps to be explicitly enabled by the program.

Another possible interpretation of p59 is that traps should be true on any implementation that supports traps regardless of whether they are enabled by default or not. I don't think such an interpretation makes the traps member very useful, even though that is how traps is implemented on several platforms. It is also the only way to implement traps on platforms that allow programs to enable and disable trapping at runtime.

Proposed resolution:

Change p59 to read:

> True if, at program startup, there exists a value of the type that would cause an arithmetic operation using that value to trap.

Rationale:

Real issue, since trapping can be turned on and off. Unclear what a static query can say about a dynamic issue. The real advice we should give users is to use cfenv for these sorts of queries. But this new proposed resolution is at least consistent and slightly better than nothing.

### 498(i). Requirements for partition() and stable_partition() too strong

Section: 26.8.5 [alg.partitions] Status: C++11 Submitter: Sean Parent, Joe Gottman Opened: 2005-05-04 Last modified: 2025-03-13

Priority: Not Prioritized

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Discussion:

Problem: The iterator requirements for partition() and stable_partition() [25.2.12] are listed as BidirectionalIterator, however, there are efficient algorithms for these functions that only require ForwardIterator that have been known since before the standard existed. The SGI implementation includes these (see https://www.boost.org/sgi/stl/partition.html and https://www.boost.org/sgi/stl/stable_partition.html).

[ 2009-04-30 Alisdair adds: ]

> Now we have concepts this is easier to express!

> Proposed resolution:

> Add the following signature to:

> Header `<algorithm>` synopsis [algorithms.syn]
> p3 Partitions 26.8.5 [alg.partitions]

> >      template<ForwardIterator Iter, Predicate<auto, Iter::value_type> Pred>
> >        requires ShuffleIterator<Iter>
> >              && CopyConstructible<Pred>
> >        Iter partition(Iter first, Iter last, Pred pred);

> Update p3 Partitions 26.8.5 [alg.partitions]:

> > Complexity: At most `(last - first)/2` swaps. Exactly `last - first` applications of the predicate are done. If `Iter` satisfies `BidirectionalIterator`, at most `(last - first)/2` swaps. Exactly `last - first` applications of the predicate are done.

> > If `Iter` merely satisfied `ForwardIterator` at most `(last - first)` swaps are done. Exactly `(last - first)` applications of the predicate are done.

> [Editorial note: I looked for existing precedent in how we might call out distinct overloads overloads from a set of constrained templates, but there is not much existing practice to lean on. advance/distance were the only algorithms I could find, and that wording is no clearer.]

[ 2009-07 Frankfurt ]

> Hinnant: if you want to partition your std::forward_list, you'll need partition() to accept ForwardIterators.

> No objection to Ready.

> Move to Ready.

Proposed resolution:

Change 25.2.12 from

>     template<class BidirectionalIterator, class Predicate>
>     BidirectionalIterator partition(BidirectionalIterato r first,
>                                     BidirectionalIterator last,
>                                     Predicate pred);

to

>     template<class ForwardIterator, class Predicate>
>     ForwardIterator partition(ForwardIterator first,
>                               ForwardIterator last,
>                               Predicate pred);

Change the complexity from

> At most (last - first)/2 swaps are done. Exactly (last - first) applications of the predicate are done.

to

> If ForwardIterator is a bidirectional_iterator, at most (last - first)/2 swaps are done; otherwise at most (last - first) swaps are done. Exactly (last - first) applications of the predicate are done.

Rationale:

Partition is a "foundation" algorithm useful in many contexts (like sorting as just one example) - my motivation for extending it to include forward iterators is foward_list - without this extension you can't partition an foward_list (without writing your own partition). Holes like this in the standard library weaken the argument for generic programming (ideally I'd be able to provide a library that would refine std::partition() to other concepts without fear of conflicting with other libraries doing the same - but that is a digression). I consider the fact that partition isn't defined to work for ForwardIterator a minor embarrassment.

[Mont Tremblant: Moved to Open, request motivation and use cases by next meeting. Sean provided further rationale by post-meeting mailing.]

### 505(i). Result_type in random distribution requirements

Section: 29.5.3 [rand.req], 99 [tr.rand.req] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 17: Random distribution requirements

Row 1 requires that each random distribution provide a nested type "input_type"; this type denotes the type of the values that the distribution consumes.

Inspection of all distributions in [tr.rand.dist] reveals that each distribution provides a second typedef ("result_type") that denotes the type of the values the distribution produces when called.

Proposed resolution:

It seems to me that this is also a requirement for all distributions and should therefore be indicated as such via a new second row to this table 17:

X::result_type | T | --- | compile-time

[ Berlin: Voted to WP. N1932 adopts the proposed resolution: see Table 5 row 1. ]

### 507(i). Missing requirement for variate_generator::operator()

Section: 29.5 [rand], 99 [tr.rand.var] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 11 of [tr.rand.var] equires that the member template

>     template<class T> result_type operator() (T value);

return

>     distribution()(e, value)

However, not all distributions have an operator() with a corresponding signature.

[ Berlin: As a working group we voted in favor of N1932 which makes this moot: variate_generator has been eliminated. Then in full committee we voted to give this issue WP status (mistakenly). ]

Proposed resolution:

We therefore recommend that we insert the following precondition before paragraph 11:

> Precondition: `distribution().operator()(e,value)` is well-formed.

### 508(i). Bad parameters for ranlux64_base_01

Section: 29.5.6 [rand.predef], 99 [tr.rand.predef] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The fifth of these engines with predefined parameters, ranlux64_base_01, appears to have an unintentional error for which there is a simple correction. The two pre-defined subtract_with_carry_01 engines are given as:

>     typedef subtract_with_carry_01<float,  24, 10, 24> ranlux_base_01;
>     typedef subtract_with_carry_01<double, 48, 10, 24> ranlux64_base_01;

We demonstrate below that ranlux64_base_01 fails to meet the intent of the random number generation proposal, but that the simple correction to

>     typedef subtract_with_carry_01<double, 48,  5, 12> ranlux64_base_01;

does meet the intent of defining well-known good parameterizations.

The ranlux64_base_01 engine as presented fails to meet the intent for predefined engines, stated in proposal N1398 (section E):

> In order to make good random numbers available to a large number of library users, this proposal not only defines generic random-number engines, but also provides a number of predefined well-known good parameterizations for those.

The predefined ranlux_base_01 engine has been proven [1,2,3] to have a very long period and so meets this criterion. This property makes it suitable for use in the excellent discard_block engines defined subsequently. The proof of long period relies on the fact (proven in [1]) that 2**(w*r) - 2**(w*s) + 1 is prime (w, r, and s are template parameters to subtract_with_carry_01, as defined in [tr.rand.eng.sub1]).

The ranlux64_base_01 engine as presented in [tr.rand.predef] uses w=48, r=24, s=10. For these numbers, the combination 2**(w*r)-2**(w*s)+1 is non-prime (though explicit factorization would be a challenge). In consequence, while it is certainly possible for some seeding states that this engine would have a very long period, it is not at all "well-known" that this is the case. The intent in the N1398 proposal involved the base of the ranlux64 engine, which finds heavy use in the physics community. This is isomorphic to the predefined ranlux_base_01, but exploits the ability of double variables to hold (at least) 48 bits of mantissa, to deliver 48 random bits at a time rather than 24.

Proposed resolution:

To achieve this intended behavior, the correct template parameteriztion would be:

>     typedef subtract_with_carry_01<double, 48, 5, 12> ranlux64_base_01;

The sequence of mantissa bits delivered by this is isomorphic (treating each double as having the bits of two floats) to that delivered by ranlux_base_01.

References:

1. F. James, Comput. Phys. Commun. 60(1990) 329
2. G. Marsaglia and A. Zaman, Ann. Appl. Prob 1(1991) 462
3. M. Luscher, Comput. Phys. Commun. 79(1994) 100-110

[ Berlin: Voted to WP. N1932 adopts the proposed resolution in 26.3.5, just above paragraph 5. ]

### 518(i). Are insert and erase stable for unordered_multiset and unordered_multimap?

Section: 23.2.8 [unord.req], 99 [tr.unord.req] Status: CD1 Submitter: Matt Austern Opened: 2005-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Issue 371 deals with stability of multiset/multimap under insert and erase (i.e. do they preserve the relative order in ranges of equal elements). The same issue applies to unordered_multiset and unordered_multimap.

[ Moved to open (from review): There is no resolution. ]

[ Toronto: We have a resolution now. Moved to Review. Some concern was noted as to whether this conflicted with existing practice or not. An additional concern was in specifying (partial) ordering for an unordered container. ]

Proposed resolution:

Wording for the proposed resolution is taken from the equivalent text for associative containers.

Change 23.2.8 [unord.req], Unordered associative containers, paragraph 6 to:

> An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. `unordered_set` and `unordered_map` support unique keys. `unordered_multiset` and `unordered_multimap` support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other. For `unordered_multiset` and `unordered_multimap`,`insert` and `erase` preserve the relative ordering of equivalent elements.

Change 23.2.8 [unord.req], Unordered associative containers, paragraph 8 to:

> The elements of an unordered associative container are organized into buckets. Keys with the same hash code appear in the same bucket. The number of buckets is automatically increased as elements are added to an unordered associative container, so that the average number of elements per bucket is kept below a bound. Rehashing invalidates iterators, changes ordering between elements, and changes which buckets elements appear in, but does not invalidate pointers or references to elements. For `unordered_multiset` and `unordered_multimap`, rehashing preserves the relative ordering of equivalent elements.

### 519(i). Data() undocumented

Section: 23.3.3 [array], 99 [tr.array.array] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`array<>::data()` is present in the class synopsis, but not documented.

Proposed resolution:

Add a new section, after 6.2.2.3:

>     T*       data()
>     const T* data() const;

Returns: `elems`.

Change 6.2.2.4/2 to:

> In the case where `N == 0`, `begin() == end()`. The return value of `data()` is unspecified.

### 520(i). Result_of and pointers to data members

Section: 22.10.15 [func.bind], 99 [tr.func.bind] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the original proposal for binders, the return type of bind() when called with a pointer to member data as it's callable object was defined to be mem_fn(ptr); when Peter Dimov and I unified the descriptions of the TR1 function objects we hoisted the descriptions of return types into the INVOKE pseudo-function and into result_of. Unfortunately, we left pointer to member data out of result_of, so bind doesn't have any specified behavior when called with a pointer to member data.

Proposed resolution:

[ Pete and Peter will provide wording. ]

In 20.5.4 [lib.func.ret] ([tr.func.ret]) p3 add the following bullet after bullet 2:

1. […]
2. […]
3. If `F` is a member data pointer type `R T::*`, `type` shall be `cv R&` when `T1` is `cv U1&`, `R` otherwise.

[ Peter provided wording. ]

### 521(i). Garbled requirements for argument_type in reference_wrapper

Section: 22.10.6 [refwrap], 99 [tr.util.refwrp.refwrp] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

2.1.2/3, second bullet item currently says that reference_wrapper<T> is derived from unary_function<T, R> if T is:

> a pointer to member function type with cv-qualifier cv and no arguments; the type T1 is cv T* and R is the return type of the pointer to member function;

The type of T1 can't be cv T*, 'cause that's a pointer to a pointer to member function. It should be a pointer to the class that T is a pointer to member of. Like this:

> a pointer to a member function R T0::f() cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*

Similarly, bullet item 2 in 2.1.2/4 should be:

> a pointer to a member function R T0::f(T2) cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*

Proposed resolution:

Change bullet item 2 in 2.1.2/3:

> - a pointer to member function type with cv-qualifier `cv` and no arguments; the type `T1` is `cv T*` and `R` is the return type of the pointer to member function `R T0::f() cv` (where `cv` represents the member function's cv-qualifiers); the type `T1` is `cv T0*`

Change bullet item 2 in 2.1.2/4:

> - a pointer to member function with cv-qualifier `cv` and taking one argument of type `T2`; the type `T1` is `cv T*` and `R` is the return type of the pointer to member function `R T0::f(T2) cv` (where `cv` represents the member function's cv-qualifiers); the type `T1` is `cv T0*`

### 522(i). Tuple doesn't define swap

Section: 22.4 [tuple], 99 [tr.tuple] Status: CD1 Submitter: Andy Koenig Opened: 2005-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Tuple doesn't define swap(). It should.

[ Berlin: Doug to provide wording. ]

[ Batavia: Howard to provide wording. ]

[ Toronto: Howard to provide wording (really this time). ]

[ Bellevue: Alisdair provided wording. ]

Proposed resolution:

Add these signatures to 22.4 [tuple]

>     template <class... Types>
>       void swap(tuple<Types...>& x, tuple<Types...>& y);
>     template <class... Types>
>       void swap(tuple<Types...>&& x, tuple<Types...>& y);
>     template <class... Types>
>       void swap(tuple<Types...>& x, tuple<Types...>&& y);

Add this signature to 22.4.4 [tuple.tuple]

>     void swap(tuple&&);

Add the following two sections to the end of the tuple clauses

> 20.3.1.7 tuple swap [tuple.swap]

>     void swap(tuple&& rhs);

> > Requires: Each type in `Types` shall be `Swappable`.

> > Effects: Calls `swap` for each element in `*this` and its corresponding element in `rhs`.

> > Throws: Nothing, unless one of the element-wise `swap` calls throw an exception.

> 20.3.1.8 tuple specialized algorithms [tuple.special]

>     template <class... Types>
>       void swap(tuple<Types...>& x, tuple<Types...>& y);
>     template <class... Types>
>       void swap(tuple<Types...>&& x, tuple<Types...>& y);
>     template <class... Types>
>       void swap(tuple<Types...>& x, tuple<Types...>&& y);

> > Effects: x.swap(y)

### 524(i). regex named character classes and case-insensitivity don't mix

Section: 28.6 [re] Status: CD1 Submitter: Eric Niebler Opened: 2005-07-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This defect is also being discussed on the Boost developers list. The full discussion can be found here: http://lists.boost.org/boost/2005/07/29546.php

-- Begin original message --

Also, I may have found another issue, closely related to the one under discussion. It regards case-insensitive matching of named character classes. The regex_traits<> provides two functions for working with named char classes: lookup_classname and isctype. To match a char class such as [[:alpha:]], you pass "alpha" to lookup_classname and get a bitmask. Later, you pass a char and the bitmask to isctype and get a bool yes/no answer.

But how does case-insensitivity work in this scenario? Suppose we're doing a case-insensitive match on [[:lower:]]. It should behave as if it were [[:lower:][:upper:]], right? But there doesn't seem to be enough smarts in the regex_traits interface to do this.

Imagine I write a traits class which recognizes [[:fubar:]], and the "fubar" char class happens to be case-sensitive. How is the regex engine to know that? And how should it do a case-insensitive match of a character against the [[:fubar:]] char class? John, can you confirm this is a legitimate problem?

I see two options:

1) Add a bool icase parameter to lookup_classname. Then, lookup_classname( "upper", true ) will know to return lower|upper instead of just upper.

2) Add a isctype_nocase function

I prefer (1) because the extra computation happens at the time the pattern is compiled rather than when it is executed.

-- End original message --

For what it's worth, John has also expressed his preference for option (1) above.

Proposed resolution:

Adopt the proposed resolution in N2409.

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 525(i). type traits definitions not clear

Section: 21.3.5 [meta.unary], 99 [tr.meta.unary] Status: Resolved Submitter: Robert Klarer Opened: 2005-07-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It is not completely clear how the primary type traits deal with cv-qualified types. And several of the secondary type traits seem to be lacking a definition.

[ Berlin: Howard to provide wording. ]

Proposed resolution:

Wording provided in N2028. A revision (N2157) provides more detail for motivation.

Rationale:

Solved by revision (N2157) in the WP.

### 527(i). `tr1::bind` has lost its Throws clause

Section: 22.10.15.4 [func.bind.bind], 99 [tr.func.bind.bind] Status: CD1 Submitter: Peter Dimov Opened: 2005-10-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The original bind proposal gives the guarantee that `tr1::bind(f, t1, ..., tN)` does not throw when the copy constructors of `f, t1, ..., tN` don't.

This guarantee is not present in the final version of TR1.

I'm pretty certain that we never removed it on purpose. Editorial omission? :-)

[ Berlin: not quite editorial, needs proposed wording. ]

[ Batavia: Doug to translate wording to variadic templates. ]

[ Toronto: We agree but aren't quite happy with the wording. The "t"'s no longer refer to anything. Alan to provide improved wording. ]

[ Pre-Bellevue: Alisdair provided wording. ]

TR1 proposed resolution:

> In 99 [tr.func.bind.bind], add a new paragraph after p2:

> > Throws: Nothing unless one of the copy constructors of `f, t1, t2, ..., tN` throws an exception.

> Add a new paragraph after p4:

> > Throws: nothing unless one of the copy constructors of `f, t1, t2, ..., tN` throws an exception.

Proposed resolution:

In 22.10.15.4 [func.bind.bind], add a new paragraph after p2:

> Throws: Nothing unless the copy constructor of `F` or of one of the types in the `BoundArgs...` pack expansion throws an exception.

In 22.10.15.4 [func.bind.bind], add a new paragraph after p4:

> Throws: Nothing unless the copy constructor of `F` or of one of the types in the `BoundArgs...` pack expansion throws an exception.

### 530(i). Must elements of a string be contiguous?

Section: 27.4.3 [basic.string] Status: CD1 Submitter: Matt Austern Opened: 2005-11-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Issue 69(i), which was incorporated into C++03, mandated that the elements of a vector must be stored in contiguous memory. Should the same also apply to `basic_string`?

We almost require contiguity already. Clause 23.4.7 [multiset] defines `operator[]` as `data()[pos]`. What's missing is a similar guarantee if we access the string's elements via the iterator interface.

Given the existence of `data()`, and the definition of `operator[]` and `at` in terms of `data`, I don't believe it's possible to write a useful and standard- conforming `basic_string` that isn't contiguous. I'm not aware of any non-contiguous implementation. We should just require it.

Proposed resolution:

Add the following text to the end of 27.4.3 [basic.string], paragraph 2.

> The characters in a string are stored contiguously, meaning that if `s` is a `basic_string<charT, Allocator>`, then it obeys the identity `&*(s.begin() + n) == &*s.begin() + n` for all `0 <= n < s.size()`.

Rationale:

Not standardizing this existing practice does not give implementors more freedom. We thought it might a decade ago. But the vendors have spoken both with their implementations, and with their voice at the LWG meetings. The implementations are going to be contiguous no matter what the standard says. So the standard might as well give string clients more design choices.

### 531(i). array forms of unformatted input functions

Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2005-11-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The array forms of unformatted input functions don't seem to have well-defined semantics for zero-element arrays in a couple of cases. The affected ones (`istream::get()` and `istream::getline()`) are supposed to terminate when `(n - 1)` characters are stored, which obviously can never be true when `(n == 0)` holds to start with. See c++std-lib-16071.

Proposed resolution:

I suggest changing 27.6.1.3, p7 (`istream::get()`), bullet 1 to read:

- `(n < 1)` is true or `(n - 1)` characters are stored;

Change 27.6.1.3, p9:

> If the function stores no characters, it calls `setstate(failbit)` (which may throw `ios_base::failure` (27.4.4.3)). In any case, if `(n > 0)` is true it then stores a null character into the next successive location of the array.

and similarly p17 (`istream::getline()`), bullet 3 to:

- `(n < 1)` is true or `(n - 1)` characters are stored (in which case the function calls `setstate(failbit)`).

In addition, to clarify that `istream::getline()` must not store the terminating NUL character unless the the array has non-zero size, Robert Klarer suggests in c++std-lib-16082 to change 27.6.1.3, p20 to read:

> In any case, provided `(n > 0)` is true, it then stores a null character (using charT()) into the next successive location of the array.

[ post-Redmond: Pete noticed that the current resolution for `get` requires writing to out of bounds memory when `n == 0`. Martin provided fix. ]

### 533(i). typo in 2.2.3.10/1

Section: 20.3.2.2.11 [util.smartptr.getdeleter], 99 [tr.util.smartptr.getdeleter] Status: CD1 Submitter: Paolo Carlini Opened: 2005-11-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I'm seeing something that looks like a typo. The Return of `get_deleter` says:

> If `*this` owns a deleter `d`...

but `get_deleter` is a free function!

Proposed resolution:

Therefore, I think should be:

> If `*this p` owns a deleter `d`...

### 534(i). Missing basic_string members

Section: 27.4.3 [basic.string] Status: CD1 Submitter: Alisdair Meredith Opened: 2005-11-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

OK, we all know std::basic_string is bloated and already has way too many members. However, I propose it is missing 3 useful members that are often expected by users believing it is a close approximation of the container concept. All 3 are listed in table 71 as 'optional'

i/ pop_back.

This is the one I feel most strongly about, as I only just discovered it was missing as we are switching to a more conforming standard library <g>

I find it particularly inconsistent to support push_back, but not pop_back.

ii/ back.

There are certainly cases where I want to examine the last character of a string before deciding to append, or to trim trailing path separators from directory names etc. *rbegin() somehow feels inelegant.

iii/ front

This one I don't feel strongly about, but if I can get the first two, this one feels that it should be added as a 'me too' for consistency.

I believe this would be similarly useful to the data() member recently added to vector, or at() member added to the maps.

Proposed resolution:

Add the following members to definition of class template basic_string, 21.3p7

>     void pop_back ()

>     const charT & front() const
>     charT & front()

>     const charT & back() const
>     charT & back()

Add the following paragraphs to basic_string description

21.3.4p5

>     const charT & front() const
>     charT & front()

> Precondition: `!empty()`

> Effects: Equivalent to `operator[](0)`.

21.3.4p6

>     const charT & back() const
>     charT & back()

> Precondition: `!empty()`

> Effects: Equivalent to `operator[]( size() - 1)`.

21.3.5.5p10

>     void pop_back ()

> Precondition: `!empty()`

> Effects: Equivalent to `erase( size() - 1, 1 )`.

Update Table 71: (optional sequence operations) Add basic_string to the list of containers for the following operations.

>     a.front()
>     a.back()
>     a.push_back()
>     a.pop_back()
>     a[n]

[ Berlin: Has support. Alisdair provided wording. ]

### 535(i). std::string::swap specification poorly worded

Section: 27.4.3.7.8 [string.swap] Status: CD1 Submitter: Beman Dawes Opened: 2005-12-14 Last modified: 2016-11-12

Priority: Not Prioritized

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Discussion:

std::string::swap currently says for effects and postcondition:

> Effects: Swaps the contents of the two strings.

> Postcondition: `*this` contains the characters that were in `s`, `s` contains the characters that were in `*this`.

Specifying both Effects and Postcondition seems redundant, and the postcondition needs to be made stronger. Users would be unhappy if the characters were not in the same order after the swap.

Proposed resolution:

> Effects: Swaps the contents of the two strings.

> Postcondition: `*this` contains the same sequence of characters that were was in `s`, `s` contains the same sequence of characters that were was in `*this`.

### 537(i). Typos in the signatures in 27.6.1.3/42-43 and 27.6.2.4

Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Paolo Carlini Opened: 2006-02-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the most recent working draft, I'm still seeing:

>     seekg(off_type& off, ios_base::seekdir dir)

and

>     seekp(pos_type& pos)

>     seekp(off_type& off, ios_base::seekdir dir)

that is, by reference off and pos arguments.

Proposed resolution:

After 27.6.1.3p42 change:

>     basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir);

After 27.6.2.4p1 change:

>     basic_ostream<charT,traits>& seekp(pos_type& pos);

After 27.6.2.4p3 change:

>     basic_ostream<charT,traits>& seekp(off_type& off, ios_base::seekdir dir);

### 538(i). 241 again: Does unique_copy() require CopyConstructible and Assignable?

Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I believe I botched the resolution of 241(i) "Does unique_copy() require CopyConstructible and Assignable?" which now has WP status.

This talks about `unique_copy` requirements and currently reads:

> -5- Requires: The ranges `[first, last)` and `[result, result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type of `InputIterator` must be CopyConstructible (20.1.3). Otherwise CopyConstructible is not required.

The problem (which Paolo discovered) is that when the iterators are at their most restrictive (`InputIterator`, `OutputIterator`), then we want `InputIterator::value_type` to be both `CopyConstructible` and `CopyAssignable` (for the most efficient implementation). However this proposed resolution only makes it clear that it is `CopyConstructible`, and that one can assign from `*first` to `*result`. This latter requirement does not necessarily imply that you can:

>     *first = *first;

Proposed resolution:

> -5- Requires: The ranges `[first, last)` and `[result, result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type `value_type` of `InputIterator` must be CopyConstructible (20.1.3) and Assignable. Otherwise CopyConstructible is not required.

### 539(i). `partial_sum` and `adjacent_difference` should mention requirements

Section: 26.10.7 [partial.sum] Status: C++11 Submitter: Marc Schoolderman Opened: 2006-02-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There are some problems in the definition of `partial_sum` and `adjacent_difference` in 26.4 [lib.numeric.ops]

Unlike `accumulate` and `inner_product`, these functions are not parametrized on a "type T", instead, 26.4.3 [lib.partial.sum] simply specifies the effects clause as;

> Assigns to every element referred to by iterator `i` in the range `[result,result + (last - first))` a value correspondingly equal to

> >     ((...(* first + *( first + 1)) + ...) + *( first + ( i - result )))

And similarly for BinaryOperation. Using just this definition, it seems logical to expect that:

>     char i_array[4] = { 100, 100, 100, 100 };
>     int  o_array[4];

>     std::partial_sum(i_array, i_array+4, o_array);

Is equivalent to

>     int o_array[4] = { 100, 100+100, 100+100+100, 100+100+100+100 };

i.e. 100, 200, 300, 400, with addition happening in the `result type`, `int`.

Yet all implementations I have tested produce 100, -56, 44, -112, because they are using an accumulator of the `InputIterator`'s `value_type`, which in this case is `char`, not `int`.

The issue becomes more noticeable when the result of the expression `*i + *(i+1)` or `binary_op(*i, *i-1)` can't be converted to the `value_type`. In a contrived example:

>     enum not_int { x = 1, y = 2 };
>     ...
>     not_int e_array[4] = { x, x, y, y };
>     std::partial_sum(e_array, e_array+4, o_array);

Is it the intent that the operations happen in the `input type`, or in the `result type`?

If the intent is that operations happen in the `result type`, something like this should be added to the "Requires" clause of 26.4.3/4 [lib.partial.sum]:

> The type of `*i + *(i+1)` or `binary_op(*i, *(i+1))` shall meet the requirements of `CopyConstructible` (20.1.3) and `Assignable` (23.1) types.

(As also required for `T` in 26.4.1 [lib.accumulate] and 26.4.2 [lib.inner.product].)

The "auto initializer" feature proposed in N1894 is not required to implement `partial_sum` this way. The 'narrowing' behaviour can still be obtained by using the `std::plus<>` function object.

If the intent is that operations happen in the `input type`, then something like this should be added instead;

> The type of *first shall meet the requirements of `CopyConstructible` (20.1.3) and `Assignable` (23.1) types. The result of `*i + *(i+1)` or `binary_op(*i, *(i+1))` shall be convertible to this type.

The 'widening' behaviour can then be obtained by writing a custom proxy iterator, which is somewhat involved.

In both cases, the semantics should probably be clarified.

26.4.4 [lib.adjacent.difference] is similarly underspecified, although all implementations seem to perform operations in the 'result' type:

>     unsigned char i_array[4] = { 4, 3, 2, 1 };
>     int o_array[4];

>     std::adjacent_difference(i_array, i_array+4, o_array);

o_array is 4, -1, -1, -1 as expected, not 4, 255, 255, 255.

In any case, `adjacent_difference` doesn't mention the requirements on the `value_type`; it can be brought in line with the rest of 26.4 [lib.numeric.ops] by adding the following to 26.4.4/2 [lib.adjacent.difference]:

> The type of `*first` shall meet the requirements of `CopyConstructible` (20.1.3) and `Assignable` (23.1) types."

[ Berlin: Giving output iterator's value_types very controversial. Suggestion of adding signatures to allow user to specify "accumulator". ]

[ Bellevue: ]

> The intent of the algorithms is to perform their calculations using the type of the input iterator. Proposed wording provided.

[ Sophia Antipolis: ]

> We did not agree that the proposed resolution was correct. For example, when the arguments are types `(float*, float*, double*)`, the highest-quality solution would use double as the type of the accumulator. If the intent of the wording is to require that the type of the accumulator must be the `input_iterator`'s `value_type`, the wording should specify it.

[ 2009-05-09 Alisdair adds: ]

> Now that we have the facility, the 'best' accumulator type could probably be deduced as:

> >     std::common_type<InIter::value_type, OutIter::reference>::type

> This type would then have additional requirements of constructability and incrementability/assignability.

> If this extracting an accumulator type from a pair/set of iterators (with additional requirements on that type) is a problem for multiple functions, it might be worth extracting into a SharedAccumulator concept or similar.

> I'll go no further in writing up wording now, until the group gives a clearer indication of preferred direction.

[ 2009-07 Frankfurt ]

> The proposed resolution isn't quite right. For example, "the type of `*first`" should be changed to "`iterator::value_type`" or similar. Daniel volunteered to correct the wording.

[ 2009-07-29 Daniel corrected wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

1. Change 26.10.7 [partial.sum]/1 as indicated:

   > Effects: Let `VT` be `InputIterator`'s value type. For a nonempty range, initializes an accumulator `acc` of type `VT` with `*first` and performs `*result = acc`. For every iterator `i` in `[first + 1, last)` in order, `acc` is then modified by `acc = acc + *i` or `acc = binary_op(acc, *i)` and is assigned to `*(result + (i - first))`. Assigns to every element referred to by iterator `i` in the range `[result,result + (last - first))` a value correspondingly equal to

   > >     ((...(*first + *(first + 1)) + ...) + *(first + (i - result)))

   > or

   > >     binary_op(binary_op(...,
   > >        binary_op(*first, *(first + 1)),...), *(first + (i - result)))

2. Change 26.10.7 [partial.sum]/3 as indicated:

   > Complexity: Exactly `max((last - first) - 1, 0)` applications of `binary_op` the binary operation.

3. Change 26.10.7 [partial.sum]/4 as indicated:

   > Requires: `VT` shall be constructible from the type of `*first`, the result of `acc + *i` or `binary_op(acc, *i)` shall be implicitly convertible to `VT`, and the result of the expression `acc` shall be writable to the `result` output iterator. In the ranges `[first,last]` and `[result,result + (last - first)]` [..]

4. Change 26.10.12 [adjacent.difference]/1 as indicated:

   > Effects: Let `VT` be `InputIterator`'s value type. For a nonempty range, initializes an accumulator `acc` of type `VT` with `*first` and performs `*result = acc`. For every iterator `i` in `[first + 1, last)` in order, initializes a value `val` of type `VT` with `*i`, assigns the result of `val - acc` or `binary_op(val, acc)` to `*(result + (i - first))` and modifies `acc = std::move(val)`. Assigns to every element referred to by iterator `i` in the range `[result + 1, result + (last - first))` a value correspondingly equal to

   > >     *(first + (i - result)) - *(first + (i - result) - 1)

   > or

   > >     binary_op(*(first + (i - result)), *(first + (i - result) - 1)).

   > result gets the value of *first.

5. Change 26.10.12 [adjacent.difference]/2 as indicated:

   > Requires: `VT` shall be `MoveAssignable` ([moveassignable]) and shall be constructible from the type of `*first`. The result of the expression `acc` and the result of the expression `val - acc` or `binary_op(val, acc)` shall be writable to the `result` output iterator. In the ranges `[first,last]` [..]

6. Change 26.10.12 [adjacent.difference]/5 as indicated:

   > Complexity: Exactly `max((last - first) - 1, 0)` applications of `binary_op` the binary operation.

### 540(i). shared_ptr<void>::operator*()

Section: 20.3.2.2.6 [util.smartptr.shared.obs], 99 [tr.util.smartptr.shared.obs] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I'm trying to reconcile the note in tr.util.smartptr.shared.obs, p6 that talks about the operator*() member function of shared_ptr:

> Notes: When T is void, attempting to instantiate this member function renders the program ill-formed. [Note: Instantiating shared_ptr<void> does not necessarily result in instantiating this member function. --end note]

with the requirement in temp.inst, p1:

> The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions...

I assume that what the note is really trying to say is that "instantiating shared_ptr<void> *must not* result in instantiating this member function." That is, that this function must not be declared a member of shared_ptr<void>. Is my interpretation correct?

Proposed resolution:

Change 2.2.3.5p6

> -6- Notes: When `T` is `void`, attempting to instantiate this member function renders the program ill-formed. [Note: Instantiating `shared_ptr<void>` does not necessarily result in instantiating this member function. --end note] it is unspecified whether this member function is declared or not, and if so, what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.

### 541(i). shared_ptr template assignment and void

Section: 20.3.2.2 [util.smartptr.shared], 99 [tr.util.smartptr.shared] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Is the void specialization of the template assignment operator taking a shared_ptr<void> as an argument supposed be well-formed?

I.e., is this snippet well-formed:

>     shared_ptr<void> p;
>     p.operator=<void>(p);

Gcc complains about auto_ptr<void>::operator*() returning a reference to void. I suspect it's because shared_ptr has two template assignment operators, one of which takes auto_ptr, and the auto_ptr template gets implicitly instantiated in the process of overload resolution.

The only way I see around it is to do the same trick with auto_ptr<void> operator*() as with the same operator in shared_ptr<void>.

PS Strangely enough, the EDG front end doesn't mind the code, even though in a small test case (below) I can reproduce the error with it as well.

>     template <class T>
>     struct A { T& operator*() { return *(T*)0; } };

>     template <class T>
>     struct B {
>         void operator= (const B&) { }
>         template <class U>
>         void operator= (const B<U>&) { }
>         template <class U>
>         void operator= (const A<U>&) { }
>     };

>     int main ()
>     {
>         B<void> b;
>         b.operator=<void>(b);
>     }

Proposed resolution:

In [lib.memory] change:

>     template<class X> class auto_ptr;
>     template<> class auto_ptr<void>;

In [lib.auto.ptr]/2 add the following before the last closing brace:

>     template<> class auto_ptr<void>
>     {
>     public:
>         typedef void element_type;
>     };

### 542(i). shared_ptr observers

Section: 20.3.2.2.6 [util.smartptr.shared.obs], 99 [tr.util.smartptr.shared.obs] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Peter Dimov wrote: To: C++ libraries mailing list Message c++std-lib-15614 [...] The intent is for both use_count() and unique() to work in a threaded environment. They are intrinsically prone to race conditions, but they never return garbage.

This is a crucial piece of information that I really wish were captured in the text. Having this in a non-normative note would have made everything crystal clear to me and probably stopped me from ever starting this discussion :) Instead, the sentence in p12 "use only for debugging and testing purposes, not for production code" very strongly suggests that implementations can and even are encouraged to return garbage (when threads are involved) for performance reasons.

How about adding an informative note along these lines:

> Note: Implementations are encouraged to provide well-defined behavior for use_count() and unique() even in the presence of multiple threads.

I don't necessarily insist on the exact wording, just that we capture the intent.

Proposed resolution:

Change 20.3.2.2.6 [util.smartptr.shared.obs] p12:

> [Note: `use_count()` is not necessarily efficient. Use only for debugging and testing purposes, not for production code. --end note]

Change 20.3.2.3.6 [util.smartptr.weak.obs] p3:

> [Note: `use_count()` is not necessarily efficient. Use only for debugging and testing purposes, not for production code. --end note]

### 543(i). valarray slice default constructor

Section: 29.6.4 [class.slice] Status: CD1 Submitter: Howard Hinnant Opened: 2005-11-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

If one explicitly constructs a slice or glice with the default constructor, does the standard require this slice to have any usable state? It says "creates a slice which specifies no elements", which could be interpreted two ways:

1. There are no elements to which the slice refers (i.e. undefined).
2. The slice specifies an array with no elements in it (i.e. defined).

Here is a bit of code to illustrate:

>     #include <iostream>
>     #include <valarray>

>     int main()
>     {
>         std::valarray<int> v(10);
>         std::valarray<int> v2 = v[std::slice()];
>         std::cout << "v[slice()].size() = " << v2.size() << '\n';
>     }

Is the behavior undefined? Or should the output be:

>     v[slice()].size() = 0

There is a similar question and wording for gslice at 26.3.6.1p1.

Proposed resolution:

[Martin suggests removing the second sentence in 29.6.4.2 [cons.slice] as well.]

Change 29.6.4.2 [cons.slice]:

> 1 - The default constructor for `slice` creates a `slice` which specifies no elements. The default constructor is equivalent to `slice(0, 0, 0)`. A default constructor is provided only to permit the declaration of arrays of slices. The constructor with arguments for a slice takes a start, length, and stride parameter.

Change 29.6.6.2 [gslice.cons]:

> 1 - The default constructor creates a `gslice` which specifies no elements. The default constructor is equivalent to `gslice(0, valarray<size_t>(), valarray<size_t>())`. The constructor with arguments builds a `gslice` based on a specification of start, lengths, and strides, as explained in the previous section.

### 545(i). When is a deleter deleted?

Section: 20.3.2.2.11 [util.smartptr.getdeleter], 99 [tr.util.smartptr.shared.dest] Status: CD1 Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The description of ~shared_ptr doesn't say when the shared_ptr's deleter, if any, is destroyed. In principle there are two possibilities: it is destroyed unconditionally whenever ~shared_ptr is executed (which, from an implementation standpoint, means that the deleter is copied whenever the shared_ptr is copied), or it is destroyed immediately after the owned pointer is destroyed (which, from an implementation standpoint, means that the deleter object is shared between instances). We should say which it is.

Proposed resolution:

Add after the first sentence of 20.3.2.2.11 [util.smartptr.getdeleter]/1:

> The returned pointer remains valid as long as there exists a `shared_ptr` instance that owns `d`.

> [Note: it is unspecified whether the pointer remains valid longer than that. This can happen if the implementation doesn't destroy the deleter until all `weak_ptr` instances in the ownership group are destroyed. -- end note]

### 550(i). What should the return type of `pow(float,int)` be?

Section: 29.7 [c.math] Status: CD1 Submitter: Howard Hinnant Opened: 2006-01-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Assuming we adopt the C compatibility package from C99 what should be the return type of the following signature be:

>     ?  pow(float, int);

C++03 says that the return type should be `float`. TR1 and C90/99 say the return type should be `double`. This can put clients into a situation where C++03 provides answers that are not as high quality as C90/C99/TR1. For example:

>     #include <math.h>

>     int main()
>     {
>         float x = 2080703.375F;
>         double y = pow(x, 2);
>     }

Assuming an IEEE 32 bit float and IEEE 64 bit double, C90/C99/TR1 all suggest:

>     y = 4329326534736.390625

which is exactly right. While C++98/C++03 demands:

>     y = 4329326510080.

which is only approximately right.

I recommend that C++0X adopt the mixed mode arithmetic already adopted by Fortran, C and TR1 and make the return type of `pow(float,int)` be `double`.

[ Kona (2007): Other functions that are affected by this issue include `ldexp`, `scalbln`, and `scalbn`. We also believe that there is a typo in 26.7/10: `float nexttoward(float, long double);` [sic] should be `float nexttoward(float, float);` Proposed Disposition: Review (the proposed resolution appears above, rather than below, the heading "Proposed resolution") ]

[Howard, post Kona:]

> Unfortunately I strongly disagree with a part of the resolution from Kona. I am moving from New to Open instead of to Review because I do not believe we have consensus on the intent of the resolution.

> This issue does not include `ldexp`, `scalbln`, and `scalbn` because the second integral parameter in each of these signatures (from C99) is not a generic parameter according to C99 7.22p2. The corresponding C++ overloads are intended (as far as I know) to correspond directly to C99's definition of generic parameter.

> For similar reasons, I do not believe that the second `long double` parameter of `nexttoward`, nor the return type of this function, is in error. I believe the correct signature is:

> >     float nexttoward(float, long double);

> which is what both the C++0X working paper and C99 state (as far as I currently understand).

> This is really only about `pow(float, int)`. And this is because C++98 took one route (with `pow` only) and C99 took another (with many math functions in `<tgmath.h>`. The proposed resolution basically says: C++98 got it wrong and C99 got it right; let's go with C99.

[ Bellevue: ]

> This signature was not picked up from C99. Instead, if one types `pow(2.0f,2)`, the promotion rules will invoke "double pow(double, double)", which generally gives special treatment for integral exponents, preserving full accuracy of the result. New proposed wording provided.

Proposed resolution:

Change 29.7 [c.math] p10:

> The added signatures are:

> >     ...
> >     float pow(float, int);
> >     ...
> >     double pow(double, int);
> >     ...
> >     long double pow(long double, int);

### 551(i). <ccomplex>

Section: 17.15 [support.c.headers] Status: CD1 Submitter: Howard Hinnant Opened: 2006-01-23 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

Previously xxx.h was parsable by C++. But in the case of C99's <complex.h> it isn't. Otherwise we could model it just like <string.h>, <cstring>, <string>:

- <string> : C++ API in namespace std
- <cstring> : C API in namespace std
- <string.h> : C API in global namespace

In the case of C's complex, the C API won't compile in C++. So we have:

- <complex> : C++ API in namespace std
- <ccomplex> : ?
- <complex.h> : ?

The ? can't refer to the C API. TR1 currently says:

- <complex> : C++ API in namespace std
- <ccomplex> : C++ API in namespace std
- <complex.h> : C++ API in global namespace

Proposed resolution:

Change 26.3.11 [cmplxh]:

> The header behaves as if it includes the header `<ccomplex>`., and provides sufficient using declarations to declare in the global namespace all function and type names declared or defined in the neader `<complex>`. [Note: `<complex.h>` does not promote any interface into the global namespace as there is no C interface to promote. --end note]

### 552(i). random_shuffle and its generator

Section: 26.7.13 [alg.random.shuffle] Status: CD1 Submitter: Martin Sebor Opened: 2006-01-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

...is specified to shuffle its range by calling swap but not how (or even that) it's supposed to use the RandomNumberGenerator argument passed to it.

Shouldn't we require that the generator object actually be used by the algorithm to obtain a series of random numbers and specify how many times its operator() should be invoked by the algorithm?

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 556(i). Is `Compare` a `BinaryPredicate`?

Section: 26.8 [alg.sorting] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 25, p8 we allow BinaryPredicates to return a type that's convertible to bool but need not actually be bool. That allows predicates to return things like proxies and requires that implementations be careful about what kinds of expressions they use the result of the predicate in (e.g., the expression in if (!pred(a, b)) need not be well-formed since the negation operator may be inaccessible or return a type that's not convertible to bool).

Here's the text for reference:

> ...if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct if (binary_pred(*first1, first2)){...}.

In 25.3, p2 we require that the Compare function object return true of false, which would seem to preclude such proxies. The relevant text is here:

> Compare is used as a function object which returns true if the first argument is less than the second, and false otherwise...

[ Portland: Jack to define "convertible to bool" such that short circuiting isn't destroyed. ]

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Move to Review once wording received. Stefanus to send proposed wording.

[ 2009-10 Santa Cruz: ]

> Move to Review once wording received. Stefanus to send proposed wording.

[ 2009-10-24 Stefanus supplied wording. ]

> Move to Review once wording received. Stefanus to send proposed wording. Old proposed wording here:

> > I think we could fix this by rewording 25.3, p2 to read somthing like:

> > > -2- `Compare` is used as a function object which returns `true` if the first argument a `BinaryPredicate`. The return value of the function call operator applied to an object of type `Compare`, when converted to type `bool`, yields `true` if the first argument of the call is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator.

[ 2010-01-17: ]

> Howard expresses concern that the current direction of the proposed wording outlaws expressions such as:

> >     if (!comp(x, y))

> Daniel provides wording which addresses that concern.

> The previous wording is saved here:

> > Change 26.8 [alg.sorting] p2:

> > > `Compare` is used as a function object. The return value of the function call operator applied to an object of type Compare, when converted to type bool, yields true if the first argument of the call which returns `true` if the first argument is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator.

[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 26.1 [algorithms.general]/7+8 as indicated. [This change is recommended to bring the return value requirements of `BinaryPredicate` and `Compare` in sync.]

   > 7 The `Predicate` parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing the corresponding iterator returns a value testable as `true`. In other words, if an algorithm takes `Predicate pred` as its argument and `first` as its iterator argument, it should work correctly in the construct if `(pred(*first)){...}` `pred(*first)` contextually converted to `bool` (7.3 [conv]). The function object `pred` shall not apply any nonconstant function through the dereferenced iterator. This function object may be a pointer to function, or an object of a type with an appropriate function call operator.

   > 8 The `BinaryPredicate` parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type `T` when `T` is part of the signature returns a value testable as `true`. In other words, if an algorithm takes `BinaryPredicate` `binary_pred` as its argument and `first1` and `first2` as its iterator arguments, it should work correctly in the construct `if (binary_pred(*first1, *first2)){...}` `binary_pred(*first1, *first2)` contextually converted to `bool` (7.3 [conv]). `BinaryPredicate` always takes the first iterator type as its first argument, that is, in those cases when `T value` is part of the signature, it should work correctly in the context of `if (binary_pred(*first1, value)){...}` construct `binary_pred(*first1, value)` contextually converted to `bool` (7.3 [conv]). `binary_pred` shall not apply any non-constant function through the dereferenced iterators.

2. Change 26.8 [alg.sorting]/2 as indicated:

   > 2 `Compare` is used as a function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of type `Compare`, when contextually converted to type `bool` (7.3 [conv]), yields `true` if the first argument of the call which returns `true` if the first argument is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator.

### 559(i). numeric_limits<const T>

Section: 17.3.5 [numeric.limits] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-19 Last modified: 2017-06-15

Priority: Not Prioritized

View all other issues in [numeric.limits].

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Discussion:

[limits], p2 requires implementations to provide specializations of the `numeric_limits` template for each scalar type. While this could be interepreted to include cv-qualified forms of such types such an interepretation is not reflected in the synopsis of the `<limits>` header.

The absence of specializations of the template on cv-qualified forms of fundamental types makes `numeric_limits` difficult to use in generic code where the constness (or volatility) of a type is not always immediately apparent. In such contexts, the primary template ends up being instantiated instead of the provided specialization, typically yielding unexpected behavior.

Require that specializations of `numeric_limits` on cv-qualified fundamental types have the same semantics as those on the unqualifed forms of the same types.

Proposed resolution:

Add to the synopsis of the `<limits>` header, immediately below the declaration of the primary template, the following:

    template <class T> class numeric_limits<const T>;
    template <class T> class numeric_limits<volatile T>;
    template <class T> class numeric_limits<const volatile T>;

Add a new paragraph to the end of 17.3.5 [numeric.limits], with the following text:

-new-para- The value of each member of a `numeric_limits` specialization on a cv-qualified T is equal to the value of the same member of `numeric_limits<T>`.

[ Portland: Martin will clarify that user-defined types get cv-specializations automatically. ]

### 561(i). inserter overly generic

Section: 24.5.2.4.3 [inserter] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

The declaration of `std::inserter` is:

>     template <class Container, class Iterator>
>     insert_iterator<Container>
>     inserter(Container& x, Iterator i);

The template parameter `Iterator` in this function is completely unrelated to the template parameter `Container` when it doesn't need to be. This causes the code to be overly generic. That is, any type at all can be deduced as `Iterator`, whether or not it makes sense. Now the same is true of `Container`. However, for every free (unconstrained) template parameter one has in a signature, the opportunity for a mistaken binding grows geometrically.

It would be much better if `inserter` had the following signature instead:

>     template <class Container>
>     insert_iterator<Container>
>     inserter(Container& x, typename Container::iterator i);

Now there is only one free template parameter. And the second argument to `inserter` must be implicitly convertible to the container's iterator, else the call will not be a viable overload (allowing other functions in the overload set to take precedence). Furthermore, the first parameter must have a nested type named `iterator`, or again the binding to `std::inserter` is not viable. Contrast this with the current situation where any type can bind to `Container` or `Iterator` and those types need not be anything closely related to containers or iterators.

This can adversely impact well written code. Consider:

>     #include <iterator>
>     #include <string>

>     namespace my
>     {

>     template <class String>
>     struct my_type {};

>     struct my_container
>     {
>     template <class String>
>     void push_back(const my_type<String>&);
>     };

>     template <class String>
>     void inserter(const my_type<String>& m, my_container& c) {c.push_back(m);}

>     }  // my

>     int main()
>     {
>         my::my_container c;
>         my::my_type<std::string> m;
>         inserter(m, c);
>     }

Today this code fails because the call to `inserter` binds to `std::inserter` instead of to `my::inserter`. However with the proposed change `std::inserter` will no longer be a viable function which leaves only `my::inserter` in the overload resolution set. Everything works as the client intends.

To make matters a little more insidious, the above example works today if you simply change the first argument to an rvalue:

>         inserter(my::my_type(), c);

It will also work if instantiated with some string type other than `std::string` (or any other `std` type). It will also work if `<iterator>` happens to not get included.

And it will fail again for such inocuous reaons as `my_type` or `my_container` privately deriving from any `std` type.

It seems unfortunate that such simple changes in the client's code can result in such radically differing behavior.

Proposed resolution:

Change 24.2:

> 24.2 Header `<iterator>` synopsis

> >     ...
> >     template <class Container, class Iterator>
> >        insert_iterator<Container> inserter(Container& x, Iterator typename Container::iterator i);
> >     ...

Change 24.4.2.5:

> 24.4.2.5 Class template `insert_iterator`

> >     ...
> >     template <class Container, class Iterator>
> >        insert_iterator<Container> inserter(Container& x, Iterator typename Container::iterator i);
> >     ...

Change 24.4.2.6.5:

> 24.4.2.6.5 `inserter`

>     template <class Container, class Inserter>
>        insert_iterator<Container> inserter(Container& x, Inserter typename Container::iterator i);

> > -1- Returns: `insert_iterator<Container>(x,typename Container::iterator(i))`.

[ Kona (2007): This issue will probably be addressed as a part of the concepts overhaul of the library anyway, but the proposed resolution is correct in the absence of concepts. Proposed Disposition: Ready ]

### 562(i). stringbuf ctor inefficient

Section: 31.8 [string.streams] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [string.streams].

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Discussion:

For better efficiency, the requirement on the stringbuf ctor that takes a string argument should be loosened up to let it set `epptr()` beyond just one past the last initialized character just like `overflow()` has been changed to be allowed to do (see issue 432). That way the first call to `sputc()` on an object won't necessarily cause a call to `overflow`. The corresponding change should be made to the string overload of the `str()` member function.

Proposed resolution:

Change 27.7.1.1, p3 of the Working Draft, N1804, as follows:

>     explicit basic_stringbuf(const basic_string<charT,traits,Allocator>& str,
>                    ios_base::openmode which = ios_base::in | ios_base::out);

> -3- Effects: Constructs an object of class `basic_stringbuf`, initializing the base class with `basic_streambuf()` (27.5.2.1), and initializing `mode` with `which`. Then calls `str(s)`. copies the content of str into the `basic_stringbuf` underlying character sequence. If `which & ios_base::out` is true, initializes the output sequence such that `pbase()` points to the first underlying character, `epptr()` points one past the last underlying character, and `pptr()` is equal to `epptr()` if `which & ios_base::ate` is true, otherwise `pptr()` is equal to `pbase()`. If `which & ios_base::in` is true, initializes the input sequence such that `eback()` and `gptr()` point to the first underlying character and `egptr()` points one past the last underlying character.

Change the Effects clause of the `str()` in 27.7.1.2, p2 to read:

> -2- Effects: Copies the contents of `s` into the `basic_stringbuf` underlying character sequence and initializes the input and output sequences according to `mode`. If `mode & ios_base::out` is true, initializes the output sequence such that `pbase()` points to the first underlying character, `epptr()` points one past the last underlying character, and `pptr()` is equal to `epptr()` if `mode & ios_base::in` is true, otherwise `pptr()` is equal to `pbase()`. If `mode & ios_base::in` is true, initializes the input sequence such that `eback()` and `gptr()` point to the first underlying character and `egptr()` points one past the last underlying character.

> -3- Postconditions: If `mode & ios_base::out` is true, `pbase()` points to the first underlying character and `(epptr() >= pbase() + s.size())` holds; in addition, if `mode & ios_base::in` is true, `(pptr() == pbase() + s.data())` holds, otherwise `(pptr() == pbase())` is true. If `mode & ios_base::in` is true, `eback()` points to the first underlying character, and `(gptr() == eback())` and `(egptr() == eback() + s.size())` hold.

[ Kona (2007) Moved to Ready. ]

### 563(i). stringbuf seeking from end

Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [stringbuf.virtuals].

View all other issues in [stringbuf.virtuals].

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Discussion:

According to Table 92 (unchanged by issue 432), when `(way == end)` the `newoff` value in out mode is computed as the difference between `epptr()` and `pbase()`.

This value isn't meaningful unless the value of `epptr()` can be precisely controlled by a program. That used to be possible until we accepted the resolution of issue 432, but since then the requirements on `overflow()` have been relaxed to allow it to make more than 1 write position available (i.e., by setting `epptr()` to some unspecified value past `pptr()`). So after the first call to `overflow()` positioning the output sequence relative to end will have unspecified results.

In addition, in `in|out` mode, since `(egptr() == epptr())` need not hold, there are two different possible values for `newoff`: `epptr() - pbase()` and `egptr() - eback()`.

Proposed resolution:

Change the `newoff` column in the last row of Table 94 to read:

> the end high mark pointer minus the beginning pointer (`xend high_mark - xbeg`).

[ Kona (2007) Moved to Ready. ]

### 564(i). `stringbuf` seekpos underspecified

Section: 31.8.2.5 [stringbuf.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [stringbuf.virtuals].

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Discussion:

The effects of the `seekpos()` member function of `basic_stringbuf` simply say that the function positions the input and/or output sequences but fail to spell out exactly how. This is in contrast to the detail in which `seekoff()` is described.

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Change 27.7.1.3, p13 to read:

> -13- Effects: Equivalent to `seekoff(off_type(sp), ios_base::beg, which)`. Alters the stream position within the controlled sequences, if possible, to correspond to the stream position stored in `sp` (as described below).

> - If `(which & ios_base::in) != 0`, positions the input sequence.
> - If `(which & ios_base::out) != 0`, positions the output sequence.
> - If `sp` is an invalid stream position, or if the function positions neither sequence, the positioning operation fails. If `sp` has not been obtained by a previous successful call to one of the positioning functions (`seekoff`, `seekpos`, `tellg`, `tellp`) the effect is undefined.

[ Kona (2007): A `pos_type` is a position in a stream by definition, so there is no ambiguity as to what it means. Proposed Disposition: NAD ]

[ Post-Kona Martin adds: I'm afraid I disagree with the Kona '07 rationale for marking it NAD. The only text that describes precisely what it means to position the input or output sequence is in `seekoff()`. The `seekpos()` Effects clause is inadequate in comparison and the proposed resolution plugs the hole by specifying `seekpos()` in terms of `seekoff()`. ]

### 565(i). `xsputn` inefficient

Section: 31.6.3.5.5 [streambuf.virt.put] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

`streambuf::xsputn()` is specified to have the effect of "writing up to `n` characters to the output sequence as if by repeated calls to `sputc(c)`."

Since `sputc()` is required to call `overflow()` when `(pptr() == epptr())` is true, strictly speaking `xsputn()` should do the same. However, doing so would be suboptimal in some interesting cases, such as in unbuffered mode or when the buffer is `basic_stringbuf`.

Assuming calling `overflow()` is not really intended to be required and the wording is simply meant to describe the general effect of appending to the end of the sequence it would be worthwhile to mention in `xsputn()` that the function is not actually required to cause a call to `overflow()`.

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Add the following sentence to the `xsputn()` Effects clause in 27.5.2.4.5, p1 (N1804):

> -1- Effects: Writes up to `n` characters to the output sequence as if by repeated calls to `sputc(c)`. The characters written are obtained from successive elements of the array whose first element is designated by `s`. Writing stops when either `n` characters have been written or a call to `sputc(c)` would return `traits::eof()`. It is uspecified whether the function calls `overflow()` when `(pptr() == epptr())` becomes true or whether it achieves the same effects by other means.

In addition, I suggest to add a footnote to this function with the same text as Footnote 292 to make it extra clear that derived classes are permitted to override `xsputn()` for efficiency.

[ Kona (2007): We want to permit a `streambuf` that streams output directly to a device without making calls to `sputc` or `overflow`. We believe that has always been the intention of the committee. We believe that the proposed wording doesn't accomplish that. Proposed Disposition: Open ]

### 566(i). array forms of unformatted input function undefined for zero-element arrays

Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.unformatted].

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Discussion:

The array forms of unformatted input functions don't have well-defined semantics for zero-element arrays in a couple of cases. The affected ones (`istream::get()` and `getline()`) are supposed to terminate when `(n - 1)` characters are stored, which obviously can never be true when `(n == 0)` to start with.

Proposed resolution:

I propose the following changes (references are relative to the Working Draft (document N1804).

Change 27.6.1.3, p8 (`istream::get()`), bullet 1 as follows:

> if `(n < 1)` is true or `(n - 1)` characters are stored;

Similarly, change 27.6.1.3, p18 (`istream::getline()`), bullet 3 as follows:

> `(n < 1)` is true or `(n - 1)` characters are stored (in which case the function calls `setstate(failbit)`).

Finally, change p21 as follows:

> In any case, provided `(n > 0)` is true, it then stores a null character (using charT()) into the next successive location of the array.

### 567(i). streambuf inserter and extractor should be unformatted

Section: 31.7 [iostream.format] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iostream.format].

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Discussion:

Issue 60 explicitly made the extractor and inserter operators that take a `basic_streambuf*` argument formatted input and output functions, respectively. I believe that's wrong, certainly in the case of the extractor, since formatted functions begin by extracting and discarding whitespace. The extractor should not discard any characters.

Proposed resolution:

I propose to change each operator to behave as unformatted input and output function, respectively. The changes below are relative to the working draft document number N1804.

Specifically, change 27.6.1.2.3, p14 as follows:

> Effects: Behaves as an unformatted input function (as described in 27.6.1.2.127.6.1.3, paragraph 1).

And change 27.6.2.5.3, p7 as follows:

> Effects: Behaves as an unformatted output function (as described in 27.6.2.5.127.6.2.6, paragraph 1).

[ Kona (2007): Proposed Disposition: Ready ]

### 574(i). DR 369 Contradicts Text

Section: 31.4 [iostream.objects] Status: CD1 Submitter: Pete Becker Opened: 2006-04-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iostream.objects].

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Discussion:

lib.iostream.objects requires that the standard stream objects are never destroyed, and it requires that they be destroyed.

DR 369 adds words to say that we really mean for `ios_base::Init` objects to force construction of standard stream objects. It ends, though, with the phrase "these stream objects shall be destroyed after the destruction of dynamically ...". However, the rule for destruction is stated in the standard: "The objects are not destroyed during program execution."

Proposed resolution:

Change 31.4 [iostream.objects]/1:

> -2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class `ios_base::Init` is constructed, and in any case before the body of main begins execution.290) The objects are not destroyed during program execution.291) If a translation unit includes `<iostream>` or explicitly constructs an `ios_base::Init` object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.

[ Kona (2007): From 31.4 [iostream.objects]/2, strike the words "...and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit." Proposed Disposition: Review ]

### 575(i). the specification of ~shared_ptr is MT-unfriendly, makes implementation assumptions

Section: 20.3.2.2.3 [util.smartptr.shared.dest], 99 [tr.util.smartptr.shared.dest] Status: CD1 Submitter: Peter Dimov Opened: 2006-04-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.smartptr.shared.dest].

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Discussion:

[tr.util.smartptr.shared.dest] says in its second bullet:

"If *this shares ownership with another shared_ptr instance (use_count() > 1), decrements that instance's use count."

The problem with this formulation is that it presupposes the existence of an "use count" variable that can be decremented and that is part of the state of a shared_ptr instance (because of the "that instance's use count".)

This is contrary to the spirit of the rest of the specification that carefully avoids to require an use count variable. Instead, use_count() is specified to return a value, a number of instances.

In multithreaded code, the usual implicit assumption is that a shared variable should not be accessed by more than one thread without explicit synchronization, and by introducing the concept of an "use count" variable, the current wording implies that two shared_ptr instances that share ownership cannot be destroyed simultaneously.

In addition, if we allow the interpretation that an use count variable is part of shared_ptr's state, this would lead to other undesirable consequences WRT multiple threads. For example,

>     p1 = p2;

would now visibly modify the state of p2, a "write" operation, requiring a lock.

Proposed resolution:

Change the first two bullets of [lib.util.smartptr.shared.dest]/1 to:

> - If `*this` is empty or shares ownership with another `shared_ptr` instance (`use_count() > 1`), there are no side effects.
> - If `*this` shares ownership with another `shared_ptr` instance (`use_count() > 1`), decrements that instance's use count.

Add the following paragraph after [lib.util.smartptr.shared.dest]/1:

> [Note: since the destruction of `*this` decreases the number of instances in `*this`'s ownership group by one, all `shared_ptr` instances that share ownership with `*this` will report an `use_count()` that is one lower than its previous value after `*this` is destroyed. --end note]

### 576(i). find_first_of is overconstrained

Section: 26.6.9 [alg.find.first.of] Status: CD1 Submitter: Doug Gregor Opened: 2006-04-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.find.first.of].

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Discussion:

In 25.1.4 Find First [lib.alg.find.first], the two iterator type parameters to find_first_of are specified to require Forward Iterators, as follows:

>     template<class ForwardIterator1, class ForwardIterator2>
>       ForwardIterator1
>       find_first_of(ForwardIterator1 first1, ForwardIterator1 last1,
>                             ForwardIterator2 first2, ForwardIterator2 last2);
>     template<class ForwardIterator1, class ForwardIterator2,
>                       class BinaryPredicate>
>     ForwardIterator1
>       find_first_of(ForwardIterator1 first1, ForwardIterator1 last1,
>                              ForwardIterator2 first2, ForwardIterator2 last2,
>                             BinaryPredicate pred);

However, ForwardIterator1 need not actually be a Forward Iterator; an Input Iterator suffices, because we do not need the multi-pass property of the Forward Iterator or a true reference.

Proposed resolution:

Change the declarations of `find_first_of` to:

>     template<class ForwardIterator1InputIterator1, class ForwardIterator2>
>       ForwardIterator1InputIterator1
>       find_first_of(ForwardIterator1InputIterator1 first1, ForwardIterator1InputIterator1 last1,
>                             ForwardIterator2 first2, ForwardIterator2 last2);
>     template<class ForwardIterator1InputIterator1, class ForwardIterator2,
>                       class BinaryPredicate>
>     ForwardIterator1InputIterator1
>       find_first_of(ForwardIterator1InputIterator1 first1, ForwardIterator1InputIterator1 last1,
>                              ForwardIterator2 first2, ForwardIterator2 last2,
>                             BinaryPredicate pred);

### 577(i). upper_bound(first, last, ...) cannot return last

Section: 26.8.4.3 [upper.bound] Status: CD1 Submitter: Seungbeom Kim Opened: 2006-05-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

ISO/IEC 14882:2003 says:

> 25.3.3.2 upper_bound

> Returns: The furthermost iterator `i` in the range `[first, last)` such that for any iterator `j` in the range `[first, i)` the following corresponding conditions hold: `!(value < *j)` or `comp(value, *j) == false`.

From the description above, upper_bound cannot return last, since it's not in the interval [first, last). This seems to be a typo, because if value is greater than or equal to any other values in the range, or if the range is empty, returning last seems to be the intended behaviour. The corresponding interval for lower_bound is also [first, last].

Proposed resolution:

Change [lib.upper.bound]:

> Returns: The furthermost iterator `i` in the range `[first, last)]` such that for any iterator `j` in the range `[first, i)` the following corresponding conditions hold: `!(value < *j)` or `comp(value, *j) == false`.

### 578(i). purpose of hint to allocator::allocate()

Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Martin Sebor Opened: 2006-05-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The description of the allocator member function `allocate()` requires that the hint argument be either 0 or a value previously returned from `allocate()`. Footnote 227 further suggests that containers may pass the address of an adjacent element as this argument.

I believe that either the footnote is wrong or the normative requirement that the argument be a value previously returned from a call to `allocate()` is wrong. The latter is supported by the resolution to issue 20-004 proposed in c++std-lib-3736 by Nathan Myers. In addition, the hint is an ordinary void* and not the `pointer` type returned by `allocate()`, with the two types potentially being incompatible and the requirement impossible to satisfy.

See also c++std-lib-14323 for some more context on where this came up (again).

Proposed resolution:

Remove the requirement in 20.6.1.1, p4 that the hint be a value previously returned from `allocate()`. Specifically, change the paragraph as follows:

Requires: hint either 0 or previously obtained from member `allocate` and not yet passed to member `deallocate`. The value hint may be used by an implementation to help improve performance 223). [Note: The value hint may be used by an implementation to help improve performance. -- end note]

> [Footnote: 223)In a container member function, the address of an adjacent element is often a good choice to pass for this argument.

### 581(i). `flush()` not unformatted function

Section: 31.7.6.4 [ostream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2006-06-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ostream.unformatted].

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Discussion:

The resolution of issue 60 changed `basic_ostream::flush()` so as not to require it to behave as an unformatted output function. That has at least two in my opinion problematic consequences:

First, `flush()` now calls `rdbuf()->pubsync()` unconditionally, without regard to the state of the stream. I can't think of any reason why `flush()` should behave differently from the vast majority of stream functions in this respect.

Second, `flush()` is not required to catch exceptions from `pubsync()` or set `badbit` in response to such events. That doesn't seem right either, as most other stream functions do so.

Proposed resolution:

I propose to revert the resolution of issue 60 with respect to `flush()`. Specifically, I propose to change 27.6.2.6, p7 as follows:

Effects: Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). If `rdbuf()` is not a null pointer, constructs a sentry object. If this object returns `true` when converted to a value of type bool the function calls `rdbuf()->pubsync()`. If that function returns -1 calls `setstate(badbit)` (which may throw `ios_base::failure` (27.4.4.3)). Otherwise, if the sentry object returns `false`, does nothing.Does not behave as an unformatted output function (as described in 27.6.2.6, paragraph 1).

[ Kona (2007): Proposed Disposition: Ready ]

### 586(i). string inserter not a formatted function

Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Martin Sebor Opened: 2006-06-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section and paragraph numbers in this paper are relative to the working draft document number N2009 from 4/21/2006.

The `basic_string` extractor in 21.3.7.9, p1 is clearly required to behave as a formatted input function, as is the `std::getline()` overload for string described in p7.

However, the `basic_string` inserter described in p5 of the same section has no such requirement. This has implications on how the operator responds to exceptions thrown from `xsputn()` (formatted output functions are required to set `badbit` and swallow the exception unless `badbit` is also set in `exceptions()`; the string inserter doesn't have any such requirement).

I don't see anything in the spec for the string inserter that would justify requiring it to treat exceptions differently from all other similar operators. (If it did, I think it should be made this explicit by saying that the operator "does not behave as a formatted output function" as has been made customary by the adoption of the resolution of issue 60).

Proposed resolution:

I propose to change the Effects clause in 21.3.7.9, p5, as follows:

> Effects: Begins by constructing a sentry object k as if k were constructed by typename `basic_ostream<charT, traits>::sentry k (os)`. If `bool(k)` is `true`, Behaves as a formatted output function (27.6.2.5.1). After constructing a `sentry` object, if this object returns `true` when converted to a value of type `bool`, determines padding as described in 22.2.2.2.2, then inserts the resulting sequence of characters `seq` as if by calling `os.rdbuf()->sputn(seq , n)`, where `n` is the larger of `os.width()` and `str.size()`; then calls `os.width(0)`. If the call to sputn fails, calls `os.setstate(ios_base::failbit)`.

This proposed resilution assumes the resolution of issue 394 (i.e., that all formatted output functions are required to set `ios_base::badbit` in response to any kind of streambuf failure), and implicitly assumes that a return value of `sputn(seq, n)` other than `n` indicates a failure.

### 589(i). Requirements on iterators of member template functions of containers

Section: 23.2 [container.requirements] Status: CD1 Submitter: Peter Dimov Opened: 2006-08-02 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

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Duplicate of: 536

Discussion:

There appears to be no requirements on the InputIterators used in sequences in 23.1.1 in terms of their value_type, and the requirements in 23.1.2 appear to be overly strict (requires InputIterator::value_type be the same type as the container's value_type).

Proposed resolution:

Change 23.1.1 p3:

> In Tables 82 and 83, `X` denotes a sequence class, `a` denotes a value of `X`, `i` and `j` denote iterators satisfying input iterator requirements and refer to elements implicitly convertible to `value_type`, `[i, j)` denotes a valid range, `n` denotes a value of `X::size_type`, `p` denotes a valid iterator to `a`, `q` denotes a valid dereferenceable iterator to `a`, `[q1, q2)` denotes a valid range in `a`, and `t` denotes a value of `X::value_type`.

Change 23.1.2 p7:

> In Table 84, `X` is an associative container class, `a` is a value of `X`, `a_uniq` is a value of `X` when `X` supports unique keys, and `a_eq` is a value of `X` when `X` supports multiple keys, `i` and `j` satisfy input iterator requirements and refer to elements of implicitly convertible to `value_type`, `[i, j)` is a valid range, `p` is a valid iterator to `a`, `q` is a valid dereferenceable iterator to `a`, `[q1, q2)` is a valid range in `a`, `t` is a value of `X::value_type`, `k` is a value of `X::key_type` and `c` is a value of type `X::key_compare`.

Rationale:

Concepts will probably come in and rewrite this section anyway. But just in case it is easy to fix this up as a safety net and as a clear statement of intent.

### 593(i). __STDC_CONSTANT_MACROS

Section: 17.4.1 [cstdint.syn], 99 [tr.c99.cstdint] Status: CD1 Submitter: Walter Brown Opened: 2006-08-28 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

Clause 18.3 of the current Working Paper (N2009) deals with the new C++ headers <cstdint> and <stdint.h>. These are of course based on the C99 header <stdint.h>, and were part of TR1.

Per 18.3.1/1, these headers define a number of macros and function macros. While the WP does not mention __STDC_CONSTANT_MACROS in this context, C99 footnotes do mention __STDC_CONSTANT_MACROS. Further, 18.3.1/2 states that "The header defines all ... macros the same as C99 subclause 7.18."

Therefore, if I wish to have the above-referenced macros and function macros defined, must I #define __STDC_CONSTANT_MACROS before I #include <cstdint>, or does the C++ header define these macros/function macros unconditionally?

Proposed resolution:

To put this issue to rest for C++0X, I propose the following addition to 18.3.1/2 of the Working Paper N2009:

> [Note: The macros defined by <cstdint> are provided unconditionally: in particular, the symbols __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS (mentioned in C99 footnotes 219, 220, and 222) play no role in C++. --end note]

### 594(i). Disadvantages of defining `Swappable` in terms of `CopyConstructible` and `Assignable`

Section: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Niels Dekker Opened: 2006-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It seems undesirable to define the `Swappable` requirement in terms of `CopyConstructible` and `Assignable` requirements. And likewise, once the `MoveConstructible` and `MoveAssignable` requirements (N1860) have made it into the Working Draft, it seems undesirable to define the `Swappable` requirement in terms of those requirements. Instead, it appears preferable to have the `Swappable` requirement defined exclusively in terms of the existence of an appropriate swap function.

Section 20.1.4 [lib.swappable] of the current Working Draft (N2009) says:

> The Swappable requirement is met by satisfying one or more of the following conditions:

> - T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);
> - T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33.

I can think of three disadvantages of this definition:

1. If a client's type T satisfies the first condition (T is both CopyConstructible and Assignable), the client cannot stop T from satisfying the Swappable requirement without stopping T from satisfying the first condition.

   A client might want to stop T from satisfying the Swappable requirement, because swapping by means of copy construction and assignment might throw an exception, and she might find a throwing swap unacceptable for her type. On the other hand, she might not feel the need to fully implement her own swap function for this type. In this case she would want to be able to simply prevent algorithms that would swap objects of type T from being used, e.g., by declaring a swap function for T, and leaving this function purposely undefined. This would trigger a link error, if an attempt would be made to use such an algorithm for this type. For most standard library implementations, this practice would indeed have the effect of stopping T from satisfying the Swappable requirement.

2. A client's type T that does not satisfy the first condition can not be made Swappable by providing a specialization of std::swap for T.

   While I'm aware about the fact that people have mixed feelings about providing a specialization of std::swap, it is well-defined to do so. It sounds rather counter-intuitive to say that T is not Swappable, if it has a valid and semantically correct specialization of std::swap. Also in practice, providing such a specialization will have the same effect as satisfying the Swappable requirement.

3. For a client's type T that satisfies both conditions of the Swappable requirement, it is not specified which of the two conditions prevails. After reading section 20.1.4 [lib.swappable], one might wonder whether objects of T will be swapped by doing copy construction and assignments, or by calling the swap function of T.

   I'm aware that the intention of the Draft is to prefer calling the swap function of T over doing copy construction and assignments. Still in my opinion, it would be better to make this clear in the wording of the definition of Swappable.

I would like to have the Swappable requirement defined in such a way that the following code fragment will correctly swap two objects of a type T, if and only if T is Swappable:

       using std::swap;
       swap(t, u);  // t and u are of type T.

This is also the way Scott Meyers recommends calling a swap function, in Effective C++, Third Edition, item 25.

Most aspects of this issue have been dealt with in a discussion on comp.std.c++ about the Swappable requirement, from 13 September to 4 October 2006, including valuable input by David Abrahams, Pete Becker, Greg Herlihy, Howard Hinnant and others.

[ San Francisco: ]

> Recommend NADResolved. Solved by N2774.

[ 2009-07 Frankfurt ]

> Moved to Open. Waiting for non-concepts draft.

[ 2009-11-08 Howard adds: ]

> This issue is very closely related to 742(i).

[ 2010-02-03 Sean Hunt adds: ]

> While reading N3000, I independently came across Issue 594. Having seen that it's an issue under discussion, I think the proposed wording needs fixing to something more like "...function call swap(t,u) that includes std::swap in its overload set is valid...", because "...is valid within the namespace std..." does not allow other libraries to simply use the Swappable requirement by referring to the standard's definition, since they cannot actually perform any calls within std.

> This wording I suggested would also make overloads visible in the same scope as the `using std::swap` valid for Swappable requirements; a more complex wording limiting the non-ADL overload set to std::swap might be required.

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial. Rationale added.

Rationale:

Solved by N3048.

Proposed resolution:

Change section 20.1.4 [lib.swappable] as follows:

> The Swappable requirement is met by satisfying one or more of the following conditions: the following condition:

> - T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);
> - T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33. T is Swappable if an unqualified function call swap(t,u) is valid within the namespace std, and has the semantics described in Table 33.

### 595(i). TR1/C++0x: `fabs(complex<T>)` redundant / wrongly specified

Section: 29.4.7 [complex.value.ops] Status: CD1 Submitter: Stefan Große Pawig Opened: 2006-09-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [complex.value.ops].

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Discussion:

TR1 introduced, in the C compatibility chapter, the function `fabs(complex<T>)`:

>     ----- SNIP -----
>     8.1.1 Synopsis                                [tr.c99.cmplx.syn]

>       namespace std {
>       namespace tr1 {
>     [...]
>       template<class T> complex<T> fabs(const complex<T>& x);
>       } // namespace tr1
>       } // namespace std

>     [...]

>     8.1.8 Function fabs                          [tr.c99.cmplx.fabs]

>     1 Effects: Behaves the same as C99 function cabs, defined in
>       subclause 7.3.8.1.
>     ----- SNIP -----

The current C++0X draft document (n2009.pdf) adopted this definition in chapter 26.3.1 (under the comment // 26.3.7 values) and 26.3.7/7.

But in C99 (ISO/IEC 9899:1999 as well as the 9899:TC2 draft document n1124), the referenced subclause reads

>     ----- SNIP -----
>     7.3.8.1 The cabs functions

>       Synopsis

>     1 #include <complex.h>
>       double cabs(double complex z);
>       float cabsf(float complex z);
>       long double cabsl(long double z);

>       Description

>     2 The cabs functions compute the complex absolute value (also called
>       norm, modulus, or magnitude) of z.

>       Returns

>     3 The cabs functions return the complex absolute value.
>     ----- SNIP -----

Note that the return type of the cabs*() functions is not a complex type. Thus, they are equivalent to the already well established template<class T> T abs(const complex<T>& x); (26.2.7/2 in ISO/IEC 14882:1998, 26.3.7/2 in the current draft document n2009.pdf).

So either the return value of fabs() is specified wrongly, or fabs() does not behave the same as C99's cabs*().

Possible Resolutions

This depends on the intention behind the introduction of fabs().

If the intention was to provide a /complex/ valued function that calculates the magnitude of its argument, this should be explicitly specified. In TR1, the categorization under "C compatibility" is definitely wrong, since C99 does not provide such a complex valued function.

Also, it remains questionable if such a complex valued function is really needed, since complex<T> supports construction and assignment from real valued arguments. There is no difference in observable behaviour between

>       complex<double> x, y;
>       y = fabs(x);
>       complex<double> z(fabs(x));

and

>       complex<double> x, y;
>       y = abs(x);
>       complex<double> z(abs(x));

If on the other hand the intention was to provide the intended functionality of C99, fabs() should be either declared deprecated or (for C++0X) removed from the standard, since the functionality is already provided by the corresponding overloads of abs().

[ Bellevue: ]

> Bill believes that `abs()` is a suitable overload. We should remove `fabs()`.

Proposed resolution:

Change the synopsis in 29.4.2 [complex.syn]:

>     template<class T> complex<T> fabs(const complex<T>&);

Remove 29.4.7 [complex.value.ops], p7:

>     template<class T> complex<T> fabs(const complex<T>& x);

> > -7- Effects: Behaves the same as C99 function `cabs`, defined in subclause 7.3.8.1.

[ Kona (2007): Change the return type of `fabs(complex)` to `T`. Proposed Disposition: Ready ]

### 596(i). 27.8.1.3 Table 112 omits "a+" and "a+b" modes

Section: 31.10.3.4 [filebuf.members] Status: CD1 Submitter: Thomas Plum Opened: 2006-09-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In testing 31.10.3.4 [filebuf.members], Table 112 (in the latest N2009 draft), we invoke

>        ostr.open("somename", ios_base::out | ios_base::in | ios_base::app)

and we expect the open to fail, because out|in|app is not listed in Table 92, and just before the table we see very specific words:

> If mode is not some combination of flags shown in the table then the open fails.

But the corresponding table in the C standard, 7.19.5.3, provides two modes "a+" and "a+b", to which the C++ modes out|in|app and out|in|app|binary would presumably apply.

We would like to argue that the intent of Table 112 was to match the semantics of 7.19.5.3 and that the omission of "a+" and "a+b" was unintentional. (Otherwise there would be valid and useful behaviors available in C file I/O which are unavailable using C++, for no valid functional reason.)

We further request that the missing modes be explicitly restored to the WP, for inclusion in C++0x.

[ Martin adds: ]

...besides "a+" and "a+b" the C++ table is also missing a row for a lone app bit which in at least two current implementation as well as in Classic Iostreams corresponds to the C stdio "a" mode and has been traditionally documented as implying ios::out. Which means the table should also have a row for in|app meaning the same thing as "a+" already proposed in the issue.

Proposed resolution:

Add to the table "File open modes" in 31.10.3.4 [filebuf.members]:

> File open modes
> `ios_base` Flag combination | `stdio` equivalent
> --- | ---
> `binary` | `in` | `out` | `trunc` | `app` | ``
> |  | `+` |  |  | `"w"`
> |  | `+` |  | `+` | `"a"`
> |  |  |  | `+` | `"a"`
> |  | `+` | `+` |  | `"w"`
> | `+` |  |  |  | `"r"`
> | `+` | `+` |  |  | `"r+"`
> | `+` | `+` | `+` |  | `"w+"`
> | `+` | `+` |  | `+` | `"a+"`
> | `+` |  |  | `+` | `"a+"`
> `+` |  | `+` |  |  | `"wb"`
> `+` |  | `+` |  | `+` | `"ab"`
> `+` |  |  |  | `+` | `"ab"`
> `+` |  | `+` | `+` |  | `"wb"`
> `+` | `+` |  |  |  | `"rb"`
> `+` | `+` | `+` |  |  | `"r+b"`
> `+` | `+` | `+` | `+` |  | `"w+b"`
> `+` | `+` | `+` |  | `+` | `"a+b"`
> `+` | `+` |  |  | `+` | `"a+b"`

[ Kona (2007) Added proposed wording and moved to Review. ]

### 598(i). [dec.tr] Conversion to integral should truncate, not round.

Section: 3.2 [dec.tr::trdec.types.types] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31

Priority: Not Prioritized

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Discussion:

In a private email, Daniel writes:

> I would like to ask, what where the reason for the decision to define the semantics of the integral conversion of the decimal types, namely

>     "operator long long() const;

>          Returns: Returns the result of the
>     conversion of *this to the type long long, as if
>     performed by the expression llrounddXX(*this)."

> where XX stands for either 32, 64, or 128, corresponding to the proper decimal type. The exact meaning of llrounddXX is not given in that paper, so I compared it to the corresponding definition given in C99, 2nd edition (ISO 9899), which says in 7.12.9.7 p. 2:

> "The lround and llround functions round their argument to the nearest integer value, rounding halfway cases away from zero, regardless of the current rounding direction. [..]"

> Now considering the fact that integral conversion of the usual floating-point types ("4.9 Floating-integral conversions") has truncation semantic I wonder why this conversion behaviour has not been transferred for the decimal types.

Robert comments:

Also, there is a further error in the Returns: clause for converting `decimal::decimal128` to `long long`. It currently calls `llroundd64`, not `llroundd128`.

Proposed resolution:

Change the Returns: clause in 3.2.2.4 to:

> Returns: Returns the result of the conversion of `*this` to the type `long long`, as if performed by the expression `llroundd32(*this)` while the decimal rounding direction mode [3.5.2] `FE_DEC_TOWARD_ZERO` is in effect.

Change the Returns: clause in 3.2.3.4 to:

> Returns: Returns the result of the conversion of `*this` to the type `long long`, as if performed by the expression `llroundd64(*this)` while the decimal rounding direction mode [3.5.2] `FE_DEC_TOWARD_ZERO` is in effect.

Change the Returns: clause in 3.2.4.4 to:

> Returns: Returns the result of the conversion of `*this` to the type `long long`, as if performed by the expression `llroundd64(*this)` `llroundd128(*this)` while the decimal rounding direction mode [3.5.2] `FE_DEC_TOWARD_ZERO` is in effect.

### 599(i). [dec.tr] Say "octets" instead of "bytes."

Section: 3.1 [dec.tr::trdec.types.encodings] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31

Priority: Not Prioritized

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Discussion:

Daniel writes in a private email:

> - 3.1 'Decimal type encodings' says in its note:

>     "this implies that
>     sizeof(std::decimal::decimal32) == 4,
>     sizeof(std::decimal::decimal64) == 8, and
>     sizeof(std::decimal::decimal128) == 16."

> This is a wrong assertion, because the definition of 'byte' in 1.7 'The C+ + memory model' of ISO 14882 (2nd edition) does not specify that a byte must be necessarily 8 bits large, which would be necessary to compare with the specified bit sizes of the types decimal32, decimal64, and decimal128.

Proposed resolution:

Change 3.1 as follows:

> The three decimal encoding formats defined in IEEE-754R correspond to the three decimal floating types as follows:

> - decimal32 is a decimal32 number, which is encoded in four consecutive bytes octets (32 bits)
> - decimal64 is a decimal64 number, which is encoded in eight consecutive bytes octets (64 bits)
> - decimal128 is a decimal128 number, which is encoded in 16 consecutive bytes octets (128 bits)

> [Note: this implies that `sizeof(std::decimal::decimal32) == 4`, `sizeof(std::decimal::decimal64) == 8`, and `sizeof(std::decimal::decimal128) == 16`. --end note]

### 600(i). [dec.tr] Wrong parameters for wcstod* functions

Section: 3.9 [dec.tr::trdec.types.cwchar] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31

Priority: Not Prioritized

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Discussion:

Daniel writes:

> - 3.9.1 'Additions to <cwchar>' provides wrong signatures to the wcstod32, wcstod64, and wcstod128 functions ([the parameters have type pointer-to-] char instead of wchar_t).

Proposed resolution:

Change "3.9.1 Additions to `<cwchar>` synopsis" to:

           namespace std {
           namespace decimal {
             // 3.9.2 wcstod functions:
             decimal32  wcstod32  (const char wchar_t * nptr, char wchar_t ** endptr);
             decimal64  wcstod64  (const char wchar_t * nptr, char wchar_t ** endptr);
             decimal128 wcstod128 (const char wchar_t * nptr, char wchar_t ** endptr);
           }
           }

### 601(i). [dec.tr] numeric_limits typos

Section: 3.3 [dec.tr::trdec.types.limits] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31

Priority: Not Prioritized

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Discussion:

Daniel writes in a private email:

> - 3.3 'Additions to header <limits>' contains two errors in the specialisation of numeric_limits<decimal::decimal128>:

> 1. The static member max() returns DEC128_MIN, this should be DEC128_MAX.
> 2. The static member digits is assigned to 384, this should be 34 (Probably mixed up with the max. exponent for decimal::decimal64).

Proposed resolution:

In "3.3 Additions to header `<limits>`" change numeric_limits<decimal::decimal128> as follows:

            template<> class numeric_limits<decimal::decimal128> {
              public:
                static const bool is_specialized = true;

                static decimal::decimal128 min() throw() { return DEC128_MIN; }
                static decimal::decimal128 max() throw() { return DEC128_MIN; DEC128_MAX; }

                static const int digits       = 384 34;
                /* ... */

### 602(i). [dec.tr] "generic floating type" not defined.

Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Daniel Krügler Opened: 2006-05-28 Last modified: 2016-01-31

Priority: Not Prioritized

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Discussion:

The document uses the term "generic floating types," but defines it nowhere.

Proposed resolution:

Change the first paragraph of "3 Decimal floating-point types" as follows:

> This Technical Report introduces three decimal floating-point types, named decimal32, decimal64, and decimal128. The set of values of type decimal32 is a subset of the set of values of type decimal64; the set of values of the type decimal64 is a subset of the set of values of the type decimal128. Support for decimal128 is optional. These types supplement the Standard C++ types `float`, `double`, and `long double`, which are collectively described as the basic floating types.

### 603(i). [dec.tr] Trivially simplifying decimal classes.

Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Martin Sebor Opened: 2006-05-28 Last modified: 2016-01-31

Priority: Not Prioritized

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Discussion:

In c++std-lib-17198, Martin writes:

> Each of the three classes proposed in the paper (decimal32, decimal64, and decimal128) explicitly declares and specifies the semantics of its copy constructor, copy assignment operator, and destructor. Since the semantics of all three functions are identical to the trivial versions implicitly generated by the compiler in the absence of any declarations it is safe to drop them from the spec. This change would make the proposed classes consistent with other similar classes already in the standard (e.g., std::complex).

Proposed resolution:

Change "3.2.2 Class `decimal32`" as follows:

          namespace std {
          namespace decimal {
            class decimal32 {
              public:
                // 3.2.2.1 construct/copy/destroy:
                decimal32();
                decimal32(const decimal32 & d32);
                decimal32 & operator=(const decimal32 & d32);
                ~decimal32();
                /* ... */

Change "3.2.2.1 construct/copy/destroy" as follows:

            decimal32();

        Effects: Constructs an object of type decimal32 with the value 0;

            decimal32(const decimal32 & d32);
            decimal32 & operator=(const decimal32 & d32);

        Effects: Copies an object of type decimal32.

            ~decimal32();

        Effects: Destroys an object of type decimal32.

Change "3.2.3 Class `decimal64`" as follows:

          namespace std {
          namespace decimal {
            class decimal64 {
              public:
                // 3.2.3.1 construct/copy/destroy:
                decimal64();
                decimal64(const decimal64 & d64);
                decimal64 & operator=(const decimal64 & d64);
                ~decimal64();
                /* ... */

Change "3.2.3.1 construct/copy/destroy" as follows:

            decimal64();

        Effects: Constructs an object of type decimal64 with the value 0;

            decimal64(const decimal64 & d64);
            decimal64 & operator=(const decimal64 & d64);

        Effects: Copies an object of type decimal64.

            ~decimal64();

        Effects: Destroys an object of type decimal64.

Change "3.2.4 Class `decimal128`" as follows:

          namespace std {
          namespace decimal {
            class decimal128 {
              public:
                // 3.2.4.1 construct/copy/destroy:
                decimal128();
                decimal128(const decimal128 & d128);
                decimal128 & operator=(const decimal128 & d128);
                ~decimal128();
                /* ... */

Change "3.2.4.1 construct/copy/destroy" as follows:

            decimal128();

        Effects: Constructs an object of type decimal128 with the value 0;

            decimal128(const decimal128 & d128);
            decimal128 & operator=(const decimal128 & d128);

        Effects: Copies an object of type decimal128.

            ~decimal128();

        Effects: Destroys an object of type decimal128.

### 604(i). [dec.tr] Storing a reference to a facet unsafe.

Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Martin Sebor Opened: 2006-05-28 Last modified: 2016-01-31

Priority: Not Prioritized

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Discussion:

In c++std-lib-17197, Martin writes:

> The extended_num_get and extended_num_put facets are designed to store a reference to a num_get or num_put facet which the extended facets delegate the parsing and formatting of types other than decimal. One form of the extended facet's ctor (the default ctor and the size_t overload) obtains the reference from the global C++ locale while the other form takes this reference as an argument.

> The problem with storing a reference to a facet in another object (as opposed to storing the locale object in which the facet is installed) is that doing so bypasses the reference counting mechanism designed to prevent a facet that is still being referenced (i.e., one that is still installed in some locale) from being destroyed when another locale that contains it is destroyed. Separating a facet reference from the locale it comes from van make it cumbersome (and in some cases might even make it impossible) for programs to prevent invalidating the reference. (The danger of this design is highlighted in the paper.)

> This problem could be easily avoided by having the extended facets store a copy of the locale from which they would extract the base facet either at construction time or when needed. To make it possible, the forms of ctors of the extended facets that take a reference to the base facet would need to be changed to take a locale argument instead.

Proposed resolution:

1. Change the `extended_num_get` synopsis in 3.10.2 as follows:

                extended_num_get(const std::num_get<charT, InputIterator> std::locale & b, size_t refs = 0);

                /* ... */

                // const std::num_get<charT, InputIterator> & base;        exposition only
                // std::locale baseloc;                                    exposition only

2. Change the description of the above constructor in 3.10.2.1:

                extended_num_get(const std::num_get<charT, InputIterator> std::locale & b, size_t refs = 0);

> Effects: Constructs an `extended_num_get` facet as if by:

>            extended_num_get(const std::num_get<charT, InputIterator> std::locale & b, size_t refs = 0)
>                     : facet(refs), baseloc(b)
>                     { /* ... */ }

> Notes: Care must be taken by the implementation to ensure that the lifetime of the facet referenced by base exceeds that of the resulting `extended_num_get` facet.

3. Change the Returns: clause for `do_get(iter_type, iter_type, ios_base &, ios_base::iostate &, bool &) const`, et al to

> Returns: `base std::use_facet<std::num_get<charT, InputIterator> >(baseloc).get(in, end, str, err, val)`.

4. Change the `extended_num_put` synopsis in 3.10.3 as follows:

                extended_num_put(const std::num_put<charT, OutputIterator> std::locale & b, size_t refs = 0);

                /* ... */

                // const std::num_put<charT, OutputIterator> & base;       exposition only
                // std::locale baseloc;                                    exposition only

5. Change the description of the above constructor in 3.10.3.1:

                extended_num_put(const std::num_put<charT, OutputIterator> std::locale & b, size_t refs = 0);

> Effects: Constructs an `extended_num_put` facet as if by:

>            extended_num_put(const std::num_put<charT, OutputIterator> std::locale & b, size_t refs = 0)
>                     : facet(refs), baseloc(b)
>                     { /* ... */ }

> Notes: Care must be taken by the implementation to ensure that the lifetime of the facet referenced by base exceeds that of the resulting `extended_num_put` facet.

6. Change the Returns: clause for `do_put(iter_type, ios_base &, char_type, bool &) const`, et al to

> Returns: `base std::use_facet<std::num_put<charT, OutputIterator> >(baseloc).put(s, f, fill, val)`.

[ Redmond: We would prefer to rename "extended" to "decimal". ]

### 605(i). [dec.tr] <decfloat.h> doesn't live here anymore.

Section: 3.4 [dec.tr::trdec.types.cdecfloat] Status: TRDec Submitter: Robert Klarer Opened: 2006-10-17 Last modified: 2016-01-31

Priority: Not Prioritized

View all issues with TRDec status.

Discussion:

In Berlin, WG14 decided to drop the <decfloat.h> header. The contents of that header have been moved into <float.h>. For the sake of C compatibility, we should make corresponding changes.

Proposed resolution:

1. Change the heading of subclause 3.4, "Headers `<cdecfloat>` and `<decfloat.h>`" to "Additions to headers `<cfloat>` and `<float.h>`."

2. Change the text of subclause 3.4 as follows:

> The standard C++ headers `<cfloat>` and `<float.h>` define characteristics of the floating-point types `float`, `double`, and `long double`. Their contents remain unchanged by this Technical Report.

> Headers `<cdecfloat>` and `<decfloat.h>` define characteristics of the decimal floating-point types `decimal32`, `decimal64`, and `decimal128`. As well, `<decfloat.h>` defines the convenience typedefs `_Decimal32`, `_Decimal64`, and `_Decimal128`, for compatibilty with the C programming language.

> The header `<cfloat>` is described in [tr.c99.cfloat]. The header `<float.h>` is described in [tr.c99.floath]. These headers are extended by this Technical Report to define characteristics of the decimal floating-point types `decimal32`, `decimal64`, and `decimal128`. As well, `<float.h>` is extended to define the convenience typedefs `_Decimal32`, `_Decimal64`, and `_Decimal128` for compatibility with the C programming language.

3. Change the heading of subclause 3.4.1, "Header `<cdecfloat>` synopsis" to "Additions to header `<cfloat>` synopsis."

4. Change the heading of subclause 3.4.2, "Header `<decfloat.h>` synopsis" to "Additions to header `<float.h>` synopsis."

5. Change the contents of 3.4.2 as follows:

          #include <cdecfloat>

          // C-compatibility convenience typedefs:

          typedef std::decimal::decimal32  _Decimal32;
          typedef std::decimal::decimal64  _Decimal64;
          typedef std::decimal::decimal128 _Decimal128;

### 607(i). Concern about short seed vectors

Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2006-10-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.util.seedseq].

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Discussion:

Short seed vectors of 32-bit quantities all result in different states. However this is not true of seed vectors of 16-bit (or smaller) quantities. For example these two seeds

>     unsigned short seed = {1, 2, 3};
>     unsigned short seed = {1, 2, 3, 0};

both pack to

>     unsigned seed = {0x20001, 0x3};

yielding the same state.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 608(i). Unclear seed_seq construction details

Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2006-10-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.util.seedseq].

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Discussion:

In 26.4.7.1 [rand.util.seedseq] /6, the order of packing the inputs into b and the treatment of signed quantities is unclear. Better to spell it out.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 609(i). missing static const

Section: 29.5.5.3 [rand.adapt.ibits], 99 [tr.rand] Status: CD1 Submitter: Walter E. Brown Opened: 2006-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

In preparing N2111, an error on my part resulted in the omission of the following line from the template synopsis in the cited section:

>     static const size_t word_size = w;

(This same constant is found, for example, in 26.4.3.3 [rand.eng.sub].)

Proposed resolution:

Add the above declaration as the first line after the comment in [rand.adapt.ibits] p4:

>     // engine characteristics
>     static const size_t word_size = w;

and accept my apologies for the oversight.

### 610(i). Suggested non-normative note for C++0x

Section: 22.10.17.3.2 [func.wrap.func.con], 99 [tr.func.wrap.func.con] Status: CD1 Submitter: Scott Meyers Opened: 2006-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.wrap.func.con].

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Discussion:

My suggestion is that implementers of both tr1::function and its official C++0x successor be explicitly encouraged (but not required) to optimize for the cases mentioned above, i.e., function pointers and small function objects. They could do this by using a small internal buffer akin to the buffer used by implementations of the small string optimization. (That would make this the small functor optimization -- SFO :-}) The form of this encouragement could be a note in the standard akin to footnote 214 of the current standard.

Dave Abrahams notes:

"shall not throw exceptions" should really be "nothing," both to be more grammatical and to be consistent with existing wording in the standard.

Doug Gregor comments: I think this is a good idea. Currently, implementations of tr1::function are required to have non-throwing constructors and assignment operators when the target function object is a function pointer or a reference_wrapper. The common case, however, is for a tr1::function to store either an empty function object or a member pointer + an object pointer.

The function implementation in the upcoming Boost 1.34.0 uses the "SFO", so that the function objects for typical bind expressions like

>     bind(&X::f, this, _1, _2, _3)

do not require heap allocation when stored in a boost::function. I believe Dinkumware's implementation also performs this optimization.

Proposed resolution:

Revise 20.5.14.2.1 p6 [func.wrap.func.con] to add a note as follows:

> Throws: shall not throw exceptions if `f`'s target is a function pointer or a function object passed via `reference_wrapper`. Otherwise, may throw `bad_alloc` or any exception thrown by the copy constructor of the stored function object.

> Note: Implementations are encouraged to avoid the use of dynamically allocated memory for "small" function objects, e.g., where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer (a "bound member function").

### 611(i). Standard library templates and incomplete types

Section: 16.4.5.8 [res.on.functions] Status: CD1 Submitter: Nicola Musatti Opened: 2006-11-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [res.on.functions].

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Discussion:

In the latest available draft standard (N2134) § 17.4.3.6 [res.on.functions] states:

> -1- In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ Standard Library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.

> -2- In particular, the effects are undefined in the following cases:

> [...]

> - if an incomplete type (3.9) is used as a template argument when instantiating a template component.

This is contradicted by § 20.6.6.2/2 [util.smartptr.shared] which states:

> [...]

> The template parameter `T` of `shared_ptr` may be an incomplete type.

Proposed resolution:

Modify the last bullet of § 17.4.3.6/2 [res.on.functions] to allow for exceptions:

> - if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for the component.

### 612(i). numeric_limits::is_modulo insufficiently defined

Section: 17.3.5.2 [numeric.limits.members] Status: CD1 Submitter: Chris Jefferson Opened: 2006-11-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [numeric.limits.members].

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Discussion:

18.2.1.2 55 states that "A type is modulo if it is possible to add two positive numbers together and have a result that wraps around to a third number that is less". This seems insufficient for the following reasons:

1. Doesn't define what that value received is.
2. Doesn't state the result is repeatable
3. Doesn't require that doing addition, subtraction and other operations on all values is defined behaviour.

[ Batavia: Related to N2144. Pete: is there an ISO definition of modulo? Underflow on signed behavior is undefined. ]

[ Bellevue: accept resolution, move to ready status. Does this mandate that is_modulo be true on platforms for which int happens to b modulo? A: the standard already seems to require that. ]

Proposed resolution:

Suggest 17.3.5.2 [numeric.limits.members], paragraph 57 is amended to:

> A type is modulo if, it is possible to add two positive numbers and have a result that wraps around to a third number that is less. given any operation involving +,- or * on values of that type whose value would fall outside the range `[min(), max()]`, then the value returned differs from the true value by an integer multiple of `(max() - min() + 1)`.

### 613(i). max_digits10 missing from numeric_limits

Section: 17.3.5.3 [numeric.special] Status: CD1 Submitter: Bo Persson Opened: 2006-11-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [numeric.special].

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Discussion:

Section 17.3.5.3 [numeric.special] starts out by saying that "All members shall be provided for all specializations."

Then it goes on to show specializations for float and bool, where one member is missing (max_digits10).

Maarten Kronenburg adds:

I agree, just adding the comment that the exact number of decimal digits is digits * ln(radix) / ln(10), where probably this real number is rounded downward for digits10, and rounded upward for max_digits10 (when radix=10, then digits10=max_digits10). Why not add this exact definition also to the standard, so the user knows what these numbers exactly mean.

Howard adds:

For reference, here are the correct formulas from N1822:

>     digits10 = floor((digits-1) * log10(2))
>     max_digits10 = ceil((1 + digits) * log10(2))

We are also missing a statement regarding for what specializations this member has meaning.

Proposed resolution:

Change and add after 17.3.5.2 [numeric.limits.members], p11:

>     static const int max_digits10;

> > -11- Number of base 10 digits required to ensure that values which differ by only one epsilon are always differentiated.

> > -12- Meaningful for all floating point types.

Change 17.3.5.3 [numeric.special], p2:

>     template<> class numeric_limits<float> {
>     public:
>       static const bool is_specialized = true;
>       ...
>       static const int digits10 = 6;
>       static const int max_digits10 = 9;
>       ...

Change 17.3.5.3 [numeric.special], p3:

>     template<> class numeric_limits<bool> {
>     public:
>       static const bool is_specialized = true;
>       ...
>       static const int digits10 = 0;
>       static const int max_digits10 = 0;
>       ...

### 616(i). missing 'typename' in ctype_byname

Section: 28.3.4.2.3 [locale.ctype.byname] Status: CD1 Submitter: Bo Persson Opened: 2006-12-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [locale.ctype.byname].

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Discussion:

Section 22.2.1.2 defines the ctype_byname class template. It contains the line

>     typedef ctype<charT>::mask   mask;

Proposed resolution:

as this is a dependent type, it should obviously be

>     typedef typename ctype<charT>::mask   mask;

### 618(i). valarray::cshift() effects on empty array

Section: 29.6.2.8 [valarray.members] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2007-01-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

I would respectfully request an issue be opened with the intention to clarify the wording for `size() == 0` for `cshift`.

Proposed resolution:

Change 29.6.2.8 [valarray.members], paragraph 10:

>     valarray<T> cshift(int n) const;

> > This function returns an object of class `valarray<T>`, of length `size()`, each of whose elements `I` is `(*this)[(I + n ) % size()]`. Thus, if element zero is taken as the leftmost element, a positive value of n shifts the elements circularly left n places. that is a circular shift of `*this`. If element zero is taken as the leftmost element, a non-negative value of n shifts the elements circularly left n places and a negative value of n shifts the elements circularly right -n places.

Rationale:

We do not believe that there is any real ambiguity about what happens when `size() == 0`, but we do believe that spelling this out as a C++ expression causes more trouble that it solves. The expression is certainly wrong when `n < 0`, since the sign of % with negative arguments is implementation defined.

[ Kona (2007) Changed proposed wording, added rationale and set to Review. ]

### 619(i). Longjmp wording problem

Section: 17.14 [support.runtime] Status: CD1 Submitter: Lawrence Crowl Opened: 2007-01-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [support.runtime].

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Discussion:

The wording for `longjmp` is confusing.

17.14 [support.runtime] -4- Other runtime support

> The function signature `longjmp(jmp_buf jbuf, int val)` has more restricted behavior in this International Standard. If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call to `longjmp(jbuf, val)` that the throw point that transfers control to the same (destination) point has undefined behavior.

Someone at Google thinks that should say "then a call to `longjmp(jbuf, val)` *at* the throw point that transfers control".

Bill Gibbons thinks it should say something like "If any automatic objects would be destroyed by an exception thrown at the point of the longjmp and caught only at the point of the setjmp, the behavior is undefined."

Proposed resolution:

In general, accept Bill Gibbons' recommendation, but add "call" to indicate that the undefined behavior comes from the dynamic call, not from its presence in the code. In 17.14 [support.runtime] paragraph 4, change

> The function signature `longjmp(jmp_buf jbuf, int val)` has more restricted behavior in this International Standard. If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call to `longjmp(jbuf, val)` that the throw point that transfers control to the same (destination) point has undefined behavior. A `setjmp`/`longjmp` call pair has undefined behavior if replacing the `setjmp` and `longjmp` by `catch` and `throw` would destroy any automatic objects.

### 620(i). valid uses of empty valarrays

Section: 29.6.2.2 [valarray.cons] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [valarray.cons].

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Discussion:

The Effects clause for the default `valarray` ctor suggests that it is possible to increase the size of an empty `valarray` object by calling other non-const member functions of the class besides `resize()`. However, such an interpretation would be contradicted by the requirement on the copy assignment operator (and apparently also that on the computed assignments) that the assigned arrays be the same size. See the reflector discussion starting with c++std-lib-17871.

In addition, Footnote 280 uses some questionable normative language.

Proposed resolution:

Reword the Effects clause and Footnote 280 as follows (29.6.2.2 [valarray.cons]):

> `valarray();`

> Effects: Constructs an object of class `valarray<T>`,279) which has zero length until it is passed into a library function as a modifiable lvalue or through a non-constant this pointer.280)

> Postcondition: `size() == 0`.

> Footnote 280: This default constructor is essential, since arrays of `valarray` are likely to prove useful. There shall also be a way to change the size of an array after initialization; this is supplied by the semantics may be useful. The length of an empty array can be increased after initialization by means of the `resize()` member function.

### 621(i). non-const copy assignment operators of helper arrays

Section: 29.6 [numarray] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [numarray].

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Discussion:

The computed and "fill" assignment operators of `valarray` helper array class templates (`slice_array`, `gslice_array`, `mask_array`, and `indirect_array`) are const member functions of each class template (the latter by the resolution of 123(i) since they have reference semantics and thus do not affect the state of the object on which they are called. However, the copy assignment operators of these class templates, which also have reference semantics, are non-const. The absence of constness opens the door to speculation about whether they really are intended to have reference semantics (existing implementations vary widely).

Pre-Kona, Martin adds:

I realized that adding the const qualifier to the functions as I suggested would break the const correctness of the classes. A few possible solutions come to mind:

1. Add the const qualifier to the return types of these functions.
2. Change the return type of all the functions to void to match the signatures of all the other assignment operators these classes define.
3. Prohibit the copy assignment of these classes by declaring the copy assignment operators private (as is done and documented by some implementations).

Proposed resolution:

Declare the copy assignment operators of all four helper array class templates const.

Specifically, make the following edits:

Change the signature in 29.6.5 [template.slice.array] and 29.6.5.2 [slice.arr.assign] as follows:

>     const slice_array& operator= (const slice_array&) const;

Change the signature in 29.6.7 [template.gslice.array] and 29.6.7.2 [gslice.array.assign] as follows:

>     const gslice_array& operator= (const gslice_array&) const;

Change the signature in 29.6.8 [template.mask.array] and 29.6.8.2 [mask.array.assign] as follows:

>     const mask_array& operator= (const mask_array&) const;

Change the signature in 29.6.9 [template.indirect.array] and 29.6.9.2 [indirect.array.assign] as follows:

>     const indirect_array& operator= (const indirect_array&) const;

[ Kona (2007) Added const qualification to the return types and set to Ready. ]

### 622(i). behavior of `filebuf` dtor and `close` on error

Section: 31.10.6.4 [fstream.members] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

`basic_filebuf` dtor is specified to have the following straightforward effects:

> Effects: Destroys an object of class `basic_filebuf`. Calls `close()`.

`close()` does a lot of potentially complicated processing, including calling `overflow()` to write out the termination sequence (to bring the output sequence to its initial shift state). Since any of the functions called during the processing can throw an exception, what should the effects of an exception be on the dtor? Should the dtor catch and swallow it or should it propagate it to the caller? The text doesn't seem to provide any guidance in this regard other than the general restriction on throwing (but not propagating) exceptions from destructors of library classes in 16.4.6.14 [res.on.exception.handling].

Further, the last thing `close()` is specified to do is call `fclose()` to close the `FILE` pointer. The last sentence of the Effects clause reads:

> ... If any of the calls to `overflow` or `std::fclose` fails then `close` fails.

This suggests that `close()` might be required to call `fclose()` if and only if none of the calls to `overflow()` fails, and avoid closing the `FILE` otherwise. This way, if `overflow()` failed to flush out the data, the caller would have the opportunity to try to flush it again (perhaps after trying to deal with whatever problem may have caused the failure), rather than losing it outright.

On the other hand, the function's Postcondition specifies that `is_open() == false`, which suggests that it should call `fclose()` unconditionally. However, since Postcondition clauses are specified for many functions in the standard, including constructors where they obviously cannot apply after an exception, it's not clear whether this Postcondition clause is intended to apply even after an exception.

It might be worth noting that the traditional behavior (Classic Iostreams `fstream::close()` and C `fclose()`) is to close the `FILE` unconditionally, regardless of errors.

[ See 397(i) and 418(i) for related issues. ]

Proposed resolution:

After discussing this on the reflector (see the thread starting with c++std-lib-17650) we propose that `close()` be clarified to match the traditional behavior, that is to close the `FILE` unconditionally, even after errors or exceptions. In addition, we propose the dtor description be amended so as to explicitly require it to catch and swallow any exceptions thrown by `close()`.

Specifically, we propose to make the following edits in 31.10.3.4 [filebuf.members]:

>     basic_filebuf<charT,traits>* close();

> Effects: If `is_open() == false`, returns a null pointer. If a put area exists, calls `overflow(traits::eof())` to flush characters. If the last virtual member function called on `*this` (between `underflow`, `overflow`, `seekoff`, and `seekpos`) was `overflow` then calls `a_codecvt.unshift` (possibly several times) to determine a termination sequence, inserts those characters and calls `overflow(traits::eof())` again. Finally, regardless of whether any of the preceding calls fails or throws an exception, the function it closes the file ("as if" by calling `std::fclose(file)`).334) If any of the calls made by the functionto `overflow` or, including `std::fclose`, fails then `close` fails by returning a null pointer. If one of these calls throws an exception, the exception is caught and rethrown after closing the file.

And to make the following edits in 31.10.3.2 [filebuf.cons].

>     virtual ~basic_filebuf();

> Effects: Destroys an object of class `basic_filebuf<charT,traits>`. Calls `close()`. If an exception occurs during the destruction of the object, including the call to `close()`, the exception is caught but not rethrown (see 16.4.6.14 [res.on.exception.handling]).

### 623(i). `pubimbue` forbidden to call `imbue`

Section: 31.2.1 [iostream.limits.imbue] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

31.2.1 [iostream.limits.imbue] specifies that "no function described in clause 27 except for `ios_base::imbue` causes any instance of `basic_ios::imbue` or `basic_streambuf::imbue` to be called."

That contradicts the Effects clause for `basic_streambuf::pubimbue()` which requires the function to do just that: call `basic_streambuf::imbue()`.

Proposed resolution:

To fix this, rephrase the sentence above to allow `pubimbue` to do what it was designed to do. Specifically. change 31.2.1 [iostream.limits.imbue], p1 to read:

> No function described in clause 27 except for `ios_base::imbue` and `basic_filebuf::pubimbue` causes any instance of `basic_ios::imbue` or `basic_streambuf::imbue` to be called. ...

### 624(i). `valarray` assignment and arrays of unequal length

Section: 29.6.2.3 [valarray.assign] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [valarray.assign].

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Discussion:

The behavior of the `valarray` copy assignment operator is defined only when both sides have the same number of elements and the spec is explicit about assignments of arrays of unequal lengths having undefined behavior.

However, the generalized subscripting assignment operators overloaded on `slice_array` et al (29.6.2.3 [valarray.assign]) don't have any such restriction, leading the reader to believe that the behavior of these overloads is well defined regardless of the lengths of the arguments.

For example, based on the reading of the spec the behavior of the snippet below can be expected to be well-defined:

        const std::slice from_0_to_3 (0, 3, 1);   // refers to elements 0, 1, 2
        const std::valarray<int> a (1, 3);        // a = { 1, 1, 1 }
        std::valarray<int>       b (2, 4);        // b = { 2, 2, 2, 2 }

        b = a [from_0_to_3];

In practice, `b` may end up being `{ 1, 1, 1 }`, `{ 1, 1, 1, 2 }`, or anything else, indicating that existing implementations vary.

Quoting from Section 3.4, Assignment operators, of Al Vermeulen's Proposal for Standard C++ Array Classes (see c++std-lib-704; N0308):

> ...if the size of the array on the right hand side of the equal sign differs from the size of the array on the left, a run time error occurs. How this error is handled is implementation dependent; for compilers which support it, throwing an exception would be reasonable.

And see more history in N0280.

It has been argued in discussions on the committee's reflector that the semantics of all `valarray` assignment operators should be permitted to be undefined unless the length of the arrays being assigned is the same as the length of the one being assigned from. See the thread starting at c++std-lib-17786.

In order to reflect such views, the standard must specify that the size of the array referred to by the argument of the assignment must match the size of the array under assignment, for example by adding a Requires clause to 29.6.2.3 [valarray.assign] as follows:

> Requires: The length of the array to which the argument refers equals `size()`.

Note that it's far from clear that such leeway is necessary in order to implement `valarray` efficiently.

Proposed resolution:

Insert new paragraph into 29.6.2.3 [valarray.assign]:

>     valarray<T>& operator=(const slice_array<T>&);
>     valarray<T>& operator=(const gslice_array<T>&);
>     valarray<T>& operator=(const mask_array<T>&);
>     valarray<T>& operator=(const indirect_array<T>&);

> > Requires: The length of the array to which the argument refers equals `size()`.

> > These operators allow the results of a generalized subscripting operation to be assigned directly to a `valarray`.

### 625(i). Mixed up Effects and Returns clauses

Section: 16 [library] Status: Resolved Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 895

Discussion:

Many member functions of `basic_string` are overloaded, with some of the overloads taking a `string` argument, others `value_type*`, others `size_type`, and others still `iterators`. Often, the requirements on one of the overloads are expressed in the form of Effects, Throws, and in the Working Paper (N2134) also Remark clauses, while those on the rest of the overloads via a reference to this overload and using a Returns clause.

The difference between the two forms of specification is that per 16.3.2.4 [structure.specifications], p3, an Effects clause specifies "actions performed by the functions," i.e., its observable effects, while a Returns clause is "a description of the return value(s) of a function" that does not impose any requirements on the function's observable effects.

Since only Notes are explicitly defined to be informative and all other paragraphs are explicitly defined to be normative, like Effects and Returns, the new Remark clauses also impose normative requirements.

So by this strict reading of the standard there are some member functions of `basic_string` that are required to throw an exception under some conditions or use specific traits members while many other otherwise equivalent overloads, while obliged to return the same values, aren't required to follow the exact same requirements with regards to the observable effects.

Here's an example of this problem that was precipitated by the change from informative Notes to normative Remarks (presumably made to address 424(i)):

In the Working Paper, `find(string, size_type)` contains a Remark clause (which is just a Note in the current standard) requiring it to use `traits::eq()`.

`find(const charT *s, size_type pos)` is specified to return `find(string(s), pos)` by a Returns clause and so it is not required to use `traits::eq()`. However, the Working Paper has replaced the original informative Note about the function using `traits::length()` with a normative requirement in the form of a Remark. Calling `traits::length()` may be suboptimal, for example when the argument is a very long array whose initial substring doesn't appear anywhere in `*this`.

Here's another similar example, one that existed even prior to the introduction of Remarks:

`insert(size_type pos, string, size_type, size_type)` is required to throw `out_of_range` if `pos > size()`.

`insert(size_type pos, string str)` is specified to return `insert(pos, str, 0, npos)` by a Returns clause and so its effects when `pos > size()` are strictly speaking unspecified.

I believe a careful review of the current Effects and Returns clauses is needed in order to identify all such problematic cases. In addition, a review of the Working Paper should be done to make sure that the newly introduced normative Remark clauses do not impose any undesirable normative requirements in place of the original informative Notes.

[ Batavia: Alan and Pete to work. ]

[ Bellevue: Marked as NAD Editorial. ]

[ Post-Sophia Antipolis: Martin indicates there is still work to be done on this issue. Reopened. ]

[ Batavia (2009-05): ]

> Tom proposes we say that, unless specified otherwise, it is always the caller's responsibility to verify that supplied arguments meet the called function's requirements. If further semantics are specified (e.g., that the function throws under certain conditions), then it is up to the implementer to check those conditions. Alan feels strongly that our current use of Requires in this context is confusing, especially now that `requires` is a new keyword.

[ 2009-07 Frankfurt ]

> Move to Tentatively NAD.

[ 2009 Santa Cruz: ]

> Move to Open. Martin will work on proposed wording.

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial, solved by revision to N3021.

Rationale:

Solved by revision to N3021.

Proposed resolution:

### 628(i). Inconsistent definition of basic_regex constructor

Section: 28.6.7 [re.regex] Status: CD1 Submitter: Bo Persson Opened: 2007-01-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 28.6.7 [re.regex] lists a constructor

>     template<class InputIterator>
>     basic_regex(InputIterator first, InputIterator last,
>                            flag_type f = regex_constants::ECMAScript);

However, in section 28.6.7.2 [re.regex.construct], this constructor takes a pair of `ForwardIterator`.

Proposed resolution:

Change 28.6.7.2 [re.regex.construct]:

>     template <class ForwardIterator InputIterator>
>       basic_regex(ForwardIterator InputIterator first, ForwardIterator InputIterator last,
>                   flag_type f = regex_constants::ECMAScript);

### 629(i). `complex<T>` insertion and locale dependence

Section: 29.4.6 [complex.ops] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2007-01-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

is there an issue opened for (0,3) as complex number with the French local? With the English local, the above parses as an imaginery complex number. With the French locale it parses as a real complex number.

Further notes/ideas from the lib-reflector, messages 17982-17984:

> Add additional entries in `num_punct` to cover the complex separator (French would be ';').

> Insert a space before the comma, which should eliminate the ambiguity.

> Solve the problem for ordered sequences in general, perhaps with a dedicated facet. Then complex should use that solution.

[ Bellevue: ]

> After much discussion, we agreed on the following: Add a footnote:

> [In a locale in which comma is being used as a decimal point character, inserting "showbase" into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]

> And move this to READY status.

[ Pre-Sophia Antipolis, Howard adds: ]

> Changed "showbase" to "showpoint" and changed from Ready to Review.

[ Post-Sophia Antipolis: ]

> I neglected to pull this issue from the formal motions page after the "showbase" to "showpoint" change. In Sophia Antipolis this change was reviewed by the LWG and the issue was set to Ready. We subsequently voted the footnote into the WP with "showbase".

> I'm changing from WP back to Ready to pick up the "showbase" to "showpoint" change.

Proposed resolution:

Add a footnote to 29.4.6 [complex.ops] p16:

> [In a locale in which comma is being used as a decimal point character, inserting `showpoint` into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]

### 630(i). arrays of `valarray`

Section: 29.6.2.2 [valarray.cons] Status: C++11 Submitter: Martin Sebor Opened: 2007-01-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [valarray.cons].

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Discussion:

Section 29.2 [numeric.requirements], p1 suggests that a `valarray` specialization on a type `T` that satisfies the requirements enumerated in the paragraph is itself a valid type on which `valarray` may be instantiated (Footnote 269 makes this clear). I.e., `valarray<valarray<T> >` is valid as long as `T` is valid. However, since implementations of `valarray` are permitted to initialize storage allocated by the class by invoking the default ctor of `T` followed by the copy assignment operator, such implementations of `valarray` wouldn't work with (perhaps user-defined) specializations of `valarray` whose assignment operator had undefined behavior when the size of its argument didn't match the size of `*this`. By "wouldn't work" I mean that it would be impossible to resize such an array of arrays by calling the `resize()` member function on it if the function used the copy assignment operator after constructing all elements using the default ctor (e.g., by invoking `new value_type[N]`) to obtain default-initialized storage) as it's permitted to do.

Stated more generally, the problem is that `valarray<valarray<T> >::resize(size_t)` isn't required or guaranteed to have well-defined semantics for every type `T` that satisfies all requirements in 29.2 [numeric.requirements].

I believe this problem was introduced by the adoption of the resolution outlined in N0857, Assignment of valarrays, from 1996. The copy assignment operator of the original numerical array classes proposed in N0280, as well as the one proposed in N0308 (both from 1993), had well-defined semantics for arrays of unequal size (the latter explicitly only when `*this` was empty; assignment of non empty arrays of unequal size was a runtime error).

The justification for the change given in N0857 was the "loss of performance [deemed] only significant for very simple operations on small arrays or for architectures with very few registers."

Since tiny arrays on a limited subset of hardware architectures are likely to be an exceedingly rare case (despite the continued popularity of x86) I propose to revert the resolution and make the behavior of all `valarray` assignment operators well-defined even for non-conformal arrays (i.e., arrays of unequal size). I have implemented this change and measured no significant degradation in performance in the common case (non-empty arrays of equal size). I have measured a 50% (and in some cases even greater) speedup in the case of assignments to empty arrays versus calling `resize()` first followed by an invocation of the copy assignment operator.

[ Bellevue: ]

> If no proposed wording by June meeting, this issue should be closed NAD.

[ 2009-07 Frankfurt ]

> Move resolution 1 to Ready.

> Howard: second resolution has been commented out (made invisible). Can be brought back on demand.

Proposed resolution:

Change 29.6.2.3 [valarray.assign], p1 as follows:

> `valarray<T>& operator=(const valarray<T>& x);`

> -1- Each element of the `*this` array is assigned the value of the corresponding element of the argument array. The resulting behavior is undefined if When the length of the argument array is not equal to the length of the *this array. resizes `*this` to make the two arrays the same length, as if by calling `resize(x.size())`, before performing the assignment.

And add a new paragraph just below paragraph 1 with the following text:

> -2- Postcondition: `size() == x.size()`.

Also add the following paragraph to 29.6.2.3 [valarray.assign], immediately after p4:

> -?- When the length, `N` of the array referred to by the argument is not equal to the length of `*this`, the operator resizes `*this` to make the two arrays the same length, as if by calling `resize(N)`, before performing the assignment.

[ pre-Sophia Antipolis, Martin adds the following compromise wording, but prefers the original proposed resolution: ]

[ Kona (2007): Gaby to propose wording for an alternative resolution in which you can assign to a `valarray` of size 0, but not to any other `valarray` whose size is unequal to the right hand side of the assignment. ]

### 634(i). `allocator.address()` doesn't work for types overloading `operator&`

Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Howard Hinnant Opened: 2007-02-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [allocator.members].

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Duplicate of: 350

Discussion:

20.2.10.2 [allocator.members] says:

>     pointer address(reference x) const;

> > -1- Returns: `&x`.

20.2.10.2 [allocator.members] defines `CopyConstructible` which currently not only defines the semantics of copy construction, but also restricts what an overloaded `operator&` may do. I believe proposals are in the works (such as concepts and rvalue reference) to decouple these two requirements. Indeed it is not evident that we should disallow overloading `operator&` to return something other than the address of `*this`.

An example of when you want to overload `operator&` to return something other than the object's address is proxy references such as `vector<bool>` (or its replacement, currently code-named `bit_vector`). Taking the address of such a proxy reference should logically yield a proxy pointer, which when dereferenced, yields a copy of the original proxy reference again.

On the other hand, some code truly needs the address of an object, and not a proxy (typically for determining the identity of an object compared to a reference object). boost has long recognized this dilemma and solved it with `boost::addressof`. It appears to me that this would be useful functionality for the default allocator. Adopting this definition for `allocator::address` would free the standard of requiring anything special from types which overload `operator&`. Issue 580(i) is expected to make use of `allocator::address` mandatory for containers.

Proposed resolution:

Change 20.2.10.2 [allocator.members]:

>     pointer address(reference x) const;

> > -1- Returns: `&x`. The actual address of object referenced by x, even in the presence of an overloaded `operator&`.

>     const_pointer address(address(const_reference x) const;

> > -2- Returns: `&x`. The actual address of object referenced by x, even in the presence of an overloaded `operator&`.

[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]

[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 635(i). domain of `allocator::address`

Section: 16.4.4.6 [allocator.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-02-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The table of allocator requirements in 16.4.4.6 [allocator.requirements] describes `allocator::address` as:

>     a.address(r)
>     a.address(s)

where `r` and `s` are described as:

> a value of type `X::reference` obtained by the expression `*p`.

and `p` is

> a value of type `X::pointer`, obtained by calling `a1.allocate`, where `a1 == a`

This all implies that to get the address of some value of type `T` that value must have been allocated by this allocator or a copy of it.

However sometimes container code needs to compare the address of an external value of type `T` with an internal value. For example `list::remove(const T& t)` may want to compare the address of the external value `t` with that of a value stored within the list. Similarly `vector` or `deque insert` may want to make similar comparisons (to check for self-referencing calls).

Mandating that `allocator::address` can only be called for values which the allocator allocated seems overly restrictive.

[ post San Francisco: ]

> Pablo recommends NAD Editorial, solved by N2768.

[ 2009-04-28 Pablo adds: ]

> Tentatively-ready NAD Editorial as fixed by N2768.

[ 2009-07 Frankfurt ]

> Fixed by N2768.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2982.

Proposed resolution:

Change 16.4.4.6 [allocator.requirements]:

> `r` : a value of type `X::reference` obtained by the expression *p.

> `s` : a value of type `X::const_reference` obtained by the expression `*q` or by conversion from a value `r`.

[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]

[ Kona (2007): This issue is section 8 of N2387. There was some discussion of it but no resolution to this issue was recorded. Moved to Open. ]

### 638(i). `deque` end invalidation during erase

Section: 23.3.5.4 [deque.modifiers] Status: CD1 Submitter: Steve LoBasso Opened: 2007-02-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The standard states at 23.3.5.4 [deque.modifiers]/4:

>     deque erase(...)

> Effects: ... An erase at either end of the deque invalidates only the iterators and the references to the erased elements.

This does not state that iterators to end will be invalidated. It needs to be amended in such a way as to account for end invalidation.

Something like:

> Any time the last element is erased, iterators to end are invalidated.

This would handle situations like:

>     erase(begin(), end())
>     erase(end() - 1)
>     pop_back()
>     resize(n, ...) where n < size()
>     pop_front() with size() == 1

[ Post Kona, Steve LoBasso notes: ]

> My only issue with the proposed resolution is that it might not be clear that `pop_front()` [where `size() == 1`] can invalidate past-the-end iterators.

[ Kona (2007): Proposed wording added and moved to Review. ]

[ Bellevue: ]

> Note that there is existing code that relies on iterators not being invalidated, but there are also existing implementations that do invalidate iterators. Thus, such code is not portable in any case. There is a `pop_front()` note, which should possibly be a separate issue. Mike Spertus to evaluate and, if need be, file an issue.

Proposed resolution:

Change 23.3.5.4 [deque.modifiers], p4:

>     iterator erase(const_iterator position);
>     iterator erase(const_iterator first, const_iterator last);

> > -4- Effects: An erase in the middle of the `deque` invalidates all the iterators and references to elements of the `deque` and the past-the-end iterator. An erase at either end of the `deque` invalidates only the iterators and the references to the erased elements, except that erasing at the end also invalidates the past-the-end iterator.

### 640(i). 27.6.2.5.2 does not handle `(unsigned) long long`

Section: 31.7.6.3.2 [ostream.inserters.arithmetic] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ostream.inserters.arithmetic].

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Discussion:

The arithmetic inserters are described in 31.7.6.3.2 [ostream.inserters.arithmetic]. Although the section starts with a listing of the inserters including the new ones:

>     operator<<(long long val );
>     operator<<(unsigned long long val );

the text in paragraph 1, which describes the corresponding effects of the inserters, depending on the actual type of val, does not handle the types `long long` and `unsigned long long`.

[ Alisdair: In addition to the (unsigned) long long problem, that whole paragraph misses any reference to extended integral types supplied by the implementation - one of the additions by core a couple of working papers back. ]

Proposed resolution:

In 31.7.6.3.2 [ostream.inserters.arithmetic]/1 change the third sentence

> When val is of type `bool`, `long`, `unsigned long`, long long, unsigned long long, `double`, `long double`, or `const void*`, the formatting conversion occurs as if it performed the following code fragment:

### 643(i). Impossible "as if" clauses

Section: 31.10.3 [filebuf], 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

The current standard 14882:2003(E) as well as N2134 have the following defects:

31.10.3 [filebuf]/5 says:

> In order to support file I/O and multibyte/wide character conversion, conversions are performed using members of a facet, referred to as `a_codecvt` in following sections, obtained "as if" by

> >     codecvt<charT,char,typename traits::state_type> a_codecvt =
> >       use_facet<codecvt<charT,char,typename traits::state_type> >(getloc());

`use_facet` returns a `const facet` reference and no facet is copyconstructible, so the codecvt construction should fail to compile.

A similar issue arises in 28.3.4.3.3.3 [facet.num.put.virtuals]/15 for `num_punct`.

Proposed resolution:

In 31.10.3 [filebuf]/5 change the "as if" code

>     const codecvt<charT,char,typename traits::state_type>& a_codecvt =
>       use_facet<codecvt<charT,char,typename traits::state_type> >(getloc());

In 28.3.4.3.3.3 [facet.num.put.virtuals]/15 (This is para 5 in N2134) change

> A local variable `punct` is initialized via

> >     const numpunct<charT>& punct = use_facet< numpunct<charT> >(str.getloc() );

(Please note also the additional provided trailing semicolon)

### 646(i). const incorrect match_result members

Section: 28.6.9.6 [re.results.form] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

28.6.9.6 [re.results.form] (root and para 3) in N2134 defines the two function template members format as non-const functions, although they are declared as const in 28.6.9 [re.results]/3.

Proposed resolution:

Add the missing `const` specifier to both `format` overloads described in section 28.6.9.6 [re.results.form].

### 650(i). regex_token_iterator and const correctness

Section: 28.6.11.2 [re.tokiter] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Both the class definition of regex_token_iterator (28.6.11.2 [re.tokiter]/6) and the latter member specifications (28.6.11.2.3 [re.tokiter.comp]/1+2) declare both comparison operators as non-const functions. Furtheron, both dereference operators are unexpectedly also declared as non-const in 28.6.11.2 [re.tokiter]/6 as well as in (28.6.11.2.4 [re.tokiter.deref]/1+2).

Proposed resolution:

1) In (28.6.11.2 [re.tokiter]/6) change the current declarations

>     bool operator==(const regex_token_iterator&) const;
>     bool operator!=(const regex_token_iterator&) const;
>     const value_type& operator*() const;
>     const value_type* operator->() const;

2) In 28.6.11.2.3 [re.tokiter.comp] change the following declarations

>     bool operator==(const regex_token_iterator& right) const;
>     bool operator!=(const regex_token_iterator& right) const;

3) In 28.6.11.2.4 [re.tokiter.deref] change the following declarations

>     const value_type& operator*() const;
>     const value_type* operator->() const;

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 651(i). Missing preconditions for regex_token_iterator c'tors

Section: 28.6.11.2.2 [re.tokiter.cnstr] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The text provided in 28.6.11.2.2 [re.tokiter.cnstr]/2+3 describes the effects of the three non-default constructors of class template regex_token_iterator but is does not clarify which values are legal values for submatch/submatches. This becomes an issue, if one takes 28.6.11.2 [re.tokiter]/9 into account, which explains the notion of a "current match" by saying:

> The current match is `(*position).prefix()` if `subs[N] == -1`, or `(*position)[subs[N]]` for any other value of `subs[N]`.

It's not clear to me, whether other negative values except -1 are legal arguments or not - it seems they are not.

Proposed resolution:

Add the following precondition paragraph just before the current 28.6.11.2.2 [re.tokiter.cnstr]/2:

> Requires: Each of the initialization values of `subs` must be `>= -1`.

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 652(i). regex_iterator and const correctness

Section: 28.6.11.1 [re.regiter] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Both the class definition of regex_iterator (28.6.11.1 [re.regiter]/1) and the latter member specification (28.6.11.1.3 [re.regiter.comp]/1+2) declare both comparison operators as non-const functions. Furtheron, both dereference operators are unexpectedly also declared as non-const in 28.6.11.1 [re.regiter]/1 as well as in (28.6.11.1.4 [re.regiter.deref]/1+2).

Proposed resolution:

1) In (28.6.11.1 [re.regiter]/1) change the current declarations

>     bool operator==(const regex_iterator&) const;
>     bool operator!=(const regex_iterator&) const;
>     const value_type& operator*() const;
>     const value_type* operator->() const;

2) In 28.6.11.1.4 [re.regiter.deref] change the following declarations

>     const value_type& operator*() const;
>     const value_type* operator->() const;

3) In 28.6.11.1.3 [re.regiter.comp] change the following declarations

>     bool operator==(const regex_iterator& right) const;
>     bool operator!=(const regex_iterator& right) const;

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 654(i). Missing IO roundtrip for random number engines

Section: 29.5.3.4 [rand.req.eng] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 98 and para 5 in 29.5.3.4 [rand.req.eng] specify the IO insertion and extraction semantic of random number engines. It can be shown, v.i., that the specification of the extractor cannot guarantee to fulfill the requirement from para 5:

> If a textual representation written via os << x was subsequently read via is >> v, then x == v provided that there have been no intervening invocations of x or of v.

The problem is, that the extraction process described in table 98 misses to specify that it will initially set the if.fmtflags to ios_base::dec, see table 104:

> dec: converts integer input or generates integer output in decimal base

Proof: The following small program demonstrates the violation of requirements (exception safety not fulfilled):

>     #include <cassert>
>     #include <ostream>
>     #include <iostream>
>     #include <iomanip>
>     #include <sstream>

>     class RanNumEngine {
>       int state;
>     public:
>       RanNumEngine() : state(42) {}

>       bool operator==(RanNumEngine other) const {
>           return state == other.state;
>       }

>       template <typename Ch, typename Tr>
>       friend std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& os, RanNumEngine engine) {
>         Ch old = os.fill(os.widen(' ')); // Sets space character
>         std::ios_base::fmtflags f = os.flags();
>         os << std::dec << std::left << engine.state; // Adds ios_base::dec|ios_base::left
>         os.fill(old); // Undo
>         os.flags(f);
>         return os;
>       }

>       template <typename Ch, typename Tr>
>       friend std::basic_istream<Ch, Tr>& operator>>(std::basic_istream<Ch, Tr>& is, RanNumEngine& engine) {
>            // Uncomment only for the fix.

>         //std::ios_base::fmtflags f = is.flags();
>         //is >> std::dec;
>         is >> engine.state;
>         //is.flags(f);
>         return is;
>       }
>     };

>     int main() {
>         std::stringstream s;
>         s << std::setfill('#'); // No problem
>             s << std::oct; // Yikes!
>             // Here starts para 5 requirements:
>         RanNumEngine x;
>         s << x;
>         RanNumEngine v;
>         s >> v;
>         assert(x == v); // Fails: 42 == 34
>     }

A second, minor issue seems to be, that the insertion description from table 98 unnecessarily requires the addition of ios_base::fixed (which only influences floating-point numbers). Its not entirely clear to me whether the proposed standard does require that the state of random number engines is stored in integral types or not, but I have the impression that this is the indent, see e.g. p. 3

> The specification of each random number engine defines the size of its state in multiples of the size of its result_type.

If other types than integrals are supported, then I wonder why no requirements are specified for the precision of the stream.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 655(i). Signature of generate_canonical not useful

Section: 29.5.8.2 [rand.util.canonical] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 29.5.2 [rand.synopsis] we have the declaration

>     template<class RealType, class UniformRandomNumberGenerator,
>       size_t bits>
>     result_type generate_canonical(UniformRandomNumberGenerator& g);

Besides the "result_type" issue (already recognized by Bo Persson at Sun, 11 Feb 2007 05:26:47 GMT in this group) it's clear, that the template parameter order is not reasonably choosen: Obviously one always needs to specify all three parameters, although usually only two are required, namely the result type RealType and the wanted bits, because UniformRandomNumberGenerator can usually be deduced.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 658(i). Two unspecified function comparators in [function.objects]

Section: 22.10 [function.objects] Status: Resolved Submitter: Daniel Krügler Opened: 2007-03-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The header `<functional>` synopsis in 22.10 [function.objects] contains the following two free comparison operator templates for the `function` class template

>     template<class Function1, class Function2>
>     void operator==(const function<Function1>&, const function<Function2>&);
>     template<class Function1, class Function2>
>     void operator!=(const function<Function1>&, const function<Function2>&);

which are nowhere described. I assume that they are relicts before the corresponding two private and undefined member templates in the function template (see 22.10.17.3 [func.wrap.func] and [func.wrap.func.undef]) have been introduced. The original free function templates should be removed, because using an undefined entity would lead to an ODR violation of the user.

Proposed resolution:

Remove the above mentioned two function templates from the header `<functional>` synopsis (22.10 [function.objects])

>     template<class Function1, class Function2>
>     void operator==(const function<Function1>&, const function<Function2>&);
>     template<class Function1, class Function2>
>     void operator!=(const function<Function1>&, const function<Function2>&);

Rationale:

Fixed by N2292 Standard Library Applications for Deleted Functions.

### 659(i). `istreambuf_iterator` should have an `operator->()`

Section: 24.6.4 [istreambuf.iterator] Status: C++11 Submitter: Niels Dekker Opened: 2007-03-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Greg Herlihy has clearly demonstrated that a user defined input iterator should have an `operator->()`, even if its value type is a built-in type (comp.std.c++, "Re: Should any iterator have an `operator->()` in C++0x?", March 2007). And as Howard Hinnant remarked in the same thread that the input iterator `istreambuf_iterator` doesn't have one, this must be a defect!

Based on Greg's example, the following code demonstrates the issue:

     #include <iostream>
     #include <fstream>
     #include <streambuf>

     typedef char C;
     int main ()
     {
       std::ifstream s("filename", std::ios::in);
       std::istreambuf_iterator<char> i(s);

       (*i).~C();  // This is well-formed...
       i->~C();  // ... so this should be supported!
     }

Of course, `operator->` is also needed when the `value_type` of `istreambuf_iterator` is a class.

The `operator->` could be implemented in various ways. For instance, by storing the current value inside the iterator, and returning its address. Or by returning a proxy, like `operator_arrow_proxy`, from http://www.boost.org/boost/iterator/iterator_facade.hpp

I hope that the resolution of this issue will contribute to getting a clear and consistent definition of iterator concepts.

[ Kona (2007): The proposed resolution is inconsistent because the return type of `istreambuf_iterator::operator->()` is specified to be `pointer`, but the proposed text also states that "`operator->` may return a proxy." ]

[ Niels Dekker (mailed to Howard Hinnant): ]

> The proposed resolution does not seem inconsistent to me. `istreambuf_iterator::operator->()` should have `istreambuf_iterator::pointer` as return type, and this return type may in fact be a proxy.

> AFAIK, the resolution of 445(i) ("`iterator_traits::reference` unspecified for some iterator categories") implies that for any iterator class `Iter`, the return type of `operator->()` is `Iter::pointer`, by definition. I don't think `Iter::pointer` needs to be a raw pointer.

> Still I wouldn't mind if the text "`operator->` may return a proxy" would be removed from the resolution. I think it's up to the library implementation, how to implement `istreambuf_iterator::operator->()`. As longs as it behaves as expected: `i->m` should have the same effect as `(*i).m`. Even for an explicit destructor call, `i->~C()`. The main issue is just: `istreambuf_iterator` should have an `operator->()`!

[ 2009-04-30 Alisdair adds: ]

> Note that `operator->` is now a requirement in the `InputIterator` concept, so this issue cannot be ignored or existing valid programs will break when compiled with an 0x library.

[ 2009-05-29 Alisdair adds: ]

> I agree with the observation that in principle the type 'pointer' may be a proxy, and the words highlighting this are redundant.

> However, in the current draught `pointer` is required to be exactly '`charT *`' by the derivation from `std::iterator`. At a minimum, the 4th parameter of this base class template should become unspecified. That permits the introduction of a proxy as a nested class in some further undocumented (not even exposition-only) base.

> It also permits the `istream_iterator` approach where the cached value is stored in the iterator itself, and the iterator serves as its own proxy for post-increment `operator++` - removing the need for the existing exposition-only nested class `proxy`.

> Note that the current `proxy` class also has exactly the right properties to serve as the pointer `proxy` too. This is likely to be a common case where an `InputIterator` does not hold internal state but delegates to another class.

> Proposed Resolution:

> In addition to the current proposal:

> 24.6.4 [istreambuf.iterator]

> >     template<class charT, class traits = char_traits<charT> >
> >     class istreambuf_iterator
> >       : public iterator<input_iterator_tag, charT,
> >                         typename traits::off_type, charT* unspecified, charT> {

[ 2009-07 Frankfurt ]

> Move the additional part into the proposed resolution, and wrap the descriptive text in a Note.

> [Howard: done.]

> Move to Ready.

Proposed resolution:

Add to the synopsis in 24.6.4 [istreambuf.iterator]:

>     charT operator*() const;
>     pointer operator->() const;
>     istreambuf_iterator<charT,traits>& operator++();

24.6.4 [istreambuf.iterator]

>     template<class charT, class traits = char_traits<charT> >
>     class istreambuf_iterator
>       : public iterator<input_iterator_tag, charT,
>                         typename traits::off_type, charT* unspecified, charT> {

Change 24.6.4 [istreambuf.iterator], p1:

> The class template `istreambuf_iterator` reads successive characters from the `streambuf` for which it was constructed. `operator*` provides access to the current input character, if any. [Note: `operator->` may return a proxy. — end note] Each time `operator++` is evaluated, the iterator advances to the next input character. If the end of stream is reached (`streambuf_type::sgetc()` returns `traits::eof()`), the iterator becomes equal to the end of stream iterator value. The default constructor `istreambuf_iterator()` and the constructor `istreambuf_iterator(0)` both construct an end of stream iterator object suitable for use as an end-of-range.

### 660(i). Missing Bitwise Operations

Section: 22.10 [function.objects] Status: CD1 Submitter: Beman Dawes Opened: 2007-04-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 22.10 [function.objects] provides function objects for some unary and binary operations, but others are missing. In a LWG reflector discussion, beginning with c++std-lib-18078, pros and cons of adding some of the missing operations were discussed. Bjarne Stroustrup commented "Why standardize what isn't used? Yes, I see the chicken and egg problems here, but it would be nice to see a couple of genuine uses before making additions."

A number of libraries, including Rogue Wave, GNU, Adobe ASL, and Boost, have already added these functions, either publicly or for internal use. For example, Doug Gregor commented: "Boost will also add ... (|, &, ^) in 1.35.0, because we need those function objects to represent various parallel collective operations (reductions, prefix reductions, etc.) in the new Message Passing Interface (MPI) library."

Because the bitwise operators have the strongest use cases, the proposed resolution is limited to them.

Proposed resolution:

To 22.10 [function.objects], Function objects, paragraph 2, add to the header <functional> synopsis:

>     template <class T> struct bit_and;
>     template <class T> struct bit_or;
>     template <class T> struct bit_xor;

At a location in clause 20 to be determined by the Project Editor, add:

> The library provides basic function object classes for all of the bitwise operators in the language ([expr.bit.and], [expr.or], [exp.xor]).

>     template <class T> struct bit_and : binary_function<T,T,T> {
>       T operator()(const T& x , const T& y ) const;
>     };

> > `operator()` returns`x & y` .

>     template <class T> struct bit_or : binary_function<T,T,T> {
>       T operator()(const T& x , const T& y ) const;
>     };

> > `operator()` returns `x | y` .

>     template <class T> struct bit_xor : binary_function<T,T,T> {
>       T operator()(const T& x , const T& y ) const;
>     };

> > `operator()` returns `x ^ y` .

### 661(i). New 27.6.1.2.2 changes make special extractions useless

Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: CD1 Submitter: Daniel Krügler Opened: 2007-04-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

To the more drastic changes of 31.7.5.3.2 [istream.formatted.arithmetic] in the current draft N2134 belong the explicit description of the extraction of the types short and int in terms of as-if code fragments.

1. The corresponding as-if extractions in paragraph 2 and 3 will never result in a change of the operator>> argument val, because the contents of the local variable lval is in no case written into val. Furtheron both fragments need a currently missing parentheses in the beginning of the if-statement to be valid C++.
2. I would like to ask whether the omission of a similar explicit extraction of unsigned short and unsigned int in terms of long - compared to their corresponding new insertions, as described in 31.7.6.3.2 [ostream.inserters.arithmetic], is a deliberate decision or an oversight.

Proposed resolution:

1. In 31.7.5.3.2 [istream.formatted.arithmetic]/2 change the current as-if code fragment

   >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
   >     iostate err = 0;
   >     long lval;
   >     use_facet<numget>(loc).get(*this, 0, *this, err, lval );
   >     if (err == 0) {
   >       && if (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval))
   >           err = ios_base::failbit;
   >       else
   >         val = static_cast<short>(lval);
   >     }
   >     setstate(err);

   Similarily in 31.7.5.3.2 [istream.formatted.arithmetic]/3 change the current as-if fragment

   >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
   >     iostate err = 0;
   >     long lval;
   >     use_facet<numget>(loc).get(*this, 0, *this, err, lval );
   >     if (err == 0) {
   >       && if (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval))
   >           err = ios_base::failbit;
   >       else
   >         val = static_cast<int>(lval);
   >     }
   >     setstate(err);

2. ---

[ Kona (2007): Note to the editor: the name lval in the call to `use_facet` is incorrectly italicized in the code fragments corresponding to `operator>>(short &)` and `operator >>(int &)`. Also, val -- which appears twice on the line with the `static_cast` in the proposed resolution -- should be italicized. Also, in response to part two of the issue: this is deliberate. ]

### 664(i). `do_unshift` for `codecvt<char, char, mbstate_t>`

Section: 28.3.4.2.5.3 [locale.codecvt.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

28.3.4.2.5.3 [locale.codecvt.virtuals], para 7 says (regarding `do_unshift`):

> Effects: Places characters starting at to that should be appended to terminate a sequence when the current `stateT` is given by `state`.237) Stores no more than `(to_limit - to)` destination elements, and leaves the `to_next` pointer pointing one beyond the last element successfully stored. `codecvt<char, char, mbstate_t>` stores no characters.

The following objection has been raised:

> Since the C++ Standard permits a nontrivial conversion for the required instantiations of `codecvt`, it is overly restrictive to say that `do_unshift` must store no characters and return `noconv`.

[Plum ref _222152Y50]

Proposed resolution:

Change 28.3.4.2.5.3 [locale.codecvt.virtuals], p7:

> Effects: Places characters starting at to that should be appended to terminate a sequence when the current `stateT` is given by state.237) Stores no more than (to_limit -to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. `codecvt<char, char, mbstate_t>` stores no characters.

### 665(i). `do_unshift` return value

Section: 28.3.4.2.5.3 [locale.codecvt.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

28.3.4.2.5.3 [locale.codecvt.virtuals], para 8 says:

> `codecvt<char,char,mbstate_t>`, returns `noconv`.

The following objection has been raised:

> Despite what the C++ Standard says, `unshift` can't always return `noconv` for the default facets, since they can be nontrivial. At least one implementation does whatever the C functions do.

[Plum ref _222152Y62]

Proposed resolution:

Change 28.3.4.2.5.3 [locale.codecvt.virtuals], p8:

> Returns: An enumeration value, as summarized in Table 76:

> ...

> `codecvt<char,char,mbstate_t>`, returns `noconv`.

### 666(i). `moneypunct::do_curr_symbol()`

Section: 28.3.4.7.4.3 [locale.moneypunct.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

28.3.4.7.4.3 [locale.moneypunct.virtuals], para 4 footnote 257 says

> 257) For international specializations (second template parameter `true`) this is always four characters long, usually three letters and a space.

The following objection has been raised:

> The international currency symbol is whatever the underlying locale says it is, not necessarily four characters long.

[Plum ref _222632Y41]

Proposed resolution:

Change footnote 253 in 28.3.4.7.4.3 [locale.moneypunct.virtuals]:

> 253) For international specializations (second template parameter `true`) this is always typically four characters long, usually three letters and a space.

### 671(i). precision of `hexfloat`

Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++11 Submitter: John Salmon Opened: 2007-04-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I am trying to understand how TR1 supports hex float (%a) output.

As far as I can tell, it does so via the following:

8.15 Additions to header `<locale>` [tr.c99.locale]

In subclause 28.3.4.3.3.3 [facet.num.put.virtuals], Table 58 Floating-point conversions, after the line:
`floatfield == ios_base::scientific %E`

add the two lines:

>     floatfield == ios_base::fixed | ios_base::scientific && !uppercase %a
>     floatfield == ios_base::fixed | ios_base::scientific %A 2

[Note: The additional requirements on print and scan functions, later in this clause, ensure that the print functions generate hexadecimal floating-point fields with a %a or %A conversion specifier, and that the scan functions match hexadecimal floating-point fields with a %g conversion specifier. end note]

Following the thread, in 28.3.4.3.3.3 [facet.num.put.virtuals], we find:

For conversion from a floating-point type, if `(flags & fixed) != 0` or if `str.precision() > 0`, then `str.precision()` is specified in the conversion specification.

This would seem to imply that when `floatfield == fixed|scientific`, the precision of the conversion specifier is to be taken from `str.precision()`. Is this really what's intended? I sincerely hope that I'm either missing something or this is an oversight. Please tell me that the committee did not intend to mandate that hex floats (and doubles) should by default be printed as if by %.6a.

[ Howard: I think the fundamental issue we overlooked was that with %f, %e, %g, the default precision was always 6. With %a the default precision is not 6, it is infinity. So for the first time, we need to distinguish between the default value of precision, and the precision value 6. ]

[ 2009-07 Frankfurt ]

> Leave this open for Robert and Daniel to work on.

> Straw poll: Disposition?

> - Default is %.6a (i.e. NAD): 2
> - Always %a (no precision): 6
> - precision(-1) == %a: 3

> Daniel and Robert have direction to write up wording for the "always %a" solution.

> [ 2009-07-15 Robert provided wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Change 28.3.4.3.3.3 [facet.num.put.virtuals], Stage 1, under p5 (near the end of Stage 1):

> For conversion from a floating-point type, `str.precision()` is specified as precision in the conversion specification if `floatfield != (ios_base::fixed | ios_base::scientific)`, else no precision is specified.

[ Kona (2007): Robert volunteers to propose wording. ]

### 672(i). Swappable requirements need updating

Section: 16.4.4.2 [utility.arg.requirements] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current `Swappable` is:

> Table 37: `Swappable` requirements [swappable]
> expression | return type | post-condition
> --- | --- | ---
> `swap(s,t)` | `void` | `t` has the value originally held by `u`, and `u` has the value originally held by `t`
> The Swappable requirement is met by satisfying one or more of the following conditions: `T` is Swappable if `T` satisfies the `CopyConstructible` requirements (Table 34) and the `CopyAssignable` requirements (Table 36); `T` is Swappable if a namespace scope function named `swap` exists in the same namespace as the definition of `T`, such that the expression `swap(t,u)` is valid and has the semantics described in this table.

With the passage of rvalue reference into the language, `Swappable` needs to be updated to require only `MoveConstructible` and `MoveAssignable`. This is a minimum.

Additionally we may want to support proxy references such that the following code is acceptable:

>     namespace Mine {

>     template <class T>
>     struct proxy {...};

>     template <class T>
>     struct proxied_iterator
>     {
>        typedef T value_type;
>        typedef proxy<T> reference;
>        reference operator*() const;
>        ...
>     };

>     struct A
>     {
>        // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable
>        void swap(A&);
>        ...
>     };

>     void swap(A&, A&);
>     void swap(proxy<A>, A&);
>     void swap(A&, proxy<A>);
>     void swap(proxy<A>, proxy<A>);

>     }  // Mine

>     ...

>     Mine::proxied_iterator<Mine::A> i(...)
>     Mine::A a;
>     swap(*i1, a);

I.e. here is a call to `swap` which the user enables swapping between a proxy to a class and the class itself. We do not need to anything in terms of implementation except not block their way with overly constrained concepts. That is, the `Swappable` concept should be expanded to allow swapping between two different types for the case that one is binding to a user-defined `swap`.

Proposed resolution:

Change 16.4.4.2 [utility.arg.requirements]:

> -1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables, `T` is a type to be supplied by a C++ program instantiating a template; `a`, `b`, and `c` are values of type `const T`; `s` and `t` are modifiable lvalues of type `T`; `u` is a value of type (possibly `const`) `T`; and `rv` is a non-`const` rvalue of type `T`.

> Table 37: `Swappable` requirements [swappable]
> expression | return type | post-condition
> --- | --- | ---
> `swap(s,t)` | `void` | `t` has the value originally held by `u`, and `u` has the value originally held by `t`
> The `Swappable` requirement is met by satisfying one or more of the following conditions: `T` is `Swappable` if `T` satisfies the `CopyConstructible` MoveConstructible requirements (Table 34 33) and the `CopyAssignable` MoveAssignable requirements (Table 36 35); `T` is `Swappable` if a namespace scope function named `swap` exists in the same namespace as the definition of `T`, such that the expression `swap(t,u)` is valid and has the semantics described in this table.

[ Kona (2007): We like the change to the `Swappable` requirements to use move semantics. The issue relating to the support of proxies is separable from the one relating to move semantics, and it's bigger than just swap. We'd like to address only the move semantics changes under this issue, and open a separated issue (742(i)) to handle proxies. Also, there may be a third issue, in that the current definition of `Swappable` does not permit rvalues to be operands to a swap operation, and Howard's proposed resolution would allow the right-most operand to be an rvalue, but it would not allow the left-most operand to be an rvalue (some swap functions in the library have been overloaded to permit left operands to swap to be rvalues). ]

### 673(i). `unique_ptr` update

Section: 20.3.1 [unique.ptr] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Since the publication of N1856 there have been a few small but significant advances which should be included into `unique_ptr`. There exists a example implementation for all of these changes.

- Even though `unique_ptr<void>` is not a valid use case (unlike for `shared_ptr<void>`), unexpected cases to crop up which require the instantiation of the interface of `unique_ptr<void>` even if it is never used. For example see 541(i) for how this accidently happened to `auto_ptr`. I believe the most robust way to protect `unique_ptr` against this type of failure is to augment the return type of `unique_ptr<T>:operator*()` with `add_lvalue_reference<T>::type`. This means that given an instantiated `unique_ptr<void>` the act of dereferencing it will simply return `void` instead of causing a compile time failure. This is simpler than creating a `unique_ptr<void>` specialization which isn't robust in the face of `cv-`qualified `void` types.

  This resolution also supports instantiations such as `unique_ptr<void, free_deleter>` which could be very useful to the client.

- Efforts have been made to better support containers and smart pointers in shared memory contexts. One of the key hurdles in such support is not assuming that a pointer type is actually a `T*`. This can easily be accomplished for `unique_ptr` by having the deleter define the pointer type: `D::pointer`. Furthermore this type can easily be defaulted to `T*` should the deleter `D` choose not to define a pointer type (example implementation here). This change has no run time overhead. It has no interface overhead on authors of custom delter types. It simply allows (but not requires) authors of custom deleter types to define a smart pointer for the storage type of `unique_ptr` if they find such functionality useful. `std::default_delete` is an example of a deleter which defaults `pointer` to `T*` by simply ignoring this issue and not including a `pointer typedef`.

- When the deleter type is a function pointer then it is unsafe to construct a `unique_ptr` without specifying the function pointer in the constructor. This case is easy to check for with a `static_assert` assuring that the deleter is not a pointer type in those constructors which do not accept deleters.

  >     unique_ptr<A, void(*)(void*)> p(new A);  // error, no function given to delete the pointer!

[ Kona (2007): We don't like the solution given to the first bullet in light of concepts. The second bullet solves the problem of supporting fancy pointers for one library component only. The full LWG needs to decide whether to solve the problem of supporting fancy pointers piecemeal, or whether a paper addressing the whole library is needed. We think that the third bullet is correct. ]

[ Post Kona: Howard adds example user code related to the first bullet: ]

>     void legacy_code(void*, std::size_t);

>     void foo(std::size_t N)
>     {
>         std::unique_ptr<void, void(*)(void*)> ptr(std::malloc(N), std::free);
>         legacy_code(ptr.get(), N);
>     }   // unique_ptr used for exception safety purposes

I.e. `unique_ptr<void>` is a useful tool that we don't want to disable with concepts. The only part of `unique_ptr<void>` we want to disable (with concepts or by other means) are the two member functions:

>     T& operator*() const;
>     T* operator->() const;

Proposed resolution:

[ I am grateful for the generous aid of Peter Dimov and Ion Gaztañaga in helping formulate and review the proposed resolutions below. ]

- Change 20.3.1.3 [unique.ptr.single]:

  >     template <class T, class D = default_delete<T>> class unique_ptr {
  >        ...
  >        T& typename add_lvalue_reference<T>::type operator*() const;
  >        ...
  >     };

  Change 20.3.1.3.5 [unique.ptr.single.observers]:

  >     T& typename add_lvalue_reference<T>::type operator*() const;

- Change 20.3.1.3 [unique.ptr.single]:

  >     template <class T, class D = default_delete<T>> class unique_ptr {
  >     public:
  >       typedef implementation (see description below) pointer;
  >        ...
  >        explicit unique_ptr(T* pointer p);
  >        ...
  >        unique_ptr(T* pointer p, implementation defined (see description below) d);
  >        unique_ptr(T* pointer p, implementation defined (see description below) d);
  >        ...
  >        T* pointer operator->() const;
  >        T* pointer get() const;
  >        ...
  >        T* pointer release();
  >        void reset(T* pointer p = 0 pointer());
  >     };

  -3- If the type `remove_reference<D>::type::pointer` exists, then `unique_ptr<T, D>::pointer` is a typedef to `remove_reference<D>::type::pointer`. Otherwise `unique_ptr<T, D>::pointer` is a typedef to `T*`. The type `unique_ptr<T, D>::pointer` shall be `CopyConstructible` and `CopyAssignable`.

  Change 20.3.1.3.2 [unique.ptr.single.ctor]:

  >     unique_ptr(T* pointer p);
  >     ...
  >     unique_ptr(T* pointer p, implementation defined d);
  >     unique_ptr(T* pointer p, implementation defined d);
  >     ...
  >     unique_ptr(T* pointer p, const A& d);
  >     unique_ptr(T* pointer p, A&& d);
  >     ...
  >     unique_ptr(T* pointer p, A& d);
  >     unique_ptr(T* pointer p, A&& d);
  >     ...
  >     unique_ptr(T* pointer p, const A& d);
  >     unique_ptr(T* pointer p, const A&& d);
  >     ...

  -23- Requires: If `D` is not a reference type, construction of the deleter `D` from an rvalue of type `E` must shall be well formed and not throw an exception. If `D` is a reference type, then `E` must shall be the same type as `D` (diagnostic required). `U*` `unique_ptr<U,E>::pointer` must shall be implicitly convertible to `T*` pointer.

  -25- Postconditions: `get() == value u.get()` had before the construction, modulo any required offset adjustments resulting from the cast from `U*` `unique_ptr<U,E>::pointer` to `T*` pointer. `get_deleter()` returns a reference to the internally stored deleter which was constructed from `u.get_deleter()`.

  Change 20.3.1.3.4 [unique.ptr.single.asgn]:

  > -8- Requires: Assignment of the deleter `D` from an rvalue `D` must shall not throw an exception. `U*` `unique_ptr<U,E>::pointer` must shall be implicitly convertible to `T*` pointer.

  Change 20.3.1.3.5 [unique.ptr.single.observers]:

  >     T* pointer operator->() const;

  > ...

  >     T* pointer get() const;

  Change 20.3.1.3.6 [unique.ptr.single.modifiers]:

  >     T* pointer release();

  > ...

  >     void reset(T* pointer p = 0 pointer());

  Change 20.3.1.4 [unique.ptr.runtime]:

  >     template <class T, class D> class unique_ptr<T[], D> {
  >     public:
  >       typedef implementation pointer;
  >        ...
  >        explicit unique_ptr(T* pointer p);
  >        ...
  >        unique_ptr(T* pointer p, implementation defined d);
  >        unique_ptr(T* pointer p, implementation defined d);
  >        ...
  >        T* pointer get() const;
  >        ...
  >        T* pointer release();
  >        void reset(T* pointer p = 0 pointer());
  >     };

  Change 20.3.1.4.2 [unique.ptr.runtime.ctor]:

  >     unique_ptr(T* pointer p);
  >     unique_ptr(T* pointer p, implementation defined d);
  >     unique_ptr(T* pointer p, implementation defined d);

  > These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to `T*` `pointer`. [Note: One implementation technique is to create private templated overloads of these members. -- end note]

  Change 20.3.1.4.5 [unique.ptr.runtime.modifiers]:

  >     void reset(T* pointer p = 0 pointer());

  > -1- Requires: Does not accept pointer types which are convertible to `T*` `pointer` (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]

- Change 20.3.1.3.2 [unique.ptr.single.ctor]:

  >     unique_ptr();

  > > Requires: `D` must shall be default constructible, and that construction must shall not throw an exception. `D` must shall not be a reference type or pointer type (diagnostic required).

  >     unique_ptr(T* pointer p);

  > > Requires: The expression `D()(p)` must shall be well formed. The default constructor of `D` must shall not throw an exception. `D` must shall not be a reference type or pointer type (diagnostic required).

### 674(i). `shared_ptr` interface changes for consistency with N1856

Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.smartptr.shared].

View all issues with CD1 status.

Discussion:

N1856 does not propose any changes to `shared_ptr`. It needs to be updated to use a rvalue reference where appropriate and to interoperate with `unique_ptr` as it does with `auto_ptr`.

Proposed resolution:

Change 20.3.2.2 [util.smartptr.shared] as follows:

>     template<class Y> explicit shared_ptr(auto_ptr<Y>&&& r);
>     template<class Y, class D> explicit shared_ptr(const unique_ptr<Y,D>& r) = delete;
>     template<class Y, class D> explicit shared_ptr(unique_ptr<Y,D>&& r);
>     ...
>     template<class Y> shared_ptr& operator=(auto_ptr<Y>&&& r);
>     template<class Y, class D> shared_ptr& operator=(const unique_ptr<Y,D>& r) = delete;
>     template<class Y, class D> shared_ptr& operator=(unique_ptr<Y,D>&& r);

Change 20.3.2.2.2 [util.smartptr.shared.const] as follows:

>     template<class Y> shared_ptr(auto_ptr<Y>&&& r);

Add to 20.3.2.2.2 [util.smartptr.shared.const]:

>     template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

> > Effects: Equivalent to `shared_ptr( r.release(), r.get_deleter() )` when `D` is not a reference type, `shared_ptr( r.release(), ref( r.get_deleter() ) )` otherwise.

> > Exception safety: If an exception is thrown, the constructor has no effect.

Change 20.3.2.2.4 [util.smartptr.shared.assign] as follows:

>     template<class Y> shared_ptr& operator=(auto_ptr<Y>&&& r);

Add to 20.3.2.2.4 [util.smartptr.shared.assign]:

>     template<class Y, class D> shared_ptr& operator=(unique_ptr<Y,D>&& r);

> > -4- Effects: Equivalent to `shared_ptr(std::move(r)).swap(*this)`.

> > -5- Returns: `*this`.

[ Kona (2007): We may need to open an issue (743(i)) to deal with the question of whether `shared_ptr` needs an rvalue `swap`. ]

### 675(i). Move assignment of containers

Section: 23.2 [container.requirements] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

View all issues with CD1 status.

Discussion:

James Hopkin pointed out to me that if `vector<T>` move assignment is O(1) (just a `swap`) then containers such as `vector<shared_ptr<ostream>>` might have the wrong semantics under move assignment when the source is not truly an rvalue, but a moved-from lvalue (destructors could run late).

>     vector<shared_ptr<ostream>> v1;
>     vector<shared_ptr<ostream>> v2;
>     ...
>     v1 = v2;               // #1
>     v1 = std::move(v2);    // #2

Move semantics means not caring what happens to the source (`v2` in this example). It doesn't mean not caring what happens to the target (`v1`). In the above example both assignments should have the same effect on `v1`. Any non-shared `ostream`'s `v1` owns before the assignment should be closed, whether `v1` is undergoing copy assignment or move assignment.

This implies that the semantics of move assignment of a generic container should be `clear, swap` instead of just swap. An alternative which could achieve the same effect would be to move assign each element. In either case, the complexity of move assignment needs to be relaxed to `O(v1.size())`.

The performance hit of this change is not nearly as drastic as it sounds. In practice, the target of a move assignment has always just been move constructed or move assigned from. Therefore under `clear, swap` semantics (in this common use case) we are still achieving O(1) complexity.

Proposed resolution:

Change 23.2 [container.requirements]:

> Table 89: Container requirements
> expression | return type | operational semantics | assertion/note pre/post-condition | complexity
> --- | --- | --- | --- | ---
> `a = rv;` | `X&` | All existing elements of `a` are either move assigned or destructed | `a` shall be equal to the value that `rv` had before this construction | (Note C) linear

> Notes: the algorithms `swap()`, `equal()` and `lexicographical_compare()` are defined in clause 25. Those entries marked "(Note A)" should have constant complexity. Those entries marked "(Note B)" have constant complexity unless `allocator_propagate_never<X::allocator_type>::value` is `true`, in which case they have linear complexity. Those entries marked "(Note C)" have constant complexity if `a.get_allocator() == rv.get_allocator()` or if either `allocator_propagate_on_move_assignment<X::allocator_type>::value` is `true` or `allocator_propagate_on_copy_assignment<X::allocator_type>::value` is `true` and linear complexity otherwise.

[ post Bellevue Howard adds: ]

> This issue was voted to WP in Bellevue, but accidently got stepped on by N2525 which was voted to WP simulataneously. Moving back to Open for the purpose of getting the wording right. The intent of this issue and N2525 are not in conflict.

[ post Sophia Antipolis Howard updated proposed wording: ]

### 676(i). Moving the unordered containers

Section: 23.5 [unord] Status: C++11 Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unord].

View all issues with C++11 status.

Discussion:

Move semantics are missing from the `unordered` containers. The proposed resolution below adds move-support consistent with N1858 and the current working draft.

The current proposed resolution simply lists the requirements for each function. These might better be hoisted into the requirements table for unordered associative containers. Futhermore a mild reorganization of the container requirements could well be in order. This defect report is purposefully ignoring these larger issues and just focusing on getting the unordered containers "moved".

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10-17 Removed rvalue-swaps from wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Review. Alisdair will review proposed wording.

[ 2009-10-29 Daniel updates wording. ]

[ 2010-01-26 Alisdair updates wording. ]

[ 2010-02-10 Howard updates wording to reference the unordered container requirements table (modified by 704(i)) as much as possible. ]

[ Voted to WP in Bellevue. ]

[ post Bellevue, Pete notes: ]

> Please remind people who are reviewing issues to check that the text modifications match the current draft. Issue 676, for example, adds two overloads for unordered_map::insert taking a hint. One takes a const_iterator and returns a const_iterator, and the other takes an iterator and returns an iterator. This was correct at the time the issue was written, but was changed in Toronto so there is only one hint overload, taking a const_iterator and returning an iterator.

> This issue is not ready. In addition to the relatively minor signature problem I mentioned earlier, it puts requirements in the wrong places. Instead of duplicating requirements throughout the template specifications, it should put them in the front matter that talks about requirements for unordered containers in general. This presentation problem is editorial, but I'm not willing to do the extensive rewrite that it requires. Please put it back into Open status.

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-24 Pete moved to Open: ]

> The descriptions of the semantics of the added `insert` functions belong in the requirements table. That's where the rest of the `insert` functions are.

[ 2010 Pittsburgh: ]

> Move issue 676 to Ready for Pittsburgh. Nico to send Howard an issue for the broader problem.

Rationale:

[ San Francisco: ]

> Solved by N2776.

[ Rationale is obsolete. ]

Proposed resolution:

`unordered_map`

Change 23.5.3 [unord.map]:

>     class unordered_map
>     {
>         ...
>         unordered_map(const unordered_map&);
>         unordered_map(unordered_map&&);
>         unordered_map(const Allocator&);
>         unordered_map(const unordered_map&, const Allocator&);
>         unordered_map(unordered_map&&, const Allocator&);
>         ...
>         unordered_map& operator=(const unordered_map&);
>         unordered_map& operator=(unordered_map&&);
>         ...
>         // modifiers
>         ...
>         std::pair<iterator, bool> insert(const value_type& obj);
>         template <class P> pair<iterator, bool> insert(P&& obj);
>         iterator       insert(const_iterator hint, const value_type& obj);
>         template <class P> iterator       insert(const_iterator hint, P&& obj);
>         ...
>         mapped_type& operator[](const key_type& k);
>         mapped_type& operator[](key_type&& k);
>         ...
>     };

Add to 23.5.3.3 [unord.map.elem]:

>     mapped_type& operator[](const key_type& k);

> > ...

> > Requires: `key_type` shall be `CopyConstructible` and `mapped_type` shall be `DefaultConstructible`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

>     mapped_type& operator[](key_type&& k);

> > Requires: `key_type` shall be `MoveConstructible` and `mapped_type` shall be `DefaultConstructible`.

> > Effects: If the `unordered_map` does not already contain an element whose key is equivalent to `k` , inserts the value `value_type(std::move(k), mapped_type())`.

> > Returns: A reference to `x.second`, where `x` is the (unique) element whose key is equivalent to `k`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

Add new section [unord.map.modifiers]:

>     template <class P>
>       pair<iterator, bool> insert(P&& x);

> > Requires: `value_type` is constructible from `std::forward<P>(x)`.

> > Effects: Inserts `x` converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(x)`.

> > Returns: The `bool` component of the returned `pair` indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key of `value_type(x)`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

> > Remarks: `P` shall be implicitly convertible to `value_type`, else this signature shall not participate in overload resolution.

>     template <class P>
>       iterator insert(const_iterator hint, P&& x);

> > Requires: `value_type` is constructible from `std::forward<P>(x)`.

> > Effects: Inserts `x` converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(x)`. The iterator `hint` is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.

> > Returns: An iterator pointing to the element with key equivalent to the key of `value_type(x)`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

> > Remarks: `P` shall be implicitly convertible to `value_type`, else this signature shall not participate in overload resolution.

`unordered_multimap`

Change 23.5.4 [unord.multimap]:

>     class unordered_multimap
>     {
>         ...
>         unordered_multimap(const unordered_multimap&);
>         unordered_multimap(unordered_multimap&&);
>         unordered_multimap(const Allocator&);
>         unordered_multimap(const unordered_multimap&, const Allocator&);
>         unordered_multimap(unordered_multimap&&, const Allocator&);
>         ...
>         unordered_multimap& operator=(const unordered_multimap&);
>         unordered_multimap& operator=(unordered_multimap&&);
>         ...
>         // modifiers
>         ...
>         iterator insert(const value_type& obj);
>         template <class P> iterator insert(P&& obj);
>         iterator       insert(const_iterator hint, const value_type& obj);
>         template <class P> iterator       insert(const_iterator hint, P&& obj);
>         ...
>     };

Add new section [unord.multimap.modifiers]:

>     template <class P>
>       iterator insert(P&& x);

> > Requires: `value_type` is constructible from `std::forward<P>(x)`.

> > Effects: Inserts `x` converted to `value_type`.

> > Returns: An iterator pointing to the element with key equivalent to the key of `value_type(x)`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

> > Remarks: `P` shall be implicitly convertible to `value_type`, else this signature shall not participate in overload resolution.

>     template <class P>
>       iterator insert(const_iterator hint, P&& x);

> > Requires: `value_type` is constructible from `std::forward<P>(x)`.

> > Effects: Inserts `x` converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(x)`. The iterator `hint` is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.

> > Returns: An iterator pointing to the element with key equivalent to the key of `value_type(x)`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

> > Remarks: `P` shall be implicitly convertible to `value_type`, else this signature shall not participate in overload resolution.

`unordered_set`

Change 23.5.6 [unord.set]:

>     class unordered_set
>     {
>         ...
>         unordered_set(const unordered_set&);
>         unordered_set(unordered_set&&);
>         unordered_set(const Allocator&);
>         unordered_set(const unordered_set&, const Allocator&);
>         unordered_set(unordered_set&&, const Allocator&);
>         ...
>         unordered_set& operator=(const unordered_set&);
>         unordered_set& operator=(unordered_set&&);
>         ...
>         // modifiers
>         ...
>         std::pair<iterator, bool> insert(const value_type& obj);
>         pair<iterator, bool> insert(value_type&& obj);
>         iterator       insert(const_iterator hint, const value_type& obj);
>         iterator       insert(const_iterator hint, value_type&& obj);
>         ...
>     };

`unordered_multiset`

Change 23.5.7 [unord.multiset]:

>     class unordered_multiset
>     {
>         ...
>         unordered_multiset(const unordered_multiset&);
>         unordered_multiset(unordered_multiset&&);
>         unordered_multiset(const Allocator&);
>         unordered_multiset(const unordered_multiset&, const Allocator&);
>         unordered_multiset(unordered_multiset&&, const Allocator&);
>         ...
>         unordered_multiset& operator=(const unordered_multiset&);
>         unordered_multiset& operator=(unordered_multiset&&);
>         ...
>         // modifiers
>         ...
>         iterator insert(const value_type& obj);
>         iterator insert(value_type&& obj);
>         iterator       insert(const_iterator hint, const value_type& obj);
>         iterator       insert(const_iterator hint, value_type&& obj);
>         ...
>     };

### 677(i). Weaknesses in seed_seq::randomize [rand.util.seedseq]

Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2007-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.util.seedseq].

View all issues with CD1 status.

Discussion:

`seed_seq::randomize` provides a mechanism for initializing random number engines which ideally would yield "distant" states when given "close" seeds. The algorithm for `seed_seq::randomize` given in the current Working Draft for C++, N2284 (2007-05-08), has 3 weaknesses

1. Collisions in state. Because of the way the state is initialized, seeds of different lengths may result in the same state. The current version of seed_seq has the following properties:

   - For a given `s <= n`, each of the 2^(32s) seed vectors results in a distinct state.

   The proposed algorithm (below) has the considerably stronger properties:

   - All of the `(2^(32n)-1)/(2^32-1)` seed vectors of lengths `s < n` result in distinct states.
   - All of the `2^(32n)` seed vectors of length `s == n` result in distinct states.

2. Poor mixing of `v'`s entropy into the state. Consider `v.size() == n` and hold `v[n/2]` thru `v[n-1]` fixed while varying `v[0]` thru `v[n/2-1]`, a total of `2^(16n)` possibilities. Because of the simple recursion used in `seed_seq`, `begin[n/2]` thru `begin[n-1]` can take on only 2^64 possible states.

   The proposed algorithm uses a more complex recursion which results in much better mixing.

3. `seed_seq::randomize` is undefined for `v.size() == 0`. The proposed algorithm remedies this.

The current algorithm for `seed_seq::randomize` is adapted by me from the initialization procedure for the Mersenne Twister by Makoto Matsumoto and Takuji Nishimura. The weakness (2) given above was communicated to me by Matsumoto last year.

The proposed replacement for `seed_seq::randomize` is due to Mutsuo Saito, a student of Matsumoto, and is given in the implementation of the SIMD-oriented Fast Mersenne Twister random number generator SFMT. http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/index.html http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/SFMT-src-1.2.tar.gz

See Mutsuo Saito, An Application of Finite Field: Design and Implementation of 128-bit Instruction-Based Fast Pseudorandom Number Generator, Master's Thesis, Dept. of Math., Hiroshima University (Feb. 2007) http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/M062821.pdf

One change has been made here, namely to treat the case of small `n` (setting `t = (n-1)/2` for `n < 7`).

Since `seed_seq` was introduced relatively recently there is little cost in making this incompatible improvement to it.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 678(i). Changes for [rand.req.eng]

Section: 29.5.3.4 [rand.req.eng] Status: CD1 Submitter: Charles Karney Opened: 2007-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.req.eng].

View all issues with CD1 status.

Discussion:

Section 29.5.3.4 [rand.req.eng] Random number engine requirements:

This change follows naturally from the proposed change to `seed_seq::randomize` in 677(i).

In table 104 the description of `X(q)` contains a special treatment of the case `q.size() == 0`. This is undesirable for 4 reasons:

1. It replicates the functionality provided by `X()`.
2. It leads to the possibility of a collision in the state provided by some other `X(q)` with `q.size() > 0`.
3. It is inconsistent with the description of the `X(q)` in paragraphs 29.5.4.2 [rand.eng.lcong] p5, 29.5.4.3 [rand.eng.mers] p8, and 29.5.4.4 [rand.eng.sub] p10 where there is no special treatment of `q.size() == 0`.
4. The proposed replacement for `seed_seq::randomize` given above allows for the case `q.size() == 0`.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 679(i). resize parameter by value

Section: 23.3 [sequences] Status: CD1 Submitter: Howard Hinnant Opened: 2007-06-11 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [sequences].

View all issues with CD1 status.

Discussion:

The C++98 standard specifies that one member function alone of the containers passes its parameter (`T`) by value instead of by const reference:

>     void resize(size_type sz, T c = T());

This fact has been discussed / debated repeatedly over the years, the first time being even before C++98 was ratified. The rationale for passing this parameter by value has been:

> So that self referencing statements are guaranteed to work, for example:

> >     v.resize(v.size() + 1, v[0]);

However this rationale is not convincing as the signature for `push_back` is:

>     void push_back(const T& x);

And `push_back` has similar semantics to `resize` (append). And `push_back` must also work in the self referencing case:

>     v.push_back(v[0]);  // must work

The problem with passing `T` by value is that it can be significantly more expensive than passing by reference. The converse is also true, however when it is true it is usually far less dramatic (e.g. for scalar types).

Even with move semantics available, passing this parameter by value can be expensive. Consider for example `vector<vector<int>>`:

>     std::vector<int> x(1000);
>     std::vector<std::vector<int>> v;
>     ...
>     v.resize(v.size()+1, x);

In the pass-by-value case, `x` is copied once to the parameter of `resize`. And then internally, since the code can not know at compile time by how much `resize` is growing the `vector`, `x` is usually copied (not moved) a second time from `resize`'s parameter into its proper place within the `vector`.

With pass-by-const-reference, the `x` in the above example need be copied only once. In this case, `x` has an expensive copy constructor and so any copies that can be saved represents a significant savings.

If we can be efficient for `push_back`, we should be efficient for `resize` as well. The resize taking a reference parameter has been coded and shipped in the CodeWarrior library with no reports of problems which I am aware of.

Proposed resolution:

Change 23.3.5 [deque], p2:

>     class deque {
>        ...
>        void resize(size_type sz, const T& c);

Change 23.3.5.3 [deque.capacity], p3:

>     void resize(size_type sz, const T& c);

Change 23.3.11 [list], p2:

>     class list {
>        ...
>        void resize(size_type sz, const T& c);

Change 23.3.11.3 [list.capacity], p3:

>     void resize(size_type sz, const T& c);

Change 23.3.13 [vector], p2:

>     class vector {
>        ...
>        void resize(size_type sz, const T& c);

Change 23.3.13.3 [vector.capacity], p11:

>     void resize(size_type sz, const T& c);

### 680(i). move_iterator `operator->` return

Section: 24.5.4.2 [move.iterator] Status: CD1 Submitter: Howard Hinnant Opened: 2007-06-11 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [move.iterator].

View all issues with CD1 status.

Discussion:

`move_iterator`'s `operator->` return type `pointer` does not consistently match the type which is returned in the description in [move.iter.op.ref].

>     template <class Iterator>
>     class move_iterator {
>     public:
>         ...
>         typedef typename iterator_traits<Iterator>::pointer pointer;
>         ...
>         pointer operator->() const {return current;}
>         ...
>     private:
>         Iterator current; // exposition only
>     };

There are two possible fixes.

1. `pointer operator->() const {return &*current;}`
2. `typedef Iterator pointer;`

The first solution is the one chosen by `reverse_iterator`. A potential disadvantage of this is it may not work well with iterators which return a proxy on dereference and that proxy has overloaded `operator&()`. Proxy references often need to overloaad `operator&()` to return a proxy pointer. That proxy pointer may or may not be the same type as the iterator's `pointer` type.

By simply returning the `Iterator` and taking advantage of the fact that the language forwards calls to `operator->` automatically until it finds a non-class type, the second solution avoids the issue of an overloaded `operator&()` entirely.

Proposed resolution:

Change the synopsis in 24.5.4.2 [move.iterator]:

>     typedef typename iterator_traits<Iterator>::pointer pointer;

### 681(i). Operator functions impossible to compare are defined in [re.submatch.op]

Section: 28.6.8.3 [re.submatch.op] Status: CD1 Submitter: Nozomu Katoo Opened: 2007-05-27 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [re.submatch.op].

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Discussion:

In 28.6.8.3 [re.submatch.op] of N2284, operator functions numbered 31-42 seem impossible to compare. E.g.:

>     template <class BiIter>
>        bool operator==(typename iterator_traits<BiIter>::value_type const& lhs,
>                         const sub_match<BiIter>& rhs);

> > -31- Returns: `lhs == rhs.str()`.

When `char*` is used as `BiIter`, `iterator_traits<BiIter>::value_type` would be `char`, so that `lhs == rhs.str()` ends up comparing a `char` value and an object of `std::basic_string<char>`. However, the behaviour of comparison between these two types is not defined in 27.4.4 [string.nonmembers] of N2284. This applies when `wchar_t*` is used as `BiIter`.

Proposed resolution:

Adopt the proposed resolution in N2409.

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 682(i). basic_regex ctor takes InputIterator or ForwardIterator?

Section: 28.6.7.2 [re.regex.construct] Status: CD1 Submitter: Eric Niebler Opened: 2007-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [re.regex.construct].

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Discussion:

Looking at N2284, 28.6.7 [re.regex], p3 `basic_regex` class template synopsis shows this constructor:

>     template <class InputIterator>
>          basic_regex(InputIterator first, InputIterator last,
>                      flag_type f = regex_constants::ECMAScript);

In 28.6.7.2 [re.regex.construct], p15, the constructor appears with this signature:

>     template <class ForwardIterator>
>          basic_regex(ForwardIterator first, ForwardIterator last,
>                      flag_type f = regex_constants::ECMAScript);

`ForwardIterator` is probably correct, so the synopsis is wrong.

[ John adds: ]

> I think either could be implemented? Although an input iterator would probably require an internal copy of the string being made.

> I have no strong feelings either way, although I think my original intent was `InputIterator`.

Proposed resolution:

Adopt the proposed resolution in N2409.

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 685(i). reverse_iterator/move_iterator difference has invalid signatures

Section: 24.5.1.9 [reverse.iter.nonmember], 24.5.4.9 [move.iter.nonmember] Status: CD1 Submitter: Bo Persson Opened: 2007-06-10 Last modified: 2021-06-06

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

In C++03 the difference between two `reverse_iterators`

>     ri1 - ri2

is possible to compute only if both iterators have the same base iterator. The result type is the `difference_type` of the base iterator.

In the current draft, the operator is defined as [reverse.iter.opdiff]

>     template<class Iterator1, class Iterator2>
>     typename reverse_iterator<Iterator>::difference_type
>        operator-(const reverse_iterator<Iterator1>& x,
>                         const reverse_iterator<Iterator2>& y);

The return type is the same as the C++03 one, based on the no longer present `Iterator` template parameter.

Besides being slightly invalid, should this operator work only when `Iterator1` and `Iterator2` has the same `difference_type`? Or should the implementation choose one of them? Which one?

The same problem now also appears in `operator-()` for `move_iterator` 24.5.4.9 [move.iter.nonmember].

Proposed resolution:

Change the synopsis in 24.5.1.2 [reverse.iterator]:

>     template <class Iterator1, class Iterator2>
>       typename reverse_iterator<Iterator>::difference_type auto operator-(
>         const reverse_iterator<Iterator1>& x,
>         const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);

Change [reverse.iter.opdiff]:

>     template <class Iterator1, class Iterator2>
>       typename reverse_iterator<Iterator>::difference_type auto operator-(
>         const reverse_iterator<Iterator1>& x,
>         const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);

> > Returns: `y.current - x.current`.

Change the synopsis in 24.5.4.2 [move.iterator]:

>     template <class Iterator1, class Iterator2>
>       typename move_iterator<Iterator>::difference_type auto operator-(
>         const move_iterator<Iterator1>& x,
>         const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());

Change 24.5.4.9 [move.iter.nonmember]:

>     template <class Iterator1, class Iterator2>
>       typename move_iterator<Iterator>::difference_type auto operator-(
>         const move_iterator<Iterator1>& x,
>         const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());

> > Returns: `x.base() - y.base()`.

[ Pre Bellevue: This issue needs to wait until the `auto -> return` language feature goes in. ]

### 687(i). shared_ptr conversion constructor not constrained

Section: 20.3.2.2.2 [util.smartptr.shared.const], 20.3.2.3.2 [util.smartptr.weak.const] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [util.smartptr.shared.const].

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Discussion:

Since all conversions from `shared_ptr<T>` to `shared_ptr<U>` have the same rank regardless of the relationship between `T` and `U`, reasonable user code that works with raw pointers fails with `shared_ptr`:

>     void f( shared_ptr<void> );
>     void f( shared_ptr<int> );

>     int main()
>     {
>       f( shared_ptr<double>() ); // ambiguous
>     }

Now that we officially have `enable_if`, we can constrain the constructor and the corresponding assignment operator to only participate in the overload resolution when the pointer types are compatible.

Proposed resolution:

In 20.3.2.2.2 [util.smartptr.shared.const], change:

> -14- Requires: For the second constructor The second constructor shall not participate in the overload resolution unless `Y*` shall be is implicitly convertible to `T*`.

In 20.3.2.3.2 [util.smartptr.weak.const], change:

>     template<class Y> weak_ptr(shared_ptr<Y> const& r);
>     weak_ptr(weak_ptr const& r);
>     template<class Y> weak_ptr(weak_ptr<Y> const& r);
>     weak_ptr(weak_ptr const& r);
>     template<class Y> weak_ptr(weak_ptr<Y> const& r);
>     template<class Y> weak_ptr(shared_ptr<Y> const& r);

> > -4- Requires: For tThe second and third constructors, shall not participate in the overload resolution unless `Y*` shall be is implicitly convertible to `T*`.

### 688(i). reference_wrapper, cref unsafe, allow binding to rvalues

Section: 22.10.6.2 [refwrap.const] Status: C++11 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [refwrap.const].

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Discussion:

A `reference_wrapper` can be constructed from an rvalue, either by using the constructor, or via `cref` (and `ref` in some corner cases). This leads to a dangling reference being stored into the `reference_wrapper` object. Now that we have a mechanism to detect an rvalue, we can fix them to disallow this source of undefined behavior.

Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.

[ 2009-05-09 Alisdair adds: ]

> Now that `ref/cref` are constained that `T` must be an `ObjectType`, I do not believe there is any risk of binding `ref` to a temporary (which would rely on deducing `T` to be an rvalue reference type)

> However, the problem for `cref` remains, so I recommend retaining that deleted overload.

[ 2009-05-10 Howard adds: ]

> Without:

> >     template <class T> void ref(const T&& t) = delete;

> I believe this program will compile:

> >     #include <functional>

> >     struct A {};

> >     const A source() {return A();}

> >     int main()
> >     {
> >        std::reference_wrapper<const A> r = std::ref(source());
> >     }

> I.e. in:

> >     template <ObjectType T> reference_wrapper<T> ref(T& t);

> this:

> >     ref(source())

> deduces `T` as `const A`, and so:

> >     ref(const A& t)

> will bind to a temporary (tested with a pre-concepts rvalue-ref enabled compiler).

> Therefore I think we still need the ref-protection. I respectfully disagree with Alisdair's comment and am in favor of the proposed wording as it stands. Also, CWG 606 (noted below) has now been "favorably" resolved.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 22.10 [function.objects], add the following two signatures to the synopsis:

>     template <class T> void ref(const T&& t) = delete;
>     template <class T> void cref(const T&& t) = delete;

[ N2292 addresses the first part of the resolution but not the second. ]

[ Bellevue: Doug noticed problems with the current wording. ]

[ post Bellevue: Howard and Peter provided revised wording. ]

[ This resolution depends on a "favorable" resolution of CWG 606: that is, the "special deduction rule" is disabled with the const T&& pattern. ]

### 689(i). reference_wrapper constructor overly constrained

Section: 22.10.6.2 [refwrap.const] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [refwrap.const].

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Discussion:

The constructor of `reference_wrapper` is currently `explicit`. The primary motivation behind this is the safety problem with respect to rvalues, which is addressed by the proposed resolution of the previous issue. Therefore we should consider relaxing the requirements on the constructor since requests for the implicit conversion keep resurfacing.

Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.

Proposed resolution:

Remove the `explicit` from the constructor of `reference_wrapper`. If the proposed resolution of the previous issue is accepted, remove the `explicit` from the `T&&` constructor as well to keep them in sync.

### 691(i). const_local_iterator cbegin, cend missing from TR1

Section: 23.5 [unord], 99 [tr.hash] Status: CD1 Submitter: Joaquín M López Muñoz Opened: 2007-06-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unord].

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Discussion:

The last version of TR1 does not include the following member functions for unordered containers:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

which looks like an oversight to me. I've checked th TR1 issues lists and the latest working draft of the C++0x std (N2284) and haven't found any mention to these menfuns or to their absence.

Is this really an oversight, or am I missing something?

Proposed resolution:

Add the following two rows to table 93 (unordered associative container requirements) in section 23.2.8 [unord.req]:

> Unordered associative container requirements (in addition to container)
> expression | return type | assertion/note pre/post-condition | complexity
> --- | --- | --- | ---
> `b.cbegin(n)` | `const_local_iterator` | `n` shall be in the range `[0, bucket_count())`. Note: `[b.cbegin(n), b.cend(n))` is a valid range containing all of the elements in the `n`th bucket. | Constant
> `b.cend(n)` | `const_local_iterator` | `n` shall be in the range `[0, bucket_count())`. | Constant

Add to the synopsis in 23.5.3 [unord.map]:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

Add to the synopsis in 23.5.4 [unord.multimap]:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

Add to the synopsis in 23.5.6 [unord.set]:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

Add to the synopsis in 23.5.7 [unord.multiset]:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

### 692(i). `get_money` and `put_money` should be formatted I/O functions

Section: 31.7.8 [ext.manip] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ext.manip].

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Discussion:

In a private email Bill Plauger notes:

> I believe that the function that implements `get_money` [from N2072] should behave as a formatted input function, and the function that implements `put_money` should behave as a formatted output function. This has implications regarding the skipping of whitespace and the handling of errors, among other things.

> The words don't say that right now and I'm far from convinced that such a change is editorial.

Martin's response:

> I agree that the manipulators should handle exceptions the same way as formatted I/O functions do. The text in N2072 assumes so but the Returns clause explicitly omits exception handling for the sake of brevity. The spec should be clarified to that effect.

> As for dealing with whitespace, I also agree it would make sense for the extractors and inserters involving the new manipulators to treat it the same way as formatted I/O.

Proposed resolution:

Add a new paragraph immediately above p4 of 31.7.8 [ext.manip] with the following text:

> Effects: The expression `in >> get_money(mon, intl)` described below behaves as a formatted input function (as described in 31.7.5.3.1 [istream.formatted.reqmts]).

Also change p4 of 31.7.8 [ext.manip] as follows:

> Returns: An object `s` of unspecified type such that if `in` is an object of type `basic_istream<charT, traits>` then the expression `in >> get_money(mon, intl)` behaves as a formatted input function that calls `f(in, mon, intl)` were called. The function `f` can be defined as...

[ post Bellevue: ]

> We recommend moving immediately to Review. We've looked at the issue and have a consensus that the proposed resolution is correct, but want an iostream expert to sign off. Alisdair has taken the action item to putt this up on the reflector for possible movement by Howard to Tenatively Ready.

### 693(i). `std::bitset::all()` missing

Section: 22.9.2 [template.bitset] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [template.bitset].

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Discussion:

The `bitset` class template provides the member function `any()` to determine whether an object of the type has any bits set, and the member function `none()` to determine whether all of an object's bits are clear. However, the template does not provide a corresponding function to discover whether a `bitset` object has all its bits set. While it is possible, even easy, to obtain this information by comparing the result of `count()` with the result of `size()` for equality (i.e., via `b.count() == b.size()`) the operation is less efficient than a member function designed specifically for that purpose could be. (`count()` must count all non-zero bits in a `bitset` a word at a time while `all()` could stop counting as soon as it encountered the first word with a zero bit).

Proposed resolution:

Add a declaration of the new member function `all()` to the defintion of the `bitset` template in 22.9.2 [template.bitset], p1, right above the declaration of `any()` as shown below:

>     bool operator!=(const bitset<N>& rhs) const;
>     bool test(size_t pos) const;
>     bool all() const;
>     bool any() const;
>     bool none() const;

Add a description of the new member function to the end of 22.9.2.3 [bitset.members] with the following text:

> `bool all() const;`

> > Returns: `count() == size()`.

In addition, change the description of `any()` and `none()` for consistency with `all()` as follows:

> `bool any() const;`

> > Returns: `true` if any bit in `*this` is one `count() != 0`.

> `bool none() const;`

> > Returns: `true` if no bit in `*this` is one `count() == 0`.

### 694(i). `std::bitset` and `long long`

Section: 22.9.2 [template.bitset] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [template.bitset].

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Discussion:

Objects of the `bitset` class template specializations can be constructed from and explicitly converted to values of the widest C++ integer type, `unsigned long`. With the introduction of `long long` into the language the template should be enhanced to make it possible to interoperate with values of this type as well, or perhaps `uintmax_t`. See c++std-lib-18274 for a brief discussion in support of this change.

Proposed resolution:

For simplicity, instead of adding overloads for `unsigned long long` and dealing with possible ambiguities in the spec, replace the `bitset` ctor that takes an `unsigned long` argument with one taking `unsigned long long` in the definition of the template as shown below. (The standard permits implementations to add overloads on other integer types or employ template tricks to achieve the same effect provided they don't cause ambiguities or changes in behavior.)

>     // [bitset.cons] constructors:
>     bitset();
>     bitset(unsigned long long val);
>     template<class charT, class traits, class Allocator>
>     explicit bitset(
>                     const basic_string<charT,traits,Allocator>& str,
>                     typename basic_string<charT,traits,Allocator>::size_type pos = 0,
>                     typename basic_string<charT,traits,Allocator>::size_type n =
>                         basic_string<charT,traits,Allocator>::npos);

Make a corresponding change in 22.9.2.2 [bitset.cons], p2:

> `bitset(unsigned long long val);`

> > Effects: Constructs an object of class bitset<N>, initializing the first `M` bit positions to the corresponding bit values in `val`. `M` is the smaller of `N` and the number of bits in the value representation (section [basic.types]) of `unsigned long long`. If `M < N` is `true`, the remaining bit positions are initialized to zero.

Additionally, introduce a new member function `to_ullong()` to make it possible to convert `bitset` to values of the new type. Add the following declaration to the definition of the template, immediate after the declaration of `to_ulong()` in 22.9.2 [template.bitset], p1, as shown below:

>     // element access:
>     bool operator[](size_t pos) const; // for b[i];
>     reference operator[](size_t pos); // for b[i];
>     unsigned long to_ulong() const;
>     unsigned long long to_ullong() const;
>     template <class charT, class traits, class Allocator>
>     basic_string<charT, traits, Allocator> to_string() const;

And add a description of the new member function to 22.9.2.3 [bitset.members], below the description of the existing `to_ulong()` (if possible), with the following text:

> `unsigned long long to_ullong() const;`

> > Throws: `overflow_error` if the integral value `x` corresponding to the bits in `*this` cannot be represented as type `unsigned long long`.

> > Returns: `x`.

### 695(i). ctype<char>::classic_table() not accessible

Section: 28.3.4.2.4 [facet.ctype.special] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

The `ctype<char>::classic_table()` static member function returns a pointer to an array of const `ctype_base::mask` objects (enums) that contains `ctype<char>::table_size` elements. The table describes the properties of the character set in the "C" locale (i.e., whether a character at an index given by its value is alpha, digit, punct, etc.), and is typically used to initialize the `ctype<char>` facet in the classic "C" locale (the protected `ctype<char>` member function `table()` then returns the same value as `classic_table()`).

However, while `ctype<char>::table_size` (the size of the table) is a public static const member of the `ctype<char>` specialization, the `classic_table()` static member function is protected. That makes getting at the classic data less than convenient (i.e., one has to create a whole derived class just to get at the masks array). It makes little sense to expose the size of the table in the public interface while making the table itself protected, especially when the table is a constant object.

The same argument can be made for the non-static protected member function `table()`.

Proposed resolution:

Make the `ctype<char>::classic_table()` and `ctype<char>::table()` member functions public by moving their declarations into the public section of the definition of specialization in 28.3.4.2.4 [facet.ctype.special] as shown below:

>       static locale::id id;
>       static const size_t table_size = IMPLEMENTATION_DEFINED;
>     protected:
>       const mask* table() const throw();
>       static const mask* classic_table() throw();
>     protected:

>     ~ctype(); // virtual
>     virtual char do_toupper(char c) const;

### 696(i). `istream::operator>>(int&)` broken

Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: C++11 Submitter: Martin Sebor Opened: 2007-06-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.formatted.arithmetic].

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Discussion:

From message c++std-lib-17897:

The code shown in 31.7.5.3.2 [istream.formatted.arithmetic] as the "as if" implementation of the two arithmetic extractors that don't have a corresponding `num_get` interface (i.e., the `short` and `int` overloads) is subtly buggy in how it deals with `EOF`, overflow, and other similar conditions (in addition to containing a few typos).

One problem is that if `num_get::get()` reaches the EOF after reading in an otherwise valid value that exceeds the limits of the narrower type (but not `LONG_MIN` or `LONG_MAX`), it will set `err` to `eofbit`. Because of the if condition testing for `(err == 0)`, the extractor won't set `failbit` (and presumably, return a bogus value to the caller).

Another problem with the code is that it never actually sets the argument to the extracted value. It can't happen after the call to `setstate()` since the function may throw, so we need to show when and how it's done (we can't just punt as say: "it happens afterwards"). However, it turns out that showing how it's done isn't quite so easy since the argument is normally left unchanged by the facet on error except when the error is due to a misplaced thousands separator, which causes `failbit` to be set but doesn't prevent the facet from storing the value.

[ Batavia (2009-05): ]

> We believe this part of the Standard has been recently adjusted and that this issue was addressed during that rewrite.

> Move to NAD.

[ 2009-05-28 Howard adds: ]

> I've moved this issue from Tentatively NAD to Open.

> The current wording of N2857 in 28.3.4.3.2.3 [facet.num.get.virtuals] p3, stage 3 appears to indicate that in parsing arithmetic types, the value is always set, but sometimes in addition to setting `failbit`.

> - If there is a range error, the value is set to min or max, else
> - if there is a conversion error, the value is set to 0, else
> - if there is a grouping error, the value is set to whatever it would be if grouping were ignored, else
> - the value is set to its error-free result.

> However there is a contradictory sentence in 28.3.4.3.2.3 [facet.num.get.virtuals] p1.

> 31.7.5.3.2 [istream.formatted.arithmetic] should mimic the behavior of 28.3.4.3.2.3 [facet.num.get.virtuals] (whatever we decide that behavior is) for `int` and `short`, and currently does not. I believe that the correct code fragment should look like:

> >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> >     iostate err = ios_base::goodbit;
> >     long lval;
> >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> >     if (lval < numeric_limits<int>::min())
> >     {
> >       err |= ios_base::failbit;
> >       val = numeric_limits<int>::min();
> >     }
> >     else if (lval > numeric_limits<int>::max())
> >     {
> >       err |= ios_base::failbit;
> >       val = numeric_limits<int>::max();
> >     }
> >     else
> >       val = static_cast<int>(lval);
> >     setstate(err);

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Change 28.3.4.3.2.3 [facet.num.get.virtuals], p1:

> -1- Effects: Reads characters from `in`, interpreting them according to `str.flags()`, `use_facet<ctype<charT> >(loc)`, and `use_facet< numpunct<charT> >(loc)`, where `loc` is `str.getloc()`. If an error occurs, `val` is unchanged; otherwise it is set to the resulting value.

Change 31.7.5.3.2 [istream.formatted.arithmetic], p2 and p3:

>     operator>>(short& val);

> > -2- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

> > >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> > >     iostate err = iostate_base::goodbit;
> > >     long lval;
> > >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> > >     if (err != 0)
> > >       ;
> > >     else if (lval < numeric_limits<short>::min()
> > >       || numeric_limits<short>::max() < lval)
> > >          err = ios_base::failbit;
> > >     if (lval < numeric_limits<short>::min())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<short>::min();
> > >     }
> > >     else if (lval > numeric_limits<short>::max())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<short>::max();
> > >     }
> > >     else
> > >       val = static_cast<short>(lval);
> > >     setstate(err);

>     operator>>(int& val);

> > -3- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

> > >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> > >     iostate err = iostate_base::goodbit;
> > >     long lval;
> > >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> > >     if (err != 0)
> > >       ;
> > >     else if (lval < numeric_limits<int>::min()
> > >       || numeric_limits<int>::max() < lval)
> > >          err = ios_base::failbit;
> > >     if (lval < numeric_limits<int>::min())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<int>::min();
> > >     }
> > >     else if (lval > numeric_limits<int>::max())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<int>::max();
> > >     }
> > >     else
> > >       val = static_cast<int>(lval);
> > >     setstate(err);

### 697(i). New `<system_error>` header leads to name clashes

Section: 19.5 [syserr] Status: Resolved Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The most recent state of N2241 as well as the current draft N2284 (section 19.5 [syserr], p.2) proposes a new enumeration type `posix_errno` immediatly in the namespace `std`. One of the enumerators has the name `invalid_argument`, or fully qualified: `std::invalid_argument`. This name clashes with the exception type `std::invalid_argument`, see 19.2 [std.exceptions]/p.3. This clash makes e.g. the following snippet invalid:

>     #include <system_error>
>     #include <stdexcept>

>     void foo() { throw std::invalid_argument("Don't call us - we call you!"); }

I propose that this enumeration type (and probably the remaining parts of `<system_error>` as well) should be moved into one additional inner namespace, e.g. `sys` or `system` to reduce foreseeable future clashes due to the great number of members that `std::posix_errno` already contains (Btw.: Why has the already proposed `std::sys` sub-namespace from N2066 been rejected?). A further clash candidate seems to be `std::protocol_error` (a reasonable name for an exception related to a std network library, I guess).

Another possible resolution would rely on the proposed strongly typed enums, as described in N2213. But maybe the forbidden implicit conversion to integral types would make these enumerators less attractive in this special case?

Proposed resolution:

Fixed by issue 7 of N2422.

### 698(i). `system_error` needs `const char*` constructors

Section: 19.5.8.1 [syserr.syserr.overview] Status: CD1 Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

In 19.5.8.1 [syserr.syserr.overview] we have the class definition of `std::system_error`. In contrast to all exception classes, which are constructible with a `what_arg string` (see 19.2 [std.exceptions], or `ios_base::failure` in [ios::failure]), only overloads with with `const string&` are possible. For consistency with the re-designed remaining exception classes this class should also provide c'tors which accept a const `char* what_arg` string.

Please note that this proposed addition makes sense even considering the given implementation hint for `what()`, because `what_arg` is required to be set as `what_arg` of the base class `runtime_error`, which now has the additional c'tor overload accepting a `const char*`.

Proposed resolution:

This proposed wording assumes issue 832(i) has been accepted and applied to the working paper.

Change 19.5.8.1 [syserr.syserr.overview] Class system_error overview, as indicated:

>     public:
>       system_error(error_code ec, const string& what_arg);
>       system_error(error_code ec, const char* what_arg);
>       system_error(error_code ec);
>       system_error(int ev, const error_category* ecat,
>           const string& what_arg);
>       system_error(int ev, const error_category* ecat,
>           const char* what_arg);
>       system_error(int ev, const error_category* ecat);

To 19.5.8.2 [syserr.syserr.members] Class system_error members add:

>     system_error(error_code ec, const char* what_arg);

> > Effects: Constructs an object of class `system_error`.

> > Postconditions: `code() == ec` and `strcmp(runtime_error::what(), what_arg) == 0`.

>     system_error(int ev, const error_category* ecat, const char* what_arg);

> > Effects: Constructs an object of class `system_error`.

> > Postconditions: `code() == error_code(ev, ecat)` and `strcmp(runtime_error::what(), what_arg) == 0`.

### 699(i). N2111 changes min/max

Section: 29.5 [rand] Status: CD1 Submitter: P.J. Plauger Opened: 2007-07-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

N2111 changes `min/max` in several places in random from member functions to static data members. I believe this introduces a needless backward compatibility problem between C++0X and TR1. I'd like us to find new names for the static data members, or perhaps change `min/max` to `constexpr`s in C++0X.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 700(i). N1856 defines struct `identity`

Section: 22.2.4 [forward] Status: CD1 Submitter: P.J. Plauger Opened: 2007-07-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

N1856 defines struct `identity` in `<utility>` which clashes with the traditional definition of struct `identity` in `<functional>` (not standard, but a common extension from old STL). Be nice if we could avoid this name clash for backward compatibility.

Proposed resolution:

Change 22.2.4 [forward]:

>     template <class T> struct identity
>     {
>         typedef T type;
>         const T& operator()(const T& x) const;
>     };

> >     const T& operator()(const T& x) const;

> > > Returns: `x`.

### 703(i). `map::at()` need a complexity specification

Section: 23.4.3.3 [map.access] Status: CD1 Submitter: Joe Gottman Opened: 2007-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`map::at()` need a complexity specification.

Proposed resolution:

Add the following to the specification of `map::at()`, 23.4.3.3 [map.access]:

> Complexity: logarithmic.

### 704(i). `MoveAssignable` requirement for container value type overly strict

Section: 23.2 [container.requirements] Status: C++11 Submitter: Howard Hinnant Opened: 2007-05-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

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Discussion:

The move-related changes inadvertently overwrote the intent of 276(i). Issue 276(i) removed the requirement of `CopyAssignable` from most of the member functions of node-based containers. But the move-related changes unnecessarily introduced the `MoveAssignable` requirement for those members which used to require `CopyAssignable`.

We also discussed (c++std-lib-18722) the possibility of dropping `MoveAssignable` from some of the sequence requirements. Additionally the in-place construction work may further reduce requirements. For purposes of an easy reference, here are the minimum sequence requirements as I currently understand them. Those items in requirements table in the working draft which do not appear below have been purposefully omitted for brevity as they do not have any requirements of this nature. Some items which do not have any requirements of this nature are included below just to confirm that they were not omitted by mistake.

Container Requirements
`X u(a)` | `value_type` must be `CopyConstructible`
`X u(rv)` | `array` requires `value_type` to be `CopyConstructible`
`a = u` | Sequences require `value_type` to be `CopyConstructible` and `CopyAssignable`. Associative containers require `value_type` to be `CopyConstructible`.
`a = rv` | `array` requires `value_type` to be `CopyAssignable`. Sequences containers with `propagate_on_container_move_assignment == false` allocators require `value_type` to be `MoveConstructible` and `MoveAssignable`. Associative containers with `propagate_on_container_move_assignment == false` allocators require `value_type` to be `MoveConstructible`.
`swap(a,u)` | `array` requires `value_type` to be `Swappable`.

Sequence Requirements
`X(n)` | `value_type` must be `DefaultConstructible`
`X(n, t)` | `value_type` must be `CopyConstructible`
`X(i, j)` | Sequences require `value_type` to be constructible from `*i`. Additionally if input_iterators are used, `vector` and `deque` require `MoveContructible` and `MoveAssignable`.
`a.insert(p, t)` | The `value_type` must be `CopyConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `CopyAssignable`.
`a.insert(p, rv)` | The `value_type` must be `MoveConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `MoveAssignable`.
`a.insert(p, n, t)` | The `value_type` must be `CopyConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `CopyAssignable`.
`a.insert(p, i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `CopyAssignable` when the iterators return an lvalue. If the iterators return an rvalue the `value_type` must be `MoveConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `MoveAssignable` when the iterators return an rvalue.
`a.erase(p)` | The sequences `vector` and `deque` require the `value_type` to be `MoveAssignable`.
`a.erase(q1, q2)` | The sequences `vector` and `deque` require the `value_type` to be `MoveAssignable`.
`a.clear()` |
`a.assign(i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible` and `CopyAssignable`. If the iterators return an rvalue the `value_type` must be `MoveConstructible` and `MoveAssignable`.
`a.assign(n, t)` | The `value_type` must be `CopyConstructible` and `CopyAssignable`.
`a.resize(n)` | The `value_type` must be `DefaultConstructible`. The sequence `vector` also requires the `value_type` to be `MoveConstructible`.
`a.resize(n, t)` | The `value_type` must be `CopyConstructible`.

Optional Sequence Requirements
`a.front()` |
`a.back()` |
`a.push_front(t)` | The `value_type` must be `CopyConstructible`.
`a.push_front(rv)` | The `value_type` must be `MoveConstructible`.
`a.push_back(t)` | The `value_type` must be `CopyConstructible`.
`a.push_back(rv)` | The `value_type` must be `MoveConstructible`.
`a.pop_front()` |
`a.pop_back()` |
`a[n]` |
`a.at[n]` |

Associative Container Requirements
`X(i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. If the iterators return an rvalue the `value_type` must be `MoveConstructible`.
`a_uniq.insert(t)` | The `value_type` must be `CopyConstructible`.
`a_uniq.insert(rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a_eq.insert(t)` | The `value_type` must be `CopyConstructible`.
`a_eq.insert(rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a.insert(p, t)` | The `value_type` must be `CopyConstructible`.
`a.insert(p, rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a.insert(i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. If the iterators return an rvalue the `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`..

Unordered Associative Container Requirements
`X(i, j, n, hf, eq)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. If the iterators return an rvalue the `value_type` must be `MoveConstructible`.
`a_uniq.insert(t)` | The `value_type` must be `CopyConstructible`.
`a_uniq.insert(rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a_eq.insert(t)` | The `value_type` must be `CopyConstructible`.
`a_eq.insert(rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a.insert(p, t)` | The `value_type` must be `CopyConstructible`.
`a.insert(p, rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a.insert(i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. If the iterators return an rvalue the `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`..

Miscellaneous Requirements
`map[lvalue-key]` | The `key_type` must be `CopyConstructible`. The `mapped_type` must be `DefaultConstructible` and `MoveConstructible`.
`map[rvalue-key]` | The `key_type` must be `MoveConstructible`. The `mapped_type` must be `DefaultConstructible` and `MoveConstructible`.

[ Kona (2007): Howard and Alan to update requirements table in issue with emplace signatures. ]

[ Bellevue: This should be handled as part of the concepts work. ]

[ 2009-07-20 Reopened by Howard: ]

> This is one of the issues that was "solved by concepts" and is now no longer solved.

> In a nutshell, concepts adopted the "minimum requirements" philosophy outlined in the discussion of this issue, and enforced it. My strong suggestion is that we translate the concepts specification into documentation for the containers.

> What this means for vendors is that they will have to implement container members being careful to only use those characteristics of a type that the concepts specification formally allowed. Note that I am not talking about `enable_if`'ing everything. I am simply suggesting that (for example) we tell the vendor he can't call `T's` copy constructor or move constructor within the `emplace` member function, etc.

> What this means for customers is that they will be able to use types within C++03 containers which are sometimes not CopyConstructible, and sometimes not even MoveConstructible, etc.

[ 2009-10 Santa Cruz: ]

> Leave open. Howard to provide wording.

[ 2010-02-06 Howard provides wording. ]

[ 2010-02-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Howard opened. I neglected to reduce the requirements on value_type for the insert function of the ordered and unordered associative containers when the argument is an rvalue. Fixed it. ]

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-03-08 Nico opens: ]

> I took the task to see whether 868(i) is covered by 704 already. However, by doing that I have the impression that 704 is a big mistake.

> Take e.g. the second change of 868(i):

> > Change 23.3.5.2 [deque.cons] para 5:

> > > Effects: Constructs a `deque` with `n` default constructed elements.

> > where "default constructed" should be replaced by "value-initialized". This is the constructor out of a number of elements:

> > >     ContType c(num)

> > 704 says:

> > > Remove the entire section 23.3.5.2 [deque.cons].

> > > > [ This section is already specified by the requirements tables. ]

> > BUT, there is no requirement table that lists this constructor at all, which means that we would lose the entire specification of this function !!!

> > In fact, I found with further investigation, if we follow 704 to remove 23.3.2.1 we

> > - have no semantics for `ContType c(num)`
> > - have no complexity and no allocator specification for `ContType c(num,val)`
> > - have no semantics for `ContType c(num,val,alloc)`
> > - - have no complexity and no allocator specification for `ContType c(beg,end)`
> > - - have no semantics for `ContType c(beg,end,alloc)`
> > - - have different wording (which might or might not give the same guarantees) for the `assign` functions

> > because all these guarantees are given in the removed section but nowhere else (as far as I saw).

> > Looks to me that 704 need a significant review before we take that change, because chances are high that there are similar flaws in other proposed changes there (provided I am not missing anything).

[ 2010 Pittsburgh: ]

> Removed the parts from the proposed wording that removed existing sections, and set to Ready for Pittsburgh.

Rationale:

[ post San Francisco: ]

> Solved by N2776.

This rationale is obsolete.

Proposed resolution:

Change 23.2.2 [container.requirements.general]/4:

> 4 In Tables 91 and 92, `X` denotes a container class containing objects of type `T`, `a` and `b` denote values of type `X`, `u` denotes an identifier, `r` denotes an lvalue or a const rvalue a non-const value of type `X`, and `rv` denotes a non-const rvalue of type `X`.

Change the following rows in Table 91 — Container requirements 23.2.2 [container.requirements.general]:

> Table 91 — Container requirements
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X::value_type` | `T` | Requires: `T` is `Destructible`. | compile time

Change 23.2.2 [container.requirements.general]/10:

> Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.2.3, and 23.3.6.4) all container types defined in this Clause meet the following additional requirements:

> - …

> - no `erase()`, `clear()`, `pop_back()` or `pop_front()` function throws an exception.

> - …

Insert a new paragraph prior to 23.2.2 [container.requirements.general]/14:

> The descriptions of the requirements of the type `T` in this section use the terms `CopyConstructible`, `MoveConstructible`, constructible from `*i`, and constructible from `args`. These terms are equivalent to the following expression using the appropriate arguments:

> >     allocator_traits<allocator_type>::construct(x.get_allocator(), q, args...);

> where `x` is a non-const lvalue of some container type `X` and `q` has type `X::value_type*`.

> [Example: The container is going to move construct a `T`, so will call:

> >     allocator_traits<allocator_type>::construct(get_allocator(), q, std::move(t));

> The default implementation of construct will call:

> >     ::new (q) T(std::forward<T>(t)); // where forward is the same as move here, cast to rvalue

> But the allocator author may override the above definition of `construct` and do the construction of `T` by some other means. — end example]

> 14 ...

Add to 23.2.2 [container.requirements.general]/14:

> 14 In Table 93, `X` denotes an allocator-aware container class with a `value_type` of `T` using allocator of type `A`, `u` denotes a variable, `a` and `b` denote non-const lvalues of type `X`, `t` denotes an lvalue or a const rvalue of type `X`, `rv` denotes a non-const rvalue of type `X`, `m` is a value of type `A`, and `Q` is an allocator type.

Change or add the following rows in Table 93 — Allocator-aware container requirements in 23.2.2 [container.requirements.general]:

> Table 93 — Allocator-aware container requirements
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X(t, m) X u(t, m);` |  | Requires: `T` is `CopyConstructible`. post: `u == t`, `get_allocator() == m` | linear
> `X(rv, m) X u(rv, m);` |  | Requires: `T` is `MoveConstructible`. post: `u` shall have the same elements, or copies of the elements, that `rv` had before this construction, `get_allocator() == m` | constant if `m == rv.get_allocator()`, otherwise linear
> `a = t` | `X&` | Requires: `T` is `CopyConstructible` and `CopyAssignable` post: `a == t`. | linear
> `a = rv` | `X&` | Requires: If `allocator_traits< allocator_type > ::propagate_on_container_move_assignment ::value` is `false`, `T` is `MoveConstructible` and `MoveAssignable`. All existing elements of `a` are either move assigned to or destroyed. `a` shall be equal to the value that `rv` had before this assignment | linear
> `a.swap(b);` | `void` | exchanges the contents of `a` and `b` | constant

Change the following rows in Table 94 — Sequence container requirements (in addition to container) in 23.2.4 [sequence.reqmts]:

> Table 94 — Sequence container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition
> --- | --- | ---
> `X(i, j) X a(i, j)` |  | Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, `T` shall be `CopyConstructible`. `T` shall be constructible from `*i`. If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), then `vector` also requires `T` to be `MoveConstructible`. Each iterator in the range `[i,j)` shall be dereferenced exactly once. post: `size() ==` distance between `i` and `j` Constructs a sequence container equal to the range `[i, j)`
> `a = il;` | `X&` | Requires: `T` is `CopyConstructible` and `CopyAssignable`. `a = X(il);` Assigns the range `[il.begin(), il.end())` into `a`. All existing elements of `a` are either assigned or destroyed. rReturns `*this;`
> `a.emplace(p, args);` | `iterator` | Requires: `ConstructibleAsElement<A, T, Args>`. `T` is constructible from `args`. `vector` and `deque` also require `T` to be `MoveConstructible` and `MoveAssignable`. Inserts an object of type `T` constructed with `std::forward<Args>(args)...` before `p`.
> `a.insert(p, t);` | `iterator` | Requires: `ConstructibleAsElement<A, T, Args>` and `T` shall be `CopyAssignable`. `T` shall be `CopyConstructible`. `vector` and `deque` also require `T` to be `CopyAssignable`. Inserts a copy `t` before `p`.
> `a.insert(p, rv);` | `iterator` | Requires: `ConstructibleAsElement<A, T, T&&>` and `T` shall be `MoveAssignable`. `T` shall be `MoveConstructible`. `vector` and `deque` also require `T` to be `MoveAssignable`. Inserts a copy `rv` before `p`.
> `a.insert(p, i, j)` | `iterator` | Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, `T` shall be `CopyConstructible`. `T` shall be constructible from `*i`. If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), then `vector` also requires `T` to be `MoveConstructible` and `MoveAssignable`. Each iterator in the range `[i,j)` shall be dereferenced exactly once. pre: `i` and `j` are not iterators into `a`. Inserts copies of elements in `[i, j)` before `p`
> `a.erase(q);` | `iterator` | Requires: `T` and `T` shall be `MoveAssignable`. `vector` and `deque` require `T` to be `MoveAssignable`. Erases the element pointed to by `q`.
> `a.erase(q1, q2);` | `iterator` | Requires: `T` and `T` shall be `MoveAssignable`. `vector` and `deque` require `T` to be `MoveAssignable`. Erases the elements in the range `[q1, q2)`.
> `a.clear();` | `void` | `erase(begin(), end())` Destroys all elements in `a`. Invalidates all references, pointers, and iterators referring to the elements of `a` and may invalidate the past-the-end iterator. post: `size() == 0 a.empty() == true`
> `a.assign(i, j)` | `void` | Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, `T` shall be `CopyConstructible` and `CopyAssignable`. `T` shall be constructible and assignable from `*i`. If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), then `vector` also requires `T` to be `MoveConstructible`. Each iterator in the range `[i,j)` shall be dereferenced exactly once. pre: `i`, `j` are not iterators into `a`. Replaces elements in `a` with a copy of `[i, j)`.

Change the following rows in Table 95 — Optional sequence container operations in 23.2.4 [sequence.reqmts]:

> Table 95 — Optional sequence container operations
> Expression | Return type | Operational semantics | Container
> --- | --- | --- | ---
> `a.emplace_front(args)` | `void` | `a.emplace(a.begin(), std::forward<Args>(args)...)` Prepends an object of type `T` constructed with `std::forward<Args>(args)...`. Requires: `ConstructibleAsElement<A, T, Args>` `T` shall be constructible from `args`. | `list`, `deque`, `forward_list`
> `a.emplace_back(args)` | `void` | `a.emplace(a.end(), std::forward<Args>(args)...)` Appends an object of type `T` constructed with `std::forward<Args>(args)...`. Requires: `ConstructibleAsElement<A, T, Args>` `T` shall be constructible from `args`. `vector` also requires `T` to be `MoveConstructible`. | `list`, `deque`, `vector`
> `a.push_front(t)` | `void` | `a.insert(a.begin(), t)` Prepends a copy of `t`. Requires: `ConstructibleAsElement<A, T, T>` and `T` shall be `CopyAssignable`. `T` shall be `CopyConstructible`. | `list`, `deque`, `forward_list`
> `a.push_front(rv)` | `void` | `a.insert(a.begin(), t)` Prepends a copy of `rv`. Requires: `ConstructibleAsElement<A, T, T&&>` and `T` shall be `MoveAssignable`. `T` shall be `MoveConstructible`. | `list`, `deque`, `forward_list`
> `a.push_back(t)` | `void` | `a.insert(a.end(), t)` Appends a copy of `t`. Requires: `ConstructibleAsElement<A, T, T>` and `T` shall be `CopyAssignable`. `T` shall be `CopyConstructible`. | `vector`, `list`, `deque`, `basic_string`
> `a.push_back(rv)` | `void` | `a.insert(a.end(), t)` Appends a copy of `rv`. Requires: `ConstructibleAsElement<A, T, T&&>` and `T` shall be `MoveAssignable`. `T` shall be `MoveConstructible`. | `vector`, `list`, `deque`, `basic_string`
> `a.pop_front()` | `void` | `a.erase(a.begin())` Destroys the first element. Requires: `a.empty()` shall be `false`. | `list`, `deque`, `forward_list`
> `a.pop_back()` | `void` | `{ iterator tmp = a.end(); --tmp; a.erase(tmp); }` Destroys the last element. Requires: `a.empty()` shall be `false`. | `vector`, `list`, `deque`, `basic_string`

Insert a new paragraph prior to 23.2.7 [associative.reqmts]/7, and edit paragraph 7:

> The associative containers meet all of the requirements of Allocator-aware containers (23.2.2 [container.requirements.general]), except for the containers `map` and `multimap`, the requirements placed on `value_type` in Table 93 apply instead directly to `key_type` and `mapped_type`. [Note: For example `key_type` and `mapped_type` are sometimes required to be `CopyAssignable` even though the `value_type` (`pair<const key_type, mapped_type>`) is not `CopyAssignable`. — end note]

> 7 In Table 96, `X` denotes an associative container class, a denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `u` denotes an identifier, `r` denotes an lvalue or a const rvalue of type `X`, `rv` denotes a non-const rvalue of type `X`, `i` and `j` satisfy input iterator requirements and refer to elements implicitly convertible to `value_type`, `[i,j)` denotes a valid range, `p` denotes a valid const iterator to `a`, `q` denotes a valid dereferenceable const iterator to `a`, `[q1, q2)` denotes a valid range of const iterators in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` denotes a value of `X::value_type`, `k` denotes a value of `X::key_type` and `c` denotes a value of type `X::key_compare`. `A` denotes the storage allocator used by `X`, if any, or `std::allocator<X::value_type>` otherwise, and `m` denotes an allocator of a type convertible to `A`.

Change or add the following rows in Table 96 — Associative container requirements (in addition to container) in 23.2.7 [associative.reqmts]:

> Table 96 — Associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X::key_type` | `Key` | Requires: `Key` is `CopyConstructible` and `CopyAssignable` `Destructible` | compile time
> `X::mapped_type` (`map` and `multimap` only) | `T` | Requires: `T` is `Destructible` | compile time
> `X(c) X a(c);` |  | Requires: `ConstructibleAsElement<A, key_compare, key_compare>`. `key_compare` is `CopyConstructible`. Constructs an empty container. Uses a copy of `c` as a comparison object. | constant
> `X() X a;` |  | Requires: `ConstructibleAsElement<A, key_compare, key_compare>`. `key_compare` is `DefaultConstructible`. Constructs an empty container. Uses `Compare()` as a comparison object. | constant
> `X(i, j, c) X a(i, j, c);` |  | Requires: `ConstructibleAsElement<A, key_compare, key_compare>`. `key_compare` is `CopyConstructible`. `value_type` shall be constructible from `*i`. Constructs an empty container ans inserts elements from the range `[i, j)` into it; uses `c` as a comparison object. | `N` log `N` in general (`N` is the distance from `i` to `j`); linear if `[i, j)` is sorted with `value_comp()`
> `X(i, j) X a(i, j);` |  | Requires: `ConstructibleAsElement<A, key_compare, key_compare>`. `value_type` shall be constructible from `*i`. `key_compare` is `DefaultConstructible`. Same as above, but uses `Compare()` as a comparison object. | same as above
> `a = il` | `X&` | `a = X(il); return *this;` Requires: `T` is `CopyConstructible` and `CopyAssignable`. Assigns the range `[il.begin(), il.end())` into `a`. All existing elements of `a` are either assigned or destroyed. | Same as ``a = X(il)``. `N` log `N` in general (`N` is `il.size()` added to the existing size of `a`); linear if `[il.begin(), il.end())` is sorted with `value_comp()`
> `a_uniq.emplace(args)` | `pair<iterator, bool>` | Requires: `T` shall be constructible from `args` inserts a `T` object `t` constructed with `std::forward<Args>(args)...` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of `t`. | logarithmic
> `a_eq.emplace(args)` | `iterator` | Requires: `T` shall be constructible from `args` inserts a `T` object `t` constructed with `std::forward<Args>(args)...` and returns the iterator pointing to the newly inserted element. | logarithmic
> `a_uniq.insert(t)` | `pair<iterator, bool>` | Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`. inserts `t` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of `t`. | logarithmic
> `a_eq.insert(t)` | `iterator` | Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`. inserts `t` and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to `t` exists in `a_eq`, `t` is inserted at the end of that range. | logarithmic
> `a.insert(p, t)` | `iterator` | Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`. inserts `t` if and only if there is no element with key equivalent to the key of `t` in containers with unique keys; always inserts `t` in containers with equivalent keys; always returns the iterator pointing to the element with key equivalent to the key of `t`. `t` is inserted as close as possible to the position just prior to `p`. | logarithmic in general, but amortized constant if `t` is inserted right before `p`.
> `a.insert(i, j)` | `void` | Requires: `T` shall be constructible from `*i`. pre: `i`, `j` are not iterators into `a`. inserts each element from the range `[i,j)` if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys. | N log(size() + N ) (N is the distance from i to j)

Insert a new paragraph prior to 23.2.8 [unord.req]/9:

> The unordered associative containers meet all of the requirements of Allocator-aware containers (23.2.2 [container.requirements.general]), except for the containers `unordered_map` and `unordered_multimap`, the requirements placed on `value_type` in Table 93 apply instead directly to `key_type` and `mapped_type`. [Note: For example `key_type` and `mapped_type` are sometimes required to be `CopyAssignable` even though the `value_type` (`pair<const key_type, mapped_type>`) is not `CopyAssignable`. — end note]

> 9 ...

Change or add the following rows in Table 98 — Unordered associative container requirements (in addition to container) in 23.2.8 [unord.req]:

> Table 98 — Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X::key_type` | `Key` | Requires: `Key` shall be `CopyAssignable` and `CopyConstructible` `Destructible` | compile time
> `X::mapped_type` (`unordered_map` and `unordered_multimap` only) | `T` | Requires:`T` is `Destructible` | compile time
> `X(n, hf, eq) X a(n, hf, eq)` | `X` | Requires: `hasher` and `key_equal` are `CopyConstructible`. Constructs an empty container with at least `n` buckets, using `hf` as the hash function and `eq` as the key equality predicate. | `O(N)`
> `X(n, hf) X a(n, hf)` | `X` | Requires: `hasher` is `CopyConstructible` and `key_equal` is `DefaultConstructible`. Constructs an empty container with at least `n` buckets, using `hf` as the hash function and `key_equal()` as the key equality predicate. | `O(N)`
> `X(n) X a(n)` | `X` | Requires: `hasher` and `key_equal` are `DefaultConstructible`. Constructs an empty container with at least `n` buckets, using `hasher()` as the hash function and `key_equal()` as the key equality predicate. | `O(N)`
> `X() X a` | `X` | Requires: `hasher` and `key_equal` are `DefaultConstructible`. Constructs an empty container an unspecified number of buckets, using `hasher()` as the hash function and `key_equal()` as the key equality predicate. | constant
> `X(i, j, n, hf, eq) X a(i, j, n, hf, eq)` | `X` | Requires: `value_type` is constructible from `*i`. `hasher` and `key_equal` are `CopyConstructible`. Constructs an empty container with at least `n` buckets, using `hf` as the hash function and `eq` as the key equality predicate, and inserts elements from `[i, j)` into it. | Average case `O(N)` (`N` is `distance(i, j)`), worst case `O(N2)`
> `X(i, j, n, hf) X a(i, j, n, hf)` | `X` | Requires: `value_type` is constructible from `*i`. `hasher` is `CopyConstructible` and `key_equal` is `DefaultConstructible`. Constructs an empty container with at least `n` buckets, using `hf` as the hash function and `key_equal()` as the key equality predicate, and inserts elements from `[i, j)` into it. | Average case `O(N)` (`N` is `distance(i, j)`), worst case `O(N2)`
> `X(i, j, n) X a(i, j, n)` | `X` | Requires: `value_type` is constructible from `*i`. `hasher` and `key_equal` are `DefaultConstructible`. Constructs an empty container with at least `n` buckets, using `hasher()` as the hash function and `key_equal()` as the key equality predicate, and inserts elements from `[i, j)` into it. | Average case `O(N)` (`N` is `distance(i, j)`), worst case `O(N2)`
> `X(i, j) X a(i, j)` | `X` | Requires: `value_type` is constructible from `*i`. `hasher` and `key_equal` are `DefaultConstructible`. Constructs an empty container with an unspecified number of buckets, using `hasher()` as the hash function and `key_equal()` as the key equality predicate, and inserts elements from `[i, j)` into it. | Average case `O(N)` (`N` is `distance(i, j)`), worst case `O(N2)`
> `X(b) X a(b)` | `X` | Copy constructor. In addition to the contained elements requirements of Table 93 (23.2.2 [container.requirements.general]), copies the hash function, predicate, and maximum load factor. | Average case linear in `b.size()`, worst case quadratic.
> `a = b` | `X&` | Copy assignment operator. In addition to the contained elements requirements of Table 93 (23.2.2 [container.requirements.general]), copies the hash function, predicate, and maximum load factor. | Average case linear in `b.size()`, worst case quadratic.
> `a = il` | `X&` | `a = X(il); return *this;` Requires: `T` is `CopyConstructible` and `CopyAssignable`. Assigns the range `[il.begin(), il.end())` into `a`. All existing elements of `a` are either assigned or destroyed. | Average case linear in `il.size()`, worst case quadratic.
> `a_uniq.emplace(args)` | `pair<iterator, bool>` | Requires: `T` shall be constructible from `args` inserts a `T` object `t` constructed with `std::forward<Args>(args)...` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of `t`. | Average case O(1), worst case O(`a_uniq.size()`).
> `a_eq.emplace(args)` | `iterator` | Requires: `T` shall be constructible from `args` inserts a `T` object `t` constructed with `std::forward<Args>(args)...` and returns the iterator pointing to the newly inserted element. | Average case O(1), worst case O(`a_eq.size()`).
> `a.emplace_hint(p, args)` | `iterator` | Requires: `T` shall be constructible from `args` equivalent to `a.emplace( std::forward<Args>(args)...)`. Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The `const_iterator p` is a hint pointing to where the search should start. Implementations are permitted to ignore the hint. | Average case O(1), worst case O(`a.size()`).
> `a_uniq.insert(t)` | `pair<iterator, bool>` | Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`. Inserts `t` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key of `t`. | Average case O(1), worst case O(`a_uniq.size()`).
> `a_eq.insert(t)` | `iterator` | Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`. Inserts `t`, and returns an iterator pointing to the newly inserted element. | Average case O(1), worst case O(`a_uniq.size()`).
> `a.insert(q, t)` | `iterator` | Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`. Equivalent to `a.insert(t)`. Return value is an iterator pointing to the element with the key equivalent to that of `t`. The iterator `q` is a hint pointing to where the search should start. Implementations are permitted to ignore the hint. | Average case O(1), worst case O(`a_uniq.size()`).
> `a.insert(i, j)` | `void` | Requires: `T` shall be constructible from `*i`. Pre: `i` and `j` are not iterators in `a`. Equivalent to `a.insert(t)` for each element in `[i,j)`. | Average case O(`N`), where `N` is `distance(i, j)`. Worst case O(`N * a.size()`).

Change [forwardlist]/2:

> 2 A `forward_list` satisfies all of the requirements of a container (table 91), except that the `size()` member function is not provided. A `forward_list` also satisfies all of the requirements of an allocator-aware container (table 93). And `forward_list` provides the `assign` member functions as specified in Table 94, Sequence container requirements, and several of the optional sequence container requirements (Table 95). Descriptions are provided here only for operations on `forward_list` that are not described in that table or for operations where there is additional semantic information.

Add a new paragraph after [forwardlist.modifiers]/23:

>     void clear();

> > 23 Effects: Erases all elements in the range `[begin(),end())`.

> > Remarks: Does not invalidate past-the-end iterators.

Change 23.3.13.3 [vector.capacity]/13:

>     void resize(size_type sz, const T& c);

> > 13 Requires: `T` shall be `CopyConstructible`. If `value_type` has a move constructor, that constructor shall not throw any exceptions.

In 23.5.6 [unord.set] and 23.5.7 [unord.multiset] substitute "`Key`" for "`Value`".

> [ The above substitution is normative as it ties into the requirements table. ]

### 705(i). type-trait `decay` incompletely specified

Section: 21.3.8.7 [meta.trans.other] Status: CD1 Submitter: Thorsten Ottosen Opened: 2007-07-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current working draft has a type-trait `decay` in 21.3.8.7 [meta.trans.other].

Its use is to turn C++03 pass-by-value parameters into efficient C++0x pass-by-rvalue-reference parameters. However, the current definition introduces an incompatible change where the cv-qualification of the parameter type is retained. The deduced type should loose such cv-qualification, as pass-by-value does.

Proposed resolution:

In 21.3.8.7 [meta.trans.other] change the last sentence:

> Otherwise the member typedef `type` equals `remove_cv<U>::type`.

In 22.4.5 [tuple.creation]/1 change:

> where each `Vi` in `VTypes` is `X&` if, for the corresponding type `Ti` in `Types`, `remove_cv<remove_reference<Ti>::type>::type` equals `reference_wrapper<X>`, otherwise `Vi` is `decay<Ti>::type`. Let `Ui` be `decay<Ti>::type` for each `Ti` in `Types`. Then each `Vi` in `VTypes` is `X&` if `Ui` equals `reference_wrapper<X>`, otherwise `Vi` is `Ui`.

### 706(i). `make_pair()` should behave as `make_tuple()` wrt. `reference_wrapper()`

Section: 22.3 [pairs] Status: CD1 Submitter: Thorsten Ottosen Opened: 2007-07-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current draft has `make_pair()` in 22.3 [pairs]/16 and `make_tuple()` in 22.4.5 [tuple.creation]. `make_tuple()` detects the presence of `reference_wrapper<X>` arguments and "unwraps" the reference in such cases. `make_pair()` would OTOH create a `reference_wrapper<X>` member. I suggest that the two functions are made to behave similar in this respect to minimize confusion.

Proposed resolution:

In 22.2 [utility] change the synopsis for make_pair() to read

>     template <class T1, class T2>
>       pair<typename decay<T1>::type V1, typename decay<T2>::type V2> make_pair(T1&&, T2&&);

In 22.3 [pairs]/16 change the declaration to match the above synopsis. Then change the 22.3 [pairs]/17 to:

> Returns: `pair<typename decay<T1>::type V1,typename decay<T2>::type V2>(forward<T1>(x),forward<T2>(y))` where `V1` and `V2` are determined as follows: Let `Ui` be `decay<Ti>::type` for each `Ti`. Then each `Vi` is `X&` if `Ui` equals `reference_wrapper<X>`, otherwise `Vi` is `Ui`.

### 709(i). `char_traits::not_eof` has wrong signature

Section: 27.2.4 [char.traits.specializations] Status: CD1 Submitter: Bo Persson Opened: 2007-08-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The changes made for `constexpr` in 27.2.4 [char.traits.specializations] have not only changed the `not_eof` function from pass by const reference to pass by value, it has also changed the parameter type from `int_type` to `char_type`.

This doesn't work for type `char`, and is inconsistent with the requirements in Table 56, Traits requirements, 27.2.2 [char.traits.require].

Pete adds:

> For what it's worth, that may not have been an intentional change. N2349, which detailed the changes for adding constant expressions to the library, has strikeout bars through the `const` and the `&` that surround the `char_type` argument, but none through `char_type` itself. So the intention may have been just to change to pass by value, with text incorrectly copied from the standard.

Proposed resolution:

Change the signature in 27.2.4.2 [char.traits.specializations.char], [char.traits.specializations.char16_t], [char.traits.specializations.char32_t], and 27.2.4.6 [char.traits.specializations.wchar.t] to

>     static constexpr int_type not_eof(char_type int_type c);

[ Bellevue: ]

> Resolution: NAD editorial - up to Pete's judgment

[ Post Sophia Antipolis ]

> Moved from Pending NAD Editorial to Review. The proposed wording appears to be correct but non-editorial.

### 710(i). Missing postconditions

Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

A discussion on comp.std.c++ has identified a contradiction in the `shared_ptr` specification. The `shared_ptr` move constructor and the cast functions are missing postconditions for the `get()` accessor.

[ Bellevue: ]

> Move to "ready", adopting the first (Peter's) proposed resolution.

> Note to the project editor: there is an editorial issue here. The wording for the postconditions of the casts is slightly awkward, and the editor should consider rewording "If w is the return value...", e. g. as "For a return value w...".

Proposed resolution:

Add to 20.3.2.2.2 [util.smartptr.shared.const]:

>     shared_ptr(shared_ptr&& r);
>     template<class Y> shared_ptr(shared_ptr<Y>&& r);

> > Postconditions: `*this` shall contain the old value of `r`. `r` shall be empty. `r.get() == 0`.

Add to 20.3.2.2.10 [util.smartptr.shared.cast]:

>     template<class T, class U> shared_ptr<T> static_pointer_cast(shared_ptr<U> const& r);

> > Postconditions: If `w` is the return value, `w.get() == static_cast<T*>(r.get()) && w.use_count() == r.use_count()`.

>     template<class T, class U> shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const& r);

> > Postconditions: If `w` is the return value, `w.get() == dynamic_cast<T*>(r.get())`.

>     template<class T, class U> shared_ptr<T> const_pointer_cast(shared_ptr<U> const& r);

> > Postconditions: If `w` is the return value, `w.get() == const_cast<T*>(r.get()) && w.use_count() == r.use_count()`.

Alberto Ganesh Barbati has written an alternative proposal where he suggests (among other things) that the casts be respecified in terms of the aliasing constructor as follows:

Change 20.3.2.2.10 [util.smartptr.shared.cast]:

> -2- Returns: If `r` is empty, an `empty shared_ptr<T>;` otherwise, a `shared_ptr<T>` object that stores `static_cast<T*>(r.get())` and shares ownership with `r`. `shared_ptr<T>(r, static_cast<T*>(r.get())`.

> -6- Returns:

> - When `dynamic_cast<T*>(r.get())` returns a nonzero value, a `shared_ptr<T>` object that stores a copy of it and shares ownership with `r`;
> - Otherwise, an empty `shared_ptr<T>` object.
> - If `p = dynamic_cast<T*>(r.get())` is a non-null pointer, `shared_ptr<T>(r, p);`
> - Otherwise, `shared_ptr<T>()`.

> -10- Returns: If `r` is empty, an `empty shared_ptr<T>;` otherwise, a `shared_ptr<T>` object that stores `const_cast<T*>(r.get())` and shares ownership with `r`. `shared_ptr<T>(r, const_cast<T*>(r.get())`.

This takes care of the missing postconditions for the casts by bringing in the aliasing constructor postcondition "by reference".

### 711(i). Contradiction in empty `shared_ptr`

Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: C++11 Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

A discussion on comp.std.c++ has identified a contradiction in the `shared_ptr` specification. The note:

> [ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -end note ]

after the aliasing constructor

>     template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);

reflects the intent of N2351 to, well, allow the creation of an empty `shared_ptr` with a non-NULL stored pointer.

This is contradicted by the second sentence in the Returns clause of 20.3.2.2.6 [util.smartptr.shared.obs]:

>     T* get() const;

> > Returns: the stored pointer. Returns a null pointer if `*this` is empty.

[ Bellevue: ]

> Adopt option 1 and move to review, not ready.

> There was a lot of confusion about what an empty `shared_ptr` is (the term isn't defined anywhere), and whether we have a good mental model for how one behaves. We think it might be possible to deduce what the definition should be, but the words just aren't there. We need to open an issue on the use of this undefined term. (The resolution of that issue might affect the resolution of issue 711(i).)

> The LWG is getting more uncomfortable with the aliasing proposal (N2351) now that we realize some of its implications, and we need to keep an eye on it, but there isn't support for removing this feature at this time.

[ Sophia Antipolis: ]

> We heard from Peter Dimov, who explained his reason for preferring solution 1.

> Because it doesn't seem to add anything. It simply makes the behavior for p = 0 undefined. For programmers who don't create empty pointers with p = 0, there is no difference. Those who do insist on creating them presumably have a good reason, and it costs nothing for us to define the behavior in this case.

> The aliasing constructor is sharp enough as it is, so "protecting" users doesn't make much sense in this particular case.

> > Do you have a use case for r being empty and r being non-null?

> I have received a few requests for it from "performance-conscious" people (you should be familiar with this mindset) who don't like the overhead of allocating and maintaining a control block when a null deleter is used to approximate a raw pointer. It is obviously an "at your own risk", low-level feature; essentially a raw pointer behind a shared_ptr facade.

> We could not agree upon a resolution to the issue; some of us thought that Peter's description above is supporting an undesirable behavior.

[ 2009-07 Frankfurt: ]

> We favor option 1, move to Ready.

> [ Howard: Option 2 commented out for clarity, and can be brought back. ]

Proposed resolution:

In keeping the N2351 spirit and obviously my preference, change 20.3.2.2.6 [util.smartptr.shared.obs]:

>     T* get() const;

> > Returns: the stored pointer. Returns a null pointer if `*this` is empty.

### 712(i). `seed_seq::size` no longer useful

Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Marc Paterno Opened: 2007-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

One of the motivations for incorporating `seed_seq::size()` was to simplify the wording in other parts of 29.5 [rand]. As a side effect of resolving related issues, all such references to `seed_seq::size()` will have been excised. More importantly, the present specification is contradictory, as "The number of 32-bit units the object can deliver" is not the same as "the result of `v.size()`."

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 713(i). `sort()` complexity is too lax

Section: 26.8.2.1 [sort] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The complexity of `sort()` is specified as "Approximately `N log(N)` (where `N == last - first` ) comparisons on the average", with no worst case complicity specified. The intention was to allow a median-of-three quicksort implementation, which is usually `O(N log N)` but can be quadratic for pathological inputs. However, there is no longer any reason to allow implementers the freedom to have a worst-cast-quadratic sort algorithm. Implementers who want to use quicksort can use a variant like David Musser's "Introsort" (Software Practice and Experience 27:983-993, 1997), which is guaranteed to be `O(N log N)` in the worst case without incurring additional overhead in the average case. Most C++ library implementers already do this, and there is no reason not to guarantee it in the standard.

Proposed resolution:

In 26.8.2.1 [sort], change the complexity to "O(N log N)", and remove footnote 266:

> Complexity: Approximately O(N log(N)) (where N == last - first ) comparisons on the average.266)

> 266) If the worst case behavior is important `stable_sort()` (25.3.1.2) or `partial_sort()` (25.3.1.3) should be used.

### 714(i). `search_n` complexity is too lax

Section: 26.6.15 [alg.search] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The complexity for `search_n` (26.6.15 [alg.search] par 7) is specified as "At most (last - first ) * count applications of the corresponding predicate if count is positive, or 0 otherwise." This is unnecessarily pessimistic. Regardless of the value of count, there is no reason to examine any element in the range more than once.

Proposed resolution:

Change the complexity to "At most (last - first) applications of the corresponding predicate".

>     template<class ForwardIterator, class Size, class T>
>       ForwardIterator
>         search_n(ForwardIterator first , ForwardIterator last , Size count ,
>                  const T& value );

>     template<class ForwardIterator, class Size, class T,
>              class BinaryPredicate>
>       ForwardIterator
>         search_n(ForwardIterator first , ForwardIterator last , Size count ,
>                  const T& value , BinaryPredicate pred );

> > Complexity: At most `(last - first ) * count` applications of the corresponding predicate if `count` is positive, or 0 otherwise.

### 715(i). `minmax_element` complexity is too lax

Section: 26.8.9 [alg.min.max] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The complexity for `minmax_element` (26.8.9 [alg.min.max] par 16) says "At most `max(2 * (last - first ) - 2, 0)` applications of the corresponding comparisons", i.e. the worst case complexity is no better than calling `min_element` and `max_element` separately. This is gratuitously inefficient. There is a well known technique that does better: see section 9.1 of CLRS (Introduction to Algorithms, by Cormen, Leiserson, Rivest, and Stein).

Proposed resolution:

Change 26.8.9 [alg.min.max] to:

>     template<class ForwardIterator>
>       pair<ForwardIterator, ForwardIterator>
>         minmax_element(ForwardIterator first , ForwardIterator last);
>     template<class ForwardIterator, class Compare>
>       pair<ForwardIterator, ForwardIterator>
>         minmax_element(ForwardIterator first , ForwardIterator last , Compare comp);

> > Returns: `make_pair(m, M)`, where `m` is `min_element(first, last)` or `min_element(first, last, comp)` the first iterator in `[first, last)` such that no iterator in the range refers to a smaller element, and where `M` is `max_element(first, last)` or `max_element(first, last, comp)` the last iterator in `[first, last)` such that no iterator in the range refers to a larger element.

> > Complexity: At most `max(2 * (last - first ) - 2, 0)` `max(⌊(3/2) (N-1)⌋, 0)` applications of the corresponding comparisons predicate, where `N` is `distance(first, last)`.

### 716(i). Production in [re.grammar] not actually modified

Section: 28.6.12 [re.grammar] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2007-08-31 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

[tr.re.grammar]/3 and C++0x WP 28.6.12 [re.grammar]/3 say:

> The following productions within the ECMAScript grammar are modified as follows:

> >     CharacterClass ::
> >     [ [lookahead ∉ {^}] ClassRanges ]
> >     [ ^ ClassRanges ]

This definition for `CharacterClass` appears to be exactly identical to that in ECMA-262.

Was an actual modification intended here and accidentally omitted, or was this production accidentally included?

[ Batavia (2009-05): ]

> We agree that what is specified is identical to what ECMA-262 specifies. Pete would like to take a bit of time to assess whether we had intended, but failed, to make a change. It would also be useful to hear from John Maddock on the issue.

> Move to Open.

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

Remove this mention of the CharacterClass production.

>     CharacterClass ::
>     [ [lookahead ∉ {^}] ClassRanges ]
>     [ ^ ClassRanges ]

### 719(i). `std::is_literal` type traits should be provided

Section: 21 [meta] Status: Resolved Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 750

Discussion:

Since the inclusion of `constexpr` in the standard draft N2369 we have a new type category "literal", which is defined in 6.9 [basic.types]/p.11:

> -11- A type is a literal type if it is:

> - a scalar type; or

> - a class type (clause 9) with

>   - a trivial copy constructor,
>   - a trivial destructor,
>   - at least one constexpr constructor other than the copy constructor,
>   - no virtual base classes, and
>   - all non-static data members and base classes of literal types; or
> - an array of literal type.

I strongly suggest that the standard provides a type traits for literal types in 21.3.5.4 [meta.unary.prop] for several reasons:

1. To keep the traits in sync with existing types.
2. I see many reasons for programmers to use this trait in template code to provide optimized template definitions for these types, see below.
3. A user-provided definition of this trait is practically impossible to write portably.

The special problem of reason (c) is that I don't see currently a way to portably test the condition for literal class types:

> - at least one constexpr constructor other than the copy constructor,

[ Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits. ]

[ 2009-07 Frankfurt: ]

> Beman, Daniel, and Alisdair will work on a paper proposing new type traits.

[ Addressed in N2947. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2984.

Proposed resolution:

In 21.3.3 [meta.type.synop] in the group "type properties", just below the line

>     template <class T> struct is_pod;

add a new one:

>     template <class T> struct is_literal;

In 21.3.5.4 [meta.unary.prop], table Type Property Predicates, just below the line for the `is_pod` property add a new line:

Template | Condition | Preconditions
--- | --- | ---
`template <class T> struct is_literal;` | `T` is a literal type (3.9) | `T` shall be a complete type, an array of unknown bound, or (possibly cv-qualified) `void`.

### 720(i). Omissions in constexpr usages

Section: 23.3.3 [array], 22.9.2 [template.bitset] Status: CD1 Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

1. The member function `bool array<T,N>::empty() const` should be a `constexpr` because this is easily to proof and to implement following it's operational semantics defined by Table 87 (Container requirements) which says: `a.size() == 0`.
2. The member function `bool bitset<N>::test() const` must be a `constexpr` (otherwise it would violate the specification of `constexpr bitset<N>::operator[](size_t) const`, because it's return clause delegates to `test()`).
3. I wonder how the constructor `bitset<N>::bitset(unsigned long)` can be declared as a `constexpr`. Current implementations usually have no such `bitset` c'tor which would fulfill the requirements of a `constexpr` c'tor because they have a non-empty c'tor body that typically contains for-loops or `memcpy` to compute the initialisation. What have I overlooked here?

[ Sophia Antipolis: ]

> We handle this as two parts

> 1. The proposed resolution is correct; move to ready.
> 2. The issue points out a real problem, but the issue is larger than just this solution. We believe a paper is needed, applying the full new features of C++ (including extensible literals) to update `std::bitset`. We note that we do not consider this new work, and that is should be handled by the Library Working Group.

> In order to have a consistent working paper, Alisdair and Daniel produced a new wording for the resolution.

Proposed resolution:

1. In the class template definition of 23.3.3 [array]/p. 3 change

   >     constexpr bool empty() const;

2. In the class template definition of 22.9.2 [template.bitset]/p. 1 change

   >     constexpr bool test(size_t pos ) const;

   and in 22.9.2.3 [bitset.members] change

   >     constexpr bool test(size_t pos ) const;

### 722(i). Missing [c.math] functions `nanf` and `nanl`

Section: 29.7 [c.math] Status: CD1 Submitter: Daniel Krügler Opened: 2007-08-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the listing of 29.7 [c.math], table 108: Header `<cmath>` synopsis I miss the following C99 functions (from 7.12.11.2):

>     float nanf(const char *tagp);
>     long double nanl(const char *tagp);

(Note: These functions cannot be overloaded and they are also not listed anywhere else)

Proposed resolution:

In 29.7 [c.math], table 108, section "Functions", add `nanf` and `nanl` just after the existing entry `nan`.

### 723(i). `basic_regex` should be moveable

Section: 28.6.7 [re.regex] Status: C++11 Submitter: Daniel Krügler Opened: 2007-08-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 316

According to the current state of the standard draft, the class template `basic_regex`, as described in 28.6.7 [re.regex]/3, is neither `MoveConstructible` nor `MoveAssignable`. IMO it should be, because typical regex state machines tend to have a rather large data quantum and I have seen several use cases, where a factory function returns regex values, which would take advantage of moveabilities.

[ Sophia Antipolis: ]

> Needs wording for the semantics, the idea is agreed upon.

[ Post Summit Daniel updated wording to reflect new "swap rules". ]

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

In the class definition of `basic_regex`, just below 28.6.7 [re.regex]/3, perform the following changes:

1. Just after `basic_regex(const basic_regex&);` insert:

   >     basic_regex(basic_regex&&);

2. Just after `basic_regex& operator=(const basic_regex&);` insert:

   >     basic_regex& operator=(basic_regex&&);

3. Just after `basic_regex& assign(const basic_regex& that);` insert:

   >     basic_regex& assign(basic_regex&& that);

4. In 28.6.7.2 [re.regex.construct], just after p.11 add the following new member definition:

   >     basic_regex(basic_regex&& e);

   > > Effects: Move-constructs a `basic_regex` instance from `e`.

   > > Postconditions: `flags()` and `mark_count()` return `e.flags()` and `e.mark_count()`, respectively, that `e` had before construction, leaving `e` in a valid state with an unspecified value.

   > > Throws: nothing.

5. Also in 28.6.7.2 [re.regex.construct], just after p.18 add the following new member definition:

   >     basic_regex& operator=(basic_regex&& e);

   > > Effects: Returns the result of `assign(std::move(e))`.

6. In 28.6.7.3 [re.regex.assign], just after p. 2 add the following new member definition:

   >     basic_regex& assign(basic_regex&& rhs);

   > > Effects: Move-assigns a `basic_regex` instance from `rhs` and returns `*this`.

   > > Postconditions: `flags()` and `mark_count()` return `rhs.flags()` and `rhs.mark_count()`, respectively, that `rhs` had before assignment, leaving `rhs` in a valid state with an unspecified value.

   > > Throws: nothing.

### 724(i). `DefaultConstructible` is not defined

Section: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Pablo Halpern Opened: 2007-09-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The `DefaultConstructible` requirement is referenced in several places in the August 2007 working draft N2369, but is not defined anywhere.

[ Bellevue: ]

> Walking into the default/value-initialization mess...

> Why two lines? Because we need both expressions to be valid.

> AJM not sure what the phrase "default constructed" means. This is unfortunate, as the phrase is already used 24 times in the library!

> Example: `const int` would not accept first line, but will accept the second.

> This is an issue that must be solved by concepts, but we might need to solve it independantly first.

> It seems that the requirements are the syntax in the proposed first column is valid, but not clear what semantics we need.

> A table where there is no post-condition seems odd, but appears to sum up our position best.

> At a minimum an object is declared and is destructible.

> Move to open, as no-one happy to produce wording on the fly.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-08-17 Daniel adds "[defaultconstructible]" to table title. 408(i) depends upon this issue. ]

[ 2009-08-18 Alisdair adds: ]

> Looking at the proposed table in this issue, it really needs two rows:

> > Table 33: `DefaultConstructible` requirements [defaultconstructible]
> > expression | post-condition
> > --- | ---
> > `T t;` | `t` is default-initialized.
> > `T{}` | Object of type `T` is value-initialized.

> Note I am using the new brace-initialization syntax that is unambiguous in all use cases (no most vexing parse.)

[ 2009-10-03 Daniel adds: ]

> The suggested definition `T{}` describing it as value-initialization is wrong, because it belongs to list-initialization which would - as the current rules are - always prefer a initializer-list constructor over a default-constructor. I don't consider this as an appropriate definition of `DefaultConstructible`. My primary suggestion is to ask core, whether the special case `T{}` (which also easily leads to ambiguity situations for more than one initializer-list in a class) would always prefer a default-constructor - if any - before considering an initializer-list constructor or to provide another syntax form to prefer value-initialization over list-initialization. If that fails I would fall back to suggest to use the expression `T()` instead of `T{}` with all it's disadvantages for the meaning of the expression

> >     T t();

[ 2009-10 Santa Cruz: ]

> Leave Open. Core is looking to make Alisdair's proposed resolution correct.

[ 2010-01-24 At Alisdair's request, moved his proposal into the proposed wording section. The old wording is preserved here: ]

> In section 16.4.4.2 [utility.arg.requirements], before table 33, add the following table:

> Table 33: `DefaultConstructible` requirements [defaultconstructible]

> expression | post-condition
> `T t;` `T()` | `T` is default constructed.

[ 2010-02-04: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Rationale:

[ San Francisco: ]

> We believe concepts will solve this problem (N2774).

[ Rationale is obsolete. ]

Proposed resolution:

In section 16.4.4.2 [utility.arg.requirements], before table 33, add the following table:

> Table 33: `DefaultConstructible` requirements [defaultconstructible]
> expression | post-condition
> --- | ---
> `T t;` | Object `t` is default-initialized.
> `T u{};` | Object `u` is value-initialized.
> `T() T{}` | A temporary object of type `T` is value-initialized.

### 727(i). `regex_replace()` doesn't accept `basic_string`s with custom traits and allocators

Section: 28.6.10.4 [re.alg.replace] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`regex_match()` and `regex_search()` take `const basic_string<charT, ST, SA>&`. `regex_replace()` takes `const basic_string<charT>&`. This prevents `regex_replace()` from accepting `basic_string`s with custom traits and allocators.

Overloads of `regex_replace()` taking `basic_string` should be additionally templated on `class ST, class SA` and take `const basic_string<charT, ST, SA>&`. Consistency with `regex_match()` and `regex_search()` would place `class ST, class SA` as the first template arguments; compatibility with existing code using TR1 and giving explicit template arguments to `regex_replace()` would place `class ST, class SA` as the last template arguments.

[ Batavia (2009-05): ]

> Bill comments, "We need to look at the depth of this change."

> Pete remarks that we are here dealing with a convenience function that saves a user from calling the iterato-based overload.

> Move to Open.

[ 2009-07 Frankfurt: ]

> Howard to ask Stephan Lavavej to provide wording.

[ 2009-07-17 Stephan provided wording. ]

[ 2009-07-25 Daniel tweaks both this issue and 726(i). ]

> One relevant part of the proposed resolution below suggests to add a new overload of the format member function in the `match_results` class template that accepts two character pointers defining the `begin` and `end` of a format range. A more general approach could have proposed a pair of iterators instead, but the used pair of char pointers reflects existing practice. If the committee strongly favors an iterator-based signature, this could be simply changed. I think that the minimum requirement should be a `BidirectionalIterator`, but current implementations take advantage (at least partially) of the `RandomAccessIterator` sub interface of the char pointers.

> Suggested Resolution:

> [Moved into the proposed resloution]

[ 2009-07-30 Stephan agrees with Daniel's wording. Howard places Daniel's wording in the Proposed Resolution. ]

[ 2009-10 Santa Cruz: ]

> Move to Review. Chair is anxious to move this to Ready in Pittsburgh.

[ 2010-01-27 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 28.6.3 [re.syn] as indicated:

   >     // 28.11.4, function template regex_replace:
   >     template <class OutputIterator, class BidirectionalIterator,
   >               class traits, class charT, class ST, class SA>
   >       OutputIterator
   >       regex_replace(OutputIterator out,
   >                     BidirectionalIterator first, BidirectionalIterator last,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, ST, SA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class OutputIterator, class BidirectionalIterator,
   >               class traits, class charT>
   >       OutputIterator
   >       regex_replace(OutputIterator out,
   >                     BidirectionalIterator first, BidirectionalIterator last,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT, class ST, class SA,
   >               class FST, class FSA>
   >       basic_string<charT, ST, SA>
   >       regex_replace(const basic_string<charT, ST, SA>& s,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, FST, FSA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT, class ST, class SA>
   >       basic_string<charT, ST, SA>
   >       regex_replace(const basic_string<charT, ST, SA>& s,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT, class ST, class SA>
   >       basic_string<charT>
   >       regex_replace(const charT* s,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, ST, SA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT>
   >       basic_string<charT>
   >       regex_replace(const charT* s,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

2. Change 28.6.9 [re.results]/3, class template `match_results` as indicated:

   >     template <class OutputIter>
   >       OutputIter
   >       format(OutputIter out,
   >              const char_type* fmt_first, const char_type* fmt_last,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   >     template <class OutputIter, class ST, class SA>
   >       OutputIter
   >       format(OutputIter out,
   >              const string_typebasic_string<char_type, ST, SA>& fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   >     template <class ST, class SA>
   >       string_typebasic_string<char_type, ST, SA>
   >       format(const string_typebasic_string<char_type, ST, SA>& fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   >     string_type
   >     format(const char_type* fmt,
   >            regex_constants::match_flag_type flags =
   >              regex_constants::format_default) const;

3. Insert at the very beginning of 28.6.9.6 [re.results.form] the following:

   >     template <class OutputIter>
   >       OutputIter
   >       format(OutputIter out,
   >              const char_type* fmt_first, const char_type* fmt_last,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   > > 1 Requires: The type `OutputIter` shall satisfy the requirements for an Output Iterator (24.3.5.4 [output.iterators]).

   > > 2 Effects: Copies the character sequence `[fmt_first,fmt_last)` to `OutputIter out`. Replaces each format specifier or escape sequence in the copied range with either the character(s) it represents or the sequence of characters within `*this` to which it refers. The bitmasks specified in `flags` determine which format specifiers and escape sequences are recognized.

   > > 3 Returns: `out`.

4. Change 28.6.9.6 [re.results.form], before p. 1 until p. 3 as indicated:

   >     template <class OutputIter, class ST, class SA>
   >       OutputIter
   >       format(OutputIter out,
   >              const string_typebasic_string<char_type, ST, SA>& fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   > > 1 Requires: The type `OutputIter` shall satisfy the requirements for an Output Iterator (24.2.3).

   > > 2 Effects: Copies the character sequence `[fmt.begin(),fmt.end())` to `OutputIter out`. Replaces each format specifier or escape sequence in `fmt` with either the character(s) it represents or the sequence of characters within `*this` to which it refers. The bitmasks specified in `flags` determines what format specifiers and escape sequences are recognized Equivalent to `return format(out, fmt.data(), fmt.data() + fmt.size(), flags)`.

   > > 3 Returns: `out`.

5. Change 28.6.9.6 [re.results.form], before p. 4 until p. 4 as indicated:

   >     template <class ST, class SA>
   >       string_typebasic_string<char_type, ST, SA>
   >       format(const string_typebasic_string<char_type, ST, SA>& fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   > > Effects: Returns a copy of the string `fmt`. Replaces each format specifier or escape sequence in `fmt` with either the character(s) it represents or the sequence of characters within `*this` to which it refers. The bitmasks specified in flags determines what format specifiers and escape sequences are recognized. Constructs an empty string `result` of type `basic_string<char_type, ST, SA>`, and calls `format(back_inserter(result), fmt, flags)`.

   > > Returns: `result`

6. At the end of 28.6.9.6 [re.results.form] insert as indicated:

   >     string_type
   >       format(const char_type* fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   > > Effects: Constructs an empty string `result` of type `string_type`, and calls `format(back_inserter(result), fmt, fmt + char_traits<char_type>::length(fmt), flags)`.

   > > Returns: `result`

7. Change 28.6.10.4 [re.alg.replace] before p. 1 as indicated:

   >     template <class OutputIterator, class BidirectionalIterator,
   >               class traits, class charT, class ST, class SA>
   >       OutputIterator
   >       regex_replace(OutputIterator out,
   >                     BidirectionalIterator first, BidirectionalIterator last,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, ST, SA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class OutputIterator, class BidirectionalIterator,
   >               class traits, class charT>
   >       OutputIterator
   >       regex_replace(OutputIterator out,
   >                     BidirectionalIterator first, BidirectionalIterator last,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   > > Effects: [..]. If any matches are found then, for each such match, if `!(flags & regex_constants::format_no_copy)` calls `std::copy(m.prefix().first, m.prefix().second, out)`, and then calls `m.format(out, fmt, flags)` for the first form of the function and `m.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags)` for the second form. [..].

8. Change 28.6.10.4 [re.alg.replace] before p. 3 as indicated:

   >     template <class traits, class charT, class ST, class SA,
   >               class FST, class FSA>
   >       basic_string<charT, ST, SA>
   >       regex_replace(const basic_string<charT, ST, SA>& s,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, FST, FSA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT, class ST, class SA>
   >       basic_string<charT, ST, SA>
   >       regex_replace(const basic_string<charT, ST, SA>& s,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   > > Effects: Constructs an empty string `result` of type `basic_string<charT, ST, SA>`, calls `regex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags)`, and then returns `result`.

9. At the end of 28.6.10.4 [re.alg.replace] add the following new prototype description:

   >     template <class traits, class charT, class ST, class SA>
   >       basic_string<charT>
   >       regex_replace(const charT* s,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, ST, SA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT>
   >       basic_string<charT>
   >       regex_replace(const charT* s,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   > > Effects: Constructs an empty string `result` of type `basic_string<charT>`, calls `regex_replace(back_inserter(result), s, s + char_traits<charT>::length(s), e, fmt, flags)`, and then returns `result`.

### 728(i). Problem in [rand.eng.mers]/6

Section: 29.5.4.3 [rand.eng.mers] Status: CD1 Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.eng.mers].

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Discussion:

The `mersenne_twister_engine` is required to use a seeding method that is given as an algorithm parameterized over the number of bits `W`. I doubt whether the given generalization of an algorithm that was originally developed only for unsigned 32-bit integers is appropriate for other bit widths. For instance, `W` could be theoretically 16 and `UIntType` a 16-bit integer, in which case the given multiplier would not fit into the `UIntType`. Moreover, T. Nishimura and M. Matsumoto have chosen a dif ferent multiplier for their 64 bit Mersenne Twister [reference].

I see two possible resolutions:

1. Restrict the parameter `W` of the `mersenne_twister_template` to values of 32 or 64 and use the multiplier from [the above reference] for the 64-bit case (my preference)
2. Interpret the state array for any `W` as a 32-bit array of appropriate length (and a specified byte order) and always employ the 32-bit algorithm for seeding

See N2424 for further discussion.

[ Bellevue: ]

> Stephan Tolksdorf has additional comments on N2424. He comments: "there is a typo in the required behaviour for mt19937_64: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025)." These values need checking.

> Take the proposed recommendation in N2424 and move to REVIEW.

Proposed resolution:

See N2424 for the proposed resolution.

[ Stephan Tolksdorf adds pre-Bellevue: ]

> I support the proposed resolution in N2424, but there is a typo in the required behaviour for `mt19937_64`: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025). The change to para. 8 proposed by Charles Karney should also be included in the proposed wording.

[ Sophia Antipolis: ]

> Note the main part of the issue is resolved by N2424.

### 732(i). Defect in [rand.dist.samp.genpdf]

Section: 99 [rand.dist.samp.genpdf] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.dist.samp.genpdf].

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Duplicate of: 795

Discussion:

99 [rand.dist.samp.genpdf] describes the interface for a distribution template that is meant to simulate random numbers from any general distribution given only the density and the support of the distribution. I'm not aware of any general purpose algorithm that would be capable of correctly and efficiently implementing the described functionality. From what I know, this is essentially an unsolved research problem. Existing algorithms either require more knowledge about the distribution and the problem domain or work only under very limited circumstances. Even the state of the art special purpose library UNU.RAN does not solve the problem in full generality, and in any case, testing and customer support for such a library feature would be a nightmare.

Possible resolution: For these reasons, I propose to delete section 99 [rand.dist.samp.genpdf].

[ Bellevue: ]

> Disagreement persists.

> Objection to this issue is that this function takes a general functor. The general approach would be to normalize this function, integrate it, and take the inverse of the integral, which is not possible in general. An example function is sin(1+n*x) — for any spatial frequency that the implementor chooses, there is a value of n that renders that choice arbitrarily erroneous.

> Correction: The formula above should instead read 1+sin(n*x).

> Objector proposes the following possible compromise positions:

> - rand.dist.samp.genpdf takes an number of points so that implementor need not guess.
> - replace rand.disk.samp.genpdf with an extension to either or both of the discrete functions to take arguments that take a functor and number of points in place of the list of probabilities. Reference issues 793 and 794.

Proposed resolution:

See N2813 for the proposed resolution.

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 734(i). Unnecessary restriction in [rand.dist.norm.chisq]

Section: 29.5.9.5.3 [rand.dist.norm.chisq] Status: CD1 Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

`chi_squared_distribution`, `fisher_f_distribution` and `student_t_distribution` have parameters for the "degrees of freedom" `n` and `m` that are specified as integers. For the following two reasons this is an unnecessary restriction: First, in many applications such as Bayesian inference or Monte Carlo simulations it is more convenient to treat the respective param- eters as continuous variables. Second, the standard non-naive algorithms (i.e. O(1) algorithms) for simulating from these distributions work with floating-point parameters anyway (all three distributions could be easily implemented using the Gamma distribution, for instance).

Similar arguments could in principle be made for the parameters `t` and `k` of the discrete `binomial_distribution` and `negative_binomial_distribution`, though in both cases continuous parameters are less frequently used in practice and in case of the `binomial_distribution` the implementation would be significantly complicated by a non-discrete parameter (in most implementations one would need an approximation of the log-gamma function instead of just the log-factorial function).

Possible resolution: For these reasons, I propose to change the type of the respective parameters to double.

[ Bellevue: ]

> In N2424. Not wildly enthusiastic, not really felt necessary. Less frequently used in practice. Not terribly bad either. Move to OPEN.

[ Sophia Antipolis: ]

> Marc Paterno: The generalizations were explicitly left out when designing the facility. It's harder to test.

> Marc Paterno: Ask implementers whether floating-point is a significant burden.

> Alisdair: It's neater to do it now, do ask Bill Plauger.

> Disposition: move to review with the option for "NAD" if it's not straightforward to implement; unanimous consent.

Proposed resolution:

See N2424 for the proposed resolution.

[ Stephan Tolksdorf adds pre-Bellevue: ]

> In 29.5.9.5.3 [rand.dist.norm.chisq]:

> > Delete ", where `n` is a positive integer" in the first paragraph.

> > Replace both occurrences of "`explicit chi_squared_distribution(int n = 1);`" with "`explicit chi_squared_distribution(RealType n = 1);`".

> > Replace both occurrences of "`int n() const;`" with "`RealType n() const;`".

> In 29.5.9.5.5 [rand.dist.norm.f]:

> > Delete ", where `m` and `n` are positive integers" in the first paragraph.

> > Replace both occurrences of

> > >     explicit fisher_f_distribution(int m = 1, int n = 1);

> > with

> > >     explicit fisher_f_distribution(RealType m = 1, RealType n = 1);

> > Replace both occurrences of "`int m() const;" with "RealType m() const;`".

> > Replace both occurrences of "`int n() const;" with "RealType n() const;`".

> In 29.5.9.5.6 [rand.dist.norm.t]:

> > Delete ", where `n` is a positive integer" in the first paragraph.

> > Replace both occurrences of "`explicit student_t_distribution(int n = 1);`" with "`explicit student_t_distribution(RealType n = 1);`".

> > Replace both occurrences of "`int n() const;`" with "`RealType n() const;`".

### 740(i). Please remove `*_ptr<T[N]>`

Section: 20.3.1 [unique.ptr] Status: CD1 Submitter: Herb Sutter Opened: 2007-10-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Please don't provide `*_ptr<T[N]>`. It doesn't enable any useful bounds-checking (e.g., you could imagine that doing `op++` on a `shared_ptr<T[N]>` yields a `shared_ptr<T[N-1]>`, but that promising path immediately falters on `op--` which can't reliably dereference because we don't know the lower bound). Also, most buffers you'd want to point to don't have a compile-time known size.

To enable any bounds-checking would require run-time information, with the usual triplet: base (lower bound), current offset, and max offset (upper bound). And I can sympathize with the point of view that you wouldn't want to require this on `*_ptr` itself. But please let's not follow the `<T[N]>` path, especially not with additional functions to query the bounds etc., because this sets wrong user expectations by embarking on a path that doesn't go all the way to bounds checking as it seems to imply.

If bounds checking is desired, consider a `checked_*_ptr` instead (e.g., `checked_shared_ptr`). And make the interfaces otherwise identical so that user code could easily `#define/typedef` between prepending `checked_` on debug builds and not doing so on release builds (for example).

Note that some may object that `checked_*_ptr` may seem to make the smart pointer more like `vector`, and we don't want two ways to spell `vector`. I don't agree, but if that were true that would be another reason to remove `*_ptr<T[N]>` which equally makes the smart pointer more like `std::array.` :-)

[ Bellevue: ]

> Suggestion that fixed-size array instantiations are going to fail at compile time anyway (if we remove specialization) due to pointer decay, at least that appears to be result from available compilers.

> So concerns about about requiring static_assert seem unfounded.

> After a little more experimentation with compiler, it appears that fixed size arrays would only work at all if we supply these explicit specialization. So removing them appears less breaking than originally thought.

> straw poll unanimous move to Ready.

Proposed resolution:

Change the synopsis under 20.3.1 [unique.ptr] p2:

>     ...
>     template<class T> struct default_delete;
>     template<class T> struct default_delete<T[]>;
>     template<class T, size_t N> struct default_delete<T[N]>;

>     template<class T, class D = default_delete<T>> class unique_ptr;
>     template<class T, class D> class unique_ptr<T[], D>;
>     template<class T, class D, size_t N> class unique_ptr<T[N], D>;
>     ...

Remove the entire section [unique.ptr.dltr.dflt2] `default_delete<T[N]>`.

Remove the entire section [unique.ptr.compiletime] `unique_ptr` for array objects with a compile time length and its subsections: [unique.ptr.compiletime.dtor], [unique.ptr.compiletime.observers], [unique.ptr.compiletime.modifiers].

### 742(i). Enabling `swap` for proxy iterators

Section: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This issue was split from 672(i). 672(i) now just deals with changing the requirements of `T` in the `Swappable` requirement from `CopyConstructible` and `CopyAssignable` to `MoveConstructible` and `MoveAssignable`.

This issue seeks to widen the `Swappable` requirement to support proxy iterators. Here is example code:

>     namespace Mine {

>     template <class T>
>     struct proxy {...};

>     template <class T>
>     struct proxied_iterator
>     {
>        typedef T value_type;
>        typedef proxy<T> reference;
>        reference operator*() const;
>        ...
>     };

>     struct A
>     {
>        // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable
>        void swap(A&);
>        ...
>     };

>     void swap(A&, A&);
>     void swap(proxy<A>, A&);
>     void swap(A&, proxy<A>);
>     void swap(proxy<A>, proxy<A>);

>     }  // Mine

>     ...

>     Mine::proxied_iterator<Mine::A> i(...)
>     Mine::A a;
>     swap(*i1, a);

The key point to note in the above code is that in the call to `swap`, `*i1` and `a` are different types (currently types can only be `Swappable` with the same type). A secondary point is that to support proxies, one must be able to pass rvalues to `swap`. But note that I am not stating that the general purpose `std::swap` should accept rvalues! Only that overloaded `swap`s, as in the example above, be allowed to take rvalues.

That is, no standard library code needs to change. We simply need to have a more flexible definition of `Swappable`.

[ Bellevue: ]

> While we believe Concepts work will define a swappable concept, we should still resolve this issue if possible to give guidance to the Concepts work.

> Would an ambiguous swap function in two namespaces found by ADL break this wording? Suggest that the phrase "valid expression" means such a pair of types would still not be swappable.

> Motivation is proxy-iterators, but facility is considerably more general. Are we happy going so far?

> We think this wording is probably correct and probably an improvement on what's there in the WP. On the other hand, what's already there in the WP is awfully complicated. Why do we need the two bullet points? They're too implementation-centric. They don't add anything to the semantics of what swap() means, which is there in the post-condition. What's wrong with saying that types are swappable if you can call swap() and it satisfies the semantics of swapping?

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Leave as Open. Dave to provide wording.

[ 2009-11-08 Howard adds: ]

> Updated wording to sync with N3000. Also this issue is very closely related to 594(i).

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added.

Rationale:

Solved by N3048.

Proposed resolution:

Change 16.4.4.2 [utility.arg.requirements]:

> -1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables, `T` and `V` are is a types to be supplied by a C++ program instantiating a template; `a`, `b`, and `c` are values of type `const T`; `s` and `t` are modifiable lvalues of type `T`; `u` is a value of type (possibly `const`) `T`; and `rv` is a non-`const` rvalue of type `T`; `w` is a value of type `T`; and `v` is a value of type `V`.

> Table 37: `Swappable` requirements [swappable]
> expression | Return type | Post-condition
> --- | --- | ---
> `swap(sw,tv)` | `void` | `t` `w` has the value originally held by `u` `v`, and `u` `v` has the value originally held by `t` `w`
> The `Swappable` requirement is met by satisfying one or more of the following conditions: `T` is `Swappable` if `T` and `V` are the same type and `T` satisfies the `MoveConstructible` requirements (Table 33) and the `MoveAssignable` requirements (Table 35); `T` is `Swappable` with `V` if a namespace scope function named `swap` exists in the same namespace as the definition of `T` or `V`, such that the expression `swap(sw,t v)` is valid and has the semantics described in this table. `T` is `Swappable` if `T` is an array type whose element type is `Swappable`.

Rationale:

[ post San Francisco: ]

> Solved by N2758.

### 743(i). rvalue `swap` for `shared_ptr`

Section: 20.3.2.2.9 [util.smartptr.shared.spec] Status: CD1 Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

When the LWG looked at 674(i) in Kona the following note was made:

> We may need to open an issue to deal with the question of whether `shared_ptr` needs an rvalue `swap`.

This issue was opened in response to that note.

I believe allowing rvalue `shared_ptr`s to `swap` is both appropriate, and consistent with how other library components are currently specified.

[ Bellevue: ]

> Concern that the three signatures for swap is needlessly complicated, but this issue merely brings shared_ptr into equal complexity with the rest of the library. Will open a new issue for concern about triplicate signatures.

> Adopt issue as written.

Proposed resolution:

Change the synopsis in 20.3.2.2 [util.smartptr.shared]:

>     void swap(shared_ptr&& r);
>     ...
>     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b);
>     template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b);
>     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);

Change 20.3.2.2.5 [util.smartptr.shared.mod]:

>     void swap(shared_ptr&& r);

Change 20.3.2.2.9 [util.smartptr.shared.spec]:

>     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b);
>     template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b);
>     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);

### 744(i). What is the lifetime of an exception pointed to by an `exception_ptr`?

Section: 17.9.7 [propagation] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Without some lifetime guarantee, it is hard to know how this type can be used. Very specifically, I don't see how the current wording would guarantee and `exception_ptr` caught at the end of one thread could be safely stored and rethrown in another thread - the original motivation for this API.

(Peter Dimov agreed it should be clearer, maybe a non-normative note to explain?)

[ Bellevue: ]

> Agree the issue is real.

> Intent is lifetime is similar to a shared_ptr (and we might even want to consider explicitly saying that it is a shared_ptr< unspecified type >).

> We expect that most implementations will use shared_ptr, and the standard should be clear that the exception_ptr type is intended to be something whose semantics are smart-pointer-like so that the user does not need to worry about lifetime management. We still need someone to draught those words - suggest emailing Peter Dimov.

> Move to Open.

Proposed resolution:

Change 17.9.7 [propagation]/7:

> -7- Returns: An `exception_ptr` object that refers to the currently handled exception or a copy of the currently handled exception, or a null `exception_ptr` object if no exception is being handled. The referenced object remains valid at least as long as there is an `exception_ptr` that refers to it. If the function needs to allocate memory and the attempt fails, it returns an `exception_ptr` object that refers to an instance of `bad_alloc`. It is unspecified whether the return values of two successive calls to `current_exception` refer to the same exception object. [Note: that is, it is unspecified whether `current_exception` creates a new copy each time it is called. --end note]

### 746(i). `current_exception` may fail with `bad_alloc`

Section: 17.9.7 [propagation] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I understand that the attempt to copy an exception may run out of memory, but I believe this is the only part of the standard that mandates failure with specifically `bad_alloc`, as opposed to allowing an implementation-defined type derived from `bad_alloc`. For instance, the Core language for a failed new expression is:

> Any other allocation function that fails to allocate storage shall indicate failure only by throwing an exception of a type that would match a handler (15.3) of type `std::bad_alloc` (18.5.2.1).

I think we should allow similar freedom here (or add a blanket compatible-exception freedom paragraph in 17)

I prefer the clause 17 approach myself, and maybe clean up any outstanding wording that could also rely on it.

Although filed against a specific case, this issue is a problem throughout the library.

[ Bellevue: ]

> Is issue bigger than library?

> No - Core are already very clear about their wording, which is inspiration for the issue.

> While not sold on the original 18.7.5 use case, the generalised 17.4.4.8 wording is the real issue.

> Accept the broad view and move to ready

Proposed resolution:

Add the following exemption clause to 16.4.6.14 [res.on.exception.handling]:

> A function may throw a type not listed in its Throws clause so long as it is derived from a class named in the Throws clause, and would be caught by an exception handler for the base type.

### 749(i). Currently `has_nothrow_copy_constructor<T>::value` is true if `T` has 'a' nothrow copy constructor.

Section: 21.3.5.4 [meta.unary.prop] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Unfortunately a class can have multiple copy constructors, and I believe to be useful this trait should only return true is ALL copy constructors are no-throw.

For instance:

>     struct awkward {
>      awkward( const awkward & ) throw() {}
>      awkward( awkward & ) { throw "oops"; } };

Proposed resolution:

Change 21.3.5.4 [meta.unary.prop]:

>     has_trivial_copy_constructor

> > `T` is a trivial type (3.9) or a reference type or a class type with a trivial copy constructor where all copy constructors are trivial (12.8).

>     has_trivial_assign

> > `T` is neither `const` nor a reference type, and `T` is a trivial type (3.9) or a class type with a trivial copy assignment operator where all copy assignment operators are trivial (12.8).

>     has_nothrow_copy_constructor

> > `has_trivial_copy_constructor<T>::value` is `true` or `T` is a class type with a where all copy constructors that is are known not to throw any exceptions or `T` is an array of such a class type

>     has_nothrow_assign

> > `T` is neither `const` nor a reference type, and `has_trivial_assign<T>::value` is `true` or `T` is a class type with a where all copy assignment operators takeing an lvalue of type `T` that is known not to throw any exceptions or `T` is an array of such a class type.

### 752(i). Allocator complexity requirement

Section: 16.4.4.6 [allocator.requirements] Status: C++11 Submitter: Hans Boehm Opened: 2007-10-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Did LWG recently discuss 16.4.4.6 [allocator.requirements]-2, which states that "All the operations on the allocators are expected to be amortized constant time."?

As I think I pointed out earlier, this is currently fiction for `allocate()` if it has to obtain memory from the OS, and it's unclear to me how to interpret this for `construct()` and `destroy()` if they deal with large objects. Would it be controversial to officially let these take time linear in the size of the object, as they already do in real life?

`Allocate()` more blatantly takes time proportional to the size of the object if you mix in GC. But it's not really a new problem, and I think we'd be confusing things by leaving the bogus requirements there. The current requirement on `allocate()` is generally not important anyway, since it takes O(size) to construct objects in the resulting space. There are real performance issues here, but they're all concerned with the constants, not the asymptotic complexity.

Proposed resolution:

Change 16.4.4.6 [allocator.requirements]/2:

> -2- Table 39 describes the requirements on types manipulated through allocators. All the operations on the allocators are expected to be amortized constant time. Table 40 describes the requirements on allocator types.

### 753(i). Move constructor in draft

Section: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Yechezkel Mett Opened: 2007-10-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The draft standard n2369 uses the term move constructor in a few places, but doesn't seem to define it.

`MoveConstructible` requirements are defined in Table 33 in 16.4.4.2 [utility.arg.requirements] as follows:

> `MoveConstructible` requirements
> expression | post-condition
> --- | ---
> `T t = rv` | `t` is equivalent to the value of `rv` before the construction
> [Note: There is no requirement on the value of `rv` after the construction. -- end note]

(where `rv` is a non-const rvalue of type `T`).

So I assume the move constructor is the constructor that would be used in filling the above requirement.

For `vector::reserve`, `vector::resize` and the `vector` modifiers given in 23.3.13.5 [vector.modifiers] we have

> Requires: If `value_type` has a move constructor, that constructor shall not throw any exceptions.

Firstly "If `value_type` has a move constructor" is superfluous; every type which can be put into a `vector` has a move constructor (a copy constructor is also a move constructor). Secondly it means that for any `value_type` which has a throwing copy constructor and no other move constructor these functions cannot be used -- which I think will come as a shock to people who have been using such types in `vector` until now!

I can see two ways to correct this. The simpler, which is presumably what was intended, is to say "If `value_type` has a move constructor and no copy constructor, the move constructor shall not throw any exceptions" or "If `value_type` has a move constructor which changes the value of its parameter,".

The other alternative is add to `MoveConstructible` the requirement that the expression does not throw. This would mean that not every type that satisfies the `CopyConstructible` requirements also satisfies the `MoveConstructible` requirements. It would mean changing requirements in various places in the draft to allow either `MoveConstructible` or `CopyConstructible`, but I think the result would be clearer and possibly more concise too.

Proposed resolution:

Add new defintions to [definitions]:

> move constructor

> a constructor which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the construction.

> move assignment operator

> an assignment operator which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the assignment.

> move assignment

> use of the move assignment operator.

[ Howard adds post-Bellevue: ]

> Unfortunately I believe the wording recommended by the LWG in Bellevue is incorrect. `reserve` et. al. will use a move constructor if one is available, else it will use a copy constructor. A type may have both. If the move constructor is used, it must not throw. If the copy constructor is used, it can throw. The sentence in the proposed wording is correct without the recommended insertion. The Bellevue LWG recommended moving this issue to Ready. I am unfortunately pulling it back to Open. But I'm drafting wording to atone for this egregious action. :-)

### 755(i). `std::vector` and `std:string` lack explicit shrink-to-fit operations

Section: 23.3.13.3 [vector.capacity], 27.4.3.5 [string.capacity] Status: CD1 Submitter: Beman Dawes Opened: 2007-10-31 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

A `std::vector` can be shrunk-to-fit via the swap idiom:

>     vector<int> v;
>     ...
>     v.swap(vector<int>(v));  // shrink to fit

> > or:

>     vector<int>(v).swap(v);  // shrink to fit

> > or:

>     swap(v, vector<int>(v));  // shrink to fit

A non-binding request for shrink-to-fit can be made to a `std::string` via:

>     string s;
>     ...
>     s.reserve(0);

Neither of these is at all obvious to beginners, and even some experienced C++ programmers are not aware that shrink-to-fit is trivially available.

Lack of explicit functions to perform these commonly requested operations makes vector and string less usable for non-experts. Because the idioms are somewhat obscure, code readability is impaired. It is also unfortunate that two similar vector-like containers use different syntax for the same operation.

The proposed resolution addresses these concerns. The proposed function takes no arguments to keep the solution simple and focused.

Proposed resolution:

To Class template basic_string 27.4.3 [basic.string] synopsis, Class template vector 23.3.13 [vector] synopsis, and Class vector<bool> 23.3.14 [vector.bool] synopsis, add:

>     void shrink_to_fit();

To basic_string capacity 27.4.3.5 [string.capacity] and vector capacity 23.3.13.3 [vector.capacity], add:

>     void shrink_to_fit();

> > Remarks: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]

[ 850(i) has been added to deal with this issue with respect to `deque`. ]

### 756(i). Container adaptors push

Section: 23.6 [container.adaptors] Status: Resolved Submitter: Paolo Carlini Opened: 2007-10-31 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.adaptors].

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Discussion:

After n2369 we have a single `push_back` overload in the sequence containers, of the "emplace" type. At variance with that, still in n2461, we have two separate overloads, the C++03 one + one taking an rvalue reference in the container adaptors. Therefore, simply from a consistency point of view, I was wondering whether the container adaptors should be aligned with the specifications of the sequence container themselves: thus have a single `push` along the lines:

>     template<typename... _Args>
>     void
>     push(_Args&&... __args)
>       { c.push_back(std::forward<_Args>(__args)...); }

[ Related to 767(i) ]

Proposed resolution:

Change 23.6.3.1 [queue.defn]:

>     void push(const value_type& x) { c.push_back(x); }
>     void push(value_type&& x) { c.push_back(std::move(x)); }
>     template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }

Change 23.6.4 [priority.queue]:

>     void push(const value_type& x) { c.push_back(x); }
>     void push(value_type&& x) { c.push_back(std::move(x)); }
>     template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }

Change 23.6.4.4 [priqueue.members]:

>     void push(const value_type& x);

> > Effects:

> > >     c.push_back(x);
> > >     push_heap(c.begin(), c.end(), comp);

>     template<class... Args> void push(value_type Args&&... x args);

> > Effects:

> > >     c.push_back(std::moveforward<Args>(x args)...);
> > >     push_heap(c.begin(), c.end(), comp);

Change 23.6.6.2 [stack.defn]:

>     void push(const value_type& x) { c.push_back(x); }
>     void push(value_type&& x) { c.push_back(std::move(x)); }
>     template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }

Rationale:

Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).

### 758(i). `shared_ptr` and `nullptr`

Section: 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Joe Gottman Opened: 2007-10-31 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.smartptr.shared].

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Discussion:

Consider the following program:

>     int main() {
>        shared_ptr<int> p(nullptr);
>        return 0;
>     }

This program will fail to compile because `shared_ptr` uses the following template constructor to construct itself from pointers:

>     template <class Y> shared_ptr(Y *);

According to N2431, the conversion from `nullptr_t` to `Y *` is not deducible, so the above constructor will not be found. There are similar problems with the constructors that take a pointer and a `deleter` or a pointer, a `deleter` and an allocator, as well as the corresponding forms of `reset()`. Note that N2435 will solve this problem for constructing from just `nullptr`, but not for constructors that use `deleters` or allocators or for `reset()`.

In the case of the functions that take deleters, there is the additional question of what argument should be passed to the deleter when it is eventually called. There are two reasonable possibilities: `nullptr` or `static_cast<T *>(0)`, where `T` is the template argument of the `shared_ptr`. It is not immediately clear which of these is better. If `D::operator()` is a template function similar to `shared_ptr`'s constructor, then `d(static_cast<T*>(0))` will compile and `d(nullptr)` will not. On the other hand, if `D::operator()()` takes a parameter that is a pointer to some type other that `T` (for instance `U*` where `U` derives from `T`) then `d(nullptr)` will compile and `d(static_cast<T *>(0))` may not.

[ Bellevue: ]

> The general idea is right, we need to be able to pass a nullptr to a shared_ptr, but there are a few borderline editorial issues here. (For example, the single-argument nullptr_t constructor in the class synopsis isn't marked explicit, but it is marked explicit in the proposed wording for 20.6.6.2.1. There is a missing empty parenthesis in the form that takes a nullptr_t, a deleter, and an allocator.)

> More seriously: this issue says that a shared_ptr constructed from a nullptr is empty. Since "empty" is undefined, it's hard to know whether that's right. This issue is pending on handling that term better.

> Peter suggests definition of empty should be "does not own anything"

> Is there an editorial issue that post-conditions should refer to get() = nullptr, rather than get() = 0?

> No strong feeling towards accept or NAD, but prefer to make a decision than leave it open.

> Seems there are no technical merits between NAD and Ready, comes down to "Do we intentially want to allow/disallow null pointers with these functions". Staw Poll - support null pointers 5 - No null pointers 0

> Move to Ready, modulo editorial comments

[ post Bellevue Peter adds: ]

> The following wording changes are less intrusive:

> In 20.3.2.2.2 [util.smartptr.shared.const], add:

> >     shared_ptr(nullptr_t);

> after:

> >     shared_ptr();

> (Absence of explicit intentional.)

> `px.reset( nullptr )` seems a somewhat contrived way to write `px.reset()`, so I'm not convinced of its utility.

> It's similarly not clear to me whether the deleter constructors need to be extended to take `nullptr`, but if they need to:

> Add

> >     template<class D> shared_ptr(nullptr_t p, D d);
> >     template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

> after

> >     template<class Y, class D> shared_ptr(Y* p, D d);
> >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);

> Note that this changes the semantics of the new constructors such that they consistently call `d(p)` instead of `d((T*)0)` when `p` is `nullptr`.

> The ability to be able to pass `0/NULL` to a function that takes a `shared_ptr` has repeatedly been requested by users, but the other additions that the proposed resolution makes are not supported by real world demand or motivating examples.

> It might be useful to split the obvious and non-controversial `nullptr_t` constructor into a separate issue. Waiting for "empty" to be clarified is unnecessary; this is effectively an alias for the default constructor.

[ Sophia Antipolis: ]

> We want to remove the reset functions from the proposed resolution.

> The remaining proposed resolution text (addressing the constructors) are wanted.

> Disposition: move to review. The review should check the wording in the then-current working draft.

Proposed resolution:

In 20.3.2.2 [util.smartptr.shared] p4, add to the definition/synopsis of `shared_ptr`:

>     template<class D> shared_ptr(nullptr_t p, D d);
>     template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

after

>     template<class Y, class D> shared_ptr(Y* p, D d);
>     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);

In 20.3.2.2.2 [util.smartptr.shared.const] add:

>     template<class D> shared_ptr(nullptr_t p, D d);
>     template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

after

>     template<class Y, class D> shared_ptr(Y* p, D d);
>     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);

(reusing the following paragraphs 20.3.2.2.2 [util.smartptr.shared.const]/9-13 that speak of p.)

In 20.3.2.2.2 [util.smartptr.shared.const]/10, change

> Effects: Constructs a `shared_ptr` object that owns the pointer object `p` and the deleter `d`. The second constructor shall use a copy of `a` to allocate memory for internal use.

Rationale:

[ San Francisco: ]

> "pointer" is changed to "object" to handle the fact that `nullptr_t` isn't a pointer.

### 759(i). A reference is not an object

Section: 23.2 [container.requirements] Status: CD1 Submitter: Jens Maurer Opened: 2007-11-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

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Discussion:

23.2 [container.requirements] says:

> -12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.

A reference is not an object, but this sentence appears to claim so.

What is probably meant here:

> An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.

Proposed resolution:

Change 23.2 [container.requirements]:

> -12- Objects passed to member functions of a container as rvalue references shall not be elements An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container.; Nno diagnostic required.

### 761(i). `unordered_map` needs an `at()` member function

Section: 23.5.3.3 [unord.map.elem] Status: CD1 Submitter: Joe Gottman Opened: 2007-11-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The new member function `at()` was recently added to `std::map()`. It acts like `operator[]()`, except it throws an exception when the input key is not found. It is useful when the `map` is `const`, the `value_type` of the key doesn't have a default constructor, it is an error if the key is not found, or the user wants to avoid accidentally adding an element to the map. For exactly these same reasons, `at()` would be equally useful in `std::unordered_map`.

Proposed resolution:

Add the following functions to the definition of `unordered_map` under "lookup" (23.5.3 [unord.map]):

>     mapped_type& at(const key_type& k);
>     const mapped_type &at(const key_type &k) const;

Add the following definitions to 23.5.3.3 [unord.map.elem]:

>     mapped_type& at(const key_type& k);
>     const mapped_type &at(const key_type &k) const;

> > Returns: A reference to `x.second`, where `x` is the (unique) element whose key is equivalent to `k`.

> > Throws: An exception object of type `out_of_range` if no such element is present.

[ Bellevue: Editorial note: the "(unique)" differs from map. ]

### 762(i). `std::unique_ptr` requires complete type?

Section: 20.3.1 [unique.ptr] Status: CD1 Submitter: Daniel Krügler Opened: 2007-11-30 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr].

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Discussion:

In contrast to the proposed `std::shared_ptr`, `std::unique_ptr` does currently not support incomplete types, because it gives no explicit grant - thus instantiating `unique_ptr` with an incomplete pointee type `T` automatically belongs to undefined behaviour according to 16.4.5.8 [res.on.functions]/2, last bullet. This is an unnecessary restriction and prevents many well-established patterns - like the bridge pattern - for `std::unique_ptr`.

[ Bellevue: ]

> Move to open. The LWG is comfortable with the intent of allowing incomplete types and making `unique_ptr` more like `shared_ptr`, but we are not comfortable with the wording. The specification for `unique_ptr` should be more like that of `shared_ptr`. We need to know, for individual member functions, which ones require their types to be complete. The `shared_ptr` specification is careful to say that for each function, and we need the same level of care here. We also aren't comfortable with the "part of the operational semantic" language; it's not used elsewhere in the standard, and it's not clear what it means. We need a volunteer to produce new wording.

Proposed resolution:

The proposed changes in the following revision refers to the current state of N2521 including the assumption that [unique.ptr.compiletime] will be removed according to the current state of 740(i).

The specialization `unique_ptr<T[]>` has some more restrictive constraints on type-completeness on `T` than `unique_ptr<T>`. The following proposed wordings try to cope with that. If the committee sees less usefulness on relaxed constraints on `unique_ptr<T[]>`, the alternative would be to stop this relaxation e.g. by adding one further bullet to 20.3.1.4 [unique.ptr.runtime]/1: "`T` shall be a complete type, if used as template argument of `unique_ptr<T[], D>`

This issue has some overlap with 673(i), but it seems not to cause any problems with this one, because 673(i) adds only optional requirements on `D` that do not conflict with the here discussed ones, provided that `D::pointer`'s operations (including default construction, copy construction/assignment, and pointer conversion) are specified not to throw, otherwise this would have impact on the current specification of `unique_ptr`.

1. In 20.3.1 [unique.ptr]/2 add as the last sentence to the existing para:

   > The `unique_ptr` provides a semantics of strict ownership. A `unique_ptr` owns the object it holds a pointer to. A `unique_ptr` is not `CopyConstructible`, nor `CopyAssignable`, however it is `MoveConstructible` and `MoveAssignable`. The template parameter `T` of `unique_ptr` may be an incomplete type. [ Note: The uses of `unique_ptr` include providing exception safety for dynamically allcoated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function. -- end note ]

2. 20.3.1.3.2 [unique.ptr.single.ctor]/1: No changes necessary.

   > [ N.B.: We only need the requirement that `D` is `DefaultConstructible`. The current wording says just this. ]

3. In 20.3.1.3.2 [unique.ptr.single.ctor]/5 change the requires clause to say:

   > Requires: The expression `D()(p)` shall be well formed. The default constructor of `D` shall not throw an exception. `D` must not be a reference type. `D` shall be default constructible, and that construction shall not throw an exception.

   > [ N.B.: There is no need that the expression `D()(p)` is well-formed at this point. I assume that the current wording is based on the corresponding `shared_ptr` wording. In case of `shared_ptr` this requirement is necessary, because the corresponding c'tor *can* fail and must invoke delete `p/d(p)` in this case. `Unique_ptr` is simpler in this regard. The *only* functions that must insist on well-formedness and well-definedness of the expression `get_deleter()(get())` are (1) the destructor and (2) `reset`. The reasoning for the wording change to explicitly require `DefaultConstructible` of `D` is to guarantee that invocation of `D`'s default c'tor is both well-formed and well-defined. Note also that we do *not* need the requirement that `T` must be complete, also in contrast to `shared_ptr`. `Shared_ptr` needs this, because it's c'tor is a template c'tor which potentially requires `Convertible<Y*, X*>`, which again requires Completeness of `Y`, if `!SameType<X, Y>` ]

4. Merge 20.3.1.3.2 [unique.ptr.single.ctor]/12+13 thereby removing the sentence of 12, but transferring the "requires" to 13:

   > Requires: If `D` is not an lvalue-reference type then[..]

   > [ N.B.: For the same reasons as for (3), there is no need that `d(p)` is well-formed/well-defined at this point. The current wording guarantees all what we need, namely that the initialization of both the `T*` pointer and the `D` deleter are well-formed and well-defined. ]

5. 20.3.1.3.2 [unique.ptr.single.ctor]/17: No changes necessary.

6. 20.3.1.3.2 [unique.ptr.single.ctor]/21:

   > Requires: If `D` is not a reference type, construction of the deleter `D` from an rvalue of type `E` shall be well formed and shall not throw an exception. If `D` is a reference type, then `E` shall be the same type as `D` (diagnostic required). `U*` shall be implicitly convertible to `T*`. [Note: These requirements imply that `T` and `U` be complete types. -- end note]

   [ N.B.: The current wording of 21 already implicitly guarantees that `U` is completely defined, because it requires that `Convertible<U*, T*>` is true. If the committee wishes this explicit requirement can be added, e.g. "`U` shall be a complete type." ]

7. 20.3.1.3.3 [unique.ptr.single.dtor]: Just before p1 add a new paragraph:

   > Requires: The expression `get_deleter()(get())` shall be well-formed, shall have well-defined behavior, and shall not throw exceptions. [Note: The use of `default_delete` requires `T` to be a complete type. -- end note]

   > [ N.B.: This requirement ensures that the whole responsibility on type-completeness of `T` is delegated to this expression. ]

8. 20.3.1.3.4 [unique.ptr.single.asgn]/1: No changes necessary, except the current editorial issue, that "must shall" has to be changed to "shall", but this change is not a special part of this resolution.

   [ N.B. The current wording is sufficient, because we can delegate all further requirements on the requirements of the effects clause ]

9. 20.3.1.3.4 [unique.ptr.single.asgn]/6:

   > Requires: Assignment of the deleter `D` from an rvalue `D` shall not throw an exception. `U*` shall be implicitly convertible to `T*`. [Note: These requirements imply that `T` and `U` be complete types. -- end note]

   [ N.B.: The current wording of p. 6 already implicitly guarantees that `U` is completely defined, because it requires that `Convertible<U*, T*>` is true, see (6)+(8). ]

10. 20.3.1.3.4 [unique.ptr.single.asgn]/11: No changes necessary.

    [ N.B.: Delegation to requirements of effects clause is sufficient. ]

11. 20.3.1.3.5 [unique.ptr.single.observers]/1+4+7+9+11:

    >     T* operator->() const;

    > > Note: Use typically requires `T` shall be complete. — end note]

12. 20.3.1.3.6 [unique.ptr.single.modifiers]/1: No changes necessary.

13. 20.3.1.3.6 [unique.ptr.single.modifiers]/4: Just before p. 4 add a new paragraph:

    > Requires: The expression `get_deleter()(get())` shall be well-formed, shall have well-defined behavior, and shall not throw exceptions.

14. 20.3.1.3.6 [unique.ptr.single.modifiers]/7: No changes necessary.

15. 20.3.1.4 [unique.ptr.runtime]: Add one additional bullet on paragraph 1:

    > A specialization for array types is provided with a slightly altered interface.

    > - ...
    > - `T` shall be a complete type.

[ post Bellevue: Daniel provided revised wording. ]

### 765(i). More on iterator validity

Section: 24.3.4 [iterator.concepts] Status: C++11 Submitter: Martin Sebor Opened: 2007-12-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iterator.concepts].

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Discussion:

Issue 278(i) defines the meaning of the term "invalid iterator" as one that may be singular.

Consider the following code:

       std::deque<int> x, y;
       std::deque<int>::iterator i = x.end(), j = y.end();
       x.swap(y);

Given that `swap()` is required not to invalidate iterators and using the definition above, what should be the expected result of comparing `i` and `j` to `x.end()` and `y.end()`, respectively, after the `swap()`?

I.e., is the expression below required to evaluate to `true`?

       i == y.end() && j == x.end()

(There are at least two implementations where the expression returns `false`.)

More generally, is the definition introduced in issue 278(i) meant to make any guarantees about whether iterators actually point to the same elements or be associated with the same containers after a non-invalidating operation as they did before?

Here's a motivating example intended to demonstrate the importance of the question:

       Container x, y ({ 1, 2});   // pseudocode to initialize y with { 1, 2 }
       Container::iterator i = y.begin() + 1;
       Container::iterator j = y.end();
       std::swap(x, y);
       std::find(i, j, 3);

`swap()` guarantees that `i` and `j` continue to be valid. Unless the spec says that even though they are valid they may no longer denote a valid range the code above must be well-defined. Expert opinions on this differ as does the behavior of popular implementations for some standard `Containers`.

[ San Francisco: ]

> Pablo: add a note to the last bullet of paragraph 11 of 23.1.1 clarifying that the `end()` iterator doesn't refer to an element and that it can therefore be invalidated.

> Proposed wording:

> > [Note: The `end()` iterator does not refer to any element and can therefore be invalidated. -- end note]

> Howard will add this proposed wording to the issue and then move it to Review.

[ Post Summit: ]

> Lawrence: suggestion: "Note: The `end()` iterator does not refer to any element"

> Walter: "Note: The `end()` iterator can nevertheless be invalidated, because it does not refer to any element."

> Nick: "The `end()` iterator does not refer to any element. It is therefore subject to being invalidated."

> Consensus: go with Nick

> With that update, Recommend Tentatively Ready.

Proposed resolution:

Add to 23.2.2 [container.requirements.general], p11:

> Unless otherwise specified (see 23.1.4.1, 23.1.5.1, 23.2.2.3, and 23.2.6.4) all container types defined in this Clause meet the following additional requirements:

> - ...
> - no `swap()` function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: The `end()` iterator does not refer to any element. It is therefore subject to being invalidated. — end note]

### 766(i). Inconsistent exception guarantees between ordered and unordered associative containers

Section: 23.2 [container.requirements], 23.2.8.2 [unord.req.except] Status: CD1 Submitter: Ion Gaztañaga Opened: 2007-12-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

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Discussion:

23.2 [container.requirements]p10 states:

> Unless otherwise specified (see 23.2.2.3 and 23.2.5.4) all container types defined in this clause meet the following additional requirements:

> - [...]
> - no `erase()`, `pop_back()` or `pop_front()` function throws an exception.

23.3.5.4 [deque.modifiers] and 23.3.13.5 [vector.modifiers] offer additional guarantees for `deque`/`vector insert()` and `erase()` members. However, 23.2 [container.requirements] p10 does not mention 23.2.8.2 [unord.req.except] that specifies exception safety guarantees for unordered containers. In addition, 23.2.8.2 [unord.req.except] p1 offers the following guaratee for `erase()`:

> No `erase()` function throws an exception unless that exception is thrown by the container's Hash or Pred object (if any).

Summary:

According to 23.2 [container.requirements] p10 no `erase()` function should throw an exception unless otherwise specified. Although does not explicitly mention 23.2.8.2 [unord.req.except], this section offers additional guarantees for unordered containers, allowing `erase()` to throw if predicate or hash function throws.

In contrast, associative containers have no exception safety guarantees section so no `erase()` function should throw, including `erase(k)` that needs to use the predicate function to perform its work. This means that the predicate of an associative container is not allowed to throw.

So:

1. `erase(k)` for associative containers is not allowed to throw. On the other hand, `erase(k)` for unordered associative containers is allowed to throw.
2. `erase(q)` for associative containers is not allowed to throw. On the other hand, `erase(q)` for unordered associative containers is allowed to throw if it uses the hash or predicate.
3. To fulfill 1), predicates of associative containers are not allowed to throw. Predicates of unordered associative containers are allowed to throw.
4. 2) breaks a widely used programming pattern (flyweight pattern) for unordered containers, where objects are registered in a global map in their constructors and unregistered in their destructors. If `erase(q)` is allowed to throw, the destructor of the object would need to rethrow the exception or swallow it, leaving the object registered.

Proposed resolution:

Create a new sub-section of 23.2.7 [associative.reqmts] (perhaps [associative.req.except]) titled "Exception safety guarantees".

> 1 For associative containers, no `clear()` function throws an exception. `erase(k)` does not throw an exception unless that exception is thrown by the container's Pred object (if any).

> 2 For associative containers, if an exception is thrown by any operation from within an `insert()` function inserting a single element, the `insert()` function has no effect.

> 3 For associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Pred object (if any).

Change 23.2.8.2 [unord.req.except]p1:

> For unordered associative containers, no `clear()` function throws an exception. No `erase(k)` function does not throws an exception unless that exception is thrown by the container's Hash or Pred object (if any).

Change 23.2 [container.requirements] p10 to add references to new sections:

> Unless otherwise specified (see [deque.modifiers], and [vector.modifiers], [associative.req.except], [unord.req.except]) all container types defined in this clause meet the following additional requirements:

Change 23.2 [container.requirements] p10 referring to `swap`:

> - no `swap()` function throws an exception unless that exception is thrown by the copy constructor or assignment operator of the container's Compare object (if any; see [associative.reqmts]).

### 767(i). Forwarding and backward compatibility

Section: 23 [containers] Status: Resolved Submitter: Sylvain Pion Opened: 2007-12-28 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [containers].

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Discussion:

Playing with g++'s C++0X mode, I noticed that the following code, which used to compile:

>     #include <vector>

>     int main()
>     {
>         std::vector<char *> v;
>         v.push_back(0);
>     }

now fails with the following error message:

> .../include/c++/4.3.0/ext/new_allocator.h: In member function 'void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, _Args&& ...) [with _Args = int, _Tp = char*]': .../include/c++/4.3.0/bits/stl_vector.h:707: instantiated from 'void std::vector<_Tp, _Alloc>::push_back(_Args&& ...) [with _Args = int, _Tp = char*, _Alloc = std::allocator<char*>]' test.cpp:6: instantiated from here .../include/c++/4.3.0/ext/new_allocator.h:114: error: invalid conversion from 'int' to 'char*'

As far as I know, g++ follows the current draft here.

Does the committee really intend to break compatibility for such cases?

[ Sylvain adds: ]

> I just noticed that `std::pair` has the same issue. The following now fails with GCC's -std=c++0x mode:

> >     #include <utility>

> >     int main()
> >     {
> >        std::pair<char *, char *> p (0,0);
> >     }

> I have not made any general audit for such problems elsewhere.

[ Related to 756(i) ]

[ Bellevue: ]

> Motivation is to handle the old-style int-zero-valued `NULL` pointers. Problem: this solution requires concepts in some cases, which some users will be slow to adopt. Some discussion of alternatives involving prohibiting variadic forms and additional library-implementation complexity.

> Discussion of "perfect world" solutions, the only such solution put forward being to retroactively prohibit use of the integer zero for a `NULL` pointer. This approach was deemed unacceptable given the large bodies of pre-existing code that do use integer zero for a `NULL` pointer.

> Another approach is to change the member names. Yet another approach is to forbid the extension in absence of concepts.

> Resolution: These issues (756(i), 767(i), 760(i), 763(i)) will be subsumed into a paper to be produced by Alan Talbot in time for review at the 2008 meeting in France. Once this paper is produced, these issues will be moved to NADResolved.

Proposed resolution:

Add the following rows to Table 90 "Optional sequence container operations", 23.2.4 [sequence.reqmts]:

> expression | return type | assertion/note pre-/post-condition | container
> --- | --- | --- | ---
> `a.push_front(t)` | `void` | `a.insert(a.begin(), t)` Requires: `T` shall be `CopyConstructible`. | `list, deque`
> `a.push_front(rv)` | `void` | `a.insert(a.begin(), rv)` Requires: `T` shall be `MoveConstructible`. | `list, deque`
> `a.push_back(t)` | `void` | `a.insert(a.end(), t)` Requires: `T` shall be `CopyConstructible`. | `list, deque, vector, basic_string`
> `a.push_back(rv)` | `void` | `a.insert(a.end(), rv)` Requires: `T` shall be `MoveConstructible`. | `list, deque, vector, basic_string`

Change the synopsis in 23.3.5 [deque]:

>     void push_front(const T& x);
>     void push_front(T&& x);
>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change 23.3.5.4 [deque.modifiers]:

>     void push_front(const T& x);
>     void push_front(T&& x);
>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change the synopsis in 23.3.11 [list]:

>     void push_front(const T& x);
>     void push_front(T&& x);
>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change 23.3.11.4 [list.modifiers]:

>     void push_front(const T& x);
>     void push_front(T&& x);
>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change the synopsis in 23.3.13 [vector]:

>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change 23.3.13.5 [vector.modifiers]:

>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Rationale:

Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).

If there is still an issue with pair, Howard should submit another issue.

### 768(i). Typos in [atomics]?

Section: 32.5.8 [atomics.types.generic] Status: CD1 Submitter: Alberto Ganesh Barbati Opened: 2007-12-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

in the latest publicly available draft, paper N2641, in section 32.5.8 [atomics.types.generic], the following specialization of the template `atomic<>` is provided for pointers:

>     template <class T> struct atomic<T*> : atomic_address {
>       T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
>       T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;

>       atomic() = default;
>       constexpr explicit atomic(T);
>       atomic(const atomic&) = delete;
>       atomic& operator=(const atomic&) = delete;

>       T* operator=(T*) volatile;
>       T* operator++(int) volatile;
>       T* operator--(int) volatile;
>       T* operator++() volatile;
>       T* operator--() volatile;
>       T* operator+=(ptrdiff_t) volatile;
>       T* operator-=(ptrdiff_t) volatile;
>     };

First of all, there is a typo in the non-default constructor which should take a `T*` rather than a `T`.

As you can see, the specialization redefine and therefore hide a few methods from the base class `atomic_address`, namely `fetch_add`, `fetch_sub`, `operator=`, `operator+=` and `operator-=`. That's good, but... what happened to the other methods, in particular these ones:

>     void store(T*, memory_order = memory_order_seq_cst) volatile;
>     T* load( memory_order = memory_order_seq_cst ) volatile;
>     T* swap( T*, memory_order = memory_order_seq_cst ) volatile;
>     bool compare_swap( T*&, T*, memory_order, memory_order ) volatile;
>     bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile;

By reading paper N2427 "C++ Atomic Types and Operations", I see that the definition of the specialization `atomic<T*>` matches the one in the draft, but in the example implementation the methods `load()`, `swap()` and `compare_swap()` are indeed present.

Strangely, the example implementation does not redefine the method `store()`. It's true that a `T*` is always convertible to `void*`, but not hiding the `void*` signature from the base class makes the class error-prone to say the least: it lets you assign pointers of any type to a `T*`, without any hint from the compiler.

Is there a true intent to remove them from the specialization or are they just missing from the definition because of a mistake?

[ Bellevue: ]

> The proposed revisions are accepted.

> Further discussion: why is the ctor labeled "constexpr"? Lawrence said this permits the object to be statically initialized, and that's important because otherwise there would be a race condition on initialization.

Proposed resolution:

Change the synopsis in 32.5.8 [atomics.types.generic]:

>     template <class T> struct atomic<T*> : atomic_address {
>       void store(T*, memory_order = memory_order_seq_cst) volatile;
>       T* load( memory_order = memory_order_seq_cst ) volatile;
>       T* swap( T*, memory_order = memory_order_seq_cst ) volatile;
>       bool compare_swap( T*&, T*, memory_order, memory_order ) volatile;
>       bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile;

>       T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
>       T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;

>       atomic() = default;
>       constexpr explicit atomic(T*);
>       atomic(const atomic&) = delete;
>       atomic& operator=(const atomic&) = delete;

>       T* operator=(T*) volatile;
>       T* operator++(int) volatile;
>       T* operator--(int) volatile;
>       T* operator++() volatile;
>       T* operator--() volatile;
>       T* operator+=(ptrdiff_t) volatile;
>       T* operator-=(ptrdiff_t) volatile;
>     };

### 769(i). std::function should use nullptr_t instead of "unspecified-null-pointer-type"

Section: 22.10.17.3 [func.wrap.func] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

N2461 already replaced in 22.10.17.3 [func.wrap.func] it's originally proposed (implicit) conversion operator to "unspecified-bool-type" by the new explicit bool conversion, but the inverse conversion should also use the new `std::nullptr_t` type instead of "unspecified-null-pointer- type".

Proposed resolution:

In 22.10 [function.objects], header `<functional>` synopsis replace:

>     template<class R, class... ArgTypes>
>       bool operator==(const function<R(ArgTypes...)>&, unspecified-null-pointer-type nullptr_t);
>     template<class R, class... ArgTypes>
>       bool operator==(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>&);
>     template<class R, class... ArgTypes>
>       bool operator!=(const function<R(ArgTypes...)>&, unspecified-null-pointer-type nullptr_t);
>     template<class R, class... ArgTypes>
>       bool operator!=(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>&);

In the class function synopsis of 22.10.17.3 [func.wrap.func] replace

>     function(unspecified-null-pointer-type nullptr_t);
>     ...
>     function& operator=(unspecified-null-pointer-type nullptr_t);

In 22.10.17.3 [func.wrap.func], "Null pointer comparisons" replace:

>     template <class R, class... ArgTypes>
>       bool operator==(const function<R(ArgTypes...)>&, unspecified-null-pointer-type nullptr_t);
>     template <class R, class... ArgTypes>
>       bool operator==(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>&);
>     template <class R, class... ArgTypes>
>       bool operator!=(const function<R(ArgTypes...)>&, unspecified-null-pointer-type nullptr_t);
>     template <class R, class... ArgTypes>
>       bool operator!=(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>&);

In 22.10.17.3.2 [func.wrap.func.con], replace

>     function(unspecified-null-pointer-type nullptr_t);
>     ...
>     function& operator=(unspecified-null-pointer-type nullptr_t);

In 22.10.17.3.7 [func.wrap.func.nullptr], replace

>     template <class R, class... ArgTypes>
>       bool operator==(const function<R(ArgTypes...)>& f, unspecified-null-pointer-type nullptr_t);
>     template <class R, class... ArgTypes>
>       bool operator==(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>& f);

and replace

>     template <class R, class... ArgTypes>
>       bool operator!=(const function<R(ArgTypes...)>& f, unspecified-null-pointer-type nullptr_t);
>     template <class R, class... ArgTypes>
>       bool operator!=(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>& f);

### 770(i). std::function should use rvalue swap

Section: 22.10.17 [func.wrap] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It is expected that typical implementations of `std::function` will use dynamic memory allocations at least under given conditions, so it seems appropriate to change the current lvalue swappabilty of this class to rvalue swappability.

Proposed resolution:

In 22.10 [function.objects], header `<functional>` synopsis, just below of

>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);
>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&);
>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);

In 22.10.17.3 [func.wrap.func] class `function` definition, change

>     void swap(function&&);

In 22.10.17.3 [func.wrap.func], just below of

>     template <class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);
>     template <class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&);
>     template <class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);

In 22.10.17.3.3 [func.wrap.func.mod] change

>     void swap(function&& other);

In 22.10.17.3.8 [func.wrap.func.alg] add the two overloads

>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&& f1, function<R(ArgTypes...)>& f2);
>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>&& f2);

### 771(i). Impossible throws clause in [string.conversions]

Section: 27.4.5 [string.conversions] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The new `to_string` and `to_wstring` functions described in 27.4.5 [string.conversions] have throws clauses (paragraphs 8 and 16) which say:

> Throws: nothing

Since all overloads return either a `std::string` or a `std::wstring` by value this throws clause is impossible to realize in general, since the `basic_string` constructors can fail due to out-of-memory conditions. Either these throws clauses should be removed or should be more detailled like:

> Throws: Nothing if the string construction throws nothing

Further there is an editorial issue in p. 14: All three `to_wstring` overloads return a `string`, which should be `wstring` instead (The header `<string>` synopsis of 27.4 [string.classes] is correct in this regard).

Proposed resolution:

In 27.4.5 [string.conversions], remove the paragraphs 8 and 16.

>     string to_string(long long val);
>     string to_string(unsigned long long val);
>     string to_string(long double val);

> > Throws: nothing

>     wstring to_wstring(long long val);
>     wstring to_wstring(unsigned long long val);
>     wstring to_wstring(long double val);

> > Throws: nothing

### 772(i). Impossible return clause in [string.conversions]

Section: 27.4.5 [string.conversions] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The return clause 27.4.5 [string.conversions] paragraph 15 of the new `to_wstring` overloads says:

> Returns: each function returns a `wstring` object holding the character representation of the value of its argument that would be generated by calling `wsprintf(buf, fmt, val)` with a format specifier of `L"%lld"`, `L"%ulld"`, or `L"%f"`, respectively.

Problem is: There does not exist any `wsprintf` function in C99 (I checked the 2nd edition of ISO 9899, and the first and the second corrigenda from 2001-09-01 and 2004-11-15). What probably meant here is the function `swprintf` from `<wchar.h>/<cwchar>`, but this has the non-equivalent declaration:

>     int swprintf(wchar_t * restrict s, size_t n,
>     const wchar_t * restrict format, ...);

therefore the paragraph needs to mention the `size_t` parameter `n`.

Proposed resolution:

Change the current wording of 27.4.5 [string.conversions] p. 15 to:

> Returns: eEach function returns a `wstring` object holding the character representation of the value of its argument that would be generated by calling `wsswprintf(buf, bufsz, fmt, val)` with a format specifier `fmt` of `L"%lld"`, `L"%ulld"`, or `L"%f"`, respectively, where `buf` designates an internal character buffer of sufficient size `bufsz`.

[Hint to the editor: The resolution also adds to mention the name of the format specifier "fmt"]

I also would like to remark that the current wording of it's equivalent paragraph 7 should also mention the meaning of `buf` and `fmt`.

Change the current wording of 27.4.5 [string.conversions] p. 7 to:

> Returns: eEach function returns a string object holding the character representation of the value of its argument that would be generated by calling `sprintf(buf, fmt, val)` with a format specifier `fmt` of `"%lld"`, `"%ulld"`, or `"%f"`, respectively, where `buf` designates an internal character buffer of sufficient size.

### 774(i). Member `swap` undefined for most containers

Section: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-01-14 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [containers].

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Discussion:

It appears most containers declare but do not define a member-swap function.

This is unfortunate, as all overload the `swap` algorithm to call the member-swap function! (required for `swappable` guarantees [Table 37] and Container Requirements [Table 87])

Note in particular that Table 87 gives semantics of `a.swap(b)` as `swap(a,b)`, yet for all containers we define `swap(a,b)` to call `a.swap(b)` - a circular definition.

A quick survey of clause 23 shows that the following containers provide a definition for member-swap:

>     array
>     queue
>     stack
>     vector

Whereas the following declare it, but do not define the semantics:

>     deque
>     list
>     map
>     multimap
>     multiset
>     priority_queue
>     set
>     unordered_map
>     unordered_multi_map
>     unordered_multi_set
>     unordered_set

Suggested resolution:

> Provide a definition for each of the affected containers...

[ Bellevue: ]

> Move to Open and ask Alisdair to provide wording.

[ 2009-07 Frankfurt: ]

> Daniel to provide wording. N2590 is no longer applicable.

[ 2009-07-28 Daniel provided wording. ]

> 1. It assumes that the proposed resolution for 883(i) is applied, which breaks the circularity of definition between member `swap` and free `swap`.
> 2. It uses the notation of the pre-concept allocator trait `allocator_propagation_map`, which might be renamed after the next refactoring phase of generalized allocators.
> 3. It requires that compare objects, key equal functions and hash functions in containers are swapped via unqualified free `swap` according to 594(i).

[ 2009-09-30 Daniel adds: ]

> The outcome of this issue should be considered with the outcome of 1198(i) both in style and in content (e.g. bullet 9 suggests to define the semantic of `void priority_queue::swap(priority_queue&)` in terms of the member `swap` of the container).

[ 2009-10 Santa Cruz: ]

> Looked at, but took no action on as it overlaps too much with N2982. Waiting for a new draft WP.

[ 2009-10 Santa Cruz: ]

> Leave as open. Pablo to provide wording.

[ 2009-10-26 Pablo updated wording. Here is the wording he replaced: ]

> 1. Add a new Throws clause just after 99 [allocator.propagation.map]/5:

>    >     static void swap(Alloc& a, Alloc& b);

>    > > Effects: [..]

>    > > Throws: Nothing.

>    [ This exception requirement is added, such that it's combination with the general container requirements of N2723 [container.requirements.general]/9 make it unambiguously clear that the following descriptions of "swaps the allocators" have the following meaning: (a) This swap is done by calling `allocator_propagation_map<allocator_type>::swap` and (b) This allocator swap does never propagate an exception ]

> 2. Change 23.2.7.2 [associative.reqmts.except]/3 as indicated:

>    > For associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator `swap` of the container's `Pred` objects (if any).

> 3. Change 23.2.8.2 [unord.req.except]/3 as indicated:

>    > For unordered associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator `swap` of the container's `Hash` or `Pred` objects, respectively (if any).

> 4. Insert a new paragraph just after 23.3 [sequences]/1:

>    > In addition to being available via inclusion of the `<algorithm>` header, the `swap` function templates in 26.7.3 [alg.swap] are also available when the header `<queue>` is included.

>    [ There is a new issue in process that will suggest a minimum header for `swap` and `move`. If this one is provided, this text can be removed and the header dependency should be added to `<queue>` ]

> 5. Add one further clause at the end of 23.3.3.4 [array.special]:

>    [This part is added, because otherwise `array::swap` would otherwise contradict the general contract of 23.2.2 [container.requirements.general] p. 10 b. 5]

>    > Throws: Nothing, unless one of the element-wise `swap` calls throws an exception.

> 6. 1. In 23.3.5 [deque], class template `deque` synopsis change as indicated:

>       >     void swap(deque<T,Alloc>&);

>    2. At the end of 23.3.5.4 [deque.modifiers] add as indicated:

>       >     void swap(deque& x);

>       > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`.

>       > > Complexity: Constant time.

> 7. 1. In [forwardlist], class template `forward_list` synopsis change as indicated:

>       >     void swap(forward_list<T,Allocator>&);

>    2. At the end of [forwardlist.modifiers] add as indicated:

>       >     void swap(forward_list& x);

>       > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`.

>       > > Complexity: Constant time.

> 8. 1. In 23.3.11 [list], class template `list` synopsis change as indicated:

>       >     void swap(list<T,Allocator>&);

>    2. At the end of 23.3.11.4 [list.modifiers] add as indicated:

>       >     void swap(list& x);

>       > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`.

>       > > Complexity: Constant time.

> 9. At the end of 23.6.4.4 [priqueue.members] add a new prototype description:

>    >     void swap(priority_queue& q);

>    > > Requires: `Compare` shall satisfy the `Swappable` requirements ( [swappable]).

>    > > [ This requirement is added to ensure that even a user defined `swap` which is found by ADL for `Compare` satisfies the `Swappable` requirements ]

>    > > Effects: `this->c.swap(q.c); swap(this->comp, q.comp);`

>    > > Throws: What and if `c.swap(q.c)` and `swap(comp, q.comp)` throws.

>    [ This part is added, because otherwise `priority_queue::swap` would otherwise contradict the general contract of 23.2.2 [container.requirements.general] p. 10 b. 5 ]

> 10. 1. In 23.3.13 [vector], class template `vector` synopsis change as indicated:

>        >     void swap(vector<T,Allocator>&);

>     2. Change 23.3.13.3 [vector.capacity] p. 8 as indicated:

>        >     void swap(vector<T,Allocator>& x);

>        > > Effects: Exchanges the contents and `capacity()` and swaps the allocators of `*this` with that of `x`.

> 11. Insert a new paragraph just before 23.4 [associative]/1:

>     > In addition to being available via inclusion of the `<algorithm>` header, the `swap` function templates in 26.7.3 [alg.swap] are also available when any of the headers `<map>` or `<set>` are included.

> 12. 1. In 23.4.3 [map], class template `map` synopsis change as indicated:

>        >     void swap(map<Key,T,Compare,Allocator>&);

>     2. At the end of 23.4.3.4 [map.modifiers] add as indicated:

>        >     void swap(map& x);

>        > > Requires: Compare shall satisfy the `Swappable` requirements ( [swappable]).

>        > > [ This requirement is added to ensure that even a user defined `swap` which is found by ADL for `Compare` satisfies the `Swappable` requirements ]

>        > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the comparison objects of `*this` and `x`.

>        > > Complexity: Constant time

> 13. 1. In 23.4.4 [multimap], class template `multimap` synopsis change as indicated:

>        >     void swap(multimap<Key,T,Compare,Allocator>&);

>     2. At the end of 23.4.4.3 [multimap.modifiers] add as indicated:

>        >     void swap(multimap& x);

>        > > Requires: Compare shall satisfy the `Swappable` requirements ( [swappable]).

>        > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the comparison objects of `*this` and `x`.

>        > > Complexity: Constant time

> 14. 1. In 23.4.6 [set], class template `set` synopsis change as indicated:

>        >     void swap(set<Key,Compare,Allocator>&);

>     2. After section 23.4.6.2 [set.cons] add a new section `set` modifiers 23.4.6.4 [set.modifiers] and add the following paragraphs:

>        >     void swap(set& x);

>        > > Requires: Compare shall satisfy the `Swappable` requirements ( [swappable]).

>        > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the comparison objects of `*this` and `x`.

>        > > Complexity: Constant time

> 15. 1. In 23.4.7 [multiset], class template `multiset` synosis, change as indicated:

>        >     void swap(multiset<Key,Compare,Allocator>&);

>     2. After section 23.4.7.2 [multiset.cons] add a new section `multiset` modifiers [multiset.modifiers] and add the following paragraphs:

>        >     void swap(multiset& x);

>        > > Requires: Compare shall satisfy the `Swappable` requirements ( [swappable]).

>        > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the comparison objects of `*this` and `x`.

>        > > Complexity: Constant time

> 16. Insert a new paragraph just before 23.5 [unord] p. 1:

>     > In addition to being available via inclusion of the `<algorithm>` header, the `swap` function templates in 26.7.3 [alg.swap] are also available when any of the headers `<unordered_map>` or `<unordered_set>` are included.

> 17. After section 23.5.3.3 [unord.map.elem] add a new section unordered_map modifiers 23.5.3.4 [unord.map.modifiers] and add the following paragraphs:

>     >     void swap(unordered_map& x);

>     > > Requires: `Hash` and `Pred` shall satisfy the `Swappable` requirements ( [swappable]).

>     > > [ This requirement is added to ensure that even a user defined `swap` which is found by ADL for `Hash` and `Pred` satisfies the `Swappable` requirements ]

>     > > Effects: Exchanges the contents and hash policy and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the `Pred` objects and an unqualified `swap` of the `Hash` objects of `*this` and `x`.

>     > > Complexity: Constant time

> 18. After section 23.5.4.2 [unord.multimap.cnstr] add a new section unordered_multimap modifiers 23.5.4.3 [unord.multimap.modifiers] and add the following paragraphs:

>     >     void swap(unordered_multimap& x);

>     > > Requires: `Hash` and `Pred` shall satisfy the `Swappable` requirements ( [swappable]).

>     > > Effects: Exchanges the contents and hash policy and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the `Pred` objects and an unqualified `swap` of the `Hash` objects of `*this` and `x`

>     > > Complexity: Constant time

> 19. After section 23.5.6.2 [unord.set.cnstr] add a new section unordered_set modifiers 23.5.6.4 [unord.set.modifiers] and add the following paragraphs:

>     >     void swap(unordered_set& x);

>     > > Requires: `Hash` and `Pred` shall satisfy the `Swappable` requirements ( [swappable]).

>     > > Effects: Exchanges the contents and hash policy and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the `Pred` objects and an unqualified `swap` of the `Hash` objects of `*this` and `x`

>     > > Complexity: Constant time

> 20. After section 23.5.7.2 [unord.multiset.cnstr] add a new section unordered_multiset modifiers [unord.multiset.modifiers] and add the following paragraphs:

>     >     void swap(unordered_multiset& x);

>     > > Requires: `Hash` and `Pred` shall satisfy the `Swappable` requirements ( [swappable]).

>     > > Effects: Exchanges the contents and hash policy and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the `Pred` objects and an unqualified `swap` of the `Hash` objects of `*this` and `x`

>     > > Complexity: Constant time

[ 2009-10-30 Pablo and Daniel updated wording. ]

[ 2010 Pittsburgh: Ready for Pittsburgh. ]

Proposed resolution:

[ This resolution is based on the September 2009 WP, N2960, except that it assumes that N2982 and issues 883(i) and 1232(i) have already been applied. Note in particular that Table 91 in N2960 is refered to as Table 90 because N2982 removed the old Table 90. This resolution also addresses issue 431(i). ]

In 23.2.2 [container.requirements.general], replace the a.swap(b) row in table 90, "container requirements" (was table 91 before the application of N2982 to the WP):

> `a.swap(b)` | `void` |  | `swap(a,b)Exchange the contents of `a` and `b`.` | (Note A)
> `swap(a,b)` | `void` | `` | `a.swap(b)` | (Note A)

Modify the notes immediately following Table 90 in 23.2.2 [container.requirements.general] as follows (The wording below is after the application of N2982 to N2960. The editor might also want to combine Notes A and B into one.):

> Notes: the algorithms swap(), equal() and lexicographical_compare() are defined in Clause 25. Those entries marked "(Note A)" or "(Note B)" should have linear complexity for array and constant complexity for all other standard containers.

In 23.2.2 [container.requirements.general], before paragraph 8, add:

> The expression `a.swap(b)`, for containers `a` and `b` of a standard container type other than `array`, exchanges the values of `a` and `b` without invoking any move, copy, or swap operations on the individual container elements. Any `Compare`, `Pred`, or `Hash` function objects belonging to `a` and `b` shall be `swappable` and are exchanged by unqualified calls to non-member `swap`. If `allocator_traits<allocator_type>::propagate_on_container_swap::value == true`, then the allocators of `a` and `b` are also exchanged using an unqualified call to non-member `swap`. Otherwise, the behavior is undefined unless `a.get_allocator() == b.get_allocator()`. Each iterator refering to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with value `a.end()` before the swap will have value `b.end()` after the swap. In addition to being available via inclusion of the `<utility>` header, the `swap` function template in 26.7.3 [alg.swap] is also available within the definition of every standard container's `swap` function.

[ Note to the editor: Paragraph 2 starts with a sentence fragment, clearly from an editing or source-control error. ]

Modify 23.2.7.2 [associative.reqmts.except] as follows:

> 23.2.4.1 Exception safety guarantees 23.2.7.2 [associative.reqmts.except]

> For associative containers, no `clear()` function throws an exception. `erase(k)` does not throw an exception unless that exception is thrown by the container's `PredCompare` object (if any).

> For associative containers, if an exception is thrown by any operation from within an `insert()` function inserting a single element, the `insert()` function has no effect.

> For associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operatorswap of the container's `PredCompare` object (if any).

Modify 23.2.8.2 [unord.req.except], paragraph 3 as follows:

> For unordered associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operatorswap of the container's `Hash` or `Pred` object (if any).

Modify section 23.3.3.4 [array.special]:

> array specialized algorithms 23.3.3.4 [array.special]

> `template <class T, size_t N> void swap(array<T,N>& x,array<T,N>& y);`

> > Effects: `swap_ranges(x.begin(), x.end(), y.begin() );x.swap(y);`

Add a new section after [array.fill] (Note to the editor: array::fill make use of a concept requirement that must be removed or changed to text.):

> array::swap [array.swap]

> `void swap(array& y);`

> > Effects: `swap_ranges(this->begin(), this->end(), y.begin() );`

> > Throws: Nothing unless one of the element-wise swap calls throws an exception.

> > [Note: Unlike other containers' `swap` functions, `array::swap` takes linear, not constant, time, may exit via an exception, and does not cause iterators to become associated with the other container. — end note]

Insert a new paragraph just after 23.6 [container.adaptors]/1:

> For container adaptors, no `swap` function throws an exception unless that exception is thrown by the swap of the adaptor's `Container` or `Compare` object (if any).

### 775(i). Tuple indexing should be unsigned?

Section: 22.4.7 [tuple.helper] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-01-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [tuple.helper].

View all issues with CD1 status.

Discussion:

The tuple element access API identifies the element in the sequence using signed integers, and then goes on to enforce the requirement that I be >= 0. There is a much easier way to do this - declare I as `unsigned`.

In fact the proposal is to use `std::size_t`, matching the type used in the `tuple_size` API.

A second suggestion is that it is hard to imagine an API that deduces and index at compile time and returns a reference throwing an exception. Add a specific Throws: Nothing paragraph to each element access API.

In addition to `tuple`, update the API applies to `pair` and `array`, and should be updated accordingly.

A third observation is that the return type of the `get` functions for `std::pair` is pseudo-code, but it is not clearly marked as such. There is actually no need for pseudo-code as the return type can be specified precisely with a call to `tuple_element`. This is already done for `std::tuple`, and `std::array` does not have a problem as all elements are of type `T`.

Proposed resolution:

Update header <utility> synopsis in 22.2 [utility]

    // 20.2.3, tuple-like access to pair:
    template <class T> class tuple_size;
    template <intsize_t I, class T> class tuple_element;

    template <class T1, class T2> struct tuple_size<std::pair<T1, T2> >;
    template <class T1, class T2> struct tuple_element<0, std::pair<T1, T2> >;
    template <class T1, class T2> struct tuple_element<1, std::pair<T1, T2> >;

    template<intsize_t I, class T1, class T2>
      Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(std::pair<T1, T2>&);
    template<intsize_t I, class T1, class T2>
      const Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(const std::pair<T1, T2>&);

Update 22.3 [pairs] Pairs

    template<intsize_t I, class T1, class T2>
      Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(pair<T1, T2>&);
    template<intsize_t I, class T1, class T2>
      const Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(const pair<T1, T2>&);

24 Return type: If `I == 0` then `P` is `T1`, if `I == 1` then `P` is `T2`, and otherwise the program is ill-formed.

25 Returns: If `I == 0` returns `p.first`, otherwise if `I == 1` returns `p.second`, and otherwise the program is ill-formed.

Throws: Nothing.

Update header <tuple> synopsis in 22.4 [tuple] with a APIs as below:

    template <intsize_t I, class T> class tuple_element; // undefined
    template <intsize_t I, class... Types> class tuple_element<I, tuple<Types...> >;

    // 20.3.1.4, element access:
    template <intsize_t I, class... Types>
      typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>&);
    template <intsize_t I, class ... types>
      typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>&);

Update 22.4.7 [tuple.helper] Tuple helper classes

    template <intsize_t I, class... Types>
    class tuple_element<I, tuple<Types...> > {
    public:
      typedef TI type;
    };

1 Requires: `0 <= I and I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

2 Type: `TI` is the type of the `I`th element of `Types`, where indexing is zero-based.

Update 22.4.8 [tuple.elem] Element access

    template <intsize_t I, class... types >
    typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>& t);

1 Requires: `0 <= I and I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

2 Returns: A reference to the `I`th element of `t`, where indexing is zero-based. Throws: Nothing.

    template <intsize_t I, class... types>
    typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>& t);

3 Requires: `0 <= I and I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

4 Returns: A const reference to the `I`th element of `t`, where indexing is zero-based.

Throws: Nothing.

Update header <array> synopsis in 22.2 [utility]

    template <class T> class tuple_size; // forward declaration
    template <intsize_t I, class T> class tuple_element; // forward declaration
    template <class T, size_t N>
      struct tuple_size<array<T, N> >;
    template <intsize_t I, class T, size_t N>
      struct tuple_element<I, array<T, N> >;
    template <intsize_t I, class T, size_t N>
      T& get(array<T, N>&);
    template <intsize_t I, class T, size_t N>
      const T& get(const array<T, N>&);

Update 23.3.3.7 [array.tuple] Tuple interface to class template array

    tuple_element<size_t I, array<T, N> >::type

3 Requires: `0 <= I < N.` The program is ill-formed if `I` is out of bounds.

4 Value: The type `T`.

    template <intsize_t I, class T, size_t N> T& get(array<T, N>& a);

5 Requires: `0 <= I < N`. The program is ill-formed if `I` is out of bounds.

Returns: A reference to the `I`th element of `a`, where indexing is zero-based.

Throws: Nothing.

    template <intsize_t I, class T, size_t N> const T& get(const array<T, N>& a);

6 Requires: `0 <= I < N`. The program is ill-formed if `I` is out of bounds.

7 Returns: A const reference to the `I`th element of `a`, where indexing is zero-based.

Throws: Nothing.

[ Bellevue: Note also that the phrase "The program is ill-formed if I is out of bounds" in the requires clauses are probably unnecessary, and could be removed at the editor's discretion. Also std:: qualification for pair is also unnecessary. ]

### 776(i). Undescribed `assign` function of `std::array`

Section: 23.3.3 [array] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [array].

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Discussion:

The class template array synopsis in 23.3.3 [array] p. 3 declares a member function

>     void assign(const T& u);

which's semantic is no-where described. Since this signature is not part of the container requirements, such a semantic cannot be derived by those.

I found only one reference to this function in the issue list, 588(i) where the question is raised:

> what's the effect of calling `assign(T&)` on a zero-sized array?

which does not answer the basic question of this issue.

If this function shall be part of the `std::array`, it's probable semantic should correspond to that of `boost::array`, but of course such wording must be added.

Proposed resolution:

Just after the section [array.data] add the following new section:

23.2.1.5 array::fill [array.fill]

>     void fill(const T& u);

> 1: Effects: `fill_n(begin(), N, u)`

[N.B: I wonder, why class `array` does not have a "modifiers" section. If it had, then `assign` would naturally belong to it]

Change the synopsis in 23.3.3 [array]/3:

>     template <class T, size_t N>
>     struct array {
>       ...
>       void assign fill(const T& u);
>       ...

[ Bellevue: ]

> Suggest substituting "fill" instead of "assign".

> Set state to Review given substitution of "fill" for "assign".

### 777(i). Atomics Library Issue

Section: 32.5.8.2 [atomics.types.operations] Status: CD1 Submitter: Lawrence Crowl Opened: 2008-01-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.types.operations].

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Discussion:

The load functions are defined as

>     C atomic_load(volatile A* object);
>     C atomic_load_explicit(volatile A* object, memory_order);
>     C A::load(memory_order order = memory_order_seq_cst) volatile;

which prevents their use in `const` contexts.

[ post Bellevue Peter adds: ]

> Issue 777(i) suggests making `atomic_load` operate on `const` objects. There is a subtle point here. Atomic loads do not generally write to the object, except potentially for the `memory_order_seq_cst` constraint. Depending on the architecture, a dummy write with the same value may be required to be issued by the atomic load to maintain sequential consistency. This, in turn, may make the following code:

> >     const atomic_int x{};

> >     int main()
> >     {
> >       x.load();
> >     }

> dump core under a straightforward implementation that puts const objects in a read-only section.

> There are ways to sidestep the problem, but it needs to be considered.

> The tradeoff is between making the data member of the atomic types mutable and requiring the user to explicitly mark atomic members as mutable, as is already the case with mutexes.

Proposed resolution:

Add the `const` qualifier to `*object` and `*this`.

>     C atomic_load(const volatile A* object);
>     C atomic_load_explicit(const volatile A* object, memory_order);
>     C A::load(memory_order order = memory_order_seq_cst) const volatile;

### 778(i). std::bitset does not have any constructor taking a string literal

Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Thorsten Ottosen Opened: 2008-01-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [bitset.cons].

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Duplicate of: 116

Discussion:

A small issue with `std::bitset`: it does not have any constructor taking a string literal, which is clumsy and looks like an oversigt when we tried to enable uniform use of `string` and `const char*` in the library.

Suggestion: Add

>     explicit bitset( const char* str );

to std::bitset.

Proposed resolution:

Add to synopsis in 22.9.2 [template.bitset]

>     explicit bitset( const char* str );

Add to synopsis in 22.9.2.2 [bitset.cons]

>     explicit bitset( const char* str );

> Effects: Constructs a `bitset` as if `bitset(string(str))`.

### 779(i). Resolution of #283 incomplete

Section: 26.7.8 [alg.remove] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.remove].

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Discussion:

The resolution of 283(i) did not resolve similar necessary changes for algorithm `remove_copy[_if]`, which seems to be an oversight.

Proposed resolution:

In 26.7.8 [alg.remove] p.6, replace the N2461 requires clause with:

> Requires: Type `T` is `EqualityComparable` (31). The ranges `[first,last)` and `[result,result + (last - first))` shall not overlap. The expression `*result = *first` shall be valid.

### 780(i). `std::merge()` specification incorrect/insufficient

Section: 26.8.6 [alg.merge] Status: C++11 Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.merge].

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Discussion:

Though issue 283(i) has fixed many open issues, it seems that some are still open:

Both 25.3.4 [lib.alg.merge] in 14882:2003 and 26.8.6 [alg.merge] in N2461 have no Requires element and the Effects element contains some requirements, which is probably editorial. Worse is that:

- no assignment requirements are specified (neither implicit nor explicit).
- the effects clause just speaks of "merges", which is badly worded near to a circular definition.
- p. 2 mentions a range `[first, last)`, which is not defined by the function arguments or otherwise.
- p. 2 says "according to the ordering defined by comp" which is both incomplete (because this excludes the first variant with <) and redundant (because the following subordinate clause mentions comp again)

[ Post Summit Alisdair adds: ]

> Suggest:

> > (where `last` is equal to `next(result, distance(first1, last1) + distance(first2, last2))`, such that resulting range will be sorted in non-decreasing order; that is, for every iterator `i` in `[result,last)` other than `result`, the condition `*i < *prev(i)` or, respectively, `comp(*i, *prev(i))` will be `false`.

> Note that this might still not be technically accurate in the case of `InputIterators`, depending on other resolutions working their way through the system (1011(i)).

[ Post Summit Daniel adds: ]

> If we want to use `prev` and `next` here (Note: `merge` is sufficiently satisfied with `InputIterator`) we should instead add more to 26 [algorithms] p. 6, but I can currently not propose any good wording for this.

[ Batavia (2009-05): ]

> Pete points out the existing wording in [algorithms] p. 4 that permits the use of + in algorithm specifications.

> Alisdair points out that that wording may not apply to input iterators.

> Move to Review.

[ 2009-07 Frankfurt: ]

> Move to Ready.

[ 2009-08-23 Daniel reopens: ]

> The proposed wording must be rephrased, because the part

> > for every iterator `i` in `[result,last)` other than `result`, the condition `*i < *(i - 1)` or, respectively, `comp(*i, *(i - 1))` will be `false`"

> isn't meaningful, because the range `[result,last)` is that of a pure `OutputIterator`, which is not readable in general.

> [Howard: Proposed wording updated by Daniel, status moved from Ready to Review.]

[ 2009-10 Santa Cruz: ]

> Matt has some different words to propose. Those words have been moved into the proposed wording section, and the original proposed wording now appears here:

> > In 26.8.6 [alg.merge] replace p.1+ 2:

> > > Effects: MergesCopies all the elements of the two sorted ranges `[first1,last1)` and `[first2,last2)` into the range `[result,result + (last1 - first1) + (last2 - first2))` , such that resulting range will be sorted in non-decreasing order; that is for every pair of iterators `i` and `j` of either input ranges, where `*i` was copied to the output range before `*j` was copied to the output range, the condition `*j < *i` or, respectively, `comp(*j, *i)` will be `false`.

> > > Requires:The resulting range shall not overlap with either of the original ranges. The list will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i` in `[first,last)` other than `first`, the condition `*i < *(i - 1)` or `comp(*i, *(i - 1))` will be `false`.

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 26.8.6 [alg.merge] 1 and 2:

> 1 Effects: Merges two sorted ranges `[first1,last1)` and `[first2,last2)` into the range `[result, result + (last1 - first1) + (last2 - first2))`.

> Effects: Copies all the elements of the two ranges `[first1,last1)` and `[first2,last2)` into the range `[result, result_last)`, where `result_last` is `result + (last1 - first1) + (last2 - first2)`, such that the resulting range satisfies `is_sorted(result, result_last)` or `is_sorted(result, result_last, comp)`, respectively.

> 2 Requires: The ranges `[first1,last1)` and `[first2,last2)` shall be sorted with respect to `operator<` or `comp`. The resulting range shall not overlap with either of the original ranges. The list will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i` in `[first,last)` other than `first`, the condition `*i < *(i - 1)` or `comp(*i, *(i - 1))` will be `false`.

Change 26.8.6 [alg.merge] p. 6+7 as indicated [This ensures harmonization between `inplace_merge` and `merge`]

> 6 Effects: Merges two sorted consecutive ranges `[first,middle)` and `[middle,last)`, putting the result of the merge into the range `[first,last)`. The resulting range will be in non-decreasing order; that is, for every iterator `i` in `[first,last)` other than `first`, the condition `*i < *(i - 1)` or, respectively, `comp(*i, *(i - 1))` will be false.

> 7 Requires: The ranges `[first,middle)` and `[middle,last)` shall be sorted with respect to `operator<` or `comp`. The type of `*first` shall satisfy the `Swappable` requirements (37), the `MoveConstructible` requirements (Table 33), and the the `MoveAssignable` requirements (Table 35).

### 781(i). `std::complex` should add missing C99 functions

Section: 29.4.7 [complex.value.ops] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [complex.value.ops].

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Discussion:

A comparision of the N2461 header `<complex>` synopsis ([complex.syn]) with the C99 standard (ISO 9899, 2nd edition and the two corrigenda) show some complex functions that are missing in C++. These are:

1. 7.3.9.4: (required elements of the C99 library)
   The `cproj` functions
2. 7.26.1: (optional elements of the C99 library)

       cerf    cerfc    cexp2
       cexpm1  clog10   clog1p
       clog2   clgamma  ctgamma

I propose that at least the required `cproj` overloads are provided as equivalent C++ functions. This addition is easy to do in one sentence (delegation to C99 function).

Please note also that the current entry `polar` in 29.4.10 [cmplx.over] p. 1 should be removed from the mentioned overload list. It does not make sense to require that a function already expecting scalar arguments should cast these arguments into corresponding `complex<T>` arguments, which are not accepted by this function.

Proposed resolution:

In 29.4.2 [complex.syn] add just between the declaration of `conj` and `fabs`:

>     template<class T> complex<T> conj(const complex<T>&);
>     template<class T> complex<T> proj(const complex<T>&);
>     template<class T> complex<T> fabs(const complex<T>&);

In 29.4.7 [complex.value.ops] just after p.6 (return clause of `conj`) add:

>     template<class T> complex<T> proj(const complex<T>& x);

> > Effects: Behaves the same as C99 function `cproj`, defined in subclause 7.3.9.4."

In 29.4.10 [cmplx.over] p. 1, add one further entry `proj` to the overload list.

> The following function templates shall have additional overloads:

> >     arg           norm
> >     conj          polar proj
> >     imag          real

### 782(i). Extended `seed_seq` constructor is useless

Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-27 Last modified: 2015-10-20

Priority: Not Prioritized

View all other issues in [rand.util.seedseq].

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Discussion:

Part of the resolution of n2423, issue 8 was the proposal to extend the `seed_seq` constructor accepting an input range as follows (which is now part of N2461):

>     template<class InputIterator,
>     size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
>     seed_seq(InputIterator begin, InputIterator end);

First, the expression `iterator_traits<InputIterator>::value_type` is invalid due to missing `typename` keyword, which is easy to fix.

Second (and worse), while the language now supports default template arguments of function templates, this customization point via the second `size_t` template parameter is of no advantage, because `u` can never be deduced, and worse - because it is a constructor function template - it can also never be explicitly provided (13.10.2 [temp.arg.explicit]/7).

The question arises, which advantages result from a compile-time knowledge of `u` versus a run time knowledge? If run time knowledge suffices, this parameter should be provided as normal function default argument [Resolution marked (A)], if compile-time knowledge is important, this could be done via a tagging template or more user-friendly via a standardized helper generator function (`make_seed_seq`), which allows this [Resolution marked (B)].

[ Bellevue: ]

> Fermilab does not have a strong opinion. Would prefer to go with solution A. Bill agrees that solution A is a lot simpler and does the job.

> Proposed Resolution: Accept Solution A.

Issue 803(i) claims to make this issue moot.

Proposed resolution:

1. In 29.5.8.1 [rand.util.seedseq]/2, class `seed_seq` synopsis replace:

   >     class seed_seq
   >     {
   >     public:
   >        ...
   >        template<class InputIterator,
   >           size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
   >               seed_seq(InputIterator begin, InputIterator end,
   >               size_t u = numeric_limits<typename iterator_traits<InputIterator>::value_type>::digits);
   >        ...
   >     };

   and do a similar replacement in the member description between p.3 and p.4.

2. In 29.5.8.1 [rand.util.seedseq]/2, class `seed_seq` synopsis and in the member description between p.3 and p.4 replace:

   >     template<class InputIterator,
   >       size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
   >           seed_seq(InputIterator begin, InputIterator end);
   >     template<class InputIterator, size_t u>
   >     seed_seq(InputIterator begin, InputIterator end, implementation-defined s);

   In 29.5.2 [rand.synopsis], header `<random>` synopsis, immediately after the class `seed_seq` declaration and in 29.5.8.1 [rand.util.seedseq]/2, immediately after the class `seed_seq` definition add:

   >     template<size_t u, class InputIterator>
   >       seed_seq make_seed_seq(InputIterator begin, InputIterator end);

   In 29.5.8.1 [rand.util.seedseq], just before p.5 insert two paragraphs:

   > The first constructor behaves as if it would provide an integral constant expression `u` of type `size_t` of value `numeric_limits<typename iterator_traits<InputIterator>::value_type>::digits`.

   > The second constructor uses an implementation-defined mechanism to provide an integral constant expression `u` of type `size_t` and is called by the function `make_seed_seq`.

   In 29.5.8.1 [rand.util.seedseq], just after the last paragraph add:

   >     template<size_t u, class InputIterator>
   >        seed_seq make_seed_seq(InputIterator begin, InputIterator end);

   > > where `u` is used to construct an object `s` of implementation-defined type.

   > > Returns: `seed_seq(begin, end, s)`;

### 783(i). `thread::id` reuse

Section: 32.4.3.2 [thread.thread.id] Status: CD1 Submitter: Hans Boehm Opened: 2008-02-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.thread.id].

View all issues with CD1 status.

Discussion:

The current working paper (N2497, integrated just before Bellevue) is not completely clear whether a given `thread::id` value may be reused once a thread has exited and has been joined or detached. Posix allows thread ids (`pthread_t` values) to be reused in this case. Although it is not completely clear whether this originally was the right decision, it is clearly the established practice, and we believe it was always the intent of the C++ threads API to follow Posix and allow this. Howard Hinnant's example implementation implicitly relies on allowing reuse of ids, since it uses Posix thread ids directly.

It is important to be clear on this point, since it the reuse of thread ids often requires extra care in client code, which would not be necessary if thread ids were unique across all time. For example, a hash table indexed by thread id may have to be careful not to associate data values from an old thread with a new one that happens to reuse the id. Simply removing the old entry after joining a thread may not be sufficient, if it creates a visible window between the join and removal during which a new thread with the same id could have been created and added to the table.

[ post Bellevue Peter adds: ]

> There is a real issue with `thread::id` reuse, but I urge the LWG to reconsider fixing this by disallowing reuse, rather than explicitly allowing it. Dealing with thread id reuse is an incredibly painful exercise that would just force the world to reimplement a non-conflicting `thread::id` over and over.

> In addition, it would be nice if a `thread::id` could be manipulated atomically in a lock-free manner, as motivated by the recursive lock example:

> http://www.decadent.org.uk/pipermail/cpp-threads/2006-August/001091.html

Proposed resolution:

Add a sentence to 32.4.3.2 [thread.thread.id]/p1:

> An object of type `thread::id` provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.threads.class]). Each thread of execution has a `thread::id` that is not equal to the `thread::id` of other threads of execution and that is not equal to the `thread::id` of `std::thread` objects that do not represent threads of execution. The library may reuse the value of a `thread::id` of a terminated thread that can no longer be joined.

### 786(i). Thread library timed waits, UTC and monotonic clocks

Section: 30 [time] Status: Resolved Submitter: Christopher Kohlhoff, Jeff Garland Opened: 2008-02-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time].

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Discussion:

The draft C++0x thread library requires that the time points of type `system_time` and returned by `get_system_time()` represent Coordinated Universal Time (UTC) (section [datetime.system]). This can lead to surprising behavior when a library user performs a duration-based wait, such as `condition_variable::timed_wait()`. A complete explanation of the problem may be found in the Rationale for the Monotonic Clock section in POSIX, but in summary:

- Operations such as `condition_variable::timed_wait()` (and its POSIX equivalent, `pthread_cond_timedwait()`) are specified using absolute times to address the problem of spurious wakeups.
- The typical use of the timed wait operations is to perform a relative wait. This may be achieved by first calculating an absolute time as the sum of the current time and the desired duration. In fact, the C++0x thread library includes duration-based overloads of `condition_variable::timed_wait()` that behave as if by calling the corresponding absolute time overload with a time point value of `get_system_time() + rel_time`.
- A UTC clock may be affected by changes to the system time, such as synchronization with an external source, leap seconds, or manual changes to the clock.
- Should the clock change during a timed wait operation, the actual duration of the wait will not be the expected length. For example, a user may intend a timed wait of one second duration but, due to an adjustment of the system clock backwards by a minute, the wait instead takes 61 seconds.

POSIX solves the problem by introducing a new monotonic clock, which is unaffected by changes to the system time. When a condition variable is initialized, the user may specify whether the monotonic clock is to be used. (It is worth noting that on POSIX systems it is not possible to use `condition_variable::native_handle()` to access this facility, since the desired clock type must be specified during construction of the condition variable object.)

In the context of the C++0x thread library, there are added dimensions to the problem due to the need to support platforms other than POSIX:

- Some environments (such as embedded systems) do not have a UTC clock, but do have a monotonic clock.
- Some environments do not have a monotonic clock, but do have a UTC clock.
- The Microsoft Windows API's synchronization functions use relative timeouts based on an implied monotonic clock. A program that switches from the Windows API to the C++0x thread library will now find itself susceptible to clock changes.

One possible minimal solution:

- Strike normative references to UTC and an epoch based on 1970-01-01.
- Make the semantics of `system_time` and `get_system_time()` implementation-defined (i.e standard library implementors may choose the appropriate underlying clock based on the capabilities of the target platform).
- Add a non-normative note encouraging use of a monotonic clock.
- Remove `system_time::seconds_since_epoch()`.
- Change the constructor `explicit system_time(time_t secs, nanoseconds ns = 0)` to `explicit system_time(nanoseconds ns)`.

Proposed resolution:

Rationale:

Addressed by N2661: A Foundation to Sleep On.

### 787(i). complexity of `binary_search`

Section: 26.8.4.5 [binary.search] Status: CD1 Submitter: Daniel Krügler Opened: 2007-09-08 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

In 26.8.4.5 [binary.search] p. 3 the complexity of `binary_search` is described as

> At most `log(last - first) + 2` comparisons.

This should be precised and brought in line with the nomenclature used for `lower_bound`, `upper_bound`, and `equal_range`.

All existing libraries I'm aware of, delegate to `lower_bound` (+ one further comparison). Since issue 384(i) has now WP status, the resolution of #787 should be brought in-line with 384(i) by changing the `+ 2` to `+ O(1)`.

[ Sophia Antipolis: ]

> Alisdair prefers to apply an upper bound instead of O(1), but that would require fixing for `lower_bound`, `upper_bound` etc. as well. If he really cares about it, he'll send an issue to Howard.

Proposed resolution:

Change 26.8.4.5 [binary.search]/3

> Complexity: At most `log2(last - first) + 2 O(1)` comparisons.

### 788(i). Ambiguity in [istream.iterator]

Section: 24.6.2 [istream.iterator] Status: C++11 Submitter: Martin Sebor Opened: 2008-02-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.iterator].

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Discussion:

Addresses UK 287

> It is not clear what the initial state of an `istream_iterator` should be. Is _value_ initialized by reading the stream, or default/value initialized? If it is initialized by reading the stream, what happens if the initialization is deferred until first dereference, when ideally the iterator value should have been that of an end-of-stream iterator which is not safely dereferencable?

> Recommendation: Specify _value_ is initialized by reading the stream, or the iterator takes on the end-of-stream value if the stream is empty.

The description of how an istream_iterator object becomes an end-of-stream iterator is a) ambiguous and b) out of date WRT issue 468(i):

> `istream_iterator` reads (using `operator>>`) successive elements from the input stream for which it was constructed. After it is constructed, and every time `++` is used, the iterator reads and stores a value of `T`. If the end of stream is reached (`operator void*()` on the stream returns `false`), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments `istream_iterator()` always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of `operator*` on an end of stream is not defined. For any other iterator value a `const T&` is returned. The result of `operator->` on an end of stream is not defined. For any other iterator value a `const T*` is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that `++` operators are not equality preserving, that is, `i == j` does not guarantee at all that `++i == ++j`. Every time `++` is used a new value is read.

`istream::operator void*()` returns null if `istream::fail()` is `true`, otherwise non-null. `istream::fail()` returns `true` if `failbit` or `badbit` is set in `rdstate()`. Reaching the end of stream doesn't necessarily imply that `failbit` or `badbit` is set (e.g., after extracting an `int` from `stringstream("123")` the stream object will have reached the end of stream but `fail()` is `false` and `operator void*()` will return a non-null value).

Also I would prefer to be explicit about calling `fail()` here (there is no `operator void*()` anymore.)

[ Summit: ]

> Moved from Ready to Open for the purposes of using this issue to address NB UK 287. Martin to handle.

[ 2009-07 Frankfurt: ]

> This improves the wording.

> Move to Ready.

Proposed resolution:

Change 24.6.2 [istream.iterator]/1:

> `istream_iterator` reads (using `operator>>`) successive elements from the input stream for which it was constructed. After it is constructed, and every time `++` is used, the iterator reads and stores a value of `T`. If the end of stream is reached the iterator fails to read and store a value of `T` (`operator void*() fail()` on the stream returns `false true`), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments `istream_iterator()` always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of `operator*` on an end of stream is not defined. For any other iterator value a `const T&` is returned. The result of `operator->` on an end of stream is not defined. For any other iterator value a `const T*` is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that `++` operators are not equality preserving, that is, `i == j` does not guarantee at all that `++i == ++j`. Every time `++` is used a new value is read.

### 789(i). `xor_combine_engine(result_type)` should be explicit

Section: 99 [rand.adapt.xor] Status: CD1 Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.adapt.xor].

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Discussion:

`xor_combine_engine(result_type)` should be `explicit`. (Obvious oversight.)

[ Bellevue: ]

> Non-controversial. Bill is right, but Fermilab believes that this is easy to use badly and hard to use right, and so it should be removed entirely. Got into TR1 by well defined route, do we have permission to remove stuff? Should probably check with Jens, as it is believed he is the originator. Broad consensus that this is not a robust engine adapter.

Proposed resolution:

Remove xor_combine_engine from synopsis of 29.5.2 [rand.synopsis].

Remove 99 [rand.adapt.xor] `xor_combine_engine`.

### 792(i). `piecewise_constant_distribution` is undefined for a range with just one endpoint

Section: 29.5.9.6.2 [rand.dist.samp.pconst] Status: CD1 Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.dist.samp.pconst].

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Discussion:

`piecewise_constant_distribution` is undefined for a range with just one endpoint. (Probably should be the same as an empty range.)

Proposed resolution:

Change 29.5.9.6.2 [rand.dist.samp.pconst] paragraph 3b:

> b) If `firstB == lastB` or the sequence `w` has the length zero,

### 793(i). `discrete_distribution` missing constructor

Section: 29.5.9.6.1 [rand.dist.samp.discrete] Status: Resolved Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.dist.samp.discrete].

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Discussion:

`discrete_distribution` should have a constructor like:

>     template<class _Fn>
>       discrete_distribution(result_type _Count, double _Low, double _High,
>                             _Fn& _Func);

(Makes it easier to fill a histogram with function values over a range.)

[ Bellevue: ]

> How do you specify the function so that it does not return negative values? If you do it is a bad construction. This requirement is already there. Where in each bin does one evaluate the function? In the middle. Need to revisit tomorrow.

[ Sophia Antipolis: ]

> Bill is not requesting this.

> Marc Paterno: `_Fn` cannot return negative values at the points where the function is sampled. It is sampled in the middle of each bin. `_Fn` cannot return 0 everywhere it is sampled.

> Jens: lambda expressions are rvalues

> Add a library issue to provide an `initializer_list<double>` constructor for `discrete_distribution`.

> Marc Paterno: dislikes reference for `_Fn` parameter. Make it pass-by-value (to use lambda), use `std::ref` to wrap giant-state function objects.

> Daniel: See `random_shuffle`, pass-by-rvalue-reference.

> Daniel to draft wording.

[ Pre San Francisco, Daniel provided wording: ]

> The here proposed changes of the WP refer to the current state of N2691. During the Sophia Antipolis meeting two different proposals came up regarding the functor argument type, either by value or by rvalue-reference. For consistence with existing conventions (state-free algorithms and the `general_pdf_distribution` c'tor signature) the author decided to propose a function argument that is provided by value. If severe concerns exists that stateful functions would be of dominant relevance, it should be possible to replace the two occurrences of `Func` by `Func&&` in this proposal as part of an editorial process.

Proposed resolution:

Non-concept version of the proposed resolution

1. In 29.5.9.6.1 [rand.dist.samp.discrete]/1, class `discrete_distribution`, just before the member declaration

   >     explicit discrete_distribution(const param_type& parm);

   insert:

   >     template<typename Func>
   >     discrete_distribution(result_type nf, double xmin, double xmax, Func fw);

2. Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::

   >     template<typename Func>
   >     discrete_distribution(result_type nf, double xmin, double xmax, Func fw);

   > Complexity: Exactly nf invocations of fw.

   > Requires:

   > 1. fw shall be callable with one argument of type double, and shall return values of a type convertible to double;
   > 2. If nf > 0, the relation `xmin` < `xmax` shall hold, and for all sample values `xk`, fw(`xk`) shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > 3. The following relations shall hold: nf ≥ 0, and 0 < S = `w0`+. . .+`wn-1`.

   > Effects:

   > 1. If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value `w0` = 1.

   > 2. Otherwise, sets n = nf, deltax = (`xmax` - `xmin`)/n and `xcent` = `xmin` + 0.5 * deltax.

   >    For each k = 0, . . . ,n-1, calculates:

   >    `xk` = `xcent` + k * deltax `wk` = fw(`xk`)

   > 3. Constructs a discrete_distribution object with probabilities:

   >    > `pk` = `wk`/S for k = 0, . . . , n-1.

Concept version of the proposed resolution

1. In 29.5.9.6.1 [rand.dist.samp.discrete]/1, class `discrete_distribution`, just before the member declaration

   >     explicit discrete_distribution(const param_type& parm);

   insert:

   >     template<Callable<auto, double> Func>
   >      requires Convertible<Func::result_type, double>
   >     discrete_distribution(result_type nf, double xmin, double xmax, Func fw);

2. Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::

   >     template<Callable<auto, double> Func>
   >      requires Convertible<Func::result_type, double>
   >     discrete_distribution(result_type nf, double xmin, double xmax, Func fw);

   > Complexity: Exactly nf invocations of fw.

   > Requires:

   > 1. If nf > 0, the relation `xmin` < `xmax` shall hold, and for all sample values `xk`, fw(`xk`) shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > 2. The following relations shall hold: nf ≥ 0, and 0 < S = `w0`+. . .+`wn-1`.

   > Effects:

   > 1. If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value `w0` = 1.

   > 2. Otherwise, sets n = nf, deltax = (`xmax` - `xmin`)/n and `xcent` = `xmin` + 0.5 * deltax.

   >    For each k = 0, . . . ,n-1, calculates:

   >    `xk` = `xcent` + k * deltax `wk` = fw(`xk`)

   > 3. Constructs a discrete_distribution object with probabilities:

   >    > `pk` = `wk`/S for k = 0, . . . , n-1.

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 794(i). `piecewise_constant_distribution` missing constructor

Section: 29.5.9.6.2 [rand.dist.samp.pconst] Status: Resolved Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.dist.samp.pconst].

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Discussion:

`piecewise_constant_distribution` should have a constructor like:

>     template<class _Fn>
>        piecewise_constant_distribution(size_t _Count,
>                 _Ty _Low, _Ty _High, _Fn& _Func);

(Makes it easier to fill a histogram with function values over a range. The two (reference 793(i)) make a sensible replacement for `general_pdf_distribution`.)

[ Sophia Antipolis: ]

> Marc: uses variable width of bins and weight for each bin. This is not giving enough flexibility to control both variables.

> Add a library issue to provide an constructor taking an `initializer_list<double>` and `_Fn` for `piecewise_constant_distribution`.

> Daniel to draft wording.

[ Pre San Francisco, Daniel provided wording. ]

> The here proposed changes of the WP refer to the current state of N2691. For reasons explained in 793(i), the author decided to propose a function argument that is provided by value. The issue proposes a c'tor signature, that does not take advantage of the full flexibility of `piecewise_constant_distribution`, because it restricts on a constant bin width, but the use-case seems to be popular enough to justify it's introduction.

Proposed resolution:

Non-concept version of the proposed resolution

1. In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class `piecewise_constant_distribution`, just before the member declaration

   >     explicit piecewise_constant_distribution(const param_type& parm);

   insert:

   >     template<typename Func>
   >     piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);

2. Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:

   >     template<typename Func>
   >     piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);

   > > [p5_1] Complexity: Exactly `nf` invocations of `fw`.

   > > [p5_2] Requires:

   > > 1. `fw` shall be callable with one argument of type `RealType`, and shall return values of a type convertible to double;
   > > 2. For all sample values `xk` defined below, fw(`xk`) shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > > 3. The following relations shall hold: `xmin` < `xmax`, and 0 < S = `w0`+. . .+`wn-1`.

   > > [p5_3] Effects:

   > > 1. If nf == 0,

   > >    1. sets deltax = `xmax` - `xmin`, and
   > >    2. lets the sequence `w` have length `n = 1` and consist of the single value `w0` = 1, and
   > >    3. lets the sequence `b` have length `n+1` with `b0` = `xmin` and `b1` = `xmax`

   > > 2. Otherwise,

   > >    1. sets `n = nf`, `deltax =` (`xmax` - `xmin`)/n, `xcent` = `xmin` + 0.5 * deltax, and

   > >    2. lets the sequences `w` and `b` have length `n` and `n+1`, resp. and

   > >       for each k = 0, . . . ,n-1, calculates:

   > >       `dxk` = k * deltax `bk` = `xmin` + `dxk` `xk` = `xcent` + `dxk` `wk` = fw(`xk`),

   > >       and

   > >    3. sets `bn` = `xmax`

   > > 3. Constructs a `piecewise_constant_distribution` object with the above computed sequence `b` as the interval boundaries and with the probability densities:

   > >    > `ρk` = `wk`/(S * deltax) for k = 0, . . . , n-1.

   > > [p5_4] [Note: In this context, the subintervals [`bk`, `bk+1`) are commonly known as the bins of a histogram. -- end note]

Concept version of the proposed resolution

1. In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class `piecewise_constant_distribution`, just before the member declaration

   >     explicit piecewise_constant_distribution(const param_type& parm);

   insert:

   >     template<Callable<auto, RealType> Func>
   >      requires Convertible<Func::result_type, double>
   >     piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);

2. Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:

   >     template<Callable<auto, RealType> Func>
   >      requires Convertible<Func::result_type, double>
   >     piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);

   > > [p5_1] Complexity: Exactly `nf` invocations of `fw`.

   > > [p5_2] Requires:

   > > 1. For all sample values `xk` defined below, fw(`xk`) shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > > 2. The following relations shall hold: `xmin` < `xmax`, and 0 < S = `w0`+. . .+`wn-1`.

   > > [p5_3] Effects:

   > > 1. If nf == 0,

   > >    1. sets deltax = `xmax` - `xmin`, and
   > >    2. lets the sequence `w` have length `n = 1` and consist of the single value `w0` = 1, and
   > >    3. lets the sequence `b` have length `n+1` with `b0` = `xmin` and `b1` = `xmax`

   > > 2. Otherwise,

   > >    1. sets `n = nf`, `deltax =` (`xmax` - `xmin`)/n, `xcent` = `xmin` + 0.5 * deltax, and

   > >    2. lets the sequences `w` and `b` have length `n` and `n+1`, resp. and

   > >       for each k = 0, . . . ,n-1, calculates: `dxk` = k * deltax `bk` = `xmin` + `dxk` `xk` = `xcent` + `dxk` `wk` = fw(`xk`),

   > >       and

   > >    3. sets `bn` = `xmax`

   > > 3. Constructs a `piecewise_constant_distribution` object with the above computed sequence `b` as the interval boundaries and with the probability densities:

   > >    > `ρk` = `wk`/(S * deltax) for k = 0, . . . , n-1.

   > > [p5_4] [Note: In this context, the subintervals [`bk`, `bk+1`) are commonly known as the bins of a histogram. -- end note]

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 798(i). Refactoring of binders lead to interface breakage

Section: 99 [depr.lib.binders] Status: CD1 Submitter: Daniel Krügler Opened: 2008-02-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [depr.lib.binders].

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Discussion:

N2521 and its earlier predecessors have moved the old binders from [lib.binders] to 99 [depr.lib.binders] thereby introducing some renaming of the template parameter names (`Operation -> Fn`). During this renaming process the protected data member `op` was also renamed to `fn`, which seems as an unnecessary interface breakage to me - even if this user access point is probably rarely used.

Proposed resolution:

Change [depr.lib.binder.1st]:

>     template <class Fn>
>     class binder1st
>       : public unary_function<typename Fn::second_argument_type,
>                               typename Fn::result_type> {
>     protected:
>       Fn fn op;
>       typename Fn::first_argument_type value;
>     public:
>       binder1st(const Fn& x,
>                 const typename Fn::first_argument_type& y);
>       typename Fn::result_type
>         operator()(const typename Fn::second_argument_type& x) const;
>       typename Fn::result_type
>         operator()(typename Fn::second_argument_type& x) const;
>     };

> > -1- The constructor initializes `fn` `op` with `x` and `value` with `y`.

> > -2- `operator()` returns `fnop(value,x)`.

Change [depr.lib.binder.2nd]:

>     template <class Fn>
>     class binder2nd
>       : public unary_function<typename Fn::first_argument_type,
>                               typename Fn::result_type> {
>     protected:
>       Fn fn op;
>       typename Fn::second_argument_type value;
>     public:
>       binder2nd(const Fn& x,
>                 const typename Fn::second_argument_type& y);
>       typename Fn::result_type
>         operator()(const typename Fn::first_argument_type& x) const;
>       typename Fn::result_type
>         operator()(typename Fn::first_argument_type& x) const;
>     };

> > -1- The constructor initializes `fn` `op` with `x` and `value` with `y`.

> > -2- `operator()` returns `fnop(value,x)`.

### 800(i). Issues in 26.4.7.1 [rand.util.seedseq](6)

Section: 29.5.8.1 [rand.util.seedseq] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2008-02-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.util.seedseq].

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Discussion:

The for-loop in the algorithm specification has `n` iterations, where `n` is defined to be `end - begin`, i.e. the number of supplied w-bit quantities. Previous versions of this algorithm and the general logic behind it suggest that this is an oversight and that in the context of the for-loop `n` should be the number of full 32-bit quantities in `b` (rounded upwards). If `w` is 64, the current algorithm throws away half of all bits in `b`. If `w` is 16, the current algorithm sets half of all elements in `v` to 0.

There are two more minor issues:

- Strictly speaking `end - begin` is not defined since `InputIterator` is not required to be a random access iterator.
- Currently all integral types are allowed as input to the `seed_seq` constructor, including `bool`. IMHO allowing `bool`s unnecessarily complicates the implementation without any real benefit to the user. I'd suggest to exclude `bool`s as input.

[ Bellevue: ]

> Move to Open: Bill will try to propose a resolution by the next meeting.

[ post Bellevue: Bill provided wording. ]

This issue is made moot if 803(i) is accepted.

Proposed resolution:

Replace 29.5.8.1 [rand.util.seedseq] paragraph 6 with:

> Effects: Constructs a `seed_seq` object by effectively concatenating the low-order `u` bits of each of the elements of the supplied sequence `[begin, end)` in ascending order of significance to make a (possibly very large) unsigned binary number `b` having a total of `n` bits, and then carrying out the following algorithm:

> > for( v.clear(); n > 0; n -= 32 ) v.push_back(b mod 232), b /= 232;

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 801(i). `tuple` and `pair` trivial members

Section: 22.4 [tuple] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-02-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Classes with trivial special member functions are inherently more efficient than classes without such functions. This efficiency is particularly pronounced on modern ABIs that can pass small classes in registers. Examples include value classes such as complex numbers and floating-point intervals. Perhaps more important, though, are classes that are simple collections, like `pair` and `tuple`. When the parameter types of these classes are trivial, the `pair`s and `tuple`s themselves can be trivial, leading to substantial performance wins.

The current working draft make specification of trivial functions (where possible) much easer through `default`ed and `delete`d functions. As long as the semantics of defaulted and deleted functions match the intended semantics, specification of defaulted and deleted functions will yield more efficient programs.

There are at least two cases where specification of an explicitly defaulted function may be desirable.

First, the `std::pair` template has a non-trivial default constructor, which prevents static initialization of the pair even when the types are statically initializable. Changing the definition to

>     pair() = default;

would enable such initialization. Unfortunately, the change is not semantically neutral in that the current definition effectively forces value initialization whereas the change would not value initialize in some contexts.

** Does the committee confirm that forced value initialization was the intent? If not, does the committee wish to change the behavior of `std::pair` in C++0x?

Second, the same default constructor issue applies to `std::tuple`. Furthermore, the `tuple` copy constructor is current non-trivial, which effectively prevents passing it in registers. To enable passing `tuples` in registers, the copy constructor should be make explicitly `default`ed. The new declarations are:

>     tuple() = default;
>     tuple(const tuple&) = default;

This changes is not implementation neutral. In particular, it prevents implementations based on pointers to the parameter types. It does however, permit implementations using the parameter types as bases.

** How does the committee wish to trade implementation efficiency versus implementation flexibility?

[ Bellevue: ]

> General agreement; the first half of the issue is NAD.

> Before voting on the second half, it was agreed that a "Strongly Favor" vote meant support for trivial tuples (assuming usual requirements met), even at the expense of other desired qualities. A "Weakly Favor" vote meant support only if not at the expense of other desired qualities.

> Concensus: Go forward, but not at expense of other desired qualities.

> It was agreed to Alisdair should fold this work in with his other pair/tuple action items, above, and that issue 801 should be "open", but tabled until Alisdair's proposals are disposed of.

[ 2009-05-27 Daniel adds: ]

> This is partly solved by 1117(i).

[ 2009-07 Frankfurt: ]

> Wait for dust to settle from fixing exception safety problem with rvalue refs.

[ 2009-07-20 Alisdair adds: ]

> Basically, this issue is what should we do with the default constructor for pairs and tuples of trivial types. The motivation of the issue was to force static initialization rather than dynamic initialization, and was rejected in the case of pair as it would change the meaning of existing programs. The advice was "do the best we can" for tuple without changing existing meaning.

> Frankfurt seems to simply wait and see the resolution on no-throw move constructors, which (I believe) is only tangentially related to this issue, but as good as any to defer until Santa Cruz.

> Looking again now, I think constant (static) initialization for pair can be salvaged by making the default construct constexpr. I have a clarification from Core that this is intended to work, even if the constructor is not trivial/constexpr, so long as no temporaries are implied in the process (even if elided).

[ 2009-10 Santa Cruz: ]

> Leave as open. Alisdair to provide wording.

[ 2010 Pittsburgh: ]

> We believe this may be NAD Editorial since both pair and tuple now have constexpr default constructors, but we're not sure.

[ 2010 Rapperswil: ]

> Daniel believes his pair/tuple paper will resolve this issue. `constexpr` will allow static initialization, and he is already changing the move and copy constructors to be defaulted.

[ 2010-10-24 Daniel adds: ]

> The proposed resolution of n3140 should resolve this issue.

Proposed resolution:

See n3140.

### 803(i). Simplification of `seed_seq::seq_seq`

Section: 29.5.8.1 [rand.util.seedseq] Status: Resolved Submitter: Charles Karney Opened: 2008-02-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`seed_seq(InputIterator begin, InputIterator end);` constructs a `seed_seq` object repacking the bits of supplied sequence `[begin, end)` into a 32-bit vector.

This repacking triggers several problems:

1. Distinctness of the output of `seed_seq::generate` required the introduction of the initial "`if (w < 32) v.push_back(n);`" (Otherwise the unsigned short vectors [1, 0] and [1] generate the same sequence.)
2. Portability demanded the introduction of the template parameter `u`. (Otherwise some sequences could not be obtained on computers where no integer types are exactly 32-bits wide.)
3. The description and algorithm have become unduly complicated.

I propose simplifying this `seed_seq` constructor to be "32-bit only". Despite it's being simpler, there is NO loss of functionality (see below).

Here's how the description would read

> 29.5.8.1 [rand.util.seedseq] Class `seed_seq`

> >     template<class InputIterator>
> >       seed_seq(InputIterator begin, InputIterator end);

> > > 5 Requires: NO CHANGE

> > > 6 Effects: Constructs a `seed_seq` object by

> > > > for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);

Discussion:

The chief virtues here are simplicity, portability, and generality.

- Simplicity — compare the above specification with the n2461 proposal.
- Portability — with `iterator_traits<InputIterator>::value_type = uint_least32_t` the user is guaranteed to get the same behavior across platforms.
- Generality — any behavior that the n2461 proposal can achieve can be obtained with this simpler proposal (albeit with a shuffling of bits in the input sequence).

Arguments (and counter-arguments) against making this change (and retaining the n2461 behavior) are:

- The user can pass an array of `unsigned char` and `seed_seq` will nicely repack it.

  Response: So what? Consider the seed string "ABC". The n2461 proposal results in

  >     v = { 0x3, 0x434241 };

  while the simplified proposal yields

  >     v = { 0x41, 0x42, 0x43 };

  The results produced by `seed_seq::generate` with the two inputs are different but nevertheless equivalently "mixed up" and this remains true even if the seed string is long.

- With long strings (e.g., with bit-length comparable to the number of bits in the state), `v` is longer (by a factor of 4) with the simplified proposal and `seed_seq::generate` will be slower.

  Response: It's unlikely that the efficiency of `seed_seq::generate` will be a big issue. If it is, the user is free to repack the seed vector before constructing `seed_seq`.

- A user can pass an array of 64-bit integers and all the bits will be used.

  Response: Indeed. However, there are many instances in the n2461 where integers are silently coerced to a narrower width and this should just be a case of the user needing to read the documentation. The user can of course get equivalent behavior by repacking his seed into 32-bit pieces. Furthermore, the unportability of the n2461 proposal with

  >     unsigned long s[] = {1, 2, 3, 4};
  >     seed_seq q(s, s+4);

  which typically results in `v = {1, 2, 3, 4}` on 32-bit machines and in `v = {1, 0, 2, 0, 3, 0, 4, 0}` on 64-bit machines is a major pitfall for unsuspecting users.

Note: this proposal renders moot issues 782(i) and 800(i).

[ Bellevue: ]

> Walter needs to ask Fermilab for guidance. Defer till tomorrow. Bill likes the proposed resolution.

[ Sophia Antipolis: ]

> Marc Paterno wants portable behavior between 32bit and 64bit machines; we've gone to significant trouble to support portability of engines and their values.

> Jens: the new algorithm looks perfectly portable

> Marc Paterno to review off-line.

> Modify the proposed resolution to read "Constructs a seed_seq object by the following algorithm ..."

> Disposition: move to review; unanimous consent.

> (moots 782(i) and 800(i))

Proposed resolution:

Change 29.5.8.1 [rand.util.seedseq]:

>     template<class InputIterator,
>       size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
>       seed_seq(InputIterator begin, InputIterator end);

> > -5- Requires: `InputIterator` shall satisfy the requirements of an input iterator (24.1.1) such that `iterator_traits<InputIterator>::value_type` shall denote an integral type.

> > -6- Constructs a `seed_seq` object by the following algorithm rearranging some or all of the bits of the supplied sequence `[begin,end)` of w-bit quantities into 32-bit units, as if by the following:

> > First extract the rightmost `u` bits from each of the `n = end - begin` elements of the supplied sequence and concatenate all the extracted bits to initialize a single (possibly very large) unsigned binary number, `b = ∑n-1i=0 (begin[i] mod 2u) · 2w·i` (in which the bits of each `begin[i]` are treated as denoting an unsigned quantity). Then carry out the following algorithm:

> > > v.clear(); if ($w$ < 32) v.push_back($n$); for( ; $n$ > 0; --$n$) v.push_back(b mod 232), b /= 232;

> > > for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 804(i). Some problems with classes `error_code`/`error_condition`

Section: 19.5 [syserr] Status: CD1 Submitter: Daniel Krügler Opened: 2008-02-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

1. 19.5.4.1 [syserr.errcode.overview]/1, class `error_code` and 19.5.5.1 [syserr.errcondition.overview]/, class `error_condition` synopses declare an expository data member `cat_`:

   >     const error_category& cat_; // exposition only

   which is used to define the semantics of several members. The decision to use a member of reference type lead to several problems:

   1. The classes are not `(Copy)Assignable`, which is probably not the intent.
   2. The post conditions of all modifiers from 19.5.4.3 [syserr.errcode.modifiers] and 19.5.5.3 [syserr.errcondition.modifiers], resp., cannot be fulfilled.

   The simple fix would be to replace the reference by a pointer member.

2. I would like to give the editorial remark that in both classes the constrained `operator=` overload (template with `ErrorCodeEnum` argument) makes in invalid usage of `std::enable_if`: By using the default value for the second `enable_if` parameter the return type would be defined to be `void&` even in otherwise valid circumstances - this return type must be explicitly provided (In `error_condition` the first declaration uses an explicit value, but of wrong type).
3. The member function `message` throws clauses ( 19.5.3.2 [syserr.errcat.virtuals]/10, 19.5.4.4 [syserr.errcode.observers]/8, and 19.5.5.4 [syserr.errcondition.observers]/6) guarantee "throws nothing", although they return a `std::string` by value, which might throw in out-of-memory conditions (see related issue 771(i)).

[ Sophia Antipolis: ]

> Part A: NAD (editorial), cleared by the resolution of issue 832(i).

> Part B: Technically correct, save for typo. Rendered moot by the concept proposal (N2620) NAD (editorial).

> Part C: We agree; this is consistent with the resolution of issue 721(i).

> Howard: please ping Beman, asking him to clear away parts A and B from the wording in the proposed resolution, so it is clear to the editor what needs to be applied to the working paper.

> Beman provided updated wording. Since issue 832(i) is not going forward, the provided wording includes resolution of part A.

Proposed resolution:

Resolution of part A:

> Change 19.5.4.1 [syserr.errcode.overview] Class error_code overview synopsis as indicated:

> >     private:
> >       int val_;                    // exposition only
> >       const error_category&* cat_; // exposition only

> Change 19.5.4.2 [syserr.errcode.constructors] Class error_code constructors as indicated:

> >     error_code();

> > > Effects: Constructs an object of type `error_code`.

> > > Postconditions: `val_ == 0` and `cat_ == &system_category`.

> > > Throws: Nothing.

> >     error_code(int val, const error_category& cat);

> > > Effects: Constructs an object of type `error_code`.

> > > Postconditions: `val_ == val` and `cat_ == &cat`.

> > > Throws: Nothing.

> Change 19.5.4.3 [syserr.errcode.modifiers] Class error_code modifiers as indicated:

> >     void assign(int val, const error_category& cat);

> > > Postconditions: `val_ == val` and `cat_ == &cat`.

> > > Throws: Nothing.

> Change 19.5.4.4 [syserr.errcode.observers] Class error_code observers as indicated:

> >     const error_category& category() const;

> > > Returns: `*cat_`.

> > > Throws: Nothing.

> Change 19.5.5.1 [syserr.errcondition.overview] Class error_condition overview synopsis as indicated:

> >     private:
> >       int val_;                    // exposition only
> >       const error_category&* cat_; // exposition only

> Change 19.5.5.2 [syserr.errcondition.constructors] Class error_condition constructors as indicated:

> [ (If the proposed resolution of issue 805(i) has already been applied, the name `posix_category` will have been changed to `generic_category`. That has no effect on this resolution.) ]

> >     error_condition();

> > > Effects: Constructs an object of type `error_condition`.

> > > Postconditions: `val_ == 0` and `cat_ == &posix_category`.

> > > Throws: Nothing.

> >     error_condition(int val, const error_category& cat);

> > > Effects: Constructs an object of type `error_condition`.

> > > Postconditions: `val_ == val` and `cat_ == &cat`.

> > > Throws: Nothing.

> Change 19.5.5.3 [syserr.errcondition.modifiers] Class error_condition modifiers as indicated:

> >     void assign(int val, const error_category& cat);

> > > Postconditions: `val_ == val` and `cat_ == &cat`.

> > > Throws: Nothing.

> Change 19.5.5.4 [syserr.errcondition.observers] Class error_condition observers as indicated:

> >     const error_category& category() const;

> > > Returns: `*cat_`.

> > > Throws: Nothing.

Resolution of part C:

> In 19.5.3.2 [syserr.errcat.virtuals], remove the throws clause p. 10.

> >     virtual string message(int ev) const = 0;

> > > Returns: A string that describes the error condition denoted by `ev`.

> > > Throws: Nothing.

> In 19.5.4.4 [syserr.errcode.observers], remove the throws clause p. 8.

> >     string message() const;

> > > Returns: `category().message(value())`.

> > > Throws: Nothing.

> In 19.5.5.4 [syserr.errcondition.observers], remove the throws clause p. 6.

> >     string message() const;

> > > Returns: `category().message(value())`.

> > > Throws: Nothing.

### 805(i). `posix_error::posix_errno` concerns

Section: 19.5 [syserr] Status: CD1 Submitter: Jens Maurer Opened: 2008-02-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

19.5 [syserr]

>     namespace posix_error {
>       enum posix_errno {
>         address_family_not_supported, // EAFNOSUPPORT
>         ...

should rather use the new scoped-enum facility (9.8.1 [dcl.enum]), which would avoid the necessity for a new `posix_error` namespace, if I understand correctly.

[ Further discussion: ]

> See N2347, Strongly Typed Enums, since renamed Scoped Enums.

> Alberto Ganesh Barbati also raised this issue in private email, and also proposed the scoped-enum solution.

> Nick Stoughton asked in Bellevue that `posix_error` and `posix_errno` not be used as names. The LWG agreed.

> The wording for the Proposed resolution was provided by Beman Dawes.

Proposed resolution:

Change System error support 19.5 [syserr] as indicated:

>     namespace posix_error {
>       enum posix_errno class errc {
>         address_family_not_supported, // EAFNOSUPPORT
>         ...
>         wrong_protocol_type, // EPROTOTYPE
>       };
>     } // namespace posix_error

>     template <> struct is_error_condition_enum<posix_error::posix_errno errc>
>       : public true_type {}

>     namespace posix_error {
>       error_code make_error_code(posix_errno errc e);
>       error_condition make_error_condition(posix_errno errc e);
>     } // namespace posix_error

Change System error support 19.5 [syserr] :

> The `is_error_code_enum` and `is_error_condition_enum` templates may be specialized for user-defined types to indicate that such a type is eligible for class `error_code` and class `error_condition` automatic conversions, respectively.

Change System error support 19.5 [syserr] and its subsections:

> - remove all occurrences of `posix_error::`
> - change all instances of `posix_errno` to `errc`
> - change all instances of `posix_category` to `generic_category`
> - change all instances of `get_posix_category` to `get_generic_category`

Change Error category objects 19.5.3.5 [syserr.errcat.objects], paragraph 2:

> Remarks: The object's `default_error_condition` and equivalent virtual functions shall behave as specified for the class `error_category`. The object's name virtual function shall return a pointer to the string "POSIX" "generic".

Change 19.5.4.5 [syserr.errcode.nonmembers] Class `error_code` non-member functions as indicated:

>     error_code make_error_code(posix_errno errc e);

> > Returns: `error_code(static_cast<int>(e), posixgeneric_category)`.

Change 19.5.5.5 [syserr.errcondition.nonmembers] Class `error_condition` non-member functions as indicated:

>     error_condition make_error_condition(posix_errno errc e);

> > Returns: `error_condition(static_cast<int>(e), posixgeneric_category)`.

Rationale:

Names Considered
`portable` | Too non-specific. Did not wish to reserve such a common word in namespace std. Not quite the right meaning, either.
`portable_error` | Too long. Explicit qualification is always required for scoped enums, so a short name is desirable. Not quite the right meaning, either. May be misleading because `*_error` in the std lib is usually an exception class name.
`std_error` | Fairly short, yet explicit. But in fully qualified names like `std::std_error::not_enough_memory`, the std_ would be unfortunate. Not quite the right meaning, either. May be misleading because `*_error` in the std lib is usually an exception class name.
`generic` | Short enough. The category could be `generic_category`. Fully qualified names like `std::generic::not_enough_memory` read well. Reserving in namespace std seems dicey.
`generic_error` | Longish. The category could be `generic_category`. Fully qualified names like `std::generic_error::not_enough_memory` read well. Misleading because `*_error` in the std lib is usually an exception class name.
`generic_err` | A bit less longish. The category could be `generic_category`. Fully qualified names like `std::generic_err::not_enough_memory` read well.
`gen_err` | Shorter still. The category could be `generic_category`. Fully qualified names like `std::gen_err::not_enough_memory` read well.
`generr` | Shorter still. The category could be `generic_category`. Fully qualified names like `std::generr::not_enough_memory` read well.
`error` | Shorter still. The category could be `generic_category`. Fully qualified names like `std::error::not_enough_memory` read well. Do we want to use this general a name?
`err` | Shorter still. The category could be `generic_category`. Fully qualified names like `std::err::not_enough_memory` read well. Although alone it looks odd as a name, given the existing `errno` and `namespace std` names, it seems fairly intuitive. Problem: `err` is used throughout the standard library as an argument name and in examples as a variable name; it seems too confusing to add yet another use of the name.
`errc` | Short enough. The "c" stands for "constant". The category could be `generic_category`. Fully qualified names like `std::errc::not_enough_memory` read well. Although alone it looks odd as a name, given the existing `errno` and `namespace std` names, it seems fairly intuitive. There are no uses of `errc` in the current C++ standard.

### 806(i). `unique_ptr::reset` effects incorrect, too permissive

Section: 20.3.1.3.6 [unique.ptr.single.modifiers] Status: CD1 Submitter: Peter Dimov Opened: 2008-03-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`void unique_ptr::reset(T* p = 0)` is currently specified as:

> Effects: If `p == get()` there are no effects. Otherwise `get_deleter()(get())`.

There are two problems with this. One, if `get() == 0` and `p != 0`, the deleter is called with a `NULL` pointer, and this is probably not what's intended (the destructor avoids calling the deleter with 0.)

Two, the special check for `get() == p` is generally not needed and such a situation usually indicates an error in the client code, which is being masked. As a data point, `boost::shared_ptr` was changed to assert on such self-resets in 2001 and there were no complaints.

One might think that self-resets are necessary for `operator=` to work; it's specified to perform

>     reset( u.release() );

and the self-assignment

>     p = move(p);

might appear to result in a self-reset. But it doesn't; the `release()` is performed first, zeroing the stored pointer. In other words, `p.reset( q.release() )` works even when `p` and `q` are the same `unique_ptr`, and there is no need to special-case `p.reset( q.get() )` to work in a similar scenario, as it definitely doesn't when `p` and `q` are separate.

Proposed resolution:

Change 20.3.1.3.6 [unique.ptr.single.modifiers]:

>     void reset(T* p = 0);

> > -4- Effects: If `p == get() == 0` there are no effects. Otherwise `get_deleter()(get())`.

Change 20.3.1.4.5 [unique.ptr.runtime.modifiers]:

>     void reset(T* p = 0);

> > ...

> > -2- Effects: If `p == get() == 0` there are no effects. Otherwise `get_deleter()(get())`.

### 807(i). `tuple` construction should not fail unless its element's construction fails

Section: 22.4.4.2 [tuple.cnstr] Status: CD1 Submitter: Howard Hinnant Opened: 2008-03-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

527(i) Added a throws clause to `bind` constructors. I believe the same throws clause should be added to `tuple` except it ought to take into account move constructors as well.

Proposed resolution:

Add to 22.4.4.2 [tuple.cnstr]:

> For each `tuple` constructor and assignment operator, an exception is thrown only if the construction or assignment of one of the types in `Types` throws an exception.

### 808(i). §[forward] incorrect redundant specification

Section: 22.2.4 [forward] Status: CD1 Submitter: Jens Maurer Opened: 2008-03-13 Last modified: 2016-02-01

Priority: Not Prioritized

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Discussion:

p4 (forward) says:

> Return type: If `T` is an lvalue-reference type, an lvalue; otherwise, an rvalue.

First of all, lvalue-ness and rvalue-ness are properties of an expression, not of a type (see 7.2.1 [basic.lval]). Thus, the phrasing "Return type" is wrong. Second, the phrase says exactly what the core language wording says for folding references in 13.4.2 [temp.arg.type]/p4 and for function return values in 7.6.1.3 [expr.call]/p10. (If we feel the wording should be retained, it should at most be a note with cross-references to those sections.)

The prose after the example talks about "forwarding as an `int&` (an lvalue)" etc. In my opinion, this is a category error: "`int&`" is a type, "lvalue" is a property of an expression, orthogonal to its type. (Btw, expressions cannot have reference type, ever.)

Similar with move:

> Return type: an rvalue.

is just wrong and also redundant.

Proposed resolution:

Change 22.2.4 [forward] as indicated:

>     template <class T> T&& forward(typename identity<T>::type&& t);

> > ...

> > Return type: If `T` is an lvalue-reference type, an lvalue; otherwise, an rvalue.

> > ...

> > -7- In the first call to `factory`, `A1` is deduced as `int`, so 2 is forwarded to `A`'s constructor as an `int&&` (an rvalue). In the second call to factory, `A1` is deduced as `int&`, so `i` is forwarded to `A`'s constructor as an `int&` (an lvalue). In both cases, `A2` is deduced as double, so 1.414 is forwarded to `A`'s constructor as `double&&` (an rvalue).

>     template <class T> typename remove_reference<T>::type&& move(T&& t);

> > ...

> > Return type: an rvalue.

### 809(i). `std::swap` should be overloaded for array types

Section: 26.7.3 [alg.swap] Status: CD1 Submitter: Niels Dekker Opened: 2008-02-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

For the sake of generic programming, the header `<algorithm>` should provide an overload of `std::swap` for array types:

    template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);

It became apparent to me that this overload is missing, when I considered how to write a swap function for a generic wrapper class template. (Actually I was thinking of Boost's value_initialized.) Please look at the following template, `W`, and suppose that is intended to be a very generic wrapper:

    template<class T> class W {
    public:
      T data;
    };

Clearly `W<T>` is CopyConstructible and CopyAssignable, and therefore Swappable, whenever `T` is CopyConstructible and CopyAssignable. Moreover, `W<T>` is also Swappable when `T` is an array type whose element type is CopyConstructible and CopyAssignable. Still it is recommended to add a custom swap function template to such a class template, for the sake of efficiency and exception safety. (E.g., Scott Meyers, Effective C++, Third Edition, item 25: Consider support for a non-throwing swap.) This function template is typically written as follows:

    template<class T> void swap(W<T>& x, W<T>& y) {
      using std::swap;
      swap(x.data, y.data);
    }

Unfortunately, this will introduce an undesirable inconsistency, when `T` is an array. For instance, `W<std::string[8]>` is Swappable, but the current Standard does not allow calling the custom swap function that was especially written for `W`!

    W<std::string[8]> w1, w2;  // Two objects of a Swappable type.
    std::swap(w1, w2);  // Well-defined, but inefficient.
    using std::swap;
    swap(w1, w2);  // Ill-formed, just because ADL finds W's swap function!!!

`W`'s `swap` function would try to call `std::swap` for an array, `std::string[8]`, which is not supported by the Standard Library. This issue is easily solved by providing an overload of `std::swap` for array types. This swap function should be implemented in terms of swapping the elements of the arrays, so that it would be non-throwing for arrays whose element types have a non-throwing swap.

Note that such an overload of `std::swap` should also support multi-dimensional arrays. Fortunately that isn't really an issue, because it would do so automatically, by means of recursion.

For your information, there was a discussion on this issue at comp.lang.c++.moderated: [Standard Library] Shouldn't std::swap be overloaded for C-style arrays?

Proposed resolution:

Add an extra condition to the definition of Swappable requirements [swappable] in 16.4.4.2 [utility.arg.requirements]:

> - `T` is `Swappable` if `T` is an array type whose element type is `Swappable`.

Add the following to 26.7.3 [alg.swap]:

>     template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);

> > Requires: Type `T` shall be `Swappable`.

> > Effects: `swap_ranges(a, a + N, b);`

### 810(i). Missing traits dependencies in operational semantics of extended manipulators

Section: 31.7.8 [ext.manip] Status: C++11 Submitter: Daniel Krügler Opened: 2008-03-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ext.manip].

View all issues with C++11 status.

Discussion:

The recent draft (as well as the original proposal n2072) uses an operational semantic for `get_money` ([ext.manip]/4) and `put_money` ([ext.manip]/6), which uses

>     istreambuf_iterator<charT>

and

>     ostreambuf_iterator<charT>

resp, instead of the iterator instances, with explicitly provided traits argument (The operational semantic defined by `f` is also traits dependent). This is an obvious oversight because both `*stream_buf` c'tors expect a `basic_streambuf<charT,traits>` as argument.

The same problem occurs within the `get_time` and `put_time` semantic where additional to the problem we have an editorial issue in `get_time` (`streambuf_iterator` instead of `istreambuf_iterator`).

[ Batavia (2009-05): ]

> This appears to be an issue of presentation.

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 31.7.8 [ext.manip]/4 within function `f` replace the first line

>     template <class charT, class traits, class moneyT>
>     void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) {
>        typedef istreambuf_iterator<charT, traits> Iter;
>        ...

In 31.7.8 [ext.manip]/5 remove the first template `charT` parameter:

>     template <class charT, class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);

In 31.7.8 [ext.manip]/6 within function `f` replace the first line

>     template <class charT, class traits, class moneyT>
>     void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) {
>       typedef ostreambuf_iterator<charT, traits> Iter;
>       ...

In 31.7.8 [ext.manip]/8 within function `f` replace the first line

>     template <class charT, class traits>
>     void f(basic_ios<charT, traits>& str, struct tm *tmb, const charT *fmt) {
>       typedef istreambuf_iterator<charT, traits> Iter;
>       ...

In 31.7.8 [ext.manip]/10 within function `f` replace the first line

>     template <class charT, class traits>
>     void f(basic_ios<charT, traits>& str, const struct tm *tmb, const charT *fmt) {
>       typedef ostreambuf_iterator<charT, traits> Iter;
>       ...

In 31.7 [iostream.format], Header `<iomanip>` synopsis change:

>     template <class charT, class moneyT> T8 put_money(const moneyT& mon, bool intl = false);

### 811(i). `pair` of pointers no longer works with literal 0

Section: 22.3 [pairs] Status: C++11 Submitter: Doug Gregor Opened: 2008-03-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [pairs].

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Discussion:

>     #include <utility>

>     int main()
>     {
>        std::pair<char *, char *> p (0,0);
>     }

I just got a bug report about that, because it's valid C++03, but not C++0x. The important realization, for me, is that the emplace proposal---which made `push_back` variadic, causing the `push_back(0)` issue---didn't cause this break in backward compatibility. The break actually happened when we added this pair constructor as part of adding rvalue references into the language, long before variadic templates or emplace came along:

>     template<class U, class V> pair(U&& x, V&& y);

Now, concepts will address this issue by constraining that `pair` constructor to only `U`'s and `V`'s that can properly construct "first" and "second", e.g. (from N2322):

>     template<class U , class V >
>     requires Constructible<T1, U&&> && Constructible<T2, V&&>
>     pair(U&& x , V&& y );

[ San Francisco: ]

> Suggested to resolve using pass-by-value for that case.

> Side question: Should pair interoperate with tuples? Can construct a tuple of a pair, but not a pair from a two-element tuple.

> Related to 885(i).

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Leave as open. Howard to provide wording.

[ 2010-02-06 Howard provided wording. ]

[ 2010-02-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Rationale:

[ San Francisco: ]

> Solved by N2770.

[ The rationale is obsolete. ]

Proposed resolution:

Add a paragraph to 22.3 [pairs]:

>     template<class U, class V> pair(U&& x, V&& y);

> > 6 Effects: The constructor initializes `first` with `std::forward<U>(x)` and second with `std::forward<V>(y)`.

> > Remarks: `U` shall be implicitly convertible to `first_type` and `V` shall be implicitly convertible to `second_type`, else this constructor shall not participate in overload resolution.

### 813(i). "empty" undefined for `shared_ptr`

Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Matt Austern Opened: 2008-02-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.smartptr.shared].

View all issues with CD1 status.

Discussion:

Several places in 20.3.2.2 [util.smartptr.shared] refer to an "empty" `shared_ptr`. However, that term is nowhere defined. The closest thing we have to a definition is that the default constructor creates an empty `shared_ptr` and that a copy of a default-constructed `shared_ptr` is empty. Are any other `shared_ptr`s empty? For example, is `shared_ptr((T*) 0)` empty? What are the properties of an empty `shared_ptr`? We should either clarify this term or stop using it.

One reason it's not good enough to leave this term up to the reader's intuition is that, in light of N2351 and issue 711(i), most readers' intuitive understanding is likely to be wrong. Intuitively one might expect that an empty `shared_ptr` is one that doesn't store a pointer, but, whatever the definition is, that isn't it.

[ Peter adds: ]

> Or, what is an "empty" `shared_ptr`?

> - Are any other `shared_ptrs` empty?

>   Yes. Whether a given `shared_ptr` instance is empty or not is (*) completely specified by the last mutating operation on that instance. Give me an example and I'll tell you whether the `shared_ptr` is empty or not.

>   > (*) If it isn't, this is a legitimate defect.

> - For example, is `shared_ptr((T*) 0)` empty?

>   No. If it were empty, it would have a `use_count()` of 0, whereas it is specified to have an `use_count()` of 1.

> - What are the properties of an empty `shared_ptr`?

>   The properties of an empty `shared_ptr` can be derived from the specification. One example is that its destructor is a no-op. Another is that its `use_count()` returns 0. I can enumerate the full list if you really like.

> - We should either clarify this term or stop using it.

>   I don't agree with the imperative tone

>   A clarification would be either a no-op - if it doesn't contradict the existing wording - or a big mistake if it does.

>   I agree that a clarification that is formally a no-op may add value.

> - However, that term is nowhere defined.

>   Terms can be useful without a definition. Consider the following simplistic example. We have a type `X` with the following operations defined:

>   >     X x;
>   >     X x2(x);
>   >     X f(X x);
>   >     X g(X x1, X x2);

>   A default-constructed value is green.
>   A copy has the same color as the original.
>   `f(x)` returns a red value if the argument is green, a green value otherwise.
>   `g(x1,x2)` returns a green value if the arguments are of the same color, a red value otherwise.

>   Given these definitions, you can determine the color of every instance of type `X`, even if you have absolutely no idea what green and red mean.

>   Green and red are "nowhere defined" and completely defined at the same time.

> Alisdair's wording is fine.

Proposed resolution:

Append the following sentance to 20.3.2.2 [util.smartptr.shared]

> The `shared_ptr` class template stores a pointer, usually obtained via `new`. `shared_ptr` implements semantics of shared ownership; the last remaining owner of the pointer is responsible for destroying the object, or otherwise releasing the resources associated with the stored pointer. A `shared_ptr` object that does not own a pointer is said to be empty.

### 814(i). `vector<bool>::swap(reference, reference)` not defined

Section: 23.3.14 [vector.bool] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [vector.bool].

View all issues with C++11 status.

Discussion:

`vector<bool>::swap(reference, reference)` has no definition.

[ San Francisco: ]

> Move to Open. Alisdair to provide a resolution.

[ Post Summit Daniel provided wording. ]

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Just after 23.3.14 [vector.bool]/5 add the following prototype and description:

> static void swap(reference x, reference y);

> > -6- Effects: Exchanges the contents of `x` and `y` as-if by:

> > >     bool b = x;
> > >     x = y;
> > >     y = b;

### 815(i). `std::function` and `reference_closure` do not use perfect forwarding

Section: 22.10.17.3.5 [func.wrap.func.inv] Status: Resolved Submitter: Alisdair Meredith Opened: 2008-03-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.wrap.func.inv].

View all issues with Resolved status.

Discussion:

`std::function` and `reference_closure` should use "perfect forwarding" as described in the rvalue core proposal.

[ Sophia Antipolis: ]

> According to Doug Gregor, as far as `std::function` is concerned, perfect forwarding can not be obtained because of type erasure. Not everyone agreed with this diagnosis of forwarding.

[ 2009-05-01 Howard adds: ]

> Sebastian Gesemann brought to my attention that the `CopyConstructible` requirement on `function`'s `ArgTypes...` is an unnecessary restriction.

> >     template<Returnable R, CopyConstructible... ArgTypes>
> >     class function<R(ArgTypes...)>
> >     ...

> On further investigation, this complaint seemed to be the same issue as this one. I believe the reason `CopyConstructible` was put on `ArgTypes` in the first place was because of the nature of the invoke member:

> >     template<class R, class ...ArgTypes>
> >     R
> >     function<R(ArgTypes...)>::operator()(ArgTypes... arg) const
> >     {
> >         if (f_ == 0)
> >             throw bad_function_call();
> >         return (*f_)(arg...);
> >     }

> However now with rvalue-refs, "by value" no longer implies `CopyConstructible` (as Sebastian correctly points out). If rvalue arguments are supplied, `MoveConstructible` is sufficient. Furthermore, the constraint need not be applied in `function` if I understand correctly. Rather the client must apply the proper constraints at the call site. Therefore, at the very least, I recommend that `CopyConstructible` be removed from the template class `function`.

> Furthermore we need to mandate that the invoker is coded as:

> >     template<class R, class ...ArgTypes>
> >     R
> >     function<R(ArgTypes...)>::operator()(ArgTypes... arg) const
> >     {
> >         if (f_ == 0)
> >             throw bad_function_call();
> >         return (*f_)(std::forward<ArgTypes>(arg)...);
> >     }

> Note that `ArgTypes&&` (the "perfect forwarding signature") is not appropriate here as this is not a deduced context for `ArgTypes`. Instead the client's arguments must implicitly convert to the non-deduced `ArgType` type. Catching these arguments by value makes sense to enable decay.

> Next `forward` is used to move the `ArgTypes` as efficiently as possible, and also with minimum requirements (not `CopyConstructible`) to the type-erased functor. For object types, this will be a `move`. For reference type `ArgTypes`, this will be a copy. The end result must be that the following is a valid program:

> >     #include <functional>
> >     #include <memory>
> >     #include <cassert>

> >     std::unique_ptr<int>
> >     f(std::unique_ptr<int> p, int& i)
> >     {
> >         ++i;
> >         return std::move(p);
> >     }

> >     int main()
> >     {
> >         int i = 2;
> >         std::function<std::unique_ptr<int>(std::unique_ptr<int>,
> >                                            int&> g(f);
> >         std::unique_ptr<int> p = g(std::unique_ptr<int>(new int(1)), i);
> >         assert(*p == 1);
> >         assert(i == 3);
> >     }

> [ Tested in pre-concepts rvalue-ref-enabled compiler. ]

> In the example above, the first `ArgType` is `unique_ptr<int>` and the second `ArgType` is `int&`. Both must work!

[ 2009-05-27 Daniel adds: ]

> in the 2009-05-01 comment of above mentioned issue Howard

> 1. Recommends to replace the `CopyConstructible` requirement by a `MoveConstructible` requirement
> 2. Says: "Furthermore, the constraint need not be applied in `function` if I understand correctly. Rather the client must apply the proper constraints at the call site"

> I'm fine with (a), but I think comment (b) is incorrect, at least in the sense I read these sentences. Let's look at Howard's example code:

> >     function<R(ArgTypes...)>::operator()(ArgTypes... arg) const
> >     {
> >        if (f_ == 0)
> >            throw bad_function_call();
> >        return (*f_)(std::forward<ArgTypes>(arg)...);
> >     }

> In the constrained scope of this `operator()` overload the expression "`(*f_)(std::forward<ArgTypes>(arg)...)`" must be valid. How can it do so, if `ArgTypes` aren't at least `MoveConstructible`?

[ 2009-07 Frankfurt: ]

> Leave this open and wait until concepts are removed from the Working Draft so that we know how to write the proposed resolution in terms of diffs to otherwise stable text.

[ 2009-10 Santa Cruz: ]

> Leave as open. Howard to provide wording. Howard welcomes any help.

[ 2009-12-12 Jonathan Wakely adds: ]

> 22.10.17.3 [func.wrap.func] says

> > 2 A function object `f` of type `F` is Callable for argument types `T1, T2, ..., TN` in `ArgTypes` and a return type `R`, if, given lvalues `t1, t2, ..., tN` of types `T1, T2, ..., TN`, respectively, `INVOKE (f, t1, t2, ..., tN)` is well formed (20.7.2) and, if `R` is not `void`, convertible to `R`.

> N.B. lvalues, which means you can't use `function<R(T&&)>` or `function<R(unique_ptr<T>)>`

> I recently implemented rvalue arguments in GCC's `std::function`, all that was needed was to use `std::forward<ArgTypes>` in a few places. The example in issue 815 works.

> I think 815 could be resolved by removing the requirement that the target function be callable with lvalues. Saying `ArgTypes` need to be `CopyConstructible` is wrong, and IMHO saying `MoveConstructible` is unnecessary, since the by-value signature implies that already, but if it is needed it should only be on `operator()`, not the whole class (you could in theory instantiate `std::function<R(noncopyable)>` as long as you don't invoke the call operator.)

> I think defining invocation in terms of `INVOKE` already implies perfect forwarding, so we don't need to say explicitly that `std::forward` should be used (N.B. the types that are forwarded are those in `ArgTypes`, which can differ from the actual parameter types of the target function. The actual parameter types have gone via type erasure, but that's not a problem - IMHO forwarding the arguments as `ArgTypes` is the right thing to do anyway.)

> Is it sufficient to simply replace "lvalues" with "values"? or do we need to say something like "lvalues when `Ti` is an lvalue-reference and rvalues otherwise"? I prefer the former, so I propose the following resolution for 815:

> Edit 22.10.17.3 [func.wrap.func] paragraph 2:

> > 2 A function object `f` of type `F` is Callable for argument types `T1, T2, ..., TN` in `ArgTypes` and a return type `R`, if, given lvalues `t1, t2, ..., tN` of types `T1, T2, ..., TN`, respectively, `INVOKE (f, t1, t2, ..., tN)` is well formed (20.7.2) and, if `R` is not `void`, convertible to `R`.

[ 2009-12-12 Daniel adds: ]

> I don't like the reduction to "values" and prefer the alternative solution suggested using "lvalues when Ti is an lvalue-reference and rvalues otherwise". The reason why I dislike the shorter version is based on different usages of "values" as part of defining the semantics of requirement tables via expressions. E.g. 16.4.4.2 [utility.arg.requirements]/1 says "`a`, `b`, and `c` are values of type `const T;`" or similar in 23.2.2 [container.requirements.general]/4 or /14 etc. My current reading of all these parts is that both rvalues and lvalues are required to be supported, but this interpretation would violate the intention of the suggested fix of #815, if I correctly understand Jonathan's rationale.

[ 2009-12-12 Howard adds: ]

> > "lvalues when Ti is an lvalue-reference and rvalues otherwise"

> doesn't quite work here because the `Ti` aren't deduced. They are specified by the `function` type. `Ti` might be `const int&` (an lvalue reference) and a valid `ti` might be `2` (a non-const rvalue). I've taken another stab at the wording using "expressions" and "bindable to".

[ 2010-02-09 Wording updated by Jonathan, Ganesh and Daniel. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Daniel opens to improve wording. ]

[ 2010-02-11 This issue is now addressed by 870(i). ]

[ 2010-02-12 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. Rationale added below. ]

Rationale:

Addressed by 870(i).

Proposed resolution:

Edit 22.10.17.3 [func.wrap.func] paragraph 2:

> 2 A function object `f` of type `F` is Callable for argument types `T1, T2, ..., TN` in `ArgTypes` and a return type `R`, if, given lvalues `t1, t2, ..., tN` of types `T1, T2, ..., TN`, respectively, the expression `INVOKE(f, declval<ArgTypes>()..., Rt1, t2, ..., tN)`, considered as an unevaluated operand (7 [expr]), is well formed (20.7.2) and, if `R` is not `void`, convertible to `R`.

### 816(i). Should `bind()`'s returned functor have a nofail copy ctor when `bind()` is nofail?

Section: 22.10.15.4 [func.bind.bind] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2008-02-08 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.bind.bind].

View all issues with Resolved status.

Discussion:

Library Issue 527(i) notes that `bind(f, t1, ..., tN)` should be nofail when `f, t1, ..., tN` have nofail copy ctors.

However, no guarantees are provided for the copy ctor of the functor returned by `bind()`. (It's guaranteed to have a copy ctor, which can throw implementation-defined exceptions: `bind()` returns a forwarding call wrapper, TR1 3.6.3/2. A forwarding call wrapper is a call wrapper, TR1 3.3/4. Every call wrapper shall be CopyConstructible, TR1 3.3/4. Everything without an exception-specification may throw implementation-defined exceptions unless otherwise specified, C++03 17.4.4.8/3.)

Should the nofail guarantee requested by Library Issue 527(i) be extended to cover both calling `bind()` and copying the returned functor?

[ Howard adds: ]

> `tuple` construction should probably have a similar guarantee.

[ San Francisco: ]

> Howard to provide wording.

[ Post Summit, Anthony provided wording. ]

[ Batavia (2009-05): ]

> Part of all of this issue appears to be rendered moot by the proposed resolution to issue 817(i) (q.v.). We recommend the issues be considered simultaneously (or possibly even merged) to ensure there is no overlap. Move to Open, and likewise for issue 817(i).

[ 2009-07 Frankfurt: ]

> Related to 817(i) (see below). Leave Open.

[ 2009-10 Santa Cruz: ]

> Move to Ready. Decoupling from issue 817(i).

[ 2010-02-11 Moved from Ready to Tentatively NAD Editorial, rationale added below. ]

Rationale:

This issue is solved as proposed by 817(i).

Proposed resolution:

Add a new sentence to the end of paragraphs 2 and 4 of 22.10.15.4 [func.bind.bind]:

> -2- Returns: A forwarding call wrapper `g` with a weak result type (20.6.2). The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(f, v1, v2, ..., vN, Callable<F cv,V1, V2, ..., VN>::result_type)`, where cv represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `F` or any of the types in `BoundArgs...` throw an exception.

> ...

> -5- Returns: A forwarding call wrapper `g` with a nested type `result_type` defined as a synonym for `R`. The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(f, v1, v2, ..., vN, R)`, where the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `F` or any of the types in `BoundArgs...` throw an exception.

### 817(i). `bind` needs to be moved

Section: 22.10.15.4 [func.bind.bind] Status: C++11 Submitter: Howard Hinnant Opened: 2008-03-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.bind.bind].

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Discussion:

Addresses US 72, JP 38 and DE 21

The functor returned by `bind()` should have a move constructor that requires only move construction of its contained functor and bound arguments. That way move-only functors can be passed to objects such as `thread`.

This issue is related to issue 816(i).

US 72:

> `bind` should support move-only functors and bound arguments.

JP 38:

> add the move requirement for bind's return type.

> For example, assume following `th1` and `th2`,

> >     void f(vector<int> v) { }

> >     vector<int> v{ ... };
> >     thread th1([v]{ f(v); });
> >     thread th2(bind(f, v));

> When function object are set to thread, `v` is moved to `th1`'s lambda expression in a Move Constructor of lambda expression because `th1`'s lambda expression has a Move Constructor. But `bind` of `th2`'s return type doesn't have the requirement of Move, so it may not moved but copied.

> Add the requirement of move to get rid of this useless copy.

> And also, add the `MoveConstructible` as well as `CopyConstructible`.

DE 21

> The specification for bind claims twice that "the values and types for the bound arguments v1, v2, ..., vN are determined as specified below". No such specification appears to exist.

[ San Francisco: ]

> Howard to provide wording.

[ Post Summit Alisdair and Howard provided wording. ]

> Several issues are being combined in this resolution. They are all touching the same words so this is an attempt to keep one issue from stepping on another, and a place to see the complete solution in one place.

> 1. `bind` needs to be "moved".
> 2. 22.10.15.4 [func.bind.bind]/p3, p6 and p7 were accidently removed from N2798.
> 3. Issue 929(i) argues for a way to pass by && for efficiency but retain the decaying behavior of pass by value for the `thread` constructor. That same solution is applicable here.

[ Batavia (2009-05): ]

> We were going to recommend moving this issue to Tentatively Ready until we noticed potential overlap with issue 816 (q.v.).

> Move to Open, and recommend both issues be considered together (and possibly merged).

[ 2009-07 Frankfurt: ]

> The proposed resolution uses concepts. Leave Open.

[ 2009-10 Santa Cruz: ]

> Leave as Open. Howard to provide deconceptified wording.

[ 2009-11-07 Howard updates wording. ]

[ 2009-11-15 Further updates by Peter, Chris and Daniel. ]

[ Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Change 22.10 [function.objects] p2:

>     template<class Fn, class... Types BoundArgs>
>       unspecified bind(Fn&&, Types BoundArgs&&...);
>     template<class R, class Fn, class... Types BoundArgs>
>       unspecified bind(Fn&&, Types BoundArgs&&...);

Change 22.10.4 [func.require]:

> 4 Every call wrapper (22.10.3 [func.def]) shall be `CopyMoveConstructible`. A simple call wrapper is a call wrapper that is `CopyConstructible` and `CopyAssignable` and whose copy constructor, move constructor and assignment operator do not throw exceptions. A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

> >     template<class... ArgTypesUnBoundsArgs>
> >     R operator()(ArgTypesUnBoundsArgs&&... unbound_args) cv-qual;

> — end note]

Change 22.10.15.4 [func.bind.bind]:

> Within this clause:

> - Let `FD` be a synonym for the type `decay<F>::type`.
> - Let `fd` be an lvalue of type `FD` constructed from `std::forward<F>(f)`.
> - Let `Ti` be a synonym for the ith type in the template parameter pack `BoundArgs`.
> - Let `TiD` be a synonym for the type `decay<Ti>::type`.
> - Let `ti` be the ith argument in the function parameter pack `bound_args`.
> - Let `tid` be an lvalue of type `TiD` constructed from `std::forward<Ti>(ti)`.
> - Let `Uj` be the jth deduced type of the `UnBoundArgs&&...` parameter of the `operator()` of the forwarding call wrapper.
> - Let `uj` be the jth argument associated with `Uj`.

>     template<class F, class... BoundArgs>
>       unspecified bind(F&& f, BoundArgs&&... bound_args);

> > -1- Requires: `is_constructible<FD, F>::value` shall be `true`. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be `true`. `F` and each `Ti` in `BoundArgs` shall be CopyConstructible. `INVOKE(fd, w1, w2, ..., wN)` (22.10.4 [func.require]) shall be a valid expression for some values w1, w2, ..., wN, where `N == sizeof...(bound_args)`.

> > -2- Returns: A forwarding call wrapper `g` with a weak result type (22.10.4 [func.require]). The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type)`, where cv represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `FD` or of any of the types `TiD` throws an exception.

> > -3- Throws: Nothing unless the copy constructionor of `Ffd` or of one of the values `tid` types in the `BoundArgs...` pack expansion throws an exception.

> > Remarks: The unspecified return type shall satisfy the requirements of `MoveConstructible`. If all of `FD` and `TiD` satisfy the requirements of `CopyConstructible` then the unspecified return type shall satisfy the requirements of `CopyConstructible`. [Note: This implies that all of `FD` and `TiD` shall be `MoveConstructible` — end note]

>     template<class R, class F, class... BoundArgs>
>       unspecified bind(F&& f, BoundArgs&&... bound_args);

> > -4- Requires: `is_constructible<FD, F>::value` shall be `true`. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be `true`. `F` and each `Ti` in `BoundArgs` shall be CopyConstructible. `INVOKE(fd, w1, w2, ..., wN)` shall be a valid expression for some values w1, w2, ..., wN, where `N == sizeof...(bound_args)`.

> > -5- Returns: A forwarding call wrapper `g` with a nested type `result_type` defined as a synonym for `R`. The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, v1, v2, ..., vN, R)`, where the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `FD` or of any of the types `TiD` throws an exception.

> > -6- Throws: Nothing unless the copy constructionor of `Ffd` or of one of the values `tid` types in the `BoundArgs...` pack expansion throws an exception.

> > Remarks: The unspecified return type shall satisfy the requirements of `MoveConstructible`. If all of `FD` and `TiD` satisfy the requirements of `CopyConstructible` then the unspecified return type shall satisfy the requirements of `CopyConstructible`. [Note: This implies that all of `FD` and `TiD` shall be `MoveConstructible` — end note]

> -7- The values of the bound arguments `v1, v2, ..., vN` and their corresponding types `V1, V2, ..., VN` depend on the types `TiD` derived from of the corresponding argument `ti` in `bound_args` of type `Ti` in `BoundArgs` in the call to `bind` and the cv-qualifiers cv of the call wrapper `g` as follows:

> - if `ti TiD` is of type `reference_wrapper<T>` the argument is `tid.get()` and its type `Vi` is `T&`;
> - if the value of `std::is_bind_expression<TiD>::value` is `true` the argument is `tid(std::forward<Uj>(uj)... u1, u2, ..., uM)` and its type `Vi` is `result_of<TiD cv (Uj... U1&, U2&, ..., UM&)>::type`;
> - if the value `j` of `std::is_placeholder<TiD>::value` is not zero the argument is `std::forward<Uj>(uj)` and its type `Vi` is `Uj&&`;
> - otherwise the value is `tid` and its type `Vi` is `TiD cv &`.

### 818(i). wording for memory ordering

Section: 32.5.4 [atomics.order] Status: CD1 Submitter: Jens Maurer Opened: 2008-03-22 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [atomics.order].

View all other issues in [atomics.order].

View all issues with CD1 status.

Discussion:

32.5.4 [atomics.order] p1 says in the table that

> Element | Meaning
> --- | ---
> `memory_order_acq_rel` | the operation has both acquire and release semantics

To my naked eye, that seems to imply that even an atomic read has both acquire and release semantics.

Then, p1 says in the table:

> Element | Meaning
> --- | ---
> `memory_order_seq_cst` | the operation has both acquire and release semantics, and, in addition, has sequentially-consistent operation ordering

So that seems to be "the same thing" as `memory_order_acq_rel`, with additional constraints.

I'm then reading p2, where it says:

> The `memory_order_seq_cst` operations that load a value are acquire operations on the affected locations. The `memory_order_seq_cst` operations that store a value are release operations on the affected locations.

That seems to imply that atomic reads only have acquire semantics. If that is intended, does this also apply to `memory_order_acq_rel` and the individual load/store operations as well?

Also, the table in p1 contains phrases with "thus" that seem to indicate consequences of normative wording in 6.10.2 [intro.multithread]. That shouldn't be in normative text, for the fear of redundant or inconsistent specification with the other normative text.

Double-check 32.5.8.2 [atomics.types.operations] that each operation clearly says whether it's a load or a store operation, or both. (It could be clearer, IMO. Solution not in current proposed resolution.)

32.5.4 [atomics.order] p2: What's a "consistent execution"? It's not defined in 6.10.2 [intro.multithread], it's just used in notes there.

And why does 32.5.8.2 [atomics.types.operations] p9 for "load" say:

> Requires: The order argument shall not be `memory_order_acquire` nor `memory_order_acq_rel`.

(Since this is exactly the same restriction as for "store", it seems to be a typo.)

And then: 32.5.8.2 [atomics.types.operations] p12:

> These operations are read-modify-write operations in the sense of the "synchronizes with" definition (6.10.2 [intro.multithread]), so both such an operation and the evaluation that produced the input value synchronize with any evaluation that reads the updated value.

This is redundant with 6.10.2 [intro.multithread], see above for the reasoning.

[ San Francisco: ]

> Boehm: "I don't think that this changes anything terribly substantive, but it improves the text."

> Note that "Rephrase the table in as [sic] follows..." should read "Replace the table in [atomics.order] with the following...."

> The proposed resolution needs more work. Crowl volunteered to address all of the atomics issues in one paper.

> This issue is addressed in N2783.

Proposed resolution:

edit 32.5.4 [atomics.order], paragraph 1 as follows.

> The enumeration `memory_order` specifies the detailed regular (non-atomic) memory synchronization order as defined in Clause 1.7 section 1.10 and may provide for operation ordering. Its enumerated values and their meanings are as follows:

> > For `memory_order_relaxed`,
> >   no operation orders memory.
> > For `memory_order_release`, `memory_order_acq_rel`, and `memory_order_seq_cst`,
> >   a store operation performs a release operation on the affected memory location.
> > For `memory_order_consume`,
> >   a load operation performs a consume operation on the affected memory location.
> > For `memory_order_acquire`, `memory_order_acq_rel`, and `memory_order_seq_cst`,
> >   a load operation performs an acquire operation on the affected memory location.

remove table 136 in 32.5.4 [atomics.order].

> Table 136 — memory_order effects
> Element | Meaning
> --- | ---
> `memory_order_relaxed` | the operation does not order memory
> `memory_order_release` | the operation performs a release operation on the affected memory location, thus making regular memory writes visible to other threads through the atomic variable to which it is applied
> `memory_order_acquire` | the operation performs an acquire operation on the affected memory location, thus making regular memory writes in other threads released through the atomic variable to which it is applied visible to the current thread
> `memory_order_consume` | the operation performs a consume operation on the affected memory location, thus making regular memory writes in other threads released through the atomic variable to which it is applied visible to the regular memory reads that are dependencies of this consume operation.
> `memory_order_acq_rel` | the operation has both acquire and release semantics
> `memory_order_seq_cst` | the operation has both acquire and release semantics, and, in addition, has sequentially-consistent operation ordering

edit 32.5.4 [atomics.order], paragraph 2 as follows.

> The `memory_order_seq_cst` operations that load a value are acquire operations on the affected locations. The `memory_order_seq_cst` operations that store a value are release operations on the affected locations. In addition, in a consistent execution, there There must be is a single total order S on all `memory_order_seq_cst` operations, consistent with the happens before order and modification orders for all affected locations, such that each `memory_order_seq_cst` operation observes either the last preceding modification according to this order S, or the result of an operation that is not `memory_order_seq_cst`. [Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the happens before ordering. —end note]

### 819(i). rethrow_if_nested

Section: 17.9.8 [except.nested] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [except.nested].

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Discussion:

Looking at the wording I submitted for `rethrow_if_nested`, I don't think I got it quite right.

The current wording says:

>     template <class E> void rethrow_if_nested(const E& e);

> > Effects: Calls `e.rethrow_nested()` only if `e` is publicly derived from `nested_exception`.

This is trying to be a bit subtle, by requiring `e` (not `E`) to be publicly derived from `nested_exception` the idea is that a `dynamic_cast` would be required to be sure. Unfortunately, if `e` is dynamically but not statically derived from `nested_exception`, `e.rethrow_nested()` is ill-formed.

[ San Francisco: ]

> Alisdair was volunteered to provide wording.

[ 2009-10 Santa Cruz: ]

> Leave as Open. Alisdair to provide wording.

[ 2009-11-09 Alisdair provided wording. ]

[ 2010-03-10 Dietmar updated wording. ]

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

Change 17.9.8 [except.nested], p8:

>     template <class E> void rethrow_if_nested(const E& e);

> > -8- Effects: Calls `e.rethrow_nested()` oOnly if the dynamic type of `e` is publicly and unambiguously derived from `nested_exception` this calls `dynamic_cast<const nested_exception&>(e).rethrow_nested()`.

### 820(i). `current_exception()`'s interaction with throwing copy ctors

Section: 17.9.7 [propagation] Status: CD1 Submitter: Stephan T. Lavavej Opened: 2008-03-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [propagation].

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Discussion:

As of N2521, the Working Paper appears to be silent about what `current_exception()` should do if it tries to copy the currently handled exception and its copy constructor throws. 17.9.7 [propagation]/7 says "If the function needs to allocate memory and the attempt fails, it returns an `exception_ptr` object that refers to an instance of `bad_alloc`.", but doesn't say anything about what should happen if memory allocation succeeds but the actual copying fails.

I see three alternatives: (1) return an `exception_ptr` object that refers to an instance of some fixed exception type, (2) return an `exception_ptr` object that refers to an instance of the copy ctor's thrown exception (but if that has a throwing copy ctor, an infinite loop can occur), or (3) call `terminate()`.

I believe that `terminate()` is the most reasonable course of action, but before we go implement that, I wanted to raise this issue.

[ Peter's summary: ]

> The current practice is to not have throwing copy constructors in exception classes, because this can lead to `terminate()` as described in 14.6.2 [except.terminate]. Thus calling `terminate()` in this situation seems consistent and does not introduce any new problems.

> However, the resolution of core issue 475 may relax this requirement:

> > The CWG agreed with the position that `std::uncaught_exception()` should return `false` during the copy to the exception object and that `std::terminate()` should not be called if that constructor exits with an exception.

> Since throwing copy constructors will no longer call `terminate()`, option (3) doesn't seem reasonable as it is deemed too drastic a response in a recoverable situation.

> Option (2) cannot be adopted by itself, because a potential infinite recursion will need to be terminated by one of the other options.

Proposed resolution:

Add the following paragraph after 17.9.7 [propagation]/7:

> Returns (continued): If the attempt to copy the current exception object throws an exception, the function returns an `exception_ptr` that refers to the thrown exception or, if this is not possible, to an instance of `bad_exception`.

> [Note: The copy constructor of the thrown exception may also fail, so the implementation is allowed to substitute a `bad_exception` to avoid infinite recursion. -- end note.]

Rationale:

[ San Francisco: ]

> Pete: there may be an implied assumption in the proposed wording that current_exception() copies the existing exception object; the implementation may not actually do that.

> Pete will make the required editorial tweaks to rectify this.

### 821(i). Minor cleanup : `unique_ptr`

Section: 20.3.1.4.5 [unique.ptr.runtime.modifiers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-30 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.runtime.modifiers].

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Discussion:

Reading resolution of LWG issue 673(i) I noticed the following:

>     void reset(T* pointer p = 0 pointer());

> -1- Requires: Does not accept pointer types which are convertible to `T*` `pointer` (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]

This could be cleaned up by mandating the overload as a public deleted function. In addition, we should probably overload `reset` on `nullptr_t` to be a stronger match than the deleted overload. Words...

Proposed resolution:

Add to class template definition in 20.3.1.4 [unique.ptr.runtime]

>     // modifiers
>     pointer release();
>     void reset(pointer p = pointer());
>     void reset( nullptr_t );
>     template< typename U > void reset( U ) = delete;
>     void swap(unique_ptr&& u);

Update 20.3.1.4.5 [unique.ptr.runtime.modifiers]

>     void reset(pointer p = pointer());
>     void reset(nullptr_t);

> -1- Requires: Does not accept pointer types which are convertible to `pointer` (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]

> Effects: If `get() == nullptr` there are no effects. Otherwise `get_deleter()(get())`.

> ...

[ Note this wording incorporates resolutions for 806(i) (New) and 673(i) (Ready). ]

### 823(i). `identity<void>` seems broken

Section: 22.2.4 [forward] Status: Resolved Submitter: Walter Brown Opened: 2008-04-09 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [forward].

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Discussion:

N2588 seems to have added an `operator()` member function to the `identity<>` helper in 22.2.4 [forward]. I believe this change makes it no longer possible to instantiate `identity<void>`, as it would require forming a reference-to-`void` type as this `operator()`'s parameter type.

Suggested resolution: Specialize `identity<void>` so as not to require the member function's presence.

[ Sophia Antipolis: ]

> Jens: suggests to add a requires clause to avoid specializing on `void`.

> Alisdair: also consider cv-qualified `void`.

> Alberto provided proposed wording.

[ 2009-07-30 Daniel reopens: ]

> This issue became closed, because the `ReferentType` requirement fixed the problem - this is no longer the case. In retrospective it seems to be that the root of current issues around `std::identity` (823, 700(i), 939(i)) is that it was standardized as something very different (an unconditional type mapper) than traditional usage indicated (a function object that should derive from `std::unary_function)`, as the SGI definition does. This issue could be solved, if `std::identity` is removed (one proposal of 939(i)), but until this has been decided, this issue should remain open. An alternative for removing it, would be, to do the following:

> 1. Let `identity` stay as a real function object, which would now properly derive from `unary_function`:

>    >     template <class T> struct identity : unary_function<T, T> {
>    >       const T& operator()(const T&) const;
>    >     };

> 2. Invent (if needed) a generic type wrapper (corresponding to concept `IdentityOf`), e.g. `identity_of`, and move it's prototype description back to 22.2.4 [forward]:

>    >     template <class T> struct identity_of {
>    >       typedef T type;
>    >     };

>    and adapt the `std::forward` signature to use `identity_of` instead of `identity`.

[ 2009-10 Santa Cruz: ]

> Mark as NAD EditorialResolved, fixed by 939(i).

Proposed resolution:

Change definition of `identity` in 22.2.4 [forward], paragraph 2, to:

>     template <class T>  struct identity {
>         typedef T type;

>         requires ReferentType<T>
>           const T& operator()(const T& x) const;
>       };

...

>       requires ReferentType<T>
>         const T& operator()(const T& x) const;

Rationale:

The point here is to able to write `T&` given `T` and `ReferentType` is precisely the concept that guarantees so, according to N2677 (Foundational concepts). Because of this, it seems preferable than an explicit check for `cv void` using `SameType/remove_cv` as it was suggested in Sophia. In particular, Daniel remarked that there may be types other than `cv void` which aren't referent types (`int[]`, perhaps?).

### 824(i). rvalue ref issue with `basic_string` inserter

Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [string.io].

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Discussion:

In the current working paper, the `<string>` header synopsis at the end of 27.4 [string.classes] lists a single `operator<<` overload for `basic_string`.

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>&& os,
>                   const basic_string<charT,traits,Allocator>& str);

The definition in 27.4.4.4 [string.io] lists two:

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>& os,
>                   const basic_string<charT,traits,Allocator>& str);

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>&& os,
>                   const basic_string<charT,traits,Allocator>& str);

I believe the synopsis in 27.4 [string.classes] is correct, and the first of the two signatures in 27.4.4.4 [string.io] should be deleted.

Proposed resolution:

Delete the first of the two signatures in 27.4.4.4 [string.io]:

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>& os,
>                   const basic_string<charT,traits,Allocator>& str);

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>&& os,
>                   const basic_string<charT,traits,Allocator>& str);

### 825(i). Missing rvalues reference stream insert/extract operators?

Section: 19.5.4.1 [syserr.errcode.overview], 20.3.2.2.12 [util.smartptr.shared.io], 99 [facets.examples], 22.9.4 [bitset.operators], 29.4.6 [complex.ops], 31.6 [stream.buffers], 28.6.8 [re.submatch] Status: Resolved Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2017-03-21

Priority: Not Prioritized

View all other issues in [syserr.errcode.overview].

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Discussion:

Addresses UK 220

Should the following use rvalues references to stream in insert/extract operators?

- 19.5.4.1 [syserr.errcode.overview]
- 20.3.2.2.12 [util.smartptr.shared.io]
- 99 [facets.examples]
- 22.9.4 [bitset.operators]
- 29.4.6 [complex.ops]
- Doubled signatures in 31.6 [stream.buffers] for character inserters (ref 31.7.6.3.4 [ostream.inserters.character]) + definition 31.7.6.3.4 [ostream.inserters.character]
- 28.6.8 [re.submatch]

[ Sophia Antipolis ]

> Agree with the idea in the issue, Alisdair to provide wording.

[ Daniel adds 2009-02-14: ]

> The proposal given in the paper N2831 apparently resolves this issue.

[ Batavia (2009-05): ]

> The cited paper is an earlier version of N2844, which changed the rvalue reference binding rules. That paper includes generic templates `operator<<` and `operator>>` that adapt rvalue streams.

> We therefore agree with Daniel's observation. Move to NAD EditorialResolved.

Proposed resolution:

### 827(i). `constexpr shared_ptr::shared_ptr()?`

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: Peter Dimov Opened: 2008-04-11 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [util.smartptr.shared.const].

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Discussion:

Would anyone object to making the default constructor of `shared_ptr` (and `weak_ptr` and `enable_shared_from_this`) `constexpr`? This would enable static initialization for `shared_ptr` variables, eliminating another unfair advantage of raw pointers.

[ San Francisco: ]

> It's not clear to us that you can initialize a pointer with the literal 0 in a constant expression. We need to ask CWG to make sure this works. Bjarne has been appointed to do this.

> Core got back to us and assured as that `nullptr` would do the job nicely here.

[ 2009-05-01 Alisdair adds: ]

> I don't believe that `constexpr` will buy anything in this case. `shared_ptr/weak_ptr/enable_shared_from_this` cannot be literal types as they have a non-trivial copy constructor. As they do not produce literal types, then the `constexpr` default constructor will not guarantee constant initialization, and so not buy the hoped for optimization.

> I recommend referring this back to Core to see if we can get static initialization for types with `constexpr` constructors, even if they are not literal types. Otherwise this should be closed as NAD.

[ 2009-05-26 Daniel adds: ]

> If Alisdair's 2009-05-01 comment is correct, wouldn't that also make `constexpr mutex()` useless, because this class has a non-trivial destructor? (828(i))

[ 2009-07-21 Alisdair adds: ]

> The feedback from core is that this and similar uses of `constexpr` constructors to force static initialization should be supported. If there are any problems with this in the working draught, we should file core issues.

> Recommend we declare the default constructor `constexpr` as the issue suggests (proposed wording added).

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2994.

Proposed resolution:

Change 20.3.2.2 [util.smartptr.shared] and 20.3.2.2.2 [util.smartptr.shared.const]:

>     constexpr shared_ptr();

Change 20.3.2.3 [util.smartptr.weak] and 20.3.2.3.2 [util.smartptr.weak.const]:

>     constexpr weak_ptr();

Change 20.3.2.7 [util.smartptr.enab] (2 places):

>     constexpr enable_shared_from_this();

### 828(i). Static initialization for `std::mutex`?

Section: 32.6.4.2.2 [thread.mutex.class] Status: Resolved Submitter: Peter Dimov Opened: 2008-04-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.mutex.class].

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Discussion:

[Note: I'm assuming here that [basic.start.init]/1 will be fixed.]

Currently `std::mutex` doesn't support static initialization. This is a regression with respect to `pthread_mutex_t`, which does. I believe that we should strive to eliminate such regressions in expressive power where possible, both to ease migration and to not provide incentives to (or force) people to forego the C++ primitives in favor of pthreads.

[ Sophia Antipolis: ]

> We believe this is implementable on POSIX, because the initializer-list feature and the constexpr feature make this work. Double-check core language about static initialization for this case. Ask core for a core issue about order of destruction of statically-initialized objects wrt. dynamically-initialized objects (should come afterwards). Check non-POSIX systems for implementability.

> If ubiquitous implementability cannot be assured, plan B is to introduce another constructor, make this constexpr, which is conditionally-supported. To avoid ambiguities, this new constructor needs to have an additional parameter.

[ Post Summit: ]

> Jens: constant initialization seems to be ok core-language wise

> Consensus: Defer to threading experts, in particular a Microsoft platform expert.

> Lawrence to send e-mail to Herb Sutter, Jonathan Caves, Anthony Wiliams, Paul McKenney, Martin Tasker, Hans Boehm, Bill Plauger, Pete Becker, Peter Dimov to alert them of this issue.

> Lawrence: What about header file shared with C? The initialization syntax is different in C and C++.

> Recommend Keep in Review

[ Batavia (2009-05): ]

> Keep in Review status pending feedback from members of the Concurrency subgroup.

[ See related comments from Alisdair and Daniel in 827(i). ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2994.

Proposed resolution:

Change 32.6.4.2.2 [thread.mutex.class]:

>     class mutex {
>     public:
>       constexpr mutex();
>       ...

### 829(i). current_exception wording unclear about exception type

Section: 17.9.7 [propagation] Status: CD1 Submitter: Beman Dawes Opened: 2008-04-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [propagation].

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Discussion:

Consider this code:

>     exception_ptr xp;

>     try {do_something(); }

>     catch (const runtime_error& ) {xp = current_exception();}

>     ...

>     rethrow_exception(xp);

Say `do_something()` throws an exception object of type `range_error`. What is the type of the exception object thrown by `rethrow_exception(xp)` above? It must have a type of `range_error`; if it were of type `runtime_error` it still isn't possible to propagate an exception and the exception_ptr/current_exception/rethrow_exception machinery serves no useful purpose.

Unfortunately, the current wording does not explicitly say that. Different people read the current wording and come to different conclusions. While it may be possible to deduce the correct type from the current wording, it would be much clearer to come right out and explicitly say what the type of the referred to exception is.

[ Peter adds: ]

> I don't like the proposed resolution of 829. The normative text is unambiguous that the `exception_ptr` refers to the currently handled exception. This term has a standard meaning, see 14.4 [except.handle]/8; this is the exception that `throw;` would rethrow, see 14.2 [except.throw]/7.

> A better way to address this is to simply add the non-normative example in question as a clarification. The term currently handled exception should be italicized and cross-referenced. A [Note: the currently handled exception is the exception that a throw expression without an operand (14.2 [except.throw]/7) would rethrow. --end note] is also an option.

Proposed resolution:

After 17.9.7 [propagation] , paragraph 7, add the indicated text:

>     exception_ptr current_exception();

> > Returns: `exception_ptr` object that refers to the currently handled exception (14.4 [except.handle]) or a copy of the currently handled exception, or a null `exception_ptr` object if no exception is being handled. If the function needs to allocate memory and the attempt fails, it returns an `exception_ptr` object that refers to an instance of `bad_alloc`. It is unspecified whether the return values of two successive calls to `current_exception` refer to the same exception object. [Note: that is, it is unspecified whether `current_exception` creates a new copy each time it is called. -- end note]

> > Throws: nothing.

### 834(i). `unique_ptr::pointer` requirements underspecified

Section: 20.3.1.3 [unique.ptr.single] Status: Resolved Submitter: Daniel Krügler Opened: 2008-05-14 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [unique.ptr.single].

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Discussion:

Issue 673(i) (including recent updates by 821(i)) proposes a useful extension point for `unique_ptr` by granting support for an optional `deleter_type::pointer` to act as pointer-like replacement for `element_type*` (In the following: `pointer`).

Unfortunately no requirements are specified for the type `pointer` which has impact on at least two key features of `unique_ptr`:

1. Operational fail-safety.
2. (Well-)Definedness of expressions.

The `unique_ptr` specification makes great efforts to require that essentially all operations cannot throw and therefore adds proper wording to the affected operations of the deleter as well. If user-provided `pointer`-emulating types ("smart pointers") will be allowed, either all throw-nothing clauses have to be replaced by weaker "An exception is thrown only if `pointer`'s {op} throws an exception"-clauses or it has to be said explicitly that all used operations of `pointer` are required not to throw. I understand the main focus of `unique_ptr` to be as near as possible to the advantages of native pointers which cannot fail and thus strongly favor the second choice. Also, the alternative position would make it much harder to write safe and simple template code for `unique_ptr`. Additionally, I assume that a general statement need to be given that all of the expressions of `pointer` used to define semantics are required to be well-formed and well-defined (also as back-end for 762(i)).

[ Sophia Antipolis: ]

> Howard: We maybe need a core concept `PointerLike`, but we don't need the arithmetic (see `shared_ptr` vs. `vector<T>::iterator`.

> Howard will go through and enumerate the individual requirements wrt. `pointer` for each member function.

[ 2009-07 Frankfurt: ]

> Move to Ready.

[ 2009-10-15 Alisdair pulls from Ready: ]

> I hate to pull an issue out of Ready status, but I don't think 834 is fully baked yet.

> For reference the proposed resolution is to add the following words:

> > `unique_ptr<T, D>::pointer`'s operations shall be well-formed, shall have well defined behavior, and shall not throw exceptions.

> This leaves me with a big question : which operations?

> Are all pointer operations required to be nothrow, including operations that have nothing to do with interactions with `unique_ptr`? This was much simpler with concepts where we could point to operations within a certain concept, and so nail down the interactions.

[ 2009-10-15 Daniel adds: ]

> I volunteer to prepare a more fine-grained solution, but I would like to ask for feedback that helps me doing so. If this question is asked early in the meeting I might be able to fix it within the week, but I cannot promise that now.

[ 2009-10 Santa Cruz: ]

> Leave in open. Daniel to provide wording as already suggested.

[ 2009-12-22 Daniel provided wording and rationale. ]

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

The here proposed resolution has considerable overlap with the requirements that are used in the allocator requirements.

This might be a convincing argument to isolate the common subset into one requirement. The reason I did not do that is basically because we might find out that they are either over-constraining or under-constraining at this late point of specification. Note also that as a result of the idea of a general requirement set I added the requirement:

> A default-initialized object may have a singular value

even though this does not play a relevant role for `unique_ptr`.

One further characteristics of the resolution is that availability of relational operators of `unique_ptr<T, D>::pointer` is not part of the basic requirements, which is in sync with the allocator requirements on pointer-like (this means that `unique_ptr` can hold a `void_pointer` or `const_void_pointer`).

Solved by N3073.

Proposed resolution:

1. Change 20.3.1.3 [unique.ptr.single] p. 1 as indicated: [The intent is to replace the coupling between `T*` and the deleter's `operator()` by a coupling between `unique_ptr<T, D>::pointer` and this `operator()`]

   > 1 - The default type for the template parameter `D` is `default_delete`. A client-supplied template argument `D` shall be a function pointer or functor for which, given a value `d` of type `D` and a pointer value `ptr` of type `T* unique_ptr<T, D>::pointer`, the expression `d(ptr)` is valid and has the effect of deallocating the pointer as appropriate for that deleter. `D` may also be an lvalue-reference to a deleter.

2. Change 20.3.1.3 [unique.ptr.single] p. 3 as indicated:

   > 3 - If the type `remove_reference<D>::type::pointer` exists, then `unique_ptr<T, D>::pointer` shall be a synonym for `remove_reference<D>::type::pointer`. Otherwise `unique_ptr<T, D>::pointer` shall be a synonym for `T*`. The type `unique_ptr<T, D>::pointer` shall be satisfy the requirements of `EqualityComparable`, `DefaultConstructible`, `CopyConstructible` (Table 34) and, `CopyAssignable` (Table 36), `Swappable`, and `Destructible` (16.4.4.2 [utility.arg.requirements]). A default-initialized object may have a singular value. A value-initialized object produces the null value of the type. The null value shall be equivalent only to itself. An object of this type can be copy-initialized with a value of type `nullptr_t`, compared for equality with a value of type `nullptr_t`, and assigned a value of type `nullptr_t`. The effect shall be as if a value-initialized object had been used in place of the null pointer constant. An object `p` of this type can be contextually converted to `bool`. The effect shall be as if `p != nullptr` had been evaluated in place of `p`. No operation on this type which is part of the above mentioned requirements shall exit via an exception.

   > [Note: Given an allocator type `X` (16.4.4.6 [allocator.requirements]), the types `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` may be used as `unique_ptr<T, D>::pointer` — end note]

   > In addition to being available via inclusion of the `<utility>` header, the `swap` function template in 22.2.2 [utility.swap] is also available within the definition of `unique_ptr`'s `swap` function.

3. Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 2+3 as indicated: [The first change ensures that we explicitly say, how the stored pointer is initialized. This is important for a `constexpr` function, because this may make a difference for user-defined pointer-like types]

   >     constexpr unique_ptr();

   > > ...

   > > 2 - Effects: Constructs a `unique_ptr` which owns nothing, value-initializing the stored pointer.

   > > 3 - Postconditions: `get() == 0 nullptr`.

4. Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 6+7 as indicated: [This is a step-by-fix to ensure consistency to the changes of N2976]

   >     unique_ptr(pointer p);

   > > ...

   > > 6 - Effects: Constructs a `unique_ptr` which owns `p`, initializing the stored pointer with `p`.

   > > 7 - Postconditions: `get() == p`. `get_deleter()` returns a reference to a default constructed value-initialized deleter `D`.

5. Insert a new effects clause in 20.3.1.3.2 [unique.ptr.single.ctor] just before p. 14: [The intent is to fix the current lack of specification in which way the stored pointer is initialized]

   >     unique_ptr(pointer p, implementation-defined see below d1);
   >     unique_ptr(pointer p, implementation-defined see below d2);

   > > ...

   > > Effects: Constructs a `unique_ptr` which owns `p`, initializing the stored pointer with `p` and the initializing the deleter as described above.

   > > 14 - Postconditions: `get() == p`. `get_deleter()` returns a reference to the internally stored deleter. If `D` is a reference type then `get_deleter()` returns a reference to the lvalue `d`.

6. Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 18+22 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]

   >     unique_ptr(unique_ptr&& u);

   > > [..]

   > > 18 - Postconditions: `get() == value u.get()` had before the construction and `u.get() == nullptr`. `get_deleter()` returns a reference to the internally stored deleter which was constructed from `u.get_deleter()`. If `D` is a reference type then `get_deleter()` and `u.get_deleter()` both reference the same lvalue deleter.

   >     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

   > > [..]

   > > 22 - Postconditions: `get() == value u.get()` had before the construction, modulo any required offset adjustments resulting from the cast from `unique_ptr<U, E>::pointer` to `pointer` and `u.get() == nullptr`. `get_deleter()` returns a reference to the internally stored deleter which was constructed from `u.get_deleter()`.

7. Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 20 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950(i)]

   >     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

   > > 20 - Requires: If `D` is not a reference type, construction of the deleter `D` from an rvalue of type `E` shall be well formed and shall not throw an exception. If `D` is a reference type, then `E` shall be the same type as `D` (diagnostic required). `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U` are complete types. — end note]

8. Change 20.3.1.3.3 [unique.ptr.single.dtor] p. 2 as indicated:

   >     ~unique_ptr();

   > > ...

   > > 2 - Effects: If `get() == 0 nullptr` there are no effects. Otherwise `get_deleter()(get())`.

9. Change 20.3.1.3.4 [unique.ptr.single.asgn] p. 3+8 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]

   >     unique_ptr& operator=(unique_ptr&& u);

   > > [..]

   > > 3 - Postconditions: This `unique_ptr` now owns the pointer which `u` owned, and `u` no longer owns it, `u.get() == nullptr`. [Note: If `D` is a reference type, then the referenced lvalue deleters are move assigned. — end note]

   >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

   > > [..]

   > > 8 - Postconditions: This `unique_ptr` now owns the pointer which `u` owned, and `u` no longer owns it, `u.get() == nullptr`.

10. Change 20.3.1.3.4 [unique.ptr.single.asgn] p. 6 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950(i)]

    >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

    > > [..]

    > > 6 - Requires: Assignment of the deleter `D` from an rvalue `D` shall not throw an exception. `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U` are complete types. — end note]

11. Change 20.3.1.3.4 [unique.ptr.single.asgn] before p. 11 and p. 12 as indicated: [The first change is a simple typo fix]

    >     unique_ptr& operator=(nullptr_t});

    > > 11 - Effects: `reset()`.

    > > 12 - Postcondition: `get() == 0 nullptr`

12. Change 20.3.1.3.5 [unique.ptr.single.observers] p. 1+4+12 as indicated:

    >     typename add_lvalue_reference<T>::type operator*() const;

    > > 1 - Requires: `get() != 0 nullptr`. The variable definition `add_lvalue_reference<T>::type t = *get()` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > [..]

    >     pointer operator->() const;

    > > 4 - Requires: `get() != 0 nullptr`.

    > > [..]

    >     explicit operator bool() const;

    > > 12 - Returns: `get() != 0nullptr`.

13. Change 20.3.1.3.6 [unique.ptr.single.modifiers] p. 1 as indicated:

    >     pointer release();

    > > 1 - Postcondition: `get() == 0 nullptr`.

14. Change 20.3.1.3.6 [unique.ptr.single.modifiers] p. 9 as indicated: [The intent is to ensure that potentially user-defined swaps are used. A side-step fix and harmonization with the specification of the the deleter is realized. Please note the additional requirement in bullet 2 of this proposed resolution regarding the availability of the generic `swap` templates within the member `swap` function.]

    >     void swap(unique_ptr& u);

    > > 8 - Requires: The deleter `D` shall be `Swappable` and shall not throw an exception under `swap`.

    > > 9 - Effects: The stored pointers of `*this` and `u` are exchanged by an unqualified call to non-member `swap`. The stored deleters are `swap`'d (unqualified) exchanged by an unqualified call to non-member `swap`.

15. Change 20.3.1.4.4 [unique.ptr.runtime.observers] p. 1 as indicated:

    >     T& operator[](size_t i) const;

    > > Requires: `i <` the size of the array to which the stored pointer points. The variable definition `T& t = get()[i]` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

16. Change 20.3.1.4.5 [unique.ptr.runtime.modifiers] p. 1 as indicated:

    >     void reset(pointer p = pointer());
    >     void reset(nullptr_t p);

    > > 1 - Effects: If `get() == 0 nullptr` there are no effects. Otherwise `get_deleter()(get())`.

17. Change 20.3.1.6 [unique.ptr.special] as indicated: [We don't add the relational operators to the basic requirement set, therefore we need special handling here]

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() == y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 2 - Returns: `x.get() == y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() != y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 3 - Returns: `x.get() != y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() < y.get()`; shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 4 - Returns: `x.get() < y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() <= y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 5 - Returns: `x.get() <= y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() > y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 6 - Returns: `x.get() > y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() >= y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 7 - Returns: `x.get() >= y.get()`.

    > > Throws: nothing.

### 835(i). Tying two streams together (correction to DR 581)

Section: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [basic.ios.members].

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Discussion:

The fix for issue 581(i), now integrated into the working paper, overlooks a couple of minor problems.

First, being an unformatted function once again, `flush()` is required to create a sentry object whose constructor must, among other things, flush the tied stream. When two streams are tied together, either directly or through another intermediate stream object, flushing one will also cause a call to `flush()` on the other tied stream(s) and vice versa, ad infinitum. The program below demonstrates the problem.

Second, as Bo Persson notes in his comp.lang.c++.moderated post, for streams with the `unitbuf` flag set such as `std::stderr`, the destructor of the sentry object will again call `flush()`. This seems to create an infinite recursion for `std::cerr << std::flush;`

>     #include <iostream>

>     int main ()
>     {
>        std::cout.tie (&std::cerr);
>        std::cerr.tie (&std::cout);
>        std::cout << "cout\n";
>        std::cerr << "cerr\n";
>     }

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Review.

[ 2009-05-26 Daniel adds: ]

> I think that the most recently suggested change in [ostream::sentry] need some further word-smithing. As written, it would make the behavior undefined, if under conditions when `pubsync()` should be called, but when in this scenario `os.rdbuf()` returns 0.

> This case is explicitly handled in `flush()` and needs to be taken care of. My suggested fix is:

> > If `((os.flags() & ios_base::unitbuf) && !uncaught_exception()` `&& os.rdbuf() != 0`) is true, calls `os.flush()` `os.rdbuf()->pubsync()`.

> Two secondary questions are:

> 1. Should `pubsync()` be invoked in any case or shouldn't a base requirement for this trial be that `os.good() == true` as required in the original `flush()` case?
> 2. Since `uncaught_exception()` is explicitly tested, shouldn't a return value of -1 of `pubsync()` produce `setstate(badbit)` (which may throw `ios_base::failure`)?

[ 2009-07 Frankfurt: ]

> Daniel volunteered to modify the proposed resolution to address his two questions.

> Move back to Open.

[ 2009-07-26 Daniel provided wording. Moved to Review. ]

[ 2009-10-13 Daniel adds: ]

> This proposed wording is written to match the outcome of 397(i).

[ 2009 Santa Cruz: ]

> Move to Open. Martin to propose updated wording that will also resolve issue 397(i) consistently.

[ 2010-02-15 Martin provided wording. ]

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

1. Just before 31.5.4.3 [basic.ios.members] p. 2 insert a new paragraph:

   > Requires: If `(tiestr != 0)` is `true`, `tiestr` must not be reachable by traversing the linked list of tied stream objects starting from `tiestr->tie()`.

2. Change [ostream::sentry] p. 4 as indicated:

   > If `((os.flags() & ios_base::unitbuf) && !uncaught_exception() && os.good())` is `true`, calls `os.flush()` `os.rdbuf()->pubsync()`. If that function returns -1 sets `badbit` in `os.rdstate()` without propagating an exception.

3. Add after [ostream::sentry] p17, the following paragraph:

   > Throws: Nothing.

### 836(i). Effects of `money_base::space` and `money_base::none` on `money_get`

Section: 28.3.4.7.2.3 [locale.money.get.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [locale.money.get.virtuals].

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Duplicate of: 670

Discussion:

In paragraph 2, 28.3.4.7.2.3 [locale.money.get.virtuals] specifies the following:

> Where `space` or `none` appears in the format pattern, except at the end, optional white space (as recognized by `ct.is`) is consumed after any required space.

This requirement can be (and has been) interpreted two mutually exclusive ways by different readers. One possible interpretation is that:

> 1. where `money_base::space` appears in the format, at least one space is required, and
> 2. where `money_base::none` appears in the format, space is allowed but not required.

The other is that:

> where either `money_base::space` or `money_base::none` appears in the format, white space is optional.

[ San Francisco: ]

> Martin will revise the proposed resolution.

[ 2009-07 Frankfurt: ]

> There is a noun missing from the proposed resolution. It's not clear that the last sentence would be helpful, even if the word were not missing:

> > In either case, any required MISSINGWORD followed by all optional whitespace (as recognized by `ct.is()`) is consumed.

> Strike this sentence and move to Review.

> [ Howard: done. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

I propose to change the text to make it clear that the first interpretation is intended, that is, to make following change to 28.3.4.7.2.3 [locale.money.get.virtuals], p. 2:

> When `money_base::space` or `money_base::none` appears as the last element in the format pattern, except at the end, optional white space (as recognized by `ct.is`) is consumed after any required space. no white space is consumed. Otherwise, where `money_base::space` appears in any of the initial elements of the format pattern, at least one white space character is required. Where `money_base::none` appears in any of the initial elements of the format pattern, white space is allowed but not required. If `(str.flags() & str.showbase)` is `false`, ...

### 838(i). Can an end-of-stream iterator become a non-end-of-stream one?

Section: 24.6.2 [istream.iterator] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.iterator].

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Discussion:

From message c++std-lib-20003...

The description of `istream_iterator` in 24.6.2 [istream.iterator], p. 1 specifies that objects of the class become the end-of-stream (EOS) iterators under the following condition (see also issue 788(i) another problem with this paragraph):

> If the end of stream is reached (`operator void*()` on the stream returns `false`), the iterator becomes equal to the end-of-stream iterator value.

One possible implementation approach that has been used in practice is for the iterator to set its `in_stream` pointer to 0 when it reaches the end of the stream, just like the default ctor does on initialization. The problem with this approach is that the Effects clause for `operator++()` says the iterator unconditionally extracts the next value from the stream by evaluating `*in_stream >> value`, without checking for `(in_stream == 0)`.

Conformance to the requirement outlined in the Effects clause can easily be verified in programs by setting `eofbit` or `failbit` in `exceptions()` of the associated stream and attempting to iterate past the end of the stream: each past-the-end access should trigger an exception. This suggests that some other, more elaborate technique might be intended.

Another approach, one that allows `operator++()` to attempt to extract the value even for EOS iterators (just as long as `in_stream` is non-0) is for the iterator to maintain a flag indicating whether it has reached the end of the stream. This technique would satisfy the presumed requirement implied by the Effects clause mentioned above, but it isn't supported by the exposition-only members of the class (no such flag is shown). This approach is also found in existing practice.

The inconsistency between existing implementations raises the question of whether the intent of the specification is that a non-EOS iterator that has reached the EOS become a non-EOS one again after the stream's `eofbit` flag has been cleared? That is, are the assertions in the program below expected to pass?

>        sstream strm ("1 ");
>        istream_iterator eos;
>        istream_iterator it (strm);
>        int i;
>        i = *it++
>        assert (it == eos);
>        strm.clear ();
>        strm << "2 3 ";
>        assert (it != eos);
>        i = *++it;
>        assert (3 == i);

Or is it intended that once an iterator becomes EOS it stays EOS until the end of its lifetime?

[ San Francisco: ]

> We like the direction of the proposed resolution. We're not sure about the wording, and we need more time to reflect on it,

> Move to Open. Detlef to rewrite the proposed resolution in such a way that no reference is made to exposition only members of `istream_iterator`.

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

The discussion of this issue on the reflector suggests that the intent of the standard is for an `istreambuf_iterator` that has reached the EOS to remain in the EOS state until the end of its lifetime. Implementations that permit EOS iterators to return to a non-EOS state may only do so as an extension, and only as a result of calling `istream_iterator` member functions on EOS iterators whose behavior is in this case undefined.

To this end we propose to change 24.6.2 [istream.iterator], p1, as follows:

> The result of operator-> on an end-of-stream is not defined. For any other iterator value a `const T*` is returned. Invoking `operator++()` on an end-of-stream iterator is undefined. It is impossible to store things into istream iterators...

Add pre/postconditions to the member function descriptions of `istream_iterator` like so:

>     istream_iterator();

> Effects: Constructs the end-of-stream iterator.
> Postcondition: `in_stream == 0`.

>     istream_iterator(istream_type &s);

> Effects: Initializes `in_stream` with &s. value may be initialized during construction or the first time it is referenced.
> Postcondition: `in_stream == &s`.

>     istream_iterator(const istream_iterator &x);

> Effects: Constructs a copy of `x`.
> Postcondition: `in_stream == x.in_stream`.

>     istream_iterator& operator++();

> Requires: `in_stream != 0`.
> Effects: `*in_stream >> value`.

>     istream_iterator& operator++(int);

> Requires: `in_stream != 0`.
> Effects:

> >     istream_iterator tmp (*this);
> >     *in_stream >> value;
> >     return tmp;

### 839(i). Maps and sets missing splice operation

Section: 23.4 [associative], 23.5 [unord] Status: Resolved Submitter: Alan Talbot Opened: 2008-05-18 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [associative].

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Discussion:

Splice is a very useful feature of `list`. This functionality is also very useful for any other node based container, and I frequently wish it were available for maps and sets. It seems like an omission that these containers lack this capability. Although the complexity for a splice is the same as for an insert, the actual time can be much less since the objects need not be reallocated and copied. When the element objects are heavy and the compare operations are fast (say a `map<int, huge_thingy>`) this can be a big win.

Suggested resolution:

Add the following signatures to map, set, multimap, multiset, and the unordered associative containers:

>     void splice(list<T,Allocator>&& x);
>     void splice(list<T,Allocator>&& x, const_iterator i);
>     void splice(list<T,Allocator>&& x, const_iterator first, const_iterator last);

Hint versions of these are also useful to the extent hint is useful. (I'm looking for guidance about whether hints are in fact useful.)

>     void splice(const_iterator position, list<T,Allocator>&& x);
>     void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i);
>     void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last);

[ Sophia Antipolis: ]

> Don't try to `splice "list"` into the other containers, it should be container-type.

> `forward_list` already has `splice_after`.

> Would "`splice`" make sense for an `unordered_map`?

> Jens, Robert: "`splice`" is not the right term, it implies maintaining ordering in `list`s.

> Howard: `adopt`?

> Jens: `absorb`?

> Alan: `subsume`?

> Robert: `recycle`?

> Howard: `transfer`? (but no direction)

> Jens: `transfer_from`. No.

> Alisdair: Can we give a nothrow guarantee? If your `compare()` and `hash()` doesn't throw, yes.

> Daniel: For `unordered_map`, we can't guarantee nothrow.

[ San Francisco: ]

> Martin: this would possibly outlaw an implementation technique that is currently in use; caching nodes in containers.

> Alan: if you cache in the allocator, rather than the individual container, this proposal doesn't interfere with that.

> Martin: I'm not opposed to this, but I'd like to see an implementation that demonstrates that it works.

[ 2009-07 Frankfurt: ]

> NAD Future.

[ 2009-09-19 Howard adds: ]

> I'm not disagreeing with the NAD Future resolution. But when the future gets here, here is a possibility worth exploring:

> > Add to the "unique" associative containers:

> > >     typedef details      node_ptr;

> > >     node_ptr             remove(const_iterator p);
> > >     pair<iterator, bool> insert(node_ptr&& nd);
> > >     iterator             insert(const_iterator p, node_ptr&& nd);

> > And add to the "multi" associative containers:

> > >     typedef details node_ptr;

> > >     node_ptr remove(const_iterator p);
> > >     iterator insert(node_ptr&& nd);
> > >     iterator insert(const_iterator p, node_ptr&& nd);

> > `Container::node_ptr` is a smart pointer much like `unique_ptr`. It owns a node obtained from the container it was removed from. It maintains a reference to the allocator in the container so that it can properly deallocate the node if asked to, even if the allocator is stateful. This being said, the `node_ptr` can not outlive the container for this reason.

> > The `node_ptr` offers "`const`-free" access to the node's `value_type`.

> > With this interface, clients have a great deal of flexibility:

> > - A client can remove a node from one container, and insert it into another (without any heap allocation). This is the splice functionality this issue asks for.
> > - A client can remove a node from a container, change its key or value, and insert it back into the same container, or another container, all without the cost of allocating a node.
> > - If the Compare function is nothrow (which is very common), then this functionality is nothrow unless modifying the value throws. And if this does throw, it does so outside of the containers involved.
> > - If the Compare function does throw, the `insert` function will have the argument `nd` retain ownership of the node.
> > - The `node_ptr` should be independent of the `Compare` parameter so that a node can be transferred from `set<T, C1, A>` to `set<T, C2, A>` (for example).

> > Here is how the customer might use this functionality:

> > - Splice a node from one container to another:

> >   >     m2.insert(m1.remove(i));

> > - Change the "key" in a `std::map` without the cost of node reallocation:

> >   >     auto p = m.remove(i);
> >   >     p->first = new_key;
> >   >     m.insert(std::move(p));

> > - Change the "value" in a `std::set` without the cost of node reallocation:

> >   >     auto p = s.remove(i);
> >   >     *p = new_value;
> >   >     s.insert(std::move(p));

> > - Move a move-only or heavy object out of an associative container (as opposed to the proposal in 1041(i)):

> >   >     MoveOnly x = std::move(*s.remove(i));

> >   1. `remove(i)` transfers ownership of the node from the set to a temporary `node_ptr`.
> >   2. The `node_ptr` is dereferenced, and that non-const reference is sent to `move` to cast it to an rvalue.
> >   3. The rvalue `MoveOnly` is move constructed into `x` from the `node_ptr`.
> >   4. `~node_ptr()` destructs the moved-from `MoveOnly` and deallocates the node.

> >   Contrast this with the 1041(i) solution:

> >   >     MoveOnly x = std::move(s.extract(i).first);

> >   The former requires one move construction for `x` while the latter requires two (one into the `pair` and then one into `x`). Either of these constructions can throw (say if there is only a copy constructor for `x`). With the former, the point of throw is outside of the container `s`, after the element has been removed from the container. With the latter, one throwing construction takes place prior to the removal of the element, and the second takes place after the element is removed.

> > The "node insertion" API maintains the API associated with inserting `value_type`s so the customer can use familiar techniques for getting an iterator to the inserted node, or finding out whether it was inserted or not for the "unique" containers.

> > Lightly prototyped. No implementation problems. Appears to work great for the client.

[08-2016, Post-Chicago]

Move to Tentatively Resolved

Proposed resolution:

This functionality is provided by P0083R3

### 842(i). `ConstructibleAsElement` and bit containers

Section: 23.2 [container.requirements], 23.3.14 [vector.bool], 22.9.2 [template.bitset] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

View all issues with CD1 status.

Discussion:

23.2 [container.requirements] p. 3 says:

> Objects stored in these components shall be constructed using `construct_element` (20.6.9). For each operation that inserts an element of type `T` into a container (`insert`, `push_back`, `push_front`, `emplace`, etc.) with arguments `args... T` shall be `ConstructibleAsElement`, as described in table 88. [Note: If the component is instantiated with a scoped allocator of type `A` (i.e., an allocator for which `is_scoped_allocator<A>::value` is `true`), then `construct_element` may pass an inner allocator argument to `T`'s constructor. -- end note]

However `vector<bool, A>` (23.3.14 [vector.bool]) and `bitset<N>` (22.9.2 [template.bitset]) store bits, not `bool`s, and `bitset<N>` does not even have an allocator. But these containers are governed by this clause. Clearly this is not implementable.

Proposed resolution:

Change 23.2 [container.requirements] p. 3:

> Objects stored in these components shall be constructed using `construct_element` (20.6.9), unless otherwise specified. For each operation that inserts an element of type `T` into a container (`insert`, `push_back`, `push_front`, `emplace`, etc.) with arguments `args... T` shall be `ConstructibleAsElement`, as described in table 88. [Note: If the component is instantiated with a scoped allocator of type `A` (i.e., an allocator for which `is_scoped_allocator<A>::value` is `true`), then `construct_element` may pass an inner allocator argument to `T`'s constructor. -- end note]

Change 23.3.14 [vector.bool]/p2:

> Unless described below, all operations have the same requirements and semantics as the primary `vector` template, except that operations dealing with the `bool` value type map to bit values in the container storage, and `construct_element` (23.2 [container.requirements]) is not used to construct these values.

Move 22.9.2 [template.bitset] to clause 20.

### 843(i). Reference Closure

Section: 99 [func.referenceclosure.cons] Status: CD1 Submitter: Lawrence Crowl Opened: 2008-06-02 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with CD1 status.

Discussion:

The `std::reference_closure` type has a deleted copy assignment operator under the theory that references cannot be assigned, and hence the assignment of its reference member must necessarily be ill-formed.

However, other types, notably `std::reference_wrapper` and `std::function` provide for the "copying of references", and thus the current definition of `std::reference_closure` seems unnecessarily restrictive. In particular, it should be possible to write generic functions using both `std::function` and `std::reference_closure`, but this generality is much harder when one such type does not support assignment.

The definition of `reference_closure` does not necessarily imply direct implementation via reference types. Indeed, the `reference_closure` is best implemented via a frame pointer, for which there is no standard type.

The semantics of assignment are effectively obtained by use of the default destructor and default copy assignment operator via

>     x.~reference_closure(); new (x) reference_closure(y);

So the copy assignment operator generates no significant real burden to the implementation.

Proposed resolution:

In [func.referenceclosure] Class template reference_closure, replace the `=delete` in the copy assignment operator in the synopsis with `=default`.

>     template<class R , class... ArgTypes >
>       class reference_closure<R (ArgTypes...)> {
>       public:
>          ...
>          reference_closure& operator=(const reference_closure&) = delete default;
>          ...

In 99 [func.referenceclosure.cons] Construct, copy, destroy, add the member function description

>     reference_closure& operator=(const reference_closure& f)

> > Postcondition: `*this` is a copy of `f`.

> > Returns: `*this`.

### 844(i). `complex pow` return type is ambiguous

Section: 29.4.10 [cmplx.over] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [cmplx.over].

View all issues with CD1 status.

Discussion:

The current working draft is in an inconsistent state.

29.4.8 [complex.transcendentals] says that:

> `pow(complex<float>(), int())` returns a `complex<float>`.

29.4.10 [cmplx.over] says that:

> `pow(complex<float>(), int())` returns a `complex<double>`.

[ Sophia Antipolis: ]

> Since `int` promotes to `double`, and C99 doesn't have an `int`-based overload for `pow`, the C99 result is `complex<double>`, see also C99 7.22, see also library issue 550(i).

> Special note: ask P.J. Plauger.

> > Looks fine.

Proposed resolution:

Strike this `pow` overload in 29.4.2 [complex.syn] and in 29.4.8 [complex.transcendentals]:

>     template<class T> complex<T> pow(const complex<T>& x, int y);

### 845(i). atomics cannot support aggregate initialization

Section: 32.5.8 [atomics.types.generic] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.types.generic].

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Discussion:

The atomic classes (and class templates) are required to support aggregate initialization (99 [atomics.types.integral] p. 2 / 99 [atomics.types.address] p. 1) yet also have user declared constructors, so cannot be aggregates.

This problem might be solved with the introduction of the proposed initialization syntax at Antipolis, but the wording above should be altered. Either strike the sentence as redundant with new syntax, or refer to 'brace initialization'.

[ Jens adds: ]

> Note that

> >     atomic_itype a1 = { 5 };

> would be aggregate-initialization syntax (now coming under the disguise of brace initialization), but would be ill-formed, because the corresponding constructor for atomic_itype is explicit. This works, though:

> >     atomic_itype a2 { 6 };

[ San Francisco: ]

> The preferred approach to resolving this is to remove the explicit specifiers from the atomic integral type constructors.

> Lawrence will provide wording.

> This issue is addressed in N2783.

Proposed resolution:

within the synopsis in 99 [atomics.types.integral] edit as follows.

>     ....
>     typedef struct atomic_bool {
>     ....
>       constexpr explicit atomic_bool(bool);
>     ....
>     typedef struct atomic_itype {
>     ....
>       constexpr explicit atomic_itype(integral);
>     ....

edit 99 [atomics.types.integral] paragraph 2 as follows.

> The atomic integral types shall have standard layout. They shall each have a trivial default constructor, a constexpr explicit value constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor. They shall each support aggregate initialization syntax.

within the synopsis of 99 [atomics.types.address] edit as follows.

>     ....
>     typedef struct atomic_address {
>     ....
>       constexpr explicit atomic_address(void*);
>     ....

edit 99 [atomics.types.address] paragraph 1 as follows.

> The type `atomic_address` shall have standard layout. It shall have a trivial default constructor, a constexpr explicit value constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor. It shall support aggregate initialization syntax.

within the synopsis of 32.5.8 [atomics.types.generic] edit as follows.

>     ....
>     template <class T> struct atomic {
>     ....
>       constexpr explicit atomic(T);
>     ....
>     template <> struct atomic<integral> : atomic_itype {
>     ....
>       constexpr explicit atomic(integral);
>     ....
>     template <> struct atomic<T*> : atomic_address {
>     ....
>       constexpr explicit atomic(T*);
>     ....

edit 32.5.8 [atomics.types.generic] paragraph 2 as follows.

> Specializations of the `atomic` template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexpr explicit value constructor.

### 846(i). No definition for constructor

Section: 32.5.8.2 [atomics.types.operations] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.types.operations].

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Discussion:

The atomic classes and class templates (99 [atomics.types.integral] / 99 [atomics.types.address]) have a constexpr constructor taking a value of the appropriate type for that atomic. However, neither clause provides semantics or a definition for this constructor. I'm not sure if the initialization is implied by use of constexpr keyword (which restricts the form of a constructor) but even if that is the case, I think it is worth spelling out explicitly as the inference would be far too subtle in that case.

[ San Francisco: ]

> Lawrence will provide wording.

> This issue is addressed in N2783.

Proposed resolution:

before the description of ...`is_lock_free`, that is before 32.5.8.2 [atomics.types.operations] paragraph 4, add the following description.

>     constexpr A::A(C desired);

> Effects:
>   Initializes the object with the value `desired`. [Note: Construction is not atomic. —end note]

### 847(i). string exception safety guarantees

Section: 27.4.3.2 [string.require] Status: C++11 Submitter: Hervé Brönnimann Opened: 2008-06-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [string.require].

View all issues with C++11 status.

Discussion:

In March, on comp.lang.c++.moderated, I asked what were the string exception safety guarantees are, because I cannot see *any* in the working paper, and any implementation I know offers the strong exception safety guarantee (string unchanged if a member throws exception). The closest the current draft comes to offering any guarantees is 27.4.3 [basic.string], para 3:

> The class template `basic_string` conforms to the requirements for a Sequence Container (23.1.1), for a Reversible Container (23.1), and for an Allocator-aware container (91). The iterators supported by `basic_string` are random access iterators (24.1.5).

However, the chapter 23 only says, on the topic of exceptions: 23.2 [container.requirements], para 10:

> Unless otherwise specified (see 23.2.2.3 and 23.2.6.4) all container types defined in this clause meet the following additional requirements:

> - if an exception is thrown by...

I take it as saying that this paragraph has *no* implication on `std::basic_string`, as `basic_string` isn't defined in Clause 23 and this paragraph does not define a *requirement* of Sequence nor Reversible Container, just of the models defined in Clause 23. In addition, LWG Issue 718(i) proposes to remove 23.2 [container.requirements], para 3.

Finally, the fact that no operation on Traits should throw exceptions has no bearing, except to suggest (since the only other throws should be allocation, `out_of_range`, or `length_error`) that the strong exception guarantee can be achieved.

The reaction in that group by Niels Dekker, Martin Sebor, and Bo Persson, was all that this would be worth an LWG issue.

A related issue is that `erase()` does not throw. This should be stated somewhere (and again, I don't think that the 23.2 [container.requirements], para 1 applies here).

[ San Francisco: ]

> Implementors will study this to confirm that it is actually possible.

[ Daniel adds 2009-02-14: ]

> The proposed resolution of paper N2815 interacts with this issue (the paper does not refer to this issue).

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

Add a blanket statement in 27.4.3.2 [string.require]:

> - if any member function or operator of `basic_string<charT, traits, Allocator>` throws, that function or operator has no effect.

> - no `erase()` or `pop_back()` function throws.

As far as I can tell, this is achieved by any implementation. If I made a mistake and it is not possible to offer this guarantee, then either state all the functions for which this is possible (certainly at least `operator+=`, `append`, `assign`, and `insert`), or add paragraphs to Effects clauses wherever appropriate.

### 848(i). Missing `std::hash` specializations for `std::bitset/std::vector<bool>`

Section: 22.10.19 [unord.hash] Status: CD1 Submitter: Thorsten Ottosen Opened: 2008-06-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unord.hash].

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Discussion:

In the current working draft, `std::hash<T>` is specialized for builtin types and a few other types. Bitsets seems like one that is missing from the list, not because it cannot not be done by the user, but because it is hard or impossible to write an efficient implementation that works on 32bit/64bit chunks at a time. For example, `std::bitset` is too much encapsulated in this respect.

Proposed resolution:

Add the following to the synopsis in 22.10 [function.objects]/2:

>     template<class Allocator> struct hash<std::vector<bool,Allocator>>;
>     template<size_t N> struct hash<std::bitset<N>>;

Modify the last sentence of 22.10.19 [unord.hash]/1 to end with:

> ... and `std::string`, `std::u16string`, `std::u32string`, `std::wstring`, `std::error_code`, `std::thread::id`, `std::bitset`, `and std::vector<bool>`.

### 850(i). Should `shrink_to_fit` apply to `std::deque`?

Section: 23.3.5.3 [deque.capacity] Status: CD1 Submitter: Niels Dekker Opened: 2008-06-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [deque.capacity].

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Discussion:

Issue 755(i) added a `shrink_to_fit` function to `std::vector` and `std::string`. It did not yet deal with `std::deque`, because of the fundamental difference between `std::deque` and the other two container types. The need for `std::deque` may seem less evident, because one might think that for this container, the overhead is a small map, and some number of blocks that's bounded by a small constant.

The container overhead can in fact be arbitrarily large (i.e. is not necessarily O(N) where N is the number of elements currently held by the `deque`). As Bill Plauger noted in a reflector message, unless the map of block pointers is shrunk, it must hold at least maxN⁄B pointers where maxN is the maximum of N over the lifetime of the `deque` since its creation. This is independent of how the map is implemented (`vector`-like circular buffer and all), and maxN bears no relation to N, the number of elements it currently holds.

Hervé Brönnimann reports a situation where a `deque` of requests grew very large due to some temporary backup (the front request hanging), and the map of the `deque` grew quite large before getting back to normal. Just to put some color on it, assuming a `deque` with 1K pointer elements in steady regime, that held, at some point in its lifetime, maxN=10M pointers, with one block holding 128 elements, the spine must be at least (maxN ⁄ 128), in that case 100K. In that case, shrink-to-fit would allow to reuse about 100K which would otherwise never be reclaimed in the lifetime of the `deque`.

An added bonus would be that it allows implementations to hang on to empty blocks at the end (but does not care if they do or not). A `shrink_to_fit` would take care of both shrinks, and guarantee that at most O(B) space is used in addition to the storage to hold the N elements and the N⁄B block pointers.

Proposed resolution:

To class template `deque` 23.3.5 [deque] synopsis, add:

>     void shrink_to_fit();

To deque capacity 23.3.5.3 [deque.capacity], add:

>     void shrink_to_fit();

> > Remarks: `shrink_to_fit` is a non-binding request to reduce memory use. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]

### 852(i). unordered containers `begin(n)` mistakenly `const`

Section: 23.5 [unord] Status: CD1 Submitter: Robert Klarer Opened: 2008-06-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unord].

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Discussion:

In 3 of the four unordered containers the local `begin` member is mistakenly declared `const`:

>     local_iterator begin(size_type n) const;

Proposed resolution:

Change the synopsis in 23.5.3 [unord.map], 23.5.4 [unord.multimap], and 23.5.7 [unord.multiset]:

>     local_iterator begin(size_type n) const;

### 853(i). `to_string` needs updating with `zero` and `one`

Section: 22.9.2 [template.bitset] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [template.bitset].

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Discussion:

Issue 396(i) adds defaulted arguments to the `to_string` member, but neglects to update the three newer `to_string` overloads.

[ post San Francisco: ]

> Daniel found problems with the wording and provided fixes. Moved from Ready to Review.

[ Post Summit: ]

> Alisdair: suggest to not repeat the default arguments in B, C, D (definition of to_string members)

> Walter: This is not really a definition.

> Consensus: Add note to the editor: Please apply editor's judgement whether default arguments should be repeated for B, C, D changes.

> Recommend Tentatively Ready.

[ 2009-05-09: See alternative solution in issue 1113(i). ]

Proposed resolution:

1. replace in 22.9.2 [template.bitset]/1 (class `bitset`)

   >     template <class charT, class traits>
   >       basic_string<charT, traits, allocator<charT> >
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;
   >     template <class charT>
   >       basic_string<charT, char_traits<charT>, allocator<charT> >
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;
   >     basic_string<char, char_traits<char>, allocator<char> >
   >       to_string(char zero = '0', char one = '1') const;

2. replace in 22.9.2.3 [bitset.members]/37

   >     template <class charT, class traits>
   >       basic_string<charT, traits, allocator<charT> >
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;

   > > 37 Returns: `to_string<charT, traits, allocator<charT> >(zero, one)`.

3. replace in 22.9.2.3 [bitset.members]/38

   >     template <class charT>
   >       basic_string<charT, char_traits<charT>, allocator<charT> >
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;

   > > 38 Returns: `to_string<charT, char_traits<charT>, allocator<charT> >(zero, one)`.

4. replace in 22.9.2.3 [bitset.members]/39

   >     basic_string<char, char_traits<char>, allocator<char> >
   >       to_string(char zero = '0', char one = '1') const;

   > > 39 Returns: `to_string<char, char_traits<char>, allocator<char> >(zero, one)`.

### 854(i). `default_delete` converting constructor underspecified

Section: 20.3.1.2.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.dltr.dflt].

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Discussion:

No relationship between `U` and `T` in the converting constructor for `default_delete` template.

Requirements: `U*` is convertible to `T*` and `has_virtual_destructor<T>`; the latter should also become a concept.

Rules out cross-casting.

The requirements for `unique_ptr` conversions should be the same as those on the deleter.

[ Howard adds 2008-11-26: ]

> I believe we need to be careful to not outlaw the following use case, and I believe the current proposed wording (`requires Convertible<U*, T*> && HasVirtualDestructor<T>`) does so:

> >     #include <memory>

> >     int main()
> >     {
> >         std::unique_ptr<int> p1(new int(1));
> >         std::unique_ptr<const int> p2(move(p1));
> >         int i = *p2;
> >     //    *p2 = i;  // should not compile
> >     }

> I've removed "`&& HasVirtualDestructor<T>`" from the `requires` clause in the proposed wording.

[ Post Summit: ]

> Alisdair: This issue has to stay in review pending a paper constraining `unique_ptr`.

> Consensus: We agree with the resolution, but `unique_ptr` needs to be constrained, too.

> Recommend Keep in Review.

[ Batavia (2009-05): ]

> Keep in Review status for the reasons cited.

[ 2009-07 Frankfurt: ]

> The proposed resolution uses concepts. Howard needs to rewrite the proposed resolution.

> Move back to Open.

[ 2009-07-26 Howard provided rewritten proposed wording and moved to Review. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add after 20.3.1.2.2 [unique.ptr.dltr.dflt], p1:

>     template <class U> default_delete(const default_delete<U>& other);

> > -1- Effects: ...

> > Remarks: This constructor shall participate in overload resolution if and only if `U*` is implicitly convertible to `T*`.

### 856(i). Removal of `aligned_union`

Section: 21.3.8.7 [meta.trans.other] Status: CD1 Submitter: Jens Maurer Opened: 2008-06-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [meta.trans.other].

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Discussion:

With the arrival of extended unions (N2544), there is no known use of `aligned_union` that couldn't be handled by the "extended unions" core-language facility.

Proposed resolution:

Remove the following signature from 21.3.3 [meta.type.synop]:

>     template <std::size_t Len, class... Types> struct aligned_union;

Remove the second row from table 51 in 21.3.8.7 [meta.trans.other], starting with:

>     template <std::size_t Len,
>     class... Types>
>     struct aligned_union;

### 857(i). `condition_variable::time_wait` return `bool` error prone

Section: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Beman Dawes Opened: 2008-06-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition.condvar].

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Discussion:

The meaning of the `bool` returned by `condition_variable::timed_wait` is so obscure that even the class' designer can't deduce it correctly. Several people have independently stumbled on this issue.

It might be simpler to change the return type to a scoped enum:

>     enum class timeout { not_reached, reached };

That's the same cost as returning a `bool`, but not subject to mistakes. Your example below would be:

>     if (cv.wait_until(lk, time_limit) == timeout::reached )
>       throw time_out();

[ Beman to supply exact wording. ]

[ San Francisco: ]

> There is concern that the enumeration names are just as confusing, if not more so, as the bool. You might have awoken because of a signal or a spurious wakeup, for example.

> Group feels that this is a defect that needs fixing.

> Group prefers returning an enum over a void return.

> Howard to provide wording.

[ 2009-06-14 Beman provided wording. ]

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

Change Condition variables 32.7 [thread.condition], Header condition_variable synopsis, as indicated:

>     namespace std {
>       class condition_variable;
>       class condition_variable_any;

>       enum class cv_status { no_timeout, timeout };
>     }

Change class `condition_variable` 32.7.4 [thread.condition.condvar] as indicated:

>     class condition_variable {
>     public:
>       ...
>       template <class Clock, class Duration>
>         bool cv_status wait_until(unique_lock<mutex>& lock,
>                         const chrono::time_point<Clock, Duration>& abs_time);
>       template <class Clock, class Duration, class Predicate>
>         bool wait_until(unique_lock<mutex>& lock,
>                         const chrono::time_point<Clock, Duration>& abs_time,
>                         Predicate pred);

>       template <class Rep, class Period>
>         bool cv_status wait_for(unique_lock<mutex>& lock,
>                       const chrono::duration<Rep, Period>& rel_time);
>       template <class Rep, class Period, class Predicate>
>         bool wait_for(unique_lock<mutex>& lock,
>                       const chrono::duration<Rep, Period>& rel_time,
>                       Predicate pred);
>       ...
>     };

>     ...

>     template <class Clock, class Duration>
>       bool cv_status wait_until(unique_lock<mutex>& lock,
>                       const chrono::time_point<Clock, Duration>& abs_time);

> > -15- Precondition: `lock` is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting threads (via `wait`, `wait_for` or `wait_until`.).

> > -16- Effects:

> > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.
> > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the current time exceeding `abs_time` if `Clock::now() >= abs_time`, or spuriously.
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> > -17- Postcondition: `lock` is locked by the calling thread.

> > -18- Returns: `Clock::now() < abs_time` `cv_status::timeout` if the function unblocked because `abs_time` was reached, otherwise `cv_status::no_timeout`.

> > -19- Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > -20- Error conditions:

> > - `operation_not_permitted` — if the thread does not own the lock.
> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

>     template <class Rep, class Period>
>       bool cv_status wait_for(unique_lock<mutex>& lock,
>                     const chrono::duration<Rep, Period>& rel_time);

> > -21- Effects Returns:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

> > -22- Returns: `false` if the call is returning because the time duration specified by `rel_time` has elapsed, otherwise `true`.

> > [ This part of the wording may conflict with 859(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

>     template <class Clock, class Duration, class Predicate>
>       bool wait_until(unique_lock<mutex>& lock,
>                       const chrono::time_point<Clock, Duration>& abs_time,
>                       Predicate pred);

> > -23- Effects:

> > >     while (!pred())
> > >       if (!wait_until(lock, abs_time) == cv_status::timeout)
> > >         return pred();
> > >     return true;

> > -24- Returns: `pred()`.

> > -25- [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. — end note].

Change Class condition_variable_any 32.7.5 [thread.condition.condvarany] as indicated:

>     class condition_variable_any {
>     public:
>       ...
>       template <class Lock, class Clock, class Duration>
>         bool cv_status wait_until(Lock& lock,
>                         const chrono::time_point<Clock, Duration>& abs_time);
>       template <class Lock, class Clock, class Duration, class Predicate>
>         bool wait_until(Lock& lock,
>                         const chrono::time_point<Clock, Duration>& abs_time,
>                         Predicate pred);

>       template <class Lock, class Rep, class Period>
>         bool cv_status wait_for(Lock& lock,
>                       const chrono::duration<Rep, Period>& rel_time);
>       template <class Lock, class Rep, class Period, class Predicate>
>         bool wait_for(Lock& lock,
>                       const chrono::duration<Rep, Period>& rel_time,
>                       Predicate pred);
>       ...
>     };

>     ...

>     template <class Lock, class Clock, class Duration>
>       bool cv_status wait_until(Lock& lock,
>                       const chrono::time_point<Clock, Duration>& abs_time);

> > -13- Effects:

> > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.
> > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the current time exceeding `abs_time` if `Clock::now() >= abs_time`, or spuriously.
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> > -14- Postcondition: `lock` is locked by the calling thread.

> > -15- Returns: `Clock::now() < abs_time` `cv_status::timeout` if the function unblocked because `abs_time` was reached, otherwise `cv_status::no_timeout`.

> > -16- Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > -17- Error conditions:

> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

>     template <class Lock, class Rep, class Period>
>       bool cv_status wait_for(Lock& lock,
>                     const chrono::duration<Rep, Period>& rel_time);

> > -18- Effects Returns:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

> > -19- Returns: `false` if the call is returning because the time duration specified by `rel_time` has elapsed, otherwise `true`.

> > [ This part of the wording may conflict with 859(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

>     template <class Lock, class Clock, class Duration, class Predicate>
>       bool wait_until(Lock& lock,
>                       const chrono::time_point<Clock, Duration>& rel_time abs_time,
>                       Predicate pred);

> > -20- Effects:

> > >     while (!pred())
> > >       if (!wait_until(lock, abs_time) == cv_status::timeout)
> > >         return pred();
> > >     return true;

> > -21- Returns: `pred()`.

> > -22- [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. — end note].

### 858(i). Wording for Minimal Support for Garbage Collection

Section: 99 [util.dynamic.safety] Status: CD1 Submitter: Pete Becker Opened: 2008-06-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.dynamic.safety].

View all issues with CD1 status.

Discussion:

The first sentence of the Effects clause for `undeclare_reachable` seems to be missing some words. I can't parse

> ... for all non-null `p` referencing the argument is no longer declared reachable...

I take it the intent is that `undeclare_reachable` should be called only when there has been a corresponding call to `declare_reachable`. In particular, although the wording seems to allow it, I assume that code shouldn't call `declare_reachable` once then call `undeclare_reachable` twice.

I don't know what "shall be live" in the Requires clause means.

In the final Note for `undeclare_reachable`, what does "cannot be deallocated" mean? Is this different from "will not be able to collect"?

For the wording on nesting of `declare_reachable` and `undeclare_reachable`, the words for locking and unlocking recursive mutexes probably are a good model.

[ San Francisco: ]

> Nick: what does "shall be live" mean?

> Hans: I can provide an appropriate cross-reference to the Project Editor to clarify the intent.

Proposed resolution:

In 99 [util.dynamic.safety] (N2670, Minimal Support for Garbage Collection)

Add at the beginning, before paragraph 39

> A complete object is declared reachable while the number of calls to `declare_reachable` with an argument referencing the object exceeds the number of `undeclare_reachable` calls with pointers to the same complete object.

Change paragraph 42 (Requires clause for `undeclare_reachable`)

> If `p` is not null, `declare_reachable(p)` was previously called the complete object referenced by `p` shall have been previously declared reachable, and shall be live (6.8.4 [basic.life]) from the time of the call until the last `undeclare_reachable(p)` call on the object.

Change the first sentence in paragraph 44 (Effects clause for `undeclare_reachable`):

> Effects: Once the number of calls to `undeclare_reachable(p)` equals the number of calls to `declare_reachable(p)` for all non-null `p` referencing the argument is no longer declared reachable. When this happens, pointers to the object referenced by p may not be subsequently dereferenced. After a call to `undeclare_reachable(p)`, if `p` is not null and the object `q` referenced by `p` is no longer declared reachable, then dereferencing any pointer to `q` that is not safely derived results in undefined behavior. ...

Change the final note:

[Note: It is expected that calls to `declare_reachable(p)` will consume a small amount of memory, in addition to that occupied by the referenced object, until the matching call to `undeclare_reachable(p)` is encountered. In addition, the referenced object cannot be deallocated during this period, and garbage collecting implementations will not be able to collect the object while it is declared reachable. Long running programs should arrange that calls for short-lived objects are matched. --end note]

### 859(i). Monotonic Clock is Conditionally Supported?

Section: 32.7 [thread.condition] Status: C++11 Submitter: Pete Becker Opened: 2008-06-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition].

View all issues with C++11 status.

Discussion:

Related to 958(i), 959(i).

N2661 says that there is a class named `monotonic_clock`. It also says that this name may be a synonym for `system_clock`, and that it's conditionally supported. So the actual requirement is that it can be monotonic or not, and you can tell by looking at `is_monotonic`, or it might not exist at all (since it's conditionally supported). Okay, maybe too much flexibility, but so be it.

A problem comes up in the threading specification, where several variants of `wait_for` explicitly use `monotonic_clock::now()`. What is the meaning of an effects clause that says

>     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

when `monotonic_clock` is not required to exist?

[ San Francisco: ]

> Nick: maybe instead of saying that `chrono::monotonic_clock` is conditionally supported, we could say that it's always there, but not necessarily supported..

> Beman: I'd prefer a typedef that identifies the best clock to use for `wait_for` locks.

> Tom: combine the two concepts; create a duration clock type, but keep the is_monotonic test.

> Howard: if we create a `duration_clock` type, is it a typedef or an entirely true type?

> There was broad preference for a typedef.

> Move to Open. Howard to provide wording to add a typedef for duration_clock and to replace all uses of monotonic_clock in function calls and signatures with duration_clock.

[ Howard notes post-San Francisco: ]

> After further thought I do not believe that creating a `duration_clock typedef` is the best way to proceed. An implementation may not need to use a `time_point` to implement the `wait_for` functions.

> For example, on POSIX systems `sleep_for` can be implemented in terms of `nanosleep` which takes only a duration in terms of nanoseconds. The current working paper does not describe `sleep_for` in terms of `sleep_until`. And paragraph 2 of 32.2.4 [thread.req.timing] has the words strongly encouraging implementations to use monotonic clocks for `sleep_for`:

> > 2 The member functions whose names end in `_for` take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions.

> I believe the approach taken in describing the effects of `sleep_for` and `try_lock_for` is also appropriate for `wait_for`. I.e. these are not described in terms of their `_until` variants.

[ 2009-07 Frankfurt: ]

> Beman will send some suggested wording changes to Howard.

> Move to Ready.

[ 2009-07-21 Beman added the requested wording changes to 962(i). ]

Proposed resolution:

Change 32.7.4 [thread.condition.condvar], p21-22:

>     template <class Rep, class Period>
>       bool wait_for(unique_lock<mutex>& lock,
>                     const chrono::duration<Rep, Period>& rel_time);

> > Precondition: `lock` is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting threads (via `wait`, `wait_for` or `wait_until`).

> > 21 Effects:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

> > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.
> > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the elapsed time `rel_time` passing (32.2.4 [thread.req.timing]), or spuriously.
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> > Postcondition: `lock` is locked by the calling thread.

> > 22 Returns: `false` if the call is returning because the time duration specified by `rel_time` has elapsed, otherwise `true`.

> > [ This part of the wording may conflict with 857(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

> > Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > Error conditions:

> > - `operation_not_permitted` -- if the thread does not own the lock.
> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

Change 32.7.4 [thread.condition.condvar], p26-p29:

>     template <class Rep, class Period, class Predicate>
>       bool wait_for(unique_lock<mutex>& lock,
>                     const chrono::duration<Rep, Period>& rel_time,
>                     Predicate pred);

> > Precondition: `lock` is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting threads (via `wait`, `wait_for` or `wait_until`).

> > 26 Effects:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))

> > > - Executes a loop: Within the loop the function first evaluates `pred()` and exits the loop if the result of `pred()` is `true`.
> > > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > > - When unblocked, calls `lock.lock()` (possibly blocking on the lock).
> > > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the elapsed time `rel_time` passing (30.1.4 [thread.req.timing]), or spuriously.
> > > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.
> > > - The loop terminates when `pred()` returns `true` or when the time duration specified by `rel_time` has elapsed.

> > 27 [Note: There is no blocking if `pred()` is initially `true`, even if the timeout has already expired. -- end note]

> > Postcondition: `lock` is locked by the calling thread.

> > 28 Returns: `pred()`

> > 29 [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. -- end note]

> > Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > Error conditions:

> > - `operation_not_permitted` -- if the thread does not own the lock.
> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

Change 32.7.5 [thread.condition.condvarany], p18-19:

>     template <class Lock, class Rep, class Period>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

> > 18 Effects:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

> > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.
> > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the elapsed time `rel_time` passing (32.2.4 [thread.req.timing]), or spuriously.
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> > Postcondition: `lock` is locked by the calling thread.

> > 19 Returns: `false` if the call is returning because the time duration specified by `rel_time` has elapsed, otherwise `true`.

> > Throws: `std::system_error` when the returned value, effects, or postcondition cannot be achieved.

> > Error conditions:

> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

Change 32.7.5 [thread.condition.condvarany], p23-p26:

>     template <class Lock, class Rep, class Period, class Predicate>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

> > Precondition: `lock` is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting threads (via `wait`, `wait_for` or `wait_until`).

> > 23 Effects:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))

> > > - Executes a loop: Within the loop the function first evaluates `pred()` and exits the loop if the result of `pred()` is `true`.
> > > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > > - When unblocked, calls `lock.lock()` (possibly blocking on the lock).
> > > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the elapsed time `rel_time` passing (30.1.4 [thread.req.timing]), or spuriously.
> > > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.
> > > - The loop terminates when `pred()` returns `true` or when the time duration specified by `rel_time` has elapsed.

> > 24 [Note: There is no blocking if `pred()` is initially `true`, even if the timeout has already expired. -- end note]

> > Postcondition: `lock` is locked by the calling thread.

> > 25 Returns: `pred()`

> > 26 [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. -- end note]

> > Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > Error conditions:

> > - `operation_not_permitted` -- if the thread does not own the lock.
> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

### 860(i). Floating-Point State

Section: 29 [numerics] Status: C++11 Submitter: Lawrence Crowl Opened: 2008-06-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [numerics].

View all issues with C++11 status.

Discussion:

There are a number of functions that affect the floating point state. These function need to be thread-safe, but I'm unsure of the right approach in the standard, as we inherit them from C.

[ San Francisco: ]

> Nick: I think we already say that these functions do not introduce data races; see 17.6.5.6/20

> Pete: there's more to it than not introducing data races; are these states maintained per thread?

> Howard: 21.5/14 says that strtok and strerror are not required to avoid data races, and 20.9/2 says the same about asctime, gmtime, ctime, and gmtime.

> Nick: POSIX has a list of not-safe functions. All other functions are implicitly thread safe.

> Lawrence is to form a group between meetings to attack this issue. Nick and Tom volunteered to work with Lawrence.

> Move to Open.

[ Post Summit: ]

> Hans: Sane oses seem ok. Sensible thing is implementable and makes sense.

> Nick: Default wording seems to cover this? Hole in POSIX, these functions need to be added to list of thread-unsafe functions.

> Lawrence: Not sufficient, not "thread-safe" per our definition, but think of state as a thread-local variable. Need something like "these functions only affect state in the current thread."

> Hans: Suggest the following wording: "The floating point environment is maintained per-thread."

> Walter: Any other examples of state being thread safe that are not already covered elsewhere?

> Have thread unsafe functions paper which needs to be updated. Should just fold in 29.3 [cfenv] functions.

> Recommend Open. Lawrence instead suggests leaving it open until we have suitable wording that may or may not include the thread local commentary.

[ 2009-09-23 Hans provided wording. ]

> If I understand the history correctly, Nick, as the Posix liaison, should probably get a veto on this, since I think it came from Posix (?) via WG14 and should probably really be addressed there (?). But I think we are basically in agreement that there is no other sane way to do this, and hence we don't have to worry too much about stepping on toes. As far as I can tell, this same issue also exists in the latest Posix standard (?).

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

Add at the end of 29.3.1 [cfenv.syn]:

> 2 The header defines all functions, types, and macros the same as C99 7.6.

> A separate floating point environment shall be maintained for each thread. Each function accesses the environment corresponding to its calling thread.

### 861(i). Incomplete specification of `EqualityComparable` for `std::forward_list`

Section: 23.2 [container.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2008-06-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

View all issues with C++11 status.

Discussion:

Table 89, Container requirements, defines `operator==` in terms of the container member function `size()` and the algorithm `std::equal`:

> `==` is an equivalence relation. `a.size() == b.size() && equal(a.begin(), a.end(), b.begin()`

The new container `forward_list` does not provide a `size` member function by design but does provide `operator==` and `operator!=` without specifying it's semantic.

Other parts of the (sequence) container requirements do also depend on `size()`, e.g. `empty()` or `clear()`, but this issue explicitly attempts to solve the missing `EqualityComparable` specification, because of the special design choices of `forward_list`.

I propose to apply one of the following resolutions, which are described as:

1. Provide a definition, which is optimal for this special container without previous size test. This choice prevents two `O(N)` calls of `std::distance()` with the corresponding container ranges and instead uses a special `equals` implementation which takes two container ranges instead of 1 1/2.
2. The simple fix where the usual test is adapted such that `size()` is replaced by `distance` with corresponding performance disadvantages.

Both proposal choices are discussed, the preferred choice of the author is to apply (A).

[ San Francisco: ]

> There's an Option C: change the requirements table to use distance().

> LWG found Option C acceptable.

> Martin will draft the wording for Option C.

[ post San Francisco: ]

> Martin provided wording for Option C.

[ 2009-07 Frankfurt ]

> Other operational semantics (see, for example, Tables 82 and 83) are written in terms of a container's size() member. Daniel to update proposed resolution C.

> [ Howard: Commented out options A and B. ]

[ 2009-07-26 Daniel updated proposed resolution C. ]

[ 2009-10 Santa Cruz: ]

> Mark NAD Editorial. Addressed by N2986.

[ 2009-10 Santa Cruz: ]

> Reopened. N2986 was rejected in full committee on procedural grounds.

[ 2010-01-30 Howard updated Table numbers. ]

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

Option (C):

> 1. In 23.2.2 [container.requirements.general] change Table 90 -- Container requirements as indicated:

>    1. Change the text in the Assertion/note column in the row for "`X u`;" as follows:

>       > post: `u.size() == 0empty() == true`

>    2. Change the text in the Assertion/note column in the row for "`X();`" as follows:

>       > `X().size() == 0empty() == true`

>    3. Change the text in the Operational Semantics column in the row for "`a == b`" as follows:

>       > `==` is an equivalence relation. `a.size()distance(a.begin(), a.end()) == b.size()distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin())`

>    4. Add text in the Ass./Note/pre-/post-condition column in the row for "`a == b`" as follows:

>       > Requires: `T` is `EqualityComparable`

>    5. Change the text in the Operational Semantics column in the row for "`a.size()`" as follows:

>       > `a.end() - a.begin()distance(a.begin(), a.end())`

>    6. Change the text in the Operational Semantics column in the row for "`a.max_size()`" as follows:

>       > `size()distance(begin(), end())` of the largest possible container

>    7. Change the text in the Operational Semantics column in the row for "`a.empty()`" as follows:

>       > `a.size() == 0a.begin() == a.end()`

> 2. In 23.2.2 [container.requirements.general] change Table 93 — Allocator-aware container requirements as indicated:

>    1. Change the text in the Assertion/note column in the row for "`X() / X u;`" as follows:

>       > Requires: `A` is `DefaultConstructible` post: `u.size() == 0u.empty() == true`, `get_allocator() == A()`

>    2. Change the text in the Assertion/note column in the row for "`X(m) / X u(m);`" as follows:

>       > post: `u.size() == 0u.empty() == true, get_allocator() == m`

> 3. In 23.2.4 [sequence.reqmts] change Table 94 — Sequence container requirements as indicated:

>    1. Change the text in the Assertion/note column in the row for "`X(n, t) / X a(n, t)`" as follows:

>       > post: `size()distance(begin(), end()) == n [..]`

>    2. Change the text in the Assertion/note column in the row for "`X(i, j) / X a(i, j)`" as follows:

>       > [..] post: `size() ==` distance between `i` and `j` `distance(begin(), end()) == distance(i, j)` [..]

>    3. Change the text in the Assertion/note column in the row for "`a.clear()`" as follows:

>       > `a.erase(a.begin(), a.end())` post: `size() == 0a.empty() == true`

> 4. In 23.2.7 [associative.reqmts] change Table 96 — Associative container requirements as indicated:

>    [ Not every occurrence of `size()` was replaced, because all current associative containers have a `size`. The following changes ensure consistency regarding the semantics of "`erase`" for all tables and adds some missing objects ]

>    1. Change the text in the Complexity column in the row for `X(i,j,c)/Xa(i,j,c);` as follows:

>       > `N` log `N` in general (`N` `== distance(i, j)`is the distance from `i` to `j`); ...

>    2. Change the text in the Complexity column in the row for "`a.insert(i, j)`" as follows:

>       > `N log(a.size() + N)` (`N` is the distance from `i` to `j`) where `N == distance(i, j)`

>    3. Change the text in the Complexity column in the row for "`a.erase(k)`" as follows:

>       > `log(a.size()) + a.count(k)`

>    4. Change the text in the Complexity column in the row for "`a.erase(q1, q2)`" as follows:

>       > `log(a.size()) + N` where `N` is the distance from `q1` to `q2` `== distance(q1, q2)`.

>    5. Change the text in the Assertion/note column in the row for "`a.clear()`" as follows:

>       > `a.erase(a.begin(),a.end())` post: `size() == 0a.empty() == true`

>    6. Change the text in the Complexity column in the row for "`a.clear()`" as follows:

>       > linear in `a.size()`

>    7. Change the text in the Complexity column in the row for "`a.count(k)`" as follows:

>       > `log(a.size()) + a.count(k)`

> 5. In 23.2.8 [unord.req] change Table 98 — Unordered associative container requirements as indicated:

>    [ The same rational as for Table 96 applies here ]

>    1. Change the text in the Assertion/note column in the row for "`a.clear()`" as follows:

>       > [..] Post: `a.size() == 0empty() == true`

### 865(i). More algorithms that throw away information

Section: 26.7.6 [alg.fill], 26.7.7 [alg.generate] Status: C++11 Submitter: Daniel Krügler Opened: 2008-07-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In regard to library defect 488(i) I found some more algorithms which unnecessarily throw away information. These are typically algorithms, which sequentially write into an `OutputIterator`, but do not return the final value of this output iterator. These cases are:

1.     template<class OutputIterator, class Size, class T>
       void fill_n(OutputIterator first, Size n, const T& value);

2.     template<class OutputIterator, class Size, class Generator>
       void generate_n(OutputIterator first, Size n, Generator gen);

In both cases the minimum requirements on the iterator are `OutputIterator`, which means according to the requirements of 24.3.5.4 [output.iterators] p. 2 that only single-pass iterations are guaranteed. So, if users of `fill_n` and `generate_n` have only an `OutputIterator` available, they have no chance to continue pushing further values into it, which seems to be a severe limitation to me.

[ Post Summit Daniel "conceptualized" the wording. ]

[ Batavia (2009-05): ]

> Alisdair likes the idea, but has concerns about the specific wording about the returns clauses.

> Alan notes this is a feature request.

> Bill notes we have made similar changes to other algorithms.

> Move to Open.

[ 2009-07 Frankfurt ]

> We have a consensus for moving forward on this issue, but Daniel needs to deconceptify it.

[ 2009-07-25 Daniel provided non-concepts wording. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

1. Replace the current declaration of `fill_n` in 26 [algorithms]/2, header `<algorithm>` synopsis and in 26.7.6 [alg.fill] by

   >     template<class OutputIterator, class Size, class T>
   >       voidOutputIterator fill_n(OutputIterator first, Size n, const T& value);

   Just after the effects clause add a new returns clause saying:

   > Returns: For `fill_n` and positive `n`, returns `first + n`. Otherwise returns `first` for `fill_n`.

2. Replace the current declaration of `generate_n` in 26 [algorithms]/2, header `<algorithm>` synopsis and in 26.7.7 [alg.generate] by

   >     template<class OutputIterator, class Size, class Generator>
   >       voidOutputIterator generate_n(OutputIterator first, Size n, Generator gen);

   Just after the effects clause add a new returns clause saying:

   > For `generate_n` and positive `n`, returns `first + n`. Otherwise returns `first` for `generate_n`.

### 866(i). Qualification of placement new-expressions

Section: 26.11 [specialized.algorithms], 20.3.2.2.7 [util.smartptr.shared.create] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-07-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

LWG issue 402(i) replaced "`new`" with "`::new`" in the placement new-expression in 20.2.10.2 [allocator.members]. I believe the rationale given in 402(i) applies also to the following other contexts:

- in 26.11 [specialized.algorithms], all four algorithms `unitialized_copy`, `unitialized_copy_n`, `unitialized_fill` and `unitialized_fill_n` use the unqualified placement new-expression in some variation of the form:

  >     new  (static_cast<void*>(&*result)) typename  iterator_traits<ForwardIterator>::value_type(*first);

- in 20.3.2.2.7 [util.smartptr.shared.create] there is a reference to the unqualified placement new-expression:

  >     new  (pv)  T(std::forward<Args>(args)...),

I suggest to add qualification in all those places. As far as I know, these are all the remaining places in the whole library that explicitly use a placement new-expression. Should other uses come out, they should be qualified as well.

As an aside, a qualified placement new-expression does not need additional requirements to be compiled in a constrained context. By adding qualification, the `HasPlacementNew` concept introduced recently in N2677 (Foundational Concepts) would no longer be needed by library and should therefore be removed.

[ San Francisco: ]

> Detlef: If we move this to Ready, it's likely that we'll forget about the side comment about the `HasPlacementNew` concept.

[ post San Francisco: ]

> Daniel: `HasPlacementNew` has been removed from N2774 (Foundational Concepts).

Proposed resolution:

Replace "`new`" with "`::new`" in:

- 26.11.5 [uninitialized.copy], paragraphs 1 and 3
- 26.11.7 [uninitialized.fill] paragraph 1
- [uninitialized.fill.n] paragraph 1
- 20.3.2.2.7 [util.smartptr.shared.create] once in paragraph 1 and twice in paragraph 2.

### 868(i). Default construction and value-initialization

Section: 23 [containers] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-07-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The term "default constructed" is often used in wording that predates the introduction of the concept of value-initialization. In a few such places the concept of value-initialization is more correct than the current wording (for example when the type involved can be a built-in) so a replacement is in order. Two of such places are already covered by issue 867(i). This issue deliberately addresses the hopefully non-controversial changes in the attempt of being approved more quickly. A few other occurrences (for example in `std::tuple`, `std::reverse_iterator` and `std::move_iterator`) are left to separate issues. For `std::reverse_iterator`, see also issue 408(i). This issue is related with issue 724(i).

[ San Francisco: ]

> The list provided in the proposed resolution is not complete. James Dennett will review the library and provide a complete list and will double-check the vocabulary.

> This issue relates to Issue 886(i) tuple construction

[ 2009-07 Frankfurt ]

> The proposed resolution is incomplete.

> Move to Tentatively NAD Future. Howard will contact Ganesh for wording. If wording is forthcoming, Howard will move it back to Review.

[ 2009-07-18 Ganesh updated the proposed wording. ]

> Howard: Moved back to Review. Note that 16.4.4.2 [utility.arg.requirements] refers to a section that is not in the current working paper, but does refer to a section that we expect to reappear after the de-concepts merge. This was a point of confusion we did not recognize when we reviewed this issue in Frankfurt.

> Howard: Ganesh also includes a survey of places in the WP surveyed for changes of this nature and purposefully not treated:

> > Places where changes are not being proposed

> > In the following paragraphs, we are not proposing changes because it's not clear whether we actually prefer value-initialization over default-initialization (now partially covered by 1012(i)):

> > - 20.3.1.3.2 [unique.ptr.single.ctor] para 3 e 7

> > - 24.5.1.4 [reverse.iter.cons] para 1

> > - [move.iter.op.const] para 1

> > In the following paragraphs, the expression "default constructed" need not be changed, because the relevant type does not depend on a template parameter and has a user-provided constructor:

> > - [func.referenceclosure.invoke] para 12, type: reference_closure

> > - 32.4.3.3 [thread.thread.constr] para 30, type: thread

> > - 32.4.3.6 [thread.thread.member] para 52, type: thread_id

> > - 32.4.5 [thread.thread.this], para 1, type: thread_id

[ 2009-08-18 Daniel adds: ]

> I have no objections against the currently suggested changes, but I also cross-checked with the list regarding intentionally excluded changes, and from this I miss the discussion of

> 1. 27.4.3.2 [string.require] p. 2:

>    > "[..] The `Allocator` object used shall be a copy of the `Allocator>` object passed to the `basic_string` object's constructor or, if the constructor does not take an `Allocator` argument, a copy of a default-constructed `Allocator` object."

> 2. N2723, 29.5.3.4 [rand.req.eng], Table 109, expression "`T()`":

>    > Pre-/post-condition: "Creates an engine with the same initial state as all other default-constructed engines of type `X`."

>    as well as in 29.5.6 [rand.predef]/1-9 (N2914), 29.5.8.1 [rand.util.seedseq]/3, 31.7.5.2.2 [istream.cons]/3, 31.7.6.2.2 [ostream.cons]/9 (N2914), 28.6.12 [re.grammar]/2, 32.4.3.5 [thread.thread.assign]/1 (N2914),

>    [ Candidates for the "the expression "default constructed" need not be changed" list ]

>    I'm fine, if these would be added to the intentionally exclusion list, but mentioning them makes it easier for other potential reviewers to decide on the relevance or not-relevance of them for this issue.

> 3. I suggest to remove the reference of [func.referenceclosure.invoke] in the "it's not clear" list, because this component does no longer exist.

> 4. I also suggest to add a short comment that all paragraphs in the resolution whether they refer to N2723 or to N2914 numbering, because e.g. "Change 23.3.5.2 [deque.cons] para 5" is an N2723 coordinate, while "Change 23.3.5.3 [deque.capacity] para 1" is an N2914 coordinate. Even better would be to use one default document for the numbering (probably N2914) and mention special cases (e.g. "Change 16.4.4.2 [utility.arg.requirements] para 2" as referring to N2723 numbering).

[ 2009-08-18 Alisdair adds: ]

> I strongly believe the term "default constructed" should not appear in the library clauses unless we very clearly define a meaning for it, and I am not sure what that would be.

> In those cases where we do not want to replace "default constructed" with "vale initialized" we should be using "default initialized". If we have a term that could mean either, we reduce portability of programs.

> I have not done an exhaustive review to clarify if that is a vendor freedom we have reason to support (e.g. value-init in debug, default-init in release) so I may yet be convinced that LWG has reason to define this new term of art, but generally C++ initialization is confusing enough without supporting further ill-defined terms.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010 Pittsburgh: ]

> Moved to review in order to enable conflict resolution with 704(i).

[ 2010-03-26 Daniel harmonized the wording with the upcoming FCD. ]

[ 2010 Rapperswil: ]

> Move to Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 16.4.4.2 [utility.arg.requirements] para 2:

> 2 In general, a default constructor is not required. Certain container class member function signatures specify the default constructor `T()` as a default argument. `T()` shall be a well-defined expression (8.5) if one of those signatures is called using the default argument (8.3.6).

Change 23.3.5.2 [deque.cons] para 3:

> 3 Effects: Constructs a deque with `n` default constructedvalue-initialized elements.

Change 23.3.5.3 [deque.capacity] para 1:

> 1 Effects: If `sz < size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` default constructedvalue-initialized elements to the sequence.

Change [forwardlist.cons] para 3:

> 3 Effects: Constructs a `forward_list` object with `n` default constructedvalue-initialized elements.

Change [forwardlist.modifiers] para 22:

> 22 Effects: [...] For the first signature the inserted elements are default constructedvalue-initialized, and for the second signature they are copies of `c`.

Change 23.3.11.2 [list.cons] para 3:

> 3 Effects: Constructs a `list` with `n` default constructedvalue-initialized elements.

Change 23.3.11.3 [list.capacity] para 1:

> 1 Effects: If `sz < size()`, equivalent to `list<T>::iterator it = begin(); advance(it, sz); erase(it, end());`. If `size() < sz`, `appends sz - size()` default constructedvalue-initialized elements to the sequence.

Change 23.3.13.2 [vector.cons] para 3:

> 3 Effects: Constructs a `vector` with `n` default constructedvalue-initialized elements.

Change 23.3.13.3 [vector.capacity] para 9:

> 9 Effects: If `sz < size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` default constructedvalue-initialized elements to the sequence.

### 869(i). Bucket (local) iterators and iterating past end

Section: 23.2.8 [unord.req] Status: C++11 Submitter: Sohail Somani Opened: 2008-07-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Is there any language in the current draft specifying the behaviour of the following snippet?

>     unordered_set<int> s;
>     unordered_set<int>::local_iterator it = s.end(0);

>     // Iterate past end - the unspecified part
>     it++;

I don't think there is anything about `s.end(n)` being considered an iterator for the past-the-end value though (I think) it should be.

[ San Francisco: ]

> We believe that this is not a substantive change, but the proposed change to the wording is clearer than what we have now.

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

Change Table 97 "Unordered associative container requirements" in 23.2.8 [unord.req]:

> Table 97: Unordered associative container requirements
> expression | return type | assertion/note pre/post-condition | complexity
> --- | --- | --- | ---
> `b.begin(n)` | `local_iterator` `const_local_iterator` for const `b`. | Pre: n shall be in the range [0,b.bucket_count()). Note: [b.begin(n), b.end(n)) is a valid range containing all of the elements in the nth bucket. `b.begin(n)` returns an iterator referring to the first element in the bucket. If the bucket is empty, then `b.begin(n) == b.end(n)`. | Constant
> `b.end(n)` | `local_iterator` `const_local_iterator` for const `b`. | Pre: n shall be in the range `[0, b.bucket_count())`. `b.end(n)` returns an iterator which is the past-the-end value for the bucket. | Constant

### 870(i). Do unordered containers not support function pointers for predicate/hasher?

Section: 23.2.8 [unord.req] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Good ol' associative containers allow both function pointers and function objects as feasible comparators, as described in 23.2.7 [associative.reqmts]/2:

> Each associative container is parameterized on `Key` and an ordering relation `Compare` that induces a strict weak ordering (25.3) on elements of Key. [..]. The object of type `Compare` is called the comparison object of a container. This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.[..]

The corresponding wording for unordered containers is not so clear, but I read it to disallow function pointers for the hasher and I miss a clear statement for the equality predicate, see 23.2.8 [unord.req]/3+4+5:

> Each unordered associative container is parameterized by `Key`, by a function object `Hash` that acts as a hash function for values of type `Key`, and by a binary predicate `Pred` that induces an equivalence relation on values of type `Key`.[..]

> A hash function is a function object that takes a single argument of type `Key` and returns a value of type `std::size_t`.

> Two values `k1` and `k2` of type `Key` are considered equal if the container's equality function object returns `true` when passed those values.[..]

and table 97 says in the column "assertion...post-condition" for the expression `X::hasher`:

> `Hash` shall be a unary function object type such that the expression `hf(k)` has type `std::size_t`.

Note that 22.10 [function.objects]/1 defines as "Function objects are objects with an `operator()` defined.[..]"

Does this restriction exist by design or is it an oversight? If an oversight, I suggest that to apply the following

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Ask Daniel to provide proposed wording that: makes it explicit that function pointers are function objects at the beginning of 22.10 [function.objects]; fixes the "requirements" for typedefs in 22.10.6 [refwrap] to instead state that the function objects defined in that clause have these typedefs, but not that these typedefs are requirements on function objects; remove the wording that explicitly calls out that associative container comparators may be function pointers.

[ 2009-12-19 Daniel updates wording and rationale. ]

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Rationale:

The below provided wording also affects some part of the library which is involved with callable types (22.10.3 [func.def]/3). Reason for this is that callable objects do have a lot in common with function objects. A simple formula seems to be:

> callable objects = function objects + pointers to member

The latter group is excluded from function objects because of the expression-based usage of function objects in the algorithm clause, which is incompatible with the notation to dereference pointers to member without a concept map available in the language.

This analysis showed some currently existing normative definition differences between the above subset of callable objects and function objects which seem to be unintended: Backed by the Santa Cruz outcome function objects should include both function pointers and "object[s] with an operator() defined". This clearly excludes class types with a conversion function to a function pointer or all similar conversion function situations described in 12.2 [over.match]/2 b. 2. In contrast to this, the wording for callable types seems to be less constrained (22.10.3 [func.def]/3):

> A callable type is a [..] class type whose objects can appear immediately to the left of a function call operator.

The rationale given in N1673 and a recent private communication with Peter Dimov revealed that the intention of this wording was to cover the above mentioned class types with conversion functions as well. To me the current wording of callable types can be read either way and I suggest to make the intention more explicit by replacing

> [..] class type whose objects can appear immediately to the left of a function call operator

by

> [..] class type whose objects can appear as the leftmost subexpression of a function call expression 7.6.1.3 [expr.call].

and to use the same definition for the class type part of function objects, because there is no reason to exclude class types with a conversion function to e.g. pointer to function from being used in algorithms.

Now this last term "function objects" itself brings us to a third unsatisfactory state: The term is used both for objects (e.g. "Function objects are objects[..]" in 22.10 [function.objects]/1) and for types (e.g. "Each unordered associative container is parameterized [..] by a function object Hash that acts as a hash function [..]" in 23.2.8 [unord.req]/3). This impreciseness should be fixed and I suggest to introduce the term function object type as the counter part to callable type. This word seems to be a quite natural choice, because the library already uses it here and there (e.g. "Hash shall be a unary function object type such that the expression `hf(k)` has type `std::size_t`." in Table 98, "`X::hasher`" or "Requires: `T` shall be a function object type [..]" in 22.10.17.3.6 [func.wrap.func.targ]/3).

Finally I would like to add that part of the issue 870 discussion related to the requirements for typedefs in 22.10.6 [refwrap] during the Santa Cruz meeting is now handled by the new issue 1290(i).

Obsolete rationale:

> [ San Francisco: ]

> > This is fixed by N2776.

Proposed resolution:

1. Change 22.10 [function.objects]/1 as indicated:

   > 1 Function objects are objects with an `operator()` defined. An object type (6.9 [basic.types]) that can be the type of the postfix-expression in a function call (7.6.1.3 [expr.call], 12.2.2.2 [over.match.call]) is called a function object type*. A function object is an object of a function object type. In the places where one would expect to pass a pointer to a function to an algorithmic template (Clause 26 [algorithms]), the interface is specified to accept an object with an `operator()` defineda function object. This not only makes algorithmic templates work with pointers to functions, but also enables them to work with arbitrary function objects.

   > > * Such a type is either a function pointer or a class type which often has a member `operator()`, but in some cases it can omit that member and provide a conversion to a pointer to function.

2. Change 22.10.3 [func.def]/3 as indicated: [The intent is to make the commonality of callable types and function object types more explicit and to get rid of wording redundancies]

   > 3 A callable type is a pointer to function, a pointer to member function, a pointer to member data, or a class type whose objects can appear immediately to the left of a function call operator function object type (22.10 [function.objects]).

3. Change [bind]/1 as indicated:

   > 1 The function template `bind` returns an object that binds a function callable object passed as an argument to additional arguments.

4. Change 22.10.15 [func.bind]/1 as indicated:

   > 1 This subclause describes a uniform mechanism for binding arguments of function callable objects.

5. Change 22.10.17 [func.wrap]/1 as indicated:

   > 1 This subclause describes a polymorphic wrapper class that encapsulates arbitrary function callable objects.

6. Change 22.10.17.3 [func.wrap.func]/2 as indicated [The reason for this change is that 22.10.17.3 [func.wrap.func]/1 clearly says that all callable types may be wrapped by `std::function` and current implementations indeed do provide support for pointer to members as well. One further suggested improvement is to set the below definition of Callable in italics]:

   > 2 A functioncallable object `f` of type `F` is Callable Callable for argument types `T1, T2, ..., TN` in `ArgTypes` and a return type `R`, if, given lvalues `t1, t2, ..., tN` of types `T1, T2, ..., TN`, respectively, the expression `INVOKE(f, declval<ArgTypes>()..., Rt1, t2, ..., tN)`, considered as an unevaluated operand (7 [expr]), is well formed (20.7.2) and, if `R` is not `void`, convertible to `R`.

7. Change 22.10.17.3.2 [func.wrap.func.con]/7 as indicated:

   >     function(const function& f);
   >     template <class A> function(allocator_arg_t, const A& a, const function& f);

   > > ...

   > > 7 Throws: shall not throw exceptions if `f`'s target is a function pointer or a function callable object passed via `reference_wrapper`. Otherwise, may throw `bad_alloc` or any exception thrown by the copy constructor of the stored function callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small function callable objects, e.g., where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

8. Change 22.10.17.3.2 [func.wrap.func.con]/11 as indicated:

   >     template<class F> function(F f);
   >     template <class F, class A> function(allocator_arg_t, const A& a, F f);

   > > ...

   > > 11 [..] [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small function callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

9. Change 22.10.17.3.5 [func.wrap.func.inv]/3 as indicated:

   >     R operator()(ArgTypes... args) const

   > > ...

   > > 3 Throws: `bad_function_call` if `!*this`; otherwise, any exception thrown by the wrapped function callable object.

10. Change 22.10.17.3.6 [func.wrap.func.targ]/3 as indicated:

    >     template<typename T>       T* target();
    >     template<typename T> const T* target() const;

    > > ...

    > > 3 Requires: `T` shall be a function object type that is Callable (22.10.17.3 [func.wrap.func]) for parameter types `ArgTypes` and return type `R`.

11. Change 23.2.7 [associative.reqmts]/2 as indicated: [The suggested removal seems harmless, because 26.8 [alg.sorting]1 already clarifies that `Compare` is a function object type. Nevertheless it is recommended, because the explicit naming of "pointer to function" is misleading]

    > 2 Each associative container is parameterized on `Key` and an ordering relation `Compare` that induces a strict weak ordering (26.8 [alg.sorting]) on elements of `Key`. In addition, `map` and `multimap` associate an arbitrary type `T` with the `Key`. The object of type `Compare` is called the comparison object of a container. This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.

12. Change 23.2.8 [unord.req]/3 as indicated:

    > 3 Each unordered associative container is parameterized by `Key`, by a function object type `Hash` that acts as a hash function for values of type `Key`, and by a binary predicate `Pred` that induces an equivalence relation on values of type Key. [..]

13. Change 26.1 [algorithms.general]/7 as indicated: [The intent is to bring this part in sync with 22.10 [function.objects]]

    > 7 The `Predicate` parameter is used whenever an algorithm expects a function object (22.10 [function.objects]) that when applied to the result of dereferencing the corresponding iterator returns a value testable as `true`. In other words, if an algorithm takes `Predicate pred` as its argument and `first` as its iterator argument, it should work correctly in the construct `if (pred(*first)){...}`. The function object `pred` shall not apply any nonconstant function through the dereferenced iterator. This function object may be a pointer to function, or an object of a type with an appropriate function call operator.

14. Change 20.3.1.3 [unique.ptr.single]/1 as indicated:

    > 1 The default type for the template parameter `D` is `default_delete`. A client-supplied template argument `D` shall be a function pointer or functor object type for which, given a value `d` of type `D` and a pointer `ptr` of type `T*`, the expression `d(ptr)` is valid and has the effect of deallocating the pointer as appropriate for that deleter. `D` may also be an lvalue-reference to a deleter.

### 871(i). Iota's requirements on `T` are too strong

Section: 26.10.13 [numeric.iota] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

According to the recent WP N2691, 26.10.13 [numeric.iota]/1, the requires clause of `std::iota` says:

> `T` shall meet the requirements of `CopyConstructible` and `Assignable` types, and shall be convertible to `ForwardIterator`'s value type.[..]

Neither `CopyConstructible` nor `Assignable` is needed, instead `MoveConstructible` seems to be the correct choice. I guess the current wording resulted as an artifact from comparing it with similar numerical algorithms like `accumulate`.

Note: If this function will be conceptualized, the here proposed `MoveConstructible` requirement can be removed, because this is an implied requirement of function arguments, see N2710/[temp.req.impl]/3, last bullet.

[ post San Francisco: ]

> Issue pulled by author prior to review.

[ 2009-07-30 Daniel reopened: ]

> with the absence of concepts, this issue (closed) is valid again and I suggest to reopen it. I also revised by proposed resolution based on N2723 wording:

[ 2009-10 Santa Cruz: ]

> Change 'convertible' to 'assignable', Move To Ready.

Proposed resolution:

Change the first sentence of 26.10.13 [numeric.iota]/1:

> Requires: `T` shall meet the requirements of `CopyConstructible` and `Assignable` types, and shall be assignable to `ForwardIterator`'s value type. [..]

### 872(i). `move_iterator::operator[]` has wrong return type

Section: 24.5.4.6 [move.iter.elem] Status: C++11 Submitter: Doug Gregor Opened: 2008-08-21 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

`move_iterator`'s `operator[]` is declared as:

>     reference operator[](difference_type n) const;

This has the same problem that `reverse_iterator`'s `operator[]` used to have: if the underlying iterator's `operator[]` returns a proxy, the implicit conversion to `value_type&&` could end up referencing a temporary that has already been destroyed. This is essentially the same issue that we dealt with for `reverse_iterator` in DR 386(i).

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-08-15 Howard adds: ]

> I recommend closing this as a duplicate of 1051(i) which addresses this issue for both `move_iterator` and `reverse_iterator`.

[ 2009-10 Santa Cruz: ]

> Move to Ready. Note that if 1051(i) is reopened, it may yield a better resolution, but 1051(i) is currently marked NAD.

Proposed resolution:

In 24.5.4.2 [move.iterator] and [move.iter.op.index], change the declaration of `move_iterator`'s `operator[]` to:

>     reference unspecified operator[](difference_type n) const;

Rationale:

[ San Francisco: ]

> NAD Editorial, see N2777.

### 874(i). Missing `initializer_list` constructor for `discrete_distribution`

Section: 29.5.9.6.1 [rand.dist.samp.discrete] Status: Resolved Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

During the Sophia Antipolis meeting it was decided to separate from 793(i) a subrequest that adds initializer list support to `discrete_distribution`, specifically, the issue proposed to add a c'tor taking a `initializer_list<double>`.

Proposed resolution:

1. In 29.5.9.6.1 [rand.dist.samp.discrete] p. 1, class `discrete_distribution`, just before the member declaration

   >     explicit discrete_distribution(const param_type& parm);

   insert

   >     discrete_distribution(initializer_list<double> wl);

2. Between p.4 and p.5 of the same section insert a new paragraph as part of the new member description:

   >     discrete_distribution(initializer_list<double> wl);

   > > Effects: Same as `discrete_distribution(wl.begin(), wl.end())`.

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 875(i). Missing `initializer_list` constructor for `piecewise_constant_distribution`

Section: 29.5.9.6.2 [rand.dist.samp.pconst] Status: Resolved Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

During the Sophia Antipolis meeting it was decided to separate from 794(i) a subrequest that adds initializer list support to `piecewise_constant_distribution`, specifically, the issue proposed to add a c'tor taking a `initializer_list<double>` and a `Callable` to evaluate weight values. For consistency with the remainder of this class and the remainder of the `initializer_list`-aware library the author decided to change the list argument type to the template parameter `RealType` instead. For the reasoning to use `Func` instead of `Func&&` as c'tor function argument see issue 793(i).

Proposed resolution:

Non-concept version of the proposed resolution

1. In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class `piecewise_constant_distribution`, just before the member declaration

   >     explicit piecewise_constant_distribution(const param_type& parm);

   insert

   >     template<typename Func>
   >     piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);

2. Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:

   >     template<typename Func>
   >     piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);

   > > [p5_1] Complexity: Exactly `nf = max(bl.size(), 1) - 1` invocations of `fw`.

   > > [p5_2] Requires:

   > > 1. `fw` shall be callable with one argument of type `RealType`, and shall return values of a type convertible to `double`;
   > > 2. The relation `0 < S = w0+. . .+wn-1` shall hold. For all sampled values `xk` defined below, `fw(xk)` shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > > 3. If `nf > 0` let `bk = *(bl.begin() + k), k = 0, . . . , bl.size()-1` and the following relations shall hold for `k = 0, . . . , nf-1: bk < bk+1`.

   > > [p5_3] Effects:

   > > 1. If `nf == 0`,

   > >    1. lets the sequence `w` have length `n = 1` and consist of the single value `w0 = 1`, and
   > >    2. lets the sequence `b` have length `n+1` with `b0 = 0` and `b1 = 1`.

   > > 2. Otherwise,

   > >    1. sets `n = nf`, and `[bl.begin(), bl.end())` shall form the sequence `b` of length `n+1`, and

   > >    2. lets the sequences `w` have length `n` and for each `k = 0, . . . ,n-1`, calculates:

   > >       > xk = 0.5*(bk+1 + bk) wk = fw(xk)

   > > 3. Constructs a `piecewise_constant_distribution` object with the above computed sequence `b` as the interval boundaries and with the probability densities:

   > >    > ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.

Concept version of the proposed resolution

1. In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class `piecewise_constant_distribution`, just before the member declaration

   >     explicit piecewise_constant_distribution(const param_type& parm);

   insert

   >     template<Callable<auto, RealType> Func>
   >      requires Convertible<Func::result_type, double>
   >     piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);

2. Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:

   >     template<Callable<auto, RealType> Func>
   >      requires Convertible<Func::result_type, double>
   >     piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);

   > > [p5_1] Complexity: Exactly `nf = max(bl.size(), 1) - 1` invocations of `fw`.

   > > [p5_2] Requires:

   > > 1. The relation `0 < S = w0+. . .+wn-1` shall hold. For all sampled values `xk` defined below, `fw(xk)` shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > > 2. If `nf > 0` let `bk = *(bl.begin() + k), k = 0, . . . , bl.size()-1` and the following relations shall hold for `k = 0, . . . , nf-1: bk < bk+1`.

   > > [p5_3] Effects:

   > > 1. If `nf == 0`,

   > >    1. lets the sequence `w` have length `n = 1` and consist of the single value `w0 = 1`, and
   > >    2. lets the sequence `b` have length `n+1` with `b0 = 0` and `b1 = 1`.

   > > 2. Otherwise,

   > >    1. sets `n = nf`, and `[bl.begin(), bl.end())` shall form the sequence `b` of length `n+1`, and

   > >    2. lets the sequences `w` have length `n` and for each `k = 0, . . . ,n-1`, calculates:

   > >       > xk = 0.5*(bk+1 + bk) wk = fw(xk)

   > > 3. Constructs a `piecewise_constant_distribution` object with the above computed sequence `b` as the interval boundaries and with the probability densities:

   > >    > ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 876(i). `basic_string` access operations should give stronger guarantees

Section: 27.4.3 [basic.string] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

During the Sophia Antipolis meeting it was decided to split-off some parts of the n2647 ("Concurrency modifications for `basic_string`") proposal into a separate issue, because these weren't actually concurrency-related. The here proposed changes refer to the recent update document n2668 and attempt to take advantage of the stricter structural requirements.

Indeed there exists some leeway for more guarantees that would be very useful for programmers, especially if interaction with transactionary or exception-unaware C API code is important. This would also allow compilers to take advantage of more performance optimizations, because more functions can have throw() specifications. This proposal uses the form of "Throws: Nothing" clauses to reach the same effect, because there already exists a different issue in progress to clean-up the current existing "schizophrenia" of the standard in this regard.

Due to earlier support for copy-on-write, we find the following unnecessary limitations for C++0x:

1. Missing no-throw guarantees: `data()` and `c_str()` simply return a pointer to their guts, which is a non-failure operation. This should be spelled out. It is also noteworthy to mention that the same guarantees should also be given by the size query functions, because the combination of pointer to content and the length is typically needed during interaction with low-level API.
2. Missing complexity guarantees: `data()` and `c_str()` simply return a pointer to their guts, which is guaranteed O(1). This should be spelled out.
3. Missing reading access to the terminating character: Only the const overload of `operator[]` allows reading access to the terminator char. For more intuitive usage of strings, reading access to this position should be extended to the non-const case. In contrast to C++03 this reading access should now be homogeneously an lvalue access.

The proposed resolution is split into a main part (A) and a secondary part (B) (earlier called "Adjunct Adjunct Proposal"). (B) extends (A) by also making access to index position size() of the at() overloads a no-throw operation. This was separated, because this part is theoretically observable in specifically designed test programs.

[ San Francisco: ]

> We oppose part 1 of the issue but hope to address `size()` in issue 877(i).

> We do not support part B. 4 of the issue because of the breaking API change.

> We support part A. 2 of the issue.

> On support part A. 3 of the issue:

> > Pete's broader comment: now that we know that `basic_string` will be a block of contiguous memory, we should just rewrite its specification with that in mind. The expression of the specification will be simpler and probably more correct as a result.

[ 2009-07 Frankfurt ]

> Move proposed resolution A to Ready.

> [ Howard: Commented out part B. ]

Proposed resolution:

1. 1. In 27.4.3.5 [string.capacity], just after p. 1 add a new paragraph:

      > Throws: Nothing.

   2. In 27.4.3.6 [string.access] replace p. 1 by the following 4 paragraghs:

      > Requires: `pos ≤ size()`.

      > Returns: If `pos < size()`, returns `*(begin() + pos)`. Otherwise, returns a reference to a `charT()` that shall not be modified.

      > Throws: Nothing.

      > Complexity: Constant time.

   3. In 27.4.3.8.1 [string.accessors] replace the now common returns clause of `c_str()` and `data()` by the following three paragraphs:

      > Returns: A pointer `p` such that `p+i == &operator[](i)` for each `i` in `[0, size()]`.

      > Throws: Nothing.

      > Complexity: Constant time.

### 878(i). `forward_list` preconditions

Section: 23.3.7 [forward.list] Status: C++11 Submitter: Martin Sebor Opened: 2008-08-23 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

`forward_list` member functions that take a `forward_list::iterator` (denoted `position` in the function signatures) argument have the following precondition:

> Requires: `position` is dereferenceable or equal to `before_begin()`.

I believe what's actually intended is this:

> Requires: `position` is in the range [`before_begin()`, `end()`).

That is, when it's dereferenceable, `position` must point into `*this`, not just any `forward_list` object.

[ San Francisco: ]

> Robert suggested alternate proposed wording which had large support.

[ Post Summit: ]

> Walter: "position is before_begin() or a dereferenceable": add "is" after the "or"

> With that minor update, Recommend Tentatively Ready.

Proposed resolution:

Change the Requires clauses [forwardlist], p21, p24, p26, p29, and, [forwardlist.ops], p39, p43, p47 as follows:

> Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range `[begin(), end())` or equal to `before_begin()`. ...

### 880(i). Missing atomic exchange parameter

Section: 32.5 [atomics] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-08-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 942

Discussion:

The `atomic_exchange` and `atomic_exchange_explicit` functions seem to be inconsistently missing parameters.

[ Post Summit: ]

> Lawrence: Need to write up a list for Pete with details.

> Detlef: Should not be New, we already talked about in Concurrency group.

> Recommend Open.

[ 2009-07 Frankfurt ]

> Lawrence will handle all issues relating to atomics in a single paper.

> LWG will defer discussion on atomics until that paper appears.

> Move to Open.

[ 2009-08-17 Handled by N2925. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2992.

Proposed resolution:

Add the appropriate parameters. For example,

>     bool atomic_exchange(volatile atomic_bool*, bool);
>     bool atomic_exchange_explicit(volatile atomic_bool*, bool, memory_order);

### 881(i). `shared_ptr` conversion issue

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++11 Submitter: Peter Dimov Opened: 2008-08-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

We've changed `shared_ptr<Y>` to not convert to `shared_ptr<T>` when `Y*` doesn't convert to `T*` by resolving issue 687(i). This only fixed the converting copy constructor though. N2351 later added move support, and the converting move constructor is not constrained.

[ San Francisco: ]

> We might be able to move this to NAD, Editorial once shared_ptr is conceptualized, but we want to revisit this issue to make sure.

[ 2009-07 Frankfurt ]

> Moved to Ready.

> This issue now represents the favored format for specifying constrained templates.

Proposed resolution:

We need to change the Requires clause of the move constructor:

>     shared_ptr(shared_ptr&& r);
>     template<class Y> shared_ptr(shared_ptr<Y>&& r);

> > Requires Remarks: For the second constructor `Y*` shall be convertible to `T*`. The second constructor shall not participate in overload resolution unless `Y*` is convertible to `T*`.

in order to actually make the example in 687(i) compile (it now resolves to the move constructor).

### 882(i). `duration` non-member arithmetic requirements

Section: 30.5.6 [time.duration.nonmember] Status: CD1 Submitter: Howard Hinnant Opened: 2008-09-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

N2661 specified the following requirements for the non-member `duration` arithmetic:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator*(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: Let `CR` represent the `common_type` of `Rep1` and `Rep2`. Both `Rep1` and `Rep2` shall be implicitly convertible to CR, diagnostic required.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator*(const Rep1& s, const duration<Rep2, Period>& d);

> > Requires: Let `CR` represent the `common_type` of `Rep1` and `Rep2`. Both `Rep1` and `Rep2` shall be implicitly convertible to `CR`, diagnostic required.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator/(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: Let `CR` represent the `common_type` of `Rep1` and `Rep2`. Both `Rep1` and `Rep2` shall be implicitly convertible to `CR`, and `Rep2` shall not be an instantiation of `duration`, diagnostic required.

During transcription into the working paper, the requirements clauses on these three functions was changed to:

> Requires: `CR(Rep1, Rep2)` shall exist. Diagnostic required.

This is a non editorial change with respect to N2661 as user written representations which are used in `duration` need not be implicitly convertible to or from arithmetic types in order to interoperate with `duration`s based on arithmetic types. An explicit conversion will do fine for most expressions as long as there exists a `common_type` specialization relating the user written representation and the arithmetic type. For example:

>     class saturate
>     {
>     public:
>       explicit saturate(long long i);
>       ...
>     };

>     namespace std {

>     template <>
>     struct common_type<saturate, long long>
>     {
>         typedef saturate type;
>     };

>     template <>
>     struct common_type<long long, saturate>
>     {
>         typedef saturate type;
>     };

>     }  // std

>     millisecond ms(3);  // integral-based duration
>     duration<saturate, milli> my_ms = ms;  // ok, even with explicit conversions
>     my_ms = my_ms + ms;                    // ok, even with explicit conversions

However, when dealing with multiplication of a duration and its representation, implicit convertibility is required between the rhs and the lhs's representation for the member `*=` operator:

>     template <class Rep, class Period = ratio<1>>
>     class duration {
>     public:
>        ...
>        duration& operator*=(const rep& rhs);
>        ...
>     };
>     ...
>     ms *= 2;               // ok, 2 is implicitly convertible to long long
>     my_ms *= saturate(2);  // ok, rhs is lhs's representation
>     my_ms *= 2;            // error, 2 is not implicitly convertible to saturate

The last line does not (and should not) compile. And we want non-member multiplication to have the same behavior as member arithmetic:

>     my_ms = my_ms * saturate(2);  // ok, rhs is lhs's representation
>     my_ms = my_ms * 2;            // should be error, 2 is not implicitly convertible to saturate

The requirements clauses of N2661 make the last line an error as expected. However the latest working draft at this time (N2723) allows the last line to compile.

All that being said, there does appear to be an error in these requirements clauses as specified by N2661.

> Requires: ... Both `Rep1` and `Rep2` shall be implicitly convertible to CR, diagnostic required.

It is not necessary for both `Rep`s to be implicitly convertible to the `CR`. It is only necessary for the rhs `Rep` to be implicitly convertible to the `CR`. The `Rep` within the `duration` should be allowed to only be explicitly convertible to the `CR`. The explicit-conversion-requirement is covered under 30.5.8 [time.duration.cast].

Proposed resolution:

Change the requirements clauses under 30.5.6 [time.duration.nonmember]:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator*(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: `CR(Rep1, Rep2)` shall exist. `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator*(const Rep1& s, const duration<Rep2, Period>& d);

> > Requiresd behavior: `CR(Rep1, Rep2)` shall exist. `Rep1` shall be implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator/(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: `CR(Rep1, Rep2)` shall exist `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be an instantiation of `duration`. Diagnostic required.

### 883(i). swap circular definition

Section: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Note in particular that Table 90 "Container Requirements" gives semantics of `a.swap(b)` as `swap(a,b)`, yet for all containers we define `swap(a,b)` to call `a.swap(b)` - a circular definition.

[ San Francisco: ]

> Robert to propose a resolution along the lines of "Postcondition: "a = b, b = a" This will be a little tricky for the hash containers, since they don't have `operator==`.

[ Post Summit Anthony Williams provided proposed wording. ]

[ 2009-07 Frankfurt ]

> Moved to Ready with minor edits (which have been made).

Proposed resolution:

In table 80 in section 23.2.2 [container.requirements.general], replace the postcondition of `a.swap(b)` with the following:

> Table 80 -- Container requirements
> Expression | Return type | Operational semantics | Assertion/note pre-/post-conidtion | Complexity
> --- | --- | --- | --- | ---
> ... | ... | ... | ... | ...
> `a.swap(b);` | `void` |  | `swap(a,b)` Exchange the contents of `a` and `b`. | (Note A)

Remove the reference to swap from the paragraph following the table.

> Notes: the algorithms `swap()`, `equal()` and `lexicographical_compare()` are defined in Clause 25. ...

### 884(i). `shared_ptr` swap

Section: 20.3.2.2.5 [util.smartptr.shared.mod] Status: Resolved Submitter: Jonathan Wakely Opened: 2008-09-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

>     #include <memory>
>     #include <cassert>

>     struct A { };
>     struct B : A { };

>     int main()
>     {
>         std::shared_ptr<A> pa(new A);
>         std::shared_ptr<B> pb(new B);
>         std::swap<A>(pa, pb);  // N.B. no argument deduction
>         assert( pa.get() == pb.get() );
>         return 0;
>     }

Is this behaviour correct (I believe it is) and if so, is it unavoidable, or not worth worrying about?

This calls the lvalue/rvalue swap overload for `shared_ptr`:

>     template<class T> void swap( shared_ptr<T> & a, shared_ptr<T> && b );

silently converting the second argument from `shared_ptr<B>` to `shared_ptr<A>` and binding the rvalue ref to the produced temporary.

This is not, in my opinion, a `shared_ptr` problem; it is a general issue with the rvalue swap overloads. Do we want to prevent this code from compiling? If so, how?

Perhaps we should limit rvalue args to swap to those types that would benefit from the "swap trick". Or, since we now have `shrink_to_fit()`, just eliminate the rvalue swap overloads altogether. The original motivation was:

>     vector<A> v = ...;
>     ...
>     swap(v, vector<A>(v));

N1690.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to NAD EditorialResolved.

Proposed resolution:

Recommend NAD EditorialResolved, fixed by N2844.

### 885(i). `pair` assignment

Section: 22.3 [pairs] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

>     20.2.3 pairs
>     Missing assignemnt operator:
>     template<class U , class V>
>       requires CopyAssignable<T1, U> && CopyAssignable<T2, V>
>         pair& operator=(pair<U , V> const & p );

Well, that's interesting. This assignment operator isn't in the current working paper, either. Perhaps we deemed it acceptable to build a temporary of type `pair` from `pair<U, V>`, then move-assign from that temporary?

It sounds more like an issue waiting to be opened, unless you want to plug it now. As written we risk moving from lvalues.

[ San Francisco: ]

> Would be NAD if better ctors fixed it.

> Related to 811(i).

[ post San Francisco: ]

> Possibly NAD Editorial, solved by N2770.

[ 2009-05-25 Alisdair adds: ]

> Issue 885(i) was something I reported while reviewing the library concepts documents ahead of San Francisco. The missing operator was added as part of the paper adopted at that meeting (N2770) and I can confirm this operator is present in the current working paper. I recommend NAD.

[ 2009-07 Frankfurt ]

> We agree with the intent, but we need to wait for the dust to settle on concepts.

[ 2010-03-11 Stefanus provided wording. ]

[ 2010 Pittsburgh: Moved to Ready for Pittsburgh. ]

Proposed resolution:

Add the following declaration 22.3.2 [pairs.pair], before the declaration of `pair& operator=(pair&& p);`:

>     template<class U, class V> pair& operator=(const pair<U, V>& p);

Add the following description to 22.3.2 [pairs.pair] after paragraph 11 (before the description of `pair& operator=(pair&& p);)`:

>     template<class U, class V> pair& operator=(const pair<U, V>& p);

> > Requires: `T1` shall satisfy the requirements of `CopyAssignable` from `U`. `T2` shall satisfy the requirements of `CopyAssignable` from `V`.

> > Effects: Assigns `p.first` to `first` and `p.second` to `second`.

> > Returns: `*this`.

### 886(i). `tuple` construction

Section: 22.4.4.2 [tuple.cnstr] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

22.4.4.2 [tuple.cnstr]:

> Effects: Default initializes each element.

Could be clarified to state each "non-trivial" element. Otherwise we have a conflict with Core deinfition of default initialization - trivial types do not get initialized (rather than initialization having no effect)

I'm going to punt on this one, because it's not an issue that's related to concepts. I suggest bringing it to Howard's attention on the reflector.

[ San Francisco: ]

> Text in draft doesn't mean anything, changing to "non-trivial" makes it meaningful.

> We prefer "value initializes". Present implementations use value-initialization. Users who don't want value initialization have alternatives.

> Request resolution text from Alisdair.

> This issue relates to Issue 868(i) default construction and value-initialization.

[ 2009-05-04 Alisdair provided wording and adds: ]

> Note: This IS a change of semantic from TR1, although one the room agreed with during the discussion. To preserve TR1 semantics, this would have been worded:

> >     requires DefaultConstructible<Types>... tuple();

> > > -2- Effects: Default-initializes each non-trivial element.

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Change p2 in Construction 22.4.4.2 [tuple.cnstr]:

>     requires DefaultConstructible<Types>... tuple();

> > -2- Effects: Default Value-initializes each element.

### 888(i). `this_thread::yield` too strong

Section: 32.4.5 [thread.thread.this] Status: C++11 Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I never thought I'd say this, but `this_thread::yield` seems to be too strong in specification. The issue is that some systems distinguish between yielding to another thread in the same process and yielding to another process. Given that the C++ standard only talks about a single program, one can infer that the specification allows yielding only to another thread within the same program. Posix has no facility for that behavior. Can you please file an issue to weaken the wording. Perhaps "Offers the operating system the opportunity to reschedule."

[ Post Summit: ]

> Recommend move to Tentatively Ready.

Proposed resolution:

Change 32.4.5 [thread.thread.this]/3:

>     void this_thread::yield();

> > Effects: Offers the operating system implementation the opportunity to reschedule. another thread.

### 889(i). `thread::id` comparisons

Section: 32.4.3.2 [thread.thread.id] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 324

The `thread::id` type supports the full set of comparison operators. This is substantially more than is required for the associative containers that justified them. Please place an issue against the threads library.

[ San Francisco: ]

> Would depend on proposed extension to POSIX, or non-standard extension. What about hash? POSIX discussing op. POSIX not known to be considering support needed for hash, op.

> Group expresses support for putting ids in both unordered and ordered containers.

[ post San Francisco: ]

> Howard: It turns out the current working paper N2723 already has `hash<thread::id>` (22.10 [function.objects], 22.10.19 [unord.hash]). We simply overlooked it in the meeting. It is a good thing we voted in favor of it (again). :-)

> Recommend NAD.

[ Post Summit: ]

> Recommend to close as NAD. For POSIX, see if we need to add a function to convert `pthread_t` to integer.

[ Post Summit, Alisdair adds: ]

> The recommendation for LWG-889/UK-324 is NAD, already specified.

> It is not clear to me that the specification is complete.

> In particular, the synopsis of `<functional>` in 22.10 [function.objects] does not mention `hash< thread::id>` nor `hash< error_code >`, although their existence is implied by 22.10.19 [unord.hash], p1.

> I am fairly uncomfortable putting the declaration for the `thread_id` specialization into `<functional>` as `id` is a nested class inside `std::thread`, so it implies that `<functional>` would require the definition of the `thread` class template in order to forward declared `thread::id` and form this specialization.

> It seems better to me that the dependency goes the other way around (`<thread>` will more typically make use of `<functional>` than vice-versa) and the `hash<thread::id>` specialization be declared in the `<thread>` header.

> I think `hash<error_code>` could go into either `<system_error>` or `<functional>` and have no immediate preference either way. However, it should clearly appear in the synopsis of one of these two.

> Recommend moving 889 back to open, and tying in a reference to UK-324.

[ Batavia (2009-05): ]

> Howard observes that `thread::id` need not be a nested class; it could be a `typedef` for a more visible type.

[ 2009-05-24 Alisdair adds: ]

> I do not believe this is correct. `thread::id` is explicitly documents as a nested class, rather than as an unspecified typedef analogous to an iterator. If the intent is that this is not implemented as a nested class (under the as-if freedoms) then this is a novel form of standardese.

[ 2009-07 Frankfurt ]

> Decided we want to move hash specialization for thread_id to the thread header. Alisdair to provide wording.

[ 2009-07-28 Alisdair provided wording, moved to Review. ]

[ 2009-10 Santa Cruz: ]

> Add a strike for `hash<thread::id>`. Move to Ready

[ 2009-11-13 The proposed wording of 1182(i) is a superset of the wording in this issue. ]

[ 2010-02-09 Moved from Ready to Open: ]

> Issue 1182(i) is not quite a superset of this issue and it is controversial whether or not the note:

> > hash template specialization allows `thread::id` objects to be used as keys in unordered containers.

> should be added to the WP.

[ 2010-02-09 Objections to moving this to NAD EditorialResolved, addressed by 1182(i) have been removed. Set to Tentatively NAD EditorialResolved. ]

Rationale:

Solved by 1182(i).

Proposed resolution:

Remove the following prototype from the synopsis in 22.10 [function.objects]:

>     template <> struct hash<std::thread::id>;

Add to 32.4 [thread.threads], p1 Header `<thread>` synopsis:

>     template <class T> struct hash;
>     template <> struct hash<thread::id>;

Add template specialization below class definition in 32.4.3.2 [thread.thread.id]

>     template <>
>     struct hash<thread::id> : public unary_function<thread::id, size_t> {
>        size_t operator()(thread::id val) const;
>     };

Extend note in p2 32.4.3.2 [thread.thread.id] with second sentence:

> [Note: Relational operators allow `thread::id` objects to be used as keys in associative containers. `hash` template specialization allows `thread::id` objects to be used as keys in unordered containers. — end note]

Add new paragraph to end of 32.4.3.2 [thread.thread.id]

>     template <> struct hash<thread::id>;

> > An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type `thread::id`.

### 890(i). Improving `<system_error>` initialization

Section: 19.5.3 [syserr.errcat] Status: C++11 Submitter: Beman Dawes Opened: 2008-09-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [syserr.errcat].

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Discussion:

The `static const error_category` objects `generic_category` and `system_category` in header `<system_error>` are currently declared:

>     const error_category& get_generic_category();
>     const error_category& get_system_category();

>     static const error_category& generic_category = get_generic_category();
>     static const error_category& system_category = get_system_category();

This formulation has several problems:

- Implementation details are exposed, since initialization is specified in the interface. This over-constrains implementations without offsetting user benefits. The form of initialization specified may be less than maximally efficient on some platforms.
- Use of the objects is more expensive in terms of number of machine level instructions. See Implementation experience below.
- Depending on the compiler, some cost may be incurred by each translation unit that includes the header, even if the objects are not used. This is a common scenario in user code, since the header is included by other standard library headers. It should be mentioned that at least one compilers is able to optimize this cost away, however.

IO streams uses a somewhat different formulation for iostream_category, but still suffer much the same problems.

The original plan was to eliminate these problems by applying the C++0x `constexpr` feature. See LWG issue 832(i). However, that approach turned out to be unimplementable, since it would require a `constexpr` object of a class with virtual functions, and that is not allowed by the core language.

The proposed resolution was developed as an alternative. It mitigates the above problems by removing initialization from the visible interface, allowing implementations flexibility.

Implementation experience:

Prototype implementations of the current WP interface and proposed resolution interface were tested with recent Codegear, GCC, Intel, and Microsoft compilers on Windows. The code generated by the Microsoft compiler was studied at length; the WP and proposal versions generated very similar code. For both versions the compiler did make use of static initialization; apparently the compiler applied an implicit `constexpr` where useful, even in cases where `constexpr` would not be permitted by the language!

Acknowledgements:

Martin Sebor, Chris Kohlhoff, and John Lakos provided useful ideas and comments on initialization issues.

[ San Francisco: ]

> Martin: prefers not to create more file-scope static objects, and would like to see `get_*` functions instead.

[Pre-Summit:]

> Beman: The proposed resolution has been reworked to remove the file-scope static objects, per Martin's suggestions. The `get_` prefix has been eliminated from the function names as no longer necessary and to conform with standard library naming practice.

[ Post Summit: ]

> Agreement that this is wise and essential, text provided works and has been implemented. Seems to be widespread consensus. Move to Tentative Ready.

Proposed resolution:

Change 16.4.6.16 [value.error.codes] Value of error codes as indicated:

> Certain functions in the C++ standard library report errors via a `std::error_code` (19.4.2.2) object. That object's `category()` member shall return a reference to `std::system_category```()`` for errors originating from the operating system, or a reference to an implementation-defined error_category object for errors originating elsewhere. The implementation shall define the possible values of value() for each of these error categories. [Example: For operating systems that are based on POSIX, implementations are encouraged to define the `std::system_category```()`` values as identical to the POSIX `errno` values, with additional values as defined by the operating system's documentation. Implementations for operating systems that are not based on POSIX are encouraged to define values identical to the operating system's values. For errors that do not originate from the operating system, the implementation may provide enums for the associated values --end example]

Change 19.5.3.1 [syserr.errcat.overview] Class `error_category` overview `error_category` synopsis as indicated:

>     const error_category& get_generic_category();
>     const error_category& get_system_category();

>     static storage-class-specifier const error_category& generic_category = get_generic_category();
>     static storage-class-specifier const error_category& system_category = get_system_category();

Change 19.5.3.5 [syserr.errcat.objects] Error category objects as indicated:

>     const error_category& get_generic_category();

> > Returns: A reference to an object of a type derived from class `error_category`.

> > Remarks: The object's `default_error_condition` and `equivalent` virtual functions shall behave as specified for the class `error_category`. The object's `name` virtual function shall return a pointer to the string `"GENERIC"`.

>     const error_category& get_system_category();

> > Returns: A reference to an object of a type derived from class `error_category`.

> > Remarks: The object's `equivalent` virtual functions shall behave as specified for class `error_category`. The object's `name` virtual function shall return a pointer to the string `"system"`. The object's `default_error_condition` virtual function shall behave as follows:

> > > If the argument `ev` corresponds to a POSIX `errno` value `posv`, the function shall return `error_condition(posv, generic_category())`. Otherwise, the function shall return `error_condition(ev, system_category())`. What constitutes correspondence for any given operating system is unspecified. [Note: The number of potential system error codes is large and unbounded, and some may not correspond to any POSIX `errno` value. Thus implementations are given latitude in determining correspondence. — end note]

Change 19.5.4.2 [syserr.errcode.constructors] Class `error_code` constructors as indicated:

>     error_code();

> > Effects: Constructs an object of type error_code.

> > Postconditions: `val_ == 0` and `cat_ == &system_category``()`.

Change 19.5.4.3 [syserr.errcode.modifiers] Class `error_code` modifiers as indicated:

>     void clear();

> > Postconditions: `value() == 0` and `category() == system_category``()`.

Change 19.5.4.5 [syserr.errcode.nonmembers] Class `error_code` non-member functions as indicated:

>     error_code make_error_code(errc e);

> > Returns: `error_code(static_cast<int>(e), generic_category``()``)`.

Change 19.5.5.2 [syserr.errcondition.constructors] Class `error_condition` constructors as indicated:

>     error_condition();

> > Effects: Constructs an object of type `error_condition`.

> > Postconditions: `val_ == 0` and `cat_ == &generic_category``()`.

Change 19.5.5.3 [syserr.errcondition.modifiers] Class `error_condition` modifiers as indicated:

>     void clear();

> > Postconditions: `value() == 0` and `category() == generic_category``()`.

Change 19.5.5.5 [syserr.errcondition.nonmembers] Class `error_condition` non-member functions as indicated:

>     error_condition make_error_condition(errc e);

> > Returns: `error_condition(static_cast<int>(e), generic_category())`.

Change 31.5 [iostreams.base] Iostreams base classes, Header `<ios>` synopsis as indicated:

>     concept_map ErrorCodeEnum<io_errc> { };
>     error_code make_error_code(io_errc e);
>     error_condition make_error_condition(io_errc e);
>     storage-class-specifier const error_category& iostream_category();

Change [ios::failure] Class ios_base::failure, paragraph 2 as indicated:

> When throwing `ios_base::failure` exceptions, implementations should provide values of `ec` that identify the specific reason for the failure. [ Note: Errors arising from the operating system would typically be reported as `system_category``()` errors with an error value of the error number reported by the operating system. Errors arising from within the stream library would typically be reported as `error_code(io_errc::stream, iostream_category())`. — end note ]

Change 31.5.6 [error.reporting] Error reporting as indicated:

>     error_code make_error_code(io_errc e);

> > Returns: `error_code(static_cast<int>(e), iostream_category`()`)`.

>     error_condition make_error_condition(io_errc e);

> > Returns: `error_condition(static_cast<int>(e), iostream_category`()`)`.

>     storage-class-specifier const error_category& iostream_category();

> > The implementation shall initialize iostream_category. Its storage-class-specifier may be static or extern. It is unspecified whether initialization is static or dynamic (3.6.2). If initialization is dynamic, it shall occur before completion of the dynamic initialization of the first translation unit dynamically initialized that includes header <system_error>.

> > Returns: A reference to an object of a type derived from class `error_category`.

> > Remarks: The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "iostream".

### 891(i). `std::thread`, `std::call_once` issue

Section: 32.4.3.3 [thread.thread.constr], 32.6.7.2 [thread.once.callonce] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [thread.thread.constr].

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Discussion:

I notice that the vararg overloads of `std::thread` and `std::call_once` (N2723 32.4.3.3 [thread.thread.constr] and 32.6.7.2 [thread.once.callonce]) are no longer specified in terms of `std::bind`; instead, some of the `std::bind` wording has been inlined into the specification.

There are two problems with this.

First, the specification (and implementation) in terms of `std::bind` allows, for example:

>     std::thread th( f, 1, std::bind( g ) );

which executes `f( 1, g() )` in a thread. This can be useful. The "inlined" formulation changes it to execute `f( 1, bind(g) )` in a thread.

Second, assuming that we don't want the above, the specification has copied the wording

> `INVOKE(func, w1, w2, ..., wN)` (20.6.2) shall be a valid expression for some values `w1, w2, ..., wN`

but this is not needed since we know that our argument list is args; it should simply be

> `INVOKE(func, args...)` (20.6.2) shall be a valid expression

[ Summit: ]

> Move to open.

[ Post Summit Anthony provided proposed wording. ]

[ 2009-07 Frankfurt ]

> Leave Open. Await decision for thread variadic constructor.

[ 2009-10 Santa Cruz: ]

> See proposed wording for 929(i) for this, for the formulation on how to solve this. 929(i) modifies the thread constructor to have "pass by value" behavior with pass by reference efficiency through the use of the `decay` trait. This same formula would be useful for `call_once`.

[ 2010-02-11 Anthony updates wording. ]

[ 2010-02-12 Moved to Tentatively Ready after 5 postive votes on c++std-lib. ]

Proposed resolution:

Modify 32.6.7.2 [thread.once.callonce] p1-p2 with the following:

>     template<class Callable, class ...Args>
>       void call_once(once_flag& flag, Callable&& func, Args&&... args);

> > Given a function as follows:

> > >     template<typename T> typename decay<T>::type decay_copy(T&& v)
> > >        { return std::forward<T>(v); }

> > 1 Requires: The template parameters `Callable` and each `Ti` in `Args` shall be `CopyConstructible` if an lvalue and otherwise satisfy the `MoveConstructible` requirements. `INVOKE(decay_copy(std::forward<Callable>(func), w1, w2, ..., wN decay_copy(std::forward<Args>(args))...)` (22.10.4 [func.require]) shall be a valid expression for some values `w1, w2, ..., wN`, where `N == sizeof...(Args)`.

> > 2 Effects: Calls to `call_once` on the same `once_flag` object are serialized. If there has been a prior effective call to `call_once` on the same `once_flag` object, the call to `call_once` returns without invoking `func`. If there has been no prior effective call to `call_once` on the same `once_flag` object, the argument `func` (or a copy thereof) is called as if by invoking `func(args)` `INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)` is executed. The call to `call_once` is effective if and only if `func(args)` `INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)` returns without throwing an exception. If an exception is thrown it is propagated to the caller.

### 893(i). `std::mutex` issue

Section: 32.6.4.2.2 [thread.mutex.class] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 905

Discussion:

32.6.4.2.2 [thread.mutex.class]/27 (in N2723) says that the behavior is undefined if:

- a thread that owns a `mutex` object calls `lock()` or `try_lock()` on that object

I don't believe that this is right. Calling `lock()` or `try_lock()` on a locked `mutex` is well defined in the general case. `try_lock()` is required to fail and return `false`. `lock()` is required to either throw an exception (and is allowed to do so if it detects deadlock) or to block until the `mutex` is free. These general requirements apply regardless of the current owner of the `mutex`; they should apply even if it's owned by the current thread.

Making double `lock()` undefined behavior probably can be justified (even though I'd still disagree with the justification), but `try_lock()` on a locked `mutex` must fail.

[ Summit: ]

> Move to open. Proposed resolution:

> - In 32.6.4 [thread.mutex.requirements] paragraph 12, change the error condition for `resource_deadlock_would_occur` to: "if the implementation detects that a deadlock would occur"
> - Strike 32.6.4.2.2 [thread.mutex.class] paragraph 3 bullet 2 "a thread that owns a mutex object calls `lock()` or `try_lock()` on that object, or"

[ 2009-07 Frankfurt ]

> Move to Review. Alisdair to provide note.

[ 2009-07-31 Alisdair provided note. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2009-11-18 Peter Opens: ]

> I don't believe that the proposed note:

> > [Note: a program may deadlock if the thread that owns a `mutex` object calls `lock()` or `try_lock()` on that object. If the program can detect the deadlock, a `resource_deadlock_would_occur` error condition may be observed. — end note]

> is entirely correct. "or `try_lock()`" should be removed, because `try_lock` is non-blocking and doesn't deadlock; it just returns `false` when it fails to lock the mutex.

> [ Howard: I've set to Open and updated the wording per Peter's suggestion. ]

[ 2009-11-18 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In 32.6.4 [thread.mutex.requirements] paragraph 12 change:

> - ...
> - `resource_deadlock_would_occur` -- if the current thread already owns the mutex and is able to detect it implementation detects that a deadlock would occur.
> - ...

Strike 32.6.4.2.2 [thread.mutex.class] paragraph 3 bullet 2:

> -3- The behavior of a program is undefined if:

> - ...
> - a thread that owns a `mutex` object calls `lock()` or `try_lock()` on that object, or
> - ...

Add the following note after p3 32.6.4.2.2 [thread.mutex.class]

> [Note: a program may deadlock if the thread that owns a `mutex` object calls `lock()` on that object. If the implementation can detect the deadlock, a `resource_deadlock_would_occur` error condition may be observed. — end note]

### 894(i). longjmp and destructors

Section: 17.14 [support.runtime] Status: C++11 Submitter: Lawrence Crowl, Alisdair Meredith Opened: 2008-09-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The interaction between `longjmp` and exceptions seems unnecessarily restrictive and not in keeping with existing practice.

Proposed resolution:

Edit paragraph 4 of 17.14 [support.runtime] as follows:

> The function signature `longjmp(jmp_buf jbuf, int val)` has more restricted behavior in this International Standard. A `setjmp/longjmp` call pair has undefined behavior if replacing the `setjmp` and `longjmp` by `catch` and `throw` would destroy invoke any non-trivial destructors for any automatic objects.

### 896(i). Library thread safety issue

Section: 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Hans Boehm Opened: 2008-09-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It is unclear whether `shared_ptr` is thread-safe in the sense that multiple threads may simultaneously copy a `shared_ptr`. However this is a critical piece of information for the client, and it has significant impact on usability for many applications. (Detlef Vollman thinks it is currently clear that it is not thread-safe. Hans Boehm thinks it currently requires thread safety, since the `use_count` is not an explicit field, and constructors and assignment take a const reference to an existing `shared_ptr`.)

Pro thread-safety:

Many multi-threaded usages are impossible. A thread-safe version can be used to destroy an object when the last thread drops it, something that is often required, and for which we have no other easy mechanism.

Against thread-safety:

The thread-safe version is well-known to be far more expensive, even if used by a single thread. Many applications, including all single-threaded ones, do not care.

[ San Francisco: ]

> Beman: this is a complicated issue, and would like to move this to Open and await comment from Peter Dimov; we need very careful and complete rationale for any decision we make; let's go slow

> Detlef: I think that `shared_ptr` should not be thread-safe.

> Hans: When you create a thread with a lambda, it in some cases makes it very difficult for the lambda to reference anything in the heap. It's currently ambiguous as to whether you can use a `shared_ptr` to get at an object.

> Leave in Open. Detlef will submit an alternative proposed resolution that makes `shared_ptr` explicitly unsafe.

> A third option is to support both threadsafe and non-safe share_ptrs, and to let the programmer decide which behavior they want.

> Beman: Peter, do you support the PR?

> Peter:

> > Yes, I support the proposed resolution, and I certainly oppose any attempts to `make shared_ptr` thread-unsafe.

> > I'd mildly prefer if

> > > [Note: This is true in spite of that fact that such functions often modify `use_count()` --end note]

> > is changed to

> > > [Note: This is true in spite of that fact that such functions often cause a change in `use_count()` --end note]

> > (or something along these lines) to emphasise that `use_count()` is not, conceptually, a variable, but a return value.

[ 2009-07 Frankfurt ]

> Vote: Do we want one thread-safe shared pointer or two? If two, one would allow concurrent construction and destruction of shared pointers, and one would not be thread-safe. If one, then it would be thread-safe.

> No concensus on that vote.

> Hans to improve wording in consultation with Pete. Leave Open.

[ 2009-10 Santa Cruz: ]

> Move to Ready. Ask Editor to clear up wording a little when integrating to make it clear that the portion after the first comma only applies for the presence of data races.

[ 2009-10-24 Hans adds: ]

> I think we need to pull 896 back from ready, unfortunately. My wording doesn't say the right thing.

> I suspect we really want to say something along the lines of:

> > For purposes of determining the presence of a data race, member functions access and modify only the `shared_ptr` and `weak_ptr` objects themselves and not objects they refer to. Changes in `use_count()` do not reflect modifications that can introduce data races.

> But I think this needs further discussion by experts to make sure this is right.

> Detlef and I agree continue to disagree on the resolution, but I think we agree that it would be good to try to expedite this so that it can be in CD2, since it's likely to generate NB comments no matter what we do. And lack of clarity of intent is probably the worst option. I think it would be good to look at this between meetings.

[ 2010-01-20 Howard: ]

> I've moved Hans' suggested wording above into the proposed resolution section and preserved the previous wording here:

> > Make it explicitly thread-safe, in this weak sense, as I believe was intended:

> > Insert in 20.3.2.2 [util.smartptr.shared], before p5:

> > > For purposes of determining the presence of a data race, member functions do not modify `const shared_ptr` and const `weak_ptr` arguments, nor any objects they refer to. [Note: This is true in spite of that fact that such functions often cause a change in `use_count()` --end note]

> > On looking at the text, I'm not sure we need a similar disclaimer anywhere else, since nothing else has the problem with the modified `use_count()`. I think Howard arrived at a similar conclusion.

[ 2010 Pittsburgh: Moved to Ready for Pittsburgh ]

Proposed resolution:

Insert a new paragraph at the end of 20.3.2.2 [util.smartptr.shared]:

> For purposes of determining the presence of a data race, member functions access and modify only the `shared_ptr` and `weak_ptr` objects themselves and not objects they refer to. Changes in `use_count()` do not reflect modifications that can introduce data races.

### 897(i). Forward_list issues... Part 2

Section: 23.3.7.5 [forward.list.modifiers] Status: Resolved Submitter: Howard Hinnant Opened: 2008-09-22 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [forward.list.modifiers].

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Discussion:

This issue was split off from 892(i) at the request of the LWG.

[ San Francisco: ]

> This issue is more complicated than it looks.

> paragraph 47: replace each `(first, last) with (first, last]`

> add a statement after paragraph 48 that complexity is O(1)

> remove the complexity statement from the first overload of splice_after

> We may have the same problems with other modifiers, like erase_after. Should it require that all iterators in the range (position, last] be dereferenceable?

There are actually 3 issues here:

1. What value should `erase_after` return? With `list`, code often looks like:

   >     for (auto i = l.begin(); i != l.end();)
   >     {
   >         // inspect *i and decide if you want to erase it
   >         // ...
   >         if (I want to erase *i)
   >             i = l.erase(i);
   >         else
   >             ++i;
   >     }

   I.e. the iterator returned from `erase` is useful for setting up the logic for operating on the next element. For `forward_list` this might look something like:

   >     auto i = fl.before_begin();
   >     auto ip1 = i;
   >     for (++ip1; ip1 != fl.end(); ++ip1)
   >     {
   >         // inspect *(i+1) and decide if you want to erase it
   >         // ...
   >         if (I want to erase *(i+1))
   >             i = fl.erase_after(i);
   >         else
   >             ++i;
   >         ip1 = i;
   >     }

   In the above example code, it is convenient if `erase_after` returns the element prior to the erased element (range) instead of the element after the erase element (range).

   Existing practice:

   - SGI slist returns an iterator referencing the element after the erased range.
   - CodeWarrior slist returns an iterator referencing the element before the erased range.

   There is not a strong technical argument for either solution over the other.

2. With all other containers, operations always work on the range `[first, last)` and/or prior to the given `position`.

   With `forward_list`, operations sometimes work on the range `(first, last]` and/or after the given `position`.

   This is simply due to the fact that in order to operate on `*first` (with `forward_list`) one needs access to `*(first-1)`. And that's not practical with `forward_list`. So the operating range needs to start with `(first`, not `[first` (as the current working paper says).

   Additionally, if one is interested in splicing the range `(first, last)`, then (with `forward_list`), one needs practical (constant time) access to `*(last-1)` so that one can set the next field in this node to the proper value. As this is not possible with `forward_list`, one must specify the last element of interest instead of one past the last element of interest. The syntax for doing this is to pass `(first, last]` instead of `(first, last)`.

   With `erase_after` we have a choice of either erasing the range `(first, last]` or `(first, last)`. Choosing the latter enables:

   >     x.erase_after(pos, x.end());

   With the former, the above statement is inconvenient or expensive due to the lack of constant time access to `x.end()-1`. However we could introduce:

   >     iterator erase_to_end(const_iterator position);

   to compensate.

   The advantage of the former (`(first, last]`) for `erase_after` is a consistency with `splice_after` which uses `(first, last]` as the specified range. But this either requires the addition of `erase_to_end` or giving up such functionality.

3. As stated in the discussion of 892(i), and reienforced by point 2 above, a `splice_after` should work on the source range `(first, last]` if the operation is to be Ο(1). When splicing an entire list `x` the algorithm needs `(x.before_begin(), x.end()-1]`. Unfortunately `x.end()-1` is not available in constant time unless we specify that it must be. In order to make `x.end()-1` available in constant time, the implementation would have to dedicate a pointer to it. I believe the design of N2543 intended a nominal overhead of `foward_list` of 1 pointer. Thus splicing one entire `forward_list` into another can not be Ο(1).

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Review.

[ 2009-07 Frankfurt ]

> We may need a new issue to correct splice_after, because it may no longer be correct to accept an rvalues as an argument. Merge may be affected, too. This might be issue 1133(i). (Howard: confirmed)

> Move this to Ready, but the Requires clause of the second form of splice_after should say "(first, last)," not "(first, last]" (there are three occurrences). There was considerable discussion on this. (Howard: fixed)

> Alan suggested removing the "foward_last<T. Alloc>&& x" parameter from the second form of splice_after, because it is redundant. PJP wanted to keep it, because it allows him to check for bad ranges (i.e. "Granny knots").

> We prefer to keep `x`.

> Beman. Whenever we deviate from the customary half-open range in the specification, we should add a non-normative comment to the standard explaining the deviation. This clarifies the intention and spares the committee much confusion in the future.

> Alan to write a non-normative comment to explain the use of fully-closed ranges.

> Move to Ready, with the changes described above. (Howard: awaiting note from Alan)

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved, addressed by N2988.

Proposed resolution:

Wording below assumes issue 878(i) is accepted, but this issue is independent of that issue.

Change [forwardlist.modifiers]:

>     iterator erase_after(const_iterator position);

> > Requires: The iterator following `position` is dereferenceable.

> > Effects: Erases the element pointed to by the iterator following `position`.

> > Returns: An iterator pointing to the element following the one that was erased, or `end()` if no such element exists An iterator equal to `position`.

>     iterator erase_after(const_iterator position, const_iterator last);

> > Requires: All iterators in the range `[(position,last)` are dereferenceable.

> > Effects: Erases the elements in the range `[(position,last)`.

> > Returns: An iterator equal to `position` `last`

Change [forwardlist.ops]:

>     void splice_after(const_iterator position, forward_list<T,Allocator>&& x);

> > Requires: `position` is `before_begin()` or a dereferenceable iterator in the range `[begin(), end))`. `&x != this`.

> > Effects: Inserts the contents of `x` after `position`, and `x` becomes empty. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

> > Throws: Nothing.

> > Complexity: Ο(1) Ο(`distance(x.begin(), x.end())`)

> ...

>     void splice_after(const_iterator position, forward_list<T,Allocator>&& x,
>                       const_iterator first, const_iterator last);

> > Requires: `position` is `before_begin()` or a dereferenceable iterator in the range `[begin(), end))`. `(first,last)` is a valid range in `x`, and all iterators in the range `(first,last)` are dereferenceable. `position` is not an iterator in the range `(first,last)`.

> > Effects: Inserts elements in the range `(first,last)` after `position` and removes the elements from `x`. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

> > Complexity: Ο(1).

### 898(i). Small contradiction in n2723 to forward to committee

Section: 23.3.7.6 [forward.list.ops] Status: C++11 Submitter: Arch Robison Opened: 2008-09-08 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [forward.list.ops].

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Discussion:

I ran across a small contradiction in working draft n2723.

> [forwardlist]p2: A `forward_list` satisfies all of the requirements of a container (table 90), except that the `size()` member function is not provided.

> [forwardlist.ops]p57: Complexity: At most `size() + x.size() - 1` comparisons.

Presumably [forwardlist.ops]p57 needs to be rephrased to not use `size()`, or note that it is used there only for sake of notational convenience.

[ 2009-03-29 Beman provided proposed wording. ]

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

Change [forwardlist.ops], forward_list operations, paragraph 19, merge complexity as indicated:

> Complexity: At most `size() + x.size() distance(begin(), end()) + distance(x.begin(), x.end()) - 1` comparisons.

### 899(i). Adjusting `shared_ptr` for `nullptr_t`

Section: 20.3.2.2.3 [util.smartptr.shared.dest] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.smartptr.shared.dest].

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Discussion:

James Dennett, message c++std-lib-22442:

> The wording below addresses one case of this, but opening an issue to address the need to sanity check uses of the term "pointer" in 20.3.2.2 [util.smartptr.shared] would be a good thing.

There's one more reference, in `~shared_ptr;` we can apply your suggested change to it, too. That is:

Change 20.3.2.2.3 [util.smartptr.shared.dest]/1 second bullet from:

> Otherwise, if *this owns a pointer p and a deleter d, d(p) is called.

to:

> Otherwise, if *this owns an object p and a deleter d, d(p) is called.

[ Post Summit: ]

> Recommend Review.

[ Batavia (2009-05): ]

> Peter Dimov notes the analogous change has already been made to "the new `nullptr_t` taking constructors in 20.3.2.2.2 [util.smartptr.shared.const] p9-13."

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.3.2.2.3 [util.smartptr.shared.dest] p1 second bullet:

> - ...
> - Otherwise, if `*this` owns a pointer an object `p` and a deleter `d`, `d(p)` is called.

### 900(i). Stream move-assignment

Section: 31.10.4 [ifstream] Status: C++11 Submitter: Niels Dekker Opened: 2008-09-20 Last modified: 2023-02-07

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

It appears that we have an issue similar to issue 675(i) regarding the move-assignment of stream types. For example, when assigning to an `std::ifstream`, `ifstream1`, it seems preferable to close the file originally held by `ifstream1`:

>     ifstream1 = std::move(ifstream2);

The current Draft (N2723) specifies that the move-assignment of stream types like `ifstream` has the same effect as a swap:

> Assign and swap [ifstream.assign]

>     basic_ifstream& operator=(basic_ifstream&& rhs);

> > Effects: `swap(rhs)`.

[ Batavia (2009-05): ]

> Howard agrees with the analysis and the direction proposed.

> Move to Open pending specific wording to be supplied by Howard.

[ 2009-07 Frankfurt: ]

> Howard is going to write wording.

[ 2009-07-26 Howard provided wording. ]

[ 2009-09-13 Niels adds: ]

> Note: The proposed change of 31.10.3.3 [filebuf.assign] p1 depends on the resolution of LWG 1204(i), which allows implementations to assume that `*this` and `rhs` refer to different objects.

[ 2009 Santa Cruz: ]

> Leave as Open. Too closely related to 911(i) to move on at this time.

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

Change 31.8.2.3 [stringbuf.assign]/1:

>     basic_stringbuf& operator=(basic_stringbuf&& rhs);

> > -1- Effects: `swap(rhs)`. After the move assignment `*this` reflects the same observable state it would have if it had been move constructed from `rhs` (31.8.2.2 [stringbuf.cons]).

Change [istringstream.assign]/1:

>     basic_istringstream& operator=(basic_istringstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change [ostringstream.assign]/1:

>     basic_ostringstream& operator=(basic_ostringstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change [stringstream.assign]/1:

>     basic_stringstream& operator=(basic_stringstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change 31.10.3.3 [filebuf.assign]/1:

>     basic_filebuf& operator=(basic_filebuf&& rhs);

> > -1- Effects: `swap(rhs)`. Begins by calling `this->close()`. After the move assignment `*this` reflects the same observable state it would have if it had been move constructed from `rhs` (31.10.3.2 [filebuf.cons]).

Change [ifstream.assign]/1:

>     basic_ifstream& operator=(basic_ifstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change [ofstream.assign]/1:

>     basic_ofstream& operator=(basic_ofstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change [fstream.assign]/1:

>     basic_fstream& operator=(basic_fstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

### 904(i). `result_of` argument types

Section: 99 [func.ret] Status: C++11 Submitter: Jonathan Wakely Opened: 2008-09-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.ret].

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Discussion:

The WP and TR1 have the same text regarding the argument types of a `result_of` expression:

> The values `ti` are lvalues when the corresponding type `Ti` is a reference type, and rvalues otherwise.

I read this to mean that this compiles:

>     typedef int (*func)(int&);
>     result_of<func(int&&)>::type i = 0;

even though this doesn't:

>     int f(int&);
>     f( std::move(0) );

Should the text be updated to say "when `Ti` is an lvalue-reference type" or am I missing something?

I later came up with this self-contained example which won't compile, but I think it should:

>     struct X {
>       void operator()(int&);
>       int operator()(int&&);
>     } x;

>     std::result_of< X(int&&) >::type i = x(std::move(0));

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

Change 99 [func.ret], p1:

> ... The values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

### 907(i). Bitset's immutable element retrieval is inconsistently defined

Section: 22.9.2.3 [bitset.members] Status: C++11 Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [bitset.members].

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Discussion:

The current standard 14882::2003(E) as well as the current draft N2723 have in common a contradiction of the operational semantics of member function `test` 22.9.2.3 [bitset.members] p.56-58 and the immutable member `operator[]` overload 22.9.2.3 [bitset.members] p.64-66 (all references are defined in terms of N2723):

1.     bool test(size_t pos) const;

   > Requires: `pos` is valid

   > Throws: `out_of_range` if `pos` does not correspond to a valid bit position.

   > Returns: `true` if the bit at position `pos` in `*this` has the value one.

2.     constexpr bool operator[](size_t pos) const;

   > Requires: `pos` shall be valid.

   > Throws: nothing.

   > Returns: `test(pos)`.

Three interpretations:

1. The `operator[]` overload is indeed allowed to throw an exception (via `test()`, if `pos` corresponds to an invalid bit position) which does not leave the call frame. In this case this function cannot be a `constexpr` function, because `test()` is not, due to 7.7 [expr.const]/2, last bullet.
2. The intend was not to throw an exception in `test` in case of an invalid bit position. There is only little evidence for this interpretation.
3. The intend was that `operator[]` should not throw any exception, but that `test` has the contract to do so, if the provided bit position is invalid.

The problem became worse, because issue 720(i) recently voted into WP argued that member `test` logically must be a `constexpr` function, because it was used to define the semantics of another `constexpr` function (the `operator[]` overload).

Three alternatives are proposed, corresponding to the three bullets (A), (B), and (C), the author suggests to follow proposal (C).

Proposed alternatives:

1. Remove the `constexpr` specifier in front of `operator[]` overload and undo that of member `test` (assuming 720(i) is accepted) in both the class declaration 22.9.2 [template.bitset]/1 and in the member description before 22.9.2.3 [bitset.members]/56 and before /64 to read:

   >     constexpr bool test(size_t pos) const;
   >     ..
   >     constexpr bool operator[](size_t pos) const;

   Change the throws clause of p. 65 to read:

   > Throws: nothing `out_of_range` if `pos` does not correspond to a valid bit position.

2. Replace the throws clause p. 57 to read:

   > Throws: `out_of_range` if `pos` does not correspond to a valid bit position nothing.

3. Undo the addition of the `constexpr` specifier to the `test` member function in both class declaration 22.9.2 [template.bitset]/1 and in the member description before 22.9.2.3 [bitset.members]/56, assuming that 720(i) was applied.

   >     constexpr bool test(size_t pos) const;

   Change the returns clause p. 66 to read:

   > Returns: `test(pos)` `true` if the bit at position `pos` in `*this` has the value one, otherwise `false`.

[ Post Summit: ]

> Lawrence: proposed resolutions A, B, C are mutually exclusive.

> Recommend Review with option C.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

1. […]
2. […]

3. Undo the addition of the `constexpr` specifier to the `test` member function in both class declaration 22.9.2 [template.bitset] p.1 and in the member description before 22.9.2.3 [bitset.members] p.56, assuming that 720(i) was applied.

   >     constexpr bool test(size_t pos) const;

   Change the returns clause p. 66 to read:

   > Returns: `test(pos)` `true` if the bit at position `pos` in `*this` has the value one, otherwise `false`.

### 908(i). Deleted assignment operators for atomic types must be volatile

Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Anthony Williams Opened: 2008-09-26 Last modified: 2017-11-29

Priority: Not Prioritized

View all other issues in [atomics.types.generic].

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Discussion:

Addresses US 90

The deleted copy-assignment operators for the atomic types are not marked as volatile in N2723, whereas the assignment operators from the associated non-atomic types are. e.g.

>     atomic_bool& operator=(atomic_bool const&) = delete;
>     atomic_bool& operator=(bool) volatile;

This leads to ambiguity when assigning a non-atomic value to a non-volatile instance of an atomic type:

>     atomic_bool b;
>     b=false;

Both assignment operators require a standard conversions: the copy-assignment operator can use the implicit `atomic_bool(bool)` conversion constructor to convert `false` to an instance of `atomic_bool`, or `b` can undergo a qualification conversion in order to use the assignment from a plain `bool`.

This is only a problem once issue 845(i) is applied.

[ Summit: ]

> Move to open. Assign to Lawrence. Related to US 90 comment.

[ 2009-08-17 Handled by N2925. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2992.

Proposed resolution:

Add volatile qualification to the deleted copy-assignment operator of all the atomic types:

>     atomic_bool& operator=(atomic_bool const&) volatile = delete;
>     atomic_itype& operator=(atomic_itype const&) volatile = delete;

etc.

This will mean that the deleted copy-assignment operator will require two conversions in the above example, and thus be a worse match than the assignment from plain `bool`.

### 909(i). `regex_token_iterator` should use `initializer_list`

Section: 28.6.11.2 [re.tokiter] Status: C++11 Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [re.tokiter].

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Discussion:

Addresses UK 319

Construction of a `regex_token_iterator` (28.6.11.2 [re.tokiter]/6+) usually requires the provision of a sequence of integer values, which can currently be done via a `std::vector<int>` or a C array of `int`. Since the introduction of `initializer_list` in the standard it seems much more reasonable to provide a corresponding constructor that accepts an `initializer_list<int>` instead. This could be done as a pure addition or one could even consider replacement. The author suggests the replacement strategy (A), but provides an alternative additive proposal (B) as a fall-back, because of the handiness of this range type:

[ Batavia (2009-05): ]

> We strongly recommend alternative B of the proposed resolution in order that existing code not be broken. With that understanding, move to Tentatively Ready.

Original proposed wording:

1. 1. In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 change the constructor declaration:

      >     template <std::size_t N>
      >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
      >                          const regex_type& re,
      >                          const int (&submatches)[N] initializer_list<int> submatches,
      >                          regex_constants::match_flag_type m =
      >                            regex_constants::match_default);

   2. In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence

      > The third constructor initializes the member `subs` to hold a copy of the sequence of integer values pointed to by the iterator range `[&submatches.begin(), &submatches.end() + N)`.

2. 1. In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 insert the following constructor declaration between the already existing ones accepting a `std::vector` and a C array of `int`, resp.:

      >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
      >                          const regex_type& re,
      >                          initializer_list<int> submatches,
      >                          regex_constants::match_flag_type m =
      >                            regex_constants::match_default);

   2. In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence

      > The third and fourth constructor initializes the member `subs` to hold a copy of the sequence of integer values pointed to by the iterator range `[&submatches,&submatches + N)` and `[submatches.begin(),submatches.end())`, respectively.

Proposed resolution:

1. […]

2. 1. In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 insert the following constructor declaration between the already existing ones accepting a `std::vector` and a C array of `int`, resp.:

      >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
      >                          const regex_type& re,
      >                          initializer_list<int> submatches,
      >                          regex_constants::match_flag_type m =
      >                            regex_constants::match_default);

   2. In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence

      > The third and fourth constructor initializes the member `subs` to hold a copy of the sequence of integer values pointed to by the iterator range `[&submatches,&submatches + N)` and `[submatches.begin(),submatches.end())`, respectively.

### 911(i). I/O streams and `move/swap` semantic

Section: 31.7.5 [input.streams], 31.7.6 [output.streams] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-09-29 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Class template `basic_istream`, `basic_ostream` and `basic_iostream` implements public move constructors, move assignment operators and `swap` method and free functions. This might induce both the user and the compiler to think that those types are `MoveConstructible`, `MoveAssignable` and `Swappable`. However, those class templates fail to fulfill the user expectations. For example:

>     std::ostream os(std::ofstream("file.txt"));
>     assert(os.rdbuf() == 0); // buffer object is not moved to os, file.txt has been closed

>     std::vector<std::ostream> v;
>     v.push_back(std::ofstream("file.txt"));
>     v.reserve(100); // causes reallocation
>     assert(v[0].rdbuf() == 0); // file.txt has been closed!

>     std::ostream&& os1 = std::ofstream("file1.txt");
>     os1 = std::ofstream("file2.txt");
>     os1 << "hello, world"; // still writes to file1.txt, not to file2.txt!

>     std::ostream&& os1 = std::ofstream("file1.txt");
>     std::ostream&& os2 = std::ofstream("file2.txt");
>     std::swap(os1, os2);
>     os1 << "hello, world"; // writes to file1.txt, not to file2.txt!

This is because the move constructor, the move assignment operator and `swap` are all implemented through calls to `std::basic_ios` member functions `move()` and `swap()` that do not move nor swap the controlled stream buffers. That can't happen because the stream buffers may have different types.

Notice that for `basic_streambuf`, the member function `swap()` is protected. I believe that is correct and all of `basic_istream`, `basic_ostream`, `basic_iostream` should do the same as the move ctor, move assignment operator and swap member function are needed by the derived `fstream`s and `stringstream`s template. The free swap functions for `basic_(i|o|io)stream` templates should be removed for the same reason.

[ Batavia (2009-05): ]

> We note that the rvalue swap functions have already been removed.

> Bill is unsure about making the affected functions protected; he believes they may need to be public.

> We are also unsure about removing the lvalue swap functions as proposed.

> Move to Open.

[ 2009-07 Frankfurt: ]

> It's not clear that the use case is compelling.

> Howard: This needs to be implemented and tested.

[ 2009-07-26 Howard adds: ]

> I started out thinking I would recommend NAD for this one. I've turned around to agree with the proposed resolution (which I've updated to the current draft). I did not fully understand Ganesh's rationale, and attempt to describe my improved understanding below.

> The move constructor, move assignment operator, and swap function are different for `basic_istream`, `basic_ostream` and `basic_iostream` than other classes. A timely conversation with Daniel reminded me of this long forgotten fact. These members are sufficiently different that they would be extremely confusing to use in general, but they are very much needed for derived clients.

> - The move constructor moves everything but the `rdbuf` pointer.
> - The move assignment operator moves everything but the `rdbuf` pointer.
> - The swap function swaps everything but the `rdbuf` pointer.

> The reason for this behavior is that for the std-derived classes (stringstreams, filestreams), the `rdbuf` pointer points back into the class itself (self referencing). It can't be swapped or moved. But this fact isn't born out at the `stream` level. Rather it is born out at the `fstream`/`sstream` level. And the lower levels just need to deal with that fact by not messing around with the `rdbuf` pointer which is stored down at the lower levels.

> In a nutshell, it is very confusing for all of those who are not so intimately related with streams that they've implemented them. And it is even fairly confusing for some of those who have (including myself). I do not think it is safe to swap or move `istreams` or `ostreams` because this will (by necessary design) separate stream state from streambuffer state. Derived classes (such as `fstream` and `stringstream` must be used to keep the stream state and stream buffer consistently packaged as one unit during a move or swap.

> I've implemented this proposal and am living with it day to day.

[ 2009 Santa Cruz: ]

> Leave Open. Pablo expected to propose alternative wording which would rename move construction, move assignment and swap, and may or may not make them protected. This will impact issue 900(i).

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

31.7.5.2 [istream]: make the following member functions protected:

>     basic_istream(basic_istream&&  rhs);
>     basic_istream&  operator=(basic_istream&&  rhs);
>     void  swap(basic_istream&  rhs);

Ditto: remove the swap free function signature

>     // swap:
>     template <class charT, class traits>
>       void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y);

31.7.5.2.3 [istream.assign]: remove paragraph 4

>     template <class charT, class traits>
>       void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y);

> > Effects: `x.swap(y)`.

31.7.5.7 [iostreamclass]: make the following member function protected:

>     basic_iostream(basic_iostream&&  rhs);
>     basic_iostream&  operator=(basic_iostream&&  rhs);
>     void  swap(basic_iostream&  rhs);

Ditto: remove the swap free function signature

>     template <class charT, class traits>
>       void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y);

31.7.5.7.4 [iostream.assign]: remove paragraph 3

>     template <class charT, class traits>
>       void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y);

> > Effects: `x.swap(y)`.

31.7.6.2 [ostream]: make the following member function protected:

>     basic_ostream(basic_ostream&&  rhs);
>     basic_ostream&  operator=(basic_ostream&&  rhs);
>     void  swap(basic_ostream&  rhs);

Ditto: remove the swap free function signature

>     // swap:
>     template <class charT, class traits>
>       void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);

31.7.6.2.3 [ostream.assign]: remove paragraph 4

>     template <class charT, class traits>
>       void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);

> > Effects: `x.swap(y)`.

### 920(i). Ref-qualification support in the library

Section: 22.10.16 [func.memfn] Status: C++11 Submitter: Bronek Kozicki Opened: 2008-10-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.memfn].

View all issues with C++11 status.

Duplicate of: 1230

Discussion:

Daniel Krügler wrote:

> Shouldn't above list be completed for &- and &&-qualified member functions This would cause to add:

> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) &);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) const &);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) volatile &);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) const volatile &);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) &&);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) const &&);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) volatile &&);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);

yes, absolutely. Thanks for spotting this. Without this change `mem_fn` cannot be initialized from pointer to ref-qualified member function. I believe semantics of such function pointer is well defined.

[ Post Summit Daniel provided wording. ]

[ Batavia (2009-05): ]

> We need to think about whether we really want to go down the proposed path of combinatorial explosion. Perhaps a Note would suffice.

> We would really like to have an implementation before proceeding.

> Move to Open, and recommend this be deferred until after the next Committee Draft has been issued.

[ 2009-10-10 Daniel updated wording to post-concepts. ]

> 1230(i) has a similar proposed resolution

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

1. Change 22.10 [function.objects]/2, header `<functional>` synopsis as follows:

   >     // 20.7.14, member function adaptors:
   >     template<class R, class T> unspecified mem_fn(R T::*);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...));
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);

2. Change the prototype list of 22.10.16 [func.memfn] as follows [NB: The following text, most notably p.2 and p.3 which discuss influence of the cv-qualification on the definition of the base class's first template parameter remains unchanged. ]:

   >     template<class R, class T> unspecified mem_fn(R T::* pm);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...));
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);

3. Remove 22.10.16 [func.memfn]/5:

   > Remarks: Implementations may implement `mem_fn` as a set of overloaded function templates.

### 921(i). Rational Arithmetic should use template aliases

Section: 21.5.3 [ratio.ratio] Status: C++11 Submitter: Pablo Halpern Opened: 2008-10-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ratio.ratio].

View all issues with C++11 status.

Discussion:

The compile-time functions that operate on `ratio<N,D>` require the cumbersome and error-prone "evaluation" of a `type` member using a meta-programming style that predates the invention of template aliases. Thus, multiplying three ratios `a`, `b`, and `c` requires the expression:

>     ratio_multiply<a, ratio_multiply<b, c>::type>::type

The simpler expression:

>     ratio_multiply<a, ratio_multiply<b, c>>

Could be used by if template aliases were employed in the definitions.

[ Post Summit: ]

> Jens: not a complete proposed resolution: "would need to make similar change"

> Consensus: We agree with the direction of the issue.

> Recommend Open.

[ 2009-05-11 Daniel adds: ]

> Personally I'm not in favor for the addition of:

> >     typedef ratio type;

> For a reader of the standard it's usage or purpose is unclear. I haven't seen similar examples of attempts to satisfy non-feature complete compilers.

[ 2009-05-11 Pablo adds: ]

> The addition of type to the `ratio` template allows the previous style (i.e., in the prototype implementations) to remain valid and permits the use of transitional library implementations for C++03 compilers. I do not feel strongly about its inclusion, however, and leave it up to the reviewers to decide.

[ Batavia (2009-05): ]

> Bill asks for additional discussion in the issue that spells out more details of the implementation. Howard points us to issue 948(i) which has at least most of the requested details. Tom is strongly in favor of overflow-checking at compile time. Pete points out that there is no change of functionality implied. We agree with the proposed resolution, but recommend moving the issue to Review to allow time to improve the discussion if needed.

[ 2009-07-21 Alisdair adds: ]

> See 1121(i) for a potentially incompatible proposal.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

1. In 21.5 [ratio] p.3 change as indicated:

   >     // ratio arithmetic
   >     template <class R1, class R2> structusing ratio_add = see below;
   >     template <class R1, class R2> structusing ratio_subtract = see below;
   >     template <class R1, class R2> structusing ratio_multiply = see below;
   >     template <class R1, class R2> structusing ratio_divide = see below;

2. In 21.5.3 [ratio.ratio], change as indicated:

   >     namespace std {
   >       template <intmax_t N, intmax_t D = 1>
   >       class ratio {
   >       public:
   >         typedef ratio type;
   >         static const intmax_t num;
   >         static const intmax_t den;
   >       };
   >     }

3. In 21.5.4 [ratio.arithmetic] change as indicated:

   >     template <class R1, class R2> structusing ratio_add = see below{
   >       typedef see below type;
   >     };

   > > 1 The nested typedef type `ratio_add<R1, R2>` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::den + R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

   >     template <class R1, class R2> structusing ratio_subtract = see below{
   >       typedef see below type;
   >     };

   > > 2 The nested typedef type `ratio_subtract<R1, R2>` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::den - R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

   >     template <class R1, class R2> structusing ratio_multiply = see below{
   >       typedef see below type;
   >     };

   > > 3 The nested typedef type `ratio_multiply<R1, R2>` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::num` and `T2` has the value `R1::den * R2::den`.

   >     template <class R1, class R2> structusing ratio_divide = see below{
   >       typedef see below type;
   >     };

   > > 4 The nested typedef type `ratio_divide<R1, R2>` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::den` and `T2` has the value `R1::den * R2::num`.

4. In 30.5.2 [time.duration.cons] p.4 change as indicated:

   > Requires: `treat_as_floating_point<rep>::value` shall be true or `ratio_divide<Period2, period>::type::den` shall be 1.[..]

5. In 30.5.8 [time.duration.cast] p.2 change as indicated:

   > Returns: Let CF be `ratio_divide<Period, typename ToDuration::period>::type`, and [..]

### 922(i). §[func.bind.place] Number of placeholders

Section: B [implimits] Status: C++11 Submitter: Sohail Somani Opened: 2008-10-11 Last modified: 2016-02-01

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Addresses DE 24

With respect to the section 22.10.15.5 [func.bind.place]:

TR1 dropped some suggested implementation quantities for the number of placeholders. The purpose of this defect is to put these back for C++0x.

[ Post Summit: ]

> see DE 24

> Recommend applying the proposed resolution from DE 24, with that Tentatively Ready.

Original proposed resolution:

Add 22.10.15.5 [func.bind.place] p.2:

> While the exact number of placeholders (`_M`) is implementation defined, this number shall be at least 10.

Proposed resolution:

Add to B [implimits]:

- Number of placeholders (22.10.15.5 [func.bind.place]) [10].

### 923(i). atomics with floating-point

Section: 32.5 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics].

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Discussion:

Right now, C++0x doesn't have `atomic<float>`. We're thinking of adding the words to support it for TR2 (note: that would be slightly post-C++0x). If we need it, we could probably add the words.

Proposed resolutions: Using `atomic<FP>::compare_exchange` (weak or strong) should be either:

1. ill-formed, or
2. well-defined.

I propose Option 1 for C++0x for expediency. If someone wants to argue for Option 2, they need to say what exactly they want `compare_exchange` to mean in this case (IIRC, C++0x doesn't even assume IEEE 754).

[ Summit: ]

> Move to open. Blocked until concepts for atomics are addressed.

[ Post Summit Anthony adds: ]

> Recommend NAD. C++0x does have `std::atomic<float>`, and both `compare_exchange_weak` and `compare_exchange_strong` are well-defined in this case. Maybe change the note in 32.5.8.2 [atomics.types.operations] paragraph 20 to:

> > [Note: The effect of the compare-and-exchange operations is

> > >     if (!memcmp(object,expected,sizeof(*object)))
> > >         *object = desired;
> > >     else
> > >         *expected = *object;

> > This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2992.

Proposed resolution:

Change the note in 32.5.8.2 [atomics.types.operations] paragraph 20 to:

> [Note: The effect of the compare-and-exchange operations is

> >     if (*object == *expected !memcmp(object,expected,sizeof(*object)))
> >         *object = desired;
> >     else
> >         *expected = *object;

> This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]

### 924(i). structs with internal padding

Section: 32.5 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics].

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Discussion:

Right now, the `compare_exchange_weak` loop should rapidly converge on the padding contents. But `compare_exchange_strong` will require a bit more compiler work to ignore padding for comparison purposes.

Note that this isn't a problem for structs with no padding, and we do already have one portable way to ensure that there is no padding that covers the key use cases: Have elements be the same type. I suspect that the greatest need is for a structure of two pointers, which has no padding problem. I suspect the second need is a structure of a pointer and some form of an integer. If that integer is `intptr_t`, there will be no padding.

Related but separable issue: For unused bitfields, or other unused fields for that matter, we should probably say it's the programmer's responsibility to set them to zero or otherwise ensure they'll be ignored by `memcmp`.

Proposed resolution: Using `atomic<struct-with-padding>::compare_exchange_strong` should be either:

1. ill-formed, or
2. well-defined.

I propose Option 1 for C++0x for expediency, though I'm not sure how to say it. I would be happy with Option 2, which I believe would mean that `compare_exchange_strong` would be implemented to avoid comparing padding bytes, or something equivalent such as always zeroing out padding when loading/storing/comparing. (Either implementation might require compiler support.)

[ Summit: ]

> Move to open. Blocked until concepts for atomics are addressed.

[ Post Summit Anthony adds: ]

> The resolution of LWG 923(i) should resolve this issue as well.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2992.

Proposed resolution:

### 925(i). `shared_ptr`'s explicit conversion from `unique_ptr`

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++11 Submitter: Rodolfo Lima Opened: 2008-10-12 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [util.smartptr.shared.const].

View all other issues in [util.smartptr.shared.const].

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Discussion:

The current working draft (N2798), section 20.3.2.2.2 [util.smartptr.shared.const] declares `shared_ptr`'s constructor that takes a rvalue reference to `unique_ptr` and `auto_ptr` as being explicit, affecting several valid smart pointer use cases that would take advantage of this conversion being implicit, for example:

>     class A;
>     std::unique_ptr<A> create();
>     void process(std::shared_ptr<A> obj);

>     int main()
>     {
>        process(create());                  // use case #1
>        std::unique_ptr<A> uobj = create();
>        process(std::move(uobj));           // use case #2
>        return 0;
>     }

If `unique_ptr` to `shared_ptr` conversions are explicit, the above lines should be written:

>     process(std::shared_ptr<A>(create()));        // use case #1
>     process(std::shared_ptr<A>(std::move(uobj))); // use case #2

The extra cast required doesn't seems to give any benefits to the user, nor protects him of any unintended conversions, this being the raison d'etre of explicit constructors.

It seems that this constructor was made explicit to mimic the conversion from `auto_ptr` in pre-rvalue reference days, which accepts both lvalue and rvalue references. Although this decision was valid back then, C++0x allows the user to express in a clear and non verbose manner when he wants move semantics to be employed, be it implicitly (use case 1) or explicitly (use case 2).

[ Batavia (2009-05): ]

> Howard and Alisdair like the motivating use cases and the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

In both 20.3.2.2 [util.smartptr.shared] paragraph 1 and 20.3.2.2.2 [util.smartptr.shared.const] change:

>     template <class Y> explicit shared_ptr(auto_ptr<Y> &&r);
>     template <class Y, class D> explicit shared_ptr(unique_ptr<Y, D> &&r);

### 929(i). Thread constructor

Section: 32.4.3.3 [thread.thread.constr] Status: C++11 Submitter: Anthony Williams Opened: 2008-10-23 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [thread.thread.constr].

View all other issues in [thread.thread.constr].

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Discussion:

Addresses UK 323

The `thread` constructor for starting a new thread with a function and arguments is overly constrained by the signature requiring rvalue references for `func` and `args` and the `CopyConstructible` requirements for the elements of `args`. The use of an rvalue reference for the function restricts the potential use of a plain function name, since the type of the bound parameter will be deduced to be a function reference and decay to pointer-to-function will not happen. This therefore complicates the implementation in order to handle a simple case. Furthermore, the use of rvalue references for args prevents the array to pointer decay. Since arrays are not `CopyConstructible` or even `MoveConstructible`, this essentially prevents the passing of arrays as parameters. In particular it prevents the passing of string literals. Consequently a simple case such as

>     void f(const char*);
>     std::thread t(f,"hello");

is ill-formed since the type of the string literal is `const char[6]`.

By changing the signature to take all parameters by value we can eliminate the `CopyConstructible` requirement and permit the use of arrays, as the parameter passing semantics will cause the necessary array-to-pointer decay. They will also cause the function name to decay to a pointer to function and allow the implementation to handle functions and function objects identically.

The new signature of the `thread` constructor for a function and arguments is thus:

>     template<typename F,typename... Args>
>     thread(F,Args... args);

Since the parameter pack `Args` can be empty, the single-parameter constructor that takes just a function by value is now redundant.

[ Howard adds: ]

> I agree with everything Anthony says in this issue. However I believe we can optimize in such a way as to get the pass-by-value behavior with the pass-by-rvalue-ref performance. The performance difference is that the latter removes a `move` when passing in an lvalue.

> This circumstance is very analogous to `make_pair` (22.3 [pairs]) where we started with passing by const reference, changed to pass by value to get pointer decay, and then changed to pass by rvalue reference, but modified with `decay<T>` to retain the pass-by-value behavior. If we were to apply the same solution here it would look like:

> >     template <class F> explicit thread(F f);
> >     template <class F, class ...Args> thread(F&& f, Args&&... args);

> > > -4- Requires: `F` and each `Ti` in `Args` shall be `CopyConstructible` if an lvalue and otherwise `MoveConstructible`. `INVOKE(f, w1, w2, ..., wN)` (22.10.4 [func.require]) shall be a valid expression for some values `w1, w2, ... , wN,` where `N == sizeof...(Args)`.

> > > -5- Effects: Constructs an object of type `thread` and executes `INVOKE(f, t1, t2, ..., tN)` in a new thread of execution, where `t1, t2, ..., tN` are the values in `args...`. Constructs the following objects in memory which is accessible to a new thread of execution as if:

> > > >     typename decay<F>::type g(std::forward<F>(f));
> > > >     tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);

> > > The new thread of execution executes `INVOKE(g, wi...)` where the `wi...` refers to the elements stored in the `tuple w`. Any return value from `g` is ignored. If `f` terminates with an uncaught exception, `std::terminate()` shall be called. If the evaluation of `INVOKE(g, wi...)` terminates with an uncaught exception, `std::terminate()` shall be called [Note: `std::terminate()` could be called before entering `g`. -- end note]. Any exception thrown before the evaluation of `INVOKE` has started shall be catchable in the calling thread.

> Text referring to when `terminate()` is called was contributed by Ganesh.

[ Batavia (2009-05): ]

> We agree with the proposed resolution, but would like the final sentence to be reworded since "catchable" is not a term of art (and is used nowhere else).

[ 2009-07 Frankfurt: ]

> This is linked to N2901.

> Howard to open a separate issue to remove (1176(i)).

> In Frankfurt there is no consensus for removing the variadic constructor.

[ 2009-10 Santa Cruz: ]

> We want to move forward with this issue. If we later take it out via 1176(i) then that's ok too. Needs small group to improve wording.

[ 2009-10 Santa Cruz: ]

> Stefanus provided revised wording. Moved to Review Here is the original wording:

> > Modify the class definition of `std::thread` in 32.4.3 [thread.thread.class] to remove the following signature:

> > >     template<class F> explicit thread(F f);
> > >     template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);

> > Modify 32.4.3.3 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:

> > > -4- Requires: `F` and each `Ti` in `Args` shall be `CopyConstructible` if an lvalue and otherwise `MoveConstructible`. `INVOKE(f, w1, w2, ..., wN)` (22.10.4 [func.require]) shall be a valid expression for some values `w1, w2, ... , wN,` where `N == sizeof...(Args)`.

> > > -5- Effects: Constructs an object of type `thread` and executes `INVOKE(f, t1, t2, ..., tN)` in a new thread of execution, where `t1, t2, ..., tN` are the values in `args...`. Constructs the following objects:

> > > >     typename decay<F>::type g(std::forward<F>(f));
> > > >     tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);

> > > and executes `INVOKE(g, wi...)` in a new thread of execution. These objects shall be destroyed when the new thread of execution completes. Any return value from `g` is ignored. If `f` terminates with an uncaught exception, `std::terminate()` shall be called. If the evaluation of `INVOKE(g, wi...)` terminates with an uncaught exception, `std::terminate()` shall be called [Note: `std::terminate()` could be called before entering `g`. -- end note]. Any exception thrown before the evaluation of `INVOKE` has started shall be catchable in the calling thread.

> > > -6- Synchronization: The invocation of the constructor happens before the invocation of `f` `g`.

[ 2010-01-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Modify the class definition of `std::thread` in 32.4.3 [thread.thread.class] to remove the following signature:

>     template<class F> explicit thread(F f);
>     template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);

Modify 32.4.3.3 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:

> Given a function as follows:

> >     template<typename T> typename decay<T>::type decay_copy(T&& v)
> >         { return std::forward<T>(v); }

> -4- Requires: `F` and each `Ti` in `Args` shall be `CopyConstructible` if an lvalue and otherwise satisfy the `MoveConstructible` requirements. `INVOKE(f, w1, w2, ..., wN)` (22.10.4 [func.require]) shall be a valid expression for some values `w1, w2, ... , wN,` where `N == sizeof...(Args)`. `INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)` (22.10.4 [func.require]) shall be a valid expression.

> -5- Effects: Constructs an object of type `thread` and executes `INVOKE(f, t1, t2, ..., tN)` in a new thread of execution, where `t1, t2, ..., tN` are the values in `args...`. Any return value from `f` is ignored. If `f` terminates with an uncaught exception, `std::terminate()` shall be called. The new thread of execution executes `INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)` with the calls to `decay_copy()` being evaluated in the constructing thread. Any return value from this invocation is ignored. [Note: this implies any exceptions not thrown from the invocation of the copy of `f` will be thrown in the constructing thread, not the new thread. — end note]. If the invocation of `INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)` terminates with an uncaught exception, `std::terminate` shall be called.

> -6- Synchronization: The invocation of the constructor happens before the invocation of the copy of `f`.

### 931(i). type trait `extent<T, I>`

Section: 21.3.5.4 [meta.unary.prop] Status: C++11 Submitter: Yechezkel Mett Opened: 2008-11-04 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

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Discussion:

The draft (N2798) says in 21.3.5.4 [meta.unary.prop] Table 44:

> Table 44 -- Type property queries
> Template | Value
> --- | ---
> `template <class T, unsigned I = 0> struct extent;` | If `T` is not an array type (8.3.4), or if it has rank less than `I`, or if `I` is 0 and `T` has type "array of unknown bound of `U`", then 0; otherwise, the size of the `I`'th dimension of `T`

Firstly it isn't clear from the wording if `I` is 0-based or 1-based ("the `I`'th dimension" sort of implies 1-based). From the following example it is clear that the intent is 0-based, in which case it should say "or if it has rank less than or equal to `I`".

Sanity check:

The example says `assert((extent<int[2], 1>::value) == 0);`

Here the rank is 1 and `I` is 1, but the desired result is 0.

[ Post Summit: ]

> Do not use "size" or "value", use "bound". Also, move the cross-reference to 8.3.4 to just after "bound".

> Recommend Tentatively Ready.

Proposed resolution:

In Table 44 of 21.3.5.4 [meta.unary.prop], third row, column "Value", change the cell content:

> Table 44 -- Type property queries
> Template | Value
> --- | ---
> `template <class T, unsigned I = 0> struct extent;` | If `T` is not an array type (8.3.4), or if it has rank less than or equal to `I`, or if `I` is 0 and `T` has type "array of unknown bound of `U`", then 0; otherwise, the size bound (8.3.4) of the `I`'th dimension of `T`, where indexing of `I` is zero-based.

[ Wording supplied by Daniel. ]

### 932(i). `unique_ptr(pointer p)` for pointer deleter types

Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2008-11-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.single.ctor].

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Discussion:

Addresses US 79

20.3.1.3.2 [unique.ptr.single.ctor]/5 no longer requires for `D` not to be a pointer type. I believe this restriction was accidently removed when we relaxed the completeness reuqirements on `T`. The restriction needs to be put back in. Otherwise we have a run time failure that could have been caught at compile time:

>     {
>     unique_ptr<int, void(*)(void*)> p1(malloc(sizeof(int)));  // should not compile
>     }  // p1.~unique_ptr() dereferences a null function pointer
>     unique_ptr<int, void(*)(void*)> p2(malloc(sizeof(int)), free);  // ok

[ Post Summit: ]

> Recommend Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be improved for `enable_if` type constraining, possibly following Robert's formula.

[ 2009-07 Frankfurt: ]

> We need to consider whether some requirements in the Requires paragraphs of [unique.ptr] should instead be Remarks.

> Leave Open. Howard to provide wording, and possibly demonstrate how this can be implemented using enable_if.

[ 2009-07-27 Howard adds: ]

> The two constructors to which this issue applies are not easily constrained with `enable_if` as they are not templated:

> >     unique_ptr();
> >     explicit unique_ptr(pointer p);

> To "SFINAE" these constructors away would take heroic effort such as specializing the entire `unique_ptr` class template on pointer deleter types. There is insufficient motivation for such heroics. Here is the expected and reasonable implementation for these constructors:

> >     unique_ptr()
> >         : ptr_(pointer())
> >         {
> >             static_assert(!is_pointer<deleter_type>::value,
> >                 "unique_ptr constructed with null function pointer deleter");
> >         }
> >     explicit unique_ptr(pointer p)
> >         : ptr_(p)
> >         {
> >             static_assert(!is_pointer<deleter_type>::value,
> >                 "unique_ptr constructed with null function pointer deleter");
> >         }

> I.e. just use `static_assert` to verify that the constructor is not instantiated with a function pointer for a deleter. The compiler will automatically take care of issuing a diagnostic if the deleter is a reference type (uninitialized reference error).

> In keeping with our discussions in Frankfurt, I'm moving this requirement on the implementation from the Requires paragraph to a Remarks paragraph.

[ 2009-08-17 Daniel adds: ]

> It is insufficient to require a diagnostic. This doesn't imply an ill-formed program as of 3.18 [defns.diagnostic] (a typical alternative would be a compiler warning), but exactly that seems to be the intend. I suggest to use the following remark instead:

> > Remarks: The program shall be ill-formed if this constructor is instantiated when `D` is a pointer type or reference type.

> Via the general standard rules of 4.1 [intro.compliance] the "diagnostic required" is implied.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

Change the description of the default constructor in 20.3.1.3.2 [unique.ptr.single.ctor]:

>     unique_ptr();

> > -1- Requires: `D` shall be default constructible, and that construction shall not throw an exception. `D` shall not be a reference type or pointer type (diagnostic required).

> > ...

> > Remarks: The program shall be ill-formed if this constructor is instantiated when `D` is a pointer type or reference type.

Add after 20.3.1.3.2 [unique.ptr.single.ctor]/8:

>     unique_ptr(pointer p);

> > ...

> > Remarks: The program shall be ill-formed if this constructor is instantiated when `D` is a pointer type or reference type.

### 934(i). `duration` is missing `operator%`

Section: 30.5 [time.duration] Status: C++11 Submitter: Terry Golubiewski Opened: 2008-11-30 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.duration].

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Discussion:

Addresses US 81

`duration` is missing `operator%`. This operator is convenient for computing where in a time frame a given `duration` lies. A motivating example is converting a `duration` into a "broken-down" time duration such as hours::minutes::seconds:

>     class ClockTime
>     {
>         typedef std::chrono::hours hours;
>         typedef std::chrono::minutes minutes;
>         typedef std::chrono::seconds seconds;
>     public:
>         hours hours_;
>         minutes minutes_;
>         seconds seconds_;

>         template <class Rep, class Period>
>           explicit ClockTime(const std::chrono::duration<Rep, Period>& d)
>             : hours_  (std::chrono::duration_cast<hours>  (d)),
>               minutes_(std::chrono::duration_cast<minutes>(d % hours(1))),
>               seconds_(std::chrono::duration_cast<seconds>(d % minutes(1)))
>               {}
>     };

[ Summit: ]

> Agree except that there is a typo in the proposed resolution. The member operators should be `operator%=`.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.

[ 2009-07 Frankfurt: ]

> Howard to open a separate issue (1177(i)) to handle the removal of member functions from overload sets, provide wording, and possibly demonstrate how this can be implemented using enable_if (see 947(i)).

> Move to Ready.

Proposed resolution:

Add to the synopsis in 30 [time]:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator%(const duration<Rep1, Period>& d, const Rep2& s);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>       typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
>       operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

Add to the synopsis of `duration` in 30.5 [time.duration]:

>     template <class Rep, class Period = ratio<1>>
>     class duration {
>     public:
>       ...
>       duration& operator%=(const rep& rhs);
>       duration& operator%=(const duration& d);
>       ...
>     };

Add to 30.5.4 [time.duration.arithmetic]:

>     duration& operator%=(const rep& rhs);

> > Effects: `rep_ %= rhs`.

> > Returns: `*this`.

>     duration& operator%=(const duration& d);

> > Effects: `rep_ %= d.count()`.

> > Returns: `*this`.

Add to 30.5.6 [time.duration.nonmember]:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator%(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be an instantiation of `duration`. Diagnostic required.

> > Returns: `duration<CR, Period>(d) %= s`.

>     template <class Rep1, class Period1, class Rep2, class Period2>
>       typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
>       operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

> > Returns: `common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type(lhs) %= rhs`.

### 936(i). Mutex type overspecified

Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: Pete Becker Opened: 2008-12-05 Last modified: 2026-04-13

Priority: 4

View all other issues in [thread.mutex.requirements].

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Duplicate of: 961

Discussion:

32.6.4 [thread.mutex.requirements] describes the requirements for a type to be a "Mutex type". A Mutex type can be used as the template argument for the `Lock` type that's passed to `condition_variable_any::wait` (although `Lock` seems like the wrong name here, since `Lock` is given a different formal meaning in 32.6.5 [thread.lock]) and, although the WD doesn't quite say so, as the template argument for `lock_guard` and `unique_lock`.

The requirements for a Mutex type include:

- `m.lock()` shall be well-formed and have [described] semantics, including a return type of `void`.
- `m.try_lock()` shall be well-formed and have [described] semantics, including a return type of `bool`.
- `m.unlock()` shall be well-formed and have [described] semantics, including a return type of `void`.

Also, a Mutex type "shall not be copyable nor movable".

The latter requirement seems completely irrelevant, and the three requirements on return types are tighter than they need to be. For example, there's no reason that `lock_guard` can't be instantiated with a type that's copyable. The rule is, in fact, that `lock_guard`, etc. won't try to copy objects of that type. That's a constraint on locks, not on mutexes. Similarly, the requirements for `void` return types are unnecessary; the rule is, in fact, that `lock_guard`, etc. won't use any returned value. And with the return type of `bool`, the requirement should be that the return type is convertible to `bool`.

[ Summit: ]

> Move to open. Related to conceptualization and should probably be tackled as part of that.

> - The intention is not only to place a constraint on what types such as `lock_guard` may do with mutex types, but on what any code, including user code, may do with mutex types. Thus the constraints as they are apply to the mutex types themselves, not the current users of mutex types in the standard.
> - This is a low priority issue; the wording as it is may be overly restrictive but this may not be a real issue.

[ Post Summit Anthony adds: ]

> Section 32.6.4 [thread.mutex.requirements] conflates the requirements on a generic Mutex type (including user-supplied mutexes) with the requirements placed on the standard-supplied mutex types in an attempt to group everything together and save space.

> When applying concepts to chapter 30, I suggest that the concepts `Lockable` and `TimedLockable` embody the requirements for *use* of a mutex type as required by `unique_lock/lock_guard/condition_variable_any`. These should be relaxed as Pete describes in the issue. The existing words in 32.6.4 [thread.mutex.requirements] are requirements on all of `std::mutex`, `std::timed_mutex`, `std::recursive_mutex` and `std::recursive_timed_mutex`, and should be rephrased as such.

[2017-03-01, Kona]

SG1: Agreement that we need a paper.

[2025-10-21; Priority set to 4 based on age of issue and lack of activity.]

[2026-04-10 Resolved by the adoption of N3197 at the November 2010 plenary. Status changed: LEWG → Resolved.]

Proposed resolution:

### 938(i). `default_delete<T[]>::operator()` should only accept `T*`

Section: 20.3.1.2.3 [unique.ptr.dltr.dflt1] Status: C++11 Submitter: Howard Hinnant Opened: 2008-12-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Consider:

>     derived* p = new derived[3];
>     std::default_delete<base[]> d;
>     d(p);  // should fail

Currently the marked line is a run time failure. We can make it a compile time failure by "poisoning" `op(U*)`.

[ Post Summit: ]

> Recommend Review.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 20.3.1.2.3 [unique.ptr.dltr.dflt1]:

>     namespace std {
>       template <class T> struct default_delete<T[]> {
>         void operator()(T*) const;
>       template <class U> void operator()(U*) const = delete;
>     };
>     }

### 939(i). Problem with `std::identity` and reference-to-temporaries

Section: 22.2.4 [forward] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-12-11 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [forward].

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Discussion:

`std::identity` takes an argument of type `T const &` and returns a result of `T const &`.

Unfortunately, this signature will accept a value of type other than `T` that is convertible-to-`T`, and then return a reference to the dead temporary. The constraint in the concepts version simply protects against returning reference-to-`void`.

Solutions:

> i/ Return-by-value, potentially slicing bases and rejecting non-copyable types

> ii/ Provide an additional overload:

> >     template< typename T >
> >     template operator( U & ) = delete;

> This seems closer on intent, but moves beyond the original motivation for the operator, which is compatibility with existing (non-standard) implementations.

> iii/ Remove the `operator()` overload. This restores the original definition of the `identity`, although now effectively a type_trait rather than part of the perfect forwarding protocol.

> iv/ Remove `std::identity` completely; its original reason to exist is replaced with the `IdentityOf` concept.

My own preference is somewhere between (ii) and (iii) - although I stumbled over the issue with a specific application hoping for resolution (i)!

[ Batavia (2009-05): ]

> We dislike options i and iii, and option ii seems like overkill. If we remove it (option iv), implementers can still provide it under a different name.

> Move to Open pending wording (from Alisdair) for option iv.

[ 2009-05-23 Alisdair provided wording for option iv. ]

[ 2009-07-20 Alisdair adds: ]

> I'm not sure why this issue was not discussed at Frankfurt (or I missed the discussion) but the rationale is now fundamentally flawed. With the removal of concepts, `std::identity` again becomes an important library type so we cannot simply remove it.

> At that point, we need to pick one of the other suggested resolutions, but have no guidance at the moment.

[ 2009-07-20 Howard adds: ]

> I believe the rationale for not addressing this issue in Frankfurt was that it did not address a national body comment.

> I also believe that removal of `identity` is still a practical option as my latest reformulation of `forward`, which is due to comments suggested at Summit, no longer uses `identity`. :-)

> >     template <class T, class U,
> >         class = typename enable_if
> >                 <
> >                     !is_lvalue_reference<T>::value ||
> >                      is_lvalue_reference<T>::value &&
> >                      is_lvalue_reference<U>::value
> >                 >::type,
> >         class = typename enable_if
> >                 <
> >                     is_same<typename remove_all<T>::type,
> >                             typename remove_all<U>::type>::value
> >                 >::type>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >         return static_cast<T&&>(t);

> >     }

> > [ The above code assumes acceptance of 1120(i) for the definition of `remove_all`. This is just to make the syntax a little more palatable. Without this trait the above is still very implementable. ]

> Paper with rationale is on the way ... really, I promise this time! ;-)

[ 2009-07-30 Daniel adds: See 823(i) for an alternative resolution. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready. Howard will update proposed wording to reflect current draft.

Proposed resolution:

Strike from 22.2 [utility]:

>     template <class T> struct identity;

Remove from 22.2.4 [forward]:

>     template <class T> struct identity {
>       typedef T type;

>       const T& operator()(const T& x) const;
>     };

>     const T& operator()(const T& x) const;

> > -2- Returns: `x`

### 940(i). `std::distance`

Section: 24.4.3 [iterator.operations] Status: Resolved Submitter: Thomas Opened: 2008-12-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 270

Regarding the `std::distance` - function, 24.4.3 [iterator.operations] p.4 says:

> Returns the number of increments or decrements needed to get from first to last.

This sentence is completely silent about the sign of the return value. 24.4.3 [iterator.operations] p.1 gives more information about the underlying operations, but again no inferences about the sign can be made. Strictly speaking, that is taking that sentence literally, I think this sentence even implies a positive return value in all cases, as the number of increments or decrements is clearly a ratio scale variable, with a natural zero bound.

Practically speaking, my implementations did what common sense and knowledge based on pointer arithmetic forecasts, namely a positive sign for increments (that is, going from `first` to `last` by `operator++`), and a negative sign for decrements (going from `first` to `last` by `operator--`).

Here are my two questions:

First, is that paragraph supposed to be interpreted in the way what I called 'common sense', that is negative sign for decrements ? I am fairly sure that's the supposed behavior, but a double-check here in this group can't hurt.

Second, is the present wording (2003 standard version - no idea about the draft for the upcoming standard) worth an edit to make it a bit more sensible, to mention the sign of the return value explicitly ?

[ Daniel adds: ]

> My first thought was that resolution 204(i) would already cover the issue report, but it seems that current normative wording is in contradiction to that resolution:

> Referring to N2798, 24.4.3 [iterator.operations]/ p.4 says:

> > Effects: Returns the number of increments or decrements needed to get from `first` to `last`.

> IMO the part " or decrements" is in contradiction to p. 5 which says

> > Requires: `last` shall be reachable from `first`.

> because "reachable" is defined in 24.3.4 [iterator.concepts]/7 as

> > An iterator `j` is called reachable from an iterator `i` if and only if there is a finite sequence of applications of the expression `++i` that makes `i == j`.[..]

> Here is wording that would be consistent with this definition of "reachable":

> Change 24.4.3 [iterator.operations] p4 as follows:

> > Effects: Returns the number of increments or decrements needed to get from `first` to `last`.

Thomas adds more discussion and an alternative view point here.

[ Summit: ]

> The proposed wording below was verbally agreed to. Howard provided.

[ Batavia (2009-05): ]

> Pete reports that a recent similar change has been made for the `advance()` function.

> We agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-07 Frankfurt: ]

> Leave Open pending arrival of a post-Concepts WD.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready, replacing the Effects clause in the proposed wording with "If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise returns the number of increments needed to get from first to list.".

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added below.

Rationale:

Solved by N3066.

Proposed resolution:

1. Change 24.3.5.7 [random.access.iterators], Table 105 as indicated [This change is not essential but it simplifies the specification] for the row with expression "`b - a`" and the column Operational semantics:

   >     (a < b) ? distance(a,b)
   >     : -distance(b,a)

2. Change 24.4.3 [iterator.operations]/4+5 as indicated:

   >     template<class InputIterator>
   >       typename iterator_traits<InputIterator>::difference_type
   >         distance(InputIterator first, InputIterator last);

   > > 4 Effects: If `InputIterator` meets the requirements of random access iterator then returns `(last - first)`, otherwise Rreturns the number of increments or decrements needed to get from `first` to `last`.

   > > 5 Requires: If `InputIterator` meets the requirements of random access iterator then `last` shall be reachable from `first` or `first` shall be reachable from `last`, otherwise `last` shall be reachable from `first`.

### 943(i). `ssize_t` undefined

Section: 99 [atomics.types.address] Status: C++11 Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There is a row in "Table 122 - Atomics for standard typedef types" in 99 [atomics.types.integral] with `atomic_ssize_t` and `ssize_t`. Unless, I'm missing something `ssize_t` is not defined by the standard.

[ Summit: ]

> Move to review. Proposed resolution: Remove the typedef. Note: `ssize_t` is a POSIX type.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Remove the row containing `ssize_t` from Table 119 "Atomics for standard typedef types" in 99 [atomics.types.address].

### 944(i). `atomic<bool>` derive from `atomic_bool`?

Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.types.generic].

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Discussion:

I think it's fairly obvious that `atomic<bool>` is supposed to be derived from `atomic_bool` (and otherwise follow the `atomic<integral>` interface), though I think the current wording doesn't support this. I raised this point along with `atomic<floating-point>` privately with Herb and I seem to recall it came up in the resulting discussion on this list. However, I don't see anything on the current libs issue list mentioning this problem.

32.5.8 [atomics.types.generic]/3 reads

> There are full specializations over the integral types on the atomic class template. For each integral type integral in the second column of table 121 or table 122, the specialization `atomic<integral>` shall be publicly derived from the corresponding atomic integral type in the first column of the table. These specializations shall have trivial default constructors and trivial destructors.

Table 121 does not include (`atomic_bool`, `bool`), so that this should probably be mentioned explicitly in the quoted paragraph.

[ Summit: ]

> Move to open. Lawrence will draft a proposed resolution. Also, ask Howard to fix the title.

[ Post Summit Anthony provided proposed wording. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2992.

Proposed resolution:

Replace paragraph 3 in 32.5.8 [atomics.types.generic] with

> -3- There are full specializations over the integral types on the `atomic` class template. For each integral type `integral` in the second column of table 121 or table 122, the specialization `atomic<integral>` shall be publicly derived from the corresponding atomic integral type in the first column of the table. In addition, the specialization `atomic<bool>` shall be publicly derived from `atomic_bool`. These specializations shall have trivial default constructors and trivial destructors.

### 947(i). `duration` arithmetic: contradictory requirements

Section: 30.5.6 [time.duration.nonmember] Status: Resolved Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.duration.nonmember].

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Discussion:

In 30.5.6 [time.duration.nonmember], paragraph 8 says that calling `dur / rep` when `rep` is an instantiation of `duration` requires a diagnostic. That's followed by an `operator/` that takes two durations. So `dur1 / dur2` is legal under the second version, but requires a diagnostic under the first.

[ Howard adds: ]

> Please see the thread starting with c++std-lib-22980 for more information.

[ Batavia (2009-05): ]

> Move to Open, pending proposed wording (and preferably an implementation).

[ 2009-07-27 Howard adds: ]

> I've addressed this issue under the proposed wording for 1177(i) which cleans up several places under 30.5 [time.duration] which used the phrase "diagnostic required".

> For clarity's sake, here is an example implementation of the constrained `operator/`:

> >     template <class _Duration, class _Rep, bool = __is_duration<_Rep>::value>
> >     struct __duration_divide_result
> >     {
> >     };

> >     template <class _Duration, class _Rep2,
> >         bool = is_convertible<_Rep2,
> >                               typename common_type<typename _Duration::rep, _Rep2>::type>::value>
> >     struct __duration_divide_imp
> >     {
> >     };

> >     template <class _Rep1, class _Period, class _Rep2>
> >     struct __duration_divide_imp<duration<_Rep1, _Period>, _Rep2, true>
> >     {
> >         typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period> type;
> >     };

> >     template <class _Rep1, class _Period, class _Rep2>
> >     struct __duration_divide_result<duration<_Rep1, _Period>, _Rep2, false>
> >         : __duration_divide_imp<duration<_Rep1, _Period>, _Rep2>
> >     {
> >     };

> >     template <class _Rep1, class _Period, class _Rep2>
> >     inline
> >     typename __duration_divide_result<duration<_Rep1, _Period>, _Rep2>::type
> >     operator/(const duration<_Rep1, _Period>& __d, const _Rep2& __s)
> >     {
> >         typedef typename common_type<_Rep1, _Rep2>::type _Cr;
> >         duration<_Cr, _Period> __r = __d;
> >         __r /= static_cast<_Cr>(__s);
> >         return __r;
> >     }

> `__duration_divide_result` is basically a custom-built `enable_if` that will contain `type` only if `Rep2` is not a `duration` and if `Rep2` is implicitly convertible to `common_type<typename Duration::rep, Rep2>::type`. `__is_duration` is simply a private trait that answers `false`, but is specialized for `duration` to answer `true`.

> The constrained `operator%` works identically.

[ 2009-10 Santa Cruz: ]

> Mark NAD EditorialResolved, fixed by 1177(i).

Proposed resolution:

### 948(i). `ratio` arithmetic tweak

Section: 21.5.4 [ratio.arithmetic] Status: C++11 Submitter: Howard Hinnant Opened: 2008-12-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ratio.arithmetic].

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Discussion:

N2800, 21.5.4 [ratio.arithmetic] lacks a paragraph from the proposal N2661:

> ratio arithmetic [ratio.arithmetic]

> ... If the implementation is unable to form the indicated `ratio` due to overflow, a diagnostic shall be issued.

The lack of a diagnostic on compile-time overflow is a significant lack of functionality. This paragraph could be put back into the WP simply editorially. However in forming this issue I realized that we can do better than that. This paragraph should also allow alternative formulations which go to extra lengths to avoid overflow when possible. I.e. we should not mandate overflow when the implementation can avoid it.

For example:

>     template <class R1, class R2> struct ratio_multiply {
>       typedef see below} type;

> > The nested typedef type shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::num` and `T2` has the value `R1::den * R2::den`.

Consider the case where `intmax_t` is a 64 bit 2's complement signed integer, and we have:

>     typedef std::ratio<0x7FFFFFFFFFFFFFFF, 0x7FFFFFFFFFFFFFF0> R1;
>     typedef std::ratio<8, 7> R2;
>     typedef std::ratio_multiply<R1, R2>::type RT;

According to the present formulation the implementaiton will multiply `0x7FFFFFFFFFFFFFFF * 8` which will result in an overflow and subsequently require a diagnostic.

However if the implementation is first allowed to divde `0x7FFFFFFFFFFFFFFF` by `7` obtaining `0x1249249249249249 / 1` and divide `8` by `0x7FFFFFFFFFFFFFF0` obtaining `1 / 0x0FFFFFFFFFFFFFFE`, then the exact result can then be computed without overflow:

>     [0x7FFFFFFFFFFFFFFF/0x7FFFFFFFFFFFFFF0] * [8/7] = [0x1249249249249249/0x0FFFFFFFFFFFFFFE]

Example implmentation which accomplishes this:

>     template <class R1, class R2>
>     struct ratio_multiply
>     {
>     private:
>         typedef ratio<R1::num, R2::den> _R3;
>         typedef ratio<R2::num, R1::den> _R4;
>     public:
>         typedef ratio<__ll_mul<_R3::num, _R4::num>::value,
>                       __ll_mul<_R3::den, _R4::den>::value> type;
>     };

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

Add a paragraph prior to p1 in 21.5.4 [ratio.arithmetic]:

> Implementations may use other algorithms to compute the indicated ratios to avoid overflow. If overflow occurs, a diagnostic shall be issued.

### 949(i). `owner_less`

Section: 20.3.2.4 [util.smartptr.ownerless] Status: C++11 Submitter: Thomas Plum Opened: 2008-12-30 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

20.3.2.4 [util.smartptr.ownerless] (class template `owner_less`) says that `operator()(x,y)` shall return `x.before(y)`.

However, `shared_ptr` and `weak_ptr` have an `owner_before()` but not a `before()`, and there's no base class to provide a missing `before()`.

Being that the class is named `owner_less` , I'm guessing that "`before()`" should be "`owner_before()`", right?

[ Herve adds: ]

> Agreed with the typo, it should be "shall return `x.owner_before(y)`".

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

Change 20.3.2.4 [util.smartptr.ownerless] p2:

> -2- `operator()(x,y)` shall return `x.owner_before(y)`. [Note: ...

### 950(i). `unique_ptr` converting ctor shouldn't accept array form

Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.single.ctor].

View all issues with Resolved status.

Discussion:

`unique_ptr`'s of array type should not convert to `unique_ptr`'s which do not have an array type.

>     struct Deleter
>     {
>        void operator()(void*) {}
>     };

>     int main()
>     {
>        unique_ptr<int[], Deleter> s;
>        unique_ptr<int, Deleter> s2(std::move(s));  // should not compile
>     }

[ Post Summit: ]

> Walter: Does the "diagnostic required" apply to both arms of the "and"?

> Tom Plum: suggest to break into several sentences

> Walter: suggest "comma" before the "and" in both places

> Recommend Review.

[ Batavia (2009-05): ]

> The post-Summit comments have been applied to the proposed resolution. We now agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be improved for `enable_if` type constraining, possibly following Robert's formula.

[ 2009-08-01 Howard updates wording and sets to Review. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010-02-27 Pete Opens: ]

> The proposed replacement text doesn't make sense.

> > If `D` is a reference type, then `E` shall be the same type as `D`, else this constructor shall not participate in overload resolution.

> This imposes two requirements. 1. If `D` is a reference type, `E` has to be `D`. 2. If `D` is not a reference type, the constructor shall not participate in overload resolution. If the latter apples, the language in the preceding paragraph that this constructor shall not throw an exception if `D` is not a reference type is superfluous. I suspect that's not the intention, but I can't parse this text any other way.

> > `U` shall not be an array type, else this constructor shall not participate in overload resolution.

> I don't know what this means.

[ 2010-02-27 Peter adds: ]

> I think that the intent is (proposed text):

> > Remarks: this constructor shall only participate in overload resolution if:

> > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer`,
> > - `U` is not an array type, and
> > - if `D` is a reference type, `E` is the same type as `D`.

[ 2010-02-28 Howard adds: ]

> I like Peter's proposal. Here is a tweak of it made after looking at my implementation. I believe this fixes a further defect not addressed by the current proposed wording:

> > Remarks: this constructor shall only participate in overload resolution if:

> > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer`, and
> > - `U` is not an array type, and
> > - if `D` is a reference type, `E` is the same type as `D`, else `E` shall be implicitly convertible to `D`.

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

Solved by N3073.

Proposed resolution:

Change 20.3.1.3.2 [unique.ptr.single.ctor]:

>     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

> > -20- Requires: If `D` is not a reference type, construction of the deleter `D` from an rvalue of type `E` shall be well formed and shall not throw an exception. If `D` is a reference type, then `E` shall be the same type as `D` (diagnostic required). `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U` are complete types. — end note]

> > Remarks: If `D` is a reference type, then `E` shall be the same type as `D`, else this constructor shall not participate in overload resolution. `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`, else this constructor shall not participate in overload resolution. `U` shall not be an array type, else this constructor shall not participate in overload resolution. [Note: These requirements imply that `T` and `U` are complete types. — end note]

Change 20.3.1.3.4 [unique.ptr.single.asgn]:

>     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

> > -6- Requires: Assignment of the deleter `D` from an rvalue `D` shall not throw an exception. `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U` are complete types. — end note]

> > Remarks: `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`, else this operator shall not participate in overload resolution. `U` shall not be an array type, else this operator shall not participate in overload resolution. [Note: These requirements imply that `T` and `U` are complete types. — end note]

### 951(i). Various threading bugs #1

Section: 30.4.1 [time.traits.is.fp] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2021-06-06

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Related to 953(i).

[time.traits.is_fp] says that the type `Rep` "is assumed to be ... a class emulating an integral type." What are the requirements for such a type?

[ 2009-05-10 Howard adds: ]

> `IntegralLike`.

[ Batavia (2009-05): ]

> As with issue 953(i), we recommend this issue be addressed in the context of providing concepts for the entire `thread` header.

> We look forward to proposed wording.

> Move to Open.

[ 2009-08-01 Howard adds: ]

> I have surveyed all clauses of [time.traits.duration_values], 30.4.3 [time.traits.specializations] and 30.5 [time.duration]. I can not find any clause which involves the use of a `duration::rep` type where the requirements on the `rep` type are not clearly spelled out. These requirements were carefully crafted to allow any arithmetic type, or any user-defined type emulating an arithmetic type.

> Indeed, `treat_as_floating_point` becomes completely superfluous if `duration::rep` can never be a class type.

> There will be some `Rep` types which will not meet the requirements of every `duration` operation. This is no different than the fact that `vector<T>` can easily be used for types `T` which are not `DefaultConstructible`, even though some members of `vector<T>` require `T` to be `DefaultConstructible`. This is why the requirements on `Rep` are specified for each operation individually.

> In [time.traits.is_fp] p1:

> >     template <class Rep> struct treat_as_floating_point
> >       : is_floating_point<Rep> { };

> > > The `duration` template uses the `treat_as_floating_point` trait to help determine if a `duration` object can be converted to another `duration` with a different tick period. If `treat_as_floating_point<Rep>::value` is `true`, then `Rep` is a floating-point type and implicit conversions are allowed among `duration`s. Otherwise, the implicit convertibility depends on the tick periods of the `duration`s. If `Rep` is a class type which emulates a floating-point type, the author of `Rep` can specialize `treat_as_floating_point` so that `duration` will treat this `Rep` as if it were a floating-point type. Otherwise `Rep` is assumed to be an integral type or a class emulating an integral type.

> The phrases "a class type which emulates a floating-point type" and "a class emulating an integral type" are clarifying phrases which refer to the summation of all the requirements on the `Rep` type specified in detail elsewhere (and should not be repeated here).

> This specification has been implemented, now multiple times, and the experience has been favorable. The current specification clearly specifies the requirements at each point of use (though I'd be happy to fix any place I may have missed, but none has been pointed out).

> I am amenable to improved wording of this paragraph (and any others), but do not have any suggestions for improved wording at this time. I am strongly opposed to changes which would significantly alter the semantics of the specification under 30 [time] without firmly grounded and documented rationale, example implementation, testing, and user experience which relates a positive experience.

> I recommend NAD unless someone wants to produce some clarifying wording.

[ 2009-10 Santa Cruz: ]

> Stefanus to provide wording to turn this into a note.

[ 2010-02-11 Stefanus provided wording. ]

[ 2010 Rapperswil: ]

> Move to Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change [time.traits.is_fp]/1:

> 1 The `duration` template uses the `treat_as_floating_point` trait to help determine if a `duration` object can be converted to another `duration` with a different tick period. If `treat_as_floating_point<Rep>::value` is `true`, then `Rep` is a floating-point type and implicit conversions are allowed among `duration`s. Otherwise, the implicit convertibility depends on the tick periods of the `duration`s. If `Rep` is a class type which emulates a floating-point type, the author of `Rep` can specialize `treat_as_floating_point` so that duration will treat this `Rep` as if it were a floating-point type. Otherwise `Rep` is assumed to be an integral type or a class emulating an integral type. [Note: The intention of this trait is to indicate whether a given class behaves like a floating point type, and thus allows division of one value by another with acceptable loss of precision. If `treat_as_floating_point<Rep>::value` is `false`, `Rep` will be treated as if it behaved like an integral type for the purpose of these conversions. — end note]

### 953(i). Various threading bugs #3

Section: 30.3 [time.clock.req] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.clock.req].

View all issues with Resolved status.

Discussion:

Related to 951(i).

30.3 [time.clock.req] says that a clock's `rep` member is "an arithmetic type or a class emulating an arithmetic type." What are the requirements for such a type?

[ 2009-05-10 Howard adds: ]

> This wording was aimed directly at the `ArithmeticLike` concept.

[ Batavia (2009-05): ]

> We recommend this issue be addressed in the context of providing concepts for the entire `thread` header.

> May resolve for now by specifying arithmetic types, and in future change to `ArithmeticLike`. However, Alisdair believes this is not feasible.

> Bill disagrees.

> We look forward to proposed wording. Move to Open.

[ 2009-08-01 Howard adds: ]

> See commented dated 2009-08-01 in 951(i).

[ 2009-10 Santa Cruz: ]

> Stefanus to provide wording to turn this into a note.

[ 2010-02-11 Stephanus provided wording for 951(i) which addresses this issue as well. ]

[ 2010 Rapperswil: ]

> Move to NAD EditorialResolved, resolved by 951(i).

Proposed resolution:

### 954(i). Various threading bugs #4

Section: 30.3 [time.clock.req] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.clock.req].

View all issues with C++11 status.

Discussion:

Table 55 — Clock Requirements (in 30.3 [time.clock.req])

1. the requirements for `C1::time_point` require `C1` and `C2` to "refer to the same epoch", but "epoch" is not defined.
2. "Different clocks may share a `time_point` definition if it is valid to compare their `time_point`s by comparing their respective `duration`s." What does "valid" mean here? And, since `C1::rep` is "THE representation type of the native `duration` and `time_point`" (emphasis added), there doesn't seem to be much room for some other representation.
3. `C1::is_monotonic` has type "`const bool`". The "`const`" should be removed.
4. `C1::period` has type `ratio`. `ratio` isn't a type, it's a template. What is the required type?

[ 2009-05-10 Howard adds: ]

1. "epoch" is purposefully not defined beyond the common English definition. The C standard also chose not to define epoch, though POSIX did. I believe it is a strength of the C standard that epoch is not defined. When it is known that two `time_point`s refer to the same epoch, then a definition of the epoch is not needed to compare the two `time_point`s, or subtract them.

   A `time_point` and a `Clock` implicitly refer to an (unspecified) epoch. The `time_point` represents an offset (`duration`) from an epoch.

2. The sentence:

   > Different clocks may share a `time_point` definition if it is valid to compare their `time_point`s by comparing their respective `duration`s.

   is redundant and could be removed. I believe the sentence which follows the above:

   > `C1` and `C2` shall refer to the same epoch.

   is sufficient. If two clocks share the same epoch, then by definition, comparing their `time_point`s is valid.

3. `is_monotonic` is meant to never change (be `const`). It is also desired that this value be usable in compile-time computation and branching.

4. This should probably instead be worded:

   > An instantiation of `ratio`.

[ Batavia (2009-05): ]

> Re (a): It is not clear to us whether "epoch" is a term of art.

> Re (b), (c), and (d): We agree with Howard's comments, and would consider adding to (c) a `static constexpr` requirement.

> Move to Open pending proposed wording.

[ 2009-05-25 Daniel adds: ]

> In regards to (d) I suggest to say "a specialization of ratio" instead of "An instantiation of ratio". This seems to be the better matching standard core language term for this kind of entity.

[ 2009-05-25 Ganesh adds: ]

> Regarding (a), I found this paper on the ISO website using the term "epoch" consistently with the current wording:

> http://standards.iso.org/ittf/PubliclyAvailableStandards/C030811e_FILES/MAIN_C030811e/text/ISOIEC_18026E_TEMPORAL_CS.HTM

> which is part of ISO/IEC 18026 "Information technology -- Spatial Reference Model (SRM)".

[ 2009-08-01 Howard: Moved to Reivew as the wording requested in Batavia has been provided. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

1. Change 30.3 [time.clock.req] p1:

   > -1- A clock is a bundle consisting of a native `duration`, a native `time_point`, and a function `now()` to get the current `time_point`. The origin of the clock's `time_point` is referred to as the clock's epoch as defined in section 6.3 of ISO/IEC 18026. A clock shall meet the requirements in Table 45.

2. Remove the sentence from the `time_point` row of the table "Clock Requirements":

   Clock requirements
   `C1::time_point` | `chrono::time_point<C1>` or `chrono::time_point<C2, C1::duration>` | The native `time_point` type of the clock. Different clocks may share a `time_point` definition if it is valid to compare their `time_point`s by comparing their respective `duration`s. `C1` and `C2` shall refer to the same epoch.

3. Change the row starting with `C1::period` of the table "Clock Requirements":

   Clock requirements
   `C1::period` | a specialization of `ratio` | The tick period of the clock in seconds.

### 956(i). Various threading bugs #6

Section: 30.3 [time.clock.req] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.clock.req].

View all issues with C++11 status.

Discussion:

30.3 [time.clock.req] uses the word "native" in several places, but doesn't define it. What is a "native `duration`"?

[ 2009-05-10 Howard adds: ]

> The standard uses "native" in several places without defining it (e.g. 5.13.3 [lex.ccon]). It is meant to mean "that which is defined by the facility", or something along those lines. In this case it refers to the nested `time_point` and `duration` types of the clock. Better wording is welcome.

[ Batavia (2009-05): ]

> Move to Open pending proposed wording from Pete.

[ 2009-10-23 Pete provides wording: ]

[ 2009-11-18 Daniel adds: ]

> I see that 32.6.4.3 [thread.timedmutex.requirements]/3 says:

> > Precondition: If the tick `period` of `rel_time` is not exactly convertible to the native tick `period`, the `duration` shall be rounded up to the nearest native tick `period`.

> I would prefer to see that adapted as well. Following the same style as the proposed resolution I come up with

> > Precondition: If the tick `period` of `rel_time` is not exactly convertible to the native tick `period` of the execution environment, the `duration` shall be rounded up to the nearest native tick `period` of the execution environment.

[ 2010-03-28 Daniel synced wording with N3092 ]

[ Post-Rapperswil, Howard provides wording: ]

> Moved to Tentatively Ready with revised wording from Howard Hinnant after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 30.3 [time.clock.req]:

> 1 A clock is a bundle consisting of a native `duration`, a native `time_point`, and a function `now()` to get the current `time_point`. The origin of the clock's `time_point` is referred to as the clock's epoch. A clock shall meet the requirements in Table 56.

> 2 ...

> Table 56 — Clock requirements
> Expression | Return type | Operational semantics
> --- | --- | ---
> `C1::rep` | An arithmetic type or a class emulating an arithmetic type | The representation type of the native `C1::duration`. and `time_point`.
> `C1::period` | ... | ...
> `C1::duration` | `chrono::duration<C1::rep, C1::period>` | The native `duration` type of the clock.
> `C1::time_point` | `chrono::time_point<C1>` or `chrono::time_point<C2, C1::duration>` | The native `time_point` type of the clock. `C1` and `C2` shall refer to the same epoch.
> ...

### 957(i). Various threading bugs #7

Section: 30.7.2 [time.clock.system] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.clock.system].

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Discussion:

30.7.2 [time.clock.system]: `to_time_t` is overspecified. It requires truncation, but should allow rounding. For example, suppose a system has a clock that gives times in milliseconds, but `time()` rounds those times to the nearest second. Then `system_clock` can't use any resolution finer than one second, because if it did, truncating times between half a second and a full second would produce the wrong `time_t` value.

[ Post Summit Anthony Williams provided proposed wording. ]

[ Batavia (2009-05): ]

> Move to Review pending input from Howard. and other stakeholders.

[ 2009-05-23 Howard adds: ]

> I am in favor of the wording provided by Anthony.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

In 30.7.2 [time.clock.system] replace paragraphs 3 and 4 with:

>     time_t to_time_t(const time_point& t);

> > -3- Returns: A `time_t` object that represents the same point in time as `t` when both values are truncated restricted to the coarser of the precisions of `time_t` and `time_point`. It is implementation defined whether values are rounded or truncated to the required precision.

>     time_point from_time_t(time_t t);

> > -4- Returns: A `time_point` object that represents the same point in time as `t` when both values are truncated restricted to the coarser of the precisions of `time_t` and `time_point`. It is implementation defined whether values are rounded or truncated to the required precision.

### 958(i). Various threading bugs #8

Section: 32.7.4 [thread.condition.condvar] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition.condvar].

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Discussion:

32.7.4 [thread.condition.condvar]: the specification for `wait_for` with no predicate has an effects clause that says it calls `wait_until`, and a returns clause that sets out in words how to determine the return value. Is this description of the return value subtly different from the description of the value returned by `wait_until`? Or should the effects clause and the returns clause be merged?

[ Summit: ]

> Move to open. Associate with LWG 859(i) and any other monotonic-clock related issues.

[ 2009-08-01 Howard adds: ]

> I believe that 859(i) (currently Ready) addresses this issue, and that this issue should be marked NAD, solved by 859(i) (assuming it moves to WP).

[ 2009-10 Santa Cruz: ]

> Mark as NAD EditorialResolved, addressed by resolution of Issue 859(i).

Proposed resolution:

### 960(i). Various threading bugs #10

Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.mutex.requirements].

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Discussion:

32.6.4 [thread.mutex.requirements]: paragraph 4 is entitled "Error conditions", but according to 16.3.2.4 [structure.specifications], "Error conditions:" specifies "the error conditions for error codes reported by the function." It's not clear what this should mean when there is no function in sight.

[ Summit: ]

> Move to open.

[ Beman provided proposed wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready. Fix the proposed wording with "functions of type Mutex" -> "functions of Mutex type"

Proposed resolution:

Change 32.6.4 [thread.mutex.requirements] Mutex requirements, paragraph 4 as indicated:

> -4- Error conditions: The error conditions for error codes, if any, reported by member functions of Mutex type shall be:

> - `not_enough_memory` — if there is not enough memory to construct the mutex object.
> - `resource_unavailable_try_again` — if any native handle type manipulated is not available.
> - `operation_not_permitted` — if the thread does not have the necessary permission to change the state of the mutex object.
> - `device_or_resource_busy` — if any native handle type manipulated is already locked.
> - `invalid_argument` — if any native handle type manipulated as part of mutex construction is incorrect.

### 961(i). Various threading bugs #11

Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2026-04-13

Priority: 4

View all other issues in [thread.mutex.requirements].

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Duplicate of: 936

Discussion:

32.6.4 [thread.mutex.requirements] describes required member functions of mutex types, and requires that they throw exceptions under certain circumstances. This is overspecified. User-defined types can abort on such errors without affecting the operation of templates supplied by standard-library.

[ Summit: ]

> Move to open. Related to conceptualization and should probably be tackled as part of that.

[ 2009-10 Santa Cruz: ]

> Would be OK to leave it as is for time constraints, could loosen later.

> Mark as NAD Future.

[2017-03-01, Kona]

SG1: Agreement that we need a paper.

[2025-10-21; Priority set to 4 based on age of issue and lack of activity.]

[2026-04-10 Resolved by the adoption of N3197 at the November 2010 plenary. Status changed: LEWG → Resolved.]

Proposed resolution:

### 962(i). Various threading bugs #12

Section: 32.6.5.4.3 [thread.lock.unique.locking] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.lock.unique.locking].

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Discussion:

32.6.5.4.3 [thread.lock.unique.locking]: `unique_lock::lock` is required to throw an object of type `std::system_error` "when the postcondition cannot be achieved." The postcondition is `owns == true`, and this is trivial to achieve. Presumably, the requirement is intended to mean something more than that.

[ Summit: ]

> Move to open.

[ Beman has volunteered to provide proposed wording. ]

[ 2009-07-21 Beman added wording to address 32.2.2 [thread.req.exception] in response to the Frankfurt notes in 859(i). ]

[ 2009-09-25 Beman: minor update to wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Change Exceptions 32.2.2 [thread.req.exception] as indicated:

> Some functions described in this Clause are specified to throw exceptions of type `system_error` (19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function from satisfying its postconditions or from returning a meaningful value meeting its specifications. Failure to allocate storage shall be reported as described in 16.4.6.14 [res.on.exception.handling].

Change thread assignment 32.4.3.6 [thread.thread.member], join(), paragraph 8 as indicated:

> Throws: `std::system_error` when the postconditions cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change thread assignment 32.4.3.6 [thread.thread.member], detach(), paragraph 13 as indicated:

> Throws: `std::system_error` when the effects or postconditions cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 11, as indicated:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 3, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 8, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 13, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 18, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 22, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change Function call_once 32.6.7.2 [thread.once.callonce], paragraph 4, as indicated

> Throws: `std::system_error` when the effects cannot be achieved an exception is required (32.2.2 [thread.req.exception]), or any exception thrown by `func`.

Change Class condition_variable 32.7.4 [thread.condition.condvar], paragraph 12, as indicated:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change Class condition_variable 32.7.4 [thread.condition.condvar], paragraph 19, as indicated:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change Class condition_variable_any 32.7.5 [thread.condition.condvarany], paragraph 10, as indicated:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Change Class condition_variable_any 32.7.5 [thread.condition.condvarany], paragraph 16, as indicated:

> Throws: `std::system_error` when the returned value, effects, or postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 32.7.4 [thread.condition.condvar] as indicated:

>     template <class Rep, class Period>
>     bool wait_for(unique_lock<mutex>& lock,
>                   const chrono::duration<Rep, Period>& rel_time);

>     ...

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required ([thread.req.exception]).

Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 32.7.4 [thread.condition.condvar] as indicated:

>     template <class Rep, class Period, class Predicate>
>       bool wait_for(unique_lock<mutex>& lock,
>                     const chrono::duration<Rep, Period>& rel_time,
>                     Predicate pred);

>     ...

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 32.7.5 [thread.condition.condvarany] as indicated:

>     template <class Lock, class Rep, class Period>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

>     ...

> Throws: `std::system_error` when the returned value, effects or postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 32.7.5 [thread.condition.condvarany] as indicated:

>     template <class Lock, class Rep, class Period, class Predicate>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

>     ...

> Throws: `std::system_error` when the returned value, effects or postcondition cannot be achieved an exception is required (32.2.2 [thread.req.exception]).

### 963(i). Various threading bugs #13

Section: 32.4.3.6 [thread.thread.member] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.4.3.6 [thread.thread.member]: `thread::detach` is required to throw an exception if the thread is "not a detachable thread". "Detachable" is never defined.

[ Howard adds: ]

> Due to a mistake on my part, 3 proposed resolutions appeared at approximately the same time. They are all three noted below in the discussion.

[ Summit, proposed resolution: ]

> In 32.4.3.6 [thread.thread.member] change:

> >     void detach();

> > > ...

> > > -14- Error conditions:

> > > - `no_such_process` -- if the thread is not a valid thread.
> > > - `invalid_argument` -- if the thread is not a detachable joinable thread.

[ Post Summit, Jonathan Wakely adds: ]

> A `thread` is detachable if it is joinable. As we've defined joinable, we can just use that.

> This corresponds to the pthreads specification, where pthread_detach fails if the thread is not joinable:

> > EINVAL: The implementation has detected that the value specified by thread does not refer to a joinable thread.

> Jonathan recommends this proposed wording:

> > In 32.4.3.6 [thread.thread.member] change:

> > >     void detach();

> > > > ...

> > > > -14- Error conditions:

> > > > - ...
> > > > - `invalid_argument` -- not a detachable joinable thread.

[ Post Summit, Anthony Williams adds: ]

> This is covered by the precondition that `joinable()` be `true`.

> Anthony recommends this proposed wording:

> > In 32.4.3.6 [thread.thread.member] change:

> > >     void detach();

> > > > ...

> > > > -14- Error conditions:

> > > > - ...
> > > > - `invalid_argument` -- not a detachable thread.

[ 2009-10 Santa Cruz: ]

> Mark as Ready with proposed resolution from Summit.

Proposed resolution:

In 32.4.3.6 [thread.thread.member] change:

>     void detach();

> > ...

> > -14- Error conditions:

> > - `no_such_process` -- if the thread is not a valid thread.
> > - `invalid_argument` -- if the thread is not a detachable joinable thread.

### 964(i). Various threading bugs #14

Section: 32.7.5 [thread.condition.condvarany] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The requirements for the constructor for `condition_variable` has several error conditions, but the requirements for the constructor for `condition_variable_any` has none. Is this difference intentional?

[ Summit: ]

> Move to open, pass to Howard. If this is intentional, a note may be helpful. If the error conditions are to be copied from `condition_variable`, this depends on LWG 965(i).

[ Post Summit Howard adds: ]

> The original intention (N2447) was to let the OS return whatever errors it was going to return, and for those to be translated into exceptions, for both `condition_variable` and `condition_variable_any`. I have not received any complaints about specific error conditions from vendors on non-POSIX platforms, but such complaints would not surprise me if they surfaced.

[ 2009-10 Santa Cruz: ]

> Leave open. Benjamin to provide wording.

[ 2010 Pittsburgh: ]

> We don't have throw clauses for condition variables.

> This issue may be dependent on LWG 1268(i).

> Leave open. Detlef will coordinate with Benjamin.

> Consider merging LWG 964, 966(i), and 1268(i) into a single paper.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 965(i). Various threading bugs #15

Section: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.7.4 [thread.condition.condvar]: the constructor for `condition_variable` throws an exception with error code `device_or_resource_busy` "if attempting to initialize a previously-initialized but as of yet undestroyed `condition_variable`." How can this occur?

[ Summit: ]

> Move to review. Proposed resolution: strike the `device_or_resource_busy` error condition from the constructor of `condition_variable`.

> - This is a POSIX error that cannot occur in this interface because the C++ interface does not separate declaration from initialization.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 32.7.4 [thread.condition.condvar] p3:

> - ...
> - `device_or_resource_busy` -- if attempting to initialize a previously-initialized but as of yet undestroyed `condition_variable`.

### 966(i). Various threading bugs #16

Section: 32.7.4 [thread.condition.condvar] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.7.4 [thread.condition.condvar]: `condition_variable::wait` and `condition_variable::wait_until` both have a postcondition that `lock` is locked by the calling thread, and a throws clause that requires throwing an exception if this postcondition cannot be achieved. How can the implementation detect that this `lock` can never be obtained?

[ Summit: ]

> Move to open. Requires wording. Agreed this is an issue, and the specification should not require detecting deadlocks.

[ 2009-08-01 Howard provides wording. ]

> The proposed wording is inspired by the POSIX spec which says:

> > [EINVAL]
> >   The value specified by cond or mutex is invalid.
> > [EPERM]
> >   The mutex was not owned by the current thread at the time of the call.

> I do not believe [EINVAL] is possible without memory corruption (which we don't specify). [EPERM] is possible if this thread doesn't own the mutex, which is listed as a precondition. "May" is used instead of "Shall" because not all OS's are POSIX.

[ 2009-10 Santa Cruz: ]

> Leave open, Detlef to provide improved wording.

[ 2009-10-23 Detlef Provided wording. ]

> Detlef's wording put in Proposed resolution. Original wording here:

> > Change 32.7.4 [thread.condition.condvar] p12, p19 and 32.7.5 [thread.condition.condvarany] p10, p16:

> > > Throws: May throw `std::system_error` if a precondition is not met. when the effects or postcondition cannot be achieved.

[ 2009-10 Santa Cruz: ]

> Leave open, Detlef to provide improved wording.

[ 2009-11-18 Anthony adds: ]

> `condition_variable::wait` takes a `unique_lock<mutex>`. We know whether or not a `unique_lock` owns a lock, through use of its `owns_lock()` member.

> I would like to propose the following resolution:

> > Modify the first sentence of 32.7.4 [thread.condition.condvar] p9:

> > >     void wait(unique_lock<mutex>& lock);

> > > > 9 Precondition: `lock` is locked by the calling thread `lock.owns_lock()` is `true`, and either

> > > > ...

> > Replace 32.7.4 [thread.condition.condvar] p11-13 with:

> > >     void wait(unique_lock<mutex>& lock);

> > > > ...

> > > > 11 Postcondition: `lock` is locked by the calling thread `lock.owns_lock()` is `true`.

> > > > 12 Throws: `std::system_error` when the effects or postcondition cannot be achieved if the implementation detects that the preconditions are not met or the effects cannot be achieved. Any exception thrown by `lock.lock()` or `lock.unlock()`.

> > > > 13 Error Conditions: The error conditions are implementation defined.

> > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

[ 2010 Pittsburgh: ]

> There are heavy conflicts with adopted papers.

> This issue is dependent on LWG 1268(i).

> Leave open pending outstanding edits to the working draft. Detlef will provide wording.

> Possibly related to 964(i).

[2011-03-24 Madrid]

Rationale:

This has been resolved since filing, with the introduction of `system_error` to the thread specification.

Proposed resolution:

Replace 32.7.4 [thread.condition.condvar] p12, p19 and 32.7.5 [thread.condition.condvarany] p10, p16:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> Error conditions:

> - equivalent error condition from `lock.lock()` or `lock.unlock()`.

> Throws: It is implementation-defined whether a `std::system_error` with implementation-defined error condition is thrown if the precondition is not met.

### 967(i). Various threading bugs #17

Section: 32.4.3.3 [thread.thread.constr] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

the error handling for the constructor for `condition_variable` distinguishes lack of memory from lack of other resources, but the error handling for the thread constructor does not. Is this difference intentional?

[ Beman has volunteered to provide proposed wording. ]

[ 2009-09-25 Beman provided proposed wording. ]

> The proposed resolution assumes 962(i) has been accepted and its proposed resolution applied to the working paper.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 4, as indicated:

> Error conditions:

> > - `not_enough_memory` — if there is not enough memory to construct the mutex object.
> > - `resource_unavailable_try_again` — if any native handle type manipulated is not available.
> > - `operation_not_permitted` — if the thread does not have the necessary permission to change the state of the mutex object.
> > - `device_or_resource_busy` — if any native handle type manipulated is already locked.
> > - `invalid_argument` — if any native handle type manipulated as part of mutex construction is incorrect.

Change Class `condition_variable` 32.7.4 [thread.condition.condvar], default constructor, as indicated:

> `condition_variable();`

> > Effects: Constructs an object of type `condition_variable`.

> > Throws: `std::system_error` when an exception is required (32.2.2 [thread.req.exception]).

> > Error conditions:

> > > - `not_enough_memory` — if a memory limitation prevents initialization.
> > > - `resource_unavailable_try_again` — if some non-memory resource limitation prevents initialization.
> > > - `device_or_resource_busy` — if attempting to initialize a previously-initialized but as of yet undestroyed `condition_variable`.

### 968(i). Various threading bugs #18

Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.6.4 [thread.mutex.requirements]: several functions are required to throw exceptions "if the thread does not have the necessary permission ...". "The necessary permission" is not defined.

[ Summit: ]

> Move to open.

[ Beman has volunteered to provide proposed wording. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready with minor word-smithing in the example.

Proposed resolution:

Change Exceptions 32.2.2 [thread.req.exception] as indicated:

> Some functions described in this Clause are specified to throw exceptions of type `system_error` (19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function from meeting its specifications. [Note: See 16.4.6.14 [res.on.exception.handling] for exceptions thrown to report storage allocation failures. —end note]

> [Example:

> > Consider a function in this clause that is specified to throw exceptions of type `system_error` and specifies Error conditions that include `operation_not_permitted` for a thread that does not have the privilege to perform the operation. Assume that, during the execution of this function, an `errno` of `EPERM` is reported by a POSIX API call used by the implementation. Since POSIX specifies an `errno` of `EPERM` when "the caller does not have the privilege to perform the operation", the implementation maps `EPERM` to an `error_condition` of `operation_not_permitted` (19.5 [syserr]) and an exception of type `system_error` is thrown.

> —end example]

> Editorial note: For the sake of exposition, the existing text above is shown with the changes proposed in issues 962 and 967. The proposed additional example is independent of whether or not the 962 and 967 proposed resolutions are accepted.

Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 4, as indicated:

> — `operation_not_permitted` — if the thread does not have the necessary permission to change the state of the mutex object privilege to perform the operation.

Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 12, as indicated:

> — `operation_not_permitted` — if the thread does not have the necessary permission to change the state of the mutex privilege to perform the operation.

### 970(i). `addressof` overload unneeded

Section: 20.2.11 [specialized.addressof] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

20.2.11 [specialized.addressof] specifies:

>     template <ObjectType T> T* addressof(T& r);
>     template <ObjectType T> T* addressof(T&& r);

The two signatures are ambiguous when the argument is an lvalue. The second signature seems not useful: what does it mean to take the address of an rvalue?

[ Post Summit: ]

> Recommend Review.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-11-18 Moved from Pending WP to WP. Confirmed in N3000. ]

Proposed resolution:

Change 20.2.11 [specialized.addressof]:

>     template <ObjectType T> T* addressof(T& r);
>     template <ObjectType T> T* addressof(T&& r);

### 974(i). `duration<double>` should not implicitly convert to `duration<int>`

Section: 30.5.2 [time.duration.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The following code should not compile because it involves implicit truncation errors (against the design philosophy of the `duration` library).

>     duration<double> d(3.5);
>     duration<int> i = d;  // implicit truncation, should not compile

This intent was codified in the example implementation which drove this proposal but I failed to accurately translate the code into the specification in this regard.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be improved for `enable_if` type constraining, possibly following Robert's formula.

[ 2009-08-01 Howard adds: ]

> Addressed by 1177(i).

[ 2009-10 Santa Cruz: ]

> Not completely addressed by 1177(i). Move to Ready.

Proposed resolution:

Change 30.5.2 [time.duration.cons], p4:

>     template <class Rep2, class Period2>
>       duration(const duration<Rep2, Period2>& d);

> > -4- Requires: `treat_as_floating_point<rep>::value` shall be `true` or both `ratio_divide<Period2, period>::type::den` shall be 1 and `treat_as_floating_point<Rep2>::value` shall be `false`. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-based `duration` types. Such a construction could easily lead to confusion about the value of the `duration`. — end note]

### 975(i). `is_convertible` cannot be instantiated for non-convertible types

Section: 21.3.7 [meta.rel] Status: C++11 Submitter: Daniel Krügler Opened: 2009-01-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 206

Related to 1114(i).

The current specification of `std::is_convertible` (reference is draft N2798) is basically defined by 21.3.7 [meta.rel] p.4:

> In order to instantiate the template `is_convertible<From, To>`, the following code shall be well formed:

> >     template <class T>
> >       typename add_rvalue_reference<T>::type create();

> >     To test() {
> >       return create<From>();
> >     }

> [Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]

The first sentence can be interpreted, that e.g. the expression

>     std::is_convertible<double, int*>::value

is ill-formed because `std::is_convertible<double, int*>` could not be instantiated, or in more general terms: The wording requires that `std::is_convertible<X, Y>` cannot be instantiated for otherwise valid argument types `X` and `Y` if `X` is not convertible to `Y`.

This semantic is both unpractical and in contradiction to what the last type traits paper N2255 proposed:

> If the following `test` function is well formed code `b` is `true`, else it is `false`.

> >     template <class T>
> >       typename add_rvalue_reference<T>::type create();

> >     To test() {
> >       return create<From>();
> >     }

> [Note: This definition gives well defined results for `reference` types, `void` types, array types, and function types. — end note]

[ Post Summit: ]

> Jens: Checking that code is well-formed and then returning `true`/`false` sounds like speculative compilation. John Spicer would really dislike this. Please find another wording suggesting speculative compilation.

> Recommend Open.

[ Post Summit, Howard adds: ]

> John finds the following wording clearer:

> > Template | Condition | Comments
> > --- | --- | ---
> > `template <class From, class To> struct is_convertible;` | see below | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.

> > Given the following function prototype:

> > >     template <class T>
> > >       typename add_rvalue_reference<T>::type create();

> > `is_convertible<From, To>::value` shall be `true` if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, else `is_convertible<From, To>::value` shall be `false`.

> > >     To test() {
> > >       return create<From>();
> > >     }

Original proposed wording:

In 21.3.7 [meta.rel]/4 change:

> In order to instantiate the template `is_convertible<From, To>`, the following code shall be well formed If the following code is well formed `is_convertible<From, To>::value` is `true`, otherwise `false`:[..]

Revision 2

> In 21.3.7 [meta.rel] change:

> > Template | Condition | Comments
> > --- | --- | ---
> > ... | ... | ...
> > `template <class From, class To> struct is_convertible;` | The code set out below shall be well formed. see below | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.

> > -4- In order to instantiate the template `is_convertible<From, To>`, the following code shall be well formed: Given the following function prototype:

> > >     template <class T>
> > >       typename add_rvalue_reference<T>::type create();

> > `is_convertible<From, To>::value` inherits either directly or indirectly from `true_type` if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, else `is_convertible<From, To>::value` inherits either directly or indirectly from `false_type`.

> > >     To test() {
> > >       return create<From>();
> > >     }

> > [Note: This requirement gives well defined results for reference types, void types, array types, and function types. -- end note]

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 21.3.7 [meta.rel] change:

> Template | Condition | Comments
> --- | --- | ---
> ... | ... | ...
> `template <class From, class To> struct is_convertible;` | The code set out below shall be well formed. see below | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.

> -4- In order to instantiate the template `is_convertible<From, To>`, the following code shall be well formed: Given the following function prototype:

> >     template <class T>
> >       typename add_rvalue_reference<T>::type create();

> the predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied, if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function.

> >     To test() {
> >       return create<From>();
> >     }

> [Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]

### 976(i). Class template `std::stack` should be movable

Section: 23.6.6.2 [stack.defn] Status: Resolved Submitter: Daniel Krügler Opened: 2009-02-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopsis given in 23.6.6.2 [stack.defn] does not show up

>     requires MoveConstructible<Cont> stack(stack&&);
>     requires MoveAssignable<Cont> stack& operator=(stack&&);

although the other container adaptors do provide corresponding members.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-08-18 Daniel updates the wording and Howard sets to Review. ]

[ 2009-08-23 Howard adds: ]

> 1194(i) also adds these move members using an editorially different style.

[ 2009-10 Santa Cruz: ]

> Mark NAD EditorialResolved, addressed by issue 1194(i).

Proposed resolution:

In the class stack synopsis of 23.6.6.2 [stack.defn] insert:

>     template <class T, class Container = deque<T> >
>     class stack {
>       [..]
>       explicit stack(const Container&);
>       explicit stack(Container&& = Container());
>       stack(stack&& s) : c(std::move(s.c)) {}
>       stack& operator=(stack&& s) { c = std::move(s.c); return *this; }
>       [..]
>     };

### 978(i). Hashing smart pointers

Section: 22.10.19 [unord.hash] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-02-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 208

I don't see an open issue on supporting `std::hash` for smart pointers (`unique_ptr` and `shared_ptr` at least).

It seems reasonable to at least expect support for the smart pointers, especially as they support comparison for use in ordered associative containers.

[ Batavia (2009-05): ]

> Howard points out that the client can always supply a custom hash function.

> Alisdair replies that the smart pointer classes are highly likely to be frequently used as hash keys.

> Bill would prefer to be conservative.

> Alisdair mentions that this issue may also be viewed as a subissue or duplicate of issue 1025(i).

> Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-05-31 Peter adds: ]

> > Howard points out that the client can always supply a custom hash function.

> Not entirely true. The client cannot supply the function that hashes the address of the control block (the equivalent of the old `operator<`, now proudly carrying the awkward name of '`owner_before`'). Only the implementation can do that, not necessarily via specializing `hash<>`, of course.

> This hash function makes sense in certain situations for `shared_ptr` (when one needs to switch from `set/map` using ownership ordering to `unordered_set/map`) and is the only hash function that makes sense for `weak_ptr`.

[ 2009-07-28 Alisdair provides wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2009-11-16 Moved from Ready to Open: ]

> Pete writes:

> > As far as I can see, "...suitable for using this type as key in unordered associative containers..." doesn't define any semantics. It's advice to the reader, and if it's present at all it should be in a note. But we have far too much of this sort of editorial commentary as it is.

> > And in the resolution of 978 it's clearly wrong: it says that if there is no hash specialization available for `D::pointer`, the implementation may provide `hash<unique_ptr<T,D>>` if the result is not suitable for use in unordered containers.

> Howard writes:

> > Is this a request to pull 978 from Ready?

> Barry writes:

> > I read this as more than a request. The PE says it's wrong, so it can't be Ready.

[ 2010-01-31 Alisdair: related to 1245(i) and 1182(i). ]

[ 2010-02-08 Beman updates wording. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add the following declarations to the synopsis of `<memory>` in 20.2 [memory]

>     // [util.smartptr.hash] hash support
>     template <class T> struct hash;
>     template <class T, class D> struct hash<unique_ptr<T,D>>;
>     template <class T> struct hash<shared_ptr<T>>;

Add a new subclause under [util.smartptr] called hash support

> ### hash support [util.smartptr.hash]

>     template <class T, class D> struct hash<unique_ptr<T,D>>;

> > Specialization meeting the requirements of class template `hash` (22.10.19 [unord.hash]). For an object `p` of type `UP`, where `UP` is a type `unique_ptr<T,D>`, `hash<UP>()(p)` shall evaluate to the same value as `hash<typename UP::pointer>()(p.get())`. The specialization `hash<typename UP::pointer>` is required to be well-formed.

>     template <class T> struct hash<shared_ptr<T>>;

> > Specialization meeting the requirements of class template `hash` (22.10.19 [unord.hash]). For an object `p` of type `shared_ptr<T>`, `hash<shared_ptr<T>>()(p)` shall evaluate to the same value as `hash<T*>()(p.get())`.

### 981(i). Unordered container requirements should add `initializer_list` support

Section: 23.2.8 [unord.req] Status: C++11 Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Refering to N2800 all container requirements tables (including those for associative containers) provide useful member function overloads accepting `std::initializer_list` as argument, the only exception is Table 87. There seems to be no reason for not providing them, because 23.5 [unord] is already `initializer_list`-aware. For the sake of library interface consistency and user-expectations corresponding overloads should be added to the table requirements of unordered containers as well.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

In 23.2.8 [unord.req]/9 insert:

> ... `[q1, q2)` is a valid range in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` is a value of type `X::value_type`, ...

In 23.2.8 [unord.req], Table 87 insert:

> Table 87 - Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X(i, j) X a(i, j)` | `X` | ... | ...
> `X(il)` | `X` | Same as `X(il.begin(), il.end())`. | Same as `X(il.begin(), il.end())`.
> ... | ... | ... | ...
> `a = b` | `X` | ... | ...
> `a = il` | `X&` | `a = X(il); return *this;` | Same as `a = X(il)`.
> ... | ... | ... | ...
> `a.insert(i, j)` | `void` | ... | ...
> `a.insert(il)` | `void` | Same as `a.insert(il.begin(), il.end())`. | Same as `a.insert(il.begin(), il.end())`.

### 982(i). Wrong complexity for initializer_list assignment in Table 85

Section: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

According to N2800, the associative container requirements table 85 says that assigning an `initializer_list` to such a container is of constant complexity, which is obviously wrong.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

In 23.2.7 [associative.reqmts], Table 85 change:

> Table 85 - Associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `a = il` | `X&` | `a = X(il); return *this;` | constantSame as `a = X(il)`.

### 983(i). `unique_ptr` reference deleters should not be moved from

Section: 20.3.1.3 [unique.ptr.single] Status: Resolved Submitter: Howard Hinnant Opened: 2009-02-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Dave brought to my attention that when a `unique_ptr` has a non-const reference type deleter, move constructing from it, even when the `unique_ptr` containing the reference is an rvalue, could have surprising results:

>     D d(some-state);
>     unique_ptr<A, D&> p(new A, d);
>     unique_ptr<A, D> p2 = std::move(p);
>     // has d's state changed here?

I agree with him. It is the `unique_ptr` that is the rvalue, not the deleter. When the deleter is a reference type, the `unique_ptr` should respect the "lvalueness" of the deleter.

Thanks Dave.

[ Batavia (2009-05): ]

> Seems correct, but complicated enough that we recommend moving to Review.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

Change 20.3.1.3.2 [unique.ptr.single.ctor], p20-21

>     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

> > -20- Requires: If `D E` is not a reference type, construction of the deleter `D` from an rvalue of type `E` shall be well formed and shall not throw an exception. Otherwise `E` is a reference type and construction of the deleter `D` from an lvalue of type `E` shall be well formed and shall not throw an exception. If `D` is a reference type, then `E` shall be the same type as `D` (diagnostic required). `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [`Note:` These requirements imply that `T` and `U` are complete types. — end note]

> > -21- Effects: Constructs a `unique_ptr` which owns the pointer which `u` owns (if any). If the deleter `E` is not a reference type, it this deleter is move constructed from `u`'s deleter, otherwise the reference this deleter is copy constructed from `u`.'s deleter. After the construction, `u` no longer owns a pointer. [Note: The deleter constructor can be implemented with `std::forward<DE>`. — end note]

Change 20.3.1.3.4 [unique.ptr.single.asgn], p1-3

>     unique_ptr& operator=(unique_ptr&& u);

> > -1- Requires: If the deleter `D` is not a reference type, Aassignment of the deleter `D` from an rvalue `D` shall not throw an exception. Otherwise the deleter `D` is a reference type, and assignment of the deleter `D` from an lvalue `D` shall not throw an exception.

> > -2- Effects: reset(u.release()) followed by an move assignment from `u`'s deleter to this deleter `std::forward<D>(u.get_deleter())`.

> > -3- Postconditions: This `unique_ptr` now owns the pointer which `u` owned, and `u` no longer owns it. [Note: If `D` is a reference type, then the referenced lvalue deleters are move assigned. — end note]

Change 20.3.1.3.4 [unique.ptr.single.asgn], p6-7

>     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

> > Requires: If the deleter `E` is not a reference type, Aassignment of the deleter `D` from an rvalue `DE` shall not throw an exception. Otherwise the deleter `E` is a reference type, and assignment of the deleter `D` from an lvalue `E` shall not throw an exception. `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U>` are complete types. — end note]

> > Effects: `reset(u.release())` followed by an move assignment from `u`'s deleter to this deleter `std::forward<E>(u.get_deleter())`. If either `D` or `E` is a reference type, then the referenced lvalue deleter participates in the move assignment.

### 984(i). Does `<cinttypes>` have macro guards?

Section: 31.13 [c.files] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The C standard says about `<inttypes.h>`:

> C++ implementations should define these macros only when `__STDC_FORMAT_MACROS`is defined before `<inttypes.h>` is included.

The C standard has a similar note about `<stdint.h>`. For `<cstdint>` we adopted a "thanks but no thanks" policy and documented that fact in 17.4.1 [cstdint.syn]:

> ... [Note: The macros defined by `<stdint>` are provided unconditionally. In particular, the symbols `__STDC_LIMIT_MACROS` and `__STDC_CONSTANT_MACROS` (mentioned in C99 footnotes 219, 220, and 222) play no role in C++. — end note]

I recommend we put a similar note in 31.13 [c.files] regarding `<cinttypes>`.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 31.13 [c.files]:

> Table 112 describes header `<cinttypes>`. [Note: The macros defined by `<cintypes>` are provided unconditionally. In particular, the symbol `__STDC_FORMAT_MACROS` (mentioned in C99 footnote 182) plays no role in C++. — end note]

### 985(i). Allowing throwing move

Section: 23.2.2 [container.requirements.general] Status: Resolved Submitter: Rani Sharoni Opened: 2009-02-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Introduction

This proposal is meant to resolve potential regression of the N2800 draft, see next section, and to relax the requirements for containers of types with throwing move constructors.

The basic problem is that some containers operations, like `push_back`, have a strong exception safety guarantee (i.e. no side effects upon exception) that are not achievable when throwing move constructors are used since there is no way to guarantee revert after partial move. For such operations the implementation can at most provide the basic guarantee (i.e. valid but unpredictable) as it does with multi copying operations (e.g. range insert).

For example, `vector<T>::push_back()` (where `T` has a move constructor) might resize the `vector` and move the objects to the new underlying buffer. If move constructor throws it might not be possible to recover the throwing object or to move the old objects back to the original buffer.

The current draft is explicit by disallowing throwing move for some operations (e.g. `vector<>::reserve`) and not clear about other operations mentioned in 23.2.2 [container.requirements.general]/10 (e.g. single element `insert`): it guarantees strong exception safety without explicitly disallowing a throwing move constructor.

Regression

This section only refers to cases in which the contained object is by itself a standard container.

Move constructors of standard containers are allowed to throw and therefore existing operations are broken, compared with C++03, due to move optimization. (In fact existing implementations like Dinkumware are actually throwing).

For example, `vector< list<int> >::reserve` yields undefined behavior since `list<int>`'s move constructor is allowed to throw. On the other hand, the same operation has strong exception safety guarantee in C++03.

There are few options to solve this regression:

1. Disallow throwing move and throwing default constructor
2. Disallow throwing move but disallowing usage after move
3. Special casing
4. Disallow throwing move and making it optional

Option 1 is suggested by proposal N2815 but it might not be applicable for existing implementations for which containers default constructors are throwing.

Option 2 limits the usage significantly and it's error prone by allowing zombie objects that are nothing but destructible (e.g. no `clear()` is allowed after move). It also potentially complicates the implementation by introducing special state.

Option 3 is possible, for example, using default construction and `swap` instead of move for standard containers case. The implementation is also free to provide special hidden operation for non throwing move without forcing the user the cope with the limitation of option-2 when using the public move.

Option 4 impact the efficiency in all use cases due to rare throwing move.

The proposed wording will imply option 1 or 3 though option 2 is also achievable using more wording. I personally oppose to option 2 that has impact on usability.

Relaxation for user types

Disallowing throwing move constructors in general seems very restrictive since, for example, common implementation of move will be default construction + `swap` so move will throw if the default constructor will throw. This is currently the case with the Dinkumware implementation of node based containers (e.g. `std::list`) though this section doesn't refer to standard types.

For throwing move constructors it seem that the implementation should have no problems to provide the basic guarantee instead of the strong one. It's better to allow throwing move constructors with basic guarantee than to disallow it silently (compile and run), via undefined behavior.

There might still be cases in which the relaxation will break existing generic code that assumes the strong guarantee but it's broken either way given a throwing move constructor since this is not a preserving optimization.

[ Batavia (2009-05): ]

> Bjarne comments (referring to his draft paper): "I believe that my suggestion simply solves that. Thus, we don't need a throwing move."

> Move to Open and recommend it be deferred until after the next Committee Draft is issued.

[ 2009-10 Santa Cruz: ]

> Should wait to get direction from Dave/Rani (N2983).

[ 2010-03-28 Daniel updated wording to sync with N3092. ]

> The suggested change of 23.3.5.4 [deque.modifiers]/2 should be removed, because the current wording does say more general things:

> > 2 Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` there are no effects. If an exception is thrown by the move constructor of a non-CopyConstructible `T`, the effects are unspecified.

> The suggested change of 23.3.13.3 [vector.capacity]/2 should be removed, because the current wording does say more general things:

> > 2 Effects: A directive that informs a `vector` of a planned change in size, so that it can manage the storage allocation accordingly. After `reserve()`, `capacity()` is greater or equal to the argument of `reserve` if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`. If an exception is thrown other than by the move constructor of a non-`CopyConstructible` type, there are no effects.

[2011-03-15: Daniel updates wording to sync with N3242 and comments]

> The issue has nearly been resolved by previous changes to the working paper, in particular all suggested changes for `deque` and `vector` are no longer necessary. The still remaining parts involve the unordered associative containers.

[2011-03-24 Madrid meeting]

It looks like this issue has been resolved already by `noexcept` paper N3050

Rationale:

Resolved by N3050

Proposed resolution:

23.2.2 [container.requirements.general] paragraph 10 add footnote:

> -10- Unless otherwise specified (see 23.2.7.2 [associative.reqmts.except], 23.2.8.2 [unord.req.except], 23.3.5.4 [deque.modifiers], and 23.3.13.5 [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:

> - …

> [Note: for compatibility with C++ 2003, when "no effect" is required, standard containers should not use the `value_type`'s throwing move constructor when the contained object is by itself a standard container. — end note]

23.2.8.2 [unord.req.except] change paragraph 2+4 to say:

> -2- For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an `insert()` function inserting a single element, the `insert()` function has no effect unless the exception is thrown by the contained object move constructor.

> […]

> -4- For unordered associative containers, if an exception is thrown from within a `rehash()` function other than by the container's hash function or comparison function, the `rehash()` function has no effect unless the exception is thrown by the contained object move constructor.

Keep 23.3.5.4 [deque.modifiers] paragraph 2 unchanged [Drafting note: The originally proposed wording did suggest to add a last sentence as follows:

> If an exception is thrown by `push_back()` or `emplace_back()` function, that function has no effects unless the exception is thrown by the move constructor of `T`.

— end drafting note ]

> -2- Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertable `T`, the effects are unspecified.

Keep 23.3.13.3 [vector.capacity] paragraph 2 unchanged [Drafting note: The originally proposed wording did suggest to change the last sentence as follows:

> If an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.

— end drafting note ]

> -2- Effects: A directive that informs a `vector` of a planned change in size, so that it can manage the storage allocation accordingly. After `reserve()`, `capacity()` is greater or equal to the argument of `reserve` if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`. If an exception is thrown other than by the move constructor of a non-`CopyInsertable` type, there are no effects.

Keep 23.3.13.3 [vector.capacity] paragraph 12 unchanged [Drafting note: The originally proposed wording did suggest to change the old paragraph as follows:

> -12- Requires: If `value_type` has a move constructor, that constructor shall not throw any exceptions. If an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.

— end drafting note ]

> -12- Requires: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

Keep 23.3.13.5 [vector.modifiers] paragraph 1 unchanged [Drafting note: The originally proposed wording did suggest to change the old paragraph as follows:

> -1- Requires: If `value_type` has a move constructor, that constructor shall not throw any exceptions. Remarks: If an exception is thrown by `push_back()` or `emplace_back()` function, that function has no effect unless the exception is thrown by the move constructor of `T`.

— end drafting note ]

> -1- Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` or by any `InputIterator` operation there are no effects. If an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

### 986(i). Generic `try_lock` contradiction

Section: 32.6.6 [thread.lock.algorithm] Status: C++11 Submitter: Chris Fairles Opened: 2009-02-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 32.6.6 [thread.lock.algorithm], the generic `try_lock` effects (p2) say that a failed `try_lock` is when it either returns `false` or throws an exception. In the event a call to `try_lock` does fail, by either returning `false` or throwing an exception, it states that `unlock` shall be called for all prior arguments. Then the returns clause (p3) goes on to state in a note that after returning, either all locks are locked or none will be. So what happens if multiple locks fail on `try_lock`?

Example:

>     #include <mutex>

>     int main() {
>      std::mutex m0, m1, m2;
>      std::unique_lock<std::mutex> l0(m0, std::defer_lock);
>      std::unique_lock<std::mutex> l1(m1); //throws on try_lock
>      std::unique_lock<std::mutex> l2(m2); //throws on try_lock

>      int result = std::try_lock(l0, l1, l2);

>      assert( !l0.owns_lock() );
>      assert( l1.owns_lock() ); //??
>      assert( l2.owns_lock() ); //??
>     }

The first lock's `try_lock` succeeded but, being a prior argument to a lock whose `try_lock` failed, it gets unlocked as per the effects clause of 32.6.6 [thread.lock.algorithm]. However, 2 locks remain locked in this case but the return clause states that either all arguments shall be locked or none will be. This seems to be a contradiction unless the intent is for implementations to make an effort to unlock not only prior arguments, but the one that failed and those that come after as well. Shouldn't the note only apply to the arguments that were successfully locked?

Further discussion and possible resolutions in c++std-lib-23049.

[ Summit: ]

> Move to review. Agree with proposed resolution.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 32.6.6 [thread.lock.algorithm], p2:

> -2- Effects: Calls `try_lock()` for each argument in order beginning with the first until all arguments have been processed or a call to `try_lock()` fails, either by returning `false` or by throwing an exception. If a call to `try_lock()` fails, `unlock()` shall be called for all prior arguments and there shall be no further calls to `try_lock()`.

Delete the note from 32.6.6 [thread.lock.algorithm], p3

> -3- Returns: -1 if all calls to `try_lock()` returned `true`, otherwise a 0-based index value that indicates the argument for which `try_lock()` returned `false`. [Note: On return, either all arguments will be locked or none will be locked. -- end note]

### 987(i). `reference_wrapper` and function types

Section: 22.10.6 [refwrap] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopsis in 22.10.6 [refwrap] says:

>     template <ObjectType T> class reference_wrapper
>     ...

And then paragraph 3 says:

> The template instantiation `reference_wrapper<T>` shall be derived from `std::unary_function<T1, R>` only if the type `T` is any of the following:

> - a function type or a pointer to function type taking one argument of type `T1` and returning `R`

But function types are not `ObjectType`s.

Paragraph 4 contains the same contradiction.

[ Post Summit: ]

> Jens: restricted reference to ObjectType

> Recommend Review.

[ Post Summit, Peter adds: ]

> In https://svn.boost.org/trac/boost/ticket/1846 however Eric Niebler makes the very reasonable point that `reference_wrapper<F>`, where `F` is a function type, represents a reference to a function, a legitimate entity. So `boost::ref` was changed to allow it.

> http://svn.boost.org/svn/boost/trunk/libs/bind/test/ref_fn_test.cpp

> Therefore, I believe an alternative proposed resolution for issue 987 could simply allow `reference_wrapper` to be used with function types.

[ Post Summit, Howard adds: ]

> I agree with Peter (and Eric). I got this one wrong on my first try. Here is code that demonstrates how easy (and useful) it is to instantiate `reference_wrapper` with a function type:

> >     #include <functional>

> >     template <class F>
> >     void test(F f);

> >     void f() {}

> >     int main()
> >     {
> >         test(std::ref(f));
> >     }

> Output (link time error shows type of `reference_wrapper` instantiated with function type):

> >     Undefined symbols:
> >       "void test<std::reference_wrapper<void ()()> >(std::reference_wrapper<void ()()>)",...

> I've taken the liberty of changing the proposed wording to allow function types and set to Open. I'll also freely admit that I'm not positive `ReferentType` is the correct concept.

[ Batavia (2009-05): ]

> Howard observed that `FunctionType`, a concept not (yet?) in the Working Paper, is likely the correct constraint to be applied. However, the proposed resolution provides an adequate approximation.

> Move to Review.

[ 2009-05-23 Alisdair adds: ]

> By constraining to `PointeeType` we rule out the ability for `T` to be a reference, and call in reference-collapsing. I'm not sure if this is correct and intended, but would like to be sure the case was considered.

> Is dis-allowing reference types and the implied reference collapsing the intended result?

[ 2009-07 Frankfurt ]

> Moved from Review to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

Change 22.10.6 [refwrap]/1 as indicated:

> `reference_wrapper<T>` is a `CopyConstructible` and `CopyAssignable` wrapper around a reference to an object or function of type `T`.

### 990(i). `monotonic_clock::is_monotonic` must be `true`

Section: 99 [time.clock.monotonic] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

There is some confusion over what the value of `monotonic_clock::is_monotonic` when `monotonic_clock` is a synonym for `system_clock`. The intent is that if `monotonic_clock` exists, then `monotonic_clock::is_monotonic` is `true`.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

Change 99 [time.clock.monotonic], p1:

> -1- Objects of class `monotonic_clock` represent clocks for which values of `time_point` never decrease as physical time advances. `monotonic_clock` may be a synonym for `system_clock` if and only if `system_clock::is_monotonic` is `true`.

### 991(i). Provide allocator for `wstring_convert`

Section: 99 [depr.conversions.string] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2017-04-22

Priority: Not Prioritized

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Discussion:

Addresses JP-50 [CD1]

Add custom allocator parameter to `wstring_convert`, since we cannot allocate memory for strings from a custom allocator.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change [conversions.string]:

>     template<class Codecvt, class Elem = wchar_t,
>              class Wide_alloc = std::allocator<Elem>,
>              class Byte_alloc = std::allocator<char> > class wstring_convert {
>       public:
>         typedef std::basic_string<char, char_traits<char>, Byte_alloc> byte_string;
>         typedef std::basic_string<Elem, char_traits<Elem>, Wide_alloc> wide_string;
>          ...

Change [conversions.string], p3:

> -3- The class template describes an ob ject that controls conversions between wide string ob jects of class `std::basic_string<Elem, char_traits<Elem>, Wide_alloc>` and byte string objects of class `std::basic_string<char, char_traits<char>, Byte_alloc>` (also known as `std::string`).

### 993(i). `_Exit` needs better specification

Section: 17.5 [support.start.term] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK-188 [CD1]

The function `_Exit` does not appear to be defined in this standard. Should it be added to the table of functions included-by-reference to the C standard?

[ 2009-05-09 Alisdair fixed some minor issues in the wording. ]

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 17.5 [support.start.term] Table 20 (Header `<cstdlib>` synopsis) Functions:

>     _Exit

Add before the description of `abort(void)`:

>     void _Exit [[noreturn]] (int status)

> > The function `_Exit(int status)` has additional behavior in this International Standard:

> > - The program is terminated without executing destructors for objects of automatic, thread, or static storage duration and without calling the functions passed to `atexit()` (6.10.3.4 [basic.start.term]).

### 994(i). `quick_exit` should terminate well-defined

Section: 17.6.4.3 [new.handler] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK-193 [CD1]

`quick_exit` has been added as a new valid way to terminate a program in a well defined way.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 17.6.4.3 [new.handler], p2:

> -2- Required behavior: ...

> - ...
> - call either `abort()` or `exit();` terminate execution of the program without returning to the caller

### 997(i). "Effects: Equivalent to" is underspecified

Section: 16.3.2.4 [structure.specifications] Status: C++11 Submitter: Thomas Plum Opened: 2009-03-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK-163 [CD1]

Many functions are defined as "Effects: Equivalent to a...", which seems to also define the preconditions, effects, etc. But this is not made clear.

After studying the occurrences of "Effects: Equivalent to", I agree with the diagnosis but disagree with the solution. In 27.4.3.3 [string.cons] we find

> 14 Effects: If `InputIterator` is an integral type, equivalent to `basic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)`

> 15 Otherwise constructs a string from the values in the range `[begin, end)`, as indicated in the Sequence Requirements table (see 23.1.3).

This would be devishly difficult to re-write with an explicit "Equivalent to:" clause. Instead, I propose the following, which will result in much less editorial re-work.

[ 2009-05-09 Alisdair adds: ]

> This issue is related to 492(i).

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add a new paragraph after 16.3.2.4 [structure.specifications], p3:

> -3- Descriptions of function semantics contain the following elements (as appropriate):154

> - Requires: the preconditions for calling the function
> - Effects: the actions performed by the function
> - Postconditions: the observable results established by the function
> - Returns: a description of the value(s) returned by the function
> - Throws: any exceptions thrown by the function, and the conditions that would cause the exception
> - Complexity: the time and/or space complexity of the function
> - Remarks: additional semantic constraints on the function
> - Error conditions: the error conditions for error codes reported by the function.
> - Notes: non-normative comments about the function

> Whenever the Effects element specifies that the semantics of some function `F` are Equivalent to some code-sequence, then the various elements are interpreted as follows. If `F`'s semantics specifies a Requires element, then that requirement is logically imposed prior to the equivalent-to semantics. Then, the semantics of the code-sequence are determined by the Requires, Effects, Postconditions, Returns, Throws, Complexity, Remarks, Error Conditions and Notes specified for the (one or more) function invocations contained in the code-sequence. The value returned from `F` is specified by `F`'s Returns element, or if `F` has no Returns element, a non-`void` return from `F` is specified by the Returns elements in code-sequence. If `F`'s semantics contains a Throws (or Postconditions, or Complexity) element, then that supersedes any occurrences of that element in the code-sequence.

### 998(i). Smart pointer referencing its owner

Section: 20.3.1.3.6 [unique.ptr.single.modifiers] Status: C++11 Submitter: Pavel Minaev Opened: 2009-02-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Consider the following (simplified) implementation of `std::auto_ptr<T>::reset()`:

>     void reset(T* newptr = 0) {
>        if (this->ptr && this->ptr != newptr) {
>          delete this->ptr;
>        }
>        this->ptr = newptr;
>     }

Now consider the following code which uses the above implementation:

>     struct foo {
>        std::auto_ptr<foo> ap;
>        foo() : ap(this) {}
>        void reset() { ap.reset(); }
>     };
>     int main() {
>        (new foo)->reset();
>     }

With the above implementation of auto_ptr, this results in U.B. at the point of auto_ptr::reset(). If this isn't obvious yet, let me explain how this goes step by step:

1. `foo::reset()` entered
2. `auto_ptr::reset()` entered
3. `auto_ptr::reset()` tries to delete `foo`
4. `foo::~foo()` entered, tries to destruct its members
5. `auto_ptr::~auto_ptr()` executed - `auto_ptr` is no longer a valid object!
6. `foo::~foo()` left
7. `auto_ptr::reset()` sets its "ptr" field to 0 <- U.B.! `auto_ptr` is not a valid object here already!

[ Thanks to Peter Dimov who recognized the connection to `unique_ptr` and brought this to the attention of the LWG, and helped with the solution. ]

[ Howard adds: ]

> To fix this behavior `reset` must be specified such that deleting the pointer is the last action to be taken within `reset`.

[ Alisdair adds: ]

> The example providing the rationale for LWG 998(i) is poor, as it relies on broken semantics of having two object believing they are unique owners of a single resource. It should not be surprising that UB results from such code, and I feel no need to go out of our way to support such behaviour.

> If an example is presented that does not imply multiple ownership of a unique resource, I would be much more ready to accept the proposed resolution.

[ Batavia (2009-05): ]

> Howard summarizes:

> > This issue has to do with circular ownership, and affects `auto_ptr`, too (but we don't really care about that). It is intended to spell out the order in which operations must be performed so as to avoid the possibility of undefined behavior in the self-referential case.

> Howard points to message c++std-lib-23175 for another example, requested by Alisdair.

> We agree with the issue and with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.3.1.3.6 [unique.ptr.single.modifiers], p5 (Effects clause for `reset`), and p6:

> -5- Effects: If `get() == nullptr` there are no effects. Otherwise `get_deleter()(get())`. Assigns `p` to the stored `pointer`, and then if the old value of the `pointer` is not equal to `nullptr`, calls `get_deleter()(`the old value of the `pointer)`. [Note: The order of these operations is significant because the call to `get_deleter()` may destroy `*this`. -- end note]

> -6- Postconditions: `get() == p`. [Note: The postcondition does not hold if the call to `get_deleter()` destroys `*this` since `this->get()` is no longer a valid expression. -- end note]

### 999(i). Taking the address of a function

Section: 26.11 [specialized.algorithms] Status: C++11 Submitter: Peter Dimov Opened: 2009-03-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The same fix (reference 987(i)) may be applied to `addressof`, which is also constrained to `ObjectType`. (That was why `boost::ref` didn't work with functions - it tried to apply `boost::addressof` and the `reinterpret_cast<char&>` implementation of `addressof` failed.)

[ Batavia (2009-05): ]

> We agree.

> Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-10 Daniel updates wording to concept-free. ]

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

[ The resolution assumes that `addressof` is reintroduced as described in n2946 ]

In 26.11 [specialized.algorithms] change as described:

>     template <class T> T* addressof(T& r);

> > Returns: The actual address of the object or function referenced by `r`, even in the presence of an overloaded `operator&`.

### 1004(i). Clarify "throws an exception"

Section: 16.4.5.8 [res.on.functions] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 179

According to the 4th bullet there is a problem if "if any replacement function or handler function or destructor operation throws an exception". There should be no problem throwing exceptions so long as they are caught within the function.

[ Batavia (2009-05): ]

> The phrasing "throws an exception" is commonly used elsewhere to mean "throws or propagates an exception." Move to Open pending a possible more general resolution.

[ 2009-07 Frankfurt: ]

> Replace "propagates" in the proposed resolution with the phrase "exits via" and move to Ready.

Proposed resolution:

Change the 4th bullet of 16.4.5.8 [res.on.functions], p2:

> - if any replacement function or handler function or destructor operation throws exits via an exception, unless specifically allowed in the applicable Required behavior: paragraph.

### 1006(i). `operator delete` in garbage collected implementation

Section: 17.6.3 [new.delete] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 190

It is not entirely clear how the current specification acts in the presence of a garbage collected implementation.

[ Summit: ]

> Agreed.

[ 2009-05-09 Alisdair adds: ]

> Proposed wording is too strict for implementations that do not support garbage collection. Updated wording supplied.

[ Batavia (2009-05): ]

> We recommend advancing this to Tentatively Ready with the understanding that it will not be moved for adoption unless and until the proposed resolution to Core issue #853 is adopted.

Proposed resolution:

(Editorial note: This wording ties into the proposed resolution for Core #853)

Add paragraphs to 17.6.3.2 [new.delete.single]:

>     void operator delete(void* ptr) throw();
>     void operator delete(void* ptr, const std::nothrow_t&) throw();

> [ The second signature deletion above is editorial. ]

> > Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -10- ...

>     void operator delete(void* ptr, const std::nothrow_t&) throw();

> > Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -15- ...

Add paragraphs to 17.6.3.3 [new.delete.array]:

>     void operator delete[](void* ptr) throw();
>     void operator delete[](void* ptr, const std::nothrow_t&) throw();

> [ The second signature deletion above is editorial. ]

> > Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -9- ...

>     void operator delete[](void* ptr, const std::nothrow_t&) throw();

> > Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -13- ...

Add paragraphs to 17.6.3.4 [new.delete.placement]:

>     void operator delete(void* ptr, void*) throw();

> > Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -7- ...

>     void operator delete[](void* ptr, void*) throw();

> > Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -9- ...

### 1011(i). `next/prev` wrong iterator type

Section: 24.4.3 [iterator.operations] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 271

`next/prev` return an incremented iterator without changing the value of the original iterator. However, even this may invalidate an `InputIterator`. A `ForwardIterator` is required to guarantee the 'multipass' property.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

1. Change header `<iterator>` synopsis 24.2 [iterator.synopsis] as indicated:

   >     // 24.4.4, iterator operations:
   >     ...
   >     template <class InputForwardIterator>
   >       InputForwardIterator
   >       next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);

2. Change 24.4.3 [iterator.operations] before p.6 as indicated:

   >     template <class InputForwardIterator>
   >       InputForwardIterator
   >       next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);

### 1012(i). `reverse_iterator` default ctor should value initialize

Section: 24.5.1.4 [reverse.iter.cons] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 277

The default constructor default-initializes current, rather than value-initializes. This means that when Iterator corresponds to a trivial type, the current member is left un-initialized, even when the user explictly requests value intialization! At this point, it is not safe to perform any operations on the reverse_iterator other than assign it a new value or destroy it. Note that this does correspond to the basic definition of a singular iterator.

[ Summit: ]

> Agree with option i.

Related issue: 408(i)

[ Batavia (2009-05): ]

> We believe this should be revisited in conjunction with issue 408(i), which nearly duplicates this issue. Move to Open.

[ 2009-07 post-Frankfurt: ]

> Change "constructed" to "initialized" in two places in the proposed resolution.

> Move to Tentatively Ready.

[ 2009 Santa Cruz: ]

> Moved to Ready for this meeting.

Proposed resolution:

Change [reverse.iter.con]:

>     reverse_iterator();

> > -1- Effects: Default Value initializes `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a default constructed value initialized iterator of type `Iterator`.

Change [move.iter.op.const]:

>     move_iterator();

> > -1- Effects: Constructs a `move_iterator`, default value initializing `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value initialized iterator of type `Iterator`.

### 1014(i). `basic_regex` should be created/assigned from initializer lists

Section: 28.6.7.2 [re.regex.construct] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 317 and JP 74

UK 317:

> `basic_string` has both a constructor and an assignment operator that accepts an initializer list, `basic_regex` should have the same.

JP 74:

> `basic_regex & operator= (initializer_list<T>);` is not defined.

[ Batavia (2009-05): ]

> UK 317 asks for both assignment and constructor, but the requested constructor is already present in the current Working Paper. We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 28.6.7 [re.regex]:

>     template <class charT,
>               class traits = regex_traits<charT> >
>     class basic_regex {
>       ...
>       basic_regex& operator=(const charT* ptr);
>       basic_regex& operator=(initializer_list<charT> il);
>       template <class ST, class SA>
>         basic_regex& operator=(const basic_string<charT, ST, SA>& p);
>       ...
>     };

Add in 28.6.7.2 [re.regex.construct]:

> > -20- ...

>     basic_regex& operator=(initializer_list<charT> il);

> > -21- Effects: returns `assign(il.begin(), il.end());`

### 1019(i). Make `integral_constant` objects useable in integral-constant-expressions

Section: 21.3.4 [meta.help] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 205 [CD1]

`integral_constant` objects should be usable in integral-constant-expressions. The addition to the language of literal types and the enhanced rules for constant expressions make this possible.

[ Batavia (2009-05): ]

> We agree that the `static` data member ought be declared `constexpr`, but do not see a need for the proposed `operator value_type()`. (A use case would be helpful.) Move to Open.

[ 2009-05-23 Alisdair adds: ]

> The motivating case in my mind is that we can then use `true_type` and `false_type` as integral Boolean expressions, for example inside a `static_assert` declaration. In that sense it is purely a matter of style.

> Note that Boost has applied the non-explicit conversion operator for many years as it has valuable properties for extension into other metaprogramming libraries, such as MPL. If additional rationale is desired I will poll the Boost lists for why this extension was originally applied. I would argue that explicit conversion is more appropriate for 0x though.

[ 2009-07-04 Howard adds: ]

> Here's a use case which demonstrates the syntactic niceness which Alisdair describes:

> >     #define requires(...) class = typename std::enable_if<(__VA_ARGS__)>::type

> >     template <class T, class U,
> >         requires(!is_lvalue_reference<T>() ||
> >                   is_lvalue_reference<T>() && is_lvalue_reference<U>()),
> >         requires(is_same<typename base_type<T>::type,
> >                          typename base_type<U>::type>)>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >         return static_cast<T&&>(t);
> >     }

[ 2009-07 post-Frankfurt: ]

> Move to Tentatively Ready.

[ 2009 Santa Cruz: ]

> Moved to Ready for this meeting.

Proposed resolution:

Add to the `integral_constant` struct definition in 21.3.4 [meta.help]:

>     template <class T, T v>
>     struct integral_constant {
>       static constexpr T value = v;
>       typedef T value_type;
>       typedef integral_constant<T,v> type;
>       constexpr operator value_type() { return value; }
>     };

### 1021(i). Allow `nullptr_t` assignments to `unique_ptr`

Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 211 [CD1]

The `nullptr_t` type was introduced to resolve the null pointer literal problem. It should be used for the assignment operator, as with the constructor and elsewhere through the library.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change the synopsis in 20.3.1.3 [unique.ptr.single]:

>     unique_ptr& operator=(unspecified-pointer-type nullptr_t);

Change 20.3.1.3.4 [unique.ptr.single.asgn]:

>     unique_ptr& operator=(unspecified-pointer-type nullptr_t);

> > Assigns from the literal 0 or `NULL`. [Note: The unspecified-pointer-type is often implemented as a pointer to a private data member, avoiding many of the implicit conversion pitfalls. — end note]

### 1030(i). Missing requirements for smart-pointer safety API

Section: 99 [depr.util.smartptr.shared.atomic] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2017-11-29

Priority: Not Prioritized

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Discussion:

Addresses JP 44 [CD1]

The 1st parameter `p` and 2nd parameter `v` is now `shared_ptr<T>*`.

It should be `shared_ptr<T>&`, or if these are `shared_ptr<T>*` then add the "`p` shall not be a null pointer" at the requires.

[ Summit: ]

> Agree. All of the functions need a requirement that `p` (or `v`) is a pointer to a valid object.

[ 2009-07 post-Frankfurt: ]

> Lawrence explained that these signatures match the regular atomics. The regular atomics must not use references because these signatures are shared with C. The decision to pass shared_ptrs by pointer rather than by reference was deliberate and was motivated by the principle of least surprise.

> Lawrence to write wording that requires that the pointers not be null.

[ 2009-09-20 Lawrence provided wording: ]

> The parameter types for atomic shared pointer access were deliberately chosen to be pointers to match the corresponding parameters of the atomics chapter. Those in turn were deliberately chosen to match C functions, which do not have reference parameters.

> We adopt the second suggestion, to require that such pointers not be null.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

In section "`shared_ptr` atomic access" [util.smartptr.shared.atomic], add to each function the following clause.

> Requires: `p` shall not be null.

### 1033(i). `thread::join()` effects?

Section: 32.4.3.6 [thread.thread.member] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

While looking at `thread::join()` I think I spotted a couple of possible defects in the specifications. I could not find a previous issue or NB comment about that, but I might have missed it.

The postconditions clause for `thread::join()` is:

> Postconditions: If `join()` throws an exception, the value returned by `get_id()` is unchanged. Otherwise, `get_id() == id()`.

and the throws clause is:

> Throws: `std::system_error` when the postconditions cannot be achieved.

Now... how could the postconditions not be achieved? It's just a matter of resetting the value of `get_id()` or leave it unchanged! I bet we can always do that. Moreover, it's a chicken-and-egg problem: in order to decide whether to throw or not I depend on the postconditions, but the postconditions are different in the two cases.

I believe the throws clause should be:

> Throws: `std::system_error` when the effects or postconditions cannot be achieved.

as it is in `detach()`, or, even better, as the postcondition is trivially satisfiable and to remove the circular dependency:

> Throws: `std::system_error` if the effects cannot be achieved.

Problem is that... ehm... `join()` has no "Effects" clause. Is that intentional?

[ See the thread starting at c++std-lib-23204 for more discussion. ]

[ Batavia (2009-05): ]

> Pete believes there may be some more general language (in frontmatter) that can address this and related issues such as 962(i).

> Move to Open.

[ 2009-11-18 Anthony provides wording. ]

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Edit 32.4.3.6 [thread.thread.member] as indicated:

>     void join();

> > 5 Precondition: `joinable()` is `true`.

> > Effects: Blocks until the thread represented by `*this` has completed.

> > 6 Synchronization: The completion of the thread represented by `*this` happens before (6.10.2 [intro.multithread]) `join()` returns. [Note: Operations on `*this` are not synchronized. — end note]

> > 7 Postconditions: If `join()` throws an exception, the value returned by `get_id()` is unchanged. Otherwise, The thread represented by `*this` has completed. `get_id() == id()`.

> > 8 ...

### 1034(i). Clarify generality of Container Requirement tables

Section: 23.2.2 [container.requirements.general] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 222 [CD1]

It is not clear what purpose the Requirement tables serve in the Containers clause. Are they the definition of a library Container? Or simply a conventient shorthand to factor common semantics into a single place, simplifying the description of each subsequent container? This becomes an issue for 'containers' like `array`, which does not meet the default-construct-to-empty requirement, or `forward_list` which does not support the size operation. Are these components no longer containers? Does that mean the remaining requirements don't apply? Or are these contradictions that need fixing, despite being a clear design decision?

Recommend:

Clarify all the tables in 23.2 [container.requirements] are there as a convenience for documentation, rather than a strict set of requirements. Containers should be allowed to relax specific requirements if they call attention to them in their documentation. The introductory text for `array` should be expanded to mention a default constructed `array` is not empty, and `forward_list` introduction should mention it does not provide the required `size` operation as it cannot be implemented efficiently.

[ Summit: ]

> Agree in principle.

[ 2009-07 post-Frankfurt: ]

> We agree in principle, but we have a timetable. This group feels that the issue should be closed as NAD unless a proposed resolution is submitted prior to the March 2010 meeting.

[ 2009-10 Santa Cruz: ]

> Looked at this and still intend to close as NAD in March 2010 unless there is proposed wording that we like.

[ 2010-02-02 Nicolai M. Josuttis updates proposed wording and adds: ]

> I just came across issue #1034 (response to UK 222), which covers the role of container requirements. The reason I found this issue was that I am wondering why `array<>` is specified to be a sequence container. For me, currently, this follows from Sequence containers 23.2.4 [sequence.reqmts] saying:

> > The library provides five basic kinds of sequence containers: `array`, `vector`, `forward_list`, `list`, and `deque`. while later on in Table 94 "Sequence container requirements" are defined.

> IMO, you can hardly argue that this is NAD. We MUST say somewhere that either array is not a sequence container or does not provide all operations of a sequence container (even not all requirements of a container in general).

> Here is the number of requirements `array<>` does not meet (AFAIK):

> general container requirements:

> - a default constructed `array` is not empty
> - `swap` has no constant complexity

> Note also that `swap` not only has linear complexity it also invalidates iterators (or to be more precise, assigns other values to the elements), which is different from the effect swap has for other containers. For this reason, I must say that i tend to propose to remove `swap()` for `arrays`.

> sequence container requirements:

> - There is no constructor and assignment for a range
> - There is no constructor and assignment for `n` copies of `t`
> - There are no `emplace`, `insert`, `erase`, `clear`, `assign` operations

> In fact, out of all sequence container requirements `array<>` only provides the following operations: from sequence requirements (Table 94):

> >     X(il);
> >     a = il;

> and from optional requirements (Table 95):

> >     [], at(), front(), back()

> This is almost nothing!

> Note in addition, that due to the fact that `array` is an aggregate and not a container with `initializer_lists` a construction or assignment with an initializer list is valid for all sequence containers but not valid for array:

> >     vector<int>  v({1,2,3});   // OK
> >     v = {4,5,6};               // OK

> >     array<int,3> a({1,2,3});   // Error
> >     array<int,3> a = {1,2,3};  // OK
> >     a = {4,5,6};               // Error

> BTW, for this reason, I am wondering, why `<array>` includes `<initializer_list>`.

> IMO, we can't really say that `array` is a sequence container. `array` is special. As the solution to this issue seemed to miss some proposed wording where all could live with, let me try to suggest some.

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: Ok with move to Ready except for "OPEN:" part. ]

Proposed resolution:

In Sequence containers 23.2.4 [sequence.reqmts] modify paragraph 1 as indicated:

> 1 A sequence container organizes a finite set of objects, all of the same type, into a strictly linear arrangement. The library provides five four basic kinds of sequence containers: `array`, `vector`, `forward_list`, `list`, and `deque`. In addition, `array` is provided as a sequence container that only provides limited sequence operations because it has a fixed number of elements. It The library also provides container adaptors that make it easy to construct abstract data types, such as `stack`s or `queue`s, out of the basic sequence container kinds (or out of other kinds of sequence containers that the user might define).

Modify paragraph 2 as follows (just editorial):

> 2 The five basic sequence containers offer the programmer different complexity trade-offs and should be used accordingly. `vector` or `array` is the type of sequence container that should be used by default. `list` or `forward_list` should be used when there are frequent insertions and deletions from the middle of the sequence. `deque` is the data structure of choice when most insertions and deletions take place at the beginning or at the end of the sequence.

In Class template array 23.3.3 [array] modify paragraph 3 as indicated:

> 3 Unless otherwise specified, all `array` operations are as described in 23.2. An array satisfies all of the requirements of a container and of a reversible container (given in two tables in 23.2 [container.requirements]) except that a default constructed `array` is not empty, `swap` does not have constant complexity, and `swap` may throw exceptions. An `array` satisfies some of the requirements of a sequence container (given in 23.2.4 [sequence.reqmts]). Descriptions are provided here only for operations on `array` that are not described in that Clause in one of these tables or for operations where there is additional semantic information.

In array specialized algorithms 23.3.3.4 [array.special] add to the specification of `swap()`:

>     template <class T, size_t N> void swap(array<T,N>& x, array<T,N>& y);

> > 1 Effects: ...

> > Complexity: Linear in `N`.

### 1037(i). Unclear status of `match_results` as library container

Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 232 [CD1]

`match_results` may follow the requirements but is not listed a general purpose library container.

Remove reference to `match_results` against `a[n]` operation.

[ Summit: ]

> Agree. `operator[]` is defined elsewhere.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.4 [sequence.reqmts] Table 84, remove reference to `match_results` in the row describing the `a[n]` operation.

### 1038(i). Sequence requirement table needs to reference several new containers

Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 233 [CD1]

Table 84 is missing references to several new container types.

[ Summit: ]

> Agree.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.4 [sequence.reqmts] Table 84, Add reference to listed containers to the following rows:

> Table 84 -- Optional sequence container operations
> Expression | Return type | Operational semantics | Container
> --- | --- | --- | ---
> `a.front()` | ... | ... | `vector, list, deque, basic_string, array, forward_list`
> `a.back()` | ... | ... | `vector, list, deque, basic_string, array`
> `a.emplace_front(args)` | ... | ... | `list, deque, forward_list`
> `a.push_front(t)` | ... | ... | `list, deque, forward_list`
> `a.push_front(rv)` | ... | ... | `list, deque, forward_list`
> `a.pop_front()` | ... | ... | `list, deque, forward_list`
> `a[n]` | ... | ... | `vector, deque, basic_string, array`
> `a.at(n)` | ... | ... | `vector, deque, basic_string, array`

### 1039(i). Sequence container `back` function should also support `const_iterator`

Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 234 [CD1]

The reference to `iterator` in semantics for `back` should also allow for `const_iterator` when called on a const-qualified container. This would be ugly to specify in the 03 standard, but is quite easy with the addition of `auto` in this new standard.

[ Summit: ]

> Agree.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.4 [sequence.reqmts] Table 84, replace iterator with auto in semantics for back:

> Table 84 — Optional sequence container operations
> Expression | Return type | Operational semantics | Container
> --- | --- | --- | ---
> `a.back()` | `reference; const_reference` for constant `a` | `{ iterator auto tmp = a.end(); --tmp; return *tmp; }` | `vector, list, deque, basic_string`

### 1040(i). Clarify possible sameness of associative container's `iterator` and `const_iterator`

Section: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 238 [CD1]

Leaving it unspecified whether or not `iterator` and `const_iterator` are the same type is dangerous, as user code may or may not violate the One Definition Rule by providing overloads for both types. It is probably too late to specify a single behaviour, but implementors should document what to expect. Observing that problems can be avoided by users restricting themselves to using `const_iterator`, add a note to that effect.

Suggest Change 'unspecified' to 'implementation defined'.

[ Summit: ]

> Agree with issue. Agree with adding the note but not with changing the normative text. We believe the note provides sufficient guidance.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.7 [associative.reqmts] p6, add:

> -6- `iterator` of an associative container meets the requirements of the `BidirectionalIterator` concept. For associative containers where the value type is the same as the key type, both `iterator` and `const_iterator` are constant iterators. It is unspecified whether or not `iterator` and `const_iterator` are the same type. [Note: `iterator` and `const_iterator` have identical semantics in this case, and `iterator` is convertible to `const_iterator`. Users can avoid violating the One Definition Rule by always using `const_iterator` in their function parameter lists -- end note]

### 1041(i). Add associative/unordered container functions that allow to extract elements

Section: 23.2.7 [associative.reqmts] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses UK 239 [CD1]

It is not possible to take a move-only key out of an unordered container, such as (`multi`)`set` or (`multi`)`map`, or the new unordered containers.

Add below `a.erase(q)`, `a.extract(q)`, with the following notation:

`a.extract(q)>`, Return type `pair<key, iterator>` Extracts the element pointed to by `q` and erases it from the `set`. Returns a `pair` containing the value pointed to by `q` and an `iterator` pointing to the element immediately following `q` prior to the element being erased. If no such element exists,returns `a.end()`.

[ Summit: ]

> We look forward to a paper on this topic. We recommend no action until a paper is available. The paper would need to address exception safety.

[ Post Summit Alisdair adds: ]

> Would `value_type` be a better return type than `key_type`?

[ 2009-07 post-Frankfurt: ]

> Leave Open. Alisdair to contact Chris Jefferson about this.

[ 2009-09-20 Howard adds: ]

> See the 2009-09-19 comment of 839(i) for an API which accomplishes this functionality and also addresses several other use cases which this proposal does not.

[ 2009-10 Santa Cruz: ]

> Mark as NAD Future. No consensus to make the change at this time.

Original resolution [SUPERSEDED]:

> In 23.2.7 [associative.reqmts] Table 85, add:

> > Table 85 -- Associative container requirements (in addition to container)
> > Expression | Return type | Assertion/note pre-/post-condition | Complexity
> > --- | --- | --- | ---
> > `a.erase(q)` | ... | ... | ...
> > `a.extract(q)` | `pair<key_type, iterator>` | Extracts the element pointed to by `q` and erases it from the `set`. Returns a `pair` containing the value pointed to by `q` and an `iterator` pointing to the element immediately following `q` prior to the element being erased. If no such element exists, returns `a.end()`. | amortized constant

> In 23.2.8 [unord.req] Table 87, add:

> > Table 87 -- Unordered associative container requirements (in addition to container)
> > Expression | Return type | Assertion/note pre-/post-condition | Complexity
> > --- | --- | --- | ---
> > `a.erase(q)` | ... | ... | ...
> > `a.extract(q)` | `pair<key_type, iterator>` | Extracts the element pointed to by `q` and erases it from the `set`. Returns a `pair` containing the value pointed to by `q` and an `iterator` pointing to the element immediately following `q` prior to the element being erased. If no such element exists, returns `a.end()`. | amortized constant

[08-2016, Post-Chicago]

Move to Tentatively Resolved

Proposed resolution:

This functionality is provided by P0083R3

### 1043(i). Clarify that `compare_exchange` is not a read-modify-write operation

Section: 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US 91 [CD1]

It is unclear whether or not a failed `compare_exchange` is a RMW operation (as used in 6.10.2 [intro.multithread]).

Suggested solution:

Make failing `compare_exchange` operations not be RMW.

[ Anthony Williams adds: ]

> In 32.5.8.2 [atomics.types.operations] p18 it says that "These operations are atomic read-modify-write operations" (final sentence). This is overly restrictive on the implementations of `compare_exchange_weak` and `compare_exchange_strong` on platforms without a native CAS instruction.

[ Summit: ]

> Group agrees with the resolution as proposed by Anthony Williams in the attached note.

[ Batavia (2009-05): ]

> We recommend the proposed resolution be reviewed by members of the Concurrency Subgroup.

[ 2009-07 post-Frankfurt: ]

> This is likely to be addressed by Lawrence's upcoming paper. He will adopt the proposed resolution.

[ 2009-08-17 Handled by N2925. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2992.

Proposed resolution:

Change 32.5.8.2 [atomics.types.operations] p18:

> -18- Effects: Atomically, compares the value pointed to by `object` or by `this` for equality with that in `expected`, and if true, replaces the value pointed to by `object` or by `this` with desired, and if false, updates the value in `expected` with the value pointed to by `object` or by `this`. Further, if the comparison is true, memory is affected according to the value of `success`, and if the comparison is false, memory is affected according to the value of `failure`. When only one `memory_order` argument is supplied, the value of `success` is `order`, and the value of `failure` is `order` except that a value of `memory_order_acq_rel` shall be replaced by the value `memory_order_acquire` and a value of `memory_order_release` shall be replaced by the value `memory_order_relaxed`. If the comparison is `true`, Tthese operations are atomic read-modify-write operations (1.10). If the comparison is `false`, these operations are atomic load operations.

### 1044(i). Empty tag types should be `constexpr` literals

Section: 32.6 [thread.mutex] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 325 [CD1]

We believe `constexpr` literal values should be a more natural expression of empty tag types than extern objects as it should improve the compiler's ability to optimize the empty object away completely.

[ Summit: ]

> Move to review. The current specification is a "hack", and the proposed specification is a better "hack".

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change the synopsis in 32.6 [thread.mutex]:

>     struct defer_lock_t {};
>     struct try_to_lock_t {};
>     struct adopt_lock_t {};

>     extern constexpr defer_lock_t defer_lock {};
>     extern constexpr try_to_lock_t try_to_lock {};
>     extern constexpr adopt_lock_t adopt_lock {};

### 1045(i). Remove unnecessary preconditions from `unique_lock` constructor

Section: 32.6.5.4.2 [thread.lock.unique.cons] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 326 [CD1]

The precondition that the mutex is not owned by this thread offers introduces the risk of unnecessary undefined behaviour into the program. The only time it matters whether the current thread owns the mutex is in the lock operation, and that will happen subsequent to construction in this case. The lock operation has the identical pre-condition, so there is nothing gained by asserting that precondition earlier and denying the program the right to get into a valid state before calling lock.

[ Summit: ]

> Agree, move to review.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Strike 32.6.5.4.2 [thread.lock.unique.cons] p7:

>     unique_lock(mutex_type& m, defer_lock_t);

> > -7- Precondition: If `mutex_type` is not a recursive mutex the calling thread does not own the mutex.

### 1046(i). Provide simple facility to start asynchronous operations

Section: 32.10 [futures] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 329 [CD1]

`future`, `promise` and `packaged_task` provide a framework for creating future values, but a simple function to tie all three components together is missing. Note that we only need a simple facility for C++0x. Advanced thread pools are to be left for TR2.

Simple Proposal:

Provide a simple function along the lines of:

>     template< typename F, typename ... Args >
>       requires Callable< F, Args... >
>         future< Callable::result_type > async( F&& f, Args && ... );

Semantics are similar to creating a `thread` object with a `packaged_task` invoking `f` with `forward<Args>(args...)` but details are left unspecified to allow different scheduling and thread spawning implementations.

It is unspecified whether a task submitted to async is run on its own thread or a thread previously used for another async task. If a call to `async` succeeds, it shall be safe to wait for it from any thread.

The state of `thread_local` variables shall be preserved during `async` calls.

No two incomplete async tasks shall see the same value of `this_thread::get_id()`.

[Note: this effectively forces new tasks to be run on a new thread, or a fixed-size pool with no queue. If the library is unable to spawn a new thread or there are no free worker threads then the `async` call should fail. --end note]

[ Summit: ]

> The concurrency subgroup has revisited this issue and decided that it could be considered a defect according to the Kona compromise. A task group was formed lead by Lawrence Crowl and Bjarne Stroustrup to write a paper for Frankfort proposing a simple asynchronous launch facility returning a `future`. It was agreed that the callable must be run on a separate thread from the caller, but not necessarily a brand-new thread. The proposal might or might not allow for an implementation that uses fixed-size or unlimited thread pools.

> Bjarne in c++std-lib-23121: I think that what we agreed was that to avoid deadlock `async()` would almost certainly be specified to launch in a different thread from the thread that executed `async()`, but I don't think it was a specific design constraint.

[ 2009-10 Santa Cruz: ]

> Proposed resolution: see N2996 (Herb's and Lawrence's paper on Async). Move state to NAD editorialResolved.

Proposed resolution:

### 1047(i). Ensure that future's `get()` blocks when not ready

Section: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Addresses UK 334 [CD1]

Behaviour of `get()` is undefined if calling `get()` while not `is_ready()`. The intent is that `get()` is a blocking call, and will wait for the future to become ready.

[ Summit: ]

> Agree, move to Review.

[ 2009-04-03 Thomas J. Gritzan adds: ]

> This issue also applies to `shared_future::get()`.

> Suggested wording:

> Add a paragraph to [futures.shared_future]:

> >     void shared_future<void>::get() const;

> > > Effects: If `is_ready()` would return `false`, block on the asynchronous result associated with `*this`.

[ Batavia (2009-05): ]

> It is not clear to us that this is an issue, because the proposed resolution's Effects clause seems to duplicate information already present in the Synchronization clause. Keep in Review status.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2997.

Proposed resolution:

Add a paragraph to [futures.unique_future]:

>     R&& unique_future::get();
>     R& unique_future<R&>::get();
>     void unique_future<void>::get();

> > Note:...

> > Effects: If `is_ready()` would return `false`, block on the asynchronous result associated with `*this`.

> > Synchronization: if `*this` is associated with a `promise` object, the completion of `set_value()` or `set_exception()` to that `promise` happens before (1.10) `get()` returns.

### 1048(i). Provide empty-state inspection for `std::unique_future`

Section: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Addresses UK 335 [CD1]

`std::unique_future` is `MoveConstructible`, so you can transfer the association with an asynchronous result from one instance to another. However, there is no way to determine whether or not an instance has been moved from, and therefore whether or not it is safe to wait for it.

>     std::promise<int> p;
>     std::unique_future<int> uf(p.get_future());
>     std::unique_future<int> uf2(std::move(uf));
>     uf.wait(); // oops, uf has no result to wait for.

Suggest we add a `waitable()` function to `unique_future` (and `shared_future`) akin to `std::thread::joinable()`, which returns `true` if there is an associated result to wait for (whether or not it is ready).

Then we can say:

>     if(uf.waitable()) uf.wait();

[ Summit: ]

> Create an issue. Requires input from Howard. Probably NAD.

[ Post Summit, Howard throws in his two cents: ]

> Here is a copy/paste of my last prototype of `unique_future` which was several years ago. At that time I was calling `unique_future` `future`:

> >     template <class R>
> >     class future
> >     {
> >     public:
> >         typedef R result_type;
> >     private:
> >         future(const future&);// = delete;
> >         future& operator=(const future&);// = delete;

> >         template <class R1, class F1> friend class prommise;
> >     public:
> >         future();
> >         ~future();

> >         future(future&& f);
> >         future& operator=(future&& f);

> >         void swap(future&& f);

> >         bool joinable() const;
> >         bool is_normal() const;
> >         bool is_exceptional() const;
> >         bool is_ready() const;

> >         R get();

> >         void join();
> >         template <class ElapsedTime>
> >             bool timed_join(const ElapsedTime&);
> >     };

> `shared_future` had a similar interface. I intentionally reused the `thread` interface where possible to lessen the learning curve std::lib clients will be faced with.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2997.

Proposed resolution:

### 1049(i). Move assignment of `promise` inverted

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 339 [CD1]

Move assignment is going in the wrong direction, assigning from `*this` to the passed rvalue, and then returning a reference to an unusable `*this`.

[ Summit: ]

> Agree, move to Review.

[ Batavia (2009-05): ]

> We recommend deferring this issue until after Detlef's paper (on futures) has been issued.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2997.

Proposed resolution:

Strike 32.10.6 [futures.promise] p6 and change p7:

>     promise& operator=(promise&& rhs);

> > -6- Effects: move assigns its associated state to `rhs`.

> > -7- Postcondition: `*this` has no associated state. associated state of `*this` is the same as the associated state of `rhs` before the call. `rhs` has no associated state.

### 1050(i). Clarify postconditions for `get_future()`

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 340 [CD1]

There is an implied postcondition for `get_future()` that the state of the `promise` is transferred into the `future` leaving the `promise` with no associated state. It should be spelled out.

[ Summit: ]

> Agree, move to Review.

[ 2009-04-03 Thomas J. Gritzan adds: ]

> `promise::get_future()` must not invalidate the state of the promise object.

> A promise is used like this:

> >     promise<int> p;
> >     unique_future<int> f = p.get_future();
> >     // post 'p' to a thread that calculates a value
> >     // use 'f' to retrieve the value.

> So `get_future()` must return an object that shares the same associated state with `*this`.

> But still, this function should throw an `future_already_retrieved` error when it is called twice.

> `packaged_task::get_future()` throws `std::bad_function_call` if its `future` was already retrieved. It should throw `future_error(future_already_retrieved)`, too.

> Suggested resolution:

> Replace p12/p13 32.10.6 [futures.promise]:

> > -12- Throws: `future_error` if `*this` has no associated state the `future` has already been retrieved.

> > -13- Error conditions: `future_already_retrieved` if `*this` has no associated state the `future` associated with the associated state has already been retrieved.

> > Postcondition: The returned object and `*this` share the associated state.

> Replace p14 32.10.10 [futures.task]:

> > -14- Throws: `std::bad_function_call future_error` if the future associated with the task has already been retrieved.

> > Error conditions: `future_already_retrieved` if the `future` associated with the task has already been retrieved.

> > Postcondition: The returned object and `*this` share the associated task.

[ Batavia (2009-05): ]

> Keep in Review status pending Detlef's forthcoming paper on futures.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2997.

Proposed resolution:

Add after p13 32.10.6 [futures.promise]:

>     unique_future<R> get_future();

> > -13- ...

> > Postcondition: `*this` has no associated state.

### 1052(i). `reverse_iterator::operator->` should also support smart pointers

Section: 24.5.1.6 [reverse.iter.elem] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06

Priority: Not Prioritized

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Duplicate of: 2775

Discussion:

Addresses UK 281 [CD1]

The current specification for return value for `reverse_iterator::operator->` will always be a true pointer type, but `reverse_iterator` supports proxy iterators where the pointer type may be some kind of 'smart pointer'.

[ Summit: ]

> `move_iterator` avoids this problem by returning a value of the wrapped Iterator type. study group formed to come up with a suggested resolution.

> `move_iterator` solution shown in proposed wording.

[ 2009-07 post-Frankfurt: ]

> Howard to deconceptize. Move to Review after that happens.

[ 2009-08-01 Howard deconceptized: ]

[ 2009-10 Santa Cruz: ]

> We can't think of any reason we can't just define reverse iterator's pointer types to be the same as the underlying iterator's pointer type, and get it by calling the right arrow directly.

> Here is the proposed wording that was replaced:

> >     template <class Iterator>
> >     class reverse_iterator {
> >       […]
> >       typedef typename iterator_traits<Iterator>::pointer pointer;

> Change [reverse.iter.opref]:

> >     pointer operator->() const;

> > > Returns:

> > > >     &(operator*());
> > > >     this->tmp = current;
> > > >     --this->tmp;
> > > >     return this->tmp;

[ 2010-03-03 Daniel opens: ]

> 1. There is a minor problem with the exposition-only declaration of the private member `deref_tmp` which is modified in a const member function (and the same problem occurs in the specification of `operator*`). The fix is to make it a mutable member.

> 2. The more severe problem is that the resolution for some reasons does not explain in the rationale why it was decided to differ from the suggested fix (using `deref_tmp` instead of `tmp`) in the [ 2009-10 Santa Cruz] comment:

>    >     this->deref_tmp = current;
>    >     --this->deref_tmp;
>    >     return this->deref_tmp;

>    combined with the change of

>    >     typedef typename iterator_traits<Iterator>::pointer pointer;

>    to

>    >     typedef Iterator pointer;

>    The problem of the agreed on wording is that the following rather typical example, that compiled with the wording before 1052 had been applied, won't compile anymore:

>    >     #include <iterator>
>    >     #include <utility>

>    >     int main() {
>    >       typedef std::pair<int, double> P;
>    >       P op;
>    >       std::reverse_iterator<P*> ri(&op + 1);
>    >       ri->first; // Error
>    >     }

>    Comeau online returns (if a correspondingly changed `reverse_iterator` is used):

>    >     "error: expression must have class type
>    >          return deref_tmp.operator->();
>    >                 ^
>    >              detected during instantiation of "Iterator
>    >                        reverse_iterator<Iterator>::operator->() const [with
>    >                        Iterator=std::pair<int, double> *]""

>    Thus the change will break valid, existing code based on `std::reverse_iterator`.

> IMO the suggestion proposed in the comment is a necessary fix, which harmonizes with the similar specification of `std::move_iterator` and properly reflects the recursive nature of the evaluation of `operator->` overloads.

> Suggested resolution:

> 1. In the class template `reverse_iterator` synopsis of 24.5.1.2 [reverse.iterator] change as indicated:

>    >     namespace std {
>    >     template <class Iterator>
>    >     class reverse_iterator : public
>    >                  iterator<typename iterator_traits<Iterator>::iterator_category,
>    >                  typename iterator_traits<Iterator>::value_type,
>    >                  typename iterator_traits<Iterator>::difference_type,
>    >                  typename iterator_traits<Iterator>::pointer,
>    >                  typename iterator_traits<Iterator>::reference> {
>    >     public:
>    >       [..]
>    >       typedef typename iterator_traits<Iterator>::pointer pointer;
>    >       [..]
>    >     protected:
>    >       Iterator current;
>    >     private:
>    >       mutable Iterator deref_tmp; // exposition only
>    >     };

> 2. Change [reverse.iter.opref]/1 as indicated:

>    >     pointer operator->() const;

>    > > 1 Returns Effects: `&(operator*())`.

>    > > >     deref_tmp = current;
>    > > >     --deref_tmp;
>    > > >     return deref_tmp;

[ 2010 Pittsburgh: ]

> We prefer to make to use a local variable instead of `deref_tmp` within `operator->()`. And although this means that the `mutable` change is no longer needed, we prefer to keep it because it is needed for `operator*()` anyway.

> Here is the proposed wording that was replaced:

> > Change [reverse.iter.opref]:

> > >     pointer operator->() const;

> > > > Returns:

> > > > >     &(operator*());
> > > > >     deref_tmp = current;
> > > > >     --deref_tmp;
> > > > >     return deref_tmp::operator->();

[ 2010-03-10 Howard adds: ]

> Here are three tests that the current proposed wording passes, and no other solution I've seen passes all three:

> 1. Proxy pointer support:

>    >     #include <iterator>
>    >     #include <cassert>

>    >     struct X { int m; };

>    >     X x;

>    >     struct IterX {
>    >         typedef std::bidirectional_iterator_tag iterator_category;
>    >         typedef X& reference;
>    >         struct pointer
>    >         {
>    >             pointer(X& v) : value(v) {}
>    >             X& value;
>    >             X* operator->() const {return &value;}
>    >         };
>    >         typedef std::ptrdiff_t difference_type;
>    >         typedef X value_type;
>    >         // additional iterator requirements not important for this issue

>    >         reference operator*() const { return x; }
>    >         pointer operator->() const { return pointer(x); }
>    >         IterX& operator--() {return *this;}

>    >     };

>    >     int main()
>    >     {
>    >         std::reverse_iterator<IterX> ix;
>    >         assert(&ix->m == &(*ix).m);
>    >     }

> 2. Raw pointer support:

>    >     #include <iterator>
>    >     #include <utility>

>    >     int main() {
>    >       typedef std::pair<int, double> P;
>    >       P op;
>    >       std::reverse_iterator<P*> ri(&op + 1);
>    >       ri->first; // Error
>    >     }

> 3. Caching iterator support:

>    >     #include <iterator>
>    >     #include <cassert>

>    >     struct X { int m; };

>    >     struct IterX {
>    >         typedef std::bidirectional_iterator_tag iterator_category;
>    >         typedef X& reference;
>    >         typedef X* pointer;
>    >         typedef std::ptrdiff_t difference_type;
>    >         typedef X value_type;
>    >         // additional iterator requirements not important for this issue

>    >         reference operator*() const { return value; }
>    >         pointer operator->() const { return &value; }
>    >         IterX& operator--() {return *this;}

>    >     private:
>    >         mutable X value;
>    >     };

>    >     int main()
>    >     {
>    >         std::reverse_iterator<IterX> ix;
>    >         assert(&ix->m == &(*ix).m);
>    >     }

[ 2010 Pittsburgh: ]

> Moved to NAD Future, rationale added.

[LEWG Kona 2017]

Recommend that we accept the "Alternate Proposed Resolution" from 2775(i).

[ Original rationale: ]

The LWG did not reach a consensus for a change to the WP.

Previous resolution [SUPERSEDED]:

> 1. In the class template `reverse_iterator` synopsis of 24.5.1.2 [reverse.iterator] change as indicated:

>    >     namespace std {
>    >     template <class Iterator>
>    >     class reverse_iterator : public
>    >                  iterator<typename iterator_traits<Iterator>::iterator_category,
>    >                  typename iterator_traits<Iterator>::value_type,
>    >                  typename iterator_traits<Iterator>::difference_type,
>    >                  typename iterator_traits<Iterator&>::pointer,
>    >                  typename iterator_traits<Iterator>::reference> {
>    >     public:
>    >       [..]
>    >       typedef typename iterator_traits<Iterator&>::pointer pointer;
>    >       [..]
>    >     protected:
>    >       Iterator current;
>    >     private:
>    >       mutable Iterator deref_tmp; // exposition only
>    >     };

> 2. Change [reverse.iter.opref]/1 as indicated:

>    >     pointer operator->() const;

>    > > 1 Returns Effects: `&(operator*())`.

>    > > >     deref_tmp = current;
>    > > >     --deref_tmp;
>    > > >     return deref_tmp;

> [Alternate Proposed Resolution from 2775(i), which was closed as a dup of this issue]

> This wording is relative to N4606.

> 1. Modify [reverse.iter.opref] as indicated:

>    >     constexpr pointer operator->() const;

>    > > -1- Returns: `addressof(operator*())`.Effects: If `Iterator` is a pointer type, as if by:

>    > > >     Iterator tmp = current;
>    > > >     return --tmp;

>    > > Otherwise, as if by:

>    > > >     Iterator tmp = current;
>    > > >     --tmp;
>    > > >     return tmp.operator->();

[2018-08-06; Daniel rebases to current working draft and reduces the proposed wording to that preferred by LEWG]

[2018-11-13; Casey Carter comments]

The acceptance of P0896R4 during the San Diego meeting resolves this issue: The wording in [reverse.iter.elem] for `operator->` is equivalent to the PR for LWG 1052.

Proposed resolution:

This wording is relative to N4762.

1. Modify 24.5.1.6 [reverse.iter.elem] as indicated:

   >     constexpr pointer operator->() const;

   > > -2- Returns: `addressof(operator*())`.Effects: If `Iterator` is a pointer type, as if by:

   > > >     Iterator tmp = current;
   > > >     return --tmp;

   > > Otherwise, as if by:

   > > >     Iterator tmp = current;
   > > >     --tmp;
   > > >     return tmp.operator->();

### 1054(i). `forward` broken

Section: 22.2.4 [forward] Status: Resolved Submitter: Howard Hinnant Opened: 2009-03-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [forward].

View all issues with Resolved status.

Discussion:

This is a placeholder issue to track the fact that we (well I) put the standard into an inconsistent state by requesting that we accept N2844 except for the proposed changes to [forward].

There will exist in the post meeting mailing N2835 which in its current state reflects the state of affairs prior to the Summit meeting. I hope to update it in time for the post Summit mailing, but as I write this issue I have not done so yet.

[ Batavia (2009-05): ]

> Move to Open, awaiting the promised paper.

[ 2009-08-02 Howard adds: ]

> My current preferred solution is:

> >     template <class T>
> >     struct __base_type
> >     {
> >        typedef typename remove_cv<typename remove_reference<T>::type>::type type;
> >     };

> >     template <class T, class U,
> >        class = typename enable_if<
> >            !is_lvalue_reference<T>::value ||
> >             is_lvalue_reference<T>::value &&
> >             is_lvalue_reference<U>::value>::type,
> >        class = typename enable_if<
> >             is_same<typename __base_type<T>::type,
> >                     typename __base_type<U>::type>::value>::type>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >        return static_cast<T&&>(t);
> >     }

> This has been tested by Bill, Jason and myself.

> It allows the following lvalue/rvalue casts:

> 1. Cast an lvalue `t` to an lvalue `T` (identity).
> 2. Cast an lvalue `t` to an rvalue `T`.
> 3. Cast an rvalue `t` to an rvalue `T` (identity).

> It disallows:

> 1. Cast an rvalue `t` to an lvalue `T`.
> 2. Cast one type `t` to another type `T` (such as `int` to `double`).

> "a." is disallowed as it can easily lead to dangling references. "b." is disallowed as this function is meant to only change the lvalue/rvalue characteristic of an expression.

> Jason has expressed concern that "b." is not dangerous and is useful in contexts where you want to "forward" a derived type as a base type. I find this use case neither dangerous, nor compelling. I.e. I could live with or without the "b." constraint. Without it, forward would look like:

> >     template <class T, class U,
> >        class = typename enable_if<
> >            !is_lvalue_reference<T>::value ||
> >             is_lvalue_reference<T>::value &&
> >             is_lvalue_reference<U>::value>::type>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >        return static_cast<T&&>(t);
> >     }

> Or possibly:

> >     template <class T, class U,
> >        class = typename enable_if<
> >            !is_lvalue_reference<T>::value ||
> >             is_lvalue_reference<T>::value &&
> >             is_lvalue_reference<U>::value>::type,
> >        class = typename enable_if<
> >             is_base_of<typename __base_type<U>::type,
> >                        typename __base_type<T>::type>::value>::type>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >        return static_cast<T&&>(t);
> >     }

> The "promised paper" is not in the post-Frankfurt mailing only because I'm waiting for the non-concepts draft. But I'm hoping that by adding this information here I can keep people up to date.

[ 2009-08-02 David adds: ]

> `forward` was originally designed to do one thing: perfect forwarding. That is, inside a function template whose actual argument can be a const or non-const lvalue or rvalue, restore the original "rvalue-ness" of the actual argument:

> >     template <class T>
> >     void f(T&& x)
> >     {
> >         // x is an lvalue here.  If the actual argument to f was an
> >         // rvalue, pass static_cast<T&&>(x) to g; otherwise, pass x.
> >         g( forward<T>(x) );
> >     }

> Attempting to engineer `forward` to accomodate uses other than perfect forwarding dilutes its idiomatic meaning. The solution proposed here declares that `forward<T>(x)` means nothing more than `static_cast<T&&>(x)`, with a patchwork of restrictions on what `T` and `x` can be that can't be expressed in simple English.

> I would be happy with either of two approaches, whose code I hope (but can't guarantee) I got right.

> 1. Use a simple definition of `forward` that accomplishes its original purpose without complications to accomodate other uses:

>    >     template <class T, class U>
>    >     T&& forward(U& x)
>    >     {
>    >         return static_cast<T&&>(x);
>    >     }

> 2. Use a definition of `forward` that protects the user from as many potential mistakes as possible, by actively preventing all other uses:

>    >     template <class T, class U>
>    >     boost::enable_if_c<
>    >         // in forward<T>(x), x is a parameter of the caller, thus an lvalue
>    >         is_lvalue_reference<U>::value
>    >         // in caller's deduced T&& argument, T can only be non-ref or lvalue ref
>    >         && !is_rvalue_reference<T>::value
>    >         // Must not cast cv-qualifications or do any type conversions
>    >         && is_same<T&,U&>::value
>    >         , T&&>::type forward(U&& a)
>    >     {
>    >         return static_cast<T&&>(a);
>    >     }

[ 2009-09-27 Howard adds: ]

> A paper, N2951, is available which compares several implementations (including David's) with respect to several use cases (including Jason's) and provides wording for one implementation.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2951.

Proposed resolution:

### 1055(i). Provide a trait that returns the underlying type of an enumeration type

Section: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [meta.trans.other].

View all issues with Resolved status.

Discussion:

Addresses UK 98 [CD1]

It would be useful to be able to determine the underlying type of an arbitrary enumeration type. This would allow safe casting to an integral type (especially needed for scoped enums, which do not promote), and would allow use of `numeric_limits`. In general it makes generic programming with enumerations easier.

[ Batavia (2009-05): ]

> Pete observes (and Tom concurs) that the proposed resolution seems to require compiler support for its implementation, as it seems necessary to look at the range of values of the enumerated type. To a first approximation, a library solution could give an answer based on the size of the type. If the user has specialized `numeric_limits` for the enumerated type, then the library might be able to do better, but there is no such requirement. Keep status as Open and solicit input from CWG.

[ 2009-05-23 Alisdair adds: ]

> Just to confirm that the BSI originator of this comment assumed it did indeed imply a compiler intrinsic. Rather than request a Core extension, it seemed in keeping with that the type traits interface provides a library API to unspecified compiler features - where we require several other traits (e.g. `has_trivial_*`) to get the 'right' answer now, unlike in TR1.

[ Addressed in N2947. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2984.

Proposed resolution:

Add a new row to the table in 21.3.8.7 [meta.trans.other]:

> Table 41 -- Other transformations
> Template | Condition | Comments
> --- | --- | ---
> `template< class T > struct enum_base;` | `T` shall be an enumeration type (9.8.1 [dcl.enum]) | The member typedef `type` shall name the underlying type of the enum `T`.

### 1065(i). Allow inline namespaces within namespace `std` for implementations

Section: 16.4.2.2 [contents] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [contents].

View all issues with C++11 status.

Discussion:

Addresses UK 168 [CD1]

We should make it clear (either by note or normatively) that namespace `std` may contain inline namespaces, and that entities specified to be defined in std may in fact be defined in one of these inline namespaces. (If we're going to use them for versioning, eg when TR2 comes along, we're going to need that.)

Replace "namespace std or namespaces nested within namespace std" with "namespace std or namespaces nested within namespace std or inline namespaces nested directly or indirectly within namespace std"

[ Summit: ]

> adopt UK words (some have reservations whether it is correct)

[ 2009-05-09 Alisdair improves the wording. ]

[ Batavia (2009-05): ]

> Bill believes there is strictly speaking no need to say that because no portable test can detect the difference. However he agrees that it doesn't hurt to say this.

> Move to Tentatively Ready.

Proposed resolution:

Change 16.4.2.2 [contents] p2:

> All library entities except macros, `operator new` and `operator delete` are defined within the namespace `std` or namespaces nested within namespace `std`. It is unspecified whether names declared in a specific namespace are declared directly in that namespace, or in an inline namespace inside that namespace. [Footnote: This gives implementers freedom to support multiple configurations of the library.]

### 1066(i). Use `[[noreturn]]` attribute in the library

Section: 17 [support] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2021-06-06

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Addresses UK 189 and JP 27 [CD1]

The addition of the `[[noreturn]]` attribute to the language will be an important aid for static analysis tools.

The following functions should be declared in C++ with the `[[noreturn]]` attribute: `abort` `exit` `quick_exit` `terminate` `unexpected` `rethrow_exception` `throw_with_nested`.

[ Summit: ]

> Agreed.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 17.5 [support.start.term] p3:

> -2- ...

>     void abort [[noreturn]] (void)

> -3- ...

> -6- ...

>     void exit [[noreturn]] (int status)

> -7- ...

> -11- ...

>     void quick_exit [[noreturn]] (int status)

> -12- ...

Change the `<exception>` synopsis in 17.9 [support.exception]:

>     void unexpected [[noreturn]] ();
>     ...
>     void terminate [[noreturn]] ();
>     ...
>     void rethrow_exception [[noreturn]] (exception_ptr p);
>     ...
>     template <class T> void throw_with_nested [[noreturn]] (T&& t); // [[noreturn]]

Change 99 [unexpected]:

>     void unexpected [[noreturn]] ();

Change 17.9.5.4 [terminate]:

>     void terminate [[noreturn]] ();

Change 17.9.7 [propagation]:

>     void rethrow_exception [[noreturn]] (exception_ptr p);

In the synopsis of 17.9.8 [except.nested] and the definition area change:

>     template <class T> void throw_with_nested [[noreturn]] (T&& t); // [[noreturn]]

### 1070(i). Ambiguous move overloads in function

Section: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-19 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.wrap.func].

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Discussion:

The synopsis in 22.10.17.3 [func.wrap.func] says:

>     template<Returnable R, CopyConstructible... ArgTypes>
>     class function<R(ArgTypes...)>
>     {
>         ...
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function(F);
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function(F&&);
>         ...
>         template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F);
>         template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&);
>         ...
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes..>
>                 && Convertible<Callable<F, ArgTypes...>::result_type
>           function& operator=(F);
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function& operator=(F&&);
>         ...
>     };

Each of the 3 pairs above are ambiguous. We need only one of each pair, and we could do it with either one. If we choose the `F&&` version we need to bring `decay` into the definition to get the pass-by-value behavior. In the proposed wording I've gotten lazy and just used the pass-by-value signature.

[ 2009-05-01 Daniel adds: ]

> 1024(i) modifies the second removed constructor.

[ Batavia (2009-05): ]

> We briefly discussed whether we ought support moveable function objects, but decided that should be a separate issue if someone cares to propose it.

> Move to Tentatively Ready.

Proposed resolution:

Change the synopsis of 22.10.17.3 [func.wrap.func], and remove the associated definitions in 22.10.17.3.2 [func.wrap.func.con]:

>     template<Returnable R, CopyConstructible... ArgTypes>
>     class function<R(ArgTypes...)>
>     {
>         ...
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function(F);
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function(F&&);
>         ...
>         template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F);
>         template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&);
>         ...
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes..>
>                 && Convertible<Callable<F, ArgTypes...>::result_type
>           function& operator=(F);
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function& operator=(F&&);
>         ...
>     };

### 1071(i). `is_bind_expression` should derive from `integral_constant<bool>`

Section: 22.10.15.2 [func.bind.isbind] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.bind.isbind].

View all issues with C++11 status.

Discussion:

Class template is_bind_expression 22.10.15.2 [func.bind.isbind]:

>     namespace std {
>       template<class T> struct is_bind_expression {
>         static const bool value = see below;
>       };
>     }

`is_bind_expression` should derive from `std::integral_constant<bool>` like other similar trait types.

[ Daniel adds: ]

> We need the same thing for the trait `is_placeholder` as well.

[ 2009-03-22 Daniel provided wording. ]

[ Batavia (2009-05): ]

> We recommend this be deferred until after the next Committee Draft is issued.

> Move to Open.

[ 2009-05-31 Peter adds: ]

> I am opposed to the proposed resolution and to the premise of the issue in general. The traits's default definitions should NOT derive from `integral_constant`, because this is harmful, as it misleads people into thinking that `is_bind_expression<E>` always derives from `integral_constant`, whereas it may not.

> `is_bind_expression` and `is_placeholder` allow user specializations, and in fact, this is their primary purpose. Such user specializations may not derive from `integral_constant`, and the places where `is_bind_expression` and `is_placeholder` are used intentionally do not require such derivation.

> The long-term approach here is to switch to `BindExpression<E>` and `Placeholder<P>` explicit concepts, of course, but until that happens, I say leave them alone.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready. We are comfortable with requiring user specializations to derive from `integral_constant`.

Proposed resolution:

1. In 22.10.15.2 [func.bind.isbind] change as indicated:

   >     namespace std {
   >      template<class T> struct is_bind_expression : integral_constant<bool, see below> { };{
   >        static const bool value = see below;
   >      };
   >     }

2. In 22.10.15.2 [func.bind.isbind]/2 change as indicated:

   >     static const bool value;

   > > -2- `true` if `T` is a type returned from `bind`, `false` otherwise. If `T` is a type returned from `bind`, `is_bind_expression<T>` shall be publicly derived from `integral_constant<bool, true>`, otherwise it shall be publicly derived from `integral_constant<bool, false>`.

3. In 22.10.15.3 [func.bind.isplace] change as indicated:

   >     namespace std {
   >      template<class T> struct is_placeholder : integral_constant<int, see below> { };{
   >        static const int value = see below;
   >      };
   >     }

4. In 22.10.15.3 [func.bind.isplace]/2 change as indicated:

   >     static const int value;

   > > -2- value is `J` if `T` is the type of `std::placeholders::_J`, 0 otherwise. If `T` is the type of `std::placeholders::_J`, `is_placeholder<T>` shall be publicly derived from `integral_constant<int, J>` otherwise it shall be publicly derived from `integral_constant<int, 0>`.

### 1073(i). Declaration of `allocator_arg` should be `constexpr`

Section: 20.2 [memory] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [memory].

View all issues with C++11 status.

Discussion:

Declaration of `allocator_arg` should be `constexpr` to ensure constant initialization.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.2 [memory] p2:

>     // 20.8.1, allocator argument tag
>     struct allocator_arg_t { };
>     constexpr allocator_arg_t allocator_arg = allocator_arg_t();

### 1075(i). Scoped allocators are too complex

Section: 22 [utilities], 23 [containers] Status: Resolved Submitter: Alan Talbot Opened: 2009-03-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [utilities].

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Discussion:

Addresses US 65 and US 74.1 [CD1]

US 65:

> Scoped allocators and allocator propagation traits add a small amount of utility at the cost of a great deal of machinery. The machinery is user visible, and it extends to library components that don't have any obvious connection to allocators, including basic concepts and simple components like `pair` and `tuple`.

> Suggested resolution:

> Sketch of proposed resolution: Eliminate scoped allocators, replace allocator propagation traits with a simple uniform rule (e.g. always propagate on copy and move), remove all mention of allocators from components that don't explicitly allocate memory (e.g. pair), and adjust container interfaces to reflect this simplification.

> Components that I propose eliminating include `HasAllocatorType`, `is_scoped_allocator`, `allocator_propagation_map`, `scoped_allocator_adaptor`, and `ConstructibleAsElement`.

US 74.1:

> Scoped allocators represent a poor trade-off for standardization, since (1) scoped-allocator--aware containers can be implemented outside the C++ standard library but used with its algorithms, (2) scoped allocators only benefit a tiny proportion of the C++ community (since few C++ programmers even use today's allocators), and (3) all C++ users, especially the vast majority of the C++ community that won't ever use scoped allocators are forced to cope with the interface complexity introduced by scoped allocators.

> In essence, the larger community will suffer to support a very small subset of the community who can already implement their own data structures outside of the standard library. Therefore, scoped allocators should be removed from the working paper.

> Some evidence of the complexity introduced by scoped allocators:

> > 22.3 [pairs], 22.4 [tuple]: Large increase in the number of pair and tuple constructors.

> > 23 [containers]: Confusing "AllocatableElement" requirements throughout.

> Suggested resolution:

> Remove support for scoped allocators from the working paper. This includes at least the following changes:

> > Remove 99 [allocator.element.concepts]

> > Remove 20.6 [allocator.adaptor]

> > Remove [construct.element]

> > In Clause 23 [containers]: replace requirements naming the `AllocatableElement` concept with requirements naming `CopyConstructible`, `MoveConstructible`, `DefaultConstructible`, or `Constructible`, as appropriate.

[ Post Summit Alan moved from NAD to Open. ]

[ 2009-05-15 Ganesh adds: ]

> The requirement `AllocatableElement` should not be replaced with `Constructible` on the `emplace_xxx()` functions as suggested. In the one-parameter case the `Constructible` requirement is not satisfied when the constructor is explicit (as per [concept.map.fct], twelfth bullet) but we do want to allow explicit constructors in emplace, as the following example shows:

> >     vector<shared_ptr<int>> v;
> >     v.emplace_back(new int); // should be allowed

> If the issue is accepted and scoped allocators are removed, I suggest to add a new pair of concepts to [concept.construct], namely:

> >     auto concept HasExplicitConstructor<typename T, typename... Args> {
> >      explicit T::T(Args...);
> >     }

> >     auto concept ExplicitConstructible<typename T, typename... Args>
> >      : HasExplicitConstructor<T, Args...>, NothrowDestructible<T>
> >     { }

> We should then use `ExplicitConstructible` as the requirement for all `emplace_xxx()` member functions.

> For coherence and consistency with the similar concepts `Convertible/ExplicitlyConvertible`, we might also consider changing `Constructible` to:

> >     auto concept Constructible<typename T, typename... Args>
> >      : HasConstructor<T, Args...>, ExplicitConstructible<T, Args...>
> >     { }

> Moreover, all emplace-related concepts in [container.concepts] should also use `ExplicitConstructible` instead of `Constructible` in the definitions of their axioms. In fact the concepts in [container.concepts] should be corrected even if the issue is not accepted.

> On the other hand, if the issue is not accepted, the scoped allocator adaptors should be fixed because the following code:

> >     template <typename T> using scoped_allocator = scoped_allocator_adaptor<allocator<T>>;

> >     vector<shared_ptr<int>, scoped_allocator<shared_ptr<int>>> v;
> >     v.emplace_back(new int); // ops! doesn't compile

> doesn't compile, as the member function `construct()` of the scoped allocator requires non-explicit constructors through concept `ConstructibleWithAllocator`. Fixing that is not difficult but probably more work than it's worth and is therefore, IMHO, one more reason in support of the complete removal of scoped allocators.

[ 2009-06-09 Alan adds: ]

> I reopened this issue because I did not think that these National Body comments were adequately addressed by marking them NAD. My understanding is that something can be marked NAD if it is clearly a misunderstanding or trivial, but a substantive issue that has any technical merit requires a disposition that addresses the concerns.

> The notes in the NB comment list (US 65 & US 74.1) say that:

> 1. this issue has not introduced any new arguments not previously discussed,
> 2. the vote (4-9-3) was not a consensus for removing scoped allocators,
> 3. the issue is resolved by N2840.

> My opinion is:

> 1. there are new arguments in both comments regarding concepts (which were not present in the library when the scoped allocator proposal was voted in),
> 2. the vote was clearly not a consensus for removal, but just saying there was a vote does not provide a rationale,
> 3. I do not believe that N2840 addresses these comments (although it does many other things and was voted in with strong approval).

> My motivation to open the issue was to ensure that the NB comments were adequately addressed in a way that would not risk a "no" vote on our FCD. If there are responses to the technical concerns raised, then perhaps they should be recorded. If the members of the NB who authored the comments are satisfied with N2840 and the other disposition remarks in the comment list, then I am sure they will say so. In either case, this issue can be closed very quickly in Frankfurt, and hopefully will have helped make us more confident of approval with little effort. If in fact there is controversy, my thought is that it is better to know now rather than later so there is more time to deal with it.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2982.

Proposed resolution:

Rationale:

Scoped allocators have been revised significantly.

### 1079(i). UK-265: `RandomAccessIterator`'s `operator-` has nonsensical effects clause

Section: 24.3.5.7 [random.access.iterators] Status: C++11 Submitter: Doug Gregor Opened: 2009-03-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [random.access.iterators].

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Discussion:

Addresses UK 265

UK-265:

This effects clause is nonesense. It looks more like an axiom stating equivalence, and certainly an effects clause cannot change the state of two arguments passed by const reference

[ 2009-09-18 Alisdair adds: ]

> For random access iterators, the definitions of `(b-a)` and `(a<b)` are circular:

> From table Table 104 -- Random access iterator requirements:

> >     b - a :==>  (a < b) ? distance(a,b) : -distance(b,a)

> >     a < b :==>  b - a > 0

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2010-02-13 Alisdair opens. ]

> Looking again at LWG 1079(i), the wording in the issue no longer exists, and appears to be entirely an artefact of the concepts wording.

> This issue is currently on our Ready list (not even Tentative!) but I think it has to be pulled as there is no way to apply the resolution.

> Looking at the current paper, I think this issue is now "NAD, solved by the removal of concepts". Unfortunately it is too late to poll again, so we will have to perform that review in Pittsburgh.

[ 2010-02-13 Daniel updates the wording to address the circularity problem. ]

> [ The previous wording is preserved here: ]

> > Modify 24.3.5.7 [random.access.iterators]p7-9 as follows:

> > >     difference_type operator-(const X& a, const X& b);

> > > > -7- Precondition: there exists a value `n` of `difference_type` such that `a == b + n`.

> > > > -8- Effects: `b == a + (b - a)`

> > > > -9- Returns: `(a < b) ? distance(a,b) : -distance(b,a)``n`

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

Modify Table 105 in 24.3.5.7 [random.access.iterators]:

> Table 105 — Random access iterator requirements (in addition to bidirectional iterator)
> Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
> --- | --- | --- | ---
> `b - a` | `Distance` | `distance(a,b)` return `n` | pre: there exists a value `n` of `Distance` such that `a + n == b`. `b == a + (b - a)`.

### 1088(i). `std::promise` should provide non-member `swap` overload

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.promise].

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Discussion:

Addresses UK 342 [CD1]

`std::promise` is missing a non-member overload of `swap`. This is inconsistent with other types that provide a `swap` member function.

Add a non-member overload `void swap(promise&& x,promise&& y){ x.swap(y); }`

[ Summit: ]

> Create an issue. Move to review, attention: Howard. Detlef will also look into it.

[ Post Summit Daniel provided wording. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2997.

Proposed resolution:

1. In 32.10.6 [futures.promise], before p.1, immediately after class template promise add:

   >     template <class R>
   >     void swap(promise<R>& x, promise<R>& y);

2. Change 32.10.6 [futures.promise]/10 as indicated (to fix a circular definition):

   > -10- Effects: swap(*this, other)Swaps the associated state of `*this` and `other`

   > Throws: Nothing.

3. After the last paragraph in 32.10.6 [futures.promise] add the following prototype description:

   >     template <class R>
   >     void swap(promise<R>& x, promise<R>& y);

   > > Effects: `x.swap(y)`

   > > Throws: Nothing.

### 1089(i). Unify "Throws: Nothing." specifications

Section: 32 [thread] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread].

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Discussion:

Addresses JP 76 [CD1]

A description for "Throws: Nothing." are not unified.

At the part without throw, "Throws: Nothing." should be described.

Add "Throws: Nothing." to the following.

- 32.4.3.7 [thread.thread.static] p1
- 32.6.5.2 [thread.lock.guard] p4
- 32.6.5.4.2 [thread.lock.unique.cons] p6
- 32.7.4 [thread.condition.condvar] p7 and p8
- 32.7.5 [thread.condition.condvarany] p6, p7, p19, p21 and p25

[ Summit: ]

> Pass on to editor.

[ Post Summit: Editor declares this non-editorial. ]

[ 2009-08-01 Howard provided wording: ]

> The definition of "Throws: Nothing." that I added is probably going to be controversial, but I beg you to consider it seriously.

> > In C++ there are three "flow control" options for a function:

> > 1. It can return, either with a value, or with `void`.
> > 2. It can call a function which never returns, such as `std::exit` or `std::terminate`.
> > 3. It can throw an exception.

> > The above list can be abbreviated with:

> > 1. Returns.
> > 2. Ends program.
> > 3. Throws exception.

> > In general a function can have the behavior of any of these 3, or any combination of any of these three, depending upon run time data.

> > 1. R
> > 2. E
> > 3. T
> > 4. RE
> > 5. RT
> > 6. ET
> > 7. RET

> > A function with no throw spec, and no documentation, is in general a RET function. It may return, it may end the program, or it may throw. When we specify a function with an empty throw spec:

> > >     void f() throw();

> > We are saying that `f()` is an RE function: It may return or end the program, but it will not throw.

> > I posit that there are very few places in the library half of the standard where we intend for functions to be able to end the program (call `terminate`). And none of those places where we do say `terminate` could be called, do we currently say "Throws: Nothing.".

> > I believe that if we define "Throws: Nothing." to mean R, we will both clarify many, many places in the standard, and give us a good rationale for choosing between "Throws: Nothing." (R) and `throw()` (RE) in the future. Indeed, this may give us motivation to change several `throw()`s to "Throws: Nothing.".

> I did not add the following changes as JP 76 requested as I believe we want to allow these functions to throw:

> > Add a paragraph under 32.6.5.2 [thread.lock.guard] p4:

> > >     explicit lock_guard(mutex_type& m);

> > > Throws: Nothing.

> > Add a paragraph under 32.6.5.4.2 [thread.lock.unique.cons] p6:

> > >     explicit unique_lock(mutex_type& m);

> > > Throws: Nothing.

> > Add a paragraph under 32.7.5 [thread.condition.condvarany] p19, p21 and p25:

> > >     template <class Lock, class Rep, class Period>
> > >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

> > > Throws: Nothing.

> > >     template <class Lock, class Duration, class Predicate>
> > >       bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& rel_time, Predicate pred);

> > > Throws: Nothing.

> > >     template <class Lock, class Rep, class Period, class Predicate>
> > >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

> > > Throws: Nothing.

[ 2009-10 Santa Cruz: ]

> Defer pending further developments with exception restriction annotations.

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-24 Pete moved to Open: ]

> A "Throws: Nothing" specification is not the place to say that a function is not allowed to call `exit()`. While I agree with the thrust of the proposed resolution, "doesn't throw exceptions" is a subset of "always returns normally". If it's important to say that most library functions don't call `exit()`, say so.

[ 2010 Pittsburgh: ]

> Move to Ready except for the added paragraph to 16.3.2.4 [structure.specifications].

Proposed resolution:

Add a paragraph under 32.4.3.7 [thread.thread.static] p1:

>     unsigned hardware_concurrency();

> -1- Returns: ...

> Throws: Nothing.

Add a paragraph under 32.7.4 [thread.condition.condvar] p7 and p8:

> [Informational, not to be incluced in the WP: The POSIX spec allows only:

> [EINVAL]
>   The value `cond` does not refer to an initialized condition variable. — end informational]

>     void notify_one();

> -7- Effects: ...

> Throws: Nothing.

>     void notify_all();

> -8- Effects: ...

> Throws: Nothing.

Add a paragraph under 32.7.5 [thread.condition.condvarany] p6 and p7:

>     void notify_one();

> -6- Effects: ...

> Throws: Nothing.

>     void notify_all();

> -7- Effects: ...

> Throws: Nothing.

### 1090(i). Missing description of `packaged_task` member `swap`, missing non-member `swap`

Section: 32.10.10 [futures.task] Status: Resolved Submitter: Daniel Krügler Opened: 2009-03-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.task].

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Discussion:

Class template `packaged_task` in 32.10.10 [futures.task] shows a member `swap` declaration, but misses to document it's effects (No prototype provided). Further on this class misses to provide a non-member swap.

[ Batavia (2009-05): ]

> Alisdair notes that paragraph 2 of the proposed resolution has already been applied in the current Working Draft.

> We note a pending `future`-related paper by Detlef; we would like to wait for this paper before proceeding.

> Move to Open.

[ 2009-05-24 Daniel removed part 2 of the proposed resolution. ]

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready, removing bullet 3 from the proposed resolution but keeping the other two bullets.

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

1. In 32.10.10 [futures.task], immediately after the definition of class template packaged_task add:

   >     template<class R, class... Argtypes>
   >     void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&);

2. At the end of 32.10.10 [futures.task] (after p. 20), add the following prototype description:

   >     template<class R, class... Argtypes>
   >     void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y);

   > > Effects: `x.swap(y)`

   > > Throws: Nothing.

### 1093(i). Multiple definitions for `random_shuffle` algorithm

Section: 26.7.13 [alg.random.shuffle] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.random.shuffle].

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Discussion:

There are a couple of issues with the declaration of the `random_shuffle` algorithm accepting a random number engine.

1. The Iterators must be shuffle iterators, yet this requirement is missing.
2. The `RandomNumberEngine` concept is now provided by the random number library (n2836) and the placeholder should be removed.

[ 2009-05-02 Daniel adds: ]

> this issue completes adding necessary requirement to the third new `random_shuffle` overload. The current suggestion is:

> >     template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand>
> >     requires ShuffleIterator<Iter>
> >     void random_shuffle(Iter first, Iter last, Rand&& g);

> IMO this is still insufficient and I suggest to add the requirement

> >     Convertible<Rand::result_type, Iter::difference_type>

> to the list (as the two other overloads already have).

> Rationale:

> > Its true that this third overload is somewhat different from the remaining two. Nevertheless we know from `UniformRandomNumberGenerator`, that it's `result_type` is an integral type and that it satisfies `UnsignedIntegralLike<result_type>`.

> > To realize it's designated task, the algorithm has to invoke the `Callable` aspect of `g` and needs to perform some algebra involving it's `min()/max()` limits to compute another index value that at this point is converted into `Iter::difference_type`. This is so, because 24.3.5.7 [random.access.iterators] uses this type as argument of it's algebraic operators. Alternatively consider the equivalent iterator algorithms in 24.4.3 [iterator.operations] with the same result.

> > This argument leads us to the conclusion that we also need `Convertible<Rand::result_type, Iter::difference_type>` here.

[ Batavia (2009-05): ]

> Alisdair notes that point (ii) has already been addressed.

> We agree with the proposed resolution to point (i) with Daniel's added requirement.

> Move to Review.

[ 2009-06-05 Daniel updated proposed wording as recommended in Batavia. ]

[ 2009-07-28 Alisdair adds: ]

> Revert to Open, with a note there is consensus on direction but the wording needs updating to reflect removal of concepts.

[ 2009-10 post-Santa Cruz: ]

> Leave Open, Walter to work on it.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved, addressed by N3056. ]

Rationale:

Solved by N3056.

Proposed resolution:

Change in [algorithms.syn] and 26.7.13 [alg.random.shuffle]:

>     concept UniformRandomNumberGenerator<typename Rand> { }
>     template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand>
>       requires ShuffleIterator<Iter> &&
>       Convertible<Rand::result_type, Iter::difference_type>
>       void random_shuffle(Iter first, Iter last, Rand&& g);

### 1094(i). Replace "unspecified-bool-type" by "`explicit operator bool() const`" in I/O library

Section: 31.5.4.4 [iostate.flags] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iostate.flags].

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Discussion:

Addresses JP 65 and JP 66 [CD1]

Switch from "unspecified-bool-type" to "explicit operator bool() const".

Replace `operator unspecified-bool-type() const;`" with `explicit operator bool() const;`

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Review.

[ 2009 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

Change the synopis in 31.5.4 [ios]:

>     explicit operator unspecified-bool-type bool() const;

Change 31.5.4.4 [iostate.flags]:

>     explicit operator unspecified-bool-type bool() const;

> > -1- Returns: `!fail()` If `fail()` then a value that will evaluate false in a boolean context; otherwise a value that will evaluate true in a boolean context. The value type returned shall not be convertible to int.

> > [Note: This conversion can be used in contexts where a bool is expected (e.g., an `if` condition); however, implicit conversions (e.g., to `int`) that can occur with `bool` are not allowed, eliminating some sources of user error. One possible implementation choice for this type is pointer-to-member. -- end note]

### 1095(i). Shared objects and the library wording unclear

Section: 16.4.5.10 [res.on.objects] Status: C++11 Submitter: Beman Dawes Opened: 2009-03-27 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [res.on.objects].

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Discussion:

N2775, Small library thread-safety revisions, among other changes, removed a note from 16.4.5.10 [res.on.objects] that read:

> [Note: This prohibition against concurrent non-const access means that modifying an object of a standard library type shared between threads without using a locking mechanism may result in a data race. --end note.]

That resulted in wording which is technically correct but can only be understood by reading the lengthy and complex 16.4.6.10 [res.on.data.races] Data race avoidance. This has the effect of making 16.4.5.10 [res.on.objects] unclear, and has already resulted in a query to the LWG reflector. See c++std-lib-23194.

[ Batavia (2009-05): ]

> The proposed wording seems to need a bit of tweaking ("really bad idea" isn't quite up to standardese). We would like feedback as to whether the original Note's removal was intentional.

> Change the phrase "is a really bad idea" to "risks undefined behavior" and move to Review status.

[ 2009-10 Santa Cruz: ]

> Note: Change to read: "Modifying...", Delete 'thus', move to Ready

Proposed resolution:

Change 16.4.5.10 [res.on.objects] as indicated:

> The behavior of a program is undefined if calls to standard library functions from different threads may introduce a data race. The conditions under which this may occur are specified in 17.6.4.7.

> [Note: Modifying an object of a standard library type shared between threads risks undefined behavior unless objects of the type are explicitly specified as being sharable without data races or the user supplies a locking mechanism. --end note]

### 1097(i). #define __STDCPP_THREADS

Section: 17.2 [support.types] Status: C++11 Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [support.types].

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Discussion:

Addresses DE 18

Freestanding implementations do not (necessarily) have support for multiple threads (see 6.10.2 [intro.multithread]). Applications and libraries may want to optimize for the absence of threads. I therefore propose a preprocessor macro to indicate whether multiple threads can occur.

There is ample prior implementation experience for this feature with various spellings of the macro name. For example, gcc implicitly defines `_REENTRANT` if multi-threading support is selected on the compiler command-line.

While this is submitted as a library issue, it may be more appropriate to add the macro in 16.8 cpp.predefined in the core language.

See also N2693.

[ Batavia (2009-05): ]

> We agree with the issue, and believe it is properly a library issue.

> We prefer that the macro be conditionally defined as part of the `<thread>` header.

> Move to Review.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010-02-25 Pete moved to Open: ]

> The proposed resolution adds a feature-test macro named `__STDCPP_THREADS`, described after the following new text:

> > The standard library defines the following macros; no explicit prior inclusion of any header file is necessary.

> The correct term here is "header", not "header file". But that's minor. The real problem is that library entities are always defined in headers. If `__STDCPP_THREADS` is defined without including any header it's part of the language and belongs with the other predefined macros in the Preprocessor clause.

> Oddly enough, the comments from Batavia say "We prefer that the macro be conditionally defined as part of the `<thread>` header." There's no mention of a decision to change this.

[ 2010-02-26 Ganesh updates wording. ]

[ 2010 Pittsburgh: Adopt Ganesh's wording and move to Review. ]

[ 2010-03-08 Pete adds: ]

> Most macros we have begin and end with with double underbars, this one only begins with double underbars.

[ 2010 Pittsburgh: Ganesh's wording adopted and moved to Ready for Pittsburgh. ]

Proposed resolution:

Change 16.4.2.5 [compliance]/3:

> 3 The supplied version of the header `<cstdlib>` shall declare at least the functions `abort()`, `atexit()`, and `exit()` (18.5). The supplied version of the header `<thread>` either shall meet the same requirements as for a hosted implementation or including it shall have no effect. The other headers listed in this table shall meet the same requirements as for a hosted implementation.

Add the following line to table 15:

> Table 15 — C++ headers for freestanding implementations
> Subclause | Header(s)
> --- | ---
> ...
> 32.4 [thread.threads] Threads | `<thread>`

Add to the `<thread>` synopsis in 32.4 [thread.threads]/1 the line:

>     namespace std {

>     #define __STDCPP_THREADS __cplusplus

>       class thread;
>       ...

### 1098(i). definition of `get_pointer_safety()`

Section: 99 [util.dynamic.safety] Status: C++11 Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.dynamic.safety].

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Discussion:

Addresses DE 18

In 99 [util.dynamic.safety], `get_pointer_safety()` purports to define behavior for non-safely derived pointers ( [basic.stc.dynamic.safety]). However, the cited core-language section in paragraph 4 specifies undefined behavior for the use of such pointer values. This seems an unfortunate near-contradiction. I suggest to specify the term relaxed pointer safety in the core language section and refer to it from the library description. This issue deals with the library part, the corresponding core issue (c++std-core-13940) deals with the core modifications.

See also N2693.

[ Batavia (2009-05): ]

> We recommend if this issue is to be moved, the issue be moved concurrently with the cited Core issue.

> We agree with the intent of the proposed resolution. We would like input from garbage collection specialists.

> Move to Open.

[ 2009-10 Santa Cruz: ]

> The core issue is 853 and is in Ready status.

Proposed resolution:

In 99 [util.dynamic.safety] p16, replace the description of `get_pointer_safety()` with:

> `pointer_safety get_pointer_safety();`

> > Returns: an enumeration value indicating the implementation's treatment of pointers that are not safely derived (3.7.4.3). Returns `pointer_safety::relaxed` if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program. Returns `pointer_safety::preferred` if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program but allows the implementation to hint that it could be desirable to avoid dereferencing pointers that are not safely derived as described. [Example: `pointer_safety::preferred` might be returned to detect if a leak detector is running to avoid spurious leak reports. -- end note] Returns `pointer_safety::strict` if pointers that are not safely derived might be treated differently than pointers that are safely derived.

> > Returns: Returns `pointer_safety::strict` if the implementation has strict pointer safety ( [basic.stc.dynamic.safety]). It is implementation-defined whether `get_pointer_safety` returns `pointer_safety::relaxed` or `pointer_safety::preferred` if the implementation has relaxed pointer safety ( [basic.stc.dynamic.safety]).Footnote

> > Throws: nothing

> > Footnote) `pointer_safety::preferred` might be returned to indicate to the program that a leak detector is running so that the program can avoid spurious leak reports.

### 1100(i). `auto_ptr` to `unique_ptr` conversion

Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.single.ctor].

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Discussion:

Message c++std-lib-23182 led to a discussion in which several people expressed interest in being able to convert an `auto_ptr` to a `unique_ptr` without the need to call `release`. Below is wording to accomplish this.

[ Batavia (2009-05): ]

> Pete believes it not a good idea to separate parts of a class's definition. Therefore, if we do this, it should be part of `unique-ptr`'s specification.

> Alisdair believes the lvalue overload may be not necessary.

> Marc believes it is more than just sugar, as it does ease the transition to `unique-ptr`.

> We agree with the resolution as presented. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-08-01 Howard deconceptifies wording: ]

> I also moved the change from 99 [depr.auto.ptr] to 20.3.1.3 [unique.ptr.single] per the Editor's request in Batavia (as long as I was making changes anyway). Set back to Review.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

Add to 20.3.1.3 [unique.ptr.single]:

>     template <class T, class D>
>     class unique_ptr
>     {
>     public:
>         template <class U>
>           unique_ptr(auto_ptr<U>& u);
>         template <class U>
>           unique_ptr(auto_ptr<U>&& u);
>     };

Add to 20.3.1.3.2 [unique.ptr.single.ctor]:

>     template <class U>
>       unique_ptr(auto_ptr<U>& u);
>     template <class U>
>       unique_ptr(auto_ptr<U>&& u);

> > Effects: Constructs a `unique_ptr` with `u.release()`.

> > Postconditions: `get() ==` the value `u.get()` had before the construciton, modulo any required offset adjustments resulting from the cast from `U*` to `T*`. `u.get() == nullptr`.

> > Throws: nothing.

> > Remarks: `U*` shall be implicitly convertible to `T*` and `D` shall be the same type as `default_delete<T>`, else these constructors shall not participate in overload resolution.

### 1103(i). `system_error` constructor postcondition overly strict

Section: 19.5.8.2 [syserr.syserr.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

19.5.8.2 [syserr.syserr.members] says:

>     system_error(error_code ec, const string& what_arg);

> > Effects: Constructs an object of class `system_error`.

> > Postconditions: `code() == ec` and `strcmp(runtime_error::what(), what_arg.c_str()) == 0`.

However the intent is for:

>     std::system_error se(std::errc::not_a_directory, "In FooBar");
>     ...
>     se.what();  // returns something along the lines of:
>                 //   "In FooBar: Not a directory"

The way the constructor postconditions are set up now, to achieve both conformance, and the desired intent in the `what()` string, the `system_error` constructor must store "In FooBar" in the base class, and then form the desired output each time `what()` is called. Or alternatively, store "In FooBar" in the base class, and store the desired `what()` string in the derived `system_error`, and override `what()` to return the string in the derived part.

Both of the above implementations seem suboptimal to me. In one I'm computing a new string every time `what()` is called. And since `what()` can't propagate exceptions, the client may get a different string on different calls.

The second solution requires storing two strings instead of one.

What I would like to be able to do is form the desired `what()` string once in the `system_error` constructor, and store that in the base class. Now I'm:

1. Computing the desired `what()` only once.
2. The base class `what()` definition is sufficient and nothrow.
3. I'm not storing multiple strings.

This is smaller code, smaller data, and faster.

`ios_base::failure` has the same issue.

[ Comments about this change received favorable comments from the `system_error` designers. ]

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

In 19.5.8.2 [syserr.syserr.members], change the following constructor postconditions:

>     system_error(error_code ec, const string& what_arg);

> > -2- Postconditions: `code() == ec` and `strcmp(runtime_error::what(), what_arg.c_str()) == 0 string(what()).find(what_arg) != string::npos`.

>     system_error(error_code ec, const char* what_arg);

> > -4- Postconditions: `code() == ec` and `strcmp(runtime_error::what(), what_arg) == 0 string(what()).find(what_arg) != string::npos`.

>     system_error(error_code ec);

> > -6- Postconditions: `code() == ec` and `strcmp(runtime_error::what(), ""`.

>     system_error(int ev, const error_category& ecat, const string& what_arg);

> > -8- Postconditions: `code() == error_code(ev, ecat)` and `strcmp(runtime_error::what(), what_arg.c_str()) == 0 string(what()).find(what_arg) != string::npos`.

>     system_error(int ev, const error_category& ecat, const char* what_arg);

> > -10- Postconditions: `code() == error_code(ev, ecat)` and `strcmp(runtime_error::what(), what_arg) == 0 string(what()).find(what_arg) != string::npos`.

>     system_error(int ev, const error_category& ecat);

> > -12- Postconditions: `code() == error_code(ev, ecat)` and `strcmp(runtime_error::what(), "") == 0`.

In 19.5.8.2 [syserr.syserr.members], change the description of `what()`:

>     const char *what() const throw();

> > -14- Returns: An NTBS incorporating `runtime_error::what()` and `code().message()` the arguments supplied in the constructor.

> > [Note: One possible implementation would be: The return NTBS might take the form: `what_arg + ": " + code().message()`

> > >     if (msg.empty()) {
> > >       try {
> > >         string tmp = runtime_error::what();
> > >         if (code()) {
> > >           if (!tmp.empty())
> > >             tmp += ": ";
> > >           tmp += code().message();
> > >         }
> > >         swap(msg, tmp);
> > >       } catch(...) {
> > >         return runtime_error::what();
> > >       }
> > >     return msg.c_str();

> > — end note]

In [ios::failure], change the synopsis:

>     namespace std {
>       class ios_base::failure : public system_error {
>       public:
>         explicit failure(const string& msg, const error_code& ec = io_errc::stream);
>         explicit failure(const char* msg, const error_code& ec = io_errc::stream);
>         virtual const char* what() const throw();
>       };
>     }

In [ios::failure], change the description of the constructors:

>     explicit failure(const string& msg, , const error_code& ec = io_errc::stream);

> > -3- Effects: Constructs an object of class `failure` by constructing the base class with `msg` and `ec`.

> > -4- Postcondition: `code() == ec` and `strcmp(what(), msg.c_str()) == 0`

>     explicit failure(const char* msg, const error_code& ec = io_errc::stream);

> > -5- Effects: Constructs an object of class `failure` by constructing the base class with `msg` and `ec`.

> > -6- Postcondition: `code() == ec and strcmp(what(), msg) == 0`

In [ios::failure], remove `what` (the base class definition need not be repeated here).

>     const char* what() const;

> > -7- Returns: The message `msg` with which the exception was created.

### 1104(i). `basic_ios::move` should accept lvalues

Section: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

With the rvalue reference changes in N2844 `basic_ios::move` no longer has the most convenient signature:

>     void move(basic_ios&& rhs);

This signature should be changed to accept lvalues. It does not need to be overloaded to accept rvalues. This is a special case that only derived clients will see. The generic `move` still needs to accept rvalues.

[ Batavia (2009-05): ]

> Tom prefers, on general principles, to provide both overloads. Alisdair agrees.

> Howard points out that there is no backward compatibility issue as this is new to C++0X.

> We agree that both overloads should be provided, and Howard will provide the additional wording. Move to Open.

[ 2009-05-23 Howard adds: ]

> Added overload, moved to Review.

[ 2009 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add a signature to the existing prototype in the synopsis of 31.5.4 [ios] and in 31.5.4.3 [basic.ios.members]:

>     void move(basic_ios& rhs);
>     void move(basic_ios&& rhs);

### 1106(i). Multiple exceptions from connected `shared_future::get()`?

Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Thomas J. Gritzan Opened: 2009-04-03 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

It is not clear, if multiple threads are waiting in a `shared_future::get()` call, if each will rethrow the stored exception.

Paragraph 9 reads:

> Throws: the stored exception, if an exception was stored and not retrieved before.

The "not retrieved before" suggests that only one exception is thrown, but one exception for each call to `get()` is needed, and multiple calls to `get()` even on the same `shared_future` object seem to be allowed.

I suggest removing "and not retrieved before" from the Throws paragraph. I recommend adding a note that explains that multiple calls on `get()` are allowed, and each call would result in an exception if an exception was stored.

[ Batavia (2009-05): ]

> We note there is a pending paper by Detlef on such `future`-related issues; we would like to wait for his paper before proceeding.

> Alisdair suggests we may want language to clarify that this `get()` function can be called from several threads with no need for explicit locking.

> Move to Open.

[ 2010-01-23 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. ]

Rationale:

Resolved by paper N2997.

Proposed resolution:

Change [futures.shared_future]:

>     const R& shared_future::get() const;
>     R& shared_future<R&>::get() const;
>     void shared_future<void>::get() const;

> > ...

> > -9- Throws: the stored exception, if an exception was stored and not retrieved before. [Note: Multiple calls on `get()` are allowed, and each call would result in an exception if an exception was stored. — end note]

### 1108(i). thread.req.exception overly constrains implementations

Section: 32.2.2 [thread.req.exception] Status: C++11 Submitter: Christopher Kohlhoff Opened: 2009-04-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current formulation of 32.2.2 [thread.req.exception]/2 reads:

> The `error_category` of the `error_code` reported by such an exception's `code()` member function is as specified in the error condition Clause.

This constraint on the code's associated `error_categor` means an implementation must perform a mapping from the system-generated error to a `generic_category()` error code. The problems with this include:

- The mapping is always performed, even if the resultant value is never used.

- The original error produced by the operating system is lost.

The latter was one of Peter Dimov's main objections (in a private email discussion) to the original `error_code`-only design, and led to the creation of `error_condition` in the first place. Specifically, `error_code` and `error_condition` are intended to perform the following roles:

- `error_code` holds the original error produced by the operating system.
- `error_condition` and the generic category provide a set of well known error constants that error codes may be tested against.

Any mapping determining correspondence of the returned error code to the conditions listed in the error condition clause falls under the "latitude" granted to implementors in 19.5.3.5 [syserr.errcat.objects]. (Although obviously their latitude is restricted a little by the need to match the right error condition when returning an error code from a library function.)

It is important that this `error_code/error_condition` usage is done correctly for the thread library since it is likely to set the pattern for future TR libraries that interact with the operating system.

[ Batavia (2009-05): ]

> Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

Change 32.2.2 [thread.req.exception] p.2:

> -2- The `error_category` (19.5.1.1) of the `error_code` reported by such an exception's `code()` member function is as specified in the error condition Clause. The `error_code` reported by such an exception's `code()` member function shall compare equal to one of the conditions specified in the function's error condition Clause. [Example: When the thread constructor fails:

> >     ec.category() == implementation-defined // probably system_category
> >     ec == errc::resource_unavailable_try_again // holds true

> — end example]

### 1110(i). Is `for_each` overconstrained?

Section: 26.6.5 [alg.foreach] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-04-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Quoting working paper for reference (26.6.5 [alg.foreach]):

>     template<InputIterator Iter, Callable<auto, Iter::reference> Function>
>       requires CopyConstructible<Function>
>       Function for_each(Iter first, Iter last, Function f);

> > 1 Effects: Applies f to the result of dereferencing every iterator in the range [first,last), starting from first and proceeding to last - 1.

> > 2 Returns: f.

> > 3 Complexity: Applies f exactly last - first times.

P2 implies the passed object `f` should be invoked at each stage, rather than some copy of `f`. This is important if the return value is to usefully accumulate changes. So the requirements are an object of type `Function` can be passed-by-value, invoked multiple times, and then return by value. In this case, `MoveConstructible` is sufficient. This would open support for move-only functors, which might become important in concurrent code as you can assume there are no other references (copies) of a move-only type and so freely use them concurrently without additional locks.

[ See further discussion starting with c++std-lib-23686. ]

[ Batavia (2009-05): ]

> Pete suggests we may want to look at this in a broader context involving other algorithms. We should also consider the implications of parallelism.

> Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-10-14 Daniel de-conceptified the proposed resolution. ]

> The note in 26.1 [algorithms.general]/9 already says the right thing:

> > Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.

> So we only need to ensure that the wording for `for_each` is sufficiently clear, which is the intend of the following rewording.

[ 2009-10-15 Daniel proposes: ]

> - Add a new Requires clause just after the prototype declaration (26.6.5 [alg.foreach]):

>   > Requires: `Function` shall be `MoveConstructible` ( [moveconstructible]), `CopyConstructible` is not required.

> - Change 26.6.5 [alg.foreach]/2 as indicated:

>   > Returns: std::move(f).

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready, using Daniel's wording without the portion saying "CopyConstructible is not required".

[ 2009-10-27 Daniel adds: ]

> I see that during the Santa Cruz meeting the originally proposed addition

> > , `CopyConstructible` is not required.

> was removed. I don't think that this removal was a good idea. The combination of 26.1 [algorithms.general] p.9

> > [Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.[..]

> with the fact that `CopyConstructible` is a refinement `MoveConstructible` makes it necessary that such an explicit statement is given. Even the existence of the usage of `std::move` in the Returns clause doesn't help much, because this would still be well-formed for a `CopyConstructible` without move constructor. Let me add that the originally proposed addition reflects current practice in the standard, e.g. 26.7.9 [alg.unique] p.5 usages a similar terminology.

> For similar wording need in case for auto_ptr see 973(i).

> [ Howard: Moved from Tentatively Ready to Open. ]

[ 2009-11-20 Howard restores "not `CopyConstructible`" to the spec. ]

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

- Add a new Requires clause just after the prototype declaration (26.6.5 [alg.foreach]):

  > Requires: `Function` shall meet the requirements of `MoveConstructible` ( [moveconstructible]). `Function` need not meet the requirements of `CopyConstructible` ( [copyconstructible]).

- Change 26.6.5 [alg.foreach]/2 as indicated:

  > Returns: std::move(f).

### 1113(i). `bitset::to_string` could be simplified

Section: 22.9.2 [template.bitset] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 853(i) our resolution is changing the signature by adding two defaulting arguments to 3 calls. In principle, this means that ABI breakage is not an issue, while API is preserved.

With that observation, it would be very nice to use the new ability to supply default template parameters to function templates to collapse all 3 signatures into 1. In that spirit, this issue offers an alternative resolution than that of 853(i).

[ Batavia (2009-05): ]

> Move to Open, and look at the issue again after 853(i) has been accepted. We further recommend this be deferred until after the next Committee Draft.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

1. In 22.9.2 [template.bitset]/1 (class bitset) ammend:

   >     template <class charT = char,
   >                 class traits = char_traits<charT>,
   >                 class Allocator = allocator<charT>>
   >       basic_string<charT, traits, Allocator>
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;
   >     template <class charT, class traits>
   >       basic_string<charT, traits, allocator<charT> > to_string() const;
   >     template <class charT>
   >       basic_string<charT, char_traits<charT>, allocator<charT> > to_string() const;
   >     basic_string<char, char_traits<char>, allocator<char> > to_string() const;

2. In 22.9.2.3 [bitset.members] prior to p35 ammend:

   >     template <class charT = char,
   >                 class traits = char_traits<charT>,
   >                 class Allocator = allocator<charT>>
   >       basic_string<charT, traits, Allocator>
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;

3. Strike 22.9.2.3 [bitset.members] paragraphs 37 -> 39 (including signature above 37)

### 1114(i). Type traits underspecified

Section: 21 [meta] Status: C++11 Submitter: Daniel Krügler Opened: 2009-05-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Related to 975(i) and 1023(i).

The current wording in 21.3.2 [meta.rqmts] is still unclear concerning it's requirements on the type traits classes regarding ambiguities. Specifically it's unclear

- if a predicate trait (21.3.5 [meta.unary], 21.3.7 [meta.rel]) could derive from both `true_type`/`false_type`.
- if any of the type traits (21.3.2 [meta.rqmts], 21.3.5 [meta.unary], 21.3.7 [meta.rel]) could ambiguously derive from the same specified result type.
- if any of the type traits (21.3.2 [meta.rqmts], 21.3.5 [meta.unary], 21.3.7 [meta.rel]) could derive from other `integral_constant` types making the contained names ambiguous
- if any of the type traits (21.3.2 [meta.rqmts], 21.3.5 [meta.unary], 21.3.7 [meta.rel]) could have other base classes that contain members hiding the name of the result type members or make the contained member names ambiguous.

[ Batavia (2009-05): ]

> Alisdair would prefer to factor some of the repeated text, but modulo a corner case or two, he believes the proposed wording is otherwise substantially correct.

> Move to Open.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

[ The usage of the notion of a BaseCharacteristic below might be useful in other places - e.g. to define the base class relation in 22.10.6 [refwrap], 22.10.16 [func.memfn], or 22.10.17.3 [func.wrap.func]. In this case it's definition should probably be moved to Clause 17 ]

1. Change 21.3.2 [meta.rqmts] p.1 as indicated:

   > [..] It shall be `DefaultConstructible`, `CopyConstructible`, and publicly and unambiguously derived, directly or indirectly, from its BaseCharacteristic, which is a specialization of the template `integral_constant` (20.6.3), with the arguments to the template `integral_constant` determined by the requirements for the particular property being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the UnaryTypeTrait.

2. Change 21.3.2 [meta.rqmts] p.2 as indicated:

   > [..] It shall be `DefaultConstructible`, `CopyConstructible`, and publicly and unambiguously derived, directly or indirectly, from an instance its BaseCharacteristic, which is a specialization of the template `integral_constant` (20.6.3), with the arguments to the template `integral_constant` determined by the requirements for the particular relationship being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the BinaryTypeTrait.

3. Change 21.3.5 [meta.unary] p.2 as indicated:

   > Each of these templates shall be a UnaryTypeTrait (20.6.1), publicly derived directly or indirectly from `true_type` if the corresponding condition is true, otherwise from `false_type` where its BaseCharacteristic shall be `true_type` if the corresponding condition is true, otherwise `false_type`.

4. Change 21.3.7 [meta.rel] p.2 as indicated:

   > Each of these templates shall be a BinaryTypeTrait (20.6.1), publicly derived directly or indirectly from `true_type` if the corresponding condition is true, otherwise from `false_type` where its BaseCharacteristic shall be `true_type` if the corresponding condition is true, otherwise `false_type`.

### 1116(i). Literal constructors for tuple

Section: 22.4.4 [tuple.tuple] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

It is not currently possible to construct `tuple` literal values, even if the elements are all literal types. This is because parameters are passed to constructor by reference.

An alternative would be to pass all constructor arguments by value, where it is known that *all* elements are literal types. This can be determined with concepts, although note that the negative constraint really requires factoring out a separate concept, as there is no way to provide an 'any of these fails' constraint inline.

Note that we will have similar issues with `pair` (and `tuple` constructors from `pair`) although I am steering clear of that class while other constructor-related issues settle.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2994.

Proposed resolution:

Ammend the tuple class template declaration in 22.4.4 [tuple.tuple] as follows

> Add the following concept:

> >     auto concept AllLiteral< typename ... Types > {
> >       requires LiteralType<Types>...;
> >     }

> ammend the constructor

> >     template <class... UTypes>
> >       requires AllLiteral<Types...>
> >             && Constructible<Types, UTypes>...
> >       explicit tuple(UTypes...);

> >     template <class... UTypes>
> >       requires !AllLiteral<Types...>
> >             && Constructible<Types, UTypes&&>...
> >       explicit tuple(UTypes&&...);

> ammend the constructor

> >     template <class... UTypes>
> >       requires AllLiteral<Types...>
> >             && Constructible<Types, UTypes>...
> >       tuple(tuple<UTypes...>);

> >     template <class... UTypes>
> >       requires !AllLiteral<Types...>
> >             && Constructible<Types, const UTypes&>...
> >       tuple(const tuple<UTypes...>&);

Update the same signatures in 22.4.4.2 [tuple.cnstr], paras 3 and 5.

### 1117(i). `tuple` copy constructor

Section: 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The copy constructor for the `tuple` template is constrained. This seems an unusual strategy, as the copy constructor will be implicitly deleted if the constraints are not met. This is exactly the same effect as requesting an `=default;` constructor. The advantage of the latter is that it retains triviality, and provides support for `tuple`s as literal types if issue 1116(i) is also accepted.

Actually, it might be worth checking with core if a constrained copy constructor is treated as a constructor template, and as such does not suppress the implicit generation of the copy constructor which would hide the template in this case.

[ 2009-05-27 Daniel adds: ]

> This would solve one half of the suggested changes in 801(i).

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2994.

Proposed resolution:

Change 22.4.4 [tuple.tuple] and 22.4.4.2 [tuple.cnstr] p4:

>     requires CopyConstructible<Types>... tuple(const tuple&) = default;

### 1118(i). `tuple` query APIs do not support cv-qualification

Section: 22.4.7 [tuple.helper] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The APIs `tuple_size` and `tuple_element` do not support cv-qualified `tuple`s, `pair`s or `array`s.

The most generic solution would be to supply partial specializations once for each cv-type in the `tuple` header. However, requiring this header for cv-qualified `pair`s/`array`s seems unhelpful. The BSI editorial suggestion (UK-198/US-69, N2533) to merge `tuple` into `<utility>` would help with `pair`, but not `array`. That might be resolved by making a dependency between the `<array>` header and `<utility>`, or simply recognising the dependency be fulfilled in a Remark.

[ 2009-05-24 Daniel adds: ]

> All `tuple_size` templates with a base class need to derive publicly, e.g.

> >     template <IdentityOf T> class tuple_size< const T > :
> >        public tuple_size<T> {};

> The same applies to the tuple_element class hierarchies.

> What is actually meant with the comment

> > this solution relies on 'metafunction forwarding' to inherit the nested typename type

> ?

> I ask, because all base classes are currently unconstrained and their instantiation is invalid in the constrained context of the `tuple_element` partial template specializations.

[ 2009-05-24 Alisdair adds: ]

> I think a better solution might be to ask Pete editorially to change all declarations of tupling APIs to use the struct specifier instead of class.

> "metafunction forwarding" refers to the MPL metafunction protocol, where a metafunction result is declared as a nested typedef with the name "type", allowing metafunctions to be chained by means of inheritance. It is a neater syntax than repeatedly declaring a typedef, and inheritance syntax is slightly nicer when it comes to additional typename keywords.

> The constrained template with an unconstrained base is a good observation though.

[ 2009-10 post-Santa Cruz: ]

> Move to Open, Alisdair to provide wording. Once wording is provided, Howard will move to Review.

[ 2010-03-28 Daniel deconceptified wording. ]

[ Post-Rapperswil - Daniel provides wording: ]

The below given P/R reflects the discussion from the Rapperswil meeting that the wording should not constrain implementation freedom to realize the actual issue target. Thus the original code form was replaced by normative words.

While preparing this wording it turned out that several `tuple_size` specializations as that of `pair` and `array` are underspecified, because the underlying type of the member value is not specified except that it is an integral type. For the specializations we could introduce a canonical one - like `size_t` - or we could use the same type as the specialization of the unqualified type uses. The following wording follows the second approach.

The wording refers to N3126.

> Moved to Tentatively Ready after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change 22.4.1 [tuple.general] p.2, header `<tuple>` synopsis, as indicated:

   >     // 20.4.2.5, tuple helper classes:
   >     template <class T> class tuple_size; // undefined
   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   >     template <class... Types> class tuple_size<tuple<Types...> >;

   >     template <size_t I, class T> class tuple_element; // undefined
   >     template <size_t I, class T> class tuple_element<I, const T>;
   >     template <size_t I, class T> class tuple_element<I, volatile T>;
   >     template <size_t I, class T> class tuple_element<I, const volatile T>;

   >     template <size_t I, class... Types> class tuple_element<I, tuple<Types...> >;

2. Add the end of subclause 22.4.7 [tuple.helper] insert the following two paragraphs:

   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   > > Let TS denote `tuple_size<T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the UnaryTypeTrait requirements (20.7.1) with a BaseCharacteristic of `integral_constant<remove_cv<decltype(TS::value)>::type, TS::value>`.

   >     template <size_t I, class T> class tuple_element<I, const T>;
   >     template <size_t I, class T> class tuple_element<I, volatile T>;
   >     template <size_t I, class T> class tuple_element<I, const volatile T>;

   > > Let TE denote `tuple_element<I, T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the TransformationTrait requirements (20.7.1) with a member typedef `type` that shall name the same type as the following type:

   > > - for the first specialization, the type `add_const<TE::type>::type`,
   > > - for the second specialization, the type `add_volatile<TE::type>::type`, and
   > > - for the third specialization, the type `add_cv<TE::type>::type`

### 1122(i). Ratio values should be `constexpr`

Section: 21.5.3 [ratio.ratio] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ratio.ratio].

View all issues with Resolved status.

Discussion:

The values `num` and `den` in the `ratio` template should be declared `constexpr`.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2994.

Proposed resolution:

21.5.3 [ratio.ratio]

>     namespace std {
>       template <intmax_t N, intmax_t D = 1>
>       class ratio {
>       public:
>         static constexpr intmax_t num;
>         static constexpr intmax_t den;
>       };
>     }

### 1123(i). No requirement that standard streams be flushed

Section: 31.5.2.2.6 [ios.init] Status: C++11 Submitter: James Kanze Opened: 2009-05-14 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [ios.init].

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Discussion:

As currently formulated, the standard doesn't require that there is ever a flush of `cout`, etc. (This implies, for example, that the classical hello, world program may have no output.) In the current draft (N2798), there is a requirement that the objects be constructed before `main`, and before the dynamic initialization of any non-local objects defined after the inclusion of `<iostream>` in the same translation unit. The only requirement that I can find concerning flushing, however, is in [ios::Init], where the destructor of the last `std::ios_base::Init` object flushes. But there is, as far as I can see, no guarantee that such an object ever exists.

Also, the wording in [iostreams.objects] says that:

> The objects are constructed and the associations are established at some time prior to or during the first time an object of class `ios_base::Init` is constructed, and in any case before the body of main begins execution.

In [ios::Init], however, as an effect of the constructor, it says that

> If `init_cnt` is zero, the function stores the value one in `init_cnt`, then constructs and initializes the objects `cin`, `cout`, `cerr`, `clog` `wcin`, `wcout`, `wcerr`, and `wclog`"

which seems to forbid earlier construction.

(Note that with these changes, the exposition only "`static int init_cnt`" in `ios_base::Init` can be dropped.)

Of course, a determined programmer can still inhibit the flush with things like:

>     new std::ios_base::Init ;       //  never deleted

or (in a function):

>     std::ios_base::Init ensureConstruction ;
>     //  ...
>     exit( EXIT_SUCCESS ) ;

Perhaps some words somewhere to the effect that all `std::ios_base::Init` objects should have static lifetime would be in order.

[ 2009 Santa Cruz: ]

> Moved to Ready. Some editorial changes are expected (in addition to the proposed wording) to remove `init_cnt` from `Init`.

Proposed resolution:

Change 31.4 [iostream.objects]/2:

> -2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class `ios_base::Init` is constructed, and in any case before the body of main begins execution.292 The objects are not destroyed during program execution.293 If a translation unit includes `<iostream>` or explicitly constructs an `ios_base::Init` object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit. The results of including `<iostream>` in a translation unit shall be as if `<iostream>` defined an instance of `ios_base::Init` with static lifetime. Similarly, the entire program shall behave as if there were at least one instance of `ios_base::Init` with static lifetime.

Change [ios::Init]/3:

>     Init();

> > -3- Effects: Constructs an object of class `Init`. If `init_cnt` is zero, the function stores the value one in `init_cnt`, then constructs and initializes the objects `cin`, `cout`, `cerr`, `clog` (27.4.1), `wcin`, `wcout`, `wcerr`, and `wclog` (27.4.2). In any case, the function then adds one to the value stored in `init_cnt`. Constructs and initializes the objects `cin`, `cout`, `cerr`, `clog`, `wcin`, `wcout`, `wcerr` and `wclog` if they have not already been constructed and initialized.

Change [ios::Init]/4:

>     ~Init();

> > -4- Effects: Destroys an object of class `Init`. The function subtracts one from the value stored in `init_cnt` and, if the resulting stored value is one, If there are no other instances of the class still in existance, calls `cout.flush()`, `cerr.flush()`, `clog.flush()`, `wcout.flush()`, `wcerr.flush()`, `wclog.flush()`.

### 1126(i). `istreambuff_iterator::equal` needs a const & parameter

Section: 24.6.4.4 [istreambuf.iterator.ops] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2017-11-29

Priority: Not Prioritized

View all other issues in [istreambuf.iterator.ops].

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Discussion:

The `equal` member function of `istreambuf_iterator` is declared `const`, but takes its argument by non-const reference.

This is not compatible with the `operator==` free function overload, which is defined in terms of calling `equal` yet takes both arguments by reference to const.

[ The proposed wording is consistent with 110(i) with status TC1. ]

[ 2009-11-02 Howard adds: ]

> Set to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Ammend in both:
24.6.4 [istreambuf.iterator]
[istreambuf.iterator::equal]

>     bool equal(const istreambuf_iterator& b) const;

### 1129(i). `istream(buf)_iterator` should support literal sentinel value

Section: 24.6.2.2 [istream.iterator.cons], 24.6.4 [istreambuf.iterator] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-30 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.iterator.cons].

View all issues with Resolved status.

Discussion:

`istream_iterator` and `istreambuf_iterator` should support literal sentinel values. The default constructor is frequently used to terminate ranges, and could easily be a literal value for `istreambuf_iterator`, and `istream_iterator` when iterating value types. A little more work using a suitably sized/aligned char-array for storage (or an updated component like `boost::optional` proposed for TR2) would allow `istream_iterator` to support `constexpr` default constructor in all cases, although we might leave this tweak as a QoI issue. Note that requiring `constexpr` be supported also allows us to place no-throw guarantees on this constructor too.

[ 2009-06-02 Daniel adds: ]

> I agree with the usefulness of the issue suggestion, but we need to ensure that `istream_iterator` can satisfy be literal if needed. Currently this is not clear, because 24.6.2 [istream.iterator]/3 declares a copy constructor and a destructor and explains their semantic in 24.6.2.2 [istream.iterator.cons]/3+4.

> The prototype semantic specification is ok (although it seems somewhat redundant to me, because the semantic doesn't say anything interesting in both cases), but for support of trivial class types we also need a trivial copy constructor and destructor as of 11 [class]/6. The current non-informative specification of these two special members suggests to remove their explicit declaration in the class and add explicit wording that says that if `T` is trivial a default constructed iterator is also literal, alternatively it would be possible to mark both as defaulted and add explicit (memberwise) wording that guarantees that they are trivial.

> Btw.: I'm quite sure that the `istreambuf_iterator` additions to ensure triviality are not sufficient as suggested, because the library does not yet give general guarantees that a defaulted special member declaration makes this member also trivial. Note that e.g. the atomic types do give a general statement!

> Finally there is a wording issue: There does not exist something like a "literal constructor". The core language uses the term "constexpr constructor" for this.

> Suggestion:

> 1. Change 24.6.2 [istream.iterator]/3 as indicated:

>    >     constexpr istream_iterator();
>    >     istream_iterator(istream_type& s);
>    >     istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;
>    >     ~istream_iterator() = default;

> 2. Change 24.6.2.2 [istream.iterator.cons]/1 as indicated:

>    >     constexpr istream_iterator();

>    > > -1- Effects: Constructs the end-of-stream iterator. If `T` is a literal type, then this constructor shall be a constexpr constructor.

> 3. Change 24.6.2.2 [istream.iterator.cons]/3 as indicated:

>    >     istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;

>    > > -3- Effects: Constructs a copy of `x`. If `T` is a literal type, then this constructor shall be a trivial copy constructor.

> 4. Change 24.6.2.2 [istream.iterator.cons]/4 as indicated:

>    >     ~istream_iterator() = default;

>    > > -4- Effects: The iterator is destroyed. If `T` is a literal type, then this destructor shall be a trivial destructor.

> 5. Change 24.6.4 [istreambuf.iterator] before p. 1 as indicated:

>    >     constexpr istreambuf_iterator() throw();
>    >     istreambuf_iterator(const istreambuf_iterator&)  throw() = default;
>    >     ~istreambuf_iterator()  throw() = default;

> 6. Change 24.6.4 [istreambuf.iterator]/1 as indicated:

>    > [..] The default constructor `istreambuf_iterator()` and the constructor `istreambuf_iterator(0)` both construct an end of stream iterator object suitable for use as an end-of-range. All specializations of `istreambuf_iterator` shall have a trivial copy constructor, a constexpr default constructor and a trivial destructor.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2994.

Proposed resolution:

24.6.2 [istream.iterator] para 3

>     constexpr istream_iterator();

24.6.2.2 [istream.iterator.cons]

>     constexpr istream_iterator();

> > -1- Effects: Constructs the end-of-stream iterator. If `T` is a literal type, then this constructor shall be a literal constructor.

24.6.4 [istreambuf.iterator]

>     constexpr istreambuf_iterator() throw();

### 1130(i). `copy_exception` name misleading

Section: 17.9.7 [propagation] Status: C++11 Submitter: Peter Dimov Opened: 2009-05-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [propagation].

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Discussion:

The naming of `std::copy_exception` misleads almost everyone (experts included!) to think that it is the function that copies an `exception_ptr`:

>     exception_ptr p1 = current_exception();
>     exception_ptr p2 = copy_exception( p1 );

But this is not at all what it does. The above actually creates an `exception_ptr p2` that contains a copy of `p1`, not of the exception to which `p1` refers!

This is, of course, all my fault; in my defence, I used `copy_exception` because I was unable to think of a better name.

But I believe that, based on what we've seen so far, any other name would be better.

Therefore, I propose `copy_exception` to be renamed to `create_exception`:

>     template<class E> exception_ptr create_exception(E e);

with the following explanatory paragraph after it:

> Creates an `exception_ptr` that refers to a copy of `e`.

[ 2009-05-13 Daniel adds: ]

> What about

> >     make_exception_ptr

> in similarity to `make_pair` and `make_tuple`, `make_error_code` and `make_error_condition`, or `make_shared`? Or, if a stronger symmetry to `current_exception` is preferred:

> >     make_exception

> We have not a single `create_*` function in the library, it was always `make_*` used.

[ 2009-05-13 Peter adds: ]

> `make_exception_ptr` works for me.

[ 2009-06-02 Thomas J. Gritzan adds: ]

> To avoid surprises and unwanted recursion, how about making a call to `std::make_exception_ptr` with an `exception_ptr` illegal?

> It might work like this:

> >     template<class E>
> >     exception_ptr make_exception_ptr(E e);
> >     template<>
> >     exception_ptr make_exception_ptr<exception_ptr>(exception_ptr e) = delete;

[ 2009 Santa Cruz: ]

> Move to Review for the time being. The subgroup thinks this is a good idea, but doesn't want to break compatibility unnecessarily if someone is already shipping this. Let's talk to Benjamin and PJP tomorrow to make sure neither objects.

[ 2009-11-16 Jonathan Wakely adds: ]

> GCC 4.4 shipped with `copy_exception` but we could certainly keep that symbol in the library (but not the headers) so it can still be found by any apps foolishly relying on the experimental C++0x mode being ABI stable.

[ 2009-11-16 Peter adopts wording supplied by Daniel. ]

[ 2009-11-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 17.9 [support.exception]/1, header `<exception>` synopsis as indicated:

   >     exception_ptr current_exception();
   >     void rethrow_exception [[noreturn]] (exception_ptr p);
   >     template<class E> exception_ptr copy_exceptionmake_exception_ptr(E e);

2. Change 17.9.7 [propagation]:

   >     template<class E> exception_ptr copy_exceptionmake_exception_ptr(E e);

   > > -11- Effects: Creates an `exception_ptr` that refers to a copy of `e`, as if

   > > >     try {
   > > >       throw e;
   > > >     } catch(...) {
   > > >       return current_exception();
   > > >     }

   > > ...

3. Change 32.10.6 [futures.promise]/7 as indicated:

   > Effects: if the associated state of `*this` is not ready, stores an exception object of type `future_error` with an error code of `broken_promise` as if by `this->set_exception(copy_exceptionmake_exception_ptr( future_error(future_errc::broken_promise))`. Destroys ...

### 1131(i). C++0x does not need `alignment_of`

Section: 21.3.5.4 [meta.unary.prop] Status: C++11 Submitter: Niels Dekker Opened: 2009-06-01 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

View all other issues in [meta.unary.prop].

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Discussion:

The `alignment_of` template is no longer necessary, now that the core language will provide `alignof`. Scott Meyers raised this issue at comp.std.c++, C++0x: alignof vs. alignment_of, May 21, 2009. In a reply, Daniel Krügler pointed out that `alignof` was added to the working paper after `alignment_of`. So it appears that `alignment_of` is only part of the current Working Draft (N2857) because it is in TR1.

Having both `alignof` and `alignment_of` would cause unwanted confusion. In general, I think TR1 functionality should not be brought into C++0x if it is entirely redundant with other C++0x language or library features.

[ 2009-11-16 Chris adds: ]

> I would like to suggest the following new wording for this issue, based on recent discussions. Basically this doesn't delete `alignment_of`, it just makes it clear that it is just a wrapper for `alignof`. This deletes the first part of the proposed resolution, changes the second part by leaving in `alignof(T)` but changing the precondition and leaves the 3rd part unchanged.

> Suggested Resolution:

> > Change the first row of Table 44 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:

> > > Table 44 — Type property queries
> > > `template <class T> struct alignment_of;` | `alignof(T)`. Precondition: `T` shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) `void`. `alignof(T)` shall be defined

> > Change text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:

> > > Table 51 — Other transformations
> > > …`aligned_storage;` | `Len` shall not be zero. `Align` shall be equal to `alignment_of<T>::value alignof(T)` for some type `T` or to default-alignment. | …

[ 2010-01-30 Alisdair proposes that Chris' wording be moved into the proposed wording section and tweaks it on the way. ]

> Original proposed wording saved here:

> > Remove from Header <type_traits> synopsis 21.3.3 [meta.type.synop]:

> > >     template <class T> struct alignment_of;

> > Remove the first row of Table 44 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:

> > > Table 44 — Type property queries
> > > `template <class T> struct alignment_of;` | `alignof(T)`. Precondition: `T` shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) `void`.

> > Change text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:

> > > Table 51 — Other transformations
> > > …`aligned_storage;` | `Len` shall not be zero. Align shall be equal to `alignment_of<T>::value alignof(T)` for some type `T` or to default-alignment. | …

[ 2010-01-30 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change the first row of Table 43 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:

> Table 43 — Type property queries
> `template <class T> struct alignment_of;` | `alignof(T)`. Precondition: `T` shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) `void`. `alignof(T)` is a valid expression (7.6.2.6 [expr.alignof])

Change text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:

> Table 51 — Other transformations
> …`aligned_storage;` | `Len` shall not be zero. `Align` shall be equal to `alignment_of<T>::value alignof(T)` for some type `T` or to default-alignment. | …

### 1133(i). Does N2844 break current specification of list::splice?

Section: 23.3.7.6 [forward.list.ops], 23.3.11.5 [list.ops] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [forward.list.ops].

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Discussion:

IIUC, N2844 means that lvalues will no longer bind to rvalue references. Therefore, the current specification of `list::splice` (list operations 23.3.11.5 [list.ops]) will be a breaking change of behaviour for existing programs. That is because we changed the signature to swallow via an rvalue reference rather than the lvalue reference used in 03.

Retaining this form would be safer, requiring an explicit move when splicing from lvalues. However, this will break existing programs. We have the same problem with `forward_list`, although without the risk of breaking programs so here it might be viewed as a positive feature.

The problem signatures:

>     void splice_after(const_iterator position, forward_list<T,Alloc>&& x);
>     void splice_after(const_iterator position, forward_list<T,Alloc>&& x,
>                       const_iterator i);
>     void splice_after(const_iterator position, forward_list<T,Alloc>&& x,
>                       const_iterator first, const_iterator last);

>     void splice(const_iterator position, list<T,Alloc>&& x);
>     void splice(const_iterator position, list<T,Alloc>&& x,
>                 const_iterator i);
>     void splice(const_iterator position, list<T,Alloc>&& x,
>                 const_iterator first, const_iterator last);

Possible resolutions:

Option A. Add an additional (non-const) lvalue-reference overload in each case

Option B. Change rvalue reference back to (non-const) lvalue-reference overload in each case

Option C. Add an additional (non-const) lvalue-reference overload in just the `std::list` cases

I think (B) would be very unfortunate, I really like the `forward_list` behaviour in (C) but feel (A) is needed for consistency.

My actual preference would be NAD, ship with this as a breaking change as it is a more explicit interface. I don't think that will fly though!

See the thread starting with c++std-lib-23725 for more discussion.

[ 2009-10-27 Christopher Jefferson provides proposed wording for Option C. ]

[ 2009-12-08 Jonathan Wakely adds: ]

> As Bill Plauger pointed out, `list::merge` needs similar treatment.

> [ Wording updated. ]

[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In 23.3.11 [list]

Add lvalue overloads before rvalue ones:

>     void splice(const_iterator position, list<T,Allocator>& x);
>     void splice(const_iterator position, list<T,Allocator>&& x);
>     void splice(const_iterator position, list<T,Allocator>& x, const_iterator i);
>     void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i);
>     void splice(const_iterator position, list<T,Allocator>& x,
>                 const_iterator first, const_iterator last);
>     void splice(const_iterator position, list<T,Allocator>&& x,
>                 const_iterator first, const_iterator last);
>     void merge(list<T,Allocator>& x);
>     template <class Compare> void merge(list<T,Allocator>& x, Compare comp);
>     void merge(list<T,Allocator>&& x);
>     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

In 23.3.11.5 [list.ops], similarly add lvalue overload before each rvalue one:

(After paragraph 2)

>     void splice(const_iterator position, list<T,Allocator>& x);
>     void splice(const_iterator position, list<T,Allocator>&& x);

(After paragraph 6)

>     void splice(const_iterator position, list<T,Allocator>& x, const_iterator i);
>     void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i);

(After paragraph 10)

>     void splice(const_iterator position, list<T,Allocator>& x,
>                 const_iterator first, const_iterator last);
>     void splice(const_iterator position, list<T,Allocator>&& x,
>                 const_iterator first, const_iterator last);

In 23.3.11.5 [list.ops], after paragraph 21

>     void merge(list<T,Allocator>& x);
>     template <class Compare> void merge(list<T,Allocator>& x, Compare comp);
>     void merge(list<T,Allocator>&& x);
>     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

### 1134(i). Redundant specification of `<stdint.h>`, `<fenv.h>`, `<tgmath.h>`, and maybe `<complex.h>`

Section: 29.7 [c.math], 99 [stdinth], 99 [fenv], 99 [cmplxh] Status: C++11 Submitter: Robert Klarer Opened: 2009-05-26 Last modified: 2017-06-15

Priority: Not Prioritized

View all other issues in [c.math].

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Discussion:

This is probably editorial.

The following items should be removed from the draft, because they're redundant with Annex D, and they arguably make some *.h headers non-deprecated:

99 [stdinth] (regarding `<stdint.h>`)

99 [fenv] (regarding `<fenv.h>`

Line 3 of 29.7 [c.math] (regarding `<tgmath.h>`)

99 [cmplxh] (regarding `<complex.h>`, though the note in this subclause is not redundant)

[ 2009-06-10 Ganesh adds: ]

> While searching for `stdint` in the CD, I found that `<stdint.h>` is also mentioned in 6.9.2 [basic.fundamental] p.5. I guess it should refer to `<cstdint>` instead.

[ 2009 Santa Cruz: ]

> Real issue. Maybe just editorial, maybe not. Move to Ready.

Proposed resolution:

Remove the section 99 [stdinth].

Remove the section 99 [fenv].

Remove 29.7 [c.math], p3:

> -3- The header `<tgmath.h>` effectively includes the headers `<complex.h>` and `<math.h>`.

Remove the section 99 [cmplxh].

### 1135(i). `exception_ptr` should support contextual conversion to `bool`

Section: 17.9.7 [propagation] Status: Resolved Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [propagation].

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Discussion:

As of N2857 17.9.7 [propagation] p.5, the implementation-defined type `exception_ptr` does provide the following ways to check whether it is a null value:

>     void f(std::exception_ptr p) {
>       p == nullptr;
>       p == 0;
>       p == exception_ptr();
>     }

This is rather cumbersome way of checking for the null value and I suggest to require support for evaluation in a boolean context like so:

>     void g(std::exception_ptr p) {
>       if (p) {}
>       !p;
>     }

[ 2009 Santa Cruz: ]

> Move to Ready. Note to editor: considering putting in a cross-reference to 7.3 [conv], paragraph 3, which defines the phrase "contextually converted to `bool`".

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

In section 17.9.7 [propagation] insert a new paragraph between p.5 and p.6:

> An object `e` of type `exception_ptr` can be contextually converted to `bool`. The effect shall be as if `e != exception_ptr()` had been evaluated in place of `e`. There shall be no implicit conversion to arithmetic type, to enumeration type or to pointer type.

### 1136(i). Incomplete specification of `nested_exception::rethrow_nested()`

Section: 17.9.8 [except.nested] Status: C++11 Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [except.nested].

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Discussion:

It was recently mentioned in a newsgroup article http://groups.google.de/group/comp.std.c++/msg/f82022aff68edf3d that the specification of the member function `rethrow_nested()` of the class `nested_exception` is incomplete, specifically it remains unclear what happens, if member `nested_ptr()` returns a null value. In 17.9.8 [except.nested] we find only the following paragraph related to that:

>     void rethrow_nested() const; // [[noreturn]]

> > -4- Throws: the stored exception captured by this `nested_exception` object.

This is a problem, because it is possible to create an object of `nested_exception` with exactly such a state, e.g.

>     #include <exception>
>     #include <iostream>

>     int main() try {
>       std::nested_exception e; // OK, calls current_exception() and stores it's null value
>       e.rethrow_nested(); // ?
>       std::cout << "A" << std::endl;
>     }
>     catch(...) {
>       std::cout << "B" << std::endl;
>     }

I suggest to follow the proposal of the reporter, namely to invoke `terminate()` if `nested_ptr()` return a null value of `exception_ptr` instead of relying on the fallback position of undefined behavior. This would be consistent to the behavior of a `throw;` statement when no exception is being handled.

[ 2009 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Change around 17.9.8 [except.nested] p.4 as indicated:

> -4- Throws: the stored exception captured by this `nested_exception` object, if `nested_ptr() != nullptr`

> - Remarks: If `nested_ptr() == nullptr`, `terminate()` shall be called.

### 1137(i). Return type of `conj` and `proj`

Section: 29.4.10 [cmplx.over] Status: C++11 Submitter: Marc Steinbach Opened: 2009-06-11 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [cmplx.over].

View all issues with C++11 status.

Discussion:

In clause 1, the Working Draft (N2857) specifies overloads of the functions

>     arg, conj, imag, norm, proj, real

for non-complex arithmetic types (`float`, `double`, `long double`, and integers). The only requirement (clause 2) specifies effective type promotion of arguments.

I strongly suggest to add the following requirement on the return types:

> All the specified overloads must return real (i.e., non-complex) values, specifically, the nested `value_type` of effectively promoted arguments.

(This has no effect on `arg`, `imag`, `norm`, `real`: they are real-valued anyway.)

Rationale:

Mathematically, `conj()` and `proj()`, like the transcendental functions, are complex-valued in general but map the (extended) real line to itself. In fact, both functions act as identity on the reals. A typical user will expect `conj()` and `proj()` to preserve this essential mathematical property in the same way as `exp()`, `sin()`, etc. A typical use of `conj()`, e.g., is the generic scalar product of n-vectors:

>     template<typename T>
>     inline T
>     scalar_product(size_t n, T const* x, T const* y) {
>       T result = 0;
>       for (size_t i = 0; i < n; ++i)
>         result += x[i] * std::conj(y[i]);
>       return result;
>     }

This will work equally well for real and complex floating-point types `T` if `conj()` returns `T`. It will not work with real types if `conj()` returns complex values.

Instead, the implementation of `scalar_product` becomes either less efficient and less useful (if a complex result is always returned), or unnecessarily complicated (if overloaded versions with proper return types are defined). In the second case, the real-argument overload of `conj()` cannot be used. In fact, it must be avoided.

Overloaded `conj()` and `proj()` are principally needed in generic programming. All such use cases will benefit from the proposed return type requirement, in a similar way as the `scalar_product` example. The requirement will not harm use cases where a complex return value is expected, because of implicit conversion to complex. Without the proposed return type guarantee, I find overloaded versions of `conj()` and `proj()` not only useless but actually troublesome.

[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Insert a new paragraph after 29.4.10 [cmplx.over]/2:

> All of the specified overloads shall have a return type which is the nested `value_type` of the effectively cast arguments.

### 1138(i). Unusual return value for `operator+`

Section: 27.4.4.1 [string.op.plus] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-06-12 Last modified: 2021-06-06

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Many of the `basic_string operator+` overloads return an rvalue-reference. Is that really intended?

I'm considering it might be a mild performance tweak to avoid making un-necessary copies of a cheaply movable type, but it opens risk to dangling references in code like:

>     auto && s = string{"x"} + string{y};

and I'm not sure about:

>     auto s = string{"x"} + string{y};

[ 2009-10-11 Howard updated Returns: clause for each of these. ]

[ 2009-11-05 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Strike the `&&` from the return type in the following function signatures:

> 27.4 [string.classes] p2 Header Synopsis

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   const basic_string<charT,traits,Allocator>& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(const basic_string<charT,traits,Allocator>& lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(const charT* lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   const charT* rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);

> [string.op+]

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   const basic_string<charT,traits,Allocator>& rhs);

> > > Returns: `std::move(lhs.append(rhs))`

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(const basic_string<charT,traits,Allocator>& lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> > > Returns: `std::move(rhs.insert(0, lhs))`

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> > > Returns: `std::move(lhs.append(rhs))` [Note: Or equivalently `std::move(rhs.insert(0, lhs))` — end note]

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(const charT* lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> > > Returns: `std::move(rhs.insert(0, lhs))`.

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs);

> > > Returns: `std::move(rhs.insert(0, 1, lhs))`.

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   const charT* rhs);

> > > Returns: `std::move(lhs.append(rhs))`.

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);

> > > Returns: `std::move(lhs.append(1, rhs))`.

### 1143(i). Atomic operations library not concept enabled

Section: 32.5 [atomics] Status: Resolved Submitter: LWG Opened: 2009-06-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US 87, UK 311

The atomics chapter is not concept enabled.

Needs to also consider issues 923(i) and 924(i).

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2992.

Proposed resolution:

### 1144(i). "thread safe" is undefined

Section: 17.5 [support.start.term] Status: C++11 Submitter: LWG Opened: 2009-06-16 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [support.start.term].

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Discussion:

Addresses UK 187

The term "thread safe" is not defined nor used in this context anywhere else in the standard.

Suggested action:

Clarify the meaning of "thread safe".

[ 2009 Santa Cruz: ]

> The "thread safe" language has already been change in the WP. It was changed to "happen before", but the current WP text is still a little incomplete: "happen before" is binary, but the current WP text only mentions one thing.

> Move to Ready.

Proposed resolution:

For the following functions in 17.5 [support.start.term].

>     extern "C" int at_quick_exit(void (*f)(void));
>     extern "C++" int at_quick_exit(void (*f)(void));

Edit paragraph 10 as follows. The intent is to provide the other half of the happens before relation; to note indeterminate ordering; and to clean up some formatting.

> Effects: The `at_quick_exit()` functions register the function pointed to by `f` to be called without arguments when `quick_exit` is called. It is unspecified whether a call to `at_quick_exit()` that does not happen-before happen before (1.10) all calls to `quick_exit` will succeed. [Note: the `at_quick_exit()` functions shall not introduce a data race (17.6.4.7). exitnote —end note] [Note: The order of registration may be indeterminate if `at_quick_exit` was called from more than one thread. —end note] [Note: The `at_quick_exit` registrations are distinct from the `atexit` registrations, and applications may need to call both registration functions with the same argument. —end note]

For the following function.

>     void quick_exit [[noreturn]] (int status)

Edit paragraph 13 as follows. The intent is to note that thread-local variables may be different.

> Effects: Functions registered by calls to `at_quick_exit` are called in the reverse order of their registration, except that a function shall be called after any previously registered functions that had already been called at the time it was registered. Objects shall not be destroyed as a result of calling `quick_exit`. If control leaves a registered function called by `quick_exit` because the function does not provide a handler for a thrown exception, `terminate()` shall be called. [Note: Functions registered by one thread may be called by any thread, and hence should not rely on the identity of thread-storage-duration objects. —end note] After calling registered functions, `quick_exit` shall call `_Exit(status)`. [Note: The standard file buffers are not flushed. See: ISO C 7.20.4.4. —end note]

### 1145(i). Inappropriate headers for atomics

Section: 32.5 [atomics] Status: Resolved Submitter: LWG Opened: 2009-06-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 312

The contents of the `<stdatomic.h>` header are not listed anywhere, and `<cstdatomic>` is listed as a C99 header in chapter 17. If we intend to use these for compatibility with a future C standard, we should not use them now.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2992.

Proposed resolution:

Remove `<cstdatomic>` from the C99 headers in table 14. Add a new header `<atomic>` to the headers in table 13. Update chapter 29 to remove reference to `<stdatomic.h>` and replace the use of `<cstdatomic>` with `<atomic>`.

[ If and when WG14 adds atomic operations to C we can add corresponding headers to table 14 with a TR. ]

### 1146(i). "lockfree" does not say enough

Section: 32.5.5 [atomics.lockfree] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2016-02-25

Priority: Not Prioritized

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Discussion:

Addresses US 88

The "lockfree" facilities do not tell the programmer enough.

There are 2 problems here. First, at least on x86, it's less important to me whether some integral types are lock free than what is the largest type I can pass to atomic and have it be lock-free. For example, if `long long`s are not lock-free, `ATOMIC_INTEGRAL_LOCK_FREE` is probably 1, but I'd still be interested in knowing whether longs are always lock-free. Or if long longs at any address are lock-free, I'd expect `ATOMIC_INTEGRAL_LOCK_FREE` to be 2, but I may actually care whether I have access to the `cmpxchg16b` instruction. None of the support here helps with that question. (There are really 2 related questions here: what alignment requirements are there for lock-free access; and what processor is the program actually running on, as opposed to what it was compiled for?)

Second, having `atomic_is_lock_free` only apply to individual objects is pretty useless (except, as Lawrence Crowl points out, for throwing an exception when an object is unexpectedly not lock-free). I'm likely to want to use its result to decide what algorithm to use, and that algorithm is probably going to allocate new memory containing atomic objects and then try to act on them. If I can't predict the lock-freedom of the new object by checking the lock-freedom of an existing object, I may discover after starting the algorithm that I can't continue.

[ 2009-06-16 Jeffrey Yasskin adds: ]

> To solve the first problem, I think 2 macros would help: `MAX_POSSIBLE_LOCK_FREE_SIZE` and `MAX_GUARANTEED_LOCK_FREE_SIZE`, which expand to the maximum value of `sizeof(T)` for which atomic may (or will, respectively) use lock-free operations. Lawrence points out that this "relies heavily on implementations using word-size compare-swap on sub-word-size types, which in turn requires address modulation." He expects that to be the end state anyway, so it doesn't bother him much.

> To solve the second, I think one could specify that equally aligned objects of the same type will return the same value from `atomic_is_lock_free()`. I don't know how to specify "equal alignment". Lawrence suggests an additional function, `atomic_is_always_lock_free()`.

[ 2009-10-22 Benjamin Kosnik: ]

> In the evolution discussion of N2925, "More Collected Issues with Atomics," there is an action item with respect to LWG 1146(i), US 88

> This is stated in the paper as:

> Relatedly, Mike Sperts will create an issue to propose adding a traits mechanism to check the compile-time properties through a template mechanism rather than macros

> Here is my attempt to do this. I don't believe that a separate trait is necessary for this, and that instead `atomic_integral::is_lock_free` can be re-purposed with minimal work as follows.

> [ Howard: Put Benjamin's wording in the proposed wording section. ]

[ 2009-10-22 Alberto Ganesh Barbati: ]

> Just a thought... wouldn't it be better to use a scoped enum instead of plain integers? For example:

> >     enum class is_lock_free
> >     {
> >         never = 0, sometimes = 1, always = 2;
> >     };

> if compatibility with C is deemed important, we could use an unscoped enum with suitably chosen names. It would still be more descriptive than 0, 1 and 2.

Previous resolution [SUPERSEDED]:

> Header `<cstdatomic>` synopsis [atomics.synopsis]

> Edit as follows:

> >     namespace std {
> >     ...
> >     // 29.4, lock-free property
> >     #define ATOMIC_INTEGRAL_LOCK_FREE unspecified
> >     #define ATOMIC_CHAR_LOCK_FREE unspecified
> >     #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
> >     #define ATOMIC_CHAR32_T_LOCK_FREE unspecified
> >     #define ATOMIC_WCHAR_T_LOCK_FREE unspecified
> >     #define ATOMIC_SHORT_LOCK_FREE unspecified
> >     #define ATOMIC_INT_LOCK_FREE unspecified
> >     #define ATOMIC_LONG_LOCK_FREE unspecified
> >     #define ATOMIC_LLONG_LOCK_FREE unspecified
> >     #define ATOMIC_ADDRESS_LOCK_FREE unspecified

> Lock-free Property 32.5.5 [atomics.lockfree]

> Edit the synopsis as follows.

> >     namespace std {
> >        #define ATOMIC_INTEGRAL_LOCK_FREE unspecified
> >        #define ATOMIC_CHAR_LOCK_FREE unspecified
> >        #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
> >        #define ATOMIC_CHAR32_T_LOCK_FREE unspecified
> >        #define ATOMIC_WCHAR_T_LOCK_FREE unspecified
> >        #define ATOMIC_SHORT_LOCK_FREE unspecified
> >        #define ATOMIC_INT_LOCK_FREE unspecified
> >        #define ATOMIC_LONG_LOCK_FREE unspecified
> >        #define ATOMIC_LLONG_LOCK_FREE unspecified
> >        #define ATOMIC_ADDRESS_LOCK_FREE unspecified
> >     }

> Edit paragraph 1 as follows.

> > The ATOMIC_...._LOCK_FREE macros ATOMIC_INTEGRAL_LOCK_FREE and ATOMIC_ADDRESS_LOCK_FREE indicate the general lock-free property of integral and address atomic the corresponding atomic integral types, with the signed and unsigned variants grouped together. The properties also apply to the corresponding specializations of the atomic template. A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.

> Operations on Atomic Types 32.5.8.2 [atomics.types.operations]

> Edit as follows.

> >     void static constexpr bool A::is_lock_free() const volatile;

> > > Returns: True if the object's types's operations are lock-free, false otherwise. [Note: In the same way that `<limits>` `std::numeric_limits<short>::max()` is related to `<limits.h>` `__LONG_LONG_MAX__`, `<atomic> std::atomic_short::is_lock_free` is related to `<stdatomic.h>` and `ATOMIC_SHORT_LOCK_FREE` — end note]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2992.

Proposed resolution:

Resolved by N2992.

### 1147(i). Non-volatile atomic functions

Section: 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US 90

The C++0X draft declares all of the functions dealing with atomics (section 32.5.8.2 [atomics.types.operations]) to take volatile arguments. Yet it also says (29.4-3),

> [ Note: Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. Thus, volatile qualified operations on non-volatile objects may be merged under some conditions. — end note ]

I was thinking about how to implement this in gcc, and I believe that we'll want to overload most of the functions on volatile and non-volatile. Here's why:

To let the compiler take advantage of the permission to merge non-volatile atomic operations and reorder atomics in certain, we'll need to tell the compiler backend about exactly which atomic operation was used. So I expect most of the functions of the form `atomic_<op>_explicit()` (e.g. `atomic_load_explicit`, `atomic_exchange_explicit`, `atomic_fetch_add_explicit`, etc.) to become compiler builtins. A builtin can tell whether its argument was volatile or not, so those functions don't really need extra explicit overloads. However, I don't expect that we'll want to add builtins for every function in chapter 29, since most can be implemented in terms of the `_explicit` free functions:

    class atomic_int {
      __atomic_int_storage value;
     public:
      int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
        // &value has type "volatile __atomic_int_storage*".
        atomic_fetch_add_explicit(&value, increment, order);
      }
      ...
    };

But now this always calls the volatile builtin version of `atomic_fetch_add_explicit()`, even if the `atomic_int` wasn't declared volatile. To preserve volatility and the compiler's permission to optimize, I'd need to write:

    class atomic_int {
      __atomic_int_storage value;
     public:
      int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
        atomic_fetch_add_explicit(&value, increment, order);
      }
      int fetch_add(int increment, memory_order order = memory_order_seq_cst) {
        atomic_fetch_add_explicit(&value, increment, order);
      }
      ...
    };

But this is visibly different from the declarations in the standard because it's now overloaded. (Consider passing `&atomic_int::fetch_add` as a template parameter.)

The implementation may already have permission to add overloads to the member functions:

> 16.4.6.5 [member.functions] An implementation may declare additional non-virtual member function signatures within a class:
> ...

> - by adding a member function signature for a member function name.

but I don't see an equivalent permission to add overloads to the free functions.

[ 2009-06-16 Lawrence adds: ]

> I recommend allowing non-volatile overloads.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2992.

Proposed resolution:

### 1150(i). wchar_t, char16_t and char32_t filenames

Section: 31.10.6 [fstream] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2018-07-02

Priority: Not Prioritized

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Discussion:

Addresses JP 73

Description

It is a problem from C++98, `fstream` cannot appoint a filename of wide character string(`const wchar_t` and `const wstring&`).

Suggestion

Add interface corresponding to `wchar_t`, `char16_t` and `char32_t`.

[ 2009-07-01 Alisdair notes that this is a duplicate of 454(i) which has more in-depth rationale. ]

[ 2009-09-21 Daniel adds: ]

> I suggest to mark this issue as NAD Future with the intend to solve the issue with a single file path c'tor template assuming a provision of a TR2 filesystem library.

[ 2009 Santa Cruz: ]

> NAD Future. This is a duplicate of 454(i).

[LEWG Kona 2017]

Recommend NAD: Needs a paper. Recommend providing overload that takes filesystem::path. Note that there are other similar issues elsewhere in the library, for example basic_filebuf. Handled by another issue.

Proposed resolution:

Resolved by the adoption of 2676(i).

### 1151(i). Behavior of the library in the presence of threads is incompletely specified

Section: 16 [library] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US 63

Description

The behavior of the library in the presence of threads is incompletely specified.

For example, if thread 1 assigns to `X`, then writes data to file `f`, which is read by thread 2, and then accesses variable `X`, is thread 2 guaranteed to be able to see the value assigned to `X` by thread 1? In other words, does the write of the data "happen before" the read?

Another example: does simultaneous access using `operator at()` to different characters in the same non-const string really introduce a data race?

Suggestion

Notes

17 SG: should go to threads group; misclassified in document

Concurrency SG: Create an issue. Hans will look into it.

[ 2009 Santa Cruz: ]

> Move to "Open". Hans and the rest of the concurrency working group will study this. We can't make progress without a thorough review and a paper.

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

Solved by N3069.

Proposed resolution:

### 1152(i). Expressions parsed differently than intended

Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++11 Submitter: Seungbeom Kim Opened: 2009-06-27 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In Table 73 — Floating-point conversions, 28.3.4.3.3.3 [facet.num.put.virtuals], in N2914, we have the following entries:

Table 73 — Floating-point conversions
State | `stdio` equivalent
--- | ---
`floatfield == ios_base::fixed | ios_base::scientific && !uppercase` | `%a`
`floatfield == ios_base::fixed | ios_base::scientific` | `%A`

These expressions are supposed to mean:

>     floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase
>     floatfield == (ios_base::fixed | ios_base::scientific)

but technically parsed as:

>     ((floatfield == ios_base::fixed) | ios_base::scientific) && (!uppercase)
>     ((floatfield == ios_base::fixed) | ios_base::scientific)

and should be corrected with additional parentheses, as shown above.

[ 2009-10-28 Howard: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change Table 83 — Floating-point conversions in 28.3.4.3.3.3 [facet.num.put.virtuals]:

Table 83 — Floating-point conversions
State | `stdio` equivalent
--- | ---
`floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase` | `%a`
`floatfield == (ios_base::fixed | ios_base::scientific)` | `%A`

### 1157(i). Local types can now instantiate templates

Section: 16.4.5.2.1 [namespace.std] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 175

Description

Local types can now be used to instantiate templates, but don't have external linkage.

Suggestion

Remove the reference to external linkage.

Notes

We accept the proposed solution. Martin will draft an issue.

[ 2009-07-28 Alisdair provided wording. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

16.4.5.2.1 [namespace.std]

Strike "of external linkage" in p1 and p2:

> -1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace `std` or to a namespace within namespace `std` unless otherwise specified. A program may add a concept map for any standard library concept or a template specialization for any standard library template to namespace `std` only if the declaration depends on a user-defined type of external linkage and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.179

> -2- The behavior of a C++ program is undefined if it declares

> - an explicit specialization of any member function of a standard library class template, or
> - an explicit specialization of any member function template of a standard library class or class template, or
> - an explicit or partial specialization of any member class template of a standard library class or class template.

> A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.

### 1158(i). Encouragement to use monotonic clock

Section: 32.2.4 [thread.req.timing] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 322, US 96

Description

Not all systems can provide a monotonic clock. How are they expected to treat a _for function?

Suggestion

Add at least a note explaining the intent for systems that do not support a monotonic clock.

Notes

Create an issue, together with UK 96. Note that the specification as is already allows a non-monotonic clock due to the word “should” rather than “shall”. If this wording is kept, a footnote should be added to make the meaning clear.

[ 2009-06-29 Beman provided a proposed resolution. ]

[ 2009-10-31 Howard adds: ]

> Set to Tentatively Ready after 5 positive votes on c++std-lib.

[ 2010-02-24 Pete moved to Open: ]

> LWG 1158's proposed resolution replaces the ISO-specified normative term "should" with "are encouraged but not required to", which presumably means the same thing, but has no ISO normative status. The WD used the latter formulation in quite a few non-normative places, but only three normative ones. I've changed all the normative uses to "should".

[ 2010-03-06 Beman updates wording. ]

[ 2010 Pittsburgh: Moved to Ready. ]

Proposed resolution:

Change Timing specifications 32.2.4 [thread.req.timing] as indicated:

The member functions whose names end in `_for` take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions. [Note: Implementations are not required to use a monotonic clock because such a clock may be unavailable. — end note]

### 1159(i). Unclear spec for `resource_deadlock_would_occur`

Section: 32.6.5.4.3 [thread.lock.unique.locking] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.lock.unique.locking].

View all issues with C++11 status.

Duplicate of: 1219

Discussion:

Addresses UK 327, UK 328

UK 327 Description

Not clear what the specification for error condition `resource_deadlock_would_occur` means. It is perfectly possible for this thread to own the mutex without setting owns to true on this specific lock object. It is also possible for lock operations to succeed even if the thread does own the mutex, if the mutex is recursive. Likewise, if the mutex is not recursive and the mutex has been locked externally, it is not always possible to know that this error condition should be raised, depending on the host operating system facilities. It is possible that 'i.e.' was supposed to be 'e.g.' and that suggests that recursive locks are not allowed. That makes sense, as the exposition-only member owns is boolean and not a integer to count recursive locks.

UK 327 Suggestion

Add a precondition `!owns`. Change the 'i.e.' in the error condition to be 'e.g.' to allow for this condition to propogate deadlock detection by the host OS.

UK 327 Notes

Create an issue. Assigned to Lawrence Crowl. Note: not sure what try_lock means for recursive locks when you are the owner. POSIX has language on this, which should ideally be followed. Proposed fix is not quite right, for example, try_lock should have different wording from lock.

UK 328 Description

There is a missing precondition that `owns` is true, or an `if(owns)` test is missing from the effect clause

UK 328 Suggestion

Add a precondition that `owns == true`. Add an error condition to detect a violation, rather than yield undefined behaviour.

UK 328 Notes

Handle in same issue as UK 327. Also uncertain that the proposed resolution is the correct one.

[ 2009-11-11 Alisdair notes that this issue is very closely related to 1219(i), if not a dup. ]

[ 2010-02-12 Anthony provided wording. ]

[ 2010 Pittsburgh: ]

> Wording updated and moved to Ready for Pittsburgh.

Proposed resolution:

Modify 32.6.5.4.3 [thread.lock.unique.locking] p3 to say:

>     void lock();

> > ...

> > 3 Throws: Any exception thrown by `pm->lock()`. `std::system_error` if an exception is required (32.2.2 [thread.req.exception]). `std::system_error` with an error condition of `operation_not_permitted` if `pm` is `0`. `std::system_error` with an error condition of `resource_deadlock_would_occur` if on entry `owns` is `true`. `std::system_error` when the postcondition cannot be achieved.

Remove 32.6.5.4.3 [thread.lock.unique.locking] p4 (Error condition clause).

Modify 32.6.5.4.3 [thread.lock.unique.locking] p8 to say:

>     bool try_lock();

> > ...

> > 8 Throws: Any exception thrown by `pm->try_lock()`. `std::system_error` if an exception is required (32.2.2 [thread.req.exception]). `std::system_error` with an error condition of `operation_not_permitted` if `pm` is `0`. `std::system_error` with an error condition of `resource_deadlock_would_occur` if on entry `owns` is `true`. `std::system_error` when the postcondition cannot be achieved.

Remove 32.6.5.4.3 [thread.lock.unique.locking] p9 (Error condition clause).

Modify 32.6.5.4.3 [thread.lock.unique.locking] p13 to say:

>     template <class Clock, class Duration>
>       bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

> > ...

> > 13 Throws: Any exception thrown by `pm->try_lock_until()`. `std::system_error` if an exception is required (32.2.2 [thread.req.exception]). `std::system_error` with an error condition of `operation_not_permitted` if `pm` is `0`. `std::system_error` with an error condition of `resource_deadlock_would_occur` if on entry `owns` is `true`. `std::system_error` when the postcondition cannot be achieved.

Remove 32.6.5.4.3 [thread.lock.unique.locking] p14 (Error condition clause).

Modify 32.6.5.4.3 [thread.lock.unique.locking] p18 to say:

>     template <class Rep, class Period>
>       bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);

> > ...

> > 18 Throws: Any exception thrown by `pm->try_lock_for()`. `std::system_error` if an exception is required (32.2.2 [thread.req.exception]). `std::system_error` with an error condition of `operation_not_permitted` if `pm` is `0`. `std::system_error` with an error condition of `resource_deadlock_would_occur` if on entry `owns` is `true`. `std::system_error` when the postcondition cannot be achieved.

Remove 32.6.5.4.3 [thread.lock.unique.locking] p19 (Error condition clause).

### 1160(i). `future_error` public constructor is 'exposition only'

Section: 32.10.4 [futures.future.error] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Addresses UK 331

Description

Not clear what it means for a public constructor to be 'exposition only'. If the intent is purely to support the library calling this constructor then it can be made private and accessed through friendship. Otherwise it should be documented for public consumption.

Suggestion

Declare the constructor as private with a note about intended friendship, or remove the exposition-only comment and document the semantics.

Notes

Create an issue. Assigned to Detlef. Suggested resolution probably makes sense.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2997.

Proposed resolution:

### 1161(i). Unnecessary `unique_future` limitations

Section: 32.10.7 [futures.unique.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [futures.unique.future].

View all issues with Resolved status.

Discussion:

Addresses UK 336

Description

It is possible to transfer ownership of the asynchronous result from one unique_future instance to another via the move-constructor. However, it is not possible to transfer it back, and nor is it possible to create a default-constructed unique_future instance to use as a later move target. This unduly limits the use of `unique_future` in code. Also, the lack of a move-assignment operator restricts the use of `unique_future` in containers such as `std::vector` - `vector::insert` requires move-assignable for example.

Suggestion

Add a default constructor with the semantics that it creates a `unique_future` with no associated asynchronous result. Add a move-assignment operator which transfers ownership.

Notes

Create an issue. Detlef will look into it.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2997.

Proposed resolution:

### 1162(i). `shared_future` should support an efficient move constructor

Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [futures.shared.future].

View all issues with Resolved status.

Discussion:

Addresses UK 337

Description

`shared_future` should support an efficient move constructor that can avoid unnecessary manipulation of a reference count, much like `shared_ptr`

Suggestion

Add a move constructor

Notes

Create an issue. Detlef will look into it.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2997.

Proposed resolution:

### 1163(i). `shared_future` is inconsistent with `shared_ptr`

Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [futures.shared.future].

View all issues with Resolved status.

Discussion:

Addresses UK 338

Description

`shared_future` is currently `CopyConstructible`, but not `CopyAssignable`. This is inconsistent with `shared_ptr`, and will surprise users. Users will then write work-arounds to provide this behaviour. We should provide it simply and efficiently as part of shared_future. Note that since the shared_future member functions for accessing the state are all declared const, the original usage of an immutable shared_future value that can be freely copied by multiple threads can be retained by declaring such an instance as "`const shared_future`".

Suggestion

Remove "=delete" from the copy-assignment operator of shared_future. Add a move-constructor `shared_future(shared_future&& rhs)`, and a move-assignment operator `shared_future& operator=(shared_future&& rhs)`. The postcondition for the copy-assignment operator is that `*this` has the same associated state as `rhs`. The postcondition for the move-constructor and move assignment is that `*this` has the same associated as `rhs` had before the constructor/assignment call and that `rhs` has no associated state.

Notes

Create an issue. Detlef will look into it.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Adressed by N2997.

Proposed resolution:

### 1165(i). Unneeded `promise` move constructor

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.promise].

View all issues with Resolved status.

Discussion:

Addresses UK 343

Description

The move constructor of a `std::promise` object does not need to allocate any memory, so the move-construct-with-allocator overload of the constructor is superfluous.

Suggestion

Remove the constructor with the signature `template <class Allocator> promise(allocator_arg_t, const Allocator& a, promise& rhs);`

Notes

Create an issue. Detlef will look into it. Will solicit feedback from Pablo. Note that "rhs" argument should also be an rvalue reference in any case.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Adressed by N2997.

Proposed resolution:

### 1166(i). Allocator-specific move/copy break model of move-constructor and move-assignment

Section: 99 [allocator.propagation], 99 [allocator.propagation.map], 23 [containers] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Addresses US 77

Description

Allocator-specific move and copy behavior for containers (N2525) complicates a little-used and already-complicated portion of the standard library (allocators), and breaks the conceptual model of move-constructor and move-assignment operations on standard containers being efficient operations. The extensions for allocator-specific move and copy behavior should be removed from the working paper.

With the introduction of rvalue references, we are teaching programmers that moving from a standard container (e.g., a `vector<string>`) is an efficient, constant-time operation. The introduction of N2525 removed that guarantee; depending on the behavior of four different traits (20.8.4), the complexity of copy and move operations can be constant or linear time. This level of customization greatly increases the complexity of standard containers, and benefits only a tiny fraction of the C++ community.

Suggestion

Remove 20.8.4.

Remove 20.8.5.

Remove all references to the facilities in 20.8.4 and 20.8.5 from clause 23.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2982.

Proposed resolution:

### 1169(i). `num_get` not fully compatible with `strto*`

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++17 Submitter: Cosmin Truta Opened: 2009-07-04 Last modified: 2017-07-30

Priority: 3

View other active issues in [facet.num.get.virtuals].

View all other issues in [facet.num.get.virtuals].

View all issues with C++17 status.

Discussion:

As specified in the latest draft, N2914, `num_get` is still not fully compatible with the following C functions: `strtoul`, `strtoull`, `strtof` and `strtod`.

In C, when conversion of a string to an unsigned integer type falls outside the representable range, `strtoul` and `strtoull` return `ULONG_MAX` and `ULLONG_MAX`, respectively, regardless whether the input field represents a positive or a negative value. On the other hand, the result of `num_get` conversion of negative values to unsigned integer types is zero. This raises a compatibility issue.

Moreover, in C, when conversion of a string to a floating-point type falls outside the representable range, `strtof`, `strtod` and `strtold` return `±HUGE_VALF`, `±HUGE_VAL` and `±HUGE_VALL`, respectively. On the other hand, the result of `num_get` conversion of such out-of-range floating-point values results in the most positive/negative representable value. Although many C library implementations do implement `HUGE_VAL` (etc.) as the highest representable (which is, usually, the infinity), this isn't required by the C standard. The C library specification makes no statement regarding the value of `HUGE_VAL` and friends, which potentially raises the same compatibility issue as in the above case of unsigned integers. In addition, neither C nor C++ define symbolic constants for the maximum representable floating-point values (they only do so only for the maximum representable finite floating-point values), which raises a usability issue (it would be hard for the programmer to check the result of `num_get` against overflow).

As such, we propose to adjust the specification of `num_get` to closely follow the behavior of all of its underlying C functions.

[ 2010 Rapperswil: ]

> Some concern that this is changing the specification for an existing C++03 function, but it was pointed out that this was underspecified as resolved by issue 23. This is clean-up for that issue in turn. Some concern that we are trying to solve the same problem in both clause 22 and 27.

> Bill: There's a change here as to whether val is stored to in an error case.

> Pablo: Don't think this changes whether val is stored to or not, but changes the value that is stored.

> Bill: Remembers having skirmishes with customers and testers as to whether val is stored to, and the resolution was not to store in error cases.

> Howard: Believes since C++03 we made a change to always store in overflow.

> Everyone took some time to review the issue.

> Pablo: C++98 definitely did not store any value during an error condition.

> Dietmar: Depends on the question of what is considered an error, and whether overflow is an error or not, which was the crux of LWG 23.

> Pablo: Yes, but given the "zero, if the conversion function fails to convert the entire field", we are requiring every error condition to store.

> Bill: When did this happen?

> Alisdair: One of the last two or three meetings.

> Dietmar: To store a value in case of failure is a very bad idea.

> Move to Open, needs more study.

[2011-03-24 Madrid meeting]

Move to deferred

[ 2011 Bloomington ]

The proposed wording looks good, no-one sure why this was held back before. Move to Review.

[2012,Kona]

Move to Open.

THe issues is what to do with `-1`. Should it match 'C' or do the "sane" thing. A fix here changes behavior, but is probably what we want.

Pablo to provide wording, with help from Howard.

[2015-05-06 Lenexa: Move to Ready]

STL: I like that this uses strtof, which I think is new in C99. that avoids truncation from using atof. I have another issue ...

MC: yes LWG 2403 (stof should call strtof)

PJP: the last line is horrible, you don't assign to err, you call setstate(ios_base::failbit). Ah, no, this is inside num_get so the caller does the setstate.

MC: we need all these words. are they the right words?

JW: I'd like to take a minute to check my impl. Technically this implies a change in behaviour (from always using strtold and checking the extracted floating point value, to using the right function). Oh, we already do exactly this.

MC: Move to Ready

6 in favor, none opposed, 1 abstention

Proposed resolution:

Change 28.3.4.3.2.3 [facet.num.get.virtuals] as follows:

> Stage 3: The sequence of `char`s accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header `<cstdlib>`:

> - For a signed integer value, the function `strtoll`.
> - For an unsigned integer value, the function `strtoull`.
> - For a `float` value, the function `strtof`.
> - For a `double` value, the function `strtod`.
> - For a floating-point `long double` value, the function `strtold`.

> The numeric value to be stored can be one of:

> - zero, if the conversion function fails to convert the entire field. `ios_base::failbit` is assigned to `err`.
> - the most positive (or negative) representable value, if the field to be converted to a signed integer type represents a value too large positive (or negative) to be represented in `val`. `ios_base::failbit` is assigned to `err`.
> - the most negative representable value or zero for an unsigned integer type, if the field represents a value too large negative to be represented in `val`. `ios_base::failbit` is assigned to `err`.
> - the most positive representable value, if the field to be converted to an unsigned integer type represents a value that cannot be represented in `val`.
> - the converted value, otherwise.

> The resultant numeric value is stored in `val`. If the conversion function fails to convert the entire field, or if the field represents a value outside the range of representable values, `ios_base::failbit` is assigned to `err`.

### 1170(i). String char-like types no longer PODs

Section: 27.1 [strings.general] Status: C++11 Submitter: Beman Dawes Opened: 2009-06-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [strings.general].

View all issues with C++11 status.

Discussion:

Addresses UK 218

Prior to the introduction of constant expressions into the library, `basic_string` elements had to be POD types, and thus had to be both trivially copyable and standard-layout. This ensured that they could be memcpy'ed and would be compatible with other libraries and languages, particularly the C language and its library.

N2349, Constant Expressions in the Standard Library Revision 2, changed the requirement in 21/1 from "POD type" to "literal type". That change had the effect of removing the trivially copyable and standard-layout requirements from `basic_string` elements.

This means that `basic_string` elements no longer are guaranteed to be memcpy'able, and are no longer guaranteed to be standard-layout types:

> 3.9/p2 and 3.9/p3 both make it clear that a "trivially copyable type" is required for memcpy to be guaranteed to work.

> Literal types (3.9p12) may have a non-trivial copy assignment operator, and that violates the trivially copyable requirements given in 9/p 6, bullet item 2.

> Literal types (3.9p12) have no standard-layout requirement, either.

This situation probably arose because the wording for "Constant Expressions in the Standard Library" was in process at the same time the C++ POD deconstruction wording was in process.

Since trivially copyable types meet the C++0x requirements for literal types, and thus work with constant expressions, it seems an easy fix to revert the `basic_string` element wording to its original state.

[ 2009-07-28 Alisdair adds: ]

> When looking for any resolution for this issue, consider the definition of "character container type" in 3.10 [defns.character.container]. This does require the character type to be a POD, and this term is used in a number of places through clause 21 and 28. This suggests the PODness constraint remains, but is much more subtle than before. Meanwhile, I suspect the change from POD type to literal type was intentional with the assumption that trivially copyable types with non-trivial-but-constexpr constructors should serve as well. I don't believe the current wording offers the right guarantees for either of the above designs.

[ 2009-11-04 Howard modifies proposed wording to disallow array types as char-like types. ]

[ 2010-01-23 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change General 27.1 [strings.general] as indicated:

> This Clause describes components for manipulating sequences of any literal non-array POD (3.9) type. In this Clause such types are called char-like types, and objects of char-like types are called char-like objects or simply characters.

### 1171(i). duration types should be literal

Section: 30.5 [time.duration] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-07-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.duration].

View all issues with C++11 status.

Discussion:

The `duration` types in 30.5 [time.duration] are exactly the sort of type that should be "literal types" in the new standard. Likewise, arithmetic operations on `duration`s should be declared `constexpr`.

[ 2009-09-21 Daniel adds: ]

> An alternative (and possibly preferable solution for potentially heap-allocating `big_int` representation types) would be to ask the core language to allow references to `const` literal types as feasible arguments for `constexpr` functions.

[ 2009-10-30 Alisdair adds: ]

> I suggest this issue moves from New to Open.

> Half of this issue was dealt with in paper n2994 on constexpr constructors.

> The other half (duration arithmetic) is on hold pending Core support for `const &` in `constexpr` functions.

[ 2010-03-15 Alisdair updated wording to be consistent with N3078. ]

[ 2010 Rapperswil: ]

> This issue was the motivation for Core adding the facility for `constexpr` functions to take parameters by `const &`. Move to Tentatively Ready.

[ Adopted at 2010-11 Batavia. ]

Proposed resolution:

Add `constexpr` to declaration of following functions and constructors:

Modify p1 30 [time], and the prototype definitions in 30.5.6 [time.duration.nonmember], 30.5.7 [time.duration.comparisons], and 30.5.8 [time.duration.cast]:

> Header `<chrono>` synopsis

>     // duration arithmetic
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
>        constexpr operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
>        constexpr operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period, class Rep2>
>        duration<typename common_type<Rep1, Rep2>::type, Period>
>        constexpr operator*(const duration<Rep1, Period>& d, const Rep2& s);
>     template <class Rep1, class Period, class Rep2>
>        duration<typename common_type<Rep1, Rep2>::type, Period>
>        constexpr operator*(const Rep1& s, const duration<Rep2, Period>& d);
>     template <class Rep1, class Period, class Rep2>
>        duration<typename common_type<Rep1, Rep2>::type, Period>
>        constexpr operator/(const duration<Rep1, Period>& d, const Rep2& s);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        typename common_type<Rep1, Rep2>::type
>        constexpr operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

>     // duration comparisons
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator==(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator!=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator< (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator<=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator> (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator>=(const  duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

>     // duration_cast
>     template <class ToDuration, class Rep, class Period>
>        constexpr ToDuration duration_cast(const duration<Rep, Period>& d);

Change 30.5 [time.duration]:

>     template <class Rep, class Period = ratio<1>>
>     class duration {
>       ...
>     public:
>       ...
>       constexpr duration(const duration&) = default;
>       ...

>     };

[ Note - this edit already seems assumed by definition of the duration static members `zero/min/max`. They cannot meaningfully be `constexpr` without this change. ]

### 1172(i). `select_on_container_(copy|move)_construction` over-constrained

Section: 99 [allocator.concepts.members] Status: Resolved Submitter: Alberto Ganesh Barbati Opened: 2009-07-08 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

I believe the two functions `select_on_container_(copy|move)_construction()` are over-constrained. For example, the return value of the "copy" version is (see 99 [allocator.concepts.members]/21):

> Returns: `x` if the allocator should propagate from the existing container to the new container on copy construction, otherwise `X()`.

Consider the case where a user decides to provide an explicit concept map for Allocator to adapt some legacy allocator class, as he wishes to provide customizations that the `LegacyAllocator` concept map template does not provide. Now, although it's true that the legacy class is required to have a default constructor, the user might have reasons to prefer a different constructor to implement `select_on_container_copy_construction()`. However, the current wording requires the use of the default constructor.

Moreover, it's not said explicitly that `x` is supposed to be the allocator of the existing container. A clarification would do no harm.

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Addressed by N2982.

Proposed resolution:

Replace 99 [allocator.concepts.members]/21 with:

>     X select_on_container_copy_construction(const X& x);

> -21- Returns: `x` if the allocator should propagate from the existing container to the new container on copy construction, otherwise `X()`. an allocator object to be used by the new container on copy construction. [Note: `x` is the allocator of the existing container that is being copied. The most obvious choices for the return value are `x`, if the allocator should propagate from the existing container, and `X()`. — end note]

Replace 99 [allocator.concepts.members]/25 with:

>     X select_on_container_move_construction(X&& x);

> -25- Returns: `move(x)` if the allocator should propagate from the existing container to the new container on move construction, otherwise `X()`. an allocator object to be used by the new container on move construction. [Note: `x` is the allocator of the existing container that is being moved. The most obvious choices for the return value are `move(x)`, if the allocator should propagate from the existing container, and `X()`. — end note]

### 1174(i). Type property predicates

Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: Jason Merrill Opened: 2009-07-16 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

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Discussion:

I've been implementing compiler support for `is_standard_layout`, and noticed a few nits about 21.3.5.4 [meta.unary.prop]:

1. There's no trait for "trivially copyable type", which is now the property that lets you do bitwise copying of a type, and therefore seems useful to be able to query. `has_trivial_assign` && `has_trivial_copy_constructor` && `has_trivial_destructor` is similar, but not identical, specifically with respect to const types.
2. `has_trivial_copy_constructor` and `has_trivial_assign` lack the "or an array of such a class type" language that most other traits in that section, including `has_nothrow_copy_constructor` and `has_nothrow_assign`, have; this seems like an oversight.

[ See the thread starting with c++std-lib-24420 for further discussion. ]

[ Addressed in N2947. ]

[ 2009-10 Santa Cruz: ]

> NAD EditorialResolved. Solved by N2984.

Proposed resolution:

### 1177(i). Improve "diagnostic required" wording

Section: 30.5 [time.duration] Status: C++11 Submitter: Howard Hinnant Opened: 2009-07-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.duration].

View all issues with C++11 status.

Discussion:

"diagnostic required" has been used (by me) for code words meaning "use `enable_if` to constrain templated functions. This needs to be improved by referring to the function signature as not participating in the overload set, and moving this wording to a Remarks paragraph.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2009-11-19 Pete opens: ]

> Oh, and speaking of 1177, most of the changes result in rather convoluted prose. Instead of saying

> > A shall be B, else C

> it should be

> > C if A is not B

> That is:

> > `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)`, else this signature shall not participate in overload resolution.

> should be

> > This signature shall not participate in overload resolution if `Rep2` is not implicitly convertible to `CR(Rep1, Rep2)`.

> That is clearer, and eliminates the false requirement that `Rep2` "shall be" convertible.

[ 2009-11-19 Howard adds: ]

> I've updated the wording to match Pete's suggestion and included bullet 16 from 1195(i).

[ 2009-11-19 Jens adds: ]

> Further wording suggestion using "unless":

> > This signature shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)`.

[ 2009-11-20 Howard adds: ]

> I've updated the wording to match Jens' suggestion.

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

[ This proposed resolution addresses 947(i) and 974(i). ]

1. Change 30.5.2 [time.duration.cons] (and reorder the Remarks paragraphs per 16.3.2.4 [structure.specifications]):

   >     template <class Rep2>
   >       explicit duration(const Rep2& r);

   > > Requires: Remarks: This constructor shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `rep` and

   > > - `treat_as_floating_point<rep>::value` shall be is `true`, or
   > > - `treat_as_floating_point<Rep2>::value` shall be is `false`.

   > > Diagnostic required [Example:

   > > >     duration<int, milli> d(3); // OK
   > > >     duration<int, milli> d(3.5); // error

   > > — end example]

   > > Effects: Constructs an object of type `duration`.

   > > Postcondition: `count() == static_cast<rep>(r)`.

   >     template <class Rep2, class Period2>
   >       duration(const duration<Rep2, Period2>& d);

   > > Requires: Remarks: This constructor shall not participate in overload resolution unless `treat_as_floating_point<rep>::value` shall be is `true` or `ratio_divide<Period2, period>::type::den` shall be is 1. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note] [Example:

   > > >     duration<int, milli> ms(3);
   > > >     duration<int, micro> us = ms; // OK
   > > >     duration<int, milli> ms2 = us; // error

   > > — end example]

   > > Effects: Constructs an object of type `duration`, constructing `rep_` from `duration_cast<duration>(d).count()`.

2. Change the following paragraphs in 30.5.6 [time.duration.nonmember]:

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator*(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator*(const Rep1& s, const duration<Rep2, Period>& d);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep1` shall be is implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator/(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be is not an instantiation of `duration`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator%(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be is not an instantiation of `duration`. Diagnostic required.

3. Change the following paragraphs in 30.5.8 [time.duration.cast]:

   >     template <class ToDuration, class Rep, class Period>
   >       ToDuration duration_cast(const duration<Rep, Period>& d);

   > > Requires Remarks: This function shall not participate in overload resolution unless `ToDuration` shall be is an instantiation of `duration`. Diagnostic required.

4. Change 30.6.2 [time.point.cons]/3 as indicated:

   > Requires: `Duration2` shall be implicitly convertible to `duration`. Diagnostic required.

   > Remarks: This constructor shall not participate in overload resolution unless `Duration2` is implicitly convertible to `duration`.

5. Change the following paragraphs in 30.6.8 [time.point.cast]:

   >     template <class ToDuration, class Clock, class Duration>
   >       time_point<Clock, ToDuration> time_point_cast(const time_point<Clock, Duration>& t);

   > > Requires Remarks: This function shall not participate in overload resolution unless `ToDuration` shall be is an instantiation of `duration`. Diagnostic required.

### 1178(i). Header dependencies

Section: 16.4.6.2 [res.on.headers] Status: C++11 Submitter: Beman Dawes Opened: 2009-07-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

See Frankfurt notes of 1001(i).

Proposed resolution:

Change 16.4.6.2 [res.on.headers], Headers, paragraph 1, as indicated:

> A C++ header may include other C++ headers.[footnote] A C++ header shall provide the declarations and definitions that appear in its synopsis (6.3 [basic.def.odr]). A C++ header shown in its synopsis as including other C++ headers shall provide the declarations and definitions that appear in the synopses of those other headers.

> [footnote] C++ headers must include a C++ header that contains any needed definition (3.2).

### 1180(i). Missing `string_type` member typedef in class `sub_match`

Section: 28.6.8.2 [re.submatch.members] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

The definition of class template `sub_match` is strongly dependent on the type `basic_string<value_type>`, both in interface and effects, but does not provide a corresponding typedef `string_type`, as e.g. class `match_results` does, which looks like an oversight to me that should be fixed.

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. In the class template `sub_match` synopsis 28.6.8 [re.submatch]/1 change as indicated:

   >     template <class BidirectionalIterator>
   >     class sub_match : public std::pair<BidirectionalIterator, BidirectionalIterator> {
   >     public:
   >       typedef typename iterator_traits<BidirectionalIterator>::value_type value_type;
   >       typedef typename iterator_traits<BidirectionalIterator>::difference_type difference_type;
   >       typedef BidirectionalIterator iterator;
   >       typedef basic_string<value_type> string_type;

   >       bool matched;

   >       difference_type length() const;
   >       operator basic_string<value_type>string_type() const;
   >       basic_string<value_type>string_type str() const;
   >       int compare(const sub_match& s) const;
   >       int compare(const basic_string<value_type>string_type& s) const;
   >       int compare(const value_type* s) const;
   >     };

2. In 28.6.8.2 [re.submatch.members]/2 change as indicated:

   >     operator basic_string<value_type>string_type() const;

   > > Returns: `matched ? basic_string<value_type> string_type(first, second) : basic_string<value_type> string_type()`.

3. In 28.6.8.2 [re.submatch.members]/3 change as indicated:

   >     basic_string<value_type>string_type str() const;

   > > Returns: `matched ? basic_string<value_type> string_type(first, second) : basic_string<value_type> string_type()`.

4. In 28.6.8.2 [re.submatch.members]/5 change as indicated:

   >     int compare(const basic_string<value_type>string_type& s) const;

### 1181(i). Invalid `sub_match` comparison operators

Section: 28.6.8.3 [re.submatch.op] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [re.submatch.op].

View all issues with C++11 status.

Discussion:

Several heterogeneous comparison operators of class template `sub_match` are specified by return clauses that are not valid in general. E.g. 28.6.8.3 [re.submatch.op]/7:

>     template <class BiIter, class ST, class SA>
>     bool operator==(
>       const basic_string<
>         typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
>       const sub_match<BiIter>& rhs);

> > Returns: `lhs == rhs.str()`.

The returns clause would be ill-formed for all cases where `ST != std::char_traits<iterator_traits<BiIter>::value_type>` or `SA != std::allocator<iterator_traits<BiIter>::value_type>`.

The generic character of the comparison was intended, so there are basically two approaches to fix the problem: The first one would define the semantics of the comparison using the traits class `ST` (The semantic of `basic_string::compare` is defined in terms of the compare function of the corresponding traits class), the second one would define the semantics of the comparison using the traits class

>     std::char_traits<iterator_traits<BiIter>::value_type>

which is essentially identical to

>     std::char_traits<sub_match<BiIter>::value_type>

I suggest to follow the second approach, because this emphasizes the central role of the `sub_match` object as part of the comparison and would also make sure that a `sub_match` comparison using some `basic_string<char_t, ..>` always is equivalent to a corresponding comparison with a string literal because of the existence of further overloads (beginning from 28.6.8.3 [re.submatch.op]/19). If users really want to take advantage of their own `traits::compare`, they can simply write a corresponding compare function that does so.

[ Post-Rapperswil ]

The following update is a result of the discussion during the Rapperswil meeting, the P/R expresses all comparisons by delegating to sub_match's compare functions. The processing is rather mechanical: Only `==` and `<` where defined by referring to `sub_match`'s compare function, all remaining ones where replaced by the canonical definitions in terms of these two.

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

1. Change 28.9.2 [re.submatch.op]/7 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator==(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 7 Returns: `lhs == rhs.str()rhs.compare(lhs.c_str()) == 0`.

2. Change 28.9.2 [re.submatch.op]/8 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator!=(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 8 Returns: `lhs != rhs.str()!(lhs == rhs)`.

3. Change 28.9.2 [re.submatch.op]/9 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator<(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 9 Returns: `lhs < rhs.str()rhs.compare(lhs.c_str()) > 0`.

4. Change 28.9.2 [re.submatch.op]/10 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator>(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 10 Returns: `lhs > rhs.str()rhs < lhs`.

5. Change 28.9.2 [re.submatch.op]/11 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator>=(
   >        const basic_string<
   >        typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >      const sub_match<BiIter>& rhs);

   > 11 Returns: `lhs >= rhs.str()!(lhs < rhs)`.

6. Change 28.9.2 [re.submatch.op]/12 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator<=(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 12 Returns: `lhs <= rhs.str()!(rhs < lhs)`.

7. Change 28.9.2 [re.submatch.op]/13 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator==(const sub_match<BiIter>& lhs,
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > 13 Returns: `lhs.str() == rhslhs.compare(rhs.c_str()) == 0`.

8. Change 28.9.2 [re.submatch.op]/14 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator!=(const sub_match<BiIter>& lhs,
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > 14 Returns: `lhs.str() != rhs!(lhs == rhs)`.

9. Change 28.9.2 [re.submatch.op]/15 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator<(const sub_match<BiIter>& lhs,
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > 15 Returns: `lhs.str() < rhslhs.compare(rhs.c_str()) < 0`.

10. Change 28.9.2 [re.submatch.op]/16 as indicated:

    >     template <class BiIter, class ST, class SA>
    >      bool operator>(const sub_match<BiIter>& lhs,
    >        const basic_string<
    >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

    > 16 Returns: `lhs.str() > rhsrhs < lhs`.

11. Change 28.9.2 [re.submatch.op]/17 as indicated:

    >     template <class BiIter, class ST, class SA>
    >      bool operator>=(const sub_match<BiIter>& lhs,
    >        const basic_string<
    >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

    > 17 Returns: `lhs.str() >= rhs!(lhs < rhs)`.

12. Change 28.9.2 [re.submatch.op]/18 as indicated:

    >     template <class BiIter, class ST, class SA>
    >      bool operator<=(const sub_match<BiIter>& lhs,
    >        const basic_string<
    >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

    > 18 Returns: `lhs.str() <= rhs!(rhs < lhs)`.

13. Change 28.9.2 [re.submatch.op]/19 as indicated:

    >     template <class BiIter>
    >      bool operator==(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 19 Returns: `lhs == rhs.str()rhs.compare(lhs) == 0`.

14. Change 28.9.2 [re.submatch.op]/20 as indicated:

    >     template <class BiIter>
    >      bool operator!=(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 20 Returns: `lhs != rhs.str()!(lhs == rhs)`.

15. Change 28.9.2 [re.submatch.op]/21 as indicated:

    >     template <class BiIter>
    >      bool operator<(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 21 Returns: `lhs < rhs.str()rhs.compare(lhs) > 0`.

16. Change 28.9.2 [re.submatch.op]/22 as indicated:

    >     template <class BiIter>
    >      bool operator>(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 22 Returns: `lhs > rhs.str()rhs < lhs`.

17. Change 28.9.2 [re.submatch.op]/23 as indicated:

    >     template <class BiIter>
    >      bool operator>=(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 23 Returns: `lhs >= rhs.str()!(lhs < rhs)`.

18. Change 28.9.2 [re.submatch.op]/24 as indicated:

    >     template <class BiIter>
    >      bool operator<=(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 24 Returns: `lhs <= rhs.str()!(rhs < lhs)`.

19. Change 28.9.2 [re.submatch.op]/25 as indicated:

    >     template <class BiIter>
    >      bool operator==(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 25 Returns: `lhs.str() == rhslhs.compare(rhs) == 0`.

20. Change 28.9.2 [re.submatch.op]/26 as indicated:

    >     template <class BiIter>
    >      bool operator!=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 26 Returns: `lhs.str() != rhs!(lhs == rhs)`.

21. Change 28.9.2 [re.submatch.op]/27 as indicated:

    >     template <class BiIter>
    >      bool operator<(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 27 Returns: `lhs.str() < rhslhs.compare(rhs) < 0`.

22. Change 28.9.2 [re.submatch.op]/28 as indicated:

    >     template <class BiIter>
    >      bool operator>(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 28 Returns: `lhs.str() > rhsrhs < lhs`.

23. Change 28.9.2 [re.submatch.op]/29 as indicated:

    >     template <class BiIter>
    >      bool operator>=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 29 Returns: `lhs.str() >= rhs!(lhs < rhs)`.

24. Change 28.9.2 [re.submatch.op]/30 as indicated:

    >     template <class BiIter>
    >      bool operator<=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 30 Returns: `lhs.str() <= rhs!(rhs < lhs)`.

25. Change 28.9.2 [re.submatch.op]/31 as indicated:

    >     template <class BiIter>
    >      bool operator==(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 31 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) == rhs.str()`.
    > 31 Returns: `rhs.compare(typename sub_match<BiIter>::string_type(1, lhs)) == 0`.

26. Change 28.9.2 [re.submatch.op]/32 as indicated:

    >     template <class BiIter>
    >      bool operator!=(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 32 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) != rhs.str()!(lhs == rhs)`.

27. Change 28.9.2 [re.submatch.op]/33 as indicated:

    >     template <class BiIter>
    >      bool operator<(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 33 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) < rhs.str()`.
    > 33 Returns: `rhs.compare(typename sub_match<BiIter>::string_type(1, lhs)) > 0`.

28. Change 28.9.2 [re.submatch.op]/34 as indicated:

    >     template <class BiIter>
    >      bool operator>(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 34 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) > rhs.str()rhs < lhs`.

29. Change 28.9.2 [re.submatch.op]/35 as indicated:

    >     template <class BiIter>
    >      bool operator>=(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 35 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) >= rhs.str()!(lhs < rhs)`.

30. Change 28.9.2 [re.submatch.op]/36 as indicated:

    >     template <class BiIter>
    >      bool operator<=(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 36 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) <= rhs.str()!(rhs < lhs)`.

31. Change 28.9.2 [re.submatch.op]/37 as indicated:

    >     template <class BiIter>
    >      bool operator==(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 37 Returns: `lhs.str() == basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)`.
    > 37 Returns: `lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) == 0`.

32. Change 28.9.2 [re.submatch.op]/38 as indicated:

    >     template <class BiIter>
    >      bool operator!=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 38 Returns: `lhs.str() != basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(lhs == rhs)`.

33. Change 28.9.2 [re.submatch.op]/39 as indicated:

    >     template <class BiIter>
    >      bool operator<(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 39 Returns: `lhs.str() < basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)`.
    > 39 Returns: `lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) < 0`.

34. Change 28.9.2 [re.submatch.op]/40 as indicated:

    >     template <class BiIter>
    >      bool operator>(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 40 Returns: `lhs.str() > basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)rhs < lhs`.

35. Change 28.9.2 [re.submatch.op]/41 as indicated:

    >     template <class BiIter>
    >      bool operator>=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 41 Returns: `lhs.str() >= basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(lhs < rhs)`.

36. Change 28.9.2 [re.submatch.op]/42 as indicated:

    >     template <class BiIter>
    >      bool operator<=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 42 Returns: `lhs.str() <= basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(rhs < lhs)`.

### 1182(i). Unfortunate hash dependencies

Section: 22.10.19 [unord.hash] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-07-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unord.hash].

View all issues with C++11 status.

Discussion:

Addresses UK 324

The implied library dependencies created by spelling out all the `hash` template specializations in the `<functional>` synopsis are unfortunate. The potential coupling is greatly reduced if the `hash` specialization is declared in the appropriate header for each library type, as it is much simpler to forward declare the primary template and provide a single specialization than it is to implement a `hash` function for a `string` or `vector` without providing a definition for the whole `string/vector` template in order to access the necessary bits.

Note that the proposed resolution purely involves moving the declarations of a few specializations, it specifically does not make any changes to 22.10.19 [unord.hash].

[ 2009-09-15 Daniel adds: ]

I suggest to add to the current existing proposed resolution the following items.

- Add to the very first strike-list of the currently suggested resolution the following lines:

  >     template <> struct hash<std::error_code>;
  >     template <> struct hash<std::thread::id>;

- Add the following declarations to 19.5 [syserr], header `<system_error>` synopsis after // 19.5.4:

  >     // 19.5.x hash support
  >     template <class T> struct hash;
  >     template <> struct hash<error_code>;

- Add a new clause 19.5.X (probably after 19.5.4):

  > 19.5.X Hash support [syserr.hash]

  >     template <> struct hash<error_code>;

  > > An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type `error_code` suitable for using this type as key in unordered associative containers (23.5 [unord]).

- Add the following declarations to 32.4.3.2 [thread.thread.id] just after the declaration of the comparison operators:

  >     template <class T> struct hash;
  >     template <> struct hash<thread::id>;

- Add a new paragraph at the end of 32.4.3.2 [thread.thread.id]:

  >     template <> struct hash<thread::id>;

  > > An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type `thread::id` suitable for using this type as key in unordered associative containers (23.5 [unord]).

- Issue 889(i) independently suggests moving the specialization `std::hash<std::thread::id>` to header `<thread>`.

[ 2009-11-13 Alisdair adopts Daniel's suggestion and the extended note from 889(i). ]

[ 2010-01-31 Alisdair: related to 1245(i) and 978(i). ]

[ 2010-02-07 Proposed wording updated by Beman, Daniel, Alisdair and Ganesh. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Strike the following specializations declared in the `<functional>` synopsis p2 22.10 [function.objects]

>     template <> struct hash<std::string>;
>     template <> struct hash<std::u16string>;
>     template <> struct hash<std::u32string>;
>     template <> struct hash<std::wstring>;

>     template <> struct hash<std::error_code>;
>     template <> struct hash<std::thread::id>;
>     template <class Allocator> struct hash<std::vector<bool, Allocator> >;
>     template <std::size_t N> struct hash<std::bitset<N> >;

Add the following at the end of 22.10.19 [unord.hash]:

>     template <> struct hash<bool>;
>     template <> struct hash<char>;
>     template <> struct hash<signed char>;
>     template <> struct hash<unsigned char>;
>     template <> struct hash<char16_t>;
>     template <> struct hash<char32_t>;
>     template <> struct hash<wchar_t>;
>     template <> struct hash<short>;
>     template <> struct hash<unsigned short>;
>     template <> struct hash<int>;
>     template <> struct hash<unsigned int>;
>     template <> struct hash<long>;
>     template <> struct hash<long long>;
>     template <> struct hash<unsigned long>;
>     template <> struct hash<unsigned long long>;
>     template <> struct hash<float>;
>     template <> struct hash<double>;
>     template <> struct hash<long double>;
>     template<class T> struct hash<T*>;

> Specializations meeting the requirements of class template `hash` 22.10.19 [unord.hash].

Add the following declarations to 19.5 [syserr], header `<system_error>` synopsis after // 19.5.4:

>     // [syserr.hash] hash support
>     template <class T> struct hash;
>     template <> struct hash<error_code>;

Add a new clause 19.5.X (probably after 19.5.4):

> 19.5.X Hash support [syserr.hash]

>     template <> struct hash<error_code>;

> Specialization meeting the requirements of class template `hash` 22.10.19 [unord.hash].

Add the following declarations to the synopsis of `<string>` in 27.4 [string.classes]

>     // [basic.string.hash] hash support
>     template <class T> struct hash;
>     template <> struct hash<string>;
>     template <> struct hash<u16string>;
>     template <> struct hash<u32string>;
>     template <> struct hash<wstring>;

Add a new clause 21.4.X

> 21.4.X Hash support [basic.string.hash]>

>     template <> struct hash<string>;
>     template <> struct hash<u16string>;
>     template <> struct hash<u32string>;
>     template <> struct hash<wstring>;

> Specializations meeting the requirements of class template `hash` 22.10.19 [unord.hash].

Add the following declarations to the synopsis of `<vector>` in 23.3 [sequences]

>     // 21.4.x hash support
>     template <class T> struct hash;
>     template <class Allocator> struct hash<vector<bool, Allocator>>;

Add a new paragraph to the end of 23.3.14 [vector.bool]

>     template <class Allocator> struct hash<vector<bool, Allocator>>;

> Specialization meeting the requirements of class template `hash` 22.10.19 [unord.hash].

Add the following declarations to the synopsis of `<bitset>` in 22.9.2 [template.bitset]

>     // [bitset.hash] hash support
>     template <class T> struct hash;
>     template <size_t N> struct hash<bitset<N> >;

Add a new subclause 20.3.7.X [bitset.hash]

> 20.3.7.X bitset hash support [bitset.hash]

>     template <size_t N> struct hash<bitset<N> >;

> Specialization meeting the requirements of class template `hash` 22.10.19 [unord.hash].

Add the following declarations to 32.4.3.2 [thread.thread.id] synopsis just after the declaration of the comparison operators:

>     template <class T> struct hash;
>     template <> struct hash<thread::id>;

Add a new paragraph at the end of 32.4.3.2 [thread.thread.id]:

>     template <> struct hash<thread::id>;

> Specialization meeting the requirements of class template `hash` 22.10.19 [unord.hash].

Change Header <typeindex> synopsis 17.7.6 [type.index.synopsis] as indicated:

>     namespace std {
>     class type_index;
>       // [type.index.hash] hash support
>       template <class T> struct hash;
>       template<> struct hash<type_index>;  : public unary_function<type_index, size_t> {
>         size_t operator()(type_index index) const;
>       }
>     }

Change Template specialization hash<type_index> [type.index.templ] as indicated:

> 20.11.4 Template specialization hash<type_index> [type.index.templ] Hash support [type.index.hash]

>     size_t operator()(type_index index) const;

> > Returns: `index.hash_code()`

>     template<> struct hash<type_index>;

> Specialization meeting the requirements of class template `hash` [unord.hash]. For an object `index` of type `type_index`, `hash<type_index>()(index)` shall evaluate to the same value as `index.hash_code()`.

### 1183(i). `basic_ios::set_rdbuf` may break class invariants

Section: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [basic.ios.members].

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Discussion:

The protected member function `set_rdbuf` had been added during the process of adding move and swap semantics to IO classes. A relevant property of this function is described by it's effects in 31.5.4.3 [basic.ios.members]/19:

> Effects: Associates the `basic_streambuf` object pointed to by sb with this stream without calling `clear()`.

This means that implementors of or those who derive from existing IO classes could cause an internal state where the stream buffer could be 0, but the IO class has the state `good()`. This would break several currently existing implementations which rely on the fact that setting a stream buffer via the currently only ways, i.e. either by calling

>     void init(basic_streambuf<charT,traits>* sb);

or by calling

>     basic_streambuf<charT,traits>* rdbuf(basic_streambuf<charT,traits>* sb);

to set `rdstate()` to `badbit`, if the buffer is 0. This has the effect that many internal functions can simply check `rdstate()` instead of `rdbuf()` for being 0.

I therefore suggest that a requirement is added for callers of `set_rdbuf` to set a non-0 value.

[ 2009-10 Santa Cruz: ]

> Moved to Open. Martin volunteers to provide new wording, where `set_rdbuf()` sets the `badbit` but does not cause an exception to be thrown like a call to `clear()` would.

[ 2009-10-20 Martin provides wording: ]

Change 31.5.4.3 [basic.ios.members] around p. 19 as indicated:

>     void set_rdbuf(basic_streambuf<charT, traits>* sb);

> > Effects: Associates the `basic_streambuf` object pointed to by `sb` with this stream without calling `clear()`. Postconditions: `rdbuf() == sb`.

> > Effects: As if:

> > >     iostate state = rdstate();
> > >     try { rdbuf(sb); }
> > >     catch(ios_base::failure) {
> > >        if (0 == (state & ios_base::badbit))
> > >            unsetf(badbit);
> > >     }

> > Throws: Nothing.

Rationale:

We need to be able to call `set_rdbuf()` on stream objects for which (`rdbuf() == 0`) holds without causing `ios_base::failure` to be thrown. We also don't want `badbit` to be set as a result of setting `rdbuf()` to 0 if it wasn't set before the call. This changed Effects clause maintains the current behavior (as of N2914) without requiring that `sb` be non-null.

[ Post-Rapperswil ]

Several reviewers and the submitter believe that the best solution would be to add a pre-condition that the buffer shall not be a null pointer value.

> Moved to Tentatively Ready with revised wording provided by Daniel after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Add a new pre-condition just before 27.5.4.2 [basic.ios.members]/23 as indicated:

   >     void set_rdbuf(basic_streambuf<charT, traits>* sb);

   > > ?? Requires: `sb != nullptr`.

   > > 23 Effects: Associates the `basic_streambuf` object pointed to by `sb` with this stream without calling `clear()`.

   > > 24 Postconditions: `rdbuf() == sb`.

   > > 25 Throws: Nothing.

Rationale:

We believe that setting a `nullptr` stream buffer can be prevented.

### 1185(i). Iterator categories and output iterators

Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-07-31 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iterator.requirements].

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Discussion:

(wording relative to N2723 pending new working paper)

According to p3 24.3 [iterator.requirements], Forward iterators, Bidirectional iterators and Random Access iterators all satisfy the requirements for an Output iterator:

> XXX iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified ...

Meanwhile, p4 goes on to contradict this:

> Besides its category, a forward, bidirectional, or random access iterator can also be mutable or constant...

> ... Constant iterators do not satisfy the requirements for output iterators

The latter seems to be the overriding concern, as the iterator tag hierarchy does not define `forward_iterator_tag` as multiply derived from both `input_iterator_tag` and `output_iterator_tag`.

The work on concepts for iterators showed us that output iterator really is fundamentally a second dimension to the iterator categories, rather than part of the linear input -> forward -> bidirectional -> random-access sequence. It would be good to clear up these words to reflect that, and separately list output iterator requirements in the requires clauses for the appropriate algorithms and operations.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

### 1187(i). `std::decay`

Section: 21.3.8.7 [meta.trans.other] Status: C++11 Submitter: Jason Merrill Opened: 2009-08-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [meta.trans.other].

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Discussion:

I notice that `std::decay` is specified to strip the cv-quals from anything but an array or pointer. This seems incorrect for values of class type, since class rvalues can have cv-qualified type (7.2.1 [basic.lval]/9).

[ 2009-08-09 Howard adds: ]

> See the thread starting with c++std-lib-24568 for further discussion. And here is a convenience link to the original proposal. Also see the closely related issue 705(i).

[ 2010 Pittsburgh: Moved to Ready. ]

Proposed resolution:

Add a note to `decay` in 21.3.8.7 [meta.trans.other]:

> [Note: This behavior is similar to the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. — end note]

### 1189(i). Awkward interface for changing the number of buckets in an unordered associative container

Section: 23.2.8 [unord.req], 23.5 [unord] Status: C++11 Submitter: Matt Austern Opened: 2009-08-10 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [unord.req].

View all other issues in [unord.req].

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Discussion:

Consider a typical use case: I create an `unordered_map` and then start adding elements to it one at a time. I know that it will eventually need to store a few million elements, so, for performance reasons, I would like to reserve enough capacity that none of the calls to `insert` will trigger a rehash.

Unfortunately, the existing interface makes this awkward. The user naturally sees the problem in terms of the number of elements, but the interface presents it as buckets. If `m` is the map and `n` is the expected number of elements, this operation is written `m.rehash(n / m.max_load_factor())` — not very novice friendly.

[ 2009-09-30 Daniel adds: ]

> I recommend to replace "`resize`" by a different name like "`reserve`", because that would better match the intended use-case. Rational: Any existing resize function has the on-success post-condition that the provided size is equal to `size()`, which is not satisfied for the proposal. Reserve seems to fit the purpose of the actual renaming suggestion.

[ 2009-10-28 Ganesh summarizes alternative resolutions and expresses a strong preference for the second (and opposition to the first): ]

> 1. In the unordered associative container requirements (23.2.8 [unord.req]), remove the row for rehash and replace it with:

>    > Table 87 — Unordered associative container requirements (in addition to container)
>    > Expression | Return type | Assertion/note pre-/post-condition | Complexity
>    > --- | --- | --- | ---
>    > `a.rehashreserve(n)` | `void` | Post: `a.bucket_count > max(a.size(), n) / a.max_load_factor()` and `a.bucket_count() >= n`. | Average case linear in `a.size()`, worst case quadratic.

>    Make the corresponding change in the class synopses in 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.6 [unord.set], and 23.5.7 [unord.multiset].

> 2. In 23.2.8 [unord.req]/9, table 98, append a new row after the last one:

>    > Table 87 — Unordered associative container requirements (in addition to container)
>    > Expression | Return type | Assertion/note pre-/post-condition | Complexity
>    > --- | --- | --- | ---
>    > `a.rehash(n)` | `void` | Post: `a.bucket_count > a.size() / a.max_load_factor()` and `a.bucket_count() >= n`. | Average case linear in `a.size()`, worst case quadratic.
>    > `a.reserve(n)` | `void` | Same as `a.rehash(ceil(n / a.max_load_factor()))` | Average case linear in `a.size()`, worst case quadratic.

>    In 23.5.3 [unord.map]/3 in the definition of class template `unordered_map`, in 23.5.4 [unord.multimap]/3 in the definition of class template `unordered_multimap`, in 23.5.6 [unord.set]/3 in the definition of class template `unordered_set` and in 23.5.7 [unord.multiset]/3 in the definition of class template `unordered_multiset`, add the following line after member function `rehash()`:

>    >     void reserve(size_type n);

[ 2009-10-28 Howard: ]

> Moved to Tentatively Ready after 5 votes in favor of Ganesh's option 2 above. The original proposed wording now appears here:

> > Informally: instead of providing `rehash(n)` provide `resize(n)`, with the semantics "make the container a good size for `n` elements".

> > In the unordered associative container requirements (23.2.8 [unord.req]), remove the row for rehash and replace it with:

> > > Table 87 — Unordered associative container requirements (in addition to container)
> > > Expression | Return type | Assertion/note pre-/post-condition | Complexity
> > > --- | --- | --- | ---
> > > `a.rehashresize(n)` | `void` | Post: `a.bucket_count > max(a.size(), n) / a.max_load_factor()` and `a.bucket_count() >= n`. | Average case linear in `a.size()`, worst case quadratic.

> > Make the corresponding change in the class synopses in 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.6 [unord.set], and 23.5.7 [unord.multiset].

Proposed resolution:

In 23.2.8 [unord.req]/9, table 98, append a new row after the last one:

> Table 87 — Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `a.rehash(n)` | `void` | Post: `a.bucket_count > a.size() / a.max_load_factor()` and `a.bucket_count() >= n`. | Average case linear in `a.size()`, worst case quadratic.
> `a.reserve(n)` | `void` | Same as `a.rehash(ceil(n / a.max_load_factor()))` | Average case linear in `a.size()`, worst case quadratic.

In 23.5.3 [unord.map]/3 in the definition of class template `unordered_map`, in 23.5.4 [unord.multimap]/3 in the definition of class template `unordered_multimap`, in 23.5.6 [unord.set]/3 in the definition of class template `unordered_set` and in 23.5.7 [unord.multiset]/3 in the definition of class template `unordered_multiset`, add the following line after member function `rehash()`:

>     void reserve(size_type n);

### 1191(i). `tuple get` API should respect rvalues

Section: 22.4.8 [tuple.elem] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-08-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [tuple.elem].

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Discussion:

The `tuple get` API should respect rvalues. This would allow for moving a single element out of a `tuple`-like type.

[ 2009-10-30 Alisdair adds: ]

> The issue of rvalue overloads of get for tuple-like types was briefly discussed in Santa Cruz.

> The feedback was this would be welcome, but we need full wording for the other types (`pair` and `array`) before advancing.

> I suggest the issue moves to Open from New as it has been considered, feedback given, and it has not (yet) been rejected as NAD.

[ 2010 Rapperswil: ]

> Note that wording has been provided, and this issue becomes more important now that we have added a function to support forwarding argument lists as `tuple`s. Move to Tentatively Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Add the following signature to p2 22.4.1 [tuple.general]

>     template <size_t I, class ... Types>
>     typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...> &&);

And again to 22.4.8 [tuple.elem].

>     template <size_t I, class ... Types>
>     typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...>&& t);

> > Effects: Equivalent to `return std::forward<typename tuple_element<I, tuple<Types...> >::type&&>(get<I>(t));`

> > [Note: If a `T` in `Types` is some reference type `X&`, the return type is `X&`, not `X&&`. However, if the element type is non-reference type `T`, the return type is `T&&`. — end note]

Add the following signature to p1 22.2 [utility]

>     template <size_t I, class T1, class T2>
>     typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&&);

And to p5 22.3.4 [pair.astuple]

>     template <size_t I, class T1, class T2>
>     typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&& p);

> > Returns: If `I == 0` returns `std::forward<T1&&>(p.first)`; if `I == 1` returns `std::forward<T2&&>(p.second)`; otherwise the program is ill-formed.

> > Throws: Nothing.

Add the following signature to 23.3 [sequences] `<array>` synopsis

>     template <size_t I, class T, size_t N>
>     T&& get(array<T,N> &&);

And after p8 23.3.3.7 [array.tuple]

>     template <size_t I, class T, size_t N>
>     T&& get(array<T,N> && a);

> > Effects: Equivalent to `return std::move(get<I>(a));`

### 1192(i). `basic_string` missing definitions for `cbegin` / `cend` / `crbegin` / `crend`

Section: 27.4.3.4 [string.iterators] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-08-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Unlike the containers in clause 23, `basic_string` has definitions for `begin()` and `end()`, but these have not been updated to include `cbegin`, `cend`, `crbegin` and `crend`.

[ 2009-10-28 Howard: ]

> Moved to Tentatively NAD after 5 positive votes on c++std-lib. Added rationale.

[ 2009-10-28 Alisdair disagrees: ]

> I'm going to have to speak up as the dissenting voice.

> I agree the issue could be handled editorially, and that would be my preference if Pete feels this is appropriate. Failing that, I really think this issue should be accepted and moved to ready. The other begin/end functions all have a semantic definition for this template, and it is confusing if a small few are missing.

> I agree that an alternative would be to strike all the definitions for `begin/end/rbegin/rend` and defer completely to the requirements tables in clause 23. I think that might be confusing without a forward reference though, as those tables are defined in a later clause than the `basic_string` template itself. If someone wants to pursue this I would support it, but recommend it as a separate issue.

> So my preference is strongly to move Ready over NAD, and a stronger preference for NAD Editorial if Pete is happy to make these changes.

[ 2009-10-29 Howard: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib. Removed rationale to mark it NAD. :-)

Proposed resolution:

Add to 27.4.3.4 [string.iterators]

>     iterator       begin();
>     const_iterator begin() const;
>     const_iterator cbegin() const;

> ...

>     iterator       end();
>     const_iterator end() const;
>     const_iterator cend() const;

> ...

>     reverse_iterator       rbegin();
>     const_reverse_iterator rbegin() const;
>     const_reverse_iterator crbegin() const;

> ...

>     reverse_iterator       rend();
>     const_reverse_iterator rend() const;
>     const_reverse_iterator crend() const;

### 1193(i). `default_delete` cannot be instantiated with incomplete types

Section: 20.3.1.2 [unique.ptr.dltr] Status: C++11 Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

According to the general rules of 16.4.5.8 [res.on.functions] p 2 b 5 the effects are undefined, if an incomplete type is used to instantiate a library template. But neither in 20.3.1.2 [unique.ptr.dltr] nor in any other place of the standard such explicit allowance is given. Since this template is intended to be instantiated with incomplete types, this must be fixed.

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2009-11-17 Alisdair Opens: ]

> LWG 1193 tries to support `unique_ptr` for incomplete types. I believe the proposed wording goes too far:

> > The template parameter `T` of `default_delete` may be an incomplete type.

> Do we really want to support `cv-void`? Suggested ammendment:

> > The template parameter `T` of `default_delete` may be an incomplete type other than `cv-void`.

> We might also consider saying something about arrays of incomplete types.

> Did we lose support for `unique_ptr<function-type>` when the concept-enabled work was shelved? If so, we might want a `default_delete` partial specialization for function types that does nothing. Alternatively, function types should not be supported by default, but there is no reason a user cannot support them via their own deletion policy.

> Function-type support might also lead to conditionally supporting a function-call operator in the general case, and that seems way too inventive at this stage to me, even if we could largely steal wording directly from `reference_wrapper`. `shared_ptr` would have similar problems too.

[ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add two new paragraphs directly to 20.3.1.2 [unique.ptr.dltr] (before 20.3.1.2.2 [unique.ptr.dltr.dflt]) with the following content:

> The class template `default_delete` serves as the default deleter (destruction policy) for the class template `unique_ptr`.

> The template parameter `T` of `default_delete` may be an incomplete type.

### 1194(i). Unintended `queue` constructor

Section: 23.6 [container.adaptors] Status: C++11 Submitter: Howard Hinnant Opened: 2009-08-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.adaptors].

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Discussion:

23.6.3.1 [queue.defn] has the following `queue` constructor:

>     template <class Alloc> explicit queue(const Alloc&);

This will be implemented like so:

>     template <class Alloc> explicit queue(const Alloc& a) : c(a) {}

The issue is that `Alloc` can be anything that a container will construct from, for example an `int`. Is this intended to compile?

>     queue<int> q(5);

Before the addition of this constructor, `queue<int>(5)` would not compile. I ask, not because this crashes, but because it is new and appears to be unintended. We do not want to be in a position of accidently introducing this "feature" in C++0X and later attempting to remove it.

I've picked on `queue`. `priority_queue` and `stack` have the same issue. Is it useful to create a `priority_queue` of 5 identical elements?

[ Daniel, Howard and Pablo collaborated on the proposed wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

[ This resolution includes a semi-editorial clean up, giving definitions to members which in some cases weren't defined since C++98. This resolution also offers editorially different wording for 976(i), and it also provides wording for 1196(i). ]

Change [ontainer.adaptor], p1:

> The container adaptors each take a `Container` template parameter, and each constructor takes a `Container` reference argument. This container is copied into the `Container` member of each adaptor. If the container takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor. Otherwise, normal copy or move construction is used for the container argument. [Note: it is not necessary for an implementation to distinguish between the one-argument constructor that takes a `Container` and the one- argument constructor that takes an allocator_type. Both forms use their argument to construct an instance of the container. — end note]

Change [ueue.def], p1:

>     template <class T, class Container = deque<T> >
>     class queue {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef Container                           container_type;
>     protected:
>       Container c;

>     public:
>       explicit queue(const Container&);
>       explicit queue(Container&& = Container());
>       queue(queue&& q); : c(std::move(q.c)) {}
>       template <class Alloc> explicit queue(const Alloc&);
>       template <class Alloc> queue(const Container&, const Alloc&);
>       template <class Alloc> queue(Container&&, const Alloc&);
>       template <class Alloc> queue(queue&&, const Alloc&);
>       queue& operator=(queue&& q); { c = std::move(q.c); return *this; }

>       bool empty() const          { return c.empty(); }
>       ...
>     };

Add a new section after 23.6.3.1 [queue.defn], [queue.cons]:

> `queue` constructors [queue.cons]

>     explicit queue(const Container& cont);

> > Effects: Initializes `c` with `cont`.

>     explicit queue(Container&& cont = Container());

> > Effects: Initializes `c` with `std::move(cont)`.

>     queue(queue&& q)

> > Effects: Initializes `c` with `std::move(q.c)`.

> For each of the following constructors, if `uses_allocator<container_type, Alloc>::value` is `false`, then the constructor shall not participate in overload resolution.

>     template <class Alloc>
>       explicit queue(const Alloc& a);

> > Effects: Initializes `c` with `a`.

>     template <class Alloc>
>       queue(const container_type& cont, const Alloc& a);

> > Effects: Initializes `c` with `cont` as the first argument and `a` as the second argument.

>     template <class Alloc>
>       queue(container_type&& cont, const Alloc& a);

> > Effects: Initializes `c` with `std::move(cont)` as the first argument and `a` as the second argument.

>     template <class Alloc>
>       queue(queue&& q, const Alloc& a);

> > Effects: Initializes `c` with `std::move(q.c)` as the first argument and `a` as the second argument.

>     queue& operator=(queue&& q);

> > Effects: Assigns `c` with `std::move(q.c)`.

> > Returns: `*this`.

Add to 23.6.4.2 [priqueue.cons]:

>     priority_queue(priority_queue&& q);

> > Effects: Initializes `c` with `std::move(q.c)` and initializes `comp` with `std::move(q.comp)`.

> For each of the following constructors, if `uses_allocator<container_type, Alloc>::value` is `false`, then the constructor shall not participate in overload resolution.

>     template <class Alloc>
>       explicit priority_queue(const Alloc& a);

> > Effects: Initializes `c` with `a` and value-initializes `comp`.

>     template <class Alloc>
>       priority_queue(const Compare& compare, const Alloc& a);

> > Effects: Initializes `c` with `a` and initializes `comp` with `compare`.

>     template <class Alloc>
>       priority_queue(const Compare& compare, const Container& cont, const Alloc& a);

> > Effects: Initializes `c` with `cont` as the first argument and `a` as the second argument, and initializes `comp` with `compare`.

>     template <class Alloc>
>       priority_queue(const Compare& compare, Container&& cont, const Alloc& a);

> > Effects: Initializes `c` with `std::move(cont)` as the first argument and `a` as the second argument, and initializes `comp` with `compare`.

>     template <class Alloc>
>       priority_queue(priority_queue&& q, const Alloc& a);

> > Effects: Initializes `c` with `std::move(q.c)` as the first argument and `a` as the second argument, and initializes `comp` with `std::move(q.comp)`.

>     priority_queue& operator=(priority_queue&& q);

> > Effects: Assigns `c` with `std::move(q.c)` and assigns `comp` with `std::move(q.comp)`.

> > Returns: `*this`.

Change 23.6.6.2 [stack.defn]:

>     template <class T, class Container = deque<T> >
>     class stack {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef Container                           container_type;
>     protected:
>       Container c;

>     public:
>       explicit stack(const Container&);
>       explicit stack(Container&& = Container());
>       stack(stack&& s);
>       template <class Alloc> explicit stack(const Alloc&);
>       template <class Alloc> stack(const Container&, const Alloc&);
>       template <class Alloc> stack(Container&&, const Alloc&);
>       template <class Alloc> stack(stack&&, const Alloc&);
>       stack& operator=(stack&& s);

>       bool empty() const          { return c.empty(); }
>       ...
>     };

Add a new section after 23.6.6.2 [stack.defn], [stack.cons]:

> `stack` constructors [stack.cons]

>     stack(stack&& s);

> > Effects: Initializes `c` with `std::move(s.c)`.

> For each of the following constructors, if `uses_allocator<container_type, Alloc>::value` is `false`, then the constructor shall not participate in overload resolution.

>     template <class Alloc>
>       explicit stack(const Alloc& a);

> > Effects: Initializes `c` with `a`.

>     template <class Alloc>
>       stack(const container_type& cont, const Alloc& a);

> > Effects: Initializes `c` with `cont` as the first argument and `a` as the second argument.

>     template <class Alloc>
>       stack(container_type&& cont, const Alloc& a);

> > Effects: Initializes `c` with `std::move(cont)` as the first argument and `a` as the second argument.

>     template <class Alloc>
>       stack(stack&& s, const Alloc& a);

> > Effects: Initializes `c` with `std::move(s.c)` as the first argument and `a` as the second argument.

>     stack& operator=(stack&& s);

> > Effects: Assigns `c` with `std::move(s.c)`.

> > Returns: `*this`.

### 1195(i). "Diagnostic required" wording is insufficient to prevent UB

Section: 16 [library] Status: C++11 Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Several parts of the library use the notion of "Diagnostic required" to indicate that in the corresponding situation an error diagnostic should occur, e.g. 20.3.1.2.2 [unique.ptr.dltr.dflt]/2

>     void operator()(T *ptr) const;

> > Effects: calls `delete` on `ptr`. A diagnostic is required if `T` is an incomplete type.

The problem with this approach is that such a requirement is insufficient to prevent undefined behavior, if this situation occurs. According to 3.18 [defns.diagnostic] a diagnostic message is defined as

> a message belonging to an implementation-defined subset of the implementation's output messages.

which doesn't indicate any relation to an ill-formed program. In fact, "compiler warnings" are a typical expression of such diagnostics. This means that above wording can be interpreted by compiler writers that they satisfy the requirements of the standard if they just produce such a "warning", if the compiler happens to compile code like this:

>     #include <memory>

>     struct Ukn; // defined somewhere else
>     Ukn* create_ukn(); // defined somewhere else

>     int main() {
>      std::default_delete<Ukn>()(create_ukn());
>     }

In this and other examples discussed here it was the authors intent to guarantee that the program is ill-formed with a required diagnostic, therefore such wording should be used instead. According to the general rules outlined in 4.1 [intro.compliance] it should be sufficient to require that these situations produce an ill-formed program and the "diagnostic required" part should be implied. The proposed resolution also suggests to remove several redundant wording of "Diagnostics required" to ensure that the absence of such saying does not cause a misleading interpretation.

[ 2009 Santa Cruz: ]

> Move to NAD.

> It's not clear that there's any important difference between "ill-formed" and "diagnostic required". From 4.1 [intro.compliance], 3.26 [defns.ill.formed], and 3.69 [defns.well.formed] it appears that an ill-formed program is one that is not correctly constructed according to the syntax rules and diagnosable semantic rules, which means that... "a conforming implementation shall issue at least one diagnostic message." The author's intent seems to be that we should be requiring a fatal error instead of a mere warning, but the standard just doesn't have language to express that distinction. The strongest thing we can ever require is a "diagnostic".

> The proposed rewording may be a clearer way of expressing the same thing that the WP already says, but such a rewording is editorial.

[ 2009 Santa Cruz: ]

> Considered again. Group disagrees that the change is technical, but likes it editorially. Moved to NAD Editorial.

[ 2009-11-19: Moved from NAD Editorial to Open. Please see the thread starting with Message c++std-lib-25916. ]

[ 2009-11-20 Daniel updated wording. ]

> The following resolution differs from the previous one by avoiding the unusual and misleading term "shall be ill-formed", which does also not follow the core language style. This resolution has the advantage of a minimum impact on the current wording, but I would like to mention that a more intrusive solution might be preferrable - at least as a long-term solution: Jens Maurer suggested the following approach to get rid of the usage of the term "ill-formed" from the library by introducing a new category to existing elements to the list of 16.3.2.4 [structure.specifications]/3, e.g. "type requirements" or "static constraints" that define conditions that can be checked during compile-time and any violation would make the program ill-formed. As an example, the currently existing phrase 22.4.7 [tuple.helper]/1

> > Requires: `I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

> could then be written as

> > Static constraints: `I < sizeof...(Types)`.

[ 2009-11-21 Daniel updated wording. ]

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 21.5 [ratio]/2 as indicated:

   > Throughout this subclause, if the template argument types `R1` and `R2` shall be are not specializations of the `ratio` template, the program is ill-formed. Diagnostic required.

2. Change 21.5.3 [ratio.ratio]/1 as indicated:

   If tThe template argument `D` shall not be is zero, and or the absolute values of the template arguments `N` and `D` shall be are not representable by type `intmax_t`, the program is ill-formed. Diagnostic required. [..]

3. Change 21.5.4 [ratio.arithmetic]/1 as indicated:

   > Implementations may use other algorithms to compute these values. If overflow occurs, the program is ill-formed a diagnostic shall be issued.

4. Change 21.5.5 [ratio.comparison]/2 as indicated:

   > [...] Implementations may use other algorithms to compute this relationship to avoid overflow. If overflow occurs, the program is ill-formed a diagnostic is required.

5. Change 20.3.1.2.2 [unique.ptr.dltr.dflt]/2 as indicated:

   > Effects: calls `delete` on `ptr`. A diagnostic is required if `T` is an incomplete type.

   > Remarks: If `T` is an incomplete type, the program is ill-formed.

6. Change 20.3.1.2.3 [unique.ptr.dltr.dflt1]/1 as indicated:

   >     void operator()(T* ptr) const;

   > > Effects: `operator()` calls `delete[]` on `ptr`. A diagnostic is required if `T` is an incomplete type.

   > > Remarks: If `T` is an incomplete type, the program is ill-formed.

7. Change 20.3.1.3.2 [unique.ptr.single.ctor] as indicated: [Note: This editorially improves the currently suggested wording of 932(i) by replacing

   > "shall be ill-formed" by "is ill-formed"]

   [If N3025 is accepted this bullet is applied identically in that paper as well.]

   > -1- Requires: `D` shall be default constructible, and that construction shall not throw an exception. `D` shall not be a reference type or pointer type (diagnostic required).

   > ...

   > Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument `D`, the program is ill-formed.

8. Change 20.3.1.3.2 [unique.ptr.single.ctor]/8 as indicated: [Note: This editorially improves the currently suggested wording of 932(i) by replacing

   > "shall be ill-formed" by "is ill-formed"]

   [If N3025 is accepted this bullet is applied identically in that paper as well.]

   >     unique_ptr(pointer p);

   > > ...

   > > Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument `D`, the program is ill-formed.

9. Change 20.3.1.3.2 [unique.ptr.single.ctor]/13 as indicated:

   > [..] If `d` is an rvalue, it will bind to the second constructor of this pair and the program is ill-formed. That constructor shall emit a diagnostic. [Note: The diagnostic could be implemented using a `static_assert` which assures that `D` is not a reference type. — end note] Else `d` is an lvalue and will bind to the first constructor of this pair. [..]

10. 20.3.1.3.2 [unique.ptr.single.ctor]/20: Solved by 950(i).

11. Change 20.3.1.4 [unique.ptr.runtime]/1 as indicated:

    > A specialization for array types is provided with a slightly altered interface.

    > - Conversions among different types of `unique_ptr<T[], D>` or to or from the non-array forms of `unique_ptr` are disallowed (diagnostic required) produce an ill-formed program.
    > - ...

12. Change 30.5 [time.duration]/2-4 as indicated:

    > 2 Requires: `Rep` shall be an arithmetic type or a class emulating an arithmetic type. If a program instantiates `duration` with a `duration` type for the template argument `Rep` a diagnostic is required.

    > 3 Remarks: If `duration` is instantiated with a `duration` type for the template argument `Rep`, the program is ill-formed.

    > 3 4 Requires Remarks: If `Period` shall be is not a specialization of `ratio`, diagnostic required the program is ill-formed.

    > 4 5 Requires Remarks: If `Period::num` shall be is not positive, diagnostic required the program is ill-formed.

13. 30.5.2 [time.duration.cons]/1+4: Apply 1177(i)
14. 30.5.6 [time.duration.nonmember]/4+6+8+11: Apply 1177(i)
15. 30.5.8 [time.duration.cast]/1: Apply 1177(i)

16. Change 30.6 [time.point]/2 as indicated:

    > If `Duration` shall be is not an instance of `duration`, the program is ill-formed. Diagnostic required.

17. 30.6.2 [time.point.cons]/3: Apply 1177(i)
18. 30.6.8 [time.point.cast]/1: Apply 1177(i)

### 1196(i). move semantics undefined for priority_queue

Section: 23.6.4.2 [priqueue.cons] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-08-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The class template `priority_queue` declares signatures for a move constructor and move assignment operator in its class definition. However, it does not provide a definition (unlike `std::queue`, and proposed resolution for `std::stack`.) Nor does it provide a text clause specifying their behaviour.

[ 2009-08-23 Daniel adds: ]

> 1194(i) provides wording that solves this issue.

[ 2009-10 Santa Cruz: ]

> Mark NAD EditorialResolved, solved by issue 1194(i).

Proposed resolution:

### 1197(i). Can unordered containers have `bucket_count() == 0`?

Section: 23.2.8 [unord.req] Status: C++11 Submitter: Howard Hinnant Opened: 2009-08-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Table 97 "Unordered associative container requirements" in 23.2.8 [unord.req] says:

> Table 97 — Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `b.bucket(k)` | `size_type` | Returns the index of the bucket in which elements with keys equivalent to `k` would be found, if any such element existed. Post: the return value shall be in the range `[0, b.bucket_count())`. | Constant

What should `b.bucket(k)` return if `b.bucket_count() == 0`?

I believe allowing `b.bucket_count() == 0` is important. It is a very reasonable post-condition of the default constructor, or of a moved-from container.

I can think of several reasonable results from `b.bucket(k)` when `b.bucket_count() == 0`:

1. Return 0.
2. Return `numeric_limits<size_type>::max()`.
3. Throw a `domain_error`.
4. Requires: `b.bucket_count() != 0`.

[ 2009-08-26 Daniel adds: ]

> A forth choice would be to add the pre-condition "`b.bucket_count() != 0`" and thus imply undefined behavior if this is violated.

> [ Howard: I like this option too, added to the list. ]

> Further on here my own favorite solution (rationale see below):

> Suggested resolution:

> [Rationale: I suggest to follow choice (1). The main reason is that all associative container functions which take a key argument, are basically free of pre-conditions and non-disrupting, therefore excluding choices (3) and (4). Option (2) seems a bit unexpected to me. It would be more natural, if several similar functions would exist which would also justify the existence of a symbolic constant like npos for this situation. The value 0 is both simple and consistent, it has exactly the same role as a past-the-end iterator value. A typical use-case is:

> >     size_type pos = m.bucket(key);
> >     if (pos != m.bucket_count()) {
> >      ...
> >     } else {
> >      ...
> >     }

> — end Rationale]

> - Change Table 97 in 23.2.8 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):

> > Table 97 — Unordered associative container requirements (in addition to container)
> > Expression | Return type | Assertion/note pre-/post-condition | Complexity
> > --- | --- | --- | ---
> > `b.bucket(k)` | `size_type` | Returns the index of the bucket in which elements with keys equivalent to `k` would be found, if any such element existed. Post: if b.bucket_count() != 0, the return value shall be in the range `[0, b.bucket_count())`, otherwise 0. | Constant

[ 2010-01-25 Choice 4 put into proposed resolution section. ]

[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change Table 97 in 23.2.8 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):

> Table 97 — Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `b.bucket(k)` | `size_type` | Pre: `b.bucket_count() > 0` Returns the index of the bucket in which elements with keys equivalent to `k` would be found, if any such element existed. Post: the return value shall be in the range `[0, b.bucket_count())`. | Constant

### 1198(i). Container adaptor swap: member or non-member?

Section: 23.6 [container.adaptors] Status: C++11 Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Under 23.6 [container.adaptors] of N2914 the member function of `swap` of `queue` and `stack` call:

>     swap(c, q.c);

But under 23.6 [container.adaptors] of N2723 these members are specified to call:

>     c.swap(q.c);

Neither draft specifies the semantics of member `swap` for `priority_queue` though it is declared.

Although the distinction between member `swap` and non-member `swap` is not important when these adaptors are adapting standard containers, it may be important for user-defined containers.

We (Pablo and Howard) feel that it is more likely for a user-defined container to support a namespace scope `swap` than a member `swap`, and therefore these adaptors should use the container's namespace scope `swap`.

[ 2009-09-30 Daniel adds: ]

> The outcome of this issue should be considered with the outcome of 774(i) both in style and in content (e.g. 774(i) bullet 9 suggests to define the semantic of `void priority_queue::swap(priority_queue&)` in terms of the member `swap` of the container).

[ 2010-03-28 Daniel update to diff against N3092. ]

[ 2010 Rapperswil: ]

> Preference to move the wording into normative text, rather than inline function definitions in the class synopsis. Move to Tenatively Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 23.6.3.1 [queue.defn]:

>     template <class T, class Container = deque<T> >
>     class queue {
>        ...
>        void swap(queue& q) { using std::swap;
>                               c.swap(c, q.c); }
>        ...
>     };

Change 23.6.4 [priority.queue]:

>     template <class T, class Container = vector<T>,
>               class Compare = less<typename Container::value_type> >
>     class priority_queue {
>         ...
>         void swap(priority_queue& q); { using std::swap;
>                                          swap(c, q.c);
>                                          swap(comp, q.comp); }
>         ...
>     };

Change 23.6.6.2 [stack.defn]:

>     template <class T, class Container = deque<T> >
>     class stack {
>        ...
>        void swap(stack& s) { using std::swap;
>                               c.swap(c, s.c); }
>        ...
>     };

### 1199(i). Missing extended copy constructor in container adaptors

Section: 23.6 [container.adaptors] Status: C++11 Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2020-11-29

Priority: Not Prioritized

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Discussion:

`queue` has a constructor:

>     template <class Alloc>
>       queue(queue&&, const Alloc&);

but it is missing a corresponding constructor:

>     template <class Alloc>
>       queue(const queue&, const Alloc&);

The same is true of `priority_queue`, and `stack`. This "extended copy constructor" is needed for consistency and to ensure that the user of a container adaptor can always specify the allocator for his adaptor.

[ 2010-02-01 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

[ This resolution has been harmonized with the proposed resolution to issue 1194(i) ]

Change 23.6.3.1 [queue.defn], p1:

>     template <class T, class Container = deque<T> >
>     class queue {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef Container                           container_type;
>     protected:
>       Container c;

>     public:
>       explicit queue(const Container&);
>       explicit queue(Container&& = Container());
>       queue(queue&& q);

>       template <class Alloc> explicit queue(const Alloc&);
>       template <class Alloc> queue(const Container&, const Alloc&);
>       template <class Alloc> queue(Container&&, const Alloc&);
>       template <class Alloc> queue(const queue&, const Alloc&);
>       template <class Alloc> queue(queue&&, const Alloc&);
>       queue& operator=(queue&& q);

>       bool empty() const          { return c.empty(); }
>       ...
>     };

To the new section 23.6.3.2 [queue.cons], introduced in 1194(i), add:

>     template <class Alloc>
>       queue(const queue& q, const Alloc& a);

> > Effects: Initializes `c` with `q.c` as the first argument and `a` as the second argument.

Change 23.6.4 [priority.queue] as follows (I've an included an editorial change to move the poorly-placed move-assignment operator):

>     template <class T, class Container = vector<T>,
>               class Compare = less<typename Container::value_type> >
>     class priority_queue {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef          Container                  container_type;
>     protected:
>       Container c;
>       Compare comp;

>     public:
>       priority_queue(const Compare& x, const Container&);
>       explicit priority_queue(const Compare& x = Compare(), Container&& = Container());
>       template <class InputIterator>
>         priority_queue(InputIterator first, InputIterator last,
>                        const Compare& x, const Container&);
>       template <class InputIterator>
>         priority_queue(InputIterator first, InputIterator last,
>                        const Compare& x = Compare(), Container&& = Container());
>       priority_queue(priority_queue&&);
>       priority_queue& operator=(priority_queue&&);
>       template <class Alloc> explicit priority_queue(const Alloc&);
>       template <class Alloc> priority_queue(const Compare&, const Alloc&);
>       template <class Alloc> priority_queue(const Compare&,
>                                             const Container&, const Alloc&);
>       template <class Alloc> priority_queue(const Compare&,
>                                             Container&&, const Alloc&);
>       template <class Alloc> priority_queue(const priority_queue&, const Alloc&);
>       template <class Alloc> priority_queue(priority_queue&&, const Alloc&);

>       priority_queue& operator=(priority_queue&&);
>       ...
>     };

Add to 23.6.4.2 [priqueue.cons]:

>     template <class Alloc>
>       priority_queue(const priority_queue& q, const Alloc& a);

> > Effects: Initializes `c` with `q.c` as the first argument and `a` as the second argument, and initializes `comp` with `q.comp`.

Change 23.6.6.2 [stack.defn]:

>     template <class T, class Container = deque<T> >
>     class stack {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef Container                           container_type;
>     protected:
>       Container c;

>     public:
>       explicit stack(const Container&);
>       explicit stack(Container&& = Container());
>       stack(stack&& s);

>       template <class Alloc> explicit stack(const Alloc&);
>       template <class Alloc> stack(const Container&, const Alloc&);
>       template <class Alloc> stack(Container&&, const Alloc&);
>       template <class Alloc> stack(const stack&, const Alloc&);
>       template <class Alloc> stack(stack&&, const Alloc&);
>       stack& operator=(stack&& s);

>       bool empty() const          { return c.empty(); }
>       ...
>     };

To the new section 23.6.6.3 [stack.cons], introduced in 1194(i), add:

>     template <class Alloc>
>       stack(const stack& s, const Alloc& a);

> > Effects: Initializes `c` with `s.c` as the first argument and `a` as the second argument.

### 1201(i). Do we always want to unwrap `ref`-wrappers in `make_tuple`

Section: 22.4.5 [tuple.creation], 22.3 [pairs] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-05 Last modified: 2018-10-05

Priority: Not Prioritized

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Discussion:

Spotting a recent thread on the boost lists regarding collapsing optional representations in `optional<optional<T>>` instances, I wonder if we have some of the same issues with `make_tuple`, and now `make_pair`?

Essentially, if my generic code in my own library is handed a `reference_wrapper` by a user, and my library in turn delegates some logic to `make_pair` or `make_tuple`, then I am going to end up with a `pair`/`tuple` holding a real reference rather than the intended reference wrapper.

There are two things as a library author I can do at this point:

1. document my library also has the same reference-wrapper behaviour as `std::make_tuple`
2. roll my own `make_tuple` that does not unwrap rereferences, a lost opportunity to re-use the standard library.

(There may be some metaprogramming approaches my library can use to wrap the `make_tuple` call, but all will be significantly more complex than simply implementing a simplified `make_tuple`.)

Now I don't propose we lose this library facility, I think unwrapping references will be the common behaviour. However, we might want to consider adding another overload that does nothing special with `ref`-wrappers. Note that we already have a second overload of `make_tuple` in the library, called `tie`.

[ 2009-09-30 Daniel adds: ]

> I suggest to change the currently proposed paragraph for `make_simple_pair`

> >     template<typename... Types>
> >       pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);

> > > Type requirements: `sizeof...(Types) == 2`. Remarks: The program shall be ill-formed, if `sizeof...(Types) != 2`.

> > > ...

> or alternatively (but with a slightly different semantic):

> > > Remarks: If `sizeof...(Types) != 2`, this function shall not participate in overload resolution.

> to follow a currently introduced style and because the library does not have yet a specific "Type requirements" element. If such thing would be considered as useful this should be done as a separate issue. Given the increasing complexity of either of these wordings it might be preferable to use the normal two-argument-declaration style again in either of the following ways:

> 1.     template<class T1, class T2>
>        pair<typename decay<T1>::type, typename decay<T2>::type>
>        make_simple_pair(T1&& t1, T2&& t2);

> 2.     template<class T1, class T2>
>        pair<V1, V2> make_simple_pair(T1&& t1, T2&& t2);

>    > Let `V1` be `typename decay<T1>::type` and `V2` be `typename decay<T2>::type`.

[ 2009-10 post-Santa Cruz: ]

> Mark as Tentatively NAD Future.

[2018-10-05 Status to Resolved]

Alisdair notes that this is solved by CTAD that was added in C++17

Rationale:

Does not have sufficient support at this time. May wish to reconsider for a future standard.

Proposed resolution:

Add the following function to 22.3 [pairs] and signature in appropriate synopses:

>     template<typename... Types>
>       pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);

> > Type requirements: `sizeof...(Types) == 2`.

> > Returns: `pair<typename decay<Types>::type...>(std::forward<Types>(t)...)`.

[ Draughting note: I chose a variadic representation similar to `make_tuple` rather than naming both types as it is easier to read through the clutter of metaprogramming this way. Given there are exactly two elements, the committee may prefer to draught with two explicit template type parameters instead ]

Add the following function to 22.4.5 [tuple.creation] and signature in appropriate synopses:

>     template<typename... Types>
>       tuple<typename decay<Types>::type...> make_simple_tuple(Types&&... t);

> > Returns: `tuple<typename decay<Types>::type...>(std::forward<Types>(t)...)`.

### 1203(i). More useful rvalue stream insertion

Section: 31.7.6.6 [ostream.rvalue], 31.7.5.6 [istream.rvalue] Status: C++20 Submitter: Howard Hinnant Opened: 2009-09-06 Last modified: 2021-02-25

Priority: 2

View all other issues in [ostream.rvalue].

View all issues with C++20 status.

Discussion:

31.7.6.6 [ostream.rvalue] was created to preserve the ability to insert into (and extract from 31.7.5.6 [istream.rvalue]) rvalue streams:

>     template <class charT, class traits, class T>
>       basic_ostream<charT, traits>&
>       operator<<(basic_ostream<charT, traits>&& os, const T& x);

> > 1 Effects: `os << x`

> > 2 Returns: `os`

This is good as it allows code that wants to (for example) open, write to, and close an `ofstream` all in one statement:

>     std::ofstream("log file") << "Some message\n";

However, I think we can easily make this "rvalue stream helper" even easier to use. Consider trying to quickly create a formatted string. With the current spec you have to write:

>     std::string s = static_cast<std::ostringstream&>(std::ostringstream() << "i = " << i).str();

This will store "`i = 10`" (for example) in the string `s`. Note the need to cast the stream back to `ostringstream&` prior to using the member `.str()`. This is necessary because the inserter has cast the `ostringstream` down to a more generic `ostream` during the insertion process.

I believe we can re-specify the rvalue-inserter so that this cast is unnecessary. Thus our customer now has to only type:

>     std::string s = (std::ostringstream() << "i = " << i).str();

This is accomplished by having the rvalue stream inserter return an rvalue of the same type, instead of casting it down to the base class. This is done by making the stream generic, and constraining it to be an rvalue of a type derived from `ios_base`.

The same argument and solution also applies to the inserter. This code has been implemented and tested.

[ 2009 Santa Cruz: ]

> NAD Future. No concensus for change.

[LEWG Kona 2017]

Recommend Open: Design looks right.

[ 2018-05-25, Billy O'Neal requests this issue be reopened and provides P/R rebased against N4750 ]

Billy O'Neal requests this issue be reopened, as changing operator>> and operator<< to use perfect forwarding as described here is necessary to implement LWG 2534(i) which was accepted. Moreover, this P/R also resolves LWG 2498(i).

Previous resolution [SUPERSEDED]:

> Change 31.7.5.6 [istream.rvalue]:

> >     template <class charT, class traits Istream, class T>
> >       basic_istream<charT, traits>& Istream&&
> >       operator>>(basic_istream<charT, traits> Istream&& is, T& x);

> > > 1 Effects: `is >> x`

> > > 2 Returns: `std::move(is)`

> > > 3 Remarks: This signature shall participate in overload resolution if and only if `Istream` is not an lvalue reference type and is derived from `ios_base`.

> Change 31.7.6.6 [ostream.rvalue]:

> >     template <class charT, class traits Ostream, class T>
> >       basic_ostream<charT, traits>& Ostream&&
> >       operator<<(basic_ostream<charT, traits> Ostream&& os, const T& x);

> > > 1 Effects: `os << x`

> > > 2 Returns: `std::move(os)`

> > > 3 Remarks: This signature shall participate in overload resolution if and only if `Ostream` is not an lvalue reference type and is derived from `ios_base`.

[2018-12-03, Ville comments]

The implementation in libstdc++ doesn't require derivation from `ios_base`, it requires convertibility to `basic_istream/basic_ostream`. This has been found to be important to avoid regressions with existing stream wrappers.

In libstdc++, the inserter/extractor also don't return a reference to the template parameter, they return a reference to the `basic_istream/basic_ostream` specialization the template parameter converts to. This was done in order to try and be closer to the earlier specification's return type, which specified `basic_ostream<charT, traits>&` and `basic_istream<charT, traits>&`. So we detected convertibility to (a type convertible to) those, and returned the result of that conversion. That doesn't seem to be necessary, and probably bothers certain chaining cases. Based on recent experiments, it seems that this return-type dance (as opposed to just returning what the p/r suggests) is unnecessary, and doesn't trigger any regressions.

[2019-01-20 Reflector prioritization]

Set Priority to 2

Previous resolution [SUPERSEDED]:

> This resolution is relative to N4750.

> Change 31.7.5.6 [istream.rvalue] as follows:

> >     template <class charT, class traits Istream, class T>
> >       basic_istream<charT, traits>& Istream&&
> >       operator>>(basic_istream<charT, traits> Istream&& is, T&& x);

> > > -1- Effects: Equivalent to:

> > >     is >> std::forward<T>(x)
> > >     return std::move(is);

> > > -2- Remarks: This function shall not participate in overload resolution unless the expression `is >> std::forward<T>(x)` is well-formed, `Istream` is not an lvalue reference type, and `Istream` is derived from `ios_base`.

> Change 31.7.6.6 [ostream.rvalue]:

> >     template <class charT, class traits Ostream, class T>
> >       basic_ostream<charT, traits>& Ostream&&
> >       operator<<(basic_ostream<charT, traits> Ostream&& os, const T& x);

> > > -1- Effects: As if by: `os << x;`

> > > -2- Returns: `std::move(os)`

> > > -3- Remarks: This signature shall not participate in overload resolution unless the expression `os << x` is well-formed, `Ostream` is not an lvalue reference type, and `Ostream` is derived from `ios_base`.

[2019-03-17; Daniel comments and provides updated wording]

After discussion with Ville it turns out that the "derived from `ios_base`" approach works fine and no breakages were found in regression tests. As result of that discussion the wording was rebased to the most recent working draft and the "overload resolution participation" wording was replaced by a corresponding Constraints: element.

[2020-02 Status to Immediate on Friday morning in Prague.]

Proposed resolution:

This wording is relative to N4810.

1. Change 31.7.5.6 [istream.rvalue] as follows:

   >     template <class charT, class traits Istream, class T>
   >       basic_istream<charT, traits>& Istream&&
   >       operator>>(basic_istream<charT, traits> Istream&& is, T&& x);

   > > -?- Constraints: The expression `is >> std::forward<T>(x)` is well-formed and `Istream` is publicly and unambiguously derived from `ios_base`.

   > > -1- Effects: Equivalent to:

   > > >     is >> std::forward<T>(x);
   > > >     return std::move(is);

   > > -2- Remarks: This function shall not participate in overload resolution unless the expression `is >> std::forward<T>(x)` is well-formed.

2. Change 31.7.6.6 [ostream.rvalue]:

   >     template <class charT, class traits Ostream, class T>
   >       basic_ostream<charT, traits>& Ostream&&
   >       operator<<(basic_ostream<charT, traits> Ostream&& os, const T& x);

   > > -?- Constraints: The expression `os << x` is well-formed and `Ostream` is publicly and unambiguously derived from `ios_base`.

   > > -1- Effects: As if by: `os << x;`

   > > -2- Returns: `std::move(os)`.

   > > -3- Remarks: This signature shall not participate in overload resolution unless the expression `os << x` is well-formed.

### 1204(i). Global permission to move

Section: 16.4.5.9 [res.on.arguments] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [res.on.arguments].

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Discussion:

When a library function binds an rvalue reference parameter to an argument, the library must be able to assume that the bound argument is a temporary, and not a moved-from lvalue. The reason for this is that the library function must be able to modify that argument without concern that such modifications will corrupt the logic of the calling code. For example:

>     template <class T, class A>
>     void
>     vector<T, A>::push_back(value_type&& v)
>     {
>         // This function should move from v, potentially modifying
>         // the object v is bound to.
>     }

If `v` is truly bound to a temporary, then `push_back` has the only reference to this temporary in the entire program. Thus any modifications will be invisible to the rest of the program.

If the client supplies `std::move(x)` to `push_back`, the onus is on the client to ensure that the value of `x` is no longer important to the logic of his program after this statement. I.e. the client is making a statement that `push_back` may treat `x` as a temporary.

> The above statement is the very foundation upon which move semantics is based.

The standard is currently lacking a global statement to this effect. I propose the following addition to 16.4.5.9 [res.on.arguments]:

> Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

> - If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer invalid for its intended use), the behavior is undefined.
> - If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
> - If a function argument binds to an rvalue reference parameter, the C++ standard library may assume that this parameter is a unique reference to this argument. If the parameter is a generic parameter of the form `T&&`, and an lvalue of type `A` is bound, then the binding is considered to be to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus not covered by this clause. [Note: If a program casts an lvalue to an rvalue while passing that lvalue to a library function (e.g. `move(x)`), then the program is effectively asking the library to treat that lvalue as a temporary. The library is at liberty to optimize away aliasing checks which might be needed if the argument were an lvalue. — end note]

Such a global statement will eliminate the need for piecemeal statements such as 23.2.2 [container.requirements.general]/13:

> An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.

Additionally this clarifies that move assignment operators need not perform the traditional `if (this != &rhs)` test commonly found (and needed) in copy assignment operators.

[ 2009-09-13 Niels adds: ]

> Note: This resolution supports the change of 31.10.3.3 [filebuf.assign]/1, proposed by LWG 900(i).

[ 2009 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add a bullet to 16.4.5.9 [res.on.arguments]:

> Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

> - If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer invalid for its intended use), the behavior is undefined.
> - If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
> - If a function argument binds to an rvalue reference parameter, the C++ standard library may assume that this parameter is a unique reference to this argument. If the parameter is a generic parameter of the form `T&&`, and an lvalue of type `A` is bound, then the binding is considered to be to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus not covered by this clause. [Note: If a program casts an lvalue to an rvalue while passing that lvalue to a library function (e.g. `move(x)`), then the program is effectively asking the library to treat that lvalue as a temporary. The library is at liberty to optimize away aliasing checks which might be needed if the argument were an lvalue. — end note]

Delete 23.2.2 [container.requirements.general]/13:

> An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.

### 1205(i). Some algorithms could more clearly document their handling of empty ranges

Section: 26 [algorithms] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-09-13 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [algorithms].

View all other issues in [algorithms].

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Discussion:

There are a number of algorithms whose result might depend on the handling of an empty range. In some cases the result is not clear, while in others it would help readers to clearly mention the result rather than require some subtle intuition of the supplied wording.

[alg.all_of]

> Returns: `true` if `pred(*i)` is `true` for every iterator `i` in the range `[first,last)`, ...

What does this mean if the range is empty?

I believe that we intend this to be `true` and suggest a non-normative note to clarify:

Add to p1 [alg.all_of]:

> [Note: Returns `true` if `[first,last)` is empty. — end note]

[alg.none_of]

> Returns: `true` if `pred(*i)` is `false` for every iterator `i` in the range `[first,last)`, ...

What does this mean if the range empty?

I believe that we intend this to be `true` and suggest a non-normative note to clarify:

Add to p1 [alg.none_of]:

> [Note: Returns `true` if `[first,last)` is empty. — end note]

[alg.any_of]

The specification for an empty range is actually fairly clear in this case, but a note wouldn't hurt and would be consistent with proposals for `all_of`/`none_of` algorithms.

Add to p1 [alg.any_of]:

> [Note: Returns `false` if `[first,last)` is empty. — end note]

26.6.8 [alg.find.end]

what does this mean if `[first2,last2)` is empty?

I believe the wording suggests the algorithm should return `last1` in this case, but am not 100% sure. Is this in fact the correct result anyway? Surely an empty range should always match and the naive expected result would be `first1`?

My proposed wording is a note to clarify the current semantic:

Add to p2 26.6.8 [alg.find.end]:

> [Note: Returns `last1` if `[first2,last2)` is empty. — end note]

I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:

Alternative wording: (NOT a note)

Add to p2 26.6.8 [alg.find.end]:

> Returns `first1` if `[first2,last2)` is empty.

26.6.9 [alg.find.first.of]

The phrasing seems precise when `[first2, last2)` is empty, but a small note to confirm the reader's understanding might still help.

Add to p2 26.6.9 [alg.find.first.of]

> [Note: Returns `last1` if `[first2,last2)` is empty. — end note]

26.6.15 [alg.search]

What is the expected result if `[first2, last2)` is empty?

I believe the wording suggests the algorithm should return `last1` in this case, but am not 100% sure. Is this in fact the correct result anyway? Surely an empty range should always match and the naive expected result would be `first1`?

My proposed wording is a note to clarify the current semantic:

Add to p2 26.6.15 [alg.search]:

> [Note: Returns `last1` if `[first2,last2)` is empty. — end note]

Again, I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:

Alternative wording: (NOT a note)

Add to p2 26.6.15 [alg.search]:

> Returns `first1` if `[first2,last2)` is empty.

26.8.5 [alg.partitions]

Is an empty range partitioned or not?

Proposed wording:

Add to p1 26.8.5 [alg.partitions]:

> [Note: Returns `true` if `[first,last)` is empty. — end note]

26.8.7.2 [includes]

> Returns: `true` if every element in the range `[first2,last2)` is contained in the range `[first1,last1)`. ...

I really don't know what this means if `[first2,last2)` is empty. I could loosely guess that this implies empty ranges always match, and my proposed wording is to clarify exactly that:

Add to p1 26.8.7.2 [includes]:

> [Note: Returns `true` if `[first2,last2)` is empty. — end note]

26.8.8.3 [pop.heap]

The effects clause is invalid if the range `[first,last)` is empty, unlike all the other heap alogorithms. The should be called out in the requirements.

Proposed wording:

Revise p2 26.8.8.3 [pop.heap]

> Requires: The range `[first,last)` shall be a valid non-empty heap.

[Editorial] Reverse order of 26.8.8.3 [pop.heap] p1 and p2.

26.8.9 [alg.min.max]

`minmax_element` does not clearly specify behaviour for an empty range in the same way that `min_element` and `max_element` do.

Add to p31 26.8.9 [alg.min.max]:

> Returns `make_pair(first, first)` if `first == last`.

26.8.11 [alg.lex.comparison]

The wording here seems quite clear, especially with the sample algorithm implementation. A note is recommended purely for consistency with the rest of these issue resolutions:

Add to p1 26.8.11 [alg.lex.comparison]:

> [Note: An empty sequence is lexicographically less than any other non-empty sequence, but not to another empty sequence. — end note]

[ 2009-11-11 Howard changes Notes to Remarks and changed `search` to return `first1` instead of `last1`. ]

[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add to [alg.all_of]:

> Remarks: Returns `true` if `[first,last)` is empty.

Add to [alg.any_of]:

> Remarks: Returns `false` if `[first,last)` is empty.

Add to [alg.none_of]:

> Remarks: Returns `true` if `[first,last)` is empty.

Add to 26.6.8 [alg.find.end]:

> Remarks: Returns `last1` if `[first2,last2)` is empty.

Add to 26.6.9 [alg.find.first.of]

> Remarks: Returns `last1` if `[first2,last2)` is empty.

Add to 26.6.15 [alg.search]:

> Remarks: Returns `first1` if `[first2,last2)` is empty.

Add to 26.8.5 [alg.partitions]:

> Remarks: Returns `true` if `[first,last)` is empty.

Add to 26.8.7.2 [includes]:

> Remarks: Returns `true` if `[first2,last2)` is empty.

Revise p2 26.8.8.3 [pop.heap]

> Requires: The range `[first,last)` shall be a valid non-empty heap.

[Editorial]

> Reverse order of 26.8.8.3 [pop.heap] p1 and p2.

Add to p35 26.8.9 [alg.min.max]:

>     template<class ForwardIterator, class Compare>
>       pair<ForwardIterator, ForwardIterator>
>         minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);

> > Returns: `make_pair(m, M)`, where `m` is the first iterator in `[first,last)` such that no iterator in the range refers to a smaller element, and where `M` is the last iterator in `[first,last)` such that no iterator in the range refers to a larger element. Returns `make_pair(first, first)` if `first == last`.

Add to 26.8.11 [alg.lex.comparison]:

> Remarks: An empty sequence is lexicographically less than any other non-empty sequence, but not less than another empty sequence.

### 1206(i). Incorrect requires for `move_backward` and `copy_backward`

Section: 26.7.2 [alg.move] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

26.7.2 [alg.move], p6 says:

>     template<class BidirectionalIterator1, class BidirectionalIterator2>
>       BidirectionalIterator2
>         move_backward(BidirectionalIterator1 first,
>                       BidirectionalIterator1 last,
>                       BidirectionalIterator2 result);

> > ...

> > Requires: `result` shall not be in the range `[first,last)`.

This is essentially an "off-by-one" error.

When `result == last`, which is allowed by this specification, then the range `[first, last)` is being move assigned into the range `[first, last)`. The `move` (forward) algorithm doesn't allow self move assignment, and neither should `move_backward`. So `last` should be included in the range which `result` can not be in.

Conversely, when `result == first`, which is not allowed by this specification, then the range `[first, last)` is being move assigned into the range `[first - (last-first), first)`. I.e. into a non-overlapping range. Therefore `first` should not be included in the range which `result` can not be in.

The same argument applies to `copy_backward` though copy assigning elements to themselves (`result == last`) should be harmless (though is disallowed by `copy`).

[ 2010 Pittsburgh: Moved to Ready. ]

Proposed resolution:

Change 26.7.2 [alg.move], p6:

>     template<class BidirectionalIterator1, class BidirectionalIterator2>
>       BidirectionalIterator2
>         move_backward(BidirectionalIterator1 first,
>                       BidirectionalIterator1 last,
>                       BidirectionalIterator2 result);

> > ...

> > Requires: `result` shall not be in the range `[(first,last])`.

Change 26.7.1 [alg.copy], p13:

>     template<class BidirectionalIterator1, class BidirectionalIterator2>
>       BidirectionalIterator2
>         copy_backward(BidirectionalIterator1 first,
>                       BidirectionalIterator1 last,
>                       BidirectionalIterator2 result);

> > ...

> > Requires: `result` shall not be in the range `[(first,last])`.

### 1207(i). Underspecified `std::list` operations?

Section: 23.3.11.5 [list.ops] Status: C++11 Submitter: Loïc Joly Opened: 2009-09-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [list.ops].

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Discussion:

It looks to me like some operations of `std::list` (`sort`, `reverse`, `remove`, `unique` & `merge`) do not specify the validity of iterators, pointers & references to elements of the list after those operations. Is it implied by some other text in the standard?

I believe `sort` & `reverse` do not invalidating anything, `remove` & `unique` only invalidates what refers to erased elements, `merge` does not invalidate anything (with the same precision as `splice` for elements who changed of container). Are those assumptions correct ?

[ 2009-12-08 Jonathan Wakely adds: ]

> 23.2.2 [container.requirements.general] paragraph 11 says iterators aren't invalidated unless specified, so I don't think it needs to be repeated on every function that doesn't invalidate iterators. `list::unique` says it "eliminates" elements, that should probably be "erases" because IMHO that term is used elsewhere and so makes it clearer that iterators to the erased elements are invalidated.

> `list::merge` coud use the same wording as `list::splice` w.r.t iterators and references to moved elements.

> Suggested resolution:

> In 23.3.11.5 [list.ops] change paragraph 19

> >                                      void unique();
> >     template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);

> > > Effects: Eliminates Erases all but the first element from every consecutive group ...

> Add to the end of paragraph 23

> >     void                          merge(list<T,Allocator>&& x);
> >     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

> > > ...

> > > Effects: ... that is, for every iterator `i`, in the range other than the first, the condition `comp(*i, *(i - 1)` will be false. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

[ 2009-12-12 Loïc adds wording. ]

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Alisdair opens: ]

> I object to the current resolution of #1207. I believe it is overly strict with regard to `list` end iterators, being the only mutating operations to require such stability.

> More importantly, the same edits need to be applied to `forward_list`, which uses slightly different words to describe some of these operations so may require subtly different edits (not checked.)

> I am prepared to pick up the `end()` iterator as a separate (new) issue, as part of the FCD ballot review (BSI might tell me 'no' first ;~) but I do want to see `forward_list` adjusted at the same time.

[ 2010-03-28 Daniel adds the first 5 bullets in an attempt to address Alisdair's concerns. ]

[ 2010 Rapperswil: ]

> The wording looks good. Move to Tentatively Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [forwardlist.ops]/12 as indicated:

   >     void remove(const T& value);
   >     template <class Predicate> void remove_if(Predicate pred);

   > > 12 Effects: Erases all the elements in the list referred by a list iterator `i` for which the following conditions hold: `*i == value (for remove()), pred(*i)` is true (`for remove_if()`). This operation shall be stable: the relative order of the elements that are not removed is the same as their relative order in the original list. Invalidates only the iterators and references to the erased elements.

2. Change [forwardlist.ops]/15 as indicated:

   >     template <class BinaryPredicate> void unique(BinaryPredicate pred);

   > > 15 Effects:: EliminatesErases all but the first element from every consecutive group of equal elements referred to by the iterator `i` in the range `[first + 1,last)` for which `*i == *(i-1)` (for the version with no arguments) or `pred(*i, *(i - 1))` (for the version with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.

3. Change [forwardlist.ops]/19 as indicated:

   >     void merge(forward_list<T,Allocator>&& x);
   >     template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp)

   > > [..]

   > > 19 Effects:: Merges `x` into `*this`. This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x`. `x` is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

4. Change [forwardlist.ops]/22 as indicated:

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > [..]

   > > 22 Effects:: Sorts the list according to the `operator<` or the `comp` function object. This operation shall be stable: the relative order of the equivalent elements is preserved. If an exception is thrown the order of the elements in `*this` is unspecified. Does not affect the validity of iterators and references.

5. Change [forwardlist.ops]/24 as indicated:

   >     void reverse();

   > > 24 Effects:: Reverses the order of the elements in the list. Does not affect the validity of iterators and references.

6. Change 23.3.11.5 [list.ops], p15:

   >                                void remove(const T& value);
   >     template <class Predicate> void remove_if(Predicate pred);

   > > Effects: Erases all the elements in the list referred by a list iterator `i` for which the following conditions hold: `*i == value, pred(*i) != false`. Invalidates only the iterators and references to the erased elements.

7. Change 23.3.11.5 [list.ops], p19:

   >                                      void unique();
   >     template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);

   > > Effects: Eliminates Erases all but the first element from every consecutive group of equal elements referred to by the iterator `i` in the range `[first + 1,last)` for which `*i == *(i-1)` (for the version of `unique` with no arguments) or `pred(*i, *(i - 1))` (for the version of `unique` with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.

8. Change 23.3.11.5 [list.ops], p23:

   >     void                          merge(list<T,Allocator>&& x);
   >     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

   > > Effects: If `(&x == this)` does nothing; otherwise, merges the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i`, in the range other than the first, the condition `comp(*i, *(i - 1)` will be false. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

9. Change 23.3.11.5 [list.ops], p26:

   >     void reverse();

   > > Effects: Reverses the order of the elements in the list. Does not affect the validity of iterators and references.

10. Change 23.3.11.5 [list.ops], p30:

    >                              void sort();
    >     template <class Compare> void sort(Compare comp);

    > > Effects: Sorts the list according to the `operator<` or a `Compare` function object. Does not affect the validity of iterators and references.

### 1208(i). `valarray initializer_list` constructor has incorrect effects

Section: 29.6.2.2 [valarray.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-23 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

29.6.2.2 [valarray.cons] says:

>     valarray(initializer_list<T> il);

> > Effects: Same as `valarray(il.begin(), il.end())`.

But there is no `valarray` constructor taking two `const T*`.

[ 2009-10-29 Howard: ]

> Moved to Tentatively Ready after 6 positive votes on c++std-lib.

Proposed resolution:

Change 29.6.2.2 [valarray.cons]:

>     valarray(initializer_list<T> il);

> > Effects: Same as `valarray(il.begin(), il.endsize())`.

### 1209(i). `match_results` should be moveable

Section: 28.6.9.2 [re.results.const] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2009-09-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In Working Draft N2914, `match_results` lacks a move constructor and move assignment operator. Because it owns dynamically allocated memory, it should be moveable.

As far as I can tell, this isn't tracked by an active issue yet; Library Issue 723(i) doesn't talk about `match_results`.

[ 2009-09-21 Daniel provided wording. ]

[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Add the following member declarations to 28.6.9 [re.results]/3:

   >     // 28.10.1, construct/copy/destroy:
   >     explicit match_results(const Allocator& a = Allocator());
   >     match_results(const match_results& m);
   >     match_results(match_results&& m);
   >     match_results& operator=(const match_results& m);
   >     match_results& operator=(match_results&& m);
   >     ~match_results();

2. Add the following new prototype descriptions to 28.6.9.2 [re.results.const] using the table numbering of N3000 (referring to the table titled "`match_results` assignment operator effects"):

   >     match_results(const match_results& m);

   > > 4 Effects: Constructs an object of class `match_results`, as a copy of `m`.

   >     match_results(match_results&& m);

   > > 5 Effects: Move-constructs an object of class `match_results` from `m` satisfying the same postconditions as Table 131. Additionally the stored `Allocator` value is move constructed from `m.get_allocator()`. After the initialization of `*this` sets `m` to an unspecified but valid state.

   > > 6 Throws: Nothing if the allocator's move constructor throws nothing.

   >     match_results& operator=(const match_results& m);

   > > 7 Effects: Assigns `m` to `*this`. The postconditions of this function are indicated in Table 131.

   >     match_results& operator=(match_results&& m);

   > > 8 Effects: Move-assigns `m` to `*this`. The postconditions of this function are indicated in Table 131. After the assignment, `m` is in a valid but unspecified state.

   > > 9 Throws: Nothing.

### 1210(i). Iterator reachability should not require a container

Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

p6 Iterator requirements 24.3 [iterator.requirements]

> An iterator `j` is called reachable from an iterator `i` if and only if there is a finite sequence of applications of the expression `++i` that makes `i == j`. If `j` is reachable from `i`, they refer to the same container.

A good example would be stream iterators, which do not refer to a container. Typically, the end iterator from a range of stream iterators will compare equal for many such ranges. I suggest striking the second sentence.

An alternative wording might be:

> If `j` is reachable from `i`, and both `i` and `j` are dereferencable iterators, then they refer to the same range.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

Change 24.3 [iterator.requirements], p6:

> An iterator `j` is called reachable from an iterator `i` if and only if there is a finite sequence of applications of the expression `++i` that makes `i == j`. If `j` is reachable from `i`, they refer to the same container.

### 1211(i). Move iterators should be restricted as input iterators

Section: 24.5.4.2 [move.iterator] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-05-14

Priority: Not Prioritized

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Discussion:

I contend that while we can support both bidirectional and random access traversal, the category of a move iterator should never be better than `input_iterator_tag`.

The contentious point is that you cannot truly have a multipass property when values are moved from a range. This is contentious if you view a moved-from object as still holding a valid value within the range.

The second reason comes from the Forward Iterator requirements table:

> Forward iterators 24.3.5.5 [forward.iterators]

> Table 102 — Forward iterator requirements

> > For expression `*a` the return type is: "`T&` if `X` is mutable, otherwise `const T&`"

There is a similar constraint on `a->m`.

There is no support for rvalue references, nor do I believe their should be. Again, opinions may vary but either this table or the definition of `move_iterator` need updating.

Note: this requirement probably need updating anyway if we wish to support proxy iterators but I am waiting to see a new working paper before filing that issue.

[ 2009-10 post-Santa Cruz: ]

> Move to Open. Howard to put his rationale mentioned above into the issue as a note.

[ 2009-10-26 Howard adds: ]

> `vector::insert(pos, iter, iter)` is significantly more effcient when `iter` is a random access iterator, as compared to when it is an input iterator.

> When `iter` is an input iterator, the best algorithm is to append the inserted range to the end of the `vector` using `push_back`. This may involve several reallocations before the input range is exhausted. After the append, then one can use `std::rotate` to place the inserted range into the correct position in the vector.

> But when `iter` is a random access iterator, the best algorithm is to first compute the size of the range to be inserted (`last - first`), do a buffer reallocation if necessary, scoot existing elements in the `vector` down to make the "hole", and then insert the new elements directly to their correct place.

> > The insert-with-random-access-iterators algorithm is considerably more efficient than the insert-with-input-iterators algorithm

> Now consider:

> >     vector<A> v;
> >     //  ... build up a large vector of A ...
> >     vector<A> temp;
> >     //  ... build up a large temporary vector of A to later be inserted ...
> >     typedef move_iterator<vector<A>::iterator> MI;
> >     //  Now insert the temporary elements:
> >     v.insert(v.begin() + N, MI(temp.begin()), MI(temp.end()));

> A major motivation for using `move_iterator` in the above example is the expectation that `A` is cheap to move but expensive to copy. I.e. the customer is looking for high performance. If we allow `vector::insert` to subtract two `MI`'s to get the distance between them, the customer enjoys substantially better performance, compared to if we say that `vector::insert` can not subtract two `MI`'s.

> I can find no rationale for not giving this performance boost to our customers. Therefore I am strongly against restricting `move_iterator` to the `input_iterator_tag` category.

> I believe that the requirement that forward iterators have a dereference that returns an lvalue reference to cause unacceptable pessimization. For example `vector<bool>::iterator` also does not return a `bool&` on dereference. Yet I am not aware of a single vendor that is willing to ship `vector<bool>::iterator` as an input iterator. Everyone classifies it as a random access iterator. Not only does this not cause any problems, it prevents significant performance problems.

Previous resolution [SUPERSEDED]:

> Class template move_iterator 24.5.4.2 [move.iterator]

> >     namespace std {
> >     template <class Iterator>
> >     class move_iterator {
> >     public:
> >      ...
> >      typedef typename iterator_traits<Iterator>::iterator_category input_iterator_tag iterator_category;

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

See N3066.

### 1212(i). result of post-increment/decrement operator

Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Forward iterator and bidirectional iterator place different requirements on the result of post-increment/decrement operator. The same form should be used in each case.

Merging row from:

>     Table 102 -- Forward iterator requirements
>     Table 103 -- Bidirectional iterator requirements

>         r++ : convertible to const X&
>         r-- : convertible to const X&

>         *r++ : T& if X is mutable, otherwise const T&
>         *r-- : convertible to T

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

### 1214(i). Insufficient/inconsistent key immutability requirements for associative containers

Section: 23.2.7 [associative.reqmts] Status: C++14 Submitter: Daniel Krügler Opened: 2009-09-20 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [associative.reqmts].

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Discussion:

Scott Meyers' mentions on a recent posting on c.s.c++ some arguments that point to an incomplete resolution of 103(i) and to an inconsistency of requirements on keys in ordered and unordered associative containers:

> 1) 103(i) introduced the term immutable without defining it in a unique manner in 23.2.7 [associative.reqmts]/5:

> > [..] Keys in an associative container are immutable.

> According to conventional dictionaries immutable is an unconditional way of saying that something cannot be changed. So without any further explicit allowance a user always runs into undefined behavior if (s)he attempts to modify such a key. IMO this was not the intend of the committee to resolve 103(i) in that way because the comments suggest an interpretation that should give any user the freedom to modify the key in an explicit way provided it would not affect the sort order in that container.

> 2) Another observation was that surprisingly no similar 'safety guards' exists against unintentional key changes for the unordered associative containers, specifically there is no such requirement as in 23.2.7 [associative.reqmts]/6 that "both `iterator` and `const_iterator` are constant iterators". But the need for such protection against unintentional changes as well as the constraints in which manner any explicit changes may be performed are both missing and necessary, because such changes could potentially change the equivalence of keys that is measured by the `hasher` and `key_equal`.

> I suggest to fix the unconditional wording involved with "immutable keys" by at least adding a hint for the reader that users may perform such changes in an explicit manner and to perform similar wording changes as 103(i) did for the ordered associative containers also for the unordered containers.

[ 2010-03-27 Daniel provides wording. ]

> This update attempts to provide normative wording that harmonizes the key and function object constraints of associative and unordered containers.

[ 2010 Batavia: ]

We're uncomfortable with the first agenda item, and we can live with the second agenda item being applied before or after Madrid.

[ 2011 Bloomington ]

Further discussion persuades us this issue is Ready (and so moved). We may need a further issue clarifying the notion of key value vs. key object, as object identity appears to be important to this wording.

Proposed resolution:

1. Change 23.2.7 [associative.reqmts]/2 as indicated: [This ensures that associative containers make better clear what this "arbitrary" type is, as the unordered containers do in 23.2.8 [unord.req]/3]

   > 2 Each associative container is parameterized on `Key` and an ordering relation `Compare` that induces a strict weak ordering (25.4) on elements of `Key`. In addition, `map` and `multimap` associate an arbitrary mapped typetype `T` with the `Key`. The object of type `Compare` is called the comparison object of a container.

2. Change 23.2.7 [associative.reqmts]/5 as indicated: [This removes the too strong requirement that keys must not be changed at all and brings this line in sync with 23.2.8 [unord.req]/7. We take care about the real constraints by the remaining suggested changes. The rationale provided by LWG 103(i) didn't really argue why that addition is necessary, and I believe the remaining additions make it clear that any user changes have strong restrictions]:

   > 5 For `set` and `multiset` the value type is the same as the key type. For `map` and `multimap` it is equal to `pair<const Key, T>`. Keys in an associative container are immutable.

3. Change 23.2.8 [unord.req]/3+4 as indicated: [The current sentence of p.4 has doesn't say something really new and this whole subclause misses to define the concepts of the container-specific hasher object and predicate object. We introduce the term key equality predicate which is already used in the requirements table. This change does not really correct part of this issue, but is recommended to better clarify the nomenclature and the difference between the function objects and the function object types, which is important, because both can potentially be stateful.]

   > 3 Each unordered associative container is parameterized by `Key`, by a function object type `Hash` that meets the `Hash` requirements (20.2.4) and acts as a hash function for argument values of type `Key`, and by a binary predicate `Pred` that induces an equivalence relation on values of type `Key`. Additionally, `unordered_map` and `unordered_multimap` associate an arbitrary mapped type `T` with the `Key`.

   > 4 The container's object of type `Hash` - denoted by `hash` - is called the hash function of the container. The container's object of type `Pred` - denoted by `pred` - is called the key equality predicate of the container.A hash function is a function object that takes a single argument of type `Key` and returns a value of type `std::size_t`.

4. Change 23.2.8 [unord.req]/5 as indicated: [This adds a similar safe-guard as the last sentence of 23.2.7 [associative.reqmts]/3]

   > 5 Two values `k1` and `k2` of type `Key` are considered equivalent if the container's key equality predicate `key_equal` function object returns `true` when passed those values. If `k1` and `k2` are equivalent, the container's hash function shall return the same value for both. [Note: thus, when an unordered associative container is instantiated with a non-default `Pred` parameter it usually needs a non-default `Hash` parameter as well. — end note] For any two keys `k1` and `k2` in the same container, calling `pred(k1, k2)` shall always return the same value. For any key `k` in a container, calling `hash(k)` shall always return the same value.

5. After 23.2.8 [unord.req]/7 add the following new paragraph: [This ensures the same level of compile-time protection that we already require for associative containers. It is necessary for similar reasons, because any change in the stored key which would change it's equality relation to others or would change it's hash value such that it would no longer fall in the same bucket, would break the container invariants]

   > 7 For `unordered_set` and `unordered_multiset` the value type is the same as the key type. For `unordered_map` and `unordered_multimap` it is `std::pair<const Key, T>`.

   > For unordered containers where the value type is the same as the key type, both `iterator` and `const_iterator` are constant iterators. It is unspecified whether or not `iterator` and `const_iterator` are the same type. [Note: `iterator` and `const_iterator` have identical semantics in this case, and `iterator` is convertible to `const_iterator`. Users can avoid violating the One Definition Rule by always using `const_iterator` in their function parameter lists. — end note]

### 1215(i). `list::merge` with unequal allocators

Section: 23.3.11.5 [list.ops] Status: C++11 Submitter: Pablo Halpern Opened: 2009-09-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In Bellevue (I think), we passed N2525, which, among other things, specifies that the behavior of `list::splice` is undefined if the allocators of the two lists being spliced do not compare equal. The same rationale should apply to `list::merge`. The intent of `list::merge` (AFAIK) is to move nodes from one sorted `list` into another sorted `list` without copying the elements. This is possible only if the allocators compare equal.

Proposed resolution:

Relative to the August 2009 WP, N2857, change 23.3.11.5 [list.ops], paragraphs 22-25 as follows:

>     void merge(list&& x);
>     template <class Compare> void merge(list&& x, Compare comp);

> > Requires: both the list and the argument list shall be sorted according to operator< or comp.

> > Effects: If `(&x == this)` does nothing; otherwise, merges the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i`, in the range other than the `first`, the condition `comp(*i, *(i - 1))` will be `false`.

> > Remarks: Stable. If `(&x != this)` the range `[x.begin(), x.end())` is empty after the merge. No elements are copied by this operation. The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

> > Complexity: At most `size() + x.size() - 1` applications of `comp` if `(&x != this)`; otherwise, no applications of `comp` are performed. If an exception is thrown other than by a comparison there are no effects.

### 1216(i). LWG 1066 Incomplete?

Section: 17.9.8 [except.nested] Status: C++11 Submitter: Pete Becker Opened: 2009-09-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

LWG 1066(i) adds `[[noreturn]]` to a bunch of things. It doesn't add it to `rethrow_nested()`, which seems like an obvious candidate. I've made the changes indicated in the issue, and haven't changed `rethrow_nested()`.

[ 2009 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add `[[noreturn]]` to `rethrow_nested()` in 17.9.8 [except.nested].

### 1218(i). mutex destructor synchronization

Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

If an object `*o` contains a mutex `mu` and a correctly-maintained reference count `c`, is the following code safe?

>     o->mu.lock();
>     bool del = (--(o->c) == 0);
>     o->mu.unlock();
>     if (del) { delete o; }

If the implementation of `mutex::unlock()` can touch the mutex's memory after the moment it becomes free, this wouldn't be safe, and "Construction and destruction of an object of a Mutex type need not be thread-safe" 32.6.4 [thread.mutex.requirements] may imply that it's not safe. Still, it's useful to allow mutexes to guard reference counts, and if it's not allowed, users are likely to write bugs.

[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

- Add a new paragraph after 32.6.4.2.2 [thread.mutex.class] p1:

  > 1 The class `mutex` provides a non-recursive mutex ...

  > [Note: After a thread `A` has called `unlock()`, releasing the mutex, it is possible for another thread `B` to lock the same mutex, observe that it is no longer in use, unlock and destroy it, before thread `A` appears to have returned from its unlock call. Implementations are required to handle such scenarios correctly, as long as thread `A` doesn't access the mutex after the unlock call returns. These cases typically occur when a reference-counted object contains a mutex that is used to protect the reference count. — end note]

### 1220(i). What does `condition_variable` wait on?

Section: 32.7 [thread.condition] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

"Class `condition_variable` provides a condition variable that can only wait on an object of type `unique_lock`" should say "...object of type `unique_lock<mutex>`"

[ 2009-11-06 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change 32.7 [thread.condition], p1:

> Condition variables provide synchronization primitives used to block a thread until notified by some other thread that some condition is met or until a system time is reached. Class `condition_variable` provides a condition variable that can only wait on an object of type `unique_lock<mutex>`, allowing maximum efficiency on some platforms. Class `condition_variable_any` provides a general condition variable that can wait on objects of user-supplied lock types.

### 1221(i). `condition_variable` wording

Section: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.7.4 [thread.condition.condvar] says:

>     ~condition_variable();

> > Precondition: There shall be no thread blocked on `*this`. [Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait. Beware that destroying a `condition_variable` object while the corresponding predicate is `false` is likely to lead to undefined behavior. — end note]

The text hasn't introduced the notion of a "corresponding predicate" yet.

[ 2010-02-11 Anthony provided wording. ]

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Modify 32.7.4 [thread.condition.condvar]p4 as follows:

>     ~condition_variable();

> > 4 Precondition: There shall be no thread blocked on `*this`. [Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait. Beware that destroying a `condition_variable` object while the corresponding predicate is false is likely to lead to undefined behavior. The user must take care to ensure that no threads wait on `*this` once the destructor has been started, especially when the waiting threads are calling the wait functions in a loop or using the overloads of `wait`, `wait_for` or `wait_until` that take a predicate. — end note]

### 1222(i). condition_variable incorrect effects for exception safety

Section: 32.7 [thread.condition] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.7.4 [thread.condition.condvar] says:

>     void wait(unique_lock<mutex>& lock);

> > ...

> > Effects:

> > - ...
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

Should that be `lock.lock()`?

[ 2009-11-17 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 32.7.4 [thread.condition.condvar] p10:

>     void wait(unique_lock<mutex>& lock);

> > ...

> > Effects:

> > - ...
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

And make a similar change in p16, and in 32.7.5 [thread.condition.condvarany], p8 and p13.

### 1225(i). C++0x `result_of` issue

Section: 99 [func.ret] Status: Resolved Submitter: Sebastian Gesemann Opened: 2009-10-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I think the text about `std::result_of` could be a little more precise. Quoting from N2960...

> 99 [func.ret] Function object return types

>     template<class> class result_of;

>     template<class Fn, class... ArgTypes>
>     class result_of<Fn(ArgTypes...)> {
>     public:
>       typedef see below type;
>     };

> Given an rvalue `fn` of type `Fn` and values `t1, t2, ..., tN` of types `T1, T2, ... TN` in `ArgTypes` respectivly, the `type` member is the result type of the expression `fn(t1,t2,...,tN)`. the values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

This text doesn't seem to consider lvalue reference types for `Fn`. Also, it's not clear whether this class template can be used for "SFINAE" like `std::enable_if`. Example:

>     template<typename Fn, typename... Args>
>     typename std::result_of<Fn(Args...)>::type
>     apply(Fn && fn, Args && ...args)
>     {
>       // Fn may be an lvalue reference, too
>       return std::forward<Fn>(fn)(std::forward<Args>(args)...);
>     }

Either `std::result_of<...>` can be instantiated and simply may not have a typedef "`type`" (-->SFINAE) or instantiating the class template for some type combinations will be a "hard" compile-time error.

[ 2010-02-14 Daniel adds: ]

> This issue should be considered resolved by 1255(i) and 1270(i). The wish to change `result_of` into a compiler-support trait was beyond the actual intention of the submitter Sebastian.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved, rationale added below. ]

Rationale:

Solved by 1270(i).

Proposed resolution:

[ These changes will require compiler support ]

Change 99 [func.ret]:

>     template<class> class result_of; // undefined

>     template<class Fn, class... ArgTypes>
>     class result_of<Fn(ArgTypes...)> {
>     public:
>       typedef see below type;
>     };

> Given an rvalue `fn` of type `Fn` and values `t1, t2, ..., tN` of types `T1, T2, ... TN` in `ArgTypes` respectivly, the `type` member is the result type of the expression `fn(t1,t2,...,tN)`. the values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

> The class template `result_of` shall meet the requirements of a TransformationTrait: Given the types `Fn`, `T1`, `T2`, ..., `TN` every template specialization `result_of<Fn(T1,T2,...,TN)>` shall define the member typedef type equivalent to `decltype(RE)` if and only if the expression `RE`

> >     value<Fn>() ( value<T1>(), value<T2>(), ... value<TN>()  )

> would be well-formed. Otherwise, there shall be no member typedef `type` defined.

[ The `value<>` helper function is a utility Daniel Krügler proposed in N2958. ]

### 1226(i). Incomplete changes of #890

Section: 32.10.3 [futures.errors] Status: Resolved Submitter: Daniel Krügler Opened: 2009-10-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Defect issue 890(i) overlooked to adapt the `future_category` from 32.10.1 [futures.overview] and 32.10.3 [futures.errors]:

>     extern const error_category* const future_category;

which should be similarly transformed into function form.

[ 2009-10-27 Howard: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ 2009-11-11 Daniel adds: ]

> I just observe that the proposed resolution of this issue is incomplete and needs to reworded. The problem is that the corresponding declarations

> >     constexpr error_code make_error_code(future_errc e);
> >     constexpr error_condition make_error_condition(future_errc e);

> as constexpr functions are incompatible to the requirements of constexpr functions given their specified implementation. Note that the incompatibility is not a result of the modifications proposed by the issue resolution, but already existed within the N2960 state where we have

> >     extern const error_category* const future_category;

> combined with

> >     constexpr error_code make_error_code(future_errc e);

> > > 3 Returns: `error_code(static_cast<int>(e), *future_category)`.

> >     constexpr error_code make_error_condition(future_errc e);

> > > 4 Returns: `error_condition(static_cast<int>(e), *future_category)`.

> Neither is any of the constructors of `error_code` and `error_condition` constexpr, nor does the expression `*future_category` satisfy the requirements for a constant expression (7.7 [expr.const]/2 bullet 6 in N3000).

> The simple solution is just to remove the constexpr qualifiers for both functions, which makes sense, because none of the remaining `make_error_*` overloads in the library is constexpr. One might consider to realize that those `make_*` functions could satisfy the constexpr requirements, but this looks not like an easy task to me, because it would need to rely on a not yet existing language feature. If such a change is wanted, a new issue should be opened after the language extension approval (if at all) [1].

> If no-one complaints I would like to ask Howard to add the following modifications to this issue, alternatively a new issue could be opened but I don't know what the best solution is that would cause as little overhead as possible.

> What-ever the route is, the following is my proposed resolution for this issue interaction part of the story:

> > In 32.10.1 [futures.overview]/1, Header `<future>` synopsis and in 32.10.3 [futures.errors]/3+4 change as indicated:

> > >     constexpr error_code make_error_code(future_errc e);
> > >     constexpr error_condition make_error_condition(future_errc e);

> [1] Let me add that we have a related NAD issue here: 832(i) so the chances for realization are little IMO.

> [ Howard: I've updated the proposed wording as Daniel suggests and set to Review. ]

[ 2009-11-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

1. Change in 32.10.1 [futures.overview], header `<future>` synopsis:

   >     extern const error_category&* const future_category();

2. In 32.10.1 [futures.overview]/1, Header `<future>` synopsis change as indicated:

   >     constexpr error_code make_error_code(future_errc e);
   >     constexpr error_condition make_error_condition(future_errc e);

3. Change in 32.10.3 [futures.errors]:

   >     extern const error_category&* const future_category();

   > > 1- `future_category` shall point to a statically initialized object of a type derived from class `error_category`.

   > > 1- Returns: A reference to an object of a type derived from class `error_category`.

   >     constexpr error_code make_error_code(future_errc e);

   > > 3 Returns: `error_code(static_cast<int>(e), *future_category())`.

   >     constexpr error_codecondition make_error_condition(future_errc e);

   > > 4 Returns: `error_condition(static_cast<int>(e), *future_category())`.

### 1227(i). `<bitset>` synopsis overspecified

Section: 22.9.2 [template.bitset] Status: C++11 Submitter: Bo Persson Opened: 2009-10-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [template.bitset].

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Discussion:

The resolutions to some library defect reports, like 1178(i) requires that `#includes` in each synopsis should be taken literally. This means that the `<bitset>` header now must include `<stdexcept>`, even though none of the exceptions are mentioned in the `<bitset>` header.

Many other classes are required to throw exceptions like `invalid_argument` and `out_of_range`, without explicitly including `<stdexcept>` in their synopsis. It is totally possible for implementations to throw the needed exceptions from utility functions, whose implementations are not visible in the headers.

I propose that `<stdexcept>` is removed from the `<bitset>` header.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

Change 22.9.2 [template.bitset]:

>     #include <cstddef>        // for size_t
>     #include <string>
>     #include <stdexcept>      // for invalid_argument,
>                               // out_of_range, overflow_error
>     #include <iosfwd>         // for istream, ostream
>     namespace std {
>     ...

### 1229(i). `error_code operator=` typo

Section: 19.5.4.3 [syserr.errcode.modifiers] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2009-10-08 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

N2960 19.5.4.1 [syserr.errcode.overview] and 19.5.4.3 [syserr.errcode.modifiers] say:

>     template <class ErrorCodeEnum>
>       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type&
>         operator=(ErrorCodeEnum e);

They should say:

>     template <class ErrorCodeEnum>
>       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code>::type&
>         operator=(ErrorCodeEnum e);

Or (I prefer this form):

>     template <class ErrorCodeEnum>
>       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code&>::type
>         operator=(ErrorCodeEnum e);

This is because `enable_if` is declared as (21.3.8.7 [meta.trans.other]):

>     template <bool B, class T = void> struct enable_if;

So, the current wording makes `operator=` return `void&`, which is not good.

19.5.4.3 [syserr.errcode.modifiers]/4 says

> Returns: `*this`.

which is correct.

Additionally,

19.5.5.1 [syserr.errcondition.overview]/1 says:

>     template<typename ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::type &
>         operator=( ErrorConditionEnum e );

Which contains several problems (`typename` versus `class` inconsistency, lack of `::value`, `error_code` instead of `error_condition`), while 19.5.5.3 [syserr.errcondition.modifiers] says:

>     template <class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type&
>         operator=(ErrorConditionEnum e);

Which returns `void&`. They should both say:

>     template <class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,
>       error_condition>::type&
>         operator=(ErrorConditionEnum e);

Or (again, I prefer this form):

>     template <class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,
>       error_condition&>::type
>         operator=(ErrorConditionEnum e);

Additionally, 19.5.5.3 [syserr.errcondition.modifiers] lacks a "Returns: `*this`." paragraph, which is presumably necessary.

[ 2009-10-18 Beman adds: ]

> The proposed resolution for issue 1237(i) makes this issue moot, so it should become NAD.

[ 2009-10 Santa Cruz: ]

> NADResolved, solved by 1237(i).

Proposed resolution:

Change 19.5.4.1 [syserr.errcode.overview] and 19.5.4.3 [syserr.errcode.modifiers]:

>     template <class ErrorCodeEnum>
>       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value,
>       error_code&>::type&
>         operator=(ErrorCodeEnum e);

Change 19.5.5.1 [syserr.errcondition.overview]:

>     template<typename class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,
>       error_conditionde&>::type &
>         operator=( ErrorConditionEnum e );

Change 19.5.5.3 [syserr.errcondition.modifiers]:

>     template <class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,
>       error_condition&>::type&
>         operator=(ErrorConditionEnum e);

> > Postcondition: `*this == make_error_condition(e)`.

> > Returns: `*this`.

> > Throws: Nothing.

### 1231(i). `weak_ptr` comparisons incompletely resolved

Section: 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.smartptr.weak.obs].

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Discussion:

The n2637 paper suggested several updates of the ordering semantics of `shared_ptr` and `weak_ptr`, among those the explicit comparison operators of `weak_ptr` were removed/deleted, instead a corresponding functor `owner_less` was added. The problem is that n2637 did not clearly enough specify, how the previous wording parts describing the comparison semantics of `weak_ptr` should be removed.

[ 2009-11-06 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

1. Change 20.3.2.3 [util.smartptr.weak]/2 as described, the intention is to fix the now no longer valid requirement that `weak_ptr` is `LessComparable` [Note the deleted comma]:

   > Specializations of `weak_ptr` shall be `CopyConstructible`, and `CopyAssignable`, and `LessThanComparable`, allowing their use in standard containers.

2. In 20.3.2.3.6 [util.smartptr.weak.obs] remove the paragraphs 9-11 including prototype:

   > template<class T, class U> bool operator<(const weak_ptr<T>& a, const weak_ptr<U>& b);

   > Returns: an unspecified value such that
   > - `operator<` is a strict weak ordering as described in 25.4;
   > - under the equivalence relation defined by `operator<`, `!(a < b) && !(b < a)`, two `weak_ptr` instances are equivalent if and only if they share ownership or are both empty.

   > Throws: nothing.

   > [Note: Allows `weak_ptr` objects to be used as keys in associative containers. — end note]

### 1234(i). "Do the right thing" and `NULL`

Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Matt Austern Opened: 2009-10-09 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [sequence.reqmts].

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Discussion:

On g++ 4.2.4 (x86_64-linux-gnu), the following file gives a compile error:

>     #include <vector>
>     void foo() { std::vector<int*> v(500L, NULL); }

Is this supposed to work?

The issue: if `NULL` happens to be defined as `0L`, this is an invocation of the constructor with two arguments of the same integral type. 23.2.4 [sequence.reqmts]/14 (N3035) says that this will behave as if the overloaded constructor

>     X(size_type, const value_type& = value_type(),
>       const allocator_type& = allocator_type())

were called instead, with the arguments `static_cast<size_type>(first)`, `last` and `alloc`, respectively. However, it does not say whether this actually means invoking that constructor with the exact textual form of the arguments as supplied by the user, or whether the standard permits an implementation to invoke that constructor with variables of the same type and value as what the user passed in. In most cases this is a distinction without a difference. In this particular case it does make a difference, since one of those things is a null pointer constant and the other is not.

Note that an implementation based on forwarding functions will use the latter interpretation.

[ 2010 Pittsburgh: Moved to Open. ]

[ 2010-03-19 Daniel provides wording. ]

> - Adapts the numbering used in the discussion to the recent working paper N3035.
> - Proposes a resolution that requires implementations to use sfinae-like means to possibly filter away the too generic template c'tor. In fact this resolution is equivalent to that used for the `pair-NULL` problem (811(i)), the only difference is, that issue 1234 was already a C++03 problem.

> This issue can be considered as a refinement of 438(i).

[ Post-Rapperswil ]

Wording was verified to match with the most recent WP. Jonathan Wakely and Alberto Barbati observed that the current WP has a defect that should be fixed here as well: The functions signatures `fx1` and `fx3` are incorrectly referring to `iterator` instead of `const_iterator`.

> Moved to Tentatively Ready with revised wording after 7 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 23.2.3 [sequence.reqmts]/14+15 as indicated:

> 14 For every sequence container defined in this Clause and in Clause 21:

> - If the constructor

>   >     template <class InputIterator>
>   >     X(InputIterator first, InputIterator last,
>   >       const allocator_type& alloc = allocator_type())

>   is called with a type `InputIterator` that does not qualify as an input iterator, then the constructor shall not participate in overload resolution.will behave as if the overloaded constructor:

>   >     X(size_type, const value_type& = value_type(),
>   >       const allocator_type& = allocator_type())

>   were called instead, with the arguments `static_cast<size_type>(first)`, `last` and `alloc`, respectively.

> - If the member functions of the forms:

>   >     template <class InputIterator> // such as insert()
>   >     rt fx1(const_iterator p, InputIterator first, InputIterator last);

>   >     template <class InputIterator> // such as append(), assign()
>   >     rt fx2(InputIterator first, InputIterator last);

>   >     template <class InputIterator> // such as replace()
>   >     rt fx3(const_iterator i1, const_iterator i2, InputIterator first, InputIterator last);

>   are called with a type `InputIterator` that does not qualify as an input iterator, then these functions shall not participate in overload resolution.will behave as if the overloaded member functions:

>   >     rt fx1(iterator, size_type, const value_type&);

>   >     rt fx2(size_type, const value_type&);

>   >     rt fx3(iterator, iterator, size_type, const value_type&);

>   were called instead, with the same arguments.

> 15 In the previous paragraph the alternative binding will fail if `first` is not implicitly convertible to `X::size_type` or if `last` is not implicitly convertible to `X::value_type`.

### 1237(i). Constrained `error_code`/`error_condition members`

Section: 19.5 [syserr] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [syserr].

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Discussion:

I'm just reflecting on the now SFINAE-constrained constructors and assignment operators of `error_code` and `error_condition`:

These are the only library components that are pro-actively announcing that they are using `std::enable_if` as constraining tool, which has IMO several disadvantages:

1. With the availability of template default arguments and decltype, using `enable_if` in the C++0x standard library, seems unnecessary restricting implementation freedom. E.g. there should be no need for a useless specification of a dummy default function argument, which only confuses the reader. A more reasonable implementation could e.g. be

   >     template <class ErrorCodeEnum
   >      class = typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type>
   >     error_code(ErrorCodeEnum e);

   As currently specified, the function signatures are so unreadable, that errors quite easily happen, see e.g. 1229(i).

2. We have a lot of constrained functions in other places, that now have a standard phrase that is easily understandable:

   > Remarks: This constructor/function shall participate in overload resolution if and only if X.

   where X describes the condition. Why should these components deviate?

3. If `enable_if` would not be explicitly specified, the standard library is much better prepared for the future. It would also be possible, that libraries with partial support for not-yet-standard-concepts could provide a much better diagnostic as is possible with `enable_if`. This again would allow for experimental concept implementations in the wild, which as a result would make concept standardization a much more natural thing, similar to the way as templates were standardized in C++.

   In summary: I consider it as a library defect that `error_code` and `error_condition` explicitly require a dependency to `enable_if` and do limit implementation freedom and I volunteer to prepare a corresponding resolution.

[ 2009-10-18 Beman adds: ]

> I support this proposed resolution, and thank Daniel for writing it up.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

[ Should this resolution be accepted, I recommend to resolve 1229(i) as NAD ]

1. In 19.5.4.1 [syserr.errcode.overview]/1, class `error_code`, change as indicated:

   >     // 19.5.2.2 constructors:
   >     error_code();
   >     error_code(int val, const error_category& cat);
   >     template <class ErrorCodeEnum>
   >       error_code(ErrorCodeEnum e,
   >         typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type * = 0);

   >     // 19.5.2.3 modifiers:
   >     void assign(int val, const error_category& cat);
   >     template <class ErrorCodeEnum>
   >       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::typeerror_code&
   >         operator=(ErrorCodeEnum e);
   >     void clear();

2. Change 19.5.4.2 [syserr.errcode.constructors] around the prototype before p. 7:

   >     template <class ErrorCodeEnum>
   >     error_code(ErrorCodeEnum e,
   >       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type * = 0);

   > > Remarks: This constructor shall not participate in overload resolution, unless `is_error_code_enum<ErrorCodeEnum>::value == true`.

3. Change 19.5.4.3 [syserr.errcode.modifiers] around the prototype before p. 3:

   >     template <class ErrorCodeEnum>
   >       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::typeerror_code&
   >         operator=(ErrorCodeEnum e);

   > > Remarks: This operator shall not participate in overload resolution, unless `is_error_code_enum<ErrorCodeEnum>::value == true`.

4. In 19.5.5.1 [syserr.errcondition.overview]/1, class `error_condition`, change as indicated:

   >     // 19.5.3.2 constructors:
   >     error_condition();
   >     error_condition(int val, const error_category& cat);
   >     template <class ErrorConditionEnum>
   >       error_condition(ErrorConditionEnum e,
   >         typename enable_if<is_error_condition_enum<ErrorConditionEnum>::type* = 0);

   >     // 19.5.3.3 modifiers:
   >     void assign(int val, const error_category& cat);
   >     template<typenameclass ErrorConditionEnum>
   >       typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::typeerror_condition &
   >         operator=( ErrorConditionEnum e );
   >     void clear();

5. Change 19.5.5.2 [syserr.errcondition.constructors] around the prototype before p. 7:

   >     template <class ErrorConditionEnum>
   >       error_condition(ErrorConditionEnum e,
   >         typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type* = 0);

   > > Remarks: This constructor shall not participate in overload resolution, unless `is_error_condition_enum<ErrorConditionEnum>::value == true`.

6. Change 19.5.5.3 [syserr.errcondition.modifiers] around the prototype before p. 3:

   >     template <class ErrorConditionEnum>
   >       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::typeerror_condition&
   >         operator=(ErrorConditionEnum e);

   > > Remarks: This operator shall not participate in overload resolution, unless `is_error_condition_enum<ErrorConditionEnum>::value == true`.

   > > Postcondition: `*this == make_error_condition(e)`.

   > > Returns: `*this`

### 1240(i). Deleted comparison functions of `std::function` not needed

Section: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.wrap.func].

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Discussion:

The class template `std::function` contains the following member declarations:

>     // deleted overloads close possible hole in the type system
>     template<class R2, class... ArgTypes2>
>       bool operator==(const function<R2(ArgTypes2...)>&) = delete;
>     template<class R2, class... ArgTypes2>
>       bool operator!=(const function<R2(ArgTypes2...)>&) = delete;

The leading comment here is part of the history of `std::function`, which was introduced with N1402. During that time no explicit conversion functions existed, and the "safe-bool" idiom (based on pointers-to-member) was a popular technique. The only disadvantage of this idiom was that given two objects `f1` and `f2` of type `std::function` the expression

>     f1 == f2;

was well-formed, just because the built-in `operator==` for pointer to member was considered after a single user-defined conversion. To fix this, an overload set of undefined comparison functions was added, such that overload resolution would prefer those ending up in a linkage error. The new language facility of deleted functions provided a much better diagnostic mechanism to fix this issue.

The central point of this issue is, that with the replacement of the safe-bool idiom by explicit conversion to bool the original "hole in the type system" does no longer exist and therefore the comment is wrong and the superfluous function definitions should be removed as well. An explicit conversion function is considered in direct-initialization situations only, which indirectly contain the so-called "contextual conversion to bool" (7.3 [conv]/3). These conversions are not considered for `==` or `!=` as defined by the core language.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

In 22.10.17.3 [func.wrap.func]/1, class function change as indicated:

>     // 20.7.15.2.3, function capacity:
>     explicit operator bool() const;

>     // deleted overloads close possible hole in the type system
>     template<class R2, class... ArgTypes2>
>       bool operator==(const function<R2(ArgTypes2...)>&) = delete;
>     template<class R2, class... ArgTypes2>
>       bool operator!=(const function<R2(ArgTypes2...)>&) = delete;

### 1241(i). `unique_copy` needs to require `EquivalenceRelation`

Section: 26.7.9 [alg.unique] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.unique].

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Discussion:

A lot of fixes were silently applied during concept-time and we should not lose them again. The Requires clause of 26.7.9 [alg.unique]/5 doesn't mention that `==` and the predicate need to satisfy an `EquivalenceRelation`, as it is correctly said for `unique`. This was intentionally fixed during conceptification, were we had:

>     template<InputIterator InIter, class OutIter>
>       requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type>
>             && EqualityComparable<InIter::value_type>
>             && HasAssign<InIter::value_type, InIter::reference>
>             && Constructible<InIter::value_type, InIter::reference>
>       OutIter unique_copy(InIter first, InIter last, OutIter result);

>     template<InputIterator InIter, class OutIter,
>              EquivalenceRelation<auto, InIter::value_type> Pred>
>       requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type>
>             && HasAssign<InIter::value_type, InIter::reference>
>             && Constructible<InIter::value_type, InIter::reference>
>             && CopyConstructible<Pred>
>       OutIter unique_copy(InIter first, InIter last, OutIter result, Pred pred);

Note that EqualityComparable implied an equivalence relation.

[ N.B. `adjacent_find` was also specified to require `EquivalenceRelation`, but that was considered as a defect in concepts, see 1000(i) ]

[ 2009-10-31 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change 26.7.9 [alg.unique]/5 as indicated:

>     template<class InputIterator, class OutputIterator>
>       OutputIterator
>         unique_copy(InputIterator first, InputIterator last, OutputIterator result);

>     template<class InputIterator, class OutputIterator, class BinaryPredicate>
>       OutputIterator
>         unique_copy(InputIterator first, InputIterator last,
>                     OutputIterator result, BinaryPredicate pred);

> > Requires: The comparison function shall be an equivalence relation. The ranges `[first,last)` and `[result,result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type of `InputIterator` shall be `CopyConstructible` (34) and `CopyAssignable` (table 36). Otherwise `CopyConstructible` is not required.

### 1244(i). `wait_*()` in `*future` for synchronous functions

Section: 32.10 [futures] Status: Resolved Submitter: Detlef Vollmann Opened: 2009-10-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

With the addition of `async()`, a `future` might be associated with a function that is not running in a different thread but is stored to by run synchronously on the `get()` call. It's not clear what the `wait()` functions should do in this case.

Suggested resolution:

Throw an exception.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

### 1245(i). `std::hash<string>` & co

Section: 22.10.19 [unord.hash] Status: C++11 Submitter: Paolo Carlini Opened: 2009-10-22 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 22.10.19 [unord.hash], `operator()` is specified as taking the argument by value. Moreover, it is said that `operator()` shall not throw exceptions.

However, for the specializations for class types, like `string`, `wstring`, etc, the former requirement seems suboptimal from the performance point of view (a specific PR has been filed about this in the GCC Bugzilla) and, together with the latter requirement, hard if not impossible to fulfill. It looks like pass by const reference should be allowed in such cases.

[ 2009-11-18: Ganesh updates wording. ]

> I've removed the list of types for which `hash` shall be instantiated because it's already explicit in the synopsis of header `<functional>` in 22.10 [function.objects]/2.

[ 2009-11-18: Original wording here: ]

> Add to 22.10.19 [unord.hash]/2:

> >     namespace std {
> >       template <class T>
> >       struct hash : public std::unary_function<T, std::size_t> {
> >         std::size_t operator()(T val) const;
> >       };
> >     }

> > The return value of `operator()` is unspecified, except that equal arguments shall yield the same result. `operator()` shall not throw exceptions. It is also unspecified whether `operator()` of `std::hash` specializations for class types takes its argument by value or const reference.

[ 2009-11-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2009-11-24 Ville Opens: ]

> I have received community requests to ask for this issue to be reopened. Some users feel that mandating the inheritance is overly constraining.

[ 2010-01-31 Alisdair: related to 978(i) and 1182(i). ]

[ 2010-02-07 Proposed resolution updated by Beman, Daniel and Ganesh. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Insert a new subclause either before or after the current 16.4.4.6 [allocator.requirements]:

> ### `Hash` Requirements [hash.requirements]

> This subclause defines the named requirement `Hash`, used in several clauses of the C++ standard library. A type `H` meets the `Hash` requirement if

> - it is a function object type (22.10 [function.objects]).

> - it satisfies the requirements of `CopyConstructible`, and `Destructible` (16.4.4.2 [utility.arg.requirements]),

> - the expressions shown in the following table are valid and have the indicated semantics, and

> - it satisfies all other requirements of this subclause.

> Given `Key` is an argument type for function objects of type `H`, in the table below `h` is a value of type (possibly `const`) `H`, `u` is an lvalue of type `Key`, and `k` is a value of a type convertible to (possibly `const`) `Key`:

> Table ? — `Hash` requirements
> Expression | Return type | Requirement
> `h(k)` | `size_t` | Shall not throw exceptions. The value returned shall depend only on the argument `k`. [Note: Thus all evaluations of the expression `h(k)` with the same value for `k` yield the same result. — end note] [Note: For `t1` and `t2` of different values, the probability that `h(t1)` and `h(t2)` compare equal should be very small, approaching `(1.0/numeric_limits<size_t>::max())`. — end note] Comment (not to go in WP): The wording for the second note is based on a similar note in 28.3.4.5.1.3 [locale.collate.virtuals]/3
> `h(u)` | `size_t` | Shall not modify `u`.

Change 22.10.19 [unord.hash] as indicated:

> 1 The unordered associative containers defined in Clause 23.5 [unord] use specializations of the class template `hash` as the default hash function. For all object types `T` for which there exists a specialization `hash<T>`, the instantiation `hash<T>` shall:

> - satisfy the `Hash` requirements([hash.requirements]), with `T` as the function call argument type, the `DefaultConstructible` requirements ([defaultconstructible]), the `CopyAssignable` requirements ([copyassignable]), and the `Swappable` requirements ([swappable]),
> - provide two nested types `result_type` and `argument_type` which shall be synonyms for `size_t` and `T`, respectively,
> - satisfy the requirement that if `k1 == k2` is `true`, `h(k1) == h(k2)` is `true`, where `h` is an object of type `hash<T>`, and `k1`, `k2` are objects of type `T`.

> This class template is only required to be instantiable for integer types (6.9.2 [basic.fundamental]), floating-point types (6.9.2 [basic.fundamental]), pointer types (9.3.4.2 [dcl.ptr]), and `std::string`, `std::u16string`, `std::u32string`, `std::wstring`, `std::error_code`, `std::thread::id`, `std::bitset`, and `std::vector<bool>`.

> >     namespace std {
> >       template <class T>
> >       struct hash : public std::unary_function<T, std::size_t> {
> >         std::size_t operator()(T val) const;
> >       };
> >     }

> 2 The return value of `operator()` is unspecified, except that equal arguments shall yield the same result. `operator()` shall not throw exceptions.

Change Unordered associative containers 23.2.8 [unord.req] as indicated:

> Each unordered associative container is parameterized by `Key`, by a function object type `Hash`([hash.requirements]) that acts as a hash function for argument values of type `Key`, and by a binary predicate `Pred` that induces an equivalence relation on values of type `Key`. Additionally, `unordered_map` and `unordered_multimap` associate an arbitrary mapped type `T` with the `Key`.

> A hash function is a function object that takes a single argument of type `Key` and returns a value of type `std::size_t`.

> Two values `k1` and `k2` of type `Key` are considered equal if the container's equality function object returns `true` when passed those values. If `k1` and `k2` are equal, the hash function shall return the same value for both. [Note: Thus supplying a non-default `Pred` parameter usually implies the need to supply a non-default `Hash` parameter. — end note]

### 1247(i). `auto_ptr` is overspecified

Section: 99 [auto.ptr] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-10-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This issue is extracted as the ongoing point-of-interest from earlier issue 463(i).

`auto_ptr` is overspecified as the `auto_ptr_ref` implementation detail is formally specified, and the technique is observable so workarounds for compiler defects can cause a working implementation of the primary `auto_ptr` template become non-conforming.

`auto_ptr_ref` is a documentation aid to describe a possible mechanism to implement the class. It should be marked exposition only, as per similar classes, e.g., `istreambuf_iterator::proxy`

[ 2009-10-25 Daniel adds: ]

> I wonder, whether the revised wording shouldn't be as straight as for `istream_buf` by adding one further sentence:

> > An implementation is permitted to provide equivalent functionality without providing a class with this name.

[ 2009-11-06 Alisdair adds Daniel's suggestion to the proposed wording. ]

[ 2009-11-06 Howard moves issue to Review. ]

[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add the term "exposition only" in the following two places:

Ammend 99 [auto.ptr]p2:

> The exposition only class Ttemplate `auto_ptr_ref` holds a reference to an `auto_ptr`. It is used by the `auto_ptr` conversions to allow `auto_ptr` objects to be passed to and returned from functions. An implementation is permitted to provide equivalent functionality without providing a class with this name.

> >     namespace std {
> >      template <class Y> struct auto_ptr_ref { }; // exposition only

### 1248(i). Equality comparison for unordered containers

Section: 23.5 [unord] Status: Resolved Submitter: Herb Sutter Opened: 2009-10-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

See N2986.

[ 2010-01-22 Alisdair Opens. ]

[ 2010-01-24 Alisdair provides wording. ]

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3068.

Proposed resolution:

Apply paper N2986.

### 1249(i). `basic_ios` default ctor

Section: 31.5.4.2 [basic.ios.cons] Status: C++11 Submitter: Martin Sebor Opened: 2009-10-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The `basic_ios` default ctor is required to leave the objects members uninitialized (see below). The paragraph says the object must be initialized by calling `basic_ios::init()` before it's destroyed but I can't find a requirement that it be initialized before calling any of the class other member functions. Am I not looking in the right place or that an issue?

[ 2009-10-25 Daniel adds: ]

> I agree, that your wording makes that clearer, but suggest to write

> > ... calling `basic_ios::init()` before ...

> Doing so, I recommend to adapt that of `ios_base();` as well, where we have:

> > Effects: Each `ios_base` member has an indeterminate value after construction. These members shall be initialized by calling `basic_ios::init`. If an `ios_base` object is destroyed before these initializations have taken place, the behavior is undefined.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 31.5.2.8 [ios.base.cons] p1:

>     ios_base();

> > Effects: Each `ios_base` member has an indeterminate value after construction. These The object's members shall be initialized by calling `basic_ios::init` before the object's first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.. If an `ios_base` object is destroyed before these initializations have taken place, the behavior is undefined.

Change 31.5.4.2 [basic.ios.cons] p2:

>     basic_ios();

> > Effects: Constructs an object of class `basic_ios` (27.5.2.7) leaving its member objects uninitialized. The object shall be initialized by calling its `basic_ios::init` before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined. member function. If it is destroyed before it has been initialized the behavior is undefined.

### 1250(i). `<bitset>` still overspecified

Section: 22.9.2 [template.bitset] Status: C++11 Submitter: Martin Sebor Opened: 2009-10-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Issue 1227(i) — `<bitset>` synopsis overspecified makes the observation that `std::bitset`, and in fact the whole library, may be implemented without needing to `#include <stdexcept>` in any library header. The proposed resolution removes the `#include <stdexcept>` directive from the header.

I'd like to add that the `<bitset>` header (as well as the rest of the library) has also been implemented without #including the `<cstddef>` header in any library header. In the case of `std::bitset`, the template is fully usable (i.e., it may be instantiated and all its member functions may be used) without ever mentioning `size_t`. In addition, just like no library header except for `<bitset>` `#includes <stdexcept>` in its synopsis, no header but `<bitset>` `#includes <cstddef>` either.

Thus I suggest that the `#include <cstddef>` directive be similarly removed from the synopsis of `<bitset>`.

[ 2010-02-08 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Change 22.9.2 [template.bitset]:

>     #include <cstddef>        // for size_t
>     #include <string>
>     #include <iosfwd>         // for istream, ostream
>     namespace std {
>     ...

### 1252(i). `wbuffer_convert::state_type` inconsistency

Section: 99 [depr.conversions.buffer] Status: C++11 Submitter: Bo Persson Opened: 2009-10-21 Last modified: 2017-04-22

Priority: Not Prioritized

View all other issues in [depr.conversions.buffer].

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Discussion:

The synopsis for `wbuffer_convert` [conversions.buffer]/2 contains

>     typedef typename Tr::state_type   state_type;

making `state_type` a synonym for (possibly) some `char_traits<x>::state_type`.

However, in paragraph 9 of the same section, we have

>     typedef typename Codecvt::state_type state_type;

> > The type shall be a synonym for `Codecvt::state_type`.

From what I can see, it might be hard to implement `wbuffer_convert` if the types were not both `std::mbstate_t`, but I cannot find a requirement that they must be the same type.

[ Batavia 2010: ]

Howard to draft wording, move to Review. Run it by Bill. Need to move this in Madrid.

[2011-03-06: Howard drafts wording]

[2011-03-24 Madrid meeting]

Moved to Immediate

Proposed resolution:

Modify the `state_type` typedef in the synopsis of [conversions.buffer] p.2 as shown [This makes the synopsis consistent with [conversions.buffer] p.9]:

>     namespace std {
>     template<class Codecvt,
>       class Elem = wchar_t,
>       class Tr = std::char_traits<Elem> >
>     class wbuffer_convert
>       : public std::basic_streambuf<Elem, Tr> {
>     public:
>       typedef typename TrCodecvt::state_type state_type;
>       […]
>     };
>     }

### 1253(i). invalidation of iterators and `emplace` vs. `insert` inconsistence in assoc. containers

Section: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Boris Dušek Opened: 2009-10-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the latest published draft N2960, section 23.2.7 [associative.reqmts], paragraph 8, it is specified that that `insert` does not invalidate any iterators. As per 23.2.2 [container.requirements.general], paragraph 12, this holds true not only for `insert`, but `emplace` as well. This gives the `insert` member a special treatment w.r.t. `emplace` member in 23.2.7 [associative.reqmts], par. 8, since both modify the container. For the sake of consistency, in 23.2.7 [associative.reqmts], par. 8: either reference to `insert` should be removed (i.e. count on 23.2.2 [container.requirements.general], par. 12), or reference to `emplace` be added (i.e. mention all members of assoc. containers that modify it).

[ 2009-11-18 Chris provided wording. ]

> This suggested wording covers both the issue discussed, and a number of other identical issues (namely `insert` being discussed without `emplace`). I'm happy to go back and split and introduce a new issue if appropriate, but I think the changes are fairly mechanical and obvious.

[ 2010-01-23 Daniel Krügler and J. Daniel García updated wording to make the use of `hint` consistent with `insert`. ]

[ 2011-02-23 Daniel Krügler adapts wording to numbering changes to match the N3225 draft. During this action it was found that 23.2.8 [unord.req] had been changed considerably due to acceptance of N3068 during the Pittsburgh meeting, such that the suggested wording change to p. 6 can no longer be applied. The new wording is more general and should now include `insert()` and `emplace()` calls as well ("mutating operations"). ]

[2011-03-06 Daniel Krügler adapts wording to numbering changes to match the N3242 draft]

Proposed resolution:

1. Modify bullet 1 of 23.2.2 [container.requirements.general], p. 10:

   10 Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:

   - if an exception is thrown by an `insert()` or `emplace()` function while inserting a single element, that function has no effects.
   - ...

2. Modify 23.2.7 [associative.reqmts], p. 4:

   > 4 An associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. The `set` and `map` classes support unique keys; the `multiset` and `multimap` classes support equivalent keys. For `multiset` and `multimap`, `insert`, `emplace`, and `erase` preserve the relative ordering of equivalent elements.

3. Modify Table 102 — Associative container requirements in 23.2.7 [associative.reqmts]:

   > Table 102 — Associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > […]
   > `a_eq.emplace(args)` | `iterator` | […] Effects: Inserts a `T` object `t` constructed with `std::forward<Args>(args)...` and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to `t` exists in `a_eq`, `t` is inserted at the end of that range. | logarithmic
   > `a.emplace_hint(p, args)` | `iterator` | equivalent to `a.emplace(std::forward<Args>(args)...)`. Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The `const_iterator p` is a hint pointing to where the search should start. The element is inserted as close as possible to the position just prior to `p`. Implementations are permitted to ignore the hint. | logarithmic in general, but amortized constant if the element is inserted right after before `p`
   > […]

4. Modify 23.2.7 [associative.reqmts], p. 9:

   > 9 The `insert` and `emplace` members shall not affect the validity of iterators and references to the container, and the `erase` members shall invalidate only iterators and references to the erased elements.

5. Modify 23.2.7.2 [associative.reqmts.except], p. 2:

   > 2 For associative containers, if an exception is thrown by any operation from within an `insert()` or `emplace()` function inserting a single element, the `insert()` function insertion has no effect.

6. Modify 23.2.8 [unord.req], p. 13 and p. 14:

   > 6 An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. `unordered_set` and `unordered_map` support unique keys. `unordered_multiset` and `unordered_multimap` support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other in the iteration order of the container. Thus, although the absolute order of elements in an unordered container is not specified, its elements are grouped into equivalent-key groups such that all elements of each group have equivalent keys. Mutating operations on unordered containers shall preserve the relative order of elements within each equivalent-key group unless otherwise specified.

   > […]

   > 13 The `insert` and `emplace` members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The erase members shall invalidate only iterators and references to the erased elements.

   > 14 The `insert` and `emplace` members shall not affect the validity of iterators if `(N+n) < z * B`, where `N` is the number of elements in the container prior to the insert operation, `n` is the number of elements inserted, `B` is the container's bucket count, and `z` is the container's maximum load factor.

7. Modify 23.2.8.2 [unord.req.except], p. 2:

   > 2 For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an `insert()` or `emplace()` function inserting a single element, the `insert()` insertion function has no effect.

### 1254(i). Misleading sentence in `vector<bool>::flip`

Section: 23.3.14 [vector.bool] Status: C++11 Submitter: Christopher Jefferson Opened: 2009-11-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The effects of `vector<bool>::flip` has the line:

> It is unspecified whether the function has any effect on allocated but unused bits.

While this is technically true, it is misleading, as any member function in any standard container may change unused but allocated memory. Users can never observe such changes as it would also be undefined behaviour to read such memory.

[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Strike second sentence from the definition of `vector<bool>::flip()`, 23.3.14 [vector.bool], paragraph 5.

> Effects: Replaces each element in the container with its complement. It is unspecified whether the function has any effect on allocated but unused bits.

### 1255(i). `declval` should be added to the library

Section: 22.2 [utility] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-03 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

During the Santa Cruz meeting it was decided to split off the provision of the library utility `value()` proposed in N2979 from the concrete request of the UK 300 comment. The provision of a new library component that allows the production of values in unevaluated expressions is considered as important to realize constrained templates in C++0x where concepts are not available.

The following proposed resolution is an improvement over that suggested in N2958, because the proposed component can now be defined without loss of general usefulness and any use by user-code will make the program ill-formed. A possible prototype implementation that satisfies the core language requirements can be written as:

>     template<class T>
>       struct declval_protector {
>         static const bool stop = false;
>         static typename std::add_rvalue_reference<T>::type delegate(); // undefined
>       };

>     template<class T>
>     typename std::add_rvalue_reference<T>::type declval() {
>       static_assert(declval_protector<T>::stop, "declval() must not be used!");
>       return declval_protector<T>::delegate();
>     }

Further-on the earlier suggested name `value()` has been changed to `declval()` after discussions with committee members.

Finally the suggestion shown below demonstrates that it can simplify existing standard wording by directly using it in the library specification, and that it also improves an overlooked corner case for `common_type` by adding support for `cv void`.

[ 2009-11-19 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

[ The proposed resolution has been updated to N3000 numbering and wording ]

1. Change 22.2 [utility], header `<utility>` synopsis as indicated:

   >     // 20.3.3, forward/move:
   >     template <class T> struct identity;
   >     template <class T, class U> T&& forward(U&&);
   >     template <class T> typename remove_reference<T>::type&& move(T&&);

   >     // 20.3.4, declval:
   >     template <class T> typename add_rvalue_reference<T>::type declval(); // as unevaluated operand

2. Immediately after the current section 22.2.4 [forward] insert a new section:

   20.3.4 Function template declval [declval]

   The library provides the function template `declval` to simplify the definition of expressions which occur as unevaluated operands (7 [expr]). The template parameter `T` of `declval` may be an incomplete type.

       template <class T> typename add_rvalue_reference<T>::type declval(); // as unevaluated operand

   > Remarks: If this function is used according to 6.3 [basic.def.odr], the program is ill-formed.

   > [Example:

   > >     template<class To, class From>
   > >     decltype(static_cast<To>(declval<From>())) convert(From&&);

   > declares a function template `convert`, which only participates in overloading if the type `From` can be explicitly cast to type `To`. For another example see class template `common_type` (21.3.8.7 [meta.trans.other]). — end example]

3. This bullet just makes clear that after applying N2984, the changes in 21.3.5.4 [meta.unary.prop], before table Type property queries should not use `declval`, because the well-formedness requirement of the specification of `is_constructible` would become more complicated, because we would need to make sure that the expression CE is checked in an unevaluated context.

4. Also 21.3.7 [meta.rel]/4 is not modified similar to the previous bullet, because with the stricter requirements of not using `declval()` the well-formedness condition would be harder to specify. The following changes are only editorial ones (e.g. the removal of the duplicate declaration of `create()`):

   > Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create();

   > the predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

   > >     template <class T>
   > >     typename add_rvalue_reference<T>::type create();
   > >     To test() {
   > >       return create<From>();
   > >     }

5. Change the entry in column "Comments" for `common_type` in Table 51 — Other transformations (21.3.8.7 [meta.trans.other]):

   [ NB: This wording change extends the type domain of `common_type` for `cv void => cv void` transformations and thus makes `common_type` usable for all binary type combinations that are supported by `is_convertible` ]

   > The member typedef `type` shall be defined as set out below. All types in the parameter pack `T` shall be complete or (possibly cv-qualified) `void`. A program may specialize this trait if at least one template parameter in the specialization is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note]

6. Change 21.3.8.7 [meta.trans.other]/3 as indicated:

   [ NB: This wording change is more than an editorial simplification of the definition of `common_type`: It also extends its usefulness for `cv void` types as outlined above ]

   > The nested typedef `common_type::type` shall be defined as follows:

   > > [..]

   > >     template <class T, class U>
   > >     struct common_type<T, U> {
   > >     private:
   > >       static T&& __t();
   > >       static U&& __u();
   > >     public:
   > >       typedef decltype(true ? __tdeclval<T>() : __udeclval<U>()) type;
   > >     };

7. Change 99 [func.ret]/1 as indicated [This part solves some main aspects of issue 1225(i)]:

   >     namespace std {
   >       template <class> class result_of; // undefined

   >       template <class Fn, class... ArgTypes>
   >       class result_of<Fn(ArgTypes...)> {
   >       public :
   >         // types
   >         typedef see belowdecltype(declval<Fn>() ( declval<ArgTypes>()... )) type;
   >       };
   >     }

   > 1 Given an rvalue `fn` of type `Fn` and values `t1, t2, ..., tN` of types `T1, T2, ..., TN` in `ArgTypes`, respectively, the `type` member is the result type of the expression `fn(t1, t2, ...,tN)`. The values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

### 1256(i). `weak_ptr` comparison functions should be removed

Section: 20.3.2.3 [util.smartptr.weak] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Additional to the necessary cleanup of the description of the the `weak_ptr` component from 20.3.2.3 [util.smartptr.weak] described in 1231(i) it turns out that the currently deleted comparison functions of `weak_ptr` are not needed at all: There is no safe-bool conversion from `weak_ptr`, and it won't silently chose a conversion to `shared_ptr`.

[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 20.3.2.3 [util.smartptr.weak]/1 as indicated:

>     namespace std {
>     template<class T> class weak_ptr {
>     public:
>     ...
>       // comparisons
>       template<class Y> bool operator<(weak_ptr<Y> const&) const = delete;
>       template<class Y> bool operator<=(weak_ptr<Y> const&) const = delete;
>       template<class Y> bool operator>(weak_ptr<Y> const&) const = delete;
>       template<class Y> bool operator>=(weak_ptr<Y> const&) const = delete;
>     };
>     ...

### 1257(i). Header <ios> still contains a `concept_map`

Section: 31.5 [iostreams.base] Status: C++11 Submitter: Beman Dawes Opened: 2009-11-04 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current WP still contains a `concept_map`.

[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change Iostreams base classes 31.5 [iostreams.base], Header <ios> synopsis, as indicated:

>     concept_map ErrorCodeEnum<io_errc> { };
>     template <> struct is_error_code_enum<io_errc> : true_type { }
>     error_code make_error_code(io_errc e);
>     error_condition make_error_condition(io_errc e);
>     const error_category& iostream_category();

### 1258(i). std::function Effects clause impossible to satisfy

Section: 22.10.17.3.3 [func.wrap.func.mod] Status: Resolved Submitter: Daniel Krügler Opened: 2009-11-05 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

As of 22.10.17.3.3 [func.wrap.func.mod]/2+ we have the following prototype description:

>     template<class F, Allocator Alloc>
>       void assign(F, const Alloc&);

> > Effects: `function(f, a).swap(*this)`

Two things: First the concept debris needs to be removed, second and much more importantly, the effects clause is now impossible to satisfy, because there is no constructor that would match the parameter sequence (`FunctionObject`, `Allocator`) [plus the fact that no `f` and no `a` is part of the signature]. The most probable candidate is

>     template<class F, class A> function(allocator_arg_t, const A&, F);

and the effects clause needs to be adapted to use this signature.

[ 2009-11-13 Daniel brought wording up to date. ]

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-11 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. It was noted that this issue was in partial conflict with 1288(i), and the two issues were merged in 1288(i). ]

Rationale:

Addressed by 1288(i).

Proposed resolution:

Change in 22.10.17.3.3 [func.wrap.func.mod] the complete prototype description as indicated

[ Question to the editor: Shouldn't there a paragraph number in front of the Effects clause? ]

>     template<class F, Allocator Allocclass A>
>       void assign(F f, const Alloc& a);

> > 3 Effects: `function(f, aallocator_arg, a, f).swap(*this)`

### 1260(i). `is_constructible<int*,void*>` reports true

Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: Peter Dimov Opened: 2009-11-07 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

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Discussion:

The specification of `is_constructible<T,Args...>` in N3000 uses

>     static_cast<T>(create<Args>()...)

for the one-argument case, but `static_cast` also permits unwanted conversions such as `void*` to `T*` and `Base*` to `Derived*`.

[ Post-Rapperswil: ]

> Moved to NAD EditorialResolved, this issue is addressed by paper n3047

Proposed resolution:

Change 21.3.5.4 [meta.unary.prop], p6:

> the predicate condition for a template specialization `is_constructible<T, Args>` shall be satisfied, if and only if the following expression CE variable definition would be well-formed:

> - if `sizeof...(Args) == 0 1`, the expression:

>   >     static_cast<T>(create<Args>()...)
>   >     T t;

> - otherwise the expression:

>   >     T t(create<Args>()...);

### 1261(i). Insufficent overloads for `to_string` / `to_wstring`

Section: 27.4.5 [string.conversions] Status: C++11 Submitter: Christopher Jefferson Opened: 2009-11-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Reported on the gcc mailing list.

> The code "`int i; to_string(i);`" fails to compile, as '`int`' is ambiguous between '`long long`' and '`long long unsigned`'. It seems unreasonable to expect users to cast numbers up to a larger type just to use `to_string`.

[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

27.4 [string.classes], change `to_string` and `to_wstring` to:

>     string to_string(int val);
>     string to_string(unsigned val);
>     string to_string(long val);
>     string to_string(unsigned long val);
>     string to_string(long long val);
>     string to_string(unsigned long long val);
>     string to_string(float val);
>     string to_string(double val);
>     string to_string(long double val);

>     wstring to_wstring(int val);
>     wstring to_wstring(unsigned val);
>     wstring to_wstring(long val);
>     wstring to_wstring(unsigned long val);
>     wstring to_wstring(long long val);
>     wstring to_wstring(unsigned long long val);
>     wstring to_wstring(float val);
>     wstring to_wstring(double val);
>     wstring to_wstring(long double val);

In 27.4.5 [string.conversions], paragraph 7, change to:

>     string to_string(int val);
>     string to_string(unsigned val);
>     string to_string(long val);
>     string to_string(unsigned long val);
>     string to_string(long long val);
>     string to_string(unsigned long long val);
>     string to_string(float val);
>     string to_string(double val);
>     string to_string(long double val);

> > 7 Returns: each function returns a `string` object holding the character representation of the value of its argument that would be generated by calling `sprintf(buf, fmt, val)` with a format specifier of `"%d"`, `"%u"`, `"%ld"`, `"%lu"`, `"%lld"`, `"%llu"`, `"%f"`, `"%f"`, or `"%Lf"`, respectively, where `buf` designates an internal character buffer of sufficient size.

In 27.4.5 [string.conversions], paragraph 14, change to:

>     wstring to_wstring(int val);
>     wstring to_wstring(unsigned val);
>     wstring to_wstring(long val);
>     wstring to_wstring(unsigned long val);
>     wstring to_wstring(long long val);
>     wstring to_wstring(unsigned long long val);
>     wstring to_wstring(float val);
>     wstring to_wstring(double val);
>     wstring to_wstring(long double val);

> > 14 Returns: Each function returns a `wstring` object holding the character representation of the value of its argument that would be generated by calling `swprintf(buf, buffsz, fmt, val)` with a format specifier of `L"%d"`, `L"%u"`, `L"%ld"`, `L"%lu"`, `L"%lld"`, `L"%llu"`, `L"%f"`, `L"%f"`, or `L"%Lf"`, respectively, where `buf` designates an internal character buffer of sufficient size `buffsz`.

### 1262(i). `std::less<std::shared_ptr<T>>` is underspecified

Section: 20.3.2.2.8 [util.smartptr.shared.cmp] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-11-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.smartptr.shared.cmp].

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Discussion:

20.3.2.2.8 [util.smartptr.shared.cmp]/5 says:

> For templates `greater`, `less`, `greater_equal`, and `less_equal`, the partial specializations for `shared_ptr` shall yield a total order, even if the built-in operators `<`, `>`, `<=`, and `>=` do not. Moreover, `less<shared_ptr<T> >::operator()(a, b)` shall return `std::less<T*>::operator()(a.get(), b.get())`.

This is necessary in order to use `shared_ptr` as the key in associate containers because n2637 changed `operator<` on `shared_ptr`s to be defined in terms of `operator<` on the stored pointers (a mistake IMHO but too late now.) By 7.6.9 [expr.rel]/2 the result of comparing builtin pointers is unspecified except in special cases which generally do not apply to `shared_ptr`.

Earlier versions of the WP (n2798, n2857) had the following note on that paragraph:

> [Editor's note: It's not clear to me whether the first sentence is a requirement or a note. The second sentence seems to be a requirement, but it doesn't really belong here, under `operator<`.]

I agree completely - if partial specializations are needed they should be properly specified.

20.3.2.2.8 [util.smartptr.shared.cmp]/6 has a note saying the comparison operator allows `shared_ptr` objects to be used as keys in associative containers, which is misleading because something else like a `std::less` partial specialization is needed. If it is not correct that note should be removed.

20.3.2.2.8 [util.smartptr.shared.cmp]/3 refers to '`x`' and '`y`' but the prototype has parameters '`a`' and '`b`' - that needs to be fixed even if the rest of the issue is NAD.

I see two ways to fix this, I prefer the first because it removes the need for any partial specializations and also fixes `operator>` and other comparisons when defined in terms of `operator<`.

1. Replace 20.3.2.2.8 [util.smartptr.shared.cmp]/3 with the following and remove p5:

   >     template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);

   > > 3 Returns: `x.get() < y.get()`. `std::less<V>()(a.get(), b.get())`, where `V` is the composite pointer type (7.6.9 [expr.rel]).

   > > 4 Throws: nothing.

   > > 5 For templates `greater`, `less`, `greater_equal`, and `less_equal`, the partial specializations for `shared_ptr` shall yield a total order, even if the built-in operators `<`, `>`, `<=`, and `>=` do not. Moreover, `less<shared_ptr<T> >::operator()(a, b)` shall return `std::less<T*>::operator()(a.get(), b.get())`.

   > > 6 [Note: Defining a comparison operator allows `shared_ptr` objects to be used as keys in associative containers. — end note]

2. Add to 20.3.2.2 [util.smartptr.shared]/1 (after the `shared_ptr` comparisons)

   >     template<class T> struct greater<shared_ptr<T>>;
   >     template<class T> struct less<shared_ptr<T>>;
   >     template<class T> struct greater_equal<shared_ptr<T>>;
   >     template<class T> struct less_equal<shared_ptr<T>>;

   Remove 20.3.2.2.8 [util.smartptr.shared.cmp]/5 and /6 and replace with:

   >     template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);

   > > 3 Returns: `xa.get() < yb.get()`.

   > > 4 Throws: nothing.

   > > 5 For templates `greater`, `less`, `greater_equal`, and `less_equal`, the partial specializations for `shared_ptr` shall yield a total order, even if the built-in operators `<`, `>`, `<=`, and `>=` do not. Moreover, `less<shared_ptr<T> >::operator()(a, b)` shall return `std::less<T*>::operator()(a.get(), b.get())`.

   > > 6 [Note: Defining a comparison operator allows `shared_ptr` objects to be used as keys in associative containers. — end note]

   >     template<class T> struct greater<shared_ptr<T>> :
   >     binary_function<shared_ptr<T>, shared_ptr<T>, bool> {
   >       bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const;
   >     };

   > > `operator()` returns `greater<T*>()(a.get(), b.get())`.

   >     template<class T> struct less<shared_ptr<T>> :
   >     binary_function<shared_ptr<T>, shared_ptr<T>, bool> {
   >       bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const;
   >     };

   > > `operator()` returns `less<T*>()(a.get(), b.get())`.

   >     template<class T> struct greater_equal<shared_ptr<T>> :
   >     binary_function<shared_ptr<T>, shared_ptr<T>, bool> {
   >       bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const;
   >     };

   > > `operator()` returns `greater_equal<T*>()(a.get(), b.get())`.

   >     template<class T> struct less_equal<shared_ptr<T>> :
   >     binary_function<shared_ptr<T>, shared_ptr<T>, bool> {
   >       bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const;
   >     };

   > > `operator()` returns `less_equal<T*>()(a.get(), b.get())`.

[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Replace 20.3.2.2.8 [util.smartptr.shared.cmp]/3 with the following and remove p5:

>     template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);

> > 3 Returns: `x.get() < y.get()`. `less<V>()(a.get(), b.get())`, where `V` is the composite pointer type (7.6.9 [expr.rel]) of `T*` and `U*`.

> > 4 Throws: nothing.

> > 5 For templates `greater`, `less`, `greater_equal`, and `less_equal`, the partial specializations for `shared_ptr` shall yield a total order, even if the built-in operators `<`, `>`, `<=`, and `>=` do not. Moreover, `less<shared_ptr<T> >::operator()(a, b)` shall return `std::less<T*>::operator()(a.get(), b.get())`.

> > 6 [Note: Defining a comparison operator allows `shared_ptr` objects to be used as keys in associative containers. — end note]

### 1264(i). `quick_exit` support for freestanding implementations

Section: 16.4.2.5 [compliance] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-11-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 172

This issue is a response to NB comment UK-172

The functions `quick_exit` and `at_quick_exit` should be added to the required features of `<cstdlib>` in a freestanding implementation.

This comment was rejected in Summit saying neither `at_exit` nor `at_quick_exit` should be required. This suggests the comment was misread, as `atexit` is already required to be supported. If the LWG really did wish to not require the registration functions be supported, then a separate issue should be opened to change the current standard.

Given both `exit` and `atexit` are required, the UK panel feels it is appropriate to require the new `quick_exit` facility is similarly supported.

[ 2009-12-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Ammend p3 Freestanding implementations 16.4.2.5 [compliance]

> 3 The supplied version of the header `<cstdlib>` shall declare at least the functions `abort()`, `atexit()`, `at_quick_exit`, and `exit()`, and `quick_exit`(17.5 [support.start.term]). The other headers listed in this table shall meet the same requirements as for a hosted implementation.

### 1266(i). `shared_future::get` and deferred `async` functions

Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

If a `shared_future` is constructed with the result of an `async` call with a deferred function, and two or more copies of that `shared_future` are created, with multiple threads calling `get()`, it is not clear which thread runs the deferred function. [futures.shared_future]p22 from N3000 says (minus editor's note):

> Effects: if the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.

In the absence of wording to the contrary, this implies that every thread that calls `wait()` will execute the deferred function.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Replace [futures.shared_future]p22 with the following:

> Effects: If the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready. was created by a `promise` or `packaged_task` object, block until the associated state is ready. If the associated state is associated with a thread created for an `async` call (32.10.9 [futures.async]), as if `associated-thread.join()`.

> If the associated state contains a deferred function, calls to `wait()` on all `shared_future` objects that share the same associated state are serialized. The first call to `wait()` that shares a given associated state executes the deferred function and stores the return value or exception in the associated state.

> Synchronization: if the associated state was created by a `promise` object, the completion of `set_value()` or `set_exception()` to that `promise` happens before (6.10.2 [intro.multithread]) `wait()` returns. If the associated state was created by a `packaged_task` object, the completion of the associated task happens before `wait()` returns. If the associated state is associated with a thread created for an `async` call (32.10.9 [futures.async]), the completion of the associated thread happens-before `wait()` returns.

> If the associated state contained a deferred function, the invocation of the deferred function happens-before any call to `wait()` on a `future` that shares that state returns.

### 1267(i). Incorrect wording for `condition_variable_any::wait_for`

Section: 32.7.5 [thread.condition.condvarany] Status: C++11 Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.7.5 [thread.condition.condvarany]p18 and 32.7.5 [thread.condition.condvarany]p27 specify incorrect preconditions for `condition_variable_any::wait_for`. The stated preconditions require that `lock` has a `mutex()` member function, and that this produces the same result for all concurrent calls to `wait_for()`. This is inconsistent with `wait()` and `wait_until()` which do not impose such a requirement.

[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Remove 32.7.5 [thread.condition.condvarany]p18 and 32.7.5 [thread.condition.condvarany]p27.

>     template <class Lock, class Rep, class Period>
>       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

> > 18 Precondition: lock is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the lock arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

> ...

>     template <class Lock, class Rep, class Period, class Predicate>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

> > 27 Precondition: lock is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the lock arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

### 1268(i). The Mutex requirements in 30.4.1 and 30.4.2 are wrong

Section: 32.6 [thread.mutex] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The `Mutex` requirements in 32.6.4 [thread.mutex.requirements] and 32.6.4.3 [thread.timedmutex.requirements] confuse the requirements on the behaviour of `std::mutex` et al with the requirements on `Lockable` types for use with `std::unique_lock`, `std::lock_guard` and `std::condition_variable_any`.

[ 2010 Pittsburgh: ]

> Concepts of threads chapter and issue presentation are: Lockable < Mutex < TimedMutex and Lockable < TimedLockable < TimedMutex.

> Typo in failed deletion of Mutex in 30.4.4 p4 edits.

> Lockable requirements are too weak for condition_variable_any, but the Mutex requirements are too strong.

> Need subset of Lockable requirements for condition_variable_any that does not include try_lock. E.g. CvLockable < Lockable.

> Text needs updating to recent draft changes.

> Needs to specify exception behavior in Lockable.

> The current standard is fine for what it says, but it places requirements that are too strong on authors of mutexes and locks.

> Move to open status. Suggest Anthony look at condition_variable_any requirements. Suggest Anthony refine requirements/concepts categories.

> Related to 964(i) and 966(i)

[ 2010-03-28 Daniel synced with N3092. ]

[ 2010-10-25 Daniel adds: ]

> Accepting n3130 would solve this issue.

[ 2010-11 Batavia: ]

> Resolved by adopting n3197.

Proposed resolution:

Add a new section to 32.2 [thread.req] after 32.2.4 [thread.req.timing] as follows:

> 30.2.5 Requirements for Lockable types

> The standard library templates `unique_lock` (32.6.5.4 [thread.lock.unique]), `lock_guard` (32.6.5.2 [thread.lock.guard]), `lock`, `try_lock` (32.6.6 [thread.lock.algorithm]) and `condition_variable_any` (32.7.5 [thread.condition.condvarany]) all operate on user-supplied `Lockable` objects. Such an object must support the member functions specified for either the `Lockable` Requirements or the `TimedLockable` requirements as appropriate to acquire or release ownership of a `lock` by a given `thread`. [Note: the nature of any lock ownership and any synchronization it may entail are not part of these requirements. — end note]

> 30.2.5.1 Lockable Requirements

> In order to qualify as a `Lockable` type, the following expressions must be supported, with the specified semantics, where `m` denotes a value of type `L` that supports the `Lockable`:

> The expression `m.lock()` shall be well-formed and have the following semantics:

> Effects:
>   Block until a lock can be acquired for the current thread.
> Return type:
>   `void`

> The expression `m.try_lock()` shall be well-formed and have the following semantics:

> Effects:
>   Attempt to acquire a lock for the current thread without blocking.
> Return type:
>   `bool`
> Returns:
>   `true` if the lock was acquired, `false` otherwise.

> The expression `m.unlock()` shall be well-formed and have the following semantics:

> Effects:
>   Release a lock on `m` held by the current thread.
> Return type:
>   `void`
> Throws:
>   Nothing if the current thread holds a lock on `m`.

> 30.2.5.2 `TimedLockable` Requirements

> For a type to qualify as `TimedLockable` it must meet the `Lockable` requirements, and additionally the following expressions must be well-formed, with the specified semantics, where `m` is an instance of a type `TL` that supports the `TimedLockable` requirements, `rel_time` denotes instantiation of `duration` (30.5 [time.duration]) and `abs_time` denotes an instantiation of `time_point` (30.6 [time.point])

> The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

> Effects:
>   Attempt to acquire a lock for the current thread within the specified time period.
> Return type:
>   `bool`
> Returns:
>   `true` if the lock was acquired, `false` otherwise.

> The expression `m.try_lock_until(abs_time)` shall be well-formed and have the following semantics:

> Effects:
>   Attempt to acquire a lock for the current thread before the specified point in time.
> Return type:
>   `bool`
> Returns:
>   `true` if the lock was acquired, `false` otherwise.

Replace 32.6.4 [thread.mutex.requirements] paragraph 2 with the following:

> 2 This section describes requirements on template argument types used to instantiate templates defined in the mutex types supplied by the C++ standard library. The template definitions in the C++ standard library refer These types shall conform to the named `Mutex` requirements whose details are set out below. In this description, `m` is an object of a `Mutex` type one of the standard library mutex types `std::mutex`, `std::recursive_mutex`, `std::timed_mutex` or `std::recursive_timed_mutex`..

Add the following paragraph after 32.6.4 [thread.mutex.requirements] paragraph 2:

> A `Mutex` type shall conform to the `Lockable` requirements (30.2.5.1).

Replace 32.6.4.3 [thread.timedmutex.requirements] paragraph 1 with the following:

> The C++ standard library `TimedMutex` types `std::timed_mutex` and `std::recursive_timed_mutex` A `TimedMutex` type shall meet the requirements for a `Mutex` type. In addition, itthey shall meet the requirements set out in this Clause 30.4.2below, where `rel_time` denotes an instantiation of `duration` (30.5 [time.duration]) and `abs_time` denotes an instantiation of `time_point` (30.6 [time.point]).

Add the following paragraph after 32.6.4.3 [thread.timedmutex.requirements] paragraph 1:

> A `TimedMutex` type shall conform to the `TimedLockable` requirements (30.2.5.1).

Add the following paragraph following 32.6.5.2 [thread.lock.guard] paragraph 1:

> The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.1).

Add the following paragraph following 32.6.5.4 [thread.lock.unique] paragraph 1:

> The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.1). `unique_lock<Mutex>` meets the `Lockable` requirements. If `Mutex` meets the `TimedLockable` requirements (30.2.5.2) then `unique_lock<Mutex>` also meets the `TimedLockable` requirements.

Replace the use of "mutex" or "mutex object" with "lockable object" throughout clause 32.6.5 [thread.lock] paragraph 1:

> 1 A lock is an object that holds a reference to a mutexlockable object and may unlock the mutexlockable object during the lock's destruction (such as when leaving block scope). A thread of execution may use a lock to aid in managing mutex ownership of a lockable object in an exception safe manner. A lock is said to own a mutexlockable object if it is currently managing the ownership of that mutexlockable object for a thread of execution. A lock does not manage the lifetime of the mutexlockable object it references. [ Note: Locks are intended to ease the burden of unlocking the mutexlockable object under both normal and exceptional circumstances. — end note ]

32.6.5 [thread.lock] paragaph 2:

> 2 Some lock constructors take tag types which describe what should be done with the mutexlockable object during the lock's constuction.

32.6.5.2 [thread.lock.guard] paragaph 1:

> 1 An object of type `lock_guard` controls the ownership of a mutexlockable object within a scope. A `lock_guard` object maintains ownership of a mutexlockable object throughout the `lock_guard` object's lifetime. The behavior of a program is undefined if the mutexlockable object referenced by `pm` does not exist for the entire lifetime (3.8) of the `lock_guard` object. `Mutex` shall meet the `Lockable` requirements (30.2.5.1).

32.6.5.4 [thread.lock.unique] paragaph 1:

> 1 An object of type `unique_lock` controls the ownership of a mutexlockable object within a scope. Mutex oOwnership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another `unique_lock` object. Objects of type `unique_lock` are not copyable but are movable. The behavior of a program is undefined if the contained pointer `pm` is not null and the mutex pointed to by `pm` does not exist for the entire remaining lifetime (3.8) of the `unique_lock` object. `Mutex` shall meet the `Lockable` requirements (30.2.5.1).

Add the following to the precondition of `unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time)` in 32.6.5.4.2 [thread.lock.unique.cons] paragraph 18:

>     template <class Clock, class Duration>
>       unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);

> > 18 Requires: If `mutex_type` is not a recursive mutex the calling thread does not own the mutex. The supplied `mutex_type` type shall meet the `TimedLockable` requirements (30.2.5.2).

Add the following to the precondition of `unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time)` in 32.6.5.4.2 [thread.lock.unique.cons] paragraph 22

> 22 Requires: If `mutex_type` is not a recursive mutex the calling thread does not own the mutex. The supplied `mutex_type` type shall meet the `TimedLockable` requirements (30.2.5.2).

Add the following as a precondition of `bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time)` before 32.6.5.4.3 [thread.lock.unique.locking] paragraph 8

>     template <class Clock, class Duration>
>       bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

> > Requires: The supplied `mutex_type` type shall meet the `TimedLockable` requirements (30.2.5.2).

Add the following as a precondition of `bool try_lock_for(const chrono::duration<Rep, Period>& rel_time)` before 32.6.5.4.3 [thread.lock.unique.locking] paragraph 12

>     template <class Rep, class Period>
>       bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);

> > Requires: The supplied `mutex_type` type shall meet the `TimedLockable` requirements (30.2.5.2).

Replace 32.6.6 [thread.lock.algorithm] p1 with the following:

>     template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);

> > 1 Requires: Each template parameter type shall meet the `Mutex Lockable` requirements (30.2.5.1)., except that a call to `try_lock()` may throw an exception. [Note: The `unique_lock` class template meets these requirements when suitably instantiated. — end note]

Replace 32.6.6 [thread.lock.algorithm] p4 with the following:

>     template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);

> > 4 Requires: Each template parameter type shall meet the `MutexMutex Lockable` requirements (30.2.5.1)., except that a call to `try_lock()` may throw an exception. [Note: The `unique_lock` class template meets these requirements when suitably instantiated. — end note]

Replace 32.7.5 [thread.condition.condvarany] paragraph 1 with:

> 1 A `Lock` type shall meet the requirements for a `Mutex` type `Lockable` requirements (30.2.5.1), except that `try_lock` is not required. [Note: All of the standard mutex types meet this requirement. — end note]

### 1269(i). Associated state doesn't account for `async`

Section: 32.10.5 [futures.state] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.state].

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Discussion:

The current description of the associated state in 32.10.5 [futures.state] does not allow for futures created by an `async` call. The description therefore needs to be extended to cover that.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Add a new sentence to 32.10.5 [futures.state] p2:

> 2 This associated state consists of some state information and some (possibly not yet evaluated) result, which can be a (possibly `void`) value or an exception. If the associated state was created by a call to `async` (32.10.9 [futures.async]) then it may also contain a deferred function or an associated `thread`.

Add an extra bullet to 32.10.5 [futures.state] p3:

> The result of an associated state can be set by calling:

> - `promise::set_value`,
> - `promise::set_exception`, or
> - packaged_task::operator()., or
> - a call to `async` (32.10.9 [futures.async]).

### 1270(i). `result_of` should be moved to `<type_traits>`

Section: 99 [func.ret] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-11-19 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.ret].

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Discussion:

Addresses UK 198

NB Comment: UK-198 makes this request among others. It refers to a more detailed issue that BSI did not manage to submit by the CD1 ballot deadline though.

`result_of` is essentially a metafunction to return the type of an expression, and belongs with the other library metafunctions in `<type_traits>` rather than lurking in `<functional>`. The current definition in `<functional>` made sense when `result_of` was nothing more than a protocol to enable several components in `<functional>` to provide their own result types, but it has become a more general tool. For instance, `result_of` is now used in the threading and futures components.

Now that `<type_traits>` is a required header for free-standing implementations it will be much more noticeable (in such environments) that a particularly useful trait is missing, unless that implementation also chooses to offer `<functional>` as an extension.

The simplest proposal is to simply move the wording (editorial direction below) although a more consistent form for type_traits would reformat this as a table.

Following the acceptance of 1255(i), `result_of` now depends on the `declval` function template, tentatively provided in `<utility>` which is not (yet) required of a free-standing implementation.

This dependency is less of an issue when `result_of` continues to live in `<functional>`.

Personally, I would prefer to clean up the dependencies so both `result_of` and `declval` are available in a free-standing implementation, but that would require slightly more work than suggested here. A minimal tweak would be to require `<utility>` in a free-standing implementation, although there are a couple of subtle issues with `make_pair`, which uses `reference_wrapper` in its protocol and that is much harder to separate cleanly from `<functional>`.

An alternative would be to enact the other half of N2979 and create a new minimal header for the new C++0x library facilities to be added to the freestanding requirements (plus `swap`.)

I have a mild preference for the latter, although there are clearly reasons to consider better library support for free-standing in general, and adding the whole of `<utility>` could be considered a step in that direction. See NB comment JP-23 for other suggestions (`array`, `ratio`)

[ 2010-01-27 Beman updated wording. ]

> The original wording is preserved here:

> > Move 99 [func.ret] to a heading below 21 [meta]. Note that in principle we should not change the tag, although this is a new tag for 0x. If it has been stable since TR1 it is clearly immutable though.

> > This wording should obviously adopt any other changes currently in (Tentatively) Ready status that touch this wording, such as 1255(i).

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

From Function objects 22.10 [function.objects], Header `<functional>` synopsis, remove:

>     // 20.7.4 result_of:
>     template <class> class result_of; // undefined
>     template <class F, class... Args> class result_of<F(ArgTypes...)>;

Remove Function object return types 99 [func.ret] in its entirety. This sub-section reads:

>     namespace std {
>       template <class> class result_of; // undefined

>       template <class Fn, class... ArgTypes>
>       class result_of<Fn(ArgTypes...)> {
>       public :
>         // types
>         typedef see below type;
>       };
>     }

> Given an rvalue `fn` of type `Fn` and values `t1, t2, ..., tN` of types T1, T2, ..., TN in `ArgTypes`, respectively, the type member is the result type of the expression `fn(t1, t2, ...,tN)`. The values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

To Header <type_traits> synopsis 21.3.3 [meta.type.synop], add at the indicated location:

>     template <class T> struct underlying_type;
>     template <class T> struct result_of; // not defined
>     template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;

To Other transformations 21.3.8.7 [meta.trans.other], Table 51 — Other transformations, add:

> Template | Condition | Comments
> `template <class T> struct underlying_type;` | `T` shall be an enumeration type (7.2) | The member typedef `type` shall name the underlying type of `T`.
> `template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;` | `Fn` shall be a function object type 22.10 [function.objects], reference to function, or reference to function object type. `decltype(declval<Fn>()(declval<ArgTypes>()...))` shall be well formed. | The member typedef `type` shall name the type `decltype(declval<Fn>()(declval<ArgTypes>()...))`.

At the end of Other transformations 21.3.8.7 [meta.trans.other] add:

> [Example: Given these definitions:

>     typedef bool(&PF1)();
>     typedef short(*PF2)(long);

>     struct S {
>       operator PF2() const;
>       double operator()(char, int&);
>      };

> the following assertions will hold:

>     static_assert(std::is_same<std::result_of<S(int)>::type, short>::value, "Error!");
>     static_assert(std::is_same<std::result_of<S&(unsigned char, int&)>::type, double>::value, "Error!");
>     static_assert(std::is_same<std::result_of<PF1()>::type, bool>::value, "Error!");

> — end example]

### 1271(i). `CR` undefined in duration operators

Section: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.duration.nonmember].

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Discussion:

IMO `CR` alone is not really defined (it should be `CR(Rep1, Rep2)`).

[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 30.5.6 [time.duration.nonmember] paragraphs 9 and 12:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator/(const duration<Rep1, Period>& d, const Rep2& s);

> > 9 Returns: `duration<CR(Rep1, Rep2), Period>(d) /= s`.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator%(const duration<Rep1, Period>& d, const Rep2& s);

> > 12 Returns: `duration<CR(Rep1, Rep2), Period>(d) %= s`.

### 1272(i). confusing declarations of `promise::set_value`

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.promise].

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Discussion:

The definitions of `promise::set_value` need tidying up, the synopsis says:

>     // setting the result
>     void set_value(const R& r);
>     void set_value(see below);

Why is the first one there? It implies it is always present for all specialisations of promise, which is not true.

The definition says:

>     void set_value(const R& r);
>     void promise::set_value(R&& r);
>     void promise<R&>::set_value(R& r);
>     void promise<void>::set_value();

The lack of qualification on the first one again implies it's present for all specialisations, again not true.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Change the synopsis in 32.10.6 [futures.promise]:

>     // setting the result
>     void set_value(const R& r);
>     void set_value(see below);

And the definition be changed by qualifying the first signature:

>     void promise::set_value(const R& r);
>     void promise::set_value(R&& r);
>     void promise<R&>::set_value(R& r);
>     void promise<void>::set_value();

### 1273(i). `future::valid` should be callable on an invalid future

Section: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [futures.unique.future].

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Discussion:

[futures.unique_future]/3 says:

> The effect of calling any member function other than the destructor or the move-assignment operator on a `future` object for which `valid() == false` is undefined.

This means calling `future::valid()` is undefined unless it will return `true`, so you can only use it if you know the answer!

[ 2009-12-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

Change [futures.unique_future]/3:

> The effect of calling any member function other than the destructor, or the move-assignment operator, or `valid`, on a `future` object for which `valid() == false` is undefined.

### 1274(i). `atomic_future` constructor

Section: 99 [futures.atomic_future] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.atomic_future].

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Discussion:

In 99 [futures.atomic_future] this constructor:

>     atomic_future(future<R>&&);

is declared in the synopsis, but not defined. Instead n2997 defines:

>     atomic_future(const future<R>&& rhs);

and n3000 defines

>     atomic_future(atomic_future<R>&& rhs);

both of which are wrong. The constructor definition should be changed to match the synopsis.

[ 2009-12-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

Adjust the signature above 99 [futures.atomic_future]/6 like so:

>     atomic_future(atomic_future<R>&& rhs);

### 1275(i). Creating and setting futures

Section: 32.10 [futures] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures].

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Discussion:

[futures.unique_future]/1 should be updated to mention `async`.

[futures.shared_future]/1 should also be updated for `async`. That paragraph also says

> ... Its value or exception can be set by use of a `shared_future`, `promise` (32.10.6 [futures.promise]), or `packaged_task` (32.10.10 [futures.task]) object that shares the same associated state.

How can the value be set by a `shared_future`?

99 [futures.atomic_future]/1 says

> An `atomic_future` object can only be created by use of a `promise` (32.10.6 [futures.promise]) or `packaged_task` (32.10.10 [futures.task]) object.

which is wrong, it's created from a `std::future`, which could have been default-constructed. That paragraph should be closer to the text of [futures.shared_future]/1, and should also mention `async`.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

### 1276(i). `forwardlist` missing allocator constructors

Section: 23.3.7 [forward.list] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-12 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [forward.list].

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Discussion:

I found that forward_list has only

>     forward_list(const forward_list<T,Allocator>& x);
>     forward_list(forward_list<T,Allocator>&& x);

but misses

>     forward_list(const forward_list& x, const Allocator&);
>     forward_list(forward_list&& x, const Allocator&);

Note to other reviewers: I also checked the container adaptors for similar inconsistencies, but as far as I can see these are already handled by the current active issues 1194(i) and 1199(i).

[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In [forwardlist]/3, class template forward_list synopsis change as indicated:

>     forward_list(const forward_list<T,Allocator>& x);
>     forward_list(forward_list<T,Allocator>&& x);
>     forward_list(const forward_list&, const Allocator&);
>     forward_list(forward_list&&, const Allocator&);

### 1277(i). `std::thread::id` should be trivially copyable

Section: 32.4.3.2 [thread.thread.id] Status: C++11 Submitter: Anthony Williams Opened: 2009-11-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.thread.id].

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Discussion:

The class definition of `std::thread::id` in N3000 is:

>     class thread::id {
>     public:
>       id();
>     };

Typically, I expect that the internal data members will either be pointers or integers, so that in practice the class will be trivially copyable. However, I don't think the current wording guarantees it, and I think it would be useful. In particular, I can see a use for `std::atomic<std::thread::id>` to allow a `thread` to claim ownership of a data structure atomicly, and `std::atomic<T>` requires that `T` is trivially copyable.

[ 2010-02-12 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]

Proposed resolution:

Add a new sentence to 32.4.3.2 [thread.thread.id] p1:

> 1 An object of type `thread::id` provides a unique identifier for each thread of execution and a single distinct value for all `thread` objects that do not represent a thread of execution (32.4.3 [thread.thread.class]). Each thread of execution has an associated `thread::id` object that is not equal to the `thread::id` object of any other thread of execution and that is not equal to the `thread::id` object of any `std::thread` object that does not represent threads of execution. The library may reuse the value of a `thread::id` of a terminated thread that can no longer be joined. `thread::id` shall be a trivially copyable class (11 [class]).

### 1278(i). Inconsistent return values for `forward_list::insert_after`

Section: 23.3.7.5 [forward.list.modifiers] Status: C++11 Submitter: Bo Persson Opened: 2009-11-25 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [forward.list.modifiers].

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Discussion:

After applying LDR149(i), `forward_list` now has 5 overloads of `insert_after`, all returning an iterator.

However, two of those - inserting a single object - return "An iterator pointing to a copy of `x` [the inserted object]" while the other three - inserting zero or more objects - return an iterator equivalent to the position parameter, pointing before any possibly inserted objects.

Is this the intended change?

I don't really know what `insert_after(position, empty_range)` should really return, but always returning `position` seems less than useful.

[ 2010-02-04 Howard adds: ]

> I agree this inconsistency will be error prone and needs to be fixed. Additionally `emplace_after`'s return value is unspecified.

[ 2010-02-04 Nico provides wording. ]

[ 2010 Pittsburgh: ]

> We prefer to return an iterator to the last inserted element. Modify the proposed wording and then set to Ready.

[ 2010-03-15 Howard adds: ]

> Wording updated and set to Ready.

Proposed resolution:

In `forward_list` modifiers [forwardlist.modifiers] make the following modifications:

>     iterator insert_after(const_iterator position, size_type n, const T& x);

> > ...

> > 10 Returns: position. An iterator pointing to the last inserted copy of `x` or `position` if `n == 0`.

>     template <class InputIterator>
>       iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

> > ...

> > 13 Returns: position. An iterator pointing to the last inserted element or `position` if `first == last`.

>     iterator insert_after(const_iterator position, initializer_list<T> il);

> > ...

> > 15 Returns: position. An iterator pointing to the last inserted element or `position` if `il` is empty.

>     template <class... Args>
>       iterator emplace_after(const_iterator position, Args&&... args);

> > ...

> > 17 ...

> > Returns: An iterator pointing to the new constructed element from args.

### 1279(i). forbid `[u|bi]nary_function` specialization

Section: 99 [depr.base] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2009-11-30 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [depr.base].

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Discussion:

A program should not be allowed to add specialization of class templates `unary_function` and `binary_function`, in force of 16.4.5.2.1 [namespace.std]/1. If a program were allowed to specialize these templates, the library could no longer rely on them to provide the intended typedefs or there might be other undesired interactions.

[ 2010-03-27 Daniel adds: ]

> Accepting issue 1290(i) would resolve this issue as NAD editorial.

[ 2010-10-24 Daniel adds: ]

> Accepting n3145 would resolve this issue as NAD editorial.

[ 2010 Batavia: ]

> Pete: Is this issue actually addressed by N3198, or did deprecating unary/binary_function?

> We determined that this issue is NOT resolved and that it must be resolved or else N3198 could break code that does specialize unary/binary function.

> Matt: don't move to NAD

> Howard: I suggest we go further and move 1279 to ready for Madrid.

> Group: Agrees move 1279 to ready for Madrid

Previous proposed resolution:

> 1 The following classes class templates are provided to simplify the typedefs of the argument and result types:. A program shall not declare specializations of these templates.

[2011-03-06 Daniel comments]

> This meeting outcome was not properly reflected in the proposed resolution. I also adapted the suggested wording to the N3242 numbering and content state. During this course of action it turned out that the first suggested wording change has already been applied.

Proposed resolution:

Change paragraph 99 [depr.base]/1 as follows:

> 1 The class templates `unary_function` and `binary_function` are deprecated. A program shall not declare specializations of these templates.

### 1280(i). Initialization of stream iterators

Section: 24.6.2.2 [istream.iterator.cons], 24.6.3.2 [ostream.iterator.cons.des] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-04 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [istream.iterator.cons].

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Discussion:

24.6.2.2 [istream.iterator.cons] describes the effects in terms of:

>     basic_istream<charT,traits>* in_stream; // exposition only

> 3 Effects: Initializes in_stream with `s`.

That should be `&s` and similarly for 24.6.3.2 [ostream.iterator.cons.des].

[ 2009-12-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Change 24.6.2.2 [istream.iterator.cons] like so:

>     istream_iterator(istream_type& s);

> > 3 Effects: Initializes in_stream with `&s`. value ...

And 24.6.3.2 [ostream.iterator.cons.des] like so:

>     ostream_iterator(ostream_type& s);

> > 1 Effects: Initializes out_stream with `&s` and delim with null.

>     ostream_iterator(ostream_type& s, const charT* delimiter);

> > 2 Effects: Initializes out_stream with `&s` and delim with `delimiter`.

### 1281(i). CopyConstruction and Assignment between ratios having the same normalized form

Section: 21.5.3 [ratio.ratio] Status: Resolved Submitter: Vicente Juan Botet Escribá Opened: 2009-12-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ratio.ratio].

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Discussion:

CopyConstruction and Assignment between `ratio`s having the same normalized form. Current N3000 do not allows to copy-construct or assign `ratio` instances of `ratio` classes having the same normalized form.

Two `ratio` classes `ratio<N1,D1>` and `ratio<N2,D2>` have the same normalized form if

>     ratio<N1, D1>::num == ratio<N2, D2>::num &&
>     ratio<N1, D1>::den == ratio<N2, D2>::den

This simple example

>     ratio<1,3> r1;
>     ratio<3,9> r2;
>     r1 = r2; // (1)

fails to compile in (1). Other example

>     ratio<1,3> r1;
>     ratio_subtract<ratio<2,3>, ratio<1,3>>::type r2;
>     r1 = r2;

The nested type of `ratio_subtract<ratio<2,3>, ratio<1,3>>` could be `ratio<3,9>` so the compilation could fail. It could also be `ratio<1,3>` and the compilation succeeds.

In 21.5.4 [ratio.arithmetic] 3 and similar clauses

> 3 The nested typedef `type` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::den - R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

the meaning of synonym let think that the result shall be a normalized `ratio` equivalent to `ratio<T1, T2>`, but there is not an explicit definition of what synonym means in this context.

Additionally we should add a typedef for accessing the normalized `ratio`, and change 21.5.4 [ratio.arithmetic] to return only this normalized result.

[ 2010 Pittsburgh: ]

> There is no consensus to add the converting copy constructor or converting copy assignment operator. However there was consensus to add the typedef.

> Proposed wording modified. Original proposed wording preserved here. Moved to Review.

> > Make `ratio` default constructible, copy-constructible and assignable from any `ratio` which has the same reduced form.

> > Add to 21.5.3 [ratio.ratio] synopsis

> > >     template <intmax_t N, intmax_t D = 1>
> > >     class ratio {
> > >     public:
> > >       static constexpr intmax_t num;
> > >       static constexpr intmax_t den;

> > >       typedef ratio<num, den> type;

> > >       ratio() = default;
> > >       template <intmax_t N2, intmax_t D2>
> > >         ratio(const ratio<N2, D2>&);
> > >       template <intmax_t N2, intmax_t D2>
> > >         ratio& operator=(const ratio<N2, D2>&);
> > >     };

> > Add to 21.5.3 [ratio.ratio]:

> > > Two ratio classes `ratio<N1,D1>` and `ratio<N2,D2>` have the same reduced form if `ratio<N1,D1>::type` is the same type as `ratio<N2,D2>::type`

> > Add a new section: [ratio.cons]

> > > Construction and assignment [ratio.cons]

> > >     template <intmax_t N2, intmax_t D2>
> > >       ratio(const ratio<N2, D2>& r);

> > > > Effects: Constructs a `ratio` object.

> > > > Remarks: This constructor shall not participate in overload resolution unless `r` has the same reduced form as `*this`.

> > >     template <intmax_t N2, intmax_t D2>
> > >       ratio& operator=(const ratio<N2, D2>& r);

> > > > Effects: None.

> > > > Returns: `*this`.

> > > > Remarks: This operator shall not participate in overload resolution unless `r` has the same reduced form as `*this`.

> > Change 21.5.4 [ratio.arithmetic]

> > > Implementations may use other algorithms to compute these values. If overflow occurs, a diagnostic shall be issued.

> > >     template <class R1, class R2> struct ratio_add {
> > >       typedef see below type;
> > >     };

> > > > The nested typedef `type` shall be a synonym for `ratio<T1, T2>::type` where `T1` has the value `R1::num * R2::den + R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

> > >     template <class R1, class R2> struct ratio_subtract {
> > >       typedef see below type;
> > >     };

> > > > The nested typedef `type` shall be a synonym for `ratio<T1, T2>::type` where `T1` has the value `R1::num * R2::den - R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

> > >     template <class R1, class R2> struct ratio_multiply {
> > >       typedef see below type;
> > >     };

> > > > The nested typedef `type` shall be a synonym for `ratio<T1, T2>::type` where `T1` has the value `R1::num * R2::num` and `T2` has the value `R1::den * R2::den`.

> > >     template <class R1, class R2> struct ratio_divide {
> > >       typedef see below type;
> > >     };

> > > > The nested typedef `type` shall be a synonym for `ratio<T1, T2>::type` where `T1` has the value `R1::num * R2::den` and `T2` has the value `R1::den * R2::num`.

[ 2010-03-27 Howard adds: ]

> Daniel brought to my attention the recent addition of the typedef `type` to the FCD N3092:

> >     typedef ratio type;

> This issue was discussed in Pittsburgh, and the decision there was to accept the typedef as proposed and move to Review. Unfortunately the issue was accidently applied to the FCD, and incorrectly. The FCD version of the typedef refers to `ratio<N, D>`, but the typedef is intended to refer to `ratio<num, den>` which in general is not the same type.

> I've updated the wording to diff against N3092.

[Batavia: NAD EditorialResolved - see rationale below]

Rationale:

Already fixed in working draft

Proposed resolution:

Add to 21.5.3 [ratio.ratio] synopsis

>     template <intmax_t N, intmax_t D = 1>
>     class ratio {
>     public:
>       static constexpr intmax_t num;
>       static constexpr intmax_t den;

>       typedef ratio<num, den> type;
>     };

### 1283(i). `MoveConstructible` and `MoveAssignable` need clarification of moved-from state

Section: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2009-12-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [utility.arg.requirements].

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Discussion:

Addresses UK 150

There is on going confusion over what one can and can not do with a moved-from object (e.g. UK 150, 910(i)). This issue attempts to clarify that moved-from objects are valid objects with an unknown state.

[ 2010-01-22 Wording tweaked by Beman. ]

[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-01-23 Alisdair opens: ]

> I'm afraid I must register an objection.

> My primary objection is that I have always been opposed to this kind of a resolution as over-constraining. My preferred example is a call implementing the pImpl idiom via `unique_ptr`. Once the pImpl has been moved from, it is no longer safe to call the vast majority of the object's methods, yet I see no reason to make such a type unusable in the standard library. I would prefer a resolution along the lines suggested in the UK comment, which only requires that the object can be safely destroyed, and serve as the target of an assignment operator (if supported.)

> However, I will not hold the issue up if I am a lone dissenting voice on this (yes, that is a call to hear more support, or I will drop that objection in Pittsburgh)

> With the proposed wording, I'm not clear what the term 'valid object' means. In my example above, is a pImpl holding a null pointer 'valid'? What about a float holding a signalling NaN? What determines if an object is valid? Without a definition of a valid/invalid object, I don't think this wording adds anything, and this is an objection that I do want resolved.

> [ 2010-01-24 Alisdair removes his objection. ]

> [ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Reopened. The wording here has been merged into 1309(i). ]

[ 2010-02-10 Moved to Tentatively NAD EditorialResolved after 5 postive votes on c++std-lib. Rationale added below. ]

Rationale:

This issue is now addressed by 1309(i).

Proposed resolution:

Change the follwing tables in 16.4.4.2 [utility.arg.requirements] as shown:

> Table 33 — `MoveConstructible` requirements [moveconstructible]
> Expression | Post-condition
> --- | ---
> `T t(rv)` | `t` is equivalent to the value of `rv` before the construction.
> [Note: There is no requirement on the value of `rv` after the construction. `rv` remains a valid object. Its state is unspecified. — end note]

> Table 35 — `MoveAssignable` requirements [moveassignable]
> Expression | Return type | Return value | Post-condition
> --- | --- | --- | ---
> `t = rv` | `T&` | `t` | `t` is equivalent to the value of `rv` before the assigment.
> [Note: There is no requirement on the value of `rv` after the assignment. `rv` remains a valid object. Its state is unspecified. — end note]

### 1284(i). `vector<bool> initializer_list` constructor missing an allocator argument

Section: 23.3.14 [vector.bool] Status: C++11 Submitter: Bo Persson Opened: 2009-12-09 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [vector.bool].

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Discussion:

The specialization for `vector<bool>` (23.3.14 [vector.bool]) has a constructor

>     vector(initializer_list<bool>);

which differs from the base template's constructor (and other containers) in that it has no `allocator` parameter.

[ 2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change the signature in the synopsis of 23.3.14 [vector.bool] to

>     vector(initializer_list<bool>, const Allocator& = Allocator());

### 1285(i). `allocator_traits` call to `new`

Section: 20.2.9.3 [allocator.traits.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-12-10 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [allocator.traits.members].

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Discussion:

LWG issue 402(i) added "`::`" to the call to `new` within `allocator::construct`. I suspect we want to retain that fix.

[ 2009-12-13 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]

Proposed resolution:

Change 16.4.4.6 [allocator.requirements], table 40 "Allocator requirements":

> Table 40 — Allocator requirements
> Expression | Return type | Assertion/note pre-/post-condition | Default
> --- | --- | --- | ---
> `a.construct(c,args)` | (not used) | Effect: Constructs an object of type `C` at `c` | `::new ((void*)c) C(forward<Args>(args)...)`

Change 20.2.9.3 [allocator.traits.members], p4:

>     template <class T, class... Args>
>       static void construct(Alloc& a, T* p, Args&&... args);

> > 4 Effects: calls `a.construct(p, std::forward<Args>(args)...)` if that call is well-formed; otherwise, invokes `::new (static_cast<void*>(p)) T(std::forward<Args>(args)...)`.

### 1286(i). `allocator_traits::select_on_container_copy_construction` type-o

Section: 20.2.9.3 [allocator.traits.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-12-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`allocator_traits::select_on_container_copy_construction` refers to an unknown "`a`":

>     static Alloc select_on_container_copy_construction(const Alloc& rhs);

> > 7 Returns: `rhs.select_on_container_copy_construction(a)` if that expression is well-formed; otherwise, `rhs`.

[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 20.2.9.3 [allocator.traits.members], p7:

>     static Alloc select_on_container_copy_construction(const Alloc& rhs);

> > 7 Returns: `rhs.select_on_container_copy_construction(a)` if that expression is well-formed; otherwise, `rhs`.

### 1287(i). `std::function` requires `CopyConstructible` target object

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I think `std::function` should require `CopyConstructible` for the target object.

I initially thought that `MoveConstructible` was enough, but it's not. If `F` is move-only then function's copy constructor cannot be called, but because function uses type erasure, `F` is not known and so the copy constructor cannot be disabled via `enable_if`. One option would be to throw an exception if you try to copy a function with a non-copyable target type, but I think that would be a terrible idea.

So although the constructors require that the target be initialised by `std::move(f)`, that's only an optimisation, and a copy constructor is required.

[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add to 22.10.17.3.2 [func.wrap.func.con] paragraph 9:

>     template<class F> function(F f);
>     template <class F, class A> function(allocator_arg_t, const A& a, F f);

> > 9 Requires: `F` shall be `CopyConstructible`. `f` shall be callable for argument types `ArgTypes` and return type `R`. The copy constructor and destructor of `A` shall not throw exceptions.

### 1288(i). `std::function` assignment from rvalues

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.wrap.func.con].

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Discussion:

In 22.10.17.3.2 [func.wrap.func.con]

>     template<class F> function& operator=(F f);

> > 20 Effects: `function(f).swap(*this);`

> > 21 Returns: `*this`

This assignment operator can be called such that `F` is an rvalue-reference e.g.

>     func.operator=<F&&>(f);

There are two issues with this.

1. the effects mean that `f` is passed as an lvalue and so there will be an unnecessary copy. The argument should be forwarded, so that the copy can be avoided.
2. It should not be necessary to use that syntax to pass an rvalue. As `F` is a deduced context it can be made to work with either lvalues or rvalues.

The same issues apply to `function::assign`.

N.B. this issue is not related to 1287(i) and applies whether that issue is resolved or not. The wording below assumes the resolution of LWG 1258(i) has been applied.

[ 2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 201002-11 Opened by Alisdair for the purpose of merging 1258(i) into this issue as there is a minor conflict. ]

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In 22.10.17.3.2 [func.wrap.func.con]

>     template<class F> function& operator=(F&& f);

> > 20 Effects: `function(std::forward<F>(f)).swap(*this);`

> > 21 Returns: `*this`

In 22.10.17.3.3 [func.wrap.func.mod]

>     template<class F, Allocator Allocclass A>
>       void assign(F&& f, const Alloc& a);

> > 3 Effects: `function(f, aallocator_arg, a, std::forward<F>(f)).swap(*this);`

Update member function signature for class template in 22.10.17.3 [func.wrap.func]

>     template<class F> function& operator=(F&&);

>     template<class F, class A> void assign(F&&, const A&);

### 1290(i). Don't require `[u|bi]nary_function` inheritance

Section: 22.10 [function.objects] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-14 Last modified: 2025-03-13

Priority: Not Prioritized

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Discussion:

This issue is a follow-up of the discussion on issue 870(i) during the 2009 Santa Cruz meeting.

The class templates `unary_function` and `binary_function` are actually very simple typedef providers,

>     namespace std {

>     template <class Arg, class Result>
>     struct unary_function {
>      typedef Arg argument_type;
>      typedef Result result_type;
>     };

>     template <class Arg1, class Arg2, class Result>
>     struct binary_function {
>      typedef Arg1 first_argument_type;
>      typedef Arg2 second_argument_type;
>      typedef Result result_type;
>     };

>     }

which may be used as base classes (similarly to the iterator template), but were originally not intended as a customization point. The SGI documentation introduced the concept Adaptable Unary Function as function objects "with nested typedefs that define its argument type and result type" and a similar definition for Adaptable Binary Function related to `binary_function`. But as of TR1 a protocol was introduced that relies on inheritance relations based on these types. 22.10.6 [refwrap]/3 b. 3 requires that a specialization of `reference_wrapper<T>` shall derive from `unary_function`, if type `T` is "a class type that is derived from `std::unary_function<T1, R>`" and a similar inheritance-based rule for `binary_function` exists as well.

As another disadvantage it has been pointed out in the TR1 issue list, N1837 (see section 10.39), that the requirements of `mem_fn` 22.10.16 [func.memfn]/2+3 to derive from `std::unary_function/std::binary_function` under circumstances, where the provision of corresponding typedefs would be sufficient, unnecessarily prevent implementations that take advantage of empty-base-class optimizations.

Both requirements should be relaxed in the sense that the `reference_wrapper` should provide typedef's `argument_type`, `first_argument_type`, and `second_argument_type` based on similar rules as the weak result type rule (22.10.4 [func.require]/3) does specify the presence of `result_type` member types.

For a related issue see also 1279(i).

[ 2010-10-24 Daniel adds: ]

> Accepting n3145 would resolve this issue as NAD editorial.

[ 2010-11 Batavia: Solved by N3198 ]

Resolved by adopting n3198.

Previous proposed resolution:

[ The here proposed resolution is an attempt to realize the common denominator of the reflector threads c++std-lib-26011, c++std-lib-26095, and c++std-lib-26124. ]

> 1. Change [base]/1 as indicated: [The intend is to provide an alternative fix for issue 1279(i) and some editorial harmonization with existing wording in the library, like 99 [iterator.basic]/1]

>    > 1 The following class templates are provided to simplify the definition of typedefs of the argument and result types for function objects. The behavior of a program that adds specializations for any of these templates is undefined.:

>    > >     namespace std {
>    > >      template <class Arg, class Result>
>    > >      struct unary_function {
>    > >        typedef Arg argument_type;
>    > >        typedef Result result_type;
>    > >      };
>    > >     }

>    > >     namespace std {
>    > >      template <class Arg1, class Arg2, class Result>
>    > >      struct binary_function {
>    > >        typedef Arg1 first_argument_type;
>    > >        typedef Arg2 second_argument_type;
>    > >        typedef Result result_type;
>    > >      };
>    > >     }

> 2. Change 22.10.6 [refwrap], class template `reference_wrapper` synopsis as indicated: [The intent is to remove the requirement that `reference_wrapper` derives from `unary_function` or `binary_function` if the situation requires the definition of the typedefs `argument_type`, `first_argument_type`, or `second_argument_type`. This change is suggested, because the new way of definition uses the same strategy as the weak result type specification applied to argument types, which provides the following advantages: It creates less potential conflicts between `[u|bi]nary_function` bases and typedefs in a function object and it ensures that user-defined function objects which provide typedefs but no such bases are handled as first class citizens.]

>    >     namespace std {
>    >      template <class T> class reference_wrapper
>    >        : public unary_function<T1, R> // see below
>    >        : public binary_function<T1, T2, R> // see below
>    >      {
>    >      public :
>    >        // types
>    >        typedef T type;
>    >        typedef see below result_type; // not always defined
>    >        typedef see below argument_type; // not always defined
>    >        typedef see below first_argument_type; // not always defined
>    >        typedef see below second_argument_type; // not always defined

>    >        // construct/copy/destroy
>    >        ...
>    >      };

> 3. Change 22.10.6 [refwrap]/3 as indicated: [The intent is to remove the requirement that `reference_wrapper` derives from `unary_function` if the situation requires the definition of the typedef `argument_type` and `result_type`. Note that this clause does concentrate on `argument_type` alone, because the `result_type` is already ruled by p. 2 via the weak result type specification. The new way of specifying `argument_type` is equivalent to the weak result type specification]

>    > 3 The template instantiation `reference_wrapper<T>` shall be derived from `std::unary_function<T1, R>`define a nested type named `argument_type` as a synonym for `T1` only if the type `T` is any of the following:

>    > - a function type or a pointer to function type taking one argument of type `T1` and returning `R`
>    > - a pointer to member function `R T0::f` cv (where cv represents the member function's cv-qualifiers); the type `T1` is cv `T0*`
>    > - a class type that is derived from `std::unary_function<T1, R>`with a member type `argument_type`; the type `T1` is `T::argument_type`

> 4. Change 22.10.6 [refwrap]/4 as indicated: [The intent is to remove the requirement that `reference_wrapper` derives from `binary_function` if the situation requires the definition of the typedef `first_argument_type`, `second_argument_type`, and `result_type`. Note that this clause does concentrate on `first_argument_type` and `second_argument_type` alone, because the `result_type` is already ruled by p. 2 via the weak result type specification. The new way of specifying `first_argument_type` and `second_argument_type` is equivalent to the weak result type specification]

>    > The template instantiation `reference_wrapper<T>` shall be derived from `std::binary_function<T1, T2, R>`define two nested types named `first_argument_type` and `second_argument_type` as a synonym for `T1` and `T2`, respectively, only if the type `T` is any of the following:

>    > - a function type or a pointer to function type taking two arguments of types `T1` and `T2` and returning `R`
>    > - a pointer to member function `R T0::f(T2)` cv (where cv represents the member function's cv-qualifiers); the type `T1` is cv `T0*`
>    > - a class type that is derived from `std::binary_function<T1, T2, R>`with member types `first_argument_type` and `second_argument_type`; the type `T1` is `T::first_argument_type` and the type `T2` is `T::second_argument_type`

> 5. Change 22.10.16 [func.memfn]/2+3 as indicated: [The intent is to remove the requirement that mem_fn's return type has to derive from `[u|bi]nary_function`. The reason for suggesting the change here is to better support empty-base-class optimization choices as has been pointed out in N1837]

>    > 2 The simple call wrapper shall be derived from `std::unary_function<cv T*, Ret>`define two nested types named `argument_type` and `result_type` as a synonym for `cv T*` and `Ret`, respectively, when `pm` is a pointer to member function with cv-qualifier cv and taking no arguments, where `Ret` is `pm`'s return type.

>    > 3 The simple call wrapper shall be derived from `std::binary_function<cv T*, T1, Ret>`define three nested types named `first_argument_type`, `second_argument_type`, and `result_type` as a synonym for `cv T*`, `T1`, and `Ret`, respectively, when `pm` is a pointer to member function with cv-qualifier cv and taking one argument of type `T1`, where `Ret` is `pm`'s return type.

Proposed resolution:

Addressed by paper n3198.

### 1291(i). Exceptions thrown during `promise::set_value`

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-12-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.promise].

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Discussion:

In 32.10.6 [futures.promise]

Does `promise<R>::set_value` return normally if the copy/move constructor of `R` throws?

The exception could be caught and set using `promise<R>::set_exception`, or it could be allowed to leave the `set_value` call, but it's not clear which is intended. I suggest the exception should not be caught.

N.B. This doesn't apply to `promise<R&>::set_value` or `promise<void>::set_value` because they don't construct a new object.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Change 32.10.6 [futures.promise]/18:

> 18 Throws: `future_error` if its associated state is already ready or, for the first version an exception thrown by the copy constructor of `R`, or for the second version an exception thrown by the move constructor of `R`.

### 1292(i). `std::function` should support all callable types

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Some parts of the specification of `std::function` is unnecessarily restricted to a subset of all callable types (as defined in 22.10.3 [func.def]/3), even though the intent clearly is to be usable for all of them as described in 22.10.17.3 [func.wrap.func]/1. This argument becomes strengthened by the fact that current C++0x-compatible compilers work fine with them:

>     #include <functional>
>     #include <iostream>

>     struct A
>     {
>       int foo(int i) const {return i+1;}
>     };

>     struct B
>     {
>       int mem;
>     };

>     int main()
>     {
>       std::function<int(const A&, int)> f(&A::foo);
>       A a;
>       std::cout << f(a, 1) << '\n';
>       std::cout << f.target_type().name() << '\n';
>       typedef int (A::* target_t)(int) const;
>       target_t* p = f.target<target_t>();
>       std::cout << (p != 0) << '\n';
>       std::function<int(B&)> f2(&B::mem);
>       B b = { 42 };
>       std::cout << f2(b) << '\n';
>       std::cout << f2.target_type().name() << '\n';
>       typedef int (B::* target2_t);
>       target2_t* p2 = f2.target<target2_t>();
>       std::cout << (p2 != 0) << '\n';
>     }

The problematic passages are 22.10.17.3.2 [func.wrap.func.con]/10:

>     template<class F> function(F f);
>     template <class F, class A> function(allocator_arg_t, const A& a, F f);

> > ...

> > 10 Postconditions: `!*this` if any of the following hold:

> > - `f` is a `NULL` function pointer.
> > - `f` is a `NULL` member function pointer.
> > - `F` is an instance of the function class template, and `!f`

because it does not consider pointer to data member and all constraints based on function objects which like 22.10.17.3 [func.wrap.func]/2 or 22.10.17.3.6 [func.wrap.func.targ]/3. The latter two will be resolved by the proposed resolution of 870(i) and are therefore not handled here.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 22.10.17.3.2 [func.wrap.func.con]/10+11 as indicated:

>     template<class F> function(F f);
>     template <class F, class A> function(allocator_arg_t, const A& a, F f);

> > ...

> > 10 Postconditions: `!*this` if any of the following hold:

> > - `f` is a `NULL` function pointer.
> > - `f` is a `NULL` pointer to member function pointer.
> > - `F` is an instance of the function class template, and `!f`

> > 11 Otherwise, `*this` targets a copy of `f` or, initialized with `std::move(f)` if `f` is not a pointer to member function, and targets a copy of `mem_fn(f)` if `f` is a pointer to member function. [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small function objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

### 1293(i). `unique_ptr<T[], D>` needs to get rid of unspecified-pointer-type

Section: 20.3.1.4 [unique.ptr.runtime] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 211

As a response to UK 211 LWG issue 1021(i) has replaced the unspecified-pointer-type by `nullptr_t` to allow assignment of type-safe null-pointer literals in the non-array form of `unique_ptr::operator=`, but did not the same for the specialization for arrays of runtime length. But without this parallel change of the signature we have a status quo, where `unique_ptr<T[], D>` declares a member function which is completely unspecified.

[ 2009-12-21 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

In 20.3.1.4 [unique.ptr.runtime], class template `unique_ptr<T[], D>` synopsis, change as indicated:

>     // assignment
>     unique_ptr& operator=(unique_ptr&& u);
>     unique_ptr& operator=(unspecified-pointer-typenullptr_t);

### 1294(i). Difference between callable wrapper and forwarding call wrapper unclear

Section: 22.10.4 [func.require] Status: C++11 Submitter: Jens Maurer Opened: 2009-12-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current wording in the standard makes it hard to discriminate the difference between a "call wrapper" as defined in 22.10.3 [func.def]/5+6:

> 5 A call wrapper type is a type that holds a callable object and supports a call operation that forwards to that object.

> 6 A call wrapper is an object of a call wrapper type.

and a "forwarding call wrapper" as defined in 22.10.4 [func.require]/4:

> 4 [..] A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

> >     template<class... ArgTypes>
> >     R operator()(ArgTypes&&... args) cv-qual;

> — end note]

Reason for this lack of clear difference seems to be that the wording adaption to variadics and rvalues that were applied after it's original proposal in N1673:

> [..] A forwarding call wrapper is a call wrapper that can be called with an argument list `t1, t2, ..., tN` where each `ti` is an lvalue. The effect of calling a forwarding call wrapper with one or more arguments that are rvalues is implementation defined. [Note: in a typical implementation forwarding call wrappers have overloaded function call operators of the form

> >     template<class T1, class T2, ..., class TN>
> >     R operator()(T1& t1, T2& t2, ..., TN& tN) cv-qual;

> — end note]

combined with the fact that the word "forward" has two different meanings in this context. This issue attempts to clarify the difference better.

[ 2010-09-14 Daniel provides improved wording and verified that it is correct against N3126. Previous resolution is shown here: ]

> 4 [..] A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and uses perfect forwarding to deliver the arguments to the wrapped callable object. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

> >     template<class... ArgTypes>
> >     R operator()(ArgTypes&&... args) cv-qual;

> — end note]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 22.10.4 [func.require]/4 as indicated:

> [..] A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments as references to the wrapped callable object. This forwarding step shall ensure that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

> >     template<class... UnBoundArgs>
> >     R operator()(UnBoundArgs&&... unbound_args) cv-qual;

> — end note ]

### 1295(i). Contradictory call wrapper requirements

Section: 22.10.4 [func.require] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-22 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [func.require].

View all other issues in [func.require].

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Discussion:

22.10.4 [func.require]/3 b 1 says

> 3 If a call wrapper (22.10.3 [func.def]) has a weak result type the type of its member type `result_type` is based on the type `T` of the wrapper's target object (22.10.3 [func.def]):

> - if `T` is a function, reference to function, or pointer to function type, `result_type` shall be a synonym for the return type of `T`;
> - [..]

The first two enumerated types (function and reference to function) can never be valid types for `T`, because

22.10.3 [func.def]/7

> 7 A target object is the callable object held by a call wrapper.

and 22.10.3 [func.def]/3

> 3 A callable type is a pointer to function, a pointer to member function, a pointer to member data, or a class type whose objects can appear immediately to the left of a function call operator.

exclude functions and references to function as "target objects".

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 22.10.4 [func.require]/3 b 1 as indicated:

> 3 If a call wrapper (22.10.3 [func.def]) has a weak result type the type of its member type `result_type` is based on the type `T` of the wrapper's target object (22.10.3 [func.def]):

> - if `T` is a function, reference to function, or pointer to function type, `result_type` shall be a synonym for the return type of `T`;
> - [..]

### 1297(i). `unique_ptr`'s relational operator functions should induce a total order

Section: 20.3.1.6 [unique.ptr.special] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.special].

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Discussion:

The comparison functions of `unique_ptr` currently directly delegate to the underlying comparison functions of `unique_ptr<T, D>::pointer`. This is disadvantageous, because this would not guarantee to induce a total ordering for native pointers and it is hard to define a total order for mixed types anyway.

The currently suggested resolution for `shared_ptr` comparison as of 1262(i) uses a normalization strategy: They perform the comparison on the composite pointer type (7.6.9 [expr.rel]). This is not exactly possible for `unique_ptr` in the presence of user-defined pointer-like types but the existing definition of `std::duration` comparison as of 30.5.7 [time.duration.comparisons] via `common_type` of both argument types demonstrates a solution of this problem. The approach can be seen as the general way to define a composite pointer type and this is the approach which is used for here suggested wording change.

For consistency reasons I would have preferred the same normalization strategy for `==` and `!=`, but Howard convinced me not to do so (now).

[ 2010-11-03 Daniel comments and adjustes the currently proposed wording changes: ]

Issue 1401(i) is remotely related. Bullet A of its proposed resolution provides an alternative solution for issue discussed here and addresses NB comment GB-99. Additionally I updated the below suggested wording in regard to the following: It is an unncessary requirement that the below defined effective composite pointer-like type `CT` satisfies the `LessThanComparable` requirements. All what is needed is, that the function object type `less<CT>` induces a strict weak ordering on the pointer values.

[2011-03-24 Madrid meeting]

Resolved by 1401(i)

Proposed resolution:

Change 20.3.1.6 [unique.ptr.special]/4-7 as indicated: [The implicit requirements and remarks imposed on the last three operators are the same as for the first one due to the normative "equivalent to" usage within a Requires element, see 16.3.2.4 [structure.specifications]/4. The effects of this change are that all real pointers wrapped in a `unique_ptr` will order like `shared_ptr` does.]

>     template <class T1, class D1, class T2, class D2>
>       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > ? Requires: Let `CT` be `common_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type`. Then the specialization `less<CT>` shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

> > 4 Returns: `less<CT>()(x.get(), y.get())x.get() < y.get()`.

> > ? Remarks: If `unique_ptr<T1, D1>::pointer` is not implicitly convertible to `CT` or `unique_ptr<T2, D2>::pointer` is not implicitly convertible to `CT`, the program is ill-formed.

>     template <class T1, class D1, class T2, class D2>
>       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > 5 Effects: Equivalent to `return !(y < x)` Returns: `x.get() <= y.get()`.

>     template <class T1, class D1, class T2, class D2>
>       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > 6 Effects: Equivalent to `return (y < x)` Returns: `x.get() > y.get()`.

>     template <class T1, class D1, class T2, class D2>
>       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > 7 Effects: Equivalent to `return !(x < y)` Returns: `x.get() >= y.get()`.

### 1298(i). Missing specialization of `ctype_byname<char>`

Section: 28.3.2 [locale.syn] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-12-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

The `<locale>` synopsis in 28.3.2 [locale.syn] calls out an explicit specialization for `ctype_byname<char>`, however no such specialization is defined in the standard. The only reference I can find to `ctype_byname<char>` is 28.3.3.1.2.2 [locale.facet]:Table 77 — Required specializations (for facets) which also refers to `ctype_byname<wchar_t>` which has no special consideration.

Is the intent an explicit instantiation which would use a slightly different syntax? Should the explicit specialization simply be struck?

[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

28.3.2 [locale.syn]

> Strike the explicit specialization for `ctype_byname<char>` from the `<locale>` synopsis

> >     ...
> >     template <class charT> class ctype_byname;
> >     template <>            class ctype_byname<char>;  // specialization
> >     ...

### 1299(i). Confusing typo in specification for `get_time`

Section: 31.7.8 [ext.manip] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-12-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ext.manip].

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Discussion:

Extended Manipulators 31.7.8 [ext.manip] p8 defines the semantics of `get_time` in terms of a function `f`.

>     template <class charT, class traits>
>     void f(basic_ios<charT, traits>& str, struct tm* tmb, const charT* fmt) {
>        typedef istreambuf_iterator<charT, traits> Iter;
>        typedef time_get<charT, Iter> TimeGet;

>        ios_base::iostate err = ios_base::goodbit;
>        const TimeGet& tg = use_facet<TimeGet>(str.getloc());

>        tm.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt));

>        if (err != ios_base::goodbit)
>            str.setstate(err):
>     }

Note the call to `tm.get`. This is clearly an error, as `tm` is a type and not an object. I believe this should be `tg.get`, rather than `tm`, but this is not my area of expertise.

[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 31.7.8 [ext.manip] p8:

>     template <class charT, class traits>
>     void f(basic_ios<charT, traits>& str, struct tm* tmb, const charT* fmt) {
>        typedef istreambuf_iterator<charT, traits> Iter;
>        typedef time_get<charT, Iter> TimeGet;

>        ios_base::iostate err = ios_base::goodbit;
>        const TimeGet& tg = use_facet<TimeGet>(str.getloc());

>        tgm.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt));

>        if (err != ios_base::goodbit)
>            str.setstate(err):
>     }

### 1300(i). Circular definition of `promise::swap`

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-12-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.promise].

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Discussion:

32.10.6 [futures.promise]/12 defines the effects of `promise::swap(promise&)` as

>     void swap(promise& other);

> > 12 Effects: `swap(*this, other)`

and 32.10.6 [futures.promise]/25 defines `swap(promise<R&>, promise<R>&)` as

>     template <class R>
>       void swap(promise<R>& x, promise<R>& y);

> > 25 Effects: `x.swap(y)`.

[ 2010-01-13 Daniel added "Throws: Nothing." ]

[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

Change 32.10.6 [futures.promise] paragraph 12

>     void swap(promise& other);

> > 12 Effects: `swap(*this, other)` Exchanges the associated states of `*this` and `other`.

> > 13 ...

> > Throws: Nothing.

### 1303(i). `shared_ptr`, `unique_ptr`, and rvalue references v2

Section: 20.3.1.3 [unique.ptr.single], 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2010-01-23 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [unique.ptr.single].

View all other issues in [unique.ptr.single].

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Discussion:

N3000 20.3.2.2 [util.smartptr.shared]/1 still says:

>     template <class Y, class D> explicit shared_ptr(const unique_ptr<Y, D>& r) = delete;
>     template <class Y, class D> shared_ptr& operator=(const unique_ptr<Y, D>& r) = delete;

I believe that this is unnecessary now that "rvalue references v2" prevents rvalue references from binding to lvalues, and I didn't see a Library Issue tracking this.

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Strike from 20.3.1.3 [unique.ptr.single]:

>     template <class T, class D = default_delete<T>> class unique_ptr {
>       ...
>       unique_ptr(const unique_ptr&) = delete;
>       template <class U, class E> unique_ptr(const unique_ptr<U, E>&) = delete;
>       unique_ptr& operator=(const unique_ptr&) = delete;
>       template <class U, class E> unique_ptr& operator=(const unique_ptr<U, E>&) = delete;
>     };

Strike from 20.3.2.2 [util.smartptr.shared]:

>     template<class T> class shared_ptr {
>       ...
>       template <class Y, class D> explicit shared_ptr(const unique_ptr<Y, D>& r) = delete;
>       ...
>       template <class Y, class D> shared_ptr& operator=(const unique_ptr<Y, D>& r) = delete;
>       ...
>     };

### 1304(i). Missing preconditions for `shared_future`

Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [futures.shared.future].

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Discussion:

The revised futures package in the current working paper simplified the `is_ready/has_exception/has_value` set of APIs, replacing them with a single 'valid' method. This method is used in many places to signal pre- and post- conditions, but that edit is not complete. Each method on a `shared_future` that requires an associated state should have a pre-condition that `valid() == true`.

[ 2010-01-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD EditorialResolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

Insert the following extra paragraphs:

In [futures.shared_future]

>     shared_future();

> > 4 Effects: constructs ...

> > Postcondition: `valid() == false`.

> > Throws: nothing.

>     void wait() const;

> > Requires: `valid() == true`.

> > 22 Effects: if the associated ...

>     template <class Rep, class Period>
>       bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;

> > Requires: `valid() == true`.

> > 23 Effects: if the associated ...

>     template <class Clock, class Duration>
>       bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

> > Requires: `valid() == true`.

> > 25 Effects: blocks until ...

### 1305(i). preconditions for `atomic_future`

Section: 99 [futures.atomic_future] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.atomic_future].

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Discussion:

The revised futures package in the current working paper simplified the `is_ready/has_exception/has_value` set of APIs, replacing them with a single 'valid' method. This method is used in many places to signal pre- and post- conditions, but that edit is not complete.

Atomic future retains the extended earlier API, and provides defined, synchronized behaviour for all calls. However, some preconditions and throws clauses are missing, which can easily be built around the new `valid()` api. Note that for consistency, I suggest `is_ready/has_exception/has_value throw` an exception if `valid()` is not `true`, rather than return `false`. I think this is implied by the existing pre-condition on `is_ready`.

[ 2010-01-23 See discussion starting with Message c++std-lib-26666. ]

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Insert the following extra paragraphs:

In 99 [futures.atomic_future]

>     bool is_ready() const;

> > 17 Precondition Requires: `valid() == true`.

> > 18 Returns: `true` only if the associated state is ready.

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     bool has_exception() const;

> > Requires: `valid() == true`.

> > 19 Returns: `true` only if the associated state is ready and contains an exception.

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     bool has_value() const;

> > Requires: `valid() == true`.

> > 20 Returns: `true` only if the associated state is ready and contains a value.

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     void wait() const;

> > Requires: `valid() == true`.

> > 22 Effects: blocks until ...

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     template <class Rep, class Period>
>       bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;

> > Requires: `valid() == true`.

> > 23 Effects: blocks until ...

> > 24 Returns: `true` only if ...

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     template <class Clock, class Duration>
>       bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

> > Requires: `valid() == true`.

> > 25 Effects: blocks until ...

> > 26 Returns: `true` only if ...

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

### 1306(i). `pointer` and `const_pointer` for `<array>`

Section: 23.3.3 [array] Status: C++11 Submitter: Nicolai Josuttis Opened: 2010-01-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [array].

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Discussion:

Class `<array>` is the only sequence container class that has no types `pointer` and `const_pointer` defined. You might argue that this makes no sense because there is no allocator support, but on the other hand, types `reference` and `const_reference` are defined for `array`.

[ 2010-02-11 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Add to Class template array 23.3.3 [array]:

>     namespace std {
>       template <class T, size_t N >
>       struct array {
>         ...
>         typedef T value_type;
>         typedef T * pointer;
>         typedef const T * const_pointer;
>         ...
>       };
>     }

### 1307(i). `exception_ptr` and `allocator` pointers don't understand !=

Section: 17.9.7 [propagation] Status: Resolved Submitter: Daniel Krügler Opened: 2010-01-26 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [propagation].

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Discussion:

The current requirements for a conforming implementation of `std::exception_ptr` (17.9.7 [propagation]/1-6) does not clarify whether the expression

>     e1 != e2
>     e1 != nullptr

with `e1` and `e2` being two values of type `std::exception_ptr` are supported or not. Reason for this oddity is that the concept `EqualityComparable` does not provide operator `!=`.

For the same reason programmers working against the types `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` of any allocator concept `X` (16.4.4.6 [allocator.requirements]/4 + Table 40) in a generic context can not rely on the availability of the != operation, which is rather unnatural and error-prone.

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

Solved by N3073.

Proposed resolution:

### 1309(i). Missing expressions for `Move/CopyConstructible`

Section: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2010-02-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [utility.arg.requirements].

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Discussion:

Table 33 — MoveConstructible requirements [moveconstructible] and Table 34 — CopyConstructible requirements [copyconstructible] support solely the following expression:

>     T t(rv)

where `rv` is defined to be as "non-const rvalue of type `T`" and `t` as a "modifiable lvalue of type `T`" in 16.4.4.2 [utility.arg.requirements]/1.

This causes two different defects:

1. We cannot move/copy-initialize a const lvalue of type `T` as in:

   >     int get_i();

   >     const int i1(get_i());

   both in Table 33 and in Table 34.

2. The single support for

   >     T t(rv)

   in case of `CopyConstructible` means that we cannot provide an lvalue as a source of a copy as in

   >     const int& get_lri();

   >     int i2(get_lri());

I believe this second defect is due to the fact that this single expression supported both initialization situations according to the old (implicit) lvalue reference -> rvalue reference conversion rules.

Finally [copyconstructible] refers to some name `u` which is not part of the expression, and both [copyconstructible] and [moveconstructible] should support construction expressions from temporaries - this would be a stylistic consequence in the light of the new `DefaultConstructible` requirements and compared with existing requirements (see e.g. Container requirements or the output/forward iterator requirements)..

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Reopened. The proposed wording of 1283(i) has been merged here. ]

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 16.4.4.2 [utility.arg.requirements]/1 as indicated: [This change suggestion is motivated to make type descriptions clearer: First, `a`, `b`, and `c` may also be non-`const T`. Second, `u` is described in a manner consistent with the container requirements tables.]

   > 1 The template definitions in the C++ standard library refer to various named requirements whose details are set out in tables 31-38. In these tables, `T` is an object or reference type to be supplied by a C++ program instantiating a template; `a`, `b`, and `c` are values of type (possibly `const) T`; `s` and `t` are modifiable lvalues of type `T`; `u` denotes an identifier; is a value of type (possibly `const`) `T`; and `rv` is an non-const rvalue of type `T`; and `v` is an lvalue of type (possibly `const`) `T` or an rvalue of type `const T`.

2. In 16.4.4.2 [utility.arg.requirements] Table 33 ([moveconstructible]) change as indicated [Note: The symbol `u` is defined to be either a const or a non-const value and is the right one we need here]:

   > Table 33 — `MoveConstructible` requirements [moveconstructible]
   > Expression | Post-condition
   > --- | ---
   > `T tu(rv);` | `tu` is equivalent to the value of `rv` before the construction
   > `T(rv)` | `T(rv)` is equivalent to the value of `rv` before the construction
   > [Note: There is no requirement on the value of `rv` after the construction. `rv` remains a valid object. Its state is unspecified. — end note]

3. In 16.4.4.2 [utility.arg.requirements] Table 34 ([copyconstructible]) change as indicated [Note: The symbol `u` is defined to be either a const or a non-const value and is the right one we need here. The expressions using `a` are recommended to ensure that lvalues are supported as sources of the copy expression]:

   > Table 34 — `CopyConstructible` requirements [copyconstructible]
   > (in addition to `MoveConstructible`)
   > Expression | Post-condition
   > --- | ---
   > `T tu(rv);` | the value of `uv` is unchanged and is equivalent to `tu`
   > `T(v)` | the value of `v` is unchanged and is equivalent to `T(v)`
   > [Note: A type that satisfies the `CopyConstructible` requirements also satisfies the `MoveConstructible` requirements. — end note]

4. In Table 35 — MoveAssignable requirements [moveassignable] change as indicated:

   > Table 35 — `MoveAssignable` requirements [moveassignable]
   > Expression | Return type | Return value | Post-condition
   > --- | --- | --- | ---
   > `t = rv` | `T&` | `t` | `t` is equivalent to the value of `rv` before the assigment.
   > [Note: There is no requirement on the value of `rv` after the assignment. `rv` remains a valid object. Its state is unspecified. — end note]

5. In 16.4.4.2 [utility.arg.requirements] change Table 36 as indicated:

   > Table 36 — `CopyAssignable` requirements [copyassignable]
   > (in addition to `MoveAssignable`)
   > Expression | Return type | Return value | Post-condition
   > --- | --- | --- | ---
   > `t = uv` | `T&` | `t` | `t` is equivalent to `uv`, the value of `uv` is unchanged
   > [Note: A type that satisfies the `CopyAssignable` requirements also satisfies the `MoveAssignable` requirements. — end note]

### 1310(i). `forward_list splice_after` from lvalues

Section: 23.3.7.6 [forward.list.ops] Status: C++11 Submitter: Howard Hinnant Opened: 2010-02-05 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [forward.list.ops].

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Discussion:

We've moved 1133(i) to Tentatively Ready and I'm fine with that.

1133(i) adds lvalue-references to the `splice` signatures for `list`. So now `list` can `splice` from lvalue and rvalue lists (which was the intent of the original move papers btw). During the discussion of this issue it was mentioned that if we want to give the same treatment to `forward_list`, that should be a separate issue.

This is that separate issue.

Consider the following case where you want to splice elements from one place in a `forward_list` to another. Currently this must be coded like so:

>     fl.splice_after(to_here, std::move(fl), from1, from2);

This looks pretty shocking to me. I would expect to be able to code instead:

>     fl.splice_after(to_here, fl, from1, from2);

but we currently don't allow it.

When I say `move(fl)`, I consider that as saying that I don't care about the value of `fl` any more (until I assign it a new value). But in the above example, this simply isn't true. I do care about the value of `fl` after the move, and I'm not assigning it a new value. I'm merely permuting its current value.

I propose adding `forward_list&` overloads to the 3 `splice_after` members. For consistency's sake (principal of least surprise) I'm also proposing to overload `merge` this way as well.

Proposed resolution:

Add to the synopsis of [forwardlist.overview]:

>     template <class T, class Allocator = allocator<T> >
>     class forward_list {
>     public:
>       ...
>       // [forwardlist.ops], forward_list operations:
>       void splice_after(const_iterator p, forward_list& x);
>       void splice_after(const_iterator p, forward_list&& x);
>       void splice_after(const_iterator p, forward_list& x, const_iterator i);
>       void splice_after(const_iterator p, forward_list&& x, const_iterator i);
>       void splice_after(const_iterator p, forward_list& x,
>                         const_iterator first, const_iterator last);
>       void splice_after(const_iterator p, forward_list&& x,
>                         const_iterator first, const_iterator last);
>       ...
>       void merge(forward_list& x);
>       void merge(forward_list&& x);
>       template <class Compare> void merge(forward_list& x, Compare comp);
>       template <class Compare> void merge(forward_list&& x, Compare comp);
>       ...
>     };

Add to the signatures of [forwardlist.ops]:

>     void splice_after(const_iterator p, forward_list& x);
>     void splice_after(const_iterator p, forward_list&& x);

> > 1 ...

>     void splice_after(const_iterator p, forward_list& x, const_iterator i);
>     void splice_after(const_iterator p, forward_list&& x, const_iterator i);

> > 4 ...

>     void splice_after(const_iterator p, forward_list& x,
>                     const_iterator first, const_iterator last);
>     void splice_after(const_iterator p, forward_list&& x,
>                     const_iterator first, const_iterator last);

> > 7 ...

>     void merge(forward_list& x);
>     void merge(forward_list&& x);
>     template <class Compare> void merge(forward_list& x, Compare comp);
>     template <class Compare> void merge(forward_list&& x, Compare comp);

> > 16 ...

### 1311(i). multi-pass property of Forward Iterator underspecified

Section: 24.3.5.5 [forward.iterators] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [forward.iterators].

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Discussion:

The following example demonstrates code that would meet the guarantees of a Forward Iterator, but only permits a single traversal of the underlying sequence:

>     template< typename ForwardIterator>
>     struct bad_iterator {
>       shared_ptr<ForwardIterator> impl;

>       bad_iterator( ForwardIterator iter ) {
>          : impl{new ForwardIterator{iter} }
>          {
>       }

>       auto operator*() const -> decltype(*ForwardIterator{}) {
>          return **impl;
>       }

>       auto operator->() const -> ForwardIterator {
>          return *impl;
>       }

>       auto operator==(bad_iterator const & rhs) const -> bool {
>          return impl == rhs.impl;
>       }

>       auto operator++() -> bad_iterator& {
>          ++(*impl);
>          return *this;
>       }
>       // other operations as necessary...
>     };

Here, we use `shared_ptr` to wrap a forward iterator, so all iterators constructed from the same original iterator share the same 'value', and incrementing any one copy increments all others.

There is a missing guarantee, expressed by the following code sequence

>     FwdIter x = seq.begin();  // obtain forward iterator from a sequence
>     FwdIter y = x;            // copy the iterator
>     assert(x == y);           // iterators must be the same
>     ++x;                      // increment *just one* iterator
>     assert(x != y);           // iterators *must now be different*
>     ++y;                      // increment the other iterator
>     assert(x == y);           // now the iterators must be the same again

That inequality in the middle is an essential guarantee. Note that this list is simplified, as each assertion should also note that they refer to exactly the same element `(&*x == &*y)` but I am not complicating the issue with tests to support proxy iterators, or value types overloading unary `operator+`.

I have not yet found a perverse example that can meet this additional constraint, and not meet the multi-pass expectations of a Forward Iterator without also violating other Forward Iterator requirements.

Note that I do not yet have standard-ready wording to resolve the problem, as saying this neatly and succinctly in 'standardese' is more difficult.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

### 1312(i). `vector::data` no longer returns a raw pointer

Section: 23.3.13.4 [vector.data] Status: C++11 Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [vector.data].

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Discussion:

The original intent of `vector::data` was to match `array::data` in providing a simple API with direct access to the contiguous buffer of elements that could be passed to a "classic" C API. At some point, the return type became the '`pointer`' typedef, which is not derived from the `allocator` via allocator traits - it is no longer specified to precisely `T *`. The return type of this function should be corrected to no longer use the typedef.

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

23.3.13 [vector]

Update the class definition in p2:

>     // 23.3.6.3 data access
>     pointerT * data();
>     const_pointerconst T * data() const;

23.3.13.4 [vector.data]

Adjust signatures:

>     pointerT * data();
>     const_pointerconst T * data() const;

### 1316(i). `scoped_allocator_adaptor operator==` has no definition

Section: 20.6 [allocator.adaptor] Status: C++11 Submitter: Pablo Halpern Opened: 2009-02-11 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [allocator.adaptor].

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Discussion:

The WP (N3000) contains these declarations:

>     template <class OuterA1, class OuterA2, class... InnerAllocs>
>       bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
>                       const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);
>     template <class OuterA1, class OuterA2, class... InnerAllocs>
>       bool operator!=(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
>                       const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);

But does not define what the behavior of these operators are.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Add a new section after 20.6.4 [allocator.adaptor.members]:

> Scoped allocator operators [scoped.adaptor.operators]

>     template <class OuterA1, class OuterA2, class... InnerAllocs>
>       bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
>                       const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);

> > Returns: `a.outer_allocator() == b.outer_allocator()` if `sizeof...(InnerAllocs)` is zero; otherwise, `a.outer_allocator() == b.outer_allocator() && a.inner_allocator() == b.inner_allocator()`.

>     template <class OuterA1, class OuterA2, class... InnerAllocs>
>       bool operator!=(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
>                       const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);

> > Returns: `!(a == b)`.

### 1319(i). Containers should require an iterator that is at least a Forward Iterator

Section: 23.2.2 [container.requirements.general] Status: C++11 Submitter: Alisdair Meredith Opened: 2010-02-16 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [container.requirements.general].

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Discussion:

The requirements on container iterators are spelled out in 23.2.2 [container.requirements.general], table 91.

> Table 91 — Container requirements
> Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | --- | ---
> `...`
> `X::iterator` | iterator type whose value type is `T` |  | any iterator category except output iterator. Convertible to `X::const_iterator`. | compile time
> `X::const_iterator` | constant iterator type whose value type is `T` |  | any iterator category except output iterator | compile time
> `...`

As input iterators do not have the multi-pass guarantee, they are not suitable for iterating over a container. For example, taking two calls to `begin()`, incrementing either iterator might invalidate the other. While data structures might be imagined where this behaviour produces interesting and useful results, it is very unlikely to meet the full set of requirements for a standard container.

[ Post-Rapperswil: ]

Daniel notes: I changed the currently suggested P/R slightly, because it is not robust in regard to new fundamental iterator catagories. I recommend to say instead that each container::iterator shall satisfy (and thus may refine) the forward iterator requirements.

> Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change Table 93 — Container requirements in [container.requirements.general] as indicated:

   > Table 93 — Container requirements
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | --- | ---
   > `...`
   > `X::iterator` | iterator type whose value type is `T` |  | any iterator category except output iterator that meets the forward iterator requirements. convertible to `X::const_iterator` | compile time
   > `X::const_iterator` | constant iterator type whose value type is `T` |  | any iterator category except output iterator that meets the forward iterator requirements. | compile time
   > `...`

### 1321(i). `scoped_allocator_adaptor construct` and `destroy` don't use `allocator_traits`

Section: 20.6.4 [allocator.adaptor.members] Status: Resolved Submitter: Howard Hinnant Opened: 2010-02-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [allocator.adaptor.members].

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Discussion:

20.6.4 [allocator.adaptor.members] p8-9 says:

>     template <class T, class... Args>
>       void construct(T* p, Args&&... args);

> > 8 Effects: let `OUTERMOST(x)` be `x` if `x` does not have an `outer_allocator()` function and `OUTERMOST(x.outer_allocator())` otherwise.

> > - If `uses_allocator<T, inner_allocator_type>::value` is `false` and `is_constructible<T, Args...>::value` is `true`, calls `OUTERMOST(*this).construct(p, std::forward<Args>(args)...)`.
> > - Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value` is `true`, calls `OUTERMOST(*this).construct(p, allocator_arg, inner_allocator(),std::forward<Args>(args)...)`.
> > - Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, Args..., inner_allocator_type>::value` is `true`, calls `OUTERMOST(*this).construct(p, std::forward<Args>(args)..., inner_allocator())`.
> > - Otherwise, the program is ill-formed. [Note: an error will result if `uses_allocator` evaluates to `true` but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]

>     template <class T>
>       void destroy(T* p);

> > 9 Effects: calls `outer_allocator().destroy(p)`.

In all other calls where applicable `scoped_allocator_adaptor` does not call members of an allocator directly, but rather does so indirectly via `allocator_traits`. For example:

>     size_type max_size() const;

> > 7 Returns: `allocator_traits<OuterAlloc>::max_size(outer_allocator())`.

Indeed, without the indirection through `allocator_traits` the definitions for `construct` and `destroy` are likely to fail at compile time since the `outer_allocator()` may not have the members `construct` and `destroy`.

[ The proposed wording is a product of Pablo, Daniel and Howard. ]

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

Solved by N3059.

Proposed resolution:

In 20.6.4 [allocator.adaptor.members] move and change p8 as indicated, and change p9 as indicated:

> Let `OUTERMOST(x)` be `x` if `x` does not have an `outer_allocator()` member function and `OUTERMOST(x.outer_allocator())` otherwise. Let `OUTERMOST_ALLOC_TRAITS(x)` be `allocator_traits<decltype(OUTERMOST(x))>`. [Note: `OUTERMOST(x)` and `OUTERMOST_ALLOC_TRAITS(x)` are recursive operations. It is incumbent upon the definition of `outer_allocator()` to ensure that the recursion terminates. It will terminate for all instantiations of `scoped_allocator_adaptor`. — end note]

>     template <class T, class... Args>
>       void construct(T* p, Args&&... args);

> > 8 Effects: let `OUTERMOST(x)` be `x` if `x` does not have an `outer_allocator()` function and `OUTERMOST(x.outer_allocator())` otherwise.

> > - If `uses_allocator<T, inner_allocator_type>::value` is `false` and `is_constructible<T, Args...>::value` is `true`, calls `OUTERMOST(*this). OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)... )`.
> > - Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value` is `true`, calls `OUTERMOST(*this). OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, allocator_arg, inner_allocator(), std::forward<Args>(args)... )`.
> > - Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, Args..., inner_allocator_type>::value` is `true`, calls `OUTERMOST(*this). OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)..., inner_allocator() )`.
> > - Otherwise, the program is ill-formed. [Note: an error will result if `uses_allocator` evaluates to `true` but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]

>     template <class T>
>       void destroy(T* p);

> > 9 Effects: calls `outer_allocator(). OUTERMOST_ALLOC_TRAITS(outer_allocator())::destroy( OUTERMOST(outer_allocator()), p)`.

### 1322(i). Explicit `CopyConstructible` requirements are insufficient

Section: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Daniel Krügler Opened: 2010-02-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [utility.arg.requirements].

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Discussion:

With the acceptance of library defect 822(i) only direct-initialization is supported, and not copy-initialization in the requirement sets `MoveConstructible` and `CopyConstructible`. This is usually a good thing, if only the library implementation needs to obey these restrictions, but the Empire strikes back quickly:

1. Affects user-code: `std::exception_ptr` is defined purely via requirements, among them `CopyConstructible`. A strict reading of the standard would make implementations conforming where `std::exception_ptr` has an explicit copy-c'tor and user-code must code defensively. This is a very unwanted effect for such an important component like `std::exception_ptr`.

2. Wrong re-use: Recently proposed requirement sets (`NullablePointer` as of N3025, Hash) or cleanup of existing requirement sets (e.g. iterator requirements as of N3046) tend to reuse existing requirement sets, so reusing `CopyConstructible` is attempting, even in cases, where the intend is to support copy-initialization as well.

3. Inconsistency: The current iterator requirements set Table 102 (output iterator requirements) and Table 103 (forward iterator requirements) demonstrate quite clearly a strong divergence of copy-semantics: The specified semantics of

   >     X u(a);
   >     X u = a;

   are underspecified compared to the most recent clarifications of the `CopyConstructible` requirements, c.f. issue 1309(i) which is very unsatisfactory. This will become worse for each further issue that involves the `CopyConstructible` specification (for possible directions see 1173(i)).

The suggested resolution is to define two further requirements `implicit-MoveConstructible` and `implicit-CopyConstructible` (or any other reasonable name like `MoveConvertible` and `CopyConvertible`) each with a very succinct but precise meaning solving all three problems mentioned above.

[Batavia: Resolved by accepting n3215.]

Proposed resolution:

1. Add the following new table ?? after Table 34 — `MoveConstructible` requirements [moveconstructible]:

   > Table ?? — `Implicit MoveConstructible` requirements [implicit.moveconstructible] (in addition to `MoveConstructible`)
   > Expression | Operational Semantics
   > --- | ---
   > `T u = rv;` | Equivalent to: `T u(rv);`

2. Add the following new table ?? after Table 35 — `CopyConstructible` requirements [copyconstructible]:

   > Table ?? — `Implicit CopyConstructible` requirements [implicit.copyconstructible] (in addition to `CopyConstructible`)
   > Expression | Operational Semantics
   > --- | ---
   > `T u = v;` | Equivalent to: `T u(v);`

3. Change 16.4.4.4 [nullablepointer.requirements]/1 as follows:

   > A `NullablePointer` type is a pointer-like type that supports null values. A type `P` meets the requirements of `NullablePointer` if:

   > - `P` satisfies the requirements of `EqualityComparable`, `DefaultConstructible`, `implicit CopyConstructible`, `CopyAssignable`, and `Destructible`,
   > - [..]

4. Change 16.4.4.5 [hash.requirements]/1 as indicated: [explicit copy-constructible functors could not be provided as arguments to any algorithm that takes these by value. Also a typo is fixed.]

   > 1 A type `H` meets the Hash requirements if:

   > - it is a function object type (20.8),
   > - it satisfiesifes the requirements of `implicit CopyConstructible` and `Destructible` (20.2.1),
   > - [..]

5. Change 21.3.2 [meta.rqmts]/1+2 as indicated:

   > 1 A UnaryTypeTrait describes a property of a type. It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the property being described. It shall be `DefaultConstructible`, `implicit CopyConstructible`, [..]

   > 2 A `BinaryTypeTrait` describes a relationship between two types. It shall be a class template that takes two template type arguments and, optionally, additional arguments that help define the relationship being described. It shall be `DefaultConstructible`, `implicit CopyConstructible`, and [..]

6. Change 22.10.4 [func.require]/4 as indicated: [explicit copy-constructible functors could not be provided as arguments to any algorithm that takes these by value]

   > 4 Every call wrapper (20.8.1) shall be `implicit MoveConstructible`. A simple call wrapper is a call wrapper that is `implicit CopyConstructible` and `CopyAssignable` and whose copy constructor, move constructor, and assignment operator do not throw exceptions. [..]

7. Change 22.10.6 [refwrap]/1 as indicated:

   > 1 `reference_wrapper<T>` is an `implicit` `CopyConstructible` and `CopyAssignable` wrapper around a reference to an object or function of type `T`.

8. Change 22.10.15.4 [func.bind.bind]/5+9 as indicated:

   > 5 Remarks: The return type shall satisfy the requirements of `implicit MoveConstructible`. If all of `FD` and `TiD` satisfy the requirements of `CopyConstructible`, then the return type shall satisfy the requirements of `implicit CopyConstructible`. [Note: this implies that all of `FD` and `TiD` are `MoveConstructible`. — end note]

   > [..]

   > 9 Remarks: The return type shall satisfy the requirements of `implicit MoveConstructible`. If all of `FD` and `TiD` satisfy the requirements of `CopyConstructible`, then the return type shall satisfy the requirements of `implicit CopyConstructible`. [Note: this implies that all of `FD` and `TiD` are `MoveConstructible`. — end note]

9. Change 22.10.15.5 [func.bind.place] as indicated:

   > 1 All placeholder types shall be `DefaultConstructible` and `implicit CopyConstructible`, and [..]

10. Change 20.3.1 [unique.ptr]/5 as indicated:

    > 5 Each object of a type `U` instantiated form the `unique_ptr` template specified in this subclause has the strict ownership semantics, specified above, of a unique pointer. In partial satisfaction of these semantics, each such `U` is `implicit MoveConstructible` and `MoveAssignable`, but is not `CopyConstructible` nor `CopyAssignable`. The template parameter `T` of `unique_ptr` may be an incomplete type.

11. Change 20.3.2.2 [util.smartptr.shared]/2 as indicated:

    > 2 Specializations of `shared_ptr` shall be `implicit CopyConstructible`, `CopyAssignable`, and `LessThanComparable`, [..]

12. Change 20.3.2.3 [util.smartptr.weak]/2 as indicated:

    > 2 Specializations of `weak_ptr` shall be `implicit CopyConstructible` and `CopyAssignable`, allowing their use in standard containers. The template parameter `T` of `weak_ptr` may be an incomplete type.

13. Change 24.3.5.2 [iterator.iterators]/2 as indicated: [This fixes a defect in the Iterator requirements. None of the usual algorithms accepting iterators would be usable with iterators with explicit copy-constructors]

    > 2 A type `X` satisfies the Iterator requirements if:

    > - `X` satisfies the `implicit CopyConstructible`, `CopyAssignable`, and `Destructible` requirements (20.2.1) and lvalues of type `X` are swappable (20.2.2), and [..]
    > - ...

14. Change 99 [auto.ptr]/3 as indicated:

    > 3 [..] Instances of `auto_ptr` meet the requirements of `implicit MoveConstructible` and `MoveAssignable`, but do not meet the requirements of `CopyConstructible` and `CopyAssignable`. — end note]

### 1323(i). `basic_string::replace` should use `const_iterator`

Section: 27.4.3.7.6 [string.replace] Status: C++11 Submitter: Daniel Krügler Opened: 2010-02-19 Last modified: 2016-11-12

Priority: Not Prioritized

View all other issues in [string.replace].

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Discussion:

In contrast to all library usages of purely positional iterator values several overloads of `std::basic_string::replace` still use iterator instead of `const_iterator` arguments. The paper N3021 quite nicely visualizes the purely positional responsibilities of the function arguments.

This should be fixed to make the library consistent, the proposed changes are quite mechanic.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. In 27.4.3 [basic.string], class template `basic_string` synopsis change as indicated:

   >     // 21.4.6 modifiers:
   >     ...
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           const basic_string& str);
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           const charT* s, size_type n);
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           const charT* s);
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           size_type n, charT c);
   >     template<class InputIterator>
   >       basic_string& replace(const_iterator i1, const_iterator i2,
   >                             InputIterator j1, InputIterator j2);
   >     basic_string& replace(const_iterator, const_iterator,
   >                           initializer_list<charT>);

2. In 27.4.3.7.6 [string.replace] before p.18, change the following signatures as indicated:

   >     basic_string& replace(const_iterator i1, const_iterator i2, const basic_string& str);

3. In 27.4.3.7.6 [string.replace] before p.21, change the following signatures as indicated:

   >     basic_string&
   >       replace(const_iterator i1, const_iterator i2, const charT* s, size_type n);

4. In 27.4.3.7.6 [string.replace] before p.24, change the following signatures as indicated:

   >     basic_string& replace(const_iterator i1, const_iterator i2, const charT* s);

5. In 27.4.3.7.6 [string.replace] before p.27, change the following signatures as indicated:

   >     basic_string& replace(const_iterator i1, const_iterator i2, size_type n,
   >                           charT c);

6. In 27.4.3.7.6 [string.replace] before p.30, change the following signatures as indicated:

   >     template<class InputIterator>
   >       basic_string& replace(const_iterator i1, const_iterator i2,
   >                             InputIterator j1, InputIterator j2);

7. In 27.4.3.7.6 [string.replace] before p.33, change the following signatures as indicated:

   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           initializer_list<charT> il);

### 1324(i). Still too many implicit conversions for `pair` and `tuple`

Section: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Daniel Krügler Opened: 2010-03-20 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [pairs.pair].

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Discussion:

In analogy to library defect 811(i), `tuple`'s variadic constructor

>     template <class... UTypes>
>     explicit tuple(UTypes&&... u);

creates the same problem as pair:

>     #include <tuple>

>     int main()
>     {
>       std::tuple<char*> p(0);
>     }

produces a similar compile error for a recent gcc implementation.

I suggest to follow the same resolution path as has been applied to `pair`'s corresponding c'tor, that is require that these c'tors should not participate in overload resolution, if the arguments are not implicitly convertible to the element types.

Further-on both `pair` and `tuple` provide converting constructors from different `pairs`/`tuples` that should be not available, if the corresponding element types are not implicitly convertible. It seems astonishing that in the following example

>     struct A {
>       explicit A(int);
>     };

>     A  a = 1; // Error

>     std::tuple<A> ta = std::make_tuple(1); // # OK?

the initialization marked with # could be well-formed.

[ Only constraints on constructors are suggested. Adding similar constraints on assignment operators is considered as QoI, because the assigments wouldn't be well-formed anyway. ]

1. Following 22.3.2 [pairs.pair]/5 add a new Remarks element:

   >     template<class U, class V> pair(const pair<U, V>& p);

   > > 5 Effects: Initializes members from the corresponding members of the argument, performing implicit conversions as needed.

   > > Remarks: This constructor shall not participate in overload resolution unless `U` is implicitly convertible to `first_type` and `V` is implicitly convertible to `second_type`.

2. Following 22.3.2 [pairs.pair]/6 add a new Remarks element:

   >     template<class U, class V> pair(pair<U, V>&& p);

   > > 6 Effects: The constructor initializes `first` with `std::move(p.first)` and second with `std::move(p.second)`.

   > > Remarks: This constructor shall not participate in overload resolution unless `U` is implicitly convertible to `first_type` and `V` is implicitly convertible to `second_type`.

3. Following 22.4.4.2 [tuple.cnstr]/7 add a new Remarks element:

   >     template <class... UTypes>
   >     explicit tuple(UTypes&&... u);

   > > 6 Requires: Each type in `Types` shall satisfy the requirements of `MoveConstructible` (Table 33) from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

   > > 7 Effects: Initializes the elements in the `tuple` with the corresponding value in `std::forward<UTypes>(u)`.

   > > Remarks: This constructor shall not participate in overload resolution unless each type in `UTypes` is implicitly convertible to its corresponding type in `Types`.

4. Following 22.4.4.2 [tuple.cnstr]/13 add a new Remarks element:

   >     template <class... UTypes> tuple(const tuple<UTypes...>& u);

   > > 12 Requires: Each type in `Types` shall be constructible from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

   > > 13 Effects: Constructs each element of `*this` with the corresponding element of `u`.

   > > Remarks: This constructor shall not participate in overload resolution unless each type in `UTypes` is implicitly convertible to its corresponding type in `Types`.

   > > 14 [Note: `enable_if` can be used to make the converting constructor and assignment operator exist only in the cases where the source and target have the same number of elements. — end note]

5. Following 22.4.4.2 [tuple.cnstr]/16 add a new Remarks element:

   >     template <class... UTypes> tuple(tuple<UTypes...>&& u);

   > > 15 Requires: Each type in `Types` shall shall satisfy the requirements of `MoveConstructible` (Table 33) from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

   > > 16 Effects: Move-constructs each element of `*this` with the corresponding element of `u`.

   > > Remarks: This constructor shall not participate in overload resolution unless each type in `UTypes` is implicitly convertible to its corresponding type in `Types`.

   > > [Note: `enable_if` can be used to make the converting constructor and assignment operator exist only in the cases where the source and target have the same number of elements. — end note]

6. Following 22.4.4.2 [tuple.cnstr]/18 add a new Remarks element:

   >     template <class U1, class U2> tuple(const pair<U1, U2>& u);

   > > 17 Requires: The first type in `Types` shall be constructible from `U1` and the second type in `Types` shall be constructible from `U2`. `sizeof...(Types) == 2`.

   > > 18 Effects: Constructs the first element with `u.first` and the second element with `u.second`.

   > > Remarks: This constructor shall not participate in overload resolution unless `U1` is implicitly convertible to the first type in `Types` and `U2` is implicitly convertible to the second type in `Types`.

7. Following 22.4.4.2 [tuple.cnstr]/20 add a new Remarks element:

   >     template <class U1, class U2> tuple(pair<U1, U2>&& u);

   > > 19 Requires: The first type in `Types` shall shall satisfy the requirements of `MoveConstructible`(Table 33) from `U1` and the second type in `Types` shall be move-constructible from `U2`. `sizeof...(Types) == 2`.

   > > 20 Effects: Constructs the first element with `std::move(u.first)` and the second element with `std::move(u.second)`

   > > Remarks: This constructor shall not participate in overload resolution unless `U1` is implicitly convertible to the first type in `Types` and `U2` is implicitly convertible to the second type in `Types`.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

Proposed resolution:

See n3140.

### 1325(i). `bitset`

Section: 22.9.2.2 [bitset.cons] Status: C++11 Submitter: Christopher Jefferson Opened: 2010-03-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [bitset.cons].

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Discussion:

As mentioned on the boost mailing list:

The following code, valid in C++03, is broken in C++0x due to ambiguity between the "`unsigned long long`" and "`char*`" constructors.

>     #include <bitset>
>     std::bitset<10> b(0);

[ The proposed resolution has been reviewed by Stephan T. Lavavej. ]

[ Post-Rapperswil ]

The proposed resolution has two problems:

- it fails to provide support for non-terminated strings, which could be easily added and constitutes an important use-case. For example, the following code would invoke UB with the current P/R:

  >     char s[4] = { '0', '1', '0', '1' }; // notice: not null-terminated!
  >     bitset<4> b(s, 0, 4);

  because it requires the evaluation (under the as-if rule, to be fair, but it doesn't matter) of `basic_string<char>(s)`

- it promotes a consistency between the two `bitset` constructors that take a `const std::string&` and a `const char*`, respectively, while practice established by `std::basic_string` would recommend a different set of parameters. In particular, the constructor of `std::basic_string` that takes a `const char*` does not have a `pos` parameter

> Moved to Tentatively Ready with revised wording provided by Alberto Ganesh Babati after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. In the synopsis of header `<bitset>` in 22.9.2 [template.bitset]/1, replace the fourth bitset constructor:

   >     explicit bitset(const char *str);
   >     template <class charT>
   >       explicit bitset(
   >         const charT *str,
   >         typename basic_string<charT>::size_type n = basic_string<charT>::npos,
   >         charT zero = charT('0'), charT one = charT('1'));

2. In 22.9.2.2 [bitset.cons]/8:

   >     explicit bitset(const char *str);
   >     template <class charT>
   >     explicit
   >     bitset(const charT *str,
   >            typename basic_string<charT>::size_type n = basic_string<charT>::npos,
   >            charT zero = charT('0'), charT one = charT('1'));

   Effects: Constructs an object of class `bitset<N>` as if by bitset(string(str)).

   >     bitset(
   >       n == basic_string<charT>::npos
   >         ? basic_string<charT>(str)
   >         : basic_string<charT>(str, n),
   >       0, n, zero, one)

### 1326(i). Missing/wrong preconditions for `pair` and `tuple` functions

Section: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Daniel Krügler Opened: 2010-03-07 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [pairs.pair].

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Discussion:

There are several constructors and creation functions of std::tuple that impose requirements on it's arguments, that are unnecessary restrictive and don't match the intention for the supported argument types. This is related to the fact that tuple is supposed to accept both object types and lvalue-references and the usual MoveConstructible and CopyConstructible requirements are bad descriptions for non-const references. Some examples:

1. 22.4.4.2 [tuple.cnstr] before p.4 and p.8, resp.:

   >     explicit tuple(const Types&...);

   > > 4 Requires: Each type in `Types` shall be copy constructible.

   >     tuple(const tuple& u) = default;

   > > 8 Requires: Each type in `Types` shall satisfy the requirements of `CopyConstructible` (Table 34).

   A tuple that contains lvalue-references to non-const can never satisfy the `CopyConstructible` requirements. `CopyConstructible` requirements refine the `MoveConstructible` requirements and this would require that these lvalue-references could bind rvalues. But the core language does not allow that. Even, if we would interpret that requirement as referring to the underlying non-reference type, this requirement would be wrong as well, because there is no reason to disallow a type such as

   >     struct NoMoveNoCopy {
   >       NoMoveNoCopy(NoMoveNoCopy&&) = delete;
   >       NoMoveNoCopy(const NoMoveNoCopy&) = delete;
   >       ...
   >     }:

   for the instantiation of `std::tuple<NoMoveNoCopy&>` and that of it's copy constructor.

   A more reasonable requirement for this example would be to require that "`is_constructible<Ti, const Ti&>::value` shall evaluate to true for all `Ti` in `Types`". In this case the special reference-folding and const-merging rules of references would make this well-formed in all cases. We could also add the further constraint "if `Ti` is an object type, it shall satisfy the `CopyConstructible` requirements", but this additional requirement seems not really to help here. Ignoring it would only mean that if a user would provide a curious object type `C` that satisfies the `std::is_constructible<C, const C&>` test, but not the "`C` is `CopyConstructible`" test would produce a `tuple<C>` that does not satisfy the `CopyConstructible` requirements as well.

2. 22.4.4.2 [tuple.cnstr] before p.6 and p.10, resp.:

   >     template <class... UTypes>
   >     explicit tuple(UTypes&&... u);

   > > 6 Requires: Each type in `Types` shall satisfy the requirements of `MoveConstructible` (Table 33) from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

   >     tuple(tuple&& u);

   > > 10 Requires: Each `type` in `Types` shall shall satisfy the requirements of `MoveConstructible` (Table 33).

   We have a similar problem as in (a): Non-const lvalue-references are intended template arguments for `std::tuple`, but cannot satisfy the `MoveConstructible` requirements. In this case the correct requirements would be

   > `is_constructible<Ti, Ui>::value` shall evaluate to true for all `Ti` in `Types` and for all `Ui` in `UTypes`

   and

   > `is_constructible<Ti, Ti>::value` shall evaluate to true for all `Ti` in `Types`

   respectively.

Many `std::pair` member functions do not add proper requirements, e.g. the default c'tor does not require anything. This is corrected within the suggested resolution. Further-on the P/R has been adapted to the FCD numbering.

[ 2010-03-25 Daniel updated wording: ]

> The issue became updated to fix some minor inconsistencies and to ensure a similarly required fix for `std::pair`, which has the same specification problem as `std::tuple`, since `pair` became extended to support reference members as well.

[Original proposed resolution:]

1. Change 22.3.2 [pairs.pair]/1 as indicated [The changes for the effects elements are not normative changes, they just ensure harmonization with existing wording style]:

   >     constexpr pair();

   > > Requires: `first_type` and `second_type` shall satisfy the `DefaultConstructible` requirements.

   > > 1 Effects: Value-initializes `first` and `second`.Initializes its members as if implemented: `pair() : first(), second() { }`.

2. Change 22.3.2 [pairs.pair]/2 as indicated:

   >     pair(const T1& x, const T2& y);

   > > Requires: `is_constructible<T1, const T1&>::value` is `true` and `is_constructible<T2, const T2&>::value` is `true`.

   > > 2 Effects: The constructor initializes `first` with `x` and `second` with `y`.

3. Change 22.3.2 [pairs.pair]/3 as indicated:

   >     template<class U, class V> pair(U&& x, V&& y);

   > > Requires: `is_constructible<first_type, U>::value` is `true` and `is_constructible<second_type, V>::value` is `true`.

   > > 3 Effects: The constructor initializes `first` with `std::forward<U>(x)` and `second` with `std::forward<V>(y)`.

   > > 4 Remarks: If `U` is not implicitly convertible to `first_type` or `V` is not implicitly convertible to `second_type` this constructor shall not participate in overload resolution.

4. Change 22.3.2 [pairs.pair]/5 as indicated [The change in the effects element should be non-normatively and is in compatible to the change suggestion of 1324(i)]:

   >     template<class U, class V> pair(const pair<U, V>& p);

   > > Requires: `is_constructible<first_type, const U&>::value` is `true` and `is_constructible<second_type, const V&>::value` is `true`.

   > > 5 Effects: Initializes members from the corresponding members of the argument, performing implicit conversions as needed.

5. Change 22.3.2 [pairs.pair]/6 as indicated:

   >     template<class U, class V> pair(pair<U, V>&& p);

   > > Requires: `is_constructible<first_type, U>::value` is `true` and `is_constructible<second_type, V>::value` is `true`.

   > > 6 Effects: The constructor initializes `first` with `std::moveforward<U>(p.first)` and `second` with `std::moveforward<V>(p.second)`.

6. Change 22.3.2 [pairs.pair]/7+8 as indicated [The deletion in the effects element should be non-normatively]:

   >     template<class... Args1, class... Args2>
   >       pair(piecewise_construct_t,
   >            tuple<Args1...> first_args, tuple<Args2...> second_args);

   > > 7 Requires: `is_constructible<first_type, Args1...>::value` is `true` and `is_constructible<second_type, Args2...>::value` is `true`. All the types in `Args1` and `Args2` shall be `CopyConstructible` (Table 35). `T1` shall be constructible from `Args1`. `T2` shall be constructible from `Args2`.

   > > 8 Effects: The constructor initializes `first` with arguments of types `Args1...` obtained by forwarding the elements of `first_args` and initializes `second` with arguments of types `Args2...` obtained by forwarding the elements of `second_args`. (Here, forwarding an element `x` of type `U` within a `tuple` object means calling `std::forward<U>(x)`.) This form of construction, whereby constructor arguments for `first` and `second` are each provided in a separate `tuple` object, is called piecewise construction.

7. Change 22.3.2 [pairs.pair] before 12 as indicated:

   >     pair& operator=(pair&& p);

   > > Requires: `first_type` and `second_type` shall satisfy the `MoveAssignable` requirements.

   > > 12 Effects: Assigns to `first` with `std::move(p.first)` and to `second` with `std::move(p.second)`.

   > > 13 Returns: `*this`.

8. Change [pairs.pair] before 14 as indicated: [The heterogeneous usage of MoveAssignable is actually not defined, but the library uses it at several places, so we follow this tradition until a better term has been agreed on. One alternative could be to write "first_type shall be assignable from an rvalue of U [..]"]

   >     template<class U, class V> pair& operator=(pair<U, V>&& p);

   > > Requires: `first_type` shall be `MoveAssignable` from `U` and `second_type` shall be `MoveAssignable` from `V`.

   > > 14 Effects: Assigns to `first` with `std::move(p.first)` and to `second` with `std::move(p.second)`.

   > > 15 Returns: `*this`.

9. Change 22.4.4.2 [tuple.cnstr]/4+5 as indicated:

   >     explicit tuple(const Types&...);

   > > 4 Requires: `is_constructible<Ti, const Ti&>::value == true` for eEach type `Ti` in `Types` shall be copy constructible.

   > > 5 Effects: Copy iInitializes each element with the value of the corresponding parameter.

10. Change 22.4.4.2 [tuple.cnstr]/6 as indicated:

    >     template <class... UTypes>
    >     explicit tuple(UTypes&&... u);

    > > 6 Requires: `is_constructible<Ti, Ui>::value == true` for eEach type `Ti` in `Types` shall satisfy the requirements of `MoveConstructible` (Table 33) fromand for the corresponding type `Ui` in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

    > > 7 Effects: Initializes the elements in the `tuple` with the corresponding value in `std::forward<UTypes>(u)`.

11. Change 22.4.4.2 [tuple.cnstr]/8+9 as indicated:

    >     tuple(const tuple& u) = default;

    > > 8 Requires: `is_constructible<Ti, const Ti&>::value == true` for eEach type `Ti` in `Types` shall satisfy the requirements of `CopyConstructible`(Table 34).

    > > 9 Effects: InitializesCopy constructs each element of `*this` with the corresponding element of `u`.

12. Change 22.4.4.2 [tuple.cnstr]/10+11 as indicated:

    >     tuple(tuple&& u);

    > > 10 Requires: Let `i` be in `[0, sizeof...(Types))` and let `Ti` be the `i`th type in `Types`. Then `is_constructible<Ti, Ti>::value` shall be `true` for all `i`. Each type in `Types` shall shall satisfy the requirements of `MoveConstructible` (Table 34).

    > > 11 Effects: For each `Ti` in `Types`, initializes the `i`th Move-constructs each element of `*this` with the corresponding element of `std::forward<Ti>(get<i>(``u``))`.

13. Change 22.4.4.2 [tuple.cnstr]/15+16 as indicated:

    >     template <class... UTypes> tuple(tuple<UTypes...>&& u);

    > > 15 Requires: Let `i` be in `[0, sizeof...(Types))`, `Ti` be the `i`th type in `Types`, and `Ui` be the `i`th type in `UTypes`. Then `is_constructible<Ti, Ui>::value` shall be `true` for all `i`. Each type in `Types` shall shall satisfy the requirements of `MoveConstructible` (Table 34) from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

    > > 16 Effects: For each type `Ti`, initializes the `i`th Move-constructs each element of `*this` with the corresponding element of `std::forward<Ui>(get<i>(``u``))`.

14. Change 22.4.4.2 [tuple.cnstr]/19+20 as indicated:

    >     template <class U1, class U2> tuple(pair<U1, U2>&& u);

    > > 19 Requires: `is_constructible<T1, U1>::value == true` for `t` `T`he first type `T1` in `Types` shall shall satisfy the requirements of `MoveConstructible`(Table 33) from `U1` and `is_constructible<T2, U2>::value == true` for the second type `T2` in `Types` shall be move-constructible from `U2`. `sizeof...(Types) == 2`.

    > > 20 Effects: InitializesConstructs the first element with `std::forward<U1>move(u.first)` and the second element with `std::forward<U2>move(u.second)`.

15. Change 22.4.5 [tuple.creation]/9-16 as indicated:

    >     template <class... TTypes, class... UTypes>
    >     tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

    > > 9 Requires: `is_constructible<Ti, const Ti&>::value == true` for each type `Ti` All the types in `TTypes` shall be `CopyConstructible` (Table 34). `is_constructible<Ui, const Ui&>::value == true` for each type `Ui` All the types in `UTypes` shall be `CopyConstructible` (Table 34).

    > > 10 Returns: A `tuple` object constructed by initializingcopy constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and initializingcopy constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

    >     template <class... TTypes, class... UTypes>
    >     tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);

    > > 11 Requires: Let `i` be in `[0, sizeof...(TTypes))`, `Ti` be the `i`th type in `Types`, `j` be in `[0, sizeof...(UTypes))`, and `Uj` be the `j`th type in `UTypes`. `is_constructible<Ti, Ti>::value` shall be `true` for each type `Ti` and `is_constructible<Uj, const Uj&>::value` shall be `true` for each type `Uj` All the types in `TTypes` shall be `MoveConstructible` (Table 34). All the types in `UTypes` shall be `CopyConstructible` (Table 35).

    > > 12 Returns: A `tuple` object constructed by initializing the `i`th element with `std::forward<Ti>(get<i>(t))` for all `Ti` in `TTypes` and initializing the `(j+sizeof...(TTypes))`th element with `get<j>(u)` for all `Uj` in `UTypes`. move constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and copy constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

    >     template <class... TTypes, class... UTypes>
    >     tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);

    > > 13 Requires: Let `i` be in `[0, sizeof...(TTypes))`, `Ti` be the `i`th type in `Types`, `j` be in `[0, sizeof...(UTypes))`, and `Uj` be the `j`th type in `UTypes`. `is_constructible<Ti, const Ti&>::value` shall be `true` for each type `Ti` and `is_constructible<Uj, Uj>::value` shall be `true` for each type `Uj` All the types in `TTypes` shall be `CopyConstructible` (Table 35). All the types in `UTypes` shall be `MoveConstructible` (Table 34).

    > > 14 Returns: A `tuple` object constructed by initializing the `i`th element with `get<i>(t)` for each type `Ti` and initializing the `(j+sizeof...(TTypes))`th element with `std::forward<Uj>(get<j>(u))` for each type `Uj` copy constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and move constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

    >     template <class... TTypes, class... UTypes>
    >     tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);

    > > 15 Requires: Let `i` be in `[0, sizeof...(TTypes))`, `Ti` be the `i`th type in `Types`, `j` be in `[0, sizeof...(UTypes))`, and `Uj` be the `j`th type in `UTypes`. `is_constructible<Ti, Ti>::value` shall be `true` for each type `Ti` and `is_constructible<Uj, Uj>::value` shall be `true` for each type `Uj` All the types in `TTypes` shall be `MoveConstructible` (Table 34). All the types in `UTypes` shall be `MoveConstructible` (Table 34).

    > > 16 Returns: A `tuple` object constructed by initializing the `i`th element with `std::forward<Ti>(get<i>(t))` for each type `Ti` and initializing the `(j+sizeof...(TTypes))`th element with `std::forward<Uj>(get<j>(u))` for each type `Uj` move constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and move constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

Proposed resolution:

See n3140.

### 1327(i). templates defined in `<cmath>` replacing C macros with the same name

Section: 29.7 [c.math] Status: Resolved Submitter: Michael Wong Opened: 2010-03-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 29.7 [c.math]p12 The templates defined in `<cmath>` replace the C99 macros with the same names. The templates have the following declarations:

>     namespace std {
>     template <class T> bool signbit(T x);
>     template <class T> int fpclassify(T x);
>     template <class T> bool isfinite(T x);
>     template <class T> bool isinf(T x);
>     template <class T> bool isnan(T x);
>     template <class T> bool isnormal(T x);
>     template <class T> bool isgreater(T x, T y);
>     template <class T> bool isgreaterequal(T x, T y);
>     template <class T> bool isless(T x, T y);
>     template <class T> bool islessequal(T x, T y);
>     template <class T> bool islessgreater(T x, T y);
>     template <class T> bool isunordered(T x, T y);
>     }

and p13:

> 13 The templates behave the same as the C99 macros with corresponding names defined in C99 7.12.3, Classification macros, and C99 7.12.14, Comparison macros in the C standard.

The C Std versions look like this:

> 7.12.14.1/p1:

> > Synopsis

> > 1 `#include <math.h>`

> >     int isgreaterequal(real-floating x, real-floating y);

which is not necessarily the same types as is required by C++ since the two parameters may be different. Would it not be better if it were truly aligned with C?

[ 2010 Pittsburgh: Bill to ask WG-14 if heterogeneous support for the two-parameter macros is intended. ]

[ 2010-09-13 Daniel comments: ]

> I recommend to resolve this issue as NAD Editorial because the accepted resolution for NB comment US-136 by motion 27 does address this.

[ 2010-09-14 Bill comments: ]

> Motion 27 directly addresses LWG 1327 and solves the problem presented there. Moreover, the solution has been aired before WG14 with no dissent. These functions now behave the same for mixed-mode calls in both C and C++

Proposed resolution:

Apply proposed resolution for US-136

### 1328(i). istream extractors not setting `failbit` if `eofbit` is already set

Section: 31.7.5.2.4 [istream.sentry] Status: Resolved Submitter: Paolo Carlini Opened: 2010-02-17 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Basing on the recent discussion on the library reflector, see c++std-lib-27728 and follow ups, I hereby formally ask for LWG 419 to be re-opened, the rationale being that according to the current specifications, per n3000, it seems actually impossible to seek away from end of file, contrary to the rationale which led 342(i) to its closure as NAD. My request is also supported by Martin Sebor, and I'd like also to add, as tentative proposed resolution for the re-opened issue, the wording suggested by Sebor, thus, change the beginning of [istream::sentry]/2, to:

> 2 Effects: If `(!noskipws && !is.good())` is `false true`, calls `is.setstate(failbit)`. Otherwise prepares for formatted or unformatted input. ...

[ 2010-10 Batavia ]

Resolved by adopting n3168.

Previous proposed resolution:

Change [istream::sentry] p.2:

> 2 Effects: If `(!noskipws && !is.good())` is `false true`, calls `is.setstate(failbit)`. Otherwise prepares for formatted or unformatted input. ...

Proposed resolution:

Addressed by paper n3168.

### 1329(i). Data races on `vector<bool>`

Section: 23.2.3 [container.requirements.dataraces] Status: Resolved Submitter: Jeffrey Yaskin Opened: 2010-03-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The common implementation of `vector<bool>` is as an unsynchronized bitfield. The addition of 23.2.3 [container.requirements.dataraces]/2 would require either a change in representation or a change in access synchronization, both of which are undesireable with respect to compatibility and performance.

[ 2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below. ]

Rationale:

Solved by N3069.

Proposed resolution:

Container data races 23.2.3 [container.requirements.dataraces]

Paragraph 1 is unchanged as follows:

> 1 For purposes of avoiding data races (17.6.4.8), implementations shall consider the following functions to be `const`: `begin`, `end`, `rbegin`, `rend`, `front`, `back`, `data`, `find`, `lower_bound`, `upper_bound`, `equal_range`, and, except in associative containers, `operator[]`.

Edit paragraph 2 as follows:

> 2 Notwithstanding (17.6.4.8), implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting `vector<bool>`, are modified concurrently.

Edit paragraph 3 as follows:

> 3 [Note: For a `vector<int> x` with a size greater than one, `x[1] = 5` and `*x.begin() = 10` can be executed concurrently without a data race, but `x[0] = 5` and `*x.begin() = 10` executed concurrently may result in a data race. As an exception to the general rule, for a `vector<bool> y`, `y[i] = true` may race with `y[j] = true`. —end note]

### 1332(i). Let Hash objects throw!

Section: 16.4.4.5 [hash.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-26 Last modified: 2025-03-13

Priority: Not Prioritized

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Discussion:

The currently added `Hash` requirements demand in Table 40 — `Hash` requirements [hash]:

> Table 40 — Hash requirements [hash]
> Expression | Return type | Requirement
> --- | --- | ---
> `h(k)` | `size_t` | Shall not throw exceptions. [..]

While it surely is a generally accepted idea that hash function objects should not throw exceptions, this basic constraint for such a fundamental requirement set does neither match the current library policy nor real world cases:

1. There are little known situations where a swap or move operation may throw an exception and in some popular domains such functions are required not to throw. But the library invested already efforts for good reasons to require "working" container implementations in the presence of throwing move or swap operations, see e.g. 23.2.7.2 [associative.reqmts.except], 23.2.8.2 [unord.req.except].
2. The container library is already specified to cope with potentially throwing comparers, predicates, and hash function objects, see above.

3. The new definition goes beyond the original hash requirements as specified by SGI library in regard to the exception requirement:

   > https://www.boost.org/sgi/stl/HashFunction.html

4. There are indeed real-world examples of potentially throwing hash functions, typically when the proxy pattern is used and when the to-be hashed proxied instance is some volatile object, e.g. a file or internet resource, that might suddenly be unavailable at the time of hashing.
5. With the new `noexcept` language facility libraries can still take advantage of no-throw guarantees of hasher functions with stricter guarantees.

Even though the majority of all known move, swap, and hash functions won't throw and in some cases must not throw, it seems like unnecessary over-constraining the definition of a Hash functor not to propagate exceptions in any case and it contradicts the general principle of C++ to impose such a requirement for this kind of fundamental requirement.

[ 2010-11-11 Daniel asks the working group whether they would prefer a replacement for the second bullet of the proposed resolution (a result of discussing this with Alberto) of the form: ]

Add to 22.10.19 [unord.hash]/1 a new bullet:

> 1 The unordered associative containers defined in Clause 23.5 use specializations of the class template `hash` as the default hash function. For all object types `Key` for which there exists a specialization `hash<Key>`, the instantiation `hash<Key>` shall:

> - satisfy the `Hash` requirements (20.2.4), with `Key` as the function call argument type, the `DefaultConstructible` requirements (33), the `CopyAssignable` requirements (37),
> - be swappable (20.2.2) for lvalues,
> - provide two nested types `result_type` and `argument_type` which shall be synonyms for `size_t` and `Key`, respectively,
> - satisfy the requirement that if `k1 == k2` is true, `h(k1) == h(k2)` is also true, where `h` is an object of type `hash<Key>` and `k1` and `k2` are objects of type `Key`,.
> - satisfy the requirement `noexcept(h(k)) == true`, where `h` is an object of type `hash<Key>` and `k` is an object of type `Key`, unless `hash<Key>` is a user-defined specialization that depends on at least one user-defined type.

[Batavia: Closed as NAD Future, then reopened. See the wiki for Tuesday.]

Proposed resolution:

1. Change Table 26 — `Hash` requirements [tab:hash] as indicated:

   > Table 26 — `Hash` requirements [tab:hash]
   > Expression | Return type | Requirement
   > --- | --- | ---
   > `h(k)` | `size_t` | Shall not throw exceptions. […]

2. Add to 22.10.19 [unord.hash] p. 1 a new bullet:

   > 1 The unordered associative containers defined in Clause 23.5 [unord] use specializations of the class template `hash` as the default hash function. For all object types `Key` for which there exists a specialization `hash<Key>`, the instantiation `hash<Key>` shall:

   > - satisfy the `Hash` requirements ([hash.requirements]), with `Key` as the function call argument type, the `DefaultConstructible` requirements (Table [defaultconstructible]), the `CopyAssignable` requirements (Table [copyassignable]),
   > - be swappable ([swappable.requirements]) for lvalues,
   > - provide two nested types `result_type` and `argument_type` which shall be synonyms for `size_t` and `Key`, respectively,
   > - satisfy the requirement that if `k1 == k2` is true, `h(k1) == h(k2)` is also true, where `h` is an object of type `hash<Key>` and `k1` and `k2` are objects of type `Key`,.
   > - satisfy the requirement that the expression `h(k)`, where `h` is an object of type `hash<Key>` and `k` is an object of type `Key`, shall not throw an exception, unless `hash<Key>` is a user-defined specialization that depends on at least one user-defined type.

### 1333(i). Missing forwarding during `std::function` invocation

Section: 22.10.17.3.5 [func.wrap.func.inv] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current wording of 22.10.17.3.5 [func.wrap.func.inv]/1:

>     R operator()(ArgTypes... args) const

> > Effects: `INVOKE(f, t1, t2, ..., tN, R)` (20.8.2), where `f` is the target object (20.8.1) of `*this` and `t1, t2, ..., tN` are the values in `args...`.

uses an unclear relation between the actual args and the used variables `ti`. It should be made clear, that `std::forward` has to be used to conserve the expression lvalueness.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 22.10.17.3.5 [func.wrap.func.inv]/1+2 as indicated:

>     R operator()(ArgTypes... args) const

> > 1 Effects:: `INVOKE(f, std::forward<ArgTypes>(args)...t1, t2, ..., tN, R)` (20.8.2), where `f` is the target object (20.8.1) of `*this` and `t1, t2, ..., tN` are the values in `args...`.

> > 2 Returns: Nothing if `R` is `void`, otherwise the return value of `INVOKE(f, std::forward<ArgTypes>(args)...t1, t2, ..., tN, R)`.

> > 3 Throws: `bad_function_call` if `!*this`; otherwise, any exception thrown by the wrapped callable object.

### 1334(i). Insert iterators are broken for some proxy containers compared to C++03

Section: 24.5.2.2.2 [back.insert.iter.ops], 24.5.2.3.2 [front.insert.iter.ops], 24.5.2.4.2 [insert.iter.ops] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-28 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

In C++03 this was valid code:

>     #include <vector>
>     #include <iterator>

>     int main() {
>       typedef std::vector<bool> Cont;
>       Cont c;
>       std::back_insert_iterator<Cont> it = std::back_inserter(c);
>       *it = true;
>     }

In C++0x this code does no longer compile because of an ambiguity error for this `operator=` overload pair:

>     back_insert_iterator<Container>&
>     operator=(typename Container::const_reference value);

>     back_insert_iterator<Container>&
>     operator=(typename Container::value_type&& value);

This is so, because for proxy-containers like `std::vector<bool>` the `const_reference` usually is a non-reference type and in this case it's identical to `Container::value_type`, thus forming the ambiguous overload pair

>     back_insert_iterator<Container>&
>     operator=(bool value);

>     back_insert_iterator<Container>&
>     operator=(bool&& value);

The same problem exists for `std::back_insert_iterator`, `std::front_insert_iterator`, and `std::insert_iterator`.

One possible fix would be to require that `const_reference` of a proxy container must not be the same as the `value_type`, but this would break earlier valid code. The alternative would be to change the first signature to

>     back_insert_iterator<Container>&
>     operator=(const typename Container::const_reference& value);

This would have the effect that this signature always expects an lvalue or rvalue, but it would not create an ambiguity relative to the second form with rvalue-references. [For all non-proxy containers the signature will be the same as before due to reference-collapsing and const folding rules]

[ Post-Rapperswil ]

This problem is not restricted to the unspeakable `vector<bool>`, but is already existing for other proxy containers like gcc's `rope` class. The following code does no longer work ([Bug libstdc++/44963]):

>     #include <iostream>
>     #include <ext/rope>

>     using namespace std;

>     int main()
>     {
>          __gnu_cxx::crope line("test");
>          auto ii(back_inserter(line));

>          *ii++ = 'm'; // #1
>          *ii++ = 'e'; // #2

>          cout << line << endl;
>     }

Both lines marked with #1 and #2 issue now an error because the library has properly implemented the current wording state (Thanks to Paolo Calini for making me aware of this real-life example).

The following P/R is a revision of the orignal P/R and was initially suggested by Howard Hinnant. Paolo verified that the approach works in gcc.

> Moved to Tentatively Ready with revised wording after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

1. Change [back.insert.iterator], class `back_insert_iterator` synopsis as indicated:

   >     template <class Container>
   >     class back_insert_iterator :
   >      public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >      Container* container;
   >     public:
   >      [..]
   >      back_insert_iterator<Container>&
   >        operator=(const typename Container::const_referencevalue_type& value);
   >      back_insert_iterator<Container>&
   >        operator=(typename Container::value_type&& value);
   >      [..]
   >     };

2. Change [back.insert.iter.op=] before p. 1 as indicated:

   >     back_insert_iterator<Container>&
   >        operator=(const typename Container::const_referencevalue_type& value);

   > > 1 Effects: `container->push_back(value)`;
   > > 2 Returns: `*this`.

3. Change [front.insert.iterator], class `front_insert_iterator` synposis as indicated:

   >     template <class Container>
   >     class front_insert_iterator :
   >      public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >      Container* container;
   >     public:
   >      [..]
   >      front_insert_iterator<Container>&
   >        operator=(const typename Container::const_referencevalue_type& value);
   >      front_insert_iterator<Container>&
   >        operator=(typename Container::value_type&& value);
   >      [..]
   >     };

4. Change [front.insert.iter.op=] before p.1 as indicated:

   >     front_insert_iterator<Container>&
   >        operator=(const typename Container::const_referencevalue_type& value);

   > > 1 Effects: `container->push_front(value)`;
   > > 2 Returns: `*this`.

5. Change [insert.iterator], class insert_iterator synopsis as indicated:

   >     template <class Container>
   >        class insert_iterator :
   >          public iterator<output_iterator_tag,void,void,void,void> {
   >        protected:
   >          Container* container;
   >          typename Container::iterator iter;
   >        public:
   >          [..]
   >          insert_iterator<Container>&
   >            operator=(const typename Container::const_referencevalue_type& value);
   >          insert_iterator<Container>&
   >            operator=(typename Container::value_type&& value);
   >          [..]
   >        };

6. Change [insert.iter.op=] before p. 1 as indicated:

   >     insert_iterator<Container>&
   >         operator=(const typename Container::const_referencevalue_type& value);

   > > 1 Effects:

   > > >       iter = container->insert(iter, value);
   > > >       ++iter;

   > > 2 Returns: `*this`.

### 1335(i). Insufficient requirements for `tuple::operator<()`

Section: 22.4.9 [tuple.rel] Status: C++11 Submitter: Joe Gottman Opened: 2010-05-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The requirements section for `std::tuple` says the following:

> Requires: For all `i`, where `0 <= i and i < sizeof...(Types)`, `get<i>(t) < get<i>(u)` is a valid expression returning a type that is convertible to `bool`. `sizeof...(TTypes) == sizeof...(UTypes)`.

This is necessary but not sufficient, as the algorithm for comparing `tuple`s also computes `get<i>(u) < get<i>(t)` (note the order)

[ Post-Rapperswil ]

> Moved to Tentatively Ready with updated wording correcting change-bars after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [tuple.rel] before p. 4 as indicated [Remark to the editor: This paragraph doesn't have a number yet, but it seems to me as if it should have one]:

   >     template<class... TTypes, class... UTypes>
   >     bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

   > > Requires: For all `i`, where `0 <= i` and `i < sizeof...(Types)`, `get<i>(t) < get<i>(u)` and `get<i>(u) < get<i>(t)`is a valid expression returning a type that is are valid expressions returning types that are convertible to `bool`. `sizeof...(TTypes) == sizeof...(UTypes)`.

### 1337(i). Swapped arguments in `regex_traits::isctype`

Section: 28.6.6 [re.traits] Status: C++11 Submitter: Howard Hinnant Opened: 2010-06-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

28.6.6 [re.traits]/12 describes `regex_traits::isctype` in terms of `ctype::is(c, m)`, where `c` is a `charT` and `m` is a `ctype_base::mask`. Unfortunately 28.3.4.2.2.2 [locale.ctype.members] specifies this function as `ctype::is(m, c)`

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 28.6.6 [re.traits] p.12:

>     bool isctype(charT c, char_class_type f) const;

> > 11 ...

> > 12 Returns: Converts `f` into a value `m` of type `std::ctype_base::mask` in an unspecified manner, and returns `true` if `use_facet<ctype<charT> >(getloc()).is(cm, mc)` is `true`. Otherwise returns `true` if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence `"w"` is not equal to 0 and `c == '_'`, or if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence `"blank"` is not equal to 0 and `c` is one of an implementation-defined subset of the characters for which `isspace(c, getloc())` returns `true`, otherwise returns `false`.

### 1338(i). LWG 1205 incorrectly applied

Section: 26.6.15 [alg.search] Status: C++11 Submitter: Howard Hinnant Opened: 2010-06-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

LWG 1205(i) (currently in WP) clarified the return value of several algorithms when dealing with empty ranges. In particular it recommended for 26.6.15 [alg.search]:

>     template<class ForwardIterator1, class ForwardIterator2>
>       ForwardIterator1
>         search(ForwardIterator1 first1, ForwardIterator1 last1,
>                ForwardIterator2 first2, ForwardIterator2 last2);

>     template<class ForwardIterator1, class ForwardIterator2,
>              class BinaryPredicate>
>       ForwardIterator1
>         search(ForwardIterator1 first1, ForwardIterator1 last1,
>                ForwardIterator2 first2, ForwardIterator2 last2,
>                BinaryPredicate pred);

> > 1 Effects: ...

> > 2 Returns: ... Returns `last1` if no such iterator is found.

> > 3 Remarks: Returns `first1` if `[first2,last2)` is empty.

Unfortunately this got translated to an incorrect specification for what gets returned when no such iterator is found (N3092):

>     template<class ForwardIterator1, class ForwardIterator2>
>       ForwardIterator1
>         search(ForwardIterator1 first1, ForwardIterator1 last1,
>                ForwardIterator2 first2, ForwardIterator2 last2);

>     template<class ForwardIterator1, class ForwardIterator2,
>              class BinaryPredicate>
>       ForwardIterator1
>         search(ForwardIterator1 first1, ForwardIterator1 last1,
>                ForwardIterator2 first2, ForwardIterator2 last2,
>                BinaryPredicate pred);

> > 1 Effects: ...

> > 2 Returns: ... Returns `first1` if `[first2,last2)` is empty or if no such iterator is found.

LWG 1205(i) is correct and N3092 is not equivalent nor correct.

I have not reviewed the other 10 recommendations of 1205(i).

[ Post-Rapperswil ]

> It was verified that none of the remaining possibly affected algorithms does have any similar problems and a concrete P/R was added that used a similar style as has been applied to the other cases.

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [alg.search] p.2 as indicated:

   >     template<class ForwardIterator1, class ForwardIterator2>
   >       ForwardIterator1
   >         search(ForwardIterator1 first1, ForwardIterator1 last1,
   >                ForwardIterator2 first2, ForwardIterator2 last2);

   >     template<class ForwardIterator1, class ForwardIterator2,
   >                 class BinaryPredicate>
   >       ForwardIterator1
   >         search(ForwardIterator1 first1, ForwardIterator1 last1,
   >                ForwardIterator2 first2, ForwardIterator2 last2,
   >                BinaryPredicate pred);

   > > 1 - [...]

   > > 2 - Returns: The first iterator `i` in the range `[first1,last1 - (last2-first2))` such that for any nonnegative integer `n` less than `last2 - first2` the following corresponding conditions hold: `*(i + n) == *(first2 + n)`, `pred(*(i + n), *(first2 + n)) != false`. Returns `first1` if `[first2,last2)` is empty or, otherwise returns `last1` if no such iterator is found.

### 1339(i). `uninitialized_fill_n` should return the end of its range

Section: 26.11.7 [uninitialized.fill] Status: C++11 Submitter: Jared Hoberock Opened: 2010-07-14 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

N3092's specification of `uninitialized_fill_n` discards useful information and is inconsistent with other algorithms such as `fill_n` which accept an iterator and a size. As currently specified, `unintialized_fill_n` requires an additional linear traversal to find the end of the range.

Instead of returning `void`, `unintialized_fill_n` should return one past the last iterator it dereferenced.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

In section 20.2 [memory] change:,

>     template <class ForwardIterator, class Size, class T>
>       void ForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);

In section [uninitialized.fill.n] change,

>     template <class ForwardIterator, class Size, class T>
>       void ForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);

> > 1 Effects:

> > >     for (; n--; ++first)
> > >       ::new (static_cast<void*>(&*first))
> > >         typename iterator_traits<ForwardIterator>::value_type(x);
> > >     return first;

### 1340(i). Why does `forward_list::resize` take the object to be copied by value?

Section: 23.3.7.5 [forward.list.modifiers] Status: C++11 Submitter: James McNellis Opened: 2010-07-16 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

In N3092 [forwardlist.modifiers], the `resize()` member function is declared as:

>     void resize(size_type sz, value_type c);

The other sequence containers (`list`, `deque`, and `vector`) take `'c'` by const reference.

Is there a reason for this difference? If not, then `resize()` should be declared as:

>     void resize(size_type sz, const value_type& c);

The declaration would need to be changed both at its declaration in the class definition at [forwardlist]/3 and where its behavior is specified at [forwardlist.modifiers]/22.

This would make `forward_list` consistent with the CD1 issue 679(i).

[ Post-Rapperswil ]

Daniel changed the P/R slightly, because one paragraph number has been changed since the issue had been submitted. He also added a similar Requires element that exists in all other containers with a `resize` member function. He deliberately did not touch the wrong usage of "default-constructed" because that will be taken care of by LWG issue 868(i).

> Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [forwardlist]/3, class template `forward_list` synopsis, as indicated:

   >     ...
   >     void resize(size_type sz);
   >     void resize(size_type sz, const value_type& c);
   >     void clear();
   >     ...

2. Change [forwardlist.modifiers]/27 as indicated:

   >     void resize(size_type sz);
   >     void resize(size_type sz, const value_type& c);

   > > 27 Effects: If `sz < distance(begin(), end())`, erases the last `distance(begin(), end()) - sz` elements from the list. Otherwise, inserts `sz - distance(begin(), end())` elements at the end of the list. For the first signature the inserted elements are default constructed, and for the second signature they are copies of `c`.

   > > 28 - Requires: `T` shall be `DefaultConstructible` for the first form and it shall be `CopyConstructible` for the second form.

### 1344(i). Replace `throw()` with `noexcept`

Section: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 1351

Discussion:

Addresses GB-60, CH-16

Dynamic exception specifications are deprecated; the library should recognise this by replacing all non-throwing exception specifications of the form `throw()` with the `noexcept` form.

[ Resolution proposed by ballot comment: ]

Replace all non-throwing exception specifications of the form 'throw()' with the 'noexcept' form.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of n3148 would satisfy this request.

[ 2010-Batavia: ]

> Resolved by adopting n3148.

Proposed resolution:

See n3148 See n3150 See n3195 See n3155 See n3156

### 1345(i). Library classes should have `noexcept` move operations

Section: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-61

All library types should have non-throwing move constructors and move-assignment operators unless wrapping a type with a potentially throwing move operation. When such a type is a class-template, these operations should have a conditional `noexcept` specification.

There are many other places where a `noexcept` specification may be considered, but the move operations are a special case that must be called out, to effectively support the `move_if_noexcept` function template.

[ Resolution proposed by ballot comment: ]

Review every class and class template in the library. If `noexcept` move constructor/assignment operators can be implicitly declared, then they should be implicitly declared, or explicitly defaulted. Otherwise, a move constructor/move assignment operator with a `noexcept` exception specification should be provided.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of n3157 would satisfy this request.

[2011-03-24 Madrid meeting]

Resolved by papers to be listed later

Proposed resolution:

See n3157

### 1346(i). Apply `noexcept` where library specification does not permit exceptions

Section: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 1352

Discussion:

Addresses GB-62, CH-17

Issues with efficiency and unsatisfactory semantics mean many library functions document they do not throw exceptions with a Throws: Nothing clause, but do not advertise it with an exception specification. The semantic issues are largely resolved with the new 'noexcept' specifications, and the noexcept operator means we will want to detect these guarantees programatically in order to construct programs taking advantage of the guarantee.

[ Resolution proposed by ballot comment: ]

Add a `noexcept` exception specification on each libary API that offers an unconditional Throws: Nothing guarantee. Where the guarantee is conditional, add the appropriate `noexcept(constant-expression)` if an appropriate constant expression exists.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of n3149 would satisfy this request.

[ 2010-Batavia: ]

> Resolved by adopting n3149.

Proposed resolution:

This issue is resolved by the adoption of n3195

### 1347(i). Apply `noexcept` judiciously throughout the library

Section: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-63, US-80

Since the newly introduced operator `noexcept` makes it easy (easier than previously) to detect whether or not a function has been declared with the empty exception specification (including `noexcept`) library functions that cannot throw should be decorated with the empty exception specification. Failing to do so and leaving it as a matter of QoI would be detrimental to portability and efficiency.

[ Resolution proposed by ballot comment ]

Review the whole library, and apply the `noexcept` specification where it is appropriate.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of the combination of n3155, n3156, n3157, n3167 would satisfy this request. The paper n3150 is related to this as well.

[ 2010 Batavia: ]

While the LWG expects to see further papers in this area, sufficient action was taken in Batavia to close the issue as Resolved by the listed papers.

Proposed resolution:

See n3155, n3156, n3157, n3167 and remotely n3150

### 1349(i). `swap` should not throw

Section: 16 [library] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-65

Nothrowing `swap` operations are key to many C++ idioms, notably the common copy/swap idiom to provide the strong exception safety guarantee.

[ Resolution proposed by ballot comment ]

Where possible, all library types should provide a `swap` operation with an exception specification guaranteeing no exception shall propagate. Where `noexcept(true)` cannot be guaranteed to not terminate the program, and the `swap` in questions is a template, an exception specification with the appropriate conditional expression could be specified.

[2011-03-13: Daniel comments and drafts wording]

During a survey of the library some main categories for potential `noexcept` `swap` function could be isolated:

1. Free `swap` functions that are specified in terms of already `noexcept` `swap` member functions, like that of `valarray`.
2. Free `swap` of `std::function`, and member and free `swap` functions of stream buffers and streams where considered but rejected as good candidates, because of the danger to potentially impose requirements of existing implementations. These functions could be reconsidered as candidates in the future.

Negative list:

1. Algorithms related to swap, like `iter_swap`, have not been touched, because there are no fundamental exceptions constraints on iterator operations in general (only for specific types, like library container iterators)

While evaluating the current state of `swap` functions of library components it was observed that several conditional `noexcept` functions have turned into unconditional ones, e.g. in the header `<utility>` synopsis:

>     template<class T> void swap(T& a, T& b) noexcept;

The suggested resolution shown below also attempts to fix these cases.

[2011-03-22 Daniel redrafts to satisfy new criteria for applying `noexcept`. Parts resolved by N3263-v2 and D3267 are not added here.]

Proposed resolution:

1. Edit 22.2 [utility] p. 2, header `<utility>` synopsis and 22.2.2 [utility.swap] before p. 1, as indicated (The intent is to fix an editorial omission):

   >     template<class T> void swap(T& a, T& b) noexcept(see below);

2. Edit the prototype declaration in 22.3.2 [pairs.pair] before p. 34 as indicated (The intent is to fix an editorial omission):

   >     void swap(pair& p) noexcept(see below);

3. Edit 22.4.1 [tuple.general] p. 2 header `<tuple>` synopsis and 22.4.12 [tuple.special] before p. 1 as indicated (The intent is to fix an editorial omission):

   >     template <class... Types>
   >     void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);

4. Edit 22.4.4 [tuple.tuple], class template `tuple` synopsis and 22.4.4.4 [tuple.swap] before p. 1 as indicated (The intent is to fix an editorial omission):

   >     void swap(tuple&) noexcept(see below);

5. Edit 20.2.2 [memory.syn] p. 1, header `<memory>` synopsis as indicated (The intent is to fix an editorial omission of the proposing paper N3195).

   >     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

6. Edit header `<valarray>` synopsis, 29.6.1 [valarray.syn] and 29.6.3.4 [valarray.special] before p. 1 as indicated [Drafting comment: The corresponding member swap is already noexcept]:

   >     template<class T> void swap(valarray<T>&, valarray<T>&) noexcept;

### 1353(i). Clarify the state of a moved-from object

Section: 16 [library] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses CH-18

The general approach on moving is that a library object after moving out is in a "valid but unspecified state". But this is stated at the single object specifications, which is error prone (especially if the move operations are implicit) and unnecessary duplication.

[ Resolution proposed by ballot comment ]

Consider putting a general statement to the same effect into clause 17.

[2010-11-05 Beman provides exact wording. The wording was inspired by Dave Abrahams' message c++std-lib-28958, and refined with help from Alisdair, Daniel, and Howard. ]

[2011-02-25 P/R wording superseded by N3241. ]

Proposed resolution:

Resolved by N3241

### 1354(i). The definition of deadlock excludes cases involving a single thread

Section: 3.16 [defns.deadlock] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-52

The definition of deadlock in 17.3.7 excludes cases involving a single thread making it incorrect.

[ Resolution proposed by ballot comment ]

The definition should be corrected.

[ 2010 Batavia Concurrency group provides a Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 3.16 [defns.deadlock] as indicated:

> deadlock

> twoone or more threads are unable to continue execution because each is blocked waiting for one or more of the others to satisfy some condition.

### 1355(i). The definition of move-assignment operator is redundant

Section: 99 [defns.move.assign.op] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-50

This definition of move-assignment operator is redundant and confusing now that the term move-assignment operator is defined by the core language in subclause 12.8p21.

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 provides a superior resolution.

[ Resolution proposed by ballot comment ]

Strike subclause 99 [defns.move.assign.op]. Add a cross-reference to (12.8) to 17.3.12.

Proposed resolution:

Resolved by paper n3142.

### 1356(i). The definition of move-constructor is redundant

Section: 99 [defns.move.ctor] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-51

This definition of move-constructor is redundant and confusing now that the term constructor is defined by the core language in subclause 12.8p3.

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 provides a superior resolution.

[ 2010 Batavia: resolved as NAD Editorial by adopting paper n3142. ]

Original proposed resolution preserved for reference:

> Strike subclause 17.3.14, [defns.move.ctor]

> > 17.3.14 [defns.move.ctor]
> > move constructor a constructor which accepts only an rvalue argument of the type being constructed and might modify the argument as a side effect during construction.

Proposed resolution:

Resolved by paper n3142.

### 1357(i). Library bitmask types to not satisfy the bimask type requirements

Section: 16.3.3.3.3 [bitmask.types] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-53

The bitmask types defined in 27.5.2 and 28.5 contradict the bitmask type requirements in 17.5.2.1.3, and have missing or incorrectly defined operators.

[ Resolution proposed by ballot comment ]

See Appendix 1 - Additional Details

[ 2010 - Rapperswil ]

The paper n3110 was made available during the meeting to resolve this comment, but withdrawn from formal motions to give others time to review the document. There was no technical objection, and it is expected that this paper will go forward at the next meeting.

Proposed resolution:

See n3110.

### 1360(i). Add `<atomic>` to free-standing implementations

Section: 16.4.2.5 [compliance] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-57

The atomic operations facility is closely tied to clause 1 and the memory model. It is not easily supplied as an after-market extension, and should be trivial to implement of a single-threaded serial machine. The consequence of not having this facility will be poor interoperability with future C++ libraries that memory model concerns seriously, and attempt to treat them in a portable way.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add `<atomic>` to table 15, headers required for a free-standing implementation.

### 1362(i). Description of binding to rvalue-references should use the new 'xvalue' vocabulary

Section: 16.4.5.9 [res.on.arguments] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-21

Priority: Not Prioritized

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Discussion:

Addresses US-82

16.4.5.9 [res.on.arguments] p.1. b.3: The second Note can benefit by adopting recent nomenclature.

[ Resolution proposed by the ballot comment: ]

Rephrase the Note in terms of xvalue.

[ Pre-Batavia: ]

Walter Brown provides wording.

[Batavia: Immediate]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Amend the note in 17.6.3.9 [res.on.arguments] p1 bullet 3.

> [ Note: If a program casts an lvalue to an rvaluexvalue while passing that lvalue to a library function (e.g. by calling the function with the argument `move(x)`), the program is effectively asking that function to treat that lvalue as a temporary. The implementation is free to optimize away aliasing checks which might be needed if the argument was anlvalue. — end note]

### 1363(i). `offsetof` should be marked `noexcept`

Section: 17.2 [support.types] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-68

There is no reason for the offsetof macro to invoke potentially throwing operations, so the result of noexcept(offsetof(type,member-designator)) should be true.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add to the end of 18.2p4:

> No operation invoked by the offsetof macro shall throw an exception, and `noexcept(offsetof(type,member-designator))` shall be true.

### 1364(i). It is not clear how `exception_ptr` is synchronized

Section: 17.9.7 [propagation] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses CH-19

It is not clear how `exception_ptr` is synchronized.

[ Resolution proposed by ballot comment ]

Make clear that accessing in different threads multiple `exception_ptr` objects that all refer to the same exception introduce a race.

[2011-03-08: Lawrence comments and drafts wording]

I think fundamentally, this issue is NAD, but clarification would not hurt.

> Proposed resolution

> Add a new paragraph to 17.9.7 [propagation] after paragraph 6 as follows:

> [Note: Exception objects have no synchronization requirements, and expressions using them may conflict (6.10.2 [intro.multithread]). — end note]

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1365(i). Thread-safety of handler functions

Section: 17.6.4 [alloc.errors] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-71

The thread safety of `std::set_new_handler()`, `std::set_unexpected()`, `std::set_terminate()`, is unspecified making the the functions impossible to use in a thread safe manner.

[ Resolution proposed by ballot comment: ]

> The thread safety guarantees for the functions must be specified and new interfaces should be provided to make it possible to query and install handlers in a thread safe way.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of n3122 partially addresses this request. This issue is related to 1366(i).

[ 2010-Batavia: ]

> Resolved by adopting n3189.

Proposed resolution:

Resolved in Batavia by accepting n3189.

### 1366(i). New-handler and data races

Section: 17.6.3.5 [new.delete.dataraces] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses DE-14

It is unclear how a user replacement function can simultaneously satisfy the race-free conditions imposed in this clause and query the new-handler in case of a failed allocation with the only available, mutating interface `std::set_new_handler`.

[ Resolution proposed by ballot comment: ]

Offer a non-mutating interface to query the current new-handler.

[ 2010-10-24 Daniel adds: ]

> Accepting n3122 would solve this issue. This issue is related to 1365(i).

[ 2010-Batavia: ]

> Resolved by adopting n3189.

Proposed resolution:

Resolved in Batavia by accepting n3189.

### 1367(i). Deprecate library support for checking dynamic exception specifications

Section: 99 [exception.unexpected] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-72

Dynamic exception specifications are deprecated, so clause 18.8.2 that describes library support for this facility should move to Annex D, with the exception of the `bad_exception` class which is retained to indicate other failures in the exception dispatch mechanism (e.g. calling `current_exception()`).

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

With the exception of 18.8.2.1 [bad.exception], move clause 18.8.2 directly to Annex D. [bad.exception] should simply become the new 18.8.2.

### 1368(i). Thread safety of `std::uncaught_exception()`

Section: 99 [depr.uncaught] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

Addresses GB-73

The thread safety `std::uncaught_exception()` and the result of the function when multiple threads throw exceptions at the same time are unspecified. To make the function safe to use in the presence of exceptions in multiple threads the specification needs to be updated.

[ Resolution proposed by ballot comment ]

Update this clause to support safe calls from multiple threads without placing synchronization requirements on the user.

[ 2010 Batavia Concurrency group provides a Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 99 [uncaught] p. 1 as follows:

Returns: `true` after the current thread has initialized initializing an exception object (15.1) until a handler for the exception (including `unexpected()` or `terminate()`) is activated (15.3). [ Note: This includes stack unwinding (15.2). — end note ]

### 1370(i). `throw_with_nested` should not use perfect forwarding

Section: 17.9.8 [except.nested] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-84

The `throw_with_nested` specification passes in its argument as `T&&` (perfect forwarding pattern), but then discusses requirements on `T` without taking into account that `T` may be an lvalue-reference type. It is also not clear in the spec that `t` is intended to be perfectly forwarded.

[ Resolution proposed by ballot comment ]

Patch [except.nested] p6-7 to match the intent with regards to requirements on `T` and the use of `std::forward<T>(t)`.

[ 2010-10-24 Daniel adds: ]

> Accepting n3144 would solve this issue.

[2010-11-10 Batavia: LWG accepts Howard's updated wording with corrected boo boos reported by Sebastian Gesemann and Pete Becker, which is approved for Immediate adoption this meeting.]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 18.8.7 nested_exception [except.nested] as indicated:

>     [[noreturn]] template <class T> void throw_with_nested(T&& t);

> > Let `U` be `remove_reference<T>::type`

> > 6 Requires: `T U` shall be `CopyConstructible`.

> > 7 Throws: If `T U` is a non-union class type not derived from `nested_exception`, an exception of unspecified type that is publicly derived from both `T U` and `nested_exception` and constructed from `std::forward<T>(t)`, otherwise throws `std::forward<T>(t)`.

### 1372(i). Adopt recommended practice for standard error categories

Section: 19.5.3.5 [syserr.errcat.objects] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-76

The C++0x FCD recommends, in a note (see 19.5.1.1/1), that users create a single `error_category` object for each user defined error category and specifies `error_category` equality comparsions based on equality of addresses (19.5.1.3). The Draft apparently ignores this when specifying standard error category objects in section 19.5.1.5, by allowing the `generic_category()` and `system_category()` functions to return distinct objects for each invocation.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Append a new sentence to 19.5.3.5 [syserr.errcat.objects]/1, and append the same sentence to 19.5.1.5/3.

> All calls of this function return references to the same object.

### 1377(i). The revised `forward` is not compatible with access-control

Section: 22.2 [utility] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-90

In n3090, at variance with previous iterations of the idea discussed in papers and incorporated in WDs, `std::forward` is constrained via `std::is_convertible`, thus is not robust wrt access control. This causes problems in normal uses as implementation detail of member functions. For example, the following snippet leads to a compile time failure, whereas that was not the case for an implementation along the lines of n2835 (using `enable_if`s instead of concepts for the constraining, of course)

    #include <utility>
    struct Base { Base(Base&&); };

    struct Derived
      : private Base
    {
      Derived(Derived&& d)
        : Base(std::forward<Base>(d)) { }
    };

In other terms, LWG 1054 can be resolved in a better way, the present status is not acceptable.

[ 2010-10-24 Daniel adds: ]

> Accepting n3143 would solve this issue.

Proposed resolution:

Resolved as NAD Editorial by paper n3143.

### 1378(i). `pair` and `tuple` have too many conversions

Section: 22.3 [pairs] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses DE-15

Several function templates of `pair` and `tuple` allow for too many implicit conversions, for example:

    #include <tuple>
    std::tuple<char*> p(0); // Error?

    struct A { explicit A(int){} };
    A a = 1; // Error
    std::tuple<A> ta = std::make_tuple(1); // OK?

[ Resolution proposed by ballot comment ]

Consider to add wording to constrain these function templates.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

Proposed resolution:

See n3140.

### 1379(i). `pair` copy-assignment not consistent for references

Section: 22.3.2 [pairs.pair] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-95

Copy-assignment for `pair` is defaulted and does not work for pairs with reference members. This is inconsistent with conversion-assignment, which deliberately succeeds even if one or both elements are reference types, just as for `tuple`. The copy-assignment operator should be consistent with the conversion-assignment operator and with `tuple`'s assignment operators.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would provide a superior resolution, because `pair` does not depend on the semantic requirements of `CopyAssignable`.

[ 2010-11 Batavia ]

Resolved by adopting n3140.

Proposed resolution:

Add to `pair` synopsis:

> `pair& operator=(const pair& p);`

Add before paragraph 9:

> `pair& operator=(const pair& p);`

> > Requires: `T1` and `T2` shall satisfy the requirements of `CopyAssignable`.

> > Effects: Assigns `p.first` to `first` and `p.second` to `second`. Returns: `*this`.

### 1380(i). `pair` and `tuple` of references need to better specify move-semantics

Section: 22.3 [pairs] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses DE-16

Several `pair` and `tuple` functions in regard to move operations are incorrectly specified if the member types are references, because the result of a `std::move` cannot be assigned to lvalue-references. In this context the usage of the requirement sets `MoveConstructible` and `CopyConstructible` also doesn't make sense, because non-const lvalue-references cannot satisfy these requirements.

[ Resolution proposed by ballot comment ]

Replace the usage of `std::move` by that of `std::forward` and replace `MoveConstructible` and `CopyConstructible` requirements by other requirements.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

[ 2010-11 Batavia: ]

> Resolved by adopting n3140.

Proposed resolution:

See n3140.

### 1381(i). Replace `pair`'s range support by proper range facility

Section: 99 [pair.range] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-85

While `std::pair` may happen to hold a pair of iterators forming a valid range, this is more likely a coincidence than a feature guaranteed by the semantics of the `pair` template. A distinct range-type should be supplied to enable the new for-loop syntax rather than overloading an existing type with a different semantic.

If a replacement facility is required for C++0x, consider n2995.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Strike 20.3.5.4 and the matching declarations in 20.3 header synopsis.

### 1382(i). `pair` and `tuple` constructors should `forward` arguments

Section: 22.3 [pairs] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-96

`pair` and `tuple` constructors and assignment operators use `std::move` when they should use `std::forward`. This causes lvalue references to be erroneously converted to rvalue references. Related requirements clauses are also wrong.

[ Resolution proposed by ballot comment ]

> See Appendix 1 - Additional Details

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

[ 2010-11 Batavia ]

Resolved by adopting n3140.

Proposed resolution:

See n3140.

### 1383(i). Inconsistent defaulted move/copy members in `pair` and `tuple`

Section: 22.3 [pairs], 22.4 [tuple] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-97

`pair`'s class definition in N3092 22.3.2 [pairs.pair] contains "`pair(const pair&) = default;`" and "`pair& operator=(pair&& p);`". The latter is described by 22.3.2 [pairs.pair] p.12-13.

"`pair(const pair&) = default;`" is a user-declared explicitly defaulted copy constructor. According to [class.copy]/10, this inhibits the implicitly-declared move constructor. `pair` should be move constructible. ( [class.copy]/7 explains that "`pair(pair<U, V>&& p)`" will never be instantiated to move `pair<T1, T2>` to `pair<T1, T2>`.)
"`pair& operator=(pair&& p);`" is a user-provided move assignment operator (according to 9.6.2 [dcl.fct.def.default]/4: "A special member function is user-provided if it is user-declared and not explicitly defaulted on its first declaration."). According to [class.copy]/20, this inhibits the implicitly-declared copy assignment operator. `pair` should be copy assignable, and was in C++98/03. (Again, [class.copy]/7 explains that "`operator=(const pair<U, V>& p)`" will never be instantiated to copy `pair<T1, T2>` to `pair<T1, T2>`.)
Additionally, "`pair& operator=(pair&& p);`" is unconditionally defined, whereas according to [class.copy]/25, defaulted copy/move assignment operators are defined as deleted in several situations, such as when non-static data members of reference type are present.

If "`pair(const pair&) = default;`" and "`pair& operator=(pair&& p);`" were removed from `pair`'s class definition in 22.3.2 [pairs.pair] and from 22.3.2 [pairs.pair]/12-13, `pair` would receive implicitly-declared copy/move constructors and copy/move assignment operators, and [class.copy]/25 would apply. The implicitly-declared copy/move constructors would be trivial when `T1` and `T2` have trivial copy/move constructors, according to [class.copy]/13, and similarly for the assignment operators, according to [class.copy]/27. Notes could be added as a reminder that these functions would be implicitly-declared, but such notes would not be necessary (the Standard Library specification already assumes a high level of familiarity with the Core Language, and casual readers will simply assume that `pair` is copyable and movable).

Alternatively, `pair` could be given explicitly-defaulted copy/move constructors and copy/move assignment operators. This is a matter of style.

`tuple` is also affected. `tuple`'s class definition in 22.4 [tuple] contains:

    tuple(const tuple&) = default;
    tuple(tuple&&);
    tuple& operator=(const tuple&);
    tuple& operator=(tuple&&);

They should all be removed or all be explicitly-defaulted, to be consistent with `pair`. Additionally, 22.4.4.2 [tuple.cnstr]/8-9 specifies the behavior of an explicitly defaulted function, which is currently inconsistent with `pair`.

[ Resolution proposed by ballot comment: ]

> Either remove "`pair(const pair&) = default;`" and "`pair& operator=(pair&& p);`" from `pair`'s class definition in 22.3.2 [pairs.pair] and from 22.3.2 [pairs.pair] p.12-13, or give pair explicitly-defaulted copy/move constructors and copy/move assignment operators.
> Change `tuple` to match.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue: The move/copy constructor will be defaulted, but the corresponding assignment operators need a non-default implementation because they are supposed to work for references as well.

Proposed resolution:

See n3140.

### 1384(i). Function `pack_arguments` is poorly named

Section: 22.4.5 [tuple.creation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-98

`pack_arguments` is poorly named. It does not reflect the fact that it is a tuple creation function and that it forwards arguments.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Rename `pack_arguments` to `forward_as_tuple` throughout the standard.

### 1385(i). `tuple_cat` should be a single variadic signature

Section: 22.4.5 [tuple.creation] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-88

The `tuple_cat` template consists of four overloads and that can concatenate only two `tuple`s. A single variadic signature that can concatenate an arbitrary number of `tuple`s would be preferred.

[ Resolution proposed by ballot comment: ]

> Adopt a simplified form of the proposal in n2975, restricted to `tuple`s and neither requiring nor outlawing support for other `tuple`-like types.

[ 2010 Rapperswil: Alisdair to provide wording. ]

[ 2010-11-06: Daniel comments and proposes some alternative wording: ]

There are some problems in the wording: First, even though the result type `tuple<see below>` implies it, the specification of the contained tuple element types is missing. Second, the term "`tuple` protocol" is not defined anywhere and I see no reason why this normative wording should not be a non-normative note. We could at least give a better approximation, maybe "tuple-like protocol" as indicated from header `<utility>` synopsis. Further, it seems to me that the effects need to contain a combination of `std::forward` with the call of `get`. Finally I suggest to replace the requirements `Move/CopyConstructible` by proper usage of `is_constructible`, as indicated by n3140.

[ 2010 Batavia ]

Moved to Ready with Daniel's improved wording.

Proposed resolution:

Note: This alternate proposed resolution works only if 1191(i) has been accepted.

1. Change 22.4.1 [tuple.general] p. 2, header `<tuple>` synopsis, as indicated:

   >     namespace std {

   >     ...

   >     // 20.4.2.4, tuple creation functions:
   >     const unspecified ignore;

   >     template <class... Types>
   >       tuple<VTypes...> make_tuple(Types&&...);
   >       template <class... Types>
   >       tuple<ATypes...> forward_as_tuple(Types&&...);

   >     template<class... Types>
   >       tuple<Types&...> tie(Types&...);

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, const tuple<UTypes...>&);
   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, const tuple<UTypes...>&);
   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, tuple<UTypes...>&&);
   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, tuple<UTypes...>&&);
   >     template <class... Tuples>
   >       tuple<CTypes...> tuple_cat(Tuples&&...);

   >     ...

2. Change 22.4.5 [tuple.creation] as indicated:

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

   > > 8 Requires: All the types in `TTypes` shall be `CopyConstructible` (Table 35). All the types in `UTypes` shall be `CopyConstructible` (Table 35).

   > > 9 Returns: A `tuple` object constructed by copy constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and copy constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);

   > > 10 Requires: All the types in `TTypes` shall be `MoveConstructible` (Table 34). All the types in `UTypes` shall be `CopyConstructible` (Table 35).

   > > 11 Returns: A `tuple` object constructed by move constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and copy constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);

   > > 12 Requires: All the types in `TTypes` shall be `CopyConstructible` (Table 35). All the types in `UTypes` shall be `MoveConstructible` (Table 34).

   > > 13 Returns: A `tuple` object constructed by copy constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and move constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);

   > > 14 Requires: All the types in `TTypes` shall be `MoveConstructible` (Table 34). All the types in `UTypes` shall be `MoveConstructible` (Table 34).

   > > 15 Returns: A `tuple` object constructed by move constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and move constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

   >     template <class... Tuples>
   >       tuple<CTypes...> tuple_cat(Tuples&&... tpls);

   > > 8 Let `Ti` be the `i`th type in `Tuples`, `Ui` be `remove_reference<Ti>::type`, and `tpi` be the `i`th parameter in the function parameter pack `tpls`, where all indexing is zero-based in the following paragraphs of this sub-clause [tuple.creation].

   > > 9 Requires: For all `i`, `Ui` shall be the type cv`i` `tuple<Argsi...>`, where cv`i` is the (possibly empty) `i`th cv-qualifier-seq, and `Argsi` is the parameter pack representing the element types in `Ui`. Let `Aik` be the `ki`th type in `Argsi`, then for all `Aik` the following requirements shall be satisfied: If `Ti` is deduced as an lvalue reference type, then `is_constructible<Aik, cvi Aik&>::value == true`, otherwise `is_constructible<Aik, cvi Aik&&>::value == true`.

   > > 10 Remarks: The types in `CTypes` shall be equal to the ordered sequence of the expanded types `Args0..., Args1..., Argsn-1...`, where `n` equals `sizeof...(Tuples)`. Let `ei...` be the `i`th ordered sequence of tuple elements of the result `tuple` object corresponding to the type sequence `Argsi`.

   > > 11 Returns: A `tuple` object constructed by initializing the `ki`th type element `eik` in `ei...` with `get<ki>(std::forward<Ti>(tpi))` for each valid `ki` and each element group `ei` in order.

   > > 12 [Note: An implementation may support additional types in the parameter pack `Tuples`, such as `pair` and `array` that support the `tuple`-like protocol. — end note]

### 1386(i). `pack_arguments` overly complex

Section: 22.4.5 [tuple.creation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-99

`pack_arguments` is overly complex.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

This issue resulted from a lack of understanding of how references are forwarded. The definition of `pack_arguments` should be simply:
`template <class... Types> tuple<ATypes&&> pack_arguments(Types&&...t);`
Types: Let `Ti` be each type in `Types`....
Effects: ...
Returns:
`tuple<ATypes&&...>(std::forward<Types>(t)...)`
The synopsis should also change to reflect this simpler signature.

### 1387(i). Range support by `tuple` should be removed

Section: 99 [tuple.range] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-87

There is no compelling reason to assume a heterogeneous tuple of two elements holds a pair of iterators forming a valid range. Unlike `std::pair`, there are no functions in the standard library using this as a return type with a valid range, so there is even less reason to try to adapt this type for the new for-loop syntax.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Strike 20.4.2.10 and the matching declarations in the header synopsis in 20.4.

### 1388(i). LWG 1281 incorrectly accepted

Section: 21.5.3 [ratio.ratio] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-100

LWG 1281 was discussed in Pittsburgh, and the decision there was to accept the typedef as proposed and move to Review. Unfortunately the issue was accidentally applied to the FCD, and incorrectly. The FCD version of the typedef refers to `ratio<N, D>`, but the typedef is intended to refer to `ratio<num, den>` which in general is not the same type.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Accept the current proposed wording of LWG 1281(i) which adds:
`typedef ratio<num, den> type;`

### 1389(i). Compile-time rational arithmetic and overflow

Section: 21.5.4 [ratio.arithmetic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-89

The alias representations of the `ratio` arithmetic templates do not allow implementations to avoid overflow, since they explicitly specify the form of the aliased template instantiation. For example `ratio_multiply`, `ratio<2, LLONG_MAX>` is required to alias `ratio<2*LLONG_MAX, LLONG_MAX*2>`, which overflows, so is ill-formed. However, this is trivially equal to `ratio<1, 1>`. It also contradicts the opening statement of 21.5.4 [ratio.arithmetic] p. 1 "implementations may use other algorithms to compute these values".

[ 2010-10-25 Daniel adds: ]

> Accepting n3131 would solve this issue.

[Batavia: Resolved by accepting n3210.]

Proposed resolution:

Change the wording in 21.5.4 [ratio.arithmetic] p. 2-5 as follows:

>     template <class R1, class R2> using ratio_add = see below;

> > 2 The type `ratio_add<R1, R2>` shall be a synonym for `ratio<T1, T2>` `ratio<U, V>` such that `ratio<U, V>::num` and `ratio<U, V>::den` are the same as the corresponding members of `ratio<T1, T2>` would be in the absence of arithmetic overflow where `T1` has the value `R1::num * R2::den + R2::num * R1::den` and `T2` has the value `R1::den * R2::den`. If the required values of `ratio<U, V>::num` and `ratio<U, V>::den` cannot be represented in `intmax_t` then the program is ill-formed.

>     template <class R1, class R2> using ratio_subtract = see below;

> > 3 The type `ratio_subtract<R1, R2>` shall be a synonym for `ratio<T1, T2>` `ratio<U, V>` such that `ratio<U, V>::num` and `ratio<U, V>::den` are the same as the corresponding members of `ratio<T1, T2>` would be in the absence of arithmetic overflow where `T1` has the value `R1::num * R2::den - R2::num * R1::den` and `T2` has the value `R1::den * R2::den`. If the required values of `ratio<U, V>::num` and `ratio<U, V>::den` cannot be represented in `intmax_t` then the program is ill-formed.

>     template <class R1, class R2> using ratio_multiply = see below;

> > 4 The type `ratio_multiply<R1, R2>` shall be a synonym for `ratio<T1, T2>` `ratio<U, V>` such that `ratio<U, V>::num` and `ratio<U, V>::den` are the same as the corresponding members of `ratio<T1, T2>` would be in the absence of arithmetic overflow where `T1` has the value `R1::num * R2::num` and `T2` has the value `R1::den * R2::den`. If the required values of `ratio<U, V>::num` and `ratio<U, V>::den` cannot be represented in `intmax_t` then the program is ill-formed.

>     template <class R1, class R2> using ratio_divide = see below;

> > 5 The type `ratio_divide<R1, R2>` shall be a synonym for `ratio<T1, T2>` `ratio<U, V>` such that `ratio<U, V>::num` and `ratio<U, V>::den` are the same as the corresponding members of `ratio<T1, T2>` would be in the absence of arithmetic overflow where `T1` has the value `R1::num * R2::den` and `T2` has the value `R1::den * R2::num`. If the required values of `ratio<U, V>::num` and `ratio<U, V>::den` cannot be represented in `intmax_t` then the program is ill-formed.

### 1390(i). Limit speculative compilation for constructible/convertible traits

Section: 21 [meta] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses DE-17

Speculative compilation for `std::is_constructible` and `std::is_convertible` should be limited, similar to the core language (see 14.8.2 paragraph 8).

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 would solve this issue.

Proposed resolution:

Resolved by paper n3142.

### 1391(i). constructible/convertible traits and access control

Section: 21 [meta] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses DE-18

Several type traits require compiler support, e.g. `std::is_constructible` or `std::is_convertible`. Their current specification seems to imply, that the corresponding test expressions should be well-formed, even in absense of access:

    class X { X(int){} };
    constexpr bool test = std::is_constructible<X, int>::value;

The specification does not clarify the context of this test and because it already goes beyond normal language rules, it's hard to argue by means of normal language rules what the context and outcome of the test should be.

[ Resolution proposed by ballot comment ]

Specify that `std::is_constructible` and `std::is_convertible` will return `true` only for public constructors/conversion functions.

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 would solve this issue.

Proposed resolution:

Resolved by paper n3142.

### 1392(i). `result_of` should support pointer-to-data-member

Section: 21.3.5 [meta.unary] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-102

Despite Library Issue 520's ("`result_of` and pointers to data members") resolution of CD1, the FCD's `result_of` supports neither pointers to member functions nor pointers to data members. It should.

[ Resolution proposed by ballot comment ]

Ensure `result_of` supports pointers to member functions and pointers to data members.

[ 2010-10-24 Daniel adds: ]

> Accepting n3123 would solve this issue.

Proposed resolution:

Resolved by n3123.

### 1393(i). Trivial traits imply `noexcept`

Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-92

Trivial functions implicitly declare a `noexcept` exception specification, so the references to `has_trivial_*` traits in the `has_nothrow_*` traits are redundant, and should be struck for clarity.

[ Resolution proposed by ballot comment ]

For each of the `has_nothrow_something` traits, remove all references to the matching `has_trivial_something` traits.

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 would solve this issue.

Proposed resolution:

Resolved by n3142.

### 1394(i). `is_constructible` reports false positives

Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses DE-19

The fundamental trait `is_constructible` reports false positives, e.g.

    is_constructible<char*, void*>::value

evaluates to true, even though a corresponding variable initialization would be ill-formed.

[ Resolved in Rapperswil by paper N3047. ]

Proposed resolution:

Remove all false positives from the domain of `is_constructible`.

### 1397(i). Deprecate '98 binders

Section: 22.10 [function.objects] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-95

The adaptable function protocol supported by `unary_function/binary_function` has been superceded by lambda expressions and `std::bind`. Despite the name, the protocol is not very adaptable as it requires intrusive support in the adaptable types, rather than offering an external traits-like adaption mechanism. This protocol and related support functions should be deprecated, and we should not make onerous requirements for the specification to support this protocol for callable types introduced in this standard revision, including those adopted from TR1. It is expected that high-quality implementations will provide such support, but we should not have to write robust standard specifications mixing this restricted support with more general components such as `function`, `bind` and `reference_wrapper`.

[ Resolution proposed by ballot comment ]

Move clauses 20.8.3, 20.8.9, 20.8.11 and 20.8.12 to Annex D.

Remove the requirements to conditionally derive from `unary/binary_function` from `function`, `reference_wrapper`, and the results of calling `mem_fn` and `bind`.

[ 2010-10-24 Daniel adds: ]

> Accepting n3145 would solve this issue.

Proposed resolution:

Resolved by paper N3198.

### 1399(i). `function` does not need an `explicit` default constructor

Section: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses JP-3

`explicit` default contructor is defined in `std::function`. Although it is allowed according to 12.3.1, it seems unnecessary to qualify the constructor as `explicit`. If it is `explicit`, there will be a limitation in `initializer_list`.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Remove `explicit`.

    namespace std {
    template<class> class function;
    // undefined
    template<class R, class... ArgTypes>
    class function<R(ArgTypes...)>
    : public unary_function<T1, R>
    // iff sizeof...(ArgTypes) == 1 and ArgTypes contains T1
    : public binary_function<T1, T2, R>
    // iff sizeof...(ArgTypes) == 2 and ArgTypes contains T1 andT2
    {
    public:typedef R result_type;
    // 20.8.14.2.1, construct/copy/destroy:
      explicit function();

### 1400(i). FCD `function` does not need an `explicit` default constructor

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses JP-4

Really does the `function` require that default constructor is `explicit`?

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Remove `explicit`.

    function();
    template <class A>
    function(allocator_arg_t, const A& a);

### 1401(i). Provide support for `unique_ptr<T> == nullptr`

Section: 20.2 [memory] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-99

One reason that the `unique_ptr` constructor taking a `nullptr_t` argument is not `explicit` is to allow conversion of `nullptr` to `unique_ptr` in contexts like equality comparison. Unfortunately `operator==` for `unique_ptr` is a little more clever than that, deducing template parameters for both arguments. This means that `nullptr` does not get deduced as `unique_ptr` type, and there are no other comparison functions to match.

[ Resolution proposed by ballot comment: ]

> Add the following signatures to 20.2 [memory] p.1, `<memory>` header synopsis:

> >     template<typename T, typename D>
> >     bool operator==(const unique_ptr<T, D> & lhs, nullptr_t);
> >     template<typename T, typename D>
> >     bool operator==(nullptr_t, const unique_ptr<T, D> & rhs);
> >     template<typename T, typename D>
> >     bool operator!=(const unique_ptr<T, D> & lhs, nullptr_t);
> >     template<typename T, typename D>
> >     bool operator!=(nullptr_t, const unique_ptr<T, D> & rhs);

[ 2010-11-02 Daniel comments and provides a proposed resolution: ]

> The same problem applies to `shared_ptr` as well: In both cases there are no conversions considered because the comparison functions are templates. I agree with the direction of the proposed resolution, but I believe it would be very surprising and inconsistent, if given a smart pointer object `p`, the expression `p == nullptr` would be provided, but not `p < nullptr` and the other relational operators. According to 7.6.9 [expr.rel] they are defined if null pointer values meet other pointer values, even though the result is unspecified for all except some trivial ones. But null pointer values are nothing special here: The Library already defines the relational operators for both `unique_ptr` and `shared_ptr` and the outcome of comparing non-null pointer values will be equally unspecified. If the idea of supporting `nullptr_t` arguments for relational operators is not what the committee prefers, I suggest at least to consider to remove the existing relational operators for both `unique_ptr` and `shared_ptr` for consistency. But that would not be my preferred resolution of this issue.

> The number of overloads triple the current number, but I think it is much clearer to provide them explicitly instead of adding wording that attempts to say that "sufficient overloads" are provided. The following proposal makes the declarations explicit.

> Additionally, the proposal adds the missing declarations for some `shared_ptr` comparison functions for consistency.

[ 2010-11-03 Daniel adds: ]

Issue 1297(i) is remotely related. The following proposed resolution splits this bullet into sub-bullets A and B. Sub-bullet A would also solve 1297(i), but sub-bullet B would not.

A further remark in regard to the proposed semantics of the ordering of `nullptr` against other pointer(-like) values: One might think that the following definition might be superior because of simplicity:

>     template<class T>
>     bool operator<(const shared_ptr<T>& a, nullptr_t);
>     template<class T>
>     bool operator>(nullptr_t, const shared_ptr<T>& a);

> > Returns: `false`.

The underlying idea behind this approach is the assumption that nullptr corresponds to the least ordinal pointer value. But this assertion does not hold for all supported architectures, therefore this approach was not followed because it would lead to the inconsistency, that the following assertion could fire:

>     shared_ptr<int> p(new int);
>     shared_ptr<int> null;
>     bool v1 = p < nullptr;
>     bool v2 = p < null;
>     assert(v1 == v2);

[2011-03-06: Daniel comments]

The current issue state is not acceptable, because the Batavia meeting did not give advice whether choice A or B of bullet 3 should be applied. Option B will now be removed and if this resolution is accepted, issue 1297(i) should be declared as resolved by 1401(i). This update also resyncs the wording with N3242.

Proposed resolution:

Wording changes are against N3242.

1. Change 20.2.2 [memory.syn] p. 1, header `<memory>` synopsis as indicated. `noexcept` specifications are only added, where the guarantee exists, that the function shall no throw an exception (as replacement of "Throws: Nothing". Note that the `noexcept` additions to the `shared_ptr` comparisons are editorial, because they are already part of the accepted paper n3195:

   >     namespace std {
   >       […]
   >       // [unique.ptr] Class unique_ptr:
   >       template <class T> class default_delete;
   >       template <class T> class default_delete<T[]>;
   >       template <class T, class D = default_delete<T>> class unique_ptr;
   >       template <class T, class D> class unique_ptr<T[], D>;

   >       template <class T1, class D1, class T2, class D2>
   >       bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   >       template <class T, class D>
   >       bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >       template <class T, class D>
   >       bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;
   >       template <class T, class D>
   >       bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >       template <class T, class D>
   >       bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;
   >       template <class T, class D>
   >       bool operator<(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator<(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator<=(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator>(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator>(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator>=(nullptr_t, const unique_ptr<T, D>& x);

   >       // [util.smartptr.weakptr], Class bad_weak_ptr:
   >       class bad_weak_ptr;

   >       // [util.smartptr.shared], Class template shared_ptr:
   >       template<class T> class shared_ptr;

   >       // [util.smartptr.shared.cmp], shared_ptr comparisons:
   >       template<class T, class U>
   >       bool operator==(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator!=(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator<(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator>(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator<=(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator>=(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;

   >       template<class T>
   >       bool operator==(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator==(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template<class T>
   >       bool operator!=(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator!=(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template<class T>
   >       bool operator<(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator<(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template>class T>
   >       bool operator>(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template>class T>
   >       bool operator>(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template<class T>
   >       bool operator<=(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator<=(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template>class T>
   >       bool operator>=(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template>class T>
   >       bool operator>=(nullptr_t, shared_ptr<T> const& a) noexcept;

   >       […]
   >     }

2. Change the synopsis just after 20.3.1 [unique.ptr] p. 6 as indicated:

   >     namespace std {
   >       […]

   >       template <class T1, class D1, class T2, class D2>
   >       bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   >       template <class T, class D>
   >       bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >       template <class T, class D>
   >       bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;
   >       template <class T, class D>
   >       bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >       template <class T, class D>
   >       bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;
   >       template <class T, class D>
   >       bool operator<(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator<(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator<=(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator>(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator>(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator>=(nullptr_t, const unique_ptr<T, D>& x);

   >     }

3. This bullet does now only suggest the first approach:

   Change 20.3.1.6 [unique.ptr.special] p. 4-7 as indicated and add a series of prototype descriptions:

   >     template <class T1, class D1, class T2, class D2>
   >       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   > > ? Requires: Let `CT` be `common_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type`. Then the specialization `less<CT>` shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

   > > 4 Returns: `less<CT>()(x.get(), y.get())x.get() < y.get()`.

   > > ? Remarks: If `unique_ptr<T1, D1>::pointer` is not implicitly convertible to `CT` or `unique_ptr<T2, D2>::pointer` is not implicitly convertible to `CT`, the program is ill-formed.

   >     template <class T1, class D1, class T2, class D2>
   >       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   > > 5 Returns: `!(y < x)x.get() <= y.get()`.

   >     template <class T1, class D1, class T2, class D2>
   >       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   > > 6 Returns: `(y < x)x.get() > y.get()`.

   >     template <class T1, class D1, class T2, class D2>
   >       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   > > 7 Returns: `!(x < y)x.get() >= y.get()`.

   >     template <class T, class D>
   >     bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >     template <class T, class D>
   >     bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;

   > > ? Returns: `!x`.

   >     template <class T, class D>
   >     bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >     template <class T, class D>
   >     bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;

   > > ? Returns: `(bool) x`.

   >     template <class T, class D>
   >     bool operator<(const unique_ptr<T, D>& x, nullptr_t);
   >     template <class T, class D>
   >     bool operator>(nullptr_t, const unique_ptr<T, D>& x);

   > > ? Requires: The specialization `less<unique_ptr<T, D>::pointer>` shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

   > > ? Returns: `less<unique_ptr<T, D>::pointer>()(x.get(), nullptr)`.

   >     template <class T, class D>
   >     bool operator<(nullptr_t, const unique_ptr<T, D>& x);
   >     template <class T, class D>
   >     bool operator>(const unique_ptr<T, D>& x, nullptr_t);

   > > ? Requires: The specialization `less<unique_ptr<T, D>::pointer>` shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

   > > ? Returns: `less<unique_ptr<T, D>::pointer>()(nullptr, x.get())`.

   >     template <class T, class D>
   >     bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
   >     template <class T, class D>
   >     bool operator>=(nullptr_t, const unique_ptr<T, D>& x);

   > > ? Returns: `!(nullptr < x)`.

   >     template <class T, class D>
   >     bool operator<=(nullptr_t, const unique_ptr<T, D>& x);
   >     template <class T, class D>
   >     bool operator>=(const unique_ptr<T, D>& x, nullptr_t);

   > > ? Returns: `!(x < nullptr)`.

4. Change 20.3.2.2 [util.smartptr.shared] p. 1, class template `shared_ptr` synopsis as indicated. For consistency reasons the remaining normal relation operators are added as well:

   >     namespace std {

   >       […]

   >       // [util.smartptr.shared.cmp], shared_ptr comparisons:
   >       template<class T, class U>
   >       bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator<=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator>=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

   >       template<class T>
   >       bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator==(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template<class T>
   >       bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator!=(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template<class T>
   >       bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator<(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template>class T>
   >       bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template>class T>
   >       bool operator>(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template<class T>
   >       bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator<=(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template>class T>
   >       bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template>class T>
   >       bool operator>=(nullptr_t, const shared_ptr<T>& a) noexcept;

   >       […]
   >     }

5. Add the following series of prototype specifications at the very end of 20.3.2.2.8 [util.smartptr.shared.cmp]. For mixed comparison the general "generation" rule of [operators] p. 10 does not apply, therefore all of them are defined. Below wording takes advantage of the simplified definition of the composite pointer type if one partner is a null pointer constant:

   >     template<class T>
   >     bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept;
   >     template<class T>
   >     bool operator==(nullptr_t, const shared_ptr<T>& a) noexcept;

   > > ? Returns: `!a`.

   >     template<class T>
   >     bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >     template<class T>
   >     bool operator!=(nullptr_t, const shared_ptr<T>& a) noexcept;

   > > ? Returns: `(bool) a`.

   >     template<class T>
   >     bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept;
   >     template<class T>
   >     bool operator>(nullptr_t, const shared_ptr<T>& a) noexcept;

   > > ? Returns: `less<T*>()(a.get(), nullptr)`.

   >     template<class T>
   >     bool operator<(nullptr_t, const shared_ptr<T>& a) noexcept;
   >     template<class T>
   >     bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept;

   > > ? Returns: `less<T*>()(nullptr, a.get())`.

   >     template<class T>
   >     bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >     template<class T>
   >     bool operator>=(nullptr_t, const shared_ptr<T>& a) noexcept;

   > > ? Returns: `!(nullptr < a)`.

   >     template<class T>
   >     bool operator<=(nullptr_t, const shared_ptr<T>& a) noexcept;
   >     template<class T>
   >     bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept;

   > > ? Returns: `!(a < nullptr)`.

### 1402(i). `nullptr` constructors for smart pointers should be `constexpr`

Section: 20.2 [memory] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [memory].

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Discussion:

Addresses GB-100

The `unique_ptr` and `shared_ptr` constructors taking `nullptr_t` delegate to a `constexpr` constructor, and could be `constexpr` themselves.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

In the 20.3.1.3 [unique.ptr.single] synopsis add "constexpr" to `unique_ptr(nullptr_t)`.
In the 20.3.1.4 [unique.ptr.runtime] synopsis add "constexpr" to `unique_ptr(nullptr_t)`.
In the 20.3.2.2 [util.smartptr.shared] synopsis add "constexpr" to `shared_ptr(nullptr_t)`.

### 1403(i). Inconsistent definitions for `allocator_arg`

Section: 20.2.7 [allocator.tag] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Addresses JP-85

There are inconsistent definitions for `allocator_arg`. In 20.2 [memory] paragraph 1,

    constexpr allocator_arg_t allocator_arg = allocator_arg_t();

and in 20.9.1,

    const allocator_arg_t allocator_arg = allocator_arg_t();

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Change "const" to "constexpr" in 20.9.1 as follows.

    constexpr allocator_arg_t allocator_arg = allocator_arg_t();

### 1404(i). `pointer_traits` should have a `size_type` member

Section: 20.2.3 [pointer.traits] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [pointer.traits].

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Discussion:

Addresses US-106

`pointer_traits` should have a `size_type` member for completeness.

Add `typedef see below size_type;` to the generic `pointer_traits` template and `typedef size_t size_type;` to `pointer_traits<T*>`. Use `pointer_traits::size_type` and `pointer_traits::difference_type` as the defaults for `allocator_traits::size_type` and `allocator_traits::difference_type`.

See Appendix 1 - Additional Details

[ Post-Rapperswil, Pablo provided wording: ]

The original ballot comment reads simply: "`pointer_traits` should have a `size_type` for completeness." The additional details reveal, however, that the desire for a `size_type` is actually driven by the needs of `allocator_traits`. The `allocator_traits` template should get its default `difference_type` from `pointer_traits` but if it did, it should get its `size_type` from the same source. Unfortunately, there is no obvious meaning for `size_type` in `pointer_traits`.

Alisdair suggested, however, that the natural relationship between `difference_type` and `size_type` can be expressed simply by the `std::make_unsigned<T>` metafunction. Using this metafunction, we can easily define `size_type` for `allocator_traits` without artificially adding `size_type` to `pointer_traits`.

> Moved to Tentatively Ready after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

In [allocator.requirements], Table 42, change two rows as follows:

> `X::size_type` | unsigned integral type | a type that can represent the size of the largest object in the allocation model | `size_t make_unsigned<X::difference_type>::type`
> `X::difference_type` | signed integral type | a type that can represent the difference between any two pointers in the allocation model | `ptrdiff_t pointer_traits<X::pointer>::difference_type`

In [allocator.traits.types], Change the definition of `difference_type` and `size_type` as follows:

> `typedef` see below `difference_type;`

> > Type: `Alloc::difference_type` if such a type exists, else `ptrdiff_t pointer_traits<pointer>::difference_type`.

> `typedef` see below `size_type;`

> > Type: `Alloc::size_type` if such a type exists, else `size_t make_unsigned<difference_type>::type`.

### 1405(i). Move `scoped_allocator_adaptor` into separate header

Section: 20.6 [allocator.adaptor] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [allocator.adaptor].

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Discussion:

Addresses US-107

`scoped_allocator_adaptor` should have its own header.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

See Appendix 1 - Additional Details

### 1407(i). Synch `shared_ptr` constructors taking movable types

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [util.smartptr.shared.const].

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Discussion:

Addresses US-108

`shared_ptr` should have the same policy for constructing from `auto_ptr` as `unique_ptr`. Currently it does not.

[ Resolved in Rapperswil by paper N3109. ]

Proposed resolution:

Add

    template <class Y> explicit shared_ptr(auto_ptr<Y>&);

to [util.smartptr.shared.const] (and to the synopsis).

### 1408(i). Allow recycling of pointers after `undeclare_no_pointers`

Section: 99 [util.dynamic.safety] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.dynamic.safety].

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Discussion:

Addresses GB-103

The precondition to calling `declare_no_pointers` is that no bytes in the range "have been previously registered" with this call. As written, this precondition includes bytes in ranges, even after they have been explicitly unregistered with a later call to `undeclare_no_pointers`.

Proposed resolution:

Update 99 [util.dynamic.safety] p.9:

>     void declare_no_pointers(char *p, size_t n);

> > `9` Requires: No bytes in the specified range have been previously registeredare currently registered with `declare_no_pointers()`. If the specified range is in an allocated object, then it must be entirely within a single allocated object. The object must be live until the corresponding `undeclare_no_pointers()` call. [..]

### 1409(i). Specify whether `monotonic_clock` is a distinct type or a typedef

Section: 99 [time.clock.monotonic] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.clock.monotonic].

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Discussion:

Addresses US-111

What it means for `monotonic_clock` to be a synonym is undefined. If it may or may not be a typedef, then certain classes of programs become unportable.

[ Resolution proposed in ballot comment: ]

Require that it be a distinct class type.

[ 2010-11-01 Daniel comments: ]

> Paper n3128 addresses this issue by replacing `monotonic_clock` with `steady_clock`, which is not a typedef.

Proposed resolution:

This is resolved by n3191.

### 1410(i). Add a feature-detect macro for `monotonic_clock`

Section: 99 [time.clock.monotonic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.clock.monotonic].

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Duplicate of: 1411

Discussion:

Addresses GB-107, DE-20

4.1 [intro.compliance] p.9 states that which conditionally supported constructs are available should be provided in the documentation for the implementation. This doesn't help programmers trying to write portable code, as they must then rely on implementation-specific means to determine the availability of such constructs. In particular, the presence or absence of `std::chrono::monotonic_clock` may require different code paths to be selected. This is the only conditionally-supported library facility, and differs from the conditionally-supported language facilities in that it has standard-defined semantics rather than implementation-defined semantics.

[ Resolution proposed in ballot comment: ]

Provide feature test macro for determining the presence of `std::chrono::monotonic_clock`. Add `_STDCPP_HAS_MONOTONIC_CLOCK` to the `<chrono>` header, which is defined if `monotonic_clock` is present, and not defined if it is not present.

[ 2010-11-01 Daniel comments: ]

> Paper n3128 addresses this issue by replacing `monotonic_clock` with `steady_clock`, which is not conditionally supported, so there is no need to detect it.

Proposed resolution:

This is resolved by n3191.

### 1412(i). Make monotonic clocks mandatory

Section: 99 [time.clock.monotonic] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.clock.monotonic].

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Discussion:

Addresses CH-21

Monotonic clocks are generally easy to provide on all systems and are implicitely required by some of the library facilities anyway.

[ 2010-11-01 Daniel comments: ]

> Paper n3128 addresses this issue by replacing `monotonic_clock` with `steady_clock`, which is mandatory.

[ 2010-11-13 Batavia meeting: ]

This is resolved by adopting n3191. The original resolution is preserved for reference:

> Make monotonic clocks mandatory.

> Strike 99 [time.clock.monotonic] p.2

> > `2` The class `monotonic_clock` is conditionally supported.

> Change 32.2.4 [thread.req.timing] p.2 accordingly

> > The member functions whose names end in `_for` take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions. [ Note: Implementations are not required to use a monotonic clock because such a clock may not be available. — end note ]

Proposed resolution:

This is resolved by n3191.

### 1414(i). Fixing remaining dead links to `POS_T` and `OFF_T`

Section: 27.2.4.4 [char.traits.specializations.char16.t] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [char.traits.specializations.char16.t].

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Duplicate of: 1444

Discussion:

Addresses GB-109, GB-123

It is not clear what the specification means for `u16streampos`, `u32streampos` or `wstreampos` when they refer to the requirements for `POS_T` in 21.2.2, as there are no longer any such requirements. Similarly the annex D.7 refers to the requirements of type `POS_T` in 27.3 that no longer exist either.

Clarify the meaning of all cross-reference to the removed type `POS_T`.

[ Post-Rapperswil, Daniel provides the wording. ]

When preparing the wording for this issue I first thought about adding both `u16streampos` and `u32streampos` to the [iostream.forward] header `<iosfwd>` synopsis similar to `streampos` and `wstreampos`, but decided not to do so, because the IO library does not yet actively support the `char16_t` and `char32_t` character types. Adding those would misleadingly imply that they would be part of the iostreams. Also, the addition would make them also similarly equal to a typedef to `fpos<mbstate_t>`, as for `streampos` and `wstreampos`, so there is no loss for users that would like to use the proper `fpos` instantiation for these character types.

Additionally the way of referencing was chosen to follow the style suggested by NB comment GB 108.

> Moved to Tentatively Ready with proposed wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The following wording changes are against N3126.

1. Change [char.traits.specializations.char16_t]p.1 as indicated:

   > 1 - The type `u16streampos` shall be an implementation-defined type that satisfies the requirements for `POS_T` in 21.2.2 `pos_type` in [iostreams.limits.pos].

2. Change [char.traits.specializations.char32_t]p.1 as indicated:

   > 1 - The type `u32streampos` shall be an implementation-defined type that satisfies the requirements for `POS_T` in 21.2.2 `pos_type` in [iostreams.limits.pos].

3. Change [char.traits.specializations.wchar.t]p.2 as indicated:

   > 2 - The type `wstreampos` shall be an implementation-defined type that satisfies the requirements for `POS_T` in 21.2.2 `pos_type` in [iostreams.limits.pos].

4. Change [fpos.operations], Table 124 — Position type requirements as indicated:

   > Table 124 — Position type requirements
   > Expression | Return type | `...`
   > --- | --- | ---
   > `...` | `...` | `...`
   > `O(p)` | `OFF_T` `streamoff` | ...
   > `...` | `...` | `...`
   > `o = p - q` | `OFF_T` `streamoff` | `...`
   > `streamsize(o)` `O(sz)` | `streamsize` `OFF_T` `streamoff` | `...`

5. Change [depr.ios.members]p.1 as indicated:

   >     namespace std {
   >      class ios_base {
   >      public:
   >        typedef T1 io_state;
   >        typedef T2 open_mode;
   >        typedef T3 seek_dir;
   >        typedef OFF_Timplementation-defined streamoff;
   >        typedef POS_Timplementation-defined streampos;
   >        // remainder unchanged
   >      };
   >     }

6. Change [depr.ios.members]p.5+6 as indicated:

   > 5 - The type `streamoff` is an implementation-defined type that satisfies the requirements of type `OFF_T` (27.5.1)`off_type` in [iostreams.limits.pos].

   > 6 - The type `streampos` is an implementation-defined type that satisfies the requirements of type `POS_T` (27.3)`pos_type` in [iostreams.limits.pos].

### 1416(i). `forward_list::erase_after` should not be allowed to throw

Section: 23.2 [container.requirements] Status: C++11 Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

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Discussion:

Addresses DE-21

23.2.1/11 provides a general no-throw guarantee for erase() container functions, exceptions from this are explicitly mentioned for individual containers. Because of its different name, forward_list's erase_after() function is not ruled by this but should so.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add a "Throws: Nothing" clause to both `erase_after` overloads in 23.3.3.4, [forwardlist.modifiers].

### 1417(i). `front/back` on a zero-sized `array` should be undefined

Section: 23.3.3.5 [array.zero] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-112

Should the effect of calling `front/back` on a zero-sized `array` really be implementation defined i.e. require the implementor to define behaviour?

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Change "implementation defined" to "undefined"

### 1418(i). Effects of `resize(size())` on a `deque`

Section: 23.3.5.3 [deque.capacity] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2017-03-21

Priority: Not Prioritized

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Discussion:

Addresses GB-113

There is no mention of what happens if `sz==size()`. While it obviously does nothing I feel a standard needs to say this explicitely.

[ 2010 Batavia ]

Accepted with a simplified resolution turning one of the `<` comparisons into `<=.`

Proposed resolution:

Ammend [deque.capacity]

`void resize(size_type sz);`

Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` default constructedvalue initialized elements to the sequence.

### 1420(i). Effects of `resize(size())` on a `list`

Section: 23.3.11.3 [list.capacity] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-115

There is no mention of what happens if `sz==size()`. While it obviously does nothing I feel a standard needs to say this explicitely.

[ Resolution proposed in ballot comment ]

Express the semantics as pseudo-code similarly to the way it is done for the copying overload that follows (in p3). Include an else clause that does nothing and covers the `sz==size()` case.

[ 2010 Batavia ]

Accepted with a simplified resolution turning one of the `<` comparisons into `<=.`

Proposed resolution:

Ammend [list.capacity] p1:

> `void resize(size_type sz);`

> > Effects: If `sz <= size()`, equivalent to `list<T>::iterator it = begin(); advance(it, sz); erase(it, end());`. If `size() < sz`, appends `sz - size()` default constructed value initialized elements to the sequence.

### 1421(i). Accidental move-only library types due to new core language rules

Section: 23.6 [container.adaptors] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.adaptors].

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Duplicate of: 1350

Discussion:

Addresses DE-22, CH-15

With the final acceptance of move operations as special members and introduction of corresponding suppression rules of implicitly generated copy operations the some library types that were copyable in C++03 are no longer copyable (only movable) in C++03, among them `queue`, `priority_queue`, and `stack`.

[ 2010-10-26: Daniel comments: ]

Accepting n3112 should fix this.

[2011-02-17: Lawrence comments:]

The only open issue in CH 15 with respect to the concurrency group was the treatment of `atomic_future`. Since we removed `atomic_future` in Batavia, I think all that remains is to remove the open issue from N3112 and adopt it.

[2011-03-23 Madrid meeting]

Resolved by N3264

Proposed resolution:

See n3112

### 1423(i). `map` constructor accepting an allocator as single parameter should be explicit

Section: 23.4.3 [map] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses JP-6

Constructor accepting an allocator as a single parameter should be qualified as explicit.

    namespace std {
    template <class Key, class T, class Compare =
    less<Key>,
    class Allocator = allocator<pair<const Key, T> > >
    class map {
    public:
    ...
    map(const Allocator&);

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key, class T, class Compare =
    less<Key>,
    class Allocator = allocator<pair<const Key, T> > >
    class map {
    public:
    ...
    explicit map(const Allocator&);

### 1424(i). `multimap` constructor accepting an allocator as a single parameter should be explicit

Section: 23.4.4 [multimap] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Addresses JP-7

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key, class T, class Compare =
    less<Key>,
    class Allocator = allocator<pair<const Key, T> > >
    class multimap {
    public:
    ...
    explicit multimap(const Allocator&);

### 1425(i). `set` constructor accepting an allocator as a single parameter should be explicit

Section: 23.4.6 [set] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [set].

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Discussion:

Addresses JP-8

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key, class Compare = less<Key>,
    class Allocator = allocator<Key> >
    class set {
    public:
    ...
    explicit set(const Allocator&);

### 1426(i). `multiset` constructor accepting an allocator as a single parameter should be explicit

Section: 23.4.7 [multiset] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Addresses JP-9

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key, class Compare = less<Key>,
    class Allocator = allocator<Key> >
    class multiset {
    public:
    ...
    explicit multiset(const Allocator&);

### 1427(i). `unordered_map` constructor accepting an allocator as a single parameter should be explicit

Section: 23.5.3 [unord.map] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unord.map].

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Discussion:

Addresses JP-10

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key,
    template <class Key,
    class T,
    class Hash = hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<std::pair<const Key,
    T> > >
    class unordered_map
    {
    public:
    ...
    explicit unordered_map(const Allocator&);

### 1428(i). `unordered_multimap` constructor accepting an allocator as a single parameter should be explicit

Section: 23.5.4 [unord.multimap] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses JP-11

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key,
    class T,
    class Hash = hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<std::pair<const Key,
    T> > >
    class unordered_multimap
    {
    public:
    ...
    explicit unordered_multimap(const Allocator&);

### 1429(i). `unordered_set` constructor accepting an allocator as a single parameter should be explicit

Section: 23.5.6 [unord.set] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Addresses JP-12

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key,
    class Hash = hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<Key> >
    class unordered_set
    {
    public:
    ...
    explicit unordered_set(const Allocator&);

### 1430(i). `unordered_multiset` constructor accepting an allocator as a single parameter should be explicit

Section: 23.5.7 [unord.multiset] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Addresses JP-13

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key,
    class Hash = hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<Key> >
    class unordered_multiset
    {
    public:
    ...
    explicit unordered_multiset(const Allocator&);

### 1431(i). `is_permutation` must be more restrictive

Section: 26.6.14 [alg.is.permutation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Addresses US-120

`is_permutation` is underspecified for anything but the simple case where both ranges have the same value type and the comparison function is an equivalence relation.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Restrict `is_permutation` to the case where it is well specified. See Appendix 1 - Additional Details

### 1432(i). `random_shuffle` signatures are inconsistent

Section: 26.7.13 [alg.random.shuffle] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.random.shuffle].

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Duplicate of: 1433

Discussion:

Addresses US-121, GB-119

`random_shuffle` and `shuffle` should be consistent in how they accept their source of randomness: either both by rvalue reference or both by lvalue reference.

[ Post-Rapperswil, Daniel provided wording ]

The signatures of the `shuffle` and `random_shuffle` algorithms are different in regard to the support of rvalues and lvalues of the provided generator:

>     template<class RandomAccessIterator, class RandomNumberGenerator>
>     void random_shuffle(RandomAccessIterator first,
>     RandomAccessIterator last,
>     RandomNumberGenerator&& rand);

>     template<class RandomAccessIterator, class UniformRandomNumberGenerator>
>     void shuffle(RandomAccessIterator first,
>     RandomAccessIterator last,
>     UniformRandomNumberGenerator& g);

The first form uses the perfect forwarding signature and that change compared to `C++03` was done intentionally as shown in the first rvalue proposal papers.

While it is true, that random generators are excellent examples of stateful functors, there still exist good reasons to support rvalues as arguments:

1. If one of the shuffle algorithms is called with the intention to shuffle items with a reproducible ordering from a given generator class, it makes sense to create a generator exactly at the call point.
2. Other algorithms with similar need for stateful functors (like `std::generate` and `std::generate_n`) accept both rvalues and lvalues as well.
3. Given the deduction rules for perfect forwarding it is hard for a user to produce code that does the wrong thing unintentionally. Any lvalue generator will deduce an lvalue-reference and behave as in `C++03`. In the specific cases, where rvalues are provided, the argument will be accepted instead of being rejected.

Arguments have been raised that accepting rvalues is error-prone or even fundamentally wrong. The author of this proposal disagrees with that position for two additional reasons:

1. Enforcing lvalues as arguments won't prevent user code to enforce what they want. So given

   >     my_generator get_generator(int size);

   instead of writing

   >     std::vector<int> v = ...;
   >     std::shuffle(v.begin(), v.end(), get_generator(v.size()));

   they will just write

   >     std::vector<int> v = ...;
   >     auto gen = get_generator(v.size());
   >     std::shuffle(v.begin(), v.end(), gen);

   and feel annoyed about the need for it.
2. Generators may be copyable and movable, and random number engines are required to be `CopyConstructible` and this is obviously a generally useful property for such objects. It is also useful and sometimes necessary to start a generator with exactly a specific seed again and again and thus to provide a new generator (or a copy) for each call. The `CopyConstructible` requirements allow providing rvalues of generators and thus this idiom must be useful as well. Therefore preventing `[random_]shuffle` to accept rvalues is an unnecessary restriction which doesn't prevent any user-error, if there would exist one.

Thus this proposal recommends to make both `shuffle` functions consistent and perfectly forward-able.

> Moved to Tentatively Ready after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [algorithms.general], header `<algorithm>` synopsis as indicated:

   >     template<class RandomAccessIterator, class UniformRandomNumberGenerator>
   >     void shuffle(RandomAccessIterator first, RandomAccessIterator last,
   >     UniformRandomNumberGenerator&& rand);

2. Change the prototype description of [alg.random.shuffle] as indicated:

   >     template<class RandomAccessIterator, class UniformRandomNumberGenerator>
   >     void shuffle(RandomAccessIterator first, RandomAccessIterator last,
   >     UniformRandomNumberGenerator&& rand);

### 1435(i). Unclear returns specifications for C99 complex number functions

Section: 29.4.7 [complex.value.ops] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [complex.value.ops].

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Discussion:

Addresses GB-120

The complex number functions added for compatibility with the C99 standard library are defined purely as a cross-reference, with no hint of what they should return. This is distinct from the style of documentation for the functions in the earlier standard. In the case of the inverse-trigonometric and hyperbolic functions, a reasonable guess of the functionality may be made from the name, this is not true of the cproj function, which apparently returns the projection on the Reimann Sphere. A single line description of each function, associated with the cross-reference, will greatly improve clarity.

[2010-11-06 Beman provides proposed resolution wording.]

[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 26.4.7 complex value operations [complex.value.ops] as indicated:

> `template<class T> complex<T> proj(const complex<T>& x);`

> > Returns: the projection of `x` onto the Riemann sphere.

> > Effects: Remarks: Behaves the same as the C function `cproj`, defined in 7.3.9.4.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> acos(const complex<T>& x);`

> > Returns: the complex arc cosine of `x`.

> > Effects: Remarks: Behaves the same as the C function `cacos`, defined in 7.3.5.1.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> asin(const complex<T>& x);`

> > Returns: the complex arc sine of `x`.

> > Effects: Remarks: Behaves the same as the C function `casin`, defined in 7.3.5.2.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> atan(const complex<T>& x);`

> > Returns: the complex arc tangent of `x`.

> > Effects: Remarks: Behaves the same as the C function `catan`, defined in 7.3.5.3.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> acosh(const complex<T>& x);`

> > Returns: the complex arc hyperbolic cosine of `x`.

> > Effects: Remarks: Behaves the same as the C function `cacosh`, defined in 7.3.6.1.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> asinh(const complex<T>& x);`

> > Returns: the complex arc hyperbolic sine of `x`.

> > Effects: Remarks: Behaves the same as the C function `casinh`, defined in 7.3.6.2.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> atanh(const complex<T>& x);`

> > Returns: the complex arc hyperbolic tangent of `x`.

> > Effects: Remarks: Behaves the same as the C function `catanh`, defined in 7.3.6.2.

### 1436(i). Random number engine constructor concerns

Section: 29.5.4 [rand.eng] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.eng].

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Discussion:

Addresses GB-121

All the random number engine types in this clause have a constructor taking an unsigned integer type, and a constructor template for seed sequences. This means that an attempt to create a random number engine seeded by an integer literal must remember to add the appropriate unsigned suffix to the literal, as a signed integer will attempt to use the seed sequence template, yielding undefined behaviour, as per 26.5.1.1p1a. It would be helpful if at least these anticipated cases produced a defined behaviour, either an erroneous program with diagnostic, or a conversion to unsigned int forwarding to the appropriate constructor.

[ 2010-11-03 Daniel comments and provides a proposed resolution: ]

I suggest to apply a similar solution as recently suggested for 1234(i). It is basically a requirement for an implementation to constrain the template.

[ 2010-11-04 Howard suggests to use `!is_convertible<Sseq, result_type>::value` as minimum requirement instead of the originally proposed `!is_scalar<Sseq>::value`. This would allow for a user-defined type `BigNum`, that is convertible to `result_type`, to be used as argument for a seed instead of a seed sequence. The wording has been updated to reflect this suggestion. ]

[ 2010 Batavia: There were some initial concerns regarding the portability and reproducibility of results when seeded with a negative signed value, but these concerns were allayed after discussion. Thus, after reviewing the issue, the working group concurred with the issue's Proposed Resolution. ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Add the following paragraph at the end of 29.5.4 [rand.eng]:

> > 5 Each template specified in this section [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.

> > ? For every random number engine and for every random number engine adaptor `X` defined in this sub-clause [rand.eng] and in sub-clause [rand.adapt]:

> > - If the constructor

> >   >     template<class Sseq> explicit X(Sseq& q);

> >   is called with a type `Sseq` that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution.

> > - If the member function

> >   >     template<class Sseq> void seed(Sseq& q);

> >   is called with a type `Sseq` that does not qualify as a seed sequence, then this function shall not participate in overload resolution.

> > The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as seed sequence, if it is implicitly convertible to `X::result_type`.

### 1437(i). Mersenne twister meaningless for word sizes less than two

Section: 29.5.4.3 [rand.eng.mers] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-124

The Mersenne twister algorithm is meaningless for word sizes less than two, as there are then insufficient bits available to be “twisted”.

[ Resolution proposed by ballot comment: ]

> Insert the following among the relations that are required to hold: `2u < w`.

[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 29.5.4.3 [rand.eng.mers] p. 4 as indicated:

> 4 The following relations shall hold: `0 < m`, `m <= n`, `2u < w`, `r <= w`, `u <= w`, `s <= w`, `t <= w`, `l <= w`, `w <= numeric_limits<UIntType>::digits`, `a <= (1u<<w) - 1u`, `b <= (1u<<w) - 1u`, `c <= (1u<<w) - 1u`, `d <= (1u<<w) - 1u`, and `f <= (1u<<w) - 1u`.

### 1438(i). No definition for `base()`

Section: 29.5.5.2 [rand.adapt.disc] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-126

Each adaptor has a member function called `base()` which has no definition.

[ Resolution proposed by ballot comment: ]

> Give it the obvious definition.

[ 2010-11-03 Daniel comments and provides a proposed resolution: ]

The following proposal adds `noexcept` specifiers to the declarations of the `base()` functions as replacement for a "Throws: Nothing" element.

[ 2010 Batavia: The working group reviewed this issue, and recommended to add the following to the Proposed Resolution. ]

- Append to paragraph 1 of [rand.req.adapt] (or at the Editor's discretion insert as a new paragraph following that paragraph): The expression `a.base()` shall be valid and shall return a const reference to `a`'s base engine.

After further review, the working group concurred with the Proposed Resolution.

[Batavia: waiting for WEB to review wording]

Proposed resolution:

1. Add the following sentence to the end of 29.5.3.5 [rand.req.adapt]/1:

   > A random number engine adaptor (commonly shortened to adaptor) `a` of type `A` is a random number engine that takes values produced by some other random number engine, and applies an algorithm to those values in order to deliver a sequence of values with different randomness properties. An engine `b` of type `B` adapted in this way is termed a base engine in this context. The expression `a.base()` shall be valid and shall return a const reference to `a`'s base engine.

2. Change in [rand.adapt.disc]/3, class template `discard_block_engine` synopsis, the following declaration:

   >     // property functions
   >     const Engine& base() const noexcept;

3. Add the following new prototype description at the end of sub-clause 29.5.5.2 [rand.adapt.disc]:

   >     const Engine& base() const noexcept;

   > > ? Returns: `e`.

4. Change in [rand.adapt.ibits]/4, class template `independent_bits_engine` synopsis, the following declaration:

   >     // property functions
   >     const Engine& base() const noexcept;

5. Add the following new prototype description at the end of sub-clause 29.5.5.3 [rand.adapt.ibits]:

   >     const Engine& base() const noexcept;

   > > ? Returns: `e`.

6. Change in 29.5.5.4 [rand.adapt.shuf]/3, class template `shuffle_order_engine` synopsis, the following declaration:

   >     // property functions
   >     const Engine& base() const noexcept;

7. Add the following new prototype description at the end of sub-clause 29.5.5.4 [rand.adapt.shuf]:

   >     const Engine& base() const noexcept;

   > > ? Returns: `e`.

### 1439(i). Return from `densities()` functions?

Section: 29.5.9.6.2 [rand.dist.samp.pconst] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-134

These two distributions have a member function called `densities()` which returns a `vector<double>`. The distribution is templated on `RealType`. The distribution also has another member called `intervals()` which returns a `vector<RealType>`. Why doesn't densities return `vector<RealType>` as well? If `RealType` is `long double`, the computed densities property isn't being computed to the precision the client desires. If `RealType` is `float`, the densities vector is taking up twice as much space as the client desires.

[ Resolution proposed by ballot comment: ]

> Change the piecewise constant and linear distributions to hold / return the densities in a `vector<result_type>`.

> If this is not done, at least correct 29.5.9.6.2 [rand.dist.samp.pconst] p. 13 which describes the return of densities as a `vector<result_type>`.

[ Batavia 2010: After reviewing this issue, the working group concurred with the first of the suggestions proposed by the NB comment: "Change the piecewise constant and linear distributions to hold/return the densities in a vector. " ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change 29.5.9.6.2 [rand.dist.samp.pconst] p. 2, class template `piecewise_constant_distribution` synopsis and the prototype description 29.5.9.6.2 [rand.dist.samp.pconst] before p. 13 as indicated:

   >     vector<doubleresult_type> densities() const;

2. Change 29.5.9.6.3 [rand.dist.samp.plinear] p. 2, class template `piecewise_linear_distribution` synopsis and the prototype description 29.5.9.6.3 [rand.dist.samp.plinear] before p. 13 as indicated:

   >     vector<doubleresult_type> densities() const;

### 1440(i). Incorrect specification for `piecewise_linear_distribution`

Section: 29.5.9.6.3 [rand.dist.samp.plinear] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-135

This paragraph says: Let bk = xmin+k·δ for k = 0,...,n, and wk = fw(bk +δ) for k = 0,...,n. However I believe that fw(bk) would be far more desirable. I strongly suspect that this is nothing but a type-o.

[ Resolution proposed by ballot comment: ]

> Change p. 10 to read:
> Let bk = xmin+k·δ for k = 0,...,n, and wk = fw(bk) for k = 0,...,n.

[ 2010-11-02 Daniel translates into a proposed resolution ]

[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 29.5.9.6.3 [rand.dist.samp.plinear] p. 10 as indicated:

> 10 Effects: Constructs a `piecewise_linear_distribution` object with parameters taken or calculated from the following values: Let `bk = xmin+k·δ` for `k = 0, . . . , n`, and `wk = fw(bk +δ)` for `k = 0, . . . , n`.

### 1441(i). Floating-point test functions are incorrectly specified

Section: 29.7 [c.math] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-136

Floating-point test functions are incorrectly specified.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

See Appendix 1 - Additional Details

### 1445(i). Several iostreams member functions incorrectly specified

Section: 31.7 [iostream.format] Status: Resolved Submitter: INCITS/PJ Plauger Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-137

Several iostreams member functions are incorrectly specified.

[ Resolution proposed by ballot comment: ]

See Appendix 1 - Additional Details

[ 2010-10-24 Daniel adds: ]

> Accepting n3168 would solve this issue.

Proposed resolution:

Addressed by paper n3168.

### 1447(i). Request to resolve issue LWG 1328

Section: 31.7 [iostream.format] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-139

Resolve issue LWG 1328 one way or the other, but preferably in the direction outlined in the proposed resolution, which, however, is not complete as-is: in any case, the sentry must not set `ok_ = false` if `is.good() == false`, otherwise `istream::seekg`, being an unformatted input function, does not take any action because the sentry object returns false when converted to type `bool`. Thus, it remains impossible to seek away from end of file.

Proposed resolution:

Addressed by paper n3168.

### 1448(i). Concerns about `basic_stringbuf::str(basic_string)` postconditions

Section: 31.8.2.4 [stringbuf.members] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-124

N3092 31.8.2.4 [stringbuf.members] contains this text specifying the postconditions of `basic_stringbuf::str(basic_string)`:

> Postconditions: If `mode & ios_base::out` is `true`, `pbase()` points to the first underlying character and `epptr() >= pbase() + s.size()` holds; in addition, if `mode & ios_base::in` is `true`, `pptr() == pbase() + s.data()` holds, otherwise `pptr() == pbase()` is `true`. [...]

Firstly, there's a simple mistake: It should be `pbase() + s.length()`, not `pbase() + s.data()`.

Secondly, it doesn't match existing implementations. As far as I can tell, GCC 4.5 does not test for `mode & ios_base::in` in the second part of that sentence, but for `mode & (ios_base::app | ios_base_ate)`, and Visual C++ 9 for `mode & ios_base::app`. Besides, the wording of the C++0x draft doesn't make any sense to me. I suggest changing the second part of the sentence to one of the following:

Replace `ios_base::in` with `(ios_base::ate | ios_base::app)`, but this would require Visual C++ to change (replacing only with `ios_base::ate` would require GCC to change, and would make `ios_base::app` completely useless with `stringstreams`):

in addition, if `mode & (ios_base::ate | ios_base::app)` is `true`, `pptr() == pbase() + s.length()` holds, otherwise `pptr() == pbase()` is `true`.

Leave `pptr()` unspecified if `mode & ios_base::app`, but not `mode & ios_base::ate` (implementations already differ in this case, and it is always possible to use `ios_base::ate` to get the effect of appending, so it is not necessary to require any implementation to change):

in addition, if `mode & ios_base::ate` is `true`, `pptr() == pbase() + s.length()` holds, if neither `mode & ios_base::ate` nor `mode & ios_base::app` is `true`, `pptr() == pbase()` holds, otherwise `pptr() >= pbase() && pptr() <= pbase() + s.length()` (which of the values in this range is unspecified).

Slightly stricter:

in addition, if `mode & ios_base::ate` is `true`, `pptr() == pbase() + s.length()` holds, if neither `mode & ios_base::ate` nor `mode & ios_base::app` is `true`, `pptr() == pbase()` holds, otherwise `pptr() == pbase() || pptr() == pbase() + s.length()` (which of these two values is unspecified). A small table might help to better explain the three cases. BTW, at the end of the postconditions is this text: "`egptr() == eback() + s.size()` hold". Is there a perference for `basic_string::length` or `basic_string::size`? It doesn't really matter, but it looks a bit inconsistent.

[2011-03-09: Nicolai Josuttis comments and drafts wording]

First, it seems the current wording is just an editorial mistake. When we added issue 432(i) to the draft standard (in n1733), the wording in the issue:

> If `mode & ios_base::out` is true, initializes the output sequence such that `pbase()` points to the first underlying character, `epptr()` points one past the last underlying character, and if `(mode & ios_base::ate)` is true, `pptr()` is set equal to `epptr()` else `pptr()` is set equal to `pbase()`.

became:

> If `mode & ios_base::out` is true, initializes the output sequence such that `pbase()` points to the first underlying character, `epptr()` points one past the last underlying character, and `pptr()` is equal to `epptr()` if `mode & ios_base::in` is true, otherwise `pptr()` is equal to `pbase()`.

which beside some changes of the order of words changed

>     ios_base::ate

into

>     ios_base::in

So, from this point of view, clearly `mode & ios_base::ate` was meant.

Nevertheless, with this proposed resolution we'd have no wording regarding `ios_base::app`. Currently the only statements about `app` in the Standard are just in two tables:

- Table 125 — "`openmode` effects" says that the effect of `app` is "seek to end before each write"
- Table 132 — "File open modes" says that the stdio equivalent for `app` is `"a"`

Indeed we seem to have different behavior currently in respect to `app`: For

>     stringstream s2(ios_base::out|ios_base::in|ios_base::app);
>     s2.str("s2 hello");
>     s1 << "more";

- Visual C++ 10 does overwrite (=> `"moreello"`)
- G++ 4.5 does append (=> `"s2 hellomore"`)

BTW, for fstreams, both implementations append when `app` is set: If `f2.txt` has contents `"xy"`,

>     fstream f2("f2.txt",ios_base::out|ios_base::in|ios_base::app);
>     f1 << "more";

appends `"more"` so that the contents is `"xymore"`.

So IMO `app` should set the write pointer to the end so that each writing appends.

I don't know whether what the standard says is enough. You can argue the statement in Table 125 clearly states that such a buffer should always append, which of course also applies to `str()` of stringbuffer.

Nevertheless, it doesn't hurt IMO if we clarify the behavior of `str()` here in respect to `app`.

[2011-03-10: P.J.Plauger comments:]

I think we should say nothing special about `app` at construction time (thus leaving the write pointer at the beginning of the buffer). Leave implementers wiggle room to ensure subsequent append writes as they see fit, but don't change existing rules for initial seek position.

[Madrid meeting: It was observed that a different issue should be opened that clarifies the meaning of `app` for `stringstream`]

Proposed resolution:

Change 31.8.2.4 [stringbuf.members] p. 3 as indicated:

>     void str(const basic_string<charT,traits,Allocator>& s);

> > 2 Effects: Copies the content of `s` into the `basic_stringbuf` underlying character sequence and initializes the input and output sequences according to `mode`.

> > 3 Postconditions: If `mode & ios_base::out` is true, `pbase()` points to the first underlying character and `epptr() >= pbase() + s.size()` holds; in addition, if `mode & ios_base::inios_base::ate` is true, `pptr() == pbase() + s.data()s.size()` holds, otherwise `pptr() == pbase()` is true. If `mode & ios_base::in` is true, `eback()` points to the first underlying character, and both `gptr() == eback() and egptr() == eback() + s.size()` hold.

### 1449(i). Incomplete specification of header `<cinttypes>`

Section: 31.8.3 [istringstream] Status: C++11 Submitter: Canada Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

Addresses CA-4

Subclause 27.9.2 [c.files] specifies that <cinttypes> has declarations for abs() and div(); however, the signatures are not present in this subclause. The signatures proposed under TR1 ([tr.c99.inttypes]) are not present in FCD (unless if intmax_t happened to be long long). It is unclear as to which, if any of the abs() and div() functions in [c.math] are meant to be declared by <cinttypes>. This subclause mentions imaxabs() and imaxdiv(). These functions, among other things, are not specified in FCD to be the functions from Subclause 7.8 of the C Standard. Finally, <cinttypes> is not specified in FCD to include <cstdint> (whereas <inttypes.h> includes <stdint.h> in C).

[ Post-Rapperswil, Daniel provides wording ]

Subclause [c.files] specifies that `<cinttypes>` has declarations for `abs()` and `div()`; however, the signatures are not present in this subclause. The signatures proposed under TR1 ([tr.c99.inttypes]) are not present in FCD (unless if `intmax_t` happened to be `long long`). It is unclear as to which, if any of the `abs()` and `div()` functions in [c.math] are meant to be declared by `<cinttypes>`. This subclause mentions `imaxabs()` and `imaxdiv()`. These functions, among other things, are not specified in FCD to be the functions from subclause 7.8 of the `C` Standard. Finally, `<cinttypes>` is not specified in FCD to include `<cstdint>` (whereas `<inttypes.h>` includes `<stdint.h>` in `C`).

> Moved to Tentatively Ready with proposed wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

1. Add the following series of new paragraphs following [c.files] p.1:

   > Table 132 describes header `<cinttypes>`. [Note: The macros defined by `<cinttypes>` are provided unconditionally. In particular, the symbol `__STDC_FORMAT_MACROS`, mentioned in footnote 182 of the `C` standard, plays no role in `C++`. — end note ]

   > 2 - The contents of header `<cinttypes>` are the same as the Standard `C` library header `<inttypes.h>`, with the following changes:

   > 3 - The header `<cinttypes>` includes the header `<cstdint>` instead of `<stdint.h>`.

   > 4 - If and only if the type `intmax_t` designates an extended integer type ([basic.fundamental]), the following function signatures are added:

   > >     intmax_t abs(intmax_t);
   > >     imaxdiv_t div(intmax_t, intmax_t);

   > which shall have the same semantics as the function signatures `intmax_t imaxabs(intmax_t)` and `imaxdiv_t imaxdiv(intmax_t, intmax_t)`, respectively.

### 1450(i). Contradiction in `regex_constants`

Section: 28.6.4.3 [re.matchflag] Status: C++14 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: 3

View all other issues in [re.matchflag].

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Discussion:

Addresses GB-127

The Bitmask Type requirements in 16.3.3.3.3 [bitmask.types] p.3 say that all elements on a bitmask type have distinct values, but 28.6.4.3 [re.matchflag] defines `regex_constants::match_default` and `regex_constants::format_default` as elements of the bitmask type `regex_constants::match_flag_type`, both with value 0. This is a contradiction.

[ Resolution proposed by ballot comment: ]

> One of the bitmask elements should be removed from the declaration and should be defined separately, in the same manner as `ios_base::adjustfield`, `ios_base::basefield` and `ios_base::floatfield` are defined by [ios::fmtflags] p.2 and Table 120. These are constants of a bitmask type, but are not distinct elements, they have more than one value set in the bitmask. `regex_constants::format_default` should be specified as a constant with the same value as `regex_constants::match_default`.

[ 2010-10-31 Daniel comments: ]

Strictly speaking, a bitmask type cannot have any element of value 0 at all, because any such value would contradict the requirement expressed in 16.3.3.3.3 [bitmask.types] p. 3:

> for any pair Ci and Cj, Ci & Ci is nonzero

So, actually both `regex_constants::match_default` and `regex_constants::format_default` are only constants of the type `regex_constants::match_flag_type`, and no bitmask elements.

[ 2010-11-03 Daniel comments and provides a proposed resolution: ]

The proposed resolution is written against N3126 and considered as a further improvement of the fixes suggested by n3110.

> Add the following sentence to 28.6.4.3 [re.matchflag] paragraph 1:

> > 1 The type `regex_constants::match_flag_type` is an implementation-defined bitmask type (17.5.2.1.3). Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set. Type `regex_constants::match_flag_type` also defines the constants `regex_constants::match_default` and `regex_constants::format_default`.

[ 2011 Bloomington ]

It appears the key problem is the phrasing of the bitmask requirements. Jeremiah supplies updated wording.

Pete Becker has also provided an alternative resolution.

Ammend 16.3.3.3.3 [bitmask.types]:

Change the list of values for "enum bit mask" in p2 from

`V0 = 1 << 0, V1 = 1 << 1, V2 = 1 << 2, V3 = 1 << 3, ...`.

to

`V0 = 0, V1 = 1 << 0, V2 = 1 << 1, V3 = 1 << 2, ...`.

Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such non-zero elements have distinct values such that, for any pair Ci and Cj where i != j, Ci & Ci is nonzero and Ci & Cj is zero.

Change bullet 3 of paragraph 4:

TheA non-zero value Y is set in the object X if the expression X & Y is nonzero.

[2014-02-13 Issaquah:]

Proposed resolution:

Ammend 16.3.3.3.3 [bitmask.types] p3:

Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such elements have distinct, nonzero values such that, for any pair Ci and Cj where i != j, Ci & Ci is nonzero and Ci & Cj is zero. Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.

Add the following sentence to 28.6.4.3 [re.matchflag] paragraph 1:

> 1 The type `regex_constants::match_flag_type` is an implementation-defined bitmask type (17.5.2.1.3). The constants of that type, except for `match_default` and `format_default`, are bitmask elements. The `match_default` and `format_default` constants are empty bitmasks. Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set.

### 1453(i). Default constructed `match_results` behavior for certain operations

Section: 28.6.9.5 [re.results.acc] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [re.results.acc].

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Discussion:

Addresses GB-126

It's unclear how match_results should behave if it has been default-constructed. The sub_match objects returned by operator[], prefix and suffix cannot point to the end of the sequence that was searched if no search was done. The iterators held by unmatched sub_match objects might be singular.

[ Resolution proposed by ballot comment: ]

> Add to match_results::operator[], match_results::prefix and match_results::suffix:
> Requires: !empty()

[ 2010-10-24 Daniel adds: ]

> Accepting n3158 would solve this issue.

Proposed resolution:

Addressed by paper n3158.

### 1455(i). C language compatibility for atomics

Section: 32.5 [atomics] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics].

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Duplicate of: 1454

Discussion:

Addresses CH-22, GB-128

WG14 currently plans to introduce atomic facilities that are intended to be compatible with the facilities of clause 29. They should be compatible.

[ Resolution proposed by ballot comment ]

Make sure the headers in clause 29 are defined in a way that is compatible with the planned C standard.

[ 2010 Batavia ]

Resolved by adoption of n3193.

Proposed resolution:

Solved by n3193.

### 1457(i). Splitting lock-free properties

Section: 32.5.2 [atomics.syn] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.syn].

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Discussion:

Addresses GB-130

The synopsis for the `<atomic>` header lists the macros `ATOMIC_INTEGRAL_LOCK_FREE` and `ATOMIC_ADDRESS_LOCK_FREE`.

The `ATOMIC_INTEGRAL_LOCK_FREE` macro has been replaced with a set of macros for each integral type, as listed in 32.5.5 [atomics.lockfree].

[Proposed resolution as of comment]

> Against FCD, N3092:

> In [atomics.syn], header `<atomic>` synopsis replace as indicated:

> >     // 29.4, lock-free property
> >     #define ATOMIC_INTEGRAL_LOCK_FREE unspecified
> >     #define ATOMIC_CHAR_LOCK_FREE implementation-defined
> >     #define ATOMIC_CHAR16_T_LOCK_FREE implementation-defined
> >     #define ATOMIC_CHAR32_T_LOCK_FREE implementation-defined
> >     #define ATOMIC_WCHAR_T_LOCK_FREE implementation-defined
> >     #define ATOMIC_SHORT_LOCK_FREE implementation-defined
> >     #define ATOMIC_INT_LOCK_FREE implementation-defined
> >     #define ATOMIC_LONG_LOCK_FREE implementation-defined
> >     #define ATOMIC_LLONG_LOCK_FREE implementation-defined
> >     #define ATOMIC_ADDRESS_LOCK_FREE unspecified

[ 2010-10-26: Daniel adds: ]

The proposed resolution below is against the FCD working draft. After application of the editorial issues US-144 and US-146 the remaining difference against the working draft is the usage of implementation-defined instead of unspecified, effectively resulting in this delta:

>     // 29.4, lock-free property
>     #define ATOMIC_CHAR_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_CHAR16_T_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_CHAR32_T_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_WCHAR_T_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_SHORT_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_INT_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_LONG_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_LLONG_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_ADDRESS_LOCK_FREE unspecified

It is my understanding that the intended wording should be unspecified as for `ATOMIC_ADDRESS_LOCK_FREE` but if this is right, we need to use the same wording in 32.5.5 [atomics.lockfree], which consequently uses the term implementation-defined. I recommend to keep 32.5.2 [atomics.syn] as it currently is and to fix 32.5.5 [atomics.lockfree] instead as indicated:

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

[2011-02-20: Daniel adapts the proposed wording to N3225 and fixes an editorial omission of applying N3193]

[2011-03-06: Daniel adapts the wording to N3242]

[Proposed Resolution]

> Change 32.5.5 [atomics.lockfree] as indicated:

> >     #define ATOMIC_CHAR_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_CHAR16_T_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_CHAR32_T_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_WCHAR_T_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_SHORT_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_INT_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_LONG_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_LLONG_LOCK_FREE implementation-definedunspecified

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1460(i). Missing lock-free property for type `bool` should be added

Section: 32.5.5 [atomics.lockfree] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.lockfree].

View all issues with Resolved status.

Discussion:

Addresses US-154

There is no `ATOMIC_BOOL_LOCK_FREE` macro.

Proposed resolution suggested by the NB comment:

Add `ATOMIC_BOOL_LOCK_FREE` to 32.5.5 [atomics.lockfree] and to 32.5.2 [atomics.syn]:

>     [..]
>     #define ATOMIC_BOOL_LOCK_FREE unspecified
>     #define ATOMIC_CHAR_LOCK_FREE unspecified
>     #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
>     #define ATOMIC_CHAR32_T_LOCK_FREE unspecified
>     [..]

[2011-03-12: Lawrence comments and drafts wording]

Point: We were missing a macro test for `bool`.

Comment: The `atomic<bool>` type is the easiest to make lock-free. There is no harm in providing a macro.

Action: Add an `ATOMIC_BOOL_LOCK_FREE`.

Point: We were missing a macro test for pointers.

Comment: The national body comment noting the missing macro for `bool` did not note the lack of a macro for pointers because `ATOMIC_ADDRESS_LOCK_FREE` was present at the time of the comment. Its removal appears to be an overzealous consequence of removing `atomic_address`.

Action: Add an `ATOMIC_POINTER_LOCK_FREE`.

Point: Presumably `atomic_is_lock_free()` will be an inline function producing a constant in those cases in which the macros are useful.

Comment: The point is technically correct, but could use some exposition. Systems providing forward binary compatibility, e.g. mainstream desktop and server systems, would likely have these functions as inline constants only when the answer is true. Otherwise, the function should defer to a platform-specific dynamic library to take advantage of future systems that do provide lock-free support.

Comment: Such functions are not useful in the preprocessor, and not portably useful in `static_assert`.

Action: Preserve the macros.

Point: The required explicit instantiations are `atomic<X>` for each of the types `X` in Table 145. Table 145 does not list `bool`, so `atomic<bool>` is not a required instantiation.

Comment: I think specialization was intended in the point instead of instantiation. In any event, 32.5.8 [atomics.types.generic] paragraph 5 does indirectly require a specialization for `atomic<bool>`. Confusion arises because the specialization for other integral types have a wider interface than the generic `atomic<T>`, but `atomic<bool>` does not.

Action: Add clarifying text.

Point: The name of Table 145, "atomic integral typedefs", is perhaps misleading, since the types listed do not contain all of the "integral" types.

Comment: Granted, though the table describe those with extra operations.

Action: Leave the text as is.

Point: The macros correspond to the types in Table 145, "with the signed and unsigned variants grouped together". That's a rather inartful way of saying that `ATOMIC_SHORT_LOCK_FREE` applies to `signed short` and `unsigned short`. Presumably this also means that `ATOMIC_CHAR_LOCK_FREE` applies to all three char types.

Comment: Yes, it is inartful.

Comment: Adding additional macros to distinguish signed and unsigned would provide no real additional information given the other constraints in the language.

Comment: Yes, it applies to all three `char` types.

Action: Leave the text as is.

Point: The standard says that "There are full specializations over the integral types (`char`, `signed char`, ...)" `bool` is not in the list. But this text is not normative. It simply tells you that "there are" specializations, not "there shall be" specializations, which would impose a requirement. The requirement, to the extent that there is one, is in the header synopsis, which, in N3242, sort of pulls in the list of types in Table 145.

Comment: The intent was for the specializations to be normative. Otherwise the extra member functions could not be present.

Action: Clarify the text.

[Proposed Resolution]

> 1. Edit header `<atomic>` synopsis 32.5.2 [atomics.syn]:

>    >     namespace std {
>    >       // 29.3, order and consistency
>    >       enum memory_order;
>    >       template <class T>
>    >       T kill_dependency(T y);

>    >       // 29.4, lock-free property
>    >       #define ATOMIC_BOOL_LOCK_FREE unspecified
>    >       #define ATOMIC_CHAR_LOCK_FREE unspecified
>    >       #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
>    >       #define ATOMIC_CHAR32_T_LOCK_FREE unspecified
>    >       #define ATOMIC_WCHAR_T_LOCK_FREE unspecified
>    >       #define ATOMIC_SHORT_LOCK_FREE unspecified
>    >       #define ATOMIC_INT_LOCK_FREE unspecified
>    >       #define ATOMIC_LONG_LOCK_FREE unspecified
>    >       #define ATOMIC_LLONG_LOCK_FREE unspecified
>    >       #define ATOMIC_POINTER_LOCK_FREE unspecified

>    >       // 29.5, generic types
>    >       template<class T> struct atomic;
>    >       template<> struct atomic<integral>;
>    >       template<class T> struct atomic<T*>;

>    >       // 29.6.1, general operations on atomic types
>    >       // In the following declarations, atomic_type is either
>    >       // atomic<T> or a named base class for T from
>    >       // Table 145 or inferred from
>    >       // Table 146 or from bool.

>    >       […]
>    >     }

> 2. Edit the synopsis of 32.5.5 [atomics.lockfree] and paragraph 1 as follows:

>    >     #define ATOMIC_BOOL_LOCK_FREE unspecified
>    >     #define ATOMIC_CHAR_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_CHAR16_T_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_CHAR32_T_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_WCHAR_T_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_SHORT_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_INT_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_LONG_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_LLONG_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_POINTER_LOCK_FREE unspecified

>    > > 1 The `ATOMIC_…_LOCK_FREE` macros indicate the lock-free property of the corresponding atomic types, with the signed and unsigned variants grouped together. The properties also apply to the corresponding (partial) specializations of the `atomic` template. A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.

> 3. Edit 32.5.8 [atomics.types.generic] paragraph 3, 4, and 6-8 as follows:

>    > 2 The semantics of the operations on specializations of `atomic` are defined in 32.5.8.2 [atomics.types.operations].

>    > 3 Specializations and instantiations of the `atomic` template shall have a deleted copy constructor, a deleted copy assignment operator, and a `constexpr` value constructor.

>    > 4 There areshall be full specializations overfor the integral types ( `char`, `signed char`, `unsigned char`, `short`, `unsigned short`, `int`, `unsigned int`, `long`, `unsigned long`, `long long`, `unsigned long long`, `char16_t`, `char32_t`, and `wchar_t`, and any other types needed by the typedefs in the header `<cstdint>`) on the `atomic` class template. For each integral type integral, the specialization `atomic<integral>` provides additional atomic operations appropriate to integral types. [Editor's note: I'm guessing that this is the correct rendering of the text in the paper; if this sentence was intended to impose a requirement, rather than a description, it will have to be changed.] There shall be a specialization `atomic<bool>` which provides the general atomic operations as specified in [atomics.types.operations.general].

>    > 5 The atomic integral specializations and the specialization `atomic<bool>` shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax.

>    > 6 There areshall be pointer partial specializations onof the `atomic` class template. These specializations shall have trivial default constructors and trivial destructors.

>    > 7 There areshall be named types corresponding to the integral specializations of `atomic`, as specified in Table 145. In addition, there shall be named type `atomic_bool` corresponding to the specialization `atomic<bool>`. Each named type is either a typedef to the corresponding specialization or a base class of the corresponding specialization. If it is a base class, it shall support the same member functions as the corresponding specialization.

>    > 8 There areshall be atomic typedefs corresponding to the typedefs in the header `<inttypes.h>` as specified in Table 146.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1462(i). Ambiguous value assignment to `atomic_bool`

Section: 99 [atomics.types.integral] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.types.integral].

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Duplicate of: 1463

Discussion:

Addresses GB-132, US-157

The `atomic_itype` types and `atomic_address` have two overloads of `operator=`; one is `volatile` qualified, and the other is not. `atomic_bool` only has the `volatile` qualified version:

>     bool operator=(bool) volatile;

On a non-`volatile`-qualified object this is ambiguous with the deleted copy-assignment operator

>     atomic_bool& operator=(atomic_bool const&) = delete;

due to the need for a single standard conversion in each case when assigning a bool to an `atomic_bool` as in:

>     atomic_bool b;
>     b = true;

The conversions are:

>     atomic_bool& → atomic_bool volatile&

vs

>     bool → atomic_bool

[ Proposed resolution as of NB comment: ]

Change 99 [atomics.types.integral] as indicated:

>     namespace std {
>       typedef struct atomic_bool {
>         [..]
>         bool operator=(bool) volatile;
>         bool operator=(bool);
>       } atomic_bool;
>       [..]
>     }

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue by replacing `atomic_bool` by `atomic<bool>`.

[ 2010 Batavia ]

Resolved by adoption of n3193.

Proposed resolution:

Solved by n3193.

### 1464(i). Underspecified typedefs for atomic integral types

Section: 99 [atomics.types.integral] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-160

The last sentence of 99 [atomics.types.integral] p.1 says:

> Table 143 shows typedefs to atomic integral classes and the corresponding `<cstdint>` typedefs.

That's nice, but nothing says these are supposed to be part of the implementation, and they are not listed in the synopsis.

[ Proposed resolution as of NB comment ]

1. Remove Table 143 — Atomics for standard typedef types.

2. Change 99 [atomics.types.integral] p.1 as indicated:

   > 1 The name `atomic_itype` and the functions operating on it in the preceding synopsis are placeholders for a set of classes and functions. Throughout the preceding synopsis, `atomic_itype` should be replaced by each of the class names in Table 142 and integral should be replaced by the integral type corresponding to the class name. Table 143 shows typedefs to atomic integral classes and the corresponding `<cstdint>` typedefs.

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue.

[ 2010-11 Batavia ]

Resolved by adopting n3193.

Proposed resolution:

Solved by n3193.

### 1465(i). Missing arithmetic operators for `atomic_address`

Section: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-161

`atomic_address` has `operator+=` and `operator-=`, but no `operator++` or `operator--`. The template specialization `atomic<Ty*>` has all of them.

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue by replacing `atomic_address` by `atomic<void*>`.

[ Resolved in Batavia by accepting n3193. ]

Proposed resolution:

Change 99 [atomics.types.address], class `atomic_address` synopsis, as indicated:

>     namespace std {
>       typedef struct atomic_address {
>         […]
>         void* operator=(const void*) volatile;
>         void* operator=(const void*);
>         void* operator++(int) volatile;
>         void* operator++(int);
>         void* operator--(int) volatile;
>         void* operator--(int);
>         void* operator++() volatile;
>         void* operator++();
>         void* operator--() volatile;
>         void* operator--();
>         void* operator+=(ptrdiff_t) volatile;
>         void* operator+=(ptrdiff_t);
>         void* operator-=(ptrdiff_t) volatile;
>         void* operator-=(ptrdiff_t);
>       } atomic_address;
>       […]
>     }

### 1466(i). Silent `const` breakage by `compare_exchange_*` member functions

Section: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-162

The `compare_exchange_weak` and `compare_exchange_strong` member functions that take `const void*` arguments lead to a silent removal of `const`, because the `load` member function and other acessors return the stored value as a `void*`.

[ Proposed resolution as of NB comment: ]

Change 99 [atomics.types.address], class `atomic_address` synopsis, as indicated:

>     namespace std {
>       typedef struct atomic_address {
>         [..]
>         bool compare_exchange_weak(const void*&, const void*,
>           memory_order, memory_order) volatile;
>         bool compare_exchange_weak(const void*&, const void*,
>           memory_order, memory_order);
>         bool compare_exchange_strong(const void*&, const void*,
>           memory_order, memory_order) volatile;
>         bool compare_exchange_strong(const void*&, const void*,
>           memory_order, memory_order);
>         bool compare_exchange_weak(const void*&, const void*,
>           memory_order = memory_order_seq_cst) volatile;
>         bool compare_exchange_weak(const void*&, const void*,
>           memory_order = memory_order_seq_cst);
>         bool compare_exchange_strong(const void*&, const void*,
>           memory_order = memory_order_seq_cst) volatile;
>         bool compare_exchange_strong(const void*&, const void*,
>           memory_order = memory_order_seq_cst);
>         [..]
>       } atomic_address;
>       [..]
>     }

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue by replacing `atomic_address` by `atomic<void*>`.

[ Resolved in Batavia by accepting n3193. ]

Proposed resolution:

Solved by n3193.

### 1467(i). Deriving `atomic<T*>` from `atomic_address` breaks type safety

Section: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-163

Requiring `atomic<T*>` to be derived from `atomic_address` breaks type safety:

>     atomic<double*> ip;
>     char ch;
>     atomic_store(&ip, &ch);
>     *ip.load() = 3.14159;

The last line overwrites `ch` with a value of type `double`.

[ 2010-10-27 Daniel adds: ]

> Resolving this issue will also solve 1469(i)

> Accepting n3164 would solve this issue by removing `atomic_address`.

[ Resolved in Batavia by accepting n3193. ]

Proposed resolution:

1. Change 32.5.8 [atomics.types.generic], class template specialization `atomic<T*>` synopsis, as indicated:

   >     namespace std {
   >       template <class T> struct atomic<T*> : atomic_address {
   >         [..]
   >       };
   >       [..]
   >     }

2. Change 32.5.8 [atomics.types.generic] p. 4 as indicated:

   > 4 There are pointer partial specializations on the `atomic` class template. These specializations shall be publicly derived from `atomic_address`. The unit of addition/subtraction for these specializations shall be the size of the referenced type. These specializations shall have trivial default constructors and trivial destructors.

### 1468(i). `atomic_address::compare_exchange_*` member functions should match `atomic_compare_exchange_*` free functions

Section: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-164

`atomic_address` has member functions `compare_exchange_weak` and `compare_exchange_strong` that take arguments of type `const void*`, in addition to the `void*` versions. If these member functions survive, there should be corresponding free functions.

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue differently by removing the overloads with `const void*` arguments, because they break type-safety.

[ Resolved in Batavia by accepting n3193. ]

Proposed resolution:

Extend the synopsis around `atomic_address` in 99 [atomics.types.address] as indicated:

>     namespace std {
>       [..]
>       bool atomic_compare_exchange_weak(volatile atomic_address*, void**, void*);
>       bool atomic_compare_exchange_weak(atomic_address*, void**, void*);
>       bool atomic_compare_exchange_strong(volatile atomic_address*, void**, void*);
>       bool atomic_compare_exchange_strong(atomic_address*, void**, void*);
>       bool atomic_compare_exchange_weak_explicit(volatile atomic_address*, void**, void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_weak_explicit(atomic_address*, void**, void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, void**, void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(atomic_address*, void**, void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_weak(volatile atomic_address*, const void**, const void*);
>       bool atomic_compare_exchange_weak(atomic_address*, const void**, const void*);
>       bool atomic_compare_exchange_strong(volatile atomic_address*, const void**, const void*);
>       bool atomic_compare_exchange_strong(atomic_address*, const void**, const void*);
>       bool atomic_compare_exchange_weak_explicit(volatile atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_weak_explicit(atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       [..]
>     }

### 1469(i). `atomic<T*>` inheritance from `atomic_address` breaks type safety

Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-133

The free functions that operate on `atomic_address` can be used to store a pointer to an unrelated type in an `atomic<T*>` without a cast. e.g.

>     int i;
>     atomic<int*> ai(&i);
>     string s;
>     atomic_store(&ai,&s);

Overload the `atomic_store`, `atomic_exchange` and `atomic_compare_exchange_[weak/strong]` operations for `atomic<T*>` to allow storing only pointers to `T`.

[ 2010-10-27 Daniel adds: ]

> Resolving this issue will also solve 1467(i)

> Accepting n3164 would solve this issue by removing `atomic_address`.

[Resolved in Batavia by accepting n3193. ]

Proposed resolution:

Add the following overloads to 32.5.8 [atomics.types.generic], the synopsis around the specialization `atomic<T*>`, as indicated:

>     namespace std {
>       [..]
>       template <class T> struct atomic<T*> : atomic_address {
>         [..]
>       };

>       template<typename T>
>       void atomic_store(atomic<T*>&,T*);
>       template<typename T>
>       void atomic_store(atomic<T*>&,void*) = delete;
>       template<typename T>
>       void atomic_store_explicit(atomic<T*>&,T*,memory_order);
>       template<typename T>
>       void atomic_store_explicit(atomic<T*>&,void*,memory_order) = delete;
>       template<typename T>
>       T* atomic_exchange(atomic<T*>&,T*);
>       template<typename T>
>       T* atomic_exchange(atomic<T*>&,void*) = delete;
>       template<typename T>
>       T* atomic_exchange_explicit(atomic<T*>&,T*,memory_order);
>       template<typename T>
>       T* atomic_exchange_explicit(atomic<T*>&,void*,memory_order) = delete;
>       template<typename T>
>       T* atomic_compare_exchange_weak(atomic<T*>&,T**,T*);
>       template<typename T>
>       T* atomic_compare_exchange_weak(atomic<T*>&,void**,void*) = delete;
>       template<typename T>
>       T* atomic_compare_exchange_weak_explicit(atomic<T*>&,T**,T*,memory_order);
>       template<typename T>
>       T* atomic_compare_exchange_weak_explicit(atomic<T*>&,void**,void*,memory_order) = delete;
>       template<typename T>
>       T* atomic_compare_exchange_strong(atomic<T*>&,T**,T*);
>       template<typename T>
>       T* atomic_compare_exchange_strong(atomic<T*>&,void**,void*) = delete;
>       template<typename T>
>       T* atomic_compare_exchange_strong_explicit(atomic<T*>&,T**,T*,memory_order);
>       template<typename T>
>       T* atomic_compare_exchange_strong_explicit(atomic<T*>&,void**,void*,memory_order) = delete;

>     }

### 1474(i). weak compare-and-exchange confusion

Section: 32.5.8.2 [atomics.types.operations] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-06-15

Priority: Not Prioritized

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Duplicate of: 1470, 1475, 1476, 1477

Discussion:

Addresses US-175, US-165, CH-23, GB-135

99 [atomics.types.operations.req] p. 25: The first sentence is grammatically incorrect.

[ 2010-10-28 Daniel adds: ]

Duplicate issue 1475(i) also has a proposed resolution, but both issues are resolved with below proposed resolution.

[ 2011-02-15 Howard fixes numbering, Hans improves the wording ]

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

1. Change 99 [atomics.types.operations.req] p. 23 as indicated:

   > [ Note: For example, tThe effect of the compare-and-exchange operations `atomic_compare_exchange_strong` is

   > >     if (memcmp(object, expected, sizeof(*object)) == 0)
   > >       memcpy(object, &desired, sizeof(*object));
   > >     else
   > >       memcpy(expected, object, sizeof(*object));

   > — end note ] [..]

2. Change 99 [atomics.types.operations.req] p. 25 as indicated:

   > 25 Remark: The weak compare-and-exchange operations may fail spuriously, that is, return false while leaving the contents of memory pointed to by `expected` before the operation is the same that same as that of the `object` and the same as that of `expected` after the operationA weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by `expected` and `object` are equal, it may return false and store back to `expected` the same memory contents that were originally there.. [ Note: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., loadlocked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

   > When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable. — end note ]

### 1478(i). Clarify race conditions in atomics initialization

Section: 32.5.8.2 [atomics.types.operations] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-136

GB requests normative clarification in 32.5.8.2 [atomics.types.operations] p.4 that concurrent access constitutes a race, as already done on p.6 and p.7.

[ Resolution proposed in ballot comment: ]

> Initialisation of atomics:

> We believe the intent is that for any atomics there is a distinguished initialisation write, but that this need not happens-before all the other operations on that atomic - specifically so that the initialisation write might be non-atomic and hence give rise to a data race, and hence undefined behaviour, in examples such as this (from Hans):

> >     atomic<atomic<int> *> p
> >     f()                      |
> >     { atomic<int>x;          | W_na x
> >       p.store(&x,mo_rlx);    | W_rlx p=&x
> >     }                        |

> (where na is nonatomic and rlx is relaxed). We suspect also that no other mixed atomic/nonatomic access to the same location is intended to be permitted. Either way, a note would probably help.

[2011-02-26: Hans comments and drafts wording]

I think the important point here is to clarify that races on atomics are possible, and can be introduced as a result of non-atomic initialization operations. There are other parts of this that remain unclear to me, such as whether there are other ways to introduce data races on atomics, or whether the races with initialization also introduce undefined behavior by the 3.8 lifetime rules. But I don't think that it is necessary to resolve those issues before releasing the standard. That's particularly true since we've introduced `atomic_init`, which allows easier ways to construct initialization races.

[2011-03 Madrid]

Accepted to be applied immediately to the WP

Proposed resolution:

1. Update 99 [atomics.types.operations.req] p. 5 as follows:

   >     constexpr A::A(C desired);

   > > 5 Effects: Initializes the object with the value `desired`. [ Note: Construction is not atomic. — end note ] Initialization is not an atomic operation (1.10) [intro.multithread]. [Note: It is possible to have an access to an atomic object `A` race with its construction, for example by communicating the address of the just-constructed object `A` to another thread via `memory_order_relaxed` atomic operations on a suitable atomic pointer variable, and then immediately accessing `A` in the receiving thread. This results in undefined behavior. — end note]

2. In response to the editor comment to 99 [atomics.types.operations.req] p. 8: The first Effects element is the correct and intended one:

   >     void atomic_init(volatile A *object, C desired);
   >     void atomic_init(A *object, C desired);

   > > 8 Effects: Non-atomically initializes `*object` with value `desired`. This function shall only be applied to objects that have been default constructed, and then only once. [ Note: these semantics ensure compatibility with C. — end note ] [ Note: Concurrent access from another thread, even via an atomic operation, constitutes a data race. — end note ] [Editor's note: The preceding text is from the WD as amended by N3196. N3193 makes different changes, marked up in the paper as follows:] Effects: Dynamically initializes an atomic variable. Non-atomically That is, non-atomically assigns the value desired to `*object`. [ Note: this operation may need to initialize locks. — end note ] Concurrent access from another thread, even via an atomic operation, constitutes a data race.

### 1479(i). Fence functions should be `extern "C"`

Section: 32.5.11 [atomics.fences] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-179

The fence functions (32.5.11 [atomics.fences] p.5 + p.6) should be `extern "C"`, for `C` compatibility.

[2011-02-16 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

1. Change 32.5.2 [atomics.syn], header `<atomic>` synopsis as indicated:

   >     namespace std {
   >       [..]
   >       // 29.8, fences
   >       extern "C" void atomic_thread_fence(memory_order);
   >       extern "C" void atomic_signal_fence(memory_order);
   >     }

2. Change 32.5.11 [atomics.fences], p. 5 and p. 6 as indicated:

   >     extern "C" void atomic_thread_fence(memory_order);

   > > 5 Effects: depending on the value of `order`, this operation: [..]

   >     extern "C" void atomic_signal_fence(memory_order);

   > > 6 Effects: equivalent to `atomic_thread_fence(order)`, except that synchronizes with relationships are established only between a thread and a signal handler executed in the same thread.

### 1480(i). Atomic fences don't have synchronizes with relation

Section: 32.5.11 [atomics.fences] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-137

Thread fence not only establish synchronizes with relationships, there are semantics of fences that are expressed not in terms of synchronizes with relationships (for example see 32.5.4 [atomics.order] p.5). These semantics also need to apply to the use of `atomic_signal_fence` in a restricted way.

[Batavia: Concurrency group discussed issue, and is OK with the proposed resolution.]

[2011-02-26 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Change 32.5.11 [atomics.fences] p. 6 as indicated:

>     void atomic_signal_fence(memory_order);

> > 6 Effects: equivalent to `atomic_thread_fence(order)`, except that synchronizes with relationshipsthe resulting ordering constraints are established only between a thread and a signal handler executed in the same thread.

### 1481(i). Missing Lockable requirements

Section: 32.2 [thread.req] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-138

The FCD combines the requirements for lockable objects with those for the standard mutex objects. These should be separate. This is LWG issue 1268(i).

[ Resolution proposed by ballot comment: ]

> See attached Appendix 1 - Additional Details

[ 2010-11-01 Daniel comments: ]

> Paper n3130 addresses this issue.

Proposed resolution:

Resolved by n3197.

### 1482(i). Timeout operations are under-specified

Section: 32.2.4 [thread.req.timing] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-181

The timeout operations are under-specified.

[ Resolution proposed by ballot comment: ]

> Define precise semantics for `timeout_until` and `timeout_for`. See n3141 page 193 - Appendix 1 - Additional Details

[ 2010-11-01 Daniel comments: ]

> Accepting n3128 would solve this issue.

Proposed resolution:

Resolved by n3191.

### 1487(i). Clock related operations exception specifications conflict

Section: 32.4.5 [thread.thread.this] Status: C++11 Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses CH-25

Clock related operations are currently not required not to throw. So "Throws: Nothing." is not always true.

[ Resolution proposed by ballot comment: ]

> Either require clock related operations not to throw (in 20.10) or change the Throws clauses in 30.3.2. Also possibly add a note that `abs_time` in the past or negative `rel_time` is allowed.

[2011-02-10: Howard Hinnant provides a resolution proposal]

[Previous proposed resolution:]

1. Change the Operational semantics of `C1::now()` in 30.3 [time.clock.req], Table 59 — `Clock` requirements as follows:

   > Table 59 — `Clock` requirements
   > Expression | Return type | Operational semantics
   > --- | --- | ---
   > `C1::now()` | `C1::time_point` | Returns a `time_point` object representing the current point in time. Shall not throw an exception.

[2011-02-19: Daniel comments and suggests an alternative wording]

Imposing the no-throw requirement on `C1::now()` of any clock time is an overly radical step: It has the indirect consequences that representation types for `C1::rep` can never by types with dynamic memory managment, e.g. my `big_int`, which are currently fully supported by the time utilities. Further-on this strong constraint does not even solve the problem described in the issue, because we are still left with the fact that any of the arithmetic operations of `C1::rep`, `C1::duration`, and `C1::time_point` may throw exceptions.

The alternative proposal uses the following strategy: The general `Clock` requirements remain untouched, but we require that any functions of the library-provided clocks from sub-clause 30.7 [time.clock] and their associated types shall not throw exceptions. Second, we replace existing `noexcept` specifications of functions from Clause 30 that depend on durations, clocks, or time points by wording that clarifies that these functions can only throw, if the operations of user-provided durations, clocks, or time points used as arguments to these functions throw exceptions.

[2011-03-23 Daniel and Peter check and simplify the proposed resolution resulting in this paper]

There is an inherent problem with `std::time_point` that it doesn't seem to have an equivalent value for `((time_t)-1)` that gets returned by C's `time()` function to signal a problem, e.g., because the underlying hardware is unavailable. After a lot of thinking and checks we came to the resolution that `timepoint::max()` should be the value to serve as a value signaling errors in cases where we prefer to stick with no-throw conditions. Of-course, user-provided representation types don't need to follow this approach if they prefer exceptions to signal such failures.

the functions `now()` and `from_time_t()` can remain `noexcept` with the solution to return `timepoint::max()` in case the current time cannot be determined or `(time_t)-1` is passed in, respectively.

Based on the previous proposed solution to LWG 1487 we decided that the new `TrivialClock` requirements should define that `now()` mustn't throw and return `timepoint::max()` to signal a problem. That is in line with the C standard where `(time_t)-1` signals a problem. Together with a fix to a - we assumed - buggy specifcation in 20.11.3 p2 which uses "happens-before" relationship with something that isn't any action:

> 2 In Table 59 `C1` and `C2` denote clock types. `t1` and `t2` are values returned by `C1::now()` where the call returning `t1` happens before (1.10) the call returning `t2` and both of these calls happen before `C1::time_point::max()`.

[2011-03-23 Review with Concurrency group suggested further simplifications and Howard pointed out, that we do not need time_point::max() as a special value.]

also the second "happens before" will be changed to "occurs before" in the english meaning. this is to allow a steady clock to wrap.

Peter updates issue accordingly to discussion.

[Note to the editor: we recommend removal of the following redundant paragraphs in 32.7.5 [thread.condition.condvarany] p. 18 to p. 21, p. 27, p. 28, p. 30, and p. 31 that are defining details for the wait functions that are given by the Effects element. ]

[Note to the editor: we recommend removal of the following redundant paragraphs in 32.7.4 [thread.condition.condvar]: p24-p26, p33-p34, and p36-p37 that are defining details for the `wait_for` functions. We believe these paragraphs are redundant with respect to the Effects clauses that define semantics based on `wait_until`. An example of such a specification is the `wait()` with a predicate. ]

Proposed resolution:

1. Change p2 in 20.11.3 [time.clock.req] as follows

   > 2 In Table 59 `C1` and `C2` denote clock types. `t1` and `t2` are values returned by `C1::now()` where the call returning `t1` happens before (1.10) the call returning `t2` and both of these calls happen occur before `C1::time_point::max()`. [ Note: This means `C1` didn't wrap around between `t1` and `t2` — end note ]

2. Add the following new requirement set at the end of sub-clause 30.3 [time.clock.req]: [Comment: This requirement set is intentionally incomplete. The reason for this incompleteness is the based on the fact, that if we would make it right for C++0x, we would end up defining something like a complete `ArithmeticLike` concept for `TC::rep`, `TC::duration`, and `TC::time_point`. But this looks out-of scope for C++0x to me. The effect is that we essentially do not exactly say, which arithmetic or comparison operations can be used in the time-dependent functions from Clause 30, even though I expect that all declared functions of `duration` and `time_point` are well-formed and well-defined. — end comment]

   > 3 [ Note: the relative difference in durations between those reported by a given clock and the SI definition is a measure of the quality of implementation. — end note ]

   > ? A type `TC` meets the `TrivialClock` requirements if:

   > - `TC` satisfies the `Clock` requirements (30.3 [time.clock.req]),

   > - the types `TC::rep`, `TC::duration`, and `TC::time_point` satisfy the requirements of `EqualityComparable` ( [equalitycomparable]), `LessThanComparable` ( [lessthancomparable]), `DefaultConstructible` ( [defaultconstructible]), `CopyConstructible` ( [copyconstructible]), `CopyAssignable` ( [copyassignable]), `Destructible` ( [destructible]), and of numeric types ([numeric.requirements]) [Note: This means in particular, that operations of these types will not throw exceptions — end note ],

   > - lvalues of the types `TC::rep`, `TC::duration`, and `TC::time_point` are swappable (16.4.4.3 [swappable.requirements]),

   > - the function `TC::now()` does not throw exceptions, and

   > - the type `TC::time_point::clock` meets the `TrivialClock` requirements, recursively.

3. Modify 30.7 [time.clock] p. 1 as follows:

   > 1 - The types defined in this subclause shall satisfy the `TrivialClock` requirements (20.11.1).

4. Modify 30.7.2 [time.clock.system] p. 1, class `system_clock` synopsis, as follows:

   >     class system_clock {
   >     public:
   >       typedef see below rep;
   >       typedef ratio<unspecified , unspecified > period;
   >       typedef chrono::duration<rep, period> duration;
   >       typedef chrono::time_point<system_clock> time_point;
   >       static const bool is_monotonic is_steady = unspecified;
   >       static time_point now() noexcept;
   >       // Map to C API
   >       static time_t to_time_t (const time_point& t) noexcept;
   >       static time_point from_time_t(time_t t) noexcept;
   >     };

5. Modify the prototype declarations in 30.7.2 [time.clock.system] p. 3 + p. 4 as indicated (This edit also fixes the miss of the `static` specifier in these prototype declarations):

   >     static time_t to_time_t(const time_point& t) noexcept;

   >     static time_point from_time_t(time_t t) noexcept;

6. Modify 30.7.7 [time.clock.steady] p. 1, class `steady_clock` synopsis, as follows:

   >     class steady_clock {
   >     public:
   >       typedef unspecified rep;
   >       typedef ratio<unspecified , unspecified > period;
   >       typedef chrono::duration<rep, period> duration;
   >       typedef chrono::time_point<unspecified, duration> time_point;
   >       static const bool is_monotonic is_steady = true;

   >       static time_point now() noexcept;
   >     };

7. Modify 30.7.8 [time.clock.hires] p. 1, class `high_resolution_clock` synopsis, as follows:

   >     class high_resolution_clock {
   >     public:
   >       typedef unspecified rep;
   >       typedef ratio<unspecified , unspecified > period;
   >       typedef chrono::duration<rep, period> duration;
   >       typedef chrono::time_point<unspecified, duration> time_point;
   >       static const bool is_monotonic is_steady = unspecified;

   >       static time_point now() noexcept;
   >     };

8. Add a new paragraph at the end of 32.2.4 [thread.req.timing]:

   > 6 The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.

   > ? Implementation-provided clocks that are used for these functions shall meet the `TrivialClock` requirements (30.3 [time.clock.req]).

9. Edit the synopsis of 32.4.5 [thread.thread.this] before p. 1. [Note: this duplicates edits also in D/N3267]:

   >     template <class Clock, class Duration>
   >     void sleep_until(const chrono::time_point<Clock, Duration>& abs_time) noexcept;
   >     template <class Rep, class Period>
   >     void sleep_for(const chrono::duration<Rep, Period>& rel_time) noexcept;

10. Modify the prototype specifications in 32.4.5 [thread.thread.this] before p. 4 and p. 6 and re-add a Throws element following the Synchronization elements at p. 5 and p. 7:

    >     template <class Clock, class Duration>
    >     void sleep_until(const chrono::time_point<Clock, Duration>& abs_time) noexcept;

    > > 4 - [...]

    > > 5 - Synchronization: None.

    > > ? - Throws: Nothing if `Clock` satisfies the `TrivialClock` requirements (30.3 [time.clock.req]) and operations of `Duration` do not throw exceptions. [Note: Instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]

    >     template <class Rep, class Period>
    >     void sleep_for(const chrono::duration<Rep, Period>& rel_time) noexcept;

    > > 6 [...]

    > > 7 Synchronization: None.

    > > ? Throws: Nothing if operations of `chrono::duration<Rep, Period>` do not throw exceptions. [Note: Instantiations of duration types supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]

11. Fix a minor incorrectness in p. 5: Duration types need to compare against `duration<>::zero()`, not `0`:

    > 3 The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

    > [...]

    > 5 Effects: The function attempts to obtain ownership of the mutex within the relative timeout (30.2.4) specified by `rel_time`. If the time specified by `rel_time` is less than or equal to `0` `rel_time.zero()`, the function attempts to obtain ownership without blocking (as if by calling `try_lock()`). The function shall return within the timeout specified by `rel_time` only if it has obtained ownership of the mutex object. [ Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note ]

12. Modify the class `timed_mutex` synopsis in 32.6.4.3.2 [thread.timedmutex.class] as indicated: [Note: this duplicates edits also in D/N3267]:

    >     class timed_mutex {
    >     public:
    >       [...]
    >       template <class Rep, class Period>
    >         bool try_lock_for(const chrono::duration<Rep, Period>& rel_time) noexcept;
    >       template <class Clock, class Duration>
    >         bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time) noexcept;
    >       [...]
    >     };

13. Modify the class `recursive_timed_mutex` synopsis in 32.6.4.3.3 [thread.timedmutex.recursive] as indicated: [Note: this duplicates edits also in D/N3267]:

    >     class recursive_timed_mutex {
    >     public:
    >       [...]
    >       template <class Rep, class Period>
    >         bool try_lock_for(const chrono::duration<Rep, Period>& rel_time) noexcept;
    >       template <class Clock, class Duration>
    >         bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time) noexcept;
    >       [...]
    >     };

14. Modify the class template `unique_lock` synopsis in 32.6.5.4 [thread.lock.unique] as indicated. [Note: this duplicates edits also in D/N3267]:

    >     template <class Mutex>
    >     class unique_lock {
    >     public:
    >       [...]
    >       template <class Clock, class Duration>
    >         unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time) noexcept;
    >       template <class Rep, class Period>
    >         unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time) noexcept;
    >       [...]
    >     };

15. Modify the constructor prototypes in 32.6.5.4.2 [thread.lock.unique.cons] before p. 14 and p. 17 [Note: this duplicates edits also in D/N3267]:

    >     template <class Clock, class Duration>
    >       unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time) noexcept;

    >     template <class Rep, class Period>
    >       unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time) noexcept;

### 1490(i). Mutex requirements too stringent

Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses CH-27

The mutex requirements force `try_lock` to be `noexcept(true)`. However, where they are used by the generic algorithms, those relax this requirement and say that `try_lock` may throw. This means the requirement is too stringent, also a non-throwing `try_lock` does not allow for a diagnostic such as `system_error` that `lock()` will give us.

[ Resolution proposed by ballot comment: ]

> delete p18, adjust 30.4.4 p1 and p4 accordingly

[ 2010-11-01 Daniel comments: ]

> Accepting n3130 would solve this issue.

Proposed resolution:

Resolved by n3197.

### 1491(i). `try_lock` does not guarantee forward progress

Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-186

`try_lock` does not provide a guarantee of forward progress because it is allowed to spuriously fail.

[ Resolution proposed by ballot comment: ]

> The standard mutex types must not fail spuriously in `try_lock`. See n3141 page 205 - Appendix 1 - Additional Details

[ 2010-11-01 Daniel comments: ]

> Paper n3152 addresses this issue.

Proposed resolution:

Resolved by n3209.

### 1492(i). Mutex requirements should not be bound to threads

Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-188

Mutex requirements should not be bound to threads.

[ Resolution proposed by ballot comment: ]

> See Appendix 1 of n3141 - Additional Details, p. 208.

[ 2010-10-24 Daniel adds: ]

> Accepting n3130 would solve this issue.

Proposed resolution:

Resolved by n3197.

### 1494(i). Term "are serialized" not defined

Section: 32.6.7.2 [thread.once.callonce] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-190

The term "are serialized" is never defined (32.6.7.2 [thread.once.callonce] p. 2).

[ Resolution proposed by ballot comment: ]

Remove the sentence with "are serialized" from paragraph 2. Add "Calls to `call_once` on the same `once_flag` object shall not introduce data races (17.6.4.8)." to paragraph 3.

[ 2010-11-01 Daniel translates NB comment into wording ]

[ 2011-02-17: Hans proposes an alternative resolution ]

[ 2011-02-25: Hans, Clark, and Lawrence update the suggested wording ]

[2011-02-26 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Change 32.6.7.2 [thread.once.callonce] p.2+3 as indicated:

>     template<class Callable, class ...Args>
>     void call_once(once_flag& flag, Callable&& func, Args&&... args);

> > [..]

> > 2 Effects: Calls to `call_once` on the same `once_flag` object are serialized. If there has been a prior effective call to `call_once` on the same `once_flag object`, the call to `call_once` returns without invoking `func`. If there has been no prior effective call to `call_once` on the same `once_flag` object, `INVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)` is executed. The call to `call_once` is effective if and only if `INVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)` returns without throwing an exception. If an exception is thrown it is propagated to the caller. An execution of `call_once` that does not call its `func` is a passive execution. An execution of `call_once` that calls its `func` is an active execution. An active execution shall call `INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)`. If such a call to `func` throws an exception, the execution is exceptional, otherwise it is returning. An exceptional execution shall propagate the exception to the caller of `call_once`. Among all executions of `call_once` for any given `once_flag`: at most one shall be a returning execution; if there is a returning execution, it shall be the last active execution; and there are passive executions only if there is a returning execution. [Note: Passive executions allow other threads to reliably observe the results produced by the earlier returning execution. — end note]

> > 3 Synchronization: The completion of an effective call to `call_once` on a `once_flag` object synchronizes with (6.10.2 [intro.multithread]) all subsequent calls to `call_once` on the same `once_flag` object.For any given `once_flag`: all active executions occur in a total order; completion of an active execution synchronizes with (6.10.2 [intro.multithread]) the start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.

### 1497(i). `lock()` postcondition can not be generally achieved

Section: 32.7 [thread.condition] Status: C++11 Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition].

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Discussion:

Addresses CH-30

If `lock.lock()` throws an exception, the postcondition can not be generally achieved.

[ Resolution proposed by ballot comment: ]

> Either state that the postcondition might not be achieved, depending on the error condition, or state that `terminate()` is called in this case.

[ 2010-08-13 Peter Sommerlad comments and provides wording ]

> 32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany]

> p. 13, last bullet, and corresponding paragraphs in all wait functions

> Problem:
> Condition variable wait might fail, because the lock cannot be acquired when notified. CH-30 says: "If lock.lock() throws an exception, the postcondition can not be generally achieved." CH-30 proposes: "Either state that the postcondition might not be achieved, depending on the error condition, or state that terminate() is called in this case."

> The discussion in Rapperswil concluded that calling `terminate()` might be too drastic in this case and a corresponding exception should be thrown/passed on and one should use a lock type that allows querying its status, which `unique_lock` allows for `std::condition_variable`

> We also had some additional observations while discussing in Rapperswil:
> - in 32.7.4 [thread.condition.condvar] `wait` with predicate and `wait_until` with predicate lack the precondition, postcondition and Error conditions sections. the lack of the precondition would allow to call `pred()` without holding the lock.
> - in 32.7.4 [thread.condition.condvar] `wait_until` and `wait_for` and 32.7.5 [thread.condition.condvarany] `wait_for` still specify an error condition for a violated precondition. This should be removed.

> and add the following proposed solution:

[2011-02-27: Daniel adapts numbering to n3225]

Proposed resolution:

1. Change 32.7.4 [thread.condition.condvar] as indicated:

   >     void wait(unique_lock<mutex>& lock);

   > 9 Requires: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait` or `timed_wait`) threads.

   [..]

   > 11 Postcondition: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   [..]

   >     template <class Predicate>
   >     void wait(unique_lock<mutex>& lock, Predicate pred);

   > ?? Requires: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait` or `timed_wait`) threads.

   > 14 Effects:

   > >     while (!pred())
   > >       wait(lock);

   > ?? Postcondition: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   > ?? Throws: `std::system_error` when an exception is required (30.2.2).

   > ?? Error conditions:

   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   >     template <class Clock, class Duration>
   >     cv_status wait_until(unique_lock<mutex>& lock,
   >       const chrono::time_point<Clock, Duration>& abs_time);

   > 15 Requires: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

   [..]

   > 17 Postcondition: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   [..]

   > 20 Error conditions:

   > - `operation_not_permitted` — if the thread does not own the lock.
   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   >     template <class Rep, class Period>
   >     cv_status wait_for(unique_lock<mutex>& lock,
   >       const chrono::duration<Rep, Period>& rel_time);

   > 21 Requires: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

   [..]

   > 24 Postcondition: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   [..]

   > 26 Error conditions:

   > - `operation_not_permitted` — if the thread does not own the lock.
   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   >     template <class Clock, class Duration, class Predicate>
   >     bool wait_until(unique_lock<mutex>& lock,
   >       const chrono::time_point<Clock, Duration>& abs_time,
   >         Predicate pred);

   > ?? Requires: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait` or `timed_wait`) threads.

   > 27 Effects:

   > >     while (!pred())
   > >       if (wait_until(lock, abs_time) == cv_status::timeout)
   > >         return pred();
   > >     return true;

   > 28 Returns: `pred()`

   > ?? Postcondition: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   > 29 [ Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note ]

   > ?? Throws: `std::system_error` when an exception is required (30.2.2).

   > ?? Error conditions:

   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   >     template <class Rep, class Period, class Predicate>
   >     bool wait_for(unique_lock<mutex>& lock,
   >       const chrono::duration<Rep, Period>& rel_time,
   >         Predicate pred);

   > 30 Requires: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

   [..]

   > 33 Postcondition: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   [..]

   > 37 Error conditions:

   > - `operation_not_permitted` — if the thread does not own the lock.
   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

2. Change 32.7.5 [thread.condition.condvarany] as indicated:

   [..]

   >     template <class Lock, class Predicate>
   >     void wait(Lock& lock, Predicate pred);

   > [Note: if any of the wait functions exits with an exception it is indeterminate if the `Lock` is held. One can use a `Lock` type that allows to query that, such as the `unique_lock` wrapper. — end note]

   > 11 Effects:

   > >     while (!pred())
   > >       wait(lock);

   [..]

   > 31 Error conditions:

   > - `operation_not_permitted` — if the thread does not own the lock.
   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

### 1498(i). Unclear specification for [thread.condition]

Section: 32.7 [thread.condition] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition].

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Discussion:

Addresses CH-29

It is unclear if a spurious wake-up during the loop and reentering of the blocked state due to a repeated execution of the loop will adjust the timer of the blocking with the respect to the previously specified `rel_time` value.

[ Resolution proposed by ballot comment: ]

> Make it clear (e.g. by a note) that when reexecuting the loop the waiting time when blocked will be adjusted with respect to the elapsed time of the previous loop executions.

[ 2010-08-13 Peter Sommerlad comments and provides wording: ]

> Problem: It is unclear if a spurious wake-up during the loop and re-entering of the blocked state due to a repeated execution of the loop will adjust the timer of the blocking with the respect to the previously specified `rel_time` value.

> Proposed Resolution from CH29:

> Make it clear (e.g. by a note) that when re-executing the loop the waiting time when blocked will be adjusted with respect to the elapsed time of the previous loop executions.

> Discussion in Rapperswil:

> Assuming the introduction of a mandatory `steady_clock` proposed by US-181 to the FCD the specification of `condition_variable::wait_for` can be defined in terms of `wait_until` using the `steady_clock`. This is also interleaving with US-181, because that touches the same paragraph (30.5.1 p 25, p34 and 30.5.2 p 20, p 28 in n3092.pdf)

> (The "as if" in the proposed solutions should be confirmed by the standardization terminology experts)

[ 2010-11 Batavia: Resolved by applying n3191 ]

> 1. Change 32.7.4 [thread.condition.condvar] paragraph 25, `wait_for` Effects as indicated:

>    >     template <class Rep, class Period>
>    >     cv_status wait_for(unique_lock<mutex>& lock,
>    >       const chrono::duration<Rep, Period>& rel_time);

>    > [..]

>    > 25 Effects: as if

>    > >     return wait_until(lock, chrono::steady_clock::now() + rel_time);

>    > - Atomically calls `lock.unlock()` and blocks on `*this`.
>    > - When unblocked, calls `lock.lock()` (possibly blocking on the lock), then returns.
>    > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4), or spuriously.
>    > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

> 2. Change 32.7.4 [thread.condition.condvar] paragraph 34, `wait_for` with predicate Effects as indicated:

>    >     template <class Rep, class Period, class Predicate>
>    >     bool wait_for(unique_lock<mutex>& lock,
>    >       const chrono::duration<Rep, Period>& rel_time,
>    >       Predicate pred);

>    > [..]

>    > 34 Effects: as if

>    > >     return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

>    > - Executes a loop: Within the loop the function first evaluates `pred()` and exits the loop if the result is `true`.
>    > - Atomically calls `lock.unlock()` and blocks on `*this`.
>    > - When unblocked, calls `lock.lock()` (possibly blocking on the lock).
>    > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4), or spuriously.
>    > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.
>    > - The loop terminates when `pred()` returns `true` or when the time duration specified by `rel_time` has elapsed.

> 3. Change 32.7.5 [thread.condition.condvarany] paragraph 20, `wait_for` Effects as indicated:

>    >     template <class Lock, class Rep, class Period>
>    >     cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

>    > 20 Effects: as if

>    > >     return wait_until(lock, chrono::steady_clock::now() + rel_time);

>    > - Atomically calls `lock.unlock()` and blocks on `*this`.
>    > - When unblocked, calls `lock.lock()` (possibly blocking on the lock), then returns.
>    > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4), or spuriously.
>    > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> 4. Change 32.7.5 [thread.condition.condvarany] paragraph 28, `wait_for` with predicate Effects as indicated:

>    >     template <class Lock, class Rep, class Period, class Predicate>
>    >     bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

>    > 28 Effects: as if

>    > >     return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

>    > - Executes a loop: Within the loop the function first evaluates `pred()` and exits the loop if the result is `true`.
>    > - Atomically calls `lock.unlock()` and blocks on `*this`.
>    > - When unblocked, calls `lock.lock()` (possibly blocking on the lock).
>    > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4), or spuriously.
>    > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.
>    > - The loop terminates when `pred()` returns `true` or when the time duration specified by `rel_time` has elapsed.

Proposed resolution:

Resolved by n3191.

### 1501(i). Specification for managing associated asynchronous state has problems

Section: 32.10 [futures] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-194

The specification for managing associated asynchronous state is confusing, sometimes omitted, and redundantly specified.

[ Resolution proposed by ballot comment: ]

> Define terms-of-art for releasing, making ready, and abandoning an associated asynchronous state. Use those terms where appropriate. See Appendix 1 - Additional Details

Proposed resolution:

Resolved in Batavia by accepting n3192.

### 1502(i). Specification of [futures.state] unclear

Section: 32.10.5 [futures.state] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-195

The intent and meaning of 32.10.5 [futures.state] p10 is not apparent.

> 10 Accesses to the same shared state conflict (1.10).

[ 2011-03-07 Jonathan Wakely adds: ]

> It's not clear which paragraph this refers to, I had to go to the ballot comments where US-195 reveals it's para 8, which in the FCD (N3092) says:

> > Accesses to the same associated asynchronous state conflict (1.10).

> This is now para 10 in N3242:

> > Accesses to the same shared state conflict (1.10).

[2011-03-07: Lawrence comments and drafts wording]

The intent of this paragraph is to deal with operations, such as `shared_future::get()`, that return a reference to a value held in the shared state. User code could potentially conflict when accessing that value.

Lawrence proposed resolution:

Modify 32.10.5 [futures.state] p10 as follows:

> 10 Accesses to the same shared state conflict (6.10.2 [intro.multithread]).Some operations, e.g. `shared_future::get()` ( [futures.shared_future]), may return a reference to a value held in their shared state. Accesses and modifications through those references by concurrent threads to the same shared state may potentially conflict (6.10.2 [intro.multithread]). [Note: As a consequence, accesses must either use read-only operations or provide additional synchronization. — end note]

[2011-03-19: Detlef suggests an alternative resolution, shown below.]

> Proposed Resolution

> Modify 32.10.5 [futures.state] p10 as follows:

> > 10 Accesses to the same shared state conflict (6.10.2 [intro.multithread]). [Note: This explicitely specifies that the shared state is visible in the objects that reference this state in the sense of data race avoidance 16.4.6.10 [res.on.data.races]. — end note]

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1504(i). Term "are serialized" is not defined

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-196

The term "are serialized" is not defined (32.10.6 [futures.promise] p. 21, 25).

[ Resolution proposed by ballot comment: ]

> Replace "are serialized" with "shall not introduce a data race (17.6.4.8)".

[ 2010-11-02 Daniel translates proposal into proper wording changes ]

[2011-03-19: Detlef comments]

The proposed resolution for 1507(i) would cover this issue as well.

[Proposed Resolution]

> 1. Change 32.10.6 [futures.promise] p. 21 as indicated:

>    > 21 Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serializedshall not introduce a data race ([res.on.data.races]). [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]

> 2. Change 32.10.6 [futures.promise] p. 25 as indicated:

>    > 25 Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serializedshall not introduce a data race ([res.on.data.races]). [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]

Proposed resolution:

Resolved 2001-03 Madrid by issue 1507.

### 1505(i). Synchronization between `promise::set_value` and `future::get`

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-197

There is no defined synchronization between `promise::set_value` and `future::get` (32.10.6 [futures.promise] p. 21, 25).

[ Resolution proposed by ballot comment: ]

> Replace "[Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note]" with the normative "They synchronize with (1.10) any operation on a future object with the same associated asynchronous state marked ready."

[ 2010-11-02 Daniel translates proposal into proper wording changes ]

[2011-03-19: Detlef comments]

The proposed resolution for 1507(i) would cover this issue as well. Effectively it will reject the request but a clarification is added that the normative wording is already in 32.10.5 [futures.state].

> 1. Change 32.10.6 [futures.promise] p. 21 as indicated:

>    > 21 Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized. [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]They synchronize with ([intro.multithread]) any operation on a future object with the same associated asynchronous state marked ready.

> 2. Change 32.10.6 [futures.promise] p. 25 as indicated:

>    > 25 Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized. [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]They synchronize with ([intro.multithread]) any operation on a future object with the same associated asynchronous state marked ready.

Proposed resolution:

Resolved 2001-03 Madrid by issue 1507.

### 1507(i). `promise::XXX_at_thread_exit` functions have no synchronization requirements

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-199

`promise::XXX_at_thread_exit` functions have no synchronization requirements. Specifying synchronization for these member functions requires coordinating with the words in 32.10.6 [futures.promise]/21 and 25, which give synchronization requirements for `promise::set_value` and `promise::set_exception` (32.10.6 [futures.promise] p. 26 ff., p. 29 ff.).

[ Resolution proposed by ballot comment: ]

> Change 32.10.6 [futures.promise]/21 to mention `set_value_at_thread_exit` and `set_exception_at_thread_exit;` with this text, replace 32.10.6 [futures.promise]/25 and add two new paragraphs, after 32.10.6 [futures.promise]/28 and 32.10.6 [futures.promise]/31.

[2011-03-8: Lawrence comments and drafts wording]

This comment applies as well to other `*_at_thread_exit` functions. The following resolution adds synchronization paragraphs to all of them and edits a couple of related synchronization paragraphs.

[2011-03-09: Hans and Anthony add some improvements]

[2011-03-19: Detlef comments]

In regard to the suggested part:

> These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

I would like this to change to:

> These operations do not provide any ordering guarantees with respect to other operations on the same promise object. [Note: They synchronize with calls to operations on objects that refer to the same shared state according to 32.10.5 [futures.state]. — end note]

The current proposed resolution has exactly the same paragraph at for places. I propose to have it only once as new paragraph 2.

This also covers 1504(i) (US-196) and 1505(i) (US-197). US-197 is essentially rejected with this resolution, but a clarification is added that the normative wording is already in 32.10.5 [futures.state].

> Proposed Resolution

> 1. Edit 32.6.4.2 [thread.mutex.requirements.mutex] paragraph 5 as follows:

>    > 5 The implementation shall provide lock and unlock operations, as described below. The implementation shall serialize those operations.For purposes of determining the existence of a data race, these behave as atomic operations (6.10.2 [intro.multithread]). The lock and unlock operations on a single mutex shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.10.2 [intro.multithread]) of the mutex. — end note] [ Note: Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization should be used to ensure that mutex objects are initialized and visible to other threads. — end note ]

> 2. Edit 32.7 [thread.condition] paragraphs 6-9 as follows:

>    >     void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);

>    > > -6- Requires: `lk` is locked by the calling thread and either

>    > > - no other thread is waiting on `cond`, or
>    > > - `lk.mutex()` returns the same value for each of the lock arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

>    > > -7- Effects: transfers ownership of the lock associated with `lk` into internal storage and schedules `cond` to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification shall be as if

>    > > >     lk.unlock();
>    > > >     cond.notify_all();

>    > > -?- Synchronization: The call to `notify_all_at_thread_exit` and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.10.2 [intro.multithread]) calls to functions waiting on `cond`.

>    > > -8- Note: The supplied lock will be held until the thread exits, and care must be taken to ensure that this does not cause deadlock due to lock ordering issues. After calling `notify_all_at_thread_exit` it is recommended that the thread should be exited as soon as possible, and that no blocking or time-consuming tasks are run on that thread.

>    > > -9- Note: It is the user's responsibility to ensure that waiting threads do not erroneously assume that the thread has finished if they experience spurious wakeups. This typically requires that the condition being waited for is satisfied while holding the lock on `lk`, and that this lock is not released and reacquired prior to calling `notify_all_at_thread_exit`.

> 3. Edit 32.10.6 [futures.promise], paragraphs 14-27 as follows:

>    >     void promise::set_value(const R& r);
>    >     void promise::set_value(R&& r);
>    >     void promise<R&>::set_value(R& r);
>    >     void promise<void>::set_value();

>    > > -14- Effects: atomically stores the value `r` in the shared state and makes that state ready (32.10.5 [futures.state]).

>    > > -15- Throws:
>    > > - `future_error` if its shared state already has a stored value or exception, or
>    > > - for the first version, any exception thrown by the copy constructor of `R`, or
>    > > - for the second version, any exception thrown by the move constructor of `R`.

>    > > -16- Error conditions:

>    > > - `promise_already_satisfied` if its shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -17- Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized. [ Note: And they synchronize and serialize with other functions through the referred shared state. — end note ]For purposes of determining the existence of a data race, `set_value`, `set_exception`, `set_value_at_thread_exit`, and `set_exception_at_thread_exit` behave as atomic operations (6.10.2 [intro.multithread]) on the memory location associated with the `promise`. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.10.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

>    >     void set_exception(exception_ptr p);

>    > > -18- Effects: atomically stores the exception pointer `p` in the shared state and makes that state ready (32.10.5 [futures.state]).

>    > > -19- Throws: `future_error` if its shared state already has a stored value or exception.

>    > > -20- Error conditions:

>    > > - `promise_already_satisfied` if its shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -21- Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized. [ Note: And they synchronize and serialize with other functions through the referred shared state. — end note ]For purposes of determining the existence of a data race, `set_value`, `set_exception`, `set_value_at_thread_exit`, and `set_exception_at_thread_exit` behave as atomic operations (6.10.2 [intro.multithread]) on the memory location associated with the `promise`. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.10.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

>    >     void promise::set_value_at_thread_exit(const R& r);
>    >     void promise::set_value_at_thread_exit(R&& r);
>    >     void promise<R&>::set_value_at_thread_exit(R& r);
>    >     void promise<void>::set_value_at_thread_exit();

>    > > -22- Effects: Stores the value `r` in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

>    > > -23- Throws: `future_error` if an error condition occurs.

>    > > -24- Error conditions:
>    > > - `promise_already_satisfied` if its shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -??- Synchronization: For purposes of determining the existence of a data race, `set_value`, `set_exception`, `set_value_at_thread_exit`, and `set_exception_at_thread_exit` behave as atomic operations (6.10.2 [intro.multithread]) on the memory location associated with the `promise`. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.10.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

>    >     void promise::set_exception_at_thread_exit(exception_ptr p);

>    > > -25- Effects: Stores the exception pointer `p` in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

>    > > -26- Throws: `future_error` if an error condition occurs.

>    > > -27- Error conditions:
>    > > - `promise_already_satisfied` if its shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -??- Synchronization: For purposes of determining the existence of a data race, `set_value`, `set_exception`, `set_value_at_thread_exit`, and `set_exception_at_thread_exit` behave as atomic operations (6.10.2 [intro.multithread]) on the memory location associated with the `promise`. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.10.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

> 4. Edit 32.10.10.2 [futures.task.members], paragraph 15-21 as follows:

>    >     void operator()(ArgTypes... args);

>    > > -15- Effects: `INVOKE(f, t1, t2, ..., tN, R)`, where `f` is the stored task of `*this` and `t1`, `t2`, `...`, `tN` are the values in `args...`. If the task returns normally, the return value is stored as the asynchronous result in the shared state of `*this`, otherwise the exception thrown by the task is stored. The shared state of `*this` is made ready, and any threads blocked in a function waiting for the shared state of `*this` to become ready are unblocked.

>    > > -16- Throws: a `future_error` exception object if there is no shared state or the stored task has already been invoked.

>    > > -17- Error conditions:
>    > > - `promise_already_satisfied` if the shared state is already ready.
>    > > - `no_state` if `*this` has no shared state.

>    > > -18- Synchronization: a successful call to `operator()` synchronizes with (6.10.2 [intro.multithread]) a call to any member function of a `future` or `shared_future` object that shares the shared state of `*this`. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with (6.10.2 [intro.multithread]) the successful return from any member function that detects that the state is set to ready. [ Note: `operator()` synchronizes and serializes with other functions through the shared state. — end note ]

>    >     void make_ready_at_thread_exit(ArgTypes... args);

>    > > -19- Effects: `INVOKE(f, t1, t2, ..., tN, R)`, where `f` is the stored task and `t1`, `t2`, `...`, `tN` are the values in `args...`. If the task returns normally, the return value is stored as the asynchronous result in the shared state of `*this`, otherwise the exception thrown by the task is stored. In either case, this shall be done without making that state ready (32.10.5 [futures.state]) immediately. Schedules the shared state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

>    > > -20- Throws: `future_error` if an error condition occurs.

>    > > -21- Error conditions:
>    > > - `promise_already_satisfied` if the shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -??- Synchronization: a successful call to `make_ready_at_thread_exit` synchronizes with (6.10.2 [intro.multithread]) a call to any member function of a `future` or `shared_future` object that shares the shared state of `*this`. The completion of

>    > > - the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state

>    > > - the destructors for all the current thread's variables of thread storage duration

>    > > synchronize with (6.10.2 [intro.multithread]) the successful return from any member function that detects that the state is set to ready. [Note: `make_ready_at_thread_exit` synchronizes and serializes with other functions through the shared state. — end note]

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1508(i). Rename `packaged_task::operator bool()`

Section: 32.10.10 [futures.task] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-201

`packaged_task` provides `operator bool()` to check whether an object has an associated asynchronous state. The various `future` types provide a member function `valid()` that does the same thing. The names of these members should be the same.

[ Resolution proposed by ballot comment: ]

> Replaced the name `packaged_task::operator bool()` with `packaged_task::valid()` in the synopsis (32.10.10 [futures.task]/2) and the member function specification (before 32.10.10.2 [futures.task.members]/15).

[ 2010-11-02 Daniel translates proposed wording changes into a proper proposed resolution and verified that no other places implicitly take advantage of `packaged_task` conversion to bool. ]

[Resolved in Batavia by accepting n3194. ]

Proposed resolution:

1. Change 32.10.10 [futures.task]/2, class template `packaged_task` synopsis as indicated:

   >     template<class R, class... ArgTypes>
   >     class packaged_task<R(ArgTypes...)> {
   >     public:
   >       typedef R result_type;
   >       [..]
   >       explicit operator bool valid() const;
   >       [..]
   >     };

2. Change 32.10.10 [futures.task] before p. 15 as indicated:

   >     explicit operator bool valid() const;

   > > 15 Returns: true only if `*this` has an associated asynchronous state.

   > > 16 Throws: nothing.

### 1513(i). 'launch' enum too restrictive

Section: 32.10 [futures] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses CH-36

Providing only three different possible values for the enum `launch` and saying that `launch::any` means either `launch::sync` or `launch::async` is very restricting. This hinders future implementors to provide clever infrastructures that can simply by used by a call to `async(launch::any,...)`. Also there is no hook for an implementation to provide additional alternatives to `launch` enumeration and no useful means to combine those (i.e. interpret them like flags). We believe something like `async(launch::sync | launch::async, ...)` should be allowed and can become especially useful if one could say also something like `async(launch::any & ~launch::sync, ....)` respectively. This flexibility might limit the features usable in the function called through `async()`, but it will allow a path to effortless profit from improved hardware/software without complicating the programming model when just using `async(launch::any,...)`

[ Resolution proposed by ballot comment: ]

Change in 32.10.1 [futures.overview] 'enum class launch' to allow further implementation defined values and provide the following bit-operators on the launch values (`operator|`, `operator&`, `operator~` delivering a `launch` value).

Note: a possible implementation might use an unsigned value to represent the `launch` enums, but we shouldn't limit the standard to just 32 or 64 available bits in that case and also should keep the launch enums in their own enum namespace.

Change [future.async] p3 according to the changes to `enum launch`. change --`launch::any` to "the implementation may choose any of the policies it provides." Note: this can mean that an implementation may restrict the called function to take all required information by copy in case it will be called in a different address space, or even, on a different processor type. To ensure that a call is either performed like `launch::async` or `launch::sync` describe one should call `async(launch::sync|launch::async,...)`

[ 2010-11-02 Daniel comments: ]

> The new paper n3113 provides concrete wording.

Proposed resolution:

Resolved by n3188.

### 1514(i). `packaged_task` constructors need review

Section: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-207

The constructor that takes `R(*)(ArgTypes...)` is not needed; the constructor that takes a callable type works for this argument type. More generally, the constructors for packaged_task should parallel those for function.

[ US-207 Suggested Resolution: ]

> Review the constructors for packaged_task and provide the same ones as function, except where inappropriate.

[ 2010-10-22 Howard provides wording, as requested by the LWG in Rapperswil. ]

[2011-02-10 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Alter the list of constructors in both 32.10.10 [futures.task] and in 32.10.10.2 [futures.task.members] as indicated:

>     template <class F>
>     explicit packaged_task(F f);
>     template <class F, class Allocator>
>     explicit packaged_task(allocator_arg_t, const Allocator& a, F f);
>     explicit packaged_task(R(*f)(ArgTypes...));
>     template <class F>
>     explicit packaged_task(F&& f);
>     template <class F, class Allocator>
>     explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);

### 1515(i). `packaged_task::make_ready_at_thread_exit` has no synchronization requirements

Section: 32.10.10.2 [futures.task.members] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-208

`packaged_task::make_ready_at_thread_exit` has no synchronization requirements.

[ Resolution proposed by ballot comment: ]

Figure out what the synchronization requirements should be and write them.

[2011-02-09 Anthony provides a proposed resolution]

[2011-02-19 Additional edits by Hans, shown in the proposed resolution section]

[2011-02-22 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

> Proposed Resolution

> Add a new paragraph following 32.10.10.2 [futures.task.members] p. 19:

> >     void make_ready_at_thread_exit(ArgTypes... args);

> > 19 - ...

> > ?? - Synchronization: Following a successful call to `make_ready_at_thread_exit`, the destruction of all objects with thread storage duration associated with the current thread happens before the associated asynchronous state is made ready. The marking of the associated asynchronous state as ready synchronizes with (6.10.2 [intro.multithread]) the successful return from any function that detects that the state is set to ready.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1516(i). No specification for which header contains `auto_ptr`

Section: 99 [depr.auto.ptr] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-142

`auto_ptr` does not appear in the `<memory>` synopsis and [depr.auto.ptr] doesn't say which header declares it. Conversely, the deprecated binders `bind1st` etc. are in the `<functional>` synopsis, this is inconsistent

Either `auto_ptr` should be declared in the `<memory>` synopsis, or the deprecated binders should be removed from the `<functional>` synopsis and appendix D should say which header declares the binders and `auto_ptr`.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Add the following lines to the synopsis of header `<memory>` in [memory]/1:

>     // [depr.auto.ptr], Class auto_ptr (deprecated):
>     template <class X> class auto_ptr;

### 1517(i). default_delete's default constructor should be trivial

Section: 20.3.1.2.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Daniel Krügler Opened: 2010-09-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current working draft does specify the default c'tor of `default_delete` in a manner to guarantee static initialization for default-constructed objects of static storage duration as a consequence of the acceptance of the proposal n2976 but this paper overlooked the fact that the suggested declaration does not ensure that the type will be a trivial type. The type `default_delete` was always considered as a simple wrapper for calling `delete` or `delete[]`, respectivly and should be a trivial type.

In agreement with the new settled core language rules this easy to realize by just changing the declaration to

>     constexpr default_delete() = default;

This proposal also automatically solves the problem, that the semantics of the default constructor of the partial specialization `default_delete<T[]>` is not specified at all. By defaulting its default constructor as well, the semantics are well-defined.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The following wording changes are against N3126.

1. Change the synopsis of the primary template definition of `default_delete` in [unique.ptr.dltr.dflt] as indicated:

   >     namespace std {
   >       template <class T> struct default_delete {
   >         constexpr default_delete() = default;
   >         template <class U> default_delete(const default_delete<U>&);
   >         void operator()(T*) const;
   >       };
   >     }

2. Remove the prototype specification of the `default_delete` default constructor in [unique.ptr.dltr.dflt]/1. This brings it in harmony with the style used in the partial specialization `default_delete<T[]>`. Since there are neither implied nor explicit members, there is no possibility to misinterpret what the constructor does:

   >     constexpr default_delete();

   > > 1 Effects: Default constructs a `default_delete` object.

3. Change the synopsis of the partial specialization of `default_delete` in [unique.ptr.dltr.dflt1] as indicated:

   >     namespace std {
   >       template <class T> struct default_delete<T[]> {
   >         constexpr default_delete() = default;
   >         void operator()(T*) const;
   >         template <class U> void operator()(U*) const = delete;
   >       };
   >     }

### 1518(i). Waiting for deferred functions

Section: 32.10 [futures] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2010-09-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current WP N3126 contains ambiguous statements about the behaviour of functions `wait_for`/`wait_until` in case the future refers to a deferred function. Moreover, I believe it describes a disputable intent, different from the one contained in the original async proposals, that may have been introduced inadvertently during the "async cleanup" that occurred recently. Consider the following case:

>     int f();
>     future<int> x = async(launch::deferred, f);
>     future_status s = x.wait_for(chrono::milliseconds(100));

This example raises two questions:

1. is `f` invoked?
2. what is the value of `s`?

According to the current WP, the answer to question 1 is yes, because 30.6.9/3 says "The first call to a function waiting for the associated asynchronous state created by this async call to become ready shall invoke the deferred function in the thread that called the waiting function". The answer to question 2, however, is not as clear. According to 30.6.6/23, s should be `future_status::deferred` because `x` refers to a deferred function that is not running, but it should also be `future_status::ready` because after executing `f` (and we saw that `f` is always executed) the state becomes ready. By the way, the expression "deferred function that is not running" is very unfortunate in itself, because it may apply to both the case where the function hasn't yet started, as well as the case where it was executed and completed.

While we clearly have a defect in the WP answering to question 2, it is my opinion that the answer to question 1 is wrong, which is even worse. Consider that the execution of the function `f` can take an arbitrarily long time. Having `wait_for()` invoke `f` is a potential violation of the reasonable expectation that the execution of `x.wait_for(chrono::milliseconds(100))` shall take at most 100 milliseconds plus a delay dependent on the quality of implementation and the quality of management (as described in paper N3128). In fact, previous versions of the WP clearly specified that only function `wait()` is required to execute the deferred function, while `wait_for()` and `wait_until()` shouldn't.

The proposed resolution captures the intent that `wait_for()` and `wait_until()` should never attempt to invoke the deferred function. In other words, the P/R provides the following answers to the two questions above:

1. no
2. `future_status::deferred`

In order to simplify the wording, the definition of deferred function has been tweaked so that the function is no longer considered deferred once its evaluation has started, as suggested by Howard.

Discussions in the reflector questioned whether `wait_for()` and `wait_until()` should return immediately or actually wait hoping for a second thread to execute the deferred function. I believe that waiting could be useful only in a very specific scenario but detrimental in the general case and would introduce another source of ambiguity: should `wait_for()` return `future_status::deferred` or `future_status::timeout` after the wait? Therefore the P/R specifies that `wait_for`/`wait_until` shall return immediately, which is simpler, easier to explain and more useful in the general case.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The proposed wording changes are relative to the Final Committee Draft, N3126.

Note to the editor: the proposed wording is meant not be in conflict with any change proposed by paper N3128 "C++ Timeout Specification". Ellipsis are deliberately used to avoid any unintended overlapping.

1. In [futures.unique_future] 30.6.6/22:

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

2. In [futures.unique_future] 30.6.6/23 first bullet:

   — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

3. In [futures.unique_future] 30.6.6/25:

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

4. In [futures.unique_future] 30.6.6/26 first bullet:

   — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

5. In [futures.shared_future] 30.6.7/27

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

6. In [futures.unique_future] 30.6.7/28 first bullet:

   — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

7. In [futures.shared_future] 30.6.6/30:

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

8. In [futures.unique_future] 30.6.7/31 first bullet:

   — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

9. In [futures.atomic_future] 30.6.8/23

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

10. In [futures.unique_future] 30.6.8/24 first bullet:

    — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

11. In [futures.atomic_future] 30.6.8/27:

    Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

12. In [futures.unique_future] 30.6.8/28 first bullet:

    — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

13. In [futures.async] 30.6.9/3 second bullet:

    [...] The first call to a function waitingrequiring a non-timed wait for the associated asynchronous state created by this async call to become ready shall invoke the deferred function in the thread that called the waiting function; once evaluation of `INVOKE(g, xyz)` begins, the function is no longer considered deferred all other calls waiting for the same associated asynchronous state to become ready shall block until the deferred function has completed.

### 1519(i). bucketsize() const only for unordered set

Section: 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.7 [unord.multiset] Status: C++11 Submitter: Nicolai Josuttis Opened: 2010-10-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

While `bucket_size()` is const for `unordered_set`, for all other unordered containers it is not defined as constant member function.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

1. Change 23.7.1 Class template unordered_map [unord.map]/3, as indicated:

   >       namespace std {
   >         template <class Key,
   >           class T,
   >           class Hash = hash<Key>,
   >           class Pred = std::equal_to<Key>,
   >           class Alloc = std::allocator<std::pair<const Key, T> > >
   >         class unordered_map
   >         {
   >         public:
   >           [..]
   >           // bucket interface
   >           size_type bucket_count() const;
   >           size_type max_bucket_count() const;
   >           size_type bucket_size(size_type n) const;
   >           [..]

2. Change 23.7.2 Class template unordered_multimap [unord.multimap]/3, as indicated:

   >       namespace std {
   >         template <class Key,
   >           class T,
   >           class Hash = hash<Key>,
   >           class Pred = std::equal_to<Key>,
   >           class Alloc = std::allocator<std::pair<const Key, T> > >
   >         class unordered_multimap
   >         {
   >         public:
   >           [..]
   >           // bucket interface
   >           size_type bucket_count() const;
   >           size_type max_bucket_count() const;
   >           size_type bucket_size(size_type n) const;
   >           [..]

3. Change 23.7.4 Class template unordered_multiset [unord.multiset]/3, as indicated:

   >       namespace std {
   >         template <class Key,
   >           class Hash = hash<Key>,
   >           class Pred = std::equal_to<Key>,
   >           class Alloc = std::allocator<Key> >
   >         class unordered_multiset
   >         {
   >         public:
   >           [..]
   >           // bucket interface
   >           size_type bucket_count() const;
   >           size_type max_bucket_count() const;
   >           size_type bucket_size(size_type n) const;
   >           [..]

### 1520(i). `INVOKE` on member data pointer with too many arguments

Section: 22.10.4 [func.require] Status: C++11 Submitter: Howard Hinnant Opened: 2010-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

20.8.2 [func.require] p1 says:

> 1 Define `INVOKE(f, t1, t2, ..., tN)` as follows:

> - `(t1.*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`;
> - `((*t1).*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is not one of the types described in the previous item;
> - `t1.*f` when `f` is a pointer to member data of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`;
> - `(*t1).*f` when `f` is a pointer to member data of a class `T` and `t1` is not one of the types described in the previous item;
> - `f(t1, t2, ..., tN)` in all other cases.

The question is: What happens in the 3rd and 4th bullets when `N > 1`?

Does the presence of `t2, ..., tN` get ignored, or does it make the `INVOKE` ill formed?

Here is sample code which presents the problem in a concrete example:

>     #include <functional>
>     #include <cassert>

>     struct S {
>        char data;
>     };

>     typedef char S::*PMD;

>     int main()
>     {
>        S s;
>        PMD pmd = &S::data;
>        std::reference_wrapper<PMD> r(pmd);
>        r(s, 3.0) = 'a';  // well formed?
>        assert(s.data == 'a');
>     }

Without the "`3.0`" the example is well formed.

[Note: Daniel provided wording to make it explicit that the above example is ill-formed. — end note ]

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

Change 20.8.2 [func.require]/1 as indicated:

> 1 Define `INVOKE(f, t1, t2, ..., tN)` as follows:

> - ...
> - ...
> - `t1.*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`;
> - `(*t1).*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is not one of the types described in the previous item;
> - ...

### 1522(i). `conj` specification is now nonsense

Section: 29.4.10 [cmplx.over] Status: C++11 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In Pittsburgh, we accepted the resolution of library issue 1137(i), to add a sentence 3 to [cmplx.over]:

> All the specified overloads shall have a return type which is the nested `value_type` of the effectively cast arguments.

This was already true for four of the six functions except `conj` and `proj`. It is not completely unreasonable to make `proj` return the real value only, but the IEC specification does call for an imaginary part of -0 in some circumstances. The people who care about these distinctions really care, and it is required by an international standard.

Making `conj` return just the real part breaks it horribly, however. It is well understood in mathematics that `conj(re + i*im)` is `(re - i*im)`, and it is widely used. The accepted new definition makes `conj` useful only for pure real operations. This botch absolutely must be fixed.

[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Remove the recently added paragraph 3 from [cmplx.over]:

> 3 All the specified overloads shall have a return type which is the nested `value_type` of the effectively cast arguments.

### 1523(i). `noexcept` for Clause 29

Section: 32.5 [atomics] Status: Resolved Submitter: Hans Boehm Opened: 2010-11-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-63 for Clause 29

Clause 29 does not specify noexcept for any of the atomic operations. It probably should, though that's not completely clear. In particular, atomics may want to throw in implementations that support transactional memory.

Proposed resolution:

Apply paper N3251, noexcept for the Atomics Library.

### 1524(i). Allocation functions are missing happens-before requirements and guarantees

Section: 17.6.3.5 [new.delete.dataraces] Status: C++11 Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US-34

Technical details:

When the same unit of storage is allocated and deallocated repeatedly, operations on it can't be allowed to race between the allocator and the user program. But I don't see any mention of happens-before in the descriptions of allocation and deallocation functions.

Proposed resolution (not wording yet):

- The call to an allocation function returning a pointer `P` must happen-before the matching deallocation call with `P` as a parameter. Otherwise the behavior is undefined. I don't know whether receiving `P` with `memory_order_consume` fits this requirement. `memory_order_relaxed` does not.

- If some memory is passed to a deallocation function, the implementation must ensure that the deallocation call happens-before any allocation call that returns the same memory address.

[2011-02-26: Hans comments and drafts wording]

The second requirement already exists, almost verbatim, as 17.6.3.5 [new.delete.dataraces] p. 1. I think this is where the statement belongs. However, this paragraph requires work to correctly address the first part of the issue.

[Adopted at Madrid, 2011-03]

Proposed resolution:

Change 17.6.3.5 [new.delete.dataraces] p. 1 as follows:

> 1 The library versions of `operator new` and `operator delete`, user replacement versions of global `operator new` and `operator delete`, and the C standard library functions `calloc`, `malloc`, `realloc`, and `free` shall not introduce data races (6.10.2 [intro.multithread]) as a result of concurrent calls from different threads. For purposes of determining the existence of data races, the library versions of `operator new`, user replacement versions of global `operator new`, and the C standard library functions `calloc` and `malloc` shall behave as though they accessed and modified only the storage referenced by the return value. The library versions of `operator delete`, user replacement versions of `operator delete`, and the C standard library function `free` shall behave as though they accessed and modified only the storage referenced by their first argument. The C standard library `realloc` function shall behave as though it accessed and modified only the storage referenced by its first argument and by its return value. Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.

### 1525(i). Effects of `resize(size())` on a `vector`

Section: 23.3.13.3 [vector.capacity] Status: C++11 Submitter: BSI Opened: 2011-03-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses GB-117

23.3.13.3 [vector.capacity] p. 9 (Same as for 23.3.5.3 [deque.capacity] p. 1 i.e. `deque::resize`). There is no mention of what happens if `sz==size()`. While it obviously does nothing I feel a standard needs to say this explicitely.

Suggested resolution:

Append "If `sz == size()`, does nothing" to the effects.

[2011-03-24 Daniel comments]

During the edit of this issue some non-conflicting overlap with 2033(i) became obvious. `CopyInsertable` should be `MoveInsertable` and there is missing the `DefaultConstructible` requirements, but this should be fixed by 2033(i).

Proposed resolution:

Change 23.3.13.3 [vector.capacity] p. 9 as follows:

>     void resize(size_type sz);

> > 9 Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` value-initialized elements to the sequence.

> > 10 Requires: `T` shall be `CopyInsertable` into `*this`.

### 1526(i). C++ should not impose thread safety requirements on C99 library implementations

Section: 16.4.6.10 [res.on.data.races] Status: Resolved Submitter: BSI Opened: 2011-03-24 Last modified: 2016-01-28

Priority: 3

View other active issues in [res.on.data.races].

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Discussion:

Addresses GB-111

Section 16.4.6.10 [res.on.data.races], Data Race Avoidance, requires the C++ Standard Library to avoid data races that might otherwise result from two threads making calls to C++ Standard Library functions on distinct objects. The C standard library is part of the C++ Standard Library and some C++ Standary library functions (parts of the Localization library, as well as Numeric Conversions in 21.5), are specified to make use of the C standard library. Therefore, the C++ standard indirectly imposes a requirement on the thread safety of the C standard library. However, since the C standard does not address the concept of thread safety conforming C implementations exist that do no provide such guarantees. This conflict needs to be reconciled.

Suggested resolution by national body comment:

> remove the requirement to make use of `strtol()` and `sprintf()` since these functions depend on the global C locale and thus cannot be made thread safe.

[2011-03-24 Madrid meeting]

Deferred

[ 2011 Bloomington ]

Alisdair: PJ, does this cause a problem in C?

PJ: Every implementation know of is thread safe.

Pete: There a couple of effects that are specified on strtol() and sprintf() which is a problem.

PJ: When C++ talks about C calls it should be "as if" calling the function.

Pete: Culprit is to string stuff. My fault.

PJ: Not your fault. You did what you were told. Distinct resolution to change wording.

Dietmar: What would we break if we change it back?

Pete: Nothing. If implemented on top of thread safe C library you are just fine.

Alisdair: Anyone want to clean up wording and put it back to what Pete gave us?

Alisdair: No volunteers. Do we want to mark as NAD? We could leave it as deferred.

Stefanus: Did original submitter care about this?

Lawrence: There is some work to make local calls thread safe. The resolution would be to call those thread safe version.

Pete: "As if called under single threaded C program"

Action Item (Alisdair): Write wording for this issue.

[2012, Kona]

Re-opened at the request of the concurrency subgroup, who feel there is an issue that needs clarifying for the (planned) 2017 standard.

Rationale:

No consensus to make a change at this time

[2012, Portland]

The concurrency subgroup decided to encourage the LWG to consider a change to 16.2 [library.c] or thereabouts to clarify that we are requiring C++-like thread-safety for `setlocale`, so that races are not introduced by C locale accesses, even when the C library allows it. This would require e.g. adding "and data race avoidance" at the end of 16.2 [library.c] p1:

"The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety and data race avoidance.",

with some further clarifications in the sections mentioned in 1526(i).

This seems to be consistent with existing implementations. This would technically not be constraining C implementation, but it would be further constraining C libraries used for both C and C++.

[Lenexa 2015-05-05: Move to Resolved]

JW: it's a bit odd that the issue title says sould not impose requirements on C libs, then the P/R does exactly that. Does make sense though, previously we imposed an implicit requirement which would not have been met. Now we say it explicitly and require it is met.

STL: I think this is Resolved, it has been fixed in the working paper [support.runtime]/6 is an example where we call out where things can race. That implies that for everything else they don't create races.

JW: I'm not sure, I think we still need the "and data race avoidance" to clarify that the features from C avoid races, even though C99 says no such thing.

STL: [library.c] says that something like sqrt is part of the C++ Standard LIbrary. [res.on.data.races] then applies to them. Would be OK with a note there, but am uncomfortable with "and data race avoidance" which sounds like it's making a very strong guarantee.

ACTION ITEM JW to editorially add note to [library.c] p1: "Unless otherwise specified, the C Standard Library functions shall meet the requirements for data race avoidance (xref [res.on.data.races])"

Move to Resolved?

10 in favor, 0 opposed, 3 abstentions

Proposed resolution:

This wording is relative to N3376.

1. Change 16.2 [library.c] p1 as indicated:

   > -1- The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety and data race avoidance.

### 2000(i). Missing definition of `packaged_task` specialization of `uses_allocator`

Section: 32.10.10.3 [futures.task.nonmembers] Status: C++11 Submitter: Howard Hinnant Opened: 2010-08-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

[futures.task.nonmembers]/3 says:

>        template <class R, class Alloc>
>          struct uses_allocator<packaged_task<R>, Alloc>;

This is a declaration, but should be a definition.

Proposed resolution:

Change [futures.task.nonmembers]/3:

>        template <class R, class Alloc>
>          struct uses_allocator<packaged_task<R>, Alloc>;
>             : true_type {};

### 2001(i). Class template `basic_regex` uses non existent `string_type`

Section: 28.6.7.3 [re.regex.assign] Status: C++11 Submitter: Volker Lukas Opened: 2010-10-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In working draft N3126, subclause 28.6.7.3 [re.regex.assign], paragraphs 12, 13 and 19, the name `string_type` is used. This is presumably a typedef for `basic_string<value_type>`, where `value_type` is the character type used by `basic_regex`. The `basic_regex` template however defines no such typedef, and neither does the `<regex>` header or the `<initializer_list>` header included by `<regex>`.

[ 2010-11-03 Daniel comments and suggests alternative wording: ]

> The proposed resolution needs to use `basic_string<charT>` instead of `basic_string<char>`

Previous Proposed Resolution:

Make the following changes to [re.regex.assign]:

>     basic_regex& assign(const charT* ptr, flag_type f = regex_constants::ECMAScript);

> > 12 Returns: `assign(string_typebasic_string<charT>(ptr), f)`.

>     basic_regex& assign(const charT* ptr, size_t len,
>       flag_type f = regex_constants::ECMAScript);

> > 13 Returns: `assign(string_typebasic_string<charT>(ptr, len), f)`.

>     [..]

>     template <class InputIterator>
>       basic_regex& assign(InputIterator first, InputIterator last,
>                               flag_type f = regex_constants::ECMAScript);

> > 18 Requires: The type `InputIterator` shall satisfy the requirements for an Input Iterator (24.2.3).

> > 19 Returns: `assign(string_typebasic_string<charT>(first, last), f)`.

[ 2010 Batavia ]

Unsure if we should just give `basic_regex` a `string_type` typedef. Looking for when `string_type` was introduced into `regex`. Howard to draft wording for `typedef typename traits::string_type string_type`, then move to Review.

[ 2011-02-16: Daniel comments and provides an alternative resolution. ]

I'm strongly in favour with the Batavia idea to provide a separate `string_type` within `basic_regex`, but it seems to me that the issue resultion should add one more important typedef, namely that of the traits type! Currently, `basic_regex` is the only template that does not publish the type of the associated traits type. Instead of opening a new issue, I added this suggestion as part of the proposed wording.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

Change the class template `basic_regex` synopsis, 28.6.7 [re.regex] p. 3, as indicated:

>     namespace std {
>       template <class charT,
>                 class traits = regex_traits<charT> >
>       class basic_regex {
>       public:
>         // types:
>         typedef charT value_type;
>         typedef traits traits_type;
>         typedef typename traits::string_type string_type;
>         typedef regex_constants::syntax_option_type flag_type;
>         typedef typename traits::locale_type locale_type;

>         [..]
>       };
>     }

### 2002(i). Class template `match_results` does not specify the semantics of `operator==`

Section: 28.6.9.9 [re.results.nonmember] Status: Resolved Submitter: Daniel Krügler Opened: 2010-10-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The Returns element of `operator==` says:

> `true` only if the two objects refer to the same match

It is not really clear what this means: The current specification would allow for an implementation to return `true`, only if the address values of `m1` and `m2` are the same. While this approach is unproblematic in terms of used operations this is also a bit unsatisfactory. With identity equality alone there seems to be no convincing reason to provide this operator at all. It could for example also refer to an comparison based on iterator values. In this case a user should better know that this will be done, because there is no guarantee at all that inter-container comparison of iterators is a feasible operation. This was a clear outcome of the resolution provided in N3066 for LWG issue 446(i). It could also mean that a character-based comparison of the individual `sub_match` elements should be done - this would be equivalent to applying `operator==` to the subexpressions, prefix and suffix.

Proposed resolution:

Addressed by paper n3158.

### 2003(i). String exception inconsistency in erase.

Section: 27.4.3.2 [string.require] Status: C++14 Submitter: José Daniel García Sánchez Opened: 2010-10-21 Last modified: 2016-11-12

Priority: 0

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Discussion:

Clause 21.4.1 [string.require]p3 states:

> No `erase()` or `pop_back()` member function shall throw any exceptions.

However in 21.4.6.5 [string.erase] p2 the first version of `erase` has

> Throws: `out_of_range` if `pos > size()`.

[2011-03-24 Madrid meeting]

Beman: Don't want to just change this, can we just say "unless otherwise specified"?

Alisdair: Leave open, but update proposed resolution to say something like "unless otherwise specified".

General agreement that it should be corrected but not a stop-ship.

Action: Update proposed wording for issue 2003 as above, but leave Open.

[2014-02-12 Issaquah meeting]

Jeffrey: Madrid meeting's proposed wording wasn't applied, and it's better than the original proposed wording. However, this sentence is only doing 3 functions' worth of work, unlike the similar paragraphs in 23.2.2 [container.requirements.general]. Suggest just putting "Throws: Nothing" on the 3 functions.

[2014-02-13 Issaquah meeting]

Move as Immmediate

Proposed resolution:

Remove [string.require]p/3:

> 3 No `erase()` or `pop_back()` member function shall throw any exceptions.

Add to the specifications of `iterator erase(const_iterator p);`, `iterator erase(const_iterator first, const_iterator last);`, and `void pop_back();` in 27.4.3.7.5 [string.erase]:

> Throws: Nothing

### 2004(i). `duration::operator*` has template parameters in funny order

Section: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In [time] and [time.duration.nonmember] we have:

>     template <class Rep1, class Period, class Rep2>
>         duration<typename common_type<Rep1, Rep2>::type, Period>
>             operator*(const Rep1& s, const duration<Rep2, Period>& d);

Everywhere else, we always have `<rep, period>` in that order for a given type. But here, we have `Period` and `Rep2` in reverse order for `<Rep2, Period>`. This is probably of little importance, since the template parameters are seldom spelled out for a function like this. But changing it now will eliminate a potential source of future errors and confusion.

Proposed resolution:

Change the signature in [time] and [time.duration.nonmember] to:

>     template <class Rep1, class PeriodRep2, class Rep2Period>
>         duration<typename common_type<Rep1, Rep2>::type, Period>
>             operator*(const Rep1& s, const duration<Rep2, Period>& d);

### 2005(i). `unordered_map::insert(T&&)` protection should apply to `map` too

Section: 23.4.3.4 [map.modifiers], 23.4.4.3 [multimap.modifiers], 23.5.3.4 [unord.map.modifiers], 23.5.4.3 [unord.multimap.modifiers] Status: C++14 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [map.modifiers].

View all issues with C++14 status.

Discussion:

In [unord.map.modifiers], the signature:

>     template <class P>
>         pair<iterator, bool> insert(P&& obj);

now has an added Remarks paragraph:

> Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type`.

The same is true for `unordered_multimap`.

But neither `map` nor `multimap` have this constraint, even though it is a Good Thing(TM) in those cases as well.

[ The submitter suggests: Add the same Remarks clause to [map.modifiers] and [multimap.modifiers]. ]

[ 2010-10-29 Daniel comments: ]

I believe both paragraphs need more cleanup: First, the current Requires element conflict with the Remark; second, it seems to me that the whole single Requires element is intended to be split into a Requires and an Effects element; third, the reference to `tuple` is incorrect (noticed by Paolo Carlini); fourth, it refers to some non-existing `InputIterator` parameter relevant for a completely different overload; sixth, the return type of the overload with hint is wrong. The following proposed resolution tries to solve these issues as well and uses similar wording as for the corresponding unordered containers. Unfortunately it has some redundancy over Table 99, but I did not remove the specification because of the more general template parameter `P` - the Table 99 requirements apply only for an argument identical to `value_type`.

Daniel's Proposed resolution (not current):

> 1. Change 23.4.3.4 [map.modifiers] around p. 1 as indicated:

>    >     template <class P> pair<iterator, bool> insert(P&& x);
>    >     template <class P> pair<iterator, bool> insert(const_iterator position, P&& x);

>    > > 1 Requires: `P` shall be convertible to `value_type` is constructible from `std::forward<P>(x)`..

>    > > If `P` is instantiated as a reference type, then the argument `x` is copied from. Otherwise `x` is considered to be an rvalue as it is converted to `value_type` and inserted into the map. Specifically, in such cases `CopyConstructible` is not required of `key_type` or `mapped_type` unless the conversion from `P` specifically requires it (e.g., if `P` is a `tuple<const key_type, mapped_type>`, then `key_type` must be `CopyConstructible`). The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type,mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.
>    > > ? Effects: Inserts `x` converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(x)`. For the second form, the iterator `position` is a hint pointing to where the search should start.

>    > > ? Returns: For the first form, the `bool` component of the returned `pair` object indicates whether the insertion took place and the iterator component - or for the second form the returned iterator - points to the element with key equivalent to the key of `value_type(x)`.

>    > > ? Complexity: Logarithmic in general, but amortized constant if `x` is inserted right before `position`.

>    > > ? Remarks: These signatures shall not participate in overload resolution unless `P` is implicitly convertible to `value_type`.

> 2. Change 23.4.4.3 [multimap.modifiers] around p. 1 as indicated:

>    >     template <class P> iterator insert(P&& x);
>    >     template <class P> iterator insert(const_iterator position, P&& x);

>    > > 1 Requires: `P` shall be convertible to `value_type` is constructible from `std::forward<P>(x)`.

>    > > If `P` is instantiated as a reference type, then the argument `x` is copied from. Otherwise `x` is considered to be an rvalue as it is converted to `value_type` and inserted into the map. Specifically, in such cases `CopyConstructible` is not required of `key_type` or `mapped_type` unless the conversion from `P` specifically requires it (e.g., if `P` is a `tuple<const key_type, mapped_type>`, then `key_type` must be `CopyConstructible`). The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type, mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.
>    > > ? Effects: Inserts `x` converted to `value_type`. For the second form, the iterator `position` is a hint pointing to where the search should start.

>    > > ? Returns: An iterator that points to the element with key equivalent to the key of `value_type(x)`.

>    > > ? Complexity: Logarithmic in general, but amortized constant if `x` is inserted right before `position`.

>    > > ? Remarks: These signatures shall not participate in overload resolution unless `P` is implicitly convertible to `value_type`.

[ 2010 Batavia: ]

We need `is_convertible`, not `is_constructible`, both in ordered and unordered containers.

[ 2011 Bloomington ]

The effects of these inserts can be concisely stated in terms of emplace(). Also, the correct term is "EmplaceConstructible", not "constructible".

New wording by Pablo, eliminating duplicate requirements already implied by the effects clause. Move to Review.

[ 2011-10-02 Daniel comments and refines the proposed wording ]

> Unfortunately the template constraints expressed as "`P` is implicitly convertible to `value_type`" reject the intended effect to support move-only key types, which was the original intention when the library became move-enabled through the rvalue-reference proposals by Howard (This can clearly be deduced from existing carefully selected wording that emphasizes that `CopyConstructible` is only required for special situations involving lvalues or const rvalues as arguments). The root of the problem is based on current core rules, where an "implicitly converted" value has copy-initialization semantics. Consider a move-only key type `KM`, some mapped type `T`, and a source value `p` of type `P` equal to `std::pair<KM, T>`, this is equivalent to:

> >     std::pair<const KM, T> dest = std::move(p);

> Now 9.5 [dcl.init] p16 b6 sb2 says that the effects of this heterogeneous copy-initialization (`p` has a different type than `dest`) are as-if a temporary of the target type `std::pair<const KM, T>` is produced from the rvalue `p` of type `P` (which is fine), and this temporary is used to initialize `dest`. This second step cannot succeed, because we cannot move from `const KM` to `const KM`. This means that `std::is_convertible<P, std::pair<const KM, T>>::value` is false.

> But the actual code that is required (with the default allocator) is simply a direct-initialization from `P` to `value_type`, so imposing an implicit conversion is more than necessary. Therefore I strongly recommend to reduce the "overload participation" constraint to `std::is_constructible<std::pair<const KM, T>, P>::value` instead. This change is the only change that has been performed to the previous proposed wording from Pablo shown below.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup, after much discussion on Daniel's analysis of Copy Initialization and move semantics, which we ultimately agreed with.

[2012, Portland: applied to WP]

Proposed resolution:

1. Change 23.4.3.4 [map.modifiers] around p. 1 as indicated:

   >     template <class P> pair<iterator, bool> insert(P&& x);
   >     template <class P> pair<iterator, bool> insert(const_iterator position, P&& x);

   > > 1 Requires: `P` shall be convertible to `value_type`.

   > > If `P` is instantiated as a reference type, then the argument `x` is copied from. Otherwise `x` is considered to be an rvalue as it is converted to `value_type` and inserted into the map. Specifically, in such cases `CopyConstructible` is not required of `key_type` or `mapped_type` unless the conversion from `P` specifically requires it (e.g., if `P` is a `tuple<const key_type, mapped_type>`, then `key_type` must be `CopyConstructible`). The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type,mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.
   > > ? Effects: The first form is equivalent to `return emplace(std::forward<P>(x))`. The second form is equivalent to `return emplace_hint(position, std::forward<P>(x))`.

   > > ? Remarks: These signatures shall not participate in overload resolution unless `std::is_constructible<value_type, P&&>::value` is true.

2. Change 23.4.4.3 [multimap.modifiers] around p. 1 as indicated:

   >     template <class P> iterator insert(P&& x);
   >     template <class P> iterator insert(const_iterator position, P&& x);

   > > 1 Requires: `P` shall be convertible to `value_type`.

   > > If `P` is instantiated as a reference type, then the argument `x` is copied from. Otherwise `x` is considered to be an rvalue as it is converted to `value_type` and inserted into the map. Specifically, in such cases `CopyConstructible` is not required of `key_type` or `mapped_type` unless the conversion from `P` specifically requires it (e.g., if `P` is a `tuple<const key_type, mapped_type>`, then `key_type` must be `CopyConstructible`). The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type, mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.
   > > ? Effects: The first form is equivalent to `return emplace(std::forward<P>(x))`. The second form is equivalent to `return emplace_hint(position, std::forward<P>(x))`.

   > > ? Remarks: These signatures shall not participate in overload resolution unless `std::is_constructible<value_type, P&&>::value` is true.

3. Change [unord.map.modifers] around p. 1 as indicated:

   >     template <class P>
   >     pair<iterator, bool> insert(P&& obj);

   > > 1 Requires: `value_type` is constructible from `std::forward<P>(obj)`.

   > > 2 Effects: equivalent to `return emplace(std::forward<P>(obj))`. Inserts obj converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(obj)`.

   > > 3 Returns: The bool component of the returned pair object indicates whether the insertion took place and the iterator component points to the element with key equivalent to the key of `value_type(obj)`.

   > > 4 Complexity: Average case O(1), worst case O(size()).

   > > 53 Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type` `std::is_constructible<value_type, P&&>::value` is true.

   >     template <class P>
   >     iterator insert(const_iterator hint, P&& obj);

   > > 6 Requires: `value_type` is constructible from `std::forward<P>(obj)`.

   > > 7? Effects: equivalent to `return emplace_hint(hint, std::forward<P>(obj))`. Inserts obj converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(obj)`. The iterator hint is a hint pointing to where the search should start.

   > > 8 Returns: An iterator that points to the element with key equivalent to the key of `value_type(obj)`.

   > > 9 Complexity: Average case O(1), worst case O(size()).

   > > 10? Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type` `std::is_constructible<value_type, P&&>::value` is true.

4. Change [unord.multimap.modifers] around p. 1 as indicated:

   >     template <class P>
   >     iterator insert(P&& obj);

   > > 1 Requires: `value_type` is constructible from `std::forward<P>(obj)`.

   > > 2 Effects: equivalent to `return emplace(std::forward<P>(obj))`. Inserts obj converted to `value_type`.

   > > 3 Returns: An iterator that points to the element with key equivalent to the key of `value_type(obj)`.

   > > 4 Complexity: Average case O(1), worst case O(size()).

   > > 53 Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type` `std::is_constructible<value_type, P&&>::value` is true.

   >     template <class P>
   >     iterator insert(const_iterator hint, P&& obj);

   > > 6 Requires: `value_type` is constructible from `std::forward<P>(obj)`.

   > > 7? Effects: equivalent to `return emplace_hint(hint, std::forward<P>(obj))`. Inserts obj converted to `value_type`. The iterator hint is a hint pointing to where the search should start.

   > > 8 Returns: An iterator that points to the element with key equivalent to the key of `value_type`(obj).

   > > 9 Complexity: Average case O(1), worst case O(size()).

   > > 10? Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type` `std::is_constructible<value_type, P&&>::value` is true.

### 2007(i). Incorrect specification of return value for `map<>::at()`

Section: 23.4.3.3 [map.access] Status: C++11 Submitter: Matt Austern Opened: 2010-11-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [map.access].

View all issues with C++11 status.

Discussion:

In [map.access]/9, the Returns clause for `map<Key, T>::at(x)` says that it returns "a reference to the element whose key is equivalent to `x`." That can't be right. The signature for `at()` says that its return type is `T`, but the elements of `map<Key, T>` have type `pair<const K, T>`. (I checked [unord.map.elem] and found that its specification of `at()` is correct. This is a problem for `map` only.)

Proposed resolution:

Change the wording in [map.access]/9 so it's identical to what we already say for `operator[]`, which is unambiguous and correct.

> Returns: A reference to the element whose key is equivalent `mapped_type` corresponding to `x` in `*this`.

### 2008(i). Conflicting Error Conditions for `packaged_task::operator()`

Section: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: Pete Becker Opened: 2010-06-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.task.members].

View all issues with C++11 status.

Discussion:

The Throws clause for `packaged_task::operator()` says that it throws "a `future_error` exception object if there is no associated asynchronous state or the stored task has already been invoked." However, the Error Conditions clause does not define an error condition when the stored task has already been invoked, only when the associated state is already ready (i.e. the invocation has completed).

[2011-02-17 Anthony provides an alternative resolution]

Previous proposed resolution:

Change the first bullet item in 32.10.10.2 [futures.task.members] /22:

>     void operator()(ArgTypes... args);

> > 20 ...

> > 21 ...

> > 22 Error conditions:

> > - `promise_already_satisfied` if the associated asynchronous state is already ready `operator()` has already been called.
> > - `no_state` if `*this` has no associated asynchronous state.

[Adopted at Madrid, 2011-03]

Proposed resolution:

1. Change the first bullet item in 32.10.10.2 [futures.task.members] p. 17:

   >     void operator()(ArgTypes... args);

   > > 15 ...

   > > 16 ...

   > > 17 Error conditions:

   > > - `promise_already_satisfied` if the associated asynchronous state is already readystored task has already been invoked.
   > > - `no_state` if `*this` has no associated asynchronous state.

2. Change the first bullet item in 32.10.10.2 [futures.task.members] p. 21:

   >     void make_ready_at_thread_exit(ArgTypes... args);

   > > 19 ...

   > > 20 ...

   > > 21 Error conditions:

   > > - `promise_already_satisfied` if the associated asynchronous state already has a stored value or exceptionstored task has already been invoked.
   > > - `no_state` if `*this` has no associated asynchronous state.

### 2009(i). Reporting out-of-bound values on numeric string conversions

Section: 27.4.5 [string.conversions] Status: C++14 Submitter: Alisdair Meredith Opened: 2010-07-19 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [string.conversions].

View all issues with C++14 status.

Discussion:

The functions (`w`)`stoi` and (`w`)`stof` are specified in terms of calling C library APIs for potentially wider types. The integer and floating-point versions have subtly different behaviour when reading values that are too large to convert. The floating point case will throw `out_of_bound` if the read value is too large to convert to the wider type used in the implementation, but behaviour is undefined if the converted value cannot narrow to a float. The integer case will throw `out_of_bounds` if the converted value cannot be represented in the narrower type, but throws `invalid_argument`, rather than `out_of_range`, if the conversion to the wider type fails due to overflow.

Suggest that the Throws clause for both specifications should be consistent, supporting the same set of fail-modes with the matching set of exceptions.

Proposed resolution:

21.5p3 [string.conversions]

>     int stoi(const string& str, size_t *idx = 0, int base = 10);
>     long stol(const string& str, size_t *idx = 0, int base = 10);
>     unsigned long stoul(const string& str, size_t *idx = 0, int base = 10);
>     long long stoll(const string& str, size_t *idx = 0, int base = 10);
>     unsigned long long stoull(const string& str, size_t *idx = 0, int base = 10);

> > ...

> > 3 Throws: `invalid_argument` if `strtol`, `strtoul`, `strtoll`, or `strtoull` reports that no conversion could be performed. Throws `out_of_range` if `strtol`, `strtoul`, `strtoll` or `strtoull` sets `errno` to `ERANGE`, or if the converted value is outside the range of representable values for the return type.

21.5p6 [string.conversions]

>     float stof(const string& str, size_t *idx = 0);
>     double stod(const string& str, size_t *idx = 0);
>     long double stold(const string& str, size_t *idx = 0);

> > ...

> > 6 Throws: `invalid_argument` if `strtod` or `strtold` reports that no conversion could be performed. Throws `out_of_range` if `strtod` or `strtold` sets `errno` to `ERANGE` or if the converted value is outside the range of representable values for the return type.

### 2010(i). `is_* traits` for binding operations can't be meaningfully specialized

Section: 22.10.15.2 [func.bind.isbind] Status: C++14 Submitter: Sean Hunt Opened: 2010-07-19 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.bind.isbind].

View all issues with C++14 status.

Discussion:

22.10.15.2 [func.bind.isbind] says for `is_bind_expression`:

> Users may specialize this template to indicate that a type should be treated as a subexpression in a `bind` call.

But it also says:

> If `T` is a type returned from `bind`, `is_bind_expression<T>` shall be publicly derived from `integral_constant<bool, true>`, otherwise from `integral_constant<bool, false>`.

This means that while the user is free to specialize, any specialization would have to be `false` to avoid violating the second requirement. A similar problem exists for `is_placeholder`.

[ 2010 Batavia (post meeting session) ]

Alisdair recognises this is clearly a bug introduced by some wording he wrote, the sole purpose of this metafunction is as a customization point for users to write their own `bind`-expression types that participate in the standard library `bind` protocol. The consensus was that this should be fixed in Madrid, moved to Open.

[2011-05-13 Jonathan Wakely comments and provides proposed wording]

The requirements are that `is_bind_expression<T>::value` is true when `T` is a type returned from `bind`, false for any other type, except when there's a specialization involving a user-defined type (N.B. 16.4.5.2.1 [namespace.std] means we don't need to say e.g. `is_bind_expression<string>` is false.)

The obvious way to meet the requirements is for the primary template to derive from `integral_constant<bool, false>` and for implementations to provide specializations for the unspecified types returned from `bind`. User-defined specializations can do whatever they like, as long as `is_bind_expression::value` is sane. There's no reason to forbid users from defining `is_bind_expression<user_defined_type>::value=false` if that's what they want to do.

Similar reasoning applies to `is_placeholder`, but a further issue is that 22.10.15.2 [func.bind.isbind] contains wording for `is_placeholder` but contains no definition of it and the sub-clause name only refers to `is_bind_expression`. The wording below proposes splitting paragraphs 3 and 4 of 22.10.15.2 [func.bind.isbind] into a new sub-clause covering `is_placeholder`.

If the template specializations added by the proposed wording are too vague then they could be preceded by "for exposition only" comments

[2011-05-18 Daniel comments and provides some refinements to the P/R]

Both `bind`-related type traits should take advantage of the UnaryTypeTrait requirements. Additionally, the updated wording does not imply that the implementation provides several specializations. Wording was used similar to the specification of the `uses_allocator` type trait (which unfortunately is not expressed in terms of BinaryTypeTrait requirements).

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

1. Change 22.10.15.2 [func.bind.isbind] to:

   >     namespace std {
   >       template<class T> struct is_bind_expression; // see below
   >         : integral_constant<bool, see below> { };
   >     }

   > > -1- `is_bind_expression` can be used to detect function objects generated by `bind`. `bind` uses `is_bind_expression` to detect subexpressions. Users may specialize this template to indicate that a type should be treated as a subexpression in a `bind` call.

   > > -2- If `T` is a type returned from `bind`, `is_bind_expression<T>` shall be publicly derived from `integral_constant<bool, true>`, otherwise from `integral_constant<bool, false>`Instantiations of the `is_bind_expression` template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic of `true_type` if `T` is a type returned from `bind`, otherwise it shall have a BaseCharacteristic of `false_type`. A program may specialize this template for a user-defined type `T` to have a BaseCharacteristic of `true_type` to indicate that `T` should be treated as a subexpression in a `bind` call..

   > > -3- `is_placeholder` can be used to detect the standard placeholders `_1`, `_2`, and so on. `bind` uses `is_placeholder` to detect placeholders. Users may specialize this template to indicate a placeholder type.

   > > -4- If `T` is the type of `std::placeholders::_J`, `is_placeholder<T>` shall be publicly derived from `integral_constant<int, J>`, otherwise from `integral_constant<int, 0>`.

2. Insert a new sub-clause immediately following sub-clause 22.10.15.2 [func.bind.isbind], the suggested sub-clause tag is [func.bind.isplace]:

   ### 20.8.9.1.? Class template `is_placeholder` [func.bind.isplace]

   >     namespace std {
   >       template<class T> struct is_placeholder; // see below
   >     }

   > > -?- `is_placeholder` can be used to detect the standard placeholders `_1`, `_2`, and so on. `bind` uses `is_placeholder` to detect placeholders.

   > > -?- Instantiations of the `is_placeholder` template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic of `integral_constant<int, J>` if `T` is the type of `std::placeholders::_J`, otherwise it shall have a BaseCharacteristic of `integral_constant<int, 0>`. A program may specialize this template for a user-defined type `T` to have a BaseCharacteristic of `integral_constant<int, N>` with `N > 0` to indicate that `T` should be treated as a placeholder type.

### 2011(i). Unexpected output required of strings

Section: 27.4.4.4 [string.io] Status: C++14 Submitter: James Kanze Opened: 2010-07-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [string.io].

View all issues with C++14 status.

Discussion:

What should the following code output?

>     #include <string>
>     #include <iostream>
>     #include <iomanip>

>     int main()
>     {
>        std::string test("0X1Y2Z");
>        std::cout.fill('*');
>        std::cout.setf(std::ios::internal, std::ios::adjustfield);
>        std::cout << std::setw(8) << test << std::endl;
>     }

I would expect "`**0X1Y2Z`", and this is what the compilers I have access to (VC++, g++ and Sun CC) do. But according to the standard, it should be "`0X**1Y2Z`":

27.4.4.4 [string.io]/5:

>     template<class charT, class traits, class Allocator>
>       basic_ostream<charT, traits>&
>         operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);

> > Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]). After constructing a `sentry` object, if this object returns `true` when converted to a value of type `bool`, determines padding as described in 28.3.4.3.3.3 [facet.num.put.virtuals], then inserts the resulting sequence of characters seq as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size()`; then calls `os.width(0)`.

28.3.4.3.3.3 [facet.num.put.virtuals]/5:

> […]

> Stage 3: A local variable is initialized as

> >     fmtflags adjustfield= (flags & (ios_base::adjustfield));

> The location of any padding is determined according to Table 88.

> If `str.width()` is nonzero and the number of `charT`'s in the sequence after stage 2 is less than `str.width()`, then enough fill characters are added to the sequence at the position indicated for padding to bring the length of the sequence to `str.width()`. `str.width(0)` is called.

> Table 88 — Fill padding
> State | Location
> --- | ---
> `adjustfield == ios_base::left` | pad after
> `adjustfield == ios_base::right` | pad before
> `adjustfield == internal` and a sign occurs in the representation | pad after the sign
> `adjustfield == internal` and representation after stage 1 began with 0x or 0X | pad after x or X
> otherwise | pad before

Although it's not 100% clear what "the sequence after stage 2" should mean here, when there is no stage 2, the only reasonable assumption is that it is the contents of the string being output. In the above code, the string being output is "`0X1Y2Z`", which starts with "`0X`", so the padding should be inserted "after x or X", and not before the string. I believe that this is a defect in the standard, and not in the three compilers I tried.

[ 2010 Batavia (post meeting session) ]

Consensus that all known implementations are consistent, and disagree with the standard. Preference is to fix the standard before implementations start trying to conform to the current spec, as the current implementations have the preferred form. Howard volunteered to drught for Madrid, move to Open.

[2011-03-24 Madrid meeting]

Daniel Krügler volunteered to provide wording, interacting with Dietmar and Bill.

[2011-06-24 Daniel comments and provides wording]

The same problem applies to the output provided by `const char*` and similar character sequences as of 31.7.6.3.4 [ostream.inserters.character] p. 5. and even for single character output (!) as described in 31.7.6.3.4 [ostream.inserters.character] p. 1, just consider the character value '-' where '-' is the sign character. In this case Table 91 — "Fill padding" requires to pad after the sign, i.e. the output for the program

>     #include <iostream>
>     #include <iomanip>

>     int main()
>     {
>        char c = '-';
>        std::cout.fill('*');
>        std::cout.setf(std::ios::internal, std::ios::adjustfield);
>        std::cout << std::setw(2) << c << std::endl;
>     }

According to the current wording this program should output "`-*`", but all tested implementations output "`*-`" instead.

I suggest to replace the reference to 28.3.4.3.3.3 [facet.num.put.virtuals] in all three places. It is not very complicated to describe the padding rules for simple character sequences "inline". A similar approach is used as for the `money_put` functions.

[ 2011 Bloomington ]

Move to Review, the resolution seems correct but it would be nice if some factoring of the common words were proposed.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

While better factoring of the common words is desirable, it is also editorial and should not hold up the progress of this issue. As the edits impact two distinct clauses, it is not entirely clear what a better factoring should look like.

[2012, Portland: applied to WP]

Proposed resolution:

The new wording refers to the FDIS numbering.

1. Change 27.4.4.4 [string.io]/5 as indicated:

   >     template<class charT, class traits, class Allocator>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os,
   >                    const basic_string<charT,traits,Allocator>& str);

   > > -5- Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]). After constructing a sentry object, if this object returns `true` when converted to a value of type `bool`, determines padding as described in [facet.num.put.virtuals],follows: A `charT` character sequence is produced, initially consisting of the elements defined by the range `[str.begin(), str.end())`. If `str.size()` is less than `os.width()`, then enough copies of `os.fill()` are added to this sequence as necessary to pad to a width of `os.width()` characters. If `(os.flags() & ios_base::adjustfield) == ios_base::left` is `true`, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. Tthen inserts the resulting sequence of characters `seq` as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size()`; then calls `os.width(0)`.

2. Change 31.7.6.3.4 [ostream.inserters.character]/1 as indicated (An additional editorial fix is suggested for the first prototype declaration):

   >     template<class charT, class traits>
   >       basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
   >                                               charT c});
   >     template<class charT, class traits>
   >       basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
   >                                               char c);
   >     // specialization
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              char c);
   >     // signed and unsigned
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              signed char c);
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              unsigned char c);

   > > -1- Effects: Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of `out`. After a sentry object is constructed it inserts characters. In case `c` has type `char` and the character type of the stream is not `char`, then the character to be inserted is `out.widen(c)`; otherwise the character is `c`. Padding is determined as described in [facet.num.put.virtuals]follows: A character sequence is produced, initially consisting of the insertion character. If `out.width()` is greater than one, then enough copies of `out.fill()` are added to this sequence as necessary to pad to a width of `out.width()` characters. If `(out.flags() & ios_base::adjustfield) == ios_base::left` is `true`, the fill characters are placed after the insertion character; otherwise, they are placed before the insertion character. `width(0)` is called. The insertion character and any required padding are inserted into `out`; then calls `os.width(0)`.

3. Change 31.7.6.3.4 [ostream.inserters.character]/5 as indicated:

   >     template<class charT, class traits>
   >       basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
   >                                               const charT* s);
   >     template<class charT, class traits>
   >       basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
   >                                               const char* s);
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              const char* s);
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              const signed char* s);
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              const unsigned char* s);

   > > […]

   > > -5- Padding is determined as described in [facet.num.put.virtuals]. The `n` characters starting at `s` are widened using `out.widen` ([basic.ios.members])follows: A character sequence is produced, initially consisting of the elements defined by the `n` characters starting at `s` widened using `out.widen` ([basic.ios.members]). If `n` is less than `out.width()`, then enough copies of `out.fill()` are added to this sequence as necessary to pad to a width of `out.width()` characters. If `(out.flags() & ios_base::adjustfield) == ios_base::left` is `true`, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. The widened characters and any required padding are inserted into `out`. Calls `width(0)`.

### 2012(i). Associative maps should insert `pair`, not `tuple`

Section: 23.4 [associative] Status: Resolved Submitter: Paolo Carlini Opened: 2010-10-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I'm seeing something strange in the paragraphs 23.4.3.4 [map.modifiers] and 23.4.4.3 [multimap.modifiers]: they both talk about `tuple<const key_type, mapped_type>` but I think they should be talking about `pair<const key_type, mapped_type>` because, among other reasons, a `tuple` is not convertible to a `pair`. If I replace `tuple` with `pair` everything makes sense to me.

The proposed resolution is obvious.

[ 2010-11-07 Daniel comments ]

This is by far not the only necessary fix within both sub-clauses. For details see the 2010-10-29 comment in 2005(i).

[2011-03-24 Madrid meeting]

Paolo: Don't think we can do it now.

Daniel K: Agrees.

[ 2011 Bloomington ]

Consensus that this issue will be resolved by 2005(i), but held open until that issue is resolved.

Proposed resolution:

Apply the resolution proposed by the 2010-10-29 comment in 2005(i).

### 2013(i). Do library implementers have the freedom to add `constexpr`?

Section: 16.4.6.7 [constexpr.functions] Status: C++14 Submitter: Matt Austern Opened: 2010-11-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Suppose that a particular function is not tagged as constexpr in the standard, but that, in some particular implementation, it is possible to write it within the constexpr constraints. If an implementer tags such a function as constexpr, is that a violation of the standard or is it a conforming extension?

There are two questions to consider. First, is this allowed under the as-if rule? Second, if it does not fall under as-if, is there (and should there be) any special license granted to implementers to do this anyway, sort of the way we allow elision of copy constructors even though it is detectable by users?

I believe that this does not fall under "as-if", so implementers probably don't have that freedom today. I suggest changing the WP to grant it. Even if we decide otherwise, however, I suggest that we make it explicit.

[ 2011 Bloomington ]

General surprise this was not already in 'Ready' status, and so moved.

[ 2012 Kona ]

Some concern expressed when presented to full committee for the vote to WP status that this issue had been resolved without sufficient thought of the consequences for diverging library implementations, as users may use SFINAE to observe different behavior from otherwise identical code. Issue moved back to Review status, and will be discussed again in Portland with a larger group. Note for Portland: John Spicer has agreed to represent Core's concerns during any such discussion within LWG.

[2013-09 Chicago]

Straw poll: LWG strongly favoured to remove from implementations the freedom to add `constexpr`.

Matt provides new wording.

[2013-09 Chicago]

Move to Immediate after reviewing Matt's new wording, apply the new wording to the Working Paper.

Proposed resolution:

In 16.4.6.7 [constexpr.functions], change paragraph 1 to:

> This standard explicitly requires that certain standard library functions are `constexpr` [dcl.constexpr]. An implementation shall not declare any standard library function signature as `constexpr` except for those where it is explicitly required. Within any header that provides any non-defining declarations of `constexpr` functions or constructors an implementation shall provide corresponding definitions.

### 2014(i). More restrictions on macro names

Section: 16.4.5.3.3 [macro.names] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2010-11-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

A program is currently forbidden to use keywords as macro names. This restriction should be strengthened to include all identifiers that could be used by the library as attribute-tokens (for example `noreturn`, which is used by header `<cstdlib>`) and the special identifiers introduced recently for override control (these are not currently used in the library public interface, but could potentially be used by the implementation or in future revisions of the library).

[2011-02-10 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Modify 16.4.5.3.3 [macro.names] paragraph 2 as follows:

> A translation unit shall not `#define` or `#undef` names lexically identical to keywords, to the identifiers listed in Table X [Identifiers with special meaning], or to the attribute-tokens described in clause 7.6 [dcl.attr].

### 2015(i). Incorrect pre-conditions for some type traits

Section: 21.3.5 [meta.unary] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2010-11-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

According to N3126 ‑ 3.9/9,

"Scalar types, trivial class types (Clause 9), arrays of such types and cv‑qualified versions of these types (3.9.3) are collectively called trivial types."

Thus, an array (possibly of unknown bound) can be trivial type, non‑trivial type, or an array type whose triviality cannot be determined because its element type is incomplete.

According to N3126 ‑ Table 45, preconditions for `std::is_trivial` are defined as follows:

"`T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound"

It seems that "an array of unknown bound" should be changed to "an array of unknown bound of a complete element type". Preconditions for some other templates (e.g., `std::is_trivially_copyable`, `std::is_standard_layout`, `std::is_pod`, and `std::is_literal_type`) should be changed similarly.

On the other hand, some preconditions look too restrictive. For example, `std::is_empty` and `std::is_polymorphic` might accept any incomplete non‑class type.

[2011-02-18: Daniel provides wording proposal]

While reviewing the individual preconditions I could find three different groups of either too weakening or too strengthening constraints:

1.     is_empty/is_polymorphic/is_abstract/has_virtual_destructor:

   These traits can only apply for non‑union class types, otherwise the result must always be false

2.     is_base_of:

   Similar to the previous bullet, but the current wording comes already near to that ideal, it only misses to add the non‑union aspect.

3.     is_trivial/is_trivially_copyable/is_standard_layout/is_pod/is_literal_type:

   These traits always require that `std::remove_all_extents<T>::type` to be `cv void` or a complete type.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

1. Modify the pre-conditions of the following type traits in 21.3.5.4 [meta.unary.prop], Table 48 — Type property predicates:

   > Table 48 — Type property predicates
   > Template | Condition | Preconditions
   > --- | --- | ---
   > ...
   > `template <class T> struct is_trivial;` | `T` is a trivial type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_trivially_copyable;` | `T` is a trivially copyable type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_standard_layout;` | `T` is a standard-layout type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_pod;` | `T` is a POD type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_literal_type;` | `T` is a literal type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_empty;` | `T` is a class type, but not a union type, with no non-static data members other than bit-fields of length 0, no virtual member functions, no virtual base classes, and no base class B for which `is_empty<B>::value` is false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown boundIf `T` is a non‑union class type, `T` shall be a complete type.
   > `template <class T> struct is_polymorphic;` | `T` is a polymorphic class (10.3) | `T` shall be a complete type, type, (possibly cv-qualified) `void`, or an array of unknown boundIf `T` is a non‑union class type, `T` shall be a complete type.
   > `template <class T> struct is_abstract;` | `T` is an abstract class (10.4) | `T` shall be a complete type, type, (possibly cv-qualified) `void`, or an array of unknown boundIf `T` is a non‑union class type, `T` shall be a complete type.
   > ...
   > `template <class T> struct has_virtual_destructor;` | `T` has a virtual destructor (12.4) | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown boundIf `T` is a non‑union class type, `T` shall be a complete type.

2. Modify the pre-conditions of the following type traits in 21.3.7 [meta.rel], Table 50 — Type relationship predicates:

   > Table 50 — Type relationship predicates
   > Template | Condition | Comments
   > --- | --- | ---
   > ...
   > `template <class Base, class Derived> struct is_base_of;` | `Base` is a base class of `Derived` (10) without regard to cv-qualifiers or `Base` and `Derived` are not unions and name the same class type without regard to cv-qualifiers | If `Base` and `Derived` are non‑union class types and are different types (ignoring possible cv-qualifiers) then `Derived` shall be a complete type. [ Note: Base classes that are private, protected, or ambigious are, nonetheless, base classes. — end note ]
   > ...

### 2016(i). `Allocators` must be no-throw swappable

Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2010-11-17 Last modified: 2017-07-30

Priority: 2

View other active issues in [allocator.requirements].

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Discussion:

During the Batavia meeting it turned out that there is a definition hole for types satisfying the `Allocators` requirements: The problem became obvious when it was discussed whether all `swap` functions of `Containers` with internal data handles can be safely tagged with `noexcept` or not. While it is correct that the implicit `swap` function of an allocator is required to be a no-throw operation (because move/copy-constructors and assignment operators are required to be no-throw functions), there are no such requirements for specialized `swap` overloads for a particular allocator.

But this requirement is essential because the `Containers` are required to support swappable `Allocators`, when the value `allocator_traits<>::propagate_on_container_swap` evaluates to `true`.

[2011-02-10 Alberto, Daniel, and Pablo collaborated on the proposed wording]

The proposed resolution (based on N3225) attempts to solve the following problems:

1. Table 44 — Allocator requirements, expression rows `X::propagate_on_container_copy_assignment`, `X::propagate_on_container_move_assignment`, and `X::propagate_on_container_swap` only describe operations, but no requirements. In fact, if and only if these compile-time predicates evaluate to `true`, the additional requirements `CopyAssignable`, no-throw `MoveAssignable`, and no-throw lvalue `Swappable`, respectively, are imposed on the allocator types.
2. 23.2.2 [container.requirements.general] p. 9 misses to refer to the correct swap conditions: The current wording does not relate to 16.4.4.3 [swappable.requirements] as it should and omits to mention that lvalues shall be swapped. Additional there is one situation described twice in p. 8 and p. 9 (undefined behaviour unless `a.get_allocator() == b.get_allocator()` or `allocator_traits<allocator_type>::propagate_on_container_swap::value == true`), which should be cleaned up.

[2011-04-08 Pablo comments]

I'm implementing a version of list now and I actually do find it impossible to write an exception-safe assignment operator unless I can assume that allocator assignment does not throw. (The problem is that I use a sentinel node and I need to allocate a new sentinel using the new allocator without destroying the old one -- then swap the allocator and sentinel pointer in atomically, without risk of an exception leaving one inconsistent with the other.

Please update the proposed resolution to add the nothrow requirement to copy-assignment.

[2014-02-14 Issaquah: Move to Ready]

Fix a couple of grammar issues related to calling `swap` and move to Ready.

Proposed resolution:

1. Adapt the following three rows from Table 44 — Allocator requirements:

   > Table 44 — Allocator requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `X::propagate_on_container_copy_assignment` | Identical to or derived from `true_type` or `false_type` | `true_type` only if an allocator of type `X` should be copied when the client container is copy-assigned. See Note B, below. | `false_type`
   > `X::propagate_on_container_move_assignment` | Identical to or derived from `true_type` or `false_type` | `true_type` only if an allocator of type `X` should be moved when the client container is move-assigned. See Note B, below. | `false_type`
   > `X::propagate_on_container_swap` | Identical to or derived from `true_type` or `false_type` | `true_type` only if an allocator of type `X` should be swapped when the client container is swapped. See Note B, below. | `false_type`

2. Following 16.4.4.6 [allocator.requirements] p. 3 insert a new normative paragraph:

   > Note B: If `X::propagate_on_container_copy_assignment::value` is true, `X` shall satisfy the `CopyAssignable` requirements (Table 39 [copyassignable]) and the copy operation shall not throw exceptions. If `X::propagate_on_container_move_assignment::value` is true, `X` shall satisfy the `MoveAssignable` requirements (Table 38 [moveassignable]) and the move operation shall not throw exceptions. If `X::propagate_on_container_swap::value` is true, lvalues of `X` shall be swappable (16.4.4.3 [swappable.requirements]) and the `swap` operation shall not throw exceptions.

3. Modify 23.2.2 [container.requirements.general] p. 8 and p. 9 as indicated:

   > 8 - [..] The allocator may be replaced only via assignment or `swap()`. Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only if `allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value`, `allocator_traits<allocator_type>::propagate_on_container_move_assignment::value`, or `allocator_traits<allocator_type>::propagate_on_container_swap::value` is true within the implementation of the corresponding container operation. The behavior of a call to a container's `swap` function is undefined unless the objects being swapped have allocators that compare equal or `allocator_traits<allocator_type>::propagate_on_container_swap::value` is true. In all container types defined in this Clause, the member `get_allocator()` returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement.

   > 9 - The expression `a.swap(b)`, for containers `a` and `b` of a standard container type other than `array`, shall exchange the values of `a` and `b` without invoking any move, copy, or swap operations on the individual container elements. Lvalues of aAny `Compare`, `Pred`, or `Hash` objectstypes belonging to `a` and `b` shall be swappable and shall be exchanged by unqualified calls to non-member calling `swap` as described in 16.4.4.3 [swappable.requirements]. If `allocator_traits<allocator_type>::propagate_on_container_swap::value` is `true`, then lvalues of `allocator_type` shall be swappable and the allocators of `a` and `b` shall also be exchanged using an unqualified call to non-memberby calling `swap` as described in 16.4.4.3 [swappable.requirements]. Otherwise, theythe allocators shall not be swapped, and the behavior is undefined unless `a.get_allocator() == b.get_allocator()`. Every iterator referring to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with value `a.end()` before the swap will have value `b.end()` after the swap.

### 2017(i). `std::reference_wrapper` makes incorrect usage of `std::result_of`

Section: 22.10.6 [refwrap] Status: C++11 Submitter: Nikolay Ivchenkov Opened: 2010-11-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`std::reference_wrapper`'s function call operator uses wrong type encoding for rvalue-arguments. An rvalue-argument of type `T` must be encoded as `T&&`, not as just `T`.

>     #include <functional>
>     #include <iostream>
>     #include <string>
>     #include <type_traits>
>     #include <utility>

>     template <class F, class... Types>
>          typename std::result_of<F (Types...)>::type
>              f1(F f, Types&&... params)
>     {
>          return f(std::forward<Types...>(params...));
>     }

>     template <class F, class... Types>
>          typename std::result_of<F (Types&&...)>::type
>              f2(F f, Types&&... params)
>     {
>          return f(std::forward<Types...>(params...));
>     }

>     struct Functor
>     {
>          template <class T>
>              T&& operator()(T&& t) const
>          {
>              return static_cast<T&&>(t);
>          }
>     };

>     int main()
>     {
>          typedef std::string const Str;
>          std::cout << f1(Functor(), Str("1")) << std::endl; // (1)
>          std::cout << f2(Functor(), Str("2")) << std::endl; // (2)
>     }

Lets consider the function template `f1` (which is similar to `std::reference_wrapper`'s function call operator). In the invocation (1) `F` is deduced as '`Functor`' and `Types` is deduced as type sequence which consists of one type '`std::string const`'. After the substitution we have the following equivalent:

>     template <>
>         std::result_of<F (std::string const)>::type
>             f1<Functor, std::string const>(Functor f, std::string const && params)
>     {
>         return f(std::forward<const std::string>(params));
>     }

The top-level cv-qualifier in the parameter type of '`F (std::string const)`' is removed, so we have

>     template <>
>         std::result_of<F (std::string)>::type
>             f1<Functor, std::string const>(Functor f, std::string const && params)
>     {
>         return f(std::forward<const std::string>(params));
>     }

Let `r` be an rvalue of type '`std::string`' and `cr` be an rvalue of type '`std::string const`'. The expression `Str("1")` is `cr`. The corresponding return type for the invocation

>     Functor().operator()(r)

is '`std::string &&`'. The corresponding return type for the invocation

>     Functor().operator()(cr)

is '`std::string const &&`'.

`std::result_of<Functor (std::string)>::type` is the same type as the corresponding return type for the invocation `Functor().operator()(r)`, i.e. it is '`std::string &&`'. As a consequence, we have wrong reference binding in the return statement in `f1`.

Now lets consider the invocation (2) of the function template `f2`. When the template arguments are substituted we have the following equivalent:

>     template <>
>         std::result_of<F (std::string const &&)>::type
>             f2<Functor, std::string const>(Functor f, std::string const && params)
>     {
>         return f(std::forward<const std::string>(params));
>     }

`std::result_of<F (std::string const &&)>::type` is the same type as '`std::string const &&`'. This is correct result.

[ 2010-12-07 Jonathan Wakely comments and suggests a proposed resolution ]

I agree with the analysis and I think this is a defect in the standard, it would be a shame if it can't be fixed.

In the following example one would expect `f(Str("1"))` and `std::ref(f)(Str("2"))` to be equivalent but the current wording makes the invocation through `reference_wrapper` ill-formed:

>     #include <functional>
>     #include <string>

>     struct Functor
>     {
>        template <class T>
>            T&& operator()(T&& t) const
>            {
>                return static_cast<T&&>(t);
>            }
>     };

>     int main()
>     {
>        typedef std::string const Str;
>        Functor f;
>        f( Str("1") );
>        std::ref(f)( Str("2") );  // error
>     }

[ 2010-12-07 Daniel comments and refines the proposed resolution ]

There is one further defect in the usage of `result_of` within `reference_wrapper`'s function call operator: According to 22.10.6.5 [refwrap.invoke] p. 1 the invokable entity of type `T` is provided as lvalue, but `result_of` is fed as if it were an rvalue. This does not only lead to potentially incorrect result types, but it will also have the effect that we could never use the function call operator with a function type, because the type encoding used in `result_of` would form an invalid function type return a function type. The following program demonstrates this problem:

>     #include <functional>

>     void foo(int) {}

>     int main()
>     {
>        std::ref(foo)(0);  // error
>     }

The correct solution is to ensure that `T` becomes `T&` within `result_of`, which solves both problems at once.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

1. Change the synopsis in 22.10.6 [refwrap] paragraph 1:

   >     namespace std {
   >       template <class T> class reference_wrapper
   >       {
   >       public :
   >         [...]
   >         // invocation
   >         template <class... ArgTypes>
   >         typename result_of<T&(ArgTypes&&...)>::type
   >         operator() (ArgTypes&&...) const;
   >       };
   >     }

2. Change the signature in 22.10.6.5 [refwrap.invoke] before paragraph 1

   >     template <class... ArgTypes>
   >     typename result_of<T&(ArgTypes&&... )>::type
   >     operator()(ArgTypes&&... args) const;

   > > 1 Returns: `INVOKE(get(), std::forward<ArgTypes>(args)...)`. (20.8.2)

### 2018(i). [CD] `regex_traits::isctype` Returns clause is wrong

Section: 28.6.6 [re.traits] Status: C++14 Submitter: Jonathan Wakely Opened: 2010-11-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [re.traits].

View all issues with C++14 status.

Discussion:

Addresses GB 10

28.6.6 [re.traits] p. 12 says:

> returns true if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence "w" is not equal to `0` and `c == '_'`

If the bitmask value corresponding to "w" has a non-zero value (which it must do) then the bitwise or with any value is also non-zero, and so `isctype('_', f)` returns true for any `f`. Obviously this is wrong, since `'_'` is not in every `ctype` category.

There's a similar problem with the following phrases discussing the "blank" char class.

[2011-05-06: Jonathan Wakely comments and provides suggested wording]

DR 2019(i) added `isblank` support to `<locale>` which simplifies the definition of `regex_traits::isctype` by removing the special case for the "blank" class.

My suggestion for 2018 is to add a new table replacing the lists of recognized names in the Remarks clause of `regex_traits::lookup_classname`. I then refer to that table in the Returns clause of `regex_traits::isctype` to expand on the "in an unspecified manner" wording which is too vague. The conversion can now be described using the "is set" term defined by 16.3.3.3.3 [bitmask.types] and the new table to convey the intented relationship between e.g. [[:digit:]] and `ctype_base::digit`, which is not actually stated in the FDIS.

The effects of `isctype` can then most easily be described in code, given an "exposition only" function prototype to do the not-quite-so-unspecified conversion from `char_class_type` to `ctype_base::mask`.

The core of LWG 2018 is the "bitwise or'ed" wording which gives the wrong result, always evaluating to true for all values of `f`. That is replaced by the condition `(f&x) == x` where `x` is the result of calling `lookup_classname` with "w". I believe that's necessary, because the "w" class could be implemented by an internal "underscore" class i.e. `x = _Alnum|_Underscore` in which case `(f&x) != 0` would give the wrong result when `f==_Alnum`.

The proposed resolution also makes use of `ctype::widen` which addresses the problem that the current wording only talks about "w" and '_' which assumes `charT` is char. There's still room for improvement here: the regex grammar in 28.6.12 [re.grammar] says that the class names in the table should always be recognized, implying that e.g. U"digit" should be recognized by `regex_traits<char32_t>`, but the specification of `regex_traits::lookup_classname` doesn't cover that, only mentioning `char` and `wchar_t`. Maybe the table should not distinguish narrow and wide strings, but should just have one column and add wording to say that `regex_traits` widens the name as if by using `use_facet<ctype<charT>>::widen()`.

Another possible improvement would be to allow additional implementation-defined extensions in `isctype`. An implementation is allowed to support additional class names in `lookup_classname`, e.g. [[:octdigit:]] for [0-7] or [[:bindigit:]] for [01], but the current definition of isctype provides no way to use them unless `ctype_base::mask` also supports them.

[2011-05-10: Alberto and Daniel perform minor fixes in the P/R]

[ 2011 Bloomington ]

Consensus that this looks to be a correct solution, and the presentation as a table is a big improvement.

Concern that the middle section wording is a little muddled and confusing, Stefanus volunteered to reword.

[ 2013-09 Chicago ]

Stefanus provides improved wording (replaced below)

[ 2013-09 Chicago ]

Move as Immediate after reviewing Stefanus's revised wording, apply the new wording to the Working Paper.

Proposed resolution:

This wording is relative to the FDIS.

1. Modify 28.6.6 [re.traits] p. 10 as indicated:

   >     template <class ForwardIterator>
   >       char_class_type lookup_classname(
   >         ForwardIterator first, ForwardIterator last, bool icase = false) const;

   > > -9- Returns: an unspecified value that represents the character classification named by the character sequence designated by the iterator range [`first`,`last`). If the parameter `icase` is true then the returned mask identifies the character classification without regard to the case of the characters being matched, otherwise it does honor the case of the characters being matched.(footnote 335) The value returned shall be independent of the case of the characters in the character sequence. If the name is not recognized then returns a value that compares equal to `0`.

   > > -10- Remarks: For `regex_traits<char>`, at least the names "d", "w", "s", "alnum", "alpha", "blank", "cntrl", "digit", "graph", "lower", "print", "punct", "space", "upper" and "xdigit"narrow character names in Table X shall be recognized. For `regex_traits<wchar_t>`, at least the names L"d", L"w", L"s", L"alnum", L"alpha", L"blank", L"cntrl", L"digit", L"graph", L"lower", L"print", L"punct", L"space", L"upper" and L"xdigit"wide character names in Table X shall be recognized.

2. Modify 28.6.6 [re.traits] p. 12 as indicated:

   >     bool isctype(charT c, char_class_type f) const;

   > > -11- Effects: Determines if the character `c` is a member of the character classification represented by `f`.

   > > -12- Returns: Converts `f` into a value `m` of type `std::ctype_base::mask` in an unspecified manner, and returns true if `use_facet<ctype<charT> >(getloc()).is(m, c)` is true. Otherwise returns true if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence "w" is not equal to `0` and `c == '_'`, or if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence "blank" is not equal to `0` and `c` is one of an implementation-defined subset of the characters for which `isspace(c, getloc())` returns true, otherwise returns false. Given an exposition-only function prototype

   > > >       template<class C>
   > > >        ctype_base::mask convert(typename regex_traits<C>::char_class_type f);

   > > that returns a value in which each `ctype_base::mask` value corresponding to a value in `f` named in Table X is set, then the result is determined as if by:

   > > >     ctype_base::mask m = convert<charT>(f);
   > > >     const ctype<charT>& ct = use_facet<ctype<charT>>(getloc());
   > > >     if (ct.is(m, c)) {
   > > >       return true;
   > > >     } else if (c == ct.widen('_')) {
   > > >       charT w[1] = { ct.widen('w') };
   > > >       char_class_type x = lookup_classname(w, w+1);

   > > >       return (f&x) == x;
   > > >     } else {
   > > >       return false;
   > > >     }

   > > [Example:

   > > >     regex_traits<char> t;
   > > >     string d("d");
   > > >     string u("upper");
   > > >     regex_traits<char>::char_class_type f;
   > > >     f = t.lookup_classname(d.begin(), d.end());
   > > >     f |= t.lookup_classname(u.begin(), u.end());
   > > >     ctype_base::mask m = convert<char>(f); // m == ctype_base::digit|ctype_base::upper

   > > — end example]

   > > [Example:

   > > >     regex_traits<char> t;
   > > >     string w("w");
   > > >     regex_traits<char>::char_class_type f;
   > > >     f = t.lookup_classname(w.begin(), w.end());
   > > >     t.isctype('A', f); // returns true
   > > >     t.isctype('_', f); // returns true
   > > >     t.isctype(' ', f); // returns false

   > > — end example]

3. At the end of 28.6.6 [re.traits] add a new "Table X — Character class names and corresponding ctype masks":

   > Table X — Character class names and corresponding ctype masks
   > Narrow character name | Wide character name | Corresponding `ctype_base::mask` value
   > --- | --- | ---
   > `"alnum"` | `L"alnum"` | `ctype_base::alnum`
   > `"alpha"` | `L"alpha"` | `ctype_base::alpha`
   > `"blank"` | `L"blank"` | `ctype_base::blank`
   > `"cntrl"` | `L"cntrl"` | `ctype_base::cntrl`
   > `"digit"` | `L"digit"` | `ctype_base::digit`
   > `"d"` | `L"d"` | `ctype_base::digit`
   > `"graph"` | `L"graph"` | `ctype_base::graph`
   > `"lower"` | `L"lower"` | `ctype_base::lower`
   > `"print"` | `L"print"` | `ctype_base::print`
   > `"punct"` | `L"punct"` | `ctype_base::punct`
   > `"space"` | `L"space"` | `ctype_base::space`
   > `"s"` | `L"s"` | `ctype_base::space`
   > `"upper"` | `L"upper"` | `ctype_base::upper`
   > `"w"` | `L"w"` | `ctype_base::alnum`
   > `"xdigit"` | `L"xdigit"` | `ctype_base::xdigit`

### 2019(i). `isblank` not supported by `std::locale`

Section: 28.3.3.3.1 [classification] Status: C++11 Submitter: Jonathan Wakely Opened: 2010-11-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

C99 added `isblank` and `iswblank` to `<locale.h>` but `<locale>` does not provide any equivalent.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

Add to 28.3.3.3.1 [classification] synopsis:

>     template <class charT> bool isgraph (charT c, const locale& loc);
>     template <class charT> bool isblank (charT c, const locale& loc);

Add to 28.3.4.2 [category.ctype] synopsis:

>     static const mask xdigit = 1 << 8;
>     static const mask blank = 1 << 9;
>     static const mask alnum = alpha | digit;
>     static const mask graph = alnum | punct;

### 2020(i). Time utility arithmetic `constexpr` functions have invalid effects

Section: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: Daniel Krügler Opened: 2010-12-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.duration.nonmember].

View all issues with C++11 status.

Discussion:

As of issue 1171(i) several time-utility functions have been marked `constexpr`. Alas this was done without adapting the corresponding return elements, which has the effect that none of current arithmetic functions of class template `duration` marked as `constexpr` can ever be `constexpr` functions (which makes them ill-formed, no diagnostics required as of recent core rules), because they invoke a non-constant expression, e.g. 30.5.6 [time.duration.nonmember]/2:

>     template <class Rep1, class Period1, class Rep2, class Period2>
>     constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type
>     operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

>     2 Returns: CD(lhs) += rhs.

The real problem is, that we cannot defer to as-if rules here: The returns element specifies an indirect calling contract of a potentially user-defined function. This cannot be the `+=` assignment operator of such a user-defined type, but must be the corresponding immutable binary `operator+` (unless we require that `+=` shall be an immutable function which does not really makes sense).

[2011-02-17 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

The suggested wording changes are against the working draft N3242. Additional to the normative wording changes some editorial fixes are suggested.

1. In 30.5.6 [time.duration.nonmember], change the following arithmetic function specifications as follows:

   >     template <class Rep1, class Period1, class Rep2, class Period2>
   >     constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type
   >     operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

   > > 2 Returns: `CD(lhs) += rhs` `CD(CD(lhs).count() + CD(rhs).count())`.

   >     template <class Rep1, class Period1, class Rep2, class Period2>
   >     constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type
   >     operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

   > > 3 Returns: `CD(lhs) -= rhs` `CD(CD(lhs).count() - CD(rhs).count())`.

   >     template <class Rep1, class Period, class Rep2>
   >     constexpr duration<typename common_type<Rep1, Rep2>::type, Period>
   >     operator*(const duration<Rep1, Period>& d, const Rep2& s);

   > > 4 Remarks: This operator shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)`.

   > > 5 Returns: `duration<CR(Rep1, Rep2), Period>(d) *= s` `CD(CD(d).count() * s)`.

   > `[...]`

   >     template <class Rep1, class Period, class Rep2>
   >     constexpr duration<typename common_type<Rep1, Rep2>::type, Period>
   >     operator/(const duration<Rep1, Period>& d, const Rep2& s);

   > > 8 Remarks: This operator shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` is not an instantiation of `duration`.

   > > 9 Returns: `duration<CR(Rep1, Rep2), Period>(d) /= s` `CD(CD(d).count() / s)`.

   > `[...]`

   >     template <class Rep1, class Period, class Rep2>
   >     constexpr duration<typename common_type<Rep1, Rep2>::type, Period>
   >     operator%(const duration<Rep1, Period>& d, const Rep2& s);

   > > 11 Remarks: This operator shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` is not an instantiation of `duration`.

   > > 12 Returns: `duration<CR(Rep1, Rep2), Period>(d) %= s` `CD(CD(d).count() % s)`

   >     template <class Rep1, class Period1, class Rep2, class Period2>
   >     constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
   >     operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

   > > 13 Returns: `common_type<duration<Rep1, Period1>, duration<Rep2, Period2> >::type(lhs) %= rhs` `CD(CD(lhs).count() % CD(rhs).count())`.

### 2021(i). Further incorrect usages of `result_of`

Section: 22.10.15.4 [func.bind.bind], 32.10.1 [futures.overview], 32.10.9 [futures.async] Status: C++14 Submitter: Daniel Krügler Opened: 2010-12-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [func.bind.bind].

View all issues with C++14 status.

Discussion:

Issue 2017(i) points out some incorrect usages of `result_of` in the declaration of the function call operator overload of `reference_wrapper`, but there are more such specification defects:

1. According to 22.10.15.4 [func.bind.bind] p. 3:

   > [..] The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type)` [..]

   but `fd` is defined as "an lvalue of type `FD` constructed from `std::forward<F>(f)`". This means that the above usage must refer to `result_of<FD cv & (V1, V2, ..., VN)>` instead.

2. Similar in 22.10.15.4 [func.bind.bind] p. 10 bullet 2 we have:

   > if the value of `is_bind_expression<TiD>::value` is true, the argument is `tid(std::forward<Uj>(uj)...)` and its type `Vi` is `result_of<TiD cv (Uj...)>::type`

   Again, `tid` is defined as "lvalue of type `TiD` constructed from `std::forward<Ti>(ti)`". This means that the above usage must refer to `result_of<TiD cv & (Uj...)>` instead. We also have similar defect as in 2017(i) in regard to the argument types, this leads us to the further corrected form `result_of<TiD cv & (Uj&&...)>`. This is not the end: Since the `Vi` are similar sensitive to the argument problem, the last part must say:

   "[..] its type `Vi` is `result_of<TiD cv & (Uj&&...)>::type &&"`

   (The bound arguments `Vi` can never be `void` types, therefore we don't need to use the more defensive `std::add_rvalue_reference` type trait)

3. The function template `async` is declared as follows (the other overload has the same problem):

   >     template <class F, class... Args>
   >     future<typename result_of<F(Args...)>::type>
   >     async(F&& f, Args&&... args);

   This usage has the some same problems as we have found in `reference_wrapper` (2017(i)) and more: According to the specification in 32.10.9 [futures.async] the effective result type is that of the call of

   >     INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)

   First, `decay_copy` potentially modifies the effective types to `decay<F>::type` and `decay<Args>::type...`. Second, the current specification is not really clear, what the value category of callable type or the arguments shall be: According to the second bullet of 32.10.9 [futures.async] p. 3:

   > Invocation of the deferred function evaluates `INVOKE(g, xyz)` where `g` is the stored value of `decay_copy(std::forward<F>(f))` and `xyz` is the stored copy of `decay_copy(std::forward<Args>(args))...`.

   This seems to imply that lvalues are provided in contrast to the direct call expression of 32.10.9 [futures.async] p. 2 which implies rvalues instead. The specification needs to be clarified.

[2011-06-13: Daniel comments and refines the proposed wording changes]

The feedback obtained following message c++std-lib-30745 and follow-ups point to the intention, that the implied provision of lvalues due to named variables in `async` should be provided as rvalues to support move-only types, but the functor type should be forwarded as lvalue in `bind`.

If `bind` were newly invented, the value strategy could be improved, because now we have a preference of ref `&` qualified function call operator overloads. But such a change seems to be too late now. User-code that needs to bind a callable object with an ref `&&` qualified function call operator (or conversion function to function pointer) needs to use a corresponding wrapper similar to `reference_wrapper` that forwards the reference as rvalue-reference instead.

The wording has been adapted to honor these observations and to fit to FDIS numbering as well.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

The suggested wording changes are against the FDIS.

1. Change 22.10.15.4 [func.bind.bind] p. 3 as indicated:

   >     template<class F, class... BoundArgs>
   >       unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > -2- Requires: `is_constructible<FD, F>::value` shall be true. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be true. `INVOKE(fd, w1, w2, ..., wN)` (20.8.2) shall be a valid expression for some values `w1`, `w2`, ..., `wN`, where `N == sizeof...(bound_args)`.

   > > -3- Returns: A forwarding call wrapper `g` with a weak result type (20.8.2). The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of<FD cv & (V1, V2, ..., VN)>::type)`, where cv represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1`, `v2`, ..., `vN` are determined as specified below. […]

2. Change 22.10.15.4 [func.bind.bind] p. 7 as indicated:

   >     template<class R, class F, class... BoundArgs>
   >        unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > -6- Requires: `is_constructible<FD, F>::value` shall be true. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be true. `INVOKE(fd, w1, w2, ..., wN)` shall be a valid expression for some values `w1`, `w2`, ..., `wN`, where `N == sizeof...(bound_args)`.

   > > -7- Returns: A forwarding call wrapper `g` with a nested type `result_type` defined as a synonym for `R`. The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), R)`, where the values and types of the bound arguments `v1`, `v2`, ..., `vN` are determined as specified below. […]

3. Change 22.10.15.4 [func.bind.bind] p. 10 as indicated:

   > -10- The values of the bound arguments `v1`, `v2`, ..., `vN` and their corresponding types `V1`, `V2`, ..., `VN` depend on the types `TiD` derived from the call to bind and the cv-qualifiers cv of the call wrapper `g` as follows:

   > - if `TiD` is `reference_wrapper<T>`, the argument is `tid.get()` and its type `Vi` is `T&`;
   > - if the value of `is_bind_expression<TiD>::value` is `true`, the argument is `tid(std::forward<Uj>(uj)...)` and its type `Vi` is `result_of<TiD cv & (Uj&&...)>::type&&`;
   > - if the value `j` of `is_placeholder<TiD>::value` is not zero, the argument is `std::forward<Uj>(uj)` and its type `Vi` is `Uj&&`;
   > - otherwise, the value is `tid` and its type `Vi` is `TiD cv &`.

4. This resolution assumes that the wording of 32.10.9 [futures.async] is intended to provide rvalues as arguments of `INVOKE`.

   Change the function signatures in header `<future>` synopsis 32.10.1 [futures.overview] p. 1 and in 32.10.9 [futures.async] p. 1 as indicated:

   >     template <class F, class... Args>
   >     future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   >     async(F&& f, Args&&... args);
   >     template <class F, class... Args>
   >     future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   >     async(launch policy, F&& f, Args&&... args);

5. Change 32.10.9 [futures.async] as indicated: (Remark: There is also a tiny editorial correction in p. 4 that completes one `::` scope specifier)

   > -3- Effects: […]

   > - […]
   > - if `policy & launch::deferred` is non-zero — Stores `DECAY_COPY(std::forward<F>(f))` and `DECAY_COPY(std::forward<Args>(args))...` in the shared state. These copies of `f` and `args` constitute a deferred function. Invocation of the deferred function evaluates `INVOKE(std::move(g), std::move(xyz))` where `g` is the stored value of `DECAY_COPY(std::forward<F>(f))` and `xyz` is the stored copy of `DECAY_COPY(std::forward<Args>(args))...`. The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4) on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called the waiting function. Once evaluation of `INVOKE(std::move(g), std::move(xyz))` begins, the function is no longer considered deferred. [ Note: If this policy is specified together with other policies, such as when using a `policy` value of `launch::async | launch::deferred`, implementations should defer invocation or the selection of the policy when no more concurrency can be effectively exploited. — end note ]

   > -4- Returns: an object of type `future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>` that refers to the associated asynchronous state created by this call to `async`.

### 2022(i). `reference_wrapper<T>::result_type` is underspecified

Section: 22.10.6 [refwrap] Status: C++11 Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [refwrap].

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Discussion:

Issue 1295(i) correctly removed function types and references to function types from the bullet 1 of 22.10.4 [func.require] p. 3 because neither function types nor function references satisfy the requirements for a target object which is defined to be an object of a callable type. This has the effect that the reference in 22.10.6 [refwrap] p. 2

> `reference_wrapper` has a weak result type (20.8.2).

is insufficient as a reference to define the member type `result_type` when the template argument `T` is a function type.

There are basically two approaches to solve the problem:

1. Extend the definition of a weak result type in 22.10.4 [func.require] p. 3 to both function types and references thereof. This extension must be specified independend from the concept of a call wrapper, though.

2. Add one extra sentence to 22.10.6 [refwrap] p. 2 that simply defines the member type `result_type` for `reference_wrapper<T>`, when `T` is a function type.

I checked the current usages of weak result type to have a base to argue for one or the other approach. It turns out, that there is no further reference to this definition in regard to function types or references thereof. The only other reference can be found in 22.10.15.4 [func.bind.bind] p. 3, where `g` is required to be a class type.

[2011-02-23 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

The suggested wording changes are against the working draft N3242.

1. Change 22.10.6 [refwrap] p. 2 as indicated:

   > 2 `reference_wrapper<T>` has a weak result type (20.8.2). If `T` is a function type, `result_type` shall be a synonym for the return type of `T`.

### 2023(i). Incorrect requirements for `lock_guard` and `unique_lock`

Section: 32.6.5.2 [thread.lock.guard], 32.6.5.4 [thread.lock.unique] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.lock.guard].

View all issues with Resolved status.

Discussion:

There are two different `*Lockable` requirements imposed on template arguments of the class template `lock_guard` as of 32.6.5.2 [thread.lock.guard] p. 1+2:

> 1 [..] The supplied `Mutex` type shall meet the `BasicLockable` requirements (30.2.5.2).

> 2 The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.3).

The `Lockable` requirements include the availability of a member function `try_lock()`, but there is no operational semantics in the specification of `lock_guard` that would rely on such a function. It seems to me that paragraph 2 should be removed.

Similarly, 32.6.5.4 [thread.lock.unique] p. 1+2 refer to exactly the same two requirements. In this case it seems as if the intention was that the template arguement `Mutex` should always provide the `try_lock()` member function, because several member functions of `unique_lock` (`unique_lock(mutex_type& m, try_to_lock_t)` or `bool try_lock()`) take advantage of such a function without adding extra requirements for this. It seems that the requirement subset `BasicLockable` should be removed.

I searched for further possible misusages of the `*Lockable` requirements, but could not find any more.

[2011-02-23]

Howard suggests an alternative approach in regard to `unique_lock`: The current minimum requirements on its template argument should better be reduced to `BasicLockable` instead of the current `Lockable`, including ammending member-wise constraints where required. This suggestions was supported by Anthony, Daniel, and Pablo.

Daniel drafts wording that follows this separation strategy.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

> The suggested wording changes are against the working draft N3242.

> 1. Remove 32.6.5.2 [thread.lock.guard] p. 2 completely:

>    > 2 The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.3).

> 2. Change 32.6.5.4 [thread.lock.unique] p. 1-3 as indicated. The intend is to make `BasicLockable` the fundamental requirement for `unique_lock`. We also update the note to reflect these changes and synchronize one remaining reference of 'mutex' by the proper term 'lockable object' in sync to the wording changes of `lock_guard`:

>    > 1 [..] The behavior of a program is undefined if the contained pointer `pm` is not null and the mutexlockable object pointed to by `pm` does not exist for the entire remaining lifetime (3.8) of the `unique_lock` object. The supplied `Mutex` type shall meet the `BasicLockable` requirements (30.2.5.2). [Editor's note: BasicLockable is redundant, since the following additional paragraph requires Lockable.]

>    > 2 The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.3).

>    > 3 [ Note: `unique_lock<Mutex>` meets the `BasicLockable` requirements. If `Mutex` meets the `Lockable` requirements ([thread.req.lockable.req]), `unique_lock<Mutex>` also meets the `Lockable` requirements and if `Mutex` meets the `TimedLockable` requirements (30.2.5.4), `unique_lock<Mutex>` also meets the `TimedLockable` requirements. — end note ]

> 3. Modify 32.6.5.4.2 [thread.lock.unique.cons] to add the now necessary member-wise additional constraints for `Lockable`:

>    >     unique_lock(mutex_type& m, try_to_lock_t) noexcept;

>    > > 8 Requires: The supplied `Mutex` type shall meet the `Lockable` requirements ([thread.req.lockable.req]). If `mutex_type` is not a recursive mutex the calling thread does not own the mutex.

>    > > 9 Effects: Constructs an object of type `unique_lock` and calls `m.try_lock()`.

> 4. Modify 32.6.5.4.3 [thread.lock.unique.locking] to add the now necessary member-wise additional constraints for `Lockable`:

>    >     bool try_lock();

>    > > ? Requires: The supplied `Mutex` type shall meet the `Lockable` requirements ([thread.req.lockable.req]).

>    > > 4 Effects: `pm->try_lock()`

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2024(i). Inconsistent implementation requirements for `atomic<integral>` and `atomic<T*>`

Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Paragraph 5 and 6 of 32.5.8 [atomics.types.generic] impose different requirements on implementations for specializations of the `atomic` class template for integral types and for pointer types:

> 5 The atomic integral specializations and the specialization `atomic<bool>` shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax.

> 6 There are pointer partial specializations on the `atomic` class template. These specializations shall have trivial default constructors and trivial destructors.

It looks like an oversight to me, that for pointer specializations the requirements for standard layout and support for aggregate initialization syntax are omitted. In fact, this been confirmed by the N3193 proposal author. I suggest to impose the same implementation requirements for pointer types as for integral types, this should not impose unrealistic requirements on implementations.

[2011-02-10 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

> Proposed Resolution

> The suggested wording changes are against the working draft N3242.

> 1. Change 32.5.8 [atomics.types.generic] p. 6 as indicated:

>    > 6 There are pointer partial specializations on the `atomic` class template. These specializations shall have standard layout, trivial default constructors, and trivial destructors. They shall each support aggregate initialization syntax.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2025(i). Incorrect semantics of move assignment operator of `packaged_task`

Section: 32.10.10.2 [futures.task.members] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

According to 32.10.10.2 [futures.task.members] p. 7 bullet 2:

>     packaged_task& operator=(packaged_task&& other);

> > 7 Effects:

> > - [...]

> > - `packaged_task<R, ArgTypes...>(other).swap(*this)`.

The argument `other` given to the move constructor is an lvalue and must be converted into an rvalue via appropriate usage of `std::move`.

> Proposed Resolution

> The suggested wording changes are against the working draft N3242.

> 1. Change 32.10.10.2 [futures.task.members] p. 7 bullet 2 as indicated:

>    >     packaged_task& operator=(packaged_task&& other);

>    > > 7 Effects:

>    > > - [...]

>    > > - `packaged_task(std::move(other)).swap(*this)`.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2027(i). Initialization of the stored task of a `packaged_task`

Section: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2011-02-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Related with LWG issue 1514(i).

The move constructor of `packaged_task` does not specify how the stored task is constructed. The obvious way is to move-construct it using the task stored in the argument. Moreover, the constructor should be provided with a throws clause similar to one used for the other constructors, as the move constructor of the stored task is not required to be nothrow.

As for the other constructors, the terms "stores a copy of `f`" do not reflect the intent, which is to allow `f` to be moved when possible.

[2011-02-25: Alberto updates wording]

[2011-02-26 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

(wording written assuming LWG 1514(i) is also accepted)

1. Change 32.10.10.2 [futures.task.members] paragraph 3:

   > 3 Effects: constructs a new `packaged_task` object with an associated asynchronous state and stores a copy of `f` as the object's stored taskinitializes the object's stored task with `std::forward<F>(f)`. The constructors that take an `Allocator` argument use it to allocate memory needed to store the internal data structures.

2. Change 32.10.10.2 [futures.task.members] paragraph 5:

   > 5 Effects: constructs a new `packaged_task` object and transfers ownership of `other`'s associated asynchronous state to `*this`, leaving `other` with no associated asynchronous state. Moves the stored task from `other` to `*this`.

### 2028(i). `messages_base::catalog` overspecified

Section: 28.3.4.8.2 [locale.messages] Status: C++14 Submitter: Howard Hinnant Opened: 2011-02-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 28.3.4.8.2 [locale.messages], `messages_base::catalog` is specified to be a typedef to `int`. This type is subsequently used to open, access and close catalogs.

However, an OS may have catalog/messaging services that are indexed and managed by types other than `int`. For example `POSIX`, publishes the following messaging API:

>     typedef unspecified nl_catd;

>     nl_catd catopen(const char* name , int oflag);
>     char*   catgets(nl_catd catd, int set_id, int msg_id, const char* s);
>     int     catclose(nl_catd catd);

I.e., the catalog is managed with an unspecified type, not necessarily an `int`. Mac OS uses a `void*` for `nl_catd` (which is conforming to the `POSIX` standard). The current `messages_base` spec effectively outlaws using the built-in OS messaging service supplied for this very purpose!

[2011-02-24: Chris Jefferson updates the proposed wording, changing unspecified to unspecified signed integral type]

[2011-03-02: Daniel updates the proposed wording, changing unspecified signed integral type to unspecified signed integer type (We don't want to allow for `bool` or `char`)]

[2011-03-24 Madrid meeting]

Consensus that this resolution is the direction we would like to see.

Proposed resolution:

1. Modify 28.3.4.8.2 [locale.messages]:

   >     namespace std {
   >       class messages_base {
   >       public:
   >         typedef intunspecified signed integer type catalog;
   >       };
   >       ...
   >     }

### 2029(i). Missing '`noexcept`' on `basic_regex` move-assignment operator

Section: 28.6.7 [re.regex] Status: C++11 Submitter: Jonathan Wakely Opened: 2011-02-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

N3149 replaced the "`Throws: nothing`" clause on `basic_regex::assign(basic_regex&&)` with the `noexcept` keyword. The effects of the move-assignment operator are defined in terms of the `assign()` function, so the "`Throws: nothing`" applied there too, and a `noexcept`-specification should be added there too.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 7 votes.

Proposed resolution:

1. Modify the `basic_regex` synopsis in 28.6.7 [re.regex] p. 3:

   >     namespace std {
   >       template <class charT,
   >                 class traits = regex_traits<charT> >
   >       class basic_regex {
   >       public:
   >         ...
   >         basic_regex& operator=(const basic_regex&);
   >         basic_regex& operator=(basic_regex&&) noexcept;
   >         basic_regex& operator=(const charT* ptr);
   >         ...
   >       };
   >     }

2. Modify 28.6.7.3 [re.regex.assign] p. 2:

   >     basic_regex& operator=(basic_regex&& e) noexcept;

   > > 2 Effects: returns `assign(std::move(e))`.

### 2030(i). `packaged_task::result_type` should be removed

Section: 32.10.10 [futures.task] Status: C++11 Submitter: Anthony Williams Opened: 2010-11-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`packaged_task::operator()` always returns `void`, regardless of the return type of the wrapped task. However, `packaged_task::result_type` is a typedef to the return type of the wrapped task. This is inconsistent with other uses of `result_type` in the standard, where it matches the return type of `operator()` (e.g. `function`, `owner_less`). This is confusing.

It also violates the TR1 `result_of` protocol, and thus makes `packaged_task` harder to use with anything that respects that protocol.

Finally, it is of little use anyway.

`packaged_task::result_type` should therefore be removed.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Alter the class definition of `packaged_task` in 32.10.10 [futures.task] p. 2 as follows:

>     template<class R, class... ArgTypes>
>     class packaged_task<R(ArgTypes...)> {
>     public:
>       typedef R result_type;
>       [...]
>     };

### 2031(i). `std::future<>::share()` only applies to rvalues

Section: 32.10.7 [futures.unique.future] Status: C++11 Submitter: Anthony Williams Opened: 2011-02-17 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

As specified, `future<>::share()` has the signature

>     shared_future<R> share() &&;

This means that it can only be applied to rvalues. One of the key benefits of `share()` is that it can be used with the new `auto` facility:

>     std::promise<some_long_winded_type_name> some_promise;
>     auto f = some_promise.get_future(); // std::future
>     auto sf = std::move(f).share();

`share()` is sufficiently explicit that the move should not be required. We should be able to write:

>     auto sf = f.share();

[2011-02-22 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Alter the declaration of `share()` to remove the "&&" rvalue qualifier in [futures.unique_future] p. 3, and [futures.unique_future] p. 11:

>     shared_future<R> share() &&;

### 2032(i). Incorrect synchronization clause of `async` function

Section: 32.10.9 [futures.async] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2011-02-17 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Clause 32.10.9 [futures.async] has undergone significant rewording in Batavia. Due to co-presence of at least three different sources of modification there is a part where changes have overlapped (marked by an Editor's note), which should be reconciled. Moreover, I believe that a few non-overlapping sentences are now incorrect and should be fixed, so the problem cannot be handled editorially. (See c++std-lib-29667.)

[Adopted in Madrid, 2011-03]

Proposed resolution:

1. Edit 32.10.5 [futures.state], paragraph 3 as follows.

   > An asynchronous return object is an object that reads results from an associated asynchronous state. A waiting function of an asynchronous return object is one that potentially blocks to wait for the associated asynchronous state to be made ready. If a waiting function can return before the state is made ready because of a timeout (30.2.5), then it is a timed waiting function, otherwise it is a non-timed waiting function.

2. Edit within 32.10.9 [futures.async] paragraph 3 bullet 2 as follows.

   > Effects: [...]

   > - if `policy & launch::deferred` is non-zero — [...] The associated asynchronous state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4 [futures.state]) requiring a non-timed wait on an asynchronous return object referring to the this associated asynchronous state created by this `async` call to become ready shall invoke the deferred function in the thread that called the waiting function;. once Once evaluation of `INVOKE(g, xyz)` begins, the function is no longer considered deferred. [...]

3. Edit 32.10.9 [futures.async] paragraph 5 as follows.

   > Synchronization: Regardless of the provided `policy` argument,

   > - the invocation of `async` synchronizes with (1.10) the invocation of `f`. [Note: this statement applies even when the corresponding future object is moved to another thread. —end note]; and
   > - the completion of the function `f` is sequenced before (1.10) the associated asynchronous state is made ready. [Note: `f` might not be called at all, so its completion might never happen. —end note]

   > If `policy & launch::async` is non-zero, If the implementation chooses the `launch::async` policy,

   > - a call to a waiting function on an asynchronous return object that shares the associated asynchronous state created by this `async` call shall block until the associated thread has completed, as if joined (30.3.1.5);
   > - the `join()` on the created thread object the associated thread completion synchronizes with (1.10) the return from the first function that successfully detects the ready status of the associated asynchronous state or with the return from the last function that releases the associated asynchronous state returns, whichever happens first. [Editor's note: N3196 changes the following sentence as indicated. N3188 removes the sentence. Please pick one.] If the invocation is deferred, the completion of the invocation of the deferred function synchronizes with the successful return from a call to a waiting function on the associated asynchronous state.

### 2033(i). Preconditions of `reserve`, `shrink_to_fit`, and `resize` functions

Section: 23.3.13.3 [vector.capacity], 23.3.5.3 [deque.capacity] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2011-02-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I have several questions with regard to the working paper N3225 (C++0x working draft):

1. Where the working draft specifies preconditions for `shrink_to_fit` member function of `std::vector` and `std::deque`?

2. Where the working draft specifies preconditions for '`void reserve(size_type n)`' member function of `std::vector`?

3. Does a call to '`void resize(size_type sz)`' of `std::vector` require the element type to be `DefaultConstructible`? If yes, why such requirement is not listed in the Requires paragraph?

4. Does a call to '`void resize(size_type sz)`' of `std::vector` require the element type to be `MoveAssignable` because the call `erase(begin() + sz, end())` mentioned in the Effects paragraph would require the element type to be `MoveAssignable`?

5. Why `CopyInsertable` requirement is used for '`void resize(size_type sz)`' of `std::vector` instead of `MoveInsertable` requirement?

[2011-06-12: Daniel comments and provides wording]

According to my understanding of the mental model of vector (and to some parts for deque) the some requirements are missing in the standard as response to above questions:

1. The preconditions of `shrink_to_fit` for both `std::vector` and `std::deque` should impose the `MoveInsertable` requirements. The reason for this is, that these containers can host move-only types. For a container type `X` the C++03 idiom `X(*this).swap(*this)` imposes the `CopyInsertable` requirements which would make the function call ill-formed, which looks like an unacceptable restriction to me. Assuming the committee decides to support the move-only case, further wording has to be added for the situation where such a move-only type could throw an exception, because this can leave the object in an unspecified state. This seems consistent with the requirements of `reserve`, which seems like a very similar function to me (for `vector`). And this brings us smoothly to the following bullet:

2. I agree that we are currently missing to specify the preconditions of the `reserve` function. My interpretation of the mental model of this function is that it should work for move-only types, which seems to be supported by the wording used in 23.3.13.3 [vector.capacity] p2:

   > […] If an exception is thrown other than by the move constructor of a non-CopyInsertable type, there are no effects.

   Given this statement, the appropriate requirement is `MoveInsertable` into the `vector`.

3. I agree that `vector::resize(size_type)` misses to list the `DefaultConstructible` requirements.
4. Unfortunately the current specification in terms of `erase` implies the `MoveAssignable` requirements. I don't think that this implication is intended. This function requires "append" and "pop-back" effects, respectively, where the former can be realized in terms of `MoveInsertable` requirements. The same fix in regard to using `pop_back` instead of `erase` is necessary for the two argument overload of `resize` as well (no `MoveAssignable` is required).
5. The `CopyInsertable` requirement is incorrect and should be `MoveInsertable` instead.

In addition to above mentioned items, the proposed resolution adds a linear complexity bound for `shrink_to_fit` and attempts to resolve the related issue 2066(i).

[ 2011 Bloomington ]

Move to Ready.

Note for editor: we do not normally refer to 'linear time' for complexity requirements, but there is agreement that any clean-up of such wording is editorial.

Proposed resolution:

This wording is relative to the FDIS.

1. Edit 23.3.5.3 [deque.capacity] as indicated [Remark: The suggested change of p4 is not redundant, because `CopyInsertable` is not necessarily a refinement of `MoveInsertable` in contrast to the fact that `CopyConstructible` is a refinement of `MoveConstructible`]:

   >     void resize(size_type sz);

   > > -1- Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end());`calling `pop_back()` `size() - sz` times. If `size() < sz`, appends `sz - size()` value-initialized elements to the sequence.

   > > -2- Requires: `T` shall be `MoveInsertable` into `*this` and `DefaultConstructible`.

   >     void resize(size_type sz, const T& c);

   > > -3- Effects: If `sz <= size()`, equivalent to calling `pop_back()` `size() - sz` times. If `size() < sz`, appends `sz - size()` copies of `c` to the sequence.

   > > >     if (sz > size())
   > > >       insert(end(), sz-size(), c);
   > > >     else if (sz < size())
   > > >       erase(begin()+sz, end());
   > > >     else
   > > >       ; // do nothing

   > > -4- Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`.

   >     void shrink_to_fit();

   > > -?- Requires: `T` shall be `MoveInsertable` into `*this`.

   > > -?- Complexity: Takes at most linear time in the size of the sequence.

   > > -5- Remarks: `shrink_to_fit` is a non-binding request to reduce memory use but does not change the size of the sequence. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ]

2. Edit 23.3.13.3 [vector.capacity] as indicated including edits that also resolve 2066(i) [Remark: The combined listing of `MoveInsertable` and `CopyInsertable` before p12 is not redundant, because `CopyInsertable` is not necessarily a refinement of `MoveInsertable` in contrast to the fact that `CopyConstructible` is a refinement of `MoveConstructible`]:

   […]

   >     void reserve(size_type n);

   > > -?- Requires: `T` shall be `MoveInsertable` into `*this`.

   > > -2- Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage allocation accordingly. After `reserve()`, `capacity()` is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`. If an exception is thrown other than by the move constructor of a non-`CopyInsertable` type, there are no effects.

   > > -3- Complexity: It does not change the size of the sequence and takes at most linear time in the size of the sequence.

   > > -4- Throws: `length_error` if `n > max_size()`.[footnote 266]

   > > -5- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to `reserve()` until the time when an insertion would make the size of the vector greater than the value of `capacity()`.

   >     void shrink_to_fit();

   > > -?- Requires: `T` shall be `MoveInsertable` into `*this`.

   > > -?- Complexity: Takes at most linear time in the size of the sequence.

   > > -6- Remarks: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

   […]

   >     void resize(size_type sz);

   > > -9- Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end());`calling `pop_back()` `size() - sz` times. If `size() < sz`, appends `sz - size()` value-initialized elements to the sequence.

   > > -10- Requires: `T` shall be `CopyMoveInsertable` into `*this` and `DefaultConstructible`.

   > > -??- Remarks: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

   >     void resize(size_type sz, const T& c);

   > > -11- Effects: If `sz <= size()`, equivalent to calling `pop_back()` `size() - sz` times. If `size() < sz`, appends `sz - size()` copies of `c` to the sequence.

   > > >     if (sz > size())
   > > >       insert(end(), sz-size(), c);
   > > >     else if (sz < size())
   > > >       erase(begin()+sz, end());
   > > >     else
   > > >       ; // do nothing

   > > -??- Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`.

   > > -12- RequiresRemarks: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

### 2034(i). Initialization of atomics is misspecified so that it doesn't preserve sequential consistency

Section: 32.5.4 [atomics.order] Status: Resolved Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This violates the core intent of the memory model, as stated in the note in 6.10.2 [intro.multithread] p. 21.

This was discovered by Mark Batty, and pointed out in their POPL 2011 paper, "Mathematizing C++ Concurrency", section 4, "Sequential consistency of SC atomics". The problem is quite technical, but well-explained in that paper.

This particular issue was not understood at the time the FCD comments were generated. But it is closely related to a number of FCD comments. It should have arisen from US-171, though that's not the actual history.

This issue has been under discussion for several months in a group that included a half dozen or so of the most interested committee members. The P/R represents a well-considered consensus among us:

[2011-03-16: Jens updates wording]

> Modify 32.5.4 [atomics.order] p.3, so that the normative part reads:

> > 3 There shall be a single total order S on all `memory_order_seq_cst` operations, consistent with the "happens before" order and modification orders for all affected locations, such that each `memory_order_seq_cst` operation that loads a value observes either the last preceding modification according to this order S, A (if any), or the result of an operation X that is notdoes not happen before A and that is not `memory_order_seq_cst`. [ Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the "happens before" ordering. — end note ]

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2037(i). `atomic` free functions incorrectly specified

Section: 32.5.2 [atomics.syn] Status: Resolved Submitter: Pete Becker Opened: 2011-03-01 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In earlier specifications of atomics the template specialization `atomic<integer>` was derived from `atomic_integer` (e.g. `atomic<int>` was derived from `atomic_int`), and the working draft required free functions such as

>     int atomic_load(const atomic_int*)

for each of the `atomic_integer` types. This worked fine with normal function overloading.

For the post-Batavia working draft, N3193 removed the requirement that `atomic<integer>` be derived from `atomic_integer` and replaced the free functions taking pointers to `atomic_integer` with template functions taking `atomic_type*`, such as

>     template <class T> T atomic_load(const atomic_type*);

and a code comment explaining that `atomic_type` can be either `atomic<T>` or a named base class of `atomic<T>`. The latter possibility is supposed to allow existing implementations based on the previous specification to continue to conform.

From history, this allowance seems to imply that functions like `atomic_load` can be non-template free functions, as they were before. The explicit requirements do not allow this, and, by requiring that they be templates, make them far more complicated. As the specification is currently written, code that uses an implementation that uses a base class would have to provide an explicit template type:

>     atomic<int> my_atomic_int;
>     atomic_load<int>(&my_atomic_int);

That type argument isn't needed when `atomic_type` is `atomic<T>`, but cautious users would always provide it to make their code portable across different implementations of the standard library.

One possibility for the implementor would be to do some template meta-programming to infer the type `T` when there are no function parameters of type `T`, but without running afoul of the prohibition on adding parameters with default values (16.4.6.4 [global.functions]/3).

So the promise that implementations of the previous specification continue to conform has not been met. The specification of these free functions should be rewritten to support library code written to the previous specification or the vacuous promise should be removed.

[2011-03-08: Lawrence comments and drafts wording:]

One of the goals is to permit atomics code to compile under both C and C++. Adding explicit template arguments would defeat that goal.

The intent was to permit the normal function overloads for `atomic_int` when `atomic_int` is distinct from `atomic<int>`. That intent was not reflected in the wording.

> Proposed Resolution

> Explicitly permit free functions.

> 1. Edit within the header `<atomic>` synopsis 32.5.2 [atomics.syn] as follows:

>    >     // 29.6.1, general operations on atomic types
>    >     // In the following declarations, atomic_type is either
>    >     // atomic<T> or a named base class for T from
>    >     // Table 145 or inferred from
>    >     // Table 146.
>    >     // In the following declarations, atomic-type is either
>    >     // atomic<T> or a named base class for T from
>    >     // Table 145 or inferred from
>    >     // Table 146.
>    >     // If it is atomic<T>, then the declaration is a template
>    >     // declaration prefixed with template <class T>
>    >     template <class T>
>    >     bool atomic_is_lock_free(const volatile atomic_typeatomic-type*);
>    >     template <class T>
>    >     bool atomic_is_lock_free(const atomic_typeatomic-type*);
>    >     template <class T>
>    >     void atomic_init(volatile atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     void atomic_init(atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     void atomic_store(volatile atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     void atomic_store(atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     void atomic_store_explicit(volatile atomic_typeatomic-type*, T, memory_order);
>    >     template <class T>
>    >     void atomic_store_explicit(atomic_typeatomic-type*, T, memory_order);
>    >     template <class T>
>    >     T atomic_load(const volatile atomic_typeatomic-type*);
>    >     template <class T>
>    >     T atomic_load(const atomic_typeatomic-type*);
>    >     template <class T>
>    >     T atomic_load_explicit(const volatile atomic_typeatomic-type*, memory_order);
>    >     template <class T>
>    >     T atomic_load_explicit(const atomic_typeatomic-type*, memory_order);
>    >     template <class T>
>    >     T atomic_exchange(volatile atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     T atomic_exchange(atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     T atomic_exchange_explicit(volatile atomic_typeatomic-type*, T, memory_order);
>    >     template <class T>
>    >     T atomic_exchange_explicit(atomic_typeatomic-type*, T, memory_order);
>    >     template <class T>
>    >     bool atomic_compare_exchange_weak(volatile atomic_typeatomic-type*, T*, T);
>    >     template <class T>
>    >     bool atomic_compare_exchange_weak(atomic_typeatomic-type*, T*, T);
>    >     template <class T>
>    >     bool atomic_compare_exchange_strong(volatile atomic_typeatomic-type*, T*, T);
>    >     template <class T>
>    >     bool atomic_compare_exchange_strong(atomic_typeatomic-type*, T*, T);
>    >     template <class T>
>    >     bool atomic_compare_exchange_weak_explicit(volatile atomic_typeatomic-type*, T*, T,
>    >       memory_order, memory_order);
>    >     template <class T>
>    >     bool atomic_compare_exchange_weak_explicit(atomic_typeatomic-type*, T*, T.
>    >       memory_order, memory_order);
>    >     template <class T>
>    >     bool atomic_compare)exchange_strong_explicit(volatile atomic_typeatomic-type*, T*, T,
>    >       memory_order, memory_order);
>    >     template <class T>
>    >     bool atomic_compare_exchange_strong_explicit(atomic_typeatomic-type*, T*, T,
>    >       memory_order, memory_order);

>    >     // 29.6.2, templated operations on atomic types
>    >     // In the following declarations, atomic_type is either
>    >     // atomic<T> or a named base class for T from
>    >     // Table 145 or inferred from
>    >     // Table 146.
>    >     template <class T>
>    >     T atomic_fetch_add(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_add(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_add_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_add_explicit(atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_sub(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_sub(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_sub_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_sub_explicit(atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_and(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_and(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_and_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_and_explicit(atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_or(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_or(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_or_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_or_explicit(atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_xor(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_xor(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_xor_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_xor_explicit(atomic-typeatomic<T>*, T, memory_order);

>    >     // 29.6.3, arithmetic operations on atomic types
>    >     // In the following declarations, atomic-integral is either
>    >     // atomic<T> or a named base class for T from
>    >     // Table 145 or inferred from
>    >     // Table 146.
>    >     // If it is atomic<T>,
>    >     // then the declaration is a template specialization declaration prefixed with
>    >     // template <>
>    >     template <>
>    >     integral atomic_fetch_add(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_add(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_add_explicit(volatile atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_add_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_sub(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_sub(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_sub_explicit(volatile atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_sub_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_and(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_and(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_and_explicit(volatile atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_and_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_or(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_or(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_or_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_or_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_xor(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_xor(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_xor_explicit(volatile atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_xor_explicit(atomic-integral*, integral, memory_order);

> 2. Edit [atomics.types.operations.general] paragraph 1+2 as follows:

>    > -1- The implementation shall provide the functions and function templates identified as "general operations on atomic types" in 32.5.2 [atomics.syn].

>    > -2- In the declarations of these functions and function templates, the name atomic-type refers to either `atomic<T>` or to a named base class for `T` from Table 145 or inferred from Table 146.

> 3. In [atomics.types.operations.templ] delete paragraph 2:

>    > -1- The implementation shall declare but not define the function templates identified as "templated operations on atomic types" in 32.5.2 [atomics.syn].

>    > -2- In the declarations of these templates, the name atomic-type refers to either `atomic<T>` or to a named base class for `T` from Table 145 or inferred from Table 146.

> 4. Edit [atomics.types.operations.arith] paragraph 1+2 as follows:

>    > -1- The implementation shall provide the functions and function template specializations identified as "arithmetic operations on atomic types" in 32.5.2 [atomics.syn].

>    > -2- In the declarations of these functions and function template specializations, the name integral refers to an integral type and the name atomic-integral refers to either `atomic<`integral`>` or to a named base class for integral from Table 145 or inferred from Table 146.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2039(i). Issues with `std::reverse` and `std::copy_if`

Section: 26.7.1 [alg.copy], 26.7.10 [alg.reverse] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2011-03-02 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [alg.copy].

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Discussion:

1. In the description of `std::reverse`

   > Effects: For each non-negative integer `i <= (last - first)/2`, applies `iter_swap` to all pairs of iterators `first + i`, `(last - i) - 1`.

   should be changed to

   > Effects: For each non-negative integer `i < (last - first)/2`, applies `iter_swap` to all pairs of iterators `first + i`, `(last - i) - 1`.

   Here `i` shall be strictly less than `(last - first)/2`.

2. In the description of `std::copy_if` Returns paragraph is missing.

[2011-03-02: Daniel drafts wording]

Proposed resolution:

1. Modify 26.7.10 [alg.reverse] p. 1 as indicated:

   > 1 Effects: For each non-negative integer `i <= (last - first)/2`, applies `iter_swap` to all pairs of iterators `first + i`, `(last - i) - 1`.

2. Add the following Returns element after 26.7.1 [alg.copy] p. 9:

   >     template<class InputIterator, class OutputIterator, class Predicate>
   >     OutputIterator copy_if(InputIterator first, InputIterator last,
   >        OutputIterator result, Predicate pred);

   > > 8 Requires: The ranges `[first,last)` and `[result,result + (last - first))` shall not overlap.

   > > 9 Effects: Copies all of the elements referred to by the iterator `i` in the range `[first,last)` for which `pred(*i)` is true.

   > > ?? Returns: The end of the resulting range.

   > > 10 Complexity: Exactly `last - first` applications of the corresponding predicate.

   > > 11 Remarks: Stable.

### 2040(i). Missing type traits related to `is_convertible`

Section: 21 [meta] Status: Resolved Submitter: Daniel Krügler Opened: 2011-03-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

When n3142 was suggested, it concentrated on constructions, assignments, and destructions, but overlooked to complement the single remaining compiler-support trait

>     template <class From, class To> struct is_convertible;

with the no-throw and triviality related aspects as it had been done with the other expression-based traits. Specifically, the current specification misses to add the following traits:

>     template <class From, class To> struct is_nothrow_convertible;
>     template <class From, class To> struct is_trivially_convertible;

In particular the lack of `is_nothrow_convertible` is severely restricting. This was recently recognized when the proposal for `decay_copy` was prepared by n3255. There does not exist a portable means to define the correct conditional `noexcept` specification for the `decay_copy` function template, which is declared as:

>     template <class T>
>     typename decay<T>::type decay_copy(T&& v) noexcept(???);

The semantics of `decay_copy` bases on an implicit conversion which again influences the overload set of functions that are viable here. In most circumstances this will have the same effect as comparing against the trait `std::is_nothrow_move_constructible`, but there is no guarantee for that being the right answer. It is possible to construct examples, where this would lead to the false result, e.g.

>     struct S {
>       S(const S&) noexcept(false);

>       template<class T>
>       explicit S(T&&) noexcept(true);
>     };

`std::is_nothrow_move_constructible` will properly honor the explicit template constructor because of the direct-initialization context which is part of the `std::is_constructible` definition and will in this case select it, such that `std::is_nothrow_move_constructible<S>::value == true`, but if we had the traits `is_nothrow_convertible`, `is_nothrow_convertible<S, S>::value` would evaluate to `false`, because it would use the copy-initialization context that is part of the `is_convertible` definition, excluding any explicit constructors and giving the opposite result.

The `decay_copy` example is surely not one of the most convincing examples, but `is_nothrow_convertible` has several use-cases, and can e.g. be used to express whether calling the following implicit conversion function could throw an exception or not:

>     template<class T, class U>
>     T implicit_cast(U&& u) noexcept(is_nothrow_convertible<U, T>::value)
>     {
>       return std::forward<U>(u);
>     }

Therefore I suggest to add the missing trait `is_nothrow_convertible` and for completeness also the missing trait `is_trivially_convertible` to 21 [meta].

[2011-03-24 Madrid meeting]

Daniel K: This is a new feature so out of scope.

Pablo: Any objections to moving 2040 to Open?

No objections.

[Bloomington, 2011]

Move to NAD Future, this would be an extension to existing functionality.

[LEWG, Kona 2017]

Fallen through the cracks since 2011, but we should discuss it. Alisdair points out that triviality is about replacing operations with `memcpy`, so be sure this is false for `int`->`float`.

[Rapperswil, 2018]

Resolved by the adoption of p0758r1.

Proposed resolution:

1. Ammend the following declarations to the header `<type_traits>` synopsis in 21.3.3 [meta.type.synop]:

   >     namespace std {
   >       …
   >       // 20.9.6, type relations:
   >       template <class T, class U> struct is_same;
   >       template <class Base, class Derived> struct is_base_of;
   >       template <class From, class To> struct is_convertible;
   >       template <class From, class To> struct is_trivially_convertible;
   >       template <class From, class To> struct is_nothrow_convertible;

   >       …
   >     }

2. Modify Table 51 — "Type relationship predicates" as indicated. The removal of the remaining traces of the trait `is_explicitly_convertible` is an editorial step, it was removed by n3047:

   > Table 51 — Type relationship predicates
   > Template | Condition | Comments
   > --- | --- | ---
   > …
   > `template <class From, class To> struct is_convertible;` | see below | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.
   > `template <class From, class To> struct is_explicitly_convertible;` | `is_constructible<To, From>::value` | a synonym for a two-argument version of `is_constructible`. An implementation may define it as an alias template.
   > `template <class From, class To> struct is_trivially_convertible;` | `is_convertible<From, To>::value` is `true` and the conversion, as defined by `is_convertible`, is known to call no operation that is not trivial ([basic.types], [special]). | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.
   > `template <class From, class To> struct is_nothrow_convertible;` | `is_convertible<From, To>::value` is `true` and the conversion, as defined by `is_convertible`, is known not to throw any exceptions ([expr.unary.noexcept]). | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.
   > …

### 2041(i). Stage 2 accumulate incompatibilty

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++11 Submitter: Howard Hinnant Opened: 2011-03-09 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [facet.num.get.virtuals].

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Discussion:

`num_get` Stage 2 accumulation changed between C++03 and the current C++0x working draft. The sentences:

> If it is not discarded, then a check is made to determine if `c` is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.

have been dropped from 28.3.4.3.2.3 [facet.num.get.virtuals], Stage 2, paragraph 3 that begins:

> If `discard` is true, […]

Consider this code:

>     #include <sstream>
>     #include <iostream>

>     int main(void)
>     {
>        std::istringstream s("8cz");
>        long i = 0;
>        char c;
>        s >> i;
>        if (!s.fail())
>            std::cout << "i = " << i << '\n';
>        else
>        {
>            std::cout << "s >> i failed\n";
>            s.clear();
>        }
>        s >> c;
>        if (!s.fail())
>            std::cout << "c = " << c << '\n';
>        else
>            std::cout << "s >> c failed\n";
>     }

C++0x currently prints out:

>     s >> i failed
>     c = z

However C++03 conforming implementations will output:

>     i = 8
>     c = c

I believe we need to restore C++03 compatibility.

Proposed resolution:

Add to 28.3.4.3.2.3 [facet.num.get.virtuals], Stage 2:

> If `discard` is true, then if `'.'` has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if `'.'` has already been accumulated, the character is discarded and Stage 2 terminates. If it is not discarded, then a check is made to determine if `c` is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.

> If the character is either discarded or accumulated then in is advanced by `++in` and processing returns to the beginning of stage 2.

### 2042(i). Comparing `forward_list::before_begin()` to `forward_list::end()`

Section: 23.3.7.3 [forward.list.iter] Status: C++11 Submitter: Joe Gottman Opened: 2011-03-13 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

For an object `c` of type `forward_list<X, Alloc>`, the iterators `c.before_begin()` and `c.end()` are part of the same underlying sequence, so the expression `c.before_begin() == c.end()` must be well-defined. But the standard says nothing about what the result of this expression should be. The forward iterator requirements says no dereferenceable iterator is equal to a non-dereferenceable iterator and that two dereferenceable iterators are equal if and only if they point to the same element. But since `before_begin()` and `end()` are both non-dereferenceable, neither of these rules applies.

Many `forward_list` methods, such as `insert_after()`, have a precondition that the iterator passed to them must not be equal to `end()`. Thus, user code might look like the following:

>     void foo(forward_list<int>& c, forward_list<int>::iterator it)
>     {
>       assert(it != c.end());
>       c.insert_after(it, 42);
>     }

Conversely, `before_begin()` was specifically designed to be used with methods like `insert_after()`, so if `c.before_begin()` is passed to this function the assertion must not fail.

[2011-03-14: Daniel comments and updates the suggested wording]

The suggested wording changes are necessary but not sufficient. Since there does not exist an equivalent semantic definition of `cbefore_begin()` as we have for `cbegin()`, this still leaves the question open whether the normative remark applies to `cbefore_begin()` as well. A simple fix is to define the operational semantics of `cbefore_begin()` in terms of `before_begin()`.

[2011-03-24 Madrid meeting]

General agreement that this is a serious bug.

Pablo: Any objections to moving 2042 to Immediate?

No objections.

Proposed resolution:

Add to the definition of `forward_list::before_begin()` [forwardlist.iter] the following:

>     iterator before_begin();
>     const_iterator before_begin() const;
>     const_iterator cbefore_begin() const;

> > -1- Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by `begin()`.

> > -?- Effects: `cbefore_begin()` is equivalent to `const_cast<forward_list const&>(*this).before_begin()`.

> > -?- Remarks: `before_begin() == end()` shall equal false.

### 2044(i). No definition of "Stable" for copy algorithms

Section: 16.4.6.8 [algorithm.stable] Status: C++14 Submitter: Pablo Halpern Opened: 2011-03-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

16.4.6.8 [algorithm.stable] specified the meaning of "stable" when applied to the different types of algorithms. The second bullet says:

> — For the remove algorithms the relative order of the elements that are not removed is preserved.

There is no description of what "stable" means for copy algorithms, even though the term is applied to `copy_if` (and perhaps others now or in the future). Thus, `copy_if` is using the term without a precise definition.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

1. In 16.4.6.8 [algorithm.stable] p. 1 change as indicated:

   > When the requirements for an algorithm state that it is “stable” without further elaboration, it means:

   > - For the sort algorithms the relative order of equivalent elements is preserved.
   > - For the remove and copy algorithms the relative order of the elements that are not removed is preserved.
   > - For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range precede the elements from the second range.

### 2045(i). `forward_list::merge` and `forward_list::splice_after` with unequal allocators

Section: 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Pablo Halpern Opened: 2011-03-24 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

See also: 1215(i)

`list::merge` and `list::splice` have the requirement that the two lists being merged or spliced must use the same allocator. Otherwise, moving list nodes from one container to the other would corrupt the data structure. The same requirement is needed for `forward_list::merge` and `forward_list::splice_after`.

[ 2011 Bloomington ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. In [forwardlist.ops] p. 1 change as indicated:

   >     void splice_after(const_iterator position, forward_list<T,Allocator>& x);
   >     void splice_after(const_iterator position, forward_list<T,Allocator>&& x);

   > 1 - Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`,`end()`). `get_allocator() == x.get_allocator()`. `&x != this`.

2. In [forwardlist.ops] p. 5 change as indicated:

   >     void splice_after(const_iterator position, forward_list<T,Allocator>& x, const_iterator i);
   >     void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator i);

   > 5 - Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`,`end()`). The iterator following `i` is a dereferenceable iterator in `x`. `get_allocator() == x.get_allocator()`.

3. In [forwardlist.ops] p. 9 change as indicated:

   >     void splice_after(const_iterator position, forward_list<T,Allocator>& x,
   >                       const_iterator first, const_iterator last);
   >     void splice_after(const_iterator position, forward_list<T,Allocator>&& x,
   >                       const_iterator first, const_iterator last);

   > 9 - Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`,`end()`). (`first`,`last`) is a valid range in `x`, and all iterators in the range (`first`,`last`) are dereferenceable. `position` is not an iterator in the range (`first`,`last`). `get_allocator() == x.get_allocator()`.

4. In [forwardlist.ops] p. 18 change as indicated:

   >     void merge(forward_list<T,Allocator>& x);
   >     void merge(forward_list<T,Allocator>&& x);
   >     template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp);
   >     template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);

   > 18 - Requires: `comp` defines a strict weak ordering ([alg.sorting]), and `*this` and `x` are both sorted according to this ordering. `get_allocator() == x.get_allocator()`.

### 2047(i). Incorrect "mixed" move-assignment semantics of `unique_ptr`

Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++14 Submitter: Daniel Krügler Opened: 2011-04-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.single.asgn].

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Discussion:

The semantics described in 20.3.1.3.4 [unique.ptr.single.asgn] p. 6

> Effects: Transfers ownership from `u` to `*this` as if […] followed by an assignment from `std::forward<D>(u.get_deleter())`.

contradicts to the pre-conditions described in p. 4:

> Requires: If E is not a reference type, assignment of the deleter from an rvalue of type `E` shall be well-formed and shall not throw an exception. Otherwise, `E` is a reference type and assignment of the deleter from an lvalue of type `E` shall be well-formed and shall not throw an exception.

Either the pre-conditions are incorrect or the semantics should be an assignment from `std::forward<E>(u.get_deleter())`, instead.

It turns out that this contradiction is due to an incorrect transcription from the proposed resolution of 983(i) to the finally accepted proposal n3073 (see bullet 12) as confirmed by Howard Hinnant, thus the type argument provided to `std::forward` must be fixed as indicated.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

1. Edit 20.3.1.3.4 [unique.ptr.single.asgn] p. 6 as indicated:

   >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

   > > 4 - Requires: If `E` is not a reference type, assignment of the deleter from an rvalue of type `E` shall be well-formed and shall not throw an exception. Otherwise, `E` is a reference type and assignment of the deleter from an lvalue of type `E` shall be well-formed and shall not throw an exception.

   > > 5 - Remarks: This operator shall not participate in overload resolution unless:
   > > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer` and
   > > - `U` is not an array type.

   > > 6 - Effects: Transfers ownership from `u` to `*this` as if by calling `reset(u.release())` followed by an assignment from `std::forward<DE>(u.get_deleter())`.

   > > 7 - Returns: `*this`.

### 2048(i). Unnecessary `mem_fn` overloads

Section: 22.10 [function.objects], 22.10.16 [func.memfn] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-04-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [function.objects].

View all issues with C++14 status.

Discussion:

The `mem_fn` overloads for member functions are redundant and misleading and should be removed from the post-C++11 WP.

I believe the history of the overloads is as follows:

In TR1 and in C++0x prior to the N2798 draft, `mem_fn` was specified by a single signature:

>     template<class R, class T> unspecified mem_fn(R T::* pm);

and was accompanied by the remark "Implementations may implement `mem_fn` as a set of overloaded function templates." This remark predates variadic templates and was presumably to allow implementations to provide overloads for a limited number of function parameters, to meet the implementation-defined limit on numbers of template parameters.

N2770 "Concepts for the C++0x Standard Library: Utilities" added separate overloads for pointers to member functions, apparently so that function parameters would require the `CopyConstructible` concept (those overloads first appeared in N2322.) The overloads failed to account for varargs member functions (i.e. those declared with an ellipsis in the parameter-declaration-clause) e.g.

>     struct S {
>      int f(int, ...);
>     };

Syntactically such a function would be handled by the original `mem_fn(R T::* pm)` signature, the only minor drawback being that there would be no `CopyConstructible` requirement on the parameter list. (Core DR 547 clarifies that partial specializations can be written to match cv-qualified and ref-qualified functions to support the case where `R T::*` matches a pointer to member function type.)

LWG issue 920(i) pointed out that additional overloads were missing for member functions with ref-qualifiers. These were not strictly necessary, because such functions are covered by the `mem_fn(R T::* pm)` signature.

Concepts were removed from the draft and N3000 restored the original single signature and accompanying remark.

LWG 1230(i) was opened to strike the remark again and to add an overload for member functions (this overload was unnecessary for syntactic reasons and insufficient as it didn't handle member functions with cv-qualifiers and/or ref-qualifiers.)

920(i) (and 1230(i)) were resolved by restoring a full set of (non-concept-enabled) overloads for member functions with cv-qualifiers and ref-qualifiers, but as in the concept-enabled draft there were no overloads for member functions with an ellipsis in the parameter-declaration-clause. This is what is present in the FDIS.

Following the thread beginning with message c++std-lib-30675, it is my understanding that all the `mem_fn` overloads for member functions are unnecessary and were only ever added to allow concept requirements. I'm not aware of any reason implementations cannot implement `mem_fn` as a single function template. Without concepts the overloads are redundant, and the absence of overloads for varargs functions can be interpreted to imply that varargs functions are not intended to work with `mem_fn`. Clarifying the intent by adding overloads for varargs functions would expand the list of 12 redundant overloads to 24, it would be much simpler to remove the 12 redundant overloads entirely.

[Bloomington, 2011]

Move to Review.

The issue and resolution appear to be correct, but there is some concern that the wording of INVOKE may be different depending on whether you pass a pointer-to-member-data or pointer-to-member-function. That might make the current wording necessary after all, and then we might need to add the missing elipsis overloads.

There was some concern that the Remark confirming implementors had freedom to implement this as a set of overloaded functions may need to be restored if we delete the specification for these overloads.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

1. Change the `<functional>` synopsis 22.10 [function.objects] p. 2 as follows:

   >     namespace std {
   >       […]
   >       // [func.memfn], member function adaptors:
   >       template<class R, class T> unspecified mem_fn(R T::*);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...));
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) volatile);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const volatile);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) &);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const &);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) volatile &);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const volatile &);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) &&);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const &&);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) volatile &&);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const volatile &&);

   >       […]
   >     }

2. Change 22.10.16 [func.memfn] as follows:

   >     template<class R, class T> unspecified mem_fn(R T::*);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...));
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) volatile);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const volatile);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) &);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const &);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) volatile &);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const volatile &);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) &&);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const &&);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) volatile &&);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const volatile &&);

### 2049(i). `is_destructible` is underspecified

Section: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2011-04-18 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

View all other issues in [meta.unary.prop].

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Discussion:

The conditions for the type trait `is_destructible` to be true are described in Table 49 — Type property predicates:

> For a complete type `T` and given
> `template <class U> struct test { U u; };`,
> `test<T>::~test()` is not deleted.

This specification does not say what the result would be for function types or for abstract types:

1. For an abstract type `X` the instantiation `test<X>` is already ill-formed, so we cannot say anything about whether the destructor would be deleted or not.
2. In regard to function types, there exists a special rule in the core language, 13.4.2 [temp.arg.type] p. 3, which excludes member functions to be declared via the type of the template parameter:

   > If a declaration acquires a function type through a type dependent on a template-parameter and this causes a declaration that does not use the syntactic form of a function declarator to have function type, the program is ill-formed.

   > [ Example:

   > >     template<class T> struct A {
   > >       static T t;
   > >     };
   > >     typedef int function();
   > >     A<function> a; // ill-formed: would declare A<function>::t
   > >                    // as a static member function

   > — end example ]

   which has the same consequence as for abstract types, namely that the corresponding instantiation of `test` is already ill-formed and we cannot say anything about the destructor.

To solve this problem, I suggest to specify function types as trivially and nothrowing destructible, because above mentioned rule is very special for templates. For non-templates, a typedef can be used to introduce a member as member function as clarified in 9.3.4.6 [dcl.fct] p. 10.

For abstract types, two different suggestions have been brought to my attention: Either declare them as unconditionally non-destructible or check whether the expression

>     std::declval<T&>().~T()

is well-formed in an unevaluated context. The first solution is very easy to specify, but the second version has the advantage for providing more information to user-code. This information could be quite useful, if generic code is supposed to invoke the destructor of a reference to a base class indirectly via a delete expression, as suggested by Howard Hinnant:

>     template <class T>
>     my_pointer<T>::~my_pointer() noexcept(is_nothrow_destructible<T>::value)
>     {
>        delete ptr_;
>     }

Additional to the `is_destructible` traits, its derived forms `is_trivially_destructible` and `is_nothrow_destructible` are similarly affected, because their wording refers to "the indicated destructor" and probably need to be adapted as well.

[ 2011 Bloomington ]

After discussion about to to handle the exceptional cases of reference types, function types (available by defererencing a function pointer) and `void` types, Howard supplied proposed wording.

[ 2011-08-20 Daniel comments and provides alternatives wording ]

The currently proposed wording would have the consequence that every array type is not destructible, because the pseudo-destructor requires a scalar type with the effect that the expression

>     std::declval<T&>().~T()

is not well-formed for e.g. `T` equal to `int[3]`. The intuitive solution to fix this problem would be to adapt the object type case to refer to the expression

>     std::declval<U&>().~U()

with `U` equal to `remove_all_extents<T>::type`, but that would have the effect that arrays of unknown bounds would be destructible, if the element type is destructible, which was not the case before (This was intentionally covered by the special "For a complete type T" rule in the FDIS).

Suggestion: Use the following definition instead:

> Let `U` be `remove_all_extents<T>::type`.
> For incomplete types and function types, `is_destructible<T>::value` is `false`.
> For object types, if the expression `std::declval<U&>().~U()` is well-formed
> when treated as an unevaluated operand (Clause 5), then `is_destructible<T>::value`
> is `true`, otherwise it is `false`.
> For reference types, `is_destructible<T>::value` is `true`.

This wording also harmonizes with the "unevaluated operand" phrase used in other places, there does not exist the definition of an "unevaluated context"

Note: In the actually proposed wording this wording has been slightly reordered with the same effects.

Howard's (old) proposed resolution:

> Update 21.3.5.4 [meta.unary.prop], table 49:

> `template <class T> struct is_destructible;` | For a complete type `T` and given `template <class U> struct test { U u; };`, `test<T>::~test()` is not deleted. For object types, if the expression: `std::declval<T&>().~T()` is well-formed in an unevaluated context then `is_destructible<T>::value` is `true`, otherwise it is `false`. For `void` types, `is_destructible<T>::value` is `false`. For reference types, `is_destructible<T>::value` is `true`. For function types, `is_destructible<T>::value` is `false`. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

[2012, Portland: applied to WP]

Proposed resolution:

Update 21.3.5.4 [meta.unary.prop], table 49:

`template <class T> struct is_destructible;` | For a complete type `T` and given `template <class U> struct test { U u; };`, `test<T>::~test()` is not deleted. For reference types, `is_destructible<T>::value` is `true`. For incomplete types and function types, `is_destructible<T>::value` is `false`. For object types and given `U` equal to `remove_all_extents<T>::type`, if the expression `std::declval<U&>().~U()` is well-formed when treated as an unevaluated operand (Clause 7 [expr]), then `is_destructible<T>::value` is `true`, otherwise it is `false`. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.

### 2050(i). Unordered associative containers do not use `allocator_traits` to define member types

Section: 23.5 [unord] Status: C++14 Submitter: Tom Zieberman Opened: 2011-04-29 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unord].

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Discussion:

The unordered associative containers define their member types `reference`, `const_reference`, `pointer`, `const_pointer` in terms of their template parameter `Allocator` (via `allocator_type` typedef). As a consequence, only the allocator types, that provide sufficient typedefs, are usable as allocators for unordered associative containers, while other containers do not have this deficiency. In addition to that, the definitions of said typedefs are different from ones used in the other containers. This is counterintuitive and introduces a certain level of confusion. These issues can be fixed by defining `pointer` and `const_pointer` typedefs in terms of `allocator_traits<Allocator>` and by defining `reference` and `const_reference` in terms of `value_type` as is done in the other containers.

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

- Change 23.5.3.1 [unord.map.overview] paragraph 3 as indicated:

  >     namespace std {
  >       template <class Key,
  >                 class T,
  >                 class Hash = hash<Key>,
  >                 class Pred = std::equal_to<Key>,
  >                 class Allocator = std::allocator<std::pair<const Key, T> > >
  >       class unordered_map
  >       {
  >       public:
  >         // types
  >         typedef Key key_type;
  >         typedef std::pair<const Key, T> value_type;
  >         typedef T mapped_type;
  >         typedef Hash hasher;
  >         typedef Pred key_equal;
  >         typedef Allocator allocator_type;
  >         typedef typename allocator_typeallocator_traits<Allocator>::pointer pointer;
  >         typedef typename allocator_typeallocator_traits<Allocator>::const_pointer const_pointer;
  >         typedef typename allocator_type::referencevalue_type& reference;
  >         typedef typename allocator_type::const_referenceconst value_type& const_reference;
  >         typedef implementation-defined size_type;
  >         typedef implementation-defined difference_type;

  >         […]
  >       };
  >     }

- Change 23.5.4.1 [unord.multimap.overview] paragraph 3 as indicated:

  >     namespace std {
  >       template <class Key,
  >                 class T,
  >                 class Hash = hash<Key>,
  >                 class Pred = std::equal_to<Key>,
  >                 class Allocator = std::allocator<std::pair<const Key, T> > >
  >       class unordered_multimap
  >       {
  >       public:
  >         // types
  >         typedef Key key_type;
  >         typedef std::pair<const Key, T> value_type;
  >         typedef T mapped_type;
  >         typedef Hash hasher;
  >         typedef Pred key_equal;
  >         typedef Allocator allocator_type;
  >         typedef typename allocator_typeallocator_traits<Allocator>::pointer pointer;
  >         typedef typename allocator_typeallocator_traits<Allocator>::const_pointer const_pointer;
  >         typedef typename allocator_type::referencevalue_type& reference;
  >         typedef typename allocator_type::const_referenceconst value_type& const_reference;
  >         typedef implementation-defined size_type;
  >         typedef implementation-defined difference_type;

  >         […]
  >       };
  >     }

- Change 23.5.6.1 [unord.set.overview] paragraph 3 as indicated:

  >     namespace std {
  >       template <class Key,
  >                 class Hash = hash<Key>,
  >                 class Pred = std::equal_to<Key>,
  >                 class Allocator = std::allocator<Key> >
  >       class unordered_set
  >       {
  >       public:
  >         // types
  >         typedef Key key_type;
  >         typedef Key value_type;
  >         typedef Hash hasher;
  >         typedef Pred key_equal;
  >         typedef Allocator allocator_type;
  >         typedef typename allocator_typeallocator_traits<Allocator>::pointer pointer;
  >         typedef typename allocator_typeallocator_traits<Allocator>::const_pointer const_pointer;
  >         typedef typename allocator_type::referencevalue_type& reference;
  >         typedef typename allocator_type::const_referenceconst value_type& const_reference;
  >         typedef implementation-defined size_type;
  >         typedef implementation-defined difference_type;

  >         […]
  >       };
  >     }

- Change 23.5.7.1 [unord.multiset.overview] paragraph 3 as indicated:

  >     namespace std {
  >       template <class Key,
  >                 class Hash = hash<Key>,
  >                 class Pred = std::equal_to<Key>,
  >                 class Allocator = std::allocator<Key> >
  >       class unordered_multiset
  >       {
  >       public:
  >         // types
  >         typedef Key key_type;
  >         typedef Key value_type;
  >         typedef Hash hasher;
  >         typedef Pred key_equal;
  >         typedef Allocator allocator_type;
  >         typedef typename allocator_typeallocator_traits<Allocator>::pointer pointer;
  >         typedef typename allocator_typeallocator_traits<Allocator>::const_pointer const_pointer;
  >         typedef typename allocator_type::referencevalue_type& reference;
  >         typedef typename allocator_type::const_referenceconst value_type& const_reference;
  >         typedef implementation-defined size_type;
  >         typedef implementation-defined difference_type;

  >         […]
  >       };
  >     }

### 2051(i). Explicit `tuple` constructors for more than one parameter

Section: 22.4.4 [tuple.tuple], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Ville Voutilainen Opened: 2011-05-01 Last modified: 2016-01-28

Priority: 2

View all other issues in [tuple.tuple].

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Discussion:

One of my constituents wrote the following:

-------snip------------

So far the only use I've found for `std::tuple` is as an ad-hoc type to emulate multiple return values. If the tuple ctor was made non-explicit one could almost think C++ supported multiple return values especially when combined with `std::tie()`.

>     // assume types line_segment and point
>     // assume function double distance(point const&, point const&)

>     std::tuple<point, point>
>     closest_points(line_segment const& a, line_segment const& b) {
>      point ax;
>      point bx;
>      /* some math */

>      return {ax, bx};
>     }

>     double
>     distance(line_segment const& a, line_segment const& b) {
>      point ax;
>      point bx;
>      std::tie(ax, bx) = closest_points(a, b);

>      return distance(ax, bx);
>     }

-------snap----------

See also the messages starting from lib-29330.

Some notes:

1. `pair` allows such a return
2. a lambda with a deduced return type doesn't allow it for any type
3. `decltype` refuses `{1, 2}`

I would recommend making non-unary `tuple` constructors non-explicit.

[Bloomington, 2011]

Move to NAD Future, this would be an extension to existing functionality.

[Portland, 2012]

Move to Open at the request of the Evolution Working Group.

[Lenexa 2015-05-05]

VV: While in the area of tuples, LWG 2051 should have status of WP, it is resolved by Daniel's "improving pair and tuple" paper.

MC: status Resolved, by N4387

Proposed resolution:

Resolved by the adoption of N4387.

### 2052(i). Mixup between `mapped_type` and `value_type` for associative containers

Section: 23.2.7 [associative.reqmts] Status: Resolved Submitter: Marc Glisse Opened: 2011-05-04 Last modified: 2016-01-28

Priority: 2

View other active issues in [associative.reqmts].

View all other issues in [associative.reqmts].

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Discussion:

(this is basically reopening the first part of issue 2006(i), as discussed in the thread starting at c++std-lib-30698 )

Section 23.2.7 [associative.reqmts]

In Table 102, several uses of `T` (which means `mapped_type` here) should be `value_type` instead. This is almost editorial. For instance:

>     a_uniq.emplace(args)

> Requires: `T` shall be `EmplaceConstructible` into `X` from args.

> Effects: Inserts a `T` object `t` constructed with `std::forward<Args>(args)...` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of `t`.

[ 2011 Bloomington ]

Not even an exhaustive list of problem locations. No reason to doubt issue.

Pablo agrees to provide wording.

[ 2011-09-04 Pablo Halpern provides improved wording ]

[2014-02-15 post-Issaquah session : move to Resolved]

AJM to replace this note with a 'Resolved By...' after tracking down the exact sequence of events, but clearly resolved in C++14 DIS.

Proposed resolution:

In both section 23.2.7 [associative.reqmts] Table 102 and 23.2.8 [unord.req], Table 103, make the following text replacements:

Original text, in FDIS | Replacement text
`T` is CopyInsertable into `X` and `CopyAssignable`. | `value_type` is `CopyInsertable` into `X`, `key_type` is `CopyAssignable`, and `mapped_type` is `CopyAssignable` (for containers having a `mapped_type`)
`T` is `CopyInsertable` | `value_type` is CopyInsertable
`T` shall be `CopyInsertable` | `value_type` shall be CopyInsertable
`T` shall be `MoveInsertable` | `value_type` shall be MoveInsertable
`T` shall be `EmplaceConstructible` | `value_type` shall be EmplaceConstructible
`T` object | `value_type` object

[ Notes to the editor: The above are carefully selected phrases that can be used for global search-and-replace within the specified sections without accidentally making changes to correct uses `T`. ]

### 2053(i). Errors in `regex` bitmask types

Section: 28.6.4 [re.const] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-05-09 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [re.const].

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Discussion:

When N3110 was applied to the WP some redundant "static" keywords were added and one form of initializer which isn't valid for enumeration types was replaced with another form of invalid initializer.

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Change 28.6.4.2 [re.synopt] as indicated:

   >     namespace std {
   >       namespace regex_constants {
   >         typedef T1 syntax_option_type;
   >         static constexpr syntax_option_type icase = unspecified ;
   >         static constexpr syntax_option_type nosubs = unspecified ;
   >         static constexpr syntax_option_type optimize = unspecified ;
   >         static constexpr syntax_option_type collate = unspecified ;
   >         static constexpr syntax_option_type ECMAScript = unspecified ;
   >         static constexpr syntax_option_type basic = unspecified ;
   >         static constexpr syntax_option_type extended = unspecified ;
   >         static constexpr syntax_option_type awk = unspecified ;
   >         static constexpr syntax_option_type grep = unspecified ;
   >         static constexpr syntax_option_type egrep = unspecified ;
   >       }
   >     }

2. Change 28.6.4.3 [re.matchflag] as indicated:

   >     namespace std {
   >       namespace regex_constants {
   >         typedef T2 match_flag_type;
   >         static constexpr match_flag_type match_default = 0{};
   >         static constexpr match_flag_type match_not_bol = unspecified ;
   >         static constexpr match_flag_type match_not_eol = unspecified ;
   >         static constexpr match_flag_type match_not_bow = unspecified ;
   >         static constexpr match_flag_type match_not_eow = unspecified ;
   >         static constexpr match_flag_type match_any = unspecified ;
   >         static constexpr match_flag_type match_not_null = unspecified ;
   >         static constexpr match_flag_type match_continuous = unspecified ;
   >         static constexpr match_flag_type match_prev_avail = unspecified ;
   >         static constexpr match_flag_type format_default = 0{};
   >         static constexpr match_flag_type format_sed = unspecified ;
   >         static constexpr match_flag_type format_no_copy = unspecified ;
   >         static constexpr match_flag_type format_first_only = unspecified ;
   >       }
   >     }

3. Change 28.6.4.4 [re.err] as indicated:

   >     namespace std {
   >       namespace regex_constants {
   >         typedef T3 error_type;
   >         static constexpr error_type error_collate = unspecified ;
   >         static constexpr error_type error_ctype = unspecified ;
   >         static constexpr error_type error_escape = unspecified ;
   >         static constexpr error_type error_backref = unspecified ;
   >         static constexpr error_type error_brack = unspecified ;
   >         static constexpr error_type error_paren = unspecified ;
   >         static constexpr error_type error_brace = unspecified ;
   >         static constexpr error_type error_badbrace = unspecified ;
   >         static constexpr error_type error_range = unspecified ;
   >         static constexpr error_type error_space = unspecified ;
   >         static constexpr error_type error_badrepeat = unspecified ;
   >         static constexpr error_type error_complexity = unspecified ;
   >         static constexpr error_type error_stack = unspecified ;
   >       }
   >     }

### 2054(i). `time_point` constructors need to be `constexpr`

Section: 30.6 [time.point] Status: Resolved Submitter: Anthony Williams Opened: 2011-05-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

In 30.6 [time.point], `time_point::min()` and `time_point::max()` are listed as `constexpr`. However, `time_point` has no `constexpr` constructors, so is not a literal type, and so these functions cannot be `constexpr` without adding a `constexpr` constructor for implementation purposes.

Proposed resolution: Add `constexpr` to the constructors of `time_point`. The effects of the constructor template basically imply that the member function `time_since_epoch()` is intended to be `constexpr` as well.

[2012, Portland]

Resolved by adopting paper n3469.

Proposed resolution:

This wording is relative to the FDIS.

1. Alter the class template definition in 30.6 [time.point] as follows:

   >     template <class Clock, class Duration = typename Clock::duration>
   >     class time_point {
   >       […]
   >     public:
   >       // 20.11.6.1, construct:
   >       constexpr time_point(); // has value epoch
   >       constexpr explicit time_point(const duration& d); // same as time_point() + d
   >       template <class Duration2>
   >         constexpr time_point(const time_point<clock, Duration2>& t);

   >       // 20.11.6.2, observer:
   >       constexpr duration time_since_epoch() const;
   >       […]
   >     };

2. Alter the declarations in 30.6.2 [time.point.cons]:

   >     constexpr time_point();

   > > -1- Effects: Constructs an object of type `time_point`, initializing `d_` with `duration::zero()`. Such a `time_point` object represents the epoch.

   >     constexpr explicit time_point(const duration& d);

   > > -2- Effects: Constructs an object of type `time_point`, initializing `d_` with `d`. Such a `time_point` object represents the epoch `+ d`.

   >     template <class Duration2>
   >       constexpr time_point(const time_point<clock, Duration2>& t);

   > > -3- Remarks: This constructor shall not participate in overload resolution unless `Duration2` is implicitly convertible to `duration`.

   > > -4- Effects: Constructs an object of type `time_point`, initializing `d_` with `t.time_since_epoch()`.

3. Alter the declaration in 30.6.3 [time.point.observer]:

   >     constexpr duration time_since_epoch() const;

   > > -1- Returns: d_.

### 2055(i). `std::move` in `std::accumulate` and other algorithms

Section: 26.10 [numeric.ops] Status: Resolved Submitter: Chris Jefferson Opened: 2011-01-01 Last modified: 2020-09-06

Priority: 3

View all other issues in [numeric.ops].

View all issues with Resolved status.

Discussion:

The C++0x draft says `std::accumulate` uses: `acc = binary_op(acc, *i)`.

Eelis van der Weegen has pointed out, on the libstdc++ mailing list, that using `acc = binary_op(std::move(acc), *i)` can lead to massive improvements (particularly, it means accumulating strings is linear rather than quadratic).

Consider the simple case, accumulating a bunch of strings of length 1 (the same argument holds for other length buffers). For strings `s` and `t`, `s+t` takes time `length(s)+length(t)`, as you have to copy both `s` and `t` into a new buffer.

So in accumulating `n` strings, step `i` adds a string of length `i-1` to a string of length 1, so takes time `i`.

Therefore the total time taken is: `1+2+3+...+n` = O(`n2`)

`std::move(s)+t`, for a "good" implementation, is amortized time `length(t)`, like `vector`, just copy `t` onto the end of the buffer. So the total time taken is:

`1+1+1+...+1` (`n` times) = O(`n`). This is the same as `push_back` on a `vector`.

I'm trying to decide if this implementation might already be allowed. I suspect it might not be (although I can't imagine any sensible code it would break). There are other algorithms which could benefit similarly (`inner_product`, `partial_sum` and `adjacent_difference` are the most obvious).

Is there any general wording for "you can use rvalues of temporaries"?

The reflector discussion starting with message c++std-lib-29763 came to the conclusion that above example is not covered by the "as-if" rules and that enabling this behaviour would seem quite useful.

[ 2011 Bloomington ]

Moved to NAD Future. This would be a larger change than we would consider for a simple TC.

[2017-02 in Kona, LEWG responds]

Like the goal.

What is broken by adding std::move() on the non-binary-op version?

A different overload might be selected, and that would be a breakage. Is it breakage that we should care about?

We need to encourage value semantics.

Need a paper. What guidance do we give?

Use std::reduce() (uses generalized sum) instead of accumulate which doesn’t suffer it.

Inner_product and adjacent_difference also. adjacent_difference solves it half-way for “val” object, but misses the opportunity for passing acc as `std::move(acc)`.

[2017-06-02 Issues Telecon]

Ville to encourage Eelis to write a paper on the algorithms in <numeric>, not just for accumulate.

Howard pointed out that this has already been done for the algorithms in <algorithm>

Status to Open; Priority 3

[2017-11-11, Albuquerque]

This was resolved by the adoption of P0616r0.

Proposed resolution:

### 2056(i). `future_errc` enums start with value 0 (invalid value for `broken_promise`)

Section: 32.10.1 [futures.overview] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-05-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.overview].

View all issues with C++14 status.

Discussion:

In 32.10.1 [futures.overview] `enum class future_errc` is defined as follows:

>     enum class future_errc {
>       broken_promise,
>       future_already_retrieved,
>       promise_already_satisfied,
>       no_state
>     };

With this declaration `broken_promise` has value 0, which means that for a `future_error f` with this code

>     f.code().operator bool()

yields false, which makes no sense. 0 has to be reserved for "no error". So, the enums defined here have to start with 1.

Howard, Anthony, and Jonathan have no objections.

[Discussion in Bloomington 2011-08-16]

Previous resolution:

> This wording is relative to the FDIS.

> 1. In 32.10.1 [futures.overview], header `<future>` synopsis, fix the declaration of `future_errc` as follows:

>    >     namespace std {
>    >       enum class future_errc {
>    >         broken_promise,
>    >         future_already_retrieved = 1,
>    >         promise_already_satisfied,
>    >         no_state,
>    >         broken_promise
>    >       };
>    >       […]
>    >     }

Is this resolution overspecified? These seem to be all implementation-defined. How do users add new values and not conflict with established error codes?

PJP proxy says: over-specified. boo.

Other error codes: look for `is_error_code_enum` specializations. Only one exists `io_errc`

Peter: I don't see any other parts of the standard that specify error codes where we have to do something similar.

Suggest that for every place where we add an error code, the following:

1. no zero values
2. all implementation defined values, so future_already_retrieved = implementation_defined
3. values are distinct

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

In 32.10.1 [futures.overview], header `<future>` synopsis, fix the declaration of `future_errc` as follows:

>     namespace std {
>       enum class future_errc {
>         broken_promise = implementation defined,
>         future_already_retrieved = implementation defined,
>         promise_already_satisfied = implementation defined,
>         no_state = implementation defined
>       };
>       […]
>     }

In 32.10.1 [futures.overview], header `<future>` synopsis, add a paragraph after paragraph 2 as follows:

The enum values of `future_errc` are distinct and not zero.

### 2057(i). `time_point + duration` semantics should be made `constexpr` conforming

Section: 30.6.6 [time.point.nonmember] Status: Resolved Submitter: Daniel Krügler Opened: 2011-05-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.point.nonmember].

View all issues with Resolved status.

Discussion:

It has been observed by LWG 2054(i) that the specification of some `time_point` member functions already imply that `time_point` needs to be a literal type and suggests to specify the constructors and the member function `time_since_epoch()` as `constexpr` functions at the minimum necessary. Adding further `constexpr` specifier to other operations should clearly be allowed and should probably be done as well. But to allow for further `constexpr` functions in the future requires that their semantics is compatible to operations allowed in `constexpr` functions. This is already fine for all operations, except this binary plus operator:

>     template <class Clock, class Duration1, class Rep2, class Period2>
>     time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type>
>     operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);

> > -1- Returns: `CT(lhs) += rhs`, where `CT` is the type of the return value.

for similar reasons as those mentioned in 2020(i). The semantics should be fixed to allow for making them `constexpr`. This issue should also be considered as a placeholder for a request to make the remaining `time_point` operations similarly `constexpr` as had been done for `duration`.

[2012, Portland]

Resolved by adopting paper n3469.

Proposed resolution:

This wording is relative to the FDIS.

1. In 30.6.6 [time.point.nonmember], p.1 change the Returns element semantics as indicated:

   >     template <class Clock, class Duration1, class Rep2, class Period2>
   >     time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type>
   >     operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);

   > > -1- Returns: `CT(lhs) += rhsCT(lhs.time_since_epoch() + rhs)`, where `CT` is the type of the return value.

### 2058(i). `valarray` and `begin/end`

Section: 29.6 [numarray] Status: C++14 Submitter: Gabriel Dos Reis Opened: 2011-05-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [numarray].

View all issues with C++14 status.

Discussion:

It was just brought to my attention that the pair of functions `begin/end` were added to `valarray` component. Those additions strike me as counter to the long standing agreement that `valarray<T>` is not yet another container. Valarray values are in general supposed to be treated as a whole, and as such has a loose specification allowing expression template techniques.

The addition of these functions sound to me as making it much harder (or close to impossible) to effectively use expression templates as implementation techniques, for no clear benefits.

My recommendation would be to drop `begin/end` - or at least for the `const valarray<T>&` version. I strongly believe those are defects.

[This issue was discussed on the library reflector starting from c++std-lib-30761. Some of the key conclusions of this discussion were:]

1. The `begin/end` members were added to allow `valarray` to participate in the new range-based for-loop by n2930 and not to make them container-like.
2. It is currently underspecified when the iterator values returned from `begin/end` become invalidated. To fix this, these invalidation rules need at least to reflect the invalidation rules of the references returned by the `operator[]` overloads of `valarray` (29.6.2.4 [valarray.access]).
3. A further problem is that the requirements expressed in 29.6.1 [valarray.syn] p.3-5 enforce an implementation to provide further overloads of `begin/end`, if the replacement type technique is used (which was clearly part of the design of `valarray`). Providing such additional overloads would also lead to life-time problems in examples like `begin(x + y)` where `x` and `y` are expressions involving `valarray` objects. To fix this, the `begin/end` overloads could be explicitly excluded from the general statements of 29.6.1 [valarray.syn] p.3-5. This would make it unspecified whether the expression `begin(x + y)` would be well-formed, portable code would need to write this as `begin(std::valarray<T>(x + y))`.

[ 2011 Bloomington ]

The intent of these overloads is entirely to support the new for syntax, and not to create new containers.

Stefanus provides suggested wording.

[2012, Kona]

Moved to Tenatively Ready by post-meeting issues processing group, after confirmation from Gaby.

[2012, Portland: applied to WP]

Proposed resolution:

In 29.6.1 [valarray.syn]/4, make the following insertion:

4 Implementations introducing such replacement types shall provide additional functions and operators as follows:

- for every function taking a `const valarray<T>&` other than `begin` and `end` (29.6.10 [valarray.range]), identical functions taking the replacement types shall be added;
- for every function taking two `const valarray<T>&` arguments, identical functions taking every combination of `const valarray<T>&` and replacement types shall be added.

In 29.6.10 [valarray.range], make the following insertion:

1 In the `begin` and `end` function templates that follow, unspecified1 is a type that meets the requirements of a mutable random access iterator (24.2.7) whose `value_type` is the template parameter `T` and whose `reference` type is `T&`. unspecified2 is a type that meets the requirements of a constant random access iterator (24.2.7) whose `value_type` is the template parameter `T` and whose `reference` type is `const T&`.

2 The iterators returned by `begin` and `end` for an array are guaranteed to be valid until the member function `resize(size_t, T)` (29.6.2.8 [valarray.members]) is called for that array or until the lifetime of that array ends, whichever happens first.

### 2059(i). C++0x ambiguity problem with `map::erase`

Section: 23.4.3 [map] Status: C++17 Submitter: Christopher Jefferson Opened: 2011-05-18 Last modified: 2017-07-30

Priority: 3

View all other issues in [map].

View all issues with C++17 status.

Discussion:

`map::erase` (and several related methods) took an iterator in C++03, but take a `const_iterator` in C++0x. This breaks code where the map's `key_type` has a constructor which accepts an iterator (for example a template constructor), as the compiler cannot choose between `erase(const key_type&)` and `erase(const_iterator)`.

>     #include <map>

>     struct X
>     {
>       template<typename T>
>       X(T&) {}
>     };

>     bool operator<(const X&, const X&) { return false; }

>     void erasor(std::map<X,int>& s, X x)
>     {
>       std::map<X,int>::iterator it = s.find(x);
>       if (it != s.end())
>         s.erase(it);
>     }

[ 2011 Bloomington ]

This issue affects only associative container `erase` calls, and is not more general, as these are the only functions that are also overloaded on another single arguement that might cause confusion - the `erase` by key method. The complete resolution should simply restore the `iterator` overload in addition to the `const_iterator` overload for all eight associative containers.

Proposed wording supplied by Alan Talbot, and moved to Review.

[2012, Kona]

Moved back to Open by post-meeting issues processing group.

Pablo very unhappy about case of breaking code with ambiguous conversion between both iterator types.

Alisdair strongly in favor of proposed resolution, this change from C++11 bit Chris in real code, and it took a while to track down the cause.

Move to open, bring in front of a larger group

Proposed wording from Jeremiah: `erase(key)` shall not participate in overload resolution if `iterator` is convertible to `key`. Note that this means making `erase(key)` a template-method

Poll Chris to find out if he already fixed his code, or fixed his library

Jeremiah - allow both overloads, but `enable_if` the `const_iterator` form as a template, requiring `is_same` to match only `const_iterator`.

Poll PJ to see if he has already applied this fix?

[2015-02 Cologne]

AM: To summarize, we changed a signature and code broke. At what point do we stop and accept breakage in increasingly obscure code? VV: libc++ is still broken, but libstdc++ works, so they've fixed this — perhaps using this PR? [Checks] Yes, libstdc++ uses this solution, and has a comment pointing to LWG 2059. AM: This issue hasn't been looked at since Kona. In any case, we already have implementation experience now.

AM: I'd say let's ship it. We already have implementation experience (libstdc++ and MSVS). MC: And "tentatively ready" lets me try to implement this and see how it works.

Proposed resolution:

Editorial note: The following things are different between 23.2.7 [associative.reqmts] p.8 and 23.2.8 [unord.req] p.10. These should probably be reconciled.

> 1. First uses the convention "denotes"; second uses the convention "is".
> 2. First redundantly says: "If no such element exists, returns a.end()." in erase table entry, second does not.

23.2.7 [associative.reqmts] Associative containers

8 In Table 102, `X` denotes an associative container class, `a` denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `u` denotes an identifier, `i` and `j` satisfy input iterator requirements and refer to elements implicitly convertible to `value_type`, `[i,j)` denotes a valid range, `p` denotes a valid const iterator to `a`, `q` denotes a valid dereferenceable const iterator to `a`, `r` denotes a valid dereferenceable iterator to a, `[q1, q2)` denotes a valid range of const iterators in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` denotes a value of `X::value_type`, `k` denotes a value of `X::key_type` and `c` denotes a value of type `X::key_compare`. `A` denotes the storage allocator used by `X`, if any, or `std::allocator<X::value_type>` otherwise, and `m` denotes an allocator of a type convertible to `A`.

23.2.7 [associative.reqmts] Associative containers Table 102

Add row:

`a.erase(r)` | `iterator` | erases the element pointed to by `r`. Returns an iterator pointing to the element immediately following `r` prior to the element being erased. If no such element exists, returns `a.end()`. | amortized constant

23.2.8 [unord.req] Unordered associative containers

10 In table 103: `X` is an unordered associative container class, `a` is an object of type `X`, `b` is a possibly const object of type `X`, `a_uniq` is an object of type `X` when `X` supports unique keys, `a_eq` is an object of type `X` when `X` supports equivalent keys, `i` and `j` are input iterators that refer to `value_type`, `[i, j)` is a valid range, `p` and `q2` are valid const iterators to `a`, `q` and `q1` are valid dereferenceable const iterators to `a`, `r` is a valid dereferenceable iterator to a, `[q1,q2)` is a valid range in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` is a value of type `X::value_type`, `k` is a value of type `key_type`, `hf` is a possibly const value of type `hasher`, `eq` is a possibly const value of type `key_equal`, `n` is a value of type `size_type`, and `z` is a value of type `float`.

23.2.8 [unord.req] Unordered associative containers Table 103

Add row:

`a.erase(r)` | `iterator` | Erases the element pointed to by `r`. Returns the iterator immediately following `r` prior to the erasure. | Average case O(1), worst case O(`a.size()`).

23.4.3.1 [map.overview] Class template map overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.4.4.1 [multimap.overview] Class template multimap overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.4.6.1 [set.overview] Class template set overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.4.7.1 [multiset.overview] Class template multiset overview

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.5.3.1 [unord.map.overview] Class template unordered_map overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.5.4.1 [unord.multimap.overview] Class template unordered_multimap overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.5.6.1 [unord.set.overview] Class template unordered_set overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.5.7.1 [unord.multiset.overview] Class template unordered_multiset overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

C.6.12 [diff.cpp03.containers] C.2.12 Clause 23: containers library

23.2.3, 23.2.4

Change: Signature changes: from iterator to const_iterator parameters

Rationale: Overspecification. Effects: The signatures of the following member functions changed from taking an iterator to taking a const_iterator:

- insert(iter, val) for vector, deque, list, set, multiset, map, multimap
- insert(pos, beg, end) for vector, deque, list, forward_list
- erase(iter) for set, multiset, map, multimap
- erase(begin, end) for set, multiset, map, multimap
- all forms of list::splice
- all forms of list::merge

Valid C++ 2003 code that uses these functions may fail to compile with this International Standard.

### 2061(i). `make_move_iterator` and arrays

Section: 24.2 [iterator.synopsis], 24.5.4 [move.iterators] Status: C++14 Submitter: Marc Glisse Opened: 2011-05-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iterator.synopsis].

View all issues with C++14 status.

Discussion:

The standard library always passes template iterators by value and never by reference, which has the nice effect that an array decays to a pointer. There is one exception: `make_move_iterator`.

>     #include <iterator>
>     int main(){
>       int a[]={1,2,3,4};
>       std::make_move_iterator(a+4);
>       std::make_move_iterator(a); // fails here
>     }

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Modify the header `<iterator>` synopsis in 24.2 [iterator.synopsis]:

   >     namespace std {
   >       […]
   >       template <class Iterator>
   >       move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i);

   >       […]
   >     }

2. Modify the class template `move_iterator` synopsis in 24.5.4.2 [move.iterator]:

   >     namespace std {
   >       […]
   >       template <class Iterator>
   >       move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i);
   >     }

3. Modify 24.5.4.9 [move.iter.nonmember]:

   >     template <class Iterator>
   >     move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i);

   > > -3- Returns: `move_iterator<Iterator>(i)`.

### 2062(i). Effect contradictions w/o no-throw guarantee of `std::function` swaps

Section: 22.10.17.3 [func.wrap.func], 22.10.17.3.3 [func.wrap.func.mod] Status: C++17 Submitter: Daniel Krügler Opened: 2011-05-28 Last modified: 2020-09-06

Priority: 2

View all other issues in [func.wrap.func].

View all issues with C++17 status.

Discussion:

Howard Hinnant observed in reflector message c++std-lib-30841 that 22.10.17.3 [func.wrap.func] makes the member swap `noexcept`, even though the non-member swap is not `noexcept`.

The latter was an outcome of the discussions during the Batavia meeting and the Madrid meeting involving LWG 1349(i), which seems to indicate that the remaining `noexcept` specifier at the member swap is incorrect and should be removed.

But if we allow for a potentially throwing member swap of `std::function`, this causes another conflict with the exception specification for the following member function:

>     template<class F> function& operator=(reference_wrapper<F> f) noexcept;

> > Effects: `function(f).swap(*this);`

Note that in this example the sub-expression `function(f)` does not cause any problems, because of the nothrow-guarantee given in 22.10.17.3.2 [func.wrap.func.con] p. 10. The problem is located in the usage of the swap which could potentially throw given the general latitude.

So, either the Madrid meeting decision need to be revised (and both member and free swap of `std::function` should be noexcept), or this function needs to be adapted as well, e.g. by taking the exception-specification away or by changing the semantics.

One argument for "swap-may-throw" would be to allow for small-object optimization techniques where the copy of the target may throw. But given the fact that the swap function has been guaranteed to be "Throws: Nothing" from TR1 on, it seems to me that that there would still be opportunities to perform small-object optimizations just restricted to the set of target copies that cannot throw.

In my opinion member swap of `std::function` has always been intended to be no-throw, because otherwise there would be no good technical reason to specify the effects of several member functions in terms of the "construct-swap" idiom (There are three functions that are defined this way), which provides the strong exception safety in this case. I suggest to enforce that both member swap and non-member swap of `std::function` are nothrow functions as it had been guaranteed since TR1 on.

[ 2011 Bloomington ]

Dietmar: May not be swappable in the first place.

Alisdair: This is wide contact. Then we should be taking noexcept off instead of putting it on. This is preferred resolution.

Pablo: This is bigger issue. Specification of assignment in terms of swap is suspect to begin with. It is over specification. How this was applied to string is a better example to work from.

Pablo: Two problems: inconsistency that should be fixed (neither should have noexcept), the other issues is that assignment should not be specified in terms of swap. There are cases where assignment should succeed where swap would fail. This is easier with string as it should follow container rules.

Action Item (Alisdair): There are a few more issues found to file.

Dave: This is because of allocators? The allocator makes this not work.

Howard: There is a type erased allocator in shared_ptr. There is a noexcept allocator in shared_ptr.

Pablo: shared_ptr is a different case. There are shared semantics and the allocator does move around. A function does not have shared semantics.

Alisdair: Function objects think they have unique ownership.

Howard: In function we specify semantics with copy construction and swap.

Action Item (Pablo): Write this up better (why assignment should not be defined in terms of swap)

Howard: Not having trouble making function constructor no throw.

Dietmar: Function must allocate memory.

Howard: Does not put stuff that will throw on copy or swap in small object optimization. Put those on heap. Storing allocator, but has to be no throw copy constructable.

Pablo: Are you allowed to or required to swap or move allocators in case or swap or move.

Dave: An allocator that is type erased should be different...

Pablo: it is

Dave: Do you need to know something about allocator types? But only at construction time.

Pablo: You could have allocators that are different types.

Dave: Swap is two ended operation.

Pablo: Opinion is that both have to say propagate on swap for them to swap.

John: It is not arbitrary. If one person says no. No is no.

Howard: Find noexcept swap to be very useful. Would like to move in that direction and bring container design along.

Dave: If you have something were allocator must not propagate you can detect that at construction time.

...

Pablo: Need to leave this open and discuss in smaller group.

Alisdair: Tried to add boost::any as TR2 proposal and ran into this issue. Only the first place where we run into issues with type erased allocators. Suggest we move it to open.

Action Item: Move to open.

Action Item (Pablo works with Howard and Daniel): Address the more fundamental issue (which may be multiple issues) and write up findings.

Previous resolution [SUPERSEDED]:

> This wording is relative to the FDIS.

> 1. Modify the header `<functional>` synopsis in 22.10 [function.objects] as indicated:

>    >     namespace std {
>    >       […]

>    >       template<class R, class... ArgTypes>
>    >       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;

>    >       […]
>    >     }

> 2. Modify the class template `function` synopsis in 22.10.17.3 [func.wrap.func] as indicated:

>    >     namespace std {
>    >       […]

>    >       // [func.wrap.func.alg], specialized algorithms:
>    >       template<class R, class... ArgTypes>
>    >       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;

>    >       […]
>    >     }

> 3. Modify 22.10.17.3.8 [func.wrap.func.alg] as indicated:

>    >     template<class R, class... ArgTypes>
>    >     void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;

>    > > -1- Effects: `f1.swap(f2);`

[2014-02-28 (Post Issaquah), Pablo provides more information]

For cross-referencing purposes: The resolution of this issue should be harmonized with any resolution to LWG 2370(i), which addresses inappropriate `noexcept`s in some function constructors.

We have the following choices:

1. `swap()` does not throw

   > Discussion: This definition is desirable, and allows assignment to be implemented with the strong exception guarantee, but it does have consequences: The implementation cannot use the small-object optimization for a function-object `F` unless `F` is `NothrowMovable` (nothrow-swappable is unimportant because `F` is not swapped with another `F`). Note that many functors written before C++11 will not have move constructors decorated with `noexcept`, so this limitation could affect a lot of code.

   > It is not clear what other implementation restrictions might be needed. Allocators are required not to throw on move or copy. Is that sufficient?

2. `swap()` can throw

   > Discussion: This definition gives maximum latitude to implementation to use small-object optimization. However, the strong guarantee on assignment is difficult to achieve. Should we consider giving up on the strong guarantee? How much are we willing to pessimize code for exceptions?

3. `swap()` will not throw if both functions have `NoThrowMoveable` functors

   > Discussion: This definition is similar to option 2, but gives slightly stronger guarantees. Here, `swap()` can throw, but the programmer can theoretically prevent that from happening. This should be straight-forward to implement and gives the implementation a lot of latitude for optimization. However, because this is a dynamic decision, the program is not as easy to reason about. Also, the strong guarantee for assignment is compromized as in option 2.

[2016-08-02, Ville, Billy, and Billy comment and reinstantiate the original P/R]

We (Ville, Billy, and Billy) propose to require that `function`'s `swap` is `noexcept` in all cases.

- libstdc++ does not throw in their `swap`. It is not `noexcept` today, but the small functor optimization only engages for trivially copyable types.

- msvc++ checks `is_nothrow_move_constructible` before engaging the small functor optimization and marks its `swap` `noexcept`

- libc++ marks `swap` `noexcept` (though I have not looked at its implementation)

Moreover, many of the concerns that were raised by providing this guarantee are no longer applicable now that P0302 has been accepted, which removes allocator support from `std::function`.

Therefore we are re-proposing the original resolution above.

[2016-08 Chicago]

Tues PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify the header `<functional>` synopsis in 22.10 [function.objects] as indicated:

   >     namespace std {
   >       […]

   >       template<class R, class... ArgTypes>
   >       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;

   >       […]
   >     }

2. Modify the class template `function` synopsis in 22.10.17.3 [func.wrap.func] as indicated:

   >     namespace std {
   >       […]

   >       // [func.wrap.func.alg], specialized algorithms:
   >       template<class R, class... ArgTypes>
   >       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;

   >       […]
   >     }

3. Modify 22.10.17.3.8 [func.wrap.func.alg] as indicated:

   >     template<class R, class... ArgTypes>
   >     void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;

   > > -1- Effects: As if by: `f1.swap(f2);`

### 2063(i). Contradictory requirements for string move assignment

Section: 27.4.3 [basic.string] Status: C++17 Submitter: Howard Hinnant Opened: 2011-05-29 Last modified: 2017-07-30

Priority: 3

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Discussion:

27.4.3.2 [string.require]/p4 says that `basic_string` is an "allocator-aware" container and behaves as described in 23.2.2 [container.requirements.general].

23.2.2 [container.requirements.general] describes move assignment in p7 and Table 99.

If `allocator_traits<allocator_type>::propagate_on_container_move_assignment::value` is false, and if the allocators stored in the lhs and rhs sides are not equal, then move assigning a string has the same semantics as copy assigning a string as far as resources are concerned (resources can not be transferred). And in this event, the lhs may have to acquire resources to gain sufficient capacity to store a copy of the rhs.

However 27.4.3.3 [string.cons]/p22 says:

>     basic_string<charT,traits,Allocator>&
>     operator=(basic_string<charT,traits,Allocator>&& str) noexcept;

> > Effects: If `*this` and `str` are not the same object, modifies `*this` as shown in Table 71. [Note: A valid implementation is `swap(str)`. — end note ]

These two specifications for `basic_string::operator=(basic_string&&)` are in conflict with each other. It is not possible to implement a `basic_string` which satisfies both requirements.

Additionally assign from an rvalue `basic_string` is defined as:

>     basic_string& assign(basic_string&& str) noexcept;

> > Effects: The function replaces the string controlled by `*this` with a string of length `str.size()` whose elements are a copy of the string controlled by `str`. [ Note: A valid implementation is `swap(str)`. — end note ]

It seems contradictory that this member can be sensitive to `propagate_on_container_swap` instead of `propagate_on_container_move_assignment`. Indeed, there is a very subtle chance for undefined behavior here: If the implementation implements this in terms of `swap`, and if `propagate_on_container_swap` is false, and if the two allocators are unequal, the behavior is undefined, and will likely lead to memory corruption. That's a lot to go wrong under a member named "assign".

[ 2011 Bloomington ]

Alisdair: Can this be conditional `noexcept`?

Pablo: We said we were not going to put in many conditional `noexcept`s. Problem is not allocator, but non-normative definition. It says swap is a valid operation which it is not.

Dave: Move assignment is not a critical method.

Alisdair: Was confusing assignment and construction.

Dave: Move construction is critical for efficiency.

Kyle: Is it possible to test for `noexcept`.

Alisdair: Yes, query the `noexcept` operator.

Alisdair: Agreed there is a problem that we cannot unconditionally mark these operations as `noexcept`.

Pablo: How come swap is not defined in alloc

Alisdair: It is in utility.

Pablo: Swap has a conditional `noexcept`. Is no throw move constructable, is no throw move assignable.

Pablo: Not critical for strings or containers.

Kyle: Why?

Pablo: They do not use the default swap.

Dave: Important for deduction in other types.

Alisdair: Would change the policy we adopted during FDIS mode.

Pablo: Keep it simple and get some vendor experience.

Alisdair: Is this wording correct? Concerned with bullet 2.

Pablo: Where does it reference containers section.

Alisdair: String is a container.

Alisdair: We should not remove redundancy piecemeal.

Pablo: I agree. This is a deviation from rest of string. Missing forward reference to containers section.

Pablo: To fix section 2. Only the note needs to be removed. The rest needs to be a forward reference to containers.

Alisdair: That is a new issue.

Pablo: Not really. Talking about adding one sentence, saying that basic string is a container.

Dave: That is not just a forward reference, it is a semantic change.

PJ: We intended to make it look like a container, but it did not satisfy all the requirements.

Pablo: Clause 1 is correct. Clause 2 is removing note and `noexcept` (do not remove the rest). Clause 3 is correct.

Alisdair: Not sure data() is correct (in clause 2).

Conclusion: Move to open, Alisdair and Pablo volunteered to provide wording

[ originally proposed wording: ]

This wording is relative to the FDIS.

1. Modify the class template `basic_string` synopsis in 27.4.3 [basic.string]:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         basic_string& operator=(basic_string&& str) noexcept;
   >         […]
   >         basic_string& assign(basic_string&& str) noexcept;
   >         […]
   >       };
   >     }

2. Remove the definition of the `basic_string` move assignment operator from 27.4.3.3 [string.cons] entirely, including Table 71 — `operator=(const basic_string<charT, traits, Allocator>&&)`. This is consistent with how we define move assignment for the containers in Clause 23:

   >     basic_string<charT,traits,Allocator>&
   >     operator=(basic_string<charT,traits,Allocator>&& str) noexcept;

   > > -22- Effects: If `*this` and `str` are not the same object, modifies `*this` as shown in Table 71. [ Note: A valid implementation is `swap(str)`. — end note ]

   > > -23- If `*this` and `str` are the same object, the member has no effect.

   > > -24- Returns: `*this`

   > Table 71 — `operator=(const basic_string<charT, traits, Allocator>&&)`
   > Element | Value
   > --- | ---
   > `data()` | points at the array whose first element was pointed at by `str.data()`
   > `size()` | previous value of `str.size()`
   > `capacity()` | a value at least as large as `size()`

3. Modify the paragraphs prior to 27.4.3.7.3 [string.assign] p.3 as indicated (The first insertion recommends a separate paragraph number for the indicated paragraph):

   >     basic_string& assign(basic_string&& str) noexcept;

   > > -?- Effects: Equivalent to `*this = std::move(str)`. The function replaces the string controlled by `*this` with a string of length `str.size()` whose elements are a copy of the string controlled by `str`. [ Note: A valid implementation is `swap(str)`. — end note ]

   > > -3- Returns: `*this`

[ 2012-08-11 Joe Gottman observes: ]

> One of the effects of `basic_string`'s move-assignment operator (27.4.3.3 [string.cons], Table 71) is

> > Element | Value
> > --- | ---
> > `data()` | points at the array whose first element was pointed at by `str.data()`

> If a string implementation uses the small-string optimization and the input string `str` is small enough to make use of it, this effect is impossible to achieve. To use the small string optimization, a string has to be implemented using something like

> >     union
> >     {
> >        char buffer[SMALL_STRING_SIZE];
> >        char *pdata;
> >     };

> When the string is small enough to fit inside `buffer`, the `data()` member function returns `static_cast<const char *>(buffer)`, and since `buffer` is an array variable, there is no way to implement move so that the moved-to string's `buffer` member variable is equal to `this->buffer`.

> Resolution proposal:

> Change Table 71 to read:

> > Element | Value
> > --- | ---
> > `data()` | points at the array whose first element was pointed at by `str.data()` that contains the same characters in the same order as `str.data()` contained before `operator=()` was called

[2015-05-07, Lenexa]

Howard suggests improved wording

Move to Immediate

Proposed resolution:

This wording is relative to N4431.

1. Modify the class template `basic_string` synopsis in 27.4.3 [basic.string]:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         basic_string& assign(basic_string&& str) noexcept(
   >              allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);
   >         […]
   >       };
   >     }

2. Change 27.4.3.3 [string.cons]/p21-23:

   >     basic_string&
   >     operator=(basic_string&& str) noexcept(
   >              allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);

   > > -21- Effects: If `*this` and `str` are not the same object, modifies `*this` as shown in Table 71. [ Note: A valid implementation is `swap(str)`. — end note ] Move assigns as a sequence container ([container.requirements]), except that iterators, pointers and references may be invalidated.

   > > -22- If `*this` and `str` are the same object, the member has no effect.

   > > -23- Returns: `*this`

   > Table 71 — `operator=(basic_string&&)` effects
   > Element | Value
   > --- | ---
   > `data()` | points at the array whose first element was pointed at by `str.data()`
   > `size()` | previous value of `str.size()`
   > `capacity()` | a value at least as large as `size()`

3. Modify the paragraphs prior to 27.4.3.7.3 [string.assign] p.3 as indicated

   >     basic_string& assign(basic_string&& str) noexcept(
   >              allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);

   > > -3- Effects: Equivalent to `*this = std::move(str)`. The function replaces the string controlled by `*this` with a string of length `str.size()` whose elements are a copy of the string controlled by `str`. [ Note: A valid implementation is `swap(str)`. — end note ]

   > > -4- Returns: `*this`

### 2064(i). More `noexcept` issues in `basic_string`

Section: 27.4.3 [basic.string] Status: C++14 Submitter: Howard Hinnant Opened: 2011-05-29 Last modified: 2016-11-12

Priority: Not Prioritized

View other active issues in [basic.string].

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Discussion:

The following inconsistencies regarding `noexcept` for `basic_string` are noted.

Member swap is not marked `noexcept`:

>     void swap(basic_string& str);

But the global swap is marked `noexcept`:

>     template<class charT, class traits, class Allocator>
>     void swap(basic_string<charT,traits,Allocator>& lhs,
>               basic_string<charT,traits,Allocator>& rhs) noexcept;

But only in the definition, not in the synopsis.

All comparison operators are marked `noexcept` in their definitions, but not in the synopsis.

The compare function that takes a pointer:

>     int compare(const charT *s) const;

is not marked `noexcept`. But some of the comparison functions which are marked `noexcept` (only in their definition) are specified to call the throwing compare operator:

>     template<class charT, class traits, class Allocator>
>     bool operator==(const basic_string<charT,traits,Allocator>& lhs,
>                     const charT* rhs) noexcept;

> > Returns: `lhs.compare(rhs) == 0`.

All functions with a narrow contract should not be declared as `noexcept` according to the guidelines presented in n3279. Among these narrow contract functions are the `swap` functions (23.2.2 [container.requirements.general] p. 8) and functions with non-`NULL` `const charT*` parameters.

[2011-06-08 Daniel provides wording]

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS. Both move-assignment operator and the moving `assign` function are not touched by this issue, because they are handled separately by issue 2063(i).

1. Modify the header `<string>` synopsis in 27.4 [string.classes] as indicated (Rationale: Adding `noexcept` to these specific overloads is in sync with applying the same rule to specific overloads of the member functions `find`, `compare`, etc. This approach deviates from that taken in n3279, but seems more consistent given similar application for comparable member functions):

   >     #include <initializer_list>

   >     namespace std {

   >       […]
   >       template<class charT, class traits, class Allocator>
   >         bool operator==(const basic_string<charT,traits,Allocator>& lhs,
   >                         const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]
   >       template<class charT, class traits, class Allocator>
   >         bool operator!=(const basic_string<charT,traits,Allocator>& lhs,
   >                         const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]

   >       template<class charT, class traits, class Allocator>
   >         bool operator<(const basic_string<charT,traits,Allocator>& lhs,
   >                        const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]
   >       template<class charT, class traits, class Allocator>
   >         bool operator>(const basic_string<charT,traits,Allocator>& lhs,
   >                        const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]

   >       template<class charT, class traits, class Allocator>
   >         bool operator<=(const basic_string<charT,traits,Allocator>& lhs,
   >                         const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]
   >       template<class charT, class traits, class Allocator>
   >         bool operator>=(const basic_string<charT,traits,Allocator>& lhs,
   >                         const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]
   >     }

2. Modify the class template `basic_string` synopsis in 27.4.3 [basic.string] as indicated (Remark 1: The `noexcept` at the move-constructor is fine, because even for a small-object optimization there is no problem here, because `basic_string::value_type` is required to be a non-array POD as of 27.1 [strings.general] p1, Remark 2: This proposal removes the `noexcept` at single character overloads of `find`, `rfind`, etc. because they are defined in terms of potentially allocating functions. It seems like an additional issue to me to change the semantics in terms of non-allocating functions and adding `noexcept` instead):

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         // [string.ops], string operations:
   >         […]
   >         size_type find (charT c, size_type pos = 0) const noexcept;
   >         […]
   >         size_type rfind(charT c, size_type pos = npos) const noexcept;
   >         […]
   >         size_type find_first_of(charT c, size_type pos = 0) const noexcept;
   >         […]
   >         size_type find_last_of (charT c, size_type pos = npos) const noexcept;
   >         […]
   >         size_type find_first_not_of(charT c, size_type pos = 0) const noexcept;
   >         […]
   >         size_type find_last_not_of (charT c, size_type pos = npos) const noexcept;
   >         […]
   >       };
   >     }

3. Modify 27.4.3.8.2 [string.find] before p5 and before p7 as indicated:

   >     size_type find(const charT* s, size_type pos = 0) const noexcept;
   >     […]
   >     size_type find(charT c, size_type pos = 0) const noexcept;

   > > -7- Returns: `find(basic_string<charT,traits,Allocator>(1,c), pos)`.

4. Modify [string.rfind] before p7 as indicated:

   >     size_type rfind(charT c, size_type pos = npos) const noexcept;

   > > -7- Returns: `rfind(basic_string<charT,traits,Allocator>(1,c),pos)`.

5. Modify [string.find.first.of] before p7 as indicated:

   >     size_type find_first_of(charT c, size_type pos = 0) const noexcept;

   > > -7- Returns: `find_first_of(basic_string<charT,traits,Allocator>(1,c), pos)`.

6. Modify [string.find.last.of] before p7 as indicated:

   >     size_type find_last_of(charT c, size_type pos = npos) const noexcept;

   > > -7- Returns: `find_last_of(basic_string<charT,traits,Allocator>(1,c),pos)`.

7. Modify [string.find.first.not.of] before p7 as indicated:

   >     size_type find_first_not_of(charT c, size_type pos = 0) const noexcept;

   > > -7- Returns: `find_first_not_of(basic_string(1, c), pos)`.

8. Modify [string.find.last.not.of] before p7 as indicated:

   >     size_type find_last_not_of(charT c, size_type pos = npos) const noexcept;

   > > -7- Returns: `find_last_not_of(basic_string(1, c), pos)`.

9. Modify [string.operator==] before p2+p3 as indicated:

   >     template<class charT, class traits, class Allocator>
   >     bool operator==(const charT* lhs,
   >                     const basic_string<charT,traits,Allocator>& rhs) noexcept;

   >     […]

   >     template<class charT, class traits, class Allocator>
   >     bool operator==(const basic_string<charT,traits,Allocator>& lhs,
   >                     const charT* rhs) noexcept;

10. Modify [string.op!=] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator!=(const charT* lhs,
    >                     const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator!=(const basic_string<charT,traits,Allocator>& lhs,
    >                     const charT* rhs) noexcept;

11. Modify [string.op<] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator<(const charT* lhs,
    >                    const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator<(const basic_string<charT,traits,Allocator>& lhs,
    >                    const charT* rhs) noexcept;

12. Modify [string.op>] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator>(const charT* lhs,
    >                    const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator>(const basic_string<charT,traits,Allocator>& lhs,
    >                    const charT* rhs) noexcept;

13. Modify [string.op<=] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator<=(const charT* lhs,
    >                     const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator<=(const basic_string<charT,traits,Allocator>& lhs,
    >                     const charT* rhs) noexcept;

14. Modify [string.op>=] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator>=(const charT* lhs,
    >                     const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator>=(const basic_string<charT,traits,Allocator>& lhs,
    >                     const charT* rhs) noexcept;

15. Modify 27.4.4.3 [string.special] as indicated (Remark: The change of the semantics guarantees as of 16.3.2.4 [structure.specifications] p4 that the "Throws: Nothing" element of member swap is implied):

    >     template<class charT, class traits, class Allocator>
    >       void swap(basic_string<charT,traits,Allocator>& lhs,
    >         basic_string<charT,traits,Allocator>& rhs) noexcept;

    > > -1- Effects: Equivalent to `lhs.swap(rhs);`

### 2065(i). Minimal allocator interface

Section: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-06-06 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [allocator.requirements].

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Discussion:

The example in 16.4.4.6 [allocator.requirements] says `SimpleAllocator` satisfies the requirements of Table 28 — Allocator requirements, but it doesn't support comparison for equality/inequality.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

1. Modify the example in 16.4.4.6 [allocator.requirements] p5 as indicated:

   > -5- […]

   > [ Example: the following is an allocator class template supporting the minimal interface that satisfies the requirements of Table 28:

   > >     template <class Tp>
   > >     struct SimpleAllocator {
   > >       typedef Tp value_type;
   > >       SimpleAllocator(ctor args);
   > >       template <class T> SimpleAllocator(const SimpleAllocator<T>& other);
   > >       Tp *allocate(std::size_t n);
   > >       void deallocate(Tp *p, std::size_t n);
   > >     };

   > >     template <class T, class U>
   > >     bool operator==(const SimpleAllocator<T>&, const SimpleAllocator<U>&);
   > >     template <class T, class U>
   > >     bool operator!=(const SimpleAllocator<T>&, const SimpleAllocator<U>&);

   > — end example ]

### 2066(i). Missing specification of `vector::resize(size_type)`

Section: 23.3.13.3 [vector.capacity] Status: Resolved Submitter: Rani Sharoni Opened: 2011-03-29 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In C++1x (N3090) there are two version of `vector::resize` — 23.3.13.3 [vector.capacity]:

>     void resize(size_type sz);
>     void resize(size_type sz, const T& c);

The text in 23.3.13.3 [vector.capacity]/12 only mentions "no effects on throw" for the two args version of resize:

> Requires: If an exception is thrown other than by the move constructor of a non-`CopyConstructible` `T` there are no effects.

This seems like unintentional oversight since `resize(size)` is semantically the same as `resize(size, T())`. Additionally, the C++03 standard only specify single version of resize with default for the second argument - 23.2.4:

>     void resize(size_type sz, T c = T());

Therefore not requiring same guarantees for both version of resize is in fact a regression.

[2011-06-12: Daniel comments]

The proposed resolution for issue 2033(i) should solve this issue as well.

[ 2011 Bloomington ]

This issue will be resolved by issue 2033(i), and closed when this issue is applied.

[2012, Kona]

Resolved by adopting the resolution in issue 2033(i) at this meeting.

Proposed resolution:

Apply the proposed resolution of issue 2033(i)

### 2067(i). `packaged_task` should have deleted copy c'tor with const parameter

Section: 32.10.10 [futures.task] Status: C++14 Submitter: Daniel Krügler Opened: 2011-06-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.task].

View all issues with C++14 status.

Discussion:

Class template `packaged_task` is a move-only type with the following form of the deleted copy operations:

>     packaged_task(packaged_task&) = delete;
>     packaged_task& operator=(packaged_task&) = delete;

Note that the argument types are non-const. This does not look like a typo to me, this form seems to exist from the very first proposing paper on N2276. Using either of form of the copy-constructor did not make much difference before the introduction of defaulted special member functions, but it makes now an observable difference. This was brought to my attention by a question on a German C++ newsgroup where the question was raised why the following code does not compile on a recent gcc:

>     #include <utility>
>     #include <future>
>     #include <iostream>
>     #include <thread>

>     int main(){
>       std::packaged_task<void()> someTask([]{ std::cout << std::this_thread::get_id() << std::endl; });
>       std::thread someThread(std::move(someTask)); // Error here
>       // Remainder omitted
>     }

It turned out that the error was produced by the instantiation of some return type of `std::bind` which used a defaulted copy-constructor, which leads to a const declaration conflict with [class.copy] p8.

Some aspects of this problem are possibly core-language related, but I consider it more than a service to programmers, if the library would declare the usual form of the copy operations (i.e. those with const first parameter type) as deleted for `packaged_task` to prevent such problems.

A similar problem exists for class template `basic_ostream` in 31.7.6.2 [ostream]:

>     namespace std {
>       template <class charT, class traits = char_traits<charT> >
>       class basic_ostream : virtual public basic_ios<charT,traits> {
>         […]

>         // 27.7.3.3 Assign/swap
>         basic_ostream& operator=(basic_ostream& rhs) = delete;
>         basic_ostream& operator=(const basic_ostream&& rhs);
>         void swap(basic_ostream& rhs);
>     };

albeit this could be considered as an editorial swap of copy and move assignment operator, I suggest to fix this as part of this issue as well.

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Modify the class template `basic_ostream` synopsis in 31.7.6.2 [ostream] as indicated (Note: The prototype signature of the move assignment operator in 31.7.6.2.3 [ostream.assign] is fine):

   >     namespace std {
   >       template <class charT, class traits = char_traits<charT> >
   >       class basic_ostream : virtual public basic_ios<charT,traits> {
   >         […]

   >         // 27.7.3.3 Assign/swap
   >         basic_ostream& operator=(const basic_ostream& rhs) = delete;
   >         basic_ostream& operator=(const basic_ostream&& rhs);
   >         void swap(basic_ostream& rhs);
   >     };

2. Modify the class template `packaged_task` synopsis in 32.10.10 [futures.task] p2 as indicated:

   >     namespace std {
   >       template<class> class packaged_task; // undefined

   >       template<class R, class... ArgTypes>
   >       class packaged_task<R(ArgTypes...)> {
   >       public:
   >         […]

   >         // no copy
   >         packaged_task(const packaged_task&) = delete;
   >         packaged_task& operator=(const packaged_task&) = delete;

   >         […]
   >       };
   >       […]
   >     }

### 2069(i). Inconsistent exception spec for `basic_string` move constructor

Section: 27.4.3.3 [string.cons] Status: C++14 Submitter: Bo Persson Opened: 2011-07-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [string.cons].

View all issues with C++14 status.

Discussion:

Sub-clause 27.4.3.3 [string.cons] contains these constructors in paragraphs 2 and 3:

>     basic_string(const basic_string<charT,traits,Allocator>& str);
>     basic_string(basic_string<charT,traits,Allocator>&& str) noexcept;

> > […]

> > -3- Throws: The second form throws nothing if the allocator's move constructor throws nothing.

How can it ever throw anything if it is marked `noexcept`?

[2011-07-11: Daniel comments and suggests wording changes]

Further, according to paragraph 18 of the same sub-clause:

>     basic_string(const basic_string& str, const Allocator& alloc);
>     basic_string(basic_string&& str, const Allocator& alloc);

> > […]

> > -18- Throws: The second form throws nothing if `alloc == str.get_allocator()` unless the copy constructor for `Allocator` throws.

The constraint "unless the copy constructor for `Allocator` throws" is redundant, because according to Table 28 — Allocator requirements, the expressions

>     X a1(a);
>     X a(b);

impose the requirement: "Shall not exit via an exception".

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Change 27.4.3.3 [string.cons] p3 as indicated (This move constructor has a wide contract and is therefore safely marked as `noexcept`):

   >     basic_string(const basic_string<charT,traits,Allocator>& str);
   >     basic_string(basic_string<charT,traits,Allocator>&& str) noexcept;

   > > -2- Effects: Constructs an object of class `basic_string` as indicated in Table 64. In the second form, `str` is left in a valid state with an unspecified value.

   > > -3- Throws: The second form throws nothing if the allocator's move constructor throws nothing.

2. Change 27.4.3.3 [string.cons] p18 as indicated (This move-like constructor may throw, if the allocators don't compare equal, but not because of a potentially throwing allocator copy constructor, only because the allocation attempt may fail and throw an exception):

   >     basic_string(const basic_string& str, const Allocator& alloc);
   >     basic_string(basic_string&& str, const Allocator& alloc);

   > > […]

   > > -18- Throws: The second form throws nothing if `alloc == str.get_allocator()` unless the copy constructor for `Allocator` throws.

### 2070(i). `allocate_shared` should use `allocator_traits<A>::construct`

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: Resolved Submitter: Jonathan Wakely Opened: 2011-07-11 Last modified: 2017-07-16

Priority: 2

View all other issues in [util.smartptr.shared.create].

View all issues with Resolved status.

Discussion:

20.3.2.2.7 [util.smartptr.shared.create] says:

> -2- Effects: Allocates memory suitable for an object of type `T` and constructs an object in that memory via the placement new expression `::new (pv) T(std::forward<Args>(args)...)`. The template `allocate_shared` uses a copy of a to allocate memory. If an exception is thrown, the functions have no effect.

This explicitly requires placement new rather than using `allocator_traits<A>::construct(a, (T*)pv, std::forward<Args>(args)...)` In most cases that would result in the same placement new expression, but would allow more control over how the object is constructed e.g. using `scoped_allocator_adaptor` to do uses-allocator construction, or using an allocator declared as a friend to construct objects with no public constructors.

[2011-08-16 Bloomington:]

Agreed to fix in principle, but believe that `make_shared` and `allocate_shared` have now diverged enough that their descriptions should be separated. Pablo and Stefanus to provide revised wording.

Daniel's (old) proposed resolution:

> This wording is relative to the FDIS.

> 1. Change the following paragraphs of 20.3.2.2.7 [util.smartptr.shared.create] as indicated (The suggested removal of the last sentence of p1 is not strictly required to resolve this issue, but is still recommended, because it does not say anything new but may give the impression that it says something new):

>    >     template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);
>    >     template<class T, class A, class... Args>
>    >       shared_ptr<T> allocate_shared(const A& a, Args&&... args);

>    > > -1- Requires: For the template `make_shared`, tThe expression `::new (pv) T(std::forward<Args>(args)...)`, where `pv` has type `void*` and points to storage suitable to hold an object of type `T`, shall be well formed. For the template `allocate_shared`, the expression `allocator_traits<A>::construct(a, pt, std::forward<Args>(args)...)`, where `pt` has type `T*` and points to storage suitable to hold an object of type `T`, shall be well formed. `A` shall be an allocator ([allocator.requirements]). The copy constructor and destructor of `A` shall not throw exceptions.

>    > > -2- Effects: Allocates memory suitable for an object of type `T` and constructs an object in that memory. The template `make_shared` constructs the object via the placement new expression `::new (pv) T(std::forward<Args>(args)...)`. The template `allocate_shared` uses a copy of `a` to allocate memory and constructs the object by calling `allocator_traits<A>::construct(a, pt, std::forward<Args>(args)...)`. If an exception is thrown, the functions have no effect.

>    > > -3- Returns: A `shared_ptr` instance that stores and owns the address of the newly constructed object of type `T`.

>    > > -4- Postconditions: `get() != 0 && use_count() == 1`

>    > > -5- Throws: `bad_alloc`, or, for the template `make_shared`, an exception thrown from the constructor of `T`, or, for the template `allocate_shared`, an exception thrown from `A::allocate` or from `allocator_traits<A>::construct` from the constructor of `T`.

>    > > -6- Remarks: Implementations are encouraged, but not required, to perform no more than one memory allocation. [ Note: This provides efficiency equivalent to an intrusive smart pointer. — end note ]

>    > > -7- [ Note: These functions will typically allocate more memory than `sizeof(T)` to allow for internal bookkeeping structures such as the reference counts. — end note ]

[2011-12-04: Jonathan and Daniel improve wording]

See also c++std-lib-31796

[2013-10-13, Ville]

This issue is related to 2089(i).

[2014-02-15 post-Issaquah session : move to Tentatively NAD]

STL: This takes an allocator, but then ignores its construct. That's squirrely.

Alisdair: The convention is when you take an allocator, you use its construct.

STL: 23.2.2 [container.requirements.general]/3, argh! This fills me with despair, but I understand it now.

STL: Ok, this is some cleanup.

STL: You're requiring `b` to be of type `A` and not being rebound, is that an overspecification?

Pablo: Good point. Hmm, that's only a requirement on what must be well-formed.

STL: If it's just a well-formed requirement, then why not just use a directly?

Pablo: Yeah, the well-formed requirement is overly complex. It's not a real call, we could just use a directly. It makes it harder to read.

Alisdair: `b` should be an allocator in the same family as `a`.

Pablo: This is a well-formed requirement, I wonder if it's the capital A that's the problem here. It doesn't matter here, this is way too much wording.

Alisdair: It's trying to tie the constructor arguments into the allocator requirements.

Pablo: `b` could be struck, that's a runtime quality. The construct will work with anything that's in the family of `A`.

Alisdair: The important part is the `forward` of `Args`.

Pablo: `A` must be an allocator, and `forward` `Args` must work with that.

Alisdair: First let's nail down `A`.

Pablo: Then replace `b` with `a`, and strike the rest.

STL: You need `pt`'s type, at least.

Pablo: There's nothing to be said about runtime constraints here, this function doesn't even take a `pt`.

STL: Looking at the Effects, I believe `b` is similarly messed up, we can use `a2` to construct an object.

Alisdair: Or any allocator in the family of `a`.

STL: We say this stuff for the deallocate too, it should be lifted up.

STL: "owns the address" is weird.

Alisdair: shared_ptr owns pointers, although it does sound funky.

Walter: "to destruct" is ungrammatical.

STL: "When ownership is given up" is not what we usually say.

Alisdair: I think the Returns clause is the right place to say this.

STL: The right place to say this is `shared_ptr`'s dtor, we don't want to use Core's "come from" convention.

Alisdair: I'm on the hook to draft cleaner wording.

[2015-10, Kona Saturday afternoon]

AM: I was going to clean up the wording, but haven't done it yet.
Defer until we have new wording.

[2016-03, Jacksonville]

Alisdair: we need to figure out whether we should call construct or not; major implementation divergence
STL: this does not grant friendship, does it?
Jonathan: some people want it.
Thomas: scoped allocator adapter should be supported, so placement new doesn't work
Alisdair: this makes the make_ functions impossible
Thomas: you don't want to use those though.
Alisdair: but people use that today, at Bloomberg
Alisdair: and what do we do about fancy pointers?
Jonathan: we constrain it to only non-fancy pointers.
STL: shared_ptr has never attempted to support fancy pointers; seems like a paper is needed.
Poll: call construct:6 operator new: 0 don't care: 4
Poll: should we support fancy pointers? Yes: 1 No: 4 don't care: 4
STL: 20.8.2.2.6p2: 'and pv->~T()' is bogus for void
STL: 20.8.2.2.6p4: is this true even if we're going to allocate a bit more?
Alisdair: yes
Alisdair: coming up with new wording

[2016-08, Chicago Monday PM]

Alisdair to provide new wording this week

[2017-07 Toronto]

Resolved by the adoption of P0674R1 in Toronto

Proposed resolution:

This wording is relative to the FDIS.

1. Change the following paragraphs of 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   >     template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);
   >     template<class T, class A, class... Args>
   >       shared_ptr<T> allocate_shared(const A& a, Args&&... args);

   -1- Requires: The expression `::new (pv) T(std::forward<Args>(args)...)`, where `pv` has type `void*` and points to storage suitable to hold an object of type `T`, shall be well formed. `A` shall be an allocator (16.4.4.6 [allocator.requirements]). The copy constructor and destructor of `A` shall not throw exceptions.

   -2- Effects: Equivalent to

   >     return allocate_shared<T>(allocator<T>(), std::forward<Args>(args)...);

   Allocates memory suitable for an object of type `T` and constructs an object in that memory via the placement new expression `::new (pv) T(std::forward<Args>(args)...)`. The template `allocate_shared` uses a copy of `a` to allocate memory. If an exception is thrown, the functions have no effect.

   -?- Remarks: An implementation may meet the effects (and the implied guarantees) without creating the allocator object [Note: That is, user-provided specializations of `std::allocator` may not be instantiated, the expressions `::new (pv) T(std::forward<Args>(args)...)` and `pv->~T()` may be evaluated directly — end note].

   -3- Returns: A `shared_ptr` instance that stores and owns the address of the newly constructed object of type `T`.

   -4- Postconditions: `get() != 0 && use_count() == 1`

   -5- Throws: `bad_alloc`, or an exception thrown from `A::allocate` or from the constructor of `T`.

   -6- Remarks: Implementations are encouraged, but not required, to perform no more than one memory allocation. [Note: This provides efficiency equivalent to an intrusive smart pointer. — end note]

   -7- [Note: These functions will typically allocate more memory than `sizeof(T)` to allow for internal bookkeeping structures such as the reference counts. — end note]

2. Add the following set of new paragraphs immediately following the previous paragraph 7 of 20.3.2.2.7 [util.smartptr.shared.create]:

   >     template<class T, class A, class... Args>
   >       shared_ptr<T> allocate_shared(const A& a, Args&&... args);

   -?- Requires: The expressions `allocator_traits<A>::construct(b, pt, std::forward<Args>(args)...)` and `allocator_traits<A>::destroy(b, pt)` shall be well-formed and well-defined, where `b` has type `A` and is a copy of `a` and where `pt` has type `T*` and points to storage suitable to hold an object of type `T`. `A` shall meet the allocator requirements (16.4.4.6 [allocator.requirements]).

   -?- Effects: Uses an object `a2` of type `allocator_traits<A>::rebind_alloc<unspecified>` that compares equal to `a` to allocate memory suitable for an object of type `T`. Uses a copy `b` of type `A` from `a` to construct an object of type `T` in that memory by calling `allocator_traits<A>::construct(b, pt, std::forward<Args>(args)...)`. If an exception is thrown, the function has no effect.

   -?- Returns: A `shared_ptr` instance that stores and owns the address of the newly constructed object of type `T`. When ownership is given up, the effects are as follows: Uses a copy `b2` of type `A` from `a` to destruct an object of type `T` by calling `allocator_traits<A>::destroy(b2, pt2)` where `pt2` has type `T*` and refers to the newly constructed object. Then uses an object of type `allocator_traits<A>::rebind_alloc<unspecified>` that compares equal to `a` to deallocate the allocated memory.

   -?- Postconditions: `get() != 0 && use_count() == 1`

   -?- Throws: Nothing unless memory allocation or `allocator_traits<A>::construct` throws an exception.

   -?- Remarks: Implementations are encouraged, but not required, to perform no more than one memory allocation. [Note: Such an implementation provides efficiency equivalent to an intrusive smart pointer. — end note]

   -?- [Note: This function will typically allocate more memory than `sizeof(T)` to allow for internal bookkeeping structures such as the reference counts. — end note]

### 2071(i). `std::valarray` move-assignment

Section: 29.6.2.3 [valarray.assign] Status: C++14 Submitter: Paolo Carlini Opened: 2011-05-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [valarray.assign].

View all issues with C++14 status.

Discussion:

Yesterday I noticed that the language we have in the FDIS about `std::valarray` move assignment is inconsistent with the resolution of LWG 675. Indeed, we guarantee constant complexity (vs linear complexity). We also want it to be noexcept, that is more subtle, but again it's at variance with all the containers.

Also, even if we suppose that LWG 675(i) applies only to the containers proper, I don't think the current "as if by calling resize(v.size())" is internally consistent with the noexcept requirement.

So, what do we really want for `std::valarray`? Shall we maybe just strike or fix the as-if, consider it some sort of pasto from the copy-assignment text, thus keep the noexcept and constant complexity requirements (essentially the whole operation would boild down to a swap of POD data members). Or LWG 675(i) should be explicitly extended to `std::valarray` too? In that case both noexcept and constant complexity would go, I think, and the operation would boil down to the moral equivalent of `clear()` (which doesn't really exist in this case) + `swap`?

Howard: I agree the current wording is incorrect. The complexity should be linear in `size()` (not `v.size()`) because the first thing this operator needs to do is `resize(0)` (or `clear()` as you put it).

I think we can keep the `noexcept`.

As for proper wording, here's a first suggestion:

> Effects: `*this` obtains the value of `v`. The value of `v` after the assignment is not specified.

> Complexity: linear.

See also reflector discussion starting with c++std-lib-30690.

[2012, Kona]

Move to Ready.

Some discussion on the types supported by `valarray` concludes that the wording is trying to say something similar to the core wording for trivial types, but significantly predates it, and does allow for types with non-trivial destructors. Howard notes that the only reason for linear complexity, rather than constant, is to support types with non-trivial destructors.

AJM suggests replacing the word 'value' with 'state', but straw poll prefers moving forward with the current wording, 5 to 2.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

In 29.6.2.3 [valarray.assign] update as follows:

>     valarray<T>& operator=(valarray<T>&& v) noexcept;

> > 3 Effects: `*this` obtains the value of `v`. If the length of `v` is not equal to the length of `*this`, resizes `*this` to make the two arrays the same length, as if by calling `resize(v.size())`, before performing the assignment.The value of `v` after the assignment is not specified.

> > 4 Complexity: ConstantLinear.

### 2072(i). Unclear wording about capacity of temporary buffers

Section: 99 [depr.temporary.buffer] Status: C++17 Submitter: Kazutoshi Satoda Opened: 2011-08-10 Last modified: 2025-03-13

Priority: 3

View all other issues in [depr.temporary.buffer].

View all issues with C++17 status.

Discussion:

According to [temporary.buffer] p1+2:

>     template <class T>
>     pair<T*, ptrdiff_t> get_temporary_buffer(ptrdiff_t n) noexcept;

> > -1- Effects: Obtains a pointer to storage sufficient to store up to `n` adjacent `T` objects. It is implementation-defined whether over-aligned types are supported (3.11).

> > -2- Returns: A pair containing the buffer's address and capacity (in the units of `sizeof(T)`), or a pair of 0 values if no storage can be obtained or if `n <= 0`.

I read this as prohibiting to return a buffer of which capacity is less than `n`, because such a buffer is not sufficient to store `n` objects.

The corresponding description in SGI STL is clear on this point, but I think it is a bit too verbose:

> (for the return value, a pair `P`) [...] the buffer pointed to by `P.first` is large enough to hold `P.second` objects of type `T`. `P.second` is greater than or equal to 0, and less than or equal to `len`.

There seems to be two different targets of the "up to n" modification: The capacity of obtained buffer, and the actual number that the caller will store into the buffer.

First I read as the latter, and got surprised seeing that libstdc++ implementation can return a smaller buffer. I started searching about `get_temporary_buffer()`. After reading a quote from TC++PL at stackoverflow, I realized that the former is intended.

Such misinterpretation seems common:

- The above question is likely started from same misinterpretation.

- JIS standard (Japanese translation of ISO/IEC standard) says nothing like "up to". I think the editor misinterpreted the original wording, and omitted words for "up to" as it is redundant. (If a buffer is sufficient to store `n` objects, it is also sufficient to store up to `n` objects.)

- Rogue Wave implementation doesn't return smaller buffer, instead, it can return larger buffer on some circumstances. Apache STDCXX is a derived version of that implementation, and publicly accessible:

  > Specializations of the `get_temporary_buffer()` function template attempt to allocate a region of storage sufficiently large to store at least `n` adjacent objects of type `T`.

  I know one commercial compiler package based on Rogue Wave implementation, and its implementation is essentially same as the above.

[2014-05-18, Daniel comments and suggests concrete wording]

The provided wording attempts to clarify the discussed capacity freedom, but it also makes it clearer that the returned memory is just "raw memory", which is currently not really clear. In addition the wording clarifies that the deallocating `return_temporary_buffer` function does not throw exceptions, which I believe is the intention when the preconditions of the functions are satisfied. Then, my understanding is that we can provide to `return_temporary_buffer` a null pointer value if that was the value, `get_temporary_buffer()` had returned. Furthermore, as STL noticed, the current wording seemingly allows multiple invocations of `return_temporary_buffer` with the same value returned by `get_temporary_buffer`; this should be constrained similar to the wording we have for `operator delete` (unfortunately we miss such wording for allocators).

[2015-05, Lenexa]

MC: move to ready? in favor: 14, opposed: 0, abstain: 0

Proposed resolution:

This wording is relative to N3936.

1. Change [temporary.buffer] as indicated:

   >     template <class T>
   >       pair<T*, ptrdiff_t> get_temporary_buffer(ptrdiff_t n) noexcept;

   > > -1- Effects: Obtains a pointer to uninitialized, contiguous storage for `N` adjacent objects of type `T`, for some non-negative number `N`.Obtains a pointer to storage sufficient to store up to `n` adjacent `T` objects. It is implementation-defined whether over-aligned types are supported (3.11).

   > > -?- Remarks: Calling `get_temporary_buffer` with a positive number `n` is a non-binding request to return storage for `n` objects of type `T`. In this case, an implementation is permitted to return instead storage for a non-negative number `N` of such objects, where `N != n` (including `N == 0`). [Note: The request is non-binding to allow latitude for implementation-specific optimizations of its memory management. — end note].

   > > -2- Returns: If `n <= 0` or if no storage could be obtained, returns a pair `P` such that `P.first` is a null pointer value and `P.second == 0`; otherwise returns a pair `P` such that `P.first` refers to the address of the uninitialized storage and `P.second` refers to its capacity `N` (in the units of `sizeof(T)`).A `pair` containing the buffer's address and capacity (in the units of `sizeof(T)`), or a pair of 0 values if no storage can be obtained or if `n <= 0`.

   >     template <class T> void return_temporary_buffer(T* p);

   > > -3- Effects: Deallocates the buffer to which `p` pointsstorage referenced by `p`.

   > > -4- Requires: The buffer shall have been previously allocated by `p` shall be a pointer value returned by an earlier call to `get_temporary_buffer` which has not been invalidated by an intervening call to `return_temporary_buffer(T*)`.

   > > -?- Throws: Nothing.

### 2074(i). Off by one error in `std::reverse_copy`

Section: 26.7.10 [alg.reverse] Status: C++14 Submitter: Peter Miller Opened: 2011-08-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.reverse].

View all issues with C++14 status.

Discussion:

The output of the program below should be:

>     "three two one null \n"

But when `std::reverse_copy` is implemented as described in N3291 26.7.10 [alg.reverse] it's:

>     "null three two one \n"

because there's an off by one error in 26.7.10 [alg.reverse]/4; the definition should read:

>     *(result + (last - first) - 1 - i) = *(first + i)

Test program:

>     #include <algorithm>
>     #include <iostream>

>     template <typename BiIterator, typename OutIterator>
>     auto
>     reverse_copy_as_described_in_N3291(
>       BiIterator first, BiIterator last, OutIterator result )
>     -> OutIterator
>     {
>       // 25.3.10/4 [alg.reverse]:
>       // "...such that for any non-negative integer i < (last - first)..."
>       for ( unsigned i = 0; i < ( last - first ); ++i )
>         // "...the following assignment takes place:"
>         *(result + (last - first) - i) = *(first + i);

>       // 25.3.10/6
>       return result + (last - first);
>     }

>     int main()
>     {
>       using std::begin;
>       using std::end;
>       using std::cout;

>       static const char*const in[3]  { "one", "two", "three" };
>       const char*             out[4] { "null", "null", "null", "null" };

>       reverse_copy_as_described_in_N3291( begin( in ), end( in ), out );

>       for ( auto s : out )
>         cout << s << ' ';

>       cout << std::endl;

>       return 0;
>     }

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 26.7.10 [alg.reverse] p4 as follows:

>     template<class BidirectionalIterator, class OutputIterator>
>       OutputIterator
>         reverse_copy(BidirectionalIterator first,
>                      BidirectionalIterator last, OutputIterator result);

> > -4- Effects: Copies the range [`first,last`) to the range [`result,result+(last-first)`) such that for any non-negative integer `i < (last - first)` the following assignment takes place: `*(result + (last - first) - 1 - i) = *(first + i)`.

> > -5- Requires: The ranges [`first,last`) and [`result,result+(last-first)`) shall not overlap.

> > -6- Returns: `result + (last - first)`.

> > -7- Complexity: Exactly `last - first` assignments.

### 2075(i). Progress guarantees, lock-free property, and scheduling assumptions

Section: 6.10.2 [intro.multithread], 32.5.5 [atomics.lockfree], 99 [atomics.types.operations.req] Status: Resolved Submitter: Torvald Riegel Opened: 2011-08-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [intro.multithread].

View all issues with Resolved status.

Discussion:

According to 6.10.2 [intro.multithread] p2:

> "Implementations should ensure that all unblocked threads eventually make progress."

- If taken literally, this cannot be achieved with lock-free atomics in general because they only guarantee that some thread makes progress (i.e., minimal progress, whereas 6.10.2 [intro.multithread] p2 seems to require maximal progress).
- What does it mean precisely to "make progress"? Does "unblocked threads" exclude live-locked threads (if so, lock-free atomics would be sufficient I suppose)?

- Which assumptions can an implementation make about the thread scheduling? This is relevant for how implementations implement compare-exchange with load-linked / store conditional (LL-SC), and atomic read-modifiy-write operations with load...compare-exchange-weak loops.

  - Do threads run long enough without being descheduled (e.g., OS timeslices are long enough, interrupt frequency is not too high, etc.)?
  - Or is this implementation-defined, and the sentence is just about stating that the progress guarantees will not hold on, for example, systems with unfair scheduling or thread priorities?

32.5.5 [atomics.lockfree] p2 declares the lock-free property for a particular object. However, "lock-free" is never defined, and in discussions that I had with committee members it seemed as if the standard's lock-free would be different from what lock-free means in other communities (eg, research, text books on concurrent programming, etc.).

- Originally, lock-freedom for an object requires minimal progress (ie, some thread makes progress, but other threads might never do) without any assumptions about the scheduling (threads could be stopped executing (so it is "nonblocking"), and threads are not guaranteed to execute in isolation, even for very small intervals of cycles).
- In contrast, obstruction-freedom, another nonblocking progress condition, guarantees progress for all threads that eventually get executed long enough in isolation (ie, without interference by other threads).
- Simple load...compare-exchange-weak loops (or LL-SC loops) to implement atomic read-modify-write operations can be just obstruction-free but not lock-free because they can livelock (depending on the hardware's LL-SC implementation, though). However, they effectively guarantee the same as lock-free iff threads will eventually run in isolation for long enough (that can be an assumption about the OS scheduler), or if the implementation adds this (e.g., probabilistically by employing randomized exponential back-off when contention is detected, in all operations that can create contention).
- Does the particular object has to be lock-free, or is it only required that threads make progress irrespective on which object? Again considering compare-exchange-weak or LL-SC here, what happens if the compare-exchange object shares a cacheline with an integer counter object that is constantly updated by other threads? The compare-exchange-weak can always fail, so the object would not be lock-free. However, if we consider progress to be overall progress for threads, it would be lock-free because other threads succeed updating the integer counter. I would have assumed the lock-free property is strictly about the atomic object, but in discussions with committee members it seemed as if progress for any object could be the intended guarantee.

Following 99 [atomics.types.operations.req] p7 `is_lock_free()` returns "true if the object is lock-free". What is returned if the object is only sometimes lock-free?

Basically, I would like to see clarifications for the progress guarantees so that users know what they can expect from implementations (and what they cannot expect!), and to give implementors a clearer understanding of which user expectations they have to implement.

1. Elaborate on the intentions of the progress guarantee in 6.10.2 [intro.multithread] p2. As I don't know about your intentions, it's hard to suggest a resolution.

   - Is it for straightforward, non-synchronizing code only?
   - Is it for blocking code only? (Is "unblocked" more than blocked on external I/O or on deadlocks?)
   - What does it mean to "make progress"?
   - Is this meant to only waive any progress guarantees if there are thread priorities?
   - Can an implementation make any assumptions about thread scheduling?

2. Define the lock-free property. The definition should probably include the following points:

   - Is it just nonblocking, or what is the distinction to just being nonblocking?
   - Does it make any assumptions about the scheduler?
   - What are the progress guarantees, minimal or maximal (some or all threads finish eventually).
   - Is progress guaranteed for all operations on the particular object, or do operations on other objects also count as "making progress"?
3. Add a note explaining that compare-exchange-weak is not necessarily lock-free (but is nonblocking)? Or is it indeed intended to be lock-free (only allowed to fail spuriously but guaranteed to not fail eventually)? Implementing the latter might be a challenge on LL-SC machines or lead to space overheads I suppose, see the cacheline sharing example above.

[2011-12-01: Hans comments]

6.10.2 [intro.multithread] p2 was an intentional compromise, and it was understood at the time that it was not a precise statement. The wording was introduced by N3209, which discusses some of the issues. There were additional reflector discussions.

This is somewhat separable from the question of what lock-free means, which is probably a more promising question to focus on.

[2012, Kona]

General direction: lock-free means obstruction-free. Leave the current "should" recommendation for progress. It would take a lot of effort to try to do better.

[2012, Portland: move to Open]

The current wording of 6.10.2 [intro.multithread] p2 doesn't really say very much. As far as we can tell the term lock-free is nowhere defined in the standard.

James: we would prefer a different way to phrase it.

Hans: the research literature includes the term abstraction-free which might be a better fit.

Detlef: does Posix define a meaning for blocking (or locking) that we could use?

Hans: things like compare-exchange-strong can wait indefinitely.

Niklas: what about spin-locks -- still making no progress.

Hans: suspect we can only give guidance, at best. The lock-free meaning from the theoretical commmunity (forard progress will be made) is probably too strong here.

Atrur: what about livelocks?

Hans: each atomic modification completes, even if the whole thing is blocked.

Moved to open.

[2013-11-06: Jason Hearne-McGuiness comments]

Related to this issue, the wording in 32.5.5 [atomics.lockfree] p2,

> In any given program execution, the result of the lock-free query shall be consistent for all pointers of the same type.

should be made clearer, because the object-specific (non-static) nature of the `is_lock_free()` functions from 32.5.8 [atomics.types.generic] and 32.5.8.2 [atomics.types.operations] imply that for different instances of pointers of the same type the returned value could be different.

Proposed resolution:

[2014-02-16 Issaquah: Resolved by paper n3927]

### 2076(i). Bad `CopyConstructible` requirement in set constructors

Section: 23.4.6.2 [set.cons] Status: C++17 Submitter: Jens Maurer Opened: 2011-08-20 Last modified: 2017-07-30

Priority: 3

View all issues with C++17 status.

Discussion:

23.4.6.2 [set.cons] paragraph 4 says:

> Requires: If the iterator's dereference operator returns an lvalue or a non-const rvalue, then `Key` shall be `CopyConstructible`.

I'm confused why a "non-const rvalue" for the return value of the iterator would require `CopyConstructible`; isn't that exactly the situation when you'd want to apply the move constructor?

The corresponding requirement for `multimap` seems better in that regard ([multimap.cons] paragraph 3):

> Requires: If the iterator's dereference operator returns an lvalue or a const rvalue `pair<key_type, mapped_type>`, then both `key_type` and mapped_type shall be `CopyConstructible`.

Obviously, if I have a const rvalue, I can't apply the move constructor (which will likely attempt modify its argument).

Dave Abrahams:

I think you are right. Proposed resolution: drop "non-" from 23.4.6.2 [set.cons] paragraph 3.

[2012, Kona]

The wording is in this area will be affected by Pablo's paper being adopted at this meeting. Wait for that paper to be applied before visiting this issue - deliberately leave in New status until the next meeting.

Proposed resolution from Kona 2012:

> This wording is relative to the FDIS.

> Change 23.4.6.2 [set.cons] p3 as follows:

> >     template <class InputIterator>
> >       set(InputIterator first, InputIterator last,
> >         const Compare& comp = Compare(), const Allocator& = Allocator());

> > > -3- Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range [`first,last`).

> > > -4- Requires: If the iterator's dereference operator returns an lvalue or a non-const rvalue, then `Key` shall be `CopyConstructible`.

> > > -5- Complexity: Linear in `N` if the range [`first,last`) is already sorted using `comp` and otherwise `N logN`, where `N` is `last - first`.

[2014-05-18, Daniel comments]

According to Pablo, the current P/R correctly incorporates the changes from his paper (which was adopted in Kona)

[2014-06-10, STL comments and suggests better wording]

N1858 was voted into WP N2284 but was "(reworded)", introducing the "non-const" damage.

N1858 wanted to add this for map:

> Requires: Does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type, mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.

And this for set:

> Requires: `Key` must be `CopyConstructible` only if the dereferenced `InputIterator` returns an lvalue or const rvalue.

(And similarly for multi.)

This was reworded to N2284 23.3.1.1 [map.cons]/3 and N2284 23.3.3.1 [set.cons]/4, and it slightly changed over the years into N3936 23.4.4.2 [map.cons]/3 and N3936 23.4.6.2 [set.cons]/4.

In 2005/2007, this was the best known way to say "hey, we should try to move this stuff", as the fine-grained element requirements were taking shape.

Then in 2010, N3173 was voted into WP N3225, adding the definition of `EmplaceConstructible` and modifying the container requirements tables to make the range constructors require `EmplaceConstructible`.

After looking at this history and double-checking our implementation (where `map`/`set` range construction goes through emplacement, with absolutely no special-casing for `map`'s pairs), I am convinced that N3173 superseded N1858 here. (Range-`insert()` and the unordered containers are unaffected.)

Previous resolution [SUPERSEDED]:

> This wording is relative to the N3936.

> Change 23.4.6.2 [set.cons] p4 as follows:

> >     template <class InputIterator>
> >       set(InputIterator first, InputIterator last,
> >         const Compare& comp = Compare(), const Allocator& = Allocator());

> > > -3- Effects: Constructs an empty `set` using the specified comparison object and allocator, and inserts elements from the range [`first,last`).

> > > -4- Requires: If the iterator's indirection operator returns an lvalue or a non-const rvalue, then `Key` shall be `CopyInsertible` into `*this`.

> > > -5- Complexity: Linear in `N` if the range [`first,last`) is already sorted using `comp` and otherwise `N logN`, where `N` is `last - first`.

[2015-02 Cologne]

GR: Do requirements supersede rather than compose [container requirements and per-function requirements]? AM: Yes, they supersede.

AM: This looks good. Ready? Agreement.

Proposed resolution:

This wording is relative to the N4296.

1. Remove 23.4.3.2 [map.cons] p3:

   >     template <class InputIterator>
   >       map(InputIterator first, InputIterator last,
   >           const Compare& comp = Compare(), const Allocator& = Allocator());

   > > -3- Requires: If the iterator's indirection operator returns an lvalue or a `const` rvalue `pair<key_type, mapped_type>`, then both `key_type` and `mapped_type` shall be `CopyInsertible` into `*this`.

   > > […]

2. Remove 23.4.4.2 [multimap.cons] p3:

   >     template <class InputIterator>
   >       multimap(InputIterator first, InputIterator last,
   >                const Compare& comp = Compare(),
   >                const Allocator& = Allocator());

   > > -3- Requires: If the iterator's indirection operator returns an lvalue or a `const` rvalue `pair<key_type, mapped_type>`, then both `key_type` and `mapped_type` shall be `CopyInsertible` into `*this`.

   > > […]

3. Remove 23.4.6.2 [set.cons] p4:

   >     template <class InputIterator>
   >       set(InputIterator first, InputIterator last,
   >           const Compare& comp = Compare(), const Allocator& = Allocator());

   > > […]

   > > -4- Requires: If the iterator's indirection operator returns an lvalue or a non-`const` rvalue, then `Key` shall be `CopyInsertible` into `*this`.

4. Remove 23.4.7.2 [multiset.cons] p3:

   >     template <class InputIterator>
   >       multiset(InputIterator first, InputIterator last,
   >                const Compare& comp = Compare(),
   >                const Allocator& = Allocator());

   > > -3- Requires: If the iterator's indirection operator returns an lvalue or a `const` rvalue, then `Key` shall be `CopyInsertible` into `*this`.

   > > […]

### 2078(i). Throw specification of `async()` incomplete

Section: 32.10.9 [futures.async] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-08-29 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.async].

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Discussion:

The current throw specification of `async()` does state:

> -6- Throws: `system_error` if policy is `launch::async` and the implementation is unable to start a new thread.

First it seems not clear whether this only applies if policy equals `launch::async` of if the `async` launch mode flag is set (if `policy|launch::async!=0`)

In the discussion Lawrence Crowl also wrote:

> More generally, I think what we want to say is that if the implementation cannot successfully execute on one of the policies allowed, then it must choose another. The principle would apply to implementation-defined policies as well.

Peter Sommerlad:

> Should not throw. That was the intent. "is async" meat exactly.

[2012, Portland: move to Tentatively NAD Editorial]

If no launch policy, it is undefined behavior.

Agree with Lawrence, should try all the allowed policies. We will rephrase so that the policy argument should be `lauch::async`. Current wording seems good enough.

We believe this choice of policy statement is really an editorial issue.

[2013-09 Chicago]

If all the implementors read it and can't get it right - it is not editorial. Nico to provide wording

No objections to revised wording, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change 32.10.9 [futures.async] p6, p7 as indicated:

   > -6- Throws: `system_error` if `policy` is `==` `launch::async` and the implementation is unable to start a new thread.

   > -7- Error conditions:

   > - `resource_unavailable_try_again` — if `policy` is `==` `launch::async` and the system is unable to start a new thread.

### 2080(i). Specify when `once_flag` becomes invalid

Section: 32.6.7 [thread.once] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-08-30 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++14 status.

Discussion:

In function `call_once` 32.6.7.2 [thread.once.callonce] paragraph 4 and 5 specify for `call_once()`:

> Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]), or any exception thrown by `func`.

> Error conditions:
> - `invalid_argument` — if the `once_flag` object is no longer valid.

However, nowhere in 32.6.7 [thread.once] is specified, when a once-flag becomes invalid.

As far as I know this happens if the flag is used for different functions. So we either have to have to insert a sentence/paragraph in

> 30.4.4.2 Function call_once [thread.once.callonce]

or

> 30.4.4 Call once [thread.once]

explaining when a `once_flag` becomes invalidated or we should state as error condition something like:

- `invalid_argument` — if the `func` used in combination with the `once_flag` is different from a previously passed `func` for the same `once_flag`

Anthony Williams:

> A `once_flag` is invalidated if you destroy it (e.g. it is an automatic object, or heap allocated and deleted, etc.)

> If the library can detect that this is the case then it will throw this exception. If it cannot detect such a case then it will never be thrown.

Jonathan Wakely:

> I have also wondered how that error can happen in C++, where the type system will reject a non-callable type being passed to `call_once()` and should prevent a `once_flag` being used after its destructor runs.

> If a `once_flag` is used after its destructor runs then it is indeed undefined behaviour, so implementations are already free to throw any exception (or set fire to a printer) without the standard saying so.

> My assumption was that it's an artefact of basing the API on pthreads, which says:

> > The `pthread_once()` function may fail if:

> > `[EINVAL]` If either `once_control` or `init_routine` is invalid.

Pete Becker:

> Yes, probably. We had to clean up several UNIXisms that were in the original design.

[2012, Kona]

Remove error conditions, move to Review.

[2012, Portland: move to Tentatively Ready]

Concurrency move to Ready, pending LWG review.

LWG did not have time to perform the final review in Portland, so moving to tentatively ready to reflect the Concurrency belief that the issue is ready, but could use a final inspection from library wordsmiths.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3337.

1. Change 32.6.7.2 [thread.once.callonce] as indicated:

   >     template<class Callable, class ...Args>
   >     void call_once(once_flag& flag, Callable&& func, Args&&... args);

   > > […]

   > > -4- Throws: `system_error` when an exception is required (30.2.2), or any exception thrown by `func`.

   > > -5- Error conditions:
   > > - `invalid_argument` — if the `once_flag` object is no longer valid.

### 2081(i). `Allocator` requirements should include `CopyConstructible`

Section: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-08-30 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [allocator.requirements].

View all other issues in [allocator.requirements].

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Discussion:

As discussed in c++std-lib-31054 and c++std-lib-31059, the `Allocator` requirements implicitly require `CopyConstructible` because `a.select_on_container_copy_construction()` and `container.get_allocator()` both return a copy by value, but the requirement is not stated explicitly anywhere.

In order to clarify that allocators cannot have 'explicit' copy constructors, the requirements should include `CopyConstructible`.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

1. Change Table 28 — Allocator requirements in 16.4.4.6 [allocator.requirements]:

   Table 28 — Allocator requirements
   Expression | Return type | Assertion/note pre-/post-condition | Default
   --- | --- | --- | ---
   `X a1(a); X a1 = a;` |  | Shall not exit via an exception. post: `a1 == a` |
   `…`
   `X a1(move(a)); X a1 = move(a);` |  | Shall not exit via an exception. post: `a1` equals the prior value of `a`. |

2. Change 16.4.4.6 [allocator.requirements] paragraph 4:

   > An allocator type `X` shall satisfy the requirements of `CopyConstructible` (16.4.4.2 [utility.arg.requirements]). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (16.4.4.4 [nullablepointer.requirements]). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.3 [iterator.requirements]).

### 2083(i). const-qualification on `weak_ptr::owner_before`

Section: 20.3.2.3 [util.smartptr.weak], 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++14 Submitter: Ai Azuma Opened: 2011-09-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [util.smartptr.weak].

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Discussion:

Is there any reason why `weak_ptr::owner_before` member function templates are not const-qualified?

Daniel Krügler:

> I don't think so. To the contrary, without these to be const member function templates, the semantics of the specializations `owner_less<weak_ptr<T>>` and `owner_less<shared_ptr<T>>` described in 20.3.2.4 [util.smartptr.ownerless] is unclear.

> It is amusing to note that this miss has remain undetected from the accepted paper n2637 on. For the suggested wording changes see below.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

1. Change the class template `weak_ptr` synopsis in 20.3.2.3 [util.smartptr.weak] as indicated:

   >     namespace std {
   >       template<class T> class weak_ptr {
   >       public:
   >         typedef T element_type;
   >         […]
   >         template<class U> bool owner_before(shared_ptr<U> const& b) const;
   >         template<class U> bool owner_before(weak_ptr<U> const& b) const;
   >       };
   >       […]
   >     }

2. Change the prototypes in 20.3.2.3.6 [util.smartptr.weak.obs] before p6 as indicated:

   >     template<class U> bool owner_before(shared_ptr<U> const& b) const;
   >     template<class U> bool owner_before(weak_ptr<U> const& b) const;

### 2085(i). Wrong description of effect 1 of `basic_istream::ignore`

Section: 31.7.5.4 [istream.unformatted] Status: C++14 Submitter: Krzysztof Żelechowski Opened: 2011-09-11 Last modified: 2016-08-09

Priority: Not Prioritized

View all other issues in [istream.unformatted].

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Discussion:

31.7.5.4 [istream.unformatted] in N3242 currently has the following to say about the semantics of `basic_istream::ignore`:

> [..]. Characters are extracted until any of the following occurs:

> - if `n != numeric_limits<streamsize>::max()` (18.3.2), `n` characters are extracted

This statement, apart from being slightly ungrammatical, indicates that if (`n == numeric_limits<streamsize>::max()`), the method returns without extracting any characters.

The description intends to describe the observable behaviour of an implementation in terms of logical assertions. Logical assertions are not "bullets" that can be "entered" but need not; they are predicates that can evaluate to true or false.

The description contains two predicates, either of them causes extraction to terminate. In the incriminated case, the first predicate is evaluates to true because its premise is false, therefore no characters will be extracted.

The intended semantics would be described by the following statement:

> [..]. Characters are extracted until any of the following occurs:

> - `(n != numeric_limits<streamsize>::max())` (18.3.2) and (`n`) characters have been extracted so far.

[2013-04-20, Bristol]

Resolution: Ready.

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to the FDIS.

Change 31.7.5.4 [istream.unformatted] p25 as indicated:

>     basic_istream<charT,traits>&
>       ignore(streamsize n = 1, int_type delim = traits::eof());

> > -25- Effects: Behaves as an unformatted input function (as described in 31.7.5.4 [istream.unformatted], paragraph 1). After constructing a `sentry` object, extracts characters and discards them. Characters are extracted until any of the following occurs:

> > - if `n != numeric_limits<streamsize>::max()` ( [limits.numeric]),and `n` characters arehave been extracted so far
> > - end-of-file occurs on the input sequence (in which case the function calls `setstate(eofbit)`, which may throw `ios_base::failure` (31.5.4.4 [iostate.flags]));
> > - `traits::eq_int_type(traits::to_int_type(c), delim)` for the next available input character `c` (in which case `c` is extracted).

### 2086(i). Overly generic type support for math functions

Section: 29.7 [c.math] Status: C++14 Submitter: Daniel Krügler Opened: 2011-09-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [c.math].

View all issues with C++14 status.

Discussion:

29.7 [c.math] ends with a description of a rule set for "sufficient overloads" in p11:

> Moreover, there shall be additional overloads sufficient to ensure:

> 1. If any argument corresponding to a `double` parameter has type `long double`, then all arguments corresponding to `double` parameters are effectively cast to `long double`.
> 2. Otherwise, if any argument corresponding to a `double` parameter has type `double` or an integer type, then all arguments corresponding to `double` parameters are effectively cast to `double`.
> 3. Otherwise, all arguments corresponding to `double` parameters are effectively cast to `float`.

My impression is that this rule set is probably more generic as intended, my assumption is that it is written to mimic the C99/C1x rule set in 7.25 p2+3 in the "C++" way:

> -2- Of the `<math.h>` and `<complex.h>` functions without an `f` (`float`) or `l` (`long double`) suffix, several have one or more parameters whose corresponding real type is `double`. For each such function, except `modf`, there is a corresponding type-generic macro. (footnote 313) The parameters whose corresponding real type is `double` in the function synopsis are generic parameters. Use of the macro invokes a function whose corresponding real type and type domain are determined by the arguments for the generic parameters. (footnote 314)

> -3- Use of the macro invokes a function whose generic parameters have the corresponding real type determined as follows:
> - First, if any argument for generic parameters has type `long double`, the type determined is `long double`.
> - Otherwise, if any argument for generic parameters has type `double` or is of integer type, the type determined is `double`.
> - Otherwise, the type determined is `float`.

where footnote 314 clarifies the intent:

> If the type of the argument is not compatible with the type of the parameter for the selected function, the behavior is undefined.

The combination of the usage of the unspecific term "cast" with otherwise no further constraints (note that C constraints the valid set to types that C++ describes as arithmetic types, but see below for one important difference) has the effect that it requires the following examples to be well-formed and well-defined:

>     #include <cmath>

>     enum class Ec { };

>     struct S { explicit operator long double(); };

>     void test(Ec e, S s) {
>      std::sqrt(e); // OK, behaves like std::sqrt((float) e);
>      std::sqrt(s); // OK, behaves like std::sqrt((float) s);
>     }

GCC 4.7 does not accept any of these examples.

I found another example where the C++ rule differs from the C set, but in this case I'm not so sure, which direction C++ should follow. The difference is located in the fact, that in C enumerated types are integer types as described in 6.2.5 p17 (see e.g. n1569 or n1256):

"The type char, the signed and unsigned integer types, and the enumerated types are collectively called integer types. The integer and real floating types are collectively called real types."

This indicates that in C the following code

>     #include <math.h>

>     enum E { e };

>     void test(void) {
>       sqrt(e); // OK, behaves like sqrt((double) e);
>     }

seems to be well-defined and `e` is cast to `double`, but in C++ referring to

>     #include <cmath>

>     enum E { e };

>     void test() {
>       std::sqrt(e); // OK, behaves like sqrt((float) e);
>     }

is also well-defined (because of our lack of constraints) but we must skip bullet 2 (because E is not an integer type) and effectively cast `e` to `float`. Accepting this, we would introduce a silent, but observable runtime difference for C and C++.

GCC 4.7 does not accept this example, but causes an ambiguity error among the three floating point overloads of sqrt.

My current suggestion to fix these problems would be to constrain the valid argument types of these functions to arithmetic types.

> Howard provided wording to solve the issue.

[2012, Kona]

Moved to Ready. The proposed wording reflects both original intent from TR1, and current implementations.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 29.7 [c.math] p11 as indicated:

> Moreover, there shall be additional overloads sufficient to ensure:

> 1. If any arithmetic argument corresponding to a `double` parameter has type `long double`, then all arithmetic arguments corresponding to `double` parameters are effectively cast to `long double`.
> 2. Otherwise, if any arithmetic argument corresponding to a `double` parameter has type `double` or an integer type, then all arithmetic arguments corresponding to `double` parameters are effectively cast to `double`.
> 3. Otherwise, all arithmetic arguments corresponding to `double` parameters are effectively cast tohave type `float`.

### 2087(i). `iostream_category()` and `noexcept`

Section: 31.5 [iostreams.base] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-09-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iostreams.base].

View all issues with C++14 status.

Discussion:

In `<system_error>` we have:

>     const error_category& generic_category() noexcept;
>     const error_category& system_category() noexcept;

In `<future>` we have:

>     const error_category& future_category() noexcept;

But in `<ios>` we have:

>     const error_category& iostream_category();

Is there any reason that `iostream_category()` is not declared with `noexcept` or is this an oversight?

Daniel:

This looks like an oversight to me. We made the above mentioned changes as part of noexcept-ifying the thread library but `iostream_category()` was skipped, so it seems to be forgotten. There should be no reason, why it cannot be `noexcept`. When doing so, we should also make these functions `noexcept` (similar to corresponding overloads):

>     error_code make_error_code(io_errc e);
>     error_condition make_error_condition(io_errc e);

Suggested wording provided by Daniel.

[2013-04-20, Bristol]

Unanimous.

Resolution: move to tentatively ready.

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to the FDIS.

1. Change [iostreams.base.overview], header `<ios>` synopsis as indicated:

   >     #include <iosfwd>
   >     namespace std {
   >       […]
   >       error_code make_error_code(io_errc e) noexcept;
   >       error_condition make_error_condition(io_errc e) noexcept;
   >       const error_category& iostream_category() noexcept;
   >     }

2. Change the prototype declarations in 31.5.6 [error.reporting] as indicated:

   >     error_code make_error_code(io_errc e) noexcept;

   > -1- Returns: `error_code(static_cast<int>(e), iostream_category())`.

   >     error_condition make_error_condition(io_errc e) noexcept;

   > -2- Returns: `error_condition(static_cast<int>(e), iostream_category())`.

   >     const error_category& iostream_category() noexcept;

   > -3- Returns: A reference to an object of a type derived from class `error_category`.

   > -4- The object’s `default_error_condition` and `equivalent` virtual functions shall behave as specified for the class `error_category`. The object’s `name` virtual function shall return a pointer to the string `"iostream"`.

### 2088(i). `std::terminate` problem

Section: 17.9.5 [exception.terminate] Status: Resolved Submitter: Daniel Krügler Opened: 2011-09-25 Last modified: 2024-11-07

Priority: 3

View all issues with Resolved status.

Discussion:

Andrzej Krzemienski reported the following on comp.std.c++:

> In N3290, which is to become the official standard, in 17.9.5.4 [terminate], paragraph 1 reads

> > Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1), in effect immediately after evaluating the throw-expression (18.8.3.1). May also be called directly by the program.

> It is not clear what is "in effect". It was clear in previous drafts where paragraphs 1 and 2 read:

> > Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1). May also be called directly by the program.

> > Effects: Calls the `terminate_handler` function in effect immediately after evaluating the throw-expression (18.8.3.1), if called by the implementation, or calls the current `terminate_handler` function, if called by the program.

> It was changed by N3189. The same applies to function unexpected (D. 11.4, paragraph 1).

> Assuming the previous wording is still intended, the wording can be read "unless `std::terminate` is called by the program, we will use the handler that was in effect immediately after evaluating the throw-expression".

> This assumes that there is some throw-expression connected to every situation that triggers the call to `std::terminate`. But this is not the case:
> - In case `std::thread` is assigned to or destroyed while being joinable there is no throw-expression involved.
> - In case `std::unexpected` is called by the program, `std::terminate` is triggered by the implementation - no throw-expression involved.
> - In case a destructor throws during stack unwinding we have two throw-expressions involved.

> Which one is referred to?

> In case `std::nested_exception::rethrow_nested` is called for an object that has captured no exception, there is no throw-expression involved directly (and may no throw be involved even indirectly).

> Next, 17.9.5.1 [terminate.handler], paragraph 2 says

> > Required behavior: A `terminate_handler` shall terminate execution of the program without returning to the caller.

> This seems to allow that the function may exit by throwing an exception (because word "return" implies a normal return).

> One could argue that words "terminate execution of the program" are sufficient, but then why "without returning to the caller" would be mentioned. In case such handler throws, noexcept specification in function `std::terminate` is violated, and `std::terminate` would be called recursively - should `std::abort` not be called in case of recursive `std::terminate` call? On the other hand some controlled recursion could be useful, like in the following technique.

The here mentioned wording changes by N3189 in regard to 17.9.5.4 [terminate] p1 were done for a better separation of effects (Effects element) and additional normative wording explanations (Remarks element), there was no meaning change intended. Further, there was already a defect existing in the previous wording, which was not updated when further situations where defined, when `std::terminate` where supposed to be called by the implementation.

The part

"in effect immediately after evaluating the throw-expression"

should be removed and the quoted reference to 17.9.5.1 [terminate.handler] need to be part of the effects element where it refers to the current `terminate_handler` function, so should be moved just after

"Effects: Calls the current `terminate_handler` function."

It seems ok to allow a termination handler to exit via an exception, but the suggested idiom should better be replaced by a more simpler one based on evaluating the current exception pointer in the terminate handler, e.g.

>     void our_terminate (void) {
>       std::exception_ptr p = std::current_exception();
>       if (p) {
>         ... // OK to rethrow and to determine it's nature
>       } else {
>         ... // Do something else
>       }
>     }

[2011-12-09: Daniel comments]

A related issue is 2111(i).

[2012, Kona]

Move to Open.

There is an interaction with Core issues in this area that Jens is already supplying wording for. Review this issue again once Jens wording is available.

Alisdair to review clause 15.5 (per Jens suggestion) and recommend any changes, then integrate Jens wording into this issue.

[2024-11-07 Status changed: Open → Resolved.]

Resolved by the resolution of LWG 2111(i).

Proposed resolution:

### 2089(i). `std::allocator::construct` should use uniform initialization

Section: 20.2.10.2 [allocator.members] Status: Resolved Submitter: David Krauss Opened: 2011-10-07 Last modified: 2020-09-06

Priority: 2

View all other issues in [allocator.members].

View all issues with Resolved status.

Discussion:

When the `EmplaceConstructible` (23.2.2 [container.requirements.general]/13) requirement is used to initialize an object, direct-initialization occurs. Initializing an aggregate or using a `std::initializer_list` constructor with emplace requires naming the initialized type and moving a temporary. This is a result of `std::allocator::construct` using direct-initialization, not list-initialization (sometimes called "uniform initialization") syntax.

Altering `std::allocator<T>::construct` to use list-initialization would, among other things, give preference to `std::initializer_list` constructor overloads, breaking valid code in an unintuitive and unfixable way — there would be no way for `emplace_back` to access a constructor preempted by `std::initializer_list` without essentially reimplementing `push_back`.

>     std::vector<std::vector<int>> v;
>     v.emplace_back(3, 4); // v[0] == {4, 4, 4}, not {3, 4} as in list-initialization

The proposed compromise is to use SFINAE with `std::is_constructible`, which tests whether direct-initialization is well formed. If `is_constructible` is false, then an alternative `std::allocator::construct` overload is chosen which uses list-initialization. Since list-initialization always falls back on direct-initialization, the user will see diagnostic messages as if list-initialization (uniform-initialization) were always being used, because the direct-initialization overload cannot fail.

I can see two corner cases that expose gaps in this scheme. One occurs when arguments intended for `std::initializer_list` satisfy a constructor, such as trying to emplace-insert a value of `{3, 4}` in the above example. The workaround is to explicitly specify the `std::initializer_list` type, as in `v.emplace_back(std::initializer_list<int>(3, 4))`. Since this matches the semantics as if `std::initializer_list` were deduced, there seems to be no real problem here.

The other case is when arguments intended for aggregate initialization satisfy a constructor. Since aggregates cannot have user-defined constructors, this requires that the first nonstatic data member of the aggregate be implicitly convertible from the aggregate type, and that the initializer list have one element. The workaround is to supply an initializer for the second member. It remains impossible to in-place construct an aggregate with only one nonstatic data member by conversion from a type convertible to the aggregate's own type. This seems like an acceptably small hole.

The change is quite small because `EmplaceConstructible` is defined in terms of whatever allocator is specified, and there is no need to explicitly mention SFINAE in the normative text.

[2012, Kona]

Move to Open.

There appears to be a real concern with initializing aggregates, that can be performed only using brace-initialization. There is little interest in the rest of the issue, given the existence of 'emplace' methods in C++11.

Move to Open, to find an acceptable solution for intializing aggregates. There is the potential that EWG may have an interest in this area of language consistency as well.

[2013-10-13, Ville]

This issue is related to 2070(i).

[2015-02 Cologne]

Move to EWG, Ville to write a paper.

[2015-09, Telecon]

Ville: N4462 reviewed in Lenexa. EWG discussion to continue in Kona.

[2016-08 Chicago]

See N4462

The notes in Lenexa say that Marshall & Jonathan volunteered to write a paper on this

[2018-08-23 Batavia Issues processing]

P0960 (currently in flight) should resolve this.

[2020-01 Resolved by the adoption of P0960 in Kona.]

Proposed resolution:

This wording is relative to the FDIS.

Change 20.2.10.2 [allocator.members] p12 as indicated:

>     template <class U, class... Args>
>       void construct(U* p, Args&&... args);

> > 12 Effects: `::new((void *)p) U(std::forward<Args>(args)...)` if `is_constructible<U, Args...>::value` is `true`, else `::new((void *)p) U{std::forward<Args>(args)...}`

### 2091(i). Misplaced effect in `m.try_lock_for()`

Section: 32.6.4.3 [thread.timedmutex.requirements] Status: C++14 Submitter: Pete Becker Opened: 2011-10-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.timedmutex.requirements].

View all issues with C++14 status.

Discussion:

32.6.4.3 [thread.timedmutex.requirements]/4 says, in part,

> "Requires: If the tick period of [the argument] is not exactly convertible … [it] shall be rounded up …"

This doesn't belong in the requires clause. It's an effect. It belongs in paragraph 5. Nitpickingly, this would be a technical change: as written it imposes an obligation on the caller, while moving it imposes an obligation on the callee. Although that's certainly not what was intended.

Peter Dimov comments:

Not to mention that it should round down, not up. :-)

Incidentally, I see that the wrong `try_lock` requirement that the caller shall not own the mutex has entered the standard, after all. Oh well. Let's hope that the real world continues to ignore it.

[2012, Kona]

Remove the offending sentence from the requirements clause. Do not add it back anywhere else. The implementation already must have license to wake up late, so the rounding is invisible.

Move to Review.

[2012, Portland]

Concurrency move to Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3337.

1. Change 32.6.4.3 [thread.timedmutex.requirements]/4 as indicated:

   > -3- The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

   > -4- Requires: If the tick period of `rel_time` is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. If `m` is of type `std::timed_mutex`, the calling thread does not own the mutex.

### 2092(i). Vague Wording for `condition_variable_any`

Section: 32.7.5 [thread.condition.condvarany] Status: C++14 Submitter: Pete Becker Opened: 2011-10-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition.condvarany].

View all issues with C++14 status.

Discussion:

32.7.5 [thread.condition.condvarany]/4 says, in part, that `condition_variable_any()` throws an exception "if any native handle type manipulated is not available".

I don't know what that means. Is this intended to say something different from the analogous words for `condition_variable()` [32.7.4 [thread.condition.condvar]/4], "if some non-memory resource limitation prevents initialization"? If not, it should be worded the same way.

[2012, Kona]

Copy the corresponding wording from the `condition_variable` constructor in 32.7.4 [thread.condition.condvar] p4.

Move to Review.

[2012, Portland]

Concurrency move to Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3337.

1. Change 32.6.4.3 [thread.timedmutex.requirements]/4 as indicated:

   >     condition_variable_any();

   > > […]

   > > -4- Error conditions:
   > > - `resource_unavailable_try_again` — if any native handle type manipulated is not available if some non-memory resource limitation prevents initialization.
   > > - `operation_not_permitted` — if the thread does not have the privilege to perform the operation.

### 2093(i). Throws clause of `condition_variable::wait` with predicate

Section: 32.7.4 [thread.condition.condvar] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2011-10-27 Last modified: 2015-10-20

Priority: Not Prioritized

View all other issues in [thread.condition.condvar].

View all issues with C++14 status.

Discussion:

the Throws: clause of `condition_variable::wait/wait_xxx` functions that take a predicate argument is:

> Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]).

If executing the predicate throws an exception, I would expect such exception to propagate unchanged to the caller, but the throws clause seems to indicate that it gets mutated into a `system_error`. That's because of 16.3.2.4 [structure.specifications]/4:

"If `F`'s semantics contains a Throws:, Postconditions:, or Complexity: element, then that supersedes any occurrences of that element in the code sequence."

Is my interpretation correct? Does it match the intent?

Daniel comments:

I don't think that this interpretation is entirely correct, the wording does not say that `std::system_error` or a derived class must be thrown, it simply is underspecified in this regard (The extreme interpretation is that the behaviour would be undefined, but that would be too far reaching I think). We have better wording for this in 32.6.7.2 [thread.once.callonce] p4, where it says:

"Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]), or any exception thrown by `func`."

or in 32.4.5 [thread.thread.this] p6/p9:

"Throws: Nothing if `Clock` satisfies the `TrivialClock` requirements (30.3 [time.clock.req]) and operations of `Duration` do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note ]"

So, the here discussed Throws elements should add lines along the lines of

"Any exception thrown by operations of `pred`."

and similar wording for time-related operations:

"Any exception thrown by operations of `Duration`",

"Any exception thrown by operations of `chrono::duration<Rep, Period>`"

[2011-11-28: Ganesh comments and suggests wording]

As for the discussion about the exception thrown by the manipulation of time-related objects, I believe the argument applies to all functions declared in 32 [thread]. Therefore, instead of adding wording to each member, I would simply move those requirements from 32.4.5 [thread.thread.this] p6/p9 to a new paragraph in 32.2.4 [thread.req.timing].

As for 32.7.5 [thread.condition.condvarany], the member functions `wait()` and `wait_until()` are described only in terms of the Effects: clause (so strictly speaking, they need no changes), however, `wait_for()` is described with a full set of clauses including Throws: and Error conditions:. Either we should add those clauses to `wait/wait_until` with changes similar to the one above, or remove paragraphs 29 to 34 entirely. By the way, even paragraph 26 could be removed IMHO.

[2012, Kona]

We like the idea behind the proposed resolution.

Modify the first addition to read instead: "Functions that specify a timeout, will throw if an operation on a clock, time point, or time duration throws an exception."

In the note near the bottom change "even if the timeout has already expired" to "or if the timeout has already expired". (This is independent, but the original doesn't seem to make sense.)

Move to Review.

[2012, Portland]

Concurrency move to Ready with slightly ammended wording.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3337.

1. Add a new paragraph at the end of 32.2.4 [thread.req.timing]:

   > […]

   > -6- The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.

   > -7- Implementation-provided clocks that are used for these functions shall meet the `TrivialClock` requirements (30.3 [time.clock.req]).

   > -?- Functions that specify a timeout, will throw if, during the execution of this function, a clock, time point, or time duration throws an exception. [ Note: instantiations of clock, time point and duration types supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]

2. Change 32.4.5 [thread.thread.this] as indicated:

   >     template <class Clock, class Duration>
   >       void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);;

   > > -4- Effects: Blocks the calling thread for the absolute timeout (32.2.4 [thread.req.timing]) specified by `abs_time`.

   > > -5- Synchronization: None.

   > > -6- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).Nothing if `Clock` satisfies the `TrivialClock` requirements (30.3 [time.clock.req]) and operations of `Duration` do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]

   >     template <class Rep, class Period>
   >       void sleep_for(const chrono::duration<Rep, Period>& rel_time);;

   > > -7- Effects: Blocks the calling thread for the relative timeout (32.2.4 [thread.req.timing]) specified by `rel_time`.

   > > -8- Synchronization: None.

   > > -9- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).Nothing if operations of `chrono::duration<Rep, Period>` do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]

3. Change 32.6.4.3 [thread.timedmutex.requirements] as indicated:

   -3- The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

   […]

   -5- Effects: The function attempts to obtain ownership of the mutex within the relative timeout (32.2.4 [thread.req.timing]) specified by `rel_time`. If the time specified by `rel_time` is less than or equal to `rel_time.zero()`, the function attempts to obtain ownership without blocking (as if by calling `try_lock()`). The function shall return within the timeout specified by `rel_time` only if it has obtained ownership of the mutex object. [Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note]

   […]

   -8- Synchronization: If `try_lock_for()` returns `true`, prior `unlock()` operations on the same object synchronize with (6.10.2 [intro.multithread]) this operation.

   -9- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).Nothing.

   -10- The expression `m.try_lock_until(abs_time)` shall be well-formed and have the following semantics:

   […]

   -12- Effects: The function attempts to obtain ownership of the mutex. If `abs_time` has already passed, the function attempts to obtain ownership without blocking (as if by calling `try_lock()`). The function shall return before the absolute timeout (32.2.4 [thread.req.timing]) specified by `abs_time` only if it has obtained ownership of the mutex object. [Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note]

   […]

   -15- Synchronization: If `try_lock_until()` returns true, prior `unlock()` operations on the same object synchronize with (6.10.2 [intro.multithread]) this operation.

   -16- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).Nothing.

4. Change 32.7.4 [thread.condition.condvar] as indicated:

   >     template <class Predicate>
   >       void wait(unique_lock<mutex>& lock, Predicate pred);

   > > […]

   > > -15- Effects: Equivalent to:

   > > >     while (!pred())
   > > >       wait(lock);

   > > […]

   > > -17- Throws: `std::system_error` when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown by `pred`.

   > > […]

   >     template <class Clock, class Duration>
   >       cv_status wait_until(unique_lock<mutex>& lock,
   >                            const chrono::time_point<Clock, Duration>& abs_time);

   > > […]

   > > -23- Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]) or timeout-related exceptions (32.2.4 [thread.req.timing]).

   > > […]

   >     template <class Rep, class Period>
   >       cv_status wait_for(unique_lock<mutex>& lock,
   >                          const chrono::duration<Rep, Period>& rel_time);

   > > […]

   > > -26- Effects: as ifEquivalent to:

   > > >     return wait_until(lock, chrono::steady_clock::now() + rel_time);

   > > […]

   > > -29- Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]) or timeout-related exceptions (32.2.4 [thread.req.timing]).

   > > […]

   >     template <class Clock, class Duration, class Predicate>
   >       bool wait_until(unique_lock<mutex>& lock,
   >                       const chrono::time_point<Clock, Duration>& abs_time,
   >                       Predicate pred);

   > > […]

   > > -32- Effects: Equivalent to:

   > > >     while (!pred())
   > > >       if (wait_until(lock, abs_time) == cv_status::timeout)
   > > >         return pred();
   > > >     return true;

   > > -33- Returns: `pred()`

   > > […]

   > > -36- Throws: `std::system_error` when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown by `pred`.

   > > […]

   >     template <class Rep, class Period, class Predicate>
   >       bool wait_for(unique_lock<mutex>& lock,
   >                     const chrono::duration<Rep, Period>& rel_time,
   >                     Predicate pred);

   > > […]

   > > -39- Effects: as ifEquivalent to:

   > > >     return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

   > > […]

   > > -42- Returns: `pred()`

   > > […]

   > > -44- Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown by `pred`.

   > > […]

5. Change 32.7.5 [thread.condition.condvarany] as indicated:

   >     template <class Lock, class Predicate>
   >       void wait(Lock& lock, Predicate pred);

   > > -14- Effects: Equivalent to:

   > > >     while (!pred())
   > > >       wait(lock);

   >     template <class Lock, class Clock, class Duration>
   >       cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);

   > > […]

   > > -18- Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]) or any timeout-related exceptions (32.2.4 [thread.req.timing]).

   > > […]

   >     template <class Lock, class Rep, class Period>
   >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

   > > […]

   > > -20- Effects: as ifEquivalent to:

   > > >     return wait_until(lock, chrono::steady_clock::now() + rel_time);

   > > […]

   > > -23- Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]) or any timeout-related exceptions (32.2.4 [thread.req.timing]).

   > > […]

   >     template <class Lock, class Clock, class Duration, class Predicate>
   >       bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);

   > > -25- Effects: Equivalent to:

   > > >     while (!pred())
   > > >       if (wait_until(lock, abs_time) == cv_status::timeout)
   > > >         return pred();
   > > >     return true;

   > > -26- Returns: `pred()`[Note: There is no blocking if `pred()` is initially `true`, or if the timeout has already expired. — end note]

   > > -27- [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. end note]

   >     template <class Lock, class Rep, class Period, class Predicate>
   >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

   > > -28- Effects: as ifEquivalent to:

   > > >     return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

   > > -29- [Note: There is no blocking if `pred()` is initially `true`, even if the timeout has already expired. — end note] -30- Postcondition: `lock` is locked by the calling thread.

   > > -31- Returns: `pred()`

   > > -32- [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. — end note]

   > > -33- Throws: `system_error` when an exception is required (32.2.2 [thread.req.exception]).

   > > -34- Error conditions:
   > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

### 2094(i). `duration` conversion overflow shouldn't participate in overload resolution

Section: 30.5.2 [time.duration.cons] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2011-10-31 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.duration.cons].

View all issues with C++14 status.

Discussion:

30.5.2 [time.duration.cons] says:

>     template <class Rep2, class Period2>
>       constexpr duration(const duration<Rep2, Period2>& d);

> > Remarks: This constructor shall not participate in overload resolution unless `treat_as_floating_point<rep>::value` is `true` or both `ratio_divide<Period2, period>::den` is `1` and `treat_as_floating_point<Rep2>::value` is `false`.

The evaluation of `ratio_divide<Period2, period>::den` could make `ratio_divide<Period2, period>::num` overflow.

This occur for example when we try to create a millisecond (`period`=`ratio<1,1000>`) from an exa-second (`Period2`=`ratio<1018>`).

`ratio_divide<ratio<1018>, ratio<1,1000>>::num` is `1021` which overflows which makes the compiler error.

If the function `f` is overloaded with milliseconds and seconds

>     void f(milliseconds);
>     void f(seconds);

The following fails to compile.

>     duration<int,exa> r(1);
>     f(r);

While the conversion to seconds work, the conversion to milliseconds make the program fail at compile time. In my opinion, this program should be well formed and the constructor from `duration<int,exa>` to milliseconds shouldn't participate in overload resolution as the result can not be represented.

I think the wording of the standard can be improved so no misinterpretations are possible by adding that "no overflow is induced by the conversion".

[2012, Kona]

Move to Review.

Pete: The wording is not right.

Howard: Will implement this to be sure it works.

Jeffrey: If ratio needs a new hook, should it be exposed to users for their own uses?

Pete: No.

Move to Review, Howard to implement in a way that mere mortals can understand.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to the FDIS.

Change the following paragraphs of 30.5.2 [time.duration.cons] p4 indicated:

>     template <class Rep2, class Period2>
>       constexpr duration(const duration<Rep2, Period2>& d);

> > Remarks: This constructor shall not participate in overload resolution unless no overflow is induced in the conversion and `treat_as_floating_point<rep>::value` is `true` or both `ratio_divide<Period2, period>::den` is `1` and `treat_as_floating_point<Rep2>::value` is `false`. [ Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note ]

### 2095(i). `promise` and `packaged_task` missing constructors needed for uses-allocator construction

Section: 32.10.6 [futures.promise], 32.10.10 [futures.task] Status: Resolved Submitter: Jonathan Wakely Opened: 2011-11-01 Last modified: 2025-06-23

Priority: 4

View all other issues in [futures.promise].

View all issues with Resolved status.

Discussion:

This example is ill-formed according to C++11 because `uses_allocator<promise<R>, A>::value` is true, but `is_constructible<promise<R>, A, promise<R>&&>::value` is false. Similarly for `packaged_task`.

>     #include <future>
>     #include <memory>
>     #include <tuple>

>     using namespace std;

>     typedef packaged_task<void()> task;
>     typedef promise<void> prom;
>     allocator<task> a;

>     tuple<task, prom> t1{ allocator_arg, a };
>     tuple<task, prom> t2{ allocator_arg, a, task{}, prom{} };

[2012, Portland]

This is an allocator issue, and should be dealt with directly by LWG.

[2013-03-06]

Jonathan suggests to make the new constructors non-explicit and makes some representational improvements.

[2013-09 Chicago]

Move to deferred.

This issue has much in common with similar problems with `std::function` that are being addressed by the polymorphic allocators proposal currently under evaluation in LEWG. Defer further discussion on this topic until the final outcome of that paper and its proposed resolution is known.

[2014-02-20 Re-open Deferred issues as Priority 4]

[2016-08 Chicago]

Fri PM: Send to LEWG - and this also applies to `function` in LFTS.

[2019-06-03 Jonathan Wakely provides updated wording]

Jonathan updates wording post-2976(i) and observes that this resolution conflicts with 3003(i).

Previous resolution [SUPERSEDED]:

> [This wording is relative to the FDIS.]

> 1. Add to 32.10.6 [futures.promise], class template `promise` synopsis, as indicated:

>    >     namespace std {
>    >       template <class R>
>    >       class promise {
>    >       public:
>    >         promise();
>    >         template <class Allocator>
>    >         promise(allocator_arg_t, const Allocator& a);
>    >         template <class Allocator>
>    >         promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
>    >         promise(promise&& rhs) noexcept;
>    >         promise(const promise& rhs) = delete;
>    >         ~promise();
>    >         […]
>    >       };
>    >       […]
>    >     }

> 2. Change 32.10.6 [futures.promise] as indicated:

>    >     promise(promise&& rhs) noexcept;
>    >     template <class Allocator>
>    >     promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;

>    > > -5- Effects: constructs a new `promise` object and transfers ownership of the shared state of `rhs` (if any) to the newly-constructed object.

>    > > -6- Postcondition: `rhs` has no shared state.

>    > > -?- [Note: `a` is not used — end note]

> 3. Add to 32.10.10 [futures.task], class template `packaged_task` synopsis, as indicated:

>    >     namespace std {
>    >       template<class> class packaged_task; // undefined

>    >       template<class R, class... ArgTypes>
>    >       class packaged_task<R(ArgTypes...)> {
>    >       public:
>    >         // construction and destruction
>    >         packaged_task() noexcept;
>    >         template <class Allocator>
>    >           packaged_task(allocator_arg_t, const Allocator& a) noexcept;
>    >         template <class F>
>    >           explicit packaged_task(F&& f);
>    >         template <class F, class Allocator>
>    >           explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
>    >         ~packaged_task();

>    >         // no copy
>    >         packaged_task(const packaged_task&) = delete;
>    >         template<class Allocator>
>    >           packaged_task(allocator_arg_t, const Allocator& a, const packaged_task&) = delete;
>    >         packaged_task& operator=(const packaged_task&) = delete;

>    >         // move support
>    >         packaged_task(packaged_task&& rhs) noexcept;
>    >         template <class Allocator>
>    >           packaged_task(allocator_arg_t, const Allocator& a, packaged_task&& rhs) noexcept;
>    >         packaged_task& operator=(packaged_task&& rhs) noexcept;
>    >         void swap(packaged_task& other) noexcept;
>    >         […]
>    >       };
>    >       […]
>    >     }

> 4. Change 32.10.10.2 [futures.task.members] as indicated:

>    >     packaged_task() noexcept;
>    >     template <class Allocator>
>    >       packaged_task(allocator_arg_t, const Allocator& a) noexcept;

>    > > -1- Effects: constructs a `packaged_task` object with no shared state and no stored task.

>    > > -?- [Note: `a` is not used — end note]

>    […]

>    >     packaged_task(packaged_task&& rhs) noexcept;
>    >     template <class Allocator>
>    >       packaged_task(allocator_arg_t, const Allocator& a, packaged_task&& rhs) noexcept;

>    > > -5- Effects: constructs a new `packaged_task` object and transfers ownership of `rhs`'s shared state to `*this`, leaving `rhs` with no shared state. Moves the stored task from `rhs` to `*this`.

>    > > -6- Postcondition: `rhs` has no shared state.

>    > > -?- [Note: `a` is not used — end note]

[2025-06-21 Status changed: LEWG → Resolved.]

Resolved by adoption of P3503R3 in Sofia.

Proposed resolution:

[This wording is relative to N4810.]

1. Add to 32.10.6 [futures.promise], class template `promise` synopsis, as indicated:

   >     namespace std {
   >       template <class R>
   >       class promise {
   >       public:
   >         promise();
   >         template <class Allocator>
   >           promise(allocator_arg_t, const Allocator& a);
   >         template <class Allocator>
   >           promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;
   >         promise(promise&& rhs) noexcept;
   >         promise(const promise& rhs) = delete;
   >         ~promise();
   >         […]
   >       };
   >       […]
   >     }

2. Change 32.10.6 [futures.promise] as indicated:

   >     promise(promise&& rhs) noexcept;
   >     template <class Allocator>
   >       promise(allocator_arg_t, const Allocator& a, promise&& rhs) noexcept;

   > > -5- Effects: constructs a new `promise` object and transfers ownership of the shared state of `rhs` (if any) to the newly-constructed object.

   > > -6- Postcondition: `rhs` has no shared state.

   > > -?- [Note: `a` is not used — end note]

### 2096(i). Incorrect constraints of `future::get` in regard to `MoveAssignable`

Section: 32.10.7 [futures.unique.future] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-02 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [futures.unique.future].

View all issues with C++14 status.

Discussion:

[futures.unique_future] paragraph 15 says the following:

>     R future::get();

…

-15- Returns:

- `future::get()` returns the value stored in the object’s shared state. If the type of the value is `MoveAssignable` the returned value is moved, otherwise it is copied.

  …

There are some problems with the description:

"If the type of the value is `MoveAssignable` the returned value is moved, otherwise it is copied."

1. It seems to impose unrealistic constraints on implementations, because how could an implementor recognize whether a user-defined type satisfies the semantics of `MoveAssignable`? This should be based solely on a pure expression-based requirement, if this is an requirement for implementations.
2. Reducing `MoveAssignable` to the plain expression part `std::is_move_assignable` would solvs (1), but raises another question, namely why a move-assignment should be relevant for a function return based on the value stored in the future state? We would better fall back to `std::is_move_constructible` instead.

3. The last criticism I have is about the part

   "the returned value is moved, otherwise it is copied"

   because an implementation won't be able to recognize what the user-defined type will do during an expression that is prepared by the implementation. I think the wording is intended to allow a move by seeding with an rvalue expression via `std::move` (or equivalent), else the result will be an effective copy construction.

[2011-11-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib.]

Proposed resolution:

This wording is relative to the FDIS.

Change [futures.unique_future] paragraph 15 as indicated:

>     R future::get();

…

-15- Returns:

- `future::get()` returns the value `v` stored in the object’s shared state as `std::move(v)`. If the type of the value is `MoveAssignable` the returned value is moved, otherwise it is copied.

  …

### 2097(i). `packaged_task` constructors should be constrained

Section: 32.10.10.2 [futures.task.members] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-02 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.task.members].

View all issues with C++14 status.

Discussion:

With the proposed resolution of 2067(i), this no longer selects the copy constructor:

>     std::packaged_task<void()> p1;
>     std::packaged_task<void()> p2(p1);

Instead this constructor is a better match:

>     template <class F>
>      explicit packaged_task(F&& f);

This attempts to package a `packaged_task`, which internally tries to copy `p2`, which fails because the copy constructor is deleted. For at least one implementation the resulting error message is much less helpful than the expected "cannot call deleted function" because it happens after instantiating several more templates rather than in the context where the constructor is called.

I believe the solution is to constrain to the template constructors so the template argument `F` cannot be deduced as (possibly cv) `packaged_task&` or `packaged_task`. It could be argued this constraint is already implied because `packaged_task` is not copyable and the template constructors require that "invoking a copy of `f` shall behave the same as invoking `f`".

Daniel points out that the variadic constructor of `std::thread` described in 32.4.3.3 [thread.thread.constr] has a similar problem and suggests a similar wording change, which has been integrated below.

An alternative is to declare `thread(thread&)` and `packaged_task(packaged_task&)` as deleted.

[2012, Portland]

This issue appears to be more about library specification than technical concurrency issues, so should be handled in LWG.

[2013, Chicago]

Move to Immediate resolution.

Howard volunteered existing implementation experience with the first change, and saw no issue that the second would introduce any new issue.

Proposed resolution:

This wording is relative to the FDIS.

1. Insert a new Remarks element to 32.4.3.3 [thread.thread.constr] around p3 as indicated:

   >     template <class F, class ...Args> explicit thread(F&& f, Args&&... args);

   -3- Requires: `F` and each `Ti` in `Args` shall satisfy the `MoveConstructible` requirements. `INVOKE(DECAY_COPY ( std::forward<F>(f)), DECAY_COPY (std::forward<Args>(args))...)` (20.8.2) shall be a valid expression.

   -?- Remarks: This constructor shall not participate in overload resolution if `decay<F>::type` is the same type as `std::thread`.

2. Insert a new Remarks element to 32.10.10.2 [futures.task.members] around p2 as indicated:

   >     template <class F>
   >       packaged_task(F&& f);
   >     template <class F, class Allocator>
   >       explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);

   -2- Requires: `INVOKE(f, t1, t2, ..., tN, R)`, where `t1, t2, ..., tN` are values of the corresponding types in `ArgTypes...`, shall be a valid expression. Invoking a copy of `f` shall behave the same as invoking `f`.

   -?- Remarks: These constructors shall not participate in overload resolution if `decay<F>::type` is the same type as `std::packaged_task<R(ArgTypes...)>`.

### 2098(i). Minor Inconsistency between `promise::set_value` and `promise::set_value_at_thread_exit`

Section: 32.10.6 [futures.promise] Status: C++14 Submitter: Pete Becker Opened: 2011-11-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.promise].

View all issues with C++14 status.

Discussion:

32.10.6 [futures.promise]/16 says that `promise::set_value(const R&)` throws any exceptions thrown by `R`'s copy constructor, and that `promise_set_value(R&&)` throws any exceptions thrown by `R`'s move constructor.

32.10.6 [futures.promise]/22 is the Throws: clause for `promise::set_value_at_thread_exit`. It has no corresponding requirements, only that these functions throw "`future_error` if an error condition occurs."

Daniel suggests wording to fix this: The approach is a bit more ambitious and also attempts to fix wording glitches of 32.10.6 [futures.promise]/16, because it would be beyond acceptable efforts of implementations to determine whether a constructor call of a user-defined type will indeed call a copy constructor or move constructor (in the first case it might be a template constructor, in the second case it might also be a copy-constructor, if the type has no move constructor).

[2012, Portland: move to Review]

Moved to Review by the concurrency working group, with no further comments.

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to the FDIS.

1. Change 32.10.6 [futures.promise]/16 as indicated:

   >     void promise::set_value(const R& r);
   >     void promise::set_value(R&& r);
   >     void promise<R&>::set_value(R& r);
   >     void promise<void>::set_value();

   > > […]

   > > -16- Throws:
   > > - `future_error` if its shared state already has a stored value or exception, or
   > > - for the first version, any exception thrown by the copy constructor ofconstructor selected to copy an object of `R`, or
   > > - for the second version, any exception thrown by the move constructor ofconstructor selected to move an object of `R`.

2. Change 32.10.6 [futures.promise]/22 as indicated:

   >     void promise::set_value_at_thread_exit(const R& r);
   >     void promise::set_value_at_thread_exit(R&& r);
   >     void promise<R&>::set_value_at_thread_exit(R& r);
   >     void promise<void>::set_value_at_thread_exit();

   > > […]

   > > -16- Throws: `future_error` if an error condition occurs.
   > > - `future_error` if its shared state already has a stored value or exception, or
   > > - for the first version, any exception thrown by the constructor selected to copy an object of `R`, or
   > > - for the second version, any exception thrown by the constructor selected to move an object of `R`.

### 2099(i). Unnecessary constraints of `va_start()` usage

Section: 17.14 [support.runtime] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [support.runtime].

View all issues with C++14 status.

Discussion:

In 17.14 [support.runtime] p3 we find (emphasis mine):

> The restrictions that ISO C places on the second parameter to the `va_start()` macro in header `<stdarg.h>` are different in this International Standard. The parameter `parmN` is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).227 If the parameter `parmN` is declared with a function, array, or reference type, or with a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.

It seems astonishing that the constraints on function types and array types imposes these on the declared parameter `parmN`, not to the adjusted one (which would not require this extra wording, because that is implicit). This seems to say that a function definition of the form (Thanks to Johannes Schaub for this example)

>     #include <stdarg.h>

>     void f(char const paramN[], ...) {
>       va_list ap;
>       va_start(ap, paramN);
>       va_end(ap);
>     }

would produce undefined behaviour when used.

Similar wording exists in C99 and in the most recent C11 draft in 7.16.1.4 p4

In my opinion the constraints in regard to array types and function types are unnecessary and should be relaxed. Are there really implementations out in the wild that would (according to my understanding incorrectly) provide the declared and not the adjusted type of `paramN` as deduced type to `va_start()`?

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 17.14 [support.runtime] p3 as indicated:

> The restrictions that ISO C places on the second parameter to the `va_start()` macro in header `<stdarg.h>` are different in this International Standard. The parameter `parmN` is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).227 If the parameter `parmN` is declared withof a function, array, or reference type, or withof a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.

### 2100(i). timed waiting functions cannot timeout if `launch::async` policy used

Section: 32.10.9 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.async].

View all issues with C++14 status.

Discussion:

32.10.9 [futures.async] p5 says

> If the implementation chooses the `launch::async` policy,

> - a call to a waiting function on an asynchronous return object that shares the shared state created by this `async` call shall block until the associated thread has completed, as if joined (32.4.3.6 [thread.thread.member]);

That should say a non-timed waiting function, otherwise, calling a timed waiting function can block indefinitely waiting for the associated thread to complete, rather than timing out after the specified time.

Since `std::thread` does not provide a `timed_join()` function (nor does Pthreads, making it impossible on many platforms) there is no way for a timed waiting function to try to join but return early due to timeout, therefore timed waiting functions either cannot guarantee to timeout or cannot be used to meet the requirement to block until the thread is joined. In order to allow timed waiting functions to timeout the requirement should only apply to non-timed waiting functions.

[2012, Portland: move to Review]

Detlef: Do we actually need this fix — is it detectable?

Yes — you will never get a timeout. Should we strike the whole paragraph?

Hans: issue with thread local destruction.

Niklas: I have a strong expectation that a timed wait will respect the timeout

agreed

Detlef: we want a timed wait that does not time out to return like a non-timed wait; but is this implementable?

Pablo: Could we simply append ", or else time out"

Detlef: the time out on the shared state needs implementing anyway, even if the underlying O/S does not support a timed join.

Hans: the net effect is the timeout does not cover the thread local destruction... ah, I see what you're doing

Detlef: happy with Pablo's proposal

Wording proposed is to append after the word "joined" add ", or else time out"

Moved to review with this wording.

[2013, Bristol]

"Non-timed" made the new wording redundant and the result overly weak. Remove it.

Attempted to move to add this to the working paper (Concurrency motion 2) without the addition of "non-timed". Motion was withdrawn after Jonathan Wakely expressed implementability concerns.

[2013-09, Chicago]

Discussion of interaction with the absence of a Posix timed join.

Jonathan Wakely withdrew his objection, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

[This wording is relative to the FDIS.]

Change 32.10.9 [futures.async] p5 as indicated:

> If the implementation chooses the `launch::async` policy,

> - a call to a waiting function on an asynchronous return object that shares the shared state created by this `async` call shall block until the associated thread has completed, as if joined, or else time out (32.4.3.6 [thread.thread.member]);

### 2101(i). Some transformation types can produce impossible types

Section: 21.3.8 [meta.trans] Status: C++17 Submitter: Daniel Krügler Opened: 2011-11-18 Last modified: 2017-07-30

Priority: 3

View all issues with C++17 status.

Discussion:

Table 53 — "Reference modifications" says in regard to the type trait `add_lvalue_reference` (emphasize mine)

> If `T` names an object or function type then the member typedef type shall name `T&`;

The problem with this specification is that function types with cv-qualifier or ref-qualifier, like

>     void() const
>     void() &

are also affected by the first part of the rule, but this would essentially mean, that instantiating `add_lvalue_reference` with such a type would attempt to form a type that is not defined in the C++ type system, namely

>     void(&)() const
>     void(&)() &

The general policy for TransformationTraits is to define always some meaningful mapping type, but this does not hold for `add_lvalue_reference`, `add_rvalue_reference`, and in addition to these two for `add_pointer` as well. The latter one would attempt to form the invalid types

>     void(*)() const
>     void(*)() &

A possible reason why those traits were specified in this way is that in C++03 (and that means for TR1), cv-qualifier were underspecified in the core language and several compilers just ignored them during template instantiations. This situation became fixed by adopting CWG issues 295 and 547.

While there is possibly some core language clarification needed (see reflector messages starting from c++std-core-20740), it seems also clear that the library should fix the specification. The suggested resolution follows the style of the specification of the support concepts `PointeeType` and `ReferentType` defined in N2914.

[2012-02-10, Kona]

Move to NAD.

These cv- and ref-qualified function types are aberrations in the type system, and do not represent any actual entity defined by the language. The notion of cv- and ref- qualification applies only to the implicit `*this` reference in a member function.

However, these types can be produced by quirks of template metaprogramming, the question remains what the library should do about it. For example, `add_reference` returns the original type if passed a reference type, or a `void` type. Conversely, `add_pointer` will return a pointer to the referenced type when passed a reference.

It is most likely that the 'right' answer in any case will depend on the context that the question is being asked, in terms of forming these obscure types. The best the LWG can do is allow an error to propagate back to the user, so they can provide their own meaningful answer in their context - with additional metaprogramming on their part. The consensus is that if anyone is dangerous enough with templates to get themselves into this problem, they will also have the skills to resolve the problem themselves. This is not going to trip up the non-expert developer.

Lastly, it was noted that this problem arises only because the language is inconsistent in providing us these nonsense types that do no really represent anything in the language. There may be some way Core or Evolution could give us a more consistent type system so that the LWG does not need to invent an answer at all, should this question need resolving. This is another reason to not specify anything at the LWG trait level at this time, leaving the other working groups free to produce the 'right' answer that we can then follow without changing the meaning of existing, well-defined programs.

[2012-02-10, post-Kona]

Move back to Open. Daniel is concerned that this is not an issue we can simply ignore, further details to follow.

[2012-10-06, Daniel comments]

This issue really should be resolved as a defect: First, the argument that "forming these obscure types" should "allow an error to propagate" is inconsistent with the exact same "obscure type" that would be formed when `std::add_lvalue_reference<void>` wouldn't have an extra rules for `void` types, which also cannot form references. The originally proposed resolution attempts to apply the same solution for the same common property of `void` types and function types with cv-qualifiers or ref-qualifier. These functions had the property of `ReferentType` during concept time (see CWG 749 bullet three for the final wording).

Core issue CWG 1417 has clarified that any attempt to form a reference of a pointer to a function type with cv-qualifiers or ref-qualifier is ill-formed. Unfortunately, many compilers don't implement this yet.

I also would like to warn about so-called "obscure" types: The problem is that these can occur as the side effect of finding a best match overload of function templates, where this type is exactly correct for one of these overloads, but causes a deep (not-sfinae-friendly) error for others where one of these traits are part of the signature.

Existing experience with `void` types shows, that this extra rule is not so unexpected. Further, any usage of the result types of these traits as argument types or return types of functions would make these ill-formed (and in a template context would be sfinaed away), so the expected effects are rarely unnoticed. Checking all existing explicit usages of the traits `add_rvalue_reference`, `add_lvalue_reference`, and `add_pointer` didn't show any example where the error would be silent: `add_rvalue_reference` is used to specify the return value of `declval()` and the instantiation of `declval<void() const>()` would be invalid, because of the attempt to return a function type. Similarly, `add_lvalue_reference` is used to specify the return type of `unique_ptr<T>::operator*()`. Again, any instantiation with `void() const` wouldn't remain unnoticed. The trait `add_pointer` is used to specify the trait `std::decay` and this is an interesting example, because it is well-formed when instantiated with `void` types, too, and is heavily used throughout the library specification. All use-cases would not be negatively affected by the suggested acceptance of function types with cv-qualifiers or ref-qualifier, because they involve types that are either function arguments, function parameters or types were references are formed from.

The alternative would be to add an additional extra rule that doesn't define a type member 'type' when we have a function type with cv-qualifiers or ref-qualifier. This is better than the current state but it is not superior than the proposal to specify the result as the original type, because both variants are sfinae-friendly. A further disadvantage of the "non-type" approach here would be that any usage of `std::decay` would require special protection against these function types, because instantiating `std::decay<void() const>` again would lead to a deep, sfinae-unfriendly error.

The following example demonstrates the problem: Even though the second `f` template is the best final match here, the first one will be instantiated. During that process `std::decay<T>::type` becomes instantiated as well and will raise a deep error, because as part of the implementation the trait `std::add_pointer<void() const>` becomes instantiated:

>     #include <type_traits>

>     template<class T>
>     typename std::decay<T>::type f(T&& t);

>     template<class T, class U>
>     U f(U u);

>     int main() {
>       f<void() const>(0);
>     }

When the here proposed resolution would be applied this program would be well-formed and selects the expected function.

Previous resolution from Daniel [SUPERSEDED]:

> 1. Change Table 53 — "Reference modifications" in 21.3.8.3 [meta.trans.ref] as indicated:

>    Table 53 — Reference modifications
>    Template | Comments
>    --- | ---
>    `…`
>    `template <class T> struct add_lvalue_reference;` | If `T` names an object `type` or if `T` names a function type that does not have cv-qualifiers or a ref-qualifier then the member typedef `type` shall name `T&`; otherwise, if `T` names a type "rvalue reference to `T1`" then the member typedef `type` shall name `T1&`; otherwise, `type` shall name `T`.
>    `template <class T> struct add_rvalue_reference;` | If `T` names an object `type` or if `T` names a function type that does not have cv-qualifiers or a ref-qualifier then the member typedef `type` shall name `T&&`; otherwise, `type` shall name `T`. [Note: This rule reflects the semantics of reference collapsing (9.3.4.3 [dcl.ref]). For example, when a type `T` names a type `T1&`, the type `add_rvalue_reference<T>::type` is not an rvalue reference. — end note]

> 2. Change Table 56 — "Pointer modifications" in 21.3.8.6 [meta.trans.ptr] as indicated:

>    Table 56 — Pointer modifications
>    Template | Comments
>    --- | ---
>    `…`
>    `template <class T> struct add_pointer;` | The member typedef `type` shall name the same type as If `T` names a function type that has cv-qualifiers or a ref-qualifier then the member typedef `type` shall name `T`; otherwise, it shall name the same type as `remove_reference<T>::type*`.

The following revised proposed resolution defines - in the absence of a proper core language definition - a new term referenceable type as also suggested by the resolution for LWG 2196(i) as an umbrella of the negation of `void` types and function types with cv-qualifiers or ref-qualifier. This simplifies and minimizes the requires wording changes.

[ 2013-09-26, Daniel synchronizes wording with recent draft ]

[ 2014-05-18, Daniel synchronizes wording with recent draft and comments ]

My impression is that this urgency of action this issue attempts to point out is partly caused by the fact that even for the most recent C++14 compilers the implementations have just recently changed to adopt the core wording. Examples for these are bug reports to gcc or clang.

Occasionally the argument has been presented to me that the suggested changes to the traits affected by this issue would lead to irregularities compared to other traits, especially the lack of guarantee that `add_pointer` might not return a pointer or that `add_(l/r)value_reference` might not return a reference type. I would like to point out that this kind of divergence is actually already present in most `add/remove` traits: For example, we have no guarantee that `add_const` returns a const type (Reference types or function types get special treatments), or that `add_rvalue_reference` returns an rvalue-reference (e.g. when applied to an lvalue-reference type).

Zhihao Yuan brought to my attention, that the originally proposing paper N1345 carefully discussed these design choices.

[2015-05, Lenexa]

MC: move to Ready: in favor: 16, opposed: 0, abstain: 1
STL: have libstdc++, libc++ implemented this? we would need to change your implementation

Proposed resolution:

This wording is relative to N3936.

1. Change Table 53 — "Reference modifications" in 21.3.8.3 [meta.trans.ref] as indicated:

   Table 53 — Reference modifications
   Template | Comments
   --- | ---
   `…`
   `template <class T> struct add_lvalue_reference;` | If `T` names an object or function typea referenceable type then the member typedef `type` shall name `T&`; otherwise, if `T` names a type "rvalue reference to `T1`" then the member typedef `type` shall name `T1&`; otherwise, `type` shall name `T`. [Note: This rule reflects the semantics of reference collapsing (9.3.4.3 [dcl.ref]). — end note]
   `template <class T> struct add_rvalue_reference;` | If `T` names an object or function typea referenceable type then the member typedef `type` shall name `T&&`; otherwise, `type` shall name `T`. [Note: This rule reflects the semantics of reference collapsing (9.3.4.3 [dcl.ref]). For example, when a type `T` names a type `T1&`, the type `add_rvalue_reference_t<T>` is not an rvalue reference. — end note]

2. Change Table 56 — "Pointer modifications" in 21.3.8.6 [meta.trans.ptr] as indicated:

   Table 56 — Pointer modifications
   Template | Comments
   --- | ---
   `…`
   `template <class T> struct add_pointer;` | If `T` names a referenceable type or a (possibly cv-qualified) `void` type then Tthe member typedef `type` shall name the same type as `remove_reference_t<T>*`; otherwise, `type` shall name `T`.

### 2102(i). Why is `std::launch` an implementation-defined type?

Section: 32.10.1 [futures.overview] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.10.1 [futures.overview] says `std::launch` is an implementation-defined bitmask type, which would usually mean the implementation can choose whether to define an enumeration type, or a `bitset`, or an integer type. But in the case of `std::launch` it's required to be a scoped enumeration type,

>     enum class launch : unspecified {
>       async = unspecified,
>       deferred = unspecified,
>       implementation-defined
>     };

so what is implementation-defined about it, and what is an implementation supposed to document about its choice?

[2011-12-02 Moved to Tentatively Ready after 6 positive votes on c++std-lib.]

Proposed resolution:

This wording is relative to the FDIS.

Change 32.10.1 [futures.overview] paragraph 2 as indicated:

> The enum type `launch` is an implementation-defineda bitmask type (16.3.3.3.3 [bitmask.types]) with `launch::async` and `launch::deferred` denoting individual bits. [ Note: Implementations can provide bitmasks to specify restrictions on task interaction by functions launched by `async()` applicable to a corresponding subset of available launch policies. Implementations can extend the behavior of the first overload of `async()` by adding their extensions to the launch policy under the “as if” rule. — end note ]

### 2103(i). `std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment`

Section: 20.2.10 [default.allocator] Status: C++14 Submitter: Ai Azuma Opened: 2011-11-08 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

"`std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment::value`" is specified as "false", according to (20.2.10 [default.allocator]) and (20.2.9.2 [allocator.traits.types]). However, according to (23.2.2 [container.requirements.general]), this specification leads to the unneeded requirements (`MoveInsertable` and `MoveAssignable` of the value type) on the move assignment operator of containers with the default allocator.

Proposed resolution:

Either of the following two changes;

1. adding the nested typedef like "`typedef std::true_type propagate_on_container_move_assignment;`" in the definition of `std::allocator` class template,
2. adding the explicit partial specialization of "`std::allocator_traits`" class template for "`std::allocator`" class template, in which "`propagate_on_container_move_assignment`" nested typedef is specified as "`std::true_type`".

Pablo prefers the first resolution.

[2011-12-02: Pablo comments]

This issue has potentially some overlap with 2108(i). Should the trait `always_compare_equal` been added, this issue's resolution should be improved based on that.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 20.2.10 [default.allocator], the class template `allocator` synopsis as indicated:

>     namespace std {
>       template <class T> class allocator;

>       // specialize for void:
>       template <> class allocator<void> {
>       public:
>         typedef void* pointer;
>         typedef const void* const_pointer;
>         // reference-to-void members are impossible.
>         typedef void value_type;
>         template <class U> struct rebind { typedef allocator<U> other; };
>       };

>       template <class T> class allocator {
>       public:
>         typedef size_t size_type;
>         typedef ptrdiff_t difference_type;
>         typedef T* pointer;
>         typedef const T* const_pointer;
>         typedef T& reference;
>         typedef const T& const_reference;
>         typedef T value_type;
>         template <class U> struct rebind { typedef allocator<U> other; };
>         typedef true_type propagate_on_container_move_assignment;

>         […]
>       };
>     }

### 2104(i). `unique_lock` move-assignment should not be `noexcept`

Section: 32.6.5.4 [thread.lock.unique] Status: C++14 Submitter: Anthony Williams Opened: 2011-11-27 Last modified: 2017-07-05

Priority: Not Prioritized

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Discussion:

I just noticed that the `unique_lock` move-assignment operator is declared `noexcept`. This function may call `unlock()` on the wrapped mutex, which may throw.

Suggested change: remove the `noexcept` specification from `unique_lock::operator=(unique_lock&&)` in 32.6.5.4 [thread.lock.unique] and 32.6.5.4.2 [thread.lock.unique.cons].

Daniel:

I think the situation is actually a bit more complex as it initially looks.

First, the effects of the move-assignment operator are (emphasize mine):

> Effects: If `owns` calls `pm->unlock()`.

Now according to the `BasicLockable` requirements:

> `m.unlock()`

> 3 Requires: The current execution agent shall hold a lock on `m`.

> 4 Effects: Releases a lock on `m` held by the current execution agent.

> Throws: Nothing.

This shows that unlock itself is a function with narrow contract and for this reasons no unlock function of a mutex or lock itself does have a noexcept specifier according to our mental model.

Now the move-assignment operator attempts to satisfy these requirement of the function and calls it only when it assumes that the conditions are ok, so from the view-point of the caller of the move-assignment operator it looks as if the move-assignment operator would in total a function with a wide contract.

The problem with this analysis so far is, that it depends on the assumed correctness of the state "owns".

Looking at the construction or state-changing functions, there do exist several ones that depend on caller-code satisfying the requirements and there is one guy, who looks most suspicious:

> `unique_lock(mutex_type& m, adopt_lock_t);`

> 11 Requires: The calling thread own the mutex.
> […]
> 13 Postconditions: `pm == &m` and `owns == true`.

because this function does not even call `lock()` (which may, but is not required to throw an exception if the calling thread does already own the mutex). So we have in fact still a move-assignment operator that might throw an exception, if the mutex was either constructed or used (call of lock) incorrectly.

The correct fix seems to me to also add a "Throws: Nothing" element to the move-assignment operator, because using it correctly shall not throw an exception.

[Issaquah 2014-02-11: Move to Immediate after SG1 review]

Proposed resolution:

This wording is relative to the FDIS.

1. Change 32.6.5.4 [thread.lock.unique], class template `unique_lock` synopsis as indicated:

   >     namespace std {
   >       template <class Mutex>
   >       class unique_lock {
   >       public:
   >         typedef Mutex mutex_type;
   >         […]
   >         unique_lock(unique_lock&& u) noexcept;
   >         unique_lock& operator=(unique_lock&& u) noexcept;
   >         […]
   >       };
   >     }

2. Change 32.6.5.4.2 [thread.lock.unique.cons] around p22 as indicated:

   >     unique_lock& operator=(unique_lock&& u) noexcept;

   > > -22- Effects: If `owns` calls `pm->unlock()`.

   > > -23- Postconditions: `pm == u_p.pm` and `owns == u_p.owns` (where `u_p` is the state of `u` just prior to this construction), `u.pm == 0` and `u.owns == false`.

   > > -24- [Note: With a recursive mutex it is possible for both `*this` and `u` to own the same mutex before the assignment. In this case, `*this` will own the mutex after the assignment and `u` will not. — end note] -??- Throws: Nothing.

### 2105(i). Inconsistent requirements on `const_iterator`'s `value_type`

Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Jeffrey Yasskin Opened: 2011-11-28 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In the FDIS, Table 96 specifies `X::const_iterator` as a "constant iterator type whose value type is `T`". However, Table 97 specifies `X::const_reverse_iterator` as an "iterator type whose value type is `const T`" and which is defined as `reverse_iterator<const_iterator>`. But `reverse_iterator::value_type` is just "`typename iterator_traits<Iterator>::value_type`" 24.5.1.2 [reverse.iterator], so `const_iterator` and `const_reverse_iterator` must have the same `value_type`.

The resolution to issue 322(i) implies that `const_reverse_iterator` should change.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change Table 97 — "Reversible container requirements" as indicated

Table 97 — Reversible container requirements
Expression | Return type | Assertion/note pre-/post-condition | Complexity
--- | --- | --- | ---
`X::reverse_- iterator` | iterator type whose value type is `T` | `reverse_iterator<iterator>` | compile time
`X::const_- reverse_- iterator` | constant iterator type whose value type is `const T` | `reverse_iterator<const_iterator>` | compile time

### 2106(i). `move_iterator` wrapping iterators returning prvalues

Section: 24.5.4 [move.iterators] Status: C++17 Submitter: Dave Abrahams Opened: 2011-11-30 Last modified: 2017-07-30

Priority: 3

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Discussion:

Shouldn't `move_iterator` be specialized so that if the iterator it wraps returns a prvalue when dereferenced, the `move_iterator` also returns by value? Otherwise, it creates a dangling reference.

Howard: I believe just changing `move_iterator<I>::reference` would do. A direction might be testing on `is_reference<iterator_traits<I>::reference>`, or `is_reference<decltype(*declval<I>())>`.

Daniel: I would prefer to use a consistent style among the iterator adaptors, so I suggest to keep with the `iterator_traits` typedefs if possible.

>     using reference = typename conditional<
>       is_reference<typename iterator_traits<Iterator>::reference>::value,
>       value_type&&,
>       value_type
>     >::type;

We might also want to ensure that if `Iterator`'s `reference` type is a reference, the referent is equal to `value_type` (after removal of cv-qualifiers). In general we have no such guarantee.

Marc: In the default case where we don't return `value_type&&`, should we use `value_type` or should we keep the `reference` type of the wrapped iterator?

Daniel: This suggestion looks appealing at first, but the problem here is that using this typedef can make it impossible for `move_iterator` to satisfy its contract, which means returning an rvalue of the value type (Currently it says rvalue-reference, but this must be fixed as of this issue anyway). I think that user-code can reasonably expect that when it has constructed an object `m` of `move_iterator<It>`, where `It` is a valid mutable iterator type, the expression

>     It::value_type&& rv = *m;

is well-formed.

Let's set `R` equal to `iterator_traits<Iterator>::reference` in the following. We can discuss the following situations:

1. `R` is a reference type: We can only return the corresponding xvalue of `R`, if `value_type` is reference-related to the referent type, else this is presumably no forward iterator and we cannot say much about it, except that it must be convertible to `value_type`, so it better should return a prvalue.
2. `R` is not a reference type: In this case we can rely on a conversion to `value_type` again, but not much more. Assume we would return `R` directly, this might turn out to have a conversion to an lvalue-reference type of the value type (for example). If that is the case, this would indirectly violate the contract of `move_iterator`.

In regard to the first scenario I suggest that implementations are simply required to check that `V2 = remove_cv<remove_reference<R>::type>::type` is equal to the value type `V1` as a criterion to return this reference as an xvalue, in all other cases it should return the value type directly as prvalue.

The additional advantage of this strategy is, that we always ensure that `reference` has the correct cv-qualification, if `R` is a real reference.

It is possible to improve this a bit by indeed supporting reference-related types, this would require to test `is_same<V1, V2>::value || is_base_of<V1, V2>::value` instead. I'm unsure whether (a) this additional effort is worth it and (b) a strict reading of the forward iterator requirements seems not to allow to return a reference-related type (Whether this is a defect or not is another question).

[2011-12-05: Marc Glisse comments and splits into two resolution alternatives]

I guess I am looking at the speed of:

>     value_type x;
>     x = *m;

(copy construction would likely trigger copy elision and thus be neutral) instead of the validity of:

>     value_type&& x = *m;

In this sense, Daniels earlier proposition that ignored `value_type` and just did `switch_lvalue_ref_to_rvalue_ref<reference>` was easier to understand (and it didn't require thinking about reference related types).

The currently proposed resolution has been split into two alternatives.

[2012, Kona]

Move to Review.

Alisdair: This only applies to input iterators, so keep that in mind when thinking about this.

STL: I see what B is doing, but not A.

Howard: I agree.

Alisdair: Should we use `add_rvalue_reference`?

STL: No, we do not want reference collapsing.

STL: Re A, messing with the CV qualification scares me.

Alisdair: Agree. That would break my intent.

STL: Actually I don't think it's actually wrong, but I still don't see what it's doing.

Alisdair: A is picking the value type, B is picking the proxy type.

Howard: I like returning the proxy type.

STL: Returning a reference (B) seems right, because the requirements say "reference". I suspect that B works correctly if you have a move iterator wrapping a move iterator wrapping a thing. I think that A would mess up the type in the middle.

Considerable discussion about which version is correct, checking various examples.

STL: Still think B is right. Still don't understand A. In A we are losing the proxyness.

Howard: Agree 100%. We don't want to lose the proxy. If it's const, so be it.

STL: B is also understandable by mortals.

Howard: Remove to review, keep A but move it out of the proposed resolution area (but keep it for rational).

Alisdair: Adding an explanatory note might be a good idea, if someone wants to write one.

Walter: Concerned about losing the word "reference" in p.1.

Howard: `move_iterator` will return an xvalue or a prvalue, both of which are rvalues.

[Proposed resolution A, rejected in preference to the currently proposed resolution (B)

1. Change 24.5.4 [move.iterators] p1 as indicated:

   > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its dereference operator implicitly converts the value returned by the underlying iterator's dereference operator to an rvalue referenceof the value type. Some generic algorithms can be called with move iterators to replace copying with moving.

2. Change 24.5.4.2 [move.iterator], class template `move_iterator` synopsis, as indicated:

   >     namespace std {
   >       template <class Iterator>
   >       class move_iterator {
   >       public:
   >         typedef Iterator iterator_type;
   >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
   >         typedef Iterator pointer;
   >         typedef typename iterator_traits<Iterator>::value_type value_type;
   >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
   >         typedef value_type&&see below reference;
   >         […]
   >       };
   >     }

3. Immediately following the class template `move_iterator` synopsis in 24.5.4.2 [move.iterator] insert a new paragraph as indicated:

   > -?- Let `R` be `iterator_traits<Iterator>::reference` and let `V` be `iterator_traits<Iterator>::value_type`. If `is_reference<R>::value` is `true` and if `remove_cv<remove_reference<R>::type>::type` is the same type as `V`, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `V`.

]

[2012, Portland: Move to Tentatively Ready]

AJM wonders if the implied trait might be useful elsewhere, and worth adding to type traits as a transformation type trait.

Suspicion that the Range SG might find such a trait useful, but wait until there is clear additional use of such a trait before standardizing.

Minor wording tweak to use `add_rvalue_reference` rather than manually adding the `&&`, then move to Tentatively Ready.

[2013-01-09 Howard Hinnant comments]

I believe the P/R for LWG 2106 is incorrect (item 3). The way it currently reads, `move_iterator<I>::reference` is always an lvalue reference. I.e. if `R` is an lvalue reference type, then reference becomes `add_rvalue_reference<R>::type` which is just `R`. And if `R` is not a reference type, then reference becomes `R` (which is also just `R` ;-)).

I believe the correct wording is what was there previously:

-?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is true, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `R`.

Additionally Marc Glisse points out that `move_iterator<I>::operator*()` should return `static_cast<reference>(*current)`, not `std::move(*current)`.

Previous resolution:

> This wording is relative to the FDIS.

> 1. Change 24.5.4 [move.iterators] p1 as indicated:

>    > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its dereference operator implicitly converts the value returned by the underlying iterator's dereference operator to an rvalue reference. Some generic algorithms can be called with move iterators to replace copying with moving.

> 2. Change 24.5.4.2 [move.iterator], class template `move_iterator` synopsis, as indicated:

>    >     namespace std {
>    >       template <class Iterator>
>    >       class move_iterator {
>    >       public:
>    >         typedef Iterator iterator_type;
>    >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
>    >         typedef Iterator pointer;
>    >         typedef typename iterator_traits<Iterator>::value_type value_type;
>    >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
>    >         typedef value_type&&see below reference;
>    >         […]
>    >       };
>    >     }

> 3. Immediately following the class template `move_iterator` synopsis in 24.5.4.2 [move.iterator] insert a new paragraph as indicated:

>    > -?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is `true`, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `add_rvalue_reference<R>::type`, otherwise as a synonym for `R`.

[2014-05-19, Daniel comments]

The term instantiation has been changed to specialization in the newly added paragraph as suggested by STL and much preferred by myself.

[2014-05-19 Library reflector vote]

The issue has been identified as Tentatively Ready based on five votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 24.5.4 [move.iterators] p1 as indicated:

   > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its indirection operator implicitly converts the value returned by the underlying iterator's indirection operator to an rvalue reference. Some generic algorithms can be called with move iterators to replace copying with moving.

2. Change 24.5.4.2 [move.iterator], class template `move_iterator` synopsis, as indicated:

   >     namespace std {
   >       template <class Iterator>
   >       class move_iterator {
   >       public:
   >         typedef Iterator iterator_type;
   >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
   >         typedef Iterator pointer;
   >         typedef typename iterator_traits<Iterator>::value_type value_type;
   >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
   >         typedef value_type&&see below reference;
   >         […]
   >       };
   >     }

3. Immediately following the class template `move_iterator` synopsis in 24.5.4.2 [move.iterator] insert a new paragraph as indicated:

   > -?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is `true`, the template specialization `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `R`.

4. Edit [move.iter.op.star] p1 as indicated:

   >     reference operator*() const;

   > > -1- Returns: `std::movestatic_cast<reference>(*current)`.

### 2108(i). No way to identify allocator types that always compare equal

Section: 16.4.4.6 [allocator.requirements] Status: Resolved Submitter: Jonathan Wakely Opened: 2011-12-01 Last modified: 2018-12-03

Priority: 3

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View all issues with Resolved status.

Discussion:

Whether two allocator objects compare equal affects the complexity of container copy and move assignments and also the possibility of an exception being thrown by container move assignments. The latter point means container move assignment cannot be `noexcept` when `propagate_on_container_move_assignment` (POCMA) is false for the allocator because there is no way to detect at compile-time if two allocators will compare equal. LWG 2013(i) means this affects all containers using `std::allocator`, but even if that is resolved, this affects all stateless allocators which do not explicitly define POCMA to `true_type`.

One solution would be to add an "always_compare_equal" trait to `allocator_traits`, but that would be duplicating information that is already defined by the type's equality operator if that operator always returns true. Requiring users to write `operator==` that simply returns true and also explicitly override a trait to repeat the same information would be unfortunate and risk user errors that allow the trait and actual `operator==` to disagree.

Dave Abrahams suggested a better solution in message c++std-lib-31532, namely to allow `operator==` to return `true_type`, which is convertible to `bool` but also detectable at compile-time. Adopting this as the recommended way to identify allocator types that always compare equal only requires a slight relaxation of the allocator requirements so that `operator==` is not required to return `bool` exactly.

The allocator requirements do not make it clear that it is well-defined to compare non-const values, that should be corrected too.

In message c++std-lib-31615 Pablo Halpern suggested an `always_compare_equal` trait that could still be defined, but with a sensible default value rather than requiring users to override it, and using that to set sensible values for other allocator traits:

> Do we still need `always_compare_equal` if we can have an `operator==` that returns `true_type`? What would its default value be? `is_empty<A> || is_convertible<decltype(a == a), true_type>::value`, perhaps? One benefit I see to such a definition is that stateless C++03 allocators that don't use the `true_type` idiom will still benefit from the new trait.

> […]

> One point that I want to ensure doesn't get lost is that if we adopt some sort of `always_compare_equal`-like trait, then `propagate_on_container_swap` and `propagate_on_container_move_assignment` should default to `always_compare_equal`. Doing this will eliminate unnecessary requirements on the container element type, as per [LWG 2103(i)].

Optionally, `operator==` for `std::allocator` could be made to return `true_type`, however if LWG 2103(i) is adopted that is less important.

Alberto Ganesh Barbati: Suggest either `always_compare_equal`, `all_objects_(are_)equivalent`, or `all_objects_compare_equal`.

[2014-11-07 Urbana]

Resolved by N4258

Proposed resolution:

This wording is relative to the FDIS.

1. Change Table 27 — "Descriptive variable definitions" in 16.4.4.6 [allocator.requirements]:

   Table 27 — Descriptive variable definitions
   Variable | Definition
   --- | ---
   `a3, a4` | an rvalue ofvalues of (possibly `const`) type `X`
   `b` | a value of (possibly `const`) type `Y`

2. Change Table 28 — "Allocator requirements" in 16.4.4.6 [allocator.requirements]:

   Table 28 — Allocator requirements
   Expression | Return type | Assertion/note pre-/post-condition | Default
   --- | --- | --- | ---
   `a1 == a2a3 == a4` | convertible to `bool` | returns true only if storage allocated from each can be deallocated via the other. `operator==` shall be reflexive, symmetric, and transitive, and shall not exit via an exception. |
   `a1 != a2a3 != a4` | convertible to `bool` | same as `!(a1 == a2)!(a3 == a4)` |
   `a3 == b` | convertible to `bool` | same as `a3 == Y::rebind<T>::other(b)` |
   `a3 != b` | convertible to `bool` | same as `!(a3 == b)` |
   `[…]`
   `a.select_on_- container_copy_- construction()` | `X` | Typically returns either `a` or `X()` | `return a;`
   `X::always_compares_equal` | Identical to or derived from `true_type` or `false_type` | `true_type` if the expression `x1 == x2` is guaranteed to be `true` for any two (possibly `const`) values `x1, x2` of type `X`, when implicitly converted to `bool`. See Note B, below. | `true_type`, if `is_empty<X>::value` is `true` or if `decltype(declval<const X&>() == declval<const X&>())` is convertible to `true_type`, otherwise `false_type`.
   `[…]`

   Note A: […]

   Note B: If `X::always_compares_equal::value` or `XX::always_compares_equal::value` evaluate to `true` and an expression equivalent to `x1 == x2` or `x1 != x2` for any two values `x1, x2` of type `X` evaluates to `false` or `true`, respectively, the behaviour is undefined.

3. Change class template `allocator_traits` synopsis, 20.2.9 [allocator.traits] as indicated:

   >     namespace std {
   >       template <class Alloc> struct allocator_traits {
   >         typedef Alloc allocator_type;
   >         […]
   >         typedef see below always_compares_equal;
   >         typedef see below propagate_on_container_copy_assignment;
   >         […]
   >       };
   >     }

4. Insert the following between 20.2.9.2 [allocator.traits.types] p6 and p7 as indicated:

   >     typedef see below always_compares_equal;

   > > -?- Type: `Alloc::always_compares_equal` if such a type exists; otherwise, `true_type` if `is_empty<Alloc>::value` is `true` or if `decltype(declval<const Alloc&>() == declval<const Alloc&>())` is convertible to `true_type`; otherwise, `false_type` .

   >     typedef see below propagate_on_container_copy_assignment;

   > > -7- Type: `Alloc::propagate_on_container_copy_assignment` if such a type exits, otherwise `false_type`.

5. Change class template `allocator` synopsis, 20.2.10 [default.allocator] as indicated:

   >     namespace std {
   >       template <class T> class allocator;

   >       // specialize for void:
   >       template <> class allocator<void> {
   >       public:
   >         typedef void* pointer;
   >         typedef const void* const_pointer;
   >         // reference-to-void members are impossible.
   >         typedef void value_type;
   >         template <class U> struct rebind { typedef allocator<U> other; };
   >       };

   >       template <class T> class allocator {
   >       public:
   >         typedef size_t size_type;
   >         typedef ptrdiff_t difference_type;
   >         typedef T* pointer;
   >         typedef const T* const_pointer;
   >         typedef T& reference;
   >         typedef const T& const_reference;
   >         typedef T value_type;
   >         template <class U> struct rebind { typedef allocator<U> other; };
   >         typedef true_type always_compares_equal;

   >         […]
   >       };
   >     }

### 2109(i). Incorrect requirements for `hash` specializations

Section: 19.5.7 [syserr.hash], 20.3.3 [util.smartptr.hash], 22.10.19 [unord.hash], 17.7.6 [type.index.synopsis], 27.4.6 [basic.string.hash], 23.3.14 [vector.bool], 32.4.3.2 [thread.thread.id] Status: C++14 Submitter: Daniel Krügler Opened: 2011-12-04 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++14 status.

Discussion:

20.3.3 [util.smartptr.hash] p2 is specified as follows:

> Requires: the template specializations shall meet the requirements of class template `hash` (20.8.12).

The problem here is the usage of a Requires element, which is actually a pre-condition that a user of a component has to satisfy. But the intent of this wording is actually to be a requirement on implementations. The Requires element should be removed here and the wording should be improved to say what it was intended for.

We have similar situations in basically all other places where the specification of library-provided `hash` specializations is defined. Usually, the Requires element is incorrect. In the special case of `hash<unique_ptr<T, D>>` the implementation depends on the behaviour of `hash` specializations, that could be user-provided. In this case the specification needs to separate the requirements on these specializations and those that are imposed on the implementation.

[2012, Kona]

Update wording and move to Review.

Believe a simpler formulation is to simply string the term Requires: and leave the current wording intact, rather than strike the whole clause and replace it.

[Originally proposed wording for reference

1. Change 19.5.7 [syserr.hash] as indicated:

   >     template <> struct hash<error_code>;

   > > -1- Requires: the template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<system_error>` provides a definition for a specialization of the template `hash<error_code>`. The requirements for the members of this specialization are given in sub-clause 22.10.19 [unord.hash].

2. Change 22.9.3 [bitset.hash] as indicated:

   >     template <size_t N> struct hash<bitset<N> >;

   > > -1- Requires: the template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<bitset>` provides a definition for a partial specialization of the `hash` class template for specializations of class template `bitset<N>`. The requirements for the members of instantiations of this specialization are given in sub-clause 22.10.19 [unord.hash].

3. Change 20.3.3 [util.smartptr.hash] as indicated:

   >     template <class T, class D> struct hash<unique_ptr<T, D> >;

   > > -1- Requires: the template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<memory>` provides a definition for a partial specialization of the `hash` class template for specializations of class template `unique_ptr<T, D>`. The requirements for the members of instantiations of this specialization are given in sub-clause 22.10.19 [unord.hash]. For an object `p` of type `UP`, where `UP` is `unique_ptr<T, D>`, `hash<UP>()(p)` shall evaluate to the same value as `hash<typename UP::pointer>()(p.get())`. The specialization `hash<typename UP::pointer>` shall be well-formed.

   > > -?- Requires: The specialization `hash<typename UP::pointer>` shall be well-formed and well-defined [Note: the general requirements of class template `hash` (22.10.19 [unord.hash]) are implied — end note].

   >     template <class T> struct hash<shared_ptr<T> >;

   > > -2- Requires: the template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<memory>` provides a definition for a partial specialization of the `hash` class template for specializations of class template `shared_ptr<T>`. The requirements for the members of instantiations of this specialization are given in sub-clause 22.10.19 [unord.hash]. For an object `p` of type `shared_ptr<T>`, `hash<shared_ptr<T> >()(p)` shall evaluate to the same value as `hash<T*>()(p.get())`.

4. Change 22.10.19 [unord.hash] p2 as indicated: [Comment: For unknown reasons the extended integer types are not mentioned here, which looks like an oversight to me and makes also the wording more complicated. See 2119(i) for this part of the problem. — end comment]

   >     template <> struct hash<bool>;
   >     template <> struct hash<char>;
   >     […]
   >     template <> struct hash<long double>;
   >     template <class T> struct hash<T*>;

   > > -2- Requires: the template specializations shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<functional>` provides definitions for specializations of the `hash` class template for each cv-unqualified arithmetic type except for the extended integer types. This header also provides a definition for a partial specialization of the `hash` class template for any pointer type. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].

5. Change [type.index.hash] p1 as indicated:

   >     template <> struct hash<type_index>;

   > > -1- Requires: the template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<typeindex>` provides a definition for a specialization of the template `hash<type_index>`. The requirements for the members of this specialization are given in sub-clause 22.10.19 [unord.hash]. For an object `index` of type `type_index`, `hash<type_index>()(index)` shall evaluate to the same result as `index.hash_code()`.

6. Change 27.4.6 [basic.string.hash] p1 as indicated:

   >     template <> struct hash<string>;
   >     template <> struct hash<u16string>;
   >     template <> struct hash<u32string>;
   >     template <> struct hash<wstring>;

   > > -1- Requires: the template specializations shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<string>` provides definitions for specializations of the `hash` class template for the types `string`, `u16string`, `u32string`, and `wstring`. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].

7. Change 23.3.14 [vector.bool] p7 as indicated:

   >     template <class Allocator> struct hash<vector<bool, Allocator> >;

   > > -7- Requires: the template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<vector>` provides a definition for a partial specialization of the `hash` class template for specializations of class template `vector<bool, Allocator>`. The requirements for the members of instantiations of this specialization are given in sub-clause 22.10.19 [unord.hash].

8. Change 32.4.3.2 [thread.thread.id] p14 as indicated:

   >     template <> struct hash<thread::id>;

   > > -14- Requires: the template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<thread>` provides a definition for a specialization of the template `hash<thread::id>`. The requirements for the members of this specialization are given in sub-clause 22.10.19 [unord.hash].

]

[2012, Portland: Move to Tentatively Ready]

No further wording issues, so move to Tentatively Ready (post meeting issues processing).

[2013-04-20 Bristol]

Proposed resolution:

1. Change 19.5.7 [syserr.hash] as indicated:

   >     template <> struct hash<error_code>;

   > > -1- Requires: tThe template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash].

2. Change 22.9.3 [bitset.hash] as indicated:

   >     template <size_t N> struct hash<bitset<N> >;

   > > -1- Requires: tThe template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash]).

3. Change 20.3.3 [util.smartptr.hash] as indicated:

   >     template <class T, class D> struct hash<unique_ptr<T, D> >;

   > > -1- Requires: tThe template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash]). For an object `p` of type `UP`, where `UP` is `unique_ptr<T, D>`, `hash<UP>()(p)` shall evaluate to the same value as `hash<typename UP::pointer>()(p.get())`. The specialization `hash<typename UP::pointer>` shall be well-formed.

   > > -?- Requires: The specialization `hash<typename UP::pointer>` shall be well-formed and well-defined, and shall meet the requirements of class template `hash` (22.10.19 [unord.hash]).

   >     template <class T> struct hash<shared_ptr<T> >;

   > > -2- Requires: tThe template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash]). For an object `p` of type `shared_ptr<T>`, `hash<shared_ptr<T> >()(p)` shall evaluate to the same value as `hash<T*>()(p.get())`.

4. Change 22.10.19 [unord.hash] p2 as indicated: [Comment: For unknown reasons the extended integer types are not mentioned here, which looks like an oversight to me and makes also the wording more complicated. See 2119(i) for this part of the problem. — end comment]

   >     template <> struct hash<bool>;
   >     template <> struct hash<char>;
   >     […]
   >     template <> struct hash<long double>;
   >     template <class T> struct hash<T*>;

   > > -2- Requires: tThe template specializations shall meet the requirements of class template `hash` (22.10.19 [unord.hash]).

5. Change [type.index.hash] p1 as indicated:

   >     template <> struct hash<type_index>;

   > > -1- Requires: tThe template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash]). For an object `index` of type `type_index`, `hash<type_index>()(index)` shall evaluate to the same result as `index.hash_code()`.

6. Change 27.4.6 [basic.string.hash] p1 as indicated:

   >     template <> struct hash<string>;
   >     template <> struct hash<u16string>;
   >     template <> struct hash<u32string>;
   >     template <> struct hash<wstring>;

   > > -1- Requires: tThe template specializations shall meet the requirements of class template `hash` (22.10.19 [unord.hash]).

7. Change 23.3.14 [vector.bool] p7 as indicated:

   >     template <class Allocator> struct hash<vector<bool, Allocator> >;

   > > -7- Requires: tThe template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash]).

8. Change 32.4.3.2 [thread.thread.id] p14 as indicated:

   >     template <> struct hash<thread::id>;

   > > -14- Requires: tThe template specialization shall meet the requirements of class template `hash` (22.10.19 [unord.hash]).

### 2110(i). `remove` can't swap but note says it might

Section: 26.7.8 [alg.remove] Status: C++14 Submitter: Howard Hinnant Opened: 2011-12-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.remove].

View all issues with C++14 status.

Discussion:

26.7.8 [alg.remove]/p1 says:

> 1 Requires: The type of `*first` shall satisfy the `MoveAssignable` requirements (Table 22).

This means that `remove`/`remove_if` can only use move assignment to permute the sequence. But then 26.7.8 [alg.remove]/p6 (non-normatively) contradicts p1:

> 6 Note: each element in the range `[ret,last)`, where `ret` is the returned value, has a valid but unspecified state, because the algorithms can eliminate elements by swapping with or moving from elements that were originally in that range.

[2012, Kona]

Move to Ready.

Alisdair notes we could extend permission to use `swap` if it is available, but there is no interest. Accept the proposed resolution as written.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 26.7.8 [alg.remove] as indicated:

>     template<class ForwardIterator, class T>
>       ForwardIterator remove(ForwardIterator first, ForwardIterator last,
>                              const T& value);

>     template<class ForwardIterator, class Predicate>
>       ForwardIterator remove_if(ForwardIterator first, ForwardIterator last,
>                                 Predicate pred);

> > […]

> > -6-Note: each element in the range `[ret,last)`, where `ret` is the returned value, has a valid but unspecified state, because the algorithms can eliminate elements by swapping with or moving from elements that were originally in that range.

### 2111(i). Which `unexpected`/`terminate` handler is called from the exception handling runtime?

Section: 17.9.5.4 [terminate], 99 [unexpected] Status: C++17 Submitter: Howard Hinnant Opened: 2011-12-06 Last modified: 2017-07-30

Priority: 3

View all other issues in [terminate].

View all issues with C++17 status.

Discussion:

Prior to N3242, modified by N3189, we said this about `unexpected()`:

> Effects: Calls the `unexpected_handler` function in effect immediately after evaluating the throw-expression (D.13.1), if called by the implementation, or calls the current `unexpected_handler`, if called by the program.

and this about `terminate()`:

> Effects: Calls the `terminate_handler` function in effect immediately after evaluating the throw-expression (18.8.3.1), if called by the implementation, or calls the current `terminate_handler` function, if called by the program.

But now in both places we say:

> Calls the current `unexpected_handler` function.

and:

> Calls the current `terminate` function.

The difference is that in C++98/03 if a destructor reset a handler during stack unwinding, that new handler was not called if the unwinding later led to `unexpected()` or `terminate()` being called. But these new words say that this new handler is called. This is an ABI-breaking change in the way exceptions are handled. Was this change intentional?

N3189 was mainly about introducing exception safety and getters for the handlers. I don't recall the issue of which handler gets called being part of the discussion.

I propose that we revert to the C++98/03 behavior in this regard, lest ABI's such as the Itanium ABI are invalidated. A mechanical way to do this is to revert bullets 9 and 12 of N3189.

[2011-12-09: Daniel comments]

There was no such semantic change intended. It was an unfortunate side effect when trying to better separate different responsibilities in the previous wording.

A related issue is 2088(i).

[2012-01-30: Howard comments]

The C++98/03 wording is somewhat ambiguous:

> Calls the terminate_handler function in effect immediately after evaluating the throw-expression...

There are potentially two throw-expressions being referred to here, and it is not clear if this sentence is referring to just the first or both:

1. `throw assignment-expression;`
2. `throw;`

There is ample evidence in current implementations that it is understood that only 1. was meant. But clearly both 1 and 2 could have been meant. We need a clarification. Does an execution of a rethrow (throw;) update which handlers can potentially be called?

1. `throw;` // update handlers to get_xxx()?

My opinion: Go with existing practice, and clarify what that practice is, if surveys find that everyone does the same thing. Gcc 4.2 and Apple do 1. only, and do not reset the handlers to the current handlers on throw;.

If current practice is not unanimously one way or the other, I have no strong opinion. I have not found a motivating use case for the use of any particular handler. Most applications set the handlers once at the beginning of the program and then do not change them, and so will not be impacted by whatever decision is made here.

[2014-02-15 Issaquah: Move to Review]

STL: Original change in N3242 came from trying to make set/get exception handler thread safe. The issue requests we revert to 98/03, which Howard notes was already ambiguous.

Alisdair: Issue author thinks we made this change in C++11 without taking into account Itanium ABI, which cannot implement the new semantic (without breaking compatibility).

Alisdair: original change in N3242 was trying to solve the problem of which handler is called when the handler is changing in another thread, but this turns out to be an issue in even the single-threaded case.

Pablo: despite wanting to make it thread safe, you are still changing a global

STL and Marshall confirm that there is real implementation divergance on the question, so we cannot pick just one behavior if we want to avoid breaking exisitng practice.

Alisdair: not sure who to talk to across all library vendors to fix, need more information for progress (IBM and Sun)

STL: Howard did identify a problem with the wording as well: `throw;` is a throw expression, but we typically want to re-activate the in-flight exception, not throw a new copy.

Pablo: wondering why all of this wording is here (N3189)? It looks like we were trying to handle another thread changing handler between a `throw` and `terminate` in current thread.

Alisdair: Anything working with exception handling should have used only thread-local resources, but that ship has sailed. We must account for the same exception object being re-thrown in multiple threads simultaneously, with no happens-before relationships.

Room: Why on earth would we care about exactly which way the program dies when the terminate calls are racing?!

Pablo: Reasonable to set the handler once (in `main`) and never change it.

Pablo: If willing to put lots of work into this, you could say at point of a `throw` these handlers become thread local but that is overkill. We want destructors to be able to change these handlers (if only for backwards compatibility).

Alisdair: the "do it right" is to do something per thread, but that is more work than vendors will want to do. Want to say setting handler while running multiple threads is unspecified.

Pablo: possible all we need to do is say it is always the current handler

STL: That prevents an implementation or single threaded program from calling a new handler after a `throw`, probably should say if `terminate` is called by the implementation (during EH), any handler that was current can be called. Leaves it up in the air as to when the handler is captured, supporting the diverging existing practices.

Jeffrey: use happens before terminology to avoid introducing races

STL: Give this to concurrency?

Jeffrey: It is in clause 18, generally LWG and not SG1 territory.

Alisdair: Concerned about introducing happens before into fundamental exception handling since it would affect single threaded performance as well. Want to give to concurrency or LEWG/EWG, we are into language design here.

Jeffrey: suspect LEWG won't have a strong opinion. I don't want it to be ours!!!

Pablo: Might be a case for core>

Alisdair: Would be happier if at least one core person were in the discussion.

STL: No sympathy for code that tries to guard the terminate handler.

Alisdair: We are back to set it once, globally. Want to be clear that if `set_terminate` is called just once, when EH is not active, and never changed again, then the user should get the handler from that specific call.

AlisdairM: "unspecified which handler is called if an exception is active when `set_terminate` is called." This supports existing behaviors, and guarantees which handler is called in non-conentious situations. Implicit assumption that a funtion becomes a handler only after a successful call to `set_handler`, so we are not leaving a door open to the implementation inventing entirely new handlers of its own.

Consensus.

Poll to confirm status as P1: new consensus is P3

Action: Alisdair provides new wording. Drop from P1 to P3, and move to Review.

[2015-05, Lenexa]

HH: we accidentally changed semantics of which handler gets called during exception unwinding. This was attempt to put it back. Discovered implementations don't really do anything. […] Fine with unspecified behavior to move this week.
STL/MC: observed different behavior
STL: legitimizes all implementations and tells users to not do this
Move to ready? 9/0/1

Proposed resolution:

Amend 17.9.5.4 [terminate] as indicated:

`[[noreturn]] void terminate() noexcept;`

> Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1) , in effect immediately after throwing the exception. May also be called directly by the program.

> Effects: Calls a `terminate_handler` function. It is unspecified which `terminate_handler` function will be called if an exception is active during a call to `set_terminate`. Otherwise cCalls the current `terminate_handler` function. [Note: A default `terminate_handler` is always considered a callable handler in this context. — end note]

Amend 99 [unexpected] as indicated:

`[[noreturn]] void unexpected();`

> Remarks: Called by the implementation when a function exits via an exception not allowed by its exception-specification (15.5.2), in effect after evaluating the throw-expression (D.11.1). May also be called directly by the program.

> Effects: Calls an `unexpected_handler` function. It is unspecified which `unexpected_handler` function will be called if an exception is active during a call to `set_unexpected`. Otherwise cCalls the current `unexpected_handler` function. [Note: A default `unexpected_handler` is always considered a callable handler in this context. — end note]

### 2112(i). User-defined classes that cannot be derived from

Section: 16.4.6 [conforming], 20.2.9 [allocator.traits], 20.6.1 [allocator.adaptor.syn] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-30 Last modified: 2016-01-28

Priority: 1

View all other issues in [conforming].

View all issues with C++14 status.

Discussion:

It is a very established technique for implementations to derive internally from user-defined class types that are used to customize some library component, e.g. deleters and allocators are typical candidates. The advantage of this approach is to possibly take advantage of the empty-base-class optimization (EBCO).

Whether or whether not libraries did take advantage of such a detail didn't much matter in C++03. Even though there did exist a portable idiom to prevent that a class type could be derived from, this idiom has never reached great popularity: The technique required to introduce a virtual base class and it did not really prevent the derivation, but only any construction of such a type. Further, such types are not empty as defined by the `std::is_empty` trait, so could easily be detected by implementations from TR1 on.

With the new C++11 feature of final classes and final member functions it is now very easy to define an empty, but not derivable from class type. From the point of the user it is quite natural to use this feature for types that he or she did not foresee to be derivable from.

On the other hand, most library implementations (including third-party libraries) often take advantage of EBCO applied to user-defined types used to instantiate library templates internally. As the time of submitting this issue the following program failed to compile on all tested library implementations:

>     #include <memory>

>     struct Noop final {
>      template<class Ptr>
>      void operator()(Ptr) const {}
>     };

>     std::unique_ptr<int, Noop> up;

In addition, many `std::tuple` implementations with empty, final classes as element types failed as well, due to a popular inheritance-based implementation technique. EBCO has also a long tradition to be used in library containers to efficiently store potentially stateless, empty allocators.

It seems that both user and library did the best they could: None of the affected types did impose explicit requirements on the corresponding user-defined types to be derivable from (This capability was not part of the required operations), and libraries did apply EBCO whereever possible to the convenience of the customer.

Nonetheless given the existence of non-derivable-from class types in C++11, libraries have to cope with failing derivations. How should that problem be solved?

It would certainly be possible to add weazel wording to the allocator requirements similar to what we had in C++03, but restricted to derivation-from requirements. I consider this as the bad solution, because it would add new requirements that never had existed before in this explicit form onto types like allocators.

Existing libraries presumably will need internal traits like `__is_final` or `__is_derivable` to make EBCO possible in the current form but excluding non-derivable class types. As of this writing this seems to happen already. Problem is that without a `std::is_derivable` trait, third-party libraries have no portable means to do the same thing as standard library implementations. This should be a good reason to make such a trait public available soon, but seems not essential to have now. Further, this issue should also be considered as a chance to recognice that EBCO has always been a very special corner case (There exist parallels to the previously existing odd core language rule that did make the interplay between `std::auto_ptr` and `std::auto_ptr_ref` possible) and that it would be better to provide explicit means for space-efficient storage, not necessarily restricted to inheritance relations, e.g. by marking data members with a special attribute.

At least two descriptions in the current standard should be fixed now for better clarification:

1. As mentioned by Ganesh, 20.2.9 [allocator.traits] p1 currently contains a (non-normative) note "Thus, it is always possible to create a derived class from an allocator." which should be removed.

2. As pointed out by Howard, the specification of `scoped_allocator_adaptor` as of 20.6.1 [allocator.adaptor.syn] already requires derivation from `OuterAlloc`, but only implies indirectly the same for the inner allocators due to the exposition-only description of member `inner`. This indirect implication should be normatively required for all participating allocators.

[2012, Kona]

What we really need is a type trait to indicate if a type can be derived from. Howard reports Clang and libc++ have had success with this approach.

Howard to provide wording, and AJM to alert Core that we may be wanting to add a new trait that requires compiler support.

[2014-02, Issaquah: Howard and Daniel comment and provide wording]

Several existing C++11 compilers do already provide an internal `__is_final` intrinsic (e.g. clang and gcc) and therefore we believe that this is evidence enough that this feature is implementable today.

We believe that both a simple and clear definition of the `is_final` query should result in a true outcome if and only if the current existing language definition holds that a complete class type (either union or non-union) has been marked with the class-virt-specifier `final` — nothing more.

The following guidelines lead to the design decision and the wording choice given below:

It has been expressed several times that a high-level trait such as "`is_derivable`" would be preferred and would be more useful for non-experts. One problem with that request is that it is astonishingly hard to find a common denominator for what the precise definition of this trait should be, especially regarding corner-cases. Another example of getting very differing points of view is to ask a bunch of C++ experts what the best definition of the `is_empty` trait should be (which can be considered as a kind of higher-level trait).

Once we have a fundamental trait like `is_final` available, we can easily define higher-level traits in the future on top of this by a proper logical combination of the low-level traits.

A critical question is whether providing such a low-level compile-time introspection might be considered as disadvantageous, because it could constrain the freedom of existing implementations even further and whether a high-level trait would solve this dilemma. We assert that since C++11 the static introspection capabilities are already very large and we believe that making the presence or absence of the `final` keyword testable does not make the current situation worse.

Below code example demonstrates the intention and the implementability of this feature:

>     #include <type_traits>

>     namespace std
>     {

>     template <class T>
>     struct is_final
>       : public integral_constant<bool, __is_final(T)>
>     {};

>     }  // std

>     // test it

>     union FinalUnion final { };

>     union NonFinalUnion { };

>     class FinalClass final { };

>     struct NonFinalClass { };

>     class Incomplete;

>     int main()
>     {
>       using std::is_final;
>       static_assert( is_final<const volatile FinalUnion>{}, "");
>       static_assert(!is_final<FinalUnion[]>{}, "");
>       static_assert(!is_final<FinalUnion[1]>{}, "");
>       static_assert(!is_final<NonFinalUnion>{}, "");
>       static_assert( is_final<FinalClass>{}, "");
>       static_assert(!is_final<FinalClass&>{}, "");
>       static_assert(!is_final<FinalClass*>{}, "");
>       static_assert(!is_final<NonFinalClass>{}, "");
>       static_assert(!is_final<void>{}, "");
>       static_assert(!is_final<int>{}, "");
>       static_assert(!is_final<Incomplete>{}, ""); // error incomplete type 'Incomplete' used in type trait expression
>     }

[2014-02-14, Issaquah: Move to Immediate]

This is an important issue, that we really want to solve for C++14.

Move to Immediate after polling LEWG, and then the NB heads of delegation.

Proposed resolution:

This wording is relative to N3797.

1. Change 21.3.3 [meta.type.synop], header `<type_traits>` synopsis, as indicated

   >     namespace std {
   >       […]
   >       // 20.10.4.3, type properties:
   >       […]
   >       template <class T> struct is_empty;
   >       template <class T> struct is_polymorphic;
   >       template <class T> struct is_abstract;
   >       template <class T> struct is_final;

   >       […]
   >     }

2. Change 21.3.5.4 [meta.unary.prop], Table 49 — Type property predicates, as indicated

   Table 49 — Type property predicates
   Template | Condition | Preconditions
   --- | --- | ---
   `template <class T>` `struct is_abstract;` | […] | […]
   `template <class T>` `struct is_final;` | `T` is a class type marked with the class-virt-specifier `final` (11 [class]). [Note: A union is a class type that can be marked with `final`. — end note] | If `T` is a class type, `T` shall be a complete type

3. After 21.3.5.4 [meta.unary.prop] p5 add one further example as indicated:

   > [Example:

   > >     // Given:
   > >     struct P final { };
   > >     union U1 { };
   > >     union U2 final { };

   > >     // the following assertions hold:
   > >     static_assert(!is_final<int>::value, "Error!");
   > >     static_assert( is_final<P>::value, "Error!");
   > >     static_assert(!is_final<U1>::value, "Error!");
   > >     static_assert( is_final<U2>::value, "Error!");

   > — end example]

### 2114(i). Incorrect "contextually convertible to `bool`" requirements

Section: 16.4.4.4 [nullablepointer.requirements], 24.3.5.3 [input.iterators], 24.3.5.7 [random.access.iterators], 26.1 [algorithms.general], 26.8 [alg.sorting], 32.2.1 [thread.req.paramname] Status: Resolved Submitter: Daniel Krügler Opened: 2011-12-09 Last modified: 2025-03-13

Priority: 3

View all other issues in [nullablepointer.requirements].

View all issues with Resolved status.

Discussion:

As of 16.4.4.2 [utility.arg.requirements] Table 17/18, the return types of the expressions

>     a == b

or

>     a < b

for types satisfying the `EqualityComparable` or `LessThanComparable` types, respectively, are required to be "convertible to `bool`" which corresponds to a copy-initialization context. But several newer parts of the library that refer to such contexts have lowered the requirements taking advantage of the new terminology of "contextually convertible to `bool`" instead, which corresponds to a direct-initialization context (In addition to "normal" direct-initialization constructions, operands of logical operations as well as `if` or `switch` conditions also belong to this special context).

One example for these new requirements are input iterators which satisfy `EqualityComparable` but also specify that the expression

>     a != b

shall be just "contextually convertible to `bool`". The same discrepancy exists for requirement set `NullablePointer` in regard to several equality-related expressions.

For random access iterators we have

> `a < b` contextually convertible to `bool`

as well as for all derived comparison functions, so strictly speaking we could have a random access iterator that does not satisfy the `LessThanComparable` requirements, which looks like an artifact to me.

Even if we keep with the existing requirements based on `LessThanComparable` or `EqualityComparable` we still would have the problem that some current specifications are actually based on the assumption of implicit convertibility instead of "explicit convertibility", e.g. 20.3.1.6 [unique.ptr.special] p3:

>     template <class T1, class D1, class T2, class D2>
>     bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > -3- Returns: `x.get() != y.get()`.

Similar examples exist in 20.3.1.3.3 [unique.ptr.single.dtor] p2, 20.3.1.3.4 [unique.ptr.single.asgn] p9, 20.3.1.3.5 [unique.ptr.single.observers] p1+3+8, etc.

In all these places the expressions involving comparison functions (but not those of the conversion of a `NullablePointer` to `bool`!) assume to be "convertible to `bool`". I think this is a very natural assumption and all delegations of the comparison functions of some type `X` to some other API type `Y` in third-party code does so assuming that copy-initialization semantics will just work.

The actual reason for using the newer terminology can be rooted back to LWG 556(i). My hypotheses is that the resolution of that issue also needs a slight correction. Why so?

The reason for opening that issue were worries based on the previous "convertible to `bool`" wording. An expressions like "`!pred(a, b)`" might not be well-formed in those situations, because `operator!` might not be accessible or might have an unusual semantics (and similarly for other logical operations). This can indeed happen with unusual proxy return types, so the idea was that the evaluation of `Predicate`, `BinaryPredicate` (26.1 [algorithms.general] p8+9), and `Compare` (26.8 [alg.sorting] p2) should be defined based on contextual conversion to `bool`. Unfortunately this alone is not sufficient: In addition, I think, we also want the predicates to be (implicitly) convertible to `bool`! Without this wording, several conditions are plain wrong, e.g. 26.6.6 [alg.find] p2, which talks about "`pred(*i) != false`" (`find_if`) and "`pred(*i) == false`" (`find_if_not`). These expressions are not within a boolean context!

While we could simply fix all these places by proper wording to be considered in a "contextual conversion to `bool`", I think that this is not the correct solution: Many third-party libraries already refer to the previous C++03 `Predicate` definition — it actually predates C++98 and is as old as the SGI specification. It seems to be a high price to pay to switch to direct initialization here instead of fixing a completely different specification problem.

A final observation is that we have another definition for a `Predicate` in 32.2.1 [thread.req.paramname] p2:

> If a parameter is `Predicate`, `operator()` applied to the actual template argument shall return a value that is convertible to `bool`.

The problem here is not that we have two different definitions of `Predicate` in the standard — this is confusing, but this fact alone is not a defect. The first (minor) problem is that this definition does not properly apply to function objects that are function pointers, because `operator()` is not defined in a strict sense. But the actually worse second problem is that this wording has the very `same` problem that has originally lead to LWG 556(i)! We only need to look at 32.7.4 [thread.condition.condvar] p15 to recognice this:

>     while (!pred())
>       wait(lock);

The negation expression here looks very familiar to the example provided in LWG 556(i) and is sensitive to the same "unusual proxy" problem. Changing the 32.2.1 [thread.req.paramname] wording to a corresponding "contextual conversion to `bool`" wouldn't work either, because existing specifications rely on "convertible to `bool`", e.g. 32.7.4 [thread.condition.condvar] p32+33+42 or 32.7.5 [thread.condition.condvarany] p25+26+32+33.

To summarize: I believe that LWG 556(i) was not completely resolved. A pessimistic interpretation is, that even with the current wording based on "contextually convertible to `bool`" the actual problem of that issue has not been fixed. What actually needs to be required here is some normative wording that basically expresses something along the lines of:

> The semantics of any contextual conversion to `bool` shall be equivalent to the semantics of any implicit conversion to `bool`.

This is still not complete without having concepts, but it seems to be a better approximation. Another way of solving this issue would be to define a minimum requirements table with equivalent semantics. The proposed wording is a bit simpler but attempts to express the same thing.

[2012, Kona]

Agree with Daniel that we potentially broke some C++03 user code, accept the changes striking "contextually" from tables. Stefan to provide revised wording for section 25, and figure out changes to section 30.

Move to open, and then to Review when updated wording from Stefan is available.

[2012-10-12, STL comments]

1. The current proposed resolution still isn't completely satisfying. It would certainly be possible for the Standard to require these various expressions to be implicitly and contextually convertible to `bool`, but that would have a subtle consequence (which, I will argue, is undesirable - regardless of the fact that it dates all the way back to C++98/03). It would allow users to provide really wacky types to the Standard Library, with one of two effects:

   1. Standard Library implementations would have to go to great lengths to respect such wacky types, essentially using `static_cast<bool>` when invoking any predicates or comparators.

   2. Otherwise, such wacky types would be de facto nonportable, because they would make Standard Library implementations explode.

   Effect B is the status quo we're living with today. What Standard Library implementations want to do with `pred(args)` goes beyond "`if (pred(args))`" (C++03), contextually converting `pred(args)` to `bool` (C++11), or implicitly and contextually converting `pred(args)` to `bool` (the current proposed resolution). Implementations want to say things like:

   >     if (pred(args))
   >     if (!pred(args))
   >     if (cond && pred(args))
   >     if (cond && !pred(args))

   These are real examples taken from Dinkumware's implementation. There are others that would be realistic ("`pred(args) && cond`", "`cond || pred(args)`", etc.)

   Although negation was mentioned in this issue's Discussion section, and in LWG 556(i)'s, the current proposed resolution doesn't fix this problem. Requiring `pred(args)` to be implicitly and contextually convertible to `bool` doesn't prevent `operator!()` from being overloaded and returning `std::string` (as a wacky example). More ominously, it doesn't prevent `operator&&()` and `operator||()` from being overloaded and destroying short-circuiting.

2. I would like LWG input before working on Standardese for a new proposed resolution. Here's an outline of what I'd like to do:

   1. Introduce a new "concept" in 16.4.4 [utility.requirements], which I would call `BooleanTestable` in the absence of better ideas.

   2. Centralize things and reduce verbosity by having everything simply refer to `BooleanTestable` when necessary. I believe that the tables could say "Return type: `BooleanTestable`", while Predicate/BinaryPredicate/Compare would need the incantation "shall satisfy the requirements of BooleanTestable".

   3. Resolve the tug-of-war between users (who occasionally want to do weird things) and implementers (who don't want to have to contort their code) by requiring that:

      1. Given a `BooleanTestable x`, `x` is both implicitly and contextually convertible to `bool`.

      2. Given a `BooleanTestable x`, `!x` is `BooleanTestable`. (This is intentionally "recursive".)

      3. Given a `BooleanTestable x`, `bool t = x, t2(x), f = !x;` has the postcondition `t == t2 && t != f`.

      4. Given a `BooleanTestable x` and a `BooleanTestable y` of possibly different types, "`x && y`" and "`x || y`" invoke the built-in `operator&&()` and `operator||()`, triggering short-circuiting.

      5. `bool` is `BooleanTestable`.

   I believe that this simultaneously gives users great latitude to use types other than `bool`, while allowing implementers to write reasonable code in order to get their jobs done. (If I'm forgetting anything that implementers would want to say, please let me know.)

3. About requirement (I): As Daniel patiently explained to me, we need to talk about both implicit conversions and contextual conversions, because it's possible for a devious type to have both "`explicit operator bool()`" and "`operator int()`", which might behave differently (or be deleted, etc.).

4. About requirement (IV): This is kind of tricky. What we'd like to say is, "`BooleanTestable` can't ever trigger an overloaded logical operator". However, given a perfectly reasonable type `Nice` - perhaps even `bool` itself! - other code (perhaps a third-party library) could overload `operator&&(Nice, Evil)`. Therefore, I believe that the requirement should be "no first use" - the Standard Library will ask for various `BooleanTestable` types from users (for example, the result of "`first != last`" and the result of "`pred(args)`"), and as long as they don't trigger overloaded logical operators with each other, everything is awesome.

5. About requirement (V): This is possibly redundant, but it's trivial to specify, makes it easier for users to understand what they need to do ("oh, I can always achieve this with `bool`"), and provides a "base case" for requirement (IV) that may or may not be necessary. Since `bool` is `BooleanTestable`, overloading `operator&&(bool, Other)` (etc.) clearly makes the `Other` type non-`BooleanTestable`.

Previous resolution from Daniel [SUPERSEDED]:

> This wording is relative to the FDIS.

> 1. Change Table 25 — "`NullablePointer` requirements" in 16.4.4.4 [nullablepointer.requirements] as indicated:

>    Table 25 — `NullablePointer` requirements
>    Expression | Return type | Operational semantics
>    --- | --- | ---
>    `[…]`
>    `a != b` | contextually convertible to `bool` | `!(a == b)`
>    `a == np np == a` | contextually convertible to `bool` | `a == P()`
>    `a != np np != a` | contextually convertible to `bool` | `!(a == np)`

> 2. Change Table 107 — "Input iterator requirements" in 24.3.5.3 [input.iterators] as indicated:

>    Table 107 — Input iterator requirements (in addition to Iterator)
>    Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
>    --- | --- | --- | ---
>    `a != b` | contextually convertible to `bool` | `!(a == b)` | pre: `(a, b)` is in the domain of `==`.
>    `[…]`

> 3. Change Table 111 — "Random access iterator requirements" in 24.3.5.7 [random.access.iterators] as indicated:

>    Table 111 — Random access iterator requirements (in addition to bidirectional iterator)
>    Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
>    --- | --- | --- | ---
>    `[…]`
>    `a < b` | contextually convertible to `bool` | `b - a > 0` | `<` is a total ordering relation
>    `a > b` | contextually convertible to `bool` | `b < a` | `>` is a total ordering relation opposite to `<`.
>    `a >= b` | contextually convertible to `bool` | `!(a < b)` |
>    `a <= b` | contextually convertible to `bool` | `!(a > b)` |

> 4. Change 26.1 [algorithms.general] p8+9 as indicated:

>    > -8- The `Predicate` parameter is used whenever an algorithm expects a function object (22.10 [function.objects]) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable as `true`. In other words, if an algorithm takes `Predicate pred` as its argument and first as its iterator argument, it should work correctly in the construct `pred(*first)` implicitly or contextually converted to `bool` (Clause 7.3 [conv]). The function object `pred` shall not apply any non-constant function through the dereferenced iterator.

>    > -9- The `BinaryPredicate` parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type `T` when `T` is part of the signature returns a value testable as `true`. In other words, if an algorithm takes `BinaryPredicate binary_pred` as its argument and `first1` and `first2` as its iterator arguments, it should work correctly in the construct `binary_pred(*first1, *first2)` implicitly or contextually converted to `bool` (Clause 7.3 [conv]). `BinaryPredicate` always takes the first iterator's `value_type` as its first argument, that is, in those cases when `T` value is part of the signature, it should work correctly in the construct `binary_pred(*first1, value)` implicitly or contextually converted to `bool` (Clause 7.3 [conv]). `binary_pred` shall not apply any non-constant function through the dereferenced iterators.

> 5. Change 26.8 [alg.sorting] p2 as indicated:

>    > -2- `Compare` is a function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of type `Compare`, when implicitly or contextually converted to `bool` (7.3 [conv]), yields `true` if the first argument of the call is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator.

> 6. Change 32.2.1 [thread.req.paramname] p2 as indicated:

>    > -2- If a parameter is `Predicate`, operator() applied to the actual template argument shall return a value that is convertible to `bool` `Predicate` is a function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of type `Predicate`, when implicitly or contextually converted to `bool` (7.3 [conv]), yields `true` if the corresponding test condition is satisfied, and `false` otherwise.

[2014-05-20, Daniel suggests concrete wording based on STL's proposal]

The presented wording follows relatively closely STL's outline with the following notable exceptions:

1. A reference to `BooleanTestable` in table "Return Type" specifications seemed very unusual to me and I found no "prior art" for this in the Standard. Instead I decided to follow the usual style to add a symbol with a specific meaning to a specific paragraph that specifies symbols and their meanings.

2. STL's requirement IV suggested to directly refer to built-in operators `&&` and `||`. In my opinion this concrete requirement isn't needed if we simply require that two `BooleanTestable` operands behave equivalently to two those operands after conversion to `bool` (each of them).

3. I couldn't find a good reason to require normatively that type `bool` meets the requirements of `BooleanTestable`: My assertion is that after having defined them, the result simply falls out of this. But to make this a bit clearer, I added also a non-normative note to these effects.

[2014-06-10, STL comments]

In the current wording I would like to see changed the suggested changes described by bullet #6:

1. In 23.2.2 [container.requirements.general] p4 undo the suggested change

2. Then change the 7 occurrences of "convertible to `bool`" in the denoted tables to "`bool`".

[2015-05-05 Lenexa]

STL: Alisdair wanted to do something here, but Daniel gave us updated wording.

[2015-07 Telecon]

Alisdair: Should specify we don't break short circuiting.
Ville: Looks already specified because that's the way it works for bool.
Geoffrey: Maybe add a note about the short circuiting.
B2/P2 is somewhat ambiguous. It implies that B has to be both implicitly convertible to bool and contextually convertible to bool.
We like this, just have nits.
Status stays Open.
Marshall to ping Daniel with feedback.

[2016-02-27, Daniel updates wording]

1. The revised wording has been updated from N3936 to N4567.

2. To satisfy the Kona 2015 committee comments, the wording in [booleantestable.requirements] has been improved to better separate the two different requirements of "can be contextually converted to `bool`" and "can be implicitly converted to `bool`. Both are necessary because it is possible to define a type that has the latter property but not the former, such as the following one:

   > 2016-08-07, Daniel: The below example has been corrected to reduce confusion about the performed conversions as indicated by the delta markers:

   >     using Bool = int;

   >     struct OddBoolean
   >     {
   >       explicit operator bool() const = delete;
   >       operator Bool() const;
   >       OddBoolean(bool) = delete;
   >       OddBoolean(Bool){}
   >     } ob;

   >     bool b2 = ob; // OK
   >     bool b1(ob);  // Error
   >     OddBoolean b2 = true; // OK
   >     OddBoolean b1(true);  // Error

3. In [booleantestable.requirements] a note has been added to ensure that an implementation is not allowed to break any short-circuiting semantics.

4. I decided to separate LWG 2587(i)/2588(i) from this issue. Both these issues aren't exactly the same but depending on the committee's position, their resolution might benefit from the new vocabulary introduced here.

[2021-06-25; Daniel comments]

The paper P2167R1 is provided to resolve this issue.

[2022-05-01; Daniel comments]

The paper P2167R2 is provided to resolve this issue.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4567.

> 1. Change 16.4.4.2 [utility.arg.requirements] p1, Table 17 — "EqualityComparable requirements", and Table 18 — "LessThanComparable requirements" as indicated:

>    > -1- […] In these tables, `T` is an object or reference type to be supplied by a C++ program instantiating a template; `a`, `b`, and `c` are values of type (possibly `const`) `T`; `s` and `t` are modifiable lvalues of type `T`; `u` denotes an identifier; `rv` is an rvalue of type `T`; and `v` is an lvalue of type (possibly `const`) `T` or an rvalue of type `const T`; and `BT` denotes a type that meets the `BooleanTestable` requirements ([booleantestable.requirements]).

>    > […]

>    > > Table 17 — `EqualityComparable` requirements [equalitycomparable]
>    > > Expression | Return type | Requirement
>    > > --- | --- | ---
>    > > `a == b` | convertible to `bool` `BT` | `==` is an equivalence relation, that is, it has the following properties: […]

>    > […]

>    > > Table 18 — `LessThanComparable` requirements [lessthancomparable]
>    > > Expression | Return type | Requirement
>    > > --- | --- | ---
>    > > `a < b` | convertible to `bool` `BT` | `<` is a strict weak ordering relation (26.8 [alg.sorting])

> 2. Between 16.4.4.3 [swappable.requirements] and 16.4.4.4 [nullablepointer.requirements] insert a new sub-clause as indicated:

>    > ?.?.?.? `BooleanTestable` requirements [booleantestable.requirements]

>    > > -?- A `BooleanTestable` type is a boolean-like type that also supports conversions to `bool`. A type `B` meets the `BooleanTestable` requirements if the expressions described in Table ?? are valid and have the indicated semantics, and if `B` also satisfies all the other requirements of this sub-clause [booleantestable.requirements].

>    > > An object `b` of type `B` can be implicitly converted to `bool` and in addition can be contextually converted to `bool` (Clause 4). The result values of both kinds of conversions shall be equivalent.

>    > > [Example: The types `bool`, `std::true_type`, and `std::bitset<>::reference` are `BooleanTestable` types. — end example]

>    > > For the purpose of Table ??, let `B2` and `Bn` denote types (possibly both equal to `B` or to each other) that meet the `BooleanTestable` requirements, let `b1` denote a (possibly `const`) value of `B`, let `b2` denote a (possibly `const`) value of `B2`, and let `t1` denote a value of type `bool`.

>    > > [Note: These rules ensure what an implementation can rely on but doesn't grant it license to break short-circuiting behavior of a `BooleanTestable` type. — end note]

> 3. Somewhere within the new sub-clause [booleantestable.requirements] insert the following new Table (?? denotes the assigned table number):

>    > Table ?? — `BooleanTestable` requirements [booleantestable]
>    > Expression | Return type | Operational semantics
>    > --- | --- | ---
>    > `bool(b1)` | `bool` | Remarks: `bool(b1) == t1` for every value `b1` implicitly converted to `t1`.
>    > `!b1` | `Bn` | Remarks: `bool(b1) == !bool(!b1)` for every value `b1`.
>    > `b1 && b2` | `bool` | `bool(b1) && bool(b2)`
>    > `b1 || b2` | `bool` | `bool(b1) || bool(b2)`

> 4. Change 16.4.4.4 [nullablepointer.requirements] p5 and Table 25 — "NullablePointer requirements" as indicated:

>    > […]

>    > -5- In Table 25, `u` denotes an identifier, `t` denotes a non-`const` lvalue of type `P`, `a` and `b` denote values of type (possibly `const`) `P`, and `np` denotes a value of type (possibly `const`) `std::nullptr_t`, and `BT` denotes a type that meets the `BooleanTestable` requirements ([booleantestable.requirements]).

>    > […]

>    > > Table 25 — `NullablePointer` requirements [nullablepointer]
>    > > Expression | Return type | Operational semantics
>    > > --- | --- | ---
>    > > `…`
>    > > `a != b` | contextually convertible to `bool` `BT` | […]
>    > > `a == np` `np == a` | contextually convertible to `bool` `BT` | […]
>    > > `a != np` `np != a` | contextually convertible to `bool` `BT` | […]

> 5. Change 22.4.9 [tuple.rel] as indicated;

>    >     template<class... TTypes, class... UTypes>
>    >     constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

>    > > -1- Requires: For all `i`, where `0 <= i` and `i < sizeof...(TTypes)`, `get<i>(t) == get<i>(u)` is a valid expression returning a type that is convertible to `bool` meets the `BooleanTestable` requirements ([booleantestable.requirements]). `sizeof...(TTypes) == sizeof...(UTypes)`.

>    > > […]

>    >     template<class... TTypes, class... UTypes>
>    >     constexpr bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

>    > > -4- Requires: For all `i`, where `0 <= i` and `i < sizeof...(TTypes)`, `get<i>(t) < get<i>(u)` and `get<i>(u) < get<i>(t)` are valid expressions returning types that are convertible to `bool` meet the `BooleanTestable` requirements ([booleantestable.requirements]). `sizeof...(TTypes) == sizeof...(UTypes)`.

>    > > […]

> 6. Change 23.2.2 [container.requirements.general], Table 95 — "Container requirements", and Table 97 — "Optional container operations" as indicated:

>    > -4- In Tables 95, 96, and 97 `X` denotes a container class containing objects of type `T`, `a` and `b` denote values of type `X`, `u` denotes an identifier, `r` denotes a non-`const` value of type `X`, and `rv` denotes a non-`const` rvalue of type `X`, and `BT` denotes a type that meets the `BooleanTestable` requirements ([booleantestable.requirements]).

>    > > Table 95 — Container requirements
>    > > Expression | Return type | […]
>    > > --- | --- | ---
>    > > `…`
>    > > `a == b` | convertible to `bool` `BT` | […]
>    > > `a != b` | convertible to `bool` `BT` | […]
>    > > `…`
>    > > `a.empty()` | convertible to `bool` `BT` | […]

>    > […]

>    > > Table 97 — Optional container requirements
>    > > Expression | Return type | […]
>    > > --- | --- | ---
>    > > `…`
>    > > `a < b` | convertible to `bool` `BT` | […]
>    > > `a > b` | convertible to `bool` `BT` | […]
>    > > `a <= b` | convertible to `bool` `BT` | […]
>    > > `a >= b` | convertible to `bool` `BT` | […]

> 7. Change 24.3.1 [iterator.requirements.general], Table 106 — "Input iterator requirements", and Table 110 — "Random access iterator requirements" as indicated:

>    > -12- In the following sections, `a` and `b` denote values of type `X` or `const X`, `difference_type` and `reference` refer to the types `iterator_traits<X>::difference_type` and `iterator_traits<X>::reference`, respectively, `n` denotes a value of `difference_type`, `u`, `tmp`, and `m` denote identifiers, `r` denotes a value of `X&`, `t` denotes a value of value type `T`, `o` denotes a value of some type that is writable to the output iterator, and `BT` denotes a type that meets the `BooleanTestable` requirements ([booleantestable.requirements]).

>    > > Table 106 — Input iterator requirements
>    > > Expression | Return type | […]
>    > > --- | --- | ---
>    > > `a != b` | contextually convertible to `bool` `BT` | […]

>    > […]

>    > > Table 110 — Random access iterator requirements
>    > > Expression | Return type | […]
>    > > --- | --- | ---
>    > > `…`
>    > > `a < b` | contextually convertible to `bool` `BT` | […]
>    > > `a > b` | contextually convertible to `bool` `BT` | […]
>    > > `a >= b` | contextually convertible to `bool` `BT` | […]
>    > > `a <= b` | contextually convertible to `bool` `BT` | […]

> 8. Change 26.1 [algorithms.general] p8+p9 as indicated:

>    > [Drafting note: The wording changes below also fix (a) unusual wording forms used ("should work") which are unclear in which sense they are imposing normative requirements and (b) the problem, that the current wording seems to allow that the predicate may mutate a call argument, if that is not a dereferenced iterator. Upon applying the new wording it became obvious that the both the previous and the new wording has the effect that currently algorithms such as `adjacent_find`, `search_n`, `unique`, and `unique_copy` are not correctly described (because they have no iterator argument named `first1`), which could give raise to a new library issue. — end drafting note]

>    > -8- The `Predicate` parameter is used whenever an algorithm expects a function object (20.9) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable as `true`. In other words, iIf an algorithm takes `Predicate pred` as its argument and `first` as its iterator argument, it should work correctly in the construct `pred(*first)` contextually converted to `bool` (Clause 4)the expression `pred(*first)` shall have a type that meets the `BooleanTestable` requirements ( [booleantestable.requirements]). The function object `pred` shall not apply any non-constant function through the dereferenced iteratorits argument.

>    > -9- The `BinaryPredicate` parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type `T` when `T` is part of the signature returns a value testable as `true`. In other words, iIf an algorithm takes `BinaryPredicate binary_pred` as its argument and `first1` and `first2` as its iterator arguments, it should work correctly in the construct `binary_pred(*first1, *first2)` contextually converted to `bool` (Clause 4)the expression `binary_pred(*first1, *first2)` shall have a type that meets the `BooleanTestable` requirements ( [booleantestable.requirements]). `BinaryPredicate` always takes the first iterator's `value_type` as its first argument, that is, in those cases when `T` value is part of the signature, it should work correctly in the construct `binary_pred(*first1, value)` contextually converted to `bool` (Clause 4)the expression `binary_pred(*first1, value)` shall have a type that meets the `BooleanTestable` requirements ( [booleantestable.requirements]). `binary_pred` shall not apply any non-constant function through the dereferenced iteratorsany of its arguments.

> 9. Change 26.8 [alg.sorting] p2 as indicated:

>    > […]

>    > -2- `Compare` is a function object type (20.9). The return value of the function call operation applied to an object of type `Compare`, when contextually converted to `bool`(Clause 4), yields `true` if the first argument of the call is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. Let `a` and `b` denote two argument values whose types depend on the corresponding algorithm. Then the expression `comp(a, b)` shall have a type that meets the `BooleanTestable` requirements ( [booleantestable.requirements]). The return value of `comp(a, b)`, converted to `bool`, yields `true` if the first argument `a` is less than the second argument `b`, and `false` otherwise. It is assumed that `comp` will not apply any non-constant function through the dereferenced iteratorany of its arguments.

>    > […]

> 10. Change 31.5.3.3 [fpos.operations] and Table 126 — "Position type requirements" as indicated:

>     > -1- Operations specified in Table 126 are permitted. In that table,

>     > - `P` refers to an instance of `fpos`,

>     > - […]

>     > - `o` refers to a value of type `streamoff`,

>     > - `BT` refers to a type that meets the `BooleanTestable` requirements ([booleantestable.requirements]),

>     > - […]

>     > > Table 126 — Position type requirements
>     > > Expression | Return type | […]
>     > > --- | --- | ---
>     > > `…`
>     > > `p == q` | convertible to `bool` `BT` | […]
>     > > `p != q` | convertible to `bool` `BT` | […]

> 11. Change 32.2.1 [thread.req.paramname] p1 as indicated:

>     > -1- Throughout this Clause, the names of template parameters are used to express type requirements. If a template parameter is named `Predicate`, `operator()` applied to the template argument shall return a value that is convertible to `bool` `Predicate` is a function object type (22.10 [function.objects]). Let `pred` denote an lvalue of type `Predicate`. Then the expression `pred()` shall have a type that meets the `BooleanTestable` requirements ( [booleantestable.requirements]). The return value of `pred()`, converted to `bool`, yields `true` if the corresponding test condition is satisfied, and `false` otherwise.

[2022-11-05; Daniel comments]

The paper P2167R3 has been reviewed and accepted by LWG and would solve this issue.

[2022-11-22 Resolved by P2167R3 accepted in Kona. Status changed: Open → Resolved.]

Proposed resolution:

### 2118(i). [CD] `unique_ptr` for array does not support cv qualification conversion of actual argument

Section: 20.3.1.4 [unique.ptr.runtime] Status: Resolved Submitter: Alf P. Steinbach Opened: 2011-12-16 Last modified: 2017-09-07

Priority: 1

View all other issues in [unique.ptr.runtime].

View all issues with Resolved status.

Discussion:

Addresses US 16

N3290 20.3.1.4.2 [unique.ptr.runtime.ctor] "`unique_ptr` constructors":

> These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to `pointer`. [Note: One implementation technique is to create private templated overloads of these members. — end note]

This language excludes even `pointer` itself as type for the actual argument.

But of more practical concern is that both Visual C++ 10.0 and MinGW g++ 4.1.1 reject the code below, where only an implicit cv qualification is needed, which cv qualification is supported by the non-array version:

>     #include <memory>
>     using namespace std;

>     struct T {};

>     T* foo() { return new T; }
>     T const* bar() { return foo(); }

>     int main()
>     {
>        unique_ptr< T const >       p1( bar() );        // OK
>        unique_ptr< T const [] >    a1( bar() );        // OK

>        unique_ptr< T const >       p2( foo() );        // OK
>        unique_ptr< T const [] >    a2( foo() );        // ? this is line #15
>     }

The intent seems to be clearly specified in 20.3.1.4 [unique.ptr.runtime]/1 second bullet:

> — Pointers to types derived from `T` are rejected by the constructors, and by `reset`.

But the following language in 20.3.1.4.2 [unique.ptr.runtime.ctor] then rejects far too much...

Proposed new wording of N3290 20.3.1.4.2 [unique.ptr.runtime.ctor] "`unique_ptr` constructors":

> These constructors behave the same as in the primary template except that actual argument pointers `p` to types derived from `T` are rejected by the constructors. [Note: One implementation technique is to create private templated overloads of these members. — end note]

This will possibly capture the intent better, and avoid the inconsistency between the non-array and array versions of `unique_ptr`, by using nearly the exact same phrasing as for the paragraph explaining the intent.

[2012-08-25 Geoffrey Romer comments in c++std-lib-32978]

1. The current P/R seems to intend to support at least two different implementation techniques — additional unusable templates that catch forbidden arguments or replacing existing constructors by templates that ensure ill-formed code inside the template body, when the requirements are not met. It seems unclear whether the current wording allows the second approach, though. It should be considered to allow both strategies or if that is not possible the note should be clearer.

2. The very same problem exists for the `reset` member function, but even worse, because the current specification is more than clear that the deleted `reset` function will catch all cases not equal to `pointer`. It seems confusing at best to have different policies for the constructor and for the `reset` function. In this case, the question in regard to implementation freedom mentioned above is even more important.

3. It's awkward to refer to "the constructors" twice in the same sentence; I suggest revising the sentence as "...except that they do not accept argument pointers `p` to types derived from `T`"

[2012-12-20: Geoffrey Romer comments and provides a revised resolution]

The array specialization of `unique_ptr` differs from the primary template in several ways, including the following:

- `unique_ptr<T[], D>` cannot be constructed from a plain pointer whose type is not exactly `unique_ptr<T[], D>::pointer` or `nullptr_t`.

- `unique_ptr<T[], D>` cannot be constructed from a `unique_ptr<U[], E>&&` unless `U` is exactly `T` and `E` is exactly `D`.

- `unique_ptr<T[], D>` cannot be moveassigned from a `unique_ptr<U[], E>&&` unless `U` is exactly `T` and `E` is exactly `D`.

- `unique_ptr<T[], D>::reset` cannot take an argument whose type is not exactly `unique_ptr<T[], D>::pointer` or `nullptr_t`.

- `default_delete<T[]>` cannot be constructed from a `default_delete<U[]>` unless `U` is exactly `T`.

- `default_delete<T[]>::operator()` cannot be called on a pointer whose type is not exactly `T*`.

The common intent of all these restrictions appears to be to disallow implicit conversions from pointer-to-derived-class to pointer-to-base-class in contexts where the pointer is known to point to an array, because such conversions are inherently unsafe; deleting or subscripting the result of such a conversion leads to undefined behavior (see also CWG 1504). However, these restrictions have the effect of disallowing all implicit conversions in those contexts, including most notably cv-qualification, but also user-defined conversions, and possibly others. This PR narrows all those restrictions, to disallow only unsafe pointer-to-derived to pointer-to-base conversions, while allowing all others.

I removed the nebulous language stating that certain functions "will not accept" certain arguments. Instead I use explicitly deleted template functions, which participate in overload resolution only for pointer-to-derived to pointer-to-base conversions. This is more consistent with the existing text and easier to express correctly than an approach based on declaring certain types of calls to be ill-formed, but may produce inferior compiler diagnostics.

Wherever possible, this PR defines the semantics of template specializations in terms of their differences from the primary template. This improves clarity and minimizes the risk of unintended differences (e.g. LWG 2169(i), which this PR also fixes). This PR also makes it explicit that the specialization inherits the description of all members, not just member functions, from the primary template and, in passing, clarifies the default definition of pointer in the specialization.

This resolution only disallows pointer-to-derived to pointer-to-base conversions between ordinary pointer types; if user-defined pointer types provide comparable conversions, it is their responsibility to ensure they are safe. This is consistent with C++'s general preference for expressive power over safety, and for assuming the user knows what they're doing; furthermore, enforcing such a restriction on user-defined types appears to be impractical without cooperation from the user.

The "base class without regard to cv-qualifiers" language is intended to parallel the specification of `std::is_base_of`.

Jonathan Wakely has a working implementation of this PR patched into libstdc++.

Previous resolution:

> This wording is relative to the FDIS.

> Change 20.3.1.4.2 [unique.ptr.runtime.ctor] as indicated:

> >     explicit unique_ptr(pointer p) noexcept;
> >     unique_ptr(pointer p, see below d) noexcept;
> >     unique_ptr(pointer p, see below d) noexcept;

> > > These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to `pointer` argument pointers `p` to types derived from `T` are rejected by the constructors. [Note: One implementation technique is to create private templated overloads of these members. — end note]

[2014-02, Issaquah]

GR: want to prevent unsafe conversions. Standard is inconsistent how it does this. for `reset()` has deleted function template capturing everything except the known-safe cases. Other functions use SFINAE. Main reason this is hard is that `unique_ptr` supports fancy pointers. Have to figure out how to handle them and what requirements to put on them. Requirements are minimal, not even required to work with `pointer_traits`.

STL surprised `pointer_traits` doesn't work

GR: ways to get fancy pointers to work is to delegate responsibility for preventing unsafe conversions to the fancy pointers themselves. Howard doesn't like that, he wants even fancy pointers to be prevented from doing unsafe conversions in `unique_ptr` contexts.

AM: Howard says `unique_ptr` was meant to be very very safe under all conditions, this open a hole in that. Howard wants to eke forward and support more, but not if we open any holes in type safety.

GR: do we need to be typesafe even for fancy types with incorrect `pointer_traits`?

AM: that would mean it's only unsafe for people who lie by providing a broken specialization of `pointer_traits`

GR: probably can't continue with ambiguity between using SFINAE and ill-formedness. Would appreciate guidance on direction used for that.

STL: difference is observable in convertibility using type traits.

STL: for `reset()` which doesn't affect convertibility ill-formed allows `static_assert`, better diagnostic. For assignment it's detectable and has traits, constraining them is better.

EN: I strongly prefer constraints than `static_asserts`

STL: if we could rely on `pointer_traits` that might be good. Alternatively could we add more machinery to deleter? make deleter say conversions are allowed, otherwise we lock down all conversions. basically want to know if converting `U` to `T` is safe.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3485.

> 1. Revise 20.3.1.2.3 [unique.ptr.dltr.dflt1] as follows

>    >     namespace std {
>    >       template <class T> struct default_delete<T[]> {
>    >         constexpr default_delete() noexcept = default;
>    >         template <class U> default_delete(const default_delete<U>&) noexcept;
>    >         void operator()(T*) const;
>    >         template <class U> void operator()(U*) const = delete;
>    >       };
>    >     }

>    -?- Descriptions are provided below only for member functions that have behavior different from the primary template.

>    >     template <class U> default_delete(const default_delete<U>&) noexcept;

>    > > -?- This constructor behaves the same as in the primary template except that it shall not participate in overload resolution unless:

>    > > - `U` is an array type, and

>    > > - `V*` is implicitly convertible to `T*`, and

>    > > - `T` is not a base class of `V` (without regard to cv-qualifiers),

>    > > where `V` is the array element type of `U`.

>    > >     void operator()(T* ptr) const;

>    > > > -1- Effects: calls `delete[]` on `ptr`.

>    > > > -2- Remarks: If `T` is an incomplete type, the program is ill-formed.

>    > >     template <class U> void operator()(U*) const = delete;

>    > > > -?- Remarks: This function shall not participate in overload resolution unless `T` is a base class of `U` (without regard to cv-qualifiers).

> 2. Revise 20.3.1.3 [unique.ptr.single]/3 as follows:

>    > If the type `remove_reference<D>::type::pointer` exists, then `unique_ptr<T, D>::pointer` shall be a synonym for `remove_reference<D>::type::pointer`. Otherwise `unique_ptr<T, D>::pointer` shall be a synonym for `Telement_type*`. The type `unique_ptr<T, D>::pointer` shall satisfy the requirements of `NullablePointer` (16.4.4.4 [nullablepointer.requirements]).

> 3. Revise 20.3.1.4 [unique.ptr.runtime] as follows:

>    >     namespace std {
>    >       template <class T, class D> class unique_ptr<T[], D> {
>    >       public:
>    >         typedef see below pointer;
>    >         typedef T element_type;
>    >         typedef D deleter_type;

>    >         // 20.7.1.3.1, constructors
>    >         constexpr unique_ptr() noexcept;
>    >         explicit unique_ptr(pointer p) noexcept;
>    >         template <class U> explicit unique_ptr(U* p) = delete;
>    >         unique_ptr(pointer p, see below d) noexcept;
>    >         template <class U> unique_ptr(U* p, see below d) = delete;
>    >         unique_ptr(pointer p, see below d) noexcept;
>    >         template <class U> unique_ptr(U* p, see below d) = delete;
>    >         unique_ptr(unique_ptr&& u) noexcept;
>    >         constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
>    >         template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;

>    >         // destructor
>    >         ~unique_ptr();

>    >         // assignment
>    >         unique_ptr& operator=(unique_ptr&& u) noexcept;
>    >         template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
>    >         unique_ptr& operator=(nullptr_t) noexcept;

>    >         // 20.7.1.3.2, observers
>    >         T& operator[](size_t i) const;
>    >         pointer get() const noexcept;
>    >         deleter_type& get_deleter() noexcept;
>    >         const deleter_type& get_deleter() const noexcept;
>    >         explicit operator bool() const noexcept;

>    >         // 20.7.1.3.3 modifiers
>    >         pointer release() noexcept;
>    >         void reset(pointer p = pointer()) noexcept;
>    >         void reset(nullptr_t) noexcept;
>    >         template <class U> void reset(U*) = delete;
>    >         void swap(unique_ptr& u) noexcept;

>    >         // disable copy from lvalue
>    >         unique_ptr(const unique_ptr&) = delete;
>    >         unique_ptr& operator=(const unique_ptr&) = delete;
>    >       };
>    >     }

>    > -1- A specialization for array types is provided with a slightly altered interface.

>    > - Conversions between different types of `unique_ptr<T[], D>`from `unique_ptr<Derived[]>` to `unique_ptr<Base[]>`, where `Base` is a base class of `Derived`, from `auto_ptr`, or to or from the non-array forms of `unique_ptr` produce an ill-formed program.

>    > - Pointers to types derived from `T` are rejected by the constructors, and by `reset`.

>    > - The observers `operator*` and `operator->` are not provided.

>    > - The indexing observer `operator[]` is provided.

>    > - The default deleter will call `delete[]`.

>    > -2- Descriptions are provided below only for member functions that have behavior differentmembers that differ from the primary template.

>    > -3- The template argument `T` shall be a complete type.

> 4. Revise 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:

>    >     explicit unique_ptr(pointer p) noexcept;
>    >     unique_ptr(pointer p, see below d) noexcept;
>    >     unique_ptr(pointer p, see below d) noexcept;
>    >     template <class U> explicit unique_ptr(U* p) = delete;
>    >     template <class U> unique_ptr(U* p, see below d) = delete;
>    >     template <class U> unique_ptr(U* p, see below d) = delete;

>    > > These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to pointer. [Note: One implementation technique is to create private templated overloads of these members. — end note]These constructors shall not participate in overload resolution unless:

>    > > - `pointer` is a pointer type, and

>    > > - `U*` is implicitly convertible to `pointer`, and

>    > > - `T` is a base class of `U` (without regard to cv-qualifiers).

>    > > The type of `d` is determined as in the corresponding non-deleted constructors.

>    >     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;

>    > > -?- This constructor behaves the same as in the primary template, except that it shall not participate in overload resolution unless:

>    > > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer`, and

>    > > - `U` is an array type, and

>    > > - either `D` is a reference type and `E` is the same type as `D`, or `D` is not a reference type and `E` is implicitly convertible to `D`, and

>    > > - either at least one of `pointer` and `unique_ptr<U, E>::pointer` is not a pointer type, or `T` is not a base class of the array element type of `U` (without regard to cv-qualifiers).

> 5. Insert a new sub-clause following 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:

>    ?? `unique_ptr` assignment [unique.ptr.runtime.asgn]

>    >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

>    > > -?- This operator behaves the same as in the primary template, except that it shall not participate in overload resolution unless:

>    > > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer`, and

>    > > - `U` is an array type, and

>    > > - either `D` is a reference type and `E` is the same type as `D`, or `D` is not a reference type and `E` is implicitly convertible to `D`, and

>    > > - either at least one of `pointer` and `unique_ptr<U, E>::pointer` is not a pointer type, or `T` is not a base class of the array element type of `U` (without regard to cv-qualifiers).

> 6. Revise 20.3.1.4.5 [unique.ptr.runtime.modifiers] as follows:

>    >     void reset(pointer p = pointer()) noexcept;
>    >     void reset(nullptr_t p) noexcept;
>    >     template <class U> void reset(U*) = delete;

>    > > -1- Effects: If `get() == nullptr` there are no effects. Otherwise `get_deleter()(get())`.

>    > > -2- Postcondition: `get() == p`.

>    > > -?- This function shall not participate in overload resolution unless:

>    > > - `pointer` is a pointer type, and

>    > > - `U*` is implicitly convertible to `pointer`, and

>    > > - `T` is a base class of `U` (without regard to cv-qualifiers).

[2014-06 Rapperswil]

Discussion of N4042 and general agreement that this paper resolves the substance of this issue and should be adopted with minor edits. Geoffrey Romer will provide an updated paper.

[2014-06 post-Rapperswil]

As described in N4089.

[2014-11-07 Urbana]

Resolved by N4089

Proposed resolution:

See proposed wording in N4089.

### 2119(i). Missing `hash` specializations for extended integer types

Section: 22.10.19 [unord.hash] Status: C++17 Submitter: Daniel Krügler Opened: 2011-12-16 Last modified: 2017-07-30

Priority: 3

View all other issues in [unord.hash].

View all issues with C++17 status.

Discussion:

According to the header `<functional>` synopsis 22.10 [function.objects] and to the explicit description in 22.10.19 [unord.hash] class template `hash` specializations shall be provided for all arithmetic types that are not extended integer types. This is not explicitly mentioned, but neither the list nor any normative wording does include them, so it follows by implication.

What are the reasons that extended integer types are excluded? E.g. for `numeric_limits` corresponding specializations are required. I would expect that an `unordered_map` with key type `std::uintmax_t` would just work, but that depends now on whether this type is an extended integer type or not.

This issue is not asking for also providing specializations for the cv-qualified arithmetic types. While this is surely a nice-to-have feature, I consider that restriction as a more secondary problem in practice.

The proposed resolution also fixes a problem mentioned in 2109(i) in regard to confusing requirements on user-defined types and those on implementations.

[2012, Kona]

Move to Open.

Agreed that it's a real issue and that the proposed wording fixes it. However, the wording change is not minimal and isn't consistent with the way we fixed hash wording elsewhere.

Alisdair will provide updated wording.

[2014-05-06 Geoffrey Romer suggests alternative wording]

Previous resolution from Daniel [SUPERSEDED]:

> This wording is relative to the FDIS.

> Change 22.10.19 [unord.hash] p2 as indicated:

> >     template <> struct hash<bool>;
> >     template <> struct hash<char>;
> >     […]
> >     template <> struct hash<long double>;
> >     template <class T> struct hash<T*>;

> > > -2- Requires: the template specializations shall meet the requirements of class template `hash` (22.10.19 [unord.hash])The header `<functional>` provides definitions for specializations of the `hash` class template for each cv-unqualified arithmetic type. This header also provides a definition for a partial specialization of the `hash` class template for any pointer type. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].

[2015-05, Lenexa]

STL: the new PR is very simple and could resolve that nicely
MC: the older PR is rather longish

- anybody have any objections to this approach?

- what people want to have as a status?

STL: I want to have Ready
MC: move to ready: in favor: 13, opposed: 0, abstain: 4

Proposed resolution:

This wording is relative to N3936.

Change 22.10.19 [unord.hash] p1 as indicated:

> The unordered associative containers defined in 23.5 use specializations of the class template `hash` as the default hash function. For all object types `Key` for which there exists a specialization `hash<Key>`, and for all integral and enumeration types (7.2) `Key`, the instantiation `hash<Key>` shall: […]

### 2120(i). What should `async` do if neither '`async`' nor '`deferred`' is set in policy?

Section: 32.10.9 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-01-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.async].

View all issues with C++14 status.

Discussion:

Implementations already disagree, one returns an invalid future with no shared state, one chooses `policy == async` and one chooses `policy == deferred`, see c++std-lib-30839, c++std-lib-30840 and c++std-lib-30844. It's not clear if returning an invalid future is allowed by the current wording.

If the intention is to allow an empty future to be returned, then 32.10.9 [futures.async] p3 and p4 should be adjusted to clarify that a shared state might not be created and an invalid future might be returned.

If the intention is that a valid future is always returned, p3 should say something about the case where none of the conditions applies.

[2012, Portland: move to Review]

We could make it undefined if no launch policy is defined.

Hans: If no launch policy is specified the behaviour is undefined

Artur: or implementation defined?

Hans: no: we don't want people to do this

[Proposed wording]

This wording is relative to N3376

Add a third bullet to the end of the list in 30.6.8p3

> "if no valid launch policy is provided the behaviour is undefined"

Moved to review

[2013-04-19, Bristol]

Detlef provides new wording

Previous wording:

> [This wording is relative to N3376]

> Add a third bullet to the end of the list in 32.10.9 [futures.async]p3

> > – if no valid launch policy is provided the behaviour is undefined

[2013-09 Chicago]

If no policy is given, it should be undefined, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

[This wording is relative to N3485]

Add a third bullet to the end of the list in 32.10.9 [futures.async]p3

> – If no value is set in the launch policy, or a value is set that is neither specified in this International Standard or by the implementation, the behaviour is undefined.

### 2122(i). `merge()` stability for lists versus forward lists

Section: 23.3.11.5 [list.ops], 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Nicolai Josuttis Opened: 2012-01-15 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [list.ops].

View all issues with C++14 status.

Discussion:

`forward_list::merge()` is specified in [forwardlist.ops], p19 as follows:

> This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x`.

But `list::merge()` is only specified in 23.3.11.5 [list.ops], p24 as follows:

> Remarks: Stable.

Note that in general we define "stable" only for algorithms (see 3.61 [defns.stable] and 16.4.6.8 [algorithm.stable]) so for member function we should explain it everywhere we use it.

Thus for lists we have to add:

> Stable: for equivalent elements in the two lists, the elements from the list always precede the elements from the argument list.

This, BTW, was the specification we had with C++03.

In addition, I wonder whether we also have some guarantees regarding stability saying that the order of equivalent elements of each list merged remains stable (which would be my interpretation of just saying "stable", BTW).

Thus, I'd expect that for equivalent elements we guarantee that

- we first have all element of `*this` (in the same order as on entry)
- and then all elements of the passed argument (in the same order as on entry).

[2012, Kona]

Move to Open.

STL says we need to fix up 17.6.5.7 to be stronger, and then the remarks for merge should just say "Remarks: Stable (see 17.6.5.7)"

Assigned to STL for word-smithing.

[ 2013-04-14 STL provides rationale and improved wording ]

Step 1: Centralize all specifications of stability to 16.4.6.8 [algorithm.stable].

Step 2: 3.61 [defns.stable] and 16.4.6.8 [algorithm.stable] talk about "algorithms", without mentioning "container member functions". There's almost no potential for confusion here, but there's a simple way to increase clarity without increasing verbosity: make the container member functions explicitly cite 16.4.6.8 [algorithm.stable]. For consistency, we can also update the non-member functions.

Step 3: Fix the "so obvious, we forgot to say it" bug in 16.4.6.8 [algorithm.stable]: a "stable" merge of equivalent elements A B C D and W X Y Z produces A B C D W X Y Z, never D C B A X W Z Y.

Step 3.1: Say "(preserving their original order)" to be consistent with "the relative order [...] is preserved" in 16.4.6.8 [algorithm.stable]'s other bullet points.

Step 4: Copy part of `list::merge()`'s wording to `forward_list::merge()`, in order to properly connect with 16.4.6.8 [algorithm.stable]'s "first range" and "second range".

[2013-04-18, Bristol]

Original wording saved here:

> This wording is relative to the FDIS.

> 1. Change 23.3.11.5 [list.ops] as indicated:

>    >     void                          merge(list<T,Allocator>& x);
>    >     void                          merge(list<T,Allocator>&& x);
>    >     template <class Compare> void merge(list<T,Allocator>& x, Compare comp);
>    >     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

>    > > […]

>    > > -24- Remarks: StableThis operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x` and the order of equivalent elements of `*this` and `x` remains stable. If `(&x != this)` the range `[x.begin(), x.end())` is empty after the merge. No elements are copied by this operation. The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

> 2. Change [forwardlist.ops] as indicated:

>    >     void merge(forward_list<T,Allocator>& x);
>    >     void merge(forward_list<T,Allocator>&& x);
>    >     template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp);
>    >     template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);

>    > > […]

>    > > -19- Effects: Merges `x` into `*this`. This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x` and the order of equivalent elements of `*this` and `x` remains stable. `x` is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

Proposed resolution:

This wording is relative to the N3485.

1. Change 16.4.6.8 [algorithm.stable]/1 as indicated:

   > When the requirements for an algorithm state that it is “stable” without further elaboration, it means:

   > […]
   > - For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).

2. Change [forwardlist.ops] as indicated:

   >     void remove(const T& value);
   >     template <class Predicate> void remove_if(Predicate pred);

   > > -12- Effects: Erases all the elements in the list referred by a list iterator `i` for which the following conditions hold: `*i == value` (for `remove()`), `pred(*i)` is true (for `remove_if()`). This operation shall be stable: the relative order of the elements that are not removed is the same as their relative order in the original list. Invalidates only the iterators and references to the erased elements.

   > > -13- Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.

   > > -??- Remarks: Stable (16.4.6.8 [algorithm.stable]).

   > > […]

   >     void merge(forward_list& x);
   >     void merge(forward_list&& x);
   >     template <class Compare> void merge(forward_list& x, Compare comp)
   >     template <class Compare> void merge(forward_list&& x, Compare comp)

   > > […]

   > > -19- Effects: Merges `x` into `*this` the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x`. `x` is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

   > > -20- Remarks: Stable (16.4.6.8 [algorithm.stable]). The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

   > > […]

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > […]

   > > -23- Effects: Sorts the list according to the `operator<` or the `comp` function object. This operation shall be stable: the relative order of the equivalent elements is preserved. If an exception is thrown the order of the elements in `*this` is unspecified. Does not affect the validity of iterators and references.

   > > -??- Remarks: Stable (16.4.6.8 [algorithm.stable]).

   > > […]

3. Change 23.3.11.5 [list.ops] as indicated:

   >     void remove(const T& value);
   >     template <class Predicate> void remove_if(Predicate pred);

   > > […]

   > > -17- Remarks: Stable (16.4.6.8 [algorithm.stable]).

   > > […]

   >     void merge(list& x);
   >     void merge(list&& x);
   >     template <class Compare> void merge(list& x, Compare comp)
   >     template <class Compare> void merge(list&& x, Compare comp)

   > > […]

   > > -24- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]

   > > […]

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > […]

   > > -30- Remarks: Stable (16.4.6.8 [algorithm.stable]).

   > > […]

4. Change 26.7.1 [alg.copy]/12 as indicated:

   >     template<class InputIterator, class OutputIterator, class Predicate>
   >     OutputIterator
   >     copy_if(InputIterator first, InputIterator last,
   >             OutputIterator result, Predicate pred);

   > > […]

   > > -12- Remarks: Stable (16.4.6.8 [algorithm.stable]).

5. Change 26.7.8 [alg.remove] as indicated:

   >     template<class ForwardIterator, class T>
   >     ForwardIterator
   >     remove(ForwardIterator first, ForwardIterator last, const T& value);
   >     template<class ForwardIterator, class Predicate>
   >     ForwardIterator
   >     remove_if(ForwardIterator first, ForwardIterator last, Predicate pred);

   > > […]

   > > -4- Remarks: Stable (16.4.6.8 [algorithm.stable]).

   > > […]

   >     template<class InputIterator, class OutputIterator, class T>
   >     OutputIterator
   >     remove_copy(InputIterator first, InputIterator last,
   >                 OutputIterator result, const T& value);
   >     template<class InputIterator, class OutputIterator, class Predicate>
   >     OutputIterator
   >     remove_copy_if(InputIterator first, InputIterator last,
   >                    OutputIterator result, Predicate pred);

   > > […]

   > > -11- Remarks: Stable (16.4.6.8 [algorithm.stable]).

6. Change 26.8.2.2 [stable.sort]/4 as indicated:

   >     template<class RandomAccessIterator>
   >     void stable_sort(RandomAccessIterator first, RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >     void stable_sort(RandomAccessIterator first, RandomAccessIterator last,
   >     Compare comp);

   > > […]

   > > -4- Remarks: Stable (16.4.6.8 [algorithm.stable]).

7. Change 26.8.6 [alg.merge] as indicated:

   >     template<class InputIterator1, class InputIterator2,
   >              class OutputIterator>
   >     OutputIterator
   >     merge(InputIterator1 first1, InputIterator1 last1,
   >           InputIterator2 first2, InputIterator2 last2,
   >           OutputIterator result);
   >     template<class InputIterator1, class InputIterator2,
   >              class OutputIterator, class Compare>
   >     OutputIterator
   >     merge(InputIterator1 first1, InputIterator1 last1,
   >           InputIterator2 first2, InputIterator2 last2,
   >           OutputIterator result, Compare comp);

   > > […]

   > > -5- Remarks: Stable (16.4.6.8 [algorithm.stable]).

   > > […]

   >     template<class BidirectionalIterator>
   >     void inplace_merge(BidirectionalIterator first,
   >                        BidirectionalIterator middle,
   >                        BidirectionalIterator last);
   >     template<class BidirectionalIterator, class Compare>
   >     void inplace_merge(BidirectionalIterator first,
   >                        BidirectionalIterator middle,
   >                        BidirectionalIterator last, Compare comp);

   > > […]

   > > -9- Remarks: Stable (16.4.6.8 [algorithm.stable]).

### 2123(i). `merge()` allocator requirements for lists versus forward lists

Section: 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Nicolai Josuttis Opened: 2012-01-15 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [forward.list.ops].

View all issues with C++14 status.

Discussion:

Sub-clause 23.3.11.5 [list.ops], p24 states for lists:

> The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

But there is nothing like that for forward lists in [forwardlist.ops], although I would expect the same undefined behavior there.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

1. Add a new paragraph after [forwardlist.ops] p19 as indicated:

   >     void merge(forward_list<T,Allocator>& x);
   >     void merge(forward_list<T,Allocator>&& x);
   >     template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp);
   >     template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);

   > > […]

   > > -19- Effects: […]

   > > -?- Remarks: The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

### 2127(i). Move-construction with `raw_storage_iterator`

Section: 99 [depr.storage.iterator] Status: C++17 Submitter: Jonathan Wakely Opened: 2012-01-23 Last modified: 2017-07-30

Priority: 3

View all other issues in [depr.storage.iterator].

View all issues with C++17 status.

Discussion:

Aliaksandr Valialkin pointed out that `raw_storage_iterator` only supports constructing a new object from lvalues so cannot be used to construct move-only types:

>     template <typename InputIterator, typename T>
>     void move_to_raw_buffer(InputIterator first, InputIterator last, T *raw_buffer)
>     {
>       std::move(first, last, std::raw_storage_iterator<T *, T>(raw_buffer));
>     }

This could easily be solved by overloading `operator=` for rvalues.

Dave Abrahams:

`raw_storage_iterator` causes exception-safety problems when used with any generic algorithm. I suggest leaving it alone and not encouraging its use.

[2014-11-11, Jonathan provides improved wording]

In Urbana LWG decided to explicitly say the value is constructed from an rvalue.

Previous resolution from Jonathan [SUPERSEDED]:

> This wording is relative to N3337.

> 1. Add a new signature to the synopsis in [storage.iterator] p1:

>    >     namespace std {
>    >       template <class OutputIterator, class T>
>    >       class raw_storage_iterator
>    >         : public iterator<output_iterator_tag,void,void,void,void> {
>    >       public:
>    >         explicit raw_storage_iterator(OutputIterator x);

>    >         raw_storage_iterator<OutputIterator,T>& operator*();
>    >         raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
>    >         raw_storage_iterator<OutputIterator,T>& operator=(T&& element);
>    >         raw_storage_iterator<OutputIterator,T>& operator++();
>    >         raw_storage_iterator<OutputIterator,T> operator++(int);
>    >     };
>    >     }

> 2. Insert the new signature and a new paragraph before p4:

>    >     raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
>    >     raw_storage_iterator<OutputIterator,T>& operator=(T&& element);

>    > > -?- Requires: For the first signature `T` shall be `CopyConstructible`. For the second signature `T` shall be `MoveConstructible`.

>    > > -4- Effects: Constructs a value from `element` at the location to which the iterator points.

>    > > -5- Returns: A reference to the iterator.

[2015-05, Lenexa]

MC: Suggestion to move it to Ready for incorporation on Friday
MC: move to ready: in favor: 12, opposed: 0, abstain: 3

Proposed resolution:

This wording is relative to N4140.

1. Add a new signature to the synopsis in [storage.iterator] p1:

   >     namespace std {
   >       template <class OutputIterator, class T>
   >       class raw_storage_iterator
   >         : public iterator<output_iterator_tag,void,void,void,void> {
   >       public:
   >         explicit raw_storage_iterator(OutputIterator x);

   >         raw_storage_iterator<OutputIterator,T>& operator*();
   >         raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
   >         raw_storage_iterator<OutputIterator,T>& operator=(T&& element);
   >         raw_storage_iterator<OutputIterator,T>& operator++();
   >         raw_storage_iterator<OutputIterator,T> operator++(int);
   >     };
   >     }

2. Insert a new paragraph before p4:

   >     raw_storage_iterator<OutputIterator,T>& operator=(const T& element);

   > > -?- Requires: `T` shall be `CopyConstructible`.

   > > -4- Effects: Constructs a value from `element` at the location to which the iterator points.

   > > -5- Returns: A reference to the iterator.

3. Insert the new signature and a new paragraph after p5:

   >     raw_storage_iterator<OutputIterator,T>& operator=(T&& element);

   > > -?- Requires: `T` shall be `MoveConstructible`.

   > > -?- Effects: Constructs a value from `std::move(element)` at the location to which the iterator points.

   > > -?- Returns: A reference to the iterator.

### 2128(i). Absence of global functions `cbegin/cend`

Section: 24.2 [iterator.synopsis], 24.7 [iterator.range] Status: C++14 Submitter: Dmitry Polukhin Opened: 2012-01-23 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iterator.synopsis].

View all issues with C++14 status.

Discussion:

All standard containers support `cbegin/cend` member functions but corresponding global functions are missing. Proposed resolution it to add global `cbegin/cend` functions by analogy with global `begin/end` functions. This addition will unify things for users.

[2012, Kona]

STL: Range-based for loops do not use global `begin`/`end` (anymore).

Alisdair: We will have to make sure these will be available through many headers.

STL: Do this, including `r` and `cr`. This won't add any additional work.

Matt: Users will find it strange if these are not all available.

Alisdair: Should we have these available everywhere begin/end are available?

Marshall: Yes. Not any extra work.

Howard: Adding all of these means we need all of `<iterator>`.

STL: We already need it all.

Matt: We have to be careful what we are requiring if we include the `r` versions.

Jeffrey: If we include `r`, should they adapt if the container does not define reverse iteration?

STL: No. No special behavior. Should fail to compile. Up to user to add the reverse code--it's easy.

Howard: Anyway it will SFINAE out.

Alisdair: Error messages due to SFINAE are harder to understand than simple failure to compile.

STL: Agrees that SFINAE makes error messages much worse.

Action: STL to provide additional wording for the `r` variants. Move to Review once that wording is availalbe.

[ 2013-04-14 STL provides rationale and improved wording ]

Step 1: Implement `std::cbegin/cend()` by calling `std::begin/end()`. This has numerous advantages:

- It automatically works with arrays, which is the whole point of these non-member functions.
- It works with C++98/03-era user containers, written before `cbegin/cend()` members were invented.
- It works with `initializer_list`, which is extremely minimal and lacks `cbegin/cend()` members.
- 23.2.2 [container.requirements.general] guarantees that this is equivalent to calling cbegin/cend() members.

Step 2: Like `std::begin/end()`, implement `std::rbegin/rend()` by calling `c.rbegin/rend()`. Note that C++98/03 had the `Reversible Container Requirements`.

Step 3: Also like `std::begin/end()`, provide overloads of `std::rbegin/rend()` for arrays.

Step 4: Provide overloads of `std::rbegin/rend()` for `initializer_list`, because it lacks `rbegin/rend()` members. These overloads follow [support.initlist.range]'s signatures. Note that because these overloads return `reverse_iterator`, they aren't being specified in `<initializer_list>`.

Step 5: Like Step 1, implement `std::crbegin/crend()` by calling `std::rbegin/rend()`.

Original wording saved here:

> This wording is relative to N3337.

> 1. In 24.2 [iterator.synopsis], header iterator synopsis, add the following declarations:

>    >     namespace std {
>    >       […]
>    >       // 24.6.5, range access:
>    >       template <class C> auto begin(C& c) -> decltype(c.begin());
>    >       template <class C> auto begin(const C& c) -> decltype(c.begin());
>    >       template <class C> auto end(C& c) -> decltype(c.end());
>    >       template <class C> auto end(const C& c) -> decltype(c.end());
>    >       template <class C> auto cbegin(const C& c) -> decltype(c.cbegin());
>    >       template <class C> auto cend(const C& c) -> decltype(c.cend());
>    >       template <class T, size_t N> T* begin(T (&array)[N]);
>    >       template <class T, size_t N> T* end(T (&array)[N]);
>    >       template <class T, size_t N> const T* cbegin(T (&array)[N]);
>    >       template <class T, size_t N> const T* cend(T (&array)[N]);
>    >     }

> 2. In 24.7 [iterator.range] after p5 add the following series of paragraphs:

>    >     template <class C> auto cbegin(const C& c) -> decltype(c.cbegin());

>    > > -?- Returns: `c.cbegin()`.

>    >     template <class C> auto cend(const C& c) -> decltype(c.cend());

>    > > -?- Returns: `c.cend()`.

>    >     template <class T, size_t N> const T* cbegin(T (&array)[N]);

>    > > -?- Returns: `array`.

>    >     template <class T, size_t N> const T* cend(T (&array)[N]);

>    > > -?- Returns: `array + N`.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3485.

1. In 24.2 [iterator.synopsis], header iterator synopsis, add the following declarations:

   >     namespace std {
   >       […]
   >       // 24.6.5, range access:
   >       template <class C> auto begin(C& c) -> decltype(c.begin());
   >       template <class C> auto begin(const C& c) -> decltype(c.begin());
   >       template <class C> auto end(C& c) -> decltype(c.end());
   >       template <class C> auto end(const C& c) -> decltype(c.end());
   >       template <class T, size_t N> T* begin(T (&array)[N]);
   >       template <class T, size_t N> T* end(T (&array)[N]);
   >       template <class C> auto cbegin(const C& c) -> decltype(std::begin(c));
   >       template <class C> auto cend(const C& c) -> decltype(std::end(c));
   >       template <class C> auto rbegin(C& c) -> decltype(c.rbegin());
   >       template <class C> auto rbegin(const C& c) -> decltype(c.rbegin());
   >       template <class C> auto rend(C& c) -> decltype(c.rend());
   >       template <class C> auto rend(const C& c) -> decltype(c.rend());
   >       template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]);
   >       template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]);
   >       template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il);
   >       template <class E> reverse_iterator<const E*> rend(initializer_list<E> il);
   >       template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c));
   >       template <class C> auto crend(const C& c) -> decltype(std::rend(c));
   >     }

2. At the end of 24.7 [iterator.range], add:

   >     template <class C> auto cbegin(const C& c) -> decltype(std::begin(c));

   > > -?- Returns: `std::begin(c)`.

   >     template <class C> auto cend(const C& c) -> decltype(std::end(c));

   > > -?- Returns: `std::end(c)`.

   >     template <class C> auto rbegin(C& c) -> decltype(c.rbegin());
   >     template <class C> auto rbegin(const C& c) -> decltype(c.rbegin());

   > > -?- Returns: `c.rbegin()`.

   >     template <class C> auto rend(C& c) -> decltype(c.rend());
   >     template <class C> auto rend(const C& c) -> decltype(c.rend());

   > > -?- Returns: `c.rend()`.

   >     template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]);

   > > -?- Returns: `reverse_iterator<T*>(array + N)`.

   >     template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]);

   > > -?- Returns: `reverse_iterator<T*>(array)`.

   >     template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il);

   > > -?- Returns: `reverse_iterator<const E*>(il.end())`.

   >     template <class E> reverse_iterator<const E*> rend(initializer_list<E> il);

   > > -?- Returns: `reverse_iterator<const E*>(il.begin())`.

   >     template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c));

   > > -?- Returns: `std::rbegin(c)`.

   >     template <class C> auto crend(const C& c) -> decltype(std::rend(c));

   > > -?- Returns: `std::rend(c)`.

### 2129(i). User specializations of `std::initializer_list`

Section: 16.4.5.2.1 [namespace.std], 17.11 [support.initlist] Status: C++17 Submitter: Richard Smith Opened: 2012-01-18 Last modified: 2017-07-30

Priority: 3

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Discussion:

Since the implementation is intended to magically synthesize instances of `std::initializer_list` (rather than by a constructor call, for instance), user specializations of this type can't generally be made to work. I can't find any wording which makes such specializations ill-formed, though, which leads me to suspect that they're technically legal under the provisions of 16.4.5.2.1 [namespace.std] p1.

[2012, Kona]

This sounds correct, but we need wording for a resolution.

Marshall Clow volunteers to produce wording.

[2014-02-19, Jonathan Wakely provides proposed wording]

[2014-03-27, Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Add new new paragraph below 17.11 [support.initlist] p2:

   > -2- An object of type `initializer_list<E>` provides access to an array of objects of type `const E`. […]

   > -?- If an explicit specialization or partial specialization of `initializer_list` is declared, the program is ill-formed.

### 2130(i). Missing ordering constraints

Section: 32.5.4 [atomics.order] Status: C++14 Submitter: Mark Batty Opened: 2012-02-22 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [atomics.order].

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Discussion:

C11 issue 407

It seems that both C11 and C++11 are missing the following two derivatives of this rule:

> For atomic modifications `A` and `B` of an atomic object `M`, if there is a `memory_order_seq_cst` fence `X` such that `A` is sequenced before `X`, and `X` precedes `B` in `S`, then `B` occurs later than `A` in the modification order of `M`.

> For atomic modifications `A` and `B` of an atomic object `M`, if there is a `memory_order_seq_cst` fence `Y` such that `Y` is sequenced before `B`, and `A` precedes `Y` in `S`, then `B` occurs later than `A` in the modification order of `M`.

Above wording has been suggested for the Technical Corrigendum of C11 via issue 407, details can be found here.

[2012-03-19: Daniel proposes a slightly condensed form to reduce wording duplications]

[2012-03-20: Hans comments]

The usage of the term atomic operations in 32.5.4 [atomics.order] p7 is actually incorrect and should better be replaced by atomic modifications as used in the C11 407 wording.

There seems to be a similar wording incorrectness used in 6.10.2 [intro.multithread] p17 which should be corrected as well.

[2012, Portland: move to Review]

Olivier: does the fence really participate in the modifications?

Hans: S is the total set of all sequentially consistent operations, and sequentially consistent fences are in S.

Olivier: this sort of combination of a pair of half-open rules seems to imply the write must make it to main memory

But not all implementations treat a fence as a memory operation; cannot observe the half-open rule.

Hans: not sure this is actually prevented here. You could defer until the next load. What the wording doesn't quite show is that the third bullet in the new wording is already in the standard.

Hans: it is the interaction between fences on one side and other memory modifications on the other that is being defined here.

Pablo: S is not directly observable; it is a hypothetic ordering.

Moved to review

Hans: to alert C liaison

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to N3376.

1. [Drafting note: The project editor is kindly asked to consider to replace in 6.10.2 [intro.multithread] p17 the phrase "before an operation B on M" by "before a modification B of M".]

2. Change 32.5.4 [atomics.order] paragraph 7 as indicated: [Drafting note: Note that the wording change intentionally does also replace the term atomic operation by atomic modification]

   -7- For atomic operations A and B on an atomic object M, if there are `memory_order_seq_cst` fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M. For atomic modifications A and B of an atomic object M, B occurs later than A in the modification order of M if:

   - there is a `memory_order_seq_cst` fence X such that A is sequenced before X, and X precedes B in S, or
   - there is a `memory_order_seq_cst` fence Y such that Y is sequenced before B, and A precedes Y in S, or
   - there are `memory_order_seq_cst` fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S.

   -8- [ Note: `memory_order_seq_cst` ensures sequential consistency only for a program that is free of data races and uses exclusively `memory_order_seq_cst` operations. Any use of weaker ordering will invalidate this guarantee unless extreme care is used. In particular, `memory_order_seq_cst` fences ensure a total order only for the fences themselves. Fences cannot, in general, be used to restore sequential consistency for atomic operations with weaker ordering specifications. — end note ]

### 2132(i). `std::function` ambiguity

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++14 Submitter: Ville Voutilainen Opened: 2012-02-28 Last modified: 2016-01-28

Priority: 2

View all other issues in [func.wrap.func.con].

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Discussion:

Consider the following:

>     #include <functional>

>     void f(std::function<void()>) {}
>     void f(std::function<void(int)>) {}

>     int main() {
>       f([]{});
>       f([](int){});
>     }

The calls to `f` in `main` are ambiguous. Apparently because the conversion sequences to `std::function` from the lambdas are identical. The standard specifies that the function object given to `std::function` "shall be Callable (20.8.11.2) for argument types `ArgTypes` and return type `R`." It doesn't say that if this is not the case, the constructor isn't part of the overload set.

Daniel: During the preparation of N3123 it turned out that there are no longer reasons to refer to INVOKE as a conceptually entity alone, its real implementation as a function template `invoke` is possible but was deferred for a later point in time. Defining a type trait for the Callable requirement would also be possible, so there seem to be no technical reasons why the template constructor of `std::function` should not be constrained. The below suggested wording does this without introducing a special trait for this. This corresponds to the way that has been used to specify the `result_of` trait. Note that the definition of the Callable requirement is perfectly suitable for this, because it is a pure syntactically based requirement and can be directly transformed into a constrained template.

The suggested resolution also applies such wording to the "perfectly forwarding" assignment operator

>     template<class F> function& operator=(F&&);

The positive side-effect of this is that it automatically implements a solution to a problem similar to that mentioned in issue 1234(i).

It would be possible to apply similar constraints to the member signatures

>     template<class F> function& operator=(reference_wrapper<F>);

>     template<class F, class A> void assign(F&&, const A&);

as well. At this point there does not seem to be a pestering reason to do so.

[2012-10 Portland: Move to Review]

STL: This is a real issue, but does not like a resolution relying on a SFINAEable metafunction that is not specified and available to the users.

`packaged_task` has the same issue.

STL strongly wants to see an `is_callable` type trait to clarify the proposed wording.

Jeremiah concerned about holding up what appears to be a correct resolution for a hypothetical better one later - the issue is real.

Why must `f` by CopyConstructible? Surely MoveConstructible would be sufficient?

Answer: because `function` is CopyConstructible, and the bound functor is type-erased so must support all the properties of `function` itself.

Replace various applications of `declval` in the proposed resolution with simply using the passed functor object, `f`.

Alisdair to apply similar changes to `packaged_task`.

[2012-11-09, Vicente J. Botet Escriba provides another example]

Consider the following:

>     class AThreadWrapper {
>     public:
>       explicit operator std::thread();
>       ...
>     };
>     std::thread th = std::thread(AThreadWrapper); // call to conversion operator intended

The call to the conversion operator is overloaded with the thread constructor. But thread constructor requirement makes it fail as `AThreadWrapper` is not a Callable and the compiler tries to instantiate the thread constructor and fails.

[2014-02-14 Issaquah meeting: Move to Immediate]

Proposed resolution:

This wording is relative to N3376.

1. Change the following paragraphs in 22.10.17.3.2 [func.wrap.func.con]: [Editorial comment: The removal of the seemingly additional no-throw requirements of copy constructor and destructor of `A` is recommended, because they are already part of the Allocator requirements. Similar clean-up has been suggested by 2070(i) — end comment]

   >     template<class F> function(F f);
   >     template<class F, class A> function(allocator_arg_t, const A& a, F f);

   > > -7- Requires: `F` shall be `CopyConstructible`. `f` shall be Callable (22.10.17.3 [func.wrap.func]) for argument types `ArgTypes` and return type `R`. The copy constructor and destructor of `A` shall not throw exceptions.

   > > -?- Remarks: These constructors shall not participate in overload resolution unless `f` is Callable (22.10.17.3 [func.wrap.func]) for argument types `ArgTypes...` and return type `R`.

   > […]

   >     template<class F> function& operator=(F&& f);

   > > -18- Effects: `function(std::forward<F>(f)).swap(*this);`

   > > -19- Returns: `*this`

   > > -?- Remarks: This assignment operator shall not participate in overload resolution unless `declval<typename decay<F>::type&>()` is Callable (22.10.17.3 [func.wrap.func]) for argument types `ArgTypes...` and return type `R`.

### 2133(i). Attitude to overloaded comma for iterators

Section: 16.4.6.4 [global.functions] Status: C++17 Submitter: Yakov Galka Opened: 2012-01-25 Last modified: 2017-07-30

Priority: 3

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Discussion:

16.4.6.4 [global.functions] says

> Unless otherwise specified, global and non-member functions in the standard library shall not use functions from another namespace which are found through argument-dependent name lookup (3.4.2).

This sounds clear enough. There are just two problems:

1. Both implementations I tested (VS2005 and GCC 3.4.3) do unqualified calls to the comma operator in some parts of the library with operands of user-defined types.

2. The standard itself does this in the description of some algorithms. E.g. `uninitialized_copy` is defined as:

   > Effects:

   > >     for (; first != last; ++result, ++first)
   > >       ::new (static_cast<void*>(&*result))
   > >         typename iterator_traits<ForwardIterator>::value_type(*first);

If understood literally, it is required to call `operator,(ForwardIterator, InputIterator)`.

For detailed discussion with code samples see here.

Proposal:

1. Add an exception to the rule in 16.4.6.4 [global.functions] by permitting the implementation to call the comma operator as much as it wants to. I doubt we want this. or
2. Fix the description of the said algorithms and perhaps add a note to 16.4.6.4 [global.functions] that brings attention of the implementers to avoid this pitfall.

[2013-03-15 Issues Teleconference]

Moved to Open.

There are real questions here, that may require a paper to explore and answer properly.

[2014-05-18, Daniel comments and suggests concrete wording]

Other issues, such as 2114(i) already follow a similar spirit as the one suggested by bullet 2 of the issue submitter. I assert that consideration of possible user-provided overloads of the comma-operator were not intended by the original wording and doing so afterwards would unnecessarily complicate a future conceptualization of the library and would needlessly restrict implementations.

I don't think that a paper is needed to solve this issue, there is a simply way to ensure that the code-semantics excludes consideration of user-provided comma operators. The provided wording below clarifies this by explicitly casting the first argument of the operator to `void`.

[2015-05, Lenexa]

DK: is putting it in the middle the right place for it?
STL: either works, but visually putting it in the middle is clearer, and for "++it1, ++i2, ++it3" it needs to be done after the second comma, so "++it1, (void) ++i2, (void) ++it3" is better than "(void) ++it1, ++i2, (void) ++it3"
ZY: for `INVOKE` yesterday we used `static_cast<void>` but here we're using C-style cast, why?
STL: for `INVOKE` I want to draw attention that there's an intentional coercion to `void` because that's the desired type. Here we only do it because that's the best way to prevent the problem, not because we specifically want a `void` type.
Move to Ready: 9 in favor, none opposed, 1 abstention

Proposed resolution:

This wording is relative to N3936.

1. Change 26.11.5 [uninitialized.copy] as indicated:

   >     template <class InputIterator, class ForwardIterator>
   >       ForwardIterator uninitialized_copy(InputIterator first, InputIterator last,
   >                                          ForwardIterator result);

   > > -1- Effects:

   > > >     for (; first != last; ++result, (void) ++first)
   > > >       ::new (static_cast<void*>(&*result))
   > > >         typename iterator_traits<ForwardIterator>::value_type(*first);

   > > […]

   >     template <class InputIterator, class Size,class ForwardIterator>
   >       ForwardIterator uninitialized_copy_n(InputIterator first, Size n,
   >                                            ForwardIterator result);

   > > -3- Effects:

   > > >     for (; n > 0; ++result, (void) ++first, --n)
   > > >       ::new (static_cast<void*>(&*result))
   > > >         typename iterator_traits<ForwardIterator>::value_type(*first);

2. Change 26.8.11 [alg.lex.comparison] p3 as indicated:

   >     template<class InputIterator1, class InputIterator2>
   >       bool
   >         lexicographical_compare(InputIterator1 first1, InputIterator1 last1,
   >                                 InputIterator2 first2, InputIterator2 last2);
   >     template<class InputIterator1, class InputIterator2, class Compare>
   >       bool
   >         lexicographical_compare(InputIterator1 first1, InputIterator1 last1,
   >                                 InputIterator2 first2, InputIterator2 last2,
   >                                 Compare comp);

   > > -3- Remarks: […]

   > > >     for ( ; first1 != last1 && first2 != last2 ; ++first1, (void) ++first2) {
   > > >       if (*first1 < *first2) return true;
   > > >       if (*first2 < *first1) return false;
   > > >     }
   > > >     return first1 == last1 && first2 != last2;

### 2135(i). Unclear requirement for exceptions thrown in `condition_variable::wait()`

Section: 32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany] Status: C++14 Submitter: Pete Becker Opened: 2012-03-06 Last modified: 2015-10-03

Priority: Not Prioritized

View all other issues in [thread.condition.condvar].

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Discussion:

`condition_varible::wait()` (and, presumably, `condition_variable_any::wait()`, although I haven't looked at it) says that it calls `lock.unlock()`, and if `condition_variable::wait()` exits by an exception it calls `lock.lock()` on the way out. But if the initial call to `lock.unlock()` threw an exception, does it make sense to call `lock.lock()`? We simply don't know the state of that lock object, and it's probably better not to touch it.

That aside, once the `wait()` call has been unblocked, it calls `lock.lock()`. If `lock.lock()` throws an exception, what happens? The requirement is:

> If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

That can be read in two different ways. One way is as if it said "`lock.lock()` shall have been called …", i.e. the original, failed, call to `lock.lock()` is all that's required. But a more natural reading is that wait has to call `lock.lock()` again, even though it already failed.

I think this wording suffers from being too general. There are two possible exception sources: the initial call to `lock.unlock()` and the final call to `lock.lock()`. Each one should have its own requirement. Lumping them together muddles things.

[2012, Portland: move to Open]

Pablo: `unlock` failing is easy -- the call leaves it locked. The second case, trying to `lock` fails -- what can you do? This is an odd state as we had it locked before was called wait. Maybe we should call `terminate` as we cannot meet the post-conditions. We could throw a different exception.

Hans: calling `terminate` makes sense as we're likely to call it soon anyway and at least we have some context.

Detlef: what kind of locks might be being used?

Pablo: condition variables are 'our' locks so this is less of a problem. `condition_variable_any` might be more problematic.

The general direction is to call `terminate` if the lock cannot be reacquired.

Pablo: Can we change the wording to 'leaves the mutex locked' ?

Hans: so if the `unlock` throws we simply propagate the exception.

Move the issue to open and add some formal wording at a later time.

[2013-09 Chicago: Resolved]

Detlef improves wording. Daniel suggests to introduce a Remarks element for the special "If the function fails to meet the postcondition..." wording and applies this to the proposed wording.

Proposed resolution:

This wording is relative to N3691.

1. Edit 32.7.4 [thread.condition.condvar] as indicated:

   >     void wait(unique_lock<mutex>& lock);

   > > […]

   > > -10- Effects:

   > > - Atomically calls `lock.unlock()` and blocks on `*this`.

   > > - When unblocked, calls `lock.lock()` (possibly blocking on the lock), then returns.

   > > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, or spuriously.

   > > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

   > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > -11- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > -12- Throws: Nothing `system_error` when an exception is required (30.2.2).

   > > -13- Error conditions:

   > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Predicate>
   > >     void wait(unique_lock<mutex>& lock, Predicate pred);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -16- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > -17- Throws: `system_error` when an exception is required (30.2.2), timeout-related exceptions (30.2.4), or any exception thrown by `pred`.

   > > > -18- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Clock, class Duration>
   > >       cv_status wait_until(unique_lock<mutex>& lock,
   > >         const chrono::time_point<Clock, Duration>& abs_time);

   > > > […]

   > > > -20- Effects:

   > > > - […]

   > > > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -21- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > […]

   > > > -23- Throws: `system_error` when an exception is required (30.2.2) or timeout-related exceptions (30.2.4).

   > > > -24- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Rep, class Period>
   > >       cv_status wait_for(unique_lock<mutex>& lock,
   > >         const chrono::duration<Rep, Period>& rel_time);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -28- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > […]

   > > > -29- Throws: `system_error` when an exception is required (30.2.2) or timeout-related exceptions (30.2.4).

   > > > -30- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Clock, class Duration, class Predicate>
   > >       bool wait_until(unique_lock<mutex>& lock,
   > >         const chrono::time_point<Clock, Duration>& abs_time,
   > >     	Predicate pred);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -33- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > […]

   > > > -35- Throws: `system_error` when an exception is required (30.2.2), timeout-related exceptions (30.2.4), or any exception thrown by `pred`.

   > > > -36- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Rep, class Period, class Predicate>
   > >       bool wait_for(unique_lock<mutex>& lock,
   > >         const chrono::duration<Rep, Period>& rel_time,
   > >     	Predicate pred);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -40- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > […]

   > > > -42- Throws: `system_error` when an exception is required (30.2.2), timeout-related exceptions (30.2.4), or any exception thrown by `pred`.

   > > > -43- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

2. Edit 32.7.5 [thread.condition.condvarany] as indicated:

   >     template<class Lock>
   >     void wait(Lock& lock);

   > > […]

   > > -10- Effects:

   > > - Atomically calls `lock.unlock()` and blocks on `*this`.

   > > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.

   > > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, or spuriously.

   > > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

   > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > -11- Postcondition: `lock` is locked by the calling thread.

   > > -12- Throws: Nothing `system_error` when an exception is required (30.2.2).

   > > -13- Error conditions:

   > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Lock, class Clock, class Duration>
   > >       cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);

   > > > […]

   > > > -15- Effects:

   > > > - […]

   > > > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -16- Postcondition: `lock` is locked by the calling thread.

   > > > […]

   > > > -18- Throws: `system_error` when an exception is required (30.2.2) or timeout-related exceptions (30.2.4).

   > > > -19- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Lock, class Rep, class Period>
   > >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -22- Postcondition: `lock` is locked by the calling thread.

   > > > […]

   > > > -23- Throws: `system_error` when an exception is required (30.2.2) or timeout-related exceptions (30.2.4).

   > > > -24- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

### 2138(i). `atomic_flag::clear` should not accept `memory_order_consume`

Section: 32.5.10 [atomics.flag] Status: C++14 Submitter: Ben Viglietta Opened: 2012-03-08 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.flag].

View all issues with C++14 status.

Discussion:

N3376 32.5.10 [atomics.flag]/7 says this about `atomic_flag::clear`:

> Requires: The `order` argument shall not be `memory_order_acquire` or `memory_order_acq_rel`.

In addition, `memory_order_consume` should be disallowed, since it doesn't meaningfully apply to store operations. It's already disallowed on the analogous `atomic<T>::store`. The proposed updated text would be:

> Requires: The `order` argument shall not be `memory_order_consume`, `memory_order_acquire`, or `memory_order_acq_rel`.

[2012, Portland: move to Review]

Hans: this is a clear oversight.

Moved to review

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

[This wording is relative to N3376.]

>     void atomic_flag_clear(volatile atomic_flag *object) noexcept;
>     void atomic_flag_clear(atomic_flag *object) noexcept;
>     void atomic_flag_clear_explicit(volatile atomic_flag *object, memory_order order) noexcept;
>     void atomic_flag_clear_explicit(atomic_flag *object, memory_order order) noexcept;
>     void atomic_flag::clear(memory_order order = memory_order_seq_cst) volatile noexcept;
>     void atomic_flag::clear(memory_order order = memory_order_seq_cst) noexcept;

> > -7- Requires: The `order` argument shall not be `memory_order_consume`, `memory_order_acquire`, or `memory_order_acq_rel`.

> > -8- Effects: Atomically sets the value pointed to by `object` or by `this` to false. Memory is affected according to the value of `order`.

### 2139(i). What is a user-defined type?

Section: 16.4.5.2.1 [namespace.std], 19.5 [syserr], 20.2.8.1 [allocator.uses.trait], 22.10.15.2 [func.bind.isbind], 22.10.15.3 [func.bind.isplace], 22.10.19 [unord.hash], 21.3.8.7 [meta.trans.other], 28.3.3.1 [locale], 28.3.4.2.5 [locale.codecvt], 28.6.11.1.5 [re.regiter.incr] Status: C++20 Submitter: Loïc Joly Opened: 2012-03-08 Last modified: 2021-02-25

Priority: 4

View other active issues in [namespace.std].

View all other issues in [namespace.std].

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Discussion:

The expression "user-defined type" is used in several places in the standard, but I'm not sure what it means. More specifically, is a type defined in the standard library a user-defined type?

From my understanding of English, it is not. From most of the uses of this term in the standard, it seem to be considered as user defined. In some places, I'm hesitant, e.g. 16.4.5.2.1 [namespace.std] p1:

> A program may add a template specialization for any standard library template to namespace `std` only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

Does it mean we are allowed to add in the namespace `std` a specialization for `std::vector<std::pair<T, U>>`, for instance?

Additional remarks from the reflector discussion: The traditional meaning of user-defined types refers to class types and enum types, but the library actually means here user-defined types that are not (purely) library-provided. Presumably a new term - like user-provided type - should be introduced and properly defined.

[ 2012-10 Portland: Move to Deferred ]

The issue is real, in that we never define this term and rely on a "know it when I see it" intuition. However, there is a fear that any attempt to pin down a definition is more likely to introduce bugs than solve them - getting the wording for this precisely correct is likely far more work than we are able to give it.

There is unease at simple closing as NAD, but not real enthusiasm to provide wording either. Move to Deferred as we are not opposed to some motivated individual coming back with full wording to review, but do not want to go out of our way to encourage someone to work on this in preference to other issues.

[2014-02-20 Re-open Deferred issues as Priority 4]

[2015-03-05 Jonathan suggests wording]

I dislike the suggestion to change to "user-provided" type because I already find the difference between user-declared / user-provided confusing for special member functions, so I think it would be better to use a completely different term. The core language uses "user-defined conversion sequence" and "user-defined literal" and similar terms for things which the library provides, so I think we should not refer to "user" at all to distinguish entities defined outside the implementation from things provided by the implementation.

I propose "program-defined type" (and "program-defined specialization"), defined below. The P/R below demonstrates the scope of the changes required, even if this name isn't adopted. I haven't proposed a change for "User-defined facets" in [locale].

[Lenexa 2015-05-06]

RS, HT: The core language uses "user-defined" in a specific way, including library things but excluding core language things, let's use a different term.

MC: Agree.

RS: "which" should be "that", x2

RS: Is std::vector<MyType> a "program-defined type"?

MC: I think it should be.

TK: std::vector<int> seems to take the same path.

JW: std::vector<MyType> isn't program-defined, we don't need it to be, anything that depends on that also depends on =MyType.

TK: The type defined by an "explicit template specialization" should be a program-defined type.

RS: An implicit instantiation of a "program-defined partial specialization" should also be a program-defined type.

JY: This definition formatting is horrible and ugly, can we do better?

RS: Checking ISO directives.

RS: Define "program-defined type" and "program-defined specialization" instead, to get rid of the angle brackets.

JW redrafting.

[2017-09-12]

Jonathan revises wording as per Lenexa discussion

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> 1. Add a new sub-clause to [definitions]:

>    17.3.? [defns.program.defined]

>    program-defined

>    <type> a class type or enumeration type which is not part of the C++ standard library and not defined by the implementation. [Note: Types defined by the implementation include extensions (4.1 [intro.compliance]) and internal types used by the library. — end note]

>    program-defined

>    <specialization> an explicit template specialization or partial specialization which is not part of the C++ standard library and not defined by the implementation.

> 2. Change 16.4.5.2.1 [namespace.std] paragraph 1+2:

>    -1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace `std` or to a namespace within namespace `std` unless otherwise specified. A program may add a template specialization for any standard library template to namespace `std` only if the declaration depends on a userprogram-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

>    -2- The behavior of a C++ program is undefined if it declares

>    […]

>    A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a userprogram-defined type and the instantiation meets the standard library requirements for the original template.

> 3. Change 19.5 [syserr] paragraph 4:

>    -4- The `is_error_code_enum` and `is_error_condition_enum` may be specialized for userprogram-defined types to indicate that such types are eligible for class `error_code` and class `error_condition` automatic conversions, respectively.

> 4. Change 20.2.8.1 [allocator.uses.trait] paragraph 1:

>    -1- Remarks: automatically detects […]. A program may specialize this template to derive from `true_type` for a userprogram-defined type `T` that does not have a nested `allocator_type` but nonetheless can be constructed with an allocator where either: […]

> 5. Change 22.10.15.2 [func.bind.isbind] paragraph 2:

>    -2- Instantiations of the `is_bind_expression` template […]. A program may specialize this template for a userprogram-defined type `T` to have a `BaseCharacteristic` of `true_type` to indicate that `T` should be treated as a subexpression in a `bind` call.

> 6. Change 22.10.15.3 [func.bind.isplace] paragraph 2:

>    -2- Instantiations of the `is_placeholder` template […]. A program may specialize this template for a userprogram-defined type `T` to have a `BaseCharacteristic` of `integral_constant<int, N>` with `N > 0` to indicate that `T` should be treated as a placeholder type.

> 7. Change 22.10.19 [unord.hash] paragraph 1:

>    The unordered associative containers defined in 23.5 use specializations of the class template `hash` […], the instantiation `hash<Key>` shall:

>    - […]

>    - […]

>    - […]

>    - […]

>    - satisfy the requirement that the expression `h(k)`, where `h` is an object of type `hash<Key>` and `k` is an object of type `Key`, shall not throw an exception unless `hash<Key>` is a userprogram-defined specialization that depends on at least one userprogram-defined type.

> 8. Change 21.3.8.6 [meta.trans.ptr] Table 57 (`common_type` row):

>    > Table 57 — Other transformations
>    > Template | Condition | Comments
>    > --- | --- | ---
>    > `…`
>    > `template <class... T> struct common_type;` |  | The member typedef `type` shall be defined or omitted as specified below. […]. A program may specialize this trait if at least one template parameter in the specialization is a userprogram-defined type. […]
>    > `…`

> 9. Change 28.3.4.2.5 [locale.codecvt] paragraph 3:

>    -3- The specializations required in Table 81 (22.3.1.1.1) […]. Other encodings can be converted by specializing on a userprogram-defined `stateT` type.[…]

> 10. Change 28.6.11.1.5 [re.regiter.incr] paragraph 8:

>     -8- [Note: This means that a compiler may call an implementation-specific search function, in which case a userprogram-defined specialization of `regex_search` will not be called. — end note]

[2018-3-14 Wednesday evening issues processing; move to Ready]

After this lands, we need to audit Annex C to find "user-defined type" Example: [diff.cpp03.containers]/3

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4687.

1. Add a new sub-clause to [definitions]:

   20.3.? [defns.program.defined.spec]

   program-defined specialization

   explicit template specialization or partial specialization that is not part of the C++ standard library and not defined by the implementation

   20.3.? [defns.program.defined.type]

   program-defined type

   class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or an instantiation of a program-defined specialization

   > [Drafting note: ISO directives say the following Note should be labelled as a "Note to entry" but the C++ WP doesn't follow that rule (yet). — end drafting note]

   [Note: Types defined by the implementation include extensions (4.1 [intro.compliance]) and internal types used by the library. — end note]

2. Change 16.4.5.2.1 [namespace.std] paragraph 1+2:

   -1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace `std` or to a namespace within namespace `std` unless otherwise specified. A program may add a template specialization for any standard library template to namespace `std` only if the declaration depends on a userprogram-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

   -2- The behavior of a C++ program is undefined if it declares

   […]

   A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a userprogram-defined type and the instantiation meets the standard library requirements for the original template.

3. Change 19.5 [syserr] paragraph 4:

   -4- The `is_error_code_enum` and `is_error_condition_enum` may be specialized for userprogram-defined types to indicate that such types are eligible for class `error_code` and class `error_condition` automatic conversions, respectively.

4. Change 20.2.8.1 [allocator.uses.trait] paragraph 1:

   -1- Remarks: automatically detects […]. A program may specialize this template to derive from `true_type` for a userprogram-defined type `T` that does not have a nested `allocator_type` but nonetheless can be constructed with an allocator where either: […]

5. Change 22.10.15.2 [func.bind.isbind] paragraph 2:

   -2- Instantiations of the `is_bind_expression` template […]. A program may specialize this template for a userprogram-defined type `T` to have a `BaseCharacteristic` of `true_type` to indicate that `T` should be treated as a subexpression in a `bind` call.

6. Change 22.10.15.3 [func.bind.isplace] paragraph 2:

   -2- Instantiations of the `is_placeholder` template […]. A program may specialize this template for a userprogram-defined type `T` to have a `BaseCharacteristic` of `integral_constant<int, N>` with `N > 0` to indicate that `T` should be treated as a placeholder type.

7. Change 22.10.19 [unord.hash] paragraph 1:

   The unordered associative containers defined in 23.5 use specializations of the class template `hash` […], the instantiation `hash<Key>` shall:

   - […]

   - […]

   - […]

   - […]

   - satisfy the requirement that the expression `h(k)`, where `h` is an object of type `hash<Key>` and `k` is an object of type `Key`, shall not throw an exception unless `hash<Key>` is a userprogram-defined specialization that depends on at least one userprogram-defined type.

8. Change 21.3.8.6 [meta.trans.ptr] Table 57 (`common_type` row):

   > Table 57 — Other transformations
   > Template | Condition | Comments
   > --- | --- | ---
   > `…`
   > `template <class... T> struct common_type;` |  | The member typedef `type` shall be defined or omitted as specified below. […]. A program may specialize this trait if at least one template parameter in the specialization is a userprogram-defined type. […]
   > `…`

9. Change 28.3.4.2.5 [locale.codecvt] paragraph 3:

   -3- The specializations required in Table 81 (22.3.1.1.1) […]. Other encodings can be converted by specializing on a userprogram-defined `stateT` type.[…]

10. Change 28.6.11.1.5 [re.regiter.incr] paragraph 8:

    -8- [Note: This means that a compiler may call an implementation-specific search function, in which case a userprogram-defined specialization of `regex_search` will not be called. — end note]

### 2140(i). Meaning of `notify_all_at_thread_exit` synchronization requirement?

Section: 32.7 [thread.condition] Status: C++14 Submitter: Pete Becker Opened: 2012-03-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition].

View all issues with C++14 status.

Discussion:

`notify_all_at_thread_exit` has the following synchronization requirement:

> Synchronization: The call to `notify_all_at_thread_exit` and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.10.2 [intro.multithread]) calls to functions waiting on `cond`.

The functions waiting on `cond` have already been called, otherwise they wouldn't be waiting. So how can a subsequent call to `notify_all_at_thread_exit` synchronize with them?

Also, "synchronizes with" is a relationship between library calls (6.10.2 [intro.multithread]/8), so it's not meaningful for completion of destructors for non-library objects. Presumably the intention wasn't so make library destructors special here.

[2012-03-09 Jeffrey Yasskin comments:]

I think the text should say that "`notify_all_at_thread_exit` and destructor calls are sequenced before the `lk.unlock()`", and leave it at that, unless there's a funny implementation I haven't thought of.

[2012-03-19 Hans Boehm comments:]

I think the synchronization clause should just be replaced with (modulo wording tweaks):

"The implied `lk.unlock()` call is sequenced after the destruction of all objects with thread storage duration associated with the current thread."

as Jeffrey suggested.

To use this correctly, the notifying thread has to essentially acquire the lock, set a variable indicating it's done, call `notify_all_at_thread_exit()`, while the waiting thread acquires the lock, and repeatedly waits on the cv until the variable is set, and then releases the lock. That ensures that we have the proper synchronizes with relationship as a result of the lock.

[2012, Portland: move to Review]

The `lk.unlock()` refers back to the wording the previous paragraph.

Moved to review

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to N3376.

1. Modify 32.7 [thread.condition] p8 as indicated:

   >     void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);

   > > […]

   > > -8- Synchronization: The call to `notify_all_at_thread_exit` and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.10.2 [intro.multithread]) calls to functions waiting on `cond` The implied `lk.unlock()` call is sequenced after the destruction of all objects with thread storage duration associated with the current thread.

### 2141(i). `common_type` trait produces reference types

Section: 21.3.8.7 [meta.trans.other] Status: C++14 Submitter: Doug Gregor Opened: 2012-03-11 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [meta.trans.other].

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Discussion:

The type computation of the `common_type` type trait is defined as

>     template <class T, class U>
>      struct common_type<T, U> {
>        typedef decltype(true ? declval<T>() : declval<U>()) type;
>      };

This means that `common_type<int, int>::type` is `int&&`, because

- `declval<int>()` returns `int&&`
- The conditional operator returns an xvalue when its second and third operands have the same type and are both xvalues (7.6.16 [expr.cond] p4)
- `decltype` returns `T&&` when its expression is an xvalue (9.2.9.3 [dcl.type.simple] p4)

Users of `common_type` do not expect to get a reference type as the result; the expectation is that `common_type` will return a non-reference type to which all of the types can be converted.

Daniel: In addition to that it should be noted that without such a fix the definition of `std::unique_ptr`'s `operator<` in 20.3.1.6 [unique.ptr.special] (around p4) is also broken: In the most typical case (with default deleter), the determination of the common pointer type CT will instantiate `std::less<CT>` which can now be `std::less<T*&&>`, which will not be the specialization of pointer types that guarantess a total order.

Given the historic constext of `common_type` original specification, the proper resolution to me seems to be using `std::decay` instead of `std::remove_reference`:

>     template <class T, class U>
>     struct common_type<T, U> {
>       typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
>     };

At that time rvalues had no identity in this construct and rvalues of non-class types have no cv-qualification. With this change we would ensure that

>     common_type<int, int>::type == common_type<const int, const int>::type == int

Note that this harmonizes with the corresponding heterogenous case, which has already the exact same effect:

>     common_type<int, long>::type == common_type<const int, const long>::type == long

[2012-10-11 Daniel comments]

While testing the effects of applying the proposed resolution I noticed that this will have the effect that the unary form of `common_type`, like

>     common_type<int>

is not symmetric to the n-ary form (n > 1). This is unfortunate, because this difference comes especially to effect when `common_type` is used with variadic templates. As an example consider the following `make_array` template:

>     #include <array>
>     #include <type_traits>
>     #include <utility>

>     template<class... Args>
>     std::array<typename std::common_type<Args...>::type, sizeof...(Args)>
>     make_array(Args&&... args)
>     {
>       typedef typename std::common_type<Args...>::type CT;
>       return std::array<CT, sizeof...(Args)>{static_cast<CT>(std::forward<Args>(args))...};
>     }

>     int main()
>     {
>       auto a1 = make_array(0); // OK: std::array<int, 1>
>       auto a2 = make_array(0, 1.2); // OK: std::array<double, 2>
>       auto a3 = make_array(5, true, 3.1415f, 'c'); // OK: std::array<float, 4>

>       int i = 0;
>       auto a1b = make_array(i); // Error, attempt to form std::array<int&, 1>

>       auto a2b = make_array(i, 1.2); // OK: std::array<double, 2>
>       auto a2c = make_array(i, 0); // OK: std::array<int, 2>
>     }

The error for `a1b` only happens in the unary case and it is easy that it remains unnoticed during tests. You cannot explain that reasonably to the user here.

Of-course it is possible to fix that in this example by applying `std::decay` to the result of the `std::common_type` deduction. But if this is necessary here, I wonder why it should also be applied to the binary case, where it gives the wrong illusion of a complete type decay? The other way around: Why is `std::decay` not also applied to the unary case as well?

This problem is not completely new and was already observed for the original `std::common_type` specification. At this time the `decltype` rules had a similar asymmetric effect when comparing

> `std::common_type<const int, const int>::type` (equal to '`int`' at this time)

with:

> `std::common_type<const int>::type` (equal to '`const int`')

and I wondered whether the unary form shouldn't also perform the same "decay" as the n-ary form.

This problem makes me think that the current resolution proposal might not be ideal and I expect differences in implementations (for those who consider to apply this proposed resolution already). I see at least three reasonable options:

1. Accept the current wording suggestion for LWG 2141 as it is and explain that to users.

2. Keep `std::common_type` as currently specified in the Standard and tell users to use `std::decay` where needed. Also fix other places in the library, e.g. the comparison functions of `std::unique_ptr` or a most of the time library functions.

3. Apply `std::decay` also in the unary specialization of `std::common_type` with the effect that `std::common_type<const int&>::type` returns `int`.

[2012-10-11 Marc Glisse comments]

If we are going with decay everywhere, I wonder whether we should also decay in the 2-argument version before and not only after. So if I specialize `common_type<mytype, double>`, `common_type<const mytype, volatile double&>` would automatically work.

[2012-10-11 Daniel provides wording for bullet 3 of his list:]

1. Change 21.3.8.7 [meta.trans.other] p3 as indicated:

   >     template <class T>
   >     struct common_type<T> {
   >       typedef typename decay<T>::type type;
   >     };

   >     template <class T, class U>
   >     struct common_type<T, U> {
   >       typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
   >     };

[2013-03-15 Issues Teleconference]

Moved to Review.

Want to carefully consider the effect of `decay` vs. `remove_reference` with respect to constness before adopting, although this proposed resolution stands for review in Bristol.

[2013-04-18, Bristol meeting]

Previous wording:

> This wording is relative to N3376.

> 1. In 21.3.8.7 [meta.trans.other] p3, change the `common_type` definition to

>    >     template <class T, class U>
>    >     struct common_type<T, U> {
>    >       typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
>    >     };

[2013-04-18, Bristol]

Move to Ready

[2013-09-29, Chicago]

Accepted for the working paper

Proposed resolution:

This wording is relative to N3485.

1. Change 21.3.8.7 [meta.trans.other] p3 as indicated:

   >     template <class T>
   >     struct common_type<T> {
   >       typedef typename decay<T>::type type;
   >     };

   >     template <class T, class U>
   >     struct common_type<T, U> {
   >       typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
   >     };

### 2142(i). `packaged_task::operator()` synchronization too broad?

Section: 32.10.10.2 [futures.task.members] Status: C++14 Submitter: Pete Becker Opened: 2012-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [futures.task.members].

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Discussion:

According to 32.10.10.2 [futures.task.members] p.18:

> [A] successful call to [`packaged_task::`]`operator()` synchronizes with a call to any member function of a `future` or `shared_future` object that shares the shared state of `*this`.

This requires that the call to `operator()` synchronizes with calls to `future::wait_for`, `future::wait_until`, `shared_future::wait_for`, and `shared_future::wait_until`, even when these functions return because of a timeout.

[2012, Portland: move to Open]

If it said "a successful return from" (or "a return from" to cover exceptions) the problem would be more obvious.

Detlef: will ask Anthony Williams to draft some wording.

Moved to open (Anthony drafted to draft)

[2013-09, Chicago: move to Ready]

Anthony's conclusion is that the offending paragraph is not needed. Already included in the statement that the state is made ready.

Recommendation: Remove 32.10.10.2 [futures.task.members] p18 (the synchronization clause). Redundant because of 32.10.5 [futures.state] p9.

Moved to Ready

Proposed resolution:

This wording is relative to N3691.

1. Remove 32.10.10.2 [futures.task.members] p18 as indicated:

   >     void operator()(ArgTypes... args);

   > > […]

   > > -18- Synchronization: a successful call to `operator()` synchronizes with (1.10) a call to any member function of a `future` or `shared_future` object that shares the shared state of `*this`. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with (1.10) the successful return from any member function that detects that the state is set to ready. [Note: `operator()` synchronizes and serializes with other functions through the shared state. — end note]

### 2143(i). `ios_base::xalloc` should be thread-safe

Section: 31.5.2 [ios.base] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2012-03-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [ios.base].

View all issues with C++14 status.

Discussion:

The static function `ios_base::xalloc()` could be called from multiple threads and is not covered by 16.4.5.10 [res.on.objects] and 16.4.6.10 [res.on.data.races]. Adding a thread-safety requirement should not impose a significant burden on implementations, as the function can be easily implemented with hopefully lock-free atomics.

[2013-04-20, Bristol]

Unanimous.

Resolution: move tentatively ready. (Inform Bill about this issue.)

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3376.

1. In 31.5.2.6 [ios.base.storage] add a new paragraph after paragraph 1:

   >     static int xalloc();

   > > -1- Returns: `index ++`.

   > > -?- Remarks: Concurrent access to this function by multiple threads shall not result in a data race (6.10.2 [intro.multithread]).

### 2144(i). Missing `noexcept` specification in `type_index`

Section: 17.7.7 [type.index] Status: C++14 Submitter: Daniel Krügler Opened: 2012-03-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [type.index].

View all issues with C++14 status.

Discussion:

The class type `type_index` is a thin wrapper of `type_info` to adapt it as a valid associative container element. Similar to `type_info`, all member functions have an effective `noexcept(true)` specification, with the exception of `hash_code()` and `name()`. The actual effects of these functions is a direct call to `type_info`'s `hash_code()` and `name` function, but according to 17.7 [support.rtti] these are both `noexcept` functions, so there is no reason for not declaring them as `noexcept`, too. In fact, one of the suggested changes of the original proposing paper N2530 specifically was to ensure that `type_info` would get a `hash_code()` function that guarantees not to throw exceptions (during that time the `hash` requirements did not allow to exit with an exception). From this we can conclude that `type_index::hash_code()` was intended to be nothrow.

It seems both consistent and technically simply to require these functions to be `noexcept`.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Modify the class `type_index` synopsis, [type.index.overview] as indicated:

   >     namespace std {
   >       class type_index {
   >       public:
   >         type_index(const type_info& rhs) noexcept;
   >         bool operator==(const type_index& rhs) const noexcept;
   >         bool operator!=(const type_index& rhs) const noexcept;
   >         bool operator< (const type_index& rhs) const noexcept;
   >         bool operator<= (const type_index& rhs) const noexcept;
   >         bool operator> (const type_index& rhs) const noexcept;
   >         bool operator>= (const type_index& rhs) const noexcept;
   >         size_t hash_code() const noexcept;
   >         const char* name() const noexcept;
   >       private:
   >         const type_info* target; // exposition only
   >         // Note that the use of a pointer here, rather than a reference,
   >         // means that the default copy/move constructor and assignment
   >         // operators will be provided and work as expected.
   >       };
   >     }

1. Modify the prototype definitions in [type.index.members] as indicated:

   >     size_t hash_code() const noexcept;

   > > -8- Returns: `target->hash_code()`

   >     const char* name() const noexcept;

   > > -9- Returns: `target->name()`

### 2145(i). `error_category` default constructor

Section: 19.5.3 [syserr.errcat] Status: C++14 Submitter: Howard Hinnant Opened: 2012-03-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [syserr.errcat].

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Discussion:

Should `error_category` have a default constructor?

If you look at the synopsis in 19.5.3.1 [syserr.errcat.overview], it appears the answer is no. There is no default constructor declared and there is another constructor declared (which should inhibit a default constructor).

However in paragraph 1 of the same section, descriptive text says:

> Classes may be derived from `error_category` to support categories of errors in addition to those defined in this International Standard.

How shall classes derived from `error_category` construct their base?

Jonathan Wakely: In N2066 `error_category` was default-constructible. That is still the case in N2241, because no other constructor is declared. Then later N2422 (issue 6) declares the copy constructor as deleted, but doesn't add a default constructor, causing it to be no longer default-constructible. That looks like an oversight to me, and I think there should be a public default constructor.

Daniel: A default-constructor indeed should be provided to allow user-derived classes as described by the standard. I suggest this one to be both `noexcept` and `constexpr`. The latter allows user-derived non-abstract classes to take advantage of the special constant initialization rule of [basic.start.init] p2 b2 for objects with static (or thread) storage duration in namespace scope. Note that a `constexpr` constructor is feasible here, even though there exists a non-trivial destructor and even though `error_category` is not a literal type (see `std::mutex` for a similar design choice).

In addition to that the proposed resolution fixes another minor glitch: According to 16.3.3.5 [functions.within.classes] virtual destructors require a semantics description.

Alberto Ganesh Barbati: I would suggest to remove `=default` from the constructor instead. Please consider that defaulting a constructor or destructor may actually define them as deleted under certain conditions (see 11.4.5 [class.ctor]/5 and 11.4.7 [class.dtor]/5). Removing `=default` is easier than providing wording to ensures that such conditions do not occur.

[2012-10 Portland: move to Ready]

The issue is real and the resolution looks good.

Are there similar issues elsewhere in this clause?

Potential to add `constexpr` to more constructors, but clearly a separable issue.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Modify the class `error_category` synopsis, 19.5.3.1 [syserr.errcat.overview] as indicated: [Drafting note: According to the general `noexcept` library guidelines destructors should not have any explicit exception specification. This destructor was overlooked during the paper analysis — end note]

   >     namespace std {
   >       class error_category {
   >       public:
   >         constexpr error_category() noexcept;
   >         virtual ~error_category() noexcept;
   >         error_category(const error_category&) = delete;
   >         error_category& operator=(const error_category&) = delete;
   >         virtual const char* name() const noexcept = 0;
   >         virtual error_condition default_error_condition(int ev) const noexcept;
   >         virtual bool equivalent(int code, const error_condition& condition) const noexcept;
   >         virtual bool equivalent(const error_code& code, int condition) const noexcept;
   >         virtual string message(int ev) const = 0;
   >         bool operator==(const error_category& rhs) const noexcept;
   >         bool operator!=(const error_category& rhs) const noexcept;
   >         bool operator<(const error_category& rhs) const noexcept;
   >       };
   >     }

2. Before 19.5.3.2 [syserr.errcat.virtuals] p1 insert a new prototype description as indicated:

   >     virtual ~error_category();

   > > -?- Effects: Destroys an object of class `error_category`.

3. Before 19.5.3.3 [syserr.errcat.nonvirtuals] p1 insert a new prototype description as indicated:

   >     constexpr error_category() noexcept;

   > > -?- Effects: Constructs an object of class `error_category`.

### 2147(i). Unclear hint type in `Allocator`'s `allocate` function

Section: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Daniel Krügler Opened: 2012-03-05 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [allocator.requirements].

View all other issues in [allocator.requirements].

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Discussion:

According to Table 28 — "Allocator requirements", the expression

>     a.allocate(n, u)

expects as second argument a value `u` that is described in Table 27 as:

> a value of type `YY::const_pointer` obtained by calling `YY::allocate`, or else `nullptr`.

This description leaves it open, whether or whether not a value of type `YY::const_void_pointer` is valid or not. The corresponding wording in C++03 is nearly the same, but in C++03 there did not exist the concept of a general `void_pointer` for allocators. There is some evidence for support of void pointers because the general `allocator_traits` template declares

>     static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);

and the corresponding function for `std::allocator<T>` is declared as:

>     pointer allocate(size_type, allocator<void>::const_pointer hint = 0);

As an additional minor wording glitch (especially when comparing with the `NullablePointer` requirements imposed on `const_pointer` and `const_void_pointer`), the wording seems to exclude lvalues of type `std::nullptr_t`, which looks like an unwanted artifact to me.

[ 2012-10 Portland: Move to Ready ]

No strong feeling that this is a big issue, but consensus that the proposed resolution is strictly better than the current wording, so move to Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change Table 27 — "Descriptive variable definitions" in 16.4.4.6 [allocator.requirements]:

   Table 27 — Descriptive variable definitions
   Variable | Definition
   --- | ---
   `u` | a value of type `YY::const_pointer` obtained by calling `YY::allocate`, or else `nullptr` `XX::const_void_pointer` obtained by conversion from a result value of `YY::allocate`, or else a value of type (possibly const) `std::nullptr_t`.

### 2148(i). Hashing enums should be supported directly by `std::hash`

Section: 22.10.19 [unord.hash] Status: C++14 Submitter: Ville Voutilainen Opened: 2012-04-10 Last modified: 2016-08-03

Priority: Not Prioritized

View all other issues in [unord.hash].

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Discussion:

The paper proposes various hashing improvements. What it doesn't mention is hashing of enums; enums are integral types, and users expect them to have built-in hashing support, rather than having to convert enums to ints for uses with unordered containers and other uses of hashes. Daniel Krügler explains in c++std-lib-32412 that this is not achievable with a SFINAEd hash specialization because it would require a partial specialization with a type parameter and a non-type parameter with a default argument, which is currently not allowed, and hence the fixes in N3333 should be adopted instead.

[2012-10 Portland: Move to Open]

We agree this is a real issue that should be resolved, by specifying such a hash.

It is not clear that we should specify this as calling hash on the `underlying_type`, or whether that is overspecification and we merely require that the hash be supplied.

STL already has shipped an implementation, and is keen to provide wording.

[ 2013-04-14 STL provides rationale and improved wording ]

Rationale:

This can be achieved by inserting a very small tweak to the Standardese. We merely have to require that `hash<Key>` be valid when `Key` is an "enumeration type" (which includes both scoped and unscoped enums). This permits, but does not require, `hash<Enum>` to behave identically to `hash<underlying_type<Enum>::type>`, following existing precedent — note that when `unsigned int` and `unsigned long` are the same size, `hash<unsigned int>` is permitted-but-not-required to behave identically to `hash<unsigned long>`.

This proposed resolution doesn't specify anything else about the primary template, allowing implementations to do whatever they want for non-enums: `static_assert` nicely, explode horribly at compiletime or runtime, etc.

While we're in the neighborhood, this proposed resolution contains an editorial fix. The 22.10 [function.objects] synopsis says "base template", which doesn't appear anywhere else in the Standard, and could confuse users into thinking that they need to derive from it. The proper phrase is "primary template".

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3485.

1. In 22.10 [function.objects], header functional synopsis, edit as indicated:

   >     namespace std {
   >       […]
   >       // 20.8.12, hash function baseprimary template:
   >       template <class T> struct hash;
   >       […]
   >     }

2. In 22.10.19 [unord.hash]/1 edit as indicated:

   > -1- The unordered associative containers defined in 23.5 [unord] use specializations of the class template `hash` as the default `hash` function. For all object types `Key` for which there exists a specialization `hash<Key>`, and for all enumeration types (9.8.1 [dcl.enum]) Key, the instantiation `hash<Key>` shall: […]

### 2149(i). Concerns about 20.8/5

Section: 22.10 [function.objects] Status: C++14 Submitter: Scott Meyers Opened: 2012-02-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [function.objects].

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Discussion:

22.10 [function.objects] p5 says:

> To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects correspondingly provide typedefs `argument_type` and `result_type` for function objects that take one argument and `first_argument_type`, `second_argument_type`, and `result_type` for function objects that take two arguments.

I have two concerns about this paragraph. First, the wording appears to prescribe a requirement for all function objects in valid C++ programs, but it seems unlikely that that is the intent. As such, the scope of the requirement is unclear. For example, there is no mention of these typedefs in the specification for closures (5.1.2), and Daniel Krügler has explained in the thread at http://tinyurl.com/856plkn that conforming implementations can detect the difference between closures with and without these typedefs. (Neither gcc 4.6 nor VC10 appear to define typedefs such as `result_type` for closure types. I have not tested other compilers.)

Second, the requirement appears to be unimplementable in some cases, notably for function objects returned from `std::bind`, as Howard Hinnant explains in the thread at http://tinyurl.com/6q5bos4.

From what I can tell, the standard already defines which adaptability typedefs must be provided by various kinds of function objects in the specifications for those objects. Examples include the function objects specified in 22.10.6 [refwrap]- [negators]. I therefore suggest that 22.10 [function.objects]/5 simply be removed from the standard. I don't think it adds anything except opportunities for confusion.

[2012-10 Portland: Move to Open]

This wording caused confusion earlier in the week when reviewing Stefan's paper on `greater<>`.

This phrasing sounds normative, but is actually descriptive but uses unfortunate wording.

The main reason this wording exists is to document the protocol required to support the legacy binders in Annex D.

Stefan points out that `unary_negate` and `binary_negate` have not been deprecated and rely on this. He plans a paper to remove this dependency.

Consensus that this wording is inadequate, confusing, and probably should be removed. However, that leaves a big hole in the specification for the legacy binders, that needs filling.

While not opposed to striking this paragraph, we will need the additional wording to fix the openning hole before this issue can move forward.

[ 2013-04-14 STL provides rationale ]

Rationale:

I've concluded that Scott's original proposed resolution was correct and complete. There are two sides to this story: the producers and the consumers of these typedefs.

Producers: As Scott noted, the Standard clearly documents which function objects must provide these typedefs. Some function objects must provide them unconditionally (e.g. `plus<T>` (for `T != void`), 22.10.7 [arithmetic.operations]/1), some conditionally (e.g. `reference_wrapper<T>`, 22.10.6 [refwrap]/2-4), and some don't have to provide them at all (e.g. lambdas, 7.5.6 [expr.prim.lambda]). These requirements are clear, so we shouldn't change them or even add informative notes. Furthermore, because these typedefs aren't needed in the C++11 world with `decltype`/perfect forwarding/etc., we shouldn't add more requirements to provide them.

Consumers: This is what we were concerned about at Portland. However, the consumers also clearly document their requirements in the existing text. For example, `reference_wrapper<T>` is also a conditional consumer, and 22.10.6 [refwrap] explains what typedefs it's looking for. We were especially concerned about the old negators and the deprecated binders, but they're okay too. [negators] clearly says that `unary_negate<Predicate>` requires `Predicate::argument_type` to be a type, and `binary_negate<Predicate>` requires `Predicate::first_argument_type` and `Predicate::second_argument_type` to be types. (`unary_negate`/`binary_negate` provide `result_type` but they don't consume it.) 99 [depr.lib.binders] behaves the same way with `Fn::first_argument_type`, `Fn::second_argument_type`, and `Fn::result_type`. No additional wording is necessary.

A careful reading of 22.10 [function.objects]/5 reveals that it wasn't talking about anything beyond the mere existence of the mentioned typedefs — for example, it didn't mention that the function object's return type should be `result_type`, or even convertible to `result_type`. As the producers and consumers are certainly talking about the existence of the typedefs (in addition to clearly implying semantic requirements), we lose nothing by deleting the unnecessary paragraph.

[2013-04-18, Bristol]

Previous wording:

> Remove 22.10 [function.objects] p5:

> > To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects correspondingly provide typedefs `argument_type` and `result_type` for function objects that take one argument and `first_argument_type`, `second_argument_type`, and `result_type` for function objects that take two arguments.

Proposed resolution:

This wording is relative to N3485.

Edit 22.10 [function.objects] p5:

> [Note:To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objectsmany of the function objects in this clause correspondingly provide typedefs `argument_type` and `result_type` for function objects that take one argument and `first_argument_type`, `second_argument_type`, and `result_type` for function objects that take two arguments.— end note]

### 2150(i). Unclear specification of `find_end`

Section: 26.6.8 [alg.find.end] Status: C++14 Submitter: Andrew Koenig Opened: 2012-03-28 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++14 status.

Discussion:

26.6.8 [alg.find.end] describes the behavior of find_end as returning:

> The last iterator `i` in the range `[first1,last1 - (last2 - first2))` such that for any nonnegative integer `n < (last2 - first2)`, the following corresponding conditions hold: `*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false`.

Does "for any" here mean "for every" or "there exists a"? I think it means the former, but it could be interpreted either way.

Daniel: The same problem exists for the following specifications from Clause 26 [algorithms]:

1. 26.6.15 [alg.search] p2 and p6
2. 26.7.10 [alg.reverse] p4
3. 26.8.5 [alg.partitions] p5 and p9
4. 26.8 [alg.sorting] p5
5. 26.8.3 [alg.nth.element] p1
6. 26.8.4.2 [lower.bound] p2
7. 26.8.4.3 [upper.bound] p2
8. 26.8.9 [alg.min.max] p21 and p23

[2013-04-20, Bristol]

Unanimous agreement on the wording.

Resolution: move to tentatively ready

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3376.

1. Change 26.6.8 [alg.find.end] p2 as indicated:

   >     template<class ForwardIterator1, class ForwardIterator2>
   >     ForwardIterator1
   >     find_end(ForwardIterator1 first1, ForwardIterator1 last1,
   >              ForwardIterator2 first2, ForwardIterator2 last2);
   >     template<class ForwardIterator1, class ForwardIterator2,
   >              class BinaryPredicate>
   >     ForwardIterator1
   >     find_end(ForwardIterator1 first1, ForwardIterator1 last1,
   >              ForwardIterator2 first2, ForwardIterator2 last2,
   >              BinaryPredicate pred);

   > > […]

   > > -2- Returns: The last iterator `i` in the range `[first1,last1 - (last2 - first2))` such that for anyevery nonnegative integer `n < (last2 - first2)`, the following corresponding conditions hold: `*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false`. Returns `last1` if `[first2,last2)` is empty or if no such iterator is found.

2. Change 26.6.15 [alg.search] p2 and p6 as indicated:

   >     template<class ForwardIterator1, class ForwardIterator2>
   >     ForwardIterator1
   >     search(ForwardIterator1 first1, ForwardIterator1 last1,
   >            ForwardIterator2 first2, ForwardIterator2 last2);
   >     template<class ForwardIterator1, class ForwardIterator2,
   >              class BinaryPredicate>
   >     ForwardIterator1
   >     search(ForwardIterator1 first1, ForwardIterator1 last1,
   >            ForwardIterator2 first2, ForwardIterator2 last2,
   >            BinaryPredicate pred);

   > > […]

   > > -2- Returns: The first iterator `i` in the range `[first1,last1 - (last2-first2))` such that for anyevery nonnegative integer `n` less than `last2 - first2` the following corresponding conditions hold: `*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false`. Returns `first1` if `[first2,last2)` is empty, otherwise returns `last1` if no such iterator is found.

   […]

   >     template<class ForwardIterator, class Size, class T>
   >     ForwardIterator
   >     search_n(ForwardIterator first, ForwardIterator last, Size count,
   >              const T& value);
   >     template<class ForwardIterator, class Size, class T,
   >              class BinaryPredicate>
   >     ForwardIterator
   >     search_n(ForwardIterator first, ForwardIterator last, Size count,
   >              const T& value, BinaryPredicate pred);

   > > […]

   > > -6- Returns: The first iterator `i` in the range `[first,last-count)` such that for anyevery non-negative integer `n` less than `count` the following corresponding conditions hold: `*(i + n) == value, pred(*(i + n),value) != false`. Returns `last` if no such iterator is found.

3. Change 26.7.10 [alg.reverse] p4 as indicated:

   >     template<class BidirectionalIterator, class OutputIterator>
   >     OutputIterator
   >     reverse_copy(BidirectionalIterator first,
   >                  BidirectionalIterator last, OutputIterator result);

   > > […]

   > > -4- Effects: Copies the range `[first,last)` to the range `[result,result+(last-first))` such that for anyevery non-negative integer `i < (last - first)` the following assignment takes place: `*(result + (last - first) - i) = *(first + i)`.

4. Change 26.8.5 [alg.partitions] p5 and p9 as indicated:

   >     template<class ForwardIterator, class Predicate>
   >     ForwardIterator
   >     partition(ForwardIterator first,
   >               ForwardIterator last, Predicate pred);

   > > […]

   > > -5- Returns: An iterator `i` such that for anyevery iterator `j` in the range `[first,i) pred(*j) != false`, and for anyevery iterator `k` in the range `[i,last), pred(*k) == false`.

   […]

   >     template<class BidirectionalIterator, class Predicate>
   >     BidirectionalIterator
   >     stable_partition(BidirectionalIterator first,
   >                      BidirectionalIterator last, Predicate pred);

   > > […]

   > > -9- Returns: An iterator `i` such that for anyevery iterator `j` in the range `[first,i), pred(*j) != false`, and for anyevery iterator `k` in the range `[i,last), pred(*k) == false`. The relative order of the elements in both groups is preserved.

5. Change 26.8 [alg.sorting] p5 as indicated:

   > -5- A sequence is sorted with respect to a comparator `comp` if for anyevery iterator `i` pointing to the sequence and anyevery non-negative integer `n` such that `i + n` is a valid iterator pointing to an element of the sequence, `comp(*(i + n), *i) == false`.

6. Change 26.8.3 [alg.nth.element] p1 as indicated:

   >     template<class RandomAccessIterator>
   >     void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                      RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >     void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                      RandomAccessIterator last, Compare comp);

   > > -1- After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for anyevery iterator `i` in the range `[first,nth)` and anyevery iterator `j` in the range `[nth,last)` it holds that: `!(*i > *j)` or `comp(*j, *i) == false`.

7. Change 26.8.4.2 [lower.bound] p2 as indicated:

   >     template<lass ForwardIterator, class T>
   >     ForwardIterator
   >     lower_bound(ForwardIterator first, ForwardIterator last,
   >                 const T& value);
   >     template<class ForwardIterator, class T, class Compare>
   >     ForwardIterator
   >     lower_bound(ForwardIterator first, ForwardIterator last,
   >                 const T& value, Compare comp);

   > > […]

   > > -2- Returns: The furthermost iterator `i` in the range `[first,last]` such that for anyevery iterator `j` in the range `[first,i)` the following corresponding conditions hold: `*j < value` or `comp(*j, value) != false`.

8. Change 26.8.4.3 [upper.bound] p2 as indicated:

   >     template<lass ForwardIterator, class T>
   >     ForwardIterator
   >     upper_bound(ForwardIterator first, ForwardIterator last,
   >                 const T& value);
   >     template<class ForwardIterator, class T, class Compare>
   >     ForwardIterator
   >     upper_bound(ForwardIterator first, ForwardIterator last,
   >                 const T& value, Compare comp);

   > > […]

   > > -2- Returns: The furthermost iterator `i` in the range `[first,last]` such that for anyevery iterator `j` in the range `[first,i)` the following corresponding conditions hold: `!(value < *j)` or `comp(value, *j) == false`.

9. Change 26.8.9 [alg.min.max] p21 and p23 as indicated:

   >     template<class ForwardIterator>
   >     ForwardIterator min_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     ForwardIterator min_element(ForwardIterator first, ForwardIterator last,
   >                                 Compare comp);

   > > -21- Returns: The first iterator `i` in the range `[first,last)` such that for anyevery iterator `j` in the range `[first,last)` the following corresponding conditions hold: `!(*j < *i)` or `comp(*j, *i) == false`. Returns `last` if `first == last`.

   […]

   >     template<class ForwardIterator>
   >     ForwardIterator max_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     ForwardIterator max_element(ForwardIterator first, ForwardIterator last,
   >                                 Compare comp);

   > > -23- Returns: The first iterator `i` in the range `[first,last)` such that for anyevery iterator `j` in the range `[first,last)` the following corresponding conditions hold: `!(*i < *j)` or `comp(*i, *j) == false`. Returns `last` if `first == last`.

### 2151(i). `basic_string<>::swap` semantics ignore allocators

Section: 27.4.3.2 [string.require] Status: Resolved Submitter: Robert Shearer Opened: 2012-04-13 Last modified: 2018-11-25

Priority: 3

View all other issues in [string.require].

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Discussion:

In C++11, `basic_string` is not described as a "container", and is not governed by the allocator-aware container semantics described in sub-clause 23.2 [container.requirements]; as a result, and requirements or contracts for the `basic_string` interface must be documented in Clause 27 [strings].

Sub-clause 27.4.3.7.8 [string.swap] defines the `swap` member function with no requirements, and with guarantees to execute in constant time without throwing. Fulfilling such a contract is not reasonable in the presence of unequal non-propagating allocators.

In contrast, 23.2.2 [container.requirements.general] p7 declares the behavior of member `swap` for containers with unequal non-propagating allocators to be undefined.

Resolution proposal:

Additional language from Clause 23 [containers] should probably be copied to Clause 27 [strings]. I will refrain from an exactly recommendation, however, as I am raising further issues related to the language in Clause 23 [containers].

[2013-03-15 Issues Teleconference]

Moved to Open.

Alisdair has offered to provide wording.

Telecon notes that 23.2.2 [container.requirements.general]p13 says that `string` is an allocator-aware container.

Resolved by the adoption of P1148 in San Diego.

Proposed resolution:

### 2154(i). What exactly does compile-time complexity imply?

Section: 29.5.3.3 [rand.req.urng] Status: Resolved Submitter: John Salmon Opened: 2012-04-26 Last modified: 2020-10-06

Priority: 4

View all other issues in [rand.req.urng].

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Discussion:

The expressions `G::min()` and `G::max()` in Table 116 in 29.5.3.3 [rand.req.urng] are specified as having "compile-time" complexity.

It is not clear what, exactly, this requirement implies. If a URNG has a method:

>     static int min();

then is the method required to have a `constexpr` qualifier? I believe the standard would benefit from clarification of this point.

[2018-12-08; Tim Song comments]

This issue was resolved by P0898R3 and the subsequent editorial rewrite of this subclause.

[2020-10-06; Reflector discussions]

Resolved by P0898R3 voted in Rapperswil.

Rationale:

Resolved by P0898R3.

Proposed resolution:

### 2155(i). Macro `__bool_true_false_are_defined` should be removed

Section: 17.14 [support.runtime] Status: Resolved Submitter: Thomas Plum Opened: 2012-04-30 Last modified: 2020-11-09

Priority: 4

View all other issues in [support.runtime].

View all issues with Resolved status.

Discussion:

Since C99, the C standard describes a macro named `__bool_true_false_are_defined`.

In the process of harmonizing C++11 with C99, this name became part of the C++ standard.

I propose that all mention of this name should be removed from the C and C++ standards.

Here's the problem: The name was originally proposed as a transition tool, so that the headers for a project could contain lines like the following.

>     #if !defined(__bool_true_false_are_defined)
>     #define bool int /* or whatever */
>     #define true 1
>     #define false 0
>     #endif

Then when the project was compiled by a "new" compiler that implemented `bool` as defined by the evolving C++98 or C99 standards, those lines would be skipped; but when compiled by an "old" compiler that didn't yet provide `bool`, `true`, and `false`, then the `#define`'s would provide a simulation that worked for most purposes.

It turns out that there is an unfortunate ambiguity in the name. One interpretation is as shown above, but a different reading says "bool, true, and false are #define'd", i.e. that the meaning of the macro is to assert that these names are macros (not built-in) ... which is true in C, but not in C++.

In C++11, the name appears in parentheses followed by a stray period, so some editorial change is needed in any event:

17.14 [support.runtime] para 1:

> Headers `<csetjmp>` (nonlocal jumps), `<csignal>` (signal handling), `<cstdalign>` (alignment), `<cstdarg>` (variable arguments), `<cstdbool>` (`__bool_true_false_are_defined`). `<cstdlib>` (runtime environment `getenv()`, `system()`), and `<ctime>` (system clock `clock()`, `time()`) provide further compatibility with C code.

However, para 2 says

> "The contents of these headers are the same as the Standard C library headers `<setjmp.h>`, `<signal.h>`, `<stdalign.h>`, `<stdarg.h>`, `<stdbool.h>`, `<stdlib.h>`, and `<time.h>`, respectively, with the following changes:",

and para 8 says

> "The header `<cstdbool>` and the header `<stdbool.h>` shall not define macros named `bool`, `true`, or `false`."

Thus para 8 doesn't exempt the C++ implementation from the arguably clear requirement of the C standard, to provide a macro named `__bool_true_false_are_defined` defined to be 1.

Real implementations of the C++ library differ, so the user cannot count upon any consistency; furthermore, the usefulness of the transition tool has faded long ago.

That's why my suggestion is that both C and C++ standards should eliminate any mention of `__bool_true_false_are_defined`. In that case, the name belongs to implementers to provide, or not, as they choose.

[2013-03-15 Issues Teleconference]

Moved to Open.

While not strictly necessary, the clean-up look good.

We would like to hear from our C liaison before moving on this issue though.

[2015-05 Lenexa]

LWG agrees. Jonathan provides wording.

[2017-03-04, Kona]

The reference to <cstdbool> in p2 needs to be struck as well. Continue the discussion on the reflector once the DIS is available.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> 1. Edit the footnote on 16.4.2.3 [headers] p7:

>    > 176) In particular, including any of the standard headers `<stdbool.h>`, `<cstdbool>`, `<iso646.h>` or `<ciso646>` has no effect.

> 2. Edit 17.14 [support.runtime] p1 as indicated (and remove the index entry for `__bool_true_false_are_defined`):

>    > -1- Headers `<csetjmp>` (nonlocal jumps), `<csignal>` (signal handling), `<cstdalign>` (alignment), `<cstdarg>` (variable arguments), `<cstdbool>`, (`__bool_true_false_are_defined`). `<cstdlib>` (runtime environment `getenv()`, `system()`), and `<ctime>` (system clock `clock()`, `time()`) provide further compatibility with C code.

> 3. Remove Table 38 — Header `<cstdbool>` synopsis [tab:support.hdr.cstdbool] from 17.14 [support.runtime]

>    > Table 38 — Header `<cstdbool>` synopsis
>    > Type | Name(s)
>    > Macro: | `__bool_true_false_are_defined`

[2019-03-18; Daniel comments and eliminates previously proposed wording]

With the approval of P0619R4 in Rapperswil, the offending wording has now been eliminated from the working draft. I suggest to close this issue as: Resolved by P0619R4.

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Rationale:

Resolved by P0619R4.

Proposed resolution:

### 2156(i). Unordered containers' `reserve(n)` reserves for `n-1` elements

Section: 23.2.8 [unord.req] Status: C++17 Submitter: Daniel James Opened: 2012-05-07 Last modified: 2017-07-30

Priority: 3

View other active issues in [unord.req].

View all other issues in [unord.req].

View all issues with C++17 status.

Discussion:

I think that unordered containers' `reserve` doesn't quite do what it should. I'd expect after calling `x.reserve(n)` to be able to insert `n` elements without invalidating iterators. But as the standard is written (I'm looking at n3376), I think the guarantee only holds for `n-1` elements.

For a container with `max_load_factor` of `1`, `reserve(n)` is equivalent to `rehash(ceil(n/1))`, ie. `rehash(n)`. `rehash(n)` requires that the bucket count is `>= n`, so it can be `n` (Table 103). The rule is that `insert` shall not affect the validity of iterators if `(N + n) < z * B` (23.2.8 [unord.req] p15). But for this case the two sides of the equation are equal, so `insert` can affect the validity of iterators.

[2013-03-16 Howard comments and provides wording]

Given the following:

>     LF := load_factor()
>     MLF := max_load_factor()
>     S := size()
>     B := bucket_count()

>     LF == S/B

The container has an invariant:

>     LF <= MLF

Therefore:

>     MLF >= S/B
>     S <= MLF * B
>     B >= S/MLF

[2013-03-15 Issues Teleconference]

Moved to Open.

Howard to provide rationale and potentally revised wording.

[2012-02-12 Issaquah : recategorize as P3]

Jonathan Wakely: submitter is Boost.Hash maintainer. Think it's right.

Marshall Clow: even if wrong it's more right than what we have now

Geoffrey Romer: issue is saying rehash should not leave container in such a state that a notional insertion of zero elements should not trigger a rehash

AJM: e.g. if you do a range insert from an empty range

AJM: we don't have enough brainpower to do this now, so not priority zero

Recategorised as P3

[Lenexa 2015-05-06: Move to Ready]

Proposed resolution:

This wording is relative to N3485.

1. In 23.2.8 [unord.req] Table 103 — Unordered associative container requirements, change the post-condition in the row for `a.rehash(n)` to:

   > Post: `a.bucket_count() >= a.size() / a.max_load_factor()` and `a.bucket_count() >= n`.

2. In 23.2.8 [unord.req]/p15 change

   > The `insert` and `emplace` members shall not affect the validity of iterators if `(N+n) <= z * B`, where `N` is the number of elements in the container prior to the insert operation, `n` is the number of elements inserted, `B` is the container's bucket count, and `z` is the container's maximum load factor.

### 2159(i). `atomic_flag` initialization

Section: 32.5.10 [atomics.flag] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2012-05-24 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [atomics.flag].

View all issues with C++14 status.

Discussion:

32.5.10 [atomics.flag]/4 describes the `ATOMIC_FLAG_INIT`, but it's not quite clear about a couple of points:

1. it's said that `ATOMIC_FLAG_INIT` "can be used to initialize an object of type `atomic_flag`" and the following example:

   >     std::atomic_flag guard = ATOMIC_FLAG_INIT;

   is presented. It's not clear whether the macro can also be used in the other initialization contexts:

   >     std::atomic_flag guard ATOMIC_FLAG_INIT;
   >     std::atomic_flag guard {ATOMIC_FLAG_INIT};

   >     struct A { std::atomic_flag flag; A(); };
   >     A::A() : flag (ATOMIC_FLAG_INIT);
   >     A::A() : flag {ATOMIC_FLAG_INIT};

   Please also note that examples are non-normative, according to the ISO directives, meaning that the wording presents no normative way to use the macro.

2. it's said that "It is unspecified whether an uninitialized `atomic_flag` object has an initial state of set or clear.". I believe the use of "uninitialized" is inappropriate. First of all, if an object is uninitialized it is obvious that we cannot assert anything about its state. Secondly, it doesn't address the following cases:

   >     std::atomic_flag a; // object is "initialized" by trivial default constructor
   >     std::atomic_flag a {}; // object is value-initialized
   >     static std::atomic_flag a; // object is zero-initialized

   strictly speaking a trivial constructor "initializes" the object, although it doesn't actually initialize the sub-objects.

3. it's said that "For a static-duration object, that initialization shall be static.". Considering the following example:

   >     struct A
   >     {
   >       A(); // user-provided, not constexpr

   >       std::atomic_flag flag = ATOMIC_FLAG_INIT;
   >       // possibly other non-static data members
   >     };

   >     static A a;

   The object `a.flag` (as a sub-object of the object `a`) has static-duration, yet the initialization has to be dynamic because `A::A` is not `constexpr`.

[2012, Portland]

We would like to be able to allow more initialisation contexts for example:

1. C struct
2. C++ constructor initializer-list

However we need further input from experts with implementation specific knowledge to identify which additional contexts (if any) would be universally valid.

Moved to open

[2013, Chicago]

Move to Immediate, following review.

Some discussion over the explicit use of only copy initialization, and not direct initialization. This is necessary to allow the implementation of `atomic_flag` as an aggregate, and may be further reviewed in the future.

Accept for Working Paper

Proposed resolution:

[This wording is relative to N3376.]

Change 32.5.10 [atomics.flag]/4 as follows:

> The macro `ATOMIC_FLAG_INIT` shall be defined in such a way that it can be used to initialize an object of type `atomic_flag` to the clear state. The macro can be used in the form:

> >     atomic_flag guard = ATOMIC_FLAG_INIT;

> It is unspecified whether the macro can be used in other initialization contexts. For a complete static-duration object, that initialization shall be static. It is unspecified whether an uninitialized Unless initialized with `ATOMIC_FLAG_INIT`, it is unspecified whether an `atomic_flag` object has an initial state of set or clear. [ Example:

> >     atomic_flag guard = ATOMIC_FLAG_INIT;

> — end example ]

### 2160(i). Unintended destruction ordering-specification of `resize`

Section: 23.3.13.3 [vector.capacity] Status: C++17 Submitter: Daniel Krügler Opened: 2012-06-07 Last modified: 2017-07-30

Priority: 1

View other active issues in [vector.capacity].

View all other issues in [vector.capacity].

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Discussion:

As part of resolving LWG issue 2033(i) a wording change was done for `resize()` to respect the problem mentioned in the question:

> Does a call to 'void resize(size_type sz)' of `std::vector` require the element type to be `MoveAssignable` because the call `erase(begin() + sz, end())` mentioned in the Effects paragraph would require the element type to be `MoveAssignable`?

The wording change was to replace in 23.3.5.3 [deque.capacity] and 23.3.13.3 [vector.capacity]:

> -1- Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end())`; […]

by:

> -1- Effects: If `sz <= size()`, equivalent to calling `pop_back() size() - sz` times. […]

The overlooked side-effect of this wording change is that this implies a destruction order of the removed elements to be in reverse order of construction, but the previous version did not impose any specific destruction order due to the way how the semantics of `erase` is specified in Table 100.

Given the program:

>     #include <vector>
>     #include <iostream>

>     struct Probe {
>       int value;
>       Probe() : value(0) {}
>       Probe(int value) : value(value) {}
>       ~Probe() { std::cout << "~Probe() of " << value << std::endl; }
>     };

>     int main() {
>       std::vector<Probe> v;
>       v.push_back(Probe(1));
>       v.push_back(Probe(2));
>       v.push_back(Probe(3));
>       std::cout << "---" << std::endl;
>       v.resize(0);
>     }

the last three lines of the output for every compiler I tested was:

>     ~Probe() of 1
>     ~Probe() of 2
>     ~Probe() of 3

but a conforming implementation would now need to change this order to

>     ~Probe() of 3
>     ~Probe() of 2
>     ~Probe() of 1

This possible stringent interpretation makes sense, because one can argue that sequence containers (or at least `std::vector`) should have the same required destruction order of it's elements, as elements of a C array or controlled by memory deallocated with an array `delete` have. I also learned that libc++ does indeed implement `std::vector::resize` in a way that the second output form is observed.

While I agree that required reverse-destruction would better mimic the natural behaviour of `std::vector` this was not required in C++03 and this request may be too strong. My current suggestion would be to restore the effects of the previous wording in regard to the destruction order, because otherwise several currently existing implementations would be broken just because of this additional requirement.

[2013-03-15 Issues Teleconference]

Moved to Open.

Jonathan says that he believes this is a valid issue.

Walter wonders if this was intended when we made the previous change - if so, this would be NAD.

Jonathan said that Issue 2033(i) doesn't mention ordering.

Walter then asked if anyone is really unhappy that we're destroying items in reverse order of construction.

Jonathan points out that this conflicts with existing practice (libstc++, but not libc++).

Jonathan asked for clarification as to whether this change was intended by 2033(i).

[2014-06 Rapperswil]

Daniel points out that the ordering change was not intended.

General agreement that implementations should not be required to change.

[2014-06-28 Daniel provides alternative wording]

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 23.3.5.3 [deque.capacity] as indicated: [Drafting note: The chosen wording form is similar to that for `forward_list`. Note that the existing Requires element already specifies the necessary operational requirements on the value type. — end drafting note]

   >     void resize(size_type sz);

   > > -1- Effects: If `sz <= size()`, erases the last `size() - sz` elements from the sequenceequivalent to calling `pop_back() size() - sz` times. OtherwiseIf `size() <= sz`, appends `sz - size()` default-inserted elements to the sequence.

   > > […]

   >     void resize(size_type sz, const T& c);

   > > -3- Effects: If `sz <= size()`, erases the last `size() - sz` elements from the sequenceequivalent to calling `pop_back() size() - sz` times. OtherwiseIf `size() < sz`, appends `sz - size()` copies of `c` to the sequence.

   > > […]

2. Change 23.3.13.3 [vector.capacity] as indicated: [Drafting note: See `deque` for the rationale of the used wording. — end drafting note]

   >     void resize(size_type sz);

   > > -12- Effects: If `sz <= size()`, erases the last `size() - sz` elements from the sequenceequivalent to calling `pop_back() size() - sz` times. OtherwiseIf `size() < sz`, appends `sz - size()` default-inserted elements to the sequence.

   > > […]

   >     void resize(size_type sz, const T& c);

   > > -15- Effects: If `sz <= size()`, erases the last `size() - sz` elements from the sequenceequivalent to calling `pop_back() size() - sz` times. OtherwiseIf `size() < sz`, appends `sz - size()` copies of `c` to the sequence.

   > > […]

### 2162(i). `allocator_traits::max_size` missing `noexcept`

Section: 16.4.4.6 [allocator.requirements], 20.2.9.3 [allocator.traits.members], 20.2.9 [allocator.traits] Status: C++14 Submitter: Bo Persson Opened: 2012-07-03 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [allocator.requirements].

View all other issues in [allocator.requirements].

View all issues with C++14 status.

Discussion:

N3376 describes in 20.2.9.3 [allocator.traits.members]/7

>     static size_type max_size(Alloc& a);

> Returns: `a.max_size()` if that expression is well-formed; otherwise, `numeric_limits<size_type>::max()`.

The `max_size` function is supposed to call one of two functions that are both `noexcept`. To make this intermediate function useful for containers, it should preserve the `noexcept` attribute.

Proposed changes:

In 20.2.9 [allocator.traits] and 20.2.9.3 [allocator.traits.members]/7, change the function signature to

>     static size_type max_size(Alloc& a) noexcept;

[2012-08-05 Daniel comments]

On the first sight this does not seem like a defect of the specification, because the Allocator requirements in 16.4.4.6 [allocator.requirements] (Table 28) do not impose a no-throw requirement onto `max_size()`; the table just describes the fall-back implementation for `max_size()` if a given allocator does not provide such a function.

`std::allocator` as a special model of this concept and is allowed to increase the exception-guarantees for `max_size()`, but this does not imply a corresponding rules for other allocators.

Furthermore, `max_size()` of Containers is not specified in terms of `Allocator::max_size()`, so again this is not a real contradiction.

Nonetheless I think that the following stronger decision should be considered:

1. Require that for all Allocators (as specified in 16.4.4.6 [allocator.requirements]) `max_size()` never throws an exception. This would it make much more useful to call this function in situations where no exception should leave the context.

2. Require that for all Allocators (as specified in 16.4.4.6 [allocator.requirements]) `max_size()` can be called on const allocator object. Together with the previous item this would allow an implementation of a container's `max_size()` function to delegate to the allocator's `max_size()` function.

In regard to the second statement it should be mentioned that there are two current specification deviations from that in the draft:

1. The synopsis of 20.2.9 [allocator.traits] uses a const allocator argument as part of the signature of the `max_size` function.

2. Both the synopsis of 20.6.1 [allocator.adaptor.syn] and the member specification in 20.6.4 [allocator.adaptor.members] p8 declare `scoped_allocator_adaptor::max_size` as const member function, but this function delegates to

   >     allocator_traits<OuterAlloc>::max_size(outer_allocator())

   where `outer_allocator()` resolves to the member function overload returning a `const outer_allocator_type&`.

The question arises whether these current defects actually point to a defect in the Allocator requirements and should be fixed there.

[ 2012-10 Portland: Move to Review ]

Consensus that the change seems reasonable, and that for any given type the template is intantiated with the contract should be 'wide' so this meets the guidelines we agreed in Madrid for C++11.

Some mild concern that while we don't imagine many allocator implementations throwing on this method, it is technically permited by current code that we would not be breaking, by turning `throw` expressions into disguised `terminate` calls. In this case, an example might be an instrumented 'logging' allocator that writes every function call to a log file or database, and might throw if that connection/file were no longer available.

Another option would be to make exception spefication a conditional no-except, much like we do for some `swap` functions and assignment operators. However, this goes against the intent of the Madrid adoption of `noexcept` which is that vendors are free to add such extensions, but we look for a clear line in the library specification, and do not want to introduce conditional-noexcept piecemeal. A change in our conventions here would require a paper addressing the library specification as a whole.

Consensus was to move forward, but move the issue only to Review rather than Ready to allow time for further comments. This issue should be considered 'Ready' next time it is reviewed unless we get such comments in the meantime.

[2013-04-18, Bristol]

Proposed resolution:

In 20.2.9 [allocator.traits] and 20.2.9.3 [allocator.traits.members]/7, change the function signature to

>     static size_type max_size(Alloc& a) noexcept;

### 2163(i). `nth_element` requires inconsistent post-conditions

Section: 26.8.3 [alg.nth.element] Status: C++14 Submitter: Peter Sommerlad Opened: 2012-07-06 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.nth.element].

View all issues with C++14 status.

Discussion:

The specification of nth_element refers to `operator<` whereas all sorting without a compare function is based on `operator<`. While it is true that for all regular types both operators should be defined accordingly, all other sorting algorithms only rely on existence of `operator<`. So I guess the paragraph p1

> After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*i > *j)` or `comp(*j, *i) == false`.

should read

> After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

Note only "`!(*i > *j)`" was changed to "`!(*j < *i)`" and it would be more symmetric with `comp(*j, *i)` as well.

In theory this might be a semantic change to the spec, but I believe the mistake is unintended.

[ 2012-10 Portland: Move to Ready ]

This is clearly correct by inspection, moved to Ready by unanimous consent.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change 26.8.3 [alg.nth.element] p1 as indicated:

   >     template<class RandomAccessIterator>
   >     void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                      RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >     void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                      RandomAccessIterator last, Compare comp);

   > > -1- After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*i > *j)!(*j < *i)` or `comp(*j, *i) == false`.

### 2164(i). What are the semantics of `vector.emplace(vector.begin(), vector.back())`?

Section: 23.3.13.5 [vector.modifiers], 23.2 [container.requirements] Status: C++20 Submitter: Howard Hinnant Opened: 2012-07-07 Last modified: 2021-02-25

Priority: 2

View all other issues in [vector.modifiers].

View all issues with C++20 status.

Discussion:

Nikolay Ivchenkov recently brought the following example on the std-discussion newsgroup, asking whether the following program well-defined:

>     #include <iostream>
>     #include <vector>

>     int main()
>     {
>       std::vector<int> v;
>       v.reserve(4);
>       v = { 1, 2, 3 };
>       v.emplace(v.begin(), v.back());
>       for (int x : v)
>         std::cout << x << std::endl;
>     }

Nikolay Ivchenkov:

I think that an implementation of `vector`'s 'emplace' should initialize an intermediate object with `v.back()` before any shifts take place, then perform all necessary shifts and finally replace the value pointed to by `v.begin()` with the value of the intermediate object. So, I would expect the following output:

>     3
>     1
>     2
>     3

GNU C++ 4.7.1 and GNU C++ 4.8.0 produce other results:

>     2
>     1
>     2
>     3

Howard Hinnant:

I believe Nikolay is correct that vector should initialize an intermediate object with `v.back()` before any shifts take place. As Nikolay pointed out in another email, this appears to be the only way to satisfy the strong exception guarantee when an exception is not thrown by `T`'s copy constructor, move constructor, copy assignment operator, or move assignment operator as specified by 23.3.13.5 [vector.modifiers]/p1. I.e. if the emplace construction throws, the vector must remain unaltered.

That leads to an implementation that tolerates objects bound to the function parameter pack of the `emplace` member function may be elements or sub-objects of elements of the container.

My position is that the standard is correct as written, but needs a clarification in this area. Self-referencing `emplace` should be legal and give the result Nikolay expects. The proposed resolution of LWG 760(i) is not correct.

[2015-02 Cologne]

LWG agrees with the analysis including the assessment of LWG 760(i) and would appreciate a concrete wording proposal.

[2015-04-07 dyp comments]

The Standard currently does not require that creation of such intermediate objects is legal. 23.2.4 [sequence.reqmts] Table 100 — "Sequence container requirements" currently specifies:

> Table 100 — Sequence container requirements
> Expression | Return type | Assertion/note pre-/post-condition
> --- | --- | ---
> `…`
> `a.emplace(p, args);` | `iterator` | Requires: `T` is `EmplaceConstructible` into `X` from `args`. For `vector` and `deque`, `T` is also `MoveInsertable` into `X` and `MoveAssignable`. […]
> `…`

The `EmplaceConstructible` concept is defined via `allocator_traits<A>::construct` in 23.2.2 [container.requirements.general] p15.5 That's surprising to me since the related concepts use the suffix `Insertable` if they refer to the allocator. An additional requirement such as `std::is_constructible<T, Args...>` is necessary to allow creation of intermediate objects.

The creation of intermediate objects also affects other functions, such as `vector.insert`. Since aliasing the vector is only allowed for the single-element forms of `insert` and `emplace` (see 526(i)), the range-forms are not affected. Similarly, aliasing is not allowed for the rvalue-reference overload. See also LWG 2266(i).

There might be a problem with a requirement of `std::is_constructible<T, Args...>` related to the issues described in LWG 2461(i). For example, a scoped allocator adapter passes additional arguments to the constructor of the value type. This is currently not done in recent implementations of libstdc++ and libc++ when creating the intermediate objects, they simply create the intermediate object by perfectly forwarding the arguments. If such an intermediate object is then moved to its final destination in the vector, a change of the allocator instance might be required — potentially leading to an expensive copy. One can also imagine worse problems, such as run-time errors (allocators not comparing equal at run-time) or compile-time errors (if the value type cannot be created without the additional arguments). I have not looked in detail into this issue, but I'd be reluctant adding a requirement such as `std::is_constructible<T, Args...>` without further investigation.

It should be noted that the creation of intermediate objects currently is inconsistent in libstdc++ vs libc++. For example, libstdc++ creates an intermediate object for `vector.insert`, but not `vector.emplace`, whereas libc++ does the exact opposite in this respect.

A live demo of the inconsistent creation of intermediate objects can be found here.

[2015-10, Kona Saturday afternoon]

HH: If it were easy, it'd have wording. Over the decades I have flipped 180 degrees on this. My current position is that it should work even if the element is in the same container.

TK: What's the implentation status? JW: Broken in GCC. STL: Broken in MSVS. Users complain about this every year.

MC: 526 says push_back has to work.

HH: I think you have to make a copy of the element anyway for reasons of exception safety. [Discussion of exception guarantees]

STL: vector has strong exception guarantees. Could we not just provide the Basic guarantee here.

HH: It would terrify me to relax that guarantee. It'd be an ugly, imperceptible runtime error.

HH: I agree if we had a clean slate that strong exception safety is costing us here, and we shouldn't provide it if it costs us.

STL: I have a mail here, "how can vector provide the strong guarantee when inserting in the middle".

HH: The crucial point is that you only get the strong guarantee if the exception is thrown by something other than the copy and move operations that are used to make the hole.

STL: I think we need to clean up the wording. But it does mandate currently that the self-emplacement must work, because nothings says that you can't do it. TK clarifies that a) self-emplacement must work, and b) you get the strong guarantee only if the operations for making the hole don't throw, otherwise basic. HH agrees. STL wants this to be clear in the Standard.

STL: Should it work for deque, too? HH: Yes.

HH: I will attempt wording for this.

TK: Maybe mail this to the reflector, and maybe someone has a good idea?

JW: I will definitely not come up with anything better, but I can critique wording.

Moved to Open; Howard to provide wording, with feedback from Jonathan.

[2017-01-25, Howard suggests wording]

[2018-1-26 issues processing telecon]

Status to 'Tentatively Ready' after adding a period to Howard's wording.

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Modify in 23.2.4 [sequence.reqmts] Table 87 — "Sequence container requirements" as indicated:

   > Table 87 — Sequence container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition
   > --- | --- | ---
   > `[…]`
   > `a.emplace(p, args)` | `iterator` | Requires: `T` is `EmplaceConstructible` into `X` from `args`. For `vector` and `deque`, `T` is also `MoveInsertable` into `X` and `MoveAssignable`. Effects: Inserts an object of type `T` constructed with `std::forward<Args>(args)...` before `p`. [Note: `args` may directly or indirectly refer to a value in `a`. — end note]

### 2165(i). `std::atomic<X>` requires `X` to be nothrow default constructible

Section: 32.5.8 [atomics.types.generic], 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Jonathan Wakely Opened: 2012-07-19 Last modified: 2016-01-28

Priority: 4

View all other issues in [atomics.types.generic].

View all issues with Resolved status.

Discussion:

As raised in c++std-lib-32781, this fails to compile even though the default constructor is not used:

>     #include <atomic>

>     struct X {
>       X() noexcept(false) {}
>       X(int) { }
>     };

>     std::atomic<X> x(3);

This is because `atomic<T>`'s default constructor is declared to be non-throwing and is explicitly-defaulted on its first declaration:

>     atomic() noexcept = default;

This is ill-formed if the implicitly-declared default constructor would not be non-throwing.

Possible solutions:

1. Add nothrow default constructible to requirements for template argument of the generic `atomic<T>`
2. Remove `atomic<T>::atomic()` from the overload set if `T` is not nothrow default constructible.
3. Remove `noexcept` from `atomic<T>::atomic()`, allowing it to be deduced (but the default constructor is intended to be always noexcept)
4. Do not default `atomic<T>::atomic()` on its first declaration (but makes the default constructor user-provided and so prevents `atomic<T>` being trivial)
5. A core change to allow the mismatched exception specification if the default constructor isn't used (see c++std-core-21990)

[2012, Portland: move to Core]

Recommend referring to core to see if the constructor `noexcept` mismatch can be resolved there. The issue is not specific to concurrency.

[2015-04-09 Daniel comments]

CWG issue 1778, which had been created in behalf of this LWG issue, has been resolved as a defect.

[2015-10, Kona]

Mark as resolved

Proposed resolution:

### 2166(i). Heap property underspecified?

Section: 26.8.8 [alg.heap.operations] Status: C++17 Submitter: Peter Sommerlad Opened: 2012-07-09 Last modified: 2017-07-30

Priority: 3

View all other issues in [alg.heap.operations].

View all issues with C++17 status.

Discussion:

Another similar issue to the `operator<` vs greater in `nth_element` but not as direct occurs in 26.8.8 [alg.heap.operations]:

> -1- A heap is a particular organization of elements in a range between two random access iterators `[a,b)`. Its two key properties are:

> 1. There is no element greater than `*a` in the range and
> 2. `*a` may be removed by `pop_heap()`, or a new element added by `push_heap()`, in O(log(`N`)) time.

As noted by Richard Smith, it seems that the first bullet should read:

> `*a` is not less than any element in the range

Even better the heap condition could be stated here directly, instead of leaving it unspecified, i.e.,

> Each element at `(a+2*i+1)` and `(a+2*i+2)` is less than the element at `(a+i)`, if those elements exist, for `i>=0`.

But may be that was may be intentional to allow other heap organizations?

See also follow-up discussion of c++std-lib-32780.

[2016-08 Chicago]

Walter provided wording

Tues PM: Alisdair & Billy(MS) to improve the wording.

[2016-08-02 Chicago LWG]

Walter provides initial Proposed Resolution. Alisdair objects to perceived complexity of the mathematical phrasing.

Previous resolution [SUPERSEDED]:

> [Note to editor: As a drive-by editorial adjustment, please replace the current enumerated list format by the numbered bullet items shown below.]

> Change [alg.heap.operations]:

> 1 A heap is a particular organization of elements in a range between two random access iterators [a, b). Its two key properties aresuch that:

> (1.1) -- There is no element greater than `*a` in the range and
> For all `i >= 0`,
> `comp(a[i], a[L])` is false whenever L = 2*i+1 < b-a,
> and
> `comp(a[i], a[R])` is false whenever R = 2*i+2 < b-a.

> (1.2) -- `*a` may be removed by `pop_heap()`, or a new element added by `push_heap()`, in O(log(N)) time.

[2016-08-03 Chicago LWG]

Walter and Billy O'Neal provide revised Proposed Resolution, superseding yesterday's.

Thurs PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change 26.8.8 [alg.heap.operations] as indicated:

   > Note to project editor: As a drive-by editorial adjustment, please replace the current enumerated list format by numbered bullet items.

   > -1- A heap is a particular organization of elements in a range between two random access iterators `[a, b)`. Its two key properties aresuch that:

   > 1. (1.1) — There is no element greater than `*a` in the range and With N=b- a, for all i, 0<i<N, `comp(a[` ⌊ i - 1 2 ⌋ `], a[i])` is `false`.

   >    > [Note to the project editor: In LaTeX the above insertion should be expressed as follows:

   >    > With $N = `b`-`a`$, for all $i$, $0 < i < N$, `comp(a[$\left \lfloor{\frac{i-1}{2}}\right \rfloor$], a[$i$])` is `false`.]

   > 2. (1.2) — `*a` may be removed by `pop_heap()`, or a new element added by `push_heap()`, in 𝒪(log(N)) time.

### 2168(i). Inconsistent specification of `uniform_real_distribution` constructor

Section: 29.5.9.2.2 [rand.dist.uni.real] Status: C++17 Submitter: Marshall Clow Opened: 2012-07-14 Last modified: 2017-07-30

Priority: 3

View all issues with C++17 status.

Discussion:

uniform_real says in 29.5.9.2.2 [rand.dist.uni.real] p1:

> A `uniform_real_distribution` random number distribution produces random numbers `x`, `a ≤ x < b`,

but also that (29.5.9.2.2 [rand.dist.uni.real] p2):

>     explicit uniform_real_distribution(RealType a = 0.0, RealType b = 1.0);

> > -2- Requires: `a ≤ b` and `b - a ≤ numeric_limits<RealType>::max()`.

If you construct a `uniform_real_distribution<RealType>(a, b)` where there are no representable numbers between 'a' and 'b' (using `RealType`'s representation) then you cannot satisfy 29.5.9.2.2 [rand.dist.uni.real].

An obvious example is when `a == b`.

[2014-11-04 Urbana]

Jonathan provides wording.

[2014-11-08 Urbana]

Moved to Ready with the note.

There remains concern that the constructors are permitting values that may (or may not) be strictly outside the domain of the function, but that is a concern that affects the design of the random number facility as a whole, and should be addressed by a paper reviewing and addressing the whole clause, not picked up in the issues list one distribution at a time. It is still not clear that such a paper would be uncontroversial.

Proposed resolution:

This wording is relative to N4140.

1. Add a note after paragraph 1 before the synopsis in 29.5.9.2.2 [rand.dist.uni.real]:

   > -1- A `uniform_real_distribution` random number distribution produces random numbers x , a≤x<b , distributed according to the constant probability density function

   > > p(x|a, b) = 1∕(b-a) .

   > [Note: This implies that p(x|a, b) is undefined when `a == b`. — end note]

   > Drafting note: p(x|a, b) should be in math font, and `a == b` should be in code font.

### 2169(i). Missing `reset()` requirements in `unique_ptr` specialization

Section: 20.3.1.4.5 [unique.ptr.runtime.modifiers] Status: C++14 Submitter: Geoffrey Romer Opened: 2012-07-16 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.runtime.modifiers].

View all issues with C++14 status.

Discussion:

In 20.3.1.4.5 [unique.ptr.runtime.modifiers]/p1-2 of N3376, the description of `reset()` in the array specialization of `unique_ptr` partially duplicates the description of the base template method (as specified in 20.3.1.3.6 [unique.ptr.single.modifiers]/p3-5), but lacks some significant requirements. Specifically, the text introduced in LWG 998(i), and item 13 of LWG 762(i), is present only in the base template, not the specialization.

This gives the appearance that these requirements specifically do not apply to the specialization, which I don't believe is correct or intended: the issue of `reset()` operation order addressed by LWG 998(i) applies just as much to the derived template as to the base template, and the derived template has just as much need to rely on `get_deleter()(get())` being well-defined, well-formed, and not throwing exceptions (arguably some of those properties follow from the fact that `T` is required to be a complete type, but not all).

Assuming the derived template's `reset()` semantics are intended to be identical to the base template's, there is no need to explicitly specify the semantics of `reset(pointer p)` at all (since 20.3.1.4 [unique.ptr.runtime]/3 specifies "Descriptions are provided below only for member functions that have behavior different from the primary template."), and `reset(nullptr_t p)` can be specified by reference to the 'pointer' overload. This is more concise, and eliminates any ambiguity about intentional vs. accidental discrepancies.

[2012-10 Portland: Move to Ready]

This resolution looks blatantly wrong, as it seems to do nothing but defer to primary template where we should describe the contract here.

Ongoing discussion points out that the primary template has a far more carefully worded semantic for `reset(p)` that we would want to copy here.

STL points out that we need the `nullptr` overload for this dynamic-array form, as there is a deleted member function template that exists to steal overloads of pointer-to-derived, avoiding undifined behavior, so we need the extra overload.

Finally notice that there is blanket wording further up the clause saying we describe only changes from the primary template, so the proposed wording is in fact exactly correct. Move to Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

Change 20.3.1.4.5 [unique.ptr.runtime.modifiers] as indicated:

>     void reset(pointer p = pointer()) noexcept;
>     void reset(nullptr_t p) noexcept;

> > -1- Effects: If `get() == nullptr` there are no effects. Otherwise `get_deleter()(get())` Equivalent to `reset(pointer())`.

> > -2- Postcondition: `get() == p`.

### 2170(i). Aggregates cannot be `DefaultConstructible`

Section: 16.4.4.2 [utility.arg.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2012-07-19 Last modified: 2017-07-30

Priority: 2

View all other issues in [utility.arg.requirements].

View all issues with C++17 status.

Discussion:

The lack of the definition of the `DefaultConstructible` requirements in C++03 was fixed by LWG 724(i) at a time where the core rules of list-initialization were slightly different than today, at that time value-initialization (shortly) was the primary rule for class types, i.e. just before applying CWG 1301, CWG 1324, and CWG 1368.

The order in 9.5.5 [dcl.init.list] p3 was changed to respect aggregate initialization, but that had the side-effect that formally aggregate types cannot satisfy the `DefaultConstructible` requirements anymore, because we require that

>     T u{};

value-initializes the object `u`.

Of-course exclusion of aggregates was not intended, therefore I suggest to extend the requirements in Table 19 (16.4.4.2 [utility.arg.requirements]) for empty aggregate-initialization as well.

[ 2012-10 Portland: Move to Core ]

We are not qualified to pick apart the Core rules quickly at this point, but the consensus is that if the core language has changed in this manner, then the fix should similarly be applied in Core - this is not something that we want users of the language to have to say every time they want to Value initialize (or aggregate initialize) an object.

More to Open until we get a clear response from Core, Alisdair to file an issue with Mike.

[2013-04 Bristol: Back to Library]

The Core Working group opened, discussed, and resolved CWG 1578 as NAD for this library-related problem: Empty aggregate initialization and value-initialization are different core language concepts, and this difference can be observed (e.g. for a type with a deleted default-constructor).

[2014-02-15 Issaquah: Move to Ready]

AM: core says still LWG issue, wording has been non-controversial, move to ready?

NJ: what about durations? think they are ok

Ville: `pair` and a few other have value initialize

AM: look at core 1578

AM: value initialize would require `()`, remove braces from third row?

STL: no

PH: core has new issue on aggregates and non-aggregates.

AM: right, they said does not affect this issue

NJ: why ok with `pair` and `tuple`?

STL: will use `()`, `tuple` of aggregates with deleted constructor is ill-formed

Ville: aggregate with reference can't have `()`

STL: `{}` would be an issue too

Ville: aggregate with reference will have `()` deleted implicitly

Move to Ready.

Proposed resolution:

This wording is relative to N3691.

Change Table 19 in 16.4.4.2 [utility.arg.requirements] as indicated:

Table 19 — `DefaultConstructible` requirements [defaultconstructible]
Expression | Post-condition
--- | ---
`T t;` | object `t` is default-initialized
`T u{};` | object `u` is value-initialized or aggregate-initialized
`T()` `T{}` | a temporary object of type `T` is value-initialized or aggregate-initialized

### 2172(i). Does `atomic_compare_exchange_*` accept `v == nullptr` arguments?

Section: 99 [depr.util.smartptr.shared.atomic] Status: C++14 Submitter: Howard Hinnant Opened: 2012-07-28 Last modified: 2017-11-29

Priority: Not Prioritized

View all other issues in [depr.util.smartptr.shared.atomic].

View all issues with C++14 status.

Discussion:

Looking at [util.smartptr.shared.atomic]/p31

>     template<class T>
>       bool
>       atomic_compare_exchange_strong_explicit(shared_ptr<T>* p,
>                                               shared_ptr<T>* v,
>                                               shared_ptr<T> w,
>                                               memory_order success, memory_order failure);

> > -31- Requires: `p` shall not be null.

What about `v`? Can it be null? And if so, what happens?

This looks closely related to C++11 issue LWG 1030(i), where we gave every signature in this section a:

> Requires: `p` shall not be null.

It looks like a simple oversight to me that we did not add for the `atomic_compare_exchange_*`:

> Requires: `p` shall not be null and `v` shall not be null.

[2012-10 Portland: Move to Ready]

This is clearly the right thing to do, and Lawrence concurs.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

Change [util.smartptr.shared.atomic] as indicated:

>     template<class T>
>       bool atomic_compare_exchange_weak(
>         shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);

> > -27- Requires: `p` shall not be null and `v` shall not be null.

> > […]

>     template<class T>
>       bool atomic_compare_exchange_weak_explicit(
>         shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
>         memory_order success, memory_order failure);
>     template<class T>
>       bool atomic_compare_exchange_strong_explicit(
>         shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
>         memory_order success, memory_order failure);

> > -31- Requires: `p` shall not be null and `v` shall not be null.

> > […]

### 2174(i). `wstring_convert::converted()` should be `noexcept`

Section: 99 [depr.conversions.string] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-04-22

Priority: Not Prioritized

View all other issues in [depr.conversions.string].

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Discussion:

There is no reason `wstring_convert::converted()` shouldn't be `noexcept`.

It might be possible for `wstring_convert::state()` and `wbuffer_convert::state()` to be `noexcept` too, depending on the requirements on `mbstate_t`.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

Defer the separate discsussion of `state()` to another issue, if anyone is ever motivated to file one.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Edit in the class template `wstring_convert` synopsis [conversions.string] p2:

   >     size_t converted() const noexcept;

2. Edit the signature before [conversions.string] p6:

   >     size_t converted() const noexcept;

### 2175(i). `wstring_convert` and `wbuffer_convert` validity

Section: 99 [depr.conversions.string], 99 [depr.conversions.buffer] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-09-07

Priority: Not Prioritized

View all other issues in [depr.conversions.string].

View all issues with C++14 status.

Discussion:

See discussion following c++std-lib-32710.

It's not specified what happens if `wstring_convert` and `wbuffer_convert` objects are constructed with null `Codecvt` pointers.

Should the constructors have preconditions that the pointers are not null? If not, are conversions expected to fail, or is it undefined to attempt conversions if the pointers are null?

There are no observer functions to check whether objects were constructed with valid `Codecvt` pointers. If the types are made movable such observers would be necessary even if the constructors require non-null pointers (see also LWG 2176(i)).

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Insert a new paragraph before [conversions.string] paragraph 16:

   >     wstring_convert(Codecvt *pcvt = new Codecvt);
   >     wstring_convert(Codecvt *pcvt, state_type state);
   >     wstring_convert(const byte_string& byte_err,
   >       const wide_string& wide_err = wide_string());

   > > -?- Requires: For the first and second constructors `pcvt != nullptr`.

   > > -16- Effects: The first constructor shall store `pcvt` in `cvtptr` and default values in `cvtstate`, `byte_err_string`, and `wide_err_string`. The second constructor shall store `pcvt` in `cvtptr`, `state` in `cvtstate`, and default values in `byte_err_string` and `wide_err_string`; moreover the stored state shall be retained between calls to `from_bytes` and `to_bytes`. The third constructor shall store `new Codecvt` in `cvtptr`, `state_type()` in cvtstate, `byte_err` in `byte_err_string`, and `wide_err` in `wide_err_string`.

2. Insert a new paragraph before [conversions.buffer] paragraph 10:

   >     wbuffer_convert(std::streambuf *bytebuf = 0,
   >       Codecvt *pcvt = new Codecvt, state_type state = state_type());

   > > -?- Requires: `pcvt != nullptr`.

   > > -10- Effects: The constructor constructs a stream buffer object, initializes `bufptr` to `bytebuf`, initializes `cvtptr` to `pcvt`, and initializes `cvtstate` to `state`.

### 2176(i). Special members for `wstring_convert` and `wbuffer_convert`

Section: 99 [depr.conversions.string], 99 [depr.conversions.buffer] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-09-07

Priority: Not Prioritized

View all other issues in [depr.conversions.string].

View all issues with C++14 status.

Discussion:

See discussion following c++std-lib-32699.

The constructors for `wstring_convert` and `wbuffer_convert` should be explicit, to avoid implicit conversions which take ownership of a `Codecvt` pointer and delete it unexpectedly.

Secondly, [conversions.buffer] p11 describes a destructor which is not declared in the class synopsis in p2.

Finally, and most importantly, the definitions in [conversions.string] and [conversions.buffer] imply implicitly-defined copy constructors and assignment operators, which would do shallow copies of the owned `Codecvt` objects and result in undefined behaviour in the destructors.

`Codecvt` is not required to be `CopyConstructible`, so deep copies are not possible. The `wstring_convert` and `wstring_buffer` types could be made move-only, but the proposed resolution below doesn't do so because of the lack of preconditions regarding null `Codecvt` pointers and the absence of observer functions that would allow users to check preconditions (see also LWG 2175(i)).

[2013-03-15 Issues Teleconference]

Moved to Review.

Jonathan pointed out that you can have an implicit constructor that takes ownership of a heap reference, which would result an unexpected deletion.

No-one really likes the 'naked new' in the interface here, either.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Edit the class template `wstring_convert` synopsis in [conversions.string] p2:

   >     explicit wstring_convert(Codecvt *pcvt = new Codecvt);
   >     wstring_convert(Codecvt *pcvt, state_type state);
   >     explicit wstring_convert(const byte_string& byte_err,
   >                              const wide_string& wide_err = wide_string());
   >     ~wstring_convert();

   >     wstring_convert(const wstring_convert&) = delete;
   >     wstring_convert& operator=(const wstring_convert&) = delete;

2. Edit the signatures before [conversions.string] p16:

   >     explicit wstring_convert(Codecvt *pcvt = new Codecvt);
   >     wstring_convert(Codecvt *pcvt, state_type state);
   >     explicit wstring_convert(const byte_string& byte_err,
   >         const wide_string& wide_err = wide_string());

3. Edit the class template `wbuffer_convert` synopsis in [conversions.buffer] p2:

   >     explicit wbuffer_convert(std::streambuf *bytebuf = 0,
   >                              Codecvt *pcvt = new Codecvt,
   >                              state_type state = state_type());

   >     ~wbuffer_convert();

   >     wbuffer_convert(const wbuffer_convert&) = delete;
   >     wbuffer_convert& operator=(const wbuffer_convert&) = delete;

4. Edit the signature before [conversions.buffer] p10:

   >     explicit wbuffer_convert(std::streambuf *bytebuf = 0,
   >         Codecvt *pcvt = new Codecvt, state_type state = state_type());

### 2177(i). Requirements on `Copy/MoveInsertable`

Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Loïc Joly Opened: 2012-08-10 Last modified: 2017-09-07

Priority: Not Prioritized

View other active issues in [container.requirements.general].

View all other issues in [container.requirements.general].

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Discussion:

See also discussion following c++std-lib-32883 and c++std-lib-32897.

The requirements on `CopyInsertable` and `MoveInsertable` are either incomplete, or complete but hard to figure out.

From e-mail c++std-lib-32897:

Pablo Halpern:

I agree that we need semantic requirements for all of the `*Insertable` concepts analogous to the requirements we have on similar concepts.

Howard Hinnant:

I've come to believe that the standard is actually correct as written in this area. But it is really hard to read. I would have no objection whatsoever to clarifications to `CopyInsertable` as you suggest (such as the post-conditions on `v`). And I do agree with you that the correct approach to the clarifications is to confirm that `CopyInsertable` implies `MoveInsertable`.

[2012, Portland: Move to Tentatively Ready]

Move to Tentatively Ready by unanimous consent.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Edit 23.2.2 [container.requirements.general] p13 as indicated:

   > -13- […] Given a container type `X` having an `allocator_type` identical to `A` and a `value_type` identical to `T` and given an lvalue `m` of type `A`, a pointer `p` of type `T*`, an expression `v` of type (possibly `const`) `T`, and an rvalue `rv` of type `T`, the following terms are defined. If `X` is not allocator-aware, the terms below are defined as if `A` were `std::allocator<T>` — no allocator object needs to be created and user specializations of `std::allocator<T>` are not instantiated:

   > - `T` is `DefaultInsertable` into `X` means that the following expression is well-formed:

   >   >     allocator_traits<A>::construct(m, p);

   > - An element of `X` is default-inserted if it is initialized by evaluation of the expression

   >   >     allocator_traits<A>::construct(m, p);

   >   where `p` is the address of the uninitialized storage for the element allocated within `X`.

   > - `T` is `CopyMoveInsertable` into `X` means that the following expression is well-formed:

   >   >     allocator_traits<A>::construct(m, p, rv);

   >   and when evaluated the following postconditions hold: The value of `*p` is equivalent to the value of `rv` before the evaluation. [Note: `rv` remains a valid object. Its state is unspecified — end note]

   > - `T` is `MoveCopyInsertable` into `X` means that, in addition to satisfying the `MoveInsertable` requirements, the following expression is well-formed:

   >   >     allocator_traits<A>::construct(m, p, rv);

   >   and when evaluated the following postconditions hold: The value of `v` is unchanged and is equivalent to `*p`.

   > - `T` is `EmplaceConstructible` into `X` from `args`, for zero or more arguments `args`, means that the following expression is well-formed:

   >   >     allocator_traits<A>::construct(m, p, args);

   > - `T` is `Erasable` from `X` means that the following expression is well-formed:

   >   >     allocator_traits<A>::destroy(m, p);

   > [Note: A container calls `allocator_traits<A>::construct(m, p, args)` to construct an element at `p` using `args`. The default construct in `std::allocator` will call `::new((void*)p) T(args)`, but specialized allocators may choose a different definition. — end note]

### 2179(i). `enable_shared_from_this` and construction from raw pointers

Section: 20.3.2.7 [util.smartptr.enab], 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: Daniel Krügler Opened: 2012-08-16 Last modified: 2017-09-07

Priority: 3

View all other issues in [util.smartptr.enab].

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Discussion:

On reflector message c++std-lib-32927, Matt Austern asked whether the following example should be well-defined or not

>     struct X : public enable_shared_from_this<X> { };
>     auto xraw = new X;
>     shared_ptr<X> xp1(xraw);
>     shared_ptr<X> xp2(xraw);

pointing out that 20.3.2.2.2 [util.smartptr.shared.const] does not seem to allow it, since `xp1` and `xp2` aren't allowed to share ownership, because each of them is required to have `use_count() == 1`. Despite this wording it might be reasonable (and technical possible) to implement that request.

On the other hand, there is the non-normative note in 20.3.2.7 [util.smartptr.enab] p11 (already part of TR1):

> The `shared_ptr` constructors that create unique pointers can detect the presence of an `enable_shared_from_this` base and assign the newly created `shared_ptr` to its `__weak_this member`.

Now according to the specification in 20.3.2.2.2 [util.smartptr.shared.const] p3-7:

>     template<class Y> explicit shared_ptr(Y* p);

the notion of creating unique pointers can be read to be included by this note, because the post-condition of this constructor is `unique() == true`. Evidence for this interpretation seems to be weak, though.

Howard Hinnant presented the counter argument, that actually the following is an "anti-idiom" and it seems questionable to teach it to be well-defined in any case:

>     auto xraw = new X;
>     shared_ptr<X> xp1(xraw);
>     shared_ptr<X> xp2(xraw);

He also pointed out that the current post-conditions of the affected `shared_ptr` constructor would need to be reworded.

It needs to be decided, which direction to follow. If this idiom seems too much broken to be supported, the note could be improved. If it should be supported, the constructors in 20.3.2.2.2 [util.smartptr.shared.const] need a careful analysis to ensure that post-conditions are correct.

Several library implementations currently do not support this example, instead they typically cause a crash. Matt points out that there are currently no explicit requirements imposed on `shared_ptr` objects to prevent them from owning the same underlying object without sharing the ownership. It might be useful to add such a requirement.

[2013-03-15 Issues Teleconference]

Moved to Open.

More discussion is needed to pick a direction to guide a proposed resolution.

[2013-05-09 Jonathan comments]

The note says the newly created `shared_ptr` is assigned to the `weak_ptr` member. It doesn't say before doing that the `shared_ptr` should check if the `weak_ptr` is non-empty and possibly share ownership with some other pre-existing `shared_ptr`.

[2015-08-26 Daniel comments]

LWG issue 2529(i) is independent but related to this issue.

[2016-03-16, Alisdair comments]

This issues should be closed as `Resolved` by paper p0033r1 at Jacksonville.

Proposed resolution:

### 2180(i). Exceptions from `std::seed_seq` operations

Section: 29.5.8.1 [rand.util.seedseq] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [rand.util.seedseq].

View all issues with C++14 status.

Discussion:

29.5.8.1 [rand.util.seedseq] p1 says upfront:

> No function described in this section 29.5.8.1 [rand.util.seedseq] throws an exception.

This constraint seems non-implementable to me when looking especially at the members

>     template<class T>
>     seed_seq(initializer_list<T> il);

>     template<class InputIterator>
>     seed_seq(InputIterator begin, InputIterator end);

which have the effect of invoking `v.push_back()` for the exposition-only member of type `std::vector` (or its equivalent) over all elements of the provided range, so out-of-memory exceptions are always possible and the `seed_seq` object doesn't seem to be constructible this way.

In addition to the potential lack-of-resources problem, the operations of `InputIterator` might also throw exceptions.

Aside to that it should me mentioned, that a default constructor of `vector<uint_least32_t>` in theory can also throw exceptions, even though this seems less of a problem to me in this context, because such an implementation could easily use a different internal container in `seed_seq` that can hold this no-throw exception guarantee.

Secondly, a slightly different problem category related to exceptions occurs for the member templates

>     template<class RandomAccessIterator>
>     void generate(RandomAccessIterator begin, RandomAccessIterator end);

>     template<class OutputIterator>
>     void param(OutputIterator dest) const;

where the actual operations performed by the implementation would never need to throw, but since they invoke operations of a user-provided customization point, the overall operation, like for example

>     copy(v.begin(), v.end(), dest);

could also throw exceptions. In this particular example we can just think of a `std::back_insert_iterator` applied to a container that needs to allocate its elements used as the type for `OutputIterator`.

Even though Clause 29 [numerics] has mostly stronger exception constraints than other parts of the library the here discussed are overrestrictive, especially since no operation of `std::seed_seq` except the template `generate` is actually needed within the library implementation, as mentioned in the discussion of LWG 2124(i).

I suggest to remove the general no-exception constraints for operations of `std::seed_seq` except for member `size()` and the default constructor and to provide specific wording for `generate()` and `param()` to ensure that the algorithm itself is a nothrow operation, which is especially for `generate()` important, because the templates specified in 29.5.4 [rand.eng] and 29.5.5 [rand.adapt] also depend on this property indirectly, which is further discussed in LWG 2181(i).

Howard:

I suggest to use a different form for the exception specification, something similar to 22.10.15.4 [func.bind.bind] p4:

> Throws: Nothing unless an operation on `RandomAccessIterator` throws an exception.

Daniel:

The currently suggested "what and when" form seems a bit more specific and harmonizes with the form used for function template `generate_canonical` from 29.5.8.2 [rand.util.canonical].

[2013-04-20, Bristol]

Open an editorial issue on the exception wording ("Throws: What and when").

Solution: move to tentatively ready.

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3376.

1. Edit 29.5.8.1 [rand.util.seedseq] p1 as indicated:

   > -1- No function described in this section 29.5.8.1 [rand.util.seedseq] throws an exception.

2. Edit 29.5.8.1 [rand.util.seedseq] around p2 as indicated:

   >     seed_seq();

   > > -2- Effects: Constructs a `seed_seq` object as if by default-constructing its member `v`.

   > > -?- Throws: Nothing.

3. Edit 29.5.8.1 [rand.util.seedseq] around p7 as indicated:

   >     template<class RandomAccessIterator>
   >       void generate(RandomAccessIterator begin, RandomAccessIterator end);

   > > -7- Requires: `RandomAccessIterator` shall meet the requirements of a mutable random access iterator (Table 111) type. Moreover, `iterator_traits<class RandomAccessIterator>::value_type` shall denote an unsigned integer type capable of accommodating 32-bit quantities.

   > > -8- Effects: Does nothing if `begin == end`. Otherwise, with `s = v.size()` and `n = end - begin`, fills the supplied range `[begin, end)` according to the following algorithm […]

   > > -?- Throws: What and when `RandomAccessIterator` operations of `begin` and `end` throw.

4. Edit 29.5.8.1 [rand.util.seedseq] around p9 as indicated:

   >     size_t size() const;

   > > -9- Returns: The number of 32-bit units that would be returned by a call to `param()`.

   > > -??- Throws: Nothing.

   > > -10- Complexity: constant time.

5. Edit 29.5.8.1 [rand.util.seedseq] around p11 as indicated:

   >     template<class OutputIterator>
   >       void param(OutputIterator dest) const;

   > > -11- Requires: `OutputIterator` shall satisfy the requirements of an output iterator (Table 108) type. Moreover, the expression `*dest = rt` shall be valid for a value `rt` of type `result_type`.

   > > -12- Effects: Copies the sequence of prepared 32-bit units to the given destination, as if by executing the following statement:

   > > >     copy(v.begin(), v.end(), dest);

   > > -??- Throws: What and when `OutputIterator` operations of `dest` throw.

### 2181(i). Exceptions from seed sequence operations

Section: 29.5.3.2 [rand.req.seedseq], 29.5.4 [rand.eng], 29.5.5 [rand.adapt] Status: C++17 Submitter: Daniel Krügler Opened: 2012-08-18 Last modified: 2017-07-30

Priority: 3

View all other issues in [rand.req.seedseq].

View all issues with C++17 status.

Discussion:

LWG issue 2180(i) points out some deficiences in regard to the specification of the library-provided type `std::seed_seq` regarding exceptions, but there is another specification problem in regard to general types satisfying the seed sequence constraints (named `SSeq`) as described in 29.5.3.2 [rand.req.seedseq].

29.5.4 [rand.eng] p3 and 29.5.5.1 [rand.adapt.general] p3 say upfront:

> Except where specified otherwise, no function described in this section 29.5.4 [rand.eng]/29.5.5 [rand.adapt] throws an exception.

This constraint causes problems, because the described templates in these sub-clauses depend on operations of `SSeq::generate()` which is a function template, that depends both on operations provided by the implementor of `SSeq` (e.g. of `std::seed_seq`), and those of the random access iterator type provided by the caller. With class template `linear_congruential_engine` we have just one example for a user of `SSeq::generate()` via:

>     template<class Sseq>
>     linear_congruential_engine<>::linear_congruential_engine(Sseq& q);

>     template<class Sseq>
>     void linear_congruential_engine<>::seed(Sseq& q);

None of these operations has an exclusion rule for exceptions.

As described in 2180(i) the wording for `std::seed_seq` should and can be fixed to ensure that operations of `seed_seq::generate()` won't throw except from operations of the provided iterator range, but there is no corresponding "safety belt" for user-provided `SSeq` types, since 29.5.3.2 [rand.req.seedseq] does not impose no-throw requirements onto operations of seed sequences.

1. A quite radical step to fix this problem would be to impose general no-throw requirements on the expression `q.generate(rb,re)` from Table 115, but this is not as simple as it looks initially, because this function again depends on general types that are mutable random access iterators. Typically, we do not impose no-throw requirements on iterator operations and this would restrict general seed sequences where exceptions are not a problem. Furthermore, we do not impose comparable constraints for other expressions, like that of the expression `g()` in Table 116 for good reasons, e.g. `random_device::operator()` explicitly states when it throws exceptions.

2. A less radical variant of the previous suggestion would be to add a normative requirement on the expression `q.generate(rb,re)` from Table 115 that says: "Throws nothing if operations of `rb` and `re` do not throw exceptions". Nevertheless we typically do not describe conditional Throws elements in proper requirement sets elsewhere (Container requirements excluded, they just describe the containers from Clause 23) and this may exclude resonable implementations of seed sequences that could throw exceptions under rare situations.

3. The iterator arguments provided to `SSeq::generate()` for operations in templates of 29.5.4 [rand.eng] and 29.5.5 [rand.adapt] are under control of implementations, so we could impose stricter exceptions requirements on `SSeq::generate()` for `SSeq` types that are used to instantiate member templates in 29.5.4 [rand.eng] and 29.5.5 [rand.adapt] solely.

4. We simply add extra wording to the introductive parts of 29.5.4 [rand.eng] and 29.5.5 [rand.adapt] that specify that operations of the engine (adaptor) templates that depend on a template parameter `SSeq` throw no exception unless `SSeq::generate()` throws an exception.

Given these options I would suggest to apply the variant described in the fourth bullet.

The proposed resolution attempts to reduce a lot of the redundancies of requirements in the introductory paragraphs of 29.5.4 [rand.eng] and 29.5.5 [rand.adapt] by introducing a new intermediate sub-clause "Engine and engine adaptor class templates" following sub-clause 29.5.2 [rand.synopsis]. This approach also solves the problem that currently 29.5.4 [rand.eng] also describes requirements that apply for 29.5.5 [rand.adapt] (Constrained templates involving the `Sseq` parameters).

[2013-04-20, Bristol]

Remove the first bullet point:

> ?- Throughout this sub-clause general requirements and conventions are described that apply to every class template specified in sub-clause 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]. Phrases of the form "in those sub-clauses" shall be interpreted as equivalent to "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".

Replace "in those sub-clauses" with "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".

Find another place for the wording.

Daniel: These are requirements on the implementation not on the types. I'm not comfortable in moving it to another place without double checking.

Improve the text (there are 4 "for"s): for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses.

Move the information of this paragraph to the paragraphs it refers to:

> "-?- Descriptions are provided in those sub-clauses only for engine operations that are not described in 29.5.3.4 [rand.req.eng], for adaptor operations that are not described in 29.5.3.5 [rand.req.adapt], or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses."

Alisdair: I prefer duplication here than consolidation/reference to these paragraphs.

The room showed weakly favjust or for duplication.

Previous resolution from Daniel [SUPERSEDED]:

> 1. Add a new sub-clause titled "Engine and engine adaptor class templates" following sub-clause 29.5.2 [rand.synopsis] (but at the same level) and add one further sub-clause "General" as child of the new sub-clause as follows:

>    Engine and engine adaptor class templates [rand.engadapt]

>    General [rand.engadapt.general]

>    > -?- Throughout this sub-clause general requirements and conventions are described that apply to every class template specified in sub-clause 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]. Phrases of the form "in those sub-clauses" shall be interpreted as equivalent to "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".

>    > -?- Except where specified otherwise, the complexity of each function specified in those sub-clauses is constant.

>    > -?- Except where specified otherwise, no function described in those sub-clauses throws an exception.

>    > -?- Every function described in those sub-clauses that has a function parameter `q` of type `SSeq&` for a template type parameter named `SSeq` that is different from type `std::seed_seq` throws what and when the invocation of `q.generate` throws.

>    > -?- Descriptions are provided in those sub-clauses only for engine operations that are not described in 29.5.3.4 [rand.req.eng], for adaptor operations that are not described in 29.5.3.5 [rand.req.adapt], or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses.

>    > -?- Each template specified in those sub-clauses requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.

>    > -?- For every random number engine and for every random number engine adaptor `X` defined in those sub-clauses:

>    > - if the constructor

>    >   >     template <class Sseq> explicit X(Sseq& q);

>    >   is called with a type `Sseq` that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution;

>    > - if the member function

>    >   >     template <class Sseq> void seed(Sseq& q);

>    >   is called with a type `Sseq` that does not qualify as a seed sequence, then this function shall not participate in overload resolution;

>    > The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as a seed sequence if it is implicitly convertible to `X::result_type`.

> 2. Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:

>    > -1- Each type instantiated from a class template specified in this section 29.5.4 [rand.eng] satisfies the requirements of a random number engine (29.5.3.4 [rand.req.eng]) type and the general implementation requirements specified in sub-clause [rand.engadapt.general].

>    > -2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant.

>    > -3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception.

>    > -4- Descriptions are provided in this section 29.5.4 [rand.eng] only for engine operations that are not described in 29.5.3.4 [rand.req.eng] […]

>    > -5- Each template specified in this section 29.5.4 [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. […]

>    > -6- For every random number engine and for every random number engine adaptor `X` defined in this subclause (29.5.4 [rand.eng]) and in sub-clause 29.5.4 [rand.eng]: […]

> 3. Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:

>    > -1- Each type instantiated from a class template specified in this section 29.5.4 [rand.eng]29.5.5 [rand.adapt] satisfies the requirements of a random number engine adaptor (29.5.3.5 [rand.req.adapt]) type and the general implementation requirements specified in sub-clause [rand.engadapt.general].

>    > -2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant.

>    > -3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception.

>    > -4- Descriptions are provided in this section 29.5.5 [rand.adapt] only for engine operations that are not described in 29.5.3.5 [rand.req.adapt] […]

>    > -5- Each template specified in this section 29.5.5 [rand.adapt] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. […]

[2014-02-09, Daniel provides alternative resolution]

[Lenexa 2015-05-07: Move to Ready]

LWG 2181 exceptions from seed sequence operations

STL: Daniel explained that I was confused. I said, oh, seed_seq says it can throw if the RanIt throws. Daniel says the RanIts are provided by the engine. Therefore if you give a seed_seq to an engine, it cannot throw, as implied by the current normative text. So what Daniel has in the PR is correct, if slightly unnecessary. It's okay to have explicitly non-overlapping Standardese even if overlapping would be okay.

Marshall: And this is a case where the std:: on seed_seq is a good thing.

STL: Meh.

STL: And that was my only concern with this PR. I like the latest PR much better than the previous.

Marshall: Yes. There's a drive-by fix for referencing the wrong section. Other than that, the two are the same.

STL: Alisdair wanted the repetition instead of centralization, and I agree.

Marshall: Any other opinions?

Jonathan: I'll buy it.

STL: For a dollar?

Hwrd: I'll buy that for a nickel.

Marshall: Any objections to Ready? I don't see a point in Immediate.

Jonathan: Absolutely agree.

Marshall: 7 for ready, 0 opposed, 0 abstain.

[2014-05-22, Daniel syncs with recent WP]

[2015-10-31, Daniel comments and simplifies suggested wording changes]

Upon Walter Brown's suggestion the revised wording does not contain any wording changes that could be considered as editorial.

Previous resolution from Daniel [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:

>    > -1- Each type instantiated from a class template specified in this section 29.5.4 [rand.eng] satisfies the requirements of a random number engine (29.5.3.4 [rand.req.eng]) type.

>    > -2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant.

>    > -3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception.

>    > -?- Every function described in this section 29.5.4 [rand.eng] that has a function parameter `q` of type `Sseq&` for a template type parameter named `Sseq` that is different from type `std::seed_seq` throws what and when the invocation of `q.generate` throws.

>    > -4- Descriptions are provided in this section 29.5.4 [rand.eng] only for engine operations that are not described in 29.5.3.4 [rand.req.eng] or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality operators, and inequality operators are not shown in the synopses.

>    > -5- Each template specified in this section 29.5.4 [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.

>    > -6- For every random number engine and for every random number engine adaptor `X` defined in this subclause (29.5.4 [rand.eng]) and in sub-clause 29.5.5 [rand.adapt]:

>    > - if the constructor

>    >   >     template <class Sseq> explicit X(Sseq& q);

>    >   is called with a type `Sseq` that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution;

>    > - if the member function

>    >   >     template <class Sseq> void seed(Sseq& q);

>    >   is called with a type `Sseq` that does not qualify as a seed sequence, then this function shall not participate in overload resolution;

>    > The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as a seed sequence if it is implicitly convertible to `X::result_type`.

> 2. Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:

>    > -1- Each type instantiated from a class template specified in this section 29.5.4 [rand.eng]29.5.5 [rand.adapt] satisfies the requirements of a random number engine adaptor (29.5.3.5 [rand.req.adapt]) type.

>    > -2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant.

>    > -3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception.

>    > -?- Every function described in this section 29.5.5 [rand.adapt] that has a function parameter `q` of type `Sseq&` for a template type parameter named `Sseq` that is different from type `std::seed_seq` throws what and when the invocation of `q.generate` throws.

>    > -4- Descriptions are provided in this section 29.5.5 [rand.adapt] only for adaptor operations that are not described in section 29.5.3.5 [rand.req.adapt] or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality operators, and inequality operators are not shown in the synopses.

>    > -5- Each template specified in this section 29.5.5 [rand.adapt] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.

> 3. Edit the contents of sub-clause 29.5.9.1 [rand.dist.general] p2 as indicated: [Drafting note: These editorial changes are just for consistency with those applied to 29.5.4 [rand.eng] and 29.5.5.1 [rand.adapt.general] — end drafting note]

>    > -2- Descriptions are provided in this section 29.5.9 [rand.dist] only for distribution operations that are not described in 29.5.3.6 [rand.req.dist] or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality operators, and inequality operators are not shown in the synopses.

Proposed resolution:

This wording is relative to N4527.

1. Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:

   > -1- […]

   > -2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant.

   > -3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception.

   > -?- Every function described in this section 29.5.4 [rand.eng] that has a function parameter `q` of type `Sseq&` for a template type parameter named `Sseq` that is different from type `std::seed_seq` throws what and when the invocation of `q.generate` throws.

   > […]

2. Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:

   > -1- […]

   > -2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant.

   > -3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception.

   > -?- Every function described in this section 29.5.5 [rand.adapt] that has a function parameter `q` of type `Sseq&` for a template type parameter named `Sseq` that is different from type `std::seed_seq` throws what and when the invocation of `q.generate` throws.

### 2182(i). `Container::[const_]reference` types are misleadingly specified

Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-20 Last modified: 2017-07-05

Priority: 0

View other active issues in [container.requirements.general].

View all other issues in [container.requirements.general].

View all issues with C++14 status.

Discussion:

According to Table 96 (Container requirements) the return type of `X::reference` and `X::const_reference` is "lvalue of `T`" and "`const` lvalue of `T`", respectively. This does not make much sense, because an lvalue is an expression category, not a type. It could also refer to an expression that has a type, but this doesn't make sense either in this context, because obviously `X::[const_]reference` are intended to refer to types.

Given the fact that `vector<bool>` has no real reference type for `X::[const_]reference` and this definition presumably is intended to cover such situations as well, one might think that the wording is just a sloppy form of "type that represents a [const] lvalue of `T`". But this is also problematic, because basically all proxy reference expressions are rvalues.

It is unclear what the intention is. A straightward way of fixing this wording could make `X::[const_]reference` identical to `[const] T&`. This holds for all Library containers except for `vector<bool>`.

Another way of solving this definition problem would be to impose a requirement that holds for both references and reference-like proxies. Both `X::reference` and `X::const_reference` would need to be convertible to `const T&`. Additionally `X::reference` would need to support for a mutable container an assignment expression of the form `declval<X::reference>() = declval<T>()` (this presentation intentionally does not require `declval<X::reference&>() = declval<T>()`).

Further, the Table 96 does not impose any relations between `X::reference` and `X::const_reference`. It seems that at least `X::reference` needs to be convertible to `X::const_reference`.

A related question is whether `X::reference` is supposed to be a mutable reference-like type, irrespective of whether the container is an immutable container or not. The way, type `match_results` defines `reference` identical to `const_reference` indicates one specific interpretation (similarly, the `initializer_list` template also defines member type `reference` equal to `const value_type&`). Note that this can be a different decision as that for `iterator` and `const_iterator`, e.g. for sets the type `X::reference` still is a mutable reference, even though `iterator` is described as constant iterator.

The proposed resolution is incomplete in regard to the last question.

[2013-03-15 Issues Teleconference]

Moved to Review.

Alisdair notes that this looks like wording in the right direction. Wonders about congruence of these typedefs and the similar ones for iterators.

[2013-09 Chicago]

Moved to Ready.

Consensus that the requirements should require real references, just like iterators, as containers are required to support at least ForwardIterators, which have the same restriction on references.

Matt will file a new issue for some additional concerns with regex `match_results`.

[2014-02-10, Daniel comments]

The new issue opened by Matt is LWG 2306(i).

[Issaquah 2014-02-11: Move to Immediate]

Issue should have been Ready in pre-meeting mailing.

Proposed resolution:

This wording is relative to N3376.

1. Change Table 96 — "Container requirements" as indicated:

   Table 96 — Container requirements
   Expression | Return type | Operational Semantics | Assertion/note pre-/post-condition | Complexity
   --- | --- | --- | --- | ---
   `X::reference` | lvalue of `T&` |  |  | compile time
   `X::const_reference` | const lvalue of `const` `T&` |  |  | compile time

### 2183(i). Muddled allocator requirements for `match_results` constructors

Section: 28.6.9.2 [re.results.const], 28.6.9.7 [re.results.all] Status: C++20 Submitter: Pete Becker Opened: 2012-08-29 Last modified: 2021-02-25

Priority: 3

View all other issues in [re.results.const].

View all issues with C++20 status.

Discussion:

28.6.9.2 [re.results.const] p1 says:

> In all `match_results` constructors, a copy of the `Allocator` argument shall be used for any memory allocation performed by the constructor or member functions during the lifetime of the object.

There are three constructors:

>     match_results(const Allocator& = Allocator());
>     match_results(const match_results& m);
>     match_results(match_results&& m) noexcept;

The second and third constructors do no have an `Allocator` argument, so despite the "all `match_results` constructors", it is not possible to use "the `Allocator` argument" for the second and third constructors.

The requirements for those two constructors also does not give any guidance. The second constructor has no language about allocators, and the third states that the stored `Allocator` value is move constructed from `m.get_allocator()`, but doesn't require using that allocator to allocate memory.

The same basic problem recurs in 28.6.9.7 [re.results.all], which gives the required return value for `get_allocator()`:

> Returns: A copy of the `Allocator` that was passed to the object's constructor or, if that allocator has been replaced, a copy of the most recent replacement.

Again, the second and third constructors do not take an `Allocator`, so there is nothing that meets this requirement when those constructors are used.

[2018-06-02, Daniel comments and provides wording]

The introductory wording of `match_results` says in 28.6.9 [re.results] p2:

> The class template `match_results` satisfies the requirements of an allocator-aware container and of a sequence container (26.2.1, 26.2.3) except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.

This wording essentially brings us to 23.2.2 [container.requirements.general] which describes in p8 in general the usage of allocators:

> […] Copy constructors for these container types obtain an allocator by calling `allocator_traits<allocator_ type>::select_on_container_copy_construction` on the allocator belonging to the container being copied. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. […]

The constructors referred to in the issue discussion are the copy constructor and move constructor of `match_results`, so we know already what the required effects are supposed to be.

23.2.2 [container.requirements.general] p8 also says more to this allocator topic a bit latter:

> […] All other constructors for these container types take a `const allocator_type&` argument. [Note: If an invocation of a constructor uses the default value of an optional allocator argument, then the Allocator type must support value-initialization. — end note] A copy of this allocator is used for any memory allocation and element construction performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. […]

Further requirements imposed on two of the three `match_results` constructors can be derived from Table 80 — "Allocator-aware container requirements" via the specified expressions

>     X()
>     X(m)
>     X(rv)

In other words: The existing wording does already say everything that it said by 28.6.9.2 [re.results.const] p1 (end even more), except for possibly the tiny problem that

>     match_results(const Allocator& a = Allocator());

uses "`const Allocator&`" instead of "`const allocator_type&`" in the signature, albeit even that deviation shouldn't change the intended outcome, which is IMO crystal-clear when looking at sub-clauses 23.2.2 [container.requirements.general] and 23.2.4 [sequence.reqmts] as a whole.

That directly makes two mutually exclusive solutions feasible:

- Either strike 28.6.9.2 [re.results.const] p1 completely; or

- Replace 28.6.9.2 [re.results.const] p1 by referring to the specification of allocators in 23.2.2 [container.requirements.general] and 23.2.4 [sequence.reqmts].

My suggestion is to favour for the first option, because attempting to provide extra wording that refers to allocators and the three constructors may lead to the false impression that no further allocator-related requirements hold for type `match_results` which are not explicitly repeated here again.

[2018-06, Rapperswil]

The group agrees with the provided resolution. Move to Ready.

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Edit 28.6.9.2 [re.results.const] as indicated:

   > -1- In all `match_results` constructors, a copy of the `Allocator` argument shall be used for any memory allocation performed by the constructor or member functions during the lifetime of the object.

### 2184(i). Muddled allocator requirements for `match_results` assignments

Section: 28.6.9.2 [re.results.const], 28.6.9.7 [re.results.all] Status: C++20 Submitter: Pete Becker Opened: 2012-08-29 Last modified: 2021-02-25

Priority: 3

View all other issues in [re.results.const].

View all issues with C++20 status.

Discussion:

The effects of the two assignment operators are specified in Table 141. Table 141 makes no mention of allocators, so, presumably, they don't touch the target object's allocator. That's okay, but it leaves the question: `match_results::get_allocator()` is supposed to return "A copy of the Allocator that was passed to the object's constructor or, if that allocator has been replaced, a copy of the most recent replacement"; if assignment doesn't replace the allocator, how can the allocator be replaced?

[2018-06-04, Daniel comments and provides wording]

Similar to the reasoning provided in the 2018-06-02 comment in LWG 2183(i), it is possible to refer to the introductory wording of `match_results` which says in 28.6.9 [re.results] p2:

> The class template `match_results` satisfies the requirements of an allocator-aware container and of a sequence container (26.2.1, 26.2.3) except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.

Again, similar to LWG 2183(i), this allows us to deduce the required effects of the copy/move assignment operators discussed here, because 23.2.2 [container.requirements.general] p8 also says:

> […] The allocator may be replaced only via assignment or `swap()`. Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only if `allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value`, `allocator_traits<allocator_type>::propagate_on_container_move_assignment::value`, or `allocator_traits<allocator_type>::propagate_on_container_swap::value` is `true` within the implementation of the corresponding container operation. In all container types defined in this Clause, the member `get_allocator()` returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement. […]

So this wording already specifies everything we need, except for the problem that 28.6.9 [re.results] p2 quoted above restricts to operations supported by a const-qualified sequence container, which of-course would exclude the copy assignment and the move assignment operators. But given that these mutable definitions are defined for `match_results`, it seems that the only fix needed is to adjust 28.6.9 [re.results] p2 a bit to ensure that both assignment operators are covered (again) by the general allocator-aware container wording.

[2018-06, Rapperswil]

The group generally likes the suggested direction, but would prefer the changed wording to say effectively "except that only copy assignment, move assignment, and operations defined...". Once applied, move to ready.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> 1. Edit 28.6.9 [re.results] as indicated:

>    > -2- The class template `match_results` satisfies the requirements of an allocator-aware container and of a sequence container (23.2.2 [container.requirements.general], 23.2.4 [sequence.reqmts]) except that besides copy assignment and move assignment only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.

[2018-06-06, Daniel updates wording]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Edit 28.6.9 [re.results] as indicated:

   > -2- The class template `match_results` satisfies the requirements of an allocator-aware container and of a sequence container (23.2.2 [container.requirements.general], 23.2.4 [sequence.reqmts]) except that only copy assignment, move assignment, and operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.

### 2185(i). Missing throws clause for `future/shared_future::wait_for/wait_until`

Section: 32.10.7 [futures.unique.future], 32.10.8 [futures.shared.future] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2012-09-20 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [futures.unique.future].

View all issues with C++14 status.

Discussion:

The functions `future::wait_for`, `future::wait_until`, `shared_future::wait_for`, and `shared_future::wait_for` can throw any timeout-related exceptions. It would be better if the wording could be more explicit. This is in line with the changes proposed in LWG 2093(i)'s Throws element of `condition_variable::wait` with predicate.

[2012, Portland: move to Review]

The phrase timeout-related exception does not exist.

2093(i) was put in review, and there is some dependency here with this issue.

If you provide a user-defined clock that throws, we need to put back an exception to allow that to be done.

We will put this in review and say that this cannot go in before 2093(i).

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

[This resolution should not be adopted before resolving 2093(i)]

[This wording is relative to N3376.]

1. Change [futures.unique_future] as indicated:

   >     template <class Rep, class Period>
   >     future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;

   > > -21- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the relative timeout (32.2.4 [thread.req.timing]) specified by `rel_time` has expired.

   > > -22- Returns: […]

   > > -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).

   >     template <class Clock, class Duration>
   >     future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

   > > -23- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout (32.2.4 [thread.req.timing]) specified by `abs_time` has expired.

   > > -24- Returns: […]

   > > -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).

2. Change [futures.shared_future] as indicated:

   >     template <class Rep, class Period>
   >     future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;

   > > -23- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the relative timeout (32.2.4 [thread.req.timing]) specified by `rel_time` has expired.

   > > -24- Returns: […]

   > > -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).

   >     template <class Clock, class Duration>
   >     future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

   > > -25- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout (32.2.4 [thread.req.timing]) specified by `abs_time` has expired.

   > > -26- Returns: […]

   > > -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).

### 2186(i). Incomplete action on `async/launch::deferred`

Section: 32.10.9 [futures.async] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2012-09-20 Last modified: 2017-07-05

Priority: Not Prioritized

View all other issues in [futures.async].

View all issues with C++14 status.

Discussion:

The description of the effects of `async` when the launch policy is `launch::deferred` doesn't state what is done with the result of the deferred function invocation and the possible exceptions as it is done for the asynchronous function when the policy is `launch::async`.

[2012, Portland: move to Open]

Detlef: agree with the problem but not with the resolution. The wording should be applied to all launch policies rather than having to be separately specified for each one.

Hans: we should redraft to factor out the proposed text outside the two bullets. Needs to be carefully worded to be compatible with the resolution of 2120(i) (see above).

Moved to open

[Issaquah 2014-02-11: Move to Immediate after SG1 review]

Proposed resolution:

[This wording is relative to N3376.]

1. Change 32.10.9 [futures.async] p3 bullet 2 as indicated:

   >     template <class F, class... Args>
   >     future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   >     async(F&& f, Args&&... args);
   >     template <class F, class... Args>
   >     future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   >     async(launch policy, F&& f, Args&&... args);

   > > -2- Requires: […]

   > > -3- Effects:: The first function behaves the same as a call to the second function with a `policy` argument of `launch::async | launch::deferred` and the same arguments for `F` and `Args`. […] The further behavior of the second function depends on the `policy` argument as follows (if more than one of these conditions applies, the implementation may choose any of the corresponding policies):

   > > - if `policy & launch::async` is non-zero […]

   > > - if `policy & launch::deferred` is non-zero — Stores `DECAY_COPY(std::forward<F>(f))` and `DECAY_COPY(std::forward<Args>(args))...` in the shared state. These copies of `f` and `args` constitute a deferred function. Invocation of the deferred function evaluates `INVOKE(std::move(g), std::move(xyz))` where `g` is the stored value of `DECAY_COPY(std::forward<F>(f))` and `xyz` is the stored copy of `DECAY_COPY(std::forward<Args>(args))...`. Any return value is stored as the result in the shared state. Any exception propagated from the execution of the deferred function is stored as the exceptional result in the shared state. The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (32.10.5 [futures.state]) on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called the waiting function. Once evaluation of `INVOKE(std::move(g), std::move(xyz))` begins, the function is no longer considered deferred. [Note: If this policy is specified together with other policies, such as when using a policy value of `launch::async | launch::deferred`, implementations should defer invocation or the selection of the `policy` when no more concurrency can be effectively exploited. — end note]

### 2187(i). `vector<bool>` is missing `emplace` and `emplace_back` member functions

Section: 23.3.14 [vector.bool] Status: C++14 Submitter: Nevin Liber Opened: 2012-09-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [vector.bool].

View all issues with C++14 status.

Discussion:

It should have them so that it more closely matches the `vector<T>` interface, as this helps when writing generic code.

[2012, Portland: Move to Tentatively Ready]

Question on whether the variadic template is really needed, but it turns out to be needed to support `emplace` of no arguments.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change the class template `vector<bool>` synopsis, 23.3.14 [vector.bool] p1, as indicated:

   >     namespace std {
   >       template <class Allocator> class vector<bool, Allocator> {
   >       public:
   >         […]
   >         // modifiers:
   >         template <class... Args> void emplace_back(Args&&... args);
   >         void push_back(const bool& x);
   >         void pop_back();
   >         template <class... Args> iterator emplace(const_iterator position, Args&&... args);
   >         iterator insert(const_iterator position, const bool& x);
   >         […]
   >       };
   >     }

### 2188(i). Reverse iterator does not fully support targets that overload `operator&`

Section: 24.5.1.6 [reverse.iter.elem] Status: C++14 Submitter: Alisdair Meredith Opened: 2012-09-23 Last modified: 2021-06-06

Priority: 1

View all other issues in [reverse.iter.elem].

View all issues with C++14 status.

Discussion:

The specification for `reverse_iterator::operator->` returns the address of the object yielded by dereferencing with `operator*`, but does not have the usual wording about returning the true address of the object. As `reverse_iterator` requires the adapted iterator have at least the bidirectional iterator category, we know that the returned reference is a true reference, and not a proxy, hence we can use `std::addressof` on the reference to get the right answer.

This will most likely show itself as an issue with a `list` or `vector` of a type with such an overloaded operator, where algorithms are likely to work with a forward iteration, but not with reverse iteration.

[2013-04-20, Bristol]

Resolution: Goes to open now and move to review as soon as Daniel proposes a new wording.

[2014-02-12 Issaquah meeting]

Use `std::addressof` as the library uses elsewhere, then move as Immediate.

Proposed resolution:

Revise [reverse.iter.opref] p1, as indicated:

Returns: `addressof&(operator*())`.

### 2190(i). Condition variable specification

Section: 32.7 [thread.condition] Status: C++14 Submitter: Hans Boehm Opened: 2012-09-25 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition].

View all issues with C++14 status.

Discussion:

The condition variable specification possibly leaves it unclear whether the effect of a `notify_one()` call can effectively be delayed, so that a call unblocks a `wait()` call that happens after the `notify_one` call. (For `notify_all()` this is not detectable, since it only results in spurious wake-ups.) Although this may at first glance seem like a contrived interpretation, it gains relevance since glibc in fact allows the analogous behavior (see here) and it is currently controversial whether this is correct and the Posix specification allows it (see here).

The following proposed resolution disallows the glibc implementation, remaining consistent with the believed intent of C++11. To make that clear, we require that the "unspecified total order" O from 32.7 [thread.condition] p4 be consistent with happens-before. We also intend that the 3 components of a wait occur in order in O, but stating that explicitly seems too pedantic. Since they are numbered, it appears clear enough that they are sequenced one after the other.

Another uncertainty with the current phrasing is whether there is a single total order that includes all c.v. accesses, or one total order per c.v. We believe it actually doesn't matter, because there is no way to tell the difference, but this requires a bit more thought. We resolved it one way, just to remove the potential ambiguity.

[2012, Portland: Move to Review]

This is linked to a glibc issue, and a POSIX specification issue.

We believe the proposed wording fixes the ambiguity in C++ and is compatible with the proposed resolution for Posix (which confirms the glibc behaviour as illegal).

Moved to review (Detlef hopes to send some improved wording to the reflector).

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to N3376.

1. Change 32.7 [thread.condition] p4 as indicated:

   > -4- The implementation shall behave as if all executions of `notify_one`, `notify_all`, and each part of the `wait`, `wait_for`, and `wait_until` executions are executed in somea single unspecified total order consistent with the "happens before" order.

### 2191(i). Incorrect specification of `match_results(match_results&&)`

Section: 28.6.9.2 [re.results.const] Status: C++23 Submitter: Pete Becker Opened: 2012-10-02 Last modified: 2023-11-22

Priority: 4

View all other issues in [re.results.const].

View all issues with C++23 status.

Discussion:

28.6.9.2 [re.results.const]/3: "Move-constructs an object of class `match_results` satisfying the same postconditions as Table 141."

Table 141 lists various member functions and says that their results should be the results of the corresponding member function calls on `m`. But `m` has been moved from, so the actual requirement ought to be based on the value that `m` had before the move construction, not on `m` itself.

In addition to that, the requirements for the copy constructor should refer to Table 141.

Ganesh:

Also, the requirements for move-assignment should refer to Table 141. Further it seems as if in Table 141 all phrases of "for all integers `n < m.size()`" should be replaced by "for all unsigned integers `n < m.size()`".

[2019-03-26; Daniel comments and provides wording]

The previous Table 141 (Now Table 128 in N4810) has been modified to cover now the effects of move/copy constructors and move/copy assignment operators. Newly added wording now clarifies that for move operations the corresponding values refer to the values of the move source before the operation has started.

Re Ganesh's proposal: Note that no further wording is needed for the move-assignment operator, because in the current working draft the move-assignment operator's Effects: element refers already to Table 128. The suggested clarification of unsigned integers has been implemented by referring to non-negative integers instead.

Upon suggestion from Casey, the wording also introduces Ensures: elements that refer to Table 128 and as drive-by fix eliminates a "Throws: Nothing." element from a `noexcept` function. Previous resolution [SUPERSEDED]:

> This wording is relative to N4810.

> 1. Add a new paragraph at the beginning of 28.6.9.2 [re.results.const] as indicated:

>    > -?- Table 128 lists the postconditions of `match_results` copy/move constructors and copy/move assignment operators. For move operations, the results of the expressions depending on the parameter `m` denote the values they had before the respective function calls.

> 2. Modify 28.6.9.2 [re.results.const] as indicated:

>    >     match_results(const match_results& m);

>    > > -3- Effects: Constructs an object of class `match_results`, as a copy of `m`.

>    > > -?- Ensures: As indicated in Table 128.

>    >     match_results(match_results&& m) noexcept;

>    > > -4- Effects: Move constructs an object of class `match_results` from `m` satisfying the same postconditions as Table 128. Additionally, the stored `Allocator` value is move constructed from `m.get_allocator()`.

>    > > -?- Ensures: As indicated in Table 128.

>    > > -5- Throws: Nothing.

>    >     match_results& operator=(const match_results& m);

>    > > -6- Effects: Assigns `m` to `*this`. The postconditions of this function are indicated in Table 128.

>    > > -?- Ensures: As indicated in Table 128.

>    >     match_results& operator=(match_results&& m);

>    > > -7- Effects: Move- assigns `m` to `*this`. The postconditions of this function are indicated in Table 128.

>    > > -?- Ensures: As indicated in Table 128.

> 3. Modify 28.6.9.2 [re.results.const], Table 128 — "`match_results` assignment operator effects", as indicated:

>    > Table 128 — `match_results` assignment operator effectscopy/move operation postconditions
>    > Element | Value
>    > --- | ---
>    > `ready()` | `m.ready()`
>    > `size()` | `m.size()`
>    > `str(n)` | `m.str(n)` for all non-negative integers `n < m.size()`
>    > `prefix()` | `m.prefix()`
>    > `suffix()` | `m.suffix()`
>    > `(*this)[n]` | `m[n]` for all non-negative integers `n < m.size()`
>    > `length(n)` | `m.length(n)` for all non-negative integers `n < m.size()`
>    > `position(n)` | `m.position(n)` for all non-negative integers `n < m.size()`

[2021-06-25; Daniel comments and provides new wording]

The revised wording has been rebased to N4892. It replaces the now obsolete Ensures: element by the Postconditions: element and also restores the copy constructor prototype that has been eliminated by P1722R2 as anchor point for the link to Table 140.

[2021-06-30; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Add a new paragraph at the beginning of 28.6.9.2 [re.results.const] as indicated:

   > -?- Table 140 [tab:re.results.const] lists the postconditions of `match_results` copy/move constructors and copy/move assignment operators. For move operations, the results of the expressions depending on the parameter `m` denote the values they had before the respective function calls.

2. Modify 28.6.9.2 [re.results.const] as indicated:

   >     explicit match_results(const Allocator& a);

   > > -1- Postconditions: `ready()` returns `false`. `size()` returns `0`.

   >     match_results(const match_results& m);

   > > -?- Postconditions: As specified in Table 140.

   >     match_results(match_results&& m) noexcept;

   > > -2- Effects: The stored `Allocator` value is move constructed from `m.get_allocator()`.

   > > -3- Postconditions: As specified in Table 140.

   >     match_results& operator=(const match_results& m);

   > > -4- Postconditions: As specified in Table 140.

   >     match_results& operator=(match_results&& m);

   > > -5- Postconditions: As specified in Table 140.

3. Modify 28.6.9.2 [re.results.const], Table 140 — "`match_results` assignment operator effects", [tab:re.results.const], as indicated:

   > Table 140 — `match_results` assignment operator effectscopy/move operation postconditions
   > Element | Value
   > --- | ---
   > `ready()` | `m.ready()`
   > `size()` | `m.size()`
   > `str(n)` | `m.str(n)` for all non-negative integers `n < m.size()`
   > `prefix()` | `m.prefix()`
   > `suffix()` | `m.suffix()`
   > `(*this)[n]` | `m[n]` for all non-negative integers `n < m.size()`
   > `length(n)` | `m.length(n)` for all non-negative integers `n < m.size()`
   > `position(n)` | `m.position(n)` for all non-negative integers `n < m.size()`

### 2192(i). Validity and return type of `std::abs(0u)` is unclear

Section: 29.7 [c.math] Status: C++17 Submitter: Daniel Krügler Opened: 2012-10-02 Last modified: 2017-07-30

Priority: 2

View all other issues in [c.math].

View all issues with C++17 status.

Discussion:

In C++03 the following two programs are invalid:

1. >     #include <cmath>

   >     int main() {
   >       std::abs(0u);
   >     }

2. >     #include <cstdlib>

   >     int main() {
   >       std::abs(0u);
   >     }

because none of the `std::abs()` overloads is a best match.

In C++11 the additional "sufficient overload" rule from 29.7 [c.math] p11 (see also LWG 2086(i)) can be read to be applicable to the `std::abs()` overloads as well, which can lead to the following possible conclusions:

1. The program

   >     #include <type_traits>
   >     #include <cmath>

   >     static_assert(std::is_same<decltype(std::abs(0u)), double>(), "Oops");

   >     int main() {
   >       std::abs(0u); // Calls std::abs(double)
   >     }

   is required to be well-formed, because of sub-bullet 2 ("[..] or an integer type [..]") of 29.7 [c.math] p11 (Note that the current resolution of LWG 2086(i) doesn't fix this problem).

2. Any translation unit including both `<cmath>` and `<cstdlib>` might be ill-formed because of two conflicting requirements for the return type of the overload `std::abs(int)`.

It seems to me that at least the second outcome is not intended, personally I think that both are unfortunate: In contrast to all other floating-point functions explicitly listed in sub-clause 29.7 [c.math], the `abs` overloads have a special and well-defined meaning for signed integers and thus have explicit overloads returning a signed integral type. I also believe that there is no problem accepting that `std::fabs(0u)` is well-defined with return type `double`, because the leading 'f' clearly signals that we have a floating point function here. But the expected return type of `std::abs(0u)` seems less than clear to me. A very reasonable answer could be that this has the same type as its argument type, alternatively it could be a reasonably chosen signed integer type, or a floating point type. It should also be noted, that the corresponding "generic type function" rule set from C99/C1x in 7.25 p2+3 is restricted to the floating-point functions from `<math.h>` and `<complex.h>`, so cannot be applied to the `abs` functions (but to the `fabs` functions!).

Selecting a signed integer return type for unsigned input values can also problematic: The directly corresponding signed integer type would give half of the possible argument values an implementation-defined result value. Choosing the first signed integer value that can represent all positive values would solve this problem for `unsigned int`, but there would be no clear answer for the input type `std::uintmax_t`.

Based on this it seems to me that the C++03 state in regard to unsigned integer values was the better situation, alerting the user that this code is ambigious at the moment (This might be change with different core-language rules as described in N3387).

[2013-04-20, Bristol]

Resolution: leave as new and bring it back in Chicago.

[2013-09 Chicago]

This issue also relates to LWG 2294(i)

STL: these two issues should be bundled

Stefanus: do what Pete says, and add overloads for unsigned to return directly

STL: agree Consensus that this is an issue

Walter: motion to move to Open

STL: no wording for 2294(i)

Stefanus: move to open and note the 2 issues are related and should be moved together

Stefanus: add and define unsigned versions of `abs()`

[2014-02-03 Howard comments]

Defining `abs()` for unsigned integers is a bad idea. Doing so would turn compile time errors into run time errors, especially in C++ where we have templates, and the types involved are not always apparent to the programmer at design time. For example, consider:

>     template <class Int>
>     Int
>     analyze(Int x, Int y)
>     {
>       // ...
>       if (std::abs(x - y) < threshold)
>       {
>         // ...
>       }
>       // ...
>     }

`std::abs(expr)` is often used to ask: Are these two numbers sufficiently close? When the assumption is that the two numbers are signed (either signed integral, or floating point), the logic is sound. But when the same logic is accidentally used with an arithmetic type not capable of representing negative numbers, and especially if unsigned overflow will silently happen, then the logic is no longer correct:

>     auto i = analyze(20u, 21u);  // Today a compile time error
>         // But with abs(unsigned) becomes a run time error

This is not idle speculation. Search the net for "`abs unsigned`" here or here.

In C++11, `chrono` `duration`s and `time_point`s are allowed to be based on unsigned integers. Taking the absolute value of the difference of two such `time_point`s would be easy to accidentally do (say in code templated on `time_point`s), and would certainly be a logic bug, caught at compile time unless we provide the error prone `abs(unsigned)`.

[2015-02, Cologne]

GR: Do we want to make the changes to both `<cmath>` and `<cstdlib>`?
AM: I think so; we should provide consistent overloads.
GR: Then we're imposing restrictions on what users put in the global namespace.
AM: I'm not so worried about that. Users already know not to use C library names.
VV: So what are we going to do about unsigned integers? AM: We will say that they are ill-formed.
AM: Does anyone volunteer to send updated wording to Daniel? GR, can you do it? GR: Sure.
GR: To clarify: we want to make unsigned types ill-formed?
AM: With promotion rank at least `unsigned int`.
GR: And NL suggests to just list those types.

Conclusion: Merge the resolution into a single issue. Previous resolution from Daniel [SUPERSEDED]:

> This wording is relative to N3376.

> 1. Change 29.7 [c.math] p11 as indicated:

>    > -11- Moreover, except for the `abs` functions, there shall be additional overloads sufficient to ensure:

>    > […]

[2015-03-03, Geoffrey Romer provides improved wording]

In the following I've drafted combined wording to resolve LWG 2192(i) and 2294(i). Note that the first two paragraphs are taken verbatim from the P/R of LWG 2294(i), but the third is newly drafted:

[2015-05-05 Lenexa: Howard to draft updated wording]

[2015-09-11: Howard updated wording]

[2015-10, Kona Saturday afternoon]

HH: abs() for unsigned types is really dangerous. People often use abs(x - y), which would be a disaster.

TK: That's why you need a two-argument abs_diff(x, y), especially for unsigned types.

JW: Lawrence has a proposal for abs_diff in the mailing.

STL: As an alternative to considering promotions, I would just ban all unsigned types, even unsigned char.

STL: Does the PR change any implementation? Is the final paragraph just a consequence?

HH: It's a consequence. It could just be a note.

VV: Ship it as is.

STL: Editorial: capitalize the first letter in the Note.

Move to Tentatively ready.

Proposed resolution:

This wording is relative to N4527.

1. Insert the following new paragraphs after 29.7 [c.math] p7:

   > -6- In addition to the `int` versions of certain math functions in `<cstdlib>`, C++ adds `long` and `long long` overloaded versions of these functions, with the same semantics.

   > -7- The added signatures are:

   > >     long abs(long); // labs()
   > >     long long abs(long long); // llabs()
   > >     ldiv_t div(long, long); // ldiv()
   > >     lldiv_t div(long long, long long); // lldiv()

   > -?- To avoid ambiguities, C++ also adds the following overloads of `abs()` to `<cstdlib>`, with the semantics defined in `<cmath>`:

   > >     float abs(float);
   > >     double abs(double);
   > >     long double abs(long double);

   > -?- To avoid ambiguities, C++ also adds the following overloads of `abs()` to `<cmath>`, with the semantics defined in `<cstdlib>`:

   > >     int abs(int);
   > >     long abs(long);
   > >     long long abs(long long);

   > -?- If `abs()` is called with an argument of type `X` for which `is_unsigned<X>::value` is `true` and if `X` cannot be converted to `int` by integral promotion (7.3.7 [conv.prom]), the program is ill-formed. [Note: arguments that can be promoted to `int` are permitted for compatibility with C. — end note]

### 2193(i). Default constructors for standard library containers are explicit

Section: 23 [containers] Status: C++14 Submitter: Richard Smith Opened: 2012-10-04 Last modified: 2017-07-05

Priority: 1

View other active issues in [containers].

View all other issues in [containers].

View all issues with C++14 status.

Discussion:

Most (all?) of the standard library containers have explicit default constructors. Consequently:

>     std::set<int> s1 = { 1, 2 }; // ok
>     std::set<int> s2 = { 1 }; // ok
>     std::set<int> s3 = {}; // ill-formed, copy-list-initialization selected an explicit constructor

Note that Clang + libc++ rejects the declaration of `s3` for this reason. This cannot possibly match the intent.

Suggested fix: apply this transformation throughout the standard library:

>     set() : set(Compare()) {}
>     explicit set(const Compare& comp = Compare(),
>                  const Allocator& = Allocator());

[ 2012-10-06: Daniel adds concrete wording. ]

[2012, Portland: Move to Open]

This may be an issue better solved by a core language tweak. Throw the issue over to EWG and see whether they believe the issue is better resolved in Core or Library.

AJM suggest we spawn a new status of 'EWG' to handle such issues - and will move this issue appropriately when the software can record such resolutions.

[2013-08-27, Joaquín M López Muñoz comments:]

For the record, I'd like to point out that the resolution proposed by the submitter, namely replacing

>     explicit basic_string(const Allocator& a = Allocator());

by

>     basic_string() : basic_string(Allocator()) {}
>     explicit basic_string(const Allocator& a);

(and similarly for other container and container-like classes) might introduce a potential backwards-compatibility problem related with explicit instantiation. Consider for instance

>     struct my_allocator
>     {
>       my_allocator(...); // no default ctor
>       ...
>     };

>     template class std::basic_string<char, std::char_traits<char>, my_allocator<char>>;

This (which I understand is currently a valid explicit instantiation of `std::basic_string`) will break if `std::basic_string` ctors are modified as proposed by this issue, since `my_allocator` doesn't have a default ctor.

[2013-10-06, Daniel comments:]

Issue 2303(i) describes the more general problem related to explicit instantiation requests in the current library and may help to solve this problem here as well.

[2014-02-13, Issaquah, Jonathan revises wording]

Previous resolution from Daniel [SUPERSEDED]:

> This wording is relative to N3376.

> The more general criterion for performing the suggested transformation was: Any type with an initializer-list constructor that also has an explicit default constructor.

> 1. Change class template `basic_string` synopsis, 27.4.3 [basic.string] p5 as indicated:

>    >     basic_string() : basic_string(Allocator()) {}
>    >     explicit basic_string(const Allocator& a = Allocator());

> 2. Change 27.4.3.3 [string.cons] before p1 as indicated:

>    >     explicit basic_string(const Allocator& a = Allocator());

> 3. Change class template `deque` synopsis, 23.3.5.1 [deque.overview] p2 as indicated:

>    >     deque() : deque(Allocator()) {}
>    >     explicit deque(const Allocator& = Allocator());

> 4. Change 23.3.5.2 [deque.cons] before p1 as indicated:

>    >     explicit deque(const Allocator& = Allocator());

> 5. Change class template `forward_list` synopsis, [forwardlist.overview] p3 as indicated:

>    >     forward_list() : forward_list(Allocator()) {}
>    >     explicit forward_list(const Allocator& = Allocator());

> 6. Change [forwardlist.cons] before p1 as indicated:

>    >     explicit forward_list(const Allocator& = Allocator());

> 7. Change class template `list` synopsis, 23.3.11.1 [list.overview] p2 as indicated:

>    >     list() : list(Allocator()) {}
>    >     explicit list(const Allocator& = Allocator());

> 8. Change 23.3.11.2 [list.cons] before p1 as indicated:

>    >     explicit list(const Allocator& = Allocator());

> 9. Change class template `vector` synopsis, 23.3.13.1 [vector.overview] p2 as indicated:

>    >     vector() : vector(Allocator()) {}
>    >     explicit vector(const Allocator& = Allocator());

> 10. Change 23.3.13.2 [vector.cons] before p1 as indicated:

>     >     explicit vector(const Allocator& = Allocator());

> 11. Change class template specialization `vector<bool>` synopsis, 23.3.14 [vector.bool] p1 as indicated:

>     >     vector() : vector(Allocator()) {}
>     >     explicit vector(const Allocator& = Allocator());

> 12. Change class template `map` synopsis, 23.4.3.1 [map.overview] p2 as indicated:

>     >     map() : map(Compare()) {}
>     >     explicit map(const Compare& comp = Compare(),
>     >                  const Allocator& = Allocator());

> 13. Change 23.4.3.2 [map.cons] before p1 as indicated:

>     >     explicit map(const Compare& comp = Compare(),
>     >                  const Allocator& = Allocator());

> 14. Change class template `multimap` synopsis, 23.4.4.1 [multimap.overview] p2 as indicated:

>     >     multimap() : multimap(Compare()) {}
>     >     explicit multimap(const Compare& comp = Compare(),
>     >                       const Allocator& = Allocator());

> 15. Change 23.4.4.2 [multimap.cons] before p1 as indicated:

>     >     explicit multimap(const Compare& comp = Compare(),
>     >                       const Allocator& = Allocator());

> 16. Change class template `set` synopsis, 23.4.6.1 [set.overview] p2 as indicated:

>     >     set() : set(Compare()) {}
>     >     explicit set(const Compare& comp = Compare(),
>     >                  const Allocator& = Allocator());

> 17. Change 23.4.6.2 [set.cons] before p1 as indicated:

>     >     explicit set(const Compare& comp = Compare(),
>     >                  const Allocator& = Allocator());

> 18. Change class template `multiset` synopsis, 23.4.7.1 [multiset.overview] p2 as indicated:

>     >     multiset() : multiset(Compare()) {}
>     >     explicit multiset(const Compare& comp = Compare(),
>     >                       const Allocator& = Allocator());

> 19. Change 23.4.7.2 [multiset.cons] before p1 as indicated:

>     >     explicit multiset(const Compare& comp = Compare(),
>     >                       const Allocator& = Allocator());

> 20. Change class template `unordered_map` synopsis, 23.5.3.1 [unord.map.overview] p3 as indicated:

>     >     unordered_map() : unordered_map(see below) {}
>     >     explicit unordered_map(size_type n = see below,
>     >                            const hasher& hf = hasher(),
>     >                            const key_equal& eql = key_equal(),
>     >                            const allocator_type& a = allocator_type());

> 21. Change 23.5.3.2 [unord.map.cnstr] before p1 as indicated:

>     >     unordered_map() : unordered_map(see below) {}
>     >     explicit unordered_map(size_type n = see below,
>     >                            const hasher& hf = hasher(),
>     >                            const key_equal& eql = key_equal(),
>     >                            const allocator_type& a = allocator_type());

> 22. Change class template `unordered_multimap` synopsis, 23.5.4.1 [unord.multimap.overview] p3 as indicated:

>     >     unordered_multimap() : unordered_multimap(see below) {}
>     >     explicit unordered_multimap(size_type n = see below,
>     >                                 const hasher& hf = hasher(),
>     >                                 const key_equal& eql = key_equal(),
>     >                                 const allocator_type& a = allocator_type());

> 23. Change 23.5.4.2 [unord.multimap.cnstr] before p1 as indicated:

>     >     unordered_multimap() : unordered_multimap(see below) {}
>     >     explicit unordered_multimap(size_type n = see below,
>     >                                 const hasher& hf = hasher(),
>     >                                 const key_equal& eql = key_equal(),
>     >                                 const allocator_type& a = allocator_type());

> 24. Change class template `unordered_set` synopsis, 23.5.6.1 [unord.set.overview] p3 as indicated:

>     >     unordered_set() : unordered_set(see below) {}
>     >     explicit unordered_set(size_type n = see below,
>     >                            const hasher& hf = hasher(),
>     >                            const key_equal& eql = key_equal(),
>     >                            const allocator_type& a = allocator_type());

> 25. Change 23.5.6.2 [unord.set.cnstr] before p1 as indicated:

>     >     unordered_set() : unordered_set(see below) {}
>     >     explicit unordered_set(size_type n = see below,
>     >                            const hasher& hf = hasher(),
>     >                            const key_equal& eql = key_equal(),
>     >                            const allocator_type& a = allocator_type());

> 26. Change class template `unordered_multiset` synopsis, 23.5.7.1 [unord.multiset.overview] p3 as indicated:

>     >     unordered_multiset() : unordered_multiset(see below) {}
>     >     explicit unordered_multiset(size_type n = see below,
>     >                                 const hasher& hf = hasher(),
>     >                                 const key_equal& eql = key_equal(),
>     >                                 const allocator_type& a = allocator_type());

> 27. Change 23.5.7.2 [unord.multiset.cnstr] before p1 as indicated:

>     >     unordered_multiset() : unordered_multiset(see below) {}
>     >     explicit unordered_multiset(size_type n = see below,
>     >                                 const hasher& hf = hasher(),
>     >                                 const key_equal& eql = key_equal(),
>     >                                 const allocator_type& a = allocator_type());

[Issaquah 2014-02-11: Move to Immediate after final review]

Proposed resolution:

This wording is relative to N3376.

The more general criterion for performing the suggested transformation was: Any type with an initializer-list constructor that also has an explicit default constructor.

1. Change class template `basic_string` synopsis, 27.4.3 [basic.string] p5 as indicated:

   >     basic_string() : basic_string(Allocator()) { }
   >     explicit basic_string(const Allocator& a = Allocator());

2. Change 27.4.3.3 [string.cons] before p1 as indicated:

   >     explicit basic_string(const Allocator& a = Allocator());

3. Change class template `deque` synopsis, 23.3.5.1 [deque.overview] p2 as indicated:

   >     deque() : deque(Allocator()) { }
   >     explicit deque(const Allocator& = Allocator());

4. Change 23.3.5.2 [deque.cons] before p1 as indicated:

   >     explicit deque(const Allocator& = Allocator());

5. Change class template `forward_list` synopsis, [forwardlist.overview] p3 as indicated:

   >     forward_list() : forward_list(Allocator()) { }
   >     explicit forward_list(const Allocator& = Allocator());

6. Change [forwardlist.cons] before p1 as indicated:

   >     explicit forward_list(const Allocator& = Allocator());

7. Change class template `list` synopsis, 23.3.11.1 [list.overview] p2 as indicated:

   >     list() : list(Allocator()) { }
   >     explicit list(const Allocator& = Allocator());

8. Change 23.3.11.2 [list.cons] before p1 as indicated:

   >     explicit list(const Allocator& = Allocator());

9. Change class template `vector` synopsis, 23.3.13.1 [vector.overview] p2 as indicated:

   >     vector() : vector(Allocator()) { }
   >     explicit vector(const Allocator& = Allocator());

10. Change 23.3.13.2 [vector.cons] before p1 as indicated:

    >     explicit vector(const Allocator& = Allocator());

11. Change class template specialization `vector<bool>` synopsis, 23.3.14 [vector.bool] p1 as indicated:

    >     vector() : vector(Allocator()) { }
    >     explicit vector(const Allocator& = Allocator());

12. Change class template `map` synopsis, 23.4.3.1 [map.overview] p2 as indicated:

    >     map() : map(Compare()) { }
    >     explicit map(const Compare& comp = Compare(),
    >                  const Allocator& = Allocator());

13. Change 23.4.3.2 [map.cons] before p1 as indicated:

    >     explicit map(const Compare& comp = Compare(),
    >                  const Allocator& = Allocator());

14. Change class template `multimap` synopsis, 23.4.4.1 [multimap.overview] p2 as indicated:

    >     multimap() : multimap(Compare()) { }
    >     explicit multimap(const Compare& comp = Compare(),
    >                       const Allocator& = Allocator());

15. Change 23.4.4.2 [multimap.cons] before p1 as indicated:

    >     explicit multimap(const Compare& comp = Compare(),
    >                       const Allocator& = Allocator());

16. Change class template `set` synopsis, 23.4.6.1 [set.overview] p2 as indicated:

    >     set() : set(Compare()) { }
    >     explicit set(const Compare& comp = Compare(),
    >                  const Allocator& = Allocator());

17. Change 23.4.6.2 [set.cons] before p1 as indicated:

    >     explicit set(const Compare& comp = Compare(),
    >                  const Allocator& = Allocator());

18. Change class template `multiset` synopsis, 23.4.7.1 [multiset.overview] p2 as indicated:

    >     multiset() : multiset(Compare()) { }
    >     explicit multiset(const Compare& comp = Compare(),
    >                       const Allocator& = Allocator());

19. Change 23.4.7.2 [multiset.cons] before p1 as indicated:

    >     explicit multiset(const Compare& comp = Compare(),
    >                       const Allocator& = Allocator());

20. Change class template `unordered_map` synopsis, 23.5.3.1 [unord.map.overview] p3 as indicated:

    >     unordered_map();
    >     explicit unordered_map(size_type n = see below,
    >                            const hasher& hf = hasher(),
    >                            const key_equal& eql = key_equal(),
    >                            const allocator_type& a = allocator_type());

21. Change 23.5.3.2 [unord.map.cnstr] before p1 as indicated:

    >     unordered_map() : unordered_map(size_type(see below)) { }
    >     explicit unordered_map(size_type n = see below,
    >                            const hasher& hf = hasher(),
    >                            const key_equal& eql = key_equal(),
    >                            const allocator_type& a = allocator_type());

    > -1- Effects: Constructs an empty `unordered_map` using the specified hash function, key equality func-
    > tion, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    > the number of buckets is implementation-defined. `max_load_factor()` returns 1.0.

22. Change class template `unordered_multimap` synopsis, 23.5.4.1 [unord.multimap.overview] p3 as indicated:

    >     unordered_multimap();
    >     explicit unordered_multimap(size_type n = see below,
    >                                 const hasher& hf = hasher(),
    >                                 const key_equal& eql = key_equal(),
    >                                 const allocator_type& a = allocator_type());

23. Change 23.5.4.2 [unord.multimap.cnstr] before p1 as indicated:

    >     unordered_multimap() : unordered_multimap(size_type(see below)) { }
    >     explicit unordered_multimap(size_type n = see below,
    >                                 const hasher& hf = hasher(),
    >                                 const key_equal& eql = key_equal(),
    >                                 const allocator_type& a = allocator_type());

    > -1- Effects: Constructs an empty `unordered_multimap` using the specified hash function, key equality
    > function, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    > the number of buckets is implementation-defined. `max_load_factor()` returns 1.0.

24. Change class template `unordered_set` synopsis, 23.5.6.1 [unord.set.overview] p3 as indicated:

    >     unordered_set();
    >     explicit unordered_set(size_type n = see below,
    >                            const hasher& hf = hasher(),
    >                            const key_equal& eql = key_equal(),
    >                            const allocator_type& a = allocator_type());

25. Change 23.5.6.2 [unord.set.cnstr] before p1 as indicated:

    >     unordered_set() : unordered_set(size_type(see below)) { }
    >     explicit unordered_set(size_type n = see below,
    >                            const hasher& hf = hasher(),
    >                            const key_equal& eql = key_equal(),
    >                            const allocator_type& a = allocator_type());

    > -1- Effects: Constructs an empty `unordered_set` using the specified hash function, key equality func-
    > tion, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    > the number of buckets is implementation-defined. `max_load_factor()` returns 1.0.

26. Change class template `unordered_multiset` synopsis, 23.5.7.1 [unord.multiset.overview] p3 as indicated:

    >     unordered_multiset();
    >     explicit unordered_multiset(size_type n = see below,
    >                                 const hasher& hf = hasher(),
    >                                 const key_equal& eql = key_equal(),
    >                                 const allocator_type& a = allocator_type());

27. Change 23.5.7.2 [unord.multiset.cnstr] before p1 as indicated:

    >     unordered_multiset() : unordered_multiset(size_type(see below)) { }
    >     explicit unordered_multiset(size_type n = see below,
    >                                 const hasher& hf = hasher(),
    >                                 const key_equal& eql = key_equal(),
    >                                 const allocator_type& a = allocator_type());

    > -1- Effects: Constructs an empty `unordered_multiset` using the specified hash function, key equality
    > function, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    > the number of buckets is implementation-defined. `max_load_factor()` returns 1.0.

### 2194(i). Impossible container requirements for adaptor types

Section: 23.6 [container.adaptors] Status: C++14 Submitter: Sebastian Mach Opened: 2012-10-05 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.adaptors].

View all issues with C++14 status.

Discussion:

The `stack` class template does not have an member type `iterator`, and therefore instantiations do not meet the general container requirements as described in 23.2.2 [container.requirements.general]. But 23.6.1 [container.adaptors.general] p1 says:

> The headers `<queue>` and `<stack>` define the container adaptors `queue`, `priority_queue`, and `stack`. These container adaptors meet the requirements for sequence containers.

Since sequence containers is a subset of general containers, this imposes requirements on the container adaptors that are not satisfied.

Daniel Krügler: The wording change was performed as an editorial reorganization as requested by GB 116 occuring first in N3242, as a side-effect it does now make the previous implicit C++03 classification to [lib.sequences]/1 more obvious. As the NB comment noticed, the adaptors really are not sequences nor containers, so this wording needs to be fixed. The most simple way to realize that is to strike the offending sentence.

[ Daniel adds concrete wording. ]

[2013-04-20, Bristol]

Unanimous consensus that `queue` and `stack` are not meant to be sequences.

Decision: move to tentatively ready

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3376.

1. Change 23.6.1 [container.adaptors.general] p1 as indicated:

   > -1- The headers `<queue>` and `<stack>` define the container adaptors `queue`, `priority_queue`, and `stack`. These container adaptors meet the requirements for sequence containers.

### 2195(i). Missing constructors for `match_results`

Section: 28.6.9 [re.results] Status: C++23 Submitter: Daniel Krügler Opened: 2012-10-06 Last modified: 2023-11-22

Priority: 3

View all other issues in [re.results].

View all issues with C++23 status.

Discussion:

The requirement expressed in 28.6.9 [re.results] p2

> The class template `match_results` shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 23.2.4 [sequence.reqmts], except that only operations defined for const-qualified sequence containers are supported.

can be read to require the existence of the described constructors from as well, but they do not exist in the synopsis.

The missing sequence constructors are:

>     match_results(initializer_list<value_type>);
>     match_results(size_type, const value_type&);
>     template<class InputIterator> match_results(InputIterator, InputIterator);

The missing allocator-aware container constructors are:

>     match_results(const match_results&, const Allocator&);
>     match_results(match_results&&, const Allocator&);

It should be clarified, whether (a) constructors are an exception of above mentioned operations or (b) whether at least some of them (like those accepting a `match_results` value and an allocator) should be added.

As visible in several places of the standard (including the core language), constructors seem usually to be considered as "operations" and they certainly can be invoked for const-qualified objects.

The below given proposed resolution applies only the minimum necessary fix, i.e. it excludes constructors from above requirement.

[2013-04-20, Bristol]

Check current implementations to see what they do and, possibly, write a paper.

[2013-09 Chicago]

Ask Daniel to update the proposed wording to include the allocator copy and move constructors.

[2014-01-18 Daniel changes proposed resolution]

Previous resolution from Daniel [SUPERSEDED]:

> 1. Change 28.6.9 [re.results] p2 as indicated:

>    > The class template `match_results` shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 23.2.4 [sequence.reqmts], except that only operations defined for const-qualified sequence containers that are not constructors are supported.

[2015-05-06 Lenexa]

MC passes important knowledge to EF.

VV, RP: Looks good.

TK: Second form should be conditionally noexcept

JY: Sequence constructors are not here, but mentioned in the issue writeup. Why?

TK: That would have been fixed by the superseded wording.

JW: How does this interact with Mike Spertus' allocator-aware regexes? [...] Perhaps it doesn't.

JW: Can't create match_results, want both old and new resolution.

JY: It's problematic that users can't create these, but not this issue.

VV: Why conditional noexcept?

MC: Allocator move might throw.

JW: Update superseded wording to "only non-constructor operations that are"?

MC: Only keep superseded, but append "and the means of constructing match_results are limited to [...]"?

JY: Bullet 4 paragraph 2 needs to address the allocator constructor.

Assigned to JW for drafting.

[2015-10, Kona Saturday afternoon]

STL: I want Mike Spertus to be aware of this issue.

Previous resolution from Daniel [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 28.6.9 [re.results] p4, class template `match_results` synopsis, as indicated:

>    >     […]
>    >     // 28.10.1, construct/copy/destroy:
>    >     explicit match_results(const Allocator& a = Allocator());
>    >     match_results(const match_results& m);
>    >     match_results(const match_results& m, const Allocator& a);
>    >     match_results(match_results&& m) noexcept;
>    >     match_results(match_results&& m, const Allocator& a) noexcept;
>    >     […]

> 2. Change 28.6.9.2 [re.results.const] as indicated: [Drafting note: Paragraph 6 as currently written, makes not much sense, because the `noexcept` does not allow any exception to propagate. Further-on, the allocator requirements do not allow for throwing move constructors. Deleting it seems to be near to editorial — end drafting note]

>    >     match_results(const match_results& m);
>    >     match_results(const match_results& m, const Allocator& a);

>    > > -4- Effects: Constructs an object of class `match_results`, as a copy of `m`.

>    >     match_results(match_results&& m) noexcept;
>    >     match_results(match_results&& m, const Allocator& a) noexcept;

>    > > -5- Effects: Move-constructs an object of class `match_results` from `m` satisfying the same postconditions as Table 142. AdditionallyFor the first form, the stored `Allocator` value is move constructed from `m.get_allocator()`.

>    > > -6- Throws: Nothing if the allocator's move constructor throws nothing.

[2019-03-27 Jonathan updates proposed resolution]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4810.

> These edits overlap with the proposed resolution of 2191(i) but it should be obvious how to resolve the conflicts. Both resolutions remove the word "Additionally" from p4. Issue 2191 removes the entire Throws: element in p5 but this issue replaces it with different text that applies to the new constructor only.

> 1. Change 28.6.9 [re.results] p4, class template `match_results` synopsis, as indicated:

>    >     […]
>    >     // 30.10.1, construct/copy/destroy:
>    >     explicit match_results(const Allocator& a = Allocator());
>    >     match_results(const match_results& m);
>    >     match_results(const match_results& m, const Allocator& a);
>    >     match_results(match_results&& m) noexcept;
>    >     match_results(match_results&& m, const Allocator& a);
>    >     […]

> 2. Change 28.6.9.2 [re.results.const] as indicated:

>    >     match_results(const match_results& m);
>    >     match_results(const match_results& m, const Allocator& a);

>    > > -3- Effects: Constructs an object of class `match_results`, as a copy of `m`. For the second form, the stored `Allocator` value is constructed from `a`.

>    >     match_results(match_results&& m) noexcept;
>    >     match_results(match_results&& m, const Allocator& a);

>    > > -4- Effects: Move-constructs an object of class `match_results` from `m` satisfying the same postconditions as Table 128. AdditionallyFor the first form, the stored `Allocator` value is move constructed from `m.get_allocator()`. For the second form, the stored `Allocator` value is constructed from `a`.

>    > > -6- Throws: Nothing. The second form throws nothing if `a == m.get_allocator()`.

[2022-11-06; Daniel syncs wording with recent working draft]

To ensure that all constructors are consistent in regard to the information about how the stored allocator is constructed, more wording is added. This harmonizes with the way how we specify the individual container constructors (Such as `vector`) even though 23.2.2.5 [container.alloc.reqmts] already provides some guarantees. For the copy-constructor we intentionally refer to 23.2.2.2 [container.reqmts] so that we don't need to repeat what is said there.

[Kona 2022-11-08; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Change 28.6.9 [re.results], class template `match_results` synopsis, as indicated:

   >     […]
   >     // 28.6.9.2 [re.results.const], construct/copy/destroy:
   >     match_results() : match_results(Allocator()) {}
   >     explicit match_results(const Allocator& a);
   >     match_results(const match_results& m);
   >     match_results(const match_results& m, const Allocator& a);
   >     match_results(match_results&& m) noexcept;
   >     match_results(match_results&& m, const Allocator& a);
   >     […]

2. Change 28.6.9.2 [re.results.const] as indicated:

   >     explicit match_results(const Allocator& a);

   > > -?- Effects: The stored `Allocator` value is constructed from `a`.

   > > -2- Postconditions: `ready()` returns `false`. `size()` returns `0`.

   >     match_results(const match_results& m);
   >     match_results(const match_results& m, const Allocator& a);

   > > -?- Effects: For the first form, the stored `Allocator` value is obtained as specified in 23.2.2.2 [container.reqmts]. For the second form, the stored `Allocator` value is constructed from `a`.

   > > -3- Postconditions: As specified in Table 142 [tab:re.results.const].

   >     match_results(match_results&& m) noexcept;
   >     match_results(match_results&& m, const Allocator& a);

   > > -4- Effects: For the first form, tThe stored `Allocator` value is move constructed from `m.get_allocator()`. For the second form, the stored `Allocator` value is constructed from `a`.

   > > -5- Postconditions: As specified in Table 142 [tab:re.results.const].

   > > -?- Throws: The second form throws nothing if `a == m.get_allocator()` is `true`.

### 2196(i). Specification of `is_*[copy/move]_[constructible/assignable]` unclear for non-referencable types

Section: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2012-10-06 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

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Discussion:

The pre-conditions for the type `is_copy_constructible` allow for template argument types were the language forbids forming a reference, namely `void` types and function types that have cv-qualifiers or a ref-qualifier.

But the current wording in Table 49 defining the predicate condition,

> `is_constructible<T, const T&>::value` is `true`.

leaves it open whether such argument types would (a) create a well-formed instantiation of the trait template or if so (b) what the outcome of the trait evaluation would be, as an example consider `std::is_copy_constructible<void>`.

Current implementations differ, e.g. gcc accepts the instantiation and returns a false result, VS 2012 also accepts the instantiation but returns true.

I would suggest that the wording clarifies that the instantiation would be valid for these types and I also would strongly prefer the outcome that the trait would always return false for these types. The latter seems rather natural to me, because there is no way to define a variable of `void` type or of function type at all, so it would be surprising to return a positive result for copy or move construction if no other construction could succeed. It is also not possible to assign to a any of these values (because there is no way to form lvalues of them), so the same argumentation can be applied to the `is_copy/move_assignable` traits as well.

To reduce the amount of wording changes and repetitions, I suggest to define the term referenceable type in sub-clause [definitions] or alternatively in the core language to describe types to which references can be created via a typedef name. This definition corresponds to what the support concept `ReferentType` intended to describe during concept time.

In addition, LWG issue 2101(i) can also take advantage of the definition of a referenceable type.

If the proposed resolution for LWG issue 2101(i) would be accepted, there is an alternative solution possible with the same effects. Now we would be able to use the now always well-formed instantiation of `std::add_lvalue_reference` to modify the current definition of `is_copy_constructible` to

> `is_constructible<T,
> typename add_lvalue_reference<
> typename add_const<T>::type>::type>::value` is true.

and similar changes for the other affected traits.

[2012-10 Portland: Move to Open]

Referencable-type should be defined as "something that can be bound into a reference" or similar, rather than a list of types where that is true today. We can then provide the list of known types that cannot be bound as examples that do not qualify in a note.

Otherwise we are happy with the wording. AJM to redraft the definition and move to Review.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Add the following new definition to [definitions] as indicated:

   referenceable type [defns.referenceable]

   > An object type, a function type that does not have cv-qualifiers or a ref-qualifier, or a reference type. [Note: The term describes a type to which a reference can be created, including reference types. — end note]

2. Change Table 49 as indicated:

   Table 49 — Type property predicates
   Template | Condition | Preconditions
   --- | --- | ---
   `template <class T>` `struct is_copy_constructible;` | For a referenceable type `T`, the same result as `is_constructible<T,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_move_constructible;` | For a referenceable type `T`, the same result as `is_constructible<T,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_copy_assignable;` | For a referenceable type `T`, the same result as `is_assignable<T&,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_move_assignable;` | For a referenceable type `T`, the same result as `is_assignable<T&,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_trivially_copy_constructible;` | For a referenceable type `T`, the same result as `is_trivially_constructible<T,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_trivially_move_constructible;` | For a referenceable type `T`, the same result as `is_trivially_constructible<T,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_trivially_copy_assignable;` | For a referenceable type `T`, the same result as `is_trivially_assignable<T&,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_trivially_move_assignable;` | For a referenceable type `T`, the same result as `is_trivially_assignable<T&,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_nothrow_copy_constructible;` | For a referenceable type `T`, the same result as `is_nothrow_constructible<T,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_nothrow_move_constructible;` | For a referenceable type `T`, the same result as `is_nothrow_constructible<T,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_nothrow_copy_assignable;` | For a referenceable type `T`, the same result as `is_nothrow_assignable<T&,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_nothrow_move_assignable;` | For a referenceable type `T`, the same result as `is_nothrow_assignable<T&,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.

### 2197(i). Specification of `is_[un]signed` unclear for non-arithmetic types

Section: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2012-10-07 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

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Discussion:

The pre-conditions for the trait `is_signed` allow for any types as template arguments, including non-arithmetic ones.

But the current wording in Table 49 defining the predicate condition,

>     is_arithmetic<T>::value && T(-1) < T(0)

looks like real code and so leaves it open whether such argument types would create a well-formed instantiation of the trait template or not. As written this definition would lead to a hard instantiation error for a non-arithmetic type like e.g.

>     struct S {};

I would suggest that the wording clarifies that the instantiation would be valid for such types as well, by means of a specification that is not an exact code pattern. This also reflects how existing implementations behave.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change Table 49 as indicated:

   Table 49 — Type property predicates
   Template | Condition | Preconditions
   --- | --- | ---
   `template <class T>` `struct is_signed;` | If `is_arithmetic<T>::value &&` is true, the same result as `integral_constant<bool, T(-1) < T(0)>::value`; otherwise, false. |
   `template <class T>` `struct is_unsigned;` | If `is_arithmetic<T>::value &&` is true, the same result as `integral_constant<bool, T(0) < T(-1)>::value`; otherwise, false. |

### 2200(i). Data race avoidance for all containers, not only for sequences

Section: 23.2.3 [container.requirements.dataraces] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements.dataraces].

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Discussion:

23.2.3 [container.requirements.dataraces]/2 says "[…] implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting `vector<bool>`, are modified concurrently."

This should say "same container" instead of "same sequence", to avoid the interpretation that it only applies to sequence containers.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change 23.2.3 [container.requirements.dataraces]/2 as indicated:

   > -2- Notwithstanding (16.4.6.10 [res.on.data.races]), implementations are required to avoid data races when the contents of the contained object in different elements in the same sequencecontainer, excepting `vector<bool>`, are modified concurrently.

   > -3- [Note: For a `vector<int> x` with a size greater than one, `x[1] = 5` and `*x.begin() = 10` can be executed concurrently without a data race, but `x[0] = 5` and `*x.begin() = 10` executed concurrently may result in a data race. As an exception to the general rule, for a `vector<bool> y, y[0] = true` may race with `y[1] = true`. — end note ]

### 2203(i). `scoped_allocator_adaptor` uses wrong argument types for piecewise construction

Section: 20.6.4 [allocator.adaptor.members] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 20.6.4 [allocator.adaptor.members] paragraph 11 the effects clause says a tuple should be constructed with `inner_allocator_type()`, but that creates an rvalue which cannot bind to `inner_allocator_type&`, and would also be wrong if `this->inner_allocator() != inner_allocator_type()`. This could be considered editorial, since the current wording doesn't even compile.

Secondly, in the same paragraph, the tuple objects `xprime` and `yprime` seem to be lvalues and might be constructed by copying `x` and `y`. This prevents using `scoped_allocator` to construct pairs from arguments of move-only types. I believe the `tuple_cast()` expressions should use `std::move(x)` and `std::move(y)` to move from the incoming arguments (which are passed by value to candidates for moving) and the final sentence of the paragraph should be:

then calls `OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST (*this), p, piecewise_construct, std::move(xprime), std::move(yprime))`.

so that the objects are passed to `std::pair`'s piecewise constructor as rvalues and are eligible for moving into the constructor arguments. This could also be considered editorial, as the current wording prevents certain uses which were intended to be supported.

I've implemented these changes and can confirm they allow code to work that can't be compiled according to the current wording.

[2013-03-15 Issues Teleconference]

Moved to Review.

The resolution looks good, with wording provided by a recent implementer. However, it will take more time than the telecon allows to review with confidence, and we would like Pablo to at least take a look over the resolution and confirm that it matches the design intent.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change 20.6.4 [allocator.adaptor.members] paragraph 11 as indicated:

   > -11- Effects: Constructs a `tuple` object `xprime` from `x` by the following rules:

   > - If `uses_allocator<T1, inner_allocator_type>::value` is `false` and `is_constructible<T1, Args1...>::value` is `true`, then `xprime` is `x`.

   > - Otherwise, if `uses_allocator<T1, inner_allocator_type>::value` is `true` and `is_constructible<T1, allocator_arg_t, inner_allocator_type, Args1...>::value` is `true`, then `xprime` is `tuple_cat(tuple<allocator_arg_t, inner_allocator_type&>( allocator_arg, inner_allocator_type()), std::move(x))`.

   > - Otherwise, if `uses_allocator<T1, inner_allocator_type>::value` is `true` and `is_constructible<T1, Args1..., inner_allocator_type>::value` is `true`, then `xprime` is `tuple_cat(std::move(x), tuple<inner_allocator_type&>(inner_allocator_type()))`.

   > - Otherwise, the program is ill-formed.

   > and constructs a `tuple` object `yprime` from `y` by the following rules:

   > - If `uses_allocator<T2, inner_allocator_type>::value` is `false` and `is_constructible<T2, Args2...>::value` is `true`, then `yprime` is `y`.

   > - Otherwise, if `uses_allocator<T2, inner_allocator_type>::value` is `true` and `is_constructible<T2, allocator_arg_t, inner_allocator_type, Args2...>::value` is `true`, then `yprime` is `tuple_cat(tuple<allocator_arg_t, inner_allocator_type&>( allocator_arg, inner_allocator_type()), std::move(y))`.

   > - Otherwise, if `uses_allocator<T2, inner_allocator_type>::value` is `true` and `is_constructible<T2, Args2..., inner_allocator_type>::value` is `true`, then `yprime` is `tuple_cat(std::move(y), tuple<inner_allocator_type&>(inner_allocator_type()))`.

   > - Otherwise, the program is ill-formed.

   > then calls `OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, piecewise_construct, std::move(xprime), std::move(yprime))`.

### 2205(i). Problematic postconditions of `regex_match` and `regex_search`

Section: 28.6.10.2 [re.alg.match], 28.6.10.3 [re.alg.search] Status: C++14 Submitter: Pete Becker Opened: 2012-10-24 Last modified: 2017-07-05

Priority: 0

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Discussion:

Table 142 lists post-conditions on the `match_results` object when a call to `regex_match` succeeds. `regex_match` is required to match the entire target sequence. The post-condition for `m[0].matched` is "true if a full match was found." Since these are conditions for a successful search which is, by definition, a full match, the post-condition should be simply "true".

There's an analogous probem in Table 143: the condition for `m[0].matched` is "true if a match was found, false otherwise." But Table 143 gives post-conditions for a successful match, so the condition should be simply "true".

Furthermore, they have explicit requirements for `m[0].first`, `m[0].second`, and `m[0].matched`. They also have requirements for the other elements of `m`, described as `m[n].first`, `m[n].second`, and `m[n].matched`, in each case qualifying the value of `n` as "for `n < m.size()`". Since there is an explicit description for `n == 0`, this qualification should be "for `0 < n < m.size()`" in all 6 places.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3376.

1. Change Table 142 as indicated:

   Table 142 — Effects of `regex_match` algorithm
   Element | Value
   --- | ---
   `…`
   `m[0].first` | `first`
   `m[0].second` | `last`
   `m[0].matched` | `true` if a full match was found.
   `m[n].first` | For all integers `0 < n < m.size()`, the start of the sequence that matched sub-expression `n`. Alternatively, if subexpression `n` did not participate in the match, then `last`.
   `m[n].second` | For all integers `0 < n < m.size()`, the end of the sequence that matched sub-expression `n`. Alternatively, if sub-expression `n` did not participate in the match, then `last`.
   `m[n].matched` | For all integers `0 < n < m.size()`, `true` if sub-expression `n` participated in the match, `false` otherwise.

2. Change Table 143 as indicated:

   Table 143 — Effects of `regex_search` algorithm
   Element | Value
   --- | ---
   `…`
   `m[0].first` | The start of the sequence of characters that matched the regular expression
   `m[0].second` | The end of the sequence of characters that matched the regular expression
   `m[0].matched` | `true` if a match was found, and false otherwise.
   `m[n].first` | For all integers `0 < n < m.size()`, the start of the sequence that matched sub-expression `n`. Alternatively, if subexpression `n` did not participate in the match, then `last`.
   `m[n].second` | For all integers `0 < n < m.size()`, the end of the sequence that matched sub-expression `n`. Alternatively, if sub-expression `n` did not participate in the match, then `last`.
   `m[n].matched` | For all integers `0 < n < m.size()`, `true` if sub-expression `n` participated in the match, `false` otherwise.

### 2207(i). `basic_string::at` should not have a Requires clause

Section: 27.4.3.6 [string.access] Status: C++14 Submitter: Nevin Liber Opened: 2012-10-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

`basic_string::at()` has a wide contract and should not have a "Requires" clause on it.

[2013-01-17, Juan Soulie comments]

This issue would also apply to every member function of `basic_string` that throws `out_of_range`, and to some cases where a `length_error` can be thrown.

[2013-03-15 Issues Teleconference]

Moved to Review.

While this could simply move to Ready on inspection, there is concern that this will not be the only such case. Alisdair volunteers to review clause 21/23 for more of such issues for Bristol, and update the proposed resolution as necessary.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Remove 27.4.3.6 [string.access] p5:

   >     const_reference at(size_type pos) const;
   >     reference at(size_type pos);

   > > -5- Requires: `pos < size()`

   > > -6- Throws: `out_of_range` if `pos >= size()`.

   > > -7- Returns: `operator[](pos)`.

### 2208(i). `std::reverse_iterator` should be a literal type

Section: 24.5.1 [reverse.iterators] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2012-10-30 Last modified: 2017-03-12

Priority: 3

View all other issues in [reverse.iterators].

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Discussion:

`std::reverse_iterator::reverse_iterator(Iterator)` should be constexpr so that other constexpr functions can return `reverse_iterator`s. Of the other methods, the other constructors, `base()`, `operator+`, `operator-`, `operator[]`, and the non-member operators can probably also be `constexpr`.

`operator*` cannot be constexpr because it involves an assignment to a member variable. Discussion starting with c++std-lib-33282 indicated that it would be useful to make reverse_iterator a literal type despite this restriction on its use at compile time.

Proposed resolution:

This issue was Resolved by paper P0031R0 adopted at Jacksonville, 2016.

### 2209(i). `assign()` overspecified for sequence containers

Section: 23.3 [sequences] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-31 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [sequences].

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Discussion:

DR 704(i) ensures allocator-aware containers can reuse existing elements during copy/move assignment, and sequence containers can do the same for `assign()`.

But apart from `std::list` (which was changed by DR 320(i)) the sequence containers define the Effects of `assign()` in terms of `clear()` followed by `insert`. A user-defined allocator can easily tell whether all old elements are cleared and then new elements inserted or whether existing elements are assigned to, so those Effects clauses cannot be ignored via the as-if rule.

The descriptions of the `assign()` members for `deque`, `forward_list` and `vector` should be removed. Their intended effects are entirely described by the sequence container requirements table, and the specific definitions of them are worse than redundant, they're contradictory (if the operations are defined in terms of `erase` and `insert` then there's no need for elements to be assignable.) The descriptions of `assign()` for `list` are correct but redundant, so should be removed too.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Edit 23.3.5.2 [deque.cons] to remove everything after paragraph 10:

   >     template <class InputIterator>
   >     void assign(InputIterator first, InputIterator last);

   > > -11- Effects:

   > > >     erase(begin(), end());
   > > >     insert(begin(), first, last);

   >     void assign(size_type n, const T& t);

   > > -12- Effects:

   > > >     erase(begin(), end());
   > > >     insert(begin(), n, t);

2. Edit [forwardlist.cons] to remove everything after paragraph 10:

   >     template <class InputIterator>
   >     void assign(InputIterator first, InputIterator last);

   > > -11- Effects: `clear(); insert_after(before_begin(), first, last);`

   >     void assign(size_type n, const T& t);

   > > -12- Effects: `clear(); insert_after(before_begin(), n, t);`

3. Edit 23.3.11.2 [list.cons] to remove everything after paragraph 10:

   >     template <class InputIterator>
   >     void assign(InputIterator first, InputIterator last);

   > > -11- Effects: Replaces the contents of the list with the range `[first, last)`.

   >     void assign(size_type n, const T& t);

   > > -12- Effects: Replaces the contents of the list with `n` copies of `t`.

4. Edit 23.3.13.2 [vector.cons] to remove everything after paragraph 10:

   >     template <class InputIterator>
   >     void assign(InputIterator first, InputIterator last);

   > > -11- Effects:

   > > >     erase(begin(), end());
   > > >     insert(begin(), first, last);

   >     void assign(size_type n, const T& t);

   > > -12- Effects:

   > > >     erase(begin(), end());
   > > >     insert(begin(), n, t);

### 2210(i). Missing allocator-extended constructor for allocator-aware containers

Section: 23.3 [sequences], 23.4 [associative], 23.5 [unord] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-11-01 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [sequences].

View all issues with C++14 status.

Discussion:

The `forward_list(size_type)` constructor has no allocator-extended equivalent, preventing the following code from compiling:

>     #include <forward_list>
>     #include <vector>
>     #include <scoped_allocator>

>     using namespace std;

>     int main()
>     {
>       using list = forward_list<int>;
>       vector<list, scoped_allocator_adaptor<list::allocator_type>> v;
>       v.emplace_back(1u);
>     }

The very same problem exists for all allocator-aware sequence containers.

In addition it exists for associative containers. For example, it's possible to construct `std::set<int>{0, 1, 2}` but not `std::set<int>{{0, 1, 2}, alloc}`, and possible to construct `std::set<int>{begin, end}` but not `std::set<int>{begin, end, alloc}`.

This makes the following program fail when `SCOPED` is defined:

>     #include <set>
>     #include <vector>
>     #include <scoped_allocator>

>     #if SCOPED
>     using A = std::scoped_allocator_adaptor<std::allocator<int>>;
>     #else
>     using A = std::allocator<int>;
>     #endif

>     int main()
>     {
>       int values[] = {0, 1, 2};
>       std::vector<std::set<int>, A> v;
>       v.emplace_back(std::begin(values), std::end(values));
>     }

[2013-03-15 Issues Teleconference]

Moved to Review.

Jonathan: There are lots of places where this is missing.

Howard: We should ping Pablo, this might be a deliberate design decision.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Edit the synopsis in 23.3.5.1 [deque.overview]/2:

   >     namespace std {
   >       template <class T, class Allocator = allocator<T> >
   >       class deque {
   >       public:
   >         […]
   >         explicit deque(const Allocator& = Allocator());
   >         explicit deque(size_type n, const Allocator& = Allocator());
   >         […]
   >       };
   >     }

2. Edit 23.3.5.2 [deque.cons]/2:

   >     explicit deque(size_type n, const Allocator& = Allocator());

   > > -3- Effects: Constructs a `deque` with `n` default-inserted elements using the specified allocator.

3. Edit the synopsis in [forwardlist.overview]/3:

   >     namespace std {
   >       template <class T, class Allocator = allocator<T> >
   >       class forward_list {
   >       public:
   >         […]
   >         explicit forward_list(const Allocator& = Allocator());
   >         explicit forward_list(size_type n, const Allocator& = Allocator());
   >         […]
   >       };
   >     }

4. Edit [forwardlist.cons]/3:

   >     explicit forward_list(size_type n, const Allocator& = Allocator());

   > > -3- Effects: Constructs a `forward_list` object with `n` default-inserted elements using the specified allocator.

5. Edit the synopsis in 23.3.11.1 [list.overview]/2:

   >     namespace std {
   >       template <class T, class Allocator = allocator<T> >
   >       class list {
   >       public:
   >         […]
   >         explicit list(const Allocator& = Allocator());
   >         explicit list(size_type n, const Allocator& = Allocator());
   >         […]
   >       };
   >     }

6. Edit 23.3.11.2 [list.cons]/3:

   >     explicit list(size_type n, const Allocator& = Allocator());

   > > -3- Effects: Constructs a `list` with `n` default-inserted elements using the specified allocator.

7. Edit the synopsis in 23.3.13.1 [vector.overview]/2:

   >     namespace std {
   >       template <class T, class Allocator = allocator<T> >
   >       class vector {
   >       public:
   >         […]
   >         explicit vector(const Allocator& = Allocator());
   >         explicit vector(size_type n, const Allocator& = Allocator());
   >         […]
   >       };
   >     }

8. Edit 23.3.13.2 [vector.cons]/3:

   >     explicit vector(size_type n, const Allocator& = Allocator());

   > > -3- Effects: Constructs a `vector` with `n` default-inserted elements using the specified allocator.

9. Edit the synopsis in 23.3.14 [vector.bool]/1:

   >     namespace std {
   >       template <class Allocator> class vector<bool, Allocator> {
   >       class vector {
   >       public:
   >         […]
   >         explicit vector(const Allocator& = Allocator());
   >         explicit vector(size_type n, const Allocator& = Allocator());
   >         explicit vector(size_type n, const bool& value = bool(),
   >                         const Allocator& = Allocator());
   >         […]
   >       };
   >     }

10. Add to the synopsis in 23.4.3.1 [map.overview] p2:

    >     namespace std {
    >       template <class Key, class T, class Compare = less<Key>,
    >         class Allocator = allocator<pair<const Key, T> > > {
    >       class map {
    >       public:
    >         […]
    >         map(initializer_list<value_type>,
    >           const Compare& = Compare(),
    >           const Allocator& = Allocator());
    >         template <class InputIterator>
    >         map(InputIterator first, InputIterator last, const Allocator& a)
    >           : map(first, last, Compare(), a) { }
    >         map(initializer_list<value_type> il, const Allocator& a)
    >           : map(il, Compare(), a) { }
    >         ~map();
    >         […]
    >       };
    >     }

11. Add to the synopsis in 23.4.4.1 [multimap.overview] p2:

    >     namespace std {
    >       template <class Key, class T, class Compare = less<Key>,
    >         class Allocator = allocator<pair<const Key, T> > > {
    >       class multimap {
    >       public:
    >         […]
    >         multimap(initializer_list<value_type>,
    >           const Compare& = Compare(),
    >           const Allocator& = Allocator());
    >         template <class InputIterator>
    >         multimap(InputIterator first, InputIterator last, const Allocator& a)
    >           : multimap(first, last, Compare(), a) { }
    >         multimap(initializer_list<value_type> il, const Allocator& a)
    >           : multimap(il, Compare(), a) { }
    >         ~multimap();
    >         […]
    >       };
    >     }

12. Add to the synopsis in 23.4.6.1 [set.overview] p2:

    >     namespace std {
    >       template <class Key, class Compare = less<Key>,
    >         class Allocator = allocator<Key> > {
    >       class set {
    >       public:
    >         […]
    >         set(initializer_list<value_type>,
    >           const Compare& = Compare(),
    >           const Allocator& = Allocator());
    >         template <class InputIterator>
    >         set(InputIterator first, InputIterator last, const Allocator& a)
    >           : set(first, last, Compare(), a) { }
    >         set(initializer_list<value_type> il, const Allocator& a)
    >           : set(il, Compare(), a) { }
    >         ~set();
    >         […]
    >       };
    >     }

13. Add to the synopsis in 23.4.7.1 [multiset.overview] p2:

    >     namespace std {
    >       template <class Key, class Compare = less<Key>,
    >         class Allocator = allocator<Key> > {
    >       class multiset {
    >       public:
    >         […]
    >         multiset(initializer_list<value_type>,
    >           const Compare& = Compare(),
    >           const Allocator& = Allocator());
    >         template <class InputIterator>
    >         multiset(InputIterator first, InputIterator last, const Allocator& a)
    >           : multiset(first, last, Compare(), a) { }
    >         multiset(initializer_list<value_type> il, const Allocator& a)
    >           : multiset(il, Compare(), a) { }
    >         ~multiset();
    >         […]
    >       };
    >     }

14. Add to the synopsis in 23.5.3.1 [unord.map.overview] p3:

    >     namespace std {
    >       template <class Key, class T,
    >         class Hash = hash<Key>,
    >         class Pred = std::equal_to<Key>,
    >         class Allocator = std::allocator<std::pair<const Key, T> > > {
    >       class unordered_map {
    >       public:
    >         […]
    >         unordered_map(initializer_list<value_type>,
    >           size_type = see below,
    >           const hasher& hf = hasher(),
    >           const key_equal& eql = key_equal(),
    >           const allocator_type& a = allocator_type());
    >         unordered_map(size_type n, const allocator_type& a)
    >           : unordered_map(n, hasher(), key_equal(), a) { }
    >         unordered_map(size_type n, const hasher& hf, const allocator_type& a)
    >           : unordered_map(n, hf, key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
    >           : unordered_map(f, l, n, hasher(), key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
    >     	    const allocator_type& a)
    >           : unordered_map(f, l, n, hf, key_equal(), a) { }
    >         unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a)
    >           : unordered_map(il, n, hasher(), key_equal(), a) { }
    >         unordered_map(initializer_list<value_type> il, size_type n, const hasher& hf,
    >     	  const allocator_type& a)
    >           : unordered_map(il, n, hf, key_equal(), a) { }
    >         ~unordered_map();
    >         […]
    >       };
    >     }

15. Add to the synopsis in 23.5.4.1 [unord.multimap.overview] p3:

    >     namespace std {
    >       template <class Key, class T,
    >         class Hash = hash<Key>,
    >         class Pred = std::equal_to<Key>,
    >         class Allocator = std::allocator<std::pair<const Key, T> > > {
    >       class unordered_multimap {
    >       public:
    >         […]
    >         unordered_multimap(initializer_list<value_type>,
    >           size_type = see below,
    >           const hasher& hf = hasher(),
    >           const key_equal& eql = key_equal(),
    >           const allocator_type& a = allocator_type());
    >         unordered_multimap(size_type n, const allocator_type& a)
    >           : unordered_multimap(n, hasher(), key_equal(), a) { }
    >         unordered_multimap(size_type n, const hasher& hf, const allocator_type& a)
    >           : unordered_multimap(n, hf, key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
    >           : unordered_multimap(f, l, n, hasher(), key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
    >     	    const allocator_type& a)
    >           : unordered_multimap(f, l, n, hf, key_equal(), a) { }
    >         unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a)
    >           : unordered_multimap(il, n, hasher(), key_equal(), a) { }
    >         unordered_multimap(initializer_list<value_type> il, size_type n, const hasher& hf,
    >     	  const allocator_type& a)
    >           : unordered_multimap(il, n, hf, key_equal(), a) { }
    >         ~unordered_multimap();
    >         […]
    >       };
    >     }

16. Add to the synopsis in 23.5.6.1 [unord.set.overview] p3:

    >     namespace std {
    >       template <class Key,
    >         class Hash = hash<Key>,
    >         class Pred = std::equal_to<Key>,
    >         class Allocator = std::allocator<Key> > {
    >       class unordered_set {
    >       public:
    >         […]
    >         unordered_set(initializer_list<value_type>,
    >           size_type = see below,
    >           const hasher& hf = hasher(),
    >           const key_equal& eql = key_equal(),
    >           const allocator_type& a = allocator_type());
    >         unordered_set(size_type n, const allocator_type& a)
    >           : unordered_set(n, hasher(), key_equal(), a) { }
    >         unordered_set(size_type n, const hasher& hf, const allocator_type& a)
    >           : unordered_set(n, hf, key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
    >           : unordered_set(f, l, n, hasher(), key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
    >     	    const allocator_type& a)
    >           : unordered_set(f, l, n, hf, key_equal(), a) { }
    >         unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a)
    >           : unordered_set(il, n, hasher(), key_equal(), a) { }
    >         unordered_set(initializer_list<value_type> il, size_type n, const hasher& hf,
    >     	  const allocator_type& a)
    >           : unordered_set(il, n, hf, key_equal(), a) { }
    >         ~unordered_set();
    >         […]
    >       };
    >     }

17. Add to the synopsis in 23.5.7.1 [unord.multiset.overview] p3:

    >     namespace std {
    >       template <class Key,
    >         class Hash = hash<Key>,
    >         class Pred = std::equal_to<Key>,
    >         class Allocator = std::allocator<Key> > {
    >       class unordered_multiset {
    >       public:
    >         […]
    >         unordered_multiset(initializer_list<value_type>,
    >           size_type = see below,
    >           const hasher& hf = hasher(),
    >           const key_equal& eql = key_equal(),
    >           const allocator_type& a = allocator_type());
    >         unordered_multiset(size_type n, const allocator_type& a)
    >           : unordered_multiset(n, hasher(), key_equal(), a) { }
    >         unordered_multiset(size_type n, const hasher& hf, const allocator_type& a)
    >           : unordered_multiset(n, hf, key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
    >           : unordered_multiset(f, l, n, hasher(), key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf,
    >     	    const allocator_type& a)
    >           : unordered_multiset(f, l, n, hf, key_equal(), a) { }
    >         unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a)
    >           : unordered_multiset(il, n, hasher(), key_equal(), a) { }
    >         unordered_multiset(initializer_list<value_type> il, size_type n, const hasher& hf,
    >     	  const allocator_type& a)
    >           : unordered_multiset(il, n, hf, key_equal(), a) { }
    >         ~unordered_multiset();
    >         […]
    >       };
    >     }

### 2211(i). Replace ambiguous use of "Allocator" in container requirements

Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-11-07 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [container.requirements.general].

View all other issues in [container.requirements.general].

View all issues with C++14 status.

Discussion:

23.2.2 [container.requirements.general]/7 says:

> All other constructors for these container types take an `Allocator&` argument (16.4.4.6 [allocator.requirements]), an allocator whose value type is the same as the container's value type.

This is a strange place to state the requirement on the allocator's `value_type`, because the allocator is a property (and template parameter) of the container type not of some of its constructors. It's also unclear whether "`Allocator&`" refers to the concept (as implied by the cross-reference to the allocator requirements in Clause 17) or to the container's template parameter (as implied by the fact it's shown as an lvalue-reference type.) I believe the latter is intended, because those constructors can't take any model of the allocator concept, they can only take the container's `allocator_type`.

I think it would be clearer to remove the value type requirement earlier in the paragraph (Table 99 already imposes that requirement) and to make it clear the constructor arguments are the container's `allocator_type`. There is already a cross-reference to the allocator requirements earlier in the paragraph, so it doesn't need to be repeated in another place where it causes confusion.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Edit 23.2.2 [container.requirements.general] paragraph 7:

   > Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see 16.4.4.6 [allocator.requirements]). Copy constructors for these container types obtain an allocator by calling `allocator_traits<allocator_type>::select_on_container_copy_construction` on their first parameters. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. Such move construction of the allocator shall not exit via an exception. All other constructors for these container types take an `Allocator&` argument (16.4.4.6 [allocator.requirements]), an allocator whose value type is the same as the container's value typea `const allocator_type&` argument. [Note: If an invocation of a constructor uses the default value of an optional allocator argument, then the `Allocator` type must support value initialization. — end note] A copy of this allocator is used for any memory allocation performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. […]

### 2212(i). `tuple_size` for `const pair` request `<tuple>` header

Section: 22.2 [utility] Status: C++17 Submitter: Alisdair Meredith Opened: 2012-11-09 Last modified: 2017-07-30

Priority: 3

View all other issues in [utility].

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Discussion:

The `<utility>` header declares sufficient of the tuple API to specialize the necessary templates for `pair`, notably `tuple_size` and `tuple_element`. However, it does not make available the partial specializations that support cv-qualified template arguments, so while I can write the following after including only `<utility>`:

>     #include <utility>

>     using TestType = std::pair<int, int>;
>     static_assert(2 == std::tuple_size<TestType>(), "Pairs have two elements");
>     std::tuple_element<0, TestType>::type var{1};

the following may fail to compile unless I also include `<tuple>`:

>     #include <utility>

>     using TestType = const std::pair<int, int>;
>     static_assert(2 == std::tuple_size<TestType>(), "Pairs have two elements");
>     std::tuple_element<0, TestType>::type var{1};

Note, however, that the latter may compile with some standard library implementations but not others, leading to subtle portability issues.

[2013-03-15 Issues Teleconference]

Moved to Open.

Howard notes that we have the same issue with array, so any resolution should apply to that header too.

[2013-10-18 Daniel provides wording]

The suggested wording uses a similar approach as we already have in 24.7 [iterator.range] to ensure that the range access templates are available when at least one of an enumerated list of header files is included.

I also think that the restricted focus on `tuple_size` of this issue is too narrow and should be extended to the similar partial template specializations of `tuple_element` as well. Therefore the suggested wording ensures this as well.

[2014-03-27 Library reflector vote]

The issue has been identified as Tentatively Ready based on eight votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 22.4.7 [tuple.helper] as indicated:

   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   > > -3- Let `TS` denote `tuple_size<T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the `UnaryTypeTrait` requirements (20.10.1) with a `BaseCharacteristic` of

   > > >     integral_constant<size_t, TS::value>

   > > -?- In addition to being available via inclusion of the `<tuple>` header, each of the three templates are available when any of the headers `<array>` or `<utility>` are included.

   >     template <size_t I, class T> class tuple_element<I, const T>;
   >     template <size_t I, class T> class tuple_element<I, volatile T>;
   >     template <size_t I, class T> class tuple_element<I, const volatile T>;

   > > -?- Let `TE` denote `tuple_element<I, T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the `TransformationTrait` requirements (20.10.1) with a member typedef `type` that names the following type:

   > > - for the first specialization, `add_const<TE::type>::type`,

   > > - for the second specialization, `add_volatile<TE::type>::type`, and

   > > - for the third specialization, `add_cv<TE::type>::type`.

   > > -?- In addition to being available via inclusion of the `<tuple>` header, each of the three templates are available when any of the headers `<array>` or `<utility>` are included.

### 2213(i). Return value of `std::regex_replace`

Section: 28.6.10.4 [re.alg.replace] Status: C++14 Submitter: Pete Becker Opened: 2012-11-08 Last modified: 2017-07-05

Priority: 0

View all other issues in [re.alg.replace].

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Discussion:

In 28.6.10.4 [re.alg.replace], the first two variants of `std::regex_replace` take an output iterator named "out" as their first argument. Paragraph 2 of that section says that the functions return "out". When I first implemented this, many years ago, I wrote it to return the value of the output iterator after all the insertions (cf. `std::copy`), which seems like the most useful behavior. But looking at the requirement now, it like the functions should return the original value of "out" (i.e. they have to keep a copy of the iterator for no reason except to return it). Is that really what was intended?

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3485.

1. Edit 28.6.10.4 [re.alg.replace] as indicated:

   >     template <class OutputIterator, class BidirectionalIterator,
   >       class traits, class charT, class ST, class SA>
   >     OutputIterator
   >     regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last,
   >       const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt,
   >       regex_constants::match_flag_type flags = regex_constants::match_default);
   >     template <class OutputIterator, class BidirectionalIterator,
   >       class traits, class charT>
   >     OutputIterator
   >     regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last,
   >       const basic_regex<charT, traits>& e, const charT* fmt,
   >       regex_constants::match_flag_type flags = regex_constants::match_default);

   > > -1- Effects: Constructs a `regex_iterator` object `i` as if by `regex_iterator<BidirectionalIterator, charT, traits> i(first, last, e, flags)`, and uses `i` to enumerate through all of the matches `m` of type `match_results<BidirectionalIterator>` that occur within the sequence `[first, last)`. If no such matches are found and `!(flags & regex_constants ::format_no_copy)` then calls `out = std::copy(first, last, out)`. If any matches are found then, for each such match, if `!(flags & regex_constants::format_no_copy)`, calls `out = std::copy(m.prefix().first, m.prefix().second, out)`, and then calls `out = m.format(out, fmt, flags)` for the first form of the function and `out = m.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags)` for the second. Finally, if such a match is found and `!(flags & regex_constants ::format_no_copy)`, calls `out = std::copy(last_m.suffix().first, last_m.suffix().second, out)` where `last_m` is a copy of the last match found. If `flags & regex_constants::format_first_only` is non-zero then only the first match found is replaced.

   > > -2- Returns: `out`.

### 2217(i). `operator==(sub_match, string)` slices on embedded `'\0'`s

Section: 28.6.8.3 [re.submatch.op] Status: C++17 Submitter: Jeffrey Yasskin Opened: 2012-11-26 Last modified: 2017-07-30

Priority: 2

View all other issues in [re.submatch.op].

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Discussion:

>     template <class BiIter, class ST, class SA>
>       bool operator==(
>         const basic_string<
>           typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
>         const sub_match<BiIter>& rhs);

is specified as:

> Returns: `rhs.compare(lhs.c_str()) == 0`.

This is odd because `sub_match::compare(basic_string)` is defined to honor embedded `'\0'` characters. This could allow a `sub_match` to `==` or `!=` a `std::string` unexpectedly.

[Daniel:]

This wording change was done intentionally as of LWG 1181(i), but the here mentioned slicing effect was not considered at that time. It seems best to use another overload of compare to fix this problem:

> Returns: `rhs.str().compare(0, rhs.length(), lhs.data(), lhs.size()) == 0`.

or

> Returns: `rhs.compare(sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0`.

[2013-10-17: Daniel provides concrete wording]

The original wording was suggested to reduce the need to allocate memory during comparisons. The specification would be very much easier, if `sub_match` would provide an additional `compare` overload of the form:

>     int compare(const value_type* s, size_t n) const;

But given the fact that currently all of `basic_string`'s `compare` overloads are defined in terms of temporary string constructions, the following proposed wording does follow the same string-construction route as `basic_string` does (where needed to fix the embedded zeros issue) and to hope that existing implementations ignore to interpret this semantics in the literal sense.

I decided to use the second replacement form

>     Returns: rhs.compare(sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0.

because it already reflects the existing style used in 28.6.8.3 [re.submatch.op] p31.

[2014-02-15 post-Issaquah session : move to Tentatively Ready]

Proposed resolution:

This wording is relative to N3691.

1. Change 28.6.8.3 [re.submatch.op] as indicated:

   >     template <class BiIter, class ST, class SA>
   >       bool operator==(
   >         const basic_string<
   >           typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >         const sub_match<BiIter>& rhs);

   > > -7- Returns: `rhs.compare(lhs.c_str()typename sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0`.

   […]

   >     template <class BiIter, class ST, class SA>
   >       bool operator<(
   >         const basic_string<
   >           typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >         const sub_match<BiIter>& rhs);

   > > -9- Returns: `rhs.compare(lhs.c_str()typename sub_match<BiIter>::string_type(lhs.data(), lhs.size())) > 0`.

   […]

   >     template <class BiIter, class ST, class SA>
   >       bool operator==(const sub_match<BiIter>& lhs,
   >                       const basic_string<
   >                         typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > > -13- Returns: `lhs.compare(rhs.c_str()typename sub_match<BiIter>::string_type(rhs.data(), rhs.size())) == 0`.

   […]

   >     template <class BiIter, class ST, class SA>
   >       bool operator<(const sub_match<BiIter>& lhs,
   >                      const basic_string<
   >                        typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > > -15- Returns: `lhs.compare(rhs.c_str()typename sub_match<BiIter>::string_type(rhs.data(), rhs.size())) < 0`.

### 2218(i). Unclear how containers use `allocator_traits::construct()`

Section: 23.2.2 [container.requirements.general] Status: C++17 Submitter: Jonathan Wakely Opened: 2012-11-27 Last modified: 2017-07-30

Priority: 3

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Discussion:

Firstly, 23.2.2 [container.requirements.general]/7 says a container's allocator is used to obtain memory, but it isn't stated explicitly that the same allocator is used to construct and destroy elements, as opposed to a value-initialized allocator of the same type.

Secondly, 23.2.2 [container.requirements.general]/3 says elements "shall be constructed using the `allocator_traits<allocator_type>::construct` function and destroyed using the `allocator_traits<allocator_type>::destroy` function" and 23.2.2 [container.requirements.general]/13 defines `CopyInsertable` etc. in terms of an allocator `A` which is identical to the container's `allocator_type`.

The intent of making `construct()` and `destroy()` function templates was that containers would be permitted to use `allocator_traits<A>::construct()` instead of `allocator_traits<allocator_type>::construct()`, where `A` is `allocator_traits<allocator_type>::rebind_alloc<U>` for some other type `U`. This allows node-based containers to store an allocator of the right type for allocating nodes and to use the same object to construct elements in aligned storage within those nodes, avoiding rebinding and copying the stored allocator every time an element needs to be constructed.

It should be made clear that a possibly-rebound copy of the container's allocator is used for object construction.

[2013-03-15 Issues Teleconference]

Moved to Open.

Jonathan: point 2 in the proposed resolution is definitely needed.

[2014-11-28, Jonathan improves wording]

In the first set of edits to paragraph 3 both pieces inserting "`rebind_alloc<U>::`" should be replaced by "`rebind_traits<U>::`"

Otherwise it implies using the allocator's functions directly, but they might not exist and it should be through the rebound traits type.

[2015-05, Lenexa]

STL: You want to permit but not require rebinding?
Wakely: The current wording forces me to use the original allocator, not the rebound one.
STL: Oh, I see. Yeah, we immediately rebind.
Wakely: The edits clarify that we don't use some other allocator. The third diff is because the definitions of EmplaceConstructible/etc. happen with the same types. The diff to the note is because it doesn't require the value of the allocator was the one passed in.
STL: After looking at this, I think I'm comfortable with the edits. The previous Standardese was nonsense so it's pretty easy to improve upon.
Marshall: Any other opinions?
Marshall: Any objections to moving it to Ready? Review? Ready in Kona?
Wakely: My preference would be Ready. We all know this is what we're doing anyways.
Nevin: The intent won't change.
STL: I think this is the right fix.
Hwrd: I third Ready. Even if Jonathan retracts his.
Marshall: Ready!

Proposed resolution:

This wording is relative to N3485.

1. Edit 23.2.2 [container.requirements.general] paragraph 3:

   > For the components affected by this subclause that declare an `allocator_type`, objects stored in these components shall be constructed using the `allocator_traits<allocator_type>::rebind_traits<U>::construct` function and destroyed using the `allocator_traits<allocator_type>::rebind_traits<U>::destroy` function (20.2.9.3 [allocator.traits.members]), where `U` is either `allocator_type::value_type` or an internal type used by the container. These functions are called only for the container's element type, not for internal types used by the container. [ Note: This means, for example, that a node-based container might need to construct nodes containing aligned buffers and call construct to place the element into the buffer. — end note ]

2. Edit 23.2.2 [container.requirements.general] paragraph 7:

   > […] A copy of this allocator is used for any memory allocation and element construction performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. […]

3. Edit 23.2.2 [container.requirements.general] paragraph 13:

   > […] Given an allocator type `A` and given a container type `X` having an `allocator_type` identical to `A` and a `value_type` identical to `T` and an `allocator_type` identical to `allocator_traits<A>::rebind_alloc<T>` and given an lvalue `m` of type `A`, a pointer `p` of type `T*`, an expression `v` of type (possibly `const`) `T`, and an rvalue `rv` of type `T`, the following terms are defined.

   > […]

   > [ Note: A container calls `allocator_traits<A>::construct(m, p, args)` to construct an element at `p` using `args`, with `m == get_allocator()`. The default `construct` in `std::allocator` will call `::new((void*)p) T(args)`, but specialized allocators may choose a different definition. — end note ]

### 2219(i). `INVOKE`-ing a pointer to member with a `reference_wrapper` as the object expression

Section: 22.10.4 [func.require] Status: C++17 Submitter: Jonathan Wakely Opened: 2012-11-28 Last modified: 2017-07-30

Priority: 2

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Discussion:

The standard currently requires this to be invalid:

>     #include <functional>

>     struct X { int i; } x;
>     auto f = &X::i;
>     auto t1 = std::ref(x);
>     int i = std::mem_fn(f)(t1);

The call expression on the last line is equivalent to `INVOKE(f, std::ref(x))` which according to 22.10.4 [func.require]p1 results in the invalid expression `(*t1).*f` because `reference_wrapper<X>` is neither an object of type `X` nor a reference to an object of type `X` nor a reference to an object of a type derived from `X`.

The same argument applies to pointers to member functions, and if they don't work with `INVOKE` it becomes harder to do all sorts of things such as:

> `call_once(o, &std::thread::join, std::ref(thr))`

or

> `async(&std::list<int>::sort, std::ref(list));`

The definition of `INVOKE` should be extended to handle reference wrappers.

[2013-03-15 Issues Teleconference]

Moved to Review.

The wording seems accurate, but verbose. If possible, we would like to define the kind of thing being specified so carefully as one of a number of potential language constructs in a single place. It is also possible that this clause is that single place.

[2013-04-18, Bristol]

Jonathan comments:

In the proposed resolution in the first bullet `(t1.*f)` is not valid if `t1` is a `reference_wrapper`, so we probably need a separate bullet to handle the `reference_wrapper` case.

[2014-02-14, Issaquah, Mike Spertus supplies wording]

Previous resolution from Jonathan [SUPERSEDED]:

> This wording is relative to N3485.

> 1. Edit 22.10.4 [func.require]:

>    > Define `INVOKE(f, t1, t2, ..., tN)` as follows:

>    > - `(t1.*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T` `U` or an object of type `reference_wrapper<U>` or a reference to an object of type `reference_wrapper<U>` where `U` is either the type `T` or a type derived from `T`;

>    > - `((*t1).*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is not one of the types described in the previous item;

>    > - `t1.*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T` `U` or an object of type `reference_wrapper<U>` or a reference to an object of type `reference_wrapper<U>` where `U` is either the type `T` or a type derived from `T`;

>    > - `(*t1).*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is not one of the types described in the previous item;

>    > - `f(t1, t2, ..., tN)` in all other cases.

[2014-10-01, STL adds discussion and provides an improved resolution]

Because neither `t1.*f` nor `(*t1).*f` will compile when `t1` is `reference_wrapper<U>` for any `U`, we don't need to inspect `U` carefully. We can bluntly detect all `reference_wrapper`s and use `get()` for them.

We would have to be more careful if we had to deal with pointers to members of `reference_wrapper` itself. Fortunately, we don't. First, it doesn't have user-visible data members. Second, users technically can't take the addresses of its member functions (this is a consequence of 16.4.6.5 [member.functions], the Implementer's Best Friend).

While we're in the neighborhood, I recommend simplifying and clarifying the wording used to detect base/derived objects.

Previous resolution from Mike Spertus [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Edit 22.10.4 [func.require]:

>    > Define `INVOKE(f, t1, t2, ..., tN)` as follows:

>    > - `(t1.*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`;

>    > - `(t1.get().*f)(t2, ..., tN)` when `f` is a pointer to a member function of class `T` and `t1` is an object of type `reference_wrapper<U>` where `U` is either the type `T` or a type derived from `T`.

>    > - `((*t1).*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is not one of the types described in the previous item;

>    > - `t1.*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`;

>    > - `t1.get().*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is an object of type `reference_wrapper<U>` where `U` is either the type `T` or a type derived from `T`.

>    > - `(*t1).*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is not one of the types described in the previous item;

>    > - `f(t1, t2, ..., tN)` in all other cases.

[2015-02, Cologne]

Waiting for implementation experience.

[2015-05, Lenexa]

STL: latest note from Cologne, waiting for implementation experience
STL: don't think this is harder than anything else we do
MC: it does involve `mem_fn` and invoke
STL: my simplication was not to attempt fine-grained
STL: can ignore pmf
STL: can't invoke pmf to reference wrapper
STL: wording dated back to TR1 when there was no `decltype`
MC: should `decay_t<decltype(t1)>` be pulled out since it is in multiple places
STL: it could be handled editorially
STL: we fix function, bind, invoke
STL: have not implemented this but believe it is fine
MC: Eric F, you have worked in invoke
EF: yes, looks ok
MC: consensus move to ready

Proposed resolution:

This wording is relative to N3936.

1. Change 22.10.4 [func.require] p1 as depicted:

   > Define `INVOKE(f, t1, t2, ..., tN)` as follows:

   > - `(t1.*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T` `is_base_of<T, decay_t<decltype(t1)>>::value` is true;

   > - `(t1.get().*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `decay_t<decltype(t1)>` is a specialization of `reference_wrapper`;

   > - `((*t1).*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is not one of the types described in the previous itemdoes not satisfy the previous two items;

   > - `t1.*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T` `is_base_of<T, decay_t<decltype(t1)>>::value` is true;

   > - `t1.get().*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `decay_t<decltype(t1)>` is a specialization of `reference_wrapper`;

   > - `(*t1).*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is not one of the types described in the previous itemdoes not satisfy the previous two items;

   > - `f(t1, t2, ..., tN)` in all other cases.

### 2221(i). No formatted output operator for `nullptr`

Section: 31.7.6 [output.streams] Status: C++17 Submitter: Matt Austern Opened: 2012-12-07 Last modified: 2017-07-30

Priority: 3

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Discussion:

When I write

>     std::cout << nullptr << std::endl;

I get a compilation error, "ambiguous overload for '`operator<<`' in '`std::cout << nullptr`'". As far as I can tell, the compiler is right to issue that error. There are inserters for `const void*`, `const char*`, `const signed char*`, and `const unsigned char*`, and none for `nullptr_t`, so the expression really is ambiguous.

Proposed wording:

The obvious library solution is to add a `nullptr_t` overload, which would be defined something like

>     template<class C, class T>
>     basic_ostream<C, T>& operator<<(basic_ostream<C, T>& os, nullptr_t)
>     {
>       return os << (void*) nullptr;
>     }

We might also consider addressing this at a core level: add a special-case language rule that addresses all cases where you write `f(nullptr)` and `f` is overloaded on multiple pointer types. (Perhaps a tiebreaker saying that `void*` is preferred in such cases.)

[2016-01-18, comments from Mike and Ville collected by Walter Brown]

Mike Miller: "Changing overload resolution sounds like something that should be considered by EWG before CWG […]"

Ville: "Agreed, such a change would be Evolutionary. Personally, I think it would also be wrong, because I don't see how `void*` is the right choice to prefer in the case of code that is currently ambiguous. Sure, it would solve this particular library issue, but it seemingly has wider repercussions. If LWG really wants to, EWG can certainly discuss this issue, but I would recommend solving it on the LWG side (which doesn't mean that the standard necessarily needs to change, I wouldn't call it far-fetched to NAD it)."

[2016-08 Chicago]

Zhihao recommends NAD:

`nullptr` is printable if being treated as `void*`, but causes UB if being treated as `char cv*`. Capturing this ambigurity at compile time and avoid a runtime UB is a good thing.

[2016-08 Chicago]

Tues PM: General agreement on providing the overload; discussion on what it should say.

Polls:
Matt's suggestion (in the issue): 2/0/6/2/2/
Unspecified output: 3/2/5/0/1
Specified output: 1/1/6/3/0

Move to Open

[2016-08 Chicago]

The group consensus is that we only output `nullptr` because it is of a fundamental type, causing problems in functions doing forwarding, and we don't want to read it back.

Fri PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606

1. Insert the signature into 31.7.6.2 [ostream], class template `basic_ostream` synopsis, as follows:

   > [Drafting notes: Why member? Don't want to define a new category of inserters just for this.]

   >     namespace std {
   >       template <class charT, class traits = char_traits<charT> >
   >       class basic_ostream
   >         : virtual public basic_ios<charT, traits> {
   >       public:
   >         […]
   >         basic_ostream<charT, traits>& operator<<(const void* p);
   >         basic_ostream<charT, traits>& operator<<(nullptr_t);
   >         basic_ostream<charT, traits>& operator<<(
   >           basic_streambuf<char_type, traits>* sb);
   >         […]
   >       };

2. Append the following new paragraphs to 31.7.6.3.3 [ostream.inserters]:

   >     basic_ostream<charT, traits>& operator<<
   >       (basic_streambuf<charT, traits>* sb);

   > > […]

   > > -10- Returns: `*this`.

   >     basic_ostream<charT, traits>& operator<<(nullptr_t);

   > > -??- Effects: Equivalent to `return *this << s;` where `s` is an implementation-defined NTCTS.

### 2222(i). Inconsistency in description of `forward_list::splice_after` single-element overload

Section: 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Edward Catmur Opened: 2012-12-11 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

[forwardlist.ops] p6 has

>     void splice_after(const_iterator position, forward_list& x, const_iterator i);
>     void splice_after(const_iterator position, forward_list&& x, const_iterator i);

> Effects: Inserts the element following `i` into `*this`, following `position`, and removes it from `x`. The result is unchanged if `position == i` or `position == ++i`. Pointers and references to `*i` continue to refer to the same element but as a member of `*this`. Iterators to `*i` (including `i` itself) continue to refer to the same element, but now behave as iterators into `*this`, not into `x`.

This overload splices the element following `i` from `x` to `*this`, so the language in the two latter sentences should refer to `++i`:

> Pointers and references to `*++i` continue to refer to the same element but as a member of `*this`. Iterators to `*++i` continue to refer to the same element, but now behave as iterators into `*this`, not into `x`.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Edit [forwardlist.ops] p6 as indicated:

   >     void splice_after(const_iterator position, forward_list& x, const_iterator i);
   >     void splice_after(const_iterator position, forward_list&& x, const_iterator i);

   > -5- Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range `[begin(),end())`. The iterator following `i` is a dereferenceable iterator in `x`. `get_allocator() == x.get_allocator()`.

   > -6- Effects: Inserts the element following `i` into `*this`, following `position`, and removes it from `x`. The result is unchanged if `position == i` or `position == ++i`. Pointers and references to `*++i` continue to refer to the same element but as a member of `*this`. Iterators to `*++i` (including `i` itself) continue to refer to the same element, but now behave as iterators into `*this`, not into `x`.

### 2223(i). `shrink_to_fit` effect on iterator validity

Section: 23.3.13.3 [vector.capacity] Status: C++17 Submitter: Juan Soulie Opened: 2012-12-17 Last modified: 2017-07-30

Priority: 2

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Discussion:

After the additions by 2033(i), it appears clear that the intended effect includes a reallocation and thus the potential effect on iterators should be explicitly added to the text in order to not contradict 23.2.2 [container.requirements.general]/11, or at the very least, explicitly state that a reallocation may happen.

Taking consistency with "reserve" into consideration, I propose:

- that the current "Remarks" are made its "Effect" instead, inserting "Reallocation happens at this point if and only if the function effectively reduces the capacity." after the note on non-bindingness.

- adding a "Remarks" paragraph, similar to that of reserve: "Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence."

BTW, while we are at it, I believe the effect on iterators should also be explicitly stated in the other instance a reallocation may happen: 23.3.13.5 [vector.modifiers]/1 — even if obvious, it only contradicts 23.2.2 [container.requirements.general]/11 implicitly.

I propose to also insert "Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence." at the appropriate location in its "Remarks".

[2012-12-19: Jonathan Wakely comments]

The described problem also affects `std::basic_string` and `std::deque`.

[2013-03-15 Issues Teleconference]

Moved to Review.

[2013-04-18, Bristol]

Daniel extends the P/R.

Rationale:

The wording in 27.4.3.5 [string.capacity] combined with 27.4.3.2 [string.require] seems to say the necessary things. We cannot impose all requirements as we do for `vector`, because we want to allow the short-string-optimization.

[2014-02-15 post-Issaquah session]

STL: I think that `shrink_to_fit` should be a no-op when called twice.

STL: Do we ever define reallocation for `deque`? Nope, all mentions of "reallocation" are in `vector`. We define what it means in `vector::reserve()`, but not for `deque`.

STL: Oh duh, they define reallocate in the PR. But I think we can do better here.

STL: Optimally, deque shrinking just allocates a new map of pointers, and drops empty blocks, but preserves pointers/references to elements.

Alisdair: That's like unordered containers, invalidating only iterators.

Pablo: It doesn't make sense to reduce `capacity()` to `size()`, because `deque` doesn't have capacity!

STL: For `vector`, "effectively reduces the capacity" is unnecessary, the capacity there is observable.

STL: There is a strong reason to provide an optimal shrink to fit for `deque`, since only the library implementer can do this.

STL: The other thing I don't like the repeated definition of reallocation for `vector`, we define it once and use it in a bunch of places. At most we can lift it up to the `vector` synopsis.

STL: I'll write new wording.

[2014-10-01, STL adds discussion and provides new wording]

Compared to the previous proposed resolution:

- I'm changing `basic_string`'s wording because (1) we should guarantee that capacity won't increase, (2) we should mention that it's linear complexity, and (3) we can provide a better invalidation guarantee than 27.4.3.2 [string.require]/5. (As previously noted, we already have the strong exception guarantee.) This introduces the term "reallocation" into `basic_string`, but immediately explains what it means for iterator validity. As far as I can tell, the Small String Optimization doesn't complicate the wording here; it's a reason why an implementation might not honor the request, but if the capacity is reduced, we are definitely reallocating buffers and will invalidate everything (including when the destination is the small buffer).

- Between N3485 and N3936, `deque`'s wording was updated to avoid talking about `capacity()` which it doesn't have. Since the container's capacity is unobservable, I'm saying that invalidation is unconditional.

- In `vector`'s wording, I'm also guaranteeing that capacity won't increase, and that iterators/etc. remain valid if the capacity is unchanged.

My wording doesn't directly say that `shrink_to_fit()` should be a no-op when called twice in a row. (Indirectly, if the first call reduces `capacity()` to `size()`, the second call must preserve iterators/etc.) I considered rewording the complexity to say "linear if reallocation happens", but that's potentially problematic (what if we copy almost all `N` elements, then one throws and we have to unwind? There are no effects, so reallocation didn't happen, yet we took longer than constant time). Implementers can always do better than the stated complexity bounds.

I chose not to modify `deque`'s requirements, so implementations remain free to reallocate the elements themselves.

I didn't attempt to centralize vector's reallocation wording. That can be done editorially, if someone is sufficiently motivated.

Previous resolution from Juan Soulie/Daniel [SUPERSEDED]:

> This wording is relative to N3485.

> 1. Keep 27.4.3.5 [string.capacity] around p14 unchanged, because we don't speak about reallocations and we give the strong exception guarantee in 27.4.3.2 [string.require] (Invalidation specification also at that place):

>    >     void shrink_to_fit();

>    > -14- Remarks: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ].

> 2. Edit 23.3.5.3 [deque.capacity] around p7 as indicated:

>    >     void shrink_to_fit();

>    > -5- Requires: `T` shall be `MoveInsertable` into `*this`.

>    > -?- Effects: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] Reallocation happens at this point if and only if the function effectively reduces the capacity. If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

>    > -6- Complexity: Linear in the size of the sequence.

>    > -7- Remarks: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence.

> 3. Edit 23.3.13.3 [vector.capacity] around p7 as indicated:

>    >     void shrink_to_fit();

>    > -7- Requires: `T` shall be `MoveInsertable` into `*this`.

>    > -?- Effects: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] Reallocation happens at this point if and only if the function effectively reduces the capacity. If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

>    > -8- Complexity: Linear in the size of the sequence.

>    > -9- Remarks: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence.

> 4. Edit 23.3.13.5 [vector.modifiers] p1 as indicated:

>    >     iterator insert(const_iterator position, const T& x);
>    >     iterator insert(const_iterator position, T&& x);
>    >     iterator insert(const_iterator position, size_type n, const T& x);
>    >     template <class InputIterator>
>    >     iterator insert(const_iterator position, InputIterator first, InputIterator last);
>    >     iterator insert(const_iterator position, initializer_list<T>);
>    >     template <class... Args> void emplace_back(Args&&... args);
>    >     template <class... Args> iterator emplace(const_iterator position, Args&&... args);
>    >     void push_back(const T& x);
>    >     void push_back(T&& x);

>    > -1- Remarks: Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` or by any `InputIterator` operation there are no effects. If an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

[2015-02 Cologne]

GR: I'm concerned that `shrink_to_fit` may cause reallocation without changing the capacity. […] It's about correctness. The statement about invalidation is useless if I cannot detect whether reallocation has happened?

AM: It seems like the logic goes the other way round: It's the capacity change that causes reallocation, so if there's no capacity change, there's no reallocation. But that's not quite how I'd like to say it... maybe this, : "If capacity does not change, no reallocation occurs."

GR: Where does it actually say that `reserve()` invalidates? AM: It should say that in the container requirements. VV: vector specifies in `reserve` that there's reallocation if and only if the capacity changes. GR: I can't find anything in the container requirements about `reserve`. DK: No, it's specified for every container separately. GR: It isn't specified for string.

GR: I'm noticing that the issue touches on `shrink_to_fit` for a bunch of containers. Anyway, I think the reserve issue [re string] is in scope for this issue. This change is touching on a lot of members.

AM: Landing this change will provide clarity for what we should do with `basic_string`. GR: We're already asking for changes; we should fix string as well. AM: If one of the changes is ready before the other, I'd like to land the finished part first, but if both are ready for Lenexa, I'm equally happy to fix them in one go.

DK will reword this.

Conclusion: Update wording, revisit in Lenexa.

[2016-08 Chicago]

Monday PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 27.4.3.5 [string.capacity] p14 as depicted:

   >     void shrink_to_fit();

   > > -14- RemarksEffects: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note] It does not increase `capacity()`, but may reduce `capacity()` by causing reallocation.

   > > -?- Complexity: Linear in the size of the sequence.

   > > -?- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, they remain valid.

2. Change 23.3.5.3 [deque.capacity] p5-p7 as depicted:

   >     void shrink_to_fit();

   > > -5- Requires: `T` shall be `MoveInsertable` into `*this`.

   > > -?- Effects: `shrink_to_fit` is a non-binding request to reduce memory use but does not change the size of the sequence. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note] If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

   > > -6- Complexity: Linear in the size of the sequence.

   > > -7- Remarks: `shrink_to_fit` is a non-binding request to reduce memory use but does not change the size of the sequence. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]`shrink_to_fit` invalidates all the references, pointers, and iterators referring to the elements in the sequence.

3. Change 23.3.13.3 [vector.capacity] p7-p9 as depicted:

   >     void shrink_to_fit();

   > > -7- Requires: `T` shall be `MoveInsertable` into `*this`.

   > > -?- Effects: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note] It does not increase `capacity()`, but may reduce `capacity()` by causing reallocation. If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

   > > -8- Complexity: Linear in the size of the sequence.

   > > -9- Remarks: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note] If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, they remain valid.

4. Change 23.3.13.5 [vector.modifiers] p1 as depicted:

   > -1- Remarks: Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, all the iterators and references before the insertion point remain valid. […]

### 2224(i). Ambiguous status of access to non-live objects

Section: 16.4.5.10 [res.on.objects] Status: C++17 Submitter: Geoffrey Romer Opened: 2012-12-17 Last modified: 2017-09-07

Priority: 2

View all other issues in [res.on.objects].

View all issues with C++17 status.

Discussion:

The standard currently does not discuss when library objects may be accessed, except in a non-normative note pertaining to synchronization in [res.on.objects], leaving it ambiguous whether single-threaded code can access a library object during its construction or destruction. For example, there is a reasonable question as to what happens if the deleter supplied to a `unique_ptr` transitively accesses the `unique_ptr` itself during `unique_ptr`'s destruction; a straightforward reading suggests that this is permitted, and that the deleter will see the `unique_ptr` still holding the originally stored pointer, but consensus on the LWG reflector indicates this was not the intent (see discussion beginning with c++std-lib-33362).

[2013-03-15 Issues Teleconference]

Moved to Open.

Geoffrey will provide an example that clearly highlights the issue.

[2013-03-19 Geoffrey provides revised resolution and an example]

I contend that the most straightforward reading of the current standard requires the following example code to print "good" (because `~unique_ptr` is not specified to modify the state of the internal pointer), but the consensus on the reflector was that its behavior should be undefined.

This example also shows that, contrary to a comment in the telecon, the PR is not tautological. 11.9.5 [class.cdtor]/p4 explicitly permits member function calls during destruction, so the behavior of this code is well-defined as far as the core language is concerned, despite the fact that it accesses a library object after the end of the object's lifetime. If we want this code to have undefined behavior, we need to specify that at the library level.

>     #include <memory>
>     #include <iostream>

>     class A;

>     struct B {
>      std::unique_ptr<A> a;
>     };

>     struct A {
>      B* b;
>      ~A() {
>        if (b->a.get() == this) {
>          std::cout << "good" << std::endl;
>        }
>      }
>     };

>     int main() {
>      B b;
>      b.a.reset(new A);
>      b.a->b = &b;
>     }

Previous resolution:

> 1. Change the title of sub-clause 16.4.5.10 [res.on.objects] as indicated:

>    Shared objects and the libraryLibrary object access [res.on.objects]

> 2. Edit 16.4.5.10 [res.on.objects] p2 as indicated:

>    -2- [Note: In particular, the program is required to ensure that completion of the constructor of any object of a class type defined in the standard library happens before any other member function invocation on that object and, unless otherwise specified, to ensure that completion of any member function invocation other than destruction on such an object happens before destruction of that object. This applies even to objects such as mutexes intended for thread synchronization. — end note] If an object of a standard library type is accessed outside of the object's lifetime (6.8.4 [basic.life]), the behavior is undefined unless otherwise specified.

[2014 Urbana]

STL: is this resolved by our change to the reeentrancy rules? [LWG 2414(i)]
GR: don't think that solves the multi-threaded case
MC: I like changing the note to normative text
GR: uses the magic "happens before" words, and "access" is magic too
JW: I like this. strict improvement, uses the right wording we have to say this properly
STL: I like the last sentence of the note, could we add that as a new note at the end?
So add "[Note: This applies even to objects such as mutexes intended for thread synchronization.]" to the end and move to Ready

Proposed resolution:

This wording is relative to N3485.

1. Change the title of sub-clause 16.4.5.10 [res.on.objects] as indicated:

   Shared objects and the libraryLibrary object access [res.on.objects]

2. Edit 16.4.5.10 [res.on.objects] p2 as indicated: [Editorial remark: The motivation, is to be more precise about the meaning of "outside the object's lifetime" in the presence of threads — end editorial remark]

   -2- [Note: In particular, the program is required to ensure that completion of the constructor of any object of a class type defined in the standard library happens before any other member function invocation on that object and, unless otherwise specified, to ensure that completion of any member function invocation other than destruction on such an object happens before destruction of that object. This applies even to objects such as mutexes intended for thread synchronization. — end note] If an object of a standard library type is accessed, and the beginning of the object's lifetime (6.8.4 [basic.life]) does not happen before the access, or the access does not happen before the end of the object's lifetime, the behavior is undefined unless otherwise specified. [Note: This applies even to objects such as mutexes intended for thread synchronization. — end note]

### 2225(i). Unrealistic header inclusion checks required

Section: 16.4.3.2 [using.headers] Status: C++14 Submitter: Richard Smith Opened: 2012-12-18 Last modified: 2025-10-15

Priority: Not Prioritized

View all other issues in [using.headers].

View all issues with C++14 status.

Discussion:

16.4.3.2 [using.headers]/3 says:

A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header.

Per 4.1 [intro.compliance]/1, programs which violate this rule are ill-formed, and a conforming implementation is required to produce a diagnostic. This does not seem to match reality. Presumably, this paragraph is missing a "no diagnostic is required".

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

[2025-10-15; related to LWG 657(i)]

Proposed resolution:

This wording is relative to N3485.

1. Edit 16.4.3.2 [using.headers] p3 as indicated:

   -3- A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header. No diagnostic is required.

### 2228(i). Missing SFINAE rule in `unique_ptr` templated assignment

Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: Resolved Submitter: Geoffrey Romer Opened: 2012-12-20 Last modified: 2016-01-28

Priority: 3

View all other issues in [unique.ptr.single.asgn].

View all issues with Resolved status.

Discussion:

20.3.1.3.4 [unique.ptr.single.asgn]/5 permits `unique_ptr`'s templated assignment operator to participate in overload resolution even when incompatibilities between `D` and `E` will render the result ill-formed, but the corresponding templated copy constructor is removed from the overload set in those situations (see the third bullet point of 20.3.1.3.2 [unique.ptr.single.ctor]/19). This asymmetry is confusing, and presumably unintended; it may lead to situations where constructing one `unique_ptr` from another is well-formed, but assigning from the same `unique_ptr` would be ill-formed.

There is a slight coupling between this and LWG 2118(i), in that my PR for LWG 2118(i) incorporates equivalent wording in the specification of the templated assignment operator for the array specialization; the two PRs are logically independent, but if my PR for 2118(i) is accepted but the above PR is not, the discrepancy between the base template and the specialization could be confusing.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3485.

> 1. Revise 20.3.1.3.4 [unique.ptr.single.asgn] p5 as follows:

>    >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

>    > > -4- Requires: If `E` is not a reference type, assignment of the deleter from an rvalue of type `E` shall be well-formed and shall not throw an exception. Otherwise, `E` is a reference type and assignment of the deleter from an lvalue of type `E` shall be well-formed and shall not throw an exception.

>    > > -5- Remarks: This operator shall not participate in overload resolution unless:

>    > > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer` and

>    > > - `U` is not an array type., and

>    > > - either `D` is a reference type and `E` is the same type as `D`, or `D` is not a reference type and `E` is implicitly convertible to `D`.

>    > > -6- Effects: Transfers ownership from `u` to `*this` as if by calling `reset(u.release())` followed by an assignment from `std::forward<E>(u.get_deleter())`.

>    > > -7- Returns: `*this`.

[2013-03-15 Issues Teleconference]

Moved to Review.

The wording looks good, but we want a little more time than the telecon permits to be truly comfortable. We expect this issue to resolve fairly easily in Bristol.

[2015-05-18, Howard comments]

Updated proposed wording has been provided in N4366.

[2015-05, Lenexa]

Straw poll: send N4366 to full committee, with both fixes from the sections "What is the correct fix?" and "`unique_ptr<T[]>` needs the correct fix too"

Proposed resolution:

Resolved by accepting N4366.

### 2229(i). Standard code conversion facets underspecified

Section: 99 [depr.locale.stdcvt] Status: C++14 Submitter: Beman Dawes Opened: 2012-12-30 Last modified: 2017-04-22

Priority: Not Prioritized

View all other issues in [depr.locale.stdcvt].

View all issues with C++14 status.

Discussion:

The only specification for the non-inherited members of classes `codecvt_utf8`, `codecvt_utf16`, and `codecvt_utf8_utf16` is a comment line in the synopsis that says `// unspecified`. There is no further indication of functionality, so a user does not know if one of these classes can be constructed or destroyed.

The proposed resolution adds a constructor that mimics the class `codecvt` constructor, and also adds a destructor. Following the practice of class `codecvt`, the semantics are not specified.

The only existing implementation I could find was libc++, and it does supply the proposed constructor and destructor for each of the three classes.

[2013-03-15 Issues Teleconference]

Moved to Review.

There was concern about the unspecified semantics - but that matches what is done in `codecvt`.

Jonathan: Should these constructor/destructors be public? Proposed wording is private. Base class constructor is public.

Howard noted that other facets do not have specified constructors.

Alisdair noted that this whole section was new in C++11.

Howard suggested looking at section 28.3.3.1.2.2 [locale.facet]p2/p3 for more info.

[2013-04-18, Bristol]

Proposed resolution:

In [locale.stdcvt] paragraph 2, Header codecvt synopsis:

>     template<class Elem, unsigned long Maxcode = 0x10ffff,
>       codecvt_mode Mode = (codecvt_mode)0>
>     class codecvt_utf8
>       : public codecvt<Elem, char, mbstate_t> {
>       // unspecified
>     public:
>       explicit codecvt_utf8(size_t refs = 0);
>       ~codecvt_utf8();
>       };

>     template<class Elem, unsigned long Maxcode = 0x10ffff,
>       codecvt_mode Mode = (codecvt_mode)0>
>     class codecvt_utf16
>       : public codecvt<Elem, char, mbstate_t> {
>       // unspecified
>     public:
>       explicit codecvt_utf16(size_t refs = 0);
>       ~codecvt_utf16();
>       };

>     template<class Elem, unsigned long Maxcode = 0x10ffff,
>       codecvt_mode Mode = (codecvt_mode)0>
>     class codecvt_utf8_utf16
>       : public codecvt<Elem, char, mbstate_t> {
>       // unspecified
>     public:
>       explicit codecvt_utf8_utf16(size_t refs = 0);
>       ~codecvt_utf8_utf16();
>       };

### 2230(i). "see below" for initializer-list constructors of unordered containers

Section: 23.5 [unord] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-01-06 Last modified: 2017-07-30

Priority: 4

View all other issues in [unord].

View all issues with C++17 status.

Discussion:

The `unordered_map` class definition in 23.5.3.1 [unord.map.overview] declares an initializer-list constructor that says "see below":

>     unordered_map(initializer_list<value_type>,
>         size_type = see below,
>         const hasher& hf = hasher(),
>         const key_equal& eql = key_equal(),
>         const allocator_type& a = allocator_type());

But that constructor isn't defined below. The same problem exists for the other unordered associative containers.

[2013-09 Chicago]

STL: ordered are also missing declarations, but issue is forthcoming

Walter: how does adding a signature address issue? — nevermind

Jayson: in his wording, isn't he just dropping the `size_type`?

Walter: partial fix is to introduce the name

Stefanus: explanation of requiring name because of n buckets

STL: solution for his issue satisfies both ordered and unordered and is simplier than provided wording

STL: patches general table instead

STL: proposes adding extra rows instead of extra declarations

Stefanus: clarify `n` in the synopsis

Walter: general rule, name is optional in declaration

Stefanus: how to proceed

Walter: significant overlap with forthcoming issue, suggestion to defer

[2014-02-20 Re-open Deferred issues as Priority 4]

[2014-03-27 Jonathan improves proposed wording]

[2014-05-20 STL and Jonathan communicate]

STL: With 2322(i) resolved, is there anything left for this issue to fix?

Jonathan: The synopsis still says "see below" and it's not immediately clear that "see below" means "see the definition of a different constructor, which defines the behaviour of this one due to a table defined much earlier".

[2014-05-23 Library reflector vote]

The issue has been identified as Tentatively Ready based on five votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Edit 23.5.3.1 [unord.map.overview], class template `unordered_map` synopsis, as follows:

   >     […]
   >     unordered_map(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     […]

2. Edit 23.5.3.2 [unord.map.cnstr] as follows:

   >     template <class InputIterator>
   >     unordered_map(InputIterator f, InputIterator l,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     unordered_map(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());

   > > -3- Effects: Constructs an empty `unordered_map` using the specified hash function, key equality function, and allocator, and using at least `n` buckets. If `n` is not provided, the number of buckets is implementation-defined. Then inserts elements from the range `[f, l)` for the first form, or from the range `[il.begin(), il.end())` for the second form. `max_load_factor()` returns `1.0`.

3. Edit 23.5.4.1 [unord.multimap.overview], class template `unordered_multimap` synopsis, as follows:

   >     […]
   >     unordered_multimap(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     […]

4. Edit 23.5.4.2 [unord.multimap.cnstr] as follows:

   >     template <class InputIterator>
   >     unordered_multimap(InputIterator f, InputIterator l,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     unordered_multimap(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());

   > > -3- Effects: Constructs an empty `unordered_multimap` using the specified hash function, key equality function, and allocator, and using at least `n` buckets. If `n` is not provided, the number of buckets is implementation-defined. Then inserts elements from the range `[f, l)` for the first form, or from the range `[il.begin(), il.end())` for the second form. `max_load_factor()` returns `1.0`.

5. Edit 23.5.6.1 [unord.set.overview], class template `unordered_set` synopsis, as follows:

   >     […]
   >     unordered_set(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     […]

6. Edit 23.5.6.2 [unord.set.cnstr] as follows:

   >     template <class InputIterator>
   >     unordered_set(InputIterator f, InputIterator l,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     unordered_set(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());

   > > -3- Effects: Constructs an empty `unordered_set` using the specified hash function, key equality function, and allocator, and using at least `n` buckets. If `n` is not provided, the number of buckets is implementation-defined. Then inserts elements from the range `[f, l)` for the first form, or from the range `[il.begin(), il.end())` for the second form. `max_load_factor()` returns `1.0`.

7. Edit 23.5.7.1 [unord.multiset.overview], class template `unordered_multiset` synopsis, as follows:

   >     […]
   >     unordered_multiset(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     […]

8. Edit 23.5.7.2 [unord.multiset.cnstr] as follows:

   >     template <class InputIterator>
   >     unordered_multiset(InputIterator f, InputIterator l,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     unordered_multiset(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());

   > > -3- Effects: Constructs an empty `unordered_multiset` using the specified hash function, key equality function, and allocator, and using at least `n` buckets. If `n` is not provided, the number of buckets is implementation-defined. Then inserts elements from the range `[f, l)` for the first form, or from the range `[il.begin(), il.end())` for the second form. `max_load_factor()` returns `1.0`.

### 2231(i). DR 704 removes complexity guarantee for `clear()`

Section: 23.2.4 [sequence.reqmts] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-12-30 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [sequence.reqmts].

View all other issues in [sequence.reqmts].

View all issues with C++14 status.

Discussion:

From the question at stackoverflow.

Were we aware that the resolution to LWG 704(i) means there is no complexity guarantee for `clear()` on most sequence containers? Previously it was implied by defining it in terms of `erase(begin(), end())` but we no longer do that.

There are explicit complexity requirements for `std::list::clear()`, but not the other sequence containers.

Daniel:

The idea was that the notion of "destroys all elements in `a`" would imply a linear complexity, but the wording needs to be clearer, because this doesn't say that this step is the actual complexity bound.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Change Table 100 as indicated:

   > Table 100 — Sequence container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition
   > --- | --- | ---
   > `…`
   > `a.clear()` | `void` | Destroys all elements in `a`. Invalidates all references, pointers, and iterators referring to the elements of `a` and may invalidate the past-the-end iterator. post: `a.empty()` returns `true` complexity: linear
   > `…`

### 2232(i). [CD] The `char_traits` specializations should declare their `length()`, `compare()`, and `find()` members constexpr

Section: 27.2.4 [char.traits.specializations] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2012-12-24 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [char.traits.specializations].

View all issues with Resolved status.

Discussion:

Addresses ES 14, US 19

These functions have easy recursive constexpr implementations that, unfortunately, aren't efficient at runtime. EWG is still figuring out how to solve this problem in general (e.g., N3444 isn't sufficient to avoid stack overflows in debug builds or to get the optimal assembly-based implementations at runtime), so users can't portably solve this problem for themselves, but implementations can use compiler-specific techniques to choose the right implementation inside their standard libraries.

The LWG is still undecided about whether individual implementations can add constexpr to these functions, so we need to add `constexpr` to the standard here for implementations to be able to improve this.

[2013-03-15 Issues Teleconference]

Moved to Open.

There are a number of people who have a strong interest in this issue not available for the telecon.

It also plays at the heart of a discussion about library freedoms for `constexpr` and specifying a library that may depend on unspecified compiler intrinsics to be implementable.

[2013-09 Chicago]

Moved to NAD Future.

While it is clear that this feature can be implemented using only C++14 `constexpr` features, there is real concern that we cannot call the efficient, highly optimized, C implementations of these functions under a C++14 `constexpr` implementation, nor implement similar ourselves as this typically involves use of inline `asm` instructions.

Clang and libc++ have some experience of using intrinsics to try to address the performance issue, but the current intrinsics are not general enough to support `char_traits`. The intrinsics support only operations on character string literals, and the string literal is no longer visible as a literal after passing as a `const char *` to the `char_traits` functions.

Additional concern was raised that these operations are unlikely to be useful anyway, as the only client is `basic_string` which relies on dynamic memory allocation, and so cannot effectively be made a literal type. Jeffrey then pointed out the pending `string_view` library that will also use `char_traits` and would most certainly benefit from being a literal type.

Given the choice of giving up performance on a critical library component, or requiring a compiler intrinsic with only unsuccessful implementation experience, the consensus is to not reject this, unless compelling implementation experience is demonstrated. NAD Future seems the appropriate resolution.

[2017-06-02 Issues Telecon]

Resolved by P0426R1, adopted in Issaquah.

Proposed resolution:

This wording is relative to N3691.

1. In 27.2.4.2 [char.traits.specializations.char], [char.traits.specializations.char16_t], [char.traits.specializations.char32_t], and 27.2.4.6 [char.traits.specializations.wchar.t]:

   >     static constexpr int compare(const char_type* s1, const char_type* s2, size_t n);
   >     static constexpr size_t length(const char_type* s);
   >     static constexpr const char_type* find(const char_type* s, size_t n, const char_type& a);

### 2233(i). `bad_function_call::what()` unhelpful

Section: 22.10.17.2 [func.wrap.badcall] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-01-05 Last modified: 2017-09-07

Priority: 3

View all issues with C++17 status.

Discussion:

A strict reading of the standard implies `std::bad_function_call{}.what()` returns the same string as `std::exception{}.what()` which doesn't help to know what happened if you catch an exception by reference to `std::exception`.

For consistency with `bad_weak_ptr::what()` it should return `"bad_function_call"`.

See c++std-lib-33515 for other details.

There was a considerable support on the reflector to instead change the specification of both `bad_weak_ptr::what()` and `bad_function_call::what()` to return an implementation-defined string instead.

[2013-03-15 Issues Teleconference]

Moved to Open.

Consensus that we want consistency in how this is treated. Less consensus on what the common direction should be.

Alisdair to provide wording proposing that all string literals held by standard exception objects are either unspecified, or implmentation defined.

[2014-02-15 Issauqah]

STL: I think it should be an implementation-defined NTBS, same on `bad_weak_ptr`. I will write a PR.

[2014-03-27, STL provides improved wording]

The new wording reflects better the general agreement of the committee, see also issue 2376(i) for similar wording.

[2014-03-28 Library reflector vote]

The issue has been identified as Tentatively Ready based on five votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Edit [func.wrap.badcall.const]:

   >     bad_function_call() noexcept;

   > > -1- Effects: constructs a `bad_function_call object`.

   > > -?- Postconditions: `what()` returns an implementation-defined NTBS.

### 2234(i). `assert()` should allow usage in constant expressions

Section: 19.3 [assertions] Status: C++17 Submitter: Daniel Krügler Opened: 2013-01-12 Last modified: 2017-07-30

Priority: 2

View all other issues in [assertions].

View all issues with C++17 status.

Discussion:

It is unclear from the current specification whether `assert()` expressions can be used in (potential) constant expressions. As an example consider the implementation of a `constexpr` function:

>     #include <cassert>

>     template<class T, unsigned N>
>     struct array {
>       T data[N];
>       constexpr const T& operator[](unsigned i) const {
>         return assert(i < N), data[i];
>       }
>     };

>     int main() {
>       constexpr array<int, 3> ai = {1, 2, 3};
>       constexpr int i = ai[0];
>       int j = ai[0];
>       // constexpr int k = ai[5];
>     }

The first question is whether this program is guaranteed well-formed? A second question is whether is would guaranteed to be ill-formed, if we uncomment the last code line in `main()`?

The wording in 19.3 [assertions] doesn't add anything significant to the C99 wording. From the C99 specification (7.2 p1 and 7.2.1.1 p2) we get already some valuable guarantees:

- The expression `assert(e)` is a `void` expression for all expressions `e` independent of the definition of `NDEBUG`.

- If `NDEBUG` is defined, `assert(e)` is equivalent to the expression `void()` (or anything that cannot be distinguished from that).

The current wording does not yet guarantee that `assert` expressions can be used in constant expressions, but all tested implementations (gcc, MSVC) would already support this use-case. It seems to me that this should be possible without giving `assert` a special meaning for the core language.

As a related comment it should be added, that there is a core language proposal that intents to relax some current constraints for `constexpr` functions and `literal` types. The most interesting one (making `void` a literal types and allowing for expression-statements) would simplify the motivating example implementation of `operator[]` to:

>     constexpr const T& operator[](unsigned i) const {
>       assert(i < N);
>       return data[i];
>     };

[2013-03-15 Issues Teleconference]

Moved to Open.

We are still gaining experience with `constexpr` as a language feature, and there may be work in Evolution that would help address some of these concerns. Defer discussion until we have a group familiar with any evolutionary direction.

[2014-06-08, Daniel comments and suggests wording]

After approval of N3652, `void` is now a literal type and `constexpr` functions can contain multiple statements, so this makes the guarantee that `assert` expressions are per-se `constexpr`-friendly even more relevant. A possible wording form could be along the lines of:

> For every core constant expression e of scalar type that evaluates to `true` after being contextually converted to `bool`, the expression `assert(e)` shall be a prvalue core constant expression of type `void`.

Richard Smith pointed out some weaknesses of this wording form, for example it would not guarantee to require the following example to work:

>     constexpr void check(bool b) { assert(b); }

because `b` is not a core constant expression in this context.

He suggested improvements that lead to the wording form presented below (any defects mine).

[Lenexa 2015-05-05]

MC : ran into this
Z : Is it guaranteed to be an expression?
MC : clarifies that assert runs at runtime, not sure what it does at compile time
STL : c standard guarantees its an expression and not a whole statement, so comma chaining it is ok
HH : Some implementations work as author wants it to
STL : also doing this as constexpr
DK/STL : discussing how this can actually work
HH : GCC 5 also implements it. We have implementor convergence
MC : Wants to do this without giving assert a special meaning
STL : NDEBUG being defined where assert appears is not how assert works. This is bug in wording. Should be "when assert is defined" or something like that. ... is a constant subexpression if NDEBUG is defined at the point where assert is last defined or redefined."
Would like to strike the "either" because ok if both debug or assertion is true. We want inclusive-or here
MC : is redefined needed?
STL : my mental model is its defined once and then redefined
HH : wants to up to P2
Z/STL : discussing how wording takes care of how/when assert is defined/redefefined
STL/WB : discussing whether to move to ready or review. -> Want to move it to ready.
ask for updated wording
p3 -> p2
plan to go to ready after checking wording

[Telecon 2015-06-30]

HH: standardizing existing practice
MC: what about the comment from Lenexa about striking "either"?
HH: all three implementations accept it
MC: update issue to strike "either" and move to Tentatively Ready

Proposed resolution:

This wording is relative to N3936.

Previous resolution [SUPERSEDED]:

> 1. Introduce the following new definition to the existing list in [definitions]: [Drafting note: If LWG 2296(i) is accepted before this issue, the accepted wording for the new definition should be used instead — end drafting note]

>    > constant subexpression [defns.const.subexpr]

>    > an expression whose evaluation as subexpression of a conditional-expression CE (7.6.16 [expr.cond]) would not prevent CE from being a core constant expression (7.7 [expr.const]).

> 2. Insert a new paragraph following 19.3 [assertions] p1 as indicated:

>    > -?- An expression `assert(E)` is a constant subexpression (3.15 [defns.const.subexpr]), if either

>    > - `NDEBUG` is defined at the point where `assert(E)` appears, or

>    > - `E` contextually converted to `bool` (7.3 [conv]), is a constant subexpression that evaluates to the value `true`.

1. Introduce the following new definition to the existing list in [definitions]: [Drafting note: If LWG 2296(i) is accepted before this issue, the accepted wording for the new definition should be used instead — end drafting note]

   > constant subexpression [defns.const.subexpr]

   > an expression whose evaluation as subexpression of a conditional-expression CE (7.6.16 [expr.cond]) would not prevent CE from being a core constant expression (7.7 [expr.const]).

2. Insert a new paragraph following 19.3 [assertions] p1 as indicated:

   > -?- An expression `assert(E)` is a constant subexpression (3.15 [defns.const.subexpr]), if

   > - `NDEBUG` is defined at the point where `assert(E)` appears, or

   > - `E` contextually converted to `bool` (7.3 [conv]), is a constant subexpression that evaluates to the value `true`.

### 2235(i). Undefined behavior without proper requirements on `basic_string` constructors

Section: 27.4.3.3 [string.cons] Status: C++14 Submitter: Juan Soulie Opened: 2013-01-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [string.cons].

View all issues with C++14 status.

Discussion:

In 27.4.3.3 [string.cons], I believe tighter requirements should be imposed on `basic_string`'s constructors taking an `s` argument (or, a behavior should be provided for the undefined cases). These requirements are properly stated in the other members functions taking `s` arguments (`append`, `assign`, `insert`,...).

>     basic_string(const charT* s, size_type n, const Allocator& a = Allocator());

Relative to N3485, 27.4.3.3 [string.cons]/6 says "Requires: `s` shall not be a null pointer and `n < npos`", where it should say: "Requires: `s` points to an array of at least `n` elements of `charT`"

>     basic_string(const charT* s, const Allocator& a = Allocator());

27.4.3.3 [string.cons]/8 says "Requires: `s` shall not be a null pointer.", where it should say: "Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`"

Daniel:

I think that 16.4.5.9 [res.on.arguments] p1 b2 basically requires this already, but the wording is indeed worth improving it.

[2013-03-15 Issues Teleconference]

Moved to Review.

The resolution could be worded more cleanly, and there is some concern about redundancy between Requirements and Effects clauses. Consensus that we do want to say something like this for the Requirements though.

[2013-04-18, Bristol]

Move to Ready

[2013-09-29, Bristol]

Apply to the Working Paper

Proposed resolution:

This wording is relative to N3485.

1. Change 27.4.3.3 [string.cons]/6 as indicated:

   >     basic_string(const charT* s, size_type n, const Allocator& a = Allocator());

   > > -6- Requires: `s` shall not be a null pointer and `n < npos` points to an array of at least `n` elements of `charT`.

2. Change 27.4.3.3 [string.cons]/8 as indicated:

   >     basic_string(const charT* s, const Allocator& a = Allocator());

   > > -8- Requires: `s` shall not be a null pointerpoints to an array of at least `traits::length(s) + 1` elements of `charT`.

### 2239(i). `min/max/minmax` requirements

Section: 26.8.9 [alg.min.max] Status: C++17 Submitter: Juan Soulie Opened: 2013-01-26 Last modified: 2017-07-30

Priority: 3

View other active issues in [alg.min.max].

View all other issues in [alg.min.max].

View all issues with C++17 status.

Discussion:

26.8.9 [alg.min.max] requires type `T` in `min`, `max`, and `minmax` to be `LessThanComparable`, but I don't believe this should be required for the versions that take a `Compare` argument.

Paragraphs 1 to 4 of 26.8 [alg.sorting] should apply anyway, although I'm not sure about `Compare` being required to induce a strict weak ordering here.

Further, `min` and `max` also lack formal complexity guarantees.

[2014-06-07 Daniel comments and provides wording]

Certainly, the functions with `Compare` should not impose `LessThanComparable` requirements.

In regard to the question whether a strict weak ordering should be required as implied by the `Compare` requirements, I would like to point out that this is requirement is in fact needed, because the specification of the normative Remarks elements (e.g. "Returns the first argument when the arguments are equivalent.") do depend on the existence of a equivalence relation that can be relied on and this is also consistent with the same strict weak ordering requirement that is indirectly imposed by the `LessThanComparable` requirement set for functions referring to `operator<` (Let me note that the very same `StrictWeakOrder` language concept had intentionally been required for similar reasons during "concept-time" in N2914).

[2015-02 Cologne]

JY: We have library-wide requirements that `Comp` induce a strict weak ordering.

JY/MC: The un-marked-up "Complexity" (p16) is wrong. DK: I'll fix that.

DK will update the wording for Lenexa.

[2015-03-30 Daniel comments]

The Complexity element of p16 is correct, but some others involving `initializer_list` arguments are wrong.

[2015-04-02 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N4296.

1. Change 26.8.9 [alg.min.max] as indicated:

   >     template<class T> constexpr const T& min(const T& a, const T& b);
   >     template<class T, class Compare>
   >       constexpr const T& min(const T& a, const T& b, Compare comp);

   > > -1- Requires: For the first form, type `T` shall beType `T` is `LessThanComparable` (Table 18).

   > > -2- Returns: The smaller value.

   > > -3- Remarks: Returns the first argument when the arguments are equivalent.

   > > -?- Complexity: Exactly one comparison.

   >     template<class T>
   >       constexpr T min(initializer_list<T> t);
   >     template<class T, class Compare>
   >       constexpr T min(initializer_list<T> t, Compare comp);

   > > -4- Requires: `T` is `LessThanComparable` andshall be `CopyConstructible` and `t.size() > 0`. For the first form, type `T` shall be `LessThanComparable`.

   > > -5- Returns: […]

   > > -6- Remarks: […]

   > > -?- Complexity: Exactly `t.size() - 1` comparisons.

   >     template<class T> constexpr const T& max(const T& a, const T& b);
   >     template<class T, class Compare>
   >       constexpr const T& max(const T& a, const T& b, Compare comp);

   > > -7- Requires: For the first form, type `T` shall beType `T` is `LessThanComparable` (Table 18).

   > > -8- Returns: […]

   > > -9- Remarks: […]

   > > -?- Complexity: Exactly one comparison.

   >     template<class T>
   >       constexpr T max(initializer_list<T> t);
   >     template<class T, class Compare>
   >       constexpr T max(initializer_list<T> t, Compare comp);

   > > -10- Requires: `T` is `LessThanComparable` andshall be `CopyConstructible` and `t.size() > 0`. For the first form, type `T` shall be `LessThanComparable`.

   > > -11- Returns: […]

   > > -12- Remarks: […]

   > > -?- Complexity: Exactly `t.size() - 1` comparisons.

   >     template<class T> constexpr pair<const T&, const T&> minmax(const T& a, const T& b);
   >     template<class T, class Compare>
   >       constexpr pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp);

   > > -13- Requires: For the first form, tType `T` shall be `LessThanComparable` (Table 18).

   > > -14- Returns: […]

   > > -15- Remarks: […]

   > > -16- Complexity: Exactly one comparison.

   >     template<class T>
   >       constexpr pair<T, T> minmax(initializer_list<T> t);
   >     template<class T, class Compare>
   >       constexpr pair<T, T> minmax(initializer_list<T> t, Compare comp);

   > > -17- Requires: `T` is `LessThanComparable` andshall be `CopyConstructible` and `t.size() > 0`. For the first form, type `T` shall be `LessThanComparable`.

   > > -18- Returns: […]

   > > -19- Remarks: […]

   > > -20- Complexity: At most `(3/2) * t.size()` applications of the corresponding predicate.

### 2240(i). Probable misuse of term "function scope" in [thread.condition]

Section: 32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany] Status: Resolved Submitter: FrankHB1989 Opened: 2013-02-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [thread.condition.condvar].

View all issues with Resolved status.

Discussion:

All usages of "function scope" in 32.7.4 [thread.condition.condvar] and 32.7.5 [thread.condition.condvarany], such as 32.7.4 [thread.condition.condvar] p10 b4:

> If the function exits via an exception, lock.lock() shall be called prior to exiting the function scope.

seem to be inappropriate compared to the actual core language definition of [basic.funscope]:

> Labels (6.1) have function scope and may be used anywhere in the function in which they are declared. Only labels have function scope.

Probably the intended meaning is "outermost block scope of the function".

[2013-09 Chicago: Resolved by proposed resolution of LWG 2135(i)]

Proposed resolution:

Resolved by proposed resolution of LWG 2135(i).

### 2241(i). `<cstdalign>` and `#define` of `alignof`

Section: 17.14 [support.runtime] Status: Resolved Submitter: Richard Smith Opened: 2013-02-14 Last modified: 2020-09-06

Priority: 2

View all other issues in [support.runtime].

View all issues with Resolved status.

Discussion:

According to 17.14 [support.runtime] p2:

> The contents of these headers are the same as the Standard C library headers [..], `<stdalign.h>`, [..]

Since our base C standard is C99, which doesn't have a `<stdalign.h>`, the reference to a non-existing C header is irritating (In this context `<stdalign.h>` doesn't refer to the deprecated C++ header `<stdalign.h>` described in [depr.c.headers]).

Furthermore, it would be also important that it doesn not define a macro named `alignof`, which C11 also defines in this header.

Currently we only have the following guarantee as part of 17.14 [support.runtime] p7:

> The header `<cstdalign>` and the header `<stdalign.h>` shall not define a macro named `alignas`.

It is unclear what the better strategy is: Striking the reference to `<stdalign.h>` in 17.14 [support.runtime] p2 or upgrading to C11 as new base C standard.

[2014-02-15 Issaquah]

STL: related to earlier issue on C4, 2201(i), and now we get a C11 header
JY: find _Alignof as keyword C11 FDIS has four defines in stdalign.h
AM: need paper for C11 as base library we should really do that
STL: really need vendor input
STL: don't think we need to do anything right now not P1
AM: any objections to downscale to P2 (no objections)

[2016-03 Jacksonville]

Walter: this is on track to go away if we adopt Clark's paper to rebase to C11
Room: tentatively resolved; revisit after C11 paper: P0063

[2016-03 Oulu]

P0063 was adopted.

Change status to Tentatively Resolved

Proposed resolution:

### 2243(i). `istream::putback` problem

Section: 31.7.5.4 [istream.unformatted] Status: C++20 Submitter: Juan Soulie Opened: 2013-03-01 Last modified: 2021-02-25

Priority: 3

View all other issues in [istream.unformatted].

View all issues with C++20 status.

Discussion:

In 31.7.5.4 [istream.unformatted] / 34, when describing `putback`, it says that "`rdbuf->sputbackc()`" is called. The problem are not the obvious typos in the expression, but the fact that it may lead to different interpretations, since nowhere is specified what the required argument to `sputbackc` is.

It can be guessed to be "`rdbuf()->sputbackc(c)`", but "`rdbuf()->sputbackc(char_type())`" or just anything would be as conforming (or non-conforming) as the first guess.

[2017-12-12, Jonathan comments and provides wording]

Fix the bogus expression, and change `sputbackc()` to just `sputbackc` since we're talking about the function, not an expression `sputbackc()` (which isn't a valid expression any more than `rdbuf->sputbackc()` is). Make the corresponding change to the equivalent wording in p36 too.

[ 2017-12-14 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Change 31.7.5.4 [istream.unformatted] as shown:

   >     basic_istream<charT, traits>& putback(char_type c);

   > > -34- Effects: Behaves as an unformatted input function (as described above), except that the function first clears `eofbit`. After constructing a sentry object, if `!good()` calls `setstate(failbit)` which may throw an exception, and return. If `rdbuf()` is not null, calls `rdbuf()->sputbackc(c)`. If `rdbuf()` is null, or if `sputbackc()` returns `traits::eof()`, calls `setstate(badbit)` (which may throw `ios_base::failure` (31.5.4.4 [iostate.flags])). [Note: This function extracts no characters, so the value returned by the next call to `gcount()` is `0`. — end note]

   > > -35- Returns: `*this`.

   >     basic_istream<charT, traits>& unget();

   > > -36- Effects: Behaves as an unformatted input function (as described above), except that the function first clears `eofbit`. After constructing a sentry object, if `!good()` calls `setstate(failbit)` which may throw an exception, and return. If `rdbuf()` is not null, calls `rdbuf()->sungetc()`. If `rdbuf()` is null, or if `sungetc()` returns `traits::eof()`, calls `setstate(badbit)` (which may throw `ios_base::failure` (31.5.4.4 [iostate.flags])). [Note: This function extracts no characters, so the value returned by the next call to `gcount()` is `0`. — end note]

   > > -37- Returns: `*this`.

### 2244(i). Issue on `basic_istream::seekg`

Section: 31.7.5.4 [istream.unformatted] Status: C++17 Submitter: Juan Soulie Opened: 2013-03-04 Last modified: 2017-07-30

Priority: 3

View all other issues in [istream.unformatted].

View all issues with C++17 status.

Discussion:

When issue 1445(i) was resolved by adopting N3168, it exposed the need to modify both overloads of `basic_istream::seekg` (by inserting "the function clears eofbit," after "except that"), but the fix applied to the text apparently forgets the second overload at 31.7.5.4 [istream.unformatted] p43.

[2013-10-17: Daniel provides concrete wording]

It seems that the tiny sentence "SIMILARLY for 27.7.1.3/43 (`seekg`)." had been overlooked. I agree that the wording needs to be applied here as well.

[2015-05-06 Lenexa: Move to Ready]

MC: This was just missed when we added "the function first clears eofbit" to the other overload, Daniel agrees. Editing mistake.

Move to Ready, consensus.

Proposed resolution:

This wording is relative to N3691.

1. Change 31.7.5.4 [istream.unformatted] p43 as indicated:

   >     basic_istream<charT,traits>& seekg(off_type off, ios_base::seekdir dir);

   > > -43- Effects: Behaves as an unformatted input function (as described in 27.7.2.3, paragraph 1), except that the function first clears `eofbit`, it does not count the number of characters extracted, and does not affect the value returned by subsequent calls to `gcount()`. […]

### 2245(i). `packaged_task::reset()` memory allocation

Section: 32.10.10.2 [futures.task.members] Status: Resolved Submitter: Jonathan Wakely Opened: 2013-03-05 Last modified: 2017-03-20

Priority: 3

View all other issues in [futures.task.members].

View all issues with Resolved status.

Discussion:

The effects of `packaged_task::reset()` result in memory allocation, but don't allow a user to provide an allocator.

`packaged_task::reset()` needs to be overloaded like so:

>     template<class Alloc>
>     void reset(const Alloc&);

Alternatively, the effects of `reset()` need to require the same allocator is used as at construction, which would require the constructor to store the allocator for later use.

I like to remark that GCC at the moment uses the second option, i.e. the allocator passed to the constructor (if any) is used to create the new shared state, because this didn't require any change to the interface.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

No strong opinions in SG1, and this is really an LWG issue. Back to you.

[2016-08-02 Chicago, Billy O'Neal comments and suggests concrete wording]

Talked this over with Alasdair, who says there's little desire to allow the `packaged_task` to be change allocators after initial construction, making what libstdc++ does already the "right thing." A clarification note is still necessary to indicate that the allocator supplied to the `allocator_arg_t` constructor is to be used.

Wed PM: Move to Tentatively Ready

[2016-09-08]

Alisdair requests change to Review.

[2017-03-03, Kona]

This was resolved by adopting 2921(i), which removed the constructors that take allocators.

Proposed resolution:

This wording is relative to N4606

1. Change 32.10.10.2 [futures.task.members] as indicated:

   >     void reset();

   > > -22- Effects:

   > > - if the shared state associated with `*this` was created via the `packaged_task(F&& f)` constructor, aAs if `*this = packaged_task(std::move(f))`, where `f` is the task stored in `*this`.

   > > - if the shared state associated with `*this` was created via the `packaged_task(allocator_arg_t, Allocator& a, F&&)` constructor, as if `*this = packaged_task(allocator_arg, a, std::move(f))`, where `a` is the allocator used to allocate the shared state associated with `*this`, and `f` is the task stored in `*this`.

   > > [Note: This constructs a new shared state for `*this`. The old state is abandoned (30.6.4). — end note]

   > > -23- Throws:

   > > - if no allocator was used, `bad_alloc` if memory for the new shared state could not be allocated.

   > > - if an allocator was used, any exception thrown by `std::allocator_traits<Allocator>::template rebind_traits<unspecified>::allocate`.

   > > - any exception thrown by the move constructor of the task stored in the shared state.

   > > - `future_error` with an error condition of `no_state` if `*this` has no shared state.

### 2246(i). `unique_ptr` assignment effects w.r.t. deleter

Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-03-13 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [unique.ptr.single.asgn].

View all issues with C++14 status.

Discussion:

The Effects clauses for `unique_ptr` assignment don't make sense, what is the target of "an assignment from `std::forward<D>(u.get_deleter())`"?

Obviously it's intended to be the deleter, but that isn't stated clearly.

[2013-04-20, Bristol]

Move to Ready

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3485.

1. Edit 20.3.1.3.4 [unique.ptr.single.asgn] paragraph 2:

   >     unique_ptr& operator=(unique_ptr&& u) noexcept;

   > > […]

   > > -2- Effects: Transfers ownership from `u` to `*this` as if by calling `reset(u.release())` followed by an assignment from `get_deleter() = std::forward<D>(u.get_deleter())`.

2. Edit 20.3.1.3.4 [unique.ptr.single.asgn] paragraph 6:

   >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

   > > […]

   > > -6- Effects: Transfers ownership from `u` to `*this` as if by calling `reset(u.release())` followed by an assignment from `get_deleter() = std::forward<E>(u.get_deleter())`.

### 2247(i). Type traits and `std::nullptr_t`

Section: 21.3.5.2 [meta.unary.cat] Status: C++14 Submitter: Joe Gottman Opened: 2013-03-15 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++14 status.

Discussion:

According to 21.3.5.2 [meta.unary.cat], for every type `T`, exactly one of the primary type traits is true. So which is true for the type `std::nullptr_t`? By 5.13.8 [lex.nullptr] `std::nullptr_t` is not a pointer type or a pointer-to-member type, so `is_pointer`, `is_member_object_pointer` and `is_member_function_pointer` can't be true for `std::nullptr_t`, and none of the other primary type traits seem to apply.

[2013-04-20, Bristol]

Rename to `is_null_pointer`, move to Ready

Previous wording:

> This wording is relative to N3485.

> 1. Edit 21.3.3 [meta.type.synop], header `<type_traits>` synopsis:

>    >     namespace std {
>    >       […]
>    >       // 20.9.4.1, primary type categories:
>    >       template <class T> struct is_void;
>    >       template <class T> struct is_nullptr;
>    >       template <class T> struct is_integral;
>    >       template <class T> struct is_floating_point;
>    >       […]
>    >     }

> 2. Edit Table 47 — "Primary type category predicates" as indicated:

>    > Table 47 — Primary type category predicates
>    > Template | Condition | Comments
>    > --- | --- | ---
>    > `…`
>    > `template <class T> struct is_nullptr;` | `T` is `std::nullptr_t` ([basic.fundamental]) |
>    > `…`

[2013-09-29, Chicago]

Apply to the Working Paper

Proposed resolution:

This wording is relative to N3485.

1. Edit 21.3.3 [meta.type.synop], header `<type_traits>` synopsis:

   >     namespace std {
   >       […]
   >       // 20.9.4.1, primary type categories:
   >       template <class T> struct is_void;
   >       template <class T> struct is_null_pointer;
   >       template <class T> struct is_integral;
   >       template <class T> struct is_floating_point;
   >       […]
   >     }

2. Edit Table 47 — "Primary type category predicates" as indicated:

   > Table 47 — Primary type category predicates
   > Template | Condition | Comments
   > --- | --- | ---
   > `…`
   > `template <class T> struct is_null_pointer;` | `T` is `std::nullptr_t` ([basic.fundamental]) |
   > `…`

### 2249(i). [CD] Remove `gets` from `<cstdio>`

Section: 31.13 [c.files] Status: Resolved Submitter: Jonathan Wakely Opened: 2013-04-17 Last modified: 2016-10-31

Priority: Not Prioritized

View all other issues in [c.files].

View all issues with Resolved status.

Discussion:

Addresses GB 9

In 31.13 [c.files] the current C++ standard claims that `<cstdio>` defines a function called "gets" but it has no declaration or semantics, because it was removed from C11, having been deprecated since C99. We should remove it for C++14.

[2013-09 Chicago]

Will resolve with the wording in the NB comment.

Proposed resolution:

Resolved by resolution as suggested by NB comment GB 9

### 2250(i). Follow-up On Library Issue 2207

Section: 22.9.2.2 [bitset.cons], 22.9.2.3 [bitset.members], 27.4.3.3 [string.cons], 27.4.3.7 [string.modifiers], 27.4.3.8 [string.ops] Status: C++17 Submitter: Frank Birbacher Opened: 2013-04-18 Last modified: 2017-07-30

Priority: 3

View all other issues in [bitset.cons].

View all issues with C++17 status.

Discussion:

Similar to LWG 2207(i) there are several other places where the "Requires" clause precludes the "Throws" condition. Searching for the `out_of_range` exception to be thrown, the following have been found (based on the working draft N3485):

1. 22.9.2.2 [bitset.cons] p3+4

2. 22.9.2.3 [bitset.members] p13+14 (`set`)

3. 22.9.2.3 [bitset.members] p19+20 (`reset`)

4. 22.9.2.3 [bitset.members] p27+28 (`flip`)

5. 22.9.2.3 [bitset.members] p41+42 (`test`)

6. 27.4.3.3 [string.cons] p3+4

7. 27.4.3.7.2 [string.append] p3+4

8. 27.4.3.7.3 [string.assign] p4+5

9. 27.4.3.7.4 [string.insert] p1+2, p5+6, p9+10 (partially)

10. 27.4.3.7.5 [string.erase] p1+2

11. 27.4.3.7.6 [string.replace] p1+2, p5+6, p9+10 (partially)

12. 27.4.3.7.7 [string.copy] p1+2

13. 27.4.3.8.3 [string.substr] p1+2

[2013-10-15: Daniel provides wording]

In addition to the examples mentioned in the discussion, a similar defect exists for `thread`'s `join()` and `detach` functions (see 32.4.3.6 [thread.thread.member]). The suggested wording applies a similar fix for these as well.

[2015-05, Lenexa]

STL : likes it
DK : does it change behavior?
Multiple : no
Move to ready? Unanimous

Proposed resolution:

This wording is relative to N3936.

1. Modify 22.9.2.2 [bitset.cons] as indicated: [Editorial comment: The wording form used to ammend the Throws element is borrowed from a similar style used in 27.4.3.7.6 [string.replace] p10]

   >     template <class charT, class traits, class Allocator>
   >     explicit
   >     bitset(const basic_string<charT, traits, Allocator>& str,
   >            typename basic_string<charT, traits, Allocator>::size_type pos = 0,
   >            typename basic_string<charT, traits, Allocator>::size_type n =
   >              basic_string<charT, traits, Allocator>::npos,
   >              charT zero = charT('0'), charT one = charT('1'));

   > > -3- Requires: `pos <= str.size()`.

   > > -4- Throws: `out_of_range` if `pos > str.size()` or `invalid_argument` if an invalid character is found (see below).

   > > -5- Effects: Determines the effective length `rlen` of the initializing string as the smaller of `n` and `str.size() - pos`.

   > > The function then throws `invalid_argument` if any of the `rlen` characters in `str` beginning at position `pos` is other than `zero` or `one`. The function uses `traits::eq()` to compare the character values.

   > > […]

2. Modify 22.9.2.3 [bitset.members] as indicated:

   >     bitset<N>& set(size_t pos, bool val = true);

   > > -13- Requires: `pos` is valid

   > > -14- Throws: `out_of_range` if `pos` does not correspond to a valid bit position.

   > > […]

   >     bitset<N>& reset(size_t pos);

   > > -19- Requires: `pos` is valid

   > > -20- Throws: `out_of_range` if `pos` does not correspond to a valid bit position.

   > > […]

   >     bitset<N>& flip(size_t pos);

   > > -27- Requires: `pos` is valid

   > > -28- Throws: `out_of_range` if `pos` does not correspond to a valid bit position.

   > > […]

   >     bool test(size_t pos) const;

   > > -41- Requires: `pos` is valid

   > > -42- Throws: `out_of_range` if `pos` does not correspond to a valid bit position.

   > > […]

3. Modify 27.4.3.3 [string.cons] as indicated:

   >     basic_string(const basic_string& str,
   >                  size_type pos, size_type n = npos,
   >                  const Allocator& a = Allocator());

   > > -3- Requires: `pos <= str.size()`

   > > -4- Throws: `out_of_range` if `pos > str.size()`.

4. Modify 27.4.3.5 [string.capacity] as indicated:

   >     void resize(size_type n, charT c);

   > > -6- Requires: `n <= max_size()`

   > > -7- Throws: `length_error` if `n > max_size()`.

5. Modify 27.4.3.7.2 [string.append] as indicated:

   >     basic_string&
   >       append(const basic_string& str, size_type pos, size_type n = npos);

   > > -3- Requires: `pos <= str.size()`

   > > -4- Throws: `out_of_range` if `pos > str.size()`.

6. Modify 27.4.3.7.3 [string.assign] as indicated:

   >     basic_string&
   >       assign(const basic_string& str, size_type pos,
   >              size_type n = npos);

   > > -5- Requires: `pos <= str.size()`

   > > -6- Throws: `out_of_range` if `pos > str.size()`.

7. Modify 27.4.3.7.4 [string.insert] as indicated: [Editorial note: The first change suggestion is also a bug fix of the current wording, because (a) the function has parameter `pos1` but the semantics refers to `pos` and (b) it is possible that this function can throw `length_error`, see p10]

   >     basic_string&
   >       insert(size_type pos1, const basic_string& str);

   > > -1- Requires: `pos <= size()`.

   > > -2- Throws: `out_of_range` if `pos > size()`.

   > > -3- Effects: CallsEquivalent to: `return insert(pos, str.data(), str.size());`.

   > > -4- Returns: `*this`.

   >     basic_string&
   >       insert(size_type pos1, const basic_string& str,
   >              size_type pos2, size_type n = npos);

   > > -5- Requires: `pos1 <= size()` and `pos2 <= str.size()`.

   > > -6- Throws: `out_of_range` if `pos1 > size()` or `pos2 > str.size()`.

   > > […]

   >     basic_string&
   >       insert(size_type pos, const charT* s, size_type n);

   > > -9- Requires: `s` points to an array of at least `n` elements of `charT` and `pos <= size()`.

   > > -10- Throws: `out_of_range` if `pos > size()` or `length_error` if `size() + n > max_size()`.

   > > […]

   >     basic_string&
   >       insert(size_type pos, const charT* s);

   > > -13- Requires: `pos <= size()` and `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -14- Effects: Equivalent to `return insert(pos, s, traits::length(s));`.

   > > -15- Returns: `*this`.

8. Modify 27.4.3.7.5 [string.erase] as indicated:

   >     basic_string& erase(size_type pos = 0, size_type n = npos);

   > > -1- Requires: `pos <= size()`

   > > -2- Throws: `out_of_range` if `pos > size()`.

   > > […]

9. Modify 27.4.3.7.6 [string.replace] as indicated: [Editorial note: The first change suggestion is also a bug fix of the current wording, because it is possible that this function can throw `length_error`, see p10]

   >     basic_string&
   >       replace(size_type pos1, size_type n1,
   >               const basic_string& str);

   > > -1- Requires: `pos1 <= size()`.

   > > -2- Throws: `out_of_range` if `pos1 > size()`.

   > > -3- Effects: CallsEquivalent to `return replace(pos1, n1, str.data(), str.size());`.

   > > -4- Returns: `*this`.

   >     basic_string&
   >       replace(size_type pos1, size_type n1,
   >               const basic_string& str,
   >               size_type pos2, size_type n = npos);

   > > -5- Requires: `pos1 <= size()` and `pos2 <= str.size()`.

   > > -6- Throws: `out_of_range` if `pos1 > size()` or `pos2 > str.size()`.

   > > […]

   >     basic_string&
   >       replace(size_type pos1, size_type n1, const charT* s, size_type n2);

   > > -9- Requires: `pos1 <= size()` and `s` points to an array of at least `n2` elements of `charT`.

   > > -10- Throws: `out_of_range` if `pos1 > size()` or `length_error` if the length of the resulting string would exceed `max_size()` (see below).

   > > […]

   >     basic_string&
   >       replace(size_type pos, size_type n, const charT* s);

   > > -13- Requires: `pos <= size()` and `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -14- Effects: Equivalent to `return replace(pos, n, s, traits::length(s));`.

   > > -15- Returns: `*this`.

10. Modify 27.4.3.7.7 [string.copy] as indicated:

    >     size_type copy(charT* s, size_type n, size_type pos = 0) const;

    > > -1- Requires: `pos <= size()`

    > > -2- Throws: `out_of_range` if `pos > size()`.

    > > […]

11. Modify 27.4.3.8.3 [string.substr] as indicated:

    >     basic_string substr(size_type pos = 0, size_type n = npos) const;

    > > -1- Requires: `pos <= size()`

    > > -2- Throws: `out_of_range` if `pos > size()`.

    > > […]

12. Modify 32.4.3.6 [thread.thread.member] as indicated:

    >     void join();

    > > -3- Requires: `joinable()` is true.

    > > […]

    > > -7- Throws: `system_error` when an exception is required (30.2.2).

    > > -8- Error conditions:

    > > - […]

    > > - `invalid_argument` — if the thread is not joinable.

    >     void detach();

    > > -9- Requires: `joinable()` is true.

    > > […]

    > > -12- Throws: `system_error` when an exception is required (30.2.2).

    > > -13- Error conditions:

    > > - […]

    > > - `invalid_argument` — if the thread is not joinable.

### 2252(i). Strong guarantee on `vector::push_back()` still broken with C++11?

Section: 23.3.13.5 [vector.modifiers] Status: C++14 Submitter: Nicolai Josuttis Opened: 2013-04-21 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [vector.modifiers].

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Discussion:

According to my understanding, the strong guarantee of `push_back()` led to the introduction of `noexcept` and to the typical implementation that vectors usually copy their elements on reallocation unless the move operations of their element type guarantees not to throw.

However, if I read the standard correctly, we still don't give the strong guarantee any more: Yes, 23.2.2 [container.requirements.general]/10 specifies:

> Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.3.4, and 23.3.6.5) all container types defined in this Clause meet the following additional requirements:

> - […]
> - if an exception is thrown by a `push_back()` or `push_front()` function, that function has no effects.

However, 23.3.13.5 [vector.modifiers] specifies for vector modifiers, including `push_back()`:

> If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` or by any `InputIterator` operation there are no effects. If an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

I would interpret this as an "otherwise specified" behavior for `push_back()`, saying that the strong guarantee is only given if constructors and assignments do not throw.

That means, the strong guarantee of C++03 is broken with C++11.

In addition to all that 23.2.2 [container.requirements.general] p10 b2 doesn't mention the corresponding functions `emplace_back()` and `emplace_front()`. These are similar single-element additions and should provide the same strong guarantee.

Daniel adds:

It seems the error came in when N2350 and N2345 became accepted and where integrated into the working draft N2369. The merge resulted in a form that changed the previous meaning and as far as I understand it, this effect was not intended.

[2013-09-16, Nico provides concrete wording]

[2013-09-26, Nico improves wording]

The new proposed resolution is driven as follows:

- In the container requirements section, there shall be general statements that single element insertions and `push_back()`, `pop_back`, `emplace_front()`, and `emplace_back()` have no effect on any exception.

  That is: we extend the first two statements by adding `emplace_front()` and `emplace_back()`, which are missing.

- Formulate only the exceptions from that (or where other general statements might lead to the impression, that the blanket statement no longer applies):

  - remove the statement in `list::push_back()` saying again that exceptions have to effect.

  - Clarify that all single-element insertions at either end of a `deque` have the strong guarantee.

  - Clarify that all single-element insertions at the end of a `vector` have the strong guarantee.

Proposed resolution:

This wording is relative to N3691.

1. Edit 23.2.2 [container.requirements.general] p10 b2 as indicated:

   - if an exception is thrown by an `insert()` or `emplace()` function while inserting a single element, that function has no effects.

   - if an exception is thrown by a `push_back()` or, `push_front()`, `emplace_back()`, or `emplace_front()` function, that function has no effects.

2. Edit 23.3.5.4 [deque.modifiers] as indicated:

   >     iterator insert(const_iterator position, const T& x);
   >     iterator insert(const_iterator position, T&& x);
   >     iterator insert(const_iterator position, size_type n, const T& x);
   >     template <class InputIterator>
   >       iterator insert(const_iterator position,
   >                       InputIterator first, InputIterator last);
   >     iterator insert(const_iterator position, initializer_list<T>);
   >     template <class... Args> void emplace_front(Args&&... args);
   >     template <class... Args> void emplace_back(Args&&... args);
   >     template <class... Args> iterator emplace(const_iterator position, Args&&... args);
   >     void push_front(const T& x);
   >     void push_front(T&& x);
   >     void push_back(const T& x);
   >     void push_back(T&& x);

   > > -1- Effects: An insertion in the middle of the deque invalidates all the iterators and references to elements of the deque. An insertion at either end of the deque invalidates all the iterators to the deque, but has no effect on the validity of references to elements of the deque.

   > > -2- Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` there are no effects. If an exception is thrown while inserting a single element at either end, there are no effects. IfOtherwise, if an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

   > > -3- Complexity: The complexity is linear in the number of elements inserted plus the lesser of the distances to the beginning and end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to a constructor of `T`.

3. Edit 23.3.13.5 [vector.modifiers] as indicated:

   >     iterator insert(const_iterator position, const T& x);
   >     iterator insert(const_iterator position, T&& x);
   >     iterator insert(const_iterator position, size_type n, const T& x);
   >     template <class InputIterator>
   >       iterator insert(const_iterator position, InputIterator first, InputIterator last);
   >     iterator insert(const_iterator position, initializer_list<T>);
   >     template <class... Args> void emplace_back(Args&&... args);
   >     template <class... Args> iterator emplace(const_iterator position, Args&&... args);
   >     void push_back(const T& x);
   >     void push_back(T&& x);

   > > -1- Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` or by any `InputIterator` operation there are no effects. If an exception is thrown while inserting a single element at the end and `T` is `CopyInsertable` or `is_nothrow_move_constructible<T>::value` is `true`, there are no effects. IfOtherwise, if an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

   > > -2- Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.

### 2257(i). Simplify container requirements with the new algorithms

Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Marshall Clow Opened: 2013-05-29 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [container.requirements.general].

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Discussion:

Currently (n3690) Table 96 says, in the row for "`a == b`", that the Operational semantics are:

> `==` is an equivalence relation.
> `distance(a.begin(), a.end()) == distance(b.begin(), b.end()) && equal(a.begin(), a.end(),b.begin())`

Given the extension of equal for C++14, this can be simplified to:

> `==` is an equivalence relation.
> `equal(a.begin(), a.end(), b.begin(), b.end())`

[ Alisdair notes that a similar edit would apply to the unordered containers requirements. ]

Previous resolution from Marshall Clow:

> Ammend the Operational Semantics for 23.2.2 [container.requirements.general], Table 96, row "`a == b`"

> > `==` is an equivalence relation.
> > `distance(a.begin(), a.end()) == distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin(), b.end())`

> Ammend 23.2.8 [unord.req] p12:

> > Two unordered containers `a` and `b` compare equal if `a.size() == b.size()` and, for every equivalent-key group `[Ea1,Ea2)` obtained from `a.equal_range(Ea1)`, there exists an equivalent-key group `[Eb1,Eb2)` obtained from `b.equal_range(Ea1)`, such that `distance(Ea1, Ea2) == distance(Eb1, Eb2)` and `is_permutation(Ea1, Ea2, Eb1, Eb2)` returns `true`. For ...

[2013-09 Chicago]

Marshall improves wording

[2013-09 Chicago (evening issues)]

Moved to ready, after confirming latest wording reflects the discussion earlier in the day.

Proposed resolution:

1. Ammend 23.2.2 [container.requirements.general], Table 96 as indicated:

   > Table 96 — Container requirements (continued)
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | --- | ---
   > `…`
   > `a == b` | convertible to `bool` | `==` is an equivalence relation. `distance(a.begin(), a.end()) == distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin(), b.end())` | Requires: `T` is `EqualityComparable` | Constant if `a.size() != b.size()`, linear otherwise
   > `…`

2. Ammend 23.2.8 [unord.req] p12:

   > Two unordered containers `a` and `b` compare equal if `a.size() == b.size()` and, for every equivalent-key group `[Ea1,Ea2)` obtained from `a.equal_range(Ea1)`, there exists an equivalent-key group `[Eb1,Eb2)` obtained from `b.equal_range(Ea1)`, such that `distance(Ea1, Ea2) == distance(Eb1, Eb2)` and `is_permutation(Ea1, Ea2, Eb1, Eb2)` returns `true`. For […]

3. Amend [forwardlist.overview] p2:

   > -2- A `forward_list` satisfies all of the requirements of a container (Table 96), except that the `size()` member function is not provided and `operator==` has linear complexity. […]

### 2258(i). `a.erase(q1, q2)` unable to directly return `q2`

Section: 23.2.7 [associative.reqmts] Status: C++14 Submitter: Geoff Alexander Opened: 2013-05-11 Last modified: 2016-01-28

Priority: 0

View other active issues in [associative.reqmts].

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Discussion:

Section 23.2.7 [associative.reqmts], Table 102, page 743 of the C++ 2011 Standard states that `a.erase(q1, q2)` returns `q2`. The problem is that `a.erase(q1, q2)` cannot directly return `q2` as the return type, `iterator`, differs from that of `q2`, `const_iterator`.

[2013-09 Chicago (evening issues group)]

The wording looks good, but is worded slightly differently to how we say the same for sequence containers, and for unordered associative containers. We should apply consistent wording in all three cases.

Alisdair to provide the wording.

[2014-02-12 Issaquah meeting]

Move a Immediate.

Proposed resolution:

1. In the specification of `a.erase(q1, q2)` in sub-clause 23.2.7 [associative.reqmts], Table 102 change as indicated:

   > Table 102 — Associative container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `a.erase(q1, q2)` | `iterator` | erases all the elements in the range `[q1,q2)`. Returns `q2` an iterator pointing to the element pointed to by `q2` prior to any elements being erased. If no such element exists, `a.end()` is returned. | `log(a.size()) + N` where `N` has the value `distance(q1, q2)`.
   > `…`

### 2259(i). Issues in 17.6.5.5 rules for member functions

Section: 16.4.6.5 [member.functions] Status: C++17 Submitter: Richard Smith Opened: 2013-05-12 Last modified: 2017-07-30

Priority: 3

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Discussion:

16.4.6.5 [member.functions] p2 says:

> "An implementation may declare additional non-virtual member function signatures within a class:

> - by adding arguments with default values to a member function signature; [Footnote: Hence, the address of a member function of a class in the C++ standard library has an unspecified type.] [Note: An implementation may not add arguments with default values to virtual, global, or non-member functions. — end note]
> - by replacing a member function signature with default values by two or more member function signatures with equivalent behavior; and
> - by adding a member function signature for a member function name."

1. This wording is not using the correct terminology. "by adding arguments with default values" presumably means "by adding parameters with default arguments", and likewise throughout.

2. This paragraph only allows an implementation to declare "additional" signatures, but the first bullet is talking about replacing a standard signature with one with additional parameters.

3. None of these bullets allows a member function with no ref-qualifier to be replaced by signatures with ref-qualifiers (a situation which was just discussed on std-proposals), and likewise for cv-qualifiers. Presumably that is not intentional, and such changes should be permissible.

I think the first two items are probably editorial, since the intent is clear.

[2013-12-11 Richard provides concrete wording]

[2015-05, Lenexa]

JW: I don't like that this loses the footnote about the address of member functions having an unspecified type, the footnote is good to be able to point to as an explicit clarification of one consequence of the normative wording.
MC: so we want to keep the footnote
STL: doesn't need to be a footnote, can be an inline Note
JW: does this have any impact on our ability to add totally different functions with unrelated names, not described in the standard?
MC: no, the old wording didn't refer to such functions anyway
Move to Ready and include in motion on Friday?
9 in favor, 0 opposed, 2 abstention

Proposed resolution:

This wording is relative to N3797.

1. Merge 16.4.6.5 [member.functions]p2+3 as indicated:

   > -2- An implementation may declare additional non-virtual member function signatures within a class:

   > - by adding arguments with default values to a member function signature;188 [Note: An implementation may not add arguments with default values to virtual, global, or non-member functions. — end note]

   > - by replacing a member function signature with default values by two or more member function signatures with equivalent behavior; and

   > - by adding a member function signature for a member function name.

   > -3- A call to a member function signature described in the C++ standard library behaves as if the implementation declares no additional member function signatures.[Footnote: A valid C++ program always calls the expected library member function, or one with equivalent behavior. An implementation may also define additional member functions that would otherwise not be called by a valid C++ program.] For a non-virtual member function described in the C++ standard library, an implementation may declare a different set of member function signatures, provided that any call to the member function that would select an overload from the set of declarations described in this standard behaves as if that overload were selected. [Note: For instance, an implementation may add parameters with default values, or replace a member function with default arguments with two or more member functions with equivalent behavior, or add additional signatures for a member function name. — end note]

### 2260(i). Missing requirement for `Allocator::pointer`

Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-05-14 Last modified: 2017-07-30

Priority: 3

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Discussion:

For an allocator `A<T>` which defines `A<T>::pointer` to a class type, i.e. not `T*`, I see no requirement that `A<T>::pointer` is convertible to `A<U>::pointer`, even if `T*` is convertible to `U*`. Such conversions are needed in containers to convert from e.g. `ListNodeBase*` to `ListNode<T>*`.

The obvious way to do such conversions appears to be `pointer_traits::pointer_to()`, but that's ill-formed if the static member function `A<T>::pointer::pointer_to()` doesn't exist and the allocator requirements don't mention that function, so you need to cast `A<T>::pointer` to `A<T>::void_pointer` then cast that to `A<U>::pointer`.

Is converting via `void_pointer` really intended, or are we missing a requirement that `pointer_traits<A<T>::pointer>::pointer_to()` be well-formed?

Proposed resolution:

Add to the Allocator requirements table the following requirement:

> The expression `pointer_traits<XX::pointer>::pointer_to(r)` is well-defined.

[2013-09 Chicago]

Pablo to come back with proposed wording

[2015-07 Telecon]

Marshall to ping Pablo for proposed wording and disable current wording.

Previous resolution [SUPERSEDED]:

> 1. Edit Table 28 as indicated:

>    > Table 28 — Allocator requirements (continued)
>    > Expression | Return type | Assertion/note pre-/post-condition | Default
>    > --- | --- | --- | ---
>    > `…`
>    > `static_cast<X::const_pointer>(z)` | `X::const_pointer` | `static_cast<X::const_pointer>(z) == q` |
>    > `pointer_traits<X::pointer>::pointer_to(r)` | `X::pointer` |  |
>    > `…`

[2016-11-12, Issaquah]

This is related to 1521(i).

Sat PM: Restore original P/R and move to tentatively ready.

Proposed resolution:

1. Edit Table 28 as indicated:

   > Table 28 — Allocator requirements (continued)
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `…`
   > `static_cast<X::const_pointer>(z)` | `X::const_pointer` | `static_cast<X::const_pointer>(z) == q` |
   > `pointer_traits<X::pointer>::pointer_to(r)` | `X::pointer` |  |
   > `…`

### 2261(i). Are containers required to use their '`pointer`' type internally?

Section: 23.2 [container.requirements] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-05-14 Last modified: 2017-07-30

Priority: 2

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Discussion:

Is a container `C` only supposed to refer to allocated memory (blocks of contiguous storage, nodes, etc.) through objects of type `C::pointer` rather than `C::value_type*`?

I don't see anything explicitly requiring this, so a container could immediately convert the result of `get_allocator().allocate(1)` to a built-in pointer of type `value_type*` and only deal with the built-in pointer until it needs to deallocate it again, but that removes most of the benefit of allowing allocators to use custom pointer types.

[2014-06-12, Jonathan comments]

This issue is basically the same issue as LWG 1521(i), which agrees it's an issue, to be dealt with in the future, so I request that 2261(i) not be closed as a dup unless we reopen 1521(i).

[2016-08, Zhihao comments]

The `pointer` types are not exposed in the container interface, and we consider that the memory allocation constraints "all containers defined in this clause obtain memory using an allocator" already implies the reasonable expectation. We propose the fix as non-normative.

[2016-08 Chicago]

Tues PM: General agreement on direction, Alisdair and Billy to update wording

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

[Drafting notes: if people prefer this to be normative, strip the "Note" markups.]

Modify 23.2.2 [container.requirements.general]/8 as indicated:

> Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see 16.4.4.6 [allocator.requirements]). [Note: In particular, containers and iterators do not store references to allocated elements other than through the allocator's pointer type, i.e., as objects of type `P` or `pointer_traits<P>::template rebind<unspecified>`, where `P` is `allocator_traits<allocator_type>::pointer`. — end note]

### 2263(i). Comparing iterators and allocator pointers with different const-character

Section: 16.4.4.6 [allocator.requirements], 23.2 [container.requirements] Status: C++14 Submitter: Howard Hinnant Opened: 2013-06-25 Last modified: 2016-01-28

Priority: 1

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Discussion:

This ancient issue 179(i) says one ought to be able to compare iterators with `const_iterators` from any given container. I'm having trouble finding words that guarantee this in C++11. This impacts not only a container's iterators, but also the allocator requirements in [llocator.requirements] surrounding `pointer`, `const_pointer`, `void_pointer` and `const_void_pointer`. E.g. can one compare a `pointer` with a `const_pointer`?

Since `allocator::pointer` and `const_pointer` are required to be random access iterators, one could expect that the 179(i) guarantees apply for them as well.

[ Daniel comments: ]

The wording for 179(i) was part of several working drafts (e.g. also in N3092) over some time and suddenly got lost in N3242, presumably by accident. Whatever we decide for allocator pointers, I expect that we need to restore the 179(i) wording as part of the overall resolution:

Reinsert after 23.2 [container.requirements] p6:

> -6- `begin()` returns an iterator referring to the first element in the container. `end()` returns an iterator which is the past-the-end value for the container. If the container is empty, then `begin() == end()`;

> -?- In the expressions

> >     i == j
> >     i != j
> >     i < j
> >     i <= j
> >     i >= j
> >     i > j
> >     i - j

> where `i` and `j` denote objects of a container's `iterator` type, either or both may be replaced by an object of the container's `const_iterator` type referring to the same element with no change in semantics.

[2014-02-13 Issaquah, Daniel comments and suggests wording]

First, I didn't originally move the seemingly lost wording to the resolution section because I wanted to ensure that the committee double-checks the reason of this loss.

Second, albeit restoring this wording will restore the comparability of `const_iterator` and `iterator` of containers specified in Clause 23, but this alone would not imply that this guarantee automatically extends to all other iterators, simply because there is no fundamental relation between a mutable iterator and a constant iterator by itself. This relation only exists under specific conditions, for example for containers which provide two such `typedefs` of that kind. Thus the wording restoration would not ensure that allocator `pointer` and `const_pointer` would be comparable with each other. To realize that, we would need additional guarantees added to the allocator requirements. In fact, it is crucial to separate these things, because allocators are not restricted to be used within containers, they have their own legitimate use for other places as well (albeit containers presumably belong to the most important use-cases), and this is also stated in the introduction of 16.4.4.6 [allocator.requirements], where it says:

> All of the string types (Clause 21), containers (Clause 23) (except array), string buffers and string streams (Clause 27), and `match_results` (Clause 28) are parameterized in terms of allocators.

[2014-02-12 Issaquah meeting]

Move a Immediate.

Proposed resolution:

1. Insert after 16.4.4.6 [allocator.requirements] p4 as indicated:

   > -4- An allocator type `X` shall satisfy the requirements of `CopyConstructible` (17.6.3.1). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.2).

   > -?- Let `x1` and `x2` denote objects of (possibly different) types `X::void_pointer`, `X::const_void_pointer`, `X::pointer`, or `X::const_pointer`. Then, `x1` and `x2` are equivalently-valued pointer values, if and only if both `x1` and `x2` can be explicitly converted to the two corresponding objects `px1` and `px2` of type `X::const_pointer`, using a sequence of `static_cast`s using only these four types, and the expression `px1 == px2` evaluates to `true`.

   > > Drafting note: This wording uses the seemingly complicated route via `X::const_pointer`, because these are (contrary to `X::const_void_pointer`) random access iterators and we can rely here for dereferenceable values on the fundamental pointee equivalence of 24.3.5.5 [forward.iterators] p6:

   > > > If `a` and `b` are both dereferenceable, then `a == b` if and only if `*a` and `*b` are bound to the same object.

   > > while for null pointer values we can rely on the special equality relation induced by 16.4.4.4 [nullablepointer.requirements].

   > -?- Let `w1` and `w2` denote objects of type `X::void_pointer`. Then for the expressions

   > >     w1 == w2
   > >     w1 != w2

   > either or both objects may be replaced by an equivalently-valued object of type `X::const_void_pointer` with no change in semantics.

   > -?- Let `p1` and `p2` denote objects of type `X::pointer`. Then for the expressions

   > >     p1 == p2
   > >     p1 != p2
   > >     p1 < p2
   > >     p1 <= p2
   > >     p1 >= p2
   > >     p1 > p2
   > >     p1 - p2

   > either or both objects may be replaced by an equivalently-valued object of type `X::const_pointer` with no change in semantics.

2. Reinsert after 23.2 [container.requirements] p6:

   > -6- `begin()` returns an iterator referring to the first element in the container. `end()` returns an iterator which is the past-the-end value for the container. If the container is empty, then `begin() == end()`;

   > -?- In the expressions

   > >     i == j
   > >     i != j
   > >     i < j
   > >     i <= j
   > >     i >= j
   > >     i > j
   > >     i - j

   > where `i` and `j` denote objects of a container's `iterator` type, either or both may be replaced by an object of the container's `const_iterator` type referring to the same element with no change in semantics.

### 2266(i). `vector` and `deque` have incorrect `insert` requirements

Section: 23.2.4 [sequence.reqmts] Status: C++17 Submitter: Ahmed Charles Opened: 2013-05-17 Last modified: 2017-07-30

Priority: 2

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Discussion:

According to Table 100 in n3485 23.2.4 [sequence.reqmts]/4 the notes for the expression `a.insert(p,i,j)` say:

> Requires: `T` shall be `EmplaceConstructible` into `X` from `*i`. For `vector`, if the iterator does not meet the forward iterator requirements (24.2.5), `T` shall also be `MoveInsertable` into `X` and `MoveAssignable`.

> Each iterator in the range `[i,j)` shall be dereferenced exactly once.

> pre: `i` and `j` are not iterators into `a`.

> Inserts copies of elements in `[i, j)` before `p`

There are two problems with that wording: First, the special constraints for `vector`, that are expressed to be valid for forward iterators only, are necessary for all iterator categories. Second, the same special constraints are needed for `deque`, too.

[2013-10-05, Stephan T. Lavavej comments and provides alternative wording]

In Chicago, we determined that the original proposed resolution was correct, except that it needed additional requirements. When `vector` `insert(p, i, j)` is called with input-only iterators, it can't know how many elements will be inserted, which is obviously problematic for insertion anywhere other than at the end. Therefore, implementations typically append elements (geometrically reallocating), followed by `rotate()`. Given forward+ iterators, some implementations append and `rotate()` when they determine that there is sufficient capacity. Additionally, `deque` `insert(p, i, j)` is typically implemented with prepending/appending, with a possible call to `reverse()`, followed by a call to `rotate()`. Note that `rotate()`'s requirements are strictly stronger than `reverse()`'s.

Therefore, when patching Table 100, we need to add `rotate()`'s requirements. Note that this does not physically affect code (implementations were already calling `rotate()` here), and even in Standardese terms it is barely noticeable — if an element is `MoveInsertable` and `MoveAssignable` then it is almost certainly `MoveConstructible` and swappable. However, this patch is necessary to be strictly correct.

Previous resolution from Ahmed Charles:

> 1. Change Table 100 as indicated:

>    > Table 100 — Sequence container requirements (in addition to container) (continued)
>    > Expression | Return type | Assertion/note pre-/post-condition
>    > --- | --- | ---
>    > `…`
>    > `a.insert(p,i,j)` | `iterator` | Requires: `T` shall be `EmplaceConstructible` into `X` from `*i`. For `vector` and `deque`, if the iterator does not meet the forward iterator requirements (24.2.5), `T` shall also be `MoveInsertable` into `X` and `MoveAssignable`. Each iterator in the range `[i,j)` shall be dereferenced exactly once. pre: `i` and `j` are not iterators into `a`. Inserts copies of elements in `[i, j)` before `p`
>    > `…`

[2014-02-15 post-Issaquah session : move to Tentatively Ready]

Pablo: We might have gone too far with the fine-grained requirements. Typically these things come in groups.

Alisdair: I think the concepts folks assumed we would take their guidance.

Move to Tentatively Ready.

Proposed resolution:

1. Change Table 100 as indicated:

   > Table 100 — Sequence container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition
   > --- | --- | ---
   > `…`
   > `a.insert(p,i,j)` | `iterator` | Requires: `T` shall be `EmplaceConstructible` into `X` from `*i`. For `vector` and `deque`, if the iterator does not meet the forward iterator requirements (24.2.5), `T` shall also be `MoveInsertable` into `X`, `MoveConstructible`, and `MoveAssignable`, and swappable (16.4.4.3 [swappable.requirements]). Each iterator in the range `[i,j)` shall be dereferenced exactly once. pre: `i` and `j` are not iterators into `a`. Inserts copies of elements in `[i, j)` before `p`
   > `…`

### 2268(i). Setting a default argument in the declaration of a member function `assign` of `std::basic_string`

Section: 27.4.3 [basic.string] Status: C++14 Submitter: Vladimir Grigoriev Opened: 2013-06-26 Last modified: 2016-11-12

Priority: Not Prioritized

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Discussion:

Constructors and member functions `assign` of class `std::basic_string` have one to one relation (except the explicit constructor that creates an empty string). The following list shows this relation:

>     explicit basic_string(const Allocator& a = Allocator());

>     basic_string(const basic_string& str);
>     basic_string& assign(const basic_string& str);

>     basic_string(basic_string&& str) noexcept;
>     basic_string& assign(basic_string&& str) noexcept;

>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>       const Allocator& a = Allocator());
>     basic_string& assign(const basic_string& str, size_type pos,
>       size_type n);

>     basic_string(const charT* s,
>       size_type n, const Allocator& a = Allocator());
>     basic_string& assign(const charT* s, size_type n);

>     basic_string(const charT* s, const Allocator& a = Allocator());
>     basic_string& assign(const charT* s);

>     basic_string(size_type n, charT c, const Allocator& a = Allocator());
>     basic_string& assign(size_type n, charT c);

>     template<class InputIterator>
>     basic_string(InputIterator begin, InputIterator end,
>       const Allocator& a = Allocator());
>     template<class InputIterator>
>     basic_string& assign(InputIterator first, InputIterator last);

>     basic_string(initializer_list<charT>, const Allocator& = Allocator());
>     basic_string& assign(initializer_list<charT>);

So in fact any creating of an object of type `std::basic_string` using any of the above constructors except the explicit constructor can be substituted for creating a (possibly non-empty) string and then applying to it the corresponding method assign.

For example these two code snippets give the same result:

>     std::string s("Hello World");

and

>     std::string s;
>     s.assign("Hello World");

However there is one exception that has no a logical support. It is the pair of the following constructor and member function `assign`

>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>                  const Allocator& a = Allocator());

>     basic_string& assign(const basic_string& str, size_type pos, size_type n);

The third parameter of the constructor has a default argument while in the `assign` function it is absent. So it is impossible one to one to substitute the following code snippet

>     std::string s("Hello World");
>     std::string t(s, 6);

by

>     std::string s("Hello World");
>     std::string t;
>     t.assign(s, 6); // error: no such function

To get an equivalent result using the `assign` function the programmer has to complicate the code that is error-prone

>     std::string s("Hello World");
>     std::string t;
>     t.assign(s, 6, s.size() - 6);

To fix that, the declaration of the member function `assign` should be changed in such a way that its declaration would be fully compatible with the declaration of the corresponding constructor, that is to specify the same default argument for the third parameter of the `assign`.

The `assign` function is not the only function that requires to be revised.

Now let include in the list of pairs constructor-assign with the modified method `assign` one more member function `append`. We will get:

>     explicit basic_string(const Allocator& a = Allocator());

>     basic_string(const basic_string& str);
>     basic_string& assign(const basic_string& str);
>     basic_string& append(const basic_string& str);

>     basic_string(basic_string&& str) noexcept;
>     basic_string& assign(basic_string&& str) noexcept;

>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>       const Allocator& a = Allocator());
>     basic_string& assign(const basic_string& str, size_type pos,
>       size_type n);
>     basic_string& append(const basic_string& str, size_type pos,
>       size_type n);

>     basic_string(const charT* s,
>       size_type n, const Allocator& a = Allocator());
>     basic_string& assign(const charT* s, size_type n);
>     basic_string& append(const charT* s, size_type n);

>     basic_string(const charT* s, const Allocator& a = Allocator());
>     basic_string& assign(const charT* s);
>     basic_string& append(const charT* s);

>     basic_string(size_type n, charT c, const Allocator& a = Allocator());
>     basic_string& assign(size_type n, charT c);
>     basic_string& append(size_type n, charT c);

>     template<class InputIterator>
>     basic_string(InputIterator begin, InputIterator end,
>       const Allocator& a = Allocator());
>     template<class InputIterator>
>     basic_string& assign(InputIterator first, InputIterator last);
>     template<class InputIterator>
>     basic_string& append(InputIterator first, InputIterator last);

>     basic_string(initializer_list<charT>, const Allocator& = Allocator());
>     basic_string& assign(initializer_list<charT>);
>     basic_string& append(initializer_list<charT>);

As it seen from this record:

>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>       const Allocator& a = Allocator());
>     basic_string& assign(const basic_string& str, size_type pos,
>       size_type n);
>     basic_string& append(const basic_string& str, size_type pos,
>       size_type n);

it is obvious that the function `append` also should have the default argument that is that it should be declared as:

>     basic_string& append(const basic_string& str, size_type pos,
>       size_type n = npos);

In fact there is no a great difference in using `assign` or `append` especially when the string is empty:

>     std::string s("Hello World");
>     std::string t;
>     t.assign(s, 6);

>     std::string s("Hello World");
>     std::string t;
>     t.append(s, 6);

In both cases the result will be the same. So the `assign` and `append` will be interchangeable from the point of view of used arguments.

There are another three member functions in class `std::basic_string` that could be brought in conformity with considered above functions. They are member functions `insert`, `replace`, and `compare`.

So it is suggested to substitute the following declarations of `insert`, `replace`, and `compare`:

>     basic_string& insert(size_type pos1, const basic_string& str,
>       size_type pos2, size_type n);

>     basic_string& replace(size_type pos1, size_type n1,
>       const basic_string& str, size_type pos2, size_type n2);

>     int compare(size_type pos1, size_type n1,
>       const basic_string& str, size_type pos2, size_type n2) const;

by the declarations:

>     basic_string& insert(size_type pos1, const basic_string& str,
>       size_type pos2, size_type n = npos);

>     basic_string& replace(size_type pos1, size_type n1,
>       const basic_string& str, size_type pos2, size_type n2 = npos);

>     int compare(size_type pos1, size_type n1,
>       const basic_string& str, size_type pos2, size_type n2 = npos) const;

[2013-09 Chicago]

Howard: Are we positive this won't conflict with any other overloads?

They all appear to be unambiguous.

Alisdair: Ok, move to Ready.

Proposed resolution:

1. Change class template `basic_string` synopsis, 27.4.3 [basic.string] p5, as indicated:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         basic_string& append(const basic_string& str, size_type pos, size_type n = npos);
   >         […]
   >         basic_string& assign(const basic_string& str, size_type pos, size_type n = npos);
   >         […]
   >         basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);
   >         […]
   >         basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos);
   >         […]
   >         int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos) const;
   >         […]
   >       };
   >     }

2. Change 27.4.3.7.2 [string.append] before p3 as indicated:

   >     basic_string& append(const basic_string& str, size_type pos, size_type n = npos);

3. Change 27.4.3.7.3 [string.assign] before p4 as indicated:

   >     basic_string& assign(const basic_string& str, size_type pos, size_type n = npos);

4. Change 27.4.3.7.4 [string.insert] before p5 as indicated:

   >     basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);

5. Change 27.4.3.7.6 [string.replace] before p5 as indicated:

   >     basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos);

6. Change 27.4.3.8.4 [string.compare] before p4 as indicated:

   >     int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos) const;

### 2271(i). `regex_traits::lookup_classname` specification unclear

Section: 28.6.6 [re.traits] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-07-02 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [re.traits].

View all issues with C++14 status.

Discussion:

28.6.6 [re.traits] p9 says that `regex_traits::lookup_classname` should return a value that compares equal to 0, but there is no requirement that a bitmask type is equality comparable with 0, e.g. 16.3.3.3.3 [bitmask.types] says bitmask types can be implemented using `std::bitset`.

Either there should be an additional requirement on the type or the function definition should be fixed.

[2013-09 Chicago]

Stefanus: Resolution looks good, doesn't seem to need fixing anywhere else from a quick look through the draft.

Any objection to Ready?

No objection.

Action: Move to Ready.

Proposed resolution:

This wording is relative to N3691.

1. Edit 28.6.6 [re.traits] p9:

   >     template <class ForwardIterator>
   >       char_class_type lookup_classname(
   >         ForwardIterator first, ForwardIterator last, bool icase = false) const;

   > > -9- Returns: an unspecified value that represents the character classification named by the character sequence designated by the iterator range `[first,last)`. If the parameter `icase` is true then the returned mask identifies the character classification without regard to the case of the characters being matched, otherwise it does honor the case of the characters being matched. The value returned shall be independent of the case of the characters in the character sequence. If the name is not recognized then returns a value that compares equal to 0 `char_class_type()`.

### 2272(i). `quoted` should use `char_traits::eq` for character comparison

Section: 31.7.9 [quoted.manip] Status: C++14 Submitter: Marshall Clow Opened: 2013-07-12 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [quoted.manip].

View all issues with C++14 status.

Discussion:

In 31.7.9 [quoted.manip] p2 b2:

> — Each character in `s`. If the character to be output is equal to `escape` or `delim`, as determined by `operator==`, first output `escape`.

In 31.7.9 [quoted.manip] p3 b1 sb1:

> — If the first character extracted is equal to `delim`, as determined by `operator==`, […]

these should both use `traits::eq`.

Also, I believe that 31.7.9 [quoted.manip] p3 implies that:

>     std::ostream _stream;
>     std::string _string;
>     _stream << _string;
>     _stream << quoted(_string);

should both compile, or both fail to compile, based on whether or not their `char_traits` match. But I believe that the standard should say that explicitly.

[ 2013-09 Chicago ]

Marshall Clow improved the wording with support from Stefanus.

[ 2013-09 Chicago (late night issues) ]

Moved to Ready, after confirming wording correctly reflects discussion earlier in the day.

Proposed resolution:

This wording is relative to N3691.

1. Change 31.7.9 [quoted.manip] p2+3 as indicated:

   >     template <class charT>
   >       unspecified quoted(const charT* s, charT delim=charT('"'), charT escape=charT('\\'));
   >     template <class charT, class traits, class Allocator>
   >       unspecified quoted(const basic_string<charT, traits, Allocator>& s,
   >                          charT delim=charT('"'), charT escape=charT('\\'));

   > > -2- Returns: An object of unspecified type such that if `out` is an instance of `basic_ostream` with member type `char_type` the same as `charT` and with member type `traits_type`, which in the second form is the same as `traits`, then the expression `out << quoted(s, delim, escape)` behaves as if it inserts the following characters into `out` using character inserter function templates (27.7.3.6.4), which may throw `ios_base::failure` (27.5.3.1.1):

   > > - `delim`

   > > - Each character in `s`. If the character to be output is equal to `escape` or `delim`, as determined by `operator==``traits_type::eq`, first output `escape`.

   > > - `delim`

   >     template <class charT, class traits, class Allocator>
   >       unspecified quoted(basic_string<charT, traits, Allocator>& s,
   >                          charT delim=charT('"'), charT escape=charT('\\'));

   > > -3- Returns: An object of unspecified type such that:

   > > - If `in` is an instance of `basic_istream` with member types `char_type` and `traits_type` the same as `charT` and `traits`, respectively, then the expression `in >> quoted(s, delim, escape)` behaves as if it extracts the following characters from in using `basic_istream::operator>>` (27.7.2.2.3) which may throw `ios_base::failure` (27.5.3.1.1):

   > >   - If the first character extracted is equal to `delim`, as determined by `operator==``traits_type::eq`, then: […]

   > > - If `out` is an instance of `basic_ostream` with member types `char_type` and `traits_type` the same as `charT` and `traits`, respectively, then the expression `out << quoted(s, delim, escape)` behaves as specified for the `const basic_string<charT, traits, Allocator>&` overload of the quoted function.

### 2273(i). `regex_match` ambiguity

Section: 28.6.10.2 [re.alg.match] Status: C++17 Submitter: Howard Hinnant Opened: 2013-07-14 Last modified: 2017-07-30

Priority: 2

View all other issues in [re.alg.match].

View all issues with C++17 status.

Discussion:

28.6.10.2 [re.alg.match] p2 in describing regex_match says:

> -2- Effects: Determines whether there is a match between the regular expression `e`, and all of the character sequence `[first,last)`. The parameter `flags` is used to control how the expression is matched against the character sequence. Returns true if such a match exists, false otherwise.

It has come to my attention that different people are interpreting the first sentence of p2 in different ways:

1. If a search of the input string using the regular expression `e` matches the entire input string, `regex_match` should return true.

2. Search the input string using the regular expression `e`. Reject all matches that do not match the entire input string. If a such a match is found, return true.

The difference between these two subtly different interpretations is found using the following ECMAScript example:

>     std::regex re("Get|GetValue");

Using `regex_search`, this `re` can never match the input string `"GetValue"`, because ECMA specifies that alternations are ordered, not greedy. As soon as `"Get"` is matched in the left alternation, the matching algorithm stops.

Using definition 1, `regex_match` would return false for an input string of `"GetValue"`.

However definition 2 alters the grammar and appears equivalent to augmenting the regex with a trailing `'$'`, which is an anchor that specifies, reject any matches which do not come at the end of the input sequence. So, using definition 2, `regex_match` would return true for an input string of `"GetValue"`.

My opinion is that it would be strange to have `regex_match` return true for a `string/regex` pair that `regex_search` could never find. I.e. I favor definition 1.

John Maddock writes:

The intention was always that `regex_match` would reject any match candidate which didn't match the entire input string. So it would find `GetValue` in this case because the `"Get"` alternative had already been rejected as not matching. Note that the comparison with ECMA script is somewhat moot, as ECMAScript defines the regex grammar (the bit we've imported), it does not define anything like `regex_match`, nor do we import from ECMAScript the behaviour of that function. So IMO the function should behave consistently regardless of the regex dialect chosen. Saying "use awk regexes" doesn't cut it, because that changes the grammar in other ways.

(John favors definition 2).

We need to clarify 28.6.10.2 [re.alg.match]/p2 in one of these two directions.

[2014-06-21, Rapperswil]

AM: I think there's a clear direction and consensus we agree with John Maddock's position, and if noone else thinks we need the other function I won't ask for it.

Marshall Clow and STL to draft.

[2015-06-10, Marshall suggests concrete wording]

[2015-01-11, Telecon]

Move to Tenatatively Ready

Proposed resolution:

This wording is relative to N4527.

1. Change 28.6.10.2 [re.alg.match]/2, as follows:

   >     template <class BidirectionalIterator, class Allocator, class charT, class traits>
   >       bool regex_match(BidirectionalIterator first, BidirectionalIterator last,
   >                        match_results<BidirectionalIterator, Allocator>& m,
   >                        const basic_regex<charT, traits>& e,
   >                        regex_constants::match_flag_type flags =
   >                          regex_constants::match_default);

   > > -1- Requires: The type `BidirectionalIterator` shall satisfy the requirements of a Bidirectional Iterator (24.2.6).

   > > -2- Effects: Determines whether there is a match between the regular expression `e`, and all of the character sequence `[first,last)`. The parameter `flags` is used to control how the expression is matched against the character sequence. When determining if there is a match, only potential matches that match the entire character sequence are considered. Returns `true` if such a match exists, `false` otherwise. [Example:

   > > >     std::regex re("Get|GetValue");
   > > >     std::cmatch m;
   > > >     regex_search("GetValue", m, re);	// returns true, and m[0] contains "Get"
   > > >     regex_match ("GetValue", m, re);	// returns true, and m[0] contains "GetValue"
   > > >     regex_search("GetValues", m, re);	// returns true, and m[0] contains "Get"
   > > >     regex_match ("GetValues", m, re);	// returns false

   > > — end example]

   > > […]

### 2274(i). Does `map::operator[]` value-initialize or default-insert a missing element?

Section: 23.4.3.3 [map.access], 23.5.3.3 [unord.map.elem] Status: Resolved Submitter: Andrzej Krzemieński Opened: 2013-07-16 Last modified: 2015-10-22

Priority: 3

View all other issues in [map.access].

View all issues with Resolved status.

Discussion:

Suppose that I provide a custom allocator for type `int`, that renders value 1 rather than 0 in default-insertion:

>     struct Allocator1 : std::allocator<int>
>     {
>       using super = std::allocator<int>;

>       template<typename Up, typename... Args>
>       void construct(Up* p, Args&&... args)
>       { super::construct(p, std::forward<Args>(args)...); }

>       template<typename Up>
>       void construct(Up* p)
>       { ::new((void*)p) Up(1); }
>     };

Now, if I use this allocator with `std::map`, and I use `operator[]` to access a not-yet-existent value, what value of the `mapped_type` should be created? 0 (value-initialization) or 1 (default-insertion):

>     map<string, int, less<string>, Allocator1> map;
>     cout << map["cat"];

N3960 is not very clear. 23.4.3.3 [map.access] in para 1 says:

> "If there is no key equivalent to `x` in the map, inserts `value_type(x, T())` into the map."

So, it requires value-initialization.

But para 2 says:

> "`mapped_type` shall be `DefaultInsertable` into `*this`."

This implies default-insertion, because if not, why the requirement. Also similar functions like `vector::resize` already require default-insertion wherever they put `DefaultInsertable` requirements.

Not to mention that default-insertion is more useful, because it allows custom allocators to "override" the default value of `mapped_type`.

[2013-09 Chicago]

Alisdair: Matters only for POD or trivial types

Marshall: issue might show up elsewhere other than `map<>`

Alisdair: initialize elements in any containers — by calling construct on allocator traits

Marshall: existing wording is clear

Alisdair: main concern is difference in wording, discusses default initialization

Nico: different requirement needed

Alisdair: gut is issue is NAD, brings up `DefaultInsertable` definition — discusses definition

Nico: why do we have the requirement?

Alisdair: other containers have this requirement

Marshall: this applies to many other containers

Nico: `deque<>` in particular

Alisdair: discusses allocator construct

Alisdair: wording raises concerns that aren't said in existing standard

Nico: sees no benefit to change

Marshall: leery of change

Alisdair: can be made clearer; might need to add note to `DefaultInsertable`; borderline editorial, comfortable without note, willing to wait until other issues arise. close issue as NAD

[2015-01-20: Tomasz Kamiński comments]

With the addition of the `try_emplace` method the behavior of the `operator[]` for the maps, may be defined as follows:

>     T& operator[](const key_type& x);

> > Effects: Equivalent to: `try_emplace(x).first->second`;

>     T& operator[](key_type&& x);

> > Effects: Equivalent to `try_emplace(std::move(x)).first->second`;

This would simplify the wording and also after resolution of the issue 2464(i), this wording would also address this issue.

[2015-02 Cologne]

Wait until 2464(i) and 2469(i) are in, which solve this.

[2015-05-06 Lenexa: This is resolved by 2469(i).]

Proposed resolution:

This wording is relative to N3691.

1. Change 23.4.3.3 [map.access] p1+p5 as indicated:

   >     T& operator[](const key_type& x);

   > > -1- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(x, T())` into the mapinto the map a value with `key_type` initialized using expression `x` and `mapped_type` initialized by default-insertion.

   > > -2- Requires: `key_type` shall be `CopyInsertable` and `mapped_type` shall be `DefaultInsertable` into `*this`.

   > > […]

   >     T& operator[](key_type&& x);

   > > -5- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(std::move(x), T())` into the mapinto the map a value with `key_type` initialized using expression `std::move(x)` and `mapped_type` initialized by default-insertion.

   > > -6- Requires: `mapped_type` shall be `DefaultInsertable` into `*this`.

2. Change 23.5.3.3 [unord.map.elem] p2 as indicated:

   >     mapped_type& operator[](const key_type& k);
   >     mapped_type& operator[](key_type&& k);

   > > -1- Requires: `mapped_type` shall be `DefaultInsertable` into `*this`. For the first operator, `key_type` shall be `CopyInsertable` into `*this`. For the second operator, `key_type` shall be `MoveConstructible`.

   > > -2- Effects: If the `unordered_map` does not already contain an element whose key is equivalent to `k`, the first operator inserts the value `value_type(k, mapped_type())`a value with `key_type` initialized using expression `x` and `mapped_type` initialized by default-insertion and the second operator inserts the value `value_type(std::move(k), mapped_type())`a value with `key_type` initialized using expression `std::move(x)` and `mapped_type` initialized by default-insertion.

### 2275(i). [CD] Why is `forward_as_tuple` not `constexpr`?

Section: 22.4.5 [tuple.creation] Status: C++14 Submitter: Marshall Clow Opened: 2013-07-30 Last modified: 2017-09-07

Priority: Not Prioritized

View all other issues in [tuple.creation].

View all issues with C++14 status.

Discussion:

Addresses ES 11

In n3471, a bunch of routines from header `<tuple>` were made `constexpr`.

`make_tuple/tuple_cat/get<>(tuple)/`relational operators — all these were "constexpr-ified".

But not `forward_as_tuple`.

Why not?

This was discussed in Portland, and STL opined that this was "an omission" (along with `tuple_cat`, which was added)

In discussion on lib@lists.isocpp.org list, Pablo agreed that `forward_as_tuple` should be `constexpr`.

[2013-09 Chicago]

Moved to Immediate, this directly addresses an NB comment and the wording is non-controversial.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change header `<tuple>` synopsis, 22.4.1 [tuple.general] as indicated:

   >     template <class... Types>
   >       constexpr tuple<Types&&...> forward_as_tuple(Types&&...) noexcept;

2. Change 22.4.5 [tuple.creation] before p5 as indicated:

   >     template <class... Types>
   >       constexpr tuple<Types&&...> forward_as_tuple(Types&&... t) noexcept;

### 2276(i). Missing requirement on `std::promise::set_exception`

Section: 32.10 [futures] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-07-30 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [futures].

View all issues with C++17 status.

Discussion:

The standard does not specify the behaviour of this program:

>     #include <future>
>     #include <cassert>

>     struct NonTrivial
>     {
>       NonTrivial() : init(true) { }
>       ~NonTrivial() { assert(init); }
>       bool init;
>     };

>     int main()
>     {
>       std::promise<NonTrivial> p;
>       auto f = p.get_future();
>       p.set_exception(std::exception_ptr());
>       f.get();
>     }

The standard doesn't forbid making the state ready with a null `exception_ptr`, so what should `get()` return? There's no stored exception to throw, but it can't return a value because none was initialized.

A careful reading of the standard shows 32.10.5 [futures.state] p8 says "A shared state is ready only if it holds a value or an exception ready for retrieval." One can infer from the fact that `set_exception()` makes the state ready that it must store a value or exception, so cannot store "nothing", but that isn't explicit.

The `promise::set_exception()` and `promise::set_exception_at_thread_exit()` members should require `p != nullptr` or should state the type of exception thrown if `p` is null.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

SG1 provides P/R and requests move to SG1-OK status: Add Requires clauses for promise (30.6.5 [futures.promise]) set_exception (before p18) and set_exception_at_thread_exit (before p24): Requires: p is not null.

[2015-10, Kona issue prioritization]

Priority 0, move to Ready

Proposed resolution:

This wording is relative to N4431.

Add Requires clauses for promise (32.10.6 [futures.promise]) `set_exception` (before p18) and `set_exception_at_thread_exit` (before p24): Requires: `p` is not null.

1. Change 32.10.6 [futures.promise] as depicted:

   >     void set_exception(exception_ptr p);

   > > -??- Requires: `p` is not null.

   > > -18- Effects: atomically stores the exception pointer `p` in the shared state and makes that state ready (30.6.4).

   >     void set_exception_at_thread_exit(exception_ptr p);

   > > -??- Requires: `p` is not null.

   > > -24- Effects: Stores the exception pointer `p` in the shared state without making that state ready immediately. […]

### 2278(i). User-defined literals for Standard Library types

Section: 30.2 [time.syn], 27.4 [string.classes] Status: C++14 Submitter: Howard Hinnant Opened: 2013-07-22 Last modified: 2017-09-07

Priority: Not Prioritized

View all other issues in [time.syn].

View all issues with C++14 status.

Discussion:

This paper adds user-defined literals for `string`, `complex` and `chrono` types. It puts each new literal signature in an inline namespace inside of std. Section 3.1 of the paper gives the rationale for doing this:

> As a common schema this paper proposes to put all suffixes for user defined literals in separate inline namespaces that are below the inline namespace `std::literals`. [Note: This allows a user either to do a `using namespace std::literals;` to import all literal operators from the standard available through header file inclusion, or to use `using namespace std::string_literals;` to just obtain the literals operators for a specific type. — end note]

This isn't how inline namespaces work.

9.9.2 [namespace.def]/p8 says in part:

> Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup (3.4.2) whenever one of them is, and a using- directive (7.3.4) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (7.3.1.1). […]

I.e. these literals will appear to the client to already be imported into namespace `std`. The rationale in the paper appears to indicate that this is not the intended behavior, and that instead the intended behavior is to require the user to say:

>     using namespace std::literals;

or:

>     using namespace std::literals::string_literals;

prior to use. To get this behavior non-inlined (normal) namespaces must be used.

Originally proposed resolution:

Strike the use of "inline" from each use associated with `literals`, `string_literals`, `chrono_literals`.

My opinion is that this must be done prior to publishing C++14, otherwise we are stuck with this (apparently unwanted) decision forever.

Marshall Clow:

> The rationale that I recall was that:

> 1. Users could write "`using namespace std::literals;`" to get all the literal suffixes, or

> 2. Users could write "`using namespace std::literals::string_literals;`" or "`using namespace std::literals::chrono_literals;`" to get a subset of the suffixes.

> To accomplish that, I believe that:

> 1. Namespace "`std::literals`" should not be `inline`

> 2. Namespaces "`std::literals::string_literals`" and "`std::literals::chrono_literals`" should be `inline`

Further details see also reflector message c++std-lib-34256.

Previous resolution from Marshall Clow:

> 1. Modify header `<chrono>` synopsis, 30.2 [time.syn], as indicated:

>    >     namespace std {
>    >     namespace chrono {
>    >     […]
>    >     } // namespace chrono
>    >     inline namespace literals {
>    >     inline namespace chrono_literals {
>    >     […]
>    >     } // namespace chrono_literals
>    >     } // namespace literals
>    >     } // namespace std

> 2. Modify header `<string>` synopsis, 27.4 [string.classes] p1, as indicated:

>    >     #include <initializer_list>

>    >     namespace std {
>    >     […]
>    >     inline namespace literals {
>    >     inline namespace string_literals {
>    >     […]
>    >     }
>    >     }
>    >     }

[2013-09 Chicago]

After a discussion about intent, the conclusion was that if you hoist a type with a "using" directive, then you should also get the associated literal suffixes with the type.

This is accomplished by marking namespace `std::literals` as `inline`, but for types in their own namespace inside `std`, then they will need to do this as well. The only case in the current library is `chrono`.

Marshall Clow provides alternative wording.

[2013-09 Chicago (late night issues)]

Moved to Ready, after confirming wording reflects the intent of the earlier discussion.

Proposed resolution:

This wording is relative to N3691.

1. Modify header `<chrono>` synopsis, 30.2 [time.syn], as indicated:

   >     namespace std {
   >     […]
   >     inline namespace literals {
   >     inline namespace chrono_literals {
   >     […]
   >     constexpr chrono::duration<unspecified , nano> operator "" ns(long double);

   >     } // namespace chrono_literals
   >     } // namespace literals

   >     namespace chrono {
   >         using namespace literals::chrono_literals;
   >     } // namespace chrono

   >     } // namespace std

### 2280(i). `begin/end` for arrays should be `constexpr` and `noexcept`

Section: 24.7 [iterator.range] Status: C++14 Submitter: Andy Sawyer Opened: 2013-08-22 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [iterator.range].

View all issues with C++14 status.

Discussion:

The array forms of `std::begin` and `std::end` should be `constexpr` and `noexcept`.

Previous resolution from Andy Sawyer:

> 1. Edit header `<iterator>` synopsis, 24.2 [iterator.synopsis] as indicated:

>    >     […]
>    >     template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;
>    >     template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;
>    >     template <class C> constexpr auto cbegin(const C& c) -> decltype(std::begin(c));
>    >     template <class C> constexpr auto cend(const C& c) -> decltype(std::end(c));
>    >     […]

> 2. Edit 24.7 [iterator.range] before p4+5 as indicated:

>    >     template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;

>    > > -4- Returns: `array`.

>    >     template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;

>    > > -5- Returns: `array + N`.

>    >     template <class C> constexpr auto cbegin(const C& c) -> decltype(std::begin(c));

>    > > -6- Returns: `std::begin(c)`.

>    >     template <class C> constexpr auto cend(const C& c) -> decltype(std::end(c));

>    > > -7- Returns: `std::end(c)`.

[2013-09 Chicago]

Add `noexcept(noexcept(std::begin/end(c)))` to `cbegin` and `cend`, move to ready

Proposed resolution:

This wording is relative to N3797.

1. Edit header `<iterator>` synopsis, 24.2 [iterator.synopsis] as indicated:

   >     […]
   >     template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;
   >     template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;
   >     template <class C> constexpr auto cbegin(const C& c) noexcept(noexcept(std::begin(c))) -> decltype(std::begin(c));
   >     template <class C> constexpr auto cend(const C& c) noexcept(noexcept(std::end(c))) -> decltype(std::end(c));
   >     […]

2. Edit 24.7 [iterator.range] before p4+5 as indicated:

   >     template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;

   > > -4- Returns: `array`.

   >     template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;

   > > -5- Returns: `array + N`.

   >     template <class C> constexpr auto cbegin(const C& c) noexcept(noexcept(std::begin(c))) -> decltype(std::begin(c));

   > > -6- Returns: `std::begin(c)`.

   >     template <class C> constexpr auto cend(const C& c) noexcept(noexcept(std::end(c))) -> decltype(std::end(c));

   > > -7- Returns: `std::end(c)`.

### 2282(i). [fund.ts] Incorrect `is_assignable` constraint in `optional::op=(U&&)`

Section: 5.3.3 [fund.ts::optional.object.assign] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-25 Last modified: 2015-10-26

Priority: Not Prioritized

View all other issues in [fund.ts::optional.object.assign].

View all issues with Resolved status.

Discussion:

Addresses: fund.ts

Minor wording nit in 5.3.3 [fund.ts::optional.object.assign]/p15:

>     template <class U> optional<T>& operator=(U&& v);

> > -15- Requires: `is_constructible<T, U>::value` is true and `is_assignable<U, T>::value` is true.

Should be:

>     template <class U> optional<T>& operator=(U&& v);

> > -15- Requires: `is_constructible<T, U>::value` is true and `is_assignable<T&, U>::value` is true.

[2013-09 Chicago:]

Move to Deferred. This feature will ship after C++14 and should be revisited then.

[2014-06-06 pre-Rapperswill]

This issue has been reopened as fundamentals-ts.

[2014-06-07 Daniel comments]

This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.

[2014-06-16 Rapperswill]

Confirmed that this issue is resolved in the current Library Fundamentals working paper.

Proposed resolution:

This wording is relative to N3691.

1. Edit 5.3.3 [fund.ts::optional.object.assign] p15 as indicated:

   >     template <class U> optional<T>& operator=(U&& v);

   > > -15- Requires: `is_constructible<T, U>::value` is true and `is_assignable<U, TT&, U>::value` is true.

### 2283(i). [fund.ts] `optional` declares and then does not define an `operator<()`

Section: 5.9 [fund.ts::optional.comp_with_t] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-26 Last modified: 2021-06-06

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Addresses: fund.ts

In 22.5.2 [optional.syn] there is:

>     template <class T> constexpr bool operator<(const T&, const optional<T>&);

But I can find no definition for this signature.

[2013-09 Chicago:]

Move to Deferred. This feature will ship after C++14 and should be revisited then.

[2014-06-06 pre-Rapperswill]

This issue has been reopened as fundamentals-ts.

[2014-06-07 Daniel comments]

This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.

[2014-06-16 Rapperswill]

Confirmed that this issue is resolved in the current Library Fundamentals working paper.

Proposed resolution:

This wording is relative to N3691.

1. Add to 5.9 [fund.ts::optional.comp_with_t]:

   >     template <class T> constexpr bool operator<(const T& v, const optional<T>& x);

   > > -?- Returns: `bool(x) ? less<T>{}(v, *x) : false`.

### 2284(i). Inconsistency in `allocator_traits::max_size`

Section: 20.2.9 [allocator.traits] Status: C++14 Submitter: Marshall Clow Opened: 2013-08-27 Last modified: 2018-08-21

Priority: Not Prioritized

View all issues with C++14 status.

Discussion:

Section 20.2.9 [allocator.traits] says:

>     static size_type max_size(const Alloc& a) noexcept;

Section 20.2.9.3 [allocator.traits.members] says:

>     static size_type max_size(Alloc& a) noexcept;

These should be the same.

Discussion:

Pablo (who I believe wrote the `allocator_traits` proposal) says "The function should take a `const` reference."

[2013-09 Chicago]

No objections, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change 20.2.9.3 [allocator.traits.members] as follows:

   >     static size_type max_size(const Alloc& a) noexcept;

### 2285(i). `make_reverse_iterator`

Section: 24.5.1 [reverse.iterators] Status: C++14 Submitter: Zhihao Yuan Opened: 2013-08-27 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [reverse.iterators].

View all issues with C++14 status.

Discussion:

We have `make_move_iterator`, but not `make_reverse_iterator`, which is also useful when dealing with some types without an `rbegin`/`rend` support (like, C strings).

[2013-09 Chicago]

Billy: reviewed it last night STL: has suggested prior, but denied for complexity

Billy: Alisdair wanted to review for `reverse(reverse())`;

STL: likes the issue, was like him

Stefanus: likes definitions, places where things should be

STL: for consistency with `make_move_iterator`

Stefanus: minor editorial issue - subdivision in these 2 sections is different from [move.iter]. See 24.5.4.9 [move.iter.nonmember]

STL: motion to move to Ready

Move to Ready

Proposed resolution:

This wording is relative to N3691.

1. Change header `<iterator>` synopsis, 24.2 [iterator.synopsis] as indicated:

   >     namespace std {
   >       […]
   >       template <class Iterator>
   >         reverse_iterator<Iterator> operator+(
   >           typename reverse_iterator<Iterator>::difference_type n,
   >           const reverse_iterator<Iterator>& x);

   >       template <class Iterator>
   >         reverse_iterator<Iterator> make_reverse_iterator(Iterator i);

   >     }

2. Change class template `reverse_iterator` synopsis, 24.5.1.2 [reverse.iterator] as indicated:

   >     namespace std {
   >       […]
   >       template <class Iterator>
   >         reverse_iterator<Iterator> operator+(
   >           typename reverse_iterator<Iterator>::difference_type n,
   >           const reverse_iterator<Iterator>& x);

   >       template <class Iterator>
   >         reverse_iterator<Iterator> make_reverse_iterator(Iterator i);

   >     }

3. After [reverse.iter.opsum] add the following new sub-clause to [reverse.iter.ops]:

   >     template <class Iterator>
   >       reverse_iterator<Iterator> make_reverse_iterator(Iterator i);

   > > -?- Returns: `reverse_iterator<Iterator>(i)`.

### 2287(i). [fund.ts] Incorrect exception safety for `optional` copy assignment operator

Section: 5.3.3 [fund.ts::optional.object.assign] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-16 Last modified: 2015-10-26

Priority: Not Prioritized

View all other issues in [fund.ts::optional.object.assign].

View all issues with Resolved status.

Discussion:

Addresses: fund.ts

The Exception safety paragraph of 5.3.3 [fund.ts::optional.object.assign] calls out `T`'s copy constructor when it should refer to `T`'s copy assignment operator.

[2013-09 Chicago:]

Move to Deferred. This feature will ship after C++14 and should be revisited then.

[2014-06-06 pre-Rapperswill]

This issue has been reopened as fundamentals-ts.

[2014-06-07 Daniel comments]

This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.

[2014-06-16 Rapperswill]

Confirmed that this issue is resolved in the current Library Fundamentals working paper.

Proposed resolution:

This wording is relative to N3691.

1. Change 5.3.3 [fund.ts::optional.object.assign] as indicated:

   >     optional<T>& operator=(const optional<T>& rhs);

   > > […]

   > > -8- Exception safety: If any exception is thrown, the values of `init` and `rhs.init` remain unchanged. If an exception is thrown during the call to `T`'s copy constructor, no effect. If an exception is thrown during the call to `T`'s copy assignment, the state of its contained value is as defined by the exception safety guarantee of `T`'s copy constructorassignment.

### 2288(i). Inconsistent requirements for shared mutexes

Section: 32.6.4.4 [thread.sharedmutex.requirements] Status: C++14 Submitter: Daniel Krügler Opened: 2013-08-30 Last modified: 2016-01-28

Priority: Not Prioritized

View all issues with C++14 status.

Discussion:

Albeit shared mutex types refine timed mutex types, the requirements imposed on the corresponding required member function expressions are inconsistent in several aspects, most probably because failing synchronisation with wording changes for timed mutexes applied by some issues:

1. Due to acceptance of N3568 a wording phrase came in 32.6.4.4 [thread.sharedmutex.requirements] p26,

   > Effects: If the tick period of `rel_time` is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. […]

   while a very similar one had been removed for 32.6.4.3 [thread.timedmutex.requirements] by LWG 2091(i).

   Having this guaranteed effect for `try_lock_shared_for` but not for `try_lock_for` seems inconsistent and astonishing.

   If the actual intended restriction imposed onto the implementation is to forbid early wakeups here, we should ensure that to hold for timed mutex's `try_lock_for` as well. Note that the rationale provided for LWG 2091(i) was a potential late wakeup situation, but it seems that there is no implementation restriction that prevents early wakeups.

2. The shared-lock requirements for any `*lock*()` functions don't provide the guarantee that "If an exception is thrown then a lock shall not have been acquired for the current execution agent.". For other mutex types this guarantee can be derived from the corresponding `TimedLockable` requirements, but there are no `SharedLockable` requirements.

3. The shared-lock requirements for `*lock_for/_until()` functions require "Throws: Nothing." instead of "Throws: Timeout-related exceptions (30.2.4)." which had been added by LWG 2093(i), because user-provided clocks, durations, or time points may throw exceptions.

4. With the addition of `std::shared_mutex`, the explicit lists of 32.6.4.2 [thread.mutex.requirements.mutex] p7+15,

   > Requires: If `m` is of type `std::mutex` or `std::timed_mutex`, the calling thread does not own the mutex.

   and of 32.6.4.3 [thread.timedmutex.requirements] p4+11,

   > Requires: If `m` is of type `std::timed_mutex`, the calling thread does not own the mutex.

   are incomplete and should add the non-recursive `std::shared_mutex` as well.

[2014-02-16: Moved as Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change 32.6.4.2 [thread.mutex.requirements.mutex] p7+15 as indicated:

   > -6- The expression `m.lock()` shall be well-formed and have the following semantics:

   > > -7- Requires: If `m` is of type `std::mutex` or, `std::timed_mutex`, or `std::shared_mutex`, the calling thread does not own the mutex.

   > […]

   > -14- The expression `m.try_lock()` shall be well-formed and have the following semantics:

   > > -15- Requires: If `m` is of type `std::mutex` or, `std::timed_mutex`, or `std::shared_mutex`, the calling thread does not own the mutex.

2. Change 32.6.4.3 [thread.timedmutex.requirements] p4+11 as indicated:

   > -3- The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

   > > -4- Requires: If `m` is of type `std::timed_mutex` or `std::shared_mutex`, the calling thread does not own the mutex.

   > […]

   > -10- The expression `m.try_lock_until(abs_time)` shall be well-formed and have the following semantics:

   > > -11- Requires: If `m` is of type `std::timed_mutex` or `std::shared_mutex`, the calling thread does not own the mutex.

3. Change 32.6.4.4 [thread.sharedmutex.requirements] as indicated:

   > -3- The expression `m.lock_shared()` shall be well-formed and have the following semantics:

   > > -4- Requires: The calling thread has no ownership of the mutex.

   > > -5- Effects: Blocks the calling thread until shared ownership of the mutex can be obtained for the calling thread. If an exception is thrown then a shared lock shall not have been acquired for the current thread.

   > > […]

   > -24- The expression `m.try_lock_shared_for(rel_time)` shall be well-formed and have the following semantics:

   > > -25- Requires: The calling thread has no ownership of the mutex.

   > > -26- Effects: If the tick period of `rel_time` is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. Attempts to obtain shared lock ownership for the calling thread within the relative timeout (30.2.4) specified by `rel_time`. If the time specified by `rel_time` is less than or equal to `rel_time.zero()`, the function attempts to obtain ownership without blocking (as if by calling `try_lock_shared()`). The function shall return within the timeout specified by `rel_time` only if it has obtained shared ownership of the mutex object. [Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note] If an exception is thrown then a shared lock shall not have been acquired for the current thread.

   > > […]

   > > -30- Throws: NothingTimeout-related exceptions (32.2.4 [thread.req.timing]).

   > -31- The expression `m.try_lock_shared_until(abs_time)` shall be well-formed and have the following semantics:

   > > -32- Requires: The calling thread has no ownership of the mutex.

   > > -33- Effects: The function attempts to obtain shared ownership of the mutex. If `abs_time` has already passed, the function attempts to obtain shared ownership without blocking (as if by calling `try_lock_shared()`). The function shall return before the absolute timeout (30.2.4) specified by `abs_time` only if it has obtained shared ownership of the mutex object. [Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note] If an exception is thrown then a shared lock shall not have been acquired for the current thread.

   > > […]

   > > -37- Throws: NothingTimeout-related exceptions (32.2.4 [thread.req.timing]).

### 2291(i). `std::hash` is vulnerable to collision DoS attack

Section: 16.4.4.5 [hash.requirements] Status: C++14 Submitter: Zhihao Yuan Opened: 2013-09-02 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [hash.requirements].

View all issues with C++14 status.

Discussion:

For a non-cryptographic hash function, it's possible to pre-calculate massive inputs with the same hashed value to algorithmically slow down the unordered containers, and results in a denial-of-service attack. Many languages with built-in hash table support have fixed this issue. For example, Perl has universal hashing, Python 3 uses salted hashes.

However, for C++, in 16.4.4.5 [hash.requirements] p2, Table 26:

> The value returned shall depend only on the argument `k`. [Note: Thus all evaluations of the expression `h(k)` with the same value for `k` yield the same result. — end note]

The wording is not clear here: does that mean all the standard library implementations must use the same hash function for a same type? Or it is not allowed for an implementation to change its hash function?

I suggest to explicitly allow the salted hash functions.

[2013-09 Chicago]

Moved to Ready.

There is some concern that the issue of better hashing, especially standardizing any kind of secure hashing, is a feature that deserves attention in LEWG

The proposed resolution is much simpler than the larger issue though, merely clarifying a permission that many implementers believe they already have, without mandating a change to more straight forward implementations.

Move to Ready, rather than Immediate, as even the permission has been contentious in reflector discussion, although the consensus in Chicago is to accept as written unless we hear a further strong objection.

Proposed resolution:

This wording is relative to N3691.

1. Edit 16.4.4.5 [hash.requirements] p2, Table 26, as indicated: [Editorial note: We can consider adding some additional guideline here. Unlike N3333, this proposed change makes the hashing per-execution instead of per-process. The standard does not discuss OS processes. And, practically, a per-process hashing makes a program unable to share an unordered container to a child process. — end note ]

   > Table 26 — Hash requirements [hash]
   > Expression | Return type | Requirement
   > --- | --- | ---
   > `h(k)` | `size_t` | The value returned shall depend only on the argument `k` for the duration of the program. [Note: Thus all evaluations of the expression `h(k)` with the same value for `k` yield the same result for a given execution of the program. — end note]
   > `…`

### 2292(i). Find a better phrasing for "shall not participate in overload resolution"

Section: 16.3.2.4 [structure.specifications] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2013-09-03 Last modified: 2020-05-12

Priority: 3

View other active issues in [structure.specifications].

View all other issues in [structure.specifications].

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Discussion:

The C++14 CD has 25 sections including the phrase "X shall not participate in overload resolution ...". Most of these uses are double negatives, which are hard to interpret. "shall not ... unless" tends to be the easiest to read, since the condition is true when the function is available, but we also have a lot of "if X is not Y, then Z shall not participate", which actually means "You can call Z if X is Y." The current wording is also clumsy and long-winded. We should find a better and more concise phrasing.

As an initial proposal, I'd suggest using "X is enabled if and only if Y" in prose and adding an "Enabled If: ..." element to 16.3.2.4 [structure.specifications].

Daniel:

I suggest to name this new specification element for 16.3.2.4 [structure.specifications] as "Template Constraints:" instead, because the mentioned wording form was intentionally provided starting with LWG 1237(i) to give implementations more freedom to realize the concrete constraints. Instead of the original `std::enable_if`-based specifications we can use better forms of "SFINAE" constraints today and it eases the path to possible language-based constraints in the future.

[2019-03-21; Daniel comments ]

Apparently the adoption of P0788R3 at the Rapperswil 2018 meeting has resolved this issue by introduction of the new Constraints: element.

[2020-05-10; Reflector discussions]

Resolved by P0788R3 and the Mandating paper series culminating with P1460R1.

Rationale:

Resolved by P0788R3 and finally by P1460R1.

Proposed resolution:

### 2293(i). Wrong facet used by `num_put::do_put`

Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++14 Submitter: Juan Soulie Opened: 2013-09-04 Last modified: 2017-07-05

Priority: 0

View other active issues in [facet.num.put.virtuals].

View all other issues in [facet.num.put.virtuals].

View all issues with C++14 status.

Discussion:

At the end of 28.3.4.3.3.3 [facet.num.put.virtuals] (in p6), the return value is said to be obtained by calling `truename` or `falsename` on the wrong facet: `ctype` should be replaced by `numpunct`.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 28.3.4.3.3.3 [facet.num.put.virtuals] p6 as indicated:

   > -6- Returns: If `(str.flags() & ios_base::boolalpha) == 0` returns `do_put(out, str, fill, (int)val)`, otherwise obtains a string `s` as if by

   > >     string_type s =
   > >       val ? use_facet<ctypenumpunct<charT> >(loc).truename()
   > >           : use_facet<ctypenumpunct<charT> >(loc).falsename();

   > and then inserts each character c of s into out via *out++ = c and returns out.

### 2294(i). `<cstdlib>` should declare `abs(double)`

Section: 29.7 [c.math] Status: Resolved Submitter: Pete Becker Opened: 2013-09-04 Last modified: 2017-03-12

Priority: 2

View all other issues in [c.math].

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Discussion:

… and `abs(float)` and `abs(long double)`. And `<cmath>` should declare `abs(int)`, `abs(long)`, and `abs(long long)`.

As things currently stand, this program is illegal:

>     #include <cstdlib>

>     int main() {
>       double d = -1.23;
>       double dd = std::abs(d);
>       return 0;
>     }

The call is ambiguous because of the various integer overloads, that's because `<cstdlib>` provides `abs(int)` but not `abs(double)`.

This lead one commenter on Stackoverflow to state that `abs` is dangerous, and to recommend using `fabs` instead.

In general, it makes sense to declare overloaded functions that take user-defined types in the same header as the definition of the user-defined types; it isn't necessary to declare all of the overloads in the same place. But here we're not dealing with any user-defined types; we're dealing with builtin types, which are always defined; all of the overloads should be defined in the same place, to avoid mysterious problems like the one in the code above.

The standard library has six overloads for `abs`:

>     int abs(int);  // <cstdlib>
>     long abs(long); // <cstdlib>
>     long long abs(long long); // <cstdlib>

>     float abs(float); // <cmath>
>     double abs(double); // <cmath>
>     long double abs(long double); // <cmath>

These should all be declared in both headers.

I have no opinion on `<stdlib.h>` and `<math.h>`.

[2013-09 Chicago]

This issue is related to LWG 2192(i)

Move to open

[2014-02-13 Issaquah — Nicolai Josuttis suggest wording]

[2015-03-03, Geoffrey Romer provides improved wording]

See proposed resolution of LWG 2192(i).

[2015-09-11, Telecon]

Geoff provided combined wording for 2192(i) after Cologne, Howard to provide updated wording for Kona.

Howard: my notes say I wanted to use `is_unsigned` instead of 'unsigned integral type'.

Previous resolution from Nicolai [SUPERSEDED]:

> 1. Edit 29.7 [c.math] after p7 as indicated:

>    > -6- In addition to the `int` versions of certain math functions in `<cstdlib>`, C++ adds `long` and `long long` overloaded versions of these functions, with the same semantics.

>    > -7- The added signatures are:

>    > >     long abs(long);                    // labs()
>    > >     long long abs(long long);          // llabs()
>    > >     ldiv_t div(long, long);            // ldiv()
>    > >     lldiv_t div(long long, long long); // lldiv()

>    > -?- To avoid ambiguities, C++ also adds the following overloads of `abs()` to `<cstdlib>`, with the semantics defined in `<cmath>`:

>    > >     float abs(float);
>    > >     double abs(double);
>    > >     long double abs(long double);

>    > -?- To avoid ambiguities, C++ also adds the following overloads of `abs()` to `<cmath>`, with the semantics defined in `<cstdlib>`:

>    > >     int abs(int);
>    > >     long abs(long);
>    > >     long long abs(long long);

[2015-08 Chicago]

Resolved by 2192(i)

Proposed resolution:

See proposed resolution of LWG 2192(i).

### 2295(i). Locale name when the provided `Facet` is a `nullptr`

Section: 28.3.3.1.3 [locale.cons] Status: C++23 Submitter: Juan Soulie Opened: 2013-09-04 Last modified: 2023-11-22

Priority: 3

View all other issues in [locale.cons].

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Discussion:

28.3.3.1.3 [locale.cons] p14 ends with:

> "[…] If `f` is null, the resulting object is a copy of `other`."

but the next line p15 says:

> "Remarks: The resulting locale has no name."

But both can't be true when `other` has a name and `f` is null.

I've tried it on two implementations (MSVC,GCC) and they are inconsistent with each other on this.

Daniel Krügler:

As currently written, the Remarks element applies unconditionally for all cases and thus should "win". The question arises whether the introduction of this element by LWG 424(i) had actually intended to change the previous Note to a Remarks element. In either case the wording should be improved to clarify this special case.

[2022-02-14; Daniel comments]

This issue seems to have some overlap with LWG 3676(i) so both should presumably be resolved in a harmonized way.

[2022-11-01; Jonathan provides wording]

This also resolves 3673(i) and 3676(i).

[2022-11-04; Jonathan revises wording after feedback]

Revert an incorrect edit to p8, which was incorrectly changed to:

"If `cats` is equal to `locale::none`, the resulting locale has the same name as `locale(std_name)`. Otherwise, the locale has a name if and only if `other` has a name."

[Kona 2022-11-08; Move to Ready status]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 28.3.3.1.3 [locale.cons] as indicated:

   > >     explicit locale(const char* std_name);

   > > -2- Effects: Constructs a locale using standard C locale names, e.g., `"POSIX"`. The resulting locale implements semantics defined to be associated with that name.

   > > -3- Throws: `runtime_error` if the argument is not valid, or is null.

   > > -4- Remarks: The set of valid string argument values is `"C"`, `""`, and any implementation-defined values.

   > >     explicit locale(const string& std_name);

   > > -5- Effects: The same asEquivalent to `locale(std_name.c_str())`.

   > >     locale(const locale& other, const char* std_name, category cats);

   > > -?- Preconditions: `cats` is a valid `category` value (28.3.3.1.2.1 [locale.category]).

   > > -6- Effects: Constructs a locale as a copy of `other` except for the facets identified by the `category` argument, which instead implement the same semantics as `locale(std_name)`.

   > > -7- Throws: `runtime_error` if the second argument is not valid, or is null.

   > > -8- Remarks: The locale has a name if and only if `other` has a name.

   > >     locale(const locale& other, const string& std_name, category cats);

   > > -9- Effects: The same asEquivalent to `locale(other, std_name.c_str(), cats)`.

   > >     template<class Facet> locale(const locale& other, Facet* f);

   > > -10- Effects: Constructs a locale incorporating all facets from the first argument except that of type `Facet`, and installs the second argument as the remaining facet. If `f` is null, the resulting object is a copy of `other`.

   > > -11- Remarks: If `f` is null, the resulting locale has the same name as `other`. Otherwise, the The resulting locale has no name.

   > >     locale(const locale& other, const locale& one, category cats);

   > > -?- Preconditions: `cats` is a valid `category` value.

   > > -12- Effects: Constructs a locale incorporating all facets from the first argument except for those that implement `cats`, which are instead incorporated from the second argument.

   > > -13- Remarks: If `cats` is equal to `locale::none`, the resulting locale has a name if and only if the first argument has a name. Otherwise, the The locale has a name if and only if the first two arguments both have names.

### 2296(i). `std::addressof` should be `constexpr`

Section: 20.2.11 [specialized.addressof] Status: C++17 Submitter: Daryle Walker Opened: 2013-09-08 Last modified: 2017-07-30

Priority: 3

View all other issues in [specialized.addressof].

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Discussion:

I'm writing a function that needs to be `constexpr` and I wanted to take the address of its input. I was thinking of using `std::addressof` to be safe, but it isn't currently `constexpr`. A sample implementation couldn't be `constexpr` under the C++11 rules, though.

Daniel Krügler:

Indeed the core language clarified by CWG 1312 and by CWG 1384, that such emulations of `std::addressof` implementations are not valid in constant expressions, therefore it seems more like a defect than a feature request to ask for the guarantee that `std::addressof` is a `constexpr` function. It should be added that a similar requirement already exists for `offsetof` indirectly via the C99 standard as of 7.17 p3:

> The macros are […]

> >     offsetof(type, member-designator)

> which expands to an integer constant expression that has type `size_t` […]

combined with the noted property in C++11 that:

> "`offsetof` is required to work as specified even if unary `operator&` is overloaded for any of the types involved"

Therefore implementations should already be able without heroic efforts to realize this functionality by some intrinsic. The wording needs at least to ensure that for any lvalue core constant expression `e` the expression `std::addressof(e)` is a core constant expression.

[2013-09 Chicago]

[2014-06-08, Daniel improves wording]

It has been ensured that the wording is in sync with the recent working paper and the usage of "any" has been improved to say "every" instead (the fix is similar to that applied by LWG 2150(i)).

Previous resolution from Daniel [SUPERSEDED]:

> 1. Change header `<memory>` synopsis, 20.2.2 [memory.syn] as indicated:

>    >     namespace std {
>    >       […]
>    >       // 26.11 [specialized.algorithms], specialized algorithms:
>    >       template <class T> constexpr T* addressof(T& r) noexcept;
>    >       […]
>    >     }

> 2. Change 20.2.11 [specialized.addressof] as indicated:

>    >     template <class T> constexpr T* addressof(T& r) noexcept;

>    > > -1- Returns: The actual address of the object or function referenced by `r`, even in the presence of an overloaded `operator&`.

>    > > -?- Remarks: For every lvalue core constant expression `e` (7.7 [expr.const]), the expression `std::addressof(e)` is a core constant expression.

[2014-06-09, further improvements]

A new wording form is now used similar to the approach used by LWG 2234(i), which is a stricter way to impose the necessary implementation requirements.

[2015-05, Lenexa]

STL: the intent of this change is good; I think the wording is good
- I'm a bit worried about asking for a compiler hook
- if every implementer says: yes they can do it we should be good
EB: there is missing the word "a" before "subexpression" (in multiple places)
MC: the editor should do - we rely on our editors
MC: move to Review with a note stating that we wait for implementation experience first
- in favor: 13, opposed: 0, abstain: 2 HB: N4430 will bring something which is addressing this issue
MC: good we didn't go to ready then

Proposed resolution:

This wording is relative to N3936.

1. Introduce the following new definition to the existing list in [definitions]: [Drafting note: If LWG 2234(i) is accepted before this issue, the accepted wording for the new definition should be used instead — end drafting note]

   > constant subexpression [defns.const.subexpr]

   > an expression whose evaluation as a subexpression of a conditional-expression CE (7.6.16 [expr.cond]) would not prevent CE from being a core constant expression (7.7 [expr.const]).

2. Change header `<memory>` synopsis, 20.2.2 [memory.syn] as indicated:

   >     namespace std {
   >       […]
   >       // 26.11 [specialized.algorithms], specialized algorithms:
   >       template <class T> constexpr T* addressof(T& r) noexcept;
   >       […]
   >     }

3. Change 20.2.11 [specialized.addressof] as indicated:

   >     template <class T> constexpr T* addressof(T& r) noexcept;

   > > -1- Returns: The actual address of the object or function referenced by `r`, even in the presence of an overloaded `operator&`.

   > > -?- Remarks: An expression `std::addressof(E)` is a constant subexpression (3.15 [defns.const.subexpr]), if `E` is an lvalue constant subexpression.

### 2298(i). [CD] `is_nothrow_constructible` is always false because of `create<>`

Section: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-24 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses US 18

The trait `is_constructible<T, Args...>` is defined in terms of a helper template, `create<>`, that is identical to `std::declval<>` except for the latter's `noexcept` clause.

If the absence of `noexcept` is critical to this definition, insert a Note of explanation; otherwise, excise `create<>` and reformulate in terms of `declval<>` the definition of `is_constructible`.

[2013-09-24 Daniel comments and provides resolution suggestion]

Replacing `create<>` by `std::declval<>` would make the situation worse, because the definition of `is_constructible` is based on a well-formed variable definition and there is no way to specify a variable definition without odr-using its initializer arguments. It should also be added, that there is another problem with the specification of all existing `is_trivially_*` traits, because neither `create<>` nor `std::declval<>` are considered as trivial functions, but this should be solved by a different issue.

[2013-09-26 Nico improves wording]

The additional change is just to keep both places were `create()` is defined consistent.

[2013-09 Chicago]

No objections, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change 21.3.5.4 [meta.unary.prop] around p6 as indicated:

   > -6- Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create() noexcept;

   > the predicate condition for a template specialization `is_constructible<T, Args...>` shall be satisfied if and only if the following variable definition would be well-formed for some invented variable `t`:

   > >     T t(create<Args>()...);

   > […]

2. Change 21.3.5.4 [meta.unary.prop] around p4 as indicated:

   > -4- Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create() noexcept;

   > the predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

   > >     To test() {
   > >       return create<From>();
   > >     }

   > […]

### 2299(i). [CD] Effects of inaccessible `key_compare::is_transparent` type are not clear

Section: 23.2.7 [associative.reqmts] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-24 Last modified: 2016-01-28

Priority: 1

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Discussion:

Addresses ES 16

The condition "`X::key_compare::is_transparent` exists" does not specify that the type be publicly accessible.

Consider the public accessibility of `X::key_compare::is_transparent` and whether its potential inaccessibility should be banned for a compliant `key_compare` type.

[2013-09-24 Daniel provides resolution suggestion]

[2013-09-25 Chicago]

Daniel's wording is good, advance to Immediate to respond to NB comment.

[2013-09-26 Chicago]

Moved back to Review as Daniel would like another look at the words, and to confirm implementability.

Previous resolution from Daniel [SUPERSEDED]:

> 1. Change 23.2.7 [associative.reqmts] p8 as indicated:

>    > -8- In Table 102, `X` denotes an associative container class, `a` denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `a_tran` denotes a value of `X` when thea publicly accessible type `X::key_compare::is_transparent` exists whose name is unambiguous and not hidden, […]

> 2. Change 23.2.7 [associative.reqmts] p13 as indicated:

>    > The member function templates `find`, `count`, `lower_bound`, `upper_bound`, and `equal_range` shall not participate in overload resolution unless thea publicly accessible type `Compare::is_transparent` exists whose name is unambiguous and not hidden.

[2014-02-10 Daniel comments provides alternative wording]

I could confirm that my previous concerns were unwarranted, because they turned out to be due to a compiler-bug. Nonetheless I would suggest to replace the previously suggested replication of core-wording situations (access, ambiguity, hidden) by a single more robust phrase based on "valid type".

[2014-02-12 Issaquah: Move to Immediate]

STL: This uses "valid type", which is a Phrase Of Power in Core, and Daniel has a citation for the term.

Jonathan: It's nice to rely on Core.

Proposed resolution:

This wording is relative to N3797.

1. Change 23.2.7 [associative.reqmts] p8 as indicated:

   > -8- In Table 102, `X` denotes an associative container class, `a` denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `a_tran` denotes a value of `X` when the typequalified-id `X::key_compare::is_transparent` existsis valid and denotes a type (13.10.3 [temp.deduct]), […]

2. Change 23.2.7 [associative.reqmts] p13 as indicated:

   > The member function templates `find`, `count`, `lower_bound`, `upper_bound`, and `equal_range` shall not participate in overload resolution unless the typequalified-id `Compare::is_transparent` existsis valid and denotes a type (13.10.3 [temp.deduct]).

### 2300(i). [CD] Redundant sections for `map` and `multimap` members should be removed

Section: 23.4.3 [map], 23.4.4 [multimap] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-25 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

Addresses ES 17

Sections are redundant with general associative container requirements at 23.2.7 [associative.reqmts], Table 102.

Suggested action:

Delete sections.

[2013-09-25 Daniel provides resolution suggestion]

[2013-09-25 Chicago]

Daniel's wording is good, move to Immediate to resolve NB comment.

[2013-09-29 Chicago]

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change the header `<map>` synopsis, 23.4.3.1 [map.overview] p2 as indicated:

   >     // 23.4.4.5, map operations:
   >     iterator find(const key_type& x);
   >     const_iterator find(const key_type& x) const;
   >     template <class K> iterator find(const K& x);
   >     template <class K> const_iterator find(const K& x) const;

2. Delete the complete sub-clause [map.ops]:

   23.4.4.5 map operations [map.ops]

   >     iterator find(const key_type& x);
   >     const_iterator find(const key_type& x) const;
   >     iterator lower_bound(const key_type& x);
   >     const_iterator lower_bound(const key_type& x) const;
   >     iterator upper_bound(const key_type& x);
   >     const_iterator upper_bound(const key_type &x) const;
   >     pair<iterator, iterator>
   >       equal_range(const key_type &x);
   >     pair<const_iterator, const_iterator>
   >       equal_range(const key_type& x) const;

   > > -1- The `find`, `lower_bound`, `upper_bound` and `equal_range` member functions each have two versions, one const and the other non-const. In each case the behavior of the two functions is identical except that the const version returns a `const_iterator` and the non-const version an `iterator` (23.2.4).

3. Delete the complete sub-clause [multimap.ops]:

   23.4.5.4 multimap operations [multimap.ops]

   >     iterator find(const key_type &x);
   >     const_iterator find(const key_type& x) const;

   >     iterator lower_bound(const key_type& x);
   >     const_iterator lower_bound(const key_type& x) const;

   >     pair<iterator, iterator>
   >       equal_range(const key_type &x);
   >     pair<const_iterator, const_iterator>
   >       equal_range(const key_type& x) const;

   > > -1- The `find`, `lower_bound`, `upper_bound` and `equal_range` member functions each have two versions, one const and one non-const. In each case the behavior of the two versions is identical except that the const version returns a `const_iterator` and the non-const version an `iterator` (23.2.4).

### 2301(i). Why is `std::tie` not `constexpr`?

Section: 22.4.5 [tuple.creation] Status: C++14 Submitter: Rein Halbersma Opened: 2013-09-11 Last modified: 2016-01-28

Priority: 2

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Discussion:

In N3471, a bunch of routines from header `<tuple>` were made `constexpr`.

`make_tuple/tuple_cat/get<>(tuple)/`relational operators — all these were "`constexpr`-ified".

But not `tie`. This is similar to Issue 2275(i), where the same observation was made about `forward_as_tuple`.

[2014-02-13 Issaquah: Move as Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change the header `<tuple>` synopsis, 22.4.1 [tuple.general] p2 as indicated:

   >     template<class... Types>
   >       constexpr tuple<Types&...> tie(Types&...) noexcept;

2. Change 22.4.5 [tuple.creation] around p7 as indicated:

   >     template<class... Types>
   >       constexpr tuple<Types&...> tie(Types&... t) noexcept;

### 2302(i). Passing null pointer to placement new

Section: 17.6.3.4 [new.delete.placement] Status: Resolved Submitter: Marc Glisse Opened: 2013-09-12 Last modified: 2026-04-13

Priority: 2

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Discussion:

Based on this discussion and as discussed in c++std-core-23998 and c++std-lib-34442, calling placement new currently forces the compiler to check if the pointer is null before initializing the object (a non-negligible cost). It seems many people were not aware of this and they consider it a user error to pass a null pointer to it.

Proposed resolution: for `operator new` and `operator new[]`, add:

> Requires: `ptr` shall not be a null pointer.

[2014-02-15 post-Issaquah session : move to Tentatively NAD]

AJM to supply the rationale...

[2026-04-09; This was resolved by CWG 1748.]

Proposed resolution:

This wording is relative to N3691.

1. Change 17.6.3.4 [new.delete.placement] as indicated:

   >     void* operator new(std::size_t size, void* ptr) noexcept;

   > > -?- Requires: `ptr` shall not be a null pointer.

   > > -2- Returns: `ptr`.

   > > -3- Remarks: Intentionally performs no other action.

   > > -4- [Example: This can be useful for constructing an object at a known address:

   > > >     void* place = operator new(sizeof(Something));
   > > >     Something* p = new (place) Something();

   > > — end example]

   >     void* operator new[](std::size_t size, void* ptr) noexcept;

   > > -?- Requires: `ptr` shall not be a null pointer.

   > > -5- Returns: `ptr`.

   > > -6- Remarks: Intentionally performs no other action.

### 2304(i). Complexity of `count` in unordered associative containers

Section: 23.2.8 [unord.req] Status: C++14 Submitter: Joaquín M López Muñoz Opened: 2013-09-20 Last modified: 2017-07-05

Priority: 0

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Discussion:

Table 103 in 23.2.8 [unord.req] states that the complexity of `b.count(k)` is average case 𝒪(`1`) rather than linear with the number of equivalent elements, which seems to be a typo as this requires holding an internal count of elements in each group of equivalent keys, something which hardly looks the intent of the standard and no (known by the submitter) stdlib implementation is currently doing.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change Table 103 as indicated:

   > Table 103 — Unordered associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `b.count(k)` | `size_type` | Returns the number of elements with key equivalent to `k`. | Average case 𝒪(`1b.count(k)`), worst case 𝒪(`b.size()`).
   > `…`

### 2306(i). `match_results::reference` should be `value_type&`, not `const value_type&`

Section: 28.6.9 [re.results] Status: C++14 Submitter: Matt Austern Opened: 2013-09-25 Last modified: 2017-07-05

Priority: 4

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Discussion:

The match_results class synopsis has

>     typedef const value_type& const_reference;
>     typedef const_reference reference;

We're getting too enthusiastic about types here by insisting that `reference` is a const reference, even though `match_results` is a read-only container. In the container requirements table (Table 96, in section 23.2.2 [container.requirements.general] we say that `Container::reference` is "lvalue of `T`" and `Container::const_reference` is "const lvalue of `T`".

That phrasing in the container requirements table is admittedly a little fuzzy and ought to be clarified (as discussed in lwg issue 2182(i)), but in context it's clear that `Container::reference` ought to be a `T&` even for constant containers. In the rest of Clause 23 we see that `Container::reference` is `T&`, not `const T&`, even for const-qualified containers and that it's `T&`, not `const T&`, even for containers like `set` and `unordered_set` that provide const iterators only.

The way we handle const containers is just that in the case of a const-qualified container (including `match_results`) there are no operations that return `Container::reference`. That's already the case, so this issue is complaining about an unused typedef.

[2013-10-17: Daniel comments]

The `std::initializer_list` synopsis, 17.11 [support.initlist] shows a similar problem:

>     template<class E> class initializer_list {
>     public:
>       typedef E value_type;
>       typedef const E& reference;
>       typedef const E& const_reference;
>       […]
>     }

Given the fact that `std::initializer_list` doesn't meet the container requirements anyway (and is such a core-language related type) I recommend to stick with the current state.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change the class template `match_results` header synopsis, 28.6.9 [re.results] p4 as indicated:

   >     typedef const value_type& const_reference;
   >     typedef const_referencevalue_type& reference;

### 2308(i). Clarify container destructor requirements w.r.t. `std::array`

Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-09-26 Last modified: 2017-07-05

Priority: 0

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Discussion:

It has been suggested that Table 96 — "Container requirements" makes confusing requirements for the destructor of `std::array`:

"note: the destructor is applied to every element of `a`; all the memory is deallocated."

Since `std::array` obtains no memory, there is none to deallocate, arguably making it unclear what the requirement means for `std::array::~array()`.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change in 23.2.2 [container.requirements.general], Table 96 — "Container requirements", the "Assertion/note/pre-/post-condition" for the expression "`(&a)->~X()`" as indicated:

   > note: the destructor is applied to every element of `a`; all theany memory obtained is deallocated.

### 2309(i). `mutex::lock()` should not throw `device_or_resource_busy`

Section: 32.6.4.2 [thread.mutex.requirements.mutex] Status: C++17 Submitter: Detlef Vollmann Opened: 2013-09-27 Last modified: 2017-07-30

Priority: 0

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Discussion:

As discussed during the Chicago meeting in SG1 the only reasonable reasons for throwing `device_or_resource_busy` seem to be:

- The thread currently already holds the mutex, the mutex is not recursive, and the implementation detects this. In this case `resource_deadlock_would_occur` should be thrown.

- Priority reasons. At least `std::mutex` (and possibly all standard mutex types) should not be setup this way, otherwise we have real problems with `condition_variable::wait()`.

[2014-06-17 Rapperswil]

Detlef provides wording

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

We believe we were already done with it. Should be in SG1-OK status.

[2015-10 pre-Kona]

SG1 hands this over to LWG for wording review

[2015-10 Kona]

Geoffrey provides new wording.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 32.6.4.2 [thread.mutex.requirements.mutex] as indicated:

>    > -13- Error conditions:

>    > - `operation_not_permitted` — if the thread does not have the privilege to perform the operation.

>    > - `resource_deadlock_would_occur` — if the implementation detects that a deadlock would occur.

>    > - `device_or_resource_busy` — if the mutex is already locked and blocking is not possible.

Proposed resolution:

This wording is relative to N4527.

1. Change 32.6.4.2 [thread.mutex.requirements.mutex] as indicated:

   > […]

   > -4- The error conditions for error codes, if any, reported by member functions of the mutex types shall be:

   > 1. (4.1) — `resource_unavailable_try_again` — if any native handle type manipulated is not available.

   > 2. (4.2) — `operation_not_permitted` — if the thread does not have the privilege to perform the operation.

   > 3. (4.3) — `device_or_resource_busy` — if any native handle type manipulated is already locked.

   > […]

   > -13- Error conditions:

   > 1. (13.1) — `operation_not_permitted` — if the thread does not have the privilege to perform the operation.

   > 2. (13.2) — `resource_deadlock_would_occur` — if the implementation detects that a deadlock would occur.

   > 3. (13.3) — `device_or_resource_busy` — if the mutex is already locked and blocking is not possible.

2. Change 32.6.4.4 [thread.sharedmutex.requirements] as indicated:

   > […]

   > -10- Error conditions:

   > 1. (10.1) — `operation_not_permitted` — if the thread does not have the privilege to perform the operation.

   > 2. (10.2) — `resource_deadlock_would_occur` — if the implementation detects that a deadlock would occur.

   > 3. (10.3) — `device_or_resource_busy` — if the mutex is already locked and blocking is not possible.

   > […]

### 2310(i). Public exposition only member in `std::array`

Section: 23.3.3.1 [array.overview] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-09-30 Last modified: 2017-07-30

Priority: 4

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Discussion:

23.3.3.1 [array.overview] shows `std::array` with an "exposition only" data member, `elems`.

The wording in 16.3.3.6 [objects.within.classes] that defines how "exposition only" is used says it applies to private members, but `std::array::elems` (or its equivalent) must be public in order for `std::array` to be an aggregate.

If the intention is that `std::array::elems` places requirements on the implementation to provide "equivalent external behavior" to a public array member, then 16.3.3.6 [objects.within.classes] needs to cover public members too, or some other form should be used in 23.3.3.1 [array.overview].

[Urbana 2014-11-07: Move to Open]

[Kona 2015-10: Link to 2516(i)]

[2015-11-14, Geoffrey Romer provides wording]

[2016-02-04, Tim Song improves the P/R]

Instead of the build-in address-operator, `std::addressof` should be used.

[2016-03 Jacksonville]

Move to Ready.

Proposed resolution:

This wording is relative to N4582.

1. Edit 23.3.3.1 [array.overview] as indicated

   > […]

   > -3- An array […]

   > >     namespace std {
   > >       template <class T, size_t N>
   > >       struct array {
   > >         […]
   > >         T elems[N]; // exposition only
   > >         […]
   > >       };
   > >     }

   > -4- [Note: The member variable `elems` is shown for exposition only, to emphasize that array is a class aggregate. The name `elems` is not part of array's interface. — end note]

2. Edit [array.data] as follows:

   >     constexpr T* data() noexcept;
   >     constexpr const T* data() const noexcept;

   > > -1- Returns: `elems` A pointer such that `[data(), data() + size())` is a valid range, and `data() == addressof(front())`.

### 2312(i). `tuple`'s constructor constraints need to be phrased more precisely

Section: 22.4.4.2 [tuple.cnstr] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-30

Priority: 2

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Discussion:

Consider the following code:

>     void meow(tuple<long, long>) { puts("Two"); }

>     void meow(tuple<long, long, long>) { puts("Three"); }

>     tuple<int, int, int> t(0, 0, 0);

>     meow(t);

This should compile and print "Three" because `tuple<long, long>`'s constructor from `const tuple<int, int, int>&` should remove itself from overload resolution. Implementations sensibly do this, but the Standard doesn't actually say it!

In this case, `Types` is "`long, long`" and `UTypes` is "`int, int, int`". 22.4.4.2 [tuple.cnstr]/3 says "let `i` be in the range `[0,sizeof...(Types))` in order", which is `[0, 2)`. Then /17 says "Remark: This constructor shall not participate in overload resolution unless `const Ui&` is implicitly convertible to `Ti` for all `i`." Interpreted literally, this is true! /15 says "Requires: `sizeof...(Types) == sizeof...(UTypes)`." but requiring the sizes to be identical doesn't help. Only the special phrase "shall not participate in overload resolution unless" mandates SFINAE/`enable_if` machinery.

The wording that we need is almost available in the Requires paragraphs, except that the Requires paragraphs say "`is_constructible`" while the Remark paragraphs say "is implicitly convertible", which is the correct thing for the SFINAE constraints to check. My proposed resolution is to unify the Requires and Remark paragraphs, after which there will be no need for Requires (when a constructor participates in overload resolution if and only if `X` is true, then there's no need for it to Require that `X` is true).

Note: 21.3.5.4 [meta.unary.prop]/6 specifies `is_constructible<To, From>` and 21.3.7 [meta.rel]/4 specifies `is_convertible<From, To>`. Both are specified in terms of "`template <class T> typename add_rvalue_reference<T>::type create();`". Therefore, passing `From` and `From&&` is equivalent, regardless of whether `From` is an object type, an lvalue reference, or an rvalue reference.

Also note that 22.4.4.2 [tuple.cnstr]/3 defines `T0` and `T1` so we don't need to repeat their definitions.

[2014-10-05, Daniel comments]

This issue is closely related to LWG 2419(i).

[2015-02, Cologne]

AM: Howard wants to do something in this space and I want to wait for him to get a paper in.

Postponed.

[2015-05, Lenexa]

MC: handled by Daniel's tuple paper N4387
STL: look at status after N4387 applied.

[2015-05-05, Daniel comments]

N4387 doesn't touch these area intentionally. I agree with Howard that a different option exists that would introduce a TupleLike concept. Some implementations currently take advantage of this choice and this P/R would forbid them, which seems unfortunate to me.

[2015-09, Telecon]

Proposed resolution is obsolete.
Howard has considered writing a paper.
Status quo gives more implementation freedom.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3691.

> 1. Edit 22.4.4.2 [tuple.cnstr] as indicated:

>    >     template <class... UTypes>
>    >       explicit constexpr tuple(UTypes&&... u);

>    > > -8- Requires: `sizeof...(Types) == sizeof...(UTypes)`. `is_constructible<Ti, Ui&&>::value` is true for all i.

>    > > […]

>    > > -10- Remark: This constructor shall not participate in overload resolution unless each type in `UTypes` is implicitly convertible to its corresponding type in `Types` `sizeof...(Types) == sizeof...(UTypes)` and both `is_constructible<Ti, Ui>::value` and `is_convertible<Ui, Ti>::value` are true for all i.

>    > […]

>    >     template <class... UTypes>
>    >       constexpr tuple(const tuple<UTypes...>& u);

>    > > -15- Requires: `sizeof...(Types) == sizeof...(UTypes)`. `is_constructible<Ti, const Ui&>::value` is true for all i.

>    > > […]

>    > > -17- Remark: This constructor shall not participate in overload resolution unless `const Ui&` is implicitly convertible to `Ti` `sizeof...(Types) == sizeof...(UTypes)` and both `is_constructible<Ti, const Ui&>::value` and `is_convertible<const Ui&, Ti>::value` are true for all i.

>    >     template <class... UTypes>
>    >       constexpr tuple(tuple<UTypes...>&& u);

>    > > -18- Requires: `sizeof...(Types) == sizeof...(UTypes)`. `is_constructible<Ti, Ui&&>::value` is true for all i.

>    > > […]

>    > > -20- Remark: This constructor shall not participate in overload resolution unless each type in `UTypes` is implicitly convertible to its corresponding type in `Types` `sizeof...(Types) == sizeof...(UTypes)` and both `is_constructible<Ti, Ui>::value` and `is_convertible<Ui, Ti>::value` are true for all i.

>    >     template <class U1, class U2> constexpr tuple(const pair<U1, U2>& u);

>    > > -21- Requires: `sizeof...(Types) == 2`. `is_constructible<T0, const U1&>::value` is true for the first type `T0` in Types and `is_constructible<T1, const U2&>::value` is true for the second type `T1` in `Types`.

>    > > […]

>    > > -23- Remark: This constructor shall not participate in overload resolution unless `const U1&` is implicitly convertible to `T0` and `const U2&` is implicitly convertible to `T1` `sizeof...(Types) == 2 && is_constructible<T0, const U1&>::value && is_constructible<T1, const U2&>::value && is_convertible<const U1&, T0>::value && is_convertible<const U2&, T1>::value` is true.

>    >     template <class U1, class U2> constexpr tuple(pair<U1, U2>&& u);

>    > > -24- Requires: `sizeof...(Types) == 2`. `is_constructible<T0, U1&&>::value` is true for the first type `T0` in Types and `is_constructible<T1, U2&&>::value` is true for the second type `T1` in `Types`.

>    > > […]

>    > > -26- Remark: This constructor shall not participate in overload resolution unless `U1` is implicitly convertible to `T0` and `U2` is implicitly convertible to `T1` `sizeof...(Types) == 2 && is_constructible<T0, U1>::value && is_constructible<T1, U2>::value && is_convertible<U1, T0>::value && is_convertible<U2, T1>::value` is true.

[2016-03, Jacksonville]

STL provides improved wording.

[2016-06 Oulu]

Tuesday: Adopt option 1, drop option B of 2549(i), and move to Ready.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4567.

1. Edit 22.4.4.2 [tuple.cnstr] as indicated:

   >     template <class... UTypes>
   >       EXPLICIT constexpr tuple(UTypes&&... u);

   > > -8- Requires: `sizeof...(Types) == sizeof...(UTypes)`.

   > > […]

   > > -10- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) >= 1` and `sizeof...(Types) == sizeof...(UTypes)` and `is_constructible<Ti, Ui&&>::value` is `true` for all `i`. The constructor is explicit if and only if `is_convertible<Ui&&, Ti>::value` is `false` for at least one `i`.

   > […]

   >     template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);

   > > -15- Requires: `sizeof...(Types) == sizeof...(UTypes)`.

   > > […]

   > > -17- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) == sizeof...(UTypes)` and `is_constructible<Ti, const Ui&>::value` is `true` for all `i`. The constructor is explicit if and only if `is_convertible<const Ui&, Ti>::value` is `false` for at least one `i`.

   >     template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);

   > > -18- Requires: `sizeof...(Types) == sizeof...(UTypes)`.

   > > […]

   > > -20- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) == sizeof...(UTypes)` and `is_constructible<Ti, Ui&&>::value` is `true` for all `i`. The constructor is explicit if and only if `is_convertible<Ui&&, Ti>::value` is `false` for at least one `i`.

   >     template <class U1, class U2> EXPLICIT constexpr tuple(const pair<U1, U2>& u);

   > > -21- Requires: `sizeof...(Types) == 2`.

   > > […]

   > > -23- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) == 2` and `is_constructible<T0, const U1&>::value` is `true` and `is_constructible<T1, const U2&>::value` is `true`. The constructor is explicit if and only if `is_convertible<const U1&, T0>::value` is `false` or `is_convertible<const U2&, T1>::value` is `false`.

   >     template <class U1, class U2> EXPLICIT constexpr tuple(pair<U1, U2>&& u);

   > > -24- Requires: `sizeof...(Types) == 2`.

   > > […]

   > > -26- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) == 2` and `is_constructible<T0, U1&&>::value` is `true` and `is_constructible<T1, U2&&>::value` is `true`. The constructor is explicit if and only if `is_convertible<U1&&, T0>::value` is `false` or `is_convertible<U2&&, T1>::value` is `false`.

### 2313(i). `tuple_size` should always derive from `integral_constant<size_t, N>`

Section: 22.4.7 [tuple.helper] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 2

View all other issues in [tuple.helper].

View all issues with C++14 status.

Discussion:

In 22.4.7 [tuple.helper], the "primary template" is depicted as:

>     template <class... Types>
>     class tuple_size<tuple<Types...> >
>       : public integral_constant<size_t, sizeof...(Types)> { };

However, 22.3.4 [pair.astuple]/1-2 and 23.3.3.7 [array.tuple]/1-2 are underspecified, saying:

>     tuple_size<pair<T1, T2> >::value

> > Returns: Integral constant expression.

> > Value: `2`.

>     tuple_size<array<T, N> >::value

> > Return type: integral constant expression.

> > Value: `N`

They should be required to behave like the "primary template". This is more than a stylistic decision — it allows `tuple_size` to be passed to a function taking `integral_constant`.

LWG 1118(i) noticed this underspecification, but instead of correcting it, the resolution changed 22.4.7 [tuple.helper]/3 to require `tuple_size<cv T>` to derive from `integral_constant<remove_cv<decltype(TS::value)>::type, TS::value>`. This is unnecessarily overgeneralized. `tuple_size` is primarily for tuples, where it is required to be `size_t`, and it has been extended to handle pairs and arrays, which (as explained above) should also be guaranteed to be `size_t`. `tuple_size<cv T>` works with cv-qualified tuples, pairs, arrays, and user-defined types that also want to participate in the `tuple_size` system. It would be far simpler and perfectly reasonable to expect that user-defined types supporting the "tuple-like protocol" should have `tuple_sizes` of `size_t`.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 22.3.4 [pair.astuple]/1-2 as indicated:

   >     tuple_size<pair<T1, T2> >::value
   >     template <class T1, class T2>
   >     struct tuple_size<pair<T1, T2>>
   >       : integral_constant<size_t, 2> { };

   > > -1- Returns: Integral constant expression.

   > > -2- Value: `2`.

2. Edit 23.3.3.7 [array.tuple]/1-2 as indicated:

   >     tuple_size<array<T, N> >::value
   >     template <class T, size_t N>
   >     struct tuple_size<array<T, N>>
   >       : integral_constant<size_t, N> { };

   > > -1- Returns: Integral constant expression.

   > > -2- Value: `N`.

3. Edit 22.4.7 [tuple.helper]/p1-p3 as indicated:

   >     template <class T> struct tuple_size;

   > > -?- Remarks: All specializations of `tuple_size<T>` shall meet the `UnaryTypeTrait` requirements (21.3.2 [meta.rqmts]) with a `BaseCharacteristic` of `integral_constant<size_t, N>` for some `N`.

   >     template <class... Types>
   >     struct tuple_size<tuple<Types...> >
   >       : integral_constant<size_t, sizeof...(Types)> { };

   >     template <size_t I, class... Types>
   >     class tuple_element<I, tuple<Types...> > {
   >     public:
   >       typedef TI type;
   >     };

   > > -1- Requires: `I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

   > > […]

   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   > > -3- Let TS denote `tuple_size<T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the `UnaryTypeTrait` requirements (21.3.2 [meta.rqmts]) with a `BaseCharacteristic` of

   > > >     integral_constant<remove_cv<decltype(TS::value)>::typesize_t, TS::value>

### 2314(i). `apply()` should return `decltype(auto)` and use `decay_t` before `tuple_size`

Section: 21.2.1 [intseq.general] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 0

View all issues with C++14 status.

Discussion:

The example in 21.2.1 [intseq.general]/2 depicts `apply_impl()` and `apply()` as returning `auto`. This is incorrect because it will trigger decay and will not preserve `F`'s return type. For example, if invoking the functor returns `const int&`, `apply_impl()` and `apply()` will return `int`. `decltype(auto)` should be used for "perfect returning".

Additionally, this depicts `apply()` as taking `Tuple&&`, then saying "`std::tuple_size<Tuple>::value`". This is incorrect because when `apply()` is called with lvalue tuples, perfect forwarding will deduce `Tuple` to be cv `tuple&`, but 22.4.7 [tuple.helper] says that `tuple_size` handles only cv `tuple`, not references to tuples. Using `remove_reference_t` would avoid this problem, but so would `decay_t`, which has a significantly shorter name. (The additional transformations that `decay_t` does are neither beneficial nor harmful here.)

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit the example code in 21.2.1 [intseq.general]/2 as indicated:

   >     template<class F, class Tuple, std::size_t... I>
   >       autodecltype(auto) apply_impl(F&& f, Tuple&& t, index_sequence<I...>) {
   >         return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...);
   >       }
   >     template<class F, class Tuple>
   >       autodecltype(auto) apply(F&& f, Tuple&& t) {
   >         using Indices = make_index_sequence<std::tuple_size<std::decay_t<Tuple>>::value>;
   >         return apply_impl(std::forward<F>(f), std::forward<Tuple>(t), Indices());
   >       }

### 2315(i). `weak_ptr` should be movable

Section: 20.3.2.3 [util.smartptr.weak] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28

Priority: 2

View all other issues in [util.smartptr.weak].

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Discussion:

Like `shared_ptr`, `weak_ptr` should be movable to avoid unnecessary atomic increments/decrements of the weak refcount.

[2014-02-13 Issaquah: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 20.3.2.3 [util.smartptr.weak]/1, class template `weak_ptr` synopsis, as indicated:

   >     namespace std {
   >       template<class T> class weak_ptr {
   >       public:
   >         typedef T element_type;

   >         // 20.9.2.3.1, constructors
   >         constexpr weak_ptr() noexcept;
   >         template<class Y> weak_ptr(shared_ptr<Y> const& r) noexcept;
   >         weak_ptr(weak_ptr const& r) noexcept;
   >         template<class Y> weak_ptr(weak_ptr<Y> const& r) noexcept;
   >         weak_ptr(weak_ptr&& r) noexcept;
   >         template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

   >         […]

   >         // 20.9.2.3.3, assignment
   >         weak_ptr& operator=(weak_ptr const& r) noexcept;
   >         template<class Y> weak_ptr& operator=(weak_ptr<Y> const& r) noexcept;
   >         template<class Y> weak_ptr& operator=(shared_ptr<Y> const& r) noexcept;
   >         weak_ptr& operator=(weak_ptr&& r) noexcept;
   >         template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;
   >       };
   >     }

2. Add the following new paragraphs at the end of sub-clause 20.3.2.3.2 [util.smartptr.weak.const]:

   >     weak_ptr(weak_ptr&& r) noexcept;
   >     template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

   > > -?- Remark: The second constructor shall not participate in overload resolution unless `Y*` is implicitly convertible to `T*`.

   > > -?- Effects: Move-constructs a `weak_ptr` instance from `r`.

   > > -?- Postconditions: `*this` shall contain the old value of `r`. `r` shall be empty. `r.use_count() == 0`.

3. Edit 20.3.2.3.4 [util.smartptr.weak.assign] as indicated:

   >     weak_ptr& operator=(const weak_ptr& r) noexcept;
   >     template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
   >     template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;

   > > -1- Effects: […]

   > > -2- Remarks: […]

   > > -?- Returns: `*this`.

   >     weak_ptr& operator=(weak_ptr&& r) noexcept;
   >     template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;

   > > -?- Effects: Equivalent to `weak_ptr(std::move(r)).swap(*this)`.

   > > -?- Returns: `*this`.

### 2316(i). `weak_ptr::lock()` should be atomic

Section: 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 0

View all other issues in [util.smartptr.weak.obs].

View all issues with C++14 status.

Discussion:

20.3.2.2 [util.smartptr.shared]/4 says: "For purposes of determining the presence of a data race, member functions shall access and modify only the `shared_ptr` and `weak_ptr` objects themselves and not objects they refer to. Changes in `use_count()` do not reflect modifications that can introduce data races." This requires `shared_ptr/weak_ptr` implementations to protect their strong and weak refcounts with atomic operations, without the Standardese having to say this elsewhere. However, 20.3.2.3.6 [util.smartptr.weak.obs]/5 describes `weak_ptr::lock()` with "Returns: `expired() ? shared_ptr<T>() : shared_ptr<T>(*this)`." Even after considering the blanket wording about data races, this specification is insufficient. If this conditional expression were literally implemented, the `use_count()` could change from nonzero to zero after testing `expired()`, causing `shared_ptr<T>(*this)` to throw `bad_weak_ptr` when the intention is for `weak_ptr::lock()` to return empty or nonempty without throwing — indeed, `weak_ptr::lock()` is marked as `noexcept`.

We all know what `weak_ptr::lock()` should do, the Standardese just doesn't say it. `shared_ptr(const weak_ptr<Y>&)`'s specification is not really affected because 20.3.2.2.2 [util.smartptr.shared.const]/23-27 describes the behavior with English instead of code.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 20.3.2.3.6 [util.smartptr.weak.obs]/5 as indicated:

   >     shared_ptr<T> lock() const noexcept;

   > > -5- Returns: `expired() ? shared_ptr<T>() : shared_ptr<T>(*this)`, executed atomically.

### 2317(i). The type property queries should be `UnaryTypeTraits` returning `size_t`

Section: 21.3.6 [meta.unary.prop.query] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 0

View all issues with C++14 status.

Discussion:

The sibling sections 21.3.5 [meta.unary], 21.3.7 [meta.rel], and 21.3.8 [meta.trans] respectively specify `UnaryTypeTraits`, `BinaryTypeTraits`, and `TransformationTraits`, as stated by each /2 paragraph. However, 21.3.6 [meta.unary.prop.query] is underspecified. `alignment_of`, `rank`, and `extent` are said to produce "Values", but the type of that Value is not specified, and the struct templates are not required to derive from `integral_constant`. Such derivation is more than stylistic — it allows the structs to be passed to functions taking `integral_constant`.

`alignment_of` returns `alignof(T)` which is `size_t` (7.6.2.6 [expr.alignof]/2). `extent` returns an array bound, which is clearly `size_t`. `rank` returns "the number of dimensions" of an array, so any type could be chosen, with `size_t` being a reasonable choice. (Another choice would be `unsigned int`, to match `extent`'s template parameter `I`.)

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Following 21.3.6 [meta.unary.prop.query]/1 add a new paragraph as indicated:

   > Each of these templates shall be a `UnaryTypeTrait` (21.3.2 [meta.rqmts]) with a `BaseCharacteristic` of `integral_constant<size_t, Value>`.

### 2318(i). `basic_string`'s wording has confusing relics from the copy-on-write era

Section: 27.4.3 [basic.string] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2018-11-25

Priority: 4

View other active issues in [basic.string].

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Discussion:

27.4.3.5 [string.capacity]/8 specifies `basic_string::resize(n, c)` with:

> Effects: Alters the length of the string designated by `*this` as follows:

> - If `n <= size()`, the function replaces the string designated by `*this` with a string of length `n` whose elements are a copy of the initial elements of the original string designated by `*this`.

> - If `n > size()`, the function replaces the string designated by `*this` with a string of length `n` whose first `size()` elements are a copy of the original string designated by `*this`, and whose remaining elements are all initialized to `c`.

This wording is a relic of the copy-on-write era. In addition to being extremely confusing, it has undesirable implications. Saying "replaces the string designated by `*this` with a string of length `n` whose elements are a copy" suggests that the trimming case can reallocate. Reallocation during trimming should be forbidden, like `vector`.

At least 7 paragraphs are affected: 27.4.3.5 [string.capacity]/8, 27.4.3.7.2 [string.append]/9, 27.4.3.7.3 [string.assign]/3 and /10, 27.4.3.7.4 [string.insert]/11, 27.4.3.7.5 [string.erase]/4, and 27.4.3.7.6 [string.replace]/11 say "replaces the string [designated/controlled] by `*this`". (27.4.3.7.7 [string.copy]/3 is different — it "replaces the string designated by `s`".)

Of the affected paragraphs, `resize()` and `erase()` are the most important to fix because they should forbid reallocation during trimming.

Resolved by the adoption of P1148 in San Diego.

Proposed resolution:

### 2320(i). `select_on_container_copy_construction()` takes allocators, not containers

Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 0

View other active issues in [container.requirements.general].

View all other issues in [container.requirements.general].

View all issues with C++14 status.

Discussion:

23.2.2 [container.requirements.general]/7 says "Copy constructors for these container types obtain an allocator by calling `allocator_traits<allocator_type>::select_on_container_copy_construction` on their first parameters." However, 20.2.9.3 [allocator.traits.members]/8 says that this takes `const Alloc&`, not a container. 23.2.2 [container.requirements.general]/7 goes on to say "Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved." so we can follow that wording.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. In 23.2.2 [container.requirements.general]/7 change as indicated:

   > -7- Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see 17.6.3.5). Copy constructors for these container types obtain an allocator by calling `allocator_traits<allocator_type>::select_on_container_copy_construction` on their first parametersthe allocator belonging to the container being copied. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. […]

### 2322(i). Associative(`initializer_list`, stuff) constructors are underspecified

Section: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 0

View other active issues in [associative.reqmts].

View all other issues in [associative.reqmts].

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Discussion:

23.2.7 [associative.reqmts] specifies both `X(i,j)` and `X(i,j,c)`, but only `X(il)`. 23.4.3.1 [map.overview] declares "`map(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator());`" but 23.4.3.2 [map.cons] intentionally doesn't explain it, relying on the big table's requirements. As a result, `map(il, c)`'s behavior is not actually specified by the Standard. (All of the other ordered associative containers also provide such constructors.)

The unordered associative containers are similarly affected, although they have more arguments. (Again, the actual containers are correctly depicted with the desired constructors, their behavior just isn't specified.)

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 23.2.7 [associative.reqmts], Table 102 — "Associative container requirements", as indicated:

   > Table 102 — Associative container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `X(il);` | `` | Same as `X(il.begin(), il.end())`. | sSame as `X(il.begin(), il.end())`.
   > `X(il, c);` | `` | Same as `X(il.begin(), il.end(), c)`. | Same as `X(il.begin(), il.end(), c)`.
   > `…`

2. Edit 23.2.8 [unord.req], Table 103 "Unordered associative container requirements", as indicated:

   > Table 103 — Unordered associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `X(il)` | `X` | Same as `X(il.begin(), il.end())`. | Same as `X(il.begin(), il.end())`.
   > `X(il, n)` | `X` | Same as `X(il.begin(), il.end(), n)`. | Same as `X(il.begin(), il.end(), n)`.
   > `X(il, n, hf)` | `X` | Same as `X(il.begin(), il.end(), n, hf)`. | Same as `X(il.begin(), il.end(), n, hf)`.
   > `X(il, n, hf, eq)` | `X` | Same as `X(il.begin(), il.end(), n, hf, eq)`. | Same as `X(il.begin(), il.end(), n, hf, eq)`.
   > `…`

### 2323(i). `vector::resize(n, t)`'s specification should be simplified

Section: 23.3.13.3 [vector.capacity], 23.3.5.3 [deque.capacity] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 0

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Discussion:

First, 23.3.5.3 [deque.capacity]/4 and 23.3.13.3 [vector.capacity]/16 say that `resize(size_type sz, const T& c)` "Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`." The `CopyInsertable` requirement is correct (because `sz` might be `size() + 2` or more), but the `MoveInsertable` requirement is redundant due to 23.2.2 [container.requirements.general]/13: "`T` is `CopyInsertable` into `X` means that, in addition to `T` being `MoveInsertable` into `X`, the [...]". (LWG 2033(i)'s resolution said that this was "not redundant, because `CopyInsertable` is not necessarily a refinement of `MoveInsertable`" which was true at the time, but then LWG 2177(i)'s resolution made it a refinement.)

Second, 23.3.13.3 [vector.capacity]/17 says "Remarks: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects." This is confusing because `T` is required to be `CopyInsertable`. (/14 says the same thing for `resize(size_type sz)`, where it is correct because that overload requires only `MoveInsertable` and `DefaultInsertable`.)

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 23.3.5.3 [deque.capacity]/4 as indicated:

   >     void resize(size_type sz, const T& c);

   > > […]

   > > -4- Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`.

2. Edit 23.3.13.3 [vector.capacity]/16+17 as indicated:

   >     void resize(size_type sz, const T& c);

   > > […]

   > > -16- Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`.

   > > -17- Remarks: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

### 2324(i). Insert iterator constructors should use `addressof()`

Section: 24.5.2.2.2 [back.insert.iter.ops], 24.5.2.3.2 [front.insert.iter.ops], 24.5.2.4.2 [insert.iter.ops] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2021-06-06

Priority: 0

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Discussion:

[back.insert.iter.cons]/1, [front.insert.iter.cons]/1, and [insert.iter.cons]/1 say "Initializes container with `&x`", which doesn't defend against containers overloading `operator&()`. Containers are now required to have such defenses for their elements, so we may as well be consistent here.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit [back.insert.iter.cons]/1 as indicated:

   >     explicit back_insert_iterator(Container& x);

   > > -1- Effects: Initializes `container` with `&xstd::addressof(x)`.

2. Edit [front.insert.iter.cons]/1 as indicated:

   >     explicit front_insert_iterator(Container& x);

   > > -1- Effects: Initializes `container` with `&xstd::addressof(x)`.

3. Edit [insert.iter.cons]/1 as indicated:

   >     insert_iterator(Container& x, typename Container::iterator i);

   > > -1- Effects: Initializes `container` with `&xstd::addressof(x)` and `iter` with `i`.

### 2325(i). `minmax_element()`'s behavior differing from `max_element()`'s should be noted

Section: 26.8.9 [alg.min.max] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-30

Priority: 3

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Discussion:

26.8.9 [alg.min.max]/23 says that `max_element()` finds the first biggest element, while /25 says that `minmax_element()` finds the last biggest element. This significant difference is unusual — it means that `minmax_element(args)` is not equivalent to `make_pair(min_element(args), max_element(args))`, whereas the other major "two for one" algorithm `equal_range(args)` is equivalent to `make_pair(lower_bound(args), upper_bound(args))`. `minmax_element()`'s behavior is intentional — it is a fundamental consequence of the `3N/2` algorithm — but the Standardese does not draw attention to this in any way. This wording came from LWG 715(i)'s resolution (which changed the semantics but didn't mention it), citing CLRS for the algorithm — but CLRS doesn't mention the behavior for equivalent elements! The wording here deeply confused me (as an STL maintainer fixing an incorrect implementation) until I walked through the algorithm by hand and figured out the fundamental reason. It would be really nice for the Standard to provide a hint that something magical is happening here.

[2014-06-06 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3691.

1. Add a footnote to 26.8.9 [alg.min.max]/25 as indicated:

   >     template<class ForwardIterator>
   >       pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >       pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);

   > > -25- Returns: `make_pair(first, first)` if `[first,last)` is empty, otherwise `make_pair(m, M)`, where `m` is the first iterator in `[first,last)` such that no iterator in the range refers to a smaller element, and where `M` is the last iterator [Footnote: This behavior intentionally differs from `max_element()`.] in `[first,last)` such that no iterator in the range refers to a larger element.

### 2328(i). Rvalue stream extraction should use perfect forwarding

Section: 31.7.5.6 [istream.rvalue] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-30

Priority: 3

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Discussion:

31.7.5.6 [istream.rvalue] declares `operator>>(basic_istream<charT, traits>&& is, T& x)`. However, [istream::extractors]/7 declares `operator>>(basic_istream<charT,traits>& in, charT* s)`, plus additional overloads for `unsigned char*` and `signed char*`. This means that "`return_rvalue_istream() >> &arr[0]`" won't compile, because `T&` won't bind to the rvalue `&arr[0]`.

[2014-02-12 Issaquah : recategorize as P3]

Jonathan Wakely: Bill was certain the change is right, I think so with less certainty

Jeffrey Yaskin: I think he's right, hate that we need this

Jonathan Wakely: is this the security issue Jeffrey raised on lib reflector?

Move to P3

[2015-05-06 Lenexa: Move to Review]

WEB, MC: Proposed wording changes one signature (in two places) to take a forwarding reference.

TK: Should be consistent with an istream rvalue?

MC: This is the case where you pass the stream by rvalue reference.

RP: I would try it before standardizing.

TK: Does it break anything?

RP, TK: It will take all arguments, will be an exact match for everything.

RS, TK: This adapts streaming into an rvalue stream to make it act like streaming into an lvalue stream.

RS: Should this really return the stream by lvalue reference instead of by rvalue reference? ⇒ new LWG issue.

RP: Security issue?

MC: No. That's `istream >> char*`, C++ version of `gets()`. Remove it, as we did for `gets()`? ⇒ new LWG issue.

RS: Proposed resolution looks correct to me.

MC: Makes me (and Jonathan Wakely) feel uneasy.

Move to Review, consensus.

[2016-01-15, Daniel comments and suggests improved wording]

It has been pointed out by Tim Song, that the initial P/R (deleting the Returns paragraph and making the Effects "equivalent to `return is >> /* stuff */;`") breaks cases where the applicable `operator>>` doesn't return a type that is convertible to a reference to the stream:

>     struct A{};
>     void operator>>(std::istream&, A&){ }

>     void f() {
>       A a;
>       std::istringstream() >> a; // was OK, now error
>     }

This seems like an unintended wording artifact of the "Equivalent to" Phrase Of Power and could be easily fixed by changing the second part of the P/R slightly as follows:

>     template <class charT, class traits, class T>
>       basic_istream<charT, traits>&
>       operator>>(basic_istream<charT, traits>&& is, T&& x);

> > -1- Effects: Equivalent to:`is >>x`

> > >     is >> std::forward<T>(x);
> > >     return is;

> > -2- Returns: `is`

Previous resolution [SUPERSEDED]:

> This wording is relative to N4567.

> 1. Edit [iostream.format.overview], header `<istream>` synopsis, as indicated:

>    >     namespace std {
>    >       […]
>    >       template <class charT, class traits, class T>
>    >         basic_istream<charT, traits>&
>    >         operator>>(basic_istream<charT, traits>&& is, T&& x);
>    >     }

> 2. Edit 31.7.5.6 [istream.rvalue] as indicated:

>    >     template <class charT, class traits, class T>
>    >       basic_istream<charT, traits>&
>    >       operator>>(basic_istream<charT, traits>&& is, T&& x);

>    > > -1- Effects: Equivalent to `return` `is >>x std::forward<T>(x)`

>    > > -2- Returns: `is`

[2016-03 Jacksonville: Move to Ready]

Proposed resolution:

This wording is relative to N4567.

1. Edit [iostream.format.overview], header `<istream>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       template <class charT, class traits, class T>
   >         basic_istream<charT, traits>&
   >         operator>>(basic_istream<charT, traits>&& is, T&& x);
   >     }

2. Edit 31.7.5.6 [istream.rvalue] as indicated:

   >     template <class charT, class traits, class T>
   >       basic_istream<charT, traits>&
   >       operator>>(basic_istream<charT, traits>&& is, T&& x);

   > > -1- Effects: Equivalent to:`is >>x`

   > > >     is >> std::forward<T>(x);
   > > >     return is;

   > > -2- Returns: `is`

### 2329(i). `regex_match()/regex_search()` with `match_results` should forbid temporary strings

Section: 28.6.3 [re.syn] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28

Priority: 2

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Discussion:

Consider the following code:

>     const regex r(R"(meow(\d+)\.txt)");
>     smatch m;
>     if (regex_match(dir_iter->path().filename().string(), m, r)) {
>       DoSomethingWith(m[1]);
>     }

This occasionally crashes. The problem is that `dir_iter->path().filename().string()` returns a temporary string, so the `match_results` contains invalidated iterators into a destroyed temporary string.

It's fine for `regex_match/regex_search(str, reg)` to accept temporary strings, because they just return `bool`. However, the overloads taking `match_results` should forbid temporary strings.

[2014-02-13 Issaquah: Move as Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 28.6.3 [re.syn], header `<regex>` synopsis, as indicated:

   >     #include <initializer_list>

   >     namespace std {

   >       […]

   >       // 28.11.2, function template regex_match:
   >       […]
   >       template <class ST, class SA, class Allocator, class charT, class traits>
   >       bool regex_match(const basic_string<charT, ST, SA>&&,
   >                        match_results<
   >                          typename basic_string<charT, ST, SA>::const_iterator,
   >                          Allocator>&,
   >                        const basic_regex<charT, traits>&,
   >                        regex_constants::match_flag_type =
   >                          regex_constants::match_default) = delete;

   >       // 28.11.3, function template regex_search:
   >       […]
   >       template <class ST, class SA, class Allocator, class charT, class traits>
   >       bool regex_search(const basic_string<charT, ST, SA>&&,
   >                         match_results<
   >                           typename basic_string<charT, ST, SA>::const_iterator,
   >                           Allocator>&,
   >                         const basic_regex<charT, traits>&,
   >                         regex_constants::match_flag_type =
   >                           regex_constants::match_default) = delete;
   >       […]
   >     }

### 2330(i). `regex("meow", regex::icase)` is technically forbidden but should be permitted

Section: 28.6.4.2 [re.synopt] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05

Priority: 0

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Discussion:

28.6.4.2 [re.synopt]/1 says "A valid value of type `syntax_option_type` shall have exactly one of the elements `ECMAScript`, `basic`, `extended`, `awk`, `grep`, `egrep`, set."

This "exactly one" wording technically forbids passing `icase` by itself! Users should not be required to pass `regex::ECMAScript | regex::icase`. (Note that the cost of an additional check for no grammar being explicitly requested is completely irrelevant, as regex construction is so much more expensive.)

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 28.6.4.2 [re.synopt] as indicated:

   > -1- The type `syntax_option_type` is an implementation-defined bitmask type (17.5.2.1.3). Setting its elements has the effects listed in table 138. A valid value of type `syntax_option_type` shall have exactlyat most one of the grammar elements `ECMAScript`, `basic`, `extended`, `awk`, `grep`, `egrep`, set. If no grammar element is set, the default grammar is `ECMAScript`.

### 2332(i). `regex_iterator/regex_token_iterator` should forbid temporary regexes

Section: 28.6.11 [re.iter] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28

Priority: 2

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Discussion:

Users can write "`for(sregex_iterator i(s.begin(), s.end(), regex("meow")), end; i != end; ++i)`", binding a temporary `regex` to `const regex&` and storing a pointer to it. This will compile silently, triggering undefined behavior at runtime. We now have the technology to prevent this from compiling, like how `reference_wrapper` refuses to bind to temporaries.

[2014-02-14 Issaquah meeting: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change 28.6.11.1 [re.regiter]/1, class template `regex_iterator` synopsis, as indicated:

   >     regex_iterator();
   >     regex_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >       const regex_type& re,
   >       regex_constants::match_flag_type m =
   >         regex_constants::match_default);
   >     regex_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >       const regex_type&& re,
   >       regex_constants::match_flag_type m =
   >         regex_constants::match_default) = delete;

2. Change 28.6.11.2 [re.tokiter]/6, class template `regex_token_iterator` synopsis, as indicated:

   >     regex_token_iterator();
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          int submatch = 0,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default);
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          const std::vector<int>& submatches,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default);
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          initializer_list<int> submatches,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default);
   >     template <std::size_t N>
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          const int (&submatches)[N],
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default);
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type&& re,
   >                          int submatch = 0,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default) = delete;
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type&& re,
   >                          const std::vector<int>& submatches,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default) = delete;
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type&& re,
   >                          initializer_list<int> submatches,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default) = delete;
   >     template <std::size_t N>
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type&& re,
   >                          const int (&submatches)[N],
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default) = delete;

### 2333(i). [fund.ts] Hashing disengaged `optional<T>` objects

Section: 5.11 [fund.ts::optional.hash] Status: Resolved Submitter: Jonathan Wakely Opened: 2013-10-03 Last modified: 2015-10-26

Priority: Not Prioritized

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Discussion:

Addresses: fund.ts

The spec for `hash<optional<T>>` doesn't say anything about disengaged objects, so 3.66 [defns.undefined] would imply it's undefined behaviour, but that's very unhelpful to users.

If hashing disengaged optional objects is undefined there should be a Requires, otherwise there should be some statement saying it's OK.

It would be possible to specify the value, e.g. saying it returns the same value as something like `std::hash<void*>()(nullptr)`, but leaving it unspecified would permit users to specialize `hash<optional<UserDefinedType>>` so that the hash value for a disengaged object is distinct from any value returned by `hash<UserDefinedType>`.

[2014-06-06 pre-Rapperswill]

This issue has been reopened as fundamentals-ts.

[2014-06-07 Daniel comments]

This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.

[2014-06-16 Rapperswill]

Confirmed that this issue is resolved in the current Library Fundamentals working paper.

Proposed resolution:

This wording is relative to N3691.

1. Add to 5.11 [fund.ts::optional.hash]/3

   >     template <class T> struct hash<optional<T>>;

   > > […]

   > > -3- For an object `o` of type `optional<T>`, if `bool(o) == true`, `hash<optional<T>>()(o)` shall evaluate to the same value as `hash<T>()(*o)` otherwise it evaluates to an unspecified value.

### 2334(i). `atomic`'s default constructor requires "uninitialized" state even for types with non-trivial default-constructor

Section: 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Daniel Krügler Opened: 2013-10-03 Last modified: 2019-11-19

Priority: Not Prioritized

View all other issues in [atomics.types.operations].

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Discussion:

According to 99 [atomics.types.operations.req] p4,

>     A::A() noexcept = default;

> > Effects: leaves the atomic object in an uninitialized state. [Note: These semantics ensure compatibility with `C`. — end note]

This implementation requirement is OK for POD types, like `int`, but 32.5.8 [atomics.types.generic] p1 intentionally allows template arguments of `atomic` with a non-trivial default constructor ("The type of the template argument `T` shall be trivially copyable (3.9)"), so this wording can be read in a way that makes the behaviour of the following code undefined:

>     #include <atomic>
>     #include <iostream>

>     struct S {
>       S() noexcept : v(42) {}
>       int v;
>     };

>     int main() {
>       std::atomic<S> as; // Default-initialization
>       std::cout << as.load().v << std::endl; // ?
>     }

For a user-defined emulation of `atomic` the expected outcome would be defined and the program would output "42", but existing implementations differ and the result value is a "random number" for at least one implementation. This seems very surprising to me.

To realize that seemingly existing requirement, an implementation is either required to violate normal language rules internally or to perform specific bit-randomization-techniques after the normal default-initialization that called the default constructor of `S`.

According to my understanding, the non-normative note in 99 [atomics.types.operations.req] p4 is intended to refer to types that are valid `C`-types, but the example type `S` is not such a type.

To make the mental model of `atomic`'s default constructor more intuitive for user-code, I suggest to clarify the wording to have the effects of default-initialization instead. The current state seems more like an unintended effect of imprecise language used here and has some similarities to wording that was incorrectly used to specify `atomic_flag` initialization as described by LWG 2159(i).

[2014-05-17, Daniel comments and provides alternative wording]

The current wording was considered controversial as expressed by reflector discussions. To me, the actual problem is not newly introduced by that wording, but instead is already present in basically all paragraphs specifying semantics of atomic types, since the wording never clearly distinguishes the value of the actual atomic type A and the value of the "underlying", corresponding non-atomic type C. The revised proposed wording attempts to improve the current ambiguity of these two kinds of values.

Previous resolution from Daniel [SUPERSEDED]:

> This wording is relative to N3691.

> 1. Modify 99 [atomics.types.operations.req] p4 as indicated: [Editorial note: There is no exposition-only member in `atomic`, which makes it a bit hard to specify what actually is initialized, but the usage of the term "value" seems consistent with similar wording used to specify the effects of the atomic `load` functions]

>    >     A ::A () noexcept = default;

>    > > -4- Effects: leaves the atomic object in an uninitialized stateThe value of the atomic object is default-initialized (9.5 [dcl.init]). [Note: These semantics ensure compatibility with `C`. — end note]

[2015-02 Cologne]

Handed over to SG1.

[2017-07 Toronto]

SG1 reviewed the PR below:

Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Modify 99 [atomics.types.operations.req] p2 as indicated: [Editorial note: This is a near-to editorial change not directly affecting this issue, but `atomic_address` does no longer exist and the pointed to definition is relevant in the context of this issue resolution.]

>    > -2- In the following operation definitions:

>    > - an A refers to one of the atomic types.

>    > - a C refers to its corresponding non-atomic type. The `atomic_address` atomic type corresponds to the `void*` non-atomic type.

>    > - […]

> 2. Modify 99 [atomics.types.operations.req] p4 and the following as indicated: [Editorial note: There is no exposition-only member in `atomic`, which makes it a bit hard to specify what actually is initialized, but the introductory wording of 99 [atomics.types.operations.req] p2 b2 defines: "a C refers to its corresponding non-atomic type." which helps to specify the semantics in terms of "the C value referred to by the atomic object"]

>    >     A::A() noexcept = default;

>    > > -4- Effects: leaves the atomic object in an uninitialized stateDefault-initializes (9.5 [dcl.init]) the C value referred to by the atomic object. [Note: These semantics ensure compatibility with `C`. — end note]

>    >     constexpr A::A(C desired) noexcept;

>    > > -5- Effects: Direct-iInitializes the C value referred to by the atomic object with the value `desired`. Initialization is not an atomic operation (1.10). […]

>    > > […]

>    >     void atomic_init(volatile A* object, C desired) noexcept;
>    >     void atomic_init(A* object, C desired) noexcept;

>    > > -8- Effects: Non-atomically initializes the C value referred to by `*object` with value `desired`. […]

>    >     void atomic_store(volatile A* object, C desired) noexcept;
>    >     […]
>    >     void A::store(C desired, memory_order order = memory_order_seq_cst) noexcept;

>    > > -9- […]

>    > > -10- Effects: Atomically replaces the C value pointed to by `object` or by `this` with the value of `desired`. […]

>    > > […]

>    >     C atomic_load(const volatile A* object) noexcept;
>    >     […]
>    >     C A::load(memory_order order = memory_order_seq_cst) const noexcept;

>    > > -13- […]

>    > > -14- […]

>    > > -15- Returns: Atomically returns the C value pointed to by `object` or by `this`.

>    > > […]

>    >     C atomic_exchange(volatile A* object, C desired) noexcept;
>    >     […]
>    >     C A::exchange(C desired, memory_order order = memory_order_seq_cst) noexcept;

>    > > -18- Effects: Atomically replaces the C value pointed to by `object` or by `this` with `desired`. […]

>    > > -19- Returns: Atomically returns the C value pointed to by `object` or by `this` immediately before the effects.

>    > > […]

>    >     C atomic_fetch_key(volatile A* object, M operand) noexcept;
>    >     […]
>    >     C A::fetch_key(M operand, memory_order order = memory_order_seq_cst) noexcept;

>    > > -28- Effects: Atomically replaces the C value pointed to by `object` or by `this` with the result of the computation applied to the C value pointed to by `object` or by `this` and the given `operand`. […]

>    > > -29- Returns: Atomically, returns the C value pointed to by `object` or by `this` immediately before the effects.

>    > > […]

> 3. Modify 32.5.10 [atomics.flag] p5 and the following as indicated:

>    >     bool atomic_flag_test_and_set(volatile atomic_flag* object) noexcept;
>    >     […]
>    >     bool atomic_flag::test_and_set(memory_order order = memory_order_seq_cst) noexcept;

>    > > -5- Effects: Atomically sets the bool value pointed to by `object` or by `this` to `true`. […]

>    > > -6- Returns: Atomically, returns the bool value of thepointed to by `object` or by `this` immediately before the effects.

>    >     void atomic_flag_clear(volatile atomic_flag* object) noexcept;
>    >     […]
>    >     void atomic_flag::clear(memory_order order = memory_order_seq_cst) noexcept;

>    > > -7- […]

>    > > -8- Effects: Atomically sets the bool value pointed to by `object` or by `this` to `false`. […]

SG1 also reviewed another PR from Lawrence Crowl. Lawrence's feedback was that turning atomic<T> into a container of T was a mistake, even if we allow the implementation of atomic to contain a T. SG1 agreed with Lawrence, but his PR (http://wiki.edg.com/bin/view/Wg21toronto2017/DefaultInitNonContainer) had massive merge conflicts caused by the adoption of P0558. Billy O'Neal supplied a new PR, which SG1 agreed to and which LWG looked at informally. This change also makes it clearer that initialization of an atomic is not an atomic operation in all forms, changes the C compatibility example to actually be compatible with C, and removes "initialization-compatible" which is not defined anywhere.

SG1 considered moving `ATOMIC_VAR_INIT` into Annex D, as their understanding at this time is that WG14 is considering removal of that macro. However, consensus was that moving things between clauses would require a paper, and that we should wait to remove that until WG14 actually does so.

[2019-02, Monday in Kona]

While discussing Richard's P1286 paper, we noted that this issue's resolution needs to be updated based on that discussion.

Also, the idea that `atomic() noexcept = default` is ok will not fly for implementors who store additional information inside the atomic variable.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4659.

> Modify 32.5.8.2 [atomics.types.operations] as indicated:

> -?- Initialization of an atomic object is not an atomic operation (6.10.2 [intro.multithread]). [Note: It is possible to have an access to an atomic object A race with its construction, for example by communicating the address of the just-constructed object A via a `memory_order_relaxed` operations on a suitable atomic pointer variable, and then immediately accessing A in the recieving thread. This results in undefined behavior. — end note]

> -1- [Note: Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. — end note]

> >     atomic() noexcept = default;

> > > -2- Effects: Leaves the atomic object in an uninitialized state. [Note: These semantics ensure compatibility with C. — end note]Initializes the atomic object with a default-initialized (9.5 [dcl.init]) value of type `T`. [Note: The default-initialized value may not be pointer-interconvertible with the atomic object. — end note]

> >     constexpr atomic(T desired) noexcept;

> > > -3- Effects: Initializes the atomic object with the value `desired`. Initialization is not an atomic operation (6.10.2 [intro.multithread]). [Note: It is possible to have an access to an atomic object A race with its construction, for example by communicating the address of the just-constructed object A to another thread via `memory_order_relaxed` operations on a suitable atomic pointer variable, and then immediately accessing A in the receiving thread. This results in undefined behavior — end note]

> >     #define ATOMIC_VAR_INIT(value) see below{value}

> > > -4- The macro expands to a token sequence suitable for constant initialization of an atomic variable of static storage duration of a type that is initialization-compatible with value. [Note: This operation may need to initialize locks. — end note] Concurrent access to the variable being initialized, even via an atomic operation, constitutes a data race. [Note: This macro ensures compatibility with C. — end note]
> > > [Example:
> > > `atomic<int>atomic_int v = ATOMIC_VAR_INIT(5);`
> > > — end example]

[2019-11; Resolved by the adoption of P0883 in Belfast.]

Proposed resolution:

Resolved by P0883.

### 2336(i). `is_trivially_constructible/is_trivially_assignable` traits are always false

Section: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: Daniel Krügler Opened: 2013-10-01 Last modified: 2017-07-30

Priority: 3

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Discussion:

In 21.3.5.4 [meta.unary.prop] we have traits to allow testing for triviality of specific operations, such as `is_trivially_constructible` and `is_trivially_assignable` (and their derived forms), which are specified in terms of the following initialization and assignment, respectively:

>     T t(create<Args>()...);

>     declval<T>() = declval<U>()

The wording that describes the way how triviality is deduced, is in both cases of the same form:

> [… ] and the variable definition/assignment, as defined by `is_constructible/is_assignable`, is known to call no operation that is not trivial (3.9, 12).

The problematic part of this wording is that both definitions are specified in terms of an "object construction" function `create` or `declval`, respectively, (The former being a conceptual function, the latter being a library function), but for none of these functions we can assume that they could be considered as trivial — only special member functions can have this property and none of these is one. This problem became obvious, when the similar issue LWG 2298(i) in regard to `is_nothrow_constructible` was opened.

A possible approach to solve this specification problem is to make a blanket statement for sub-clause 21.3.5.4 [meta.unary.prop] that these helper functions are considered trivial for the purpose of defining these traits.

Using this kind of wording technique can also be used to get rid of the additional helper function template `create`, which is currently needed for the `is_convertible` and the `is_constructible` traits, because both traits are specified in terms of contexts where technically the corresponding "object construction" function would be considered as odr-used. This is problematic, because these traits are defined in terms of well-formed code and odr-using `declval` would make the program ill-formed (see 22.2.6 [declval]). So extending above blanket statement to consider `std::declval<T>()` as not odr-used in the context of the corresponding trait definition would allow for replacing `create` by `declval`.

[2015-05, Lenexa]

STL: would you consider moving the change to 20.10 as editorial or are you uncomfortable with it?
JW: this sounds a viable editorial change
VV: I guarantee you that moving it doesn't change anything
MC: how about this: we move it to Ready as is and if we conclude moving it is editorial we can do it and if not open an issue
STL: I would like to guarantee that the lifting happens
JW: I do that! If it goes in I move it up
MC: move to Ready: in favor: 15, opposed: 0, abstain: 1

Proposed resolution:

This wording is relative to N3936.

1. Add a new paragraph after 21.3.5.4 [meta.unary.prop] p3 as indicated: [Editorial note: The first change in 21.3.5.4 [meta.unary.prop] p3 is recommended, because technically a Clause is always a "main chapter" — such as Clause 20 — but every child of a Clause or sub-clause is a sub-clause]

   > […]

   > -3- For all of the class templates `X` declared in this Clausesub-clause, instantiating that template with a template-argument that is a class template specialization may result in the implicit instantiation of the template argument if and only if the semantics of `X` require that the argument must be a complete type.

   > -?- For the purpose of defining the templates in this sub-clause, a function call expression `declval<T>()` for any type `T` is considered to be a trivial (6.9 [basic.types], 11.4.4 [special]) function call that is not an odr-use (6.3 [basic.def.odr]) of `declval` in the context of the corresponding definition notwithstanding the restrictions of 22.2.6 [declval].

   > […]

2. Modify 21.3.5.4 [meta.unary.prop] p7 as indicated:

   > -7- Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create();

   > tThe predicate condition for a template specialization `is_constructible<T, Args...>` shall be satisfied if and only if the following variable definition would be well-formed for some invented variable `t`:

   > >     T t(createdeclval<Args>()...);

   > […]

3. Add a new paragraph after 21.3.7 [meta.rel] p2 as indicated: [Editorial note: Technically we don't need the guarantee of "a trivial function call" for the type relationship predicates at the very moment, but it seems more robust and consistent to have the exact same guarantee here as well]

   > […]

   > -2- […]

   > -?- For the purpose of defining the templates in this sub-clause, a function call expression `declval<T>()` for any type `T` is considered to be a trivial (6.9 [basic.types], 11.4.4 [special]) function call that is not an odr-use (6.3 [basic.def.odr]) of `declval` in the context of the corresponding definition notwithstanding the restrictions of 22.2.6 [declval].

   > […]

4. Modify 21.3.7 [meta.rel] p4 as indicated:

   > -4- Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create();

   > tThe predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

   > >     To test() {
   > >       return createdeclval<From>();
   > >     }

   > […]

### 2339(i). Wording issue in `nth_element`

Section: 26.8.3 [alg.nth.element] Status: C++14 Submitter: Christopher Jefferson Opened: 2013-10-19 Last modified: 2017-07-05

Priority: 0

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Discussion:

The wording of `nth_element` says:

>     template<class RandomAccessIterator>
>       void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
>                        RandomAccessIterator last);

> > After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for every iterator `i` in the range `[first,nth)` and every iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

That wording, to me, implies that there must be an element at '`nth`'. However, gcc at least accepts `nth == last`, and returns without effect (which seems like the sensible option).

Is it intended to accept `nth == last`? If so, then I would suggest adding this to the wording explicitly, say:

> After `nth_element` the element in the position pointed to by `nth`, if any, is the element that would be in that position if the whole range were sorted. Also for every iterator `i` in the range `[first,nth)` and every iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 26.8.3 [alg.nth.element]/1 as indicated:

   >     template<class RandomAccessIterator>
   >       void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                        RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >       void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                        RandomAccessIterator last, Compare comp);

   > > -1- After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted, unless `nth == last`. Also for every iterator `i` in the range `[first,nth)` and every iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

### 2340(i). Replacement allocation functions declared as inline

Section: 16.4.5.6 [replacement.functions] Status: C++17 Submitter: David Majnemer Opened: 2013-10-20 Last modified: 2017-07-30

Priority: 2

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Discussion:

N3290 16.4.5.6 [replacement.functions]/p3 says:

> The program's definitions shall not be specified as `inline`.

This seems to permit declarations of replacement allocation functions that are specified as `inline` so long as they aren't used. This behavior seems more like a bug than a feature, I propose that we do the following:

> The program's definitionsdeclarations shall not be specified as `inline`.

[2014-02-15 Issaquah : Move to Ready]

Proposed resolution:

This wording is relative to N3797.

1. Modify 16.4.5.6 [replacement.functions]/3 as indicated:

   > -3- The program's definitions are used instead of the default versions supplied by the implementation (18.6). Such replacement occurs prior to program startup (3.2, 3.6). The program's definitionsdeclarations shall not be specified as `inline`. No diagnostic is required.

### 2341(i). Inconsistency between `basic_ostream::seekp(pos)` and `basic_ostream::seekp(off, dir)`

Section: 31.7.6.2.5 [ostream.seeks] Status: C++14 Submitter: Marshall Clow Opened: 2013-10-21 Last modified: 2017-07-05

Priority: 0

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Discussion:

In 31.7.6.2.5 [ostream.seeks], we have:

>     basic_ostream<charT,traits>& seekp(pos_type pos);

> > -3- Effects: If `fail() != true`, executes `rdbuf()->pubseekpos(pos, ios_base::out)`. In case of failure, the function calls `setstate(failbit)` (which may throw `ios_base::failure`).

> > -4- Returns: `*this`.

>     basic_ostream<charT,traits>& seekp(off_type off, ios_base::seekdir dir);

> > -5- Effects: If `fail() != true`, executes `rdbuf()->pubseekoff(off, dir, ios_base::out)`.

> > -6- Returns: `*this`.

The first call is required to set the `failbit` on failure, but the second is not

So (given two ostreams, `os1` and `os2`) the following code (confusingly) works:

>     os1.seekp(-1);
>     assert(os1.fail());

>     os2.seekp(-1, std::ios_base::beg);
>     assert(os2.good());

Note that the description of `basic_istream<charT,traits>& seekg(off_type off, ios_base::seekdir dir)` in 31.7.5.4 [istream.unformatted] p43 does require setting `failbit`.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 31.7.6.2.5 [ostream.seeks]p5 as indicated:

   >     basic_ostream<charT,traits>& seekp(off_type off, ios_base::seekdir dir);

   > > -5- Effects: If `fail() != true`, executes `rdbuf()->pubseekoff(off, dir, ios_base::out)`. In case of failure, the function calls `setstate(failbit)` (which may throw `ios_base::failure`).

   > > -6- Returns: `*this`.

### 2343(i). Is the value of the ECMA-262 RegExp object's multiline property really false?

Section: 28.6.12 [re.grammar] Status: Resolved Submitter: Nayuta Taga Opened: 2013-10-30 Last modified: 2017-03-12

Priority: 2

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Discussion:

In the following "Multiline" is the value of the ECMA-262 RegExp object's multiline property.

In ECMA-262, there are some definitions that relate to Multiline:

- ECMA-262 15.10.2.6:

  > If Multiline is true, ^ matches just after LineTerminator.

  > If Multiline is false, ^ does not match just after LineTerminator.

  > If Multiline is true, $ matches just before LineTerminator.

  > If Multiline is false, $ does not match just before LineTerminator.

- ECMA-262 15.10.4.1, 15.10.7.4:

  > By default, Multiline is false.

So, the C++11 standard says that Multiline is false. As it is false, ^ matches only the beginning of the string, and $ matches only the end of the string.

However, two flags are defined in 28.6.4.3 [re.matchflag] Table 139:

> `match_not_bol`: the character ^ in the regular expression shall not match `[first,first)`.

> `match_not_eol`: the character "$" in the regular expression shall not match `[last,last)`.

As Multiline is false, the `match_not_bol` and the `match_not_eol` are meaningless because they only make ^ and $ match none.

In my opinion, Multiline should be true.

FYI, Multiline of the existing implementations are as follows:

Multiline=false:

- libstdc++ r206594

- libc++ r199174

Multiline=true:

- Visual Studio Express 2013

- boost 1.55

[2015-05-22, Daniel comments]

This issue interacts with LWG 2503(i).

[2016-08 Chicago]

Resolving 2503(i) will resolve this as well.

Proposed resolution:

Resolved by LWG 2503(i).

### 2344(i). `quoted()`'s interaction with padding is unclear

Section: 31.7.9 [quoted.manip] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-11-01 Last modified: 2016-01-28

Priority: 1

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Discussion:

Given this code:

>     cout << "[" << left << setfill('x') << setw(20) << R"("AB \"CD\" EF")" << "]" << endl;
>     cout << "[" << left << setfill('y') << setw(20) << quoted(R"(GH "IJ" KL)") << "]" << endl;

The first line prints `["AB \"CD\" EF"xxxxxx]`. The second line should probably print `["GH \"IJ\" KL"yyyyyy]`, but 31.7.9 [quoted.manip]/2 doesn't say whether or how `quoted()` should interact with padding. All it says is that

> "`out << quoted(s, delim, escape)` behaves as if it inserts the following characters into out using character inserter function templates (27.7.3.6.4)".

31.7.6.3.4 [ostream.inserters.character] specifies both single-character and null-terminated inserters, both referring to 31.7.6.3.1 [ostream.formatted.reqmts]/3 for padding. Literally implementing `quoted()` with single-character inserters would result in padding being emitted after the first character, with undesirable effects for `ios_base::left`.

It appears that 27.4.4.4 [string.io]/5 has the appropriate incantations to follow here. It says that `os << str`

> "Behaves as a formatted output function (27.7.3.6.1) of `os`. Forms a character sequence `seq`, initially consisting of the elements defined by the range `[str.begin(), str.end())`. Determines padding for `seq` as described in 27.7.3.6.1. Then inserts `seq` as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size();` then calls `os.width(0)`."

Additionally, saying that it's a "formatted output function" activates 31.7.6.3.1 [ostream.formatted.reqmts]/1's wording for sentry objects.

[2014-02-14 Issaquah meeting: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Edit 31.7.9 [quoted.manip] as follows:

   >     template <class charT>
   >       unspecified quoted(const charT* s, charT delim=charT('"'), charT escape=charT('\\'));
   >     template <class charT, class traits, class Allocator>
   >       unspecified quoted(const basic_string<charT, traits, Allocator>& s,
   >                          charT delim=charT('"'), charT escape=charT('\\'));

   > > -2- Returns: An object of unspecified type such that if `out` is an instance of `basic_ostream` with member type `char_type` the same as `charT`, then the expression `out << quoted(s, delim, escape)` behaves as if it inserts the following characters into `out` using character inserter function templates (27.7.3.6.4), which may throw `ios_base::failure` (27.5.3.1.1)a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of `out`. This forms a character sequence `seq`, initially consisting of the following elements:

   > > - `delim`.

   > > - Each character in `s`. If the character to be output is equal to escape or `delim`, as determined by `operator==`, first output `escape`.

   > > - `delim`.

   > > Let `x` be the number of elements initially in `seq`. Then padding is determined for `seq` as described in 31.7.6.3.1 [ostream.formatted.reqmts], `seq` is inserted as if by calling `out.rdbuf()->sputn(seq, n)`, where `n` is the larger of `out.width()` and `x`, and `out.width(0)` is called. The expression `out << quoted(s, delim, escape)` shall have type `basic_ostream<charT, traits>&` and value `out`.

### 2346(i). `integral_constant`'s member functions should be marked `noexcept`

Section: 21.3.4 [meta.help] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-11-05 Last modified: 2017-07-05

Priority: 0

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Discussion:

Obvious.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Edit 21.3.4 [meta.help] as indicated:

   >     namespace std {
   >       template<class T, T v>
   >       struct integral_constant {
   >         static constexpr T value = v;
   >         typedef T value_type;
   >         typedef integral_constant<T,v> type;
   >         constexpr operator value_type() const noexcept { return value; }
   >         constexpr value_type operator()() const noexcept { return value; }
   >       };
   >       […]
   >     }

### 2349(i). Clarify input/output function rethrow behavior

Section: 31.7.5.3.1 [istream.formatted.reqmts] Status: Resolved Submitter: Zhihao Yuan Opened: 2013-12-06 Last modified: 2022-11-22

Priority: 3

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Discussion:

The formatted input function requirement says in 31.7.5.3.1 [istream.formatted.reqmts]:

> "If an exception is thrown during input then `ios::badbit` is turned on in `*this`'s error state. If `(exceptions()&badbit) != 0` then the exception is rethrown."

while some formatted function may throw an exception from `basic_ios::clear`, for example in 22.9.4 [bitset.operators] p6:

> "If no characters are stored in `str`, calls `is.setstate(ios_base::failbit)` (which may throw `ios_base::failure`)"

So should this exception be considered as "an exception [...] thrown during input"? And here is an implementation divergence (or you can read the following as "a bug libc++ only has" :)

>     cin.exceptions(ios_base::failbit);
>     bitset<N> b;
>     try {
>       cin >> b;  // type 'a' and return
>     } catch (...)
>     {}

Now `cin.rdstate()` is just `failbit` in libstdc++ (and Dinkumware, by PJ), but `failbit & badbit` libc++. Similar difference found in other places, like `eofbit & badbid` after `std::getline`.

PJ and Matt both agree that the intention (of `badbit` + rethrow) is "to signify an exception arising in user code, not the iostreams package".

In addition, I found the following words in unformatted input function's requirements (31.7.5.4 [istream.unformatted]):

> If an exception is thrown during input then `ios::badbit` is turned on in `*this`'s error state. (Exceptions thrown from `basic_ios<>::clear()` are not caught or rethrown.) If `(exceptions()&badbit) != 0` then the exception is rethrown.

The content within the parenthesis is added by LWG defect 61(i), and does fix the ambiguity. However, it only fixed the 1 of 4 requirements, and it lost some context (the word "rethrown" is not seen before this sentence within this section).

[Lenexa 2015-05-07: Marshall to research and report]

[Kona 2022-11-08; this would be resolved by P1264]

[2022-11-22 Resolved by P1264R2 accepted in Kona. Status changed: Open → Resolved.]

Proposed resolution:

This wording is relative to N3797.

[Drafting note: The editor is kindly asked to introduce additional spaces at the following marked occurrences of `operator&` — end drafting note]

1. Modify 31.7.5.3.1 [istream.formatted.reqmts] p1 as indicated:

   > -1- Each formatted input function begins execution by constructing an object of class `sentry` with the `noskipws` (second) argument false. If the `sentry` object returns true, when converted to a value of type `bool`, the function endeavors to obtain the requested input. If an exception, other than the ones thrown from `clear()`, if any, is thrown during input then `ios::badbit` is turned on[Footnote 314] in `*this`'s error state. If `(exceptions() & badbit) != 0` then the exception is rethrown. In any case, the formatted input function destroys the `sentry` object. If no exception has been thrown, it returns `*this`.

2. Modify 31.7.6.3.1 [ostream.formatted.reqmts] p1 as indicated:

   > -1- Each formatted output function begins execution by constructing an object of class `sentry`. If this object returns true when converted to a value of type `bool`, the function endeavors to generate the requested output. If the generation fails, then the formatted output function does `setstate(ios_base::failbit)`, which might throw an exception. If an exception, other than the ones thrown from `clear()`, if any, is thrown during output, then `ios::badbit` is turned on[Footnote 327] in `*this`'s error state. If `(exceptions() & badbit) != 0` then the exception is rethrown. Whether or not an exception is thrown, the `sentry` object is destroyed before leaving the formatted output function. If no exception is thrown, the result of the formatted output function is `*this`.

3. Modify 31.7.6.4 [ostream.unformatted] p1 as indicated:

   > -1- Each unformatted output function begins execution by constructing an object of class `sentry`. If this object returns true, while converting to a value of type `bool`, the function endeavors to generate the requested output. If an exception, other than the ones thrown from `clear()`, if any, is thrown during output, then ios::badbit is turned on[Footnote 330] in `*this`'s error state. If `(exceptions() & badbit) != 0` then the exception is rethrown. In any case, the unformatted output function ends by destroying the `sentry` object, then, if no exception was thrown, returning the value specified for the unformatted output function.

4. Modify 31.7.5.4 [istream.unformatted] p1 as indicated:

   > -1- Each unformatted input function begins execution by constructing an object of class `sentry` with the default argument `noskipws` (second) argument true. If the `sentry` object returns true, when converted to a value of type `bool`, the function endeavors to obtain the requested input. Otherwise, if the `sentry` constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type `bool`, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to `0`; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using `charT()`) in the first location of the array. If an exception, other than the ones thrown from `clear()`, if any, is thrown during input then `ios::badbit` is turned on[Footnote 317] in `*this`'s error state. (Exceptions thrown from `basic_ios<>::clear()` are not caught or rethrown.) If `(exceptions() & badbit) != 0` then the exception is rethrown. It also counts the number of characters extracted. If no exception has been thrown it ends by storing the count in a member object and returning the value specified. In any event the `sentry` object is destroyed before leaving the unformatted input function.

### 2350(i). `min`, `max`, and `minmax` should be `constexpr`

Section: 26.8.9 [alg.min.max] Status: C++14 Submitter: Ville Voutilainen Opened: 2013-12-15 Last modified: 2016-01-28

Priority: 1

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Discussion:

Having `min`, `max`, and `minmax` `constexpr` was a large part of the motivation to allow reference-to-const arguments for `constexpr` functions as per N3039. Furthermore, `initializer_lists` are immutable and not-movable-from for large part in order to allow using them in `constexpr` contexts and other hoisting-optimizations. In N3797 version of the draft none of these functions are `constexpr`, and they should be made `constexpr`.

Proposed resolution:

This wording is relative to N3797.

1. In 26.1 [algorithms.general], header `<algorithm>` synopsis, and 26.8.9 [alg.min.max], change as indicated (add `constexpr` to every signature before `min_element`):

   >     template<class T> constexpr const T& min(const T& a, const T& b);
   >     template<class T, class Compare>
   >     constexpr const T& min(const T& a, const T& b, Compare comp);
   >     […]
   >     template<class T>
   >     constexpr T min(initializer_list<T> t);
   >     template<class T, class Compare>
   >     constexpr T min(initializer_list<T> t, Compare comp);
   >     […]
   >     template<class T> constexpr const T& max(const T& a, const T& b);
   >     template<class T, class Compare>
   >     constexpr const T& max(const T& a, const T& b, Compare comp);
   >     […]
   >     template<class T>
   >     constexpr T max(initializer_list<T> t);
   >     template<class T, class Compare>
   >     constexpr T max(initializer_list<T> t, Compare comp);
   >     […]
   >     template<class T> constexpr pair<const T&, const T&> minmax(const T& a, const T& b);
   >     template<class T, class Compare>
   >     constexpr pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp);
   >     […]
   >     template<class T>
   >     constexpr pair<T, T> minmax(initializer_list<T> t);
   >     template<class T, class Compare>
   >     constexpr pair<T, T> minmax(initializer_list<T> t, Compare comp);

### 2353(i). `std::next` is over-constrained

Section: 24.4.3 [iterator.operations] Status: C++17 Submitter: Eric Niebler Opened: 2013-12-24 Last modified: 2017-09-07

Priority: 4

View other active issues in [iterator.operations].

View all other issues in [iterator.operations].

View all issues with C++17 status.

Discussion:

In LWG 1011(i), `std::next` and `std::prev` were changed from accepting `InputIterator` to accepting `ForwardIterator`. This needlessly excludes perfectly legitimate use cases. Consider the following hypothetical range-based implementation of drop, which creates a view of a range without the first `n` elements:

>     template<typename Distance, typename InputRange>
>     iterator_range<range_iterator_t<InputRange>>
>     drop(Distance n, InputRange& rng)
>     {
>       return make_iterator_range(
>         std::next(std::begin(rng), n),
>         std::end(rng)
>       );
>     }

I believe this to be a legitimate use case that is currently outlawed by the standard without cause. See the discussion beginning at c++std-lib-35313 for an in-depth discussion of the issue, in which Howard Hinnant agreed that it was a defect.

(Some discussion then ensued about whether an overload should be added that only accepts rvalue `InputIterators` to avoid the surprise that issue 1011(i) sought to address. I make no such attempt, nor do I believe it to be necessary.)

Suggested resolution:

Back out the resolution of 1011(i).

[Lenexa 2015-05-07: Move to Ready]

Proposed resolution:

This wording is relative to N3797.

1. Change 24.2 [iterator.synopsis], header `<iterator>` synopsis, and 24.4.3 [iterator.operations] before p.6 as indicated:

   >     template <class ForwardInputIterator>
   >       ForwardInputIterator next(ForwardInputIterator x,
   >         typename std::iterator_traits<ForwardInputIterator>::difference_type n = 1);

### 2354(i). Unnecessary copying when inserting into maps with braced-init syntax

Section: 23.4.3.1 [map.overview], 23.4.4.1 [multimap.overview], 23.5.3.1 [unord.map.overview], 23.5.4.1 [unord.multimap.overview] Status: C++17 Submitter: Geoffrey Romer Opened: 2014-01-08 Last modified: 2017-10-13

Priority: 2

View all other issues in [map.overview].

View all issues with C++17 status.

Discussion:

The rvalue-reference `insert()` members of `map`, `multimap`, `unordered_map`, and `unordered_multimap` are specified as function templates, where the rvalue-reference parameter type depends on the template parameter. As a consequence, these overloads cannot be invoked via braced-initializer syntax (e.g. `my_map.insert({key, value})`), because the template argument cannot be deduced from a braced-init-list. Such calls instead resolve to the const lvalue reference overload, which forces a non-elidable copy of the argument, despite the fact that the argument is an rvalue, and so should be eligible for moving and copy elision.

This leads to sub-optimal performance for copyable values, and makes this syntax unusable with noncopyable values. This is particularly problematic because sources such as Josuttis's "C++ Standard Library" recommend this syntax as the preferred way to insert into a map in C++11.

I think this can be fixed by adding an equivalent non-template `value_type&&` overload for each affected member template. Simply declaring these members in the class synopses should be sufficient; their semantics are already dictated by the container concepts (c.f. the corresponding lvalue-reference overloads, which have no additional discussion beyond being listed in the synopsis).

[2014-02-13 Issaquah]

AJM: Is this not better solved by `emplace`?

Nico: `emplace` was a mistake, it breaks a uniform pattern designed into the STL. Hence, this fix is important, it should be the preferred way to do this.

JonW: `emplace` is still more efficient, as this form must make a non-elidable copy.

GeoffR: Also, cannot move from a `const` key, must always make a copy.

Poll for adopting the proposed wording:

SF: 1 WF: 4 N: 4 WA: 1 SA: 0

Move to Ready, pending implementation experience.

Proposed resolution:

This wording is relative to N3797.

1. Change 23.4.3.1 [map.overview], class template `map` synopsis, as indicated:

   >     […]
   >     pair<iterator, bool> insert(const value_type& x);
   >     pair<iterator, bool> insert(value_type&& x);
   >     template <class P> pair<iterator, bool> insert(P&& x);
   >     iterator insert(const_iterator position, const value_type& x);
   >     iterator insert(const_iterator position, value_type&& x);
   >     template <class P>
   >       iterator insert(const_iterator position, P&&);
   >     […]

2. Change 23.4.4.1 [multimap.overview], class template `multimap` synopsis, as indicated:

   >     […]
   >     iterator insert(const value_type& x);
   >     iterator insert(value_type&& x);
   >     template <class P> iterator insert(P&& x);
   >     iterator insert(const_iterator position, const value_type& x);
   >     iterator insert(const_iterator position, value_type&& x);
   >     template <class P> iterator insert(const_iterator position, P&& x);
   >     […]

3. Change 23.5.3.1 [unord.map.overview], class template `unordered_map` synopsis, as indicated:

   >     […]
   >     pair<iterator, bool> insert(const value_type& obj);
   >     pair<iterator, bool> insert(value_type&& obj);
   >     template <class P> pair<iterator, bool> insert(P&& obj);
   >     iterator insert(const_iterator hint, const value_type& obj);
   >     iterator insert(const_iterator hint, value_type&& obj);
   >     template <class P> iterator insert(const_iterator hint, P&& obj);
   >     […]

4. Change 23.5.4.1 [unord.multimap.overview], class template `unordered_multimap` synopsis, as indicated:

   >     […]
   >     iterator insert(const value_type& obj);
   >     iterator insert(value_type&& obj);
   >     template <class P> iterator insert(P&& obj);
   >     iterator insert(const_iterator hint, const value_type& obj);
   >     iterator insert(const_iterator hint, value_type&& obj);
   >     template <class P> iterator insert(const_iterator hint, P&& obj);
   >     […]

### 2356(i). Stability of erasure in unordered associative containers

Section: 23.2.8 [unord.req] Status: C++14 Submitter: Joaquín M López Muñoz Opened: 2014-01-21 Last modified: 2016-01-28

Priority: 2

View other active issues in [unord.req].

View all other issues in [unord.req].

View all issues with C++14 status.

Discussion:

Issue 518(i) resolution for unordered associative containers, modelled after that of issue 371(i), which is related to associative containers, states that insertion, erasure and rehashing preserve the relative ordering of equivalent elements. Unfortunately, this is not sufficient to guarantee the validity of code such as this:

>     std::unordered_multimap<int, int> m;
>     auto i = m.begin();
>     while (i != m.end()) {
>       if (pred(i))
>         m.erase(i++);
>       else
>         ++i;
>     }

(which is a direct translation from `multimap` to `unordered_multimap` of the motivating example in 371(i)), or even this:

>     std::unordered_multimap<int, int> m;
>     auto p = m.equal_range(k);
>     while (p.first != p.second) {
>       if (pred(p.first))
>         m.erase((p.first)++);
>       else
>         ++(p.first);
>     }

because the relative ordering of non-equivalent elements elements could potentially change after erasure (not that any actual implementation does that, anyway). Such an underspecification does not happen for regular associative containers, where the relative ordering of non-equivalent elements is kept by design.

[2014-02-13 Issaquah: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 23.2.8 [unord.req], p14 as indicated:

   > -14- The `insert` and `emplace` members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The `erase` members shall invalidate only iterators and references to the erased elements, and preserve the relative order of the elements that are not erased.

### 2357(i). Remaining "`Assignable`" requirement

Section: 26.8.5 [alg.partitions] Status: C++14 Submitter: Daniel Krügler Opened: 2014-02-01 Last modified: 2017-07-05

Priority: 0

View all other issues in [alg.partitions].

View all issues with C++14 status.

Discussion:

The Requires element of `partition_copy` says (emphasis mine):

> Requires: `InputIterator`'s value type shall be `Assignable`, and …

The C++03 term `Assignable` was replaced by `CopyAssignable`, remaining cleanups happened via LWG issue 972(i), but algorithm `partition_copy` was not affected at that time (during that time the requirements of `partition_copy` didn't mention writable nor assignable, but I cannot track down at the moment where these requirements had been added). Presumably this requirement should be corrected similarly to the approach used in 972(i).

Another question is whether a `CopyAssignable` is needed here, given the fact that we already require "writable to" an `OutputIterator` which is defined in 24.3.1 [iterator.requirements.general] and does already impose the necessary statement

>     *out = *in;

Given the fact that `partition_copy` never touches any input value twice, there is no reason why anything more than writable to should be necessary.

The below suggested primary resolution does not respond to the second part of this question.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 26.8.5 [alg.partitions], p12 as indicated:

   > -12- Requires: `InputIterator`'s value type shall be `CopyAssignable`, and shall be writable to the `out_true` and `out_false` `OutputIterator`s, and shall be convertible to `Predicate`'s argument type. The input range shall not overlap with either of the output ranges.

### 2359(i). How does `regex_constants::nosubs` affect `basic_regex::mark_count()`?

Section: 28.6.4.2 [re.synopt] Status: C++14 Submitter: Jonathan Wakely Opened: 2014-02-01 Last modified: 2017-09-07

Priority: 0

View all other issues in [re.synopt].

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Discussion:

As discussed in c++std-lib-35399 and its replies, I can see two possible interpretations of the effects of `regex_constants::nosubs`:

1. The effect of `nosubs` only applies during matching. Parentheses are still recognized as marking a sub-expression by the `basic_regex` compiler, and `basic_regex::mark_count()` still returns the number of marked sub-expressions, but anything they match is not stored in the results. This means it is not always true that `results.size() == r.mark_count() + 1` for a successful match.

2. `nosubs` affects how a regular expression is compiled, altering the state of the `std::basic_regex` object so that `mark_count()` returns zero. This also affects any subsequent matching.

The definition of `nosubs` should make this clear.

The wording in 28.6.4.2 [re.synopt]/1 seems to imply that `nosubs` only has effects during matching, which is (1), but all known implementations do (2). John Maddock confirmed that (2) was intended.

[Issaquah 2014-02-12: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Apply the following edit to the table in 28.6.4.2 [re.synopt]/1

   > Specifies that no sub-expressions shall be considered to be marked, so that when a regular expression is matched against a character container sequence, no sub-expression matches shall be stored in the supplied `match_results` structure.

### 2360(i). `reverse_iterator::operator*()` is unimplementable

Section: 24.5.1.2 [reverse.iterator] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2014-02-07 Last modified: 2014-02-27

Priority: 1

View all other issues in [reverse.iterator].

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Discussion:

Previously, C++03 24.4.1.3.3 [lib.reverse.iter.op.star] required:

>     reference operator*() const;

> > Effects:

> > >     Iterator tmp = current;
> > >     return *--tmp;

Now, N3797 24.5.1.1 [reverse.iterator] depicts:

>     private:
>       Iterator deref_tmp; // exposition only
>     };

And 24.5.1.3.4 [reverse.iter.op.star] requires:

>     reference operator*() const;

> > Effects:

> > >     deref_tmp = current;
> > >     --deref_tmp;
> > >     return *deref_tmp;

> > [Note: This operation must use an auxiliary member variable rather than a temporary variable to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.2.) — end note]

As written, this won't compile, because `operator*()` is `const` yet it's modifying (via assignment and decrement) the `deref_tmp` data member. So what happens if you say "`mutable Iterator deref_tmp;`"?

DANGER: WARP CORE BREACH IMMINENT.

The Standard requires `const` member functions to be callable from multiple threads simultaneously. This is 16.4.6.10 [res.on.data.races]/3: "A C++ standard library function shall not directly or indirectly modify objects (1.10) accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's non-const arguments, including `this`."

Multiple threads simultaneously modifying `deref_tmp` will trigger data races, so both `mutable` and some form of synchronization (e.g. `mutex` or `atomic`) are actually necessary!

Here's what implementations currently do: Dinkumware/VC follows C++03 and doesn't use `deref_tmp` (attempting to implement it is what led me to file this issue). According to Jonathan Wakely, libstdc++ also follows C++03 (see PR51823 which is suspended until LWG 2204(i) is resolved). According to Marshall Clow, libc++ uses `deref_tmp` with `mutable` but without synchronization, so it can trigger data races.

This `deref_tmp` Standardese was added by LWG 198(i) "Validity of pointers and references unspecified after iterator destruction" and is present in Working Papers going back to N1638 on April 11, 2004, long before C++ recognized the existence of multithreading and developed the "`const` means simultaneously readable" convention.

A related issue is LWG 1052(i) "`reverse_iterator::operator->` should also support smart pointers" which mentioned the need to depict `mutable` in the Standardese, but it was resolved NAD Future and no change was made.

Finally, LWG 2204(i) "`reverse_iterator` should not require a second copy of the base iterator" talked about removing `deref_tmp`, but without considering multithreading.

I argue that `deref_tmp` must be removed. Its existence has highly undesirable consequences: either no synchronization is used, violating the Standard's usual multithreading guarantees, or synchronization is used, adding further costs for all users that benefit almost no iterators.

`deref_tmp` is attempting to handle iterators that return references to things "inside themselves", which I usually call "stashing iterators" (as they have a secret stash). Note that these are very unusual, and are different from proxy iterators like `vector<bool>::iterator`. While `vector<bool>::iterator`'s `operator*()` does not return a true reference, it refers to a bit that is unrelated to the iterator's lifetime.

[2014-02-14 Issaquah meeting: Move to Immediate]

Strike superfluous note to avoid potential confusion, and move to Immediate.

Proposed resolution:

This wording is relative to N3797.

1. Change class template `reverse_iterator` synopsis, 24.5.1.2 [reverse.iterator], as indicated:

   >     […]
   >     protected:
   >       Iterator current;
   >     private:
   >       Iterator deref_tmp; // exposition only
   >     };

2. Change [reverse.iter.op.star] as indicated:

   >     reference operator*() const;

   > > -1- Effects:

   > > >     deref_tmp = current;
   > > >     --deref_tmp;
   > > >     return *deref_tmp;
   > > >     Iterator tmp = current;
   > > >     return *--tmp;

   > > -2- [Note: This operation must use an auxiliary member variable rather than a temporary variable to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.2.) — end note]

### 2361(i). Apply 2299 resolution throughout library

Section: 20.3.1.3 [unique.ptr.single], 20.2.3.2 [pointer.traits.types], 20.2.8.1 [allocator.uses.trait], 20.2.9.2 [allocator.traits.types], 23.2.4 [sequence.reqmts] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-02-14 Last modified: 2017-07-30

Priority: Not Prioritized

View other active issues in [unique.ptr.single].

View all other issues in [unique.ptr.single].

View all issues with C++17 status.

Discussion:

LWG 2299(i) addressed a N.B. comment pointing out that recently added wording about a type existing was not clear what happens if the type exists but is inaccessible. There are 16 pre-existing uses of the same language in the library that should use the same wording used to resolve 2299.

The relevant paragraphs are:

20.3.1.3 [unique.ptr.single]

20.2.3.2 [pointer.traits.types]

20.2.8.1 [allocator.uses.trait]

20.2.9.2 [allocator.traits.types]

23.2.4 [sequence.reqmts]

[2014-05-16, Daniel provides wording]

[2014-05-18 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 20.2.3.2 [pointer.traits.types] as indicated:

   >     typedef see below element_type;

   > > -1- Type: `Ptr::element_type` if such a type existsthe qualified-id `Ptr::element_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `T` if `Ptr` is a class template instantiation of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the specialization is ill-formed.

   >     typedef see below difference_type;

   > > -2- Type: `Ptr::difference_type` if such a type existsthe qualified-id `Ptr::difference_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `std::ptrdiff_t`.

   >     template <class U> using rebind = see below;

   > > -3- Alias template: `Ptr::rebind<U>` if such a type existsthe qualified-id `Ptr::rebind<U>` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `SomePointer<U, Args>` if `Ptr` is a class template instantiation of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the instantiation of `rebind` is ill-formed.

2. Change 20.2.8.1 [allocator.uses.trait] p1 as indicated:

   >     template <class T, class Alloc> struct uses_allocator;

   > > -1- Remarks: automatically detects whether `T` has a nested `allocator_type` that is convertible from `Alloc`. Meets the `BinaryTypeTrait` requirements (20.10.1). The implementation shall provide a definition that is derived from `true_type` if a typethe qualified-id `T::allocator_type` existsis valid and denotes a type (13.10.3 [temp.deduct]) and `is_convertible<Alloc, T::allocator_type>::value != false`, otherwise it shall be derived from `false_type`. […]

3. Change 20.2.9.2 [allocator.traits.types] as indicated:

   >     typedef see below pointer;

   > > -1- Type: `Alloc::pointer` if such a type existsthe qualified-id `Alloc::pointer` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `value_type*`.

   >     typedef see below const_pointer;

   > > -2- Type: `Alloc::const_pointer` if such a type existsthe qualified-id `Alloc::const_pointer` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `pointer_traits<pointer>::rebind<const value_type>`.

   >     typedef see below void_pointer;

   > > -3- Type: `Alloc::void_pointer` if such a type existsthe qualified-id `Alloc::void_pointer` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `pointer_traits<pointer>::rebind<void>`.

   >     typedef see below const_void_pointer;

   > > -4- Type: `Alloc::const_void_pointer` if such a type existsthe qualified-id `Alloc::const_void_pointer` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `pointer_traits<pointer>::rebind<const void>`.

   >     typedef see below difference_type;

   > > -5- Type: `Alloc::difference_type` if such a type existsthe qualified-id `Alloc::difference_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `pointer_traits<pointer>::difference_type`.

   >     typedef see below size_type;

   > > -6- Type: `Alloc::size_type` if such a type existsthe qualified-id `Alloc::size_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `make_unsigned_t<difference_type>`.

   >     typedef see below propagate_on_container_copy_assignment;

   > > -7- Type: `Alloc::propagate_on_container_copy_assignment` if such a type existsthe qualified-id `Alloc::propagate_on_container_copy_assignment` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `false_type`.

   >     typedef see below propagate_on_container_move_assignment;

   > > -8- Type: `Alloc::propagate_on_container_move_assignment` if such a type existsthe qualified-id `Alloc::propagate_on_container_move_assignment` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `false_type`.

   >     typedef see below propagate_on_container_swap;

   > > -9- Type: `Alloc::propagate_on_container_swap` if such a type existsthe qualified-id `Alloc::propagate_on_container_swap` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `false_type`.

   >     template <class T> using rebind_alloc = see below;

   > > -10- Alias template: `Alloc::rebind<T>::other` if such a type existsthe qualified-id `Alloc::rebind<T>::other` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `Alloc<T, Args>` if `Alloc` is a class template instantiation of the form `Alloc<U, Args>`, where `Args` is zero or more type arguments; otherwise, the instantiation of `rebind_alloc` is ill-formed.

4. Change 20.3.1.3 [unique.ptr.single] p3 as indicated:

   > -3- If the typequalified-id `remove_reference_t<D>::pointer` existsis valid and denotes a type (13.10.3 [temp.deduct]), then `unique_ptr<T, D>::pointer` shall be a synonym for `remove_reference_t<D>::pointer`. […]

5. Change 23.2.4 [sequence.reqmts] p3 as indicated:

   > -3- In Tables 100 and 101, `X` denotes a sequence container class, `a` denotes a value of `X` containing elements of type `T`, `A` denotes `X::allocator_type` if it existsthe qualified-id `X::allocator_type` is valid and denotes a type (13.10.3 [temp.deduct]) and `std::allocator<T>` if it doesn't, […]

### 2363(i). Defect in 30.4.1.4.1 [thread.sharedtimedmutex.class]

Section: 32.6.4.5.2 [thread.sharedtimedmutex.class] Status: Resolved Submitter: Richard Smith Opened: 2014-02-16 Last modified: 2021-05-18

Priority: 2

View all issues with Resolved status.

Discussion:

32.6.4.5.2 [thread.sharedtimedmutex.class] paragraph 2:

> The class `shared_timed_mutex` shall satisfy all of the `SharedTimedMutex` requirements (30.4.1.4). It shall be a standard layout class (Clause 9).

There's no `SharedTimedMutex` requirements; this name doesn't appear anywhere else in the standard. (Prior to N3891, this was `SharedMutex`, which was equally undefined.)

I assume this concept should be defined somewhere?

Also, n3891 changes 32.6.4.5 [thread.sharedtimedmutex.requirements] from defining "shared mutex type" to defining "shared timed mutex type", but its paragraph 2 still talks about "shared mutex type". Is that OK? I think you could argue that it's clear enough what it means, but presumably it should use the term that paragraph 1 defined.

32.6.5.5 [thread.lock.shared] paragraph 1 talks about the "shared mutex requirements", which again is a term that isn't defined, and presumably means "the requirements on a shared timed mutex type" or similar (maybe if `SharedMutex` or `SharedTimedMutex` were defined it could be reused here).

[2014-05-22, Daniel comments]

As for `SharedTimedMutex`, there exists a similar problem in regard to `TimedMutex` referred to in 32.6.4.3.2 [thread.timedmutex.class] p2 and in 32.6.4.3.3 [thread.timedmutex.recursive] p2, but nowhere defined.

Another problem is, that according to 32.6.4.2.2 [thread.mutex.class] p3, "The class `mutex` shall satisfy all the `Mutex` requirements (32.6.4 [thread.mutex.requirements]).", but there are no concrete `Mutex` requirements, 32.6.4 [thread.mutex.requirements] — titled as "Mutex requirements" — describes mutex types, timed mutex types, and shared timed mutex types.

[2014-06-08, Daniel comments and provides wording]

The presented wording adds to the existing mutex types, timed mutex types, and shared timed mutex types terms a new set of corresponding `MutexType`, `TimedMutexType`, and `SharedTimedMutexType` requirements.

The reason for the change of requirement names is two-fold: First, the new name better matches the intention to have a concrete name for the requirements imposed on the corresponding mutex types (This kind of requirement deviate from the more general `Lockable` requirements, which are not restricted to a explicitly enumerated set of library types). Second, using `**MutexType` over `**Mutex` provides the additional advantage that it reduces the chances of confusing named requirements from template parameters named `Mutex` (such as for `unique_lock` or `shared_lock`).

Nonetheless the here presented wording has one unfortunate side-effect: Once applied it would have the effect that types used to instantiate `std::shared_lock` cannot be user-defined shared mutex types due to 32.6.5.5 [thread.lock.shared]. The reason is based on the currently lack of an existing `SharedLockable` requirement set, which would complete the existing `BasicLockable` and `Lockable` requirements (which are "real" requirements). This restriction is not actually a problem introduced by the provided resolution but instead one that existed before but becomes more obvious now.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

Thanks to Daniel, and please put it in SG1-OK status. Perhaps open another issue for the remaining problem Daniel points out?

[2015-10 pre-Kona]

SG1 hands this over to LWG for wording review

[2015-10-21 Kona, Daniel comments and adjusts wording to to untimed shared mutex types]

The new wording reflects the addition of the new shared mutex types. The approach used for `shared_lock` is similar to the one used for `unique_lock`: The template argument `Mutex` has a reduced requirement set that is not sufficient for all operations. Only those members that require stronger requirements of `SharedTimedMutexType` specify that additionally in the Requires element of the corresponding prototype specifications.

The proposed wording could be more general if we would introduce more fundamental requirements set for `SharedLockable` and `SharedTimedLockable` types which could be satisfied by user-provided types as well, because the `SharedMutexType` and `SharedTimedMutexType` requirements are essentially restricted to an enumerated set of types provided by the Standard Library. But this extension seemed too large for this issue and can be easily fixed later without any harm.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 32.6.4.2 [thread.mutex.requirements.mutex] as indicated:

>    > -1- The mutex types are the standard library types `std::mutex`, `std::recursive_mutex`, `std::timed_mutex`, `std::recursive_timed_mutex`, and `std::shared_timed_mutex`. They shall meet the `MutexType` requirements set out in this section. In this description, `m` denotes an object of a mutex type.

> 2. Change 32.6.4.2.2 [thread.mutex.class] as indicated:

>    > -3- The class `mutex` shall satisfy all the `MutexType` requirements (32.6.4.2 [thread.mutex.requirements.mutex]32.6.4 [thread.mutex.requirements]). It shall be a standard-layout class (Clause 9).

> 3. Change 32.6.4.2.3 [thread.mutex.recursive] as indicated:

>    > -2- The class `recursive_mutex` shall satisfy all the Mutex `MutexType` requirements (32.6.4.2 [thread.mutex.requirements.mutex]32.6.4 [thread.mutex.requirements]). It shall be a standard-layout class (Clause 9).

> 4. Change 32.6.4.3 [thread.timedmutex.requirements] as indicated:

>    > -1- The timed mutex types are the standard library types `std::timed_mutex`, `std::recursive_timed_mutex`, and `std::shared_timed_mutex`. They shall meet the `TimedMutexType` requirements set out below. In this description, `m` denotes an object of a mutex type, `rel_time` denotes an object of an instantiation of `duration` (20.12.5), and `abs_time` denotes an object of an instantiation of `time_point` (20.12.6).

> 5. Change 32.6.4.3.2 [thread.timedmutex.class] as indicated:

>    > -2- The class `timed_mutex` shall satisfy all of the `TimedMutexType` requirements (32.6.4.3 [thread.timedmutex.requirements]). It shall be a standard-layout class (Clause 9).

> 6. Change 32.6.4.3.3 [thread.timedmutex.recursive] as indicated:

>    > -2- The class `recursive_timed_mutex` shall satisfy all of the `TimedMutexType` requirements (32.6.4.3 [thread.timedmutex.requirements]). It shall be a standard-layout class (Clause 9).

> 7. Change 32.6.4.5 [thread.sharedtimedmutex.requirements] as indicated: [Drafting note: The reference to the timed mutex types requirements has been moved after introducing the new requirement set to ensure that `SharedTimedMutexType` refine `TimedMutexType`.]

>    > -1- The standard library type `std::shared_timed_mutex` is a shared timed mutex type. Shared timed mutex types shall meet the `SharedTimedMutexType` requirements of timed mutex types (32.6.4.3 [thread.timedmutex.requirements]), and additionally shall meet the requirements set out below. In this description, `m` denotes an object of a mutex type, `rel_type` denotes an object of an instantiation of `duration` (20.12.5), and `abs_time` denotes an object of an instantiation of `time_point` (20.12.6).

>    > -?- The shared timed mutex types shall meet the `TimedMutexType` requirements (32.6.4.3 [thread.timedmutex.requirements]).

> 8. Change 32.6.4.5.2 [thread.sharedtimedmutex.class] as indicated:

>    > -2- The class `shared_timed_mutex` shall satisfy all of the `SharedTimedMutexType` requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]). It shall be a standard-layout class (Clause 9).

> 9. Change 32.6.5.5 [thread.lock.shared] as indicated: [Drafting note: Once N3995 has been applied, the following reference should be changed to the new `SharedMutexType` requirements ([thread.sharedmutex.requirements]) or even better to some new `SharedLockable` requirements (to be defined) — end drafting note]

>    > -1- […] The supplied `Mutex` type shall meet the shared mutex `SharedTimedMutexType` requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]).

>    > -2- [Note: `shared_lock<Mutex>` meets the `TimedLockable` requirements (30.2.5.4). — end note]

[2016-02 Jacksonville]

Marshall to review wording.

[2018-08-23 Batavia Issues processing]

Tim to redraft.

[2021-05-18 Resolved by the adoption of P2160R1 at the February 2021 plenary. Status changed: Open → Resolved.]

Proposed resolution:

This wording is relative to N4527.

1. Change 32.6.4.2 [thread.mutex.requirements.mutex] as indicated:

   > -1- The mutex types are the standard library types `std::mutex`, `std::recursive_mutex`, `std::timed_mutex`, `std::recursive_timed_mutex`, `std::shared_mutex`, and `std::shared_timed_mutex`. They shall meet the `MutexType` requirements set out in this section. In this description, `m` denotes an object of a mutex type.

   > -2- The mutex types shall meet the `Lockable` requirements (32.2.5.3 [thread.req.lockable.req]).

2. Change 32.6.4.2.2 [thread.mutex.class] as indicated:

   > -3- The class `mutex` shall satisfy all the `MutexType` requirements (32.6.4.2 [thread.mutex.requirements.mutex]32.6.4 [thread.mutex.requirements]). It shall be a standard-layout class (Clause 9).

3. Change 32.6.4.2.3 [thread.mutex.recursive] as indicated:

   > -2- The class `recursive_mutex` shall satisfy all the Mutex `MutexType` requirements (32.6.4.2 [thread.mutex.requirements.mutex]32.6.4 [thread.mutex.requirements]). It shall be a standard-layout class (Clause 9).

4. Change 32.6.4.3 [thread.timedmutex.requirements] as indicated:

   > -1- The timed mutex types are the standard library types `std::timed_mutex`, `std::recursive_timed_mutex`, and `std::shared_timed_mutex`. They shall meet the `TimedMutexType` requirements set out below. In this description, `m` denotes an object of a mutex type, `rel_time` denotes an object of an instantiation of `duration` (20.12.5), and `abs_time` denotes an object of an instantiation of `time_point` (20.12.6).

   > -2- The timed mutex types shall meet the `TimedLockable` requirements (32.2.5.4 [thread.req.lockable.timed]).

5. Change 32.6.4.3.2 [thread.timedmutex.class] as indicated:

   > -2- The class `timed_mutex` shall satisfy all of the `TimedMutexType` requirements (32.6.4.3 [thread.timedmutex.requirements]). It shall be a standard-layout class (Clause 9).

6. Change 32.6.4.3.3 [thread.timedmutex.recursive] as indicated:

   > -2- The class `recursive_timed_mutex` shall satisfy all of the `TimedMutexType` requirements (32.6.4.3 [thread.timedmutex.requirements]). It shall be a standard-layout class (Clause 9).

7. Change 32.6.4.4 [thread.sharedmutex.requirements] as indicated: [Drafting note: The reference to the mutex types requirements has been moved after introducing the new requirement set to ensure that `SharedMutexType` refines `MutexType`.]

   > -1- The standard library types `std::shared_mutex` and `std::shared_timed_mutex` are shared mutex types. Shared mutex types shall meet the `SharedMutexType` requirements of mutex types (32.6.4.2 [thread.mutex.requirements.mutex]), and additionally shall meet the requirements set out below. In this description, `m` denotes an object of a shared mutex type.

   > -?- The shared mutex types shall meet the `MutexType` requirements (32.6.4.2 [thread.mutex.requirements.mutex]).

8. Change 32.6.4.4.2 [thread.sharedmutex.class] as indicated:

   > -2- The class `shared_mutex` shall satisfy all of the `SharedMutexType` requirements for shared mutexes (32.6.4.4 [thread.sharedmutex.requirements]). It shall be a standard-layout class (Clause 9).

9. Change 32.6.4.5 [thread.sharedtimedmutex.requirements] as indicated: [Drafting note: The reference to the timed mutex types requirements has been moved after introducing the new requirement set to ensure that `SharedTimedMutexType` refines `TimedMutexType` and `SharedMutexType`.]

   > -1- The standard library type `std::shared_timed_mutex` is a shared timed mutex type. Shared timed mutex types shall meet the `SharedTimedMutexType` requirements of timed mutex types (32.6.4.3 [thread.timedmutex.requirements]), shared mutex types (32.6.4.4 [thread.sharedmutex.requirements]), and additionally shall meet the requirements set out below. In this description, `m` denotes an object of a shared timed mutex type, `rel_type` denotes an object of an instantiation of `duration` (20.12.5), and `abs_time` denotes an object of an instantiation of `time_point` (20.12.6).

   > -?- The shared timed mutex types shall meet the `TimedMutexType` requirements (32.6.4.3 [thread.timedmutex.requirements]) and the `SharedMutexType` requirements (32.6.4.4 [thread.sharedmutex.requirements]).

10. Change 32.6.4.5.2 [thread.sharedtimedmutex.class] as indicated:

    > -2- The class `shared_timed_mutex` shall satisfy all of the `SharedTimedMutexType` requirements for shared timed mutexes (32.6.4.5 [thread.sharedtimedmutex.requirements]). It shall be a standard-layout class (Clause 9).

11. Change 32.6.5.5 [thread.lock.shared] as indicated:

    > -1- […] The supplied `Mutex` type shall meet the shared mutex `SharedMutexType` requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]32.6.4.4 [thread.sharedmutex.requirements]).

    > -2- [Note: `shared_lock<Mutex>` meets the `TimedLockable` requirements (30.2.5.4). — end note]

12. Change 32.6.5.5.2 [thread.lock.shared.cons] as indicated:

    >     template <class Clock, class Duration>
    >       shared_lock(mutex_type& m,
    >                   const chrono::time_point<Clock, Duration>& abs_time);

    > > -14- Requires: The supplied `Mutex` type shall meet the `SharedTimedMutexType` requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]). The calling thread does not own the mutex for any ownership mode.

    > > -15- Effects: Constructs an object of type `shared_lock` and calls `m.try_lock_shared_until(abs_time)`.

    > > […]

    >     template <class Rep, class Period>
    >       shared_lock(mutex_type& m,
    >                   const chrono::duration<Rep, Period>& rel_time);

    > > -17- Requires: The supplied `Mutex` type shall meet the `SharedTimedMutexType` requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]). The calling thread does not own the mutex for any ownership mode.

    > > -18- Effects: Constructs an object of type `shared_lock` and calls `m.try_lock_shared_for(rel_time)`.

    > > […]

13. Change 32.6.5.5.3 [thread.lock.shared.locking] as indicated:

    >     template <class Clock, class Duration>
    >       bool
    >       try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

    > > -?- Requires: The supplied `Mutex` type shall meet the `SharedTimedMutexType` requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]).

    > > -8- Effects: `pm->try_lock_shared_until(abs_time)`.

    > > […]

    >     template <class Rep, class Period>
    >       bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);

    > > -?- Requires: The supplied `Mutex` type shall meet the `SharedTimedMutexType` requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]).

    > > -12- Effects: `pm->try_lock_shared_for(rel_time)`.

    > > […]

### 2364(i). `deque` and `vector` `pop_back` don't specify iterator invalidation requirements

Section: 23.3.5.4 [deque.modifiers], 23.3.13.5 [vector.modifiers] Status: C++17 Submitter: Deskin Miller Opened: 2014-02-17 Last modified: 2017-07-30

Priority: 0

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Discussion:

I think it's obvious that `vector::pop_back` invalidates the path-the-end iterator, but I cannot find language that says so to my satisfaction in the Standard. N3797 23.2.4 [sequence.reqmts] Table 101 lists `a.pop_back()` semantics as "Destroys the last element", but nowhere do I see this required to invalidate the end iterator (or iterators previously referring to the last element). [container.reqmts.general]/11 states "Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container." 23.3.13.5 [vector.modifiers]/3 says that each flavor of `vector::erase` "Invalidates iterators and references at or after the point of the erase", but `pop_back` isn't discussed, and it wasn't specified in terms of `erase`.

Similarly for `std::deque`, 23.2.4 [sequence.reqmts] Table 101 and [container.reqmts.general]/11 both apply. Yet 23.3.5.4 [deque.modifiers] likewise doesn't discuss `pop_back` nor `pop_front`. Furthermore paragraph 4 fails to specify the iterator-invalidation guarantees when erasing the first element but not the last.

Both `std::vector` and `std::deque` are in contrast to `std::list`, which says in 23.3.11.4 [list.modifiers]/3 regarding `pop_back` (as well as all forms of `erase`, `pop_front`, and `clear`) "Effects: Invalidates only the iterators and references to the erased elements."

[2014-06-16 Jonathan comments and improves wording]

I believe this reflects our preferred form discussed earlier, specifically putting the signatures with the `erase` signatures, so that the full specification of `erase()` applies to the `pop_xxx()` functions. This covers the case for `deque` where `pop_front()` erases the only element (which is both the first and last element).

Open question: the "erase" wording talks about "An erase operation" — are `pop_front` and `pop_back` clearly covered by "erase operations"? I believe so, as 23.3.5.1 [deque.overview]/1 and other places talk about "insert and erase operations" which covers `push`/`pop` functions too. I've added a note which could be used to clarify that if desired. Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 23.3.5.4 [deque.modifiers] as indicated:

>    >     iterator erase(const_iterator position);
>    >     iterator erase(const_iterator first, const_iterator last);

>    > > -4- Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements. An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements. An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque.

>    > > -5- […]

>    > > -6- […]

>    >     void pop_front();
>    >     void pop_back();

>    > > -?- Effects: `pop_front` invalidates iterators and references to the first element of the `deque`. `pop_back` invalidates the past-the-end iterator, and all iterators and references to the last element of the `deque`.

> 2. Change 23.3.13.5 [vector.modifiers] as indicated:

>    > > -5- […]

>    >     void pop_back();

>    > > -?- Effects: Invalidates the past-the-end iterator, and iterators and references to the last element of the vector.

[2014-06-21 Rapperswil]

Tony van Eerd: Would be good to define "an erase operation is ..." somewhere.

AM: The containers clause is known to be suboptimal in many ways.

Looks good

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 23.3.5.4 [deque.modifiers] as indicated:

   >     iterator erase(const_iterator position);
   >     iterator erase(const_iterator first, const_iterator last);
   >     void pop_front();
   >     void pop_back();

   > > -4- Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements. An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements. An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque. [Note: `pop_front` and `pop_back` are erase operations — end note]

2. Change 23.3.13.5 [vector.modifiers] as indicated:

   >     iterator erase(const_iterator position);
   >     iterator erase(const_iterator first, const_iterator last);
   >     void pop_back();

   > > -3- Effects: Invalidates iterators and references at or after the point of the erase.

### 2365(i). Missing `noexcept` in `shared_ptr::shared_ptr(nullptr_t)`

Section: 20.3.2.2 [util.smartptr.shared] Status: C++17 Submitter: Cassio Neri Opened: 2014-02-13 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

The declaration and definition of `shared_ptr::shared_ptr(nullptr_t)`, given in 20.3.2.2 [util.smartptr.shared], is

>     constexpr shared_ptr(nullptr_t) : shared_ptr() { }

The intention seems clear: this constructor should have the same semantics of the default constructor. However, contrarily to the default constructor, this one is not `noexcept`. In contrast, `unique_ptr::unique_ptr(nullptr_t)` is `noexcept`, as per 20.3.1.3 [unique.ptr.single]:

>     constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }

Both libstdc++ and libc++ have added `noexcept` to `shared_ptr::shared_ptr(nullptr_t)`. Microsoft's STL has not.

[2014-03-26 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change class template `shared_ptr` synopsis, 20.3.2.2 [util.smartptr.shared], as indicated:

   >     constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }

### 2367(i). `pair` and `tuple` are not correctly implemented for `is_constructible` with no args

Section: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: Howard Hinnant Opened: 2014-02-19 Last modified: 2017-07-30

Priority: 3

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Discussion:

Consider:

>     struct X
>     {
>       X() = delete;
>     };

>     int main()
>     {
>       typedef std::pair<int, X> P;
>       static_assert(!std::is_constructible<P>::value, "");
>       static_assert(!std::is_default_constructible<P>::value, "");
>       typedef std::tuple<int, X> T;
>       static_assert(!std::is_constructible<T>::value, "");
>       static_assert(!std::is_default_constructible<T>::value, "");
>     }

For me these `static_asserts` fail. And worse than that, even asking the question fails (as opposed to gets the wrong answer):

>     assert(!std::is_constructible<P>::value);

In file included from test.cpp:2:

>     error:
>           call to deleted constructor of 'X'
>        pair() : first(), second() {}
>                          ^
>     note: function has been explicitly marked deleted here
>         X() = delete;
>         ^
>     1 error generated.

This can be solved by specializing `is_constructible` on `pair` and `tuple` for zero Args:

>     template <class T, class U>
>     struct is_constructible<pair<T, U>>
>       : integral_constant<bool, is_default_constructible<T>::value &&
>                                 is_default_constructible<U>::value>
>     {};

>     template <class ...T>
>     struct is_constructible<tuple<T...>>
>       : integral_constant<bool,
>                           __all<is_default_constructible<T>::value...>::value>
>     {};

Now everything just works.

[2014-05-14, Daniel comments]

The proposed resolution is incomplete, because it wouldn't work for cv-qualified objects of `pair` or for references of them during reference-initialization.

I would like to point out that the approach suggested in N3739 can be easily extended to solve the problem without need to muddle with specializing `is_constructible`:

>     template<class U1 = T1, class U2 = T2,
>       typename enable_if<
>         is_default_constructible<U1>::value && is_default_constructible<U2>::value
>       , bool>::type = false
>     >
>     constexpr pair();

The new wording proposal represents an alternative wording change that I would strongly prefer.

Previous resolution from Howard [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Add to 22.3.3 [pairs.spec]:

>    >     template <class T, class U>
>    >     struct is_constructible<pair<T, U>>
>    >       : integral_constant<bool, is_default_constructible<T>::value &&
>    >                                 is_default_constructible<U>::value>
>    >     {};

> 2. Add to 22.4.12 [tuple.special]:

>    >     template <class ...T>
>    >     struct is_constructible<tuple<T...>>
>    >       : integral_constant<bool, see below>
>    >     {};

>    > > -?- The second argument to `integral_constant` shall be true if for each `T`, `is_default_constructible<T>::value` is true.

[2015-05, Lenexa]

STL: I object to this resolution due to British spelling of behavior
JW: we already have other places of this spelling
VV: the easy resolution is to remove the notes
MC: if that's all we want to change: put it in and make the editorial change of removing the note
VV: the other paper doesn't make any of these changes so it would be consistent
JW: this make me want even more the features of having constructors doing the Right Thing - I haven't written up the request to do something like that
VV: so it would be an aggregate reflecting the properties of the constituting types
JW: I should write that up
MC: any objection to move to ready? in favor: 16, opposed: 0, abstain: 1

Proposed resolution:

This wording is relative to N3936.

1. Change 22.3.2 [pairs.pair] around p3 as indicated:

   >     constexpr pair();

   > > -3- Requires: `is_default_constructible<first_type>::value` is true and `is_default_constructible<second_type>::value` is true.

   > > -4- Effects: Value-initializes first and second.

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_default_constructible<first_type>::value` is true and `is_default_constructible<second_type>::value` is true. [Note: This behaviour can be implemented by a constructor template with default template arguments — end note].

2. Change 22.4.4.2 [tuple.cnstr] around p4 as indicated:

   >     constexpr tuple();

   > > -4- Requires: `is_default_constructible<Ti>::value` is true for all i.

   > > -5- Effects: Value initializes each element.

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_default_constructible<Ti>::value` is true for all i. [Note: This behaviour can be implemented by a constructor template with default template arguments — end note].

### 2368(i). Replacing global `operator new`

Section: 17.6.3 [new.delete] Status: Resolved Submitter: Stephen Clamage Opened: 2014-02-20 Last modified: 2020-09-06

Priority: 2

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Discussion:

Section 17.6.3 [new.delete] and subsections shows:

>     void* operator new(std::size_t size);
>     void* operator new[](std::size_t size);

That is, without exception-specifications. (Recall that C++03 specified these functions with `throw(std::bad_alloc)`.)

Section 16.4.6.14 [res.on.exception.handling] the end of paragraph 4 says:

> Any other functions defined in the C++ standard library that do not have an exception-specification may throw implementation-defined exceptions unless otherwise specified. An implementation may strengthen this implicit exception-specification by adding an explicit one.

For example, an implementation could provide C++03-compatible declarations of `operator new`.

Programmers are allowed to replace these `operator new` functions. But how can you write the definition of these functions when the exception specification can vary among implementations? For example, the declarations

>     void* operator new(std::size_t size) throw(std::bad_alloc);
>     void* operator new(std::size_t size);

are not compatible.

From what I have been able to determine, gcc has a hack for the special case of `operator new` to ignore the differences in (at least) the two cases I show above. But can users expect all compilers to quietly ignore the incompatibility?

The blanket permission to add any explicit exception specification could cause a problem for any user-overridable function. Different implementations could provide incompatible specifications, making portable code impossible to write.

[2016-03, Jacksonville]

STL: Core changes to remove dynamic exception specs would make this moot
Room: This is on track to be resolved by P0003, or may be moot.

[2016-07, Toronto Thursday night issues processing]

Resolved by P0003.

Proposed resolution:

### 2369(i). `constexpr max(initializer_list)` vs `max_element`

Section: 26.8.9 [alg.min.max] Status: C++17 Submitter: Marc Glisse Opened: 2014-02-21 Last modified: 2017-07-30

Priority: 3

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Discussion:

As part of the resolution for LWG issue 2350(i), `max(initializer_list)` was marked as `constexpr`. Looking at two implementations of this function (libstdc++ and libc++), both implement it in terms of `max_element`, which is not marked as `constexpr`. This is inconsistent and forces some small amount of code duplication in the implementation. Unless we remove `constexpr` from this overload of `max`, I believe we should add `constexpr` to `max_element`.

[2015-02 Cologne]

AM: Can we implement this with the C++14 `constexpr` rules? JM: Yes. AM: Ready? [Yes]

Accepted.

Proposed resolution:

This wording is relative to N3936.

1. In 26.1 [algorithms.general], header `<algorithm>` synopsis, and 26.8.9 [alg.min.max], change as indicated (add `constexpr` to every signature from the first `min_element` to the second `minmax_element`)::

   >     template<class ForwardIterator>
   >     constexpr ForwardIterator min_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     constexpr ForwardIterator min_element(ForwardIterator first, ForwardIterator last,
   >                                           Compare comp);
   >     […]
   >     template<class ForwardIterator>
   >     constexpr ForwardIterator max_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     constexpr ForwardIterator max_element(ForwardIterator first, ForwardIterator last,
   >                                           Compare comp);
   >     […]
   >     template<class ForwardIterator>
   >     constexpr pair<ForwardIterator, ForwardIterator>
   >     minmax_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     constexpr pair<ForwardIterator, ForwardIterator>
   >     minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);

### 2370(i). Operations involving type-erased allocators should not be `noexcept` in `std::function`

Section: 22.10.17.3 [func.wrap.func] Status: Resolved Submitter: Pablo Halpern Opened: 2014-02-27 Last modified: 2020-09-06

Priority: 3

View all other issues in [func.wrap.func].

View all issues with Resolved status.

Discussion:

The following constructors in 22.10.17.3 [func.wrap.func] are declared `noexcept`, even though it is not possible for an implementation to guarantee that they will not throw:

>     template <class A> function(allocator_arg_t, const A&) noexcept;
>     template <class A> function(allocator_arg_t, const A&, nullptr_t) noexcept;

In addition, the following functions are guaranteed not to throw if the target is a function pointer or a `reference_wrapper`:

>     template <class A> function(allocator_arg_t, const A& a, const function& f);
>     template <class F, class A> function(allocator_arg_t, const A& a, F f);

In all of the above cases, the function object might need to allocate memory (an operation that can throw) in order to hold a copy of the type-erased allocator itself. The first two constructors produce an empty function object, but the allocator is still needed in case the object is later assigned to. In this case, we note that the propagation of allocators on assignment is underspecified for `std::function`. There are three possibilities:

1. The allocator is never copied on copy-assignment, moved on move-assignment, or swapped on swap.

2. The allocator is always copied on copy-assignment, moved on move-assignment, and swapped on swap.

3. Whether or not the allocator is copied, moved, or swapped is determined at run-time based on the `propagate_on_container_copy_assignment` and `propagate_on_container_move_assignment` traits of the allocators at construction of the source function, the target function, or both.

Although the third option seems to be the most consistent with existing wording in the containers section of the standard, it is problematic in a number of respects. To begin with, the propagation behavior is determined at run time based on a pair of type-erased allocators, instead of at compile time. Such run-time logic is not consistent with the rest of the standard and is hard to reason about. Additionally, there are two allocator types involved, rather than one. Any set of rules that attempts to rationally interpret the propagation traits of both allocators is likely to be arcane at best, and subtly wrong for some set of codes at worst.

The second option is a non-starter. Historically, and in the vast majority of existing code, an allocator does not change after an object is constructed. The second option, if adopted, would undermine the programmer's ability to construct, e.g., an array of function objects, all using the same allocator.

The first option is (in Pablo's opinion) the simplest and best. It is consistent with historical use of allocators, is easy to understand, and requires minimal wording. It is also consistent with the wording in N3916, which formalizes type-erased allocators.

For cross-referencing purposes: The resolution of this issue should be harmonized with any resolution to LWG 2062(i), which questions the `noexcept` specification on the following member functions of std::function:

>     template <class F> function& operator=(reference_wrapper<F>) noexcept;
>     void swap(function&) noexcept;

[2015-05 Lenexa]

MC: change to P3 and status to open.

STL: note that `noexcept` is an issue and large chunks of allocator should be destroyed.

[2015-12-16, Daniel comments]

See 2564(i) for a corresponding issue addressing library fundamentals v2.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 22.10.17.3 [func.wrap.func], class template `function` synopsis, as indicated:

>    >     template <class A> function(allocator_arg_t, const A&) noexcept;
>    >     template <class A> function(allocator_arg_t, const A&, nullptr_t) noexcept;

> 2. Change 22.10.17.3.2 [func.wrap.func.con] as indicated:

>    > -1- When any function constructor that takes a first argument of type `allocator_arg_t` is invoked, the second argument shall have a type that conforms to the requirements for `Allocator` (Table 17.6.3.5). A copy of the allocator argument is used to allocate memory, if necessary, for the internal data structures of the constructed function object. For the remaining constructors, an instance of `allocator<T>`, for some suitable type `T`, is used to allocate memory, if necessary, for the internal data structures of the constructed function object.

>    >     function() noexcept;
>    >     template <class A> function(allocator_arg_t, const A&) noexcept;

>    > > -2- Postconditions: `!*this`.

>    >     function(nullptr_t) noexcept;
>    >     template <class A> function(allocator_arg_t, const A&, nullptr_t) noexcept;

>    > > -3- Postconditions: `!*this`.

>    >     function(const function& f);
>    >     template <class A> function(allocator_arg_t, const A& a, const function& f);

>    > > -4- Postconditions: `!*this` if `!f`; otherwise, `*this` targets a copy of `f.target()`.

>    > > -5- Throws: shall not throw exceptions if `f`'s target is a callable object passed via `reference_wrapper` or a function pointer. Otherwise, may throw `bad_alloc` or any exception thrown by the copy constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

>    >     template <class A> function(allocator_arg_t, const A& a, const function& f);

>    > > -?- Postconditions: `!*this` if `!f`; otherwise, `*this` targets a copy of `f.target()`.

>    >     function(function&& f);
>    >     template <class A> function(allocator_arg_t, const A& a, function&& f);

>    > > -6- Effects: If `!f`, `*this` has no target; otherwise, move-constructs the target of `f` into the target of `*this`, leaving `f` in a valid state with an unspecified value. If an allocator is not specified, the constructed function will use the same allocator as `f`.

>    >     template<class F> function(F f);
>    >     template <class F, class A> function(allocator_arg_t, const A& a, F f);

>    > > -7- Requires: `F` shall be `CopyConstructible`.

>    > > -8- Remarks: These constructors shall not participate in overload resolution unless `f` is Callable (20.9.11.2) for argument types `ArgTypes...` and return type `R`.

>    > > -9- Postconditions: `!*this` if any of the following hold:

>    > > - `f` is a null function pointer value.

>    > > - `f` is a null member pointer value.

>    > > - `F` is an instance of the function class template, and `!f`

>    > > -10- Otherwise, `*this` targets a copy of `f` initialized with `std::move(f)`. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

>    > > -11- Throws: shall not throw exceptions when an allocator is not specified and `f` is a function pointer or a `reference_wrapper<T>` for some `T`. Otherwise, may throw `bad_alloc` or any exception thrown by `F`'s copy or move constructor or by `A`'s allocate function.

[2016-08 Chicago]

Tues PM: Resolved by P0302R1

Proposed resolution:

Resolved by acceptance of P0302R1.

### 2371(i). [fund.ts] No template aliases defined for new type traits

Section: 3.3.1 [fund.ts::meta.type.synop] Status: TS Submitter: Joe Gottman Opened: 2014-03-07 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts

The library fundamentals specification defines two new type trait template classes: `invocation_type` and `raw_invocation_type`. But it does not define the corresponding template aliases. Note that both of these classes define a member typedef `type` and no other public members, so according to the argument in N3887 the template aliases should be defined.

[2013-06-21 Rapperswil]

Accept for Library Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N3908.

1. Add the following to section 3.3.1[meta.type.synop] of the Library Fundamentals specification as indicated:

   >     namespace std {
   >     namespace experimental {
   >     inline namespace fundamentals_v1 {
   >       […]
   >       // 3.3.2, Other type transformations
   >       template <class> class invocation_type; // not defined
   >       template <class F, class... ArgTypes> class invocation_type<F(ArgTypes...)>;
   >       template <class> class raw_invocation_type; // not defined
   >       template <class F, class... ArgTypes> class raw_invocation_type<F(ArgTypes...)>;

   >       template <class T>
   >         using invocation_type_t = typename invocation_type<T>::type;
   >       template <class T>
   >         using raw_invocation_type_t = typename raw_invocation_type<T>::type;

   >     } // namespace fundamentals_v1
   >     } // namespace experimental
   >     } // namespace std

### 2374(i). [fund.ts] Remarks for `optional::to_value` are too restrictive

Section: 5.3.5 [fund.ts::optional.object.observe] Status: TS Submitter: Jonathan Wakely Opened: 2014-03-25 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts

In Bristol I think I claimed that the remarks for `optional::to_value()` were unimplementable and the function could only be `constexpr` if both constructors that could be called were `constexpr`, but I was wrong. The remarks should be reverted to the original pre-n3793 form.

[2013-06-21 Rapperswil]

Accept for Library Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N3908.

1. Change [optional.object.observe] p23 of the Library Fundamentals specification as indicated:

   >     template <class U> constexpr T value_or(U&& v) const &;

   > > […]

   > > -23- Remarks: If both constructors of `T` which could be selected are `constexpr` constructorsthe selected constructor of `T` is a `constexpr` constructor, this function shall be a `constexpr` function.

### 2375(i). Is [iterator.requirements.general]/9 too broadly applied?

Section: 24.3.1 [iterator.requirements.general] Status: Resolved Submitter: Marshall Clow Opened: 2014-03-25 Last modified: 2021-06-23

Priority: 3

View all other issues in [iterator.requirements.general].

View all issues with Resolved status.

Discussion:

24.3.1 [iterator.requirements.general] p9 says:

> Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.

But the resolution of LWG issue 2360(i) specifically advocates returning `*--temp;` where `temp` is a local variable.

And 24.3.5.5 [forward.iterators] p6 says:

> If `a` and `b` are both dereferenceable, then `a == b` if and only if `*a` and `*b` are bound to the same object.

which disallows "stashing" iterators (i.e, iterators that refer to data inside themselves).

So, I suspect that the restriction in p9 should only apply to input iterators, and can probably be moved into 24.3.5.3 [input.iterators] instead of 24.3.1 [iterator.requirements.general].

[2014-05-22, Daniel comments]

Given that forward iterators (and beyond) are refinements of input iterator, moving this constraint to input iterators won't help much because it would still hold for all refined forms.

[2021-06-23 Resolved by adoption of P0896R4 in San Diego. Status changed: New → Resolved.]

Proposed resolution:

### 2376(i). `bad_weak_ptr::what()` overspecified

Section: 20.3.2.1 [util.smartptr.weak.bad] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-03-27 Last modified: 2017-07-30

Priority: Not Prioritized

View all issues with C++17 status.

Discussion:

[util.smartptr.weakptr] p2 requires `bad_weak_ptr` to return precisely the string `"bad_weak_ptr"`.

There was general agreement on the reflector and at the Issaquah meeting that this is over-constrained and implementation should be free to return something more descriptive if desired.

The proposed resolution makes `bad_weak_ptr` consistent with other exception types such as `bad_alloc` and `bad_cast`.

If accepted, the P/R for issue 2233(i), which currently uses similar wording to `bad_weak_ptr`, could be updated appropriately.

[2014-03-27 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Edit [util.smartptr.weakptr]:

   >     bad_weak_ptr() noexcept;

   > > -2- Postconditions: `what()` returns `"bad_weak_ptr"`an implementation-defined NTBS.

### 2377(i). `std::align` requirements overly strict

Section: 20.2.5 [ptr.align] Status: C++17 Submitter: Peter Dimov Opened: 2014-03-30 Last modified: 2017-07-30

Priority: 0

View all other issues in [ptr.align].

View all issues with C++17 status.

Discussion:

`std::align` requires that its `alignment` argument shall be "a fundamental alignment value or an extended alignment value supported by the implementation in this context".

This requirement is overly strict. There are use cases that require a buffer aligned at values that are not tied to the C++ implementation, such as page size, cache line size, sector size. These come from the hardware or the OS and are generally not known until run time. The implementation of `std::align` does not depend on the requirement that `alignment` be a fundamental or an extended alignment value; any power of two would be handled the same way.

In addition, it is not possible for the user to even determine whether a value is "a fundamental alignment value or an extended alignment value supported by the implementation in this context". One would expect values coming from `alignof` to be fine, but I'm not sure whether even that is guaranteed in the presence of `alignas`.

Therefore, I propose that

> Requires:

> - `alignment` shall be a fundamental alignment value or an extended alignment value supported by the implementation in this context

be changed to

> Requires:

> - `alignment` shall be a power of two

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Edit 20.2.5 [ptr.align] p2 as indicated:

   >     void* align(std::size_t alignment, std::size_t size,
   >       void*& ptr, std::size_t& space);

   > > -1- […]

   > > -2- Requires:

   > > - `alignment` shall be a fundamental alignment value or an extended alignment value supported by the implementation in this contextpower of two

### 2378(i). Behaviour of standard exception types

Section: 17.6.4.1 [bad.alloc], 17.6.4.2 [new.badlength], 17.7.4 [bad.cast], 17.7.5 [bad.typeid], 17.9.4 [bad.exception] Status: C++17 Submitter: Andy Sawyer Opened: 2014-03-31 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

I think we have an issue with the specification of some of the standard exception types. In particular, several of them have default constructors with remarks to the effect that "The result of calling `what()` on the newly constructed object is implementation-defined". (In some cases this is contradictory to a further specification of `what()`, which is specified to return an implementation-defined NTBS.)

Previous resolution from Andy [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Edit 17.6.4.1 [bad.alloc] p3 as indicated:

>    >     bad_alloc() noexcept;

>    > > […]

>    > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined `what()` returns an implementation-defined NTBS.

> 2. Edit 17.6.4.2 [new.badlength] p3 as indicated: [Drafting note: Added the Postcondition, since we don't say anything else about `bad_array_new_length::what()` — end of note]

>    >     bad_array_new_length() noexcept;

>    > > […]

>    > > -3- RemarksPostcondition: The result of calling `what()` on the newly constructed object is implementation-defined `what()` returns an implementation-defined NTBS.

> 3. Edit 17.7.4 [bad.cast] p3 as indicated:

>    >     bad_cast() noexcept;

>    > > […]

>    > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

> 4. Edit 17.7.5 [bad.typeid] p3 as indicated:

>    >     bad_typeid() noexcept;

>    > > […]

>    > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

> 5. Edit 17.9.4 [bad.exception] p3 as indicated:

>    >     bad_exception() noexcept;

>    > > […]

>    > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

[2014-06-17, Rapperswil]

Jonathan provides alternative wording.

[2015-02, Cologne]

NJ: I don't know why we need the explict statement about `what()` here, since `bad_array_new_length` already derives.
AM: yes.
NJ: Then "what()" is missing from the synopsis.
AM: Yes, that's an error and it needs to be added.

Conclusion: Update the wording to add the missing entry in the synopsis.

AM: The issue needs another update; we need to add missing "Remarks". DK updates the paper.
AM: Any objections to "tentatively ready"? No objections.

Proposed resolution:

This wording is relative to N4296.

1. Edit 17.6.4.1 [bad.alloc] p3 as indicated:

   >     bad_alloc() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined.

2. Edit 17.6.4.1 [bad.alloc] p5 as indicated:

   >     virtual const char* what() const noexcept;

   > > -5- Returns: An implementation-defined NTBS.

   > > -?- Remarks: The message may be a null-terminated multibyte string (17.5.2.1.4.2), suitable for conversion and display as a `wstring` (21.3, 22.4.1.4).

3. Edit class `bad_array_new_length` synopsis 17.6.4.2 [new.badlength] as indicated:

   >     namespace std {
   >       class bad_array_new_length : public bad_alloc {
   >       public:
   >         bad_array_new_length() noexcept;
   >         virtual const char* what() const noexcept;
   >       };
   >     }

4. Edit 17.6.4.2 [new.badlength] as indicated:

   >     bad_array_new_length() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined.

   >     virtual const char* what() const noexcept;

   > > -?- Returns: An implementation-defined NTBS.

   > > -?- Remarks: The message may be a null-terminated multibyte string (17.5.2.1.4.2), suitable for conversion and display as a `wstring` (21.3, 22.4.1.4).

5. Edit 17.7.4 [bad.cast] p3 as indicated:

   >     bad_cast() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

6. Edit 17.7.5 [bad.typeid] p3 as indicated:

   >     bad_typeid() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

7. Edit 17.9.4 [bad.exception] p3 as indicated:

   >     bad_exception() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

### 2380(i). May `<cstdlib>` provide `long ::abs(long)` and `long long ::abs(long long)`?

Section: 16.4.2.3 [headers] Status: C++17 Submitter: Richard Smith Opened: 2014-03-31 Last modified: 2017-07-30

Priority: 2

View all other issues in [headers].

View all issues with C++17 status.

Discussion:

[depr.c.headers] p3 says:

> [Example: The header `<cstdlib>` assuredly provides its declarations and definitions within the namespace `std`. It may also provide these names within the global namespace. The header `<stdlib.h>` assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespace `std`. — end example]

This suggests that `<cstdlib>` may provide `::abs(long)` and `::abs(long long)`. But this seems like it might contradict the normative wording of 16.4.2.3 [headers] p4:

> Except as noted in Clauses 18 through 30 and Annex D, the contents of each header `cname` shall be the same as that of the corresponding header `name.h`, as specified in the C standard library (1.2) or the C Unicode TR, as appropriate, as if by inclusion. In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope (3.3.6) of the namespace `std`. It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespace `std` by explicit using-declarations (7.3.3).

Note that this allows `<cstdlib>` to provide `::abs(int)`, but does not obviously allow `::abs(long)` nor `::abs(long long)`, since they are not part of the header `stdlib.h` as specified in the C standard library.

29.7 [c.math] p7 adds signatures `std::abs(long)` and `std::abs(long long)`, but not in a way that seems to allow `::abs(long)` and `::abs(long long)` to be provided.

I think the right approach here would be to allow `<cstdlib>` to either provide no `::abs` declaration, or to provide all three declarations from namespace `std`, but it should not be permitted to provide only `int abs(int)`. Suggestion:

Change in 16.4.2.3 [headers] p4:

> […]. It is unspecified whether these names (including any overloads added in Clauses 18 through 30 and Annex D) are first declared within the global namespace scope and are then injected into namespace `std` by explicit using-declarations (7.3.3).

[2015-05, Lenexa]

MC: do we need to defer this?
PJP: just need to get my mind around it, already playing dirty games here, my reaction is just do it as it will help C++
STL: this is safe
TP: would be surprising if using abs didn't bring in all of the overloads
MC: that's Richard's argument
MC: move to ready

Proposed resolution:

This wording is relative to N3936.

1. Modify 16.4.2.3 [headers] p4 as indicated:

   > Except as noted in Clauses 18 through 30 and Annex D, the contents of each header `cname` shall be the same as that of the corresponding header `name.h`, as specified in the C standard library (1.2) or the C Unicode TR, as appropriate, as if by inclusion. In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope (3.3.6) of the namespace `std`. It is unspecified whether these names (including any overloads added in Clauses 18 through 30 and Annex D) are first declared within the global namespace scope and are then injected into namespace `std` by explicit using-declarations (7.3.3).

### 2381(i). Inconsistency in parsing floating point numbers

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++23 Submitter: Marshall Clow Opened: 2014-04-30 Last modified: 2023-11-22

Priority: 2

View other active issues in [facet.num.get.virtuals].

View all other issues in [facet.num.get.virtuals].

View all issues with C++23 status.

Discussion:

In 28.3.4.3.2.3 [facet.num.get.virtuals] we have:

> Stage 3: The sequence of chars accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header `<cstdlib>`:

> - For a signed integer value, the function `strtoll`.

> - For an unsigned integer value, the function `strtoull`.

> - For a floating-point value, the function `strtold`.

This implies that for many cases, this routine should return true:

>     bool is_same(const char* p)
>     {
>       std::string str{p};
>       double val1 = std::strtod(str.c_str(), nullptr);
>       std::stringstream ss(str);
>       double val2;
>       ss >> val2;
>       return std::isinf(val1) == std::isinf(val2) &&                 // either they're both infinity
>              std::isnan(val1) == std::isnan(val2) &&                 // or they're both NaN
>              (std::isinf(val1) || std::isnan(val1) || val1 == val2); // or they're equal
>     }

and this is indeed true, for many strings:

>     assert(is_same("0"));
>     assert(is_same("1.0"));
>     assert(is_same("-1.0"));
>     assert(is_same("100.123"));
>     assert(is_same("1234.456e89"));

but not for others

>     assert(is_same("0xABp-4")); // hex float
>     assert(is_same("inf"));
>     assert(is_same("+inf"));
>     assert(is_same("-inf"));
>     assert(is_same("nan"));
>     assert(is_same("+nan"));
>     assert(is_same("-nan"));

>     assert(is_same("infinity"));
>     assert(is_same("+infinity"));
>     assert(is_same("-infinity"));

These are all strings that are correctly parsed by `std::strtod`, but not by the stream extraction operators. They contain characters that are deemed invalid in stage 2 of parsing.

If we're going to say that we're converting by the rules of `strtold`, then we should accept all the things that `strtold` accepts.

[2016-04, Issues Telecon]

People are much more interested in round-tripping hex floats than handling `inf` and `nan`. Priority changed to P2.

Marshall says he'll try to write some wording, noting that this is a very closely specified part of the standard, and has remained unchanged for a long time. Also, there will need to be a sample implementation.

[2016-08, Chicago]

Zhihao provides wording

The `src` array in Stage 2 does narrowing only. The actual input validation is delegated to `strtold` (independent from the parsing in Stage 3 which is again being delegated to `strtold`) by saying:

[...] If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by Stage 1.

So a conforming C++11 `num_get` is supposed to magically accept an hexfloat without an exponent

0x3.AB

because we refers to C99, and the fix to this issue should be just expanding the `src` array.

Support for Infs and NaNs are not proposed because of the complexity of nan(n-chars).

[2016-08, Chicago]

Tues PM: Move to Open

[2016-09-08, Zhihao Yuan comments and updates proposed wording]

Examples added.

[2018-08-23 Batavia Issues processing]

Needs an Annex C entry. Tim to write Annex C.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:

>    > `static const char src[] = "0123456789abcdefpxABCDEFPX+-";`

> 2. Append the following examples to 28.3.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:

>    > [Example:

>    > > Given an input sequence of `"0x1a.bp+07p"`,

>    > > - if Stage 1 returns `%d`, `"0"` is accumulated;

>    > > - if Stage 1 returns `%i`, `"0x1a"` are accumulated;

>    > > - if Stage 1 returns `%g`, `"0x1a.bp+07"` are accumulated.

>    > > In all cases, leaving the rest in the input.

>    > — end example]

[2021-05-18 Tim updates wording]

Based on the git history, libc++ appears to have always included `p` and `P` in `src`.

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:

   > — Stage 2:

   > If `in == end` then stage 2 terminates. Otherwise a `charT` is taken from `in` and local variables are initialized as if by

   > >     char_type ct = *in;
   > >     char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms];
   > >     if (ct == use_facet<numpunct<charT>>(loc).decimal_point())
   > >     c = '.';
   > >     bool discard =
   > >       ct == use_facet<numpunct<charT>>(loc).thousands_sep()
   > >       && use_facet<numpunct<charT>>(loc).grouping().length() != 0;

   > where the values `src` and `atoms` are defined as if by:

   > >     static const char src[] = "0123456789abcdefpxABCDEFPX+-";
   > >     char_type atoms[sizeof(src)];
   > >     use_facet<ctype<charT>>(loc).widen(src, src + sizeof(src), atoms);

   > for this value of `loc`.

   > If `discard` is true, then if `'.'` has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if `'.'` has already been accumulated, the character is discarded and Stage 2 terminates. If it is not discarded, then a check is made to determine if `c` is allowed as the next character of an input field of the conversion specifier returned by Stage 1. If so, it is accumulated.

   > If the character is either discarded or accumulated then `in` is advanced by `++in` and processing returns to the beginning of stage 2.

   > [Example:

   > > Given an input sequence of `"0x1a.bp+07p"`,

   > > - if the conversion specifier returned by Stage 1 is `%d`, `"0"` is accumulated;

   > > - if the conversion specifier returned by Stage 1 is `%i`, `"0x1a"` are accumulated;

   > > - if the conversion specifier returned by Stage 1 is `%g`, `"0x1a.bp+07"` are accumulated.

   > > In all cases, the remainder is left in the input.

   > — end example]

2. Add the following new subclause to C.6 [diff.cpp03]:

   > C.4.? [locale]: localization library [diff.cpp03.locale]

   > Affected subclause: 28.3.4.3.2.3 [facet.num.get.virtuals]
   > Change: The `num_get` facet recognizes hexadecimal floating point values.
   > Rationale: Required by new feature.
   > Effect on original feature: Valid C++2003 code may have different behavior in this revision of C++.

### 2382(i). Unclear order of container update versus object destruction on removing an object

Section: 16.4.6.9 [reentrancy] Status: Resolved Submitter: Peter Kasting Opened: 2014-05-06 Last modified: 2026-04-13

Priority: 2

View all other issues in [reentrancy].

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Discussion:

The standard does not seem to discuss reentrant access to a container during removal of an element, leaving it unclear whether a removed object is destroyed before or after it is removed from the container. For example, the behavior of the following code seems to be unspecified:

>     #include <iostream>
>     #include <map>
>     #include <memory>

>     struct T;
>     typedef std::map<int, std::shared_ptr<T>> TMap;

>     struct T {
>       T(TMap* t_map, int index) : t_map(t_map), index(index) {}
>       ~T() {
>         std::cout << "Object " << index << " is ";
>         if (t_map->count(index))
>           std::cout << "destroyed before being removed from the map" << std::endl;
>         else
>           std::cout << "removed from the map before being destroyed" << std::endl;
>       }

>       static void AddToMap(TMap* map, int index) {
>         (*map)[index] = std::make_shared<T>(map, index);
>       }

>       TMap* t_map;
>       int index;
>     };

>     int main()
>     {
>       TMap t_map;
>       T::AddToMap(&t_map, 0);
>       T::AddToMap(&t_map, 1);
>       t_map.erase(1);
>       t_map.erase(0);
>     }

The output of this program in Visual Studio 2013 is:

>     Object 1 is removed from the map before being destroyed
>     Object 0 is destroyed before being removed from the map

The core issue here is whether an object removed from a container should be destroyed before or after it is removed from the container. The current standard seems to be silent on this issue. The above output demonstrates that the behavior is actually inconsistent. (It's difficult to fully describe Visual Studio's behavior; for example, changing `main()` in the above example to the following:)

>     int main()
>     {
>       TMap t_map;
>       T::AddToMap(&t_map, 0);
>       T::AddToMap(&t_map, 1);
>       T::AddToMap(&t_map, 2);
>       T::AddToMap(&t_map, 3);
>       t_map.erase(3);
>       t_map.clear();
>     }

(...gives this output:)

>     Object 3 is removed from the map before being destroyed
>     Object 2 is destroyed before being removed from the map
>     Object 1 is destroyed before being removed from the map
>     Object 0 is removed from the map before being destroyed

In my opinion, the standard should explicitly describe when objects are destroyed as part of removal from a container. To me, it makes the most sense to say that objects should be removed from the container before they are destroyed.

[2014-05-07, Jeffrey Yasskin comments]

I think there are two main points here beyond this writeup:

1. We can't make recursive use of a standard library container valid in all cases.

2. If recursion through especially `erase()` is undefined behavior, that's pretty scary for existing large applications with code in destructors. Of course, "scary" doesn't mean we have to define the behavior.

I'll add a third: The language in 16.4.6.9 [reentrancy] nearly makes this undefined behavior already. I think any fix is probably going to live there, and extend the current "implementation-defined" on recursive reentrancy for individual functions to recursive reentrancy on class instances. I'm not sure exactly how to word that.

[2014-06 Rapperswil]

STL: We need more wording about how container methods can be reentrency.

Jeffrey: The title for this issue is confusing, what we really want is "reentrancy for objects".

Alisdair: Should we then close 2382 as NAD with a link to the new issue?

[2026-04-09; This was resolved by LWG 2414(i).]

Proposed resolution:

### 2384(i). Allocator's `deallocate` function needs better specification

Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2014-05-19 Last modified: 2017-07-30

Priority: 3

View other active issues in [allocator.requirements].

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Discussion:

According to Table 28, 16.4.4.6 [allocator.requirements], an `Allocator`'s `deallocate` function is specified as follows:

> All `n` `T` objects in the area pointed to by `p` shall be destroyed prior to this call. `n` shall match the value passed to allocate to obtain this memory. Does not throw exceptions. [Note: `p` shall not be singular. — end note]

This wording is confusing in regard to the following points:

1. This specification does not make clear that the result of an `allocate` call can only be returned once to the `deallocate` function. This is much clearer expressed for `operator delete` (17.6.3.2 [new.delete.single] p12, emphasis mine):

   > Requires: `ptr` shall be a null pointer or its value shall be a value returned by an earlier call to the (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t,const std::nothrow_t&)` which has not been invalidated by an intervening call to `operator delete(void*)`.

2. The intended meaning of that wording was to say that `deallocate` shall accept every result value that had been returned by a corresponding call to `allocate`, this includes also a possible result of a null pointer value, which is possible ("[Note: If `n == 0`, the return value is unspecified. — end note]"). Unfortunately the `deallocate` function uses a non-normative note ("`p` shall not be singular.") which refers to the fuzzy term `singular`, that is one of the most unclear and misunderstood terms of the library, as pointed out in 1213(i). The occurrence of this term has lead to the possible understanding, that this function would never allow null pointer values. Albeit for allocators the intention had not been to require the support in general that a null pointer value can be provided to `deallocate` (as it is allowed for `std::free` and `operator delete`), the mental model was that every returned value of `allocate` shall be an acceptable argument type of the corresponding `deallocate` function.

This issue is not intending to enforce a specific meaning of singular iterator values, but the assertion is that this note does more harm than good. In addition to wording from `operator delete` there is no longer any need to obfuscate the normative wording.

[2014-05-24 Alisdair comments]

Now that I am reading it very precisely, there is another mis-stated assumption as a precondition for `deallocate`:

> All `n T` objects in the area pointed to by `p` shall be destroyed prior to this call.

This makes a poor assumption that every possible object in the allocated buffer was indeed constructed, but this is often not the case, e.g., a `vector` that is not filled to capacity. We should require calling the destructor for only those objects actually constructed in the buffer, which may be fewer than `n`, or even 0.

I wonder if we really require all objects to be destroyed before calling `deallocate` though. Are we really so concerned about leaking objects that might not manage resources? Should this not be the proper concern of the library managing the objects and memory?

[2014-06-05 Daniel responds and improves wording]

I fully agree with the last comment and I think that this requirement should be removed. We have no such requirements for comparable functions such as `operator delete` or `return_temporary_buffer()`, and this wording seems to be a wording rudiment that exists since C++98.

[2015-05, Lenexa]

Marshall: What do people think about this?
PJP: Sure.
Wakely: Love it.
Marshall: Ready?
Everyone agrees.

Proposed resolution:

This wording is relative to N3936.

1. Change Table 28 ("`Allocator` requirements") as indicated:

   > Table 28 — `Allocator` requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `…`
   > `a.deallocate(p,n)` | (not used) | Pre: `p` shall be a value returned by an earlier call to `allocate` which has not been invalidated by an intervening call to `deallocate`. `n` shall match the value passed to `allocate` to obtain this memory. All `n T` objects in the area pointed to by `p` shall be destroyed prior to this call. Throws: Nothing.`n` shall match the value passed to allocate to obtain this memory. Does not throw exceptions. [Note: `p` shall not be singular. — end note] |
   > `…`

### 2385(i). `function::assign` allocator argument doesn't make sense

Section: 22.10.17.3 [func.wrap.func] Status: C++17 Submitter: Pablo Halpern Opened: 2014-05-23 Last modified: 2017-07-30

Priority: 2

View all other issues in [func.wrap.func].

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Discussion:

The definition of `function::assign` in N3936 is:

>     template<class F, class A>
>       void assign(F&& f, const A& a);

> > Effects: `function(allocator_arg, a, std::forward<F>(f)).swap(*this)`

This definition is flawed in several respects:

1. The interface implies that the intent is to replace the allocator in `*this` with the specified allocator, `a`. Such functionality is unique in the standard and is problematic when creating, e.g. a container of function objects, all using the same allocator.

2. The current definition of `swap()` makes it unclear whether the objects being swapped can have different allocators. The general practice is that allocators must be equal in order to support swap, and this practice is reinforced by the proposed library TS. Thus, the definition of `assign` would have undefined behavior unless the allocator matched the allocator already within function.

3. The general rule for members of function is to supply the allocator before the functor, using the `allocator_arg` prefix. Supplying the allocator as a second argument, without the `allocator_arg` prefix is error prone and confusing.

I believe that this ill-conceived interface was introduced in the effort to add allocators to parts of the standard where it had been missing, when we were unpracticed in the right way to accomplish that. Allocators were added to function at a time when the allocator model was in flux and it was the first class in the standard after `shared_ptr` to use type-erased allocators, so it is not surprising to see some errors in specification here. (`shared_ptr` is a special case because of its shared semantics, and so is not a good model.)

The main question is whether this member should be specified with better precision or whether it should be deprecated/removed. The only way I can see to give a "reasonable" meaning to the existing interface is to describe it in terms of destroying and re-constructing `*this`:

>     function temp(allocator_arg, a, std::forward<F>(f));
>     this->~function();
>     ::new(this) function(std::move(temp));

(The temp variable is needed for exception safety). The ugliness of this specification underscores the ugliness of the concept. What is the purpose of this member other than to reconstruct the object from scratch, a facility that library classes do not generally provide? Programmers are always free to destroy and re-construct objects — there is no reason why function should make that especially easy.

I propose, therefore, that we make no attempt at giving the current interface a meaningful definition of questionable utility, but simply get rid of it all together. This leaves us with only two questions:

1. Should we deprecate the interface or just remove it?

2. Should we replace it with an `assign(f)` member that doesn't take an allocator?

Of these four combinations of binary answers to the above questions, I think the ones that make the most sense are (remove, no) and (deprecate, yes). The proposed new interface provides nothing that `operator=` does not already provide. However, if the old (deprecated) interface remains, then having the new interface will guide the programmer away from it.

The proposed wording below assumes deprecation. If we choose removal, then there is no wording needed; simply remove the offending declaration and definition.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change class template `function` synopsis, 22.10.17.3 [func.wrap.func], as indicated:

>    >     template<class R, class... ArgTypes>
>    >     class function<R(ArgTypes...)> {
>    >       […]
>    >       // 20.9.11.2.2, function modifiers:
>    >       void swap(function&) noexcept;
>    >       template<class F, class A> void assign(F&&, const A&);
>    >       […]
>    >     };

> 2. Change 22.10.17.3.3 [func.wrap.func.mod] as indicated:

>    >     template<class F, class A> void assign(F&& f, const A& a);

>    > > Effects: `*this = forward<F>(f);function(allocator_arg, a, std::forward<F>(f)).swap(*this)`

> 3. To deprecation section [depr.function.objects], add the following new sub-clause:

>    > Old `assign` member of polymorphic function wrappers [depr.function.objects.assign]

>    > >     namespace std{
>    > >       template<class R, class... ArgTypes>
>    > >       class function<R(ArgTypes...)> {
>    > >         // remainder unchanged
>    > >         template<class F, class A> void assign(F&& f, const A& a);
>    > >         […]
>    > >       };
>    > >     }

>    > The two-argument form of `assign` is defined as follows:

>    >     template<class F, class A> void assign(F&& f, const A& a);

>    > > Requires: `a` shall be equivalent to the allocator used to construct `*this`.

>    > > Effects: `this->assign(forward<F>(f));`

[2015-05, Lenexa]

STL: I would ask, does anybody oppose removing this outright?
Wakely: I don't even have this signature.
Hwrd: And I think this doesn't go far enough, even more should be removed. This is a step in the right direction.
PJP: I'm in favor of removal.
Wakely: We've already got TS1 that has a new function that does it right. We could wait for feedback on that. I think this issue should be taken now.
Marshall: Then the goal will be to move to ready.

Proposed resolution:

This wording is relative to N4431.

1. Change class template `function` synopsis, 22.10.17.3 [func.wrap.func], as indicated:

   >     template<class R, class... ArgTypes>
   >     class function<R(ArgTypes...)> {
   >       […]
   >       // 20.9.12.2.2, function modifiers:
   >       void swap(function&) noexcept;
   >       template<class F, class A> void assign(F&&, const A&);
   >       […]
   >     };

2. Change 22.10.17.3.3 [func.wrap.func.mod] as indicated:

   >     template<class F, class A>
   >       void assign(F&& f, const A& a);

   > > -2- Effects: `function(allocator_arg, a, std::forward<F>(f)).swap(*this)`

### 2387(i). More nested types that must be accessible and unambiguous

Section: 22.10.4 [func.require], 22.10.6 [refwrap] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-05-23 Last modified: 2017-07-30

Priority: Not Prioritized

View other active issues in [func.require].

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Discussion:

Further to 2299(i) and 2361(i), 22.10.4 [func.require] p3 and 22.10.6 [refwrap] p3 and p4 talk about member types without any mention of being accessible and unambiguous.

[2014-06-05 Daniel provides wording]

[2014-06-06 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 22.10.4 [func.require] p3 as indicated:

   > -3- If a call wrapper (20.9.1) has a weak result type the type of its member type `result_type` is based on the type `T` of the wrapper's target object (20.9.1):

   > - if `T` is a pointer to function type, `result_type` shall be a synonym for the return type of `T`;

   > - if `T` is a pointer to member function, `result_type` shall be a synonym for the return type of `T`;

   > - if `T` is a class type and the qualified-id `T::result_type` is valid and denotes a type (13.10.3 [temp.deduct])with a member type `result_type`, then `result_type` shall be a synonym for `T::result_type`;

   > - otherwise `result_type` shall not be defined.

2. Change 22.10.6 [refwrap] p3+p4 as indicated:

   > -3- The template instantiation `reference_wrapper<T>` shall define a nested type named `argument_type` as a synonym for `T1` only if the type `T` is any of the following:

   > - a function type or a pointer to function type taking one argument of type `T1`

   > - a pointer to member function `R T0::f` cv (where cv represents the member function_s cv-qualifiers); the type `T1` is cv `T0*`

   > - a class type where the qualified-id `T::argument_type` is valid and denotes a type (13.10.3 [temp.deduct])with a member type `argument_type`; the type `T1` is `T::argument_type`.

   > -4- The template instantiation `reference_wrapper<T>` shall define two nested types named `first_argument_type` and `second_argument_type` as synonyms for `T1` and `T2`, respectively, only if the type `T` is any of the following:

   > - a function type or a pointer to function type taking two arguments of types `T1` and `T2`

   > - a pointer to member function `R T0::f(T2)` cv (where cv represents the member function's cv-qualifiers); the type `T1` is cv `T0*`

   > - a class type where the qualified-ids `T::first_argument_type` and `T::second_argument_type` are both valid and both denote types (13.10.3 [temp.deduct])with member types `first_argument_type` and `second_argument_type`; the type `T1` is `T::first_argument_type`. and the type `T2` is `T::second_argument_type`.

### 2389(i). [fund.ts] `function::operator=` is over-specified and handles allocators incorrectly

Section: 4.2.1 [fund.ts::func.wrap.func.con] Status: TS Submitter: Pablo Halpern Opened: 2014-05-23 Last modified: 2017-07-30

Priority: 2

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Discussion:

Addresses: fund.ts

This issue against the TS is similar to LWG 2386(i), which is against the standard. The Effects clauses for the assignment operator for class template `function` are written as code that constructs a temporary `function` and then swaps it with `*this`. The intention appears to be that assignment should have the strong exception guarantee, i.e., `*this` is not modified if an exception is thrown. The description in the standard is incorrect when `*this` was originally constructed using an allocator. The TS attempts to correct the problem, but the correction is incomplete.

The wording in the TS uses `get_memory_resource()` to construct a temporary `function` object with the same allocator as the left-hand size (lhs) of the assignment. The intended result of using this pattern was that the allocator for `*this` would be unchanged, but it doesn't quite work. The problem is that the allocator returned by `get_memory_resource()` is not the same type as the type-erased allocator used to construct the `function` object, but rather a type-erased distillation of that type that is insufficient for making a true copy of the allocator. The rules for type-erased allocators in the TS ([memory.type.erased.allocator]) specify that the lifetime of the object returned by `get_memory_resource()` is sometimes tied to the lifetime of `*this`, which might cause the (single copy of) the allocator to be destroyed if the `swap` operation destroys and reconstructs `*this`, as some implementations do (and are allowed to do).

The desired behavior is that assignment would leave the allocator of the lhs unchanged. The way to achieve this behavior is to construct the temporary `function` using the original allocator. Unfortunately, we cannot describe the desired behavior in pure code, because `get_memory_resource()` does not really name the type-erased allocator, as mentioned above. The PR below, therefore, uses pseudo-code, inventing a fictitious `ALLOCATOR_OF(f)` expression that evaluates to the actual allocator type, even if that allocator was type erased. I have implemented this PR successfully.

[2014-06-21, Rapperswil]

Apply to Library Fundamentals TS (after removing the previous "Throws: Nothing" element to prevent an editorial conflict with 2401(i)).

Proposed resolution:

This wording is relative to N3908.

1. Change in [mods.func.wrap] in the Library TS as indicated:

   > In the following descriptions, let `ALLOCATOR_OF(f)` be the allocator specified in the construction of `function` `f`, or `allocator<char>()` if no allocator was specified.

   >     function& operator=(const function& f);

   > > -5- Effects: `function(allocator_arg, get_memory_resource()ALLOCATOR_OF(*this), f).swap(*this);`

   > > […]

   >     function& operator=(function&& f);

   > > -8- Effects: `function(allocator_arg, get_memory_resource()ALLOCATOR_OF(*this), std::move(f)).swap(*this);`

   > > […]

   >     function& operator=(nullptr_t);

   > > -11- Effects: If `*this != NULL`, destroys the target of `this`.

   > > -12- Postconditions: `!(*this)`. The memory resource returned by `get_memory_resource()` after the assignment is equivalent to the memory resource before the assignment. [Note: the address returned by `get_memory_resource()` might change — end note]

   > > -13- Returns: `*this`

   >     template<class F> function& operator=(F&& f);

   > > -15- Effects: `function(allocator_arg, get_memory_resource()ALLOCATOR_OF(*this), std::forward<F>(f)).swap(*this);`

   > > […]

   >     template<class F> function& operator=(reference_wrapper<F> f);

   > > -18- Effects: `function(allocator_arg, get_memory_resource()ALLOCATOR_OF(*this), f).swap(*this);`

   > > […]

### 2390(i). [fund.ts] Invocation types and rvalues

Section: 3.3.2 [fund.ts::meta.trans.other] Status: TS Submitter: Michael Spertus Opened: 2014-05-26 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: fund.ts

`invocation_type` falls short of its stated goals in the following case. Using the notation of Invocation type traits, consider

>     void f(int const& i);
>     more_perfect_forwarding_async(f, int(7)); // Oops. Dangling reference because rvalue gone when async runs

This was always the advertised intent of the proposal, but while the language reflects this for the first parameter in the case of a member pointer, it failed to include corresponding language for other parameters.

[2014-06-18, Rapperswil]

Mike Spertus, Richard Smith, Jonathan Wakely, and Jeffrey Yasskin suggest improved wording.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3908.

> 1. Change Table 3, [meta.trans.other] in the Library TS as indicated:

>    > Table 3 — Other type transformations
>    > Template | Condition | Comments
>    > --- | --- | ---
>    > `…`
>    > `template <class Fn, class... ArgTypes> struct invocation_type<Fn(ArgTypes...)>;` | `Fn` and all types in the parameter pack `ArgTypes` shall be complete types, (possibly cv-qualified) `void`, or arrays of unknown bound. | If `A1, A2,...` denotes `ArgTypes...` and `raw_invocation_type<Fn(ArgTypes...)>::type` is the function type `R(T1, T2, ...)` then let `Ui` be `decay<Ai>::type` if `Ai` is an rvalue otherwise `Ti`. If and `Fn` is a pointer to member type and `T1` is an rvalue reference, then let `U1` be `R(decay<T1>::type, T2, ...)`. Otherwise, `raw_invocation_type<Fn(ArgTypes...)>::type` The member typedef `type` shall equal `R(U1, U2, ...)`.

[2013-06-21 Rapperswil]

Accept for Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N4023.

1. Change Table 3, [meta.trans.other] in the Library TS as indicated:

   > Table 3 — Other type transformations
   > Template | Condition | Comments
   > --- | --- | ---
   > `…`
   > `template <class Fn, class... ArgTypes> struct invocation_type<Fn(ArgTypes...)>;` | `Fn` and all types in the parameter pack `ArgTypes` shall be complete types, (possibly cv-qualified) `void`, or arrays of unknown bound. | The nested typedef `invocation_type<Fn(ArgTypes...)>::type` shall be defined as follows. If `raw_invocation_type<Fn(ArgTypes...)>::type` does not exist, there shall be no member typedef `type`. Otherwise: Let `A1, A2,` … denote `ArgTypes...` Let `R(T1, T2, …)` denote `raw_invocation_type_t<Fn(ArgTypes...)>` Then the member typedef `type` shall name the function type `R(U1, U2, …)` where `Ui` is `decay_t<Ai>` if `declval<Ai>()` is an rvalue otherwise `Ti`. If `raw_invocation_type<Fn(ArgTypes...)>::type` is the function type `R(T1, T2, …)` and `Fn` is a pointer to member type and `T1` is an rvalue reference, then `R(decay<T1>::type, T2, …)`. Otherwise, `raw_invocation_type<Fn(ArgTypes...)>::type`.

### 2391(i). `basic_string` is missing non-`const` `data()`

Section: 27.4.3 [basic.string] Status: Resolved Submitter: Michael Bradshaw Opened: 2014-05-27 Last modified: 2017-03-12

Priority: 3

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Discussion:

Regarding 27.4.3 [basic.string], `std::basic_string<charT>::data()` returns a `const charT*` 27.4.3.8.1 [string.accessors]. While this method is convenient, it doesn't quite match `std::array<T>::data()` [array.data] or `std::vector<T>::data()` 23.3.13.4 [vector.data], both of which provide two versions (that return `T*` or `const T*`). An additional `data()` method can be added to `std::basic_string` that returns a `charT*` so it can be used in similar situations that `std::array` and `std::vector` can be used. Without a non-`const` `data()` method, `std::basic_string` has to be treated specially in code that is otherwise oblivious to the container type being used.

Adding a `charT*` return type to `data()` would be equivalent to doing `&str[0]` or `&str.front()`.

Small discussion on the issue can be found here and in the std-discussion thread (which didn't get too much attention).

This requires a small change to `std::basic_string`'s definition in 27.4.3 [basic.string] to add the method to `std::basic_string`, and another small change in 27.4.3.8.1 [string.accessors] to define the new method.

[2015-02 Cologne]

Back to LEWG.

[2016-05-22]

Marshall says: this issue has been resolved by P0272R1.

Proposed resolution:

This wording is relative to N3936.

1. Change class template `basic_string` synopsis, 27.4.3 [basic.string], as indicated:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >       class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         // 21.4.7, string operations:
   >         const charT* c_str() const noexcept;
   >         const charT* data() const noexcept;
   >         charT* data() noexcept;
   >         allocator_type get_allocator() const noexcept;
   >         […]
   >       };
   >     }

2. Add the following sequence of paragraphs following 27.4.3.8.1 [string.accessors] p3, as indicated:

   >     charT* data() noexcept;

   > > -?- Returns: A pointer `p` such that `p + i == &operator[](i)` for each `i` in `[0,size()]`.

   > > -?- Complexity: Constant time.

   > > -?- Requires: The program shall not alter the value stored at `p + size()`.

### 2392(i). "character type" is used but not defined

Section: 3.36 [defns.ntcts], 28.3.3.1.2.1 [locale.category], 31.2.3 [iostreams.limits.pos], 31.7.6.3.1 [ostream.formatted.reqmts], 31.7.6.3.4 [ostream.inserters.character] Status: C++26 Submitter: Jeffrey Yasskin Opened: 2014-06-01 Last modified: 2026-06-09

Priority: 3

View all issues with C++26 status.

Discussion:

The term "character type" is used in 3.36 [defns.ntcts], 28.3.3.1.2.1 [locale.category], 31.2.3 [iostreams.limits.pos], 31.7.6.3.1 [ostream.formatted.reqmts], and 31.7.6.3.4 [ostream.inserters.character], but the core language only defines "narrow character types" (6.9.2 [basic.fundamental]).

"wide-character type" is used in 99 [depr.locale.stdcvt], but the core language only defines a "wide-character set" and "wide-character literal".

[2023-06-14; Varna; Daniel comments and provides wording]

Given the resolution of P2314 which had introduced to 6.9.2 [basic.fundamental] p11 a definition of "character type":

> The types `char`, `wchar_t`, `char8_t`, `char16_t`, `char32_t` are collectively called character types.

one might feel tempted to have most parts of this issue resolved here, but I think that this actually is a red herring.

First, as Jonathan already pointed out, for two places, 31.7.6.3.1 [ostream.formatted.reqmts] and 31.7.6.3.4 [ostream.inserters.character], this clearly doesn't work, instead it seems as if we should replace "character type of the stream" here by "`char_type` of the stream".

To me "`char_type` of the stream" sounds a bit odd (we usually refer to `char_type` in terms of a qualified name such as `X::char_type` instead unless we are specifying a member of some `X`, where we can omit the qualification) and in the suggested wording below I'm taking advantage of the already defined term "character container type" (3.10 [defns.character.container]) instead, which seems to fit its intended purpose here.

Second, on further inspection it turns out that actually only one usage of the term "character type" seems to be intended to refer to the actual core language meaning (See the unchanged wording for 28.3.4.3.3.3 [facet.num.put.virtuals] in the proposed wording below), all other places quite clearly must refer to the above mentioned "character container type".

For the problem related to the missing definition of "wide-character type" (used two times in 99 [depr.locale.stdcvt]) I would like to suggest a less general and less inventive approach to solve the definition problem here, because it only occurs in an already deprecated component specification: My suggestion is to simply get rid of that term by just identifying `Elem` with being one of `wchar_t`, `char16_t`, or, `char32_t`. (This result is identical to identifying "wide-character type" with a "character type that is not a narrow character type (6.9.2 [basic.fundamental])", but this seemingly more general definition doesn't provide a real advantage.)

[Varna 2023-06-14; Move to Ready]

[2023-06-25; Daniel comments]

During the Varna LWG discussions of this issue it had been pointed out that the wording change applied to [depr.locale.stdcvt.req] bullet (1.1) could exclude now the previously allowed support of narrow character types as a "wide-character" with e.g. a `Maxcode` value of 255. First, I don't think that the revised wording really forbids this. Second, the originating proposal N2401 doesn't indicate what the actual intend here was and it seems questionable to assign LEWG to this issue given that the relevant wording is part of deprecated components, especially given their current position expressed here to eliminate the specification of the affected components as suggested by P2871.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

> [Drafting note: All usages of "character type" in 28.5 [format] seem to be without problems.]

1. Modify 28.3.3.1.2.1 [locale.category] as indicated:

   > [Drafting note: The more general interpretation of "character container type" instead of character type by the meaning of the core language seems safe here. It seems reasonable that an implementation allows more than the core language character types, but still could impose additional constraints imposed on them. Even if an implementation does never intend to support anything beyond `char` and `wchar_t`, the wording below is harmless. One alternative could be here to use the even more general term "char-like types" from 27.1 [strings.general], but I'm unconvinced that this buys us much]

   > -6- […] A template parameter with name `C` represents the set of types containing `char`, `wchar_t`, and any other implementation-defined character container types (3.10 [defns.character.container]) that meet the requirements for a character on which any of the iostream components can be instantiated. […]

2. Keep 28.3.4.3.3.3 [facet.num.put.virtuals] of Stage 1 following p4 unchanged:

   > [Drafting note: The wording here seems to refer to the pure core language wording meaning of a character type.]

   > […] For conversion from an integral type other than a character type, the function determines the integral conversion specifier as indicated in Table 110.

3. Modify 31.2.3 [iostreams.limits.pos] as indicated:

   > [Drafting note: Similar to 28.3.3.1.2.1 [locale.category] above the more general interpretation of "character container type" instead of character type by the meaning of the core language seems safe here. ]

   > -3- In the classes of Clause 31, a template parameter with name `charT` represents a member of the set of types containing `char`, `wchar_t`, and any other implementation-defined character container types (3.10 [defns.character.container]) that meet the requirements for a character on which any of the iostream components can be instantiated.

4. Modify 31.7.6.3.1 [ostream.formatted.reqmts] as indicated:

   > -3- If a formatted output function of a stream `os` determines padding, it does so as follows. Given a `charT` character sequence `seq` where `charT` is the character container type of the stream, […]

5. Modify 31.7.6.3.4 [ostream.inserters.character] as indicated:

   >     template<class charT, class traits>
   >       basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, charT c);
   >     template<class charT, class traits>
   >       basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, char c);
   >     // specialization
   >     template<class traits>
   >       basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, char c);
   >     // signed and unsigned
   >     template<class traits>
   >       basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, signed char c);
   >     template<class traits>
   >       basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, unsigned char c);

   > > -1- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of `out`. Constructs a character sequence `seq`. If `c` has type `char` and the character container type of the stream is not `char`, then `seq` consists of `out.widen(c)`; otherwise `seq` consists of `c`. Determines padding for `seq` as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Inserts `seq` into `out`. Calls `os.width(0)`.

6. Modify [depr.locale.stdcvt.req] as indicated:

   > 1. (1.1) — `Elem` is one ofthe wide-character type, such as `wchar_t`, `char16_t`, or `char32_t`.

   > 2. (1.2) — `Maxcode` is the largest wide-character code value of `Elem` converted to `unsigned long` that the facet will read or write without reporting a conversion error.

   > 3. […]

### 2393(i). `std::function`'s Callable definition is broken

Section: 22.10.17.3 [func.wrap.func] Status: C++17 Submitter: Daniel Krügler Opened: 2014-06-03 Last modified: 2017-07-30

Priority: 2

View all other issues in [func.wrap.func].

View all issues with C++17 status.

Discussion:

The existing definition of `std::function`'s Callable requirements provided in 22.10.17.3 [func.wrap.func] p2,

> A callable object `f` of type `F` is Callable for argument types `ArgTypes` and return type `R` if the expression `INVOKE(f, declval<ArgTypes>()..., R)`, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).

is defective in several aspects:

1. The wording can be read to be defined in terms of callable objects, not of callable types.

2. Contrary to that, 22.10.17.3.6 [func.wrap.func.targ] p2 speaks of "`T` shall be a type that is Callable (20.9.11.2) for parameter types `ArgTypes` and return type `R`."

3. The required value category of the callable object during the call expression (lvalue or rvalue) strongly depends on an interpretation of the expression `f` and therefore needs to be specified unambiguously.

The intention of original proposal (see IIIa. Relaxation of target requirements) was to refer to both types and values ("we say that the function object `f` (and its type `F`) is Callable […]"), but that mental model is not really deducible from the existing wording. An improved type-dependence wording would also make the sfinae-conditions specified in 22.10.17.3.2 [func.wrap.func.con] p8 and p21 ("[…] shall not participate in overload resolution unless `f` is Callable (20.9.11.2) for argument types `ArgTypes...` and return type `R`.") easier to interpret.

My understanding always had been (see e.g. Howard's code example in the 2009-05-01 comment in LWG 815(i)), that `std::function` invokes the call operator of its target via an lvalue. The required value-category is relevant, because it allows to reflect upon whether an callable object such as

>     struct RVF
>     {
>       void operator()() const && {}
>     };

would be a feasible target object for `std::function<void()>` or not.

Clarifying the current Callable definition seems also wise to make a future transition to language-based concepts easier. A local fix of the current wording is simple to achieve, e.g. by rewriting it as follows:

> A callable object `f` of type (22.10.3 [func.def]) `F` is Callable for argument types `ArgTypes` and return type `R` if the expression `INVOKE(fdeclval<F&>(), declval<ArgTypes>()..., R)`, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).

It seems appealing to move such a general Callable definition to a more "fundamental" place (e.g. as another paragraph of 22.10.3 [func.def]), but the question arises, whether such a more general concept should impose the requirement that the call expression is invoked on an lvalue of the callable object — such a special condition would also conflict with the more general definition of the `result_of` trait, which is defined for either lvalues or rvalues of the callable type `Fn`. In this context I would like to point out that "Lvalue-Callable" is not the one and only Callable requirement in the library. Counter examples are `std::thread`, `call_once`, or `async`, which depend on "Rvalue-Callable", because they all act on functor rvalues, see e.g. 32.4.3.3 [thread.thread.constr]:

> […] The new thread of execution executes `INVOKE(DECAY_COPY(std::forward<F>(f)), DECAY_COPY(std::forward<Args>(args))...)` […]

For every callable object `F`, the result of `DECAY_COPY` is an rvalue. These implied rvalue function calls are no artifacts, but had been deliberately voted for by a Committee decision (see LWG 2021(i), 2011-06-13 comment) and existing implementations respect these constraints correctly. Just to give an example,

>     #include <thread>

>     struct LVF
>     {
>       void operator()() & {}
>     };

>     int main()
>     {
>       LVF lf;
>       std::thread t(lf);
>       t.join();
>     }

is supposed to be rejected.

The below presented wording changes are suggested to be minimal (still local to `std::function`), but the used approach would simplify a future (second) conceptualization or any further generalization of Callable requirements of the Library.

[2015-02 Cologne]

Related to N4348. Don't touch with a barge pole.

[2015-09 Telecon]

N4348 not going anywhere, can now touch with or without barge poles
Ville: where is Lvalue-Callable defined?
Jonathan: this is the definition. It's replacing Callable with a new term and defining that. Understand why it's needed, hate the change.
Geoff: punt to an LWG discussion in Kona

[2015-10 Kona]

STL: I like this in general. But we also have an opportunity here to add a precondition. By adding static assertions, we can make implementations better. Accept the PR but reinstate the requirement.

MC: Status Review, to be moved to TR at the next telecon.

[2015-10-28 Daniel comments and provides alternative wording]

The wording has been changed as requested by the Kona result. But I would like to provide the following counter-argument for this changed resolution: Currently the following program is accepted by three popular Standard libraries, Visual Studio 2015, gcc 6 libstdc++, and clang 3.8.0 libc++:

>     #include <functional>
>     #include <iostream>
>     #include <typeinfo>
>     #include "boost/function.hpp"

>     void foo(int) {}

>     int main() {
>       std::function<void(int)> f(foo);
>       std::cout << f.target<double>() << std::endl;
>       boost::function<void(int)> f2(foo);
>       std::cout << f2.target<double>() << std::endl;
>     }

and outputs the implementation-specific result for two null pointer values.

Albeit this code is not conforming, it is probable that similar code exists in the wild. The current boost documentation does not indicate any precondition for calling the `target` function, so it is natural that programmers would expect similar specification and behaviour.

Standardizing the suggested change requires a change of all implementations and I don't see any advantage for the user. With that change previously working code could now cause instantiation errors, I don't see how this could be considered as an improvement of the status quo. The result value of `target` is always a pointer, so a null-check by the user code is already required, therefore I really see no reason what kind of problem could result out of the current implementation behaviour, since the implementation never is required to perform a C cast to some funny type.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 22.10.17.3 [func.wrap.func] p2 as indicated:

>    > -2- A callable object `f` of type (22.10.3 [func.def]) `F` is Lvalue-Callable for argument types `ArgTypes` and return type `R` if the expression `INVOKE(fdeclval<F&>(), declval<ArgTypes>()..., R)`, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).

> 2. Change 22.10.17.3.2 [func.wrap.func.con] p8+p21 as indicated:

>    >     template<class F> function(F f);
>    >     template <class F, class A> function(allocator_arg_t, const A& a, F f);

>    > > […]

>    > > -8- Remarks: These constructors shall not participate in overload resolution unless `fF` is `Lvalue-Callable` (20.9.11.2) for argument types `ArgTypes...` and return type `R`.

>    > […]

>    >     template<class F> function& operator=(F&& f);

>    > > […]

>    > > -21- Remarks: This assignment operator shall not participate in overload resolution unless `declval<typename decay<F>::type&>()decay_t<F>` is `Lvalue-Callable` (20.9.11.2) for argument types `ArgTypes...` and return type `R`.

> 3. Change 22.10.17.3.6 [func.wrap.func.targ] p2 as indicated: [Editorial comment: Instead of adapting the preconditions for the naming change I recommend to strike it completely, because the `target()` functions do not depend on it; the corresponding wording exists since its initial proposal and it seems without any advantage to me. Assume that some template argument `T` is provided, which does not satisfy the requirements: The effect will be that the result is a null pointer value, but that case can happen in other (valid) situations as well. — end comment]

>    >     template<class T> T* target() noexcept;
>    >     template<class T> const T* target() const noexcept;

>    > > -2- Requires: `T` shall be a type that is `Callable` (20.9.11.2) for parameter types `ArgTypes` and return type `R`.

>    > > -3- Returns: If `target_type() == typeid(T)` a pointer to the stored function target; otherwise a null pointer.

[2015-10, Kona Saturday afternoon]

GR explains the current short-comings. There's no concept in the standard that expresses rvalue member function qualification, and so, e.g. std::function cannot be forbidden from wrapping such functions. TK: Although it wouldn't currently compile.

GR: Implementations won't change as part of this. We're just clearing up the wording.

STL: I like this in general. But we also have an opportunity here to add a precondition. By adding static assertions, we can make implementations better. Accept the PR but reinstate the requirement.

JW: I hate the word "Lvalue-Callable". I don't have a better suggestion, but it'd be terrible to teach. AM: I like the term. I don't like that we need it, but I like it. AM wants the naming not to get in the way with future naming. MC: We'll review it.

TK: Why don't we also add Rvalue-Callable? STL: Because nobody consumes it.

Discussion whether "tentatively ready" or "review". The latter would require one more meeting. EF: We already have implementation convergence. MC: I worry about a two-meeting delay. WEB: All that being said, I'd be slightly more confident with a review since we'll have new wording, but I wouldn't object. MC: We can look at it in a telecon and move it.

STL reads out email to Daniel.

Status Review, to be moved to TR at the next telecon.

[2016-01-31, Daniel comments and suggests less controversive resolution]

It seems that specifically the wording changes for 22.10.17.3.6 [func.wrap.func.targ] p2 prevent this issue from making make progress. Therefore the separate issue LWG 2591(i) has been created, that focuses solely on this aspect. Furtheron the current P/R of this issue has been adjusted to the minimal possible one, where the term "Callable" has been replaced by the new term "Lvalue-Callable".

Previous resolution II [SUPERSEDED]:

> This wording is relative to N4527.

> 1. Change 22.10.17.3 [func.wrap.func] p2 as indicated:

>    > -2- A callable object `f` of type (22.10.3 [func.def]) `F` is Lvalue-Callable for argument types `ArgTypes` and return type `R` if the expression `INVOKE(fdeclval<F&>(), declval<ArgTypes>()..., R)`, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).

> 2. Change 22.10.17.3.2 [func.wrap.func.con] p8+p21 as indicated:

>    >     template<class F> function(F f);
>    >     template <class F, class A> function(allocator_arg_t, const A& a, F f);

>    > > […]

>    > > -8- Remarks: These constructors shall not participate in overload resolution unless `fF` is `Lvalue-Callable` (20.9.11.2) for argument types `ArgTypes...` and return type `R`.

>    > […]

>    >     template<class F> function& operator=(F&& f);

>    > > […]

>    > > -21- Remarks: This assignment operator shall not participate in overload resolution unless `declval<typename decay<F>::type&>()decay_t<F>` is `Lvalue-Callable` (20.9.11.2) for argument types `ArgTypes...` and return type `R`.

> 3. Change 22.10.17.3.6 [func.wrap.func.targ] p2 as indicated:

>    >     template<class T> T* target() noexcept;
>    >     template<class T> const T* target() const noexcept;

>    > > -2- Remarks: If `T` is a type that is not `Lvalue-Callable` (20.9.11.2) for parameter types `ArgTypes` and return type `R`, the program is ill-formedRequires: `T` shall be a type that is `Callable` (20.9.11.2) for parameter types `ArgTypes` and return type `R`.

>    > > -3- Returns: If `target_type() == typeid(T)` a pointer to the stored function target; otherwise a null pointer.

[2016-03 Jacksonville]

Move to Ready.

Proposed resolution:

This wording is relative to N4567.

1. Change 22.10.17.3 [func.wrap.func] p2 as indicated:

   > -2- A callable object `f` of type (22.10.3 [func.def]) `F` is Lvalue-Callable for argument types `ArgTypes` and return type `R` if the expression `INVOKE(fdeclval<F&>(), declval<ArgTypes>()..., R)`, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).

2. Change 22.10.17.3.2 [func.wrap.func.con] p8+p21 as indicated:

   >     template<class F> function(F f);
   >     template <class F, class A> function(allocator_arg_t, const A& a, F f);

   > > […]

   > > -8- Remarks: These constructors shall not participate in overload resolution unless `fF` is `Lvalue-Callable` (22.10.17.3 [func.wrap.func]) for argument types `ArgTypes...` and return type `R`.

   > […]

   >     template<class F> function& operator=(F&& f);

   > > […]

   > > -21- Remarks: This assignment operator shall not participate in overload resolution unless `declval<typename decay<F>::type&>()decay_t<F>` is `Lvalue-Callable` (22.10.17.3 [func.wrap.func]) for argument types `ArgTypes...` and return type `R`.

3. Change 22.10.17.3.6 [func.wrap.func.targ] p2 as indicated:

   >     template<class T> T* target() noexcept;
   >     template<class T> const T* target() const noexcept;

   > > -2- Requires: `T` shall be a type that is `Lvalue-Callable` (22.10.17.3 [func.wrap.func]) for parameter types `ArgTypes` and return type `R`.

   > > -3- Returns: If `target_type() == typeid(T)` a pointer to the stored function target; otherwise a null pointer.

### 2394(i). `locale::name` specification unclear — what is implementation-defined?

Section: 28.3.3.1.4 [locale.members] Status: C++17 Submitter: Richard Smith Opened: 2014-06-09 Last modified: 2017-07-30

Priority: 3

View all other issues in [locale.members].

View all issues with C++17 status.

Discussion:

28.3.3.1.4 [locale.members] p5 says:

> Returns: The name of `*this`, if it has one; otherwise, the string `"*"`. If `*this` has a name, then `locale(name().c_str())` is equivalent to `*this`. Details of the contents of the resulting string are otherwise implementation-defined.

So… what is implementation-defined here, exactly? The first sentence completely defines the behavior of this function in all cases.

Also, the second sentence says (effectively) that all locales with the same name are equivalent: given `L1` and `L2` that have the same name `N`, they are both equivalent to `locale(N)`, and since there is no definition of "equivalent" specific to `locale`, I assume it's the normal transitive equivalence property, which would imply that `L1` is equivalent to `L2`. I'm not sure why this central fact is in the description of `locale::name`, nor why it's written in this roundabout way.

[2016-08-03 Chicago LWG]

Walter, Nevin, and Jason provide initial Proposed Resolution.

[2016-08 - Chicago]

Thurs PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change 28.3.3.1.4 [locale.members] as indicated:

   >     basic_string<char> name() const;

   > > -5- Returns: The name of `*this`, if it has one; otherwise, the string `"*"`. If `*this` has a name, then `locale(name().c_str())` is equivalent to `*this`. Details of the contents of the resulting string are otherwise implementation-defined.

### 2395(i). [fund.ts] Preconditions: is defined nowhere

Section: 4.2 [fund.ts::func.wrap.func], 8.5.3 [fund.ts::memory.resource.priv], 8.6.2 [fund.ts::memory.polymorphic.allocator.ctor], 8.6.3 [fund.ts::memory.polymorphic.allocator.mem], 8.9.3 [fund.ts::memory.resource.pool.ctor], 8.10.2 [fund.ts::memory.resource.monotonic.buffer.ctor] Status: TS Submitter: Zhihao Yuan Opened: 2014-06-09 Last modified: 2017-07-30

Priority: 2

View all issues with TS status.

Discussion:

Addresses: fund.ts

This element has been introduced by N3916, but the standard does not define it. The standard defines Requires: to indicate a precondition (16.3.2.4 [structure.specifications] p3).

Proposed wording:

Substitute all Preconditions: with Requires:.

[2013-06-21 Rapperswil]

Accept for Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N4023.

1. Substitute all Preconditions: with Requires:.

### 2396(i). `underlying_type` doesn't say what to do for an incomplete enumeration type

Section: 21.3.8.7 [meta.trans.other] Status: C++17 Submitter: Richard Smith Opened: 2014-06-12 Last modified: 2017-07-30

Priority: 0

View all other issues in [meta.trans.other].

View all issues with C++17 status.

Discussion:

Consider:

>     enum E {
>       e = std::underlying_type<E>::type(1)
>     };

Clearly this should be ill-formed, but the library section doesn't appear to ban it. Suggestion:

Change in 21.3.8.7 [meta.trans.other] Table 57:

> Template: `template<class T> struct underlying_type;`

> Condition: `T` shall be a complete an enumeration type (7.2)

> Comments: […]

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change Table 57 — "Other transformations" as indicated:

   > Table 3 — Other type transformations
   > Template | Condition | Comments
   > --- | --- | ---
   > `…`
   > `template <class T> struct underlying_type;` | T shall be a complete an enumeration type (7.2) | […]

### 2397(i). `map<K, V>::emplace` and explicit `V` constructors

Section: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Peter Dimov Opened: 2014-06-12 Last modified: 2015-05-05

Priority: 1

View all other issues in [meta.trans.other].

View all issues with Resolved status.

Discussion:

Please consider the following example:

>     #include <map>
>     #include <atomic>

>     int main()
>     {
>        std::map<int, std::atomic<int>> map_;

>        map_.emplace(1, 0);  // fail
>        map_.emplace(1);     // fail
>        map_.emplace(1, {}); // fail

>        map_.emplace(std::piecewise_construct,
>            std::tuple<int>(1), std::tuple<>()); // OK
>     }

The first three calls represent attempts by an ordinary programmer (in which role I appear today) to construct a map element. Since `std::atomic<int>` is non-copyable and immovable, I was naturally drawn to `emplace()` because it constructs in-place and hence doesn't need to copy or move. The logic behind the attempts was that `K=int` would be constructed from '`1`', and `V=std::atomic<int>` would be (directly) constructed by '`0`', default constructed, or constructed by '`{}`'.

Yet none of the obvious attempts worked.

I submit that at least two of the three ought to have worked, and that we have therefore a defect in either `map::emplace` or `pair`.

Ville:

There exists a related EWG issue for this.

Daniel:

If the proposal N4387 would be accepted, it would solve the first problem mentioned above.

[2015-02, Cologne]

AM: I think Peter's expectation is misguided that the second and third "//fail" cases should work.
DK: Howard's paper [note: which hasn't been written yet] will make the second case work... AM: ...but the third one will never work without core changes.

Case 1 is solved by DK's paper, cases 2 and 3 are not defects; at best they are extensions.

[2015-05, Lenexa]

STL: think this is covered with N4387
MC: this was accepted in Cologne
STL: only want to fix the first emplace
MC: leave alone and mark as closed by N4387

Proposed resolution:

Resolved by acceptance of N4387.

### 2399(i). `shared_ptr`'s constructor from `unique_ptr` should be constrained

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30

Priority: 0

View other active issues in [util.smartptr.shared.const].

View all other issues in [util.smartptr.shared.const].

View all issues with C++17 status.

Discussion:

Consider the following code:

>     #include <iostream>
>     #include <memory>
>     #include <string>

>     using namespace std;

>     void meow(const shared_ptr<int>& sp) {
>       cout << "int: " << *sp << endl;
>     }

>     void meow(const shared_ptr<string>& sp) {
>       cout << "string: " << *sp << endl;
>     }

>     int main() {
>       meow(make_unique<int>(1729));
>       meow(make_unique<string>("kitty"));
>     }

This fails to compile due to ambiguous overload resolution, but we can easily make this work. (Note: `shared_ptr`'s constructor from `auto_ptr` is also affected, but I believe that it's time to remove `auto_ptr` completely.)

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 20.3.2.2.2 [util.smartptr.shared.const] around p33 as indicated:

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > -?- Remark: This constructor shall not participate in overload resolution unless `unique_ptr<Y, D>::pointer` is convertible to `T*`.

   > -33- Effects: Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`.

### 2400(i). `shared_ptr`'s `get_deleter()` should use `addressof()`

Section: 20.3.2.2.11 [util.smartptr.getdeleter] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30

Priority: 0

View all other issues in [util.smartptr.getdeleter].

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Discussion:

The Standard Library should consistently use `addressof()` to defend itself against overloaded `operator&()`.

While we're in the neighbourhood, we should editorially change `0` to `nullptr`.

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 20.3.2.2.11 [util.smartptr.getdeleter] as indicated:

   >     template <class D, class T> get_deleter(const shared_ptr<T>& p) noexcept;

   > -1- Returns: If `p` owns a deleter `d` of type cv-unqualified `D`, returns `&std::addressof(d)`; otherwise returns `0nullptr`. […]

### 2401(i). `std::function` needs more `noexcept`

Section: 22.10.17.3 [func.wrap.func] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30

Priority: 0

View all other issues in [func.wrap.func].

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Discussion:

There are two issues here:

1. `std::function`'s constructor from `nullptr_t` is marked as `noexcept`, but its assignment operator from `nullptr_t` isn't. This assignment can and should be marked as `noexcept`.

2. `std::function`'s comparisons with `nullptr_t` are marked as `noexcept` in two out of three places.

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 22.10 [function.objects] p2, header `<functional>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 20.9.11 polymorphic function wrappers:
   >       […]
   >       template<class R, class... ArgTypes>
   >       bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
   >       template<class R, class... ArgTypes>
   >       bool operator==(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
   >       template<class R, class... ArgTypes>
   >       bool operator!=(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
   >       template<class R, class... ArgTypes>
   >       bool operator!=(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
   >       […]
   >     }

2. Change 22.10.17.3 [func.wrap.func], class template `function` synopsis, as indicated:

   >     […]
   >     // 20.9.11.2.1, construct/copy/destroy:
   >     […]
   >     function& operator=(nullptr_t) noexcept;
   >     […]

3. Change 22.10.17.3.2 [func.wrap.func.con] before p16 as indicated:

   >     function& operator=(nullptr_t) noexcept;

### 2403(i). `stof()` should call `strtof()` and `wcstof()`

Section: 27.4.5 [string.conversions] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30

Priority: 2

View all other issues in [string.conversions].

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Discussion:

`stof()` is currently specified to call `strtod()`/`wcstod()` (which converts the given string to `double`) and then it's specified to convert that `double` to `float`. This performs rounding twice, which introduces error. Here's an example written up by James McNellis:

Consider the following number `X`:

>     1.999999821186065729339276231257827021181583404541015625 (X)

This number is exactly representable in binary as:

>     1.111111111111111111111101000000000000000000000000000001
>     * ^1st                  ^23rd                        ^52nd

I've marked the 23rd and 52nd fractional bits. These are the least significant bits for `float` and `double`, respectively.

If we convert this number directly to `float`, we take the 23 most significant bits:

>     1.11111111111111111111110

The next bit is a one and the tail is nonzero (the 54th fractional bit is a one), so we round up. This gives us the correctly rounded result:

>     1.11111111111111111111111

So far so good. But... If we convert `X` to `double`, we take the 52 most significant bits:

>     1.1111111111111111111111010000000000000000000000000000 (Y)

The next bit is a zero, so we round down (truncating the value). If we then convert `Y` to `float`, we take its 23 most significant bits:

>     1.11111111111111111111110

The next bit is a one and the tail is zero, so we round to even (leaving the value unchanged). This is off by 1ulp from the correctly rounded result.

[2014-06 Rapperswil]

Marshall Clow will look at this.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 27.4.5 [string.conversions] p4+p6 as indicated:

   >     float stof(const string& str, size_t* idx = 0);
   >     double stod(const string& str, size_t* idx = 0);
   >     long double stold(const string& str, size_t* idx = 0);

   > > -4- Effects: the first twoThese functions call `strtof(str.c_str(), ptr)`, `strtod(str.c_str(), ptr)`, and the third function calls `strtold(str.c_str(), ptr)`, respectively. Each function returns the converted result, if any. […]

   > > […]

   > > -6- Throws: `invalid_argument` if `strtof`, `strtod`, or `strtold` reports that no conversion could be performed. Throws `out_of_range` if `strtof`, `strtod`, or `strtold` sets `errno` to `ERANGE` or if the converted value is outside the range of representable values for the return type.

2. Change 27.4.5 [string.conversions] p11+p13 as indicated:

   >     float stof(const wstring& str, size_t* idx = 0);
   >     double stod(const wstring& str, size_t* idx = 0);
   >     long double stold(const wstring& str, size_t* idx = 0);

   > > -11- Effects: the first twoThese functions call `wcstof(str.c_str(), ptr)`, `wcstod(str.c_str(), ptr)`, and the third function calls `wcstold(str.c_str(), ptr)`, respectively. Each function returns the converted result, if any. […]

   > > […]

   > > -13- Throws: `invalid_argument` if `wcstof`, `wcstod`, or `wcstold` reports that no conversion could be performed. Throws `out_of_range` if `wcstof`, `wcstod`, or `wcstold` sets `errno` to `ERANGE`.

### 2404(i). `mismatch()`'s complexity needs to be updated

Section: 26.6.12 [alg.mismatch] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2025-11-11

Priority: 0

View all other issues in [alg.mismatch].

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Discussion:

N3671 updated the complexities of `equal()` and `is_permutation()`, but not `mismatch()`.

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change [mismatch] p3 as indicated:

   > -3- Complexity: At most `min(last1 - first1, last2 - first2)` applications of the corresponding predicate.

### 2406(i). `negative_binomial_distribution` should reject `p == 1`

Section: 29.5.9.3.4 [rand.dist.bern.negbin] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30

Priority: 3

View all other issues in [rand.dist.bern.negbin].

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Discussion:

29.5.9.3.4 [rand.dist.bern.negbin] p2 requires "`0 < p <= 1`". Consider what happens when `p == 1`. The discrete probability function specified by p1 involves "`* p^k * (1 - p)^i`". For `p == 1`, this is "`* 1^k * 0^i`", so every integer `i >= 0` is produced with zero probability. (Let's avoid thinking about `0^0`.)

Wikipedia states that `p` must be within `(0, 1)`, exclusive on both sides. Previous resolution [SUPERSEDED]:

> 1. Change 29.5.9.3.4 [rand.dist.bern.negbin] p2 as indicated: [Drafting note: This should be read as: Replace the symbol "`≤`" by "`<`" — end drafting note]

>    >     explicit negative_binomial_distribution(IntType k = 1, double p = 0.5);

>    > > -2- Requires: `0 < p ≤< 1` and `0 < k`.

[2014-11 Urbana]

SG6 suggests better wording.

[2014-11-08 Urbana]

Moved to Ready with the node.

There remains concern that the constructors are permitting values that may (or may not) be strictly outside the domain of the function, but that is a concern that affects the design of the random number facility as a whole, and should be addressed by a paper reviewing and addressing the whole clause, not picked up in the issues list one distribution at a time. It is still not clear that such a paper would be uncontroversial.

Proposed resolution:

This wording is relative to N4140.

1. Add a note after paragraph 1 before the synopsis in 29.5.9.3.4 [rand.dist.bern.negbin]:

   > A `negative_binomial_distribution` random number distribution produces random integers i≥0 distributed according to the discrete probability function

   > > P(i|k, p) = k + i - 1 i · p k · (1-p) i .

   > [Note: This implies that P(i|k, p) is undefined when `p == 1`. — end note]

   > Drafting note: P(i|k, p) should be in math font, and `p == 1` should be in code font.

### 2407(i). `packaged_task(allocator_arg_t, const Allocator&, F&&)` should neither be constrained nor `explicit`

Section: 32.10.10.2 [futures.task.members] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [futures.task.members].

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Discussion:

LWG 2097(i)'s resolution was slightly too aggressive. It constrained `packaged_task(allocator_arg_t, const Allocator&, F&&)`, but that's unnecessary because `packaged_task` doesn't have any other three-argument constructors. Additionally, it's marked as `explicit` (going back to WP N2798 when `packaged_task` first appeared) which is unnecessary.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

Back to LWG; not an SG1 issue.

[2015-05 Lenexa]

STL improves proposed wording by restoring the constraint again.

Proposed resolution:

This wording is relative to N3936.

1. Change 32.10.10 [futures.task] p2, class template `packaged_task` as indicated:

   >     template <class F>
   >     explicit packaged_task(F&& f);
   >     template <class F, class Allocator>
   >     explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);

2. Change 32.10.10.2 [futures.task.members] as indicated:

   >     template <class F>
   >     packaged_task(F&& f);
   >     template <class F, class Allocator>
   >     explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);

   > > […]

   > > -3- Remarks: These constructors shall not participate in overload resolution if `decay_t<F>` is the same type as `std::packaged_task<R(ArgTypes...)>`.

### 2408(i). SFINAE-friendly `common_type/iterator_traits` is missing in C++14

Section: 21.3.8.7 [meta.trans.other], 24.3.2.3 [iterator.traits] Status: C++17 Submitter: Daniel Krügler Opened: 2014-06-19 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [meta.trans.other].

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Discussion:

During the Rapperswil meeting the proposal N4041 had been discussed and there seemed to be strong consensus to apply the SFINAE-friendly definitions that currently exist within the fundamentals-ts to the C++ Standard working draft. This issue requests this change to happen.

Proposed resolution:

This wording is relative to N3936.

1. Change 21.3.8.7 [meta.trans.other] p3 as indicated:

   > -3- For the `common_type` trait applied to a parameter pack `T` of types, the member `type` shall be either defined or not present as follows:

   > - If `sizeof...(T)` is zero, there shall be no member `type`.

   > - If `sizeof...(T)` is one, let `T0` denote the sole type comprising `T`. The member typedef `type` shall denote the same type as `decay_t<T0>`.

   > - If `sizeof...(T)` is greater than one, let `T1`, `T2`, and `R`, respectively, denote the first, second, and (pack of) remaining types comprising `T`. [Note: `sizeof...(R)` may be zero. — end note] Finally, let `C` denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `T1`, and whose third operand is an xvalue of type `T2`. If there is such a type `C`, the member typedef `type` shall denote the same type, if any, as `common_type_t<C,R...>`. Otherwise, there shall be no member `type`.

   > The nested typedef `common_type::type` shall be defined as follows:

   > >     template <class ...T> struct common_type;

   > >     template <class T>
   > >     struct common_type<T> {
   > >       typedef decay_t<T> type;
   > >     };

   > >     template <class T, class U>
   > >     struct common_type<T, U> {
   > >       typedef decay_t<decltype(true ? declval<T>() : declval<U>())> type;
   > >     };

   > >     template <class T, class U, class... V>
   > >     struct common_type<T, U, V...> {
   > >       typedef common_type_t<common_type_t<T, U>, V...> type;
   > >     };

2. Change 24.3.2.3 [iterator.traits] p2 as indicated:

   > -2- The template `iterator_traits<Iterator>` is defined asshall have the following as publicly accessible members, and have no other members, if and only if `Iterator` has valid (13.10.3 [temp.deduct]) member types `difference_type`, `value_type`, `pointer`, `reference`, and `iterator_category`; otherwise, the template shall have no members:

   > >     namespace std {
   > >       template<class Iterator> struct iterator_traits {
   > >         typedef typename Iterator::difference_type difference_type;
   > >         typedef typename Iterator::value_type value_type;
   > >         typedef typename Iterator::pointer pointer;
   > >         typedef typename Iterator::reference reference;
   > >         typedef typename Iterator::iterator_category iterator_category;
   > >       };
   > >     }

### 2409(i). [fund.ts] SFINAE-friendly `common_type/iterator_traits` should be removed from the fundamental-ts

Section: 3.3.2 [fund.ts::meta.trans.other], 99 [fund.ts::iterator.traits] Status: TS Submitter: Daniel Krügler Opened: 2014-06-19 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [fund.ts::meta.trans.other].

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Discussion:

Addresses: fund.ts

During the Rapperswil meeting the proposal N4041 had been discussed and there seemed to be strong consensus to apply the SFINAE-friendly definitions that currently exist within the fundamentals-ts to the C++17 working draft. If this happens, the fundamentals-ts needs to remove its own specification regarding these templates. This issue requests this change to happen.

[2013-06-21 Rapperswil]

Accept for Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N4023 in regard to fundamental-ts changes.

1. In fundamental-ts, change Table 2 — "Significant features in this technical specification" as indicated:

   > Table 2 — Significant features in this technical specification
   > Doc. No. | Title | Primary Section | Macro Name Suffix | Value | Header
   > --- | --- | --- | --- | --- | ---
   > `…`
   > N3843 | A SFINAE- Friendly `common_type` | 2.4 [mods.meta.trans.other] | `common_type_sfinae` | `201402` | `<type_traits>`
   > N3843 | A SFINAE- Friendly `iterator_traits` | 2.5 [mods.iterator.traits] | `iterator_traits_sfinae` | `201402` | `<iterator>`
   > `…`

2. In fundamental-ts, remove the existing sub-clause 2.4 [mods.meta.trans.other] in its entirety:

   2.4 Changes to `std::common_type` [mods.meta.trans.other]

   > -1- […]

3. In fundamental-ts, remove the existing sub-clause 2.5 [mods.iterator.traits] in its entirety:

   2.5 Changes to `std::iterator_traits` [mods.iterator.traits]

   > -1- […]

### 2410(i). [fund.ts] `shared_ptr<array>`'s constructor from `unique_ptr` should be constrained

Section: 8.2.1.1 [fund.ts::memory.smartptr.shared.const] Status: TS Submitter: Jeffrey Yasskin Opened: 2014-06-16 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts

The proposed resolution for LWG 2399(i) doesn't apply cleanly to the Fundamentals TS, but the issue is still present.

[2015-02, Cologne]

Unanimous consent.

Proposed resolution:

This wording is relative to N4023 in regard to fundamental-ts changes.

1. In fundamental-ts, change [mods.util.smartptr.shared.const] p34 as indicated:

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > > -34- RequiresRemarks: This constructor shall not participate in overload resolution unless `Y*` shall beis compatible with `T*`.

   > > -35- Effects: Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`.

   > > -36- Exception safety: If an exception is thrown, the constructor has no effect.

### 2411(i). `shared_ptr` is only contextually convertible to `bool`

Section: 20.3.2.2 [util.smartptr.shared] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-06-21 Last modified: 2017-07-30

Priority: 0

View all other issues in [util.smartptr.shared].

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Discussion:

N3920 made this edit, which is correct but unrelated to the support for arrays:

> Change 20.7.2.2 [util.smartptr.shared] p2 as follows:

> > Specializations of `shared_ptr` shall be `CopyConstructible`, `CopyAssignable`, and `LessThanComparable`, allowing their use in standard containers. Specializations of `shared_ptr` shall be contextually convertible to `bool`, allowing their use in boolean expressions and declarations in conditions. […]

That change is actually fixing a defect in the current wording and should be applied directly to the working paper, not just to the Library Fundamentals TS. The declarations of the conversion operator in 20.3.2.2 [util.smartptr.shared] and 20.3.2.2.6 [util.smartptr.shared.obs] are `explicit` which contradicts the "convertible to `bool`" statement. The intention is definitely for `shared_ptr` to only be contextually convertible to `bool`.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 20.3.2.2 [util.smartptr.shared] p2 as indicated:

   > -2- Specializations of `shared_ptr` shall be `CopyConstructible`, `CopyAssignable`, and `LessThanComparable`, allowing their use in standard containers. Specializations of `shared_ptr` shall be contextually convertible to `bool`, allowing their use in boolean expressions and declarations in conditions. The template parameter `T` of `shared_ptr` may be an incomplete type.

### 2412(i). `promise::set_value()` and `promise::get_future()` should not race

Section: 32.10.6 [futures.promise], 32.10.10.2 [futures.task.members] Status: C++20 Submitter: Jonathan Wakely Opened: 2014-06-23 Last modified: 2021-02-25

Priority: 3

View all other issues in [futures.promise].

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Discussion:

The following code has a data race according to the standard:

>     std::promise<void> p;
>     std::thread t{ []{
>       p.get_future().wait();
>     }};
>     p.set_value();
>     t.join();

The problem is that both `promise::set_value()` and `promise::get_future()` are non-const member functions which modify the same object, and we only have wording saying that the `set_value()` and `wait()` calls (i.e. calls setting and reading the shared state) are synchronized.

The calls don't actually access the same memory locations, so the standard should allow it. My suggestion is to state that calling `get_future()` does not conflict with calling the various functions that make the shared state ready, but clarify with a note that this does not imply any synchronization or "happens before", only being free from data races.

[2015-02 Cologne]

Handed over to SG1.

[2016-10-21, Nico comments]

After creating a promise or packaged task one thread can call `get_future()` while another thread can set values/exceptions (either directly or via function call). This happens very easily.

Consider:

>     promise<string> p;
>     thread t(doSomething, ref(p));
>     cout << "result: " << p.get_future().get() << endl;

AFAIK, this is currently UB due to a data race (calling `get_future()` for the promise might happen while setting the value in the promise).

Yes, a fix is pretty easy:

>     promise<string> p;
>     future<string> f(p.get_future());
>     thread t(doSomething, ref(p));
>     cout << "result: " << f.get() << endl;

but I would like to have `get_future()` and setters be synchronized to avoid this UB.

This would especially make the use of packaged tasks a lot easier. Consider:

>     vector<packaged_task<int(char)>> tasks;
>     packaged_task<int(char)> t1(func);

>     // start separate thread to run all tasks:
>     auto futCallTasks = async(launch::async, callTasks, ref(tasks));

>     for (auto& fut : tasksResults) {
>       cout << "result: " << fut.get_future().get() << endl; // OOPS: UB
>     }

Again, AFAIK, this program currently is UB due to a data race. Instead, currently I'd have to program, which is a lot less convenient:

>     vector<packaged_task<int(char)>> tasks;
>     vector<future<int>> tasksResults;
>     packaged_task<int(char)> t1(func);
>     tasksResults.push_back(t1.getFuture()));
>     tasks.push_back(move(t1));

>     // start separate thread to run all tasks:
>     auto futCallTasks = async(launch::async, callTasks, ref(tasks));

>     for (auto& fut : tasksResults) {
>       cout << "result: " << fut.get() << endl;
>     }

With my naive thinking I see not reason not to guarantee that these calls synchronize (as `get_future` returns an "address/reference" while all setters set the values there).

Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 32.10.6 [futures.promise] around p12 as indicated:

>    >     future<R> get_future();

>    > > -12- Returns: A `future<R>` object with the same shared state as `*this`.

>    > > -?- Synchronization: Calls to this function do not conflict (6.10.2 [intro.multithread]) with calls to `set_value`, `set_exception`, `set_value_at_thread_exit`, or `set_exception_at_thread_exit`. [Note: Such calls need not be synchronized, but implementations must ensure they do not introduce data races. — end note]

>    > > -13- Throws: `future_error` if `*this` has no shared state or if `get_future` has already been called on a `promise` with the same shared state as `*this`.

>    > > -14- Error conditions: […]

> 2. Change 32.10.10.2 [futures.task.members] around p13 as indicated:

>    >     future<R> get_future();

>    > > -13- Returns: A `future<R>` object that shares the same shared state as `*this`.

>    > > -?- Synchronization: Calls to this function do not conflict (6.10.2 [intro.multithread]) with calls to `operator()` or `make_ready_at_thread_exit`. [Note: Such calls need not be synchronized, but implementations must ensure they do not introduce data races. — end note]

>    > > -14- Throws: a `future_error` object if an error occurs.

>    > > -15- Error conditions: […]

[2017-02-28, Kona]

SG1 has updated wording for LWG 2412. SG1 voted to move this to Ready status by unanimous consent.

[2017-03-01, Kona, SG1]

GeoffR to forward revised wording.

[2018-06, Rapperswil, Wednesday evening session]

JW: lets move on and I'll file another issue to make the wording better
BO: the current wording is better than what there before
JM: ACTION I should file an editorial issue to clean up on how to refer to [res.on.data.races]: raised editorial issue 2097
ACTION: move to Ready

Daniel rebases wording to N4750.

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Change 32.10.6 [futures.promise] around p12 as indicated:

   >     future<R> get_future();

   > > -12- Returns: A `future<R>` object with the same shared state as `*this`.

   > > -?- Synchronization: Calls to this function do not introduce data races (6.10.2 [intro.multithread]) with calls to `set_value`, `set_exception`, `set_value_at_thread_exit`, or `set_exception_at_thread_exit`. [Note: Such calls need not synchronize with each other. — end note]

   > > -13- Throws: `future_error` if `*this` has no shared state or if `get_future` has already been called on a `promise` with the same shared state as `*this`.

   > > -14- Error conditions: […]

2. Change 32.10.10.2 [futures.task.members] around p13 as indicated:

   >     future<R> get_future();

   > > -13- Returns: A `future` object that shares the same shared state as `*this`.

   > > -?- Synchronization: Calls to this function do not introduce data races (6.10.2 [intro.multithread]) with calls to `operator()` or `make_ready_at_thread_exit`. [Note: Such calls need not synchronize with each other. — end note]

   > > -14- Throws: a `future_error` object if an error occurs.

   > > -15- Error conditions: […]

### 2414(i). Member function reentrancy should be implementation-defined

Section: 16.4.6.9 [reentrancy] Status: C++26 Submitter: Stephan T. Lavavej Opened: 2014-07-01 Last modified: 2026-06-09

Priority: 3

View all other issues in [reentrancy].

View all issues with C++26 status.

Discussion:

N3936 16.4.6.9 [reentrancy]/1 talks about "functions", but that doesn't address the scenario of calling different member functions of a single object. Member functions often have to violate and then re-establish invariants. For example, vectors often have "holes" during insertion, and element constructors/destructors/etc. shouldn't be allowed to observe the vector while it's in this invariant-violating state. The [reentrancy] Standardese should be extended to cover member functions, so that implementers can either say that member function reentrancy is universally prohibited, or selectively allowed for very specific scenarios.

(For clarity, this issue has been split off from LWG 2382(i).)

[2014-11-03 Urbana]

AJM confirmed with SG1 that they had no special concerns with this issue, and LWG should retain ownership.

AM: this is too overly broad as it also covers calling the exact same member function on a different object
STL: so you insert into a map, and copying the value triggers another insertion into a different map of the same type
GR: reentrancy seems to imply the single-threaded case, but needs to consider the multi-threaded case

Needs more wording.

Move to Open

[2015-07 Telecon Urbana]

Marshall to ping STL for updated wording.

[2016-05 email from STL]

I don't have any better suggestions than my original PR at the moment.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 16.4.6.9 [reentrancy] p1 as indicated:

>    > -1- Except where explicitly specified in this standard, it is implementation-defined which functions (including different member functions called on a single object) in the Standard C++ library may be recursively reentered.

[2021-07-29 Tim suggests new wording]

The "`this` pointer" restriction is modeled on 11.9.5 [class.cdtor] p2. It allows us to continue to specify a member function `f` as calling some other member function `g`, since any such call would use something obtained from the first member function's `this` pointer.

In all other cases, this wording disallows such "recursion on object" unless both member functions are `const` (or are treated as such for the purposes of data race avoidance). Using "access" means that we also cover direct access to the object representation, such as the following pathological example from Arthur O'Dwyer, which is now undefined:

>     std::string s = "hello world";
>     char *first = (char*)&s;
>     char *last = (char*)(&s + 1);
>     s.append(first, last);

Previous resolution [SUPERSEDED]

This wording is relative to N4892.

1. Add the following paragraph to 16.4.6.9 [reentrancy]:

   > -?- During the execution of a standard library non-static member function F on an object, if that object is accessed through a glvalue that is not obtained, directly or indirectly, from the `this` pointer of F, in a manner that can conflict (6.10.2.2 [intro.races]) with any access that F is permitted to perform (16.4.6.10 [res.on.data.races]), the behavior is undefined unless otherwise specified.

[2026-02-20; Tim provides new wording]

Refer to "object parameter" not "`this` pointer".

[2026-02-20; LWG telecon; Status changed: Open → Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Append the following paragraph to 16.4.6.9 [reentrancy]:

   > -?- During the execution of a standard library non-static member function F on an object, if that object is accessed through a glvalue that is not obtained, directly or indirectly, from the object parameter of F, in a manner that can conflict (6.10.2.2 [intro.races]) with any access that F is permitted to perform (16.4.6.10 [res.on.data.races]), the behavior is undefined unless otherwise specified.

### 2415(i). Inconsistency between `unique_ptr` and `shared_ptr`

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-03 Last modified: 2017-07-30

Priority: 2

View other active issues in [util.smartptr.shared.const].

View all other issues in [util.smartptr.shared.const].

View all issues with C++17 status.

Discussion:

`unique_ptr` guarantees that it will not invoke its deleter if it stores a null pointer, which is useful for deleters that must not be called with a null pointer e.g.

>     unique_ptr<FILE, int(*)(FILE*)> fptr(file, &::fclose);

However, `shared_ptr` does invoke the deleter if it owns a null pointer, which is a silent change in behaviour when transferring ownership from `unique_ptr` to `shared_ptr`. That means the following leads to undefined behaviour:

>     std:shared_ptr<FILE> fp = std::move(fptr);

Peter Dimov's suggested fix is to construct an empty `shared_ptr` from a `unique_ptr` that contains a null pointer.

[2015-01-18 Library reflector vote]

The issue has been identified as Tentatively Ready based on eight votes in favour.

Proposed resolution:

This wording is relative to N4296.

1. Change 20.3.2.2.2 [util.smartptr.shared.const] p29 as indicated:

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > > […]

   > > -29- Effects: If `r.get() == nullptr`, equivalent to `shared_ptr()`. Otherwise, if `D` is not a reference type, equivalent to `shared_ptr(r.release(), r.get_deleter())`. Otherwise, equivalent to `shared_ptr(r.release(), ref(r.get_deleter()))`Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`.

### 2416(i). [fund.ts] `std::experimental::any allocator` support is unimplementable

Section: 6.3 [fund.ts::any.class] Status: Resolved Submitter: Jonathan Wakely Opened: 2014-06-16 Last modified: 2015-10-26

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Addresses: fund.ts

The allocator-extended copy constructor for `any` requires an arbitrary template parameter to be available in a type-erased context where the dynamic type of the contained object is known. This is not believed to be possible in C++.

If the allocator-extended copy constructor cannot be defined it questions the usefulness of the other allocator-extended constructors.

[Urbana, 2014-11]

Resolved by paper N4270.

Proposed resolution:

### 2418(i). [fund.ts] `apply` does not work with member pointers

Section: 3.2.2 [fund.ts::tuple.apply] Status: TS Submitter: Zhihao Yuan Opened: 2014-07-08 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts

The definition of `apply` present in §3.2.2 [tuple.apply] prevents this function template to be used with pointer to members type passed as the first argument.

> Effects:

> […]

> `return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...);`

This makes this utility inconsistent with other standard library components and limits its usability.

We propose to define its functionally in terms of `INVOKE`.

[2015-02, Cologne]

DK: We should use the new `std::invoke`.
TK: Is this a defect?
AM: `std::invoke` goes into C++17, and this is a defect against a TS based on C++14. We can change this later, but now leave it as INVOKE.
GR: The TS lets you have Editor's Notes, so leave a note to make that change for C++17.

[…]

GR: I can't see how we can assume this is part of the design. I cannot believe it was ever intended for this design to exclude function pointers.
AM: I can give you the exact evolution: We had "apply" as an example explaining the usefulness of `index_sequence`. Then someone looked at it and said, "why isn't this in the Standard". NJ to VV: Why are you against useful steps? We are trying to converge on a consistent standard across multiple documents. The alternative is to reopen this in a later discussion.
VV: All I said is that this is not defect, whether or not people like it.
AM: So you'd be fine with the issue, but not as a DR?
Straw poll: Who's happy to make this tentatively ready as a DR against the Fundamentals TS? Lots of agreement, no opposition, 3 neutrals

Proposed resolution:

This wording is relative to N4081 in regard to fundamental-ts changes.

1. Edit §3.2.2 [tuple.apply] paragraph 2:

   >     template <class F, class Tuple>
   >     constexpr decltype(auto) apply(F&& f, Tuple&& t);

   > > -2- Effects: Given the exposition only function

   > > >     template <class F, class Tuple, size_t... I>
   > > >     constexpr decltype(auto) apply_impl(  // exposition only
   > > >         F&& f, Tuple&& t, index_sequence<I...>) {
   > > >       return INVOKE(std::forward<F>(f)(, std::get<I>(std::forward<Tuple>(t))...);
   > > >     }

   > > > […]

### 2419(i). Clang's libc++ extension to `std::tuple`

Section: 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Akim Demaille Opened: 2014-07-11 Last modified: 2018-06-23

Priority: Not Prioritized

View other active issues in [tuple.cnstr].

View all other issues in [tuple.cnstr].

View all issues with Resolved status.

Discussion:

The issue has been submitted after exchanges with the clang++ team as a consequence of two PR I sent:

Issue 20174

Issue 20175

The short version is shown in the program below:

>     #include <iostream>
>     #include <tuple>

>     struct base
>     {
>       void out(const std::tuple<char, char>& w) const
>       {
>         std::cerr << "Tuple: " << std::get<0>(w) << std::get<1>(w) << '\n';
>       }
>     };

>     struct decorator
>     {
>       base b_;

>       template <typename... Args>
>       auto
>       out(Args&&... args)
>         -> decltype(b_.out(args...))
>       {
>         return b_.out(args...);
>       }

>       void out(const char& w)
>       {
>         std::cerr << "char: " << w << '\n';
>       }
>     };

>     int main()
>     {
>       decorator d{base{}};
>       char l = 'a';
>       d.out(l);
>     }

This is a stripped down version of a real world case where I wrap objects in decorators. These decorators contributes some functions, and forward all the rest of the API to the wrapped object using perfect forwarding. There can be overloaded names.

Here the inner object provides an

>     out(const std::tuple<char, char>&) -> void

function, and the wrappers, in addition to perfect forwarding, provides

>     out(const char&) -> void

The main function then call `out(l)` where `l` is a `char` lvalue.

With (GCC's) libstdc++ I get the expected result: the `char` overload is run. With (clang++'s) libc++ it is the tuple version which is run.

>     $ g++-mp-4.9 -std=c++11 bar.cc && ./a.out
>     char: a
>     $ clang++-mp-3.5 -std=c++11 bar.cc -Wall && ./a.out
>     Tuple: a

It turns out that this is the result of an extension of `std::tuple` in libc++ where they accept constructors with fewer values that tuple elements.

The purpose of this issue is to ask the standard to forbid that this extension be allowed to participate in overload resolution.

[2014-10-05, Daniel comments]

This issue is closely related to LWG 2312(i).

[2014-11 Urbana]

Moved to LEWG.

Extensions to `tuple`'s design are initially a question for LEWG.

Proposed resolution:

This was resolved by the adoption of 2312(i) and 2549(i).

### 2420(i). `function<void(ArgTypes...)>` does not discard the return value of the target object

Section: 22.10.17.3 [func.wrap.func] Status: C++17 Submitter: Agustín Bergé Opened: 2014-07-12 Last modified: 2017-07-30

Priority: 1

View all other issues in [func.wrap.func].

View all issues with C++17 status.

Discussion:

`function<void(ArgTypes...)>` should discard the return value of the target object. This behavior was in the original proposal, and it was removed (accidentally?) by the resolution of LWG 870(i).

Previous resolution [SUPERSEDED]:

> 1. Edit 22.10.17.3 [func.wrap.func] paragraph 2:

>    > A callable object `f` of type `F` is Callable for argument types `ArgTypes` and return type `R` if the expression `INVOKE(f, declval<ArgTypes>()..., R)`, considered as an unevaluated operand (Clause 5), is well formed (22.10.4 [func.require]) and, if `R` is not `void`, implicitly convertible to `R`.

[2014-10-05 Daniel comments]

This side-effect was indeed not intended by 870(i).

[2015-05, Lenexa]

STL provides improved wording. It replaces the current PR, and intentionally leaves 22.10.17.3 [func.wrap.func] unchanged.

Due to 7 [expr]/6, `static_cast<void>` is correct even when `R` is `const void`.

Proposed resolution:

This wording is relative to N4431.

1. Edit 22.10.4 [func.require] as depicted:

   > -2- Define `INVOKE(f, t1, t2, ..., tN, R)` as `static_cast<void>(INVOKE(f, t1, t2, ..., tN))` if `R` is cv `void`, otherwise `INVOKE(f, t1, t2, ..., tN)` implicitly converted to `R`.

2. Change 22.10.17.3.5 [func.wrap.func.inv] as depicted:

   >     R operator()(ArgTypes... args) const;

   > > -1- EffectsReturns: `INVOKE(f, std::forward<ArgTypes>(args)..., R)` (20.9.2), where `f` is the target object (20.9.1) of `*this`.

   > > -2- Returns: Nothing if `R` is `void`, otherwise the return value of `INVOKE(f, std::forward<ArgTypes>(args)..., R)`.

### 2422(i). `std::numeric_limits<T>::is_modulo` description: "most machines" errata

Section: 17.3.5.2 [numeric.limits.members] Status: C++17 Submitter: Melissa Mears Opened: 2014-08-06 Last modified: 2017-07-30

Priority: 2

View all other issues in [numeric.limits.members].

View all issues with C++17 status.

Discussion:

The seemingly non-normative (?) paragraph 61 (referring to N3936) describing how "most machines" define `std::numeric_limits<T>::is_modulo` in [numeric.limits.members] appears to have some issues, in my opinion.

> -61- On most machines, this is `false` for floating types, `true` for unsigned integers, and `true` for signed integers.

Issues I see:

1. Very minor: change clause 2 to "this is `false` for floating point types". Other uses of the term say "floating point types" rather than just "floating types" — see nearby `is_iec559`, `tinyness_before`, etc.

2. `is_modulo` must be `true` for unsigned integers in order to be compliant with the Standard; this is not just for "most machines". For reference, this requirement is from [basic.fundamental] paragraph 4 with its footnote 48.

3. Depending on the definition of "most machines", `is_modulo` could be `false` for most machines' signed integer types. GCC, Clang and Visual Studio, the 3 most popular C++ compilers by far, by default treat signed integer overflow as undefined.

As an additional note regarding the definition of `is_modulo`, it seems like it should be explicitly mentioned that on an implementation for which signed integer overflow is undefined, `is_modulo` shall be `false` for signed integer types. It took bugs filed for all three of these compilers before they finally changed (or planned to change) `is_modulo` to `false` for signed types.

[2014-12 telecon]

HH: agree with the proposal, don't like the phrasing
AM: second note feels a bit wooly
WB: not even happy with the first note, notes shouldn't say "shall"
JW: the original isn't very prescriptive because "on most machines" is not something the standard controls.
AM: "On most machines" should become a note too?
AM: first note is repeating something defined in core, shouldn't say it normatively here. Change "shall" to "is"?
MC: don't like "signed integer overflow is left undefined" ... it's just plain undefined.
AM: implementations can define what they do in that case and provide guarantees.
WB: in paragraph 61, would like to see "this" replaced by "is_modulo"
AM: Move to Open

[2015-05-05 Lenexa]

Marshall: I will contact the submitter to see if she can re-draft the Proposed Resolution

Previous resolution [SUPERSEDED]:

> 1. Edit 17.3.5.2 [numeric.limits.members] around p60 as indicated:

>    >     static constexpr bool is_modulo;

>    > -60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving `+`, `-`, or `*` on values of that type whose result would fall outside the range `[min(), max()]`, the value returned differs from the true value by an integer multiple of `max() - min() + 1`.

>    > -??- [Note: `is_modulo` shall be `true` for unsigned integer types (6.9.2 [basic.fundamental]). — end note]

>    > -??- [Note: `is_modulo` shall be `false` for types for which overflow is undefined on the implementation, because such types cannot meet the modulo requirement. Often, signed integer overflow is left undefined on implementations. — end note]

>    > -61- On most machines, this is `false` for floating point types, `true` for unsigned integers, and `true` for signed integers.

>    > -62- Meaningful for all specializations.

[2016-05-21 Melissa Mears comments and provides improved wording]

GCC and Clang have `-fwrapv` and MSVC (as of a May 2016 preview build) has `/d2UndefIntOverflow-` as compiler command line flags to define signed integer overflow as wrapping. Such implementations can't set `is_modulo` to `true` for signed integer types when these options are enabled, because if translation units using opposite settings were linked together, the One Definition Rule would be violated.

Previous resolution [SUPERSEDED]:

> 1. Edit 17.3.5.2 [numeric.limits.members] around p60 as indicated:

>    >     static constexpr bool is_modulo;

>    > -60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving `+`, `-`, or `*` on values of that type whose result would fall outside the range `[min(), max()]`, the value returned differs from the true value by an integer multiple of `max() - min() + 1`.

>    > -??- [Note: `is_modulo` is `true` for unsigned integer types (6.9.2 [basic.fundamental]). — end note]

>    > -??- [Note: `is_modulo` is `false` for signed integer types (6.9.2 [basic.fundamental]) unless an implementation, as an extension to this International Standard, defines signed integer overflow to wrap as specified by the reference mentioned in footnote 217. — end note]

>    > -??- [Note: `is_modulo` is `false` for floating point types on most machines. — end note]

>    > -61- On most machines, this is `false` for floating types, `true` for unsigned integers, and `true` for signed integers.

>    > -62- Meaningful for all specializations.

[2016-06, Oulu]

We believe that the notes about unsigned integer types and floating point types are already evident from what the standard describes and should be removed. Furthermore the suggested note for signed integer types really shouldn't refer to a footnote in the own document, because footnotes have no stable identifiers. The below given revised resolution has been changed accordingly.

[2016-06 Oulu]

Added rationalization for changes. Moved to Ready.

Friday: status to Immediate

Proposed resolution:

1. Edit 17.3.5.2 [numeric.limits.members] around p60 as indicated:

   >     static constexpr bool is_modulo;

   > -60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving `+`, `-`, or `*` on values of that type whose result would fall outside the range `[min(), max()]`, the value returned differs from the true value by an integer multiple of `max() - min() + 1`.

   > -??- [Example: `is_modulo` is `false` for signed integer types (6.9.2 [basic.fundamental]) unless an implementation, as an extension to this International Standard, defines signed integer overflow to wrap. — end example]

   > -61- On most machines, this is `false` for floating types, `true` for unsigned integers, and `true` for signed integers.

   > -62- Meaningful for all specializations.

### 2424(i). 29.5 should state that atomic types are not trivially copyable

Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Jens Maurer Opened: 2014-08-14 Last modified: 2017-03-12

Priority: 2

View all other issues in [atomics.types.generic].

View all issues with Resolved status.

Discussion:

Otherwise, one could use `memcpy` to save and restore the value according to 3.9p2.

It seems the core language rules in 11 [class]p6 with [class.copy]p12 (trivial copy constructor) etc. and 9.6.2 [dcl.fct.def.default]p5 (user-provided) say that the atomic types are trivially copyable, which is bad. We shouldn't rely on future core changes in that area and simply say in the library section 32.5.8 [atomics.types.generic] that these very special types are not trivially copyable.

[2014-11 Urbana]

Lawrence:Definition of "trivially copyable" has been changing.

Doesn't hurt to add proposed change, even if the sentence is redundant

Move to Review.

[2015-02 Cologne]

GR has a minor problem with the style of the wording. VV has major issues with implementability.

[2015-03-22, Jens Maurer responses to Cologne discussion]

A library implementation could provide a partial specialization for `is_trivially_copyable<atomic<T>>`, to ensure that any such type query would return `false`.

Assuming such a specialization would be provided, how could a conforming program observe that per language rules an `atomic` specialization would actually be trivially copyable if there is no way to call the (deleted) copy constructor or copy assignment operator?

The sole effect of the suggested addition of the constraining sentence is that it would make a user program non-conforming that attempts to invoke `memcpy` (and the like) on `atomic` types, since that would invoke undefined behaviour.

[2015-05 Lenexa, SG1 response]

SG1 is fine with P/R (and agrees it's needed), but LWG may want to check the details; it's not entirely an SG1 issue.

[2015-05-05 Lenexa]

Marshall: This was discussed on the telecon. Alisdair was going to write something to Mike and send it to Core.

Hwrd: Core says that deleted copies are trivially copyable, which makes no sense to Library people.

STL: There doesn't appear to be a Core issue about it.

[2015-09-11 Telecon]

Howard: currently `std::is_trivially_copyable<std::atomic>` is true, so this resolution would contradict reality

Jonathan: changing that is good, we don't want it to be trivially copyable, otherwise users can memcpy them, which we really don't want

Howard: is it reasonable to memcpy something that isn't trivially copy constructible or trivially assignable?

Jonathan: no, it's not, but Core says you can, so this resolution is needed to stop people memcpying atomic

Howard: we should fix the core rule

Marshall: there is a separate issue of whether trivially-copyable makes sense, but this resolution is a net good purely because it stops memcpy of atomics

Howard: so should implementations specialize `is_trivially_copyable` the trait to meet this?

Jonathan: or add an empty, user-defined destructor.

Howard: should the spec specify that then?

Over-specification.

Howard: without that I fear implementation divergence.

Ville and Jonathan to investigate potential implementation options.

Ville: request a note on the issue saying we need review other types such as `condition_variable` to see if they are also unintentionally trivially-copyable. N4460 mentions some such types.

[2016-03 Jacksonville]

We think there is something coming from Core to resolve that, and that this will be NAD.
Until then, defer.

[2016-03 Jacksonville]

This was resolved by Core Issue 1496

[2017-01-19 Jens Maurer comments, issue state to Review]

The previous entry "[2016-03 Jacksonville] This was resolved by Core Issue 1496" is wrong; Core issue 1496 only modified the definition of "trivial class", not of "trivially copyable". However, as Ville Voutilainen observed, Core Issue 1734 made all atomics not trivially copyable, because they do not have at least one non-deleted copy/move constructor or copy/move assignment operator.

Previous resolution [SUPERSEDED]:

> 1. Change 32.5.8 [atomics.types.generic]p3 as indicated:

>    > Specializations and instantiations of the `atomic` template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexpr value constructor. They are not trivially copyable types (6.9 [basic.types]).

[2017-01-27 Telecon]

Resolved as NAD.

[2017-02-02 Daniel comments and adjusts status]

The NAD resolution is inappropriate, because the group didn't argue against the actual issue, instead the situation was that core wording changes in an unrelated area had resolved the previous problem indirectly. In this cases the correct resolution is Resolved by core wording changes as described by Jens Maurer in the 2017-01-19 comment.

Proposed resolution:

### 2425(i). `operator delete(void*, size_t)` doesn't invalidate pointers sufficiently

Section: 17.6.3 [new.delete] Status: C++17 Submitter: Richard Smith Opened: 2014-08-29 Last modified: 2017-07-30

Priority: 0

View all other issues in [new.delete].

View all issues with C++17 status.

Discussion:

17.6.3.2 [new.delete.single]/12 says:

> Requires: `ptr` shall be a null pointer or its value shall be a value returned by an earlier call to the (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t,const std::nothrow_t&)` which has not been invalidated by an intervening call to `operator delete(void*)`.

This should say:

> […] by an intervening call to `operator delete(void*)` or `operator delete(void*, std::size_t)`.

Likewise at the end of 17.6.3.3 [new.delete.array]/11, `operator delete[](void*, std::size_t)`.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

1. Change 17.6.3.2 [new.delete.single]p12 as indicated:

   > -12- Requires: `ptr` shall be a null pointer or its value shall be a value returned by an earlier call to the (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t,const std::nothrow_t&)` which has not been invalidated by an intervening call to `operator delete(void*)` or `operator delete(void*, std::size_t)`.

2. Change 17.6.3.3 [new.delete.array]p11 as indicated:

   > -11- Requires: `ptr` shall be a null pointer or its value shall be the value returned by an earlier call to `operator new[](std::size_t)` or `operator new[](std::size_t,const std::nothrow_t&)` which has not been invalidated by an intervening call to `operator delete[](void*)` or `operator delete[](void*, std::size_t)`.

### 2426(i). Issue about `compare_exchange`

Section: 32.5.8.2 [atomics.types.operations] Status: C++17 Submitter: Hans Boehm Opened: 2014-08-25 Last modified: 2017-07-30

Priority: 1

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Discussion:

The standard is either ambiguous or misleading about the nature of accesses through the `expected` argument to the `compare_exchange_*` functions in 99 [atomics.types.operations.req]p21.

It is unclear whether the access to `expected` is itself atomic (intent clearly no) and exactly when the implementation is allowed to read or write it. These affect the correctness of reasonable code.

Herb Sutter, summarizing a complaint from Duncan Forster wrote:

> Thanks Duncan,

> I think we have a bug in the standardese wording and the implementations are legal, but let's check with the designers of the feature.

> Let me try to summarize the issue as I understand it:

> 1. What I think was intended: Lawrence, I believe you championed having `compare_exchange_*` take the `expected` value by reference, and update `expected` on failure to expose the old value, but this was only for convenience to simplify the calling loops which would otherwise always have to write an extra "reload" line of code. Lawrence, did I summarize your intent correctly?

> 2. What I think Duncan is trying to do: However, it turns out that, now that `expected` is an lvalue, it has misled(?) Duncan into trying to use the success of `compare_exchange_*` to hand off ownership of `expected` itself to another thread. For that to be safe, if the `compare_exchange_*` succeeds then the thread that performed it must no longer read or write from `expected` else his technique contains a race. Duncan, did I summarize your usage correctly? Is that the only use that is broken?

> 3. What the standard says: I can see why Duncan thinks the standard supports his use, but I don't think this was intended (I don't remember this being discussed but I may have been away for that part) and unless you tell me this was intended I think it's a defect in the standard. From 99 [atomics.types.operations.req]/21:

>    > -21- Effects: Atomically, compares the contents of the memory pointed to by `object` or by `this` for equality with that in `expected`, and if true, replaces the contents of the memory pointed to by `object` or by `this` with that in `desired`, and if false, updates the contents of the memory in `expected` with the contents of the memory pointed to by `object` or by `this`. […]

>    I think we have a wording defect here in any case, because the "atomically" should not apply to the entire sentence — I'm pretty sure we never intended the atomicity to cover the write to `expected`.

>    As a case in point, borrowing from Duncan's mail below, I think the following implementation is intended to be legal:

>    >     inline int _Compare_exchange_seq_cst_4(volatile _Uint4_t *_Tgt, _Uint4_t *_Exp, _Uint4_t _Value)
>    >     { /* compare and exchange values atomically with
>    >          sequentially consistent memory order */
>    >       int _Res;
>    >       _Uint4_t _Prev = _InterlockedCompareExchange((volatile long *)_Tgt, _Value, *_Exp);
>    >       if (_Prev == *_Exp) //!!!!! Note the unconditional read from *_Exp here
>    >         _Res = 1;
>    >       else
>    >       { /* copy old value */
>    >         _Res = 0;
>    >         *_Exp = _Prev;
>    >       }
>    >       return (_Res);
>    >     }

>    I think this implementation is intended to be valid — I think the only code that could be broken with the "!!!!!" read of `*_Exp` is Duncan's use of treating `a.compare_exchange_*(expected, desired) == true` as implying `expected` got handed off, because then another thread could validly be using `*_Exp` — but we never intended this use, right?

In a different thread Richard Smith wrote about the same problem:

> The `atomic_compare_exchange` functions are described as follows:

> > "Atomically, compares the contents of the memory pointed to by `object` or by `this` for equality with that in `expected`, and if true, replaces the contents of the memory pointed to by `object` or by `this` with that in `desired`, and if false, updates the contents of the memory in `expected` with the contents of the memory pointed to by `object` or by `this`. Further, if the comparison is true, memory is affected according to the value of `success`, and if the comparison is false, memory is affected according to the value of `failure`."

> I think this is less clear than it could be about the effects of these operations on `*expected` in the failure case:

> 1. We have "Atomically, compares […] and updates the contents of the memory in `expected` […]". The update to the memory in `expected` is clearly not atomic, and yet this wording parallels the success case, in which the memory update is atomic.

> 2. The wording suggests that memory (including `*expected`) is affected according to the value of `failure`. In particular, the failure order could be `memory_order_seq_cst`, which might lead someone to incorrectly think they'd published the value of `*expected`.

> I think this can be clarified with no change in meaning by reordering the wording a little:

> > "Atomically, compares the contents of the memory pointed to by `object` or by `this` for equality with that in `expected`, and if true, replaces the contents of the memory pointed to by `object` or by `this` with that in `desired`, and if. If the comparison is true, memory is affected according to the value of `success`, and if the comparison is false, memory is affected according to the value of `failure`. Further, if the comparison is false, updatesreplaces the contents of the memory in `expected` with the contents ofvalue that was atomically read from the memory pointed to by `object` or by `this`. Further, if the comparison is true, memory is affected according to the value of `success`, and if the comparison is false, memory is affected according to the value of `failure`."

Jens Maurer add:

> I believe this is an improvement.

> I like to see the following additional improvements:

> - "contents of the memory" is strange phrasing, which doesn't say how large the memory block is. Do we compare the values or the value representation of the lvalue `*object` (or `*this`)?

> - 32.5.4 [atomics.order] defines memory order based on the "affected memory location". It would be better to say something like "If the comparison is true, the memory synchronization order for the affected memory location `*object` is […]"

There was also a discussion thread involving Herb Sutter, Hans Boehm, and Lawrence Crowl, resulting in proposed wording along the lines of:

> > -21- Effects: Atomically with respect to `expected` and the memory pointed to by `object` or by `this`, compares the contents of the memory pointed to by `object` or by `this` for equality with that in `expected`, and if and only if true, replaces the contents of the memory pointed to by `object` or by `this` with that in desired, and if and only if false, updates the contents of the memory in expected with the contents of the memory pointed to by `object` or by `this`.

> At the end of paragraph 23, perhaps add

> > [Example: Because the `expected` value is updated only on failure, code releasing the memory containing the `expected` value on success will work. E.g. list head insertion will act atomically and not have a data race in the following code.

> > >     do {
> > >       p->next = head; // make new list node point to the current head
> > >     } while(!head.compare_exchange_weak(p->next, p)); // try to insert

> > — end example]

Hans objected that this still gives the misimpression that the update to `expected` is atomic.

[2014-11 Urbana]

Proposed resolution was added after Redmond.

Recommendations from SG1:

1. Change wording to if trueif and only if true, and change if falseif and only if false.
2. If they want to add "respect to" clause, say "respect to object or this".
3. In example, load from head should be "head.load(memory_order_relaxed)", because people are going to use example as example of good code.

(wording edits not yet applied)

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

We believed we were done with it, but it was kicked back to us, with the wording we suggested not yet applied. It may have been that our suggestions were unclear. Was that the concern?

[2016-02 Jacksonville]

Applied the other half of the "if and only if" response from SG1, and moved to Ready.

Proposed resolution:

1. Edit 99 [atomics.types.operations.req] p21 as indicated:

   > -21- Effects: Retrieves the value in `expected`. It then aAtomically, compares the contents of the memory pointed to by `object` or by `this` for equality with that inpreviously retrieved from `expected`, and if true, replaces the contents of the memory pointed to by `object` or by `this` with that in `desired`, and if false, updates the contents of the memory in expected with the contents of the memory pointed to by `object` or by `this`. Further, if. If and only if the comparison is true, memory is affected according to the value of `success`, and if the comparison is false, memory is affected according to the value of `failure`. When only one `memory_order` argument is supplied, the value of `success` is `order`, and the value of `failure` is `order` except that a value of `memory_order_acq_rel` shall be replaced by the value `memory_order_acquire` and a value of `memory_order_release` shall be replaced by the value `memory_order_relaxed`. If and only if the comparison is false then, after the atomic operation, the contents of the memory in `expected` are replaced by the value read from `object` or by `this` during the atomic comparison. If the operation returns true, these operations are atomic read-modify-write operations (1.10) on the memory pointed to by `this` or `object`. Otherwise, these operations are atomic load operations on that memory.

2. Add the following example to the end of 99 [atomics.types.operations.req] p23:

   > -23- [Note: […] — end note] [Example: […] — end example]

   > [Example: Because the expected value is updated only on failure, code releasing the memory containing the `expected` value on success will work. E.g. list head insertion will act atomically and would not introduce a data race in the following code:

   > >     do {
   > >       p->next = head; // make new list node point to the current head
   > >     } while(!head.compare_exchange_weak(p->next, p)); // try to insert

   > — end example]

### 2427(i). Container adaptors as sequence containers, redux

Section: 23.3.1 [sequences.general] Status: C++17 Submitter: Tim Song Opened: 2014-08-29 Last modified: 2017-07-30

Priority: 0

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Discussion:

LWG 2194(i) removed "These container adaptors meet the requirements for sequence containers." from 23.6.1 [container.adaptors.general].

However, N3936 23.3.1 [sequences.general]/p2 still says "The headers `<queue>` and `<stack>` define container adaptors (23.6) that also meet the requirements for sequence containers." I assume this is just an oversight.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

1. Delete paragraph 2 of 23.3.1 [sequences.general] as indicated:

   > -2- The headers `<queue>` and `<stack>` define container adaptors (23.6) that also meet the requirements for sequence containers.

### 2428(i). "External declaration" used without being defined

Section: 16.4.3.2 [using.headers] Status: C++17 Submitter: Tim Song Opened: 2014-09-03 Last modified: 2025-10-15

Priority: 0

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Discussion:

16.4.3.2 [using.headers]/3 says

> A translation unit shall include a header only outside of any external declaration or definition […]

This wording appears to be borrowed from the C standard. However, the term "external declaration" is not defined in the C++ standard, and in fact is only used here as far as I can tell, so it is unclear what it means. The C standard does define external declarations as (WG14 N1570 6.9 External definitions/4-5):

> As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit, which consists of a sequence of external declarations. These are described as "external" because they appear outside any function (and hence have file scope). [...] An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object.

The corresponding description of a translation unit in C++ is "A translation unit consists of a sequence of declarations." (6.7 [basic.link]/3).

So it appears that the C++ counterpart of "external declaration" in C is simply a "declaration" at file scope. There is no need to specifically limit the statement in 16.4.3.2 [using.headers]/3 to file-scope declarations, however, since every non-file-scope declaration is necessarily inside a file-scope declaration, so banning including a header inside file-scope declarations necessarily bans including one inside non-file-scope declarations as well.

[Urbana 2014-11-07: Move to Ready]

[2025-10-15; related to LWG 657(i)]

Proposed resolution:

This wording is relative to N3936.

1. Edit 16.4.3.2 [using.headers] as indicated:

   > A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header. No diagnostic is required.

### 2433(i). `uninitialized_copy()`/etc. should tolerate overloaded `operator&`

Section: 26.11 [specialized.algorithms] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 0

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Discussion:

This restriction isn't necessary anymore. In fact, this is the section that defines `addressof()`.

(Editorial note: We can depict these algorithms as calling `addressof()` instead of `std::addressof()` thanks to 16.4.2.2 [contents]/3 "Whenever a name `x` defined in the standard library is mentioned, the name `x` is assumed to be fully qualified as `::std::x`, unless explicitly described otherwise.")

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 26.11 [specialized.algorithms] p1 as depicted:

   > -1- All the iterators that are used as formal template parameters in the following algorithms are required to have their `operator*` return an object for which `operator&` is defined and returns a pointer to `T`. In the algorithm `uninitialized_copy`, the formal template parameter `InputIterator` is required to satisfy the requirements of an input iterator (24.2.3). In all of the following algorithms, the formal template parameter `ForwardIterator` is required to satisfy the requirements of a forward iterator (24.2.5), and is required to have the property that no exceptions are thrown from increment, assignment, comparison, or indirection through valid iterators. In the following algorithms, if an exception is thrown there are no effects.

2. Change 26.11.5 [uninitialized.copy] p1 as depicted:

   > -1- Effects:

   > >     for (; first != last; ++result, ++first)
   > >       ::new (static_cast<void*>(addressof(&*result)))
   > >         typename iterator_traits<ForwardIterator>::value_type(*first);

3. Change 26.11.5 [uninitialized.copy] p3 as depicted:

   > -3- Effects:

   > >     for (; n > 0; ++result, ++first, --n) {
   > >       ::new (static_cast<void*>(addressof(&*result)))
   > >         typename iterator_traits<ForwardIterator>::value_type(*first);
   > >     }

4. Change 26.11.7 [uninitialized.fill] p1 as depicted:

   > -1- Effects:

   > >     for (; first != last; ++first)
   > >       ::new (static_cast<void*>(addressof(&*first)))
   > >         typename iterator_traits<ForwardIterator>::value_type(x);

5. Change [uninitialized.fill.n] p1 as depicted:

   > -1- Effects:

   > >     for (; n--; ++first)
   > >       ::new (static_cast<void*>(addressof(&*first)))
   > >         typename iterator_traits<ForwardIterator>::value_type(x);
   > >     return first;

### 2434(i). `shared_ptr::use_count()` is efficient

Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 0

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Discussion:

`shared_ptr` and `weak_ptr` have Notes that their `use_count()` might be inefficient. This is an attempt to acknowledge reflinked implementations (which can be used by Loki smart pointers, for example). However, there aren't any `shared_ptr` implementations that use reflinking, especially after C++11 recognized the existence of multithreading. Everyone uses atomic refcounts, so `use_count()` is just an atomic load.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 20.3.2.2.6 [util.smartptr.shared.obs] p7-p10 as depicted:

   >     long use_count() const noexcept;

   > > -7- Returns: the number of `shared_ptr` objects, `*this` included, that share ownership with `*this`, or 0 when `*this` is empty.

   > > -8- [Note: `use_count()` is not necessarily efficient. — end note]

   >     bool unique() const noexcept;

   > > -9- Returns: `use_count() == 1`.

   > > -10- [Note: `unique()` may be faster than `use_count()`. If you are using `unique()` to implement copy on write, do not rely on a specific value when `get() == 0`. — end note]

2. Change 20.3.2.3.6 [util.smartptr.weak.obs] p1-p4 as depicted:

   >     long use_count() const noexcept;

   > > -1- Returns: 0 if `*this` is empty; otherwise, the number of `shared_ptr` instances that share ownership with `*this`.

   > > -2- [Note: `use_count()` is not necessarily efficient. — end note]

   >     bool expired() const noexcept;

   > > -3- Returns: `use_count() == 0`.

   > > -4- [Note: `expired()` may be faster than `use_count()`. — end note]

### 2435(i). `reference_wrapper::operator()`'s Remark should be deleted

Section: 22.10.6.5 [refwrap.invoke] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 4

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Discussion:

22.10.6.5 [refwrap.invoke]/2 is no longer useful. (It was originally TR1 2.1.2.4 [tr.util.refwrp.invoke]/2.) First, we already have the As If Rule (6.10.1 [intro.execution]/1) and the STL Implementers Can Be Sneaky Rule (16.4.6.5 [member.functions]). Second, with variadic templates and other C++11/14 tech, this can be implemented exactly as depicted.

[2015-05, Lenexa]

DK: I don't see a defect here
STL: the issue is that the standard is overly verbose, we don't need this sentence. It's redundant.
MC: does anyone think this paragraph has value?
JW: it has negative value. reading it makes me wonder if there's some reason I would want to provide a set of overloaded functions, maybe there's some problem with doing it the obvious way that I'm not clever enough to see.
Move to Ready status: 8 in favor, none against.

Proposed resolution:

This wording is relative to N3936.

1. Change 22.10.6.5 [refwrap.invoke] p2 as depicted:

   >     template <class... ArgTypes>
   >       result_of_t<T&(ArgTypes&&...)>
   >         operator()(ArgTypes&&... args) const;

   > > -1- Returns: `INVOKE(get(), std::forward<ArgTypes>(args)...)`. (20.9.2)

   > > -2- Remark: `operator()` is described for exposition only. Implementations are not required to provide an actual `reference_wrapper::operator()`. Implementations are permitted to support `reference_wrapper` function invocation through multiple overloaded operators or through other means.

### 2436(i). Comparators for associative containers should always be `CopyConstructible`

Section: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2018-06-23

Priority: 2

View other active issues in [associative.reqmts].

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Discussion:

The associative container requirements attempt to permit comparators that are `DefaultConstructible` but non-`CopyConstructible`. However, the Standard contradicts itself. 23.4.3.1 [map.overview] depicts `map() : map(Compare()) { }` which requires both `DefaultConstructible` and `CopyConstructible`.

Unlike fine-grained element requirements (which are burdensome for implementers, but valuable for users), such fine-grained comparator requirements are both burdensome for implementers (as the Standard's self-contradiction demonstrates) and worthless for users. We should unconditionally require `CopyConstructible` comparators. (Note that `DefaultConstructible` should remain optional; this is not problematic for implementers, and allows users to use lambdas.)

Key equality predicates for unordered associative containers are also affected. However, 16.4.4.5 [hash.requirements]/1 already requires hashers to be `CopyConstructible`, so 23.2.8 [unord.req]'s redundant wording should be removed.

[2015-02, Cologne]

GR: I prefer to say "`Compare`" rather than "`X::key_compare`", since the former is what the user supplies. JY: It makes sense to use "`Compare`" when we talk about requirements but "`key_compare`" when we use it.

AM: We're adding requirements here, which is a breaking change, even though nobody will ever have had a non-`CopyConstructible` comparator. But the simplification is probably worth it.

GR: I don't care about unmovable containers. But I do worry that people might want to move they comparators. MC: How do you reconcile that with the function that says "give me the comparator"? GR: That one returns by value? JY: Yes. [To MC] You make it a requirement of that function. [To GR] And it [the `key_comp()` function] is missing its requirements. We need to add them everywhere. GR: map already has the right requirements.

JM: I dispute this. If in C++98 a type wasn't copyable, it had some interesting internal state, but in C++98 you wouldn't have been able to pass it into the container since you would have had to make a copy. JY: No, you could have default-constructed it and never moved it, e.g. a mutex. AM: So, it's a design change, but one that we should make. That's probably an LEWG issue. AM: There's a contradiction in the Standard here, and we need to fix it one way or another.

Conclusion: Move to LEWG

[LEWG: 2016-03, Jacksonville]

Adding `CopyConstructible` requirement OK.

Unanimous yes.

We discussed allowing `MoveConstructible`. A moved-from `set<>` might still contain elements, and using them would become undefined if the comparator changed behavior.

Proposed resolution:

This wording is relative to N3936.

1. Change 23.2.7 [associative.reqmts] Table 102 as indicated (Editorial note: For "expression" `X::key_compare` "defaults to" is redundant with the class definitions for map/etc.):

   > Table 102 — Associative container requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `X::key_compare` | `Compare` | defaults to `less<key_type>` Requires: `key_compare` is `CopyConstructible`. | compile time
   > `…`
   > `X(c) X a(c);` |  | Requires: `key_compare` is `CopyConstructible`. Effects: Constructs an empty container. Uses a copy of `c` as a comparison object. | constant
   > `…`
   > `X(i,j,c) X a(i,j,c);` |  | Requires: `key_compare` is `CopyConstructible`. `value_type` is `EmplaceConstructible` into `X` from `*i`. Effects: Constructs […] | […]
   > `…`

2. Change 23.2.8 [unord.req] Table 103 as indicated:

   > Table 103 — Unordered associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `X::key_equal` | `Pred` | Requires: `Pred` is `CopyConstructible`. `Pred` shall be a binary predicate that takes two arguments of type `Key`. `Pred` is an equivalence relation. | compile time
   > `…`
   > `X(n, hf, eq) X a(n, hf, eq);` | `X` | Requires: `hasher` and `key_equal` are `CopyConstructible`. Effects: Constructs […] | […]
   > `…`
   > `X(n, hf) X a(n, hf);` | `X` | Requires: `hasher` is `CopyConstructible` and `key_equal` is `DefaultConstructible` Effects: Constructs […] | […]
   > `…`
   > `X(i, j, n, hf, eq) X a(i, j, n, hf, eq);` | `X` | Requires: `hasher` and `key_equal` are `CopyConstructible`. `value_type` is `EmplaceConstructible` into `X` from `*i`. Effects: Constructs […] | […]
   > `…`
   > `X(i, j, n, hf) X a(i, j, n, hf);` | `X` | Requires: `hasher` is `CopyConstructible` and `key_equal` is `DefaultConstructible` `value_type` is `EmplaceConstructible` into `X` from `*i`. Effects: Constructs […] | […]
   > `…`

### 2437(i). `iterator_traits<OutIt>::reference` can and can't be `void`

Section: 24.3.5.2 [iterator.iterators] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 3

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Discussion:

24.3.5.2 [iterator.iterators]/2 requires an `Iterator`'s `*r` to return `reference`, i.e. `iterator_traits<X>::reference` according to 24.3.1 [iterator.requirements.general]/11.

24.3.5.4 [output.iterators]/1 requires an `OutputIterator`'s `*r = o` to do its job, so `*r` clearly can't return `void`.

24.3.2.3 [iterator.traits]/1 says: "In the case of an output iterator, the types

>     iterator_traits<Iterator>::difference_type
>     iterator_traits<Iterator>::value_type
>     iterator_traits<Iterator>::reference
>     iterator_traits<Iterator>::pointer

may be defined as `void`."

This is contradictory. I suggest fixing this by moving the offending requirement down from `Iterator` to `InputIterator`, and making `Iterator` say that `*r` returns an unspecified type. This will have the following effects:

- Output-only iterators will inherit Iterator's "`*r` returns unspecified" requirement, while 24.3.2.3 [iterator.traits]/1 clearly permits reference/etc. to be `void`.

- Input-or-stronger iterators (whether constant or mutable) are unaffected — they still have to satisfy "`*r` returns reference", they're just getting that requirement from `InputIterator` instead of `Iterator`.

[2015-02 Cologne]

EF: This is related to 2438(i). MC: I'd like to take up 2438 right after this.

AM: Does anyone think this is wrong?

GR: Why do we give output iterators to have reference type void? AM: we've mandated that certain output iterators define it as void since 1998. GR: Oh OK, I'm satisfied.

Accepted. And 2438(i) is already Ready.

Proposed resolution:

This wording is relative to N3936.

1. In 24.3.5.2 [iterator.iterators] Table 106 "Iterator requirements" change as indicated:

   > Table 106 — Iterator requirements
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
   > --- | --- | --- | ---
   > `*r` | `reference` unspecified | `` | pre: `r` is dereferenceable.
   > `…`

2. In 24.3.5.3 [input.iterators] Table 107 "Input iterator requirements" change as indicated:

   > Table 107 — Input iterator requirements (in addition to Iterator)
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
   > --- | --- | --- | ---
   > `…`
   > `*a` | `reference`, convertible to `T` | `` | […]
   > `…`

### 2438(i). `std::iterator` inheritance shouldn't be mandated

Section: D.18 [depr.iterator] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2023-02-07

Priority: 3

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Discussion:

For LWG convenience, nine STL iterators are depicted as deriving from `std::iterator` to get their `iterator_category`/etc. typedefs. Unfortunately (and unintentionally), this also mandates the inheritance, which is observable (not just through `is_base_of`, but also overload resolution). This is unfortunate because it confuses users, who can be misled into thinking that their own iterators must derive from `std::iterator`, or that overloading functions to take `std::iterator` is somehow meaningful. This is also unintentional because the STL's most important iterators, the container iterators, aren't required to derive from `std::iterator`. (Some are even allowed to be raw pointers.) Finally, this unnecessarily constrains implementers, who may not want to derive from `std::iterator`. (For example, to simplify debugger views.)

We could add wording to 99 [iterator.basic] saying that any depicted inheritance is for exposition only, but that wouldn't really solve reader confusion. Replacing the depicted inheritance with direct typedefs will prevent confusion. Note that implementers won't be required to change their code — they are free to continue deriving from `std::iterator` if they want.

(Editorial note: The order of the typedefs follows the order of `std::iterator`'s template parameters.)

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change [storage.iterator], class template `raw_storage_iterator` synopsis, as depicted:

   >     template <class OutputIterator, class T>
   >     class raw_storage_iterator
   >       : public iterator<output_iterator_tag,void,void,void,void> {
   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;

   >       explicit raw_storage_iterator(OutputIterator x);
   >       […]
   >     };

2. Change 24.5.1.2 [reverse.iterator], class template `reverse_iterator` synopsis, as depicted (editorial note: this reorders "`reference`, `pointer`" to "`pointer`, `reference`" and aligns whitespace):

   >     template <class Iterator>
   >     class reverse_iterator : public
   >       iterator<typename iterator_traits<Iterator>::iterator_category,
   >       typename iterator_traits<Iterator>::value_type,
   >       typename iterator_traits<Iterator>::difference_type,
   >       typename iterator_traits<Iterator>::pointer,
   >       typename iterator_traits<Iterator>::reference> {
   >     public:
   >       typedef Iterator iterator_type;
   >       typedef typename iterator_traits<Iterator>::difference_type difference_type;
   >       typedef typename iterator_traits<Iterator>::reference reference;
   >       typedef typename iterator_traits<Iterator>::pointer pointer;
   >       typedef Iterator                                              iterator_type;
   >       typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
   >       typedef typename iterator_traits<Iterator>::value_type        value_type;
   >       typedef typename iterator_traits<Iterator>::difference_type   difference_type;
   >       typedef typename iterator_traits<Iterator>::pointer           pointer;
   >       typedef typename iterator_traits<Iterator>::reference         reference;

   >       reverse_iterator();
   >       […]
   >     };

3. Change 24.5.2.2 [back.insert.iterator], class template `back_insert_iterator` synopsis, as depicted:

   >     template <class Container>
   >     class back_insert_iterator :
   >       public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >       Container* container;

   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       typedef Container container_type;
   >       explicit back_insert_iterator(Container& x);
   >       […]
   >     };

4. Change 24.5.2.3 [front.insert.iterator], class template `front_insert_iterator` synopsis, as depicted:

   >     template <class Container>
   >     class front_insert_iterator :
   >       public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >       Container* container;

   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       typedef Container container_type;
   >       explicit front_insert_iterator(Container& x);
   >       […]
   >     };

5. Change 24.5.2.4 [insert.iterator], class template `insert_iterator` synopsis, as depicted:

   >     template <class Container>
   >     class insert_iterator :
   >       public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >       Container* container;
   >       typename Container::iterator iter;

   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       typedef Container container_type;
   >       insert_iterator(Container& x, typename Container::iterator i);
   >       […]
   >     };

6. Change 24.6.2 [istream.iterator], class template `istream_iterator` synopsis, as depicted:

   >     template <class T, class charT = char, class traits = char_traits<charT>,
   >       class Distance = ptrdiff_t>
   >     class istream_iterator :
   >       public iterator<input_iterator_tag, T, Distance, const T*, const T&> {
   >     public:
   >       typedef input_iterator_tag iterator_category;
   >       typedef T value_type;
   >       typedef Distance difference_type;
   >       typedef const T* pointer;
   >       typedef const T& reference;
   >       […]
   >     };

7. Change 24.6.3 [ostream.iterator], class template `ostream_iterator` synopsis, as depicted:

   >     template <class T, class charT = char, class traits = char_traits<charT>>
   >     class ostream_iterator :
   >       public iterator<output_iterator_tag, void, void, void, void> {
   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       […]
   >     };

8. Change 24.6.4 [istreambuf.iterator], class template `istreambuf_iterator` synopsis, as depicted:

   >     template <class charT = char, class traits = char_traits<charT> >
   >     class istreambuf_iterator :
   >       public iterator<input_iterator_tag, charT,
   >                       typename traits::off_type, unspecified, charT> {
   >     public:
   >       typedef input_iterator_tag iterator_category;
   >       typedef charT value_type;
   >       typedef typename traits::off_type difference_type;
   >       typedef unspecified pointer;
   >       typedef charT reference;
   >       […]
   >     };

9. Change 24.6.5 [ostreambuf.iterator], class template `ostreambuf_iterator` synopsis, as depicted (editorial note: this removes a redundant "public:"):

   >     template <class charT = char, class traits = char_traits<charT>>
   >     class ostreambuf_iterator :
   >       public iterator<output_iterator_tag, void, void, void, void> {
   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       […]
   >     public:
   >       […]
   >     };

### 2439(i). `unique_copy()` sometimes can't fall back to reading its output

Section: 26.7.9 [alg.unique] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 3

View all other issues in [alg.unique].

View all issues with C++17 status.

Discussion:

`unique_copy()`'s wording says that if it's given input-only and output-only iterators, it needs the input's value type to be copyable. This is correct, because in this case the algorithm must have a local element copy in order to detect duplicates.

The wording also says that if it's given an `InputIterator` that's forward or stronger, the input's value type doesn't have to be copyable. This is also correct, because in this case the algorithm can reread the input in order to detect duplicates.

Finally, the wording says that if it's given an input-only iterator with an `OutputIterator` that's forward or stronger, the input's value type doesn't have to be copyable. This is telling the algorithm to compare its input to its output in order to detect duplicates, but that isn't always possible! If the input and output have the same value type, then they can be compared (as long as `*result = *first` behaves sanely; see below). If they have different value types, then we can't compare them.

This could be resolved by requiring heterogeneous value types to be comparable in this situation, but that would be extremely tricky to wordsmith (as it would challenge the concept of "group of equal elements" used by the Effects). It will be vastly simpler and more effective to extend the "local element copy" requirement to this scenario.

Note that the input-only, output forward-or-stronger, identical value types scenario needs a bit of work too. We always require *result = *first to be "valid", but in this case we need to additionally require that the assignment actually transfers the value. (Otherwise, we'd be allowing an `op=()` that ignores `*first` and always sets `*result` to zero, or other unacceptable behavior.) This is just `CopyAssignable`.

(What happens when `unique_copy()` is given a `move_iterator` is a separate issue.)

To summarize:

> input forward+: no additional requirements

> input-only, output forward+, same value types: needs `CopyAssignable`

> input-only, output forward+, different value types: needs `CopyConstructible` and `CopyAssignable`

> input-only, output-only: needs `CopyConstructible` and `CopyAssignable`

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 26.7.9 [alg.unique] p5, as depicted:

   >     template<class InputIterator, class OutputIterator>
   >     OutputIterator
   >     unique_copy(InputIterator first, InputIterator last,
   >                 OutputIterator result);
   >     template<class InputIterator, class OutputIterator,
   >              class BinaryPredicate>
   >     OutputIterator
   >     unique_copy(InputIterator first, InputIterator last,
   >                 OutputIterator result, BinaryPredicate pred);

   > > -5- Requires: The comparison function shall be an equivalence relation. The ranges [`first,last`) and [`result,result+(last-first)`) shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type of `InputIterator` shall be `CopyConstructible` (Table 21) and `CopyAssignable` (Table 23). Otherwise `CopyConstructible` is not required.Let `T` be the value type of `InputIterator`. If `InputIterator` meets the forward iterator requirements, then there are no additional requirements for `T`. Otherwise, if `OutputIterator` meets the forward iterator requirements and its value type is the same as `T`, then `T` shall be `CopyAssignable` (Table 23). Otherwise, `T` shall be both `CopyConstructible` (Table 21) and `CopyAssignable`.

### 2440(i). `seed_seq::size()` should be `noexcept`

Section: 29.5.8.1 [rand.util.seedseq] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 0

View all other issues in [rand.util.seedseq].

View all issues with C++17 status.

Discussion:

Obvious.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 29.5.8.1 [rand.util.seedseq], class `seed_seq` synopsis, as depicted:

   >     class seed_seq
   >     {
   >     public:
   >       […]
   >       size_t size() const noexcept;
   >       […]
   >     };

2. Change 29.5.8.1 [rand.util.seedseq] around p10, as depicted:

   >     size_t size() const noexcept;

   > > -10- Returns: The number of 32-bit units that would be returned by a call to `param()`.

   > > -11- Throws: Nothing.

   > > -12- Complexity: Constant time.

### 2441(i). Exact-width atomic typedefs should be provided

Section: 32.5.8 [atomics.types.generic] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 0

View all other issues in [atomics.types.generic].

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Discussion:

`<atomic>` doesn't provide counterparts for `<inttypes.h>`'s most useful typedefs, possibly because they're quasi-optional. We can easily fix this.

[2014-11, Urbana]

Typedefs were transitional compatibility hack. Should use `_Atomic` macro or template. E.g. `_Atomic(int8_t)`. BUT `_Atomic` disappeared!

Detlef will look for `_Atomic` macro. If missing, will open issue.

[2014-11-25, Hans comments]

There is no `_Atomic` in C++. This is related to the much more general unanswered question of whether C++17 should reference C11, C99, or neither.

[2015-02 Cologne]

AM: I think this is still an SG1 issue; they need to deal with it before we do.

[2015-05 Lenexa, SG1 response]

Move to SG1-OK status. This seems like an easy short-term fix. We probably need a paper on C/C++ atomics compatibility to deal with `_Atomic`, but that's a separable issue.

[2015-10 pre-Kona]

SG1 hands this over to LWG for wording review

Proposed resolution:

This wording is relative to N3936.

1. Change 32.5.8 [atomics.types.generic] p8 as depicted:

   > -8- There shall be atomic typedefs corresponding to the typedefs in the header `<inttypes.h>` as specified in Table 147. `atomic_intN_t`, `atomic_uintN_t`, `atomic_intptr_t`, and `atomic_uintptr_t` shall be defined if and only if `intN_t`, `uintN_t`, `intptr_t`, and `uintptr_t` are defined, respectively.

2. Change 99 [atomics.types.operations.req], Table 147 ("`atomic` `<inttypes.h>` typedefs"), as depicted:

   > Table 147 — `atomic` `<inttypes.h>` typedefs
   > Atomic typedef | `<inttypes.h>` type
   > --- | ---
   > `atomic_int8_t` | `int8_t`
   > `atomic_uint8_t` | `uint8_t`
   > `atomic_int16_t` | `int16_t`
   > `atomic_uint16_t` | `uint16_t`
   > `atomic_int32_t` | `int32_t`
   > `atomic_uint32_t` | `uint32_t`
   > `atomic_int64_t` | `int64_t`
   > `atomic_uint64_t` | `uint64_t`
   > `…`

### 2442(i). `call_once()` shouldn't `DECAY_COPY()`

Section: 32.6.7.2 [thread.once.callonce] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [thread.once.callonce].

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Discussion:

When LWG 891(i) overhauled `call_once()`'s specification, it used `decay_copy()`, following LWG 929(i)'s overhaul of `thread`'s constructor.

In `thread`'s constructor, this is necessary and critically important. 32.4.3.3 [thread.thread.constr]/5 "The new thread of execution executes `INVOKE(DECAY_COPY(std::forward<F>(f)), DECAY_COPY(std::forward<Args>(args))...)` with the calls to `DECAY_COPY` being evaluated in the constructing thread." requires the parent thread to copy arguments for the child thread to access.

In `call_once()`, this is unnecessary and harmful. It's unnecessary because `call_once()` doesn't transfer arguments between threads. It's harmful because:

- `decay_copy()` returns a prvalue. Given `meow(int&)`, `meow(i)` can be called directly, but `call_once(flag, meow, i)` won't compile.

- `decay_copy()` moves from modifiable rvalues. Given `purr(const unique_ptr<int>&)`, `purr(move(up))` won't modify `up`. (This is observable, because moved-from `unique_ptr`s are guaranteed empty.) However, `call_once(flag, purr, move(up))` will leave `up` empty after the first active execution. Observe the behavioral difference — if `purr()` is directly called like this repeatedly until it doesn't throw an exception, each call will observe `up` unchanged. With `call_once()`, the second active execution will observe `up` to be empty.

`call_once()` should use perfect forwarding without `decay_copy()`, in order to avoid interfering with the call like this.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

Looks good to us, but this is really an LWG issue.

[2015-05-07 Lenexa: Move Immediate]

LWG 2442 call_once shouldn't decay_copy

STL summarizes the SG1 minutes.

Marshall: Jonathan updated all the issues with SG1 status last night. Except this one.

STL summarizes the issue.

Dietmar: Of course, call_once has become useless.

STL: With magic statics.

Jonathan: Magic statics can't be per object, which I use in future.

Marshall: I see why you are removing the MoveConstructible on the arguments, but what about Callable?

STL: That's a type named Callable, which we will no longer decay_copy. We're still requiring the INVOKE expression to be valid.

Marshall: Okay. Basically, ripping the decay_copy out of here.

STL: I recall searching the Standard for other occurrences and I believe this is the only inappropriate use of decay_copy.

Marshall: We do the decay_copy.

Jonathan: Us too.

Marshall: What do people think?

Jonathan: I think STL's right. In the use I was mentioning inside futures, I actually pass them by reference_wrapper and pointers, to avoid the decay causing problems. Inside the call_once, I then extract the args. So I've had to work around this and didn't realize it was a defect.

Marshall: What do people think is the right resolution?

STL: I would like to see Immediate.

Hwrd: No objections to Immediate.

Marshall: Bill is nodding.

PJP: He said it. Everything STL says applies to our other customers.

Marshall: Any objections to Immediate?

Jonathan: I can't see any funky implementations where a decay_copy would be necessary?

Marshall: 6 votes for Immediate, 0 opposed, 0 abstaining.

Proposed resolution:

This wording is relative to N3936.

1. Change 32.6.7.2 [thread.once.callonce] p1+p2 as depicted:

   >     template<class Callable, class ...Args>
   >       void call_once(once_flag& flag, Callable&& func, Args&&... args);

   > > -1- Requires: `Callable` and each `Ti` in `Args` shall satisfy the `MoveConstructible` requirements. `INVOKE(DECAY_COPY(std::forward<Callable>(func)), DECAY_COPY(std::forward<Args>(args))...)` (20.9.2) shall be a valid expression.

   > > -2- Effects; […] An active execution shall call `INVOKE(DECAY_COPY(std::forward<Callable>(func)), DECAY_COPY(std::forward<Args>(args))...)`. […]

### 2443(i). `std::array` member functions should be `constexpr`

Section: 23.3.3 [array] Status: Resolved Submitter: Peter Sommerlad Opened: 2014-10-06 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [array].

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Discussion:

When experimenting with C++14 relaxed `constexpr` functions I made the observation that I couldn't use `std::array` to create a table of data at compile time directly using loops in a function. However, a simple substitute I could use instead:

>     template <typename T, size_t n>
>     struct ar {
>       T a[n];
>       constexpr ar() : a{{}}{}
>       constexpr auto data() const { return &a[0];}
>       constexpr T const & operator[](size_t i) const { return a[i]; }
>       constexpr T & operator[](size_t i) { return a[i]; }
>     };

>     template <size_t n>
>     using arr = ar<size_t, n>; // std::array<size_t, n>;

>     template <size_t n>
>     constexpr auto make_tab(){
>       arr<n> result;
>       for(size_t i=0; i < n; ++i)
>         result[i] = (i+1)*(i+1); // cannot define operator[] for mutable array...
>       return result;
>     }

>     template <size_t n>
>     constexpr auto squares=make_tab< n>();

>     int main() {
>       int dummy[squares<5>[3]];
>     }

Therefore, I suggest that all member functions of `std::array` should be made `constexpr` to make the type usable in `constexpr` functions.

Wording should be straight forward, may be with the exception of `fill`, which would require `fill_n` to be `constexpr` as well.

[2014-11 Urbana]

Move to LEWG

The extent to which `constexpr` becomes a part of the Library design is a policy matter best handled initially by LEWG.

[08-2016, Post-Chicago]

Move to Tentatively Resolved

Proposed resolution:

This functionality is provided by P0031R0

### 2444(i). Inconsistent complexity for `std::sort_heap`

Section: 26.8.8.5 [sort.heap] Status: C++20 Submitter: François Dumont Opened: 2014-10-07 Last modified: 2021-02-25

Priority: 3

View all issues with C++20 status.

Discussion:

While creating complexity tests for the GNU libstdc++ implementation I stumbled across a surprising requirement for the `std::sort_heap` algorithm.

In 26.8.8.5 [sort.heap] p3 the Standard states:

> Complexity: At most `N log(N)` comparisons (where `N == last - first`).

As stated on the libstdc++ mailing list by Marc Glisse `sort_heap` can be implemented by `N` calls to `pop_heap`. As max number of comparisons of `pop_heap` is `2 * log(N)` then `sort_heap` max limit should be `2 * log(1) + 2 * log(2) + .... + 2 * log(N)` that is to say `2 * log(N!)`. In terms of `log(N)` we can also consider that this limit is also cap by `2 * N * log(N)` which is surely what the Standard wanted to set as a limit.

This is why I would like to propose to replace paragraph 3 by:

> Complexity: At most `2N log(N)` comparisons (where `N == last - first`).

[2015-02 Cologne]

Marshall will research the maths and report back in Lenexa.

[2015-05-06 Lenexa]

STL: I dislike exact complexity requirements, they prevent one or two extra checks in debug mode. Would it be better to say O(N log(N)) not at most?

[2017-03-04, Kona]

Move to Tentatively Ready. STL may write a paper (with Thomas & Robert) offering guidance about Big-O notation vs. exact requirements.

Proposed resolution:

This wording is relative to N3936.

1. In 26.8.8.5 [sort.heap] p3 the Standard states:

   >     template<class RandomAccessIterator>
   >       void sort_heap(RandomAccessIterator first, RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >       void sort_heap(RandomAccessIterator first, RandomAccessIterator last,
   >                      Compare comp);

   > > […]

   > > -3- Complexity: At most `2N log(N)` comparisons (where `N == last - first`).

### 2445(i). "Stronger" memory ordering

Section: 99 [depr.util.smartptr.shared.atomic], 99 [atomics.types.operations.req] Status: Resolved Submitter: JF Bastien Opened: 2014-10-08 Last modified: 2017-11-29

Priority: Not Prioritized

View all other issues in [depr.util.smartptr.shared.atomic].

View all issues with Resolved status.

Discussion:

The definitions of compare and exchange in [util.smartptr.shared.atomic] p32 and 99 [atomics.types.operations.req] p20 state:

> Requires: The failure argument shall not be `memory_order_release` nor `memory_order_acq_rel`. The `failure` argument shall be no stronger than the `success` argument.

The term "stronger" isn't defined by the standard.

It is hinted at by 99 [atomics.types.operations.req] p21:

> When only one `memory_order` argument is supplied, the value of `success` is `order`, and the value of `failure` is `order` except that a value of `memory_order_acq_rel` shall be replaced by the value `memory_order_acquire` and a value of `memory_order_release` shall be replaced by the value `memory_order_relaxed`.

Should the standard define a partial ordering for memory orders, where consume and acquire are incomparable with release?

[2014-11 Urbana]

Move to SG1.

[2016-11-12, Issaquah]

Resolved by P0418R2

Proposed resolution:

### 2447(i). Allocators and `volatile`-qualified value types

Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2014-10-16 Last modified: 2017-07-30

Priority: 4

View other active issues in [allocator.requirements].

View all other issues in [allocator.requirements].

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Discussion:

According to Table 27 — "Descriptive variable definitions" which is used to define the symbols used in the allocator requirements table within 16.4.4.6 [allocator.requirements] we have the following constraints for the types `T, U, C`:

> any non-const object type (3.9)

This wording can be read to allow instead a `volatile`-qualified value type such as `volatile int`.

The nearest-by way of fixing this would be to add "non-`volatile`" as additional constraint to this table row.

Another choice would be to think of requiring that allocators must be capable to handle any `cv`-qualified value types. This would make all currently existing allocators non-conforming that can't handle `cv`-qualified value types, so I'm not suggesting to follow that route.

A less radical step would be to allow `cv`-qualified types just for `C` (which is used to specify the functions `construct` and `destroy` and where does not even exist any requirement that `C` actually is related to the value type of the allocator at all). This seemingly extension would be harmless because as of p8 of the same sub-clause "An allocator may constrain the types on which it can be instantiated and the arguments for which its `construct` member may be called."

This differs from the requirements imposed on the types `T` and `U` which both refer to value types of allocators.

The proposed wording attempts to separate the two classes of requirements.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4140.

> 1. Change 16.4.4.6 [allocator.requirements], Table 27 — "Descriptive variable definitions", as indicated:

>    > Table 27 — Descriptive variable definitions
>    > Variable | Definition
>    > --- | ---
>    > `T, U, C` | any non-const `const` and non-`volatile` object type (3.9)
>    > `C` | any object type
>    > `…`

> 2. Change 16.4.4.6 [allocator.requirements] p8 as indicated: (This wording change is intended to fix an obvious asymmetry between `construct` and `destroy` which I believe is not intended)

>    > -8- An allocator may constrain the types on which it can be instantiated and the arguments for which its `construct` or `destroy` members may be called. If a type cannot be used with a particular allocator, the allocator class or the call to `construct` or `destroy` may fail to instantiate.

[2014-11, Urbana]

JW: say "cv-unqualified" instead?
JW: very nervous about allowing construct on const-types, because of the cast to (non-const) `void*`
MA: should we just make the minimal fix?
STL: don't break `C` out for special treatment
New proposed resolution: just change "non-const" to "cv-unqualified". Keep addition of `destroy` later.

[2015-02 Cologne]

GR: It makes me nervous that someone at some point decided to not add "non-`volatile`".
AM: That was over ten years ago. It was a deliberate, minuted choice to support `volatile`. We are now reversing that decision. It would be good to poll our vendors, none of which are in the room. This is a bit more work than we expect of a P0 issue.
VV: libstdc++ and libc++ seem to support `volatile` template parameters for the standard allocator.
AM: To clarify, the proposed resolution here would remove the requirement to support `volatile`. Implementations could still choose to support `volatile`.
DK: I'm happy to drop this and open a new issue in regard to the `destroy` member specification.
AM: I just think this is harder than a P0. Let's reprioritize.

[2015-04-01 Daniel comments]

The less controversial part of the issue related to constraints imposed on `destroy` has be handed over to the new issue 2470(i).

[2015-05-06 Lenexa: Move to Ready]

Proposed resolution:

This wording is relative to N4431.

1. Change 16.4.4.6 [allocator.requirements], Table 27 — "Descriptive variable definitions", as indicated:

   > Table 27 — Descriptive variable definitions
   > Variable | Definition
   > --- | ---
   > `T, U, C` | any non-constcv-unqualified object type (3.9)
   > `…`

### 2448(i). Non-normative Container destructor specification

Section: 23.2.2 [container.requirements.general] Status: C++17 Submitter: Daniel Krügler Opened: 2014-10-18 Last modified: 2025-03-13

Priority: 0

View other active issues in [container.requirements.general].

View all other issues in [container.requirements.general].

View all issues with C++17 status.

Discussion:

According to Table 96 — "Container requirements" the specification:

> note: the destructor is applied to every element of `a`; any memory obtained is deallocated.

The initial "note:" can be read as if that part of the specification would not be normative (This note form differs from footnotes in tables, which have normative meaning).

It seems that this initial part of the specification exists since C++98. But comparing with the similar SGI Container specification there is no evidence for that being intended to be non-normative.

[2015-02, Cologne]

NJ: If we fix this, we should also fix it elsewhere. Oh, this is the only place?
GR: If this is intended to be different from elsewhere, we should make sure.
AM: `valarray` specifies this without the "note:".
DK: `valarray` requires trivially destructible types!
GR: That's good enough for me.
NJ: First time `valarray` has been useful for something!

Proposed resolution:

This wording is relative to N4140.

1. Change 23.2.2 [container.requirements.general], Table 96 — "Container requirements", as indicated:

   > Table 96 — Container requirements
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | --- | ---
   > `…`
   > `(&a)->~X()` | `void` |  | note: the destructor is applied to every element of `a`; any memory obtained is deallocated. | linear
   > `…`

### 2450(i). `(greater|less|greater_equal|less_equal)<void>` do not yield a total order for pointers

Section: 22.10.8 [comparisons] Status: C++17 Submitter: Joaquín M López Muñoz Opened: 2014-10-30 Last modified: 2017-07-30

Priority: 2

View other active issues in [comparisons].

View all other issues in [comparisons].

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Discussion:

`less<void>::operator(t, u)` (and the same applies to the rest of `void` specializations for standard comparison function objects) returns `t < u` even if `t` and `u` are pointers, which by 7.6.9 [expr.rel]/3 is undefined except if both pointers point to the same array or object. This might be regarded as a specification defect since the intention of N3421 is that `less<>` can substitute for `less<T>` in any case where the latter is applicable. `less<void>` can be rewritten in the following manner to cope with pointers:

>     template<> struct less<void>
>     {

>       typedef unspecified is_transparent;

>       template <class T, class U>
>       struct pointer_overload : std::is_pointer<std::common_type_t<T, U>>
>       {};

>       template <
>         class T, class U,
>         typename std::enable_if<!pointer_overload<T, U>::value>::type* = nullptr
>       >
>       auto operator()(T&& t, U&& u) const
>         -> decltype(std::forward<T>(t) < std::forward<U>(u))
>       {
>         return std::forward<T>(t) < std::forward<U>(u);
>       }

>       template <
>         class T, class U,
>         typename std::enable_if<pointer_overload<T, U>::value>::type* = nullptr
>       >
>       auto operator()(T&& t, U&& u) const
>         -> decltype(std::declval<std::less<std::common_type_t<T, U>>>()(std::forward<T>(t), std::forward<U>(u)))
>       {
>         std::less<std::common_type_t<T, U>> l;
>         return l(std::forward<T>(t), std::forward<U>(u));
>       }

>     };

> This wording is relative to N4140.

> 1. Change 22.10.8 [comparisons] p14 as indicated:

>    > -14- For templates `greater`, `less`, `greater_equal`, and `less_equal`, the specializations for any pointer type yield a total order, even if the built-in operators `<`, `>`, `<=`, `>=` do not. For template specializations `greater<void>`, `less<void>`, `greater_equal<void>`, and `less_equal<void>`, the call operator with arguments whose common type `CT` is a pointer yields the same value as the corresponding comparison function object class specialization for `CT`.

[2015-02, Cologne]

AM: Is there any way this will be resolved elsewhere? VV: No. AM: Then we should bite the bullet and deal with it here.

MC: These diamond operators are already ugly. Making them more ugly isn't a big problem.

JY found some issue with types that are convertible, and will reword.

Jeffrey suggests improved wording.

[2015-05, Lenexa]

STL: when diamond functions designed, this was on purpose
STL: this does go against the original design
STL: library is smarter and can give a total order
MC: given that the original design rejected this, give back to LEWG
STL: original proposal did not talk about total order
STL: don't feel strongly about changing the design
STL: no objections to taking this issue with some wording changes if people want it
MC: not happy with wording, comparing pointers — what does that mean?
STL: needs careful attention to wording
STL: want to guarantee that `nullptr` participates in total ordering
STL: all hooks into composite pointer type
MC: move from new to open with better wording
STL: to check updates to issue after Lenexa

[2015-06, Telecon]

MC: STL on the hook to update. He's shipping something today so not here.
MC: also add link to N4229

[2015-10, Kona Saturday afternoon]

STL was on the hook for wording, but STL: I don't care. The architecture on which this is an issue does not exist.
STL: We will also need to incorporate nullptr. TK: I think that's implied, since the wording is in terms of the resulting operation, not the deduced types.
STL: Seems legit. MC: I guess I'm OK with this. TK: I'm weakly in favour, so that we can get people to use transparent comparators without worrying.
STL: There's no change to implementations.
Move to Tentatively ready.

Proposed resolution:

This wording is relative to N4296.

1. Change 22.10.8 [comparisons] p14 as indicated:

   > -14- For templates `greater`, `less`, `greater_equal`, and `less_equal`, the specializations for any pointer type yield a total order, even if the built-in operators `<`, `>`, `<=`, `>=` do not. For template specializations `greater<void>`, `less<void>`, `greater_equal<void>`, and `less_equal<void>`, if the call operator calls a built-in operator comparing pointers, the call operator yields a total order.

### 2451(i). [fund.ts.v2] `optional<T>` should 'forward' `T`'s implicit conversions

Section: 5.3 [fund.ts.v2::optional.object] Status: TS Submitter: Geoffrey Romer Opened: 2014-10-31 Last modified: 2018-07-08

Priority: Not Prioritized

View all other issues in [fund.ts.v2::optional.object].

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Discussion:

Addresses: fund.ts.v2

Code such as the following is currently ill-formed (thanks to STL for the compelling example):

>     optional<string> opt_str = "meow";

This is because it would require two user-defined conversions (from `const char*` to `string`, and from `string` to `optional<string>`) where the language permits only one. This is likely to be a surprise and an inconvenience for users.

`optional<T>` should be implicitly convertible from any `U` that is implicitly convertible to `T`. This can be implemented as a non-explicit constructor template `optional(U&&)`, which is enabled via SFINAE only if `is_convertible_v<U, T>` and `is_constructible_v<T, U>`, plus any additional conditions needed to avoid ambiguity with other constructors (see N4064, particularly the "Odd" example, for why `is_convertible` and `is_constructible` are both needed; thanks to Howard Hinnant for spotting this).

In addition, we may want to support explicit construction from `U`, which would mean providing a corresponding explicit constructor with a complementary SFINAE condition (this is the single-argument case of the "perfect initialization" pattern described in N4064).

[2015-10, Kona Saturday afternoon]

STL: This has status LEWG, but it should be priority 1, since we cannot ship an IS without this.

TK: We assigned our own priorities to LWG-LEWG issues, but haven't actually processed any issues yet.

MC: This is important.

[2016-02-17, Ville comments and provides concrete wording]

I have prototype-implemented this wording in libstdc++. I didn't edit the copy/move-assignment operator tables into the new `operator=` templates that take `optional`s of a different type; there's a drafting note that suggests copying them from the existing tables.

[LEWG: 2016-03, Jacksonville]

Discussion of whether `variant` supports this. We think it does.

Take it for C++17.

Unanimous yes.

Proposed resolution:

This wording is relative to N4562.

1. Edit 22.5.3 [optional.optional] as indicated:

   >     template <class T>
   >     class optional
   >     {
   >     public:
   >       typedef T value_type;

   >       // 5.3.1, Constructors
   >       constexpr optional() noexcept;
   >       constexpr optional(nullopt_t) noexcept;
   >       optional(const optional&);
   >       optional(optional&&) noexcept(see below);
   >       constexpr optional(const T&);
   >       constexpr optional(T&&);
   >       template <class... Args> constexpr explicit optional(in_place_t, Args&&...);
   >       template <class U, class... Args>
   >         constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...);
   >       template <class U> constexpr optional(U&&);
   >       template <class U> constexpr optional(const optional<U>&);
   >       template <class U> constexpr optional(optional<U>&&);

   >       […]

   >       // 5.3.3, Assignment
   >       optional& operator=(nullopt_t) noexcept;
   >       optional& operator=(const optional&);
   >       optional& operator=(optional&&) noexcept(see below);
   >       template <class U> optional& operator=(U&&);
   >       template <class U> optional& operator=(const optional<U>&);
   >       template <class U> optional& operator=(optional<U>&&);
   >       template <class... Args> void emplace(Args&&...);
   >       template <class U, class... Args>
   >         void emplace(initializer_list<U>, Args&&...);

   >       […]

   >     };

2. In 5.3.1 [fund.ts.v2::optional.object.ctor], insert new signature specifications after p33:

   > [Note: The following constructors are conditionally specified as `explicit`. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]

   >     template <class U>
   >     constexpr optional(U&& v);

   > > -?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::forward<U>(v)`.

   > > -?- Postconditions: `*this` contains a value.

   > > -?- Throws: Any exception thrown by the selected constructor of `T`.

   > > -?- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true` and `U` is not the same type as `T`. The constructor is `explicit` if and only if `is_convertible_v<U&&, T>` is `false`.

   >     template <class U>
   >     constexpr optional(const optional<U>& rhs);

   > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `*rhs`.

   > > -?- Postconditions: `bool(rhs) == bool(*this)`.

   > > -?- Throws: Any exception thrown by the selected constructor of `T`.

   > > -?- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, const U&>` is `true`, `is_same<decay_t<U>, T>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false` and `is_convertible_v<const optional<U>&, T>` is `false`. The constructor is `explicit` if and only if `is_convertible_v<const U&, T>` is `false`.

   >     template <class U>
   >     constexpr optional(optional<U>&& rhs);

   > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::move(*rhs)`. `bool(rhs)` is unchanged.

   > > -?- Postconditions: `bool(rhs) == bool(*this)`.

   > > -?- Throws: Any exception thrown by the selected constructor of `T`.

   > > -?- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `is_same<decay_t<U>, T>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false` and `is_convertible_v<optional<U>&&, T>` is `false` and `U` is not the same type as `T`. The constructor is `explicit` if and only if `is_convertible_v<U&&, T>` is `false`.

3. In 5.3.3 [fund.ts.v2::optional.object.assign], change as indicated:

   >     template <class U> optional<T>& operator=(U&& v);

   > > -22- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `v` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `decay_t<U>` is not `nullopt_t` and `decay_t<U>` is not a specialization of `optional` `is_same_v<decay_t<U>, T>` is `true`.

   > > -23- Notes: The reason for providing such generic assignment and then constraining it so that effectively `T == U` is to guarantee that assignment of the form `o = {}` is unambiguous.

   >     template <class U> optional<T>& operator=(const optional<U>& rhs);

   > > -?- Requires: `is_constructible_v<T, const U&>` is `true` and `is_assignable_v<T&, const U&>` is `true`.

   > > -?- Effects:

   > > Table ? — `optional::operator=(const optional<U>&)` effects
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*rhs` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `*rhs`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

   > > -?- Returns: `*this`.

   > > -?- Postconditions: `bool(rhs) == bool(*this)`.

   > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `is_same_v<decay_t<U>, T>` is `false`.

   >     template <class U> optional<T>& operator=(optional<U>&& rhs);

   > > -?- Requires: `is_constructible_v<T, U>` is `true` and `is_assignable_v<T&, U>` is `true`.

   > > -?- Effects: The result of the expression `bool(rhs)` remains unchanged.

   > > Table ? — `optional::operator=(optional<U>&&)` effects
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `std::move(*rhs)` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `std::move(*rhs)`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

   > > -?- Returns: `*this`.

   > > -?- Postconditions: `bool(rhs) == bool(*this)`.

   > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `is_same_v<decay_t<U>, T>` is `false`.

### 2454(i). Add `raw_storage_iterator::base()` member

Section: 99 [depr.storage.iterator] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-11-11 Last modified: 2017-07-30

Priority: 0

View all other issues in [depr.storage.iterator].

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Discussion:

Eric Niebler pointed out that `raw_storage_iterator` should give access to the `OutputIterator` it wraps.

This helps alleviate the exception-safety issue pointed out in the discussion of LWG 2127(i), as an exception can be caught and then destructors can be run for the constructed elements in the range `[begin, raw.base())`

[2015-02 Cologne]

NJ: Is this "const" correct [in "base()"]? DK: Yes, we always do that. NJ: And the output iterator is not qualifying in any way? AM/DK: That wouldn't make sense. NJ: OK.

VV: What did LEWG say about this feature request? In other words, why is this a library issue? AM: LEWG/JY thought this wouldn't be a contentious issue.

NJ: I really hope the split of LEWG and LWG will be fixed soon, since it's only wasting time. VV: So you want to spend even more of your time on discussions that LEWG has?

AM: I think this specified correctly. I'm not wild about it. But no longer bothered to stand in its way.

GR: Why do we need to repeat the type in "Returns" even though it's part of the synopsis? AM: Good point, but not worth fixing.

NJ: Why is "`base()`" for `reverse_iterator` commented with "// explicit"? AM: I guess in 1998 that was the only way to say this.

AM: So, it's tentatively ready.

Proposed resolution:

This wording is relative to N4140.

1. Add a new function to the synopsis in [storage.iterator] p1:

   >     namespace std {
   >       template <class OutputIterator, class T>
   >       class raw_storage_iterator
   >         : public iterator<output_iterator_tag,void,void,void,void> {
   >       public:
   >         explicit raw_storage_iterator(OutputIterator x);

   >         raw_storage_iterator<OutputIterator,T>& operator*();
   >         raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
   >         raw_storage_iterator<OutputIterator,T>& operator++();
   >         raw_storage_iterator<OutputIterator,T> operator++(int);
   >         OutputIterator base() const;
   >     };
   >     }

2. Insert the new function and a new paragraph series after p7:

   >     OutputIterator base() const;

   > > -?- Returns: An iterator of type `OutputIterator` that points to the same value as `*this` points to.

### 2455(i). Allocator default construction should be allowed to throw

Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Pablo Halpern Opened: 2014-11-11 Last modified: 2017-07-30

Priority: Not Prioritized

View other active issues in [allocator.requirements].

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Discussion:

16.4.4.6 [allocator.requirements]/4 in the 2014-10 WP (N4140), says:

> An allocator type `X` shall satisfy the requirements of `CopyConstructible` (17.6.3.1). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.2).

The words "these types" would normally apply only to the previous sentence only, i.e., only to the pointer types. However, an alternative reading would be that the allocator constructors themselves cannot throw. The change to the `vector` and `string` default constructors, making them unconditionally `noexcept` depends on this alternative reading.

I believe that the wording in the standard is not intended to forbid throwing default constructors for allocators. Indeed, I believe that allocators do not require default constructors and that if they provide a default constructor they should be allowed to throw.

In addition, the `noexcept` specifications for the `string` and `vector` default constructors should be changed to make them conditional.

[2015-01-18 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

1. Change 16.4.4.6 [allocator.requirements] p4 as indicated:

   > An allocator type `X` shall satisfy the requirements of `CopyConstructible` (17.6.3.1). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these pointer types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.2).

2. Change 27.4.3 [basic.string] following p5, class template `basic_string` synopsis, as indicated: (This change assumes that N4258 has been applied, as voted on in Urbana on 2014-11-08)

   >     // 21.4.2, construct/copy/destroy:
   >     basic_string() noexcept(noexcept(Allocator())) : basic_string(Allocator()) { }

   > An alternative formulation of the above would be:

   >     // 21.4.2, construct/copy/destroy:
   >     basic_string() noexcept(is_nothrow_default_constructible<Allocator>{}) : basic_string(Allocator()) { }

3. Change 23.3.13.1 [vector.overview] following p2, class template `vector` synopsis, as indicated: (This change assumes that N4258 has been applied, as voted on in Urbana on 2014-11-08)

   >     // 23.3.6.2, construct/copy/destroy:
   >     vector() noexcept(noexcept(Allocator())) : vector(Allocator()) { }

   > An alternative formulation of the above would be:

   >     // 23.3.6.2, construct/copy/destroy:
   >     vector() noexcept(is_nothrow_default_constructible<Allocator>{}) : vector(Allocator()) { }

### 2456(i). Incorrect exception specifications for '`swap`' throughout library

Section: 22.2 [utility], 22.3.2 [pairs.pair], 22.4 [tuple], 23.3.3 [array], 23.6.3 [queue], 23.6.4 [priority.queue], 23.6.6 [stack] Status: Resolved Submitter: Richard Smith Opened: 2014-11-14 Last modified: 2016-03-07

Priority: 1

View all other issues in [utility].

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Discussion:

We have this antipattern in various library classes:

>     void swap(priority_queue& q) noexcept(
>         noexcept(swap(c, q.c)) && noexcept(swap(comp, q.comp)))

This doesn't work. The unqualified lookup for 'swap' finds the member named 'swap', and that suppresses ADL, so the exception specification is always ill-formed because you can't call the member 'swap' with two arguments.

Relevant history on the core language side:

This used to be ill-formed due to 6.4.7 [basic.scope.class] p1 rule 2: "A name `N` used in a class `S` shall refer to the same declaration in its context and when re-evaluated in the completed scope of `S`. No diagnostic is required for a violation of this rule."

Core issue 1330 introduced delay-parsing for exception specifications. Due to the 6.4.7 [basic.scope.class] rules, this shouldn't have changed the behavior of any conforming programs. But it changes the behavior in the non-conforming case from "no diagnostic required" to "diagnostic required", so implementations that implement core issue 1330 are now required to diagnose the ill-formed declarations in the standard library.

Suggested resolution:

Add an `is_nothrow_swappable` trait, and use it throughout the library in place of these `noexcept` expressions.

[2015-02, Cologne]

No action for now; we intend to have papers for Lenexa.

[2015-05, Lenexa]

Move to Open.

Daniel: A first paper (N4426) exists to suggest some ways of solving this issue.

[2015-10, Kona, Daniel comments]

A revised paper (N4511) has been provided

[2015-12-16, Daniel comments]

Revision 2 (P0185R0) will available for the mid February 2016 mailing.

[2016-03, Jacksonville]

P0185R1 was adopted in Jacksonville.

Proposed resolution:

### 2458(i). N3778 and new library deallocation signatures

Section: 17.6 [support.dynamic], 17.6.3.2 [new.delete.single], 17.6.3.3 [new.delete.array] Status: C++17 Submitter: Richard Smith Opened: 2014-11-23 Last modified: 2017-07-30

Priority: 2

View all other issues in [support.dynamic].

View all issues with C++17 status.

Discussion:

N3778 added the following sized deallocation signatures to the library:

>     void operator delete(void* ptr, std::size_t size) noexcept;
>     void operator delete[](void* ptr, std::size_t size) noexcept;

>     void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept;
>     void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;

The former two are an essential part of the proposal. The latter two seem spurious — they are not called when `new (std::nothrow) X` fails due to `X::X()` throwing, because the core language rules for selecting a placement deallocation function do not consider passing a `size` argument. Instead, the above would be the matching deallocation functions for:

>     void *operator new(std::size_t size, std::size_t size_again, const std::nothrow_t&) noexcept;
>     void *operator new[](std::size_t size, std::size_t size_again, const std::nothrow_t&) noexcept;

... which don't exist.

Since they're not implicitly called, the only other possible use for those functions would be to perform an explicitly non-throwing deallocation. But... the first two overloads are already explicitly non-throwing and are required to be semantically identical to the second two. So there's no point in making an explicit call to the second pair of functions either.

It seems to me that we should remove the `(void*, size_t, nothrow_t)` overloads, because the core working group decided during the Urbana 2014 meeting, that no change to the core language was warranted.

[2014-11-23, Daniel suggests concrete wording changes]

[2015-02 Cologne]

Nobody can call those overloads, since the nothrow allocation functions cannot throw. JY: Ship it. GR: Should we do due diligence and make sure we're deleting what we mean to be deleting? [Some checking, everything looks good.]

Accepted.

Proposed resolution:

This wording is relative to N4140.

1. Change 17.6 [support.dynamic], header `<new>` synopsis, as indicated:

   >     […]
   >     void operator delete(void* ptr, std::size_t size) noexcept;
   >     void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept;
   >     […]
   >     void operator delete[](void* ptr, std::size_t size) noexcept;
   >     void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;
   >     […]

2. Change 17.6.3.2 [new.delete.single], starting before p19, as indicated:

   >     void operator delete(void* ptr, const std::nothrow_t&) noexcept;
   >     void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept;

   > > […]

   > > -20- Replaceable: a C++ program may define a function with signature `void operator delete(void* ptr, const std::nothrow_t&) noexcept` that displaces the default version defined by the C++ standard library. If this function (without `size` parameter) is defined, the program should also define `void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept`. If this function with `size` parameter is defined, the program shall also define the version without the `size` parameter. [Note: The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function. — end note]

   > > […]

   > > -22- Requires: If present, the `std::size_t size` argument must equal the `size` argument passed to the allocation function that returned `ptr`.

   > > -23- Required behavior: Calls to `operator delete(void* ptr, std::size_t size, const std::nothrow_t&)` may be changed to calls to `operator delete(void* ptr, const std::nothrow_t&)` without affecting memory allocation. [Note: A conforming implementation is for `operator delete(void* ptr, std::size_t size, const std::nothrow_t&)` to simply call `operator delete(void* ptr, const std::nothrow_t&)`. — end note]

   > > -24- Default behavior: `operator delete(void* ptr, std::size_t size, const std::nothrow_t&)` calls `operator delete(ptr, std::nothrow)`, and `operator delete(void* ptr, const std::nothrow_t&)` calls `operator delete(ptr)`.

3. Change 17.6.3.3 [new.delete.array], starting before p16, as indicated:

   >     void operator delete[](void* ptr, const std::nothrow_t&) noexcept;
   >     void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;

   > > […]

   > > -17- Replaceable: a C++ program may define a function with signature `void operator delete[](void* ptr, const std::nothrow_t&) noexcept` that displaces the default version defined by the C++ standard library. If this function (without `size` parameter) is defined, the program should also define `void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept`. If this function with `size` parameter is defined, the program shall also define the version without the `size` parameter. [Note: The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function. — end note]

   > > […]

   > > -19- Requires: If present, the `std::size_t size` argument must equal the `size` argument passed to the allocation function that returned `ptr`.

   > > -20- Required behavior: Calls to `operator delete[](void* ptr, std::size_t size, const std::nothrow_t&)` may be changed to calls to `operator delete[](void* ptr, const std::nothrow_t&)` without affecting memory allocation. [Note: A conforming implementation is for `operator delete[](void* ptr, std::size_t size, const std::nothrow_t&)` to simply call `operator delete[](void* ptr, const std::nothrow_t&)`. — end note]

   > > -21- Default behavior: `operator delete[](void* ptr, std::size_t size, const std::nothrow_t&)` calls `operator delete[](ptr, std::nothrow)`, and `operator delete[](void* ptr, const std::nothrow_t&)` calls `operator delete[](ptr)`.

### 2459(i). `std::polar` should require a non-negative rho

Section: 29.4.7 [complex.value.ops] Status: C++17 Submitter: Marshall Clow Opened: 2014-10-22 Last modified: 2017-07-30

Priority: 0

View all other issues in [complex.value.ops].

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Discussion:

Different implementations give different answers for the following code:

>     #include <iostream>
>     #include <complex>

>     int main ()
>     {
>       std::cout << std::polar(-1.0, -1.0) << '\n';
>       return 0;
>     }

One implementation prints:

>     (nan, nan)

Another:

>     (-0.243068, 0.243068)

Which is correct? Or neither?

In this list, Howard Hinnant wrote:

> I've read this over several times. I've consulted C++11, C11, and IEC 10967-3. [snip]

> I'm finding:

> 1. The magnitude of a complex number `== abs(c) == hypot(c.real(), c.imag())` and is always non-negative (by all three of the above standards).

> 2. Therefore no complex number exists for which `abs(c) < 0`.

> 3. Therefore when the first argument to `std::polar` (which is called `rho`) is negative, no complex number can be formed which meets the post-conidtion that `abs(c) == rho`.

> One could argue that this is already covered in 29.4 [complex.numbers]/3, but I think it's worth making explicit.

[2015-02, Cologne]

Discussion on whether theta should also be constrained.
TK: infinite theta doesn't make sense, whereas infinite rho does (theta is on a compact domain, rho is on a non-compact domain).
AM: We already have a narrow contract, so I don't mind adding further requirements. Any objections to requiring that theta be finite?
Some more discussion, but general consensus. Agreement that if someone finds the restrictions problematic, they should write a proper paper to address how `std::polar` should behave. For now, we allow infinite rho (but not NaN and not negative), and require finite theta.

No objections to tentatively ready.

Proposed resolution:

This wording is relative to N4296.

1. Change 29.4.7 [complex.value.ops] around p9 as indicated

   >     template<class T> complex<T> polar(const T& rho, const T& theta = 0);

   > > -?- Requires: `rho` shall be non-negative and non-NaN. `theta` shall be finite.

   > > -9- Returns: The complex value corresponding to a complex number whose magnitude is `rho` and whose phase angle is `theta`.

### 2460(i). LWG issue 2408 and value categories

Section: 21.3.8.7 [meta.trans.other], 24.3.2.3 [iterator.traits] Status: C++17 Submitter: Richard Smith Opened: 2014-11-19 Last modified: 2017-07-30

Priority: 2

View all other issues in [meta.trans.other].

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Discussion:

LWG issue 2408(i) changes the meat of the specification of `common_type` to compute:

> […] the type, if any, of an unevaluated conditional expression (5.16) whose first operand is an arbitrary value of type `bool`, whose second operand is an `xvalue` of type `T1`, and whose third operand is an xvalue of type `T2`.

This has an effect on the specification that I think was unintended. It used to be the case that `common_type<T&, U&&>` would consider the type of a conditional between an lvalue of type `T` and an xvalue of type `U`. It's now either invalid (because there is no such thing as an xvalue of reference type) or considers the type of a conditional between an xvalue of type `T` and an xvalue of type `U`, depending on how you choose to read it.

Put another way, this has the effect of changing the usual definition from:

>     typedef decay_t<decltype(true ? declval<T>() : declval<U>())> type;

to:

>     typedef decay_t<decltype(true ? declval<remove_reference_t<T>>() : declval<remove_reference_t<U>>())> type;

It also makes `common_type` underspecified in the case where one of the operands is of type `void`; in that case, the resulting type depends on whether the expression is a throw-expression, which is not specified (but used to be).

Also on the subject of this wording: the changes to 24.3.2.3 [iterator.traits] say that `iterator_traits<T>` "shall have no members" in some cases. That's wrong. It's a class type; it always has at least a copy constructor, a copy assignment operator, and a destructor. Plus this removes the usual library liberty to add additional members with names that don't collide with normal usage (for instance, if a later version of the standard adds members, they can't be present here as a conforming extension). Perhaps this should instead require that the class doesn't have members with any of those five names? That's what 2408(i) does for `common_type`'s type member.

[2016-08 Chicago]

This issue has two parts, one dealing with `common_type`, the other with `iterator_traits`. The first of these is resolved by 2465(i). See below for the proposed resolution for the other one.

Wed PM: Move to Tentatively Ready

Proposed resolution:

Change 24.3.2.3 [iterator.traits] p.2:

[…] as publicly accessible members and no other members:

[…]

Otherwise, `iterator_traits<Iterator>` shall have no members by any of the above names.

### 2462(i). `std::ios_base::failure` is overspecified

Section: 31.5.2 [ios.base], 31.5.2.2.1 [ios.failure] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-12-15 Last modified: 2023-02-07

Priority: 3

View all other issues in [ios.base].

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Discussion:

31.5.2 [ios.base] defines `ios_base::failure` as a nested class:

>     namespace std {
>       class ios_base {
>       public:
>         class failure;
>         […]
>       };
>       […]
>     }

This means it is valid to use an elaborated-type-specifier to refer to `ios_base::failure`:

>     using F = class std::ios_base::failure;
>     throw F();

Therefore implementations are not permitted to define `ios_base::failure` as a typedef e.g.

>      class ios_base {
>      public:
>     #if __cplusplus < 201103L
>        class failure_cxx03 : public exception {...};
>        typedef failure_cxx03 failure;
>     #else
>        class failure_cxx11 : public system_error {...};
>        typedef failure_cxx11 failure;
>     #endif
>        […]
>      };

This constrains implementations, making it harder to manage the ABI change to `ios_base::failure` between C++03 and C++11.

[2015-05-06 Lenexa: Move to Ready]

JW: the issue is that users are currently allowed to write "class failure" with an elaborated-type-specifier and it must be well-formed, I want the freedom to make that type a typedef, so they can't necessarily use an elaborated-type-specifier (which there is no good reason to use anyway)

JW: ideally I'd like this everywhere for all nested classes, but that's a paper not an issue, I only need this type fixed right now.

RD: is a synonym the same as an alias?

JW: dcl.typedef says a typedef introduces a synonym for another type, so I think this is the right way to say this

JW: I already shipped this last month

PJP: we're going to have to break ABIs again some time, we need all the wiggle room we can get to make that easier. This helps.

MC: do we want this at all? Ready?

9 in favor, none opose or abstaining

Proposed resolution:

This wording is relative to N4296.

1. Change the synopsis in 31.5.2 [ios.base] as indicated:

   >     namespace std {
   >       class ios_base {
   >       public:
   >         class failure; // see below
   >         […]
   >       };
   >       […]
   >     };

2. Change 31.5.2 [ios.base] paragraph 1:

   > `ios_base` defines several member types:

   > - a class `failure` type `failure`, defined as either a class derived from `system_error` or a synonym for a class derived from `system_error`;

3. Change [ios::failure] paragraph 1:

   > -1- An implementation is permitted to define `ios_base::failure` as a synonym for a class with equivalent functionality to class `ios_base::failure` shown in this subclause. [Note: When `ios_base::failure` is a synonym for another type it shall provide a nested type `failure`, to emulate the injected class name. — end note] The class `failure` defines the base class for the types of all objects thrown as exceptions, by functions in the iostreams library, to report errors detected during stream buffer operations.

### 2463(i). [fund.ts] Incorrect complexity for `sample()` algorithm

Section: 10.3 [fund.ts::alg.random.sample] Status: TS Submitter: Joe Gottman Opened: 2014-12-17 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts

According to paragraph 10.1 of the Library Fundamentals 1 draft, the complexity of the new `std::experimental::sample` template function is 𝒪(`n`). Note that `n` is actually a parameter of this function, corresponding to the sample size. But both common algorithms for sampling, the selection algorithm and the reservoir algorithm, are linear with respect to the population size, which is often many orders of magnitude bigger than the sample size.

[2015-02, Cologne]

AM: I suggest we make this a DR against the Fundamentals TS.
GR: Agreed, this is a no-brainer.

Proposed resolution:

This wording is relative to N4335 in regard to fundamental-ts changes.

1. Change 10.3 [fund.ts::alg.random.sample] p5 to read:

   > -5- Complexity: 𝒪(`nlast - first`).

### 2464(i). `try_emplace` and `insert_or_assign` misspecified

Section: 23.4.3.4 [map.modifiers], 23.5.3.4 [unord.map.modifiers] Status: C++17 Submitter: Thomas Koeppe Opened: 2014-12-17 Last modified: 2017-07-30

Priority: 2

View all other issues in [map.modifiers].

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Discussion:

The specification of the `try_emplace` and `insert_or_assign` member functions in N4279 contains the following errors and omissions:

1. In `insert_or_assign`, each occurrence of `std::forward<Args>(args)...` should be `std::forward<M>(obj)`; this is was a mistake introduced in editing.

2. In `try_emplace`, the construction of the `value_type` is misspecified, which is a mistake that was introduced during the evolution from a one-parameter to a variadic form. As written, `value_type(k, std::forward<Args>(args)...)` does not do the right thing; it can only be used with a single argument, which moreover must be convertible to a `mapped_type`. The intention is to allow direct-initialization from an argument pack, and the correct constructor should be `value_type(piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...)`.

3. Both `try_emplace` and `insert_or_assign` are missing requirements on the argument types. Since the semantics of these functions are specified independent of other functions, they need to include their requirements.

[2015-02, Cologne]

This issue is related to 2469(i).

AM: The repeated references to "first and third forms" and "second and fourth forms" is a bit cumbersome. Maybe split the four functions?
GR: We don't have precendent for "EmplaceConstructible from a, b, c". I don't like the ambiguity between code commas and text commas.
TK: What's the danger?
GR: It's difficult to follow standardese.
AM: It seems fine with code commas. What's the problem?
GR: It will lead to difficulties when we use a similar construction that's not at the end of a sentence.
AM: That's premature generalization. DK: When that happens, let's look at this again.
AM: Clean up "if the map does contain"
TK: Can we call both containers "map"? DK/GR: yes.
TK will send updated wording to DK.

Conclusion: Update wording, then poll for tentatively ready

[2015-03-26, Thomas provides improved wording]

The approach is to split the descriptions of the various blocks of four functions into two blocks each so as to make the wording easier to follow.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> 1. Apply the following changes to section 23.4.3.4 [map.modifiers] p3:

>    >     template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
>    >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
>    >     template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
>    >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

>    > > -?- Requires: For the first and third forms, `value_type` shall be `EmplaceConstructible` into map from `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`. For the second and fourth forms, `value_type` shall be `EmplaceConstructible` into map from `piecewise_construct`, `forward_as_tuple(move(k))`, `forward_as_tuple(forward<Args>(args)...)`.

>    > > -3- Effects: If the key `k` already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k` If the map does already contain an element whose key is equivalent to `k`, there is no effect. Otherwise for the first and third forms inserts a `value_type` object `t` constructed with `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`, for the second and fourth forms inserts a `value_type` object `t` constructed with `piecewise_construct`, `forward_as_tuple(move(k))`, `forward_as_tuple(forward<Args>(args)...)`.

>    > > -?- Returns: In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

> 2. Apply the following changes to section 23.4.3.4 [map.modifiers] p5:

>    >     template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
>    >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
>    >     template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
>    >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

>    > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be true. For the first and third forms, `value_type` shall be `EmplaceConstructible` into map from `k`, `forward<M>(obj)`. For the second and fourth forms, `value_type` shall be `EmplaceConstructible` into map from `move(k), forward<M>(obj)`.

>    > > -5- Effects: If the key `k` does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. If the key already exists, `std::forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. In the first two overloads, the `bool` component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updated If the map does already contain an element whose key is equivalent to `k`, `forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. Otherwise the first and third forms inserts a `value_type` object `t` constructed with `k`, `forward<M>(obj)`, the second and fourth forms inserts a `value_type` object `t` constructed with `move(k), forward<M>(obj)`.

>    > > -?- Returns: In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k`.

> 3. Apply the following changes to section 23.5.3.4 [unord.map.modifiers] p5:

>    >     template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
>    >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
>    >     template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
>    >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

>    > > -?- Requires: For the first and third forms, `value_type` shall be `EmplaceConstructible` into unordered_map from `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`. For the second and fourth forms, `value_type` shall be `EmplaceConstructible` into unordered_map from `piecewise_construct`, `forward_as_tuple(move(k))`, `forward_as_tuple(forward<Args>(args)...)`.

>    > > -5- Effects: If the key `k` already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k` If the unordered_map does already contain an element whose key is equivalent to `k`, there is no effect. Otherwise for the first and third forms inserts a `value_type` object `t` constructed with `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`, for the second and fourth forms inserts a `value_type` object `t` constructed with `piecewise_construct`, `forward_as_tuple(move(k))`, `forward_as_tuple(forward<Args>(args)...)`.

>    > > -?- Returns: In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the unordered_map whose key is equivalent to `k`.

> 4. Apply the following changes to section 23.5.3.4 [unord.map.modifiers] p7:

>    >     template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
>    >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
>    >     template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
>    >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

>    > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be true. For the first and third forms, `value_type` shall be `EmplaceConstructible` into unordered_map from `k`, `forward<M>(obj)`. For the second and fourth forms, `value_type` shall be `EmplaceConstructible` into unordered_map from `move(k), forward<M>(obj)`.

>    > > -7- Effects: If the key `k` does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. If the key already exists, `std::forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. In the first two overloads, the `bool` component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updated If the unordered_map does already contain an element whose key is equivalent to `k`, `forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. Otherwise the first and third forms inserts a `value_type` object `t` constructed with `k`, `forward<M>(obj)`, the second and fourth forms inserts a `value_type` object `t` constructed with `move(k), forward<M>(obj)`.

>    > > -?- Returns: In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the unordered_map whose key is equivalent to `k`.

[2015-05, Lenexa]

STL: existing wording is horrible, this is Thomas' wording and his issue
STL: already implemented the piecewise part
MC: ok with changes
STL: changes are mechanical
STL: believe this is P1, it must be fixed, we have wording
PJP: functions are sensible
STL: has been implemented
MC: consensus is to move to ready

Proposed resolution:

This wording is relative to N4296.

1. Apply the following changes to 23.4.3.4 [map.modifiers] p3+p4:

   >     template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
   >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

   > > -?- Requires: `value_type` shall be `EmplaceConstructible` into `map` from `piecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)`.

   > > -3- Effects: If the key `k` already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k` If the map already contains an element whose key is equivalent to `k`, there is no effect. Otherwise inserts an object of type `value_type` constructed with `piecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -4- Complexity: The same as `emplace` and `emplace_hint`, respectively.

   >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

   > > -?- Requires: `value_type` shall be `EmplaceConstructible` into `map` from `piecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Effects: If the map already contains an element whose key is equivalent to `k`, there is no effect. Otherwise inserts an object of type `value_type` constructed with `piecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -?- Complexity: The same as `emplace` and `emplace_hint`, respectively.

2. Apply the following changes to 23.4.3.4 [map.modifiers] p5+p6:

   >     template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
   >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

   > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be `true`. `value_type` shall be `EmplaceConstructible` into `map` from `k, forward<M>(obj)`.

   > > -5- Effects: If the key `k` does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. If the key already exists, `std::forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. In the first two overloads, the `bool` component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updated If the map already contains an element `e` whose key is equivalent to `k`, assigns `forward<M>(obj)` to `e.second`. Otherwise inserts an object of type `value_type` constructed with `k, forward<M>(obj)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -6- Complexity: The same as `emplace` and `emplace_hint`, respectively.

   >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

   > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be `true`. `value_type` shall be `EmplaceConstructible` into `map` from `move(k), forward<M>(obj)`.

   > > -?- Effects: If the map already contains an element `e` whose key is equivalent to `k`, assigns `forward<M>(obj)` to `e.second`. Otherwise inserts an object of type `value_type` constructed with `move(k), forward<M>(obj)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -?- Complexity: The same as `emplace` and `emplace_hint`, respectively.

3. Apply the following changes to 23.5.3.4 [unord.map.modifiers] p5+p6:

   >     template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
   >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

   > > -?- Requires: `value_type` shall be `EmplaceConstructible` into `unordered_map` from `piecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)`.

   > > -5- Effects: If the key `k` already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k` If the map already contains an element whose key is equivalent to `k`, there is no effect. Otherwise inserts an object of type `value_type` constructed with `piecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -6- Complexity: The same as `emplace` and `emplace_hint`, respectively.

   >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

   > > -?- Requires: `value_type` shall be `EmplaceConstructible` into `unordered_map` from `piecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Effects: If the map already contains an element whose key is equivalent to `k`, there is no effect. Otherwise inserts an object of type `value_type` constructed with `piecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -?- Complexity: The same as `emplace` and `emplace_hint`, respectively.

4. Apply the following changes to 23.5.3.4 [unord.map.modifiers] p7+p8:

   >     template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
   >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

   > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be `true`. `value_type` shall be `EmplaceConstructible` into `unordered_map` from `k, forward<M>(obj)`.

   > > -7- Effects: If the key `k` does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. If the key already exists, `std::forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. In the first two overloads, the `bool` component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updated If the map already contains an element `e` whose key is equivalent to `k`, assigns `forward<M>(obj)` to `e.second`. Otherwise inserts an object of type `value_type` constructed with `k, forward<M>(obj)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -8- Complexity: The same as `emplace` and `emplace_hint`, respectively.

   >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

   > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be `true`. `value_type` shall be `EmplaceConstructible` into `unordered_map` from `move(k), forward<M>(obj)`.

   > > -?- Effects: If the map already contains an element `e` whose key is equivalent to `k`, assigns `forward<M>(obj)` to `e.second`. Otherwise inserts an object of type `value_type` constructed with `move(k), forward<M>(obj)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -?- Complexity: The same as `emplace` and `emplace_hint`, respectively.

### 2465(i). SFINAE-friendly `common_type` is nearly impossible to specialize correctly and regresses key functionality

Section: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Eric Niebler Opened: 2015-01-12 Last modified: 2017-09-07

Priority: 2

View all other issues in [meta.trans.other].

View all issues with Resolved status.

Discussion:

I think there's a defect regarding `common_type` and its specializations. Unless I've missed it, there is nothing preventing folks from instantiating `common_type` with cv-qualified types or reference types. In fact, the wording in N3797 explicitly mentions cv `void`, so presumably at least cv qualifications are allowed.

Users are given license to specialize `common_type` when at least of of the types is user-defined. (A separate issue is the meaning of user-defined. In core, I believe this is any class/struct/union/enum, but in lib, I think it means any type not defined in std, right?) There is at least one place in the standard that specializes `common_type` (time.traits.specializations) on time_point and duration. But the specializations are only for non-cv-qualified and non-reference specializations of `time_point` and `duration`.

If the user uses, say, `common_type<duration<X,Y> const, duration<A,B> const>`, they're not going to get the behavior they expect.

Suggest we clarify the requirements of `common_type`'s template parameters. Also, perhaps we can add blanket wording that `common_type<A [cv][&], B [cv][&]>` is required to be equivalent to `common_type<A,B>` (if that is in fact the way we intent this to work).

Also, the change to make `common_type` SFINAE-friendly regressed key functionality, as noted by Agustín K-ballo Bergé in c++std-lib-37178. Since `decay_t` is not applied until the very end of the type computation, user specializations are very likely to to be found.

Agustín says:

> Consider the following snippet:

> >     struct X {};
> >     struct Y { explicit Y(X){} };

> >     namespace std {
> >       template<> struct common_type<X, Y> { typedef Y type; };
> >       template<> struct common_type<Y, X> { typedef Y type; };
> >     }

> >     static_assert(is_same<common_type_t<X, Y>, Y>()); // (A)
> >     static_assert(is_same<common_type_t<X, Y, Y>, Y>()); // (B)
> >     static_assert(is_same<common_type_t<X, X, Y>, Y>()); // (C)

> Under the original wording, all three assertion holds. Under the current wording,

> - (A) picks the user-defined specialization, so the assertion holds.

> - (B) goes to the third bullet and, ignoring the user-defined specialization, looks for `decltype(true ? declval<X>() : declval<Y>())`; since it is ill-formed there is no common type.

> - (C) goes to the third bullet and yields `common_type_t<X&&, Y>`, which again misses the user-defined specialization.

The discussion following c++std-lib-35636 seemed to cohere around the idea that the primary `common_type` specialization should have the effect of stripping top-level ref and cv qualifiers by applying `std::decay_t` to its arguments and, if any of them change as a result of that transformation, re-dispatching to `common_type` on those transformed arguments, thereby picking up any user-defined specializations. This change to `common_type` would make the specializations in time.traits.specializations sufficient.

Suggested wording:

I'm afraid I don't know enough to suggest wording. But for exposition, the following is my best shot at implementing the suggested resolution. I believe it also fixes the regression noted by Agustín K-ballo Bergé in c++std-lib-37178.

>     namespace detail
>     {
>         template<typename T, typename U>
>         using default_common_t =
>             decltype(true? std::declval<T>() : std::declval<U>());

>         template<typename T, typename U, typename Enable = void>
>         struct common_type_if
>         {};

>         template<typename T, typename U>
>         struct common_type_if<T, U,
>           void_t<default_common_t<T, U>>>
>         {
>           using type = decay_t<default_common_t<T, U>>;
>         };

>         template<typename T, typename U,
>            typename TT = decay_t<T>, typename UU = decay_t<U>>
>         struct common_type2
>           : common_type<TT, UU> // Recurse to catch user specializations
>         {};

>         template<typename T, typename U>
>         struct common_type2<T, U, T, U>
>           : common_type_if<T, U>
>         {};

>         template<typename Meta, typename Enable = void>
>         struct has_type
>           : std::false_type
>         {};

>         template<typename Meta>
>         struct has_type<Meta, void_t<typename Meta::type>>
>           : std::true_type
>         {};

>         template<typename Meta, typename...Ts>
>         struct common_type_recurse
>           : common_type<typename Meta::type, Ts...>
>         {};

>         template<typename Meta, typename...Ts>
>         struct common_type_recurse_if
>           : std::conditional<
>               has_type<Meta>::value,
>               common_type_recurse<Meta, Ts...>,
>               empty
>             >::type
>         {};
>     }

>     template<typename ...Ts>
>     struct common_type
>     {};

>     template<typename T>
>     struct common_type<T>
>     {
>       using type = std::decay_t<T>;
>     };

>     template<typename T, typename U>
>     struct common_type<T, U>
>       : detail::common_type2<T, U>
>     {};

>     template<typename T, typename U, typename... Vs>
>     struct common_type<T, U, Vs...>
>       : detail::common_type_recurse_if<common_type<T, U>, Vs...>
>     {};

[2016-08 Chicago]

Walter and Nevin provide wording.

Previous resolution [SUPERSEDED]:

> [This also resolves the first part of 2460(i)]

> In Table 46 of N4604, entry for `common_type`:

> > ... may specialize this trait if at least one template parameter in the specialization is a user-defined type and no template parameter is cv-qualified.

> In [meta.trans.other] bullet 3.3:

> > ... whose second operand is an xvalue of type `T1` `decay_t<T1>`, and whose third operand is an xvalue of type `T2` `decay_t<T2>`. If ...

[2016-08-02, Chicago: Walt, Nevin, Rob, and Hal provide revised wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> [This also resolves the first part of LWG 2460(i)]

> 1. In Table 46 — "Other transformations" edit the entry for `common_type`:

>    > Table 46 — Other transformations
>    > Template | Comments
>    > --- | ---
>    > `…`
>    > `template <class... T> struct common_type;` | The member typedef `type` shall be defined or omitted as specified below. If it is omitted, there shall be no member `type`. All types in the parameter pack `T` shall be complete or (possibly cv) `void`. A program may specialize this trait for two cv-unqualified non-reference types if at least one template parameter in the specializationof them is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note]
>    > `…`

> 2. Edit 21.3.8.7 [meta.trans.other] p3 (and its subbullets) as shown below

>    > For the `common_type` trait applied to a parameter pack `T` of types, the member `type` shall be either defined or not present as follows:

>    > - If `sizeof...(T)` is zero, there shall be no member `type`.

>    > - If `sizeof...(T)` is one, let `T0` denote the sole type in the pack `T`. The member typedef `type` shall denote the same type as `decay_t<T0>`.

>    > - If `sizeof...(T)` is two, let `T1` and `T2`, respectively, denote the first and second types comprising `T`, and let `D1` and `D2`, respectively, denote `decay_t<T1>` and `decay_t<T2>`.

>    >   - If `is_same_v<T1, D1>` and `is_same_v<T2, D2>`, and if there is no specialization `common_type<T1, T2>`, let `C` denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `D1`, and whose third operand is an xvalue of type `D2`. If there is such a type `C`, the member typedef `type` shall denote `C`. Otherwise, there shall be no member `type`.

>    >   - If `not is_same_v<T1, D1>` or `not is_same_v<T2, D2>`, the member typedef `type` shall denote the same type, if any, as `common_type_t<D1, D2>`. Otherwise, there shall be no member `type`.

>    > - If `sizeof...(T)` is greater than onetwo, let `T1`, `T2`, and `R`, respectively, denote the first, second, and (pack of) remaining types comprising `T`. [Note: `sizeof...(R)` may be zero. — end note] Let `C` denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `T1`, and whose third operand is an xvalue of type `T2`. Let `C` denote `common_type_t<T1, T2>`. If there is such a type `C`, the member typedef `type` shall denote the same type, if any, as `common_type_t<C, R...>`. Otherwise, there shall be no member `type`.

[2016-08-03 Chicago LWG]

LWG asks for minor wording tweaks and for an added Note. Walter revises the Proposed Resolution accordingly.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> [This also resolves the first part of LWG 2460(i)]

> 1. In Table 46 — "Other transformations" edit the entry for `common_type`:

>    > Table 46 — Other transformations
>    > Template | Comments
>    > --- | ---
>    > `…`
>    > `template <class... T> struct common_type;` | The member typedef `type` shall be defined or omitted as specified below. If it is omitted, there shall be no member `type`. All types in the parameter pack `T` shall be complete or (possibly cv) `void`. A program may specialize this trait for two cv-unqualified non-reference types if at least one template parameter in the specializationof them is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note]
>    > `…`

> 2. Edit 21.3.8.7 [meta.trans.other] p3 (and its subbullets) as shown below

>    > For the `common_type` trait applied to a parameter pack `T` of types, the member `type` shall be either defined or not present as follows:

>    > 1. (3.1) — If `sizeof...(T)` is zero, there shall be no member `type`.

>    > 2. (3.2) — If `sizeof...(T)` is one, let `T0` denote the sole type in the pack `T`. The member typedef `type` shall denote the same type as `decay_t<T0>`.

>    > 3. (3.3) — If `sizeof...(T)` is two, let `T1` and `T2`, respectively, denote the first and second types comprising `T`, and let `D1` and `D2`, respectively, denote `decay_t<T1>` and `decay_t<T2>`.

>    >    1. (3.3.1) — If `is_same_v<T1, D1>` and `is_same_v<T2, D2>`, let `C` denote the type of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `D1`, and whose third operand is an xvalue of type `D2`. [Note: This will not apply if there is a specialization `common_type<D1, D2>`. — end note]

>    >    2. (3.3.2) — Otherwise, let `C` denote the type `common_type_t<D1, D2>`.

>    >    In either case, if there is such a type `C`, the member typedef `type` shall denote `C`. Otherwise, there shall be no member `type`.

>    > 4. (3.4) — If `sizeof...(T)` is greater than onetwo, let `T1`, `T2`, and `R`, respectively, denote the first, second, and (pack of) remaining types comprising `T`. [Note: `sizeof...(R)` may be zero. — end note] Let `C` denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `T1`, and whose third operand is an xvalue of type `T2`. Let `C` denote `common_type_t<T1, T2>`. If there is such a type `C`, the member typedef `type` shall denote the same type, if any, as `common_type_t<C, R...>`. Otherwise, there shall be no member `type`.

[2016-08-04 Chicago LWG]

Alisdair notes that 16.4.5.2.1 [namespace.std] p.1 seems to prohibit some kinds of specializations that we want to permit here and asks that the Table entry be augmented so as to specify the precise rules that a specialization is required to obey. Walter revises Proposed Resolution accordingly.

[2016-08-03 Chicago]

Fri PM: Move to Tentatively Ready

[2016-08-11 Daniel comments]

LWG 2763(i) presumably provides a superiour resolution that also fixes another bug in the Standard.

[2016-08-12]

Howard request to reopen this issue because of the problem pointed out by LWG 2763(i).

[2016-08-13 Tim Song comments]

In addition to the issue pointed out in LWG 2763(i), the current P/R no longer decays the type of the conditional expression. However, that seems harmless since 7 [expr]/5 means that the "type of an expression" is never a reference type, and 7.6.16 [expr.cond]'s rules appear to ensure that the type of the conditional expression will never be "decay-able" when fed with two xvalues of cv-unqualified non-array object type. Nonetheless, a note along the lines of "[Note: `C` is never a reference, function, array, or cv-qualified type. — end note]" may be appropriate, similar to the note at the end of [dcl.decomp]/1.

[2016-11-12, Issaquah]

Resolved by P0435R1

Proposed resolution:

This wording is relative to N4606.

[This also resolves the first part of LWG 2460(i)]

1. In Table 46 — "Other transformations" edit the entry for `common_type`:

   > Table 46 — Other transformations
   > Template | Comments
   > --- | ---
   > `…`
   > `template <class... T> struct common_type;` | Unless this trait is specialized (as specified in Note B, below), tThe member typedef `type` shall be defined or omitted as specified in Note A, below. If it is omitted, there shall be no member `type`. All types in the parameter pack `T` shall be complete or (possibly cv) `void`. A program may specialize this trait if at least one template parameter in the specialization is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note]
   > `…`

2. Edit 21.3.8.7 [meta.trans.other] p3 (and its subbullets) as shown below

   > -3- Note A: For the `common_type` trait applied to a parameter pack `T` of types, the member `type` shall be either defined or not present as follows:

   > 1. (3.1) — If `sizeof...(T)` is zero, there shall be no member `type`.

   > 2. (3.2) — If `sizeof...(T)` is one, let `T0` denote the sole type in the pack `T`. The member typedef `type` shall denote the same type as `decay_t<T0>`.

   > 3. (3.3) — If `sizeof...(T)` is two, let `T1` and `T2`, respectively, denote the first and second types comprising `T`, and let `D1` and `D2`, respectively, denote `decay_t<T1>` and `decay_t<T2>`.

   >    1. (3.3.1) — If `is_same_v<T1, D1>` and `is_same_v<T2, D2>`, let `C` denote the type of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `D1`, and whose third operand is an xvalue of type `D2`. [Note: This will not apply if there is a specialization `common_type<D1, D2>`. — end note]

   >    2. (3.3.2) — Otherwise, let `C` denote the type `common_type_t<D1, D2>`.

   >    In either case, if there is such a type `C`, the member typedef `type` shall denote `C`. Otherwise, there shall be no member `type`.

   > 4. (3.4) — If `sizeof...(T)` is greater than onetwo, let `T1`, `T2`, and `R`, respectively, denote the first, second, and (pack of) remaining types comprising `T`. [Note: `sizeof...(R)` may be zero. — end note] Let `C` denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `T1`, and whose third operand is an xvalue of type `T2`. Let `C` denote `common_type_t<T1, T2>`. If there is such a type `C`, the member typedef `type` shall denote the same type, if any, as `common_type_t<C, R...>`. Otherwise, there shall be no member `type`.

   > -?- Note B: A program may specialize the `common_type` trait for two cv-unqualified non-reference types if at least one of them is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note] Such a specialization need not have a member named `type`, but if it does, that member shall be a typedef-name for a cv-unqualified non-reference type that need not otherwise meet the specification set forth in Note A, above.

   > -4- [Example: Given these definitions: […]

### 2466(i). `allocator_traits::max_size()` default behavior is incorrect

Section: 16.4.4.6 [allocator.requirements], 20.2.9.3 [allocator.traits.members] Status: C++17 Submitter: Howard Hinnant Opened: 2015-01-17 Last modified: 2017-07-30

Priority: 3

View other active issues in [allocator.requirements].

View all other issues in [allocator.requirements].

View all issues with C++17 status.

Discussion:

Table 28 — "Allocator requirements" says that default behavior for `a.max_size()` is `numeric_limits<size_type>::max()`. And this is consistent with the matching statement for `allocator_traits` in 20.2.9.3 [allocator.traits.members]/p7:

>     static size_type max_size(const Alloc& a) noexcept;

> > Returns: `a.max_size()` if that expression is well-formed; otherwise, `numeric_limits<size_type>::max()`.

However, when allocating memory, an allocator must allocate `n*sizeof(value_type)` bytes, for example:

>     value_type*
>     allocate(std::size_t n)
>     {
>       return static_cast<value_type*>(::operator new (n * sizeof(value_type)));
>     }

When `n == numeric_limits<size_type>::max(), n * sizeof(value_type)` is guaranteed to overflow except when `sizeof(value_type) == 1`.

A more useful default would be `numeric_limits<size_type>::max() / sizeof(value_type)`.

[2015-05, Lenexa]

Marshall: Is this the right solution?
PJP: I think it's gilding the lily.
STL: I think this is right, and it doesn't interact with the incomplete container stuff because it's in a member function.
Marshall: Objections to this?
STL: Spaces around binary operators.
Hwrd: It's completely wrong without spaces.
Marshall: All in favor of Ready?
Lots.

Proposed resolution:

This wording is relative to N4296.

1. Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements", as indicated:

   > Table 28 — Allocator requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `…`
   > `a.max_size()` | `X::size_type` | the largest value that can meaningfully be passed to `X::allocate()` | `numeric_limits<size_type>::max()/sizeof(value_type)`
   > `…`

2. Change 20.2.9.3 [allocator.traits.members]/p7 as indicated:

   >     static size_type max_size(const Alloc& a) noexcept;

   > > Returns: `a.max_size()` if that expression is well-formed; otherwise, `numeric_limits<size_type>::max()/sizeof(value_type)`.

### 2467(i). `is_always_equal` has slightly inconsistent default

Section: 16.4.4.6 [allocator.requirements], 20.2.9.2 [allocator.traits.types] Status: C++17 Submitter: Howard Hinnant Opened: 2015-01-18 Last modified: 2017-07-30

Priority: 0

View other active issues in [allocator.requirements].

View all other issues in [allocator.requirements].

View all issues with C++17 status.

Discussion:

Table 28 — "Allocator requirements" says that `X::is_always_equal` has a default value of `is_empty<X>`, and this is consistent with the return type description:

> Identical to or derived from `true_type` or `false_type`

`is_empty<X>` is guaranteed to be derived from either `true_type` or `false_type`. So far so good.

20.2.9.2 [allocator.traits.types]/p10 says:

>     typedef see below is_always_equal;

> > Type: `Alloc::is_always_equal` if the qualified-id `Alloc::is_always_equal` is valid and denotes a type (14.8.2); otherwise `is_empty<Alloc>::type`.

This is subtly different than what Table 28 says is the default: `is_empty<Alloc>::type` is not `is_empty<Alloc>`, but is rather one of `true_type` or `false_type`.

There are two ways to fix this:

1. Change Table 28 to say: `is_empty<X>::type`.

   or

2. Change 20.2.9.2 [allocator.traits.types]/p10:

   > Type: `Alloc::is_always_equal` if the qualified-id `Alloc::is_always_equal` is valid and denotes a type (14.8.2); otherwise `is_empty<Alloc>::type`.

Both options are correct, and I see no reason to prefer one fix over the other. But Table 28 and 20.2.9.2 [allocator.traits.types]/p10 should be consistent with one another.

[2015-02 Cologne]

DK: We should accept the first bullet. GR: Why does `is_empty` even have a type member? AM: All type traits have a type member. I agree with DK's preference for the first type.

Proposed resolution:

This wording is relative to N4296.

1. Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as presented:

   > Table 28 — Allocator requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `…`
   > `X::is_always_equal` | Identical to or derived from `true_type` or `false_type` | […] | `is_empty<X>::type`
   > `…`

### 2468(i). Self-move-assignment of library types

Section: 16.4.5.9 [res.on.arguments], 16.4.4.2 [utility.arg.requirements], 16.4.6.17 [lib.types.movedfrom], 23.2.2 [container.requirements.general] Status: C++17 Submitter: Matt Austern Opened: 2015-01-22 Last modified: 2017-07-30

Priority: 2

View all other issues in [res.on.arguments].

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Discussion:

Suppose we write

>     vector<string> v{"a", "b", "c", "d"};
>     v = move(v);

What should be the state of `v` be? The standard doesn't say anything specific about self-move-assignment. There's relevant text in several parts of the standard, and it's not clear how to reconcile them.

16.4.5.9 [res.on.arguments] writes that, for all functions in the standard library, unless explicitly stated otherwise, "If a function argument binds to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument." The `MoveAssignable` requirements table in 16.4.4.2 [utility.arg.requirements] writes that, given `t = rv`, `t`'s state is equivalent to `rv`'s from before the assignment and `rv`'s state is unspecified (but valid). For containers specifically, the requirements table in 23.2.2 [container.requirements.general] says that, given `a = rv`, `a` becomes equal to what `rv` was before the assignment (and doesn't say anything about `rv`'s state post-assignment).

Taking each of these pieces in isolation, without reference to the other two:

- 16.4.5.9 [res.on.arguments] would clearly imply that the effect of `v = move(v)` is undefined.

- 16.4.4.2 [utility.arg.requirements] would clearly imply that `v = move(v)` has defined behavior. It might be read to imply that this is a no-op, or might be read to imply that it leaves `v` in a valid but unspecified state; I'm not sure which reading is more natural.

- 23.2.2 [container.requirements.general] would clearly imply that `v = move(v)` is a no-op.

It's not clear from the text how to put these pieces together, because it's not clear which one takes precedence. Maybe 16.4.5.9 [res.on.arguments] wins (it imposes an implicit precondition that isn't mentioned in the `MoveAssignable` requirements, so `v = move(v)` is undefined), or maybe 23.2.2 [container.requirements.general] wins (it explicitly gives additional guarantees for `Container::operator=` beyond what's guaranteed for library functions in general, so `v = move(v)` is a no-op), or maybe something else.

On the existing implementations that I checked, for what it's worth, `v = move(v)` appeared to clear the vector; it didn't leave the vector unchanged and it didn't cause a crash.

Proposed wording:

Informally: change the `MoveAssignable` and Container requirements tables (and any other requirements tables that mention move assignment, if any) to make it explicit that `x = move(x)` is defined behavior and it leaves `x` in a valid but unspecified state. That's probably not what the standard says today, but it's probably what we intended and it's consistent with what we've told users and with what implementations actually do.

[2015-10, Kona Saturday afternoon]

JW: So far, the library forbids self-assignment since it assumes that anything bound to an rvalue reference has no aliases. But self-assignment can happen in real code, and it can be implemented. So I want to add an exception to the Standard that this should be allowed and leave the object in a valid-but-unspecified state.

STL: When this is resolved, I want to see a) VBU for library types after self-move, but also b) requirements on user types for self-moves. E.g. should algorithms be required to avoid self-assignments (since a user-defined type might blow up)? HH: In other words, should we require that you can assign from moved-from values.

WEB: What can one generally do with moved-from values?

VV: Call any member function that has no preconditions.

JW: That's certainly the library requirement, and it's also good guidance for user types.

JW: I'm writing wording. I care about this.

Move to Open; Jonathan to provide wording

[2016-08-01, Howard provided wording]

[2016-08 Chicago]

Tuesday AM: Move to Tentatively Ready

Previous resolution [SUPERSEDED]:

> In 16.4.4.3 [swappable.requirements], modify Table 23 — `MoveAssignable` requirements [moveassignable]:

> > Table 23 — `MoveAssignable` requirements [moveassignable]
> > Expression | Return type | Return value | Post-condition
> > --- | --- | --- | ---
> > `t = rv` | `T&` | `t` | If `addressof(t) != addressof(rv)`, `t` is equivalent to the value of `rv` before the assignment
> > `rv`'s state is unspecified. [Note: `rv` must still meet the requirements of the library component that is using it, whether or not `addressof(t) == addressof(rv)`. The operations listed in those requirements must work as specified whether `rv` has been moved from or not. — end note]

[2016-08-07, Daniel reopens]

With the acceptance of LWG 2598(i), the proposed wording is invalid code, because it attempts to call `std::addressof` with an rvalue argument. It should be pointed out that the new restriction caused by 2598(i) doesn't affect real code, because any identity test within a move assignment operator (or any comparable function) would act on the current function argument, which is an lvalue in the context of the function body. The existing wording form of the issue could still be kept, if a helper variable would be introduced such as:

> Let `refrv` denote a reference initialized as if by `const T& refrv = rv;`. Then if `addressof(t) != addressof(refrv)`, `t` is equivalent to the value of `rv` before the assignment

But it seems to me that the same effect could be much easier realized by replacing the code form by a non-code English phrase that realizes the same effect.

[2016-09-09 Issues Resolution Telecon]

Move to Tentatively Ready

[2016-10-05, Tim Song comments]

The current P/R of LWG 2468 simply adds to `MoveAssignable` the requirement to tolerate self-move-assignment, but that doesn't actually do much about self-move-assignment of library types. Very few types in the library are explicitly required to satisfy `MoveAssignable`, so as written the restriction in 16.4.5.9 [res.on.arguments] would seem to still apply for any type that's not explicitly required to be `CopyAssignable` or `MoveAssignable`.

The current P/R also doesn't address the issue with 23.2.2 [container.requirements.general] noted in the issue discussion.

Proposed resolution:

This wording is relative to N4606.

1. In 16.4.4.3 [swappable.requirements], modify Table 23 — `MoveAssignable` requirements [moveassignable]:

   > Table 23 — `MoveAssignable` requirements [moveassignable]
   > Expression | Return type | Return value | Post-condition
   > --- | --- | --- | ---
   > `t = rv` | `T&` | `t` | If `t` and `rv` do not refer to the same object, `t` is equivalent to the value of `rv` before the assignment
   > `rv`'s state is unspecified. [Note: `rv` must still meet the requirements of the library component that is using it, whether or not `t` and `rv` refer to the same object. The operations listed in those requirements must work as specified whether `rv` has been moved from or not. — end note]

### 2469(i). Wrong specification of Requires clause of `operator[]` for `map` and `unordered_map`

Section: 23.4.3.3 [map.access], 23.5.3.3 [unord.map.elem] Status: C++17 Submitter: Tomasz Kamiński Opened: 2015-01-21 Last modified: 2017-07-30

Priority: 3

View all other issues in [map.access].

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Discussion:

The "Requires:" clause for the `operator[]` for the `unordered_map` and `map`, are defining separate requirements for insertability into container of `mapped_type` and `key_type`.

23.4.3.3 [map.access] p2: // `T& operator[](const key_type& x);`

Requires: `key_type` shall be `CopyInsertable` and `mapped_type` shall be `DefaultInsertable` into `*this`.

23.4.3.3 [map.access] p6: // `T& operator[](key_type&& x)`

Requires: `mapped_type` shall be `DefaultInsertable` into `*this`.

23.5.3.3 [unord.map.elem] p1: // `mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k);`

Requires: `mapped_type` shall be `DefaultInsertable` into `*this`. For the first operator, `key_type` shall be `CopyInsertable` into `*this`. For the second operator, `key_type` shall be `MoveConstructible`.

Definition of the appropriate requirements: 23.2.2 [container.requirements.general] p15.

`T` is `DefaultInsertable` into `X` means that the following expression is well-formed: //p15.1

`allocator_traits<A>::construct(m, p)`

`T` is `MoveInsertable` into `X` means that the following expression is well-formed: //p15.3

`allocator_traits<A>::construct(m, p, rv)`

`T` is `CopyInsertable` into `X` means that, in addition to `T` being `MoveInsertable` into `X`, the following expression is well-formed: //p15.4

`allocator_traits<A>::construct(m, p, v)`

In the context of above definition the requirement "`key_type` shall be `CopyInsertable` into `*this`" would mean that the key element of the `pair<const key_type, mapped_type>` (`value_type` of the map) should be constructed using separate call to the `construct` method, the same applies for the `mapped_type`. Such behavior is explicitly prohibited by 23.2.2 [container.requirements.general] p3.

> For the components affected by this sub-clause that declare an allocator_type, objects stored in these components shall be constructed using the `allocator_traits<allocator_type>::construct` function and destroyed using the `allocator_traits<allocator_type>::destroy` function (20.7.8.2). These functions are called only for the container's element type, not for internal types used by the container.

It clearly states that `element_type` of the map, must be constructed using allocator for value type, which disallows using of separate construction of first and second element, regardless of the fact if it can be actually performed without causing undefined behavior.

That means that the `MoveInsertable` and similar requirements may only be expressed in terms of `value_type`, not its members types.

[2015-02 Cologne]

This issue is related to 2464(i).

GR: Effects should say "returns ...". DK: Or just have a Returns clause? MC: A Returns clause is a directive to implementers.

TK/DK: This PR fails to address the requirements about which it complained in the first place. DK: I can reword this. TK can help.

[2015-03-29, Daniel provides improved wording]

The revised wording fixes the proper usage of the magic "Equivalent to" wording, which automatically induces Requires:, Returns:, and Complexity: elements (and possibly more). This allows us to strike all the remaining elements, because they fall out from the semantics of the wording defined by 2464(i). In particular it is important to realize that the wording form

> `value_type` shall be `EmplaceConstructible` into `map` from `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`

degenerates for the empty pack expansion `args` to:

> `value_type` shall be `EmplaceConstructible` into `map` from `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple()`

which again means that such a `pair` construction (assuming `std::allocator`) would copy `k` into member `first` and would value-initialize member `second`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> Accept resolution of the issue issue 2464(i) and define `operator[]` as follows (This would also address issue 2274(i)):

> 1. Change 23.4.3.3 [map.access] as indicated:

>    >     T& operator[](const key_type& x);

>    > > -1- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(x, T())` into the mapEquivalent to: `try_emplace(x).first->second`.

>    > > […]

>    >     T& operator[](key_type&& x);

>    > > -5- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(std::move(x), T())` into the mapEquivalent to: `try_emplace(move(x)).first->second`.

> 2. Change 23.5.3.3 [unord.map.elem] as indicated:

>    >     mapped_type& operator[](const key_type& k);
>    >     mapped_type& operator[](key_type&& k);

>    > > […]

>    > > -2- Effects: If the `unordered_map` does not already contain an element whose key is equivalent to `k`, the first operator inserts the value `value_type(k, mapped_type())` and the second operator inserts the value `value_type(std::move(k), mapped_type())`For the first operator, equivalent to: `try_emplace(k).first->second`; for the second operator, equivalent to: `try_emplace(move(k)).first->second`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> Accept resolution of the issue issue 2464(i) and define `operator[]` as follows (This would also address issue 2274(i)):

> 1. Change 23.4.3.3 [map.access] as indicated:

>    >     T& operator[](const key_type& x);

>    > > -1- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(x, T())` into the map.Equivalent to: `return try_emplace(x).first->second;`

>    > > -2- Requires: `key_type` shall be `CopyInsertable` and `mapped_type` shall be `DefaultInsertable` into `*this`.

>    > > -3- Returns: A reference to the `mapped_type` corresponding to `x` in `*this`.

>    > > -4- Complexity: Logarithmic.

>    >     T& operator[](key_type&& x);

>    > > -5- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(std::move(x), T())` into the map.Equivalent to: `return try_emplace(move(x)).first->second;`

>    > > -6- Requires: `mapped_type` shall be `DefaultInsertable` into `*this`.

>    > > -7- Returns: A reference to the `mapped_type` corresponding to `x` in `*this`.

>    > > -8- Complexity: Logarithmic.

> 2. Change 23.5.3.3 [unord.map.elem] as indicated:

>    >     mapped_type& operator[](const key_type& k);
>    >     mapped_type& operator[](key_type&& k);

>    > > -1- Requires: `mapped_type` shall be `DefaultInsertable` into `*this`. For the first operator, `key_type` shall be `CopyInsertable` into `*this`. For the second operator, `key_type` shall be `MoveConstructible`.

>    > > -2- Effects: If the `unordered_map` does not already contain an element whose key is equivalent to `k`, the first operator inserts the value `value_type(k, mapped_type())` and the second operator inserts the value `value_type(std::move(k), mapped_type())`For the first operator, equivalent to:

>    > > >     return try_emplace(k).first->second;

>    > > for the second operator, equivalent to:

>    > > >     return try_emplace(move(k)).first->second;

>    > > -3- Returns: A reference to `x.second`, where `x` is the (unique) element whose key is equivalent to `k`.

>    > > -4- Complexity: Average case 𝒪(`1`), worst case 𝒪(`size()`).

Proposed resolution:

This wording is relative to N4431.

Accept resolution of the issue issue 2464(i) and define `operator[]` as follows (This would also address issue 2274(i)):

1. Change 23.4.3.3 [map.access] as indicated:

   >     T& operator[](const key_type& x);

   > > -1- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(x, T())` into the map.Equivalent to: `return try_emplace(x).first->second;`

   > > -2- Requires: `key_type` shall be `CopyInsertable` and `mapped_type` shall be `DefaultInsertable` into `*this`.

   > > -3- Returns: A reference to the `mapped_type` corresponding to `x` in `*this`.

   > > -4- Complexity: Logarithmic.

   >     T& operator[](key_type&& x);

   > > -5- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(std::move(x), T())` into the map.Equivalent to: `return try_emplace(move(x)).first->second;`

   > > -6- Requires: `mapped_type` shall be `DefaultInsertable` into `*this`.

   > > -7- Returns: A reference to the `mapped_type` corresponding to `x` in `*this`.

   > > -8- Complexity: Logarithmic.

2. Change 23.5.3.3 [unord.map.elem] as indicated:

   >     mapped_type& operator[](const key_type& k);
   >     mapped_type& operator[](key_type&& k);

   > > -1- Requires: `mapped_type` shall be `DefaultInsertable` into `*this`. For the first operator, `key_type` shall be `CopyInsertable` into `*this`. For the second operator, `key_type` shall be `MoveConstructible`.

   > > -2- Effects: Equivalent to `return try_emplace(k).first->second;` If the `unordered_map` does not already contain an element whose key is equivalent to `k`, the first operator inserts the value `value_type(k, mapped_type())` and the second operator inserts the value `value_type(std::move(k), mapped_type())`

   > > -3- Returns: A reference to `x.second`, where `x` is the (unique) element whose key is equivalent to `k`.

   > > -4- Complexity: Average case 𝒪(`1`), worst case 𝒪(`size()`).

   >     mapped_type& operator[](key_type&& k);

   > > -?- Effects: Equivalent to `return try_emplace(move(k)).first->second;`

### 2470(i). Allocator's `destroy` function should be allowed to fail to instantiate

Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2015-03-22 Last modified: 2017-07-30

Priority: Not Prioritized

View other active issues in [allocator.requirements].

View all other issues in [allocator.requirements].

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Discussion:

This issue is a spin-off of issue LWG 2447(i): It focuses on the observation that 16.4.4.6 [allocator.requirements] p9 (based on the numbering of working draft N4296) gives the template member `construct` more relaxations than the template member `destroy`:

> An allocator may constrain the types on which it can be instantiated and the arguments for which its `construct` member may be called. If a type cannot be used with a particular allocator, the allocator class or the call to `construct` may fail to instantiate.

Construction and destruction of a type `T` are usually intimately related to each other, so it seems similarly useful to allow the `destroy` member to fail to instantiate for a possible sub-set of instantiation types.

[2015-04-01 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N4296.

1. Change 16.4.4.6 [allocator.requirements] p9 as indicated:

   > -8- An allocator may constrain the types on which it can be instantiated and the arguments for which its `construct` or `destroy` members may be called. If a type cannot be used with a particular allocator, the allocator class or the call to `construct` or `destroy` may fail to instantiate.

### 2473(i). `basic_filebuf`'s relation to C `FILE` semantics

Section: 31.10.3.5 [filebuf.virtuals] Status: C++17 Submitter: Aaron Ballman Opened: 2015-02-09 Last modified: 2017-07-30

Priority: 2

View all other issues in [filebuf.virtuals].

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Discussion:

The restrictions on reading and writing a sequence controlled by an object of class `basic_filebuf<charT, traits>` are the same as for reading and writing with the Standard C library `FILE`s. One of the restrictions placed by C is on the behavior of a stream that is opened for input and output. See the C99 standard, 7.19.5.3p6 for more details, but the gist is that when opened in update mode, reads and writes must have an intervening file positioning or flushing call to not trigger UB.

31.10.3.5 [filebuf.virtuals] p13 specifies that `basic_filebuf::seekoff()` calls `std::fseek()`. However, there is no mention of `std::fseek()` in `basic_filebuf::seekpos()`, and no mention of `std::fflush()` in `basic_filebuf::sync()`, which seem like an oversight.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> 1. Change 31.10.3.5 [filebuf.virtuals] p16 as follows [Editorial note: A footnote referring to `fseek` is not needed, because this is already covered by the existing footnote 334]:

>    > -16- Alters the file position, if possible, to correspond to the position stored in `sp` (as described below). Altering the file position performs as follows:

>    > 1. if `(om & ios_base::out) != 0`, then update the output sequence and write any unshift sequence;

>    > 2. set the file position to `sp` as if by calling `std::fseek(file, sp, SEEK_SET)`;

>    > 3. if `(om & ios_base::in) != 0`, then update the input sequence;

>    > where `om` is the open mode passed to the last call to `open()`. The operation fails if `is_open()` returns false.

> 2. Change 31.10.3.5 [filebuf.virtuals] p19 as follows and add a new footnote that mimics comparable footnotes in 31.10.3.4 [filebuf.members] and 31.10.3.5 [filebuf.virtuals]:

>    > -19- Effects: If a put area exists, calls `filebuf::overflow` to write the characters to the file, then flushes the file as if by calling `std::fflush(file)` [Footnote: The function signature `fflush(FILE*)` is declared in `<cstdio>` (27.9.2).]. If a get area exists, the effect is implementation-defined.

[2015-05, Lenexa]

Aaron provides improved wording by removing the params from `std::fseek()` due to the concerns regarding the parameters on systems where `fseek` uses 32-bit parameters.

Second wording improvement, replacing the new one see below. It

1. drops the `std::`

2. drops the footnote for `fflush`

3. replaces `fseek` with `fsetpos`

Previous resolution [SUPERSEDED]:

> This wording is relative to N4431.

> 1. Change 31.10.3.5 [filebuf.virtuals] p16 as follows [Editorial note: A footnote referring to `fseek` is not needed, because this is already covered by the existing footnote 334]:

>    > -16- Alters the file position, if possible, to correspond to the position stored in `sp` (as described below). Altering the file position performs as follows:

>    > 1. if `(om & ios_base::out) != 0`, then update the output sequence and write any unshift sequence;

>    > 2. set the file position to `sp` as if by a call to `std::fseek`;

>    > 3. if `(om & ios_base::in) != 0`, then update the input sequence;

>    > where `om` is the open mode passed to the last call to `open()`. The operation fails if `is_open()` returns false.

> 2. Change 31.10.3.5 [filebuf.virtuals] p19 as follows and add a new footnote that mimics comparable footnotes in 31.10.3.4 [filebuf.members] and 31.10.3.5 [filebuf.virtuals]:

>    > -19- Effects: If a put area exists, calls `filebuf::overflow` to write the characters to the file, then flushes the file as if by calling `std::fflush(file)` [Footnote: The function signature `fflush(FILE*)` is declared in `<cstdio>` (27.9.2).]. If a get area exists, the effect is implementation-defined.

Proposed resolution:

This wording is relative to N4431.

1. Change 31.10.3.5 [filebuf.virtuals] p16 as follows:

   > -16- Alters the file position, if possible, to correspond to the position stored in `sp` (as described below). Altering the file position performs as follows:

   > 1. if `(om & ios_base::out) != 0`, then update the output sequence and write any unshift sequence;

   > 2. set the file position to `sp` as if by a call to `fsetpos`;

   > 3. if `(om & ios_base::in) != 0`, then update the input sequence;

   > where `om` is the open mode passed to the last call to `open()`. The operation fails if `is_open()` returns false.

2. Change 31.10.3.5 [filebuf.virtuals] p19 as follows:

   > -19- Effects: If a put area exists, calls `filebuf::overflow` to write the characters to the file, then flushes the file as if by calling `fflush(file)`. If a get area exists, the effect is implementation-defined.

### 2475(i). Allow overwriting of `std::basic_string` terminator with `charT()` to allow cleaner interoperation with legacy APIs

Section: 27.4.3.6 [string.access] Status: C++17 Submitter: Matt Weber Opened: 2015-02-21 Last modified: 2017-07-30

Priority: 3

View all other issues in [string.access].

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Discussion:

It is often desirable to use a `std::basic_string` object as a buffer when interoperating with libraries that mutate null-terminated arrays of characters. In many cases, these legacy APIs write a null terminator at the specified end of the provided buffer. Providing such a function with an appropriately-sized `std::basic_string` results in undefined behavior when the `charT` object at the `size()` position is overwritten, even if the value remains unchanged.

Absent the ability to allow for this, applications are forced into pessimizations such as: providing appropriately-sized `std::vectors` of `charT` for interoperating with the legacy API, and then copying the `std::vector` to a `std::basic_string`; providing an oversized `std::basic_string` object and then calling `resize()` later.

A trivial example:

>     #include <string>
>     #include <vector>

>     void legacy_function(char *out, size_t count) {
>       for (size_t i = 0; i < count; ++i) {
>         *out++ = '0' + (i % 10);
>       }
>       *out = '\0'; // if size() == count, this results in undefined behavior
>     }

>     int main() {
>       std::string s(10, '\0');
>       legacy_function(&s[0], s.size()); // undefined behavior

>       std::vector<char> buffer(11);
>       legacy_function(&buffer[0], buffer.size() - 1);
>       std::string t(&buffer[0], buffer.size() - 1); // potentially expensive copy

>       std::string u(11, '\0');
>       legacy_function(&u[0], u.size() - 1);
>       u.resize(u.size() - 1); // needlessly complicates the program's logic
>     }

A slight relaxation of the requirement on the returned object from the element access operator would allow for this interaction with no semantic change to existing programs.

[2016-08 Chicago]

Tues PM: This should also apply to non-const `data()`. Billy to update wording.

Fri PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4296.

1. Edit 27.4.3.6 [string.access] as indicated:

   >     const_reference operator[](size_type pos) const;
   >     reference operator[](size_type pos);

   > > -1- Requires: […]

   > > -2- Returns: `*(begin() + pos)` if `pos < size()`. Otherwise, returns a reference to an object of type `charT` with value `charT()`, where modifying the object to any value other than `charT()` leads to undefined behavior.

   > > […]

### 2476(i). `scoped_allocator_adaptor` is not assignable

Section: 20.6.1 [allocator.adaptor.syn] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-03-02 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

The class definition in 20.6.1 [allocator.adaptor.syn] declares a move constructor, which means that the copy assignment operator is defined as deleted, and no move assignment operator is declared.

This means a `scoped_allocator_adaptor` is not assignable, and a container using `scoped_allocator_adaptor<A...>` may not be `CopyAssignable` or `MoveAssignable` (depending on the `propagate_on_container_xxxx_assignment` traits of the outer and inner allocator types).

[2015-04-03 Howard comments]

If the contained allocators are not assignable, I think we need the ability of `= default` to automagically become `= delete`. My concern is that `is_copy_assignable<scoped_allocator_adaptor<CustomAllocator>>::value` get the right answer for both cases:

1. `is_copy_assignable<CustomAllocator>::value` is `true`.

2. `is_copy_assignable<CustomAllocator>::value` is `false`.

If we allow the vendor to declare and provide the copy assignment operator, the chance of getting #2 correct goes to zero.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> 1. Add to the synopsis in 20.6.1 [allocator.adaptor.syn]/1 [Editorial remark: The proposed wording does not explicitly specify the semantics of the added copy/move assignment operators, based on 16.3.3.5 [functions.within.classes] p1, which says:

>    "For the sake of exposition, Clauses 18 through 30 and Annex D do not describe copy/move constructors, assignment operators, or (non-virtual) destructors with the same apparent semantics as those that can be generated by default (12.1, 12.4, 12.8)."

>    — end remark]:

>    >     […]
>    >     template <class OuterA2>
>    >       scoped_allocator_adaptor(
>    >         scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;

>    >     scoped_allocator_adaptor& operator=(const scoped_allocator_adaptor&);
>    >     scoped_allocator_adaptor& operator=(scoped_allocator_adaptor&&);

>    >     ~scoped_allocator_adaptor();
>    >     […]

[2015-05, Lenexa]

Move to Immediate.

Proposed resolution:

This wording is relative to N4296.

1. Add to the synopsis in 20.6.1 [allocator.adaptor.syn]/1:

   >     […]
   >     template <class OuterA2>
   >       scoped_allocator_adaptor(
   >         scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;

   >     scoped_allocator_adaptor& operator=(const scoped_allocator_adaptor&) = default;
   >     scoped_allocator_adaptor& operator=(scoped_allocator_adaptor&&) = default;

   >     ~scoped_allocator_adaptor();
   >     […]

### 2477(i). Inconsistency of wordings in `std::vector::erase()` and `std::deque::erase()`

Section: 23.3.5.4 [deque.modifiers], 23.3.13.5 [vector.modifiers] Status: C++17 Submitter: Anton Savin Opened: 2015-03-03 Last modified: 2017-07-30

Priority: 0

View other active issues in [deque.modifiers].

View all other issues in [deque.modifiers].

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Discussion:

In the latest draft N4296, and in all drafts up to at least N3337:

23.3.5.4 [deque.modifiers]/5 (regarding `deque::erase()`):

> Complexity: The number of calls to the destructor is the same as the number of elements erased, but the number of calls to the assignment operator is no more than the lesser of the number of elements before the erased elements and the number of elements after the erased elements.

23.3.13.5 [vector.modifiers]/4 (regarding `vector::erase()`):

> Complexity: The destructor of `T` is called the number of times equal to the number of the elements erased, but the move assignment operator of `T` is called the number of times equal to the number of elements in the vector after the erased elements.

Is there any reason for explicit mentioning of move assignment for `std::vector::erase()`? Shouldn't these two wordings be the same with this regard?

Also, for `std::deque`, it's not clear from the text which destructors and assignment operators are called.

[2015-05, Lenexa]

Move to Immediate.

Proposed resolution:

This wording is relative to N4296.

1. Change 23.3.5.4 [deque.modifiers]/5 to:

   -5- Complexity: The number of calls to the destructor of `T` is the same as the number of elements erased, but the number of calls to the assignment operator of `T` is no more than the lesser of the number of elements before the erased elements and the number of elements after the erased elements.

2. Change 23.3.13.5 [vector.modifiers]/4 to:

   -4- Complexity: The destructor of `T` is called the number of times equal to the number of the elements erased, but the move assignment operator of `T` is called the number of times equal to the number of elements in the vector after the erased elements.

### 2478(i). Unclear how `wstring_convert` uses `cvtstate`

Section: 99 [depr.conversions.string] Status: Resolved Submitter: Jonathan Wakely Opened: 2015-03-04 Last modified: 2025-11-11

Priority: 4

View all other issues in [depr.conversions.string].

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Discussion:

How do `wstring_convert::from_bytes` and `wstring_convert::to_bytes` use the `cvtstate` member?

Is it passed to the `codecvt` member functions? Is a copy of it passed to the member functions? "Otherwise it shall be left unchanged" implies a copy is used, but if that's really what's intended there are simpler ways to say so.

[2025-11-10 Resolved by the removal of `wstring_convert` via paper P2872R3 in Tokyo, 2024. Status changed: New → Resolved.]

Proposed resolution:

### 2479(i). Unclear how `wbuffer_convert` uses `cvtstate`

Section: 99 [depr.conversions.buffer] Status: Resolved Submitter: Jonathan Wakely Opened: 2015-03-04 Last modified: 2025-11-11

Priority: 4

View all other issues in [depr.conversions.buffer].

View all issues with Resolved status.

Discussion:

How does `wbuffer_convert` use the `cvtstate` member?

Is the same conversion state object used for converting both the get and put areas? That means a read which runs out of bytes halfway through a multibyte character will leave some shift state in cvtstate, which would then be used by a following write, even though the shift state of the get area is unrelated to the put area.

[2025-11-10 Resolved by the removal of `wbuffer_convert` via paper P2872R3 in Tokyo, 2024. Status changed: New → Resolved.]

Proposed resolution:

### 2480(i). Error handling of `wbuffer_convert` unclear

Section: 99 [depr.conversions.buffer] Status: Resolved Submitter: Jonathan Wakely Opened: 2015-03-04 Last modified: 2025-11-11

Priority: 4

View all other issues in [depr.conversions.buffer].

View all issues with Resolved status.

Discussion:

If a `codecvt` conversion returns `codecvt_base::error` should that be treated as `EOF`? An exception? Should all the successfully converted characters before a conversion error be available to the users of the `wbuffer_convert` and/or the internal `streambuf`, or does a conversion error lose information?

[2025-11-10 Resolved by the removal of `wbuffer_convert` via paper P2872R3 in Tokyo, 2024. Status changed: New → Resolved.]

Proposed resolution:

### 2481(i). `wstring_convert` should be more precise regarding "byte-error string" etc.

Section: 99 [depr.conversions.string] Status: Resolved Submitter: Jonathan Wakely Opened: 2015-03-04 Last modified: 2025-11-11

Priority: 4

View all other issues in [depr.conversions.string].

View all issues with Resolved status.

Discussion:

Paragraph 4 of 99 [depr.conversions.string] introduces `byte_err_string` as "a byte string to display on errors". What does display mean? The string is returned on error, it's not displayed anywhere.

Paragraph 14 says "Otherwise, if the object was constructed with a byte-error string, the member function shall return the byte-error string." The term byte-error string is not used anywhere else.

Paragraph 17 talks about storing "default values in `byte_err_string`". What default value? Is "Hello, world!" allowed? If it means default-construction it should say so. If paragraph 14 says it won't be used what does it matter how it's initialized? The end of the paragraph refers to storing "`byte_err` in `byte_err_string`". This should be more clearly related to the wording in paragraph 14.

It might help if the constructor (and destructor) was specified before the other member functions, so it can more formally define the difference between being "constructed with a byte-error string" and not.

All the same issues apply to the `wide_err_string` member.

[2025-11-10 Resolved by the removal of `wstring_convert` via paper P2872R3 in Tokyo, 2024. Status changed: New → Resolved.]

Proposed resolution:

### 2482(i). §[c.strings] Table 73 mentions nonexistent functions

Section: 27.5 [c.strings] Status: C++17 Submitter: S. B.Tam Opened: 2015-01-18 Last modified: 2017-07-30

Priority: Not Prioritized

View other active issues in [c.strings].

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Discussion:

N4296 Table 73 mentions the functions `mbsrtowc` and `wcsrtomb`, which are not defined in ISO C or ISO C++. Presumably they should be `mbsrtowcs` and `wcsrtombs` instead.

[2015-04-02 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N4296.

1. > Table 33 — Potential `mbstate_t` data races
   > `mbrlen` | `mbrtowc` | `mbsrtowcs` | `mbtowc` | `wcrtomb`
   > `wcsrtombs` | `wctomb`

### 2483(i). `throw_with_nested()` should use `is_final`

Section: 17.9.8 [except.nested] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 2

View all other issues in [except.nested].

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Discussion:

When N2559 was voted into the Working Paper, it said "This function template must take special case to handle non-class types, unions and `[[final]]` classes that cannot be derived from, and [...]". However, its Standardese didn't handle `final` classes, and this was never revisited. Now that we have `is_final`, we can achieve this proposal's original intention.

Additionally, we need to handle the case where `U` is `nested_exception` itself. `is_base_of`'s wording handles this and ignores cv-qualifiers. (Note that `is_class` detects "non-union class type".)

[2015-05, Lenexa]

STL, MC and JW already do this
MC: move to Ready, bring to motion on Friday
7 in favor, none opposed

Proposed resolution:

This wording is relative to N4296.

1. Change 17.9.8 [except.nested] as depicted:

   >     template <class T> [[noreturn]] void throw_with_nested(T&& t);

   > > -6- Let `U` be `remove_reference_t<T>`.

   > > -7- Requires: `U` shall be `CopyConstructible`.

   > > -8- Throws: if `U` is a non-union class type not derived from `nested_exception` `is_class<U>::value && !is_final<U>::value && !is_base_of<nested_exception, U>::value` is `true`, an exception of unspecified type that is publicly derived from both `U` and `nested_exception` and constructed from `std::forward<T>(t)`, otherwise `std::forward<T>(t)`.

### 2484(i). `rethrow_if_nested()` is doubly unimplementable

Section: 17.9.8 [except.nested] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 2

View all other issues in [except.nested].

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Discussion:

`rethrow_if_nested()` wants to determine "If the dynamic type of `e` is publicly and unambiguously derived from `nested_exception`", but 7.6.1.7 [expr.dynamic.cast] specifies that `dynamic_cast` has a couple of limitations.

First, nonpolymorphic inputs. These could be non-classes, or nonpolymorphic classes. The Standardese handles non-classes, although implementers need special logic. (If `E` is `int`, the dynamic type can't possibly derive from `nested_exception`. Implementers need to detect this and avoid `dynamic_cast`, which would be ill-formed due to 7.6.1.7 [expr.dynamic.cast]/2.) The Standardese is defective when `E` is a nonpolymorphic class. Consider the following example:

>     struct Nonpolymorphic { };
>     struct MostDerived : Nonpolymorphic, nested_exception { };
>     MostDerived md;
>     const Nonpolymorphic& np = md;
>     rethrow_if_nested(np);

According to 3.19 [defns.dynamic.type], the dynamic type of `np` is `MostDerived`. However, it's physically impossible to discover this, and attempting to do so will lead to an ill-formed `dynamic_cast` (7.6.1.7 [expr.dynamic.cast]/6). The Standardese must be changed to say that if `E` is nonpolymorphic, nothing happens.

Second, statically good but dynamically bad inputs. Consider the following example:

>     struct Nested1 : nested_exception { };
>     struct Nested2 : nested_exception { };
>     struct Ambiguous : Nested1, Nested2 { };
>     Ambiguous amb;
>     const Nested1& n1 = amb;
>     rethrow_if_nested(n1);

Here, the static type `Nested1` is good (i.e. publicly and unambiguously derived from `nested_exception`), but the dynamic type `Ambiguous` is bad. The Standardese currently says that we have to detect the dynamic badness, but `dynamic_cast` won't let us. 7.6.1.7 [expr.dynamic.cast]/3 and /5 are special cases (identity-casting and upcasting, respectively) that activate before the "run-time check" behavior that we want (/6 and below). Statically good inputs succeed (regardless of the dynamic type) and statically bad inputs are ill-formed (implementers must use type traits to avoid this).

It might be possible to implement this with clever trickery involving virtual base classes, but implementers shouldn't be asked to do that. It would definitely be possible to implement this with a compiler hook (a special version of `dynamic_cast`), but implementers shouldn't be asked to do so much work for such an unimportant case. (This case is pathological because the usual way of adding `nested_exception` inheritance is `throw_with_nested()`, which avoids creating bad inheritance.) The Standardese should be changed to say that statically good inputs are considered good.

Finally, we want `is_base_of`'s "base class or same class" semantics. If the static type is `nested_exception`, we have to consider it good due to `dynamic_cast`'s identity-casting behavior. And if the dynamic type is `nested_exception`, it is definitely good.

[2015-05, Lenexa]

WB: so the `is_polymorphic` trait must be used?
STL and JW: yes, that must be used to decide whether to try using `dynamic_cast` or not.
JW: I'd already made this fix in our implementation
STL: the harder case also involves `dynamic_cast`. should not try using `dynamic_cast` if we can statically detect it is OK, doing the `dynamic_cast` might fail.
STL: finally, want "is the same or derived from" behaviour of `is_base_of`
WB: should there be an "else no effect" at the end? We have "Otherwise, if ..." and nothing saying what if the condition is false.
TP I agree.
MC: move to Ready and bring to motion on Friday
7 in favor, none opposed

Proposed resolution:

This wording is relative to N4296.

1. Change 17.9.8 [except.nested] as depicted:

   >     template <class E> void rethrow_if_nested(const E& e);

   > > -9- Effects: If `E` is not a polymorphic class type, there is no effect. Otherwise, if the static type or the dynamic type of `e` is `nested_exception` or is publicly and unambiguously derived from `nested_exception`, calls `dynamic_cast<const nested_exception&>(e).rethrow_nested()`.

### 2485(i). `get()` should be overloaded for `const tuple&&`

Section: 22.4.8 [tuple.elem] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 1

View all other issues in [tuple.elem].

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Discussion:

`const` rvalues are weird, but they're part of the type system. Consider the following code:

>     #include <functional>
>     #include <string>
>     #include <tuple>

>     using namespace std;

>     string str1() { return "one"; }
>     const string str2() { return "two"; }
>     tuple<string> tup3() { return make_tuple("three"); }
>     const tuple<string> tup4() { return make_tuple("four"); }

>     int main() {
>       // cref(str1()); // BAD, properly rejected
>       // cref(str2()); // BAD, properly rejected
>       // cref(get<0>(tup3())); // BAD, properly rejected
>       cref(get<0>(tup4())); // BAD, but improperly accepted!
>     }

As `tuple` is a fundamental building block (and the only convenient way to have variadic data members), it should not open a hole in the type system. `get()` should imitate 7.6.1.5 [expr.ref]'s rules for accessing data members. (This is especially true for `pair`, where both `get<0>()` and `.first` are available.)

While we're in the neighborhood, we can dramatically simplify the wording here. All we need to do is follow 22.4.8 [tuple.elem]/9's example of saying "Returns: A reference to STUFF", and allow implementers to figure out how to achieve that with the given return types.

[2015-05, Lenexa]

TP: for the existing overloads there's no change to the code, just descriptions?
STL: right.
JW: I love it
MC: in favor of moving to Ready and bringing up for vote on Friday
7 in favor, none opposed

Proposed resolution:

This wording is relative to N4296.

1. Change 22.2 [utility]/2 "Header `<utility>` synopsis" as depicted:

   >     […]
   >     template<size_t I, class T1, class T2>
   >       constexpr tuple_element_t<I, pair<T1, T2>>&
   >         get(pair<T1, T2>&) noexcept;
   >     template<size_t I, class T1, class T2>
   >       constexpr tuple_element_t<I, pair<T1, T2>>&&
   >         get(pair<T1, T2>&&) noexcept;
   >     template<size_t I, class T1, class T2>
   >       constexpr const tuple_element_t<I, pair<T1, T2>>&
   >         get(const pair<T1, T2>&) noexcept;
   >     template<size_t I, class T1, class T2>
   >       constexpr const tuple_element_t<I, pair<T1, T2>>&&
   >         get(const pair<T1, T2>&&) noexcept;
   >     template <class T, class U>
   >       constexpr T& get(pair<T, U>& p) noexcept;
   >     template <class T, class U>
   >       constexpr const T& get(const pair<T, U>& p) noexcept;
   >     template <class T, class U>
   >       constexpr T&& get(pair<T, U>&& p) noexcept;
   >     template <class T, class U>
   >       constexpr const T&& get(const pair<T, U>&& p) noexcept;
   >     template <class T, class U>
   >       constexpr T& get(pair<U, T>& p) noexcept;
   >     template <class T, class U>
   >       constexpr const T& get(const pair<U, T>& p) noexcept;
   >     template <class T, class U>
   >       constexpr T&& get(pair<U, T>&& p) noexcept;
   >     template <class T, class U>
   >       constexpr const T&& get(const pair<U, T>&& p) noexcept;
   >     […]

2. Change 22.4.1 [tuple.general]/2 "Header `<tuple>` synopsis" as depicted:

   >     […]
   >     // 20.4.2.6, element access:
   >     template <size_t I, class... Types>
   >     constexpr tuple_element_t<I, tuple<Types...>>&
   >     get(tuple<Types...>&) noexcept;
   >     template <size_t I, class... Types>
   >     constexpr tuple_element_t<I, tuple<Types...>>&&
   >     get(tuple<Types...>&&) noexcept;
   >     template <size_t I, class... Types>
   >     constexpr const tuple_element_t<I, tuple<Types...>>&
   >     get(const tuple<Types...>&) noexcept;
   >     template <size_t I, class... Types>
   >     constexpr const tuple_element_t<I, tuple<Types...>>&&
   >     get(const tuple<Types...>&&) noexcept;
   >     template <class T, class... Types>
   >     constexpr T& get(tuple<Types...>& t) noexcept;
   >     template <class T, class... Types>
   >     constexpr T&& get(tuple<Types...>&& t) noexcept;
   >     template <class T, class... Types>
   >     constexpr const T& get(const tuple<Types...>& t) noexcept;
   >     template <class T, class... Types>
   >     constexpr const T&& get(const tuple<Types...>&& t) noexcept;
   >     […]

3. Change 23.3.1 [sequences.general]/2 "Header `<array>` synopsis" as depicted:

   >     […]
   >     template <size_t I, class T, size_t N>
   >     constexpr T& get(array<T, N>&) noexcept;
   >     template <size_t I, class T, size_t N>
   >     constexpr T&& get(array<T, N>&&) noexcept;
   >     template <size_t I, class T, size_t N>
   >     constexpr const T& get(const array<T, N>&) noexcept;
   >     template <size_t I, class T, size_t N>
   >     constexpr const T&& get(const array<T, N>&&) noexcept;
   >     […]

4. Change 22.3.4 [pair.astuple] as depicted:

   >     template<size_t I, class T1, class T2>
   >     constexpr tuple_element_t<I, pair<T1, T2>>&
   >     get(pair<T1, T2>& p) noexcept;
   >     template<size_t I, class T1, class T2>
   >     constexpr const tuple_element_t<I, pair<T1, T2>>&
   >     get(const pair<T1, T2>& p) noexcept;

   > > -3- Returns: If `I == 0` returns `p.first`; if `I == 1` returns `p.second`; otherwise the program is ill-formed.

   >     template<size_t I, class T1, class T2>
   >     constexpr tuple_element_t<I, pair<T1, T2>>&&
   >     get(pair<T1, T2>&& p) noexcept;
   >     template<size_t I, class T1, class T2>
   >     constexpr const tuple_element_t<I, pair<T1, T2>>&&
   >     get(const pair<T1, T2>&& p) noexcept;

   > > -4- Returns: If `I == 0` returns a reference to `std::forward<T1&&>(p.first)`; if `I == 1` returns a reference to `std::forward<T2&&>(p.second)`; otherwise the program is ill-formed.

   >     template <class T, class U>
   >     constexpr T& get(pair<T, U>& p) noexcept;
   >     template <class T, class U>
   >     constexpr const T& get(const pair<T, U>& p) noexcept;

   > > -5- Requires: `T` and `U` are distinct types. Otherwise, the program is ill-formed.

   > > -6- Returns: `get<0>(p)`;

   >     template <class T, class U>
   >     constexpr T&& get(pair<T, U>&& p) noexcept;
   >     template <class T, class U>
   >     constexpr const T&& get(const pair<T, U>&& p) noexcept;

   > > -7- Requires: `T` and `U` are distinct types. Otherwise, the program is ill-formed.

   > > -8- Returns: A reference to `p.first`.`get<0>(std::move(p))`;

   >     template <class T, class U>
   >     constexpr T& get(pair<U, T>& p) noexcept;
   >     template <class T, class U>
   >     constexpr const T& get(const pair<U, T>& p) noexcept;

   > > -9- Requires: `T` and `U` are distinct types. Otherwise, the program is ill-formed.

   > > -10- Returns: `get<1>(p)`;

   >     template <class T, class U>
   >     constexpr T&& get(pair<U, T>&& p) noexcept;
   >     template <class T, class U>
   >     constexpr const T&& get(const pair<U, T>&& p) noexcept;

   > > -11- Requires: `T` and `U` are distinct types. Otherwise, the program is ill-formed.

   > > -12- Returns: A reference to `p.second`.`get<1>(std::move(p))`;

5. Change 22.4.8 [tuple.elem] as depicted:

   >     template <size_t I, class... Types>
   >       constexpr tuple_element_t<I, tuple<Types...> >& get(tuple<Types...>& t) noexcept;

   > > -1- Requires: `I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

   > > -2- Returns: A reference to the `I`th element of `t`, where indexing is zero-based.

   >     template <size_t I, class... Types>
   >       constexpr tuple_element_t<I, tuple<Types...> >&& get(tuple<Types...>&& t) noexcept; // Note A

   > > -3- Effects: Equivalent to return std::forward<typename tuple_element<I, tuple<Types...> >::type&&>(get<I>(t));

   > > -4- Note: if a `T` in `Types` is some reference type `X&`, the return type is `X&`, not `X&&`. However, if the element type is a non-reference type `T`, the return type is `T&&`.

   >     template <size_t I, class... Types>
   >       constexpr tuple_element_t<I, tuple<Types...> > const& get(const tuple<Types...>& t) noexcept; // Note B
   >     template <size_t I, class... Types>
   >       constexpr const tuple_element_t<I, tuple<Types...> >&& get(const tuple<Types...>&& t) noexcept;

   > > -5- Requires: `I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

   > > -6- Returns: A const reference to the `I`th element of `t`, where indexing is zero-based.

   > > -?- [Note A: if a `T` in `Types` is some reference type `X&`, the return type is `X&`, not `X&&`. However, if the element type is a non-reference type `T`, the return type is `T&&`. — end note]

   > > -7- [Note B: Constness is shallow. If a `T` in `Types` is some reference type `X&`, the return type is `X&`, not `const X&`. However, if the element type is non-reference type `T`, the return type is `const T&`. This is consistent with how constness is defined to work for member variables of reference type. — end note]

   >     template <class T, class... Types>
   >       constexpr T& get(tuple<Types...>& t) noexcept;
   >     template <class T, class... Types>
   >       constexpr T&& get(tuple<Types...>&& t) noexcept;
   >     template <class T, class... Types>
   >       constexpr const T& get(const tuple<Types...>& t) noexcept;
   >     template <class T, class... Types>
   >       constexpr const T&& get(const tuple<Types...>&& t) noexcept;

   > > -8- Requires: The type `T` occurs exactly once in `Types...`. Otherwise, the program is ill-formed.

   > > -9- Returns: A reference to the element of `t` corresponding to the type `T` in `Types...`.

   > > […]

6. Change 23.3.3.7 [array.tuple] as depicted:

   >     template <size_t I, class T, size_t N>
   >       constexpr T& get(array<T, N>& a) noexcept;

   > > -3- Requires: `I < N`. The program is ill-formed if `I` is out of bounds.

   > > -4- Returns: A reference to the `I`th element of `a`, where indexing is zero-based.

   >     template <size_t I, class T, size_t N>
   >       constexpr T&& get(array<T, N>&& a) noexcept;

   > > -5- Effects: Equivalent to `return std::move(get<I>(a));`

   >     template <size_t I, class T, size_t N>
   >       constexpr const T& get(const array<T, N>& a) noexcept;
   >     template <size_t I, class T, size_t N>
   >       constexpr const T&& get(const array<T, N>&& a) noexcept;

   > > -6- Requires: `I < N`. The program is ill-formed if `I` is out of bounds.

   > > -7- Returns: A const reference to the `I`th element of `a`, where indexing is zero-based.

### 2486(i). `mem_fn()` should be required to use perfect forwarding

Section: 22.10.4 [func.require] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 0

View other active issues in [func.require].

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Discussion:

22.10.4 [func.require]/4 defines "simple call wrapper" and "forwarding call wrapper". Only `mem_fn()` is specified to be a "simple call wrapper", by 22.10.16 [func.memfn]/1: "A simple call wrapper (20.9.1) `fn` such that the expression `fn(t, a2, ..., aN)` is equivalent to `INVOKE(pm, t, a2, ..., aN)` (20.9.2)."

This suggests, but doesn't outright state, that perfect forwarding is involved. It matters for PMFs like `R (T::*)(Arg)` where `Arg` is passed by value — if the `mem_fn()` wrapper's function call operator takes `Arg` by value, an extra copy/move will be observable. We should require perfect forwarding here.

[2015-05, Lenexa]

Move to Immediate.

Proposed resolution:

This wording is relative to N4296.

1. Change 22.10.4 [func.require] as depicted [Editorial remark: This simply adds "A simple call wrapper is a forwarding call wrapper", then moves the sentence. — end of remark]:

   > -4- Every call wrapper (20.9.1) shall be `MoveConstructible`. A simple call wrapper is a call wrapper that is `CopyConstructible` and `CopyAssignable` and whose copy constructor, move constructor, and assignment operator do not throw exceptions. A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments to the wrapped callable object as references. This forwarding step shall ensure that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references. A simple call wrapper is a forwarding call wrapper that is `CopyConstructible` and `CopyAssignable` and whose copy constructor, move constructor, and assignment operator do not throw exceptions. [Note: In a typical implementation […] — end note]

### 2487(i). `bind()` should be `const`-overloaded, not cv-overloaded

Section: 22.10.15.4 [func.bind.bind] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 2

View all other issues in [func.bind.bind].

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Discussion:

The Standard currently requires `bind()` to return something with a cv-overloaded function call operator. `const` is great, but `volatile` is not. First, the Library almost always ignores `volatile`'s existence (with `<type_traits>` and `<atomic>` being rare exceptions). Second, implementations typically store bound arguments in a `tuple`, but `get()` isn't overloaded for `volatile tuple`. Third, when a bound argument is a `reference_wrapper`, we have to call `tid.get()`, but that won't compile for a `volatile reference_wrapper`. Finally, `const` and `volatile` don't always have to be handled symmetrically — for example, lambda function call operators are `const` by default, but they can't ever be `volatile`.

Implementers shouldn't be required to provide cv-overloading here. (They can provide it as a conforming extension if they want.)

[2015-05, Lenexa]

JW: why would a `reference_wrapper` be volatile?
STL: if a bound argument is a `reference_wrapper` then in a volatile-qualified operator() that member will be volatile so you can't call get() on it
STL: worded like this it's a conforming extension to kep overloading on volatile
HH: libc++ doesn't overload on volatile
JW: libstdc++ does overload for volatile
MC: move to Ready and bring motion on Friday
10 in favor, none opposed

Proposed resolution:

This wording is relative to N4296.

1. Change 22.10.15.4 [func.bind.bind] as depicted:

   >     template<class F, class... BoundArgs>
   >       unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > -2- Requires: `is_constructible<FD, F>::value` shall be `true`. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be `true`. `INVOKE(fd, w1, w2, ..., wN)` (20.9.2) shall be a valid expression for some values `w1`, `w2`, ..., `wN`, where `N == sizeof...(bound_args)`. The cv-qualifiers cv of the call wrapper `g`, as specified below, shall be neither `volatile` nor `const volatile`.

   > > […]

   >     template<class R, class F, class... BoundArgs>
   >       unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > -6- Requires: `is_constructible<FD, F>::value` shall be `true`. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be `true`. `INVOKE(fd, w1, w2, ..., wN)` shall be a valid expression for some values `w1`, `w2`, ..., `wN`, where `N == sizeof...(bound_args)`. The cv-qualifiers cv of the call wrapper `g`, as specified below, shall be neither `volatile` nor `const volatile`.

   > > […]

### 2488(i). Placeholders should be allowed and encouraged to be `constexpr`

Section: 22.10.15.5 [func.bind.place] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 2

View all other issues in [func.bind.place].

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Discussion:

`piecewise_construct` (22.3.5 [pair.piecewise]), `allocator_arg` (20.2.7 [allocator.tag]), and `adopt_lock`/`defer_lock`/`try_to_lock` (32.6.5 [thread.lock]) are all required to be `constexpr` with internal linkage. `bind()`'s placeholders should be allowed to follow this modern practice, for increased consistency and reduced implementer headaches (header-only is easier than separately-compiled).

[2015-05-07 Lenexa: Move to Immediate]

STL: I'd like this one immediate.

Jonathan: I want to think about forcing constexpr, but the current issue, I have no objections. I'd say ready, but I won't object to immediate if STL wants it.

Marshall: You should report in Kona how it worked out.

STL: We went around a bit on the reflector about how to phrase the encouragement.

Jonathan: I think the shall may be not quite right.

Marshall: I see, you can change your implementation, but you don't.

Jonathan: There's precedent for the shall, but it's wrong, see editorial issue 493.

STL: I would prefer not to ask Daniel to reword, and 493 can fix it later.

Marshall: I remove my objection to immediate because it doesn't force implementations to change.

Marshall: Any other discussion?

Marshall: Immediate vote: 6. Opposed, 0. Abstain, 1.

Proposed resolution:

This wording is relative to N4296.

1. Change 22.10 [function.objects] p2 "Header `<functional>` synopsis" as depicted:

   >     namespace placeholders {
   >       // M is the implementation-defined number of placeholders
   >       see belowextern unspecified _1;
   >       see belowextern unspecified _2;
   >       ...
   >       see belowextern unspecified _M;
   >     }

2. Change 22.10.15.5 [func.bind.place] p2 as depicted:

   >     namespace std::placeholders {
   >       // M is the implementation-defined number of placeholders
   >       see belowextern unspecified _1;
   >       see belowextern unspecified _2;
   >                  .
   >                  .
   >                  .
   >       see belowextern unspecified _M;
   >     }

   > -1- All placeholder types shall be `DefaultConstructible` and `CopyConstructible`, and their default constructors and copy/move constructors shall not throw exceptions. It is implementation-defined whether placeholder types are `CopyAssignable`. `CopyAssignable` placeholders' copy assignment operators shall not throw exceptions.

   > -?- Placeholders should be defined as:

   > >     constexpr unspecified _1{};

   > If they are not, they shall be declared as:

   > >     extern unspecified _1;

### 2489(i). `mem_fn()` should be `noexcept`

Section: 22.10.16 [func.memfn] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 0

View all other issues in [func.memfn].

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Discussion:

`mem_fn()` is wide contract and doesn't do anything that could throw exceptions, so it should be marked `noexcept`.

Note that `mem_fn()` is perfectly happy to wrap a null PMF/PMD, it just can't be invoked later. This is exactly like `std::function`, which can be constructed from null PMFs/PMDs. Therefore, `mem_fn()` will remain wide contract forever.

[2015-05, Lenexa]

Move to Immediate.

Proposed resolution:

This wording is relative to N4296.

1. Change 22.10 [function.objects] p2 "Header `<functional>` synopsis" as depicted:

   >     […]
   >     // 20.9.11, member function adaptors:
   >     template<class R, class T> unspecified mem_fn(R T::*) noexcept;
   >     […]

2. Change 22.10.16 [func.memfn] as depicted:

   >     template<class R, class T> unspecified mem_fn(R T::* pm) noexcept;

   > > […]

   > > -4- Throws: Nothing.

### 2492(i). Clarify requirements for `comp`

Section: 26.8 [alg.sorting] Status: C++17 Submitter: Anton Savin Opened: 2015-04-14 Last modified: 2017-07-30

Priority: 0

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Discussion:

N4296 26.8 [alg.sorting]/3 reads:

> For all algorithms that take `Compare`, there is a version that uses `operator<` instead. That is, `comp(*i,*j) != false` defaults to `*i < *j != false`. For algorithms other than those described in 25.4.3 to work correctly, `comp` has to induce a strict weak ordering on the values.

So it's not specified clearly what happens if `comp` or `operator<` don't induce a strict weak ordering. Is it undefined or implementation-defined behavior? It seems that it should be stated more clearly that the behavior is undefined.

[2015-05, Lenexa]

Move to Immediate.

Proposed resolution:

This wording is relative to N4431.

1. Change 26.8 [alg.sorting]/3 to the following:

   > For all algorithms that take `Compare`, there is a version that uses `operator<` instead. That is, `comp(*i, *j) != false` defaults to `*i < *j != false`. For algorithms other than those described in 25.4.3 to work correctly, `comp` shallhas to induce a strict weak ordering on the values.

### 2494(i). [fund.ts.v2] `ostream_joiner` needs `noexcept`

Section: 10.2 [fund.ts.v2::iterator.ostream.joiner] Status: TS Submitter: Nate Wilson Opened: 2015-05-03 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

In Library Fundamentals 2 N4481, [iterator.ostream.joiner], all operations are no-ops other than the assignment operator.

So, they should be marked as `noexcept`.

[2015-05, Lenexa]

Move to Immediate.

Proposed resolution:

This wording is relative to N4481 in regard to fundamental-ts-2 changes.

1. Change class template `ostream_joiner` synopsis, [iterator.ostream.joiner] p2, as indicated:

   >     namespace std {
   >     namespace experimental {
   >     inline namespace fundamentals_v2 {

   >     template <class DelimT, class charT = char, class traits = char_traits<charT> >
   >       class ostream_joiner {
   >       public:
   >         […]
   >         ostream_joiner<DelimT, charT,traits>& operator*() noexcept;
   >         ostream_joiner<DelimT, charT,traits>& operator++() noexcept;
   >         ostream_joiner<DelimT, charT,traits>& operator++(int) noexcept;
   >         […]
   >       };

   >     } // inline namespace fundamentals_v2
   >     } // namespace experimental
   >     } // namespace std

2. Change [iterator.ostream.joiner.ops] p3+5, as indicated:

   >     ostream_joiner<DelimT, charT, traits>& operator*() noexcept;

   > […]

   >     ostream_joiner<DelimT, charT, traits>& operator++() noexcept;
   >     ostream_joiner<DelimT, charT, traits>& operator++(int) noexcept;

### 2495(i). There is no such thing as an Exception Safety element

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-05-05 Last modified: 2017-07-30

Priority: 0

View other active issues in [util.smartptr.shared.const].

View all other issues in [util.smartptr.shared.const].

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Discussion:

20.3.2.2.2 [util.smartptr.shared.const] includes several "Exception safety" elements, but that is not one of the elements defined in 17.5.1.4 16.3.2.4 [structure.specifications]. We should either define what it means, or just move those sentences into the Effects: clause.

[2015-06, Telecon]

Move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4431.

1. Change 20.3.2.2.2 [util.smartptr.shared.const] as follows:

   >     template<class Y> explicit shared_ptr(Y* p);

   > > […]

   > > -4- Effects: Constructs a `shared_ptr` object that owns the pointer `p`. If an exception is thrown, `delete p` is called.

   > > […]

   > > -7- Exception safety: If an exception is thrown, `delete p` is called.

   >     template <class Y, class D> shared_ptr(Y* p, D d);
   >     template <class Y, class D, class A> shared_ptr(Y* p, D d, A a);
   >     template <class D> shared_ptr(nullptr_t p, D d);
   >     template <class D, class A> shared_ptr(nullptr_t p, D d, A a);

   > > […]

   > > -9- Effects: Constructs a `shared_ptr` object that owns the object `p` and the deleter `d`. The second and fourth constructors shall use a copy of `a` to allocate memory for internal use. If an exception is thrown, `d(p)` is called.

   > > […]

   > > -12- Exception safety: If an exception is thrown, `d(p)` is called.

   > […]

   >     template <class Y> explicit shared_ptr(const weak_ptr<Y>& r);

   > > […]

   > > -24- Effects: Constructs a `shared_ptr` object that shares ownership with `r` and stores a copy of the pointer stored in `r`. If an exception is thrown, the constructor has no effect.

   > > […]

   > > -27- Exception safety: If an exception is thrown, the constructor has no effect.

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > > […]

   > > -29- Effects: Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`. If an exception is thrown, the constructor has no effect.

   > > -30- Exception safety: If an exception is thrown, the constructor has no effect.

### 2498(i). `operator>>(basic_istream&&, T&&)` returns `basic_istream&`, but should probably return `basic_istream&&`

Section: 31.7.5.6 [istream.rvalue] Status: Resolved Submitter: Richard Smith Opened: 2015-05-08 Last modified: 2020-11-09

Priority: 3

View all other issues in [istream.rvalue].

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Discussion:

Consider:

>     auto& is = make_istream() >> x; // oops, istream object is already gone

With a `basic_istream&&` return type, the above would be ill-formed, and generally we'd preserve the value category properly.

[2015-06, Telecon]

JW: think this needs proper consideration, it would make

    stream() >> x >> y >> z

go from 3 operator>> calls to 6 operator>> calls, and wouldn't prevent dangling references (change the example to auto&&)

[2020-02 Resolved by the adoption of 1203(i) in Prague.]

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

### 2499(i). `operator>>(basic_istream&, CharT*)` makes it hard to avoid buffer overflows

Section: 31.7.5.3.3 [istream.extractors] Status: Resolved Submitter: Richard Smith Opened: 2015-05-08 Last modified: 2018-11-12

Priority: 2

View all other issues in [istream.extractors].

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Discussion:

We removed `gets()` (due to an NB comment and C11 — bastion of backwards compatibility — doing the same). Should we remove this too?

Unlike `gets()`, there are legitimate uses:

>     char buffer[32];
>     char text[32] = // ...
>     ostream_for_buffer(text) >> buffer; // ok, can't overrun buffer

… but the risk from constructs like "`std::cin >> buffer`" seems to outweigh the benefit.

The issue had been discussed on the library reflector starting around c++std-lib-35541.

[2015-06, Telecon]

VV: Request a paper to deprecate / remove anything

[2015-10, Kona Saturday afternoon]

STL: This overload is evil and should probably die.

VV: I agree with that, even though I don't care.

STL: Say that we either remove it outright following the gets() rationale, or at least deprecate it.

Move to Open; needs a paper.

[2016-08, Chicago: Zhihao Yuan comments and provides wording]

Rationale:

1. I would like to keep some reasonable code working;

2. Reasonable code includes two cases:

   1. `width() > 0`, any pointer argument

   2. `width() >= 0`, array argument

3. For a), banning bad code will become a silent behavior change at runtime; for b), it breaks at compile time.

I propose to replace these signatures with references to arrays. An implementation may want to ship the old instantiatations in the binary without exposing the old signatures.

[2016-08, Chicago]

Tues PM: General agreement on deprecating the unsafe call, but no consensus for the P/R.

General feeling that implementation experience would be useful.

[2018-08-23 Batavia Issues processing]

Will be resolved by the adoption of P0487.

[2018-11-11 Resolved by P0487R1, adopted in San Diego.]

Proposed resolution:

This wording is relative to N4606.

1. Modify 31.7.5.3.3 [istream.extractors] as indicated:

   >     template<class charT, class traits, size_t N>
   >       basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& in,
   >                                                charT* scharT (&s)[N]);
   >     template<class traits, size_t N>
   >       basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in,
   >                                               unsigned char* sunsigned char (&s)[N]);
   >     template<class traits, size_t N>
   >       basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in,
   >                                               signed char* ssigned char (&s)[N]);

   > > -7- Effects: Behaves like a formatted input member (as described in 31.7.5.3.1 [istream.formatted.reqmts]) of `in`. After a `sentry` object is constructed, `operator>>` extracts characters and stores them into successive locations of an array whose first element is designated by `s`. If `width()` is greater than zero, `n` is `width()min(size_t(width()), N)`. Otherwise `n` is the number of elements of the largest array of `char_type` that can store a terminating `charT()``N`. `n` is the maximum number of characters stored.

### 2500(i). [fund.ts.v2] fundts.memory.smartptr.shared.obs/6 should apply to cv-unqualified `void`

Section: 8.2.1.2 [fund.ts.v2::memory.smartptr.shared.obs] Status: TS Submitter: Jeffrey Yasskin Opened: 2015-05-11 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

S. B. Tam reported this here.

N3920 changed `operator*()` in [util.smartptr.shared.obs] as:

> Remarks: When `T` is an array type or cv-qualified `void`, it is unspecified whether this member function is declared. …

This excludes cv-unqualified `void`, which is probably unintended.

[2015-09-11, Telecon]

Move to Tentatively Ready

Proposed resolution:

1. In the library fundamentals v2, [memory.smartptr.shared.obs] p2, change as indicated:

   > Remarks: When `T` is an array type or (possibly cv-qualified) `void`, it is unspecified whether this member function is declared. If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well formed.

### 2501(i). `std::function` requires POCMA/POCCA

Section: 22.10.17.3 [func.wrap.func] Status: Resolved Submitter: David Krauss Opened: 2015-05-20 Last modified: 2020-09-06

Priority: 3

View all other issues in [func.wrap.func].

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Discussion:

The idea behind `propagate_on_container_move_assignment` is that you can keep an allocator attached to a container. But it's not really designed to work with polymorphism, which introduces the condition where the current allocator is non-POCMA and the RHS of assignment, being POCMA, wants to replace it. If function were to respect the literal meaning, any would-be attached allocator is at the mercy of every assignment operation. So, `std::function` is inherently POCMA, and passing a non-POCMA allocator should be ill-formed.

The other alternative, and the status quo, is to ignore POCMA and assume it is true. This seems just dangerous enough to outlaw. It is, in theory, possible to properly support POCMA as far as I can see, albeit with difficulty and brittle results. It would require function to keep a throwing move constructor, which otherwise can be `noexcept`.

The same applies to `propagate_on_container_copy_assignment`. This presents more difficulty because `std::allocator` does not set this to true. Perhaps it should. For function to respect this would require inspecting the POCCA of the source allocator, slicing the target from the erasure of the source, slicing the allocation from the erasure of the destination, and performing a copy with the destination's allocator with the source's target. This comes out of the blue for the destination allocator, which might not support the new type anyway. Theoretically possible, but brittle and not very practical. Again, current implementations quietly ignore the issue but this isn't very clean.

The following code example is intended to demonstrate the issue here:

>     #include <functional>
>     #include <iostream>
>     #include <vector>

>     template <typename T>
>     struct diag_alloc
>     {
>       std::string name;

>       T* allocate(std::size_t n) const
>       {
>         std::cout << '+' << name << '\n';
>         return static_cast<T*>(::operator new(n * sizeof(T)));
>       }

>       void deallocate(T* p, std::size_t) const
>       {
>         std::cout << '-' << name << '\n';
>         return ::operator delete(p);
>       }

>       template <typename U>
>       operator diag_alloc<U>() const { return {name}; }

>       friend bool operator==(const diag_alloc& a, const diag_alloc& b)
>       { return a.name == b.name; }

>       friend bool operator!=(const diag_alloc& a, const diag_alloc& b)
>       { return a.name != b.name; }

>       typedef T value_type;

>       template <typename U>
>       struct rebind { typedef diag_alloc<U> other; };
>     };

>     int main() {
>       std::cout << "VECTOR\n";
>       std::vector<int, diag_alloc<int>> foo({1, 2}, {"foo"}); // +foo
>       std::vector<int, diag_alloc<int>> bar({3, 4}, {"bar"}); // +bar

>       std::cout << "move\n";
>       foo = std::move(bar); // no message

>       std::cout << "more foo\n";
>       foo.reserve(40); // +foo -foo
>       std::cout << "more bar\n";
>       bar.reserve(40); // +bar -bar

>       std::cout << "\nFUNCTION\n";
>       int bigdata[100];
>       auto bigfun = [bigdata]{};
>       typedef decltype(bigfun) ft;
>       std::cout << "make fizz\n";
>       std::function<void()> fizz(std::allocator_arg, diag_alloc<ft>{"fizz"}, bigfun); // +fizz
>       std::cout << "another fizz\n";
>       std::function<void()> fizz2;
>       fizz2 = fizz; // +fizz as if POCCA
>       std::cout << "make buzz\n";
>       std::function<void()> buzz(std::allocator_arg, diag_alloc<ft>{"buzz"}, bigfun); // +buzz
>       std::cout << "move\n";
>       buzz = std::move(fizz); // -buzz as if POCMA

>       std::cout << "\nCLEANUP\n";
>     }

[2016-08, Chicago]

Tues PM: Resolved by P0302R1.

Proposed resolution:

Resolved by P0302R1.

### 2502(i). `std::function` does not use `allocator::construct`

Section: 22.10.17.3 [func.wrap.func] Status: Resolved Submitter: David Krauss Opened: 2015-05-20 Last modified: 2020-09-06

Priority: 3

View all other issues in [func.wrap.func].

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Discussion:

It is impossible for `std::function` to construct its target object using the `construct` method of a type-erased allocator. More confusingly, it is possible when the allocator and the target are created at the same time. The means of target construction should be specified.

[2016-08 Chicago]

Tues PM: Resolved by P0302R1.

Proposed resolution:

Resolved by P0302R1.

### 2503(i). multiline option should be added to `syntax_option_type`

Section: 28.6.4.2 [re.synopt] Status: C++17 Submitter: Nozomu Katō Opened: 2015-05-22 Last modified: 2017-07-30

Priority: 2

View all other issues in [re.synopt].

View all issues with C++17 status.

Discussion:

The specification of ECMAScript defines the Multiline property for its RegExp and the regular expressions ^ and $ behave differently according to the value of this property. Thus, this property should be available also in the ECMAScript compatible engine in `std::regex`.

[2015-05-22, Daniel comments]

This issue interacts somewhat with LWG 2343(i).

[Telecon 2015-07]

Set the priority to match LWG 2343(i).

[2016-08, Chicago]

Monday PM: Moved to Tentatively Ready. This also resolves 2343(i)

Proposed resolution:

This wording is relative to N4431.

1. Change 28.6.4.2 [re.synopt] as indicated:

   >     namespace std::regex_constants {
   >       typedef T1 syntax_option_type;
   >       constexpr syntax_option_type icase = unspecified ;
   >       constexpr syntax_option_type nosubs = unspecified ;
   >       constexpr syntax_option_type optimize = unspecified ;
   >       constexpr syntax_option_type collate = unspecified ;
   >       constexpr syntax_option_type ECMAScript = unspecified ;
   >       constexpr syntax_option_type basic = unspecified ;
   >       constexpr syntax_option_type extended = unspecified ;
   >       constexpr syntax_option_type awk = unspecified ;
   >       constexpr syntax_option_type grep = unspecified ;
   >       constexpr syntax_option_type egrep = unspecified ;
   >       constexpr syntax_option_type multiline = unspecified ;
   >     }

2. Change 28.6.4.3 [re.matchflag], Table 138 — "`syntax_option_type` effects" as indicated:

   > Table 138 — `syntax_option_type` effects
   > Element | Effect(s) if set
   > --- | ---
   > `…`
   > `multiline` | Specifies that `^` shall match the beginning of a line and `$` shall match the end of a line, if the ECMAScript engine is selected.
   > `…`

### 2505(i). `auto_ptr_ref` creation requirements underspecified

Section: 99 [auto.ptr.conv] Status: Resolved Submitter: Hubert Tong Opened: 2015-05-28 Last modified: 2020-09-06

Priority: 4

View all other issues in [auto.ptr.conv].

View all issues with Resolved status.

Discussion:

In C++14 sub-clause 99 [auto.ptr.conv], there appears to be no requirement that the formation of an `auto_ptr_ref<Y>` from an `auto_ptr<X>` is done only when `X*` can be implicitly converted to `Y*`.

For example, I expect formation of the `auto_ptr_ref<A>` from the prvalue of type `auto_ptr<B>` to be invalid in the case below (but the wording does not seem to be there):

>     #include <memory>

>     struct A { };
>     struct B { } b;

>     std::auto_ptr<B> apB() { return std::auto_ptr<B>(&b); }
>     int main() {
>       std::auto_ptr<A> apA(apB());
>       apA.release();
>     }

The behaviour of the implementation in question on the case presented above is to compile and execute it successfully (which is what the C++14 wording implies). The returned value from `apA.release()` is essentially `reinterpret_cast<A*>(&b)`.

There is nothing in the specification of

>     template <class X>
>     template <class Y> operator auto_ptr<X>::auto_ptr_ref<Y>() throw();

which implies that `X*` should be implicitly convertible to `Y*`.

The implementation in question uses the `reinterpret_cast` interpretation even when `Y` is an accessible, unambiguous base class of `X`; the result thereof is that no offset adjustment is performed.

[2015-07, Telecon]

Marshall to resolve.

[2016-03-16, Alisdair Meredith comments]

This issue is a defect in a component we have actively removed from the standard. I can't think of a clearer example of something that is no longer a defect!

[2016-08-03, Alisdair Meredith comments]

As C++17 removes `auto_ptr`, I suggest closing this issue as closed by paper N4190.

Previous resolution [SUPERSEDED]:

> This wording is relative to ISO/IEC 14882:2014(E).

> 1. Change 99 [auto.ptr.conv] as indicated:

>    >     template<class Y> operator auto_ptr_ref<Y>() throw();

>    > > -?- Requires: `X*` can be implicitly converted to `Y*`.

>    > > -3- Returns: An `auto_ptr_ref<Y>` that holds `*this`.

>    > > -?- Notes: Because `auto_ptr_ref` is present for exposition only, the only way to invoke this function is by calling one of the `auto_ptr` conversions which take an `auto_ptr_ref` as an argument. Since all such conversions will call `release()` on `*this` (in the form of the `auto_ptr` that the `auto_ptr_ref` holds a reference to), an implementation of this function may cause instantiation of said `release()` function without changing the semantics of the program.

>    >     template<class Y> operator auto_ptr<Y>() throw();

>    > > -?- Requires: `X*` can be implicitly converted to `Y*`.

>    > > -4- Effects: Calls `release()`.

>    > > -5- Returns: An `auto_ptr<Y>` that holds the pointer returned from `release()`.

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Resolved

Proposed resolution:

Resolved by acceptance of N4190.

### 2507(i). `codecvt_mode` should be a bitmask type

Section: 99 [depr.locale.stdcvt] Status: Resolved Submitter: Jonathan Wakely Opened: 2015-06-08 Last modified: 2025-11-11

Priority: 3

View all other issues in [depr.locale.stdcvt].

View all issues with Resolved status.

Discussion:

The enumeration type `codecvt_mode` is effectively a bitmask type (16.3.3.3.3 [bitmask.types]) with three elements, but isn't defined as such.

This harms usability because bitmask types are required to work well with bitwise operators, but `codecvt_mode` doesn't have overloaded operators, making it very inconvenient to combine values:

>     std::codecvt_utf16<char32_t, 0x10FFFF,
>       static_cast<std::codecvt_mode>(std::little_endian|std::generate_header)>
>     cvt;

The `static_cast` harms readability and should not be necessary.

I suggest that either `codecvt_mode` is specified to be a bitmask type, or as a minimal fix we provide an overloaded `operator|` that returns the right type.

[2025-11-10 Resolved by the removal of `codecvt_mode` via paper P2871R3 in Kona, 2023. Status changed: New → Resolved.]

Proposed resolution:

### 2509(i). [fund.ts.v2] `any_cast` doesn't work with rvalue reference targets and cannot move with a value target

Section: 6.4 [fund.ts.v2::any.nonmembers] Status: TS Submitter: Ville Voutilainen Opened: 2015-06-13 Last modified: 2017-07-30

Priority: 2

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

In Library Fundamentals v1, [any.nonmembers]/5 says:

> For the first form, `*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)`. For the second and third forms, `*any_cast<remove_reference_t<ValueType>>(&operand)`.

1. This means that

   >     any_cast<Foo&&>(whatever_kind_of_any_lvalue_or_rvalue);

   is always ill-formed. That's unfortunate, because forwarding such a cast result of an `any` is actually useful, and such uses do not want to copy/move the underlying value just yet.

2. Another problem is that that same specification prevents an implementation from moving to the target when

   >     ValueType any_cast(any&& operand);

   is used. The difference is observable, so an implementation can't perform an optimization under the as-if rule. We are pessimizing every `CopyConstructible` and `MoveConstructible` type because we are not using the move when we can. This unfortunately includes types such as the library containers, and we do not want such a pessimization!

[2015-07, Telecon]

Jonathan to provide wording

[2015-10, Kona Saturday afternoon]

Eric offered to help JW with wording

Move to Open

[2016-01-30, Ville comments and provides wording]

Drafting note: the first two changes add support for types that have explicitly deleted move constructors. Should we choose not to support such types at all, the third change is all we need. For the second change, there are still potential cases where Requires is fulfilled but Effects is ill-formed, if a suitably concocted type is thrown into the mix.

Proposed resolution:

This wording is relative to N4562.

1. In 6.3.1 [fund.ts.v2::any.cons] p11+p12, edit as follows:

   >     template<class ValueType>
   >       any(ValueType&& value);

   > > -10- Let `T` be equal to `decay_t<ValueType>`.

   > > -11- Requires: `T` shall satisfy the `CopyConstructible` requirements, except for the requirements for `MoveConstructible`. If `is_copy_constructible_v<T>` is `false`, the program is ill-formed.

   > > -12- Effects: If `is_constructible_v<T, ValueType&&>` is true, cConstructs an object of type `any` that contains an object of type `T` direct-initialized with `std::forward<ValueType>(value)`. Otherwise, constructs an object of type `any` that contains an object of type `T` direct-initialized with `value`.

   > > […]

2. In 6.4 [fund.ts.v2::any.nonmembers] p5, edit as follows:

   >     template<class ValueType>
   >       ValueType any_cast(const any& operand);
   >     template<class ValueType>
   >       ValueType any_cast(any& operand);
   >     template<class ValueType>
   >       ValueType any_cast(any&& operand);

   > > -4- Requires: `is_reference_v<ValueType>` is `true` or `is_copy_constructible_v<ValueType>` is `true`. Otherwise the program is ill-formed.

   > > -5- Returns: For the first form, `*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)`. For the second and third forms, `*any_cast<remove_reference_t<ValueType>>(&operand)`. For the third form, if `is_move_constructible_v<ValueType>` is `true` and `is_lvalue_reference_v<ValueType>` is `false`, `std::move(*any_cast<remove_reference_t<ValueType>>(&operand))`, otherwise, `*any_cast<remove_reference_t<ValueType>>(&operand)`.

   > > […]

### 2510(i). Tag types should not be `DefaultConstructible`

Section: 17.6 [support.dynamic], 22.2 [utility], 22.3.5 [pair.piecewise], 20.2.2 [memory.syn], 20.2.7 [allocator.tag], 32.6 [thread.mutex] Status: C++17 Submitter: Ville Voutilainen Opened: 2015-06-13 Last modified: 2017-07-30

Priority: 2

View all other issues in [support.dynamic].

View all issues with C++17 status.

Discussion:

`std::experimental::optional`, for certain reasons, specifies its `nullopt` type to not be `DefaultConstructible`. It doesn't do so for its tag type `in_place_t` and neither does the standard proper for any of its tag types. That turns out to be very unfortunate, consider the following:

>     #include <memory>
>     #include <array>

>     void f(std::array<int, 1>, int) {} // #1
>     void f(std::allocator_arg_t, int) {} // #2

>     int main()
>     {
>       f({}, 666); // #3
>     }

The call at #3 is ambiguous. What's even worse is that if the overload #1 is removed, the call works just fine. The whole point of a tag type is that it either needs to mentioned in a call or it needs to be a forwarded argument, so being able to construct a tag type like that makes no sense.

Making the types have an explicit default constructor might have helped, but CWG 1518 is going against that idea.

[optional.nullopt]/3 solves this problem for `nullopt`:

> Type `nullopt_t` shall not have a default constructor. It shall be a literal type. Constant `nullopt` shall be initialized with an argument of literal type.

[2015-06, Telecon]

Move to Tentatively Ready.

[2015-10, Kona Saturday afternoon]

Move back to Open

JW: The linked Core issue (CWG 1518) gives us a better tool to solve this (explicit default constructors). [The CWG Issue means that an explicit default constructor will no longer match "{}".] JW explains that it's important that tag types cannot be constructed from "{}" (e.g. the allocator tag in the tuple constructors).

WEB: Should we now go back and update our constructors? JW: For tag types, yes.

VV: The guideline is that anything that does not mention the type name explicitly should not invoke an explicit constructor.

Ville will provide wording.

Discussion about pair/tuple's default constructor - should they now be explicit?

[2016-01-31]

Ville provides revised wording.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4527.

> 1. In 17.6 [support.dynamic]/1, change the header `<new>` synopsis:

>    >     […]
>    >     struct nothrow_t {}; see below
>    >     extern const nothrow_t nothrow;
>    >     […]

> 2. Add a new paragraph after 17.6 [support.dynamic]/1 (following the header `<new>` synopsis):

>    > -?- Type `nothrow_t` shall not have a default constructor.

> 3. In 22.2 [utility]/2, change the header `<utility>` synopsis:

>    >     […]
>    >     // 20.3.5, pair piecewise construction
>    >     struct piecewise_construct_t { }; see below
>    >     constexpr piecewise_construct_t piecewise_construct{ unspecified };
>    >     […]

> 4. Add a new paragraph after 22.2 [utility]/2 (following the header `<utility>` synopsis):

>    > -?- Type `piecewise_construct_t` shall not have a default constructor. It shall be a literal type. Constant `piecewise_construct` shall be initialized with an argument of literal type.

> 5. In 22.3.5 [pair.piecewise], apply the following edits:

>    >     struct piecewise_construct_t { };
>    >     constexpr piecewise_construct_t piecewise_construct{ unspecified };

> 6. In 20.2.2 [memory.syn]/1, change the header `<memory>` synopsis:

>    >     […]
>    >     // 20.7.6, allocator argument tag
>    >     struct allocator_arg_t { }; see below
>    >     constexpr allocator_arg_t allocator_arg{ unspecified };
>    >     […]

> 7. Add a new paragraph after 20.2.2 [memory.syn]/1 (following the header `<memory>` synopsis):

>    > -?- Type `allocator_arg_t` shall not have a default constructor. It shall be a literal type. Constant `allocator_arg` shall be initialized with an argument of literal type.

> 8. In 20.2.7 [allocator.tag], apply the following edits:

>    >     namespace std {
>    >       struct allocator_arg_t { };
>    >       constexpr allocator_arg_t allocator_arg{ unspecified };
>    >     }

>    > Editorial drive-by: `piecewise_construct_t` is written, in 22.3.5 [pair.piecewise] like

>    > >     struct piecewise_construct_t { };
>    > >     constexpr piecewise_construct_t piecewise_construct{};

>    > whereas other tag types such as `allocator_construct_t` are, in e.g. 20.2.7 [allocator.tag], written like

>    > >     namespace std {
>    > >       struct allocator_arg_t { };
>    > >       constexpr allocator_arg_t allocator_arg{};
>    > >     }

>    > We should decide whether or not to write out the `std` namespace in such paragraphs. I would suggest not to write it out.

> 9. In 32.6 [thread.mutex]/1, change the header `<mutex>` synopsis:

>    >     […]
>    >     struct defer_lock_t { }; see below
>    >     struct try_to_lock_t { }; see below
>    >     struct adopt_lock_t { }; see below

>    >     constexpr defer_lock_t defer_lock { unspecified  };
>    >     constexpr try_to_lock_t try_to_lock { unspecified  };
>    >     constexpr adopt_lock_t adopt_lock { unspecified  };
>    >     […]

> 10. Add three new paragraphs after [thread.mutex]/1 (following the header `<mutex>` synopsis):

>     > -?- Type `defer_lock_t` shall not have a default constructor. It shall be a literal type. Constant `defer_lock` shall be initialized with an argument of literal type.

>     > -?- Type `try_to_lock_t` shall not have a default constructor. It shall be a literal type. Constant `try_to_lock` shall be initialized with an argument of literal type.

>     > -?- Type `adopt_lock_t` shall not have a default constructor. It shall be a literal type. Constant `adopt_lock` shall be initialized with an argument of literal type.

[2016-03 Jacksonville]

AM: should have note about compatibility in Annex C
HH: like this idiom well enough that I've started using it in my own code
AM: why are pair and tuple involved here?
GR: they are the only types which forward explicitness with EXPLICIT
AM: British spelling of behaviour
AM: happy to drop my issue about Annex C

[2016-06 Oulu]

This is waiting on Core issue 1518

Saturday: Core 1518 was resolved in Oulu

[2016-07 Chicago]

This is related to 2736(i)

Monday PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4567.

1. In 17.6 [support.dynamic]/1, change the header `<new>` synopsis:

   >     […]
   >     struct nothrow_t { explicit nothrow_t() = default; };
   >     extern const nothrow_t nothrow;
   >     […]

2. In 22.2 [utility]/2, change the header `<utility>` synopsis:

   >     […]
   >     // 20.3.5, pair piecewise construction
   >     struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
   >     constexpr piecewise_construct_t piecewise_construct{};
   >     […]

3. In 22.3.2 [pairs.pair], change the class template `pair` synopsis:

   >     […]
   >     pair(pair&&) = default;
   >     EXPLICIT constexpr pair();
   >     EXPLICIT constexpr pair(const T1& x, const T2& y);
   >     […]

4. Around 22.3.2 [pairs.pair] p3, apply the following edits:

   >     EXPLICIT constexpr pair();

   > > -3- Effects: Value-initializes `first` and `second`.

   > > -4- Remarks: This constructor shall not participate in overload resolution unless `is_default_constructible<first_type>::value` is `true` and `is_default_constructible<second_type>::value` is `true`. [Note: This behaviour can be implemented by a constructor template with default template arguments. — end note] The constructor is explicit if and only if either `first_type` or `second_type` is not implicitly default-constructible. [Note: This behaviour can be implemented with a trait that checks whether a `const first_type&` or a `const second_type&` can be initialized with `{}`. — end note]

5. In 22.3.5 [pair.piecewise], apply the following edits:

   >     struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
   >     constexpr piecewise_construct_t piecewise_construct{};

6. In 22.4.4 [tuple.tuple], change the class template `tuple` synopsis:

   >     […]
   >     // 20.4.2.1, tuple construction
   >     EXPLICIT constexpr tuple();
   >     EXPLICIT constexpr tuple(const Types&...); // only if sizeof...(Types) >= 1
   >     […]

7. Around 22.4.4.2 [tuple.cnstr] p4, apply the following edits:

   >     EXPLICIT constexpr tuple();

   > > -4- Effects: Value initializes each element.

   > > -5- Remarks: This constructor shall not participate in overload resolution unless `is_default_constructible<Ti>::value` is `true` for all i. [Note: This behaviour can be implemented by a constructor template with default template arguments. — end note] The constructor is explicit if and only if `Ti` is not implicitly default-constructible for at least one i. [Note: This behaviour can be implemented with a trait that checks whether a `const Ti&` can be initialized with `{}`. — end note]

8. In 20.2.2 [memory.syn]/1, change the header `<memory>` synopsis:

   >     […]
   >     // 20.7.6, allocator argument tag
   >     struct allocator_arg_t { explicit allocator_arg_t() = default; };
   >     constexpr allocator_arg_t allocator_arg{};
   >     […]

9. In 20.2.7 [allocator.tag], apply the following edits:

   >     namespace std {
   >       struct allocator_arg_t { explicit allocator_arg_t() = default; };
   >       constexpr allocator_arg_t allocator_arg{};
   >     }

   > Editorial drive-by: `piecewise_construct_t` is written, in 22.3.5 [pair.piecewise] like

   > >     struct piecewise_construct_t { };
   > >     constexpr piecewise_construct_t piecewise_construct{};

   > whereas other tag types such as `allocator_construct_t` are, in e.g. 20.2.7 [allocator.tag], written like

   > >     namespace std {
   > >       struct allocator_arg_t { };
   > >       constexpr allocator_arg_t allocator_arg{};
   > >     }

   > We should decide whether or not to write out the `std` namespace in such paragraphs. I would suggest not to write it out.

10. In 32.6 [thread.mutex]/1, change the header `<mutex>` synopsis:

    >     […]
    >     struct defer_lock_t { explicit defer_lock_t() = default; };
    >     struct try_to_lock_t { explicit try_to_lock_t() = default; };
    >     struct adopt_lock_t { explicit adopt_lock_t() = default; };

    >     constexpr defer_lock_t defer_lock { };
    >     constexpr try_to_lock_t try_to_lock { };
    >     constexpr adopt_lock_t adopt_lock { };
    >     […]

### 2511(i). `scoped_allocator_adaptor` piecewise construction does not require `CopyConstructible`

Section: 20.6.4 [allocator.adaptor.members] Status: Resolved Submitter: David Krauss Opened: 2015-06-16 Last modified: 2020-09-06

Priority: 3

View all other issues in [allocator.adaptor.members].

View all issues with Resolved status.

Discussion:

20.6.4 [allocator.adaptor.members]/10 requires that the argument types in the piecewise-construction tuples all be `CopyConstructible`. These tuples are typically created by `std::forward_as_tuple`, such as in ¶13. So they will be a mix of lvalue and rvalue references, the latter of which are not `CopyConstructible`.

My guess is that `CopyConstructible` was specified to feed the `tuple_cat`, before that function could handle rvalues. Since the argument tuple is already moved in ¶11, the requirement is obsolete. It should either be changed to `MoveConstructible`, or perhaps better, convert the whole tuple to references (i.e. form `tuple<Args1&&...>`) so nothing needs to be moved. After all, this is a facility for handling non-movable types.

It appears that the resolution of DR 2203(i), which added `std::move` to ¶11, simply omitted the change to ¶10.

[2016-11-08, Jonathan comments]

My paper P0475R0 provides a proposed resolution.

[2018-06 set to 'Resolved']

P0475R1 was adopted in Rapperswil.

Proposed resolution:

### 2514(i). Type traits must not be `final`

Section: 21.3.2 [meta.rqmts] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-07-03 Last modified: 2017-07-30

Priority: 3

View all other issues in [meta.rqmts].

View all issues with C++17 status.

Discussion:

We should make it clear that all standard `UnaryTypeTraits`, `BinaryTypeTraits` and `TransformationTraits` are not `final`.

Otherwise it is not safe to use them as arguments to a template like this:

>     template<typename C1, typename C2>
>     struct conjunction
>       : conditional_t<C1::value, C2, C1>
>     { };

[2016-08-03 Chicago LWG]

Walter, Nevin, and Jason provide initial Proposed Resolution.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Change 21.3.2 [meta.rqmts] as indicated:

>    > -1- A UnaryTypeTrait describes a property of a type. It shall be a non-`final` class template […]

>    > -2- A BinaryTypeTrait describes a relationship between two types. It shall be a non-`final` class template […]

>    > -3- A TransformationTrait modifies a property of a type. It shall be a non-`final` class template […]

[2016-08-04 Chicago LWG]

LWG discusses and expresses preference for a more general, Library-wide, resolution. Walter and Nevin provide a new Proposed Resolution consistent with such guidance.

[2016-08 - Chicago]

Thurs PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Add a new paragraph add the end of 16.4.6.13 [derivation] as indicated:

   > -?- All types specified in the C++ standard library shall be non-`final` types unless otherwise specified.

### 2515(i). [fund.ts.v2] Certain comparison operators of `observer_ptr` do not match synopsis

Section: 8.12.6 [fund.ts.v2::memory.observer.ptr.special] Status: TS Submitter: Tim Song Opened: 2015-07-07 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

In N4529 [memory.observer.ptr.special] paragraphs 15, 17 and 19, the `>`, `<=` and `>=` operators of `observer_ptr` are shown as

>     template <class W>
>     bool operator>(observer_ptr<W> p1, observer_ptr<W> p2);

whereas in [header.memory.synop] they are shown as

>     template <class W1, class W2>
>     bool operator>(observer_ptr<W1> p1, observer_ptr<W2> p2);

Given that the specification of `operator<` took special care to support hetergeneous types, presumably the second version is intended.

[2015-07, Telecon]

Move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4529.

1. Edit 8.12.6 [fund.ts.v2::memory.observer.ptr.special] as indicated:

   >     -15- template <class W1, class W2>
   >     bool operator>(observer_ptr<W1> p1, observer_ptr<W2> p2);

   > > […]

   >     -17- template <class W1, class W2>
   >     bool operator<=(observer_ptr<W1> p1, observer_ptr<W2> p2);

   > > […]

   >     -19- template <class W1, class W2>
   >     bool operator>=(observer_ptr<W1> p1, observer_ptr<W2> p2);

### 2516(i). [fund.ts.v2] Public "exposition only" members in `observer_ptr`

Section: 8.12.1 [fund.ts.v2::memory.observer.ptr.overview] Status: TS Submitter: Tim Song Opened: 2015-07-07 Last modified: 2017-07-30

Priority: 2

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

In N4529 [memory.observer.ptr.overview], the public member typedefs `pointer` and `reference` are marked "//exposition-only".

Clause 17 of the standard, which the TS incorporates by reference, only defines "exposition only" for private members (see 16.3.3.6 [objects.within.classes], also compare LWG 2310(i)). These types should be either made private, or not exposition only.

[2015-07, Telecon]

Geoffrey: Will survey "exposition-only" use and come up with wording to define "exposition-only" for data members, types, and concepts.

[2015-10, Kona Saturday afternoon]

GR: I cannot figure out what "exposition only" means. JW explained it to me once as meaning that "it has the same normative effect as if the member existed, without the effect of there actually being a member". But I don't know what is the "normative effect of the existence of a member" is.

MC: The "exposition-only" member should be private. GR: But it still has normative effect.

GR, STL: Maybe the member is public so it can be used in the public section? WEB (original worder): I don't know why the member is public. I'm not averse to changing it. GR: A telecon instructed me to find out how [exposition-only] is used elsewhere, and I came back with this issue because I don't understand what it means.

STL: We use exposition-only in many places. We all know what it means. GR: I don't know precisely what means.

STL: I know exactly what it means. STL suggests a rewording: the paragraph already says what we want, it's just a little too precise. GR not sure.

WEB: Are there any non-members in the Standard that are "exposition only"? STL: DECAY_COPY and INVOKE.

GR: There are a few enums.

HH: We have nested class types that are exposition-only. HH: We're about to make "none_t" an exposition-only type. [Everyone lists some examples from the Standard that are EO.] GR: "bitmask" contains some hypothetical types. That potentially contains user requirements (since regexp requires certain trait members to be "bitmask types").

STL: What's the priority of this issue? Does this have any effect on implementations? Is there anything mysterious that could happen?

AM: 2.

STL: How did that happen? It's at best priority 4. We have so many better things we could be doing.

WEB: "exposition only" in [???] is just used as plain English, not as words of power. GR: That gives me enough to make wording to fix this.

STL: I wouldn't mind if we didn't fix this ever.

MC: @JW, please write up wording for 2310(i) to make the EO members private. JW: I can't do that, because that'd make the class a non-aggregate.

WEB: Please cross-link both issues and move 2516 to "open".

[2015-11-14, Geoffrey Romer provides wording]

[2016-03 Jacksonville]

Move to ready. Note that this modifies both the Draft Standard and LFTS 2

Proposed resolution:

1. This part of the wording is against the working draft of the standard for Programming Language C++, the wording is relative to N4567.

   1. Edit 16.3.3.6 [objects.within.classes]/p2 as follows:

      > -2- Objects of certain classes are sometimes required by the external specifications of their classes to store data, apparently in member objects. For the sake of exposition, some subclauses provide representative declarations, and semantic requirements, for private members objects of classes that meet the external specifications of the classes. The declarations for such members objects and the definitions of related member types are followed by a comment that ends with exposition only, as in:

      > >     streambuf* sb; // exposition only

2. This part of the wording is against the working draft of the C++ Extensions for Library Fundamentals, Version 2, the wording is relative to N4529

   1. Edit the synopsis in 8.12.1 [fund.ts.v2::memory.observer.ptr.overview] as follows:

      >     namespace std {
      >     namespace experimental {
      >     inline namespace fundamentals_v2 {

      >       template <class W> class observer_ptr {
      >         using pointer = add_pointer_t<W>;            // exposition-only
      >         using reference = add_lvalue_reference_t<W> // exposition-only
      >       public:
      >         // publish our template parameter and variations thereof
      >         using element_type = W;
      >         using pointer = add_pointer_t<W>;            // exposition-only
      >         using reference = add_lvalue_reference_t<W> // exposition-only

      >         // 8.12.2, observer_ptr constructors
      >         // default c'tor
      >         constexpr observer_ptr() noexcept;

      >         […]
      >       }; // observer_ptr<>

      >     } // inline namespace fundamentals_v2
      >     } // namespace experimental
      >     } // namespace std

### 2517(i). [fund.ts.v2] Two `propagate_const` assignment operators have incorrect return type

Section: 3.7.5 [fund.ts.v2::propagate_const.assignment] Status: TS Submitter: Tim Song Opened: 2015-07-08 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

N4529 [propagate_const.assignment] depicts the two `operator=`s described as returning by value. This is obviously incorrect. The class synopsis correctly shows them as returning by reference.

[2015-06, Telecon]

Move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4529.

1. Edit [propagate_const.assignment] as indicated:

   >     -1- template <class U>
   >     constexpr propagate_const& operator=(propagate_const<U>&& pu)

   > > […]

   >     -5- template <class U>
   >     constexpr propagate_const& operator=(U&& u)

### 2518(i). [fund.ts.v2] Non-member `swap` for `propagate_const` should call member `swap`

Section: 3.7.10 [fund.ts.v2::propagate_const.algorithms] Status: TS Submitter: Tim Song Opened: 2015-07-08 Last modified: 2017-07-30

Priority: 3

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

For consistency with the rest of the standard library, the non-member `swap` for `propagate_const` should call member `swap`.

[2015-07, Telecon]

Both P3 and NAD were suggested.

[2016-02-20, Ville comments]

Feedback from an implementation:

The implementation of `propagate_const` in libstdc++ calls `propagate_const`'s member `swap` from the non-member `swap`.

[2016-11-08, Issaquah]

Adopted during NB comment resolution

Proposed resolution:

This wording is relative to N4529.

1. Edit [propagate_const.algorithms] as indicated:

   >     -1- template <class T>
   >     constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see below)

   > > -2- The constant-expression in the exception-specification is `noexcept(swap(pt1.t_, pt2.t_)pt1.swap(pt2))`.

   > > -3- Effects: `swap(pt1.t_, pt2.t_)pt1.swap(pt2)`.

### 2519(i). Iterator `operator-=` has gratuitous undefined behaviour

Section: 24.3.5.7 [random.access.iterators] Status: C++17 Submitter: Hubert Tong Opened: 2015-07-15 Last modified: 2017-07-30

Priority: 2

View all other issues in [random.access.iterators].

View all issues with C++17 status.

Discussion:

In subclause 24.3.5.7 [random.access.iterators], Table 110, the operational semantics for the expression "`r -= n`" are defined as

>     return r += -n;

Given a `difference_type` of a type `int` with range [-32768, 32767], if the value of `n` is -32768, then the evaluation of `-n` causes undefined behaviour (Clause 5 [expr] paragraph 4).

The operational semantics may be changed such that the undefined behaviour is avoided.

Suggested wording:

Replace the operational semantics for "`r -= n`" with:

>     {
>       difference_type m = n;
>       if (m >= 0)
>         while (m--)
>           --r;
>       else
>         while (m++)
>           ++r;
>       return r;
>     }

Jonathan Wakely:

I'm now convinced we don't want to change the definition of `-=` and instead we should explicitly state the (currently implicit) precondition that `n != numeric_limits<difference_type>::min()`.

[2016-08, Chicago]

Monday PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4527.

1. Change Table 110 "Random access iterator requirements (in addition to bidirectional iterator)" as indicated:

   > Table 110 — Random access iterator requirements (in addition to bidirectional iterator)
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
   > --- | --- | --- | ---
   > `…`
   > `r -= n` | `X&` | `return r += -n;` | pre: the absolute value of `n` is in the range of representable values of `difference_type`.
   > `…`

### 2520(i). N4089 broke initializing `unique_ptr<T[]>` from a `nullptr`

Section: 20.3.1.4.2 [unique.ptr.runtime.ctor] Status: C++17 Submitter: Ville Voutilainen Opened: 2015-07-19 Last modified: 2017-07-30

Priority: 2

View all issues with C++17 status.

Discussion:

According to the wording in 20.3.1.4.2 [unique.ptr.runtime.ctor]/1, this won't work:

>     unique_ptr<int[], DeleterType> x{nullptr, DeleterType{}};

`U` is not the same type as `pointer`, so the first bullet will not do.
`U` is not a pointer type, so the second bullet will not do.

An easy fix would be to add a new bullet after the first bullet, like so:

- `U` is the same type as `pointer`, or

- `U` is `nullptr_t`, or

- `pointer` is the same type as `element_type*`, […]

[2015-10, Kona Saturday afternoon]

MC: Is it the right fix?

GR: It'd be awefully surprising if we had an interface that accepts null pointer values but not std::nullptr_t. I think the PR is good.

STL: Are any of the assignments and reset affected? [No, they don't operate on explicit {pointer, deleter} pairs.]

VV: This is already shipping, has been implemented, has been tested and works fine.

Move to Tentatively ready.

Proposed resolution:

This wording is relative to N4527.

1. Change 20.3.1.4.2 [unique.ptr.runtime.ctor] as indicated:

   >     template <class U> explicit unique_ptr(U p) noexcept;
   >     template <class U> unique_ptr(U p, see below d) noexcept;
   >     template <class U> unique_ptr(U p, see below d) noexcept;

   > > -1- These constructors behave the same as the constructors that take a `pointer` parameter in the primary template except that they shall not participate in overload resolution unless either

   > > - `U` is the same type as `pointer`, or

   > > - `U` is `nullptr_t`, or

   > > - `pointer` is the same type as `element_type*`, `U` is a pointer type `V*`, and `V(*)[]` is convertible to `element_type(*)[]`.

### 2521(i). [fund.ts.v2] `weak_ptr`'s converting move constructor should be modified as well for array support

Section: 8.2.2.1 [fund.ts.v2::memory.smartptr.weak.const] Status: TS Submitter: Tim Song Opened: 2015-07-25 Last modified: 2017-07-30

Priority: 2

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

[memory.smartptr.weak.const] altered the constraints on `weak_ptr`'s constructor from `const weak_ptr<Y>&` and `const shared_ptr<Y>&`. The constraints on the converting move constructor from `weak_ptr<Y>&&` was not, but should be, similarly modified.

[2015-10-26]

Daniel adjusts wording to lib. fund. v2. As link to the originating proposal: The discussion in this issue refers to wording changes that were requested by N3920.

[2016-11-08, Issaquah]

Adopted during NB comment resolution

Proposed resolution:

This wording is relative to N4529.

1. At the end of [memory.smartptr.weak.const], add:
   [Drafting note: The current paragraph [memory.smartptr.weak.const] p2 is incorrectly declared as Requires element, but it does not describe a requirement, instead it describes a "template constraint" which are elsewhere always specified within a Remarks element because it describes constraints that an implementation (and not the user) has to meet. See LWG 2292(i) for a suggestion to introduce a separate new specification element for this situation. This has also been fixed in the current working draft. — end drafting note]

   >     weak_ptr(weak_ptr&& r) noexcept;
   >     template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

   > > -?- Remark: The second constructor shall not participate in overload resolution unless `Y*` is compatible with `T*`.

   > > -?- Effects: Move-constructs a `weak_ptr` instance from `r`.

   > > -?- Postconditions: `*this` shall contain the old value of `r`. `r` shall be empty. `r.use_count() == 0`.

### 2522(i). [fund.ts.v2] Contradiction in `set_default_resource` specification

Section: 8.8 [fund.ts.v2::memory.resource.global] Status: TS Submitter: Tim Song Opened: 2015-07-28 Last modified: 2017-07-30

Priority: 2

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

[memory.resource.global]/p7-8 says that the effects of `set_default_resource(r)` are

> If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

and the operation has the postcondition

> `get_default_resource() == r`.

When `r` is null, however, the postcondition cannot be met, since the call sets the default memory resource pointer to `new_delete_resource()`, and so `get_default_resource()` would return the value of `new_delete_resource()`, which is obviously not null and so cannot compare equal to `r`.

Previous resolution from Tim Song [SUPERSEDED]:

> This wording is relative to N4480.

> 1. Edit [memory.resource.global]/p8 as follows:

>    >     -6- memory_resource* set_default_resource(memory_resource* r) noexcept;

>    > > -7- Effects: If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

>    > > -8- Postconditions: `get_default_resource() == r` if `r` is non-null; otherwise, `get_default_resource() == new_delete_resource()`.

>    > > […]

[2015-09-15 Geoffrey Romer comments and suggests alternative wording]

Let's just strike 8.8 [fund.ts.v2::memory.resource.global]/p8. The problem is that p8 is restating p7 incorrectly, but the solution is not to restate p7 correctly, it's to stop trying to restate p7 at all.

[2015-10, Kona Saturday afternoon]

Move to Tentatively ready

[2015-10-26]

Daniel adjusts wording to lib. fund. v2.

Proposed resolution:

This wording is relative to N4529.

1. Edit [memory.resource.global]/p8 as follows:

   >     -6- memory_resource* set_default_resource(memory_resource* r) noexcept;

   > > -7- Effects: If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

   > > -8- Postconditions: `get_default_resource() == r`.

   > > […]

### 2523(i). `std::promise` synopsis shows two `set_value_at_thread_exit()`'s for no apparent reason

Section: 32.10.6 [futures.promise] Status: C++17 Submitter: Tim Song Opened: 2015-07-31 Last modified: 2017-07-30

Priority: 0

View all other issues in [futures.promise].

View all issues with C++17 status.

Discussion:

In 32.10.6 [futures.promise], the class synopsis shows

>     void set_value_at_thread_exit(const R& r);
>     void set_value_at_thread_exit(see below);

There's no apparent reason for having `void set_value_at_thread_exit(const R& r);`, especially as that signature isn't really present in the specializations (particularly `promise<void>`). Note that the similar `set_value` only has a `void set_value(see below);`

While we are here, 32.10.6 [futures.promise]/p1 says that the specializations "differ only in the argument type of the member function `set_value`", which missed `set_value_at_thread_exit`.

[2015-10, Kona issue prioritization]

Priority 0, move to Ready

Proposed resolution:

This wording is relative to N4527.

1. Edit 32.10.6 [futures.promise], class template `promise` synopsis, as indicated:

   >     namespace std {
   >       template <class R>
   >       class promise {
   >       public:
   >         […]

   >         // setting the result
   >         void set_value(see below);
   >         void set_exception(exception_ptr p);

   >         // setting the result with deferred notification
   >         void set_value_at_thread_exit(const R& r);
   >         void set_value_at_thread_exit(see below);
   >         void set_exception_at_thread_exit(exception_ptr p);
   >       };
   >     }

2. Edit 32.10.6 [futures.promise]/1 as indicated:

   > -1- The implementation shall provide the template `promise` and two specializations, `promise<R&>` and `promise<void>`. These differ only in the argument type of the member functions `set_value` and `set_value_at_thread_exit`, as set out in its descriptiontheir descriptions, below.

### 2524(i). `generate_canonical` can occasionally return 1.0

Section: 29.5.9.4.2 [rand.dist.pois.exp] Status: Resolved Submitter: Michael Prähofer Opened: 2015-08-20 Last modified: 2024-04-02

Priority: 2

View all issues with Resolved status.

Discussion:

Original title was: `exponential_distribution<float>` sometimes returns inf.

The random number distribution class template `exponential_distribution<float>` may return "inf" as can be seen from the following example program:

>     // compiled with
>     // g++ -std=c++11 Error_exp_distr.cpp

>     #include <iostream>
>     #include <random>
>     #include <bitset>

>     int main(){
>       unsigned long long h;
>       std::mt19937_64 mt1(1);
>       std::mt19937_64 mt2(1);
>       mt1.discard(517517);
>       mt2.discard(517517);
>       std::exponential_distribution<float> dis(1.0);
>       h = mt2();
>       std::cout << std::bitset<64>(h) << " " << (float) -log(1 - h/pow(2, 64)) << " "
>                 << -log(1 - (float) h/pow(2, 64)) << " " << dis(mt1) << std::endl;
>       h = mt2();
>       std::cout << std::bitset<64>(h) << " " << (float) -log(1 - h/pow(2, 64)) << " "
>                 << -log(1 - (float) h/pow(2, 64)) << " " << dis(mt1) << std::endl;
>     }

output:

>     0110010110001001010011000111000101001100111110100001110011100001 0.505218 0.505218 0.505218
>     1111111111111111111111111101010011000110011110011000110101100110 18.4143 inf inf

The reason seems to be that converting a `double x` in the range `[0, 1)` to `float` may result in `1.0f` if `x` is close enough to `1`. I see two possibilities to fix that:

1. use internally `double` (or `long double`?) and then convert the result at the very end to `float`.

2. take only 24 random bits and convert them to a `float x` in the range `[0, 1)` and then return `-log(1 - x)`.

I have not checked if `std::exponential_distribution<double>` has the same problem: For `float` on the average 1 out of 224 (~107) draws returns "inf", which is easily confirmed. For `double` on the average 1 out of 253 (~1016) draws might return "inf", which I have not tested.

Marshall:
I don't think the problem is in `std::exponential_distribution`; but rather in `generate_canonical`.

Consider:

>     std::mt19937_64 mt2(1);
>     mt2.discard(517517);
>     std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl;
>     std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl;
>     std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl;

which outputs:

>     0x1.962532p-2
>     0x1p+0
>     0x1.20d0cap-3

but `generate_canonical` is defined to return a result in the range [0, 1).

[2015-10, Kona Saturday afternoon]

Options:

- 1) NAD, make it work. E.g. nudge slightly, making the hightest result return the highest allowable value (next_after). This might change the distribution slightly.
- 2) Re-run the algorithm when you get a 1. This changes the specification in that it makes extra calls to the random engine.
- 3) Something else

WEB: The one thing we cannot tolerate is any output range other than [0, 1).

WEB: I believe there may be a documented algorithm for the generator, and perhaps it's possible to discover en-route that the algorithm produces the wrong result and fix it.

MC: No. I analyzed this once, and here it is: the algorithm is in [rand.util.canonical], and it's all fine until p5. The expression `S/R^k` is mathematically less than one, but it may round to one.

GR: Could we change the rounding mode for the computation?

HH: No, because the rounding mode is global, not thread-local.

AM: SG1 wants to get rid of the floating point environment.

STL: The problem is that the standard specifies the implementation, and the implementation doesn't work.

MC: I'm not sure if nudging it down will introduce a subtle bias.

EF: I worry about how the user's choice of floating point environment affects the behaviour.

MS offers to run the topic past colleagues.

MC: Will set the status to open. STL wants to rename the issue. WEB wants to be able to find the issue by its original name still.

Mike Spertus to run the options past his mathematical colleagues, and report back.

[2017-11 Albuquerque Wednesday issue processing]

Choice: Rerun the algorithm if it gets 1.0.

Thomas K to provide wording; Marshall and STL to review.

[2018-08 Batavia Monday issue discussion]

Davis has a paper P0952 which resolves this.

[2024-04-02 Kona 2023; Resolved by P0952R2. Status changed: Open → Resolved.]

Proposed resolution:

### 2525(i). [fund.ts.v2] `get_memory_resource` should be `const` and `noexcept`

Section: 4.2 [fund.ts.v2::func.wrap.func], 11.2 [fund.ts.v2::futures.promise], 11.3 [fund.ts.v2::futures.task] Status: TS Submitter: Tim Song Opened: 2015-08-04 Last modified: 2017-07-30

Priority: 3

View all other issues in [fund.ts.v2::func.wrap.func].

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

There doesn't seem to be any reason why this member function cannot be called on a `const` object, or why it would ever throw. I discussed this with Pablo Halpern, the author of N3916, and he agrees that this appears to have been an oversight.

[2015-10-26]

Daniel adjusts wording to lib. fund. v2.

[2016-11-08, Issaquah]

Adopted during NB comment resolution

Proposed resolution:

This wording is relative to N4529.

1. Edit each of the synposes in 4.2 [fund.ts.v2::func.wrap.func], 11.2 [fund.ts.v2::futures.promise], and 11.3 [fund.ts.v2::futures.task] as indicated:

   >     pmr::memory_resource* get_memory_resource() const noexcept;

### 2526(i). [fund.ts.v2] Incorrect precondition for `experimental::function::swap`

Section: 4.2.2 [fund.ts.v2::func.wrap.func.mod] Status: TS Submitter: Tim Song Opened: 2015-08-04 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

4.2.2 [fund.ts.v2::func.wrap.func.mod] says that the precondition of `swap` is

>     this->get_memory_resource() == other->get_memory_resource()

That compares two pointers and so requires the memory resource used by `*this` and `other` to be the exact same object. That doesn't seem correct (for one, it would essentially outlaw swapping all `function`s constructed with "plain" allocators, since they would each have their own `resource_adaptor` object and so the pointers will not compare equal). Presumably the intent is to compare the `memory_resource`s for equality.

Also, `other` is a reference, not a pointer.

[2015-09-11, Telecon]

Move to Tentatively Ready

[2015-10-26]

Daniel adjusts wording to lib. fund. v2.

Proposed resolution:

This wording is relative to N4529.

1. Edit 4.2.2 [fund.ts.v2::func.wrap.func.mod] as indicated:

   >     void swap(function& other);

   > > -2- Requires: `*this->get_memory_resource() == *other->.get_memory_resource()`.

   > > -3- Effects: Interchanges the targets of `*this` and `other`.

   > > -4- Remarks: The allocators of `*this` and `other` are not interchanged.

### 2527(i). [fund.ts.v2] `ALLOCATOR_OF` for `function::operator=` has incorrect default

Section: 4.2.1 [fund.ts.v2::func.wrap.func.con] Status: TS Submitter: Tim Song Opened: 2015-08-04 Last modified: 2017-07-30

Priority: 3

View all other issues in [fund.ts.v2::func.wrap.func.con].

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

According to the table in 8.3 [fund.ts.v2::memory.type.erased.allocator], if no allocator argument is specified at the time of construction, the memory resource pointer used is the value of `experimental::pmr::get_default_resource()` at the time of construction.

Yet in 4.2.1 [fund.ts.v2::func.wrap.func.con], `ALLOCATOR_OF` is specified to return `allocator<char>()` if no allocator was specified at the time of construction, which seems incorrect, especially as the user can change the default memory resource pointer to something other than `new_delete_resource()`.

[2015-10-26]

Daniel adjusts wording to lib. fund. v2.

[2016-11-08, Issaquah]

Adopted during NB comment resolution

Proposed resolution:

This wording is relative to N4529.

1. Edit 4.2.1 [fund.ts.v2::func.wrap.func.con]/p2 as indicated:

   > -2- In the following descriptions, let `ALLOCATOR_OF(f)` be the allocator specified in the construction of `function` `f`, or `allocator<char>()`the value of `experimental::pmr::get_default_resource()` at the time of the construction of `f` if no allocator was specified.

### 2529(i). Assigning to `enable_shared_from_this::__weak_this` twice

Section: 20.3.2.7 [util.smartptr.enab] Status: Resolved Submitter: Jonathan Wakely Opened: 2015-08-26 Last modified: 2020-09-06

Priority: 3

View all other issues in [util.smartptr.enab].

View all issues with Resolved status.

Discussion:

It is unclear what should happen if a pointer to an object with an `enable_shared_from_this` base is passed to two different `shared_ptr` constructors.

>     #include <memory>

>     using namespace std;

>     int main()
>     {
>       struct X : public enable_shared_from_this<X> { };
>       auto xraw = new X;
>       shared_ptr<X> xp1(xraw);  // #1
>       {
>         shared_ptr<X> xp2(xraw, [](void*) { });  // #2
>       }
>       xraw->shared_from_this();  // #3
>     }

This is similar to LWG 2179(i), but involves no undefined behaviour due to the no-op deleter, and the question is not whether the second `shared_ptr` should share ownership with the first, but which `shared_ptr` shares ownership with the `enable_shared_from_this::__weak_this` member.

With all three of the major `std::shared_ptr` implementations the `xp2` constructor modifies the `__weak_this` member so the last line of the program throws `bad_weak_ptr`, even though all the requirements on the `shared_from_this()` function are met (20.3.2.7 [util.smartptr.enab])/7:

> Requires: `enable_shared_from_this<T>` shall be an accessible base class of `T`. `*this` shall be a subobject of an object `t` of type `T`. There shall be at least one `shared_ptr` instance `p` that owns `&t`.

Boost doesn't update `__weak_this`, leaving it sharing with `xp1`, so the program doesn't throw. That change was made to `boost::enable_shared_from_this` because someone reported exactly this issue as a bug, see Boost issue 2584.

On the reflector Peter Dimov explained that there are real-world use cases that rely on the Boost behaviour, and none which rely on the behaviour of the current `std::shared_ptr` implementations. We should specify the behaviour of `enable_shared_from_this` more precisely, and resolve this issue one way or another.

[2016-03-16, Alisdair comments]

This issues should be closed as `Resolved` by paper p0033r1 at Jacksonville.

Proposed resolution:

### 2531(i). `future::get` should explicitly state that the shared state is released

Section: 32.10.7 [futures.unique.future] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2015-09-03 Last modified: 2021-06-06

Priority: 3

View all other issues in [futures.unique.future].

View all issues with C++17 status.

Discussion:

The standard is usually very explicit on when a shared state is released, except for `future::get` for which it only states `valid() == false` as a postcondition.

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4527.

1. Modify [futures.unique_future] as indicated:

   >     R future::get();
   >     R& future<R&>::get();
   >     void future<void>::get();

   > > -14- Note: as described above, the template and its two required specializations differ only in the return type and return value of the member function `get`.

   > > -15- Effects:

   > > - `wait()`s until the shared state is ready, then retrieves the value stored in the shared state.;

   > > - releases any shared state (32.10.5 [futures.state]).

   > > […]

### 2534(i). Constrain rvalue stream operators

Section: 31.7.6.6 [ostream.rvalue], 31.7.5.6 [istream.rvalue] Status: C++17 Submitter: Robert Haberlach Opened: 2015-09-08 Last modified: 2017-07-30

Priority: 3

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Discussion:

The rvalue stream insertion and extraction operators should be constrained to not participate in overload resolution unless the expression they evaluate is well-formed. Programming code that tests the validity of stream insertions (or extractions) using SFINAE can result in false positives, as the present declarations accept virtually any right-hand side argument. Moreover, there is no need for pollution of the candidate set with ill-formed specializations.

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4527.

1. Modify 31.7.6.6 [ostream.rvalue] as indicated:

   >     template <class charT, class traits, class T>
   >       basic_ostream<charT, traits>&
   >       operator<<(basic_ostream<charT, traits>&& os, const T& x);

   > > -1- Effects: `os << x`

   > > -2- Returns: `os`

   > > -?- Remarks: This function shall not participate in overload resolution unless the expression `os << x` is well-formed.

2. Modify 31.7.5.6 [istream.rvalue] as indicated:

   >     template <class charT, class traits, class T>
   >       basic_istream<charT, traits>&
   >       operator>>(basic_istream<charT, traits>&& is, T& x);

   > > -1- Effects: `is >> x`

   > > -2- Returns: `is`

   > > -?- Remarks: This function shall not participate in overload resolution unless the expression `is >> x` is well-formed.

### 2536(i). What should `<complex.h>` do?

Section: 17.15 [support.c.headers] Status: C++17 Submitter: Richard Smith Opened: 2015-09-10 Last modified: 2023-02-07

Priority: 2

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Discussion:

LWG issue 1134(i) removed the resolution of LWG 551(i), leaving an incorrect specification for the behavior of `<complex.h>`. This header is currently required to make `std::complex` (and associated functions) visible in the global namespace, but should not be so required.

[2016-09-09 Issues Resolution Telecon]

Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4527.

1. Add a new paragraph before [depr.c.headers]/2:

   > -?- The header `<complex.h>` behaves as if it simply includes the header `<ccomplex>`.

2. Change in [depr.c.headers]/2:

   > -2- Every other C header, each of which has a name of the form `name.h`, behaves as if each name placed in the standard library namespace by the corresponding `cname` header is placed within the global namespace scope. It is unspecified whether these names are first declared or defined within namespace scope (3.3.6) of the namespace `std` and are then injected into the global namespace scope by explicit using-declarations (7.3.3).

### 2537(i). Constructors for `priority_queue` taking allocators should call `make_heap`

Section: 23.6.4.3 [priqueue.cons.alloc] Status: C++17 Submitter: Eric Schmidt Opened: 2015-09-19 Last modified: 2017-07-30

Priority: 0

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Discussion:

`priority_queue` constructors taking both `Container` and `Alloc` arguments should finish by calling `make_heap`, just as with the constructors that do not have allocator parameters.

The current reading of 23.6.4.3 [priqueue.cons.alloc], if followed strictly, would effectively require calling code to ensure that the container argument is already a heap, which is probably not what was intended.

[2015-10, Kona issue prioritization]

Priority 0, move to Ready

Proposed resolution:

This wording is relative to N4527.

1. Change 23.6.4.3 [priqueue.cons.alloc] as indicated:

   >     template <class Alloc>
   >     priority_queue(const Compare& compare, const Container& cont, const Alloc& a);

   > > -4- Effects: Initializes `c` with `cont` as the first argument and `a` as the second argument, and initializes `comp` with `compare`; calls `make_heap(c.begin(), c.end(), comp)`.

   >     template <class Alloc>
   >     priority_queue(const Compare& compare, Container&& cont, const Alloc& a);

   > > -5- Effects: Initializes `c` with `std::move(cont)` as the first argument and `a` as the second argument, and initializes `comp` with `compare`; calls `make_heap(c.begin(), c.end(), comp)`.

### 2539(i). [fund.ts.v2] `invocation_trait` definition definition doesn't work for surrogate call functions

Section: 3.3.2 [fund.ts.v2::meta.trans.other] Status: TS Submitter: Mike Spertus Opened: 2015-09-25 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: fund.ts.v2

In Library Fundamentals 2 (N4529) 3.3.2p3 [meta.trans.other], the definition of invocation traits for a class object `f` considers when `f` is called via a function call operator that is matched by the arguments but ignores the possibility that `f` may be called via a surrogate call function (C++14 12.2.2.2.3 [over.call.object] p2), in which case, the definition of the invocation parameters may be either incorrect or even unsatisfiable.

[2015-10, Kona Saturday afternoon]

AM: Do we have this trait yet? JW: No, it cannot be implemented without compiler support.

Move to tentatively ready

Proposed resolution:

This wording is relative to N4529.

1. In Library Fundamentals 2, change [meta.trans.other] as indicated:

   > -3- Within this section, define the invocation parameters of `INVOKE(f, t1, t2, ..., tN)` as follows, in which `T1` is the possibly cv-qualified type of `t1` and `U1` denotes `T1&` if `t1` is an lvalue or `T1&&` if `t1` is an rvalue:

   > - […]

   > - If `f` is a class object, the invocation parameters are the parameters matching `t1, ..., tN` of the best viable function (C++14 §13.3.3) for the arguments `t1, ..., tN` among the function call operators and surrogate call functions of `f`.

   > - […]

### 2540(i). `unordered_multimap::insert` hint iterator

Section: 23.2.8 [unord.req] Status: C++17 Submitter: Isaac Hier Opened: 2015-09-16 Last modified: 2017-07-30

Priority: 3

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Discussion:

I have been wondering about the C++ standard requirements regarding the hint iterator for insertion into an `unordered_multimap` (and I imagine a similar question could be asked of `unordered_map`, but I have not researched that topic). As far as I can tell, it seems perfectly valid for an implementation to allow only valid dereferencable iterators to be used as the hint argument for this member function. If that is correct, it means that one could not expect the end iterator to be used as a valid hint nor could one use the begin iterator of an empty `unordered_multimap` as the hint. However, this essentially precludes all uses of inserter on an empty `unordered_multimap` seeing as the inserter functor requires that a hint iterator be passed to its constructor.

Howard Hinnant:

> The intent of the standard is that the iterator produced from container `c` by `c.end()` is a valid (but non-dereferenceable) iterator into container `c`. It is reachable by every other iterator into `c`.

> It appears to me that you and the Bloomberg implementation have fallen victim to a type-o in the Unordered associative container requirements, Table 102. The row containing:

> >     a.insert(q, t);

> should read instead:

> >     a.insert(p, t);

> The distinction is that `p` is valid, and `q` is both valid and dereferenceable. The correction of this type-o would make unordered container `insert` consistent with unordered `emplace_hint`, associative `insert`, and associative `emplace_hint`.

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Proposed resolution:

Change the insert-with-hint row in Table 102 Unordered associative container requirements like so:

> `a.insert(qp, t);` | `iterator` | Requires: If `t` is a non-const ... | Average Case ...

### 2541(i). [parallel.ts] Headers for `ExecutionPolicy` algorithm overloads

Section: 99 [parallel.ts::parallel.alg.overloads] Status: Resolved Submitter: Tim Song Opened: 2015-09-26 Last modified: 2017-11-13

Priority: 1

View all issues with Resolved status.

Discussion:

Addresses: parallel.ts

99 [parallel.ts::parallel.alg.overloads] provides parallel algorithm overloads for many algorithms in the standard library, but I can't find any normative wording specifying which headers these new overloads live in. Presumably, if the original algorithm is in `<meow>`, the new overloads should be in `<experimental/meow>`.

[2017-11 Albuquerque Saturday issues processing]

Resolved by P0776R1.

Proposed resolution:

### 2542(i). Missing `const` requirements for associative containers

Section: 23.2.7 [associative.reqmts] Status: C++17 Submitter: Daniel Krügler Opened: 2015-09-26 Last modified: 2017-07-30

Priority: 1

View other active issues in [associative.reqmts].

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Discussion:

Table 101 — "Associative container requirements" and its associated legend paragraph 23.2.7 [associative.reqmts] p8 omits to impose constraints related to `const` values, contrary to unordered containers as specified by Table 102 and its associated legend paragraph p11.

Reading these requirements strictly, a feasible associative container could declare several conceptual observer members — for example `key_comp()`, `value_comp()`, or `count()` — as non-`const` functions. In Table 102, "possibly `const`" values are exposed by different symbols, so the situation for unordered containers is clear that these functions may be invoked by `const` container objects.

For the above mentioned member functions this problem is only minor, because the synopses of the actual Standard associative containers do declare these members as `const` functions, but nonetheless a wording fix is recommended to clean up the specification asymmetry between associative containers and unordered containers.

The consequences of the ignorance of `const` becomes much worse when we consider a code example such as the following one from a recent libstdc++ bug report:

>     #include <set>

>     struct compare
>     {
>       bool operator()(int a, int b) // Notice the non-const member declaration!
>       {
>         return a < b;
>       }
>     };

>     int main() {
>       const std::set<int, compare> s;
>       s.find(0);
>     }

The current wording in 23.2.7 [associative.reqmts] can be read to require this code to be well-formed, because there is no requirement that an object `comp` of the ordering relation of type `Compare` might be a `const` value when the function call expression `comp(k1, k2)` is evaluated.

Current implementations differ: While Clangs libc++ and GCCs libstdc++ reject the above example, the Dinkumware library associated with Visual Studio 2015 accepts it.

I believe the current wording unintentionally misses the constraint that even `const` comparison function objects of associative containers need to support the predicate call expression. This becomes more obvious when considering the member `value_compare` of `std::map` which provides (only) a `const operator()` overload which invokes the call expression of data member `comp`.

[2016-02-20, Daniel comments and extends suggested wording]

It has been pointed out to me, that the suggested wording is a potentially breaking change and should therefore be mentioned in Annex C.

First, let me emphasize that this potentially breaking change is solely caused by the wording change in 23.2.7 [associative.reqmts] p8:

> […] and `c` denotes a possibly `const` value of type `X::key_compare`; […]

So, even if that proposal would be rejected, the rest of the suggested changes could (and should) be considered for further evaluation, because the remaining parts do just repair an obvious mismatch between the concrete associative containers (`std::set`, `std::map`, …) and the requirement tables.

Second, I believe that the existing wording was never really clear in regard to require a Standard Library to accept comparison functors with non-`const operator()`. If the committee really intends to require a Library to support comparison functors with non-`const operator()`, this should be clarified by at least an additional note to e.g. 23.2.7 [associative.reqmts] p8.

[2016-03, Jacksonville]

Move to Ready with Daniel's updated wording

Proposed resolution:

This wording is relative to N4567.

1. Change 23.2.7 [associative.reqmts] p8 as indicated:

   > -8- In Table 101, `X` denotes an associative container class, `a` denotes a value of type `X`, `b` denotes a possibly `const` value of type `X`, `u` denotes the name of a variable being declared, `a_uniq` denotes a value of type `X` when `X` supports unique keys, `a_eq` denotes a value of type `X` when `X` supports multiple keys, `a_tran` denotes a possibly `const` value of type `X` when the qualified-id `X::key_compare::is_transparent` is valid and denotes a type (14.8.2), `i` and `j` satisfy input iterator requirements and refer to elements implicitly convertible to `value_type`, `[i, j)` denotes a valid range, `p` denotes a valid const iterator to `a`, `q` denotes a valid dereferenceable const iterator to `a`, `r` denotes a valid dereferenceable iterator to `a`, `[q1, q2)` denotes a valid range of const iterators in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` denotes a value of type `X::value_type`, `k` denotes a value of type `X::key_type` and `c` denotes a possibly `const` value of type `X::key_compare`; `kl` is a value such that `a` is partitioned (25.4) with respect to `c(r, kl)`, with `r` the key value of `e` and `e` in `a`; `ku` is a value such that `a` is partitioned with respect to `!c(ku, r)`; `ke` is a value such that `a` is partitioned with respect to `c(r, ke)` and `!c(ke, r)`, with `c(r, ke)` implying `!c(ke, r)`. `A` denotes the storage allocator used by `X`, if any, or `std::allocator<X::value_type>` otherwise, and `m` denotes an allocator of a type convertible to `A`.

2. Change 23.2.7 [associative.reqmts], Table 101 — "Associative container requirements" as indicated:

   [Editorial note: This issue doesn't touch the note column entries for the expressions related to `key_comp()` and `value_comp()` (except for the symbolic correction), since these are already handled by LWG issues 2227(i) and 2215(i) — end editorial note]

   > Table 101 — Associative container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `ab.key_comp()` | `X::key_compare` | returns the comparison object out of which `ab` was constructed. | constant
   > `ab.value_comp()` | `X::value_compare` | returns an object of `value_compare` constructed out of the comparison object | constant
   > `…`
   > `ab.find(k)` | `iterator`; `const_iterator` for constant `ab`. | returns an iterator pointing to an element with the key equivalent to `k`, or `ab.end()` if such an element is not found | logarithmic
   > `…`
   > `ab.count(k)` | `size_type` | returns the number of elements with key equivalent to `k` | `log(ab.size()) + ab.count(k)`
   > `…`
   > `ab.lower_bound(k)` | `iterator`; `const_iterator` for constant `ab`. | returns an iterator pointing to the first element with key not less than `k`, or `ab.end()` if such an element is not found. | logarithmic
   > `…`
   > `ab.upper_bound(k)` | `iterator`; `const_iterator` for constant `ab`. | returns an iterator pointing to the first element with key greater than `k`, or `ab.end()` if such an element is not found. | logarithmic
   > `…`
   > `ab.equal_range(k)` | `pair<iterator, iterator>`; `pair<const_iterator, const_iterator>` for constant ab. | equivalent to `make_- pair(ab.lower_bound(k), ab.upper_bound(k))`. | logarithmic
   > `…`

3. Add a new entry to Annex C, C.4 [diff.cpp14], as indicated:

   > C.4.4 Clause 23: containers library [diff.cpp14.containers]

   > 23.2.4

   > Change: Requirements change:

   > Rationale: Increase portability, clarification of associative container requirements.

   > Effect on original feature: Valid 2014 code that attempts to use associative containers having a comparison object with non-`const` function call operator may fail to compile:

   > >     #include <set>

   > >     struct compare
   > >     {
   > >       bool operator()(int a, int b)
   > >       {
   > >         return a < b;
   > >       }
   > >     };

   > >     int main() {
   > >       const std::set<int, compare> s;
   > >       s.find(0);
   > >     }

### 2543(i). LWG 2148 (hash support for enum types) seems under-specified

Section: 22.10.19 [unord.hash] Status: Resolved Submitter: Ville Voutilainen Opened: 2015-09-27 Last modified: 2017-02-02

Priority: 2

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Discussion:

The rationale in issue 2148(i) says:

> This proposed resolution doesn't specify anything else about the primary template, allowing implementations to do whatever they want for non-enums: `static_assert` nicely, explode horribly at compiletime or runtime, etc.

libc++ seems to implement it by defining the primary template and `static_assert`ing `is_enum` inside it. However, that brings forth a problem; there are reasonable SFINAE uses broken by it:

>     #include <type_traits>
>     #include <functional>

>     class S{}; // No hash specialization

>     template<class T>
>     auto f(int) -> decltype(std::hash<T>(), std::true_type());

>     template<class T>
>     auto f(...) -> decltype(std::false_type());

>     static_assert(!decltype(f<S>(0))::value, "");

MSVC doesn't seem to accept that code either.

There is a way to implement LWG 2148(i) so that `hash` for enums is supported without breaking that sort of SFINAE uses:

1. Derive the main `hash` template from a library-internal uglified-named base template that takes a type and a `bool`, pass as argument for the base the result of `is_enum`.

2. Partially specialize that base template so that the false-case has a suitable set of private special member function declarations so that it's not an aggregate nor usable in almost any expression.

[2015-10, Kona Saturday afternoon]

EricWF to come back with wording; move to Open

[2016-05-08, Eric Fiselier & Ville provide wording]

[2016-05-25, Tim Song comments]

I see two issues with this P/R:

1. "for which neither the library nor the user provides an explicit specialization" should probably be "for which neither the library nor the user provides an explicit or partial specialization".

2. Saying that the specialization "is not `DefaultConstructible` nor `MoveAssignable`" is not enough to guarantee that common SFINAE uses will work. Both of those requirements have several parts, and it's not too hard to fail only some of them. For instance, not meeting the assignment postcondition breaks `MoveAssignable`, but is usually not SFINAE-detectible. And for `DefaultConstructible`, it's easy to write something in a way that breaks `T()` but not `T{}` (due to aggregate initialization in the latter case).

[2016-06-14, Daniel comments]

The problematic part of the P/R is that it describes constraints that would be suitable if they were constraints for user-code, but they are not suitable as requirements imposed on implementations to provide certain guarantees for clients of the Library. The guarantees should be written in terms of testable compile-time expressions, e.g. based on negative results of `is_default_constructible<hash<X>>::value`, `std::is_copy_constructible<hash<X>>::value`, and possibly also `std::is_destructible<hash<X>>::value`. How an implementation realizes these negative results shouldn't be specified, though, but the expressions need to be well-formed and well-defined.

[2016-08-03, Ville provides revised wording as response to Daniel's previous comment]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4582.

> 1. Insert a new paragraph after 22.10.19 [unord.hash]/2

>    > -2- The template specializations shall meet the requirements of class template `hash` (20.12.14).

>    > -?- For any type that is not of integral or enumeration type, or for which neither the library nor the user provides an explicit specialization of the class template `hash`, the specialization of `hash` does not meet any of the `Hash` requirements, and is not `DefaultConstructible` nor `MoveAssignable`. [Note: this means that the specialization of `hash` exists, but any attempts to use it as a Hash will be ill-formed. — end note]

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Insert a new paragraph after 22.10.19 [unord.hash]/2

>    > [Drafting note: I see no reason to specify whether `H<T>` is destructible. There's no practical use case for which that would need to be covered. libstdc++ makes it so that `H<T>` is destructible.]

>    > -2- The template specializations shall meet the requirements of class template `hash` (20.12.14).

>    > -?- For any type `T` that is not of integral or enumeration type, or for which neither the library nor the user provides an explicit or partial specialization of the class template hash, the specialization of `hash<T>` has the following properties:
>    > - `is_default_constructible_v<hash<T>>` is `false`
>    > - `is_copy_constructible_v<hash<T>>` is `false`
>    > - `is_move_constructible_v<hash<T>>` is `false`
>    > - `is_copy_assignable_v<hash<T>>` is `false`
>    > - `is_move_assignable_v<hash<T>>` is `false`
>    > - `is_callable_v<hash<T>, T&>` is `false`
>    > - `is_callable_v<hash<T>, const T&>` is `false`

>    > [Note: this means that the specialization of `hash` exists, but any attempts to use it as a `Hash` will be ill-formed. — end note]

[2016-08-09 Daniel reopens]

As pointed out by Eric, the usage of `is_callable` is incorrect. Eric provides new wording.

[2016-09-09 Issues Resolution Telecon]

Move to Tentatively Ready

[2016-11-12, Issaquah]

Resolved by P0513R0

Proposed resolution:

This wording is relative to N4606.

1. Insert a new paragraph after 22.10.19 [unord.hash]/2

   > [Drafting note: I see no reason to specify whether `H<T>` is destructible. There's no practical use case for which that would need to be covered. libstdc++ makes it so that `H<T>` is destructible.]

   > -2- The template specializations shall meet the requirements of class template `hash` (20.12.14).

   > -?- For any type `T` that is not of integral or enumeration type, or for which neither the library nor the user provides an explicit or partial specialization of the class template hash, the specialization of `hash<T>` has the following properties:
   > - `is_default_constructible_v<hash<T>>` is `false`
   > - `is_copy_constructible_v<hash<T>>` is `false`
   > - `is_move_constructible_v<hash<T>>` is `false`
   > - `is_copy_assignable_v<hash<T>>` is `false`
   > - `is_move_assignable_v<hash<T>>` is `false`
   > - `hash<T>` is not a function object type (22.10 [function.objects])

   > [Note: this means that the specialization of `hash` exists, but any attempts to use it as a `Hash` will be ill-formed. — end note]

### 2544(i). `istreambuf_iterator(basic_streambuf<charT, traits>* s)` effects unclear when `s` is `0`

Section: 24.6.4.3 [istreambuf.iterator.cons] Status: C++17 Submitter: S. B. Tam Opened: 2015-10-05 Last modified: 2017-07-30

Priority: 3

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Discussion:

N4527 24.6.4.3 [istreambuf.iterator.cons] does not mention what the effect of calling `istreambuf_iterator(basic_streambuf<charT, traits>* s)` is when `s` is a null pointer. It should be made clear that this case is well-formed and the result is a end-of-stream iterator.

Daniel:

According to 24.6.4 [istreambuf.iterator] p1:

> […] The default constructor `istreambuf_iterator()` and the constructor `istreambuf_iterator(0)` both construct an end-of-stream iterator object suitable for use as an end-of-range. […]

This indicates that the described constructor creates an end-of-stream iterator, but this wording is part of the introductory wording and I recommend to make 24.6.4.3 [istreambuf.iterator.cons] clearer, because the existing specification is already flawed, e.g. it never specifies when and how the exposition-only-member `sbuf_` is initialized. The proposed wording below attempts to solve these problems as well.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4527.

> 1. Change 24.6.4.3 [istreambuf.iterator.cons] as indicated:

>    [Editorial note: The proposed wording changes also performs some editorial clean-up of the existing mismatches of the declarations in the class template synopsis and the individual member specifications. The below wording intentionally does not say anything about the concrete value of `sbuf_` for end-of-stream iterator values, because that was never specified before; in theory, this could be some magic non-null pointer that can be used in constant expressions. But the wording could be drastically simplified by requiring `sbuf_` to be a null pointer for an end-of-stream iterator value, since I have not yet seen any implementation where this requirement does not hold. — end editorial note]

>    >     constexpr istreambuf_iterator() noexcept;

>    > > -1- Effects: Constructs the end-of-stream iterator.

>    >     istreambuf_iterator(basic_istream<charT,traits>istream_type& s) noexcept;
>    >     istreambuf_iterator(basic_streambuf<charT,traits>* s) noexcept;

>    > > -2- Effects: If `s.rdbuf()` is a null pointer, constructs an end-of-stream iterator; otherwise initializes `sbuf_` with `s.rdbuf()` and constructs an `istreambuf_iterator` that uses the `streambuf_type` object `*sbuf_`Constructs an `istreambuf_iterator<>` that uses the `basic_streambuf<>` object `*(s.rdbuf())`, or `*s`, respectively. Constructs an end-of-stream iterator if `s.rdbuf()` is null.

>    >     istreambuf_iterator(streambuf_type* s) noexcept;

>    > > -?- Effects: If `s` is a null pointer, constructs an end-of-stream iterator; otherwise initializes `sbuf_` with `s` and constructs an `istreambuf_iterator` that uses the `streambuf_type` object `*sbuf_`.

>    >     istreambuf_iterator(const proxy& p) noexcept;

>    > > -3- Effects: Initializes `sbuf_` with `p.sbuf_` and constructs an `istreambuf_iterator` that uses the `streambuf_type` object `*sbuf_`Constructs a `istreambuf_iterator<>` that uses the `basic_streambuf<>` object pointed to by the `proxy` object's constructor argument `p`.

[2015-10-20, Daniel provides alternative wording]

[2016-08-03 Chicago]

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change 24.6.4.3 [istreambuf.iterator.cons] as indicated:

   > [Drafting note: The proposed wording changes also performs some editorial clean-up of the existing mismatches of the declarations in the class template synopsis and the individual member specifications. The below wording is simplified by requiring `sbuf_` to be a null pointer for an end-of-stream iterator value, since I have not yet seen any implementation where this requirement does not hold. Even if there were such an implementation, this would still be conforming, because concrete exposition-only member values are not part of public API. — end drafting note]

   > For each `istreambuf_iterator` constructor in this section, an end-of-stream iterator is constructed if and only if the exposition-only member `sbuf_` is initialized with a null pointer value.

   >     constexpr istreambuf_iterator() noexcept;

   > > -1- Effects: Initializes `sbuf_` with `nullptr` Constructs the end-of-stream iterator.

   >     istreambuf_iterator(basic_istream<charT,traits>istream_type& s) noexcept;
   >     istreambuf_iterator(basic_streambuf<charT,traits>* s) noexcept;

   > > -2- Effects: Initializes `sbuf_` with `s.rdbuf()`Constructs an `istreambuf_iterator<>` that uses the `basic_streambuf<>` object `*(s.rdbuf())`, or `*s`, respectively. Constructs an end-of-stream iterator if `s.rdbuf()` is null.

   >     istreambuf_iterator(streambuf_type* s) noexcept;

   > > -?- Effects: Initializes `sbuf_` with `s`.

   >     istreambuf_iterator(const proxy& p) noexcept;

   > > -3- Effects: Initializes `sbuf_` with `p.sbuf_`Constructs a `istreambuf_iterator<>` that uses the `basic_streambuf<>` object pointed to by the `proxy` object's constructor argument `p`.

### 2545(i). Simplify wording for `bind` without explicitly specified return type

Section: 22.10.15.4 [func.bind.bind] Status: C++17 Submitter: Tomasz Kamiński Opened: 2015-10-05 Last modified: 2017-07-30

Priority: 3

View all other issues in [func.bind.bind].

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Discussion:

The specification of the `bind` overload without return type as of 22.10.15.4 [func.bind.bind] p3, uses the following expression `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of_t<FD cv & (V1, V2, ..., VN)>)` to describe effects of invocation of returned function.

According to the definition from 21.3.8.7 [meta.trans.other] `result_of_t<FD cv & (V1, V2, ..., VN)>` is equivalent to `decltype(INVOKE(declval<FD cv &>(), declval<V1>(), declval<V2>(), ..., declval<VN>()))`. When combined with the definition of `INVOKE` from 22.10.4 [func.require] p2, the expression `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of_t<FD cv & (V1, V2, ...., VN)>)` is equivalent to `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN))` implicitly converted to `decltype(INVOKE(declval<FD cv &>(), declval<V1>(), declval<V2>(), ..., declval<VN>()))` (itself).

It is also worth to notice that specifying the result type (`R`) in `INVOKE(f, args..., R)` does not in any way affect the selected call. As a consequence the use of wording of the form `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of_t<FD cv & (V1, V2, ..., VN)>)` does not and cannot lead to call of different overload than one invoked by `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN))`.

In summary the form `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of_t<FD cv & (V1, V2, ..., VN)>)` is a convoluted way of expressing `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN))`, that only confuses reader.

[2015-10, Kona Saturday afternoon]

STL: I most recently reimplemented std::bind from scratch, and I think this issue is correct and the solution is good.

Move to Tentatively ready.

Proposed resolution:

This wording is relative to N4527.

1. Change 22.10.15.4 [func.bind.bind] p3 as indicated:

   >     template<class F, class... BoundArgs>
   >     unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > […]

   > > -3- Returns: A forwarding call wrapper `g` with a weak result type (20.9.2). The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of_t<FD cv & (V1, V2, ..., VN)>)`, where `cv` represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `FD` or of any of the types `TiD` throws an exception.

   > > […]

### 2548(i). Missing `vfscanf` from `<cstdio>`

Section: 31.13 [c.files] Status: Resolved Submitter: Richard Smith Opened: 2015-10-09 Last modified: 2017-03-12

Priority: 3

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Discussion:

C's `vfscanf` function is not present in C++'s `<cstdio>`, and presumably should be. It looks like this is the only missing member of C's `[v]{f,s,sn}[w]{printf,scanf}` family.

[2016-08 - Chicago]

Resolved by P0175R1

Thurs AM: Moved to Tentatively Resolved

Proposed resolution:

This wording is relative to N4527.

Modify Table 133 as follows:

`ungetc`
`vfprintf`
`vfscanf`
`vprintf`
`vscanf`

### 2549(i). Tuple EXPLICIT constructor templates that take `tuple` parameters end up taking references to temporaries and will create dangling references

Section: 22.4.4.2 [tuple.cnstr] Status: C++17 Submitter: Ville Voutilainen Opened: 2015-10-11 Last modified: 2017-07-30

Priority: 2

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Discussion:

Consider this example:

>     #include <utility>
>     #include <tuple>

>     struct X {
>       int state; // this has to be here to not allow tuple
>                  // to inherit from an empty X.
>       X() {
>       }

>       X(X const&) {
>       }

>       X(X&&) {
>       }

>       ~X() {
>       }
>     };

>     int main()
>     {
>       X v;
>       std::tuple<X> t1{v};
>       std::tuple<std::tuple<X>&&> t2{std::move(t1)}; // #1
>       std::tuple<std::tuple<X>> t3{std::move(t2)}; // #2
>     }

The line marked with `#1` will use the constructor `template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);` and will construct a temporary and bind an rvalue reference to it. The line marked with `#2` will move from a dangling reference.

In order to solve the problem, the constructor templates taking tuples as parameters need additional SFINAE conditions that SFINAE those constructor templates away when `Types...` is constructible or convertible from the incoming tuple type and `sizeof...(Types)` equals one. libstdc++ already has this fix applied.

There is an additional check that needs to be done, in order to avoid infinite meta-recursion during overload resolution, for a case where the element type is itself constructible from the target tuple. An example illustrating that problem is as follows:

>     #include <tuple>

>     struct A
>     {
>       template <typename T>
>       A(T)
>       {
>       }

>       A(const A&) = default;
>       A(A&&) = default;
>       A& operator=(const A&) = default;
>       A& operator=(A&&) = default;
>       ~A() = default;
>     };

>     int main()
>     {
>       auto x = A{7};
>       std::make_tuple(x);
>     }

I provide two proposed resolutions, one that merely has a note encouraging trait-based implementations to avoid infinite meta-recursion, and a second one that avoids it normatively (implementations can still do things differently under the as-if rule, so we are not necessarily overspecifying how to do it.)

[2016-02-17, Ville comments]

It was pointed out at gcc bug 69853 that the fix for LWG 2549 is a breaking change. That is, it breaks code that expects constructors inherited from `tuple` to provide an implicit base-to-derived conversion. I think that's just additional motivation to apply the fix; that conversion is very much undesirable. The example I wrote into the bug report is just one example of a very subtle temporary being created.

Previous resolution from Ville [SUPERSEDED]:

> 1. Alternative 1:

>    1. In 22.4.4.2 [tuple.cnstr]/17, edit as follows:

>       >     template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);

>       > > […]

>       > > -17- Remarks: This constructor shall not participate in overload resolution unless

>       > > - `is_constructible<Ti, const Ui&>::value` is true for all `i`., and

>       > > - `sizeof...(Types) != 1`, or `is_convertible<const tuple<UTypes...>&, Types...>::value` is false and `is_constructible<Types..., const tuple<UTypes...>&>::value` is false.

>       > > [Note: to avoid infinite template recursion in a trait-based implementation for the case where `UTypes...` is a single type that has a constructor template that accepts `tuple<Types...>`, implementations need to additionally check that `remove_cv_t<remove_reference_t<const tuple<UTypes...>&>>` and `tuple<Types...>` are not the same type. — end note]

>       > > The constructor is explicit if and only if `is_convertible<const Ui&, Ti>::value` is false for at least one `i`.

>    2. In 22.4.4.2 [tuple.cnstr]/20, edit as follows:

>       >     template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);

>       > > […]

>       > > -20- Remarks: This constructor shall not participate in overload resolution unless

>       > > - `is_constructible<Ti, Ui&&>::value` is true for all `i`., and

>       > > - `sizeof...(Types) != 1`, or `is_convertible<tuple<UTypes...>&&, Types...>::value` is false and `is_constructible<Types..., tuple<UTypes...>&&>::value` is false.

>       > > [Note: to avoid infinite template recursion in a trait-based implementation for the case where `UTypes...` is a single type that has a constructor template that accepts `tuple<Types...>`, implementations need to additionally check that `remove_cv_t<remove_reference_t<tuple<UTypes...>&&>>` and `tuple<Types...>` are not the same type. — end note]

>       > > The constructor is explicit if and only if `is_convertible<Ui&&, Ti>::value` is false for at least one `i`.

> 2. Alternative 2 (do the sameness-check normatively):

>    1. In 22.4.4.2 [tuple.cnstr]/17, edit as follows:

>       >     template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);

>       > > […]

>       > > -17- Remarks: This constructor shall not participate in overload resolution unless

>       > > - `is_constructible<Ti, const Ui&>::value` is true for all `i`., and

>       > > - `sizeof...(Types) != 1`, or `is_convertible<const tuple<UTypes...>&, Types...>::value` is false and `is_constructible<Types..., const tuple<UTypes...>&>::value` is false and `is_same<tuple<Types...>, remove_cv_t<remove_reference_t<const tuple<UTypes...>&>>>::value` is false.

>       > > The constructor is explicit if and only if `is_convertible<const Ui&, Ti>::value` is false for at least one `i`.

>    2. In 22.4.4.2 [tuple.cnstr]/20, edit as follows:

>       >     template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);

>       > > […]

>       > > -20- Remarks: This constructor shall not participate in overload resolution unless

>       > > - `is_constructible<Ti, Ui&&>::value` is true for all `i`., and

>       > > - `sizeof...(Types) != 1`, or `is_convertible<tuple<UTypes...>&&, Types...>::value` is false and `is_constructible<Types..., tuple<UTypes...>&&>::value` is false and `is_same<tuple<Types...>, remove_cv_t<remove_reference_t<tuple<UTypes...>&&>>>::value` is false.

>       > > The constructor is explicit if and only if `is_convertible<Ui&&, Ti>::value` is false for at least one `i`.

[2016-03, Jacksonville]

STL provides a simplification of Ville's alternative #2 (with no semantic changes), and it's shipping in VS 2015 Update 2.

Proposed resolution:

This wording is relative to N4567.

1. This approach is orthogonal to the Proposed Resolution for LWG 2312(i):

   1. Edit 22.4.4.2 [tuple.cnstr] as indicated:

      >     template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);

      > > […]

      > > -17- Remarks: This constructor shall not participate in overload resolution unless

      > > - `is_constructible<Ti, const Ui&>::value` is `true` for all `i`, and

      > > - `sizeof...(Types) != 1`, or (when `Types...` expands to `T` and `UTypes...` expands to `U`) `!is_convertible_v<const tuple<U>&, T> && !is_constructible_v<T, const tuple<U>&> && !is_same_v<T, U>` is `true`.

      > > The constructor is explicit if and only if `is_convertible<const Ui&, Ti>::value` is `false` for at least one `i`.

      >     template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);

      > > […]

      > > -20- Remarks: This constructor shall not participate in overload resolution unless

      > > - `is_constructible<Ti, Ui&&>::value` is `true` for all `i`, and

      > > - `sizeof...(Types) != 1`, or (when `Types...` expands to `T` and `UTypes...` expands to `U`) `!is_convertible_v<tuple<U>, T> && !is_constructible_v<T, tuple<U>> && !is_same_v<T, U>` is `true`.

      > > The constructor is explicit if and only if `is_convertible<Ui&&, Ti>::value` is `false` for at least one `i`.

2. This approach is presented as a merge with the parts of the Proposed Resolution for LWG 2312(i) with overlapping modifications in the same paragraph, to provide editorial guidance if 2312(i) would be accepted.

   1. Edit 22.4.4.2 [tuple.cnstr] as indicated:

      >     template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);

      > > […]

      > > -17- Remarks: This constructor shall not participate in overload resolution unless

      > > - `sizeof...(Types) == sizeof...(UTypes)`, and

      > > - `is_constructible<Ti, const Ui&>::value` is `true` for all `i`, and

      > > - `sizeof...(Types) != 1`, or (when `Types...` expands to `T` and `UTypes...` expands to `U`) `!is_convertible_v<const tuple<U>&, T> && !is_constructible_v<T, const tuple<U>&> && !is_same_v<T, U>` is `true`.

      > > The constructor is explicit if and only if `is_convertible<const Ui&, Ti>::value` is `false` for at least one `i`.

      >     template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);

      > > […]

      > > -20- Remarks: This constructor shall not participate in overload resolution unless

      > > - `sizeof...(Types) == sizeof...(UTypes)`, and

      > > - `is_constructible<Ti, Ui&&>::value` is `true` for all `i`, and

      > > - `sizeof...(Types) != 1`, or (when `Types...` expands to `T` and `UTypes...` expands to `U`) `!is_convertible_v<tuple<U>, T> && !is_constructible_v<T, tuple<U>> && !is_same_v<T, U>` is `true`.

      > > The constructor is explicit if and only if `is_convertible<Ui&&, Ti>::value` is `false` for at least one `i`.

### 2550(i). Wording of unordered container's `clear()` method complexity

Section: 23.2.8 [unord.req] Status: C++17 Submitter: Yegor Derevenets Opened: 2015-10-11 Last modified: 2017-07-30

Priority: 2

View other active issues in [unord.req].

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Discussion:

I believe, the wording of the complexity specification for the standard unordered containers' `clear()` method should be changed from "Linear" to "Linear in `a.size()`". As of N4527, the change should be done in the Complexity column of row "`a.clear()`..." in "Table 102 — Unordered associative container requirements" in section Unordered associative containers 23.2.8 [unord.req].

From the current formulation it is not very clear, whether the complexity is linear in the number of buckets, in the number of elements, or both. cppreference is also not very specific: it mentions the size of the container without being explicit about what exactly the size is.

The issue is inspired by a performance bug in libstdc++. The issue is related to LWG 1175(i).

[2016-03 Jacksonville]

GR: erase(begin. end) has to touch every element. clear() has the option of working with buckets instead. will be faster in some cases, slower in some. clear() has to be at least linear in size as it has to run destructors.
MC: wording needs to say what it's linear in, either elements or buckets.
HH: my vote is the proposed resolution is correct.
Move to Ready.

Proposed resolution:

This wording is relative to N4527.

1. Change 23.2.8 [unord.req], Table 102 as indicated:

   > Table 102 — Unordered associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `a.clear()` | `void` | Erases all elements in the container. Post: `a.empty()` returns `true` | Linear in `a.size()`.
   > `…`

### 2551(i). [fund.ts.v2] "Exception safety" cleanup in library fundamentals required

Section: 8.2.1.1 [fund.ts.v2::memory.smartptr.shared.const] Status: TS Submitter: Daniel Krügler Opened: 2015-10-24 Last modified: 2017-07-30

Priority: 0

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Discussion:

Addresses: fund.ts.v2

Similar to LWG 2495(i), the current library fundamentals working paper refers to several "Exception safety" elements without providing a definition for this element type.

Proposed resolution:

This wording is relative to N4529.

1. Change 8.2.1.1 [fund.ts.v2::memory.smartptr.shared.const] as indicated:

   >     template<class Y> explicit shared_ptr(Y* p);

   > > […]

   > > -3- Effects: When `T` is not an array type, constructs a `shared_ptr` object that owns the pointer `p`. Otherwise, constructs a `shared_ptr` that owns `p` and a deleter of an unspecified type that calls `delete[] p`. If an exception is thrown, `delete p` is called when `T` is not an array type, `delete[] p` otherwise.

   > > […]

   > > -6- Exception safety: If an exception is thrown, `delete p` is called when `T` is not an array type, `delete[] p` otherwise.

   >     template<class Y, class D> shared_ptr(Y* p, D d);
   >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
   >     template <class D> shared_ptr(nullptr_t p, D d);
   >     template <class D, class A> shared_ptr(nullptr_t p, D d, A a);

   > > […]

   > > -9- Effects: Constructs a `shared_ptr` object that owns the object `p` and the deleter `d`. The second and fourth constructors shall use a copy of `a` to allocate memory for internal use. If an exception is thrown, `d(p)` is called.

   > > […]

   > > -12- Exception safety: If an exception is thrown, `d(p)` is called.

   > […]

   >     template<class Y> explicit shared_ptr(const weak_ptr<Y>& r);

   > > […]

   > > -28- Effects: Constructs a `shared_ptr` object that shares ownership with `r` and stores a copy of the pointer stored in `r`. If an exception is thrown, the constructor has no effect.

   > > […]

   > > -31- Exception safety: If an exception is thrown, the constructor has no effect.

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > > […]

   > > -34- Effects: Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`. If an exception is thrown, the constructor has no effect.

   > > -35- Exception safety: If an exception is thrown, the constructor has no effect.

### 2554(i). Swapping multidimensional arrays is never `noexcept`

Section: 22.2.2 [utility.swap] Status: Resolved Submitter: Orson Peters Opened: 2015-11-01 Last modified: 2020-09-06

Priority: 2

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Discussion:

The `noexcept` specification for the `std::swap` overload for arrays has the effect that all multidimensional arrays — even those of build-in types — would be considered as non-`noexcept` swapping, as described in the following Stackoverflow article.

Consider the following example code:

>     #include <utility>
>     #include <iostream>

>     int main()
>     {
>       int x[2][3];
>       int y[2][3];

>       using std::swap;
>       std::cout << noexcept(swap(x, y)) << "\n";
>     }

Both clang 3.8.0 and gcc 5.2.0 return `0`.

The reason for this unexpected result seems to be a consequence of both core wording rules (6.4.2 [basic.scope.pdecl] says that "The point of declaration for a name is immediately after its complete declarator (Clause 8) and before its initializer (if any)" and the exception specification is part of the declarator) and the fact that the exception-specification of the `std::swap` overload for arrays uses an expression and not a type trait. At the point where the expression is evaluated, only the non-array `std::swap` overload is in scope whose `noexcept` specification evaluates to `false` since arrays are neither move-constructible nor move-assignable.

Daniel:

The here described problem is another example for the currently broken `swap` exception specifications in the Standard library as pointed out by LWG 2456(i). The paper N4511 describes a resolution that would address this problem. If the array `swap` overload would be declared instead as follows,

>     template <class T, size_t N>
>     void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable<T>::value);

the expected outcome is obtained.

Revision 2 (P0185R0) of above mentioned paper will available for the mid February 2016 mailing.

[2016-03-06, Daniel comments]

With the acceptance of revision 3 P0185R1 during the Jacksonville meeting, this issue should be closed as "resolved": The expected program output is now `1`. The current gcc 6.0 trunk has already implemented the relevant parts of P0185R1.

Proposed resolution:

### 2555(i). [fund.ts.v2] No handling for over-aligned types in `optional`

Section: 5.3 [fund.ts.v2::optional.object] Status: TS Submitter: Marshall Clow Opened: 2015-11-03 Last modified: 2018-07-08

Priority: 0

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Discussion:

Addresses: fund.ts.v2

22.5.3 [optional.optional] does not specify whether over-aligned types are supported. In other places where we specify allocation of user-supplied types, we state that "It is implementation-defined whether over-aligned types are supported (3.11)." (Examples: 7.6.2.8 [expr.new]/p1, 20.2.10.2 [allocator.members]/p5, [temporary.buffer]/p1). We should presumably do the same thing here.

Proposed resolution:

This wording is relative to N4562.

1. Edit 22.5.3 [optional.optional]/p1 as follows::

   > […] The contained value shall be allocated in a region of the `optional<T>` storage suitably aligned for the type `T`. It is implementation-defined whether over-aligned types are supported (C++14 §3.11). When an object of type `optional<T>` is contextually converted to `bool`, the conversion returns `true` if the object contains a value; otherwise the conversion returns `false`.

### 2556(i). Wide contract for `future::share()`

Section: 32.10.7 [futures.unique.future] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2015-11-05 Last modified: 2021-06-06

Priority: 3

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Discussion:

`future::share()` is not `noexcept`, it has a narrow contact requiring `valid()` as per the blanket wording in [futures.unique_future] p3. Its effects, however, are `return shared_future<R>(std::move(*this))`, which is `noexcept` as it has a wide contract. If the source `future` isn't valid then the target `shared_future` simply won't be valid either. There appears to be no technical reason preventing `future::share()` from having a wide contract, and thus being `noexcept`.

[2016-08-03 Chicago]

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4567.

1. Change [futures.unique_future] as indicated:

   > -3- The effect of calling any member function other than the destructor, the move-assignment operator, `share`, or `valid` on a `future` object for which `valid() == false` is undefined. [Note: Implementations are encouraged to detect this case and throw an object of type `future_error` with an error condition of `future_errc::no_state`. — end note]

   > >     namespace std {
   > >       template <class R>
   > >       class future {
   > >       public:
   > >         […]
   > >         shared_future<R> share() noexcept;
   > >         […]
   > >       };
   > >     }

   > […]

   >     shared_future<R> share() noexcept;

   > > -12- Returns: `shared_future<R>(std::move(*this))`.

   > > -13- Postcondition: `valid() == false`.

   > […]

### 2557(i). Logical operator traits are broken in the zero-argument case

Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Geoffrey Romer Opened: 2015-11-05 Last modified: 2017-07-30

Priority: 0

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Discussion:

The conjunction trait in 21.3.9 [meta.logical] seems intended to support invocation with zero arguments, e.g. `conjunction<>::value`, which is likely to be a useful feature. However, the specification doesn't actually make sense in the zero-argument case. See 21.3.9 [meta.logical]/p3:

> The BaseCharacteristic of a specialization `conjunction<B1, …, BN>` is the first type `B` in the list `true_type`, `B1`, …, `BN` for which `B::value == false`, or if every `B::value != false` the BaseCharacteristic is `BN`.

If "`B1, ..., BN`" is an empty list, then every `B::value != false`, so the BaseCharacteristic is `BN`, but there is no `BN` in this case.

(If LWG concludes that `conjunction` intentionally requires at least one argument, I would appreciate their confirmation that I can editorially remove the mention of `true_type`, which seems to have no normative impact outside the zero-argument case.)

Similar comments apply to the `disjunction` trait, and to the corresponding traits in the Fundamentals working paper, see LWG 2558(i).

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Revise 21.3.9 [meta.logical] as follows:

   >     template<class... B> struct conjunction : see below { };

   > > […]

   > > -3- The BaseCharacteristic of a specialization `conjunction<B1, ..., BN>` is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value == false`, or if every `Bi::value != false`, the BaseCharacteristic is `BN` the last type in the list. [Note: This means a specialization of `conjunction` does not necessarily have a BaseCharacteristic of either `true_type` or `false_type`. — end note]

   > > […]

   >     template<class... B> struct disjunction : see below { };

   > > […]

   > > -6- The BaseCharacteristic of a specialization `disjunction<B1, ..., BN>` is the first type `Bi` in the list `false_type, B1, ..., BN` for which `Bi::value != false`, or if every `Bi::value == false`, the BaseCharacteristic is `BN` the last type in the list. [Note: This means a specialization of `disjunction` does not necessarily have a BaseCharacteristic of either `true_type` or `false_type`. — end note]

   > > […]

### 2558(i). [fund.ts.v2] Logical operator traits are broken in the zero-argument case

Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Geoffrey Romer Opened: 2015-11-05 Last modified: 2017-07-30

Priority: 0

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Discussion:

Addresses: fund.ts.v2

The conjunction trait in 3.3.3 [fund.ts.v2::meta.logical] seems intended to support invocation with zero arguments, e.g. `conjunction<>::value`, which is likely to be a useful feature. However, the specification doesn't actually make sense in the zero-argument case. See 3.3.3 [fund.ts.v2::meta.logical]/p3:

> The BaseCharacteristic of a specialization `conjunction<B1, …, BN>` is the first type `B` in the list `true_type`, `B1`, …, `BN` for which `B::value == false`, or if every `B::value != false` the BaseCharacteristic is `BN`.

If "`B1, ..., BN`" is an empty list, then every `B::value != false`, so the BaseCharacteristic is `BN`, but there is no `BN` in this case.

(If LWG concludes that `conjunction` intentionally requires at least one argument, I would appreciate their confirmation that I can editorially remove the mention of `true_type`, which seems to have no normative impact outside the zero-argument case.)

Similar comments apply to the `disjunction` trait, and to the corresponding traits in the C++ working paper, see LWG 2557(i).

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4562.

1. Revise 3.3.3 [fund.ts.v2::meta.logical] as follows:

   >     template<class... B> struct conjunction : see below { };

   > > […]

   > > -3- The BaseCharacteristic of a specialization `conjunction<B1, ..., BN>` is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value == false`, or if every `Bi::value != false`, the BaseCharacteristic is `BN` the last type in the list. [Note: This means a specialization of `conjunction` does not necessarily have a BaseCharacteristic of either `true_type` or `false_type`. — end note]

   > > […]

   >     template<class... B> struct disjunction : see below { };

   > > […]

   > > -6- The BaseCharacteristic of a specialization `disjunction<B1, ..., BN>` is the first type `Bi` in the list `false_type, B1, ..., BN` for which `Bi::value != false`, or if every `Bi::value == false`, the BaseCharacteristic is `BN` the last type in the list. [Note: This means a specialization of `disjunction` does not necessarily have a BaseCharacteristic of either `true_type` or `false_type`. — end note]

   > > […]

### 2559(i). Error in LWG 2234's resolution

Section: 19.3 [assertions] Status: C++17 Submitter: Tim Song Opened: 2015-11-07 Last modified: 2017-07-30

Priority: 0

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Discussion:

The resolution of LWG 2234(i) says that `assert(E)` is a constant subexpression if "`NDEBUG` is defined at the point where `assert(E)` appears".

This is incorrect, as noted in one of STL's comments in that issue's discussion, but was apparently overlooked.

The proposed resolution below just borrows STL's phrasing from the discussion.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Change 19.3 [assertions] p2 as indicated:

   > An expression `assert(E)` is a constant subexpression (3.15 [defns.const.subexpr]), if

   > - `NDEBUG` is defined at the point where `assert(E)` appears `assert` is last defined or redefined, or

   > - […]

### 2560(i). `is_constructible` underspecified when applied to a function type

Section: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: Richard Smith Opened: 2015-11-14 Last modified: 2017-07-30

Priority: 0

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Discussion:

What is `is_constructible<void()>::value`? Per 21.3.5.4 [meta.unary.prop] p8:

> The predicate condition for a template specialization `is_constructible<T, Args...>` shall be satisfied if and only if the following variable definition would be well-formed for some invented variable `t`:

> >     T t(declval<Args>()...);

> [Note: These tokens are never interpreted as a function declaration. — end note]

The problem here is that substituting in `T` as a function type doesn't give a variable definition that's not well-formed (by 3.69 [defns.well.formed], well-formed means that it doesn't violate any syntactic or diagnosable semantic rules, and it does not). Instead, it gives a logical absurdity: this wording forces us to imagine a variable of function type, which contradicts the definition of "variable" in 3/6, but does so without violating any diagnosable language rule. So presumably the result must be undefined behavior.

It seems that we need an explicit rule requiring `T` to be an object or reference type.

Daniel:

As one of the authors of N3142 I would like to express that at least according to my mental model the intention for this trait was to be well-defined for `T` being a function type with the result of `false` regardless of what the other type arguments are. It would seem like a very unfortunate and unnecessary complication to keep the result as being undefined. First, this result value is symmetric to the result of `is_destructible<T>::value` (where the word covers function types explicitly). Second, if such a resolution would be applied to the working paper, it wouldn't break existing implementations. I have tested clang 3.8.0, gcc 5.x until gcc 6.0, and Visual Studio 2015, all of these implementations evaluate `is_constructible<void()>::value` to `false`.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Change 21.3.5.4 [meta.unary.prop], Table 49 — "Type property predicates", as indicated:

   > Table 49 — Type property predicates
   > Template | Condition | Preconditions
   > --- | --- | ---
   > `…`
   > `template <class T, class... Args> struct is_constructible;` | For a function type `T`, `is_constructible<T, Args...>::value` is `false`, otherwise see below | `T` and all types in the parameter pack `Args` shall be complete types, (possibly cv-qualified) `void`, or arrays of unknown bound.
   > `…`

### 2561(i). [fund.ts.v2] Incorrect exception specifications for 'swap' in C++ Extensions for Library Fundamentals

Section: 5.3.4 [fund.ts.v2::optional.object.swap], 3.7.8 [fund.ts.v2::propagate_const.modifiers] Status: Resolved Submitter: Daniel Krügler Opened: 2015-11-14 Last modified: 2022-07-28

Priority: 3

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Discussion:

Addresses: fund.ts.v2

As pointed out in N4511, the Library fundamentals are affected by a similar problem as described in LWG 2456(i). First, it is caused by `optional`'s member `swap` (5.3.4 [fund.ts.v2::optional.object.swap]):

>     void swap(optional<T>& rhs) noexcept(see below);

with

> The expression inside `noexcept` is equivalent to:

> >     is_nothrow_move_constructible_v<T> && noexcept(swap(declval<T&>(), declval<T&>()))

Again, the unqualified lookup for `swap` finds the member `swap` instead of the result of a normal argument-depending lookup, making this ill-formed.

A second example of such a problem recently entered the arena with the addition of the `propagate_const` template with another member `swap` (3.7.8 [fund.ts.v2::propagate_const.modifiers]):

>     constexpr void swap(propagate_const& pt) noexcept(see below);

> > -2- The constant-expression in the exception-specification is `noexcept(swap(t_, pt.t_))`.

A working approach is presented in N4511. By adding a new trait to the standard library and referencing this by the library fundamentals (A similar approach had been applied in the file system specification where the `quoted` manipulator from C++14 had been referred to, albeit the file system specification is generally based on the C++11 standard), `optional`'s member `swap` exception specification could be rephrased as follows:

> The expression inside `noexcept` is equivalent to:

> >     is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>noexcept(swap(declval<T&>(), declval<T&>()))

and `propagate_const`'s member `swap` exception specification could be rephrased as follows:

>     constexpr void swap(propagate_const& pt) noexcept(see below);

> > -2- The constant-expression in the exception-specification is `is_nothrow_swappable_v<T>noexcept(swap(t_, pt.t_))`.

[2016-02-20, Ville comments]

Feedback from an implementation:

libstdc++ already applies the proposed resolution for `propagate_const`, but not for `optional`.

[2016-02-20, Daniel comments]

A recent paper update has been provided: P0185R0.

[2016-03, Jacksonville]

Add a link to 2456(i)

[2016-11-08, Issaquah]

Not adopted during NB comment resolution

[2020-03-30; Daniel comments]

This has strong overlap with LWG 3413(i), which describes a sub-set of the problem here. Rebasing of the library fundamentals on C++20 has removed the mentioned problem for `optional`s free `swap`, so there are now no longer any further free `swap` function templates with conditionally `noexcept` specifications except for `propagate_const` (but now handled by LWG 3413(i)).

[2022-07-28 Resolved by P0966R1 and LWG 3413(i). Status changed: New → Resolved.]

Proposed resolution:

### 2562(i). Consistent total ordering of pointers by comparison functors

Section: 22.10.8 [comparisons] Status: C++17 Submitter: Casey Carter Opened: 2015-11-18 Last modified: 2017-07-30

Priority: 3

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Discussion:

N4567 22.10.8 [comparisons]/14 specifies that the comparison functors provide a total ordering for pointer types:

> For templates `greater`, `less`, `greater_equal`, and `less_equal`, the specializations for any pointer type yield a total order, even if the built-in operators `<`, `>`, `<=`, `>=` do not.

It notably does not specify:

- whether the specializations of all of the named templates for a given pointer type yield the same total order

- whether the total order imposed respects the partial order imposed by the built-in operators

- whether the total order imposed is consistent with the partition induced by `==`

All of which are important for sane semantics and provided by common implementations, since the built-in operators provide a total order and the comparison functors yield that same order.

It would be extremely confusing — if not outright insane — for e.g.:

- `less<int*>` and `greater<int*>` to yield different orders

- `less<int*>` to disagree with `<` on the relative order of two pointers for which `<` is defined

- `less<int*>` to order `a` before `b` when `a == b`, i.e., not preserve equality.

Consistent semantics for the various comparison functors and the built-in operators is so intuitive as to be assumed by most programmers.

Related issues: 2450(i), 2547(i).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4567.

> 1. Alter 22.10.8 [comparisons]/14 to read:

>    > For templates `greater`, `less`, `greater_equal`, and `less_equal`, the specializations for any pointer type yield athe same total order, even if the built-in operators `<`, `>`, `<=`, `>=` do not. The total order shall respect the partial order imposed by the built-in operators.

[2016-05-20, Casey Carter comments and suggests revised wording]

The new proposed wording is attempting to address the issue raised in the 2016-02-04 telecon.

The real issue I'm trying to address here is ensure that "weird" implementations provide the same kind of consistency for pointer orderings as "normal" implementations that use a flat address spaces and have totally ordered `<`. If `a < b` is true for `int` pointers `a` and `b`, then `less<int*>(a, b)`, `less_equal<int*>(a, b)`, `less<char*>(a, b)`, `less<void*>(a, b)`, and `greater<int*>(b, a)` should all hold. I think this wording is sufficient to provide that.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4582.

> 1. Alter 22.10.8 [comparisons] to read:

>    > -14- For templates `greater`, `less`, `greater_equal`, and `less_equal`, the specializations for any pointer type yield athe same total order. That total order is consistent with the partial order imposed by, even if the built-in operators `<`, `>`, `≤`, and `>` do not. [Note: When `a < b` is well-defined for pointers `a` and `b` of type `P`, this implies `(a < b) == less<P>(a, b)`, `(a > b) == greater<P>(a, b)`, and so forth. — end note] For template specializations `greater<void>`, `less<void>`, `greater_equal<void>`, and `less_equal<void>`, if the call operator calls a built-in operator comparing pointers, the call operator yields a total order.

[2016-08-04 Chicago LWG]

LWG discusses and concludes that we are trying to accomplish the following:

1. >     T* a = /* ... */;
   >     T* b = /* ... */;

   if `a < b` is valid, `a < b == less<T*>(a, b)`, and analogously for `>`, `<=`, `>=`.

2. >     less<void>(a, b) == less<T*>(a, b);
   >     less<T*>(a, b) == greater<T*>(b, a);

   etc.

3. `less<T*>` produces a strict total ordering with which the other three function objects are consistent

4. `less<void>` when applied to pointers produces a strict total ordering with which the other three are consistent

5. `less<void>` when applied to pointers of the same type produces the same strict total ordering as `less<T*>`, and analogously for the other three

6. we are not addressing `less<void>` (and the other three) when applied to pointers of differing types

Walter and Nevin revise Proposed Wording accordingly.

[2016-08 - Chicago]

Thurs PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change 22.10.8 [comparisons] p14 as indicated:

   > -14- For templates `greater`, `less`, `greater_equal`, and `less_equal` `less`, `greater`, `less_equal`, and `greater_equal`, the specializations for any pointer type yield a strict total order that is consistent among those specializations and is also consistent with the partial order imposed by , even if the built-in operators `<`, `>`, `<=`, `>=` do not. [Note: When `a < b` is well-defined for pointers `a` and `b` of type `P`, this implies `(a < b) == less<P>(a, b)`, `(a > b) == greater<P>(a, b)`, and so forth. — end note] For template specializations `greater<void>`, `less<void>`, `greater_equal<void>`, and `less_equal<void>``less<void>`, `greater<void>`, `less_equal<void>`, and `greater_equal<void>`, if the call operator calls a built-in operator comparing pointers, the call operator yields a strict total order that is consistent among those specializations and is also consistent with the partial order imposed by those built-in operators.

### 2564(i). [fund.ts.v2] `std::experimental::function` constructors taking allocator arguments may throw exceptions

Section: 4.2 [fund.ts.v2::func.wrap.func] Status: Resolved Submitter: Tim Song Opened: 2015-12-05 Last modified: 2022-11-22

Priority: 3

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Discussion:

Addresses: fund.ts.v2

[This is essentially LWG 2370(i), but deals with the fundamentals TS version rather than the one in the standard]

In 4.2 [fund.ts.v2::func.wrap.func] of library fundamentals TS, the constructors

>     template<class A> function(allocator_arg_t, const A&) noexcept;
>     template<class A> function(allocator_arg_t, const A&, nullptr_t) noexcept;

must type-erase and store the provided allocator, since the `operator=` specification requires using the "allocator specified in the construction of" the `std::experimental::function` object. This may require a dynamic allocation and so cannot be noexcept. Similarly, the following constructors

>     template<class A> function(allocator_arg_t, const A&, const function&);
>     template<class A> function(allocator_arg_t, const A&, function&&);
>     template<class F, class A> function(allocator_arg_t, const A&, F);

cannot satisfy the C++14 requirement that they "shall not throw exceptions if [the `function` object to be stored] is a callable object passed via `reference_wrapper` or a function pointer" if they need to type-erase and store the allocator.

[2016-11-08, Issaquah]

Not adopted during NB comment resolution

[2022-10-12 LWG telecon]

Set status to "Open". This would be resolved by P0987R2.

[2022-11-22 Resolved by P0897R2 accepted in Kona. Status changed: Open → Resolved.]

Proposed resolution:

This wording is relative to N4562.

1. Edit 4.2 [fund.ts.v2::func.wrap.func], class template `function` synopsis, as follows:

   >     namespace std {
   >       namespace experimental {
   >       inline namespace fundamentals_v2 {

   >         […]

   >         template<class R, class... ArgTypes>
   >         class function<R(ArgTypes...)> {
   >         public:
   >           […]
   >           template<class A> function(allocator_arg_t, const A&) noexcept;
   >           template<class A> function(allocator_arg_t, const A&,
   >             nullptr_t) noexcept;
   >           […]
   >         };

   >         […]

   >       } // namespace fundamentals_v2
   >       } // namespace experimental

   >       […]

   >     } // namespace std

2. Insert the following paragraphs after 4.2.1 [fund.ts.v2::func.wrap.func.con]/1:

   > [Drafting note: This just reproduces the wording from C++14 with the "shall not throw exceptions for `reference_wrapper`/function pointer" provision deleted. — end drafting note]

   > -1- When a `function` constructor that takes a first argument of type `allocator_arg_t` is invoked, the second argument is treated as a type-erased allocator (8.3). If the constructor moves or makes a copy of a function object (C++14 §20.9), including an instance of the `experimental::function` class template, then that move or copy is performed by using-allocator construction with allocator `get_memory_resource()`.

   > >     template <class A> function(allocator_arg_t, const A& a);
   > >     template <class A> function(allocator_arg_t, const A& a, nullptr_t);

   > > > -?- Postconditions: `!*this`.

   > >     template <class A> function(allocator_arg_t, const A& a, const function& f);

   > > > -?- Postconditions: `!*this if !f`; otherwise, `*this` targets a copy of `f.target()`.

   > > > -?- Throws: May throw `bad_alloc` or any exception thrown by the copy constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

   > >     template <class A> function(allocator_arg_t, const A& a, function&& f);

   > > > -?- Effects: If `!f`, `*this` has no target; otherwise, move-constructs the target of `f` into the target of `*this`, leaving `f` in a valid state with an unspecified value.

   > >     template <class F, class A> function(allocator_arg_t, const A& a, F f);

   > > > -?- Requires: `F` shall be `CopyConstructible`.

   > > > -?- Remarks: This constructor shall not participate in overload resolution unless `f` is Callable (C++14 §20.9.11.2) for argument types `ArgTypes...` and return type `R`.

   > > > -?- Postconditions: `!*this` if any of the following hold:

   > > > - `f` is a null function pointer value.

   > > > - `f` is a null member pointer value.

   > > > - `F` is an instance of the `function` class template, and `!f`.

   > > > -?- Otherwise, `*this` targets a copy of `f` initialized with `std::move(f)`. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

   > > > -?- Throws: May throw `bad_alloc` or any exception thrown by `F`'s copy or move constructor.

   > -2- In the following descriptions, let `ALLOCATOR_OF(f)` be the allocator specified in the construction of `function f`, or `allocator<char>()` if no allocator was specified.

   > […]

### 2565(i). `std::function`'s move constructor should guarantee nothrow for `reference_wrapper`s and function pointers

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++17 Submitter: Tim Song Opened: 2015-12-05 Last modified: 2017-07-30

Priority: 0

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Discussion:

22.10.17.3.2 [func.wrap.func.con]/5 guarantees that copying a `std::function` whose "target is a callable object passed via `reference_wrapper` or a function pointer" does not throw exceptions, but the standard doesn't provide this guarantee for the move constructor, which makes scant sense.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

> [Drafting note: The inserted paragraph is a copy of 22.10.17.3.2 [func.wrap.func.con]/5, only changing "copy constructor" to "copy or move constructor". It does not attempt to fix the issue identified in LWG 2370(i), whose P/R will likely need updating if this wording is adopted.]

1. Insert after 22.10.17.3.2 [func.wrap.func.con]/6:

   >     function(function&& f);
   >     template <class A> function(allocator_arg_t, const A& a, function&& f);

   > > -6- Effects: If `!f`, `*this` has no target; otherwise, move-constructs the target of `f` into the target of `*this`, leaving `f` in a valid state with an unspecified value.

   > > -?- Throws: Shall not throw exceptions if `f`'s target is a callable object passed via `reference_wrapper` or a function pointer. Otherwise, may throw `bad_alloc` or any exception thrown by the copy or move constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

### 2566(i). Requirements on the first template parameter of container adaptors

Section: 23.6 [container.adaptors] Status: C++17 Submitter: Tim Song Opened: 2015-12-08 Last modified: 2017-07-30

Priority: 0

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Discussion:

As noted in this StackOverflow question, 23.6 [container.adaptors] doesn't seem to place any requirement on the first template parameter (`T`) of `stack`, `queue`, and `priority_queue`: the only use of `T` is in the default template argument (which need not be used) for the second template parameter (`Container`), while all of the operations of the adaptors are defined using `Container`'s member typedefs.

This permits confusing and arguably nonsensical types like `queue<double, deque<std::string>>` or `priority_queue<std::nothrow_t, vector<int>>`, which presumably wasn't intended.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Edit 23.6.1 [container.adaptors.general]/2 as indicated:

   > -2- The container adaptors each take a `Container` template parameter, and each constructor takes a `Container` reference argument. This container is copied into the `Container` member of each adaptor. If the container takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor. Otherwise, normal copy or move construction is used for the container argument. The first template parameter `T` of the container adaptors shall denote the same type as `Container::value_type`.

### 2567(i). Specification of logical operator traits uses `BaseCharacteristic`, which is defined only for `UnaryTypeTraits` and `BinaryTypeTraits`

Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Tim Song Opened: 2015-12-10 Last modified: 2017-07-30

Priority: 2

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Discussion:

The specification of `conjunction` and `disjunction` uses the term `BaseCharacteristic`, which is problematic in several ways:

- That term is defined in 21.3.2 [meta.rqmts], but only for `UnaryTypeTraits` and `BinaryTypeTraits`. `conjunction` and `disjunction` seem to be neither.

- 21.3.2 [meta.rqmts] also requires the `BaseCharacteristic` for both `UnaryTypeTraits` and `BinaryTypeTraits` to be a specialization of `integral_constant`, which is inconsistent with the current design of `conjunction` and `disjunction`.

- The requirement in 21.3.2 [meta.rqmts] that "member names of the `BaseCharacteristic` shall not be hidden and shall be unambiguously available" seems impossible to meet in every case, since the arbitrary base class from which a specialization of `conjunction` or `disjunction` derives may contain members called `conjunction` or `disjunction` that will necessarily be hidden.

[2016-08 Chicago]

Ville provided wording for both 2567(i) and 2568(i)

Previous resolution [SUPERSEDED]:

> In [meta.logical]/3, edit as follows:

> The BaseCharacteristic of a specialization `conjunction<B1, ..., BN>` has a public and unambiguous base that is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value == false`, or if every `Bi::value != false`, the aforementioned baseBaseCharacteristic is the last type in the list. [ Note: This means a specialization of conjunction does not necessarily have a BaseCharacteristic of derive from either true_type or false_type. — end note ]

> In [meta.logical]/6, edit as follows:

> The BaseCharacteristic of a specialization `disjunction<B1, ..., BN>` has a public and unambiguous base that is the first type `Bi` in the list `false_type, B1, ..., BN` for which `Bi::value != false`, or if every `Bi::value == false`, the aforementioned baseBaseCharacteristic is the last type in the list. [ Note: This means a specialization of disjunction does not necessarily have a BaseCharacteristic of derive from either true_type or false_type. — end note ]

Previous resolution [SUPERSEDED]:

> In [meta.logical]/3, edit as follows:

> The BaseCharacteristic of a specialization `conjunction<B1, ..., BN>` has a public and unambiguous base that is either
> * the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value == false`, or
> * if there is no such `Bi`, the last type in the list.
> is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value == false`, or if every `Bi::value != false`, the BaseCharacteristic is the last type in the list.
> [ Note: This means a specialization of `conjunction` does not necessarily have a BaseCharacteristic of derive from either true_type or false_type. — end note ]

> In [meta.logical]/6, edit as follows:

> The BaseCharacteristic of a specialization `disjunction<B1, ..., BN>` has a public and unambiguous base that is either
> * the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value != false`, or
> * if there is no such `Bi`, the last type in the list.
> is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value != false`, or if every `Bi::value == false`, the BaseCharacteristic is the last type in the list.
> [ Note: This means a specialization of `disjunction` does not necessarily have a BaseCharacteristic of derive from either true_type or false_type. — end note ]

Merged the resolution of 2587(i) with this issue. This proposed resolution resolves both, and includes fixes from Daniel for negation. Last review of this with LWG turned up a true_type typo in the definition of disjunction, and some editorial changes.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. In 21.3.9 [meta.logical] p3, edit as follows:

>        template<class... B> struct conjunction : see below { };

>    > -3- The BaseCharacteristic of a specialization `conjunction<B1, ..., BN>` has a public and unambiguous base that is either

>    > - the first type `Bi` in the list `true_type, B1, ..., BN` for which `bool(Bi::value)` is `false`, or
>    > - if there is no such `Bi`, the last type in the list.
>    > is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value == false`, or if every `Bi::value != false`, the BaseCharacteristic is the last type in the list.

>    > -?- The member names of the base class, other than `conjunction` and `operator=`, shall not be hidden and shall be unambiguously available in `conjunction`. [Note: This means a specialization of `conjunction` does not necessarily have a BaseCharacteristic of inherit from either `true_type` or `false_type`. —end note]

> 2. In 21.3.9 [meta.logical] p6, edit as follows:

>        template<class... B> struct disjunction : see below { };

>    > -6- The BaseCharacteristic of a specialization `disjunction<B1, ..., BN>` has a public and unambiguous base that is either

>    > - the first type `Bi` in the list `true_type, B1, ..., BN` for which `bool(Bi::value)` is `true`, or,
>    > - if there is no such `Bi`, the last type in the list.
>    > is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value != false`, or if every `Bi::value == false`, the BaseCharacteristic is the last type in the list.

>    > -?- The member names of the base class, other than `disjunction` and `operator=`, shall not be hidden and shall be unambiguously available in `disjunction`. [Note: This means a specialization of `disjunction` does not necessarily have a BaseCharacteristic of inherit from either `true_type` or `false_type`. —end note]

> 3. In 21.3.9 [meta.logical] p8, edit as follows

>        template<class B> struct negation : bool_constant<!bool(B::value)> { };

>    > -8- The class template negation forms the logical negation of its template type argument. The type `negation<B>` is a UnaryTypeTrait with a BaseCharacteristic of `bool_constant<!bool(B::value)>`.

[2016-08-03 Chicago]

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. In 21.3.9 [meta.logical] p3, edit as follows:

       template<class... B> struct conjunction : see below { };

   > […]

   > -3- The BaseCharacteristic of a specialization `conjunction<B1, ..., BN>` has a public and unambiguous base that is either
   > 1. — the first type `Bi` in the list `true_type, B1, ..., BN` for which `bool(Bi::value)` is `false`, or
   > 2. — if there is no such `Bi`, the last type in the list.
   > is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value == false`, or if every `Bi::value != false`, the BaseCharacteristic is the last type in the list.[Note: This means a specialization of `conjunction` does not necessarily have a BaseCharacteristic of inherit from either `true_type` or `false_type`. —end note]

   > -?- The member names of the base class, other than `conjunction` and `operator=`, shall not be hidden and shall be unambiguously available in `conjunction`.

2. In 21.3.9 [meta.logical] p6, edit as follows:

       template<class... B> struct disjunction : see below { };

   > […]

   > -6- The BaseCharacteristic of a specialization `disjunction<B1, ..., BN>` has a public and unambiguous base that is either
   > 1. — the first type `Bi` in the list `false_type, B1, ..., BN` for which `bool(Bi::value)` is `true`, or,
   > 2. — if there is no such `Bi`, the last type in the list.
   > is the first type `Bi` in the list `false_type, B1, ..., BN` for which `Bi::value != false`, or if every `Bi::value == false`, the BaseCharacteristic is the last type in the list.[Note: This means a specialization of `disjunction` does not necessarily have a BaseCharacteristic of inherit from either `true_type` or `false_type`. —end note]

   > -?- The member names of the base class, other than `disjunction` and `operator=`, shall not be hidden and shall be unambiguously available in `disjunction`.

3. In 21.3.9 [meta.logical] p8, edit as follows

       template<class B> struct negation : see belowbool_constant<!B::value> { };

   > -8- The class template negation forms the logical negation of its template type argument. The type `negation<B>` is a UnaryTypeTrait with a BaseCharacteristic of `bool_constant<!bool(B::value)>`.

### 2568(i). [fund.ts.v2] Specification of logical operator traits uses `BaseCharacteristic`, which is defined only for `UnaryTypeTraits` and `BinaryTypeTraits`

Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2015-12-10 Last modified: 2020-09-06

Priority: 2

View all other issues in [fund.ts.v2::meta.logical].

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Discussion:

Addresses: fund.ts.v2

The specification of `conjunction` and `disjunction` uses the term `BaseCharacteristic`, which is problematic in several ways:

- That term is defined in 21.3.2 [meta.rqmts], but only for `UnaryTypeTraits` and `BinaryTypeTraits`. `conjunction` and `disjunction` seem to be neither.

- 21.3.2 [meta.rqmts] also requires the `BaseCharacteristic` for both `UnaryTypeTraits` and `BinaryTypeTraits` to be a specialization of `integral_constant`, which is inconsistent with the current design of `conjunction` and `disjunction`.

- The requirement in 21.3.2 [meta.rqmts] that "member names of the `BaseCharacteristic` shall not be hidden and shall be unambiguously available" seems impossible to meet in every case, since the arbitrary base class from which a specialization of `conjunction` or `disjunction` derives may contain members called `conjunction` or `disjunction` that will necessarily be hidden.

[2016-08 Chicago]

Ville provided wording for both 2567(i) and 2568(i).

[2016-08-07 Daniel provides wording borrowed from 2567(i)]

[2016-11-08, Issaquah]

Adopted during NB comment resolution

Proposed resolution:

This wording is relative to N4600.

1. In 3.3.3 [fund.ts.v2::meta.logical] p3, edit as follows:

   >     template<class... B> struct conjunction : see below { };

   > > -2- The class template `conjunction` forms the logical conjunction of its template type arguments. Every template type argument shall be usable as a base class and shall have a static data member `value` which is convertible to `bool`, is not hidden, and is unambiguously available in the type.

   > > -3- The BaseCharacteristic of a specialization `conjunction<B1, …, BN>` has a public and unambiguous base that is either
   > > 1. — the first type `Bi` in the list `true_type, B1, ..., BN` for which `bool(Bi::value)` is `false`, or
   > > 2. — if there is no such `Bi`, the last type in the list.

   > > is the first type `B` in the list `true_type, B1, …, BN` for which `B::value == false`, or if `every B::value != false` the BaseCharacteristic is the last type in the list. [Note: This means a specialization of conjunction does not necessarily have a BaseCharacteristic ofinherit from either `true_type` or `false_type`. — end note]

   > > -?- The member names of the base class, other than `conjunction` and `operator=`, shall not be hidden and shall be unambiguously available in `conjunction`.

2. In 3.3.3 [fund.ts.v2::meta.logical] p6, edit as follows:

   >     template<class... B> struct disjunction : see below { };

   > > -5- The class template `disjunction` forms the logical disjunction of its template type arguments. Every template type argument shall be usable as a base class and shall have a static data member `value` which is convertible to `bool`, is not hidden, and is unambiguously available in the type.

   > > -6- The BaseCharacteristic of a specialization `disjunction<B1, …, BN>` has a public and unambiguous base that is either
   > > 1. — the first type `Bi` in the list `false_type, B1, ..., BN` for which `bool(Bi::value)` is `true`, or,
   > > 2. — if there is no such `Bi`, the last type in the list.

   > > is the first type `B` in the list `false_type, B1, …, BN` for which `B::value != false`, or if `every B::value == false` the BaseCharacteristic is the last type in the list. [Note: This means a specialization of disjunction does not necessarily have a BaseCharacteristic ofinherit from either `true_type` or `false_type`. — end note]

   > > -?- The member names of the base class, other than `disjunction` and `operator=`, shall not be hidden and shall be unambiguously available in `disjunction`.

3. In 3.3.3 [fund.ts.v2::meta.logical] p8, edit as follows:

   >     template<class B> struct negation : integral_constant<bool, !B::value>see below { };

   > > -8- The class template `negation` forms the logical negation of its template type argument. The type `negation<B>` is a UnaryTypeTrait with a BaseCharacteristic of `integral_constant<bool, !bool(B::value)>`.

### 2569(i). `conjunction` and `disjunction` requirements are too strict

Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Tim Song Opened: 2015-12-11 Last modified: 2017-12-05

Priority: 2

View all other issues in [meta.logical].

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Discussion:

21.3.9 [meta.logical]/2 and /5 impose the following requirement on the arguments of `conjunction` and `disjunction`:

> Every template type argument shall be usable as a base class and shall have a static data member value which is convertible to `bool`, is not hidden, and is unambiguously available in the type.

Since the requirement is unconditional, it applies even to type arguments whose instantiation is not required due to short circuiting. This seems contrary to the design intent, expressed in P0013R1, that it is valid to write `conjunction_v<is_class<T>, is_foo<T>>` even if instantiating `is_foo<T>::value` is ill-formed for non-class types.

[2016-08 Chicago]

Ville provided wording for both 2569(i) and 2570(i).

Tuesday AM: Move to Tentatively Ready

[2016-11-15, Reopen upon request of Dawn Perchik ]

The proposed wording requires an update, because the referenced issue LWG 2568(i) is still open.

[Dec 2017 - The resolution for this issue shipped in the C++17 standard; setting status to 'C++17']

Proposed resolution:

[We recommend applying the proposed resolution for LWG issues 2567 and 2568 before this proposed resolution, lest the poor editor gets confused.]

In [meta.logical],

- insert a new paragraph before paragraph 2:

The class template `conjunction` forms the logical conjunction of its template type arguments.

- move paragraph 4 before paragraph 2, and edit paragraph 2 as follows:

The class template `conjunction` forms the logical conjunction of its template type arguments. Every template type argument for which `Bi::value` is instantiated shall be usable as a base class and shall have a member value which is convertible to `bool`, is not hidden, and is unambiguously available in the type.

- insert a new paragraph before paragraph 5:

The class template `disjunction` forms the logical disjunction of its template type arguments.

- move paragraph 7 before paragraph 5, and edit paragraph 5 as follows:

The class template `disjunction` forms the logical disjunction of its template type arguments. Every template type argument for which `Bi::value` is instantiated shall be usable as a base class and shall have a member value which is convertible to `bool`, is not hidden, and is unambiguously available in the type.

### 2570(i). [fund.ts.v2] `conjunction` and `disjunction` requirements are too strict

Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2015-12-11 Last modified: 2017-07-30

Priority: 2

View all other issues in [fund.ts.v2::meta.logical].

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Discussion:

Addresses: fund.ts.v2

3.3.3 [fund.ts.v2::meta.logical]/2 and /5 impose the following requirement on the arguments of `conjunction` and `disjunction`:

> Every template type argument shall be usable as a base class and shall have a static data member value which is convertible to `bool`, is not hidden, and is unambiguously available in the type.

Since the requirement is unconditional, it applies even to type arguments whose instantiation is not required due to short circuiting. This seems contrary to the design intent, expressed in P0013R1, that it is valid to write `conjunction_v<is_class<T>, is_foo<T>>` even if instantiating `is_foo<T>::value` is ill-formed for non-class types.

[2016-06 Oulu]

Alisdair has a paper in progress addressing this

[2016-08 Chicago]

Ville provided wording for both 2569(i) and 2570(i).

Tuesday AM: Move to Tentatively Ready

Proposed resolution:

### 2571(i). §[map.modifiers]/2 imposes nonsensical requirement on `insert(InputIterator, InputIterator)`

Section: 23.4.3.4 [map.modifiers] Status: C++17 Submitter: Tim Song Opened: 2015-12-12 Last modified: 2017-07-30

Priority: 0

View all other issues in [map.modifiers].

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Discussion:

The initial paragraphs of 23.4.3.4 [map.modifiers] currently read:

>     template <class P> pair<iterator, bool> insert(P&& x);
>     template <class P> iterator insert(const_iterator position, P&& x);
>     template <class InputIterator>
>     void insert(InputIterator first, InputIterator last);

> > -1- Effects: The first form is equivalent to `return emplace(std::forward<P>(x))`. The second form is equivalent to `return emplace_hint(position, std::forward<P>(x))`.

> > -2- Remarks: These signatures shall not participate in overload resolution unless `std::is_constructible<value_type, P&&>::value` is `true`.

Clearly, p2's requirement makes no sense for `insert(InputIterator, InputIterator)` - it doesn't even have a template parameter called `P`.

This paragraph used to have text saying "The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-`const` rvalue `pair<key_type,mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`", but that was removed by LWG 2005(i), whose PR was written as if that overload didn't exist in the text.

It looks like the text addressing this overload is redundant to the requirements on `a.insert(i, j)` in Table 102 that `value_type` be `EmplaceConstructible` from `*i`. If so, then the signature should just be deleted from this section.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Edit 23.4.3.4 [map.modifiers] as indicated:

   >     template <class P> pair<iterator, bool> insert(P&& x);
   >     template <class P> iterator insert(const_iterator position, P&& x);
   >     template <class InputIterator>
   >     void insert(InputIterator first, InputIterator last);

   > > -1- Effects: The first form is equivalent to `return emplace(std::forward<P>(x))`. The second form is equivalent to `return emplace_hint(position, std::forward<P>(x))`.

   > > -2- Remarks: These signatures shall not participate in overload resolution unless `std::is_constructible<value_type, P&&>::value` is `true`.

### 2572(i). The remarks for `shared_ptr::operator*` should apply to cv-qualified `void` as well

Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: C++17 Submitter: Tim Song Opened: 2015-12-13 Last modified: 2017-07-30

Priority: 0

View all other issues in [util.smartptr.shared.obs].

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Discussion:

20.3.2.2.6 [util.smartptr.shared.obs]/4 says for `shared_ptr::operator*`

> Remarks: When `T` is `void`, it is unspecified whether this member function is declared. If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well formed.

This remark should also apply when `T` is cv-qualified `void` (compare LWG 2500(i)).

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Edit 20.3.2.2.6 [util.smartptr.shared.obs]/4 as indicated:

   >     T& operator*() const noexcept;

   > > […]

   > > -4- Remarks: When `T` is (possibly cv-qualified) `void`, it is unspecified whether this member function is declared. If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well formed.

### 2573(i). [fund.ts.v2] `std::hash<std::experimental::shared_ptr<T>>` does not work for arrays

Section: 8.2.1 [fund.ts.v2::memory.smartptr.shared] Status: TS Submitter: Tim Song Opened: 2015-12-13 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

The library fundamentals TS does not provide a separate specification for `std::hash<std::experimental::shared_ptr<T>>`, deferring to the C++14 specification in §20.8.2.7/3:

>     template <class T> struct hash<shared_ptr<T> >;

> > -3- The template specialization shall meet the requirements of class template `hash` (20.9.13). For an object `p` of type `shared_ptr<T>`, `hash<shared_ptr<T> >()(p)` shall evaluate to the same value as `hash<T*>()(p.get())`.

That specification doesn't work if `T` is an array type (`U[N]` or `U[]`), as in this case `get()` returns `U*`, which cannot be hashed by `std::hash<T*>`.

Proposed resolution:

This wording is relative to N4562.

1. Insert a new subclause after 8.2.1.3 [fund.ts.v2::memory.smartptr.shared.cast]:

   > [Note for the editor: The synopses in [header.memory.synop] and [memory.smartptr.shared] should be updated to refer to the new subclause rather than C++14 §20.8.2.7]

   > ?.?.?.? `shared_ptr` hash support [memory.smartptr.shared.hash]

   >     template <class T> struct hash<experimental::shared_ptr<T>>;

   > > -1- The template specialization shall meet the requirements of class template `hash` (C++14 §20.9.12). For an object `p` of type `experimental::shared_ptr<T>`, `hash<experimental::shared_ptr<T>>()(p)` shall evaluate to the same value as `hash<typename experimental::shared_ptr<T>::element_type*>()(p.get())`.

### 2574(i). [fund.ts.v2] `std::experimental::function::operator=(F&&)` should be constrained

Section: 4.2.1 [fund.ts.v2::func.wrap.func.con] Status: TS Submitter: Tim Song Opened: 2015-12-05 Last modified: 2017-07-30

Priority: 0

View all other issues in [fund.ts.v2::func.wrap.func.con].

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Discussion:

Addresses: fund.ts.v2

LWG 2132(i) constrained `std::function`'s constructor and assignment operator from callable objects for C++14. The constructors of `std::experimental::function` isn't separately specified in the fundamentals TS and so inherited the constraints from C++14, but the assignment operator is separately specified and presumably needs to be constrained.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4562.

1. Insert a paragraph after 4.2.1 [fund.ts.v2::func.wrap.func.con]/15 as indicated:

   >     template<class F> function& operator=(F&& f);

   > > -14- Effects: `function(allocator_arg, ALLOCATOR_OF(*this), std::forward<F>(f)).swap(*this);`

   > > -15- Returns: `*this`.

   > > -?- Remarks: This assignment operator shall not participate in overload resolution unless `declval<decay_t<F>&>()` is Callable (C++14 §20.9.11.2) for argument types `ArgTypes...` and return type `R`.

### 2575(i). [fund.ts.v2] `experimental::function::assign` should be removed

Section: 4.2 [fund.ts.v2::func.wrap.func] Status: TS Submitter: Tim Song Opened: 2015-12-20 Last modified: 2017-07-30

Priority: 0

View all other issues in [fund.ts.v2::func.wrap.func].

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Discussion:

Addresses: fund.ts.v2

Following the lead of LWG 2385(i), the `assign(F&&, const A&)` member function template in `std::experimental::function` makes no sense (it causes undefined behavior unless the allocator passed compares equal to the one already used by `*this`) and should be removed.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4562.

1. Edit 4.2 [fund.ts.v2::func.wrap.func], class template `function` synopsis, as indicated:

   >     namespace std {
   >       namespace experimental {
   >       inline namespace fundamentals_v2 {
   >         […]
   >         template<class R, class... ArgTypes>
   >         class function<R(ArgTypes...)> {
   >         public:
   >           […]
   >           void swap(function&);
   >           template<class F, class A> void assign(F&&, const A&);
   >           […]
   >         };
   >         […]
   >       } // namespace fundamentals_v2
   >       } // namespace experimental
   >       […]
   >     } // namespace std

### 2576(i). `istream_iterator` and `ostream_iterator` should use `std::addressof`

Section: 24.6 [stream.iterators] Status: C++17 Submitter: Tim Song Opened: 2016-01-01 Last modified: 2017-07-30

Priority: 0

View all other issues in [stream.iterators].

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Discussion:

To defend against overloaded unary `&`. This includes the constructors of both iterators, and `istream_iterator::operator->`.

Note that `{i,o}stream_type` are specializations of `basic_{i,o}stream`, but the constructors might still pick up an overloaded `&` via the `traits` template parameter. This change also provides consistency with `std::experimental::ostream_joiner` (which uses `std::addressof`).

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Edit 24.6.2.2 [istream.iterator.cons]/3+4 as indicated:

   >     istream_iterator(istream_type& s);

   > > -3- Effects: Initializes `in_stream` with `&saddressof(s)`. `value` may be initialized during construction or the first time it is referenced.

   > > -4- Postcondition: `in_stream == &saddressof(s)`.

2. Edit 24.6.2.3 [istream.iterator.ops]/2 as indicated:

   >     const T* operator->() const;

   > > -2- Returns: `&addressof(operator*())`.

3. Edit 24.6.3.2 [ostream.iterator.cons.des]/1+2 as indicated:

   >     ostream_iterator(ostream_type& s);

   > > -1- Effects: Initializes `out_stream` with `&saddressof(s)` and `delim` with null.

   >     ostream_iterator(ostream_type& s, const charT* delimiter);

   > > -2- Effects: Initializes `out_stream` with `&saddressof(s)` and `delim` with `delimiter`.

### 2577(i). `{shared,unique}_lock` should use `std::addressof`

Section: 32.6.5.4.2 [thread.lock.unique.cons], 32.6.5.5.2 [thread.lock.shared.cons] Status: C++17 Submitter: Tim Song Opened: 2016-01-01 Last modified: 2017-07-30

Priority: 0

View all other issues in [thread.lock.unique.cons].

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Discussion:

So that they work with user-defined types that have overloaded unary `&`.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Edit 32.6.5.4.2 [thread.lock.unique.cons] as indicated:

   >     explicit unique_lock(mutex_type& m);

   > > […]

   > > -5- Postconditions: `pm == &maddressof(m)` and `owns == true`.

   >     unique_lock(mutex_type& m, defer_lock_t) noexcept;

   > > […]

   > > -7- Postconditions: `pm == &maddressof(m)` and `owns == false`.

   >     unique_lock(mutex_type& m, try_to_lock_t);

   > > […]

   > > -10- Postconditions: `pm == &maddressof(m)` and `owns == res`, where `res` is the value returned by the call to `m.try_lock()`.

   >     unique_lock(mutex_type& m, adopt_lock_t);

   > > […]

   > > -13- Postconditions: `pm == &maddressof(m)` and `owns == true`.

   > > -14- Throws: Nothing.

   >     template <class Clock, class Duration>
   >       unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);

   > > […]

   > > -17- Postconditions: `pm == &maddressof(m)` and `owns == res`, where `res` is the value returned by the call to `m.try_lock_until(abs_time)`.

   >     template <class Rep, class Period>
   >       unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);

   > > […]

   > > -20- Postconditions: `pm == &maddressof(m)` and `owns == res`, where `res` is the value returned by the call to `m.try_lock_for(rel_time)`.

2. Edit 32.6.5.5.2 [thread.lock.shared.cons] as indicated:

   >     explicit shared_lock(mutex_type& m);

   > > […]

   > > -5- Postconditions: `pm == &maddressof(m)` and `owns == true`.

   >     shared_lock(mutex_type& m, defer_lock_t) noexcept;

   > > […]

   > > -7- Postconditions: `pm == &maddressof(m)` and `owns == false`.

   >     shared_lock(mutex_type& m, try_to_lock_t);

   > > […]

   > > -10- Postconditions: `pm == &maddressof(m)` and `owns == res` where `res` is the value returned by the call to `m.try_lock_shared()`.

   >     shared_lock(mutex_type& m, adopt_lock_t);

   > > […]

   > > -13- Postconditions: `pm == &maddressof(m)` and `owns == true`.

   >     template <class Clock, class Duration>
   >       shared_lock(mutex_type& m,
   >                   const chrono::time_point<Clock, Duration>& abs_time);

   > > […]

   > > -16- Postconditions: `pm == &maddressof(m)` and `owns == res` where `res` is the value returned by the call to `m.try_lock_shared_until(abs_time)`.

   >     template <class Rep, class Period>
   >       shared_lock(mutex_type& m,
   >                   const chrono::duration<Rep, Period>& rel_time);

   > > […]

   > > -19- Postconditions: `pm == &maddressof(m)` and `owns == res` where `res` is the value returned by the call to `m.try_lock_shared_for(rel_time)`.

### 2578(i). Iterator requirements should reference iterator traits

Section: 24.3 [iterator.requirements], 24.3.2.3 [iterator.traits] Status: C++17 Submitter: Ville Voutilainen Opened: 2016-01-05 Last modified: 2017-09-07

Priority: 3

View all other issues in [iterator.requirements].

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Discussion:

See this reflector discussion for background.

24.3 [iterator.requirements] attempts to establish requirements for iterators, but 24.3.2.3 [iterator.traits]/1 establishes further requirements that must be met in order to author a portable iterator that works with existing implementations. Failing to meet the requirements of the latter will fail to work in practice. The former requirements should reference the latter, normatively.

[2016-08-03 Chicago]

Fri AM: Moved to Tentatively Ready

Proposed resolution:

After [iterator.requirements.general]/5, insert the following new paragraph:

> -?- In addition to the requirements in this sub-clause, the nested typedef-names specified in ([iterator.traits]) shall be provided for the iterator type. [Note: Either the iterator type must provide the typedef-names directly (in which case iterator_traits pick them up automatically), or an iterator_traits specialization must provide them. -end note]

### 2579(i). Inconsistency wrt Allocators in `basic_string` assignment vs. `basic_string::assign`

Section: 27.4.3.7.3 [string.assign] Status: C++17 Submitter: Marshall Clow Opened: 2016-01-05 Last modified: 2017-07-30

Priority: 0

View all other issues in [string.assign].

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Discussion:

In issue 2063(i), we changed the Effects of `basic_string::assign(basic_string&&)` to match the behavior of `basic_string::operator=(basic_string&&)`, making them consistent.

We did not consider `basic_string::assign(const basic_string&)`, and its Effects differ from `operator=(const basic_string&)`.

Given the following definition:

>     typedef std::basic_string<char, std::char_traits<char>, MyAllocator<char>> MyString;

>     MyAllocator<char> alloc1, alloc2;
>     MyString string1("Alloc1", alloc1);
>     MyString string2(alloc2);

the following bits of code are not equivalent:

>     string2 = string1;       // (a) calls operator=(const MyString&)
>     string2.assign(string1); // (b) calls MyString::assign(const MyString&)

What is the allocator for `string2` after each of these calls?

1. If `MyAllocator<char>::propagate_on_container_copy_assignment` is true, then it should be `alloc2`, otherwise it should be `alloc1`.

2. `alloc2`

27.4.3.7.3 [string.assign]/1 says that (b) is equivalent to `assign(string1, 0, npos)`, which eventually calls `assign(str.data() + pos, rlen)`. No allocator transfer there.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Modify 27.4.3.7.3 [string.assign] p.1 as indicated:

   >     basic_string& assign(const basic_string& str);

   > > -1- Effects: Equivalent to `*this = strassign(str, 0, npos)`.

   > > -2- Returns: `*this`.

### 2581(i). Specialization of `<type_traits>` variable templates should be prohibited

Section: 21.3.3 [meta.type.synop] Status: C++17 Submitter: Tim Song Opened: 2016-01-07 Last modified: 2017-07-30

Priority: 0

View other active issues in [meta.type.synop].

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Discussion:

21.3.3 [meta.type.synop]/1 only prohibits adding specializations of class templates in `<type_traits>`. Now that we have `_v` variable templates, this prohibition should apply to them as well.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Edit 21.3.3 [meta.type.synop]/1 as indicated:

   > -1- The behavior of a program that adds specializations for any of the class templates defined in this subclause is undefined unless otherwise specified.

### 2582(i). §[res.on.functions]/2's prohibition against incomplete types shouldn't apply to type traits

Section: 21 [meta] Status: C++17 Submitter: Tim Song Opened: 2016-01-07 Last modified: 2017-07-30

Priority: 0

View other active issues in [meta].

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Discussion:

16.4.5.8 [res.on.functions]/2.5 says that the behavior is undefined "if an incomplete type is used as a template argument when instantiating a template component, unless specifically allowed for that component."

This rule should not apply to type traits — a literal application would make `is_same<void, void>` undefined behavior, since nothing in 21 [meta] (or elsewhere) "specifically allows" instantiating `is_same` with incomplete types.

Traits that require complete types are already explicitly specified as such, so the proposed wording below simply negates 16.4.5.8 [res.on.functions]/2.5 for 21 [meta].

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Insert a new paragraph after 21.3.3 [meta.type.synop]/1:

   > -?- Unless otherwise specified, an incomplete type may be used to instantiate a template in this subclause.

### 2583(i). There is no way to supply an allocator for `basic_string(str, pos)`

Section: 27.4.3.3 [string.cons] Status: C++17 Submitter: Pablo Halpern Opened: 2016-01-05 Last modified: 2017-07-30

Priority: 0

View all other issues in [string.cons].

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Discussion:

Container and `string` constructors in the standard follow two general rules:

1. Every constructor needs a version with and without an allocator argument (possibly through the use of default arguments).

2. Every constructor except the copy constructor for which an allocator is not provided uses a default-constructed allocator.

The first rule ensures emplacing a `string` into a container that uses a scoped allocator will correctly propagate the container's allocator to the new element.

The current standard allows constructing a `string` as `basic_string(str, pos)` but not `basic_string(str, pos, alloc)`. This omission breaks the first rule and causes something like the following to fail:

>     typedef basic_string<char, char_traits<char>, A<char>> stringA;
>     vector<stringA, scoped_allocator_adaptor<A<stringA>>> vs;
>     stringA s;

>     vs.emplace_back(s, 2); // Ill-formed

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Change 27.4.3 [basic.string], class template `basic_string` synopsis, as indicated

   >     basic_string(const basic_string& str, size_type pos, size_type n = npos,
   >                  const Allocator& a = Allocator());
   >     basic_string(const basic_string& str, size_type pos, size_type n,
   >                  const Allocator& a = Allocator());

2. Change 27.4.3.3 [string.cons] as indicated

   >     basic_string(const basic_string& str,
   >                  size_type pos, size_type n = npos,
   >                  const Allocator& a = Allocator());

   > > -3- Throws: `out_of_range` if `pos > str.size()`.

   > > -4- Effects: Constructs an object of class `basic_string` and determines the effective length `rlen` of the initial string value as the smaller of `n` and `str.size() - pos`, as indicated in Table 65.

   >     basic_string(const basic_string& str, size_type pos, size_type n,
   >                  const Allocator& a = Allocator());

   > > -?- Throws: `out_of_range` if `pos > str.size()`.

   > > -?- Effects: Constructs an object of class `basic_string` and determines the effective length `rlen` of the initial string value as the smaller of `n` and `str.size() - pos`, as indicated in Table 65.

   > > Table 65 — `basic_string(const basic_string&, size_type, size_type, const Allocator&)` and `basic_string(const basic_string&, size_type, size_type, const Allocator&)` effects
   > > Element | Value
   > > --- | ---
   > > `data()` | points at the first element of an allocated copy of `rlen` consecutive elements of the string controlled by `str` beginning at position `pos`
   > > `size()` | `rlen`
   > > `capacity()` | a value at least as large as `size()`

### 2584(i). `<regex>` ECMAScript `IdentityEscape` is ambiguous

Section: 28.6.12 [re.grammar] Status: C++17 Submitter: Billy O'Neal III Opened: 2016-01-13 Last modified: 2017-07-30

Priority: 2

View other active issues in [re.grammar].

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Discussion:

Stephan and I are seeing differences in implementation for how non-special characters should be handled in the `IdentityEscape` part of the ECMAScript grammar. For example:

>     #include <stdio.h>
>     #include <iostream>
>     #ifdef USE_BOOST
>     #include <boost/regex.hpp>
>     using namespace boost;
>     #else
>     #include <regex>
>     #endif
>     using namespace std;

>     int main() {
>       try {
>         const regex r("\\z");
>         cout << "Constructed \\z." << endl;
>         if (regex_match("z", r))
>           cout << "Matches z" << endl;
>       } catch (const regex_error& e) {
>           cout << e.what() << endl;
>       }
>     }

libstdc++, boost, and browsers I tested with (Microsoft Edge, Google Chrome) all happily interpret `\z`, which otherwise has no meaning, as an identity character escape for the letter `z`. libc++ and msvc++ say that this is invalid, and throw `regex_error` with `error_escape`.

ECMAScript 3 (which is what C++ currently points to) seems to agree with libc++ and msvc++:

>     IdentityEscape ::
>       SourceCharacter but not IdentifierPart

>     IdentifierPart ::
>       IdentifierStart
>       UnicodeCombiningMark
>       UnicodeDigit
>       UnicodeConnectorPunctuation
>       \ UnicodeEscapeSequence

>     IdentifierStart ::
>       UnicodeLetter
>       $
>       _
>       \ UnicodeEscapeSequence

But this doesn't make any sense — it prohibits things like `\$` which users absolutely need to be able to escape. So let's look at ECMAScript 6. I believe this says much the same thing, but updates the spec to better handle Unicode by referencing what the Unicode standard says is an identifier character:

>     IdentityEscape ::
>       SyntaxCharacter
>       /
>       SourceCharacter but not UnicodeIDContinue

>     UnicodeIDContinue ::
>       any Unicode code point with the Unicode property "ID_Continue", "Other_ID_Continue", or "Other_ID_Start"

However, ECMAScript 6 has an appendix B defining "additional features for web browsers" which says:

>     IdentityEscape ::
>       SourceCharacter but not c

which appears to agree with what libstdc++, boost, and browsers are doing.

What should be the correct behavior here?

[2016-08, Chicago]

Monday PM: Move to tentatively ready

Proposed resolution:

This wording is relative to N4567.

1. Change 28.6.12 [re.grammar]/3 as indicated:

   > -3- The following productions within the ECMAScript grammar are modified as follows:

   > >     ClassAtom ::
   > >       -
   > >       ClassAtomNoDash
   > >       ClassAtomExClass
   > >       ClassAtomCollatingElement
   > >       ClassAtomEquivalence

   > >     IdentityEscape ::
   > >       SourceCharacter but not c

### 2585(i). `forward_list::resize(size_type, const value_type&)` effects incorrect

Section: 23.3.7.5 [forward.list.modifiers] Status: C++17 Submitter: Tim Song Opened: 2016-01-14 Last modified: 2023-02-07

Priority: 0

View all other issues in [forward.list.modifiers].

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Discussion:

[forwardlist.modifiers]/29 says that the effects of `forward_list::resize(size_type sz, const value_type& c)` are:

> Effects: If `sz < distance(begin(), end())`, erases the last `distance(begin(), end()) - sz` elements from the list. Otherwise, inserts `sz - distance(begin(), end())` elements at the end of the list such that each new element, `e`, is initialized by a method equivalent to calling `allocator_traits<allocator_type>::construct(get_allocator(), std::addressof(e), c)`.

In light of LWG 2218(i), the use of `allocator_traits<allocator_type>::construct` is incorrect, as a rebound allocator may be used. There's no need to repeat this information, in any event — no other specification of `resize()` does it.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

[2016-02-11, Alisdair requests reopening]

I believe the standard is correct as written, and that by removing the clear direction to make the copy with uses-allocator-construction, we open ourselves to disputing this very point again at some point in the future.

The issue seems to be complaining that a rebound allocator may be used instead of the allocator returned by `get_allocator()` call, and nailing us down to exactly which instantiation of `allocator_traits` is used. Given the requirements on allocators being constructible from within the same template "family" though, and specifically that copies compare equal and can allocate/deallocate on each other's behalf, this should clearly fall under existing as-if freedom. The construct call is even more clear, as there is no requirement that the allocator to construct be of a kind that can allocate the specific type being constructed — a freedom granted precisely so this kind of code can be written, and be correct, regardless of internal node type of any container and the actual rebound allocator used internally.

I think the new wording is less clear than the current wording, and would prefer to resolve as NAD.

Proposed resolution:

This wording is relative to N4567.

1. Edit [forwardlist.modifiers]/29 as indicated:

   > [Drafting note: "copies of `c`" is the phrase used by `vector::resize` and `deque::resize`.]

   >     void resize(size_type sz, const value_type& c);

   > > -29- Effects: If `sz < distance(begin(), end())`, erases the last `distance(begin(), end()) - sz` elements from the list. Otherwise, inserts `sz - distance(begin(), end())` elementscopies of `c` at the end of the list such that each new element, `e`, is initialized by a method equivalent to calling `allocator_traits<allocator_type>::construct(get_allocator(), std::addressof(e), c)`.

### 2586(i). Wrong value category used in `scoped_allocator_adaptor::construct()`

Section: 20.6.4 [allocator.adaptor.members], 20.2.8.2 [allocator.uses.construction] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-01-15 Last modified: 2017-07-30

Priority: 0

View all other issues in [allocator.adaptor.members].

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Discussion:

20.6.4 [allocator.adaptor.members] p9 says that the `is_constructible` tests are done using `inner_allocator_type`, which checks for construction from an rvalue, but then the constructor is passed `inner_allocator()` which returns a non-`const` lvalue reference. The value categories should be consistent, otherwise this fails to compile:

>     #include <memory>
>     #include <scoped_allocator>

>     struct X {
>       using allocator_type = std::allocator<X>;
>       X(std::allocator_arg_t, allocator_type&&) { }
>       X(allocator_type&) { }
>     };

>     int main() {
>       std::scoped_allocator_adaptor<std::allocator<X>> sa;
>       sa.construct(sa.allocate(1));
>     }

`uses_allocator<X, decltype(sa)::inner_allocator_type>>` is true, because it can be constructed from an rvalue of the allocator type, so bullet (9.1) doesn't apply.

`is_constructible<X, allocator_arg_t, decltype(sa)::inner_allocator_type>` is true, so bullet (9.2) applies. That means we try to construct the object passing it `sa.inner_allocator()` which is an lvalue reference, so it fails.

The `is_constructible` checks should use an lvalue reference, as that's what's actually going to be used.

I don't think the same problem exists in the related wording in 20.2.8.2 [allocator.uses.construction] if we assume that the value categories of `v1, v2, ..., vN` and `alloc` are meant to be preserved, so that the `is_constructible` traits and the initialization expressions match. However, it does say "an allocator `alloc` of type `Alloc`" and if `Alloc` is an reference type then it's not an allocator, so I suggest a small tweak there too.

[2016-02, Issues Telecon]

Strike first paragraph of PR, and move to Tentatively Ready.

Original Resolution [SUPERSEDED]:

> 1. Change 20.2.8.2 [allocator.uses.construction] p1:

>    > -1- Uses-allocator construction with allocator `Alloc` refers to the construction of an object `obj` of type `T`, using constructor arguments `v1, v2, ..., vN` of types `V1, V2, ..., VN`, respectively, and an allocator (or reference to an allocator) `alloc` of type `Alloc`, according to the following rules:

> 2. Change the 2nd and 3rd bullets in 20.6.4 [allocator.adaptor.members] p9 to add two lvalue-references:

>    > 1. (9.2) — Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, allocator_arg_t, inner_allocator_type&, Args...>::value` is `true`, calls `OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, allocator_arg, inner_allocator(), std::forward<Args>(args)...)`.

>    > 2. (9.3) — Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, Args..., inner_allocator_type&>::value` is `true`, calls `OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, std::forward<Args>(args)..., inner_allocator())`.

> 3. Change the 2nd, 3rd, 6th, and 7th bullets in 20.6.4 [allocator.adaptor.members] p11 to add four lvalue-references:

>    > 1. (11.2) — Otherwise, if `uses_allocator<T1, inner_allocator_type>::value` is `true` and `is_constructible<T1, allocator_arg_t, inner_allocator_type&, Args1...>::value` is `true`, then `xprime` is `tuple_cat(tuple<allocator_arg_t, inner_allocator_type&>(allocator_arg, inner_allocator()), std::move(x))`.

>    > 2. (11.3) — Otherwise, if `uses_allocator<T1, inner_allocator_type>::value` is `true` and `is_constructible<T1, Args1..., inner_allocator_type&>::value` is `true`, then `xprime` is `tuple_cat(std::move(x), tuple<inner_allocator_type&>(inner_allocator()))`.

>    > 3. […]

>    > 4. (11.6) — Otherwise, if `uses_allocator<T2, inner_allocator_type>::value` is `true` and `is_constructible<T2, allocator_arg_t, inner_allocator_type&, Args2...>::value` is `true`, then `yprime` is `tuple_cat(tuple<allocator_arg_t, inner_allocator_type&>(allocator_arg, inner_allocator()), std::move(y))`.

>    > 5. (11.7) — Otherwise, if `uses_allocator<T2, inner_allocator_type>::value` is `true` and `is_constructible<T2, Args2..., inner_allocator_type&>::value` is `true`, then `yprime` is `tuple_cat(std::move(y), tuple<inner_allocator_type&>(inner_allocator()))`.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Change the 2nd and 3rd bullets in 20.6.4 [allocator.adaptor.members] p9 to add two lvalue-references:

   > 1. (9.2) — Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, allocator_arg_t, inner_allocator_type&, Args...>::value` is `true`, calls `OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, allocator_arg, inner_allocator(), std::forward<Args>(args)...)`.

   > 2. (9.3) — Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, Args..., inner_allocator_type&>::value` is `true`, calls `OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, std::forward<Args>(args)..., inner_allocator())`.

2. Change the 2nd, 3rd, 6th, and 7th bullets in 20.6.4 [allocator.adaptor.members] p11 to add four lvalue-references:

   > 1. (11.2) — Otherwise, if `uses_allocator<T1, inner_allocator_type>::value` is `true` and `is_constructible<T1, allocator_arg_t, inner_allocator_type&, Args1...>::value` is `true`, then `xprime` is `tuple_cat(tuple<allocator_arg_t, inner_allocator_type&>(allocator_arg, inner_allocator()), std::move(x))`.

   > 2. (11.3) — Otherwise, if `uses_allocator<T1, inner_allocator_type>::value` is `true` and `is_constructible<T1, Args1..., inner_allocator_type&>::value` is `true`, then `xprime` is `tuple_cat(std::move(x), tuple<inner_allocator_type&>(inner_allocator()))`.

   > 3. […]

   > 4. (11.6) — Otherwise, if `uses_allocator<T2, inner_allocator_type>::value` is `true` and `is_constructible<T2, allocator_arg_t, inner_allocator_type&, Args2...>::value` is `true`, then `yprime` is `tuple_cat(tuple<allocator_arg_t, inner_allocator_type&>(allocator_arg, inner_allocator()), std::move(y))`.

   > 5. (11.7) — Otherwise, if `uses_allocator<T2, inner_allocator_type>::value` is `true` and `is_constructible<T2, Args2..., inner_allocator_type&>::value` is `true`, then `yprime` is `tuple_cat(std::move(y), tuple<inner_allocator_type&>(inner_allocator()))`.

### 2587(i). "Convertible to `bool`" requirement in `conjunction` and `disjunction`

Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Tim Song Opened: 2016-01-18 Last modified: 2017-12-05

Priority: 3

View all other issues in [meta.logical].

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Discussion:

The specification of `conjunction` and `disjunction` in 21.3.9 [meta.logical] p2 and p5 requires `Bi::value` to be convertible to `bool`, but nothing in the specification of the actual behavior of the templates, which instead uses the expressions `Bi::value == false` and `Bi::value != false` instead, actually requires this conversion.

If the intention of this requirement is to allow implementations to pass `Bi::value` directly to `std::conditional`, like the sample implementation in P0013R1:

>     template<class B1, class B2>
>     struct and_<B1, B2> : conditional_t<B1::value, B2, B1> { };

then it's insufficient in at least two ways:

1. Nothing in the specification requires the result of comparing `Bi::value` with `false` to be consistent with the result of the implicit conversion. This is similar to LWG 2114(i), though I don't think the `BooleanTestable` requirements in that issue's P/R covers `Bi::value == false` and `Bi::value != false`.

2. More importantly, the above implementation is ill-formed for, e.g., `std::conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>`, because converting 2 to `bool` is a narrowing conversion that is not allowed for non-type template arguments (see 7.7 [expr.const]/4). (Note that GCC currently doesn't diagnose this error at all, and Clang doesn't diagnose it inside system headers.) It's not clear whether such constructs are intended to be supported, but if they are not, the current wording doesn't prohibit it.

[2016-08-03 Chicago LWG]

Walter, Nevin, and Jason provide initial Proposed Resolution.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Change 21.3.9 [meta.logical] as indicated:

>    >     template<class... B> struct conjunction : see below { };

>    > > […]

>    > > -3- The BaseCharacteristic of a specialization `conjunction<B1, ..., BN>` is the first type `Bi` in the list `true_type, B1, ..., BN` for which `Bi::value == false! bool(Bi::value)`, or if every `Bi::value != falsebool(Bi::value)`, the BaseCharacteristic is the last type in the list. […]

>    > > -4- For a specialization `conjunction<B1, ..., BN>`, if there is a template type argument `Bi` with `Bi::value == false! bool(Bi::value)`, then instantiating […]

>    >     template<class... B> struct disjunction : see below { };

>    > > […]

>    > > -6- The BaseCharacteristic of a specialization `disjunction<B1, ..., BN>` is the first type `Bi` in the list `false_type, B1, ..., BN` for which `Bi::value != falsebool(Bi::value)`, or if every `Bi::value == false! bool(Bi::value)`, the BaseCharacteristic is the last type in the list. […]

>    > > -7- For a specialization `disjunction<B1, ..., BN>`, if there is a template type argument `Bi` with `Bi::value != falsebool(Bi::value)`, then instantiating […]

>    >     template<class B> struct negation : bool_constant<!bool(B::value)> { };

>    > > -8- The class template negation forms the logical negation of its template type argument. The type `negation<B>` is a UnaryTypeTrait with a BaseCharacteristic of `bool_constant<!bool(B::value)>`.

[Dec 2017 - The resolution for this issue shipped in the C++17 standard; setting status to 'C++17']

Proposed resolution:

The resolution for this issue was combined with the resolution for LWG 2567(i), so 2567(i) resolves this issue here as well.

### 2588(i). [fund.ts.v2] "Convertible to `bool`" requirement in `conjunction` and `disjunction`

Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2016-01-18 Last modified: 2017-07-30

Priority: 3

View all other issues in [fund.ts.v2::meta.logical].

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

The specification of `conjunction` and `disjunction` in 3.3.3 [fund.ts.v2::meta.logical] p2 and p5 requires `Bi::value` to be convertible to `bool`, but nothing in the specification of the actual behavior of the templates, which instead uses the expressions `Bi::value == false` and `Bi::value != false` instead, actually requires this conversion.

If the intention of this requirement is to allow implementations to pass `Bi::value` directly to `std::conditional`, like the sample implementation in P0013R1:

>     template<class B1, class B2>
>     struct and_<B1, B2> : conditional_t<B1::value, B2, B1> { };

then it's insufficient in at least two ways:

1. Nothing in the specification requires the result of comparing `Bi::value` with `false` to be consistent with the result of the implicit conversion. This is similar to LWG 2114(i), though I don't think the `BooleanTestable` requirements in that issue's P/R covers `Bi::value == false` and `Bi::value != false`.

2. More importantly, the above implementation is ill-formed for, e.g., `std::conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>`, because converting 2 to `bool` is a narrowing conversion that is not allowed for non-type template arguments (see 7.7 [expr.const]/4). (Note that GCC currently doesn't diagnose this error at all, and Clang doesn't diagnose it inside system headers.) It's not clear whether such constructs are intended to be supported, but if they are not, the current wording doesn't prohibit it.

[2016-11-08, Issaquah]

Adopted during NB comment resolution

Proposed resolution:

The resolution for this issue was combined with the resolution for LWG 2568(i), so 2568(i) resolves this issue here as well.

### 2589(i). `match_results` can't satisfy the requirements of a container

Section: 28.6.9 [re.results] Status: C++17 Submitter: S. B. Tam Opened: 2016-01-24 Last modified: 2017-07-30

Priority: 3

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Discussion:

N4567 28.6.9 [re.results] p2 mentions

> The class template `match_results` shall satisfy the requirements of an allocator-aware container and of a sequence container, as specifed in 23.2.3, except that only operations defined for const-qualified sequence containers are supported.

However, this is impossible because `match_results` has a `operator==` whose semantics differs from the one required in Table 95 — "Container requirements".

Table 95 requires that `a == b` is an equivalence relation and means `equal(a.begin(), a.end(), b.begin(), b.end())`. But for `match_results`, `a == b` and `equal(a.begin(), a.end(), b.begin(), b.end())` can give different results. For example:

>     #include <iostream>
>     #include <regex>
>     #include <string>
>     #include <algorithm>

>     int main()
>     {
>       std::regex re("a*");
>       std::string target("baaab");
>       std::smatch a;

>       std::regex_search(target, a, re);

>       std::string target2("raaau");
>       std::smatch b;

>       std::regex_search(target2, b, re);

>       std::cout << std::boolalpha;
>       std::cout << (a == b) << '\n'; // false
>       std::cout << std::equal(a.begin(), a.end(), b.begin(), b.end()) << '\n'; // true
>     }

[2016-02, Issues Telecon]

Marshall: The submitter is absolutely right, but the proposed resolution is insufficient. We should avoid "shall", for once.
Jonathan: This is NAD, because the container comparison functions say "unless otherwise stated", 23.3.1p14 and table 97.
Ville: wrong, table 95 is relevant for ==.
Jonathan: good point

2016-05: Marshall cleans up the wording around the change

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Previous resolution [SUPERSEDED]:

> This wording is relative to N4567.

> 1. Change 28.6.9 [re.results] p2 as indicated:

>    > -2- The class template `match_results` shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 23.2.3, except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.

Proposed resolution:

This wording is relative to N4567.

1. Change 28.6.9 [re.results] p2 as indicated:

   > -2- The class template `match_results` shall satisfysatisfies the requirements of an allocator-aware container and of a sequence container, as specified in (23.2.3), except that only operations defined for const-qualified sequence containers are supported and the semantics of comparison functions are different from those required for a container.

> [Drafting note: (Post-Issaquah) Due to the outdated N4567 wording the project editor accepted the following merge suggestion into N4606 wording: — end drafting note]

> > This wording is relative to N4606.

> > 1. Change 28.6.9 [re.results] p2 as indicated:

> >    > -2- The class template `match_results` satisfies the requirements of an allocator-aware container and of a sequence container, as specified in (23.2.2 [container.requirements.general] and, 23.2.4 [sequence.reqmts]) respectively, except that only operations defined for const-qualified sequence containers are supported and the semantics of comparison functions are different from those required for a container.

### 2590(i). Aggregate initialization for `std::array`

Section: 23.3.3.1 [array.overview] Status: C++17 Submitter: Robert Haberlach Opened: 2016-01-30 Last modified: 2017-07-30

Priority: 0

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Discussion:

Similar to core issue 1270's resolution, 23.3.3.1 [array.overview]/2 should cover aggregate-initialization in general. As it stands, that paragraph solely mentions copy-list-initialization — i.e. it is unclear whether the following notation is (guaranteed to be) well-formed:

>     std::array<int, 1> arr{0};

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Change 23.3.3.1 [array.overview] p2 as indicated:

   > -2- An `array` is an aggregate (8.5.1) that can be list-initialized with the syntax

   > >     array<T, N> a = { initializer-list };

   > where initializer-list is a comma-separated list of up to `N` elements whose types are convertible to `T`.

### 2591(i). `std::function`'s member template `target()` should not lead to undefined behaviour

Section: 22.10.17.3.6 [func.wrap.func.targ] Status: C++17 Submitter: Daniel Krügler Opened: 2016-01-31 Last modified: 2017-09-07

Priority: 3

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Discussion:

This issue is a spin-off of LWG 2393(i), it solely focuses on the pre-condition of 22.10.17.3.6 [func.wrap.func.targ] p2:

> Requires: `T` shall be a type that is Callable (20.9.12.2) for parameter types `ArgTypes` and return type `R`.

Originally, the author of this issue here had assumed that simply removing the precondition as a side-step of fixing LWG 2393(i) would be uncontroversial. Discussions on the library reflector indicated that this is not the case, although it seemed that there was agreement on removing the undefined behaviour edge-case.

There exist basically the following positions:

1. The constraint should be removed completely, the function is considered as having a wide contract.

2. The pre-condition should be replaced by a Remarks element, that has the effect of making the code ill-formed, if `T` is a type that is not Lvalue-Callable (20.9.11.2) for parameter types `ArgTypes` and return type `R`. Technically this approach is still conforming with a wide contract function, because the definition of this contract form depends on runtime constraints.

Not yet explicitly discussed, but a possible variant of bullet (2) could be:

3. The pre-condition should be replaced by a Remarks element, that has the effect of SFINAE-constraining this member: "This function shall not participate in overload resolution unless `T` is a type that is Lvalue-Callable (20.9.11.2) for parameter types `ArgTypes` and return type `R`".

The following describes a list of some selected arguments that have been provided for one or the other position using corresponding list items. Unless explicitly denoted, no difference has been accounted for option (3) over option (2).

1. 1. It reflects existing implementation practice, Visual Studio 2015 SR1, gcc 6 libstdc++, and clang 3.8.0 libc++ do accept the following code:

      >     #include <functional>
      >     #include <iostream>
      >     #include <typeinfo>
      >     #include "boost/function.hpp"

      >     void foo(int) {}

      >     int main() {
      >       std::function<void(int)> f(foo);
      >       std::cout << f.target<void(*)()>() << std::endl;
      >       boost::function<void(int)> f2(foo);
      >       std::cout << f2.target<void(*)()>() << std::endl;
      >     }

      and consistently output the implementation-specific result for two null pointer values.

   2. The current Boost documentation does not indicate any precondition for calling the `target` function, so it is natural that programmers would expect similar specification and behaviour for the corresponding standard component.

   3. There is a consistency argument in regard to the free function template `get_deleter`

      >     template<class D, class T>
      >     D* get_deleter(const shared_ptr<T>& p) noexcept;

      This function also does not impose any pre-conditions on its template argument `D`.

2. 1. Programmers have control over the type they're passing to `target<T>()`. Passing a non-callable type can't possibly retrieve a non-null target, so it seems highly likely to be programmer error. Diagnosing that at compile time seems highly preferable to allowing this to return null, always, at runtime.

   2. If `T` is a reference type then the return type `T*` is ill-formed anyway. This implies that one can't blindly call `target<T>` without knowing what `T` is.

   3. It has been pointed out that some real world code, boiling down to

      >     void foo() {}

      >     int main() {
      >       std::function<void()> f = foo;
      >       if (f.target<decltype(foo)>()) {
      >         // fast path
      >       } else {
      >         // slow path
      >       }
      >     }

      had manifested as a performance issue and preparing a patch that made the library `static_assert` in that case solved this problem (Note that `decltype(foo)` evaluates to `void()`, but a proper argument of `target()` would have been the function pointer type `void(*)()`, because a function type `void()` is not any Callable type).

It might be worth adding that if use case (2 c) is indeed an often occurring idiom, it would make sense to consider to provide an explicit conversion to a function pointer (w/o template parameters that could be provided incorrectly), if the `std::function` object at runtime conditions contains a pointer to a real function, e.g.

>     R(*)(ArgTypes...) target_func_ptr() const noexcept;

[2016-08 Chicago]

Tues PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4567.

1. Change 22.10.17.3.6 [func.wrap.func.targ] p2 as indicated:

   >     template<class T> T* target() noexcept;
   >     template<class T> const T* target() const noexcept;

   > > -2- Requires: `T` shall be a type that is `Callable` (22.10.17.3 [func.wrap.func]) for parameter types `ArgTypes` and return type `R`.

   > > -3- Returns: If `target_type() == typeid(T)` a pointer to the stored function target; otherwise a null pointer.

### 2593(i). Moved-from state of Allocators

Section: 16.4.4.6 [allocator.requirements] Status: C++20 Submitter: David Krauss Opened: 2016-02-19 Last modified: 2021-02-25

Priority: 4

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Discussion:

16.4.4.6 [allocator.requirements] suggests that the moved-from state of an allocator may be unequal to its previous state. Such a move constructor would break most container implementations, which move-construct the embedded allocator along with a compressed pair. Even if a moved-from container is empty, it should still subsequently allocate from the same resource pool as it did before.

>     std::vector<int, pool> a(500, my_pool);
>     auto b = std::move(a); // b uses my_pool too.
>     a.resize(500); // should still use my_pool.

[2016-02, Jacksonville]

Marshall will see if this can be resolved editorially.

After discussion, the editors and I decided that this could not be handled editorially. The bit about a moved-from state of an allocator being the same as the original state is a normative change. I submitted a pull request to handle the mismatched variables in the table.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4567.

> 1. Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as indicated:

>    > Note there's an editorial error in Table 28 in that line and the surrounding ones. The left column was apparently updated to use `u` and the right column is still using `a`/`a1`/`b`.

>    > Table 28 — Allocator requirements
>    > Expression | Return type | Assertion/note pre-/post-condition | Default
>    > --- | --- | --- | ---
>    > `…`
>    > `X u(move(a)); X u = move(a);` |  | Shall not exit via an exception. post: `u` is equal to `a` and equal to the prior value of `a` `a1` equals the prior value of `a`. |
>    > `…`

[2016-06-20, Oulu, Daniel comments]

According to the current working draft, the situation has changed due to changes performed by the project editor, the revised resolution has been adjusted to N4594.

[2016-08 - Chicago]

Thurs AM: Moved to LEWG, as this decision (should allocators only be copyable, not movable) is design.

[2017-02 in Kona, LEWG responds]

Alisdair Meredith says that if you have a do-not-propagate-on-move-assignment, then the move of the allocator must compare equal to the original.

Have a data structure where all allocators are equal. Construct an element somewhere else moving from inside the container (which doesn't have a POCMA trait); you don't want the allocator of the moved-from element to now be different. So in that case, the allocator's move constructor must behave the same as copy.

We don't need to go as far as this issue, but going that far is ok for Bloomberg.

[2017-06-02 Issues Telecon]

We discussed containers that have sentinel nodes, etc, and so might have to allocate/deallocate using a moved-from allocator - and decided that we didn't want any part of that

Adjusted the wording slightly, and moved to Tentatively Ready

Previous resolution [SUPERSEDED]:

> This wording is relative to N4594.

> 1. Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as indicated:

>    > Table 28 — Allocator requirements
>    > Expression | Return type | Assertion/note pre-/post-condition | Default
>    > --- | --- | --- | ---
>    > `…`
>    > `X u(std::move(a)); X u = std::move(a);` |  | Shall not exit via an exception. post: `u` is equal to `a` and equal to the prior value of `a` `u` is equal to the prior value of `a`.. |
>    > `…`

Proposed resolution:

This wording is relative to N4594.

1. Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as indicated:

   > Table 28 — Allocator requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `…`
   > `X u(std::move(a)); X u = std::move(a);` |  | Shall not exit via an exception. post: The value of `a` is unchanged and is equal to `u` `u` is equal to the prior value of `a`. |
   > `…`

### 2596(i). `vector::data()` should use `addressof`

Section: 23.3.13.4 [vector.data] Status: C++17 Submitter: Marshall Clow Opened: 2016-02-29 Last modified: 2017-07-30

Priority: 0

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Discussion:

In 23.3.13.4 [vector.data], we have:

> Returns: A pointer such that `[data(),data() + size())` is a valid range. For a non-empty vector, `data() == &front()`.

This should be:

> Returns: A pointer such that `[data(),data() + size())` is a valid range. For a non-empty vector, `data() == addressof(front())`.

Proposed resolution:

This wording is relative to N4582.

1. Change 23.3.13.4 [vector.data] p1 as indicated:

   >     T* data() noexcept;
   >     const T* data() const noexcept;

   > > -1- Returns: A pointer such that `[data(), data() + size())` is a valid range. For a non-empty vector, `data() == addressof(&front())`.

### 2597(i). `std::log` misspecified for complex numbers

Section: 29.4.8 [complex.transcendentals] Status: C++20 Submitter: Thomas Koeppe Opened: 2016-03-01 Last modified: 2021-02-25

Priority: 3

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Discussion:

The current specification of `std::log` is inconsistent for complex numbers, specifically, the Returns clause (29.4.8 [complex.transcendentals]). On the one hand, it states that the imaginary part of the return value lies in the closed interval `[-i π, +i π]`. On the other hand, it says that "the branch cuts are along the negative real axis" and "the imaginary part of `log(x)` is `+π` when `x` is a negative real number".

The inconsistency lies in the difference between the mathematical concept of a branch cut and the nature of floating point numbers in C++. The corresponding specification in the C standard makes it clearer that if `x` is a real number, then `log(x + 0i) = +π`, but `log(x - 0i) = -π`, i.e. they consider positive and negative zero to represent the two different limits of approaching the branch cut from opposite directions. In other words, the term "negative real number" is misleading, and in fact there are two distinct real numbers, `x + 0i` and `x - 0i`, that compare equal but whose logarithms differ by `2 π i`.

The resolution should consist of two parts:

1. Double-check that our usage and definition of "branch cut" is sufficiently unambiguous. The C standard contains a lot more wording around this that we don't have in C++.

2. Change the Returns clause of `log` appropriately. For example: "When `x` is a negative real number, `imag(log(x + 0i))` is `π`, and `imag(log(x - 0i))` is `-π`."

Current implementations seem to behave as described in (2). (Try-it-at-home link)

[2016-11-12, Issaquah]

Move to Open - Thomas to provide wording

[2016-11-15, Thomas comments and provides wording]

Following LWG discussion in Issaquah, I now propose to resolve this issue by removing the normative requirement on the function limits, and instead adding a note that the intention is to match the behaviour of C. This allows implementations to use the behaviour of C without having to specify what floating point numbers really are.

The change applies to both `std::log` and `std::sqrt`.

Updated try-at-home link, see here.

[2017-03-04, Kona]

Minor wording update and status to Tentatively Ready.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Change the "returns" element for `std::log` (29.4.8 [complex.transcendentals] p17):

>    >     template<class T> complex<T> log(const complex<T>& x);

>    > > -16- Remarks: The branch cuts are along the negative real axis.

>    > > -17- Returns: The complex natural (base-ℯ) logarithm of `x`. For all `x`, `imag(log(x))` lies in the interval `[`-π, π`]`, and when `x` is a negative real number, `imag(log(x))` is π. [Note: The semantics of `std::log` are intended to be the same in C++ as they are for `clog` in C. — end note]

> 2. Change the "returns" element for `std::sqrt` (29.4.8 [complex.transcendentals] p25):

>    >     template<class T> complex<T> sqrt(const complex<T>& x);

>    > > -24- Remarks: The branch cuts are along the negative real axis.

>    > > -25- Returns: The complex square root of `x`, in the range of the right half-plane. If the argument is a negative real number, the value returned lies on the positive imaginary axis.[Note: The semantics of `std::sqrt` are intended to be the same in C++ as they are for `csqrt` in C. — end note]

Proposed resolution:

This wording is relative to N4606.

1. Change the "returns" element for `std::log` (29.4.8 [complex.transcendentals] p17):

   >     template<class T> complex<T> log(const complex<T>& x);

   > > -16- Remarks: The branch cuts are along the negative real axis.

   > > -17- Returns: The complex natural (base-ℯ) logarithm of `x`. For all `x`, `imag(log(x))` lies in the interval `[`-π, π`]`, and when `x` is a negative real number, `imag(log(x))` is π. [Note: the semantics of this function are intended to be the same in C++ as they are for `clog` in C. — end note]

2. Change the "returns" element for `std::sqrt` (29.4.8 [complex.transcendentals] p25):

   >     template<class T> complex<T> sqrt(const complex<T>& x);

   > > -24- Remarks: The branch cuts are along the negative real axis.

   > > -25- Returns: The complex square root of `x`, in the range of the right half-plane. If the argument is a negative real number, the value returned lies on the positive imaginary axis.[Note: The semantics of this function are intended to be the same in C++ as they are for `csqrt` in C. — end note]

### 2598(i). `addressof` works on temporaries

Section: 20.2.11 [specialized.addressof] Status: C++17 Submitter: Brent Friedman Opened: 2016-03-06 Last modified: 2017-07-30

Priority: 3

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Discussion:

LWG issue 970(i) removed the rvalue reference overload for `addressof`. This allows const prvalues to bind to a call to `addressof`, which is dissimilar from the behavior of `operator&`.

>     const vector<int> a();

>     void b()
>     {
>       auto x = addressof(a()); // "ok"
>       auto y = addressof<const int>(0); // "ok"
>       auto z = &a(); //error: cannot take address of a temporary
>     }

[2016-08 Chicago]

Tues PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4582.

1. Change 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     […]
   >     // 20.9.12, specialized algorithms:
   >     template <class T> constexpr T* addressof(T& r) noexcept;
   >     template <class T> const T* addressof(const T&& elem) = delete;
   >     […]

2. Change 20.2.11 [specialized.addressof] p1 as indicated:

   >     template <class T> constexpr T* addressof(T& r) noexcept;
   >     template <class T> const T* addressof(const T&& elem) = delete;

   > > -1- Returns: The actual address of the object or function referenced by `r`, even in the presence of an overloaded `operator&`.

### 2601(i). [filesys.ts] [PDTS] Make namespaces consistent with Library TS policy

Section: 1 [filesys.ts::fs.scope] Status: TS Submitter: FI-5, US-5, GB-3, CH-6 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

The PDTS used a placeholder namespace "tbs" since the Standard Library policy for TS namespaces had not yet been fully articulated.

[2014-02-11 Issaquah: Project editor to make indicated changes to WP, post notice on lib and SG3 reflectors.]

Proposed resolution:

[2014-02-07: Beman Dawes]

Throughout the WP, change namespace "tbs" to "experimental" as described in the Library Fundamentals TS working paper, 1.3 Namespaces and headers [general.namespaces].

### 2602(i). [filesys.ts] [PDTS] Tighten specification when there is no reasonable behavior

Section: 2.1 [filesys.ts::fs.conform.9945] Status: TS Submitter: FI-1 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

It is unfortunate that error reporting for inability to provide reasonable behaviour is completely implementation-defined. This hurts portability in the sense that programmers have no idea how errors will be reported and cannot anticipate anything.

Change "If an implementation cannot provide any reasonable behavior, the implementation shall report an error in an implementation-defined manner." to "If an implementation cannot provide any reasonable behavior, the code using the facilities for which reasonable behaviour cannot be provided shall be ill-formed." and strike the note.

[2014-02-07, Beman Dawes suggests wording]

[2014-02-12, Daniel Krügler comments:]

> In our code bases we routinely have to target different filesystems, notably POSIX-based and Windows. While on Windows there is no meaning in setting the `owner_exec` permission, for example, this is required on POSIX systems when the corresponding file should be executable. Still, attempting to invoke this operation should be possible. It would be OK, if we got a defined runtime response to this, such as a specifically defined error code value that could be tested either via the error_code& overload, but it would be IMO unacceptable for end-users to tell them "Well, this code may not compile". I don't think that we can teach people that code written using the filesystem operations might or might not compile. It would be very valuable to have at least a clear indication that implementers are required to give a defined runtime-response if they do believe that this operation cannot be implemented resulting in "reasonable behaviour".

[2014-02-12, Proposed wording updated to reflect LWG/SG-3 discussion in Issaquah.

Since the standardese to carry out the LWG/SG-3 "throw or error code" intent is best achieved by reference to the Error reporting section, a note was added by the project editor. Please review carefully. ]

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

1. Change 2.1:

   > If an implementation cannot provide any reasonable behavior, the implementation shall report an error in an implementation-defined manner as specified in § 7 [fs.err.report]. [Note: This allows users to rely on an exception being thrown or an error code being set when an implementation cannot provide any reasonable behavior. — end note] .

### 2603(i). [filesys.ts] [PDTS] Filename length needs bullet item

Section: 4.7 [filesys.ts::fs.def.filename] Status: TS Submitter: CH-2 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Filename lengths are also implementation dependent. This is not the same as `FILENAME_MAX` that specifies the maximum length of pathnames.

Add a bullet: "Length of filenames."

[2014-02-07, Beman Dawes provides wording]

Proposed resolution:

Change 4.7 [fs.def.filename]:

The name of a file. Filenames dot and dot-dot have special meaning. The following characteristics of filenames are operating system dependent:

- The permitted characters. [Example: Some operating systems prohibit the ASCII control characters (0x00-0x1F) in filenames. — end example].

- The maximum permitted length.

- Filenames that are not permitted.

- Filenames that have special meaning.

- Case awareness and sensitivity during path resolution.

- Special rules that may apply to file types other than regular files, such as directories.

### 2605(i). [filesys.ts] [PDTS] Parent of root directory unspecified

Section: 8.1 [filesys.ts::path.generic] Status: TS Submitter: CH-4 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

8.1 [path.generic] says: "The filename dot-dot is treated as a reference to the parent directory." So it must be specified what "/.." and "/../.." refer to.

Add a statement what the parent directory of the root directory is.

[2014-02-07, Beman Dawes suggests wording]

[ 2014-02-11 Issaquah: Implementation defined. See wiki notes for rationale. Beman to provide wording for review next meeting. ]

[ 2014-05-22 Beman provides wording, taken directly from POSIX. See pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html#tag_21_04_12 ]

Proposed resolution:

1. Change 8.1 [path.generic]:

   > The filename dot is treated as a reference to the current directory. The filename dot-dot is treated as a reference to the parent directory. What the filename dot-dot refers to relative to root-directory is implementation-defined. Specific filenames may have special meanings for a particular operating system.

### 2606(i). [filesys.ts] [PDTS] Path depth is underspecified

Section: 4.15 [filesys.ts::fs.def.path] Status: TS Submitter: CH-5 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Path depth is implementation dependent.

Add a paragraph: "The maximum length of the sequence (i.e. the maximum depth) is implementation dependent.

[2014-02-07, Beman Dawes comments]

"implementaton defined" and "operating system dependent" are well defined terms in this TS, but "implementation dependent" is not well defined. The path depth is operating system dependent, so that's the form used in the proposed wording.

[2014-02-07, Beman Dawes provides wording]

Proposed resolution:

Change 4.15 [fs.def.path]:

> 4.15 path [fs.def.path]

> A sequence of elements that identify the location of a file within a filesystem. The elements are the root-nameopt , root-directoryopt , and an optional sequence of filenames.

> The maximum number of elements in the sequence is operating system dependent.

### 2607(i). [filesys.ts] [PDTS] Unhelpful comment for `struct space_info`

Section: 6 [filesys.ts::fs.filesystem.synopsis], 15.32 [filesys.ts::fs.op.space] Status: TS Submitter: GB-4 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Use of the term a 'non-privileged' process. The comment for available in the struct space_info refers to: free space available to a non-privileged process. This seems quite specific to a POSIX implementation (on Windows, for instance, the equivalent data would be user-specific but not directly related to privilege)

Remove the comment and add a note to 15.32 [fs.op.space]: [Note: the precise meaning of available space is implementation dependent. — end note]

[2014-02-07, Beman Dawes comments]

"implementaton defined" and "operating system dependent" are well defined terms in this TS, but "implementation dependent" is not well defined. The meaning of `available` is operating system dependent, so that's the form used in the proposed wording.

[2014-02-07, Beman Dawes provides wording]

Proposed resolution:

1. Change 6 [fs.filesystem.synopsis]:

   > `uintmax_t available; // free space available to a non-privileged process`

2. Add Remarks to 15.32 [fs.op.space]:

   > Remarks: The value of member `space_info::available` is operating system dependent. [Note: `available` may be less than `free`. — end note]

### 2608(i). [filesys.ts] [PDTS] `file_time_type` underspecified

Section: 6 [filesys.ts::fs.filesystem.synopsis] Status: TS Submitter: CH-7 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Must the `file_time_type` hold times before 1960 and after 2050?

Specify the requirements to unspecified-trivial-clock for `file_time_type`.

[2014-02-10, Daniel suggests wording]

[ 2014-02-11 Issaquah: (1)Implementation-defined. See wiki notes for rationale. (2) Leave other additions in place, but insert "the" before "resolution" (3) Strike "unspecified-" from "unspecified-trivial-type" in two places. Beman to provide wording for review next meeting. ]

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

1. Modify 6 [fs.filesystem.synopsis] as indicated:

   >         typedef chrono::time_point<unspecified-trivial-clock>  file_time_type;

   > unspecified-trivial-clock is an unspecified type provided by the implementation implementation-defined type that satisfies the TrivialClock requirements (C++11ISO 14882:2011 §20.12.3) and that is capable of representing and measuring file time values. Implementations should ensure that the resolution and range of `file_time_type` reflect the operating system dependent resolution and range of file time values.

### 2609(i). [filesys.ts] [PDTS] Unclear why range-based-for functions return different types

Section: 6 [filesys.ts::fs.filesystem.synopsis] Status: TS Submitter: FI-2 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

It is unclear why the range-for support functions (begin()/end()) for `directory_iterator` and `recursive_directory_iterator` return different types for `begin()` and `end()`, namely that `begin()` returns a reference to `const` and `end()` returns a value.

[2014-02-07: Beman Dawes provides comments from the Boost implementation:]

      //  begin() and end() are only used by a range-based for statement in the context of
      //  auto - thus the top-level const is stripped - so returning const is harmless and
      //  emphasizes begin() is just a pass through.

[2014-02-08: Daniel responds to Beman]

The difference in return types becomes relevant, when testing whether e.g. `directory_iterator` would satisfy the `Traversable` requirements as currently specified in N3763. Expressed in code form these requirements impose that the following assertion holds:

>       static_assert(std::is_same<
>           decltype(std::range_begin(std::declval<directory_iterator>())),
>           decltype(std::range_end(std::declval<directory_iterator>()))
>         >::value, "No Traversable type");

Both `directory_iterator` and `recursive_directory_iterator` won't satisfy this requirement currently.

[ 2014-02-11 Issaquah: Change begin() argument and return to pass-by-value. See wiki notes for rationale. Beman to provide wording for review next meeting. ]

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

1. Change 6 [fs.filesystem.synopsis]:

         class directory_iterator;

         // enable directory_iterator range-based for statements
         const directory_iterator& begin(const directory_iterator& iter) noexcept;
         directory_iterator end(const directory_iterator&) noexcept;

         class recursive_directory_iterator;

         // enable recursive_directory_iterator range-based for statements
         const recursive_directory_iterator& begin(const recursive_directory_iterator& iter) noexcept;
         recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;

2. Change 13.2 [directory_iterator.nonmembers]:

   These functions enable use of `directory_iterator` with C++11 range-based for statements.

       const directory_iterator& begin(constdirectory_iterator& iter) noexcept;

   > Returns: `iter`.

       directory_iterator end(const directory_iterator&) noexcept;

   > Returns: `directory_iterator()`.

3. Change 14.2 [rec.dir.itr.nonmembers]:

   These functions enable use of `recursive_directory_iterator` with C++11 range-based for statements.

       const recursive_directory_iterator& begin(constrecursive_directory_iterator& iter) noexcept;

   > Returns: `iter`.

       recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;

   > Returns: `recursive_directory_iterator()`.

### 2614(i). [filesys.ts] [PDTS] Incorrect postconditions for `path` copy/move constructor

Section: 8.4.1 [filesys.ts::path.construct] Status: TS Submitter: GB-7, CH-10 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

The postconditions for the copy/move constructor for path are shown as "empty()". This appears to have been incorrectly copied from the default ctor.

Remove the 'postconditions' clause from the copy/move ctor.

[2014-02-07, Beman Dawes suggests wording]

Proposed resolution:

Change 8.4.1 [path.construct]:

>           path(const path& p);
>           path(path&& p) noexcept;

> > Effects: Constructs an object of class `path` with `pathname` having the original value of `p.pathname`. In the second form, `p` is left in a valid but unspecified state.

> > Postconditions: empty().

### 2615(i). [filesys.ts] [PDTS] Missing behavior for characters with no representation

Section: 8.2.2 [filesys.ts::path.type.cvt], 8.4.1 [filesys.ts::path.construct] Status: TS Submitter: GB-8 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

No specification for characters with no representation. The example at the end of 8.4.1 refers to "for other native narrow encodings some characters may have no representation" - what happens in such cases?

Suggested action:

Add some definition of the behaviour for characters with no representation.

[2014-02-12 Applied LWG/SG-3 Issaquah wording tweaks to make characters referred to plural.]

[2014-02-08, Beman Dawes provides wording]

Proposed resolution:

Add a paragraph at the end of 8.2.2 [path.type.cvt]:

> If the encoding being converted to has no representation for source characters, the resulting converted characters, if any, are unspecified.

### 2616(i). [filesys.ts] [PDTS] Append behavior underspecified if target is empty

Section: 8.4.3 [filesys.ts::path.append] Status: TS Submitter: CH-11 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Is the added separator redundant in p1 /= p2, where p1 is empty? I.e. does the result start with a separator?

Suggested action:

Specify what behaviour is required.

[2014-02-07: Beman Dawes comments]

The second bullet item is supposed to deal with the `empty()` condition.

[2014-02-12 LWG/SG-3 Issaquah: The text is correct as written, however adding a note will clarify this and address the NB comment.]

Proposed resolution:

Change 8.4.3 [path.append]:

> Effects:

> > Appends `path::preferred_separator` to `pathname` unless:

> > - an added separator would be redundant, or

> > - would change an relative path to an absolute path [Note: An empty path is relative. — end note], or

> > - `p.empty()`, or

> > - `*p.native().cbegin()` is a directory separator.

> > Then appends `p.native()` to `pathname`.

### 2618(i). [filesys.ts] [PDTS] `is_absolute()` return clause confusing

Section: 8.4.10 [filesys.ts::path.query] Status: TS Submitter: FI-7 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`is_absolute` says: "Returns: true if the elements of `root_path()` uniquely identify a file system location, else false." The "uniquely identify a location" seems confusing in presence of symlinks.

Suggested action:

Clarify the returns clause so that there's no confusion about symlinks and 'location'.

[2014-02-10 Beman Dawes provides wording]

Proposed resolution:

Change 8.4.10 path query [path.query]:

>     bool is_absolute() const;

> > Returns: `true` if the elements of `root_path()` uniquely identify a file system location `pathname` contains an absolute path (4.1 [fs.def.absolute-path]) , else `false`.

> > [Example: `path("/").is_absolute()` is `true` for POSIX based operating systems, and `false` for Windows based operating systems. — end example]

### 2619(i). [filesys.ts] [PDTS] Consider using quoted manipulators

Section: 8.6.1 [filesys.ts::path.io] Status: TS Submitter: FI-8 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

"[Note: Pathnames containing spaces require special handling by the user to avoid truncation when read by the extractor. — end note]" sounds like a quoted manipulator as specified in the C++14 draft in [quoted.manip] would be useful.

Consider using quoted manipulators for stream insertion and extraction.

[2014-02-10, Daniel suggests wording]

[2014-02-12 Applied LWG/SG-3 Issaquah wording tweak to use ISO doc number for reference to C++14.]

Proposed resolution:

1. Change 8.6.1 [path.io] as indicated:

   >     template <class charT, class traits>
   >     basic_ostream<charT, traits>&
   >     operator<<(basic_ostream<charT, traits>& os, const path& p);

   > > Effects: `os << quoted(p.string<charT, traits>())`.

   > > [Note: Pathnames containing spaces require special handling by the user to avoid truncation when read by the extractor. — end note]

   > > [Note: The `quoted` function is described in ISO 14882:2014 §27.7.6. — end note]

   > > Returns: `os`

   >     template <class charT, class traits>
   >     basic_istream<charT, traits>&
   >     operator>>(basic_istream<charT, traits>& is, path& p);

   > > Effects:

   > > >     basic_string<charT, traits> tmp;
   > > >     is >> quoted(tmp);
   > > >     p = tmp;

### 2621(i). [filesys.ts] [PDTS] `directory_entry operator==` needs clarification

Section: 12.3 [filesys.ts::directory_entry.obs] Status: TS Submitter: GB-12 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Since `operator==` for `directory_entry` does not check status, it would be worth highlighting that `operator==` only checks that the paths match.

Suggested action:

Add: [Note: does not include status values — end note]

[2014-02-09, Beman Dawes suggest wording]

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted. Typo /no/not/ fixed per suggestion.]

Proposed resolution:

To 12.3 [directory_entry.obs] `operator==` add:

[Note: Status members do not participate in determining equality. — end note]

### 2622(i). [filesys.ts] [PDTS] `directory_iterator` underspecified

Section: 13.1 [filesys.ts::directory_iterator.members] Status: TS Submitter: CH-13 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

The behaviour of increment is underspecified: What happens if the implementation detects an endless loop (e.g. caused by links)? What happens with automounting and possible race conditions?

More information on this can be found here.

Suggested action:

Specify the required behaviour in these cases.

[2014-02-13 LWG/SG-3 Issaquah: STL will provide wording for next meeting for the endless loop case. The other cases are covered by existing wording in the front matter.]

[17 Jun 2014 At the request of the LWG, Beman provides wording for a note.]

Proposed resolution:

In 14 Class recursive_directory_iterator [class.rec.dir.itr], add:

> [Note: If the directory structure being iterated over contains cycles then the end iterator may be unreachable. --end note]

### 2624(i). [filesys.ts] [PDTS] Incorrect effects clause for path `copy`

Section: 15.3 [filesys.ts::fs.op.copy] Status: TS Submitter: GB-14 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Incorrect effects clause for path copy — the effect clause for `copy` [fs.op.copy] includes "`equivalent(f, t)`" — there is no `equivalent()` function defined for variables of this type (`file_status`)

Suggested action:

Replace with "`equivalent(from, to)`"

[2014-02-09, Beman Dawes suggests wording]

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

Change 15.3 [fs.op.copy]:

> Report an error as specified in Error reporting if:

> - `!exists(f)`.
> - `equivalent(f, t from, to)`.
> - `is_other(f) || is_other(t)`.
> - `is_directory(f) && is_regular_file(t)`.

### 2625(i). [filesys.ts] [PDTS] Copying equivalent paths effects not specified

Section: 15.4 [filesys.ts::fs.op.copy_file] Status: TS Submitter: CH-15 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Even if `to` and `from` are different paths, they may be equivalent.

Specify what happens if `(options & copy_options::overwrite_existing)` but `from` and `to` resolve to the same file.

[2014-02-09, Beman Dawes: Need advice on this issue:]

What do existing implentations do?

Possible resolutions:

1. Treat it as an error.
2. Once equivalence is determined, take no further action and return true? false?
3. Don't bother to detect a file overwriting itself. This will likely result in an error, anyhow.

[2014-02-13 LWG/SG-3 Issaquah: LWG/SG-3 decided to treat equivalence in this case as an error. Beman to provide wording.]

[2014-04-09 Beman provided wording as requested. The Effects were rewritten to increase clarity. Behavior remains unchanged except for treating equivalence as an error.]

[17 Jun 2014 Rapperswil LWG moves to Immediate. Jonathan Wakely will provide editorial changes to improve the presentation of bitmask values.]

Proposed resolution:

Change 15.4 [fs.op.copy_file]:

> Precondition: At most one constant from each `copy_options` option group ([enum.copy_options]) is present in `options`.

> Effects:

> > If `exists(to) &&` `!(options & (copy_options::skip_existing | copy_options::overwrite_existing | copy_options::update_existing))` report a file already exists error as specified in Error reporting (7).

> > If `!exists(to) || (options & copy_options::overwrite_existing) || ((options & copy_options::update_existing) && last_write_time(from) > last_write_time(to)) || !(options & (copy_options::skip_existing | copy_options::overwrite_existing | copy_options::update_existing))` copy the contents and attributes of the file `from` resolves to the file `to` resolves to.

> > Report a file already exists error as specified in Error reporting (7) if:

> > - `exists(to)` and `equivalent(from, to)`, or
> > - `exists(to)` and `(options & (copy_options::skip_existing | copy_options::overwrite_existing | copy_options::update_existing)) == copy_options::none.`

> > Otherwise copy the contents and attributes of the file `from` resolves to to the file `to` resolves to if:

> > - `!exists(to)`, or
> > - `exists(to)` and `(options & copy_options::overwrite_existing) != copy_options::none` , or
> > - `exists(to)` and `(options & copy_options::update_existing) != copy_options::none` and `from` is more recent than `to`, determined as if by use of the `last_write_time` function.

> > Otherwise no effects.

> Returns: `true` if the `from` file was copied, otherwise `false`. The signature with argument `ec` return `false` if an error occurs.

> Throws: As specified in Error reporting (7).

> Complexity: At most one direct or indirect invocation of `status(to)`.

### 2627(i). [filesys.ts] [PDTS] Return value of `uintmax_t` on error?

Section: 15.14 [filesys.ts::fs.op.file_size] Status: TS Submitter: FI-9 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

"The signature with argument `ec` returns `static_cast<uintmax_t>(-1)` if an error occurs.", one would expect that both signatures return that if an error occurs?

Clarify the Returns clause, and apply the same for every function that returns an `uintmax_t` where applicable.

[2014-02-13 LWG/SG-3 Issaquah:]

Discussion when around in circles for a while, until someone suggested the reference to 15.15 was wrong, and that the issue applied to the previous function in the WP, 15.14 File size [fs.op.file_size].

The NB Comment makes much more sense if it applies to file_size(), so the chair was directed to change the reference from 15.15 [fs.op.hard_lk_ct] to 15.14 [fs.op.file_size].

The intent that file_size() is only meaningful for regular_files. Beman to strike the the static_cast, changing it to "Otherwise an error is reported.".

Proposed resolution:

Change 15.14 [fs.op.file_size]:

    uintmax_t file_size(const path& p);
    uintmax_t file_size(const path& p, error_code& ec) noexcept;

> Returns: If `!exists(p) && || !is_regular_file(p)` an error is reported (7). Otherwise, the size in bytes of the file `p` resolves to, determined as if by the value of the POSIX `stat` structure member `st_size` obtained as if by POSIX `stat()`. Otherwise, `static_cast<uintmax_t>(-1)`. The signature with argument `ec` returns `static_cast<uintmax_t>(-1)` if an error occurs.

> Throws: As specified in Error reporting (7).

### 2629(i). [filesys.ts] [PDTS] Unclear semantics of `read_symlink` on error

Section: 15.27 [filesys.ts::fs.op.read_symlink] Status: TS Submitter: GB-16 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

Unclear semantics of read_symlink on error: 15.27 [fs.op.read_symlink] has: Returns: If p resolves to a symbolic link, a path object containing the contents of that symbolic link. Otherwise path(). and also [Note: It is an error if p does not resolve to a symbolic link. -- end note]

I do not believe path() can be a valid return for the overload not taking error_code.

Strike "Otherwise `path()`."

[2014-02-09, Beman Dawes provides wording]

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

Change 15.27 [fs.op.read_symlink]:

> Returns: If `p` resolves to a symbolic link, a `path` object containing the contents of that symbolic link. Otherwise `path()`. The signature with argument `ec` returns `path()` if an error occurs.

> Throws: As specified in Error reporting. [Note: It is an error if `p` does not resolve to a symbolic link. — end note]

### 2632(i). [filesys.ts] [PDTS] `system_complete()` example needs clarification

Section: 15.36 [filesys.ts::fs.op.system_complete] Status: TS Submitter: FI-10 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

"[Example: For POSIX based operating systems, `system_complete(p)` has the same semantics as `complete(p, current_path())`." What is this `complete` that is referred here?

Clarify the example.

[2014-02-10 Beman Dawes suggests wording]

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

Change 15.36 [fs.op.system_complete]:

> [Example: For POSIX based operating systems, `system_complete(p)` has the same semantics as `completeabsolute(p, current_path())`.

### 2633(i). [filesys.ts] [PDTS] `unique_path()` is a security vulnerability

Section: 15 [filesys.ts::fs.op.funcs] Status: TS Submitter: CH-19 Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`unique_path()` is a security vulnerability. As the Linux manual page for the similar function `tmpnam()` writes in the "BUGS" section: "Never use this function. Use `mkstemp`(3) or `tmpfile`(3) instead." `mkstemp()` and `tmpfile()` avoid the inherent race condition of `unique_path()` by returning an open file descriptor or `FILE*`.

[Beman Dawes comments: 10 Feb 2014:]

> There are two issues here:

> - Confusion over what unique_path does and how it is used. The function is misleadingly named. These issue have arisen in the past, but apparently not been fully corrected. The suggested fix is to (1) rename the function and (2) provide an example of how to use the function safely with fstreams or even C I/O. See below for proposed wording.
> - Very real security concerns. See 2654(i). The security concerns are probably best dealt with in the next File System TS, since a full-blown proposal is needed and will likely take several years to develop.

[ 2014-02-11 Issaquah: Strike the function. ]

[2014-02-12 The following Proposed resolution from CH-19 was moved here to avoid confusion with the final Proposed resolution wording from the WG/SG3.]

Remove this function. Consider providing a function `create_unique_directory()`. If it fits the scope of the proposed TS, consider providing functions `create_unique_file()` that returns `ifstream`, `ofstream` and `iofstream`.

[ 2014-02-12 The following Proposed wording was moved here to avoid confusion with the final Proposed resolution wording from the WG/SG3. ]

[2014-02-10 Beman Dawes]

Previous resolution from Beman [SUPERSEDED]:

> Change 15.38 [fs.op.unique_path]:

>         path unique_pathgenerate_random_filename(const path& model="%%%%-%%%%-%%%%-%%%%");
>         path unique_pathgenerate_random_filename(const path& model, error_code& ec);

> > The `unique_path generate_random_filename` function generates a name suitable for temporary files, including directories. The name is based on a model that uses the percent sign character to specify replacement by a random hexadecimal digit.

> > [Note: The more bits of randomness in the generated name, the less likelihood of prior existence or being guessed. Each replacement hexadecimal digit in the model adds four bits of randomness. The default model thus provides 64 bits of randomness. --end note]

> > Returns: A path identical to `model`, except that each occurrence of the percent sign character is replaced by a random hexadecimal digit character in the range 0-9, a-f. The signature with argument `ec` returns `path()` if an error occurs.

> > Throws: As specified in Error reporting.

> > Remarks: Implementations are encouraged to obtain the required randomness via a cryptographically secure pseudo-random number generator, such as one provided by the operating system. [Note: Such generators may block until sufficient entropy develops. --end note]

> > Replace this example with one that opens a std::ofstream:

> > [Example:

> > >             cout << unique_pathgenerate_random_filename("test-%%%%%%%%%%%.txt") << endl;

> > Typical output would be `"test-0db7f2bf57a.txt"`. Because 11 hexadecimal output characters are specified, 44 bits of randomness are supplied. -- end example]

Proposed resolution:

Remove the two `unique_path` function signatures from 6 [fs.filesystem.synopsis].

Remove 15.38 [fs.op.unique_path] in its entirety.

[This removes all references the function from the working draft.]

### 2634(i). [filesys.ts] [PDTS] `enum class directory_options` has no summary

Section: 6 [filesys.ts::fs.filesystem.synopsis] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`enum class directory_options` has no summary.

Proposed resolution:

Change 6 [fs.filesystem.synopsis]:

>           enum class directory_options;
>           {
>           none,
>           follow_directory_symlink,
>           skip_permission_denied
>           };

Add the following sub-section:

> ### 10.4 Enum class `directory_options` [enum.directory_options]

> The `enum class` type `directory_options` is a bitmask type (C++11 §17.5.2.1.3) that specifies bitmask constants used to identify directory traversal options.

> Name | Value | Meaning
> `none` | `0` | (Default) Skip directory symlinks, permission denied is error.
> `follow_directory_symlink` | `1` | Follow rather than skip directory symlinks.
> `skip_permission_denied` | `2` | Skip directories that would otherwise result in permission denied errors.

### 2635(i). [filesys.ts] [PDTS] `directory_options::skip_permission_denied` is not used

Section: 6 [filesys.ts::fs.filesystem.synopsis] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`directory_options::skip_permission_denied` is not used.

[2014-04-13 Beman: `skip_permissions_denied` not being used is a symptom of a more serious problem; two `directory_itorator` constructors are missing `directory_options` arguments and a description of how they are used. Proposed wording provided.]

[17 Jun 2014 LWG requests two signatures rather than one with default argument. Beman updates wording.]

Proposed resolution:

Change 13 [class.directory_iterator]:

>           directory_iterator() noexcept;
>           explicit directory_iterator(const path& p);
>           directory_iterator(const path& p, directory_options options);
>           directory_iterator(const path& p, error_code& ec) noexcept;
>           directory_iterator(const path& p,
>           directory_options options, error_code& ec) noexcept;
>           directory_iterator(const directory_iterator&) = default;
>           directory_iterator(directory_iterator&&) = default;
>           ~directory_iterator();

Change 13.1 directory_iterator members [directory_iterator.members]:

>           explicit directory_iterator(const path& p);
>           directory_iterator(const path& p, directory_options options);
>           directory_iterator(const path& p, error_code& ec) noexcept;
>           directory_iterator(const path& p,
>           directory_options options, error_code& ec) noexcept;

> Effects: For the directory that `p` resolves to, constructs an iterator for the first element in a sequence of `directory_entry` elements representing the files in the directory, if any; otherwise the end iterator.

> However, if `options & directory_options::skip_permissions_denied != directory_options::none` and construction encounters an error indicating that permission to access `p` is denied, constructs the end iterator and does not report an error.

Change 14 Class `recursive_directory_iterator` [class.rec.dir.itr] :

>           explicit recursive_directory_iterator(const path& p,
>           directory_options options = directory_options::none);
>           recursive_directory_iterator(const path& p, directory_options options);
>           recursive_directory_iterator(const path& p,
>           directory_options options, error_code& ec) noexcept;
>           recursive_directory_iterator(const path& p, error_code& ec) noexcept;

Change 14.1 recursive_directory_iterator members [rec.dir.itr.members]:

>           explicit recursive_directory_iterator(const path& p,
>           directory_options options = directory_options::none);
>           recursive_directory_iterator(const path& p, directory_options options);
>           recursive_directory_iterator(const path& p,
>           directory_options options, error_code& ec) noexcept;
>           recursive_directory_iterator(const path& p, error_code& ec) noexcept;

> Effects: Constructs a iterator representing the first entry in the directory `p` resolves to, if any; otherwise, the end iterator.

> However, if `options & directory_options::skip_permissions_denied != directory_options::none` and construction encounters an error indicating that permission to access `p` is denied, constructs the end iterator and does not report an error.

Change 14.1 recursive_directory_iterator members [rec.dir.itr.members]:

>           recursive_directory_iterator& operator++();
>           recursive_directory_iterator& increment(error_code& ec);

> Requires: `*this != recursive_directory_iterator()`.

> Effects: As specified by C++11 § 24.1.1 Input iterators, except that:

> - If there are no more entries at this depth, then if `depth()!= 0` iteration over the parent directory resumes; otherwise `*this = recursive_directory_iterator()` .
> - Otherwise if `recursion_pending() && is_directory(this->status()) && (!is_symlink(this->symlink_status()) || (options() & directory_options::follow_directory_symlink) != 0 directory_options::none)` then either directory `(*this)->path()` is recursively iterated into or, if `options() & directory_options::skip_permissions_denied != directory_options::none` and an error occurs indicating that permission to access directory `(*this)->path()` is denied, then directory `(*this)->path()` is treated as an empty directory and no error is reported .

### 2636(i). [filesys.ts] [PDTS] `copy_options::copy_symlinks` is not used

Section: 10.2 [filesys.ts::enum.copy_options] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [filesys.ts::enum.copy_options].

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Discussion:

Addresses: filesys.ts

`copy_options::copy_symlinks` is not used (should test it before calling `copy_symlinks` in `copy`).

[20 May 2014 Beman Dawes provides proposed wording.]

Proposed resolution:

Change 15.3 Copy [fs.op.copy]:

> > If `is_symlink(f)`, then:

> > - If `options & copy_options::skip_symlinks`, then return. Missing != copy_options::none fixed by issue 59
> > - Otherwise if `!exists(t) && (options & copy_options::copy_symlinks) != copy_options::none` , then `copy_symlink(from, to, options)` .
> > - Otherwise report an error as specified in Error reporting (7).

### 2637(i). [filesys.ts] [PDTS] All functions with `error_code` arguments should be `noexcept`

Section: 15.2 [filesys.ts::fs.op.canonical], 15.11 [filesys.ts::fs.op.current_path], 15.27 [filesys.ts::fs.op.read_symlink], 15.36 [filesys.ts::fs.op.system_complete], 15.37 [filesys.ts::fs.op.temp_dir_path], 99 [filesys.ts::fs.op.unique_path], 8.4 [filesys.ts::path.member] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

all functions with `error_code` arguments should be `noexcept` (see `canonical`, `current_path`, `read_symlink`, `system_complete`, `temp_directory_path`, `unique_path`, plus member functions).

[2014-02-03: Stephan T. Lavavej comments:]

The declaration and definition of "`recursive_directory_iterator& increment(error_code& ec);`" should almost certainly be marked noexcept.

[2014-02-08: Daniel comments]

All functions that return a `path` value such as `canonical`, `current_path`, `read_symlink`, `system_complete`, `temp_directory_path`, `unique_path`, or any member function returning a `path` value or `basic_string` value might legally throw an exception, if the allocator of the underlying string complains about insufficient memory. This is an anti-pattern for `noexcept`, because the exception-specification includes the return statement of a function and given the fact that the language currently does declare RVO as an optional action, we cannot rely on it.

The Standard is currently very careful not to specify functions as `noexcept`, if this case can happen, see e.g. `std::locale::name()`. To the contrary, enforcing them to be `noexcept`, would cause a similar problem as the unconditional `noexcept` specifier of the value-returning `kill_dependency` as denoted by LWG 2236(i).

Basically this requirement conflicts with the section "Adopted Guidelines", second bullet of N3279.

[Beman Dawes 2014-02-27]

Issues 2637(i), 2638(i), 2641(i), and 2649(i) are concerned with signatures which should or should not be `noexcept`. I will provide unified proposed wording for these issues, possibly in a separate paper.

[21 May 2014 Beman Dawes reviewed all functions with `error_code&` arguments, and provided proposed wording for the one case found where the above guidelines were not met. This was the case previously identified by Stephan T. Lavavej. ]

Proposed resolution:

Change 14 Class recursive_directory_iterator [class.rec.dir.itr]:

>     recursive_directory_iterator& increment(error_code& ec) noexcept;

Change 14.1 recursive_directory_iterator members [rec.dir.itr.members]:

>     recursive_directory_iterator& increment(error_code& ec) noexcept;

### 2640(i). [filesys.ts] [PDTS] `class directory_entry` should retain `operator const path&()` from V2

Section: 12 [filesys.ts::class.directory_entry] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`class directory_entry` should retain `operator const path&()` from V2.

[2014-05-19 Beman Dawes supplied proposed wording.]

[2014-06-02 Beman Dawes provides rationale:]

> This conversion operator was removed when `status()` and `symlink_status()` were added during the transition from V2 to V3 as it seemed a bit unusual to have a conversion operator for one of the several values held. Users complained as they had found the automatic conversion convenient in the most common `directory_entry` use case.

[17 Jun 2014 Rapperswil LWG discusses in depth, accepts proposed wording.]

Proposed resolution:

Change 12 Class directory_entry [class.directory_entry]:

>     // observers
>     const path&  path() const noexcept;
>     operator const path&() const noexcept;
>     file_status  status() const;
>     file_status  status(error_code& ec) const noexcept;
>     file_status  symlink_status() const;
>     file_status  symlink_status(error_code& ec) const noexcept;

Change 12.3 directory_entry observers [directory_entry.obs]:

>     const path& path() const noexcept;
>     operator const path&() const noexcept;

> > Returns: `m_path`

### 2641(i). [filesys.ts] [PDTS] `directory_iterator`, `recursive_directory_iterator`, move construct/assign should be `noexcept`

Section: 13 [filesys.ts::class.directory_iterator], 14 [filesys.ts::class.rec.dir.itr] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`class directory_iterator` move construct/assign should be `noexcept`.
`class recursive_directory_iterator` move construct/assign should be `noexcept`.

[Beman Dawes 2014-02-27]

Issues 2637(i), 2638(i), 2641(i), and 2649(i) are concerned with signatures which should or should not be `noexcept`. I will provide unified proposed wording for these issues, possibly in a separate paper.

[Daniel Krügler 2014-02-28]

    directory_iterator begin(directory_iterator iter) noexcept;
    directory_iterator end(const directory_iterator&) noexcept;

are noexcept, but we have no guarantee that at least the move-constructor is noexcept:

    directory_iterator(const directory_iterator&) = default;
    directory_iterator(directory_iterator&&) = default;

This means that either the above noexcept specifications are not warranted or that at least the move-constructor of `directory_iterator` is required to be noexcept.

The same applies to `recursive_directory_iterator`.

[21 May 2014 Beman Dawes provided proposed resolution wording.]

[18 Jun 2014 "= default" removed per LWG discussion]

Proposed resolution:

Change 13 Class directory_iterator [class.directory_iterator]:

>     directory_iterator(const directory_iterator& rhs) = default;
>     directory_iterator(directory_iterator&& rhs) noexcept = default;
>     ...
>     directory_iterator& operator=(const directory_iterator& rhs) = default;
>     directory_iterator& operator=(directory_iterator&& rhs) noexcept = default;

To 13.1 directory_iterator members [directory_iterator.members] add:

>     directory_iterator(const directory_iterator& rhs);
>     directory_iterator(directory_iterator&& rhs) noexcept;

> > Effects: Constructs an object of class `directory_iterator`.

> > Postconditions: `*this` has the original value of `rhs`.

>     directory_iterator& operator=(const directory_iterator& rhs);
>     directory_iterator& operator=(directory_iterator&& rhs) noexcept;

> > Effects: If `*this` and `rhs` are the same object, the member has no effect.

> > Postconditions: `*this` has the original value of `rhs`.

> > Returns: `*this`.

Change 14 Class recursive_directory_iterator [class.rec.dir.itr]:

>     recursive_directory_iterator(const recursive_directory_iterator& rhs) = default;
>     recursive_directory_iterator(recursive_directory_iterator&& rhs) noexcept = default;
>     ...
>     recursive_directory_iterator& operator=(const recursive_directory_iterator& rhs) = default;
>     recursive_directory_iterator& operator=(recursive_directory_iterator&& rhs) noexcept = default;

To 14.1 recursive_directory_iterator members [rec.dir.itr.members] add:

>     recursive_directory_iterator(const recursive_directory_iterator& rhs);

> > Effects: Constructs an object of class `recursive_directory_iterator`.

> > Postconditions: `this->options() == rhs.options() && this->depth() == rhs.depth() && this->recursion_pending() == rhs.recursion_pending()`.

>     recursive_directory_iterator(recursive_directory_iterator&& rhs) noexcept;

> > Effects: Constructs an object of class `recursive_directory_iterator`.

> > Postconditions: `this->options()`, `this->depth()`, and `this->recursion_pending()` return the values that `rhs.options()`, `rhs.depth()`, and `rhs.recursion_pending()`, respectively, had before the function call.

>     recursive_directory_iterator& operator=(const recursive_directory_iterator& rhs);

> > Effects: If `*this` and `rhs` are the same object, the member has no effect.

> > Postconditions: `this->options() == rhs.options() && this->depth() == rhs.depth() && this->recursion_pending() == rhs.recursion_pending()` .

> > Returns: `*this`.

>     recursive_directory_iterator& operator=(recursive_directory_iterator&& rhs) noexcept;

> > Effects: If `*this` and `rhs` are the same object, the member has no effect.

> > Postconditions: `this->options()`, `this->depth()`, and `this->recursion_pending()` return the values that `rhs.options()`, `rhs.depth()`, and `rhs.recursion_pending()`, respectively, had before the function call.

> > Returns: `*this`.

### 2644(i). [filesys.ts] [PDTS] enum classes `copy_options` and `perms` should be bitmask types

Section: 10.2 [filesys.ts::enum.copy_options], 10.3 [filesys.ts::enum.perms] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

enum classes `copy_options` and `perms` should be bitmask types.

[2014-02-08 Daniel comments and suggests wording]

Since both enum types contain enumeration values that denote the value zero, they are currently not allowed to be described as bitmask types. This issue depends on LWG issue 1450, which — if it would be accepted — would allow for applying this concept to these enumeration types.

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

This wording is relative to SG3 working draft and assumes that LWG 1450 has been resolved.

1. Change [enum.copy_options] as indicated:

   > This enumerationThe `enum class` type `copy_options` is a bitmask type (C++11 §17.5.2.1.3) that specifies bitmask constants used to control the semantics of copy operations. The constants are specified in option groups. […]

2. Change [enum.perms] as indicated:

   > This enumerationThe `enum class` type `perms` is a bitmask type (C++11 §17.5.2.1.3) that specifies bitmask constants used to identify file permissions.

### 2645(i). [filesys.ts] [PDTS] `create_directory` should refer to `perms::all` instead of Posix `S_IRWXU|S_IRWXG|S_IRWXO`

Section: 15.7 [filesys.ts::fs.op.create_directory] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`create_directory` should refer to `perms::all` instead of the Posix `S_IRWXU|S_IRWXG|S_IRWXO`.

[2014-02-28 Beman provided proposed wording.]

Proposed resolution:

Effects: Establishes the postcondition by attempting to create the directory `p` resolves to, as if by POSIX `mkdir()` with a second argument of S_IRWXU|S_IRWXG|S_IRWXO `static_cast<int>(perms::all)`. Creation failure because `p` resolves to an existing directory shall not be treated as an error.

### 2647(i). [filesys.ts] [PDTS] `last_write_time()` uses ill-formed cast

Section: 15.25 [filesys.ts::fs.op.last_write_time] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

In `last_write_time`, `static_cast<file_time_type>(-1)` is ill formed, (possibly should be `chrono<unspecified-trivial-clock>::from_time_t(-1))`.

[2014-02-08 Daniel comments and provides wording]

I agree that the current wording needs to be fixed, but it is unclear to me whether invoking `from_time_t` (which is only guaranteed to exist for `std::chrono::system_clock`) with a value of `time_t(-1)` is a well-defined operation. Reason is that the C Standard makes a difference between a calendar time and the value `time_t(-1)` as an error indication. But the specification of `system_clock::from_time_t` only says "A `time_point` object that represents the same point in time as `t` […]" and it is not clear whether `time_t(-1)` can be considered as a "point in time". Instead of relying on such a potentially subtle semantics of the conversion result of `time_t(-1)` to `std::chrono::system_clock::time_point` with a further conversion to `file_time_type` (which in theory could led to overflow or underflow and thus another potential source of undefined behaviour), I suggest to change the current error value of `last_write_time` to one of the well-define limiting values of `file_time_type`, e.g. `file_time_type::min()`.

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

This wording is relative to SG3 working draft.

1. Change the last write time prototype specification, 15.25 [fs.op.last_write_time], as indicated:

   >     file_time_type last_write_time(const path& p);
   >     file_time_type last_write_time(const path& p, error_code& ec) noexcept;

   > > Returns: The time of last data modification of `p`, determined as if by the value of the POSIX `stat` structure member `st_mtime` obtained as if by POSIX `stat()`. The signature with argument `ec` returns `static_cast<file_time_type>(-1)file_time_type::min()` if an error occurs.

### 2648(i). [filesys.ts] [PDTS] `path::template<class charT>string()` conversion rules

Section: 8.4.6 [filesys.ts::path.native.obs] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`path::template<class charT>string()` should promise to convert by the same rules as `u16string` for `string<char16_t>`, etc. and one-for-one otherwise.

What if `charT` is `signed char` (or even `float`)? I don't see where that choice is disallowed, so we should either disallow it or make it be something that is quasi-sensible.

[2014-02-08 Daniel comments]

There are two relevant places in the wording that seem to clarify what is required here:

1. Most importantly, 5 [fs.req] says:

   > Throughout this Technical Specification, `char`, `wchar_t`, `char16_t`, and `char32_t` are collectively called encoded character types.

   > Template parameters named `charT` shall be one of the encoded character types.

   > […] [Note: Use of an encoded character type implies an associated encoding. Since `signed char` and `unsigned char` have no implied encoding, they are not included as permitted types. — end note]

2. For both the mentioned `string` function template and the specific non-template functions (such as `u16string()`) the same `Remarks` element exists, that refers to character conversion:

   > Conversion, if any, is performed as specified by 8.2 [path.cvt].

The first quote excludes arbitrary types such as `signed char` or `float` as feasible template arguments. The second quote makes clear that both the function template and the non-template functions do normatively refer to the same wording in 8.2 [path.cvt].

Based on this interpretation of the issue discussion I recommend resolving it as NAD.

In addition to that recommendation I recommend to rename the current usage of the symbol `charT` in this technical specification, because 5 [fs.req] assigns a very special meaning to the constraints on `charT` types that does not exist to this extend in the rest of the standard library. A similar approach is used in Clause 22, where a special symbol `C` is used for constrained character types (C++11 §22.3.1.1.1 [locale.category]):

> A template formal parameter with name `C` represents the set of types containing `char`, `wchar_t`, and any other implementation-defined character types that satisfy the requirements for a character on which any of the `iostream` components can be instantiated.

[2014-02-28 Beman provides proposed resolution wording]

[18 Jun 2014 Beman changes proposed name from C to EcharT in response to tentative vote concern that C was insuficiently informative as a name.]

Proposed resolution:

In the entire Working Paper, except in the synopsis for path inserter and extractor and 8.6.1 path inserter and extractor [path.io], change each instance of "`charT`" to "`EcharT`".

For example, in 5 [fs.req]:

> Template parameters named `charT` `EcharT` shall be one of the encoded character types.

### 2649(i). [filesys.ts] [PDTS] `path` and `directory_entry` move ctors should not be `noexcept`

Section: 8.4.1 [filesys.ts::path.construct], 12 [filesys.ts::class.directory_entry] Status: TS Submitter: Stephan T. Lavavej Opened: 2014-02-03 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [filesys.ts::path.construct].

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Discussion:

Addresses: filesys.ts

`path`'s move ctor is marked `noexcept`, but it contains a `basic_string`. Similarly, `directory_entry`'s move ctor is marked `noexcept`, but it contains a `path`. This is affected by LWG 2319 "`basic_string`'s move constructor should not be `noexcept`".

[Beman Dawes 2014-02-27]

Issues 2637(i), 2638(i), 2641(i), and 2649(i) are concerned with signatures which should or should not be `noexcept`. I will provide unified proposed wording for these issues, possibly in a separate paper.

[21 May 2014 Beman Dawes provides wording. See 2319(i) for rationale. ]

[19 Jun 2014 LWG revises P/R to stay consistent with LWG issues.]

Proposed resolution:

Change 8 Class path [class.path]:

>     path() noexcept;

Change 8.4.1 path constructors [path.construct]:

>     path() noexcept;

Change 12 Class directory_entry [class.directory_entry]:

>     directory_entry() noexcept = default;

### 2650(i). [filesys.ts] [PDTS] `path::compare(const string& s)` wrong argument type

Section: 8 [filesys.ts::class.path] Status: TS Submitter: Stephan T. Lavavej Opened: 2014-02-03 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

`path` has "`int compare(const string& s) const;`" but it should almost certainly take `const string_type&`, since the third overload takes `const value_type*`.

[2014-02-08 Daniel comments and provides wording]

This issue is the same as 2643(i) and resolves that issue as well.

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

This wording is relative to SG3 working draft.

1. Change class `path` synopsis, 8 [class.path], as indicated:

   >     // compare
   >     int compare(const path& p) const noexcept;
   >     int compare(const string_type& s) const;
   >     int compare(const value_type* s) const;

2. Change `path compare` prototype specification, 8.4.8 [path.compare], as indicated:

   >     int compare(const string_type& s) const;

### 2652(i). [filesys.ts] [PDTS] Better to avoid deriving from `std::iterator`

Section: 13 [filesys.ts::class.directory_iterator], 14 [filesys.ts::class.rec.dir.itr] Status: TS Submitter: Stephan T. Lavavej Opened: 2014-02-03 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [filesys.ts::class.directory_iterator].

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Discussion:

Addresses: filesys.ts

Although the Standard has made this mistake almost a dozen times, I recommend not depicting `directory_iterator` and `recursive_directory_iterator` as deriving from `std::iterator`, since that's a binding requirement on implementations. Instead they should be depicted as having the appropriate typedefs, and leave it up to implementers to decide how to provide them. (The difference is observable to users with `is_base_of`, not that they should be asking that question.)

[2014-02-08 Daniel comments and provides wording]

This issue is basically similar to the kind of solution that had been used to remove the requirement to derive from `unary_function` and friends as described by N3198 and I'm strongly in favour to follow that spirit here as well. I'd like to add that basically all "newer" iterator types (such as the `regex` related iterator don't derive from `std::iterator` either.

Another reason to change the current specification is the fact that it currently says that the member types `pointer` and `reference` would be determined to `value_type*` and `value_type&` (that is mutable pointers and references), which conflicts with the specification of the return types of the following members:

>     const directory_entry& operator*() const;
>     const directory_entry* operator->() const;

The proposed fording fixes this by correcting these `typedef` to corresponding `const` access.

The very same objections had been expressed by issue 2651(i) and the below given wording resolves this issue as well.

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

This wording is relative to SG3 working draft.

1. Change class `directory_iterator` synopsis, [class.directory_iterator], as indicated:

   >     namespace std { namespace tbd { namespace filesystem {

   >           class directory_iterator :
   >             public iterator<input_iterator_tag, directory_entry>
   >           {
   >           public:
   >             typedef directory_entry        value_type;
   >             typedef ptrdiff_t              difference_type;
   >             typedef const directory_entry* pointer;
   >             typedef const directory_entry& reference;
   >             typedef input_iterator_tag     iterator_category;

   >             // member functions
   >             […]
   >           };

   >     } } }  // namespaces std::tbd::filesystem

2. Change class `recursive_directory_iterator` synopsis, [class.rec.dir.itr], as indicated:

   >     namespace std { namespace tbd { namespace filesystem {

   >           class recursive_directory_iterator :
   >             public iterator<input_iterator_tag, directory_entry>
   >           {
   >           public:
   >             typedef directory_entry        value_type;
   >             typedef ptrdiff_t              difference_type;
   >             typedef const directory_entry* pointer;
   >             typedef const directory_entry& reference;
   >             typedef input_iterator_tag     iterator_category;

   >             // constructors and destructor
   >             […]

   >             // modifiers
   >             recursive_directory_iterator& operator=(const recursive_directory_iterator&) = default;
   >             recursive_directory_iterator& operator=(recursive_directory_iterator&&) = default;

   >             const directory_entry& operator*() const;
   >             const directory_entry* operator->() const;

   >             recursive_directory_iterator& operator++();
   >             recursive_directory_iterator& increment(error_code& ec);

   >             […]
   >           };

   >     } } }  // namespaces std::tbd::filesystem

### 2653(i). [filesys.ts] [PDTS] `directory_entry` multithreading concerns

Section: 12 [filesys.ts::class.directory_entry] Status: TS Submitter: Stephan T. Lavavej Opened: 2014-02-03 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

12 [class.directory_entry] depicts `directory_entry` as having `mutable m_status` and `m_symlink_status` data members. This is problematic because of C++11's multithreading guarantees, which say that `const` member functions are simultaneously callable. As a result, `mutable` data members must be protected with synchronization, which was probably not considered during this design. The complexity and expense may be acceptable in `directory_entry` (since it can save filesystem queries, apparently) but it would be very very nice to have a note about this.

[2014-02-13 LWG/SG-3 discussed in Issaquah: Beman and STL will work together to better understand the concerns, and to formulate a solution. This is not a blocking issue for finishing the TS, but is a blocking issue for merging into the IS]

[2014-04-17 Beman provides proposed wording]

[2014-06-19 Rapperswil LWG decides to remove mutable private members. Beman provides wording.]

Proposed resolution:

Change 12 [class.directory_entry] as indicated:

        namespace std { namespace experimental { namespace filesystem { inline namespace v1 {

        class directory_entry
        {
        public:

        // constructors and destructor
        directory_entry() = default;
        directory_entry(const directory_entry&) = default;
        directory_entry(directory_entry&&) noexcept = default;
        explicit directory_entry(const path& p);
        explicit directory_entry(const path& p, file_status st=file_status(),
          file_status symlink_st=file_status());
        ~directory_entry();

        // modifiers
        directory_entry& operator=(const directory_entry&) = default;
        directory_entry& operator=(directory_entry&&) noexcept = default;
        void assign(const path& p);
        void assign(const path& p, file_status st=file_status(),
          file_status symlink_st=file_status());
        void replace_filename(const path& p);
        void replace_filename(const path& p, file_status st=file_status(),
          file_status symlink_st=file_status());

        // observers
        const path&  path() const noexcept;
        file_status  status() const;
        file_status  status(error_code& ec) const noexcept;
        file_status  symlink_status() const;
        file_status  symlink_status(error_code& ec) const noexcept;

        bool operator< (const directory_entry& rhs) const noexcept;
        bool operator==(const directory_entry& rhs) const noexcept;
        bool operator!=(const directory_entry& rhs) const noexcept;
        bool operator<=(const directory_entry& rhs) const noexcept;
        bool operator> (const directory_entry& rhs) const noexcept;
        bool operator>=(const directory_entry& rhs) const noexcept;
        private:
        path                 m_path;           // for exposition only
        mutable file_status  m_status;         // for exposition only; stat()-like
        mutable file_status  m_symlink_status; // for exposition only; lstat()-like
        };

        } } } }  // namespaces std::experimental::filesystem::v1

A `directory_entry` object stores a `path` object, a `file_status` object for non-symbolic link status, and a `file_status` object for symbolic link status. The `file_status` objects act as value caches.

> [Note: Because `status()`on a pathname may be a relatively expensive operation, some operating systems provide status information as a byproduct of directory iteration. Caching such status information can result in significant time savings. Cached and non-cached results may differ in the presence of file system races. —end note]

### 12.1 `directory_entry` constructors [directory_entry.cons]

Add the description of this constructor in its entirety:

    explicit directory_entry(const path& p);

> Effects: Constructs an object of type `directory_entry`

> Postcondition: `path() == p`.

    explicit directory_entry(const path& p, file_status st=file_status(),
        file_status symlink_st=file_status());

> Strike the description

### 12.2 `directory_entry` modifiers [directory_entry.mods]

Add the description of this function in its entirety:

    void assign(const path& p);

> Postcondition: `path() == p`.

    void assign(const path& p, file_status st=file_status(),
      file_status symlink_st=file_status());

> Strike the description

Add the description of this function in its entirety:

    void replace_filename(const path& p);

> Postcondition: `path() == x.parent_path() / p` where `x` is the value of `path()` before the function is called.

    void replace_filename(const path& p, file_status st=file_status(),
      file_status symlink_st=file_status());

> Strike the description

### 12.3 `directory_entry` observers [directory_entry.obs]

    const path& path() const noexcept;

> Returns: `m_path`

    file_status status() const;
    file_status status(error_code& ec) const noexcept;

> Effects: As if,

> >     if (!status_known(m_status))
> >     {
> >       if (status_known(m_symlink_status) && !is_symlink(m_symlink_status))
> >         { m_status = m_symlink_status; }
> >       else { m_status = status(m_path[, ec]); }
> >     }

> Returns: `m_status status(path()[, ec])` .

> Throws: As specified in Error reporting (7).

    file_status  symlink_status() const;
    file_status  symlink_status(error_code& ec) const noexcept;

> Effects: As if,

> >             if (!status_known(m_symlink_status))
> >             {
> >             m_symlink_status = symlink_status(m_path[, ec]);
> >             }

> Returns: `m_symlink_status symlink_status(path()[, ec])` .

> Throws: As specified in Error reporting (7).

    bool operator==(const directory_entry& rhs) const noexcept;

> Returns: `m_path == rhs.m_path`.

> [Note: Status members do not participate in determining equality. — end note]

    bool operator!=(const directory_entry& rhs) const noexcept;

> Returns: `m_path != rhs.m_path`.

    bool operator< (const directory_entry& rhs) const noexcept;

> Returns: `m_path < rhs.m_path`.

    bool operator<=(const directory_entry& rhs) const noexcept;

> Returns: `m_path <= rhs.m_path`.

    bool operator> (const directory_entry& rhs) const noexcept;

> Returns: `m_path > rhs.m_path`.

    bool operator>=(const directory_entry& rhs) const noexcept;

> Returns: `m_path >= rhs.m_path`.

### 2655(i). [filesys.ts] [PDTS] Clarify Error reporting

Section: 7 [filesys.ts::fs.err.report] Status: TS Submitter: Beman Dawes Opened: 2014-01-20 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [filesys.ts::fs.err.report].

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Discussion:

Addresses: filesys.ts

The proposed change below was suggested in Issaquah as part of the resolution of issue 13 to clarify the Error reporting section. LWG/SG3 liked the change, but since issue 13 was NAD requested that a separate issue be opened.

[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]

Proposed resolution:

Change 7 [fs.err.report]:

> Functions not having an argument of type `error_code&` report errors as follows, unless otherwise specified:

> - When a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, an exception of type `filesystem_error` shall be thrown. For functions with a single path argument, that argument shall be passed to the `filesystem_error` constructor with a single path argument. For functions with two path arguments, the first of these arguments shall be passed to the `filesystem_error` constructor as the `path1` argument, and the second shall be passed as the `path2` argument. The `filesystem_error` constructor's `error_code` argument is set as appropriate for the specific operating system dependent error.
> - Failure to allocate storage is reported by throwing an exception as described in C++11 § 17.6.4.10.
> - Destructors throw nothing.

> Functions having an argument of type `error_code&` report errors as follows, unless otherwise specified:

> - If a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, the `error_code&` argument is set as appropriate for the specific operating system dependent error. Otherwise, `clear()` is called on the `error_code&` argument.

### 2656(i). [filesys.ts] [PDTS] Feature test macro for TS version

Section: 5 [filesys.ts::fs.req] Status: TS Submitter: Clark Nelson Opened: 2014-02-10 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [filesys.ts::fs.req].

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Discussion:

Addresses: filesys.ts

SD-6: SG10 Feature Test Recommendations recommends that each library header provide feature test macros. For Technical Specifications, the TS itself should specify recommended names for the feature test macros.

[Beman Dawes 2014-02-28 provided the proposed resolution. Thanks to Vicente J. Botet Escriba, Richard Smith, and Clark Nelson for suggestions and corrections.]

Proposed resolution:

Add a new sub-section:

> 5.2 Feature test macros [fs.req.macros]

> This macro allows users to determine which version of this Technical Specification is supported by header `<experimental/filesystem>`.

> Header `<experimental/filesystem>` shall supply the following macro definition:

> >     #define __cpp_lib_experimental_filesystem     201406

> [Note: The value of macro `__cpp_lib_experimental_filesystem` is yyyymm where yyyy is the year and mm the month when the version of the Technical Specification was completed. — end note]

### 2657(i). [filesys.ts] [PDTS] Inappropriate use of "No diagnostic is required"

Section: 2.1 [filesys.ts::fs.conform.9945] Status: TS Submitter: LWG/SG-3 Opened: 2014-02-13 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

§ 2.1 [fs.conform.9945] specifes "No diagnostic is required" for races. That prescription is used by the standard for the core language; it is not appropriate in library specifications. LWG/SG-3 wishes this be changed to undefined behavior.

[Beman comments: The replacment wording is similar to that used by C++14 17.6.4.10]

Proposed resolution:

Change the following paragraph from § 2.1 [fs.conform.9945]

> The behavior of functions described in this Technical Specification may differ from their specification in the presence of file system races ([fs.def.race]). No diagnostic is required.

> Behavior is undefined if calls to functions provided by this Technical Specification introduce a file system race (4.6 [fs.def.race]).

Insert a new sub-section head before the modified paragraph:

> 2.1 File system race behavior [fs.race.behavior]

### 2658(i). [filesys.ts] [PDTS] POSIX `utime()` is obsolescent

Section: 15.25 [filesys.ts::fs.op.last_write_time] Status: TS Submitter: LWG/SG-3 Opened: 2014-02-13 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

POSIX now says: "Since the `utimbuf` structure only contains `time_t` variables and is not accurate to fractions of a second, applications should use the `utimensat()` function instead of the obsolescent `utime()` function."

Suggested resolution: Rewrite the `last_write_time` setter description to utilize `utimensat() or similar.`

[20 May 2014 Beman Dawes supplies proposed wording. He comments:]

POSIX supplies two functions, `futimens()` and `utimensat()`, as replacements for `utime()`. `futimens()` is appropriate for the current TS. `utimensat()` will be appropriate for a future File System TS that adds the functionality of the whole family of POSIX *`at()` functions.

Proposed resolution:

Change 15.25 Last write time [fs.op.last_write_time]:

> Effects: Sets the time of last data modification of the file resolved to by `p` to `new_time`, as if by POSIX `stat()` followed by POSIX `utime()` `futimens()` .

### 2660(i). [filesys.ts] [PDTS] Incorrect Throws specification for `absolute()`

Section: 15.1 [filesys.ts::fs.op.absolute] Status: TS Submitter: Daniel Krügler Opened: 2014-02-28 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

The Throws element [fs.op.absolute] says:

> "If `base.is_absolute()` is true, throws only if memory allocation fails."

We can strike:

> "If `base.is_absolute()` is true," and the wording will still hold. Note that:

> 1. None of the involved functions has requirements.
> 2. In every case potentially memory allocation occurs, even for "`return p`", so this allocation can fail.

[2014-03-02 Beman Dawes comments and provides P/R]

The Throws element should follow the same form as similar Throws elements in the TS. There isn't enough special about this function to justify special wording and by referencing Error reporting (7) we ensure `absolute()` follows the overall policy for handling memory allocation failures.

Proposed resolution:

[Change 15.1 [fs.op.absolute]:]

> Throws: If `base.is_absolute()` is true, throws only if memory allocation fails. As specified in Error reporting (7).

### 2662(i). [filesys.ts] Allocator requirements unspecified

Section: 5 [filesys.ts::fs.req] Status: TS Submitter: Daniel Krügler Opened: 2014-05-19 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

Addresses: filesys.ts

The SG3 WP refers in several places to a template parameter named "Allocator" but does not impose requirements on this type.

Proposed resolution:

Add the following to 5 Requirements [fs.req]:

> Template parameters named `Allocator` shall meet the C++ Standard's library Allocator requirements (C++11 §17.6.3.5)

### 2663(i). Enable efficient retrieval of file size from `directory_entry`

Section: 31.12.13.15 [fs.op.file.size] Status: Resolved Submitter: Gor Nishanov Opened: 2014-05-22 Last modified: 2021-06-06

Priority: 2

View all issues with Resolved status.

Discussion:

On Windows, the `FindFileData WIN32_FIND_DATA` structure, which is the underlying data type for `directory_entry`, contains the file size as one of the fields. Thus efficient enumeration of files and getting their sizes is possible without doing a separate query for the file size.

[17 Jun 2014 Rapperswil LWG will investigate issue at a subsequent meeting.]

[23 Nov 2015 Editorally correct name of data structure mentioned in discussion.]

[Mar 2016 Jacksonville Beman to provide paper about this]

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

Previous resolution [SUPERSEDED]

> In [fs.class.directory_entry] Class `directory_entry` add the following observer declarations:

> >           uintmax_t file_size();
> >           uintmax_t file_size(error_code& ec) noexcept;

> In `directory_entry` observers 31.12.10.4 [fs.dir.entry.obs] add the following:

> >           uintmax_t file_size();
> >           uintmax_t file_size(error_code& ec) noexcept;

> > > Returns: if `*this` contains a cached file size, return it. Otherwise return `file_size(path())` or `file_size(path(), ec)` respectively.

> > > Throws: As specified in Error reporting (7).

[2016-08, Beman comments]

This will be resolved by P0317R1, Directory Entry Caching for Filesystem.

Fri AM: Moved to Tentatively Resolved

Proposed resolution:

### 2664(i). `operator/` (and other append) semantics not useful if argument has root

Section: 31.12.6.5.3 [fs.path.append], 31.12.6.8 [fs.path.nonmember] Status: C++17 Submitter: Peter Dimov Opened: 2014-05-30 Last modified: 2017-07-30

Priority: 2

View all other issues in [fs.path.append].

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Discussion:

In a recent discussion on the Boost developers mailing list, the semantics of `operator /` and other append operations were questioned:

In brief, currently `p1 / p2` is required to concatenate the lexical representation of `p1` and `p2`, inserting a preferred separator as needed.

This means that, for example, `"c:\x" / "d:\y"` gives `"c:\x\d:\y"`, and that `"c:\x" / "\\server\share"` gives `"c:\x\\server\share"`. This is rarely, if ever, useful.

An alternative interpretation of `p1 / p2` could be that it yields a path that is the approximation of what `p2` would mean if interpreted in an environment in which `p1` is the starting directory. Under this interpretation, `"c:\x" / "d:\y"` gives `"d:\y"`, which is more likely to match what was intended.

I am not saying that this second interpretation is the right one, but I do say that we have reasons to suspect that the first one (lexical concatenation using a separator) may not be entirely correct. This leads me to think that the behavior of `p1 / p2`, when `p2` has a root, needs to be left implementation-defined, so that implementations are not required to do the wrong thing, as above.

This change will not affect the ordinary use case in which `p2` is a relative, root-less, path.

[17 Jun 2014 Rapperswil LWG will investigate issue at a subsequent meeting.]

[2016-02, Jacksonville]

Beman to provide wording.

[2016-06-13, Beman provides wording and rationale]

Rationale: The purpose of the append operations is to provide a simple concatenation facility for users wishing to extend a path by appending one or more additional elements, and to do so without worrying about the details of when a separator is needed. In that context it makes no sense to provide an argument that has a root-name. The simplest solution is simply to require `!p.has_root_name()`. The other suggested solutions IMO twist the functions into something harder to reason about yet any advantages for users are purely speculative. The concatenation functions can be used instead for corner cases.

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

[2016-07-03, Daniel comments]

The same wording area is touched by LWG 2732(i).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4594.

> 1. Change 31.12.6.5.3 [fs.path.append] path appends as indicated:

>    > `path& operator/=(const path& p);`

>    > > -?- Requires: `!p.has_root_name()`.

>    > > -2- Effects: Appends `path::preferred_separator` to `pathname` unless:

>    > > - an added directory-separator would be redundant, or

>    > > - an added directory-separator would change a relative path to an absolute path [Note: An empty path is relative. — end note], or

>    > > - `p.empty()` is `true`, or

>    > > - `*p.native().cbegin()` is a directory-separator.

>    > > Then appends `p.native()` to `pathname`.

>    > > -3- Returns: `*this`.

>    > `template <class Source>
>    > path& operator/=(const Source& source);
>    > template <class Source>
>    > path& append(const Source& source);
>    > template <class InputIterator>
>    > path& append(InputIterator first, InputIterator last);`

>    > > -?- Requires: `!source.has_root_name()` or `!*first.has_root_name()`, respectively.

>    > > -4- Effects: Appends `path::preferred_separator` to `pathname`, converting format and encoding if required (31.12.6.3 [fs.path.cvt]), unless:

>    > > - an added directory-separator would be redundant, or

>    > > - an added directory-separator would change a relative path to an absolute path, or

>    > > - `source.empty()` is `true`, or

>    > > - `*source.native().cbegin()` is a directory-separator.

>    > > Then appends the effective range of `source` (31.12.6.4 [fs.path.req]) or the range `[first, last)` to `pathname`, converting format and encoding if required (31.12.6.3 [fs.path.cvt]).

>    > > -5- Returns: `*this`.

> 2. Change 31.12.6.8 [fs.path.nonmember] path non-member functions as indicated:

>    > `path operator/(const path& lhs, const path& rhs);`

>    > > -?- Requires: `!rhs.has_root_name()`.

>    > > -13- Returns: `path(lhs) /= rhs`.

[2016-08-03 Chicago]

After discussion on 2732(i), it was determined that the PR for that issue should be applied to this issue before it is accepted. That PR changes all the path appends to go through operator/=, so only one requires element remains necessary.

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606, and assumes that the PR for 2732(i) is applied.

1. Change 31.12.6.5.3 [fs.path.append] as indicated:

       path& operator/=(const path& p);

   > -?- Requires: `!p.has_root_name()`.

   > -2- Effects: Appends `path::preferred_separator` to `pathname` unless:

   > 1. — an added directory-separator would be redundant, or
   > 2. — an added directory-separator would change a relative path to an absolute path [Note: An empty path is relative. — end note], or
   > 3. — `p.empty()` is `true`, or
   > 4. — `*p.native().cbegin()` is a directory-separator.

   > Then appends `p.native()` to `pathname`.

   > -3- Returns: `*this`.

2. Change 31.12.6.8 [fs.path.nonmember] p13 as indicated:

       path operator/(const path& lhs, const path& rhs);

   > -13- ReturnsEffects: Equivalent to `return path(lhs) /= rhs;`.

### 2665(i). `remove_filename()` post condition is incorrect

Section: 31.12.6.5.5 [fs.path.modifiers] Status: Resolved Submitter: Eric Fiselier Opened: 2014-06-07 Last modified: 2017-07-17

Priority: 1

View all issues with Resolved status.

Discussion:

`remove_filename()` specifies `!has_filename()` as the post condition. This post condition is not correct. For example the path `"/foo"` has a filename of `"foo"`. If we remove the filename we get `"/"`, and `"/"` has a filename of `"/"`.

[2014-06-08 Beman supplies an Effects: element.]

[2014-06-17 Rapperswil LWG will investigate issue at a subsequent meeting.]

[2016-04 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

[2016-04, Issues Telecon]

There was concern that the effects wording is not right. Jonathan provided updated wording.

Previous resolution [SUPERSEDED]:

>         path& remove_filename();

> > Postcondition: `!has_filename()`.

> > Effects: `*this = parent_path()`, except that if `parent_path() == root_path()`, `clear()`.

> > Returns: `*this`.

> > [Example:

> > >             std::cout << path("/foo").remove_filename();  // outputs "/"
> > >             std::cout << path("/").remove_filename();     // outputs ""

> > —end example]

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Previous resolution [SUPERSEDED]:

>       path& remove_filename();

> > Postcondition: !has_filename().

> > Effects: If `*this == root_path()`, then `clear()`. Otherwise, `*this = parent_path()`.

> > Returns: `*this`.

> > [Example:

> > >           std::cout << path("/foo").remove_filename();  // outputs "/"
> > >           std::cout << path("/").remove_filename();     // outputs ""

> > —end example]

[2016-10-16, Eric reopens and provides improved wording]

The suggested PR is incorrect. `root_path()` removes redundant directory separators.

Therefore the condition `*this == root_path()` will fail for `"//foo///"` because `root_path()` returns `"//foo/"`. However using `path::compare` instead would solve this problem.

[2016-11-21, Beman comments]

This issue is closely related to CD NB comments US 25, US 37, US 51, US 52, US 53, US 54, and US 60. The Filesystem SG in Issaquah recommended that these all be handled together, as they all revolve around the exact meaning of "filename" and path decomposition.

[2017-07-14, Davis Herring comments]

Changes in P0492R2 changed `remove_filename()` and `has_filename()` in such a fashion that the postcondition is now satisfied. (In the example in the issue description, `"/foo"` does gets mapped to `"/"`, but `path("/").has_filename()` is `false`.)

[2017-07, Toronto Thursday night issues processing]

This was resolved by the adoption of P0492R2.

Proposed resolution:

This wording is relative to N4606.

1. Modify 31.12.6.5.5 [fs.path.modifiers] as indicated:

   >     path& remove_filename();

   > > -5- PostconditionEffects: `!has_filename()`If `this->compare(root_path()) == 0`, then `clear()`. Otherwise, `*this = parent_path()`..

   > > -6- Returns: `*this`.

   > > -7- [Example:

   > > >     std::cout << path("/foo").remove_filename(); // outputs "/"
   > > >     std::cout << path("/").remove_filename(); // outputs ""

   > > — end example]

### 2667(i). `path::root_directory()` description is confusing

Section: 31.12.6.5.9 [fs.path.decompose] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-03 Last modified: 2017-07-30

Priority: 0

View all other issues in [fs.path.decompose].

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Discussion:

31.12.6.5.9 [fs.path.decompose] p5 says:

> If root-directory is composed of slash name, slash is excluded from the returned string.

but the grammar says that root-directory is just slash so that makes no sense.

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

Proposed resolution:

Change 31.12.6.5.9 [fs.path.decompose] as indicated:

>     path root_directory() const;

> > Returns: root-directory, if `pathname` includes root-directory, otherwise `path()`.

> > If root-directory is composed of slash name, slash is excluded from the returned string.

### 2669(i). `recursive_directory_iterator` effects refers to non-existent functions

Section: 31.12.12.2 [fs.rec.dir.itr.members] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-10 Last modified: 2017-07-30

Priority: 0

View all other issues in [fs.rec.dir.itr.members].

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Discussion:

`operator++/increment`, second bullet item, ¶39 says "Otherwise if `recursion_pending() && is_directory(this->status()) && (!is_symlink(this->symlink_status())`..." but `recursive_directory_iterator` does not have `status` or `symlink_status` members.

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

Proposed resolution:

Change 31.12.12.2 [fs.rec.dir.itr.members] ¶39 as indicated:

> Otherwise if `recursion_pending() && is_directory((*this)->status()) && (!is_symlink((*this)->symlink_status())` ..."

### 2670(i). `system_complete` refers to undefined variable 'base'

Section: 99 [fs.op.system_complete] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-20 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

The example says "...or `p` and `base` have the same `root_name()`.", but `base` is not defined. I believe it refers to the value returned by `current_path()`.

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

Proposed resolution:

Change 99 [fs.op.system_complete] as indicated:

> For Windows based operating systems, `system_complete(p)` has the same semantics as `absolute(p, current_path())` if `p.is_absolute() || !p.has_root_name()` or `p` and `base current_path()` have the same `root_name()`. Otherwise it acts like `absolute(p, cwd)` is the current directory for the `p.root_name()` drive. This will be the current directory for that drive the last time it was set, and thus may be residue left over from a prior program run by the command processor. Although these semantics are useful, they may be surprising.

### 2671(i). Errors in Copy

Section: 31.12.13.4 [fs.op.copy] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-28 Last modified: 2017-07-30

Priority: 0

View all other issues in [fs.op.copy].

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Discussion:

31.12.13.4 [fs.op.copy] paragraph 16 says:

> Otherwise if `!exists(t) & (options & copy_options::copy_symlinks) != copy_options::none`, then `copy_symlink(from, to, options)`.

But there is no overload of `copy_symlink` that takes a `copy_options`; it should be `copy_symlink(from, to)`.

31.12.13.4 [fs.op.copy] Paragraph 26 says:

> as if by `for (directory_entry& x : directory_iterator(from))`

but the result of dereferencing a `directory_iterator` is const; it should be:

> as if by `for (const directory_entry& x : directory_iterator(from))`

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

Proposed resolution:

Change 31.12.13.4 [fs.op.copy] paragraph 16 as indicated:

> Otherwise if `!exists(t) & (options & copy_options::copy_symlinks) != copy_options::none` , then `copy_symlink(from, to, options)`.

Change 31.12.13.4 [fs.op.copy] paragraph 26 as indicated:

> as if by `for (const directory_entry& x : directory_iterator(from))`

### 2672(i). Should is_empty use error_code in its specification?

Section: 31.12.13.20 [fs.op.is.empty] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-08-01 Last modified: 2021-06-06

Priority: 3

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Discussion:

The descriptions of most functions are explicit about the use of the `ec` argument, either saying something like "`foo(p)` or `foo(p, ec)`, respectively" or using the `ec` argument like `foo(p[, ec])`, but `is_empty` does not.

[fs.op.is_empty]/2 refers to `ec` unconditionally, but more importantly [fs.op.is_empty]/3 doesn't pass `ec` to the `directory_iterator` constructor or the `file_size` function.

[ 9 Oct 2014 Beman supplies proposed wording. ]

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

Previous resolution [SUPERSEDED]:

> 1. >         bool is_empty(const path& p);
>    >         bool is_empty(const path& p, error_code& ec) noexcept;

>    > > Effects:

>    > > - Determine `file_status s`, as if by `status(p)` or `status(p, ec)`, respectively.
>    > > - For the signature with argument `ec`, return `false` if an error occurred.
>    > > - Otherwise, if `is_directory(s)`:
>    > >   - Create `directory_iterator itr`, as if by `directory_iterator(p)` or `directory_iterator(p, ec)`, respectively.
>    > >   - For the signature with argument `ec`, return `false` if an error occurred.
>    > >   - Otherwise, return `itr == directory_iterator()`.
>    > > - Otherwise:
>    > >   - Determine `uintmax_t sz`, as if by `file_size(p)` or`file_size(p, ec)`, respectively .
>    > >   - For the signature with argument `ec`, return `false` if an error occurred.
>    > >   - Otherwise, return `sz == 0`.

>    > > Remarks: The temporary objects described in Effects are for exposition only. Implementations are not required to create them.

>    > > Returns: `is_directory(s) ? directory_iterator(p) == directory_iterator() : file_size(p) == 0;` See Effects.

>    > > > The signature with argument `ec` returns `false` if an error occurs.

>    > > Throws: As specified in Error reporting (7).

[2016-08 Chicago]

Wed PM: Move to Tentatively Ready

Proposed resolution:

1. >         bool is_empty(const path& p);
   >         bool is_empty(const path& p, error_code& ec) noexcept;

   > > Effects:

   > > - Determine `file_status s`, as if by `status(p)` or `status(p, ec)`, respectively.
   > > - For the signature with argument `ec`, return `false` if an error occurred.
   > > - Otherwise, if `is_directory(s)`:
   > >   - Create `directory_iterator itr`, as if by `directory_iterator(p)` or `directory_iterator(p, ec)`, respectively.
   > >   - For the signature with argument `ec`, return `false` if an error occurred.
   > >   - Otherwise, return `itr == directory_iterator()`.
   > > - Otherwise:
   > >   - Determine `uintmax_t sz`, as if by `file_size(p)` or`file_size(p, ec)`, respectively .
   > >   - For the signature with argument `ec`, return `false` if an error occurred.
   > >   - Otherwise, return `sz == 0`.

   > > Returns: `is_directory(s) ? directory_iterator(p) == directory_iterator() : file_size(p) == 0;`

   > > > The signature with argument `ec` returns `false` if an error occurs.

   > > Throws: As specified in Error reporting (7).

### 2673(i). `status()` effects cannot be implemented as specified

Section: 31.12.13.36 [fs.op.status] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-09-15 Last modified: 2017-07-30

Priority: 0

View all other issues in [fs.op.status].

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Discussion:

31.12.13.36 [fs.op.status] paragraph 2 says:

> Effects: As if:

>     error_code ec;
>     file_status result = status(p, ec);
>     if (result == file_type::none)
>     ...

This won't compile, there is no comparison operator for `file_status` and `file_type`, and the conversion from `file_type` to `file_status` is explicit.

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

Proposed resolution:

Change 31.12.13.36 [fs.op.status] paragraph 2:

> Effects: As if:

>     error_code ec;
>     file_status result = status(p, ec);
>     if (result.type() == file_type::none)
>     ...

### 2674(i). Bidirectional iterator requirement on `path::iterator` is very expensive

Section: 31.12.6.6 [fs.path.itr] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-09-15 Last modified: 2017-07-30

Priority: 2

View all other issues in [fs.path.itr].

View all issues with C++17 status.

Discussion:

31.12.6.6 [fs.path.itr] requires `path::iterator` to be a BidirectionalIterator, which also implies the ForwardIterator requirement in [forward.iterators] p6 for the following assertion to pass:

>     path p("/");
>     auto it1 = p.begin();
>     auto it2 = p.begin();
>     assert( &*it1 == &*it2 );

This prevents iterators containing a `path`, or constructing one on the fly when dereferenced, the object they point to must exist outside the iterators and potentially outlive them. The only practical way to meet the requirement is for `p` to hold a container of child `path` objects so the iterators can refer to those children. This makes a `path` object much larger than would naïvely be expected.

The Boost and MSVC implementations of Filesystem fail to meet this requirement. The GCC implementation meets it, but it makes `sizeof(path) == 64` (for 64-bit) or `sizeof(path) == 40` for 32-bit, and makes many path operations more expensive.

[21 Nov 2015 Beman comments:]

The ForwardIterator requirement in [forward.iterators] "If `a` and `b` are both dereferenceable, then `a == b` if and only if `*a` and `*b` are bound to the same object." will be removed by N4560, Working Draft, C++ Extensions for Ranges. I see no point in requiring something for the File System TS that is expensive, has never to my knowledge been requested by users, and is going to go away soon anyhow. The wording I propose below removes the requirement.

[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]

Proposed resolution:

This wording is relative to N4582.

1. Change 31.12.6.6 [fs.path.itr] paragraph 2:

   > A `path::iterator` is a constant iterator satisfying all the requirements of a bidirectional iterator (C++14 §24.1.4 Bidirectional iterators) except that there is no requirement that two equal iterators be bound to the same object. Its `value_type` is `path`.

### 2676(i). Provide `filesystem::path` overloads for File-based streams

Section: 31.10 [file.streams] Status: C++17 Submitter: Beman Dawes Opened: 2016-03-16 Last modified: 2017-07-30

Priority: 2

View all other issues in [file.streams].

View all issues with C++17 status.

Discussion:

The constructor and `open` functions for File-based streams in 27.9 [file.streams] currently provide overloads for `const char*` and `const string&` arguments that specify the path for the file to be opened. With the addition of the File System TS to the standard library, these constructors and `open` functions need to be overloaded for `const filesystem::path&` so that file-based streams can take advantage of class `filesystem::path` features such as support for strings of character types `wchar_t`, `char16_t`, and `char32_t`.

The `const filesystem::path& p` overload for these functions is like the existing `const string&` overload; it simply calls the overload of the same function that takes a C-style string.

For operating systems like POSIX that traffic in `char` strings for filenames, nothing more is required. For operating systems like Windows that traffic in `wchar_t` strings for filenames, an additional C-style string overload is required. The overload's character type needs to be specified as `std::filesystem::path::value_type` to also support possible future operating systems that traffic in `char16_t` or `char32_t` characters.

Not recommended:

Given the proposed constructor and `open` signatures taking `const filesystem::path&`, it would in theory be possible to remove some of the other signatures. This is not proposed because it would break ABI's, break user code depending on user-defined automatic conversions to the existing argument types, and invalidate existing documentation, books, and tutorials.

Implementation experience:

The Boost Filesystem library has provided header `<boost/filesystem/fstream.hpp>` implementing the proposed resolution for over a decade.

The Microsoft/Dinkumware implementation of standard library header `<fstream>` has provided the `const wchar_t*` overloads for many years.

[2016-08-03 Chicago]

Fri PM: Move to Review

Proposed resolution:

At the end of 27.9.1 File streams [fstreams] add a paragraph:

In this subclause, member functions taking arguments of `const std::filesystem::path::value_type*` shall only be provided on systems where `std::filesystem::path::value_type` ([fs.class.path]) is not `char`. [Note: These functions enable class `path` support for systems with a wide native path character type, such as `wchar_t`. — end note]

Change 27.9.1.1 Class template basic_filebuf [filebuf] as indicated:

    basic_filebuf<charT,traits>* open(const char* s,
        ios_base::openmode mode);
    basic_filebuf<charT,traits>* open(const std::filesystem::path::value_type* s,
        ios_base::openmode mode);  // wide systems only; see [fstreams]
    basic_filebuf<charT,traits>* open(const string& s,
       ios_base::openmode mode);
    basic_filebuf<charT,traits>* open(const filesystem::path& p,
       ios_base::openmode mode);

Change 27.9.1.4 Member functions [filebuf.members] as indicated:

    basic_filebuf<charT,traits>* open(const char* s,
       ios_base::openmode mode);
    basic_filebuf<charT,traits>* open(const std::filesystem::path::value_type* s,
       ios_base::openmode mode);  // wide systems only; see [fstreams]

To 27.9.1.4 Member functions [filebuf.members] add:

    basic_filebuf<charT,traits>* open(const filesystem::path& p,
       ios_base::openmode mode);

> Returns: `open(p.c_str(), mode);`

Change 27.9.1.6 Class template basic_ifstream [ifstream] as indicated:

    explicit basic_ifstream(const char* s,
        ios_base::openmode mode = ios_base::in);
    explicit basic_ifstream(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::in);  // wide systems only; see [fstreams]
    explicit basic_ifstream(const string& s,
        ios_base::openmode mode = ios_base::in);
    explicit basic_ifstream(const filesystem::path& p,
        ios_base::openmode mode = ios_base::in);
    ...
    void open(const char* s,
        ios_base::openmode mode = ios_base::in);
    void open(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::in);  // wide systems only; see [fstreams]
    void open(const string& s,
        ios_base::openmode mode = ios_base::in);
    void open(const filesystem::path& p,
        ios_base::openmode mode = ios_base::in);

Change 27.9.1.7 basic_ifstream constructors [ifstream.cons] as indicated:

    explicit basic_ifstream(const char* s,
        ios_base::openmode mode = ios_base::in);
    explicit basic_ifstream(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::in);  // wide systems only; see [fstreams]

To 27.9.1.7 basic_ifstream constructors [ifstream.cons] add:

    explicit basic_ifstream(const filesystem::path& p,
        ios_base::openmode mode = ios_base::in);

> Effects: the same as `basic_ifstream(p.c_str(), mode)`.

Change 27.9.1.9 Member functions [ifstream.members] as indicated:

    void open(const char* s,
        ios_base::openmode mode = ios_base::in);
    void open(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::in);  // wide systems only; see [fstreams]

To 27.9.1.9 Member functions [ifstream.members] add:

    void open(const filesystem::path& p,
        ios_base::openmode mode = ios_base::in);

> Effects: calls `open(p.c_str(), mode)`.

Change 27.9.1.10 Class template basic_ofstream [ofstream] as indicated:

    explicit basic_ofstream(const char* s,
        ios_base::openmode mode = ios_base::out);
    explicit basic_ofstream(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::out);  // wide systems only; see [fstreams]
    explicit basic_ofstream(const string& s,
        ios_base::openmode mode = ios_base::out);
    explicit basic_ofstream(const filesystem::path& p,
        ios_base::openmode mode = ios_base::out);
    ...
    void open(const char* s,
        ios_base::openmode mode = ios_base::out);
    void open(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::out);  // wide systems only; see [fstreams]
    void open(const string& s,
        ios_base::openmode mode = ios_base::out);
    void open(const filesystem::path& p,
        ios_base::openmode mode = ios_base::out);

Change 27.9.1.11 basic_ofstream constructors [ofstream.cons] as indicated:

    explicit basic_ofstream(const char* s,
        ios_base::openmode mode = ios_base::out);
    explicit basic_ofstream(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::out);  // wide systems only; see [fstreams]

To 27.9.1.11 basic_ofstream constructors [ofstream.cons] add:

    explicit basic_ofstream(const filesystem::path& p,
        ios_base::openmode mode = ios_base::out);

> Effects: the same as `basic_ofstream(p.c_str(), mode)`.

Change 27.9.1.13 Member functions [ofstream.members] as indicated:

    void open(const char* s,
        ios_base::openmode mode = ios_base::out);
    void open(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::out);  // wide systems only; see [fstreams]

To 27.9.1.13 Member functions [ofstream.members] add:

    void open(const filesystem::path& p,
        ios_base::openmode mode = ios_base::out);

> Effects: calls `open(p.c_str(), mode)`.

Change 27.9.1.14 Class template basic_fstream [fstream] as indicated:

    explicit basic_fstream(const char* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);
    explicit basic_fstream(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);  // wide systems only; see [fstreams]
    explicit basic_fstream(const string& s,
        ios_base::openmode mode = ios_base::in|ios_base::out);
    explicit basic_fstream(const filesystem::path& p,
        ios_base::openmode mode = ios_base::in|ios_base::out);
    ...
    void open(const char* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);
    void open(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);  // wide systems only; see [fstreams]
    void open(const string& s,
        ios_base::openmode mode = ios_base::in|ios_base::out);
    void open(const filesystem::path& p,
        ios_base::openmode mode = ios_base::in|ios_base::out);

Change 27.9.1.15 basic_fstream constructors [fstream.cons] as indicated:

    explicit basic_fstream(const char* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);
    explicit basic_fstream(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);  // wide systems only; see [fstreams]

To 27.9.1.15 basic_fstream constructors [fstream.cons] add:

    explicit basic_fstream(const filesystem::path& p,
        ios_base::openmode mode = ios_base::in|ios_base::out);

> Effects: the same as `basic_fstream(p.c_str(), mode)`.

Change 27.9.1.17 Member functions [fstream.members] as indicated:

    void open(const char* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);
    void open(const std::filesystem::path::value_type* s,
        ios_base::openmode mode = ios_base::in|ios_base::out);  // wide systems only; see [fstreams]

To 27.9.1.17 Member functions [fstream.members] add:

    void open(const filesystem::path& p,
        ios_base::openmode mode = ios_base::in|ios_base::out);

> Effects: calls `open(p.c_str(), mode)`.

### 2677(i). `directory_entry::status` is not allowed to be cached as a quality-of-implementation issue

Section: 31.12.10.4 [fs.dir.entry.obs] Status: Resolved Submitter: Billy O'Neal Opened: 2016-03-03 Last modified: 2020-09-06

Priority: 2

View all other issues in [fs.dir.entry.obs].

View all issues with Resolved status.

Discussion:

To fix multi-threading problems in `directory_entry`, caching behavior was removed from the type. This is bad for performance reasons, because the values can no longer be cached from the result of `readdir` on POSIX platforms, or from `FindFirstFile/FindNextFile` on Windows.

It appears that the intent was to allow implementers to fill in the values for `directory_entry::status` inside `directory_iterator`, but as currently specified:

> Returns: `status(path()[, ec])`.

This is not allowed to be cached, because the target of the path can change. For example, consider the following program:

    #include <stdio.h>
    #include <stdlib.h>
    #include <filesystem>
    #include <fstream>

    using namespace std;
    namespace fs = ::std::filesystem;

    void verify(const bool b, const char * const msg) {
        if (!b) {
            printf("fail: %s\n", msg);
            exit(1);
        }
    }

    void touch_file(const char * const filename) {
        ofstream f(filename);
        f << '\n' << endl;
        verify(f.good(), "File write failed");
    }

    int main() {
        fs::remove_all("subDir");
        fs::create_directory("subDir");
        fs::create_directory("subDir/child");
        touch_file("subDir/child/child");
        fs::current_path("./subDir");
        fs::directory_iterator dir(".");
        ++dir;
        fs::directory_entry entry = *dir;

        verify(entry == "./child",
          "unexpected subdirectory"); //enumerating "subDir" returned the directory "child"

        fs::file_status status = entry.status();
        verify(status.type() == fs::file_type::directory,
            "subDir/child was not a directory");
        fs::current_path("./child");
        status = entry.status(); // REQUIRED to re-stat() on POSIX,
                                 // GetFileAttributes() on Windows
        verify(status.type() == fs::file_type::regular,
            "subDir/child/child was not a regular file");
        return 0;
    }

`directory_entry` should be re-specified to allow implementers to cache the value of `status(path)` at the time `irectory_iterator` was incremented to avoid repeated `stat()` / `GetFileAttributes` calls. (This may mean additional constructors are necessary for `directory_entry` as well)

[2016-04, Issues Telecon]

Beman is working on a paper to address this.

[2016-08, Beman comments]

This will be resolved by P0317R1, Directory Entry Caching for Filesystem.

Fri AM: Moved to Tentatively Resolved

Proposed resolution:

### 2678(i). `std::filesystem` enum classes overspecified

Section: 31.12.8.2 [fs.enum.file.type], 31.12.8.3 [fs.enum.copy.opts], 31.12.8.6 [fs.enum.dir.opts] Status: C++17 Submitter: Richard Smith Opened: 2016-03-19 Last modified: 2021-06-06

Priority: 3

View all other issues in [fs.enum.file.type].

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Discussion:

The enum class `std::filesystem::file_type` specifies a collection of enumerators for describing the type of a file, but also specifies (1) the numeric values of the enumerators, and (2) that an implementation cannot extend the list with additional types known to it (and by extension, the standard cannot extend the list in future versions without breakage).

These both seem like mistakes in the design. I would suggest we remove the specification of numerical values, and maybe also

- we allow implementations to add new implementation-defined value

- we replace `std::filesystem::file_type::unknown` with a value that only means "exists but type could not be determined" and use implementation-specific values for other possibilities.

Similar overspecification exists for `std::filesystem::copy_options` and `std::filesystem::directory_options`, where again precise numerical values are specified.

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Proposed resolution:

1. Strike the "Value" column from:

   - [fs.enum.file_type], Table 145 Enum class `file_type`

   - 31.12.8.3 [fs.enum.copy.opts], Table 146 Enum class `copy_options`

   - 31.12.8.6 [fs.enum.dir.opts], Table 148 Enum class `enum.directory_options`.

2. Change [fs.enum.file_type] Table 145 Enum class `file_type` as indicated:

   > Constant | Meaning
   > `none` | The type of the file has not been determined or an error occurred while trying to determine the type.
   > `not_found` | Pseudo-type indicating the file was not found. [ Note: The file not being found is not considered an error while determining the type of a file. —end note ]
   > `regular` | Regular file
   > `directory` | Directory file
   > `symlink` | Symbolic link file
   > `block` | Block special file
   > `character` | Character special file
   > `fifo` | FIFO or pipe file
   > `socket` | Socket file
   > implementation- defined | Implementations that support file systems having file types in addition to the above file_type types shall supply implementation-defined `file_type` constants to separately identify each of those additional file types
   > `unknown` | The file does exist, but is of an operating system dependent type not covered by any of the other cases or the process does not have permission to query the file type The file exists but the type could not be determined

### 2679(i). Inconsistent Use of Effects and Equivalent To

Section: 16.3.2.4 [structure.specifications] Status: C++17 Submitter: Dawn Perchik Opened: 2016-04-14 Last modified: 2017-07-30

Priority: 3

View other active issues in [structure.specifications].

View all other issues in [structure.specifications].

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Discussion:

- Some Effects: were intended to include the "Equivalent to" phrasing where no or different words were used. See examples in `std::filesystem` and throughout the library.

- The wording in [structure.specifications]/4 is incorrect and Effects: throughout the library which use (or were intended to use) the "Equivalent to" phrasing need to be checked to make sure they fit the intended wording.

[2016-04, Issues Telecon]

The PR is fine; but bullet #1 in the report really should have a list of places to change.

Jonathan checked throughout the library for bullet #2 and found two problems in [string.access], which have been added to the PR.

[2016-08-03 Chicago]

Fri PM: Move to Tentatively Ready

Proposed resolution:

1. Change [structure.specifications]/4 as indicated:

   > "[…] if `F` has no Returns: element, a non-`void` return from `F` is specified by the Returns: elementsreturn statements in the code sequence."

2. Add two `return` keywords to [string.access]:

   >     const charT& front() const;
   >     charT& front();

   > > -7- Requires: `!empty()`.

   > > -8- Effects: Equivalent to `return operator[](0)`.

   >     const charT& back() const;
   >     charT& back();

   > > -9- Requires: `!empty()`.

   > > -10- Effects: Equivalent to `return operator[](size() - 1)`.

### 2680(i). Add "Equivalent to" to filesystem

Section: 31.12 [filesystems] Status: C++17 Submitter: Beman Dawes Opened: 2016-04-15 Last modified: 2017-07-30

Priority: 2

View all other issues in [filesystems].

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Discussion:

The IS project editors have requested that filesystem Effects elements specified purely as C++ code include the phrase "Equivalent to" if this is the intent. They do not consider this change to be editorial.

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Proposed resolution:

31.12.6.5.5 [fs.path.modifiers]

>     path& replace_filename(const path& replacement);

> > Effects: Equivalent to:

> >     remove_filename();
> >     operator/=(replacement);

31.12.6.8 [fs.path.nonmember]

>     void swap(path& lhs, path& rhs) noexcept;

> > Effects: Equivalent to: `lhs.swap(rhs)`

31.12.6.7 [fs.path.io]

>     template <class charT, class traits>
>     basic_ostream<charT, traits>&
>     operator<<(basic_ostream<charT, traits>& os, const path& p);

> > Effects: Equivalent to: `os << quoted(p.string<charT, traits>())`.

31.12.6.7 [fs.path.io]

>     template <class charT, class traits>
>     basic_istream<charT, traits>&
>     operator>>(basic_istream<charT, traits>& is, path& p);

> > Effects: Equivalent to:

> >     basic_string<charT, traits> tmp;
> >     is >> quoted(tmp);
> >     p = tmp;

31.12.13.4 [fs.op.copy]

>     void copy(const path& from, const path& to);
>     void copy(const path& from, const path& to, error_code& ec) noexcept;

> > Effects: Equivalent to: `copy(from, to, copy_options::none)` or `copy(from, to, copy_options::none, ec)`, respectively.

[fs.op.copy_symlink]

>     void copy_symlink(const path& existing_symlink, const path& new_symlink);
>     void copy_symlink(const path& existing_symlink, const path& new_symlink,
>       error_code& ec) noexcept;

> > Effects: Equivalent to: `function(read_symlink(existing_symlink), new_symlink)` or `function(read_symlink(existing_symlink, ec), new_symlink, ec)`, respectively, where `function` is `create_symlink` or `create_directory_symlink`, as appropriate.

### 2681(i). `filesystem::copy()` cannot copy symlinks

Section: 31.12.13.4 [fs.op.copy] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-04-19 Last modified: 2017-07-30

Priority: 2

View all other issues in [fs.op.copy].

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Discussion:

31.12.13.4 [fs.op.copy] paragraph 3 bullet (3.4) says that if `is_symlink(f)` and `copy_options` is set in options then it should copy a symlink, but that case cannot be reached.

`is_symlink(f)` can only be true if f was set using `symlink_status(from)`, but that doesn't happen unless one of `create_symlinks` or `skip_symlinks` is set in options. It should depend on `copy_symlinks` too.

I'm not sure what the correct behaviour is, but I don't think we want to simply add a check for `copy_symlinks` in bullet (3.1) because that would mean that `t = symlink_status(to)`, and I don't think we want that. Consider `'touch file; mkdir dir; ln -s dir link;'` and then `filesystem::copy("file", "link", filesystem::copy_options::copy_symlinks)`. If `t = symlink_status(to)` then `is_directory(t) == false`, and we don't use bullet (3.5.4) but go to (3.5.5) instead and fail. So when `copy_symlinks` is set we still need to use `t = status(to)` as specified today.

[2016-05 Issues Telecon]

This is related to 2682(i); and should be considered together.

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Proposed resolution:

Modify paragraph 3 of 31.12.13.4 [fs.op.copy] as shown:

> Effects: Before the first use of `f` and `t`:
> — If `(options & copy_options::create_symlinks) != copy_options::none || (options & copy_options::skip_symlinks) != copy_options::none` then `auto f = symlink_status(from)` and if needed `auto t = symlink_status(to)`.
> — Otherwise, if `(options & copy_options::copy_symlinks) != copy_options::none` then `auto f = symlink_status(from)` and if needed `auto t = status(to)`.
> — Otherwise, `auto f = status(from)` and if needed `auto t = status(to)`.

### 2682(i). `filesystem::copy()` won't create a symlink to a directory

Section: 31.12.13.4 [fs.op.copy] Status: C++20 Submitter: Jonathan Wakely Opened: 2016-04-19 Last modified: 2021-02-25

Priority: 2

View all other issues in [fs.op.copy].

View all issues with C++20 status.

Discussion:

(First raised in c++std-lib-38544)

`filesystem::copy` doesn't create a symlink to a directory in this case:

> `copy("/", "root", copy_options::create_symlinks);`

If the first path is a file then a symlink is created, but I think my implementation is correct to do nothing for a directory. We get to bullet 31.12.13.4 [fs.op.copy] (3.6) where `is_directory(f)` is true, but `options == create_symlinks`, so we go to the next bullet (3.7) which says "Otherwise, no effects."

I think the case above should either create a symlink, or should report an error. GNU cp -s gives an error in this case, printing "omitting directory '/'". An error seems reasonable, you can use `create_symlink` to create a symlink to a directory.

[2016-05 Issues Telecon]

This is related to 2681(i); and should be considered together.

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Previous resolution [SUPERSEDED]:

> Add a new bullet following (3.6) in 31.12.13.4 [fs.op.copy] as shown:

> > - If `!exists(t)`, then `create_directory(to, from)`.

> > - Then, iterate over the files in `from`, as if by `for (directory_entry& x : directory_iterator(from))`, and for each iteration `copy(x.path(), to/x.path().filename(), options | copy_options::unspecified )`

> > - Otherwise, if `is_directory(f) && (options & copy_options::create_symlinks) != copy_options::none`, then report an error with an `error_code` argument equal to `make_error_code(errc::is_a_directory)`.

> > - Otherwise, no effects.

[2016-10-16, Eric reopens and provides improved wording]

The current PR makes using `copy(...)` to copy/create a directory symlink an error. For example, the following is now an error:

>     copy("/", "root", copy_options::create_symlinks);

However the current PR doesn't handle the case where both `copy_options::create_symlinks` and `copy_options::recursive` are specified. This case is still incorrectly handled by bullet (3.6) [fs.op.copy].

I suggest we move the PR before this bullet so that it catches the recursive copy case, since currently the conditions are ordered:

> 3.6 Otherwise if `is_directory(f) && (bool(options & copy_options::recursive) || ...)`
> 3.X Otherwise if `is_directory(f) && bool(options & copy_options::create_symlinks)`

So 3.6 catches `create_symlinks | recursive` but I believe we want 3.X to handle it instead.

[2018-01-26 issues processing telecon]

Status to 'Review'; we think this is OK but want some implementation experience before adopting it.

[2018-01-29 Jonathan says]

The proposed resolution for LWG 2682 has been in GCC's Filesystem TS implementation for more than a year. It's also in our `std::filesystem` implementation on the subversion trunk.

[2018-06; Rapperswil Wednesday evening]

JW: can we use the words we are shipping already since two years?
BO: what we got is better than what we had before
no objection to moving to Ready
ACTION move to Ready
ACTION link 2682 and LWG 3057(i) and set a priority 2 and look at 3057 in San Diego

Daniel rebases wording to N4750.

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Add a new bullet before (4.8) in 31.12.13.4 [fs.op.copy] as shown:

   > 1. (4.7) — Otherwise, if `is_regular_file(f)`, then:

   >    > […]

   > 2. (4.?) — Otherwise, if

   >    > >     is_directory(f) &&
   >    > >     (options & copy_options::create_symlinks) != copy_options::none

   >    > then report an error with an `error_code` argument equal to `make_error_code(errc::is_a_directory)`.

   > 3. (4.8) — Otherwise, if

   >    > >     is_directory(f) &&
   >    > >     ((options & copy_options::recursive) != copy_options::none ||
   >    > >     options == copy_options::none)

   >    > then:

   >    1. (4.8.1) — If `exists(t)` is `false`, then `create_directory(to, from)`.

   >    2. (4.8.2) — Then, iterate over the files in `from`, as if by

   >       >     for (const directory_entry& x : directory_iterator(from))
   >       >       copy(x.path(), to/x.path().filename(),
   >       >         options | copy_options::in-recursive-copy);

   >       where `in-recursive-copy` is a bitmask element of `copy_options` that is not one of the elements in 31.12.8.3 [fs.enum.copy.opts].

   > 4. (4.9) — Otherwise, for the signature with argument `ec`, `ec.clear()`.

   > 5. (4.10) — Otherwise, no effects.

### 2683(i). `filesystem::copy()` says "no effects"

Section: 31.12.13.4 [fs.op.copy] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-04-19 Last modified: 2017-07-30

Priority: 3

View all other issues in [fs.op.copy].

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Discussion:

In 31.12.13.4 [fs.op.copy]/8 the final bullet says "Otherwise, no effects" which implies there is no call to `ec.clear()` if nothing happens, nor any error condition, is that right?

[2016-06 Oulu]

Moved to P0/Ready during issues prioritization.

Friday: status to Immediate

Proposed resolution:

Change 31.12.13.4 [fs.op.copy]/8 as indicated:

> Otherwise no effects. For the signature with argument `ec`, `ec.clear()`.

### 2684(i). priority_queue lacking comparator typedef

Section: 23.6.4 [priority.queue] Status: C++17 Submitter: Robert Haberlach Opened: 2016-05-02 Last modified: 2017-07-30

Priority: 0

View all other issues in [priority.queue].

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Discussion:

The containers that take a comparison functor (`set`, `multiset`, `map`, and `multimap`) have a typedef for the comparison functor. `priority_queue` does not.

Proposed resolution:

Augment [priority.queue] as indicated:

     typedef Container container_type;
     typedef Compare value_compare;

### 2685(i). `shared_ptr` deleters must not not throw on move construction

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-05-03 Last modified: 2017-07-30

Priority: 0

View other active issues in [util.smartptr.shared.const].

View all other issues in [util.smartptr.shared.const].

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Discussion:

In 20.3.2.2.2 [util.smartptr.shared.const] p8 the `shared_ptr` constructors taking a deleter say:

> The copy constructor and destructor of `D` shall not throw exceptions.

It's been pointed out that this doesn't forbid throwing moves, which makes it difficult to avoid a leak here:

    struct D {
      D() = default;
      D(const D&) noexcept = default;
      D(D&&) { throw 1; }
      void operator()(int* p) const { delete p; }
    };

    shared_ptr<int> p{new int, D{}};

"The copy constructor" should be changed to reflect that the chosen constructor might not be a copy constructor, and that copies made using any constructor must not throw.

N.B. the same wording is used for the allocator argument, but that's redundant because the `Allocator` requirements already forbid exceptions when copying or moving.

Proposed resolution:

[Drafting note: the relevant expressions we're concerned about are enumerated in the `CopyConstructible` and `MoveConstructible` requirements, so I see no need to repeat them by saying something clunky like "Initialization of an object of type `D` from an expression of type (possibly const) `D` shall not throw exceptions", we can just refer to them. An alternative would be to define `NothrowCopyConstructible`, which includes `CopyConstructible` but requires that construction and destruction do not throw.]

Change 20.3.2.2.2 [util.smartptr.shared.const] p8:

> `D` shall be `CopyConstructible` and such construction shall not throw exceptions. The copy constructor and destructor of `D` shall not throw exceptions.

### 2686(i). Why is `std::hash` specialized for `error_code`, but not `error_condition`?

Section: 19.5.2 [system.error.syn] Status: C++17 Submitter: Tim Song Opened: 2016-05-03 Last modified: 2021-06-06

Priority: 3

View all other issues in [system.error.syn].

View all issues with C++17 status.

Discussion:

Both `error_code` and `error_condition` have an `operator<` overload, enabling their use in associative containers without having to write a custom comparator.

However, only `error_code` has a `std::hash` specialization. So it's possible to have a `set<error_code>`, a `set<error_condition>`, an `unordered_set<error_code>`, but not an `unordered_set<error_condition>`. This seems...odd.

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4582.

1. Edit [system_error.syn], header `<system_error>` synopsis, as indicated:

   >     namespace std {
   >         // ...

   >         // 19.5.6 Hash support
   >         template<class T> struct hash;
   >         template<> struct hash<error_code>;
   >         template<> struct hash<error_condition>;

   >        // ...
   >     }

2. Edit 19.5.7 [syserr.hash] as indicated:

   >     template <> struct hash<error_code>;
   >     template <> struct hash<error_condition>;

   > > -1- The template specializations shall meet the requirements of class template `hash` (20.12.14).

### 2687(i). `{inclusive,exclusive}_scan` misspecified

Section: 26.10.8 [exclusive.scan], 26.10.9 [inclusive.scan], 26.10.10 [transform.exclusive.scan], 26.10.11 [transform.inclusive.scan] Status: C++17 Submitter: Tim Song Opened: 2016-03-21 Last modified: 2017-07-30

Priority: 1

View all other issues in [exclusive.scan].

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Discussion:

When P0024R2 changed the description of `{inclusive,exclusive}_scan` (and the `transform_` version), it seems to have introduced an off-by-one error. Consider what N4582 26.10.9 [inclusive.scan]/2 says of `inclusive_scan`:

> Effects: Assigns through each iterator `i` in `[result, result + (last - first))` the value of

> - `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, *j, ...)` for every `j` in `[first, first + (i - result))` if `init` is provided, or

> - `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, *j, ...)` for every `j` in `[first, first + (i - result))` otherwise.

When `i == result`, `i - result = 0`, so the range `[first, first + (i - result))` is `[first, first)` — which is empty. Thus according to the specification, we should assign `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init)` if `init` is provided, or `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op)` otherwise, which doesn't seem "inclusive" — and isn't even defined in the second case.

The parallelism TS's version uses `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, *first, ..., *(first + (i - result)))` — which is a closed range, not a half-open one.

Similar issues affect `exclusive_scan`, `transform_inclusive_scan`, and `transform_exclusive_scan`.

[2016-06 Oulu]

Voted to Ready 11-0 Tuesday evening in Oulu

Proposed resolution:

This wording is relative to N4582.

1. Edit 26.10.8 [exclusive.scan]/2 as indicated:

   > [Drafting note: when `i` is `result`, `[first, first + (i - result))` is an empty range, so the value to be assigned reduces to `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init)`, which is `init`, so there's no need to special case this.]

   >     template<class InputIterator, class OutputIterator, class T, class BinaryOperation>
   >       OutputIterator exclusive_scan(InputIterator first, InputIterator last,
   >                                     OutputIterator result,
   >                                     T init, BinaryOperation binary_op);

   > > -2- Effects: Assigns through each iterator `i` in `[result, result + (last - first))` the value of `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, *j, ...)` for every `j` in `[first, first + (i - result))`.

   > > - `init`, if `i` is `result`, otherwise

   > > - `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, *j, ...)` for every `j` in `[first, first + (i - result) - 1)`.

2. Edit 26.10.9 [inclusive.scan]/2 as indicated:

   >     template<class InputIterator, class OutputIterator, class BinaryOperation>
   >       OutputIterator inclusive_scan(InputIterator first, InputIterator last,
   >                                     OutputIterator result,
   >                                     BinaryOperation binary_op);
   >     template<class InputIterator, class OutputIterator, class BinaryOperation>
   >       OutputIterator inclusive_scan(InputIterator first, InputIterator last,
   >                                     OutputIterator result,
   >                                     BinaryOperation binary_op, T init);

   > > -2- Effects: Assigns through each iterator `i` in `[result, result + (last - first))` the value of

   > > - `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, *j, ...)` for every `j` in `[first, first + (i - result + 1))` if `init` is provided, or

   > > - `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, *j, ...)` for every `j` in `[first, first + (i - result + 1))` otherwise.

3. Edit 26.10.10 [transform.exclusive.scan]/1 as indicated:

   >     template<class InputIterator, class OutputIterator,
   >              class UnaryOperation,
   >              class T, class BinaryOperation>
   >       OutputIterator transform_exclusive_scan(InputIterator first, InputIterator last,
   >                                               OutputIterator result,
   >                                               UnaryOperation unary_op,
   >                                               T init, BinaryOperation binary_op);

   > > -1- Effects: Assigns through each iterator `i` in `[result, result + (last - first))` the value of `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, unary_op(*j), ...)` for every `j` in `[first, first + (i - result))`.

   > > - `init`, if `i` is `result`, otherwise

   > > - `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, unary_op(*j), ...)` for every `j` in `[first, first + (i - result) - 1)`.

4. Edit 26.10.11 [transform.inclusive.scan]/1 as indicated:

   >     template<class InputIterator, class OutputIterator,
   >              class UnaryOperation,
   >              class BinaryOperation>
   >       OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last,
   >                                               OutputIterator result,
   >                                               UnaryOperation unary_op,
   >                                               BinaryOperation binary_op);
   >     template<class InputIterator, class OutputIterator,
   >              class UnaryOperation,
   >              class BinaryOperation, class T>
   >       OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last,
   >                                               OutputIterator result,
   >                                               UnaryOperation unary_op,
   >                                               BinaryOperation binary_op, T init);

   > > -1- Effects: Assigns through each iterator `i` in `[result, result + (last - first))` the value of

   > > - `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, unary_op(*j), ...)` for every `j` in `[first, first + (i - result + 1))` if `init` is provided, or

   > > - `GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, unary_op(*j), ...)` for every `j` in `[first, first + (i - result + 1))` otherwise.

### 2688(i). `clamp` misses preconditions and has extraneous condition on result

Section: 26.8.10 [alg.clamp] Status: C++17 Submitter: Martin Moene Opened: 2016-03-23 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

In Jacksonville (2016), P0025R0 was voted in instead of the intended P0025R1. This report contains the necessary mending along with two other improvements.

This report:

- adds the precondition that misses from P0025R0 but is in P0025R1,

- corrects the returns: specification that contains an extraneous condition,

- replaces the now superfluous remark with a note on usage of clamp with `float` or `double`.

Thanks to Carlo Assink and David Gaarenstroom for making us aware of the extraneous condition in the returns: specification and for suggesting the fix and to Jeffrey Yasskin for suggesting to add a note like p3.

Previous resolution [SUPERSEDED]:

> 1. Edit 26.8.10 [alg.clamp] as indicated:

>    >     template<class T>
>    >       constexpr const T& clamp(const T& v, const T& lo, const T& hi);
>    >     template<class T, class Compare>
>    >       constexpr const T& clamp(const T& v, const T& lo, const T& hi, Compare comp);

>    > > -1- Requires: The value of `lo` shall be no greater than `hi`. For the first form, type `T` shall be `LessThanComparable` (Table 18).

>    > > -2- Returns: `lo` if `v` is less than `lo`, `hi` if `hi` is less than `v`, otherwise `v` The larger value of `v` and `lo` if `v` is smaller than `hi`, otherwise the smaller value of `v` and `hi`.

>    > > -3- NoteRemarks: If NaN is avoided, `T` can be `float` or `double` Returns the first argument when it is equivalent to one of the boundary arguments.

>    > > -4- Complexity: At most two comparisons.

[2016-05 Issues Telecon]

Reworded p3 slightly.

Proposed resolution:

This wording is relative to N4582.

1. Edit 26.8.10 [alg.clamp] as indicated:

   >     template<class T>
   >       constexpr const T& clamp(const T& v, const T& lo, const T& hi);
   >     template<class T, class Compare>
   >       constexpr const T& clamp(const T& v, const T& lo, const T& hi, Compare comp);

   > > -1- Requires: The value of `lo` shall be no greater than `hi`. For the first form, type `T` shall be `LessThanComparable` (Table 18).

   > > -2- Returns: `lo` if `v` is less than `lo`, `hi` if `hi` is less than `v`, otherwise `v` The larger value of `v` and `lo` if `v` is smaller than `hi`, otherwise the smaller value of `v` and `hi`.

   > > -3- NoteRemarks: If NaN is avoided, `T` can be a floating point type Returns the first argument when it is equivalent to one of the boundary arguments.

   > > -4- Complexity: At most two comparisons.

### 2689(i). Parallel versions of `std::copy` and `std::move` shouldn't be in order

Section: 26.7.1 [alg.copy], 26.7.2 [alg.move] Status: C++17 Submitter: Tim Song Opened: 2016-03-23 Last modified: 2017-07-30

Priority: 0

View other active issues in [alg.copy].

View all other issues in [alg.copy].

View all issues with C++17 status.

Discussion:

26.3.5 [algorithms.parallel.overloads]/2 says that "Unless otherwise specified, the semantics of `ExecutionPolicy` algorithm overloads are identical to their overloads without."

There's no "otherwise specified" for the `ExecutionPolicy` overloads for `std::copy` and `std::move`, so the requirement that they "start[] from first and proceed[] to last" in the original algorithm's description would seem to apply, which defeats the whole point of adding a parallel overload.

[2016-05 Issues Telecon]

Marshall noted that all three versions of copy have subtly different wording, and suggested that they should not.

Proposed resolution:

This wording is relative to N4582.

1. Insert the following paragraphs after 26.7.1 [alg.copy]/4:

   >     template<class ExecutionPolicy, class InputIterator, class OutputIterator>
   >       OutputIterator copy(ExecutionPolicy&& policy, InputIterator first, InputIterator last,
   >                           OutputIterator result);

   > > -?- Requires: The ranges `[first, last)` and `[result, result + (last - first))` shall not overlap.

   > > -?- Effects: Copies elements in the range `[first, last)` into the range `[result, result + (last - first))`. For each non-negative integer `n < (last - first)`, performs `*(result + n) = *(first + n)`.

   > > -?- Returns: `result + (last - first)`.

   > > -?- Complexity: Exactly `last - first` assignments.

2. Insert the following paragraphs after 26.7.2 [alg.move]/4:

   >     template<class ExecutionPolicy, class InputIterator, class OutputIterator>
   >       OutputIterator move(ExecutionPolicy&& policy, InputIterator first, InputIterator last,
   >                           OutputIterator result);

   > > -?- Requires: The ranges `[first, last)` and `[result, result + (last - first))` shall not overlap.

   > > -?- Effects: Moves elements in the range `[first, last)` into the range `[result, result + (last - first))`. For each non-negative integer `n < (last - first)`, performs `*(result + n) = std::move(*(first + n))`.

   > > -?- Returns: `result + (last - first)`.

   > > -?- Complexity: Exactly `last - first` assignments.

### 2690(i). `invoke<R>`

Section: 22.10.5 [func.invoke] Status: Resolved Submitter: Zhihao Yuan Opened: 2016-03-25 Last modified: 2021-06-12

Priority: Not Prioritized

View all other issues in [func.invoke].

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Discussion:

In N4169 the author dropped the `invoke<R>` support by claiming that it's an unnecessary cruft in TR1, obsoleted by C++11 type inference. But now we have some new business went to *`INVOKE`*`(f, t1, t2, ..., tN, R)`, that is to discard the return type when `R` is `void`. This form is very useful, or possible even more useful than the basic form when implementing a call wrapper. Also note that the optional `R` support is already in `std::is_callable` and `std::is_nothrow_callable`.

[2016-07-31, Tomasz Kamiński comments]

The lack of `invoke<R>` was basically a result of the concurrent publication of the never revision of the paper and additional special semantics of `INVOKE(f, args..., void)`.

In contrast to existing `std::invoke` function, the proposed `invoke<R>` version is not SFINAE friendly, as elimination of the standard version of invoke is guaranteed by `std::result_of_t` in the result type that is missing for proposed `invoke<R>` version. To provide this guarantee, following remarks shall be added to the specification:

> Remarks: This function shall not participate in overload resolution unless `is_callable_v<F(Args...), R>` is `true`.

[2016-08-01, Tomasz Kamiński and Zhihao Yuan update the proposed wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.10 [function.objects]/2, header `<functional>` synopsis, as indicated:

>    >     namespace std {
>    >       // 20.12.3, invoke:
>    >       template <class F, class... Args> result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args);
>    >       template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);

> 2. Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:

>    >     template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);

>    > > -?- Returns: `INVOKE(std::forward<F>(f), std::forward<Args>(args)..., R)` (22.10.4 [func.require]).

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_callable_v<F(Args...), R>` is `true`.

[2016-09-04, Tomasz Kamiński comments and improves wording]

The usage of `is_callable_v<F(Args...), R>` causes problem in situation when either `F` or `Args` is an abstract type and the function type `F(Args...)` cannot be formed or when one of the args is cv-qualified, as top-level cv-qualification for function parameters is dropped by language rules. It should use `is_callable_v<F&&(Args&&...), R>` instead.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.10 [function.objects]/2, header `<functional>` synopsis, as indicated:

>    >     namespace std {
>    >       // 20.12.3, invoke:
>    >       template <class F, class... Args> result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args);
>    >       template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);

> 2. Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:

>    >     template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);

>    > > -?- Returns: `INVOKE(std::forward<F>(f), std::forward<Args>(args)..., R)` (22.10.4 [func.require]).

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_callable_v<F&&(Args&&...), R>` is `true`.

[2018-08-22, Zhihao Yuan provides improved wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4762.

> 1. Modify 22.10.2 [functional.syn], header `<functional>` synopsis, as indicated:

>    >     namespace std {
>    >       // 22.10.5 [func.invoke], invoke
>    >       template<class F, class... Args>
>    >         invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
>    >           noexcept(is_nothrow_invocable_v<F, Args...>);
>    >       template <class R, class F, class... Args>
>    >         R invoke(F&& f, Args&&... args)
>    >           noexcept(is_nothrow_invocable_r_v<R, F, Args...>);

> 2. Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:

>    >     template <class R, class F, class... Args>
>    >       R invoke(F&& f, Args&&... args)
>    >         noexcept(is_nothrow_invocable_r_v<R, F, Args...>);

>    > > -?- Constraints: `is_invocable_r_v<R, F, Args...>`.

>    > > -?- Returns: `INVOKE<R>(std::forward<F>(f), std::forward<Args>(args)...)` (22.10.4 [func.require]).

[2021-06-12; Resolved by accepting P2136R3.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 22.10.2 [functional.syn], header `<functional>` synopsis, as indicated:

   >     namespace std {
   >       // 22.10.5 [func.invoke], invoke
   >       template<class F, class... Args>
   >         constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
   >           noexcept(is_nothrow_invocable_v<F, Args...>);
   >       template <class R, class F, class... Args>
   >         constexpr R invoke(F&& f, Args&&... args)
   >           noexcept(is_nothrow_invocable_r_v<R, F, Args...>);

2. Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:

   >     template <class R, class F, class... Args>
   >       constexpr R invoke(F&& f, Args&&... args)
   >         noexcept(is_nothrow_invocable_r_v<R, F, Args...>);

   > > -?- Constraints: `is_invocable_r_v<R, F, Args...>`.

   > > -?- Returns: `INVOKE<R>(std::forward<F>(f), std::forward<Args>(args)...)` (22.10.4 [func.require]).

### 2693(i). `constexpr` for various `std::complex` arithmetic and value operators

Section: 29.4 [complex.numbers] Status: Resolved Submitter: Oliver Rosten Opened: 2016-04-14 Last modified: 2018-06-12

Priority: 3

View all other issues in [complex.numbers].

View all issues with Resolved status.

Discussion:

This modification will allow complex-number arithmetic to be performed at compile time. From a mathematical standpoint, it is natural (and desirable) to treat complex numbers on the same footing as the reals. From a programming perspective, this change will broaden the scope in which `std::complex` can be used, allowing it to be smoothly incorporated into classes exploiting `constexpr`.

Suggested resolution:

The following functions in the `std::complex` namespace should be made `constexpr`:

1. Section 29.4.5 [complex.member.ops]: The member (arithmetic) operators {`+=`, `-=`, `/=`, `*=`}

2. Section 29.4.6 [complex.ops]: The arithmetic operators unary operators {`+`, `-`} and binary operators {`+`, `-`, `/`, `*`};

3. Section 29.4.7 [complex.value.ops]: The 'value' operators `abs`, `norm`, and `conj`.

In terms of modification of the standard, all that is required is the insertion of the `constexpr` specifier in the relevant parts of:

1. Section 29.4.2 [complex.syn] (the Header synopsis), together with the corresponding entries in sections 29.4.6 [complex.ops] and 29.4.7 [complex.value.ops].

2. Sections 29.4.3 [complex], [complex.special] (class template and specializations), together with the corresponding entries in section 29.4.5 [complex.member.ops].

[2016-05 Issues Telecon]

This kind of work (new feature) has been being done via papers rather than via the issues list.

Walter believes that he knows someone who would be willing to write such a paper.

[2018-06 Rapperswil Wednesday issues processing]

This was resolved by P0415, which was adopted in Albequerque.

Proposed resolution:

### 2694(i). Application of LWG 436 accidentally deleted definition of "facet"

Section: 28.3.3.1.2.2 [locale.facet] Status: C++17 Submitter: Tim Song Opened: 2016-04-15 Last modified: 2017-07-30

Priority: 3

View all other issues in [locale.facet].

View all issues with C++17 status.

Discussion:

[locale.facet]/1 (then-called [lib.locale.facet]) read in C++03:

> Class `facet` is the base class for locale feature sets. A class is a facet if it is publicly derived from another facet, or if it is a class derived from `locale::facet` and containing a publicly-accessible declaration as follows: [Footnote: This is a complete list of requirements; there are no other requirements. Thus, a facet class need not have a public copy constructor, assignment, default constructor, destructor, etc.]

> >     static ::std::locale::id id;

> Template parameters in this clause which are required to be facets are those named `Facet` in declarations. A program that passes a type that is not a facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed.

LWG 436(i)'s intent appears to be to ban `volatile`-qualified facets and permitting `const`-qualified ones. The PR was somewhat poorly worded, however, and the editor in applying it deleted the whole first half of the paragraph, including the definition of facet and the requirement for a static data member named `id`.

As a result, we have things like 28.3.3.1.2.3 [locale.id]/2 and 28.3.3.2 [locale.global.templates]/1 referring to the `id` member that's not defined anywhere in the working draft.

[2016-08-03 Chicago]

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4582.

1. Insert the following paragraph before 28.3.3.1.2.2 [locale.facet]/1:

   > -?- Class `facet` is the base class for locale feature sets. A class is a facet if it is publicly derived from another facet, or if it is a class derived from `locale::facet` and containing a publicly accessible declaration as follows: [Footnote: This is a complete list of requirements; there are no other requirements. Thus, a facet class need not have a public copy constructor, assignment, default constructor, destructor, etc.]

   > >     static ::std::locale::id id;

   > -1- Template parameters in this Clause which are required to be facets are those named `Facet` in declarations. A program that passes a type that is not a facet, or a type that refers to a volatile-qualified facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed. A const-qualified facet is a valid template argument to any locale function that expects a Facet template parameter.

### 2696(i). Interaction between `make_shared` and `enable_shared_from_this` is underspecified

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++17 Submitter: Joe Gottman Opened: 2016-04-02 Last modified: 2017-07-30

Priority: 2

View all other issues in [util.smartptr.shared.create].

View all issues with C++17 status.

Discussion:

For each public constructor of `std::shared_ptr`, the standard says that constructor enables `shared_from_this` if that constructor is expected to initialize the internal `weak_ptr` of a contained `enable_shared_from_this<X>` object. But there are other ways to construct a `shared_ptr` than by using a public constructor. The template functions `make_shared` and `allocate_shared` both require calling a private constructor, since no public constructor can fulfill the requirement that at most one allocation is made. The standard does not specify that that private constructor enables `shared_from_this`; therefore in the following code:

>     struct Foo : public std::enable_shared_from_this<Foo> {};

>     int main() {
>       auto p = std::make_shared<Foo>();
>       assert(p == p->shared_from_this());
>       return 0;
>     }

it is unspecified whether the assertion will fail.

[2016-05 Issues Telecon]

Jonathan Wakely to provide updated wording.

[2016-08 Chicago]

Monday PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4582.

1. Change 20.3.2.2.7 [util.smartptr.shared.create] indicated:

   >     template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);
   >     template<class T, class A, class... Args>
   >       shared_ptr<T> allocate_shared(const A& a, Args&&... args);

   > > […]

   > > -6- Remarks: The `shared_ptr` constructor called by this function enables `shared_from_this` with the address of the newly constructed object of type `T`. Implementations should perform no more than one memory allocation. [Note: This provides efficiency equivalent to an intrusive smart pointer. — end note]

### 2697(i). [concurr.ts] Behavior of `future/shared_future` unwrapping constructor when given an invalid `future`

Section: 99 [concurr.ts::futures.unique_future], 99 [concurr.ts::futures.shared_future] Status: WP Submitter: Tim Song Opened: 2016-04-22 Last modified: 2026-02-21

Priority: 2

View all issues with WP status.

Discussion:

Addresses: concurr.ts

In the concurrency TS, the future/shared_future unwrapping constructors

>     future(future<future<R>>&&) noexcept;
>     shared_future(future<shared_future<R>>&& rhs) noexcept;

appear to implicitly require `rhs` be valid (e.g., by referring to its shared state, and by requiring a `valid() == true` postcondition). However, they are also marked `noexcept`, suggesting that they are wide-contract, and also makes the usual suggested handling for invalid futures, throwing a `future_error`, impossible.

Either the `noexcept` should be removed, or the behavior with an invalid future should be specified.

Original resolution alternative #1 [NOT CHOSEN]:

> This wording is relative to N4577.

> Strike the `noexcept` on these constructors in 99 [concurr.ts::futures.unique_future]/1-2 and 99 [concurr.ts::futures.shared_future]/1-2, and optionally add a Requires: `rhs.valid() == true` paragraph.

[2016-11-12, Issaquah]

Sat PM: We prefer alternative #2 - Move to review

[2018-06; Rapperswil, Wednesday evening session]

DR: there is a sentence ended followed by an entirely new sentence
JM: so the period should be a semicolon in both edits
MC: ACTION I can make the change editorially
ACTION move to Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4577.

> Alternative #2: Specify that an empty (`shared_`)`future` object is constructed if `rhs` is invalid, and adjust the postcondition accordingly.

1. Edit 99 [concurr.ts::futures.unique_future] as indicated:

   >     future(future<future<R>>&& rhs) noexcept;

   > > -3- Effects: If `rhs.valid() == false`, constructs an empty `future` object that does not refer to a shared state. Otherwise, cConstructs a `future` object from the shared state referred to by `rhs`. T; the `future` becomes ready when one of the following occurs:

   > > - Both the `rhs` and `rhs.get()` are ready. The value or the exception from `rhs.get()` is stored in the `future`'s shared state.

   > > - `rhs` is ready but `rhs.get()` is invalid. An exception of type `std::future_error`, with an error condition of `std::future_errc::broken_promise` is stored in the `future`'s shared state.

   > > -4- Postconditions:

   > > - `valid() == true` `valid()` returns the same value as `rhs.valid()` prior to the constructor invocation..

   > > - `rhs.valid() == false`.

2. Edit 99 [concurr.ts::futures.shared_future] as indicated:

   >     shared_future(future<shared_future<R>>&& rhs) noexcept;

   > > -3- Effects: If `rhs.valid() == false`, constructs an empty `shared_future` object that does not refer to a shared state. Otherwise, cConstructs a `shared_future` object from the shared state referred to by `rhs`. T; the `shared_future` becomes ready when one of the following occurs:

   > > - Both the `rhs` and `rhs.get()` are ready. The value or the exception from `rhs.get()` is stored in the `shared_future`'s shared state.

   > > - `rhs` is ready but `rhs.get()` is invalid. The `shared_future` stores an exception of type `std::future_error`, with an error condition of `std::future_errc::broken_promise`.

   > > -4- Postconditions:

   > > - `valid() == true` `valid()` returns the same value as `rhs.valid()` prior to the constructor invocation..

   > > - `rhs.valid() == false`.

### 2698(i). Effect of `assign()` on iterators/pointers/references

Section: 23.2.4 [sequence.reqmts] Status: C++17 Submitter: Tim Song Opened: 2016-04-25 Last modified: 2017-07-30

Priority: 0

View other active issues in [sequence.reqmts].

View all other issues in [sequence.reqmts].

View all issues with C++17 status.

Discussion:

The sequence container requirements table says nothing about the effect of `assign()` on iterators, pointers or references into the container. Before LWG 2209(i) (and LWG 320(i) for `std::list`), `assign()` was specified as "erase everything then `insert`", which implies wholesale invalidation from the "erase everything" part. With that gone, the blanket "no invalidation" wording in 23.2.2 [container.requirements.general]/12 would seem to apply, which makes absolutely no sense.

The proposed wording below simply spells out the invalidation rule implied by the previous "erase everything" wording.

[2016-05 Issues Telecon]

This is related to 2256(i)

Proposed resolution:

This wording is relative to N4582.

1. In 23.2.4 [sequence.reqmts], edit Table 107 (Sequence container requirements) as indicated:

   > Table 107 — Sequence container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition
   > --- | --- | ---
   > `[…]`
   > `a.assign(i, j)` | `void` | Requires: `T` shall be `EmplaceConstructible` into `X` from `*i` and assignable from `*i`. For `vector`, if the iterator does not meet the forward iterator requirements (24.2.5), `T` shall also be `MoveInsertable` into `X`. Each iterator in the range `[i, j)` shall be dereferenced exactly once. pre: `i`, `j` are not iterators into `a`. Replaces elements in `a` with a copy of `[i, j)`. Invalidates all references, pointers and iterators referring to the elements of `a`. For `vector` and `deque`, also invalidates the past-the-end iterator.
   > `[…]`
   > `a.assign(n, t)` | `void` | Requires: `T` shall be `CopyInsertable` into `X` and `CopyAssignable`. pre: `t` is not a reference into `a`. Replaces elements in `a` with `n` copies of `t`. Invalidates all references, pointers and iterators referring to the elements of `a`. For `vector` and `deque`, also invalidates the past-the-end iterator.

### 2699(i). Missing restriction in [numeric.requirements]

Section: 29.2 [numeric.requirements] Status: C++17 Submitter: Hubert Tong Opened: 2016-05-03 Last modified: 2017-07-30

Priority: 3

View all other issues in [numeric.requirements].

View all issues with C++17 status.

Discussion:

In N4582 subclause 29.2 [numeric.requirements], the "considerable flexibility in how arrays are initialized" do not appear to allow for replacement of calls to the default constructor with calls to the copy constructor with an appropriate source value.

[2016-08-03 Chicago]

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4582.

1. Adjust 29.2 [numeric.requirements]/1 as indicated:

   > -1- The `complex` and `valarray` components are parameterized by the type of information they contain and manipulate. […]

   > 1. (1.1) — `T` is not an abstract class (it has no pure virtual member functions);

   > 2. […]

   > 3. (1.8) — If `T` is a class, its assignment operator, copy and default constructors, and destructor shall correspond to each other in the following sense: Initialization of raw storage using the copy constructor on the value of `T()`, however obtained, is semantically equivalent to value initialization of the same raw storage. Initialization of raw storage using the default constructor, followed by assignment, is semantically equivalent to initialization of raw storage using the copy constructor. Destruction of an object, followed by initialization of its raw storage using the copy constructor, is semantically equivalent to assignment to the original object. [Note: This rule states, in part, that there shall not be any subtle differences in the semantics of initialization versus assignment. This gives an implementation considerable flexibility in how arrays are initialized. [Example: An implementation is allowed to initialize a `valarray` by allocating storage using the `new` operator (which implies a call to the default constructor for each element) and then assigning each element its value. Or the implementation can allocate raw storage and use the copy constructor to initialize each element. — end example] If the distinction between initialization and assignment is important for a class, or if it fails to satisfy any of the other conditions listed above, the programmer should use `vector` (23.3.11) instead of `valarray` for that class; — end note]

### 2704(i). `recursive_directory_iterator`'s members should require '`*this` is dereferenceable'

Section: 31.12.12.2 [fs.rec.dir.itr.members], 31.12.11.2 [fs.dir.itr.members] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-08 Last modified: 2017-07-30

Priority: 1

View all other issues in [fs.rec.dir.itr.members].

View all issues with C++17 status.

Discussion:

In 31.12.12.2 [fs.rec.dir.itr.members] the following members are specified as having the requirement "`*this != recursive_directory_iterator{}`":

- `options()`

- `depth()`

- `recursion_pending()`

- `operator++`

- `increment(...)`

- `pop()`

- `disable_recursion_pending()`

This requirement is not strong enough since it still allows non-dereferenceable iterators to invoke these methods. For example:

>     recursive_directory_iterator it(".");
>     recursive_directory_iterator it_copy(it);
>     assert(it_copy.depth() == 0); // OK
>     ++it;
>     assert(it_copy.depth() == ???); // Not OK
>     auto x = *it_copy; // Is this OK?

I believe these should instead require that `*this` is dereferenceable, however the current specification seems to say that all previous copies of `it` are still dereferenceable although not what they dereference to.

[fs.class.directory_iterator] p4:

> The result of `operator*` on an end iterator is undefined behavior. For any other iterator value a `const recursive_directory_entry&` is returned. The result of `operator->` on an end iterator is undefined behavior. For any other iterator value a `const directory_entry*` is returned.

Is the intention of this clause to make all non-end iterators dereferenceable?

One further complication with these methods comes from the specification of `recursive_directory_iterator`'s copy/move constructors and assignment operators which specify the following post conditions:

- `this->options() == rhs.options()`

- `this->depth() == rhs.depth()`

- `this->recursion_pending() == rhs.recursion_pending()`

If `rhs` is the end iterator these post conditions are poorly stated.

[2016-06, Oulu — Daniel comments]

The loss of information caused by bullet three of the suggested wording below is corrected by 2726(i)'s wording.

Voted to Ready 7-0 Monday morning in Oulu

Proposed resolution:

This wording is relative to N4582.

> [Drafting note: I have not attempted to fix the specification of the copy/move constructors and assignment operators for `recursive_directory_iterator`]

> [Drafting note: `increment` directly specifies "Effects: As specified by Input iterators (24.2.3)", so no additional specification is needed.]

1. Change [fs.class.directory_iterator] p4 as indicated:

   > -4- The result of `operator*` on an end iterator is undefined behavior. For any other iterator value a `const directory_entry&` is returned. The result of `operator->` on an end iterator is undefined behavior. For any other iterator value a `const directory_entry*` is returnedThe end iterator is not dereferenceable.

2. Add a new bullet after the class synopsis in 31.12.12 [fs.class.rec.dir.itr]:

   > -?- Calling `options`, `depth`, `recursion_pending`, `pop` or `disable_recursion_pending` on an iterator that is not dereferencable results in undefined behavior.

3. Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:

   >     directory_options options() const;

   > > -17- Requires: `*this != recursive_directory_iterator()`.

   > > […]

   >     int depth() const;

   > > -20- Requires: `*this != recursive_directory_iterator()`.

   > > […]

   >     bool recursion_pending() const;

   > > -23- Requires: `*this != recursive_directory_iterator()`.

   > > […]

   >     recursive_directory_iterator& operator++();
   >     recursive_directory_iterator& increment(error_code& ec) noexcept;

   > > -26- Requires: `*this != recursive_directory_iterator()`.

   > > […]

   >     void pop();

   > > -30- Requires: `*this != recursive_directory_iterator()`.

   > > […]

   >     void disable_recursion_pending();

   > > -32- Requires: `*this != recursive_directory_iterator()`.

   > > […]

### 2706(i). Error reporting for `recursive_directory_iterator::pop()` is under-specified

Section: 31.12.12 [fs.class.rec.dir.itr] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-09 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

Unlike `increment`, `pop()` does not specify how it reports errors nor does it provide a `std::error_code` overload. However implementing `pop()` all but requires performing an increment, so it should handle errors in the same way.

Proposed resolution:

This wording is relative to N4582.

1. Change 31.12.12 [fs.class.rec.dir.itr], class `recursive_directory_iterator` synopsis, as indicated:

   >     namespace std::filesystem {
   >       class recursive_directory_iterator {
   >       public:
   >         […]
   >         void pop();
   >         void pop(error_code& ec);
   >         void disable_recursion_pending();
   >         […]
   >       };
   >     }

2. Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:

   >     void pop();
   >     void pop(error_code& ec);

   > > -30- Requires: `*this != recursive_directory_iterator()`.

   > > -31- Effects: If `depth() == 0`, set `*this` to `recursive_directory_iterator()`. Otherwise, cease iteration of the directory currently being iterated over, and continue iteration over the parent directory.

   > > -?- Throws: As specified in Error reporting (31.5.6 [error.reporting]).

### 2707(i). `path` construction and assignment should have "`string_type&&`" overloads

Section: 31.12.6 [fs.class.path] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-09 Last modified: 2017-07-30

Priority: 0

View all other issues in [fs.class.path].

View all issues with C++17 status.

Discussion:

Currently construction of a `path` from the native `string_type` always performs a copy, even when that string is passed as a rvalue. This is a large pessimization as paths are commonly constructed from temporary strings.

One pattern I frequently see is:

>     path foo(path const& p) {
>       auto s = p.native();
>       mutateString(s);
>       return s;
>     }

Implementations should be allowed to move from `s` and avoid an unnecessary allocation. I believe `string_type&&` constructor and assignment operator overloads should be added to support this.

Proposed resolution:

This wording is relative to N4582.

1. Change 31.12.6 [fs.class.path], class `path` synopsis, as indicated:

   > [Drafting note: Making the `string_type&&` constructors and assignment operators `noexcept` would over-constrain implementations which may need to perform construct additional state]

   >     namespace std::filesystem {
   >       class path {
   >       public:
   >         […]
   >         // 27.10.8.4.1, constructors and destructor
   >         path() noexcept;
   >         path(const path& p);
   >         path(path&& p) noexcept;
   >         path(string_type&& source);
   >         template <class Source>
   >         path(const Source& source);
   >         […]

   >         // 27.10.8.4.2, assignments
   >         path& operator=(const path& p);
   >         path& operator=(path&& p) noexcept;
   >         path& operator=(string_type&& source);
   >         path& assign(string_type&& source);
   >         template <class Source>
   >         path& operator=(const Source& source);
   >         template <class Source>
   >         path& assign(const Source& source)
   >         template <class InputIterator>
   >         path& assign(InputIterator first, InputIterator last);
   >         […]
   >       };
   >     }

2. Add a new paragraph following 31.12.6.5.1 [fs.path.construct]/3:

   >     path(string_type&& source);

   > > -?- Effects: Constructs an object of class `path` with `pathname` having the original value of `source`. `source` is left in a valid but unspecified state.

3. Add a new paragraph following 31.12.6.5.2 [fs.path.assign]/4:

   >     path& operator=(string_type&& source);
   >     path& assign(string_type&& source);

   > > -?- Effects: Modifies `pathname` to have the original value of `source`. `source` is left in a valid but unspecified state.

   > > -?- Returns:`*this`

### 2709(i). `offsetof` is unnecessarily imprecise

Section: 17.2 [support.types] Status: C++17 Submitter: Richard Smith Opened: 2016-05-10 Last modified: 2017-07-30

Priority: 2

View all other issues in [support.types].

View all issues with C++17 status.

Discussion:

Per 17.2 [support.types]/4:

> "The macro `offsetof(type, member-designator)` accepts a restricted set of type arguments in this International Standard. If type is not a standard-layout class (Clause 9), the results are undefined. [...]"

Implementations in practice interpret this as meaning that the program is ill-formed for classes that are not standard-layout, but several implementations allow additional types as an extension (rejected in their "strictly conforming" modes).

It would seem superior to specify that implementation-defined extensions are permitted, and that the implementation must give a correct answer for any type that it chooses to accept.

[2016-05 Issues Telecon]

People were worried about the requirement to report errors implied by 'conditionally supported'.

[2016-06 Oulu]

The concern about errors appears to be less severe that we thought. Moving to Ready.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4582.

1. Change in 17.2 [support.types]/4:

   > -4- The macro `offsetof(type, member-designator)` accepts a restricted set of type arguments in this International Standard. If `type` is notUse of the `offsetof` macro with a `type` other than a standard-layout class (Clause 9), the results are undefinedis conditionally-supported.193 The expression `offsetof(type, member-designator)` is never type-dependent (14.6.2.2) and it is value-dependent (14.6.2.3) if and only if `type` is dependent. The result of applying the `offsetof` macro to a static data member or a function member is undefined. No operation invoked by the `offsetof` macro shall throw an exception and `noexcept(offsetof(type, member-designator))` shall be `true`.

### 2710(i). "Effects: Equivalent to ..." doesn't count "Synchronization:" as determined semantics

Section: 16.3.2.4 [structure.specifications] Status: C++17 Submitter: Kazutoshi Satoda Opened: 2016-05-08 Last modified: 2017-07-30

Priority: 0

View other active issues in [structure.specifications].

View all other issues in [structure.specifications].

View all issues with C++17 status.

Discussion:

From N4582 17.5.1.4 [structure.specifications] p3 and p4

> -3- Descriptions of function semantics contain the following elements (as appropriate):

> - Requires: the preconditions for calling the function
> - Effects: the actions performed by the function
> - Synchronization: the synchronization operations (1.10) applicable to the function
> - Postconditions: the observable results established by the function
> - Returns: a description of the value(s) returned by the function
> - Throws: any exceptions thrown by the function, and the conditions that would cause the exception
> - Complexity: the time and/or space complexity of the function
> - Remarks: additional semantic constraints on the function
> - Error conditions: the error conditions for error codes reported by the function.
> - Notes: non-normative comments about the function

> -4- Whenever the Effects: element specifies that the semantics of some function `F` are Equivalent to some code sequence, then the various elements are interpreted as follows. If `F`'s semantics specifies a Requires: element, then that requirement is logically imposed prior to the equivalent-to semantics. Next, the semantics of the code sequence are determined by the Requires:, Effects:, Postconditions:, Returns:, Throws:, Complexity:, Remarks:, Error conditions:, and Notes: specified for the function invocations contained in the code sequence. The value returned from `F` is specified by `F`'s Returns: element, or if `F` has no Returns: element, a non-`void` return from `F` is specified by the Returns: elements in the code sequence. If `F`'s semantics contains a Throws:, Postconditions:, or Complexity: element, then that supersedes any occurrences of that element in the code sequence.

The third sentence of p4 says "the semantics of the code sequence are determined by ..." and lists all elements in p3 except "Synchronization:".

I think it was just an oversight because p4 was added by library issue 997(i), and its proposed resolution was drafted at the time (2009) before "Synchronization:" was added into p3 for C++11.

However, I'm not definitely sure that it is really intended and safe to just supply "Synchronization:" in the list. (Could a library designer rely on this in writing new specifications, or could someone rely on this in writing user codes, after some years after C++11?)

Proposed resolution:

This wording is relative to N4582.

1. Change 16.3.2.4 [structure.specifications] as indicated:

   > -4- Whenever the Effects: element specifies that the semantics of some function `F` are Equivalent to some code sequence, then the various elements are interpreted as follows. If `F`'s semantics specifies a Requires: element, then that requirement is logically imposed prior to the equivalent-to semantics. Next, the semantics of the code sequence are determined by the Requires:, Effects:, Synchronization:, Postconditions:, Returns:, Throws:, Complexity:, Remarks:, Error conditions:, and Notes: specified for the function invocations contained in the code sequence. The value returned from `F` is specified by `F`'s Returns: element, or if `F` has no Returns: element, a non-`void` return from `F` is specified by the Returns: elements in the code sequence. If `F`'s semantics contains a Throws:, Postconditions:, or Complexity: element, then that supersedes any occurrences of that element in the code sequence.

### 2711(i). `path` is convertible from approximately everything under the sun

Section: 31.12.6.5.1 [fs.path.construct] Status: C++17 Submitter: Tim Song Opened: 2016-05-10 Last modified: 2017-07-30

Priority: 1

View all issues with C++17 status.

Discussion:

The unconstrained `template<class Source> path(const Source& source);` constructor defines an implicit conversion from pretty much everything to `path`. This can lead to surprising ambiguities in overload resolution.

For example, given LWG 2676(i)'s proposed resolution, the following code appears to be ambiguous:

>     struct meow {
>       operator const char *() const;
>     };

>     std::ifstream purr(meow{});

because a `meow` can be converted to either a `path` or a `const char*` by a user-defined conversion, even though part of the stated goal of LWG 2676(i)'s P/R is to avoid "break[ing] user code depending on user-defined automatic conversions to the existing argument types".

Previous resolution [SUPERSEDED]:

> This wording is relative to N4582.

> > [Drafting note: `decay_t<Source>` handles both the input iterator case (31.12.6.4 [fs.path.req]/1.2) and the array case (31.12.6.4 [fs.path.req]/1.3). The level of constraints required is debatable; the following wording limits the constraint to "is a `basic_string` or an iterator", but perhaps stronger constraints (e.g., an `iterator_category` check in the second case, and/or a `value_type` check for both cases) would be desirable.]

> 1. Change 31.12.6.5.1 [fs.path.construct] as indicated:

>    >     template <class Source>
>    >       path(const Source& source);
>    >     template <class InputIterator>
>    >       path(InputIterator first, InputIterator last);

>    > > -4- Effects: Constructs an object of class `path`, storing the effective range of `source` (27.10.8.3) or the range `[first, last)` in `pathname`, converting format and encoding if required (27.10.8.2.1).

>    > > -?- Remarks: The first overload shall not participate in overload resolution unless either `Source` is a specialization of `basic_string` or the qualified-id `iterator_traits<decay_t<Source>>::value_type` is valid and denotes a type (13.10.3 [temp.deduct]).

[2016-05-28, Eric Fiselier comments suggests alternative wording]

Functions taking `EcharT` or `Source` parameter types often provide additional overloads with the same arity and concrete types. In order to allow conversions to these concrete types in the interface we need to explicitly disable the `EcharT` and `Source` overloads.

[2016-06-19, Eric and Tim improve the wording]

[2016-06, Oulu]

Voted to Ready 8-0 Monday morning in Oulu

Proposed resolution:

This wording is relative to N4594.

> [Drafting note: Functions taking `EcharT` or `Source` parameter types often provide additional overloads with the same arity and concrete types. In order to allow conversions to these concrete types in the interface we need to explicitly disable the `EcharT` and `Source` overloads.]

1. Change 31.12.3 [fs.req] as indicated:

   > -2- Template parameters named `EcharT` shall beFunctions with template parameters named `EcharT` shall not participate in overload resolution unless `EcharT` is one of the encoded character types.

2. Insert a new paragraph between 31.12.6.4 [fs.path.req] p1 and p2 as indicated:

   > -?- Functions taking template parameters named `Source` shall not participate in overload resolution unless either `Source` is a specialization of `basic_string` or the qualified-id `iterator_traits<decay_t<Source>>::value_type` is valid and denotes a possibly `const` encoded character type (13.10.3 [temp.deduct]).

### 2712(i). `copy_file(from, to, ...)` has a number of unspecified error conditions

Section: 31.12.13.5 [fs.op.copy.file] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-10 Last modified: 2021-06-06

Priority: 2

View all other issues in [fs.op.copy.file].

View all issues with C++17 status.

Discussion:

There are a number of error cases that `copy_file(from, to, ...)` does not take into account. Specifically the cases where:

1. `from` does not exist
2. `from` is not a regular file
3. `to` exists and is not a regular file

These error cases should be specified as such.

[2016-05 Issues Telecon]

Eric to provide wording.

[2016-05-28, Eric Fiselier provides wording]

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4582.

1. Modify [fs.op.copy_file] as indicated:

   >     bool copy_file(const path& from, const path& to, copy_options options);
   >     bool copy_file(const path& from, const path& to, copy_options options,
   >                    error_code& ec) noexcept;

   > > -3- Requires: At most one constant from each `copy_options` option group (27.10.10.2) is present in `options`.

   > > -4- Effects: Report a file already exists error as specified in Error reporting (27.5.6.5) if:
   > > - `!is_regular_file(from)`, or
   > > - `exists(to)` and `!is_regular_file(to)`, or
   > > - `exists(to)` and `equivalent(from, to)`, or
   > > - `exists(to)` and `(options & (copy_options::skip_existing | copy_options::overwrite_existing | copy_options::update_existing)) == copy_options::none`.

### 2715(i). What is 'aggregate initialization syntax'?

Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: S. B. Tam Opened: 2016-05-24 Last modified: 2017-03-20

Priority: 3

View all other issues in [atomics.types.generic].

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Discussion:

32.5.8 [atomics.types.generic] mentions 'aggregate initialization syntax'. It's unclear what it stands for, especially since atomic types are actually not aggregate types (they have user-provided constructors).

[2016-11-13, Jonathan comments and provides wording]

LWG agreed to simply strike those sentences. We believe their purpose is to require that the same syntax as C programs use is valid, e.g.

    atomic_int i = { 1 };

But such syntax is already required to work by the constructor definitions, so there is no need to require it again here with redundant wording. It's unnecessary to require that syntax for the `atomic<T*>` specializations because they have no equivalent in C anyway (unlike the named atomic types for integers, there are no typedefs for `atomic<T*>`).

If wanted, we could add an example demonstrating the syntax that works for both C and C++ but the preference was to not do so:

[Example: For compatibility with the related types in the C library the following syntax can be used:

>      atomic_int i = { 1 };

— end example]

[2016-11-12, Issaquah]

Sat PM: The key here is that we need to support `= { ... }`

JW to provide wording

[2017-03-04, Kona]

This is resolved by P0558.

Proposed resolution:

This wording is relative to N4606.

1. Edit 32.5.8 [atomics.types.generic] as indicated:

   > -5- The atomic integral specializations and the specialization `atomic<bool>` shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax.

   > -6- There shall be pointer partial specializations of the `atomic` class template. These specializations shall have standard layout, trivial default constructors, and trivial destructors. They shall each support aggregate initialization syntax.

### 2716(i). Specification of `shuffle` and `sample` disallows lvalue `URNG`s

Section: 26.7.12 [alg.random.sample], 26.7.13 [alg.random.shuffle] Status: C++17 Submitter: Tim Song Opened: 2016-05-24 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

`std::shuffle` and `std::sample` each accepts a `UniformRandomNumberGenerator&&` argument and says that "`UniformRandomNumberGenerator` shall meet the requirements of a uniform random number generator (29.5.3.3 [rand.req.urng]) type".

29.5.3.3 [rand.req.urng]/2 in turn starts with "A class `G` satisfies the requirements of a uniform random number generator if […]".

If an lvalue is passed, `UniformRandomNumberGenerator` is a reference type, not a class, and in fact expressions like `G::min()` will not compile if `G` is a reference type.

[2016-06 Oulu]

Moved to P0/Ready during issues prioritization.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4582.

1. Edit 26.7.12 [alg.random.sample]/1 as indicated:

   >     template<class PopulationIterator, class SampleIterator,
   >              class Distance, class UniformRandomNumberGenerator>
   >       SampleIterator sample(PopulationIterator first, PopulationIterator last,
   >                             SampleIterator out, Distance n,
   >                             UniformRandomNumberGenerator&& g);

   > > -1- Requires::

   > > 1. […]
   > > 2. (1.6) — `remove_reference_t<UniformRandomNumberGenerator>` shall meet the requirements of a uniform random number generator type (26.6.1.3) whose return type is convertible to `Distance`.
   > > 3. […]

2. Edit 26.7.13 [alg.random.shuffle]/2 as indicated:

   >     template<class RandomAccessIterator, class UniformRandomNumberGenerator>
   >       void shuffle(RandomAccessIterator first,
   >                    RandomAccessIterator last,
   >                    UniformRandomNumberGenerator&& g);

   > > -1- […]

   > > -2- Requires: `RandomAccessIterator` shall satisfy the requirements of `ValueSwappable` (17.6.3.2). The type `remove_reference_t<UniformRandomNumberGenerator>` shall meet the requirements of a uniform random number generator (26.6.1.3) type whose return type is convertible to `iterator_traits<RandomAccessIterator>::difference_type`.

### 2718(i). Parallelism bug in [algorithms.parallel.exec] p2

Section: 26.3.3 [algorithms.parallel.exec] Status: C++17 Submitter: Jared Hoberock Opened: 2016-05-25 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [algorithms.parallel.exec].

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Discussion:

The paragraph describing the effect of composing `sequential_execution_policy` with parallel algorithms says:

> The invocations of element access functions in parallel algorithms invoked with an execution policy object of type `sequential_execution_policy` are indeterminately sequenced (1.9) in the calling thread.

The intended behavior of `sequential_execution_policy` is to match the classic-style algorithm semantics, cf. 26.3.5 [algorithms.parallel.overloads] p2:

> Unless otherwise specified, the semantics of `ExecutionPolicy` algorithm overloads are identical to their overloads without.

Because many classic-style algorithms execute element access functions in a specified sequence, 26.3.3 [algorithms.parallel.exec] p2 introduces unintentionally different semantics between classic-style algorithms and parallel algorithms invoked with `sequential_execution_policy`.

The unintentional change to 26.3.3 [algorithms.parallel.exec] p2 was introduced in the revision from P0024R1 to P0024R0 when the wording was changed from

> The invocations of element access functions in parallel algorithms invoked with an execution policy object of type `sequential_execution_policy` execute in sequential order in the calling thread.

to

> The invocations of element access functions in parallel algorithms invoked with an execution policy object of type `sequential_execution_policy` are indeterminately sequenced (1.9) in the calling thread.

Suggested resolution:

To restore the originally intended behavior of `sequential_execution_policy`, Jens Maurer suggests replacing 26.3.3 [algorithms.parallel.exec] p2 with:

> The invocations of element access functions in parallel algorithms invoked with an execution policy object of type `sequential_execution_policy` all occur in the calling thread. [Note: The invocations are not interleaved; see 6.10.1 [intro.execution] — end note]

[2016-06 Oulu]

Could be P0 after SG1 gives OK

Tuesday, Oulu: Hans Ok'd this

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4582.

1. Change 26.3.3 [algorithms.parallel.exec] as indicated:

   > The invocations of element access functions in parallel algorithms invoked with an execution policy object of type `sequential_execution_policy` all occurare indeterminately sequenced (1.9) in the calling thread. [Note: The invocations are not interleaved; see 6.10.1 [intro.execution] — end note]

### 2719(i). `permissions` function should not be `noexcept` due to narrow contract

Section: 31.12.13.27 [fs.op.permissions] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2017-07-30

Priority: 0

View all other issues in [fs.op.permissions].

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Discussion:

Currently the signatures for `permissions` are:

>     void permissions(const path& p, perms prms);
>     void permissions(const path& p, perms prms, error_code& ec) noexcept;

However both overloads have a narrow contract since due to the requires clause:

> Requires: `!((prms & perms::add_perms) != perms::none && (prms & perms::remove_perms) != perms::none)`.

For this reason I believe the second overload of `permissions` should not be marked `noexcept`.

[2016-06 Oulu]

Moved to P0/Ready during issues prioritization.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4582.

1. Change 31.12.4 [fs.filesystem.syn] as indicated:

   >     namespace std::filesystem {
   >       […]

   >       void permissions(const path& p, perms prms);
   >       void permissions(const path& p, perms prms, error_code& ec) noexcept;

   >       […]
   >     }

2. Change 31.12.13.27 [fs.op.permissions] as indicated:

   >     void permissions(const path& p, perms prms);
   >     void permissions(const path& p, perms prms, error_code& ec) noexcept;

   > > -1- Requires: `!((prms & perms::add_perms) != perms::none && (prms & perms::remove_perms) != perms::none)`.

### 2720(i). `permissions` function incorrectly specified for symlinks

Section: 31.12.13.27 [fs.op.permissions] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2017-07-30

Priority: 2

View all other issues in [fs.op.permissions].

View all issues with C++17 status.

Discussion:

Currently when adding or removing permissions the `permissions(p, prms, [...])` function always determines the current permissions for a file `p` using `status(p).permissions()`. This means that it resolves symlinks even when `perms::resolve_symlinks` was not specified.

I believe this is the incorrect behavior. Instead `symlink_status(p).permissions()` should be used unless `perms::resolve_symlinks` is specified.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4582.

> 1. In 31.12.13.27 [fs.op.permissions] change Table 150 — "Effects of permission bits" as indicated:

>    > Table 150 — Effects of permission bits
>    > Bits present in `prms` | Effective bits applied
>    > --- | ---
>    > Neither `add_perms` nor `remove_perms` | `prms & perms::mask`
>    > `add_perms` and `resolve_symlinks` | `status(p).permissions() | (prms & perms::mask)`
>    > `remove_perms` and `resolve_symlinks` | `status(p).permissions() & (prms & perms::mask)`
>    > `add_perms` and not `resolve_symlinks` | `symlink_status(p).permissions() | (prms & perms::mask)`
>    > `remove_perms` and not `resolve_symlinks` | `symlink_status(p).permissions() & ~(prms & perms::mask)`

[2016-06, Oulu — Jonathan comments and provides alternative wording]

We agree there is an issue here, but I don't like the proposed resolution. If Eric's P/R is accepted then it changes the default behaviour (when users do not set the `perms::resolve_symlinks` bit) to modify the permissions of the symlink itself.

I claim that modifying the permissions of a symlink (rather than what it points to) is not a sensible default. It is not supported by the POSIX `chmod` system call. To change permissions of a symlink with POSIX you must use the newer `fchmodat` function and the `AT_SYMLINK_NOFOLLOW` flag, see here.

Changing permissions of a symlink is not possible using the GNU `chmod` util, see here:

> "`chmod` never changes the permissions of symbolic links, since the `chmod` system call cannot change their permissions. This is not a problem since the permissions of symbolic links are never used."

BSD `chmod` does provide a switch to change a symlink's permissions, but it's not the default.

I suggest that we should replace the `filesystem::perms::resolve_symlinks` enumerator with `filesystem::perms::symlink_nofollow` (paint the bikeshed!), so that the default is sensible, and the uncommon, useless alternative of changing the symlink itself requires setting a bit in the flags explicitly.

`resolve_symlinks` is unused in the spec today, the only mention is its definition in Table 147.

[2016-06, Oulu]

There exists a slightly related issue, 2728(i).

[2016-06 Oulu]

Tuesday: Move to Ready. JW and Eric to implement and report back if problems found.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4594.

1. Change Table 147 — "Enum class `perms`" as indicated:

   > Table 147 — Enum class `perms`
   > Name | Value (octal) | `POSIX` macro | Definition or notes
   > --- | --- | --- | ---
   > `symlink_nofollowresolve_symlinks` | `0x40000` |  | `permissions()` shall change the permissions of symbolic linksresolve symlinks

2. Edit 31.12.13.27 [fs.op.permissions]:

   >     void permissions(const path& p, perms prms);
   >     void permissions(const path& p, perms prms, error_code& ec) noexcept;

   > > -1- Requires: `!((prms & perms::add_perms) != perms::none && (prms & perms::remove_perms) != perms::none)`.

   > > -2- Effects: Applies the effective permissions bits from `prms` to the file `p` resolves to, or if that file is a symbolic link and `symlink_nofollow` is not set in `prms`, the file that it points to, as if by POSIX `fchmodat()`. The effective permission bits are determined as specified in Table 150, where `s` is the result of `(prms & perms::symlink_nofollow) != perms::none ? symlink_status(p) : status(p)`.

3. Change Table 150 — "Effects of permission bits" as indicated:

   > [Drafting note: Very recently the project editor had already fixed a typo in Table 150 editorially, the applied change effectively was:

   > >     status(p).permissions() & ~(prms & perms::mask)

   > ]

   > Table 150 — Effects of permission bits
   > Bits present in `prms` | Effective bits applied
   > --- | ---
   > Neither `add_perms` nor `remove_perms` | `prms & perms::mask`
   > `add_perms` | `status(p).permissions() | (prms & perms::mask)`
   > `remove_perms` | `status(p).permissions() & (prms & perms::mask)`

### 2721(i). `remove_all` has incorrect post conditions

Section: 31.12.13.32 [fs.op.remove.all] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2021-06-06

Priority: 3

View all issues with C++17 status.

Discussion:

The current post condition for `remove_all(p, [...])` states:

> Postcondition: `!exists(p)`

This is not correct when `p` is a symlink, since `!exists(p)` reads through the symlink. The postcondition should be changed to match that of `remove` which states `!exists(symlink_status(p))`.

[2016-06, Oulu — Eric clarifies the importance of the suggested change]

With the current post conditions `remove_all(p)` could just not remove dangling symlinks and still meet the post conditions.

Moved to Ready after Eric convinced the room.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4582.

1. Change [fs.op.remove_all] as indicated:

   >     uintmax_t remove_all(const path& p);
   >     uintmax_t remove_all(const path& p, error_code& ec) noexcept;

   > > -1- Effects: Recursively deletes the contents of `p` if it exists, then deletes file `p` itself, as if by POSIX `remove()`.

   > > -2- [Note: A symbolic link is itself removed, rather than the file it resolves to being removed. — end note]

   > > -3- Postcondition: `!exists(symlink_status(p))`.

### 2722(i). `equivalent` incorrectly specifies throws clause

Section: 31.12.13.13 [fs.op.equivalent] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2017-07-30

Priority: 3

View all other issues in [fs.op.equivalent].

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Discussion:

The spec for `equivalent` has a throws clause which reads: [fs.op.equivalent]/5

> Throws: `filesystem_error` if `(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))`, otherwise as specified in Error reporting (27.10.7).

This explicit requirement to throw is incorrect for the `equivalent` overload which takes an `error_code`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4582.

> 1. Modify 31.12.13.13 [fs.op.equivalent] as follows:

>    >     bool equivalent(const path& p1, const path& p2);
>    >     bool equivalent(const path& p1, const path& p2, error_code& ec) noexcept;

>    > > -1- Effects: Determines `file_status s1` and `s2`, as if by `status(p1)` and `status(p2)`, respectively.

>    > > -2- Returns: If `(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))` an error is reported (31.12.5 [fs.err.report]). Otherwise `true`, if `s1 == s2` and `p1` and `p2` resolve to the same file system entity, else `false`. The signature with argument `ec` returns `false` if an error occurs.

>    > > -3- Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. This is determined as if by the values of the POSIX `stat` structure, obtained as if by `stat()` for the two paths, having equal `st_dev` values and equal `st_ino` values.

>    > > -4- Throws: `filesystem_error` if `(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))`, otherwise aAs specified in Eerror reporting (31.12.5 [fs.err.report]).

[2016-06 Oulu — Daniel provides wording improvements]

mc: do we have an error reporting clause?
jw: there is no such clause
gr: it should go into the effects clause
dk: we have the same issue for `file_size`
dk: the right place is the effects clause

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4594.

1. Modify 31.12.13.13 [fs.op.equivalent] as follows:

   >     bool equivalent(const path& p1, const path& p2);
   >     bool equivalent(const path& p1, const path& p2, error_code& ec) noexcept;

   > > -1- Let `s1` and `s2` be `file_status`s, determined as if by `status(p1)` and `status(p2)`, respectively.

   > > -2- Effects: Determines `s1` and `s2`. If `(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))` an error is reported (31.12.5 [fs.err.report]).

   > > -3- Returns: `true`, if `s1 == s2` and `p1` and `p2` resolve to the same file system entity, else `false`. The signature with argument `ec` returns `false` if an error occurs.

   > > -4- Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. This is determined as if by the values of the POSIX `stat` structure, obtained as if by `stat()` for the two paths, having equal `st_dev` values and equal `st_ino` values.

   > > -5- Throws: `filesystem_error` if `(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))`, otherwise aAs specified in Eerror reporting (31.12.5 [fs.err.report]).

### 2723(i). Do `directory_iterator` and `recursive_directory_iterator` become the end iterator upon error?

Section: 31.12.11 [fs.class.directory.iterator], 31.12.12 [fs.class.rec.dir.itr] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2021-06-06

Priority: 0

View all other issues in [fs.class.directory.iterator].

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Discussion:

Constructing or performing an increment on directory iterator types can result in an error. Currently there is implementation divergence regarding the value of the iterator after an error occurs. Both boost and libc++ construct the end iterator. libstdc++ constructs a singular iterator which is not equal to the end iterator. For this reason we should clarify the state of the iterators after an error occurs.

[2016-06 Oulu]

Moved to P0/Ready during issues prioritization.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4582.

1. Modify [fs.class.directory_iterator] as follows:

   > -3- If an iterator of type `directory_iterator` reports an error or is advanced past the last directory element, that iterator shall become equal to the end iterator value. The `directory_iterator` default constructor shall create an iterator equal to the end iterator value, and this shall be the only valid iterator for the end condition.

### 2724(i). The protected virtual member functions of `memory_resource` should be private

Section: 20.5.2 [mem.res.class] Status: C++17 Submitter: Ville Voutilainen Opened: 2016-06-04 Last modified: 2017-07-30

Priority: 4

View all other issues in [mem.res.class].

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Discussion:

`memory_resource` doesn't define any behavior, it's just an interface. Furthermore, we don't say whether the functions at [memory.resource.prot] should or should not be defined by implementations. Presumably they should not. Those functions are not designed to be called by derived classes, and thus should not be protected.

[2016-06 Oulu]

Looks fine, check with Pablo to make sure that was his intent.

Pablo replied that this was correct.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4582.

1. Modify [memory.resource.class] as indicated:

   >     class memory_resource {
   >       […]
   >     protectedprivate:
   >       virtual void* do_allocate(size_t bytes, size_t alignment) = 0;
   >       virtual void do_deallocate(void* p, size_t bytes, size_t alignment) = 0;
   >       virtual bool do_is_equal(const memory_resource& other) const noexcept = 0;
   >     };

2. Modify [memory.resource.prot] as indicated:

   > [Drafting note: I don't know whether it's too late to change the section mnemonic [memory.resource.prot] to e.g. [memory.resource.priv] or perhaps [memory.resource.virt].]

   > `memory_resource` protectedprivate virtual member functions [memory.resource.prot]

### 2725(i). `filesystem::exists(const path&, error_code&)` error reporting

Section: 31.12.13.14 [fs.op.exists] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-06-06 Last modified: 2017-07-30

Priority: 1

View all issues with C++17 status.

Discussion:

The `filesystem::exists(const path&)` function does not throw an exception if the file doesn't exist, but the corresponding function taking an `error_code&` argument does set it to indicate an error.

It seems sensible for `filesystem::exists(const path&, error_code&)` to call `ec.clear()` if `status(p, ec).type() == file_type::not_found`.

[2016-06, Oulu — Jonathan comments and provides wording]

The sentence "The signature with argument `ec` returns `false` if an error occurs." means that given a file such that `status(p).type() == file_type::unknown`, `exists(p)` is `true` but `exists(p, ec)` is `false`.

I believe we should make the behaviour of `exists(p)` and `exists(p, ec)` consistent, so that the latter clears `ec` except when the former would throw an exception, which is only for the `file_type::none` case.

[2016-06, Oulu]

Prioritized as P1

Voted to Ready 7-0 Tuesday evening in Oulu

Proposed resolution:

This wording is relative to N4594.

1. Insert a new paragraph before 31.12.13.14 [fs.op.exists] p2 and edit it as shown:

   >     bool exists(const path& p);
   >     bool exists(const path& p, error_code& ec) noexcept;

   > > -?- Let `s` be a `file_status`, determined as if by `status(p)` or `status(p, ec)`, respectively.

   > > -?- Effects: The signature with argument `ec` calls `ec.clear()` if `status_known(s)`.

   > > -2- Returns: `exists(status(p))` or `exists(status(p, ec))`, respectively `exists(s)`. The signature with argument `ec` returns `false` if an error occurs.

### 2726(i). `[recursive_]directory_iterator::increment(error_code&)` is underspecified

Section: 31.12.11.2 [fs.dir.itr.members], 31.12.12.2 [fs.rec.dir.itr.members] Status: C++17 Submitter: Daniel Krügler Opened: 2016-06-20 Last modified: 2017-07-30

Priority: 0

View all other issues in [fs.dir.itr.members].

View all issues with C++17 status.

Discussion:

Setting `X` as being either `directory_iterator` or `recursive_directory_iterator` there exists a member function in `X`,

>     X& increment(error_code& ec) noexcept;

whose effects are described as:

> As specified by Input iterators (24.2.3).

which is somewhat surprising, because for input iterators there is no call expression naming `increment` specified.

The intention here is to consider `increment` as a another name for the prefix increment operator of iterators, but that needs to be expressed somewhere.

[2016-06 Oulu]

Moved to P0/Ready during issues prioritization.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4594.

> [Drafting note: The suggested wording for this issue also repairs the information loss that had been caused by the third bullet of the proposed resolution of 2704(i)]

1. Change 31.12.11.2 [fs.dir.itr.members] as indicated:

   >     directory_iterator& operator++();
   >     directory_iterator& increment(error_code& ec) noexcept;

   > > -10- Effects: As specified by for the prefix increment operation of Input iterators (24.3.5.3 [input.iterators]).

2. Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:

   >     recursive_directory_iterator& operator++();
   >     recursive_directory_iterator& increment(error_code& ec) noexcept;

   > > […]

   > > -27- Effects: As specified by for the prefix increment operation of Input iterators (24.3.5.3 [input.iterators]), except that: […]

### 2727(i). Parallel algorithms with `constexpr` specifier

Section: 26.1 [algorithms.general] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-06-21 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

In LEWG we noticed some parallel algorithms are `constexpr`. Jared said:

I think this is an oversight, and it also applies to `std::max_element`/`std::minmax_element`. To my knowledge, neither SG1 nor LWG ever explicitly considered whether a parallel algorithm should be `constexpr`. I think the assumption was that parallel algorithms would be regular old function templates without additional specifiers such as `constexpr`.

[2016-06 Oulu]

Moved to P0/Ready during issues prioritization.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4594.

1. Change the `<algorithm>` header synopsis, 26.1 [algorithms.general], as indicated, to remove "`constexpr`" from the six `{min,max,minmax}_element` overloads with an `ExecutionPolicy` argument:

   >     namespace std {

   >     […]
   >     // 25.5.7, minimum and maximum:
   >     […]
   >     template<class ExecutionPolicy, class ForwardIterator>
   >       constexpr ForwardIterator min_element(ExecutionPolicy&& exec, // see 25.2.5
   >                                             ForwardIterator first, ForwardIterator last);
   >     template<class ExecutionPolicy, class ForwardIterator, class Compare>
   >       constexpr ForwardIterator min_element(ExecutionPolicy&& exec, // see 25.2.5
   >                                             ForwardIterator first, ForwardIterator last,
   >                                             Compare comp);
   >     […]
   >     template<class ExecutionPolicy, class ForwardIterator>
   >       constexpr ForwardIterator max_element(ExecutionPolicy&& exec, // see 25.2.5
   >                                             ForwardIterator first, ForwardIterator last);
   >     template<class ExecutionPolicy, class ForwardIterator, class Compare>
   >       constexpr ForwardIterator max_element(ExecutionPolicy&& exec, // see 25.2.5
   >                                             ForwardIterator first, ForwardIterator last,
   >                                             Compare comp);
   >     […]
   >     template<class ExecutionPolicy, class ForwardIterator>
   >       constexpr pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ExecutionPolicy&& exec, // see 25.2.5
   >                        ForwardIterator first, ForwardIterator last);
   >     template<class ExecutionPolicy, class ForwardIterator, class Compare>
   >       constexpr pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ExecutionPolicy&& exec, // see 25.2.5
   >                        ForwardIterator first, ForwardIterator last, Compare comp);
   >     […]
   >     }

### 2728(i). `status(p).permissions()` and `symlink_status(p).permissions()` are not specified

Section: 31.12.13.36 [fs.op.status], 31.12.13.38 [fs.op.symlink.status] Status: C++17 Submitter: Eric Fiselier Opened: 2016-06-19 Last modified: 2023-02-07

Priority: 0

View all other issues in [fs.op.status].

View all issues with C++17 status.

Discussion:

The current specification for `status(p)` and `symlink_status(p)` omits any mention on setting `permissions()` on the returned `file_status`. Obviously they should be set, but as currently specified the `permissions()` will always be `perms::unknown`.

[2016-06, Oulu]

2720(i) is a related issue.

[2016-06 Oulu]

Moved to P0/Ready during issues prioritization.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4594.

1. Change 31.12.13.36 [fs.op.status] as indicated:

   >     file_status status(const path& p, error_code& ec) noexcept;

   > > -4- Effects: If possible, determines the attributes of the file `p` resolves to, as if by POSIX `stat()`.using POSIX `stat()` to obtain a POSIX `struct stat`.. […]

   > > -?- Let `prms` denote the result of `(m & perms::mask)`, where `m` is determined as if by converting the `st_mode` member of the obtained `struct stat` to the type `perms`.

   > > -5- Returns:

   > > - If `ec != error_code()`:

   > >   - […]

   > > - Otherwise

   > >   - If the attributes indicate a regular file, as if by POSIX `S_ISREG`, returns `file_status(file_type::regular, prms)`. […]

   > >   - Otherwise, if the attributes indicate a directory, as if by POSIX `S_ISDIR`, returns `file_status(file_type::directory, prms)`. […]

   > >   - Otherwise, if the attributes indicate a block special file, as if by POSIX `S_ISBLK`, returns `file_status(file_type::block, prms)`.

   > >   - Otherwise, if the attributes indicate a character special file, as if by POSIX `S_ISCHR`, returns `file_status(file_type::character, prms)`.

   > >   - Otherwise, if the attributes indicate a fifo or pipe file, as if by POSIX `S_ISFIFO`, returns `file_status(file_type::fifo, prms)`.

   > >   - Otherwise, if the attributes indicate a socket, as if by POSIX `S_ISSOCK`, returns `file_status(file_type::socket, prms)`.

   > >   - Otherwise, returns `file_status(file_type::unknown, prms)`.

2. Change [fs.op.symlink_status] as indicated:

   >     file_status symlink_status(const path& p);
   >     file_status symlink_status(const path& p, error_code& ec) noexcept;

   > > -1- Effects: Same as `status()`, above, except that the attributes of `p` are determined as if by POSIX `lstat()` using POSIX `lstat()` to obtain a POSIX `struct stat`.

   > > -?- Let `prms` denote the result of `(m & perms::mask)`, where `m` is determined as if by converting the `st_mode` member of the obtained `struct stat` to the type `perms`.

   > > -2- Returns: Same as `status()`, above, except that if the attributes indicate a symbolic link, as if by POSIX `S_ISLNK`, returns `file_status(file_type::symlink, prms)`. The signature with argument `ec` returns `file_status(file_type::none)` if an error occurs.

### 2729(i). Missing SFINAE on `std::pair::operator=`

Section: 22.3.2 [pairs.pair], 22.4.4.3 [tuple.assign] Status: C++17 Submitter: Richard Smith Opened: 2016-06-07 Last modified: 2017-07-30

Priority: 2

View other active issues in [pairs.pair].

View all other issues in [pairs.pair].

View all issues with C++17 status.

Discussion:

`std::is_copy_assignable<std::pair<int, std::unique_ptr<int>>>::value` is `true`, and should be `false`. We're missing a "shall not participate in overload resolution unless" for `pair`'s `operator=`, and likewise for `tuple`.

[2016-08-03 Chicago LWG]

Inspired by Eric Fiselier and Ville, Walter and Nevin provide initial Proposed Resolution.

[2016-08 - Chicago]

Thurs PM: Moved to Tentatively Ready

Lots of discussion, but no one had a better idea.

Proposed resolution:

This wording is relative to N4606.

1. Change 22.3.2 [pairs.pair] as indicated:

   >     pair& operator=(const pair& p);

   > > -15- RequiresRemarks: This operator shall be defined as deleted unless `is_copy_assignable_v<first_type>` is `true` and `is_copy_assignable_v<second_type>` is `true`.

   > > […]

   >     template<class U, class V> pair& operator=(const pair<U, V>& p);

   > > -18- RequiresRemarks: This operator shall not participate in overload resolution unless `is_assignable_v<first_type&, const U&>` is `true` and `is_assignable_v<second_type&, const V&>` is `true`.

   > > […]

   >     pair& operator=(pair&& p) noexcept(see below);

   > > -21- Remarks: The expression inside `noexcept` is equivalent to:

   > > >     is_nothrow_move_assignable_v<T1> && is_nothrow_move_assignable_v<T2>

   > > -22- RequiresRemarks: This operator shall be defined as deleted unless `is_move_assignable_v<first_type>` is `true` and `is_move_assignable_v<second_type>` is `true`.

   > > […]

   >     template<class U, class V> pair& operator=(pair<U, V>&& p);

   > > -25- RequiresRemarks: This operator shall not participate in overload resolution unless `is_assignable_v<first_type&, U&&>` is `true` and `is_assignable_v<second_type&, V&&>` is `true`.

2. Change 22.4.4.3 [tuple.assign] as indicated:

   >     tuple& operator=(const tuple& u);

   > > -2- RequiresRemarks: This operator shall be defined as deleted unless `is_copy_assignable_v<Ti>` is `true` for all `i`.

   > > […]

   >     tuple& operator=(tuple&& u) noexcept(see below);

   > > -5- Remark: The expression inside `noexcept` is equivalent to the logical AND of the following expressions:

   > > >     is_nothrow_move_assignable_v<Ti>

   > > where `Ti` is the `ith` type in `Types`.

   > > -6- RequiresRemarks: This operator shall be defined as deleted unless `is_move_assignable_v<Ti>` is `true` for all `i`.

   > > […]

   >     template <class... UTypes>
   >       tuple& operator=(const tuple<UTypes...>& u);

   > > -9- RequiresRemarks: This operator shall not participate in overload resolution unless `sizeof...(Types) == sizeof...(UTypes)` and `is_assignable_v<Ti&, const Ui&>` is `true` for all `i`.

   > > […]

   >     template <class... UTypes>
   >       tuple& operator=(tuple<UTypes...>&& u);

   > > -12- RequiresRemarks: This operator shall not participate in overload resolution unless `is_assignable_v<Ti&, Ui&&> == true` for all `i`. and `sizeof...(Types) == sizeof...(UTypes)`.

   > > […]

   >     template <class U1, class U2> tuple& operator=(const pair<U1, U2>& u);

   > > -15- RequiresRemarks: This operator shall not participate in overload resolution unless `sizeof...(Types) == 2`. and `is_assignable_v<T0&, const U1&>` is `true` for the first type `T0` in `Types` and `is_assignable_v<T1&, const U2&>` is `true` for the second type `T1` in `Types`.

   > > […]

   >     template <class U1, class U2> tuple& operator=(pair<U1, U2>&& u);

   > > -18- RequiresRemarks: This operator shall not participate in overload resolution unless `sizeof...(Types) == 2`. and `is_assignable_v<T0&, U1&&>` is `true` for the first type `T0` in `Types` and `is_assignable_v<T1&, U2&&>` is `true` for the second type `T1` in `Types`.

### 2731(i). Existence of `lock_guard<MutexTypes...>::mutex_type` typedef unclear

Section: 32.6.5.2 [thread.lock.guard] Status: C++23 Submitter: Eric Fiselier Opened: 2016-06-13 Last modified: 2023-11-22

Priority: 3

View all other issues in [thread.lock.guard].

View all issues with C++23 status.

Discussion:

In the synopsis of 32.6.5.3 [thread.lock.scoped] the `mutex_type` typedef is specified as follows:

>     template <class... MutexTypes>
>     class scoped_lock {
>     public:
>       typedef Mutex mutex_type; // If MutexTypes... consists of the single type Mutex
>       […]
>     };

The comment seems ambiguous as it could mean either:

1. `sizeof...(MutexTypes) == 1`.
2. `sizeof...(MutexTypes) >= 1` and every type in `MutexTypes...` is the same type.

I originally took the language to mean (2), but upon further review it seems that (1) is the intended interpretation, as suggested in the LEWG discussion in Lenexa.

I think the language should be clarified to prevent implementation divergence.

[2016-07, Toronto Saturday afternoon issues processing]

General feeling that `sizeof(MutexTypes...) == 1` is a better way to state the requirement.

Reworked the text to refer to `scoped_lock` instead of `lock_guard`

Marshall and Eric to reword and discuss on reflector. Status to Open

[2018-3-14 Wednesday evening issues processing; general agreement to adopt once the wording is updated.]

2018-03-18 Marshall provides updated wording.

Previous resolution: [SUPERSEDED]

> This wording is relative to N4594.

> 1. Edit 32.6.5.2 [thread.lock.guard]/1, class template `lock_guard` synopsis, as indicated:

>    >     template <class... MutexTypes>
>    >     class lock_guard {
>    >     public:
>    >       typedef Mutex mutex_type; // Only iIf MutexTypes... consists of theexpands to a single type Mutex
>    >       […]
>    >     };

Previous resolution: [SUPERSEDED]

> This wording is relative to N4727.

> 1. Edit 32.6.5.2 [thread.lock.guard]/1, class template `lock_guard` synopsis, as indicated:

>    >     template <class... MutexTypes>
>    >     class scoped_lock {
>    >     public:
>    >       using mutex_type = Mutex; // Only iIf sizeof(MutexTypes...) == 1 MutexTypes... consists of the single type Mutex
>    >       […]
>    >     };

[2020-05-11; Daniel provides improved wording]

[2020-05-16 Reflector discussions]

Status to Tentatively Ready after five positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Edit 32.6.5.3 [thread.lock.scoped], class template `scoped_lock` synopsis, as indicated:

   > >     template <class... MutexTypes>
   > >     class scoped_lock {
   > >     public:
   > >       using mutex_type = Mutex; // If MutexTypes... consists of the single type Mutex
   > >       using mutex_type = see below; // Only if  sizeof...(MutexTypes) == 1
   > >       […]
   > >     };

   > -1- An object of type `scoped_lock` controls the ownership of lockable objects within a scope. A `scoped_lock` object maintains ownership of lockable objects throughout the `scoped_lock` object's lifetime (6.8.4 [basic.life]). The behavior of a program is undefined if the lockable objects referenced by `pm` do not exist for the entire lifetime of the `scoped_lock` object.

   > - WhenIf `sizeof...(MutexTypes)` is `1` one, let `Mutex` denote the sole type constituting the pack `MutexTypes`., the supplied `Mutex` type shall meet the Cpp17BasicLockable requirements (32.2.5.2 [thread.req.lockable.basic]). The member typedef-name `mutex_type` denotes the same type as `Mutex`.

   > - Otherwise, each of the mutex typesall types in the template parameter pack `MutexTypes` shall meet the Cpp17Lockable requirements (32.2.5.3 [thread.req.lockable.req]) and there is no member `mutex_type`.

### 2732(i). Questionable specification of `path::operator/=` and `path::append`

Section: 31.12.6.5.3 [fs.path.append] Status: C++17 Submitter: Tim Song Opened: 2016-06-14 Last modified: 2017-07-30

Priority: 2

View all other issues in [fs.path.append].

View all issues with C++17 status.

Discussion:

The current specification of `operator/=` taking a `const Source&` parameter, and of `path::append` in 31.12.6.5.3 [fs.path.append] appears to require `Source` to have a `native()` and an `empty()` member, and seemingly requires different behavior for `append(empty_range)` and `append(first, last)` when `first == last` (the last two bullet points being specified with `source` alone), which doesn't make any sense.

It appears that these overloads can just be specified using the `operator/=(const path&)` overload.

[2016-07-03, Daniel comments]

The same wording area is affected by LWG 2664(i).

[2016-08 Chicago]

Wed AM: Move to Tentatively Ready

Friday AM, in discussing 2664(i) a comment about missing "equivalent to" language was made, so PR updated.

Previous Resolution [SUPERSEDED]

> This wording is relative to N4594.

> 1. Edit 31.12.6.5.3 [fs.path.append]/4-5 as indicated:

>    >     template <class Source>
>    >       path& operator/=(const Source& source);
>    >     template <class Source>
>    >       path& append(const Source& source);

>    > > -?- Effects: `operator/=(path(source))`

>    > > -?- Returns: `*this`.

>    >     template <class InputIterator>
>    >       path& append(InputIterator first, InputIterator last);

>    > > -4- Effects: Appends `path::preferred_separator` to `pathname`, converting format and encoding if required (31.12.6.3 [fs.path.cvt]), unless:

>    > > - an added directory-separator would be redundant, or

>    > > - an added directory-separator would change an relative path to an absolute path, or

>    > > - `source.empty()` is `true`, or

>    > > - `*source.native().cbegin()` is a directory-separator.

>    > > Then appends the effective range of `source` (31.12.6.4 [fs.path.req]) or the range `[first, last)` to `pathname`, converting format and encoding if required (31.12.6.3 [fs.path.cvt])`operator/=(path(first, last))`.

>    > > -5- Returns: `*this`.

Proposed resolution:

This wording is relative to N4606.

1. Edit 31.12.6.5.3 [fs.path.append]/4-5 as indicated:

   >     template <class Source>
   >       path& operator/=(const Source& source);
   >     template <class Source>
   >       path& append(const Source& source);

   > > -?- Effects: Equivalent to `return operator/=(path(source));`.

   >     template <class InputIterator>
   >       path& append(InputIterator first, InputIterator last);

   > > -4- Effects: Equivalent to `return operator/=(path(first, last));`.Appends `path::preferred_separator` to `pathname`, converting format and encoding if required (31.12.6.3 [fs.path.cvt]), unless:

   > > 1. — an added directory-separator would be redundant, or
   > > 2. — an added directory-separator would change an relative path to an absolute path, or
   > > 3. — `source.empty()` is `true`, or
   > > 4. — `*source.native().cbegin()` is a directory-separator.

   > > Then appends the effective range of `source` (31.12.6.4 [fs.path.req]) or the range `[first, last)` to `pathname`, converting format and encoding if required (31.12.6.3 [fs.path.cvt]).

   > > -5- Returns: `*this`.

### 2733(i). [fund.ts.v2] `gcd` / `lcm` and `bool`

Section: 13.1.2 [fund.ts.v2::numeric.ops.gcd], 13.1.3 [fund.ts.v2::numeric.ops.lcm] Status: TS Submitter: Richard Smith Opened: 2016-06-15 Last modified: 2017-07-30

Priority: 4

View all other issues in [fund.ts.v2::numeric.ops.gcd].

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

According to N4562, `gcd` and `lcm` support `bool` as the operand type. The wording doesn't appear to cover the behavior for that case, since `bool` does not have a zero value and `gcd` / `lcm` are not normally mathematically defined over `{false, true}`.

Presumably `gcd` and `lcm` shouldn't accept arguments of type `bool`.

[2016-08-01, Walter Brown suggests wording]

A corresponding issue has been added addressing the WP, see LWG 2759(i).

[2016-08, Chicago]

Monday PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4600.

1. Adjust 13.1.2 [fund.ts.v2::numeric.ops.gcd] p3 as indicated:

   >     template<class M, class N>
   >       constexpr common_type_t<M, N> gcd(M m, N n);

   > > […]

   > > -3- Remarks: If either `M` or `N` is not an integer type, or if either is (possibly cv-qualified) `bool`, the program is ill-formed.

2. Adjust 13.1.3 [fund.ts.v2::numeric.ops.lcm] p3 as indicated:

   >     template<class M, class N>
   >       constexpr common_type_t<M, N> lcm(M m, N n);

   > > […]

   > > -3- Remarks: If either `M` or `N` is not an integer type, or if either is (possibly cv-qualified) `bool`, the program is ill-formed.

### 2734(i). Questionable specification in [fs.path.concat]

Section: 31.12.6.5.4 [fs.path.concat] Status: Resolved Submitter: Tim Song Opened: 2016-06-16 Last modified: 2018-01-24

Priority: 2

View all other issues in [fs.path.concat].

View all issues with Resolved status.

Discussion:

31.12.6.5.4 [fs.path.concat] specifies that the postcondition for

>     path& operator+=(const path& x);
>     path& operator+=(const string_type& x);
>     path& operator+=(const value_type* x);
>     path& operator+=(value_type x);
>     template<class Source>
>     path& operator+=(const Source& x);
>     template<class EcharT>
>     path& operator+=(EcharT x);
>     template<class Source>
>     path& concat(const Source& x);
>     template<class InputIterator>
>     path& concat(InputIterator first, InputIterator last);

is

> `native() == prior_native` + effective-argument

where effective-argument is

1. if `x` is present and is `const path&`, `x.native()`; otherwise
2. if `source` is present, the effective range of `source` (31.12.6.4 [fs.path.req]); otherwise,
3. if `first` and `last` are present, the range `[first, last)`; otherwise,
4. `x`

It also says that

> If the value type of effective-argument would not be `path::value_type`, the actual argument or argument range is first converted (31.12.6.3.2 [fs.path.type.cvt]) so that effective-argument has value type `path::value_type`.

There are several problems with this specification:

First, there is no overload taking "`source`" (note the lower case); all single-argument overloads take "`x`". Second, there's nothing that defines what it means to use `operator+` on a string and an iterator range; clearly concatentation is intended but there is no wording to that effect. Third, the final portion uses "value type", but the "value type" of a single character is not a defined concept.

Also, the reference only to 31.12.6.3.2 [fs.path.type.cvt] seems to imply that any format conversion specified in 31.12.6.3.1 [fs.path.fmt.cvt] will not be performed, in seeming contradiction to the rule that `native()` is to return the native pathname format (31.12.6.5.6 [fs.path.native.obs]/1). Is that intended?

[2016-11-10, Billy suggests wording]

The wording for LWG 2798(i) resolves this issue as well.

Proposed resolution:

This is resolved by p0492r2.

### 2735(i). `std::abs(short)`, `std::abs(signed char)` and others should return `int` instead of `double` in order to be compatible with C++98 and C

Section: 29.7 [c.math] Status: C++17 Submitter: Jörn Heusipp Opened: 2016-06-16 Last modified: 2017-07-30

Priority: 3

View all other issues in [c.math].

View all issues with C++17 status.

Discussion:

Consider this C++98 program:

>     #include <cmath>
>     #include <cstdlib>

>     int main() {
>       return std::abs(static_cast<short>(23)) % 42;
>     }

This works fine with C++98 compilers. At the `std::abs(short)` call, short gets promoted to `int` and `std::abs(int)` is called.

C++11 added the following wording on page 1083 §26.9 p15 b2 [c.math]:

> Otherwise, if any argument of arithmetic type corresponding to a `double` parameter has type `double` or an integer type, then all arguments of arithmetic type corresponding to `double` parameters are effectively cast to `double`.

C++17 draft additionally adds on page 1080 §26.9 p10 [c.math]:

> If `abs()` is called with an argument of type `X` for which `is_unsigned<X>::value` is `true` and if `X` cannot be converted to `int` by integral promotion (4.5), the program is ill-formed. [Note: Arguments that can be promoted to `int` are permitted for compatibility with C. — end note]

It is somewhat confusing and probably even contradictory to on the one hand specify `abs()` in terms of integral promotion in §26.9 p10 and on the other hand demand all integral types to be converted to `double` in §26.9 p15 b2.

Most compilers (each with their own respective library implementation) I tested (MSVC, Clang, older GCC) appear to not consider §26.9 p15 b2 for `std::abs` and compile the code successfully. GCC 4.5-5.3 (for `std::abs` but not for `::abs`) as well as GCC >=6.0 (for both `std::abs` and `::abs`) fail to compile in the following way: Taking §26.9 p15 b2 literally and applying it to `abs()` (which is listed in §26.9 p12) results in `abs(short)` returning `double`, and with `operator%` not being specified for `double`, this makes the programm ill-formed.

I do acknowledge the reason for the wording and semantics demanded by §26.9 p15 b2, i.e. being able to call math functions with integral types or with partly floating point types and partly integral types. Converting integral types to `double` certainly makes sense here for all the other floating point math functions. However, `abs()` is special. `abs()` has overloads for the 3 wider integral types which return integral types. `abs()` originates in the C standard in `stdlib.h` and had originally been specified for integral types only. Calling it in C with a short argument returns an `int`. Calling `std::abs(short)` in C++98 also returns an `int`. Calling `std::abs(short)` in C++11 and later with §26.9 p15 b2 applied to `abs()` suddenly returns a `double`.

Additionally, this behaviour also breaks third-party C headers which contain macros or inline functions calling `abs(short)`.

As per discussion on std-discussion, my reading of the standard as well as GCC's interpretation seem valid. However, as can be seen, this breaks existing code.

In addition to the compatibilty concerns, having `std::abs(short)` return `double` is also very confusing and unintuitive.

The other (possibly, depending on their respective size relative to `int`) affected types besides `short` are `signed char`, `unsigned char` and `unsigned short`, and also `char`, `char16_t`, `char32_t` and `wchar_t`, (all of these are or may be promotable to `int`). Wider integral types are not affected because explicit overloads are specified for those types by §26.9 p6, §26.9 p7 and §26.9 p9. `div()` is also not affected because it is neither listed in §26.9 p12, nor does it actually provide any overload for `double` at all.

As far as I can see, the proposed or implemented solutions for LWG 2294(i), 2192(i) and/or 2086(i) do not resolve this issue.

I think both, §26.9 p10 [c.math] and §26.9 p15 [c.math] need some correction and clarification.

(Note: These changes would explicitly render the current implementation in GCC's libstdc++ non-conforming, which would be a good thing, as outlined above.)

Previous resolution [SUPERSEDED]:

> This wording is relative to N4594.

> 1. Modify 29.7 [c.math] as indicated:

>    > -10- If `abs()` is called with an argument of type `X` for which `is_unsigned<X>::value` is `true` and if `X` cannot be converted to `int` by integral promotion (4.5), the program is ill-formed. If `abs()` is called with an argument of type `X` which can be converted to `int` by integral promotion (4.5), the argument is promoted to `int`. [Note: Arguments that can be promoted to `int` are promoted to `int` in order to keeppermitted for compatibility with C. — end note]

>    > […]

>    > -15- Moreover, there shall be additional overloads for these functions, with the exception of `abs()`, sufficient to ensure:

>    > 1. If any argument of arithmetic type corresponding to a `double` parameter has type `long double`, then all arguments of arithmetic type (3.9.1) corresponding to `double` parameters are effectively cast to `long double`.

>    > 2. Otherwise, if any argument of arithmetic type corresponding to a `double` parameter has type `double` or an integer type, then all arguments of arithmetic type corresponding to `double` parameters are effectively cast to `double`.

>    > 3. Otherwise, all arguments of arithmetic type corresponding to `double` parameters have type `float`.

>    > See also: ISO C 7.5, 7.10.2, 7.10.6.

>    > [Note: `abs()` is exempted from these rules in order to stay compatible with C. — end note]

[2016-07 Chicago]

Monday: Some of this has been changed in N4606; the rest of the changes may be editorial.

Fri PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 29.7.1 [cmath.syn] as indicated:

   > -2- For each set of overloaded functions within `<cmath>`, with the exception of `abs`, there shall be additional overloads sufficient to ensure:

   > 1. If any argument of arithmetic type corresponding to a `double` parameter has type `long double`, then all arguments of arithmetic type (3.9.1) corresponding to `double` parameters are effectively cast to `long double`.

   > 2. Otherwise, if any argument of arithmetic type corresponding to a `double` parameter has type `double` or an integer type, then all arguments of arithmetic type corresponding to `double` parameters are effectively cast to `double`.

   > 3. Otherwise, all arguments of arithmetic type corresponding to `double` parameters have type `float`.

   > [Note: `abs` is exempted from these rules in order to stay compatible with C. — end note]

   > See also: ISO C 7.5, 7.10.2, 7.10.6.

### 2736(i). `nullopt_t` insufficiently constrained

Section: 22.5.5 [optional.nullopt] Status: C++17 Submitter: Tim Song Opened: 2016-06-17 Last modified: 2017-07-30

Priority: 2

View all other issues in [optional.nullopt].

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Discussion:

22.5.5 [optional.nullopt]/2 requires of `nullopt_t` that

> Type `nullopt_t` shall not have a default constructor. It shall be a literal type. Constant `nullopt` shall be initialized with an argument of literal type.

This does not appear sufficient to foreclose the following implementation:

>     struct nullopt_t
>     {
>       constexpr nullopt_t(const nullopt_t&) = default;
>     };

>     constexpr nullopt_t nullopt(nullopt_t{});

- `nullopt_t` has no default constructor because it has a user-declared (copy) constructor;
- `nullopt_t` has a trivial destructor, is an aggregate, and is a literal type;
- `nullopt` has been initialized with an argument of literal type, to wit, `nullopt_t`.

But such a `nullopt_t` is still constructible from `{}` and so still makes `opt = {}` ambiguous.

[2016-08 Chicago]

This is related to LWG 2510(i).

Monday PM: Ville to provide updated wording

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Edit 22.5.5 [optional.nullopt]/2 as indicated:

   > [Drafting note: `{}` can do one of three things for a class type: it may be aggregate initialization, it may call a default constructor, or it may call an initializer-list constructor (see 9.5.5 [dcl.init.list], 12.2.2.8 [over.match.list]). The wording below forecloses all three possibilities. — end drafting note]

   > -2- Type `nullopt_t` shall not have a default constructor or an initializer-list constructor. It shall not be an aggregate and shall be a literal type. Constant `nullopt` shall be initialized with an argument of literal type.

### 2738(i). `is_constructible` with `void` types

Section: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: S. B. Tam Opened: 2016-06-22 Last modified: 2017-07-30

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

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Discussion:

LWG 2560(i) mention that there is no variable of function type. There's also no variable of `void` type, so should 21.3.5.4 [meta.unary.prop] also explicitly say that for a `void` type `T`, `is_constructible<T, Args...>::value` is `false`?

[2016-07-03, Daniel provides wording]

[2016-08 Chicago]

Wed PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4594.

1. Change 21.3.5.4 [meta.unary.prop], Table 52 — "Type property predicates", as indicated:

   > Table 52 — Type property predicates
   > Template | Condition | Preconditions
   > --- | --- | ---
   > `…`
   > `template <class T, class... Args> struct is_constructible;` | For a function type `T` or for a (possibly cv-qualified) `void` type `T`, `is_constructible<T, Args...>::value` is `false`, otherwise see below | `T` and all types in the parameter pack `Args` shall be complete types, (possibly cv-qualified) `void`, or arrays of unknown bound.
   > `…`

### 2739(i). Issue with `time_point` non-member subtraction with an unsigned duration

Section: 30.6.6 [time.point.nonmember] Status: C++17 Submitter: Michael Winterberg Opened: 2016-06-23 Last modified: 2017-07-30

Priority: 0

View all other issues in [time.point.nonmember].

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Discussion:

In N4594, 30.6.6 [time.point.nonmember], `operator-(time_point, duration)` is specified as:

>     template <class Clock, class Duration1, class Rep2, class Period2>
>       constexpr time_point<Clock, common_type_t<Duration1, duration<Rep2, Period2>>>
>       operator-(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);

> > -3- Returns: `lhs + (-rhs)`.

When `Rep2` is an unsigned integral type, the behavior is quite different with arithmetic of the underlying integral types because of the requirement to negate the incoming duration and then add that. It also ends up producing different results than the underlying durations as well as the non-member `time_point::operator-=`.

Consider this program:

>     #include <chrono>
>     #include <iostream>
>     #include <cstdint>

>     using namespace std;
>     using namespace std::chrono;

>     int main()
>     {
>       const duration<uint32_t> unsignedSeconds{5};

>       auto someValue = system_clock::from_time_t(200);
>       cout << system_clock::to_time_t(someValue) << '\n';
>       cout << system_clock::to_time_t(someValue - unsignedSeconds) << '\n';
>       someValue -= unsignedSeconds;
>       cout << system_clock::to_time_t(someValue) << '\n';

>       std::chrono::seconds signedDur{200};
>       cout << signedDur.count() << '\n';
>       cout << (signedDur - unsignedSeconds).count() << '\n';
>       signedDur -= unsignedSeconds;
>       cout << signedDur.count() << '\n';
>     }

The goal of the program is to compare the behavior of `time_point` non-member `operator-`, `time_point` member `operator-=`, `duration` non-member `operator-`, and `duration` member `operator-=` with basically the same inputs.

libc++ produces this output, which appears mandated by the standard:

>     200
>     4294967491
>     195
>     200
>     195
>     195

On the other hand, libstdc++ produces this output, which is what I "intuitively" expect and behaves more consistently:

>     200
>     195
>     195
>     200
>     195
>     195

Given the seemingly brief coverage of durations with unsigned representations in the standard, this seems to be an oversight rather than a deliberate choice for this behavior. Additionally, there may be other "unexpected" behaviors with durations with an unsigned representation, this is just the one that I've come across.

[2016-07 Chicago]

Monday: P0 - tentatively ready

Proposed resolution:

This wording is relative to N4594.

1. Change 30.6.6 [time.point.nonmember] as indicated:

   >     template <class Clock, class Duration1, class Rep2, class Period2>
   >       constexpr time_point<Clock, common_type_t<Duration1, duration<Rep2, Period2>>>
   >       operator-(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);

   > > -3- Returns: `lhs + (-rhs)``CT(lhs.time_since_epoch() - rhs)`, where `CT` is the type of the return value.

### 2740(i). `constexpr optional<T>::operator->`

Section: 22.5.3.7 [optional.observe] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2016-07-02 Last modified: 2017-07-30

Priority: 0

View other active issues in [optional.observe].

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Discussion:

`optional<T>::operator->`s are constrained to be `constexpr` functions only when `T` is not a type with an overloaded unary `operator&`. This constrain comes from the need to use `addressof` (or a similar mechanism), and the inability to do so in a constant expression in C++14. Given that `addressof` is now `constexpr`, this constrain is no longer needed.

[2016-07 Chicago]

Monday: P0 - tentatively ready

Proposed resolution:

This wording is relative to N4594.

1. Modify [optional.object.observe] as indicated:

   >     constexpr T const* operator->() const;
   >     constexpr T* operator->();

   > > -1- Requires: `*this` contains a value.

   > > -2- Returns: `val`.

   > > -3- Throws: Nothing.

   > > -4- Remarks: Unless `T` is a user-defined type with overloaded unary `operator&`, tThese functions shall be `constexpr` functions.

### 2741(i). `is_partitioned` requirements need updating

Section: 26.8.5 [alg.partitions] Status: Resolved Submitter: Jonathan Wakely Opened: 2016-07-06 Last modified: 2020-05-12

Priority: 3

View all other issues in [alg.partitions].

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Discussion:

> Requires: `InputIterator`'s value type shall be convertible to `Predicate`'s argument type.

This seems to date from the days of adaptable function objects with an `argument_type` typedef, but in modern C++ the predicate might not have an argument type. It could have a function template that accepts various arguments, so it doesn't make sense to state requirements in terms of a type that isn't well defined.

[2016-07, Toronto Saturday afternoon issues processing]

The proposed resolution needs to be updated because the underlying wording has changed. Also, since the sequence is homogeneous, we shouldn't have to say that the expression is well-formed for all elements in the range; that implies that it need not be well-formed if the range is empty.

Marshall and JW to reword. Status to Open

Previous resolution [SUPERSEDED]:

> This wording is relative to N4594.

> 1. Edit 26.8.5 [alg.partitions] as indicated:

>    >     template <class InputIterator, class Predicate>
>    >     bool is_partitioned(InputIterator first, InputIterator last, Predicate pred);

>    > > -1- Requires: `InputIterator`'s value type shall be convertible to `Predicate`'s argument typeThe expression `pred(*i)` shall be well-formed for all `i` in `[first, last)`.

>    > > […]

>    >     template <class InputIterator, class OutputIterator1,
>    >               class OutputIterator2, class Predicate>
>    >       pair<OutputIterator1, OutputIterator2>
>    >       partition_copy(InputIterator first, InputIterator last,
>    >                      OutputIterator1 out_true, OutputIterator2 out_false,
>    >                      Predicate pred);

>    > > -12- Requires: `InputIterator`'s value type shall be `CopyAssignable`, and shall be writable (24.3.1 [iterator.requirements.general]) to the `out_true` and `out_false` `OutputIterator`s, and shall be convertible to `Predicate`'s argument typethe expression `pred(*i)` shall be well-formed for all `i` in `[first, last)`. The input range shall not overlap with either of the output ranges.

>    > > […]

>    >     template<class ForwardIterator, class Predicate>
>    >       ForwardIterator partition_point(ForwardIterator first,
>    >                                       ForwardIterator last,
>    >                                       Predicate pred);

>    > > -16- Requires: `ForwardIterator`'s value type shall be convertible to `Predicate`'s argument typeThe expression `pred(*i)` shall be well-formed for all `i` in `[first, last)`. `[first, last)` shall be partitioned by `pred`, i.e. all elements that satisfy `pred` shall appear before those that do not.

>    > > […]

[2019-03-17; Daniel comments and removes previous wording]

In the recent working draft N4810 all the "shall be convertible to `Predicate`'s argument type" are gone - they were cleaned up when "The One Range" proposal P0896R4 had been accepted in San Diego 2018.

I also believe that we don't need the extra wording that was suggested in the previous P/R (effectively the need to say that the expression `pred(*i)` is well-formed), because this follows (in a more general way) by the expression `invoke(pred, invoke(proj, e))` that is applied to the elements `e` of `[first, last)`.

Therefore I'm suggesting that the resolution for this issue should be: Resolved by P0896R4.

[2020-05-12; Reflector discussions]

Resolved by P0896R4.

Rationale:

Resolved by P0896R4.

Proposed resolution:

### 2742(i). Inconsistent `string` interface taking `string_view`

Section: 27.4.3.3 [string.cons] Status: C++17 Submitter: Richard Smith Opened: 2016-07-06 Last modified: 2020-09-06

Priority: 1

View all other issues in [string.cons].

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Discussion:

Generally, `basic_string` has a constructor matching each `assign` function and vice versa (except the constructor takes an allocator where `assign` does not). P0254R2 violates this by adding an `assign(basic_string_view, size_type pos, size_type n = npos)` but no corresponding constructor.

[2016-08-04 Chicago LWG]

Robert Douglas provides initial wording.

We decided against another constructor overload to avoid the semantic confusion between:

>     basic_string(char const*, size_type length, Allocator = Allocator())

and

>     template<class T, class Foo = is_convertible_v<T const&, basic_string_view<charT, traits>>
>     basic_string(T const&, size_type pos, Allocator = Allocator())

where someone might call:

>     basic_string("HelloWorld", 5, 5);

and get "World", but then call

>     basic_string("HelloWorld", 5);

and instead get "Hello". The second parameter changes between length and position.

[08-2016, Chicago]

Fri PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. In 27.4.3 [basic.string] add the following constructor overload:

   >     […]
   >     basic_string(const basic_string& str, size_type pos,
   >                  const Allocator& a = Allocator());
   >     basic_string(const basic_string& str, size_type pos, size_type n,
   >                  const Allocator& a = Allocator());
   >     template<class T>
   >     basic_string(const T& t, size_type pos, size_type n, const Allocator& a = Allocator());
   >     explicit basic_string(basic_string_view<charT, traits> sv,
   >                           const Allocator& a = Allocator());
   >     […]

2. In 27.4.3.3 [string.cons] add the following ctor definition:

   >     template<class T>
   >     basic_string(const T& t, size_type pos, size_type n, const Allocator& a = Allocator());

   > > -?- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then behaves the same as:

   > > >     basic_string(sv.substr(pos, n), a)

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true`.

### 2743(i). p0083r3 `node_handle` private members missing "exposition only" comment

Section: 23.2.5.1 [container.node.overview] Status: C++23 Submitter: Richard Smith Opened: 2016-07-08 Last modified: 2023-11-22

Priority: 3

View all other issues in [container.node.overview].

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Discussion:

The private members of `node_handle` are missing the usual "exposition only" comment. As a consequence, `ptr_` and `alloc_` now appear to be names defined by the library (so programs defining these names as macros before including a library header have undefined behavior).

Presumably this is unintentional and these members should be considered to be for exposition only.

It's also not clear whether the name `node_handle` is reserved for library usage or not; 23.2.5.1 [container.node.overview]/3 says the implementation need not provide a type with this name, but doesn't seem to rule out the possibility that an implementation will choose to do so regardless.

Daniel:

A similar problem seems to exist for the exposition-only type `call_wrapper` from p0358r1, which exposes a private data member named `fd` and a typedef `FD`.

[2016-07 Chicago]

Jonathan says that we need to make clear that the name `node_handle` is not reserved

[2019-03-17; Daniel comments and provides wording]

Due to an editorial step, the previous name `node_handle`/`node_handle` has been replaced by the artificial `node-handle` name, so I see no longer any reason to talk about a name `node_handle` reservation. The provided wording therefore only takes care of the private members.

[2020-05-16 Reflector discussions]

Status to Tentatively Ready after five positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4810.

1. Change 23.2.5.1 [container.node.overview], exposition-only class template `node-handle` synopsis, as indicated:

   >     template<unspecified>
   >     class node-handle {
   >     public:
   >       […]
   >     private:
   >       using container_node_type = unspecified; // exposition only
   >       using ator_traits = allocator_traits<allocator_type>; // exposition only
   >       typename ator_traits::template rebind_traits<container_node_type>::pointer ptr_; // exposition only
   >       optional<allocator_type> alloc_; // exposition only

   >     public:
   >       […]
   >     };

### 2744(i). `any`'s `in_place` constructors

Section: 22.7.4.2 [any.cons] Status: C++17 Submitter: Ville Voutilainen Opened: 2016-07-10 Last modified: 2017-07-30

Priority: 0

View all other issues in [any.cons].

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Discussion:

The `in_place` constructor that takes an `initializer_list` has both a Requires: for `is_constructible` and a Remarks: for `is_constructible`. The one that takes just a pack has just a Requires: for `is_constructible`.

I think both of those should be Remarks:, i.e. SFINAEable constraints. Otherwise querying is_constructible for an any with `in_place_t` will not give a reasonable answer, and I utterly fail to see any implementation burden in SFINAEing those constructors.

[2016-07 Chicago]

Monday: P0 - tentatively ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 22.7.4.2 [any.cons] as indicated:

   >     template<class ValueType>
   >       any(ValueType&& value);

   > > […]

   > > -7- Requires: `T` shall satisfy the `CopyConstructible` requirements. If `is_copy_constructible_v<T>` is `false`, the program is ill-formed.

   > > -8- Effects: Constructs an object of type `any` that contains an object of type `T` direct-initialized with `std::forward<ValueType>(value)`.

   > > -9- Remarks: This constructor shall not participate in overload resolution if `decay_t<ValueType>` is the same type as `any` or if `ValueType` is a specialization of `in_place_type_t`.

   > > […]

   >     template <class T, class... Args>
   >       explicit any(in_place_type_t<T>, Args&&... args);

   > > -11- Requires: `is_constructible_v<T, Args...>` is `true`.

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, Args...>` is `true`

   > > […]

   >     template <class T, class U, class... Args>
   >       explicit any(in_place_type_t<T>, initializer_list<U> il, Args&&... args);

   > > -15- Requires: `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

   > > […]

   > > -19- Remarks: The functionThis constructor shall not participate in overload resolution unless `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

### 2745(i). [fund.ts.v2] Implementability of LWG 2451

Section: 5.3 [fund.ts.v2::optional.object] Status: TS Submitter: Casey Carter Opened: 2016-07-10 Last modified: 2018-07-08

Priority: 0

View all other issues in [fund.ts.v2::optional.object].

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Discussion:

Addresses: fund.ts.v2

LWG 2451(i) adds conditionally explicit converting constructors to `optional<T>` that accept:

1. Types convertible to `T`: `template <class U> constexpr optional(T&&);`
2. Rvalue `optional<U>` when `U&&` is convertible to `T`: `template <class U> constexpr optional(optional<U>&&);`
3. Lvalue `const optional<U>` when `const U&` is convertible to `T`: `template <class U> constexpr optional(const optional<U>&);`

All three of these constructors are required to be `constexpr` "If `T`'s selected constructor is a `constexpr` constructor". While this is not problematic for #1, it is not possible in the current language to implement signatures #2 and #3 as `constexpr` functions for the same reasons that `optional`'s non-converting constructors from `optional<T>&&` and `const optional<T>&` cannot be `constexpr`.

We should remove the "`constexpr`" specifier from the declarations of the conditionally explicit converting constructors that accept `optional<U>&&` and `const optional<U>&`, and strike the remarks requiring these constructors to be `constexpr`.

[2016-07 Chicago]

Monday: P0 - tentatively ready

This needs to be considered for C++17 as well

Proposed resolution:

This wording is relative to N4600.

> Wording relative to N4600 + LWG 2451(i), although it should be noted that this resolution should be applied wherever LWG 2451(i) is applied, be that to the fundamentals TS or the specification of `optional` in the C++ Working Paper.

1. Edit 22.5.3 [optional.optional] as indicated:

   >     template <class T>
   >     class optional
   >     {
   >     public:
   >       typedef T value_type;

   >       // 5.3.1, Constructors
   >       […]
   >       template <class U> constexpr optional(U&&);
   >       template <class U> constexpr optional(const optional<U>&);
   >       template <class U> constexpr optional(optional<U<&&);
   >       […]
   >     };

2. In 5.3.1 [fund.ts.v2::optional.object.ctor], modify the new signature specifications added by LWG 2451(i)

   >     template <class U>
   >       constexpr optional(const optional<U>& rhs);

   > > […]

   > > -48- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless […]

   >     template <class U>
   >       constexpr optional(optional<U>&& rhs);

   > > […]

   > > -53- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless […]

### 2746(i). Inconsistency between requirements for `emplace` between `optional` and `variant`

Section: 22.5.3.4 [optional.assign], 22.6.3.5 [variant.mod], 22.7.4.4 [any.modifiers] Status: C++26 Submitter: Richard Smith Opened: 2016-07-13 Last modified: 2026-06-09

Priority: 3

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Discussion:

Referring to N4604:

In [optional.object.assign]: `emplace` (normal form) has a Requires that the construction works.

> Requires: `is_constructible_v<T, Args&&...>` is `true`.

`emplace` (`initializer_list` form) has a SFINAE condition:

> Remarks: […] unless `is_constructible_v<T, initializer_list<U>&, Args&&...>` is `true`.

In 22.7.4.4 [any.modifiers]: `emplace` (normal form) has a Requires that the construction works:

> Requires: `is_constructible_v<T, Args...>` is `true`.

`emplace` (`initializer_list` form) has a SFINAE condition:

> Remarks: […] unless `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

In 22.6.3.5 [variant.mod]: `emplace` (`T`, normal form) has a SFINAE condition:

> Remarks: […] unless `is_constructible_v<T, Args...>` is `true`, and `T` occurs exactly once in `Types...`.

`emplace` (Idx, normal form) has a both a Requires and a SFINAE condition:

> Requires: `I < sizeof...(Types)`

> Remarks: […] unless `is_constructible_v<T, Args...>` is `true`, and `T` occurs exactly once in `Types...`.

`emplace` (`T`, `initializer_list` form) has a SFINAE condition:

> Remarks: […] unless `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`, and `T` occurs exactly once in `Types...`.

`emplace` (Idx, `initializer_list` form) has a both a Requires and a SFINAE condition:

> Requires: `I < sizeof...(Types)`

> Remarks: […] unless `is_constructible_v<T, Args...>` is true, and `T` occurs exactly once in `Types...`.

Why the inconsistency? Should all the cases have a SFINAE requirement?

I see that variant has an additional requirement (`T` occurs exactly once in `Types...`), but that only agues that it must be a SFINAE condition — doesn't say that the other cases (any/variant) should not.

`map`/`multimap`/`unordered_map`/`unordered_multimap` have SFINAE'd versions of `emplace` that don't take `initializer_list`s, but they don't have any `emplace` versions that take ILs.

Suggested resolution:

Add SFINAE requirements to `optional::emplace(Args&&... args)` and `any::emplace(Args&&... args);`

[2016-08 Chicago]

During issue prioritization, people suggested that this might apply to `any` as well.

Ville notes that 2746(i), 2754(i) and 2756(i) all go together.

[2020-05-10; Daniel comments and provides wording]

The inconsistency between the two `any::emplace` overloads have been removed by resolving LWG 2754(i) to use Constraints: elements. The last Mandating paper (P1460R1), adopted in Prague, changed the Requires: elements for `variant::emplace`, "`I < sizeof...(Types)`" to Mandates:, but that paper was focused on fixing inappropriate preconditions, not searching for consistency here. Given that the `in_place_index_t` constructors of `variant` uses SFINAE-conditions for this form of static precondition violation, I recommend that its `emplace` functions use the same style, which would bring them also in consistency with their corresponding type-based `emplace` forms that are Mandates:-free but delegate to the index-based forms.

Previous resolution [SUPERSEDED]

This wording is relative to N4861.

1. Modify 22.5.3.4 [optional.assign], as indicated:

   >     template<class... Args> T& emplace(Args&&... args);

   > > -29- MandatesConstraints: `is_constructible_v<T, Args...>` is `true`.

   > > […]

   >     template<class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);

   > > -35- Constraints: `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

   > > […]

2. Modify 22.6.3.5 [variant.mod], as indicated:

   >     template<class T, class... Args> T& emplace(Args&&... args);

   > > -1- Constraints: `is_constructible_v<T, Args...>` is `true`, and `T` occurs exactly once in `Types`.

   > > […]

   >     template<class T, class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);

   > > -3- Constraints: `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`, and `T` occurs exactly once in `Types`.

   > > […]

   >     template<size_t I, class... Args>
   >       variant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);

   > > -5- Mandates: `I < sizeof...(Types)`.

   > > -6- Constraints: `is_constructible_v<TI, Args...>` is `true` and `I < sizeof...(Types)` is `true`.

   > > […]

   >     template<size_t I, class U, class... Args>
   >       variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U> il, Args&&... args);

   > > -12- Mandates: `I < sizeof...(Types)`.

   > > -13- Constraints: `is_constructible_v<TI, initializer_list<U>&, Args...>` is `true` and `I < sizeof...(Types)` is `true`.

   > > […]

[2025-12-05; Jonathan provides new wording]

There's no need to use SFINAE to check if a `variant` with N alternatives allows you to emplace the N+1th alternative.

[2025-12-05; LWG telecon. Moved to Ready]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.3.4 [optional.assign], as indicated:

   >     template<class... Args> T& emplace(Args&&... args);

   > > -29- MandatesConstraints: `is_constructible_v<T, Args...>` is `true`.

   > > […]

   >     template<class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);

   > > -35- Constraints: `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

   > > […]

### 2747(i). Possibly redundant `std::move` in [alg.foreach]

Section: 26.6.5 [alg.foreach] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-07-15 Last modified: 2017-07-30

Priority: 0

View other active issues in [alg.foreach].

View all other issues in [alg.foreach].

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Discussion:

26.6.5 [alg.foreach] p3 says Returns: `std::move(f)`.

[class.copy] says that since `f` is a function parameter overload resolution to select the constructor for the return value is first performed as if for an rvalue, so the `std::move` is redundant.

It could be argued that it isn't entirely redundant, because it says that implementations can't do something slightly different like return an lvalue reference that is bound to `f`, which would prevent it being treated as an rvalue. We should discuss it.

[2016-07 Chicago]

Monday: P0 - tentatively ready

Proposed resolution:

This wording is relative to N4606.

1. Change 26.6.5 [alg.foreach] as indicated:

   >     template<class InputIterator, class Function>
   >       Function for_each(InputIterator first, InputIterator last, Function f);

   > > […]

   > > -3- Returns: `std::move(f)`.

   > > […]

### 2748(i). `swappable` traits for `optional`s

Section: 22.5.3.5 [optional.swap], 22.5.10 [optional.specalg] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2016-07-19 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

`optional` didn't benefit from the wording modifications by P0185 "Adding [`nothrow_`]`swappable` traits"; as such, it suffers from LWG 2456(i), and does not play nice with swappable traits.

[2016-07 Chicago]

Monday: P0 - tentatively ready

Proposed resolution:

This wording is relative to N4606.

1. Modify [optional.object.swap] as indicated:

   >     void swap(optional<T>& rhs) noexcept(see below);

   > > […]

   > > -4- Remarks: The expression inside `noexcept` is equivalent to:

   > > >     is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>noexcept(swap(declval<T&>(), declval<T&>()))

2. Modify 22.5.10 [optional.specalg] as indicated:

   >     template <class T> void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)));

   > > -1- Effects: Calls `x.swap(y)`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true` and `is_swappable_v<T>` is `true`.

### 2749(i). `swappable` traits for `variant`s

Section: 22.6.3.7 [variant.swap], 22.6.10 [variant.specalg] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2016-07-19 Last modified: 2017-07-30

Priority: 1

View all issues with C++17 status.

Discussion:

`variant` does not play nice with swappable traits, the non-member specialized `swap` overload is not SFINAE friendly. On the other hand, the member `swap` is SFINAE friendly, albeit with an incomplete condition, when arguably it shouldn't be. Given the Effects, Throws, and Remarks clauses, the SFINAE condition should include `is_move_constructible_v` and `is_move_assignable_v` to account for the involvement of `variant`'s move constructor and move assignment operator (the `noexcept` specification is correct as is, since the move assignment operator would only be called for `variant`s with different alternatives). This SFINAE condition should apply to the non-member `swap` overload, while the member `swap` should require all alternatives are swappable (as defined by 16.4.4.3 [swappable.requirements]).

[2016-07 Chicago]

Monday: P1 - review later in the week

Fri PM: Move to Tentatively Ready

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.6.3.7 [variant.swap] as indicated:

>    >     void swap(variant& rhs) noexcept(see below);

>    > > -?- Requires: Lvalues of type `Ti` shall be swappable and `is_move_constructible_v<Ti> && is_move_assignable_v<Ti>` is `true` for all `i`.

>    > > […]

>    > > -3- Remarks: This function shall not participate in overload resolution unless `is_swappable_v<Ti>` is `true` for all `i`. If an exception is thrown during the call to function `swap(get<i>(*this), get<i>(rhs))`, the states of the contained values of `*this` and of `rhs` are determined by the exception safety guarantee of swap for lvalues of `Ti` with `i` being `index()`. If an exception is thrown during the exchange of the values of `*this` and `rhs`, the states of the values of `*this` and of `rhs` are determined by the exception safety guarantee of `variant`'s move constructor and move assignment operator. The expression inside `noexcept` is equivalent to the logical AND of `is_nothrow_move_constructible_v<Ti> && is_nothrow_swappable_v<Ti>` for all `i`.

> 2. Modify 22.6.10 [variant.specalg] as indicated:

>    >     template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);

>    > > -1- Effects: Equivalent to `v.swap(w)`.

>    > > -2- Remarks: This function shall not participate in overload resolution unless `is_move_constructible_v<Ti> && is_move_assignable_v<Ti> && is_swappable_v<Ti>` is `true` for all `i`. The expression inside `noexcept` is equivalent to `noexcept(v.swap(w))`.

[2016-08-13, Reopened by Casey Carter]

It is possible to exchange the value of two variants using only move construction on the alternative types, as if by

    auto tmp = move(x);
    x.emplace<i>(move(y));
    y.emplace<j>(move(tmp));

where i is `y.index()` and j is `tmp.index()`. Consequently, `variant`'s member swap need not require move assignable alternatives.

[2016-09-09 Issues Resolution Telecon]

Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 22.6.3.7 [variant.swap] as indicated:

   >     void swap(variant& rhs) noexcept(see below);

   > > -?- Requires: Lvalues of type `Ti` shall be swappable and `is_move_constructible_v<Ti>` shall be `true` for all `i`.

   > > […]

   > > -2- Throws: If `index() == rhs.index()`, aAny exception thrown by `swap(get<i>(*this), get<i>(rhs))` with i being `index()` and `variant`'s move constructor and assignment operator. Otherwise, any exception thrown by the move constructor of `Ti` or `Tj` with `i` being `index()` and `j` being `rhs.index()`.

   > > -3- Remarks: This function shall not participate in overload resolution unless `is_swappable_v<Ti>` is `true` for all `i`. If an exception is thrown during the call to function `swap(get<i>(*this), get<i>(rhs))`, the states of the contained values of `*this` and of `rhs` are determined by the exception safety guarantee of swap for lvalues of `Ti` with `i` being `index()`. If an exception is thrown during the exchange of the values of `*this` and `rhs`, the states of the values of `*this` and of `rhs` are determined by the exception safety guarantee of `variant`'s move constructor and move assignment operator. The expression inside `noexcept` is equivalent to the logical AND of `is_nothrow_move_constructible_v<Ti> && is_nothrow_swappable_v<Ti>` for all `i`.

2. Modify 22.6.10 [variant.specalg] as indicated:

   >     template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);

   > > -1- Effects: Equivalent to `v.swap(w)`.

   > > -2- Remarks: This function shall not participate in overload resolution unless `is_move_constructible_v<Ti> && is_swappable_v<Ti>` is `true` for all `i`. The expression inside `noexcept` is equivalent to `noexcept(v.swap(w))`.

### 2750(i). [fund.ts.v2] LWG 2451 conversion constructor constraint

Section: 5.3.1 [fund.ts.v2::optional.object.ctor] Status: TS Submitter: Casey Carter Opened: 2016-07-20 Last modified: 2017-07-30

Priority: 0

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

LWG 2451(i) adds a converting constructor to `optional` with signature:

>     template <class U>
>     constexpr optional(U&& v);

and specifies that "This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true` and `U` is not the same type as `T`." This suffices to avoid this constructor being selected by overload resolution for arguments that should match the move constructor, but not for arguments that should match the copy constructor. The recent churn around tuple's constructors suggests that we want this constructor to not participate in overload resolution if `remove_cv_t<remove_reference_t<U>>` is the same type as `T`.

[2016-07 Chicago]

Monday: P0 - tentatively ready

Proposed resolution:

This wording is relative to N4600.

> Wording relative to N4600 + LWG 2451(i), although it should be noted that this resolution should be applied wherever LWG 2451(i) is applied, be that to the fundamentals TS or the specification of `optional` in the C++ Working Paper.

1. In 5.3.1 [fund.ts.v2::optional.object.ctor], modify as indicated:

   >     template <class U>
   >       constexpr optional(U&& v);

   > > […]

   > > -43- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true` and `decay_t<U>` is not the same type as `T`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.

### 2752(i). "Throws:" clauses of `async` and `packaged_task` are unimplementable

Section: 32.10.9 [futures.async], 32.10.10.2 [futures.task.members] Status: C++17 Submitter: Billy Robert O'Neal III Opened: 2016-07-07 Last modified: 2017-07-30

Priority: 3

View all other issues in [futures.async].

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Discussion:

`std::async` is a request from the user for type erasure; as any given function gets passed to `async` which returns only `future<ReturnType>`. Therefore, it needs to be able to allocate memory, as other issues (e.g. LWG 2202(i)) indicate. However, async's Throws clause doesn't allow an implementation to do this, as it permits only `future_error`.

`std::packaged_task`'s constructor allocates memory using a user supplied allocator. An implementation needs to call the user's `allocate` to allocate such memory. The user's allocate function is not constrained to throwing only `bad_alloc`; it can raise whatever it wants, but `packaged_task`'s constructor prohibits this.

[2016-07 Chicago]

Alisdair thinks the third bullet is not quite right.

Previous resolution [SUPERSEDED]:

> 1. Change 32.10.9 [futures.async] p6 to:

>    > Throws: `system_error` if `policy == launch::async` and the implementation is unable to start a new thread, or `std::bad_alloc` if memory for the internal data structures could not be allocated.

> 2. Change 32.10.10.2 [futures.task.members] p5 to:

>    >     template <class F>
>    >       packaged_task(F&& f);
>    >     template <class F, class Allocator>
>    >       packaged_task(allocator_arg_t, const Allocator& a, F&& f);

>    > > Throws:

>    > > 1. (?) — Aany exceptions thrown by the copy or move constructor of `f`., or
>    > > 2. (?) — For the first version, `std::bad_alloc` if memory for the internal data structures could not be allocated.
>    > > 3. (?) — For the second version, any exceptions thrown by `std::allocator_traits<Allocator>::template rebind<unspecified>::allocate`.

[2016-08 Chicago]

Wed PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change 32.10.9 [futures.async] p6 to:

   > Throws: `system_error` if `policy == launch::async` and the implementation is unable to start a new thread, or `std::bad_alloc` if memory for the internal data structures could not be allocated.

2. Change 32.10.10.2 [futures.task.members] p5 to:

   >     template <class F>
   >       packaged_task(F&& f);
   >     template <class F, class Allocator>
   >       packaged_task(allocator_arg_t, const Allocator& a, F&& f);

   > > Throws:

   > > 1. (?) — Aany exceptions thrown by the copy or move constructor of `f`., or
   > > 2. (?) — For the first version, `std::bad_alloc` if memory for the internal data structures could not be allocated.
   > > 3. (?) — For the second version, any exceptions thrown by `std::allocator_traits<Allocator>::template rebind_traits<unspecified>::allocate`.

### 2753(i). Optional's constructors and assignments need constraints

Section: 22.5.3.2 [optional.ctor], 22.5.3.4 [optional.assign] Status: Resolved Submitter: Casey Carter Opened: 2016-07-22 Last modified: 2017-06-15

Priority: 0

View all other issues in [optional.ctor].

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Discussion:

To use `optional<T>` as if it were a `T` in generic contexts, `optional<T>`'s "generic" operations must behave as do those of `T` under overload resolution. At minimum, `optional`'s constructors and assignment operators should not participate in overload resolution with argument types that cannot be used to construct/assign the contained `T` so that `is_constructible_v<optional<T>, Args...>` (respectively `is_assignable_v<optional<T>&, RHS>`) is equivalent to `is_constructible_v<T, Args...>` (respectively `is_assignable_v<T&, RHS>`).

In passing, note that the Requires element for `optional`'s in-place `initializer_list` constructor unnecessarily duplicates its Remarks element; it should be removed.

It should also be noted that the resolution of LWG 2451(i) adds constructors to `optional` with appropriate constraints, but does not constrain the additional assignment operators. If LWG chooses to apply the resolution of 2451 to the WP, the Requires elements of the additional assignment operators should also be converted to constraints as the wording herein does for the assignment operators in N4606.

[2016-07 Chicago]

Monday: P0 - tentatively ready

[2016-11-28 Post-Issaquah]

Resolved by the adoption of 2756(i)

Proposed resolution:

This wording is relative to N4606.

1. Remove [optional.object.ctor] p3, and add a new paragraph after p6:

   >     optional(const optional<T>& rhs);

   > > -3- Requires: `is_copy_constructible_v<T>` is `true`.

   > > […]

   > > -?- Remarks: The function shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true`.

2. Remove [optional.object.ctor] p7, and change p11 to:

   >     optional(optional<T>&& rhs) noexcept(see below);

   > > -7- Requires: `is_move_constructible_v<T>` is `true`.

   > > […]

   > > -11- Remarks: The expression inside `noexcept` is equivalent to `is_nothrow_move_constructible_v<T>`. The function shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true`.

3. Remove [optional.object.ctor] p12, and change p16 to:

   >     constexpr optional(const T& v);

   > > -12- Requires: `is_copy_constructible_v<T>` is `true`.

   > > […]

   > > -16- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. The function shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true`.

4. Remove [optional.object.ctor] p17, and change p21 to:

   >     constexpr optional(T&& v);

   > > -17- Requires: `is_move_constructible_v<T>` is `true`.

   > > […]

   > > -21- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. The function shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true`.

5. Remove [optional.object.ctor] p22, and change p26 to:

   >     template <class... Args>
   >       constexpr explicit optional(in_place_t, Args&&... args);

   > > -22- Requires: `is_constructible_v<T, Args&&...>` is `true`.

   > > […]

   > > -26- Remarks: If `T`'s constructor selected for the initialization is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. The function shall not participate in overload resolution unless `is_constructible_v<T, Args...>` is `true`.

6. Remove [optional.object.ctor] p27.

   >     template <class U, class... Args>
   >       constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

   > > -27- Requires: `is_constructible_v<T, initializer_list<U>&, Args&&...>` is `true`.

   > > […]

7. Remove [optional.object.assign] p4, and change p8 to:

   >     optional<T>& operator=(const optional<T>& rhs);

   > > -4- Requires: `is_copy_constructible_v<T>` is `true` and `is_copy_assignable_v<T>` is `true`.

   > > […]

   > > -8- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s copy constructor, no effect. If an exception is thrown during the call to `T`'s copy assignment, the state of its contained value is as defined by the exception safety guarantee of `T`'s copy assignment. The function shall not participate in overload resolution unless `is_copy_constructible_v<T> && is_copy_assignable_v<T>` is `true`.

8. Remove [optional.object.assign] p9, and add a new paragraph after p14:

   >     optional<T>& operator=(optional<T>&& rhs) noexcept(see below);

   > > -9- Requires: `is_move_constructible_v<T>` is `true` and `is_move_assignable_v<T>` is `true`.

   > > […]

   > > -14- Remarks: […] If an exception is thrown during the call to `T`'s move assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s move assignment.

   > > The function shall not participate in overload resolution unless `is_move_constructible_v<T> && is_move_assignable_v<T>` is `true`.

9. Remove [optional.object.assign] p15, and change p19 to (yes, this wording is odd - the intent is that it will "do the right thing" after incorporation of LWG 2451(i)):

   >     template <class U> optional<T>& operator=(U&& v);

   > > -15- Requires: `is_constructible_v<T, U>` is `true` and `is_assignable_v<T&, U>` is `true`.

   > > […]

   > > -19- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `v` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `is_same_v<decay_t<U>, T> && is_constructible_v<T, U> && is_assignable_v<T&, U>` is `true`.

### 2754(i). The `in_place` constructors and `emplace` functions added by P0032R3 don't require `CopyConstructible`

Section: 22.7.4.2 [any.cons], 22.7.4.3 [any.assign], 22.7.4.4 [any.modifiers] Status: Resolved Submitter: Ville Voutilainen Opened: 2016-07-05 Last modified: 2020-09-06

Priority: 1

View all other issues in [any.cons].

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Discussion:

The `in_place` constructors and `emplace` functions added by P0032R3 don't require `CopyConstructible`.

They must. Otherwise copying an `any` that's made to hold a non-`CopyConstructible` type must fail with a run-time error. Since that's crazy, we want to prevent storing non-`CopyConstructible` types in an `any`.

Previously, the requirement for `CopyConstructible` was just on the converting constructor template and the converting assignment operator template on `any`. Now that we are adding two `in_place` constructor overloads and two `emplace` overloads, it seems reasonable to require `CopyConstructible` in some more general location, in order to avoid repeating that requirement all over the place.

[2016-07 — Chicago]

Monday: P1

Tuesday: Ville/Billy/Billy provide wording

[2016-08-02: Daniel comments]

The P/R wording of this issue brought to my intention that the recently added `emplace` functions of `std::any` introduced a breakage of a previous class invariant that only a `decay`ed type could be stored as object into an `any`, this prevented storing arrays, references, functions, and cv-qualified types. The new constraints added my Ville do prevent some of these types (e.g. neither arrays nor functions meet the `CopyConstructible` requirements), but we need to cope with cv-qualified types and reference types.

[2016-08-02: Agustín K-ballo Bergé comments]

Presumably the constructors `any(in_place_type_t<T>, ...)` would need to be modified in the same way the `emplace` overloads were.

[2016-08-02: Ville adjusts the P/R to cope with the problems pointed out by Daniel's and Agustín's comments]

Ville notes that 2746(i), 2754(i) and 2756(i) all go together.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> > Drafting note: this P/R doesn't turn the Requires-clauses into Remarks-clauses. We might want to do that separately, because SFINAEing the constructors allows users to query for `is_constructible` and get the right answer. Failing to mandate the SFINAE will lead to non-portable answers for `is_constructible`. Currently, libstdc++ SFINAEs. That should be done as a separate issue, as this issue is an urgent bug-fix but the mandated SFINAE is not.

> 1. Change 22.7.4 [any.class], class `any` synopsis, as indicated:

>    >     class any {
>    >     public:
>    >       […]
>    >       template <class TValueType, class... Args>
>    >         explicit any(in_place_type_t<TValueType>, Args&&...);
>    >       template <class TValueType, class U, class... Args>
>    >         explicit any(in_place_type_t<TValueType>, initializer_list<U>, Args&&...);

>    >       […]
>    >       template <class TValueType, class... Args>
>    >         void emplace(Args&& ...);
>    >       template <class TValueType, class U, class... Args>
>    >         void emplace(initializer_list<U>, Args&&...);
>    >       […]
>    >     };

> 2. Change 22.7.4.2 [any.cons] as indicated:

>    >     template<class ValueType>
>    >       any(ValueType&& value);

>    > > -6- Let `T` be equal to `decay_t<ValueType>`.

>    > > -7- Requires: `T` shall satisfy the `CopyConstructible` requirements. If `is_copy_constructible_v<T>` is `false`, the program is ill-formed.

>    > > […]

>    > > -9- Remarks: This constructor shall not participate in overload resolution ifunless `decay_t<ValueType>` is not the same type as `any` and `is_copy_constructible_v<T>` is `true`.

>    >     template <class TValueType, class... Args>
>    >       explicit any(in_place_type_t<TValueType>, Args&&... args);

>    > > -?- Let `T` be equal to `remove_cv_t<ValueType>`.

>    > > -11- Requires: `T` shall satisfy the `CopyConstructible` requirements `is_constructible_v<T, Args...>` is `true`.

>    > > […]

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_reference_v<T>` is `false`, `is_array_v<T>` is `false`, `is_function_v<T>` is `false`, `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, Args...>` is `true`.

>    >     template <class TValueType, class U, class... Args>
>    >       explicit any(in_place_type_t<TValueType>, initializer_list<U> il, Args&&... args);

>    > > -?- Let `T` be equal to `remove_cv_t<ValueType>`.

>    > > -15- Requires: `T` shall satisfy the `CopyConstructible` requirements `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

>    > > […]

>    > > -19- Remarks: The function shall not participate in overload resolution unless `is_reference_v<T>` is `false`, `is_array_v<T>` is `false`, `is_function_v<T>` is `false`, `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

> 3. Change 22.7.4.3 [any.assign] as indicated:

>    >     template<class ValueType>
>    >       any& operator=(ValueType&& rhs);

>    > > -7- Let `T` be equal to `decay_t<ValueType>`.

>    > > -8- Requires: `T` shall satisfy the `CopyConstructible` requirements. If `is_copy_constructible_v<T>` is `false`, the program is ill-formed.

>    > > […]

>    > > -11- Remarks: This operator shall not participate in overload resolution ifunless `decay_t<ValueType>` is not the same type as `any` and `is_copy_constructible_v<T>` is `true`.

> 4. Change 22.7.4.4 [any.modifiers] as indicated:

>    >     template <class TValueType, class... Args>
>    >       void emplace(Args&&... args);

>    > > -?- Let `T` be equal to `remove_cv_t<ValueType>`.

>    > > -1- Requires: `T` shall satisfy the `CopyConstructible` requirements `is_constructible_v<T, Args...>` is `true`.

>    > > […]

>    > > -5- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and any previously contained object has been destroyed. This function shall not participate in overload resolution unless `is_reference_v<T>` is `false`, `is_array_v<T>` is `false`, `is_function_v<T>` is `false`, `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, Args...>` is `true`.

>    >     template <class TValueType, class U, class... Args>
>    >       void emplace(initializer_list<U> il, Args&&... args);

>    > > -?- Let `T` be equal to `remove_cv_t<ValueType>`.

>    > > -?- Requires: `T` shall satisfy the `CopyConstructible` requirements.

>    > > -6- Effects: […]

>    > > […]

>    > > -9- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and any previously contained object has been destroyed. The function shall not participate in overload resolution unless `is_reference_v<T>` is `false`, `is_array_v<T>` is `false`, `is_function_v<T>` is `false`, `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

[2016-08-03: Ville comments and revises his proposed wording]

After discussing the latest P/R, here's an update. What this update does is that:

1. It strikes the Requires-clauses and does not add `CopyConstructible` to the Requires-clauses.

   Rationale: `any` doesn't care whether the type it holds satisfies the semantic requirements of the `CopyConstructible` concept. The syntactic requirements are now SFINAE constraints in Requires-clauses.

2. It reverts back towards `decay_t` rather than `remove_cv_t`, and does not add the suggested SFINAE constraints for `is_reference`/`is_array`/`is_function`.

   Rationale:

   1. `any` decays by design. It's to some extent inconsistent to not protect against decay in the `ValueType` constructor/assignment operator, but to protect against decay in the `in_place_t` constructors and `emplace` functions

   2. I think it's saner to just decay than to potentially run into situations where I need to `remove_reference` inside `in_place_t`.

Based on that, this P/R should supersede the previous one. We want to look at this new P/R in LWG and potentially send it to LEWG for verification. Personally, I think this P/R is the more conservative one, doesn't add significant new functionality, and is consistent, and is thus not really Library-Evolutionary.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Change 22.7.4 [any.class], class `any` synopsis, as indicated:

>    >     class any {
>    >     public:
>    >       […]
>    >       template <class TValueType, class... Args>
>    >         explicit any(in_place_type_t<TValueType>, Args&&...);
>    >       template <class TValueType, class U, class... Args>
>    >         explicit any(in_place_type_t<TValueType>, initializer_list<U>, Args&&...);

>    >       […]
>    >       template <class TValueType, class... Args>
>    >         void emplace(Args&& ...);
>    >       template <class TValueType, class U, class... Args>
>    >         void emplace(initializer_list<U>, Args&&...);
>    >       […]
>    >     };

> 2. Change 22.7.4.2 [any.cons] as indicated:

>    >     template<class ValueType>
>    >       any(ValueType&& value);

>    > > -6- Let `T` be equal to `decay_t<ValueType>`.

>    > > -7- Requires: `T` shall satisfy the `CopyConstructible` requirements. If `is_copy_constructible_v<T>` is `false`, the program is ill-formed.

>    > > […]

>    > > -9- Remarks: This constructor shall not participate in overload resolution ifunless `decay_t<ValueType>` is not the same type as `any` and `is_copy_constructible_v<T>` is `true`.

>    >     template <class TValueType, class... Args>
>    >       explicit any(in_place_type_t<TValueType>, Args&&... args);

>    > > -?- Let `T` be equal to `decay_t<ValueType>`.

>    > > -11- Requires: `is_constructible_v<T, Args...>` is `true`.

>    > > […]

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, Args...>` is `true`.

>    >     template <class TValueType, class U, class... Args>
>    >       explicit any(in_place_type_t<TValueType>, initializer_list<U> il, Args&&... args);

>    > > -?- Let `T` be equal to `decay_t<ValueType>`.

>    > > -15- Requires: `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

>    > > […]

>    > > -19- Remarks: The function shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

> 3. Change 22.7.4.3 [any.assign] as indicated:

>    >     template<class ValueType>
>    >       any& operator=(ValueType&& rhs);

>    > > -7- Let `T` be equal to `decay_t<ValueType>`.

>    > > -8- Requires: `T` shall satisfy the `CopyConstructible` requirements. If `is_copy_constructible_v<T>` is `false`, the program is ill-formed.

>    > > […]

>    > > -11- Remarks: This operator shall not participate in overload resolution ifunless `decay_t<ValueType>` is not the same type as `any` and `is_copy_constructible_v<T>` is `true`.

> 4. Change 22.7.4.4 [any.modifiers] as indicated:

>    >     template <class TValueType, class... Args>
>    >       void emplace(Args&&... args);

>    > > -?- Let `T` be equal to `decay_t<ValueType>`.

>    > > -1- Requires: `is_constructible_v<T, Args...>` is `true`.

>    > > […]

>    > > -5- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and any previously contained object has been destroyed. This function shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, Args...>` is `true`.

>    >     template <class TValueType, class U, class... Args>
>    >       void emplace(initializer_list<U> il, Args&&... args);

>    > > -?- Let `T` be equal to `decay_t<ValueType>`.

>    > > -6- Effects: […]

>    > > […]

>    > > -9- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and any previously contained object has been destroyed. The function shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

[2016-08-03: Ville comments and revises his proposed wording]

This P/R brings back the `CopyConstructible` parts of the relevant Requires-clauses but removes the other parts of the Requires-clauses.

[2016-08 - Chicago]

Thurs PM: Moved to Tentatively Ready

[2016-11 - Issaquah]

Approved in plenary.

After plenary, there was concern about applying both this and 2744(i), so it was moved back to "Open". Then, when the concerns were resolved, moved to "Resolved".

Proposed resolution:

This wording is relative to N4606.

1. Change 22.7.4 [any.class], class `any` synopsis, as indicated:

   >     class any {
   >     public:
   >       […]
   >       template <class TValueType, class... Args>
   >         explicit any(in_place_type_t<TValueType>, Args&&...);
   >       template <class TValueType, class U, class... Args>
   >         explicit any(in_place_type_t<TValueType>, initializer_list<U>, Args&&...);

   >       […]
   >       template <class TValueType, class... Args>
   >         void emplace(Args&& ...);
   >       template <class TValueType, class U, class... Args>
   >         void emplace(initializer_list<U>, Args&&...);
   >       […]
   >     };

2. Change 22.7.4.2 [any.cons] as indicated:

   >     template<class ValueType>
   >       any(ValueType&& value);

   > > -6- Let `T` be `decay_t<ValueType>`.

   > > -7- Requires: `T` shall satisfy the `CopyConstructible` requirements. If `is_copy_constructible_v<T>` is `false`, the program is ill-formed.

   > > […]

   > > -9- Remarks: This constructor shall not participate in overload resolution ifunless `Tdecay_t<ValueType>` is not the same type as `any` and `is_copy_constructible_v<T>` is `true`.

   >     template <class TValueType, class... Args>
   >       explicit any(in_place_type_t<TValueType>, Args&&... args);

   > > -?- Let `T` be `decay_t<ValueType>`.

   > > -11- Requires: `T` shall satisfy the `CopyConstructible` requirements `is_constructible_v<T, Args...>` is `true`.

   > > […]

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, Args...>` is `true`.

   >     template <class TValueType, class U, class... Args>
   >       explicit any(in_place_type_t<TValueType>, initializer_list<U> il, Args&&... args);

   > > -?- Let `T` be `decay_t<ValueType>`.

   > > -15- Requires: `T` shall satisfy the `CopyConstructible` requirements `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

   > > […]

   > > -19- Remarks: The function shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

3. Change 22.7.4.3 [any.assign] as indicated:

   >     template<class ValueType>
   >       any& operator=(ValueType&& rhs);

   > > -7- Let `T` be `decay_t<ValueType>`.

   > > -8- Requires: `T` shall satisfy the `CopyConstructible` requirements. If `is_copy_constructible_v<T>` is `false`, the program is ill-formed.

   > > […]

   > > -11- Remarks: This operator shall not participate in overload resolution ifunless `Tdecay_t<ValueType>` is not the same type as `any` and `is_copy_constructible_v<T>` is `true`.

4. Change 22.7.4.4 [any.modifiers] as indicated:

   >     template <class TValueType, class... Args>
   >       void emplace(Args&&... args);

   > > -?- Let `T` be `decay_t<ValueType>`.

   > > -1- Requires: `T` shall satisfy the `CopyConstructible` requirements `is_constructible_v<T, Args...>` is `true`.

   > > […]

   > > -5- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and any previously contained object has been destroyed. This function shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, Args...>` is `true`.

   >     template <class TValueType, class U, class... Args>
   >       void emplace(initializer_list<U> il, Args&&... args);

   > > -?- Let `T` be `decay_t<ValueType>`.

   > > -?- Requires: `T` shall satisfy the `CopyConstructible` requirements.

   > > -6- Effects: […]

   > > […]

   > > -9- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and any previously contained object has been destroyed. The function shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true` and `is_constructible_v<T, initializer_list<U>&, Args...>` is `true`.

### 2755(i). §[string.view.io] uses non-existent `basic_string_view::to_string` function

Section: 27.3.5 [string.view.io], 27.4.4.4 [string.io] Status: C++17 Submitter: Billy Baker Opened: 2016-07-26 Last modified: 2020-09-06

Priority: 0

View all issues with C++17 status.

Discussion:

In looking at N4606, [string.view.io] has an Effects clause that references `basic_string_view::to_string` which no longer exists after the application of P0254R2.

[2016-07-26, Marshall suggests concrete wording]

[2016-07 Chicago LWG]

Monday: P0 - tentatively ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 27.4.4.4 [string.io] as indicated:

   >     template<class charT, class traits, class Allocator>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os,
   >                    const basic_string<charT, traits, Allocator>& str);

   > > -5- Effects: Equivalent to: `return os << basic_string_view<charT, traits>(str);`Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of `os`. Forms a character sequence `seq`, initially consisting of the elements defined by the range `[str.begin(), str.end())`. Determines padding for `seq` as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Then inserts `seq` as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size()`; then calls `os.width(0)`.

   > > -6- Returns: `os`

2. Modify 27.3.5 [string.view.io] as indicated:

   >     template<class charT, class traits>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os,
   >                    basic_string_view<charT, traits> str);

   > > -1- Effects: Equivalent to: `return os << str.to_string();`Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of `os`. Forms a character sequence `seq`, initially consisting of the elements defined by the range `[str.begin(), str.end())`. Determines padding for `seq` as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Then inserts `seq` as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size()`; then calls `os.width(0)`.

   > > -?- Returns: `os`

### 2756(i). C++ WP `optional<T>` should 'forward' `T`'s implicit conversions

Section: 22.5.3 [optional.optional] Status: C++17 Submitter: Casey Carter Opened: 2016-07-26 Last modified: 2017-07-30

Priority: 1

View all other issues in [optional.optional].

View all issues with C++17 status.

Discussion:

LWG 2451(i) adds converting constructors and assignment operators to `optional`. The committee voted to apply it to the Library Fundamentals 2 TS WP in Oulu as part of LWG Motion 3. In both LWG and LEWG discussion of this issue, it was considered to be critical to apply to the specification of `optional` before shipping C++17 — it was an oversight on the part of LWG that there was no motion brought to apply this resolution to the C++ WP.

LWG 2745(i) proposes removal of the `constexpr` specifier from the declarations of the converting constructors from `const optional<U>&` and `optional<U>&&` since they are not implementable as `constexpr` constructors in C++17.

This issue proposes application of the resolution of LWG 2451(i) as amended by LWG 2745(i) to the specification of `optional` in the C++ WP.

[2016-07 — Chicago]

Monday: Priority set to 1; will re-review later in the week

[2016-08-03, Tomasz comments]

1. Value forwarding constructor (`template<typename U> optional(U&&)`) is underspecified.

   For the following use code:

   >     optional<T> o1;
   >     optional<T> o2(o1);

   The second constructor will invoke value forwarding `(U = optional<T>&)` constructor (better match) instead of the `optional<T>` copy constructor, in case if `T` can be constructed from `optional<T>`. This happens for any type `T` that has unconstrained perfect forwarding constructor, especially `optional<any>`.

2. The behavior of the construction of the `optional<T> ot` from `optional<U> ou` is inconsistent for classes `T` than can be constructed both from `optional<U>` and `U`. There are two possible semantics for such operation:

   - unwrapping: if `ou` is initialized (`bool(ou)`), initialize contained value of `ot` from `*ou`, otherwise make `ot` uninitialized (`!bool(ot)`)

   - value forwarding: initialize contained value of `ot` from `ou`, `ot` is always initialized (`bool(ot)`).

     With the proposed resolution, construction is preferring value forwarding, while assignment is always using unwrapping.

   For example, if we consider following class:

   >     struct Widget
   >     {
   >       Widget(int);
   >       Widget(optional<int>);
   >     };

   Notice, that such set of constructor is pretty common in situation when the construction of the `Widget` from known value is common and usage of `optional` version is rare. In such situation, presence of `Widget(int)` construction is an optimization used to avoid unnecessary empty checks and construction `optional<int>`.

   For the following declarations:

   >     optional<Widget> w1(make_optional(10));
   >     optional<Widget> w2;
   >     w2 = make_optional(10);

   The `w1` will contain a value created using `Widget(optional<int>)` constructor, as corresponding unwrapping constructor (`optional<U> const&`) is eliminated by `is_constructible_v<T, const optional<U>&>` (`is_constructible_v<Widget, const optional<int>&>`) having a `true` value. In contrast `w2` will contain a value created using `Widget(int)` constructor, as corresponding value forwarding assignment (`U&&`) is eliminated by the fact that `std::decay_t<U>` (`optional<int>`) is specialization of `optional`.

   In addition, the construction is having a preference for value forwarding and assignment is always using unwrapping. That means that for the following class:

   >     struct Thingy
   >     {
   >        Thingy(optional<string>);
   >     };

   >     optional<Thingy> t1(optional<string>("test"));

   The `t1` has a contained value constructed from using `Thingy(optional<std::string>)`, as unwrapping constructor (`optional<U> const&`) are eliminated by the `is_constructible<T, U const&>` (`is_constructible<Thingy, std::string const&>`) being `false`. However the following:

   >     t1 = optional<std::string>("test2");

   will not compile, because, the value forwarding assignment (`U&&`) is eliminated by `std::decay_t<U>` (`optional<std::string>`) being optional specialization and the unwrapping assignment (`optional<U> const&`) is ill-formed because `is_constructible<T, U const&>` (`is_constructible<Thingy, std::string const&>`) is `false`.

3. The semantics of construction and assignment, of `optional<optional<V>>` from `optional<U>` where `U` is convertible to/ same as `T` is also inconsistent. Firstly, in this situation the `optional<V>` is a type that can be constructed both from `optional<U>` and `U` so it fails into set of problem described above. However in addition we have following non-template constructor in `optional<T>`:

   >     optional(T const&);

   Which for `optional<optional<V>>` is equivalent to:

   >     optional(optional<V> const&);

   While there is no corresponding non-template assignment from `T const&`, to make sure that `o = {};` always clear an `optional o`.

   So for the following declarations:

   >     optional<int> oi;
   >     optional<short> os;

   >     optional<optional<int>> ooi1(oi);
   >     optional<optional<int>> ooi2(os);

   The `ooi1` uses from-`T` constructor, while `ooi2` uses value forwarding constructor. In this case both `ooi1` and `ooi2` are initialized and their contained values `*ooi1`, `*ooi2` are uninitialized `optional`s. However, if we will attempt to make construction consistent with assignment, by preferring unwrapping (`optional<U> const&`), then `ooi2` will end up being uninitialized.

   In summary, I believe that relation between unwrapping, value forwarding and from-`T` construction/assignment is to subtle to being handled as defect report and requires a full paper analyzing possible design and their pros/cons.

Tuesday PM: Ville and Eric to implement and report back in Issaquah. Moved to Open

Ville notes that 2746(i), 2754(i) and 2756(i) all go together.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.5.3 [optional.optional] as indicated:

>    >     template <class T> class optional
>    >     {
>    >     public:
>    >       using value_type = T;

>    >       // 20.6.3.1, Constructors
>    >       constexpr optional() noexcept;
>    >       constexpr optional(nullopt_t) noexcept;
>    >       optional(const optional &);
>    >       optional(optional &&) noexcept(see below);
>    >       constexpr optional(const T &);
>    >       constexpr optional(T &&);
>    >       template <class... Args> constexpr explicit optional(in_place_t, Args &&...);
>    >       template <class U, class... Args>
>    >         constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...);
>    >       template <class U> constexpr optional(U &&);
>    >       template <class U> optional(const optional<U> &);
>    >       template <class U> optional(optional<U> &&);

>    >       […]

>    >       // 20.6.3.3, Assignment
>    >       optional &operator=(nullopt_t) noexcept;
>    >       optional &operator=(const optional &);
>    >       optional &operator=(optional &&) noexcept(see below);
>    >       template <class U> optional &operator=(U &&);
>    >       template <class U> optional& operator=(const optional<U> &);
>    >       template <class U> optional& operator=(optional<U> &&);
>    >       template <class... Args> void emplace(Args &&...);
>    >       template <class U, class... Args>
>    >         void emplace(initializer_list<U>, Args &&...);

>    >       […]

>    >     };

> 2. In [optional.object.ctor], insert new signature specifications after p31:

>    > [Note: The following constructors are conditionally specified as `explicit`. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]

>    >     template <class U>
>    >     constexpr optional(U&& v);

>    > > -?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::forward<U>(v)`.

>    > > -?- Postconditions: `*this` contains a value.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true` and `U` is not the same type as `T`. The constructor is `explicit` if and only if `is_convertible_v<U&&, T>` is `false`.

>    >     template <class U>
>    >     optional(const optional<U>& rhs);

>    > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `*rhs`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, const U&>` is `true`, `is_same<decay_t<U>, T>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false` and `is_convertible_v<const optional<U>&, T>` is `false`. The constructor is `explicit` if and only if `is_convertible_v<const U&, T>` is `false`.

>    >     template <class U>
>    >     optional(optional<U>&& rhs);

>    > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::move(*rhs)`. `bool(rhs)` is unchanged.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `is_same<decay_t<U>, T>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false` and `is_convertible_v<optional<U>&&, T>` is `false` and `U` is not the same type as `T`. The constructor is `explicit` if and only if `is_convertible_v<U&&, T>` is `false`.

> 3. In [optional.object.assign], change as indicated:

>    >     template <class U> optional<T>& operator=(U&& v);

>    > > -22- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `v` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `decay_t<U>` is not `nullopt_t` and `decay_t<U>` is not a specialization of `optional` `is_same_v<decay_t<U>, T>` is `true`.

>    > > -23- Notes: The reason for providing such generic assignment and then constraining it so that effectively `T == U` is to guarantee that assignment of the form `o = {}` is unambiguous.

>    >     template <class U> optional<T>& operator=(const optional<U>& rhs);

>    > > -?- Requires: `is_constructible_v<T, const U&>` is `true` and `is_assignable_v<T&, const U&>` is `true`.

>    > > -?- Effects:

>    > > Table ? — `optional::operator=(const optional<U>&)` effects
>    > > | `*this` contains a value | `*this` does not contain a value
>    > > --- | --- | ---
>    > > `rhs` contains a value | assigns `*rhs` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `*rhs`
>    > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

>    > > -?- Returns: `*this`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `is_same_v<decay_t<U>, T>` is `false`.

>    >     template <class U> optional<T>& operator=(optional<U>&& rhs);

>    > > -?- Requires: `is_constructible_v<T, U>` is `true` and `is_assignable_v<T&, U>` is `true`.

>    > > -?- Effects: The result of the expression `bool(rhs)` remains unchanged.

>    > > Table ? — `optional::operator=(optional<U>&&)` effects
>    > > | `*this` contains a value | `*this` does not contain a value
>    > > --- | --- | ---
>    > > `rhs` contains a value | assigns `std::move(*rhs)` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `std::move(*rhs)`
>    > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

>    > > -?- Returns: `*this`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `is_same_v<decay_t<U>, T>` is `false`.

[2016-08-05 Chicago LWG]

Ville provides revised wording, that also fixes LWG 2753(i).

Rationale:

1. The resolution of LWG 2753(i) makes special member functions defined as deleted in case the desired constraints aren't met.

2. There is no decay for the converting constructor `optional(U&&)`, there is a `remove_reference` instead. The target type may hold a cv-qualified type, and the incoming type may hold a cv-qualified type, but neither can hold a reference. Thus, `remove_reference` is what we need, `remove_cv` would be wrong, and `decay` would be wrong.

3. There is no `decay` or `remove_reference` for converting constructors like `optional(optional<U>)`, because none is needed.

4. For `optional(U&&)`, an added constraint is that `U` is not a specialization of `optional`

[2016-08, Chicago]

Fri PM: Move to Tentatively Ready

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.5.3 [optional.optional] as indicated:

>    >     template <class T> class optional
>    >     {
>    >     public:
>    >       using value_type = T;

>    >       // 20.6.3.1, Constructors
>    >       constexpr optional() noexcept;
>    >       constexpr optional(nullopt_t) noexcept;
>    >       optional(const optional &);
>    >       optional(optional &&) noexcept(see below);
>    >       constexpr optional(const T &);
>    >       constexpr optional(T &&);
>    >       template <class... Args> constexpr explicit optional(in_place_t, Args &&...);
>    >       template <class U, class... Args>
>    >         constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...);
>    >       template <class U> EXPLICIT constexpr optional(U &&);
>    >       template <class U> EXPLICIT optional(const optional<U> &);
>    >       template <class U> EXPLICIT optional(optional<U> &&);

>    >       […]

>    >       // 20.6.3.3, Assignment
>    >       optional &operator=(nullopt_t) noexcept;
>    >       optional &operator=(const optional &);
>    >       optional &operator=(optional &&) noexcept(see below);
>    >       template <class U> optional &operator=(U &&);
>    >       template <class U> optional& operator=(const optional<U> &);
>    >       template <class U> optional& operator=(optional<U> &&);
>    >       template <class... Args> void emplace(Args &&...);
>    >       template <class U, class... Args>
>    >         void emplace(initializer_list<U>, Args &&...);

>    >       […]

>    >     };

> 2. Change [optional.object.ctor] as indicated:

>    >     optional(const optional<T>& rhs);

>    > > -3- Requires: `is_copy_constructible_v<T>` is `true`.

>    > > […]

>    > > -?- Remarks: This constructor shall be defined as deleted unless `is_copy_constructible_v<T>` is `true`.

>    >     optional(optional<T>&& rhs) noexcept(see below);

>    > > -7- Requires: `is_move_constructible_v<T>` is `true`.

>    > > […]

>    > > -11- Remarks: The expression inside `noexcept` is equivalent to `is_nothrow_move_constructible_v<T>`. This constructor shall be defined as deleted unless `is_move_constructible_v<T>` is `true`.

>    >     constexpr optional(const T& v);

>    > > -12- Requires: `is_copy_constructible_v<T>` is `true`.

>    > > […]

>    > > -16- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_copy_constructible_v<T>` is `true`.

>    >     constexpr optional(T&& v);

>    > > -17- Requires: `is_move_constructible_v<T>` is `true`.

>    > > […]

>    > > -21- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true`.

>    >     template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);

>    > > -22- Requires: `is_constructible_v<T, Args&&...>` is `true`.

>    > > […]

>    > > -26- Remarks: If `T`'s constructor selected for the initialization is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, Args...>` is `true`.

>    >     template <class U, class... Args>
>    >       constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

>    > > -27- Requires: `is_constructible_v<T, initializer_list<U>&, Args&&...>` is `true`.

>    > > […]

>    > > -31- Remarks: The functionconstructor shall not participate in overload resolution unless `is_constructible_v<T, initializer_list<U>&, Args&&...>` is `true`. If `T`'s constructor selected for the initialization is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

>    > [Note: The following constructors are conditionally specified as explicit. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]

>    >     template <class U>
>    >       EXPLICIT constexpr optional(U&& v);

>    > > -?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::forward<U>(v)`.

>    > > -?- Postconditions: `*this` contains a value.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `remove_reference_t<U>` is not the same type as `T`, and `U` is not a specialization of `optional`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.

>    >     template <class U>
>    >       EXPLICIT optional(const optional<U>& rhs);

>    > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `*rhs`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, const U&>` is `true` and `is_same_v<U, T>` is `false`. The constructor is explicit if and only if `is_convertible_v<const U&, T>` is `false`.

>    >     template <class U>
>    >       EXPLICIT optional(optional<U>&& rhs);

>    > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::move(*rhs)`. `bool(rhs)` is unchanged.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true` and `is_same_v<U, T>` is `false`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.

> 3. Change [optional.object.assign] as indicated:

>    >     optional<T>& operator=(const optional<T>& rhs);

>    > > -4- Requires: `is_copy_constructible_v<T>` is `true` and `is_copy_assignable_v<T>` is `true`.

>    > > […]

>    > > -8- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s copy constructor, no effect. If an exception is thrown during the call to `T`'s copy assignment, the state of its contained value is as defined by the exception safety guarantee of `T`'s copy assignment. This operator shall be defined as deleted unless `is_copy_constructible_v<T>` is `true` and `is_copy_assignable_v<T>` is `true`.

>    >     optional<T>& operator=(optional<T>&& rhs) noexcept(see below);

>    > > -9- Requires: `is_move_constructible_v<T>` is `true` and `is_move_assignable_v<T>` is `true`.

>    > > […]

>    > > -13- Remarks: The expression inside `noexcept` is equivalent to:

>    > > >     is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>

>    > > -14- If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s move constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s move constructor. If an exception is thrown during the call to `T`'s move assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s move assignment. This operator shall be defined as deleted unless `is_move_constructible_v<T>` is `true` and `is_move_assignable_v<T>` is `true`.

>    >     template <class U> optional<T>& operator=(U&& v);

>    > > -15- Requires: `is_constructible_v<T, U>` is `true` and `is_assignable_v<T&, U>` is `true`.

>    > > […]

>    > > -19- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `v` is determined by the exception safety guarantee of `T`'s assignment. TheThis function shall not participate in overload resolution unless `is_same_v<decay_t<U>, T>` `decay_t<U>` is not `nullopt_t`, `decay_t<U>` is not a specialization of `optional`, `is_constructible_v<T, U>` is `true` and `is_assignable_v<T&, U>` is `true`.

>    > > -20- Notes: The reason for providing such generic assignment and then constraining it so that effectively `T == U` is to guarantee that assignment of the form `o = {}` is unambiguous.

>    >     template <class U> optional<T>& operator=(const optional<U>& rhs);

>    > > -?- Effects:

>    > > Table ? — `optional::operator=(const optional<U>&)` effects
>    > > | `*this` contains a value | `*this` does not contain a value
>    > > --- | --- | ---
>    > > `rhs` contains a value | assigns `*rhs` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `*rhs`
>    > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

>    > > -?- Returns: `*this`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `is_constructible_v<T, const U&>` is `true` and `is_assignable_v<T&, const U&>` is `true` and `is_same_v<U, T>` is `false`.

>    >     template <class U> optional<T>& operator=(optional<U>&& rhs);

>    > > -?- Effects: The result of the expression `bool(rhs)` remains unchanged.

>    > > Table ? — `optional::operator=(optional<U>&&)` effects
>    > > | `*this` contains a value | `*this` does not contain a value
>    > > --- | --- | ---
>    > > `rhs` contains a value | assigns `std::move(*rhs)` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `std::move(*rhs)`
>    > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

>    > > -?- Returns: `*this`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. The function shall not participate in overload resolution unless `is_constructible_v<T, U>` is `true` and `is_assignable_v<T&, U>` is `true` and `is_same_v<U, T>` is `false`.

[2016-08-08 Ville reopens and provides improved wording]

This alternative proposed wording also resolves 2753(i).

The constructors that take a const `T&` or `T&&` are replaced by a constructor template that takes a `U&&` and defaults `U = T`. This allows copy-list-initialization with empty braces to still work:

>     optional<whatever> o = {}; // equivalent to initializing optional with nullopt

This resolution makes converting constructors and assignments have the same capabilities, including using arguments that can't be deduced. That is achieved by using a perfect-forwarding constructor and an assignment operator that default their argument to `T`. We don't need separate overloads for `T`, the overload for `U` does the job:

>     optional<vector<int>> ovi{{1, 2, 3}}; // still works
>     ovi = {4, 5, 6, 7}; // now works, didn't work before

Furthermore, this proposed wording makes `optional` "always unwrap". That is, the result of the following initializations is the same:

>     optional<optional<int>> oi = optional<int>();
>     optional<optional<int>> oi = optional<short>();

Both of those initializations initialize the `optional` wrapping another `optional` as if initializing with `nullopt`. Assignments do the same. These changes solve the issues pointed out by Tomasz Kamiński.

This P/R has been implemented and tested as a modification on top of libstdc++'s `optional`.

[2016-08-08 Ville and Tomasz collaborate and improve wording]

The suggested wording retains `optional`'s converting constructors and assignment operators, but provides sane results for the types Tomasz Kaminski depicts in previous discussions.

As opposed to the current P/R of this issue, which does "always unwrap", this P/R does "always value-forward unless the incoming type is exactly a type that a special member function takes by reference, and don't unwrap if a value-forwarder can take an optional by any kind of reference".

I and Tomasz believe this is the best compromise between the different desires, and thus the best outcome so far.

This P/R has been implemented and tested on libstdc++.

[2016-09-08 Casey Carter finetunes existing resolution for move members]

[2016-09-09 Issues Resolution Telecon]

Move to Tentatively Ready

[2016-10-06 Ville Voutilainen finetunes the resolution for assignment from scalars]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.5.3 [optional.optional] as indicated:

>    >     template <class T> class optional
>    >     {
>    >     public:
>    >       using value_type = T;

>    >       // 20.6.3.1, Constructors
>    >       constexpr optional() noexcept;
>    >       constexpr optional(nullopt_t) noexcept;
>    >       optional(const optional &);
>    >       optional(optional &&) noexcept(see below);
>    >       constexpr optional(const T &);
>    >       constexpr optional(T &&);
>    >       template <class... Args> constexpr explicit optional(in_place_t, Args &&...);
>    >       template <class U, class... Args>
>    >         constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...);
>    >       template <class U = T> EXPLICIT constexpr optional(U &&);
>    >       template <class U> EXPLICIT optional(const optional<U> &);
>    >       template <class U> EXPLICIT optional(optional<U> &&);

>    >       […]

>    >       // 20.6.3.3, Assignment
>    >       optional &operator=(nullopt_t) noexcept;
>    >       optional &operator=(const optional &);
>    >       optional &operator=(optional &&) noexcept(see below);
>    >       template <class U = T> optional &operator=(U &&);
>    >       template <class U> optional& operator=(const optional<U> &);
>    >       template <class U> optional& operator=(optional<U> &&);
>    >       template <class... Args> void emplace(Args &&...);
>    >       template <class U, class... Args>
>    >         void emplace(initializer_list<U>, Args &&...);

>    >       […]

>    >     };

> 2. Change [optional.object.ctor] as indicated:

>    >     optional(const optional<T>& rhs);

>    > > -3- Requires: `is_copy_constructible_v<T>` is `true`.

>    > > […]

>    > > -?- Remarks: This constructor shall be defined as deleted unless `is_copy_constructible_v<T>` is `true`.

>    >     optional(optional<T>&& rhs) noexcept(see below);

>    > > -7- Requires: `is_move_constructible_v<T>` is `true`.

>    > > […]

>    > > -11- Remarks: The expression inside `noexcept` is equivalent to `is_nothrow_move_constructible_v<T>`. This constructor shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true`.

>    >     constexpr optional(const T& v);

>    > > -12- Requires: `is_copy_constructible_v<T>` is `true`.

>    > > -13- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `v`.

>    > > -14- Postcondition: `*this` contains a value.

>    > > -15- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -16- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

>    >     constexpr optional(T&& v);

>    > > -17- Requires: `is_move_constructible_v<T>` is `true`.

>    > > -18- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::move(v)`.

>    > > -19- Postcondition: `*this` contains a value.

>    > > -20- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -21- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

>    >     template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);

>    > > -22- Requires: `is_constructible_v<T, Args&&...>` is `true`.

>    > > […]

>    > > -26- Remarks: If `T`'s constructor selected for the initialization is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, Args...>` is `true`.

>    >     template <class U, class... Args>
>    >       constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

>    > > -27- Requires: `is_constructible_v<T, initializer_list<U>&, Args&&...>` is `true`.

>    > > […]

>    > > -31- Remarks: The functionThis constructor shall not participate in overload resolution unless `is_constructible_v<T, initializer_list<U>&, Args&&...>` is `true`. If `T`'s constructor selected for the initialization is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

>    > [Note: The following constructors are conditionally specified as explicit. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]

>    >     template <class U = T>
>    >       EXPLICIT constexpr optional(U&& v);

>    > > -?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::forward<U>(v)`.

>    > > -?- Postconditions: `*this` contains a value.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `is_same_v<U, in_place_t>` is `false`, and `is_same_v<optional<T>, decay_t<U>>` is `false`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.

>    >     template <class U>
>    >       EXPLICIT optional(const optional<U>& rhs);

>    > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `*rhs`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, const U&>` is `true`, `is_constructible_v<T, optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&&>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false`, `is_convertible_v<optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&&, T>` is `false`, and `is_convertible_v<optional<U>&&, T>` is `false`. The constructor is explicit if and only if `is_convertible_v<const U&, T>` is `false`.

>    >     template <class U>
>    >       EXPLICIT optional(optional<U>&& rhs);

>    > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::move(*rhs)`. `bool(rhs)` is unchanged.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Throws: Any exception thrown by the selected constructor of `T`.

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `is_constructible_v<T, optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&&>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false`, `is_convertible_v<optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&&, T>` is `false`, and `is_convertible_v<optional<U>&&, T>` is `false`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.

> 3. Change [optional.object.assign] as indicated:

>    >     optional<T>& operator=(const optional<T>& rhs);

>    > > -4- Requires: `is_copy_constructible_v<T>` is `true` and `is_copy_assignable_v<T>` is `true`.

>    > > […]

>    > > -8- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s copy constructor, no effect. If an exception is thrown during the call to `T`'s copy assignment, the state of its contained value is as defined by the exception safety guarantee of `T`'s copy assignment. This operator shall be defined as deleted unless `is_copy_constructible_v<T>` is `true` and `is_copy_assignable_v<T>` is `true`.

>    >     optional<T>& operator=(optional<T>&& rhs) noexcept(see below);

>    > > -9- Requires: `is_move_constructible_v<T>` is `true` and `is_move_assignable_v<T>` is `true`.

>    > > […]

>    > > -13- Remarks: The expression inside `noexcept` is equivalent to:

>    > > >     is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>

>    > > -14- If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s move constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s move constructor. If an exception is thrown during the call to `T`'s move assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s move assignment. This operator shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true` and `is_move_assignable_v<T>` is `true`.

>    >     template <class U = T> optional<T>& operator=(U&& v);

>    > > -15- Requires: `is_constructible_v<T, U>` is `true` and `is_assignable_v<T&, U>` is `true`.

>    > > […]

>    > > -19- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `v` is determined by the exception safety guarantee of `T`'s assignment. TheThis function shall not participate in overload resolution unless `is_same_v<decay_t<U>, T>` `is_same_v<optional<T>, decay_t<U>>` is `false`, `is_constructible_v<T, U>` is `true`, and `is_assignable_v<T&, U>` is `true`.

>    > > -20- Notes: The reason for providing such generic assignment and then constraining it so that effectively `T == U` is to guarantee that assignment of the form `o = {}` is unambiguous.

>    >     template <class U> optional<T>& operator=(const optional<U>& rhs);

>    > > -?- Effects: See Table ?.

>    > > Table ? — `optional::operator=(const optional<U>&)` effects
>    > > | `*this` contains a value | `*this` does not contain a value
>    > > --- | --- | ---
>    > > `rhs` contains a value | assigns `*rhs` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `*rhs`
>    > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

>    > > -?- Returns: `*this`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. This function shall not participate in overload resolution unless `is_constructible_v<T, const U&>` is `true`, `is_assignable_v<T&, const U&>` is `true`, `is_constructible_v<T, optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&&>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false`, `is_convertible_v<optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&&, T>` is `false`, `is_convertible_v<optional<U>&&, T>` is `false`, `is_assignable_v<T&, optional<U>&>` is `false`, `is_assignable_v<T&, const optional<U>&>` is `false`, `is_assignable_v<T&, const optional<U>&&>` is `false`, and `is_assignable_v<T&, optional<U>&&>` is `false`.

>    >     template <class U> optional<T>& operator=(optional<U>&& rhs);

>    > > -?- Effects: See Table ?. The result of the expression `bool(rhs)` remains unchanged.

>    > > Table ? — `optional::operator=(optional<U>&&)` effects
>    > > | `*this` contains a value | `*this` does not contain a value
>    > > --- | --- | ---
>    > > `rhs` contains a value | assigns `std::move(*rhs)` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `std::move(*rhs)`
>    > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

>    > > -?- Returns: `*this`.

>    > > -?- Postconditions: `bool(rhs) == bool(*this)`.

>    > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. This function shall not participate in overload resolution unless `is_constructible_v<T, U>` is `true`, `is_assignable_v<T&, U>` is `true`, `is_constructible_v<T, optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&&>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false`, `is_convertible_v<optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&, T>` is `false`, `is_convertible<const optional<U>&&, T>` is `false`, `is_convertible<optional<U>&&, T>` is `false`, `is_assignable_v<T&, optional<U>&>` is `false`, `is_assignable_v<T&, const optional<U>&>` is `false`, `is_assignable_v<T&, const optional<U>&&>` is `false`, and `is_assignable_v<T&, optional<U>&&>` is `false`.

Proposed resolution:

This wording is relative to N4606.

1. Modify 22.5.3 [optional.optional] as indicated:

   >     template <class T> class optional
   >     {
   >     public:
   >       using value_type = T;

   >       // 20.6.3.1, Constructors
   >       constexpr optional() noexcept;
   >       constexpr optional(nullopt_t) noexcept;
   >       optional(const optional &);
   >       optional(optional &&) noexcept(see below);
   >       constexpr optional(const T &);
   >       constexpr optional(T &&);
   >       template <class... Args> constexpr explicit optional(in_place_t, Args &&...);
   >       template <class U, class... Args>
   >         constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...);
   >       template <class U = T> EXPLICIT constexpr optional(U &&);
   >       template <class U> EXPLICIT optional(const optional<U> &);
   >       template <class U> EXPLICIT optional(optional<U> &&);

   >       […]

   >       // 20.6.3.3, Assignment
   >       optional &operator=(nullopt_t) noexcept;
   >       optional &operator=(const optional &);
   >       optional &operator=(optional &&) noexcept(see below);
   >       template <class U = T> optional &operator=(U &&);
   >       template <class U> optional& operator=(const optional<U> &);
   >       template <class U> optional& operator=(optional<U> &&);
   >       template <class... Args> void emplace(Args &&...);
   >       template <class U, class... Args>
   >         void emplace(initializer_list<U>, Args &&...);

   >       […]

   >     };

2. Change [optional.object.ctor] as indicated:

   >     optional(const optional<T>& rhs);

   > > -3- Requires: `is_copy_constructible_v<T>` is `true`.

   > > […]

   > > -?- Remarks: This constructor shall be defined as deleted unless `is_copy_constructible_v<T>` is `true`.

   >     optional(optional<T>&& rhs) noexcept(see below);

   > > -7- Requires: `is_move_constructible_v<T>` is `true`.

   > > […]

   > > -11- Remarks: The expression inside `noexcept` is equivalent to `is_nothrow_move_constructible_v<T>`. This constructor shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true`.

   >     constexpr optional(const T& v);

   > > -12- Requires: `is_copy_constructible_v<T>` is `true`.

   > > -13- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `v`.

   > > -14- Postcondition: `*this` contains a value.

   > > -15- Throws: Any exception thrown by the selected constructor of `T`.

   > > -16- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

   >     constexpr optional(T&& v);

   > > -17- Requires: `is_move_constructible_v<T>` is `true`.

   > > -18- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::move(v)`.

   > > -19- Postcondition: `*this` contains a value.

   > > -20- Throws: Any exception thrown by the selected constructor of `T`.

   > > -21- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

   >     template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);

   > > -22- Requires: `is_constructible_v<T, Args&&...>` is `true`.

   > > […]

   > > -26- Remarks: If `T`'s constructor selected for the initialization is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, Args...>` is `true`.

   >     template <class U, class... Args>
   >       constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

   > > -27- Requires: `is_constructible_v<T, initializer_list<U>&, Args&&...>` is `true`.

   > > […]

   > > -31- Remarks: The functionThis constructor shall not participate in overload resolution unless `is_constructible_v<T, initializer_list<U>&, Args&&...>` is `true`. If `T`'s constructor selected for the initialization is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

   > [Note: The following constructors are conditionally specified as explicit. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]

   >     template <class U = T>
   >       EXPLICIT constexpr optional(U&& v);

   > > -?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::forward<U>(v)`.

   > > -?- Postconditions: `*this` contains a value.

   > > -?- Throws: Any exception thrown by the selected constructor of `T`.

   > > -?- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `is_same_v<U, in_place_t>` is `false`, and `is_same_v<optional<T>, decay_t<U>>` is `false`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.

   >     template <class U>
   >       EXPLICIT optional(const optional<U>& rhs);

   > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `*rhs`.

   > > -?- Postconditions: `bool(rhs) == bool(*this)`.

   > > -?- Throws: Any exception thrown by the selected constructor of `T`.

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, const U&>` is `true`, `is_constructible_v<T, optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&&>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false`, `is_convertible_v<optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&&, T>` is `false`, and `is_convertible_v<optional<U>&&, T>` is `false`. The constructor is explicit if and only if `is_convertible_v<const U&, T>` is `false`.

   >     template <class U>
   >       EXPLICIT optional(optional<U>&& rhs);

   > > -?- Effects: If `rhs` contains a value, initializes the contained value as if direct-non-list-initializing an object of type `T` with the expression `std::move(*rhs)`. `bool(rhs)` is unchanged.

   > > -?- Postconditions: `bool(rhs) == bool(*this)`.

   > > -?- Throws: Any exception thrown by the selected constructor of `T`.

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `is_constructible_v<T, optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&&>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false`, `is_convertible_v<optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&&, T>` is `false`, and `is_convertible_v<optional<U>&&, T>` is `false`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.

3. Change [optional.object.assign] as indicated:

   >     optional<T>& operator=(const optional<T>& rhs);

   > > -4- Requires: `is_copy_constructible_v<T>` is `true` and `is_copy_assignable_v<T>` is `true`.

   > > […]

   > > -8- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s copy constructor, no effect. If an exception is thrown during the call to `T`'s copy assignment, the state of its contained value is as defined by the exception safety guarantee of `T`'s copy assignment. This operator shall be defined as deleted unless `is_copy_constructible_v<T>` is `true` and `is_copy_assignable_v<T>` is `true`.

   >     optional<T>& operator=(optional<T>&& rhs) noexcept(see below);

   > > -9- Requires: `is_move_constructible_v<T>` is `true` and `is_move_assignable_v<T>` is `true`.

   > > […]

   > > -13- Remarks: The expression inside `noexcept` is equivalent to:

   > > >     is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>

   > > -14- If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s move constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s move constructor. If an exception is thrown during the call to `T`'s move assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s move assignment. This operator shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true` and `is_move_assignable_v<T>` is `true`.

   >     template <class U = T> optional<T>& operator=(U&& v);

   > > -15- Requires: `is_constructible_v<T, U>` is `true` and `is_assignable_v<T&, U>` is `true`.

   > > […]

   > > -19- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `v` is determined by the exception safety guarantee of `T`'s assignment. TheThis function shall not participate in overload resolution unless `is_same_v<decay_t<U>, T>` `is_same_v<optional<T>, decay_t<U>>` is `false`, conjunction_v<is_scalar<T>, is_same<T, decay_t<U>>> is `false`, `is_constructible_v<T, U>` is `true`, and `is_assignable_v<T&, U>` is `true`.

   > > -20- Notes: The reason for providing such generic assignment and then constraining it so that effectively `T == U` is to guarantee that assignment of the form `o = {}` is unambiguous.

   >     template <class U> optional<T>& operator=(const optional<U>& rhs);

   > > -?- Effects: See Table ?.

   > > Table ? — `optional::operator=(const optional<U>&)` effects
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*rhs` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `*rhs`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

   > > -?- Returns: `*this`.

   > > -?- Postconditions: `bool(rhs) == bool(*this)`.

   > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. This function shall not participate in overload resolution unless `is_constructible_v<T, const U&>` is `true`, `is_assignable_v<T&, const U&>` is `true`, `is_constructible_v<T, optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&&>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false`, `is_convertible_v<optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&&, T>` is `false`, `is_convertible_v<optional<U>&&, T>` is `false`, `is_assignable_v<T&, optional<U>&>` is `false`, `is_assignable_v<T&, const optional<U>&>` is `false`, `is_assignable_v<T&, const optional<U>&&>` is `false`, and `is_assignable_v<T&, optional<U>&&>` is `false`.

   >     template <class U> optional<T>& operator=(optional<U>&& rhs);

   > > -?- Effects: See Table ?. The result of the expression `bool(rhs)` remains unchanged.

   > > Table ? — `optional::operator=(optional<U>&&)` effects
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `std::move(*rhs)` to the contained value | initializes the contained value as if direct-non-list-initializing an object of type `T` with `std::move(*rhs)`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->T::~T()` | no effect

   > > -?- Returns: `*this`.

   > > -?- Postconditions: `bool(rhs) == bool(*this)`.

   > > -?- Remarks: If any exception is thrown, the result of the expression `bool(*this)` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.val` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `*val` and `*rhs.val` is determined by the exception safety guarantee of `T`'s assignment. This function shall not participate in overload resolution unless `is_constructible_v<T, U>` is `true`, `is_assignable_v<T&, U>` is `true`, `is_constructible_v<T, optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&>` is `false`, `is_constructible_v<T, const optional<U>&&>` is `false`, `is_constructible_v<T, optional<U>&&>` is `false`, `is_convertible_v<optional<U>&, T>` is `false`, `is_convertible_v<const optional<U>&, T>` is `false`, `is_convertible<const optional<U>&&, T>` is `false`, `is_convertible<optional<U>&&, T>` is `false`, `is_assignable_v<T&, optional<U>&>` is `false`, `is_assignable_v<T&, const optional<U>&>` is `false`, `is_assignable_v<T&, const optional<U>&&>` is `false`, and `is_assignable_v<T&, optional<U>&&>` is `false`.

### 2757(i). `std::string{}.insert(3, "ABCDE", 0, 1)` is ambiguous

Section: 27.4.3.7.4 [string.insert] Status: Resolved Submitter: Marshall Clow Opened: 2016-07-30 Last modified: 2020-09-06

Priority: 1

View all other issues in [string.insert].

View all issues with Resolved status.

Discussion:

Before C++17, we had the following signature to `std::basic_string`:

>     basic_string&
>       insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);

Unlike most of the other member functions on `std::basic_string`, there were not corresponding versions that take a `charT*` or `(charT *, size)`.

In p0254r2, we added:

>     basic_string&
>       insert(size_type pos1, basic_string_view<charT, traits> sv, size_type pos2, size_type n = npos);

which made the code above ambiguous. There are two conversions from "`const charT*`", one to `basic_string`, and the other to `basic_string_view`, and they're both equally good (in the view of the compiler).

This ambiguity also occurs with the calls

>     assign(const basic_string& str,             size_type pos, size_type n = npos);
>     assign(basic_string_view<charT, traits> sv, size_type pos, size_type n = npos);

but I will file a separate issue (2758(i)) for that.

A solution is to add even more overloads to `insert`, to make it match all the other member functions of `basic_string`, which come in fours (`string`, `pointer`, `pointer + size`, `string_view`).

[2016-08-03, Chicago, Robert Douglas provides wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. In 27.4.3 [basic.string] modify the synopsis for `basic_string` as follows:

>    >     namespace std {
>    >       template<class charT, class traits = char_traits<charT>,
>    >         class Allocator = allocator<charT>>
>    >       class basic_string {
>    >       public:
>    >         […]
>    >         template<class T>
>    >         basic_string& insert(size_type pos1, basic_string_view<charT, traits>T sv,
>    >                              size_type pos2, size_type n = npos);
>    >         […]
>    >       };
>    >     }

> 2. In 27.4.3.7.4 [string.insert], modify `basic_string_view` overload as follows:

>    >     template<class T>
>    >     basic_string& insert(size_type pos1, basic_string_view<charT, traits>T sv,
>    >                          size_type pos2, size_type n = npos);

>    > > […]

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_same_v<T, basic_string_view<charT, traits>>` is `true`.

[2016-08-04, Chicago, Robert Douglas comments]

For the sake of simplicity, the previous wording suggestion has been merged into the proposed wording of LWG 2758(i).

[08-2016, Chicago]

Fri PM: Move to Tentatively Ready (along with 2758(i)).

[2016-09-09 Issues Resolution Telecon]

Since 2758(i) has been moved back to Open, move this one, too

[2016-10 Telecon]

Ville's wording for 2758(i) has been implemented in libstdc++ and libc++. Move 2758(i) to Tentatively Ready and this one to Tentatively Resolved

Proposed resolution:

This issue is resolved by the proposed wording for LWG 2758(i).

### 2758(i). `std::string{}.assign("ABCDE", 0, 1)` is ambiguous

Section: 27.4.3.7.3 [string.assign] Status: C++17 Submitter: Marshall Clow Opened: 2016-07-30 Last modified: 2020-09-06

Priority: 1

View all other issues in [string.assign].

View all issues with C++17 status.

Discussion:

Before C++17, we had the following signature to `std::basic_string`:

>     basic_string&
>       assign(const basic_string& str, size_type pos, size_type n = npos);

Unlike most of the other member functions on `std::basic_string`, there were not corresponding versions that take a `charT*` or `(charT *, size)`.

In p0254r2, we (I) added:

>     basic_string&
>       assign(basic_string_view<charT, traits> sv, size_type pos, size_type n = npos);

which made the code above ambiguous. There are two conversions from "`const charT*`", one to `basic_string`, and the other to `basic_string_view`, and they're both equally good (in the view of the compiler).

This ambiguity also occurs with the calls

>     insert(size_type pos1, const basic_string& str,             size_type pos2, size_type n = npos);
>     insert(size_type pos1, basic_string_view<charT, traits> sv, size_type pos2, size_type n = npos);

but I will file a separate issue (2757(i)) for that.

A solution is to add even more overloads to `assign`, to make it match all the other member functions of `basic_string`, which come in fours (`string`, `pointer`, `pointer + size`, `string_view`).

[2016-08-03, Chicago, Robert Douglas provides wording]

[2016-08-05, Tim Song comments]

On the assumption that the `basic_string` version is untouchable, I like the updated P/R, with a couple comments:

1. If it's constraining on `is_convertible` to `basic_string_view`, then I think it should take by reference to avoid copying `T`, which can be arbitrarily expensive. Both `const T&` and `T&&` should work; the question is whether to accommodate non-const `operator basic_string_view()`s (which arguably shouldn't exist).

2. Minor issue: `compare` tests `is_convertible` and then uses direct-initialization syntax (which is `is_constructible`). They should match, because it's possible to have `basic_string_view sv = t;` succeed yet `basic_string_view sv(t);` fail.

[2016-08-05, Chicago LWG]

1. breaking ABI was a non-starter
2. we want to convert `char const*` to `basic_string_view` if possible
3. we want to take the original type by reference, to avoid new user string types from being copied
4. it is unfortunate that each length of string literal will generate a new entry in the symbol table

Given feedback from discussion, we wish to go with the `is_convertible_v` method, but change:

1. the function parameter type to `T const&` for each overload
2. the parameter type in `is_convertible_v` to `T const&`
3. the initialization of `sv(t)` to `sv = t`

[2016-08, Chicago]

Fri PM: Move to Tentatively Ready

[2016-08-16, Jonathan Wakely reopens]

The P/R is not correct, the new overloads get chosen in preference to the overloads taking `const char*` when passed a `char*`:

>     #include <string>

>     int main () {
>       std::string str("a");
>       char c = 'b';
>       str.replace(0, 1, &c, 1);
>       if (str[0] != 'b')
>         __builtin_abort();
>     }

With the resolution of 2758 this is now equivalent to:

>     str.replace(0, 1, string_view{&c, 1}, 1);

which replaces the character with `string_view{"b", 1}.substr(1, npos)` i.e. an empty string.

The SFINAE constraints need to disable the new overloads for (at least) the case of non-const `value_type*` arguments.

I've implemented an alternative resolution, which would be specified as:

> Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

All the overloads have the same problem.

This program prints no output in C++14, and we should make sure it prints no output in C++17 too:

>     #include <string>

>     int main()
>     {
>       std::string str("a");
>       char c[1] = { 'b' };
>       str.append(c, 1);
>       if (str != "ab")
>         puts("bad append");
>       str.assign(c, 1);
>       if (str != "b")
>         puts("bad assign");
>       str.insert(0, c, 1);
>       if (str != "bb")
>         puts("bad insert");
>       str.replace(0, 2, c, 1);
>       if (str != "b")
>         puts("bad replace");
>       if (str.compare(0, 1, c, 1))
>         puts("bad compare");
>     }

Ville and I considered "`is_same_v<decay_t<T>, char*>` is `false`" but that would still select the wrong overload for an array of const `char`, because the new function template would be preferred to doing an array-to-pointer conversion to call the old overload.

[2016-09-09 Issues Resolution Telecon]

Ville to provide updated wording; Marshall to implement

[2016-10-05]

Ville provides revised wording.

[2016-10 Telecon]

Ville's wording has been implemented in libstdc++ and libc++. Move this to Tentatively Ready and 2757(i) to Tentatively Resolved

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. In 27.4.3 [basic.string] modify the synopsis for `basic_string` as follows:

>    >     namespace std {
>    >       template<class charT, class traits = char_traits<charT>,
>    >         class Allocator = allocator<charT>>
>    >       class basic_string {
>    >       public:
>    >         […]
>    >         template<class T>
>    >         basic_string& append(basic_string_view<charT, traits> svconst T& t,
>    >                              size_type pos, size_type n = npos);
>    >         […]
>    >         template<class T>
>    >         basic_string& assign(basic_string_view<charT, traits> svconst T& t,
>    >                              size_type pos, size_type n = npos);
>    >         […]
>    >         template<class T>
>    >         basic_string& insert(size_type pos1, basic_string_view<charT, traits> svconst T& t,
>    >                              size_type pos2, size_type n = npos);
>    >         […]
>    >         template<class T>
>    >         basic_string& replace(size_type pos1, size_type n1,
>    >                               basic_string_view<charT, traits> svconst T& t,
>    >                               size_type pos2, size_type n2 = npos);
>    >         […]
>    >         template<class T>
>    >         int compare(size_type pos1, size_type n1,
>    >                     basic_string_view<charT, traits> svconst T& t,
>    >                     size_type pos2, size_type n2 = npos) const;
>    >         […]
>    >       };
>    >     }

> 2. In 27.4.3.7.2 [string.append], modify `basic_string_view` overload as follows:

>    >     template<class T>
>    >     basic_string& append(basic_string_view<charT, traits> svconst T& t,
>    >                          size_type pos, size_type n = npos);

>    > > -7- Throws: `out_of_range` if `pos > sv.size()`.

>    > > -8- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t`. Determines the effective length `rlen` of the string to append as the smaller of `n` and `sv.size() - pos` and calls `append(sv.data() + pos, rlen)`.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true`.

>    > > -9- Returns: `*this`.

> 3. In 27.4.3.7.3 [string.assign], modify `basic_string_view` overload as follows:

>    >     template<class T>
>    >     basic_string& assign(basic_string_view<charT, traits> svconst T& t,
>    >                          size_type pos, size_type n = npos);

>    > > -9- Throws: `out_of_range` if `pos > sv.size()`.

>    > > -10- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t`. Determines the effective length `rlen` of the string to assign as the smaller of `n` and `sv.size() - pos` and calls `assign(sv.data() + pos, rlen)`.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true`.

>    > > -11- Returns: `*this`.

> 4. In 27.4.3.7.4 [string.insert], modify `basic_string_view` overload as follows:

>    >     template<class T>
>    >     basic_string& insert(size_type pos1, basic_string_view<charT, traits> svconst T& t,
>    >                          size_type pos2, size_type n = npos);

>    > > -6- Throws: `out_of_range` if `pos1 > size()` or `pos2 > sv.size()`.

>    > > -7- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t`. Determines the effective length `rlen` of the string to assign as the smaller of `n` and `sv.size() - pos2` and calls `insert(pos1, sv.data() + pos2, rlen)`.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true`.

>    > > -8- Returns: `*this`.

> 5. In 27.4.3.7.6 [string.replace], modify `basic_string_view` overload as follows:

>    >     template<class T>
>    >     basic_string& replace(size_type pos1, size_type n1,
>    >                          basic_string_view<charT, traits> svconst T& t,
>    >                          size_type pos2, size_type n2 = npos);

>    > > -6- Throws: `out_of_range` if `pos1 > size()` or `pos2 > sv.size()`.

>    > > -7- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t`. Determines the effective length `rlen` of the string to be inserted as the smaller of `n2` and `sv.size() - pos2` and calls `replace(pos1, n1, sv.data() + pos2, rlen)`.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true`.

>    > > -8- Returns: `*this`.

> 6. In 27.4.3.8.4 [string.compare], modify `basic_string_view` overload as follows:

>    >     template<class T>
>    >     int compare(size_type pos1, size_type n1,
>    >                 basic_string_view<charT, traits> svconst T& t,
>    >                 size_type pos2, size_type n2 = npos) const;

>    > > -4- Effects: Equivalent to:

>    > > >     {
>    > > >       basic_string_view<charT, traits> sv = t;
>    > > >       return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv, pos2, n2);
>    > > >     }

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true`.

Proposed resolution:

This wording is relative to N4606.

1. In 27.4.3 [basic.string] modify the synopsis for `basic_string` as follows:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT>>
   >       class basic_string {
   >       public:
   >         […]
   >         template<class T>
   >         basic_string& append(basic_string_view<charT, traits> svconst T& t,
   >                              size_type pos, size_type n = npos);
   >         […]
   >         template<class T>
   >         basic_string& assign(basic_string_view<charT, traits> svconst T& t,
   >                              size_type pos, size_type n = npos);
   >         […]
   >         template<class T>
   >         basic_string& insert(size_type pos1, basic_string_view<charT, traits> svconst T& t,
   >                              size_type pos2, size_type n = npos);
   >         […]
   >         template<class T>
   >         basic_string& replace(size_type pos1, size_type n1,
   >                               basic_string_view<charT, traits> svconst T& t,
   >                               size_type pos2, size_type n2 = npos);
   >         […]
   >         template<class T>
   >         int compare(size_type pos1, size_type n1,
   >                     basic_string_view<charT, traits> svconst T& t,
   >                     size_type pos2, size_type n2 = npos) const;
   >         […]
   >       };
   >     }

2. In 27.4.3.7.2 [string.append], modify `basic_string_view` overload as follows:

   >     template<class T>
   >     basic_string& append(basic_string_view<charT, traits> svconst T& t,
   >                          size_type pos, size_type n = npos);

   > > -7- Throws: `out_of_range` if `pos > sv.size()`.

   > > -8- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t`. Determines the effective length `rlen` of the string to append as the smaller of `n` and `sv.size() - pos` and calls `append(sv.data() + pos, rlen)`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

   > > -9- Returns: `*this`.

3. In 27.4.3.7.3 [string.assign], modify `basic_string_view` overload as follows:

   >     template<class T>
   >     basic_string& assign(basic_string_view<charT, traits> svconst T& t,
   >                          size_type pos, size_type n = npos);

   > > -9- Throws: `out_of_range` if `pos > sv.size()`.

   > > -10- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t`. Determines the effective length `rlen` of the string to assign as the smaller of `n` and `sv.size() - pos` and calls `assign(sv.data() + pos, rlen)`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

   > > -11- Returns: `*this`.

4. In 27.4.3.7.4 [string.insert], modify `basic_string_view` overload as follows:

   >     template<class T>
   >     basic_string& insert(size_type pos1, basic_string_view<charT, traits> svconst T& t,
   >                          size_type pos2, size_type n = npos);

   > > -6- Throws: `out_of_range` if `pos1 > size()` or `pos2 > sv.size()`.

   > > -7- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t`. Determines the effective length `rlen` of the string to assign as the smaller of `n` and `sv.size() - pos2` and calls `insert(pos1, sv.data() + pos2, rlen)`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

   > > -8- Returns: `*this`.

5. In 27.4.3.7.6 [string.replace], modify `basic_string_view` overload as follows:

   >     template<class T>
   >     basic_string& replace(size_type pos1, size_type n1,
   >                          basic_string_view<charT, traits> svconst T& t,
   >                          size_type pos2, size_type n2 = npos);

   > > -6- Throws: `out_of_range` if `pos1 > size()` or `pos2 > sv.size()`.

   > > -7- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t`. Determines the effective length `rlen` of the string to be inserted as the smaller of `n2` and `sv.size() - pos2` and calls `replace(pos1, n1, sv.data() + pos2, rlen)`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

   > > -8- Returns: `*this`.

6. In 27.4.3.8.4 [string.compare], modify `basic_string_view` overload as follows:

   > [Drafting note: The wording changes below are for the same part of the standard as 2771(i). However, they do not conflict. This one changes the definition of the routine, while the other changes the "Effects". — end drafting note]

   >     template<class T>
   >     int compare(size_type pos1, size_type n1,
   >                 basic_string_view<charT, traits> svconst T& t,
   >                 size_type pos2, size_type n2 = npos) const;

   > > -4- Effects: Equivalent to:

   > > >     {
   > > >       basic_string_view<charT, traits> sv = t;
   > > >       return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv, pos2, n2);
   > > >     }

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

### 2759(i). `gcd` / `lcm` and `bool` for the WP

Section: 26.10.14 [numeric.ops.gcd], 26.10.15 [numeric.ops.lcm] Status: C++17 Submitter: Walter Brown Opened: 2016-08-01 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [numeric.ops.gcd].

View all issues with C++17 status.

Discussion:

With the acceptance of `gcd` and `lcm` in the working draft, the same problem as pointed out by LWG 2733(i) exists here as well and should be fixed accordingly.

[2016-08, Chicago]

Monday PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Adjust 26.10.14 [numeric.ops.gcd] p2 as indicated:

   >     template<class M, class N>
   >       constexpr common_type_t<M, N> gcd(M m, N n);

   > > […]

   > > -2- Remarks: If either `M` or `N` is not an integer type, or if either is (possibly cv-qualified) `bool`, the program is ill-formed.

2. Adjust 26.10.15 [numeric.ops.lcm] p2 as indicated:

   >     template<class M, class N>
   >       constexpr common_type_t<M, N> lcm(M m, N n);

   > > […]

   > > -2- Remarks: If either `M` or `N` is not an integer type, or if either is (possibly cv-qualified) `bool`, the program is ill-formed.

### 2760(i). non-const `basic_string::data` should not invalidate iterators

Section: 27.4.3.2 [string.require] Status: C++17 Submitter: Billy Baker Opened: 2016-08-03 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [string.require].

View all issues with C++17 status.

Discussion:

27.4.3.2 [string.require]/4 does not list non-const `basic_string::data()` as being a function that may be called with the guarantee that it will not invalidate references, pointers, and iterators to elements of a `basic_string` object.

[2016-08 Chicago]

Wed PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change 27.4.3.2 [string.require]/4 as indicated:

   > -4- References, pointers, and iterators referring to the elements of a `basic_string` sequence may be invalidated by the following uses of that `basic_string` object:

   > - as an argument to any standard library function taking a reference to non-const `basic_string` as an argument.(footnote 230)

   > - Calling non-const member functions, except `operator[]`, `at`, data, `front`, `back`, `begin`, `rbegin`, `end`, and `rend`.

### 2762(i). `unique_ptr operator*()` should be `noexcept`

Section: 20.3.1.3.5 [unique.ptr.single.observers] Status: C++23 Submitter: Ville Voutilainen Opened: 2016-08-04 Last modified: 2023-11-22

Priority: 3

View all other issues in [unique.ptr.single.observers].

View all issues with C++23 status.

Discussion:

See LWG 2337(i). Since we aren't removing `noexcept` from `shared_ptr`'s `operator*`, we should consider adding `noexcept` to `unique_ptr`'s `operator*`.

[2016-08 — Chicago]

Thurs PM: P3, and status to 'LEWG'

[2016-08-05 Chicago]

Ville provides an initial proposed wording.

[LEWG Kona 2017]

->Open: Believe these should be `noexcept` for consistency. We like these. We agree with the proposed resolution. `operator->()` already has `noexcept`.

Also adds `optional::operator*`

Alisdair points out that fancy pointers might intentionally throw from `operator*`, and we don't want to prohibit that.

Go forward with conditional `noexcept(noexcept(*decltype()))`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> > [Drafting note: since this issue is all about consistency, `optional`'s pointer-like operators are additionally included.]

> 1. In 20.3.1.3 [unique.ptr.single] synopsis, edit as follows:

>    >     add_lvalue_reference_t<T> operator*() const noexcept;

> 2. Before 20.3.1.3.5 [unique.ptr.single.observers]/1, edit as follows:

>    >     add_lvalue_reference_t<T> operator*() const noexcept;

> 3. In 22.5.3 [optional.optional] synopsis, edit as follows:

>    >     constexpr T const *operator->() const noexcept;
>    >     constexpr T *operator->() noexcept;
>    >     constexpr T const &operator*() const & noexcept;
>    >     constexpr T &operator*() & noexcept;
>    >     constexpr T &&operator*() && noexcept;
>    >     constexpr const T &&operator*() const && noexcept;

> 4. Before [optional.object.observe]/1, edit as follows:

>    >     constexpr T const* operator->() const noexcept;
>    >     constexpr T* operator->() noexcept;

> 5. Before [optional.object.observe]/5, edit as follows:

>    >     constexpr T const& operator*() const & noexcept;
>    >     constexpr T& operator*() & noexcept;

> 6. Before [optional.object.observe]/9, edit as follows:

>    >     constexpr T&& operator*() && noexcept;
>    >     constexpr const T&& operator*() const && noexcept;

[2021-06-19 Tim updates wording]

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

> [Drafting note: since this issue is all about consistency, `optional`'s pointer-like operators are additionally included.]

1. Edit 20.3.1.3.1 [unique.ptr.single.general], class template `unique_ptr` synopsis, as indicated:

   >     namespace std {
   >       template<class T, class D = default_delete<T>> class unique_ptr {
   >       public:
   >         […]

   >         // 20.3.1.3.5 [unique.ptr.single.observers], observers
   >         add_lvalue_reference_t<T> operator*() const noexcept(see below);
   >         […]
   >       };
   >     }

2. Edit 20.3.1.3.5 [unique.ptr.single.observers] as indicated:

   >     add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));

   > > -1- Preconditions: `get() != nullptr`.

   > > -2- Returns: `*get()`.

3. Edit 22.5.3.1 [optional.optional.general], class template `optional` synopsis, as indicated:

   >     namespace std {
   >       template<class T>
   >       class optional {
   >       public:
   >         […]

   >         // 22.5.3.7 [optional.observe], observers
   >         constexpr const T* operator->() const noexcept;
   >         constexpr T* operator->() noexcept;
   >         constexpr const T& operator*() const & noexcept;
   >         constexpr T& operator*() & noexcept;
   >         constexpr T&& operator*() && noexcept;
   >         constexpr const T&& operator*() const && noexcept;

   >         […]
   >       };
   >     }

4. Edit 22.5.3.7 [optional.observe] as indicated:

   >     constexpr const T* operator->() const noexcept;
   >     constexpr T* operator->() noexcept;

   > > -1- Preconditions: `*this` contains a value.

   > > -2- Returns: `val`.

   > > -3- Throws: Nothing.

   > > -4- Remarks: These functions are constexpr functions.

   >     constexpr const T& operator*() const & noexcept;
   >     constexpr T& operator*() & noexcept;

   > > -5- Preconditions: `*this` contains a value.

   > > -6- Returns: `*val`.

   > > -7- Throws: Nothing.

   > > -8- Remarks: These functions are constexpr functions.

   >     constexpr T&& operator*() && noexcept;
   >     constexpr const T&& operator*() const && noexcept;

   > > -9- Preconditions: `*this` contains a value.

   > > -10- Effects: Equivalent to: `return std::move(*val);`

### 2763(i). `common_type_t<void, void>` is undefined

Section: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Tim Song Opened: 2016-08-10 Last modified: 2016-11-21

Priority: 2

View all other issues in [meta.trans.other].

View all issues with Resolved status.

Discussion:

There are no xvalues of type cv `void` (see [basic.lval]/6), so the current wording appears to mean that there is no `common_type_t<void, void>`. Is that intended?

[2016-08-11, Daniel comments]

This is strongly related to LWG 2465(i). It should be considered to resolve 2465(i) by this revised wording.

[2016-11-12, Issaquah]

Resolved by P0435R1

Proposed resolution:

This wording is relative to N4606.

1. Edit 21.3.8.7 [meta.trans.other]/3 as indicated:

   > [Drafting note: The proposed wording below simply goes back to using `declval`, which already does the right thing. To describe this in words would be something like "if `D1` is `void`, a prvalue of type `void` that is not a (possibly parenthesized) throw-expression, otherwise an xvalue of type `D1`", which seems unnecessarily convoluted at best. — end drafting note]

   > For the `common_type` trait applied to a parameter pack `T` of types, the member `type` shall be either defined or not present as follows:

   > 1. (3.1) — If `sizeof...(T)` is zero, there shall be no member `type`.

   > 2. (3.2) — If `sizeof...(T)` is one, let `T0` denote the sole type in the pack `T`. The member typedef `type` shall denote the same type as `decay_t<T0>`.

   > 3. (3.3) — If `sizeof...(T)` is greater than two, let `T1`, `T2`, and `R`, respectively, denote the first, second, and (pack of) remaining types comprising `T`. [Note: `sizeof...(R)` may be zero. — end note] Let `C` denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `T1` `declval<T1>()`, and whose third operand is an xvalue of type `T2` `declval<T2>()`. If there is such a type `C`, the member typedef `type` shall denote the same type, if any, as `common_type_t<C, R...>`. Otherwise, there shall be no member `type`.

2. The following wording is a merge of the above with the current proposed resolution of 2465(i), to provide editorial guidance if both proposed resolutions are accepted:

   > -3- Note A: For the `common_type` trait applied to a parameter pack `T` of types, the member `type` shall be either defined or not present as follows:

   > 1. (3.1) — If `sizeof...(T)` is zero, there shall be no member `type`.

   > 2. (3.2) — If `sizeof...(T)` is one, let `T0` denote the sole type in the pack `T`. The member typedef `type` shall denote the same type as `decay_t<T0>`.

   > 3. (3.3) — If `sizeof...(T)` is two, let `T1` and `T2`, respectively, denote the first and second types comprising `T`, and let `D1` and `D2`, respectively, denote `decay_t<T1>` and `decay_t<T2>`.

   >    1. (3.3.1) — If `is_same_v<T1, D1>` and `is_same_v<T2, D2>`, let `C` denote the type of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is `declval<D1>()`, and whose third operand is `declval<D2>()`. [Note: This will not apply if there is a specialization `common_type<D1, D2>`. — end note]

   >    2. (3.3.2) — Otherwise, let `C` denote the type `common_type_t<D1, D2>`.

   >    In either case, if there is such a type `C`, the member typedef `type` shall denote `C`. Otherwise, there shall be no member `type`.

   > 4. (3.4) — If `sizeof...(T)` is greater than onetwo, let `T1`, `T2`, and `R`, respectively, denote the first, second, and (pack of) remaining types comprising `T`. [Note: `sizeof...(R)` may be zero. — end note] Let `C` denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `T1`, and whose third operand is an xvalue of type `T2`. Let `C` denote `common_type_t<T1, T2>`. If there is such a type `C`, the member typedef `type` shall denote the same type, if any, as `common_type_t<C, R...>`. Otherwise, there shall be no member `type`.

   > -?- Note B: A program may specialize the `common_type` trait for two cv-unqualified non-reference types if at least one of them is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note] Such a specialization need not have a member named `type`, but if it does, that member shall be a typedef-name for a cv-unqualified non-reference type that need not otherwise meet the specification set forth in Note A, above.

   > -4- [Example: Given these definitions: […]

### 2765(i). Did LWG 1123 go too far?

Section: 31.5.2.2.6 [ios.init] Status: C++17 Submitter: Richard Smith Opened: 2016-08-13 Last modified: 2021-06-06

Priority: 0

View all other issues in [ios.init].

View all issues with C++17 status.

Discussion:

1123(i) fixed a bug where users of `<iostream>` were not guaranteed to have their streams flushed on program shutdown. However, it also added this rule:

> "Similarly, the entire program shall behave as if there were at least one instance of `ios_base::Init` with static lifetime."

This seems pointless: it only affects the behavior of programs that never include `<iostream>` (because programs that do include it are already guaranteed at least one such instance), and those programs do not need an implicit flush because they cannot have written to the relevant streams.

It's also actively harmful, because it requires the iostreams component to be linked into programs that do not use it, apparently even including freestanding implementations! Fortunately, C++ implementations appear to uniformly ignore this rule.

Can it be removed?

[2016-09-09 Issues Resolution Telecon]

P0; move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify [ios::Init] p3 as indicated:

   > -3- The objects are constructed and the associations are established at some time prior to or during the first time an object of class `ios_base::Init` is constructed, and in any case before the body of `main` begins execution.(footnote 293) The objects are not destroyed during program execution.(footnote 294) The results of including `<iostream>` in a translation unit shall be as if `<iostream>` defined an instance of `ios_base::Init` with static storage duration. Similarly, the entire program shall behave as if there were at least one instance of `ios_base::Init` with static storage duration.

### 2767(i). `not_fn call_wrapper` can form invalid types

Section: 22.10.13 [func.not.fn] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-08-19 Last modified: 2021-06-06

Priority: 0

View all other issues in [func.not.fn].

View all issues with C++17 status.

Discussion:

The definition of the `call_wrapper` type in the C++17 CD means this fails to compile:

>     #include <functional>

>     struct abc { virtual void f() const = 0; };
>     struct derived : abc { void f() const { } };
>     struct F { bool operator()(abc&) { return false; } };
>     derived d;
>     bool b = std::not_fn(F{})(static_cast<abc&&>(d));

The problem is that the return types use `result_of_t<F(abc)>` and `F(abc)` is not a valid function type, because it takes an abstract class by value.

The return types should use `result_of_t<F(Args&&...)>` instead.

[2016-09-09 Issues Resolution Telecon]

P0; move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify [func.not_fn], class `call_wrapper` synopsis, as indicated:

   >     class call_wrapper
   >     {
   >       […]
   >       template<class... Args>
   >         auto operator()(Args&&...) &
   >           -> decltype(!declval<result_of_t<FD&(Args&&...)>>());
   >       template<class... Args>
   >         auto operator()(Args&&...) const&
   >           -> decltype(!declval<result_of_t<FD const&(Args&&...)>>());
   >       template<class... Args>
   >         auto operator()(Args&&...) &&
   >           -> decltype(!declval<result_of_t<FD(Args&&...)>>());
   >       template<class... Args>
   >         auto operator()(Args&&...) const&&
   >           -> decltype(!declval<result_of_t<FD const(Args&&...)>>());
   >       […]
   >     };

2. Modify the prototype declarations of [func.not_fn] as indicated:

   >     template<class... Args>
   >       auto operator()(Args&&... args) &
   >         -> decltype(!declval<result_of_t<FD&(Args&&...)>>());
   >     template<class... Args>
   >       auto operator()(Args&&... args) const&
   >         -> decltype(!declval<result_of_t<FD const&(Args&&...)>>());

   > > […]

   >     template<class... Args>
   >       auto operator()(Args&&... args) &&
   >         -> decltype(!declval<result_of_t<FD(Args&&...)>>());
   >     template<class... Args>
   >       auto operator()(Args&&... args) const&&
   >         -> decltype(!declval<result_of_t<FD const(Args&&...)>>());

### 2768(i). `any_cast` and move semantics

Section: 22.7.5 [any.nonmembers] Status: C++17 Submitter: Casey Carter Opened: 2016-08-27 Last modified: 2017-07-30

Priority: 0

View all other issues in [any.nonmembers].

View all issues with C++17 status.

Discussion:

LWG 2509(i) made two changes to the specification of `any` in v2 of the library fundamentals TS:

1. It altered the effects of the `any_cast(any&&)` overload to enable moving the value out of the `any` object and/or obtaining an rvalue reference to the contained value.
2. It made changes to support pathological copyable-but-not-movable contained values, which is madness.

Change 1 has very desirable effects; I propose that we apply the sane part of LWG 2509(i) to `any` in the C++17 WP, for all of the reasons cited in the discussion of LWG 2509(i).

[2016-09-09 Issues Resolution Telecon]

P0; move to Tentatively Ready

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. In 22.7.5 [any.nonmembers] p5, edit as follows:

>    >     template<class ValueType>
>    >       ValueType any_cast(const any& operand);
>    >     template<class ValueType>
>    >       ValueType any_cast(any& operand);
>    >     template<class ValueType>
>    >       ValueType any_cast(any&& operand);

>    > > -4- Requires: `is_reference_v<ValueType>` is `true` or `is_copy_constructible_v<ValueType>` is `true`. Otherwise the program is ill-formed.

>    > > -5- Returns: For the first form, `*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)`. For the second and third forms, `*any_cast<remove_reference_t<ValueType>>(&operand)`. For the third form, `std::forward<ValueType>(*any_cast<remove_reference_t<ValueType>>(&operand))`.

>    > > […]

[Issues Telecon 16-Dec-2016]

Move to Tentatively Ready

Proposed resolution:

Resolved by the wording provided by LWG 2769(i).

### 2769(i). Redundant `const` in the return type of `any_cast(const any&)`

Section: 22.7.5 [any.nonmembers] Status: C++17 Submitter: Casey Carter Opened: 2016-09-02 Last modified: 2017-07-30

Priority: 0

View all other issues in [any.nonmembers].

View all issues with C++17 status.

Discussion:

The overload of `any_cast` that accepts a reference to constant `any`:

>     template<class ValueType>
>       ValueType any_cast(const any& operand);

is specified to return `*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)` in [any.nonmembers]/5. This calls the pointer-to-constant overload of `any_cast`:

>     template<class ValueType>
>       const ValueType* any_cast(const any* operand) noexcept;

which is specified as:

> Returns: If `operand != nullptr && operand->type() == typeid(ValueType)`, a pointer to the object contained by `operand`; otherwise, `nullptr`.

Since `typeid(T) == typeid(const T)` for all types `T`, `any_cast<add_const_t<T>>(&operand)` is equivalent to `any_cast<T>(&operand)` for all types `T` when operand is a constant lvalue `any`. The `add_const_t` in the return specification of the first overload above is therefore redundant.

[2016-09-09 Issues Resolution Telecon]

P0; move to Tentatively Ready

Casey will provide combined wording for this and 2768(i), since they modify the same paragraph.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.7.5 [any.nonmembers] as indicated:

>    > >     template<class ValueType>
>    > >       ValueType any_cast(const any& operand);
>    > >     template<class ValueType>
>    > >       ValueType any_cast(any& operand);
>    > >     template<class ValueType>
>    > >       ValueType any_cast(any&& operand);

>    > -4- Requires: `is_reference_v<ValueType>` is `true` or `is_copy_constructible_v<ValueType>` is `true`. Otherwise the program is ill-formed.

>    > -5- Returns: For the first form, `*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)`. For the second and third forms, `*any_cast<remove_reference_t<ValueType>>(&operand)`.

>    > […]

[2016-09-09 Casey improves wording as determined by telecon]

The presented resolution is intended as the common wording for both LWG 2768(i) and LWG 2769(i).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.7.5 [any.nonmembers] as indicated:

>    > >     template<class ValueType>
>    > >       ValueType any_cast(const any& operand);
>    > >     template<class ValueType>
>    > >       ValueType any_cast(any& operand);
>    > >     template<class ValueType>
>    > >       ValueType any_cast(any&& operand);

>    > -4- Requires: `is_reference_v<ValueType>` is `true` or `is_copy_constructible_v<ValueType>` is `true`. Otherwise the program is ill-formed.

>    > -5- Returns: For the first form, `*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)`. For the and second and third forms, `*any_cast<remove_reference_t<ValueType>>(&operand)`. For the third form, `std::forward<ValueType>(*any_cast<remove_reference_t<ValueType>>(&operand))`.

>    > […]

[2016-10-05, Tomasz and Casey reopen and improve the wording]

The constraints placed on the non-pointer `any_cast` overloads are neither necessary nor sufficient to guarantee that the specified effects are well-formed. The current PR for LWG 2769(i) also makes it possible to retrieve a dangling lvalue reference to a temporary any with e.g. `any_cast<int&>(any{42})`, which should be forbidden.

[2016-10-16, Eric made some corrections to the wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Modify 22.7.5 [any.nonmembers] as indicated:

>    > >     template<class ValueType>
>    > >       ValueType any_cast(const any& operand);
>    > >     template<class ValueType>
>    > >       ValueType any_cast(any& operand);
>    > >     template<class ValueType>
>    > >       ValueType any_cast(any&& operand);

>    > -4- Requires: `is_reference_v<ValueType>` is `true` or `is_copy_constructible_v<ValueType>` is `true`.For the first overload, `is_constructible_v<ValueType, const remove_cv_t<remove_reference_t<ValueType>>&>` is `true`. For the second overload, `is_constructible_v<ValueType, remove_cv_t<remove_reference_t<ValueType>>&>` is `true`. For the third overload, `is_constructible_v<ValueType, remove_cv_t<remove_reference_t<ValueType>>>` is `true`. Otherwise the program is ill-formed.

>    > -5- Returns: For the first form, `*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)`. For the second and third forms, `*any_cast<remove_reference_t<ValueType>>(&operand)`.For the first and second overload, `static_cast<ValueType>(*any_cast<remove_cv_t<remove_reference_t<ValueType>>>(&operand))`. For the third overload, `static_cast<ValueType>(std::move(*any_cast<remove_cv_t<remove_reference_t<ValueType>>>(&operand)))`.

>    > […]

[2016-11-10, LWG asks for simplification of the wording]

[Issues Telecon 16-Dec-2016]

Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 22.7.5 [any.nonmembers] as indicated:

   > >     template<class ValueType>
   > >       ValueType any_cast(const any& operand);
   > >     template<class ValueType>
   > >       ValueType any_cast(any& operand);
   > >     template<class ValueType>
   > >       ValueType any_cast(any&& operand);

   > -?- Let `U` be the type `remove_cv_t<remove_reference_t<ValueType>>`.

   > -4- Requires: `is_reference_v<ValueType>` is `true` or `is_copy_constructible_v<ValueType>` is `true`.For the first overload, `is_constructible_v<ValueType, const U&>` is `true`. For the second overload, `is_constructible_v<ValueType, U&>` is `true`. For the third overload, `is_constructible_v<ValueType, U>` is `true`. Otherwise the program is ill-formed.

   > -5- Returns: For the first form, `*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)`. For the second and third forms, `*any_cast<remove_reference_t<ValueType>>(&operand)`.For the first and second overload, `static_cast<ValueType>(*any_cast<U>(&operand))`. For the third overload, `static_cast<ValueType>(std::move(*any_cast<U>(&operand)))`.

   > […]

### 2770(i). `tuple_size<const T>` specialization is not SFINAE compatible and breaks decomposition declarations

Section: 22.4.7 [tuple.helper] Status: C++17 Submitter: Richard Smith Opened: 2016-08-15 Last modified: 2017-07-30

Priority: 1

View all other issues in [tuple.helper].

View all issues with C++17 status.

Discussion:

Consider:

>     #include <utility>

>     struct X { int a, b; };
>     const auto [x, y] = X();

This is ill-formed: it triggers the instantiation of `std::tuple_size<const X>`, which hits a hard error because it tries to use `tuple_size<X>::value`, which does not exist. The code compiles if `<utility>` (or another header providing `tuple_size`) is not included.

It seems that we either need a different strategy for decomposition declarations, or we need to change the library to make the `tuple_size` partial specializations for cv-qualifiers SFINAE-compatible.

The latter seems like the better option to me, and like a good idea regardless of decomposition declarations.

[2016-09-05, Daniel comments]

This is partially related to LWG 2446(i).

[2016-09-09 Issues Resolution Telecon]

Geoffrey to provide wording

[2016-09-14 Geoffrey provides wording]

[2016-10 Telecon]

Alisdair to add his concerns here before Issaquah. Revisit then. Status to 'Open'

Proposed resolution:

This wording is relative to N4606.

1. Edit 22.4.7 [tuple.helper] as follows:

   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   > > -4- Let `TS` denote `tuple_size<T>` of the cv-unqualified type `T`. If the expression `TS::value` is well-formed when treated as an unevaluated operand, tThen each of the three templates shall meet the `UnaryTypeTrait` requirements (20.15.1) with a `BaseCharacteristic` of

   > > >     integral_constant<size_t, TS::value>

   > > Otherwise, they shall have no member `value`.

   > > Access checking is performed as if in a context unrelated to `TS` and `T`. Only the validity of the immediate context of the expression is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]

   > > -5- In addition to being available via inclusion of the `<tuple>` header, the three templates are available when either of the headers `<array>` or `<utility>` are included.

### 2771(i). Broken Effects of some `basic_string::compare` functions in terms of `basic_string_view`

Section: 27.4.3.8.4 [string.compare] Status: C++17 Submitter: Daniel Krügler Opened: 2016-09-05 Last modified: 2017-07-30

Priority: 1

View all issues with C++17 status.

Discussion:

Some `basic_string::compare` functions are specified in terms of a non-existing `basic_string_view` constructor, namely 27.4.3.8.4 [string.compare] p3,

>     return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv);

and 27.4.3.8.4 [string.compare] p4:

>     return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv, pos2, n2);

because there doesn't exist a `basic_string_view` constructor with three arguments.

Albeit this can be easily fixed by a proper combination of the existing constructor

>     constexpr basic_string_view(const charT* str, size_type len);

with the additional member function

>     constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const;

it should be decided whether adding the seemingly natural constructor

>     constexpr basic_string_view(const charT* str, size_type pos, size_type n);

could simplify matters. A counter argument for this addition might be, that `basic_string` doesn't provide this constructor either.

Another problem is related to the specification of 27.4.3.8.4 [string.compare] p4, which attempts to call a non-existing `basic_string_view::compare` overload that would match the signature:

>     constexpr int compare(basic_string_view str, size_type pos1, size_type n1) const;

[2016-09-09 Issues Resolution Telecon]

Marshall to investigate using P/R vs. adding the missing constructor.

[2016-10 Issues Resolution Telecon]

Marshall reports that P/R is better. Status to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

> [Drafting note: The wording changes below are for the same part of the standard as 2758(i). However, they do not conflict. This one changes the "Effects" of the routine, while the other changes the definition. — end drafting note]

1. Change 27.4.3.8.4 [string.compare] as indicated:

   >     int compare(size_type pos1, size_type n1,
   >                 basic_string_view<charT, traits> sv) const;

   > > -3- Effects: Equivalent to:

   > > >     return basic_string_view<charT, traits>(this.data(), size()).substr(pos1, n1).compare(sv);

   >     int compare(size_type pos1, size_type n1,
   >                 basic_string_view<charT, traits> sv,
   >                 size_type pos2, size_type n2 = npos) const;

   > > -4- Effects: Equivalent to:

   > > >     return basic_string_view<charT, traits>(this.data(), size()).substr(pos1, n1).compare(sv,.substr(pos2, n2));

### 2773(i). Making `std::ignore constexpr`

Section: 22.4.1 [tuple.general] Status: C++17 Submitter: Vincent Reverdy Opened: 2016-09-10 Last modified: 2017-07-30

Priority: 0

View all other issues in [tuple.general].

View all issues with C++17 status.

Discussion:

Currently `std::ignore` is not specified `constexpr` according to the C++ draft N4606 in the paragraph 22.4.1 [tuple.general]. It prevents some use in `constexpr` context: for example declaring a `constexpr` variable equals to the result of a function to which `std::ignore` has been passed as a parameter:

>     constexpr int i = f(std::ignore); // Won't compile

If there is no fundamental reason preventing `std::ignore` to be `constexpr`, then we propose to declare it as `constexpr` instead of as `const`.

[Issues processing Telecon 2016-10-7]

P0; set to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 22.4.1 [tuple.general] as indicated:

   >     // 20.5.2.4, tuple creation functions:
   >     constconstexpr unspecified ignore;

### 2774(i). `std::function` construction vs assignment

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++23 Submitter: Barry Revzin Opened: 2016-09-14 Last modified: 2023-11-22

Priority: 3

View all other issues in [func.wrap.func.con].

View all issues with C++23 status.

Discussion:

I think there's a minor defect in the `std::function` interface. The constructor template is:

>     template <class F> function(F f);

while the assignment operator template is

>     template <class F> function& operator=(F&& f);

The latter came about as a result of LWG 1288(i), but that one was dealing with a specific issue that wouldn't have affected the constructor. I think the constructor should also take `f` by forwarding reference, this saves a move in the lvalue/xvalue cases and is also just generally more consistent. Should just make sure that it's stored as `std::decay_t<F>` instead of `F`.

Is there any reason to favor a by-value constructor over a forwarding-reference constructor?

[2019-07-26 Tim provides PR.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4820.

> 1. Edit 22.10.17.3 [func.wrap.func], class template `function` synopsis, as indicated:

>    >     namespace std {
>    >       template<class> class function; // not defined
>    >       template<class R, class... ArgTypes> {
>    >       public:
>    >         using result_type = R;

>    >         // 22.10.17.3.2 [func.wrap.func.con], construct/copy/destroy
>    >         function() noexcept;
>    >         function(nullptr_t) noexcept;
>    >         function(const function&);
>    >         function(function&&) noexcept;
>    >         template<class F> function(F&&);

>    >         […]
>    >       };

>    >       […]
>    >     }

> 2. Edit 22.10.17.3.2 [func.wrap.func.con] p7-11 as indicated:

>    >     template<class F> function(F&& f);

>    > > -7- Requires: `F` shall be Cpp17CopyConstructibleLet `FD` be `decay_t<F>`.

>    > > -8- Remarks: This constructor template shall not participate in overload resolution unless `F` Constraints:

>    > > 1. (8.1) — `is_same_v<FD, function>` is `false`; and

>    > > 2. (8.2) — `FD` is Lvalue-Callable (22.10.17.3 [func.wrap.func]) for argument types `ArgTypes...` and return type `R`.

>    > > -?- Expects: `FD` meets the Cpp17CopyConstructible requirements.

>    > > -9- Ensures: `!*this` if any of the following hold:

>    > > 1. (9.1) — `f` is a null function pointer value.

>    > > 2. (9.2) — `f` is a null member pointer value.

>    > > 3. (9.3) — `F` is an instance `remove_cvref_t<F>` is a specialization of the `function` class template, and `!f` is `true`.

>    > > -10- Otherwise, `*this` targets a copy of `f` an object of type `FD` direct-non-list-initialized with `std::move(f)` `std::forward<F>(f)`. [Note: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where `f` is refers to an object holding only a pointer or reference to an object and a member function pointer. — end note]

>    > > -11- Throws: Shall Does not throw exceptions when `f` `FD` is a function pointer type or a specialization of `reference_wrapper``<T>` for some `T`. Otherwise, may throw `bad_alloc` or any exception thrown by `F`’s copy or move constructor the initialization of the target object.

[2020-11-01; Daniel comments and improves the wording]

The proposed wording should — following the line of Marshall's "Mandating" papers — extract from the Cpp17CopyConstructible precondition a corresponding Constraints: element and in addition to that the wording should replace old-style elements such as Expects: by the recently agreed on elements.

See also the related issue LWG 3493(i).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4868.

> 1. Edit 22.10.17.3 [func.wrap.func], class template `function` synopsis, as indicated:

>    >     namespace std {
>    >       template<class> class function; // not defined

>    >       template<class R, class... ArgTypes> {
>    >       public:
>    >         using result_type = R;

>    >         // 22.10.17.3.2 [func.wrap.func.con], construct/copy/destroy
>    >         function() noexcept;
>    >         function(nullptr_t) noexcept;
>    >         function(const function&);
>    >         function(function&&) noexcept;
>    >         template<class F> function(F&&);

>    >         […]
>    >       };

>    >       […]
>    >     }

> 2. Edit 22.10.17.3.2 [func.wrap.func.con] as indicated:

>    >     template<class F> function(F&& f);

>    > > Let `FD` be `decay_t<F>`.

>    > > -8- Constraints:

>    > > 1. (8.1) — `is_same_v<remove_cvref_t<F>, function>` is `false`,

>    > > 2. (8.2) — `FD` `F` is Lvalue-Callable (22.10.17.3.1 [func.wrap.func.general]) for argument types `ArgTypes...` and return type `R`,

>    > > 3. (8.3) — `is_copy_constructible_v<FD>` is `true`, and

>    > > 4. (8.4) — `is_constructible_v<FD, F>` is `true`.

>    > > -9- Preconditions: `FFD` meets the Cpp17CopyConstructible requirements.

>    > > -10- Postconditions: `!*this` if any of the following hold:

>    > > 1. (10.1) — `f` is a null function pointer value.

>    > > 2. (10.2) — `f` is a null member pointer value.

>    > > 3. (10.3) — `F` is an instance `remove_cvref_t<F>` is a specialization of the function class template, and `!f` is `true`.

>    > > -11- Otherwise, `*this` targets a copy of `f` an object of type `FD` direct-non-list-initialized with `std::move(f)std::forward<F>(f)`.

>    > > -12- Throws: Nothing if `fFD` is a specialization of `reference_wrapper` or a function pointer type. Otherwise, may throw `bad_alloc` or any exception thrown by `F`'s copy or move constructorthe initialization of the target object.

>    > > -13- Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where `f` isrefers to an object holding only a pointer or reference to an object and a member function pointer.

[2021-05-17; Tim comments and revises the wording]

The additional constraints added in the previous wording can induce constraint recursion, as noted in the discussion of LWG 3493(i). The wording below changes them to Mandates: instead to allow this issue to make progress independently of that issue.

The proposed resolution below has been implemented and tested on top of libstdc++.

[2021-05-20; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Edit 22.10.17.3.1 [func.wrap.func.general], class template `function` synopsis, as indicated:

   >     namespace std {
   >       template<class> class function; // not defined

   >       template<class R, class... ArgTypes> {
   >       public:
   >         using result_type = R;

   >         // 22.10.17.3.2 [func.wrap.func.con], construct/copy/destroy
   >         function() noexcept;
   >         function(nullptr_t) noexcept;
   >         function(const function&);
   >         function(function&&) noexcept;
   >         template<class F> function(F&&);

   >         […]
   >       };

   >       […]
   >     }

2. Edit 22.10.17.3.2 [func.wrap.func.con] as indicated:

   >     template<class F> function(F&& f);

   > > Let `FD` be `decay_t<F>`.

   > > -8- Constraints:

   > > 1. (8.1) — `is_same_v<remove_cvref_t<F>, function>` is `false`, and

   > > 2. (8.2) — `FD` `F` is Lvalue-Callable (22.10.17.3.1 [func.wrap.func.general]) for argument types `ArgTypes...` and return type `R`.

   > > -?- Mandates:

   > > 1. (?.1) — `is_copy_constructible_v<FD>` is `true`, and

   > > 2. (?.2) — `is_constructible_v<FD, F>` is `true`.

   > > -9- Preconditions: `FFD` meets the Cpp17CopyConstructible requirements.

   > > -10- Postconditions: `!*this` is `true` if any of the following hold:

   > > 1. (10.1) — `f` is a null function pointer value.

   > > 2. (10.2) — `f` is a null member pointer value.

   > > 3. (10.3) — `F` is an instance `remove_cvref_t<F>` is a specialization of the `function` class template, and `!f` is `true`.

   > > -11- Otherwise, `*this` targets a copy of `f` an object of type `FD` direct-non-list-initialized with `std::move(f)std::forward<F>(f)`.

   > > -12- Throws: Nothing if `fFD` is a specialization of `reference_wrapper` or a function pointer type. Otherwise, may throw `bad_alloc` or any exception thrown by `F`'s copy or move constructorthe initialization of the target object.

   > > -13- Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where `f` isrefers to an object holding only a pointer or reference to an object and a member function pointer.

### 2776(i). `shared_ptr unique()` and `use_count()`

Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: Resolved Submitter: Hans Boehm Opened: 2016-09-22 Last modified: 2018-06-12

Priority: 2

View all other issues in [util.smartptr.shared.obs].

View all issues with Resolved status.

Discussion:

The removal of the "debug only" restriction for `use_count()` and `unique()` in `shared_ptr` by LWG 2434(i) introduced a bug. In order for `unique()` to produce a useful and reliable value, it needs a synchronize clause to ensure that prior accesses through another reference are visible to the successful caller of `unique()`. Many current implementations use a relaxed load, and do not provide this guarantee, since it's not stated in the standard. For debug/hint usage that was OK. Without it the specification is unclear and probably misleading.

I would vote for making `unique()` use `memory_order_acquire`, and specifying that reference count decrement operations synchronize with `unique()`. That still doesn't give us sequential consistency by default, like we're supposed to have. But the violations seem sufficiently obscure that I think it's OK. All uses that anybody should care about will work correctly, and the bad uses are clearly bad. I agree with Peter that this version of `unique()` may be quite useful.

I would prefer to specify `use_count()` as only providing an unreliable hint of the actual count (another way of saying debug only). Or deprecate it, as JF suggested. We can't make `use_count()` reliable without adding substantially more fencing. We really don't want someone waiting for `use_count() == 2` to determine that another thread got that far. And unfortunately, I don't think we currently say anything to make it clear that's a mistake.

This would imply that `use_count()` normally uses `memory_order_relaxed`, and `unique` is neither specified nor implemented in terms of `use_count()`.

[2016-10-27 Telecon]

Priority set to 2

[2018-06 Rapperswil Thursday issues processing]

This was resolved by P0521, which was adopted in Jacksonville.

Proposed resolution:

### 2777(i). `basic_string_view::copy` should use `char_traits::copy`

Section: 27.3.3.8 [string.view.ops], 27.4.3.7.7 [string.copy] Status: C++17 Submitter: Billy Robert O'Neal III Opened: 2016-09-27 Last modified: 2017-07-30

Priority: 0

View all other issues in [string.view.ops].

View all issues with C++17 status.

Discussion:

`basic_string_view::copy` is inconsistent with `basic_string::copy`, in that the former uses `copy_n` and the latter uses `traits::copy`. We should have this handling be consistent.

Moreover, the `basic_string::copy` description is excessively roundabout due to copy-on-write era wording.

[Issues processing Telecon 2016-10-7]

P0; set to Tentatively Ready

Removed "Note to project editor", since the issue there has been fixed in the current draft.

Proposed resolution:

This wording is relative to N4606.

1. Change 27.4.3.7.7 [string.copy] as indicated:

   >     size_type copy(charT* s, size_type n, size_type pos = 0) const;

   -?- Let `rlen` be the smaller of `n` and `size() - pos`.

   -2- Throws: `out_of_range` if `pos > size()`.

   -?- Requires: `[s, s + rlen)` is a valid range.

   -3- Effects: Determines the effective length `rlen` of the string to copy as the smaller of `n` and `size() - pos`. `s` shall designate an array of at least `rlen` elements.Equivalent to: `traits::copy(s, data() + pos, rlen)`. [Note: This does not terminate `s` with a null object. — end note]

   The function then replaces the string designated by `s` with a string of length `rlen` whose elements are a copy of the string controlled by `*this` beginning at position `pos`.

   The function does not append a null object to the string designated by `s`.

   -4- Returns: `rlen`.

2. Change 27.3.3.8 [string.view.ops] as indicated:

   >     size_type copy(charT* s, size_type n, size_type pos = 0) const;

   -1- Let `rlen` be the smaller of `n` and `size() - pos`.

   -2- Throws: `out_of_range` if `pos > size()`.

   -3- Requires: `[s, s + rlen)` is a valid range.

   -4- Effects: Equivalent to: `copy_n(begin() + pos, rlen, s)``traits::copy(s, data() + pos, rlen)`

   -5- Returns: `rlen`.

   -6- Complexity: 𝒪(`rlen`).

### 2778(i). `basic_string_view` is missing `constexpr`

Section: 27.3 [string.view] Status: C++17 Submitter: Billy Robert O'Neal III Opened: 2016-09-30 Last modified: 2017-07-30

Priority: 0

View other active issues in [string.view].

View all other issues in [string.view].

View all issues with C++17 status.

Discussion:

`basic_string_view` was not updated to account for other library machinery made `constexpr` in Oulu. Now that `reverse_iterator` is `constexpr` there's no reason the reverse range functions can't be. Also, now that we have C++14 relaxed `constexpr`, we can also take care of the assignment operator and `copy`.

[Issues processing Telecon 2016-10-7]

split off the copy into its' own issue 2780(i)

P0; set what's left to Tentatively Ready

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. In 27.3.3 [string.view.template], add `constexpr` to the assignment operator:

>    >     constexpr basic_string_view& operator=(const basic_string_view&) noexcept = default;

> 2. In 27.3.3 [string.view.template], add `constexpr` to the reverse range functions:

>    >     constexpr const_reverse_iterator rbegin() const noexcept;
>    >     constexpr const_reverse_iterator rend() const noexcept;
>    >     constexpr const_reverse_iterator crbegin() const noexcept;
>    >     constexpr const_reverse_iterator crend() const noexcept;

> 3. In 27.3.3.4 [string.view.iterators], add `constexpr` to the reverse range functions:

>    >     constexpr const_reverse_iterator rbegin() const noexcept;
>    >     constexpr const_reverse_iterator crbegin() const noexcept;

>    > > -6- Returns: `const_reverse_iterator(end())`

>    >     constexpr const_reverse_iterator rend() const noexcept;
>    >     constexpr const_reverse_iterator crend() const noexcept;

>    > > -7- Returns: `const_reverse_iterator(begin())`

> 4. In 27.3.3 [string.view.template], add `constexpr` to `copy`:

>    >     constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;

> 5. In 27.3.3.8 [string.view.ops], add `constexpr` to `copy`:

>    >     constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;

Proposed resolution:

This wording is relative to N4606.

1. In 27.3.3 [string.view.template], add `constexpr` to the assignment operator:

   >     constexpr basic_string_view& operator=(const basic_string_view&) noexcept = default;

2. In 27.3.3 [string.view.template], add `constexpr` to the reverse range functions:

   >     constexpr const_reverse_iterator rbegin() const noexcept;
   >     constexpr const_reverse_iterator rend() const noexcept;
   >     constexpr const_reverse_iterator crbegin() const noexcept;
   >     constexpr const_reverse_iterator crend() const noexcept;

3. In 27.3.3.4 [string.view.iterators], add `constexpr` to the reverse range functions:

   >     constexpr const_reverse_iterator rbegin() const noexcept;
   >     constexpr const_reverse_iterator crbegin() const noexcept;

   > > -6- Returns: `const_reverse_iterator(end())`

   >     constexpr const_reverse_iterator rend() const noexcept;
   >     constexpr const_reverse_iterator crend() const noexcept;

   > > -7- Returns: `const_reverse_iterator(begin())`

### 2779(i). [networking.ts] Relax requirements on buffer sequence iterators

Section: 16.2.1 [networking.ts::buffer.reqmts.mutablebuffersequence] Status: WP Submitter: Vinnie Falco Opened: 2016-10-05 Last modified: 2026-02-21

Priority: Not Prioritized

View all issues with WP status.

Discussion:

Addresses: networking.ts

We propose to relax the ForwardIterator requirements of buffer sequences in [networking.ts] by allowing buffer sequence iterators to return rvalues when dereferenced, and skip providing `operator->`. A paraphrased explanation of why the referential equality rules of ForwardIterator are harmful to the buffer sequence requirements comes from N4128, 3.3.7 "Ranges For The Standard Library":

> The [networking.ts] dependence on ForwardIterator in the buffer sequence requirements ties together the traversal and access properties of iterators. For instance, no forward iterator may return an rvalue proxy when it is dereferenced; the ForwardIterator concept requires that unary `operator*` return an lvalue. This problem has serious consequences for lazy evaluation that applies transformations to buffer sequence elements on the fly. If the transformation function does not return an lvalue, the range's iterator can model no concept stronger than InputIterator, even if the resulting iterator could in theory support BidirectionalIterator. The result in practice is that most range adaptors today will not be compatible with [networking.ts], thereby limiting the types that [networking.ts] can be passed, for no good reason.

Consider a user defined function `trim` which lazily adapts a `ConstBufferSequence`, such that when iterating the buffers in the new sequence, each buffer appears one byte shorter than in the underlying sequence:

>     #include <boost/range/adaptor/transformed.hpp>

>     struct trim
>     {
>       using result_type = const_buffer;
>       result_type operator()(const_buffer b)
>       {
>         return const_buffer{b.data(), b.size() - 1};
>       }
>     };

>     template <ConstBufferSequence>
>     auto
>     trim(ConstBufferSequence const& buffers)
>     {
>       using namespace boost::adaptors;
>       return buffers | transformed(trim{});
>     }

`trim` returns a BidirectionalRange, whose `const_iterator` returns an rvalue when dereferenced. This breaks the requirements of ForwardIterator. A solution that meets the referential equality rules of ForwardIterator, would be to evaluate the transformed sequence upon construction (for example, by storing each transformed `const_buffer` in a `vector`). Unfortunately this work-around is more expensive since it would add heap allocation which the original example avoids.

The requirement of InputIterator `operator->` is also unnecessary for buffer sequence iterators, and should be removed. Because [networking.ts] only requires that a buffer sequence iterator's `value_type` be convertible to `const_buffer` or `mutable_buffer`, implementations of [networking.ts] cannot assume the existence of any particular member functions or data members other than an implicit conversion to `const_buffer` or `mutable_buffer`. Removing the requirement for `operator->` to be present, provides additional relief from the referential equality requirements of ForwardIterator and allows transformations of buffer sequences to meet the requirements of buffer sequences.

This proposal imposes no changes on existing implementations of [networking.ts]. It does not change anything in the standard. The proposal is precise, minimal, and allows range adapters to transform buffer sequences in optimized, compatible ways.

[Issues processing Telecon 2016-10-07]

Status set to LEWG

[2017-02-21, Jonathan comments]

The use of the term "strict aliasing" in the issue discussion is misleading as that refers to type-based alias analysis in compilers, but the rule for `ForwardIterator`s is related to referential equality and not strict aliasing.

[2017-02-22, Vinnie Falco comments]

We have eliminated the use of the term "strict aliasing" from the discussion.

[2017-07-10, Toronto, LEWG comments]

Status change: LEWG → Open.

Forward to LWG with the note that they may want to use "input +" instead of "bidirectional -". Unanimous yes.

[2017-07 Toronto Wednesday night issue processing]

Status to Ready

Proposed resolution:

This wording is relative to N4588.

1. Modify 16.2.1 [networking.ts::buffer.reqmts.mutablebuffersequence] as indicated:

   > An iterator type meeting the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) whose value type is convertible to `mutable_buffer`

   > An iterator type whose `reference` type is convertible to `mutable_buffer`, and which satisfies all the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) except that:

   > 1. there is no requirement that `operator->` is provided, and
   > 2. there is no requirement that `reference` be a reference type.

2. Modify 16.2.2 [networking.ts::buffer.reqmts.constbuffersequence] as indicated:

   > An iterator type meeting the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) whose value type is convertible to `const_buffer`.

   > An iterator type whose `reference` type is convertible to `const_buffer`, and which satisfies all the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) except that:

   > 1. there is no requirement that `operator->` is provided, and
   > 2. there is no requirement that `reference` be a reference type.

### 2780(i). `basic_string_view::copy` is missing `constexpr`

Section: 27.3 [string.view] Status: Resolved Submitter: Billy Robert O'Neal III Opened: 2016-10-07 Last modified: 2018-11-25

Priority: 2

View other active issues in [string.view].

View all other issues in [string.view].

View all issues with Resolved status.

Discussion:

Now that we have C++14 relaxed `constexpr`, we can also take care of the assignment operator and `copy`.

[Issues processing Telecon 2016-10-07]

Split off from 2778(i)

[2016-11-12, Issaquah]

Sat PM: Status LEWG

[LEWG Kona 2017]

Recommend NAD: Don't want to spend the committee time. Can call std::copy if you want this, after LEWG159 goes into C++20.

[2018-06 Rapperswil Wednesday issues processing]

This will be addressed by P1032.

Resolved by the adoption of P1032 in San Diego.

Proposed resolution:

This wording is relative to N4606.

1. In 27.3.3 [string.view.template], add `constexpr` to `copy`:

   >     constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;

2. In 27.3.3.8 [string.view.ops], add `constexpr` to `copy`:

   >     constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;

### 2781(i). Contradictory requirements for `std::function` and `std::reference_wrapper`

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-13 Last modified: 2017-07-30

Priority: 0

View all other issues in [func.wrap.func.con].

View all issues with C++17 status.

Discussion:

`template<class F> function(F f)` says that the effects are "`*this` targets a copy of `f`" which seems pretty clear that if `F` is `reference_wrapper<CallableType>` then the target is a `reference_wrapper<CallableType>`.

But the function copy and move constructors say "shall not throw exceptions if `f`'s target is a callable object passed via `reference_wrapper` or a function pointer." From the requirement above it's impossible for the target to be "a callable object passed via `reference_wrapper`" because if the function was constructed with such a type then the target is the `reference_wrapper` not the callable object it wraps.

This matters because it affects the result of `function::target_type()`, and we have implementation divergence. VC++ and libc++ store the `reference_wrapper` as the target, but libstdc++ and Boost.Function (both written by Doug Gregor) unwrap it, so the following fails:

>     #include <functional>
>     #include <cassert>

>     int main()
>     {
>      auto f = []{};
>      std::function<void()> fn(std::ref(f));
>      assert(fn.target<std::reference_wrapper<decltype(f)>>() != nullptr);
>     }

If `std::function` is intended to deviate from `boost::function` this way then the Throws element for the copy and move constructors is misleading, and should be clarified.

[2016-11-12, Issaquah]

Sat AM: Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 22.10.17.3.2 [func.wrap.func.con] p4 and p6 the same way, as shown:

   >     function(const function& f);

   > > -3- Postconditions: `!*this` if `!f`; otherwise, `*this` targets a copy of `f.target()`.

   > > -4- Throws: shall not throw exceptions if `f`'s target is a callable object passed viaspecialization of `reference_wrapper` or a function pointer. Otherwise, may throw `bad_alloc` or any exception thrown by the copy constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

   >     function(function&& f);

   > > -5- Effects: If `!f`, `*this` has no target; otherwise, move constructs the target of `f` into the target of `*this`, leaving `f` in a valid state with an unspecified value.

   > > -6- Throws: shall not throw exceptions if `f`'s target is a callable object passed viaspecialization of `reference_wrapper` or a function pointer. Otherwise, may throw `bad_alloc` or any exception thrown by the copy or move constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

### 2782(i). `scoped_allocator_adaptor` constructors must be constrained

Section: 20.6.3 [allocator.adaptor.cnstr] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-14 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

The templated constructors of `scoped_allocator_adaptor` need to be constrained, otherwise uses-allocator construction gives the wrong answer and causes errors for code that should compile.

Consider two incompatible allocator types:

>     template<class T> struct Alloc1 { ... };
>     template<class T> struct Alloc2 { ... };
>     static_assert(!is_convertible_v<Alloc1<int>, Alloc2<int>>);

The unconstrained constructors give this bogus answer:

>     template<class T> using scoped = scoped_allocator_adaptor<T>;
>     static_assert(is_convertible_v<scoped<Alloc1<int>>, scoped<Alloc2<int>>>);

This causes `uses_allocator` to give the wrong answer for any specialization involving incompatible `scoped_allocator_adaptors`, which makes `scoped_allocator_adaptor::construct()` take an ill-formed branch e.g.

>     struct X
>     {
>       using allocator_type = scoped<Alloc2<int>>;
>       X(const allocator_type&);
>       X();
>     };
>     scoped<Alloc1<int>>{}.construct((X*)0);

This fails to compile, because `uses_allocator<X, scoped_allocator_adaptor<Alloc2<int>>>` is `true`, so the allocator is passed to the `X` constructor, but the conversion fails. The error is outside the immediate context, and so is a hard error.

[2016-11-12, Issaquah]

Sat AM: Priority 0; move to Ready

Billy to open another issue about the confusion with the ctor

Proposed resolution:

This wording is relative to N4606.

1. Modify 20.6.3 [allocator.adaptor.cnstr] by converting "Requires" elements to "Remarks: shall not participate ..." constraints as shown:

   >     template <class OuterA2>
   >       scoped_allocator_adaptor(OuterA2&& outerAlloc,
   >                                const InnerAllocs&... innerAllocs) noexcept;

   > > -2- Requires: `OuterAlloc` shall be constructible from `OuterA2`.

   > > -3- Effects: Initializes the `OuterAlloc` base class with `std::forward<OuterA2>(outerAlloc)` and inner with `innerAllocs...` (hence recursively initializing each allocator within the adaptor with the corresponding allocator from the argument list).

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<OuterAlloc, OuterA2>` is `true`.

   > > […]

   >     template <class OuterA2>
   >       scoped_allocator_adaptor(const scoped_allocator_adaptor<OuterA2,
   >                                InnerAllocs...>& other) noexcept;

   > > -6- Requires: `OuterAlloc` shall be constructible from `OuterA2`.

   > > -7- Effects: Initializes each allocator within the adaptor with the corresponding allocator from `other`.

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<OuterAlloc, const OuterA2&>` is `true`.

   >     template <class OuterA2>
   >       scoped_allocator_adaptor(scoped_allocator_adaptor<OuterA2,
   >                                InnerAllocs...>&& other) noexcept;

   > > -8- Requires: `OuterAlloc` shall be constructible from `OuterA2`.

   > > -9- Effects: Initializes each allocator within the adaptor with the corresponding allocator rvalue from `other`.

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_constructible_v<OuterAlloc, OuterA2>` is `true`.

### 2783(i). `stack::emplace()` and `queue::emplace()` should return `decltype(auto)`

Section: 23.6.3.1 [queue.defn], 23.6.6.2 [stack.defn] Status: C++20 Submitter: Jonathan Wakely Opened: 2016-10-14 Last modified: 2021-02-25

Priority: 2

View all other issues in [queue.defn].

View all issues with C++20 status.

Discussion:

The `stack` and `queue` adaptors are now defined as:

>     template <class... Args>
>     reference emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }

This breaks any code using `queue<UserDefinedSequence>` or `stack<UserDefinedSequence>` until the user-defined containers are updated to meet the new C++17 requirements.

If we defined them as returning `decltype(auto)` then we don't break any code. When used with `std::vector` or `std::deque` they will return `reference`, as required, but when used with C++14-conforming containers they will return `void`, as before.

[2016-11-12, Issaquah]

Sat AM: P2

[2017-03-04, Kona]

Status to Tentatively Ready.

Proposed resolution:

This wording is relative to N4606.

1. Change return type of `emplace` in class definition in 23.6.3.1 [queue.defn]:

   >     template <class... Args>
   >       referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }

2. Change return type of `emplace` in class definition in 23.6.6.2 [stack.defn]:

   >     template <class... Args>
   >       referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }

### 2784(i). Resolution to LWG 2484 is missing "otherwise, no effects" and is hard to parse

Section: 17.9.8 [except.nested] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-15 Last modified: 2017-07-30

Priority: 0

View all other issues in [except.nested].

View all issues with C++17 status.

Discussion:

The discussion notes for LWG 2484(i) point out there should be an "else, no effects" at the end, which didn't make it into the resolution. Without this it's unclear whether it should do nothing, or be ill-formed, or undefined.

Additionally, the precise effects of the Effects are hard to determine, because the conditions on the static type and dynamic type are intermingled, but must actually be checked separately (the static checks must be done statically and the dynamic checks must be done dynamically!) Furthermore, the obvious way to know if "the dynamic type of `e` is `nested_exception` or is publicly and unambiguously derived from `nested_exception`" is to use `dynamic_cast`, so we have to use `dynamic_cast` to find out whether to perform the `dynamic_cast` expression specified in the Effects. It would make more sense to specify it in terms of a `dynamic_cast` to a pointer type, and only call `rethrow_nested()` if the result is not null.

The entire spec can be expressed in C++17 as:

>     if constexpr(is_polymorphic_v<E> && (!is_base_of_v<nested_exception, E> || is_convertible_v<E*, nested_exception*>))
>       if (auto p = dynamic_cast<const nested_exception*>(addressof(e)))
>         p->rethrow_nested();

This uses traits to perform checks on the static type, then uses `dynamic_cast` to perform the checks on the dynamic type. I think the spec would be clearer if it had the same structure.

[2016-11-12, Issaquah]

Sat AM: Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 17.9.8 [except.nested] p9:

   >     template <class E> void rethrow_if_nested(const E& e);

   > > -9- Effects: If `E` is not a polymorphic class type, or if `nested_exception` is an inaccessible or ambiguous base class of `E`, there is no effect. Otherwise, if the static type or the dynamic type of `e` is `nested_exception` or is publicly and unambiguously derived from `nested_exception`, callsperforms:

   > > >     dynamic_cast<const nested_exception&>(e).rethrow_nested();
   > > >     if (auto p = dynamic_cast<const nested_exception*>(addressof(e)))
   > > >       p->rethrow_nested();

### 2785(i). `quoted` should work with `basic_string_view`

Section: 31.7.9 [quoted.manip] Status: C++17 Submitter: Marshall Clow Opened: 2016-10-27 Last modified: 2017-07-30

Priority: 0

View all other issues in [quoted.manip].

View all issues with C++17 status.

Discussion:

Quoted output for strings was added for C++14. But when we merged `string_view` from the Library Fundamentals TS, we did not add support for quoted output of `basic_string_view`

[2016-11-12, Issaquah]

Sat AM: Priority 0; move to Ready

13 -> 13 in paragraph modification; and T14 -> T15

Proposed resolution:

This wording is relative to N4606.

Add to the end of the <iomanip> synopsis in [iostream.format.overview]

    template <class charT, class traits>
      T15 quoted(basic_string_view<charT, traits> s,
                 charT delim = charT(’"’), charT escape = charT(’\\’));

Add to [quoted.manip] at the end of p2:

    template <class charT, class traits>
      unspecified quoted(basic_string_view<charT, traits> s,
                                charT delim = charT(’"’), charT escape = charT(’\\’));

Modify [quoted.manip]/3 as follows:

Returns: An object of unspecified type such that if out is an instance of `basic_ostream` with member type `char_type` the same as `charT` and with member type `traits_type` which in the second and third forms is the same as `traits`, then the expression `out << quoted(s, delim, escape)` behaves as a formatted output function (27.7.3.6.1) of `out`. This forms a character sequence `seq`, initially consisting of the following elements:

### 2786(i). Annex C should mention `shared_ptr` changes for array support

Section: C.4.9 [diff.cpp14.utilities] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-18 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

This is valid in C++11 and C++14:

>     shared_ptr<int> s{unique_ptr<int[]>{new int[1]}};

The `shared_ptr` copies the `default_delete<int[]>` deleter from the `unique_ptr`, which does the right thing on destruction.

In C++17 it won't compile, because `!is_convertible_v<int(*)[], int*>`.

The solution is to use `shared_ptr<int[]>`, which doesn't work well in C++14, so there's no transition path. This should be called out in Annex C.

[2016-11-12, Issaquah]

Sat AM: Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4606.

1. Add to C.4.9 [diff.cpp14.utilities]:

   > 20.3.2.2 [util.smartptr.shared]

   > Change: Different constraint on conversions from `unique_ptr`.

   > Rationale: Adding array support to `shared_ptr`, via the syntax `shared_ptr<T[]>` and `shared_ptr<T[N]>`.

   > Effect on original feature: Valid code may fail to compile or change meaning in this International Standard. For example:

   > >     #include <memory>
   > >     std::unique_ptr<int[]> arr(new int[1]);
   > >     std::shared_ptr<int> ptr(std::move(arr)); // error: int(*)[] is not compatible with int*

### 2787(i). §[fs.file_status.cons] doesn't match class definition

Section: 31.12.9 [fs.class.file.status], 31.12.9.2 [fs.file.status.cons] Status: C++17 Submitter: Tim Song Opened: 2016-10-21 Last modified: 2021-06-06

Priority: 0

View all issues with C++17 status.

Discussion:

[fs.class.file_status] depicts:

>     explicit file_status(file_type ft = file_type::none, perms prms = perms::unknown) noexcept;

while [fs.file_status.cons] describes two constructors:

>     explicit file_status() noexcept;
>     explicit file_status(file_type ft, perms prms = perms::unknown) noexcept;

It's also not clear why the default constructor needs to be explicit. Unlike tag types, there doesn't seem to be a compelling reason to disallow constructing a `file_status` without naming the type.

[2016-11-12, Issaquah]

Sat AM: Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4606.

1. Edit [fs.class.file_status] as indicated:

   >     class file_status {
   >     public:
   >       // 27.10.11.1, constructors and destructor:
   >       file_status() noexcept : file_status(file_type::none) {}
   >       explicit file_status(file_type ft = file_type::none,
   >                            perms prms = perms::unknown) noexcept;
   >       […]
   >     };

2. Edit [fs.file_status.cons] as indicated:

   >     explicit file_status() noexcept;

   > > -1- Postconditions: `type() == file_type::none` and `permissions() == perms::unknown`.

### 2788(i). `basic_string` range mutators unintentionally require a default constructible allocator

Section: 27.4.3.7.2 [string.append], 27.4.3.7.3 [string.assign], 27.4.3.7.4 [string.insert], 27.4.3.7.6 [string.replace] Status: C++17 Submitter: Billy O'Neal III Opened: 2016-10-25 Last modified: 2017-07-30

Priority: 2

View all other issues in [string.append].

View all issues with C++17 status.

Discussion:

Email discussion occurred on the lib reflector.

`basic_string`'s mutation functions show construction of temporary `basic_string` instances, without passing an allocator parameter. This says that `basic_string` needs to use a default-initialized allocator, which is clearly unintentional. The temporary needs to use the same allocator the current `basic_string` instance uses, if an implmentation needs to create a temporary at all.

libc++ already does this; I believe libstdc++ does as well (due to the bug report we got from a user that brought this to our attention), but have not verified there. I implemented this in MSVC++'s STL and this change is scheduled to ship in VS "15" RTW.

[2016-11-12, Issaquah]

Sat AM: Priority 2

Alisdair to investigate and (possibly) provide an alternate P/R

[2017-02-13 Alisdair responds:]

Looks good to me - no suggested alternative

[Kona 2017-02-28]

Accepted as Immediate.

Proposed resolution:

This wording is relative to N4606.

1. In 27.4.3.7.2 [string.append], add the allocator parameter to the range overload temporary:

   >     template<class InputIterator>
   >       basic_string& append(InputIterator first, InputIterator last);

   > > -19- Requires: `[first, last)` is a valid range.

   > > -20- Effects: Equivalent to `append(basic_string(first, last, get_allocator()))`.

   > > -21- Returns: `*this`.

2. In 27.4.3.7.3 [string.assign], add the allocator parameter to the range overload temporary:

   >     template<class InputIterator>
   >       basic_string& assign(InputIterator first, InputIterator last);

   > > -23- Effects: Equivalent to `assign(basic_string(first, last, get_allocator()))`.

   > > -24- Returns: `*this`.

3. In 27.4.3.7.4 [string.insert], add the allocator parameter to the range overload temporary:

   >     template<class InputIterator>
   >       iterator insert(const_iterator p, InputIterator first, InputIterator last);

   > > -23- Requires: `p` is a valid iterator on `*this`. `[first, last)` is a valid range.

   > > -24- Effects: Equivalent to `insert(p - begin(), basic_string(first, last, get_allocator()))`.

   > > -25- Returns: An iterator which refers to the copy of the first inserted character, or `p` if `first == last`.

4. In 27.4.3.7.6 [string.replace], add the allocator parameter to the range overload temporary:

   >     template<class InputIterator>
   >       basic_string& replace(const_iterator i1, const_iterator i2,
   >                             InputIterator j1, InputIterator j2);

   > > -32- Requires: `[begin(), i1)`, `[i1, i2)` and `[j1, j2)` are valid ranges.

   > > -33- Effects: Calls `replace(i1 - begin(), i2 - i1, basic_string(j1, j2, get_allocator()))`.

   > > -34- Returns: `*this`.

### 2789(i). Equivalence of contained objects

Section: 22.7.4 [any.class] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-11-09 Last modified: 2017-07-30

Priority: 0

View all other issues in [any.class].

View all issues with C++17 status.

Discussion:

Addresses US 29

What does it mean for (the contained) objects to be "equivalent"?

Suggested resolution:

Add definition (note that using `operator==()` involves complicated questions of overload resolution).

[2016-11-08, Jonathan comments and suggests wording]

We can rephrase the copy constructor in terms of equivalence to construction from the contained object. We need to use in-place construction to avoid recursion in the case where the contained object is itself an any.

For the move constructor we don't simply want to construct from the contrained object, because when the contained object is stored in dynamic memory we don't actually construct anything, we just transfer ownership of a pointer.

[Issues Telecon 16-Dec-2016]

Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change [any.cons] p2:

   >     any(const any& other);

   > > Effects: Constructs an object of type `any` with an equivalent state as `other`.If `other.has_value()` is `false`, constructs an object that has no value. Otherwise, equivalent to `any(in_place<T>, any_cast<const T&>(other))` where `T` is the type of the contained object.

2. Change [any.cons] p4:

   >     any(any&& other);

   > > Effects: Constructs an object of type `any` with a state equivalent to the original state of `other`.If `other.has_value()` is `false`, constructs an object that has no value. Otherwise, constructs an object of type `any` that contains either the contained object of `other`, or contains an object of the same type constructed from the contained object of other considering that contained object as an rvalue.

### 2790(i). Missing specification of `istreambuf_iterator::operator->`

Section: 24.6.4 [istreambuf.iterator] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-11-09 Last modified: 2017-07-30

Priority: 3

View other active issues in [istreambuf.iterator].

View all other issues in [istreambuf.iterator].

View all issues with C++17 status.

Discussion:

Addresses GB 59

There is no specification for `istreambuf_iterator::operator->`. This operator appears to have been added for C++11 by LWG issue 659(i), which gave the signature, but also lacked specification.

[2016-11-08, Jonathan comments and suggests wording]

There is no good option here, and implementations either return `nullptr`, or return the address of a temporary, or don't even provide the member at all. We took polls to decide whether to remove `istreambuf_iterator::operator->`, or specify it to return `nullptr`, and the preferred option was to remove it. It was noted that in the Ranges TS input iterators no longer require `operator->` anyway, and the library never tries to use it.

[Issues Telecon 16-Dec-2016]

Move to Review

Proposed resolution:

This wording is relative to N4606.

> This reverts LWG 659(i).

1. Remove the note in paragraph 1 of 24.6.4 [istreambuf.iterator]:

   > The class template `istreambuf_iterator` defines an input iterator (24.2.3) that reads successive characters from the streambuf for which it was constructed. `operator*` provides access to the current input character, if any. [Note: `operator->` may return a proxy. — end note] Each time `operator++` is evaluated, the iterator advances to the next input character. […]

2. Remove the member from the class synopsis in 24.6.4 [istreambuf.iterator]:

   >     charT operator*() const;
   >     pointer operator->() const;
   >     istreambuf_iterator& operator++();
   >     proxy operator++(int);

### 2791(i). `string_view` objects and strings should yield the same hash values

Section: 27.3.6 [string.view.hash] Status: Resolved Submitter: Nicolai Josuttis Opened: 2016-11-09 Last modified: 2016-11-21

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Under certain conditions we want to be able to use `string_view` instead of `string`. As a consequence both types should behave the same as long we talk about value specific behavior (if possible). For this reason, we should require that both strings and `string_view` yield the same hash values.

Should be solved in C++17 to ensure that following this use does not require to change hash values (which is allowed but possibly unfortunate).

[2016-11-12, Issaquah]

Resolved by P0513R0

Proposed resolution:

This wording is relative to N4606.

> A more formal formulation would be:

> > For any `sv` of type `SV` (being `string_view`, `u16string_view`, `u32string_view`, or `wstring_view`) and `s` of the corresponding type `S` (being `string`, `u16string`, `u32string`, or `wstring`), `hash<SV>()(sv) == hash<S>()(s)`.

1. Edit 27.3.6 [string.view.hash] as indicated:

   >     template<> struct hash<string_view>;
   >     template<> struct hash<u16string_view>;
   >     template<> struct hash<u32string_view>;
   >     template<> struct hash<wstring_view>>;

   > > -1- The template specializations shall meet the requirements of class template `hash` (22.10.19 [unord.hash]). The hash values shall be the same as the hash values for the corresponding string class (27.4.6 [basic.string.hash]).

### 2792(i). [fund.ts.v2] `gcd` and `lcm` should support a wider range of input values

Section: 13.1.2 [fund.ts.v2::numeric.ops.gcd], 13.1.3 [fund.ts.v2::numeric.ops.lcm] Status: TS Submitter: Marshall Clow Opened: 2016-11-09 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [fund.ts.v2::numeric.ops.gcd].

View all issues with TS status.

Discussion:

Addresses fund.ts.v2: JP 010, JP 011

By the current definition, `gcd((int64_t)1234, (int32_t)-2147483648)` is ill-formed (because `2147483648` is not representable as a value of `int32_t`.) We want to change this case to be well-formed. As long as both `|m|` and `|n|` are representable as values of the common type, absolute values can be calculate `d` without causing unspecified behavior, by converting `m` and `n` to the common type before taking the negation.

Suggested resolution:

> `|m|` shall be representable as a value of type `M` and `|n|` shall be representable as a value of type `N``|m|` and `|n|` shall be representable as a value of `common_type_t<M, N>`.

[Issues Telecon 16-Dec-2016]

Resolved by N4616

Proposed resolution:

This wording is relative to N4600.

1. Edit 13.1.2 [fund.ts.v2::numeric.ops.gcd] as indicated:

   >     template<class M, class N>
   >       constexpr common_type_t<M, N> gcd(M m, N n);

   > > -2- Requires: `|m|` shall be representable as a value of type `M` and `|n|` shall be representable as a value of type `N``|m|` and `|n|` shall be representable as a value of `common_type_t<M, N>`. [Note: These requirements ensure, for example, that `gcd(m, m) = |m|` is representable as a value of type `M`. — end note]

2. Edit 13.1.3 [fund.ts.v2::numeric.ops.lcm] as indicated:

   >     template<class M, class N>
   >       constexpr common_type_t<M, N> lcm(M m, N n);

   > > -2- Requires: `|m|` shall be representable as a value of type `M` and `|n|` shall be representable as a value of type `N``|m|` and `|n|` shall be representable as a value of `common_type_t<M, N>`. The least common multiple of `|m|` and `|n|` shall be representable as a value of type `common_type_t<M, N>`.

### 2793(i). Awkward conflation of trivial special members of `istream_iterator`

Section: 24.6.2.2 [istream.iterator.cons] Status: C++17 Submitter: Erich Keane Opened: 2016-11-09 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [istream.iterator.cons].

View all issues with C++17 status.

Discussion:

Addresses GB 68, US 154, US 155

The term 'literal type' is dangerous and misleading, as text using this term really wants to require that a constexpr constructor/initialization is called with a constant expression, but does not actually tie the selected constructor to the type being 'literal'.

Suggested resolution:

Verify the uses of the term in the Core and Library specifications and replace with something more precise where appropriate.

The conflation of trivial copy constructor and literal type is awkward. Not all literal types have trivial copy constructors, and not all types with trivial copy constructors are literal.

Suggested resolution:

Revise p5 as:

> Effects: Constructs a copy of `x`. If `T` has a trivial copy constructor, then this constructor shall be a trivial copy constructor. If `T` has a constexpr copy constructor, then this constructor shall be `constexpr`.

The requirement that the destructor is trivial if `T` is a literal type should be generalized to any type `T` with a trivial destructor — this encompasses all literal types, as they are required to have a trivial destructor.

Suggested resolution:

Revise p7 as:

> Effects: The iterator is destroyed. If `T` has a trivial destructor, then this destructor shall be a trivial destructor.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Change 24.6.2.2 [istream.iterator.cons] p1 as indicated:

>    >     see below istream_iterator();

>    > > -1- Effects: Constructs the end-of-stream iterator. If `T` is a literal type `is_trivially_constructible_v<T> == true`, then this constructor shall be ais a trivial, `constexpr` constructor.

> 2. Change 24.6.2.2 [istream.iterator.cons] p5 as indicated:

>    >     istream_iterator(const istream_iterator& x) = default;

>    > > -5- Effects: Constructs a copy of `x`. If `T` is a literal type `is_trivially_copyable_v<T> == true`, then this constructor shall beis a trivial copy constructor.

> 3. Change 24.6.2.2 [istream.iterator.cons] p7 as indicated:

>    >     ~istream_iterator() = default;

>    > > -7- Effects: The iterator is destroyed. If `T` is a literal type `is_trivially_destructible_v<T> == true`, then this destructor shall beis a trivial destructor.

[Issues Telecon 16-Dec-2016]

Resolved by the adoption of P0503R0

Proposed resolution:

Resolve by accepting the wording suggested by P0503R0.

### 2794(i). Missing requirements for allocator pointers

Section: 23.2.2 [container.requirements.general] Status: C++17 Submitter: Billy O'Neal III Opened: 2016-11-09 Last modified: 2017-07-30

Priority: 0

View other active issues in [container.requirements.general].

View all other issues in [container.requirements.general].

View all issues with C++17 status.

Discussion:

Addresses US 146

An allocator-aware contiguous container must require an allocator whose pointer type is a contiguous iterator. Otherwise, functions like data for `basic_string` and `vector` do not work correctly, along with many other expectations of the contiguous guarantee.

Suggested resolution:

Add a second sentence to 23.2.2 [container.requirements.general] p13:

> An allocator-aware contiguous container requires `allocator_traits<Allocator>::pointer` is a contiguous iterator.

[2016-11-12, Issaquah]

Sat PM: Move to 'Tentatively Ready'

Proposed resolution:

This wording is relative to N4606.

1. In 16.4.4.6 [allocator.requirements]/5, edit as follows:

   > -5- An allocator type `X` shall satisfy the requirements of `CopyConstructible` (17.6.3.1). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these pointer types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.3 [iterator.requirements]24.3.5.7 [random.access.iterators]) and of a contiguous iterator (24.3.1 [iterator.requirements.general]).

### 2795(i). §[global.functions] provides incorrect example of ADL use

Section: 16.4.6.4 [global.functions] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-11-09 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [global.functions].

View all issues with C++17 status.

Discussion:

Addresses GB 39

The example is supposed to highlight the 'otherwise specified' aspect of invoking ADL, yet there is no such specification. It is unlikely that we intend to explicitly qualify calls to operator functions, so they probably should be exempted from this restriction.

Suggested resolution:

Fix example (and referenced clause) to specify use of ADL, or exempt operators from this clause, and find a better example, probably using `swap`.

[2016-11-09, Jonathan comments and provides wording]

The current wording was added by DR 225(i).

[2016-11-10, Tim Song comments]

The "non-operator" seems to have been added at the wrong spot. The problem at issue is permission to call operator functions found via ADL, not permission for operator functions in the standard library to ADL all over the place. The problem is not unique to operator functions in the standard library — a significant portion of `<algorithm>` and `<numeric>` uses some operator (`==`, `<`, `+`, `*`, etc.) that may be picked up via ADL.

There is also an existing problem in that the example makes no sense anyway: the constraint in this paragraph only applies to non-members, yet `ostream_iterator::operator=` is a member function.

[2016-11-10, Tim Song and Jonathan agree on new wording]

The new wording still doesn't quite get it right:

> "calls to non-operator, non-member functions in the standard library do not use functions from another namespace which are found through argument-dependent name lookup" can be interpreted as saying that if a user writes "`std::equal(a, b, c, d)`", that call will not use ADL'd "`operator==`" because "`std::equal(a, b, c, d)`" is a "call" to a "non-operator, non-member function in the standard library".

The key point here is that "in the standard library" should be modifying "calls", not "function".

Previous resolution [SUPERSEDED]:

> This wording is relative to N4606.

> 1. Change 16.4.6.4 [global.functions] p4:

>    > Unless otherwise specified, non-operator, non-member functions in the standard library shall not use functions from another namespace which are found through argument-dependent name lookup (3.4.2). [Note: The phrase "unless otherwise specified" applies to cases such as the swappable with requirements (16.4.4.3 [swappable.requirements]). The exception for overloaded operators allowsis intended to allow argument-dependent lookup in cases like that of `ostream_iterator::operator=` (24.6.2.2):

>    > > Effects:

>    > > >     *out_stream << value;
>    > > >     if (delim != 0)
>    > > >       *out_stream << delim ;
>    > > >     return *this;

>    > > — end note]

[Issues Telecon 16-Dec-2016]

Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Change 16.4.6.4 [global.functions] p4:

   > Unless otherwise specified, calls made by functions in the standard library to non-operator, non-member functions in the standard library shalldo not use functions from another namespace which are found through argument-dependent name lookup (3.4.2). [Note: The phrase "unless otherwise specified" applies to cases such as the swappable with requirements (16.4.4.3 [swappable.requirements]). The exception for overloaded operators allowsis intended to allow argument-dependent lookup in cases like that of `ostream_iterator::operator=` (24.6.2.2):

   > > Effects:

   > > >     *out_stream << value;
   > > >     if (delim != 0)
   > > >       *out_stream << delim ;
   > > >     return *this;

   > > — end note]

### 2796(i). `tuple` should be a literal type

Section: 22.4.1 [tuple.general] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-11-09 Last modified: 2017-07-30

Priority: 2

View all other issues in [tuple.general].

View all issues with C++17 status.

Discussion:

Addresses US 109

`tuple` should be a literal type if its elements are literal types; it fails because the destructor is not necessarily trivial. It should follow the form of optional and variant, and mandate a trivial destructor if all types in `Types...` have a trivial destructor. It is not clear if `pair` has the same issue, as `pair` specifies data members `first` and `second`, and appears to have an implicitly declared and defined destructor.

Suggested resolution:

Document the destructor for `tuple`, and mandate that it is trivial if each of the elements in the `tuple` has a trivial destructor. Consider whether the same specification is needed for `pair`.

[2016-11-09, Jonathan provides wording]

[Issues Telecon 16-Dec-2016]

Move to Review; we think this is right, but are awaiting implementation experience.

Proposed resolution:

This wording is relative to N4606.

1. Add a new paragraph after 22.3.2 [pairs.pair] p2:

   > -2- The defaulted move and copy constructor, respectively, of `pair` shall be a `constexpr` function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a `constexpr` function. The destructor of `pair` shall be a trivial destructor if `(is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>)` is `true`.

2. Add a new paragraph after the class synopsis in 22.4.4 [tuple.tuple]:

   > -?- The destructor of `tuple` shall be a trivial destructor if `(is_trivially_destructible_v<Types> && ...)` is `true`.

### 2797(i). Trait precondition violations

Section: 21.3.3 [meta.type.synop] Status: Resolved Submitter: Russia Opened: 2016-11-09 Last modified: 2018-11-12

Priority: 2

View other active issues in [meta.type.synop].

View all other issues in [meta.type.synop].

View all issues with Resolved status.

Discussion:

Addresses RU 2

Failed prerequirement for the type trait must result in ill-formed program. Otherwise hard detectable errors will happen:

>     #include <type_traits>

>     struct foo;

>     void damage_type_trait() {
>       // must be ill-formed
>       std::is_constructible<foo, foo>::value;
>     }

>     struct foo{};

>     int main() {
>       static_assert(
>         // produces invalid result
>         std::is_constructible<foo, foo>::value,
>         "foo must be constructible from foo"
>       );
>     }

Suggested resolution:

Add to the end of the [meta.type.synop] section:

> Program is ill-formed if precondition for the type trait is violated.

[2016-11-09, Jonathan provides wording]

[Issues Telecon 16-Dec-2016]

Priority 2

[2017-03-02, Daniel comments]

LWG 2939(i) has been created to signal that some of our current type trait constraints are not quite correct and I recommend not to enforce the required diagnostics for traits that are sensitive to mismatches of the current approximate rules.

[2017-03-03, Kona Friday morning]

Unanimous consent to adopt this for C++17, but due to a misunderstanding, it wasn't on the ballot

Setting status to 'Ready' so we'll get it in immediately post-C++17

[2017-06-15 request from Daniel]

I don't believe that this should be "Ready"; I added the extra note to LWG 2797 *and* added the new issue 2939(i) exactly to *prevent* 2797 being accepted for C++17

Setting status back to 'Open'

[2018-08 Batavia Monday issue discussion]

Issues 2797(i), 2939(i), 3022(i), and 3099(i) are all closely related. Walter to write a paper resolving them.

[2018-11-11 Resolved by P1285R0, adopted in San Diego.]

Proposed resolution:

This wording is relative to N4606.

1. Add a new paragraph after 21.3.5.4 [meta.unary.prop] paragraph 3:

   > -?- If an instantiation of any template declared in this subclause fails to meet the preconditions, the program is ill-formed.

2. Change the specification for `alignment_of` in Table 39 in 21.3.6 [meta.unary.prop.query]:

   > Table 39 — Type property queries
   > `template <class T> struct alignment_of;` | `alignof(T)`. Requires: `alignof(T)` shall be a valid expression (5.3.6), otherwise the program is ill-formed

3. Change the specification for `is_base_of`, `is_convertible`, `is_callable`, and `is_nothrow_callable` in Table 40 in 21.3.7 [meta.rel]:

   > Table 40 — Type relationship predicates
   > `[…]`
   > `template <class Base, class Derived> struct is_base_of;` | […] | If `Base` and `Derived` are non-union class types and are different types (ignoring possible cv-qualifiers) then `Derived` shall be a complete type, otherwise the program is ill-formed. [Note: Base classes that are private, protected, or ambiguous are, nonetheless, base classes. — end note]
   > `template <class From, class To> struct is_convertible;` | see below | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types, otherwise the program is ill-formed.
   > `template <class Fn, class... ArgTypes, class R> struct is_callable< Fn(ArgTypes...), R>;` | […] | `Fn`, `R`, and all types in the parameter pack `ArgTypes` shall be complete types, (possibly cv-qualified) `void`, or arrays of unknown bound, otherwise the program is ill-formed.
   > `template <class Fn, class... ArgTypes, class R> struct is_nothrow_callable< Fn(ArgTypes...), R>;` | […] | `Fn`, `R`, and all types in the parameter pack `ArgTypes` shall be complete types, (possibly cv-qualified) `void`, or arrays of unknown bound, otherwise the program is ill-formed.

4. Add a new paragraph after 21.3.8 [meta.trans] paragraph 2:

   > -2- Each of the templates in this subclause shall be a `TransformationTrait` (20.15.1).

   > -?- If an instantiation of any template declared in this subclause fails to meet the Requires: preconditions, the program is ill-formed.

### 2798(i). Definition of path in terms of a string

Section: 31.12.6 [fs.class.path] Status: Resolved Submitter: Billy O'Neal III Opened: 2016-11-10 Last modified: 2017-07-18

Priority: 2

View all other issues in [fs.class.path].

View all issues with Resolved status.

Discussion:

Addresses US 44, LWG 2734(i)

The explicit definition of `path` in terms of a string requires that the abstraction be leaky. Consider that the meaning of the expression `p += '/'` has very different behavior in the case that `p` is empty; that a `path` can uselessly contain null characters; and that iterators must be constant to avoid having to reshuffle the packed string.

Suggested resolution:

Define member functions to express a `path` as a string, but define its state in terms of the abstract sequence of components (including the leading special components) already described by the iterator interface. Remove members that rely on arbitrary manipulation of a string value.

[2016-11-12, Issaquah]

Sat PM: "Looks good"

[Issues Telecon 16-Dec-2016]

Priority 2; should be addressed by omnibus Filesystem paper.

[Kona, 2017-03]

This is resolved by p0492r2.

Proposed resolution:

This wording is relative to N4606.

1. Edit [fs.path.concat] as follows:

   >     path& operator+=(const path& x);
   >     path& operator+=(const string_type& x);
   >     path& operator+=(basic_string_view<value_type> x);
   >     path& operator+=(const value_type * x);
   >     path& operator+=(value_type x);
   >     template <class Source>
   >         path& operator+=(const Source& x);
   >     template <class EcharT>
   >         path& operator+=(EcharT x);
   >     template <class Source>
   >         path& concat(const Source& x);
   >     template <class InputIterator>
   >         path& concat(InputIterator first, InputIterator last);

   > > -1- Postcondition: `native() == prior_native + effective-argument`, where `prior_native` is `native()` prior to the call to `operator+=`, and `effective-argument` is:

   > > - if `x` is present and is `const path&`, `x.native()`; otherwise,
   > > - if `source` is present, the effective range of `source` [ath.re]; otherwise,
   > > - >if `first` and `last` are present, the range `[first, last)`; otherwise,
   > > - `x`.

   > > If the value type of `effective-argument` would not be `path::value_type`, the acctual argument or argument range is first converted [ath.type.cv] so that `effective-argument` has value type `path::value_type`. Effects: Equivalent to `pathname.append(path(x).native())` [Note: This directly manipulates the value of `native()` and may not be portable between operating systems. — end note]

   > > -2- Returns: `*this`.

   >     template <class InputIterator>
   >         path& concat(InputIterator first, InputIterator last);

   > > -?- Effects: Equivalent to `pathname.append(path(first, last).native())` [Note: This directly manipulates the value of `native()` and may not be portable between operating systems. — end note]

   > > -?- Returns: `*this`.

### 2799(i). `noexcept`-specifications in `shared_future`

Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Zhihao Yuan Opened: 2016-11-11 Last modified: 2021-06-06

Priority: 2

View all other issues in [futures.shared.future].

View all issues with Resolved status.

Discussion:

Addresses GB 62

There is an implicit precondition on most `shared_future` operations that `valid() == true`, 30.6.7p3. The list of exempted functions seems copied directly from class `future`, and would also include copy operations for `shared_futures`, which are copyable. Similarly, this would be a wide contract that cannot throw, so those members would be marked `noexcept`.

Suggested resolution:

Revise p3:

"The effect of calling any member function other than the move constructor, the copy constructor, the destructor, the move-assignment operator, the copy-assignment operator, or `valid()` on a `shared_future` object for which `valid() == false` is undefined." […]

Add `noexcept` specification to the copy constructor and copy-assignment operator, in the class definition and where those members are specified.

[2016-11-10, Zhihao Yuan comments and provides wording]

This resolution should close LWG 2697(i) as well, but that one was filed against concurrent TS.

[Issues Telecon 16-Dec-2016]

This is also addressed (but in a slightly different way by P0516R0

[2017-11 Albuquerque Wednesday issue processing]

Resolved by P0516, adopted in Issaquah.

Proposed resolution:

This wording is relative to N4606.

> [Drafting note: This change supersedes the 1st among 3 edits in LWG 2556(i), please omit that edit and apply the one below.]

1. Modify [futures.unique_future]/3 as follows:

   > -3- The effect of calling any member function other than the destructor, the move-assignment operator, or `valid` the member functions `get`, `wait`, `wait_for`, or `wait_until` on a `future` object for which `valid() == false` is undefined. [Note: Implementations are encouraged to detect this case and throw an object of type `future_error` with an error condition of `future_errc::no_state`. — end note]

2. Change [futures.shared_future] as indicated:

   > -3- The effect of calling any member function other than the destructor, the move-assignment operator, or `valid()`the member functions `get`, `wait`, `wait_for`, or `wait_until` on a `shared_future` object for which `valid() == false` is undefined. [Note: Implementations are encouraged to detect this case and throw an object of type `future_error` with an error condition of `future_errc::no_state`. — end note]

   > >     namespace std {
   > >       template <class R>
   > >       class shared_future {
   > >       public:
   > >         shared_future() noexcept;
   > >         shared_future(const shared_future& rhs) noexcept;
   > >         shared_future(future<R>&&) noexcept;
   > >         shared_future(shared_future&& rhs) noexcept;
   > >         ~shared_future();
   > >         shared_future& operator=(const shared_future& rhs) noexcept;
   > >         shared_future& operator=(shared_future&& rhs) noexcept;
   > >         […]
   > >       };
   > >       […]
   > >     }

   > […]

   >     shared_future(const shared_future& rhs) noexcept;

   > > -7- Effects: […]

   > […]

   >     shared_future& operator=(const shared_future& rhs) noexcept;

   > > -14- Effects: […]

### 2800(i). `constexpr swap`

Section: 22.2.2 [utility.swap] Status: Resolved Submitter: United States Opened: 2016-11-09 Last modified: 2020-09-06

Priority: 3

View all other issues in [utility.swap].

View all issues with Resolved status.

Discussion:

Addresses US 108

`swap` is a critical function in the standard library, and should be declared `constexpr` to support more widespread support for `constexpr` in libraries. This was proposed in P0202R1 which was reviewed favourably at Oulu, but the widespread changes to the `<algorithm>` header were too risky and unproven for C++17. We should not lose `constexpr` support for the much simpler (and more important) `<utility>` functions because they were attached to a larger paper. Similarly, the fundamental value wrappers, `pair` and `tuple`, should have `constexpr swap` functions, and the same should be considered for `optional` and `variant`. It is not possible to mark `swap` for `std::array` as `constexpr` without adopting the rest of the P0202R1 though, or rewriting the specification for array swap to not use `swap_ranges`.

Suggested resolution:

Adopt the changes to the `<utility>` header proposed in P0202R1, i.e., only bullets C, D, and E. In addition, mark the `swap` functions of `pair` and `tuple` as `constexpr`, and consider doing the same for `optional` and `variant`.

[Issues Telecon 16-Dec-2016]

Priority 3

[2017-11 Albuquerque Wednesday issue processing]

Status to Open; we don't want to do this yet; gated on Core issue 1581. See also 2897(i).

[2017-11 Albuquerque Thursday]

It looks like 1581 is going to be resolved this week, so we should revisit soon.

[2018-06]

Resolved by the adoption of P0879R0

Proposed resolution:

### 2801(i). Default-constructibility of `unique_ptr`

Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++17 Submitter: United States Opened: 2016-11-09 Last modified: 2017-07-30

Priority: 2

View all other issues in [unique.ptr.single.ctor].

View all issues with C++17 status.

Discussion:

Addresses US 122

`unique_ptr` should not satisfy `is_constructible_v<unique_ptr<T, D>>` unless `D` is `DefaultConstructible` and not a pointer type. This is important for interactions with `pair`, `tuple`, and `variant` constructors that rely on the `is_default_constructible` trait.

Suggested resolution:

Add a Remarks: clause to constrain the default constructor to not exist unless the Requires clause is satisfied.

[Issues Telecon 16-Dec-2016]

Priority 2; Howard and Ville to provide wording.

[2016-12-24: Howard and Ville provided wording.]

[2017-03-02, Kona, STL comments and tweaks the wording]

LWG discussed this issue, and we liked it, but we wanted to tweak the PR. I ran this past Ville (who drafted the PR with Howard), and he was in favor of tweaking this.

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

1. Modify the synopsis in 20.3.1.3 [unique.ptr.single] as follows:

   >     […]
   >     constexpr unique_ptr(nullptr_t) noexcept;
   >         : unique_ptr() { }
   >     […]

2. Modify 20.3.1.3.2 [unique.ptr.single.ctor] as follows:

   >     constexpr unique_ptr() noexcept;
   >     constexpr unique_ptr(nullptr_t) noexcept;

   > > -1- Requires: `D` shall satisfy the requirements of `DefaultConstructible` (Table 22), and that construction shall not throw an exception.

   > > -2- Effects: Constructs a `unique_ptr` object that owns nothing, value-initializing the stored pointer and the stored deleter.

   > > -3- Postconditions: `get() == nullptr`. `get_deleter()` returns a reference to the stored deleter.

   > > -4- Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument `D`, the program is ill-formed. If `is_pointer_v<deleter_type>` is `true` or `is_default_constructible_v<deleter_type>` is `false`, this constructor shall not participate in overload resolution.

   >     explicit unique_ptr(pointer p) noexcept;

   > > […]

   > > -8- Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument `D`, the program is ill-formed. If `is_pointer_v<deleter_type>` is `true` or `is_default_constructible_v<deleter_type>` is `false`, this constructor shall not participate in overload resolution.

3. Modify the synopsis in 20.3.1.4 [unique.ptr.runtime] as follows:

   >     […]
   >     constexpr unique_ptr(nullptr_t) noexcept; : unique_ptr() { }
   >     […]

4. Modify 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:

   >     template <class U> explicit unique_ptr(U p) noexcept;

   > > This constructor behaves the same as the constructor that takes a pointer parameter in the primary template except that the following constraints are added for it to participate in overload resolution

   > > - `U` is the same type as `pointer`, or

   > > - `pointer` is the same type as `element_type*`, `U` is a pointer type `V*`, and `V(*)[]` is convertible to `element_type(*)[]`.

   >     template <class U> unique_ptr(U p, see below d) noexcept;
   >     template <class U> unique_ptr(U p, see below d) noexcept;

   > > 1 These constructors behave the same as the constructors that take a pointer parameter in the primary template except that they shall not participate in overload resolution unless either

### 2802(i). `shared_ptr` constructor requirements for a deleter

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2016-11-09 Last modified: 2020-09-06

Priority: 2

View other active issues in [util.smartptr.shared.const].

View all other issues in [util.smartptr.shared.const].

View all issues with C++17 status.

Discussion:

Addresses US 127

It should suffice for the deleter `D` to be nothrow move-constructible. However, to avoid potentially leaking the pointer `p` if `D` is also copy-constructible when copying the argument by-value, we should continue to require the copy constructor does not throw if `D` is `CopyConstructible`.

Proposed change:

Relax the requirement the `D` be `CopyConstructible` to simply require that `D` be `MoveConstructible`. Clarify the requirement that construction of any of the arguments passed by-value shall not throw exceptions. Note that we have library-wide wording in clause 17 that says any type supported by the library, not just this delete, shall not throw exceptions from its destructor, so that wording could be editorially removed. Similarly, the requirements that `A` shall be an allocator satisfy that neither constructor nor destructor for `A` can throw.

[2016-12-16, Issues Telecon]

Priority 3; Jonathan to provide wording.

[2017-02-23, Jonathan comments and suggests wording]

I don't think the Clause 17 wording in [res.on.functions] is sufficient to require that the delete expression is well-formed. A class-specific deallocation function ([class.free]) would not be covered by [res.on.functions] and so could throw:

>     struct Y { void operator delete(void*) noexcept(false) { throw 1; } };

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   >     template<class Y, class D> shared_ptr(Y* p, D d);
   >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
   >     template <class D> shared_ptr(nullptr_t p, D d);
   >     template <class D, class A> shared_ptr(nullptr_t p, D d, A a);

   > > -8- Requires: `D` shall be `CopyMoveConstructible` and such construction of `d` and a deleter of type `D` initialized with `std::move(d)` shall not throw exceptions. The destructor of `D` shall not throw exceptions. The expression `d(p)` shall be well formed, shall have well -defined behavior, and shall not throw exceptions. `A` shall be an allocator (17.5.3.5). The copy constructor and destructor of `A` shall not throw exceptions. When `T` is […].

### 2803(i). `hash` for arithmetic, pointer and standard library types should not throw

Section: 22.10.19 [unord.hash] Status: Resolved Submitter: United States Opened: 2016-11-09 Last modified: 2020-09-06

Priority: 3

View all other issues in [unord.hash].

View all issues with Resolved status.

Discussion:

Addresses US 140

Specializations of `std::hash` for arithmetic, pointer, and standard library types should not be allowed to throw. The constructors, assignment operators, and function call operator should all be marked as `noexcept`. It might be reasonable to consider making this a binding requirement on user specializations of the `hash` template as well (in p1) but that may be big a change to make at this stage.

[Issues Telecon 16-Dec-2016]

Priority 2, Nico to provide wording.

[2017-02-07, Nico comments]

Concrete wording is provided in P0599.

[2017-03-12, post-Kona]

Resolved by P0599R0.

Proposed resolution:

### 2804(i). Unconditional `constexpr` default constructor for `istream_iterator`

Section: 24.6.2.2 [istream.iterator.cons] Status: C++17 Submitter: United States Opened: 2016-11-09 Last modified: 2017-07-30

Priority: 0

View all other issues in [istream.iterator.cons].

View all issues with C++17 status.

Discussion:

Addresses US 152

see below for the default constructor should simply be spelled `constexpr`. The current declaration looks like a member function, not a constructor, and the `constexpr` keyword implicitly does not apply unless the instantiation could make it so, under the guarantees al ready present in the Effects clause.

Proposed change:

Replace see below with `constexpr` in the declaration of the default constructor for `istream_iterator` in the class definition, and function specification.

[Issues Telecon 16-Dec-2016]

Jonathan provides wording, Move to Tentatively Ready

Proposed resolution:

In the class synopsis in 24.6.1 [istream.iterator] change the default constructor:

>     see belowconstexpr istream_iterator();
>     istream_iterator(istream_type& s);
>     istream_iterator(const istream_iterator& x) = default;
>     ~istream_iterator() = default;

Change [istream.iterator.cons] before paragraph 1:

>     see belowconstexpr istream_iterator();

>     -1- Effects: ...

### 2805(i). `void` and reference type alternatives in `variant`, `variant<>` and `index()`

Section: 22.6 [variant], 22.6.3 [variant.variant], 22.6.3.2 [variant.ctor] Status: Resolved Submitter: Switzerland Opened: 2016-11-10 Last modified: 2020-09-06

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Addresses CH 3, CH 4, CH 5, CH 6, CH 8

1. `variant` allows reference types as alternatives; `optional` explicitly forbids to be instantiated for reference types. This is inconsistent.

2. `variant<int, void>` should be as usable as `variant<int>`.

3. `variant<>` should not have an `index()` function.

4. Clarify the intended behavior of `variant` for alternative types that are references.

5. Clarify `variant` construction.

Proposed change:

1. Consider allowing reference types for both or none.

2. —

3. Consider specifying a specialization for `variant<>` like:

   >     template<>
   >     class variant<>
   >     {
   >     public:
   >       variant() = delete;
   >       variant(const variant&) = delete;
   >       variant& operator=(variant const&) = delete;
   >     };

4. Add a respective note.

5. Add a note that `variant<>` cannot be constructed.

[Issues Telecon 16-Dec-2016]

Resolved by the adoption of P0501R0.

Proposed resolution:

### 2806(i). Base class of `bad_optional_access`

Section: 22.5.6 [optional.bad.access] Status: C++17 Submitter: Great Britain Opened: 2016-11-09 Last modified: 2017-07-30

Priority: 1

View all issues with C++17 status.

Discussion:

Addresses GB 49

LEWG 72 suggests changing the base class of `std::bad_optional_access`, but the issue appears to have been forgotten.

Proposed change:

Address LEWG issue 72, either changing it for C++17 or closing the issue.

[Issues Telecon 16-Dec-2016]

Priority 1; Marshall provides wording

Not the most important bug in the world, but we can't change it after we ship C++17, hence the P1.

[Kona 2017-03-01]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

Changes are relative to N4604.

In the class synopsis in 20.6.5 [optional.bad_optional_access] change:

>         class bad_optional_access : public exceptionlogic_error {

### 2807(i). `std::invoke` should use `std::is_nothrow_callable`

Section: 22.10.5 [func.invoke] Status: C++17 Submitter: Great Britain Opened: 2016-11-09 Last modified: 2020-09-06

Priority: 3

View all other issues in [func.invoke].

View all issues with C++17 status.

Discussion:

Addresses GB 53

`std::invoke` can be made trivially `noexcept` using the new `std::is_nothrow_callable` trait:

Proposed change:

Add the exception specifier `noexcept(is_nothrow_callable_v<F&&(Args&&...)>)` to `std::invoke`.

[Issues Telecon 16-Dec-2016]

Priority 3

[2017-02-28, Daniel comments and provides wording]

The following wording assumes that D0604R0 would be accepted, therefore uses `is_nothrow_invocable_v` instead of the suggested current trait `is_nothrow_callable_v`

[Kona 2017-03-01]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Change 22.10.2 [functional.syn], header `<functional>` synopsis, as indicated:

   >     // 20.14.4, invoke
   >     template <class F, class... Args>
   >       invoke_result_t<F, Args...>result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_v<F, Args...>);

2. Change 22.10.5 [func.invoke] as indicated:

   >     template <class F, class... Args>
   >       invoke_result_t<F, Args...>result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_v<F, Args...>);

### 2808(i). Requirements for `fpos` and `stateT`

Section: 31.5.3.3 [fpos.operations] Status: Resolved Submitter: Great Britain Opened: 2016-11-09 Last modified: 2020-05-12

Priority: 4

View all other issues in [fpos.operations].

View all issues with Resolved status.

Discussion:

Addresses GB 60

The requirements on the `stateT` type used to instantiate class template `fpos` are not clear, and the following Table 108 — "Position type requirements" is a bit of a mess. This is old wording, and should be cleaned up with better terminology from the Clause 17 Requirements. For example, `stateT` might be require `CopyConstructible`?, `CopyAssignable`?, and `Destructible`. Several entries in the final column of the table appear to be post-conditions, but without the `post` markup to clarify they are not assertions or preconditions. They frequently refer to identifiers that do not apply to all entries in their corresponding `Expression` column, leaving some expressions without a clearly defined semantic. If `stateT` is a trivial type, is `fpos` also a trivial type, or is a default constructor not required/supported?

Proposed change:

Clarify the requirements and the table.

[Issues Telecon 16-Dec-2016]

Priority 4; no consensus for any concrete change

[2019-03-17; Daniel comments]

With the acceptance of P0759R1 at the Rapperswil 2018 meeting this issue should be closed as Resolved (Please note that this paper resolves a historic NB comment that was originally written against C++17 but was at that time responded: "invite a paper if anybody wants to change it - no concensus for change").

[2020-05-12; Reflector discussions]

Resolved by P0759R1.

Rationale:

Resolved by P0759R1.

Proposed resolution:

### 2809(i). `variant` hash requirements

Section: 22.6.12 [variant.hash] Status: Resolved Submitter: Great Britain Opened: 2016-11-09 Last modified: 2020-09-06

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Addresses GB 69

The paragraph is really trying to say two different things, and should be split into two paragraphs, using standard terminology.

Proposed change:

The first sentence should become a Requires: clause, as it dictates requirements to callers.

The second sentence should be a Remarks: clause, at is a normative requirement on the implementation.

[Issues Telecon 16-Dec-2016]

Resolved by the adoption of P0513R0.

Proposed resolution:

### 2810(i). `use_count` and `unique` in `shared_ptr`

Section: 20.3.2.2 [util.smartptr.shared] Status: Resolved Submitter: Marshall Clow Opened: 2016-11-09 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [util.smartptr.shared].

View all issues with Resolved status.

Discussion:

Addresses CA 14

The removal of the "debug only" restriction for `use_count()` and `unique()` in `shared_ptr` introduced a bug: in order for `unique()` to produce a useful and reliable value, it needs a synchronize clause to ensure that prior accesses through another reference are visible to the successful caller of `unique()`. Many current implementations use a relaxed load, and do not provide this guarantee, since it's not stated in the Standard. For debug/hint usage that was OK. Without it the specification is unclear and misleading.

Proposed change:

A solution could make `unique()` use `memory_order_acquire`, and specifying that reference count decrement operations synchronize with `unique()`. This won't provide sequential consistency but may be useful. We could also specify `use_count()` as only providing an unreliable hint of the actual count, or deprecate it.

[Issues Telecon 16-Dec-2016]

Resolved by adoption of P0521R0.

Proposed resolution:

### 2812(i). Range access is available with `<string_view>`

Section: 24.7 [iterator.range], 27.3.2 [string.view.synop] Status: C++17 Submitter: Johel Ernesto Guerrero Peña Opened: 2016-10-29 Last modified: 2017-07-30

Priority: 0

View all other issues in [iterator.range].

View all issues with C++17 status.

Discussion:

27.3.2 [string.view.synop]/1 states

> The function templates defined in 20.2.2 and 24.7 are available when `<string_view>` is included.

24.7 [iterator.range], in p1 is missing `<string_view>`.

[Issues Telecon 16-Dec-2016]

Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Edit 24.7 [iterator.range] p1 as indicated:

   > -1- In addition to being available via inclusion of the `<iterator>` header, the function templates in 24.7 are available when any of the following headers are included: `<array>`, `<deque>`, `<forward_list>`, `<list>`, `<map>`, `<regex>`, `<set>`, `<string>`, `<string_view>`, `<unordered_map>`, `<unordered_set>`, and `<vector>`.

### 2813(i). `std::function` should not return dangling references

Section: 22.10.17.3.2 [func.wrap.func.con] Status: Resolved Submitter: Brian Bi Opened: 2016-11-03 Last modified: 2022-11-25

Priority: 2

View all other issues in [func.wrap.func.con].

View all issues with Resolved status.

Discussion:

If a `std::function` has a reference as a return type, and that reference binds to a prvalue returned by the callable that it wraps, then the reference is always dangling. Because any use of such a reference results in undefined behaviour, the `std::function` should not be allowed to be initialized with such a callable. Instead, the program should be ill-formed.

A minimal example of well-formed code under the current standard that exhibits this issue:

>     #include <functional>

>     int main()
>     {
>       std::function<const int&()> F([]{ return 42; });
>       int x = F(); // oops!
>     }

[2016-11-22, David Krauss comments and suggests wording]

Indirect bindings may also introduce temporaries inside `std::function`, e.g.:

>     void f(std::function<long const&()>); // Retains an observer to a long.

>     void g() {
>       int v;
>       f([&]()->int& { return v; } ); // int lvalue binds to long const& through a temporary.
>     }

A fix has been implemented. Conversions that may be conversion operators are allowed, though, because those can produce legitimate glvalues. Before adopting this, it need to be considered considered whether there should be SFINAE or a hard error.

[Issues Telecon 16-Dec-2016]

Priority 2

[2016-07, Toronto Saturday afternoon issues processing]

Billy to work with Brian to rework PR. Status to Open

[2018-08-23 Batavia Issues processing]

Really needs a language change to fix this. Status to EWG.

[2022-08-24 Resolved by P2255R2. Status changed: EWG → Resolved.]

[2022-11-25; see EWG 1370]

Proposed resolution:

This wording is relative to N4618.

1. Add a second paragraph to the remarks section of 22.10.17.3.2 [func.wrap.func.con]:

   >     template<class F> function(F f);

   > > -7- Requires: `F` shall be `CopyConstructible`.

   > > -8- Remarks: This constructor template shall not participate in overload resolution unless

   > > - `F` is Lvalue-Callable (22.10.17.3 [func.wrap.func]) for argument types `ArgTypes...` and return type `R`, and

   > > - If `R` is type "reference to `T`" and `INVOKE(ArgTypes...)` has value category `V` and type `U`:

   > >   - `V` is a prvalue, `U` is a class type, and `T` is not reference-related (9.5.4 [dcl.init.ref]) to `U`, and

   > >   - `V` is an lvalue or xvalue, and either `U` is a class type or `T` is reference-related to `U`.

   > > […]

### 2814(i). [fund.ts.v2] `to_array` should take rvalue reference as well

Section: 4.2.1 [fund.ts.v2::func.wrap.func.con] Status: Resolved Submitter: Zhihao Yuan Opened: 2016-11-07 Last modified: 2020-09-06

Priority: 3

View all other issues in [fund.ts.v2::func.wrap.func.con].

View all issues with Resolved status.

Discussion:

Addresses: fund.ts.v2

C++ doesn't have a prvalue expression of array type, but rvalue arrays can still come from different kinds of sources:

1. C99 compound literals `(int[]) {2, 4}`,

2. `std::move(arr)`,

3. Deduction `to_array<int const>({ 2, 4 })`.

   > See also CWG 1591: Deducing array bound and element type from initializer list.

For 3), users are "abusing" `to_array` to get access to uniform initialization to benefit from initializing elements through braced-init-list and/or better narrowing conversion support.

We should just add rvalue reference support to `to_array`.

[Issues Telecon 16-Dec-2016]

Status to LEWG

[2017-02 in Kona, LEWG responds]

Would like a small paper; see examples before and after. How does this affect overload resolution?

[2017-06-02 Issues Telecon]

Leave status as LEWG; priority 3

Previous resolution [SUPERSEDED]:

> This wording is relative to N4600.

> 1. Add the following signature to 9.2.1 [fund.ts.v2::header.array.synop]:

>    >     // 9.2.2, Array creation functions
>    >     template <class D = void, class... Types>
>    >       constexpr array<VT, sizeof...(Types)> make_array(Types&&... t);
>    >     template <class T, size_t N>
>    >       constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]);
>    >     template <class T, size_t N>
>    >       constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);

> 2. Modify 9.2.2 [fund.ts.v2::container.array.creation] as follows:

>    >     template <class T, size_t N>
>    >       constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]);
>    >     template <class T, size_t N>
>    >       constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);

>    > > -6- Returns: For all `i`, `0 ≤ i < N`, aAn `array<remove_cv_t<T>, N>` such that each element is copy-initialized with the corresponding element of `a` initialized with `{ a[i]... }` for the first form, or `{ std::move(a[i])... }` for the second form..

[2020-05-10; Reflector discussions]

Resolved by adoption of P0325R4 and P2081R1.

Rationale:

Resolved by P0325R4 and P2081R1.

Proposed resolution:

### 2816(i). `resize_file` has impossible postcondition

Section: 31.12.13.34 [fs.op.resize.file] Status: C++20 Submitter: Richard Smith Opened: 2016-11-07 Last modified: 2021-06-06

Priority: 3

View all issues with C++20 status.

Discussion:

`resize_file` has this postcondition (after resolving late comment 42, see P0489R0):

> Postcondition: `file_size(p) == new_size`.

This is impossible for an implementation to satisfy, due to the possibility of file system races. This is not actually a postcondition; rather, it is an effect that need no longer hold when the function returns.

[Issues Telecon 16-Dec-2016]

Priority 3

[2018-01-16, Jonathan provides wording]

[2018-1-26 issues processing telecon]

Status to 'Tentatively Ready'

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

> [Drafting note: I considered a slightly more verbose form: "Causes the size in bytes of the file `p` resolves to, as determined by `file_size` ( [fs.op.file_size]), to be equal to `new_size`, as if by POSIX `truncate`." but I don't think it's an improvement. The intent of the proposed wording is that if either `file_size(p)` or `truncate(p.c_str())` would fail then an error occurs, but no call to `file_size` is required, and file system races might change the size before any such call does occur.]

1. Modify [fs.op.resize_file] as indicated:

   >     void resize_file(const path& p, uintmax_t new_size);
   >     void resize_file(const path& p, uintmax_t new_size, error_code& ec) noexcept;

   > > -1- Postconditions: `file_size(p) == new_size` Effects: Causes the size that would be returned by `file_size(p)` to be equal to `new_size`, as if by POSIX `truncate`.

   > > -2- Throws: As specified in 31.12.5 [fs.err.report].

   > > -3- Remarks: Achieves its postconditions as if by POSIX `truncate()`.

### 2817(i). `std::hash` for `nullptr_t`

Section: 22.10.19 [unord.hash] Status: Resolved Submitter: Marshall Clow Opened: 2016-11-10 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [unord.hash].

View all issues with Resolved status.

Discussion:

In the list of types that the library provides `std::hash` specializations, there is an entry for `hash<T*>`, but none entry for `hash<nullptr_t>`.

[2016-11-13, Daniel provides wording]

[2016-11-28]

Resolved by P0513R0

Proposed resolution:

This wording is relative to N4606.

1. Change 22.10 [function.objects], header `<functional>` synopsis, as indicated:

   >     […]

   >     // Hash function specializations
   >     template <> struct hash<bool>;
   >     […]
   >     template <> struct hash<long double>;
   >     template <> struct hash<nullptr_t>;

   >     template<class T> struct hash<T*>;
   >     […]

2. Change 22.10.19 [unord.hash] before p2, as indicated:

   >     template <> struct hash<bool>;
   >     […]
   >     template <> struct hash<long double>;
   >     template <> struct hash<nullptr_t>;
   >     template<class T> struct hash<T*>;

### 2818(i). `"::std::"` everywhere rule needs tweaking

Section: 16.4.2.2 [contents] Status: C++23 Submitter: Tim Song Opened: 2016-11-11 Last modified: 2023-11-22

Priority: 2

View all other issues in [contents].

View all issues with C++23 status.

Discussion:

[contents]/3 says

> Whenever a name `x` defined in the standard library is mentioned, the name `x` is assumed to be fully qualified as `::std::x`, unless explicitly described otherwise. For example, if the Effects section for library function `F` is described as calling library function `G`, the function `::std::G` is meant.

With the introduction of nested namespaces inside `std`, this rule needs tweaking. For instance, `time_point_cast`'s Returns clause says "`time_point<Clock, ToDuration>(duration_cast<ToDuration>(t.time_since_epoch()))`"; that reference to `duration_cast` obviously means `::std::chrono::duration_cast`, not `::std::duration_cast`, which doesn't exist.

[Issues Telecon 16-Dec-2016]

Priority 2; Jonathan to provide wording

[2019 Cologne Wednesday night]

Geoffrey suggested editing 16.4.2.2 [contents]/2 to mention the case when we're defining things in a sub-namespace.

Jonathan to word this.

[2020-02-14, Prague; Walter provides wording]

[2020-10-02; Issue processing telecon: new wording from Jens]

Use "Simplified suggestion" in 13 June 2020 email from Jens.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 16.4.2.2 [contents] as indicated:

>    > -3- Whenever a name `x` defined in the standard library is mentioned, the name `x` is assumed to be fully qualified as `::std::x`, unless explicitly described otherwise. For example, if the Effects: element for library function `F` is described as calling library function `G`, the function `::std::G` is meant.Let `x` be a name specified by the standard library via a declaration in namespace `std` or in a subnamespace of namespace `std`. Whenever `x` is used as an unqualified name in a further specification, it is assumed to correspond to the same `x` that would be found via unqualified name lookup (6.5.3 [basic.lookup.unqual]) performed at that point of use. Similarly, whenever `x` is used as a qualified name in a further specification, it is assumed to correspond to the same `x` that would be found via qualified name lookup (6.5.5 [basic.lookup.qual]) performed at that point of use. [Note: Such lookups can never fail in a well-formed program. — end note] [Example: If an Effects: element for a library function `F` specifies that library function `G` is to be used, the function `::std::G` is intended. — end example]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 16.4.2.2 [contents] as indicated:

>    > [Drafting note: Consider adding a note clarifying that the unqualified lookup does not perform ADL. ]

>    > -3- Whenever a name `x` defined in the standard library is mentioned, the name `x` is assumed to be fully qualified as `::std::x`, unless explicitly described otherwise. For example, if the Effects: element for library function `F` is described as calling library function `G`, the function `::std::G` is meant. Whenever an unqualified name `x` is used in the specification of a declaration `D` in clauses 16-32, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of `D`. Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of `D`. [Example: The reference to `is_array_v` in the specification of `std::to_array` (23.3.3.6 [array.creation]) refers to `::std::is_array_v`. -- end example] [Note: Operators in expressions 12.2.2.3 [over.match.oper] are not so constrained; see 16.4.6.4 [global.functions]. -- end note]

[2020-11-04; Jens provides improved wording]

[2020-11-06; Reflector discussion]

Casey suggests to insert "or Annex D" after "in clauses 16-32". This insertion has been performed during reflector discussions immediately because it seemed editorial.

[2020-11-15; Reflector poll]

Set priority status to Tentatively Ready after seven votes in favour during reflector discussions.

[2020-11-22, Tim Song reopens]

The references to get in 25.5.4.1 [range.subrange.general] and 25.7.23.2 [range.elements.view] need to be qualified as they would otherwise refer to `std::ranges::get` instead of `std::get`. Additionally, [expos.only.func] needs to clarify that the lookup there also takes place from within namespace `std`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4868.

> 1. Modify 16.4.2.2 [contents] as indicated:

>    > -3- Whenever a name `x` defined in the standard library is mentioned, the name `x` is assumed to be fully qualified as `::std::x`, unless explicitly described otherwise. For example, if the Effects: element for library function `F` is described as calling library function `G`, the function `::std::G` is meant. Whenever an unqualified name `x` is used in the specification of a declaration `D` in clauses 16-32 or Annex D, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of `D`. [Note ?: Argument-dependent lookup is not performed. — end note] Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of `D`. [Example: The reference to `is_array_v` in the specification of `std::to_array` (23.3.3.6 [array.creation]) refers to `::std::is_array_v`. — end example] [Note ?: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]

> 2. Remove [fs.req.namespace] in its entirety:

>    > 29.11.3.2 Namespaces and headers [fs.req.namespace]

>    > -1- Unless otherwise specified, references to entities described in subclause 31.12 [filesystems] are assumed to be qualified with `::std::filesystem::`.

[2021-05-20; Jens Maurer provides an updated proposed resolution]

[2021-05-23; Daniel provides some additional tweaks to the updated proposed resolution]

[2021-05-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify [expos.only.func] as indicated:

   > -2- The following are defined for exposition only to aid in the specification of the library:

   > >     namespace std {
   > >       template<class T> constexpr decay_t<T> decay-copy(T&& v)
   > >           noexcept(is_nothrow_convertible_v<T, decay_t<T>>) // exposition only
   > >         { return std::forward<T>(v); }

   > >       constexpr auto synth-three-way =
   > >         []<class T, class U>(const T& t, const U& u)
   > >           requires requires {
   > >           { t < u } -> boolean-testable;
   > >           { u < t } -> boolean-testable;
   > >           }
   > >         {
   > >           if constexpr (three_way_comparable_with<T, U>) {
   > >             return t <=> u;
   > >           } else {
   > >             if (t < u) return weak_ordering::less;
   > >             if (u < t) return weak_ordering::greater;
   > >             return weak_ordering::equivalent;
   > >           }
   > >         };

   > >       template<class T, class U=T>
   > >       using synth-three-way-result = decltype(synth-three-way(declval<T&>(), declval<U&>()));
   > >     }

2. Modify 16.4.2.2 [contents] as indicated:

   > -3- Whenever a name `x` defined in the standard library is mentioned, the name `x` is assumed to be fully qualified as `::std::x`, unless explicitly described otherwise. For example, if the Effects: element for library function `F` is described as calling library function `G`, the function `::std::G` is meant. Whenever an unqualified name `x` is used in the specification of a declaration `D` in clauses 16-32 or Annex D, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of `D`. [Note ?: Argument-dependent lookup is not performed. — end note] Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of `D`. [Example: The reference to `is_array_v` in the specification of `std::to_array` (23.3.3.6 [array.creation]) refers to `::std::is_array_v`. — end example] [Note ?: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]

3. Modify 25.5.4.1 [range.subrange.general] as indicated:

   >     template<class T>
   >       concept pair-like =              // exposition only
   >         !is_reference_v<T> && requires(T t) {
   >           typename tuple_size<T>::type; // ensures tuple_size<T> is complete
   >           requires derived_from<tuple_size<T>, integral_constant<size_t, 2>>;
   >           typename tuple_element_t<0, remove_const_t<T>>;
   >           typename tuple_element_t<1, remove_const_t<T>>;
   >           { std::get<0>(t) } -> convertible_to<const tuple_element_t<0, T>&>;
   >           { std::get<1>(t) } -> convertible_to<const tuple_element_t<1, T>&>;
   >         };

4. Modify 25.7.23.2 [range.elements.view] as indicated:

   >     template<class T, size_t N>
   >       concept has-tuple-element = // exposition only
   >         requires(T t) {
   >           typename tuple_size<T>::type;
   >           requires N <tuple_size_v<T>;
   >           typename tuple_element_t<N, T>;
   >           { std::get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
   >         };

5. Modify 25.7.23.3 [range.elements.iterator] as indicated:

   > -2- The member typedef-name `iterator_category` is defined if and only if `Base` models `forward_range`. In that case, `iterator_category` is defined as follows: […]

   > 1. (2.1) — If `std::get<N>(*current_)` is an rvalue, `iterator_category` denotes `input_iterator_tag`.

   > 2. […]

   >     static constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition only

   > > -3- Effects: Equivalent to:

   > > >     if constexpr (is_reference_v<range_reference_t<Base>>) {
   > > >       return std::get<N>(*i);
   > > >     } else {
   > > >       using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>;
   > > >       return static_cast<E>(std::get<N>(*i));
   > > >     }

6. Remove [fs.req.namespace] in its entirety:

   > 29.11.3.2 Namespaces and headers [fs.req.namespace]

   > -1- Unless otherwise specified, references to entities described in subclause 31.12 [filesystems] are assumed to be qualified with `::std::filesystem::`.

### 2820(i). Clarify `<cstdint>` macros

Section: 17.4.1 [cstdint.syn] Status: C++23 Submitter: Thomas Köppe Opened: 2016-11-12 Last modified: 2023-11-22

Priority: 3

View all other issues in [cstdint.syn].

View all issues with C++23 status.

Discussion:

I would like clarification from LWG regarding the various limit macros like `INT_8_MIN` in `<cstdint>`, in pursuit of editorial cleanup of this header's synopsis. I have two questions:

1. At present, macros like `INT_8_MIN` that correspond to the optional type `int8_t` are required (unconditionally), whereas the underlying type to which they appertain is only optional. Is this deliberate? Should the macros also be optional?

2. Is it deliberate that C++ only specifies sized aliases for the sizes 8, 16, 32 and 64, whereas the corresponding C header allows type aliases and macros for arbitrary sizes for implementations that choose to provide extended integer types? Is the C++ wording more restrictive by accident?

[2017-01-27 Telecon]

Priority 3

[2017-03-04, Kona]

C11 ties the macro names to the existence of the types. Marshall to research the second question.

Close 2764(i) as a duplicate of this issue.

[2017-03-18, Thomas comments and provides wording]

This is as close as I can get to the C wording without resolving part (a) of the issue (whether we deliberately don't allow sized type aliases for sizes other than 8, 16, 32, 64, a departure from C). Once we resolve part (a), we need to revisit `<cinttypes>` and fix up the synopsis (perhaps to get rid of `N`) and add similar wording as the one below to make the formatting macros for the fixed-width types optional. For historical interest, this issue is related to LWG 553(i) and LWG 841(i).

[2016-07, Toronto Saturday afternoon issues processing]

Status to Open

Previous resolution: [SUPERSEDED]

> This wording is relative to N4640.

> 1. Append the following content to 17.4.1 [cstdint.syn] p2:

>    > -2- The header defines all types and macros the same as the C standard library header `<stdint.h>`. In particular, for each of the fixed-width types (`int8_t`, `int16_t`, `int32_t`, `int64_t`, `uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`) the type alias and the corresponding limit macros are defined if and only if the implementation provides the corresponding type.

[2017-10-21, Thomas Köppe provides improved wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4687.

> 1. Change 17.4.1 [cstdint.syn], header `<cstdint>` synopsis, as indicated:

>    >     […]
>    >     using int64_t = signed integer type; // optional
>    >     using intN_t = see below; // optional, see below
>    >     […]
>    >     using int_fast64_t = signed integer type;
>    >     using int_fastN_t = see below; // optional, see below
>    >     […]
>    >     using int_least64_t = signed integer type;
>    >     using int_leastN_t = see below; // optional, see below
>    >     […]
>    >     using uint64_t = unsigned integer type; // optional
>    >     using uintN_t = see below; // optional, see below
>    >     […]
>    >     using uint_fast64_t = unsigned integer type;
>    >     using uint_fastN_t = see below; // optional, see below
>    >     […]
>    >     using uint_least64_t = unsigned integer type;
>    >     using uint_leastN_t = see below; // optional, see below

>    >     using uintmax_t = unsigned integer type;
>    >     using uintptr_t = unsigned integer type; // optional

>    >     #define INT_N_MIN  see below;
>    >     #define INT_N_MAX  see below;
>    >     #define UINT_N_MAX  see below;

>    >     #define INT_FASTN_MIN  see below;
>    >     #define INT_FASTN_MAX  see below;
>    >     #define UINT_FASTN_MAX  see below;

>    >     #define INT_LEASTN_MIN  see below;
>    >     #define INT_LEASTN_MAX  see below;
>    >     #define UINT_LEASTN_MAX  see below;

>    >     #define INTMAX_MIN  see below;
>    >     #define INTMAX_MAX  see below;
>    >     #define UINTMAX_MAX  see below;

>    >     #define INTPTR_MIN  see below;
>    >     #define INTPTR_MAX  see below;
>    >     #define UINTPTR_MAX  see below;

>    >     #define PTRDIFF_MIN  see below;
>    >     #define PTRDIFF_MAX  see below;
>    >     #define SIZE_MAX  see below;

>    >     #define SIGATOMIC_MIN  see below;
>    >     #define SIGATOMIC_MAX  see below;

>    >     #define WCHAR_MIN  see below;
>    >     #define WCHAR_MAX  see below;

>    >     #define WINT_MIN  see below;
>    >     #define WINT_MAX  see below;

>    >     #define INTN_C(value)  see below;
>    >     #define UINTN_C(value)  see below;
>    >     #define INTMAX_C(value)  see below;
>    >     #define UINTMAX_C(value)  see below;

>    > -1- The header also defines numerous macros of the form:

>    > >     INT_[FAST LEAST]{8 16 32 64}_MIN
>    > >     [U]INT_[FAST LEAST]{8 16 32 64}_MAX
>    > >     INT{MAX PTR}_MIN
>    > >     [U]INT{MAX PTR}_MAX
>    > >     {PTRDIFF SIG_ATOMIC WCHAR WINT}{_MAX _MIN}
>    > >     SIZE_MAX

>    > plus function macros of the form:

>    > >     [U]INT{8 16 32 64 MAX}_C

>    > -2- The header defines all types and macros the same as the C standard library header `<stdint.h>`. See also: ISO C 7.20

>    > -?- In particular, all types that use the placeholder `N` are optional when `N` is not 8, 16, 32 or 64. The exact-width types `intN_t` and `uintN_t` for `N` = 8, 16, 32, 64 are also optional; however, if an implementation provides integer types with the corresponding width, no padding bits, and (for the signed types) that have a two's complement representation, it defines the corresponding typedef names. Only those macros are defined that correspond to typedef names that the implementation actually provides. [Note: The macros `INTN_C` and `UINTN_C` correspond to the typedef names `int_leastN_t` and `uint_leastN_t`, respectively. — end note]

> 2. Change 31.13.2 [cinttypes.syn] as indicated:

>    >     #define PRIdNN see below
>    >     #define PRIiNN see below
>    >     #define PRIoNN see below
>    >     #define PRIuNN see below
>    >     #define PRIxNN see below
>    >     #define PRIXNN see below
>    >     #define SCNdNN see below
>    >     #define SCNiNN see below
>    >     #define SCNoNN see below
>    >     #define SCNuNN see below
>    >     #define SCNxNN see below
>    >     #define PRIdLEASTNN see below
>    >     #define PRIiLEASTNN see below
>    >     #define PRIoLEASTNN see below
>    >     #define PRIuLEASTNN see below
>    >     #define PRIxLEASTNN see below
>    >     #define PRIXLEASTNN see below
>    >     #define SCNdLEASTNN see below
>    >     #define SCNiLEASTNN see below
>    >     #define SCNoLEASTNN see below
>    >     #define SCNuLEASTNN see below
>    >     #define SCNxLEASTNN see below
>    >     #define PRIdFASTNN see below
>    >     #define PRIiFASTNN see below
>    >     #define PRIoFASTNN see below
>    >     #define PRIuFASTNN see below
>    >     #define PRIxFASTNN see below
>    >     #define PRIXFASTNN see below
>    >     #define SCNdFASTNN see below
>    >     #define SCNiFASTNN see below
>    >     #define SCNoFASTNN see below
>    >     #define SCNuFASTNN see below
>    >     #define SCNxFASTNN see below
>    >     […]

>    > -1- The contents and meaning of the header `<cinttypes>` […]

>    > -?- In particular, macros that use the placeholder `N` are defined if and only if the implementation actually provides the corresponding typedef name in 17.4.1 [cstdint.syn], and moreover, the `fscanf` macros are provided unless the implementation does not have a suitable `fscanf` length modifier for the type.

[2018-04-03; Geoffrey Romer suggests improved wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4727.

> 1. Change 17.4.1 [cstdint.syn], header `<cstdint>` synopsis, as indicated:

>    >     […]
>    >     using int64_t = signed integer type; // optional
>    >     using intN_t = see below; // optional, see below
>    >     […]
>    >     using int_fast64_t = signed integer type;
>    >     using int_fastN_t = see below; // optional, see below
>    >     […]
>    >     using int_least64_t = signed integer type;
>    >     using int_leastN_t = see below; // optional, see below
>    >     […]
>    >     using uint64_t = unsigned integer type; // optional
>    >     using uintN_t = see below; // optional, see below
>    >     […]
>    >     using uint_fast64_t = unsigned integer type;
>    >     using uint_fastN_t = see below; // optional, see below
>    >     […]
>    >     using uint_least64_t = unsigned integer type;
>    >     using uint_leastN_t = see below; // optional, see below

>    >     using uintmax_t = unsigned integer type;
>    >     using uintptr_t = unsigned integer type; // optional

>    >     #define INT_N_MIN  see below;
>    >     #define INT_N_MAX  see below;
>    >     #define UINT_N_MAX  see below;

>    >     #define INT_FASTN_MIN  see below;
>    >     #define INT_FASTN_MAX  see below;
>    >     #define UINT_FASTN_MAX  see below;

>    >     #define INT_LEASTN_MIN  see below;
>    >     #define INT_LEASTN_MAX  see below;
>    >     #define UINT_LEASTN_MAX  see below;

>    >     #define INTMAX_MIN  see below;
>    >     #define INTMAX_MAX  see below;
>    >     #define UINTMAX_MAX  see below;

>    >     #define INTPTR_MIN  see below;
>    >     #define INTPTR_MAX  see below;
>    >     #define UINTPTR_MAX  see below;

>    >     #define PTRDIFF_MIN  see below;
>    >     #define PTRDIFF_MAX  see below;
>    >     #define SIZE_MAX  see below;

>    >     #define SIGATOMIC_MIN  see below;
>    >     #define SIGATOMIC_MAX  see below;

>    >     #define WCHAR_MIN  see below;
>    >     #define WCHAR_MAX  see below;

>    >     #define WINT_MIN  see below;
>    >     #define WINT_MAX  see below;

>    >     #define INTN_C(value)  see below;
>    >     #define UINTN_C(value)  see below;
>    >     #define INTMAX_C(value)  see below;
>    >     #define UINTMAX_C(value)  see below;

>    > -1- The header also defines numerous macros of the form:

>    > >     INT_[FAST LEAST]{8 16 32 64}_MIN
>    > >     [U]INT_[FAST LEAST]{8 16 32 64}_MAX
>    > >     INT{MAX PTR}_MIN
>    > >     [U]INT{MAX PTR}_MAX
>    > >     {PTRDIFF SIG_ATOMIC WCHAR WINT}{_MAX _MIN}
>    > >     SIZE_MAX

>    > plus function macros of the form:

>    > >     [U]INT{8 16 32 64 MAX}_C

>    > -2- The header defines all types and macros the same as the C standard library header `<stdint.h>`. See also: ISO C 7.20

>    > -?- In particular, all types that use the placeholder `N` are optional when `N` is not 8, 16, 32 or 64. The exact-width types `intN_t` and `uintN_t` for `N` = 8, 16, 32, 64 are also optional; however, if an implementation provides integer types with the corresponding width, no padding bits, and (for the signed types) that have a two's complement representation, it defines the corresponding typedef names. Only those macros are defined that correspond to typedef names that the implementation actually provides. [Note: The macros `INTN_C` and `UINTN_C` correspond to the typedef names `int_leastN_t` and `uint_leastN_t`, respectively. — end note]

> 2. Change 31.13.2 [cinttypes.syn] as indicated:

>    >     #define PRIdNN see below
>    >     #define PRIiNN see below
>    >     #define PRIoNN see below
>    >     #define PRIuNN see below
>    >     #define PRIxNN see below
>    >     #define PRIXNN see below
>    >     #define SCNdNN see below
>    >     #define SCNiNN see below
>    >     #define SCNoNN see below
>    >     #define SCNuNN see below
>    >     #define SCNxNN see below
>    >     #define PRIdLEASTNN see below
>    >     #define PRIiLEASTNN see below
>    >     #define PRIoLEASTNN see below
>    >     #define PRIuLEASTNN see below
>    >     #define PRIxLEASTNN see below
>    >     #define PRIXLEASTNN see below
>    >     #define SCNdLEASTNN see below
>    >     #define SCNiLEASTNN see below
>    >     #define SCNoLEASTNN see below
>    >     #define SCNuLEASTNN see below
>    >     #define SCNxLEASTNN see below
>    >     #define PRIdFASTNN see below
>    >     #define PRIiFASTNN see below
>    >     #define PRIoFASTNN see below
>    >     #define PRIuFASTNN see below
>    >     #define PRIxFASTNN see below
>    >     #define PRIXFASTNN see below
>    >     #define SCNdFASTNN see below
>    >     #define SCNiFASTNN see below
>    >     #define SCNoFASTNN see below
>    >     #define SCNuFASTNN see below
>    >     #define SCNxFASTNN see below
>    >     […]

>    > -1- The contents and meaning of the header `<cinttypes>` […]

>    > -?- `PRI` macros that use the placeholder `N` are defined if and only if the implementation actually provides the corresponding typedef name in 17.4.1 [cstdint.syn]. `SCN` macros that use the placeholder `N` are defined if and only if the implementation actually provides the corresponding typedef name and the implementation has a suitable `fscanf` length modifier for the type.

[2019-03-11; Reflector review and improved wording]

Wording simplifications due to new general two's complement requirements of integer types; removal of wording redundancies and applying some typo fixes in macro names.

[2019-03-16; Daniel comments and updates wording]

Hubert Tong pointed out that we do not have a statement about `[U]INTPTR_{MIN|MAX}` being optional. Interestingly, the C11 Standard does not say directly that the `[U]INTPTR_{MIN|MAX}` macros are optional, but this follows indirectly from the fact that `intptr_t` and `uintptr_t` are indeed optional. The updated wording therefore realizes Hubert's suggestion.

In addition, the reference document has been rebased to N4810, because that draft version contains an editorial change, which renames the term "range exponent" of integer types to "width", which is the vocabulary used below and also matches C's use.

Finally, Hubert Tong suggested the following rewording replacements of

> If and only if the implementation defines such a typedef name, it also defines the corresponding macros.

to:

> Each of the macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef name.

and of

> `PRI` macros that use the placeholder N are defined if and only if the implementation actually defines the corresponding typedef name in 17.4.1 [cstdint.syn]. `SCN` macros that use the placeholder `N` are defined if and only if the implementation actually defines the corresponding typedef name and the implementation has a suitable `fscanf` length modifier for the type.

to:

> Each of the macros listed in this subclause is defined if and only if the implementation actually defines the corresponding typedef name in 17.4.1 [cstdint.syn].

Those changes have been applied as well.

[2019-03-26; Reflector discussion and minor wording update]

Geoffrey pointed out that the revised wording has the effect that it requires an implementation to define `SCN` macros for all mentioned typedefs, but the C11 standard says "the corresponding `fscanf` macros shall be defined unless the implementation does not have a suitable `fscanf` length modifier for the type.". An additional wording update repairs this problem below.

[2020-02-22; Reflector discussion]

Status set to Tentatively Ready after seven positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Change 17.4.1 [cstdint.syn], header `<cstdint>` synopsis, as indicated:

   >     […]
   >     using int64_t = signed integer type; // optional
   >     using intN_t = see below; // optional, see below
   >     […]
   >     using int_fast64_t = signed integer type;
   >     using int_fastN_t = see below; // optional, see below
   >     […]
   >     using int_least64_t = signed integer type;
   >     using int_leastN_t = see below; // optional, see below
   >     […]
   >     using uint64_t = unsigned integer type; // optional
   >     using uintN_t = see below; // optional, see below
   >     […]
   >     using uint_fast64_t = unsigned integer type;
   >     using uint_fastN_t = see below; // optional, see below
   >     […]
   >     using uint_least64_t = unsigned integer type;
   >     using uint_leastN_t = see below; // optional, see below

   >     using uintmax_t = unsigned integer type;
   >     using uintptr_t = unsigned integer type; // optional

   >     #define INTN_MIN  see below
   >     #define INTN_MAX  see below
   >     #define UINTN_MAX  see below

   >     #define INT_FASTN_MIN  see below
   >     #define INT_FASTN_MAX  see below
   >     #define UINT_FASTN_MAX  see below

   >     #define INT_LEASTN_MIN  see below
   >     #define INT_LEASTN_MAX  see below
   >     #define UINT_LEASTN_MAX  see below

   >     #define INTMAX_MIN  see below
   >     #define INTMAX_MAX  see below
   >     #define UINTMAX_MAX  see below

   >     #define INTPTR_MIN  optional, see below
   >     #define INTPTR_MAX  optional, see below
   >     #define UINTPTR_MAX  optional, see below

   >     #define PTRDIFF_MIN  see below
   >     #define PTRDIFF_MAX  see below
   >     #define SIZE_MAX  see below

   >     #define SIG_ATOMIC_MIN  see below
   >     #define SIG_ATOMIC_MAX  see below

   >     #define WCHAR_MIN  see below
   >     #define WCHAR_MAX  see below

   >     #define WINT_MIN  see below
   >     #define WINT_MAX  see below

   >     #define INTN_C(value)  see below
   >     #define UINTN_C(value)  see below
   >     #define INTMAX_C(value)  see below
   >     #define UINTMAX_C(value)  see below

   > -1- The header also defines numerous macros of the form:

   > >     INT_[FAST LEAST]{8 16 32 64}_MIN
   > >     [U]INT_[FAST LEAST]{8 16 32 64}_MAX
   > >     INT{MAX PTR}_MIN
   > >     [U]INT{MAX PTR}_MAX
   > >     {PTRDIFF SIG_ATOMIC WCHAR WINT}{_MAX _MIN}
   > >     SIZE_MAX

   > plus function macros of the form:

   > >     [U]INT{8 16 32 64 MAX}_C

   > -2- The header defines all types and macros the same as the C standard library header `<stdint.h>`. See also: ISO C 7.20

   > -?- All types that use the placeholder `N` are optional when `N` is not 8, 16, 32 or 64. The exact-width types `intN_t` and `uintN_t` for `N` = 8, 16, 32, 64 are also optional; however, if an implementation defines integer types with the corresponding width and no padding bits, it defines the corresponding typedef names. Each of the macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef name. [Note: The macros `INTN_C` and `UINTN_C` correspond to the typedef names `int_leastN_t` and `uint_leastN_t`, respectively. — end note]

2. Change 31.13.2 [cinttypes.syn] as indicated:

   >     #define PRIdNN see below
   >     #define PRIiNN see below
   >     #define PRIoNN see below
   >     #define PRIuNN see below
   >     #define PRIxNN see below
   >     #define PRIXNN see below
   >     #define SCNdNN see below
   >     #define SCNiNN see below
   >     #define SCNoNN see below
   >     #define SCNuNN see below
   >     #define SCNxNN see below
   >     #define PRIdLEASTNN see below
   >     #define PRIiLEASTNN see below
   >     #define PRIoLEASTNN see below
   >     #define PRIuLEASTNN see below
   >     #define PRIxLEASTNN see below
   >     #define PRIXLEASTNN see below
   >     #define SCNdLEASTNN see below
   >     #define SCNiLEASTNN see below
   >     #define SCNoLEASTNN see below
   >     #define SCNuLEASTNN see below
   >     #define SCNxLEASTNN see below
   >     #define PRIdFASTNN see below
   >     #define PRIiFASTNN see below
   >     #define PRIoFASTNN see below
   >     #define PRIuFASTNN see below
   >     #define PRIxFASTNN see below
   >     #define PRIXFASTNN see below
   >     #define SCNdFASTNN see below
   >     #define SCNiFASTNN see below
   >     #define SCNoFASTNN see below
   >     #define SCNuFASTNN see below
   >     #define SCNxFASTNN see below
   >     […]

   > -1- The contents and meaning of the header `<cinttypes>` […]

   > -?- Each of the `PRI` macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef name in 17.4.1 [cstdint.syn]. Each of the `SCN` macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef name in 17.4.1 [cstdint.syn] and has a suitable `fscanf` length modifier for the type.

### 2821(i). `std::launder()` should be marked as `[[nodiscard]]`

Section: 17.6.5 [ptr.launder] Status: Resolved Submitter: Tony van Eerd Opened: 2016-11-13 Last modified: 2020-09-06

Priority: 3

View all other issues in [ptr.launder].

View all issues with Resolved status.

Discussion:

As pointed out by Nevin: A use of `std::launder` that does not make use of its return value is always pointless; the function has no side effects.

[2017-01-27 Telecon]

Priority 3; Nico's upcoming paper P0532 should address this and other issues around launder.

[2017-03-04, Kona]

This should be handled post-C++17 by Nico's paper P0600.

[2017-11-11, Albuquerque]

This was resolved by the adoption of P0600R1.

Proposed resolution:

### 2824(i). `list::sort` should say that the order of elements is unspecified if an exception is thrown

Section: 23.3.11.5 [list.ops] Status: C++17 Submitter: Tim Song Opened: 2016-11-16 Last modified: 2017-07-30

Priority: 0

View all other issues in [list.ops].

View all issues with C++17 status.

Discussion:

Sometime between N1638 and N1804 the sentence "If an exception is thrown the order of the elements in the list is indeterminate." in the specification of `list::sort` went missing. This suspiciously coincided with the editorial change that "consolidated definitions of "Stable" in the library clauses" (N1805).

`forward_list::sort` says that "If an exception is thrown the order of the elements in `*this` is unspecified"; `list::sort` should do the same.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4606.

1. Edit 23.3.11.5 [list.ops] p29 as indicated:

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > -28- Requires: `operator<` (for the first version) or `comp` (for the second version) shall define a strict weak ordering (26.8 [alg.sorting]).

   > > -29- Effects: Sorts the list according to the `operator<` or a `Compare` function object. If an exception is thrown, the order of the elements in `*this` is unspecified. Does not affect the validity of iterators and references.

   > > […]

### 2825(i). LWG 2756 breaks class template argument deduction for `optional`

Section: 22.5.3 [optional.optional] Status: Resolved Submitter: Richard Smith Opened: 2016-11-24 Last modified: 2020-11-09

Priority: 2

View all other issues in [optional.optional].

View all issues with Resolved status.

Discussion:

LWG 2756(i) applies these changes:

>     constexpr optional(const T&);
>     constexpr optional(T&&);
>     template <class... Args> constexpr explicit optional(in_place_t, Args&&...);
>     template <class U, class... Args>
>       constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...);
>     template <class U = T> EXPLICIT constexpr optional(U&&);
>     template <class U> EXPLICIT optional(const optional<U>&);
>     template <class U> EXPLICIT optional(optional<U>&&);

These break the ability for `optional` to perform class template argument deduction, as there is now no way to map from `optional`'s argument to the template parameter `T`.

[2017-01-27 Telecon]

Priority 2

[2017-01-30 Ville comments:]

Seems like the problem is resolved by a simple deduction guide:

    template <class T> optional(T) -> optional<T>;

The paper p0433r0 seems to suggest a different guide,

    template<class T> optional(T&& t) -> optional<remove_reference_t<T>>;

but I don't think the paper is up to speed with LWG 2756(i). There's no reason to use such an universal reference in the guide and `remove_reference` in its target, just guide with `T`, with the target `optional<T>`, `optional`'s constructors do the right thing once the type has been deduced.

[2020-05-29; Billy Baker comments]

At Kona 2017, P0433R2 was accepted and added Ville's deduction guide rather than one proposed in P0433R0.

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Rationale:

Resolved by P0433R2.

Proposed resolution:

### 2826(i). `string_view` iterators use old wording

Section: 27.3.3.4 [string.view.iterators] Status: C++17 Submitter: Alisdair Meredith Opened: 2016-11-17 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

Discussion:

The wording for `basic_string_view` was written before the definition of a contiguous iterator was added to C++17 to avoid repeating redundant wording.

Suggested modification of 27.3.3.4 [string.view.iterators] p1 (stealing words from `valarray begin`/`end`):

> -1- A constant random-access iterator type such that, for a `const_iterator it`, if `&*(it + N)` is valid, then `&*(it + N) == (&*it) + N` A type that meets the requirements of a constant random access iterator (24.2.7) and of a contiguous iterator (24.2.1) whose `value_type` is the template parameter `charT`.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4606.

1. Modify 27.3.3.4 [string.view.iterators] as indicated:

   >     using const_iterator = implementation-defined;

   > > -1- A constant random-access iterator type such that, for a `const_iterator it`, if `&*(it + N)` is valid, then `&*(it + N) == (&*it) + N` type that meets the requirements of a constant random access iterator (24.3.5.7 [random.access.iterators]) and of a contiguous iterator (24.3.1 [iterator.requirements.general]) whose `value_type` is the template parameter `charT`.

   > > -2- For a `basic_string_view str`, any operation that invalidates a pointer in the range `[str.data(), str.data() + str.size())` invalidates pointers, iterators, and references returned from `str`'s methods.

   > > -3- All requirements on container iterators (23.2 [container.requirements]) apply to `basic_string_view::const_iterator` as well.

### 2830(i). `insert_return_type` is only defined for containers with unique keys

Section: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: Resolved Submitter: Daniel James Opened: 2016-11-24 Last modified: 2020-09-06

Priority: Not Prioritized

View other active issues in [associative.reqmts].

View all other issues in [associative.reqmts].

View all issues with Resolved status.

Discussion:

The requirements for associative and unordered associative containers list `insert_return_type` for all containers, but looking at the class description in 23.4.3.1 [map.overview], 23.4.4.1 [multimap.overview] etc. it's only defined for containers with unique keys. Jonathan Wakely pointed out that this depends on the resolution of LWG 2772(i), which might remove `insert_return_type` completely.

A possible resolution would be to put in 23.2.7 [associative.reqmts]: '`insert_return_type` (`map` and `set` only)' and in 23.2.8 [unord.req]: '`insert_return_type` (`unordered_map` and `unordered_set` only)'. Or maybe something like '(containers with unique keys only)'.

[Issues Telecon 16-Dec-2016]

Resolved by adoption of P0508R0.

Proposed resolution:

### 2831(i). Equality can be defined when `Hash` function objects have different behaviour

Section: 23.2.8 [unord.req] Status: Resolved Submitter: Daniel James Opened: 2016-11-24 Last modified: 2020-09-06

Priority: Not Prioritized

View other active issues in [unord.req].

View all other issues in [unord.req].

View all issues with Resolved status.

Discussion:

In 23.2.8 [unord.req] paragraph 12, it says that the behaviour of `operator==` is undefined unless the `Hash` and `Pred` function objects respectively have the same behaviour. This makes comparing containers with randomized hashes with different seeds undefined behaviour, but I think that's a valid use case. It's not much more difficult to support it when the `Hash` function objects behave differently. I did a little testing and both libstdc++ and libc++ appear to support this correctly.

I suggest changing the appropriate sentence in 23.2.8 [unord.req] paragraph 12: "The behavior of a program that uses `operator==` or `operator!=` on unordered containers is undefined unless the `Hash` and `Pred` function objects respectively haveobject has the same behavior for both containers and the equality comparison operator for Key is a `refinement`"

[2017-01-27 Telecon]

This is a design issue; send to LEWG

[2018-3-17 Resolved by P0809R0, which was adopted in Jacksonville.]

Proposed resolution:

This wording is relative to N4618.

1. Change 23.2.8 [unord.req] as indicated:

   > -12- Two unordered containers `a` and `b` compare equal if `a.size() == b.size()` and, for every equivalent-key group `[Ea1, Ea2)` obtained from `a.equal_range(Ea1)`, there exists an equivalent-key group `[Eb1, Eb2)` obtained from `b.equal_range(Ea1)`, such that `is_permutation(Ea1, Ea2, Eb1, Eb2)` returns `true`. For `unordered_set` and `unordered_map`, the complexity of `operator==` (i.e., the number of calls to the `==` operator of the `value_type`, to the predicate returned by `key_eq()`, and to the hasher returned by `hash_function()`) is proportional to `N` in the average case and to `N2` in the worst case, where `N` is `a.size()`. For `unordered_multiset` and `unordered_multimap`, the complexity of `operator==` is proportional to ∑ `Ei2` in the average case and to `N2` in the worst case, where `N` is a.size(), and `Ei` is the size of the `i`th equivalent-key group in `a`. However, if the respective elements of each corresponding pair of equivalent-key groups `Eai` and `Ebi` are arranged in the same order (as is commonly the case, e.g., if `a` and `b` are unmodified copies of the same container), then the average-case complexity for `unordered_multiset` and `unordered_multimap` becomes proportional to `N` (but worst-case complexity remains 𝒪(`N2`), e.g., for a pathologically bad hash function). The behavior of a program that uses `operator==` or `operator!=` on unordered containers is undefined unless the `Hash` and `Pred` function objects respectively havehas the same behavior for both containers and the equality comparison operator for `Key` is a refinement(footnote 258) of the partition into equivalent-key groups produced by `Pred`.

### 2832(i). §[fpos.operations] strange requirement for `P(i)`

Section: 31.5.3.3 [fpos.operations] Status: Resolved Submitter: Jens Maurer Opened: 2016-11-24 Last modified: 2020-05-12

Priority: 3

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Discussion:

This is from editorial issue #1031.

The first row in Table 112 "Position type requirements" talks about the expression `P(i)` and then has an assertion `p == P(i)`. However, there are no constraints on `p` other than being of type `P`, so (on the face of it) this seems to require that `operator==` on `P` always returns true, which is non-sensical.

[2017-01-27 Telecon]

Priority 3

[2019-03-17; Daniel comments]

With the acceptance of P0759R1 at the Rapperswil 2018 meeting this issue should be closed as Resolved.

[2020-05-12; Reflector discussions]

Resolved by P0759R1.

Rationale:

Resolved by P0759R1.

Proposed resolution:

### 2834(i). Resolution LWG 2223 is missing wording about end iterators

Section: 27.4.3.5 [string.capacity], 23.3.5.3 [deque.capacity], 23.3.13.3 [vector.capacity] Status: C++17 Submitter: Thomas Koeppe Opened: 2016-11-29 Last modified: 2020-09-06

Priority: 0

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Discussion:

The resolution of LWG 2223(i) added the wording "Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence." to a number of `shrink_to_fit` operations.

This seems to be missing any mention of end iterators. Surely end iterators are invalidated, too?

Suggested resolution:

For `string`, `deque`, and `vector` each, append as follows:

Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit 27.4.3.5 [string.capacity] as indicated:

   >     void shrink_to_fit();

   > > […]

   > > -15- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator. If no reallocation happens, they remain valid.

2. Edit 23.3.5.3 [deque.capacity] as indicated:

   >     void shrink_to_fit();

   > > […]

   > > -8- Remarks: `shrink_to_fit` invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator.

3. Edit 23.3.13.3 [vector.capacity] as indicated:

   >     void shrink_to_fit();

   > > […]

   > > -10- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator. If no reallocation happens, they remain valid.

### 2835(i). LWG 2536 seems to misspecify `<tgmath.h>`

Section: 17.15 [support.c.headers], 99 [depr.ctgmath.syn] Status: C++17 Submitter: Thomas Koeppe Opened: 2016-11-29 Last modified: 2023-02-07

Priority: 0

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Discussion:

The resolution of LWG 2536(i) touches on the specification of C headers (of the form `foo.h`), but while it fixes the specification of `complex.h`, it fails to address the specification of `tgmath.h`.

Just like `complex.h`, `tgmath.h` is not defined by C. Rather, our `tgmath.h` behaves like `<ctgmath>`, which is specified in [ctgmath.syn].

Suggested resolution:

Amend [depr.c.headers] p3 as follows:

> 3. The header `<complex.h>` behaves as if it simply includes the header `<ccomplex>`. The header `<tgmath.h>` behaves as if it simply includes the header `<ctgmath>`.

> 4. Every other C header […]

See also LWG 2828(i).

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit [depr.c.headers] as indicated:

   > […]

   > -2- The use of any of the C++ headers […] is deprecated.

   > -3- The header `<complex.h>` behaves as if it simply includes the header `<ccomplex>`. The header `<tgmath.h>` behaves as if it simply includes the header `<ctgmath>`.

   > -4- Every other C header, […]

### 2836(i). More string operations should be `noexcept`

Section: 27.4.3 [basic.string], 27.4.3.8.2 [string.find], 27.4.3.8.4 [string.compare] Status: Resolved Submitter: Jonathan Wakely Opened: 2016-12-05 Last modified: 2023-02-07

Priority: 2

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Discussion:

Currently some overloads of `basic_string::find` are `noexcept` and some are not. Historically this was because some were specified in terms of constructing a temporary `basic_string`, which could throw. In practice creating a temporary (and potentially allocating memory) is a silly implementation, and so they could be `noexcept`. In the C++17 draft most of them have been changed to create a temporary `basic_string_view` instead, which can't throw anyway (P0254R2 made those changes).

This is confusing for users, as they need to carefully check which overload their code will resolve to, and consider whether that overload can throw. Refactoring code can change whether it calls a throwing or non-throwing overload. This is an unnecessary burden on users when realistically none of the functions will ever throw.

The `find`, `rfind`, `find_first_of`, `find_last_of`, `find_first_not_of` and `find_last_not_of` overloads that are defined in terms of `basic_string_view` should be `noexcept`, or "Throws: Nothing." for the ones with narrow contracts (even though those narrow contracts are not enforcable or testable).

The remaining overloads that are still specified in terms of a temporary string could also be `noexcept`. They construct `basic_string` of length one (which won't throw for an SSO implementation anyway), but can easily be defined in terms of `basic_string_view` instead.

There's one `basic_string::compare` overload that is still defined in terms of a temporary `basic_string`, which should be `basic_string_view` and so can also be `noexcept` (the other `compare` overloads can throw `out_of_range`).

[2016-12-15, Tim Song comments]

The following overloads invoking `basic_string_view<charT>(s, n)` are implicitly narrow-contract (the `basic_string_view` constructor requires `[s, s+n)` to be a valid range) and should be "Throws: Nothing" rather than `noexcept`:

>     size_type find(const charT* s, size_type pos, size_type n) const;
>     size_type rfind(const charT* s, size_type pos, size_type n) const;
>     size_type find_first_of(const charT* s, size_type pos, size_type n) const;
>     size_type find_last_of(const charT* s, size_type pos, size_type n) const;
>     size_type find_first_not_of(const charT* s, size_type pos, size_type n) const;
>     size_type find_last_not_of(const charT* s, size_type pos, size_type n) const;

Similarly, the `basic_string_view` constructor invoked by this overload of `compare` requires `[s, s + traits::length(s))` to be a valid range, and so it should be "Throws: Nothing" rather than `noexcept`:

>     int compare(const charT* s) const;

[2017-01-27 Telecon]

Priority 2; Jonathan to provide updated wording

[2017-10-22, Marshall comments]

libc++ already has all of these member fns marked as `noexcept`

[2017-11 Albuquerque Saturday issues processing]

All the fns that take a `const char *` are narrow contract, and so can't be noexcept. Should be "Throws: Nothing" instead. Alisdair to re-word.

[2018-08 mailing list discussion]

This will be resolved by Tim's string rework paper.

Resolved by the adoption of P1148 in San Diego.

Proposed resolution:

This wording is relative to N4618.

1. Change class template synopsis `std::basic_string`, 27.4.3 [basic.string], as indicated:

   >     size_type find (basic_string_view<charT, traits> sv,
   >                     size_type pos = 0) const noexcept;
   >     size_type find (const basic_string& str, size_type pos = 0) const noexcept;
   >     size_type find (const charT* s, size_type pos, size_type n) const noexcept;
   >     size_type find (const charT* s, size_type pos = 0) const;
   >     size_type find (charT c, size_type pos = 0) const noexcept;
   >     size_type rfind(basic_string_view<charT, traits> sv,
   >                     size_type pos = npos) const noexcept;
   >     size_type rfind(const basic_string& str, size_type pos = npos) const noexcept;
   >     size_type rfind(const charT* s, size_type pos, size_type n) const noexcept;
   >     size_type rfind(const charT* s, size_type pos = npos) const;
   >     size_type rfind(charT c, size_type pos = npos) const noexcept;
   >     size_type find_first_of(basic_string_view<charT, traits> sv,
   >                             size_type pos = 0) const noexcept;
   >     size_type find_first_of(const basic_string& str,
   >                             size_type pos = 0) const noexcept;
   >     size_type find_first_of(const charT* s,
   >                             size_type pos, size_type n) const noexcept;
   >     size_type find_first_of(const charT* s, size_type pos = 0) const;
   >     size_type find_first_of(charT c, size_type pos = 0) const noexcept;
   >     size_type find_last_of (basic_string_view<charT, traits> sv,
   >                             size_type pos = npos) const noexcept;
   >     size_type find_last_of (const basic_string& str,
   >                             size_type pos = npos) const noexcept;
   >     size_type find_last_of (const charT* s,
   >                             size_type pos, size_type n) const noexcept;
   >     size_type find_last_of (const charT* s, size_type pos = npos) const;
   >     size_type find_last_of (charT c, size_type pos = npos) const noexcept;
   >     size_type find_first_not_of(basic_string_view<charT, traits> sv,
   >                                 size_type pos = 0) const noexcept;
   >     size_type find_first_not_of(const basic_string& str,
   >                                 size_type pos = 0) const noexcept;
   >     size_type find_first_not_of(const charT* s, size_type pos,
   >                                 size_type n) const noexcept;
   >     size_type find_first_not_of(const charT* s, size_type pos = 0) const;
   >     size_type find_first_not_of(charT c, size_type pos = 0) const noexcept;
   >     size_type find_last_not_of (basic_string_view<charT, traits> sv,
   >                                 size_type pos = npos) const noexcept;
   >     size_type find_last_not_of (const basic_string& str,
   >                                 size_type pos = npos) const noexcept;
   >     size_type find_last_not_of (const charT* s, size_type pos,
   >                                 size_type n) const noexcept;
   >     size_type find_last_not_of (const charT* s,
   >                                 size_type pos = npos) const;
   >     size_type find_last_not_of (charT c, size_type pos = npos) const noexcept;

   >     basic_string substr(size_type pos = 0, size_type n = npos) const;
   >     int compare(basic_string_view<charT, traits> sv) const noexcept;
   >     int compare(size_type pos1, size_type n1,
   >                 basic_string_view<charT, traits> sv) const;
   >     template<class T>
   >       int compare(size_type pos1, size_type n1, const T& t,
   >                   size_type pos2, size_type n2 = npos) const;
   >     int compare(const basic_string& str) const noexcept;
   >     int compare(size_type pos1, size_type n1,
   >                 const basic_string& str) const;
   >     int compare(size_type pos1, size_type n1,
   >                 const basic_string& str,
   >                 size_type pos2, size_type n2 = npos) const;
   >     int compare(const charT* s) const noexcept;
   >     int compare(size_type pos1, size_type n1,
   >                 const charT* s) const;
   >     int compare(size_type pos1, size_type n1,
   >                 const charT* s, size_type n2) const;

2. Change 27.4.3.8.2 [string.find] as indicated:

   >     size_type find(const charT* s, size_type pos, size_type n) const noexcept;

   > > -5- Returns: `find(basic_string_view<charT, traits>(s, n), pos)`.

   >     size_type find(const charT* s, size_type pos = 0) const;

   > > -6- Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -7- Returns: `find(basic_string_view<charT, traits>(s), pos)`.

   > > -?- Throws: Nothing.

   >     size_type find(charT c, size_type pos = 0) const noexcept;

   > > -8- Returns: `find(basic_string_view<charT, traits>(addressof(c), 1)(1, c), pos)`.

3. Change [string.rfind] as indicated:

   >     size_type rfind(const charT* s, size_type pos, size_type n) const noexcept;

   > > -5- Returns: `rfind(basic_string_view<charT, traits>(s, n), pos)`.

   >     size_type rfind(const charT* s, size_type pos = npos) const;

   > > -6- Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -7- Returns: `rfind(basic_string_view<charT, traits>(s), pos)`.

   > > -?- Throws: Nothing.

   >     size_type rfind(charT c, size_type pos = npos) const noexcept;

   > > -8- Returns: `rfind(basic_string_view<charT, traits>(addressof(c), 1)(1, c), pos)`.

4. Change [string.find.first.of] as indicated:

   >     size_type
   >       find_first_of(const charT* s, size_type pos, size_type n) const noexcept;

   > > -5- Returns: `find_first_of(basic_string_view<charT, traits>(s, n), pos)`.

   >     size_type find_first_of(const charT* s, size_type pos = 0) const;

   > > -6- Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -7- Returns: `find_first_of(basic_string_view<charT, traits>(s), pos)`.

   > > -?- Throws: Nothing.

   >     size_type find_first_of(charT c, size_type pos = 0) const noexcept;

   > > -8- Returns: `find_first_of(basic_string_view<charT, traits>(addressof(c), 1)(1, c), pos)`.

5. Change [string.find.last.of] as indicated:

   >     size_type find_last_of(const charT* s, size_type pos, size_type n) const noexcept;

   > > -5- Returns: `find_last_of(basic_string_view<charT, traits>(s, n), pos)`.

   >     size_type find_last_of(const charT* s, size_type pos = npos) const;

   > > -6- Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -7- Returns: `find_last_of(basic_string_view<charT, traits>(s), pos)`.

   > > -?- Throws: Nothing.

   >     size_type find_last_of(charT c, size_type pos = npos) const noexcept;

   > > -8- Returns: `find_last_of(basic_string_view<charT, traits>(addressof(c), 1)(1, c), pos)`.

6. Change [string.find.first.not.of] as indicated:

   >     size_type
   >       find_first_not_of(const charT* s, size_type pos, size_type n) const noexcept;

   > > -5- Returns: `find_first_not_of(basic_string_view<charT, traits>(s, n), pos)`.

   >     size_type find_first_not_of(const charT* s, size_type pos = 0) const;

   > > -6- Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -7- Returns: `find_first_not_of(basic_string_view<charT, traits>(s), pos)`.

   > > -?- Throws: Nothing.

   >     size_type find_first_not_of(charT c, size_type pos = 0) const noexcept;

   > > -8- Returns: `find_first_not_of(basic_string_view<charT, traits>(addressof(c), 1)(1, c), pos)`.

7. Change [string.find.last.not.of] as indicated:

   >     size_type find_last_not_of(const charT* s, size_type pos,
   >                                size_type n) const noexcept;

   > > -5- Returns: `find_last_not_of(basic_string_view<charT, traits>(s, n), pos)`.

   >     size_type find_last_not_of(const charT* s, size_type pos = npos) const;

   > > -6- Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -7- Returns: `find_last_not_of(basic_string_view<charT, traits>(s), pos)`.

   > > -?- Throws: Nothing.

   >     size_type find_last_not_of(charT c, size_type pos = npos) const noexcept;

   > > -8- Returns: `find_last_not_of(basic_string_view<charT, traits>(addressof(c), 1)(1, c), pos)`.

8. Change 27.4.3.8.4 [string.compare] as indicated:

   >     int compare(const charT* s) const noexcept;

   > > -9- Returns: `compare(basic_string_view<charT, traits>(s))`.

### 2837(i). `gcd` and `lcm` should support a wider range of input values

Section: 26.10.14 [numeric.ops.gcd], 26.10.15 [numeric.ops.lcm] Status: C++17 Submitter: Marshall Clow Opened: 2016-12-16 Last modified: 2020-09-06

Priority: 0

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Discussion:

This is a duplicate of 2792(i), which addressed LFTS 2.

By the current definition, `gcd((int64_t)1234, (int32_t)-2147483648)` is ill-formed (because `2147483648` is not representable as a value of `int32_t`.) We want to change this case to be well-formed. As long as both `|m|` and `|n|` are representable as values of the common type, absolute values can be calculate `d` without causing unspecified behavior, by converting `m` and `n` to the common type before taking the negation.

Suggested resolution:

> `|m|` shall be representable as a value of type `M` and `|n|` shall be representable as a value of type `N``|m|` and `|n|` shall be representable as a value of `common_type_t<M, N>`.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4604.

1. Edit 26.10.14 [numeric.ops.gcd] as indicated:

   >     template<class M, class N>
   >       constexpr common_type_t<M, N> gcd(M m, N n);

   > > -2- Requires: `|m|` shall be representable as a value of type `M` and `|n|` shall be representable as a value of type `N``|m|` and `|n|` shall be representable as a value of `common_type_t<M, N>`. [Note: These requirements ensure, for example, that `gcd(m, m) = |m|` is representable as a value of type `M`. — end note]

2. Edit 26.10.15 [numeric.ops.lcm] as indicated:

   >     template<class M, class N>
   >       constexpr common_type_t<M, N> lcm(M m, N n);

   > > -2- Requires: `|m|` shall be representable as a value of type `M` and `|n|` shall be representable as a value of type `N``|m|` and `|n|` shall be representable as a value of `common_type_t<M, N>`. The least common multiple of `|m|` and `|n|` shall be representable as a value of type `common_type_t<M, N>`.

### 2838(i). `is_literal_type` specification needs a little cleanup

Section: D.13 [depr.meta.types] Status: C++17 Submitter: Tim Song Opened: 2016-12-09 Last modified: 2020-09-06

Priority: 0

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Discussion:

D.13 [depr.meta.types]/3 currently reads:

> Effects: `is_literal_type` has a base-characteristic of `true_type` if `T` is a literal type ([basic.types]), and a base-characteristic of `false_type` otherwise.

First, this doesn't seem like an Effects clause. Second, this wording fails to say that `is_literal_type` is an `UnaryTypeTrait`, and misspells `BaseCharacteristic` — which is only defined for `UnaryTypeTraits` and `BinaryTypeTraits`. Third, moving this to Annex D means that the general prohibition against specializing type traits in [meta.type.synop]/1 no longer applies, which is presumably unintended.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit D.13 [depr.meta.types] as indicated:

   > The header `<type_traits>` has the following addition:

   > >     namespace std {
   > >       template <class T> struct is_literal_type;

   > >       template <class T> constexpr bool is_literal_type_v = is_literal_type<T>::value;
   > >     }

   > -2- Requires: `remove_all_extents_t<T>` shall be a complete type or (possibly cv-qualified) `void`.

   > -3- Effects: `is_literal_type` has a base-characteristic of `true_type` if `T` is a literal type (3.9), and a basecharacteristic of `false_type` otherwise `is_literal_type<T>` is a `UnaryTypeTrait` (21.3.2 [meta.rqmts]) with a `BaseCharacteristic` of `true_type` if `T` is a literal type (6.9 [basic.types]), and `false_type` otherwise.

   > -?- The behavior of a program that adds specializations for `is_literal_type` or `is_literal_type_v` is undefined.

### 2839(i). Self-move-assignment of library types, again

Section: 16.4.6.17 [lib.types.movedfrom], 16.4.5.9 [res.on.arguments], 23.2.2 [container.requirements.general] Status: C++23 Submitter: Tim Song Opened: 2016-12-09 Last modified: 2023-11-22

Priority: 2

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Discussion:

LWG 2468(i)'s resolution added to `MoveAssignable` the requirement to tolerate self-move-assignment, but that does nothing for library types that aren't explicitly specified to meet `MoveAssignable` other than make those types not meet `MoveAssignable` any longer.

To realize the intent here, we need to carve out an exception to 16.4.5.9 [res.on.arguments]'s restriction for move assignment operators and specify that self-move-assignment results in valid but unspecified state unless otherwise specified. The proposed wording below adds that to 16.4.6.17 [lib.types.movedfrom] since it seems to fit well with the theme of the current paragraph in that section.

In addition, to address the issue with 23.2.2 [container.requirements.general] noted in LWG 2468(i)'s discussion, the requirement tables in that subclause will need to be edited in a way similar to LWG 2468(i).

[2017-01-27 Telecon]

Priority 2

[2018-1-26 issues processing telecon]

Status to 'Open'; Howard to reword using 'MoveAssignable'.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4618.

> 1. Add a new paragraph at the end of 16.4.6.17 [lib.types.movedfrom]:

>    > -1- Objects of types defined in the C++ standard library may be moved from (12.8). Move operations may be explicitly specified or implicitly generated. Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state.

>    > -?- An object of a type defined in the C++ standard library may be move-assigned (11.4.6 [class.copy.assign]) to itself. Such an assignment places the object in a valid but unspecified state unless otherwise specified.

> 2. Add a note at the end of 16.4.5.9 [res.on.arguments]/1, bullet 3, as indicated:

>    > -1- Each of the following applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

>    > 1. (1.1) — […]

>    > 2. (1.2) — […]

>    > 3. (1.3) — If a function argument binds to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument. [Note: If the parameter is a generic parameter of the form `T&&` and an lvalue of type `A` is bound, the argument binds to an lvalue reference (14.8.2.1) and thus is not covered by the previous sentence. — end note] [Note: If a program casts an lvalue to an xvalue while passing that lvalue to a library function (e.g. by calling the function with the argument `std::move(x)`), the program is effectively asking that function to treat that lvalue as a temporary. The implementation is free to optimize away aliasing checks which might be needed if the argument was an lvalue. — end note] [Note: This does not apply to the argument passed to a move assignment operator (16.4.6.17 [lib.types.movedfrom]). — end note]

> 3. Edit Table 83 "Container requirements" in 23.2.2 [container.requirements.general] as indicated:

>    > Table 83 — Container requirements
>    > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
>    > --- | --- | --- | --- | ---
>    > `…`
>    > `a = rv` | `T&` | All existing elements of `a` are either move assigned to or destroyed | post: If `a` and `rv` do not refer to the same object, `a` shall be equal to the value that `rv` had before this assignment | linear
>    > `…`

> 4. Edit Table 86 "Allocator-aware container requirements" in 23.2.2 [container.requirements.general] as indicated:

>    > Table 86 — Allocator-aware container requirements
>    > Expression | Return type | Assertion/note pre-/post-condition | Complexity
>    > --- | --- | --- | ---
>    > `…`
>    > `a = rv` | `T&` | Requires: If `allocator_traits<allocator_type >::propagate_on_container_move_assignment::value` is `false`, `T` is `MoveInsertable` into `X` and `MoveAssignable`. All existing elements of `a` are either move assigned to or destroyed. post: If `a` and `rv` do not refer to the same object, `a` shall be equal to the value that `rv` had before this assignment | linear
>    > `…`

[2018-08-16, Howard comments and provides updated wording]

I agreed to provide proposed wording for LWG 2839 that was reworded to use `MoveAssignable`. The advantage of this is that `MoveAssignable` specifies the self-assignment case, thus we do not need to repeat ourselves.

[2018-08-23 Batavia Issues processing]

Howard and Tim to discuss a revised P/R.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4762.

> 1. Add a new subsection to 16.4.6 [conforming] after 16.4.6.5 [member.functions]:

>    > Special members [conforming.special]

>    > Class types defined by the C++ standard library and specified to be default constructible, move constructible, copy constructible, move assignable, copy assignable, or destructible, shall meet the associated requirements Cpp17DefaultConstructible, Cpp17MoveConstructible, Cpp17CopyConstructible, Cpp17MoveAssignable, Cpp17CopyAssignable, and Cpp17Destructible, respectively (16.4.4.2 [utility.arg.requirements]).

[2020-06-06 Tim restores and updates P/R following 2020-05-29 telecon discussion]

The standard doesn't define phrases like "default constructible" used in the previous P/R. Moreover, the library provides a variety of wrapper types, and whether these types meet the semantic requirements of Cpp17Meowable (and maybe even syntactic, depending on how "copy constructible" is interpreted) depends on the property of their underlying wrapped types, which might not even be an object type (e.g., `tuple` or `pair` of references). This is a large can of worms (see LWG 2146(i)) that we don't want to get into.

There is a suggestion in the telecon to blanket-exempt move-assignment operators from the 16.4.5.9 [res.on.arguments] 1.3 requirement. The revised wording below does not do so, as that would carve out not just self-move-assignment but also other aliasing scenarios in which the target object owns the source object. Whether such scenarios should be permitted is outside the scope of this issue, though notably `assignable_from` (18.4.8 [concept.assignable]) contains a note alluding to these cases and suggesting that they should be considered to be outside the domain of `=` entirely.

[2020-07-17; issue processing telecon]

LWG reviewed the latest proposed resolution. Unanimous consent to move to Ready.

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Add a new paragraph at the end of 16.4.6.17 [lib.types.movedfrom]:

   > -1- Objects of types defined in the C++ standard library may be moved from ( [clss.copy.ctor]). Move operations may be explicitly specified or implicitly generated. Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state.

   > -?- An object of a type defined in the C++ standard library may be move-assigned (11.4.6 [class.copy.assign]) to itself. Unless otherwise specified, such an assignment places the object in a valid but unspecified state.

2. Edit 16.4.5.9 [res.on.arguments]/1, bullet 3, as indicated:

   > -1- Each of the following applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

   > 1. (1.1) — […]

   > 2. (1.2) — […]

   > 3. (1.3) — If a function argument binds to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument, except that the argument passed to a move-assignment operator may be a reference to `*this` (16.4.6.17 [lib.types.movedfrom]). [Note: If the type of a parameter is a generic parameter of the form `T&&` and an lvalue of type `A` is bound, the argument binds to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus is not covered by the previous sentence. forwarding reference (13.10.3.2 [temp.deduct.call]) that is deduced to an lvalue reference type, then the argument is not bound to an rvalue reference. — end note] [Note: If a program casts an lvalue to an xvalue while passing that lvalue to a library function (e.g. by calling the function with the argument `std::move(x)`), the program is effectively asking that function to treat that lvalue as a temporary. The implementation is free to optimize away aliasing checks which might be needed if the argument was an lvalue. — end note]

3. Edit Table 73 "Container requirements" in 23.2.2 [container.requirements.general] as indicated:

   > Table 73 — Container requirements
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | --- | ---
   > `…`
   > `a = rv` | `T&` | All existing elements of `a` are either move assigned to or destroyed | Postconditions: If `a` and `rv` do not refer to the same object, `a` is equal to the value that `rv` had before this assignment. | linear
   > `…`

4. Edit Table 76 "Allocator-aware container requirements" in 23.2.2 [container.requirements.general] as indicated:

   > Table 86 — Allocator-aware container requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `a = rv` | `T&` | Preconditions: If `allocator_traits<allocator_type> ::propagate_on_container_move_assignment::value` is `false`, `T` is Cpp17MoveInsertable into `X` and Cpp17MoveAssignable. Effects: All existing elements of `a` are either move assigned to or destroyed. Postconditions: If `a` and `rv` do not refer to the same object, `a` is equal to the value that `rv` had before this assignment. | linear
   > `…`

### 2840(i). `directory_iterator::increment` is seemingly narrow-contract but marked `noexcept`

Section: 31.12.11.2 [fs.dir.itr.members], 31.12.12.2 [fs.rec.dir.itr.members] Status: Resolved Submitter: Tim Song Opened: 2016-12-09 Last modified: 2020-09-06

Priority: 2

View all other issues in [fs.dir.itr.members].

View all issues with Resolved status.

Discussion:

`directory_iterator::increment` and `recursive_directory_iterator::increment` are specified by reference to the input iterator requirements, which is narrow-contract: it has a precondition that the iterator is dereferenceable. Yet they are marked as `noexcept`.

Either the `noexcept` (one of which was added by LWG 2637(i)) should be removed, or the behavior of `increment` when given a nondereferenceable iterator should be specified.

Currently, libstdc++ and MSVC report an error via the `error_code` argument for a nondereferenceable `directory_iterator`, while libc++ and `boost::filesystem` assert.

[2017-01-27 Telecon]

Priority 2; there are some problems with the wording in alternative B.

[2018-01-24 Tim Song comments]

LWG 3013(i) will remove this `noexcept` (for a different reason). The behavior of calling `increment` on a nondereferenceble iterator should still be clarified, as I was informed that LWG wanted it to be well-defined.

[2018-1-26 issues processing telecon]

Part A is already done by 3013(i).

Marshall to talk to Tim about Part B; it would be odd if `operator++` was undefined behaviour, but `increment` on the same iterator returned an error

[2018-8-15 Offline discussions]

Tim and Marshall recommend NAD for about Part B

[2018-08-20 Batavia Issues processing]

Status to resolved.

Proposed resolution:

This wording is relative to N4618.

1. Alternative A (remove the `noexcept`):

   1. Edit [fs.class.directory_iterator], class `directory_iterator` synopsis, as indicated:

      >     directory_iterator& operator++();
      >     directory_iterator& increment(error_code& ec) noexcept;

   2. Edit 31.12.11.2 [fs.dir.itr.members] before p10 as indicated:

      >     directory_iterator& operator++();
      >     directory_iterator& increment(error_code& ec) noexcept;

      > > -10- Effects: As specified for the prefix increment operation of Input iterators (24.2.3).

      > > -11- Returns: `*this`.

      > > -12- Throws: As specified in 27.10.7.

   3. Edit 31.12.12 [fs.class.rec.dir.itr], class `recursive_directory_iterator` synopsis, as indicated:

      >     recursive_directory_iterator& operator++();
      >     recursive_directory_iterator& increment(error_code& ec) noexcept;

   4. Edit 31.12.12.2 [fs.rec.dir.itr.members] before p23 as indicated:

      >     recursive_directory_iterator& operator++();
      >     recursive_directory_iterator& increment(error_code& ec) noexcept;

      > > -23- Effects: As specified for the prefix increment operation of Input iterators (24.2.3), except that: […]

      > > -24- Returns: `*this`.

      > > -25- Throws: As specified in 27.10.7.

2. Alternative B (specify that `increment` reports an error if the iterator is not dereferenceable):

   1. Edit 31.12.11.2 [fs.dir.itr.members] p10 as indicated:

      >     directory_iterator& operator++();
      >     directory_iterator& increment(error_code& ec) noexcept;

      > > -10- Effects: As specified for the prefix increment operation of Input iterators (24.2.3), except that `increment` reports an error if `*this` is not dereferenceable.

      > > -11- Returns: `*this`.

      > > -12- Throws: As specified in 27.10.7.

   2. Edit 31.12.12.2 [fs.rec.dir.itr.members] p23 as indicated:

      >     recursive_directory_iterator& operator++();
      >     recursive_directory_iterator& increment(error_code& ec) noexcept;

      > > -23- Effects: As specified for the prefix increment operation of Input iterators (24.2.3), except that:

      > > - `increment` reports an error if `*this` is not dereferenceable.

      > > - If there are no more entries at the current depth, […]

      > > - Otherwise if […]

      > > -24- Returns: `*this`.

      > > -25- Throws: As specified in 27.10.7.

### 2841(i). Use of "Equivalent to" in [strings]

Section: 27 [strings] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2016-12-09 Last modified: 2018-11-25

Priority: 3

View all other issues in [strings].

View all issues with Resolved status.

Discussion:

Reported via editorial issue #165:

The following places should probably use the "Equivalent to" wording introduced in [structure.specifications]/4.

> 1. [string.cons]/18

>    > Effects: Same as `basic_string(il.begin(), il.end(), a)`.

> 2. [string.capacity]/3

>    > Returns: `size()`.

> 3. [string.capacity]/8

>    > Effects: As if by `resize(n, charT())`.

> 4. [string.capacity]/16

>    > Effects: Behaves as if the function calls: `erase(begin(), end());`

> 5. [string.capacity]/17

>    > Returns: `size() == 0`.

> 6. [string.insert]/27

>    > Effects: As if by `insert(p, il.begin(), il.end())`.

>    > Returns: An iterator which refers to the copy of the first inserted character, or `p` if `i1` is empty.

[2017-01-27 Telecon]

Priority 3; Marshall to provide wording.

Resolved by the adoption of P1148 in San Diego.

Proposed resolution:

### 2842(i). `in_place_t` check for `optional::optional(U&&)` should decay `U`

Section: 22.5.3.2 [optional.ctor] Status: C++17 Submitter: Tim Song Opened: 2016-12-13 Last modified: 2020-09-06

Priority: 0

View all other issues in [optional.ctor].

View all issues with C++17 status.

Discussion:

As `in_place_t` is a normal tag type again, we need to decay `U` before doing the `is_same_v` check.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit 22.5.3.2 [optional.ctor] as indicated:

   >     template <class U = T>
   >       EXPLICIT constexpr optional(U&& v);

   > > […]

   > > -22- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `is_same_v<decay_t<U>, in_place_t>` is `false`, and `is_same_v<optional<T>, decay_t<U>>` is `false`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.

### 2843(i). Unclear behavior of `std::pmr::memory_resource::do_allocate()`

Section: 20.5.2.3 [mem.res.private] Status: C++20 Submitter: Jens Maurer Opened: 2016-12-13 Last modified: 2021-02-25

Priority: 3

View all other issues in [mem.res.private].

View all issues with C++20 status.

Discussion:

The specification of `do_allocate()` (20.5.2.3 [mem.res.private] p2+p3) says:

> Returns: A derived class shall implement this function to return a pointer to allocated storage (3.7.4.2) with a size of at least `bytes`. The returned storage is aligned to the specified alignment, if such alignment is supported (3.11); otherwise it is aligned to `max_align`.

> Throws: A derived class implementation shall throw an appropriate exception if it is unable to allocate memory with the requested size and alignment.

It is unclear whether a request for an unsupported alignment (e.g. larger than `max_align`) yields an exception or the returned storage is silently aligned to `max_align`.

This is editorial issue #966.

[2017-01-27 Telecon]

Priority 3; Marshall to ping Pablo for intent and provide wording.

[2017-02-12 Pablo responds and provides wording]

The original intent was:

- If the alignment is supported by the implementation (see below), then it must use that alignment or else throw.
- If the alignment is not supported it must use max_align else throw.

However, the description of do_allocate might have gone stale as the aligned-allocation proposal made its way into the standard.

The understanding I take from the definition of extended alignment in (the current text of) 3.11/3 [basic.align] and "assembling an argument list" in 5.3.4/14 [expr.new] is that it is intended that, when allocating space for an object with extended alignment in a well-formed program, the alignment will be honored and will not be truncated to max_align. I think this is a change from earlier drafts of the extended-alignment proposal, where silent truncation to `max_align` was permitted (I could be wrong). Anyway, it seems wrong to ever ignore the alignment parameter in `do_allocate()`.

[2017-11 Albuquerque Wednesday issue processing]

Move to Ready.

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

Change the specification of `do_allocate()` (20.5.2.3 [mem.res.private] p2+p3) as follows:

> Returns: A derived class shall implement this function to return a pointer to allocated storage (3.7.4.2) with a size of at least `bytes`, aligned to the specified `alignment`. The returned storage is aligned to the specified alignment, if such alignment is supported; otherwise it is aligned to `max_align`.

> Throws: A derived class implementation shall throw an appropriate exception if it is unable to allocate memory with the requested size and alignment.

### 2849(i). Why does `!is_regular_file(from)` cause `copy_file` to report a "file already exists" error?

Section: 31.12.13.5 [fs.op.copy.file] Status: C++20 Submitter: Tim Song Opened: 2016-12-17 Last modified: 2021-06-06

Priority: 2

View all other issues in [fs.op.copy.file].

View all issues with C++20 status.

Discussion:

[fs.op.copy_file]/4 says that `copy_file` reports "a file already exists error as specified in [fs.err.report] if" any of several error conditions exist.

It's not clear how some of those error conditions, such as `!is_regular_file(from)`, can be sensibly described as "file already exists". Pretty much everywhere else in the filesystem specification just says "an error" without further elaboration.

[2017-01-27 Telecon]

Priority 2; Jonathan to provide updated wording.

[2018-01-16, Jonathan comments]

I said I'd provide updated wording because I wanted to preserve the requirement that the reported error is "file already exists" for some of the error cases. I no longer think that's necessary, so I think the current P/R is fine. Please note that I don't think new wording is needed.

[ 2018-01-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Edit [fs.op.copy_file]/4 as indicated:

   >     bool copy_file(const path& from, const path& to, copy_options options);
   >     bool copy_file(const path& from, const path& to, copy_options options,
   >                    error_code& ec) noexcept;

   > > -4- Effects: As follows:

   > > 1. (4.1) — Report a file already existsan error as specified in 31.12.5 [fs.err.report] if:

   > > 2. 1. (4.1.1) —[…]

   > > 3. (4.2) —[…]

### 2850(i). `std::function` move constructor does unnecessary work

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++17 Submitter: Geoffrey Romer Opened: 2016-12-15 Last modified: 2020-09-06

Priority: 0

View all other issues in [func.wrap.func.con].

View all issues with C++17 status.

Discussion:

Consider [func.wrap.func.con]/p5:

>     function(function&& f);

> > Effects: If `!f`, `*this` has no target; otherwise, move constructs the target of `f` into the target of `*this`, leaving `f` in a valid state with an unspecified value.

By my reading, this wording requires the move constructor of `std::function` to construct an entirely new target object. This is silly: in cases where the target is held in separately allocated memory (i.e. where the target doesn't fit in `std::function`'s internal buffer, if any), `std::function`'s move constructor can be implemented by simply transferring ownership of the existing target object (which is a simple pointer assignment), so this requirement forces an unnecessary constructor invocation and probably an unnecessary allocation (the latter can be avoided with something like double-buffering, but ew). Fixing this would technically be a visible change, but I have a hard time imagining reasonable code that would be broken by it, especially since both libstdc++ and libc++ already do the sensible thing, constructing a new target only if the target is held in an internal buffer, and otherwise assigning pointers.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit 22.10.17.3.2 [func.wrap.func.con]/5 as indicated:

   > Drafting note: The "equivalent to ... before the construction" wording is based on the wording for `MoveConstructible`.

   >     function(function&& f);

   > > -5- EffectsPostconditions: If `!f`, `*this` has no target; otherwise, move constructs the target of `f` into the target of `*this`, leaving `f` the target of `*this` is equivalent to the target of `f` before the construction, and `f` is in a valid state with an unspecified value.

### 2851(i). `std::filesystem` enum classes are now underspecified

Section: 31.12.8.2 [fs.enum.file.type], 31.12.8.3 [fs.enum.copy.opts], 31.12.8.6 [fs.enum.dir.opts] Status: C++20 Submitter: Tim Song Opened: 2016-12-18 Last modified: 2021-06-06

Priority: 2

View all other issues in [fs.enum.file.type].

View all issues with C++20 status.

Discussion:

LWG 2678(i) stripped the numerical values of the enumerators from three enum classes in 31.12.8 [fs.enum]; in doing so it also removed the implicit specification 1) of the bitmask elements for the two bitmask types (`copy_options` and `directory_options`) and 2) that the `file_type` constants are distinct.

[2017-01-27 Telecon]

Priority 2; Jonathan to work with Tim to tweak wording.

[2018-01-16, Jonathan comments]

I no longer remember what I didn't like about Tim's P/R so I think we should accept the original P/R.

[2018-1-26 issues processing telecon]

Status to 'Tentatively Ready'

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Edit [fs.enum.file_type]/1 as indicated:

   > This enum class specifies constants used to identify file types, with the meanings listed in Table 123. The values of the constants are distinct.

2. Edit 31.12.8.3 [fs.enum.copy.opts]/1 as indicated:

   > The `enum class` type `copy_options` is a bitmask type (16.3.3.3.3 [bitmask.types]) that specifies bitmask constants used to control the semantics of copy operations. The constants are specified in option groups with the meanings listed in Table 124. The constant `none` represents the empty bitmask, and Constant `none` is shown in each option group for purposes of exposition; implementations shall provide only a single definition. Every other constant in the table represents a distinct bitmask element. Calling a library function with more than a single constant for an option group results in undefined behavior.

3. Edit 31.12.8.6 [fs.enum.dir.opts]/1 as indicated:

   > The `enum class` type `directory_options` is a bitmask type (16.3.3.3.3 [bitmask.types]) that specifies bitmask constants used to identify directory traversal options, with the meanings listed in Table 127. The constant `none` represents the empty bitmask; every other constant in the table represents a distinct bitmask element.

### 2853(i). Possible inconsistency in specification of `erase` in [vector.modifiers]

Section: 23.3.13.5 [vector.modifiers] Status: C++17 Submitter: Gerard Stone Opened: 2017-01-16 Last modified: 2020-09-06

Priority: 0

View all other issues in [vector.modifiers].

View all issues with C++17 status.

Discussion:

In Table 87 (Sequence container requirements) `erase(q)` and `erase(q1, q2)` functions have the following requirements:

> For `vector` and `deque`, `T` shall be `MoveAssignable`.

On the other hand, section [vector.modifiers] has the following specification for erase functions (emphasis mine):

> Throws: Nothing unless an exception is thrown by the copy constructor, move constructor, assignment operator, or move assignment operator of `T`.

Note that Table 87 requires `T` to be only `MoveAssignable`, it says nothing about `T` being copy- or move-constructible. It also says nothing about `T` being `CopyInsertable` and `MoveInsertable`, so why is this even there? The only reason might be so that vector could shrink, but in this case `T` should be required to be `MoveInsertable` or `CopyInsertable` into `vector`.

On the other hand, we expect that vector will neither allocate, nor deallocate any memory during this operation, because in Effects it is specified that iterators/references shall be invalidated at or after the point of the erase.

So to avoid any confusion, the proposed resolution is to remove mentions of `T`'s copy/move constructors from [vector.modifiers] paragraph 5.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit 23.3.13.5 [vector.modifiers] p5 as indicated:

   >     iterator erase(const_iterator position);
   >     iterator erase(const_iterator first, const_iterator last);
   >     void pop_back();

   > > -3- Effects: Invalidates iterators and references at or after the point of the erase.

   > > -4- Complexity: The destructor of `T` is called the number of times equal to the number of the elements erased, but the assignment operator of `T` is called the number of times equal to the number of elements in the vector after the erased elements.

   > > -5- Throws: Nothing unless an exception is thrown by the copy constructor, move constructor, assignment operator, or move assignment operator of `T`.

### 2855(i). `std::throw_with_nested("string_literal")`

Section: 17.9.8 [except.nested] Status: C++17 Submitter: Jonathan Wakely Opened: 2017-01-17 Last modified: 2020-09-06

Priority: 0

View all other issues in [except.nested].

View all issues with C++17 status.

Discussion:

[except.nested] says:

>     template <class T> [[noreturn]] void throw_with_nested(T&& t);

> > Let `U` be `remove_reference_t<T>`.

> > Requires: `U` shall be `CopyConstructible`.

This forbids `std::throw_with_nested("string literal")` because `T` gets deduced as `const char(&)[15]` and so `U` is `const char[15]` which is not `CopyConstructible`.

A throw expression decays an array argument to a pointer (7.6.18 [expr.throw] p2) and so works fine with string literals. GCC's `throw_with_nested` also worked fine until I added a `static_assert` to enforce the `CopyConstructible` requirement.

The same problem exists when throwing a function type, which should also decay:

>     #include <exception>

>     void f() { }

>     int main() {
>       std::throw_with_nested(f);
>     }

(Note: LWG 1370(i) added the `remove_reference`, which was a step in the right direction but not far enough.)

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit 17.9.8 [except.nested] as indicated:

   >     template <class T> [[noreturn]] void throw_with_nested(T&& t);

   > > -6- Let `U` be `remove_referencedecay_t<T>`.

   > > -7- Requires: `U` shall be `CopyConstructible`.

### 2856(i). `std::async` should be marked as `[[nodiscard]]`

Section: 32.10.9 [futures.async] Status: Resolved Submitter: Andrey Davydov Opened: 2017-01-20 Last modified: 2020-09-06

Priority: 2

View all other issues in [futures.async].

View all issues with Resolved status.

Discussion:

Because the destructor of the `std::future` returned from the `std::async` blocks until the asynchronous operation completes, discarding the `std::async` return value leads to the synchronous code execution, which is pointless. For example, in the following code

>     void task1();
>     void task2();

>     void foo()
>     {
>       std::async(std::launch::async,  task1),
>       std::async(std::launch::async,  task2);
>     }

>     void bar()
>     {
>       std::async(std::launch::async,  task1);
>       std::async(std::launch::async,  task2);
>     }

`task1` and `task2` will be concurrently executed in the function 'foo', but sequentially in the function 'bar'.

[2017-01-27 Telecon]

Priority 2; Nico to provide wording.

[2017-03-04, Kona]

This should be handled by Nico's paper P0600.

Proposed resolution:

Resolved by adoption of P0600 in Albuquerque

### 2857(i). `{variant,optional,any}::emplace` should return the constructed value

Section: 22.5 [optional], 22.6 [variant], 22.7 [any] Status: C++17 Submitter: Zhihao Yuan Opened: 2017-01-25 Last modified: 2020-09-06

Priority: 1

View all other issues in [optional].

View all issues with C++17 status.

Discussion:

When you want to continue operate on the new contained object constructed by `emplace`, you probably don't want to go through the type-safe machinery again.

[2017-01-27 Telecon]

Priority 1; send to LEWG.

[Kona 2017-03-02]

Accepted as Immediate because P1.

Proposed resolution:

This wording is relative to N4618.

1. Update the following signatures in 22.5.3 [optional.optional], class template `optional` synopsis, as indicated:

   >     […]
   >     // 22.5.3.4 [optional.assign], assignment
   >     […]
   >     template <class... Args> voidT& emplace(Args&&...);
   >     template <class U, class... Args>
   >       voidT& emplace(initializer_list<U>, Args&&...);
   >     […]

2. Modify 22.5.3.4 [optional.assign] as indicated:

   >     template <class... Args> voidT& emplace(Args&&... args);

   > > […]

   > > -27- Postconditions: `*this` contains a value.

   > > -?- Returns: A reference to the new contained value.

   > > -28- Throws: Any exception thrown by the selected constructor of `T`.

   >     template <class U, class... Args> voidT& emplace(initializer_list<U> il, Args&&... args);

   > > […]

   > > -31- Postconditions: `*this` contains a value.

   > > -?- Returns: A reference to the new contained value.

   > > -32- Throws: Any exception thrown by the selected constructor of `T`.

3. Modify the following signatures in 22.6.3 [variant.variant], class template `variant` synopsis, as indicated:

   >     […]
   >     // 22.6.3.5 [variant.mod], modifiers
   >     template <class T, class... Args> voidT& emplace(Args&&...);
   >     template <class T, class U, class... Args>
   >       voidT& emplace(initializer_list<U>, Args&&...);
   >     template <size_t I, class... Args> voidvariant_alternative_t<I, variant<Types...>>& emplace(Args&&...);
   >     template <size_t I, class U, class... Args>
   >       voidvariant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U>, Args&&...);
   >     […]

4. Modify 22.6.3.5 [variant.mod] as indicated:

   >     template <class T, class... Args> voidT& emplace(Args&&... args);

   > > -1- Effects: Equivalent to `return emplace<I>(std::forward<Args>(args)...);` where `I` is the zero-based index of `T` in `Types...`.

   > > […]

   >     template <class T, class U, class... Args>
   >       voidT& emplace(initializer_list<U> il, Args&&... args);

   > > -3- Effects: Equivalent to `return emplace<I>(il, std::forward<Args>(args)...);` where `I` is the zero-based index of `T` in `Types...`.

   > > […]

   >     template <size_t I, class... Args> voidvariant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);

   > > […]

   > > -7- Postconditions: `index()` is `I`.

   > > -?- Returns: A reference to the new contained value.

   > > -8- Throws: Any exception thrown during the initialization of the contained value.

   > > […]

   >     template <size_t I, class U, class... Args>
   >       voidvariant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U> il, Args&&... args);

   > > […]

   > > -12- Postconditions: `index()` is `I`.

   > > -?- Returns: A reference to the new contained value.

   > > -13- Throws: Any exception thrown during the initialization of the contained value.

   > > […]

5. Modify the following signatures in 22.7.4 [any.class], class `any` synopsis, as indicated:

   >     […]
   >     // 22.7.4.4 [any.modifiers], modifiers
   >     template <class ValueType, class... Args>
   >       voiddecay_t<ValueType>& emplace(Args&& ...);
   >     template <class ValueType, class U, class... Args>
   >       voiddecay_t<ValueType>& emplace(initializer_list<U>, Args&&...);
   >     […]

6. Modify 22.7.4.4 [any.modifiers] as indicated:

   >     template <class ValueType, class... Args>
   >       voiddecay_t<ValueType>& emplace(Args&&... args);

   > > […]

   > > -4- Postconditions: `*this` contains a value.

   > > -?- Returns: A reference to the new contained value.

   > > -5- Throws: Any exception thrown by the selected constructor of `T`.

   > > […]

   >     template <class ValueType, class U, class... Args>
   >       voiddecay_t<ValueType>& emplace(initializer_list<U> il, Args&&... args);

   > > […]

   > > -10- Postconditions: `*this` contains a value.

   > > -?- Returns: A reference to the new contained value.

   > > -11- Throws: Any exception thrown by the selected constructor of `T`.

   > > […]

### 2859(i). Definition of reachable in [ptr.launder] misses pointer arithmetic from pointer-interconvertible object

Section: 17.6.5 [ptr.launder] Status: C++20 Submitter: Hubert Tong Opened: 2017-01-31 Last modified: 2021-02-25

Priority: 2

View all other issues in [ptr.launder].

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Discussion:

Given:

>     struct A { int x, y; };
>     A a[100];

The bytes which compose `a[3]` can be reached from `&a[2].x`: `reinterpret_cast<A *>(&a[2].x) + 1` points to `a[3]`, however, the definition of "reachable" in [ptr.launder] does not encompass this case.

[2017-03-04, Kona]

Set priority to 2. Assign this (and 2860(i)) to Core.

[2017-08-14, CWG telecon note]

CWG is fine with the proposed resolution.

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4618.

1. Modify 17.6.5 [ptr.launder] as indicated:

   >     template <class T> constexpr T* launder(T* p) noexcept;

   > > […]

   > > -3- Remarks: An invocation of this function may be used in a core constant expression whenever the value of its argument may be used in a core constant expression. A byte of storage `b` is reachable through a pointer value that points to an object `Y` if there is an object `Z`, pointer-interconvertible with `Y`, such that `b` it is within the storage occupied by `ZY`, an object that is pointer-interconvertible with `Y`, or the immediately-enclosing array object if `ZY` is an array element. The program is ill-formed if `T` is a function type or (possibly cv-qualified) `void`.

### 2861(i). `basic_string` should require that `charT` match `traits::char_type`

Section: 27.4.3.2 [string.require] Status: C++17 Submitter: United States Opened: 2017-02-02 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [string.require].

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Discussion:

Addresses US 145

There is no requirement that `traits::char_type` is `charT`, although there is a requirement that `allocator::value_type` is `charT`. This means that it might be difficult to honour both methods returning reference (such as `operator[]`) and `charT&` (like `front`/`back`) when traits has a surprising `char_type`. It seems that the allocator should not rebind in such cases, making the reference-returning signatures the problematic ones.

Suggested resolution: Add a requirement that `is_same_v<typename traits::char_type, charT>` is true, and simplify so that value_type is just an alias for `charT`.

[2017-02-02 Marshall adds]

In [string.require]/3, there's already a note that the types shall be the same. In [string.view.template]/1, it says "In every specialization `basic_string_view<charT, traits, Allocator>`, the type `traits` shall satisfy the character traits requirements (21.2), and the type `traits::char_type` shall name the same type as `charT`".

[Kona 2017-02-28]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

Changes are based off of N4618

1. Modify [basic.string] as indicated (in the synopsis):

   >     class basic_string {
   >     public:
   >     // types:
   >       using traits_type      = traits;
   >       using value_type       = charTtypename traits::char_type;
   >       using allocator_type   = Allocator

2. Change [string.require]/3 as indicated:

   -3- In every specialization `basic_string<charT, traits, Allocator>`, the type `allocator_traits<Allocator>::value_type` shall name the same type as `charT`. Every object of type `basic_string<charT, traits, Allocator>` shall use an object of type `Allocator` to allocate and free storage for the contained `charT` objects as needed. The `Allocator` object used shall be obtained as described in 23.2.1. [ Note: In every specialization `basic_string<charT, traits, Allocator>`, the type `traits` shall satisfy the character traits requirements (21.2), and the type `traits::char_type` shall namebe the same type as `charT`; see 21.2. — end note ]

### 2862(i). LWG 2756 should be accepted

Section: 22.5 [optional] Status: Resolved Submitter: Finland Opened: 2017-02-03 Last modified: 2017-02-21

Priority: Not Prioritized

View all other issues in [optional].

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Discussion:

Addresses FI 10

Adopt the proposed resolution of LWG 2756(i) into C++17, to provide converting constructors and assignment operators for `optional`.

Proposed change:

Adopt the latest proposed resolution of LWG 2756(i), which should be available by Issaquah.

Proposed resolution:

Resolved by resolving LWG 2756(i).

### 2863(i). Undo `default_order` changes of maps and sets

Section: 99 [func.default.traits] Status: Resolved Submitter: Finland Opened: 2017-02-03 Last modified: 2017-03-20

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Addresses FI 18

It was thought that using `default_order` as the default comparison for maps and sets was not abi-breaking but this is apparently not the case.

Proposed change:

Revert the change to the default comparison of maps and sets.

[2016-10 Issaquah]

STL and AM want to revert whole paper.

[2017-03-12, post-Kona]

Resolved reverting P0181R1.

Proposed resolution:

### 2864(i). Merge `shared_ptr` changes from Library Fundamentals to C++17

Section: 20.3.2.2 [util.smartptr.shared], 20.3.2.3 [util.smartptr.weak] Status: Resolved Submitter: Finland Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [util.smartptr.shared].

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Discussion:

Addresses FI 19

The changes in the paper P0414 should be adopted into C++17.

Proposed change:

Adopt the changes in P0414.

Resolved by the adoption of P0414 in Issaquah

Proposed resolution:

### 2866(i). Incorrect derived classes constraints

Section: 16.4.6.13 [derivation] Status: C++17 Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [derivation].

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Discussion:

Addresses GB 36

For bullet (3.2), no base classes are described as non-virtual. Rather, base classes are not specified as virtual, a subtly different negative.

Proposed change:

Rewrite bullet 3.2:

> Every base class not specified as virtual shall not be virtual;

[Kona 2017-03-01]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 16.4.6.13 [derivation] paragraph 3.2 as indicated:

   > Every base class described as non-`virtual` shall not be virtual; Every base class not specified as `virtual` shall not be virtual;

### 2867(i). Bad footnote about explicit exception-specification

Section: 16.4.6.14 [res.on.exception.handling] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View other active issues in [res.on.exception.handling].

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Discussion:

Addresses GB 40

The freedom referenced in footnote 189 was curtailed in C++11 to allow only non-throwing specifications. The footnote is both wrong, and unnecessary.

Proposed change:

Strike footnote 189.

[2017-02-03, Daniel comments]

The referenced footnote had the following content in the ballot document N4604:

> That is, an implementation may provide an explicit exception-specification that defines the subset of "any" exceptions thrown by that function. This implies that the implementation may list implementation-defined types in such an exception-specification.

In N4618 this footnote has already been removed, I therefore suggest to resolve the issue as "Resolved by editorial change".

[2017-02-13, Alisdair comments]

Just to confirm that issue 2867 was filed when it looked like the paper to remove exception specifications from the language had missed C++17. The very specific edit requested by the issue can be found in the paper P0003R5 that was applied.

I suggest we mark as resolved by the above paper, rather than "resolved editorially".

Proposed resolution:

Resolved by accepting P0003R5.

### 2868(i). Missing specification of `bad_any_cast::what()`

Section: 22.7.3 [any.bad.any.cast] Status: C++17 Submitter: Great Britain Opened: 2017-02-03 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Addresses GB 54

There is no specification for `bad_any_cast.what`.

Proposed change:

Add a paragraphs:

>     const char* what() const noexcept override;

> > Returns: An implementation-defined NTBS.

> > Remarks: The message may be a null-terminated multibyte string (17.5.2.1.4.2), suitable for conversion and display as a `wstring` (21.3, 22.4.1.4).

[Kona 2017-03-01]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4618.

1. Insert the following series of paragraphs to [any.bad_any_cast] as indicated:

   >     const char* what() const noexcept override;

   > > -?- Returns: An implementation-defined NTBS.

   > > -?- Remarks: The message may be a null-terminated multibyte string (16.3.3.3.4.3 [multibyte.strings]), suitable for conversion and display as a `wstring` (27.4 [string.classes], 28.3.4.2.5 [locale.codecvt]).

### 2869(i). Deprecate sub-clause [locale.stdcvt]

Section: 99 [depr.locale.stdcvt] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2017-04-22

Priority: Not Prioritized

View all other issues in [depr.locale.stdcvt].

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Discussion:

Addresses GB 57

The contents of `<codecvt>` are underspecified, and will take a reasonable amount of work to identify and correct all of the issues. There appears to be a general feeling that this is not the best way to address unicode transcoding in the first place, and this library component should be retired to Annex D, along side `<strstream>`, until a suitable replacement is standardized.

Proposed change:

Deprecate and move the whole of clause [locale.stdcvt] to Annex D.

[2017-02 pre-Kona]

LEWG says Accept.

[2017-03-12, post-Kona]

Resolved by P0618R0.

Proposed resolution:

### 2870(i). Default value of parameter `theta` of `polar` should be dependent

Section: 29.4.7 [complex.value.ops] Status: C++20 Submitter: Japan Opened: 2017-02-03 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [complex.value.ops].

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Discussion:

Addresses JP 25

Parameter `theta` of `polar` has the type of the template parameter. Therefore, it needs to change the default initial value to `T()`. The change of the declaration of this function in 29.4.2 [complex.syn] is accompanied by this change.

Proposed change:

>     template<class T> complex<T> polar(const T& rho, const T& theta = 0T());

[2017-02 pre-Kona]

(twice)

[ 2017-06-27 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]

Proposed resolution:

This wording is relative to N4659.

1. Modify 29.4.2 [complex.syn], header `<complex>` synopsis, as indicated:

   >     template<class T> complex<T> polar(const T&, const T& = 0T());

2. Modify 29.4.7 [complex.value.ops] as indicated:

   >     template<class T> complex<T> polar(const T& rho, const T& theta = 0T());

### 2872(i). Add definition for direct-non-list-initialization

Section: 3 [intro.defs] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2021-06-06

Priority: Not Prioritized

View all issues with C++17 status.

Discussion:

Addresses US 107

The term 'direct non-list initialization' needs to be incorporated from the Library Fundamentals TS, as several components added to C++17 rely on this definition.

Proposed change:

Add:

> 17.3.X direct-non-list-initialization [defns.direct-non-list-init]

> A direct-initialization that is not list-initialization.

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4618.

1. Add the following to [definitions] as indicated:

   > 17.3.? direct-non-list-initialization [defns.direct-non-list-init]

   > A direct-initialization that is not list-initialization.

### 2873(i). Add `noexcept` to several `shared_ptr` related functions

Section: 20.3.2.2 [util.smartptr.shared] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [util.smartptr.shared].

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Discussion:

Addresses US 124

Several `shared_ptr` related functions have wide contracts and cannot throw, so should be marked unconditionally `noexcept`.

Proposed change:

Add `noexcept` to:

>     template<class U>
>     bool
>     shared_ptr::owner_before(shared_ptr<U> const& b) const noexcept;

>     template<class U>
>     bool
>     shared_ptr::owner_before(weak_ptr<U> const& b) const noexcept;

>     template<class U>
>     bool
>     weak_ptr::owner_before(shared_ptr<U> const& b) const noexcept;

>     template<class U>
>     bool
>     weak_ptr::owner_before(weak_ptr<U> const& b) const noexcept;

>     bool owner_less::operator()(A,B) const noexcept; // all versions

[2017-02-20, Marshall adds wording]

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 20.3.2.2 [util.smartptr.shared] as indicated:

   >       template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept;
   >       template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;

2. Modify 20.3.2.2.6 [util.smartptr.shared.obs] as indicated:

   >     template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept;
   >     template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;

3. Modify 20.3.2.4 [util.smartptr.ownerless] as indicated:

   >     template<class T> struct owner_less<shared_ptr<T>> {
   >        bool operator()(const shared_ptr<T>&, const shared_ptr<T>&) const noexcept;
   >        bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
   >        bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
   >     };

   >     template<class T> struct owner_less<weak_ptr<T>> {
   >        bool operator()(const weak_ptr<T>&, const weak_ptr<T>&) const noexcept;
   >        bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
   >        bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
   >     };

   >     template<> struct owner_less<void> {
   >        template<class T, class U>
   >           bool operator()(const shared_ptr<T>&, const shared_ptr<U>&) const noexcept;
   >        template<class T, class U>
   >           bool operator()(const shared_ptr<T>&, const weak_ptr<U>&) const noexcept;
   >        template<class T, class U>
   >           bool operator()(const weak_ptr<T>&, const shared_ptr<U>&) const noexcept;
   >        template<class T, class U>
   >           bool operator()(const weak_ptr<T>&, const weak_ptr<U>&) const noexcept;
   >        using is_transparent = unspecified;
   >     };

### 2874(i). Constructor `shared_ptr::shared_ptr(Y*)` should be constrained

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 125

Paragraph 4: This constructor should not participate in overload resolution unless the Requires clause is satisfied. Note that this would therefore apply to some assignment operator and `reset` overloads, via Effects: equivalent to some code wording.

Proposed change:

Add a Remarks: clause to constrain this constructor not to participate in overload resolution unless the Requires clause is satisfied.

[2017-02-23, Jonathan provides wording]

Previous resolution [SUPERSEDED]:

This wording is relative to N4640.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   > [Drafting note: This also adds a hyphen to "well defined"]

   >     template<class Y> explicit shared_ptr(Y* p);

   > > -4- Requires: `Y` shall be a complete type. The expression `delete[] p`, when `T` is an array type, or `delete p`, when `T` is not an array type, shall be well formed, shall have well -defined behavior, and shall not throw exceptions. When `T` is `U[N]`, `Y(*)[N]` shall be convertible to `T*`; when `T` is `U[]`, `Y(*)[]` shall be convertible to `T*`; otherwise, `Y*` shall be convertible to `T*`.

   > > -5- Effects: […]

   > > -6- Postconditions: […]

   > > -7- Throws: […]

   > > -?- Remarks: When `T` is an array type, this constructor shall not participate in overload resolution unless the expression `delete[] p` is well-formed and either `T` is `U[N]` and `Y(*)[N]` is convertible to `T*`, or `Y(*)[]` is convertible to `T*`. When `T` is not an array type, this constructor shall not participate in overload resolution unless the expression `delete p` is well-formed and `Y*` is convertible to `T*`.

[Kona 2017-02-27: Jonathan updates wording after LWG review]

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   > [Drafting note: This also adds a hyphen to "well defined"]

   >     template<class Y> explicit shared_ptr(Y* p);

   > > -4- Requires: `Y` shall be a complete type. The expression `delete[] p`, when `T` is an array type, or `delete p`, when `T` is not an array type, shall be well formed, shall have well -defined behavior, and shall not throw exceptions. When `T` is `U[N]`, `Y(*)[N]` shall be convertible to `T*`; when `T` is `U[]`, `Y(*)[]` shall be convertible to `T*`; otherwise, `Y*` shall be convertible to `T*`.

   > > -5- Effects: […]

   > > -6- Postconditions: […]

   > > -7- Throws: […]

   > > -?- Remarks: When `T` is an array type, this constructor shall not participate in overload resolution unless the expression `delete[] p` is well-formed and either `T` is `U[N]` and `Y(*)[N]` is convertible to `T*`, or `T` is `U[]` and `Y(*)[]` is convertible to `T*`. When `T` is not an array type, this constructor shall not participate in overload resolution unless the expression `delete p` is well-formed and `Y*` is convertible to `T*`.

### 2875(i). `shared_ptr::shared_ptr(Y*, D, […])` constructors should be constrained

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 126

Paragraph 8: This constructor should not participate in overload resolution unless the Requires clause is satisfied. Note that this would therefore apply to some assignment operator and `reset` overloads, via Effects: equivalent to some code wording.

Proposed change:

Add a Remarks: clause to constrain this constructor not to participate in overload resolution unless the Requires clause is satisfied.

[2017-02 pre-Kona]

See US 125: LWG 2874(i).

[2017-02-23, Jonathan provides wording]

This wording is relative to N4640.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   1. If the proposed resolution of LWG 2802(i) has been accepted modify p8 as shown:

      >     template<class Y, class D> shared_ptr(Y* p, D d);
      >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
      >     template <class D> shared_ptr(nullptr_t p, D d);
      >     template <class D, class A> shared_ptr(nullptr_t p, D d, A a);

      > > -8- Requires: `D` shall be `MoveConstructible` and cConstruction of `d` and a deleter of type `D` initialized with `std::move(d)` shall not throw exceptions. The expression `d(p)` shall be well formed, shall have well-defined behavior, and shall not throw exceptions. `A` shall be an allocator (17.5.3.5). When `T` is `U[N]`, `Y(*)[N]` shall be convertible to `T*`; when `T` is `U[]`, `Y(*)[]` shall be convertible to `T*`; otherwise, `Y*` shall be convertible to `T*`.

   2. If the proposed resolution of LWG 2802(i) has not been accepted modify p8 as shown:

      >     template<class Y, class D> shared_ptr(Y* p, D d);
      >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
      >     template <class D> shared_ptr(nullptr_t p, D d);
      >     template <class D, class A> shared_ptr(nullptr_t p, D d, A a);

      > > -8- Requires: `D` shall be `CopyConstructible` and such cConstruction of `d` and a copy of `d` shall not throw exceptions. The destructor of `D` shall not throw exceptions. The expression `d(p)` shall be well formed, shall have well defined behavior, and shall not throw exceptions. `A` shall be an allocator (17.5.3.5). The copy constructor and destructor of `A` shall not throw exceptions. When `T` is `U[N]`, `Y(*)[N]` shall be convertible to `T*`; when `T` is `U[]`, `Y(*)[]` shall be convertible to `T*`; otherwise, `Y*` shall be convertible to `T*`.

1. In either case, add a Remarks paragraph after p11:

   > [Drafting note: If LWG 2802(i) is not accepted, replace `is_move_constructible_v` with `is_copy_constructible_v`.]

   >     template<class Y, class D> shared_ptr(Y* p, D d);
   >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
   >     template <class D> shared_ptr(nullptr_t p, D d);
   >     template <class D, class A> shared_ptr(nullptr_t p, D d, A a);

   > > -8- Requires: […]

   > > […]

   > > -11- Throws: […]

   > > -?- Remarks: When `T` is an array type, this constructor shall not participate in overload resolution unless `is_move_constructible_v<D>` is `true`, the expression `d(p)` is well-formed, and either `T` is `U[N]` and `Y(*)[N]` is convertible to `T*`, or `Y(*)[]` is convertible to `T*`. When `T` is not an array type, this constructor shall not participate in overload resolution unless `is_move_constructible_v<D>` is `true`, the expression `d(p)` is well-formed, and `Y*` is convertible to `T*`.

[Kona 2017-02-27: Jonathan updates wording after LWG review]

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640 as modified by the proposed resolution of LWG 2802(i).

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   >     template<class Y, class D> shared_ptr(Y* p, D d);
   >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
   >     template <class D> shared_ptr(nullptr_t p, D d);
   >     template <class D, class A> shared_ptr(nullptr_t p, D d, A a);

   > > -8- Requires: `D` shall be `MoveConstructible` and cConstruction of `d` and a deleter of type `D` initialized with `std::move(d)` shall not throw exceptions. The expression `d(p)` shall be well formed, shall have well-defined behavior, and shall not throw exceptions. `A` shall be an allocator (17.5.3.5). When `T` is `U[N]`, `Y(*)[N]` shall be convertible to `T*`; when `T` is `U[]`, `Y(*)[]` shall be convertible to `T*`; otherwise, `Y*` shall be convertible to `T*`.

2. Add a Remarks paragraph after p11:

   >     template<class Y, class D> shared_ptr(Y* p, D d);
   >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
   >     template <class D> shared_ptr(nullptr_t p, D d);
   >     template <class D, class A> shared_ptr(nullptr_t p, D d, A a);

   > > -8- Requires: […]

   > > […]

   > > -11- Throws: […]

   > > -?- Remarks: When `T` is an array type, this constructor shall not participate in overload resolution unless `is_move_constructible_v<D>` is `true`, the expression `d(p)` is well-formed, and either `T` is `U[N]` and `Y(*)[N]` is convertible to `T*`, or `T` is `U[]` and `Y(*)[]` is convertible to `T*`. When `T` is not an array type, this constructor shall not participate in overload resolution unless `is_move_constructible_v<D>` is `true`, the expression `d(p)` is well-formed, and `Y*` is convertible to `T*`.

### 2876(i). `shared_ptr::shared_ptr(const weak_ptr<Y>&)` constructor should be constrained

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View other active issues in [util.smartptr.shared.const].

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Discussion:

Addresses US 129

Paragraph 22: This constructor should not participate in overload resolution unless the requirements are satisfied, in order to give correct results from the `is_constructible` trait.

Proposed change:

Add a Remarks: clause to constrain this constructor not to participate in overload resolution unless the Requires clause is satisfied.

[2017-02-23, Jonathan provides wording]

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   >     template<class Y> explicit shared_ptr(const weak_ptr<Y>& r);

   > > -22- Requires: `Y*` shall be compatible with `T*`.

   > > […]

   > > -25- Throws: […]

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `Y*` is compatible with `T*`.

### 2877(i). Strengthen meaning of "empty `shared_ptr<T>`" in `dynamic_pointer_cast`

Section: 20.3.2.2.10 [util.smartptr.shared.cast] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [util.smartptr.shared.cast].

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Discussion:

Addresses US 137

Paragraph (6.2): It is intuitive, but not specified, that the empty pointer returned by a `dynamic_pointer_cast` should point to null.

Proposed change:

Rephrase as:

> Otherwise, `shared_ptr<T>()`.

Proposed resolution:

Resolved by P0414R2, which was adopted in Issaquah.

### 2878(i). Missing `DefaultConstructible` requirement for `istream_iterator` default constructor

Section: 24.6.2.2 [istream.iterator.cons] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [istream.iterator.cons].

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Discussion:

Addresses US 153

`istream_iterator` default constructor requires a `DefaultConstructible T`.

Proposed change:

Add a new p1:

> Requires: `T` is `DefaultConstructible`.

Previous resolution [SUPERSEDED]:

This wording is relative to N4618.

1. Modify 24.6.2.2 [istream.iterator.cons] as indicated:

   >     see below istream_iterator();

   > > -?- Requires: `T` is `DefaultConstructible`.

   > > -1- Effects: Constructs the end-of-stream iterator. If `is_trivially_default_constructible_v<T>` is `true`, then this constructor is a `constexpr` constructor.

   > > -2- Postconditions: `in_stream == 0`.

[Kona 2017-02-28: Jonathan provides updated wording as requested by LWG.]

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4618.

1. Modify 24.6.2 [istream.iterator] as indicated:

   > -1- The class template `istream_iterator` is an input iterator (24.2.3) that reads (using `operator>>`) successive elements from the input stream for which it was constructed. After it is constructed, and every time `++` is used, the iterator reads and stores a value of `T`. If the iterator fails to read and store a value of `T` (`fail()` on the stream returns `true`), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments `istream_iterator()` always constructs an end-of-stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of `operator*` on an end-of-stream iterator is not defined. For any other iterator value a `const T&` is returned. The result of `operator->` on an end-of-stream iterator is not defined. For any other iterator value a `const T*` is returned. The behavior of a program that applies `operator++()` to an end-of-stream iterator is undefined. It is impossible to store things into istream iterators. The type `T` shall meet the `DefaultConstructible`, `CopyConstructible`, and `CopyAssignable` requirements.

### 2879(i). Removing C dependencies from signal handler wording

Section: 17.14.4 [csignal.syn], 17.5 [support.start.term] Status: Resolved Submitter: Canada Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

Addresses CA 1

P0270R1 went through SG1 and LWG but was too late to make it to the straw polls.

The problems it addresses stem from referring to C11, which came into C++17 at the last minute.

P0270R1 should have made it in with the C11 change.

Proposed change: Apply all of P0270R1, "Removing C dependencies from signal handler wording", to C++17.

[Kona 2017-03-04]

P0270R1 was adopted in Kona

Proposed resolution:

### 2880(i). Relax complexity specifications for non-sequenced policies

Section: 26.3.3 [algorithms.parallel.exec] Status: Resolved Submitter: Switzerland Opened: 2017-02-03 Last modified: 2017-06-26

Priority: Not Prioritized

View all other issues in [algorithms.parallel.exec].

View all issues with Resolved status.

Discussion:

Addresses CH 10

Parallel implementations of algorithms may be faster if not restricted to the complexity specifications of serial implementations.

Proposed change: Add a relaxation of complexity specifications for non-sequenced policies.

Proposed resolution:

Resolved by the adoption of P0523r1 in Kona

### 2882(i). Clarify `variant` construction

Section: 22.6.3.2 [variant.ctor] Status: Resolved Submitter: Switzerland Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View other active issues in [variant.ctor].

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Discussion:

Addresses CH 8

Clarify `variant` construction

Proposed change: Add a note that `variant<>` cannot be constructed.

[2017-02-03, Marshall comments]

[variant]/3 now says "A program that instantiates the definition of `variant` with no template arguments is ill-formed."

[2017-02-21, Marshall comments]

It appears that the changes requested by this NB comment have already been applied — as part of P0510R0, adopted in Issaquah.

The NB comment is already marked as "accepted", so I suggest we mark the issue as "Resolved".

Proposed resolution:

Resolved by applying corresponding wording changes through P0510R0.

### 2887(i). Revert the changes from P0156R0: variadic `lock_guard`

Section: 32.6.5.2 [thread.lock.guard] Status: Resolved Submitter: Finland, Great Britain Opened: 2017-02-03 Last modified: 2017-04-22

Priority: Not Prioritized

View all other issues in [thread.lock.guard].

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Discussion:

Addresses FI 8, GB 61

The class template `lock_guard` was made variadic. This is abi-breaking, and confusing because one-argument `lock_guard`s have a typedef `mutex_type` but `lock_guard`s with more than one argument don't. There's no need to try to shoehorn this functionality into one type.

Proposed change: Revert the changes to `lock_guard`, and introduce a new variadic class template `vlock_guard` that doesn't have the `mutex_type` typedef at all.

[2017-02-02, Marshall notes]

This was the subject of intense discussion in Issaquah, and a joint LEG/LEWG session agreed on this approach.

[2017-03-12, post-Kona]

Resolved by P0156R2.

Proposed resolution:

### 2888(i). Variables of library tag types need to be inline variables

Section: 22 [utilities], 32 [thread] Status: Resolved Submitter: Finland Opened: 2017-02-03 Last modified: 2017-03-20

Priority: Not Prioritized

View all other issues in [utilities].

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Discussion:

Addresses FI 9

The variables of library tag types need to be inline variables. Otherwise, using them in inline functions in multiple translation units is an ODR violation.

Proposed change: Make `piecewise_construct`, `allocator_arg`, `nullopt`, (the `in_place_tags` after they are made regular tags), `defer_lock`, `try_to_lock` and `adopt_lock` inline.

[2017-02-03, Marshall notes]

See also GB 28 (LWG 2889(i))

[2017-02-25, Daniel comments]

There will be the paper p0607r0 provided for the Kona meeting that solves this issue.

[2017-03-12, post-Kona]

Resolved by p0607r0.

Proposed resolution:

### 2889(i). Mark `constexpr` global variables as `inline`

Section: 22 [utilities], 26.3.6 [execpol] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2017-03-20

Priority: Not Prioritized

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Discussion:

Addresses GB 28

The C++ standard library provides many `constexpr` global variables. These all create the risk of ODR violations for innocent user code. This is especially bad for the new `ExecutionPolicy` algorithms, since their constants are always passed by reference, so any use of those algorithms from an inline function results in an ODR violation.

This can be avoided by marking the globals as `inline`.

Proposed change: Add `inline` specifier to: `bind` placeholders `_1, _2, ...`, `nullopt`, `piecewise_construct`, `allocator_arg`, `ignore`, `seq`, `par`, `par_unseq` in `<execution>`

[2017-02-03, Marshall notes]

See also FI 9 (LWG 2888(i)).

[2017-02-25, Daniel comments]

There will be the paper p0607r0 provided for the Kona meeting that solves this issue.

[2017-03-12, post-Kona]

Resolved by p0607r0.

Proposed resolution:

### 2890(i). The definition of 'object state' applies only to class types

Section: 99 [defns.obj.state] Status: C++17 Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [defns.obj.state].

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Discussion:

Addresses GB 30

The definition of 'object state' applies only to class types, implying that fundamental types and arrays do not have this property.

Proposed change: Replacing "an object state" with "a value of an object" in 3.68 [defns.valid] and dropping the definition of "object state" in 99 [defns.obj.state].

[2017-02-26, Scott Schurr provides wording]

[Kona 2017-03-01]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 99 [defns.obj.state] as indicated:

   > 17.3.16 [defns.obj.state]

   > object state

   > the current value of all non-static class members of an object (9.2) [Note: The state of an object can be obtained by using one or more observer functions. — end note]

2. Modify 3.68 [defns.valid] as indicated:

   > 17.3.25 [defns.valid]

   > valid but unspecified state

   > an object statea value of an object that is not specified except that the object's invariants are met and operations on the object behave as specified for its type

### 2894(i). The function template `std::apply()` is required to be `constexpr`, but `std::invoke()` isn't

Section: 22.10.5 [func.invoke] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06

Priority: 3

View all other issues in [func.invoke].

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Discussion:

Addresses GB 51

The function template `std::apply()` in 22.4.6 [tuple.apply] is required to be `constexpr`, but `std::invoke()` in 22.10.5 [func.invoke] isn't. The most sensible implementation of `apply_impl()` is exactly equivalent to `std::invoke()`, so this requires implementations to have a `constexpr` version of `invoke()` for internal use, and the public API `std::invoke`, which must not be `constexpr` even though it is probably implemented in terms of the internal version.

Proposed change: Add `constexpr` to `std::invoke`.

[2017-02-20, Marshall adds wording]

[Kona 2017-03-01]

We think this needs CWG 1581 to work; accepted as Immediate to resolve NB comment.

Friday: CWG 1581 was not moved in Kona. Status back to Open.

[2017-07 Toronto Tuesday PM issue prioritization]

Priority 3

[2020-01 Resolved by the adoption of P1065 in Cologne.]

Proposed resolution:

This wording is relative to N4640.

1. Modify 22.10.5 [func.invoke] as indicated:

   >     template <class F, class... Args>
   >          constexpr result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args);

### 2895(i). Passing function types to `result_of` and `is_callable`

Section: 21.3.7 [meta.rel] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View other active issues in [meta.rel].

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Discussion:

Addresses GB 55

It is becoming more and more apparent that using a function type as the template argument to `result_of` causes annoying problems. That was done because C++03 didn't have variadic templates, so it allowed an arbitrary number of types to be smuggled into the template via a single parameter, but it's a hack and unnecessary in C++ today. `result_of<F(Args...)>` has absolutely nothing to do with a function type that returns `F`, and the syntactic trickery using a function type has unfortunate consequences such as top-level cv-qualifiers and arrays decaying (because those are the rules for function types).

It might be too late to change `result_of`, but we should not repeat the same mistake for `std::is_callable`.

Proposed change: Possibly get rid of the `is_callable<Fn(ArgTypes?...), R>` specialization. Change the primary template `is_callable<class, class R = void> to is_callable<class Fn, class.. ArgTypes?>` and define a separate template such as `is_callable_r<class R, class Fn, class... ArgTypes?>` for the version that checks the return type. The resulting inconsistency might need to be resolved/improved upon.

[2017-02, pre-Kona]

See also LWG 2927(i).

[2017-02-22, Daniel comments and provides concrete wording]

The approach chosen to resolve this issue is a merger with LWG 2928(i), that is the `callable` traits are also renamed to `invocable`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Modify 21.3.3 [meta.type.synop], header `<type_traits>` synopsis, as indicated:

>    >     […]
>    >     // 20.15.6, type relations
>    >     […]

>    >     template <class, class R = void> struct is_callable; // not defined
>    >     template <class Fn, class... ArgTypes, class R>
>    >     struct is_callable<Fn(ArgTypes...), R>;
>    >     template <class Fn, class... ArgTypes> struct is_invocable;
>    >     template <class R, class Fn, class... ArgTypes> struct is_invocable_r;

>    >     template <class, class R = void> struct is_nothrow_callable; // not defined
>    >     template <class Fn, class... ArgTypes, class R>
>    >     struct is_nothrow_callable<Fn(ArgTypes...), R>;
>    >     template <class Fn, class... ArgTypes> struct is_nothrow_invocable;
>    >     template <class R, class Fn, class... ArgTypes> struct is_nothrow_invocable_r;

>    >     […]

>    >     // 20.15.6, type relations
>    >     […]
>    >     template <class T, class R = void> constexpr bool is_callable_v
>    >     = is_callable<T, R>::value;
>    >     template <class T, class R = void> constexpr bool is_nothrow_callable_v
>    >     = is_nothrow_callable<T, R>::value;
>    >     template <class Fn, class... ArgTypes> constexpr bool is_invocable_v
>    >     = is_invocable<Fn, ArgTypes...>::value;
>    >     template <class R, class Fn, class... ArgTypes> constexpr bool is_invocable_r_v
>    >     = is_invocable_r<R, Fn, ArgTypes...>::value;
>    >     template <class Fn, class... ArgTypes> constexpr bool is_nothrow_invocable_v
>    >     = is_nothrow_invocable<Fn, ArgTypes...>::value;
>    >     template <class R, class Fn, class... ArgTypes> constexpr bool is_nothrow_invocable_r_v
>    >     = is_nothrow_invocable_r<R, Fn, ArgTypes...>::value;
>    >     […]

> 2. Modify 21.3.7 [meta.rel], Table 44 — "Type relationship predicates", as indicated:

>    > Table 44 — Type relationship predicates
>    > `[…]`
>    > `template <class Fn, class... ArgTypes, class R> struct is_invocablecallable< Fn(ArgTypes...), R>;` | The expression `INVOKE(declval<Fn>(), declval<ArgTypes>()..., R)` is well formed when treated as an unevaluated operand | `Fn`, `R`, and all types in the parameter pack `ArgTypes` shall be complete types, cv `void`, or arrays of unknown bound.
>    > `template <class R, class Fn, class... ArgTypes> struct is_invocable_r;` | The expression `INVOKE(declval<Fn>(), declval<ArgTypes>()..., R)` is well formed when treated as an unevaluated operand | `Fn`, `R`, and all types in the parameter pack `ArgTypes` shall be complete types, cv `void`, or arrays of unknown bound.
>    > `template <class Fn, class... ArgTypes, class R> struct is_nothrow_invocablecallable< Fn(ArgTypes...), R>;` | `is_invocablecallable_v< Fn, ArgTypes...Fn(ArgTypes...), R>` is `true` and the expression `INVOKE(declval<Fn>(), declval<ArgTypes>()..., R)` is known not to throw any exceptions | `Fn`, `R`, and all types in the parameter pack `ArgTypes` shall be complete types, cv `void`, or arrays of unknown bound.
>    > `template <class R, class Fn, class... ArgTypes, class R> struct is_nothrow_invocable_r;` | `is_invocable_r_v< R, Fn, ArgTypes...>` is `true` and the expression `INVOKE(declval<Fn>(), declval<ArgTypes>()..., R)` is known not to throw any exceptions | `Fn`, `R`, and all types in the parameter pack `ArgTypes` shall be complete types, cv `void`, or arrays of unknown bound.

[2017-02-24, Daniel comments]

I suggest to apply the paper d0604r0 instead, available on the Kona LWG wiki.

[2017-03-12, post-Kona]

Resolved by p0604r0.

Proposed resolution:

### 2897(i). `array::iterator` and `array::const_iterator` should be literal types

Section: 23.3.3.1 [array.overview] Status: Resolved Submitter: Russia Opened: 2017-02-03 Last modified: 2020-09-06

Priority: 2

View other active issues in [array.overview].

View all other issues in [array.overview].

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Discussion:

Addresses RU 3

Force the literal type requirement for the `iterator` and `const_iterator` in the `std::array` so that iterators of `std::array` could be used in `constexpr` functions.

Proposed change: Add to the end of the [array.overview] section: `iterator` and `const_iterator` shall be literal types.

[2017-02-20 Marshall adds wording]

I used the formulation "are literal types", rather than "shall be", since that denotes a requirement on the user.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Add a new paragraph at the end of 23.3.3.1 [array.overview]:

>    > -?- `iterator` and `const_iterator` are literal types.

[2017-03-01, Kona]

Antony Polukhin provides revised wording.

[2017-03-02, Kona]

Wording tweaks suggested by LWG applied.

[2017-03-02, Tim Song comments]

I don't believe the blanket "all operations" wording is quite correct.

First, `T t;` (required by `DefaultConstructible`) isn't usable in a constant expression if the iterator is a pointer, since it would leave it uninitialized.

Second, an explicit destructor call `u.~T()` (required by `Destructible`) isn't usable if the iterator is a class type because it explicitly invokes a non-`constexpr` function (the destructor); see [expr.const]/2.2.

[2017-03-04, Kona]

Set priority to 2. Lisa and Alisdair to work with Antony to come up with better wording. The same wording can be used for 2938(i).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Add a new paragraph at the end of 23.3.3.1 [array.overview]:

>    > -3- An `array` satisfies all of the requirements of a container […]

>    > -?- All operations on `iterator` and `const_iterator` that are required to satisfy the random access iterator requirements are usable in constant expressions.

[2017-04-22, Antony Polukhin provides improved wording]

[2017-11 Albuquerque Wednesday issue processing]

Status to Open; We don't want to do this yet; gated on Core issue 1581 (affects swap). See also 2800(i).

Thomas to talk to Anthony about writing a paper detailing what it takes to be a constexpr iterator.

[2017-11 Albuquerque Thursday]

It looks like 1581 is going to be resolved this week, so we should revisit soon.

[2017-11 Albuquerque Saturday issues processing]

P0858R0 (adopted on Sat; to be moved in Jacksonville) will resolve this.

[2018-06 Rapperswil Wednesday issues processing]

This was resolved by P0858, which was adopted in Jacksonville.

Proposed resolution:

This wording is relative to N4659.

1. Add a new paragraph at the end of 24.3.1 [iterator.requirements.general] as indicated:

   > -12- An invalid iterator is an iterator that may be singular.(footnote: […])

   > Iterators are called constexpr iterators if all defined iterator category operations, except a pseudo-destructor call ( [expr.pseudo]) or the construction of an iterator with a singular value, are usable as constant expressions. [Note: For example, the types "pointer to int" and `reverse_iterator<int*>` are constexpr random access iterators. — end note]

   > -13- In the following sections, `a` and `b` denote […]

2. Add a new paragraph at the end of 23.3.3.1 [array.overview]:

   > -3- An `array` satisfies all of the requirements of a container […]

   > -?- `iterator` and `const_iterator` satisfy the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

### 2899(i). `is_(nothrow_)move_constructible` and `tuple`, `optional` and `unique_ptr`

Section: 22.4 [tuple], 22.5 [optional], 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++20 Submitter: United States Opened: 2017-02-03 Last modified: 2021-02-25

Priority: 2

View all other issues in [tuple].

View all issues with C++20 status.

Discussion:

Addresses US 110

The move constructors for `tuple`, `optional`, and `unique_ptr` should return false for `is_(nothrow_)move_constructible_v<TYPE>` when their corresponding Requires clauses are not satisfied, as there are now several library clauses that are defined in terms of these traits. The same concern applies to the move-assignment operator. Note that `pair` and `variant` already satisfy this constraint.

[2017-02-26, Scott Schurr provides wording]

[ 2017-06-27 P2 after 5 positive votes on c++std-lib. ]

[2016-07, Toronto Thursday night issues processing]

The description doesn't match the resolution; Alisdair to investigate. Status to Open

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Modify 22.4.4 [tuple.tuple] as indicated:

>    >     // 20.5.3.1, tuple construction
>    >     EXPLICIT constexpr tuple();
>    >     EXPLICIT constexpr tuple(const Types&...); // only if sizeof...(Types) >= 1
>    >     template <class... UTypes>
>    >     EXPLICIT constexpr tuple(UTypes&&...) noexcept(see below); // only if sizeof...(Types) >= 1

>    >     tuple(const tuple&) = default;
>    >     tuple(tuple&&) = default;

>    >     template <class... UTypes>
>    >     EXPLICIT constexpr tuple(const tuple<UTypes...>&);
>    >     template <class... UTypes>
>    >     EXPLICIT constexpr tuple(tuple<UTypes...>&&) noexcept(see below);

>    >     template <class U1, class U2>
>    >     EXPLICIT constexpr tuple(const pair<U1, U2>&); // only if sizeof...(Types) == 2
>    >     template <class U1, class U2>
>    >     EXPLICIT constexpr tuple(pair<U1, U2>&&) noexcept(see below); // only if sizeof...(Types) == 2

>    >     […]

>    >     // 20.5.3.2, tuple assignment
>    >     tuple& operator=(const tuple&);
>    >     tuple& operator=(tuple&&) noexcept(see below);

>    >     template <class... UTypes>
>    >     tuple& operator=(const tuple<UTypes...>&);
>    >     template <class... UTypes>
>    >     tuple& operator=(tuple<UTypes...>&&) noexcept(see below);

>    >     template <class U1, class U2>
>    >     tuple& operator=(const pair<U1, U2>&); // only if sizeof...(Types) == 2
>    >     template <class U1, class U2>
>    >     tuple& operator=(pair<U1, U2>&&) noexcept(see below); // only if sizeof...(Types) == 2

> 2. Modify 22.4.4.2 [tuple.cnstr] as indicated:

>    >     template <class... UTypes> EXPLICIT constexpr tuple(UTypes&&... u) noexcept(see below);

>    > > -8- Effects: Initializes the elements in the tuple with the corresponding value in `std::forward<UTypes>(u)`.

>    > > -9- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) == sizeof...(UTypes)` and `sizeof...(Types) >= 1` and `is_constructible_v<Ti, Ui&&>` is `true` for all `i`. The constructor is explicit if and only if `is_convertible_v<Ui&&, Ti>` is `false` for at least one `i`. The expression inside `noexcept` is equivalent to the logical AND of the following expressions:

>    > > >     is_nothrow_constructible_v<Ti, Ui&&>

>    > > where `Ti` is the `i`th type in `Types`, and `Ui` is the `i`th type in `UTypes`.

>    > […]

>    >     template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u) noexcept(see below);

>    > > -16- Effects: For all `i`, initializes the `i`th element of `*this` with `std::forward<Ui>(get<i>(u))`.

>    > > -17- Remarks: This constructor shall not participate in overload resolution unless

>    > > […]

>    > > The constructor is explicit if and only if `is_convertible_v<Ui&&, Ti>` is `false` for at least one `i`. The expression inside `noexcept` is equivalent to the logical AND of the following expressions:

>    > > >     is_nothrow_constructible_v<Ti, Ui&&>

>    > > where `Ti` is the `i`th type in `Types`, and `Ui` is the `i`th type in `UTypes`.

>    > […]

>    >     template <class U1, class U2> EXPLICIT constexpr tuple(pair<U1, U2>&& u) noexcept(see below);

>    > > -21- Effects: Initializes the first element with `std::forward<U1>(u.first)` and the second element with `std::forward<U2>(u.second)`.

>    > > -22- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) == 2`, `is_constructible_v<T0, U1&&>` is `true` and `is_constructible_v<T1, U2&&>` is `true`.

>    > > -23- The constructor is explicit if and only if `is_convertible_v<U1&&, T0>` is `false` or `is_convertible_v<U2&&, T1>` is `false`. The expression inside `noexcept` is equivalent to:

>    > > >     is_nothrow_constructible_v<T0, U1&&> && is_nothrow_constructible_v<T1, U2&&>

> 3. Modify 22.4.4.3 [tuple.assign] as indicated:

>    >     template <class... UTypes> tuple& operator=(tuple<UTypes...>&& u) noexcept(see below);

>    > > -12- Effects: For all `i`, assigns `std::forward<Ui>(get<i>(u))` to `get<i>(*this)`.

>    > > -13- Remarks: This operator shall not participate in overload resolution unless `is_assignable_v<Ti&, Ui&&> == true` for all `i` and `sizeof...(Types) == sizeof...(UTypes)`. The expression inside `noexcept` is equivalent to the logical AND of the following expressions:

>    > > >     is_nothrow_assignable_v<Ti&, Ui&&>

>    > > where `Ti` is the `i`th type in `Types`, and `Ui` is the `i`th type in `UTypes`.

>    > > -14- Returns: `*this`.

>    > […]

>    >     template <class U1, class U2> tuple& operator=(pair<U1, U2>&& u) noexcept(see below);

>    > > -18- Effects: Assigns `std::forward<U1>(u.first)` to the first element of `*this` and `std::forward<U2>(u.second)` to the second element of `*this`.

>    > > -19- Remarks: This operator shall not participate in overload resolution unless `sizeof...(Types) == 2` and `is_assignable_v<T0&, U1&&>` is `true` for the first type `T0` in `Types` and `is_assignable_v<T1&, U2&&>` is `true` for the second type `T1` in `Types`. The expression inside `noexcept` is equivalent to:

>    > > >     is_nothrow_assignable_v<T0&, U1&&> && is_nothrow_assignable_v<T1&, U2&&>

>    > > -20- Returns: `*this`.

[2019-02-14; Jonathan comments and provides revised wording]

The suggested change was already made to `std::optional` by LWG 2756(i). The current P/R for 2899 doesn't resolve the issue for `std::tuple` or `std::unique_ptr`. I hope the following alternative does.

[2019-02; Kona Wednesday night issue processing]

Status to Ready

Proposed resolution:

This wording is relative to N4800.

1. Modify 22.4.4.2 [tuple.cnstr] as indicated:

   >     tuple(tuple&& u) = default;

   > > -13- RequiresConstraints: `is_move_constructible_v<Ti>` is `true` for all `i`.

   > > -14- Effects: For all `i`, initializes the `i`th element of `*this` with `std::forward<Ti>(get<i>(u))`.

2. Modify 20.3.1.3.2 [unique.ptr.single.ctor] as indicated:

   >     unique_ptr(unique_ptr&& u) noexcept;

   > > -?- Constraints: `is_move_constructible_v<D>` is `true`.

   > > -15- Requires: If `D` is not a reference type, `D` shall satisfy the Cpp17MoveConstructible requirements (Table 26). Construction of the deleter from an rvalue of type `D` shall not throw an exception.

   > > […]

3. Modify 20.3.1.3.4 [unique.ptr.single.asgn] as indicated:

   >     unique_ptr& operator=(unique_ptr&& u) noexcept;

   > > -?- Constraints: `is_move_assignable_v<D>` is `true`.

   > > -1- Requires: If `D` is not a reference type, `D` shall satisfy the Cpp17MoveAssignable requirements (Table 28) and assignment of the deleter from an rvalue of type `D` shall not throw an exception. Otherwise, `D` is a reference type; `remove_reference_t<D>` shall satisfy the Cpp17CopyAssignable requirements and assignment of the deleter from an lvalue of type `D` shall not throw an exception.

   > > -2- Effects: Calls `reset(u.release())` followed by `get_deleter() = std::forward<D>(u.get_deleter())`.

   > > […]

### 2900(i). The copy and move constructors of `optional` are not `constexpr`

Section: 22.5.3 [optional.optional] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [optional.optional].

View all issues with C++17 status.

Discussion:

Addresses US 111

The copy and move constructors of `optional` are not `constexpr`. However, the constructors taking a `const T&` or `T&&` are `constexpr`, and there is a precedent for having a `constexpr` copy constructor in 29.4.3 [complex]. The defaulted copy and move constructors of `pair` and `tuple` are also conditionally `constexpr` (see 20.4.2 [pairs.pair] p2 and 20.5.2.1 [tuple.cnstr] p2).

A strong motivating use-case is `constexpr` functions returning optional values. This issue was discovered while working on a library making heavy use of such.

Proposed change: Add constexpr to:

>     optional(const optional &);
>     optional(optional &&) noexcept(see below);

[2017-02-23, Casey comments and suggests wording]

This issue corresponds to NB comment US 111, which requests that the move and copy constructors of `std::optional` be declared `constexpr`. The PR simply suggests adding the `constexpr` specifier to the declarations of the constructors. The PR fails to specify the most important thing — and this has been a failing of Library in general — under what conditions is the thing that we've declared constexpr actually expected to be usable in constant expression context? (I think the proper standardese here is "under what conditions is the full expression of an initialization that would invoke these constructors a constant subexpression?")

It is, I believe, well-known that conforming implementations of `optional<T>` must store a `T` in a union to provide constexpr constructors that either do [`optional(T const&)`] or do not [`optional()`] initialize the contained `T`. A general implementation of `optional`'s copy/move constructors must statically choose which union member, if any, to activate in each constructor. Since there is no way to change the active member of a union in a constant expression, and a constructor must statically choose a union member to activate (i.e., without being affected by the runtime state of the copy/move constructor's argument) it's not possible to implement a general constexpr copy/move constructor for `optional`.

It is, however, possible to copy/move construct a trivially copy constructible/trivially move constructible union in `constexpr` context, which effectively copies the union's object representation, resulting in a union whose active member is the same as the source union's. Indeed, at least two major implementations of `optional` (MSVC and libc++) already provide that behavior as a conforming optimization when `T` is a trivially copyable type. If we are to declare `optional<T>`'s copy and move constructors `constexpr`, we should additionally specify that those constructors are only required to have the "constexpr mojo" when `T` is trivially copyable. (Note that I suggest "trivially copyable" here rather than "trivially copy constructible or trivially move constructible" since the simpler requirement is simpler to implement, and I don't believe the more complicated requirement provides any additional benefit: I've never seen a trivially copy constructible or trivially move constructible type outside of a test suite that was not also trivially copyable.)

Previous resolution [SUPERSEDED]:

> This wording is relative to N4618.

> 1. Edit 22.5.3 [optional.optional] as indicated:

>    >     	constexpr optional(const optional &);
>    >     	constexpr optional(optional &&) noexcept(see below);

> 2. Edit 22.5.3.2 [optional.ctor] paragraph as indicated:

>    >     	constexpr optional(const optional &);

>    and

>    >     	constexpr optional(optional &&) noexcept(see below);

[2017-02-23, Marshall comments]

This is related to LWG 2745(i).

[2017-02-28, Kona, Casey comments and improves wording]

Amended PR per LWG discussion in Kona: replace the "is trivially copyable" requirement with the more specific `is_trivially_copy/move_constructible<T>`. LWG was concerned that `tuple` is a counter-example to the assumption that all three traits are equivalent for real-world types.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Change the synopsis of class template `optional` in 22.5.3 [optional.optional] as follows:

>    >     […]
>    >     // 20.6.3.1, constructors
>    >     constexpr optional() noexcept;
>    >     constexpr optional(nullopt_t) noexcept;
>    >     constexpr optional(const optional&);
>    >     constexpr optional(optional&&) noexcept(see below);
>    >     […]

> 2. Modify 22.5.3.2 [optional.ctor] as indicated:

>    >     constexpr optional(const optional& rhs);

>    > > […]

>    > > -6- Remarks: This constructor shall be defined as deleted unless `is_copy_constructible_v<T>` is `true`. If `T` is a trivially copyable type, this constructor shall be a `constexpr` constructor.

>    >     constexpr optional(optional&& rhs) noexcept(see below);

>    > > […]

>    > > -10- Remarks: The expression inside `noexcept` is equivalent to `is_nothrow_move_constructible_v<T>`. This constructor shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true`. If `T` is a trivially copyable type, this constructor shall be a `constexpr` constructor.

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Change the synopsis of class template `optional` in 22.5.3 [optional.optional] as follows:

   >     […]
   >     // 20.6.3.1, constructors
   >     constexpr optional() noexcept;
   >     constexpr optional(nullopt_t) noexcept;
   >     constexpr optional(const optional&);
   >     constexpr optional(optional&&) noexcept(see below);
   >     […]

2. Modify 22.5.3.2 [optional.ctor] as indicated:

   >     constexpr optional(const optional& rhs);

   > > […]

   > > -6- Remarks: This constructor shall be defined as deleted unless `is_copy_constructible_v<T>` is `true`. If `is_trivially_copy_constructible_v<T>` is `true`, this constructor shall be a `constexpr` constructor.

   >     constexpr optional(optional&& rhs) noexcept(see below);

   > > […]

   > > -10- Remarks: The expression inside `noexcept` is equivalent to `is_nothrow_move_constructible_v<T>`. This constructor shall not participate in overload resolution unless `is_move_constructible_v<T>` is `true`. If `is_trivially_move_constructible_v<T>` is `true`, this constructor shall be a `constexpr` constructor.

### 2901(i). Variants cannot properly support allocators

Section: 22.6.3 [variant.variant] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: 0

View all other issues in [variant.variant].

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Discussion:

Addresses US 113

Variants cannot properly support allocators, as any assignment of a subsequent value throws away the allocator used at construction. This is not an easy problem to solve, so `variant` would be better served dropping the illusion of allocator support for now, leaving open the possibility to provide proper support once the problems are fully understood.

Proposed change: Strike the 8 allocator aware constructor overloads from the class definition, and strike 20.7.2.1 [variant.ctor] p34/35. Strike clause 20.7.12 [variant.traits]. Strike the specialization of `uses_allocator` for `variant` in the `<variant>` header synopsis, 20.7.1 [variant.general].

[2017-02-28, Kona, Casey provides wording]

[2017-06-29 Moved to Tentatively Ready after 7 positive votes on c++std-lib.]

[2017-07 Toronto Moved to Immediate]

Proposed resolution:

This wording is relative to N4659.

1. Change 22.6.2 [variant.syn], header `<variant>` synopsis, as follows:

   >     […]
   >     //  [variant.traits], allocator-related traits
   >     template <class T, class Alloc> struct uses_allocator;
   >     template <class... Types, class Alloc> struct uses_allocator<variant<Types...>, Alloc>;

2. Change 22.6.3 [variant.variant], class template `variant` synopsis, as follows:

   >     […]
   >     // allocator-extended constructors
   >     template <class Alloc>
   >       variant(allocator_arg_t, const Alloc&);
   >     template <class Alloc>
   >       variant(allocator_arg_t, const Alloc&, const variant&);
   >     template <class Alloc>
   >       variant(allocator_arg_t, const Alloc&, variant&&);
   >     template <class Alloc, class T>
   >       variant(allocator_arg_t, const Alloc&, T&&);
   >     template <class Alloc, class T, class... Args>
   >       variant(allocator_arg_t, const Alloc&, in_place_type_t<T>, Args&&...);
   >     template <class Alloc, class T, class U, class... Args>
   >       variant(allocator_arg_t, const Alloc&, in_place_type_t<T>,
   >               initializer_list<U>, Args&&...);
   >     template <class Alloc, size_t I, class... Args>
   >       variant(allocator_arg_t, const Alloc&, in_place_index_t<I>, Args&&...);
   >     template <class Alloc, size_t I, class U, class... Args>
   >       variant(allocator_arg_t, const Alloc&, in_place_index_t<I>,
   >               initializer_list<U>, Args&&...);

3. Modify 22.6.3.2 [variant.ctor] as indicated:

   >     // allocator-extended constructors
   >     template <class Alloc>
   >       variant(allocator_arg_t, const Alloc& a);
   >     template <class Alloc>
   >       variant(allocator_arg_t, const Alloc& a, const variant& v);
   >     template <class Alloc>
   >       variant(allocator_arg_t, const Alloc& a, variant&& v);
   >     template <class Alloc, class T>
   >       variant(allocator_arg_t, const Alloc& a, T&& t);
   >     template <class Alloc, class T, class... Args>
   >       variant(allocator_arg_t, const Alloc& a, in_place_type_t<T>, Args&&... args);
   >     template <class Alloc, class T, class U, class... Args>
   >       variant(allocator_arg_t, const Alloc& a, in_place_type_t<T>,
   >               initializer_list<U> il, Args&&... args);
   >     template <class Alloc, size_t I, class... Args>
   >       variant(allocator_arg_t, const Alloc& a, in_place_index_t<I>, Args&&... args);
   >     template <class Alloc, size_t I, class U, class... Args>
   >       variant(allocator_arg_t, const Alloc& a, in_place_index_t<I>,
   >               initializer_list<U> il, Args&&... args);

   > > -34- Requires: `Alloc` shall meet the requirements for an Allocator (16.4.4.6 [allocator.requirements]).

   > > -35- Effects: Equivalent to the preceding constructors except that the contained value is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

4. Modify [variant.traits] as indicated:

   >     template <class... Types, class Alloc>
   >     struct uses_allocator<variant<Types...>, Alloc> : true_type { };

   > > -1- Requires: `Alloc` shall be an Allocator (16.4.4.6 [allocator.requirements]).

   > > -2- [Note: Specialization of this trait informs other library components that variant can be constructed with an allocator, even though it does not have a nested `allocator_type`. — end note]

### 2903(i). The form of initialization for the emplace-constructors is not specified

Section: 22.6.3.2 [variant.ctor] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View other active issues in [variant.ctor].

View all other issues in [variant.ctor].

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Discussion:

Addresses US 118

The form of initialization for the emplace-constructors is not specified. We are very clear to mandate "as if by direct non-list initialization" for each constructor in `optional`, so there is no ambiguity regarding parens vs. braces. That wording idiom should be followed by `variant`.

Proposed change: Insert the phrase "as if direct-non-list-initializing" at appropriate locations in paragraphs 19, 23, 27, and 31

[2017-02-20, Marshall adds wording]

[2017-02-27, Marshall adds wording to cover two more cases]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Modify 22.6.3.2 [variant.ctor] paragraph 19 as indicated:

>    > Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the arguments `std::forward<Args>(args)...`.

> 2. Modify 22.6.3.2 [variant.ctor] paragraph 23 as indicated:

>    > Effects: Initializes the contained value as if direct-non-list-initializingconstructing an object of type `T` with the arguments `il, std::forward<Args>(args)...`.

> 3. Modify 22.6.3.2 [variant.ctor] paragraph 27 as indicated:

>    > Effects: Initializes the contained value as if direct-non-list-initializingconstructing an object of type `TI` with the arguments `std::forward<Args>(args)...`.

> 4. Modify 22.6.3.2 [variant.ctor] paragraph 31 as indicated:

>    > Effects: Initializes the contained value as if direct-non-list-initializingconstructing an object of type `TI` with the arguments `il, std::forward(args)...`.

[Kona 2017-02-28]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 22.6.3.2 [variant.ctor] paragraph 19 as indicated:

   > Effects: Initializes the contained value as if direct-non-list-initializing an object of type `T` with the arguments `std::forward<Args>(args)...`.

2. Modify 22.6.3.2 [variant.ctor] paragraph 23 as indicated:

   > Effects: Initializes the contained value as if direct-non-list-initializingconstructing an object of type `T` with the arguments `il, std::forward<Args>(args)...`.

3. Modify 22.6.3.2 [variant.ctor] paragraph 27 as indicated:

   > Effects: Initializes the contained value as if direct-non-list-initializingconstructing an object of type `TI` with the arguments `std::forward<Args>(args)...`.

4. Modify 22.6.3.2 [variant.ctor] paragraph 31 as indicated:

   > Effects: Initializes the contained value as if direct-non-list-initializingconstructing an object of type `TI` with the arguments `il, std::forward(args)...`.

5. Modify 22.6.3.5 [variant.mod] paragraph 6 as indicated:

   > Effects: Destroys the currently contained value if `valueless_by_exception()` is false. Then direct-initializes the contained value as if direct-non-list-initializingconstructing a value of type `TI` with the arguments `std::forward<Args>(args)...`.

6. Modify 22.6.3.5 [variant.mod] paragraph 11 as indicated:

   > Effects: Destroys the currently contained value if `valueless_by_exception()` is false. Then direct-initializes the contained value as if direct-non-list-initializingconstructing a value of type `TI` with the arguments `il, std::forward<Args>(args)...`.

### 2904(i). Make `variant` move-assignment more exception safe

Section: 22.6.3.4 [variant.assign] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View other active issues in [variant.assign].

View all other issues in [variant.assign].

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Discussion:

Addresses US 119 and CH 7

The copy-assignment operator is very careful to not destroy the contained element until after a temporary has been constructed, which can be safely moved from.

This makes the `valueless_by_exception` state extremely rare, by design.

However, the same care and attention is not paid to the move-assignment operator, nor the assignment-from-deduced-value assignment template. This concern should be similarly important in these cases, especially the latter.

Proposed change: —

[2017-03-02, Kona, Casey comments and suggests wording]

The wording below has been developed with much input from Tomasz.

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 22.6.3.4 [variant.assign] as indicated:

   > [Drafting note: Presentation of para 9 immediately below has been split into individual bullets.]

   >     variant& operator=(const variant& rhs);

   > > Let `j` be `rhs.index()`.

   > > -1- Effects:

   > > 1. (1.1) — If neither `*this` nor `rhs` holds a value, there is no effect. Otherwise,

   > > 2. (1.2) — if `*this` holds a value but `rhs` does not, destroys the value contained in `*this` and sets `*this` to not hold a value. Otherwise,

   > > 3. (1.3) — if `index() == jrhs.index()`, assigns the value contained in `rhs` to the value contained in `*this`. Otherwise,

   > > 4. (1.?) — if `is_nothrow_copy_constructible_v<Tj> || !is_nothrow_move_constructible_v<Tj>` is `true`, equivalent to `emplace<j>(get<j>(rhs))`. Otherwise,

   > > 5. (1.4) — equivalent to `operator=(variant(rhs))`copies the value contained in `rhs` to a temporary, then destroys any value contained in `*this`. Sets `*this` to hold the same alternative index as `rhs` and initializes the value contained in `*this` as if direct-non-list-initializing an object of type `Tj` with `std::forward<Tj>(TMP)`, with `TMP` being the temporary and `j` being `rhs.index()`.

   > > -2- Returns: `*this.`

   > > -3- Postconditions: `index() == rhs.index()`.

   > > -4- Remarks: This function shall not participate in overload resolution unless `is_copy_constructible_v<Ti> && is_move_constructible_v<Ti> && is_copy_assignable_v<Ti>` is `true` for all `i`.

   > > 1. (4.1) — If an exception is thrown during the call […]

   > > 2. (4.2) — If an exception is thrown during the call […]

   > > 3. (4.3) — If an exception is thrown during the call […]

   >     variant& operator=(variant&& rhs) noexcept(see below);

   > > Let `j` be `rhs.index()`.

   > > -5- Effects:

   > > 1. (5.1) — If neither `*this` nor `rhs` holds a value, there is no effect. Otherwise,

   > > 2. (5.2) — if `*this` holds a value but `rhs` does not, destroys the value contained in `*this` and sets `*this` to not hold a value. Otherwise,

   > > 3. (5.3) — if `index() == jrhs.index()`, assigns `get<j>(std::move(rhs))` to the value contained in `*this`, with `j` being `index()`. Otherwise,

   > > 4. (5.4) — equivalent to `emplace<j>(get<j>(std::move(rhs)))`destroys any value contained in `*this`. Sets `*this` to hold the same alternative index as `rhs` and initializes the value contained in `*this` as if direct-non-list-initializing an object of type `Tj` with `get<j>(std::move(rhs))` with `j` being `rhs.index()`.

   > > […]

   > […]

   >     template <class T> variant& operator=(T&& t) noexcept(see below);

   > > -8- […]

   > > -9- Effects:

   > > 1. (9.1) — If *this holds a `Tj`, assigns `std::forward<T>(t)` to the value contained in `*this`. Otherwise,

   > > 2. (9.?) — if `is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj>` is `true`, equivalent to `emplace<j>(std::forward<T>(t))`. Otherwise,

   > > 3. (9.3) — equivalent to `operator=(variant(std::forward<T>(t)))`destroys any value contained in `*this`, sets `*this` to hold the alternative type `Tj` as selected by the imaginary function overload resolution described above, and direct-initializes the contained value as if direct-non-list-initializing it with `std::forward<T>(t)`.

### 2905(i). `is_constructible_v<unique_ptr<P, D>, P, D const &>` should be false when `D` is not copy constructible

Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2017-07-30

Priority: Not Prioritized

View all other issues in [unique.ptr.single.ctor].

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Discussion:

Addresses US 123

`is_constructible_v<unique_ptr<P, D>, P, D const &>` should be false when `D` is not copy constructible, and similarly for `D&&` when `D` is not move constructible. This could be achieved by the traditional 'does not participate in overload resolution' wording, or similar.

Proposed change: Add a Remarks: clause to constrain the appropriate constructors.

[2017-02-28, Jonathan comments and provides concrete wording]

As well as addressing the NB comment, this attempts to make some further improvements to the current wording, which is a little strange. It incorrectly uses "`d`" to mean the constructor argument that initializes the parameter `d`, and unnecessarily explains how overload resolution works for lvalues and rvalues. It refers to the copy/move constructor of `D`, but the constructor that is selected to perform the initialization may not be a copy/move constructor (e.g. initializing a deleter object from an rvalue might use a copy constructor if there is no move constructor). The condition "`d` shall be reference compatible with one of the constructors" is bogus: reference compatible is a property of two types, not a value and a constructor, and again is trying to talk about the argument not the parameter.

Note that we could replace the "see below" in the signatures and paragraphs 9, 10 and 11 by declaring the constructors as:

>     unique_ptr(pointer p, const D& d) noexcept;
>     unique_ptr(pointer p, remove_reference_t<D>&& d) noexcept;

I think this produces the same signatures in all cases. I haven't proposed that here, it could be changed separately if desired.

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

Modify [unique.ptr.single.ctor] paragraphs 9-11 as shown:

>     unique_ptr(pointer p, see below d1) noexcept;
>     unique_ptr(pointer p, see below d2) noexcept;

> -9- The signature of these constructors depends upon whether `D` is a reference type. If `D` is a non-reference type `A`, then the signatures are

>       unique_ptr(pointer p, const A& d) noexcept;
>       unique_ptr(pointer p, A&& d) noexcept;

> -10- If `D` is an lvalue reference type `A&`, then the signatures are:

>       unique_ptr(pointer p, A& d) noexcept;
>       unique_ptr(pointer p, A&& d) = delete;

> -11- If `D` is an lvalue reference type `const A&`, then the signatures are:

>       unique_ptr(pointer p, const A& d) noexcept;
>       unique_ptr(pointer p, const A&& d) = delete;

Remove paragraph 12 entirely:

> -12- Requires:

> - If `D` is not an lvalue reference type then
>   - If `d` is an lvalue or const rvalue then the first constructor of this pair will be selected. `D` shall satisfy the requirements of `CopyConstructible` (Table 24), and the copy constructor of `D` shall not throw an exception. This `unique_ptr` will hold a copy of `d`.
>   - Otherwise, `d` is a non-const rvalue and the second constructor of this pair will be selected. `D` shall satisfy the requirements of `MoveConstructible` (Table 23), and the move constructor of `D` shall not throw an exception. This `unique_ptr` will hold a value move constructed from `d`.
> - Otherwise `D` is an lvalue reference type. `d` shall be reference-compatible with one of the constructors. If `d` is an rvalue, it will bind to the second constructor of this pair and the program is ill-formed. [Note: The diagnostic could be implemented using a static_assert which assures that `D` is not a reference type. — end note] Else `d` is an lvalue and will bind to the first constructor of this pair. The type which `D` references need not be `CopyConstructible` nor `MoveConstructible`. This `unique_ptr` will hold a `D` which refers to the lvalue `d`. [Note: `D` may not be an rvalue reference type. — end note]

Modify paragraph 13 as shown:

> -13- Effects: Constructs a `unique_ptr` object which owns `p`, initializing the stored pointer with `p` and initializing the deleter as described above from `std::forward<decltype(d)>(d)`.

Add a new paragraph after paragraph 14 (Postconditions):

> -?- Remarks: These constructors shall not participate in overload resolution unless `is_constructible_v<D, decltype(d)>` is `true`.

### 2908(i). The less-than operator for shared pointers could do more

Section: 20.3.2.2.8 [util.smartptr.shared.cmp] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

View all other issues in [util.smartptr.shared.cmp].

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Discussion:

Addresses US 135

The less-than operator for shared pointers compares only those combinations that can form a composite pointer type. With the C++17 wording for the diamond functor, `less<>`, we should be able to support comparison of a wider range of shared pointers, such that `less<>::operator(shared_ptr<A>, shared_ptr<B>)` is consistent with `less<>::operator(A *, B *)`.

Proposed change: Replace `less<V>` with just `less<>`, and drop the reference to composite pointer types.

[2017-03-02, Kona, STL comments and provides wording]

We talked about this in LWG, and I talked to Alisdair. The status quo is already maximally general (operator less-than in Core always forms a composite pointer type), but we can use diamond-less to significantly simplify the specification. (We can't use less-than directly, due to the total ordering issue, which diamond-less was recently "upgraded" to handle.)

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Change 20.3.2.2.8 [util.smartptr.shared.cmp] as depicted:

   >     template<class T, class U>
   >     bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

   > > -2- Returns: `less<V>()(a.get(), b.get())`, where `V` is the composite pointer type (Clause 5) of `shared_ptr<T>::element_type*` and `shared_ptr<U>::element_type*`.

### 2911(i). An `is_aggregate` type trait is needed

Section: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 143

An `is_aggregate` type trait is needed. The emplace idiom is now common throughout the library, but typically relies on direct non-list initalization, which does not work for aggregates. With a suitable type trait, we could extend direct non-list-initlaization to perform aggregate-initalization on aggregate types.

Proposed change:

Add a new row to Table 38:

>     template <class T>
>     struct is_aggregate;

> `T` is an aggregate type ([dcl.init.aggr])

> `remove_all_extents_t<T>` shall be a complete type, an array type, or (possibly cv-qualified) `void`.

[2017-02-26, Daniel comments and completes wording]

The proposed wording is incomplete, since it doesn't update the `<type_traits>` header synopsis, also ensuring that the corresponding `is_aggregate_v` variable template is defined.

[2017-03-01, Daniel comments and adjusts wording]

There is only one minor change performed, namely to mark the constant in the `is_aggregate_v` variable template as `inline` following the LWG preferences for "full inline" bullet list (B2) from P0607R0.

[2017-03-03, Kona, LEWG]

No interest in doing the is-list-initializable instead. No objection to getting that proposal also.

Want this for C++17?

SF | F | N | A | SA

2 | 8 | 1 | 3 | 0

Want this for C++20?

SF | F | N | A | SA

6 | 5 | 2 | 0 | 0

Remember to correct the wording to: "`remove_all_extents_t<T>` shall be a complete type or cv-`void`."

[2017-03-03, Daniel updates wording]

In sync with existing writing, this was changed to "`remove_all_extents_t<T>` shall be a complete type or cv `void`"

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

This will require a new complier intrinsic; Alisdair checked with Core and they're OK with that

Proposed resolution:

This wording is relative to N4640.

1. Modify 21.3.3 [meta.type.synop], `<type_traits>` header synopsis, as indicated:

   >     // 20.15.4.3, type properties
   >     […]
   >     template <class T> struct is_final;
   >     template <class T> struct is_aggregate;

   >     […]
   >     // 20.15.4.3, type properties
   >     […]
   >     template <class T> constexpr bool is_final_v
   >       = is_final<T>::value;
   >     template <class T> inline constexpr bool is_aggregate_v
   >       = is_aggregate<T>::value;
   >     […]

2. Add a new row to Table 42 — "Type property predicates":

   >     template <class T> struct is_aggregate;

   > `T` is an aggregate type (9.5.2 [dcl.init.aggr])

   > `remove_all_extents_t<T>` shall be a complete type or cv `void`.

### 2912(i). Add a deduction guide for class template `duration`

Section: 30.5 [time.duration] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 144

Proposed change:

Add to `<chrono>` synopsis:

>     template <class Rep, class Period>
>     duration(const Rep &) -> duration<Rep>;

[2017-02-21; via email]

This should be addressed by P0433R2.

[2017-03-12, post-Kona]

Resolved by P0433R2.

Proposed resolution:

This wording is relative to N4618.

1. Add to the synopsis of `<chrono>`:

   >     template <class Rep, class Period> duration(const Rep &) -> duration<Rep>;

### 2913(i). Containers need deduction guides

Section: 23 [containers] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20

Priority: Not Prioritized

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Discussion:

Addresses US 147

One of the motivating features behind deduction guides was constructing containers from a pair of iterators, yet the standard library does not provide any such deduction guides. They should be provided in header synopsis for each container in clause 23. It is expected that the default arguments from the called constructors will provide the context to deduce any remaining class template arguments, such as the Allocator type, and default comparators/hashers for (unordered) associative containers. At this stage, we do not recommend adding additional guides to deduce a (rebound) allocator, comparator etc. due to the likely large number of such guides. It is noted that the requirements on `iterator_traits` to be an empty type will produce a SFINAE condition to allow correct deduction for vector in the case of the Do-The-Right-Thing clause, resolving ambiguity between two integers, and two iterators.

Proposed change: For each container in clause 23, add to the header synopsis a deduction guide of the form

>     template <class Iterator>
>     container(Iterator, Iterator) -> container<typename iterator_traits<Iterator>::value_type>;

[2017-03-12, post-Kona]

Resolved by P0433R2.

Proposed resolution:

### 2914(i). `std::array` does not support class-template deduction from initializers

Section: 23.3.2 [array.syn] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 148

`std::array` does not support class-template deduction from initializers without a deduction guide.

Proposed change:

Add to `<array>` synopsis:

>     template <class TYPES>
>     array(TYPES&&...) -> array<common_type_t<TYPES...>, sizeof...(TYPES)>;

[2017-03-12, post-Kona]

Resolved by P0433R2.

Proposed resolution:

This wording is relative to N4618.

1. Add to the synopsis of `<array>`:

   >     template <class TYPES>
   >     array(TYPES&&...) -> array<common_type_t<TYPES...>, sizeof...(TYPES)>;

### 2915(i). The three container adapters should each have a deduction guide

Section: 23.6 [container.adaptors] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20

Priority: Not Prioritized

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Discussion:

Addresses US 150

The three container adapters should each have a deduction guide allowing the deduction of the value type `T` from the supplied container, potentially constrained to avoid confusion with deduction from a copy/move constructor.

Proposed change: For each container adapter, add a deduction guide of the form

>     template <class Container>
>     adapter(const Container&) -> adapter<typename Container::value_type, Container>;

[2017-03-12, post-Kona]

Resolved by P0433R2.

Proposed resolution:

### 2917(i). Parallel algorithms cannot easily work with `InputIterator`s

Section: 26 [algorithms], 26.10 [numeric.ops] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 156

Parallel algorithms cannot easily work with `InputIterator`s, as any attempt to partition the work is going to invalidate iterators used by other sub-tasks. While this may work for the sequential execution policy, the goal of that policy is to transparently switch between serial and parallel execution of code without changing semantics, so there should not be a special case extension for this policy. There is a corresponding concern for writing through `OutputIterator`s. Note that the input iterator problem could be mitigated, to some extent, by serially copying/moving data out of the input range and into temporary storage with a more favourable iterator category, and then the work of the algorithm can be parallelized. If this is the design intent, a note to confirm that in the standard would avoid future issues filed in this area. However, the requirement of an algorithm that must copy/move values into intermediate storage may not be the same as those acting immediately on a dereferenced input iterator, and further issues would be likely. It is not clear that anything can be done to improve the serial nature of writing to a simple output iterator though.

Proposed change: All algorithms in the `<algorithm>` and `<numeric>` headers that take an execution policy and an `InputIterator` type should update that iterator to a `ForwardIterator`, and similarly all such overloads taking an `OutputIterator` should update that iterator to a `ForwardIterator`.

(Conversely, if the design intent is confirmed to support input and output iterators, add a note to state that clearly and avoid confusion and more issues by future generations of library implementers.)

[2017-02-13, Alisdair comments]

The pre-Kona mailing has two competing papers that provide wording to address #2917, sequential constraints on parallel algorithms. They should probably be cross-refrenced by the issue:

1. P0467R1: Iterator Concerns for Parallel Algorithms

2. P0574R0: Algorithm Complexity Constraints and Parallel Overloads

[2017-03-12, post-Kona]

Resolved by P0467R2.

Proposed resolution:

### 2918(i). Possible need for extra storage in `inner_product`

Section: 26.10.5 [inner.product] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20

Priority: Not Prioritized

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Discussion:

Addresses US 161

There is a surprising sequential operation applying `BinaryOp1` in `inner_product` that may, for example, require additional storage for the parallel algorithms to enable effective distribution of work, and is likely to be a performance bottleneck. `GENERALIZED_SUM` is probably intended here for the parallel version of the algorithm, with the corresponding strengthening on constraints on `BinaryOp1` to allow arbitrary order of evaluation.

Proposed change: For the overloads taking an execution policy, copy the current specification, but replace algorithm in Effects with

>     GENERALIZED_SUM(plus<>(), init, multiplies<>(*i1, *i2), ...)
>     GENERALIZED_SUM(binary_op1, init, binary_op2(*i1, *i2), ...)

[2017-03-12, post-Kona]

Resolved by P0623R0.

Proposed resolution:

### 2919(i). The specification for `adjacent_difference` has baked-in sequential semantics

Section: 26.10.12 [adjacent.difference] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 162

The specification for `adjacent_difference` has baked-in sequential semantics, in order to support reading/writing through input/output iterators. There should a second specification more amenable to parallelization for the overloads taking an execution policy.

Proposed change: Provide a specification for the overloads taking an execution policy this is more clearly suitable for parallel execution. (i.e., one that does not refer to an accumulated state.)

[2017-02-25, Alisdair comments]

Anthony Williams's paper on parallel algorithm complexity, p0574r0, includes wording that would resolve LWG issue 2919, and I suggest we defer initial triage to handling that paper.

[2017-03-12, post-Kona]

Resolved by P0467R2.

Proposed resolution:

### 2920(i). Add a deduction guide for creating a `shared_future` from a `future` rvalue

Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Addresses US 164

Add a deduction guide for creating a shared future from a future rvalue.

Proposed change:

Add to the `<future>` synopsis:

>     template <class R>
>     shared_future(future<R>&&) -> shared_future<R>;

[2017-03-12, post-Kona]

Resolved by P0433R2.

Proposed resolution:

This wording is relative to N4618.

1. Add to the synopsis of `<future>`:

   >     template <class R>
   >     shared_future(future<R>&&) -> shared_future<R>;

### 2921(i). `packaged_task` and type-erased allocators

Section: 32.10.10 [futures.task] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2025-06-23

Priority: Not Prioritized

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Discussion:

Addresses US 165

The constructor that type-erases an allocator has all of the problems of the similar function constructor that was removed for this CD. This constructor from `packaged_task` should similarly be removed as well. If we prefer to keep this constructor, the current wording is underspecified, as the `Allocator` argument is not required to be type satisfying the Allocator requirements, nor is `allocator_traits` used.

Proposed change:

Strike

>     template <class F, class Allocator>
>     packaged_task(allocator_arg_t, const Allocator& a, F&& f);

from the class definition in p2, and from 30.6.9.1 [futures.task.members] p2. Strike the last sentence of 30.6.9.1p4. In p3, revise "These constructors" to "This constructor"

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

[Sofia 2025-06-21; this was reverted by P3503R3]

Proposed resolution:

This wording is relative to N4618.

1. Modify 32.10.10 [futures.task] as follows:

   Strike

   >     template <class F, class Allocator>
   >     packaged_task(allocator_arg_t, const Allocator& a, F&& f);

   from the class definition in p2, and from [futures.task.members] p2.

2. Modify 32.10.10.2 [futures.task.members]/3:

   > Remarks: These constructorsThis constructor shall not participate in overload resolution if `decay_t<F>` is the same type as `packaged_task<R(ArgTypes...)>`.

3. Strike the last sentence of 32.10.10.2 [futures.task.members]/4:

   > The constructors that take an `Allocator` argument use it to allocate memory needed to store the internal data structures.

### 2922(i). The `*_constant<>` templates do not make use of `template<auto>`

Section: 21.3.3 [meta.type.synop] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2025-10-16

Priority: Not Prioritized

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Discussion:

Addresses US 171

The `*_constant<>` templates (including the proposed addition, `bool_constant<>`) do not make use of the new `template<auto>` feature.

Proposed change: Add a `constant<>` (subject to bikeshedding) template which uses `template<auto>`.

Define `integral_constant<>` as `using integral_constant<T, V> = constant<T(V)>` or `integral_constant<T, V> = constant<V>`.

Either remove `bool_constant`, define it as using `bool_constant = constant<bool(B)>` or `using bool_constant = constant<B>`.

[2017-03-03, Kona, LEWG]

Straw polls:

Name:
`constant` | 3
`numeric_constant` | 8
`static_constant` | 1
`scalar_constant` | 7
`integer_constant` (Over LWG's dead body) | 1
`auto_constant` | 4
`integral_c` | 7
`int_` | 0
`​`
`scalar_constant` | 6
`numeric_constant` | 3
`integral_c` | 5

Accept P0377 with "`scalar_constant`" for C++17 to address LWG 2922 and US 171:

SF | F | N | A | SA

0 | 1 | 3 | 7 | 5

[2017-07 Toronto Thurs Issue Prioritization]

Status LEWG with P0377

[2025-10-16 Status changed: LEWG → Resolved.]

The topic was addressed by P2781R9.

Proposed resolution:

### 2924(i). An `ExecutionPolicy` overload for `inner_product()` seems impractical

Section: 26.10 [numeric.ops] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20

Priority: Not Prioritized

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Discussion:

Addresses US 184

An `ExecutionPolicy` overload for `inner_product()` is specified in the synopsis of `<numeric>`. Such an overload seems impractical. `inner_product()` is ordered and cannot be parallelized; this was the motivation for the introduction of `transform_reduce()`.

Proposed change: Delete the `ExecutionPolicy` overload for `inner_product()`.

[2017-03-12, post-Kona]

Resolved by P0623R0.

Proposed resolution:

### 2925(i). Template argument deduction is not used in the standard library

Section: 16 [library] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 7

P0091R3 "Template argument deduction for class templates (Rev. 6)" was adopted for the core language, but the Standard Library makes no explicit use of this new feature, even though the promise of such use provided strong motivation for the feature.

Proposed change: Analyze the Standard Library's constructors to determine which classes would profit from explicit deduction guides. Formulate the appropriate guides for those classes and insert them in their respective types.

[2017-02-03, Marshall notes]

P0433 is an attempt to do exactly this.

[2017-03-12, post-Kona]

Resolved by P0433R2.

Proposed resolution:

### 2926(i). `INVOKE(f, t1, t2,... tN)` and `INVOKE(f, t1, t2,... tN, R)` are too similar

Section: 22.10.4 [func.require] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20

Priority: Not Prioritized

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Discussion:

Addresses US 84

The distinction between `INVOKE(f, t1, t2,... tN)` and `INVOKE(f, t1, t2,... tN, R)` is too subtle. If the last argument is an expression, it represents `tN`, if it's a type, then it represents `R`. Very clumsy.

Proposed change: Rename `INVOKE(f, t1, t2,... tN, R)` to `INVOKE_R(R, f, t1, t2,... tN)` and adjust all uses of this form. (Approximately 10 occurrences of invoke would need to change.)

[2017-02-24, Daniel comments]

I suggest to apply the paper d0604r0, available on the Kona LWG wiki, implements this suggestions.

[2017-03-12, post-Kona]

Resolved by p0604r0.

Proposed resolution:

### 2927(i). Encoding a functor and argument types as a function signature for `is_callable` and `result_of` is fragile

Section: 21.3.3 [meta.type.synop], 21.3.7 [meta.rel] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-21

Priority: Not Prioritized

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Discussion:

Addresses US 85

The trick of encoding a functor and argument types as a function signature for `is_callable` and `result_of` loses cv information on argument types, fails for non-decayed function types, and is confusing. E.g.,

>     typedef int MyClass::*mp; result_of_t<mp(const MyClass)>;
>     // should be const, but isn't
>     typedef int F(double); is_callable<F(float)>; // ill-formed

Minimal change:

Replace `is_callable<Fn(ArgTypes...)>` with `is_callable<Fn, ArgTypes...>` and replace `is_callable<Fn(ArgTypes...), R>` with `is_callable_r<R, Fn, ArgTypes...>`. Do the same for `is_nothrow_callable`.

Preferred change:

All of the above, plus `deprecate result_of<Fn(ArgTypes...)>` and replace it with `result_of_invoke<Fn, ArgTypes...>`

See also LWG 2895(i).

[2017-02-22, Daniel comments]

LWG 2895(i) provides now wording for this issue and for LWG 2928(i) as well.

[2017-02-24, Daniel comments]

I suggest to apply the paper d0604r0 instead, available on the Kona LWG wiki.

[2017-03-12, post-Kona]

Resolved by p0604r0.

Proposed resolution:

### 2928(i). `is_callable` is not a good name

Section: 21.3.3 [meta.type.synop], 21.3.7 [meta.rel] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 86

`is_callable` is not a good name because it implies `F(A...)` instead of `INVOKE(F, A...)`

Proposed change: Rename `is_callable` to `is_invocable` and rename `is_nothrow_callable` to `is_nothrow_invocable`.

[2017-02-22, Daniel comments and provides concrete wording]

I'm strongly in favour for this change to possibly allow for a future "pure" `is_callable` trait that solely describes function call-like expressions.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Modify 21.3.3 [meta.type.synop], header `<type_traits>` synopsis, as indicated:

>    >     […]
>    >     // 20.15.6, type relations
>    >     […]

>    >     template <class, class R = void> struct is_invocablecallable; // not defined
>    >     template <class Fn, class... ArgTypes, class R>
>    >     struct is_invocablecallable<Fn(ArgTypes...), R>;

>    >     template <class, class R = void> struct is_nothrow_invocablecallable; // not defined
>    >     template <class Fn, class... ArgTypes, class R>
>    >     struct is_nothrow_invocablecallable<Fn(ArgTypes...), R>;

>    >     […]

>    >     // 20.15.6, type relations
>    >     […]
>    >     template <class T, class R = void> constexpr bool is_invocablecallable_v
>    >     = is_invocablecallable<T, R>::value;
>    >     template <class T, class R = void> constexpr bool is_nothrow_invocablecallable_v
>    >     = is_nothrow_invocablecallable<T, R>::value;
>    >     […]

> 2. Modify 21.3.7 [meta.rel], Table 44 — "Type relationship predicates", as indicated:

>    > Table 44 — Type relationship predicates
>    > `[…]`
>    > `template <class Fn, class... ArgTypes, class R> struct is_invocablecallable< Fn(ArgTypes...), R>;` | […] | […]
>    > `template <class Fn, class... ArgTypes, class R> struct is_nothrow_invocablecallable< Fn(ArgTypes...), R>;` | `is_invocablecallable_v< Fn(ArgTypes...), R>` is `true` […] | […]

[2017-02-24, Daniel comments]

I suggest to apply the paper d0604r0 instead, available on the Kona LWG wiki.

[2017-03-12, post-Kona]

Resolved by p0604r0.

Proposed resolution:

### 2929(i). `basic_string` misuses "Effects: Equivalent to"

Section: 27.4.3.7.3 [string.assign] Status: Resolved Submitter: Jonathan Wakely Opened: 2017-02-03 Last modified: 2020-09-06

Priority: 3

View all other issues in [string.assign].

View all issues with Resolved status.

Discussion:

`basic_string::assign(size_type n, charT c);` says:

> Effects: Equivalent to `assign(basic_string(n, c))`.

This requires that a new `basic_string` is constructed, using a default-constructed allocator, potentially allocating memory, and then that new string is copy-assigned to `*this`, potentially propagating the allocator. This must be done even if `this->capacity() > n`, because memory allocation and allocator propagation are observable side effects. If the allocator doesn't propagate and isn't equal to `this->get_allocator()` then a second allocation may be required. This can't be right; it won't even compile if the allocator isn't default constructible.

`basic_string::assign(InputIterator first, InputIterator last)` has a similar problem, even if the iterators are random access and `this->capacity() > distance(first, last)`.

`basic_string::assign(std::initializer_list<charT>` doesn't say "Equivalent to" so maybe it's OK to not allocate anything if the list fits in the existing capacity.

`basic_string::append(size_type, charT)` and `basic_string::append(InputIterator, InputIterator)` have the same problem, although they don't propagate the allocator, but still require at least one, maybe two allocations.

A partial fix would be to ensure all the temporaries are constructed with `get_allocator()` so that they don't require default constructible allocators, and so propagation won't alter allocators. The problem of observable side effects is still present (the temporary might need to allocate memory, even if `this->capacity()` is large) but arguably it's unspecified when construction allocates, to allow for small-string optimisations.

[2017-03-04, Kona]

Set priority to 3. Thomas to argue on reflector that this is NAD.

[2017-03-07, LWG reflector discussion]

Thomas and Jonathan remark that LWG 2788(i) fixed most cases except the allocator respectance and provide wording for this:

Resolved by the adoption of P1148 in San Diego.

Proposed resolution:

This wording is relative to N4659.

1. Change 27.4.3.7.3 [string.assign] as indicated:

   >     basic_string& assign(size_type n, charT c);

   > > -22- Effects: Equivalent to `assign(basic_string(n, c, get_allocator()))`.

### 2932(i). Constraints on parallel algorithm implementations are underspecified

Section: 26.3.3 [algorithms.parallel.exec] Status: C++20 Submitter: Dietmar Kühl Opened: 2017-02-05 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [algorithms.parallel.exec].

View all issues with C++20 status.

Discussion:

Section 26.3.3 [algorithms.parallel.exec] specifies constraints a user of the parallel algorithms has to obey. Notably, it specifies in paragraph 3 that executions of element access functions are indeterminately sequenced with respect to each other. Correspondingly, it is the user's obligation to ensure that these calls do not introduce data races (this is also clarified in a note on this section).

Unfortunately, there is no constraint that, at least, mutating element access functions like `operator++()` on an iterator are called on different objects. An odd implementation of a parallel algorithm could increment a shared iterator from two threads without synchronisation of its own and the user would be obliged to make sure there is no data race!

For example:

>     template <typename FwdIt>
>     FwdIt adjacent_find(std::execution::parallel_policy, FwdIt it, FwdIt end)
>     {
>       if (2 <= std::distance(it, end)) {
>         FwdIt tmp(it);
>         auto f1 = std::async([&tmp](){ ++tmp; });
>         auto f2 = std::async([&tmp](){ ++tmp; });
>         f1.get();
>         f2.get();
>       }
>       return std::adjancent_find(it, end);
>     }

This code is, obviously, a contrived example but with the current specification a legal implementation of `adjacent_find()`. The problem is that, e.g., for pointers there is a data race when incrementing `tmp`, i.e., the function can't be used on pointers. I don't think any of the containers makes a guarantee that their iterators can be incremented without synchronisation, i.e., the standard library doesn't have any iterators which could be used with this algorithm!

Of course, no sane implementation would do anything like that. However, they are allowed to be implemented as above, making it unnecessarily hard and probably inefficient to use the algorithms: in portable code any user of the parallel algorithms needs to deal with the possibility that mutating operations on the same object are called from different threads. There is a constraint missing for the parallel algorithm implementations which limits calls to, at least, some element access functions to be applied only to different objects if there is synchronisation done by the algorithm. At least, obviously mutating operations like the iterator increment/decrement operators need to be correspondingly constrained.

On the other hand, it seems reasonable that a shared data structure stores, e.g., a predicate used concurrently by all threads. This use is quite reasonable and there is nothing wrong. That is, demanding that all element access functions are called on different objects between different threads would possibly adversely over-constraining the algorithms. Only the element access functions deliberately changing the object need to be constrained.

Based on checking all algorithms in the standard library taking an `ExecutionPolicy` template parameter there are broadly four groups of template parameters:

1. Parameters with a known set of possible arguments: `ExecutionPolicy` (execution policies listed in 26.3.6 [execpol]).

2. Parameters specifying types of objects which are expected not to change: `BinaryOperation`, `BinaryPredicate`, `Compare`, `Function`, `Predicate`, `UnaryFunction`, `UnaryOperation`, and `T` (all but the last one are function objects although I couldn't locate concepts for some of them — that may be a separate issue).

3. Parameters of mutable types which are also meant to be mutated: `InputIterator`, `OutputIterator`, `ForwardIterator`, `BidirectionalIterator`, `RandomAccessIterator`, and `Size` (the concept for `Size` also seems to be unspecified).

4. Some algorithms use `Generator` which seems to be a mutable function object. However, I couldn't locate a concept for this parameter.

The problematic group is 3 and possibly 4: mutations on the objects are expected. It seem the general approach of disallowing calling non-const functions without synchronisation applies. Note, however, that prohibiting calling of any non-const function from the algorithms would put undue burden on the implementation of algorithms: any of the accessor functions may be non-const although the concept assume that the function would be const. The constraint should, thus, only apply to functions which may mutate the object according to their respective concept.

Suggested Resolution:

Add a statement prohibiting unsequenced calls to element access functions on the same object which are not applicable to const objects according to the corresponding concept. I'm not sure how to best specify the constraint in general, though.

Since the current algorithms use relatively few concepts there are fairly few operations actually affected. It may be reasonable at least for the initial version (and until we could refer to constructs in concepts in the language) to explicitly list the affected operations. I haven't done a full audit but iterator `++`, `--`, `@=` (for `@` being any of the operators which can be combined with an assignment), and assignments on all objects may be the set of affected element access functions whose use needs to be constrained.

Here is a concrete proposal for the change: In 26.3.2 [algorithms.parallel.user] add a paragraph:

Parallel algorithms are constrained when calling mutating element access functions without synchronisation: if any mutating element access function is called on an object there shall be no other unsynchronised accesses to this object. The mutating element access functions are those which are specified as mutating object in the concept, notably assignment on any object, operators `++`, `--`, `+=`, and `-=` on any of the iterator or `Size` parameters, and any `@=` operators on the `Size` parameters.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Modify 26.3.2 [algorithms.parallel.user] as indicated:

>    > -1- Function objects passed into parallel algorithms as objects of type `Predicate`, `BinaryPredicate`, `Compare`, and `BinaryOperation` shall not directly or indirectly modify objects via their arguments.

>    > -?- Parallel algorithms are constrained when calling mutating element access functions without synchronisation: If any mutating element access function is called on an object there shall be no other unsynchronised accesses to this object. The mutating element access functions are those which are specified as mutating object in the concept, notably assignment on any object, operators `++`, `--`, `+=`, and `-=` on any of the iterator or `Size` parameters, and any `@=` operators on the `Size` parameters.

[2017-03-03, Kona]

Dietmar provides improved wording. Issues with the PR before the change:

- The part before the colon is redundant: we don't need to state that.

- Replace "notably" with "specifically"

- `swap()` needs to be in the list.

- Not sure what "called on an object means"

- The assignment side is overconstrained: the right hand side is allowed.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Modify 26.3.2 [algorithms.parallel.user] as indicated:

>    > -1- Function objects passed into parallel algorithms as objects of type `Predicate`, `BinaryPredicate`, `Compare`, and `BinaryOperation` shall not directly or indirectly modify objects via their arguments.

>    > -?- If an object is mutated by an element access function the algorithm will perform no other unsynchronized accesses to that object. The mutating element access functions are those which are specified as mutating the object in the relevant concept, such as `swap()`, `++`, `--`, `@=`, and assignments. For the assignment and `@=` operators only the left argument is mutated.

[2017-03-03, Kona]

Dietmar finetunes wording after review by SG1.

[2017-03-03, Kona]

Move to Ready

Proposed resolution:

This wording is relative to N4640.

1. Add a new paragraph following 26.3.3 [algorithms.parallel.exec] p1 as indicated:

   > -1- Parallel algorithms have template parameters named `ExecutionPolicy` (20.19) which describe the manner in which the execution of these algorithms may be parallelized and the manner in which they apply the element access functions.

   > -?- If an object is modified by an element access function the algorithm will perform no other unsynchronized accesses to that object. The modifying element access functions are those which are specified as modifying the object in the relevant concept [Note: For example, `swap()`, `++`, `--`, `@=`, and assignments modify the object. For the assignment and `@=` operators only the left argument is modified. — end note].

   > -2- […]

### 2933(i). PR for LWG 2773 could be clearer

Section: 22.4.5 [tuple.creation] Status: Resolved Submitter: Eric Fiselier Opened: 2017-02-06 Last modified: 2024-07-25

Priority: 3

View all other issues in [tuple.creation].

View all issues with Resolved status.

Discussion:

The current PR for LWG 2773(i) changes `std::ignore` to be a `constexpr` variable. However it says nothing about whether using `std::ignore` in `std::tie` is a constant expression. I think the intent was clearly to allow this. Therefore I suggest we update the resolution to explicitly call this out in a note. (I don't think new normative wording is needed).

I propose we update the current PR as follows:

1. Keep the current changes proposed by the PR.

2. Add a note after [tuple.creation]/p7 (`std::tie`):

   > [Note: The constructors and assignment operators provided by `ignore` shall be `constexpr`]

Perhaps LWG feels the existing wording is clear enough, but if not I think the above changes sufficiently clarify it.

The ability to `constexpr` assign to `std::ignore` can be important: Here is an extremely contrived example:

>     constexpr bool foo() {
>       auto res = std::tie(std::ignore);
>       std::get<0>(res) =42;
>       return true;
>     }
>     static_assert(foo());

[2017-03-04, Kona]

Set priority to 3. P/R is incorrect; it should NOT be a note. Marshall to work with Eric to get better wording. STL says "use an exposition-only class".

[2024-07-25 Status changed: New → Resolved.]

Resolved by P2968R2, approved in St. Louis.

Proposed resolution:

This wording is relative to N4640.

1. Modify 22.4.5 [tuple.creation] as indicated:

   >     template<class... TTypes>
   >       constexpr tuple<TTypes&...> tie(TTypes&... t) noexcept;

   > > -7- Returns: […]

   > > -?- [Note: The constructors and assignment operators provided by `ignore` shall be `constexpr`. — end note]

   > > -8- [Example: […] — end example]

### 2934(i). `optional<const T>` doesn't compare with `T`

Section: 22.5.9 [optional.comp.with.t] Status: C++17 Submitter: Ville Voutilainen Opened: 2017-02-17 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [optional.comp.with.t].

View all issues with C++17 status.

Discussion:

Consider:

>     optional<const int> x = 42;
>     int y = 666;
>     x == y; // ill-formed

The comparison is ill-formed, because in [optional.comp_with_t]/1,

    template <class T> constexpr bool operator==(const optional<T>& x, const T& v);

the `T` is deduced to be both `const int` and `int`, which is ill-formed.

Since it became apparent that the root cause of this issue is also causing difficulties with e.g. comparing `optional<string>` with literals etc., here's a p/r that

1. adds requirements for `optional`'s comparisons with `T`

2. turns `optional`-`optional` comparisons into two-template-parameter templates, allowing comparing mixed optionals

3. turns `optional`-`T` comparisons into two-template-parameter templates, allowing comparing optionals with `T` and types comparable with `T`

[Kona 2017-02-28]

Accepted as Immediate to avoid having break ABI later.

Proposed resolution:

This wording is relative to N4640.

1. Modify 22.5.2 [optional.syn], header `<optional>` synopsis, as indicated:

   >     […]

   >     // 22.5.7 [optional.relops], relational operators
   >     template <class T, class U>
   >     constexpr bool operator==(const optional<T>&, const optional<TU>&);
   >     template <class T, class U>
   >     constexpr bool operator!=(const optional<T>&, const optional<TU>&);
   >     template <class T, class U>
   >     constexpr bool operator<(const optional<T>&, const optional<TU>&);
   >     template <class T, class U>
   >     constexpr bool operator>(const optional<T>&, const optional<TU>&);
   >     template <class T, class U>
   >     constexpr bool operator<=(const optional<T>&, const optional<TU>&);
   >     template <class T, class U>
   >     constexpr bool operator>=(const optional<T>&, const optional<TU>&);

   >     […]

   >     //  [optional.comp_with_t], comparison with T
   >     template <class T, class U> constexpr bool operator==(const optional<T>&, const TU&);
   >     template <class T, class U> constexpr bool operator==(const TU&, const optional<T>&);
   >     template <class T, class U> constexpr bool operator!=(const optional<T>&, const TU&);
   >     template <class T, class U> constexpr bool operator!=(const TU&, const optional<T>&);
   >     template <class T, class U> constexpr bool operator<(const optional<T>&, const TU&);
   >     template <class T, class U> constexpr bool operator<(const TU&, const optional<T>&);
   >     template <class T, class U> constexpr bool operator<=(const optional<T>&, const TU&);
   >     template <class T, class U> constexpr bool operator<=(const TU&, const optional<T>&);
   >     template <class T, class U> constexpr bool operator>(const optional<T>&, const TU&);
   >     template <class T, class U> constexpr bool operator>(const TU&, const optional<T>&);
   >     template <class T, class U> constexpr bool operator>=(const optional<T>&, const TU&);
   >     template <class T, class U> constexpr bool operator>=(const TU&, const optional<T>&);

2. Modify 22.5.7 [optional.relops] as indicated:

   >     template <class T, class U> constexpr bool operator==(const optional<T>& x, const optional<TU>& y);

   > > […]

   >     template <class T, class U> constexpr bool operator!=(const optional<T>& x, const optional<TU>& y);

   > > […]

   >     template <class T, class U> constexpr bool operator<(const optional<T>& x, const optional<TU>& y);

   > > […]

   >     template <class T, class U> constexpr bool operator>(const optional<T>& x, const optional<TU>& y);

   > > […]

   >     template <class T, class U> constexpr bool operator<=(const optional<T>& x, const optional<TU>& y);

   > > […]

   >     template <class T, class U> constexpr bool operator>=(const optional<T>& x, const optional<TU>& y);

   > > […]

3. Modify [optional.comp_with_t] as indicated:

   >     template <class T, class U> constexpr bool operator==(const optional<T>& x, const TU& v);

   > > -?- Requires: The expression `*x == v` shall be well-formed and its result shall be convertible to `bool`. [Note: `T` need not be `EqualityComparable`. — end note]

   > > -1- Effects: Equivalent to: `return bool(x) ? *x == v : false;`

   >     template <class T, class U> constexpr bool operator==(const TU& v, const optional<T>& x);

   > > -?- Requires: The expression `v == *x` shall be well-formed and its result shall be convertible to `bool`.

   > > -2- Effects: Equivalent to: `return bool(x) ? v == *x : false;`

   >     template <class T, class U> constexpr bool operator!=(const optional<T>& x, const TU& v);

   > > -?- Requires: The expression `*x != v` shall be well-formed and its result shall be convertible to `bool`.

   > > -3- Effects: Equivalent to: `return bool(x) ? *x != v : true;`

   >     template <class T, class U> constexpr bool operator!=(const TU& v, const optional<T>& x);

   > > -?- Requires: The expression `v != *x` shall be well-formed and its result shall be convertible to `bool`.

   > > -4- Effects: Equivalent to: `return bool(x) ? v != *x : true;`

   >     template <class T, class U> constexpr bool operator<(const optional<T>& x, const TU& v);

   > > -?- Requires: The expression `*x < v` shall be well-formed and its result shall be convertible to `bool`.

   > > -5- Effects: Equivalent to: `return bool(x) ? *x < v : true;`

   >     template <class T, class U> constexpr bool operator<(const TU& v, const optional<T>& x);

   > > -?- Requires: The expression `v < *x` shall be well-formed and its result shall be convertible to `bool`.

   > > -6- Effects: Equivalent to: `return bool(x) ? v < *x : false;`

   >     template <class T, class U> constexpr bool operator<=(const optional<T>& x, const TU& v);

   > > -?- Requires: The expression `*x <= v` shall be well-formed and its result shall be convertible to `bool`.

   > > -7- Effects: Equivalent to: `return bool(x) ? *x <= v : true;`

   >     template <class T, class U> constexpr bool operator<=(const TU& v, const optional<T>& x);

   > > -?- Requires: The expression `v <= *x` shall be well-formed and its result shall be convertible to `bool`.

   > > -8- Effects: Equivalent to: `return bool(x) ? v <= *x : false;`

   >     template <class T, class U> constexpr bool operator>(const optional<T>& x, const TU& v);

   > > -?- Requires: The expression `*x > v` shall be well-formed and its result shall be convertible to `bool`.

   > > -9- Effects: Equivalent to: `return bool(x) ? *x > v : false;`

   >     template <class T, class U> constexpr bool operator>(const TU& v, const optional<T>& x);

   > > -?- Requires: The expression `v > *x` shall be well-formed and its result shall be convertible to `bool`.

   > > -10- Effects: Equivalent to: `return bool(x) ? v > *x : true;`

   >     template <class T, class U> constexpr bool operator>=(const optional<T>& x, const TU& v);

   > > -?- Requires: The expression `*x >= v` shall be well-formed and its result shall be convertible to `bool`.

   > > -11- Effects: Equivalent to: `return bool(x) ? *x >= v : false;`

   >     template <class T, class U> constexpr bool operator>=(const TU& v, const optional<T>& x);

   > > -?- Requires: The expression `v >= *x` shall be well-formed and its result shall be convertible to `bool`.

   > > -12- Effects: Equivalent to: `return bool(x) ? v >= *x : true;`

### 2935(i). What should `create_directories` do when `p` already exists but is not a directory?

Section: 31.12.13.7 [fs.op.create.directories], 31.12.13.8 [fs.op.create.directory] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-02-15 Last modified: 2021-06-06

Priority: 0

View all issues with C++20 status.

Discussion:

The `create_directory` and `create_directories` functions have a postcondition that says `is_directory(p)`, but it is unclear how they are supposed to provide this. Both of their effects say that they create a directory and return whether it was actually created. It is possible to interpret this as "if creation fails due to the path already existing, issue another system call to see if the path is a directory, and change the result if so" — but it seems unfortunate to require both Windows and POSIX to issue more system calls in this case.

In email discussion Davis Herring and Billy O'Neal discussed this issue and agreed that this was probably unintentional. Special thanks for Jonathan Wakely's suggested change to `create_directories`' Returns clause.

[2017-07 Toronto Thurs Issue Prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4640.

1. Make the following edits to [fs.op.create_directories]:

   >     bool create_directories(const path& p);
   >     bool create_directories(const path& p, error_code& ec) noexcept;

   -1- Effects: Establishes the postcondition by calling Calls `create_directory()` for anyeach element of `p` that does not exist.

   -2- Postconditions: `is_directory(p)`.

   -3- Returns: `true` if a new directory was created for the directory `p` resolves to, otherwise `false`. The signature with argument `ec` returns `false` if an error occurs.

   -4- Throws: As specified in 31.12.5 [fs.err.report].

   -5- Complexity: 𝒪(n) where n is the number of elements of `p` that do not exist.

2. Make the following edits to [fs.op.create_directory]:

   >     bool create_directory(const path& p);
   >     bool create_directory(const path& p, error_code& ec) noexcept;

   -1- Effects: Establishes the postcondition by attempting to createCreates the directory `p` resolves to, as if by POSIX `mkdir()` with a second argument of `static_cast<int>(perms::all)`. Creation failure because `p` resolves to an existing directory shall not be treated asalready exists is not an error.

   -2- Postconditions: `is_directory(p)`.

   -3- Returns: `true` if a new directory was created, otherwise `false`. The signature with argument `ec` returns `false` if an error occurs.

   -4- Throws: As specified in 31.12.5 [fs.err.report].

### 2936(i). Path comparison is defined in terms of the generic format

Section: 31.12.6.5.8 [fs.path.compare] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-02-21 Last modified: 2021-02-25

Priority: 2

View all issues with C++20 status.

Discussion:

Currently, path comparison is defined elementwise, which implies a conversion from the native format (implied by `native()` returning `const string&`). However, the conversion from the native format to the generic format might not be information preserving. This would allow two paths `a` and `b` to say `a.compare(b) == 0`, but `a.native().compare(b.native()) != 0` as a result of this missing information, which is undesirable. We only want that condition to happen if there are redundant directory separators. We also don't want to change the path comparison to be in terms of the native format, due to Peter Dimov's example where we want `path("a/b")` to sort earlier than `path("a.b")`, and we want `path("a/b") == path("a//////b")`.

Citing a Windows example, conversion to the generic format is going to have to drop alternate data streams. This might give `path("a/b:ads") == path("a/b")`. I think I should consider the alternate data streams as part of the path element though, so this case might be fine, so long as I make `path("b:ads").native()` be `"b:ads"`. This might not work for our z/OS friends though, or for folks where the native format looks nothing like the generic format.

Additionally, this treats root-directory specially. For example, the current spec wants `path("c:/a/b") == path("c:/a////b")`, but `path("c:/a/b") != path("c:///a/b")`, because `native()` for the root-directory path element will literally be the slashes or preferred separators.

This addresses similar issues to those raised in US 57 — it won't make absolute paths sort at the beginning or end but it will make paths of the same kind sort together.

[2017-03-04, Kona Saturday morning]

We decided that this had seen so much churn that we would postpone looking at this until Toronto

[2017-07 Toronto Thurs Issue Prioritization]

Priority 2

[2016-07, Toronto Saturday afternoon issues processing]

Billy to reword after Davis researches history about ordering. Status to Open.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Make the following edits to 31.12.6.5.8 [fs.path.compare]:

>    >     int compare(const path& p) const noexcept;

>    > > -1- Returns:

>    > > > — Let `rootNameComparison` be the result of `this->root_name().native().compare(p.root_name().native())`. If `rootNameComparison` is not `0`, `rootNameComparison`; otherwise,

>    > > > — If `this->has_root_directory()` and `!p.has_root_directory()`, a value less than `0`; otherwise,

>    > > > — If `!this->has_root_directory()` and `p.has_root_directory()`, a value greater than `0`; otherwise,

>    > > > — A value greater than, less than, or equal to 0, ordering the paths in a depth-first traversal order.

>    > > -?- [Note: For POSIX and Windows platforms, this is accomplished by lexicographically ordering the half-open ranges `[begin(), end())` of `this->relative_path()` and `p.relative_path()` as follows:

>    > > > — A value less than `0`, if `native()` for the elements of `*this->relative_path()` are lexicographically less than `native()` for the elements of `p.relative_path()`; otherwise,

>    > > > — a value greater than `0`, if `native()` for the elements of `*this->relative_path()` are lexicographically greater than `native()` for the elements of `p.relative_path()`; otherwise,

>    > > > — `0`.

>    > > — end note]

>    > > -2- Remarks: The elements are determined as if by iteration over the half-open range `[begin(), end())` for `*this` and `p`.

>    >     int compare(const string_type& s) const
>    >     int compare(basic_string_view<value_type> s) const;

>    > > -3- Returns: `compare(path(s))`

>    > > > [Editor's note: Delete paragraph 3 entirely and merge the `value_type` overload with those above.]

>    >     int compare(const value_type* s) const

>    > > -4- ReturnsEffects: Equivalent to `return` `compare(path(s));.`

[2018-01-26 issues processing telecon]

Status set to 'Review'. We like the wording, but would like to see some implementation experience.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4659.

> 1. Make the following edits to 31.12.6.5.8 [fs.path.compare]:

>    >     int compare(const path& p) const noexcept;

>    > > -1- Returns:

>    > > > — Let `rootNameComparison` be the result of `this->root_name().native().compare(p.root_name().native())`. If `rootNameComparison` is not `0`, `rootNameComparison`; otherwise,

>    > > > — If `this->has_root_directory()` and `!p.has_root_directory()`, a value less than `0`; otherwise,

>    > > > — If `!this->has_root_directory()` and `p.has_root_directory()`, a value greater than `0`; otherwise,

>    > > > — a value less than `0`, iIf `native()` for the elements of `*this->relative_path()` are lexicographically less than `native()` for the elements of `p.relative_path()`, a value less than `0`; otherwise,

>    > > > — a value greater than `0`, iIf `native()` for the elements of `*this->relative_path()` are lexicographically greater than `native()` for the elements of `p.relative_path()`, a value greater than `0`; otherwise,

>    > > > — `0`.

>    > > -2- Remarks: The elements are determined as if by iteration over the half-open range `[begin(), end())` for `*this` and `p`.

>    >     int compare(const string_type& s) const
>    >     int compare(basic_string_view<value_type> s) const;

>    > > -3- Returns: `compare(path(s))`

>    > > > [Editor's note: Delete paragraph 3 entirely and merge the `value_type` overload with those above.]

>    >     int compare(const value_type* s) const

>    > > -4- ReturnsEffects: Equivalent to `return` `compare(path(s));.`

[2018-02-13 Billy improves wording]

The revised wording has the effect to invert the ordering of the added new bullets (2) and (3), the effect of this change is that

>     path("c:/").compare("c:")

compares greater, not less.

[2018-06, Rapperswil Wednesday evening]

Agreement to move that to Ready, Daniel rebased to N4750.

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Make the following edits to 31.12.6.5.8 [fs.path.compare]:

   >     int compare(const path& p) const noexcept;

   > > -1- Returns:

   > > > — Let `rootNameComparison` be the result of `this->root_name().native().compare(p.root_name().native())`. If `rootNameComparison` is not `0`, `rootNameComparison`; otherwise,

   > > > — If `!this->has_root_directory()` and `p.has_root_directory()`, a value less than `0`; otherwise,

   > > > — If `this->has_root_directory()` and `!p.has_root_directory()`, a value greater than `0`; otherwise,

   > > > — a value less than `0`, iIf `native()` for the elements of `*this->relative_path()` are lexicographically less than `native()` for the elements of `p.relative_path()`, a value less than `0`; otherwise,

   > > > — a value greater than `0`, iIf `native()` for the elements of `*this->relative_path()` are lexicographically greater than `native()` for the elements of `p.relative_path()`, a value greater than `0`; otherwise,

   > > > — `0`.

   > > -2- Remarks: The elements are determined as if by iteration over the half-open range `[begin(), end())` for `*this` and `p`.

   >     int compare(const string_type& s) const
   >     int compare(basic_string_view<value_type> s) const;

   > > -3- Returns: `compare(path(s))`

   > > > [Editor's note: Delete paragraph 3 entirely and merge the `value_type` overload with those above.]

   >     int compare(const value_type* s) const

   > > -4- ReturnsEffects: Equivalent to: `return` `compare(path(s));.`

### 2937(i). Is `equivalent("existing_thing", "not_existing_thing")` an error?

Section: 31.12.13.13 [fs.op.equivalent] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-02-27 Last modified: 2021-02-25

Priority: 0

View all other issues in [fs.op.equivalent].

View all issues with C++20 status.

Discussion:

See discussion on the LWG mailing list with subject "Is equivalent("existing_thing", "not_existing_thing") an error?", abreviated below:

Billy O'Neal:

> The existing "an error is reported" effects say that an error is reported for `!exists(p1) && !exists(p2)`, but I'm not sure that treating `equivalent("existing_thing", "not_existing_thing")` as "`false`, no error" makes any more sense than for `equivalent("not_existing_thing", "not_existing_thing")`.

> It's also unfortunate that the current spec requires reporting an error for `is_other(p1) && is_other(p2)` — there's no reason that you can't give a sane answer for paths to NT pipes. (Do POSIX FIFOs give garbage answers here?)

Davis Herring:

> I'm fine with an error if either path does not exist. See also Late 29: I would much prefer

> >     file_identity identity(const path&, bool resolve = true);

> which would of course produce an error if the path did not exist (or, with the default `resolve`, was a broken symlink).

> See Late 30 and 32 (31 has been resolved). FIFOs pose no trouble: you can even `fstat(2)` on the naked file descriptors produced by `pipe(2)`. (That said, I observe the strange inconsistency that Linux but not macOS gives both ends of a pipe the same `st_ino`.)

> POSIX has no reason that I know of to treat any file type specially for `equivalent()`.

Billy O'Neal:

> I think such a `file_identity` feature would be useful but we can always add it in addition to `equivalent` post-C++17.

Beman Dawes:

> Looks good to me. Maybe submit this as an issue right away in the hopes it can go in C++17?

[2017-03-04, Kona]

Set priority to 0; Tentatively Ready

Proposed resolution:

This wording is relative to N4640.

1. Make the following edits to 31.12.13.13 [fs.op.equivalent]:

   >     bool equivalent(const path& p1, const path& p2);
   >     bool equivalent(const path& p1, const path& p2, error_code& ec) noexcept;

   > > -1- Let `s1` and `s2` be `file_status`s determined as if by `status(p1)` and `status(p2)`, respectively.

   > > -2- Effects: Determines `s1` and `s2`. If `(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))` an error is reported (27.10.7).

   > > -3- Returns: `true`, if `s1 == s2` and `p1` and `p2` resolve to the same file system entity, else `false`. The signature with argument `ec` returns `false` if an error occurs.

   > > -4- Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. [Note: On POSIX platforms, tThis is determined as if by the values of the POSIX `stat` structure, obtained as if by `stat()` for the two paths, having equal `st_dev` values and equal `st_ino` values. — end note]

   > > -?- Remarks: `!exists(p1) || !exists(p2)` is an error.

   > > -5- Throws: As specified in 27.10.7.

### 2938(i). `basic_string_view::const_iterator` should be literal types

Section: 27.3.3 [string.view.template] Status: Resolved Submitter: Antony Polukhin Opened: 2017-03-01 Last modified: 2020-09-06

Priority: 2

View all other issues in [string.view.template].

View all issues with Resolved status.

Discussion:

Force the literal type requirement for the `const_iterator` in the `std::basic_string_view` so that iterators of `std::basic_string_view` could be used in `constexpr` functions.

[2017-03-02, Kona]

Wording tweaks suggested by LWG applied.

[2017-03-02, Tim Song comments]

I don't believe the blanket "all operations" wording is quite correct.

First, `T t;` (required by `DefaultConstructible`) isn't usable in a constant expression if the iterator is a pointer, since it would leave it uninitialized.

Second, an explicit destructor call `u.~T()` (required by `Destructible`) isn't usable if the iterator is a class type because it explicitly invokes a non-`constexpr` function (the destructor); see [expr.const]/2.2.

[2017-03-04, Kona]

Set priority to 2. Lisa and Alisdair to work with Antony to come up with better wording. The same wording can be used for 2897(i).

[2017-11 Albuquerque Saturday issues processing]

P0858R0 (adopted on Sat; to be moved in Jacksonville) will resolve this.

[2018-06 Rapperswil Wednesday issues processing]

This was resolved by P0858, which was adopted in Jacksonville.

Proposed resolution:

This wording is relative to N4640.

1. Add to the end of the 27.3.3 [string.view.template] section:

   > -1- In every specialization `basic_string_view<charT, traits>`, the type traits shall satisfy the character traits requirements (21.2), and the type `traits::char_type` shall name the same type as `charT`.

   > -?- All operations on `iterator` and `const_iterator` that are required to satisfy the random access iterator requirements are usable in constant expressions.

### 2940(i). `result_of` specification also needs a little cleanup

Section: D.13 [depr.meta.types] Status: C++20 Submitter: Daniel Krügler Opened: 2017-03-04 Last modified: 2021-02-25

Priority: 3

View all other issues in [depr.meta.types].

View all issues with C++20 status.

Discussion:

P0604R0 missed a similar adjustment that was performed for the deprecated type trait templates `is_literal_type` and `is_literal_type_v` via LWG 2838(i):

Moving the `result_of` to Annex D means that the general prohibition against specializing type traits in [meta.type.synop]/1 does no longer exist, so should be explicitly spelled out. Wording will be provided after publication of the successor of N4640.

[2017-03-04, Kona]

Set priority to 3

[2017-04-24, Daniel provides wording]

Instead of enumerating all the templates where adding a specialization is valid, a general exclusion rule is provided. Albeit currently not needed for the templates handled by this sub-clause, a potential exception case is constructed to ensure correctness for the first deprecated trait template that would permit specializations by the user.

[2017-06-10, Moved to Tentatively Ready after 6 positive votes on c++std-lib]

Proposed resolution:

This wording is relative to N4659.

1. Change D.13 [depr.meta.types] as indicated:

   > -4- The behavior of a program that adds specializations for any of the templates defined in this subclause `is_literal_type` or `is_literal_type_v` is undefined, unless explicitly permitted by the specification of the corresponding template.

### 2941(i). §[thread.req.timing] wording should apply to both member and namespace-level functions

Section: 32.2.4 [thread.req.timing] Status: C++20 Submitter: Jonathan Mcdougall Opened: 2017-03-07 Last modified: 2021-02-25

Priority: 0

View all other issues in [thread.req.timing].

View all issues with C++20 status.

Discussion:

In 32.2.4 [thread.req.timing], both /3 and /4 talk about "member functions whose names end in `_for`" and "`_until`", but these clauses also apply to `this_thread::sleep_for()` and `this_thread::sleep_until()`, which are namespace-level functions (30.3.2).

[2017-07 Toronto Wed Issue Prioritization]

Priority 0; Move to Ready

Proposed resolution:

This wording is relative to N4640.

1. Modify 32.2.4 [thread.req.timing] as indicated::

   > […]

   > -3- The member functions whose names end in `_for` take an argument that specifies a duration. […]

   > -4- The member functions whose names end in `_until` take an argument that specifies a time point. […]

### 2942(i). LWG 2873's resolution missed `weak_ptr::owner_before`

Section: 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++20 Submitter: Tim Song Opened: 2017-03-09 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [util.smartptr.weak.obs].

View all issues with C++20 status.

Discussion:

The NB comment asked for `noexcept` on `shared_ptr::owner_before`, `weak_ptr::owner_before`, and `owner_less::operator()`, but the PR of LWG 2873(i) only added it to the first and the third one.

[2017-06-03, Moved to Tentatively Ready after 6 positive votes on c++std-lib]

Proposed resolution:

This wording is relative to N4659.

1. Edit 20.3.2.3 [util.smartptr.weak], class template `weak_ptr` synopsis, as indicated:

   >     […]
   >     // 20.3.2.3.6 [util.smartptr.weak.obs], observers
   >     […]
   >     template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept;
   >     template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;

2. Edit 20.3.2.3.6 [util.smartptr.weak.obs] immediately before p4 as indicated:

   >     template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept;
   >     template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;

   > > -4- Returns: An unspecified value such that […]

### 2943(i). Problematic specification of the wide version of `basic_filebuf::open`

Section: 31.10.3.4 [filebuf.members] Status: C++20 Submitter: Tim Song Opened: 2017-03-09 Last modified: 2021-02-25

Priority: 2

View all other issues in [filebuf.members].

View all issues with C++20 status.

Discussion:

LWG 2676(i) specified `basic_filebuf::open(const std::filesystem::path::value_type* s, ios_base::openmode mode)` by simply reusing the specification for the `const char*` overload, but that specification is incorrect for the wide overload: it says that `s` is an NTBS — a null-terminated byte string — which it isn't. Moreover, it specifies that the file is opened as if by calling `fopen(s, modstr)`, but that call is ill-formed if `s` isn't a `const char*`.

[2017-07 Toronto Wed Issue Prioritization]

Priority 2

[2017-11 Albuquerque Wednesday issue processing]

Status to Open; Jonathan to provide wording.

[2018-01-16; Jonathan and Tim Song provide wording]

We'll have to ask the Microsoft guys if "as by a call to `fopen`" is OK for them. There are paths that can be represented as a wide character string that can't reliably be converted to narrow characters (because they become dependent on the current codepage, or some other Windows nonsense) so they definitely won't use `fopen`. But as long as they call something that behaves like it (which should allow `_fwopen`), I think they'll still meet the spirit of the wording.

[2018-08-14; Marshall corrects a grammar nit in the P/R]

The Microsoft guys note that "as by a call to `fopen`" is OK by them.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4713.

> 1. Edit 31.10.3.4 [filebuf.members] as indicated:

>    >     basic_filebuf* open(const char* s, ios_base::openmode mode);
>    >     basic_filebuf* open(const filesystem::path::value_type* s,
>    >                         ios_base::openmode mode); // wide systems only; see 31.10.1 [fstream.syn]

>    > > -?- Requires: `s` shall point to a NTCTS (3.36 [defns.ntcts]).

>    > > -2- Effects: If `is_open() != false`, returns a null pointer. Otherwise, initializes the `filebuf` as required. It then opens the file to which `s` resolves, if possible, as if by a call to `fopen` with the second argumenta file, if possible, whose name is the ntbs `s` (as if by calling `fopen(s, modstr))`. The ntbs `modstr` is determined from `mode & ~ios_base::ate` as indicated in Table 117. If `mode` is not some combination of flags shown in the table then the open fails.

>    > > -3- If the open operation succeeds and `(mode & ios_base::ate) != 0`, positions the file to the end (as if by calling `fseek(file, 0, SEEK_END)`, where `file` is the pointer returned by calling `fopen`).(footnote 330)

>    > > -4- If the repositioning operation fails, calls `close()` and returns a null pointer to indicate failure.

>    > > -5- Returns: `this` if successful, a null pointer otherwise.

[2018-08-23 Batavia Issues processing]

Adopted after changing 'Requires' -> 'Expects' in the P/R.

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4713.

1. Edit 31.10.3.4 [filebuf.members] as indicated:

   >     basic_filebuf* open(const char* s, ios_base::openmode mode);
   >     basic_filebuf* open(const filesystem::path::value_type* s,
   >                         ios_base::openmode mode); // wide systems only; see 31.10.1 [fstream.syn]

   > > -?- Expects: `s` shall point to a NTCTS (3.36 [defns.ntcts]).

   > > -2- Effects: If `is_open() != false`, returns a null pointer. Otherwise, initializes the `filebuf` as required. It then opens the file to which `s` resolves, if possible, as if by a call to `fopen` with the second argumenta file, if possible, whose name is the ntbs `s` (as if by calling `fopen(s, modstr))`. The ntbs `modstr` is determined from `mode & ~ios_base::ate` as indicated in Table 117. If `mode` is not some combination of flags shown in the table then the open fails.

   > > -3- If the open operation succeeds and `(mode & ios_base::ate) != 0`, positions the file to the end (as if by calling `fseek(file, 0, SEEK_END)`, where `file` is the pointer returned by calling `fopen`).(footnote 330)

   > > -4- If the repositioning operation fails, calls `close()` and returns a null pointer to indicate failure.

   > > -5- Returns: `this` if successful, a null pointer otherwise.

### 2944(i). LWG 2905 accidentally removed requirement that construction of the deleter doesn't throw an exception

Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++20 Submitter: Tim Song Opened: 2017-03-11 Last modified: 2021-02-25

Priority: 0

View all other issues in [unique.ptr.single.ctor].

View all issues with C++20 status.

Discussion:

The wording simplification in LWG 2905(i) accidentally deleted the requirement that construction of the deleter doesn't throw an exception. While this isn't the end of the world since any exception will just run into the noexcept on the constructor and result in a call to `std::terminate()`, the other `unique_ptr` constructors still have a similar no-exception Requires: clause, leaving us in the odd situation where throwing an exception results in undefined behavior for some constructors and `terminate()` for others. If guaranteeing `std::terminate()` on exception is desirable, that should be done across the board.

The proposed wording below simply restores the nothrow requirement along with the `Copy/MoveConstructible` requirement. Wording for the other alternative (guaranteed `std::terminate()`) can be produced if desired.

[2017-03-16, Daniel comments]

The publication of the new working draft is awaited, before proposed wording against that new working draft is formally possible.

[2017-05-03, Tim comments]

The suggested wording has been moved to the PR section now that the new working draft is available.

[2017-07 Toronto Wed Issue Prioritization]

Priority 0; Move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Insert a paragraph after 20.3.1.3.2 [unique.ptr.single.ctor] p11:

   >     unique_ptr(pointer p, see below d1) noexcept;
   >     unique_ptr(pointer p, see below d2) noexcept;

   > > -9- The signature of these constructors depends upon whether `D` is a reference type. If `D` is a non-reference type `A`, then the signatures are:

   > > […]

   > > -10- If `D` is an lvalue reference type `A&`, then the signatures are:

   > > […]

   > > -11- If `D` is an lvalue reference type `const A&`, then the signatures are:

   > > […]

   > > -??- Requires: For the first constructor, if `D` is not a reference type, `D` shall satisfy the requirements of `CopyConstructible` and such construction shall not exit via an exception. For the second constructor, if `D` is not a reference type, `D` shall satisfy the requirements of `MoveConstructible` and such construction shall not exit via an exception.

### 2945(i). Order of template parameters in `optional` comparisons

Section: 22.5.9 [optional.comp.with.t] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-03-13 Last modified: 2021-06-06

Priority: 2

View all other issues in [optional.comp.with.t].

View all issues with C++20 status.

Discussion:

LWG 2934(i) added an additional template parameter to the comparison operators for `std::optional`, but the ones that compare `U` with `optional<T>` have the parameters backwards compared to the function parameters:

>     template <class T, class U>
>     constexpr bool operator==(const U&, const optional<T>&);

Ville confirmed there's no particular reason for this, it's just how he wrote the proposed resolution, but as this has normative effect we should consider if we really want the template parameters and function parameters to be in different orders or not.

[2017-07-13, Casey Carter provides wording]

[2016-07, Toronto Thursday night issues processing]

Status to Ready

Proposed resolution:

This wording is relative to N4659.

1. Modify 22.5.2 [optional.syn], `<optional>` synopsis, as indicated:

   >     //  [optional.comp_with_t], comparison with T
   >     template <class T, class U> constexpr bool operator==(const optional<T>&, const U&);
   >     template <class T, class U> constexpr bool operator==(const UT&, const optional<TU>&);
   >     template <class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
   >     template <class T, class U> constexpr bool operator!=(const UT&, const optional<TU>&);
   >     template <class T, class U> constexpr bool operator<(const optional<T>&, const U&);
   >     template <class T, class U> constexpr bool operator<(const UT&, const optional<TU>&);
   >     template <class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
   >     template <class T, class U> constexpr bool operator<=(const UT&, const optional<TU>&);
   >     template <class T, class U> constexpr bool operator>(const optional<T>&, const U&);
   >     template <class T, class U> constexpr bool operator>(const UT&, const optional<TU>&);
   >     template <class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
   >     template <class T, class U> constexpr bool operator>=(const UT&, const optional<TU>&);

2. Modify [optional.comp_with_t] as indicated:

   >     template <class T, class U> constexpr bool operator==(const UT& v, const optional<TU>& x);

   > > -3- […]

   > […]

   >     template <class T, class U> constexpr bool operator!=(const UT& v, const optional<TU>& x);

   > > -7- […]

   > […]

   >     template <class T, class U> constexpr bool operator<(const UT& v, const optional<TU>& x);

   > > -11- […]

   > […]

   >     template <class T, class U> constexpr bool operator<=(const UT& v, const optional<TU>& x);

   > > -15- […]

   > […]

   >     template <class T, class U> constexpr bool operator>(const UT& v, const optional<TU>& x);

   > > -19- […]

   > […]

   >     template <class T, class U> constexpr bool operator>=(const UT& v, const optional<TU>& x);

   > > -23- […]

   > […]

### 2946(i). LWG 2758's resolution missed further corrections

Section: 27.4.3.3 [string.cons], 27.4.3.7.2 [string.append], 27.4.3.7.3 [string.assign], 27.4.3.8 [string.ops] Status: C++20 Submitter: Daniel Krügler Opened: 2017-03-17 Last modified: 2021-02-25

Priority: 2

View all other issues in [string.cons].

View all issues with C++20 status.

Discussion:

LWG 2758(i) corrected newly introduced ambiguities of `std::string::assign` and other functions that got new overloads taking a `basic_string_view` as argument, but the assignment operator of `basic_string` and other functions taking a parameter of type `basic_string_view<charT, traits>` were not corrected. Similar to the previous issue the following operations lead now to an ambiguity as well:

>     #include <string>

>     int main()
>     {
>       std::string s({"abc", 1});
>       s = {"abc", 1};
>       s += {"abc", 1};
>       s.append({"abc", 1});
>       s.assign({"abc", 1});
>       s.insert(0, {"abc", 1});
>       s.replace(0, 1, {"abc", 1});
>       s.replace(s.cbegin(), s.cbegin(), {"abc", 1});
>       s.find({"abc", 1});
>       s.rfind({"abc", 1});
>       s.find_first_of({"abc", 1});
>       s.find_last_of({"abc", 1});
>       s.find_first_not_of({"abc", 1});
>       s.find_last_not_of({"abc", 1});
>       s.compare({"abc", 1});
>       s.compare(0, 1, {"abc", 1});
>     }

The right fix is to convert all member functions taken a `basic_string_view<charT, traits>` parameter into constrained function templates.

When doing so, it turns out that there occurs an additional problem: The functions that had been massaged by LWG 2758(i) are all functions that are not specified to be `noexcept`, but the wider range of "string operation" functions taking a `basic_string_view<charT, traits>` parameter are mostly `noexcept` because they had a wide contract. Now with the approach of LWG 2758(i), there are all types allowed that are convertible to `basic_string_view<charT, traits>`, but the conversion occurs now in the function body, not outside of it. So, if these conversion would be potentially exception-throwing, this would lead to a call to `std::terminate`, which is a semantic change compared to the previous specification. There are several options to handle this situation:

1. Ignore that and let `std::terminate` come into action. This is a different way of saying that we impose the requirement of a nothrowing operation.

2. Remove `noexcept` from all the affected functions.

3. Make these functions conditionally `noexcept`.

The submitter has a personal preference for option (3), except that this would complicate the wording a bit, because unfortunately there exists yet no trait `std::is_nothrow_convertible` (See LWG 2040(i)). A seemingly low-hanging fruit would be the attempt to use `std::is_nothrow_constructible` instead, but this trait describes a potentially different initialization context and is therefore inappropriate. Option (1) would conserve the existing `noexcept` guarantee for all non-throwing conversions, but now these functions become narrow-contract functions and at least according to the current `noexcept` guidelines such functions should not be marked as `noexcept`. But there are exceptions possible for that rule, and the initially suggested proposed wording below argues that this exception is reasonable here, because the required wording fixes just an unintended side-effects of transforming the functions into functions templates, but it doesn't intend to change the actual functionality.

Some of the below suggested overload exclusion constraints technically don't require the additional `is_convertible_v<const T&, const charT*> == false` requirement, but the submitter of this issue suggests a more advanced approach that should be applied in a synchronous wording adjustment combined with the existing LWG 2758(i) wording: It would presumably life easier for implementations (which are allowed to provide additional member function overloads as conforming extensions), when we would define a mini requirement set for template parameter type `T` below:

- `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true`.

- `is_convertible_v<const T&, const charT*>` is `false`.

- The implicit conversion to `basic_string_view<charT, traits>` shall not throw an exception.

But the corresponding slightly revised wording taking advantage of this "concept-like" requirements set will not be suggested before the upcoming working draft has been published to allow a simpler coordinated adjustment together with the LWG 2758(i) wording.

It should also be noted that these changes have impact on deduction behaviour and therefore may require further adjustments of the deduction rules.

[2017-07-13, Toronto]

LWG preferred to remove in all functions with added nothrow constraints the `noexcept` specifier (and the constraint) and to possibly improve the situation later.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4640.

> 1. Edit 27.4.3 [basic.string], class template `basic_string` synopsis, as indicated:

>    >     […]

>    >     // 21.3.2.2, construct/copy/destroy
>    >     […]
>    >     template<class T>
>    >     explicit basic_string(basic_string_view<charT, traits> svconst T& t,
>    >                           const Allocator& a = Allocator());
>    >     […]
>    >     template<class T>
>    >     basic_string& operator=(basic_string_view<charT, traits> svconst T& t);
>    >     basic_string& operator=(const charT* s);
>    >     […]

>    >     // 21.3.2.6, modifiers
>    >     […]
>    >     template<class T>
>    >     basic_string& operator+=(basic_string_view<charT, traits> svconst T& t);
>    >     […]
>    >     template<class T>
>    >     basic_string& append(basic_string_view<charT, traits> svconst T& t);
>    >     […]
>    >     template<class T>
>    >     basic_string& assign(basic_string_view<charT, traits> svconst T& t);
>    >     […]
>    >     template<class T>
>    >     basic_string& insert(size_type pos, basic_string_view<charT, traits> svconst T& t);
>    >     […]
>    >     template<class T>
>    >     basic_string& replace(size_type pos1, size_type n1,
>    >                           basic_string_view<charT, traits> svconst T& t);
>    >     […]
>    >     template<class T>
>    >     basic_string& replace(const_iterator i1, const_iterator i2,
>    >                           basic_string_view<charT, traits> svconst T& t);
>    >     […]

>    >     // 21.3.2.7, string operations
>    >     […]
>    >     template<class T>
>    >     size_type find (basic_string_view<charT, traits> svconst T& t,
>    >                     size_type pos = 0) const noexcept;
>    >     […]
>    >     template<class T>
>    >     size_type rfind(basic_string_view<charT, traits> svconst T& t,
>    >                     size_type pos = npos) const noexcept;
>    >     […]
>    >     template<class T>
>    >     size_type find_first_of(basic_string_view<charT, traits> svconst T& t,
>    >                             size_type pos = 0) const noexcept;
>    >     […]
>    >     template<class T>
>    >     size_type find_last_of (basic_string_view<charT, traits> svconst T& t,
>    >                             size_type pos = npos) const noexcept;
>    >     […]
>    >     template<class T>
>    >     size_type find_first_not_of(basic_string_view<charT, traits> svconst T& t,
>    >                                 size_type pos = 0) const noexcept;
>    >     […]
>    >     template<class T>
>    >     size_type find_last_not_of (basic_string_view<charT, traits> svconst T& t,
>    >                                 size_type pos = npos) const noexcept;
>    >     […]
>    >     template<class T>
>    >     int compare(basic_string_view<charT, traits> svconst T& t) const noexcept;
>    >     […]
>    >     template<class T>
>    >     int compare(size_type pos1, size_type n1, basic_string_view<charT, traits> svconst T& t) const;
>    >     […]

> 2. Edit 27.4.3.3 [string.cons] as indicated:

>    >     template<class T>
>    >     explicit basic_string(basic_string_view<charT, traits> svconst T& t,
>    >                           const Allocator& a = Allocator());

>    > > -9- Effects: Same as `basic_string(sv.data(), sv.size(), a)`.Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then behaves the same as `basic_string(sv.data(), sv.size(), a)`.

>    > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

>    > […]

>    >     template<class T>
>    >     basic_string& operator=(basic_string_view<charT, traits> svconst T& t);

>    > > -25- Effects: Equivalent to:

>    > > >     {
>    > > >       basic_string_view<charT, traits> sv = t;
>    > > >       return assign(sv);
>    > > >     }

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 3. Edit [string.op+=] as indicated:

>    >     template<class T>
>    >     basic_string& operator+=(basic_string_view<charT, traits> svconst T& t);

>    > > -3- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then cCalls `append(sv)`.

>    > > -4- Returns: `*this`.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 4. Edit 27.4.3.7.2 [string.append] as indicated:

>    >     template<class T>
>    >     basic_string& append(basic_string_view<charT, traits> svconst T& t);

>    > > -6- Effects: Equivalent to:

>    > > >     {
>    > > >       basic_string_view<charT, traits> sv = t;
>    > > >       return append(sv.data(), sv.size());
>    > > >     }

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 5. Edit 27.4.3.7.3 [string.assign] as indicated:

>    >     template<class T>
>    >     basic_string& assign(basic_string_view<charT, traits> svconst T& t);

>    > > -8- Effects: Equivalent to:

>    > > >     {
>    > > >       basic_string_view<charT, traits> sv = t;
>    > > >       return assign(sv.data(), sv.size());
>    > > >     }

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 6. Edit 27.4.3.7.4 [string.insert] as indicated:

>    >     template<class T>
>    >     basic_string& insert(size_type pos, basic_string_view<charT, traits> svconst T& t);

>    > > -5- Effects: Equivalent to:

>    > > >     {
>    > > >       basic_string_view<charT, traits> sv = t;
>    > > >       return insert(pos, sv.data(), sv.size());
>    > > >     }

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 7. Edit 27.4.3.7.6 [string.replace] as indicated:

>    >     template<class T>
>    >     basic_string& replace(size_type pos1, size_type n1,
>    >                           basic_string_view<charT, traits> svconst T& t);

>    > > -5- Effects: Equivalent to:

>    > > >     {
>    > > >       basic_string_view<charT, traits> sv = t;
>    > > >       return replace(pos1, n1, sv.data(), sv.size());
>    > > >     }

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

>    > […]

>    >     template<class T>
>    >     basic_string& replace(const_iterator i1, const_iterator i2,
>    >                           basic_string_view<charT, traits> svconst T& t);

>    > > -21- Requires: `[begin(), i1)` and `[i1, i2)` are valid ranges.

>    > > -22- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then cCalls `replace(i1 - begin(), i2 - i1, sv)`.

>    > > -23- Returns: `*this`.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 8. Edit 27.4.3.8.2 [string.find] as indicated:

>    >     template<class T>
>    >     size_type find(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept;

>    > > -?- Requires: The initialization of `sv`, as specified below, shall not throw an exception.

>    > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the lowest position `xpos`, if possible, such that both of the following conditions hold: […]

>    > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 9. Edit [string.rfind] as indicated:

>    >     template<class T>
>    >     size_type rfind(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept;

>    > > -?- Requires: The initialization of `sv`, as specified below, shall not throw an exception.

>    > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the highest position `xpos`, if possible, such that both of the following conditions hold: […]

>    > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 10. Edit [string.find.first.of] as indicated:

>     >     template<class T>
>     >     size_type find_first_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept;

>     > > -?- Requires: The initialization of `sv`, as specified below, shall not throw an exception.

>     > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the lowest position `xpos`, if possible, such that both of the following conditions hold: […]

>     > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

>     > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 11. Edit [string.find.last.of] as indicated:

>     >     template<class T>
>     >     size_type find_last_of(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept;

>     > > -?- Requires: The initialization of `sv`, as specified below, shall not throw an exception.

>     > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the highest position `xpos`, if possible, such that both of the following conditions hold: […]

>     > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

>     > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 12. Edit [string.find.first.not.of] as indicated:

>     >     template<class T>
>     >     size_type find_first_not_of(basic_string_view<charT, traits> svconst T& t,
>     >                                 size_type pos = 0) const noexcept;

>     > > -?- Requires: The initialization of `sv`, as specified below, shall not throw an exception.

>     > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the lowest position `xpos`, if possible, such that both of the following conditions hold: […]

>     > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

>     > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 13. Edit [string.find.last.not.of] as indicated:

>     >     template<class T>
>     >     size_type find_last_not_of(basic_string_view<charT, traits> svconst T& t,
>     >                                size_type pos = npos) const noexcept;

>     > > -?- Requires: The initialization of `sv`, as specified below, shall not throw an exception.

>     > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the highest position `xpos`, if possible, such that both of the following conditions hold: […]

>     > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

>     > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

> 14. Edit 27.4.3.8.4 [string.compare] as indicated:

>     >     template<class T>
>     >     int compare(basic_string_view<charT, traits> svconst T& t) const noexcept;

>     > > -?- Requires: The initialization of `sv`, as specified below, shall not throw an exception.

>     > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the effective length `rlen` of the strings to compare as the smaller of `size()` and `sv.size()`. The function then compares the two strings by calling `traits::compare(data(), sv.data(), rlen)`.

>     > > -2- Returns: The nonzero result if the result of the comparison is nonzero. Otherwise, returns a value as indicated in Table 63. […]

>     > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

>     >     template<class T>
>     >     int compare(size_type pos1, size_type n1, basic_string_view<charT, traits> svconst T& t) const;

>     > > -3- Effects: Equivalent to:

>     > > >     {
>     > > >       basic_string_view<charT, traits> sv = t;
>     > > >       return basic_string_view<charT, traits>(data(), size()).substr(pos1, n1).compare(sv);
>     > > >     }

>     > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

[2017-07-14, Toronto, Daniel refines wording]

To balance the loss of information about the removed `noexcept` specifications, all affected functions should get a new Throws: element saying that they won't throw unless this is caused by the conversion to the local `basic_string_view` object. The existing P/R has been updated to reflect that suggestion.

[2017-07 Toronto Wed Issue Prioritization]

Priority ; Marshall to investigate and if OK, will propose Tentatively Ready on reflector.

[2018-03-03: STL reported a related issue, LWG 3075(i).]

[2018-14: Wednesday night issues processing: both this and 3075(i) to status "Immediate".]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4656.

1. Edit 27.4.3 [basic.string], class template `basic_string` synopsis, as indicated:

   >     […]

   >     // 21.3.2.2, construct/copy/destroy
   >     […]
   >     template<class T>
   >     explicit basic_string(basic_string_view<charT, traits> svconst T& t,
   >                           const Allocator& a = Allocator());
   >     […]
   >     template<class T>
   >     basic_string& operator=(basic_string_view<charT, traits> svconst T& t);
   >     basic_string& operator=(const charT* s);
   >     […]

   >     // 21.3.2.6, modifiers
   >     […]
   >     template<class T>
   >     basic_string& operator+=(basic_string_view<charT, traits> svconst T& t);
   >     […]
   >     template<class T>
   >     basic_string& append(basic_string_view<charT, traits> svconst T& t);
   >     […]
   >     template<class T>
   >     basic_string& assign(basic_string_view<charT, traits> svconst T& t);
   >     […]
   >     template<class T>
   >     basic_string& insert(size_type pos, basic_string_view<charT, traits> svconst T& t);
   >     […]
   >     template<class T>
   >     basic_string& replace(size_type pos1, size_type n1,
   >                           basic_string_view<charT, traits> svconst T& t);
   >     […]
   >     template<class T>
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           basic_string_view<charT, traits> svconst T& t);
   >     […]

   >     // 21.3.2.7, string operations
   >     […]
   >     template<class T>
   >     size_type find (basic_string_view<charT, traits> svconst T& t,
   >                     size_type pos = 0) const noexcept;
   >     […]
   >     template<class T>
   >     size_type rfind(basic_string_view<charT, traits> svconst T& t,
   >                     size_type pos = npos) const noexcept;
   >     […]
   >     template<class T>
   >     size_type find_first_of(basic_string_view<charT, traits> svconst T& t,
   >                             size_type pos = 0) const noexcept;
   >     […]
   >     template<class T>
   >     size_type find_last_of (basic_string_view<charT, traits> svconst T& t,
   >                             size_type pos = npos) const noexcept;
   >     […]
   >     template<class T>
   >     size_type find_first_not_of(basic_string_view<charT, traits> svconst T& t,
   >                                 size_type pos = 0) const noexcept;
   >     […]
   >     template<class T>
   >     size_type find_last_not_of (basic_string_view<charT, traits> svconst T& t,
   >                                 size_type pos = npos) const noexcept;
   >     […]
   >     template<class T>
   >     int compare(basic_string_view<charT, traits> svconst T& t) const noexcept;
   >     […]
   >     template<class T>
   >     int compare(size_type pos1, size_type n1, basic_string_view<charT, traits> svconst T& t) const;
   >     […]

2. Edit 27.4.3.3 [string.cons] as indicated:

   >     template<class T>
   >     explicit basic_string(basic_string_view<charT, traits> svconst T& t,
   >                           const Allocator& a = Allocator());

   > > -9- Effects: Same as `basic_string(sv.data(), sv.size(), a)`.Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then behaves the same as `basic_string(sv.data(), sv.size(), a)`.

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

   > […]

   >     template<class T>
   >     basic_string& operator=(basic_string_view<charT, traits> svconst T& t);

   > > -25- Effects: Equivalent to:

   > > >     {
   > > >       basic_string_view<charT, traits> sv = t;
   > > >       return assign(sv);
   > > >     }

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

3. Edit [string.op+=] as indicated:

   >     template<class T>
   >     basic_string& operator+=(basic_string_view<charT, traits> svconst T& t);

   > > -3- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then cCalls `append(sv)`.

   > > -4- Returns: `*this`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

4. Edit 27.4.3.7.2 [string.append] as indicated:

   >     template<class T>
   >     basic_string& append(basic_string_view<charT, traits> svconst T& t);

   > > -6- Effects: Equivalent to:

   > > >     {
   > > >       basic_string_view<charT, traits> sv = t;
   > > >       return append(sv.data(), sv.size());
   > > >     }

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

5. Edit 27.4.3.7.3 [string.assign] as indicated:

   >     template<class T>
   >     basic_string& assign(basic_string_view<charT, traits> svconst T& t);

   > > -8- Effects: Equivalent to:

   > > >     {
   > > >       basic_string_view<charT, traits> sv = t;
   > > >       return assign(sv.data(), sv.size());
   > > >     }

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

6. Edit 27.4.3.7.4 [string.insert] as indicated:

   >     template<class T>
   >     basic_string& insert(size_type pos, basic_string_view<charT, traits> svconst T& t);

   > > -5- Effects: Equivalent to:

   > > >     {
   > > >       basic_string_view<charT, traits> sv = t;
   > > >       return insert(pos, sv.data(), sv.size());
   > > >     }

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

7. Edit 27.4.3.7.6 [string.replace] as indicated:

   >     template<class T>
   >     basic_string& replace(size_type pos1, size_type n1,
   >                           basic_string_view<charT, traits> svconst T& t);

   > > -5- Effects: Equivalent to:

   > > >     {
   > > >       basic_string_view<charT, traits> sv = t;
   > > >       return replace(pos1, n1, sv.data(), sv.size());
   > > >     }

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

   > […]

   >     template<class T>
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           basic_string_view<charT, traits> svconst T& t);

   > > -21- Requires: `[begin(), i1)` and `[i1, i2)` are valid ranges.

   > > -22- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then cCalls `replace(i1 - begin(), i2 - i1, sv)`.

   > > -23- Returns: `*this`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

8. Edit 27.4.3.8.2 [string.find] as indicated:

   >     template<class T>
   >     size_type find(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept;

   > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the lowest position `xpos`, if possible, such that both of the following conditions hold: […]

   > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

   > > -?- Throws: Nothing unless the initialization of `sv` throws an exception.

9. Edit [string.rfind] as indicated:

   >     template<class T>
   >     size_type rfind(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept;

   > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the highest position `xpos`, if possible, such that both of the following conditions hold: […]

   > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

   > > -?- Throws: Nothing unless the initialization of `sv` throws an exception.

10. Edit [string.find.first.of] as indicated:

    >     template<class T>
    >     size_type find_first_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept;

    > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the lowest position `xpos`, if possible, such that both of the following conditions hold: […]

    > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

    > > -?- Throws: Nothing unless the initialization of `sv` throws an exception.

11. Edit [string.find.last.of] as indicated:

    >     template<class T>
    >     size_type find_last_of(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept;

    > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the highest position `xpos`, if possible, such that both of the following conditions hold: […]

    > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

    > > -?- Throws: Nothing unless the initialization of `sv` throws an exception.

12. Edit [string.find.first.not.of] as indicated:

    >     template<class T>
    >     size_type find_first_not_of(basic_string_view<charT, traits> svconst T& t,
    >                                 size_type pos = 0) const noexcept;

    > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the lowest position `xpos`, if possible, such that both of the following conditions hold: […]

    > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

    > > -?- Throws: Nothing unless the initialization of `sv` throws an exception.

13. Edit [string.find.last.not.of] as indicated:

    >     template<class T>
    >     size_type find_last_not_of(basic_string_view<charT, traits> svconst T& t,
    >                                size_type pos = npos) const noexcept;

    > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the highest position `xpos`, if possible, such that both of the following conditions hold: […]

    > > -2- Returns: `xpos` if the function can determine such a value for `xpos`. Otherwise, returns `npos`.

    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

    > > -?- Throws: Nothing unless the initialization of `sv` throws an exception.

14. Edit 27.4.3.8.4 [string.compare] as indicated:

    >     template<class T>
    >     int compare(basic_string_view<charT, traits> svconst T& t) const noexcept;

    > > -1- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then dDetermines the effective length `rlen` of the strings to compare as the smaller of `size()` and `sv.size()`. The function then compares the two strings by calling `traits::compare(data(), sv.data(), rlen)`.

    > > -2- Returns: The nonzero result if the result of the comparison is nonzero. Otherwise, returns a value as indicated in Table 63. […]

    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

    > > -?- Throws: Nothing unless the initialization of `sv` throws an exception.

    >     template<class T>
    >     int compare(size_type pos1, size_type n1, basic_string_view<charT, traits> svconst T& t) const;

    > > -3- Effects: Equivalent to:

    > > >     {
    > > >       basic_string_view<charT, traits> sv = t;
    > > >       return basic_string_view<charT, traits>(data(), size()).substr(pos1, n1).compare(sv);
    > > >     }

    > > -?- Remarks: This function shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and `is_convertible_v<const T&, const charT*>` is `false`.

### 2948(i). `unique_ptr` does not define `operator<<` for stream output

Section: 20.3.1 [unique.ptr] Status: C++20 Submitter: Peter Dimov Opened: 2017-03-19 Last modified: 2021-02-25

Priority: 0

View all other issues in [unique.ptr].

View all issues with C++20 status.

Discussion:

`shared_ptr` does define `operator<<`, and `unique_ptr` should too, for consistency and usability reasons.

[2017-07 Toronto Wed Issue Prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Change 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     namespace std {
   >       […]

   >       // 20.3.1 [unique.ptr], class template unique_ptr
   >       […]
   >       template <class T, class D>
   >       bool operator>=(nullptr_t, const unique_ptr<T, D>& y);

   >       template<class E, class T, class Y, class D>
   >       basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);
   >       […]
   >     }

2. Change 20.3.1 [unique.ptr], class template `unique_ptr` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       template <class T, class D>
   >       bool operator>=(nullptr_t, const unique_ptr<T, D>& y);

   >       template<class E, class T, class Y, class D>
   >       basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);
   >     }

3. Add a new subclause following subclause 20.3.1.6 [unique.ptr.special] as indicated:

   > 23.11.1.?? `unique_ptr` I/O [unique.ptr.io]

   >     template<class E, class T, class Y, class D>
   >       basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);

   > > -?- Effects: Equivalent to `os << p.get();`

   > > -?- Returns: `os`.

   > > -?- Remarks: This function shall not participate in overload resolution unless `os << p.get()` is a valid expression.

### 2950(i). `std::byte` operations are misspecified

Section: 17.2.5 [support.types.byteops] Status: C++20 Submitter: Thomas Köppe Opened: 2017-03-24 Last modified: 2021-02-25

Priority: 1

View all issues with C++20 status.

Discussion:

The operations for `std::byte` (17.2.5 [support.types.byteops]) are currently specified to have undefined behaviour in general cases, since the type of the expression expr that appears in `return byte(expr)` is obtained by the arithmetic conversion rules and has higher conversion rank than `unsigned char`. Therefore, the value of the expression may be outside the range of the enum (for example, consider `~0`), and by 7.6.1.9 [expr.static.cast] p10 the conversion results in undefined behaviour.

I believe the original intent of the specification could be expressed correctly with the following, more verbose sequence of casts. I will only give one representative example:

>     byte operator<<(byte b, IntType shift)

> > Equivalent to: `return byte(static_cast<unsigned char>(static_cast<unsigned char>(b) << shift));`

[ 2017-06-27 P1 after 5 positive votes on c++std-lib. ]

[2017-06-28, STL comments and provides wording]

This proposed resolution performs its work in `unsigned int`, which is immune to promotion. For `op=`, I'm avoiding unnecessary verbosity.

It stylistically uses `static_cast`s instead of functional-style casts. All of the `static_cast`s are intentional, although not all of them are strictly necessary. I felt that it was simpler to always follow the same pattern for type conversions, instead of skipping `static_cast`s by taking advantage of the possible ranges of values. (I could prepare an alternative PR to avoid unnecessary casts.) I'm not `static_cast`ing the shift arguments, because of how 7.6.7 [expr.shift] works.

For `to_integer()`, there's a tiny question. According to 7.6.1.9 [expr.static.cast]/9, `static_cast`ing from `[128, 255]` bytes to `signed` (behavior) `char`s triggers unspecified behavior, whereas using `unsigned char` as an intermediate type would produce implementation-defined behavior, 7.3.9 [conv.integral]/3. This question is basically theoretical, and it's unaffected by this proposed resolution (which is just changing a functional-style cast to a `static_cast`).

[2016-07, Toronto Thursday night issues processing]

Status to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit 17.2.5 [support.types.byteops] as indicated:

   >     template <class IntType>
   >       constexpr byte& operator<<=(byte& b, IntType shift) noexcept;

   > > -1- Remarks: This function shall not participate in overload resolution unless `is_integral_v<IntType>` is `true`.

   > > -2- Effects: Equivalent to: `return b = b << shiftbyte(static_cast<unsigned char>(b) << shift);`

   >     template <class IntType>
   >       constexpr byte operator<<(byte b, IntType shift) noexcept;

   > > -3- Remarks: This function shall not participate in overload resolution unless `is_integral_v<IntType>` is `true`.

   > > -4- Effects: Equivalent to: `return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(b) << shift))byte(static_cast<unsigned char>(b) << shift);`

   >     template <class IntType>
   >       constexpr byte& operator>>=(byte& b, IntType shift) noexcept;

   > > -5- Remarks: This function shall not participate in overload resolution unless `is_integral_v<IntType>` is `true`.

   > > -6- Effects: Equivalent to: `return b = b >> shiftbyte(static_cast<unsigned char>(b) >> shift);`

   >     template <class IntType>
   >       constexpr byte operator>>(byte b, IntType shift) noexcept;

   > > -7- Remarks: This function shall not participate in overload resolution unless `is_integral_v<IntType>` is `true`.

   > > -8- Effects: Equivalent to: `return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(b) >> shift))byte(static_cast<unsigned char>(b) >> shift);`

   >     constexpr byte& operator|=(byte& l, byte r) noexcept;

   > > -9- Effects: Equivalent to:

   > > >     return l = l | rbyte(static_cast<unsigned char>(l) | static_cast<unsigned char>(r));

   >     constexpr byte operator|(byte l, byte r) noexcept;

   > > -10- Effects: Equivalent to:

   > > >     return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(l) |
   > > >     static_cast<unsigned int>(r)))byte(static_cast<unsigned char>(l) |
   > > >     static_cast<unsigned char>(r));

   >     constexpr byte& operator&=(byte& l, byte r) noexcept;

   > > -11- Effects: Equivalent to:

   > > >     return l = l & rbyte(static_cast<unsigned char>(l) & static_cast<unsigned char>(r));

   >     constexpr byte operator&(byte l, byte r) noexcept;

   > > -12- Effects: Equivalent to:

   > > >     return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(l) &
   > > >     static_cast<unsigned int>(r)))byte(static_cast<unsigned char>(l) &
   > > >     static_cast<unsigned char>(r));

   >     constexpr byte& operator^=(byte& l, byte r) noexcept;

   > > -13- Effects: Equivalent to:

   > > >     return l = l ^ rbyte(static_cast<unsigned char>(l) ^ static_cast<unsigned char>(r));

   >     constexpr byte operator^(byte l, byte r) noexcept;

   > > -14- Effects: Equivalent to:

   > > >     return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(l) ^
   > > >     static_cast<unsigned int>(r)))byte(static_cast<unsigned char>(l) ^
   > > >     static_cast<unsigned char>(r));

   >     constexpr byte operator~(byte b) noexcept;

   > > -15- Effects: Equivalent to: `return static_cast<byte>(static_cast<unsigned char>(~static_cast<unsigned int>(b)))byte(~static_cast<unsigned char>(b));`

   >     template <class IntType>
   >       constexpr IntType to_integer(byte b) noexcept;

   > > -16- Remarks: This function shall not participate in overload resolution unless `is_integral_v<IntType>` is `true`.

   > > -17- Effects: Equivalent to: `return static_cast<IntType>IntType(b);`

### 2951(i). `iterator_traits` should SFINAE for `void*` and function pointers

Section: 24.3.2.3 [iterator.traits] Status: Resolved Submitter: Billy Robert O'Neal III Opened: 2017-03-24 Last modified: 2020-09-06

Priority: 3

View all other issues in [iterator.traits].

View all issues with Resolved status.

Discussion:

A customer recently triggered an unexpected SFINAE condition with class `path`. We constrain the constructor that takes `const Source&` by asking if `iterator_traits<Source>::value_type` is one that's OK. This forms `iterator_traits<void*>`, which is a hard error, as it tries to form `void&`.

Pointers-to-non-object-type can never act as iterators, as they don't support pointer arithmetic (as we recently discovered in `atomic<void*>` / `atomic<int(*)(int)>`).

[2017-07 Toronto Wed Issue Prioritization]

Priority 3; Billy to look into arrays of unknown bounds

[2019-02; Kona Wednesday night issue processing]

This was resolved by the adoption of P0896 in San Diego.

Proposed resolution:

This wording is relative to N4659.

1. Change 24.3.2.3 [iterator.traits] as indicated:

   > -3- It is specialized for pointers as

   > >     namespace std {
   > >       template<class T> struct iterator_traits<T*> {
   > >         using difference_type = ptrdiff_t;
   > >         using value_type = T;
   > >         using pointer = T*;
   > >         using reference = T&;
   > >         using iterator_category = random_access_iterator_tag;
   > >       };
   > >     }

   > and for pointers to `const` as

   > >     namespace std {
   > >       template<class T> struct iterator_traits<const T*> {
   > >         using difference_type = ptrdiff_t;
   > >         using value_type = T;
   > >         using pointer = const T*;
   > >         using reference = const T&;
   > >         using iterator_category = random_access_iterator_tag;
   > >       };
   > >     }

   > -?- These partial specializations for pointers apply only when `T` is an object type.

### 2952(i). `iterator_traits` should work for pointers to cv `T`

Section: 24.3.2.3 [iterator.traits] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-03-27 Last modified: 2021-02-25

Priority: 0

View all other issues in [iterator.traits].

View all issues with C++20 status.

Discussion:

`iterator_traits` accepts pointer to `volatile T*`, but then says that the `value_type` is `volatile T`, instead of `T`, which is inconsistent for what it does for pointer to `const T`. We should either reject `volatile` outright or give the right answer.

[2017-03-30, David Krauss comments]

`volatile` pointers may not be well-behaved random-access iterators. When simple access incurs side effects, the multiple-pass guarantee depends on underlying (hardware) semantics.

[2017-07 Toronto Wed Issue Prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Change 24.2 [iterator.synopsis] as indicated:

   >     // 24.4 [iterator.primitives], primitives
   >     template<class Iterator> struct iterator_traits;
   >     template<class T> struct iterator_traits<T*>;
   >     template<class T> struct iterator_traits<const T*>;

2. Change 24.3.2.3 [iterator.traits] as indicated:

   > -3- It is specialized for pointers as

   > >     namespace std {
   > >       template<class T> struct iterator_traits<T*> {
   > >         using difference_type = ptrdiff_t;
   > >         using value_type = remove_cv_t<T>;
   > >         using pointer = T*;
   > >         using reference = T&;
   > >         using iterator_category = random_access_iterator_tag;
   > >       };
   > >     }

   > and for pointers to `const` as

   > >     namespace std {
   > >       template<class T> struct iterator_traits<const T*> {
   > >         using difference_type = ptrdiff_t;
   > >         using value_type = T;
   > >         using pointer = const T*;
   > >         using reference = const T&;
   > >         using iterator_category = random_access_iterator_tag;
   > >       };
   > >     }

### 2953(i). LWG 2853 should apply to `deque::erase` too

Section: 23.3.5.4 [deque.modifiers] Status: C++20 Submitter: Tim Song Opened: 2017-03-30 Last modified: 2021-02-25

Priority: 0

View other active issues in [deque.modifiers].

View all other issues in [deque.modifiers].

View all issues with C++20 status.

Discussion:

Most of the discussion of LWG 2853(i) applies, mutatis mutandis, to `deque::erase`. The relevant requirements table requires neither `Copy/MoveInsertable` nor `Copy/MoveConstructible` for the erase operations, so there's no way a copy/move constructor can safely be called.

And "assignment operator or move assignment operator" is just "assignment operator", since "move assignment operator" is just a species of "assignment operator".

[2017-07 Toronto Wed Issue Prioritization]

Priority 0; Move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Change 23.3.5.4 [deque.modifiers] as indicated:

   >     iterator erase(const_iterator position);
   >     iterator erase(const_iterator first, const_iterator last);
   >     void pop_front();
   >     void pop_back();

   > > -4- Effects: […]

   > > -5- Complexity: The number of calls to the destructor of `T` is the same as the number of elements erased, but the number of calls to the assignment operator of `T` is no more than the lesser of the number of elements before the erased elements and the number of elements after the erased elements.

   > > -6- Throws: Nothing unless an exception is thrown by the copy constructor, move constructor, assignment operator, or move assignment operator of `T`.

### 2954(i). Specialization of the convenience variable templates should be prohibited

Section: 16.4.5 [constraints] Status: C++20 Submitter: Tim Song Opened: 2017-03-31 Last modified: 2021-02-25

Priority: Not Prioritized

View all issues with C++20 status.

Discussion:

There's currently no rule against specializing the various `_v` convenience variable templates outside of `<type_traits>`.

There should be; `foo_v<T>` should be always equal to `foo<T>::value`. The correct way to influence, say, `is_placeholder_v<T>` should be to specialize `is_placeholder`, not `is_placeholder_v`. Otherwise, the editorial changes to use the `_v` form to the specification would no longer be editorial but have normative impact.

Adding a global prohibition in 16.4.5.2.1 [namespace.std] seems preferable to adding individual prohibitions to each affected template; the PR below carves out an exception for variable templates that are intended to be specialized by users. As far as I know there are no such templates in the current WP, but the Ranges TS does use them.

[2017-06-14, Moved to Tentatively Ready after 6 positive votes on c++std-lib]

Proposed resolution:

This wording is relative to N4659.

1. Add a paragraph to 16.4.5.2.1 [namespace.std], before p2:

   > -1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace `std` or to a namespace within namespace `std` unless otherwise specified. A program may add a template specialization for any standard library template to namespace `std` only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.(footnote: […])

   > -?- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.

   > -2- The behavior of a C++ program is undefined if it declares […]

### 2955(i). `to_chars` / `from_chars` depend on `std::string`

Section: 22.2 [utility] Status: Resolved Submitter: Jens Maurer Opened: 2017-04-11 Last modified: 2017-07-16

Priority: Not Prioritized

View all other issues in [utility].

View all issues with Resolved status.

Discussion:

We added `from_chars_result` and `to_chars_result` into `<utility>`, but those contain `error_code` instances, which come from `<system_error>`. This creates a circular dependency issue which is somewhat difficult to resolve, as `error_code` depends on `error_category` which has virtuals that return `string` — it would effectively force putting all of `<string>` into `<utility>`.

Since (I believe) nobody has implemented this yet, would there be support for changing the member of `from_chars_result` / `to_chars_result` to an `errc` instead of an `error_code` (as a fast-track DR against 17)?

[2017-06-27, Jens comments]

The paper P0682R0 "Repairing elementary string conversions" is available in the pre-Toronto mailing and provides wording.

[2017-07 Toronto]

Resolved by the adoption of P0682R1 in Toronto.

Proposed resolution:

### 2956(i). `filesystem::canonical()` still defined in terms of `absolute(p, base)`

Section: 31.12.13.3 [fs.op.canonical] Status: C++17 Submitter: Sergey Zubkov Opened: 2017-04-21 Last modified: 2020-09-06

Priority: 1

View all issues with C++17 status.

Discussion:

This is from editorial issue #1620:

Since the resolution of US-78 was applied (as part of P0492R2), `std::filesystem::absolute(path, base)` no longer exists. However, `std::filesystem::canonical` is still defined in terms of `absolute(p, base)`.

[2017-06-27 P1 after 5 positive votes on c++std-lib]

Davis Herring: This needs to be P1 — due to a wording gap in P0492R2, 2 out of the 3 overloads of `filesystem::canonical()` have bad signatures and are unimplementable.

[2017-07-14, Toronto, Davis Herring provides wording]

[2017-07-14, Toronto, Moved to Immediate]

Proposed resolution:

This wording is relative to N4659.

1. Edit 31.12.4 [fs.filesystem.syn] as indicated:

   >     path canonical(const path& p, const path& base = current_path());
   >     path canonical(const path& p, error_code& ec);
   >     path canonical(const path& p, const path& base, error_code& ec);

2. Edit 31.12.13.3 [fs.op.canonical] as indicated:

   >     path canonical(const path& p, const path& base = current_path());
   >     path canonical(const path& p, error_code& ec);
   >     path canonical(const path& p, const path& base, error_code& ec);

   > > -1- Effects: Converts `p`, which must exist, to an absolute path that has no symbolic link, dot, or dot-dot elements in its pathname in the generic format.

   > > -2- Returns: A path that refers to the same file system object as `absolute(p, base)`. For the overload without a `base` argument, `base` is `current_path()`. SignaturesThe signature with argument `ec` returns `path()` if an error occurs.

   > > […]

### 2957(i). `bind`'s specification doesn't apply the cv-qualification of the call wrapper to the callable object

Section: 22.10.15.4 [func.bind.bind] Status: Resolved Submitter: Tim Song Opened: 2017-05-04 Last modified: 2020-09-06

Priority: 3

View all other issues in [func.bind.bind].

View all issues with Resolved status.

Discussion:

According to 22.10.15.4 [func.bind.bind]/1.2,

> `fd` is an lvalue of type `FD` constructed from `std::forward<F>(f)`,

and then uses `fd` throughout the specification, seemingly without regard to the cv-qualification of the call wrapper `g`. But this definition means that `fd` is always cv-unqualified, rather than having its cv-qualification change with that of `g` as intended.

LWG 2545(i) accidentally exacerbated the problem by removing any hint that `fd`'s cv-qualifier followed that of the call wrapper.

A similar issue affects the type of `tdi` for nested binds.

[2017-07 Toronto Wed Issue Prioritization]

Priority 3

[2020-01 Resolved by the adoption of P1065 in Cologne.]

Proposed resolution:

This wording is relative to N4659.

1. Edit 22.10.15.4 [func.bind.bind] as indicated:

   >     template<class F, class... BoundArgs>
   >       unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > […]

   > > -3- Returns: A forwarding call wrapper `g` (22.10.4 [func.require]). The effect of `g(u1, u2, ..., uM)` shall be

   > > >     INVOKE(static_cast<FD cv &>(fd), std::forward<V1>(v1),
   > > >       std::forward<V2>(v2), ..., std::forward<VN>(vN))

   > > where cv represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1, v2, . . . , vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `FD` or of any of the types `TDi` throws an exception.

   > > […]

   >     template<class R, class F, class... BoundArgs>
   >       unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > […]

   > > -7- Returns: A forwarding call wrapper `g` (22.10.4 [func.require]). The effect of `g(u1, u2, ..., uM)` shall be

   > > >     INVOKE<R>(static_cast<FD cv &>(fd), std::forward<V1>(v1),
   > > >       std::forward<V2>(v2), ..., std::forward<VN>(vN))

   > > where cv represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1, v2, . . . , vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `FD` or of any of the types `TDi` throws an exception.

   > > […]

   > > -10- The values of the bound arguments `v1, v2, ... , vN` and their corresponding types `V1, V2, ... , VN` depend on the types `TDi` derived from the call to `bind` and the cv-qualifiers cv of the call wrapper `g` as follows:

   > > 1. (10.1) — […]

   > > 2. (10.2) — if the value of `is_bind_expression_v<TDi>` is `true`, the argument is `static_cast<TDi cv &>(tdi)(std::forward<Uj>(uj)...)` and its type `Vi` is `invoke_result_t<TDi cv &, Uj...>&&`;

   > > 3. (10.3) — […]

### 2958(i). Moves improperly defined as deleted

Section: 22.3.2 [pairs.pair], 22.4.4.3 [tuple.assign] Status: C++20 Submitter: Casey Carter Opened: 2017-05-05 Last modified: 2021-02-25

Priority: 2

View other active issues in [pairs.pair].

View all other issues in [pairs.pair].

View all issues with C++20 status.

Discussion:

LWG 2729(i) constrained many `pair` & `tuple` assignment operators to "not participate in overload resolution" and "be defined as deleted if." As discussed when LWG reviewed 2756(i), it is undesirable to require that a move constructor or assignment operator be "defined as deleted," since it is unclear whether that operation should be implicitly deleted, and therefore not participate in overload resolution, or explicitly deleted and therefore impede overload resolution for usages that would otherwise find copies.

[2017-07 Toronto Wed Issue Prioritization]

Priority 2

[2017-11 Albuquerque Wednesday issue processing]

Move to Immediate

Proposed resolution:

This wording is relative to N4659.

1. Edit 22.3.2 [pairs.pair] as indicated:

   >     pair& operator=(pair&& p) noexcept(see below);

   > > -21- Effects: Assigns to `first` with `std::forward<first_type>(p.first)` and to `second` with `std::forward<second_type>(p.second)`.

   > > -22- Remarks: This operator shall be defined as deletednot participate in overload resolution unless `is_move_assignable_v<first_type>` is `true` and `is_move_assignable_v<second_type>` is `true`.

   > > […]

2. Edit 22.4.4.3 [tuple.assign] as indicated:

   >     tuple& operator=(tuple&& u) noexcept(see below);

   > > -5- Effects: For all `i`, assigns `std::forward<Ti>(get<i>(u))` to `get<i>(*this)`.

   > > -6- Remarks: This operator shall be defined as deletednot participate in overload resolution unless `is_move_assignable_v<Ti>` is `true` for all `i`.

   > > […]

### 2960(i). [fund.ts.v3] `nonesuch` is insufficiently useless

Section: 6.3 [fund.ts.v3::meta] Status: WP Submitter: Tim Song Opened: 2017-05-08 Last modified: 2026-02-21

Priority: 2

View all issues with WP status.

Discussion:

Addresses: fund.ts.v3

The definition of `std::experimental::nonesuch` (with a deleted default constructor, destructor, copy constructor, and copy assignment operator) means that it is an aggregate, which means that it can be initialized from `{}` in contexts where the availability of the destructor is not considered (e.g., overload resolution or a new-expression).

The deleted default constructor also has this effect standing alone, because it doesn't affect the formation of implicit conversion sequences (and hence overload resolution). The net result is ambiguities in situations like:

>     struct such {};
>     void f(const such&);
>     void f(const nonesuch&);
>     f({});

For a real-life example of such ambiguity, see GCC bug 79141, involving libstdc++'s internal `__nonesuch` type defined just like the one in the fundamentals TS.

I believe that `nonesuch` would be substantially more useful if the ICS from `{}` is gone. `nonesuch` should have no default constructor (rather than a deleted one), and it shouldn't be an aggregate.

[2017-11-20 Priority set to 2 after discussion on the reflector.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4617.

> 1. Edit 99 [fund.ts.v3::meta.type.synop] as indicated, moving the definition of `nonesuch` to 6.3.3 [fund.ts.v3::meta.detect]:

>    >     //6.3.3 [fund.ts.v3::meta.detect], Detection idiom
>    >     […]

>    >     struct nonesuch{
>    >       nonesuch() = delete;
>    >       ~nonesuch() = delete;
>    >       nonesuch(nonesuch const&) = delete;
>    >       void operator=(nonesuch const&) = delete;
>    >     };
>    >     […]

> 2. Insert at the beginning of 6.3.3 [fund.ts.v3::meta.detect] the following paragraphs:

>    > [Drafting note: The seemingly redundant statement about default and initializer-list constructors is intended to negate the usual leeway for implementations to declare additional member function signatures granted in 16.4.6.5 [member.functions]. — end drafting note]

>    >     struct nonesuch {
>    >       ~nonesuch() = delete;
>    >       nonesuch(nonesuch const&) = delete;
>    >       void operator=(nonesuch const&) = delete;
>    >     };

>    > > -?- `nonesuch` has no default constructor (C++14 §[class.ctor]) or initializer-list constructor (C++14 §[dcl.init.list]), and is not an aggregate (C++14 §[dcl.init.aggr]).

[2018-08-23 Batavia Issues processing]

Change C++14 references to C++17, and all the LFTS2 references to LFTS3

Status to Tentatively Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4617.

1. Edit 99 [fund.ts.v3::meta.type.synop] as indicated, moving the definition of `nonesuch` to 6.3.3 [fund.ts.v3::meta.detect]:

   >     //6.3.3 [fund.ts.v3::meta.detect], Detection idiom
   >     […]

   >     struct nonesuch{
   >       nonesuch() = delete;
   >       ~nonesuch() = delete;
   >       nonesuch(nonesuch const&) = delete;
   >       void operator=(nonesuch const&) = delete;
   >     };
   >     […]

2. Insert at the beginning of 6.3.3 [fund.ts.v3::meta.detect] the following paragraphs:

   > [Drafting note: The seemingly redundant statement about default and initializer-list constructors is intended to negate the usual leeway for implementations to declare additional member function signatures granted in 16.4.6.5 [member.functions]. — end drafting note]

   >     struct nonesuch {
   >       ~nonesuch() = delete;
   >       nonesuch(nonesuch const&) = delete;
   >       void operator=(nonesuch const&) = delete;
   >     };

   > > -?- `nonesuch` has no default constructor (C++17 §[class.ctor]) or initializer-list constructor (C++17 §[dcl.init.list]), and is not an aggregate (C++17 §[dcl.init.aggr]).

### 2961(i). Bad postcondition for `set_default_resource`

Section: 20.5.4 [mem.res.global] Status: C++20 Submitter: Casey Carter Opened: 2017-05-09 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [mem.res.global].

View all issues with C++20 status.

Discussion:

The specification of `set_default_resource` in N4659 20.5.4 [mem.res.global] reads:

>     memory_resource* set_default_resource(memory_resource* r) noexcept;

> > -4- Effects: If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

> > -5- Postconditions: `get_default_resource() == r`.

> > -6- Returns: The previous value of the default memory resource pointer.

> > -7- Remarks: […]

It is apparent that the effects specified in para 4 cannot — and indeed should not — achieve the postcondition specified in para 5 when `r` is null.

[2017-05-13, Tim comments]

This is the same issue as LWG 2522(i), which just missed the Fundamentals TS working draft used for the merge. A similar resolution (simply striking the Postconditions: paragraph) might be better.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4659.

> 1. Modify 20.5.4 [mem.res.global] as follows:

>    >     memory_resource* set_default_resource(memory_resource* r) noexcept;

>    > > -4- Effects: If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

>    > > -5- Postconditions: If `r` is non-null, `get_default_resource() == r`. Otherwise, `get_default_resource() == new_delete_resource()`.

>    > > -6- Returns: The previous value of the default memory resource pointer.

>    > > -7- Remarks: […]

[2017-06-13, Casey Carter revises proposed wording]

I suggest to strike the Postconditions paragraph ([mem.res.global]/5) completely, as in LWG 2522(i)

[2017-06-15, Moved to Tentatively Ready after 6 positive votes on c++std-lib]

Proposed resolution:

This wording is relative to N4659.

1. Modify 20.5.4 [mem.res.global] as follows:

   >     memory_resource* set_default_resource(memory_resource* r) noexcept;

   > > -4- Effects: If `r` is non-null, sets the value of the default memory resource pointer to `r`, otherwise sets the default memory resource pointer to `new_delete_resource()`.

   > > -5- Postconditions: `get_default_resource() == r`.

   > > -6- Returns: The previous value of the default memory resource pointer.

   > > -7- Remarks: […]

### 2964(i). Apparently redundant requirement for `dynamic_pointer_cast`

Section: 20.3.2.2.10 [util.smartptr.shared.cast] Status: C++20 Submitter: Tim Song Opened: 2017-05-11 Last modified: 2021-02-25

Priority: 0

View all other issues in [util.smartptr.shared.cast].

View all issues with C++20 status.

Discussion:

Currently 20.3.2.2.10 [util.smartptr.shared.cast]/4 says:

> Requires: The expression `dynamic_cast<T*>((U*)nullptr)` shall be well formed and shall have well defined behavior.

A `dynamic_cast` of a null pointer, if well-formed, always has well-defined behavior: it returns a null pointer. The second part is therefore redundant as currently worded. The C++14 version, on the other hand, requires `dynamic_cast<T*>(r.get())` to have well-defined behavior, which actually adds something: it requires the user to not trigger the undefined case in [class.cdtor]/5, for instance.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit 20.3.2.2.10 [util.smartptr.shared.cast] as indicated:

   >     shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;

   > > -4- Requires: The expression `dynamic_cast<T*>((U*)0)` shall be well formed. The expression `dynamic_cast<typename shared_ptr<T>::element_type*>(r.get())` shall be well formed and shall have well defined behavior.

   > > […]

### 2965(i). Non-existing `path::native_string()` in `filesystem_error::what()` specification

Section: 31.12.7.2 [fs.filesystem.error.members] Status: C++20 Submitter: Daniel Krügler Opened: 2017-05-22 Last modified: 2021-06-06

Priority: 0

View all other issues in [fs.filesystem.error.members].

View all issues with C++20 status.

Discussion:

As pointed out by Jonathan Wakely and Bo Persson, [filesystem_error.members]/7 refers to a non-existing function `path::native_string`:

> Returns: A string containing `runtime_error::what()`. The exact format is unspecified. Implementations are encouraged but not required to include `path1.native_string()` if not empty, `path2.native_string()` if not empty, and `system_error::what()` strings in the returned string.

Existing implementations differ, as Jonathan also determined:

- Boost.Filesystem uses `path::string()`.

- Libstdc++ uses `path::string()`.

- MSVC++/Dinkumware uses `path::u8string()`.

- It seems that libc++ doesn't include the paths in `what()`.

We've had `native_string()` in the spec since N3239 (where it already didn't match any existing `path` function at that time).

Before that it was `file_string()` in N1975 (within that specification `path` was a template that was parametrized in the character type).

Since it can't be `path::native()` because that might be the wrong type, one of `path::string()` or `path::u8string()` seems appropriate.

Albeit the wording is just a non-binding encouragement to implementations, the decision on this matter should not be considered editorially due to the existing implementation variance. Any official resolution of the current state could cause a reconsideration of existing implementations, and therefore it should be documented.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4659.

> 1. Edit [filesystem_error.members] as indicated:

>    >     const char* what() const noexcept override;

>    > > -7- Returns: A string containing `runtime_error::what()`. The exact format is unspecified. Implementations are encouraged but not required to include `path1.native_string()` if not empty, `path2.native_string()` if not empty, and `system_error::what()` strings in the returned string.

[2017-05-25, Jonathan comments and suggests an alternative resolution]

The revised wording changes leave it up to the implementation which of the native format observers to use. The "if not empty" seems redundant, because if the path is empty then there's nothing to include anyway, but the proposed resolution preserves it.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit [filesystem_error.members] as indicated:

   >     const char* what() const noexcept override;

   > > -7- Returns: A string containing `runtime_error::what()`. The exact format is unspecified. Implementations are encouraged but not required to include `path1.native_string()` if not empty, `path2.native_string()` if not empty, and `system_error::what()` stringsthe `system_error::what()` string and the pathnames of `path1` and `path2` in the native format in the returned string.

### 2966(i). Incomplete resolution of US 74

Section: 31.12.6.5.6 [fs.path.native.obs] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-05-25 Last modified: 2021-02-25

Priority: Not Prioritized

View all issues with C++20 status.

Discussion:

P0492R1 US-74 failed to replace one occurrence of `pathstring`, in [fs.path.native.obs] p8. It should be changed to `native()`, as with the other native format observers.

[2017-06-12, Moved to Tentatively Ready after 6 positive votes on c++std-lib]

Proposed resolution:

This wording is relative to N4659.

1. Edit 31.12.6.5.6 [fs.path.native.obs] as indicated:

   >     std::string string() const;
   >     std::wstring wstring() const;
   >     std::string u8string() const;
   >     std::u16string u16string() const;
   >     std::u32string u32string() const;

   > > -8- Returns: `pathstringnative()`.

   > > -9- Remarks: Conversion, if any, is performed as specified by 31.12.6.3 [fs.path.cvt]. The encoding of the string returned by `u8string()` is always UTF-8.

### 2968(i). Inconsistencies between `basic_string reserve` and `vector/unordered_map/unordered_set reserve` functions

Section: 27.4.3.5 [string.capacity] Status: Resolved Submitter: Andrew Luo Opened: 2017-05-30 Last modified: 2020-09-06

Priority: 3

View all other issues in [string.capacity].

View all issues with Resolved status.

Discussion:

According to 27.4.3.5 [string.capacity] paragraph 11:

> -11- Effects: After `reserve()`, `capacity()` is greater or equal to the argument of `reserve`. [Note: Calling `reserve()` with a `res_arg` argument less than `capacity()` is in effect a non-binding shrink request. A call with `res_arg <= size()` is in effect a non-binding shrink-to-fit request. — end note]

A call to `basic_string`'s `reserve` function with `res_arg <= size()` is taken as a non-binding request to shrink the capacity, whereas for `vector` (and similarly for `unordered_map` and `unordered_set`) according to 23.3.13.3 [vector.capacity] p3:

> -3- Effects: A directive that informs a `vector` of a planned change in size, so that it can manage the storage allocation accordingly. After `reserve()`, `capacity()` is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`. If an exception is thrown other than by the move constructor of a non-`CopyInsertable` type, there are no effects.

The problem here is that the different behavior makes it that writing template code where the template argument type is a container type (for example `std::string` or `std::vector<char>`) calls to `reserve` can have different meaning depending on which container type the template is instantiated with. It might be a minor issue but it would be nice to fix the inconsistency. I ran into an issue around this when I was porting code from MSVC++ to G++ (For `basic_string`, MSVC++'s STL implementation, based on Dinkumware, ignores the call if `res_arg < capacity()` whereas GCC's STL implementation, libstdc++ will actually shrink the string. For the code I wrote this caused a huge performance issue since we were reallocating the entire string with every call to `reserve`. Of course we could have worked around it by doing the `res_arg < capacity()` check ourselves, but I think this inconsistency in the standard isn't desirable).

My proposal is to change 27.4.3.5 [string.capacity] paragraph 11 to read:

> -11- Effects: After `reserve()`, `capacity()` is greater or equal to the argument of `reserve` if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`.

I realize that this causes the `basic_string::reserve` to no longer have the secondary property of shrinking, but this is what `shrink_to_fit` is for.

[2017-07 Toronto Monday issue prioritization]

Priority 3; status to LEWG

[2018-3-17 Resolved by P0966, which was adopted in Jacksonville.]

Proposed resolution:

This wording is relative to N4659.

1. Edit 27.4.3.5 [string.capacity] as indicated:

   >     void reserve(size_type res_arg=0);

   > > -10- The member function `reserve()` is a directive that informs a `basic_string` object of a planned change in size, so that it can manage the storage allocation accordingly.

   > > -11- Effects: After `reserve()`, `capacity()` is greater or equal to the argument of `reserve`, if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`. [Note: Calling `reserve()` with a `res_arg` argument less than `capacity()` is in effect a non-binding shrink request. A call with `res_arg <= size()` is in effect a non-binding shrink-to-fit request. — end note]

   > > -12- Throws: `length_error` if `res_arg > max_size()`.

### 2969(i). `polymorphic_allocator::construct()` shouldn't pass `resource()`

Section: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Pablo Halpern Opened: 2017-05-30 Last modified: 2021-02-25

Priority: 2

View all other issues in [mem.poly.allocator.mem].

View all issues with C++20 status.

Discussion:

Section 20.5.3.3 [mem.poly.allocator.mem] defines the effect of `polymorphic_allocator<T>::construct` as:

> Effects: Construct a `T` object in the storage whose address is represented by `p` by uses-allocator construction with allocator `resource()` and constructor arguments `std::forward<Args>(args)...`.

The use of `resource()` is a hold-over from the LFTS, which contains a modified definition of uses-allocator construction. This revised definition was not carried over into the C++17 WP when allocator_resource and polymorphic_allocator were moved over.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4659.

> 1. Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:

>    >     template <class T, class... Args>
>    >       void construct(T* p, Args&&... args);

>    > > -5- Requires: Uses-allocator construction of `T` with allocator `resource()*this` (see 20.2.8.2 [allocator.uses.construction]) and constructor arguments `std::forward<Args>(args)...` is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators. — end note]

>    > > -6- Effects: Construct a `T` object in the storage whose address is represented by `p` by uses-allocator construction with allocator `resource()*this` and constructor arguments `std::forward<Args>(args)...`.

>    > > -7- Throws: Nothing unless the constructor for `T` throws.

>    >     template <class T1, class T2, class... Args1, class... Args2>
>    >       void construct(pair<T1,T2>* p, piecewise_construct_t,
>    >                      tuple<Args1...> x, tuple<Args2...> y);

>    > > -8- [Note: This method and the `construct` methods that follow are overloads for piecewise construction of pairs (22.3.2 [pairs.pair]). — end note]

>    > > -9- Effects: Let `xprime` be a `tuple` constructed from `x` according to the appropriate rule from the following list. [Note: The following description can be summarized as constructing a `pair<T1, T2>` object in the storage whose address is represented by `p`, as if by separate uses-allocator construction with allocator `resource()*this` (20.2.8.2 [allocator.uses.construction]) of `p->first` using the elements of `x` and `p->second` using the elements of `y`. — end note]

>    > > […]

[2017-06-12, Pablo comments]

The current description is correct and does not depend on changes to uses-allocator construction. It relies on the fact that `memory_resource*` is convertible to `polymorphic_allocator`.

[2017-06-13, Tim Song reopens]

While it is true that `memory_resource*` is convertible to `polymorphic_allocator`, uses-allocator construction still requires allocators, and a `memory_resource*` isn't an allocator.

To take a concrete example from the current WP, a `pmr::vector<std::promise<int>>`, as specified, will be attempting to uses-allocator construct a `promise<int>` with a `memory_resource*`, but `std::promise`'s allocator-taking constructor expects something that satisfies the allocator requirements, rather than a `memory_resource*`.

[2017-06-13, Daniel and Tim restore and improve the previously proposed wording]

[2017-07 Toronto Monday issue prioritization]

Priority 2; Dietmar to check the P/R before Albuquerque.

[2017-11 Albuquerque Wednesday issue processing]

Move to Ready.

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4659.

1. Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:

   >     template <class T, class... Args>
   >       void construct(T* p, Args&&... args);

   > > -5- Requires: Uses-allocator construction of `T` with allocator `resource()*this` (see 20.2.8.2 [allocator.uses.construction]) and constructor arguments `std::forward<Args>(args)...` is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators. — end note]

   > > -6- Effects: Construct a `T` object in the storage whose address is represented by `p` by uses-allocator construction with allocator `resource()*this` and constructor arguments `std::forward<Args>(args)...`.

   > > -7- Throws: Nothing unless the constructor for `T` throws.

   >     template <class T1, class T2, class... Args1, class... Args2>
   >       void construct(pair<T1,T2>* p, piecewise_construct_t,
   >                      tuple<Args1...> x, tuple<Args2...> y);

   > > -8- [Note: This method and the `construct` methods that follow are overloads for piecewise construction of pairs (22.3.2 [pairs.pair]). — end note]

   > > -9- Effects: Let `xprime` be a `tuple` constructed from `x` according to the appropriate rule from the following list. [Note: The following description can be summarized as constructing a `pair<T1, T2>` object in the storage whose address is represented by `p`, as if by separate uses-allocator construction with allocator `resource()*this` (20.2.8.2 [allocator.uses.construction]) of `p->first` using the elements of `x` and `p->second` using the elements of `y`. — end note]

   > > 1. (9.1) — If `uses_allocator_v<T1,memory_resource*polymorphic_allocator>` is `false` and `is_constructible_v<T1,Args1...>` is `true`, then `xprime` is `x`.

   > > 2. (9.2) — Otherwise, if `uses_allocator_v<T1,memory_resource*polymorphic_allocator>` is `true` and `is_constructible_v<T1,allocator_arg_t,memory_resource*polymorphic_allocator,Args1...>` is `true`, then `xprime` is `tuple_cat(make_tuple(allocator_arg, resource()*this), std::move(x))`.

   > > 3. (9.3) — Otherwise, if `uses_allocator_v<T1,memory_resource*polymorphic_allocator>` is `true` and `is_constructible_v<T1,Args1...,memory_resource*polymorphic_allocator>` is `true`, then `xprime` is `tuple_cat(std::move(x), make_tuple(resource()*this))`.

   > > 4. (9.4) — Otherwise the program is ill formed.

   > > Let `yprime` be a tuple constructed from `y` according to the appropriate rule from the following list:

   > > 1. (9.5) — If `uses_allocator_v<T2,memory_resource*polymorphic_allocator>` is `false` and `is_constructible_v<T2,Args2...>` is `true`, then `yprime` is `y`.

   > > 2. (9.6) — Otherwise, if `uses_allocator_v<T2,memory_resource*polymorphic_allocator>` is `true` and `is_constructible_v<T2,allocator_arg_t,memory_resource*polymorphic_allocator,Args2...>` is `true`, then `yprime` is `tuple_cat(make_tuple(allocator_arg, resource()*this), std::move(y))`.

   > > 3. (9.7) — Otherwise, if `uses_allocator_v<T2,memory_resource*polymorphic_allocator>` is `true` and `is_constructible_v<T2,Args2...,memory_resource*polymorphic_allocator>` is `true`, then `yprime` is `tuple_cat(std::move(y), make_tuple(resource()*this))`.

   > > 4. (9.8) — Otherwise the program is ill formed.

### 2970(i). Return type of `std::visit` misspecified

Section: 22.6.7 [variant.visit] Status: C++20 Submitter: Tim Song Opened: 2017-05-31 Last modified: 2021-02-25

Priority: 2

View all other issues in [variant.visit].

View all issues with C++20 status.

Discussion:

[variant.visit]/1 correctly uses "type and value category", but then p3 describes the return type of `visit` to be "the common type of all possible `INVOKE` expressions of the Effects: element." The type of an expression is never a reference type, due to [expr]/5 removing the referenceness "prior to any further analysis", so this wording as written says that `visit` always returns a non-reference type, which is presumably not the intent.

[2017-07 Toronto Monday issue prioritization]

Priority 2; Matt to provide wording

[2018-01-11, Thomas Köppe comments and suggests wording]

The return type of `std::visit` (originating by P0088R3 accepted during the Oulo 2016 meeting) is currently misspecified and refers only to the common type of all the possible visitation calls, without attention to the value category. This seems unintended, and we should preserve the value category.

[2017-01-24, Daniel comments]

This issue should be reviewed in common with LWG 3052(i).

[ 2018-02-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4727.

1. Modify 22.6.7 [variant.visit] as indicated:

   >     template<class Visitor, class... Variants>
   >       constexpr see below visit(Visitor&& vis, Variants&&... vars);

   > > […]

   > > -3- Returns: `e(m)`, where `m` is the pack for which `mi` is `varsi.index()` for all `0 <= i < n`. The return type is the type of `e(m)``decltype(e(m))`.

   > > […]

### 2972(i). What is `is_trivially_destructible_v<int>`?

Section: 21.3.5.4 [meta.unary.prop] Status: C++20 Submitter: Richard Smith Opened: 2017-06-01 Last modified: 2021-02-25

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

View all other issues in [meta.unary.prop].

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Discussion:

The spec for `is_trivially_destructible` says the value is true if "`is_destructible_v<T>` is `true` and the indicated destructor is known to be trivial."

For a case like `is_trivially_destructible_v<int>`, there is no indicated destructor, so it's unclear what value the trait would have but the most plausible reading of these words is that it should be `false`. However, I'm confident the intent is that this trait should yield `true` in that situation, and that's what all the implementations I can find actually do.

[2017-06-14, Daniel and Jonathan provide wording]

[2017-07-05 Moved to Tentatively Ready after 5 positive votes on c++std-lib.]

Proposed resolution:

This wording is relative to N4659.

1. Change 21.3.5.4 [meta.unary.prop], Table 42 — "Type property predicates", as indicated:

   > Table 22 — Type property predicates
   > Template | Condition | Preconditions
   > --- | --- | ---
   > `…`
   > `template <class T> struct is_trivially_destructible;` | `is_destructible_v<T>` is `true` and the indicated destructor is known to be trivial `remove_all_extents_t<T>` is either a non-class type or a class type with a trivial destructor. | `T` shall be a complete type, cv `void`, or an array of unknown bound.
   > `…`

### 2974(i). Diagnose out of bounds `tuple_element`/`variant_alternative`

Section: 22.3.4 [pair.astuple], 22.6.4 [variant.helper] Status: C++20 Submitter: Agustín K-ballo Bergé Opened: 2017-06-10 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [pair.astuple].

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Discussion:

Instantiating `tuple_element` with an out-of-bounds index requires a diagnostic for `tuple` and `array`, but not for `pair`. The specification requires an out-of-bounds index for `pair` to go to the base template instead, which is undefined.

Similarly, instantiating `variant_alternative` with an out-of-bounds index violates a requirement, but it is not required to be diagnosed.

[2017-06-12, Moved to Tentatively Ready after 5 positive votes on c++std-lib]

Proposed resolution:

This wording is relative to N4659.

1. Modify 22.2.1 [utility.syn], header `<utility>` synopsis, as indicated:

   >     […]

   >     // 22.3.4 [pair.astuple], tuple-like access to pair
   >     template <class T> class tuple_size;
   >     template <size_t I, class T> class tuple_element;
   >     template <class T1, class T2> struct tuple_size<pair<T1, T2>>;
   >     template <size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>>;
   >     template <class T1, class T2> struct tuple_element<0, pair<T1, T2>>;
   >     template <class T1, class T2> struct tuple_element<1, pair<T1, T2>>;

   >     […]

2. Modify 22.3.4 [pair.astuple] as indicated:

   >     tuple_element<0, pair<T1, T2>>::type

   > > -1- Value: The type `T1`.

   >     tuple_element<1, pair<T1, T2>>::type

   > > -2- Value: The type `T2`.

   >     tuple_element<I, pair<T1, T2>>::type

   > > -?- Requires: `I < 2`. The program is ill-formed if `I` is out of bounds.

   > > -?- Value: The type `T1` if `I == 0`, otherwise the type `T2`.

3. Modify 22.6.4 [variant.helper] as indicated:

   >     variant_alternative<I, variant<Types...>>::type

   > > -4- Requires: `I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

   > > -5- Value: The type `TI`.

### 2975(i). Missing case for `pair` construction in scoped and polymorphic allocators

Section: 20.5.3.3 [mem.poly.allocator.mem], 20.6.4 [allocator.adaptor.members] Status: C++20 Submitter: Casey Carter Opened: 2017-06-13 Last modified: 2021-02-25

Priority: 3

View all other issues in [mem.poly.allocator.mem].

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Discussion:

`scoped_allocator_adaptor` ([allocator.adaptor.syn]) and `polymorphic_allocator` ([mem.poly.allocator.class]) have identical families of members named `construct`:

>     template <class T, class... Args>
>       void construct(T* p, Args&&... args);

>     template <class T1, class T2, class... Args1, class... Args2>
>       void construct(pair<T1,T2>* p, piecewise_construct_t,
>                      tuple<Args1...> x, tuple<Args2...> y);
>     template <class T1, class T2>
>       void construct(pair<T1,T2>* p);
>     template <class T1, class T2, class U, class V>
>       void construct(pair<T1,T2>* p, U&& x, V&& y);
>     template <class T1, class T2, class U, class V>
>       void construct(pair<T1,T2>* p, const pair<U, V>& pr);
>     template <class T1, class T2, class U, class V>
>       void construct(pair<T1,T2>* p, pair<U, V>&& pr);

Both allocators perform `uses_allocator` construction, and therefore need special handling for `pair` constructions since `pair` doesn't specialize `uses_allocator` (`tuple` gets all of that magic and `pair` is left out in the cold). Presumably, the intent is that the `construct` overloads whose first argument is a pointer to `pair` capture all `pair` constructions. This is not the case: invoking `construct` with a `pair` pointer and a non-constant lvalue `pair` resolves to the first overload when it is viable: it's a better match than the `pair`-pointer-and-`const`-lvalue-`pair` overload. The first overload notably does not properly perform piecewise `uses_allocator` construction for `pair`s as intended.

[2017-07 Toronto Monday issue prioritization]

Priority 2; Marshall to work with Casey to reduce the negations in the wording.

Previous resolution [SUPERSEDED]:

> 1. Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:

>    >     template <class T, class... Args>
>    >       void construct(T* p, Args&&... args);

>    > > -5- Requires: Uses-allocator construction of `T` with allocator `resource()` (see 20.2.8.2 [allocator.uses.construction]) and constructor arguments `std::forward<Args>(args)...` is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators. — end note]

>    > > -6- Effects: Construct a `T` object in the storage whose address is represented by `p` by uses-allocator construction with allocator `resource()` and constructor arguments `std::forward<Args>(args)...`.

>    > > -7- Throws: Nothing unless the constructor for `T` throws.

>    > > -?- Remarks: This function shall not participate in overload resolution unless `T` is not a specialization of `pair`.

> 2. Modify 20.6.4 [allocator.adaptor.members] as indicated:

>    >     template <class T, class... Args>
>    >       void construct(T* p, Args&&... args);

>    > > -9- Effects: […]

>    > > -?- Remarks: This function shall not participate in overload resolution unless `T` is not a specialization of `pair`.

[2017-11-02 Marshall and Casey provide updated wording]

[2017-11 Albuquerque Wednesday issue processing]

Move to Ready.

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4659.

1. Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:

   >     template <class T, class... Args>
   >       void construct(T* p, Args&&... args);

   > > -5- Requires: Uses-allocator construction of `T` with allocator `resource()` (see 20.2.8.2 [allocator.uses.construction]) and constructor arguments `std::forward<Args>(args)...` is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators. — end note]

   > > -6- Effects: Construct a `T` object in the storage whose address is represented by `p` by uses-allocator construction with allocator `resource()` and constructor arguments `std::forward<Args>(args)...`.

   > > -7- Throws: Nothing unless the constructor for `T` throws.

   > > -?- Remarks: This function shall not participate in overload resolution if `T` is a specialization of `pair`.

2. Modify 20.6.4 [allocator.adaptor.members] as indicated:

   >     template <class T, class... Args>
   >       void construct(T* p, Args&&... args);

   > > -9- Effects: […]

   > > -?- Remarks: This function shall not participate in overload resolution if `T` is a specialization of `pair`.

### 2976(i). Dangling `uses_allocator` specialization for `packaged_task`

Section: 32.10.10 [futures.task], 32.10.2 [future.syn], 32.10.10.3 [futures.task.nonmembers] Status: C++20 Submitter: Tim Song Opened: 2017-06-13 Last modified: 2021-02-25

Priority: 3

View all other issues in [futures.task].

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Discussion:

When LWG 2921(i) removed allocator support from `packaged_task`, it forgot to remove the `uses_allocator` partial specialization.

[ 2017-06-26 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2017-06-26, Billy O'Neal reopens]

I think 2921(i) was resolved in error. If `promise<T>` can have an allocator, there's no reason for `packaged_task<T>` to not have one. If we remove it from `packaged_task` we should remove it from `promise` as well.

Note that I am not objecting to removing allocator support here, I'm objecting to the "remove it because this looks like `std::function`" case. `packaged_task` has none of the `std::function` problems because the function inside a given `packaged_task` is not reassignable.

If LWG decides to remove allocator support here then there are more bits that need to be struck, e.g. [futures.task.members] (5.3).

[2017-06-26, Tim updates P/R to remove more dangling bits.]

The additional point in the P/R effectively reverts the second part of the resolution of 2752(i).

The alternative resolution for this issue is, of course, to just revert the resolution of 2921. In that case 2245(i) needs to be reopened.

[2016-07, Toronto Saturday afternoon issues processing]

Status to Ready

Proposed resolution:

This wording is relative to N4659.

1. Modify 32.10.2 [future.syn], header `<future>` synopsis, and 32.10.10 [futures.task], class template `packaged_task` synopsis, as indicated:

   >     template <class R, class Alloc>
   >     struct uses_allocator<packaged_task<R>, Alloc>;

2. Modify 32.10.10.3 [futures.task.nonmembers] as indicated:

   >     template <class R, class Alloc>
   >       struct uses_allocator<packaged_task<R>, Alloc>
   >         : true_type { };

   > > -2- Requires: `Alloc` shall be an Allocator (20.5.3.5).

3. Modify 32.10.10.2 [futures.task.members]/5 as indicated:

   >     template <class F>
   >       packaged_task(F&& F);

   > > -2- Requires: […]

   > > -3- Remarks: […]

   > > -4- Effects: […]

   > > -5- Throws:

   > > > — Aany exceptions thrown by the copy or move constructor of `f`., or

   > > > — For the first version, `bad_alloc` if memory for the internal data structures could not be allocated.

   > > > — For the second version, any exceptions thrown by `allocator_traits<Allocator>::template rebind_traits<unspecified>::allocate`.

### 2977(i). `unordered_meow::merge()` has incorrect Throws: clause

Section: 23.2.8 [unord.req] Status: C++20 Submitter: Tim Song Opened: 2017-06-14 Last modified: 2021-02-25

Priority: 0

View other active issues in [unord.req].

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Discussion:

As pointed out in this StackOverflow question, `unordered_{map,multimap,set,multiset}::merge()` may need to rehash to maintain its `max_load_factor` invariant, which may require allocation, which may throw.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. In 23.2.8 [unord.req], edit Table 91 "Unordered associative container requirements" as indicated:

   > Table 91 — Unordered associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `a.merge(a2)` | `void` | Requires: `a.get_allocator() == a2.get_allocator()`. Attempts to extract each element in `a2` and insert it into `a` using the hash function and key equality predicate of `a`. In containers with unique keys, if there is an element in `a` with key equivalent to the key of an element from `a2`, then that element is not extracted from `a2`. Postconditions: Pointers and references to the transferred elements of `a2` refer to those same elements but as members of `a`. Iterators referring to the transferred elements and all iterators referring to `a` will be invalidated, but iterators to elements remaining in `a2` will remain valid. Throws: Nothing unless the hash function or key equality predicate throws. | Average case 𝒪(N), where N is `a2.size()`. Worst case 𝒪(N`*a.size()+`N).
   > `…`

### 2978(i). Hash support for `pmr::string` and friends

Section: 27.4.6 [basic.string.hash], 27.4.2 [string.syn] Status: C++20 Submitter: Tim Song Opened: 2017-06-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [basic.string.hash].

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Discussion:

In most cases, `std::pmr::meow` is a drop-in replacement for `std::meow`. The exception is `std::pmr::{,w,u16,u32}string`, because unlike their `std::` counterparts, they don't come with enabled `std::hash` specializations.

The P/R below simply adds `std::hash` specializations for those four typedefs. An alternative approach, for which wording can be produced if desired, is to make the `hash` specializations for `basic_string` allocator-agnostic, similar to the partial specialization of `hash` for `vector<bool>`.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit 27.4.2 [string.syn], header `<string>` synopsis, as indicated:

   >       namespace pmr {
   >         template <class charT, class traits = char_traits<charT>>
   >           using basic_string = std::basic_string<charT, traits, polymorphic_allocator<charT>>;

   >         using string    = basic_string<char>;
   >         using u16string = basic_string<char16_t>;
   >         using u32string = basic_string<char32_t>;
   >         using wstring   = basic_string<wchar_t>;
   >       }

   >       // 27.4.6 [basic.string.hash], hash support
   >       template<class T> struct hash;
   >       template<> struct hash<string>;
   >       template<> struct hash<u16string>;
   >       template<> struct hash<u32string>;
   >       template<> struct hash<wstring>;
   >       template<> struct hash<pmr::string>;
   >       template<> struct hash<pmr::u16string>;
   >       template<> struct hash<pmr::u32string>;
   >       template<> struct hash<pmr::wstring>;

   >       namespace pmr {
   >         template <class charT, class traits = char_traits<charT>>
   >           using basic_string = std::basic_string<charT, traits, polymorphic_allocator<charT>>;

   >         using string    = basic_string<char>;
   >         using u16string = basic_string<char16_t>;
   >         using u32string = basic_string<char32_t>;
   >         using wstring   = basic_string<wchar_t>;
   >       }

2. Edit 27.4.6 [basic.string.hash] as indicated:

   >       template<> struct hash<string>;
   >       template<> struct hash<u16string>;
   >       template<> struct hash<u32string>;
   >       template<> struct hash<wstring>;
   >       template<> struct hash<pmr::string>;
   >       template<> struct hash<pmr::u16string>;
   >       template<> struct hash<pmr::u32string>;
   >       template<> struct hash<pmr::wstring>;

   > > -1- If `S` is one of these string types, `SV` is the corresponding string view type, and `s` is an object of type `S`, then `hash<S>()(s) == hash<SV>()(SV(s))`.

### 2979(i). `aligned_union` should require complete object types

Section: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Tim Song Opened: 2017-06-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [meta.trans.other].

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Discussion:

`aligned_union`'s description doesn't, but should, require the types provided to be complete object types.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. In 21.3.8.7 [meta.trans.other], edit Table 50 "Other transformations" as indicated:

   > Table 50 — Other transformations
   > Template | Comments
   > --- | ---
   > `[…]`
   > `template <size_t Len, class... Types> struct aligned_union;` | The member typedef `type` shall be a POD type suitable for use as uninitialized storage for any object whose type is listed in `Types`; its size shall be at least `Len`. The static member `alignment_value` shall be an integral constant of type `size_t` whose value is the strictest alignment of all types listed in `Types`. Requires: At least one type is provided. Each type in the parameter pack `Types` shall be a complete object type.
   > `[…]`

### 2980(i). Cannot `compare_exchange` empty pointers

Section: 99 [depr.util.smartptr.shared.atomic] Status: C++20 Submitter: Alisdair Meredith Opened: 2017-06-15 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [depr.util.smartptr.shared.atomic].

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Discussion:

[util.smartptr.shared.atomic] p35 states that two shared pointers are equivalent if they store the same pointer value, and share ownership. As empty shared pointers never share ownership, it is not possible to replace an empty shared pointer using the atomic `compare_exchange` API.

Note that through aliasing, empty shared pointers may still point to different objects, and any resolution must allow for that case too.

[ 2017-06-26 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

This wording is relative to N4659.

1. Edit [util.smartptr.shared.atomic] as indicated:

   >     template<class T>
   >       bool atomic_compare_exchange_weak_explicit(
   >         shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
   >         memory_order success, memory_order failure);
   >     template<class T>
   >       bool atomic_compare_exchange_strong_explicit(
   >         shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
   >         memory_order success, memory_order failure);

   > > […]

   > > -35- Remarks: Two `shared_ptr` objects are equivalent if they store the same pointer value and share ownership, or if they store the same pointer value and both are empty. The weak form may fail spuriously. See 32.6.1.

### 2981(i). Remove redundant deduction guides from standard library

Section: 22.10.6 [refwrap], 32.6.5.2 [thread.lock.guard], 32.6.5.3 [thread.lock.scoped], 32.6.5.4 [thread.lock.unique], 32.6.5.5 [thread.lock.shared] Status: C++20 Submitter: Mike Spertus Opened: 2017-06-15 Last modified: 2021-02-25

Priority: 0

View all other issues in [refwrap].

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Discussion:

There are several deduction guides added to the standard library by P0433R2 that have no effect probably because LWG had not considered late changes to core wording that automatically adds a "copy deduction candidate" (12.2.2.9 [over.match.class.deduct]) that renders these explicit guides moot.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit 22.10.6 [refwrap], end of class template `reference_wrapper` synopsis, as indicated:

   >     template<class T>
   >       reference_wrapper(reference_wrapper<T>) -> reference_wrapper<T>;

2. Edit 32.6.5.2 [thread.lock.guard], end of class template `lock_guard` synopsis, as indicated:

   >     template<class Mutex> lock_guard(lock_guard<Mutex>) -> lock_guard<Mutex>;

3. Edit 32.6.5.3 [thread.lock.scoped], end of class template `scoped_lock` synopsis, as indicated:

   >     template<class... MutexTypes>
   >       scoped_lock(scoped_lock<MutexTypes...>) -> scoped_lock<MutexTypes...>;

4. Edit 32.6.5.4 [thread.lock.unique], end of class template `unique_lock` synopsis, as indicated:

   >     template<class Mutex> unique_lock(unique_lock<Mutex>) -> unique_lock<Mutex>;

5. Edit 32.6.5.5 [thread.lock.shared], end of class template `shared_lock` synopsis, as indicated:

   >     template<class Mutex> shared_lock(shared_lock<Mutex>) -> shared_lock<Mutex>;

### 2982(i). Making `size_type` consistent in associative container deduction guides

Section: 23.5.6.1 [unord.set.overview], 23.5.7.1 [unord.multiset.overview] Status: C++20 Submitter: Mike Spertus Opened: 2017-06-16 Last modified: 2021-02-25

Priority: 2

View all issues with C++20 status.

Discussion:

Due to an incompletely implemented change in Kona, some of the size_type deduction guides say something like:

> A `size_type` parameter type in an `unordered_set` deduction guide refers to the `size_type` member type of the primary `unordered_set` template

while others say

> A `size_type` parameter type in an `unordered_map` deduction guide refers to the `size_type` member type of the type deduced by the deduction guide.

Clearly they should both be the same. My recollection is that the intent of the committee was to change them all to be the latter. Note, however, that this issue may be mooted if the suggestions in the upcoming P0433R3 paper are adopted as a DR in Toronto.

[2017-07 Toronto Monday issue prioritization]

Priority 2; Mike is preparing an updated paper — currently named P0433R3.

[2016-07, Toronto Saturday afternoon issues processing]

Status to Ready; Marshall to check with Mike about his paper

Proposed resolution:

This wording is relative to N4659.

1. Edit 23.5.6.1 [unord.set.overview] as indicated:

   > -4- A `size_type` parameter type in an `unordered_set` deduction guide refers to the `size_type` member type of the primary `unordered_set` templatetype deduced by the deduction guide.

2. Edit 23.5.7.1 [unord.multiset.overview] as indicated:

   > -4- A `size_type` parameter type in an `unordered_multiset` deduction guide refers to the `size_type` member type of the primary `unordered_multiset` templatetype deduced by the deduction guide.

### 2985(i). `std::reverse` should be permitted to be vectorized

Section: 26.7.10 [alg.reverse] Status: Resolved Submitter: Billy O'Neal III Opened: 2017-06-24 Last modified: 2025-10-16

Priority: Not Prioritized

View all other issues in [alg.reverse].

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Discussion:

The fine folks on our backend team suggested that we special case `std::reverse` of 1/2/4/8 to take advantage of vector units. Unfortunately, at present `std::reverse` says it does `N/2 iter_swap`s, which doesn't permit our vector implementation even if the iterator inputs are pointers to trivially copyable `T`s.

The vectorized version for pointers to `short`s is ~8x faster on Skylake than the serial version, and about 7x faster for `unsigned long long`s; and users don't actually care whether or not we call `swap` here.

[2017-07 Toronto Monday issue prioritization]

Status to LEWG; this is similar to 2973(i)

[2018-04-02, Billy comments]

This issue should be resolved by P0551, because it prohibits user specialization of `std::swap` and `std::iter_swap`, which means the proposed vectorization optimization for pointers-to-trivially-copyable is now implementable without changes to `reverse`'s specification (We can detect if the user has provided an alternate `swap` in their own namespace, but not if they explicitly specialized `swap` or `iter_swap`).

[2025-10-16 Status changed: LEWG → Resolved.]

Resolved by P0551.

Proposed resolution:

This wording is relative to N4659.

1. Edit 26.7.10 [alg.reverse] as indicated:

   >     template<class BidirectionalIterator>
   >       void reverse(BidirectionalIterator first, BidirectionalIterator last);
   >     template<class ExecutionPolicy, class BidirectionalIterator>
   >       void reverse(ExecutionPolicy&& exec,
   >                    BidirectionalIterator first, BidirectionalIterator last);

   > > -1- Requires: `*first` shall be swappable (16.4.4.3 [swappable.requirements]).

   > > -2- Effects: For each non-negative integer `i < (last - first) / 2`, applies `iter_swap` to all pairs of iterators `first + i, (last - i) - 1`. If `is_trivially_copyable_v<typename iterator_traits<BidirectionalIterator>::value_type>` is `true`, an implementation may permute the elements by making temporary copies, rather than by calling `iter_swap`. [Note: this allows the implementation to be vectorized. — end note]

### 2988(i). Clause 32 cleanup missed one `typename`

Section: 32.5.2 [atomics.syn] Status: C++20 Submitter: Jens Maurer Opened: 2017-06-25 Last modified: 2021-02-25

Priority: 0

View all other issues in [atomics.syn].

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Discussion:

P0558R1 missed updating one of the `std::atomic_exchange` signatures to avoid independent deduction for `T` on the second parameter.

[ 2017-06-26 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

This wording is relative to N4659.

1. Edit 32.5.2 [atomics.syn], header `<atomic>` synopsis, as indicated:

   >     template<class T>
   >     T atomic_exchange(volatile atomic<T>*, typename atomic<T>::value_type) noexcept;

### 2989(i). `path`'s stream insertion operator lets you insert everything under the sun

Section: 31.12.6.7 [fs.path.io] Status: C++20 Submitter: Billy O'Neal III Opened: 2017-06-27 Last modified: 2021-02-25

Priority: 2

View all issues with C++20 status.

Discussion:

The rules for converting a `path` to a narrow character sequence aren't necessarily the same as that iostreams should use. Note that this program creates a temporary path and stream inserts that, which likely destroys information.

>     #include <iostream>
>     #include <filesystem>
>     #include <string>

>     void foo() {
>       using namespace std;
>       using namespace std::experimental::filesystem::v1;
>       wstring val(L"abc");
>       std::cout << val;
>     }

Using the godbolt online compiler we get:

>     foo PROC
>       sub      rsp, 104   ; 00000068H
>       lea      rdx, OFFSET FLAT:$SG44895
>       lea      rcx, QWORD PTR val$[rsp]
>       call     std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t>
>                >::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >
>       lea      rdx, QWORD PTR val$[rsp]
>       lea      rcx, QWORD PTR $T1[rsp]
>       call     ??$?0_WU?$char_traits@_W@std@@V?$allocator@_W@1@@path@v1@filesystem@experimental@std@@QEAA@AEBV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@4@@Z
>       lea      rdx, QWORD PTR $T1[rsp]
>       lea      rcx, OFFSET FLAT:std::cout
>       call     std::experimental::filesystem::v1::operator<<<char,std::char_traits<char> >
>       lea      rcx, QWORD PTR $T1[rsp]
>       call     std::experimental::filesystem::v1::path::~path
>       lea      rcx, QWORD PTR val$[rsp]
>       call     std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t>
>                >::~basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >
>       add      rsp, 104   ; 00000068H
>       ret      0
>     foo ENDP

This should either be disabled with a SFINAE constraint, use the `auto_ptr` user-defined conversion trick, or the stream insertion operators should be made "hidden friends" to prevent conversions to `path` from being considered here.

[2017-07 Toronto Monday issue prioritization]

Priority 2

[2017-07 Toronto Saturday afternoon]

LWG confirmed they want the hidden friend solution, Billy O'Neal to provide wording.

[2018-1-26 issues processing telecon]

Status to 'Tentatively Ready'

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4659.

1. Edit 31.12.4 [fs.filesystem.syn], header `<filesystem>` synopsis, as indicated:

   >     // 31.12.6.7 [fs.path.io], path inserter and extractor
   >     template <class charT, class traits>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os, const path& p);
   >     template <class charT, class traits>
   >       basic_istream<charT, traits>&
   >         operator>>(basic_istream<charT, traits>& is, path& p);

2. Edit 31.12.6.7 [fs.path.io] as indicated:

   > [Drafting note: The project editor is kindly asked to consider to move sub-clause 31.12.6.7 [fs.path.io] before sub-clause 31.12.6.8 [fs.path.nonmember] (as a peer of it) — end drafting note]

   >     template <class charT, class traits>
   >       friend basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os, const path& p);

   >     […]

   >     template <class charT, class traits>
   >       friend basic_istream<charT, traits>&
   >         operator>>(basic_istream<charT, traits>& is, path& p);

3. Edit 31.12.6 [fs.class.path] p2, class `path` synopsis, as indicated:

   >     namespace std::filesystem {
   >       class path {
   >       public:
   >         […]
   >         iterator begin() const;
   >         iterator end() const;

   >         // 31.12.6.7 [fs.path.io] path inserter and extractor
   >         template <class charT, class traits>
   >           friend basic_ostream<charT, traits>&
   >             operator<<(basic_ostream<charT, traits>& os, const path& p);
   >         template <class charT, class traits>
   >           friend basic_istream<charT, traits>&
   >             operator>>(basic_istream<charT, traits>& is, path& p);
   >       };
   >     }

### 2991(i). `variant` copy constructor missing `noexcept(see below)`

Section: 22.6.3.2 [variant.ctor] Status: C++26 Submitter: Peter Dimov Opened: 2017-06-27 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [variant.ctor].

View all other issues in [variant.ctor].

View all issues with C++26 status.

Discussion:

The copy constructor of `std::variant` is not conditionally `noexcept` (I think it was in the original proposal.)

It should be, for two reasons: first, this would be consistent with the other three constructors

>     constexpr variant() noexcept(see below);

>     variant(variant&&) noexcept(see below);

>     template <class T>
>     constexpr variant(T&&) noexcept(see below);

and second, `variant` itself makes use of `is_nothrow_copy_constructible`, so it's inconsistent for it to take a stance against it.

[2017-07 Toronto Tuesday PM issue prioritization]

Status to LEWG

[Wrocław 2024-11-18; LEWG approves the direction]

In P0088R1 the copy constructor was conditionally noexcept in the synopsis, but not the detailed description. This was pointed out during LWG review in Jacksonville. The approved paper, P008R3, doesn't have it in either place.

Previous resolution [SUPERSEDED]

This wording is relative to N4659.

1. Edit 22.6.3 [variant.variant], class template `variant` synopsis, as indicated:

   >     template <class... Types>
   >       class variant {
   >       public:
   >         // 23.7.3.1, constructors
   >         constexpr variant() noexcept(see below);
   >         variant(const variant&) noexcept(see below);
   >         variant(variant&&) noexcept(see below);
   >         […]
   >       };

2. Edit 22.6.3.2 [variant.ctor] as indicated:

   >     variant(const variant& w) noexcept(see below);

   > > […]

   > > -8- Remarks: This function shall not participate in overload resolution unless `is_copy_constructible_v<Ti>` is `true` for all `i`. The expression inside `noexcept` is equivalent to the logical AND of `is_nothrow_copy_constructible_v<Ti>` for all `i`.

[2025-10-20; Jonathan provides updated wording]

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to P5014.

1. Edit 22.6.3 [variant.variant], class template `variant` synopsis, as indicated:

   >     template <class... Types>
   >       class variant {
   >       public:
   >         // 23.7.3.1, constructors
   >         constexpr variant() noexcept(see below);
   >         variant(const variant&) noexcept(see below);
   >         variant(variant&&) noexcept(see below);
   >         […]
   >       };

2. Edit 22.6.3.2 [variant.ctor] as indicated:

   >     variant(const variant& w) noexcept(see below);

   > > […]

   > > -8- Remarks: This function is defined as deleted unless `is_copy_constructible_v<Ti>` is `true` for all `i`. The exception specification is equivalent to the logical and of `is_nothrow_copy_constructible_v<Ti>` for all `i`.

### 2993(i). `reference_wrapper<T>` conversion from `T&&`

Section: 22.10.6 [refwrap] Status: C++20 Submitter: Tim Song Opened: 2017-06-28 Last modified: 2021-02-25

Priority: 3

View all other issues in [refwrap].

View all issues with C++20 status.

Discussion:

`reference_wrapper<T>` has a deleted constructor taking `T&&` in order to prevent accidentally wrapping an rvalue (which can otherwise happen with the `reference_wrapper(T&)` constructor if `T` is a non-`volatile` `const`-qualified type). Unfortunately, a deleted constructor can still be used to form implicit conversion sequences, so the deleted `T&&` constructor has the (presumably unintended) effect of creating an implicit conversion sequence from a `T` rvalue to a `reference_wrapper<T>`, even though such a conversion would be ill-formed if actually used. This is visible in overload resolution:

>     void meow(std::reference_wrapper<int>); //#1
>     void meow(convertible_from_int); //#2
>     meow(0); // error, ambiguous; would unambiguously call #2 if #1 instead took int&

and in conditional expressions (and hence `std::common_type`) after core issue 1895:

>     std::reference_wrapper<int> purr();

>     auto x = true? purr() : 0; // error, ambiguous: ICS exists from int prvalue to
>                                // reference_wrapper<int> and from reference_wrapper<int> to int

>     using t = std::common_type_t<std::reference_wrapper<int>, int>; // error: no member 'type' because the conditional
>                                                                     // expression is ill-formed

The latter in turn interferes with the use of `reference_wrapper` as a proxy reference type with proxy iterators.

We should ensure that there is no implicit conversion sequence from `T` rvalues to `reference_wrapper<T>`, not just that the conversion will be ill-formed when used. This can be done by using a suitably constrained constructor template taking a forwarding reference instead of the current pair of constructors taking `T&` and `T&&`.

[2017-06-29, Tim adds P/R and comments]

The draft P/R below uses a conditional `noexcept` specification to ensure that converting a `T&` to a `reference_wrapper<T>` remains `noexcept` and make it not usable when the source type is a `reference_wrapper` of the same type so as to avoid affecting `is_trivially_constructible`. It adds a deduction guide as the new constructor template will not support class template argument deduction.

The constructor template has the additional effect of making `reference_wrapper<T>` convertible from everything that is convertible to `T&`. This implies, for instance, that `reference_wrapper<int>` is now convertible to `reference_wrapper<const int>` when it wasn't before (the conversion would have required two user-defined conversions previously). This more closely emulates the behavior of an actual reference, but does represent a change to the existing behavior.

If perfectly emulating the existing behavior is desired, a conditionally-explicit constructor that is only implicit if `T` is reference-compatible with `remove_reference_t<U>` (see 9.5.4 [dcl.init.ref]) can be used.

[2017-07 Toronto Tuesday PM issue prioritization]

Priority 3; what else in the library does this affect? `ref` or `cref`?

[2016-07, Toronto Saturday afternoon issues processing]

Status to Ready.

Proposed resolution:

This wording is relative to N4659.

1. Edit 22.10.6 [refwrap], class template `reference_wrapper` synopsis, as indicated:

   >     namespace std {
   >       template <class T> class reference_wrapper {
   >         […]
   >         // construct/copy/destroy
   >         reference_wrapper(T&) noexcept;
   >         reference_wrapper(T&&) = delete;    // do not bind to temporary objects
   >         template <class U>
   >           reference_wrapper(U&&) noexcept(see below);
   >         […]
   >       };
   >       template <class T>
   >       reference_wrapper(T&) -> reference_wrapper<T>;
   >       […]
   >     }

2. Edit 22.10.6.2 [refwrap.const]/1 as indicated:

   >     reference_wrapper(T& t) noexcept;

   > > -1- Effects: Constructs a `reference_wrapper` object that stores a reference to `t`.

   >     template<class U>
   >       reference_wrapper(U&& u) noexcept(see below);

   > > -?- Remarks: Let `FUN` denote the exposition-only functions

   > > >     void FUN(T&) noexcept;
   > > >     void FUN(T&&) = delete;

   > > This constructor shall not participate in overload resolution unless the expression `FUN(declval<U>())` is well-formed and `is_same_v<decay_t<U>, reference_wrapper>` is `false`. The expression inside `noexcept` is equivalent to `noexcept(FUN(declval<U>()))`.

   > > -?- Effects: Creates a variable `r` as if by `T& r = std::forward<U>(u)`, then constructs a `reference_wrapper` object that stores a reference to `r`.

### 2994(i). Needless UB for `basic_string` and `basic_string_view`

Section: 27.2 [char.traits], 27.4.3.2 [string.require], 27.3.3 [string.view.template] Status: C++26 Submitter: Gennaro Prota Opened: 2017-07-03 Last modified: 2026-06-09

Priority: 3

View all other issues in [char.traits].

View all issues with C++26 status.

Discussion:

`basic_string` and `basic_string_view` involve undefined behavior in a few cases where it's simple for the implementation to add a `static_assert` and make the program ill-formed.

With regards to basic_string, 27.2 [char.traits]/3 states:

> `Traits::char_type` shall be the same as `CharT`.

Here, the implementation can add a `static_assert` using the `is_same` type trait. Similar issues exist in 27.4.3.2 [string.require] and, for `basic_string_view`, in 27.3.3 [string.view.template]/1.

[2017-07 Toronto Tuesday PM issue prioritization]

Priority 3; need to check general container requirements

Partially by the adoption of P1148 in San Diego.

Tim opines: "the remainder deals with allocator value type mismatch, which I think is NAD."

Previous resolution [SUPERSEDED]

This wording is relative to N4659.

1. Edit 27.2 [char.traits] as indicated:

   > -3- To specialize those templates to generate a string or iostream class to handle a particular character container type `CharT`, that and its related character traits class `Traits` are passed as a pair of parameters to the string or iostream template as parameters `charT` and `traits`. If `Traits::char_type` shall be the same is not the same type as `CharT`, the program is ill-formed.

2. Edit 27.4.3.2 [string.require] as indicated:

   > -3- In every specialization `basic_string<charT, traits, Allocator>`, if the type `allocator_traits<Allocator>::value_type` shall name the same type is not the same type as `charT`, the program is ill-formed. Every object of type `basic_string<charT, traits, Allocator>` shall use an object of type `Allocator` to allocate and free storage for the contained `charT` objects as needed. The `Allocator` object used shall be obtained as described in 23.2.2 [container.requirements.general]. In every specialization `basic_string<charT, traits, Allocator>`, the type `traits` shall satisfy the character traits requirements (27.2 [char.traits]). If, and the type `traits::char_type` shall name the same type is not the same type as `charT`, the program is ill-formed.

3. Edit 27.3.3 [string.view.template] as indicated:

   > -1- In every specialization `basic_string_view<charT, traits>`, the type `traits` shall satisfy the character traits requirements (27.2 [char.traits]). If, and the type `traits::char_type` shall name the same type is not the same type as `charT`, the program is ill-formed.

[2023-05-15; Jonathan Wakely provides improved wording]

As noted above, most of this issue was resolved by P1148R0. The remainder is the change to make it ill-formed if the allocator has the wrong `value_type`, but since P1463R1 that is already part of the Allocator-aware container requirements, which apply to `basic_string`.

[2023-06-01; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

There was a request to editorially move the added text in Note 1 into Note 2 after this is approved.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Edit 27.4.3.2 [string.require] as indicated:

   > -3- In every specialization `basic_string<charT, traits, Allocator>`, the type `allocator_traits<Allocator>::value_type` shall name the same type as `charT`. Every object of type `basic_string<charT, traits, Allocator>` shall use an object of type `Allocator` to allocate and free storage for the contained `charT` objects as needed. The In every specialization `basic_string<charT, traits, Allocator>`, the type `traits` shall satisfy the character traits requirements (27.2 [char.traits]).

   > [Note 1: Every specialization `basic_string<charT, traits, Allocator>` is an allocator-aware container, but does not use the allocator’s `construct` and `destroy` member functions (23.2.2 [container.requirements.general]). The program is ill-formed if `Allocator::value_type` is not the same type as `charT`. — end note]

   > [Note 2: The program is ill-formed if `traits::char_type` is not the same type as `charT`. — end note]

### 2995(i). `basic_stringbuf` default constructor forbids it from using SSO capacity

Section: 31.8.2.2 [stringbuf.cons] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-07-07 Last modified: 2021-02-25

Priority: 3

View all other issues in [stringbuf.cons].

View all issues with C++20 status.

Discussion:

[stringbuf.cons] says that the default constructor initializes the base class as `basic_streambuf()` which means the all the pointers to the input and output sequences (`pbase`, `eback` etc) are all required to be null.

This means that a `stringbuf` that is implemented in terms of a Small String Optimised `std::basic_string` cannot make us of the string's initial capacity, and so cannot avoid a call to the overflow virtual function even for small writes. In other words, the following assertions must pass:

>     #include <sstream>
>     #include <cassert>

>     bool overflowed = false;

>     struct SB : std::stringbuf
>     {
>       int overflow(int c) {
>         assert( pbase() == nullptr );
>         overflowed = true;
>         return std::stringbuf::overflow(c);
>       }
>     };

>     int main()
>     {
>       SB sb;
>       sb.sputc('1');
>       assert(overflowed);
>     }

This is an unnecessary pessimisation. Implementations should be allowed to use the SSO buffer immediately and write to it without calling overflow. Libc++ already does this, so is non-conforming.

[2017-07 Toronto Tuesday PM issue prioritization]

Priority 3; is this affected by Peter Sommerlad's paper P0407R1?

[2018-06 Rapperswil Wednesday issues processing]

Status to Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4659.

1. Edit 31.8.2.2 [stringbuf.cons] as indicated:

   >     explicit basic_stringbuf(
   >       ios_base::openmode which = ios_base::in | ios_base::out);

   > > -1- Effects: Constructs an object of class `basic_stringbuf`, initializing the base class with `basic_streambuf()` (31.6.3.2 [streambuf.cons]), and initializing `mode` with `which`. It is implementation-defined whether the sequence pointers (`eback()`, `gptr()`, `egptr()`, `pbase()`, `pptr()`, `epptr()`) are initialized to null pointers.

   > > -2- Postconditions: `str() == ""`.

### 2996(i). Missing rvalue overloads for `shared_ptr` operations

Section: 20.3.2.2 [util.smartptr.shared], 20.3.2.2.10 [util.smartptr.shared.cast] Status: C++20 Submitter: Geoffrey Romer Opened: 2017-07-07 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [util.smartptr.shared].

View all issues with C++20 status.

Discussion:

The `shared_ptr` aliasing constructor and the `shared_ptr` casts are specified to take a `shared_ptr` by `const` reference and construct a new `shared_ptr` that shares ownership with it, and yet they have no corresponding rvalue reference overloads. That results in an unnecessary refcount increment/decrement when those operations are given an rvalue. Rvalue overloads can't be added as a conforming extension because they observably change semantics (but mostly only for code that does unreasonable things like pass an argument by move and then rely on the fact that it's unchanged), and [res.on.arguments]/p1.3 doesn't help because it only applies to rvalue reference parameters.

This issue is related to P0390R0.

[2017-07 Toronto Tuesday PM issue prioritization]

Status LEWG

[2018-06 Rapperswil Monday AM]

Move to Ready; choosing the PR in the issue as opposed to P0390R0 and rebase wording to most recent working draft

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Edit 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     […]
   >     // 20.3.2.2.10 [util.smartptr.shared.cast], shared_ptr casts
   >     template<class T, class U>
   >     shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
   >     template<class T, class U>
   >     shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept;
   >     template<class T, class U>
   >     shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
   >     template<class T, class U>
   >     shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept;
   >     template<class T, class U>
   >     shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
   >     template<class T, class U>
   >     shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept;
   >     template<class T, class U>
   >     shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;
   >     template<class T, class U>
   >     shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept;
   >     […]

2. Edit 20.3.2.2 [util.smartptr.shared], class template `shared_ptr` synopsis, as indicated:

   >     template<class T> class shared_ptr {
   >     public:
   >       […]
   >       // 20.3.2.2.2 [util.smartptr.shared.const], constructors
   >       […]
   >       template <class D, class A> shared_ptr(nullptr_t p, D d, A a);
   >       template<class Y> shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
   >       template<class Y> shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept;
   >       shared_ptr(const shared_ptr& r) noexcept;
   >       […]
   >     };
   >     […]

3. Edit 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   > [Drafting note: the `use_count()` postcondition can safely be deleted because it is redundant with the "shares ownership" wording in the Effects. — end drafting note]

   >     template<class Y> shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
   >     template<class Y> shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept;

   > > -14- Effects: Constructs a `shared_ptr` instance that stores `p` and shares ownership with the initial value of `r`.

   > > -15- Postconditions: `get() == p && use_count() == r.use_count()`. For the second overload, `r` is empty and `r.get() == nullptr`.

   > > -16- [Note: To avoid the possibility of a dangling pointer, the user of this constructor must ensure that `p` remains valid at least until the ownership group of `r` is destroyed. — end note]

   > > -17- [Note: This constructor allows creation of an empty `shared_ptr` instance with a non-null stored pointer. — end note]

4. Edit 20.3.2.2.10 [util.smartptr.shared.cast] as indicated:

   >     template<class T, class U>
   >       shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
   >     template<class T, class U>
   >       shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept;

   > > -1- Requires: The expression `static_cast<T*>((U*)nullptr)` shall be well-formed.

   > > -2- Returns:

   > > > `shared_ptr<T>(rR, static_cast<typename shared_ptr<T>::element_type*>(r.get()))`

   > > , where `R` is `r` for the first overload, and `std::move(r)` for the second.

   > > -3- [Note: The seemingly equivalent expression `shared_ptr<T>(static_cast<T*>(r.get()))` will eventually result in undefined behavior, attempting to delete the same object twice. — end note]

   >     template<class T, class U>
   >       shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
   >     template<class T, class U>
   >       shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept;

   > > -4- Requires: The expression `dynamic_cast<T*>((U*)nullptr)` shall be well-formed. The expression `dynamic_cast<typename shared_ptr<T>::element_type*>(r.get())` shall be well formed and shall have well-defined behavior.

   > > -5- Returns:

   > > 1. (5.1) — When `dynamic_cast<typename shared_ptr<T>::element_type*>(r.get())` returns a non-null value `p`, `shared_ptr<T>(rR, p)`, where `R` is `r` for the first overload, and `std::move(r)` for the second.

   > > 2. (5.2) — Otherwise, `shared_ptr<T>()`.

   > > -6- [Note: The seemingly equivalent expression `shared_ptr<T>(dynamic_cast<T*>(r.get()))` will eventually result in undefined behavior, attempting to delete the same object twice. — end note]

   >     template<class T, class U>
   >       shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
   >     template<class T, class U>
   >       shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept;

   > > -7- Requires: The expression `const_cast<T*>((U*)nullptr)` shall be well-formed.

   > > -8- Returns:

   > > > `shared_ptr<T>(rR, const_cast<typename shared_ptr<T>::element_type*>(r.get()))`

   > > , where `R` is `r` for the first overload, and `std::move(r)` for the second.

   > > -9- [Note: The seemingly equivalent expression `shared_ptr<T>(const_cast<T*>(r.get()))` will eventually result in undefined behavior, attempting to delete the same object twice. — end note]

   >     template<class T, class U>
   >       shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;
   >     template<class T, class U>
   >       shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept;

   > > -10- Requires: The expression `reinterpret_cast<T*>((U*)nullptr)` shall be well-formed.

   > > -11- Returns:

   > > > `shared_ptr<T>(rR, reinterpret_cast<typename shared_ptr<T>::element_type*>(r.get()))`

   > > , where `R` is `r` for the first overload, and `std::move(r)` for the second.

   > > -12- [Note: The seemingly equivalent expression `shared_ptr<T>(reinterpret_cast<T*>(r.get()))` will eventually result in undefined behavior, attempting to delete the same object twice. — end note]

### 2997(i). LWG 491 and the specification of `{forward_,}list::unique`

Section: 23.3.11.5 [list.ops], 23.3.7.6 [forward.list.ops] Status: C++23 Submitter: Tim Song Opened: 2017-07-07 Last modified: 2023-11-22

Priority: 3

View all other issues in [list.ops].

View all issues with C++23 status.

Discussion:

There are various problems with the specification of `list::unique` and its `forward_list` counterpart, some of which are obvious even on cursory inspection:

- It refers to the identifiers `first` and `last`, which aren't even defined.
- It uses `i - 1` on non-random-access iterators - in the case of `forward_list`, on forward-only iterators.
- The resolution of LWG 202(i), changing the specification of `std::unique` to require an equivalence relation and adjusting the order of comparison, weren't applied to these member functions.

LWG 491(i), which pointed out many of those problems with the specification of `list::unique`, was closed as NAD with the rationale that

> "All implementations known to the author of this Defect Report comply with these assumption", and "no impact on current code is expected", i.e. there is no evidence of real-world confusion or harm.

That implementations somehow managed to do the right thing in spite of obviously defective standardese doesn't seem like a good reason to not fix the defects.

[2017-07 Toronto Tuesday PM issue prioritization]

Priority 3; by the way, there's general wording in 26.2 [algorithms.requirements] p10 that lets us specify iterator arithmetic as if we were using random access iterators.

[2017-07-11 Tim comments]

I drafted the P/R fully aware of the general wording in 26.2 [algorithms.requirements] p10. However, that general wording is limited to Clause 28, so to make use of the shorthand permitted by that wording, we would need additional wording importing it to these subclauses.

Moreover, that general wording only defines `a+n` and `b-a`; it notably doesn't define `a-n`, which is needed here. And one cannot merely define `a-n` as `a+(-n)` since that has undefined behavior for forward iterators.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4659.

> 1. Edit 23.3.11.5 [list.ops] as indicated:

>    >     void unique();
>    >     template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);

>    > > -?- Requires: The comparison function shall be an equivalence relation.

>    > > -19- Effects: If `empty()`, has no effects. Otherwise, eErases all but the first element from every consecutive group of equalequivalent elements referred to by the iterator `i` in the range `[first + 1, last)[next(begin()), end())` for which `*i == *(i-1)*j == *i` (for the version of `unique` with no arguments) or `pred(*i, *(i - 1))pred(*j, *i)` (for the version of `unique` with a predicate argument) holds, where `j` is an iterator in `[begin(), end())` such that `next(j) == i`. Invalidates only the iterators and references to the erased elements.

>    > > -20- Throws: Nothing unless an exception is thrown by `*i == *(i-1)` or `pred(*i, *(i - 1))` the equality comparison or the predicate.

>    > > -21- Complexity: If the range `[first, last)` is not empty`!empty()`, exactly `(last - first) - 1size() - 1` applications of the corresponding predicate, otherwise no applications of the predicate.

> 2. Edit [forwardlist.ops] as indicated:

>    >     void unique();
>    >     template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);

>    > > -?- Requires: The comparison function shall be an equivalence relation.

>    > > -16- Effects: If `empty()`, has no effects. Otherwise, eErases all but the first element from every consecutive group of equalequivalent elements referred to by the iterator `i` in the range `[first + 1, last)[next(begin()), end())` for which `*i == *(i-1)*j == *i` (for the version with no arguments) or `pred(*i, *(i - 1))pred(*j, *i)` (for the version with a predicate argument) holds, where `j` is an iterator in `[begin(), end())` such that `next(j) == i`. Invalidates only the iterators and references to the erased elements.

>    > > -17- Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.

>    > > -18- Complexity: If the range `[first, last)` is not empty`!empty()`, exactly `(last - first) - 1distance(begin(), end()) - 1` applications of the corresponding predicate, otherwise no applications of the predicate.

[2021-02-21 Tim redrafts]

The wording below incorporates editorial pull request 4465.

[2021-05-21; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Edit 23.3.11.5 [list.ops] as indicated:

   > -1- Since lists allow fast insertion and erasing from the middle of a list, certain operations are provided specifically for them.222 In this subclause, arguments for a template parameter named `Predicate` or `BinaryPredicate` shall meet the corresponding requirements in 26.2 [algorithms.requirements]. The semantics of `i + n` and `i - n`, where `i` is an iterator into the list and `n` is an integer, are the same as those of `next(i, n)` and `prev(i, n)`, respectively. For `merge` and `sort`, the definitions and requirements in 26.8 [alg.sorting] apply.

   > […]

   >     void unique();
   >     template<class BinaryPredicate> void unique(BinaryPredicate binary_pred);

   > > -?- Let `binary_pred` be `equal_to<>{}` for the first overload.

   > > -?- Preconditions: `binary_pred` is an equivalence relation.

   > > -20- Effects: Erases all but the first element from every consecutive group of equalequivalent elements. That is, for a nonempty list, erases all elements referred to by the iterator `i` in the range `[first + 1, last)[begin() + 1, end())` for which `*i == *(i-1)` (for the version of `unique` with no arguments) or `binary_pred(*i, *(i - 1))` is `true`(for the version of `unique` with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.

   > > -21- Returns: The number of elements erased.

   > > -22- Throws: Nothing unless an exception is thrown by `*i == *(i-1)` or `pred(*i, *(i - 1))`the predicate.

   > > -23- Complexity: If the range `[first, last)` is not empty `empty()` is `false`, exactly `(last - first) - 1size() - 1` applications of the corresponding predicate, otherwise no applications of the predicate.

2. Edit [forwardlist.ops] as indicated:

   > -1- In this subclause, arguments for a template parameter named `Predicate` or `BinaryPredicate` shall meet the corresponding requirements in 26.2 [algorithms.requirements]. The semantics of `i + n`, where `i` is an iterator into the list and `n` is an integer, are the same as those of `next(i, n)`. The expression `i - n`, where `i` is an iterator into the list and `n` is an integer, means an iterator `j` such that `j + n == i` is `true`. For `merge` and `sort`, the definitions and requirements in 26.8 [alg.sorting] apply.

   > […]

   >     void unique();
   >     template<class BinaryPredicate> void unique(BinaryPredicate binary_pred);

   > > -?- Let `binary_pred` be `equal_to<>{}` for the first overload.

   > > -?- Preconditions: `binary_pred` is an equivalence relation.

   > > -18- Effects: Erases all but the first element from every consecutive group of equalequivalent elements. That is, for a nonempty list, erases all elements referred to by the iterator `i` in the range `[first + 1, last)[begin() + 1, end())` for which `*i == *(i-1)` (for the version with no arguments) or `binary_pred(*i, *(i - 1))` is `true` (for the version with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.

   > > -19- Returns: The number of elements erased.

   > > -20- Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.

   > > -21- Complexity: If the range `[first, last)` is not empty `empty()` is `false`, exactly `(last - first) - 1distance(begin(), end()) - 1` applications of the corresponding predicate, otherwise no applications of the predicate.

### 2998(i). Requirements on function objects passed to `{forward_,}list`-specific algorithms

Section: 23.3.11.5 [list.ops], 23.3.7.6 [forward.list.ops] Status: C++20 Submitter: Tim Song Opened: 2017-07-07 Last modified: 2023-02-07

Priority: 0

View all other issues in [list.ops].

View all issues with C++20 status.

Discussion:

Some specialized algorithms for `forward_list` and `list` take template parameters named `Predicate`, `BinaryPredicate`, or `Compare`. However, there's no wording importing the full requirements for template type parameters with such names from 26.2 [algorithms.requirements] and 26.8 [alg.sorting], which means, for instance, that there appears to be no rule prohibiting `Compare` from modifying its arguments, because we only refer to 26.8 [alg.sorting] for the definition of strict weak ordering. Is that intended?

[2017-07 Toronto Tuesday PM issue prioritization]

Priority 0; status to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit 23.3.11.5 [list.ops] as indicated:

   > -1- Since lists allow fast insertion and erasing from the middle of a list, certain operations are provided specifically for them.259) In this subclause, arguments for a template parameter named `Predicate` or `BinaryPredicate` shall meet the corresponding requirements in 26.2 [algorithms.requirements]. For `merge` and `sort`, the definitions and requirements in 26.8 [alg.sorting] apply.

2. Edit 23.3.11.5 [list.ops] as indicated:

   >     void merge(list& x);
   >     void merge(list&& x);
   >     template <class Compare> void merge(list& x, Compare comp);
   >     template <class Compare> void merge(list&& x, Compare comp);

   > > -22- Requires: `comp` shall define a strict weak ordering (26.8 [alg.sorting]), and bBoth the list and the argument list shall be sorted according to this orderingwith respect to the comparator `operator<` (for the first two overloads) or `comp` (for the last two overloads).

3. Delete 23.3.11.5 [list.ops]/28 as redundant:

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > -28- Requires: `operator<` (for the first version) or `comp` (for the second version) shall define a strict weak ordering (26.8 [alg.sorting]).

4. Insert a new paragraph at the beginning of [forwardlist.ops]:

   > -?- In this subclause, arguments for a template parameter named `Predicate` or `BinaryPredicate` shall meet the corresponding requirements in 26.2 [algorithms.requirements]. For `merge` and `sort`, the definitions and requirements in 26.8 [alg.sorting] apply.

   >     void splice_after(const_iterator position, forward_list& x);
   >     void splice_after(const_iterator position, forward_list&& x);

   > […]

5. Edit [forwardlist.ops] as indicated:

   >     void merge(forward_list& x);
   >     void merge(forward_list&& x);
   >     template <class Compare> void merge(forward_list& x, Compare comp);
   >     template <class Compare> void merge(forward_list&& x, Compare comp);

   > > -22- Requires: `comp` defines a strict weak ordering (26.8 [alg.sorting]), and `*this` and `x` are both sorted according to this orderingwith respect to the comparator `operator<` (for the first two overloads) or `comp` (for the last two overloads). `get_allocator() == x.get_allocator()`.

6. Delete [forwardlist.ops]/23 as redundant:

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > -23- Requires: `operator<` (for the version with no arguments) or `comp` (for the version with a comparison argument) defines a strict weak ordering (26.8 [alg.sorting]).

### 2999(i). §[thread.decaycopy] issue

Section: 16.3.3.2 [expos.only.entity] Status: Resolved Submitter: Marshall Clow Opened: 2017-07-11 Last modified: 2023-02-07

Priority: 3

View all other issues in [expos.only.entity].

View all issues with Resolved status.

Discussion:

[thread.decaycopy] says:

> In several places in this Clause the operation `DECAY_COPY(x)` is used. All such uses mean call the function `decay_copy(x)` and use the result, where `decay_copy` is defined as follows:

but `decay_copy` is not defined in any synopsis.

The Ranges TS has a similar use of `DECAY_COPY`, except that theirs is also `constexpr` and `noexcept`.

We should mark the function `decay_copy` as "exposition only" and `constexpr` and `noexcept`.

[2017-07-16, Daniel comments]

Currently there exists no proper way to mark `decay_copy` as conditionally `noexcept` as explained in N3255 section "Adding to the Standard". This is also slighly related to the request to add an `is_nothrow_convertible` trait, as requested by LWG 2040(i).

[ 2017-11-01 P3 as result of c++std-lib online vote. ]

[2019-03-22; Daniel comments]

Starting with N4800 have now a `constexpr` and conditionally `noexcept` `decay-copy` in the working draft ( [expos.only.func]). The pre-condition for that specification helper became possible by adoption of P0758R1, which introduced the missing `is_nothrow_convertible` trait.

[2020-05-03; Reflector discussions]

Resolved by editorial action starting with N4800.

Rationale:

Resolved by editorial creation of `decay-copy` after acceptance of P0758R1.

Proposed resolution:

### 3000(i). `monotonic_memory_resource::do_is_equal` uses `dynamic_cast` unnecessarily

Section: 20.5.6.3 [mem.res.monotonic.buffer.mem] Status: C++20 Submitter: Pablo Halpern Opened: 2017-07-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [mem.res.monotonic.buffer.mem].

View all issues with C++20 status.

Discussion:

Section [mem.res.monotonic.buffer.mem], paragraph 11 says

>     bool do_is_equal(const memory_resource& other) const noexcept override;

> > Returns: `this == dynamic_cast<const monotonic_buffer_resource*>(&other)`.

The `dynamic_cast` adds nothing of value. It is an incorrect cut-and-paste from an example `do_is_equal` for a more complex resource.

[2017-07-16, Tim Song comments]

The pool resource classes appear to also have this issue.

[2017-09-18, Casey Carter expands PR to cover the pool resources.]

Previous resolution: [SUPERSEDED]

> 1. Edit 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:

>    >     bool do_is_equal(const memory_resource& other) const noexcept override;

>    > > Returns: `this == dynamic_cast<const monotonic_buffer_resource*>(&other)`.

[ 2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4687.

1. Edit 20.5.5.4 [mem.res.pool.mem] as indicated:

   >     bool synchronized_pool_resource::do_is_equal(const memory_resource& other) const noexcept override;
   >         const memory_resource& other) const noexcept override;

   > > Returns: `this == dynamic_cast<const synchronized_pool_resource*>(&other)`.

2. Strike 20.5.5.4 [mem.res.pool.mem] paragraph 10, and the immediately preceding declaration of `unsynchronized_pool_resource::do_is_equal`.

3. Edit 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:

   >     bool do_is_equal(const memory_resource& other) const noexcept override;

   > > Returns: `this == dynamic_cast<const monotonic_buffer_resource*>(&other)`.

### 3001(i). `weak_ptr::element_type` needs `remove_extent_t`

Section: 20.3.2.3 [util.smartptr.weak] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2017-07-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [util.smartptr.weak].

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Discussion:

C++17's `shared_ptr<T>::element_type` is `remove_extent_t<T>`, but `weak_ptr<T>::element_type` is `T`. They should be the same, but this was lost over time.

First, N4562 "Working Draft, C++ Extensions for Library Fundamentals, Version 2" 8.2.2 [memory.smartptr.weak] specified:

>     namespace std {
>     namespace experimental {
>     inline namespace fundamentals_v2 {

>       template<class T> class weak_ptr {
>       public:
>         typedef typename remove_extent_t<T> element_type;

(The `typename` here was spurious.)

Then, P0220R1 "Adopt Library Fundamentals V1 TS Components for C++17 (R1)" listed:

> - 8.2.2 Class template `weak_ptr`

>   - 8.2.2.1 `weak_ptr` constructors

This obscured the fact that the Library Fundamentals TS had altered `weak_ptr::element_type`.

Finally, P0414R2 "Merging `shared_ptr` changes from Library Fundamentals to C++17" missed the change to `weak_ptr::element_type`, so it wasn't applied to C++17.

Peter Dimov has confirmed that this was unintentionally lost, and that "`boost::weak_ptr` defines `element_type` in the same way as `shared_ptr`".

[ 2017-07-17 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

This resolution is relative to N4659.

1. Edit 20.3.2.3 [util.smartptr.weak], class template `weak_ptr` synopsis, as indicated:

   >     template<class T> class weak_ptr {
   >     public:
   >       using element_type = remove_extent_t<T>;
   >       […]
   >     };

### 3002(i). [networking.ts] `basic_socket_acceptor::is_open()` isn't `noexcept`

Section: 18.9.4 [networking.ts::socket.acceptor.ops] Status: WP Submitter: Jonathan Wakely Opened: 2017-07-14 Last modified: 2026-02-21

Priority: 0

View all issues with WP status.

Discussion:

Addresses: networking.ts

`basic_socket::is_open()` is `noexcept`, but the corresponding function on `basic_socket_acceptor` is not. This is a simple observer with a wide contract and cannot fail.

[ 2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4656.

1. Edit 18.9 [networking.ts::socket.acceptor], class template `basic_socket_acceptor` synopsis, as indicated:

   >     template<class AcceptableProtocol>
   >     class basic_socket_acceptor {
   >     public:
   >       […]
   >       bool is_open() const noexcept;
   >       […]
   >     };

2. Edit 18.9.4 [networking.ts::socket.acceptor.ops] as indicated:

   >     bool is_open() const noexcept;

   > > -10- Returns: A `bool` indicating whether this acceptor was opened by a previous call to `open` or `assign`.

### 3003(i). `<future>` still has type-erased allocators in `promise`

Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Billy O'Neal III Opened: 2017-07-16 Last modified: 2025-06-23

Priority: 2

View all other issues in [futures.promise].

View all issues with Resolved status.

Discussion:

In Toronto Saturday afternoon LWG discussed LWG 2976(i) which finishes the job of removing allocator support from `packaged_task`. LWG confirmed that, despite the removal of `packaged_task` allocators "because it looks like `std::function`" was incorrect, they wanted to keep the allocator removals anyway, in large part due to this resolution being a response to an NB comment.

If we don't want the type erased allocator situation at all, then we should remove them from the remaining place they exist in `<future>`, namely, in `promise`.

This change also resolves potential implementation divergence on whether `allocator::construct` is intended to be used on elements constructed in the shared state, and allows the emplace-construction-in-`future` paper, P0319, to be implemented without potential problems there.

[28-Nov-2017 Mailing list discussion - set priority to P2]

Lots of people on the ML feel strongly about this; the suggestion was made that a paper would be welcomed laying out the rationale for removing allocator support here (and in other places).

[2018-1-26 issues processing telecon]

Status to 'Open'; Billy to write a paper.

[2019-06-03]

Jonathan observes that this resolution conflicts with 2095(i).

[Varna 2023-06-13; Change status to "LEWG"]

Previous resolution [SUPERSEDED]

This resolution is relative to N4659.

1. Edit 32.10.6 [futures.promise], class template `promise` synopsis, as indicated:

   >     template<class R>
   >     class promise {
   >     public:
   >       promise();
   >       template <class Allocator>
   >         promise(allocator_arg_t, const Allocator& a);
   >       […]
   >     };
   >     template <class R>
   >       void swap(promise<R>& x, promise<R>& y) noexcept;
   >     template <class R, class Alloc>
   >       struct uses_allocator<promise<R>, Alloc>;

   > […]

   >     template <class R, class Alloc>
   >       struct uses_allocator<promise<R>, Alloc>
   >         : true_type { };

   > > -3- Requires: `Alloc` shall be an Allocator (16.4.4.6 [allocator.requirements]).

   >     promise();
   >     template <class Allocator>
   >       promise(allocator_arg_t, const Allocator& a);

   > > -4- Effects: constructs a `promise` object and a shared state. The second constructor uses the allocator a to allocate memory for the shared state.

[2024-09-19; Jonathan provides improved wording]

In July 2023 LEWG considered this and LWG issue 2095(i) and requested a new proposed resolution that kept the existing constructor (which is useful for controlling how the shared state is allocated) but removed the `uses_allocator` specialization that makes `promise` incorrectly claim to be allocator-aware. Some of the rationale in P2787R1 is applicable here too.

Without the `uses_allocator` specialization, there's no reason to provide an allocator-extended move constructor, resolving issue 2095(i).

And if we're going to continue supporting `std::promise` construction with an allocator, we could restore that for `std::packaged_task` too. That was removed by issue 2921(i), but issue 2976(i) argues that there was no good reason to do that. Removing `uses_allocator` for `packaged_task` would have made sense (as proposed below for `promise`) but 2921 didn't do that (which is why 2976 was needed). We can restore the `packaged_task` constructor that takes an allocator, and just not restore the `uses_allocator` specialization that implies it should be fully allocator-aware. Finally, if we restore that `packaged_task` constructor then we need to fix `reset()` as discussed in issue 2245(i).

In summary:

- Keep support for using an allocator for shared state of a `promise`.
- Remove `uses_allocator` specialization for `promise`.
- Restore support for using an allocator for the shared state of `packaged_task`.
- Do not restore `uses_allocator` specialization for `packaged_task`.
- Update `packaged_task::reset()` to deal with allocators.

[Wrocław 2024-11-18; LEWG would prefer a paper for this]

[2025-06-21 Status changed: LEWG → Resolved.]

Resolved by adoption of P3503R3 in Sofia.

Proposed resolution:

This wording is relative to N4988.

1. Modify 32.10.6 [futures.promise] as indicated:

   >     template <class R, class Alloc>
   >       struct uses_allocator<promise<R>, Alloc>;

   > […]

   >     template <class R, class Alloc>
   >       struct uses_allocator<promise<R>, Alloc>
   >         : true_type { };

   > > -4- Preconditions: `Alloc` meets the Cpp17Allocator (16.4.4.6.1 [allocator.requirements.general]).

2. Modify 32.10.10.1 [futures.task.general] as indicated:

   >       template<class R, class... ArgTypes>
   >       class packaged_task<R(ArgTypes...)> {
   >       public:
   >         // construction and destruction
   >         packaged_task() noexcept;
   >         template<class F>
   >           explicit packaged_task(F&& f);
   >         template<class F, class Allocator>
   >           explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
   >         ~packaged_task();

3. Modify 32.10.10.2 [futures.task.members] as indicated:

   >     template<class F>
   >       explicit packaged_task(F&& f);

   > -?- Effects: Equivalent to `packaged_task(allocator_arg, std::allocator<int>(), std::forward<F>(f))`.

   > > [Drafting note: Uses of `std::allocator<int>` and `std::allocator<unspecified>` are not observable so this constructor can be implemented without delegating to the other constructor and without using `std::allocator`.]

   >     template<class F, class Allocator>
   >       packaged_task(allocator_arg_t, const Allocator& a, F&& f);

   > -2- Constraints: `remove_cvref_t<F>` is not the same type as `packaged_task<R(ArgTypes...)>`.

   > -3- Mandates: `is_invocable_r_v<R, F&, ArgTypes...>` is `true`.

   > > [Drafting note: Issue 4154(i) alters these Mandates: and Effects: but the two edits should combine cleanly.]

   > -4- Preconditions: Invoking a copy of `f` behaves the same as invoking `f`. `Allocator` meets the Cpp17Allocator requirements (16.4.4.6.1 [allocator.requirements.general]).

   > -5- Effects: Let `A2` be `allocator_traits<Allocator>::rebind_alloc<unspecified>` and let `a2` be an lvalue of type `A2` initialized with `A2(a)`. Creates a shared state and initializes the object's stored task with `std::forward<F>(f)`. Uses `a2` to allocate storage for the shared state and stores a copy of `a2` in the shared state.

   > -6- Throws: Any exceptions thrown by the copy or move constructor of `f`, or bad_alloc if memory for the internal data structures cannot be allocated. Any exceptions thrown by the initialization of the stored task. If storage for the shared state cannot be allocated, any exception thrown by `A2::allocate`.

   > …

   >     void reset();

   > -26- Effects: As if Equivalent to:

   >     if (!valid())
   >       throw future_error(future_errc::no_state);
   >     *this = packaged_task(allocator_arg, a, std::move(f));

   > where `f` is the task stored in `*this` and `a` is the allocator stored in the shared state.

   > [Note 2: This constructs a new shared state for `*this`. The old state is abandoned (32.10.5 [futures.state]). — end note]

   > -27- Throws:

   > 1. (27.1) — bad_alloc if memory for the new shared state cannot be allocated.
   > 2. (27.2) — Any exception thrown by the `packaged_task` constructor move constructor of the task stored in the shared state.
   > 3. (27.3) — `future_error` with an error condition of `no_state` if `*this` has no shared state.

### 3004(i). §[string.capacity] and §[vector.capacity] should specify time complexity for `capacity()`

Section: 27.4.3.5 [string.capacity], 23.3.13.3 [vector.capacity] Status: C++20 Submitter: Andy Giese Opened: 2017-07-24 Last modified: 2021-02-25

Priority: 0

View all other issues in [string.capacity].

View all issues with C++20 status.

Discussion:

`basic_string` and `vector` both have a `capacity` function that returns the size of the internally allocated buffer. This function does not specify a time complexity nor does it have an implied time complexity. However, given the complexities for `data()` to be `𝒪(1)` and `size()` to be `𝒪(1)`, we can imagine that it's reasonable to also require `capacity()` to be `𝒪(1)`, since the implementation will most likely be caching the size of the allocated buffer so as to check for the need to reallocate on insertion.

[ 2017-11-01 Moved to Tentatively Ready after 10 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4659.

1. Edit 27.4.3.5 [string.capacity] as indicated:

   >     size_type capacity() const noexcept;

   > > -9- Returns: The size of the allocated storage in the string.

   > > -?- Complexity: Constant time.

2. Edit 23.3.13.3 [vector.capacity] as indicated:

   >     size_type capacity() const noexcept;

   > > -1- Returns: The total number of elements that the vector can hold without requiring reallocation.

   > > -?- Complexity: Constant time.

### 3005(i). Destruction order of arrays by `make_shared/allocate_shared` only recommended?

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++20 Submitter: Richard Smith Opened: 2017-08-01 Last modified: 2021-02-25

Priority: 0

View all other issues in [util.smartptr.shared.create].

View all issues with C++20 status.

Discussion:

In [util.smartptr.shared.create]/7.9 we find this:

> "When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements should be destroyed in the reverse order of their construction."

Why is this only a "should be" and not a "shall be" (or, following usual conventions for how we write requirements on the implementation, "are")? Is there some problem that means we can't require an implementation to destroy in reverse construction order in all cases?

Previous resolution: [SUPERSEDED]

> This resolution is relative to N4687.

> 1. Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

>    >     template<class T, ...>
>    >     shared_ptr<T> make_shared(args);
>    >     template<class T, class A, ...>
>    >     shared_ptr<T> allocate_shared(const A& a, args);

>    > > […]

>    > > -7- Remarks:

>    > > 1. […]

>    > > 2. (7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements areshould be destroyed in decreasing index orderthe reverse order of their construction.

[2017-11-01, Alisdair comments and suggests wording improvement]

I dislike the change of how we specify the order of destruction, which is clear (but non-normative) from the preceding paragraph making the order of construction explicit. We are replacing that with a different specification, so now I need to match up ""decreasing index order" to "ascending order of address" to infer the behavior that is worded more directly today.

P0 to change "should be" to "are" though. Previous resolution: [SUPERSEDED]

> This resolution is relative to N4700.

> 1. Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

>    >     template<class T, ...>
>    >     shared_ptr<T> make_shared(args);
>    >     template<class T, class A, ...>
>    >     shared_ptr<T> allocate_shared(const A& a, args);

>    > > […]

>    > > -7- Remarks:

>    > > 1. […]

>    > > 2. (7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements areshould be destroyed in the reverse order of their construction.

[2017-11-01]

The rationale for the "decreasing index order" change in the original P/R suggested by Richard Smith had been presented as that user code may destroy one or more array elements and construct new ones in their place. In those cases `shared_ptr` has no way of knowing the construction order, so it cannot ensure that the destruction order reverses it.
Richard: Perhaps something like:

> the initialized elements are should be destroyed in the reverse order of their original construction.

would work?

[ 2017-11-02 Moved to Tentatively Ready after 10 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4700.

1. Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   >     template<class T, ...>
   >     shared_ptr<T> make_shared(args);
   >     template<class T, class A, ...>
   >     shared_ptr<T> allocate_shared(const A& a, args);

   > > […]

   > > -7- Remarks:

   > > 1. […]

   > > 2. (7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements areshould be destroyed in the reverse order of their original construction.

### 3006(i). Constructing a `basic_stringbuf` from a `string` — where does the allocator come from?

Section: 31.8.2.2 [stringbuf.cons] Status: Resolved Submitter: Marshall Clow Opened: 2017-08-02 Last modified: 2020-11-09

Priority: 3

View all other issues in [stringbuf.cons].

View all issues with Resolved status.

Discussion:

Consider the following constructor from 31.8.2.2 [stringbuf.cons] before p3:

>     explicit basic_stringbuf(
>       const basic_string<charT, traits, Allocator>& s,
>       ios_base::openmode which = ios_base::in | ios_base::out);

In [stringbuf.cons]/3, it says:

> Effects: Constructs an object of class `basic_stringbuf`, initializing the base class with `basic_streambuf()` (30.6.3.1), and initializing `mode` with `which`. Then calls `str(s)`.

But that doesn't mention where the allocator for the `basic_stringbuf` comes from. Until recently, most implementations just default constructed the allocator. But that requires that the allocator be default constructible.

This bug was filed against libc++ for this case.

I think that the `basic_stringbuf` should be constructed with a copy of the allocator from the string.

[2017-11-01, Marshall comments]

There exist two related proposals, namely P0407R1 and P0408R2.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 3

[2020-05-02 Daniel comments]

It seems that this issue is now resolved by the proposal P0408R7, accepted in Cologne 2019.

The paper clarified that the allocator is provided based on the effect of initializing the exposition-only `basic_string buf` with the provided `basic_string argument s` (Which again is specified in terms of the allocator-related specification(27.4.3.2 [string.require] p3).

[2020-05-03 Reflector discussion]

Tentatively Resolved by P0408R7 adopted in Cologne 2019.

The change we approved is not what the issue recommended (and not what some implementations do), so we decided with leaving it as Tentatively Resolved to give people a chance to review it.

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Rationale:

Resolved by P0408R7.

Proposed resolution:

### 3007(i). `allocate_shared` should rebind allocator to cv-unqualified `value_type` for construction

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++20 Submitter: Glen Joseph Fernandes Opened: 2017-08-06 Last modified: 2021-02-25

Priority: 0

View all other issues in [util.smartptr.shared.create].

View all issues with C++20 status.

Discussion:

The remarks for the `allocate_shared` family of functions specify that when constructing a (sub)object of type `U`, it uses a rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `U`. However `U` can be a `const` or `volatile` qualified type, and [allocator.requirements] specify that the `value_type` must be cv-unqualified.

[ 2017-11-01 Moved to Tentatively Ready after 6 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4687.

1. Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   >     template<class T, ...>
   >     shared_ptr<T> make_shared(args);
   >     template<class T, class A, ...>
   >     shared_ptr<T> allocate_shared(const A& a, args);

   > > […]

   > > -7- Remarks:

   > > 1. […]

   > > 2. (7.5) — When a (sub)object of a non-array type `U` is specified to have an initial value of `v`, or `U(l...)`, where `l...` is a list of constructor arguments, `allocate_shared` shall initialize this (sub)object via the expression

   > >    1. (7.5.1) — `allocator_traits<A2>::construct(a2, pv, v)` or

   > >    2. (7.5.2) — `allocator_traits<A2>::construct(a2, pv, l...)`

   > >    respectively, where `pv` points to storage suitable to hold an object of type `U` and `a2` of type `A2` is a rebound copy of the allocator a passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

   > > 3. (7.6) — When a (sub)object of non-array type `U` is specified to have a default initial value, `make_shared` shall initialize this (sub)object via the expression `::new(pv) U()`, where `pv` has type `void*` and points to storage suitable to hold an object of type `U`.

   > > 4. (7.7) — When a (sub)object of non-array type `U` is specified to have a default initial value, `allocate_shared` shall initialize this (sub)object via the expression `allocator_traits<A2>::construct(a2, pv)`, where `pv` points to storage suitable to hold an object of type `U` and `a2` of type `A2` is a rebound copy of the allocator a passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

   > > 5. […]

### 3008(i). `make_shared` (sub)object destruction semantics are not specified

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++20 Submitter: Glen Joseph Fernandes Opened: 2017-08-06 Last modified: 2021-02-25

Priority: 2

View all other issues in [util.smartptr.shared.create].

View all issues with C++20 status.

Discussion:

The remarks for the `make_shared` and `allocate_shared` functions do not specify how the objects managed by the returned `shared_ptr` are destroyed. It is implied that when objects are constructed via a placement new expression, they are destroyed by calling the destructor, and that when objects are constructed via an allocator, they are destroyed using that allocator. This should be explicitly specified.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 2

Previous resolution [SUPERSEDED]:

> This resolution is relative to N4687.

> 1. Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

>    >     template<class T, ...>
>    >     shared_ptr<T> make_shared(args);
>    >     template<class T, class A, ...>
>    >     shared_ptr<T> allocate_shared(const A& a, args);

>    > > […]

>    > > -7- Remarks:

>    > > 1. […]

>    > > 2. (7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements should be destroyed in the reverse order of their construction.

>    > > 3. (7.?) — When a (sub)object of a non-array type `U` that was initialized by `make_shared` is to be destroyed, it shall be destroyed via the expression `pv->~U()` where `pv` points to that object of type `U`.

>    > > 4. (7.?) — When a (sub)object of a non-array type `U` that was initialized by `allocate_shared` is to be destroyed, it shall be destroyed via the expression `allocator_traits<A2>::destroy(a2, pv)` where `pv` points to that object of type cv-unqualified `U` and `a2` of type `A2` is a rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

[2018-06 Rapperswil Wednesday night issues processing]

CC: what is "of type cv-unqualified `U`" and "`remove_cv_T<U>`" about?
DK: again, it isn't new wording; it is in p 7.5.2
JW: but none of the words use "of type cv-unqualified `U`"
CT: so we should also used `remove_cv_T<U>` instead?
JW: I would like to talk to Glen
FB: does anybody know how it works for an array of arrays? It seems to cover the case
JW: we could leave it vague as it is now or specify it to exactly what it does
DK: I think we should split the thing into two parts and start with definitions
DK: ACTION I can refactor the wording
MC: there was a fairly long message thread when we talked about this

Daniel comments and improves wording:

The currently allocator requirements support only the construction of cv-unqualified object types (See Table 30 type `C` and pointer variable `c` as well as Table 31 expressions "`a.construct(c, args)`" and "`a.destroy(c)`"), therefore a conforming implementation needs to effectively construct an object pointer that holds an object of type `remove_cv_T<U>` and similarly destroy such an object. Albeit it seems to be an artificial restriction to construct and destroy only non-cv-qualified object types, this is, if any, a different issue. But given this current state, the wording for `allocate_shared` needs to make a special wording dance via `remove_cv_T<U>`. For `construct` the existing wording prevents to speak about that detail by using the more indirect phrase "where `pv` points to storage suitable to hold an object of type `U`", but since object types `U` and `const U` have exactly the same storage and alignment requirements, this sentence is correct for `remove_cv_T<U>` as well.

[2018-08-23 Batavia Issues processing]

Status to Tentatively Ready.

[2018-11, Adopted in San Diego]

Proposed resolution:

This resolution is relative to N4750.

1. Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   >     template<class T, ...>
   >     shared_ptr<T> make_shared(args);
   >     template<class T, class A, ...>
   >     shared_ptr<T> allocate_shared(const A& a, args);

   > > […]

   > > -7- Remarks:

   > > 1. […]

   > > 2. (7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements are destroyed in the reverse order of their original construction.

   > > 3. (7.?) — When a (sub)object of a non-array type `U` that was initialized by `make_shared` is to be destroyed, it is destroyed via the expression `pv->~U()` where `pv` points to that object of type `U`.

   > > 4. (7.?) — When a (sub)object of a non-array type `U` that was initialized by `allocate_shared` is to be destroyed, it is destroyed via the expression `allocator_traits<A2>::destroy(a2, pv)` where `pv` points to that object of type `remove_cv_t<U>` and `a2` of type `A2` is a rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

### 3009(i). Including `<string_view>` doesn't provide `std::size/empty/data`

Section: 24.7 [iterator.range] Status: C++20 Submitter: Tim Song Opened: 2017-08-11 Last modified: 2021-06-06

Priority: 0

View all other issues in [iterator.range].

View all issues with C++20 status.

Discussion:

`basic_string_view` has `size()`, `empty()`, and `data()` members, but including `<string_view>` isn't guaranteed to give you access to the corresponding free function templates. This seems surprising.

[ 2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4687.

1. Edit [iterator.container] as indicated:

   > -1- In addition to being available via inclusion of the `<iterator>` header, the function templates in 27.8 are available when any of the following headers are included: `<array>`, `<deque>`, `<forward_list>`, `<list>`, `<map>`, `<regex>`, `<set>`, `<string>`, `<string_view>`, `<unordered_map>`, `<unordered_set>`, and `<vector>`.

### 3010(i). [networking.ts] `uses_executor` says "if a type `T::executor_type` exists"

Section: 13.11.1 [networking.ts::async.uses.executor.trait] Status: WP Submitter: Jonathan Wakely Opened: 2017-08-17 Last modified: 2026-02-21

Priority: 0

View all issues with WP status.

Discussion:

Addresses: networking.ts

[async.uses.executor.trait] p1 says "if a type `T::executor_type` exists" but we don't want it to be required to detect private or ambiguous types.

[ 2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4656.

1. Edit 13.11.1 [networking.ts::async.uses.executor.trait] as indicated:

   > -1- Remark: Detects whether `T` has a nested `executor_type` that is convertible from `Executor`. Meets the `BinaryTypeTrait` requirements (C++Std [meta.rqmts]). The implementation provides a definition that is derived from `true_type` if a typethe qualified-id `T::executor_type` existsis valid and denotes a type and `is_convertible<Executor, T::executor_type>::value != false`, otherwise it is derived from `false_type`. A program may specialize this template […].

### 3012(i). `atomic<T>` is unimplementable for non-`is_trivially_copy_constructible T`

Section: 32.5.8 [atomics.types.generic] Status: C++20 Submitter: Billy O'Neal III Opened: 2017-08-16 Last modified: 2021-02-25

Priority: 2

View all other issues in [atomics.types.generic].

View all issues with C++20 status.

Discussion:

32.5.8 [atomics.types.generic] requires that `T` for `std::atomic` is trivially copyable. Unfortunately, that's not sufficient to implement `atomic`. Consider `atomic<T>::load`, which wants to look something like this:

>     template<class T>
>     struct atomic {
>       __compiler_magic_storage_for_t storage;

>       T load(memory_order = memory_order_seq_cst) const {
>         return __magic_intrinsic(storage);
>       }

>     };

Forming this return statement, though, requires that `T` is copy constructible — trivially copyable things aren't necessarily copyable! For example, the following is trivially copyable but breaks libc++, libstdc++, and msvc++:

>     struct NonAssignable {
>       int i;
>       NonAssignable() = delete;
>       NonAssignable(int) : i(0) {}
>       NonAssignable(const NonAssignable&) = delete;
>       NonAssignable(NonAssignable&&) = default;
>       NonAssignable& operator=(const NonAssignable&) = delete;
>       NonAssignable& operator=(NonAssignable&&) = delete;
>       ~NonAssignable() = default;
>     };

All three standard libraries are happy as long as `T` is trivially copy constructible, assignability is not required. Casey Carter says that we might want to still require trivially copy assignable though, since what happens when you do an `atomic<T>::store` is morally an "assignment" even if it doesn't use the user's assignment operator.

[2017-11 Albuquerque Wednesday issue processing]

Status to Open; Casey and STL to work with Billy for better wording.

Should this include trivially copyable as well as trivially copy assignable?

2017-11-09, Billy O'Neal provides updated wording.

Previous resolution [SUPERSEDED]:

> This resolution is relative to N4687.

> 1. Edit 32.5.8 [atomics.types.generic] as indicated:

>    > -1- If `is_trivially_copy_constructible_v<T>` is `false`, the program is ill-formedThe template argument for `T` shall be trivially copyable (6.9 [basic.types]). [Note: Type arguments that are not also statically initializable may be difficult to use. — end note]

Previous resolution [SUPERSEDED]:

> This resolution is relative to N4687.

> 1. Edit 32.5.8 [atomics.types.generic] as indicated:

>    > -1- If `is_copy_constructible_v<T>` is `false` or if `is_trivially_copyable_v<T>` is `false`, the program is ill-formedThe template argument for `T` shall be trivially copyable (6.9 [basic.types]). [Note: Type arguments that are not also statically initializable may be difficult to use. — end note]

[2017-11-12, Tomasz comments and suggests alternative wording]

According to my understanding during Albuquerque Saturday issue processing we agreed that we want the type used with the atomics to have non-deleted and trivial copy/move construction and assignment.

Wording note: `CopyConstructible` and `CopyAssignable` include semantic requirements that are not checkable at compile time, so these are requirements imposed on the user and cannot be validated by an implementation without heroic efforts.

[2018-11 San Diego Thursday night issue processing]

Status to Ready.

Proposed resolution:

This resolution is relative to N4700.

1. Edit 32.5.8 [atomics.types.generic] as indicated:

   > -1- The template argument for `T` shall meet the `CopyConstructible` and `CopyAssignable` requirements. If `is_trivially_copyable_v<T> && is_copy_constructible_v<T> && is_move_constructible_v<T> && is_copy_assignable_v<T> && is_move_assignable_v<T>` is `false`, the program is ill-formedbe trivially copyable (6.9 [basic.types]). [Note: Type arguments that are not also statically initializable may be difficult to use. — end note]

### 3013(i). `(recursive_)directory_iterator` construction and traversal should not be `noexcept`

Section: 31.12.11.2 [fs.dir.itr.members], 31.12.12.2 [fs.rec.dir.itr.members], 31.12.13.4 [fs.op.copy], 31.12.13.20 [fs.op.is.empty] Status: C++20 Submitter: Tim Song Opened: 2017-08-23 Last modified: 2023-02-07

Priority: 0

View all other issues in [fs.dir.itr.members].

View all issues with C++20 status.

Discussion:

Constructing a `(recursive_)directory_iterator` from a `path` requires, at a minimum, initializing its underlying `directory_entry` object with the `path` formed from the supplied `path` and the name of the first entry, which requires a potentially throwing memory allocation; every implementation I've looked at also allocates memory to store additional data as well.

Similarly, `increment()` needs to update the `path` stored in `directory_entry` object to refer to the name of the next entry, which may require a memory allocation. While it might conceivably be possible to postpone the update in this case until the iterator is dereferenced (the dereference operation is not `noexcept` due to its narrow contract), it seems highly unlikely that such an implementation is intended (not to mention that it would require additional synchronization as the dereference operations are const).

This further calls into question whether the `error_code` overloads of `copy` and `is_empty`, whose specification uses `directory_iterator`, should be `noexcept`. There might be a case for keeping the `noexcept` for `is_empty`, although that would require changes in all implementations I checked (libstdc++, libc++, and Boost). `copy` appears to be relentlessly hostile to `noexcept`, since its specification forms a `path` via `operator/` in two places (bullets 4.7.4 and 4.8.2) in addition to the `directory_iterator` usage. The proposed resolution below removes both.

[ 2017-11-03 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4700.

1. Edit [fs.class.directory_iterator], class `directory_iterator` synopsis, as indicated:

   >     […]
   >     explicit directory_iterator(const path& p);
   >     directory_iterator(const path& p, directory_options options);
   >     directory_iterator(const path& p, error_code& ec) noexcept;
   >     directory_iterator(const path& p, directory_options options,
   >                        error_code& ec) noexcept;
   >     […]

   >     directory_iterator& operator++();
   >     directory_iterator& increment(error_code& ec) noexcept;

2. Edit 31.12.11.2 [fs.dir.itr.members] before p2 as indicated:

   >     explicit directory_iterator(const path& p);
   >     directory_iterator(const path& p, directory_options options);
   >     directory_iterator(const path& p, error_code& ec) noexcept;
   >     directory_iterator(const path& p, directory_options options, error_code& ec) noexcept;

   > > -2- Effects: […]

   > > -3- Throws: As specified in 31.12.5 [fs.err.report].

   > > -4- [Note: […] — end note]

3. Edit 31.12.11.2 [fs.dir.itr.members] before p10 as indicated:

   >     directory_iterator& operator++();
   >     directory_iterator& increment(error_code& ec) noexcept;

   > > -10- Effects: As specified for the prefix increment operation of Input iterators (24.2.3).

   > > -11- Returns: `*this`.

   > > -12- Throws: As specified in 31.12.5 [fs.err.report].

4. Edit 31.12.12 [fs.class.rec.dir.itr], class `recursive_directory_iterator` synopsis, as indicated:

   >     […]
   >     explicit recursive_directory_iterator(const path& p);
   >     recursive_directory_iterator(const path& p, directory_options options);
   >     recursive_directory_iterator(const path& p, directory_options options,
   >                                  error_code& ec) noexcept;
   >     recursive_directory_iterator(const path& p, error_code& ec) noexcept;
   >     […]

   >     recursive_directory_iterator& operator++();
   >     recursive_directory_iterator& increment(error_code& ec) noexcept;

5. Edit 31.12.12.2 [fs.rec.dir.itr.members] before p2 as indicated:

   >     explicit recursive_directory_iterator(const path& p);
   >     recursive_directory_iterator(const path& p, directory_options options);
   >     recursive_directory_iterator(const path& p, directory_options options, error_code& ec) noexcept;
   >     recursive_directory_iterator(const path& p, error_code& ec) noexcept;

   > > -2- Effects: […]

   > > -3- Postconditions: […]

   > > -4- Throws: As specified in 31.12.5 [fs.err.report].

   > > -5- [Note: […] — end note]

   > > -6- [Note: […] — end note]

6. Edit 31.12.12.2 [fs.rec.dir.itr.members] before p23 as indicated:

   >     recursive_directory_iterator& operator++();
   >     recursive_directory_iterator& increment(error_code& ec) noexcept;

   > > -23- Effects: As specified for the prefix increment operation of Input iterators (24.2.3), except that: […]

   > > -24- Returns: `*this`.

   > > -25- Throws: As specified 31.12.5 [fs.err.report].

7. Edit 31.12.4 [fs.filesystem.syn], header `<filesystem>` synopsis, as indicated:

   >     namespace std::filesystem {

   >       […]

   >       void copy(const path& from, const path& to);
   >       void copy(const path& from, const path& to, error_code& ec) noexcept;
   >       void copy(const path& from, const path& to, copy_options options);
   >       void copy(const path& from, const path& to, copy_options options,
   >                 error_code& ec) noexcept;

   >       […]

   >       bool is_empty(const path& p);
   >       bool is_empty(const path& p, error_code& ec) noexcept;

   >       […]

   >     }

8. Edit 31.12.13.4 [fs.op.copy] as indicated:

   >     void copy(const path& from, const path& to, error_code& ec) noexcept;

   > > -2- Effects: Equivalent to `copy(from, to, copy_options::none, ec)`.

   >     void copy(const path& from, const path& to, copy_options options);
   >     void copy(const path& from, const path& to, copy_options options,
   >               error_code& ec) noexcept;

   > > -3- Requires: […]

   > > -4- Effects: […]

   > > -5- Throws: […]

   > > -6- Remarks: […]

   > > -7- [Example: […] — end example]

9. Edit [fs.op.is_empty] as indicated:

   >     bool is_empty(const path& p);
   >     bool is_empty(const path& p, error_code& ec) noexcept;

   > > -1- Effects: […]

   > > -2- Throws: […]

### 3014(i). More `noexcept` issues with filesystem operations

Section: 31.12.13.5 [fs.op.copy.file], 31.12.13.7 [fs.op.create.directories], 31.12.13.32 [fs.op.remove.all] Status: C++20 Submitter: Tim Song Opened: 2017-08-23 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [fs.op.copy.file].

View all issues with C++20 status.

Discussion:

`create_directories` may need to create temporary `path`s, and `remove_all` may need to create temporary `path`s and/or `directory_iterator`s. These operations may require a potentially throwing memory allocation.

Implementations of `copy_file` may wish to dynamically allocate the buffer used for copying when the underlying OS doesn't supply a copy API directly. This can happen indirectly, e.g., by using `<fstream>` facilities to perform the copying without supplying a custom buffer. Unless LWG wishes to prohibit using a dynamically allocated buffer in this manner, the `noexcept` should be removed.

[2017-11 Albuquerque Wednesday night issues processing]

Moved to Ready

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4687.

1. Edit 31.12.4 [fs.filesystem.syn], header `<filesystem>` synopsis, as indicated:

   >     namespace std::filesystem {

   >       […]

   >       bool copy_file(const path& from, const path& to);
   >       bool copy_file(const path& from, const path& to, error_code& ec) noexcept;
   >       bool copy_file(const path& from, const path& to, copy_options option);
   >       bool copy_file(const path& from, const path& to, copy_options option,
   >                      error_code& ec) noexcept;

   >       […]

   >       bool create_directories(const path& p);
   >       bool create_directories(const path& p, error_code& ec) noexcept;

   >       […]

   >       uintmax_t remove_all(const path& p);
   >       uintmax_t remove_all(const path& p, error_code& ec) noexcept;

   >       […]
   >     }

2. Edit [fs.op.copy_file] as indicated:

   >     bool copy_file(const path& from, const path& to);
   >     bool copy_file(const path& from, const path& to, error_code& ec) noexcept;

   > > -1- Returns: […]

   > > -2- Throws: […]

   >     bool copy_file(const path& from, const path& to, copy_options options);
   >     bool copy_file(const path& from, const path& to, copy_options options,
   >                    error_code& ec) noexcept;

   > > -3- Requires: […]

   > > -4- Effects: […]

   > > -5- Returns: […]

   > > -6- Throws: […]

   > > -7- Complexity: […]

3. Edit [fs.op.create_directories] as indicated:

   >     bool create_directories(const path& p);
   >     bool create_directories(const path& p, error_code& ec) noexcept;

   > > -1- Effects: […]

   > > -2- Postconditions: […]

   > > -3- Returns: […]

   > > -4- Throws: […]

   > > -5- Complexity: […]

4. Edit [fs.op.remove_all] as indicated:

   >     uintmax_t remove_all(const path& p);
   >     uintmax_t remove_all(const path& p, error_code& ec) noexcept;

   > > -1- Effects: […]

   > > -2- Postconditions: […]

   > > -3- Returns: […]

   > > -4- Throws: […]

### 3015(i). `copy_options::unspecified` underspecified

Section: 31.12.13.4 [fs.op.copy] Status: C++20 Submitter: Tim Song Opened: 2017-08-24 Last modified: 2021-02-25

Priority: 3

View all other issues in [fs.op.copy].

View all issues with C++20 status.

Discussion:

31.12.13.4 [fs.op.copy]/4.8.2 says in the copy-a-directory case `filesystem::copy` performs:

>     for (const directory_entry& x : directory_iterator(from))
>       copy(x.path(), to/x.path().filename(), options | copy_options::unspecified);

Presumably this does not actually mean that the implementation is free to set whatever `copy_option` element it wishes (`directories_only`? `recursive`? `create_hard_links`?), or `none` at all, or – since unspecified behavior corresponds to the nondeterministic aspects of the abstract machine (6.10.1 [intro.execution]/3) – a nondeterministically picked element for every iteration of the loop. That would be outright insane.

I'm fairly sure that what's intended here is to set an otherwise-unused bit in `options` so as to prevent recursion in the `options == copy_options::none` case.

[2017-11-08]

Priority set to 3 after five votes on the mailing list

Previous resolution: [SUPERSEDED]

> This wording is relative to N4687.

> 1. Edit 31.12.13.4 [fs.op.copy] p4, bullet 4.8.2 as indicated:

>    > 1. (4.7) — Otherwise, if `is_regular_file(f)`, then:

>    >    > […]

>    > 2. (4.8) — Otherwise, if

>    >    > >     is_directory(f) &&
>    >    > >     ((options & copy_options::recursive) != copy_options::none ||
>    >    > >     options == copy_options::none)

>    >    > then:

>    >    1. (4.8.1) — If `exists(t)` is `false`, then `create_directory(to, from)`.

>    >    2. (4.8.2) — Then, iterate over the files in `from`, as if by

>    >       > >     for (const directory_entry& x : directory_iterator(from))
>    >       > >       copy(x.path(), to/x.path().filename(), options | copy_options::unspecifiedin-recursive-copy);

>    >       > where `in-recursive-copy` is an exposition-only bitmask element of `copy_options` that is not one of the elements in 31.12.8.3 [fs.enum.copy.opts].

>    > 3. (4.9) — Otherwise, for the signature with argument `ec`, `ec.clear()`.

>    > 4. (4.10) — Otherwise, no effects.

[2018-1-26 issues processing telecon]

Status to 'Tentatively Ready' after striking the words 'Exposition-only' from the added text

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4687.

1. Edit 31.12.13.4 [fs.op.copy] p4, bullet 4.8.2 as indicated:

   > 1. (4.7) — Otherwise, if `is_regular_file(f)`, then:

   >    > […]

   > 2. (4.8) — Otherwise, if

   >    > >     is_directory(f) &&
   >    > >     ((options & copy_options::recursive) != copy_options::none ||
   >    > >     options == copy_options::none)

   >    > then:

   >    1. (4.8.1) — If `exists(t)` is `false`, then `create_directory(to, from)`.

   >    2. (4.8.2) — Then, iterate over the files in `from`, as if by

   >       > >     for (const directory_entry& x : directory_iterator(from))
   >       > >       copy(x.path(), to/x.path().filename(), options | copy_options::unspecifiedin-recursive-copy);

   >       > where `in-recursive-copy` is an bitmask element of `copy_options` that is not one of the elements in 31.12.8.3 [fs.enum.copy.opts].

   > 3. (4.9) — Otherwise, for the signature with argument `ec`, `ec.clear()`.

   > 4. (4.10) — Otherwise, no effects.

### 3017(i). `list splice` functions should use `addressof`

Section: 23.3.7.6 [forward.list.ops], 23.3.11.5 [list.ops] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-09-05 Last modified: 2023-02-07

Priority: 0

View all other issues in [forward.list.ops].

View all issues with C++20 status.

Discussion:

[forwardlist.ops] p1 and 23.3.11.5 [list.ops] p3 say `&x != this`, but should use `addressof`. We really need front matter saying that when the library says `&x` it means `std::addressof(x)`.

[ 2017-11-03 Moved to Tentatively Ready after 9 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4687.

1. Edit [forwardlist.ops] p1 as indicated:

   >     void splice_after(const_iterator position, forward_list& x);
   >     void splice_after(const_iterator position, forward_list&& x);

   > > -1- Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range `[begin(), end())`. `get_allocator() == x.get_allocator()`. `&addressof(x) != this`.

2. Edit 23.3.11.5 [list.ops] p3 as indicated:

   >     void splice(const_iterator position, list& x);
   >     void splice(const_iterator position, list&& x);

   > > -3- Requires: `&addressof(x) != this`.

3. Edit 23.3.11.5 [list.ops] p3 as indicated:

   >     template <class Compare> void merge(list& x, Compare comp);
   >     template <class Compare> void merge(list&& x, Compare comp);

   > > -22- Requires: `comp` shall define a strict weak ordering (26.8 [alg.sorting]), and both the list and the argument list shall be sorted according to this ordering.

   > > -23- Effects: If `(&addressof(x) == this)` does nothing; otherwise, merges the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i`, in the range other than the first, the condition `comp(*i, *(i - 1))` will be `false`. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

   > > -24- Remarks: Stable (16.4.6.8 [algorithm.stable]). If `(&addressof(x) != this)` the range `[x.begin(), x.end())` is empty after the merge. No elements are copied by this operation. The behavior is undefined if `get_allocator() != x.get_allocator()`.

   > > -25- Complexity: At most `size() + x.size() - 1` applications of `comp` if `(&addressof(x) != this)`; otherwise, no applications of `comp` are performed. If an exception is thrown other than by a comparison there are no effects.

### 3018(i). `shared_ptr` of function type

Section: 20.3.2.2 [util.smartptr.shared] Status: C++20 Submitter: Agustín K-ballo Bergé Opened: 2017-09-13 Last modified: 2021-02-25

Priority: 3

View all other issues in [util.smartptr.shared].

View all issues with C++20 status.

Discussion:

`shared_ptr` has been designed to support use cases where it owns a pointer to function, and whose deleter does something with it. This can be used, for example, to keep a dynamic library loaded for as long as their exported functions are referenced.

Implementations have overlooked that the `T` in `shared_ptr<T>` can be a function type. It isn't immediately obvious from the standard, and it's not possible to tell from the wording that this is intentional.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 3

Previous resolution [SUPERSEDED]:

> This wording is relative to N4687.

> 1. Edit 20.3.2.2 [util.smartptr.shared] as indicated:

>    > […]

>    > -2- Specializations of `shared_ptr` shall be `CopyConstructible`, `CopyAssignable`, and `LessThanComparable`, allowing their use in standard containers. Specializations of `shared_ptr` shall be contextually convertible to `bool`, allowing their use in boolean expressions and declarations in conditions. The template parameter `T` of `shared_ptr` may be an incomplete type.

>    > -?- The template parameter `T` of `shared_ptr` may be an incomplete type. `T*` shall be an object pointer type or a function pointer type.

>    > […]

[2020-02-13; Prague]

LWG would prefer to make the new constraint a Mandates-like thing.

Original resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Edit 20.3.2.2 [util.smartptr.shared] as indicated:

>    > […]

>    > -2- Specializations of `shared_ptr` shall be Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17LessThanComparable, allowing their use in standard containers. Specializations of `shared_ptr` shall be contextually convertible to `bool`, allowing their use in boolean expressions and declarations in conditions. The template parameter `T` of `shared_ptr` may be an incomplete type.

>    > -?- The template parameter `T` of `shared_ptr` may be an incomplete type. The program is ill-formed unless `T*` is an object pointer type or a function pointer type.

>    > […]

[2020-02 Friday AM discussion in Prague.]

Marshall provides updated wording; status to Immediate

Proposed resolution:

This wording is relative to N4849.

1. Edit 20.3.2.2 [util.smartptr.shared] as indicated:

   > […]

   > -2- Specializations of `shared_ptr` shall be Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17LessThanComparable, allowing their use in standard containers. Specializations of `shared_ptr` shall be contextually convertible to `bool`, allowing their use in boolean expressions and declarations in conditions. The template parameter `T` of `shared_ptr` may be an incomplete type.

   > -?- The template parameter `T` of `shared_ptr` may be an incomplete type. [Note: T may be a function type. -- end note]

   > […]

### 3020(i). [networking.ts] Remove spurious nested `value_type` buffer sequence requirement

Section: 16.2 [networking.ts::buffer.reqmts] Status: WP Submitter: Vinnie Falco Opened: 2017-09-20 Last modified: 2026-02-21

Priority: 0

View all other issues in [networking.ts::buffer.reqmts].

View all issues with WP status.

Discussion:

Addresses: networking.ts

The post-condition requirements for `ConstBufferSequence` and `MutableBufferSequence` refer to `X::value_type`, but no such nested type is required. The lambda expression passed to `equal` can use `auto const&` parameter types instead.

Previous resolution: [SUPERSEDED]

> This wording is relative to N4588.

> 1. Modify 16.2.1 [networking.ts::buffer.reqmts.mutablebuffersequence] Table 12 "`MutableBufferSequence` requirements" as indicated:

>    > Table 12 — `MutableBufferSequence` requirements
>    > expression | return type | assertion/note pre/post-condition
>    > --- | --- | ---
>    > `[…]`
>    > `X u(x);` |  | post:
>    >     equal(
>    >       net::buffer_sequence_begin(x),
>    >       net::buffer_sequence_end(x),
>    >       net::buffer_sequence_begin(u),
>    >       net::buffer_sequence_end(u),
>    >       [](const typename X::value_typeauto& v1,
>    >          const typename X::value_typeauto& v2)
>    >         {
>    >           mutable_buffer b1(v1);
>    >           mutable_buffer b2(v2);
>    >           return b1.data() == b2.data()
>    >               && b1.size() == b2.size();
>    >         })

> 2. Modify 16.2.2 [networking.ts::buffer.reqmts.constbuffersequence] Table 13 "`ConstBufferSequence` requirements" as indicated:

>    > Table 13 — `ConstBufferSequence` requirements
>    > expression | return type | assertion/note pre/post-condition
>    > --- | --- | ---
>    > `[…]`
>    > `X u(x);` |  | post:
>    >     equal(
>    >       net::buffer_sequence_begin(x),
>    >       net::buffer_sequence_end(x),
>    >       net::buffer_sequence_begin(u),
>    >       net::buffer_sequence_end(u),
>    >       [](const typename X::value_typeauto& v1,
>    >          const typename X::value_typeauto& v2)
>    >         {
>    >           const_buffer b1(v1);
>    >           const_buffer b2(v2);
>    >           return b1.data() == b2.data()
>    >               && b1.size() == b2.size();
>    >         })

[2017-10-19, Peter Dimov provides improved wording]

The alternative wording prevents the need for `auto` parameters because it takes advantage of the "convertible to" requirement.

[ 2017-10-20 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4588.

1. Modify 16.2.1 [networking.ts::buffer.reqmts.mutablebuffersequence] Table 12 "`MutableBufferSequence` requirements" as indicated:

   > Table 12 — `MutableBufferSequence` requirements
   > expression | return type | assertion/note pre/post-condition
   > --- | --- | ---
   > `[…]`
   > `X u(x);` |  | post:
   >     equal(
   >       net::buffer_sequence_begin(x),
   >       net::buffer_sequence_end(x),
   >       net::buffer_sequence_begin(u),
   >       net::buffer_sequence_end(u),
   >       [](const typename X::value_type& v1mutable_buffer& b1,
   >          const typename X::value_type& v2mutable_buffer& b2)
   >         {
   >           mutable_buffer b1(v1);
   >           mutable_buffer b2(v2);
   >           return b1.data() == b2.data()
   >               && b1.size() == b2.size();
   >         })

2. Modify 16.2.2 [networking.ts::buffer.reqmts.constbuffersequence] Table 13 "`ConstBufferSequence` requirements" as indicated:

   > Table 13 — `ConstBufferSequence` requirements
   > expression | return type | assertion/note pre/post-condition
   > --- | --- | ---
   > `[…]`
   > `X u(x);` |  | post:
   >     equal(
   >       net::buffer_sequence_begin(x),
   >       net::buffer_sequence_end(x),
   >       net::buffer_sequence_begin(u),
   >       net::buffer_sequence_end(u),
   >       [](const typename X::value_type& v1const_buffer& b1,
   >          const typename X::value_type& v2const_buffer& v2)
   >         {
   >           const_buffer b1(v1);
   >           const_buffer b2(v2);
   >           return b1.data() == b2.data()
   >               && b1.size() == b2.size();
   >         })

### 3022(i). `is_convertible<derived*, base*>` may lead to ODR

Section: 21.3.7 [meta.rel] Status: Resolved Submitter: Alisdair Meredith Opened: 2017-09-24 Last modified: 2018-11-12

Priority: 2

View other active issues in [meta.rel].

View all other issues in [meta.rel].

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Discussion:

Given two incomplete types, `base` and `derived`, that will have the expected base/derived relationship when complete, the trait `is_convertible` claims to support instantiation with pointers to these types (as pointers to incomplete types are, themselves, complete), yet will give a different answer when the types are complete vs. when they are incomplete.

We should require pointers (and pointers to pointers etc.) point to a complete type, unless one is a pointer to cv-`void`. We may also want some weasel-wording to permit pointers to arrays-of-unknown-bound, and pointers to cv-qualified variants of the same incomplete type.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 2

[2018-08 Batavia Monday issue discussion]

Issues 2797(i), 2939(i), 3022(i), and 3099(i) are all closely related. Walter to write a paper resolving them.

[2018-11-11 Resolved by P1285R0, adopted in San Diego.]

Proposed resolution:

### 3023(i). Clarify unspecified call wrappers

Section: 22.10.16 [func.memfn], 22.10.13 [func.not.fn], 22.10.15 [func.bind] Status: Resolved Submitter: Detlef Vollmann Opened: 2017-10-07 Last modified: 2021-06-06

Priority: 3

View all other issues in [func.memfn].

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Discussion:

Even after the discussion on the reflector, starting with this reflector message it's not completely clear that unspecified as return type of `mem_fn` really means 'unspecified, but always the same'. The same problem exists for `bind()` and `not_fn()`.

Possible solution:

Specify in 22.10.3 [func.def] or 22.10.4 [func.require] that a call wrapper type is always the same for forwarding call wrappers if the object is returned by a function with the same parameter types. And also put into [func.not_fn] that a `call_wrapper` object is a simple call wrapper.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 3. Tomasz to write a paper that will address this issue. See also 3015(i)

[2017-11-10, Tomasz comments and provides wording together with STL]

From the core language rules it is already required that same function template specialization have the same return type. Given that the invocation of `mem_fn`/`bind`/`not_fn` will always return the same wrapper type, if they are instantiated (called with) same parameters type. However, the existence of this issue, shows that some library-wide clarification note would be welcomed.

[2019-05-12; Tomasz comments]

I have realized that this issue indicates an real problem with the usability of `bind` as the replacement of the `binder1st/binder2nd`. Currently it is not required that a binding functor of the same type with same argument, produces the same result, as the type of the call wrapper may depend on the cv ref qualifiers of arguments. For example we are not requiring that the types of `f1, f2, f3, f4` are the same (and actually they are not for clang):

>     auto func = [](std::string) {};
>     std::string s("foo");
>     auto f1 = std::bind(func, s);
>     auto f2 = std::bind(std::as_const(func), std::as_const(s));
>     auto f3 = std::bind(func, std::string("bar"));
>     auto f4 = std::bind(std::move(func), std::move(s));
>     // online link: https://wandbox.org/permlink/dcXJaITMJCnBWt7R

As a consequence, if the user creates a `std::vector<decltype(std::bind(func, std::string(), _2))>` (instead of `std::vector<std::binder1st<FuncType, std::string>>`) he may not be able to store the result of the binding `func` with `std::string` instance, if an copy of `std::string` is made. That leads me to conclusion that this issue actually require wording change, to provide such guarantee, and is materially different from LWG 3015(i).

During migration from `std::bind1st/std::bind2nd` (removed in C++17) to `std::bind`, the user may need to replace `std::binder1st/std::binder2nd` with an appropriate `decltype` of `std::bind` invocation. For example:

>     FuncType func; std::string s;

>     std::vector<std::binder1st<FuncType>> v;
>     v.push_back(std::bind1st(func, s));
>     v.push_back(std::bind1st(func, std::string("text")));
>     needs to be replaced with:
>     std::vector<decltype(std::bind(func, s, _1))> v;
>     v.push_back(std::bind(func, s, _1));
>     v.push_back(std::bind(func, std::string("text"), _1));

but the last statement is not guaranteed to be well-formed.

Therefore I would like to withdraw my previously suggested wording change. Previous resolution [SUPERSEDED]:

> This wording is relative to N4700.

> 1. After section [expos.only.types] "Exposition-only types" add the following new section:

>    > ?.?.?.?.? unspecified types [unspecified.types]

>    > [Note: Whenever the return type of a function template is declared as unspecified, the return type depends only on the template arguments of the specialization. Given the example:

>    > >     template<class T> unspecified f(T);

>    > the expressions `f(0)` and `f(1)` have the same type. — end note]

[2020-01 Resolved by the adoption of P1065 in Cologne.]

Proposed resolution:

### 3024(i). `variant`'s copies must be deleted instead of disabled via SFINAE

Section: 22.6.3.2 [variant.ctor] Status: C++20 Submitter: Casey Carter Opened: 2017-10-10 Last modified: 2021-02-25

Priority: Not Prioritized

View other active issues in [variant.ctor].

View all other issues in [variant.ctor].

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Discussion:

The specification of `variant`'s copy constructor and copy assignment operator require that those functions do not participate in overload resolution unless certain conditions are satisfied. There is no mechanism in C++ that makes it possible to prevent a copy constructor or copy assignment operator from participating in overload resolution. These functions should instead be specified to be defined as deleted unless the requisite conditions hold, as we did for the copy constructor and copy assignment operator of `optional` in LWG 2756(i).

[ 2017-10-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

This wording is relative to N4687.

1. Change 22.6.3.2 [variant.ctor] as indicated:

   >     variant(const variant& w);

   > > -6- Effects: If `w` holds a value, initializes the `variant` to hold the same alternative as `w` and direct-initializes the contained value with `get<j>(w)`, where `j` is `w.index()`. Otherwise, initializes the `variant` to not hold a value.

   > > -7- Throws: Any exception thrown by direct-initializing any `Ti` for all i.

   > > -8- Remarks: This function shall not participate in overload resolutionconstructor shall be defined as deleted unless `is_copy_constructible_v<Ti>` is true for all i.

2. Change 22.6.3.4 [variant.assign] as indicated:

   >     variant& operator=(const variant& rhs);

   > > […]

   > > -4- Postconditions: `index() == rhs.index()`.

   > > -5- Remarks: This function shall not participate in overload resolutionoperator shall be defined as deleted unless `is_copy_constructible_v<Ti> && is_copy_assignable_v<Ti>` is true for all i.

### 3025(i). Map-like container deduction guides should use `pair<Key, T>`, not `pair<const Key, T>`

Section: 23.4.3.1 [map.overview], 23.4.4.1 [multimap.overview], 23.5.3.1 [unord.map.overview], 23.5.4.1 [unord.multimap.overview] Status: C++20 Submitter: Ville Voutilainen Opened: 2017-10-08 Last modified: 2021-02-25

Priority: 2

View all other issues in [map.overview].

View all issues with C++20 status.

Discussion:

With the deduction guides as specified currently, code like this doesn't work:

>     map m{pair{1, 1}, {2, 2}, {3, 3}};

Same problem occurs with `multimap`, `unordered_map` and `unordered_multimap`. The problem is in deduction guides like

>     template<class Key, class T, class Compare = less<Key>,
>               class Allocator = allocator<pair<const Key, T>>>
>     map(initializer_list<pair<const Key, T>>, Compare = Compare(),
>         Allocator = Allocator()) -> map<Key, T, Compare, Allocator>;

The `pair<const Key, T>` is not matched by a `pair<int, int>`, because `int` can't match a `const Key`. Dropping the `const` from the parameter of the deduction guide makes it work with no loss of functionality.

[2017-11-03, Zhihao Yuan comments]

The fix described here prevents

>     std::map m2{m0.begin(), m0.end()};

from falling back to direct-non-list-initialization. Treating a uniform initialization with >1 clauses of the same un-cvref type as `std::initializer_list` is the only consistent interpretation I found so far.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 2

[2018-08-23 Batavia Issues processing]

Status to Tentatively Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4687.

1. Change 23.4.3.1 [map.overview] p3, class template `map` synopsis, as indicated:

   >     […]
   >     template<class Key, class T, class Compare = less<Key>,
   >              class Allocator = allocator<pair<const Key, T>>>
   >       map(initializer_list<pair<const Key, T>>, Compare = Compare(), Allocator = Allocator())
   >         -> map<Key, T, Compare, Allocator>;
   >     […]
   >     template<class Key, class T, class Allocator>
   >       map(initializer_list<pair<const Key, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>;
   >     […]

2. Change 23.4.4.1 [multimap.overview] p3, class template `multimap` synopsis, as indicated:

   >     […]
   >     template<class Key, class T, class Compare = less<Key>,
   >              class Allocator = allocator<pair<const Key, T>>>
   >       multimap(initializer_list<pair<const Key, T>>, Compare = Compare(), Allocator = Allocator())
   >         -> multimap<Key, T, Compare, Allocator>;
   >     […]
   >     template<class Key, class T, class Allocator>
   >       multimap(initializer_list<pair<const Key, T>>, Allocator)
   >         -> multimap<Key, T, less<Key>, Allocator>;
   >     […]

3. Change 23.5.3.1 [unord.map.overview] p3, class template `unordered_map` synopsis, as indicated:

   >     […]
   >     template<class Key, class T, class Hash = hash<Key>,
   >              class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>>
   >       unordered_map(initializer_list<pair<const Key, T>>, typename see below::size_type = see below, Hash = Hash(),
   >                     Pred = Pred(), Allocator = Allocator())
   >         -> unordered_map<Key, T, Hash, Pred, Allocator>;
   >     […]
   >     template<class Key, class T, typename Allocator>
   >       unordered_map(initializer_list<pair<const Key, T>>, typename see below::size_type,
   >                     Allocator)
   >         -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

   >     template<class Key, class T, typename Allocator>
   >       unordered_map(initializer_list<pair<const Key, T>>, Allocator)
   >         -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

   >     template<class Key, class T, class Hash, class Allocator>
   >       unordered_map(initializer_list<pair<const Key, T>>, typename see below::size_type, Hash,
   >                     Allocator)
   >         -> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>;
   >     […]

4. Change 23.5.4.1 [unord.multimap.overview] p3, class template `unordered_multimap` synopsis, as indicated:

   >     […]
   >     template<class Key, class T, class Hash = hash<Key>,
   >              class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>>
   >       unordered_multimap(initializer_list<pair<const Key, T>>,
   >                          typename see below::size_type = see below,
   >                          Hash = Hash(), Pred = Pred(), Allocator = Allocator())
   >         -> unordered_multimap<Key, T, Hash, Pred, Allocator>;
   >     […]
   >     template<class Key, class T, typename Allocator>
   >       unordered_multimap(initializer_list<pair<const Key, T>>, typename see below::size_type,
   >                          Allocator)
   >         -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>;

   >     template<class Key, class T, typename Allocator>
   >       unordered_multimap(initializer_list<pair<const Key, T>>, Allocator)
   >         -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>;

   >     template<class Key, class T, class Hash, class Allocator>
   >       unordered_multimap(initializer_list<pair<const Key, T>>, typename see below::size_type,
   >                          Hash, Allocator)
   >         -> unordered_multimap<Key, T, Hash, equal_to<Key>, Allocator>;
   >     […]

### 3026(i). `filesystem::weakly_canonical` still defined in terms of `canonical(p, base)`

Section: 31.12.13.40 [fs.op.weakly.canonical] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-10-14 Last modified: 2021-06-06

Priority: 0

View all other issues in [fs.op.weakly.canonical].

View all issues with C++20 status.

Discussion:

LWG 2956(i) fixed `canonical` to no longer use a base path, but `weakly_canonical` should have been changed too:

> Effects: Using `status(p)` or `status(p, ec)`, respectively, to determine existence, return a path composed by `operator/=` from the result of calling `canonical()` without a `base` argument and with a […]

Since `canonical` doesn't accept a `base` argument, it doesn't make sense to talk about calling it without one.

[ 2017-10-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4687.

1. Change [fs.op.weakly_canonical] as indicated:

   >     path weakly_canonical(const path& p);
   >     path weakly_canonical(const path& p, error_code& ec);

   > > -1- Returns: […]

   > > -2- Effects: Using `status(p)` or `status(p, ec)`, respectively, to determine existence, return a path composed by `operator/=` from the result of calling `canonical()` without a `base` argument and with a path argument composed of the leading elements of `p` that exist, if any, followed by the elements of `p` that do not exist, if any. For the first form, `canonical()` is called without an `error_code` argument. For the second form, `canonical()` is called with ec as an `error_code` argument, and `path()` is returned at the first error occurrence, if any.

   > > […]

### 3028(i). Container requirements tables should distinguish `const` and non-`const` variables

Section: 23.2.2 [container.requirements.general] Status: C++23 Submitter: Jonathan Wakely Opened: 2017-10-17 Last modified: 2023-11-22

Priority: 3

View other active issues in [container.requirements.general].

View all other issues in [container.requirements.general].

View all issues with C++23 status.

Discussion:

[container.requirements.general] p4 says:

> In Tables 83, 84, and 85 `X` denotes a container class containing objects of type `T`, `a` and `b` denote values of type `X`, `u` denotes an identifier, `r` denotes a non-`const` value of type `X`, and `rv` denotes a non-`const` rvalue of type `X`.

This doesn't say anything about whether `a` and `b` are allowed to be `const`, or must be non-`const`. In fact Table 83 uses them inconsistently, e.g. the rows for "`a = rv`" and "`a.swap(b)`" most certainly require them to be non-`const`, but all other uses are valid for either `const` or non-`const X`.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 3; Jonathan to provide updated wording.

Wording needs adjustment - could use "possibly const values of type X"

Will distinguish between lvalue/rvalue

Previous resolution [SUPERSEDED]:

> This wording is relative to N4687.

> 1. Change 23.2.2 [container.requirements.general] p4 as indicated:

>    > -4- In Tables 83, 84, and 85 `X` denotes a container class containing objects of type `T`, `a` and `b` denote values of type `X`, `u` denotes an identifier, `r` and s denotes a non-`const` values of type `X`, and `rv` denotes a non-`const` rvalue of type `X`.

> 2. Change 23.2.2 [container.requirements.general], Table 83 "Container requirements", as indicated:

>    > Table 83 — Container requirements
>    > Expression | Return type | Operational semantics | Assertion/note pre/post-condition | Complexity
>    > --- | --- | --- | --- | ---
>    > `[…]`
>    > `ar = rv` | `X&` | All existing elements of `ar` are either move assigned to or destroyed | `ar` shall be equal to the value that `rv` had before this assignment | linear
>    > `[…]`
>    > `ar.swap(bs)` | `void` |  | exchanges the contents of `ar` and `bs` | (Note A)
>    > `[…]`
>    > `swap(ar, bs)` | `void` | `ar.swap(bs)` |  | (Note A)

[2020-05-03; Daniel provides alternative wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Change 23.2.2 [container.requirements.general] as indicated:

>    > [Drafting note:

>    > - The following presentation also transforms the current list into a bullet list as we already have in 23.2.8 [unord.req] p11

>    > - It has been decided to replace the symbol `r` by `s`, because it is easy to confuse with `rv` but means an lvalue instead, and the other container tables use it rarely and for something completely different (iterator value)

>    > - A separate symbol `v` is introduced to unambigiously distinguish the counterpart of a non-`const` rvalue (See 16.4.4.2 [utility.arg.requirements])

>    > - Two separate symbols `b` and `c` represent now "(possibly `const`) values, while the existing symbol `a` represents an unspecified value, whose meaning becomes defined when context is provided, e.g. for overloads like `begin()` and `end`

>    > -4- In Tables 73, 74, and 75:

>    > 1. (4.1) — `X` denotes a container class containing objects of type `T`,

>    > 2. (4.2) — `a` and `b` denotes a values of type `X`,

>    > 3. (4.2) — `b` and `c` denote (possibly `const`) values of type `X`,

>    > 4. (4.3) — `i` and `j` denote values of type (possibly `const`) `X::iterator`,

>    > 5. (4.4) — `u` denotes an identifier,

>    > 6. (?.?) — `v` denotes an lvalue of type (possibly `const`) `X` or an rvalue of type `const X`,

>    > 7. (4.5) — `r` `s` and `t` denotes a non-`const` valuelvalues of type `X`, and

>    > 8. (4.6) — `rv` denotes a non-`const` rvalue of type `X`.

> 2. Change 23.2.2 [container.requirements.general], Table 73 "Container requirements" [tab:container.req], as indicated:

>    > [Drafting note: The following presentation also moves the copy-assignment expression just before the move-assignment expression]

>    > Table 73: — Container requirements [tab:container.req]
>    > Expression | Return type | Operational semantics | Assertion/note pre/post-condition | Complexity
>    > --- | --- | --- | --- | ---
>    > `[…]`
>    > `X(av)` |  |  | Preconditions: `T` is Cpp17CopyInsertable into `X` (see below). Postconditions: `av == X(av)`. | linear
>    > `X u(av); X u = av;` |  |  | Preconditions: `T` is Cpp17CopyInsertable into `X` (see below). Postconditions: `u == av`. | linear
>    > `X u(rv); X u = rv;` |  |  | Postconditions: `u` is equal to the value that `rv` had before this construction | (Note B)
>    > `t = v` | `X&` |  | Postconditions: `t == v`. | linear
>    > `at = rv` | `X&` | All existing elements of `at` are either move assigned to or destroyed | `at` shall be equal to the value that `rv` had before this assignment | linear
>    > `[…]`
>    > `ac == b` | convertible to `bool` | `==` is an equivalence relation. `equal(ac.begin(), ac.end(), b.begin(), b.end())` | Preconditions: `T` meets the Cpp17EqualityComparable requirements | Constant if `ac.size() != b.size()`, linear otherwise
>    > `ac != b` | convertible to `bool` | Equivalent to `!(ac == b)` |  | linear
>    > `at.swap(bs)` | `void` |  | exchanges the contents of `at` and `bs` | (Note A)
>    > `swap(at, bs)` | `void` | `at.swap(bs)` |  | (Note A)
>    > `r = a` | `X&` |  | Postconditions: `r == a`. | linear
>    > `ac.size()` | `size_type` | `distance(ac.begin(), ac.end())` |  | constant
>    > `ac.max_size()` | `size_type` | `distance(begin(), end())` for the largest possible container |  | constant
>    > `ac.empty()` | convertible to `bool` | `ac.begin() == ac.end()` |  | constant

[2022-04-20; Jonathan rebases the wording on the latest draft]

[2022-09-05; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll in April 2022.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Change 23.2.2 [container.requirements.general] as indicated:

   > [Drafting note:

   > - It has been decided to replace the symbol `r` by `s`, because it is easy to confuse with `rv` but means an lvalue instead, and the other container tables use it rarely and for something completely different (iterator value)

   > - A separate symbol `v` is introduced to unambigiously distinguish the counterpart of a non-`const` rvalue (See 16.4.4.2 [utility.arg.requirements])

   > - Two separate symbols `b` and `c` represent now "(possibly `const`) values, while the existing symbol `a` represents an unspecified value, whose meaning becomes defined when context is provided, e.g. for overloads like `begin()` and `end`

   > -1- In subclause [container.gen.reqmts],

   > 1. (1.1) — `X` denotes a container class containing objects of type `T`,

   > 2. (1.2) — `a` and `b` denote values denotes a value of type `X`,

   > 3. (?.?) — `b` and `c` denote values of type (possibly `const`) `X`,

   > 4. (1.3) — `i` and `j` denote values of type (possibly `const`) `X::iterator`,

   > 5. (1.4) — `u` denotes an identifier,

   > 6. (?.?) — `v` denotes an lvalue of type (possibly `const`) `X` or an rvalue of type `const X`,

   > 7. (1.5) — `r` denotes a `s` and `t` denote non-`const` valuelvalues of type `X`, and

   > 8. (1.6) — `rv` denotes a non-`const` rvalue of type `X`.

2. Change 23.2.2.2 [container.reqmts] as indicated:

   > [Drafting note: The following presentation also moves the copy-assignment expression just before the move-assignment expression]

   >       X u(av);
   >       X u = av;

   > -12- Preconditions: `T` is Cpp17CopyInsertable into `X` (see below).

   > -13- Postconditions: `u == av`.

   > -14- Complexity: Linear.

   >       X u(rv);
   >       X u = rv;

   > -15- Postconditions: `u` is equal to the value that `rv` had before this construction.

   > -14- Complexity: Linear for `array` and constant for all other standard containers.

   >       t = v

   > -?- Result: `X&`.

   > -?- Postconditions: `t == v`.

   > -?- Complexity: Linear.

   >       at = rv

   > -17- Result: `X&`.

   > -18- Effects: All existing elements of `at` are either move assigned to or destroyed.

   > -19- Postconditions: `at` shall be equal to the value that `rv` had before this assignment.

   > -20- Complexity: Linear.

   > […]

   >       ab.begin()

   > -24- Result: `iterator`; `const_iterator` for constant ab.

   > -25- Returns: An iterator referring to the first element in the container.

   > -26- Complexity: Constant.

   >       ab.end()

   > -27- Result: `iterator`; `const_iterator` for constant ab.

   > -28- Returns: An iterator which is the past-the-end value for the container.

   > -29- Complexity: Constant.

   >       ab.cbegin()

   > -30- Result: `const_iterator`.

   > -31- Returns: `const_cast<X const&>(ab).begin()`

   > -32- Complexity: Constant.

   >       ab.cend()

   > -33- Result: `const_iterator`.

   > -34- Returns: `const_cast<X const&>(ab).end()`

   > -35- Complexity: Constant.

   > […]

   >       ac == b

   > -39- Preconditions: `T` meets the Cpp17EqualityComparable requirements.

   > -40- Result: Convertible to `bool`.

   > -41- Returns: `equal(ac.begin(), ac.end(), b.begin(), b.end())`.

   > [Note 1: The algorithm `equal` is defined in 26.6.13 [alg.equal]. — end note]

   > -42- Complexity: Constant if `ac.size() != b.size()`, linear otherwise.

   > -43- Remarks: `==` is an equivalence relation.

   >       ac != b

   > -44- Effects: Equivalent to `!(ac == b)`.

   >       at.swap(bs)

   > -45- Result: `void`.

   > -46- Effects: Exchanges the contents of `at` and `bs`.

   > -47- Complexity: Linear for `array` and constant for all other standard containers.

   >       swap(at, bs)

   > -48- Effects: Equivalent to `at.swap(bs)`

   >       r = a

   > -49- Result: `X&`.

   > -50- Postconditions: `r == a`.

   > -51- Complexity: Linear.

   >       ac.size()

   > -52- Result: `size_type`.

   > -53- Returns: `distance(ac.begin(), ac.end())`, i.e. the number of elements in the container.

   > -54- Complexity: Constant.

   > -55- Remarks: The number of elements is defined by the rules of constructors, inserts, and erases.

   >       ac.max_size()

   > -56- Result: `size_type`.

   > -57- Returns: `distance(begin(), end())` for the largest possible container.

   > -58- Complexity: Constant.

   >       ac.empty()

   > -59- Result: Convertible to `bool`.

   > -60- Returns: `ac.begin() == ac.end())`

   > -61- Complexity: Constant.

   > -62- Remarks: If the container is empty, then `ac.empty()` is `true`.

### 3030(i). Who shall meet the requirements of `try_lock`?

Section: 32.6.6 [thread.lock.algorithm] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-11-07 Last modified: 2021-02-25

Priority: 0

View all other issues in [thread.lock.algorithm].

View all issues with C++20 status.

Discussion:

32.6.6 [thread.lock.algorithm] says:

> "If a call to `try_lock()` fails, `unlock()` shall be called for all prior arguments and there shall be no further calls to `try_lock()`."

We try to use "shall" for requirements on the user (e.g. as in the previous paragraph) which is absolutely not what is meant here.

[2017-11 Albuquerque Wednesday night issues processing]

Moved to Ready

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4700.

1. Change 32.6.6 [thread.lock.algorithm] as indicated:

   >     template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);

   > > -1- Requires: […]

   > > -2- Effects: Calls `try_lock()` for each argument in order beginning with the first until all arguments have been processed or a call to `try_lock()` fails, either by returning `false` or by throwing an exception. If a call to `try_lock()` fails, `unlock()` shall beis called for all prior arguments and there shall bewith no further calls to `try_lock()`.

   > > […]

   >     template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);

   > > -4- Requires: […]

   > > -5- Effects: All arguments are locked via a sequence of calls to `lock()`, `try_lock()`, or `unlock()` on each argument. The sequence of calls shalldoes not result in deadlock, but is otherwise unspecified. [Note: A deadlock avoidance algorithm such as try-and-back-off must be used, but the specific algorithm is not specified to avoid over-constraining implementations. — end note] If a call to `lock()` or `try_lock()` throws an exception, `unlock()` shall beis called for any argument that had been locked by a call to `lock()` or `try_lock()`.

### 3031(i). Algorithms and predicates with non-const reference arguments

Section: 26.8 [alg.sorting] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-11-08 Last modified: 2021-02-25

Priority: 2

View all other issues in [alg.sorting].

View all issues with C++20 status.

Discussion:

This doesn't compile with any major implementation:

>     int i[1] = { };
>     std::stable_sort(i, i, [](int& x, int& y) { return x < y; });

The problem is that the `Compare` expects non-const references. We say "It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator" But that isn't sufficient to forbid the example.

My first thought was to modify [alg.sorting] to make the `Compare` requirements use `comp(as_const(x), as_const(x))` but that would get very verbose to add to every expression using `comp`.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 2; Jonathan to improve the statement of the problem.

[2018-02 David Jones provided this truly awful example:]

>     #include <algorithm>
>     #include <iostream>
>     #include <vector>

>     struct Base {
>         Base(int value) : v(value) {}
>         friend bool operator<(const Base& l, const Base& r) { return l.v < r.v; }
>         int v;
>     };

>     struct Derived : public Base {
>         using Base::Base;
>         bool operator<(const Derived& o) /* no const here */ { return v > o.v; }
>     };

>     int main(void) {
>         std::vector<Base> b = {{1}, {5}, {0}, {3}};
>         std::vector<Derived> d = {{0}, {1}, {3}, {5}};

>         std::cout << std::lower_bound(d.begin(), d.end(), 4)->v << std::endl;

>         std::sort(b.begin(), b.end());
>         for (const auto &x : b) std::cout << x.v << " ";
>         std::cout << std::endl;

>         std::sort(d.begin(), d.end());
>         for (const auto &x : d) std::cout << x.v << " ";
>         std::cout << std::endl;
>     }

>     libc++:
>     =====
>     $ bin/clang++ -std=c++11 -stdlib=libc++ tmp/ex.cc && ./a.out
>     5
>     0 1 3 5
>     0 1 3 5
>     =====

>     libstdc++:
>     =====
>     $ bin/clang++ -std=c++11 -stdlib=libstdc++ tmp/ex.cc && ./a.out
>     0
>     0 1 3 5
>     5 3 1 0
>     =====

[2018-08 Batavia Monday issue discussion]

Tim to provide wording; status to 'Open'

[ 2018-08-20, Tim adds P/R based on Batavia discussion.]

Similar to the Ranges TS design, the P/R below requires `Predicate`, `BinaryPredicate`, and `Compare` to accept all mixes of `const` and non-`const` arguments.

[2018-08-23 Batavia Issues processing]

Status to Tentatively Ready after minor wording nit (corrected in place)

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

1. Edit 26.2 [algorithms.requirements] p6-7 as indicated:

   > -6- The `Predicate` parameter is used whenever an algorithm expects a function object (22.10 [function.objects]) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable as `true`. In other words, if an algorithm takes `Predicate pred` as its argument and `first` as its iterator argument with value type `T`, it should work correctly in the construct `pred(*first)` contextually converted to `bool` (7.3 [conv]). The function object `pred` shall not apply any non-constant function through the dereferenced iterator. Given a glvalue `u` of type (possibly `const`) `T` that designates the same object as `*first`, `pred(u)` shall be a valid expression that is equal to `pred(*first)`.

   > -7- The `BinaryPredicate` parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type `T` when `T` is part of the signature returns a value testable as `true`. In other words, if an algorithm takes `BinaryPredicate binary_pred` as its argument and `first1` and `first2` as its iterator arguments with respective value types `T1` and `T2`, it should work correctly in the construct `binary_pred(*first1, *first2)` contextually converted to `bool` (7.3 [conv]). Unless otherwise specified, `BinaryPredicate` always takes the first iterator's `value_type` as its first argument, that is, in those cases when `T value` is part of the signature, it should work correctly in the construct `binary_pred(*first1, value)` contextually converted to `bool` (7.3 [conv]). `binary_pred` shall not apply any non-constant function through the dereferenced iterators. Given a glvalue `u` of type (possibly `const`) `T1` that designates the same object as `*first1`, and a glvalue `v` of type (possibly `const`) `T2` that designates the same object as `*first2`, `binary_pred(u, *first2)`, `binary_pred(*first1, v)`, and `binary_pred(u, v)` shall each be a valid expression that is equal to `binary_pred(*first1, *first2)`, and `binary_pred(u, value)` shall be a valid expression that is equal to `binary_pred(*first1, value)`.

2. Edit 26.8 [alg.sorting] p2 as indicated:

   > `Compare` is a function object type (22.10 [function.objects]) that meets the requirements for a template parameter named `BinaryPredicate` (26.2 [algorithms.requirements]). The return value of the function call operation applied to an object of type `Compare`, when contextually converted to `bool` (7.3 [conv]), yields `true` if the first argument of the call is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator.

### 3032(i). `ValueSwappable` requirement missing for `push_heap` and `make_heap`

Section: 26.8.8 [alg.heap.operations] Status: C++23 Submitter: Robert Douglas Opened: 2017-11-08 Last modified: 2023-11-22

Priority: 3

View all other issues in [alg.heap.operations].

View all issues with C++23 status.

Discussion:

In discussion of D0202R3 in Albuquerque, it was observed that `pop_heap` and `sort_heap` had `constexpr` removed for their requirement of `ValueSwappable`. It was then observed that `push_heap` and `make_heap` were not similarly marked as having the `ValueSwappable` requirement. The room believed this was likely a specification error, and asked to open an issue to track it.

[2017-11 Albuquerque Wednesday night issues processing]

Priority set to 3; Marshall to investigate

Previous resolution [SUPERSEDED]:

> This wording is relative to N4700.

> 1. Change 26.8.8.2 [push.heap] as indicated:

>    >     template<class RandomAccessIterator>
>    >       void push_heap(RandomAccessIterator first, RandomAccessIterator last);
>    >     template<class RandomAccessIterator, class Compare>
>    >       void push_heap(RandomAccessIterator first, RandomAccessIterator last,
>    >                      Compare comp);

>    > > -1- Requires: The range `[first, last - 1)` shall be a valid heap. `RandomAccessIterator` shall satisfy the requirements of `ValueSwappable` (16.4.4.3 [swappable.requirements]). The type of `*first` shall satisfy the `MoveConstructible` requirements (Table 23) and the `MoveAssignable` requirements (Table 25).

> 2. Change 26.8.8.4 [make.heap] as indicated:

>    >     template<class RandomAccessIterator>
>    >       void make_heap(RandomAccessIterator first, RandomAccessIterator last);
>    >     template<class RandomAccessIterator, class Compare>
>    >       void make_heap(RandomAccessIterator first, RandomAccessIterator last,
>    >                      Compare comp);

>    > > -1- Requires: `RandomAccessIterator` shall satisfy the requirements of `ValueSwappable` (16.4.4.3 [swappable.requirements]). The type of `*first` shall satisfy the `MoveConstructible` requirements (Table 23) and the `MoveAssignable` requirements (Table 25).

[2022-11-06; Daniel comments and syncs wording with recent working draft]

For reference, the finally accepted paper was P0202R3 and the constexpr-ification of swap-related algorithms had been realized later by P0879R0 after resolution of CWG 1581 and more importantly CWG 1330.

[Kona 2022-11-09; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Change 26.8.8.2 [push.heap] as indicated:

   >     template<class RandomAccessIterator>
   >       constexpr void push_heap(RandomAccessIterator first, RandomAccessIterator last);

   >     template<class RandomAccessIterator, class Compare>
   >       constexpr void push_heap(RandomAccessIterator first, RandomAccessIterator last,
   >                                Compare comp);

   >     template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
   >              class Proj = identity>
   >       requires sortable<I, Comp, Proj>
   >       constexpr I
   >         ranges::push_heap(I first, S last, Comp comp = {}, Proj proj = {});
   >     template<random_access_range R, class Comp = ranges::less, class Proj = identity>
   >       requires sortable<iterator_t<R>, Comp, Proj>
   >       constexpr borrowed_iterator_t<R>
   >         ranges::push_heap(R&& r, Comp comp = {}, Proj proj = {});

   > > -1- Let `comp` be `less{}` and `proj` be `identity{}` for the overloads with no parameters by those names.

   > > -2- Preconditions: The range `[first, last - 1)` is a valid heap with respect to `comp` and `proj`. For the overloads in namespace `std`, `RandomAccessIterator` meets the Cpp17ValueSwappable requirements (16.4.4.3 [swappable.requirements]) and the type of `*first` meets the Cpp17MoveConstructible requirements (Table 32) and the Cpp17MoveAssignable requirements (Table 34).

   > > -3- Effects: Places the value in the location `last - 1` into the resulting heap `[first, last)`.

   > > -4- Returns: `last` for the overloads in namespace `ranges`.

   > > -5- Complexity: At most log(`last - first`) comparisons and twice as many projections.

2. Change 26.8.8.4 [make.heap] as indicated:

   >     template<class RandomAccessIterator>
   >       constexpr void make_heap(RandomAccessIterator first, RandomAccessIterator last);

   >     template<class RandomAccessIterator, class Compare>
   >       constexpr void make_heap(RandomAccessIterator first, RandomAccessIterator last,
   >                                Compare comp);

   >     template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
   >              class Proj = identity>
   >       requires sortable<I, Comp, Proj>
   >       constexpr I
   >         ranges::make_heap(I first, S last, Comp comp = {}, Proj proj = {});
   >     template<random_access_range R, class Comp = ranges::less, class Proj = identity>
   >       requires sortable<iterator_t<R>, Comp, Proj>
   >       constexpr borrowed_iterator_t<R>
   >         ranges::make_heap(R&& r, Comp comp = {}, Proj proj = {});

   > > -1- Let `comp` be `less{}` and `proj` be `identity{}` for the overloads with no parameters by those names.

   > > -2- Preconditions: For the overloads in namespace `std`, `RandomAccessIterator` meets the Cpp17ValueSwappable requirements (16.4.4.3 [swappable.requirements]) and the type of `*first` meets the Cpp17MoveConstructible (Table 32) and Cpp17MoveAssignable (Table 34) requirements.

   > > -3- Effects: Constructs a heap with respect to `comp` and `proj` out of the range `[first, last)`.

   > > -4- Returns: `last` for the overloads in namespace `ranges`.

   > > -5- Complexity: At most 3(`last - first`) comparisons and twice as many projections.

### 3034(i). P0767R1 breaks previously-standard-layout types

Section: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Casey Carter Opened: 2017-11-12 Last modified: 2021-02-25

Priority: 0

View all other issues in [meta.trans.other].

View all issues with C++20 status.

Discussion:

P0767R1 "Expunge POD" changed the requirement for several library types from "POD" to "trivial." Since these types no longer provide/require the standard-layout portion of "POD," the change breaks:

- user classes with a member of such a type that were standard-layout in C++17
- implementations of `basic_string` and `basic_string_view` that expect character types to be both trivial and standard layout.

It appears this breakage was not intentional and not discussed in LWG.

The fix is straight-forward: apply an additional standard-layout requirement to the affected types:

- `max_align_t`
- the `type` member of specializations of `aligned_storage`
- the `type` member of specializations of `aligned_union`
- the char-like objects used by `basic_string` and `basic_string_view`.

(Albeit the potential for breakage with `max_align_t` is admittedly small.)

[ 2017-11-14 Moved to Tentatively Ready after 8 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4700 + P0767R1.

1. Change in 17.2.4 [support.types.layout] paragraph 5:

   The type `max_align_t` is a trivial standard-layout type whose alignment requirement is at least as great as that of every scalar type, and whose alignment requirement is supported in every context (6.8.3 [basic.align]).

2. Change the table in 21.3.8.7 [meta.trans.other] as indicated:

   `aligned_storage`
   The member typedef `type` shall be a trivial standard-layout type suitable for use as uninitialized storage for any object whose size is at most `Len` and whose alignment is a divisor of `Align`.

   `aligned_union`
   The member typedef `type` shall be a trivial standard-layout type suitable for use as uninitialized storage for any object whose type is listed in `Types`; its size shall be at least `Len`.

3. Change 27.1 [strings.general] paragraph 1 as indicated:

   This Clause describes components for manipulating sequences of any non-array trivial standard-layout (6.9 [basic.types]) type. Such types are called char-like types, and objects of char-like types are called char-like objects or simply characters.

### 3035(i). `std::allocator`'s constructors should be `constexpr`

Section: 20.2.10 [default.allocator] Status: C++20 Submitter: Geoffrey Romer Opened: 2017-11-11 Last modified: 2021-02-25

Priority: 0

View other active issues in [default.allocator].

View all other issues in [default.allocator].

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Discussion:

`std::allocator`'s constructors should be `constexpr`. It's expected to be an empty class as far as I know, so this should impose no implementation burden, and it would be useful to permit guaranteed static initialization of objects that need to hold a `std::allocator`, but don't have to actually use it until after construction.

[ 2017-11-25 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4700.

1. Change in 20.2.10 [default.allocator] as indicated:

   >     namespace std {
   >       template <class T> class allocator {
   >       public:
   >         using value_type = T;
   >         using propagate_on_container_move_assignment = true_type;
   >         using is_always_equal = true_type;
   >         constexpr allocator() noexcept;
   >         constexpr allocator(const allocator&) noexcept;
   >         constexpr template <class U> allocator(const allocator<U>&) noexcept;
   >         ~allocator();
   >         T* allocate(size_t n);
   >         void deallocate(T* p, size_t n);
   >       };
   >     }

### 3036(i). `polymorphic_allocator::destroy` is extraneous

Section: 20.5.3 [mem.poly.allocator.class] Status: C++23 Submitter: Casey Carter Opened: 2017-11-15 Last modified: 2023-11-22

Priority: 3

View all other issues in [mem.poly.allocator.class].

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Discussion:

`polymorphic_allocator`'s member function `destroy` is exactly equivalent to the default implementation of `destroy` in `allocator_traits` (20.2.9.3 [allocator.traits.members] para 6). It should be struck from `polymorphic_allocator` as it provides no value.

[28-Nov-2017 Mailing list discussion - set priority to P3]

PJ says that Dinkumware is shipping an implementation of `polymorphic_allocator` with `destroy`, so removing it would be a breaking change for him.

[2019-02; Kona Wednesday night issue processing]

Status to Open; revisit once P0339 lands. Poll taken was 5-3-2 in favor of removal.

[2020-10-05; Jonathan provides new wording]

Previous resolution [SUPERSEDED]:

> Wording relative to N4700.

> 1. Strike the declaration of `destroy` from the synopsis of class `polymorphic_allocator` in 20.5.3 [mem.poly.allocator.class]:

>    >     template <class T1, class T2, class U, class V>
>    >       void construct(pair<T1,T2>* p, pair<U, V>&& pr);

>    >     template <class T>
>    >       void destroy(T* p);

>    >     polymorphic_allocator select_on_container_copy_construction() const;

> 2. Strike the specification of `destroy` in 20.5.3.3 [mem.poly.allocator.mem]:

>    > […]

>    >     template <class T>
>    >       void destroy(T* p);

>    > 14 Effects: As if by `p->~T()`.

>    > […]

[2020-10-11; Reflector poll]

Moved to Tentatively Ready after seven votes in favour.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

Wording relative to N4861.

1. Strike the declaration of `destroy` from the synopsis of class `polymorphic_allocator` in 20.5.3 [mem.poly.allocator.class]:

   >     template <class T1, class T2, class U, class V>
   >       void construct(pair<T1,T2>* p, pair<U, V>&& pr);

   >     template <class T>
   >       void destroy(T* p);

   >     polymorphic_allocator select_on_container_copy_construction() const;

2. Adjust the specification of `delete_object` in 20.5.3.3 [mem.poly.allocator.mem]:

   >     template <class T>
   >       void delete_object(T* p);

   > -13- Effects: Equivalent to:

   > >       allocator_traits<polymorphic_allocator>::destroy(*this, p);
   > >       deallocate_object(p);

3. Strike the specification of `destroy` in 20.5.3.3 [mem.poly.allocator.mem]:

   > […]

   >     template <class T>
   >       void destroy(T* p);

   > -17- Effects: As if by `p->~T()`.

   > […]

4. Add a new subclause to Annex D:

   > D.?? Deprecated `polymorphic_allocator` member function

   > -1- The following member is declared in addition to those members specified in 20.5.3.3 [mem.poly.allocator.mem]:

   > >     namespace std::pmr {
   > >       template<class Tp = byte>
   > >       class polymorphic_allocator {
   > >       public:
   > >         template <class T>
   > >           void destroy(T* p);
   > >       };
   > >     }

   >     template <class T>
   >       void destroy(T* p);

   > -1- Effects: As if by `p->~T()`.

### 3037(i). `polymorphic_allocator` and incomplete types

Section: 20.5.3 [mem.poly.allocator.class] Status: C++20 Submitter: Casey Carter Opened: 2017-11-15 Last modified: 2021-02-25

Priority: 2

View all other issues in [mem.poly.allocator.class].

View all issues with C++20 status.

Discussion:

`polymorphic_allocator` can trivially support the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]) just as does the default allocator. Doing so imposes no implementation burden, and enables `pmr::forward_list`, `pmr::list`, and `pmr::vector` to support incomplete types as do the non-`pmr` equivalents.

[2018-01; Priority set to 2 after mailing list discussion]

[2018-08-23 Batavia Issues processing]

Status to Tentatively Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

Wording relative to N4700.

1. Add a new paragraph in 20.5.3 [mem.poly.allocator.class] after para 1:

   > 1 A specialization of class template `pmr::polymorphic_allocator` conforms to the `Allocator` requirements [...]

   > -?- All specializations of class template `pmr::polymorphic_allocator` satisfy the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]).

### 3038(i). `polymorphic_allocator::allocate` should not allow integer overflow to create vulnerabilities

Section: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Billy O'Neal III Opened: 2017-11-16 Last modified: 2021-02-25

Priority: 2

View all other issues in [mem.poly.allocator.mem].

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Discussion:

At the moment `polymorphic_allocator` is specified to do `sizeof(T) * n` directly; this may allow an attacker to cause this calculation to overflow, resulting in `allocate()` not meeting its postcondition of returning a buffer suitable to store `n` copies of `T`; this is a common bug described in CWE-190.

Making this into a saturating multiply should be sufficient to avoid this problem; any `memory_resource` underneath `polymorphic_allocator` is going to have to throw `bad_alloc` (or another exception) for a request of `SIZE_MAX`.

(There's also a minor editorial thing here that Returns should be Effects)

[2018-06 Rapperswil Thursday issues processing]

Consensus was that the overflow should be detected and an exception thrown rather than leaving that to the underlying memory resource. Billy to reword, and then get feedback on the reflector. Status to Open.

Previous resolution [SUPERSEDED]:

> Wording relative to N4700.

> 1. Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:

>    >     Tp* allocate(size_t n);

>    > > -1- ReturnsEffects: Equivalent to

>    > > >     return static_cast<Tp*>(memory_rsrc->allocate(SIZE_MAX / sizeof(Tp) < n ? SIZE_MAX : n * sizeof(Tp), alignof(Tp)));

[2018-08-23 Batavia Issues processing]

Status to Tentatively Ready with updated wording

Previous resolution [SUPERSEDED]:

> Wording relative to N4762.

> 1. Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:

>    >     Tp* allocate(size_t n);

>    > > -1- Effects: If `SIZE_MAX / sizeof(Tp) < n`, throws `length_error`, then Eequivalent to:

>    > > >     return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));

[2018-11, Adopted in San Diego]

Proposed resolution:

Wording relative to N4762.

1. Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:

   >     Tp* allocate(size_t n);

   > > -1- Effects: If `SIZE_MAX / sizeof(Tp) < n`, throws `length_error`. Otherwise Eequivalent to:

   > > >     return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));

### 3039(i). Unnecessary `decay` in `thread` and `packaged_task`

Section: 32.4.3.3 [thread.thread.constr], 32.10.10.2 [futures.task.members] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2017-11-17 Last modified: 2021-02-25

Priority: 0

View other active issues in [thread.thread.constr].

View all other issues in [thread.thread.constr].

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Discussion:

Following P0777R1 "Treating Unnecessary decay", more occurrences can be fixed. When constraints are checking for the same type as `thread` or a specialization of `packaged_task`, decaying functions to function pointers and arrays to object pointers can't affect the result.

[28-Nov-2017 Moved to Tentatively Ready after five positive votes on the ML.]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

Wording relative to N4700.

1. Edit 32.4.3.3 [thread.thread.constr] as indicated:

   >     template <class F, class... Args> explicit thread(F&& f, Args&&... args);

   > > -3- Requires: […]

   > > -4- Remarks: This constructor shall not participate in overload resolution if `decay_tremove_cvref_t<F>` is the same type as `std::thread`.

2. Edit 32.10.10.2 [futures.task.members] as indicated:

   >     template <class F>
   >       packaged_task(F&& f);

   > > -2- Requires: […]

   > > -3- Remarks: This constructor shall not participate in overload resolution if `decay_tremove_cvref_t<F>` is the same type as `packaged_task<R(ArgTypes...)>`.

### 3040(i). `basic_string_view::starts_with` Effects are incorrect

Section: 27.3.3.8 [string.view.ops] Status: C++20 Submitter: Marshall Clow Opened: 2017-11-29 Last modified: 2021-02-25

Priority: 0

View all other issues in [string.view.ops].

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Discussion:

The effects of `starts_with` are described as equivalent to `return compare(0, npos, x) == 0`.

This is incorrect, because it returns `false` when you check to see if any sequence begins with the empty sequence. (There are other failure cases, but that one's easy)

As a drive-by fix, we can make the Effects: for `starts_with` and `ends_with` clearer.

Those are the second and proposed third changes, and they are not required.

[ 2017-12-13 Moved to Tentatively Ready after 8 positive votes for P0 on c++std-lib. ]

Previous resolution: [SUPERSEDED]

> This wording is relative to N4713.

> 1. Change 27.3.3.8 [string.view.ops] p20 as indicated:

>    >     constexpr bool starts_with(basic_string_view x) const noexcept;

>    > > -20- Effects: Equivalent to: `return size() >= x.size() && compare(0, nposx.size(), x) == 0;`

> 2. Change 27.3.3.8 [string.view.ops] p21 as indicated:

>    >     constexpr bool starts_with(charT x) const noexcept;

>    > > -21- Effects: Equivalent to: `return !empty() && traits::eq(front(), x)starts_with(basic_string_view(&x, 1));`

> 3. Change 27.3.3.8 [string.view.ops] p24 as indicated:

>    >     constexpr bool ends_with(charT x) const noexcept;

>    > > -24- Effects: Equivalent to: `return !empty() && traits::eq(back(), x)ends_with(basic_string_view(&x, 1));`

[2018-01-23, Reopening due to a comment of Billy Robert O'Neal III requesting a change of the proposed wording]

The currently suggested wording has:

> Effects: Equivalent to: `return size() >= x.size() && compare(0, x.size(), x) == 0;`

but `compare()` already does the `size() >= x.size()` check.

It seems like it should say:

> Effects: Equivalent to: `return substr(0, x.size()) == x;`

[ 2018-10-29 Moved to Tentatively Ready after 5 positive votes for P0 on c++std-lib. ]

Proposed resolution:

This wording is relative to N4713.

1. Change 27.3.3.8 [string.view.ops] p20 as indicated:

   >     constexpr bool starts_with(basic_string_view x) const noexcept;

   > > -20- Effects: Equivalent to: `return substr(0, x.size()) == xcompare(0, npos, x) == 0;`

2. Change 27.3.3.8 [string.view.ops] p21 as indicated:

   >     constexpr bool starts_with(charT x) const noexcept;

   > > -21- Effects: Equivalent to: `return !empty() && traits::eq(front(), x)starts_with(basic_string_view(&x, 1));`

3. Change 27.3.3.8 [string.view.ops] p24 as indicated:

   >     constexpr bool ends_with(charT x) const noexcept;

   > > -24- Effects: Equivalent to: `return !empty() && traits::eq(back(), x)ends_with(basic_string_view(&x, 1));`

### 3041(i). Unnecessary `decay` in `reference_wrapper`

Section: 22.10.6.2 [refwrap.const] Status: C++20 Submitter: Agustín K-ballo Bergé Opened: 2017-12-04 Last modified: 2021-02-25

Priority: 0

View all other issues in [refwrap.const].

View all issues with C++20 status.

Discussion:

Another occurrence of unnecessary decay (P0777) was introduced by the resolution of LWG 2993(i). A decayed function/array type will never be the same type as `reference_wrapper`.

[ 2018-01-09 Moved to Tentatively Ready after 9 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Change 22.10.6.2 [refwrap.const] as indicated:

   >     template<class U>
   >     reference_wrapper(U&& u) noexcept(see below);

   > > -1- Remarks: Let `FUN` denote the exposition-only functions

   > >     void FUN(T&) noexcept;
   > >     void FUN(T&&) = delete;

   > > This constructor shall not participate in overload resolution unless the expression `FUN(declval<U>())` is well-formed and `is_same_v<decay_tremove_cvref_t<U>, reference_wrapper>` is `false`. The expression inside `noexcept` is equivalent to `noexcept(FUN(declval<U>()))`.

### 3042(i). `is_literal_type_v` should be inline

Section: D.13 [depr.meta.types] Status: C++20 Submitter: Tim Song Opened: 2017-12-06 Last modified: 2021-02-25

Priority: 0

View all other issues in [depr.meta.types].

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Discussion:

P0607R0 forgot to look at D.13 [depr.meta.types] and make `is_literal_type_v` inline.

[ 2018-01-08 Moved to Tentatively Ready after 8 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Change D.13 [depr.meta.types]p1 as indicated:

   -1- The header `<type_traits>` has the following addition:

   >     namespace std {
   >       template<class T> struct is_literal_type;
   >       template<class T> inline constexpr bool is_literal_type_v = is_literal_type<T>::value;

   >       template<class> struct result_of;    // not defined
   >       template<class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;
   >       template<class T> using result_of_t = typename result_of<T>::type;

   >       template<class T> struct is_pod;
   >       template<class T> inline constexpr bool is_pod_v = is_pod<T>::value;
   >     }

### 3043(i). Bogus postcondition for `filesystem_error` constructor

Section: 31.12.7.2 [fs.filesystem.error.members] Status: C++20 Submitter: Tim Song Opened: 2017-12-06 Last modified: 2021-06-06

Priority: 0

View all other issues in [fs.filesystem.error.members].

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Discussion:

[fs.filesystem_error.members] says that constructors of `filesystem_error` have the postcondition that `runtime_error::what()` has the value `what_arg.c_str()`. That's obviously incorrect: these are pointers to distinct copies of the string in any sane implementation and cannot possibly compare equal.

The requirement seems suspect for a further reason: it mandates the content of the string returned by `runtime_error::what()`, but `filesystem_error` has no direct control over the construction of its indirect non-virtual base class `runtime_error`. Instead, what is passed to `runtime_error`'s constructor is determined by `system_error`'s constructor, which in many implementations is an eagerly crafted error string. This is permitted by the specification of `system_error` (see 19.5.8 [syserr.syserr]) but would make the requirement unimplementable.

The proposed wording below adjusts the postcondition using the formula of `system_error`'s constructor. As an editorial change, it also replaces the postcondition tables with normal postcondition clauses, in the spirit of editorial issue 1875.

[ 2018-01-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Replace [fs.filesystem_error.members] p2-4, including Tables 119 through 121, with the following:

       filesystem_error(const string& what_arg, error_code ec);

   > -2- Postconditions: `code() == ec`, `path1().empty() == true`, `path2().empty() == true`, and `string_view(what()).find(what_arg) != string_view::npos`.

       filesystem_error(const string& what_arg, const path& p1, error_code ec);

   > -3- Postconditions: `code() == ec`, `path1()` returns a reference to the stored copy of `p1`, `path2().empty() == true`, and `string_view(what()).find(what_arg) != string_view::npos`.

       filesystem_error(const string& what_arg, const path& p1, const path& p2, error_code ec);

   > -4- Postconditions: `code() == ec`, `path1()` returns a reference to the stored copy of `p1`, `path2()` returns a reference to the stored copy of `p2`, and `string_view(what()).find(what_arg) != string_view::npos`.

2. Edit [fs.filesystem_error.members] p7 as indicated:

       const char* what() const noexcept override;

   > -7- Returns: A string containing `runtime_error::what()`.An ntbs that incorporates the `what_arg` argument supplied to the constructor. The exact format is unspecified. Implementations should include the `system_error::what()` string and the pathnames of `path1` and `path2` in the native format in the returned string.

### 3045(i). `atomic<floating-point>` doesn't have `value_type` or `difference_type`

Section: 32.5.8.4 [atomics.types.float] Status: C++20 Submitter: Tim Song Opened: 2017-12-11 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

The `atomic<floating-point>` specialization doesn't have the `value_type` and `difference_type` member typedefs, making it unusable with most of the nonmember function templates. This doesn't seem to be the intent.

[ 2018-01-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Edit 32.5.8.4 [atomics.types.float] after p1, class template specialization `atomic<floating-point>` synopsis, as indicated:

         namespace std {
           template<> struct atomic<floating-point> {
             using value_type = floating-point;
             using difference_type = value_type;
             static constexpr bool is_always_lock_free = implementation-defined;
             […]
           };
         }

### 3048(i). `transform_reduce(exec, first1, last1, first2, init)` discards execution policy

Section: 26.10.6 [transform.reduce] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-12-15 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

Since there exists only one common Effects element for both the parallel and the non-parallel form of `transform_reduce` without explicit operation parameters, the current specification of a function call `std::transform_reduce(exec, first1, last1, first2, init)` has the same effect as if the `ExecutionPolicy` would have been ignored. Presumably this effect is unintended.

[ 2018-01-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Modify 26.10.6 [transform.reduce] as indicated:

   >     template<class InputIterator1, class InputIterator2, class T>
   >       T transform_reduce(InputIterator1 first1, InputIterator1 last1,
   >                          InputIterator2 first2,
   >                          T init);

   > > -?- Effects: Equivalent to:

   > > >     return transform_reduce(first1, last1, first2, init, plus<>(), multiplies<>());

   >     template<class ExecutionPolicy,
   >              class ForwardIterator1, class ForwardIterator2, class T>
   >       T transform_reduce(ExecutionPolicy&& exec,
   >                          ForwardIterator1 first1, ForwardIterator1 last1,
   >                          ForwardIterator2 first2,
   >                          T init);

   > > -1- Effects: Equivalent to:

   > > >     return transform_reduce(std::forward<ExecutionPolicy>(exec), first1, last1, first2, init, plus<>(),
   > > >     multiplies<>());

### 3050(i). Conversion specification problem in `chrono::duration` constructor

Section: 30.5.2 [time.duration.cons], 30.5.6 [time.duration.nonmember] Status: C++20 Submitter: Barry Revzin Opened: 2018-01-22 Last modified: 2021-02-25

Priority: 3

View all other issues in [time.duration.cons].

View all issues with C++20 status.

Discussion:

The converting constructor in `std::chrono::duration` is currently specified as (30.5.2 [time.duration.cons] p1):

>     template<class Rep2>
>       constexpr explicit duration(const Rep2& r);

> > Remarks: This constructor shall not participate in overload resolution unless `Rep2` is implicitly convertible to `rep` and […]

But the parameter is of type `Rep2 const`, so we should check that `Rep2 const` is implicitly convertible to `rep`, not just `Rep2`. This means that for a type like:

>     struct X { operator int64_t() /* not const */; };

`std::is_constructible_v<std::chrono::seconds, X>` is `true`, but actual construction will fail to compile.

Further analysis of sub-clause 30.5 [time.duration] revealed similar specification problems in some other functions.

[2018-06 Rapperswil Thursday issues processing]

P3; Status to Open

[2018-11 San Diego Thursday night issue processing]

Jonathan to provide updated wording; the underlying text has changed.

[2018-12-05 Jonathan provides new wording]

In San Diego Geoff noticed that the current WP does not use `CR`. Jonathan provides new wording consistent with the editorial changes that removed `CR`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4713.

> 1. Modify 30.5.2 [time.duration.cons] as indicated:

>    >     template<class Rep2>
>    >       constexpr explicit duration(const Rep2& r);

>    > > -1- Remarks: This constructor shall not participate in overload resolution unless `is_convertible_v<const Rep2&, rep>` is `true` `Rep2` is implicitly convertible to `rep` and

>    > > 1. (1.1) — `treat_as_floating_point_v<rep>` is `true` or

>    > > 2. (1.2) — `treat_as_floating_point_v<Rep2>` is `false`.

>    > > […]

>    > > -2- Effects: Constructs an object of type `duration`.

>    > > -3- Postconditions: `count() == static_cast<rep>(r)`.

> 2. Modify 30.5.6 [time.duration.nonmember] as indicated:

>    >     template<class Rep1, class Period, class Rep2>
>    >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
>    >         operator*(const duration<Rep1, Period>& d, const Rep2& s);

>    > > -4- Remarks: This operator shall not participate in overload resolution unless `is_convertible_v<const Rep2&, CR(Rep1, Rep2)>` is `true` `Rep2` is implicitly convertible to `CR(Rep1, Rep2)`.

>    > > […]

>    >     template<class Rep1, class Rep2, class Period>
>    >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
>    >         operator*(const Rep1& s, const duration<Rep2, Period>& d);

>    > > -6- Remarks: This operator shall not participate in overload resolution unless `is_convertible_v<const Rep1&, CR(Rep1, Rep2)>` is `true` `Rep1` is implicitly convertible to `CR(Rep1, Rep2)`.

>    > > […]

>    >     template<class Rep1, class Period, class Rep2>
>    >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
>    >         operator/(const duration<Rep1, Period>& d, const Rep2& s);

>    > > -8- Remarks: This operator shall not participate in overload resolution unless `is_convertible_v<const Rep2&, CR(Rep1, Rep2)>` is `true` `Rep2` is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` is not a specialization of `duration`.

>    > […]

>    >     template<class Rep1, class Period, class Rep2>
>    >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
>    >         operator%(const duration<Rep1, Period>& d, const Rep2& s);

>    > > -11- Remarks: This operator shall not participate in overload resolution unless `is_convertible_v<const Rep2&, CR(Rep1, Rep2)>` is `true` `Rep2` is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` is not a specialization of `duration`.

>    > > […]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4791.

> 1. Modify 30.5.2 [time.duration.cons] as indicated:

>    >     template<class Rep2>
>    >       constexpr explicit duration(const Rep2& r);

>    > > -1- Remarks: This constructor shall not participate in overload resolution unless `is_convertible_v<const Rep2&, rep>` is `true` `Rep2` is implicitly convertible to `rep` and

>    > > 1. (1.1) — `treat_as_floating_point_v<rep>` is `true` or

>    > > 2. (1.2) — `treat_as_floating_point_v<Rep2>` is `false`.

>    > > […]

>    > > -2- Effects: Constructs an object of type `duration`.

>    > > -3- Postconditions: `count() == static_cast<rep>(r)`.

> 2. Modify 30.5.6 [time.duration.nonmember] as indicated:

>    >     template<class Rep1, class Period, class Rep2>
>    >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
>    >         operator*(const duration<Rep1, Period>& d, const Rep2& s);

>    > > -4- Remarks: This operator shall not participate in overload resolution unless `is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>` is `true` `Rep2` is implicitly convertible to `common_type_t<Rep1, Rep2>`.

>    > > […]

>    >     template<class Rep1, class Rep2, class Period>
>    >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
>    >         operator*(const Rep1& s, const duration<Rep2, Period>& d);

>    > > -6- Remarks: This operator shall not participate in overload resolution unless `is_convertible_v<const Rep1&, common_type_t<Rep1, Rep2>>` is `true` `Rep1` is implicitly convertible to `common_type_t<Rep1, Rep2>`.

>    > > […]

>    >     template<class Rep1, class Period, class Rep2>
>    >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
>    >         operator/(const duration<Rep1, Period>& d, const Rep2& s);

>    > > -8- Remarks: This operator shall not participate in overload resolution unless `is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>` is `true` `Rep2` is implicitly convertible to `common_type_t<Rep1, Rep2>` and `Rep2` is not a specialization of `duration`.

>    > […]

>    >     template<class Rep1, class Period, class Rep2>
>    >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
>    >         operator%(const duration<Rep1, Period>& d, const Rep2& s);

>    > > -11- Remarks: This operator shall not participate in overload resolution unless `is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>` is `true` `Rep2` is implicitly convertible to `common_type_t<Rep1, Rep2>` and `Rep2` is not a specialization of `duration`.

>    > > […]

[2020-02-13, Prague]

Rebase to most recent working draft

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 30.5.2 [time.duration.cons] as indicated:

   >     template<class Rep2>
   >       constexpr explicit duration(const Rep2& r);

   > > -1- Constraints: `is_convertible_v<const Rep2&, rep>` is `true` and

   > > 1. (1.1) — `treat_as_floating_point_v<rep>` is `true` or

   > > 2. (1.2) — `treat_as_floating_point_v<Rep2>` is `false`.

   > > […]

   > > -2- Postconditions: `count() == static_cast<rep>(r)`.

2. Modify 30.5.6 [time.duration.nonmember] as indicated:

   >     template<class Rep1, class Period, class Rep2>
   >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
   >         operator*(const duration<Rep1, Period>& d, const Rep2& s);

   > > -4- Constraints: `is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>` is `true`.

   > > […]

   >     template<class Rep1, class Rep2, class Period>
   >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
   >         operator*(const Rep1& s, const duration<Rep2, Period>& d);

   > > -6- Constraints: `is_convertible_v<const Rep1&, common_type_t<Rep1, Rep2>>` is `true`.

   > > […]

   >     template<class Rep1, class Period, class Rep2>
   >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
   >         operator/(const duration<Rep1, Period>& d, const Rep2& s);

   > > -8- Constraints: `is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>` is `true` and `Rep2` is not a specialization of `duration`.

   > […]

   >     template<class Rep1, class Period, class Rep2>
   >       constexpr duration<common_type_t<Rep1, Rep2>, Period>
   >         operator%(const duration<Rep1, Period>& d, const Rep2& s);

   > > -12- Constraints: `is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>` is `true` and `Rep2` is not a specialization of `duration`.

   > > […]

### 3051(i). Floating point classifications were inadvertently changed in P0175

Section: 29.7.1 [cmath.syn] Status: C++20 Submitter: Thomas Köppe Opened: 2018-01-23 Last modified: 2021-02-25

Priority: 0

View other active issues in [cmath.syn].

View all other issues in [cmath.syn].

View all issues with C++20 status.

Discussion:

The paper P0175 was meant to be a purely editorial change to spell out the synopses of the "C library" headers. However it contained the following inadvertent normative change: The floating point classification functions `isinf`, `isfinite`, `signbit`, etc. (but not `fpclassify`) used to be specified to return "`bool`" in C++14. In C, those are macros, but in C++ they have always been functions. During the preparation of P0175, I recreated the function signatures copying the return type "`int`" from C, but failed to notice that we had already specified those functions differently, so the return type was changed to "`int`".

To restore the intended specification, we should change the return types of all the `is...` and `signbit` classification functions back to `bool`. Alternatively, we could decide that the return type should actually be `int`, but that would be a larger discussion.

Proposed resolution for restoring the original wording: Replace return type "`int`" with return type "`bool`" and for all the classification/comparison functions after "// classification/comparison functions" in [cmath.syn] except the `fpclassify` functions.

Related previous issue was LWG 1327(i) and the corresponding NB comment US-136 resolution.

[ 2018-01-29 Moved to Tentatively Ready after 8 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Modify 29.7.1 [cmath.syn] as indicated:

   >     […]
   >     namespace std {
   >       […]
   >       // 29.7.5 [c.math.fpclass], classification / comparison functions
   >       int fpclassify(float x);
   >       int fpclassify(double x);
   >       int fpclassify(long double x);
   >       boolint isfinite(float x);
   >       boolint isfinite(double x);
   >       boolint isfinite(long double x);
   >       boolint isinf(float x);
   >       boolint isinf(double x);
   >       boolint isinf(long double x);
   >       boolint isnan(float x);
   >       boolint isnan(double x);
   >       boolint isnan(long double x);
   >       boolint isnormal(float x);
   >       boolint isnormal(double x);
   >       boolint isnormal(long double x);
   >       boolint signbit(float x);
   >       boolint signbit(double x);
   >       boolint signbit(long double x);
   >       boolint isgreater(float x, float y);
   >       boolint isgreater(double x, double y);
   >       boolint isgreater(long double x, long double y);
   >       boolint isgreaterequal(float x, float y);
   >       boolint isgreaterequal(double x, double y);
   >       boolint isgreaterequal(long double x, long double y);
   >       boolint isless(float x, float y);
   >       boolint isless(double x, double y);
   >       boolint isless(long double x, long double y);
   >       boolint islessequal(float x, float y);
   >       boolint islessequal(double x, double y);
   >       boolint islessequal(long double x, long double y);
   >       boolint islessgreater(float x, float y);
   >       boolint islessgreater(double x, double y);
   >       boolint islessgreater(long double x, long double y);
   >       boolint isunordered(float x, float y);
   >       boolint isunordered(double x, double y);
   >       boolint isunordered(long double x, long double y);
   >       […]
   >     }

### 3052(i). `visit` is underconstrained

Section: 22.6.7 [variant.visit] Status: Resolved Submitter: Casey Carter Opened: 2018-01-23 Last modified: 2021-05-18

Priority: 2

View all other issues in [variant.visit].

View all issues with Resolved status.

Discussion:

`std::visit` accepts a parameter pack of forwarding references named `vars` whose types are the parameter pack `Variants`. Despite that:

1. the names of both packs are variants of "variant",
2. para 1 passes the types in `Variants` (modified) to `variant_size_v`,
3. para 3 uses the expression `varsi.index()`,
4. para 4 says "...if any `variant` in `vars` is `valueless_by_exception`, and
5. para 5 mentions "..the number of alternative types of `Variants0`"

the Requires element imposes no explicit requirements on the types in `Variants`. Notably, the `Variants` are not required to be `variant`s. This lack of constraints appears to be simply an oversight.

[2018-01-24, Daniel comments]

This issue should be reviewed in common with LWG 2970(i).

[2018-06-18 after reflector discussion]

Priority set to 2; status to LEWG

[2020-11-18; this will be resolved by P2162.]

[2021-04-19 P2162R2 was adopted at February 2021 plenary. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

This wording is relative to N4727.

1. Modify 22.6.7 [variant.visit] as indicated:

   >     template<class Visitor, class... Variants>
   >       constexpr see below visit(Visitor&& vis, Variants&&... vars);

   > > […]

   > > -4- Throws: `bad_variant_access` if any `variant` in `vars` is `valueless_by_exception()``(vars.valueless_by_exception() || ...)` is `true`.

   > > -5- Complexity: […]

   > > -?- Remarks: This function shall not participate in overload resolution unless `remove_cvref_t<Variantsi>` is a specialization of `variant` for all `0 <= i < n`.

### 3054(i). `uninitialized_copy` appears to not be able to meet its exception-safety guarantee

Section: 26.11.5 [uninitialized.copy] Status: C++20 Submitter: Jon Cohen Opened: 2018-01-24 Last modified: 2021-02-25

Priority: 2

View all other issues in [uninitialized.copy].

View all issues with C++20 status.

Discussion:

I believe that `uninitialized_copy` is unable to meet its exception-safety guarantee in the presence of throwing move constructors:

26.11 [specialized.algorithms]/1 has two statements of note for the specialized algorithms such as `uninitialized_copy`:

- the provided iterators satisfy the `InputIterator` requirements (24.3.5.3 [input.iterators])

- if an exception is thrown during the algorithm then there are no effects

Suppose we have an input iterator `Iter`. Then `std::move_iterator<Iter>` appears to also be an input iterator. Notably, it still satisfies that `(void)*a, *a` is equivalent to `*a` for move iterator `a` since the dereference only forms an rvalue reference, it doesn't actually perform the move operation (24.3.5.3 [input.iterators] Table 95 — "Input iterator requirements").

Suppose also that we have a type `T` whose move constructor can throw, a range of `T`'s `[tbegin, tend)`, and a pointer to an uninitialized buffer of `T`'s `buf`. Then `std::uninitialized_copy(std::make_move_iterator(tbegin), std::make_move_iterator(tend), buf)` can't possibly satisfy the property that it has no effects if one of the moves throws — we'll have a `T` left in a moved-from state with no way of recovering.

See here for an example in code.

It seems like the correct specification for `uninitialized_copy` should be that if `InputIterator`'s `operator*` returns an rvalue reference and `InputIterator::value_type`'s move constructor is not marked `noexcept`, then `uninitialized_copy` will leave the objects in the underlying range in a valid but unspecified state.

[2018-01-24, Casey comments and provides wording]

This issue points out a particular hole in the "..if an exception is thrown in the following algorithms there are no effects." wording for the "uninitialized" memory algorithms (26.11 [specialized.algorithms]/1) and suggests a PR to patch over said hole. The true problem here is that "no effects" is not and never has been implementable. For example, "`first != last`" may have observable effects that an implementation is required to somehow reverse if some later operation throws an exception.

Rather than finding problem case after problem case and applying individual patches, we should fix the root cause. If we alter the problematic sentence from [specialized.algorithms]/1 we can fix the issue once and for all and have implementable algorithms.

[2018-02-05, Priority set to 2 after mailing list discussion]

[2018-06 Rapperswil Thursday issues processing]

Status to Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4713.

1. Modify 26.11 [specialized.algorithms] as indicated:

   > -1- […]

   > Unless otherwise specified, if an exception is thrown in the following algorithms objects constructed by a placement new-expression (7.6.2.8 [expr.new]) are destroyed in an unspecified order before allowing the exception to propagatethere are no effects.

2. Modify 26.11.6 [uninitialized.move] as indicated (The removed paragraphs are now unnecessary):

   >     template<class InputIterator, class ForwardIterator>
   >       ForwardIterator uninitialized_move(InputIterator first, InputIterator last,
   >                                          ForwardIterator result);

   > > […]

   > > -2- Remarks: If an exception is thrown, some objects in the range `[first, last)` are left in a valid but unspecified state.

   >     template<class InputIterator, class Size, class ForwardIterator>
   >       pair<InputIterator, ForwardIterator>
   >         uninitialized_move_n(InputIterator first, Size n, ForwardIterator result);

   > > […]

   > > -4- Remarks: If an exception is thrown, some objects in the range `[first, std::next(first, n))` are left in a valid but unspecified state.

### 3055(i). `path::operator+=(single-character)` misspecified

Section: 31.12.6.5.4 [fs.path.concat] Status: C++20 Submitter: Tim Song Opened: 2018-01-24 Last modified: 2021-02-25

Priority: 3

View all other issues in [fs.path.concat].

View all issues with C++20 status.

Discussion:

31.12.6.5.4 [fs.path.concat] uses the expression `path(x).native()` to specify the effects of concatenating a single character `x`. However, there is no `path` constructor taking a single character.

[2018-06-18 after reflector discussion]

Priority set to 3

[2018-10-12 Tim updates PR to avoid suggesting the creation of a temporary `path`.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4713.

> 1. Modify 31.12.6.5.4 [fs.path.concat] as indicated:

>    >     path& operator+=(const path& x);
>    >     path& operator+=(const string_type& x);
>    >     path& operator+=(basic_string_view<value_type> x);
>    >     path& operator+=(const value_type* x);
>    >     path& operator+=(value_type x);
>    >     template<class Source>
>    >       path& operator+=(const Source& x);
>    >     template<class EcharT>
>    >       path& operator+=(EcharT x);
>    >     template<class Source>
>    >       path& concat(const Source& x);

>    > > -1- Effects: […]

>    > > -2- Returns: `*this`.

>    >     path& operator+=(value_type x);
>    >     template<class EcharT>
>    >       path& operator+=(EcharT x);

>    > > -?- Effects: Equivalent to: `return *this += path(&x, &x + 1);`.

[2019-02; Kona Wednesday night issue processing]

Status to Ready

Proposed resolution:

This wording is relative to N4762.

1. Modify 31.12.6.5.4 [fs.path.concat] as indicated:

   >     path& operator+=(const path& x);
   >     path& operator+=(const string_type& x);
   >     path& operator+=(basic_string_view<value_type> x);
   >     path& operator+=(const value_type* x);
   >     path& operator+=(value_type x);
   >     template<class Source>
   >       path& operator+=(const Source& x);
   >     template<class EcharT>
   >       path& operator+=(EcharT x);
   >     template<class Source>
   >       path& concat(const Source& x);

   > > -1- Effects: […]

   > > -2- Returns: `*this`.

   >     path& operator+=(value_type x);
   >     template<class EcharT>
   >       path& operator+=(EcharT x);

   > > -?- Effects: Equivalent to: `return *this += basic_string_view(&x, 1);`

### 3058(i). Parallel `adjacent_difference` shouldn't require creating temporaries

Section: 26.10.12 [adjacent.difference] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-02-02 Last modified: 2021-02-25

Priority: 3

View all other issues in [adjacent.difference].

View all issues with C++20 status.

Discussion:

Parallel `adjacent_difference` is presently specified to "create a temporary object whose type is `ForwardIterator1`'s value type". Serial `adjacent_difference` does that because it needs to work with input iterators, and needs to work when the destination range exactly overlaps the input range. The parallel version requires forward iterators and doesn't allow overlap, so it can avoid making these temporaries.

[2018-02-13, Priority set to 3 after mailing list discussion]

[2018-3-14 Wednesday evening issues processing; remove 'const' before `minus` and move to Ready.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4713.

> 1. Modify 26.10.12 [adjacent.difference] as indicated:

>    >     template<class InputIterator, class OutputIterator>
>    >       OutputIterator
>    >         adjacent_difference(InputIterator first, InputIterator last, OutputIterator result);
>    >     template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2>
>    >       ForwardIterator2
>    >         adjacent_difference(ExecutionPolicy&& exec,
>    >                             ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result);

>    >     template<class InputIterator, class OutputIterator, class BinaryOperation>
>    >       OutputIterator
>    >         adjacent_difference(InputIterator first, InputIterator last,
>    >                             OutputIterator result, BinaryOperation binary_op);
>    >     template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2,
>    >              class BinaryOperation>
>    >       ForwardIterator2
>    >         adjacent_difference(ExecutionPolicy&& exec,
>    >                             ForwardIterator1 first, ForwardIterator1 last,
>    >                             ForwardIterator2 result, BinaryOperation binary_op);

>    > > -?- Let `T` be the value type of `decltype(first)`. For the overloads that do not take an argument `binary_op`, let `binary_op` be an lvalue that denotes an object of type `const minus<>`.

>    > > -1- Requires:

>    > > 1. (1.1) — For the overloads with no `ExecutionPolicy`, `InputIterator`’s value type `T` shall be `MoveAssignable` (Table 25) and shall be constructible from the type of `*first`. `acc` (defined below) shall be writable (24.3.1 [iterator.requirements.general]) to the `result` output iterator. The result of the expression `val - std::move(acc)` or `binary_op(val, std::move(acc))` shall be writable to the `result` output iterator.

>    > > 2. (1.2) — For the overloads with an `ExecutionPolicy`, the value type of `ForwardIterator1` shall be `CopyConstructible` (Table 24), constructible from the expression `*first - *first` or `binary_op(*first, *first)`, and assignable to the value type of `ForwardIterator2` result of the expressions `binary_op(*first, *first)` and `*first` shall be writable to `result`.

>    > > 3. (1.3) — […]

>    > > -2- Effects: For the overloads with no `ExecutionPolicy` and a non-empty range, the function creates an accumulator `acc` whose type is `InputIterator`’s value type of type `T`, initializes it with `*first`, and assigns the result to `*result`. For every iterator `i` in `[first + 1, last)` in order, creates an object `val` whose type is `InputIterator`’s value type `T`, initializes it with `*i`, computes `val - std::move(acc)` or `binary_op(val, std::move(acc))`, assigns the result to `*(result + (i - first))`, and move assigns from `val` to `acc`.

>    > > -3- For the overloads with an `ExecutionPolicy` and a non-empty range, first the function creates an object whose type is `ForwardIterator1`'s value type, initializes it with `*first`, and assigns the result to `*result`. Then for every `d` in `[1, last - first - 1]`, creates an object `val` whose type is `ForwardIterator1`'s value type, initializes it with `*(first + d) - *(first + d - 1)` or `binary_op(*(first + d), *(first + d - 1))`, and assigns the result to `*(result + d)`performs `*result = *first`. Then, for every `d` in `[1, last - first - 1]`, performs `*(result + d) = binary_op(*(first + d), *(first + (d - 1)))`.

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4713.

1. Modify 26.10.12 [adjacent.difference] as indicated:

   >     template<class InputIterator, class OutputIterator>
   >       OutputIterator
   >         adjacent_difference(InputIterator first, InputIterator last, OutputIterator result);
   >     template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2>
   >       ForwardIterator2
   >         adjacent_difference(ExecutionPolicy&& exec,
   >                             ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result);

   >     template<class InputIterator, class OutputIterator, class BinaryOperation>
   >       OutputIterator
   >         adjacent_difference(InputIterator first, InputIterator last,
   >                             OutputIterator result, BinaryOperation binary_op);
   >     template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2,
   >              class BinaryOperation>
   >       ForwardIterator2
   >         adjacent_difference(ExecutionPolicy&& exec,
   >                             ForwardIterator1 first, ForwardIterator1 last,
   >                             ForwardIterator2 result, BinaryOperation binary_op);

   > > -?- Let `T` be the value type of `decltype(first)`. For the overloads that do not take an argument `binary_op`, let `binary_op` be an lvalue that denotes an object of type `minus<>`.

   > > -1- Requires:

   > > 1. (1.1) — For the overloads with no `ExecutionPolicy`, `InputIterator`’s value type `T` shall be `MoveAssignable` (Table 25) and shall be constructible from the type of `*first`. `acc` (defined below) shall be writable (24.3.1 [iterator.requirements.general]) to the `result` output iterator. The result of the expression `val - std::move(acc)` or `binary_op(val, std::move(acc))` shall be writable to the `result` output iterator.

   > > 2. (1.2) — For the overloads with an `ExecutionPolicy`, the value type of `ForwardIterator1` shall be `CopyConstructible` (Table 24), constructible from the expression `*first - *first` or `binary_op(*first, *first)`, and assignable to the value type of `ForwardIterator2` result of the expressions `binary_op(*first, *first)` and `*first` shall be writable to `result`.

   > > 3. (1.3) — […]

   > > -2- Effects: For the overloads with no `ExecutionPolicy` and a non-empty range, the function creates an accumulator `acc` whose type is `InputIterator`’s value type of type `T`, initializes it with `*first`, and assigns the result to `*result`. For every iterator `i` in `[first + 1, last)` in order, creates an object `val` whose type is `InputIterator`’s value type `T`, initializes it with `*i`, computes `val - std::move(acc)` or `binary_op(val, std::move(acc))`, assigns the result to `*(result + (i - first))`, and move assigns from `val` to `acc`.

   > > -3- For the overloads with an `ExecutionPolicy` and a non-empty range, first the function creates an object whose type is `ForwardIterator1`'s value type, initializes it with `*first`, and assigns the result to `*result`. Then for every `d` in `[1, last - first - 1]`, creates an object `val` whose type is `ForwardIterator1`'s value type, initializes it with `*(first + d) - *(first + d - 1)` or `binary_op(*(first + d), *(first + d - 1))`, and assigns the result to `*(result + d)`performs `*result = *first`. Then, for every `d` in `[1, last - first - 1]`, performs `*(result + d) = binary_op(*(first + d), *(first + (d - 1)))`.

### 3061(i). What is the return type of `compare_3way`?

Section: 26.8.12 [alg.three.way] Status: Resolved Submitter: Richard Smith Opened: 2018-02-07 Last modified: 2020-09-06

Priority: 2

View all other issues in [alg.three.way].

View all issues with Resolved status.

Discussion:

The P0768R1 specification of compare_3way says:

>     template<class T, class U> constexpr auto compare_3way(const T& a, const U& b);

> > -1- Effects: Compares two values and produces a result of the strongest applicable comparison category type:

> > 1. (1.1) — Returns `a <=> b` if that expression is well-formed.

> > 2. (1.2) — Otherwise, if the expressions `a == b` and `a < b` are each well-formed and convertible to `bool`, returns `strong_ordering::equal` when `a == b` is `true`, otherwise returns `strong_ordering::less` when `a < b` is `true`, and otherwise returns `strong_ordering::greater`.

> > 3. (1.3) — Otherwise, if the expression `a == b` is well-formed and convertible to `bool`, returns `strong_equality::equal` when `a == b` is `true`, and otherwise returns `strong_equality::nonequal`.

> > 4. (1.4) — Otherwise, the function shall be defined as deleted.

So, it returns `strong_ordering::...` or `strong_equality::` or `a <=> b`. By the normal core deduction rules, that means it's always ill-formed, because one return type deduction deduces `strong_ordering` and another deduces `strong_equality`.

I'm guessing the idea was actually that the above happens as if by four separate overloads / `constexpr if` / else. But I think you need to actually say that.

[Tomasz suggests proposed wording]

[2018-06-18 after reflector discussion]

Priority set to 2

[2018-11 San Diego Thursday night issue processing]

Spaceship is still in flux; revisit in Kona. Status to Open

[2020-01 Resolved by the adoption of P1614 in Cologne.]

Proposed resolution:

This wording is relative to N4713.

1. Modify 26.4 [algorithm.syn], header `<algorithm>` synopsis, as indicated:

   >     // 26.8.12 [alg.three.way], three-way comparison algorithms
   >     template<class T, class U>
   >     constexpr autosee below compare_3way(const T& a, const U& b);

2. Modify 26.8.12 [alg.three.way] as indicated:

   >     template<class T, class U> constexpr autosee below compare_3way(const T& a, const U& b);

   > > -1- Effects: Compares two values and produces a result of the strongest applicable comparison category type:

   > > 1. (1.1) — Returns `a <=> b` if that expression is well-formedIf the expression `a <=> b` is well-formed, returns a value of type `decay_t<decltype(a <=> b)>` initialized from `a <=> b`.

   > > 2. (1.2) — Otherwise, if the expressions `a == b` and `a < b` are each well-formed and convertible to `bool`, returns a value of type `strong_ordering` equal to `strong_ordering::equal` when `a == b` is `true`, otherwise returns `strong_ordering::less` when `a < b` is `true`, and otherwise returns `strong_ordering::greater`.

   > > 3. (1.3) — Otherwise, if the expression `a == b` is well-formed and convertible to `bool`, returns a value of type `strong_equality` equal to `strong_equality::equal` when `a == b` is `true`, and otherwise returns `strong_equality::nonequal`.

   > > 4. (1.4) — Otherwise, the return type is `void` and the function is defined as deleted.

### 3062(i). Unnecessary `decay_t` in `is_execution_policy_v` should be `remove_cvref_t`

Section: 26.3.5 [algorithms.parallel.overloads] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-02-07 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

Our compiler throughput friends were hissing at us about throughput regressions in C++17 mode caused by the addition of the parallel algorithms' signatures. One change to reduce the throughput impact would be to remove unnecessary decay here, as LWG has done in other places recently.

[ 2018-02-13 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4713.

1. Modify 26.3.5 [algorithms.parallel.overloads] as indicated:

   > -4- Parallel algorithms shall not participate in overload resolution unless `is_execution_policy_v<decayremove_cvref_t<ExecutionPolicy>>` is `true`.

### 3064(i). How do uninitialized memory algorithms obtain pointer without undefined behavior?

Section: 26.11 [specialized.algorithms] Status: Resolved Submitter: Alisdair Meredith Opened: 2018-02-12 Last modified: 2024-11-28

Priority: 3

View other active issues in [specialized.algorithms].

View all other issues in [specialized.algorithms].

View all issues with Resolved status.

Discussion:

A typical specification of the algorithms for initializing raw memory in `<memory>` looks like:

> Effects: Equivalent to:

> >     for (; first != last; ++first)
> >       ::new (static_cast<void*>(addressof(*first)))
> >         typename iterator_traits<ForwardIterator>::value_type;

However, this hides a nasty question:

How do we bind a reference to an uninitialized object when dereferencing our iterator, so that `static_cast<void*>(addressof(*first))` does not trigger undefined behavior on the call to `*first`?

When pointers are the only iterators we cared about, we could simply cast the iterator value to `void*` without dereferencing. I don't see how to implement this spec safely without introducing another customization point for iterators that performs the same function as casting a pointer to `void*` in order to get the address of the element.

[2018-02-20, Priority set to 3 after mailing list discussion]

[2024-11-28 Status changed: New → Resolved.]

Resolved by CWG 453, accepted as a DR in March 2024.

Proposed resolution:

### 3065(i). LWG 2989 missed that all `path`'s other operators should be hidden friends as well

Section: 31.12.6.8 [fs.path.nonmember] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-02-13 Last modified: 2021-02-25

Priority: 2

View all issues with C++20 status.

Discussion:

Consider the following program:

>     // See godbolt link
>     #include <assert.h>
>     #include <string>
>     #include <filesystem>

>     using namespace std;
>     using namespace std::filesystem;

>     int main() {
>       bool b = L"a//b" == std::string("a/b");
>       assert(b); // passes. What?!
>       return b;
>     }

`L"a"` gets converted into a `path`, and the string gets converted into a `path`, and then those paths are compared for equality. But `path` equality comparison doesn't work anything like string equality comparison, leading to surprises.

`path`'s other operators should be made hidden friends as well, so that one side or the other of a given operator is of type `path` before those conversions apply.

[2018-02-20, Priority set to 2 after mailing list discussion]

[2018-08-23 Batavia Issues processing]

Status to Tentatively Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4713.

All drafting notes from LWG 2989(i) apply here too.

1. Modify 31.12.4 [fs.filesystem.syn], header `<filesystem>` synopsis, as indicated:

   >     […]
   >     // 31.12.6.8 [fs.path.nonmember], path non-member functions
   >     void swap(path& lhs, path& rhs) noexcept;
   >     size_t hash_value(const path& p) noexcept;

   >     bool operator==(const path& lhs, const path& rhs) noexcept;
   >     bool operator!=(const path& lhs, const path& rhs) noexcept;
   >     bool operator< (const path& lhs, const path& rhs) noexcept;
   >     bool operator<=(const path& lhs, const path& rhs) noexcept;
   >     bool operator> (const path& lhs, const path& rhs) noexcept;
   >     bool operator>=(const path& lhs, const path& rhs) noexcept;

   >     path operator/ (const path& lhs, const path& rhs);
   >     […]

2. Modify 31.12.6.8 [fs.path.nonmember] as indicated:

   >     […]
   >     friend bool operator< (const path& lhs, const path& rhs) noexcept;
   >     […]
   >     friend bool operator<=(const path& lhs, const path& rhs) noexcept;
   >     […]
   >     friend bool operator> (const path& lhs, const path& rhs) noexcept;
   >     […]
   >     friend bool operator>=(const path& lhs, const path& rhs) noexcept;
   >     […]
   >     friend bool operator==(const path& lhs, const path& rhs) noexcept;
   >     […]
   >     friend bool operator!=(const path& lhs, const path& rhs) noexcept;
   >     […]
   >     friend path operator/ (const path& lhs, const path& rhs);
   >     […]

3. Modify 31.12.6 [fs.class.path], class `path` synopsis, as indicated:

   >     class path {
   >     public:
   >       […]
   >       // 31.12.6.5.5 [fs.path.modifiers], modifiers
   >       […]

   >       // 31.12.6.8 [fs.path.nonmember], non-member operators
   >       friend bool operator< (const path& lhs, const path& rhs) noexcept;
   >       friend bool operator<=(const path& lhs, const path& rhs) noexcept;
   >       friend bool operator> (const path& lhs, const path& rhs) noexcept;
   >       friend bool operator>=(const path& lhs, const path& rhs) noexcept;
   >       friend bool operator==(const path& lhs, const path& rhs) noexcept;
   >       friend bool operator!=(const path& lhs, const path& rhs) noexcept;

   >       friend path operator/ (const path& lhs, const path& rhs);

   >       // 31.12.6.5.6 [fs.path.native.obs], native format observers
   >       […]
   >     };

### 3067(i). `recursive_directory_iterator::pop` must invalidate

Section: 31.12.12.2 [fs.rec.dir.itr.members] Status: C++20 Submitter: Casey Carter Opened: 2018-02-25 Last modified: 2021-02-25

Priority: 0

View all other issues in [fs.rec.dir.itr.members].

View all issues with C++20 status.

Discussion:

`recursive_directory_iterator::pop` is effectively a "supercharged" `operator++`: it advances the iterator forward as many steps as are necessary to reach the next entry in the parent directory. Just as is the case for `operator++`, `pop` must be allowed to invalidate iterator copies to allow efficient implementation. The most efficient fix seems to be borrowing the invalidation wording from 24.3.5.3 [input.iterators] Table 87's specification for the required `++r` expression for input iterators.

[ 2018-03-06 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4727.

1. Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:

   >     void pop();
   >     void pop(error_code& ec);

   > > -26- Effects: If `depth() == 0`, set `*this` to `recursive_directory_iterator()`. Otherwise, cease iteration of the directory currently being iterated over, and continue iteration over the parent directory.

   > > -?- Postconditions: Any copies of the previous value of `*this` are no longer required either to be dereferenceable or to be in the domain of `==`.

   > > -27- Throws: As specified in 31.12.5 [fs.err.report].

### 3070(i). `path::lexically_relative` causes surprising results if a filename can also be a root-name

Section: 31.12.6.5.11 [fs.path.gen] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-02-23 Last modified: 2021-02-25

Priority: 2

View all other issues in [fs.path.gen].

View all issues with C++20 status.

Discussion:

`path::lexically_relative` constructs the resulting path with `operator/=`. If any of the filename elements from `*this` are themselves acceptable root-names, `operator/=` will destroy any previous value, and take that `root_name()`. For example:

>     path("/a:/b:").lexically_relative("/a:/c:")

On a POSIX implementation, this would return `path("../b:")`, but on a Windows implementation, the `"b:"` element is interpreted as a root-name, and clobbers the entire result `path`, giving `path("b:")`. We should detect this problematic condition and fail (by returning `path()`).

[2019-01-20 Reflector prioritization]

Set Priority to 2

[2019 Cologne Wednesday night]

Status to Ready

Proposed resolution:

This wording is relative to N4727.

1. Change 31.12.6.5.11 [fs.path.gen] as indicated:

   >     path lexically_relative(const path& base) const;

   > > -3- […]

   > > -4- Effects: If `root_name() != base.root_name()` is `true` or `is_absolute() != base.is_absolute()` is `true` or `!has_root_directory() && base.has_root_directory()` is `true` or if any filename in `relative_path()` or `base.relative_path()` can be interpreted as a root-name, returns `path()`. [Note: On a POSIX implementation, no filename in a relative-path is acceptable as a root-name — end note] Determines the first mismatched element of `*this` and `base` as if by:

   > > >     auto [a, b] = mismatch(begin(), end(), base.begin(), base.end());

   > > Then,

   > > 1. (4.1) — if `a == end()` and `b == base.end()`, returns `path(".")`; otherwise

   > > 2. (4.2) — let `n` be the number of filename elements in `[b, base.end())` that are not dot or dot-dot minus the number that are dot-dot. If `n < 0`, returns `path()`; otherwise

   > > 3. (4.3) — returns an object of class `path` that is default-constructed, followed by

   > >    1. (4.3.1) — application of `operator/=(path(".."))` `n` times, and then

   > >    2. (4.3.2) — application of `operator/=` for each element in `[a, end())`.

### 3071(i). [networking.ts] `read_until` still refers to "input sequence"

Section: 17.9 [networking.ts::buffer.read.until] Status: WP Submitter: Christopher Kohlhoff Opened: 2018-02-26 Last modified: 2026-02-21

Priority: 0

View all issues with WP status.

Discussion:

Addresses: networking.ts

When specifying `DynamicBuffers` and their related operations, early drafts of the Networking TS described the buffers in terms of their "input sequence" and "output sequence". This was changed to "readable bytes" and "writable bytes" respectively. Unfortunately, some instances of "input sequence" were missed in section 17.9 [networking.ts::buffer.read.until].

[ 2018-03-06 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4711.

1. Change 17.9 [networking.ts::buffer.read.until] as indicated:

   >     template<class SyncReadStream, class DynamicBuffer>
   >       size_t read_until(SyncReadStream& s, DynamicBuffer&& b, char delim);
   >     template<class SyncReadStream, class DynamicBuffer>
   >       size_t read_until(SyncReadStream& s, DynamicBuffer&& b,
   >                         char delim, error_code& ec);
   >     template<class SyncReadStream, class DynamicBuffer>
   >       size_t read_until(SyncReadStream& s, DynamicBuffer&& b, string_view delim);
   >     template<class SyncReadStream, class DynamicBuffer>
   >       size_t read_until(SyncReadStream& s, DynamicBuffer&& b,
   >                         string_view delim, error_code& ec);

   > > -1- Effects: Reads data from the buffer-oriented synchronous read stream (17.1.1 [networking.ts::buffer.stream.reqmts.syncreadstream]) object stream by performing zero or more calls to the stream's `read_some` member function, until the input sequencereadable bytes of the dynamic buffer (16.2.4 [networking.ts::buffer.reqmts.dynamicbuffer]) object `b` contains the specified delimiter `delim`.

   > > -2- Data is placed into the dynamic buffer object b. A mutable buffer sequence (16.2.1) is obtained prior to each `read_some` call using `b.prepare(N)`, where `N` is an unspecified value such that `N <= max_size() - size()`. [Note: Implementations are encouraged to use `b.capacity()` when determining `N`, to minimize the number of `read_some` calls performed on the stream. — end note] After each `read_some` call, the implementation performs `b.commit(n)`, where `n` is the return value from `read_some`.

   > > -3- The synchronous `read_until` operation continues until:

   > > 1. (3.1) — the input sequencereadable bytes of `b` contains the delimiter `delim`; or

   > > 2. (3.2) — `b.size() == b.max_size()`; or

   > > 3. (3.3) — an asynchronous `read_some` operation fails.

   > > -4- On exit, if the input sequencereadable bytes of `b` contains the delimiter, `ec` is set such that `!ec` is `true`. Otherwise, if `b.size() == b.max_size()`, `ec` is set such that `ec == stream_errc::not_found`. If `b.size() < b.max_size()`, `ec` contains the `error_code` from the most recent `read_some` call.

   > > -5- Returns: The number of bytes in the input sequence ofreadable bytes in `b` up to and including the delimiter, if present. [Note: On completion, the buffer may contain additional bytes following the delimiter. — end note] Otherwise returns `0`.

### 3074(i). Non-member functions for `valarray` should only deduce from the `valarray`

Section: 29.6.3 [valarray.nonmembers] Status: C++20 Submitter: Jonathan Wakely Opened: 2018-02-28 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

The expression `(std::valarray<double>{} * 2)` is ill-formed, because argument deduction fails for:

>     template<class T>
>       valarray<T> operator*(const valarray<T>&, const T&);

Is there any reason to try and deduce the argument from the scalar, instead of only deducing from the valarray and allowing implicit conversions to the scalar? i.e.

>     template<class T>
>       valarray<T> operator*(const valarray<T>&, const typename valarray<T>::value_type&);

[ 2018-03-07 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4727.

1. Edit 29.6.1 [valarray.syn], header `<valarray>` synopsis, as indicated:

   >     […]
   >     template<class T> valarray<T> operator* (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator* (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator* (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator/ (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator/ (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator/ (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator% (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator% (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator% (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator+ (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator+ (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator+ (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator- (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator- (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator- (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator^ (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator^ (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator^ (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator& (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator& (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator& (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator| (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator| (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator| (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator<<(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator<<(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator<<(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> operator>>(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> operator>>(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator>>(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<bool> operator&&(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<bool> operator&&(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator&&(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<bool> operator||(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<bool> operator||(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator||(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<bool> operator==(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<bool> operator==(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator==(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<bool> operator!=(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<bool> operator!=(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator!=(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<bool> operator< (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<bool> operator< (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator< (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<bool> operator> (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<bool> operator> (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator> (const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<bool> operator<=(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<bool> operator<=(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator<=(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<bool> operator>=(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<bool> operator>=(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator>=(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> abs (const valarray<T>&);
   >     template<class T> valarray<T> acos (const valarray<T>&);
   >     template<class T> valarray<T> asin (const valarray<T>&);
   >     template<class T> valarray<T> atan (const valarray<T>&);

   >     template<class T> valarray<T> atan2(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> atan2(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> atan2(const Ttypename valarray<T>::value_type&, const valarray<T>&);

   >     template<class T> valarray<T> cos (const valarray<T>&);
   >     template<class T> valarray<T> cosh (const valarray<T>&);
   >     template<class T> valarray<T> exp (const valarray<T>&);
   >     template<class T> valarray<T> log (const valarray<T>&);
   >     template<class T> valarray<T> log10(const valarray<T>&);

   >     template<class T> valarray<T> pow(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> pow(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> pow(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     […]

2. Edit 29.6.3.1 [valarray.binary] as indicated:

   >     […]
   >     template<class T> valarray<T> operator* (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator* (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator/ (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator/ (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator% (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator% (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator+ (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator+ (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator- (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator- (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator^ (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator^ (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator& (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator& (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator| (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator| (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator<<(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator<<(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> operator>>(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> operator>>(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     […]

3. Edit 29.6.3.2 [valarray.comparison] as indicated:

   >     […]
   >     template<class T> valarray<bool> operator==(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator==(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<bool> operator!=(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator!=(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<bool> operator< (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator< (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<bool> operator> (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator> (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<bool> operator<=(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator<=(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<bool> operator>=(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator>=(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<bool> operator&&(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator&&(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<bool> operator||(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<bool> operator||(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     […]

4. Edit 29.6.3.3 [valarray.transcend] as indicated:

   >     template<class T> valarray<T> abs (const valarray<T>&);
   >     template<class T> valarray<T> acos (const valarray<T>&);
   >     template<class T> valarray<T> asin (const valarray<T>&);
   >     template<class T> valarray<T> atan (const valarray<T>&);
   >     template<class T> valarray<T> atan2(const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> atan2(const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> atan2(const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     template<class T> valarray<T> cos (const valarray<T>&);
   >     template<class T> valarray<T> cosh (const valarray<T>&);
   >     template<class T> valarray<T> exp (const valarray<T>&);
   >     template<class T> valarray<T> log (const valarray<T>&);
   >     template<class T> valarray<T> log10(const valarray<T>&);
   >     template<class T> valarray<T> pow (const valarray<T>&, const valarray<T>&);
   >     template<class T> valarray<T> pow (const valarray<T>&, const Ttypename valarray<T>::value_type&);
   >     template<class T> valarray<T> pow (const Ttypename valarray<T>::value_type&, const valarray<T>&);
   >     […]

### 3075(i). `basic_string` needs deduction guides from `basic_string_view`

Section: 27.4.3 [basic.string], 27.4.3.3 [string.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-03-03 Last modified: 2021-02-25

Priority: Not Prioritized

View other active issues in [basic.string].

View all other issues in [basic.string].

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Discussion:

The Proposed Resolution for LWG 2946(i) appears to be correct and we've implemented it in MSVC, but it worsens a pre-existing problem with basic_string class template argument deduction.

The following `s1` and `s2` compiled in C++17 before LWG 2946's PR, fail to compile after LWG 2946's PR, and are fixed by my PR:

>     basic_string s1("cat"sv);
>     basic_string s2("cat"sv, alloc);

The following s4 failed to compile in C++17, and is fixed by my PR:

>     // basic_string s3("cat"sv, 1, 1);
>     basic_string s4("cat"sv, 1, 1, alloc);

(s3 failed to compile in C++17, and would be fixed by my PR, but it is affected by a pre-existing and unrelated ambiguity which I am not attempting to fix here.)

As C++17 and LWG 2946's PR introduced templated constructors for `basic_string` from `basic_string_view`, we need to add corresponding deduction guides.

The constructors take `const T&` that's convertible to `basic_string_view` (the additional constraint about not converting to `const charT*` is irrelevant here). However, CTAD can't deduce `charT` and `traits` from arbitrary user-defined types, so the deduction guides need `T` to be exactly `basic_string_view`.

Additionally, we need to handle the `size_type` parameters in the same way that the unordered containers do. This PR has been implemented in MSVC.

[2018-14: Wednesday night issues processing: both this and 2946(i) to status "Immediate".]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4727.

1. Edit 27.4.3 [basic.string], class template `basic_string` synopsis, as indicated:

   > >     […]

   > >     template<class InputIterator,
   > >              class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
   > >       basic_string(InputIterator, InputIterator, Allocator = Allocator())
   > >         -> basic_string<typename iterator_traits<InputIterator>::value_type,
   > >                         char_traits<typename iterator_traits<InputIterator>::value_type>,
   > >                          Allocator>;

   > >     template<class charT,
   > >              class traits,
   > >              class Allocator = allocator<charT>>
   > >       explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
   > >         -> basic_string<charT, traits, Allocator>;

   > >     template<class charT,
   > >              class traits,
   > >              class Allocator = allocator<charT>>
   > >       basic_string(basic_string_view<charT, traits>, typename see below::size_type, typename see below::size_type,
   > >                    const Allocator& = Allocator())
   > >         -> basic_string<charT, traits, Allocator>;

   > >     }

   > -?- A `size_type` parameter type in a `basic_string` deduction guide refers to the `size_type` member type of the type deduced by the deduction guide.

2. Edit 27.4.3.3 [string.cons] as indicated:

   >     template<class InputIterator,
   >              class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
   >       basic_string(InputIterator, InputIterator, Allocator = Allocator())
   >         -> basic_string<typename iterator_traits<InputIterator>::value_type,
   >                         char_traits<typename iterator_traits<InputIterator>::value_type>,
   >                          Allocator>;

   > > -25- Remarks: Shall not participate in overload resolution if `InputIterator` is a type that does not qualify as an input iterator, or if `Allocator` is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]).

   >     template<class charT,
   >              class traits,
   >              class Allocator = allocator<charT>>
   >       explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
   >         -> basic_string<charT, traits, Allocator>;

   >     template<class charT,
   >              class traits,
   >              class Allocator = allocator<charT>>
   >       basic_string(basic_string_view<charT, traits>, typename see below::size_type, typename see below::size_type,
   >                    const Allocator& = Allocator())
   >         -> basic_string<charT, traits, Allocator>;

   > > -?- Remarks: Shall not participate in overload resolution if `Allocator` is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]).

### 3076(i). `basic_string` CTAD ambiguity

Section: 27.4.3.3 [string.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-03-03 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [string.cons].

View all issues with C++20 status.

Discussion:

The following code fails to compile for surprising reasons.

>     #include <string>
>     #include <string_view>

>     using namespace std;

>     int main()
>     {
>        string s0;
>        basic_string s1(s0, 1, 1);
>        // WANT: basic_string(const basic_string&, size_type, size_type, const Allocator& = Allocator())
>        // CONFLICT: basic_string(size_type, charT, const Allocator&)

>        basic_string s2("cat"sv, 1, 1);
>        // WANT: basic_string(const T&, size_type, size_type, const Allocator& = Allocator())
>        // CONFLICT: basic_string(size_type, charT, const Allocator&)

>        basic_string s3("cat", 1);
>        // WANT: basic_string(const charT *, size_type, const Allocator& = Allocator())
>        // CONFLICT: basic_string(const charT *, const Allocator&)
>     }

For `s1` and `s2`, the signature `basic_string(size_type, charT, const Allocator&)` participates in CTAD. `size_type` is non-deduced (it will be substituted later, so the compiler can't immediately realize that `s0` or `"cat"sv` are totally non-viable arguments). `charT` is deduced to be `int` (weird, but not the problem). Finally, `Allocator` is deduced to be `int`. Then the compiler tries to substitute for `size_type`, but this ends up giving int to `allocator_traits` in a non-SFINAE context, so compilation fails.

`s3` fails for a slightly different reason. `basic_string(const charT *, const Allocator&)` participates in CTAD, deducing `charT` to be `char` (good) and `Allocator` to be `int`. This is an exact match, which is better than the constructor that the user actually wants (where `int` would need to be converted to `size_type`, which is unsigned). So CTAD deduces `basic_string<char, char_traits<char>, int>`, which is the wrong type.

This problem appears to be unique to `basic_string` and its heavily overloaded set of constructors. I haven't figured out how to fix it by adding (non-greedy) deduction guides. The conflicting constructors are always considered during CTAD, regardless of whether deduction guides are provided that correspond to the desired or conflicting constructors. (That's because deduction guides are preferred as a late tiebreaker in overload resolution; if a constructor provides a better match it will be chosen before tiebreaking.) It appears that we need to constrain the conflicting constructors themselves; this will have no effect on actual usage (where `Allocator` will be an allocator) but will prevent CTAD from considering them for non-allocators. As this is unusual, I believe it deserves a Note. This has been implemented in MSVC.

[2018-3-14 Wednesday evening issues processing; move to Ready]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4727.

1. Edit 27.4.3.3 [string.cons] as indicated:

   >     basic_string(const charT* s, const Allocator& a = Allocator());

   > > -14- Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`.

   > > -15- Effects: Constructs an object of class `basic_string` and determines its initial string value from the array of `charT` of length `traits::length(s)` whose first element is designated by `s`.

   > > -16- Postconditions: `data()` points at the first element of an allocated copy of the array whose first element is pointed at by `s`, `size()` is equal to `traits::length(s)`, and `capacity()` is a value at least as large as `size()`.

   > > -?- Remarks: Shall not participate in overload resolution if `Allocator` is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]). [Note: This affects class template argument deduction. — end note]

   >     basic_string(size_type n, charT c, const Allocator& a = Allocator());

   > > -17- Requires: `n < npos`.

   > > -18- Effects: Constructs an object of class `basic_string` and determines its initial string value by repeating the char-like object `c` for all `n` elements.

   > > -19- Postconditions: `data()` points at the first element of an allocated array of `n` elements, each storing the initial value `c`, `size()` is equal to `n`, and `capacity()` is a value at least as large as `size()`.

   > > -?- Remarks: Shall not participate in overload resolution if `Allocator` is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]). [Note: This affects class template argument deduction. — end note]

### 3077(i). `(push|emplace)_back` should invalidate the `end` iterator

Section: 23.3.13.5 [vector.modifiers] Status: C++20 Submitter: Casey Carter Opened: 2018-03-10 Last modified: 2021-02-25

Priority: 3

View all other issues in [vector.modifiers].

View all issues with C++20 status.

Discussion:

23.3.13.5 [vector.modifiers] paragraph 1 specifies that `emplace_back` and `push_back` do not invalidate iterators before the insertion point when reallocation is unnecessary:

> Remarks: Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, all the iterators and references before the insertion point remain valid. […]

This statement is redundant, given the blanket wording in 23.2.2 [container.requirements.general] paragraph 12:

> Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.

It seems that this second sentence (1) should be a note that reminds us that the blanket wording applies here when no reallocation occurs, and/or (2) actually intends to specify that iterators at and after the insertion point are invalidated.

Also, it seems intended that reallocation should invalidate the `end` iterator as well.

[2018-06-18 after reflector discussion]

Priority set to 3

Previous resolution [SUPERSEDED]:

> 1. Edit 23.3.13.5 [vector.modifiers] as indicated:

>    > -1- Remarks: Invalidates the past-the-end iterator. Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. [Note: If no reallocation happens, all the iterators and references before the insertion point remain valid.—end note] If an exception is thrown […]

[2018-11-28 Casey provides an updated P/R]

Per discussion in the prioritization thread on the reflector.

[2018-12-01 Status to Tentatively Ready after seven positive votes on the reflector.]

Proposed resolution:

This wording is relative to the post-San Diego working draft.

1. Change 27.4.3.5 [string.capacity] as indicated:

   > >     void shrink_to_fit();

   > -11- Effects: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. —end note] It does not increase `capacity()`, but may reduce `capacity()` by causing reallocation.

   > -12- Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.

   > -13- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator. [ Note: If no reallocation happens, they remain valid. — end note ]

2. Change 23.3.5.3 [deque.capacity] as indicated:

   > >     void shrink_to_fit();

   > -5- Requires: `T` shall be `Cpp17MoveInsertable` into `*this`.

   > -6- Effects: `shrink_to_fit` is a non-binding request to reduce memory use but does not change the size of the sequence. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. —end note] If the size is equal to the old capacity, or if an exception is thrown other than by the move constructor of a non-`Cpp17CopyInsertable` `T`, then there are no effects.

   > -7- Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.

   > -8- Remarks: `shrink_to_fit` If the size is not equal to the old capacity, then invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

3. Change 23.3.13.3 [vector.capacity] as indicated:

   > >     void reserve(size_type n);

   > […]

   > -7- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator. [ Note: If no reallocation happens, they remain valid. — end note ] No reallocation shall take place during insertions that happen after a call to `reserve()` until the time when an insertion would make the size of the vector greater than the value of `capacity()`.

   > >     void shrink_to_fit();

   > […]

   > -10- Complexity: If reallocation happens, linear in the size of the sequence.

   > -11- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator. [ Note: If no reallocation happens, they remain valid. — end note ]

4. Change 23.3.13.5 [vector.modifiers] as indicated:

   > -1- Remarks: Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator. If no reallocation happens, all the iterators and references then references, pointers, and iterators before the insertion point remain valid but those at or after the insertion point, including the past-the-end iterator, are invalidated. If an exception is thrown […]

   > -2- Complexity: The complexity is If reallocation happens, linear in the number of elements of the resulting vector; otherwise linear in the number of elements inserted plus the distance to the end of the vector.

### 3079(i). LWG 2935 forgot to fix the `existing_p` overloads of `create_directory`

Section: 31.12.13.8 [fs.op.create.directory] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-03-07 Last modified: 2021-06-06

Priority: 0

View all issues with C++20 status.

Discussion:

LWG 2935(i) clarified that `create_directory` is not supposed to report an error if `exists(p)`, even if `p` is not a directory. However, the P/R there missed the `existing_p` overloads.

[ 2018-03-27 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4727.

1. Edit [fs.op.create_directory] as indicated:

   >     bool create_directory(const path& p, const path& existing_p);
   >     bool create_directory(const path& p, const path& existing_p, error_code& ec) noexcept;

   > > -4- Effects: Establishes the postcondition by attempting to createCreates the directory `p` resolves to, with attributes copied from directory `existing_p`. The set of attributes copied is operating system dependent. Creation failure because `p` resolves to an existing directory shall not be treated asalready exists is not an error. [Note: For POSIX-based operating systems, the attributes are those copied by native API `stat(existing_p.c_str(), &attributes_stat)` followed by `mkdir(p.c_str(), attributes_stat.st_mode)`. For Windows-based operating systems, the attributes are those copied by native API `CreateDirectoryExW(existing_p.c_str(), p.c_str(), 0)`. — end note]

   > > -5- Postconditions: `is_directory(p)`.

   > > […]

### 3080(i). Floating point `from_chars` pattern specification breaks round-tripping

Section: 28.2.3 [charconv.from.chars] Status: C++20 Submitter: Greg Falcon Opened: 2018-03-12 Last modified: 2021-02-25

Priority: 0

View other active issues in [charconv.from.chars].

View all other issues in [charconv.from.chars].

View all issues with C++20 status.

Discussion:

`from_chars` specifies that the `'+'` character is never matched, but `to_chars` specifies its output format in terms of `printf()`, which puts a `'+'` sign before positive exponents.

Since `strtod()` matches `'+'` signs, it is also desirable to accept `'+'` in exponents, so that code currently using `strtod()` can be migrated to `from_chars()` without a breaking semantic change.

[ 2018-03-27 Moved to Tentatively Ready after 9 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4727.

1. Edit 28.2.3 [charconv.from.chars] as indicated:

   >     from_chars_result from_chars(const char* first, const char* last, float& value,
   >                                  chars_format fmt = chars_format::general);
   >     from_chars_result from_chars(const char* first, const char* last, double& value,
   >                                  chars_format fmt = chars_format::general);
   >     from_chars_result from_chars(const char* first, const char* last, long double& value,
   >                                  chars_format fmt = chars_format::general);

   > > -6- Requires: `fmt` has the value of one of the enumerators of `chars_format`.

   > > -7- Effects: The pattern is the expected form of the subject sequence in the `"C"` locale, as described for `strtod`, except that

   > > 1. (7.1) — the only sign `'+'` that may only appear is `'-'`in the exponent part;

   > > 2. (7.2) […]

   > > 3. (7.3) […]

   > > 4. (7.4) […]

### 3083(i). What should `ios::iword(-1)` do?

Section: 31.5.2.6 [ios.base.storage] Status: C++20 Submitter: Jonathan Wakely Opened: 2018-03-16 Last modified: 2021-02-25

Priority: 0

View all other issues in [ios.base.storage].

View all issues with C++20 status.

Discussion:

Is calling `iword` and `pword` with a negative argument undefined, or should it cause a failure condition (and return a valid reference)? What about `INT_MAX`? What about `0`?

Using arbitrary indices isn't safe, because the implementation could be already using them for something else. Some replies on the reflector suggested the only reliable argument is one obtained from `ios_base::xalloc()`. Others pointed out that the iwords and pwords could be stored in sparse arrays, so that any value from `INT_MIN` to `INT_MAX` could be a valid key (which might require the implementation to use keys outside that range for its own entries in the arrays).

If it's undefined we should add a Requires element to the spec. If invalid indices are supposed to cause a failure we need to define which indices are invalid (and ensure that's something the implementation can check), and specify that it causes a failure.

[ 2018-03-27 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4727.

1. Edit 31.5.2.6 [ios.base.storage] as indicated:

   >     long& iword(int idx);

   > > -?- Requires: `idx` is a value obtained by a call to `xalloc`.

   > > -3- Effects: If `iarray` is a null pointer, […]

   > > […]

   >     void*& pword(int idx);

   > > -?- Requires: `idx` is a value obtained by a call to `xalloc`.

   > > -5- Effects: If `iarray` is a null pointer, […]

   > > […]

### 3085(i). `char_traits::copy` precondition too weak

Section: 27.2.2 [char.traits.require] Status: C++23 Submitter: Jonathan Wakely Opened: 2018-03-16 Last modified: 2023-11-22

Priority: 2

View other active issues in [char.traits.require].

View all other issues in [char.traits.require].

View all issues with C++23 status.

Discussion:

Table 54, Character traits requirements, says that `char_traits::move` allows the ranges to overlap, but `char_traits::copy` requires that `p` is not in the range `[s, s + n)`. This appears to be an attempt to map to the requirements of `memmove` and `memcpy` respectively, allowing those to be used to implement the functions, however the requirements for `copy` are weaker than those for `memcpy`. The C standard says for `memcpy` "If copying takes place between objects that overlap, the behavior is undefined" which is a stronger requirement than the start of the source range not being in the destination range.

All of libstdc++, libc++ and VC++ simply use `memcpy` for `char_traits<char>::copy`, resulting in undefined behaviour in this example:

>     char p[] = "abc";
>     char* s = p + 1;
>     std::char_traits<char>::copy(s, p, 2);
>     assert(std::char_traits<char>::compare(p, "aab", 3) == 0);

If the intention is to allow `memcpy` as a valid implementation then the precondition is wrong (unfortunately nobody realized this when fixing `char_traits::move` in LWG DR 7(i)). If the intention is to require `memmove` then it is strange to have separate `copy` and `move` functions that both use `memmove`.

N.B. `std::copy` and `std::copy_backward` are not valid implementations of `char_traits::copy` either, due to different preconditions.

Changing the precondition implicitly applies to `basic_string::copy` (27.4.3.7.7 [string.copy]), and `basic_string_view::copy` (27.3.3.8 [string.view.ops]), which are currently required to support partially overlapping ranges:

>     std::string s = "abc";
>     s.copy(s.data() + 1, s.length() - 1);
>     assert(s == "aab");

[2018-04-03 Priority set to 2 after discussion on the reflector.]

[2018-08-23 Batavia Issues processing]

No consensus for direction; revisit in San Diego. Status to Open.

[2022-04-25; Daniel rebases wording on N4910]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> Option A:

> > 1. Edit 27.2.2 [char.traits.require], Table 75 — "Character traits requirements" [tab:char.traits.req], as indicated:

> >    > Table 75 — Character traits requirements [tab:char.traits.req]
> >    > Expression | Return type | Assertion/note pre/post-condition | Complexity
> >    > --- | --- | --- | ---
> >    > `[…]`
> >    > `X::copy(s,p,n)` | `X::char_type*` | Preconditions: `p` not in `[s,s+n)`The ranges `[p,p+n)` and `[s,s+n)` do not overlap. Returns: `s`. for each `i` in `[0,n)`, performs `X::assign(s[i],p[i])`. | linear
> >    > `[…]`

> Option B:

> > NAD (i.e. implementations need to be fixed, in practice `char_traits::copy` and `char_traits::move` might be equivalent).

[Kona 2022-11-11; Move to Ready]

LWG voted for Option A (6 for, 0 against, 1 netural)

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

1. Edit 27.2.2 [char.traits.require], Table 75 — "Character traits requirements" [tab:char.traits.req], as indicated:

   > Table 75 — Character traits requirements [tab:char.traits.req]
   > Expression | Return type | Assertion/note pre/post-condition | Complexity
   > --- | --- | --- | ---
   > `[…]`
   > `X::copy(s,p,n)` | `X::char_type*` | Preconditions: `p` not in `[s,s+n)`The ranges `[p,p+n)` and `[s,s+n)` do not overlap. Returns: `s`. for each `i` in `[0,n)`, performs `X::assign(s[i],p[i])`. | linear
   > `[…]`

### 3087(i). One final `&x` in §[list.ops]

Section: 23.3.11.5 [list.ops] Status: C++20 Submitter: Tim Song Opened: 2018-03-19 Last modified: 2021-02-25

Priority: 3

View all other issues in [list.ops].

View all issues with C++20 status.

Discussion:

LWG 3017(i) missed an instance of `&x` in 23.3.11.5 [list.ops] p14.

[2018-06-18 after reflector discussion]

Priority set to 3

[2018-10-15 Status to Tentatively Ready after seven positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4727.

1. Edit 23.3.11.5 [list.ops] as indicated:

   >     void splice(const_iterator position, list& x, const_iterator first,
   >                 const_iterator last);
   >     void splice(const_iterator position, list&& x, const_iterator first,
   >                 const_iterator last);

   > > -11- Requires: […]

   > > -12- Effects: […]

   > > -13- Throws: Nothing.

   > > -14- Complexity: Constant time if `&addressof(x) == this`; otherwise, linear time.

### 3088(i). `forward_list::merge` behavior unclear when passed `*this`

Section: 23.3.7.6 [forward.list.ops] Status: C++23 Submitter: Tim Song Opened: 2018-03-19 Last modified: 2023-11-22

Priority: 3

View all other issues in [forward.list.ops].

View all issues with C++23 status.

Discussion:

LWG 300(i) changed `list::merge` to be a no-op when passed `*this`, but there's no equivalent rule for `forward_list::merge`. Presumably the `forward_list` proposal predated the adoption of LWG 300's PR and was never updated for the change. Everything in the discussion of that issue applies mutatis mutandis to the current specification of `forward_list::merge`.

[2018-06-18 after reflector discussion]

Priority set to 3

[2019-07-30 Tim provides updated PR]

Per the comments during issue prioritization, the new PR tries to synchronize the wording between `list::merge` and `forward_list::merge`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4727.

> 1. Edit [forwardlist.ops] as indicated:

>    >     void merge(forward_list& x);
>    >     void merge(forward_list&& x);
>    >     template<class Compare> void merge(forward_list& x, Compare comp);
>    >     template<class Compare> void merge(forward_list&& x, Compare comp);

>    > > -20- Requires: `*this` and `x` are both sorted with respect to the comparator `operator<` (for the first two overloads) or `comp` (for the last two overloads), and `get_allocator() == x.get_allocator()` is `true`.

>    > > -21- Effects: If `addressof(x) == this`, does nothing. Otherwise, mMerges the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. The result is a range that is sorted with respect to the comparator `operator<` (for the first two overloads) or `comp` (for the last two overloads). `x` is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

>    > > -22- Remarks: Stable (16.4.6.8 [algorithm.stable]). The behavior is undefined if `get_allocator() != x.get_allocator()`.

>    > > -23- Complexity: At most `distance(begin(), end()) + distance(x.begin(), x.end()) - 1` comparisons if `addressof(x) != this`; otherwise, no comparisons are performed.

[2021-05-22 Tim syncs wording to the current working draft]

[2022-01-31; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Edit 23.3.7.6 [forward.list.ops] as indicated:

   >     void merge(forward_list& x);
   >     void merge(forward_list&& x);
   >     template<class Compare> void merge(forward_list& x, Compare comp);
   >     template<class Compare> void merge(forward_list&& x, Compare comp);

   > > -?- Let `comp` be `less<>{}` for the first two overloads.

   > > -24- Preconditions: `*this` and `x` are both sorted with respect to the comparator `operator<` (for the first two overloads) or `comp` (for the last two overloads), and `get_allocator() == x.get_allocator()` is `true`.

   > > -25- Effects: If `addressof(x) == this`, there are no effects. Otherwise, mMerges the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. The result is a range that is sorted with respect to the comparator `comp`. `x` is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

   > > -26- Complexity: At most `distance(begin(), end()) + distance(x.begin(), x.end()) - 1` comparisons if `addressof(x) != this`; otherwise, no comparisons are performed.

   > > -27- Remarks: Stable (16.4.6.8 [algorithm.stable]). If `addressof(x) != this`, `x` is empty after the merge. No elements are copied by this operation. If an exception is thrown other than by a comparison there are no effects.

2. Edit 23.3.11.5 [list.ops] as indicated:

   >     void merge(list& x);
   >     void merge(list&& x);
   >     template<class Compare> void merge(list& x, Compare comp);
   >     template<class Compare> void merge(list&& x, Compare comp);

   > > -?- Let `comp` be `less<>{}` for the first two overloads.

   > > -26- Preconditions: Both the list and the argument list shall be`*this` and `x` are both sorted with respect to the comparator `operator<` (for the first two overloads) or `comp` (for the last two overloads), and `get_allocator() == x.get_allocator()` is `true`.

   > > -27- Effects: If `addressof(x) == this`, does nothing; othere are no effects. Otherwise, merges the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i`, in the range other than the first, the condition `comp(*i, *(i - 1))` will be `false` that is sorted with respect to the comparator `comp`. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

   > > -28- Complexity: At most `size() + x.size() - 1` applications of `comp` comparisons if `addressof(x) != this`; otherwise, no applications of `comp` comparisons are performed. If an exception is thrown other than by a comparison there are no effects.

   > > -29- Remarks: Stable (16.4.6.8 [algorithm.stable]). If `addressof(x) != this`, the range `[x.begin(), x.end())``x` is empty after the merge. No elements are copied by this operation. If an exception is thrown other than by a comparison there are no effects.

### 3090(i). What is §[time.duration.cons]p4's "no overflow is induced in the conversion" intended to mean?

Section: 30.5.2 [time.duration.cons] Status: C++26 Submitter: Richard Smith Opened: 2018-03-22 Last modified: 2026-06-09

Priority: 3

View all other issues in [time.duration.cons].

View all issues with C++26 status.

Discussion:

30.5.2 [time.duration.cons] p4 says:

>     template<class Rep2, class Period2>
>       constexpr duration(const duration<Rep2, Period2>& d);

> > Remarks: This constructor shall not participate in overload resolution unless no overflow is induced in the conversion and `treat_as_floating_point_v<rep>` is `true` or both `ratio_divide<Period2, period>::den` is 1 and `treat_as_floating_point_v<Rep2>` is `false`.

with this example:

>     duration<int, milli> ms(3);
>     duration<int, micro> us = ms;  // OK
>     duration<int, milli> ms2 = us; // error

It's unclear to me what "no overflow is induced in the conversion" means in the above. What happens here:

>     duration<int, milli> ms(INT_MAX);
>     duration<int, micro> us = ms;  // ???

An overflow is clearly induced in the conversion here: internally, we'll multiply `INT_MAX` by 1000. But that cannot be determined statically (in general), and so can't affect the result of overload resolution.

So what's actually supposed to happen? Are we actually just supposed to check that `Rep2` is no larger than `Rep`? (If so, what happens on overflow? Undefined behavior?)

It has been pointed out by Howard Hinnant:

> This refers to the compile-time conversion factor to convert `Period2` to `Period`. If that conversion factor is not representable as a (reduced) `ratio<N, D>`, then the constructor is SFINAE'd out. This might happen (for example) converting years to picoseconds.

I would not have guessed that from the wording. Maybe replacing "no overflow is induced in the conversion" with "the result of `ratio_divide<Period2, Period>` is representable as a `ratio`" or similar would help?

[2018-06-18 after reflector discussion]

Priority set to 3

[2020-09-12 Jonathan adds a proposed resolution]

Since the result of the `ratio_divide` has to be a `ratio`, if it's not representable then the result simply isn't a valid type. Implementations are not required to make `ratio_divide` SFINAE-friendly to implement this constraint. They can perform the equivalent calculations to check if they would overflow, without actually using `ratio_divide`.

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 30.5.2 [time.duration.cons] as indicated:

   >     template<class Rep2, class Period2>
   >       constexpr duration(const duration<Rep2, Period2>& d);

   -3- Constraints: `is_convertible_v<const Rep2&, rep>` is `true`. `ratio_divide<typename Period2::type, period>` is a valid `ratio` specialization. Either:

   - `treat_as_floating_point_v<rep>` is `true`; or
   - `ratio_divide<Period2, period>::den` is `1` and `treat_as_floating_point_v<Rep2>` is `false`.

   No overflow is induced in the conversion and `treat_as_floating_point_v<rep>` is `true` or both `ratio_divide<Period2, period>::den` is `1` and `treat_as_floating_point_v<Rep2>` is `false`. [Note: This requirement prevents implicit truncation errors when converting between integral-based `duration` types. Such a construction could easily lead to confusion about the value of the `duration`. — end note]

### 3091(i). subsecond-precision `time_of_day` and durations that `seconds` cannot convert to

Section: 30.9 [time.hms] Status: Resolved Submitter: Richard Smith Opened: 2018-03-24 Last modified: 2021-06-06

Priority: 2

View all issues with Resolved status.

Discussion:

What should happen here:

>     const int bpm = 100;
>     using beats = duration<int, ratio<60, 100>>;
>     auto v = time_of_day<beats>(beats{2}).subseconds();

? 2 beats at 100bpm is `1.2` seconds. The `time_of_day` constructor specification says:

> `seconds()` returns the integral number of seconds `since_midnight` is after `(00:00:00 + hours() + minutes())`. `subseconds()` returns the integral number of fractional precision seconds `since_midnight` is after `(00:00:00 + hours() + minutes() + seconds())`.

But that's impossible. If `seconds()` returns 1, we need to return a `subseconds()` value representing `0.2s` of type `precision`, but type `precision` can only represent multiples of `0.6s`.

Should this `time_of_day` specialization only be available for the case where `seconds` is convertible to `precision`? Or should the `precision` type used by this specialization be `common_type_t<seconds, duration<Rep, Period>>` rather than merely `duration<Rep, Period>`?

Either way I think we need a wording update to specify one of those two behaviors.

[2018-04-09 Priority set to 2 after discussion on the reflector.]

[2019 Cologne Wednesday night]

Status to Resolved (group voted on NAD, but Marshall changed it to Resolved)

Resolved by the adoption of P1466 in Cologne.

`hh_mm_ss` is now specified such that subseconds must be a non-positive power of 10 (e.g. 1/10s, 1/100s, milliseconds, etc.). In this example 60/100 simplifies to 3/5, which can be exactly represented with 1 fractional decimal digit. So in this example `subseconds()` has the value of `2ds` (2 deciseconds).

Proposed resolution:

### 3094(i). §[time.duration.io]p4 makes surprising claims about encoding

Section: 30.5.11 [time.duration.io] Status: C++20 Submitter: Richard Smith Opened: 2018-04-02 Last modified: 2021-02-25

Priority: 0

View all other issues in [time.duration.io].

View all issues with C++20 status.

Discussion:

[time.duration.io]p4 says:

> For streams where `charT` has an 8-bit representation, `"µs"` should be encoded as UTF-8. Otherwise UTF-16 or UTF-32 is encouraged. The implementation may substitute other encodings, including `"us"`.

This choice of encoding is not up to the `<chrono>` library to decide or encourage. The basic execution character set determines how a mu should be encoded in type `char`, for instance, and it would be truly bizarre to use a UTF-8 encoding if that character set is, say, Latin-1 or EBCDIC.

I suggest we strike at least the first two sentences of this paragraph, as the meaning of the prior wording is unambiguous without them and confusing with them, and they do not providing any normative requirements (although they do provide recommendations). The third sentence appears to have a normative impact, but it's hard to see how it's legitimate to call `"us"` an "encoding" of `"µs"`; it's really just an alternative unit suffix. So how about replacing that paragraph with this:

> If `Period::type` is `micro`, but the character U+00B5 cannot be represented in the encoding used for `charT`, the unit suffix `"us"` is used instead of `"µs"`.

(This also removes the permission for an implementation to choose an arbitrary alternative "encoding", which seems undesirable.)

[ 2018-04-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4741.

1. Edit 30.5.11 [time.duration.io] as indicated:

   >     template<class charT, class traits, class Rep, class Period>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);

   > > -1- Requires: […]

   > > -2- Effects: […]

   > > -3- The units suffix depends on the type `Period::type` as follows:

   > > 1. […]

   > > 2. (3.5) — Otherwise, if `Period::type` is `micro`, the suffix is `"µs"` (`"\u00b5\u0073"`).

   > > 3. […]

   > > 4. (3.21) — Otherwise, the suffix is `"[num/den]s"`.

   > > […]

   > > -4- For streams where `charT` has an 8-bit representation, `"µs"` should be encoded as UTF-8. Otherwise UTF-16 or UTF-32 is encouraged. The implementation may substitute other encodings, including `"us"`If `Period::type` is `micro`, but the character U+00B5 cannot be represented in the encoding used for `charT`, the unit suffix `"us"` is used instead of `"µs"`.

   > > -5- Returns: `os`.

### 3095(i). `strstreambuf` refers to nonexistent member of `fpos`, `fpos::offset`

Section: 99 [depr.strstreambuf.virtuals] Status: Resolved Submitter: Billy O'Neal III Opened: 2018-04-04 Last modified: 2025-11-11

Priority: 4

View all other issues in [depr.strstreambuf.virtuals].

View all issues with Resolved status.

Discussion:

`strstreambuf` refers to a nonexistent member function of `fpos` in the specification of the member function `seekpos`, 99 [depr.strstreambuf.virtuals]/18 (emphasize mine):

> For a sequence to be positioned, if its next pointer is a null pointer, the positioning operation fails. Otherwise, the function determines `newoff` from `sp.offset()`:

The intent is clearly to get the corresponding `streamoff` from the `fpos`, as p19 says "the resultant offset `newoff` (of type `off_type`)". The mechanism to make that conversion is a normal explicit conversion, as indicated in the last row of the table in [fpos.operations].

[2018-06-18 after reflector discussion]

Priority set to 4

[2025-11-10 Resolved by the removal of `strstreambuf` via paper P2867R2 in Tokyo, 2024. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4727.

1. Edit 99 [depr.strstreambuf.virtuals] as indicated:

   >     pos_type seekpos(pos_type sp, ios_base::openmode which
   >                      = ios_base::in | ios_base::out) override;

   > > -17- Effects: […]

   > > -18- For a sequence to be positioned, if its next pointer is a null pointer, the positioning operation fails. Otherwise, the function determines `newoff` from `static_cast<off_type>(sp).offset()`:

   > > […]

### 3096(i). `path::lexically_relative` is confused by trailing slashes

Section: 31.12.6.5.11 [fs.path.gen] Status: C++20 Submitter: Jonathan Wakely Opened: 2018-04-04 Last modified: 2021-02-25

Priority: 2

View all other issues in [fs.path.gen].

View all issues with C++20 status.

Discussion:

`filesystem::proximate("/dir", "/dir/")` returns `"."` when `"/dir"` exists, and `".."` otherwise. It should always be `"."` because whether it exists shouldn't matter.

The problem is in `filesystem::path::lexically_relative`, as shown by:

>     path("/dir").lexically_relative("/dir/.");  // yields ""
>     path("/dir").lexically_relative("/dir/");   // yields ".."

The two calls should yield the same result, and when iteration of a `path` with a trailing slash gave "." as the final element they did yield the same result. In the final C++17 spec the trailing slash produces an empty filename in the iteration sequence, and `lexically_relative` doesn't handle that correctly.

[2018-04-10, Jonathan comments]

There are more inconsistencies with paths that are "obviously" equivalent to the human reader:

>     path("a/b/c").lexically_relative("a/b/c")    // yields "."
>     path("a/b/c").lexically_relative("a/b/c/")   // yields ".."
>     path("a/b/c").lexically_relative("a/b/c/.")  // yields ""
>     path("a/b/c/").lexically_relative("a/b/c")   // yields ""
>     path("a/b/c/.").lexically_relative("a/b/c")  // yields "."
>     path("a/b/c/.").lexically_relative("a/b/c/") // yields "../."

I think the right solution is:

1. when counting `[b, base.end())` in bullet (4.2) handle empty filename elements (which can only occur as the last element, due to a trailing slash) equivalently to dot elements; and

2. add a new condition for the case where `n == 0` and `[a, end())` contains no non-empty elements, i.e. the paths are equivalent except for final dots or a final slash, which don't introduce any relative difference between the paths.

[2018-06-18 after reflector discussion]

Priority set to 2

[2018-08-23 Batavia Issues processing]

Status to Tentatively Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4727.

1. Edit 31.12.6.5.11 [fs.path.gen] as indicated:

   >     path lexically_relative(const path& base) const;

   > > -3- Returns: `*this` made relative to `base`. Does not resolve (31.12.6 [fs.class.path]) symlinks. Does not first normalize (31.12.6.2 [fs.path.generic]) `*this` or `base`.

   > > -4- Effects: If `root_name() != base.root_name()` is `true` or `is_absolute() != base.is_absolute()` is `true` or `!has_root_directory() && base.has_root_directory()` is `true`, returns `path()`. Determines the first mismatched element of `*this` and `base` as if by:

   > > >     auto [a, b] = mismatch(begin(), end(), base.begin(), base.end());

   > > Then,

   > > 1. (4.1) — if `a == end()` and `b == base.end()`, returns `path(".")`; otherwise

   > > 2. (4.2) — let `n` be the number of filename elements in `[b, base.end())` that are not dot or dot-dot or empty, minus the number that are dot-dot. If `n<0`, returns `path()`; otherwise

   > > 3. (4.?) — if `n == 0` and `(a == end() || a->empty())`, returns `path(".")`; otherwise

   > > 4. (4.3) — returns an object of class `path` that is default-constructed, followed by […]

### 3100(i). Unnecessary and confusing "empty `span`" wording

Section: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-04-12 Last modified: 2021-02-25

Priority: 0

View all other issues in [span.cons].

View all issues with C++20 status.

Discussion:

The `span` constructors have wording relics that mention an "empty `span`". It's unnecessary (the behavior is fully specified by the postconditions), but I left it there because I thought it was harmless. It was later pointed out to me that this is actually confusing. Talking about an "empty `span`" implies that there's just one such thing, but `span` permits `empty()` to be `true` while `data()` can vary (being null or non-null). (This behavior is very useful; consider how `equal_range()` behaves.)

To avoid confusion, the "empty `span`" wording should simply be removed, leaving the constructor behavior unchanged. Editorially, there's also a missing paragraph number.

[ 2018-04-24 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4741.

> 1. Edit 23.7.2.2.2 [span.cons] as indicated:

>    >     constexpr span() noexcept;

>    > > -1- Effects: Constructs an empty `span`.

>    > > -2- Postconditions: `size() == 0 && data() == nullptr`.

>    > > -3- Remarks: This constructor shall not participate in overload resolution unless `Extent <= 0` is `true`.

>    >     constexpr span(pointer ptr, index_type count);

>    > > -4- Requires: `[ptr, ptr + count)` shall be a valid range. If `extent` is not equal to `dynamic_extent`, then `count` shall be equal to `extent`.

>    > > -5- Effects: Constructs a `span` that is a view over the range `[ptr, ptr + count)`. If `count` is `0` then an empty span is constructed.

>    > > -6- Postconditions: `size() == count && data() == ptr`.

>    > > -?- Throws: Nothing.

>    >     constexpr span(pointer first, pointer last);

>    > > -7- Requires: `[first, last)` shall be a valid range. If `extent` is not equal to `dynamic_extent`, then `last - first` shall be equal to `extent`.

>    > > -8- Effects: Constructs a `span` that is a view over the range `[first, last)`. If `last - first == 0` then an empty `span` is constructed.

>    > > -9- Postconditions: `size() == last - first && data() == first`.

>    > > -10- Throws: Nothing.

### 3101(i). `span`'s `Container` constructors need another constraint

Section: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-04-12 Last modified: 2021-02-25

Priority: 1

View all other issues in [span.cons].

View all issues with C++20 status.

Discussion:

When I overhauled `span`'s constructor constraints, I was careful about the built-in array, `std::array`, and converting `span` constructors. These types contain bounds information, so we can achieve safety at compile-time by permitting implicit conversions if and only if the destination extent is dynamic (this accepts anything by recording the size at runtime) or the source and destination extents are identical. However, I missed the fact that the `Container` constructors are the opposite case. A `Container` (e.g. a `vector`) has a size that's known only at runtime. It's safe to convert this to a `span` with `dynamic_extent`, but for consistency and safety, this shouldn't implicitly convert to a `span` with fixed extent. (The more verbose `(ptr, count)` and `(first, last)` constructors are available to construct fixed extent spans from runtime-length ranges. Note that debug precondition checks are equally possible with the `Container` and `(ptr, count)`/`(first, last)` constructors. The issue is that implicit conversions are notoriously problematic, so they should be permitted only when they are absolutely known to be safe.)

[2018-04-24 Priority set to 1 after discussion on the reflector.]

[2018-06 Rapperswil Thursday issues processing]

Status to LEWG. Should this be ill-formed, or fail at runtime if the container is too small? Discussion on the reflector here.

[2018-11 San Diego Saturday]

LEWG said that they're fine with the proposed resolution. Status to Tentatively Ready.

Proposed resolution:

This wording is relative to N4741.

> 1. Edit 23.7.2.2.2 [span.cons] as indicated:

>    >     template<class Container> constexpr span(Container& cont);
>    >     template<class Container> constexpr span(const Container& cont);

>    > > -14- Requires: `[data(cont), data(cont) + size(cont))` shall be a valid range. If `extent` is not equal to `dynamic_extent`, then `size(cont)` shall be equal to `extent`.

>    > > -15- Effects: Constructs a `span` that is a view over the range `[data(cont), data(cont) + size(cont))`.

>    > > -16- Postconditions: `size() == size(cont) && data() == data(cont)`.

>    > > -17- Throws: What and when `data(cont)` and `size(cont)` throw.

>    > > -18- Remarks: These constructors shall not participate in overload resolution unless:

>    > > 1. (18.?) — `extent == dynamic_extent`,

>    > > 2. (18.1) — `Container` is not a specialization of `span`,

>    > > 3. (18.2) — `Container` is not a specialization of `array`,

>    > > 4. […]

### 3102(i). Clarify `span iterator` and `const_iterator` behavior

Section: 23.7.2.2.1 [span.overview] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-04-12 Last modified: 2021-02-25

Priority: 0

View all other issues in [span.overview].

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Discussion:

There are multiple issues with how span specifies its iterators:

- `const_iterator` isn't mentioned.

- The relationship between `iterator` and `const_iterator` isn't specified. (`span` isn't a container, so it doesn't receive this automatically.)

- The iterators should be specified to be constexpr.

By imitating 27.3.3.4 [string.view.iterators]/3 "All requirements on container iterators ([container.requirements]) apply to `basic_string_view::const_iterator` as well.", we can specify that `iterator` is convertible to `const_iterator`.

[ 2018-04-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4741.

> 1. Edit 23.7.2.2.1 [span.overview] as indicated:

>    > -4- The iterator types for span is a random access iterator and a contiguous iterator `span::iterator` and `span::const_iterator` are random access iterators (24.3.5.7 [random.access.iterators]), contiguous iterators (24.3.1 [iterator.requirements.general]), and constexpr iterators (24.3.1 [iterator.requirements.general]). All requirements on container iterators (23.2 [container.requirements]) apply to `span::iterator` and `span::const_iterator` as well.

### 3103(i). Errors in taking subview of `span` should be ill-formed where possible

Section: 23.7.2.2.4 [span.sub] Status: C++20 Submitter: Tomasz Kamiński Opened: 2018-04-13 Last modified: 2021-02-25

Priority: 3

View all other issues in [span.sub].

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Discussion:

Currently all out-of-bound/inputs errors in the functions taking an subview of `span` lead to undefined behavior, even in the situation when they could be detected at compile time. This is inconsistent with the behavior of the `span` constructors, which make similar constructs ill-formed.

Furthermore, with the current specification of the `subspan` function, the following invocation:

>     span<T, N> s;   // N > 0
>     s.subspan<O>(); // with O > 0

is ill-formed when `O > N + 1`, as the return of the function is `span<T, K>` with `K < -1`. However in case when `O == N + 1`, runtime sized `span` is returned (`span<T, -1>`) instead and the behavior of the function is undefined.

Firstly, for either run time sized (`N == dynamic_extent`) and fixed sized (`N > 0`) object `s` of type `span<T, N>`, the following constructs should be ill-formed, instead of having undefined behavior:

1. `s.first<C>()` with `C < 0`

2. `s.last<C>()` with `C < 0`

3. `s.subspan<O, E>` with `O < 0` or `E < 0` and `E != dynamic_extent`.

This would follow `span` specification, that make instantiation of `span<T, N>` ill-formed for `N < 0` and `N != dynamic_extent`.

In addition the following constructs should be made ill-formed for fixed size span `s` of type `span<T, N>` (with `N > 0`):

1. `s.first<C>()` with `C > N`

2. `s.last<C>()` with `C > N`

3. `s.subspan<O, dynamic_extent>()` with `O > N`

4. `s.subspan<O, C>()` with `O + C > N`

This will match the `span` constructor that made construction of fixed size `span<T, N>` from fixed size `span` of different size ill-formed.

[2018-04-24 Priority set to 3 after discussion on the reflector.]

[2018-11 San Diego Thursday night issue processing]

Tomasz to provide updated wording.

Previous resolution: [SUPERSEDED]

> This wording is relative to N4741.

> > 1. Edit 23.7.2.2.4 [span.sub] as indicated:

> >    >     template<ptrdiff_t Count> constexpr span<element_type, Count> first() const;

> >    > > -?- Remarks: If `Count < 0 || (Extent != dynamic_extent && Count > Extent)`, the program is ill-formed.

> >    > > -1- Requires: `0 <= Count && Count <= size()`.

> >    > > -2- Effects: Equivalent to: `return {data(), Count};`

> >    >     template<ptrdiff_t Count> constexpr span<element_type, Count> last() const;

> >    > > -?- Remarks: If `Count < 0 || (Extent != dynamic_extent && Count > Extent)`, the program is ill-formed.

> >    > > -3- Requires: `0 <= Count && Count <= size()`.

> >    > > -4- Effects: Equivalent to: `return {data() + (size() - Count), Count};`

> >    >     template<ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent>
> >    >       constexpr span<element_type, see below> subspan() const;

> >    > > -?- Remarks: The program is ill-formed if:

> >    > > - `Offset < 0 || (Count < 0 && Count != dynamic_extent)`, or

> >    > > - `Extend != dynamic_extent && (Offset > Extent || (Count != dynamic_extent && Offset + Count > Extent))`.

> >    > > -5- Requires: `(0 <= Offset && Offset <= size()) && (Count == dynamic_extent || Count >= 0 && Offset + Count <= size())`.

> >    > > -6- Effects: Equivalent to: `return span<ElementType, see below>( data() + Offset, Count != dynamic_extent ? Count : size() - Offset);`

> >    > > -7- Remarks: The second template argument of the returned `span` type is:

> >    > > >     Count != dynamic_extent ? Count
> >    > > >                             : (Extent != dynamic_extent ? Extent - Offset
> >    > > >                                                         : dynamic_extent)

[2018-11-09; Tomasz provides updated wording]

I have decided to replace all Requires: elements in the section 23.7.2.2.4 [span.sub] to preserve consistency.

Previous resolution: [SUPERSEDED]

> This wording is relative to N4778.

> > 1. Edit 23.7.2.2.4 [span.sub] as indicated:

> >    >     template<ptrdiff_t Count> constexpr span<element_type, Count> first() const;

> >    > > -?- Mandates: `Count >= 0 && (Extent == dynamic_extent || Count <= Extent)`.

> >    > > -1- RequiresExpects: `0 <= Count && Count <= size()`.

> >    > > -2- Effects: Equivalent to: `return {data(), Count};`

> >    >     template<ptrdiff_t Count> constexpr span<element_type, Count> last() const;

> >    > > -?- Mandates: `Count >= 0 && (Extent == dynamic_extent || Count <= Extent)`.

> >    > > -3- RequiresExpects: `0 <= Count && Count <= size()`.

> >    > > -4- Effects: Equivalent to: `return {data() + (size() - Count), Count};`

> >    >     template<ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent>
> >    >       constexpr span<element_type, see below> subspan() const;

> >    > > -?- Mandates: `Offset >= 0 && (Count >= 0 || Count == dynamic_extent) && (Extent == dynamic_extent || (Offset <= Extent && (Count == dynamic_extent || Offset + Count <= Extent)))`.

> >    > > -5- RequiresExpects: `(0 <= Offset && Offset <= size()) && (Count == dynamic_extent || Count >= 0 && Offset + Count <= size())`.

> >    > > -6- Effects: Equivalent to: `return span<ElementType, see below>( data() + Offset, Count != dynamic_extent ? Count : size() - Offset);`

> >    > > -7- Remarks: The second template argument of the returned `span` type is:

> >    > > >     Count != dynamic_extent ? Count
> >    > > >                             : (Extent != dynamic_extent ? Extent - Offset
> >    > > >                                                         : dynamic_extent)

> >    >     constexpr span<element_type, dynamic_extent> first(index_type count) const;

> >    > > -8- RequiresExpects: `0 <= count && count <= size()`.

> >    > > -9- Effects: Equivalent to: `return {data(), count};`

> >    >     constexpr span<element_type, dynamic_extent> last(index_type count) const;

> >    > > -10- RequiresExpects: `0 <= count && count <= size()`.

> >    > > -11- Effects: Equivalent to: `return {data() + (size() - count), count};`

> >    >     constexpr span<element_type, dynamic_extent> subspan(
> >    >       index_type offset, index_type count = dynamic_extent) const;

> >    > > -12- RequiresExpects: `(0 <= offset && offset <= size()) && (count == dynamic_extent || count >= 0 && offset + count <= size())`

> >    > > -13- Effects: Equivalent to: `return {data() + offset, count == dynamic_extent ? size() - offset : count};`

[2019-06-23; Tomasz comments and provides updated wording]

The current proposed resolution no longer applies to the newest revision of the standard (N4820), due changes introduced in P1227 (making `size()` and template parameters of `span` unsigned).

[2019 Cologne Wednesday night]

Status to Ready

Proposed resolution:

This wording is relative to N4820.

> [Drafting note: This wording relies on observation, that the condition in form `Extent == dynamic_extent || Count <= Extent`, can be simplified into `Count <= Extent`, because `dynamic_extent` is equal to `numeric_limits<size_t>::max()`, thus `size() <= Extent` is always true, and `Extent == dynamic_extent` implies that `Count <= Extent`.

> Furthermore we check that `Count != dynamic_extent || Count <= Extent - Offset`, as the `Offset + Count <= Extent` may overflow (defined for unsigned integers) and produce false positive result. This change is also applied to Expects clause. ]

> 1. Edit 23.7.2.2.4 [span.sub] as indicated:

>    >     template<size_t Count> constexpr span<element_type, Count> first() const;

>    > > -?- Mandates: `Count <= Extent` is `true`.

>    > > -1- Expects: `Count <= size()` is `true`.

>    > > -2- Effects: Equivalent to: `return {data(), Count};`

>    >     template<size_t Count> constexpr span<element_type, Count> last() const;

>    > > -?- Mandates: `Count <= Extent` is `true`.

>    > > -3- Expects: `Count <= size()` is `true`.

>    > > -4- Effects: Equivalent to: `return {data() + (size() - Count), Count};`

>    >     template<size_t Offset, size_t Count = dynamic_extent>
>    >       constexpr span<element_type, see below> subspan() const;

>    > > -?- Mandates: `Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset)` is `true`.

>    > > -5- Expects: `Offset <= size() && (Count == dynamic_extent || Offset + Count <= size()Count <= size() - Offset)` is `true`.

>    > > -6- Effects: Equivalent to: `return span<ElementType, see below>(data() + Offset, Count != dynamic_extent ? Count : size() - Offset);`

>    > > -7- Remarks: The second template argument of the returned `span` type is:

>    > > >     Count != dynamic_extent ? Count
>    > > >                             : (Extent != dynamic_extent ? Extent - Offset
>    > > >                                                         : dynamic_extent)

>    > […]

>    >     constexpr span<element_type, dynamic_extent> subspan(
>    >       index_type offset, index_type count = dynamic_extent) const;

>    > > -12- Expects: `offset <= size() && (count == dynamic_extent || offset + count <= size()count <= size() - offset)` is `true`.

>    > > -13- Effects: Equivalent to: `return {data() + offset, count == dynamic_extent ? size() - offset : count};`

### 3104(i). Fixing `duration` division

Section: 30.5.6 [time.duration.nonmember] Status: C++20 Submitter: Johel Ernesto Guerrero Peña Opened: 2018-04-17 Last modified: 2021-02-25

Priority: 0

View all other issues in [time.duration.nonmember].

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Discussion:

[time.duration.nonmember]/1 states

> In the function descriptions that follow, `CD` represents the return type of the function.

From what I could find, many definitions of `CD` in the paragraphs of [time.duration.nonmember] were lifted to [time.duration.nonmember]/1 as cited above. That works for all other paragraphs, but not for [time.duration.nonmember]/10, which the change rendered ill-formed:

>     template<class Rep1, class Period1, class Rep2, class Period2>
>       constexpr common_type_t<Rep1, Rep2>
>         operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

> > Returns: `CD(lhs).count() / CD(rhs).count()`.

In this case, we want `CD` to mean `common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>`. That way, the division has the expected semantics of dividing two quantities of the same dimension.

[ 2018-04-24 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4741.

> 1. Edit 30.5.6 [time.duration.nonmember] as indicated:

>    > -1- In the function descriptions that follow, unless stated otherwise, let `CD` represents the return type of the function.

>    > […]

>    >     template<class Rep1, class Period1, class Rep2, class Period2>
>    >       constexpr common_type_t<Rep1, Rep2>
>    >         operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

>    > > Let `CD` be `common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>`.

>    > > -10- Returns: `CD(lhs).count() / CD(rhs).count()`.

### 3109(i). `strstreambuf` is copyable

Section: 99 [depr.strstreambuf] Status: Resolved Submitter: Jonathan Wakely Opened: 2018-05-02 Last modified: 2025-11-11

Priority: 4

View all issues with Resolved status.

Discussion:

In C++03 `strstreambuf` was not copyable, because `basic_streambuf` wasn't copyable. In C++11 we made `basic_streambuf` copyable by derived classes, and `strstreambuf` doesn't define any special members, so it (unintentionally?) became copyable, with completely unspecified semantics.

VC++ and libc++ make it movable not copyable, and libstdc++ still follows C++03, so it's neither movable nor copyable. Making it movable seems to be the sane option, and consistent with `filebuf` and `stringbuf`.

[2018-06-18 after reflector discussion]

Priority set to 4

[2025-11-10 Resolved by the removal of `strstreambuf` via paper P2867R2 in Tokyo, 2024. Status changed: New → Resolved.]

Proposed resolution:

### 3110(i). Contiguous Iterators should always be Random-Access

Section: 24.3.1 [iterator.requirements.general] Status: Resolved Submitter: Marc Aldorasi Opened: 2018-05-07 Last modified: 2020-09-06

Priority: 3

View all other issues in [iterator.requirements.general].

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Discussion:

In [iterator.requirements.general] paragraph 6, contiguous iterators are defined in terms of general iterators, not random-access iterators. Since the defining expressions require random-access and the original paper's introduction describes contiguous iterators as a refinement of random-access iterators, contiguous iterators should be defined in terms of random-access iterators.

[2018-06-18 after reflector discussion]

Priority set to 3

Previous resolution [SUPERSEDED]:

> This wording is relative to N4741.

> 1. Edit 24.3.1 [iterator.requirements.general] as indicated:

>    > -6- Random-access iIterators that further satisfy the requirement that, for integral values `n` and dereferenceable iterator values `a` and `(a + n)`, `*(a + n)` is equivalent to `*(addressof(*a) + n)`, are called contiguous iterators. [Note: For example, the type "pointer to `int`" is a contiguous iterator, but `reverse_iterator<int *>` is not. For a valid iterator range `[a, b)` with dereferenceable a, the corresponding range denoted by pointers is `[addressof(*a), addressof(*a) + (b - a))`; `b` might not be dereferenceable. — end note]

[2020-05-03 Reflector discussion]

Resolved by P0894R4.

Rationale:

Resolved by P0894R4

Proposed resolution:

### 3111(i). Too strong precondition on `basic_string` constructor

Section: 27.4.3.3 [string.cons] Status: Resolved Submitter: Andrzej Krzemienski Opened: 2018-05-09 Last modified: 2020-09-06

Priority: 2

View all other issues in [string.cons].

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Discussion:

The following is the spec for `basic_string` constructor taking a pointer and a size in N4741 ([string.cons]/12-14):

> `basic_string(const charT* s, size_type n, const Allocator& a = Allocator());`

> (12) Requires: `s` points to an array of at least `n` elements of `charT`.

> (13) Effects: Constructs an object of class `basic_string` and determines its initial string value from the array of `charT` of length `n` whose first element is designated by `s`.

> (14) Postconditions: `data()` points at the first element of an allocated copy of the array whose first element is pointed at by `s`, `size()` is equal to `n`, and `capacity()` is a value at least as large as `size()`.

This implies that passing a null pointer and a zero size to this constructor is violating the precondition, as null pointer cannot be described as "pointing to an array of at least `n` elements of `charT`". On the other hand, being able to pass `{nullptr, 0}` is essential for `basic_string` to be able to inter-operate with other containers that are allowed to use the null pointer value to represent sequences of size zero:

>     std::vector<char> v{};
>     assert(v.data() == nullptr); // on some implementations
>     std::string s(v.data(), v.size()); // nullptr on some implementations

This has been already acknowledged as a defect in issue 2235(i) and applied, but the resolution still implies a too strong precondition.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4741.

> 1. Edit 27.4.3.3 [string.cons] as indicated:

>    >     basic_string(const charT* s, size_type n, const Allocator& a = Allocator());

>    > > -12- Requires: Unless `n == 0`, `s` points to an array of at least `n` elements of `charT`.

>    > > -13- Effects: Constructs an object of class `basic_string` and, unless `n == 0`, determines its initial string value from the array of `charT` of length `n` whose first element is designated by `s`.

>    > > -14- Postconditions: If `n != 0`, then `data()` points at the first element of an allocated copy of the array whose first element is pointed at by `s`,; `size()` is equal to `n`, and `capacity()` is a value at least as large as `size()`.

[2016-06-04 Marshall provides alternate resolution]

[2018-06-18 after reflector discussion]

Priority set to 2

[2018-08 mailing list discussion]

This will be resolved by Tim's string rework paper.

Resolved by the adoption of P1148 in San Diego.

Proposed resolution:

This wording is relative to N4750.

1. Edit 27.4.3.3 [string.cons] as indicated:

   >     basic_string(const charT* s, size_type n, const Allocator& a = Allocator());

   > > -12- Requires: `[s, s + n)` is a valid range `s` points to an array of at least `n` elements of `charT`.

   > > -13- Effects: Constructs an object of class `basic_string` and determines its initial string value from the range `[s, s + n)`array of `charT` of length `n` whose first element is designated by `s`.

   > > -14- Postconditions: `data()` points at the first element of an allocated copy of the array whose first element is pointed at by `s`, `size()` is equal to `n`, and `capacity()` is a value at least as large as `size()`, and `traits::compare(data(), s, n) == 0`.

### 3112(i). `system_error` and `filesystem_error` constructors taking a `string` may not be able to meet their postconditions

Section: 19.5.8.2 [syserr.syserr.members], 31.12.7.2 [fs.filesystem.error.members] Status: C++20 Submitter: Tim Song Opened: 2018-05-10 Last modified: 2021-06-06

Priority: 0

View all other issues in [syserr.syserr.members].

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Discussion:

The constructors of `system_error` and `filesystem_error` taking a `std::string` `what_arg` are specified to have a postcondition of `string(what()).find(what_arg) != string::npos` (or the equivalent with `string_view`). This is not possible if `what_arg` contains an embedded null character.

[2019-01-20 Reflector prioritization]

Set Priority to 0 and status to Tentatively Ready

Proposed resolution:

This wording is relative to N4727.

> Drafting note: This contains a drive-by editorial change to use `string_view` for these postconditions rather than `string`.

1. Edit 19.5.8.2 [syserr.syserr.members] p1-4 as indicated:

   >     system_error(error_code ec, const string& what_arg);

   > > -1- Effects: Constructs an object of class `system_error`.

   > > -2- Postconditions: `code() == ec` and `string_view(what()).find(what_arg.c_str()) != string_view::npos`.

   >     system_error(error_code ec, const char* what_arg);

   > > -3- Effects: Constructs an object of class `system_error`.

   > > -4- Postconditions: `code() == ec` and `string_view(what()).find(what_arg) != string_view::npos`.

2. Edit 19.5.8.2 [syserr.syserr.members] p7-10 as indicated:

   >     system_error(int ev, const error_category& ecat, const std::string& what_arg);

   > > -7- Effects: Constructs an object of class `system_error`.

   > > -8- Postconditions: `code() == error_code(ev, ecat)` and `string_view(what()).find(what_arg.c_str()) != string_view::npos`.

   >     system_error(int ev, const error_category& ecat, const char* what_arg);

   > > -9- Effects: Constructs an object of class `system_error`.

   > > -10- Postconditions: `code() == error_code(ev, ecat)` and `string_view(what()).find(what_arg) != string_view::npos`.

3. Edit [fs.filesystem_error.members] p2-4 as indicated:

   >     filesystem_error(const string& what_arg, error_code ec);

   > > -2- Postconditions:

   > > - `code() == ec`,
   > > - `path1().empty() == true`,
   > > - `path2().empty() == true`, and
   > > - `string_view(what()).find(what_arg.c_str()) != string_view::npos`.

   >     filesystem_error(const string& what_arg, const path& p1, error_code ec);

   > > -3- Postconditions:

   > > - `code() == ec`,
   > > - `path1()` returns a reference to the stored copy of `p1`,
   > > - `path2().empty() == true`, and
   > > - `string_view(what()).find(what_arg.c_str()) != string_view::npos`.

   >     filesystem_error(const string& what_arg, const path& p1, const path& p2, error_code ec);

   > > -4- Postconditions:

   > > - `code() == ec`,
   > > - `path1()` returns a reference to the stored copy of `p1`,
   > > - `path2()` returns a reference to the stored copy of `p2`,
   > > - `string_view(what()).find(what_arg.c_str()) != string_view::npos`.

### 3113(i). `polymorphic_allocator::construct()` should more closely match `scoped_allocator_adaptor::construct()`

Section: 20.5.3.3 [mem.poly.allocator.mem] Status: Resolved Submitter: Arthur O'Dwyer Opened: 2018-05-14 Last modified: 2020-09-06

Priority: 3

View all other issues in [mem.poly.allocator.mem].

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Discussion:

Based on my new understanding of how `uses_allocator` and `is_constructible` are supposed to play together, I think there was a minor defect in the resolution of LWG 2969(i) "`polymorphic_allocator::construct()` shouldn't pass `resource()`".

The calls to `is_constructible` in 20.5.3.3 [mem.poly.allocator.mem] (10.2), which were added to resolve LWG 2969(i), are missing an lvalue-qualification to match the lvalue-qualification of the parallel calls in [llocator.adaptor.member] (9.2).

The latter talks about constructibility from `inner_allocator_type&`; the former (after LWG 2969) talks about constructibility from `polymorphic_allocator` with no ref-qualification. But since we are perfect-forwarding `*this` through a tuple of references, we definitely are going to try to construct the target object from a `polymorphic_allocator&` and not from a `polymorphic_allocator`. I believe that the wording in 20.5.3.3 [mem.poly.allocator.mem] (10.2) needs to be updated to make the program ill-formed in cases where that construction is going to fail.

Orthogonally, I believe we need additional `std::move`'s in the sentence following 20.5.3.3 [mem.poly.allocator.mem] (10.8) for two reasons:

- We don't want to copy-construct the user's arguments that came in by-value as part of `x` and/or `y`.

- The value category of the argument to `pair`'s constructor is currently unspecified; it could reasonably be either an xvalue or a prvalue. Adding the `std::move` makes it clearer that we really want it to be an xvalue.

[2018-06-18 after reflector discussion]

Priority set to 3

[2019-02; Kona Wednesday night issue processing]

This was resolved by the adoption of P0591 in San Diego.

Proposed resolution:

This wording is relative to N4750.

1. Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:

   >     template<class T1, class T2, class... Args1, class... Args2>
   >       void construct(pair<T1, T2>* p, piecewise_construct_t, tuple<Args1...> x, tuple<Args2...> y);

   > > -9- […]

   > > -10- Effects:: […]

   > > 1. (10.1) — […]

   > > 2. (10.2) — Otherwise, if `uses_allocator_v<T1,polymorphic_allocator>` is `true` and `is_constructible_v<T1, allocator_arg_t, polymorphic_allocator&, Args1...>` is `true`, then `xprime` is `tuple_cat(make_tupletuple<allocator_arg_t, polymorphic_allocator&>(allocator_arg, *this), std::move(x))`.

   > > 3. (10.3) — Otherwise, if `uses_allocator_v<T1, polymorphic_allocator>` is `true` and `is_constructible_v<T1, Args1..., polymorphic_allocator&>` is `true`, then `xprime` is `tuple_cat(std::move(x), make_tupletuple<polymorphic_allocator&>(*this))`.

   > > 4. (10.4) — Otherwise the program is ill formed.

   > > Let `yprime` be a tuple constructed from `y` according to the appropriate rule from the following list:

   > > 1. (10.5) — […]

   > > 2. (10.6) — Otherwise, if `uses_allocator_v<T2, polymorphic_allocator>` is `true` and `is_constructible_v<T2, allocator_arg_t, polymorphic_allocator&, Args2...>` is `true`, then `yprime` is `tuple_cat(make_tupletuple<allocator_arg_t, polymorphic_allocator&>(allocator_arg, *this), std::move(y))`.

   > > 3. (10.7) — Otherwise, if `uses_allocator_v<T2, polymorphic_allocator>` is `true` and `is_constructible_v<T2, Args2..., polymorphic_allocator&>` is `true`, then `yprime` is `tuple_cat(std::move(y), make_tupletuple<polymorphic_allocator&>(*this))`.

   > > 4. (10.8) — Otherwise the program is ill formed.

   > > Then, using `piecewise_construct`, `std::move(xprime)`, and `std::move(yprime)` as the constructor arguments, this function constructs a `pair<T1, T2>` object in the storage whose address is represented by `p`.

### 3115(i). Unclear description for algorithm `includes`

Section: 26.8.7.2 [includes] Status: Resolved Submitter: Casey Carter Opened: 2018-05-24 Last modified: 2020-09-06

Priority: 3

View all other issues in [includes].

View all issues with Resolved status.

Discussion:

26.8.7.2 [includes]/1 states:

> Returns: `true` if `[first2, last2)` is empty or if every element in the range `[first2, last2)` is contained in the range `[first1, last1)`. Returns `false` otherwise.

but this program:

>     #include <algorithm>
>     #include <array>

>     int main() {
>       std::array<int, 1> a{1};
>       std::array<int, 3> b{1,1,1};
>       return std::includes(a.begin(), a.end(), b.begin(), b.end());
>     }

returns `0` on every implementation I can find, despite that every element in the range `b` is contained in the range `a`. The design intent of the algorithm is actually to determine if the sorted intersection of the elements from the two ranges — as would be computed by the `set_intersection` algorithm — is the same sequence as the range `[first2, last2)`. The specification should say so.

The complexity bound in 26.8.7.2 [includes]/2 is also unnecessarily high: straightforward implementations perform at most `2 * (last1 - first1)` comparisons.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> 1. Modify 26.8.7.2 [includes] as indicated:

>    >     template<class InputIterator1, class InputIterator2>
>    >       constexpr bool includes(InputIterator1 first1, InputIterator1 last1,
>    >                               InputIterator2 first2, InputIterator2 last2);
>    >     […]

>    > > -1- Returns: `true` if and only if `[first2, last2)` is empty or if every element in the range `[first2, last2)` is contained in the range `[first1, last1)`. Returns `false` otherwisea subsequence of `[first1, last1)`.

>    > > -2- Complexity: At most `2 * ((last1 - first1) + (last2 - first2)) - 1` comparisons.

[2018-06-27 after reflector discussion]

Priority set to 3. Improved wording as result of that discussion.

[2018-11-13; Casey Carter comments]

The acceptance of P0896R4 during the San Diego meeting resolves this issue: The wording in [includes] includes the PR for LWG 3115.

Proposed resolution:

This wording is relative to N4750.

1. Modify 26.8.7.2 [includes] as indicated:

   >     template<class InputIterator1, class InputIterator2>
   >       constexpr bool includes(InputIterator1 first1, InputIterator1 last1,
   >                               InputIterator2 first2, InputIterator2 last2);
   >     […]

   > > -1- Returns: `true` if and only if `[first2, last2)` is empty or if every element in the range `[first2, last2)` is contained in the range `[first1, last1)`. Returns `false` otherwisea subsequence of `[first1, last1)`. [Note: A sequence `S` is a subsequence of another sequence `T` if `S` can be obtained from `T` by removing some, all, or none of `T`'s elements and keeping the remaining elements in the same order. — end note].

   > > -2- Complexity: At most `2 * ((last1 - first1) + (last2 - first2)) - 1` comparisons.

### 3116(i). `OUTERMOST_ALLOC_TRAITS` needs `remove_reference_t`

Section: 20.6.4 [allocator.adaptor.members] Status: C++20 Submitter: Tim Song Opened: 2018-06-04 Last modified: 2021-02-25

Priority: 0

View all other issues in [allocator.adaptor.members].

View all issues with C++20 status.

Discussion:

`OUTERMOST_ALLOC_TRAITS(x)` is currently defined in 20.6.4 [allocator.adaptor.members]p1 as `allocator_traits<decltype(OUTERMOST(x))>`. However, `OUTERMOST(x)`, as defined and used in this subclause, is an lvalue for which `decltype` produces an lvalue reference. That referenceness needs to be removed before the type can be used with `allocator_traits`.

While we are here, the current wording for `OUTERMOST` uses the imprecise "if `x` does not have an `outer_allocator()` member function". What we meant to check is the validity of the expression `x.outer_allocator()`, not whether `x` has some (possibly ambiguous and/or inaccessible) member function named `outer_allocator`.

[2018-06 Rapperswil Thursday issues processing]

Status to Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

> [Drafting note: The subclause only uses `OUTERMOST_ALLOC_TRAITS(*this)` and only in non-`const` member functions, so the result is also non-`const`. Thus, `remove_reference_t` is sufficient; there's no need to further remove cv-qualification. — end drafting note]

1. Modify 20.6.4 [allocator.adaptor.members]p1 as indicated:

   > -1- In the `construct` member functions, `OUTERMOST(x)` is `x` if `x` does not have an `outer_allocator()` member function and `OUTERMOST(x.outer_allocator())` if the expression `x.outer_allocator()` is valid (13.10.3 [temp.deduct]) and `x` otherwise; `OUTERMOST_ALLOC_TRAITS(x)` is `allocator_traits<remove_reference_t<decltype(OUTERMOST(x))>>`. [Note: […] — end note]

### 3117(i). Missing `packaged_task` deduction guides

Section: 32.10.10 [futures.task] Status: C++23 Submitter: Marc Mutz Opened: 2018-06-08 Last modified: 2023-11-22

Priority: 3

View all other issues in [futures.task].

View all issues with C++23 status.

Discussion:

`std::function` has deduction guides, but `std::packaged_task`, which is otherwise very similar, does not. This is surprising to users and I can think of no reason for the former to be treated differently from the latter. I therefore propose to add deduction guides for packaged task with the same semantics as the existing ones for function.

[2018-06-23 after reflector discussion]

Priority set to 3

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> 1. Modify 32.10.10 [futures.task], class template `packaged_task` synopsis, as indicated:

>    >     namespace std {
>    >       […]
>    >       template<class R, class... ArgTypes>
>    >       class packaged_task<R(ArgTypes...)> {
>    >         […]
>    >       };

>    >       template<class R, class... ArgTypes>
>    >       packaged_task(R (*)( ArgTypes ...)) -> packaged_task<R( ArgTypes...)>;

>    >       template<class F> packaged_task(F) -> packaged_task<see below>;

>    >       template<class R, class... ArgTypes>
>    >         void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
>    >     }

> 2. Modify 32.10.10.2 [futures.task.members] as indicated:

>    >     template<class F>
>    >       packaged_task(F&& f);

>    > […]

>    >     template<class F> packaged_task(F) -> packaged_task<see below>;

>    > > -?- Remarks: This deduction guide participates in overload resolution only if `&F::operator()` is well-formed when treated as an unevaluated operand. In that case, if `decltype(&F::operator())` is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G`, then the deduced type is `packaged_task<R(A...)>`.

>    > […]

>    >     packaged_task(packaged_task&& rhs) noexcept;

[2020-02-13; Prague]

LWG improves wording matching Marshall's Mandating paper.

[2020-02-14; Prague]

Do we want a feature test macro for this new feature?

>     F N A
>     1 7 6

[Status to Ready on Friday in Prague.]

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 32.10.10 [futures.task], class template `packaged_task` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       template<class R, class... ArgTypes>
   >       class packaged_task<R(ArgTypes...)> {
   >         […]
   >       };

   >       template<class R, class... ArgTypes>
   >       packaged_task(R (*)(ArgTypes...)) -> packaged_task<R(ArgTypes...)>;

   >       template<class F> packaged_task(F) -> packaged_task<see below>;

   >       template<class R, class... ArgTypes>
   >         void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
   >     }

2. Modify 32.10.10.2 [futures.task.members] as indicated:

   >     template<class F>
   >       packaged_task(F&& f);

   > […]

   >     template<class F> packaged_task(F) -> packaged_task<see below>;

   > > -?- Constraints: `&F::operator()` is well-formed when treated as an unevaluated operand and `decltype(&F::operator())` is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G`.

   > > -?- Remarks: The deduced type is `packaged_task<R(A...)>`.

   > […]

   >     packaged_task(packaged_task&& rhs) noexcept;

### 3118(i). `fpos` equality comparison unspecified

Section: 31.5.3.3 [fpos.operations] Status: C++23 Submitter: Jonathan Wakely Opened: 2018-06-04 Last modified: 2023-11-22

Priority: 4

View all other issues in [fpos.operations].

View all issues with C++23 status.

Discussion:

The `fpos` requirements do not give any idea what is compared by `operator==` (even after Daniel's P0759R1 paper). I'd like something to make it clear that `return true;` is not a valid implementation of `operator==(const fpos<T>&, const fpos<T>&)`. Maybe in the `P(o)` row state that "`p == P(o)`" and "`p != P(o + 1)`", i.e. two `fpos` objects constructed from the same `streamoff` values are equal, and two `fpos` objects constructed from two different `streamoff` values are not equal.

[2018-06-23 after reflector discussion]

Priority set to 4

[2022-05-01; Daniel comments and provides wording]

The proposed wording does intentionally not use a form involving addition or subtraction to prevent the need for extra wording that ensures that this computed value is well-defined. According to 31.2.2 [stream.types], `streamoff` is a signed basic integral type, so we know what equality means for such values.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> - Modify in 31.5.3.3 [fpos.operations] as indicated:

>   > [Drafting note: The return type specification of `operator==` should be resolved in sync with D2167R2; see also LWG 2114(i).]

>   > 1. (1.1) — […]

>   > 2. […]

>   > 3. (1.5) — `o` and `o2` refers to a values of type `streamoff` or `const streamoff`.

>   > Table 119: Position type requirements [tab:fpos.operations]
>   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
>   > --- | --- | --- | ---
>   > …
>   > `P p(o); P p = o;` |  |  | Effects: Value-initializes the state object. Postconditions: `p == P(o)` is `true`.
>   > …
>   > `O(p)` | `streamoff` | converts to `offset` | `P(O(p)) == p`
>   > `p == q` | convertible to `bool` |  | Remarks: For any two values `o` and `o2`, if `p` is obtained from `o` converted to `P` or from a copy of such `P` value and if `q` is obtained from `o2` converted to `P` or from a copy of such `P` value, then `bool(p == q)` is `true` only if `o == o2` is `true`.
>   > `p != q` | convertible to `bool` | `!(p == q)` |
>   > …

[2022-11-02; Daniel comments and improves wording]

LWG discussion of P2167R2 has shown preference to require that the equality operations of `fpos` should be specified to have type `bool` instead of being specified as "convertible to `bool`". This has been reflected in the most recent paper revision P2167R3. The below wording changes follow that direction to reduce the wording mismatch to a minimum.

[2022-11-02; LWG telecon]

Moved to Ready. For: 6, Against: 0, Neutral: 0

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

- Modify in 31.5.3.3 [fpos.operations] as indicated:

  > 1. (1.1) — […]

  > 2. […]

  > 3. (1.5) — `o` and `o2` refers to a values of type `streamoff` or `const streamoff`.

  > Table 124: Position type requirements [tab:fpos.operations]
  > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
  > --- | --- | --- | ---
  > …
  > `P p(o); P p = o;` |  |  | Effects: Value-initializes the state object. Postconditions: `p == P(o)` is `true`.
  > …
  > `O(p)` | `streamoff` | converts to `offset` | `P(O(p)) == p`
  > `p == q` | `bool` |  | Remarks: For any two values `o` and `o2`, if `p` is obtained from `o` converted to `P` or from a copy of such `P` value and if `q` is obtained from `o2` converted to `P` or from a copy of such `P` value, then `p == q` is `true` only if `o == o2` is `true`.
  > `p != q` | convertible to `bool` | `!(p == q)` |
  > …

### 3119(i). Program-definedness of closure types

Section: 3.42 [defns.prog.def.spec] Status: C++20 Submitter: Hubert Tong Opened: 2018-06-09 Last modified: 2021-06-06

Priority: 2

View all issues with C++20 status.

Discussion:

The description of closure types in 7.5.6.2 [expr.prim.lambda.closure] says:

> An implementation may define the closure type differently […]

The proposed resolution to LWG 2139(i) defines a "program-defined type" to be a

> class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or an instantiation of a program-defined specialization

I am not sure that the intent of whether closure types are or are not program-defined types is clearly conveyed by the wording.

[2018-06-23 after reflector discussion]

Priority set to 2

[2018-08-14 Casey provides additional discussion and a Proposed Resolution]

We use the term "program-defined" in the library specification to ensure that two users cannot create conflicts in a component in namespace `std` by specifying different behaviors for the same type. For example, we allow users to specialize `common_type` when at least one of the parameters is a program-defined type. Since two users cannot define the same program-defined type, this rule prevents two users (or libraries) defining the same specialization of `std::common_type`.

Since it's guaranteed that even distinct utterances of identical lambda expressions produce closures with distinct types (7.5.6.2 [expr.prim.lambda.closure]), adding closure types to our term "program-defined type" is consistent with the intended use despite that such types are technically defined by the implementation.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4762.

> - Modify 3.43 [defns.prog.def.type] as follows:

>   > program-defined type
>   > class type or enumeration type that is not part of the C++ standard library and not defined by the implementation (except for closure types (7.5.6.2 [expr.prim.lambda.closure]) for program-defined lambda expressions), or an instantiation of a program-defined specialization

[2018-08-23 Batavia Issues processing]

Updated wording

[2018-11 San Diego Thursday night issue processing]

Status to Ready.

Proposed resolution:

This wording is relative to N4762.

- Modify 3.43 [defns.prog.def.type] as follows:

  > program-defined type
  > non-closure class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or a closure type of a non-implementation-provided lambda expression, or an instantiation of a program-defined specialization

### 3120(i). Unclear behavior of `monotonic_buffer_resource::release()`

Section: 20.5.6.3 [mem.res.monotonic.buffer.mem] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2018-06-10 Last modified: 2023-11-22

Priority: 2

View all other issues in [mem.res.monotonic.buffer.mem].

View all issues with C++23 status.

Discussion:

The effects of `monotonic_buffer_resource::release()` are defined as:

> Calls `upstream_rsrc->deallocate()` as necessary to release all allocated memory.

This doesn't give any instruction on what to do with the memory controlled by the `monotonic_buffer_resource` which was not allocated, i.e., what to do with the initial buffer provided to its constructor.

Boost.Container's pmr implementation expels its initial buffer after a `release()`. Arthur O'Dwyer's proposed pmr implementation for libc++ reuses the initial buffer after a `release()`, on the assumption that this is what the average library user will be expecting.

>     #include <memory_resource>

>     int main()
>     {
>       char buffer[100];
>       {
>         std::pmr::monotonic_buffer_resource mr(buffer, 100, std::pmr::null_memory_resource());
>         mr.release();
>         mr.allocate(60);  // A
>       }
>       {
>         std::pmr::monotonic_buffer_resource mr(buffer, 100, std::pmr::null_memory_resource());
>         mr.allocate(60);  // B
>         mr.release();
>         mr.allocate(60);  // C
>       }
>     }

Assume that allocation "B" always succeeds.
With the proposed libc++ implementation, allocations "A" and "C" both succeed.
With Boost.Container's implementation, allocations "A" and "C" both fail.
Using another plausible implementation strategy, allocation "A" could succeed but allocation "C" could fail. I have been informed that MSVC's implementation does this.

Which of these strategies should be permitted by the Standard?

Arthur considers "A and C both succeed" to be the obviously most user-friendly strategy, and really really hopes it's going to be permitted. Requiring "C" to succeed is unnecessary (and would render MSVC's current implementation non-conforming) but could help programmers concerned with portability between different implementations.

Another side-effect of `release()` which goes underspecified by the Standard is the effect of `release()` on `next_buffer_size`. As currently written, my interpretation is that `release()` is not permitted to decrease `current_buffer_size`; I'm not sure if this is a feature or a bug.

Consider this test case (taken from here):

>     std::pmr::monotonic_buffer_resource mr(std::pmr::new_delete_resource());
>     for (int i=0; i < 100; ++i) {
>       mr.allocate(1);  // D
>       mr.release();
>     }

Arthur believes it is important that the 100th invocation of line "D" does not attempt to allocate 2100 bytes from the upstream resource.

[2018-06-23 after reflector discussion]

Priority set to 2

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> > [Drafting note: The resolution depicted below would make MSVC's and my-proposed-libc++'s implementations both conforming.]

> 1. Modify 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:

>    >     void release();

>    > > -1- Effects: Calls `upstream_rsrc->deallocate()` as necessary to release all allocated memory. Resets the state of the initial buffer.

>    > > -2- [Note: The memory is released back to `upstream_rsrc` even if some blocks that were allocated from this have not been deallocated from this. This function has an unspecified effect on `next_buffer_size`. — end note]

[2018-08-23 Batavia Issues processing]

We liked Pablo's wording from the reflector discussion. Status to Open.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> 1. Modify 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:

>    >     void release();

>    > > -1- Effects: Calls `upstream_rsrc->deallocate()` as necessary to release all allocated memory. Resets `*this` to its initial state at construction.

[2020-10-03; Daniel comments and provides improved wording]

The recent wording introduces the very generic term "state" without giving a concrete definition of that term. During reflector discussions different interpretations of that term were expressed. The revised wording below gets rid of that word and replaces it by the actually involved exposition-only members.

[2020-10-06; moved to Tentatively Ready after seven votes in favour in reflector poll]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:

   >     void release();

   > > -1- Effects: Calls `upstream_rsrc->deallocate()` as necessary to release all allocated memory. Resets `current_buffer` and `next_buffer_size` to their initial values at construction.

### 3121(i). `tuple` constructor constraints for `UTypes&&...` overloads

Section: 22.4.4.2 [tuple.cnstr] Status: C++23 Submitter: Matt Calabrese Opened: 2018-06-12 Last modified: 2023-11-22

Priority: 2

View other active issues in [tuple.cnstr].

View all other issues in [tuple.cnstr].

View all issues with C++23 status.

Discussion:

Currently the `tuple` constructors of the form:

>     template<class... UTypes>
>     EXPLICIT constexpr tuple(UTypes&&...);

are not properly constrained in that in the 1-element `tuple` case, the constraints do no short-circuit when the constructor would be (incorrectly) considered as a possible copy/move constructor candidate. libc++ has a workaround for this, but the additional short-circuiting does not actually appear in the working draft.

As an example of why this lack of short circuiting is a problem in practice, consider the following line:

>     bool a = std::is_copy_constructible_v<std::tuple<any>>;

The above code will cause a compile error because of a recursive trait definition. The copy constructibility check implies doing substitution into the `UTypes&&...` constructor overloads, which in turn will check if `tuple<any>` is convertible to any, which in turn will check if `tuple<any>` is copy constructible (and so the trait is dependent on itself).

I do not provide wording for the proposed fix in anticipation of requires clauses potentially changing how we do the specification, however, the basic solution should be similar to what we've done for other standard library types, which is to say that the very first constraint should be to check that if `sizeof...(UTypes) == 1` and the type, after applying `remove_cvref_t`, is the `tuple` type itself, then we should force substitution failure rather than checking any further constraints.

[2018-06-23 after reflector discussion]

Priority set to 3

[2018-08-20, Jonathan provides wording]

[2018-08-20, Daniel comments]

The wording changes by this issue are very near to those suggested for LWG 3155(i).

[2018-11 San Diego Thursday night issue processing]

Jonathan to update wording - using conjunction. Priority set to 2

Previous resolution [SUPERSEDED]:

> This wording is relative to N4762.

> - Modify 22.4.4.2 [tuple.cnstr] as indicated:

>   >     template<class... UTypes> explicit(see below) constexpr tuple(UTypes&&... u);

>   > > -9- Effects: Initializes the elements in the tuple with the corresponding value in `std::forward<UTypes>(u)`.

>   > > -10- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) == sizeof...(UTypes)` and `sizeof...(Types) >= 1` and `(sizeof...(Types) > 1 || !is_same_v<remove_cvref_t<U0>, tuple>)` and `is_constructible_v<Ti, Ui&&>` is `true` for all `i`. The expression inside `explicit` is equivalent to:

>   > > `!conjunction_v<is_convertible<UTypes, Types>...>`

[2021-05-20 Tim updates wording]

The new wording below also resolves LWG 3155(i), relating to an `allocator_arg_t` tag argument being treated by this constructor template as converting to the first tuple element instead of as a tag. To minimize collateral damage, this wording takes this constructor out of overload resolution only if the tuple is of size 2 or 3, the first argument is an `allocator_arg_t`, but the first tuple element isn't of type `allocator_arg_t` (in both cases after removing cv/ref qualifiers). This avoids damaging tuples that actually contain an `allocator_arg_t` as the first element (which can be formed during uses-allocator construction, thanks to `uses_allocator_construction_args`).

The proposed wording has been implemented and tested on top of libstdc++.

[2021-08-20; LWG telecon]

Set status to Tentatively Ready after telecon review.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885, and also resolves LWG 3155(i).

1. Modify 22.4.4.2 [tuple.cnstr] as indicated:

   >     template<class... UTypes> explicit(see below) constexpr tuple(UTypes&&... u);

   > > -?- Let `disambiguating-constraint` be:

   > > 1. (?.1) — `negation<is_same<remove_cvref_t<U0>, tuple>>` if `sizeof...(Types)` is 1;

   > > 2. (?.2) — otherwise, `bool_constant<!is_same_v<remove_cvref_t<U0>, allocator_arg_t> || is_same_v<remove_cvref_t<T0>, allocator_arg_t>>` if `sizeof...(Types)` is 2 or 3;

   > > 3. (?.3) — otherwise, `true_type`.

   > > -12- Constraints:

   > > 1. (12.1) — `sizeof...(Types)` equals `sizeof...(UTypes)`, and

   > > 2. (12.2) — `sizeof...(Types)` ≥ 1, and

   > > 3. (12.3) — `conjunction_v<disambiguating-constraint, is_constructible<Types, UTypes>...>` is `true` `is_constructible_v<Ti, Ui>` is `true` for all i.

   > > -13- Effects: Initializes the elements in the tuple with the corresponding value in `std::forward<UTypes>(u)`.

   > > -14- Remarks: The expression inside `explicit` is equivalent to:

   > > `!conjunction_v<is_convertible<UTypes, Types>...>`

### 3122(i). `__cpp_lib_chrono_udls` was accidentally dropped

Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-06-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [support.limits.general].

View all issues with C++20 status.

Discussion:

Between P0941R0 and P0941R1/P0941R2, the feature-test macro `__cpp_lib_chrono_udls` was dropped. It wasn't mentioned in the changelog, and Jonathan Wakely and I believe that this was unintentional.

[2018-06-23 Moved to Tentatively Ready after 5 positive votes on c++std-lib.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to the post-Rapperswil 2018 working draft.

In 17.3.1 [support.limits.general], "Table ??? - Standard library feature-test macros", add the following row:

> Table ??? — Standard library feature-test macros
> Macro name | Value | Headers
> --- | --- | ---
> `[…]`
> `__cpp_lib_chrono_udls` | `201304L` | `<chrono>`
> `[…]`

### 3123(i). `duration` constructor from representation shouldn't be effectively non-throwing

Section: 30.5 [time.duration] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2018-06-22 Last modified: 2023-11-22

Priority: 3

View all other issues in [time.duration].

View all issues with C++23 status.

Discussion:

[time.duration]/4 states:

> Members of duration shall not throw exceptions other than those thrown by the indicated operations on their representations.

Where representation is defined in the non-normative, brief description at [time.duration]/1:

> […] A duration has a representation which holds a count of ticks and a tick period. […]

[time.duration.cons]/2 doesn't indicate the operation undergone by its representation, merely stating a postcondition in [time.duration.cons]/3:

> Effects: Constructs an object of type `duration`.

> Postconditions: `count() == static_cast<rep>(r)`.

I suggest this reformulation that follows the format of [time.duration.cons]/5.

> Effects: Constructs an object of type duration, constructing `rep_` from `r`.

Now it is clear why the constructor would throw.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> Change 30.5.2 [time.duration.cons] as indicated:

> >     template<class Rep2>
> >       constexpr explicit duration(const Rep2& r);

> > > -1- Remarks: This constructor shall not participate in overload resolution unless […]

> > > -2- Effects: Constructs an object of type `duration`, constructing `rep_` from `r`.

> > > -3- Postconditions: `count() == static_cast<rep>(r)`.

[2018-06-27 after reflector discussion]

Priority set to 3. Improved wording as result of that discussion.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> Change 30.5.2 [time.duration.cons] as indicated:

> >     template<class Rep2>
> >       constexpr explicit duration(const Rep2& r);

> > > -1- Remarks: This constructor shall not participate in overload resolution unless […]

> > > -2- Effects: Constructs an object of type `duration` Initializes `rep_` with `r`.

> > > -3- Postconditions: `count() == static_cast<rep>(r)`.

[2020-05-02; Daniel resyncs wording with recent working draft]

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4861.

Change 30.5.2 [time.duration.cons] as indicated:

>     template<class Rep2>
>       constexpr explicit duration(const Rep2& r);

> > -1- Constraints: `is_convertible_v<const Rep2&, rep>` is `true` and

> > 1. (1.1) — `treat_as_floating_point_v<rep>` is `true` or

> > 2. (1.2) — `treat_as_floating_point_v<Rep2>` is `false`.

> > [Example: […] end example]

> > -?- Effects: Initializes `rep_` with `r`.

> > -2- Postconditions: `count() == static_cast<rep>(r)`.

### 3125(i). `duration` streaming precondition should be a SFINAE condition

Section: 30.5.11 [time.duration.io] Status: Resolved Submitter: Johel Ernesto Guerrero Peña Opened: 2018-06-23 Last modified: 2020-11-09

Priority: 2

View all other issues in [time.duration.io].

View all issues with Resolved status.

Discussion:

30.5.11 [time.duration.io]/1 states:

>     template<class charT, class traits, class Rep, class Period>
>       basic_ostream<charT, traits>&
>         operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);

> > Requires: `Rep` is an integral type whose integer conversion rank (6.9.6 [conv.rank]) is greater than or equal to that of `short`, or a floating point type. `charT` is `char` or `wchar_t`.

I think the intention was to make this a compile-time error, since all the information to make it so is available at compile-time. But the wording doesn't suggest that.

[2018-06-29; Daniel comments]

The wording will be significantly simplified by the application of the new Library element Constraints: introduced by P0788R3 and available with the post-Rapperswil working draft.

[2018-07-20 Priority set to 2 after reflector discussion. NAD and P0 were also mentioned.]

[2018-08-23 Batavia Issues processing]

Marshall to talk to Howard about his intent. Status to Open

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> Change 30.5.11 [time.duration.io] as indicated:

> > [Drafting note: The suggested wording changes include the insertion of two bullets into the existing running text to improve clarification of the logic arm of the "unless"].

> >     template<class charT, class traits, class Rep, class Period>
> >       basic_ostream<charT, traits>&
> >         operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);

> > > -1- RequiresRemarks: This function shall not participate in overload resolution unless:

> > > - `Rep` is an integral type whose integer conversion rank (6.9.6 [conv.rank]) is greater than or equal to that of `short`, or a floating point type., and

> > > - `charT` is `char` or `wchar_t`

> > > […]

[2020-02-13, Prague]

Will be resolved by LWG 3317(i).

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

### 3127(i). `basic_osyncstream::rdbuf` needs a `const_cast`

Section: 31.11.3.1 [syncstream.osyncstream.overview] Status: C++20 Submitter: Tim Song Opened: 2018-06-29 Last modified: 2021-02-25

Priority: 0

View all other issues in [syncstream.osyncstream.overview].

View all issues with C++20 status.

Discussion:

The current specification of `basic_osyncstream::rdbuf()` is

>         syncbuf_type* rdbuf() const noexcept { return &sb; }

This is ill-formed because the exposition-only member `sb` is `const` inside this `const` member function, but the return type is a pointer to non-`const` `syncbuf_type`. It needs to cast away the constness, consistent with the other streams with embedded stream buffers (such as string and file streams).

[2018-07-20 Status set to Tentatively Ready after five positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Change 31.11.3.1 [syncstream.osyncstream.overview], class template `basic_osyncstream` synopsis, as indicated:

   >     namespace std {
   >       template<class charT, class traits, class Allocator>
   >       class basic_osyncstream : public basic_ostream<charT, traits> {
   >       public:
   >         […]

   >         // 31.11.3.3 [syncstream.osyncstream.members], member functions
   >         void emit();
   >         streambuf_type* get_wrapped() const noexcept;
   >         syncbuf_type* rdbuf() const noexcept { return const_cast<syncbuf_type*>(&sb); }
   >         […]
   >       };
   >     }

### 3128(i). `strstream::rdbuf` needs a `const_cast`

Section: 99 [depr.strstream.oper] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

`strstream::rdbuf` has the same issue with a missing `const_cast` on `&sb`.

Somewhat amusingly, `istrstream::rdbuf` and `ostrstream::rdbuf` got this right, but each with a different style (see issue 252(i)).

[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Change 99 [depr.strstream.oper] p1 as indicated:

   >     strstreambuf* rdbuf() const;

   > > -1- Returns: `const_cast<strstreambuf*>(&sb)`.

### 3129(i). `regex_token_iterator` constructor uses wrong pointer arithmetic

Section: 28.6.11.2.2 [re.tokiter.cnstr] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25

Priority: 0

View all other issues in [re.tokiter.cnstr].

View all issues with C++20 status.

Discussion:

The specification of `regex_token_iterator` for the overload taking a `const int (&submatchs)[N]` uses the range `[&submatches, &submatches + N)`. This is obviously incorrect; we want to perform pointer arithmetic on a pointer to the first element of that array, not a pointer to the whole array.

[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Change 28.6.11.2.2 [re.tokiter.cnstr] p3 as indicated:

   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          int submatch = 0,
   >                          regex_constants::match_flag_type m = regex_constants::match_default);

   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          const vector<int>& submatches,
   >                          regex_constants::match_flag_type m = regex_constants::match_default);

   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          initializer_list<int> submatches,
   >                          regex_constants::match_flag_type m = regex_constants::match_default);

   >     template<size_t N>
   >       regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                            const regex_type& re,
   >                            const int (&submatches)[N],
   >                            regex_constants::match_flag_type m = regex_constants::match_default);

   > > -2- Requires: […]

   > > -3- Effects: The first constructor initializes the member `subs` to hold the single value `submatch`. The second constructor initializes the member `subs` to hold a copy of the argument `submatches`. The second, third and fourth constructors initialize the member `subs` to hold a copy of the sequence of integer values pointed to by the iterator range `[submatches.begin(), submatches.end())` and `[&submatches, &submatches + N)`, respectively`[begin(submatches), end(submatches))`.

   > > -4- […]

### 3130(i). §[input.output] needs many `addressof`

Section: 31 [input.output] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25

Priority: 0

View other active issues in [input.output].

View all other issues in [input.output].

View all issues with C++20 status.

Discussion:

There are 27 instances of `&sb` and one instance of `&rhs` in Clause 31 [input.output], each of which needs to use `addressof` because the operand has a user-provided template type parameter as an associated class and so the use of unary `&` is subject to ADL hijacking.

[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

1. Change 31.5.4.3 [basic.ios.members] p16 as indicated:

   >     basic_ios& copyfmt(const basic_ios& rhs);

   > > -16- Effects: If `(this == &addressof(rhs))` does nothing. […]

2. Change 31.8.3.2 [istringstream.cons] as indicated:

   >     explicit basic_istringstream(ios_base::openmode which);

   > > -1- Effects: Constructs an object of class `basic_istringstream<charT, traits>`, initializing the base class with `basic_istream<charT, traits>(&addressof(sb))` (31.7.5.2 [istream]) and initializing `sb` with `basic_stringbuf<charT, traits, Allocator>(which | ios_base::in)` (31.8.2.2 [stringbuf.cons]).

   >     explicit basic_istringstream(
   >       const basic_string<charT, traits, Allocator>& str,
   >       ios_base::openmode which = ios_base::in);

   > > -2- Effects: Constructs an object of class `basic_istringstream<charT, traits>`, initializing the base class with `basic_istream<charT, traits>(&addressof(sb))` (31.7.5.2 [istream]) and initializing `sb` with `basic_stringbuf<charT, traits, Allocator>(str, which | ios_base::in)` (31.8.2.2 [stringbuf.cons]).

   >     basic_istringstream(basic_istringstream&& rhs);

   > > -3- Effects: Move constructs from the rvalue `rhs`. This is accomplished by move constructing the base class, and the contained `basic_stringbuf`. Next `basic_istream<charT, traits>::set_rdbuf(&addressof(sb))` is called to install the contained `basic_stringbuf`.

3. Change 31.8.3.4 [istringstream.members] p1 as indicated:

   >     basic_stringbuf<charT, traits, Allocator>* rdbuf() const;

   > > -1- Returns: `const_cast<basic_stringbuf<charT, traits, Allocator>*>(&addressof(sb))`.

4. Change 31.8.4.2 [ostringstream.cons] as indicated:

   >     explicit basic_ostringstream(ios_base::openmode which);

   > > -1- Effects: Constructs an object of class `basic_ostringstream<charT, traits>`, initializing the base class with `basic_ostream<charT, traits>(&addressof(sb))` (31.7.6.2 [ostream]) and initializing `sb` with `basic_stringbuf<charT, traits, Allocator>(which | ios_base::out)` (31.8.2.2 [stringbuf.cons]).

   >     explicit basic_ostringstream(
   >       const basic_string<charT, traits, Allocator>& str,
   >       ios_base::openmode which = ios_base::out);

   > > -2- Effects: Constructs an object of class `basic_ostringstream<charT, traits>`, initializing the base class with `basic_ostream<charT, traits>(&addressof(sb))` (31.7.6.2 [ostream]) and initializing `sb` with `basic_stringbuf<charT, traits, Allocator>(str, which | ios_base::out)` (31.8.2.2 [stringbuf.cons]).

   >     basic_ostringstream(basic_ostringstream&& rhs);

   > > -3- Effects: Move constructs from the rvalue `rhs`. This is accomplished by move constructing the base class, and the contained `basic_stringbuf`. Next `basic_ostream<charT, traits>::set_rdbuf(&addressof(sb))` is called to install the contained `basic_stringbuf`.

5. Change 31.8.4.4 [ostringstream.members] p1 as indicated:

   >     basic_stringbuf<charT, traits, Allocator>* rdbuf() const;

   > > -1- Returns: `const_cast<basic_stringbuf<charT, traits, Allocator>*>(&addressof(sb))`.

6. Change 31.8.5.2 [stringstream.cons] as indicated:

   >     explicit basic_stringstream(ios_base::openmode which);

   > > -1- Effects: Constructs an object of class `basic_stringstream<charT, traits>`, initializing the base class with `basic_iostream<charT, traits>(&addressof(sb))` (31.7.5.7.2 [iostream.cons]) and initializing `sb` with `basic_stringbuf<charT, traits, Allocator>(which)`.

   >     explicit basic_stringstream(
   >       const basic_string<charT, traits, Allocator>& str,
   >       ios_base::openmode which = ios_base::out | ios_base::in);

   > > -2- Effects: Constructs an object of class `basic_stringstream<charT, traits>`, initializing the base class with `basic_iostream<charT, traits>(&addressof(sb))` (31.7.5.7.2 [iostream.cons]) and initializing `sb` with `basic_stringbuf<charT, traits, Allocator>(str, which)`.

   >     basic_stringstream(basic_stringstream&& rhs);

   > > -3- Effects: Move constructs from the rvalue `rhs`. This is accomplished by move constructing the base class, and the contained `basic_stringbuf`. Next `basic_istream<charT, traits>::set_rdbuf(&addressof(sb))` is called to install the contained `basic_stringbuf`.

7. Change 31.8.5.4 [stringstream.members] p1 as indicated:

   >     basic_stringbuf<charT, traits, Allocator>* rdbuf() const;

   > > -1- Returns: `const_cast<basic_stringbuf<charT, traits, Allocator>*>(&addressof(sb))`.

8. Change 31.10.4.2 [ifstream.cons] as indicated:

   >     basic_ifstream();

   > > -1- Effects: Constructs an object of class `basic_ifstream<charT, traits>`, initializing the base class with `basic_istream<charT, traits>(&addressof(sb))` (31.7.5.2.2 [istream.cons]) and initializing `sb` with `basic_filebuf<charT, traits>()` (31.10.3.2 [filebuf.cons]).

   >     explicit basic_ifstream(const char* s,
   >                             ios_base::openmode mode = ios_base::in);
   >     explicit basic_ifstream(const filesystem::path::value_type* s,
   >                             ios_base::openmode mode = ios_base::in);  // wide systems only; see 31.10.1 [fstream.syn]

   > > -2- Effects: Constructs an object of class `basic_ifstream<charT, traits>`, initializing the base class with `basic_istream<charT, traits>(&addressof(sb))` (31.7.5.2.2 [istream.cons]) and initializing `sb` with `basic_filebuf<charT, traits>()` (31.10.3.2 [filebuf.cons]), then calls `rdbuf()->open(s, mode | ios_base::in)`. If that function returns a null pointer, calls `setstate(failbit)`.

   > […]

   >     basic_ifstream(basic_ifstream&& rhs);

   > > -4- Effects: Move constructs from the rvalue `rhs`. This is accomplished by move constructing the base class, and the contained `basic_filebuf`. Next `basic_istream<charT, traits>::set_rdbuf(&addressof(sb))` is called to install the contained `basic_filebuf`.

9. Change 31.10.4.4 [ifstream.members] p1 as indicated:

   >     basic_filebuf<charT, traits>* rdbuf() const;

   > > -1- Returns: `const_cast<basic_filebuf<charT, traits>*>(&addressof(sb))`.

10. Change 31.10.5.2 [ofstream.cons] as indicated:

    >     basic_ofstream();

    > > -1- Effects: Constructs an object of class `basic_ofstream<charT, traits>`, initializing the base class with `basic_ostream<charT, traits>(&addressof(sb))` (31.7.6.2.2 [ostream.cons]) and initializing `sb` with `basic_filebuf<charT, traits>()` (31.10.3.2 [filebuf.cons]).

    >     explicit basic_ofstream(const char* s,
    >                             ios_base::openmode mode = ios_base::out);
    >     explicit basic_ofstream(const filesystem::path::value_type* s,
    >                             ios_base::openmode mode = ios_base::out);  // wide systems only; see 31.10.1 [fstream.syn]

    > > -2- Effects: Constructs an object of class `basic_ofstream<charT, traits>`, initializing the base class with `basic_ostream<charT, traits>(&addressof(sb))` (31.7.6.2.2 [ostream.cons]) and initializing `sb` with `basic_filebuf<charT, traits>()` (31.10.3.2 [filebuf.cons]), then calls `rdbuf()->open(s, mode | ios_base::out)`. If that function returns a null pointer, calls `setstate(failbit)`.

    > […]

    >     basic_ofstream(basic_ofstream&& rhs);

    > > -4- Effects: Move constructs from the rvalue `rhs`. This is accomplished by move constructing the base class, and the contained `basic_filebuf`. Next `basic_ostream<charT, traits>::set_rdbuf(&addressof(sb))` is called to install the contained `basic_filebuf`.

11. Change 31.10.5.4 [ofstream.members] p1 as indicated:

    >     basic_filebuf<charT, traits>* rdbuf() const;

    > > -1- Returns: `const_cast<basic_filebuf<charT, traits>*>(&addressof(sb))`.

12. Change 31.10.6.2 [fstream.cons] as indicated:

    >     basic_fstream();

    > > -1- Effects: Constructs an object of class `basic_fstream<charT, traits>`, initializing the base class with `basic_iostream<charT, traits>(&addressof(sb))` (31.7.5.7.2 [iostream.cons]) and initializing `sb` with `basic_filebuf<charT, traits>()`.

    >     explicit basic_fstream(
    >       const char* s,
    >       ios_base::openmode mode = ios_base::in | ios_base::out);
    >     explicit basic_fstream(
    >       const filesystem::path::value_type* s,
    >       ios_base::openmode mode = ios_base::in | ios_base::out);  // wide systems only; see 31.10.1 [fstream.syn]

    > > -2- Effects: Constructs an object of class `basic_fstream<charT, traits>`, initializing the base class with `basic_iostream<charT, traits>(&addressof(sb))` (31.7.5.7.2 [iostream.cons]) and initializing `sb` with `basic_filebuf<charT, traits>()`, then calls `rdbuf()->open(s, mode)`. If that function returns a null pointer, calls `setstate(failbit)`.

    > […]

    >     basic_fstream(basic_fstream&& rhs);

    > > -4- Effects: Move constructs from the rvalue `rhs`. This is accomplished by move constructing the base class, and the contained `basic_filebuf`. Next `basic_istream<charT, traits>::set_rdbuf(&addressof(sb))` is called to install the contained `basic_filebuf`.

13. Change 31.10.6.4 [fstream.members] p1 as indicated:

    >     basic_filebuf<charT, traits>* rdbuf() const;

    > > -1- Returns: `const_cast<basic_filebuf<charT, traits>*>(&addressof(sb))`.

14. Change 31.11.3.1 [syncstream.osyncstream.overview], class template `basic_osyncstream` synopsis, as indicated:

    > [Drafting note: The text shown below assumes the application of the proposed resolution of issue 3127(i).]

    >     namespace std {
    >       template<class charT, class traits, class Allocator>
    >       class basic_osyncstream : public basic_ostream<charT, traits> {
    >       public:
    >         […]

    >         // 31.11.3.3 [syncstream.osyncstream.members], member functions
    >         void emit();
    >         streambuf_type* get_wrapped() const noexcept;
    >         syncbuf_type* rdbuf() const noexcept { return const_cast<syncbuf_type*>(&addressof(sb)); }
    >         […]
    >       };
    >     }

15. Change 31.11.3.2 [syncstream.osyncstream.cons] p1 and p4 as indicated:

    >     basic_osyncstream(streambuf_type* buf, const Allocator& allocator);

    > > -1- Effects: Initializes `sb` from `buf` and `allocator`. Initializes the base class with `basic_ostream<charT, traits>(&addressof(sb))`.

    > > -2- […]

    > > -3- […]

    >     basic_osyncstream(basic_osyncstream&& other) noexcept;

    > > -4- Effects: Move constructs the base class and `sb` from the corresponding subobjects of `other`, and calls `basic_ostream<charT, traits>::set_rdbuf(&addressof(sb))`.

    > > -5- […]

### 3131(i). `addressof` all the things

Section: 30.13 [time.parse], 27.4.3.8.1 [string.accessors], 27.3.3 [string.view.template], 23.2.2 [container.requirements.general], 24.3.5.4 [output.iterators], 24.3.5.6 [bidirectional.iterators], 28.6.6 [re.traits], 28.6.11.1 [re.regiter], 32.6.5.2 [thread.lock.guard] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25

Priority: 0

View other active issues in [time.parse].

View all other issues in [time.parse].

View all issues with C++20 status.

Discussion:

Some additional instances where the library specification applies unary operator `&` when it should use `addressof`.

[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

> [Drafting note: Two uses of `&` in 24.5.1 [reverse.iterators] are not included in the wording below because the entire sentence is slated to be removed by a revision of P0896, the One Ranges Proposal.]

1. Change 30.13 [time.parse] p4-5 as indicated:

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   >               basic_string<charT, traits, Alloc>& abbrev);

   > > -4- Remarks: This function shall not participate in overload resolution unless

   > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, &addressof(abbrev))

   > > is a valid expression.

   > > -5- Returns: A manipulator that, when extracted from a `basic_istream<charT, traits> is`, calls `from_stream(is, fmt.c_str(), tp, &addressof(abbrev))`.

2. Change 30.13 [time.parse] p8-9 as indicated:

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   >               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

   > > -8- Remarks: This function shall not participate in overload resolution unless

   > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, &addressof(abbrev), &offset)

   > > is a valid expression.

   > > -9- Returns: A manipulator that, when extracted from a `basic_istream<charT, traits> is`, calls `from_stream(is, fmt.c_str(), tp, &addressof(abbrev), &offset)`.

3. Change 27.4.3.8.1 [string.accessors] p1 and p4 as indicated:

   >     const charT* c_str() const noexcept;
   >     const charT* data() const noexcept;

   > > -1- Returns: A pointer `p` such that `p + i == &addressof(operator[](i))` for each `i` in `[0, size()]`.

   > > -2- Complexity: Constant time.

   > > -3- Requires: The program shall not alter any of the values stored in the character array.

   >     charT* data() noexcept;

   > > -4- Returns: A pointer `p` such that `p + i == &addressof(operator[](i))` for each `i` in `[0, size()]`.

   > > -5- Complexity: Constant time.

   > > -6- Requires: The program shall not alter the value stored at `p + size()`.

4. Change 27.3.3.4 [string.view.iterators] p4 as indicated:

   >     constexpr const_iterator begin() const noexcept;
   >     constexpr const_iterator cbegin() const noexcept;

   > > -4- Returns: An iterator such that

   > > (4.1) — if `!empty()`, `&addressof(*begin()) == data_`,

   > > (4.2) — otherwise, an unspecified value such that `[begin(), end())` is a valid range.

5. Change 27.3.3.8 [string.view.ops] p21 and p24 as indicated:

   >     constexpr bool starts_with(charT x) const noexcept;

   > > -21- Effects: Equivalent to: `return starts_with(basic_string_view(&addressof(x), 1));`

   > […]

   >     constexpr bool ends_with(charT x) const noexcept;

   > > -24- Effects: Equivalent to: `return ends_with(basic_string_view(&addressof(x), 1));`

6. Change 27.3.3.9 [string.view.find] p5 as indicated:

   > -5- Each member function of the form

   > >     constexpr return-type F(charT c, size_type pos);

   > is equivalent to `return F(basic_string_view(&addressof(c), 1), pos);`

7. Edit 23.2.2 [container.requirements.general], Table 77 — "Container requirements", as indicated:

   > Table 77 — Container requirements
   > Expression | Return type | Operational semantics | Assertion/note pre/post-condition | Complexity
   > --- | --- | --- | --- | ---
   > `[…]`
   > `(&a)->a.~X()` | `void` |  | the destructor is applied to every element of `a`; any memory obtained is deallocated. | linear
   > `[…]`

8. Edit 24.3.5.4 [output.iterators], Table 90 — "Output iterator requirements (in addition to Iterator)", as indicated:

   > Table 90 — Output iterator requirements (in addition to Iterator)
   > Expression | Return type | Operational semantics | Assertion/note pre/post-condition
   > --- | --- | --- | ---
   > `[…]`
   > `++r` | `X&` |  | `&addressof(r) == &addressof(++r)`. […]
   > `[…]`

9. Edit 24.3.5.6 [bidirectional.iterators], Table 92 — "Bidirectional iterator requirements (in addition to forward iterator)", as indicated:

   > Table 92 — Bidirectional iterator requirements (in addition to forward iterator)
   > Expression | Return type | Operational semantics | Assertion/note pre/post-condition
   > --- | --- | --- | ---
   > `--r` | `X&` |  | […] `&addressof(r) == &addressof(--r)`.
   > `[…]`

10. Change 28.6.6 [re.traits] p6 as indicated:

    >     template<class ForwardIterator>
    >       string_type transform(ForwardIterator first, ForwardIterator last) const;

    > > -6- Effects: As if by:

    > > >     string_type str(first, last);
    > > >     return use_facet<collate<charT>>(
    > > >       getloc()).transform(&*str.begindata(), &*str.begindata() + str.length());

11. Change 28.6.11.1.2 [re.regiter.cnstr] p2 as indicated:

    >     regex_iterator(BidirectionalIterator a, BidirectionalIterator b,
    >                    const regex_type& re,
    >                    regex_constants::match_flag_type m = regex_constants::match_default);

    > > -2- Effects: Initializes `begin` and `end` to `a` and `b`, respectively, sets `pregex` to `&addressof(re)`, sets `flags` to `m`, then calls `regex_search(begin, end, match, *pregex, flags)`. If this call returns `false` the constructor sets `*this` to the end-of-sequence iterator.

12. Change 28.6.11.1.4 [re.regiter.deref] p2 as indicated:

    >     const value_type* operator->() const;

    > > -2- Returns: `&addressof(match)`.

13. Change 32.6.5.2 [thread.lock.guard] p2-7 as indicated:

    >     explicit lock_guard(mutex_type& m);

    > > -2- Requires: If `mutex_type` is not a recursive mutex, the calling thread does not own the mutex `m`.

    > > -3- Effects: As if byInitializes `pm` with `m`. Calls `m.lock()`.

    > > -4- Postconditions: `&pm == &m`

    >     lock_guard(mutex_type& m, adopt_lock_t);

    > > -5- Requires: The calling thread owns the mutex `m`.

    > > -6- Postconditions: `&pm == &m` Effects: Initializes `pm` with `m`.

    > > -7- Throws: Nothing.

### 3132(i). Library needs to ban macros named `expects` or `ensures`

Section: 16.4.5.3.3 [macro.names] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25

Priority: 0

View all other issues in [macro.names].

View all issues with C++20 status.

Discussion:

`expects` and `ensures` are not technically described as attribute-tokens when used in a contract-attribute-specifier, so the existing prohibition in 16.4.5.3.3 [macro.names] doesn't apply to them.

The remaining special identifiers used by the contract attributes are all already covered by existing wording: `assert` is also a library name so falls under p1, `default` is a keyword, and both `axiom` and `audit` were added to Table 4.

[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

1. Change 16.4.5.3.3 [macro.names] p2 as indicated:

   > -2- A translation unit shall not `#define` or `#undef` names lexically identical to keywords, to the identifiers listed in Table 4, or to the attribute-tokens described in 9.13 [dcl.attr], or to the identifiers `expects` or `ensures`.

### 3133(i). Modernizing numeric type requirements

Section: 29.2 [numeric.requirements] Status: C++20 Submitter: Tim Song Opened: 2018-07-05 Last modified: 2021-02-25

Priority: 0

View all other issues in [numeric.requirements].

View all issues with C++20 status.

Discussion:

29.2 [numeric.requirements] contains some very old wording that hasn't been changed since C++98 except for issue 2699(i). As a result, it is at once over- and under-restrictive. For example:

- It requires a class to have various member functions, but not that said member functions be actually callable (non-ambiguous, non-deleted) or called by the applicable operations;
- It doesn't disallow function or array types;
- It disallows ref-qualifiers on the assignment operator by restricting the signature;
- It bans unary `operator&` for class types, but not enumeration types; and
- It tries to impose semantic equivalence requirements on `T`'s special member functions, but doesn't require that copying produces an equivalent value to the original.

We can significantly clean up this wording by using the existing named requirements. For ease of review, the following table provides a side-by-side comparison of the current and proposed wording.

Before | After
--- | ---
A C++ program shall instantiate these components only with a type `T` that satisfies the following requirements: [Footnote … ] | A C++ program shall instantiate these components only with a cv-unqualified object type `T` that satisfies the Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructiblefollowing requirements (16.4.4.2 [utility.arg.requirements]).:
(1.1) — `T` is not an abstract class (it has no pure virtual member functions); | Cpp17DefaultConstructible
(1.2) — `T` is not a reference type; (1.3) — `T` is not cv-qualified; | Implied by "cv-unqualified object type"
(1.4) — If `T` is a class, it has a public default constructor; | Cpp17DefaultConstructible
(1.5) — If `T` is a class, it has a public copy constructor with the signature `T::T(const T&)`; | Cpp17CopyConstructible
(1.6) — If `T` is a class, it has a public destructor; | Cpp17Destructible
(1.7) — If `T` is a class, it has a public copy assignment operator whose signature is either `T& T::operator=(const T&)` or `T& T::operator=(T);` | Cpp17CopyAssignable
(1.8) — If `T` is a class, its assignment operator, copy and default constructors, and destructor shall correspond to each other in the following sense: (1.8.1) — Initialization of raw storage using the copy constructor on the value of `T()`, however obtained, is semantically equivalent to value-initialization of the same raw storage. (1.8.2) — Initialization of raw storage using the default constructor, followed by assignment, is semantically equivalent to initialization of raw storage using the copy constructor. (1.8.3) — Destruction of an object, followed by initialization of its raw storage using the copy constructor, is semantically equivalent to assignment to the original object. [Note: […] — end note] | These requirements are implied by Cpp17CopyConstructible and Cpp17CopyAssignable's requirement that the value of the copy is equivalent to the source.
(1.9) — If `T` is a class, it does not overload unary `operator&`. | omitted now that we have `std::addressof`

[2019-01-20 Reflector prioritization]

Set Priority to 0 and status to Tentatively Ready

Proposed resolution:

This wording is relative to the post-Rapperswil 2018 working draft.

1. Edit 29.2 [numeric.requirements] p1 as indicated, striking the entire bulleted list:

   > -1- The `complex` and `valarray` components are parameterized by the type of information they contain and manipulate. A C++ program shall instantiate these components only with a cv-unqualified object type `T` that satisfies the Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructiblefollowing requirements (16.4.4.2 [utility.arg.requirements]). :[Footnote: … ]

   > > (1.1) — `T` is not an abstract class (it has no pure virtual member functions);

   > > […]

   > > (1.9) — If `T` is a class, it does not overload unary `operator&`.

2. Edit 29.6.2.4 [valarray.access] p3-4 as indicated:

   >     const T&  operator[](size_t n) const;
   >     T& operator[](size_t n);

   > > -1- Requires: `n < size()`.

   > > -2- Returns: […]

   > > -3- Remarks: The expression `&addressof(a[i+j]) == &addressof(a[i]) + j` evaluates to `true` for all `size_t i` and `size_t j` such that `i+j < a.size()`.

   > > -4- The expression `&addressof(a[i]) != &addressof(b[j])` evaluates to `true` for any two arrays `a` and `b` and for any `size_t i` and `size_t j` such that `i < a.size()` and `j < b.size()`. [Note: […] — end note ]

### 3134(i). [fund.ts.v3] LFTSv3 contains extraneous [meta] variable templates that should have been deleted by P09961

Section: 99 [fund.ts.v3::meta.type.synop] Status: Resolved Submitter: Thomas Köppe Opened: 2018-07-02 Last modified: 2020-09-06

Priority: 0

View all other issues in [fund.ts.v3::meta.type.synop].

View all issues with Resolved status.

Discussion:

Addresses: fund.ts.v3

The LFTSv3 prospective-working-paper N4758 lists a large number of type trait variable templates in [meta.type.synop] that are duplicates of corresponding ones in C++17. The paper P0996R1 that was meant to rebase the LFTS on C++17 appears to have missed them.

[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]

[2018-11-11 Resolved by P1210R0, adopted in San Diego.]

Proposed resolution:

This wording is relative to N4758.

1. Delete from 99 [fund.ts.v3::meta.type.synop] all variable templates starting at `is_void_v` up to and including `is_convertible_v` as indicated:

   >     #include <type_traits>

   >     namespace std::experimental {
   >     inline namespace fundamentals_v3 {

   >       // See C++17 §23.15.4.1, primary type categories
   >       template <class T> constexpr bool is_void_v
   >         = is_void<T>::value;
   >       […]
   >       template <class From, class To> constexpr bool is_convertible_v
   >         = is_convertible<From, To>::value;

   >       // 3.3.2, Other type transformations
   >       template <class> class invocation_type; // not defined
   >       […]
   >     } // inline namespace fundamentals_v3
   >     } // namespace std::experimental

### 3135(i). [fund.ts.v3] LFTSv3 contains two redundant alias templates

Section: 99 [fund.ts.v3::meta.type.synop], 99 [fund.ts.v3::header.memory.synop] Status: Resolved Submitter: Thomas Köppe Opened: 2018-07-02 Last modified: 2020-05-03

Priority: 3

View all other issues in [fund.ts.v3::meta.type.synop].

View all issues with Resolved status.

Discussion:

Addresses: fund.ts.v3

The LFTSv3 prospective-working-paper N4758 contains two aliases that are already in C++17:

- `void_t` in 99 [fund.ts.v3::meta.type.synop]

- `uses_allocator_v` in 99 [fund.ts.v3::header.memory.synop]

I'd like to propose deleting both, but separate discussion is warranted:

`void_t` belongs with the larger "detection idiom", which has not been merged into C++17. We may prefer to keep our own local version of the alias.

`uses_allocator_v` aliases a version of `uses_allocator` that is modified by this TS. However, as specified the alias may actually end up referring to `std::uses_allocator` (because `<memory>` is included), not to `std::experimental::uses_allocator`, as may have been intended.

[2018-07-20 Priority set to 3 after reflector discussion]

[2020-05-03 Reflector discussion]

Resolved by P1210R0 accepted during the San Diego 2018 meeting.

Rationale:

Resolved by P1210R0

Proposed resolution:

### 3136(i). [fund.ts.v3] LFTSv3 awkward wording in `propagate_const` requirements

Section: 6.1.2.3 [fund.ts.v3::propagate_const.class_type_requirements] Status: WP Submitter: Thomas Köppe Opened: 2018-07-02 Last modified: 2026-02-21

Priority: 3

View all issues with WP status.

Discussion:

Addresses: fund.ts.v3

In the LFTSv3 prospective-working-paper N4758, [propagate_const.class_type_requirements] uses a strange turn of phrase:

> "In this sub-clause, `t` denotes a non-`const` lvalue of type `T`, `ct` is a `const T&` bound to `t`, […]"

The last bit is strange: "`ct` is a `const T&` bound to `t`" is not how we usually say things. The specification-variables usually denote values, and values can't be references. Perhaps we could just say, "`ct` is `as_const(t)`"?

[2018-07-20 Priority set to 3 after reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4758.

> 1. Edit 6.1.2.3 [fund.ts.v3::propagate_const.class_type_requirements] as indicated:

>    > -1- If `T` is class type then it shall satisfy the following requirements. In this sub-clause `t` denotes a non-`const` lvalue of type `T`, `ct` is a `const T&` bound to `t` `as_const(t)`, `element_type` denotes an object type.

[2022-10-12; Jonathan provides improved wording]

[2022-10-19; Reflector poll]

Set status to "Tentatively Ready" after eight votes in favour in reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4920.

1. Edit 6.1.2.2 [fund.ts.v3::propagate_const.requirements] as indicated:

   > -1- `T` shall be a cv-unqualified pointer-to-object type an object pointer type or a cv-unqualified class type for which `decltype(*declval<T&>())` is an lvalue reference to object type; otherwise the program is ill-formed.

   > -2- If `T` is an array type, reference type, pointer to function type or pointer to (possibly cv-qualified) `void`, then the program is ill-formed.

   > -3- [Note: `propagate_const<const int*>` is well-formed but propagate_const<int* const> is not. — end note]

2. Edit 6.1.2.3 [fund.ts.v3::propagate_const.class_type_requirements] as indicated:

   > -1- If `T` is class type then it shall satisfy the following requirements. In this sub-clause `t` denotes a non-`const` an lvalue of type `T`, `ct` is a `const T&` bound to `t`, `element_type` denotes an object type. denotes `as_const(t)`.

### 3137(i). Header for `__cpp_lib_to_chars`

Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: S. B. Tam Opened: 2018-07-03 Last modified: 2022-08-23

Priority: 0

View all other issues in [support.limits.general].

View all issues with C++20 status.

Discussion:

After acceptance of P0941R2 into the working draft, in [support.limits.general], `__cpp_lib_to_chars` is required to be in `<utility>`. Since the relevant feature (`std::to_chars` and `std::from_chars`) is now in the header `<charconv>`, should the macro be defined in `<charconv>` instead of `<utility>`?

[Marshall provides P/R and context]

`std::to_chars`, etc were originally proposed for the header `<utility>` and SD-6 reflected that. Somewhere along the way, they was put into their own header `<charconv>`, but the document was never updated to reflect that.

When these macros were added to the standard, the (now incorrect) value was copied as well.

[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to (the Rapperswil post-mailing standard).

Change 17.3.1 [support.limits.general] (Table 35) as indicated:

Macro name | Value | Header(s)
--- | --- | ---
`__cpp_lib_to_chars` | `201611L` | `<charconvutility>`

### 3140(i). `COMMON_REF` is unimplementable as specified

Section: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Casey Carter Opened: 2018-07-07 Last modified: 2021-02-25

Priority: 0

View all other issues in [meta.trans.other].

View all issues with C++20 status.

Discussion:

21.3.8.7 [meta.trans.other]/3.2 states:

> [Let] `XREF(A)` denote a unary class template `T` such that `T<U>` denotes the same type as `U` with the addition of `A`’s cv and reference qualifiers, for a non-reference cv-unqualified type `U`,

which is nonsensical: a specialization of a class template cannot possibly be a cv-qualified type or reference type. `XREF(A)` must be a unary alias template.

[2018-09-11; Status set to Tentatively Ready after five positive votes on the reflector]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

Change 21.3.8.7 [meta.trans.other] as indicated:

> -3- Let:

> > (3.1) — `CREF(A)` be `add_lvalue_reference_t<const remove_reference_t<A>>`,

> > (3.2) — `XREF(A)` denote a unary classalias template `T` such that `T<U>` denotes the same type as `U` with the addition of `A`’s cv and reference qualifiers, for a non-reference cv-unqualified type `U`,

> > (3.3) — `COPYCV(FROM, TO)` be an alias for type `TO` with the addition of `FROM`’s top-level cv-qualifiers. [Example: `COPYCV(const int, volatile short)` is an alias for `const volatile short`. —end example]

### 3141(i). `CopyConstructible` doesn't preserve source values

Section: 18.6 [concepts.object] Status: C++20 Submitter: Casey Carter Opened: 2018-07-07 Last modified: 2021-02-25

Priority: 2

View all issues with C++20 status.

Discussion:

The design intent of the `CopyConstructible` concept has always been that the source of the constructions it requires be left unmodified. Before P0541R1 reformulated the `Constructible` concept in terms of `is_constructible`, the wording which is now in 18.2 [concepts.equality]/5:

> This document uses a notational convention to specify which expressions declared in a requires-expression modify which inputs: except where otherwise specified, […] Operands that are constant lvalues or rvalues are required to not be modified.

indirectly enforced that requirement. Unfortunately, nothing in the current wording in 18.4.14 [concept.copyconstructible] enforces that requirement.

[2018-11 Reflector prioritization]

Set Priority to 2

Previous resolution: [SUPERSEDED]

> This wording is relative to N4762.

> Change 18.4.14 [concept.copyconstructible] as indicated:

> >     template<class T>
> >       concept CopyConstructible =
> >         MoveConstructible<T> &&
> >         Constructible<T, T&> && ConvertibleTo<T&, T> &&
> >         Constructible<T, const T&> && ConvertibleTo<const T&, T> &&
> >         Constructible<T, const T> && ConvertibleTo<const T, T>;

> > -1- If `T` is an object type, then let `v` be an lvalue of type (possibly `const`) `T` or an rvalue of type `const T`. `CopyConstructible<T>` is satisfied only if

> > (1.1) — After the definition `T u = v;`, `u` is equal to `v` and `v` is unmodified.

> > (1.2) — `T(v)` is equal to `v` and does not modify `v`.

[2020-02-13 Per LWG review, Casey provides updated P/R wording.]

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4849.

Change 18.4.14 [concept.copyconstructible] as indicated:

>     template<class T>
>       concept copy_constructible =
>         move_constructible<T> &&
>         constructible_from<T, T&> && convertible_to<T&, T> &&
>         constructible_from<T, const T&> && convertible_to<const T&, T> &&
>         constructible_from<T, const T> && convertible_to<const T, T>;

> -1- If `T` is an object type, then let `v` be an lvalue of type (possibly `const`) `T` or an rvalue of type `const T`. `T` models `copy_constructible` only if

> (1.1) — After the definition `T u = v;`, `u` is equal to `v` (18.2 [concepts.equality]) and `v` is not modified.

> (1.2) — `T(v)` is equal to `v` and does not modify `v`.

### 3143(i). `monotonic_buffer_resource` growth policy is unclear

Section: 20.5.6 [mem.res.monotonic.buffer] Status: C++23 Submitter: Jonathan Wakely Opened: 2018-07-20 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

During the discussion of LWG 3120(i) it was pointed out that the current wording in 20.5.6 [mem.res.monotonic.buffer] is contradictory. The introductory text for the class says "Each additional buffer is larger than the previous one, following a geometric progression" but the spec for `do_allocate` doesn't agree.

Firstly, it's impossible for the implementation to ensure a single geometric progression, because the size of the next buffer can be arbitrarily large. If the caller asks for an allocation that is `N` times bigger than the previous buffer, the next buffer will be at least `N` times larger than the previous one. If `N` is larger than the implementation-defined growth factor it's not a geometric progression.

Secondly, it's not even clear that each additional buffer will be larger than the previous one. Given a `monotonic_buffer_resource` object with little remaining space in `current_buffer`, a request to allocate `10*next_buffer_size` will:

"set `current_buffer` to `upstream_rsrc->allocate(n, m)`, where `n` is not less than `max(bytes, next_buffer_size)` and `m` is not less than `alignment`, and increase `next_buffer_size` by an implementation-defined growth factor (which need not be integral), then allocate the return block from the newly-allocated `current_buffer`."

The effects are to allocate a new buffer of at least `max(10*next_buffer_size, next_buffer_size)` bytes, and then do `next_buffer_size *= growth_factor`. If `growth_factor < 10` then the next allocated buffer might be smaller than the last one. This means that although `next_buffer_size` itself follows a geometric progression, the actual size of any single allocated buffer can be much larger than `next_buffer_size`. A graph of the allocated sizes looks like a geometric progression with spikes where an allocation size is larger than `next_buffer_size`.

If the intention is to set `next_buffer_size = max(n, next_buffer_size * growth_factor)` so that every allocation from upstream is larger than the previous one, then we need a change to the Effects: to actually say that. Rather than a geometric progression with anomalous spikes, this would produce a number of different geometric progressions with discontinuous jumps between them.

If the spiky interpretation is right then we need to weaken the "Each additional buffer is larger" statement. Either way, we need to add a caveat to the "following a geometric progression" text because that isn't true for the spiky interpretation or the jumpy interpretation.

Thirdly, the Effects: says that the size of the allocated block, `n`, is not less than `max(bytes, next_buffer_size)`. This seems to allow an implementation to choose to do `n = ceil2(max(bytes, next_buffer_size))` if it wishes (maybe because allocating sizes that are a power of 2 simplifies the `monotonic_buffer_resource` implementation, or allows reducing the bookkeeping overhead). This still results in an approximate geometric progression (under either the spiky or jumpy interpretation) but the graph has steps rather than being a smooth curve (but always above the curve). This is another way that "Each additional buffer is larger than the previous one" is not guaranteed. Even if `max(bytes, next_buffer_size)` is greater on every call, for a growth factor between `1.0` and `2.0` the result of `ceil2` might be the same for two successive buffers. I see no reason to forbid this, but Pablo suggested it's not allowed because it doesn't result in exponential growth (which I disagree with). If this is supposed to be forbidden, the wording needs to be fixed to forbid it.

[2019-01-20 Reflector prioritization]

Set Priority to 2

[2020-02-13, Prague]

LWG looked at the issue and a suggestion was presented to eliminate most of 20.5.6 [mem.res.monotonic.buffer] to solve the problem the current requirements impose.

[2020-02-16; Prague]

Reviewed revised wording and moved to Ready for Varna.

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 20.5.6 [mem.res.monotonic.buffer], as indicated:

   > -1- A `monotonic_buffer_resource` is a special-purpose memory resource intended for very fast memory allocations in situations where memory is used to build up a few objects and then is released all at once when the memory resource object is destroyed. It has the following qualities:

   > 1. (1.1) — A call to `deallocate` has no effect, thus the amount of memory consumed increases monotonically until the resource is destroyed.

   > 2. (1.2) — The program can supply an initial buffer, which the allocator uses to satisfy memory requests.

   > 3. (1.3) — When the initial buffer (if any) is exhausted, it obtains additional buffers from an upstream memory resource supplied at construction. Each additional buffer is larger than the previous one, following a geometric progression.

   > 4. (1.4) — It is intended for access from one thread of control at a time. Specifically, calls to `allocate` and `deallocate` do not synchronize with one another.

   > 5. (1.5) — It frees the allocated memory on destruction, even if `deallocate` has not been called for some of the allocated blocks.

### 3144(i). `span` does not have a `const_pointer` typedef

Section: 23.7.2.2.1 [span.overview] Status: C++20 Submitter: Louis Dionne Opened: 2018-07-23 Last modified: 2021-02-25

Priority: 0

View all other issues in [span.overview].

View all issues with C++20 status.

Discussion:

`std::span` does not have a typedef for `const_pointer` and `const_reference`. According to Marshall Clow, this is merely an oversight.

[2019-01-20 Reflector prioritization]

Set Priority to 0 and status to Tentatively Ready

Proposed resolution:

This wording is relative to N4750.

1. Change 23.7.2.2.1 [span.overview], class template `span` synopsis, as indicated:

   >     namespace std {
   >       template<class ElementType, ptrdiff_t Extent = dynamic_extent>
   >       class span {
   >       public:
   >         // constants and types
   >         using element_type = ElementType;
   >         using value_type = remove_cv_t<ElementType>;
   >         using index_type = ptrdiff_t;
   >         using difference_type = ptrdiff_t;
   >         using pointer = element_type*;
   >         using const_pointer = const element_type*;
   >         using reference = element_type&;
   >         using const_reference = const element_type&;
   >         using iterator = implementation-defined;
   >         using const_iterator = implementation-defined;
   >         using reverse_iterator = reverse_iterator<iterator>;
   >         using const_reverse_iterator = reverse_iterator<const_iterator>;
   >         static constexpr index_type extent = Extent;

   >         […]
   >       };
   >       […]
   >     }

### 3145(i). `file_clock` breaks ABI for C++17 implementations

Section: 30.7.6 [time.clock.file] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2018-07-26 Last modified: 2021-02-25

Priority: 1

View all issues with C++20 status.

Discussion:

It was pointed out in one of Eric's changes to libc++ here that P0355 adds `file_clock`, which is intended to be the clock used for `std::filesystem::file_time_type`'s clock.

Unfortunately, this is an ABI break for implementations that are already shipping C++17 filesystem that did not call their clock type `std::file_clock`. For example, MSVC++'s is called `std::filesystem::_File_time_clock`.

We can keep much the same interface of P0355 by making `file_clock` a typedef for an unspecified type. This technically changes the associated namespaces for expressions using that clock for the sake of ADL, but I can't imagine a user who cares, as clocks aren't generally called in ADL-able expressions, `duration`s and `time_point`s are.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> 1. Change 30.2 [time.syn], header `<chrono>` synopsis, as indicated:

>    >     […]
>    >     // 30.7.6 [time.clock.file], classtype file_clock
>    >     classusing file_clock = unspecified;
>    >     […]

> 2. Change 30.7.6 [time.clock.file] as indicated:

>    > 23.17.7.5 ClassType `file_clock` [time.clock.file]

> 3. Change 30.7.6.1 [time.clock.file.overview], class `file_clock` synopsis, as indicated:

>    >     namespace std::chrono {
>    >       using file_clock = see below;
>    >       class file_clock {
>    >       public:
>    >         using rep = a signed arithmetic type;
>    >         using period = ratio<unspecified, unspecified>;
>    >         using duration = chrono::duration<rep, period>;
>    >         using time_point = chrono::time_point<file_clock>;
>    >         static constexpr bool is_steady = unspecified;

>    >         static time_point now() noexcept;

>    >         // Conversion functions, see below
>    >       };
>    >     }

>    > -1- The clock `file_clock` is an alias for a type meeting the `TrivialClock` requirements (30.3 [time.clock.req]), uses a signed arithmetic type for `file_clock::rep`, and is used to create the `time_point` system used for `file_time_type` (31.12 [filesystems]). Its epoch is unspecified. [Note: The type `file_clock` denotes may be in a different namespace than `std::chrono`, such as `std::filesystem`. — end note]

> 4. Change 30.7.6.2 [time.clock.file.members] as indicated:

>    >     static time_point now();

>    > > -1- Returns: A `file_clock::time_point` indicating the current time.

>    > > -2- The class `file_clock` shalltype denoted by `file_clock` provides precisely one of the following two sets of static member functions: […]

[2018-07-30 Priority set to 1 after reflector discussion; wording updates based on several discussion contributions.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4762.

> 1. Change 30.2 [time.syn], header `<chrono>` synopsis, as indicated:

>    >     […]
>    >     // 30.7.6 [time.clock.file], classtype file_clock
>    >     classusing file_clock = see below;
>    >     […]

> 2. Change 30.7.6 [time.clock.file] as indicated:

>    > 23.17.7.5 ClassType `file_clock` [time.clock.file]

> 3. Change 30.7.6.1 [time.clock.file.overview], class `file_clock` synopsis, as indicated:

>    >     namespace std::chrono {
>    >       using file_clock = see below;
>    >       class file_clock {
>    >       public:
>    >         using rep = a signed arithmetic type;
>    >         using period = ratio<unspecified, unspecified>;
>    >         using duration = chrono::duration<rep, period>;
>    >         using time_point = chrono::time_point<file_clock>;
>    >         static constexpr bool is_steady = unspecified;

>    >         static time_point now() noexcept;

>    >         // Conversion functions, see below
>    >       };
>    >     }

>    > -1- The clock `file_clock` is an alias for a type meeting the `TrivialClock` requirements (30.3 [time.clock.req]), which uses a signed arithmetic type for `file_clock::rep`. `file_clock` is used to create the `time_point` system used for `file_time_type` (31.12 [filesystems]). Its epoch is unspecified, and `noexcept(file_clock::now())` is `true`. [Note: The type `file_clock` denotes may be in a different namespace than `std::chrono`, such as `std::filesystem`. — end note]

> 4. Change 30.7.6.2 [time.clock.file.members] as indicated:

>    >     static time_point now();

>    > > -1- Returns: A `file_clock::time_point` indicating the current time.

>    > > -2- The class `file_clock` shalltype denoted by `file_clock` provides precisely one of the following two sets of static member functions: […]

[2018-08-23 Batavia Issues processing: Minor wording changes, and status to "Tentatively Ready".]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

1. Change 30.2 [time.syn], header `<chrono>` synopsis, as indicated:

   >     […]
   >     // 30.7.6 [time.clock.file], classtype file_clock
   >     classusing file_clock = see below;
   >     […]

2. Change 30.7.6 [time.clock.file] as indicated:

   > 23.17.7.5 ClassType `file_clock` [time.clock.file]

3. Change 30.7.6.1 [time.clock.file.overview], class `file_clock` synopsis, as indicated:

   >     namespace std::chrono {
   >       using file_clock = see below;
   >       class file_clock {
   >       public:
   >         using rep = a signed arithmetic type;
   >         using period = ratio<unspecified, unspecified>;
   >         using duration = chrono::duration<rep, period>;
   >         using time_point = chrono::time_point<file_clock>;
   >         static constexpr bool is_steady = unspecified;

   >         static time_point now() noexcept;

   >         // Conversion functions, see below
   >       };
   >     }

   > -1- The clock `file_clock` is an alias for a type meeting the `Cpp17TrivialClock` requirements (30.3 [time.clock.req]), and using a signed arithmetic type for `file_clock::rep`. `file_clock` is used to create the `time_point` system used for `file_time_type` (31.12 [filesystems]). Its epoch is unspecified, and `noexcept(file_clock::now())` is `true`. [Note: The type that `file_clock` denotes may be in a different namespace than `std::chrono`, such as `std::filesystem`. — end note]

4. Change 30.7.6.2 [time.clock.file.members] as indicated:

   >     static time_point now();

   > > -1- Returns: A `file_clock::time_point` indicating the current time.

   > > -2- The type `file_clock` shalltype denoted by `file_clock` provides precisely one of the following two sets of static member functions: […]

### 3146(i). Excessive unwrapping in `std::ref`/`cref`

Section: 22.10.6.7 [refwrap.helpers] Status: C++23 Submitter: Agustín K-ballo Bergé Opened: 2018-07-10 Last modified: 2023-11-22

Priority: 3

View all issues with C++23 status.

Discussion:

The overloads of `std::ref`/`cref` that take a `reference_wrapper` as argument are defined as calling `std::ref`/`cref` recursively, whereas the return type is defined as unwrapping just one level. Calling these functions with arguments of multiple level of wrapping leads to ill-formed programs:

>     int i = 0;
>     std::reference_wrapper<int> ri(i);
>     std::reference_wrapper<std::reference_wrapper<int>> rri(ri);
>     std::ref(rri); // error within 'std::ref'

[Note: these overloads were added by issue resolution 10.29 for TR1, which can be found at N1688, at Redmond 2004]

[2018-08-20 Priority set to 3 after reflector discussion]

[2021-05-22 Tim syncs wording to the current working draft]

[2021-08-20; LWG telecon]

Set status to Tentatively Ready after telecon review.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Change 22.10.6.7 [refwrap.helpers] as indicated:

   >     template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;

   > > -3- Returns: `ref(t.get())t`.

   > […]

   >     template<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;

   > > -5- Returns: `cref(t.get())t`.

### 3147(i). Definitions of "likely" and "unlikely" are likely to cause problems

Section: 16.4.5.3.3 [macro.names] Status: C++20 Submitter: Casey Carter Opened: 2018-08-01 Last modified: 2021-02-25

Priority: 0

View all other issues in [macro.names].

View all issues with C++20 status.

Discussion:

16.4.5.3.3 [macro.names]/2 forbids a translation unit to define names "lexically identical to […] the attribute-tokens described in 9.13 [dcl.attr]." We recently added the attribute-tokens `likely` and `unlikely` (9.13.7 [dcl.attr.likelihood]). These names are in extremely wide use as function-like macros in the open source community, forbidding users to define them breaks large amounts of code. (Reportedly Chromium contains 19 definitions each of "likely" and "unlikely" as function-like macros.)

Indeed, this issue came up during EWG discussion of P0479R1 "Attributes for Likely and Unlikely Statements" in Kona, and EWG decided to keep the names "likely" and "unlikely" for the attribute tokens since the usage wouldn't conflict with defining them as function-like macros. 16.4.5.3.3 [macro.names]/2 should not break large amounts of existing code that doesn't actually conflict with the use of the `[[likely]]` and `[[unlikely]]` attributes.

[2018-08-20 Status to Tentatively Ready after five positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

1. Change 16.4.5.3.3 [macro.names] as indicated:

   > [Drafting Note: If both this proposed resolution and the proposed resolution of LWG 3132(i) are accepted, the text inserted by LWG 3132(i) should precede the text added here.]

   > -2- A translation unit shall not `#define` or `#undef` names lexically identical to keywords, to the identifiers listed in Table 4, or to the attribute-tokens described in 9.13 [dcl.attr], except that the names `likely` and `unlikely` may be defined as function-like macros (15.7 [cpp.replace]).

### 3148(i). `<concepts>` should be freestanding

Section: 16.4.2.5 [compliance] Status: C++20 Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2021-02-25

Priority: 0

View all other issues in [compliance].

View all issues with C++20 status.

Discussion:

The design intent of the `<concepts>` header is that it contains only fundamental concept definitions and implementations of customization points that are used by those concept definitions. There should never be components in the header that require operating system support. Consequently, freestanding implementations can and should provide it. It is an oversight on the part of LWG - and in particular the author of P0898R3 "Standard Libary Concepts" - that the `<concepts>` header is not required to be provided by freestanding implementations.

[2018-08 Batavia Monday issue prioritization]

Priority set to 0, status to 'Tentatively Ready'

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

1. In 16.4.2.5 [compliance], add a new row to Table 21:

   > Table 21 — C++ headers for freestanding implementations
   > Subclause | Header(s)
   > --- | ---
   > […] | […] | […]
   > 17.14 [support.runtime] | Other runtime support | `<cstdarg>`
   > 18 [concepts] | Concepts library | `<concepts>`
   > 21 [meta] | Type traits | `<type_traits>`
   > […] | […] | […]

### 3149(i). `DefaultConstructible` should require default initialization

Section: 18.4.12 [concept.default.init] Status: C++20 Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2021-06-06

Priority: 2

View all other issues in [concept.default.init].

View all issues with C++20 status.

Discussion:

`DefaultConstructible<T>` is equivalent to `Constructible<T>` (18.4.11 [concept.constructible]), which is equivalent to `is_constructible_v<T>` (21.3.5.4 [meta.unary.prop]). Per 21.3.5.4 [meta.unary.prop] paragraph 8:

> The predicate condition for a template specialization `is_constructible<T, Args...>` shall be satisfied if and only if the following variable definition would be well-formed for some invented variable `t`:

> >     T t(declval<Args>()...);

`DefaultConstructible<T>` requires that objects of type `T` can be value-initialized, rather than default-initialized as intended.

The library needs a constraint that requires object types to be default-initializable: the "rangified" versions of the algorithms in 26.11.3 [uninitialized.construct.default] proposed in P0896 "The One Ranges Proposal", for example. Users will also want a mechanism to provide such a constraint, and they're likely to choose `DefaultConstructible` despite its subtle unsuitability.

There are two alternative solutions: (1) change `DefaultConstructible` to require default-initialization, (2) change `is_default_constructible_v` to require default-initializaton and specify the concept in terms of the trait. (2) is probably too breaking a change to be feasible.

[2018-08-20 Priority set to 2 after reflector discussion]

Previous resolution [SUPERSEDED]:

> 1. Modify [concept.defaultconstructible] as follows:

>    >     template<class T>
>    >       concept DefaultConstructible = Constructible<T> && see below;

>    > -?- Type `T` models `DefaultConstructible` only if the variable definition

>    > >     T t;

>    > is well-formed for some invented variable `t`. Access checking is performed as if in a context unrelated to `T`. Only the validity of the immediate context of the variable initialization is considered.

[2018-08-23 Tim provides updated P/R based on Batavia discussion]

[2018-10-28 Casey expands the problem statement and the P/R]

During Batavia review of P0896R3, Tim Song noted that `{}` is not necessarily a valid initializer for a `DefaultConstructible` type. In this sample program (see Compiler Explorer):

>     struct S0 { explicit S0() = default; };
>     struct S1 { S0 x; }; // Note: aggregate
>     S1 x;   // Ok
>     S1 y{}; // ill-formed; copy-list-initializes x from {}

`S1` can be default-initialized, but not list-initialized from an empty braced-init-list. The consensus among those present was that `DefaultConstructible` should prohibit this class of pathological types by requiring that initialization form to be valid.

[2019 Cologne Wednesday night]

Status to Ready

[2019-10-07 Casey rebases P/R onto N4830 and incorporates WG21-approved changes from P1754R1]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4762.

> 1. Modify [concept.defaultconstructible] as follows:

>    >     template<class T>
>    >       inline constexpr bool is-default-initializable = see below; // exposition only

>    >     template<class T>
>    >       concept DefaultConstructible = Constructible<T> && requires { T{}; } && is-default-initializable<T>;

>    > -?- For a type `T`, `is-default-initializable<T>` is `true` if and only if the variable definition

>    > >     T t;

>    > is well-formed for some invented variable `t`; otherwise it is `false`. Access checking is performed as if in a context unrelated to `T`. Only the validity of the immediate context of the variable initialization is considered.

P1754R1 "Rename concepts to `standard_case` for C++20" - as approved by both LEWG and LWG in Cologne - contained instructions to rename the `DefaultConstructible` concept to `default_initializable` "If LWG 3151 is accepted." 3151(i) is the unrelated "`ConvertibleTo` rejects conversion from array and function types"; this issue is intended by P1754R1. Since P1754R1 was applied to the working draft in Cologne, whereas this issue was only made Ready, we should apply the desired renaming to the P/R of this issue.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 18.3 [concepts.syn], header `<concepts>` synopsis, as indicated:

>    >     […]
>    >     //  [concept.defaultconstructible], concept default_constructibleinitializable
>    >     template<class T>
>    >     concept default_constructibleinitializable = see below;
>    >     […]

> 2. Modify [concept.defaultconstructible] as indicated:

>    > 18.4.12 Concept `default_constructibleinitializable` [concept.defaultconstructibleinitializable]

>    >     template<class T>
>    >       inline constexpr bool is-default-initializable = see below; // exposition only

>    >     template<class T>
>    >       concept default_constructibleinitializable = constructible_from<T> && requires { T{}; } && is-default-initializable<T>;

>    > -?- For a type `T`, `is-default-initializable<T>` is `true` if and only if the variable definition

>    > >     T t;

>    > is well-formed for some invented variable `t`; otherwise it is `false`. Access checking is performed as if in a context unrelated to `T`. Only the validity of the immediate context of the variable initialization is considered.

> 3. Modify 18.6 [concepts.object] as indicated:

>    > -1- This subclause describes concepts that specify the basis of the value-oriented programming style on which the library is based.

>    >     template<class T>
>    >     concept movable = is_object_v<T> && move_constructible<T> &&
>    >     […]
>    >     template<class T>
>    >     concept semiregular = copyable<T> && default_constructibleinitializable<T>;
>    >     […]

> 4. Modify 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

>    >     […]
>    >     namespace ranges {
>    >       template<no-throw-forward-iterator I, no-throw-sentinel<I> S>
>    >         requires default_constructibleinitializable<iter_value_t<I>>
>    >           I uninitialized_default_construct(I first, S last);
>    >       template<no-throw-forward-range R>
>    >         requires default_constructibleinitializable<range_value_t<R>>
>    >           safe_iterator_t<R> uninitialized_default_construct(R&& r);

>    >       template<no-throw-forward-iterator I>
>    >         requires default_constructibleinitializable<iter_value_t<I>>
>    >           I uninitialized_default_construct_n(I first, iter_difference_t<I> n);
>    >     }
>    >     […]
>    >     namespace ranges {
>    >       template<no-throw-forward-iterator I, no-throw-sentinel<I> S>
>    >         requires default_constructibleinitializable<iter_value_t<I>>
>    >          I uninitialized_value_construct(I first, S last);
>    >       template<no-throw-forward-range R>
>    >         requires default_constructibleinitializable<range_value_t<R>>
>    >           safe_iterator_t<R> uninitialized_value_construct(R&& r);

>    >       template<no-throw-forward-iterator I>
>    >         requires default_constructibleinitializable<iter_value_t<I>>
>    >           I uninitialized_value_construct_n(I first, iter_difference_t<I> n);
>    >     }
>    >     […]

> 5. Modify 26.11.3 [uninitialized.construct.default] as indicated:

>    >     namespace ranges {
>    >       template<no-throw-forward-iterator I, no-throw-sentinel<I> S>
>    >         requires default_constructibleinitializable<iter_value_t<I>>
>    >           I uninitialized_default_construct(I first, S last);
>    >       template<no-throw-forward-range R>
>    >         requires default_constructibleinitializable<range_value_t<R>>
>    >           safe_iterator_t<R> uninitialized_default_construct(Ramp;& r);
>    >     }

>    > > -2- Effects: Equivalent to:

>    > > […]

>    >     namespace ranges {
>    >       template<no-throw-forward-iterator I>
>    >         requires default_constructibleinitializable<iter_value_t<I>>
>    >           I uninitialized_default_construct_n(I first, iter_difference_t<I> n);
>    >     }

>    > > -4- Effects: Equivalent to:

>    > > […]

> 6. Modify 26.11.4 [uninitialized.construct.value] as indicated:

>    >     namespace ranges {
>    >       template<no-throw-forward-iterator I, no-throw-sentinel<I> S>
>    >         requires default_constructibleinitializable<iter_value_t<I>>
>    >           I uninitialized_value_construct(I first, S last);
>    >       template<no-throw-forward-range R>
>    >         requires default_constructibleinitializable<range_value_t<R>>
>    >           safe_iterator_t<R> uninitialized_value_construct(R&& r);
>    >     }

>    > > -2- Effects: Equivalent to:

>    > > […]

>    >     namespace ranges {
>    >       template<no-throw-forward-iterator I>
>    >         requires default_constructibleinitializable<iter_value_t<I>>
>    >           I uninitialized_value_construct_n(I first, iter_difference_t<I> n);
>    >     }

>    > > -4- Effects: Equivalent to:

>    > > […]

> 7. Modify [range.semi.wrap] as indicated:

>    > -1- Many types in this subclause are specified in terms of an exposition-only class template `semiregular-box`. `semiregular-box<T>` behaves exactly like `optional<T>` with the following differences:

>    > 1. (1.1) — […]

>    > 2. (1.2) — If `T` models `default_constructibleinitializable`, the default constructor of `semiregular-box<T>` is equivalent to:

>    >    >     constexpr semiregular-box() noexcept(is_nothrow_default_constructible_v<T>)
>    >    >       : semiregular-box{in_place}
>    >    >     { }

>    > 3. (1.3) — […]

> 8. Modify 25.6.6.2 [range.istream.view], Class template `basic_istream_view` synopsis, as indicated:

>    >     namespace std::ranges {
>    >       […]
>    >       template<movable Val, class CharT, class Traits>
>    >         requires default_constructibleinitializable<Val> &&
>    >           stream-extractable<Val, CharT, Traits>
>    >       class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> {
>    >         […]
>    >       }
>    >       […]
>    >     }

[2019-11-17; Daniel comments and restores wording]

During the Belfast 2019 meeting the concept renaming was not voted in by this issue, but separately, the accepted wording can be found in P1917R0#3149. To prevent confusion, the here presented proposed wording has been synchronized with that of the voted in document.

Proposed resolution:

This wording is relative to N4762.

1. Modify [concept.defaultconstructible] as follows:

   >     template<class T>
   >       inline constexpr bool is-default-initializable = see below; // exposition only

   >     template<class T>
   >       concept DefaultConstructible = Constructible<T> && requires { T{}; } && is-default-initializable<T>;

   > -?- For a type `T`, `is-default-initializable<T>` is `true` if and only if the variable definition

   > >     T t;

   > is well-formed for some invented variable `t`; otherwise it is `false`. Access checking is performed as if in a context unrelated to `T`. Only the validity of the immediate context of the variable initialization is considered.

### 3150(i). `UniformRandomBitGenerator` should validate `min` and `max`

Section: 29.5.3.3 [rand.req.urng] Status: C++20 Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2021-02-25

Priority: 3

View all other issues in [rand.req.urng].

View all issues with C++20 status.

Discussion:

29.5.3.3 [rand.req.urng] paragraph 2 specifies axioms for the `UniformRandomBitGenerator` concept:

> 2 Let `g` be an object of type `G`. `G` models `UniformRandomBitGenerator` only if

> (2.1) — both `G::min()` and `G::max()` are constant expressions (7.7 [expr.const]),

> (2.2) — `G::min() < G::max()`,

> (2.3) — `G::min() <= g()`,

> (2.4) — `g() <= G::max()`, and

> (2.5) — `g()` has amortized constant complexity.

Bullets 2.1 and 2.2 are both compile-time requirements that ought to be validated by the concept.

[2018-08-20 Priority set to 3 after reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4791.

> 1. Modify 29.5.3.3 [rand.req.urng] as follows:

>    > 1 A uniform random bit generator `g` of type `G` is a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability of being returned. [Note: The degree to which `g`'s results approximate the ideal is often determined statistically.—end note]

>    > >     template<auto> struct require-constant; // exposition-only

>    > >     template<class G>
>    > >       concept UniformRandomBitGenerator =
>    > >         Invocable<G&> && UnsignedIntegral<invoke_result_t<G&>> &&
>    > >         requires {
>    > >           { G::min() } -> Same<invoke_result_t<G&>>;
>    > >           { G::max() } -> Same<invoke_result_t<G&>>;
>    > >           typename require-constant<G::min()>;
>    > >           typename require-constant<G::max()>;
>    > >           requires G::min() < G::max();
>    > >         };

>    > 2 Let `g` be an object of type `G`. `G` models `UniformRandomBitGenerator` only if

>    > (2.1) — both `G​::​min()` and `G​::​max()` are constant expressions (7.7 [expr.const]),

>    > (2.2) — `G​::​min() < G​::​max()`,

>    > (2.3) — `G​::​min() <= g()`,

>    > (2.4) — `g() <= G​::​max()`, and

>    > (2.5) — `g()` has amortized constant complexity.

>    > 3 A class `G` meets the uniform random bit generator requirements if `G` models `UniformRandomBitGenerator`, `invoke_result_t<G&>` is an unsigned integer type (6.9.2 [basic.fundamental]), and `G` provides a nested typedef-name `result_type` that denotes the same type as `invoke_result_t<G&>`.

[2020-02-13; Prague]

LWG provided some improved wording.

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 29.5.3.3 [rand.req.urng] as follows:

   > 1 A uniform random bit generator `g` of type `G` is a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability of being returned. [Note: The degree to which `g`'s results approximate the ideal is often determined statistically.—end note]

   > >     template<class G>
   > >       concept uniform_random_bit_generator =
   > >         invocable<G&> && unsigned_integral<invoke_result_t<G&>> &&
   > >         requires {
   > >           { G::min() } -> same_as<invoke_result_t<G&>>;
   > >           { G::max() } -> same_as<invoke_result_t<G&>>;
   > >           requires bool_constant<(G::min() < G::max())>::value;
   > >         };

   > -2- Let `g` be an object of type `G`. `G` models `uniform_random_bit_generator` only if

   > (2.1) — both `G​::​min()` and `G​::​max()` are constant expressions (7.7 [expr.const]),

   > (2.2) — `G​::​min() < G​::​max()`,

   > (2.3) — `G​::​min() <= g()`,

   > (2.4) — `g() <= G​::​max()`, and

   > (2.5) — `g()` has amortized constant complexity.

   > 3 A class `G` meets the uniform random bit generator requirements if `G` models `uniform_random_bit_generator`, `invoke_result_t<G&>` is an unsigned integer type (6.9.2 [basic.fundamental]), and `G` provides a nested typedef-name `result_type` that denotes the same type as `invoke_result_t<G&>`.

### 3151(i). `ConvertibleTo` rejects conversions from array and function types

Section: 18.4.4 [concept.convertible] Status: Resolved Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2020-11-09

Priority: 3

View other active issues in [concept.convertible].

View all other issues in [concept.convertible].

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Discussion:

In the definition of `ConvertibleTo` in 18.4.4 [concept.convertible]:

>     template<class From, class To>
>       concept ConvertibleTo =
>         is_convertible_v<From, To> &&
>         requires(From (&f)()) {
>           static_cast<To>(f());
>         };

`f` is an arbitrary function that returns type `From`. Since functions cannot return array or function types (9.3.4.6 [dcl.fct] paragraph 11), `ConvertibleTo` cannot be satisfied when `From` is an array or function type regardless of the type of `To`. This is incompatibility with `is_convertible_v` was not an intentional design feature, so it should be corrected. (Note that any change made here must take care to avoid breaking the `ConvertibleTo<T, void>` cases.)

[2018-08-20 Priority set to 3 after reflector discussion]

Previous resolution [SUPERSEDED]:

> [Drafting Note: I've used `declval` here, despite that "Concepts mean we never have to use `declval` again!" because the alternative is less readable:]

> >     requires(add_rvalue_reference_t<From> (&f)()) {
> >       static_cast<To>(f());
> >     };

> This wording is relative to N4762.

> 1. Modify 18.4.4 [concept.convertible] as follows:

>    >     template<class From, class To>
>    >       concept ConvertibleTo =
>    >         is_convertible_v<From, To> &&
>    >         requires(From (&f)()) {
>    >           static_cast<To>(f() declval<From>());
>    >         };

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

This issue is resolved by the resolution of issue 3194(i).

### 3152(i). `common_type` and `common_reference` have flaws in common

Section: 21.3.8.7 [meta.trans.other] Status: C++23 Submitter: Casey Carter Opened: 2018-08-10 Last modified: 2023-11-22

Priority: 3

View all other issues in [meta.trans.other].

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Discussion:

21.3.8.7 [meta.trans.other] p5 characterizes the requirements for program-defined specializations of `common_type` with the sentence:

> Such a specialization need not have a member named `type`, but if it does, that member shall be a typedef-name for an accessible and unambiguous cv-unqualified non-reference type `C` to which each of the types `T1` and `T2` is explicitly convertible.

This sentence - which 21.3.8.7 [meta.trans.other] p7 largely duplicates to specify requirements on program-defined specializations of `basic_common_reference` - has two problems:

1. The grammar term "typedef-name" is overconstraining; there's no reason to prefer a typedef-name here to an actual type, and

2. "accessible" and "unambiguous" are not properties of types, they are properties of names and base classes.

3. While we're here, we may as well strike the unused name `C` which both Note B and Note D define for the type denoted by `type`.

[2018-08 Batavia Monday issue prioritization]

Priority set to 3

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4762.

1. Modify 21.3.8.7 [meta.trans.other] p5 as follows:

   > -5- Note B: Notwithstanding the provisions of 21.3.3 [meta.type.synop], and pursuant to 16.4.5.2.1 [namespace.std], a program may specialize `common_type<T1, T2>` for types `T1` and `T2` such that `is_same_v<T1, decay_t<T1>>` and `is_same_v<T2, decay_t<T2>>` are each true. [Note: …] Such a specialization need not have a member named `type`, but if it does, that member shall be a typedef-name for an accessible and unambiguous the qualified-id `common_type<T1, T2>::type` shall denote a cv-unqualified non-reference type `C` to which each of the types `T1` and `T2` is explicitly convertible. Moreover, […]

2. Modify 21.3.8.7 [meta.trans.other] p7 similarly:

   > -7- Note D: Notwithstanding the provisions of 21.3.3 [meta.type.synop], and pursuant to 16.4.5.2.1 [namespace.std], a program may partially specialize `basic_common_reference<T, U, TQual, UQual>` for types `T` and `U` such that `is_same_v<T, decay_t<T>>` and `is_same_v<U, decay_t<U>>` are each true. [Note: …] Such a specialization need not have a member named `type`, but if it does, that member shall be a typedef-name for an accessible and unambiguous the qualified-id `basic_common_reference<T, U, TQual, UQual>::type` shall denote a type `C` to which each of the types `TQual<T>` and `UQual<U>` is convertible. Moreover, […]

### 3153(i). `Common` and `common_type` have too little in common

Section: 18.4.6 [concept.common] Status: C++20 Submitter: Casey Carter Opened: 2018-08-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

The `Common` concept when applied to types `T` and `U` requires that `T` and `U` are each `ConvertibleTo` ( [concept.convertibleto]) their common type `common_type_t<T, U>`. `ConvertibleTo` requires both implicit and explicit conversions with equivalent results. The requirement for implicit conversion is notably not a requirement for specializing `common_type` as detailed in 21.3.8.7 [meta.trans.other]:

> -5- Such a specialization need not have a member named `type`, but if it does, that member shall be a typedef-name for an accessible and unambiguous cv-unqualified non-reference type `C` to which each of the types `T1` and `T2` is explicitly convertible.

which only requires explicit conversion to be valid. While it's not inconsistent that the `Common` concept's requirements are a refinement of the requirements for `common_type`, there's no good reason for this additional requirement. The stated design intent is to enable writing monomorphic predicates that can compare `T`s with `U`s (and vice versa) by accepting two arguments of type `common_type_t<T, U>`, but this role has been superseded by the addition of `CommonReference` and `common_reference_t` to the ranges design. The existence of pairs of types that are only explicitly convertible to their common type suggests that using `Common` in this way would never be a fully generic solution in any case.

The only existing use of the `Common` concept in either the working draft or the Ranges proposal is as a soundness check on the comparison and difference operators of `counted_iterator`, none of which actually convert any argument to the common type in their normal operation. It would seem that we could strike the additional requirement without impacting the Ranges design, which would allow for future uses of the `Common` concept with types like `chrono::duration` (30.5 [time.duration]) which sometimes provide only explicit conversion to a common type.

Notably, removing the requirement for implicit conversion will also make the `Common` concept consistent with the description in 18.4.6 [concept.common] p1: "If `T` and `U` can both be explicitly converted to some third type, `C`, then `T` and `U` share a common type, `C`."

[2018-08 Batavia Monday issue prioritization]

P0; Status to 'Tentatively Ready' after adding two semicolons to the P/R.

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

1. Modify the definition of `Common` in 18.4.6 [concept.common] as follows:

   >     template<class T, class U>
   >       concept Common =
   >         Same<common_type_t<T, U>, common_type_t<U, T>> &&
   >         ConvertibleTo<T, common_type_t<T, U>> &&
   >         ConvertibleTo<U, common_type_t<T, U>> &&
   >         requires {
   >           static_cast<common_type_t<T, U>>(declval<T>());
   >           static_cast<common_type_t<T, U>>(declval<U>());
   >         } &&
   >         CommonReference<
   >           add_lvalue_reference_t<const T>,
   >           add_lvalue_reference_t<const U>> &&
   >         CommonReference<
   >           add_lvalue_reference_t<common_type_t<T, U>>,
   >           common_reference_t<
   >             add_lvalue_reference_t<const T>,
   >             add_lvalue_reference_t<const U>>>;

### 3154(i). `Common` and `CommonReference` have a common defect

Section: 18.4.6 [concept.common] Status: C++20 Submitter: Casey Carter Opened: 2018-08-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

The semantic requirements of both `Common` (18.4.6 [concept.common]):

> -2- Let `C` be `common_type_t<T, U>`. Let `t` be a function whose return type is `T`, and let `u` be a function whose return type is `U`. `Common<T, U>` is satisfied only if:

> (2.1) — `C(t())` equals `C(t())` if and only if `t()` is an equality-preserving expression (18.2 [concepts.equality]).

> (2.2) — `C(u())` equals `C(u())` if and only if `u()` is an equality-preserving expression (18.2 [concepts.equality]).

and similarly `CommonReference` ( [concept.commonreference]):

> -2- Let `C` be `common_reference_t<T, U>`. Let `t` be a function whose return type is `T`, and let `u` be a function whose return type is `U`. `CommonReference<T, U>` is satisfied only if:

> (2.1) — `C(t())` equals `C(t())` if and only if `t()` is an equality-preserving expression (18.2 [concepts.equality]).

> (2.2) — `C(u())` equals `C(u())` if and only if `u()` is an equality-preserving expression.

don't properly reflect the intended design that conversions to the common type / common reference type are identity-preserving: in other words, that converting two values to the common type produces equal results if and only if the values were initially equal. The phrasing "`C(E)` equals `C(E)` if and only if `E` is an equality-preserving expression" is also clearly defective regardless of the intended design: the assertion "`E` is not equality-preserving" does not imply that every evaluation of `E` produces different results.

[2018-08 Batavia Monday issue prioritization]

Priority set to 0, status to 'Tentatively Ready'

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

1. Modify 18.4.5 [concept.commonref] p2 as follows:

   > -2- Let `C` be `common_reference_t<T, U>`. Let `t` be a function whose return type is `t1` and `t2` be equality-preserving expressions (18.2 [concepts.equality]) such that `decltype((t1))` and `decltype((t2))` are each `T`, and let `u` be a function whose return type is `u1` and `u2` be equality-preserving expressions such that `decltype((u1))` and `decltype((u2))` are each `U`. `T` and `U` model `CommonReference<T, U>` is satisfied only if:

   > (2.1) — `C(t1())` equals `C(t2())` if and only if `t1()` equals `t2`, and is an equality-preserving expression (18.2 [concepts.equality]).

   > (2.2) — `C(u1())` equals `C(u2())` if and only if `u1()` equals `u2` is an equality-preserving expression.

2. Modify 18.4.6 [concept.common] p2 similarly:

   > -2- Let `C` be `common_type_t<T, U>`. Let `t` be a function whose return type is `t1` and `t2` be equality-preserving expressions (18.2 [concepts.equality]) such that `decltype((t1))` and `decltype((t2))` are each `T`, and let `u` be a function whose return type is `u1` and `u2` be equality-preserving expressions such that `decltype((u1))` and `decltype((u2))` are each `U`. `T` and `U` model `Common<T, U>` is satisfied only if:

   > (2.1) — `C(t1())` equals `C(t2())` if and only if `t1()` equals `t2`, and is an equality-preserving expression (18.2 [concepts.equality]).

   > (2.2) — `C(u1())` equals `C(u2())` if and only if `u1()` equals `u2` is an equality-preserving expression (18.2 [concepts.equality]).

### 3155(i). `tuple<any, any>{allocator_arg_t, an_allocator}`

Section: 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Jonathan Wakely Opened: 2018-08-18 Last modified: 2021-10-23

Priority: 3

View other active issues in [tuple.cnstr].

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Discussion:

For a 2-element `std::tuple`, attempting to call the "allocator-extended default constructor" might actually pass the `allocator_arg` tag and the allocator to the tuple element constructors:

>     tuple<any, any> t{allocator_arg, allocator<int>{}};
>     assert(std::get<0>(t).has_value());

This assertion should pass according to the standard, but users might expect the elements to be default constructed. If you really wanted to construct the elements with the tag and the allocator, you could do:

>     tuple<any, any> t{{allocator_arg}, {allocator<int>{}}};

or

>     tuple<any, any> t{tuple<allocator_arg_t, allocator<int>>{allocator_arg, allocator<int>{}}};

The deduction guides for `std::tuple` always treat `{allocator_arg_t, an_allocator}` as the allocator-extended default constructor, so this creates an empty tuple:

>     tuple t{allocator_arg, allocator<int>{}};

And this is needed to create `tuple<any, any>`:

>     tuple t{allocator_arg, allocator<int>{}, any{}, any{}};

The proposed resolution seems consistent with that, always calling an allocator-extended constructor for `{allocator_arg_t, a}`, instead of the `tuple(UTypes&&...)` constructor.

Ville Voutilainen:

This was discussed in this reflector thread, where Andrzej convinced me to change libstdc++ tuple.

[2018-08-20, Daniel comments]

The wording changes by this issue are very near to those suggested for LWG 3121(i).

[2018-08 Batavia Monday issue prioritization]

Priority set to 0, status to 'Tentatively Ready'. Alisdair to write a paper about SFINAE constraints on the Allocator-aware tuple constructors.

[2018-08 Batavia Friday]

Tim Song found a 3-element case of this issue. Status back to 'Open'

`tuple<any,any,any>(allocator_arg_t, a, tuple)`

[2018-09 Reflector prioritization]

Set Priority to 3

Previous resolution [SUPERSEDED]:

> This wording is relative to N4762.

> 1. Modify 22.4.4.2 [tuple.cnstr] as indicated:

>    >     template<class... UTypes> explicit(see below) constexpr tuple(UTypes&&... u);

>    > > -9- Effects: Initializes the elements in the tuple with the corresponding value in `std::forward<UTypes>(u)`.

>    > > -10- Remarks: This constructor shall not participate in overload resolution unless `sizeof...(Types) == sizeof...(UTypes)` and `sizeof...(Types) >= 1` and `is_constructible_v<Ti, Ui&&>` is `true` for all `i` and `(sizeof...(Types) != 2 || !is_same_v<remove_cvref_t<U0>, allocator_arg_t>)`. The expression inside `explicit` is equivalent to:

>    > > `!conjunction_v<is_convertible<UTypes, Types>...>`

[2021-08-20; LWG telecon]

Status changed to Tentatively Resolved, by 3121(i).

[2021-10-23 Resolved by 3121(i), approved at October 2021 virtual plenary. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

This issue is resolved by the resolution of issue 3121(i).

### 3156(i). `ForwardIterator` should only mean forward iterator

Section: 26.11 [specialized.algorithms] Status: Resolved Submitter: Casey Carter Opened: 2018-09-06 Last modified: 2020-05-02

Priority: 3

View other active issues in [specialized.algorithms].

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Discussion:

26.11 [specialized.algorithms] para 1.2 describes how the specialized algorithms with a template parameter named `ForwardIterator` impose requirements on the type passed as argument for that parameter: it must meet the `Cpp17ForwardIterator` requirements, which is consistent with how the rest of the Library uses the template parameter name `ForwardIterator`, and many of the required operations on that type must not throw exceptions, which is not consistent with how the rest of the Library uses that name.

To avoid confusion and keep the meaning of requirements imposed by template parameter names crisp, the specialized memory algorithms should use a different template parameter name for this different set of requirements.

Note that the proposed change has no normative effect; it's simply a clarification of the existing wording.

[2018-09 Reflector prioritization]

Set Priority to 3

Previous resolution [SUPERSEDED]:

> This wording is relative to N4762.

> 1. Modify 20.2.2 [memory.syn] as indicated:

>    >     […]

>    >     // 26.11 [specialized.algorithms], specialized algorithms
>    >     template<class T>
>    >       constexpr T* addressof(T& r) noexcept;
>    >     template<class T>
>    >       const T* addressof(const T&&) = delete;
>    >     template<class NoThrowForwardIterator>
>    >       void uninitialized_default_construct(NoThrowForwardIterator first, NoThrowForwardIterator last);
>    >     template<class ExecutionPolicy, class NoThrowForwardIterator>
>    >       void uninitialized_default_construct(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                            NoThrowForwardIterator first, NoThrowForwardIterator last);
>    >     template<class NoThrowForwardIterator, class Size>
>    >       NoThrowForwardIterator uninitialized_default_construct_n(NoThrowForwardIterator first, Size n);
>    >     template<class ExecutionPolicy, class NoThrowForwardIterator, class Size>
>    >       NoThrowForwardIterator uninitialized_default_construct_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                                         NoThrowForwardIterator first, Size n);
>    >     template<class NoThrowForwardIterator>
>    >       void uninitialized_value_construct(NoThrowForwardIterator first, NoThrowForwardIterator last);
>    >     template<class ExecutionPolicy, class NoThrowForwardIterator>
>    >       void uninitialized_value_construct(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                          NoThrowForwardIterator first, NoThrowForwardIterator last);
>    >     template<class NoThrowForwardIterator, class Size>
>    >       NoThrowForwardIterator uninitialized_value_construct_n(NoThrowForwardIterator first, Size n);
>    >     template<class ExecutionPolicy, class NoThrowForwardIterator, class Size>
>    >       NoThrowForwardIterator uninitialized_value_construct_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                                       NoThrowForwardIterator first, Size n);
>    >     template<class InputIterator, class NoThrowForwardIterator>
>    >       NoThrowForwardIterator uninitialized_copy(InputIterator first, InputIterator last,
>    >                                          NoThrowForwardIterator result);
>    >     template<class ExecutionPolicy, class InputIterator, class NoThrowForwardIterator>
>    >       NoThrowForwardIterator uninitialized_copy(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                          InputIterator first, InputIterator last,
>    >                                          NoThrowForwardIterator result);
>    >     template<class InputIterator, class Size, class NoThrowForwardIterator>
>    >       NoThrowForwardIterator uninitialized_copy_n(InputIterator first, Size n,
>    >                                            NoThrowForwardIterator result);
>    >     template<class ExecutionPolicy, class InputIterator, class Size, class NoThrowForwardIterator>
>    >       NoThrowForwardIterator uninitialized_copy_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                            InputIterator first, Size n,
>    >                                            NoThrowForwardIterator result);
>    >     template<class InputIterator, class NoThrowForwardIterator>
>    >       NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last,
>    >                                          NoThrowForwardIterator result);
>    >     template<class ExecutionPolicy, class InputIterator, class NoThrowForwardIterator>
>    >       NoThrowForwardIterator uninitialized_move(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                          InputIterator first, InputIterator last,
>    >                                          NoThrowForwardIterator result);
>    >     template<class InputIterator, class Size, class NoThrowForwardIterator>
>    >       pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(InputIterator first, Size n,
>    >                                                                 NoThrowForwardIterator result);
>    >     template<class ExecutionPolicy, class InputIterator, class Size, class NoThrowForwardIterator>
>    >       pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                                                 InputIterator first, Size n,
>    >                                                                 NoThrowForwardIterator result);
>    >     template<class NoThrowForwardIterator, class T>
>    >       void uninitialized_fill(NoThrowForwardIterator first, NoThrowForwardIterator last, const T& x);
>    >     template<class ExecutionPolicy, class NoThrowForwardIterator, class T>
>    >       void uninitialized_fill(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                               NoThrowForwardIterator first, NoThrowForwardIterator last, const T& x);
>    >     template<class NoThrowForwardIterator, class Size, class T>
>    >       NoThrowForwardIterator uninitialized_fill_n(NoThrowForwardIterator first, Size n, const T& x);
>    >     template<class ExecutionPolicy, class NoThrowForwardIterator, class Size, class T>
>    >       NoThrowForwardIterator uninitialized_fill_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                            NoThrowForwardIterator first, Size n, const T& x);
>    >     template<class T>
>    >       void destroy_at(T* location);
>    >     template<class NoThrowForwardIterator>
>    >       void destroy(NoThrowForwardIterator first, NoThrowForwardIterator last);
>    >     template<class ExecutionPolicy, class NoThrowForwardIterator>
>    >       void destroy(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                    NoThrowForwardIterator first, NoThrowForwardIterator last);
>    >     template<class NoThrowForwardIterator, class Size>
>    >       NoThrowForwardIterator destroy_n(NoThrowForwardIterator first, Size n);
>    >     template<class ExecutionPolicy, class NoThrowForwardIterator, class Size>
>    >       NoThrowForwardIterator destroy_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
>    >                                 NoThrowForwardIterator first, Size n);

>    >     // 20.3.1 [unique.ptr], class template unique_ptr
>    >     […]

> 2. Modify 26.11 [specialized.algorithms] as indicated:

>    > […]

>    > (1.2) — If an algorithm's template parameter is named `NoThrowForwardIterator`, the template argument shall satisfy the `Cpp17ForwardIterator` requirements (24.3.5.5 [forward.iterators]), and is required to have the property that no exceptions are thrown from increment, assignment, comparison, or indirection through valid iterators.

>    > […]

> 3. Modify the declarations of the specialized algorithms in the remainder of 26.11 [specialized.algorithms] to agree with the proposed changes to 20.2.2 [memory.syn] above.

[2020-05-02; Reflector discussions]

The issue has been resolved by accepting P1963R0 in Prague 2020.

Proposed resolution:

Resolved by P1963R0.

### 3158(i). `tuple(allocator_arg_t, const Alloc&)` should be conditionally explicit

Section: 22.4.4.2 [tuple.cnstr] Status: C++20 Submitter: Jonathan Wakely Opened: 2018-08-21 Last modified: 2021-02-25

Priority: 3

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Discussion:

`std::tuple`'s allocator-extended constructors say "Effects: Equivalent to the preceding constructors except that each element is constructed with uses-allocator construction". That's not true for the first one, as shown by:

>     #include <tuple>

>     struct X { explicit X() { } };

>     std::tuple<X> f() { return {}; }
>     std::tuple<X> g() { return { std::allocator_arg, std::allocator<int>{} }; }

The function `f()` doesn't compile because of the explicit constructor, but `g()` does, despite using the same constructor for `X`. The conditional explicit-ness is not equivalent.

Also, the editor requested that we change "implicitly default-constructible" to use words that mean something. He suggested "copy-list-initializable from an empty list".

[2018-09 Reflector prioritization]

Set Priority to 3

[2019-02; Kona Wednesday night issue processing]

Status to Ready

Proposed resolution:

This wording is relative to N4762.

1. Modify 22.4.4 [tuple.tuple], class template `tuple` synopsis, as indicated:

   >     […]
   >     // allocator-extended constructors
   >     template<class Alloc>
   >       explicit(see below) tuple(allocator_arg_t, const Alloc& a);
   >     template<class Alloc>
   >       explicit(see below) tuple(allocator_arg_t, const Alloc& a, const Types&...);
   >     […]

2. Modify 22.4.4.2 [tuple.cnstr], as indicated:

   >     explicit(see below) constexpr tuple();

   > > -5- Effects: Value-initializes each element.

   > > -6- Remarks: This constructor shall not participate in overload resolution unless `is_default_constructible_v<Ti>` is `true` for all `i`. [Note: This behavior can be implemented by a constructor template with default template arguments. — end note] The expression inside `explicit` evaluates to `true` if and only if `Ti` is not implicitly default-constructible copy-list-initializable from an empty list for at least one `i`. [Note: This behavior can be implemented with a trait that checks whether a `const Ti&` can be initialized with `{}`. — end note]

   > […]

   >     template<class Alloc>
   >       explicit(see below) tuple(allocator_arg_t, const Alloc& a);
   >     template<class Alloc>
   >       explicit(see below) tuple(allocator_arg_t, const Alloc& a, const Types&...);
   >     […]
   >     template<class Alloc, class U1, class U2>
   >       explicit(see below) tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);

   > > -25- Requires: `Alloc` shall satisfy the `Cpp17Allocator` requirements (Table 33).

   > > […]

### 3160(i). `atomic_ref() = delete;` should be deleted

Section: 32.5.7 [atomics.ref.generic] Status: C++20 Submitter: Tim Song Opened: 2018-10-01 Last modified: 2021-02-25

Priority: 0

View all other issues in [atomics.ref.generic].

View all issues with C++20 status.

Discussion:

`atomic_ref` has a deleted default constructor, which causes pointless ambiguities in cases like:

>     void meow(atomic_ref<int>);
>     void meow(some_default_constructible_struct);

>     meow({});

It should have no default constructor rather than a deleted one. (Note that it has other user-defined constructors and so cannot be an aggregate under any definition.)

[2018-10-06 Status to Tentatively Ready after seven positive votes on the reflector.]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

- Edit 32.5.7 [atomics.ref.generic], class template `atomic_ref` synopsis, as indicated:

  >     […]
  >     atomic_ref() = delete;
  >     atomic_ref& operator=(const atomic_ref&) = delete;
  >     […]

- Edit 32.5.7.3 [atomics.ref.int], class template specialization `atomic_ref<integral>` synopsis, as indicated:

  >     […]
  >     atomic_ref() = delete;
  >     atomic_ref& operator=(const atomic_ref&) = delete;
  >     […]

- Edit 32.5.7.4 [atomics.ref.float], class template specialization `atomic_ref<floating-point>` synopsis, as indicated:

  >     […]
  >     atomic_ref() = delete;
  >     atomic_ref& operator=(const atomic_ref&) = delete;
  >     […]

- Edit 32.5.7.5 [atomics.ref.pointer], class template specialization `atomic_ref<T*>` synopsis, as indicated:

  >     […]
  >     atomic_ref() = delete;
  >     atomic_ref& operator=(const atomic_ref&) = delete;
  >     […]

### 3168(i). Expects: element should be specified in one place

Section: 16.3.2.4 [structure.specifications], 99 [res.on.required] Status: Resolved Submitter: Geoffrey Romer Opened: 2018-11-21 Last modified: 2020-07-17

Priority: 2

View other active issues in [structure.specifications].

View all other issues in [structure.specifications].

View all issues with Resolved status.

Discussion:

16.3.2.4 [structure.specifications]/p3.4 specifies the Expects: element as "the conditions (sometimes termed preconditions) that the function assumes to hold whenever it is called". This is nonsensical (functions can't "assume" things because they're not sentient), and more to the point it says nothing about what happens if those conditions don't hold. This is a serious problem because the whole point of introducing Expects: was to correct the vagueness of Requires: on exactly that point.

99 [res.on.required]/p2 is more explicit: "Violation of any preconditions specified in a function's Expects: element results in undefined behavior." However, I think putting the actual meaning of the Expects: element in a subclause called "Requires paragraph", 21 pages away from where Expects: is nominally specified, is asking too much of the reader. Splitting the specification of Requires: into two places 21 pages apart also seems needlessly obtuse, but that's less pressing since Requires: should be going away soon.

[2018-11 Reflector prioritization]

Set Priority to 2

[2019-02; Kona Wednesday night issue processing]

Alternate wording discussed; Marshall to check with Geoffrey and vote on reflector.

[2019 Cologne Wednesday night]

Revisit once the appropriate "Mandating" paper has landed

[2020-06; telecon and reflector discussion]

Changing Expects: to Preconditions: doesn't change the fact that the specification is split across two subclauses. Resolved editorially by integrating [res.on.expects] into [structure.specifications].

Proposed resolution:

This wording is relative to N4778.

> [Drafting Note: I have prepared two mutually exclusive options, depicted below by Option A and Option B, respectively. In case the committee would prefer to leave "Requires" alone, Option B describes just the "Expects" edits, as an alternate P/R]

Option A

1. Change 16.3.2.4 [structure.specifications] as indicated:

   > -3- Descriptions of function semantics contain the following elements (as appropriate):(footnote)

   > 1. (3.1) — Requires: the preconditions for calling the function.the conditions that are required to hold when the function is called in order for the call to successfully complete. [Note: When these conditions are violated, the function's Throws: element may specify throwing an exception. Otherwise, the behavior is undefined. — end note]

   > 2. (3.2) — Constraints: […]

   > 3. (3.3) — Mandates: […]

   > 4. (3.4) — Expects: the conditions (sometimes termed preconditions) that the function assumes to hold whenever it is calledare required to hold when the function is called in order for the call to have well-defined behavior. [Example: An implementation might express such conditions via an attribute such as `[[expects]]` ( [dcl.attr.contract]). However, some such conditions might not lend themselves to expression via code. — end example]

2. Delete 99 [res.on.required] in it's entirety as indicated:

   > 15.5.4.11 Requires paragraph [res.on.required]

   > -1- Violation of any preconditions specified in a function's Requires: element results in undefined behavior unless the function's Throws: element specifies throwing an exception when the precondition is violated.

   > -2- Violation of any preconditions specified in a function's Expects: element results in undefined behavior.

Option B

1. Change 16.3.2.4 [structure.specifications] as indicated:

   > -3- Descriptions of function semantics contain the following elements (as appropriate):(footnote)

   > 1. (3.1) — Requires: the preconditions for calling the function.

   > 2. (3.2) — Constraints: […]

   > 3. (3.3) — Mandates: […]

   > 4. (3.4) — Expects: the conditions (sometimes termed preconditions) that the function assumes to hold whenever it is calledare required to hold when the function is called in order for the call to have well-defined behavior. [Example: An implementation might express such conditions via an attribute such as `[[expects]]` ( [dcl.attr.contract]). However, some such conditions might not lend themselves to expression via code. — end example]

2. Change 99 [res.on.required] as indicated:

   > -1- Violation of any preconditions specified in a function's Requires: element results in undefined behavior unless the function's Throws: element specifies throwing an exception when the precondition is violated.

   > -2- Violation of any preconditions specified in a function's Expects: element results in undefined behavior.

### 3169(i). `ranges` permutation generators discard useful information

Section: 26.8.13 [alg.permutation.generators] Status: C++20 Submitter: Casey Carter Opened: 2018-11-26 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

In the Ranges design, algorithms that necessarily traverse an entire range - consequently discovering the end iterator value - return that iterator value unless the algorithm's sole purpose is to return a derived value, for example, `ranges::count`. `ranges::next_permutation` and `ranges::prev_permutation` necessarily traverse the entirety of their range argument, but are currently specified to discard the end iterator value and return only a `bool` indicating whether they found a next (respectively previous) permutation or "reset" the range to the first (respectively last) permutation. They should instead return an aggregate composed of both that `bool` and the end iterator value to be consistent with the other `range` algorithms.

[2019-01-22; Daniel comments and updates wording]

During the reflector discussion it had been noticed that an additional update of 26.2 [algorithms.requirements] p.16 is necessary for the new type `next_permutation_result` and two missing occurrences of `iterator_t<>` where added. The proposed wording has been updated.

[2019-02-02 Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]

Previous resolution [SUPERSEDED]:

> 1. Modify 26.2 [algorithms.requirements] as follows:

>    > -16- The class templates `binary_transform_result`, `for_each_result`, `minmax_result`, `mismatch_result`, `next_permutation_result`, `copy_result`, and `partition_copy_result` have the template parameters, data members, and special members specified above. They have no base classes or members other than those specified.

> 2. Modify 26.4 [algorithm.syn] as follows:

>    >       // 26.8.13 [alg.permutation.generators], permutations
>    >       template<class BidirectionalIterator>
>    >         constexpr bool next_permutation(BidirectionalIterator first,
>    >                                         BidirectionalIterator last);
>    >       template<class BidirectionalIterator, class Compare>
>    >         constexpr bool next_permutation(BidirectionalIterator first,
>    >                                         BidirectionalIterator last, Compare comp);

>    >       namespace ranges {
>    >         template<class I>
>    >         struct next_permutation_result {
>    >           bool found;
>    >           I in;
>    >         };

>    >         template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>,
>    >                  class Proj = identity>
>    >           requires Sortable<I, Comp, Proj>
>    >           constexpr boolnext_permutation_result<I>
>    >             next_permutation(I first, S last, Comp comp = {}, Proj proj = {});
>    >         template<BidirectionalRange R, class Comp = ranges::less<>,
>    >                  class Proj = identity>
>    >           requires Sortable<iterator_t<R>, Comp, Proj>
>    >           constexpr boolnext_permutation_result<iterator_t<R>>
>    >             next_permutation(R&& r, Comp comp = {}, Proj proj = {});
>    >       }

>    >       template<class BidirectionalIterator>
>    >         constexpr bool prev_permutation(BidirectionalIterator first,
>    >                                         BidirectionalIterator last);
>    >       template<class BidirectionalIterator, class Compare>
>    >         constexpr bool prev_permutation(BidirectionalIterator first,
>    >                                         BidirectionalIterator last, Compare comp);

>    >       namespace ranges {
>    >         template<class I>
>    >         using prev_permutation_result = next_permutation_result<I>;

>    >         template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>,
>    >                  class Proj = identity>
>    >           requires Sortable<I, Comp, Proj>
>    >           constexpr boolprev_permutation_result<I>
>    >             prev_permutation(I first, S last, Comp comp = {}, Proj proj = {});
>    >         template<BidirectionalRange R, class Comp = ranges::less<>,
>    >                  class Proj = identity>
>    >           requires Sortable<iterator_t<R>, Comp, Proj>
>    >           constexpr boolprev_permutation_result<iterator_t<R>>
>    >             prev_permutation(R&& r, Comp comp = {}, Proj proj = {});
>    >       }
>    >     }

> 3. Modify 26.8.13 [alg.permutation.generators] as follows:

>    > >     template<class BidirectionalIterator>
>    > >       constexpr bool next_permutation(BidirectionalIterator first,
>    > >                                       BidirectionalIterator last);
>    > >     template<class BidirectionalIterator, class Compare>
>    > >       constexpr bool next_permutation(BidirectionalIterator first,
>    > >                                       BidirectionalIterator last, Compare comp);

>    > >     namespace ranges {
>    > >       template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>,
>    > >                class Proj = identity>
>    > >         requires Sortable<I, Comp, Proj>
>    > >         constexpr boolnext_permutation_result<I>
>    > >           next_permutation(I first, S last, Comp comp = {}, Proj proj = {});
>    > >       template<BidirectionalRange R, class Comp = ranges::less<>,
>    > >                class Proj = identity>
>    > >         requires Sortable<iterator_t<R>, Comp, Proj>
>    > >         constexpr boolnext_permutation_result<iterator_t<R>>
>    > >           next_permutation(R&& r, Comp comp = {}, Proj proj = {});
>    > >     }

>    > […]

>    > -4- Returns: Let `B` be `true` if and only if a next permutation was found and otherwise `false`. Returns:

>    > - `B` for the overloads in namespace `std`, or

>    > - `{ B, last }` for the overloads in namespace `ranges`.

>    > -5- Complexity: […]

>    > >     template<class BidirectionalIterator>
>    > >       constexpr bool prev_permutation(BidirectionalIterator first,
>    > >                                       BidirectionalIterator last);
>    > >     template<class BidirectionalIterator, class Compare>
>    > >       constexpr bool prev_permutation(BidirectionalIterator first,
>    > >                                       BidirectionalIterator last, Compare comp);

>    > >     namespace ranges {
>    > >       template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>,
>    > >                class Proj = identity>
>    > >         requires Sortable<I, Comp, Proj>
>    > >         constexpr boolprev_permutation_result<I>
>    > >           prev_permutation(I first, S last, Comp comp = {}, Proj proj = {});
>    > >       template<BidirectionalRange R, class Comp = ranges::less<>,
>    > >                class Proj = identity>
>    > >         requires Sortable<iterator_t<R>, Comp, Proj>
>    > >         constexpr boolprev_permutation_result<iterator_t<R>>
>    > >           prev_permutation(R&& r, Comp comp = {}, Proj proj = {});
>    > >     }

>    > […]

>    > -9- Returns: Let `B` be `true` if and only if a previous permutation was found and otherwise `false`. Returns:

>    > - `B` for the overloads in namespace `std`, or

>    > - `{ B, last }` for the overloads in namespace `ranges`.

>    > -10- Complexity: […]

[2019-02-10 Tomasz comments; Casey updates the P/R and resets status to "Review."]

Shouldn't the range overloads for an algorithms return `safe_iterator_t<R>` instead of `iterator_t<R>`? Other algorithms are consistently returning the `safe_iterator_t`/`safe_subrange_t` in situation, when range argument is an rvalue (temporary) and returned iterator may be dangling.

[2019-02; Kona Wednesday night issue processing]

Status to Ready

Proposed resolution:

This wording is relative to N4800.

1. Modify 26.2 [algorithms.requirements] as follows:

   > -16- The class templates `binary_transform_result`, `for_each_result`, `minmax_result`, `mismatch_result`, `next_permutation_result`, `copy_result`, and `partition_copy_result` have the template parameters, data members, and special members specified above. They have no base classes or members other than those specified.

2. Modify 26.4 [algorithm.syn] as follows:

   >       // 26.8.13 [alg.permutation.generators], permutations
   >       template<class BidirectionalIterator>
   >         constexpr bool next_permutation(BidirectionalIterator first,
   >                                         BidirectionalIterator last);
   >       template<class BidirectionalIterator, class Compare>
   >         constexpr bool next_permutation(BidirectionalIterator first,
   >                                         BidirectionalIterator last, Compare comp);

   >       namespace ranges {
   >         template<class I>
   >         struct next_permutation_result {
   >           bool found;
   >           I in;
   >         };

   >         template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>,
   >                  class Proj = identity>
   >           requires Sortable<I, Comp, Proj>
   >           constexpr boolnext_permutation_result<I>
   >             next_permutation(I first, S last, Comp comp = {}, Proj proj = {});
   >         template<BidirectionalRange R, class Comp = ranges::less<>,
   >                  class Proj = identity>
   >           requires Sortable<iterator_t<R>, Comp, Proj>
   >           constexpr boolnext_permutation_result<safe_iterator_t<R>>
   >             next_permutation(R&& r, Comp comp = {}, Proj proj = {});
   >       }

   >       template<class BidirectionalIterator>
   >         constexpr bool prev_permutation(BidirectionalIterator first,
   >                                         BidirectionalIterator last);
   >       template<class BidirectionalIterator, class Compare>
   >         constexpr bool prev_permutation(BidirectionalIterator first,
   >                                         BidirectionalIterator last, Compare comp);

   >       namespace ranges {
   >         template<class I>
   >         using prev_permutation_result = next_permutation_result<I>;

   >         template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>,
   >                  class Proj = identity>
   >           requires Sortable<I, Comp, Proj>
   >           constexpr boolprev_permutation_result<I>
   >             prev_permutation(I first, S last, Comp comp = {}, Proj proj = {});
   >         template<BidirectionalRange R, class Comp = ranges::less<>,
   >                  class Proj = identity>
   >           requires Sortable<iterator_t<R>, Comp, Proj>
   >           constexpr boolprev_permutation_result<safe_iterator_t<R>>
   >             prev_permutation(R&& r, Comp comp = {}, Proj proj = {});
   >       }
   >     }

3. Modify 26.8.13 [alg.permutation.generators] as follows:

   > >     template<class BidirectionalIterator>
   > >       constexpr bool next_permutation(BidirectionalIterator first,
   > >                                       BidirectionalIterator last);
   > >     template<class BidirectionalIterator, class Compare>
   > >       constexpr bool next_permutation(BidirectionalIterator first,
   > >                                       BidirectionalIterator last, Compare comp);

   > >     namespace ranges {
   > >       template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>,
   > >                class Proj = identity>
   > >         requires Sortable<I, Comp, Proj>
   > >         constexpr boolnext_permutation_result<I>
   > >           next_permutation(I first, S last, Comp comp = {}, Proj proj = {});
   > >       template<BidirectionalRange R, class Comp = ranges::less<>,
   > >                class Proj = identity>
   > >         requires Sortable<iterator_t<R>, Comp, Proj>
   > >         constexpr boolnext_permutation_result<safe_iterator_t<R>>
   > >           next_permutation(R&& r, Comp comp = {}, Proj proj = {});
   > >     }

   > […]

   > -4- Returns: Let `B` be `true` if and only if a next permutation was found and otherwise `false`. Returns:

   > - `B` for the overloads in namespace `std`, or

   > - `{ B, last }` for the overloads in namespace `ranges`.

   > -5- Complexity: […]

   > >     template<class BidirectionalIterator>
   > >       constexpr bool prev_permutation(BidirectionalIterator first,
   > >                                       BidirectionalIterator last);
   > >     template<class BidirectionalIterator, class Compare>
   > >       constexpr bool prev_permutation(BidirectionalIterator first,
   > >                                       BidirectionalIterator last, Compare comp);

   > >     namespace ranges {
   > >       template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>,
   > >                class Proj = identity>
   > >         requires Sortable<I, Comp, Proj>
   > >         constexpr boolprev_permutation_result<I>
   > >           prev_permutation(I first, S last, Comp comp = {}, Proj proj = {});
   > >       template<BidirectionalRange R, class Comp = ranges::less<>,
   > >                class Proj = identity>
   > >         requires Sortable<iterator_t<R>, Comp, Proj>
   > >         constexpr boolprev_permutation_result<safe_iterator_t<R>>
   > >           prev_permutation(R&& r, Comp comp = {}, Proj proj = {});
   > >     }

   > […]

   > -9- Returns: Let `B` be `true` if and only if a previous permutation was found and otherwise `false`. Returns:

   > - `B` for the overloads in namespace `std`, or

   > - `{ B, last }` for the overloads in namespace `ranges`.

   > -10- Complexity: […]

### 3170(i). `is_always_equal` added to `std::allocator` makes the standard library treat derived types as always equal

Section: 20.2.10 [default.allocator] Status: C++23 Submitter: Billy O'Neal III Opened: 2018-11-29 Last modified: 2023-11-22

Priority: 2

View other active issues in [default.allocator].

View all other issues in [default.allocator].

View all issues with C++23 status.

Discussion:

I (Billy O'Neal) attempted to change MSVC++'s standard library to avoid instantiating allocators' `operator==` for allocators that are declared `is_always_equal` to reduce the number of template instantiations emitted into .objs.

In so doing I introduced an unrelated bug related to POCMA handling, but it brought my attention to this allocator. This allocator doesn't meet the allocator requirements because it is getting `std::allocator`'s `operator==` and `operator!=` which don't compare the root member. However, if this had been a conforming C++14 allocator with its own `==` and `!=` we would still be treating it as `is_always_equal`, as it picks that up by deriving from `std::allocator`.

`std::allocator` doesn't actually need `is_always_equal` because the defaults provided by `allocator_traits` will say `true_type` for it, since implementers don't make `std::allocator` stateful.

Billy O'Neal thinks this is NAD on the grounds that we need to be able to add things or change the behavior of standard library types.

Stephan T Lavavej thinks we should resolve this anyway because we don't know of an implementation for which this would change the default answer provided by `allocator_traits`.

[2019-02 Priority set to 2 after reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4778.

> 1. Modify 20.2.10 [default.allocator] as follows:

>    > -1- All specializations of the default allocator satisfy the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]).

>    >     namespace std {
>    >       template<class T> class allocator {
>    >       public:
>    >         using value_type = T;
>    >         using size_type = size_t;
>    >         using difference_type = ptrdiff_t;
>    >         using propagate_on_container_move_assignment = true_type;
>    >         using is_always_equal = true_type;
>    >         constexpr allocator() noexcept;
>    >         constexpr allocator(const allocator&) noexcept;
>    >         template<class U> constexpr allocator(const allocator<U>&) noexcept;
>    >         ~allocator();
>    >         allocator& operator=(const allocator&) = default;
>    >         [[nodiscard]] T* allocate(size_t n);
>    >         void deallocate(T* p, size_t n);
>    >       };
>    >     }

>    > -?- `allocator_traits<allocator<T>>::is_always_equal::value` is `true` for any `T`.

[2019-07 Cologne]

Jonathan provides updated wording.

[2020-10-02; Issue processing telecon: Moved to Tentatively Ready.]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4820.

1. Modify 20.2.10 [default.allocator] as follows:

   > -1- All specializations of the default allocator satisfy the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]).

   >     namespace std {
   >       template<class T> class allocator {
   >       public:
   >         using value_type = T;
   >         using size_type = size_t;
   >         using difference_type = ptrdiff_t;
   >         using propagate_on_container_move_assignment = true_type;
   >         using is_always_equal = true_type;
   >         constexpr allocator() noexcept;
   >         constexpr allocator(const allocator&) noexcept;
   >         template<class U> constexpr allocator(const allocator<U>&) noexcept;
   >         ~allocator();
   >         allocator& operator=(const allocator&) = default;
   >         [[nodiscard]] T* allocate(size_t n);
   >         void deallocate(T* p, size_t n);
   >       };
   >     }

   > -?- `allocator_traits<allocator<T>>::is_always_equal::value` is `true` for any `T`.

2. Add a new subclause in Annex D after 99 [depr.str.strstreams]:

   > D.? The default allocator [depr.default.allocator]

   > -?- The following member is defined in addition to those specified in 20.2.10 [default.allocator]:

   >     namespace std {
   >       template <class T> class allocator {
   >       public:
   >         using is_always_equal = true_type;
   >       };
   >     }

### 3171(i). LWG 2989 breaks `directory_entry` stream insertion

Section: 31.12.10 [fs.class.directory.entry] Status: C++23 Submitter: Tim Song Opened: 2018-12-03 Last modified: 2023-11-22

Priority: 2

View all other issues in [fs.class.directory.entry].

View all issues with C++23 status.

Discussion:

`directory_entry` has a conversion function to `const path&` and depends on `path`'s stream insertion operator for stream insertion support, which is now broken after LWG 2989(i) made it a hidden friend.

This does not appear to be intended.

[2018-12-21 Reflector prioritization]

Set Priority to 2

[2019-02; Kona Wednesday night issue processing]

Status to Open; Marshall to move definition inline and re-vote on reflector.

Jonathan to write a paper about how to specify "hidden friends".

Previous resolution [SUPERSEDED]:

> This wording is relative to N4778.

> 1. Modify [fs.class.directory_entry], class `directory_entry` synopsis, as follows:

>    >     namespace std::filesystem {
>    >       class directory_entry {
>    >       public:
>    >         […]
>    >       private:
>    >         filesystem::path pathobject;     // exposition only
>    >         friend class directory_iterator; // exposition only

>    >         template<class charT, class traits>
>    >           friend basic_ostream<charT, traits>&
>    >             operator<<(basic_ostream<charT, traits>& os, const directory_entry& d);
>    >       };
>    >     }

> 2. Add a new subclause at the end of [fs.class.directory_entry], as follows:

>    > 28.11.11.4 Inserter [fs.dir.entry.io]

>    > >     template<class charT, class traits>
>    > >       friend basic_ostream<charT, traits>&
>    > >         operator<<(basic_ostream<charT, traits>& os, const directory_entry& d);

>    > > > -1- Effects: Equivalent to: `return os << d.path();`

[2020-05-02; Daniel resyncs wording with recent working draft and comments]

We have now the paper P1965R0, which introduced a specification of what friend functions in the library specification (see 16.4.6.6 [hidden.friends]) are supposed to mean, there is no longer an inline definition needed to clarify the meaning. In addition to updating the change of section names the provided wording has moved the friend declaration into the public part of the class definition as have done in all other cases where we take advantage of "hidden friends" declarations.

[2020-08-21 Issue processing telecon: moved to Tentatively Ready]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 31.12.10 [fs.class.directory.entry], class `directory_entry` synopsis, as follows:

   >     namespace std::filesystem {
   >       class directory_entry {
   >       public:
   >         […]
   >         bool operator==(const directory_entry& rhs) const noexcept;
   >         strong_ordering operator<=>(const directory_entry& rhs) const noexcept;

   >         // 29.11.11.? [fs.dir.entry.io], inserter
   >         template<class charT, class traits>
   >           friend basic_ostream<charT, traits>&
   >             operator<<(basic_ostream<charT, traits>& os, const directory_entry& d);
   >       private:
   >         […]
   >       };
   >     }

2. Add a new subclause at the end of 31.12.10 [fs.class.directory.entry], as indicated:

   > 29.11.11.? Inserter [fs.dir.entry.io]

   > >     template<class charT, class traits>
   > >       friend basic_ostream<charT, traits>&
   > >         operator<<(basic_ostream<charT, traits>& os, const directory_entry& d);

   > > > -?- Effects: Equivalent to: `return os << d.path();`

### 3173(i). Enable CTAD for `ref-view`

Section: 25.7.6.2 [range.ref.view] Status: C++20 Submitter: Casey Carter Opened: 2018-12-09 Last modified: 2021-06-06

Priority: 0

View all issues with C++20 status.

Discussion:

In the specification of `view::all` in 25.7.6 [range.all], paragraph 2.2 states that `view::all(E)` is sometimes expression-equivalent to "`ref-view{E}` if that expression is well-formed". Unfortunately, the expression `ref-view{E}` is never well-formed: `ref-view`'s only non-default constructor is a perfect-forwarding-ish constructor template that accepts only arguments that convert to lvalues of the `ref-view`'s template argument type, and either do not convert to rvalues or have a better lvalue conversion (similar to the `reference_wrapper` converting constructor (22.10.6.2 [refwrap.const]) after issue 2993(i)).

Presumably this breakage was not intentional, and we should add a deduction guide to enable class template argument deduction to function as intended by paragraph 2.2.

[2018-12-16 Status to Tentatively Ready after six positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4791.

1. Modify the `ref-view` class synopsis in [ranges.view.ref] as follows:

   >     namespace std::ranges {
   >       template<Range R>
   >         requires is_object_v<R>
   >       class ref-view : public view_interface<ref-view<R>> {
   >         […]
   >       };

   >       template<class R>
   >         ref_view(R&) -> ref_view<R>;
   >     }

### 3175(i). The `CommonReference` requirement of concept `SwappableWith` is not satisfied in the example

Section: 18.4.9 [concept.swappable] Status: C++20 Submitter: Kostas Kyrimis Opened: 2018-12-14 Last modified: 2021-02-25

Priority: 1

View other active issues in [concept.swappable].

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Discussion:

The defect stems from the example found in sub-clause 18.4.9 [concept.swappable] p5:

>     […]

>     template<class T, std::SwappableWith<T> U>
>     void value_swap(T&& t, U&& u) {
>       ranges::swap(std::forward<T>(t), std::forward<U>(u));
>     }

>     […]
>     namespace N {
>       struct A { int m; };
>       struct Proxy { A* a; };
>       Proxy proxy(A& a) { return Proxy{ &a }; }

>       void swap(A& x, Proxy p) {
>         ranges::swap(x.m, p.a->m);
>       }
>       void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement
>     }

>     int main() {
>       […]
>       N::A a1 = { 5 }, a2 = { -5 };
>       value_swap(a1, proxy(a2)); // diagnostic manifests here(#1)
>       assert(a1.m == -5 && a2.m == 5);
>     }

The call to `value_swap(a1, proxy(a2))` resolves to [`T` = `N::A&`, `U` = `N::Proxy`] The compiler will issue a diagnostic for #1 because:

1. rvalue `proxy(a2)` is not swappable

2. concept `SwappableWith<T, U>` requires `N::A` and `Proxy` to model `CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&>` It follows from the example that there is no common reference for [`T` = `N::A&`, `U` = `N::Proxy`]

[2019-06-20; Casey Carter comments and provides improved wording]

The purpose of the `CommonReference` requirements in the cross-type concepts is to provide a sanity check. The fact that two types satisfy a single-type concept, have a common reference type that satisfies that concept, and implement cross-type operations required by the cross-type flavor of that concept very strongly suggests the programmer intends them to model the cross-type concept. It's an opt-in that requires some actual work, so it's unlikely to be inadvertent.

The `CommonReference<const T&, const U&>` pattern makes sense for the comparison concepts which require that all variations of `const` and value category be comparable: we use `const` lvalues to trigger the "implicit expression variation" wording in 18.2 [concepts.equality]. `SwappableWith`, however, doesn't care about implicit expression variations: it only needs to witness that we can exchange the values denoted by two reference-y expressions `E1` and `E2`. This suggests that `CommonReference<decltype((E1)), decltype((E2))>` is a more appropriate requirement than the current `CommonReference<const remove_reference_t<…>` mess that was blindly copied from the comparison concepts.

We must change the definition of "exchange the values" in 18.4.9 [concept.swappable] — which refers to the common reference type — consistently. Previous resolution [SUPERSEDED]:

> This wording is relative to N4791.

> 1. Change 18.4.9 [concept.swappable] as indicated:

>    > -3- […]

>    > >     template<class T>
>    > >       concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); };

>    > >     template<class T, class U>
>    > >       concept SwappableWith =
>    > >       CommonReference<T, Uconst remove_reference_t<T>&, const remove_reference_t<U>&> &&
>    > >       requires(T&& t, U&& u) {
>    > >         ranges::swap(std::forward<T>(t), std::forward<T>(t));
>    > >         ranges::swap(std::forward<U>(u), std::forward<U>(u));
>    > >         ranges::swap(std::forward<T>(t), std::forward<U>(u));
>    > >         ranges::swap(std::forward<U>(u), std::forward<T>(t));
>    > >       };

>    > -4- […]

>    > -5- [Example: User code can ensure that the evaluation of `swap` calls is performed in an appropriate context under the various conditions as follows:

>    > >     #include <cassert>
>    > >     #include <concepts>
>    > >     #include <utility>

>    > >     namespace ranges = std::ranges;

>    > >     template<class T, std::SwappableWith<T> U>
>    > >     void value_swap(T&& t, U&& u) {
>    > >       ranges::swap(std::forward<T>(t), std::forward<U>(u));
>    > >     }

>    > >     template<std::Swappable T>
>    > >     void lv_swap(T& t1, T& t2) {
>    > >       ranges::swap(t1, t2);
>    > >     }

>    > >     namespace N {
>    > >       struct A { int m; };
>    > >       struct Proxy {
>    > >         A* a;
>    > >         Proxy(A& a) : a{&a} {}
>    > >         friend void swap(Proxy&& x, Proxy&& y) {
>    > >           ranges::swap(x.a, y.a);
>    > >         }
>    > >       };
>    > >       Proxy proxy(A& a) { return Proxy{ &a }; }
>    > >       void swap(A& x, Proxy p) {
>    > >         ranges::swap(x.m, p.a->m);
>    > >       }
>    > >       void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement
>    > >     }

>    > >     int main() {
>    > >       int i = 1, j = 2;
>    > >       lv_swap(i, j);
>    > >       assert(i == 2 && j == 1);
>    > >       N::A a1 = { 5 }, a2 = { -5 };
>    > >       value_swap(a1, proxy(a2));
>    > >       assert(a1.m == -5 && a2.m == 5);
>    > >     }

[2020-01-16 Priority set to 1 after discussion on the reflector.]

[2020-02-10 Move to Immediate Monday afternoon in Prague]

Proposed resolution:

This wording is relative to N4820.

1. Change 18.4.9 [concept.swappable] as indicated:

   > -1- Let `t1` and `t2` be equality-preserving expressions that denote distinct equal objects of type `T`, and let `u1` and `u2` similarly denote distinct equal objects of type `U`. [Note: `t1` and `u1` can denote distinct objects, or the same object. — end note] An operation exchanges the values denoted by `t1` and `u1` if and only if the operation modifies neither `t2` nor `u2` and:

   > 1. (1.1) — If `T` and `U` are the same type, the result of the operation is that `t1` equals `u2` and `u1` equals `t2`.

   > 2. (1.2) — If `T` and `U` are different types that model `CommonReference<const T&, const U&>`and `CommonReference<decltype((t1)), decltype((u1))>` is modeled, the result of the operation is that `C(t1)` equals `C(u2)` and `C(u1)` equals `C(t2)` where `C` is `common_reference_t<const T&, const U&decltype((t1)), decltype((u1))>`.

   > -2- […]

   > -3- […]

   > >     template<class T>
   > >       concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); };

   > >     template<class T, class U>
   > >       concept SwappableWith =
   > >       CommonReference<T, Uconst remove_reference_t<T>&, const remove_reference_t<U>&> &&
   > >       requires(T&& t, U&& u) {
   > >         ranges::swap(std::forward<T>(t), std::forward<T>(t));
   > >         ranges::swap(std::forward<U>(u), std::forward<U>(u));
   > >         ranges::swap(std::forward<T>(t), std::forward<U>(u));
   > >         ranges::swap(std::forward<U>(u), std::forward<T>(t));
   > >       };

   > -4- […]

   > -5- [Example: User code can ensure that the evaluation of `swap` calls is performed in an appropriate context under the various conditions as follows:

   > >     #include <cassert>
   > >     #include <concepts>
   > >     #include <utility>

   > >     namespace ranges = std::ranges;

   > >     template<class T, std::SwappableWith<T> U>
   > >     void value_swap(T&& t, U&& u) {
   > >       ranges::swap(std::forward<T>(t), std::forward<U>(u));
   > >     }

   > >     template<std::Swappable T>
   > >     void lv_swap(T& t1, T& t2) {
   > >       ranges::swap(t1, t2);
   > >     }

   > >     namespace N {
   > >       struct A { int m; };
   > >       struct Proxy {
   > >         A* a;
   > >         Proxy(A& a) : a{&a} {}
   > >         friend void swap(Proxy x, Proxy y) {
   > >           ranges::swap(*x.a, *y.a);
   > >         }
   > >       };
   > >       Proxy proxy(A& a) { return Proxy{ &a }; }
   > >       void swap(A& x, Proxy p) {
   > >         ranges::swap(x.m, p.a->m);
   > >       }
   > >       void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement
   > >     }

   > >     int main() {
   > >       int i = 1, j = 2;
   > >       lv_swap(i, j);
   > >       assert(i == 2 && j == 1);
   > >       N::A a1 = { 5 }, a2 = { -5 };
   > >       value_swap(a1, proxy(a2));
   > >       assert(a1.m == -5 && a2.m == 5);
   > >     }

### 3176(i). Underspecified behavior of unordered containers when `Container::key_equal` differs from `Pred`

Section: 23.2.8 [unord.req] Status: Resolved Submitter: S. B. Tam Opened: 2018-11-27 Last modified: 2020-01-13

Priority: 2

View other active issues in [unord.req].

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Discussion:

After acceptance of P0919R3 into the new working draft (N4791), it is possible that an unordered container's member type `key_equal` is different from its template argument for `Pred` (the former being `Hash::transparent_key_equal` while the latter being `std::equal_to<Key>`). However, it is unclear which is stored in the container and used as the predicate in this case.

In particular, [unord.req]/4 says:

> […] The container's object of type `Pred` — denoted by `pred` — is called the key equality predicate of the container.

In Table 70, the row for `X::key_equal` places requirements to `Pred` and not to `Hash::transparent_key_equal`:

> Requires: `Pred` is Cpp17CopyConstructible. `Pred` shall be a binary predicate that takes two arguments of type `Key`. `Pred` is an equivalence relation.

The specification of `operator==` and `operator!=` in [unord.req]/12 uses `Pred`:

> […] The behavior of a program that uses `operator==` or operator!= on unordered containers is undefined unless the `Pred` function object has the same behavior for both containers and the equality comparison function for `Key` is a refinement(footnote 227) of the partition into equivalent-key groups produced by `Pred`.

The exception safety guarantees in [unord.req.except] mentions "the container's `Pred` object" twice.

The noexcept-specifiers of unordered containers' move assignment operator and `swap` member function are all in terms of `Pred`.

I think the intent is to make `Hash::transparent_key_equal` override `Pred`. If that's true, then all the abovementioned uses of `Pred` in the specification should probably be changed to uses `key_equal`.

[2018-12-21 Reflector prioritization]

Set Priority to 2

[2020-01 Resolved by the adoption of P1690 in Belfast.]

Proposed resolution:

### 3177(i). Limit permission to specialize variable templates to program-defined types

Section: 16.4.5.2.1 [namespace.std] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2018-12-11 Last modified: 2023-11-22

Priority: 3

View other active issues in [namespace.std].

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Discussion:

The permission denoted by [namespace.std]/3 should be limited to program-defined types.

[2018-12-21 Reflector prioritization]

Set Priority to 3

Previous resolution [SUPERSEDED]:

> This wording is relative to N4791.

> 1. Change 16.4.5.2.1 [namespace.std] as indicated:

>    > -2- Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace `std` provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.(footnote 174)

>    > -3- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template, provided that the added declaration depends on at least one program-defined type.

[2022-08-24; LWG telecon]

Each variable template that grants permission to specialize already states requirements more precisely than proposed here anyway. For example, `disable_sized_range` only allows it for cv-unqualified program-defined types. Adding less precise wording here wouldn't be an improvement. Add a note to make it clear we didn't just forget to say something here, and to remind us to state requirements for each variable template in future.

[2022-08-25; Jonathan Wakely provides improved wording]

[2022-09-28; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

- Change 16.4.5.2.1 [namespace.std] as indicated:

  > -2- Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace `std` provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.(footnote 163)

  > -3- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.

  > [Note 1: The requirements on an explicit or partial specialization are stated by each variable template that grants such permission. — end note]

### 3178(i). `std::mismatch` is missing an upper bound

Section: 26.6.12 [alg.mismatch] Status: Resolved Submitter: Geoffrey Romer Opened: 2018-12-20 Last modified: 2025-11-11

Priority: 0

View all other issues in [alg.mismatch].

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Discussion:

Consider the following code:

>     std::vector<int> v1 = {1, 2, 3, 4};
>     std::vector<int> v2 = {1, 2, 3, 5};
>     auto result = std::mismatch(v1.begin(), v1.begin() + 2, v2.begin(), v2.begin() + 2);

The current wording of [mismatch] seems to require `result` to be `{v1.begin() + 3, v2.begin() + 3}`, because 3 is the smallest integer `n` such that `*(v1.begin() + n) != *(v2.begin + n)`. In other words, if there's a mismatch that's reachable from `first1` and `first2`, then `std::mismatch` must find and return it, even if it's beyond the end iterators passed by the user.

This is doubly unimplementable: the library has no way of knowing that it's safe to keep going past the end of the user-supplied range, and even if it could, doing so would violate the complexity requirements. More importantly, it would violate the fundamental convention that STL algorithms operate on user-supplied ranges, not on the underlying containers.

[2019-01-26 Priority to 0 and Status to Tentatively Ready after discussions on the reflector]

During that reflector discussion several contributers argued in favour for changing the current wording in [mismatch] p3 from "smallest integer" to "smallest nonnegative integer". This minor wording delta has also been added to the original proposed wording.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4791.

> 1. Change [mismatch] as indicated:

>    >     template<class InputIterator1, class InputIterator2>
>    >       constexpr pair<InputIterator1, InputIterator2>
>    >         mismatch(InputIterator1 first1, InputIterator1 last1,
>    >                  InputIterator2 first2);
>    >     […]
>    >     namespace ranges {
>    >       template<InputIterator I1, Sentinel<I1> S1, InputIterator I2, Sentinel<I2> S2,
>    >                class Proj1 = identity, class Proj2 = identity,
>    >                IndirectRelation<projected<I1, Proj1>,
>    >                projected<I2, Proj2>> Pred = ranges::equal_to<>>
>    >         constexpr mismatch_result<I1, I2>
>    >           mismatch(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
>    >                    Proj1 proj1 = {}, Proj2 proj2 = {});
>    >       template<InputRange R1, InputRange R2,
>    >                class Proj1 = identity, class Proj2 = identity,
>    >                IndirectRelation<projected<iterator_t<R1>, Proj1>,
>    >                projected<iterator_t<R2>, Proj2>> Pred = ranges::equal_to<>>
>    >         constexpr mismatch_result<safe_iterator_t<R1>, safe_iterator_t<R2>>
>    >           mismatch(R1&& r1, R2&& r2, Pred pred = {},
>    >                    Proj1 proj1 = {}, Proj2 proj2 = {});
>    >     }

>    > > -1- Let `last2` be `first2 + (last1 - first1)` for the overloads with no parameter `last2` or `r2`.

>    > > -2- Let `E` be:

>    > > 1. (2.1) — `!(*(first1 + n) == *(first2 + n))` for the overloads with no parameter `pred`,

>    > > 2. (2.2) — `pred(*(first1 + n), *(first2 + n)) == false` for the overloads with a parameter `pred` and no parameter `proj1`,

>    > > 3. (2.3) — `!invoke(pred, invoke(proj1, *(first1 + n)), invoke(proj2, *(first2 + n)))` for the overloads with both parameters `pred` and `proj1`.

>    > > -?- Let `N` be `min(last1 - first1, last2 - first2)`.

>    > > -3- Returns: `{ first1 + n, first2 + n }`, where `n` is the smallest nonnegative integer such that `E` holds, or `min(last1 - first1, last2 - first2)N` if no such integer less than `N` exists.

>    > > -4- Complexity: At most `min(last1 - first1, last2 - first2)N` applications of the corresponding predicate and any projections.

[2019-03-15; Daniel comments]

The editorial issue #2611 had been resolved via this pull request #2613. The editorial changes should make the suggested wording changes obsolete and I recommend to close this issue as Resolved.

[2020-05-02; Reflector discussions]

It seems that the editorial change has fixed the issue already. If the issue author objects, we can reopen it. Therefore:

Resolved by editorial pull request #2613.

Rationale:

Resolved by editorial pull request #2613.

Proposed resolution:

### 3179(i). `subrange` should always model `Range`

Section: 25.5.4.2 [range.subrange.ctor] Status: C++20 Submitter: Casey Carter Opened: 2018-12-21 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

The constructors of `subrange` place no requirements on the iterator and sentinel values from which a `subrange` is constructed. They allow constructions like `subrange{myvec.end(), myvec.begin()}` in which the resulting `subrange` isn't in the domain of the `Range` concept which requires that "`[ranges::begin(r), ranges::end(r))` denotes a range" ([range.range]/3.1). We should forbid the construction of abominable values like this to enforce the useful semantic that values of `subrange` are always in the domain of `Range`.

(From ericniebler/stl2#597.)

[2019-01-20 Reflector prioritization]

Set Priority to 0 and status to Tentatively Ready

Proposed resolution:

This wording is relative to N4791.

1. Change 25.5.4.2 [range.subrange.ctor] as indicated:

   > >     constexpr subrange(I i, S s) requires (!StoreSize);

   > -?- Expects: `[i, s)` is a valid range.

   > -1- Effects: Initializes `begin_` with `i` and `end_` with `s`.

   > >     constexpr subrange(I i, S s, iter_difference_t<I> n)
   > >       requires (K == subrange_kind::sized);

   > -2- Expects: `[i, s)` is a valid range, and `n == ranges::distance(i, s)`.

### 3180(i). Inconsistently named return type for `ranges::minmax_element`

Section: 26.4 [algorithm.syn] Status: C++20 Submitter: Casey Carter Opened: 2018-12-21 Last modified: 2021-02-25

Priority: 0

View all other issues in [algorithm.syn].

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Discussion:

The overloads of `std::ranges::minmax_element` are specified to return `std::ranges::minmax_result`, which is inconsistent with the intended design. When an algorithm `foo` returns an aggregate of multiple results, the return type should be named `foo_result`. The spec should introduce an alias `minmax_element_result` for `minmax_result` and use that alias as the return type of the `std::ranges::minmax_element` overloads.

[2019-01-11 Status to Tentatively Ready after five positive votes on the reflector.]

During that reflector discussion several contributers questioned the choice of alias templates to denote algorithm result types or particular aspects of it. Since this approach had been approved by LEWG before, it was suggested to those opponents to instead write a paper, because changing this as part of this issue would be a design change that would require a more global fixing approach.

Proposed resolution:

This wording is relative to N4791.

1. Change 26.4 [algorithm.syn] as indicated, and adjust the declarations of `std::ranges::minmax_element` in 26.8.9 [alg.min.max] to agree:

   >     […]
   >     template<class ExecutionPolicy, class ForwardIterator, class Compare>
   >       pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
   >                        ForwardIterator first, ForwardIterator last, Compare comp);

   >     namespace ranges {
   >       template<class I>
   >       using minmax_element_result = minmax_result<I>;

   >       template<ForwardIterator I, Sentinel<I> S, class Proj = identity,
   >                IndirectStrictWeakOrder<projected<I, Proj>> Comp = ranges::less<>>
   >         constexpr minmax_element_result<I>
   >           minmax_element(I first, S last, Comp comp = {}, Proj proj = {});
   >       template<ForwardRange R, class Proj = identity,
   >                IndirectStrictWeakOrder<projected<iterator_t<R>, Proj>> Comp = ranges::less<>>
   >         constexpr minmax_element_result<safe_iterator_t<R>>
   >           minmax_element(R&& r, Comp comp = {}, Proj proj = {});
   >     }

   >     // 26.8.10 [alg.clamp], bounded value
   >     […]

### 3182(i). Specification of `Same` could be clearer

Section: 18.4.2 [concept.same] Status: C++20 Submitter: Casey Carter Opened: 2019-01-05 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

The specification of the `Same` concept in 18.4.2 [concept.same]:

>     template<class T, class U>
>       concept Same = is_same_v<T, U>;

> -1- `Same<T, U>` subsumes `Same<U, T>` and vice versa.

seems contradictory. From the concept definition alone, it is not the case that `Same<T, U>` subsumes `Same<U, T>` nor vice versa. Paragraph 1 is trying to tell us that there's some magic that provides the stated subsumption relationship, but to a casual reader it appears to be a mis-annotated note. We should either add a note to explain what's actually happening here, or define the concept in such a way that it naturally provides the specified subsumption relationship.

Given that there's a straightforward library implementation of the symmetric subsumption idiom, the latter option seems preferable.

[2019-01-20 Reflector prioritization]

Set Priority to and status to Tentatively Ready

Proposed resolution:

This wording is relative to N4791.

1. Change 18.4.2 [concept.same] as follows:

   >     template<class T, class U>
   >       concept same-impl = // exposition only
   >         is_same_v<T, U>;

   >     template<class T, class U>
   >       concept Same = is_same_v<T, U>same-impl<T, U> && same-impl<U, T>;

   > -1- [Note: `Same<T, U>` subsumes `Same<U, T>` and vice versa.—end note]

### 3183(i). Normative permission to specialize Ranges variable templates

Section: 24.3.4.8 [iterator.concept.sizedsentinel] Status: C++20 Submitter: Casey Carter Opened: 2019-01-14 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

P0896R4 "The One Ranges Proposal" added boolean variable templates `std::disable_sized_sentinel` and `std::ranges::disable_sized_range` which users are intended to specialize to `false` for program-defined `Iterator`-`Sentinel` pairs / `Range` types which meet the syntax but do not model the semantics of the `SizedSentinel` / `SizedRange` concepts, respectively. Specializing these traits allows the use of such types with the library which would otherwise treat them as if they model `SizedSentinel` / `SizedRange`. The wording in P0896R4 failed, however, to provide normative permission to specialize these variable templates as is required by 16.4.5.2.1 [namespace.std] after the application of P0551R3.

Furthermore, 16.4.5.2.1 [namespace.std] notably does not require that program-defined specializations of standard library variable templates meet the requirements on the primary template (as is the case for class templates) or indeed any requirements. P0896R4 also added the `enable_view` variable template which is used to explicitly opt in or out of the `View` concept 25.4.5 [range.view] when the default chosen by the heuristic is incorrect. P0896R4 did include normative permission to specialize `enable_view`, but the wording does not place sufficient requirements on such user specializations so as to make them usable by the `View` concept definition. Specializations must be required to be usable as constant expressions of type `bool` to avoid hard errors in the concept.

[2019-02-03 Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4791.

> [Drafting Note: This wording uses the recently-defined core language term "usable in constant expressions" from 7.7 [expr.const] paragraph 3 which may be unfamiliar to reviewers.]

1. Change 24.3.4.8 [iterator.concept.sizedsentinel] as follows:

   > […]

   > (2.2) — If `−N` is representable by `iter_difference_t<I>`, then `i - s` is well-defined and equals `−N`.

   > -?- Pursuant to 16.4.5.2.1 [namespace.std], users may specialize `disable_sized_sentinel` for cv-unqualified non-array object types `S` and `I` at least one of which is a program-defined type. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have type `const bool`.

   > 3 [Note: `disable_sized_sentinel` allows use of sentinels and iterators with the library that satisfy but do not in fact model `SizedSentinel`.—end note]

   > […]

2. Add an index entry for `disable_sized_sentinel` that points to [iterator.concepts.sizedsentinel].

3. Change 25.4.4 [range.sized] as follows:

   > […]

   > 3 [Note: The complexity requirement for the evaluation of `ranges::size` is non-amortized, unlike the case for the complexity of the evaluations of `ranges::begin` and `ranges::end` in the `Range` concept.—end note]

   > -?- Pursuant to 16.4.5.2.1 [namespace.std], users may specialize `disable_sized_range` for cv-unqualified program-defined types. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have type `const bool`.

   > 4 [Note: `disable_sized_range` allows use of range types with the library that satisfy but do not in fact model `SizedRange`.—end note]

4. Add an index entry for `disable_sized_range` that points to 25.4.4 [range.sized].

5. Change 25.4.5 [range.view] as follows:

   > […]

   > 5 Pursuant to 16.4.5.2.1 [namespace.std], users may specialize `enable_view` to `true` for cv-unqualified program-defined types which model `View`, and `false` for types which do not. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have type `const bool`.

### 3184(i). Inconsistencies in `bind_front` wording

Section: 22.10.14 [func.bind.partial] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-01-16 Last modified: 2023-02-07

Priority: 0

View all issues with C++20 status.

Discussion:

During the merge of the P0356R5, following "oddities" of the new wording was pointed out by Jens Maurer:

1. The initialization of the state entities of the `bind_front/not_fn` is specified using formulation "`xx` initialized with the initializer `(yyy)`". Per author knowledge this specification is correct, however inconsistent with the other parts of the of the standard, that direct-non-list-initialization term in such context.

2. The specification of the Mandates element for `bind_front` uses `conjunction_v` to specify conjunction of the requirements, while corresponding element of the `not_fn` specifies it using `&&`. As `conjuction_v` implies order of evaluation that is not necessary in this case (for every valid program, all provided traits must evaluate to true), it may be replaced with usage of fold expression with operator `&&`.

[2019-01-26 Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4791.

1. Change [func.not_fn] as indicated:

   >     template<class F> unspecified not_fn(F&& f);

   > > -1- In the text that follows:

   > > 1. (1.1) — […]

   > > 2. (1.2) — […]

   > > 3. (1.3) — `fd` is the target object of `g` (22.10.3 [func.def]) of type `FD` direct-non-list-initialized withinitialized with the initializer `(std::forward<F>(f))` (9.5 [dcl.init]),

   > > 4. (1.4) — […]

2. Change [func.bind_front] as indicated:

   >     template <class F, class... Args>
   >     unspecified bind_front(F&& f, Args&&... args);

   > > -1- In the text that follows:

   > > 1. (1.1) — […]

   > > 2. (1.2) — […]

   > > 3. (1.3) — `fd` is the target object of `g` (22.10.3 [func.def]) of type `FD` direct-non-list-initialized withinitialized with the initializer `(std::forward<F>(f))` (9.5 [dcl.init]),

   > > 4. (1.4) — […]

   > > 5. (1.5) — `bound_args` is a pack of bound argument entities of `g` (22.10.3 [func.def]) of types `BoundArgs...`, direct-non-list-initialized withinitialized with initializers `(std::forward<Args>(args))...`, respectively, and

   > > 6. (1.6) — […]

   > > -2- Mandates:

   > > >     is_constructible_v<FD, F> && is_move_constructible_v<FD> &&
   > > >     (is_constructible_v<BoundArgs, Args> && ...) && (is_move_constructible_v<BoundArgs> && ...)conjunction_v<is_constructible<FD, F>, is_move_constructible<FD>,
   > > >     is_constructible<BoundArgs, Args>..., is_move_constructible<BoundArgs>...>

   > > shall be true.

### 3185(i). Uses-allocator construction functions missing `constexpr` and `noexcept`

Section: 20.2.8.2 [allocator.uses.construction] Status: C++20 Submitter: Pablo Halpern Opened: 2019-01-29 Last modified: 2021-02-25

Priority: 0

View all other issues in [allocator.uses.construction].

View all issues with C++20 status.

Discussion:

The uses-allocator construction functions introduced into WP when P0591r4 was accepted (Nov 2018, San Diego) should all be `constexpr`. All but two should also be `noexcept`. Getting this right is an important part of correctly adding `constexpr` memory allocation into the WP.

The minimal change is to add the `constexpr` to all of the new functions except `uninitialized_construct_using_allocator` and `noexcept` to all of the overloads of `uses_allocator_construction_args`. Optionally, we could consider adding conditional `noexcept` to the remaining two functions. If p0784 is accepted, then also add `constexpr` to `uninitialized_construct_using_allocator`.

[2019-02-12 Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4800.

1. Change header `<memory>` synopsis, 20.2.2 [memory.syn], as indicated:

   >     […]
   >     // 20.2.8.2 [allocator.uses.construction], uses-allocator construction
   >     template <class T, class Alloc, class... Args>
   >     constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept -> see below;
   >     template <class T, class Alloc, class Tuple1, class Tuple2>
   >     constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
   >                                           Tuple1&& x, Tuple2&& y) noexcept -> see below;
   >     template <class T, class Alloc>
   >     constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below;
   >     template <class T, class Alloc, class U, class V>
   >     constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u, V&& v) noexcept -> see below;
   >     template <class T, class Alloc, class U, class V>
   >     constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U,V>& pr) noexcept -> see below;
   >     template <class T, class Alloc, class U, class V>
   >     constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U,V>&& pr) noexcept -> see below;
   >     template <class T, class Alloc, class... Args>
   >     constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args);
   >     template <class T, class Alloc, class... Args>
   >     T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);
   >     […]

2. Change 20.2.8.2 [allocator.uses.construction] as indicated:

   >     template <class T, class Alloc, class... Args>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept -> see below;

   > > […]

   >     template <class T, class Alloc, class Tuple1, class Tuple2>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
   >                                             Tuple1&& x, Tuple2&& y) noexcept -> see below;

   > > […]

   >     template <class T, class Alloc>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below;

   > > […]

   >     template <class T, class Alloc, class U, class V>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u, V&& v) noexcept -> see below;

   > > […]

   >     template <class T, class Alloc, class U, class V>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U,V>& pr) noexcept -> see below;

   > > […]

   >     template <class T, class Alloc, class U, class V>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U,V>&& pr) noexcept -> see below;

   > > […]

   >     template <class T, class Alloc, class... Args>
   >       constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args);

   > > […]

   >     template <class T, class Alloc, class... Args>
   >       T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);

   > > […]

### 3186(i). `ranges` removal, partition, and `partial_sort_copy` algorithms discard useful information

Section: 26.7.8 [alg.remove], 26.7.9 [alg.unique], 26.8.2.4 [partial.sort.copy], 26.8.5 [alg.partitions] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-02-05 Last modified: 2021-02-25

Priority: 1

View all other issues in [alg.remove].

View all issues with C++20 status.

Discussion:

This is direct follow-up on the LWG issue 3169(i), that proposed to change additional algorithms that drop the iterator value equal to sentinel, that needs to be always computed. These set include removal (`remove`, `remove_if`, and `unique`), partition (`partition`, `stable_partition`), and `partial_sort_copy`.

For removal algorithms, the end of "not-erased" objects, and the "end-of-range" iterator forms a valid range of objects with unspecified value (that can be overwritten), thus we propose to return `subrange`.

For partition algorithms, the end of "true" object, and the "end-of-range" iterator forms a valid range of objects for which predicate returns "false", thus we propose to return `subrange`.

For `partial_sort_copy` we propose to return `partial_sort_copy_result` as an alias to `copy_result` to match other copy algorithms.

[2019-02-12; Tomasz comments and improves proposed wording]

Proposed wording is updated to incorporate wording comments from Casey Carter:

- We don't need to repeat the definition of `partial_sort_copy_result` in 26.8.2.4 [partial.sort.copy]; the single definition in the synopsis is sufficient.

- `e` is a potentially confusing choice of placeholder name for the end of the output range, given that we use a placeholder `E` for predicates in the algorithm specifications.

The placeholder `e` is replaced with `j` that seems not to be used in the specification of above algorithms.

[2019-02 Priority set to 1 after reflector discussion]

[2019-02; Kona Wednesday night issue processing]

Status to Ready

Proposed resolution:

This wording is relative to N4800.

1. Change header `<algorithm>` synopsis, 26.4 [algorithm.syn], as indicated:

   >     […]
   >     //26.7.8 [alg.remove], remove
   >     […]
   >     namespace ranges {
   >     template<Permutable I, Sentinel<I> S, class T, class Proj = identity>
   >       requires IndirectRelation<ranges::equal_to<>, projected<I, Proj>, const T*>
   >       constexpr subrange<I> remove(I first, S last, const T& value, Proj proj = {});
   >     template<ForwardRange R, class T, class Proj = identity>
   >       requires Permutable<iterator_t<R>> &&
   >                IndirectRelation<ranges::equal_to<>, projected<iterator_t<R>, Proj>, const T*>
   >       constexpr safe_subrangeiterator_t<R>
   >         remove(R&& r, const T& value, Proj proj = {});
   >     template<Permutable I, Sentinel<I> S, class Proj = identity,
   >              IndirectUnaryPredicate<projected<I, Proj>> Pred>
   >       constexpr subrange<I> remove_if(I first, S last, Pred pred, Proj proj = {});
   >     template<ForwardRange R, class Proj = identity,
   >              IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires Permutable<iterator_t<R>>
   >       constexpr safe_subrangeiterator_t<R>
   >         remove_if(R&& r, Pred pred, Proj proj = {});
   >     }
   >     […]
   >     // 26.7.9 [alg.unique], unique
   >     […]
   >     namespace ranges {
   >       template<Permutable I, Sentinel<I> S, class Proj = identity,
   >                IndirectRelation<projected<I, Proj>> C = ranges::equal_to<>>
   >         constexpr subrange<I> unique(I first, S last, C comp = {}, Proj proj = {});
   >       template<ForwardRange R, class Proj = identity,
   >                IndirectRelation<projected<iterator_t<R>, Proj>> C = ranges::equal_to<>>
   >         requires Permutable<iterator_t<R>>
   >         constexpr safe_subrangeiterator_t<R>
   >           unique(R&& r, C comp = {}, Proj proj = {});
   >     }
   >     […]
   >     // 26.8.2 [alg.sort], sorting
   >     […]
   >     namespace ranges {
   >       template<class I, class O> using partial_sort_copy_result = copy_result<I, O>;

   >       template<InputIterator I1, Sentinel<I1> S1, RandomAccessIterator I2, Sentinel<I2> S2,
   >                class Comp = ranges::less<>, class Proj1 = identity, class Proj2 = identity>
   >         requires IndirectlyCopyable<I1, I2> && Sortable<I2, Comp, Proj2> &&
   >                  IndirectStrictWeakOrder<Comp, projected<I1, Proj1>, projected<I2, Proj2>>
   >         constexpr partial_sort_copy_result<I1, I2>
   >           partial_sort_copy(I1 first, S1 last, I2 result_first, S2 result_last,
   >                             Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >       template<InputRange R1, RandomAccessRange R2, class Comp = ranges::less<>,
   >                class Proj1 = identity, class Proj2 = identity>
   >         requires IndirectlyCopyable<iterator_t<R1>, iterator_t<R2>> &&
   >                  Sortable<iterator_t<R2>, Comp, Proj2> &&
   >                  IndirectStrictWeakOrder<Comp, projected<iterator_t<R1>, Proj1>,
   >                                          projected<iterator_t<R2>, Proj2>>
   >         constexpr partial_sort_copy_result<safe_iterator_t<R1>, safe_iterator_t<R2>>
   >           partial_sort_copy(R1&& r, R2&& result_r, Comp comp = {},
   >                             Proj1 proj1 = {}, Proj2 proj2 = {});
   >     }
   >     […]
   >     // 26.8.5 [alg.partitions], partitions
   >     […]
   >     namespace ranges {
   >       template<Permutable I, Sentinel<I> S, class Proj = identity,
   >                IndirectUnaryPredicate<projected<I, Proj>> Pred>
   >         constexpr subrange<I>
   >           partition(I first, S last, Pred pred, Proj proj = {});
   >       template<ForwardRange R, class Proj = identity,
   >                IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred>
   >         requires Permutable<iterator_t<R>>
   >         constexpr safe_subrangeiterator_t<R>
   >           partition(R&& r, Pred pred, Proj proj = {});
   >     }
   >     […]
   >     namespace ranges {
   >       template<BidirectionalIterator I, Sentinel<I> S, class Proj = identity,
   >                IndirectUnaryPredicate<projected<I, Proj>> Pred>
   >         requires Permutable<I>
   >           subrange<I> stable_partition(I first, S last, Pred pred, Proj proj = {});
   >       template<BidirectionalRange R, class Proj = identity,
   >                IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred>
   >         requires Permutable<iterator_t<R>>
   >           safe_subrangeiterator_t<R> stable_partition(R&& r, Pred pred, Proj proj = {});
   >     }
   >     […]

2. Change 26.7.8 [alg.remove] as indicated:

   >     […]
   >     namespace ranges {
   >     template<Permutable I, Sentinel<I> S, class T, class Proj = identity>
   >       requires IndirectRelation<ranges::equal_to<>, projected<I, Proj>, const T*>
   >       constexpr subrange<I> remove(I first, S last, const T& value, Proj proj = {});
   >     template<ForwardRange R, class T, class Proj = identity>
   >       requires Permutable<iterator_t<R>> &&
   >                IndirectRelation<ranges::equal_to<>, projected<iterator_t<R>, Proj>, const T*>
   >       constexpr safe_subrangeiterator_t<R>
   >         remove(R&& r, const T& value, Proj proj = {});
   >     template<Permutable I, Sentinel<I> S, class Proj = identity,
   >              IndirectUnaryPredicate<projected<I, Proj>> Pred>
   >       constexpr subrange<I> remove_if(I first, S last, Pred pred, Proj proj = {});
   >     template<ForwardRange R, class Proj = identity,
   >              IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires Permutable<iterator_t<R>>
   >       constexpr safe_subrangeiterator_t<R>
   >         remove_if(R&& r, Pred pred, Proj proj = {});
   >     }

   > > […]

   > > -4- Returns: Let `j` be tThe end of the resulting range. Returns:

   > > 1. (4.?) — `j` for the overloads in namespace `std`, or

   > > 2. (4.?) — `{j, last}` for the overloads in namespace `ranges`.

   > > […]

3. Change 26.7.9 [alg.unique] as indicated:

   >     […]
   >     namespace ranges {
   >       template<Permutable I, Sentinel<I> S, class Proj = identity,
   >                IndirectRelation<projected<I, Proj>> C = ranges::equal_to<>>
   >         constexpr subrange<I> unique(I first, S last, C comp = {}, Proj proj = {});
   >       template<ForwardRange R, class Proj = identity,
   >                IndirectRelation<projected<iterator_t<R>, Proj>> C = ranges::equal_to<>>
   >         requires Permutable<iterator_t<R>>
   >         constexpr safe_subrangeiterator_t<R>
   >           unique(R&& r, C comp = {}, Proj proj = {});
   >     }

   > > […]

   > > -4- Returns: Let `j` be tThe end of the resulting range. Returns:

   > > 1. (4.?) — `j` for the overloads in namespace `std`, or

   > > 2. (4.?) — `{j, last}` for the overloads in namespace `ranges`.

   > > […]

4. Change 26.8.2.4 [partial.sort.copy] as indicated:

   >     […]
   >     namespace ranges {
   >       template<InputIterator I1, Sentinel<I1> S1, RandomAccessIterator I2, Sentinel<I2> S2,
   >                class Comp = ranges::less<>, class Proj1 = identity, class Proj2 = identity>
   >         requires IndirectlyCopyable<I1, I2> && Sortable<I2, Comp, Proj2> &&
   >                  IndirectStrictWeakOrder<Comp, projected<I1, Proj1>, projected<I2, Proj2>>
   >         constexpr partial_sort_copy_result<I1, I2>
   >           partial_sort_copy(I1 first, S1 last, I2 result_first, S2 result_last,
   >                             Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >       template<InputRange R1, RandomAccessRange R2, class Comp = ranges::less<>,
   >                class Proj1 = identity, class Proj2 = identity>
   >         requires IndirectlyCopyable<iterator_t<R1>, iterator_t<R2>> &&
   >                  Sortable<iterator_t<R2>, Comp, Proj2> &&
   >                  IndirectStrictWeakOrder<Comp, projected<iterator_t<R1>, Proj1>,
   >                                          projected<iterator_t<R2>, Proj2>>
   >         constexpr partial_sort_copy_result<safe_iterator_t<R1>, safe_iterator_t<R2>>
   >           partial_sort_copy(R1&& r, R2&& result_r, Comp comp = {},
   >                             Proj1 proj1 = {}, Proj2 proj2 = {});
   >     }

   > > […]

   > > -4- Returns:

   > > 1. (4.?) — `result_first + N` for the overloads in namespace `std`, or

   > > 2. (4.?) — `{last, result_first + N}` for the overloads in namespace `ranges`.

   > > […]

5. Change 26.8.5 [alg.partitions] as indicated:

   >     […]
   >     namespace ranges {
   >       template<Permutable I, Sentinel<I> S, class Proj = identity,
   >                IndirectUnaryPredicate<projected<I, Proj>> Pred>
   >         constexpr subrange<I>
   >           partition(I first, S last, Pred pred, Proj proj = {});
   >       template<ForwardRange R, class Proj = identity,
   >                IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred>
   >         requires Permutable<iterator_t<R>>
   >         constexpr safe_subrangeiterator_t<R>
   >           partition(R&& r, Pred pred, Proj proj = {});
   >     }

   > > […]

   > > -7- Returns: Let `i` be aAn iterator `i` such that `E(*j)` is `true` for every iterator `j` in `[first, i)` and `false` for every iterator `j` in `[i, last)`. Returns:

   > > 1. (7.?) — `i` for the overloads in namespace `std`, or

   > > 2. (7.?) — `{i, last}` for the overloads in namespace `ranges`.

   > > […]

   >     […]
   >     namespace ranges {
   >       template<BidirectionalIterator I, Sentinel<I> S, class Proj = identity,
   >                IndirectUnaryPredicate<projected<I, Proj>> Pred>
   >         requires Permutable<I>
   >           subrange<I> stable_partition(I first, S last, Pred pred, Proj proj = {});
   >       template<BidirectionalRange R, class Proj = identity,
   >                IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred>
   >         requires Permutable<iterator_t<R>>
   >           safe_subrangeiterator_t<R> stable_partition(R&& r, Pred pred, Proj proj = {});
   >     }

   > > […]

   > > -11- Returns: Let `i` be aAn iterator `i` such that for every iterator `j` in `[first, i)`, `E(*j)` is `true`, and for every iterator `j` in the range `[i, last)`, `E(*j)` is `false`,. Returns:

   > > 1. (11.?) — `i` for the overloads in namespace `std`, or

   > > 2. (11.?) — `{i, last}` for the overloads in namespace `ranges`.

   > > […]

### 3187(i). P0591R4 reverted DR 2586 fixes to `scoped_allocator_adaptor::construct()`

Section: 20.2.8.2 [allocator.uses.construction] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-02-14 Last modified: 2021-02-25

Priority: Not Prioritized

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Discussion:

2586(i) fixed the value category in the uses-allocator checks done by `scoped_allocator_adaptor`. When we made that use `uses_allocator_construction_args` we reintroduced the problem, because that function has the same bug.

>     #include <memory>

>     struct X {
>       using allocator_type = std::allocator<X>;
>       X(std::allocator_arg_t, allocator_type&&) { }
>       X(const allocator_type&) { }
>     };

>     int main() {
>       std::allocator<X> a;
>       std::make_obj_using_allocator<X>(a);
>     }

This will fail to compile, because `uses_allocator_construction_args` will check `is_constructible` using an rvalue allocator, but then return `tuple<allocator_arg_t, const allocator<X>&>` containing an lvalue allocator. Those args cannot be used to construct an `X`.

[2019-02; Kona Wednesday night issue processing]

Status to Ready

Proposed resolution:

This wording is relative to N4800.

1. Change 20.2.8.2 [allocator.uses.construction] as indicated:

   > [Drafting Note: Arguably the `uses_allocator` specialization should also use `const Alloc&` but in practice that doesn't matter, except for even more contrived cases than the very contrived example above.]

   >     template <class T, class Alloc, class... Args>
   >       auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) -> see below;

   > > […]

   > > -5- Returns: A `tuple` value determined as follows:

   > > 1. (5.1) — If `uses_allocator_v<T, Alloc>` is `false` and `is_constructible_v<T, Args...>` is `true`, return `forward_as_tuple(std::forward<Args>(args)...)`.

   > > 2. (5.2) — Otherwise, if `uses_allocator_v<T, Alloc>` is `true` and `is_constructible_v<T, allocator_arg_t, const Alloc&, Args...>` is `true`, return

   > >    >     tuple<allocator_arg_t, const Alloc&, Args&&...>(
   > >    >       allocator_arg, alloc, std::forward<Args>(args)...)

   > > 3. (5.3) — Otherwise, if `uses_allocator_v<T, Alloc>` is `true` and `is_constructible_v<T, Args..., const Alloc&>` is `true`, return `forward_as_tuple(std::forward<Args>(args)..., alloc)`.

   > > 4. (5.4) — Otherwise, the program is ill-formed.

   > > […]

### 3190(i). `std::allocator::allocate` sometimes returns too little storage

Section: 20.2.10.2 [allocator.members] Status: C++20 Submitter: Casey Carter Opened: 2019-02-20 Last modified: 2021-02-25

Priority: 3

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Discussion:

20.2.10.2 [allocator.members]/2 says:

> -2- Returns: A pointer to the initial element of an array of storage of size `n * sizeof(T)`, aligned appropriately for objects of type `T`.

As in LWG 3038(i), we should not return too little storage for n objects of size `sizeof(T)`, e.g. when `n` is `SIZE_MAX / 2` and `T` is `short`.

[2019-03-05 Priority set to 3 after reflector discussion]

[2019 Cologne Wednesday night]

Status to Ready; will open additional issue to reconcile this and 3038(i)

Proposed resolution:

This wording is relative to N4800.

1. Change 20.2.10.2 [allocator.members] as indicated:

   >     [[nodiscard]] T* allocate(size_t n);

   > > […]

   > > -4- Throws: `bad_array_new_length` if `SIZE_MAX / sizeof(T) < n`, or `bad_alloc` if the storage cannot be obtained.

### 3191(i). `std::ranges::shuffle` synopsis does not match algorithm definition

Section: 26.7.13 [alg.random.shuffle] Status: C++20 Submitter: Christopher Di Bella Opened: 2019-03-02 Last modified: 2021-02-25

Priority: 1

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Discussion:

26.4 [algorithm.syn] declares `std::ranges::shuffle` like so:

>     namespace ranges {
>       template<RandomAccessIterator I, Sentinel<I> S, class Gen>
>         requires Permutable<I> &&
>                  UniformRandomBitGenerator<remove_reference_t<Gen>> &&
>                  ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<I>>
>       I shuffle(I first, S last, Gen&& g);

>       template<RandomAccessRange R, class Gen>
>         requires Permutable<iterator_t<R> &&
>                  UniformRandomBitGenerator<remove_reference_t<Gen>> &&
>                  ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<iterator_t<R>>>
>       safe_iterator_t<R> shuffle(R&& r, Gen&& g);
>     }

26.7.13 [alg.random.shuffle] defines the algorithm like so:

>     namespace ranges {
>       template<RandomAccessIterator I, Sentinel<I> S, class Gen>
>         requires Permutable<I> &&
>                  UniformRandomBitGenerator<remove_reference_t<Gen>>
>       I shuffle(I first, S last, Gen&& g);

>       template<RandomAccessRange R, class Gen>
>         requires Permutable<iterator_t<R>> &&
>                  UniformRandomBitGenerator<remove_reference_t<Gen>>
>       safe_iterator_t<R> shuffle(R&& r, Gen&& g);
>     }

Notice the missing `ConvertibleTo` requirements in the latter. Looking at the Ranges TS, [alg.random.shuffle] includes this requirement, albeit in the Ranges TS-format.

[2019-03-03; Daniel comments]

Given that the accepted proposal P0896R4 voted in San Diego did contain the same error I decided to open this issue instead of submitting an editorial change request.

[2019-03-05 Priority set to 1 after reflector discussion]

Casey: The correct fix here is to remove the `ConvertibleTo` requirement from the header synopsis. `UniformRandomBitGenerator`s have integral result types, and the core language guarantees that all integral types are convertible to all other integral types. We don't need to validate the core language in the associated constraints of `ranges::shuffle`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4800.

> 1. Change 26.7.13 [alg.random.shuffle] as indicated:

>    >     […]
>    >     namespace ranges {
>    >       template<RandomAccessIterator I, Sentinel<I> S, class Gen>
>    >         requires Permutable<I> &&
>    >                  UniformRandomBitGenerator<remove_reference_t<Gen>> &&
>    >                  ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<I>>
>    >         I shuffle(I first, S last, Gen&& g);
>    >       template<RandomAccessRange R, class Gen>
>    >         requires Permutable<iterator_t<R>> &&
>    >                  UniformRandomBitGenerator<remove_reference_t<Gen>> &&
>    >                  ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<iterator_t<R>>>
>    >         safe_iterator_t<R> shuffle(R&& r, Gen&& g);
>    >     }

[2019-03-05 Updated proposed wording according to Casey's suggestion]

[2019-06-16 Set to "Tentatively Ready" after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4800.

1. Change 26.4 [algorithm.syn] as indicated:

   >     // 26.7.13 [alg.random.shuffle], shuffle
   >     […]
   >     namespace ranges {
   >       template<RandomAccessIterator I, Sentinel<I> S, class Gen>
   >         requires Permutable<I> &&
   >                  UniformRandomBitGenerator<remove_reference_t<Gen>> &&
   >                  ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<I>>
   >       I shuffle(I first, S last, Gen&& g);

   >       template<RandomAccessRange R, class Gen>
   >         requires Permutable<iterator_t<R> &&
   >                  UniformRandomBitGenerator<remove_reference_t<Gen>> &&
   >                  ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<iterator_t<R>>>
   >       safe_iterator_t<R> shuffle(R&& r, Gen&& g);
   >     }
   >     […]

### 3192(i). §[allocator.uses.construction] functions misbehave for `const` types

Section: 20.2.8.2 [allocator.uses.construction] Status: Resolved Submitter: Jonathan Wakely Opened: 2019-02-28 Last modified: 2026-06-09

Priority: 3

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Discussion:

The new functions added by P0591R4 misbehave for cv-qualified types. A specialization `std::uses_allocator<X, Alloc>` will not match `const X`, so `std::uses_allocator_construction_args<const X>` will return a different result from `std::uses_allocator_construction_args<X>`. It makes no sense to construct `X` and `const X` differently, either the type wants to use an allocator or it doesn't. I think `std::uses_allocator_construction_args<T>` should remove cv-qualifiers before checking `uses_allocator`, so that it works consistently.

We could consider changing `std::make_obj_using_allocator` to also strip cv-qualifiers, but it's not necessary as C++17 guaranteed elision works even for prvalues of `const` types. We only need to make the construction args ignore cv-qualifiers. We don't want to make cv-qualified types ill-formed, because that would require users of uses-allocator construction to strip cv-qualifiers before using these functions, e.g. in cases like `std::tuple<const int> t(allocator_arg, alloc, 1);`

[2019-03-15 Priority set to 3 after reflector discussion]

Previous resolution [SUPERSEDED]

This wording is relative to N4800.

1. Change 20.2.8.2 [allocator.uses.construction] as indicated:

   >     template <class T, class Alloc, class... Args>
   >       auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) -> see below;

   > > -4- Constraints: `T` is not a specialization of `pair`.

   > > -5- Returns: A `tuple` value determined as follows, where `U` denotes the type `remove_cv_t<T>`:

   > > 1. (5.1) — If `uses_allocator_v<TU, Alloc>` is `false` and `is_constructible_v<T, Args...>` is `true`, return `forward_as_tuple(std::forward<Args>(args)...)`.

   > > 2. (5.2) — Otherwise, if `uses_allocator_v<TU, Alloc>` is `true` and `is_constructible_v<T, allocator_arg_t, Alloc, Args...>` is `true`, return

   > >    >     tuple<allocator_arg_t, const Alloc&, Args&&...>(
   > >    >       allocator_arg, alloc, std::forward<Args>(args)...)

   > > 3. (5.3) — Otherwise, if `uses_allocator_v<TU, Alloc>` is `true` and `is_constructible_v<T, Args..., Alloc>` is `true`, return `forward_as_tuple(std::forward<Args>(args)..., alloc)`.

   > > 4. (5.4) — Otherwise, the program is ill-formed.

   > […]

   >     template <class T, class Alloc, class Tuple1, class Tuple2>
   >       auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
   >                                             Tuple1&& x, Tuple2&& y) -> see below;

   > > -6- Constraints: `T` is a specialization of `pair`.

   > > -7- Effects: For `T` specified as (possibly `const`) `pair<T1, T2>`, equivalent to:

   > > […]

   >     template <class T, class Alloc, class... Args>
   >       T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);

   > > -17- Effects: Equivalent to:

   > > >     return ::new(static_cast<void*>voidify(*p))
   > > >       T(make_obj_using_allocator<T>(alloc, std::forward<Args>(args)...));

[2020-05-01; Daniel syncs wording with recent working draft]

The previously needed change for `uninitialized_construct_using_allocator` is no longer required, because the reworded call to `construct_at` does do the right thing now.

Previous resolution [SUPERSEDED]

This wording is relative to N4861.

1. Change 20.2.8.2 [allocator.uses.construction] as indicated:

   >     template <class T, class Alloc, class... Args>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                             Args&&... args) noexcept -> see below;

   > > -4- Constraints: `T` is not a specialization of `pair`.

   > > -5- Returns: A `tuple` value determined as follows, where `U` denotes the type `remove_cv_t<T>`:

   > > 1. (5.1) — If `uses_allocator_v<TU, Alloc>` is `false` and `is_constructible_v<T, Args...>` is `true`, return `forward_as_tuple(std::forward<Args>(args)...)`.

   > > 2. (5.2) — Otherwise, if `uses_allocator_v<TU, Alloc>` is `true` and `is_constructible_v<T, allocator_arg_t, const Alloc&, Args...>` is `true`, return

   > >    >     tuple<allocator_arg_t, const Alloc&, Args&&...>(
   > >    >       allocator_arg, alloc, std::forward<Args>(args)...)

   > > 3. (5.3) — Otherwise, if `uses_allocator_v<TU, Alloc>` is `true` and `is_constructible_v<T, Args..., const Alloc&>` is `true`, return `forward_as_tuple(std::forward<Args>(args)..., alloc)`.

   > > 4. (5.4) — Otherwise, the program is ill-formed.

   > […]

   >     template <class T, class Alloc, class Tuple1, class Tuple2>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
   >                                             Tuple1&& x, Tuple2&& y)
   >                                             noexcept -> see below;

   > > -6- Constraints: `T` is a specialization of `pair`.

   > > -7- Effects: For `T` specified as (possibly `const`) `pair<T1, T2>`, equivalent to:

   > > […]

[2026-06-09 Resolved by the adoption of issue 3677(i) at Kona, November 2022. Status changed: New → Resolved.]

Proposed resolution:

### 3194(i). `ConvertibleTo` prose does not match code

Section: 18.4.4 [concept.convertible] Status: C++20 Submitter: Hubert Tong Opened: 2019-03-05 Last modified: 2021-02-25

Priority: 1

View other active issues in [concept.convertible].

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Discussion:

The prose in N4800 subclause [concept.convertibleto] indicates that the requirement is for an expression of a particular type and value category to be both implicitly and explicitly convertible to some other type. However, for a type

>     struct A { A(const A&) = delete; };

`ConvertibleTo<const A, A>` would be `false` despite the following being okay:

>     const A f();

>     A test() {
>       static_cast<A>(f());
>       return f();
>     }

[2019-03-15 Priority set to 1 after reflector discussion]

[2019-07-14 Tim adds PR based on discussion in 2019-07-09 LWG telecon]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4820, and also resolves LWG 3151(i).

> 1. Modify [concept.convertibleto] as indicated:

>    > -1- The `ConvertibleTo` concept requires ana glvalue expression of a particular type and value category to be both implicitly and explicitly convertible to some other type. The implicit and explicit conversions are required to produce equal results.

>    >     template<class From, class To>
>    >       concept ConvertibleTo =
>    >         is_convertible_v<From, To> &&
>    >         requires(add_rvalue_reference_t<From> (&f)()) {
>    >           static_cast<To>(f());
>    >         };

>    > > -2- Let `test` be the invented function:

>    > > >     To test(add_rvalue_reference_t<From> (&f)()) {
>    > > >       return f();
>    > > >     }

>    > > for some types `From` and `To`, and let `f` be a function with no arguments and return type `add_rvalue_reference_t<From>` such that `f()` is equality-preserving. `From` and `To` model `ConvertibleTo<From, To>` only if:

>    > > 1. (2.1) — `To` is not an object or reference-to-object type, or `static_cast<To>(f())` is equal to `test(f)`.

>    > > 2. (2.2) — `add_rvalue_reference_t<From>` is not a reference-to-object type, or

>    > >    1. (2.2.1) — If `add_rvalue_reference_t<From>` is an rvalue reference to a non const-qualified type, the resulting state of the object referenced by `f()` after either above expression is valid but unspecified (16.4.6.17 [lib.types.movedfrom]).

>    > >    2. (2.2.2) — Otherwise, the object referred to by `f()` is not modified by either above expression.

[2019-09-23; Daniel adjusts wording to working draft changes]

Due to the concept renaming caused by P1754R1 the proposed wording is outdated and needs adjustments.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830, and also resolves LWG 3151(i).

> 1. Modify 18.4.4 [concept.convertible] as indicated:

>    > -1- The `convertible_to` concept requires ana glvalue expression of a particular type and value category to be both implicitly and explicitly convertible to some other type. The implicit and explicit conversions are required to produce equal results.

>    >     template<class From, class To>
>    >       concept convertible_to =
>    >         is_convertible_v<From, To> &&
>    >         requires(add_rvalue_reference_t<From> (&f)()) {
>    >           static_cast<To>(f());
>    >         };

>    > > -2- Let `test` be the invented function:

>    > > >     To test(add_rvalue_reference_t<From> (&f)()) {
>    > > >       return f();
>    > > >     }

>    > > for some types `From` and `To`, and let `f` be a function with no arguments and return type `add_rvalue_reference_t<From>` such that `f()` is equality-preserving. `From` and `To` model `convertible_to<From, To>` only if:

>    > > 1. (2.1) — `To` is not an object or reference-to-object type, or `static_cast<To>(f())` is equal to `test(f)`.

>    > > 2. (2.2) — `add_rvalue_reference_t<From>` is not a reference-to-object type, or

>    > >    1. (2.2.1) — If `add_rvalue_reference_t<From>` is an rvalue reference to a non const-qualified type, the resulting state of the object referenced by `f()` after either above expression is valid but unspecified (16.4.6.17 [lib.types.movedfrom]).

>    > >    2. (2.2.2) — Otherwise, the object referred to by `f()` is not modified by either above expression.

[2019-11-06 Tim updates PR based on discussion in Belfast LWG evening session]

"glvalue" is incorrect because we want to allow testing `convertible_to<void, void>`. It's also less than clear how the "expression" and "a particular type" in the first sentence correspond to the parameters of the concept.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4835, and also resolves LWG 3151(i).

> 1. Modify 18.4.4 [concept.convertible] as indicated:

>    > -1- The `convertible_to` concept for types `From` and `To` requires an expression `E` such that `decltype((E))` is `add_rvalue_reference_t<From>` of a particular type and value category to be both implicitly and explicitly convertible to some other type `To`. The implicit and explicit conversions are required to produce equal results.

>    >     template<class From, class To>
>    >       concept convertible_to =
>    >         is_convertible_v<From, To> &&
>    >         requires(add_rvalue_reference_t<From> (&f)()) {
>    >           static_cast<To>(f());
>    >         };

>    > > -2- Let `FromR` be `add_rvalue_reference_t<From>` and `test` be the invented function:

>    > > >     To test(FromR (&f)()) {
>    > > >       return f();
>    > > >     }

>    > > for some types `From` and `To`, and let `f` be a function with no arguments and return type `FromR` such that `f()` is equality-preserving. `From` and `To` model `convertible_to<From, To>` only if:

>    > > 1. (2.1) — `To` is not an object or reference-to-object type, or `static_cast<To>(f())` is equal to `test(f)`.

>    > > 2. (2.2) — `FromR` is not a reference-to-object type, or

>    > >    1. (2.2.1) — If `FromR` is an rvalue reference to a non const-qualified type, the resulting state of the object referenced by `f()` after either above expression is valid but unspecified (16.4.6.17 [lib.types.movedfrom]).

>    > >    2. (2.2.2) — Otherwise, the object referred to by `f()` is not modified by either above expression.

[2019-11-09 Tim rephrased first sentence based on discussion in Belfast LWG Saturday session]

[Status to Tentatively ready after Belfast LWG Saturday session]

Proposed resolution:

This wording is relative to N4835, and also resolves LWG 3151(i).

1. Modify 18.4.4 [concept.convertible] as indicated:

   > -1- Given types `From` and `To` and an expression `E` such that `decltype((E))` is `add_rvalue_reference_t<From>`, `convertible_to<From, To>` The `convertible_to` concept requires `E` an expression of a particular type and value category to be both implicitly and explicitly convertible to some other type `To`. The implicit and explicit conversions are required to produce equal results.

   >     template<class From, class To>
   >       concept convertible_to =
   >         is_convertible_v<From, To> &&
   >         requires(add_rvalue_reference_t<From> (&f)()) {
   >           static_cast<To>(f());
   >         };

   > > -2- Let `FromR` be `add_rvalue_reference_t<From>` and `test` be the invented function:

   > > >     To test(FromR (&f)()) {
   > > >       return f();
   > > >     }

   > > for some types `From` and `To`, and let `f` be a function with no arguments and return type `FromR` such that `f()` is equality-preserving. `From` and `To` model `convertible_to<From, To>` only if:

   > > 1. (2.1) — `To` is not an object or reference-to-object type, or `static_cast<To>(f())` is equal to `test(f)`.

   > > 2. (2.2) — `FromR` is not a reference-to-object type, or

   > >    1. (2.2.1) — If `FromR` is an rvalue reference to a non const-qualified type, the resulting state of the object referenced by `f()` after either above expression is valid but unspecified (16.4.6.17 [lib.types.movedfrom]).

   > >    2. (2.2.2) — Otherwise, the object referred to by `f()` is not modified by either above expression.

### 3195(i). What is the stored pointer value of an empty `weak_ptr`?

Section: 20.3.2.3.2 [util.smartptr.weak.const] Status: C++23 Submitter: Casey Carter Opened: 2019-03-15 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

20.3.2.3.2 [util.smartptr.weak.const] specifies `weak_ptr`'s default constructor:

>     constexpr weak_ptr() noexcept;

> > 1 Effects: Constructs an empty `weak_ptr` object.

> > 2 Ensures: `use_count() == 0`.

and `shared_ptr` converting constructor template:

>     weak_ptr(const weak_ptr& r) noexcept;
>     template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept;
>     template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;

> > 3 Remarks: The second and third constructors shall not participate in overload resolution unless `Y*` is compatible with `T*`.

> > 4 Effects: If `r` is empty, constructs an empty `weak_ptr` object; otherwise, constructs a `weak_ptr` object that shares ownership with `r` and stores a copy of the pointer stored in `r`.

> > 5 Ensures: `use_count() == r.use_count()`.

Note that neither specifies the value of the stored pointer when the resulting `weak_ptr` is empty. This didn't matter — the stored pointer value was unobservable for an empty `weak_ptr` — until we added `atomic<weak_ptr>`. 32.5.8.7.3 [util.smartptr.atomic.weak]/15 says:

> Remarks: Two `weak_ptr` objects are equivalent if they store the same pointer value and either share ownership, or both are empty. The weak form may fail spuriously. See 32.5.8.2 [atomics.types.operations].

Two empty `weak_ptr` objects that store different pointer values are not equivalent. We could correct this by changing 32.5.8.7.3 [util.smartptr.atomic.weak]/15 to "Two `weak_ptr` objects are equivalent if they are both empty, or if they share ownership and store the same pointer value." In practice, an implementation of `atomic<weak_ptr>` will CAS on both the ownership (control block pointer) and stored pointer value, so it seems cleaner to pin down the stored pointer value of an empty `weak_ptr`.

[2019-06-09 Priority set to 2 after reflector discussion]

Previous resolution [SUPERSEDED]

> This wording is relative to N4810.

> 1. Modify 20.3.2.3.2 [util.smartptr.weak.const] as indicated (note the drive-by edit to cleanup the occurrences of "constructs an object of class foo"):

>    >     constexpr weak_ptr() noexcept;

>    > > -1- Effects: Constructs an empty `weak_ptr` object that stores a null pointer value.

>    > > -2- Ensures: `use_count() == 0`.

>    >     weak_ptr(const weak_ptr& r) noexcept;
>    >     template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept;
>    >     template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;

>    > > -3- Remarks: The second and third constructors shall not participate in overload resolution unless `Y*` is compatible with `T*`.

>    > > -4- Effects: If `r` is empty, constructs an empty `weak_ptr` object that stores a null pointer value; otherwise, constructs a `weak_ptr` object that shares ownership with `r` and stores a copy of the pointer stored in `r`.

>    > > -5- Ensures: `use_count() == r.use_count()`.

[2020-02-14 Casey updates P/R per LWG instruction]

While reviewing the P/R in Prague, Tim Song noticed that the stored pointer value of a moved-from `weak_ptr` must also be specified.

[2020-02-16; Prague]

Reviewed revised wording and moved to Ready for Varna.

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 20.3.2.3.2 [util.smartptr.weak.const] as indicated:

   >     constexpr weak_ptr() noexcept;

   > > -1- Effects: Constructs an empty `weak_ptr` object that stores a null pointer value.

   > > -2- Postconditions: `use_count() == 0`.

   >     weak_ptr(const weak_ptr& r) noexcept;
   >     template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept;
   >     template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;

   > > -3- Remarks: The second and third constructors shall not participate in overload resolution unless `Y*` is compatible with `T*`.

   > > -4- Effects: If `r` is empty, constructs an empty `weak_ptr` object that stores a null pointer value; otherwise, constructs a `weak_ptr` object that shares ownership with `r` and stores a copy of the pointer stored in `r`.

   > > -5- Postconditions: `use_count() == r.use_count()`.

   >     weak_ptr(weak_ptr&& r) noexcept;
   >     template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

   > > -6- Remarks: The second constructor shall not participate in overload resolution unless `Y*` is compatible with `T*`.

   > > -7- Effects: Move constructs a `weak_ptr` instance from `r`.

   > > -8- Postconditions: `*this` shall containcontains the old value of `r`. `r` shall beis empty., stores a null pointer value, and `r.use_count() == 0`.

### 3196(i). `std::optional<T>` is ill-formed is `T` is an array

Section: 22.5.3 [optional.optional] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-03-18 Last modified: 2021-02-25

Priority: 0

View all other issues in [optional.optional].

View all issues with C++20 status.

Discussion:

22.5.3 [optional.optional] appears to allow arrays:

> `T` shall be an object type other than cv `in_place_t` or cv `nullopt_t` and shall satisfy the Cpp17Destructible requirements (Table 30).

But instantiating it fails, because `value_or` attempts to return `T` by value, which isn't possible for an array type.

Existing practice seems to be to reject array types. Libstdc++ and libc++ give an error for the signature of `value_or`, and MSVC fails a static assert saying the type needs to be destructible (which is misleading, because `int[2]` is destructible, but either way it's ill-formed). Previous resolution [SUPERSEDED]:

> This wording is relative to N4810.

> 1. Modify 22.5.3 [optional.optional] as indicated:

>    > -3- `T` shall be an non-array object type other than cv `in_place_t` or cv `nullopt_t` and shall satisfy the Cpp17Destructible requirements (Table 30).

[2019-03-26 Marshall provides updated resolution based on reflector discussion]

[2019-06-16 Moved to "Tentatively Ready" based on five positive votes on the reflector]

Proposed resolution:

This wording is relative to N4810.

1. In 16.4.4.2 [utility.arg.requirements], edit Table 30 — "Cpp17Destructible requirements" as indicated:

   > Table 30 — Cpp17Destructible requirements
   > Expression | Post-condition
   > --- | ---
   > `u.~T()` | All resources owned by `u` are reclaimed, no exception is propagated.
   > [Note: Array types and non-object types are not Cpp17Destructible. — end note]

1. Modify 22.5.3 [optional.optional] as indicated:

   > -3- `T` shall be a object type other than cv `in_place_t` or cv `nullopt_t` and shall satisfythat meets the Cpp17Destructible requirements (Table 30).

1. Modify 22.6.3 [variant.variant] as indicated:

   > -2-All types in `Types` shall be (possibly cv-qualified) object types that are not arraysmeet the Cpp17Destructible requirements (Table 30).

### 3198(i). Bad constraint on `std::span::span()`

Section: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Lars Gullik Bjønnes Opened: 2019-04-03 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [span.cons].

View all issues with C++20 status.

Discussion:

It seems that this was left out of P1089. The constraint on `span()` (23.7.2.2.2 [span.cons]) in the current draft is:

> Constraints: `Extent <= 0` is `true`.

This does not seem to make much sense.

The proposal is to change the constraint to be:

> Constraints: `Extent == dynamic_extent || Extent == 0` is `true`.

[2019-06-09; Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 23.7.2.2.2 [span.cons] as indicated:

   >     constexpr span() noexcept;

   > > -1- Constraints: `Extent <== dynamic_extent || Extent == 0` is `true`.

### 3199(i). `istream >> bitset<0>` fails

Section: 22.9.4 [bitset.operators] Status: C++20 Submitter: Davis Herring Opened: 2019-04-05 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [bitset.operators].

View all issues with C++20 status.

Discussion:

From a StackOverflow question:

[bitset.operators]/5.1 says that extracting a `bitset<0>` stops after reading 0 characters. /6 then says that, since no characters were stored, `failbit` is set.

[2019-06-09; Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 22.9.4 [bitset.operators] as indicated:

   >     template<class charT, class traits, size_t N>
   >       basic_istream<charT, traits>&
   >         operator>>(basic_istream<charT, traits>& is, bitset<N>& x);

   > > […]

   > > -6- If `N > 0` and no characters are stored in `str`, calls `is.setstate(ios_base::failbit)` (which may throw `ios_base::failure` (31.5.4.4 [iostate.flags])).

### 3200(i). `midpoint` should not constrain `T` is complete

Section: 26.10.16 [numeric.ops.midpoint] Status: C++20 Submitter: Paolo Torres Opened: 2019-04-10 Last modified: 2021-02-25

Priority: 2

View all other issues in [numeric.ops.midpoint].

View all issues with C++20 status.

Discussion:

The constraint of the `midpoint` overload in 26.10.16 [numeric.ops.midpoint]:

>     template<class T>
>     constexpr T* midpoint(T* a, T* b);

> > -4- Constraints: `T` is a complete object type.

is incorrect. Paragraph 4 states `T` is constrained to be a complete object type, however it does not seem to be possible to implement `T` is complete. This means behavior is conditioned dependent on whether a type is complete, and this seems inconsistent with the library precedent.

[2019-06-12 Priority set to 2 after reflector discussion]

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 26.10.16 [numeric.ops.midpoint] as indicated:

   >     template<class T>
   >       constexpr T* midpoint(T* a, T* b);

   > > -4- Constraints: `T` is an complete object type.

   > > -?- Mandates: `T` is a complete type.

   > > […]

### 3201(i). `lerp` should be marked as `noexcept`

Section: 29.7.4 [c.math.lerp] Status: C++20 Submitter: Paolo Torres Opened: 2019-04-10 Last modified: 2021-02-25

Priority: 2

View all issues with C++20 status.

Discussion:

The overloads of `lerp` should be marked as `noexcept`, and this can be explained through the Lakos Rule. This function does not specify any undefined behaviour, and as such has no preconditions. This implies it has a wide contract, meaning it cannot throw, and thus can be marked as `noexcept`.

[2020-02 Moved to Immediate on Thursday afternoon in Prague.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 29.7.1 [cmath.syn], header `<cmath>` synopsis, as indicated:

   >     // 29.7.4 [c.math.lerp], linear interpolation
   >     constexpr float lerp(float a, float b, float t) noexcept;
   >     constexpr double lerp(double a, double b, double t) noexcept;
   >     constexpr long double lerp(long double a, long double b, long double t) noexcept;

2. Modify 29.7.4 [c.math.lerp] as indicated:

   >     constexpr float lerp(float a, float b, float t) noexcept;
   >     constexpr double lerp(double a, double b, double t) noexcept;
   >     constexpr long double lerp(long double a, long double b, long double t) noexcept;

### 3202(i). P0318R1 was supposed to be revised

Section: 22.10.2 [functional.syn] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-04-23 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

P0318R1 was discussed in Batavia and the requested changes were made in D0318R2. In San Diego the R1 paper was voted into the WP, despite not having the requested changes. There were also changes to D0318R2 suggested on the reflector, which are incorporated below.

[2019-04-30 Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 22.10.2 [functional.syn], header `<functional>` synopsis, as indicated:

   >     template<class T> struct unwrap_reference;
   >     template<class T> using unwrap_reference_t = typename unwrap_reference<T>::type;
   >     template<class T> struct unwrap_ref_decay : unwrap_reference<decay_t<T>> {};
   >     template<class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;

2. Modify 99 [refwrap.unwrapref] as indicated:

   >     template<class T>
   >       struct unwrap_reference;

   > > -1- If `T` is a specialization […]

   >     template<class T>
   >       struct unwrap_ref_decay;

   > > -?- The member typedef `type` of `unwrap_ref_decay<T>` denotes the type `unwrap_reference_t<decay_t<T>>`.

### 3203(i). `span` element access invalidation

Section: 23.7.2.2.1 [span.overview] Status: C++26 Submitter: Johel Ernesto Guerrero Peña Opened: 2019-05-04 Last modified: 2026-06-09

Priority: 2

View all other issues in [span.overview].

View all issues with C++26 status.

Discussion:

`span` doesn't explicitly point out when its accessed elements are invalidated like `string_view` does in 27.3.3.4 [string.view.iterators] p2.

[2019-06-12 Priority set to 2 after reflector discussion]

Previous resolution [SUPERSEDED]

This wording is relative to N4910.

1. Modify 23.7.2.2.1 [span.overview] as indicated:

   > -4- `ElementType` is required to be a complete object type that is not an abstract class type.

   > -?- For a `span s`, any operation that invalidates a pointer in the range `[s.data(), s.data() + s.size())` invalidates pointers, iterators, and references other than `*this` returned from `s`'s member functions.

[2023-06-13; Varna]

The reference to N4910 27.3.3.4 [string.view.iterators] p2 is located now in N4950 27.3.3.1 [string.view.template.general] p2 where its says:

> For a `basic_string_view str`, any operation that invalidates a pointer in the range

> >     [str.data(), str.data() + str.size())

> invalidates pointers, iterators, and references returned from `str`'s member functions.

The group also suggested to adjust 27.3.3.1 [string.view.template.general] p2 similarly.

[2023-06-14 Varna; Move to Ready]

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 27.3.3.1 [string.view.template.general] as indicated:

   > [Drafting note: The proposed wording also removes the extra code block that previously defined the range]

   > -2- For a `basic_string_view str`, any operation that invalidates a pointer in the range `[str.data(), str.data() + str.size())` invalidates pointers, iterators, and references to elements of `str` returned from `str`'s member functions.

2. Modify 23.7.2.2.1 [span.overview] as indicated:

   > -4- `ElementType` is required to be a complete object type that is not an abstract class type.

   > -?- For a `span s`, any operation that invalidates a pointer in the range `[s.data(), s.data() + s.size())` invalidates pointers, iterators, and references to elements of `s`.

### 3204(i). `sub_match::swap` only swaps the base class

Section: 28.6.8 [re.submatch] Status: C++23 Submitter: Jonathan Wakely Opened: 2019-05-07 Last modified: 2023-11-22

Priority: 3

View all other issues in [re.submatch].

View all issues with C++23 status.

Discussion:

`sub_match<I>` derives publicly from `pair<I,I>`, and so inherits `pair::swap(pair&)`. This means that the following program fails:

>     #include <regex>
>     #include <cassert>

>     int main()
>     {
>       std::sub_match<const char*> a, b;
>       a.matched = true;
>       a.swap(b);
>       assert(b.matched);
>     }

The `pair::swap(pair&)` member should be hidden by a `sub_match::swap(sub_match&)` member that does the right thing.

[2019-06-12 Priority set to 3 after reflector discussion]

[2020-05-01; Daniel adjusts wording to recent working draft]

[2022-04-25; Daniel adjusts wording to recent working draft]

In addition the revised wording uses the new standard phrase "The exception specification is equivalent to"

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> 1. Modify 28.6.8 [re.submatch], class template `sub_match` synopsis, as indicated:

>    >     template<class BidirectionalIterator>
>    >     class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> {
>    >     public:
>    >       […]
>    >       int compare(const sub_match& s) const;
>    >       int compare(const string_type& s) const;
>    >       int compare(const value_type* s) const;

>    >       void swap(sub_match& s) noexcept(see below);
>    >     };

> 2. Modify 28.6.8.2 [re.submatch.members] as indicated:

>    >     int compare(const value_type* s) const;

>    > > […]

>    >     void swap(sub_match& s) noexcept(see below);

>    > > [Drafting note: The swappable requirement should really be unnecessary because Cpp17Iterator requires it, but there is no wording that requires `BidirectionalIterator` in Clause 28.6 [re] in general meets the bidirectional iterator requirements. Note that the definition found in 26.2 [algorithms.requirements] does not extend to 28.6 [re] normatively. — end drafting note]

>    > > -?- Preconditions: Lvalues of type `BidirectionalIterator` are swappable (16.4.4.3 [swappable.requirements]).

>    > > -?- Effects: Equivalent to:

>    > > >     this->pair<BidirectionalIterator, BidirectionalIterator>::swap(s);
>    > > >     std::swap(matched, s.matched);

>    > > -?- Remarks: The exception specification is equivalent to `is_nothrow_swappable_v<BidirectionalIterator>`.

[2022-11-06; Daniel comments and improves wording]

With the informal acceptance of P2696R0 by LWG during a pre-Kona telecon, we should use the new requirement set Cpp17Swappable instead of the "LValues are swappable" requirements.

[2022-11-30; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917 and assumes the acceptance of P2696R0.

1. Modify 28.6.8 [re.submatch], class template `sub_match` synopsis, as indicated:

   >     template<class BidirectionalIterator>
   >     class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> {
   >     public:
   >       […]
   >       int compare(const sub_match& s) const;
   >       int compare(const string_type& s) const;
   >       int compare(const value_type* s) const;

   >       void swap(sub_match& s) noexcept(see below);
   >     };

2. Modify 28.6.8.2 [re.submatch.members] as indicated:

   >     int compare(const value_type* s) const;

   > > […]

   >     void swap(sub_match& s) noexcept(see below);

   > > [Drafting note: The Cpp17Swappable requirement should really be unnecessary because Cpp17Iterator requires it, but there is no wording that requires `BidirectionalIterator` in Clause 28.6 [re] in general meets the bidirectional iterator requirements. Note that the definition found in 26.2 [algorithms.requirements] does not extend to 28.6 [re] normatively. — end drafting note]

   > > -?- Preconditions: `BidirectionalIterator` meets the Cpp17Swappable requirements (16.4.4.3 [swappable.requirements]).

   > > -?- Effects: Equivalent to:

   > > >     this->pair<BidirectionalIterator, BidirectionalIterator>::swap(s);
   > > >     std::swap(matched, s.matched);

   > > -?- Remarks: The exception specification is equivalent to `is_nothrow_swappable_v<BidirectionalIterator>`.

### 3206(i). `year_month_day` conversion to `sys_days` uses not-existing member function

Section: 30.8.14.2 [time.cal.ymd.members] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-05-19 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

The current specification of the `year_month_day` conversion function to `sys_days`, uses the `day` member function on the `sys_days` (a.k.a. `time_point<system_clock, days>`), that does not exist.

In 30.8.14.2 [time.cal.ymd.members] p18, the expression `sys_days{y_/m_/last}.day()` is ill-formed:

> […] Otherwise, if `y_.ok() && m_.ok()` is `true`, returns a `sys_days` which is offset from `sys_days{y_/m_/last}` by the number of days `d_` is offset from `sys_days{y_/m_/last}.day()`. Otherwise the value returned is unspecified.

[2019-06-08; Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 30.8.14.2 [time.cal.ymd.members] as indicated:

   >     constexpr operator sys_days() const noexcept;

   > > -18- Returns: If `ok()`, returns a `sys_days` holding a count of days from the `sys_days` epoch to `*this` (a negative value if `*this` represents a date prior to the `sys_days` epoch). Otherwise, if `y_.ok() && m_.ok()` is `true`, returns `sys_days{y_/m_/1d} + (d_ - 1d)`a `sys_days` which is offset from `sys_days{y_/m_/last}` by the number of days `d_` is offset from `sys_days{y_/m_/last}.day()`. Otherwise the value returned is unspecified.

### 3208(i). `Boolean`'s expression requirements are ordered inconsistently

Section: 24.7 [iterator.range] Status: C++20 Submitter: Casey Carter Opened: 2019-06-06 Last modified: 2021-02-25

Priority: 0

View all other issues in [iterator.range].

View all issues with C++20 status.

Discussion:

For consistency of presentation, we should group and order the `&&` and `||` expression requirements similarly to the `==` and `!=` expression requirements. Note that the suggested change is not quite editorial: evaluation of requirements for satisfaction has short-circuiting behavior, so the declaration order of requirements is normatively significant in general.

[2019-06-13; Priority to 0 and Status to Tentatively Ready after seven positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4810.

1. Modify [concept.boolean] as indicated:

   >     […]
   >       { b1 } -> ConvertibleTo<bool>;
   >       { !b1 } -> ConvertibleTo<bool>;
   >       { b1 &&  a } -> Same<bool>;
   >       { b1 ||  a } -> Same<bool>;
   >       { b1 && b2 } -> Same<bool>;
   >       { b1 &&  a } -> Same<bool>;
   >       {  a && b2 } -> Same<bool>;
   >       { b1 || b2 } -> Same<bool>;
   >       { b1 ||  a } -> Same<bool>;
   >       {  a || b2 } -> Same<bool>;
   >       { b1 == b2 } -> ConvertibleTo<bool>;
   >       { b1 ==  a } -> ConvertibleTo<bool>;
   >       {  a == b2 } -> ConvertibleTo<bool>;
   >       { b1 != b2 } -> ConvertibleTo<bool>;
   >       { b1 !=  a } -> ConvertibleTo<bool>;
   >       {  a != b2 } -> ConvertibleTo<bool>;
   >     };

### 3209(i). Expression in `year::ok()` returns clause is ill-formed

Section: 30.8.5.2 [time.cal.year.members] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-05-28 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

The expression in the Returns element of `year::ok` in [time.cal.year.members] p18:

>     min() <= y_ && y_ <= max()

is ill-formed, as it attempts to compare type `short` (type of `y_` member) with type `year` (type of `year::min()` and `year::max()`).

[2019-06-13; Priority to 0 and Status to Tentatively Ready after seven positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 30.8.5.2 [time.cal.year.members] as indicated:

   >     constexpr bool ok() const noexcept;

   > > -18- Returns: `min().y_ <= y_ && y_ <= max().y_`.

### 3210(i). `allocate_shared` is inconsistent about removing `const` from the pointer passed to allocator `construct` and `destroy`

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: Resolved Submitter: Billy O'Neal III Opened: 2019-05-29 Last modified: 2025-11-11

Priority: 3

View all other issues in [util.smartptr.shared.create].

View all issues with Resolved status.

Discussion:

I implemented the fix for LWG 3008(i) and Stephan pointed out there's an inconsistency here for `allocate_shared<const T>`.

20.3.2.2.7 [util.smartptr.shared.create] p3 says that the allocator construct call is done without removing cv qualifiers, but 20.3.2.2.7 [util.smartptr.shared.create] p7.12 says that the `destroy` call is done with removed cv qualifiers.

The fallback for `allocator_traits::construct` rejects `const T*` (since it `static_casts` to `void*`), so the most likely outcome of attempting to do this today is to fail to compile, which is a break with C++17.

Our options are:

1. Fix the allocator model to deal with `const` elements somehow, which breaks compatibility with existing allocators unprepared for `const` elements here. We would need to extend the allocator requirements to allow `const T*` to be passed here, and fix our default to `const_cast`.

2. Fix `allocate_shared` to remove `const` before calling `construct`, which changes the experience for C++17 customers because `allocate_shared` constructs a `T` instead of a `const T`, but not in a way substantially different to edits P0674 already made here.

3. Back out `allocate_shared`'s interaction with this part of the allocator model (reverting this part of P0674 and reopening LWG 3008(i)).

4. Go around the problem by prohibiting `allocate_shared<const T>`, which breaks existing C++17 customers.

Billy O'Neal argues that only (2) preserves the design intent P0674 while maintaining compatibility for most allocators and most C++17 customers.

Peter Dimov argues that (1) isn't likely to break enough to matter.

[2019-06-16 Priority set to 3 based on reflector discussion]

Previous resolution [SUPERSEDED]

This wording is relative to N4810.

> [Drafting note: As the issue submitter prefers option (2), this is wording for that.]

1. Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   >     template<class T, ...>
   >       shared_ptr<T> make_shared(args);
   >     template<class T, class A, ...>
   >       shared_ptr<T> allocate_shared(const A& a, args);
   >     template<class T, ...>
   >       shared_ptr<T> make_shared_default_init(args);
   >     template<class T, class A, ...>
   >       shared_ptr<T> allocate_shared_default_init(const A& a, args);

   > > -2- Requires: […]

   > > […]

   > > -7- Remarks:

   > > 1. (7.1) — […]

   > > 2. […]

   > > 3. (7.5) — When a (sub)object of a non-array type `U` is specified to have an initial value of `v`, or `U(l...)`, where `l...` is a list of constructor arguments, `allocate_shared` shall initialize this (sub)object via the expression

   > >    1. (7.5.1) — `allocator_traits<A2>::construct(a2, pv, v)` or

   > >    2. (7.5.2) — `allocator_traits<A2>::construct(a2, pv, l...)`

   > >    respectively, where `pv` points to storage suitable to hold an object of type `remove_cv_t<U>` and `a2` of type `A2` is a rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

[2024-04-13; Jiang An comments and provides improved wording]

The currently proposed resolution is meaningless, because "(allocated) storage suitable to hold an object of type `remove_cv_t<U>`" is always "storage suitable to hold an object of type `U`", and vice versa. Also, the current specification doesn't seem to specify the type of `pv` in the cases of `allocator_shared`, because `pv` is merely specified to point some storage instead of an object.

[2024-10-02; will be resolved by issue 3216(i).]

[2025-11-11; LWG 3216(i) was accepted in Wrocław. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   > [Drafting note: As the issue submitter prefers option (2), this is wording for that.]

   >     template&lt;class T, ...>
   >       shared_ptr&lt;T&gt; make_shared(args);
   >     template&lt;class T, class A, ...>
   >       shared_ptr&lt;T&gt; allocate_shared(const A& a, args);
   >     template&lt;class T, ...>
   >       shared_ptr&lt;T&gt; make_shared_for_overwrite(args);
   >     template&lt;class T, class A, ...>
   >       shared_ptr&lt;T&gt; allocate_shared_for_overwrite(const A& a, args);

   > > -2- Preconditions: […]

   > > […]

   > > -7- Remarks:

   > > 1. (7.1) — […]

   > > 2. […]

   > > 3. (7.5) — When a (sub)object of a non-array type `U` is specified to have an initial value of `v`, or `U(l...)`, where `l...` is a list of constructor arguments, `allocate_shared` shall initialize this (sub)object via the expression

   > >    1. (7.5.1) — `allocator_traits<A2>::construct(a2, pv, v)` or

   > >    2. (7.5.2) — `allocator_traits<A2>::construct(a2, pv, l...)`

   > >    respectively, where `pv` has type `remove_cv_t<U>*` and points to storage suitable to hold an object of type `U` and `a2` of type `A2` is a rebound copy of the allocator a passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

   > > 4. (7.6) — […]

   > > 5. (7.7) — When a (sub)object of non-array type `U` is specified to have a default initial value, `allocate_shared` shall initialize this (sub)object via the expression `allocator_traits<A2>::construct(a2, pv)`, where `pv` has type `remove_cv_t<U>*` and points to storage suitable to hold an object of type `U` and `a2` of type `A2` is a rebound copy of the allocator a passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

### 3211(i). `std::tuple<>` should be trivially constructible

Section: 22.4.4.2 [tuple.cnstr] Status: C++23 Submitter: Louis Dionne Opened: 2019-05-29 Last modified: 2023-11-22

Priority: 3

View other active issues in [tuple.cnstr].

View all other issues in [tuple.cnstr].

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Discussion:

That requirement is really easy to enforce, and it has been requested by users (e.g. libc++ bug 41714).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4810.

> 1. Modify 22.4.4.2 [tuple.cnstr] as indicated:

>    > -4- If `is_trivially_destructible_v<Ti>` is `true` for all `Ti`, then the destructor of `tuple` is trivial. The default constructor of `tuple<>` is trivial.

[2020-02-13, Prague]

LWG discussion revealed that all where happy that we want this, except that the new wording should become a separate paragraph.

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 22.4.4.2 [tuple.cnstr] as indicated:

   > -4- If `is_trivially_destructible_v<Ti>` is `true` for all `Ti`, then the destructor of `tuple` is trivial.

   > -?- The default constructor of `tuple<>` is trivial.

### 3212(i). `tuple_element_t<1, const span<int, 42>>` is `const int`

Section: 99 [span.tuple] Status: Resolved Submitter: Tim Song Opened: 2019-05-31 Last modified: 2020-09-06

Priority: 2

View all issues with Resolved status.

Discussion:

As currently specified, it uses the cv-qualified partial specialization, which incorrectly adds cv-qualification to the element type.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4810.

> 1. Modify 23.7.2.1 [span.syn], header `<span>` synopsis, as indicated:

>    >     […]
>    >     // 99 [span.tuple], tuple interface
>    >     template<class T> class tuple_size;
>    >     template<size_t I, class T> class tuple_element;
>    >     template<class ElementType, size_t Extent>
>    >       struct tuple_size<span<ElementType, Extent>>;
>    >     template<class ElementType>
>    >       struct tuple_size<span<ElementType, dynamic_extent>>; // not defined
>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, span<ElementType, Extent>>;
>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, const span<ElementType, Extent>>;
>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, volatile span<ElementType, Extent>>;
>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, const volatile span<ElementType, Extent>>;
>    >     […]

> 2. Modify 99 [span.tuple] before p1 as indicated:

>    > [Drafting note: The representation style for the newly added wording follows the new style for `tuple_element<I, span<ElementType, Extent>>::type`, which had recently been changed editorially.]

>    >     template<class ElementType, size_t Extent>
>    >       struct tuple_size<span<ElementType, Extent>>
>    >         : integral_constant<size_t, Extent> { };

>    >     tuple_element<I, span<ElementType, Extent>>::type

>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, const span<ElementType, Extent>> {
>    >         using type = ElementType;
>    >       };

>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, volatile span<ElementType, Extent>> {
>    >         using type = ElementType;
>    >       };

>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, const volatile span<ElementType, Extent>> {
>    >         using type = ElementType;
>    >       };

>    > > -1- Mandates: `Extent != dynamic_extent && I < Extent` is `true`.

[2020-02-13, Prague]

Wording needs to be adjusted, because we accepted P1831R1.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 23.7.2.1 [span.syn], header `<span>` synopsis, as indicated:

>    >     […]
>    >     // 99 [span.tuple], tuple interface
>    >     template<class T> class tuple_size;
>    >     template<size_t I, class T> class tuple_element;
>    >     template<class ElementType, size_t Extent>
>    >       struct tuple_size<span<ElementType, Extent>>;
>    >     template<class ElementType>
>    >       struct tuple_size<span<ElementType, dynamic_extent>>; // not defined
>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, span<ElementType, Extent>>;
>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, const span<ElementType, Extent>>;
>    >     […]

> 2. Modify 99 [span.tuple] before p1 as indicated:

>    >     template<class ElementType, size_t Extent>
>    >       struct tuple_size<span<ElementType, Extent>>
>    >         : integral_constant<size_t, Extent> { };

>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, span<ElementType, Extent>> {
>    >         using type = ElementType;
>    >       };

>    >     template<size_t I, class ElementType, size_t Extent>
>    >       struct tuple_element<I, const span<ElementType, Extent>> {
>    >         using type = ElementType;
>    >       };

>    > > -1- Mandates: `Extent != dynamic_extent && I < Extent` is `true`.

> 3. Add the following wording to Annex D:

>    > D.? Deprecated `span` tuple interface [depr.span.syn]

>    > 1 The header `<span>` (23.7.2.1 [span.syn]) has the following additions:

>    > >     namespace std {
>    > >       template<size_t I, class ElementType, size_t Extent>
>    > >         struct tuple_element<I, volatile span<ElementType, Extent>>;
>    > >       template<size_t I, class ElementType, size_t Extent>
>    > >         struct tuple_element<I, const volatile span<ElementType, Extent>>;
>    > >     }

>    > D.1.? tuple interface [depr.span.tuple]

>    > >     template<size_t I, class ElementType, size_t Extent>
>    > >       struct tuple_element<I, volatile span<ElementType, Extent>> {
>    > >         using type = ElementType;
>    > >       };

>    > >     template<size_t I, class ElementType, size_t Extent>
>    > >       struct tuple_element<I, const volatile span<ElementType, Extent>> {
>    > >         using type = ElementType;
>    > >       };

>    > -?- Mandates: `Extent != dynamic_extent && I < Extent` is `true`.

[2020-02-14, Prague]

Lengthy discussion about the intended design of `span`'s tuple interface. Has been send to LEWG, who made several polls and concluded to remove the tuple interface for `span` for C++20.

Will be resolved by P2116R0.

[2020-05-01; reflector discussions]

Resolved by P2116R0 voted in in Prague.

Proposed resolution:

Resolved by P2116R0.

### 3213(i). `for_each_n` and `copy_n` missing requirements for `Size`

Section: 26.6.5 [alg.foreach], 26.7.1 [alg.copy] Status: Resolved Submitter: Jonathan Wakely Opened: 2019-05-31 Last modified: 2020-11-09

Priority: 3

View other active issues in [alg.foreach].

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Discussion:

`search_n` and `fill_n` and `generate_n` require that "The type `Size` shall be convertible to integral type", but `for_each_n` and `copy_n` have no requirements on `Size`. Presumably it should be convertible to an integral type.

[2019-07 Issue Prioritization]

Priority to 3 after discussion on the reflector.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4810.

> > [Drafting note: Clause [algorithms] has not yet gone through a "Mandating" cleanup of our new wording style for requirements expressed in Requires and Expects elements. The below wording changes perform this fix for the touched paragraphs and also replaces here Requires by Expects to prevent papers that address such wording changes to keep tracking of additional wording changes caused by proposed wording of issues.]

> 1. Modify 26.6.5 [alg.foreach] as indicated:

>    >     template<class InputIterator, class Size, class Function>
>    >       constexpr InputIterator for_each_n(InputIterator first, Size n, Function f);

>    > > -16- Requires: shall satisfy the Cpp17MoveConstructible requirements […]

>    > > -17- RequiresExpects: The type `Size` is convertible to integral type (7.3.9 [conv.integral], 11.4.8 [class.conv]). `n >= 0`.

>    > > […]

>    >     template<class ExecutionPolicy, class ForwardIterator, class Size, class Function>
>    >       ForwardIterator for_each_n(ExecutionPolicy&& exec, ForwardIterator first, Size n,
>    >                                  Function f);

>    > > -21- Requires: shall satisfy the Cpp17CopyConstructible requirements […]

>    > > -22- RequiresExpects: The type `Size` is convertible to integral type (7.3.9 [conv.integral], 11.4.8 [class.conv]). `n >= 0`.

>    > > […]

> 2. Modify 26.7.1 [alg.copy] as indicated:

>    >     template<class InputIterator, class Size, class OutputIterator>
>    >       constexpr OutputIterator copy_n(InputIterator first, Size n,
>    >                                       OutputIterator result);
>    >     template<class ExecutionPolicy, class ForwardIterator1, class Size, class ForwardIterator2>
>    >       ForwardIterator2 copy_n(ExecutionPolicy&& exec,
>    >                               ForwardIterator1 first, Size n,
>    >                               ForwardIterator2 result);
>    >     template<InputIterator I, WeaklyIncrementable O>
>    >       requires IndirectlyCopyable<I, O>
>    >       constexpr ranges::copy_n_result<I, O>
>    >         ranges::copy_n(I first, iter_difference_t<I> n, O result);

>    > > -10- Let `M` be `max(n, 0)`.

>    > > -?- Expects: The type `Size` is convertible to integral type (7.3.9 [conv.integral], 11.4.8 [class.conv]).

>    > > […]

[2020-05-01; Reflector discussions]

There was consensus that this issue has been resolved by P1718R2, voted in in Belfast 2019.

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

Resolved by P1718R2

### 3216(i). Rebinding the allocator before calling `construct`/`destroy` in `allocate_shared`

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++26 Submitter: Billy O'Neal III Opened: 2019-06-11 Last modified: 2026-06-09

Priority: 3

View all other issues in [util.smartptr.shared.create].

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Discussion:

The new `allocate_shared` wording says we need to rebind the allocator back to `T`'s type before we can call `construct` or `destroy`, but this is suboptimal (might make extra unnecessary allocator copies), and is inconsistent with the containers' behavior, which call allocator `construct` on whatever `T` they want. (For example, `std::list<T, alloc<T>>` rebinds to `alloc<_ListNode<T>>`, but calls `construct(T*)` without rebinding back)

It seems like we should be consistent with the containers and not require a rebind here. PR would look something like this, relative to N4810; I'm still not super happy with this wording because it looks like it might be saying a copy of the allocator must be made we would like to avoid…

[2019-07 Issue Prioritization]

Priority to 3 after discussion on the reflector.

Previous resolution [SUPERSEDED]

This wording is relative to N4810.

1. Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   > [Drafting note: The edits to change `pv` to `pu` were suggested by Jonathan Wakely (thanks!). This wording also has the `remove_cv_t` fixes specified by LWG 3210(i) — if that change is rejected some of those have to be stripped here.]

   >     template<class T, ...>
   >       shared_ptr<T> make_shared(args);
   >     template<class T, class A, ...>
   >       shared_ptr<T> allocate_shared(const A& a, args);
   >     template<class T, ...>
   >       shared_ptr<T> make_shared_default_init(args);
   >     template<class T, class A, ...>
   >       shared_ptr<T> allocate_shared_default_init(const A& a, args);

   > > -2- Requires: […]

   > > […]

   > > -7- Remarks:

   > > 1. (7.1) — […]

   > > 2. […]

   > > 3. (7.5) — When a (sub)object of a non-array type `U` is specified to have an initial value of `v`, or `U(l...)`, where `l...` is a list of constructor arguments, `allocate_shared` shall initialize this (sub)object via the expression

   > >    1. (7.5.1) — `allocator_traits<A2>::construct(a2, pvu, v)` or

   > >    2. (7.5.2) — `allocator_traits<A2>::construct(a2, pvu, l...)`

   > >    respectively, where `pvu` is a pointer of type `remove_cv_t<U>*` pointsing to storage suitable to hold an object of type `remove_cv_t<U>` and `a2` of type `A2` is a potentially rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

   > > 4. (7.6) — […]

   > > 5. (7.7) — When a (sub)object of non-array type `U` is specified to have a default initial value, `allocate_shared` shall initializes this (sub)object via the expression `allocator_traits<A2>::construct(a2, pvu)`, where `pvu` is a pointer of type `remove_cv_t<U>*` pointsing to storage suitable to hold an object of type `remove_cv_t<U>` and `a2` of type `A2` is a potentially rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

   > > 6. […]

   > > 7. (7.12) — When a (sub)object of non-array type `U` that was initialized by `allocate_shared` is to be destroyed, it is destroyed via the expression `allocator_traits<A2>::destroy(a2, pvu)` where `pvu` is a pointer of type `remove_cv_t<U>*` pointsing to that object of type `remove_cv_t<U>` and `a2` of type `A2` is a potentially rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

[2024-08-23; Jonathan provides updated wording]

`make_shared_default_init` and `allocate_shared_default_init` were renamed by P1973R1 so this needs a rebase. The edit to (7.11) is just for consistency, so that `pv` is always `void*` and `pu` is `remove_cv_t<U>*`. Accepting this proposed resolution would also resolve issue 3210(i).

[2024-10-02; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   >     template<class T, ...>
   >       shared_ptr<T> make_shared(args);
   >     template<class T, class A, ...>
   >       shared_ptr<T> allocate_shared(const A& a, args);
   >     template<class T, ...>
   >       shared_ptr<T> make_shared_for_overwrite(args);
   >     template<class T, class A, ...>
   >       shared_ptr<T> allocate_shared_for_overwrite(const A& a, args);

   > > -2- Preconditions: […]

   > > […]

   > > -7- Remarks:

   > > 1. (7.1) — […]

   > > 2. […]

   > > 3. (7.5) — When a (sub)object of a non-array type `U` is specified to have an initial value of `v`, or `U(l...)`, where `l...` is a list of constructor arguments, `allocate_shared` shall initialize this (sub)object via the expression

   > >    1. (7.5.1) — `allocator_traits<A2>::construct(a2, pvu, v)` or

   > >    2. (7.5.2) — `allocator_traits<A2>::construct(a2, pvu, l...)`

   > >    respectively, where `pvu` is a pointer of type `remove_cv_t<U>*` pointsing to storage suitable to hold an object of type `remove_cv_t<U>` and `a2` of type `A2` is a potentially rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

   > > 4. (7.6) — […]

   > > 5. (7.7) — When a (sub)object of non-array type `U` is specified to have a default initial value, `allocate_shared` shall initializes this (sub)object via the expression `allocator_traits<A2>::construct(a2, pvu)`, where `pvu` is a pointer of type `remove_cv_t<U>*` pointsing to storage suitable to hold an object of type `remove_cv_t<U>` and `a2` of type `A2` is a potentially rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

   > > 6. […]

   > > 7. > [Drafting note: Issue 4024(i) will add `make_shared_for_overwrite` and `allocate_shared_for_overwrite` to (7.11) but that doesn't conflict with this next edit.]

   > >    (7.11) — When a (sub)object of non-array type `U` that was initialized by `make_shared` is to be destroyed, it is destroyed via the expression `pvu->~U()` where `pvu` points to that object of type `U`.

   > > 8. (7.12) — When a (sub)object of non-array type `U` that was initialized by `allocate_shared` is to be destroyed, it is destroyed via the expression `allocator_traits<A2>::destroy(a2, pvu)` where `pvu` is a pointer of type `remove_cv_t<U>*` pointsing to that object of type `remove_cv_t<U>` and `a2` of type `A2` is a potentially rebound copy of the allocator `a` passed to `allocate_shared` such that its `value_type` is `remove_cv_t<U>`.

### 3217(i). `<memory>` and `<execution>` should define `__cpp_lib_parallel_algorithm`

Section: 17.3.1 [support.limits.general] Status: Resolved Submitter: Jonathan Wakely Opened: 2019-06-12 Last modified: 2026-06-10

Priority: 3

View all other issues in [support.limits.general].

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Discussion:

There are parallel overloads of algorithms in `<memory>`, so it should define the macro. Also, `<execution>` defines the exec policies for use with the algos, so that should define the macro too.

[2019-07 Issue Prioritization]

Priority to 3 after discussion on the reflector.

[Brno 2026-06-10; Status changed: New → Resolved by P3179R9.]

Proposed resolution:

This wording is relative to N4810.

1. Modify 17.3.1 [support.limits.general], Table 36 — "Standard library feature-test macros", as indicated:

   > Table 36 — Standard library feature-test macros
   > Macro name | Value | Header(s)
   > --- | --- | ---
   > `[…]`
   > `__cpp_lib_parallel_algorithm` | `201603L` | `<algorithm> <execution> <memory> <numeric>`
   > `[…]`

### 3218(i). Modifier for `%d` parse flag does not match POSIX and `format` specification

Section: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-13 Last modified: 2021-02-25

Priority: 0

View other active issues in [time.parse].

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Discussion:

Currently, the '`%d`' parse flags accepts the '`E`' modifier to parse the locale's alternative representation, see Table 88 — "Meaning of `parse` flags":

> The modified command `%Ed` interprets the locale's alternative representation of the day of the month.

This is inconsistent with the POSIX `strftime` specification and the `format` functions, that uses '`O`' to output alternate locale representation. Per Table 87 — "Meaning of `format` conversion specifiers":

> The modified command `%Od` produces the locale's alternative representation.

[2019-06-24; Howard comments]

This was simply a type-o in my documentation that infected the proposal and subsequently the C++ working draft.

None of `std::time_put`, C's `strftime`, or POSIX's `strftime` support `%Ed` but all support `%Od`. Furthermore the existing example implementation supports `%Od` but not `%Ed`. And all the existing example implementation does is forward to `std::time_put`.

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4810.

1. Modify 30.13 [time.parse], Table 88 — "Meaning of `parse` flags", as indicated:

   > Table 88 — Meaning of `parse` flags
   > Flag | Parsed value
   > --- | ---
   > `[…]`
   > `%d` | The day of the month as a decimal number. The modified command `%Nd` specifies the maximum number of characters to read. If `N` is not specified, the default is 2. Leading zeroes are permitted but not required. The modified command `%EOd` interprets the locale's alternative representation of the day of the month.
   > `[…]`

### 3221(i). Result of `year_month` arithmetic with `months` is ambiguous

Section: 30.8.13.3 [time.cal.ym.nonmembers] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-16 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

The current specification of the addition of `year_month` and `months` does not define a unique result value.

To illustrate, both `year(2019)/month(1)` and `year(2018)/month(13)` are valid results of `year(2018)/month(12) + months(1)` addition, according to the spec in 30.8.13.3 [time.cal.ym.nonmembers].

[2019-06-24; LWG discussion]

During discussions on the LWG reflector there was a preference to add "is `true`" at the end of the modified Returns: element. This additional edit has been applied to Tomasz' original wording below.

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4810.

1. Modify 30.8.13.3 [time.cal.ym.nonmembers] as indicated:

   >     constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;

   > > -3- Returns: A `year_month` value `z` such that `z.ok() && z - ym == dm` is `true`.

   > > Complexity: `𝒪(1)` with respect to the value of `dm`.

### 3222(i). P0574R1 introduced preconditions on non-existent parameters

Section: 26.10.9 [inclusive.scan], 26.10.11 [transform.inclusive.scan] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-06-18 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

after applying P0574R1 to the working draft, 26.10.11 [transform.inclusive.scan] bullet 1.1 says "If `init` is provided, […]; otherwise, `ForwardIterator1`'s value type shall be […]." 26.10.11 [transform.inclusive.scan] bullet 1.2 says "If `init` is provided, […]; otherwise, […] shall be convertible to `ForwardIterator1`'s value type."

For the first overload `init` is not provided, but there is no `ForwardIterator1`, so these requirements cannot be met. The requirements for the first overload need to be stated in terms of `InputIterator`'s value type, not `ForwardIterator1`'s value type.

The same problem exists in 26.10.9 [inclusive.scan]. 26.10.12 [adjacent.difference] solves this problem by saying "Let `T` be the value type of `decltype(first)`".

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4820.

1. Modify 26.10.9 [inclusive.scan] as indicated:

   >     template<class InputIterator, class OutputIterator, class BinaryOperation>
   >       OutputIterator inclusive_scan(InputIterator first, InputIterator last,
   >                                     OutputIterator result, BinaryOperation binary_op);
   >     template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2,
   >              class BinaryOperation>
   >       ForwardIterator2 inclusive_scan(ExecutionPolicy&& exec,
   >                                       ForwardIterator1 first, ForwardIterator1 last,
   >                                       ForwardIterator2 result, BinaryOperation binary_op);
   >     template<class InputIterator, class OutputIterator, class BinaryOperation, class T>
   >       OutputIterator inclusive_scan(InputIterator first, InputIterator last,
   >                                     OutputIterator result, BinaryOperation binary_op, T init);
   >     template<class ExecutionPolicy,
   >              class ForwardIterator1, class ForwardIterator2, class BinaryOperation, class T>
   >       ForwardIterator2 inclusive_scan(ExecutionPolicy&& exec,
   >                                       ForwardIterator1 first, ForwardIterator1 last,
   >                                       ForwardIterator2 result, BinaryOperation binary_op, T init);

   > > -?- Let `U` be the value type of `decltype(first)`.

   > > -3- Requires:

   > > 1. (3.1) — If `init` is provided, `T` shall be Cpp17MoveConstructible (Table 26); otherwise, `ForwardIterator1`'s value type `U` shall be Cpp17MoveConstructible.

   > > 2. (3.2) — If `init` is provided, all of `binary_op(init, init)`, `binary_op(init, *first)`, and `binary_op(*first, *first)` shall be convertible to `T`; otherwise, `binary_op(*first, *first)` shall be convertible to `ForwardIterator1`'s value type `U`.

   > > 3. (3.3) — `binary_op` shall neither invalidate iterators or subranges, nor modify elements in the ranges `[first, last]` or `[result, result + (last - first)]`.

2. Modify 26.10.11 [transform.inclusive.scan] as indicated:

   >     template<class InputIterator, class OutputIterator,
   >              class BinaryOperation, class UnaryOperation>
   >       OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last,
   >                                               OutputIterator result,
   >                                               BinaryOperation binary_op, UnaryOperation unary_op);
   >     template<class ExecutionPolicy,
   >              class ForwardIterator1, class ForwardIterator2,
   >              class BinaryOperation, class UnaryOperation>
   >       ForwardIterator2 transform_inclusive_scan(ExecutionPolicy&& exec,
   >                                                 ForwardIterator1 first, ForwardIterator1 last,
   >                                                 ForwardIterator2 result,
   >                                                 BinaryOperation binary_op, UnaryOperation unary_op);
   >     template<class InputIterator, class OutputIterator,
   >              class BinaryOperation, class UnaryOperation, class T>
   >       OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last,
   >                                               OutputIterator result,
   >                                               BinaryOperation binary_op, UnaryOperation unary_op,
   >                                               T init);
   >     template<class ExecutionPolicy,
   >              class ForwardIterator1, class ForwardIterator2,
   >              class BinaryOperation, class UnaryOperation, class T>
   >       ForwardIterator2 transform_inclusive_scan(ExecutionPolicy&& exec,
   >                                                 ForwardIterator1 first, ForwardIterator1 last,
   >                                                 ForwardIterator2 result,
   >                                                 BinaryOperation binary_op, UnaryOperation unary_op,
   >                                                 T init);

   > > -?- Let `U` be the value type of `decltype(first)`.

   > > -1- Requires:

   > > 1. (1.1) — If `init` is provided, `T` shall be Cpp17MoveConstructible (Table 26); otherwise, `ForwardIterator1`'s value type `U` shall be Cpp17MoveConstructible.

   > > 2. (1.2) — If `init` is provided, all of

   > >    1. (1.2.1) — `binary_op(init, init)`,

   > >    2. (1.2.2) — `binary_op(init, unary_op(*first))`, and

   > >    3. (1.2.3) — `binary_op(unary_op(*first), unary_op(*first))`

   > >    shall be convertible to `T`; otherwise, `binary_op(unary_op(*first), unary_op(*first))` shall be convertible to `ForwardIterator1`'s value type `U`.

   > > 3. (1.3) — Neither `unary_op` nor `binary_op` shall invalidate iterators or subranges, nor modify elements in the ranges `[first, last]` or `[result, result + (last - first)]`.

### 3223(i). `lerp` should not add the "sufficient additional overloads"

Section: 29.7.1 [cmath.syn] Status: Resolved Submitter: Billy O'Neal III Opened: 2019-06-20 Last modified: 2023-02-07

Priority: 2

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Discussion:

It seems that `lerp` can't give a sensible answer for integer inputs. It's new machinery, not the legacy C stuff extended with extra overloads.

I would prefer to fix this by explicitly annotating the functions that are supposed to get additional overloads because we keep getting this wrong, but we could also exempt `lerp` in 29.7.1 [cmath.syn] p2.

[2019-07 Issue Prioritization]

Priority to 2 after discussion on the reflector.

[2023-02-07 Status changed: New → Resolved.]

Resolved by the application of P1467R9. It's now clear that `lerp` should have "sufficient additional overloads".

Proposed resolution:

### 3224(i). `zoned_time` constructor from `TimeZonePtr` does not specify initialization of `tp_`

Section: 30.11.7.2 [time.zone.zonedtime.ctor] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-20 Last modified: 2021-02-25

Priority: 0

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Discussion:

The `zoned_time(TimeZonePtr z)` does not specify how the `tp_` sub-element is initialized. It should be consistent with `zoned_time(string_view)` that default initializes `tp_`.

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4820.

1. Modify 30.11.7.2 [time.zone.zonedtime.ctor] as indicated:

   >     explicit zoned_time(TimeZonePtr z);

   > > -5- Requires: `z` refers to a time zone.

   > > -6- Effects: Constructs a `zoned_time` by initializing `zone_` with `std::move(z)` and default constructing `tp_`.

### 3225(i). `zoned_time` converting constructor shall not be `noexcept`

Section: 30.11.7.2 [time.zone.zonedtime.ctor] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-20 Last modified: 2021-02-25

Priority: 0

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Discussion:

The `zoned_time` constructor from `zoned_time<Duration2, TimeZonePtr>` (preserving same time zone, different precision of representation) is currently marked `noexcept`. Given that the exposition-only member `tp_` of type `sys_time<duration>` has essentially type `time_point<system_clock, Duration>`, this is incompatible with the invoked `time_point` constructor, which is not marked as `noexcept`.

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4820.

1. Modify 30.11.7.1 [time.zone.zonedtime.overview], class template `zoned_time` synopsis, as indicated:

   >     template<class Duration2>
   >       zoned_time(const zoned_time<Duration2, TimeZonePtr>& zt) noexcept;

2. Modify 30.11.7.2 [time.zone.zonedtime.ctor] as indicated:

   >     template<class Duration2>
   >       zoned_time(const zoned_time<Duration2, TimeZonePtr>& y) noexcept;

   > > -9- Requires: Does not participate in overload resolution unless `sys_time<Duration2>` is implicitly convertible to `sys_time<Duration>`.

   > > -10- Effects: Constructs a `zoned_time` by initializing `zone_` with `y.zone_` and `tp_` with `y.tp_`.

### 3226(i). `zoned_time` constructor from `string_view` should accept `zoned_time<Duration2, TimeZonePtr2>`

Section: 30.11.7.2 [time.zone.zonedtime.ctor] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-23 Last modified: 2021-02-25

Priority: 2

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Discussion:

`zoned_time` constructors with `string_view` and another `zoned_time` are not accepting `zoned_time` instances that use a different time zone representation (`TimeZonePtr` parameter):

>     zoned_time(string_view name, const zoned_time<Duration>& zt);
>     zoned_time(string_view name, const zoned_time<Duration>& zt, choose);

This makes them inconsistent with the constructors from `TimeZonePtr` and `zoned_time`, that they delegate to:

>     template<class Duration2, class TimeZonePtr2>
>       zoned_time(TimeZonePtr z, const zoned_time<Duration2, TimeZonePtr2>& zt);
>     template<class Duration2, class TimeZonePtr2>
>       zoned_time(TimeZonePtr z, const zoned_time<Duration2, TimeZonePtr2>& zt, choose);

Furthermore it forces the creation of a temporary `zoned_time` object in case when the source uses the same `TimeZonePtr`, but different `Duration`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4820.

> 1. Modify 30.11.7.1 [time.zone.zonedtime.overview], class template `zoned_time` synopsis, as indicated:

>    >     template<class Duration2, class TimeZonePtr2>
>    >     zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& zt);
>    >     template<class Duration2, class TimeZonePtr2>
>    >     zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& zt, choose);

> 2. Modify 30.11.7.2 [time.zone.zonedtime.ctor] as indicated:

>    >     template<class Duration2, class TimeZonePtr2>
>    >     zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y);

>    > > -32- Remarks: This constructor does not participate in overload resolution unless `zoned_time` is constructible from the return type of `traits::locate_zone(name)` and `zoned_time<Duration2, TimeZonePtr2>`.

>    > > -33- Effects: Equivalent to construction with `{traits::locate_zone(name), y}`.

>    >     template<class Duration2, class TimeZonePtr2>
>    >     zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y, choose c););

>    > > -34- Remarks: This constructor does not participate in overload resolution unless `zoned_time` is constructible from the return type of `traits::locate_zone(name)`, `zoned_time<Duration2, TimeZonePtr2>`, and `choose`.

>    > > -35- Effects: Equivalent to construction with `{traits::locate_zone(name), y, c}`.

>    > > -36- [Note: The `choose` parameter has no effect. — end note]

[2020-02-13, Prague]

During LWG discussions it was suggested to rebase the wording to reduce the chances for confusion.

[2020-02 Status to Immediate on Thursday morning in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 30.11.7.1 [time.zone.zonedtime.overview], class template `zoned_time` synopsis, as indicated:

   >     template<class Duration2, class TimeZonePtr2>
   >     zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& zt);
   >     template<class Duration2, class TimeZonePtr2>
   >     zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& zt, choose);

2. Modify 30.11.7.2 [time.zone.zonedtime.ctor] as indicated:

   >     template<class Duration2, class TimeZonePtr2>
   >     zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y);

   > > -32- Constraints: `zoned_time` is constructible from the return type of `traits::locate_zone(name)` and `zoned_time<Duration2, TimeZonePtr2>`.

   > > -33- Effects: Equivalent to construction with `{traits::locate_zone(name), y}`.

   >     template<class Duration2, class TimeZonePtr2>
   >     zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y, choose c););

   > > -34- Constraints: `zoned_time` is constructible from the return type of `traits::locate_zone(name)`, `zoned_time<Duration2, TimeZonePtr2>`, and `choose`.

   > > -35- Effects: Equivalent to construction with `{traits::locate_zone(name), y, c}`.

   > > -36- [Note: The `choose` parameter has no effect. — end note]

### 3228(i). Surprising `variant` construction

Section: 22.6.3.2 [variant.ctor] Status: Resolved Submitter: Barry Revzin Opened: 2019-06-25 Last modified: 2020-11-09

Priority: 2

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Discussion:

User mcencora on reddit today posted this example:

>     #include <variant>

>     struct ConvertibleToBool
>     {
>       constexpr operator bool() const { return true; }
>     };

>     static_assert(std::holds_alternative<bool>(std::variant<int, bool>(ConvertibleToBool{})));

Before P0608, the `variant` holds `bool`. After P0608, the `variant` holds `int` so the static assertion fires.

I don't know what the right answer is between (a) ill-formed (b) hold `bool` and (c) hold `int` is, but I think (a) and (b) are better options than (c).

[2019-07 Issue Prioritization]

Priority to 2 after discussion on the reflector.

[This is US212; status set to "LEWG" for guidance on desired behavior.]

[2020-05-28; LEWG issue reviewing]

P1957R2 was accepted in Prague as CWG motion 5 and resolves LWG 3228.

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Rationale:

Resolved by P1957R2.

Proposed resolution:

### 3230(i). Format specifier `%y/%Y` is missing locale alternative versions

Section: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-29 Last modified: 2021-02-25

Priority: 0

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Discussion:

The year format specifier `('y', 'Y')` are missing the locale alternative version `('%EY', '%Ey' and '%Oy')`. That makes it inconsistent with the POSIX `strftime` specification:

1. `%Ey`Replaced by the offset from `%EC` (year only) in the locale's alternative representation.

2. `%EY`Replaced by the full alternative year representation.

3. `%Oy`Replaced by the year (offset from `%C`) using the locale's alternative numeric symbols.

and `parse` specifiers 30.13 [time.parse] that accepts these modified command.

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4820.

> [Drafting note: For the `'%Oy'` specifier we preserve consistency with the current specification for `'%Od'` and `'%Oe'` from Table 87 "Meaning of `format` conversion specifier" [tab:time.format.spec]:

> 1. `%d`[…] The modified command `%Od` produces the locale's alternative representation.

> 2. `%e`[…] The modified command `%Oe` produces the locale's alternative representation.

> as their corresponding POSIX specification is matching one for `'%Oy'`:

> 1. `%Od`Replaced by the day of the month, using the locale's alternative numeric symbols, filled as needed with leading zeros if there is any alternative symbol for zero; otherwise, with leading `<space>` characters.

> 2. `%Oe`Replaced by the day of the month, using the locale's alternative numeric symbols, filled as needed with leading `<space>` characters.

> ]

1. Modify "Table 87 — Meaning of `format` conversion specifiers" [tab:time.format.spec] as indicated:

   > Table 87 — Meaning of `format` conversion specifiers [tab:time.format.spec]
   > Specifier | Replacement
   > --- | ---
   > `[…]`
   > `%y` | The last two decimal digits of the year. If the result is a single digit it is prefixed by `0`. The modified command `%Oy` produces the locale's alternative representation. The modified command `%Ey` produces the locale's alternative representation of offset from `%EC` (year only).
   > `%Y` | The year as a decimal number. If the result is less than four digits it is left-padded with `0` to four digits. The modified command `%EY` produces the locale's alternative full year representation.
   > `[…]`

### 3231(i). `year_month_day_last::day` specification does not cover `!ok()` values

Section: 30.8.15.2 [time.cal.ymdlast.members] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-29 Last modified: 2021-02-25

Priority: 0

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Discussion:

The current specification of the `year_month_day_last::day` function does not cover the behaviour in the situation when `year_month_day_last` value is not `ok()`. To illustrate the sentence from 30.8.15.2 [time.cal.ymdlast.members] p13:

> A `day` representing the last day of the (`year`, `month`) pair represented by `*this`.

is unclear in the situation when `month` member has `!ok` value, e.g. what is last day of 14th month of 2019.

The proposed resolution makes the value of `ymdl.day()` (and by consequence conversion to `sys_days`/`local_days`) unspecified if `ymdl.ok()` is `false`. This make is consistent with rest of the library, that produces unspecified values in similiar situation, e.g.: 30.8.14.2 [time.cal.ymd.members] p18, 30.8.16.2 [time.cal.ymwd.members] p19, 30.8.17.2 [time.cal.ymwdlast.members] p14.

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4820.

1. Modify 30.8.15.2 [time.cal.ymdlast.members] as indicated:

   >     constexpr chrono::day day() const noexcept;

   > > -13- Returns: If `ok()` is `true`, returns aA `day` representing the last day of the (`year, month`) pair represented by `*this`. Otherwise the returned value is unspecified.

   > > -14- [Note: This value may be computed on demand. — end note]

### 3232(i). Inconsistency in `zoned_time` deduction guides

Section: 30.11.7.1 [time.zone.zonedtime.overview] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-30 Last modified: 2021-02-25

Priority: 0

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Discussion:

Currently, the `time_zone` is always storing the time with the precision not coarser that `seconds`, as consequence any instance with the duration with coarser precision with `seconds`, is de-facto equivalent to one instantiated to the duration of seconds. This is caused by the fact, that time zone offset can be defined up to seconds precision. To illustrate both of following specialization has the same behavior (keep `seconds`):

>     zoned_time<minutes> zt(current_zone(), floor<minutes>(system_clock::now());
>     zoned_time<hours>   zt(current_zone(), floor<hours>(system_clock::now());
>     zoned_time<seconds> zt(current_zone(), floor<seconds>(system_clock::now());

To avoid unnecessary code bloat caused by above, most deduction guides are instantiating `zoned_time` with at least `seconds` precision (i.e. `zoned_time<seconds>` will be deduced for all of above):

>     template<class TimeZonePtr, class Duration>
>       zoned_time(TimeZonePtr, zoned_time<Duration>, choose = choose::earliest)
>         -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;

However there is single exception:

>     template<class Duration, class TimeZonePtr, class TimeZonePtr2>
>       zoned_time(TimeZonePtr, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest)
>          -> zoned_time<Duration, TimeZonePtr>;

This deduction guide should be updated to preserve the consistency of design.

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4820.

1. Modify 30.11.7.1 [time.zone.zonedtime.overview] as indicated:

   >     […]
   >     template<class Duration, class TimeZonePtr, class TimeZonePtr2>
   >       zoned_time(TimeZonePtr, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest)
   >         -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;
   >     […]

### 3233(i). Broken requirements for `shared_ptr` converting constructors

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++20 Submitter: Casey Carter Opened: 2019-07-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

Issue 2875(i) added 20.3.2.2.2 [util.smartptr.shared.const] paragraph 13:

> Remarks: When `T` is an array type, this constructor shall not participate in overload resolution unless `is_move_constructible_v<D>` is `true`, the expression `d(p)` is well-formed, and either `T` is `U[N]` and `Y(*)[N]` is convertible to `T*`, or `T` is `U[]` and `Y(*)[]` is convertible to `T*`. When `T` is not an array type, this constructor shall not participate in overload resolution unless `is_move_constructible_v<D>` is `true`, the expression `d(p)` is well-formed, and `Y*` is convertible to `T*`.

which pertains to the four constructor overloads:

>     template<class Y, class D> shared_ptr(Y* p, D d);
>     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
>     template<class D> shared_ptr(nullptr_t p, D d);
>     template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

which is fine (ignoring for now that two occurrences of "this constructor" should read "these constructors") for the two overloads with a template parameter `Y`, but not so much for the two with no such template parameter.

MSVC ignores the constraints on `Y` for the overloads with no such template parameter, whereas libstdc++ and libc++ seem to ignore all of the constraints for those overloads (See Compiler Explorer). We should fix the broken wording, ideally by requiring the MSVC interpretation - the `nullptr_t` constructors participate in overload resolution only when `is_movable_v<D>` is `true` and `d(p)` is well-formed - so concepts and traits that check constructibility are correct.

[2019-11-17 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4835.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   > >     template<class Y, class D> shared_ptr(Y* p, D d);
   > >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
   > >     template<class D> shared_ptr(nullptr_t p, D d);
   > >     template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

   > -?- Constraints: `is_move_constructible_v<D>` is `true`, and `d(p)` is a well-formed expression. For the first two overloads:

   > - If `T` is an array type, then either `T` is `U[N]` and `Y(*)[N]` is convertible to `T*`, or `T` is `U[]` and `Y(*)[]` is convertible to `T*`.

   > - If `T` is not an array type, then `Y*` is convertible to `T*`.

   > -9- RequiresExpects: Construction of `d` and a deleter of type […]

   > […]

   > -13- Remarks: When `T` is an array type, this constructor shall not participate in overload resolution unless `is_move_constructible_v<D>` is `true`, the expression `d(p)` is well-formed, and either `T` is `U[N]` and `Y(*)[N]` is convertible to `T*`, or `T` is `U[]` and `Y(*)[]` is convertible to `T*`. When `T` is not an array type, this constructor shall not participate in overload resolution unless `is_move_constructible_v<D>` is `true`, the expression `d(p)` is well-formed, and `Y*` is convertible to `T*`.

### 3234(i). Sufficient Additional Special Math Overloads

Section: 29.7.1 [cmath.syn] Status: Resolved Submitter: Casey Carter Opened: 2019-07-11 Last modified: 2023-02-07

Priority: 3

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Discussion:

The "sufficient additional overloads" wording in 29.7.1 [cmath.syn] paragraph 2 does not apply to the special math functions, since they are not "overloaded functions". The lack of "sufficient additional overloads" doesn't agree with N3060 (the final draft of ISO/IEC 29124 which standardized the mathematical special functions) [sf.cmath] paragraphs 3 and 4:

> -3- Each of the functions specified above that has one or more `double` parameters (the `double` version) shall have two additional overloads:

> 1. a version with each `double` parameter replaced with a `float` parameter (the `float` version), and

> 2. a version with each `double` parameter replaced with a `long double` parameter (the `long double` version).

> The return type of each such `float` version shall be `float`, and the return type of each such `long double` version shall be `long double`.

> -4- Moreover, each `double` version shall have sufficient additional overloads to determine which of the above three versions to actually call, by the following ordered set of rules:

> 1. First, if any argument corresponding to a `double` parameter in the `double` version has type `long double`, the `long double` version is called.

> 2. Otherwise, if any argument corresponding to a `double` parameter in the `double` version has type `double` or has an integer type, the `double` version is called.

> 3. Otherwise, the `float` version is called.

P226R1 "Mathematical Special Functions for C++17" notably states: "At the level of and following [c.math], create a new subclause with heading and initial content the same as IS 29124:2010's clause [sf.cmath], 'Additions to header `<cmath>`,' renumbering as appropriate to the new context." which suggests the change between 29124 and C++17 was an oversight and not intentional.

Notably there is implementation divergence: MSVC implements precisely the wording in C++17, whereas libstdc++ provides additional overloads as specified in 29124, so they disagree e.g. on whether `std::sph_neumann({}, {})` is well-formed.

[2020-04-07 Issue Prioritization]

Priority to 3 after reflector discussion.

[2023-02-07 Status changed: New → Resolved.]

Resolved by the application of P1467R9. It's now clear that the special functions should have "sufficient additional overloads".

Proposed resolution:

### 3235(i). `parse` manipulator without abbreviation is not callable

Section: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-07-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

The `parse` overload that does not accept the abbreviation but does accept an offset, because the expression in the Remarks: clause:

>     from_stream(declval<basic_istream<charT, traits>*>(), fmt.c_str(), tp, nullptr, &offset)

is not valid. This is caused by deduction failure for the `basic_string<charT, traits, Alloc>*` from `nullptr` (see this link):

[2019-08-17 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

> [Drafting note: As a drive-by fix the Remarks element is also converted to a Constraints element.]

1. Modify 30.13 [time.parse] as indicated:

   > >     template<class charT, class traits, class Alloc, class Parsable>
   > >       unspecified
   > >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   > >               minutes& offset);

   > -6- RemarksConstraints: This function shall not participate in overload resolution unlessThe expression

   > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp,
   > >       declval<basic_string<charT, traits, Alloc>*>()nullptr, &offset)

   > is a validwell-formed expressionwhen treated as an unevaluated operand.

   > -7- Returns: A manipulator that, when extracted from a `basic_istream<charT, traits> is`, calls `from_stream(is, fmt.c_str(), tp, static_cast<basic_string<charT, traits, Alloc>*>(nullptr), &offset)`.

### 3236(i). Random access iterator requirements lack limiting relational operators domain to comparing those from the same range

Section: 24.3.5.7 [random.access.iterators] Status: C++23 Submitter: Peter Sommerlad Opened: 2019-07-15 Last modified: 2023-11-22

Priority: 3

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Discussion:

For forward iterators we have very clear wording regarding the restricted domain of `operator==` in 24.3.5.5 [forward.iterators] p2:

> The domain of `==` for forward iterators is that of iterators over the same underlying sequence. However, value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type. [Note: Value-initialized iterators behave as if they refer past the end of the same empty sequence. — end note]

But for the relational operators of random access iterators specified in 24.3.5.7 [random.access.iterators], Table [tab:randomaccessiterator], no such domain constraints are clearly defined, except that we can infer that they are similarly constrained as the difference of the compared iterators by means of the operational semantics of `operator<`.

[2019-07-29; Casey comments and provides wording]

Change the "Operational Semantics" column of the "`a < b`" row of [tab:randomaccessiterator] to "Effects: Equivalent to: `return` `b - a > 0`;

It then follows that `a < b` is required to be well-defined over the domain for which `b - a` is required to be well-defined, which is the set of pairs `(x, y)` such that there exists a value `n` of type difference_type such that `x + n == b`.

[2020-02-13, Prague]

P3, but some hesitation to make it Immediate, therefore moving to Ready.

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 24.3.5.7 [random.access.iterators] as indicated:

   > Table 87: Cpp17RandomAccessIterator requirements (in addition to Cpp17BidirectionalIterator) [tab:randomaccessiterator]
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
   > --- | --- | --- | ---
   > `[…]`
   > `a < b` | contextually convertible to `bool` | Effects: Equivalent to: `return` `b - a > 0;` | `<` is a total ordering relation

### 3237(i). LWG 3038 and 3190 have inconsistent PRs

Section: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Casey Carter Opened: 2019-07-18 Last modified: 2021-02-25

Priority: 2

View all other issues in [mem.poly.allocator.mem].

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Discussion:

Both LWG 3038(i) and LWG 3190(i) deal with how to respond to requests to allocate "`n * sizeof(T)`" bytes of memory when `n * sizeof(T)` is not sufficient storage for `n` objects of type `T`, i.e., when `n > SIZE_MAX / sizeof(T)`. LWG 3038(i) changed `polymorphic_allocator::allocate` to throw `length_error` upon detecting this condition, whereas LWG 3190(i) changed `allocator::allocate` to throw `bad_array_new_length`. It's peculiar that two standard library components which allocate memory both detect this condition but handle it by throwing different exception types; for consistency, the two should be harmonized.

Reflector discussion of 3190 seemed to achieve consensus that `bad_array_new_length` was the better option. Unlike `length_error`, `bad_array_new_length` derives from `bad_alloc` so we can make this change without altering the invariant that allocation functions either succeed or throw an exception derived from `bad_alloc`.

Further, P0339R6 "`polymorphic_allocator<>` as a vocabulary type" recently added the function template "`template<class T> T* allocate_object(size_t n = 1);`" to `std::pmr::polymorphic_allocator`, which is another instance of the "allocate memory for `n` objects of type `T`" pattern. 20.5.3.3 [mem.poly.allocator.mem] paragraph 8.1 specifies that `allocate_object` throws `length_error` when `SIZE_MAX / sizeof(T) < n`, presumably for consistency with `std::pmr::polymorphic_allocator::allocate` specified in paragraph 1. `allocate_object`'s behavior should be consistent with `allocator::allocate` and `polymorphic_allocator::allocate` so we have a single means of communicating "request for allocation of unrepresentable size" errors in the Standard Library.

[2020-02 Moved to Immediate on Thursday afternoon in Prague.]

Proposed resolution:

This wording is relative to N4820.

1. Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:

   >     [[nodiscard]] Tp* allocate(size_t n);

   > > -1- Effects: If `SIZE_MAX / sizeof(Tp) < n`, throws `length_errorbad_array_new_length`. Otherwise equivalent to:

   > > >     return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));

   > […]

   >     template<class T>
   >       T* allocate_object(size_t n = 1);

   > > -8- Effects: Allocates memory suitable for holding an array of `n` objects of type `T`, as follows:

   > > 1. (8.1) — if `SIZE_MAX / sizeof(T) < n`, throws `length_errorbad_array_new_length`,

   > > 2. (8.2) — otherwise equivalent to:

   > >    >     return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));

### 3238(i). Insufficiently-defined behavior of `std::function` deduction guides

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++20 Submitter: Louis Dionne Opened: 2019-07-17 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [func.wrap.func.con].

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Discussion:

The following code is currently undefined behavior:

>     #include <functional>

>     struct R { };
>     struct f0 { R operator()() && { return {}; } };

>     int main() { std::function f = f0{}; }

The reason is that 22.10.17.3.2 [func.wrap.func.con]/12 says:

> This deduction guide participates in overload resolution only if `&F::operator()` is well-formed when treated as an unevaluated operand. In that case, if `decltype(&F::operator())` is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G`, then the deduced type is `function<R(A...)>`.

However, it does not define the behavior when `&F::operator()` is well-formed but not of the required form (in the above example it's of the form `R(G::*)(A...) &&`, which is rvalue-reference qualified instead of optionally-lvalue-reference qualified). libc++'s implementation of the deduction guide SFINAE's out when either `&F::operator()` is not well-formed, or it is not of the required form. It seems like mandating that behavior in the Standard is the way to go.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4820.

> 1. Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:

>    >     template<class F> function(F) -> function<see below>;

>    > > -12- Remarks: This deduction guide participates in overload resolution only if `&F::operator()` is well-formed when treated as an unevaluated operand, and its type is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G` and a sequence of types `A...`. In that case, if `decltype(&F::operator())` is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G`, then the deduced type is `function<R(A...)>`.

[2020-02-13; Prague]

LWG improves wording matching Marshall's Mandating paper.

[Status to Immediate on Friday in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:

   > [Drafting note: This edit should be used instead of the corresponding edit in P1460]

   >     template<class F> function(F) -> function<see below>;

   > > -?- Constraints: `&F::operator()` is well-formed when treated as an unevaluated operand and `decltype(&F::operator())` is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G`.

   > > -12- Remarks: This deduction guide participates in overload resolution only if `&F::operator()` is well-formed when treated as an unevaluated operand. In that case, if `decltype(&F::operator())` is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G`, then tThe deduced type is `function<R(A...)>`.

### 3239(i). Hidden friends should be specified more narrowly

Section: 17.12.2.2 [cmp.partialord], 17.12.2.3 [cmp.weakord], 17.12.2.4 [cmp.strongord], 31.12.6 [fs.class.path], 23.2.5.1 [container.node.overview] Status: Resolved Submitter: Daniel Sunderland Opened: 2019-07-19 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

LWG has been moving papers which use hidden friends to restrict an entity's lookup to be via ADL only. However, the current wording does not prevent an implementation from making these entities from also being available via (un)qualified lookup.

Walter Brown and I have an unreviewed paper (P1601R0) in the Post Kona mailing specifying that entities which are intended to be found via ADL only include a Remarks element which states something equivalent to the following:

"Remarks: This function is to be found via argument-dependent lookup only."

Adding this element after the draft ships will be a semantic change. Marshall suggested that I file an LWG issue to add/modify Remarks elements where needed to prevent (un)qualified lookup.

The following stable names are places in the pre Cologne draft (N4820) which potentially use hidden friends. Furthermore, there are papers that LWG added to the straw polls which also potentially use hidden friends. LWG should review each of these subsections/papers to determine if they should include the text equivalent to the Remarks above. [Note: Not all these subsections should restrict lookup to ADL only. It is very likely that I missed a paper or subsection — end note].

- [cmp.weakeq]: Comparisons

- [cmp.strongeq]: Comparisons

- 17.12.2.2 [cmp.partialord]: Comparisons

- 17.12.2.3 [cmp.weakord]: Comparisons

- 17.12.2.4 [cmp.strongord]: Comparisons

- 31.12.6 [fs.class.path]: `operator<<`, `operator>>`

- 23.2.5.1 [container.node.overview]: `swap`

- P0660R10 (Stop Token and Joining Thread): Comparisons, `swap`

- P1614R2 (The Mothership has Landed): Comparisons

[2019-11-17; Daniel comments]

The acceptance of P1965R0 at the Belfast 2019 meeting should resolve this issue.

[2019-11; Resolved by the adoption of P1965 in Belfast]

Proposed resolution:

### 3241(i). `chrono-spec` grammar ambiguity in §[time.format]

Section: 30.12 [time.format] Status: C++20 Submitter: Victor Zverovich Opened: 2019-07-24 Last modified: 2021-02-25

Priority: 0

View other active issues in [time.format].

View all other issues in [time.format].

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Discussion:

The `chrono-spec` grammar introduced by P1361R2 "Integration of chrono with text formatting" in section [time.format] is ambiguous because `'%'` can be interpreted as either `literal-char` or `conversion-spec`:

>     chrono-spec     ::= literal-char | conversion-spec
>     literal-char    ::= <a character other than '{' or '}'>
>     conversion-spec ::= '%' [modifier] type

[2019-08-17 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify the `literal-char` grammar in 30.12 [time.format] as indicated:

   >     literal-char:
   >               any character other than {, or }, or %

### 3242(i). `std::format`: missing rules for `arg-id` in `width` and `precision`

Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Richard Smith Opened: 2019-07-31 Last modified: 2021-02-25

Priority: 1

View other active issues in [format.string.std].

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Discussion:

According to 28.5.2 [format.string] we have:

> If the numeric `arg-id`s in a format string are `0, 1, 2, ...` in sequence, they can all be omitted (not just some) and the numbers `0, 1, 2, ...` will be automatically used in that order. A format string does not contain a mixture of automatic and manual indexing.

… but what does that mean in the presence of `arg-id` in `width` and `precision`? Can one replace

>     "{0:{1}} {2}"

with

>     "{:{}} {}"

? The grammar says the answer is no, because the `arg-id` in `width` is not optional, but the normative wording says the answer is yes.

Victor Zverovich:

That's a bug in the grammar. The `arg-id` should be optional in `width` and `precision`:

>     width     ::= nonzero-digit [integer] | '{' [arg-id] '}'
>     precision ::= integer | '{' [arg-id] '}'

[2020-02 Status to Immediate on Thursday morning in Prague.]

Proposed resolution:

This wording is relative to N4830.

1. Modify the `width` and `precision` grammar in the syntax of format specifications of 28.5.2.2 [format.string.std] as indicated:

   > >     […]
   > >     width:
   > >              positive-integer
   > >              { arg-idopt }
   > >     precision:
   > >              . nonnegative-integer
   > >              . { arg-idopt }
   > >     […]

2. Modify 28.5.2.2 [format.string.std] as indicated:

   > -6- The # option causes […]

   > -?- If `{ arg-idopt }` is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integral type, or its value is negative for precision or non-positive for width, an exception of type `format_error` is thrown.

   > -7- The positive-integer in width is a decimal integer defining the minimum field width. If width is not specified, there is no minimum field width, and the field width is determined based on the content of the field.

### 3243(i). `std::format` and negative zeroes

Section: 28.5.2 [format.string] Status: C++20 Submitter: Richard Smith Opened: 2019-07-31 Last modified: 2021-02-25

Priority: 2

View all other issues in [format.string].

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Discussion:

What are these:

>     std::format("{}", -0.0);
>     std::format("{:+}", -0.0);
>     std::format("{:-}", -0.0);
>     std::format("{: }", -0.0);

with

>     "{:{}} {}"

A negative zero is not a negative number, so I think the answer for an implementation that supports signed zeroes is `"0"`, `"-0"`, `"0"`, `" 0"`. Is that the intent? (Note that this doesn't satisfy `to_chars`' round-trip guarantee.)

Or should the behavior actually be that `"+"` means "insert a leading `+` if `to_chars`' output does not start with `-"` and `" "` actually means "insert a leading space if `to_chars`' output does not start with `-"` (that is, the same as `"%+f"` and `"% f"`) — so that the result of all four calls is `"-0"`?

Victor Zverovich:

The latter. `std::format` is specified in terms of `to_chars` and the intent is to have similar round trip guarantee here, so the result should be `"-0"`, `"-0"`, `"-0"`, `"-0"`. This has also been extensively discussed in LEWG and the outcome of that discussion was to print `'-'` for negative zeros.

So I think it should be clarified that `'-'` and space apply to negative numbers and negative zero.

[2019-08-17 Priority set to 2 based on reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:

>    > Table 59: Meaning of sign options [tab:format.sign]
>    > Option | Meaning
>    > --- | ---
>    > `'+'` | Indicates that a sign should be used for both non-negative and negative numbers.
>    > `'-'` | Indicates that a sign should be used only for negative numbers and negative zero (this is the default behavior).
>    > space | Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.

[2020-02-13, Prague]

Rebased and some wording finetuning by LWG.

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:

   > Table 58: Meaning of sign options [tab:format.sign]
   > Option | Meaning
   > --- | ---
   > `'+'` | Indicates that a sign should be used for both non-negative and negative numbers.
   > `'-'` | Indicates that a sign should be used only for negative numbers and negative zero only (this is the default behavior).
   > space | Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.

### 3244(i). Constraints for `Source` in §[fs.path.req] insufficiently constrainty

Section: 31.12.6.4 [fs.path.req] Status: C++20 Submitter: Casey Carter Opened: 2019-08-02 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

`std::filesystem::path` has a number of functions - notably including a conversion constructor template (31.12.6.5.1 [fs.path.construct]) and assignment operator template (31.12.6.5.2 [fs.path.assign]) - that accept `const Source&`. Per 31.12.6.4 [fs.path.req] paragraph 2:

> -2- Functions taking template parameters named `Source` shall not participate in overload resolution unless either

> (2.1) — `Source` is a specialization of `basic_string` or `basic_string_view`, or

> (2.2) — the qualified-id `iterator_traits<decay_t<Source>>::value_type` is valid and denotes a possibly `const` encoded character type.

`iterator_traits<decay_t<path>>::value_type` is not valid in C++17, so this specification was sufficient to guard against the conversion constructor template (respectively assignment operator template) "pretending" to be copy constructor (respectively copy assignment operator). P0896R4 "The One Ranges Proposal", however, altered the definition of `iterator_traits` in the working draft. It now has some convenient default behaviors for types that meet (roughly) the syntax of the `Cpp17InputIterator` requirements. Notably those requirements include copy construction and copy assignment.

In the working draft, to determine the copyability of `std::filesystem::path` we must perform overload resolution to determine if we can initialize a `path` from a constant lvalue of type `path`. The conversion constructor template that accepts `const Source&` is a candidate, since its second argument is defaulted, so we must perform template argument deduction to see if this constructor is viable. `Source` is deduced to `path` and we then must check the constraint from 31.12.6.4 [fs.path.req] paragraph 2.2 (above). Checking the constraint requires us to specialize `iterator_traits<path>`, which (per 24.3.2.3 [iterator.traits] paragraph 3.2) requires us to determine if `path` satisfies the exposition-only `cpp17-input-iterator` concept, which requires `path` to be copyable.

We've completed a cycle: determining if `path` is copyable requires us to first determine if `path` is copyable. This unfortunate constraint recursion can be broken by explicitly specifying that `path` is not a valid `Source`.

[2019-08-17 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 31.12.6.4 [fs.path.req] as indicated:

   > -2- Functions taking template parameters named `Source` shall not participate in overload resolution unless `Source` denotes a type other than `path`, and either

   > […]

### 3245(i). Unnecessary restriction on `'%p'` parse specifier

Section: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-07-31 Last modified: 2021-02-25

Priority: 0

View other active issues in [time.parse].

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Discussion:

The current specification for the `'%p'` flag in "[tab:time.parse.spec] Meaning of `parse` flags" places a restriction of it's placement with regards to the `'%I'` command:

> The locale's equivalent of the AM/PM designations associated with a 12-hour clock. The command `%I` must precede `%p` in the format string.

This restriction makes the migration to new API more difficult, as it is not present for the POSIX `strptime` nor in the example implementation of the library. Per Howard's comment:

> Actually this is an obsolete requirement and it should be struck. The first time I implemented this I didn't know how to do it without this requirement. I've since reimplemented it without needing this.

[2019-08-17 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify Table 99 "Meaning of `parse` flags [tab:time.parse.spec]" in 30.13 [time.parse] as indicated:

   > Table 99: Meaning of `parse` flags [tab:time.parse.spec]
   > Flag | Parsed value
   > --- | ---
   > `[…]`
   > `%p` | The locale's equivalent of the AM/PM designations associated with a 12-hour clock. The command `%I` must precede `%p` in the format string.
   > `[…]`

### 3246(i). What are the constraints on the template parameter of `basic_format_arg`?

Section: 28.5.8.1 [format.arg] Status: C++20 Submitter: Richard Smith Opened: 2019-08-01 Last modified: 2021-02-25

Priority: 0

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Discussion:

In P0645R10 20.?.5.1/ we find:

> Constraints: `typename Context::template formatter_type<T>` is enabled.

… which doesn't mean anything, because that is an arbitrary type. Presumably the intent is that that type will always be a specialization of formatter, but there appear to be no constraints whatsoever on the `Context` template parameter, so there seems to be no requirement that that is the case.

Should `basic_format_arg` place some constraints on its `Context` template parameter? E.g., should it be required to be a specialization of `basic_format_context`?

Victor Zverovich:

The intent here is to allow different context types provide their own formatter extension types. The default formatter context and extension are `basic_format_context` and formatter respectively, but it's possible to have other. For example, in the `fmt` library there is a formatter context that supports `printf` formatting for legacy code. It cannot use the default formatter specializations because of the different syntax `(%...` vs `{...})`.

Richard Smith:

In either case, the specification here seems to be missing the rules for what is a valid `Context` parameter.

I'm happy to editorially change "is enabled" to "is an enabled specialization of formatter", since there's nothing else that this could mean, but we still need a wording fix for the broader issue here. Here's what I have so far for this wording:

> Constraints: The template specialization `typename Context::template formatter_type<T>` is an enabled specialization of formatter ([formatter.requirements]).

Tim Song:

I think what we actually want here is "`typename Context::template formatter_type<T>` meets the Formatter requirements".

[2019-08-17 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 28.5.8.1 [format.arg] as indicated:

   >     template<class T> explicit basic_format_arg(const T& v) noexcept;

   > > -4- Constraints: The template specialization

   > > >     typename Context::template formatter_type<T>

   > > is an enabled specialization of `formatter` meets the Formatter requirements (28.5.6 [format.formatter]). The extent to which an implementation determines that the specialization is enabledmeets the Formatter requirements is unspecified, except that as a minimum the expression

   > > >     typename Context::template formatter_type<T>()
   > > >       .format(declval<const T&>(), declval<Context&>())

   > > shall be well-formed when treated as an unevaluated operand.

### 3247(i). `ranges::iter_move` should perform ADL-only lookup of `iter_move`

Section: 24.3.3.1 [iterator.cust.move] Status: C++20 Submitter: Casey Carter Opened: 2019-07-29 Last modified: 2021-02-25

Priority: 2

View all other issues in [iterator.cust.move].

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Discussion:

The specification of the `ranges::iter_move` customization point in [iterator.cust.move] doesn't include a deleted poison pill overload. There is no `std::iter_move` to avoid, so such a poison pill is not needed. This is fine, except that it suggests that unqualified lookup for `iter_move` in the `iter_move(E)` expression in paragraph 1.1 should find declarations of iter_move in the global namespace via normal unqualified lookup, when the design intent is that only ADL be used to find overloads of `iter_move`.

Absent a more idiomatic wording to specify an ADL-only lookup, we can correct the problem by specifying the lookup in paragraph 1.1 is performed in a context that includes the declaration "`void iter_move();`" which has the desired effect.

[2020-02 Status to Immediate on Thursday morning in Prague.]

Proposed resolution:

This wording is relative to N4820.

> [Drafting Note: There's a drive-by fix here to change "valid" — which suggests runtime behavior which cannot be validated at compile time — to "well-formed".]

1. Modify 24.3.3.1 [iterator.cust.move] as indicated:

   > -1- The name `ranges::iter_move` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::iter_move(E)` for some subexpression `E` is expression-equivalent to:

   > 1. (1.1) — `iter_move(E)`, if that expression is validwell-formed when treated as an unevaluated operand, with overload resolution performed in a context that does not include a declaration of `ranges::iter_move`. but does include the declaration:

   >           void iter_move();

   > 2. […]

### 3248(i). `std::format` `#b`, `#B`, `#o`, `#x`, and `#X` presentation types misformat negative numbers

Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Richard Smith Opened: 2019-08-01 Last modified: 2021-02-25

Priority: 2

View other active issues in [format.string.std].

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Discussion:

According to both the specification for `'#'` and the presentation types `b`, `B`, `o`, `x`, and `X` (which redundantly duplicate the rule for the relevant prefixes), the string `0b`, `0B`, `0`, `0x`, or `0X` is prefixed to the result of `to_chars`. That means:

>     std::format("{0:#b} {0:#B} {0:#o} {0:#x} {0:#X}", -1)

produces

>     "0b-1 0B-1 0-1 0x-1 0X-1"

I assume that's a bug?

(Additionally, if the `+` specifier is used, there appears to be no specification of where the sign is inserted into the output.)

Victor Zverovich:

Yes. I suggest replacing

> adds the respective prefix `"0b"` (`"0B"`), `"0"`, or `"0x"` (`"0X"`) to the output value

with something like

> inserts the respective prefix `"0b"` (`"0B"`), `"0"`, or `"0x"` (`"0X"`) to the output value after the sign, if any,

I think the duplicate wording in the presentation types `b`, `B`, `o`, `x`, and `X` can be dropped.

Regarding the `+` specifier problem: How about adding

> The sign is inserted before the output of `to_chars`.

?

[2019-08-21 Priority set to 2 based on reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:

>    > Table 59: Meaning of sign options [tab:format.sign]
>    > Option | Meaning
>    > --- | ---
>    > `'+'` | Indicates that a sign should be used for both non-negative and negative numbers. The sign is inserted before the output of `to_chars`.
>    > `'-'` | Indicates that a sign should be used only for negative numbers (this is the default behavior).
>    > space | Indicates that a leading space should be used for non-negative numbers, and a minus sign for negative numbers.

> 2. Modify [format.string] as indicated:

>    > -6 The `#` option causes the alternate form to be used for the conversion. This option is only valid for arithmetic types other than `charT` and `bool` or when an integer presentation type is specified. For integral types, the alternate form addsinserts the base prefix (if any) specified in Table [tab:format.type.int] to the output value after the sign, if any. […]

[2019-08-21; Victor Zverovich provides improved wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:

>    > Table 59: Meaning of sign options [tab:format.sign]
>    > Option | Meaning
>    > --- | ---
>    > `'+'` | Indicates that a sign should be used for both non-negative and negative numbers. The `+` sign is inserted before the output of `to_chars` for non-negative numbers other than the negative zero. [Note: For negative numbers and the negative zero the output of `to_chars` will already contain the sign so no additional transformation is performed. — end note]
>    > `'-'` | Indicates that a sign should be used only for negative numbers (this is the default behavior).
>    > space | Indicates that a leading space should be used for non-negative numbers, and a minus sign for negative numbers.

> 2. Modify [format.string] as indicated:

>    > -6- The `#` option causes the alternate form to be used for the conversion. This option is only valid for arithmetic types other than `charT` and `bool` or when an integer presentation type is specified. For integral types, the alternate form addsinserts the base prefix (if any) specified in Table [tab:format.type.int] to the output value after the sign character (possibly space) if there is one, or before the output of `to_chars` otherwise. […]

[2020-02-13, Prague]

LWG made some improvements to the wording.

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:

   > Table 58: Meaning of sign options [tab:format.sign]
   > Option | Meaning
   > --- | ---
   > `'+'` | Indicates that a sign should be used for both non-negative and negative numbers. The `+` sign is inserted before the output of `to_chars` for non-negative numbers other than negative zero. [Note: For negative numbers and negative zero the output of `to_chars` will already contain the sign so no additional transformation is performed. — end note]
   > `'-'` | Indicates that a sign should be used only for negative numbers (this is the default behavior).
   > space | Indicates that a leading space should be used for non-negative numbers, and a minus sign for negative numbers.

2. Modify [format.string] as indicated:

   > -6- The `#` option causes the alternate form to be used for the conversion. This option is only valid for arithmetic types other than `charT` and `bool` or when an integer presentation type is specified, and not otherwise. For integral types, the alternate form addsinserts the base prefix (if any) specified in Table [tab:format.type.int] tointo the output value after the sign character (possibly space) if there is one, or before the output of `to_chars` otherwise. […]

### 3249(i). There are no 'pointers' in §[atomics.lockfree]

Section: 32.5.5 [atomics.lockfree] Status: C++23 Submitter: Billy O'Neal III Opened: 2019-08-03 Last modified: 2023-11-22

Priority: 4

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Discussion:

According to SG1 experts, the requirement in [atomics.lockfree]/2 is intended to require that the answer for `is_lock_free()` be the same for a given `T` for a given run of the program. The wording does not achieve that because it's described in terms of 'pointers', but there are no pointers in an `atomic<int>`.

[2020-02 Status to Ready on Thursday morning in Prague.]

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 32.5.5 [atomics.lockfree] as indicated:

   > -2- The functions `atomic<T>::is_lock_free`, and `atomic_is_lock_free` (32.5.8.2 [atomics.types.operations]) indicates whether the object is lock-free. In any given program execution, the result of the lock-free query is the same for all atomic objects shall be consistent for all pointers of the same type.

### 3250(i). `std::format`: `#` (alternate form) for NaN and inf

Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Richard Smith Opened: 2019-08-05 Last modified: 2021-02-25

Priority: 0

View other active issues in [format.string.std].

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Discussion:

We have:

> "For floating-point numbers, the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it."

So does that mean that infinity is formatted as `"inf."` and NaN as `"nan."`? (Or something like that? Where exactly do we add the decimal point in this case?) Or does this affect infinity but not NaN (because we can handwave that a NaN value is not a "floating-point number")?

I suspect that this should only cover finite floating-point numbers.

Victor Zverovich:

Right. So infinity and NaN should be still formatted as `"inf"` and `"nan"` without a decimal point.

[2020-02 Status to Immediate on Thursday morning in Prague.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > -6- […] For floating-point types, the alternate form causes the result of the conversion of finite values to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for `g` and `G` conversions, trailing zeros are not removed from the result.

### 3251(i). Are `std::format` alignment specifiers applied to string arguments?

Section: 28.5.2 [format.string] Status: C++20 Submitter: Richard Smith Opened: 2019-08-02 Last modified: 2021-02-25

Priority: 2

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Discussion:

We are told:

> Formatting of objects of arithmetic types and `const void*` is done as if by calling `to_chars` (unless otherwise specified) and copying the output through the output iterator of the format context with additional padding and adjustments as specified by the format specifiers.

… but there is no corresponding rule for strings. Is an alignment specifier intended to be applied to strings or not? The wording as-is is ambiguous.

(The above also doesn't cover formatting `void*` or `std::nullptr_t`. Presumably at least those two should have the relevant adjustments applied to them!)

The wording never actually anywhere says that the `basic_format_args` are in any way involved in the formatting process, or how formatting actually happens. (The wording doesn't say that `basic_format_arg::handle::format` is ever called, for example.)

Victor Zverovich:

An alignment specifier is intended to be applied to strings as well, `void*` and `std::nullptr_t` are converted into `const void*` when constructing `basic_format_arg`.

The wording for `vformat` and similar functions says that `basic_format_args` is involved:

> Returns: A string object holding the character representation of formatting arguments provided by `args` formatted according to specifications given in `fmt`.

but I admit that it is hand-wavy. Perhaps we could add something along the lines of

> For each replacement field referring to the argument with index `(arg-id) i`, the `basic_format_arg` object referring to the argument is obtained via `args.get(i)` and the `parse` and `format` functions of the `formatter` specialization for the underlying argument type are called to parse the format specification and format the value.

to clarify how we format `args` (`basic_format_args`).

[2019-08-21 Priority set to 2 based on reflector discussion]

[2019-08-21; Victor Zverovich suggests wording]

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > -3- The align specifier applies to all argument types. The meaning of the various alignment options is as specified in Table [tab:format.align]. [Example: […

### 3252(i). Parse locale's aware modifiers for commands are not consistent with POSIX spec

Section: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-08-06 Last modified: 2021-02-25

Priority: 2

View other active issues in [time.parse].

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Discussion:

The current specification of the locale modifiers for the parse flags in "[tab:time.parse.spec] Meaning of `parse` flags" is inconsistent with the POSIX `strptime` specification:

- New flags `%OC`, `%Ou` are added

- Flags `%OI`, `%OU`, `%OW` are missing

Per Howard's comment:

> I only invented in a couple places for these flags, and none of them involved locale-dependent stuff. If we are inconsistent with POSIX/C on this, it's a bug. Rationale, `std::get_time` is already specified in terms of `strptime`, and we can't afford to be inventive with locale-dependent things.

Note that, due above, the inconsistency between POSIX `strftime` specification that supports `%Ou` and `%OV` that are not handled by `strptime` should be (by design) reflected in "[tab:time.format.spec] Meaning of conversion specifiers" and "[tab:time.parse.spec] Meaning of `parse` flags" tables.

The `%d` modifier was addressed by LWG 3218(i).

[2020-02 Status to Immediate on Thursday morning in Prague.]

Proposed resolution:

This wording is relative to N4830.

1. Modify Table 99 "Meaning of `parse` flags [tab:time.parse.spec]" in 30.13 [time.parse] as indicated:

   > Table 99: Meaning of `parse` flags [tab:time.parse.spec]
   > Flag | Parsed value
   > --- | ---
   > `[…]`
   > `%C` | The century as a decimal number. The modified command `%NC` specifies the maximum number of characters to read. If `N` is not specified, the default is `2`. Leading zeroes are permitted but not required. The modified commands `%EC` and `%OC` interprets the locale's alternative representation of the century.
   > `[…]`
   > `%I` | The hour (12-hour clock) as a decimal number. The modified command `%NI` specifies the maximum number of characters to read. If `N` is not specified, the default is `2`. Leading zeroes are permitted but not required. The modified command `%OI` interprets the locale's alternative representation.
   > `[…]`
   > `%u` | The ISO weekday as a decimal number (`1-7`), where Monday is `1`. The modified command `%Nu` specifies the maximum number of characters to read. If `N` is not specified, the default is `1`. Leading zeroes are permitted but not required. The modified command `%Ou` interprets the locale's alternative representation.
   > `%U` | The week number of the year as a decimal number. The first Sunday of the year is the first day of week `01`. Days of the same year prior to that are in week `00`. The modified command `%NU` specifies the maximum number of characters to read. If `N` is not specified, the default is `2`. Leading zeroes are permitted but not required. The modified command `%OU` interprets the locale's alternative representation.
   > `[…]`
   > `%W` | The week number of the year as a decimal number. The first Monday of the year is the first day of week `01`. Days of the same year prior to that are in week `00`. The modified command `%NW` specifies the maximum number of characters to read. If `N` is not specified, the default is `2`. Leading zeroes are permitted but not required. The modified command `%OW` interprets the locale's alternative representation.
   > `[…]`

### 3253(i). `basic_syncbuf::basic_syncbuf()` should not be explicit

Section: 31.11.2.1 [syncstream.syncbuf.overview] Status: C++20 Submitter: Nevin Liber Opened: 2019-08-06 Last modified: 2021-02-25

Priority: 0

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Discussion:

When P0935 "Eradicating unnecessarily explicit default constructors from the standard library" was applied, `basic_syncbuf` was not in the working paper. `basic_syncbuf` should not have an explicit default constructor.

[2019-09-02 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 31.11.2.1 [syncstream.syncbuf.overview], class template `basic_syncbuf` synopsis, as indicated:

   >     template<class charT, class traits, class Allocator>
   >     class basic_syncbuf : public basic_streambuf<charT, traits> {
   >     public:
   >       […]
   >       // 31.11.2.2 [syncstream.syncbuf.cons], construction and destruction
   >       basic_syncbuf()
   >         : basic_syncbuf(nullptr) {}
   >       explicit basic_syncbuf(streambuf_type* obuf = nullptr)
   >         : basic_syncbuf(obuf, Allocator()) {}
   >       […]
   >     };

### 3254(i). Strike `stop_token`'s `operator!=`

Section: 32.3.4 [stoptoken] Status: C++20 Submitter: Casey Carter Opened: 2019-08-06 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

In the shiny new world of the future, we need not declare overloads of `operator!=` that `return !(x == y);` the rewrite rule in 12.2.2.3 [over.match.oper] para 3.4.3 achieves the same effect. Consequently, we should not declare such unnecessary `operator!=` overloads. The synopsis of class `stop_token` in 32.3.4 [stoptoken] declares exactly such an `operator!=` friend which should be struck.

[01-2020 Moved to Tentatively Ready after 5 positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 32.3.4 [stoptoken], class `stop_token` synopsis, as indicated:

   >     namespace std {
   >       class stop_token {
   >       public:
   >         […]
   >         [[nodiscard]] friend bool operator==(const stop_token& lhs, const stop_token& rhs) noexcept;
   >         [[nodiscard]] friend bool operator!=(const stop_token& lhs, const stop_token& rhs) noexcept;
   >         friend void swap(stop_token& lhs, stop_token& rhs) noexcept;
   >       };
   >     }

2. Modify [stoptoken.cmp] and [stoptoken.special] as indicated:

   > 32.3.3.3 Non-member functionsComparisons [stoptoken.nonmemberscmp]

   >     [[nodiscard]] bool operator==(const stop_token& lhs, const stop_token& rhs) noexcept;

   > > -1- Returns: `true` if `lhs` and `rhs` have ownership of the same stop state or if both `lhs` and `rhs` do not have ownership of a stop state; otherwise `false`.

   >     [[nodiscard]] bool operator!=(const stop_token& lhs, const stop_token& rhs) noexcept;

   > > -2- Returns: `!(lhs==rhs)`.

   > 32.3.3.4 Specialized algorithms [stoptoken.special]

   >     friend void swap(stop_token& x, stop_token& y) noexcept;

   > > -1- Effects: Equivalent to: `x.swap(y)`.

### 3255(i). `span`'s `array` constructor is too strict

Section: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Jean Guegant & Barry Revzin Opened: 2019-08-10 Last modified: 2021-02-25

Priority: 2

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Discussion:

Barry Revzin:

From StackOverflow:

This compiles:

>     std::vector<int*> v = {nullptr, nullptr};
>     std::span<const int* const> s{v};

This does not:

>     std::array<int*, 2> a = {nullptr, nullptr};
>     std::span<const int* const> s{a};

The problem is that `span`'s constructors include

- A constructor template that takes any `Container` that is neither a raw array nor a `std::array`

- A constructor template that takes an `array<value_type, N>&`

- A constructor template that takes a `const array<value_type, N>&`

So the first is excluded, and the other two don't match. We can change the array constructor templates to take an `array<T, N>` with the requirement that `T(*)[]` is convertible to `ElementType(*)[]`?

Jean Guegant:

It is impossible to create a `std::span` from a `std::array<const T, X>` given the current set of constructors of `std::span` (23.7.2.2.2 [span.cons]):

>     std::array<const int, 4> a = {1, 2, 3, 4};
>     std::span<const int> s{a}; // No overload can be found.
>     std::span s{a}; // CTAD doesn't help either.

Both constructors accepting a `std::array` (23.7.2.2.2 [span.cons] p11) require the first template parameter of the `std::array` parameter to be `value_type`:

>     template<size_t N> constexpr span(array<value_type, N>& arr) noexcept;
>     template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;

`value_type` being defined as `remove_cv_t<ElementType>` — this constrains the first template parameter not to be `const`.

Both constructors accepting a generic `Container` (23.7.2.2.2 [span.cons] p14) have a constraint — (p14.3) `Container` is not a specialization of `array` — rejecting `std::array`.

While you can call `std::array<const T, X>::data` and `std::array<const T, X>::size` to manually create a `std::span`, we should, in my opinion, offer a proper overload for this scenario. Two reasons came to my mind:

1. `std::span` handles C-arrays and `std::arrays` in an asymmetric way. The constructor taking a C-array (23.7.2.2.2 [span.cons] p11) is using `element_type` and as such can work with `const T`:

   >     const int a[] = {1, 2, 3, 4};
   >     std::span<const int> s{a}; // It works

   If a user upgrades her/his code from C-arrays to a `std::array`s and literally take the type `const T` and use it as the first parameter of `std::array`, he/she will face an error.

2. Even if the user is aware that `const std::array<T, X>` is more idiomatic than `std::array<const T, X>`, the second form may appear in the context of template instantiation.

At the time this issue is written `gls::span`, from which `std::span` is partly based on, does not suffer from the same issue: Its constructor taking a generic `const Container&` does not constraint the `Container` not to be a `std::array` (although its constructor taking a generic `Container&` does). For the users willing to upgrade from `gsl::span` to `std::span`, this could be a breaking change.

[2019-09-01 Priority set to 2 based on reflector discussion]

[2020-02-13 Tim updates PR]

The previous PR's change to the raw array constructor is both 1) unnecessary and 2) incorrect; it prevents `span<const int>` from being initialized with an `int[42]` xvalue.

Previous resolution: [SUPERSEDED]

> This wording is relative to N4830.

> > The only change is to make the constructors templated on the element type of the array as well. We already have the right constraints in place. It's just that the 2nd constraint is trivially satisfied today by the raw array constructor and either always or never satisfied by the `std::array` one.

> 1. Modify 23.7.2.2.1 [span.overview], class template `span` synopsis, as indicated:

>    >     template<class ElementType, size_t Extent = dynamic_extent>
>    >     class span {
>    >     public:
>    >       […]
>    >       // 23.7.2.2.2 [span.cons], constructors, copy, and assignment
>    >       constexpr span() noexcept;
>    >       constexpr span(pointer ptr, index_type count);
>    >       constexpr span(pointer first, pointer last);
>    >       template<class T, size_t N>
>    >         constexpr span(Telement_type (&arr)[N]) noexcept;
>    >       template<class T, size_t N>
>    >         constexpr span(array<Tvalue_type, N>& arr) noexcept;
>    >       template<class T, size_t N>
>    >         constexpr span(const array<Tvalue_type, N>& arr) noexcept;
>    >       […]
>    >     };

> 2. Modify 23.7.2.2.2 [span.cons] as indicated:

>    >     template<class T, size_t N>
>    >       constexpr span(Telement_type (&arr)[N]) noexcept;
>    >     template<class T, size_t N>
>    >       constexpr span(array<Tvalue_type, N>& arr) noexcept;
>    >     template<class T, size_t N>
>    >       constexpr span(const array<Tvalue_type, N>& arr) noexcept;

>    > > -11- Constraints:

>    > > 1. (11.1) — `extent == dynamic_extent || N == extent` is `true`, and

>    > > 2. (11.2) — `remove_pointer_t<decltype(data(arr))>(*)[]` is convertible to `ElementType(*)[]`.

>    > > -12- Effects: Constructs a `span` that is a view over the supplied array.

>    > > -13- Ensures: `size() == N && data() == data(arr)` is `true`.

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 23.7.2.2.1 [span.overview], class template `span` synopsis, as indicated:

   >     template<class ElementType, size_t Extent = dynamic_extent>
   >     class span {
   >     public:
   >       […]
   >       // 23.7.2.2.2 [span.cons], constructors, copy, and assignment
   >       constexpr span() noexcept;
   >       […]
   >       template<size_t N>
   >         constexpr span(element_type (&arr)[N]) noexcept;
   >       template<class T, size_t N>
   >         constexpr span(array<Tvalue_type, N>& arr) noexcept;
   >       template<class T, size_t N>
   >         constexpr span(const array<Tvalue_type, N>& arr) noexcept;
   >       […]
   >     };

2. Modify 23.7.2.2.2 [span.cons] as indicated:

   >     template<size_t N>
   >       constexpr span(element_type (&arr)[N]) noexcept;
   >     template<class T, size_t N>
   >       constexpr span(array<Tvalue_type, N>& arr) noexcept;
   >     template<class T, size_t N>
   >       constexpr span(const array<Tvalue_type, N>& arr) noexcept;

   > > -11- Constraints:

   > > 1. (11.1) — `extent == dynamic_extent || N == extent` is `true`, and

   > > 2. (11.2) — `remove_pointer_t<decltype(data(arr))>(*)[]` is convertible to `ElementType(*)[]`.

   > > -12- Effects: Constructs a `span` that is a view over the supplied array.

   > > -13- Postconditions: `size() == N && data() == data(arr)` is `true`.

### 3256(i). Feature testing macro for `constexpr` algorithms

Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: Antony Polukhin Opened: 2019-08-14 Last modified: 2021-02-25

Priority: 0

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Discussion:

Feature testing macro from P0202 "Add Constexpr Modifiers to Functions in `<algorithm>` and `<utility>` Headers" is missing in the WD.

For user convenience and to reduce feature testing macro count it would be better to stick to initial version of P0202 that was providing only the `__cpp_lib_constexpr_algorithms`.

So remove `__cpp_lib_constexpr_swap_algorithms`, define `__cpp_lib_constexpr_algorithms` to `201703L` if P0202 is implemented, to `201806L` if P0202+P0879 are implemented.

[2019-09-02 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify the Table 36 "Standard library feature-test macros" [tab:support.ft] in 17.3.1 [support.limits.general] as indicated:

   > Table 36: Standard library feature-test macros [tab:support.ft]
   > Macro name | Value | Header(s)
   > --- | --- | ---
   > `[…]`
   > `__cpp_lib_constexpr_swap_algorithms` | `201806L` | `<algorithm>`
   > `[…]`

### 3257(i). Missing feature testing macro update from P0858

Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: Antony Polukhin Opened: 2019-08-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [support.limits.general].

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Discussion:

P0858 "Constexpr iterator requirements" suggested to update the feature-testing macros `__cpp_lib_string_view` and `__cpp_lib_array_constexpr` to the date of adoption.

That did not happen.

[2019-09-02 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify the Table 36 "Standard library feature-test macros" [tab:support.ft] in 17.3.1 [support.limits.general] as indicated:

   > Table 36: Standard library feature-test macros [tab:support.ft]
   > Macro name | Value | Header(s)
   > --- | --- | ---
   > `[…]`
   > `__cpp_lib_array_constexpr` | `2016803L` | `<iterator> <array>`
   > `[…]`
   > `__cpp_lib_string_view` | `201606803L` | `<string> <string_view>`
   > `[…]`

### 3258(i). Range access and `initializer_list`

Section: 25.3.2 [range.access.begin], 25.3.3 [range.access.end], 25.3.6 [range.access.rbegin], 25.3.7 [range.access.rend] Status: Resolved Submitter: Casey Carter Opened: 2019-08-15 Last modified: 2021-06-23

Priority: 3

View all issues with Resolved status.

Discussion:

The specification of `ranges::begin` in 25.3.2 [range.access.begin] includes a "poison pill" overload:

>     template<class T> void begin(initializer_list<T>&&) = delete;

which exists to create an ambiguity with the non-member `initializer_list` overload of `begin` in namespace `std` (17.11.2 [initializer.list.syn]) when performing unqualified lookup, since specializations of `initializer_list` should not satisfy `forwarding-range` (25.4.2 [range.range]). The design intent is that `const` specializations of `initializer_list` should also not satisfy `forwarding-range`, although they are rare enough beasts that they were overlooked when this wording is written.

`ranges::end` (25.3.3 [range.access.end]) has a similar poison pill for `initializer_list`, which should be changed consistently.

Notably `ranges::rbegin` (25.3.6 [range.access.rbegin]) and `ranges::rend` (25.3.6 [range.access.rbegin]) as currently specified accept rvalue `initializer_list` arguments; they find the `initializer_list` overloads of `std::rbegin` and `std::rend` (24.7 [iterator.range]) via ADL. While I can't put my finger on anything in particular that's broken by this behavior, it seems wise to make `rbegin` and `rend` consistent with `begin` and `end` for `initializer_list` until and unless we discover a reason to do otherwise.

[2019-10 Priority set to 3 after reflector discussion]

[2021-06-23 Resolved by adoption of P2091R0 in Prague. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.3.2 [range.access.begin] as follows:

   > -1- The name `ranges::begin` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::begin(E)` for some subexpression `E` is expression-equivalent to:

   > (1.1) — `E + 0` if `E` is an lvalue of array type (6.9.4 [basic.compound]).

   > (1.2) — Otherwise, if `E` is an lvalue, `decay-copy(E.begin())` if it is a valid expression and its type `I` models `input_or_output_iterator`.

   > (1.3) — Otherwise, `decay-copy(begin(E))` if it is a valid expression and its type `I` models `input_or_output_iterator` with overload resolution performed in a context that includes the declarations:

   > >     template<class T> void begin(T&&) = delete;
   > >     template<class T> void begin(initializer_list<T>&&) = delete;

   > and does not include a declaration of `ranges::begin`.

   > […]

2. Modify 25.3.3 [range.access.end] as follows:

   > -1- The name `ranges::end` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::end(E)` for some subexpression `E` is expression-equivalent to:

   > (1.1) — `E + extent_v<T>` if `E` is an lvalue of array type (6.9.4 [basic.compound]) `T`.

   > (1.2) — Otherwise, if `E` is an lvalue, `decay-copy(E.end())` if it is a valid expression and its type `S` models `sentinel_for<decltype(ranges::begin(E))>`

   > (1.3) — Otherwise, `decay-copy(end(E))` if it is a valid expression and its type `S` models `sentinel_for<decltype(ranges::begin(E))>` with overload resolution performed in a context that includes the declarations:

   > >     template<class T> void end(T&&) = delete;
   > >     template<class T> void end(initializer_list<T>&&) = delete;

   > and does not include a declaration of `ranges::end`.

   > […]

3. Modify 25.3.6 [range.access.rbegin] as follows:

   > -1- The name `ranges::rbegin` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::rbegin(E)` for some subexpression `E` is expression-equivalent to:

   > (1.1) — If `E` is an lvalue, `decay-copy(E.rbegin())` if it is a valid expression and its type `I` models `input_or_output_iterator`.

   > (1.2) — Otherwise, `decay-copy(rbegin(E))` if it is a valid expression and its type `I` models `input_or_output_iterator` with overload resolution performed in a context that includes the declarations:

   > >     template<class T> void rbegin(T&&) = delete;
   > >     template<class T> void rbegin(initializer_list<T>) = delete;

   > and does not include a declaration of `ranges::rbegin`.

   > […]

4. Modify 25.3.7 [range.access.rend] as follows:

   > -1- The name `ranges::rend` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::rend(E)` for some subexpression `E` is expression-equivalent to:

   > (1.1) — If `E` is an lvalue, `decay-copy(E.rend())` if it is a valid expression and its type `S` models

   > >     sentinel_for<decltype(ranges::rbegin(E))>

   > (1.2) — Otherwise, `decay-copy(rend(E))` if it is a valid expression and its type `S` models

   > >     sentinel_for<decltype(ranges::rbegin(E))>

   > with overload resolution performed in a context that includes the declarations:

   > >     template<class T> void rend(T&&) = delete;
   > >     template<class T> void rend(initializer_list<T>) = delete;

   > and does not include a declaration of `ranges::rend`.

   > […]

### 3259(i). The definition of constexpr iterators should be adjusted

Section: 24.3.1 [iterator.requirements.general] Status: C++20 Submitter: Daniel Krügler Opened: 2019-08-18 Last modified: 2021-02-25

Priority: 0

View all other issues in [iterator.requirements.general].

View all issues with C++20 status.

Discussion:

The current definition of constexpr iterators is specified in 24.3.1 [iterator.requirements.general] p16 as follows:

> Iterators are called constexpr iterators if all operations provided to meet iterator category requirements are constexpr functions, except for

> 1. (16.1) — a pseudo-destructor call (7.5.5.5 [expr.prim.id.dtor]), and

> 2. (16.2) — the construction of an iterator with a singular value.

With the acceptance of some proposals during the Cologne 2019 meeting, these additional requirements become mostly obsolete, as it had already been pointed out during that meeting:

With the acceptance of P0784R7, destructors can be declared `constexpr` and it is possible to perform a pseudo-destructor call within a constant expression, so bullet (16.1) is no longer a necessary requirement.

With the acceptance of P1331R2, trivial default initialization in constexpr contexts is now possible, and there is no longer a requirement to initialize all sub-objects of a class object within a constant expression.

It seems to me that we should simply strike the above two constraining requirements of the definition of constexpr iterators for C++20.

[2019-09-14 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 24.3.1 [iterator.requirements.general] as indicated:

   > -16- Iterators are called constexpr iterators if all operations provided to meet iterator category requirements are constexpr functions., except for

   > 1. (16.1) — a pseudo-destructor call (7.5.5.5 [expr.prim.id.dtor]), and

   > 2. (16.2) — the construction of an iterator with a singular value.

### 3260(i). `year_month*` arithmetic rejects durations convertible to years

Section: 30.8 [time.cal] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-08-15 Last modified: 2021-02-25

Priority: 2

View all issues with C++20 status.

Discussion:

Currently, the `year_month*` types (`year_month`, `year_month_day`) provide separate arithmetic operators with duration type of `years` or `months`. This is an intentional optimization that avoids performing modulo arithmetic in the former case.

However, these make the arithmetic of `year_month*` types with durations that are convertible to `years` (and by consequence to `months`) ambiguous. For example, the following code is ambiguous:

>     using decades = duration<int, ratio_multiply<ratio<10>, years::period>>;
>     auto ymd = 2001y/January/1d;
>     ymd += decades(1); // error, ambiguous

while less usual durations that are only convertible to `months` work correctly:

>     using decamonths = duration<int, ratio_multiply<ratio<10>, months::period>>;
>     auto ymd = 2001y/January/1d;
>     ymd += decamonths(1);

The example implementation resolves the issues by making sure that the `years` overload will be preferred in case of ambiguity, by declaring the `months` overload a function template with a default argument for its parameter (suggested by Tim Song):

>     template<class = unspecified>
>     constexpr year_month_weekday& operator+=(const months& m) noexcept;
>     constexpr year_month_weekday& operator+=(const years& m) noexcept;

[2019-09-14 Priority set to 2 based on reflector discussion]

[2019-09-14; Tomasz and Howard provide concrete wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> > [Drafting note: Suggested wording below assumes that we can add a Constraints: to a signature where the constraint does not apply to a deduced template. We have examples of such constraints in other parts of the WD (e.g. 20.3.1.3.2 [unique.ptr.single.ctor]/p15, 20.3.1.3.4 [unique.ptr.single.asgn]/p1). And we have the old form "does not participate …" being used for non-deduced templates in several places as well (e.g. 22.3.2 [pairs.pair]/p5).

> > There are several ways of implementing such a constraint, such as adding a gratuitous template parameter.]

> 1. Modify 30.8.13.2 [time.cal.ym.members] as indicated:

>    >     constexpr year_month& operator+=(const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -4- Effects: `*this = *this + dm`.

>    > > […]

>    >     constexpr year_month& operator-=(const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -6- Effects: `*this = *this - dm`.

>    > > […]

> 2. Modify 30.8.13.3 [time.cal.ym.nonmembers] as indicated:

>    >     constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -3- Returns: A `year_month` value `z` such that `z - ym == dm`.

>    > > […]

>    >     constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -5- Returns: `ym + dm`.

>    >     constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -6- Returns: `ym + -dm`.

> 3. Modify 30.8.14.2 [time.cal.ymd.members] as indicated:

>    >     constexpr year_month_day& operator+=(const months& m) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -7- Effects: `*this = *this + m`.

>    > > […]

>    >     constexpr year_month_day& operator-=(const months& m) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -9- Effects: `*this = *this - m`.

>    > > […]

> 4. Modify 30.8.14.3 [time.cal.ymd.nonmembers] as indicated:

>    >     constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -3- Returns: `(ymd.year() / ymd.month() + dm) / ymd.day()`.

>    > > […]

>    >     constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -5- Returns: `ymd + dm`.

>    >     constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -6- Returns: `ymd + (-dm)`.

> 5. Modify 30.8.15.2 [time.cal.ymdlast.members] as indicated:

>    >     constexpr year_month_day_last& operator+=(const months& m) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -2- Effects: `*this = *this + m`.

>    > > […]

>    >     constexpr year_month_day_last& operator-=(const months& m) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -4- Effects: `*this = *this - m`.

>    > > […]

> 6. Modify 30.8.15.3 [time.cal.ymdlast.nonmembers] as indicated:

>    >     constexpr year_month_day_last
>    >       operator+(const year_month_day_last& ymdl, const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -3- Returns: `(ymdl.year() / ymdl.month() + dm) / last`.

>    >     constexpr year_month_day_last
>    >       operator+(const months& dm, const year_month_day_last& ymdl) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -4- Returns: `ymdl + dm`.

>    >     constexpr year_month_day_last
>    >       operator-(const year_month_day_last& ymdl, const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -5- Returns: `ymdl + (-dm)`.

> 7. Modify 30.8.16.2 [time.cal.ymwd.members] as indicated:

>    >     constexpr year_month_weekday& operator+=(const months& m) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -6- Effects: `*this = *this + m`.

>    > > […]

>    >     constexpr year_month_weekday& operator-=(const months& m) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -8- Effects: `*this = *this - m`.

>    > > […]

> 8. Modify 30.8.16.3 [time.cal.ymwd.nonmembers] as indicated:

>    >     constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -2- Returns: `(ymwd.year() / ymwd.month() + dm) / ymwd.weekday_indexed()`.

>    >     constexpr year_month_weekday operator+(const months& dm, const year_month_weekday& ymwd) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -3- Returns: `ymwd + dm`.

>    >     constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const months& dm) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -4- Returns: `ymwd + (-dm)`.

> 9. Modify 30.8.17.2 [time.cal.ymwdlast.members] as indicated:

>    >     constexpr year_month_weekday_last& operator+=(const months& m) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -2- Effects: `*this = *this + m`.

>    > > […]

>    >     constexpr year_month_weekday_last& operator-=(const months& m) noexcept;

>    > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>    > > -4- Effects: `*this = *this - m`.

>    > > […]

> 10. Modify 30.8.17.3 [time.cal.ymwdlast.nonmembers] as indicated:

>     >     constexpr year_month_weekday_last
>     >       operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;

>     > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>     > > -2- Returns: `(ymwdl.year() / ymwdl.month() + dm) / ymwdl.weekday_last()`.

>     >     constexpr year_month_weekday_last
>     >       operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;

>     > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>     > > -3- Returns: `ymwdl + dm`.

>     >     constexpr year_month_weekday_last
>     >       operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;

>     > > -?- Constraints: The argument supplied by the caller for the `months` parameter is not implicitly convertible to `years`.

>     > > -4- Returns: `ymwdl + (-dm)`.

[2020-02-13, Prague]

Tim Song found a wording problem that we would like to resolve:

Given a class like

>     struct C : months {
>       operator years();
>     };

The previous wording requires calls with a `C` argument to use the `years` overload, which would require implementation heroics since its conversion sequence to `months` is better than `years`.

[2020-02 Status to Immediate on Friday morning in Prague.]

Proposed resolution:

This wording is relative to N4849.

> [Drafting note: Suggested wording below assumes that we can add a Constraints: to a signature where the constraint does not apply to a deduced template. We have examples of such constraints in other parts of the WD (e.g. 20.3.1.3.2 [unique.ptr.single.ctor]/p15, 20.3.1.3.4 [unique.ptr.single.asgn]/p1). And we have the old form "does not participate …" being used for non-deduced templates in several places as well (e.g. 22.3.2 [pairs.pair]/p5).

> There are several ways of implementing such a constraint, such as adding a gratuitous template parameter.]

1. Modify 30.8.13.2 [time.cal.ym.members] as indicated:

   >     constexpr year_month& operator+=(const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -4- Effects: `*this = *this + dm`.

   > > […]

   >     constexpr year_month& operator-=(const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -6- Effects: `*this = *this - dm`.

   > > […]

2. Modify 30.8.13.3 [time.cal.ym.nonmembers] as indicated:

   >     constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -3- Returns: A `year_month` value `z` such that `z - ym == dm`.

   > > […]

   >     constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -5- Returns: `ym + dm`.

   >     constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -6- Returns: `ym + -dm`.

3. Modify 30.8.14.2 [time.cal.ymd.members] as indicated:

   >     constexpr year_month_day& operator+=(const months& m) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -7- Effects: `*this = *this + m`.

   > > […]

   >     constexpr year_month_day& operator-=(const months& m) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -9- Effects: `*this = *this - m`.

   > > […]

4. Modify 30.8.14.3 [time.cal.ymd.nonmembers] as indicated:

   >     constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -3- Returns: `(ymd.year() / ymd.month() + dm) / ymd.day()`.

   > > […]

   >     constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -5- Returns: `ymd + dm`.

   >     constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -6- Returns: `ymd + (-dm)`.

5. Modify 30.8.15.2 [time.cal.ymdlast.members] as indicated:

   >     constexpr year_month_day_last& operator+=(const months& m) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -2- Effects: `*this = *this + m`.

   > > […]

   >     constexpr year_month_day_last& operator-=(const months& m) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -4- Effects: `*this = *this - m`.

   > > […]

6. Modify 30.8.15.3 [time.cal.ymdlast.nonmembers] as indicated:

   >     constexpr year_month_day_last
   >       operator+(const year_month_day_last& ymdl, const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -3- Returns: `(ymdl.year() / ymdl.month() + dm) / last`.

   >     constexpr year_month_day_last
   >       operator+(const months& dm, const year_month_day_last& ymdl) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -4- Returns: `ymdl + dm`.

   >     constexpr year_month_day_last
   >       operator-(const year_month_day_last& ymdl, const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -5- Returns: `ymdl + (-dm)`.

7. Modify 30.8.16.2 [time.cal.ymwd.members] as indicated:

   >     constexpr year_month_weekday& operator+=(const months& m) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -6- Effects: `*this = *this + m`.

   > > […]

   >     constexpr year_month_weekday& operator-=(const months& m) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -8- Effects: `*this = *this - m`.

   > > […]

8. Modify 30.8.16.3 [time.cal.ymwd.nonmembers] as indicated:

   >     constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -2- Returns: `(ymwd.year() / ymwd.month() + dm) / ymwd.weekday_indexed()`.

   >     constexpr year_month_weekday operator+(const months& dm, const year_month_weekday& ymwd) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -3- Returns: `ymwd + dm`.

   >     constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const months& dm) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -4- Returns: `ymwd + (-dm)`.

9. Modify 30.8.17.2 [time.cal.ymwdlast.members] as indicated:

   >     constexpr year_month_weekday_last& operator+=(const months& m) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -2- Effects: `*this = *this + m`.

   > > […]

   >     constexpr year_month_weekday_last& operator-=(const months& m) noexcept;

   > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

   > > -4- Effects: `*this = *this - m`.

   > > […]

10. Modify 30.8.17.3 [time.cal.ymwdlast.nonmembers] as indicated:

    >     constexpr year_month_weekday_last
    >       operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;

    > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

    > > -2- Returns: `(ymwdl.year() / ymwdl.month() + dm) / ymwdl.weekday_last()`.

    >     constexpr year_month_weekday_last
    >       operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;

    > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

    > > -3- Returns: `ymwdl + dm`.

    >     constexpr year_month_weekday_last
    >       operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;

    > > -?- Constraints: If the argument supplied by the caller for the `months` parameter is convertible to `years`, its implicit conversion sequence to `years` is worse than its implicit conversion sequence to `months` (12.2.4.3 [over.ics.rank]).

    > > -4- Returns: `ymwdl + (-dm)`.

### 3262(i). Formatting of negative durations is not specified

Section: 30.12 [time.format] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-08-18 Last modified: 2021-02-25

Priority: 2

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Discussion:

The current specification of the formatting for `std::chrono::duration` and `std::hh_mm_ss` types is unclear in regards the the handling of negative values. To illustrate:

>     std::cout << std::format("%H:%M:%S", -10'000s); // prints either -02:46:40 or -02:-46:-40

The indented behavior (and currently implemented, see here) is to apply the sign once, before the leftmost converted field.

[2019-09-14 Priority set to 2 based on reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> > [Drafting note: With the above clarification, the specification of the `operator<<` for `hh_mm_ss` may be simplified to `format("{:%T}", hms)`.]

> 1. Modify 30.12 [time.format] as indicated:

>    > -2- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table [tab:time.format.spec]. Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise. If the formatted object does not contain the information the conversion specifier refers to, an exception of type `format_error` is thrown.

>    > -?- The result of formatting a `std::chrono::duration` instance holding a negative value, or of an `hh_mm_ss` object `h` for which `h.is_negative()` is `true`, is equivalent to the output of the corresponding positive value, with a `-` character placed before the replacement of the leftmost conversion specifier.

>    > [Example:

>    > >     cout << format("%T", -10'000s); // prints: -02:46:40
>    > >     cout << format("%H:%M:%S", -10'000s); // prints: -02:46:40
>    > >     cout << format("minutes %M, hours %H, seconds %S", -10'000s); // prints: minutes -46, hours 02, seconds 40

>    > — end example]

>    > -3- Unless explicitly requested, […]

> 2. Modify 30.9.3 [time.hms.nonmembers] as indicated:

>    >     template<class charT, class traits, class Duration>
>    >     basic_ostream<charT, traits>&
>    >     operator<<(basic_ostream<charT, traits>& os, const hh_mm_ss<Duration>& hms);

>    > > -1- Effects: Equivalent to:

>    > > >     return os << format(os.getloc(),
>    > > >                  hms.is_negative() ? STATICALLY-WIDEN<charT>("-{:%T}")
>    > > >                                    : STATICALLY-WIDEN<charT>("{:%T}"),
>    > > >                  abs(hms.to_duration()));

[2019-09-14; Howard improves wording]

[2020-02; Status set to Immediate after LWG discussion Thursday in Prague. (Minor example wording cleanup)]

Proposed resolution:

This wording is relative to N4830.

> [Drafting note: With the above clarification, the specification of the `operator<<` for `hh_mm_ss` may be simplified to `format("{:%T}", hms)`.]

1. Modify 30.12 [time.format] as indicated:

   > -2- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table [tab:time.format.spec]. Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise. If the formatted object does not contain the information the conversion specifier refers to, an exception of type `format_error` is thrown.

   > -?- The result of formatting a `std::chrono::duration` instance holding a negative value, or of an `hh_mm_ss` object `h` for which `h.is_negative()` is `true`, is equivalent to the output of the corresponding positive value, with a `STATICALLY-WIDEN<charT>("-")` character sequence placed before the replacement of the leftmost conversion specifier.

   > [Example:

   > >     cout << format("{%:T}", -10'000s); // prints: -02:46:40
   > >     cout << format("{:%H:%M:%S}", -10'000s); // prints: -02:46:40
   > >     cout << format("{:minutes %M, hours %H, seconds %S}", -10'000s); // prints: minutes -46, hours 02, seconds 40

   > — end example]

   > -3- Unless explicitly requested, […]

2. Modify 30.9.3 [time.hms.nonmembers] as indicated:

   >     template<class charT, class traits, class Duration>
   >     basic_ostream<charT, traits>&
   >     operator<<(basic_ostream<charT, traits>& os, const hh_mm_ss<Duration>& hms);

   > > -1- Effects: Equivalent to:

   > > >     return os << format(os.getloc(),
   > > >                  hms.is_negative() ? STATICALLY-WIDEN<charT>("-{:%T}")
   > > >                                    : STATICALLY-WIDEN<charT>("{:%T}"),
   > > >                  abs(hms.to_duration()));

### 3264(i). `sized_range` and `ranges::size` redundantly use `disable_sized_range`

Section: 25.4.4 [range.sized] Status: C++20 Submitter: Casey Carter Opened: 2019-08-26 Last modified: 2021-02-25

Priority: 1

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Discussion:

`disable_sized_range` (25.4.4 [range.sized]) is an opt-out trait that users may specialize when their `range` type conforms to the syntax of `sized_range` but not its semantics, or when the type is so poorly suited to the Standard Library that even testing the validity of the expressions `r.size()` or `size(r)` for a range `r` is impossible. The library inspects `disable_sized_range` in two places. (1) In the definition of the `sized_range` concept:

>     template<class T>
>       concept sized_range =
>         range<T> &&
>         !disable_sized_range<remove_cvref_t<T>> &&
>         requires(T& t) { ranges::size(t); };

If the pertinent specialization of `disable_sized_range` is `true`, we avoid checking the validity of the expression `ranges::size(t)` in the requires-expression. (2) In the definition of the `ranges::size` CPO itself (25.3.10 [range.prim.size]), the validity of the expressions `decay-copy(E.size())` and `decay-copy(size(E))` is not checked if the pertinent specialization of `disable_sized_range` is `true`.

`disable_sized_range` is effectively checked twice when evaluating `sized_range`. This redundancy could be forgiven, if it did not permit the existence of non-`sized_range`s for which `ranges::size` returns a valid size:

>     struct mytype {};
>     using A = mytype[42];

>     template <>
>     constexpr bool std::ranges::disable_sized_range<A> = true;

>     static_assert(std::ranges::range<A>);
>     static_assert(!std::ranges::sized_range<A>);
>     static_assert(std::ranges::size(A{}) == 42);

>     struct myrange {
>         constexpr int* begin() const { return nullptr; }
>         constexpr int* end() const { return nullptr; }
>     };

>     template <>
>     constexpr bool std::ranges::disable_sized_range<myrange> = true;

>     static_assert(std::ranges::range<myrange>);
>     static_assert(!std::ranges::sized_range<myrange>);
>     static_assert(std::ranges::size(myrange{}) == 0);

We should remove this gap between `ranges::size` and `sized_range` by checking `disable_sized_range` only in the definition of `ranges::size`, and continuing to rely on the validity of `ranges::size` in the `sized_range` concept.

[2019-09-14 Priority set to 1 based on reflector discussion]

[2019-11 Wednesday night issue processing in Belfast.]

Status to Ready

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.4.4 [range.sized] as follows:

   >     template<class T>
   >       concept sized_range =
   >         range<T> &&
   >         !disable_sized_range<remove_cvref_t<T>> &&
   >         requires(T& t) { ranges::size(t); };

### 3265(i). `move_iterator`'s conversions are more broken after P1207

Section: 24.5.4.4 [move.iter.cons] Status: C++23 Submitter: Casey Carter Opened: 2019-08-23 Last modified: 2023-11-22

Priority: 2

View all other issues in [move.iter.cons].

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Discussion:

The converting constructor and assignment operator specified in 24.5.4.4 [move.iter.cons] were technically broken before P1207:

- 24.5.4.4 [move.iter.cons] para 3 (and 5 for that matter) is an instance of LWG 3105(i); it should instead mandate that `u.base()` is convertible to `Iterator`.

- 24.5.4.4 [move.iter.cons] para 5 uses "is convertible" to guard an assignment operation instead of a conversion; it should instead mandate.

After applying P1207R4 "Movability of Single-pass Iterators", `u.base()` is not always well-formed, exacerbating the problem. These operations must ensure that `u.base()` is well-formed.

Drive-by:

- Let's burninate "Constructs a `move_iterator`" while we're touching this subclause.

- We'll also burninate "Iterator operations applied..." since the requirement it wants to impose is covered (and indeed must be covered) by the specifications of those other operations.

[2019-09-14 Priority set to 2 based on reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 24.5.4.4 [move.iter.cons] as indicated:

>    >     constexpr move_iterator();

>    > > -1- Effects: Constructs a `move_iterator`, vValue-initializesing `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value-initialized iterator of type `Iterator`.

>    >     constexpr explicit move_iterator(Iterator i);

>    > > -2- Effects: Constructs a move_iterator, iInitializesing `current` with `std::move(i)`.

>    >     template<class U> constexpr move_iterator(const move_iterator<U>& u);

>    > > -3- Mandates: `Uu.base()` is well-formed and convertible to `Iterator`.

>    > > -4- Effects: Constructs a `move_iterator`, iInitializesing `current` with `u.base()`.

>    >     template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);

>    > > -5- Mandates: `U` is convertible to `Iterator` `u.base()` is well-formed and `is_assignable_v<Iterator&, const U&>` is `true`.

>    > > -6- Effects: Assigns `u.base()` to `current`.

[2020-02-14; Prague]

LWG Review. Some wording improvements have been made and lead to revised wording.

[2020-02-16; Prague]

Reviewed revised wording and moved to Ready for Varna.

[2020-07-17; superseded by 3435(i)]

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 24.5.4.4 [move.iter.cons] as indicated:

   >     constexpr move_iterator();

   > > -1- Effects: Constructs a `move_iterator`, vValue-initializesing `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value-initialized iterator of type `Iterator`.

   >     constexpr explicit move_iterator(Iterator i);

   > > -2- Effects: Constructs a move_iterator, iInitializesing `current` with `std::move(i)`.

   >     template<class U> constexpr move_iterator(const move_iterator<U>& u);

   > > -3- Mandates: `Uu.base()` is well-formed and convertible to `Iterator`.

   > > -4- Effects: Constructs a `move_iterator`, iInitializesing `current` with `u.base()`.

   >     template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);

   > > -5- Mandates: `U` is convertible to `Iterator` `u.base()` is well-formed and `is_assignable_v<Iterator&, U>` is `true`.

   > > -6- Effects: Assigns `u.base()` to `current`.

### 3266(i). `to_chars(bool)` should be deleted

Section: 28.2.1 [charconv.syn] Status: C++20 Submitter: Jens Maurer Opened: 2019-08-23 Last modified: 2021-02-25

Priority: 0

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Discussion:

28.2.2 [charconv.to.chars] does not present an overload for `bool` (because it is neither a signed nor unsigned integer type), so an attempt to call `to_chars` with a bool argument would promote it to `int` and unambiguously call the `int` overload of `to_chars`.

This was not intended, since it is not obvious that the correct textual representation of a `bool` is `0/1` (as opposed to, say, `"true"/"false"`).

The user should cast explicitly if he wants the `0/1` behavior. (Correspondingly, there is no `bool` overload for `from_chars` in the status quo, and conversions do not apply there because of the reference parameter.)

[2019-09-14 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 28.2.1 [charconv.syn], header `<charconv>` synopsis, as indicated:

   >     […]
   >     // 28.2.2 [charconv.to.chars], primitive numerical output conversion
   >     struct to_chars_result {
   >       char* ptr;
   >       errc ec;
   >       friend bool operator==(const to_chars_result&, const to_chars_result&) = default;
   >     };

   >     to_chars_result to_chars(char* first, char* last, see below value, int base = 10);
   >     to_chars_result to_chars(char* first, char* last, bool value, int base = 10) = delete;
   >     to_chars_result to_chars(char* first, char* last, float value);
   >     […]

### 3269(i). Parse manipulators do not specify the result of the extraction from stream

Section: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-09-01 Last modified: 2021-02-25

Priority: 2

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Discussion:

None of the `parse` manipulators for the `chrono` types specifies the result of the extraction from the stream, as consequence they cannot be chained with the other read operations (at least portably). For example the following code is not required to work:

>     std::chrono::sys_stime s;
>     int x;
>     std::cin >> std::chrono::parse("%X", s) >> x;

[2019-10 Priority set to 2 after reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> > [Drafting note: As a drive-by fix the Remarks element is also converted to a Constraints element. The wording integrates the resolution for LWG 3235(i).

> 1. Modify 30.13 [time.parse] as indicated:

>    > -1- Each parse overload specified in this subclause calls `from_stream` unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, let `is` denote an object of type `basic_istream<charT, traits>` and let `I` be `basic_istream<charT, traits>&`, where `charT` and `traits` are template parameters in that context.

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);

>    > > -2- RemarksConstraints: This function shall not participate in overload resolution unlessThe expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp)

>    > > is a valid expressionwell-formed when treated as an unevaluated operand.

>    > > -3- Returns: A manipulator such that, when extracted from a `basic_istream<charT, traits> is`,the expression `is >> parse(fmt, tp)` has type `I`, value `is`, and calls `from_stream(is, fmt.c_str(), tp)`.

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
>    >               basic_string<charT, traits, Alloc>& abbrev);

>    > > -4- RemarksConstraints: This function shall not participate in overload resolution unlessThe expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, addressof(abbrev))

>    > > is a valid expressionwell-formed when treated as an unevaluated operand.

>    > > -5- Returns: A manipulator such that, when extracted from a `basic_istream<charT, traits> is`,the expression `is >> parse(fmt, tp, abbrev)` has type `I`, value `is`, and calls `from_stream(is, fmt.c_str(), tp, addressof(abbrev))`.

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
>    >               minutes& offset);

>    > > -6- RemarksConstraints: This function shall not participate in overload resolution unlessThe expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp,
>    > > >                 declval<basic_string<charT, traits, Alloc>*>()nullptr, &offset)

>    > > is a valid expressionwell-formed when treated as an unevaluated operand.

>    > > -7- Returns: A manipulator such that, when extracted from a `basic_istream<charT, traits> is`,the expression `is >> parse(fmt, tp, offset)` has type `I`, value `is`, and calls `from_stream(is, fmt.c_str(), tp, static_cast<basic_string<charT, traits, Alloc>*>(nullptr), &offset)`.

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
>    >               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

>    > > -8- RemarksConstraints: This function shall not participate in overload resolution unlessThe expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(),
>    > > >                 fmt.c_str(), tp, addressof(abbrev), &offset)

>    > > is a valid expressionwell-formed when treated as an unevaluated operand.

>    > > -9- Returns: A manipulator such that, when extracted from a `basic_istream<charT, traits> is`,the expression `is >> parse(fmt, tp, abbrev, offset)` has type `I`, value `is`, and calls `from_stream(is, fmt.c_str(), tp, addressof(abbrev), &offset)`.

[2020-02-13, Prague]

Issue wording has been rebased.

[2020-02 Status to Immediate on Friday morning in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 30.13 [time.parse] as indicated:

   > -1- Each `parse` overload specified in this subclause calls `from_stream` unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, let `is` denote an object of type `basic_istream<charT, traits>` and let `I` be `basic_istream<charT, traits>&`, where `charT` and `traits` are template parameters in that context.

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);

   > > -2- Constraints: The expression

   > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp)

   > > is a valid expressionwell-formed when treated as an unevaluated operand.

   > > -3- Returns: A manipulator such that, when extracted from a `basic_istream<charT, traits> is`,the expression `is >> parse(fmt, tp)` has type `I`, value `is`, and calls `from_stream(is, fmt.c_str(), tp)`.

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   >               basic_string<charT, traits, Alloc>& abbrev);

   > > -4- Constraints: The expression

   > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, addressof(abbrev))

   > > is a valid expressionwell-formed when treated as an unevaluated operand.

   > > -5- Returns: A manipulator such that, when extracted from a `basic_istream<charT, traits> is`,the expression `is >> parse(fmt, tp, abbrev)` has type `I`, value `is`, and calls `from_stream(is, fmt.c_str(), tp, addressof(abbrev))`.

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   >               minutes& offset);

   > > -6- Constraints: The expression

   > > >     from_stream(declval<basic_istream<charT, traits>&>(),
   > > >                 fmt.c_str(), tp,
   > > >                 declval<basic_string<charT, traits, Alloc>*>(),
   > > >                 &offset)

   > > is well-formed when treated as an unevaluated operand.

   > > -7- Returns: A manipulator such that, when extracted from a `basic_istream<charT, traits> is`,the expression `is >> parse(fmt, tp, offset)` has type `I`, value `is`, and calls

   > > >     from_stream(is,
   > > >                 fmt.c_str(), tp,
   > > >                 static_cast<basic_string<charT, traits, Alloc>*>(nullptr),
   > > >                 &offset)

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   >               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

   > > -8- Constraints: The expression

   > > >     from_stream(declval<basic_istream<charT, traits>&>(),
   > > >                 fmt.c_str(), tp, addressof(abbrev), &offset)

   > > is a valid expressionwell-formed when treated as an unevaluated operand.

   > > -9- Returns: A manipulator such that, when extracted from a `basic_istream<charT, traits> is`,the expression `is >> parse(fmt, tp, abbrev, offset)` has type `I`, value `is`, and calls `from_stream(is, fmt.c_str(), tp, addressof(abbrev), &offset)`.

### 3270(i). Parsing and formatting `%j` with `duration`s

Section: 30.12 [time.format], 30.13 [time.parse] Status: C++20 Submitter: Howard Hinnant Opened: 2019-09-02 Last modified: 2021-02-25

Priority: 2

View other active issues in [time.format].

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Discussion:

`%j` represents the day number of the year when formatting and parsing `time_point`s. It is also handy to interpret this flag consistently when formatting and parsing durations. That is if there is not enough information in the stream to represent a `time_point`, and if the target of the format/parse is a `duration`, `%j` represents a number of days.

>     #include <chrono>
>     #include <iostream>
>     #include <sstream>
>     #include <string>

>     int main()
>     {
>       using namespace std;
>       using namespace std::chrono;
>       // Parse %j as number of days into a duration
>       istringstream in{"222"};
>       hours d;
>       in >> parse("%j", d);
>       cout << d << '\n';
>       cout << format("{:%j}", d) << '\n';
>     }

Output:

>     5328h
>     222

[2019-10 Priority set to 2 after reflector discussion]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify "Table 98 — Meaning of conversion specifiers" [tab:time.format.spec] as indicated:

>    > Table 98 — Meaning of conversion specifiers [tab:time.format.spec]
>    > Specifier | Replacement
>    > --- | ---
>    > `[…]`
>    > `%j` | The day of the year as a decimal number. Jan 1 is `001`. If the result is less than three digits, it is left-padded with `0` to three digits. If the type being formatted is a specialization of `duration`, it is formatted as a decimal number of `days`.
>    > `[…]`

> 2. Modify "Table 99 — Meaning of `parse` flags" [tab:time.parse.spec] as indicated:

>    > Table 99 — Meaning of `parse` flags [tab:time.parse.spec]
>    > Flag | Parsed value
>    > --- | ---
>    > `[…]`
>    > `%j` | The day of the year as a decimal number. Jan 1 is `1`. The modified command `%Nj` specifies the maximum number of characters to read. If `N` is not specified, the default is `3`. Leading zeroes are permitted but not required. If the type being parsed is a specialization of `duration`, it is parsed as a decimal number of `days`.
>    > `[…]`

[2020-02-13 After Thursday afternoon discussion in Prague, Marshall provides updated wording.]

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4830.

1. Modify "Table 98 — Meaning of conversion specifiers" [tab:time.format.spec] as indicated:

   > Table 98 — Meaning of conversion specifiers [tab:time.format.spec]
   > Specifier | Replacement
   > --- | ---
   > `[…]`
   > `%j` | If the type being formatted is a specialization of `duration`, the decimal number of `days` without padding. Otherwise, theThe day of the year as a decimal number. Jan 1 is `001`. If the result is less than three digits, it is left-padded with `0` to three digits.
   > `[…]`

2. Modify "Table 99 — Meaning of `parse` flags" [tab:time.parse.spec] as indicated:

   > Table 99 — Meaning of `parse` flags [tab:time.parse.spec]
   > Flag | Parsed value
   > --- | ---
   > `[…]`
   > `%j` | If the type being parsed is a specialization of `duration`, a decimal number of `days`. Otherwise, theThe day of the year as a decimal number. Jan 1 is `1`. In either case, theThe modified command `%Nj` specifies the maximum number of characters to read. If `N` is not specified, the default is `3`. Leading zeroes are permitted but not required.
   > `[…]`

### 3272(i). `%I%p` should parse/format `duration` since midnight

Section: 30.12 [time.format], 30.13 [time.parse] Status: C++20 Submitter: Howard Hinnant Opened: 2019-09-02 Last modified: 2021-02-25

Priority: 0

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Discussion:

It is clear how `"%I%p"` parses and formats time points. What is not clear is how these flags interact with durations. We should treat durations as "elapsed time since midnight". For example:

>     #include <chrono>
>     #include <iostream>
>     #include <string>
>     #include <sstream>

>     int main()
>     {
>       using namespace std;
>       using namespace std::chrono;
>       // Format
>       {
>         // format time_point with %I%p
>         cout << format("{:%F %I%p}", sys_days{2019_y/August/10}+14h) << '\n';
>       }
>       {
>         // format duration with %I%p
>         cout << format("{:%I%p}", 14h) << '\n';
>       }

>       // Parse
>       {
>         // Parse %I%p as day-of-year combined with an hour into a time_point
>         istringstream in{"2019-08-10 2pm"};
>         system_clock::time_point tp;
>         in >> parse("%F %I%p", tp);
>         cout << tp << '\n';
>       }
>       {
>         // Parse %I%p as number of hours into a duration
>         istringstream in{"2pm"};
>         hours d;
>         in >> parse("%I%p", d);
>         cout << d << '\n';
>       }
>     }

Output:

>     2019-08-10 02PM
>     02PM
>     2019-08-10 14:00:00.000000
>     14h

[2019-10 Status set to 'Tentatively Ready' after reflector discussion]

Proposed resolution:

This wording is relative to N4830.

1. Modify 30.12 [time.format] as indicated:

   > -3- Unless explicitly requested, the result of formatting a chrono type does not contain time zone abbreviation and time zone offset information. If the information is available, the conversion specifiers `%Z` and `%z` will format this information (respectively). [Note: If the information is not available and a `%Z` or `%z` conversion specifier appears in the chrono-format-spec, an exception of type `format_error` is thrown, as described above. — end note]

   > -?- If the type being formatted does not contain the information that the format flag needs, an exception of type `format_error` is thrown. [Example: A `duration` does not contain enough information to format as a `weekday` — end example]. However if a flag refers to a "time of day" (e.g. `%H`, `%I`, `%p`, etc.), then a specialization of `duration` is interpreted as the time of day elapsed since midnight.

2. Modify 30.13 [time.parse] as indicated:

   > [Drafting note: The modification of 30.13 [time.parse] p1 is intended to be non-conflictingly mergeable with the change suggested by LWG 3269(i) and LWG 3271(i) at the same paragraph.]

   > -1- Each parse overload specified in this subclause calls `from_stream` unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, let `is` denote an object of type `basic_istream<charT, traits>`, where `charT` and `traits` are template parameters in that context.

   > […]

   > -10- All `from_stream` overloads behave as unformatted input functions, except that they have an unspecified effect on the value returned by subsequent calls to `basic_istream<>::gcount()`. Each overload takes a format string containing ordinary characters and flags which have special meaning. Each flag begins with a `%`. Some flags can be modified by `E` or `O`. During parsing each flag interprets characters as parts of date and time types according to Table [tab:time.parse.spec]. Some flags can be modified by a width parameter given as a positive decimal integer called out as `N` below which governs how many characters are parsed from the stream in interpreting the flag. All characters in the format string that are not represented in Table [tab:time.parse.spec], except for white space, are parsed unchanged from the stream. A white space character matches zero or more white space characters in the input stream.

   > -?- If the type being parsed can not represent the information that the format flag refers to, `is.setstate(ios_base::failbit)` is called. [Example: A `duration` cannot represent a `weekday` — end example]. However if a flag refers to a "time of day" (e.g. `%H`, `%I`, `%p`, etc.), then a specialization of `duration` is parsed as the time of day elapsed since midnight.

### 3273(i). Specify `weekday_indexed` to range of `[0, 7]`

Section: 30.8.7.2 [time.cal.wdidx.members] Status: C++20 Submitter: Howard Hinnant Opened: 2019-09-02 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

On one hand, we say that if you try to construct a `weekday_indexed` with index 0, you get an unspecified index instead (30.8.7.2 [time.cal.wdidx.members]/p1),:

> The values held are unspecified if `!wd.ok()` or index is not in the range `[1, 5]`.

OTOH, we take pains to pin down `year_month_weekday`'s conversion to `sys_days` when the index is zero (30.8.7.2 [time.cal.wdidx.members]/p19):

> If `index()` is `0` the returned `sys_days` represents the date `7` days prior to the first `weekday()` of `year()/month()`.

This is inconsistent. We should allow a slightly wider range (say, `[0, 7]`, since you need at least 3 bits anyway to represent the 5 distinct valid values, and the `0` value referred to by 30.8.7.2 [time.cal.wdidx.members]/p19.

[2019-09-24 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 30.8.7.2 [time.cal.wdidx.members] as indicated:

   > [Drafting note: As a drive-by fix a cleanup of "Constructs an object of type `weekday_indexed`" wording has been applied as well. ]

   >     constexpr weekday_indexed(const chrono::weekday& wd, unsigned index) noexcept;

   > > -1- Effects: Constructs an object of type `weekday_indexed` by initializingInitializes `wd_` with `wd` and `index_` with `index`. The values held are unspecified if `!wd.ok()` or `index` is not in the range `[10, 57]`.

### 3274(i). Missing feature test macro for `<span>`

Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-09-05 Last modified: 2021-02-25

Priority: 0

View all other issues in [support.limits.general].

View all issues with C++20 status.

Discussion:

There is no feature test macro for `std::span`.

For the purposes of SD-6, I think we want two values: `201803L` for the original addition of `<span>` by P0122R7 (Jacksonville, 2018) and then `201902L` for the API changes from P1024R3 (Kona, 2019). The C++ working draft only needs the newer value.

[2019-09-24 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify the Table 36 "Standard library feature-test macros" [tab:support.ft] in 17.3.1 [support.limits.general] as indicated:

   > Table 36: Standard library feature-test macros [tab:support.ft]
   > Macro name | Value | Header(s)
   > --- | --- | ---
   > `[…]`
   > `__cpp_lib_span` | `201902L` | `<span>`
   > `[…]`

### 3276(i). Class `split_view::outer_iterator::value_type` should inherit from `view_interface`

Section: 25.7.16.4 [range.lazy.split.outer.value] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2023-02-07

Priority: 0

View all other issues in [range.lazy.split.outer.value].

View all issues with C++20 status.

Discussion:

It is a view. It should have all the view goodies. Suggested priority P1 because it affects ABI.

The proposed change has been implemented and tested in range-v3.

[2019-09-24 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify [range.split.outer.value], class `split_view::outer_iterator::value_type` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<class V, class Pattern>
   >       template<bool Const>
   >       struct split_view<V, Pattern>::outer_iterator<Const>::value_type
   >         : view_interface<value_type> {
   >       private:
   >         outer_iterator i_ = outer_iterator(); // exposition only
   >       public:
   >         value_type() = default;
   >         constexpr explicit value_type(outer_iterator i);

   >         constexpr inner_iterator<Const> begin() const;
   >         constexpr default_sentinel_t end() const;
   >       };
   >     }

### 3277(i). Pre-increment on prvalues is not a requirement of `weakly_incrementable`

Section: 24.3.4.13 [iterator.concept.random.access] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2021-02-25

Priority: 0

View all other issues in [iterator.concept.random.access].

View all issues with C++20 status.

Discussion:

See 24.3.4.13 [iterator.concept.random.access]/2.6, which shows `++` being applied to a prvalue iterator.

A similar change has already been made to 25.6.4.2 [range.iota.view]/4.6.

Suggest priority P0 or P1 because it effects the definition of a concept.

[2019-09-24 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 24.3.4.13 [iterator.concept.random.access] as indicated:

   > -2- Let `a` and `b` be valid iterators of type `I` such that `b` is reachable from `a` after `n` applications of `++a`, let `D` be `iter_difference_t<I>`, and let `n` denote a value of type `D`. `I` models `random_access_iterator` only if

   > 1. (2.1) — `(a += n)` is equal to `b`.

   > 2. […]

   > 3. (2.6) — If `(a + D(n - 1))` is valid, then `(a + n)` is equal to `++[](I c){ return ++c; }(a + D(n - 1))`.

   > 4. […]

### 3278(i). `join_view<V>::iterator<true>` tries to write through `const join_view` ptr

Section: 25.7.14.2 [range.join.view] Status: Resolved Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2020-09-06

Priority: 2

View all other issues in [range.join.view].

View all issues with Resolved status.

Discussion:

The non-`const` `join_view::begin()` returns `iterator<simple-view<V>>`. If `simple-view<V>` is `true`, then the iterator stores a `const join_view*` named `parent_`. `iterator::satisfy()` will try to write to `parent_->inner_` if `ref_is_glvalue` is `false`. That doesn't work because the `inner_` field is not marked mutable.

[2019-10 Priority set to 2 after reflector discussion]

[2020-02-10, Prague]

Would be resolved by P1983R0.

[2020-08-21 Issue processing telecon: resolved by P1983R0 §2.4. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.7.14.2 [range.join.view], class `template join_view` synopsis, as indicated:

   > [Drafting note: Changing the `join_view<V>::inner_` member to be mutable is safe because this exposition-only member is only used when the `join_view` is single-pass and only modified by operations that invalidate other iterators]

   >     namespace std::ranges {
   >       template<input_range V>
   >         requires view<V> && input_range<range_reference_t<V>>> &&
   >                 (is_reference_v<range_reference_t<V>> ||
   >                 view<range_value_t<V>>)
   >       class join_view : public view_interface<join_view<V>> {
   >       private:
   >         […]
   >         V base_ = V(); // exposition only
   >         mutable all_view<InnerRng> inner_ = // exposition only, present only when
   >                                             // !is_reference_v<InnerRng>
   >           all_view<InnerRng>();
   >       public:
   >         […]
   >       };
   >     }

### 3279(i). `shared_ptr<int>&` does not not satisfy `readable`

Section: 24.3.4.2 [iterator.concept.readable] Status: Resolved Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2020-09-06

Priority: 1

View all issues with Resolved status.

Discussion:

In the current spec, `shared_ptr<int>` is `readable`, but `shared_ptr<int>&` is not. That is because `readable_traits` is not stripping top-level references before testing for nested typedefs.

Either `readable_traits` should see through cv- and ref-qualifiers, or else the `readable` concept should strip top-level references when building the `iter_value_t` associated type (e.g., `iter_value_t<remove_reference_t<In>>`).

Suggest priority P1 because it effects the definition of a concept which cannot change after C++20.

[2019-10 Priority set to 1 after reflector discussion]

[2019-11 This should be resolved by P1878]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 24.3.4.2 [iterator.concept.readable], concept `readable` synopsis, as indicated:

>    >     template<class In>
>    >       concept readable =
>    >         requires {
>    >           typename iter_value_t<remove_reference_t<In>>;
>    >           typename iter_reference_t<In>;
>    >           typename iter_rvalue_reference_t<In>;
>    >         } &&
>    >         common_reference_with<iter_reference_t<In>&&, iter_value_t<remove_reference_t<In>>&> &&
>    >         common_reference_with<iter_reference_t<In>&&, iter_rvalue_reference_t<In>&&> &&
>    >         common_reference_with<iter_rvalue_reference_t<In>&&, const iter_value_t<remove_reference_t<In>>&>;

[2019-11; Resolved by the adoption of P1878 in Belfast]

Proposed resolution:

Resolved by P1878.

### 3280(i). View converting constructors can cause constraint recursion and are unneeded

Section: 25.7.8.2 [range.filter.view], 25.7.9.2 [range.transform.view], 25.7.10.2 [range.take.view], 25.7.14.2 [range.join.view], 25.7.17.2 [range.split.view], 25.7.21.2 [range.reverse.view] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2021-02-25

Priority: 1

View all issues with C++20 status.

Discussion:

The following program fails to compile:

>     #include <ranges>

>     int main() {
>       namespace ranges = std::ranges;
>       int a[] = {1, 7, 3, 6, 5, 2, 4, 8};
>       auto r0 = ranges::view::reverse(a);
>       auto is_even = [](int i) { return i % 2 == 0; };
>       auto r1 = ranges::view::filter(r0, is_even);
>       int sum = 0;
>       for (auto i : r1) {
>         sum += i;
>       }
>       return sum - 20;
>     }

The problem comes from constraint recursion, caused by the following constructor:

>     template<viewable_range R>
>       requires bidirectional_range<R> && constructible_from<V, all_view<R>>
>     constexpr explicit reverse_view(R&& r);

This constructor owes its existence to class template argument deduction; it is the constructor we intend to use to resolve `reverse_view{r}`, which (in accordance to the deduction guide) will construct an object of type `reverse_view<all_view<decltype(r)>>`.

However, we note that `all_view<R>` is always one of:

- `decay_t<R>`

- `ref_view<remove_reference_t<R>>`

- `subrange<iterator_t<R>, sentinel_t<R>, [sized?]>`

In all cases, there is a conversion from `r` to the destination type. As a result, the following non-template `reverse_view` constructor can fulfill the duty that the above constructor was meant to fulfill, and does not cause constraint recursion:

>     constexpr explicit reverse_view(V r);

In short, the problematic constructor can simply be removed with no negative impact on the design. And the similar constructors from the other range adaptors should similarly be stricken.

Suggested priority P1. The view types are unusable without this change.

This proposed resolution has been implemented in range-v3 and has been shipping for some time.

[2019-10 Priority set to 1 after reflector discussion]

[2019-10 Status set to ready Wednesday night discussion in Belfast.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.7.8.2 [range.filter.view] as indicated:

   >     namespace std::ranges {
   >       template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
   >         requires view<V> && is_object_v<Pred>
   >       class filter_view : public view_interface<filter_view<V, Pred>> {
   >       […]
   >       public:
   >         filter_view() = default;
   >         constexpr filter_view(V base, Pred pred);
   >         template<input_range R>
   >           requires viewable_range<R> && constructible_from<V, all_view<R>>
   >         constexpr filter_view(R&& r, Pred pred);
   >         […]
   >       };
   >       […]
   >     }

   > […]

   >     template<input_range R>
   >       requires viewable_range<R> && constructible_from<V, all_view<R>>
   >     constexpr filter_view(R&& r, Pred pred);

   > > -2- Effects: Initializes `base_` with `views::all(std::forward<R>(r))` and initializes `pred_` with `std::move(pred)`.

2. Modify 25.7.9.2 [range.transform.view] as indicated:

   >     namespace std::ranges {
   >       template<input_range V, copy_constructible F>
   >         requires view<V> && is_object_v<F> &&
   >         regular_invocable<F&, range_reference_t<V>>
   >       class transform_view : public view_interface<transform_view<V, F>> {
   >       private:
   >         […]
   >       public:
   >         transform_view() = default;
   >         constexpr transform_view(V base, F fun);
   >         template<input_range R>
   >           requires viewable_range<R> && constructible_from<V, all_view<R>>
   >         constexpr transform_view(R&& r, F fun);
   >         […]
   >       };
   >       […]
   >     }

   > […]

   >     template<input_range R>
   >       requires viewable_range<R> && constructible_from<V, all_view<R>>
   >     constexpr transform_view(R&& r, F fun);

   > > -2- Effects: Initializes `base_` with `views::all(std::forward<R>(r))` and `fun_` with `std::move(fun)`.

3. Modify 25.7.10.2 [range.take.view] as indicated:

   >     namespace std::ranges {
   >       template<view V>
   >       class take_view : public view_interface<take_view<V>> {
   >       private:
   >         […]
   >       public:
   >         take_view() = default;
   >         constexpr take_view(V base, range_difference_t<V> count);
   >         template<viewable_range R>
   >           requires constructible_from<V, all_view<R>>
   >         constexpr take_view(R&& r, range_difference_t<V> count);
   >         […]
   >       };
   >     […]
   >     }

   > […]

   >     template<viewable_range R>
   >       requires constructible_from<V, all_view<R>>
   >     constexpr take_view(R&& r, range_difference_t<V> count);

   > > -2- Effects: Initializes `base_` with `views::all(std::forward<R>(r))` and `count_` with `count`.

4. Modify 25.7.14.2 [range.join.view] as indicated:

   >     namespace std::ranges {
   >       template<input_range V>
   >         requires view<V> && input_range<range_reference_t<V>> &&
   >                  (is_reference_v<range_reference_t<V>> ||
   >                  view<range_value_t<V>>)
   >       class join_view : public view_interface<join_view<V>> {
   >       private:
   >         […]
   >       public:
   >         join_view() = default;
   >         constexpr explicit join_view(V base);
   >         template<input_range R>
   >           requires viewable_range<R> && constructible_from<V, all_view<R>>
   >         constexpr explicit join_view(R&& r);
   >         […]
   >       };
   >     […]
   >     }

   > […]

   >     template<input_range R>
   >       requires viewable_range<R> && constructible_from<V, all_view<R>>
   >     constexpr explicit join_view(R&& r);

   > > -2- Effects: Initializes `base_` with `views::all(std::forward<R>(r))`.

5. Modify 25.7.17.2 [range.split.view] as indicated:

   >     namespace std::ranges {
   >     […]
   >       template<input_range V, forward_range Pattern>
   >         requires view<V> && view<Pattern> &&
   >                  indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
   >                  (forward_range<V> || tiny-range<Pattern>)
   >       class split_view : public view_interface<split_view<V, Pattern>> {
   >       private:
   >         […]
   >       public:
   >         split_view() = default;
   >         constexpr split_view(V base, Pattern pattern);
   >         template<input_range R, forward_range P>
   >           requires constructible_from<V, all_view<R>> &&
   >                    constructible_from<Pattern, all_view<P>>
   >         constexpr split_view(R&& r, P&& p);
   >         […]
   >       };
   >     […]
   >     }

   > […]

   >     template<input_range R, forward_range P>
   >       requires constructible_from<V, all_view<R>> &&
   >                constructible_from<Pattern, all_view<P>>
   >     constexpr split_view(R&& r, P&& p);

   > > -2- Effects: Initializes `base_` with `views::all(std::forward<R>(r))`, and `pattern_` with `views::all(std::forward<P>(p))`.

6. Modify 25.7.21.2 [range.reverse.view] as indicated:

   >     namespace std::ranges {
   >       template<view V>
   >         requires bidirectional_range<V>
   >       class reverse_view : public view_interface<reverse_view<V>> {
   >       private:
   >         […]
   >       public:
   >         reverse_view() = default;
   >         constexpr explicit reverse_view(V r);
   >         template<viewable_range R>
   >           requires bidirectional_range<R> && constructible_from<V, all_view<R>>
   >         constexpr explicit reverse_view(R&& r);
   >         […]
   >       };
   >     […]
   >     }

   > […]

   >     template<viewable_range R>
   >       requires bidirectional_range<R> && constructible_from<V, all_view<R>>
   >     constexpr explicit reverse_view(R&& r);

   > > -2- Effects: Initializes `base_` with `views::all(std::forward<R>(r))`.

### 3281(i). Conversion from `pair-like` types to `subrange` is a silent semantic promotion

Section: 25.5.4 [range.subrange] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25

Priority: 1

View all other issues in [range.subrange].

View all issues with C++20 status.

Discussion:

Just because a `pair` is holding two iterators, it doesn't mean those two iterators denote a valid range. Implicitly converting such `pair-like` types to a `subrange` is dangerous and should be disallowed.

Suggested priority P1.

Implementation experience: range-v3's subrange lacks these constructors.

[2019-10 Priority set to 1 and status to LEWG after reflector discussion]

[2019-11 Status to Ready after LWG discussion Friday in Belfast.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.5.4 [range.subrange] as indicated:

   >     namespace std::ranges {
   >     […]
   >       template<class T, class U, class V>
   >         concept pair-like-convertible-to = // exposition only
   >           !range<T> && pair-like<remove_reference_t<T>> &&
   >           requires(T&& t) {
   >             { get<0>(std::forward<T>(t)) } -> convertible_to<U>;
   >             { get<1>(std::forward<T>(t)) } -> convertible_to<V>;
   >           };
   >     […]
   >       template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K =
   >                sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized>
   >         requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
   >       class subrange : public view_interface<subrange<I, S, K>> {
   >       private:
   >         […]
   >       public:
   >         […]
   >         template<not-same-as<subrange> PairLike>
   >           requires pair-like-convertible-to<PairLike, I, S>
   >         constexpr subrange(PairLike&& r) requires (!StoreSize)
   >           : subrange{std::get<0>(std::forward<PairLike>(r)),
   >                      std::get<1>(std::forward<PairLike>(r))}
   >         {}
   >         template<pair-like-convertible-to<I, S> PairLike>
   >         constexpr subrange(PairLike&& r, make-unsigned-like-t(iter_difference_t<I>) n)
   >           requires (K == subrange_kind::sized)
   >           : subrange{std::get<0>(std::forward<PairLike>(r)),
   >                      std::get<1>(std::forward<PairLike>(r)), n}
   >         {}
   >       […]
   >       };
   >     […]
   >     }

### 3282(i). `subrange` converting constructor should disallow derived to base conversions

Section: 25.5.4 [range.subrange] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25

Priority: 1

View all other issues in [range.subrange].

View all issues with C++20 status.

Discussion:

The following code leads to slicing and general badness:

>     struct Base {};
>     struct Derived : Base {};
>     subrange<Derived*> sd;
>     subrange<Base*> sb = sd;

Traversal operations on iterators that are pointers do pointer arithmetic. If a `Base*` is actually pointing to a `Derived*`, then pointer arithmetic is invalid. `subrange`'s constructors can easily flag this invalid code, and probably should.

The following PR incorporates the suggested fix to issue LWG 3281(i) I previously reported.

Suggested priority: P1, since it will be hard to fix this after C++20 ships.

[2019-10 Priority set to 1 and status to LEWG after reflector discussion]

[2019-10; Marshall comments]

This issue would resolve US-285.

[2019-11 LEWG says OK; Status to Open. Friday PM discussion in Belfast. Casey to investigate and report back.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 25.5.4 [range.subrange] as indicated:

>    >     namespace std::ranges {
>    >       template<class From, class To>
>    >         concept convertible-to-non-slicing = // exposition only
>    >           convertible_to<From, To> &&
>    >           !(is_pointer_v<decay_t<From>> &&
>    >           is_pointer_v<decay_t<To>> &&
>    >           not-same-as<remove_pointer_t<decay_t<From>>, remove_pointer_t<decay_t<To>>>);

>    >       template<class T>
>    >         concept pair-like = // exposition only
>    >           […]

>    >       template<class T, class U, class V>
>    >         concept pair-like-convertible-to = // exposition only
>    >           !range<T> && pair-like<remove_reference_t<T>> &&
>    >           requires(T&& t) {
>    >             { get<0>(std::forward<T>(t)) } -> convertible_to<U>;
>    >             { get<1>(std::forward<T>(t)) } -> convertible_to<V>;
>    >           };

>    >        template<class T, class U, class V>
>    >          concept pair-like-convertible-from = // exposition only
>    >            !range<T> && pair-like<T> &&
>    >            constructible_from<T, U, V> &&
>    >            convertible-to-non-slicing<U, tuple_element_t<0, T>> &&
>    >            convertible_to<V, tuple_element_t<1, T>>;

>    >     […]
>    >     […]
>    >       template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K =
>    >                sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized>
>    >         requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
>    >       class subrange : public view_interface<subrange<I, S, K>> {
>    >       private:
>    >         […]
>    >       public:
>    >         subrange() = default;

>    >         constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize);

>    >         constexpr subrange(convertible-to-non-slicing<I> auto i, S s,
>    >                            make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized);

>    >         template<not-same-as<subrange> R>
>    >           requires forwarding-range<R> &&
>    >             convertible_toconvertible-to-non-slicing<iterator_t<R>, I> &&
>    >             convertible_to<sentinel_t<R>, S>
>    >         constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);

>    >         template<forwarding-range R>
>    >           requires convertible_to<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S>
>    >         constexpr subrange(R&& r, make-unsigned-like-t(iter_difference_t<I>) n)
>    >           requires (K == subrange_kind::sized)
>    >             : subrange{ranges::begin(r), ranges::end(r), n}
>    >         {}

>    >         template<not-same-as<subrange> PairLike>
>    >           requires pair-like-convertible-to<PairLike, I, S>
>    >         constexpr subrange(PairLike&& r) requires (!StoreSize)
>    >           : subrange{std::get<0>(std::forward<PairLike>(r)),
>    >                      std::get<1>(std::forward<PairLike>(r))}
>    >         {}

>    >         template<pair-like-convertible-to<I, S> PairLike>
>    >         constexpr subrange(PairLike&& r, make-unsigned-like-t(iter_difference_t<I>) n)
>    >           requires (K == subrange_kind::sized)
>    >           : subrange{std::get<0>(std::forward<PairLike>(r)),
>    >                      std::get<1>(std::forward<PairLike>(r)), n}
>    >         {}
>    >       […]
>    >       };

>    >       template<input_or_output_iterator I, sentinel_for<I> S>
>    >       subrange(I, S) -> subrange<I, S>;

>    >       […]
>    >     }

> 2. Modify 25.5.4.2 [range.subrange.ctor] as indicated:

>    >     constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize);

>    > > -1- Expects: […]

>    > > […]

>    >     constexpr subrange(convertible-to-non-slicing<I> auto i, S s,
>    >                        make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized);

>    > > -2- Expects: […]

>    > > […]

>    >     template<not-same-as<subrange> R>
>    >       requires forwarding-range<R> &&
>    >         convertible_toconvertible-to-non-slicing<iterator_t<R>, I> &&
>    >         convertible_to<sentinel_t<R>, S>
>    >     constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);

>    > > -6- Effects: […]

>    > > […]

[2020-02-10; Prague]

The group identified minor problems that have been fixed in the revised wording.

[2020-02-10 Move to Immediate Monday afternoon in Prague]

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.5.4 [range.subrange] as indicated:

   >     namespace std::ranges {
   >       template<class From, class To>
   >         concept convertible-to-non-slicing = // exposition only
   >           convertible_to<From, To> &&
   >           !(is_pointer_v<decay_t<From>> &&
   >           is_pointer_v<decay_t<To>> &&
   >           not-same-as<remove_pointer_t<decay_t<From>>, remove_pointer_t<decay_t<To>>>);

   >       template<class T>
   >         concept pair-like = // exposition only
   >           […]

   >       template<class T, class U, class V>
   >         concept pair-like-convertible-to = // exposition only
   >           !range<T> && pair-like<remove_reference_t<T>> &&
   >           requires(T&& t) {
   >             { get<0>(std::forward<T>(t)) } -> convertible_to<U>;
   >             { get<1>(std::forward<T>(t)) } -> convertible_to<V>;
   >           };

   >        template<class T, class U, class V>
   >          concept pair-like-convertible-from = // exposition only
   >            !range<T> && pair-like<T> &&
   >            constructible_from<T, U, V> &&
   >            convertible-to-non-slicing<U, tuple_element_t<0, T>> &&
   >            convertible_to<V, tuple_element_t<1, T>>;

   >     […]
   >     […]
   >       template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K =
   >                sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized>
   >         requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
   >       class subrange : public view_interface<subrange<I, S, K>> {
   >       private:
   >         […]
   >       public:
   >         subrange() = default;

   >         constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize);

   >         constexpr subrange(convertible-to-non-slicing<I> auto i, S s,
   >                            make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized);

   >         template<not-same-as<subrange> R>
   >           requires forwarding-range<R> &&
   >             convertible_toconvertible-to-non-slicing<iterator_t<R>, I> &&
   >             convertible_to<sentinel_t<R>, S>
   >         constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);

   >         template<forwarding-range R>
   >           requires convertible_toconvertible-to-non-slicing<iterator_t<R>, I> &&
   >     	    convertible_to<sentinel_t<R>, S>
   >         constexpr subrange(R&& r, make-unsigned-like-t(iter_difference_t<I>) n)
   >           requires (K == subrange_kind::sized)
   >             : subrange{ranges::begin(r), ranges::end(r), n}
   >         {}

   >         template<not-same-as<subrange> PairLike>
   >           requires pair-like-convertible-to<PairLike, I, S>
   >         constexpr subrange(PairLike&& r) requires (!StoreSize)
   >           : subrange{std::get<0>(std::forward<PairLike>(r)),
   >                      std::get<1>(std::forward<PairLike>(r))}
   >         {}

   >         template<pair-like-convertible-to<I, S> PairLike>
   >         constexpr subrange(PairLike&& r, make-unsigned-like-t(iter_difference_t<I>) n)
   >           requires (K == subrange_kind::sized)
   >           : subrange{std::get<0>(std::forward<PairLike>(r)),
   >                      std::get<1>(std::forward<PairLike>(r)), n}
   >         {}
   >       […]
   >       };

   >       template<input_or_output_iterator I, sentinel_for<I> S>
   >       subrange(I, S) -> subrange<I, S>;

   >       […]
   >     }

2. Modify 25.5.4.2 [range.subrange.ctor] as indicated:

   >     constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize);

   > > -1- Expects: […]

   > > […]

   >     constexpr subrange(convertible-to-non-slicing<I> auto i, S s,
   >                        make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized);

   > > -2- Expects: […]

   > > […]

   >     template<not-same-as<subrange> R>
   >       requires forwarding-range<R> &&
   >         convertible_toconvertible-to-non-slicing<iterator_t<R>, I> &&
   >         convertible_to<sentinel_t<R>, S>
   >     constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);

   > > -6- Effects: […]

   > > […]

### 3283(i). Types satisfying `input_iterator` but not `equality_comparable` look like C++17 output iterators

Section: 24.3.2.3 [iterator.traits] Status: Resolved Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-05-18

Priority: 2

View all other issues in [iterator.traits].

View all issues with Resolved status.

Discussion:

In C++20, if an iterator doesn't define all of the associated iterator types (value, category, reference, and difference), the primary `std::iterator_traits` template picks a category based on structural conformance to a set of implementation-defined concepts that capture the old iterator requirements tables. (See 24.3.2.3 [iterator.traits].) In C++17, input iterators were required to be equality-comparable with themselves. In C++20 that is not the case, so such iterators must not be given `intput_iterator_tag` as a category. They don't, so that's all well and good.

However, 24.3.2.3 [iterator.traits] concludes that, since such an iterator cannot be an input iterator, it must therefor be an output iterator, and it assigns it a category of `std::output_iterator_tag`. It does this even if there is a nested `iterator_category` typedef declaring the iterator to be input. (This will happen frequently as C++20 iterators don't require iterators to declare their reference type, for instance.) This will be extremely confusing to users who, understandably, will be at a loss to understand why the legacy STL algorithms think their iterator is an output iterator when they have clearly stated that the iterator is input!

The fix is to tweak the specification such that the output category is assigned to an iterator only (a) if it declares its category to be output, or (b) it doesn't specify a category at all. The result, for the user, is that their iterator simply won't look like a C++17 iterator at all, because it isn't!

Suggested priority: P1. We can't make this change after C++20 because it would be an observable change.

This fix has been implemented in range-v3.

[2019-10-12 Priority set to 1 after reflector discussion]

[2019-11 Wednesday night Issue processing in Belfast]

Much discussion along with 3289(i). CC to write rationale for NAD.

[2020-02-13, Prague; Priority reduced to 2 after LWG discussion]

[2021-05-18 Resolved by the adoption of P2259R1 at the February 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4830.

1. Modify 24.3.2.3 [iterator.traits] as indicated:

   > -3- The members of a specialization `iterator_traits<I>` generated from the `iterator_traits` primary template are computed as follows:

   > 1. (3.1) — If `I` has valid […]

   > 2. […]

   > 3. (3.3) — Otherwise, if `I` satisfies the exposition-only concept `cpp17-iterator` and either

   >    1. `I::iterator_category` is valid and denotes `output_iterator_tag` or a type publicly and unambiguously derived from `output_iterator_tag`, or

   >    2. there is no type `I::iterator_category`

   >    then `iterator_traits<I>` has the following publicly accessible members:

   > 4. […]

### 3284(i). `random_access_iterator` semantic constraints accidentally promote difference type using unary negate

Section: 24.3.4.13 [iterator.concept.random.access] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25

Priority: 0

View all other issues in [iterator.concept.random.access].

View all issues with C++20 status.

Discussion:

24.3.4.13 [iterator.concept.random.access]/p2.7 says:

> `(b += -n)` is equal to `a`

Unary minus can do integer promotion. That is not the intent here.

[2019-10-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 24.3.4.13 [iterator.concept.random.access] as indicated:

   > -2- Let `a` and `b` be valid iterators of type `I` such that `b` is reachable from `a` after `n` applications of `++a`, let `D` be `iter_difference_t<I>`, and let `n` denote a value of type `D`. `I` models `random_access_iterator` only if:

   > 1. (2.1) — `(a += n)` is equal to `b`.

   > 2. […]

   > 3. (2.7) — `(b += D(-n))` is equal to `a`.

   > 4. […]

### 3285(i). The type of a customization point object shall satisfy `semiregular`

Section: 16.3.3.3.5 [customization.point.object] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25

Priority: 0

View all other issues in [customization.point.object].

View all issues with C++20 status.

Discussion:

We should be testing the un-cv-qualified type of a customization point object.

[2019-10-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 16.3.3.3.5 [customization.point.object] as indicated:

   > -2- The type of a customization point object ignoring cv-qualifiers shall model `semiregular` (18.6 [concepts.object]).

### 3286(i). `ranges::size` is not required to be valid after a call to `ranges::begin` on an input range

Section: 25.7.10.2 [range.take.view], 25.5.4.2 [range.subrange.ctor] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25

Priority: 0

View all other issues in [range.take.view].

View all issues with C++20 status.

Discussion:

On an input (but not forward) range, `begin(rng)` is not required to be an equality-preserving expression (25.4.2 [range.range]/3.3). If the range is also sized, then it is not valid to call `size(rng)` after `begin(rng)` (25.4.4 [range.sized]/2.2). In several places in the ranges clause, this precondition is violated. A trivial re-expression of the effects clause fixes the problem.

[2019-10-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.7.10.2 [range.take.view], class template `take_view` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<view V>
   >       class take_view : public view_interface<take_view<V>> {
   >       private:
   >         […]
   >       public:
   >         […]
   >         constexpr auto begin() requires (!simple-view<V>) {
   >           if constexpr (sized_range<V>) {
   >             if constexpr (random_access_range<V>)
   >               return ranges::begin(base_);
   >             else {
   >               auto sz = size();
   >               return counted_iterator{ranges::begin(base_), size()sz};
   >             }
   >           } else
   >             return counted_iterator{ranges::begin(base_), count_};
   >         }

   >         constexpr auto begin() const requires range<const V> {
   >           if constexpr (sized_range<const V>) {
   >             if constexpr (random_access_range<const V>)
   >               return ranges::begin(base_);
   >             else {
   >               auto sz = size();
   >               return counted_iterator{ranges::begin(base_), size()sz};
   >             }
   >           } else
   >             return counted_iterator{ranges::begin(base_), count_};
   >         }

   >         […]
   >       };
   >       […]
   >     }

2. Modify 25.5.4.2 [range.subrange.ctor] as indicated:

   >     template<not-same-as<subrange> R>
   >       requires forwarding-range<R> &&
   >         convertible_to<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S>
   >     constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);

   > > -6- Effects: Equivalent to:

   > > 1. (6.1) — If `StoreSize` is `true`, `subrange{ranges::begin(r), ranges::end(r)r, ranges::size(r)}`.

   > > 2. (6.2) — Otherwise, `subrange{ranges::begin(r), ranges::end(r)}`.

### 3289(i). Cannot opt out of C++17 iterator-ness without also opting out of C++20 iterator-ness

Section: 24.3.2.3 [iterator.traits], 24.5.5.2 [common.iter.types] Status: Resolved Submitter: Eric Niebler Opened: 2019-09-11 Last modified: 2021-05-18

Priority: 2

View all other issues in [iterator.traits].

View all issues with Resolved status.

Discussion:

The way to non-intrusively say that a type doesn't satisfy the C++17 iterator requirements is to specialize `std::iterator_traits` and not provide the nested typedefs. However, if a user were to do that, she would also be saying that the type is not a C++20 iterator. That is because `readable` and `weakly_incrementable` are specified in terms of `iter_value_t<I>` and `iter_difference_t<I>`. Those aliases check to see if `std::iterator_traits<I>` has been specialized (answer: yes), and if so resolve to `std::iterator_traits<I>::value_type` and `std::iterator_traits<I>::difference_type` respectively.

The proper way to opt out of C++17 iterator-ness while opting in to C++20 iterator-ness would be to specialize `std::iterator_traits` and specify all the nested typedefs except `::iterator_category`. That's a bit weird and may throw off code that is expecting all the typedefs to be there, or none of them, so instead we can suggest users to set the `iterator_category` typedef to denote `output_iterator_tag`, which is a harmless lie; generic C++17 code will get the message: this iterator is not a c++17 input iterator, which is the salient bit.

We then must fix up all the places in the Ranges clause that make faulty assumptions about an iterator's `iterator_category` typedef (as distinct from the iterator concept that it models).

[2019-10-19 Issue Prioritization]

Priority to 1 after reflector discussion.

[2019-11 Wednesday night Issue processing in Belfast]

Much discussion; no consensus that this is a good approach. Need to coordinate between this and 3283(i)

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 24.3.2.3 [iterator.traits] as indicated:

>    > -4- Explicit or partial specializations of `iterator_traits` may have a member type `iterator_concept` that is used to indicate conformance to the iterator concepts (24.3.4 [iterator.concepts]). [Example: To indicate conformance to the `input_iterator` concept but a lack of conformance to the Cpp17InputIterator requirements (24.3.5.3 [input.iterators]), an `iterator_traits` specialization might have `iterator_concept` denote `input_iterator_tag` and `iterator_category` denote `output_iterator_tag`. — end example]

> 2. Modify 24.5.5.2 [common.iter.types] as indicated:

>    > -1- The nested typedef-names of the specialization of `iterator_traits` for `common_iterator<I, S>` are defined as follows.

>    > 1. (1.1) — `iterator_concept` denotes `forward_iterator_tag` if `I` models `forward_iterator`; otherwise it denotes `input_iterator_tag`.

>    > 2. (1.2) — `iterator_category` denotes `forward_iterator_tag` if `iterator_traits<I>::iterator_category` models `derived_from<forward_iterator_tag>`; otherwise it denotes `input_iterator_tagiterator_traits<I>::iterator_category`.

>    > 3. (1.3) — If the expression `a.operator->()` is well-formed, where `a` is an lvalue of type `const common_iterator<I, S>`, then `pointer` denotes the type of that expression. Otherwise, pointer denotes `void`.

> 3. Modify 25.7.8.3 [range.filter.iterator] as indicated:

>    > -3- `iterator::iterator_category` is defined as follows:

>    > 1. (3.1) — Let `C` denote the type `iterator_traits<iterator_t<V>>::iterator_category`.

>    > 2. (3.2) — If `C` models `derived_from<bidirectional_iterator_tag>`, then `iterator_category` denotes `bidirectional_iterator_tag`.

>    > 3. (3.3) — Otherwise, if `C` models `derived_from<forward_iterator_tag>`, then `iterator_category` denotes `forward_iterator_tag`.

>    > 4. (3.4) — Otherwise, `iterator_category` denotes `input_iterator_tagC`.

> 4. Modify 25.7.14.3 [range.join.iterator] as indicated:

>    > -3- `iterator::iterator_category` is defined as follows:

>    > 1. (3.1) — Let `OUTERC` denote `iterator_traits<iterator_t<Base>>::iterator_category`, and let `INNERC` denote `iterator_traits<iterator_t<range_reference_t<Base>>>::iterator_category`.

>    > 2. (3.?) — If `OUTERC` does not model `derived_from<input_iterator_tag>`, `iterator_category` denotes `OUTERC`.

>    > 3. (3.?) — Otherwise, if `INNERC` does not model `derived_from<input_iterator_tag>`, `iterator_category` denotes `INNERC`.

>    > 4. (3.2) — Otherwise, iIf `ref_is_glvalue` is `true`,

>    >    1. (3.2.1) — If `OUTERC` and `INNERC` each model `derived_from<bidirectional_iterator_tag>`, `iterator_category` denotes `bidirectional_iterator_tag`.

>    >    2. (3.2.2) — Otherwise, if `OUTERC` and `INNERC` each model `derived_from<forward_iterator_tag>`, `iterator_category` denotes `forward_iterator_tag`.

>    > 5. (3.3) — Otherwise, `iterator_category` denotes `input_iterator_tag`.

> 5. Modify [range.split.outer] as indicated:

>    > >     namespace std::ranges {
>    > >       template<class V, class Pattern>
>    > >       template<bool Const>
>    > >       struct split_view<V, Pattern>::outer_iterator {
>    > >       private:
>    > >         […]
>    > >       public:
>    > >         using iterator_concept =
>    > >           conditional_t<forward_range<Base>, forward_iterator_tag, input_iterator_tag>;
>    > >         using iterator_category = see belowinput_iterator_tag;
>    > >         […]
>    > >       };
>    > >       […]
>    > >     }

>    > -?- The typedef-name `iterator_category` denotes `input_iterator_tag` if `iterator_traits<iterator_t<Base>>::iterator_category` models `derived_from<input_iterator_tag>`, and `iterator_traits<iterator_t<Base>>::iterator_category` otherwise.

>    > -1- Many of the following specifications refer to the notional member `current` of `outer_iterator`. `current` is equivalent to `current_` if `V` models `forward_range`, and `parent_->current_` otherwise.

> 6. Modify [range.split.inner] as indicated:

>    > -1- The typedef-name `iterator_category` denotes `forward_iterator_tag` if `iterator_traits<iterator_t<Base>>::iterator_category` models `derived_from<forward_iterator_tag>`, and `input_iterator_tagiterator_traits<iterator_t<Base>>::iterator_category` otherwise.

[2020-02-10, Prague]

The issue is out of sync with the current working draft, Daniel provides a synchronized merge.

[2020-02-13, Prague; Priority reduced to 2 after LWG discussion]

[2021-05-18 Resolved by the adoption of P2259R1 at the February 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 24.3.2.3 [iterator.traits] as indicated:

   > -4- Explicit or partial specializations of `iterator_traits` may have a member type `iterator_concept` that is used to indicate conformance to the iterator concepts (24.3.4 [iterator.concepts]). [Example: To indicate conformance to the `input_iterator` concept but a lack of conformance to the Cpp17InputIterator requirements (24.3.5.3 [input.iterators]), an `iterator_traits` specialization might have `iterator_concept` denote `input_iterator_tag` and `iterator_category` denote `output_iterator_tag`. — end example]

2. Modify 24.5.5.2 [common.iter.types] as indicated:

   > -1- The nested typedef-names of the specialization of `iterator_traits` for `common_iterator<I, S>` are defined as follows.

   > 1. (1.1) — `iterator_concept` denotes `forward_iterator_tag` if `I` models `forward_iterator`; otherwise it denotes `input_iterator_tag`.

   > 2. (1.2) — `iterator_category` denotes `forward_iterator_tag` if `iterator_traits<I>::iterator_category` models `derived_from<forward_iterator_tag>`; otherwise it denotes `input_iterator_tagiterator_traits<I>::iterator_category`.

   > 3. (1.3) — If the expression `a.operator->()` is well-formed, where `a` is an lvalue of type `const common_iterator<I, S>`, then `pointer` denotes the type of that expression. Otherwise, pointer denotes `void`.

3. Modify 25.7.8.3 [range.filter.iterator] as indicated:

   > -3- `iterator::iterator_category` is defined as follows:

   > 1. (3.1) — Let `C` denote the type `iterator_traits<iterator_t<V>>::iterator_category`.

   > 2. (3.2) — If `C` models `derived_from<bidirectional_iterator_tag>`, then `iterator_category` denotes `bidirectional_iterator_tag`.

   > 3. (3.3) — Otherwise, if `C` models `derived_from<forward_iterator_tag>`, then `iterator_category` denotes `forward_iterator_tag`.

   > 4. (3.4) — Otherwise, `iterator_category` denotes `C`.

4. Modify 25.7.14.3 [range.join.iterator] as indicated:

   > -2- `iterator::iterator_category` is defined as follows:

   > 1. (2.1) — Let `OUTERC` denote `iterator_traits<iterator_t<Base>>::iterator_category`, and let `INNERC` denote `iterator_traits<iterator_t<range_reference_t<Base>>>::iterator_category`.

   > 2. (2.?) — If `OUTERC` does not model `derived_from<input_iterator_tag>`, `iterator_category` denotes `OUTERC`.

   > 3. (2.?) — Otherwise, if `INNERC` does not model `derived_from<input_iterator_tag>`, `iterator_category` denotes `INNERC`.

   > 4. (2.2) — Otherwise, iIf `ref-is-glvalue` is `true` and `OUTERC` and `INNERC` each model `derived_from<bidirectional_iterator_tag>`, `iterator_category` denotes `bidirectional_iterator_tag`.

   > 5. (2.3) — Otherwise, if `ref-is-glvalue` is `true` and `OUTERC` and `INNERC` each model `derived_from<forward_iterator_tag>`, `iterator_category` denotes `forward_iterator_tag`.

   > 6. (2.4) — Otherwise, if `OUTERC` and `INNERC` each model `derived_from<input_iterator_tag>`, `iterator_category` denotes `input_iterator_tag`.

   > 7. (2.5) — Otherwise, `iterator_category` denotes `output_iterator_tag`.

5. Modify [range.split.outer] as indicated:

   > [Drafting note: The previous wording change has been adjusted to follow the pattern used in [range.split.inner] p1.]

   > >     namespace std::ranges {
   > >       template<class V, class Pattern>
   > >       template<bool Const>
   > >       struct split_view<V, Pattern>::outer_iterator {
   > >       private:
   > >         […]
   > >       public:
   > >         using iterator_concept =
   > >           conditional_t<forward_range<Base>, forward_iterator_tag, input_iterator_tag>;
   > >         using iterator_category = see belowinput_iterator_tag;
   > >         […]
   > >       };
   > >       […]
   > >     }

   > -?- The typedef-name `iterator_category` denotes:

   > 1. (?.?) — `input_iterator_tag` if `iterator_traits<iterator_t<Base>>::iterator_category` models `derived_from<input_iterator_tag>`;

   > 2. (?.?) — otherwise, `iterator_traits<iterator_t<Base>>::iterator_category`.

   > -1- Many of the following specifications refer to the notional member `current` of `outer-iterator`. `current` is equivalent to `current_` if `V` models `forward_range`, and `parent_->current_` otherwise.

### 3290(i). Are `std::format` field widths code units, code points, or something else?

Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Tom Honermann Opened: 2019-09-08 Last modified: 2021-02-25

Priority: Not Prioritized

View other active issues in [format.string.std].

View all other issues in [format.string.std].

View all issues with C++20 status.

Discussion:

28.5.2.2 [format.string.std] p7 states:

> The positive-integer in width is a decimal integer defining the minimum field width. If width is not specified, there is no minimum field width, and the field width is determined based on the content of the field.

Is field width measured in code units, code points, or something else?

Consider the following example assuming a UTF-8 locale:

>     std::format("{}", "\xC3\x81");     // U+00C1        { LATIN CAPITAL LETTER A WITH ACUTE }
>     std::format("{}", "\x41\xCC\x81"); // U+0041 U+0301 { LATIN CAPITAL LETTER A } { COMBINING ACUTE ACCENT }

In both cases, the arguments encode the same user-perceived character (Á). The first uses two UTF-8 code units to encode a single code point that represents a single glyph using a composed Unicode normalization form. The second uses three code units to encode two code points that represent the same glyph using a decomposed Unicode normalization form.

How is the field width determined? If measured in code units, the first has a width of 2 and the second of 3. If measured in code points, the first has a width of 1 and the second of 2. If measured in grapheme clusters, both have a width of 1. Is the determination locale dependent? Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 28.5.2.2 [format.string.std] as indicated:

>    > -7- The positive-integer in width is a decimal integer defining the minimum field width. If width is not specified, there is no minimum field width, and the field width is determined based on the content of the field. Field width is measured in code units. Each byte of a multibyte character contributes to the field width.

[2020-02-13, Prague]

Resolved by P1868R2

[2020-04-07 Voted into the WP in Prague. Status changed: New → WP.]

Proposed resolution:

### 3291(i). `iota_view::iterator` has the wrong `iterator_category`

Section: 25.6.4.3 [range.iota.iterator] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-13 Last modified: 2021-02-25

Priority: 0

View other active issues in [range.iota.iterator].

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Discussion:

In the old way of looking at the world, forward iterators need to return real references. Since dereferencing `iota_view`'s iterators returns by value, it cannot be a C++17 forward iterator. (It can, however, be a C++20 `forward_iterator`.) However, `iota_view`'s iterator has an `iterator_category` that (sometimes) falsely claims that it is forward or better (depending on the properties of the `weakly_incrementable` type it wraps).

[2019-10-19 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.6.4.3 [range.iota.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<class W, class Bound>
   > >       struct iota_view<W, Bound>::iterator {
   > >       private:
   > >         […]
   > >       public:
   > >         using iterator_conceptategory = see below;
   > >         using iterator_category = input_iterator_tag;
   > >         using value_type = W;
   > >         using difference_type = IOTA-DIFF-T(W);
   > >         […]
   > >       };
   > >       […]
   > >     }

   > -1- `iterator::iterator_conceptategory` is defined as follows:

   > 1. (1.1) — If `W` models `advanceable`, then `iterator_conceptategory` is `random_access_iterator_tag`.

   > 2. (1.2) — Otherwise, if `W` models `decrementable`, then `iterator_conceptategory` is `bidirectional_iterator_tag`.

   > 3. (1.3) — Otherwise, if `W` models `incrementable`, then `iterator_conceptategory` is `forward_iterator_tag`.

   > 4. (1.4) — Otherwise, `iterator_conceptategory` is `input_iterator_tag`.

### 3292(i). `iota_view` is under-constrained

Section: 25.6.4.2 [range.iota.view] Status: C++20 Submitter: Barry Revzin Opened: 2019-09-13 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [range.iota.view].

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Discussion:

P1207R4 changed `weakly_incrementable` from requiring `semiregular` to requiring `default_constructible && movable`.

`iota_view` currently is specified to require that `W` be just `weakly_incrementable`. But we have to copy the `W` from the view into its iterator and also in `operator*()` to return a `W`.

The shortest resolution is just to add "`&& semiregular<W>`" to the class constraints.

[Status to ready after discussion Friday morning in Belfast]

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.6.4.2 [range.iota.view] as indicated:

   >     namespace std::ranges {
   >       […]
   >       template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t>
   >         requires weakly-equality-comparable-with<W, Bound> && semiregular<W>
   >       class iota_view : public view_interface<iota_view<W, Bound> {
   >         […]
   >       };
   >       […]
   >     }

### 3293(i). `move_iterator operator+()` has incorrect constraints

Section: 24.5.4.9 [move.iter.nonmember] Status: C++23 Submitter: Bo Persson Opened: 2019-09-13 Last modified: 2023-11-22

Priority: 3

View all other issues in [move.iter.nonmember].

View all issues with C++23 status.

Discussion:

Section 24.5.4.9 [move.iter.nonmember]/2-3 says:

>     template<class Iterator>
>       constexpr move_iterator<Iterator>
>         operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);

> > Constraints: `x + n` is well-formed and has type `Iterator`.

> > Returns: `x + n`.

However, the return type of this operator is `move_iterator<Iterator>`, so the expression `x + n` ought to have that type. Also, there is no `operator+` that matches the constraints, so it effectively disables the addition.

[2019-10-31 Issue Prioritization]

Priority to 3 after reflector discussion.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 24.5.4.9 [move.iter.nonmember] as indicated:

>    >     template<class Iterator>
>    >       constexpr move_iterator<Iterator>
>    >         operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);

>    > > -2- Constraints: `x + n` is well-formed and has type `move_iterator<Iterator>`.

>    > > -3- Returns: `x + n`.

[2019-11-04; Casey comments and provides revised wording]

After applying the P/R the Constraint element requires `x + n` to be well-formed (it always is, since that operation is unconstrained) and requires `x + n` to have type `move_iterator<Iterator>` (which it always does). Consequently, this Constraint is always satisfied and it has no normative effect. The intent of the change in P0896R4 was that this operator be constrained to require addition on the base iterator to be well-formed and have type `Iterator`, which ensures that the other semantics of this operation are implementable.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 24.5.4.9 [move.iter.nonmember] as indicated:

   >     template<class Iterator>
   >       constexpr move_iterator<Iterator>
   >         operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);

   > > -2- Constraints: `x.base() + n` is well-formed and has type `Iterator`.

   > > -3- Returns: `x + n`.

### 3294(i). `zoned_time` deduction guides misinterprets `string`/`char*`

Section: 30.11.7.1 [time.zone.zonedtime.overview] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-09-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [time.zone.zonedtime.overview].

View all issues with C++20 status.

Discussion:

The current deduction guide for `zoned_time` for the following declarations

>     zoned_time zpc("America/New_York", std::chrono::system_clock::now());
>     zoned_time zps(std::string("America/New_York"), std::chrono::system_clock::now());

will attempt to produce a `zoned_time` instance with `const char*` (for `zpc`) and with `std::string` (for `zps`), respectively, as the deduced type for the `TimeZonePtr` template parameter. This is caused by the fact that the unconstrained `TimeZonePtr` deduction guide template will produce better candidates and will be selected by overload resolution.

The proposed resolution merges the deduction of the `std::string_view`/`TimeZonePtr` deduction guides into one guide, that deduces `const time_zone*` for any type convertible to `string_view`. This is necessary to override the deduction from `TimeZonePtr` constructor candidates.

In addition, we disable the deduction from `string_view` constructors, that would produce better candidates than the deduction guides and create `zoned_time` instances with durations coarser than `seconds` (causing similar issue as LWG 3232(i)):

>     std::chrono::local_time<hours> lh(10h);
>     std::chrono::zoned_time zt1("Europe/Helsinki", lh);
>     std::chrono::zoned_time zt2(std::string("Europe/Helsinki"), lh);
>     std::chrono::zoned_time zt3(std::string_view("Europe/Helsinki"), lh);

Without disabling the deduction from the `string_view` constructor, the type of the `zt3` variable would be deduced to `zoned_time<hours>`, with the proposed change the types of the variables `zt1`, `zt2`, and `zt3` are consistently deduced as `zoned_time<seconds>`.

Finally, the wording eliminates the unnecessary `zoned_time<Duration>` guide (covered by `zoned_time<Duration, TimeZonePtr2>`).

The change was implemented in the example implementation. The dedicated test can be found here.

[2019-10-31 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 30.11.7.1 [time.zone.zonedtime.overview], class template `zoned_time` synopsis, as indicated:

   > >     namespace std::chrono {
   > >       […]
   > >       zoned_time() -> zoned_time<seconds>;

   > >       template<class Duration>
   > >         zoned_time(sys_time<Duration>)
   > >           -> zoned_time<common_type_t<Duration, seconds>>;

   > >       template<class TimeZonePtrOrName>
   > >         using time-zone-representation =
   > >           conditional_t<is_convertible_v<TimeZonePtrOrName, string_view>,
   > >             const time_zone*,
   > >             remove_cv_ref<TimeZonePtrOrName>>; // exposition only

   > >       template<class TimeZonePtrOrName>
   > >         zoned_time(TimeZonePtrOrName&&)
   > >           -> zoned_time<seconds, time-zone-representation<TimeZonePtr>>;

   > >       template<class TimeZonePtrOrName, class Duration>
   > >         zoned_time(TimeZonePtrOrName&&, sys_time<Duration>)
   > >           -> zoned_time<common_type_t<Duration, seconds>,
   > >             time-zone-representation<TimeZonePtrOrName>>;

   > >       template<class TimeZonePtrOrName, class Duration>
   > >         zoned_time(TimeZonePtrOrName&&, local_time<Duration>, choose = choose::earliest)
   > >           -> zoned_time<common_type_t<Duration, seconds>,
   > >             time-zone-representation<TimeZonePtrOrName>>;

   > >       template<class TimeZonePtr, class Duration>
   > >         zoned_time(TimeZonePtr, zoned_time<Duration>, choose = choose::earliest)
   > >           ->> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;

   > >       zoned_time(string_view) -> zoned_time<seconds>;

   > >       template<class Duration>
   > >         zoned_time(string_view, sys_time<Duration>)
   > >           -> zoned_time<common_type_t<Duration, seconds>>;

   > >       template<class Duration>
   > >         zoned_time(string_view, local_time<Duration>, choose = choose::earliest)
   > >           -> zoned_time<common_type_t<Duration, seconds>>;

   > >       template<class Duration, class TimeZonePtrOrName, class TimeZonePtr2>
   > >         zoned_time(TimeZonePtrOrName&&, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest)
   > >          -> zoned_time<Duration, time-zone-representation<TimeZonePtrOrName>>;
   > >     }

   > -1- `zoned_time` represents a logical pairing of a `time_zone` and a `time_point` with precision `Duration`. `zoned_time<Duration>` maintains the invariant that it always refers to a valid time zone and represents a point in time that exists and is not ambiguous in that time zone.

   > -2- If `Duration` is not a specialization of `chrono::duration`, the program is ill-formed.

   > -?- Every constructor of `zoned_time` that accepts a `string_view` as first parameter does not participate in class template argument deduction (12.2.2.9 [over.match.class.deduct]).

### 3295(i). Comparison category `operator==` are mis-specified

Section: 17.12.2 [cmp.categories] Status: Resolved Submitter: Barry Revzin Opened: 2019-09-14 Last modified: 2019-12-29

Priority: 1

View all other issues in [cmp.categories].

View all issues with Resolved status.

Discussion:

All the defaulted `operator==`s in 17.12.2 [cmp.categories] are currently specified as:

>     friend constexpr bool operator==(strong_ordering v, strong_ordering w) noexcept = default;

But the rule for defaulting `operator==` requires that the arguments be `const&`. All five should all look like:

>     friend constexpr bool operator==(const strong_ordering& v, const strong_ordering& w) noexcept = default;

[2019-10-31 Issue Prioritization]

Priority to 1 after reflector discussion.

[2019-11 Wednesday night issue processing - status to Open]

Our preference is for CWG to fix this. JW to provide wording in case CWG cannot.

[Resolved by the adoption of P1946R0 in Belfast]

Proposed resolution:

### 3296(i). Inconsistent default argument for `basic_regex<>::assign`

Section: 28.6.7 [re.regex] Status: C++20 Submitter: Mark de Wever Opened: 2019-09-16 Last modified: 2021-02-25

Priority: 0

View all other issues in [re.regex].

View all issues with C++20 status.

Discussion:

The declaration of the overload of `basic_regex<>::assign(const charT* p, size_t len, flag_type f)` has an inconsistent default argument for the `flag_type f` parameter.

28.6.7 [re.regex] p3:

>     basic_regex& assign(const charT* p, size_t len, flag_type f);

28.6.7.3 [re.regex.assign] before p12:

>     basic_regex& assign(const charT* ptr, size_t len, flag_type f = regex_constants::ECMAScript);

Since all other overloads have a default argument in both 28.6.7 [re.regex] and 28.6.7.3 [re.regex.assign] I propose to add a default argument for this overload in the declaration in 28.6.7 [re.regex].

It should be pointed out that there exists implementation divergence due to the current wording state: libc++ and libstdc++ do not implement the default argument. The MS STL library does have the default argument.

[2019-10-31 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 28.6.7 [re.regex], class template `basic_regex` synopsis, as indicated:

   >     […]
   >     // 28.6.7.3 [re.regex.assign], assign
   >     […]
   >     basic_regex& assign(const charT* ptr, flag_type f = regex_constants::ECMAScript);
   >     basic_regex& assign(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript);
   >     template<class string_traits, class A>
   >       basic_regex& assign(const basic_string<charT, string_traits, A>& s,
   >                           flag_type f = regex_constants::ECMAScript);
   >     template<class InputIterator>
   >       basic_regex& assign(InputIterator first, InputIterator last,
   >                           flag_type f = regex_constants::ECMAScript);
   >     basic_regex& assign(initializer_list<charT>,
   >                         flag_type = regex_constants::ECMAScript);
   >     […]

### 3299(i). Pointers don't need customized iterator behavior

Section: 24.3.3.1 [iterator.cust.move], 24.3.3.2 [iterator.cust.swap] Status: C++20 Submitter: Casey Carter Opened: 2019-10-07 Last modified: 2021-02-25

Priority: 0

View all other issues in [iterator.cust.move].

View all issues with C++20 status.

Discussion:

It is not intentional design that users may customize the behavior of `ranges::iter_move` (24.3.3.1 [iterator.cust.move]) and `ranges::iter_swap` (24.3.3.2 [iterator.cust.swap]) for pointers to program-defined type by defining e.g. `iter_move(my_type*)` or `iter_swap(my_type*, my_type*)` in a namespace associated with `my_type`. The intent of customization points is that users may define behavior for types they define, not that users may mess with the well-defined semantics for existing types like pointers.

We should forbid such silliness by constraining the "finds an overload via ADL" cases for customization points to only trigger with argument expressions of class or enumeration type. Note that WG21 made a similar change to `ranges::swap` shortly before merging it into the working draft to forbid users customizing behavior for pointers to program-defined types or arrays of program-defined types.

[2019-11-16 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

> [Drafting note: 3247(i) touches the same wording in 24.3.3.1 [iterator.cust.move]; if both are resolved simultaneously the changes should be reconciled before passing them on to the Editor.]

1. Modify 24.3.3.1 [iterator.cust.move] as follows:

   > (1.1) — `iter_move(E)`, if that expression is valid, `E` has class or enumeration type and `iter_move(E)` is a well-formed expression with overload resolution performed in a context that does not include a declaration of `ranges::iter_move`.

2. Modify 24.3.3.2 [iterator.cust.swap] as follows:

   > (4.1) — `(void)iter_swap(E1, E2)`, if that expression is valid, either `E1` or `E2` has class or enumeration type and `iter_swap(E1, E2)` is a well-formed expression with overload resolution performed in a context that includes the declaration

   > >     template<class I1, class I2>
   > >     void iter_swap(I1, I2) = delete;

   > and does not include a declaration of `ranges::iter_swap`. If the function selected by overload resolution does not exchange the values denoted by `E1` and `E2`, the program is ill-formed with no diagnostic required.

### 3300(i). Non-array `ssize` overload is underconstrained

Section: 24.7 [iterator.range] Status: C++20 Submitter: Casey Carter Opened: 2019-09-27 Last modified: 2021-02-25

Priority: 3

View all other issues in [iterator.range].

View all issues with C++20 status.

Discussion:

The overload of `ssize` specified in 24.7 [iterator.range]/18 has no constraints, yet it specializes `make_signed_t` which has a precondition that its type parameter is an integral type or enumeration but not `bool` (21.3.8.4 [meta.trans.sign]). This precondition needs to be propagated to `ssize` as "Mandates [or Constraints]: `decltype(c.size())` [meets the requirements for the type argument to `make_signed`]". "Mandates" seems to be more in line with LWG guidance since there are no traits nor concepts that observe `ssize`.

[2019-11-16 Issue Prioritization]

Priority to 3 after reflector discussion.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 24.7 [iterator.range] as indicated:

>    >     template<class C> constexpr auto ssize(const C& c)
>    >       -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;

>    > > -?- Mandates: `decltype(c.size())` is a (possibly cv-qualified) integral or enumeration type but not a `bool` type.

>    > > -18- Returns:

>    > > >     static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size())

[2019-10-28; Tim provides improved wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4835.

> 1. Modify 24.7 [iterator.range] as indicated:

>    >     template<class C> constexpr auto ssize(const C& c)
>    >       -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;

>    > > -?- Mandates: `decltype(c.size())` is an integral or enumeration type other than `bool`.

>    > > -18- Returns:

>    > > >     static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size())

[2019-11-18; Casey comments and improves wording]

It would be better to provided the Mandates: guarantee in [tab:meta.trans.sign] instead of one special place where the `make_signed` template is used. The wording below attempts to realize that.

[2019-11-23 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4835.

1. Change Table 52 — "Sign modifications" in [tab:meta.trans.sign] as indicated:

   Table 52 — Sign modifications [tab:meta.trans.sign]
   Template | Comments
   --- | ---
   `template <class T> struct make_signed;` | If `T` names a (possibly cv-qualified) signed integer type (6.9.2 [basic.fundamental]) then the member typedef `type` names the type `T`; otherwise, if `T` names a (possibly cv-qualified) unsigned integer type then `type` names the corresponding signed integer type, with the same cv-qualifiers as `T`; otherwise, `type` names the signed integer type with smallest rank (6.9.6 [conv.rank]) for which `sizeof(T) == sizeof(type)`, with the same cv-qualifiers as `T`. RequiresMandates: `T` shall beis an (possibly cv-qualified) integral type or enumeration type other than cv but not a `bool` type.
   `template <class T> struct make_unsigned;` | If `T` names a (possibly cv-qualified) unsigned integer type (6.9.2 [basic.fundamental]) then the member typedef `type` names the type `T`; otherwise, if `T` names a (possibly cv-qualified) signed integer type then `type` names the corresponding unsigned integer type, with the same cv-qualifiers as `T`; otherwise, `type` names the unsigned integer type with smallest rank (6.9.6 [conv.rank]) for which `sizeof(T) == sizeof(type)`, with the same cv-qualifiers as `T`. RequiresMandates: `T` shall beis an (possibly cv-qualified) integral type or enumeration type other than cv but not a `bool` type.

2. Change 24.7 [iterator.range] as indicated:

   >     template<class C> constexpr auto ssize(const C& c)
   >       -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;

   > > -18- ReturnsEffects: Equivalent to:

   > > >     return static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size());

### 3301(i). `transform_view::iterator` has incorrect `iterator_category`

Section: 25.7.9.3 [range.transform.iterator] Status: C++20 Submitter: Michel Morin Opened: 2019-10-03 Last modified: 2021-02-25

Priority: 1

View all other issues in [range.transform.iterator].

View all issues with C++20 status.

Discussion:

When the transformation function returns an rvalue, `transform_view::iterator` cannot model `cpp17-forward-iterator`. However, similar to LWG 3291(i), the current wording on `transform_view::iterator::iterator_category` does not consider this.

As Casey Carter pointed out here, the proposed wording below does not consider `input_iterator` that is not `cpp17-input-iterator` (this problem is not specific to the PR; it's pervasive in adapted iterators) and concepts-based determination would be a better fix for issues around `iterator_category`. But anyway, I consider this PR as a minimal fix at the moment.

[2019-10-31 Issue Prioritization]

Priority to 1 after reflector discussion.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4830.

> 1. Modify 25.7.9.3 [range.transform.iterator] as indicated:

>    > -2- `iterator::iterator_category` is defined as follows: Let `C` denote the type `iterator_traits<iterator_t<Base>>::iterator_category`.

>    > 1. (2.?) — If `is_lvalue_reference_v<iter_reference_t<iterator_t<Base>>>` is `true`,

>    >    1. (2.?.?) — If `C` models `derived_from<contiguous_iterator_tag>`, then `iterator_category` denotes `random_access_iterator_tag`;

>    >    2. (2.?.?) — Ootherwise, `iterator_category` denotes `C`.

>    > 2. (2.?) — Otherwise, `iterator_category` denotes `input_iterator_tag`.

[2019-11-06, Tim updates P/R based on Belfast LWG evening session discussion]

The check in the original P/R is incorrect; we want to check the transformation's result, not the base iterator.

[2020-02-10 Move to Immediate Monday afternoon in Prague]

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > -2- `iterator::iterator_category` is defined as follows: Let `C` denote the type `iterator_traits<iterator_t<Base>>::iterator_category`.

   > 1. (2.?) — If `is_lvalue_reference_v<invoke_result_t<F&, range_reference_t<Base>>>` is `true`,

   >    1. (2.?.?) — If `C` models `derived_from<contiguous_iterator_tag>`, then `iterator_category` denotes `random_access_iterator_tag`;

   >    2. (2.?.?) — Ootherwise, `iterator_category` denotes `C`.

   > 2. (2.?) — Otherwise, `iterator_category` denotes `input_iterator_tag`.

### 3302(i). Range adaptor objects `keys` and `values` are unspecified

Section: 25.2 [ranges.syn] Status: C++20 Submitter: Michel Morin Opened: 2019-10-04 Last modified: 2021-02-25

Priority: 1

View other active issues in [ranges.syn].

View all other issues in [ranges.syn].

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Discussion:

This issue was submitted as editorial issue cplusplus/draft#3231 but had been classified as non-editorial.

`keys` and `values` are listed in 25.2 [ranges.syn], but not specified. It seems that P1035R7 forgot to specify them (as `elements<0>` and `elements<1>`).

[2019-10-31 Issue Prioritization]

Priority to 1 after reflector discussion.

[2019-11 Wednesday night issue processing in Belfast.]

Status to Ready.

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     namespace std::ranges {
   >       […]

   >       template<class R>
   >         using keys_view = elements_view<all_view<R>, 0>;
   >       template<class R>
   >         using values_view = elements_view<all_view<R>, 1>;
   >       namespace views {
   >         template<size_t N>
   >           inline constexpr unspecified elements = unspecified;
   >         inline constexpr autounspecified keys = elements<0>unspecified;
   >         inline constexpr autounspecified values = elements<1>unspecified;
   >       }
   >     }
   >     […]

### 3303(i). Bad "`constexpr`" marker for `destroy/destroy_n`

Section: 20.2.2 [memory.syn] Status: C++20 Submitter: Jens Maurer Opened: 2019-10-10 Last modified: 2021-02-25

Priority: 0

View all other issues in [memory.syn].

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Discussion:

This issue was submitted as editorial issue cplusplus/draft#3181 but is considered non-editorial.

P0784R7, approved in Cologne, added "`constexpr`" markers to the overloads of `destroy` and `destroy_n` taking an `ExecutionPolicy` parameter. This seems to be in error; parallel algorithms should not be marked "`constexpr`". (None of the parallel algorithms in `<algorithm>` is marked "`constexpr`".)

[2019-11 Marked as 'Ready' during Monday issue prioritization in Belfast]

Proposed resolution:

This wording is relative to N4830.

1. Modify 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     namespace std {
   >       […]

   >       // 26.11.9 [specialized.destroy], destroy
   >       template<class T>
   >         constexpr void destroy_at(T* location);
   >       template<class ForwardIterator>
   >         constexpr void destroy(ForwardIterator first, ForwardIterator last);
   >       template<class ExecutionPolicy, class ForwardIterator>
   >         constexpr void destroy(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
   >                                ForwardIterator first, ForwardIterator last);
   >       template<class ForwardIterator, class Size>
   >         constexpr ForwardIterator destroy_n(ForwardIterator first, Size n);
   >       template<class ExecutionPolicy, class ForwardIterator, class Size>
   >         constexpr ForwardIterator destroy_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
   >                                             ForwardIterator first, Size n);
   >       […]
   >     }

### 3304(i). Allocate functions of `std::polymorphic_allocator` should require `[[nodiscard]]`

Section: 20.5.3 [mem.poly.allocator.class] Status: C++20 Submitter: Hiroaki Ando Opened: 2019-10-16 Last modified: 2021-02-25

Priority: 3

View all other issues in [mem.poly.allocator.class].

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Discussion:

`[[nodiscard]]` is specified for `std::polymorphic_allocator<>::allocate()`.

But the allocate functions added with P0339R6 doesn't have it.

Isn't `[[nodiscard]]` necessary for these functions?

[2019-11 Priority to 3 during Monday issue prioritization in Belfast]

[2019-11 After discussion with LEWG, assigning to LEWG]

[2019-11-4; Daniel comments]

This issue is related to LWG 3312(i).

[2019-11; Friday AM in Belfast. Status changed to "Ready"]

Proposed resolution:

This wording is relative to N4835.

1. Modify 20.5.3 [mem.poly.allocator.class], class template `polymorphic_allocator` synopsis, as indicated:

   >     namespace std::pmr {
   >       template<class Tp = byte> class polymorphic_allocator {
   >         […]
   >         // 20.5.3.3 [mem.poly.allocator.mem], member functions
   >         [[nodiscard]] Tp* allocate(size_t n);
   >         void deallocate(Tp* p, size_t n);

   >         [[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));
   >         void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t));
   >         template<class T> [[nodiscard]] T* allocate_object(size_t n = 1);
   >         template<class T> void deallocate_object(T* p, size_t n = 1);
   >         template<class T, class... CtorArgs> [[nodiscard]] T* new_object(CtorArgs&&... ctor_args);
   >         template<class T> void delete_object(T* p);
   >         […]
   >       };
   >     }

2. Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:

   >     [[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));

   > > -5- Effects: Equivalent to: `return memory_rsrc->allocate(nbytes, alignment);`

   > > […]

   > […]

   >     template<class T>
   >       [[nodiscard]] T* allocate_object(size_t n = 1);

   > > -8- Effects: Allocates memory suitable for holding an array of `n` objects of type `T`, as follows:

   > > 1. (8.1) — if `SIZE_MAX / sizeof(T) < n`, throws `length_error`,

   > > 2. (8.2) — otherwise equivalent to:

   > >    >     return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));

   > > […]

   >     template<class T, class CtorArgs...>
   >       [[nodiscard]] T* new_object(CtorArgs&&... ctor_args);

   > > -11- Effects: Allocates and constructs an object of type `T`, as follows. Equivalent to:

   > > >     T* p = allocate_object<T>();
   > > >     try {
   > > >       construct(p, std::forward<CtorArgs>(ctor_args)...);
   > > >     } catch (...) {
   > > >       deallocate_object(p);
   > > >       throw;
   > > >     }
   > > >     return p;

   > > […]

### 3305(i). `any_cast<void>`

Section: 22.7.5 [any.nonmembers] Status: C++26 Submitter: John Shaw Opened: 2019-10-16 Last modified: 2026-06-09

Priority: 2

View all other issues in [any.nonmembers].

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Discussion:

>     any foo;
>     void* p = any_cast<void>(&foo);

Per 22.7.5 [any.nonmembers]/9, since the operand isn't `nullptr` and `operand->type() == typeid(T)` (because `T = void` in this case), we should return a pointer to the object contained by `operand`. But there is no such object.

We need to handle the `T = void` case, probably by just explicitly returning `nullptr`.

[2019-11 Priority to 2 during Monday issue prioritization in Belfast. There is implementation divergence here.]

[2020-02 LWG discussion in Prague did not reach consensus. Status to Open.]

There was discussion about whether or not `any_cast<void>(a)` should be ill-formed, or return `nullptr`.

Poll "should it return nullptr" was 0-4-5-5-1.

[2022-02 Currently ill-formed in MSVC ("error C2338: std::any cannot contain void") and returns null pointer in libstdc++ and libc++.]

Previous resolution [SUPERSEDED]

This wording is relative to N4835.

1. Modify 22.7.5 [any.nonmembers] as indicated:

   >     template<class T>
   >       const T* any_cast(const any* operand) noexcept;
   >     template<class T>
   >       T* any_cast(any* operand) noexcept;

   > > -9- Returns: If `operand != nullptr && operand->type() == typeid(T) && is_object_v<T>`, a pointer to the object contained by `operand`; otherwise, `nullptr`.

   > > […]

[2023-06-14 Varna; Jonathan provides improved wording]

[2023-06-14 Varna; Move to Ready]

Poll: 7-0-1

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 22.7.5 [any.nonmembers] as indicated:

   >     template<class T>
   >       const T* any_cast(const any* operand) noexcept;
   >     template<class T>
   >       T* any_cast(any* operand) noexcept;

   > > -8- Mandates: `is_void_v<T>` is `false`.

   > > -9- Returns: If `operand != nullptr && operand->type() == typeid(T)` is `true`, a pointer to the object contained by `operand`; otherwise, `nullptr`.

   > > […]

### 3306(i). `ranges::advance` violates its preconditions

Section: 24.4.4.2 [range.iter.op.advance] Status: C++23 Submitter: Casey Carter Opened: 2019-10-27 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.iter.op.advance].

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Discussion:

Recall that "`[i, s)` denotes a range" for an iterator `i` and sentinel `s` means that either `i == s` holds, or `i` is dereferenceable and `[++i, s)` denotes a range ( [iterator.requirements.genera]).

The three-argument overload `ranges::advance(i, n, bound)` is specified in 24.4.4.2 [range.iter.op.advance] paragraphs 5 through 7. Para 5 establishes a precondition that `[bound, i)` denotes a range when `n < 0` (both `bound` and `i` must have the same type in this case). When `sized_sentinel_for<S, I>` holds and `n < bound - i`, para 6.1.1 says that `ranges::advance(i, n, bound)` is equivalent to `ranges::advance(i, bound)`. Para 3, however, establishes a precondition for `ranges::advance(i, bound)` that `[i, bound)` denotes a range. `[bound, i)` and `[i, bound)` cannot both denote ranges unless `i == bound`, which is not the case for all calls that reach 6.1.1.

The call in para 6.1.1 wants the effects of either 4.1 - which really has no preconditions - or 4.2, which is well-defined if either `[i, bound)` or `[bound, i)` denotes a range. Para 3's stronger precondition is actually only required by Para 4.3, which increments `i` blindly looking for `bound`. The straight-forward fix here seems to be to relax para 3's precondition to only apply when 4.3 will be reached.

[2019-11 Priority to 2 during Monday issue prioritization in Belfast]

[2020-08-21 Issue processing telecon: moved to Tentatively Ready]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 24.4.4.2 [range.iter.op.advance] as indicated:

   >     template<input_or_output_iterator I, sentinel_for<I> S>
   >       constexpr void ranges::advance(I& i, S bound);

   > -3- Expects: Either `assignable_from<I&, S> || sized_sentinel_for<S, I>` is modeled, or `[i, bound)` denotes a range.

   > -4- Effects:

   > (4.1) — If `I` and `S` model `assignable_from<I&, S>`, equivalent to `i = std::move(bound)`.

   > (4.2) — Otherwise, if `S` and `I` model `sized_sentinel_for<S, I>`, equivalent to `ranges::advance(i, bound - i)`.

   > (4.3) — Otherwise, while `bool(i != bound)` is `true`, increments `i`.

### 3307(i). `std::allocator<void>().allocate(n)`

Section: 20.2.10 [default.allocator] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-10-25 Last modified: 2021-02-25

Priority: 0

View other active issues in [default.allocator].

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Discussion:

In C++20 the `std::allocator<void>` explicit specialization is gone, which means it uses the primary template, which has allocate and deallocate members.

Although it's implied by the use of `sizeof(T)`, `std::allocator<T>::allocate` doesn't have an explicit precondition that the value type is complete.

[2019-11 Status to 'Ready' in Monday issue prioritization in Belfast]

Proposed resolution:

This wording is relative to N4835.

1. Modify 20.2.10.2 [allocator.members] as indicated:

   >     [[nodiscard]] constexpr T* allocate(size_t n);

   > -?- Mandates: `T` is not an incomplete type (6.9 [basic.types]).

   > -2- Returns: A pointer to the initial element of an array of storage of size `n * sizeof(T)`, aligned appropriately for objects of type `T`.

   > […]

### 3310(i). Replace `SIZE_MAX` with `numeric_limits<size_t>::max()`

Section: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Japan Opened: 2019-11-04 Last modified: 2021-02-25

Priority: 0

View all other issues in [mem.poly.allocator.mem].

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Discussion:

Addresses JP 218/219

It's better to use a C++ property than C standard library macro, `SIZE_MAX`.

[2019-11 Status to Ready during Tuesday morning issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:

   >     [[nodiscard]] Tp* allocate(size_t n);

   > > -1- Effects: If `SIZE_MAXnumeric_limits<size_t>::max() / sizeof(Tp) < n`, throws `length_error`. […]

   > […]

   >     template<class T>
   >       T* allocate_object(size_t n = 1);

   > > -8- Effects: Allocates memory suitable for holding an array of `n` objects of type `T`, as follows:

   > > 1. (8.1) — if `SIZE_MAXnumeric_limits<size_t>::max() / sizeof(T) < n`, throws `length_error`,

   > > 2. (8.2) — otherwise equivalent to:

   > >    >     return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));

### 3313(i). `join_view::iterator::operator--` is incorrectly constrained

Section: 25.7.14.3 [range.join.iterator] Status: C++20 Submitter: United States Opened: 2019-11-04 Last modified: 2021-02-25

Priority: 0

View all other issues in [range.join.iterator].

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Discussion:

Addresses US 294

`join_view::iterator::operator--` is improperly constrained. In the Effects: clause in paragraph 14, we see the statement:

>     inner_ = ranges::end(*--outer_);

However, this only well-formed when `end` returns an iterator, not a sentinel. This requirement is not reflected in the constraints of the function(s).

Eric Niebler:

From the WD, `join_view::iterator::operator--` is specified as:

>     constexpr iterator& operator--()
>       requires ref_is_glvalue && bidirectional_range<Base> &&
>         bidirectional_range<range_reference_t<Base>>;

> -14- Effects: Equivalent to:

> >     if (outer_ == ranges::end(parent_->base_))
> >       inner_ = ranges::end(*--outer_);
> >     while (inner_ == ranges::begin(*outer_))
> >       inner_ = ranges::end(*--outer_);
> >     --inner_;
> >     return *this;

The trouble is from the lines that do:

>       inner_ = ranges::end(*--outer_);

Clearly this will only compile when `*--outer` returns a `common_range`, but nowhere is that requirement stated.

[2019-11 Status to Ready during Tuesday morning issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 25.7.14.3 [range.join.iterator], class template `join_view::iterator` synopsis, as indicated:

   >     constexpr iterator& operator--()
   >       requires ref_is_glvalue && bidirectional_range<Base> &&
   >                bidirectional_range<range_reference_t<Base>> &&
   >                common_range<range_reference_t<Base>>;

   >     constexpr iterator operator--(int)
   >       requires ref_is_glvalue && bidirectional_range<Base> &&
   >                bidirectional_range<range_reference_t<Base>> &&
   >                common_range<range_reference_t<Base>>;

2. Modify 25.7.14.3 [range.join.iterator] as indicated:

   >     constexpr iterator& operator--()
   >       requires ref_is_glvalue && bidirectional_range<Base> &&
   >                bidirectional_range<range_reference_t<Base>> &&
   >                common_range<range_reference_t<Base>>;

   > > -14- Effects: Equivalent to:

   > > >     if (outer_ == ranges::end(parent_->base_))
   > > >       inner_ = ranges::end(*--outer_);
   > > >     while (inner_ == ranges::begin(*outer_))
   > > >       inner_ = ranges::end(*--outer_);
   > > >     --inner_;
   > > >     return *this;

   >     constexpr iterator operator--(int)
   >       requires ref_is_glvalue && bidirectional_range<Base> &&
   >                bidirectional_range<range_reference_t<Base>> &&
   >                common_range<range_reference_t<Base>>;

   > > -15- Effects: Equivalent to:

   > > >     auto tmp = *this;
   > > >     --*this;
   > > >     return tmp;

### 3314(i). Is stream insertion behavior locale dependent when `Period::type` is `micro`?

Section: 30.5.11 [time.duration.io] Status: C++20 Submitter: Tom Honermann Opened: 2019-11-04 Last modified: 2021-02-25

Priority: 2

View all other issues in [time.duration.io].

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Discussion:

30.5.11 [time.duration.io] states:

>     template<class charT, class traits, class Rep, class Period>
>       basic_ostream<charT, traits>&
>         operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);

> > […]

> > -3- The units suffix depends on the type `Period::type` as follows:

> > 1. […]

> > 2. (3.5) — Otherwise, if `Period::type` is `micro`, the suffix is `"µs"` (`"\u00b5\u0073"`).

> > 3. […]

> > […]

> > -4- If `Period::type` is `micro`, but the character U+00B5 cannot be represented in the encoding used for `charT`, the unit suffix `"us"` is used instead of `"µs"`.

> > […]

Which encoding is intended by "the encoding used for `charT`"? There are two candidates:

1. The associated execution character set as defined by 5.3.1 [lex.charset] p3 used to encode character and string literals (e.g., the "execution wide-character set" for `wchar_t`).

2. The locale dependent character set used by the `std::locale ctype` and `codecvt` facets as specified in 28.3.4.2 [category.ctype], sometimes referred to as the "native character set".

The behavior should not be dependent on `locale` and should therefore be specified in terms of the execution character sets.

The execution character set is implementation defined and some implementations allow the choice of execution character set to be specified via a compiler option or determined based on the locale active when the compiler is run. For example, the Microsoft compiler, when run on a Windows system with regional language settings configured for "English (United States)", will use Windows-1252 for the execution character set, but allows this choice to be overridden with the `/execution-charset` compiler option. The Microsoft compiler might therefore use `"us"` by default, but `"µs"` when invoked with the `/execution-charset:utf-8` or `/execution-charset:.437` options. In the latter two cases, the string contents would contain `"\xb5\x73"` and `"\xe6\x73"` respectively (Unicode and Windows code page 437 map µ (U+00B5, MICRO SIGN) to different code points).

This resolution relies on the character set for the locale used at run-time being compatible with the execution character set if the produced string is to be displayed correctly when written to a terminal or console. This is a typical requirement for character and string literals but is more strongly relevant for this issue since µ lacks representation in many character sets. Additionally, if the stream is imbued with a `std::codecvt` facet, the facet must provide appropriate conversion support for behavior to be well defined.

[2019-11 Priority to 2 during Tuesday morning issue processing in Belfast.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4835.

> 1. Modify 30.5.11 [time.duration.io] as indicated:

>    > [Drafting note: "implementation's native character set" is used in 28.3.4.2.2 [locale.ctype] and 28.3.4.2.5 [locale.codecvt] to refer to the locale dependent character encoding.]

>    >     template<class charT, class traits, class Rep, class Period>
>    >       basic_ostream<charT, traits>&
>    >         operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);

>    > > […]

>    > > -3- The units suffix depends on the type `Period::type` as follows:

>    > > 1. […]

>    > > 2. (3.5) — Otherwise, if `Period::type` is `micro`, the suffix is `"µs"` (`"\u00b5\u0073"`).

>    > > 3. […]

>    > > […]

>    > > -4- If `Period::type` is `micro`, but the character U+00B5 cannot be represented in the encoding usedlacks representation in the execution character set for `charT`, the unit suffix `"us"` is used instead of `"µs"`. If `"µs"` is used but the implementation's native character set lacks representation for U+00B5 and the stream is associated with a terminal or console, or if the stream is imbued with a `std::codecvt` facet that lacks conversion support for the character, then the result is unspecified.

>    > > […]

[2019-11-12; Tom Honermann improves wording]

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 30.5.11 [time.duration.io] as indicated:

   >     template<class charT, class traits, class Rep, class Period>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);

   > > […]

   > > -3- The units suffix depends on the type `Period::type` as follows:

   > > 1. […]

   > > 2. (3.5) — Otherwise, if `Period::type` is `micro`, it is implementation-defined whether the suffix is `"µs"` (`"\u00b5\u0073"`) or `"us"`.

   > > 3. […]

   > > […]

   > > -4- If `Period::type` is `micro`, but the character U+00B5 cannot be represented in the encoding used for `charT`, the unit suffix `"us"` is used instead of `"µs"`.

   > > […]

### 3315(i). Correct Allocator Default Behavior

Section: 16.4.4.6 [allocator.requirements] Status: C++20 Submitter: United States Opened: 2019-11-04 Last modified: 2021-02-25

Priority: 0

View other active issues in [allocator.requirements].

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Discussion:

Addresses US 162/US 163

US 162:

The default behavior for `a.destroy` is now to call `destroy_at`

Proposed change:

Replace "default" entry with: `destroy_at(c)`

US 163:

The default behavior for `a.construct` is now to call `construct_at`

Proposed change:

Replace "default" entry with: `construct_at(c, std::forward<Args>(args)...)`

Dietmar Kühl:

In Table 34 [tab:cpp17.allocator] the behavior of `a.construct(c, args)` and `a.destroy(c)` are described to have a default behavior of `::new ((void*)c) C(forward<Args>(args))` and `c->~C()`, respectively. However, this table doesn't actually define what is happening if these operations are omitted: The behavior is provided when using an allocator is used via `std::allocator_traits` and is, thus, defined by the corresponding `std::allocator_traits` functions. These functions are specified in 20.2.9.3 [allocator.traits.members] paragraphs 5 and 6 to call `construct_at(c, std::forward<Args>(args)` and `destroy_at(p)`, respectively. The text in the table should be updated to match the actual behavior.

[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 16.4.4.6 [allocator.requirements], Table [tab:cpp17.allocator] "`Cpp17Allocator` requirements" as indicated:

   > Table 34 — `Cpp17Allocator` requirements [tab:cpp17.allocator]
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `…`
   > `a.construct(c, args)` | (not used) | Effects: Constructs an object of type `C` at `c` | `::new ((void*)c) C(construct_at(c, std::forward<Args>(args)...)`
   > `a.destroy(c)` | (not used) | Effects: Destroys the object at `c` | `c->~C()destroy_at(c)`
   > `…`

### 3316(i). Correctly define epoch for `utc_clock` / `utc_timepoint`

Section: 30.7.3.1 [time.clock.utc.overview] Status: C++20 Submitter: Great Britain Opened: 2019-11-05 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

Addresses GB 333

UTC epoch is not correctly defined UTC has an officially recorded epoch of 1/1/1972 00:00:00 and is 10 seconds behind TAI. This can be confirmed through reference to the BIPM (the body that oversees international metrology)

> "The defining epoch of 1 January 1972, `0 h 0m 0 s` UTC was set 10 s behind TAI, which was the approximate accumulated difference between TAI and UT1 since the inception of TAI in 1958, and a unique fraction of a second adjustment was applied so that UTC would differ from TAI by an integral number of seconds. The recommended maximum departure of UTC from UT1 was 0.7 s. The term "leap second" was introduced for the stepped second."

Proposed change:

`utc_clock` and `utc_timepoint` should correctly report relative to the official UTC epoch. 27.2.2.1 footnote 1 should read:

> In contrast to `sys_time`, which does not take leap seconds into account, `utc_clock` and its associated `time_point`, `utc_time`, count time, including leap seconds, since 1972-01-01 00:00:00 UTC. [Example: `clock_cast<utc_clock>(sys_seconds{sys_days{1972y/January/1}}).time_since_epoch()` is `0s`. `clock_cast<utc_clock>(sys_seconds{sys_days{2000y/January/1}}).time_since_epoch()` is `883'612'822`, which is `10'197 * 86'400s + 22s`. — end example]

Howard Hinnant:

Clarify that the epoch of `utc_clock` is intended to be `1970-01-01`.

Rationale: The main use case of `utc_clock` is to get the correct number of seconds when subtracting time points straddling a leap second insertion point, and this computation is independent of the epoch. Furthermore learning/teaching that `utc_clock` is `system_clock` except that `utc_clock` includes leap seconds is easier. And this fact is more easily understood when comparing the underlying `.time_since_epoch()` of equivalent time points from each clock.

[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 30.7.3.1 [time.clock.utc.overview] as indicated:

   > -1- In contrast to `sys_time`, which does not take leap seconds into account, `utc_clock` and its associated `time_point`, `utc_time`, count time, including leap seconds, since 1970-01-01 00:00:00 UTC. [Note: The UTC time standard began on 1972-01-01 00:00:10 TAI. To measure time since this epoch instead, one can add/subtract the constant `sys_days{1972y/1/1} - sys_days{1970y/1/1}` (`63'072'000s`) from the `utc_time` — end note] [Example: `clock_cast<utc_clock>(sys_seconds{sys_days{1970y/January/1}}).time_since_epoch()` is `0s`. `clock_cast<utc_clock>(sys_seconds{sys_days{2000y/January/1}}).time_since_epoch()` is `946'684'822s`, which is `10'957 * 86'400s + 22s`. — end example]

### 3317(i). Incorrect `operator<<` for floating-point durations

Section: 30.5.11 [time.duration.io] Status: C++20 Submitter: United States Opened: 2019-11-05 Last modified: 2021-02-25

Priority: 0

View all other issues in [time.duration.io].

View all issues with C++20 status.

Discussion:

Addresses US 334

`operator<<` for floating-point durations always produces output with six digits after the decimal point, and doesn't use the stream's locale either.

Proposed change:

Rewrite the specification to not rely on `to_string()` for floating-point formatting.

[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 30.5.11 [time.duration.io] as indicated:

   >     template<class charT, class traits, class Rep, class Period>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);

   > > -1- Requires: `Rep` is an integral type whose integer conversion rank (6.9.6 [conv.rank]) is greater than or equal to that of `short`, or a floating-point type. `charT` is `char` or `wchar_t`.

   > > -2- Effects: Forms a `basic_string<charT, traits>` from `d.count()` using `to_string` if `charT` is `char`, or `to_wstring` if `charT` is `wchar_t`. Appends the units suffix described below to the `basic_string`. Inserts the resulting `basic_string` into `os`. [Note: This specification ensures that the result of this streaming operation will obey the width and alignment properties of the stream. — end note]Inserts the duration `d` onto the stream `os` as if it were implemented as follows:

   > > >     basic_ostringstream<charT, traits> s;
   > > >     s.flags(os.flags());
   > > >     s.imbue(os.getloc());
   > > >     s.precision(os.precision());
   > > >     s << d.count() << units_suffix;
   > > >     return os << s.str();

   > > -3- The units suffix `units_suffix` depends on the type `Period::type` as follows:

   > > 1. (3.1) — If `Period::type` is `atto`, the suffix `units_suffix` is `"as"`.

   > > 2. (3.2) — Otherwise, if `Period::type` is `femto`, the suffix `units_suffix` is `"fs"`.

   > > 3. (3.3) — Otherwise, if `Period::type` is `pico`, the suffix `units_suffix` is `"ps"`.

   > > 4. (3.4) — Otherwise, if `Period::type` is `nano`, the suffix `units_suffix` is `"ns"`.

   > > 5. (3.5) — Otherwise, if `Period::type` is `micro`, the suffix `units_suffix` is `"µs"` (`"\u00b5\u0073"`).

   > > 6. (3.6) — Otherwise, if `Period::type` is `milli`, the suffix `units_suffix` is `"ms"`.

   > > 7. (3.7) — Otherwise, if `Period::type` is `centi`, the suffix `units_suffix` is `"cs"`.

   > > 8. (3.8) — Otherwise, if `Period::type` is `deci`, the suffix `units_suffix` is `"ds"`.

   > > 9. (3.9) — Otherwise, if `Period::type` is `ratio<1>`, the suffix `units_suffix` is `"s"`.

   > > 10. (3.10) — Otherwise, if `Period::type` is `deca`, the suffix `units_suffix` is `"das"`.

   > > 11. (3.11) — Otherwise, if `Period::type` is `hecto`, the suffix `units_suffix` is `"hs"`.

   > > 12. (3.12) — Otherwise, if `Period::type` is `kilo`, the suffix `units_suffix` is `"ks"`.

   > > 13. (3.13) — Otherwise, if `Period::type` is `mega`, the suffix `units_suffix` is `"Ms"`.

   > > 14. (3.14) — Otherwise, if `Period::type` is `giga`, the suffix `units_suffix` is `"Gs"`.

   > > 15. (3.15) — Otherwise, if `Period::type` is `tera`, the suffix `units_suffix` is `"Ts"`.

   > > 16. (3.16) — Otherwise, if `Period::type` is `peta`, the suffix `units_suffix` is `"Ps"`.

   > > 17. (3.17) — Otherwise, if `Period::type` is `exa`, the suffix `units_suffix` is `"Es"`.

   > > 18. (3.18) — Otherwise, if `Period::type` is `ratio<60>`, the suffix `units_suffix` is `"min"`.

   > > 19. (3.19) — Otherwise, if `Period::type` is `ratio<3600>`, the suffix `units_suffix` is `"h"`.

   > > 20. (3.20) — Otherwise, if `Period::type` is `ratio<86400>`, the suffix `units_suffix` is `"d"`.

   > > 21. (3.21) — Otherwise, if `Period::type::den == 1`, the suffix `units_suffix` is `"[num]s"`.

   > > 22. (3.22) — Otherwise, the suffix `units_suffix` is `"[num/den]s"`.

   > > In the list above the use of `num` and `den` refer to the static data members of `Period::type`, which are converted to arrays of `charT` using a decimal conversion with no leading zeroes.

   > > -4- If `Period::type` is `micro`, but the character U+00B5 cannot be represented in the encoding used for `charT`, the unit suffix `units_suffix` `"us"` is used instead of `"µs"`.

   > > -5- Returns: `os`.

### 3318(i). Clarify whether clocks can represent time before their epoch

Section: 30.7.2.1 [time.clock.system.overview] Status: C++20 Submitter: Great Britain Opened: 2019-11-05 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

Addresses GB 335

Wording for clocks should be unified unless they are intended to behave differently In 27.7.1.1 note 1 for `system_clock` it is stated:

> "Objects of type `system_clock` represent wall clock time from the system-wide realtime clock. Objects of type `sys_time<Duration>` measure time since (and before) 1970-01-01 00:00:00 UTC"

The express statement of "since (and before)" is important given the time epoch of these clocks. If all the clocks support time prior to their zero-time then this should be stated explicitly. If not then likewise that should be noted. No change is proposed yet, clarification required over the intended behaviour when using values prior to a given clock's epoch is needed before the appropriate change can be suggested.

Proposed change:

Unify the wording.

Howard Hinnant:

The clocks that are specified to have a signed `rep` imply that they will support negative time points, but not how negative. For example if `system_clock::duration` is `nanoseconds` represented with 64 bits, then `system_clock::time_point` can't possibly represent dates prior to 1677-09-21 00:12:43.145224192. This is a negative `time_point` since it is prior to 1970-01-01 00:00:00. But it is not very negative compared to (for example) `sys_time<microseconds>::min()`.

Those clocks with a signed `rep` are:

- `system_clock`

- `utc_clock`

- `tai_clock`

- `gps_clock`

- `file_clock`

Those clocks where the signed-ness of `rep` is unspecified are:

- `steady_clock`

- `high_resolution_clock`

Therefore this response emphasizes the "Unify the wording" part of this NB comment.

[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 30.7.2.1 [time.clock.system.overview] as indicated:

   > -1- Objects of type `system_clock` represent wall clock time from the system-wide realtime clock. Objects of type `sys_time<Duration>` measure time since (and before) 1970-01-01 00:00:00 UTC excluding leap seconds. This measure is commonly referred to as Unix time. This measure facilitates an efficient mapping between `sys_time` and calendar types (30.8 [time.cal]). [Example: `sys_seconds{sys_days{1970y/January/1}}.time_since_epoch()` is `0s`. `sys_seconds{sys_days{2000y/January/1}}.time_since_epoch()` is `946'684'800s`, which is `10'957 * 86'400s`. — end example]

### 3319(i). Properly reference specification of IANA time zone database

Section: 30.11.1 [time.zone.general] Status: C++20 Submitter: Germany Opened: 2019-11-05 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

Addresses DE 344

This paragraph says

> "27.11 describes an interface for accessing the IANA Time Zone database described in RFC 6557, …"

However, RFC 6557 does not describe the database itself; it only describes the maintenance procedures for that database, as its title implies (quoted in clause 2).

Proposed change:

Add a reference to a specification of the database itself, or excise all references to the IANA time zone database.

Howard Hinnant:

We can not entirely remove the reference to IANA because we need portable `time_zone` names (e.g. "America/New_York") and definitions. However the NB comment is quite accurate and fixed with the proposed resolution.

[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 2 [intro.refs] as indicated:

   > -1- The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

   > 1. […]

   > 2. (1.2) — INTERNET ENGINEERING TASK FORCE (IETF). RFC 6557: Procedures for Maintaining the Time Zone Database [online]. Edited by E. Lear, P. Eggert. February 2012 [viewed 2018-03-26]. Available at https://www.ietf.org/rfc/rfc6557.txt

   > 3. […]

2. Modify 30.11.1 [time.zone.general] as indicated:

   > -1- 30.11 [time.zone] describes an interface for accessing the IANA Time Zone dDatabase described in RFC 6557, that interoperates with `sys_time` and `local_time`. This interface provides time zone support to both the civil calendar types (30.8 [time.cal]) and to user-defined calendars.

3. Modify section "Bibliography" as indicated:

   > The following documents are cited informatively in this document.

   > 1. — IANA Time Zone Database. Available at https://www.iana.org/time-zones

   > 2. — ISO/IEC 10967-1:2012, Information technology — Language independent arithmetic — Part 1: Integer and floating point arithmetic

   > 3. […]

### 3320(i). `span::cbegin/cend` methods produce different results than `std::[ranges::]cbegin/cend`

Section: 23.7.2.2.7 [span.iterators] Status: C++20 Submitter: Poland Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

Addresses PL 247

`span<T>` provides a `const`-qualified `begin()` method and `cbegin()` method that produces a different result if `T` is not `const`-qualifed:

1. `begin()` produces mutable iterator over `T` (as if `T*`)

2. `cbegin()` produces `const` iterator over `T` (as if `T const*`)

As consequence for the object `s` of type `span<T>`, the call to the `std::cbegin(s)/std::ranges::cbegin(s)` produces different result than `s.cbegin()`.

Proposed change:

Change `span<T>` members `cbegin()/cend()/crbegin()/crend()/const_iterator` to be equivalent to `begin()/end()/rbegin()/rend()/iterator` respectively.

Tomasz Kamiński:

Per LEWG discussion in Belfast these methods and related typedefs should be removed.

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 23.7.2.2.1 [span.overview], class template `span` synopsis, as indicated:

   >     namespace std {
   >       template<class ElementType, size_t Extent = dynamic_extent>
   >       class span {
   >       public:
   >         // constants and types
   >         using element_type = ElementType;
   >         using value_type = remove_cv_t<ElementType>;
   >         using index_type = size_t;
   >         using difference_type = ptrdiff_t;
   >         using pointer = element_type*;
   >         using const_pointer = const element_type*;
   >         using reference = element_type&;
   >         using const_reference = const element_type&;
   >         using iterator = implementation-defined; // see 23.7.2.2.7 [span.iterators]
   >         using const_iterator = implementation-defined;
   >         using reverse_iterator = std::reverse_iterator<iterator>;
   >         using const_reverse_iterator = std::reverse_iterator<const_iterator>;
   >         static constexpr index_type extent = Extent;

   >         […]
   >         // 23.7.2.2.7 [span.iterators], iterator support
   >         constexpr iterator begin() const noexcept;
   >         constexpr iterator end() const noexcept;
   >         constexpr const_iterator cbegin() const noexcept;
   >         constexpr const_iterator cend() const noexcept;
   >         constexpr reverse_iterator rbegin() const noexcept;
   >         constexpr reverse_iterator rend() const noexcept;
   >         constexpr const_reverse_iterator crbegin() const noexcept;
   >         constexpr const_reverse_iterator crend() const noexcept;
   >         friend constexpr iterator begin(span s) noexcept { return s.begin(); }
   >         friend constexpr iterator end(span s) noexcept { return s.end(); }
   >         […]
   >       };
   >     […]
   >     }

2. Modify 23.7.2.2.7 [span.iterators] as indicated:

   >     using iterator = implementation-defined;
   >     using const_iterator = implementation-defined;

   > > -1- The types models `contiguous_iterator` (24.3.4.14 [iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]), and meets the requirements for constexpr iterators (24.3.1 [iterator.requirements.general]). All requirements on container iterators (23.2 [container.requirements]) apply to `span::iterator` and `span::const_iterator` as well.

   > […]

   >     constexpr const_iterator cbegin() const noexcept;

   > > -6- Returns: A constant iterator referring to the first element in the span. If `empty()` is `true`, then it returns the same value as `cend()`.

   >     constexpr const_iterator cend() const noexcept;

   > > -7- Returns: A constant iterator which is the past-the-end value.

   >     constexpr const_reverse_iterator crbegin() const noexcept;

   > > -8- Effects: Equivalent to: `return const_reverse_iterator(cend());`

   >     constexpr const_reverse_iterator crend() const noexcept;

   > > -9- Effects: Equivalent to: `return const_reverse_iterator(cbegin());`

### 3321(i). `uninitialized_construct_using_allocator` should use `construct_at`

Section: 20.2.8.2 [allocator.uses.construction] Status: C++20 Submitter: United States Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 0

View all other issues in [allocator.uses.construction].

View all issues with C++20 status.

Discussion:

Addresses US 213

`uninitialized_construct_using_allocator` should use `construct_at` instead of `operator new`

Proposed change:

> Effects: Equivalent to:

> >     return ::new(static_cast<void*>(p))
> >     construct_at(p,
> >     T(make_obj_using_allocator<T>(alloc,
> >     std::forward<Args>(args)...)));

Tim Song:

The proposed wording in the NB comment is incorrect, because it prevents guaranteed elision.

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 20.2.8.2 [allocator.uses.construction] as indicated:

   >     template<class T, class Alloc, class... Args>
   >       constexpr T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);

   > > -17- Effects: Equivalent to:

   > > >     return ::new(static_cast<void*>(p))
   > > >       T(make_obj_using_allocator<T>(apply([&](auto&&...xs) {
   > > >              return construct_at(p, std::forward<decltype(xs)>(xs)...);
   > > >          }, uses_allocator_construction_args<T>(alloc, std::forward<Args>(args)...));

### 3322(i). Add `join_view::base()` member function

Section: 25.7.14.2 [range.join.view] Status: Resolved Submitter: United States Opened: 2019-11-06 Last modified: 2020-11-09

Priority: 0

View all other issues in [range.join.view].

View all issues with Resolved status.

Discussion:

Addresses US 293

`join_view` is missing a `base()` member for returning the underlying view. All the other range adaptors provide this.

Proposed change:

To the `join_view` class template add the member:

>     constexpr V base() const { return base_; }

Jonathan Wakely:

The NB comment says "`join_view` is missing a `base()` member for returning the underlying view. All the other range adaptors provide this."

In fact, `split_view` and `istream_view` do not provide `base()` either. Of the views that do define `base()`, all except `all_view` do so out-of-line, so the proposed resolution adds it out-of-line too.

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

[2019-12-16; Casey comments]

This issue has been resolved by P1456R1 "Move-only views", which added no less than two member functions named "base" to `join_view`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4835.

> 1. Modify 25.7.14.2 [range.join.view], class template `join_view` synopsis, as indicated:

>    >     […]
>    >     template<input_range R>
>    >       requires viewable_range<R> && constructible_from<V, all_view<R>>
>    >     constexpr explicit join_view(R&& r);

>    >     constexpr V base() const;

>    >     constexpr auto begin() {
>    >       return iterator<simple-view<V>>{*this, ranges::begin(base_)};
>    >     }
>    >     […]

> 2. Modify 25.7.14.2 [range.join.view] as indicated:

>    >     template<input_range R>
>    >       requires viewable_range<R> && constructible_from<V, all_view<R>>
>    >     constexpr explicit join_view(R&& r);

>    > > -2- Effects: […]

>    >     constexpr V base() const;

>    > > -?- Effects: Equivalent to: `return base_;`

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

Resolved by accepting P1456R1.

### 3323(i). `has-tuple-element` helper concept needs `convertible_to`

Section: 25.7.23.2 [range.elements.view] Status: C++20 Submitter: Great Britain Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 0

View all other issues in [range.elements.view].

View all issues with C++20 status.

Discussion:

Addresses GB 299

`has-tuple-element` helper concept needs `convertible_to`

The exposition-only `has-tuple-element` concept (for `elements_view`) is defined as

>     template<class T, size_t N>
>     concept has-tuple-element = exposition only
>       requires(T t) {
>         typename tuple_size<T>::type;
>         requires N < tuple_size_v<T>;
>         typename tuple_element_t<N, T>;
>         { get<N>(t) } -> const tuple_element_t<N, T>&;
>       };

However, the return type constraint for `{ get<N>(t) }` is no longer valid under the latest concepts changes

Proposed change:

Change to:

>     template<class T, size_t N>
>     concept has-tuple-element = exposition only
>       requires(T t) {
>         typename tuple_size<T>::type;
>         requires N < tuple_size_v<T>;
>         typename tuple_element_t<N, T>;
>         { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
>       };

Jonathan Wakely:

The NB comment says "The return type constraint for `{ get(t) }` is no longer valid under the latest concepts changes." The changes referred to are those in P1452R2.

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 25.7.23.2 [range.elements.view], class template `elements_view` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<class T, size_t N>
   >       concept has-tuple-element = // exposition only
   >         requires(T t) {
   >           typename tuple_size<T>::type;
   >           requires N < tuple_size_v<T>;
   >           typename tuple_element_t<N, T>;
   >           { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
   >         };
   >     […]
   >     }

### 3324(i). Special-case `std::strong/weak/partial_order` for pointers

Section: 17.12.6 [cmp.alg] Status: C++20 Submitter: Canada Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 0

View other active issues in [cmp.alg].

View all other issues in [cmp.alg].

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Discussion:

Addresses CA 178

`std::strong_order`, `weak_order`, and `partial_order` have special cases for floating point, but are missing special casing for pointers. `compare_three_way` and `std::less` have the special casing for pointers.

Proposed change:

Change [cmp.alg] bullet 1.4 from
"Otherwise, `strong_ordering(E <=> F)` if it is a well-formed expression."
to
"Otherwise, `strong_ordering(compare_three_way()(E, F))` if it is a well-formed expression."

Change [cmp.alg] bullet 2.4 from
"Otherwise, `weak_ordering(E <=> F)` if it is a well-formed expression."
to
"Otherwise, `weak_ordering(compare_three_way()(E, F))` if it is a well-formed expression."
Change [cmp.alg] bullet 3.3 from
"Otherwise, `partial_ordering(E <=> F)` if it is a well-formed expression."
to
"Otherwise, `partial_ordering(compare_three_way()(E, F))` if it is a well-formed expression."

Dietmar Kühl:

Use `compare_three_way` instead of `<=>` for the various comparison algorithms.

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Change 17.12.6 [cmp.alg] as indicated:

   > -1- The name `strong_order` […]

   > > 1. […]

   > > 2. (1.4) — Otherwise, `strong_ordering(E <=> Fcompare_three_way()(E, F))` if it is a well-formed expression.

   > > 3. […]

   > -2- The name `weak_order` […]

   > > 1. […]

   > > 2. (2.4) — Otherwise, `weak_ordering(E <=> Fcompare_three_way()(E, F))` if it is a well-formed expression.

   > > 3. […]

   > -3- The name `partial_order` […]

   > > 1. […]

   > > 2. (3.3) — Otherwise, `partial_ordering(E <=> Fcompare_three_way()(E, F))` if it is a well-formed expression.

   > > 3. […]

### 3325(i). Constrain return type of transformation function for `transform_view`

Section: 25.7.9.2 [range.transform.view] Status: C++20 Submitter: United States Opened: 2019-11-06 Last modified: 2022-01-15

Priority: 0

View all issues with C++20 status.

Discussion:

Addresses US 303

The `transform_view` does not constrain the return type of the transformation function. It is invalid to pass a `void`-returning transformation function to the `transform_view`, which would cause its `iterator`s' `operator*` member to return `void`.

Proposed change:

Change the constraints on `transform_view` to the following:

>     template<input_range V, copy_constructible F>
>       requires view<V> && is_object_v<F> &&
>                regular_invocable<F&, range_reference_t<V>> &&
>                can-reference<invoke_result_t<F&, range_reference_t<V>>>
>     class transform_view;

Jonathan Wakely:

The NB comment says "The `transform_view` does not constrain the return type of the transformation function. It is invalid to pass a `void`-returning transformation function to the `transform_view`, which would cause its `iterator`s' `operator*` member to return `void`."

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 25.7.9.2 [range.transform.view], class template `transform_view` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<input_range V, copy_constructible F>
   >         requires view<V> && is_object_v<F> &&
   >                  regular_invocable<F&, range_reference_t<V>> &&
   >                  can-reference<invoke_result_t<F&, range_reference_t<V>>>
   >       class transform_view : public view_interface<transform_view<V, F>> {
   >         […]
   >       };
   >     […]
   >     }

### 3326(i). `enable_view` has false positives

Section: 25.4.5 [range.view] Status: C++20 Submitter: Germany Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 0

View all other issues in [range.view].

View all issues with C++20 status.

Discussion:

Addresses DE 282

"Since the difference between range and view is largely semantic, the two are differentiated with the help of `enable_view`." (§3)

`enable_view` is designed as on opt-in trait to specify that a type is a view. It defaults to true for types derived from `view_base` (§4.2) which is clearly a form of opt-in. But it also employs a heuristic assuming that anything with `iterator == const_iterator` is also view (§4.3).

This is a very poor heuristic, the same paragraph already needs to define six exceptions from this rule for standard library types (§4.2).

Experience in working with range-v3 has revealed multiple of our own library types as being affected from needing to opt-out from the "auto-opt-in", as well. This is counter-intuitive: something that was never designed to be a view shouldn't go through hoops so that it isn't treated as a view.

Proposed change:

Make `enable_view` truly be opt-in by relying only on explicit specialisation or inheritance from `view_base`. This means removing 24.4.4 §4.2 - §4.4 and introducing new §4.2 "Otherwise, false".

Double-check if existing standard library types like `basic_string_view` and `span` need to opt-in to being a view now.

Casey Carter:

`enable_view` (25.4.5 [range.view]) is designed as on opt-in trait to specify that a type is a view. It defaults to `true` for types derived from `view_base` — which is a form of opt-in — and it also employs a heuristic. Unfortunately, the heuristic has false positives. The working draft itself includes six exceptions to the heuristic for standard library types. Since false positives are much more problematic for users than false negatives, we should eliminate the heuristic.

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 25.4.5 [range.view] as indicated:

   >     template<class T>
   >       inline constexpr bool enable_view = see belowderived_from<T, view_base>;

   > > -4- Remarks: For a type `T`, the default value of `enable_view<T>` is:

   > > 1. (4.1) — If `derived_from<T, view_base>` is `true`, `true`.

   > > 2. (4.2) — Otherwise, if `T` is a specialization of class template `initializer_list` (17.11 [support.initlist]), `set` (23.4.6 [set]), `multiset` (23.4.7 [multiset]), `unordered_set` (23.5.6 [unord.set]), `unordered_multiset` (23.5.7 [unord.multiset]), or `match_results` (28.6.9 [re.results]), `false`.

   > > 3. (4.3) — Otherwise, if both `T` and `const T` model `range` and `range_reference_t<T>` is not the same type as `range_reference_t<const T>`, `false`. [Note: Deep `const`-ness implies element ownership, whereas shallow const-ness implies reference semantics. — end note]

   > > 4. (4.4) — Otherwise, true.

2. Modify 27.3.2 [string.view.synop], header `<string_view>` synopsis, as indicated:

   >     namespace std {
   >       // 27.3.3 [string.view.template], class template basic_string_view
   >       template<class charT, class traits = char_traits<charT>>
   >       class basic_string_view;

   >       template<class charT, class traits>
   >         inline constexpr bool ranges::enable_view<basic_string_view<charT, traits>> = true;

   >       […]
   >     }

3. Modify 23.7.2.1 [span.syn], header `<span>` synopsis, as indicated:

   >     namespace std {
   >       // constants
   >       inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();

   >       // 23.7.2.2 [views.span], class template span
   >       template<class ElementType, size_t Extent = dynamic_extent>
   >       class span;

   >       template<class ElementType, size_t Extent>
   >         inline constexpr bool ranges::enable_view<span<ElementType, Extent>> = Extent == 0 ||
   >           Extent == dynamic_extent;

   >       […]
   >     }

4. Modify [range.split.outer.value], class `split_view::outer_iterator::value_type` synopsis, as indicated:

   > [Drafting note: The following applies the proposed wording for LWG 3276(i)]

   >     namespace std::ranges {
   >       template<class V, class Pattern>
   >       template<bool Const>
   >       struct split_view<V, Pattern>::outer_iterator<Const>::value_type
   >         : view_interface<value_type> {
   >       private:
   >         outer_iterator i_ = outer_iterator(); // exposition only
   >       public:
   >         value_type() = default;
   >         constexpr explicit value_type(outer_iterator i);

   >         constexpr inner_iterator<Const> begin() const;
   >         constexpr default_sentinel_t end() const;
   >       };
   >     }

### 3327(i). Format alignment specifiers vs. text direction

Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Great Britain Opened: 2019-11-07 Last modified: 2021-02-25

Priority: 0

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Discussion:

Addresses GB 225

`std::format()` alignment specifiers should be independent of text direction The align specifiers for formatting standard integer and string types are expressed in terms of "left" and "right". However, "left alignment" as currently defined in the `format()` specification might end up being right-aligned when the resulting string is displayed in a RTL or bidirectional locale. This ambiguity can be resolved by removing "left" and "right" and replacing with "start" and "end", without changing any existing implementation and without changing the intent of the feature.

Proposed change:

In [tab:format.align]: Forces the field to be left-aligned within aligned to the start of the available space and Forces the field to be right-aligned within aligned to the end of the available space

Jeff Garland:

Wiki notes from Belfast Wed:

> # GB225

> JG: SG16 approved this.

> JG: If you scroll back up, you'll see see it's very tiny. Two line change.

> JG: I'm willing to submit an LWG issue to suggest we make a wording change to take it off our plate.

> CC: Is this the one that changes left/right to beginning/end?

> Some people: yes

> MC: Any problem with Jeff's proposed direction and this proposed fix?

> MC: I hear none.

[2019-11 Moved to Ready on Friday AM in Belfast]

Proposed resolution:

This wording is relative to N4835.

1. Modify "Table 57 — Meaning of align options" [tab:format.align] as indicated:

   > Table 57 — Meaning of align options [tab:format.align]
   > Option | Meaning
   > --- | ---
   > `<` | Forces the field to be left-aligned withinaligned to the start of the available space. This is the default for non-arithmetic types, `charT`, and `bool`, unless an integer presentation type is specified.
   > `>` | Forces the field to be right-aligned withinaligned to the end of the available space. This is the default for arithmetic types other than `charT` and `bool` or when an integer presentation type is specified.
   > `[…]`

### 3328(i). Clarify that `std::string` is not good for UTF-8

Section: D.23.1 [depr.fs.path.factory] Status: C++20 Submitter: The Netherlands Opened: 2019-11-07 Last modified: 2021-02-25

Priority: 0

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Discussion:

Addresses NL 375

Example in deprecated section implies that `std::string` is the type to use for utf8 strings.

> [Example: A string is to be read from a database that is encoded in UTF-8, and used to create a directory using the native encoding for filenames:

>     namespace fs = std::filesystem;
>     std::string utf8_string = read_utf8_data();
>     fs::create_directory(fs::u8path(utf8_string));

Proposed change:

Add clarification that `std::string` is the wrong type for utf8 strings

Jeff Garland:

SG16 in Belfast: Recommend to accept with a modification to update the example in D.23.1 [depr.fs.path.factory] p4 to state that `std::u8string` should be preferred for UTF-8 data.

Rationale: The example code is representative of historic use of `std::filesystem::u8path` and should not be changed to use `std::u8string`. The recommended change is to a non-normative example and may therefore be considered editorial.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4835.

> 1. Modify D.23.1 [depr.fs.path.factory] as indicated:

>    > -4- [Example: A string is to be read from a database that is encoded in UTF-8, and used to create a directory using the native encoding for filenames:

>    > >     namespace fs = std::filesystem;
>    > >     std::string utf8_string = read_utf8_data();
>    > >     fs::create_directory(fs::u8path(utf8_string));

>    > For POSIX-based operating systems with the native narrow encoding set to UTF-8, no encoding or type conversion occurs.

>    > For POSIX-based operating systems with the native narrow encoding not set to UTF-8, a conversion to UTF-32 occurs, followed by a conversion to the current native narrow encoding. Some Unicode characters may have no native character set representation.

>    > For Windows-based operating systems a conversion from UTF-8 to UTF-16 occurs. — end example]

>    > [Note: The example above is representative of historic use of `filesystem` `u8path`. New code should use `std::u8string` in place of `std::string`. — end note]

LWG Belfast Friday Morning

Requested changes:

- Historic => historical.
- Add missing :: before u8path.
- Remove ISO rules forbidden 'should' in a note.
- Use language describing why new code should use the u8string constructor rather than preaching that new code should do something.

Billy O'Neal provides updated wording.

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

This wording is relative to N4835.

1. Modify D.23.1 [depr.fs.path.factory] as indicated:

   > -4- [Example: A string is to be read from a database that is encoded in UTF-8, and used to create a directory using the native encoding for filenames:

   > >     namespace fs = std::filesystem;
   > >     std::string utf8_string = read_utf8_data();
   > >     fs::create_directory(fs::u8path(utf8_string));

   > For POSIX-based operating systems with the native narrow encoding set to UTF-8, no encoding or type conversion occurs.

   > For POSIX-based operating systems with the native narrow encoding not set to UTF-8, a conversion to UTF-32 occurs, followed by a conversion to the current native narrow encoding. Some Unicode characters may have no native character set representation.

   > For Windows-based operating systems a conversion from UTF-8 to UTF-16 occurs. — end example]

   > [Note: The example above is representative of a historical use of `filesystem::u8path`. Passing a `std::u8string` to `path`'s constructor is preferred for an indication of UTF-8 encoding more consistent with `path`'s handling of other encodings. — end note]

### 3329(i). `totally_ordered_with` both directly and indirectly requires `common_reference_with`

Section: 18.5.5 [concept.totallyordered] Status: C++20 Submitter: United States Opened: 2019-11-07 Last modified: 2021-02-25

Priority: 0

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Discussion:

Addresses US 201

The `totally_ordered_with<T, U>` redundantly requires both `common_reference_with<const remove_reference_t&, const remove_reference_t&>` and `equality_comparable_with<T, U>` (which also has the `common_reference_with` requirement). The redundant requirement should be removed.

Proposed change:

Change the definition of `totally_ordered_with` to:

>     template<class T, class U>
>       concept totally_ordered_with =
>         totally_ordered<T> && totally_ordered<U> &&
>         equality_comparable_with<T, U> &&
>         totally_ordered<
>           common_reference_t<
>             const remove_reference_t<T>&,
>             const remove_reference_t<U<&>> &&
>         requires(const remove_reference_t<T<& t,
>                         const remove_reference_t>U>& u) {
>           [… as before …]

[2019-11 Moved to Ready on Friday AM in Belfast]

Proposed resolution:

This wording is relative to N4835.

1. Change 18.5.5 [concept.totallyordered] as indicated:

   > For some type `T`, let `a`, `b`, and `c` be lvalues of type `const remove_reference_t<T>`. `T` models `totally_ordered` only if

   > 1. (1.1) — Exactly one of `bool(a < b)`, `bool(a > b)`, or `bool(a == b)` is `true`.

   > 2. (1.2) — If `bool(a < b)` and `bool(b < c)`, then `bool(a < c)`.

   > 3. (1.3) — `bool(a > b) == bool(b < a)`.

   > 4. (1.4) — `bool(a <= b) == !bool(b < a)`.

   > 5. (1.5) — `bool(a >= b) == !bool(a < b)`.

   >     template<class T, class U>
   >       concept totally_ordered_with =
   >         totally_ordered<T> && totally_ordered<U> &&
   >         common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
   >         equality_comparable_with<T, U> &&
   >         totally_ordered<
   >           common_reference_t<
   >             const remove_reference_t<T>&,
   >             const remove_reference_t<U>&>> &&
   >         equality_comparable_with<T, U> &&
   >         requires(const remove_reference_t<T>& t,
   >                  const remove_reference_t<U>& u) {
   >           { t <  u } -> boolean;
   >           { t >  u } -> boolean;
   >           { t <= u } -> boolean;
   >           { t >= u } -> boolean;
   >           { u <  t } -> boolean;
   >           { u >  t } -> boolean;
   >           { u <= t } -> boolean;
   >           { u >= t } -> boolean;
   >         };

### 3330(i). Include `<compare>` from most library headers

Section: 17.13.2 [coroutine.syn], 19.5.2 [system.error.syn], 22.2.1 [utility.syn], 22.4.2 [tuple.syn], 22.5.2 [optional.syn], 22.6.2 [variant.syn], 20.2.2 [memory.syn], 17.7.6 [type.index.synopsis], 27.4.2 [string.syn], 27.3.2 [string.view.synop], 23.3.2 [array.syn], 23.3.4 [deque.syn], 23.3.6 [forward.list.syn], 23.3.10 [list.syn], 23.3.12 [vector.syn], 23.4.2 [associative.map.syn], 23.4.5 [associative.set.syn], 23.5.2 [unord.map.syn], 23.5.5 [unord.set.syn], 23.6.2 [queue.syn], 23.6.5 [stack.syn], 24.2 [iterator.synopsis], 25.2 [ranges.syn], 30.2 [time.syn], 31.12.4 [fs.filesystem.syn], 28.6.3 [re.syn], 32.4.2 [thread.syn] Status: C++20 Submitter: United States Opened: 2019-11-07 Last modified: 2021-02-25

Priority: 0

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Discussion:

Addresses US 181

The spaceship `operator<=>` is typically not usable unless the library header `<compare>` is directly included by the user. Many standard library headers provide overloads for this operator. Worse, several standard classes have replaced their existing definition for comparison operators with a reliance on the spaceship operator, and existing code will break if the necessary header is not (transitively) included. In a manner similar to the mandated library headers transitively `#include`-ing `<initializer_list>` in C++11, these headers should mandate a transitive `#include <compare>`.

Proposed change:

Add:

`#include <compare>`

to the header synopsis for each of the following headers:

>     <array>
>     <chrono>
>     <coroutine>
>     <deque>
>     <forward_list>
>     <filesystem>
>     <iterator>
>     <list>
>     <map>
>     <memory>
>     <optional>
>     <queue>
>     <ranges>
>     <regex>
>     <set>
>     <stack>
>     <string>
>     <string_view>
>     <system_error>
>     <thread>
>     <tuple>
>     <type_index>
>     <unordered_map>
>     <unordered_set>
>     <utility>
>     <variant>
>     <vector>

[2019-11 Moved to Ready on Friday AM in Belfast]

Proposed resolution:

This wording is relative to N4835.

1. Add

   >     #include <compare>

   to the following header synopses:

   1. 17.13.2 [coroutine.syn]
   2. 19.5.2 [system.error.syn]
   3. 22.2.1 [utility.syn]
   4. 22.4.2 [tuple.syn]
   5. 22.5.2 [optional.syn]
   6. 22.6.2 [variant.syn]
   7. 20.2.2 [memory.syn]
   8. 17.7.6 [type.index.synopsis]
   9. 27.4.2 [string.syn]
   10. 27.3.2 [string.view.synop]
   11. 23.3.2 [array.syn]
   12. 23.3.4 [deque.syn]
   13. 23.3.6 [forward.list.syn]
   14. 23.3.10 [list.syn]
   15. 23.3.12 [vector.syn]
   16. 23.4.2 [associative.map.syn]
   17. 23.4.5 [associative.set.syn]
   18. 23.5.2 [unord.map.syn]
   19. 23.5.5 [unord.set.syn]
   20. 23.6.2 [queue.syn]
   21. 23.6.5 [stack.syn]
   22. 24.2 [iterator.synopsis]
   23. 25.2 [ranges.syn]
   24. 30.2 [time.syn]
   25. 31.12.4 [fs.filesystem.syn]
   26. 28.6.3 [re.syn]
   27. 32.4.2 [thread.syn]

### 3331(i). Define `totally_ordered/_with` in terms of `partially-ordered-with`

Section: 18.5.5 [concept.totallyordered] Status: C++20 Submitter: Great Britain Opened: 2019-11-08 Last modified: 2021-02-25

Priority: 0

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Discussion:

Addresses GB 202

Define `totally_ordered[_with]` in terms of `partially-ordered-with`. This will simplify the definition of both concepts (particularly `totally_ordered_with`), and make them in-line with `equality_comparable[_with]`. Now that we've defined `partially-ordered-with` for 17.12.4 [cmp.concept], we should consider utilising it in as many locations as possible.

Proposed change:

    template<class T>
      concept totally_ordered =
        equality_comparable<T> &&
        partially-ordered-with<T, T>;

    template<class T, class U>
      concept totally_ordered_with =
        totally_ordered<T> &&
        totally_ordered<U> &&
        common_reference_with<
          const remove_reference_t<T>&,
          const remove_reference_t<U>&> &&
        totally_ordered<
          common_reference_t<
            const remove_reference_t<T>&,
            const remove_reference_t<U>&>> &&
        equality_comparable_with<T, U> &&
        partially-ordered-with<T, U>;

LWG discussion in Belfast notes that 3329(i) also touches the definition of `totally_ordered_with`; the two sets of changes are consistent.

[2019-11 Status to Ready Friday afternoon LWG in Belfast]

Proposed resolution:

This wording is relative to N4835.

1. Change 18.5.5 [concept.totallyordered] as follows:

   >     template<class T>
   >       concept totally_ordered =
   >         equality_comparable<T> && partially-ordered-with<T, T>;
   >         requires(const remove_reference_t<T>& a,
   >                  const remove_reference_t<T>& b) {
   >           { a <  b } -> boolean;
   >           { a >  b } -> boolean;
   >           { a <= b } -> boolean;
   >           { a >= b } -> boolean;
   >         };

   > -1- For some type `T`, let `a`, `b`, and `c` be lvalues of type `const remove_reference_t<T>`. `T` models `totally_ordered` only if

   > (1.1) — Exactly one of `bool(a < b)`, `bool(a > b)`, or `bool(a == b)` is `true`.

   > (1.2) — If `bool(a < b)` and `bool(b < c)`, then `bool(a < c)`.

   > (1.3) — `bool(a > b) == bool(b < a)`.

   > (1.4) — `bool(a <= b) == !bool(b < a)`.

   > (1.5) — `bool(a >= b) == !bool(a < b)`.

   >     template<class T, class U>
   >       concept totally_ordered_with =
   >         totally_ordered<T> && totally_ordered<U> &&
   >         common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
   >         totally_ordered<
   >           common_reference_t<
   >             const remove_reference_t<T>&,
   >             const remove_reference_t<U>&>> &&
   >           equality_comparable_with<T, U> &&
   >           partially-ordered-with<T, U>;
   >           requires(const remove_reference_t<T>& t,
   >                    const remove_reference_t<U>& u) {
   >             { t <  u } -> boolean;
   >             { t >  u } -> boolean;
   >             { t <= u } -> boolean;
   >             { t >= u } -> boolean;
   >             { u <  t } -> boolean;
   >             { u >  t } -> boolean;
   >             { u <= t } -> boolean;
   >             { u >= t } -> boolean;
   >           };

   > […]

### 3332(i). Issue in §[time.format]

Section: 30.12 [time.format] Status: C++20 Submitter: Mateusz Pusz Opened: 2019-11-05 Last modified: 2021-02-25

Priority: 0

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Discussion:

Table 97 [tab:time.format.spec] enumerates `'q'` and `'Q'` but those are not specified in chrono-format-spec provided in paragraph 1 of sub-clause 30.12 [time.format].

[2019-11-23 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4835.

1. Change the type grammar element in 30.12 [time.format] p1 as indicated:

   >     type: one of
   >              a A b B c C d D e F g G h H I j m M n
   >              p q Q r R S t T u U V w W x X y Y z Z %

### 3334(i). `basic_osyncstream` move assignment and destruction calls `basic_syncbuf::emit()` twice

Section: 31.11.3 [syncstream.osyncstream] Status: C++20 Submitter: Tim Song Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 3

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Discussion:

These functions are specified to call `emit()`, which calls `emit()` on the `basic_syncbuf` and sets `badbit` if it fails. Then, the move assignment is specified to move-assign the `basic_syncbuf`, while the destructor implicitly needs to destroy the `basic_syncbuf` data member. This calls `emit()` on the `basic_syncbuf` again.

Is this intended?

[2020-02-13 Tim adds wording after discussion with Peter]

[2020-02 Status to Immediate Thursday afternoon in Prague.]

Proposed resolution:

This wording is relative to N4849.

> [Drafting note: There is no need to explicitly call `emit` at all in these functions; memberwise move-assignment/destruction is sufficient, so we can strike the specification entirely and rely on the wording in 16.3.3.5 [functions.within.classes]. — end drafting note]

1. Edit 31.11.3.2 [syncstream.osyncstream.cons] as indicated:

   >     ~basic_osyncstream();

   > > -6- Effects: Calls `emit()`. If an exception is thrown from `emit()`, that exception is caught and ignored.

2. Strike [syncstream.osyncstream.assign]:

   >     basic_osyncstream& operator=(basic_osyncstream&& rhs) noexcept;

   > > -1- Effects: First, calls `emit()`. If an exception is thrown from `emit()`, that exception is caught and ignored. Move assigns `sb` from `rhs.sb`. [ Note: This disassociates `rhs` from its wrapped stream buffer ensuring destruction of `rhs` produces no output. — end note ]

   > > -2- Postconditions: `nullptr == rhs.get_wrapped()` is `true`. `get_wrapped()` returns the value previously returned by `rhs.get_wrapped()`.

### 3335(i). Resolve C++20 NB comments US 273 and GB 274

Section: 25.2 [ranges.syn] Status: C++20 Submitter: United States/Great Britain Opened: 2019-11-08 Last modified: 2021-02-25

Priority: 1

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Discussion:

Addresses US 273/GB 274

US 273:

`all_view` is not a view like the others. For the other view types, `foo_view{args...}` is a valid way to construct an instance of type foo_view. However, `all_view` is just an alias to the type of `view::all(arg)`, which could be one of several different types. `all_view` feels like the wrong name.

Proposed change:

Suggest renaming `all_view` to `all_t` and moving it into the `views::` namespace.

GB 274:

Add `range_size_t`.

LEWG asked that `range_size_t` be removed from P1035, as they were doing a good job of being neutral w.r.t whether or not size-types were signed or unsigned at the time. Now that we've got a policy on what size-types are, and that P1522 and P1523 have been adopted, it makes sense for there to be a `range_size_t`.

Proposed change:

Add to [ranges.syn]:

>     template<range R>
>       using range_difference_t = iter_difference_t<iterator_t<R>>;
>     template<sized_range R>
>       using range_size_t = decltype(ranges::size(declval<R&>()));

David Olsen:

The proposed wording has been approved by LEWG and LWG in Belfast.

[2019-11-23 Issue Prioritization]

Priority to 1 after reflector discussion.

[2020-02-10 Move to Immediate Monday afternoon in Prague]

Proposed resolution:

This wording is relative to N4835.

1. Change 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     #include <initializer_list>
   >     #include <iterator>

   >     namespace std::ranges {
   >       […]
   >       // 25.4.2 [range.range], ranges
   >       template<class T>
   >       concept range = see below;
   >       […]
   >       template<range R>
   >         using range_difference_t = iter_difference_t<iterator_t<R>>;
   >       template<sized_range R>
   >         using range_size_t = decltype(ranges::size(declval<R&>()));
   >       template<range R>
   >         using range_value_t = iter_value_t<iterator_t<R>>;
   >       […]
   >       // 25.7.6.2 [range.ref.view], all view
   >       namespace views { inline constexpr unspecified all = unspecified; }
   >         inline constexpr unspecified all = unspecified;

   >         template<viewable_range R>
   >           using all_tview = decltype(views::all(declval<R>()));
   >       }
   >       […]
   >     }

2. Globally replace all occurrences of `all_view` with `views::all_t`. There are 36 occurrences in addition to the definition in the `<ranges>` synopsis that was changed above.

### 3336(i). How does `std::vformat` handle exception thrown by formatters?

Section: 28.5.5 [format.functions] Status: Resolved Submitter: Tam S. B. Opened: 2019-11-11 Last modified: 2020-09-06

Priority: 2

View all other issues in [format.functions].

View all issues with Resolved status.

Discussion:

The specification for `std::vformat` in 28.5.5 [format.functions]/7 says

> Throws: `format_error` if `fmt` is not a format string.

It seems unclear whether `vformat` throws when an exception is thrown by the `formatter`, e.g. when the format string is valid, but the corresponding argument cannot be formatted with the given format string.

For example, the `"c"` format specifier is specified to throw `format_error` if the corresponding argument is not in the range of representable values for `charT` (Table 60 [tab:format.type.int]). It seems unclear whether `vformat` propagates this exception.

It also appears unclear whether `vformat` may throw other types of exception (e.g. `bad_alloc`) when the formatter or the constructor of `std::string` result throws.

[2019-11-20 Issue Prioritization]

Priority to 2 after reflector discussion.

[2019-11-20; Victor Zverovich provides initial wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4835.

> > [Drafting Note: LWG 3340(i) has considerable wording overlap with this issue. If LWG 3336(i) is applied at the same meeting or later than LWG 3340(i), please refer to Option A in LWG 3340(i) as a guideline how to apply the merge.]

> 1. Before 28.5.5 [format.functions] insert a new sub-clause as indicated:

>    > 20.20.? Error reporting [format.err.report]

>    > -?- Formatting functions throw exceptions to report formatting and other errors. They throw `format_error` if an argument `fmt` is passed that is not a format string and propagate exceptions thrown by `formatter` specializations and iterator operations. Failure to allocate storage is reported by throwing an exception as described in 16.4.6.14 [res.on.exception.handling].

> 2. Modify 28.5.5 [format.functions] as indicated:

>    >     string vformat(string_view fmt, format_args args);
>    >     wstring vformat(wstring_view fmt, wformat_args args);
>    >     string vformat(const locale& loc, string_view fmt, format_args args);
>    >     wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);

>    > > -6- […]

>    > > -7- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

>    > […]

>    >     template<class Out>
>    >       Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, const locale& loc, string_view fmt,
>    >                      format_args_t<Out, char> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, const locale& loc, wstring_view fmt,
>    >                      format_args_t<Out, wchar_t> args);

>    > > […]

>    > > -15- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

>    > […]

>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           string_view fmt, const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           wstring_view fmt, const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           const locale& loc, string_view fmt,
>    >                                           const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           const locale& loc, wstring_view fmt,
>    >                                           const Args&... args);

>    > > […]

>    > > -21- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

>    > […]

>    >     template<class... Args>
>    >       size_t formatted_size(string_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(wstring_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(const locale& loc, string_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);

>    > > […]

>    > > -25- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

[2020-02-12, Prague; LWG discussion]

Resolved by LWG 3340(i).

Proposed resolution:

Resolved by LWG 3340(i).

### 3338(i). Rename `default_constructible` to `default_initializable`

Section: 18.4.12 [concept.default.init] Status: C++20 Submitter: Casey Carter Opened: 2019-11-18 Last modified: 2021-06-06

Priority: 0

View all other issues in [concept.default.init].

View all issues with C++20 status.

Discussion:

WG21 merged P1754R1 "Rename concepts to `standard_case` for C++20" into the working draft as LWG Motion 11 in 2019 Cologne. That proposal contains editorial instructions to rename what was the `DefaultConstructible` concept:

>     IF LWG3151 ACCEPTED:
>       default_initializable
>     ELSE
>       default_constructible

Notably LWG 3151(i) "`ConvertibleTo` rejects conversions from array and function types" is not the intended issue number, LWG 3149(i) "`DefaultConstructible` should require default initialization" is. It was made clear during discussion in LEWG that 3149 would change the concept to require default-initialization to be valid rather than value-initialization which the `is_default_constructible` trait requires. LEWG agreed that it would be confusing to have a trait and concept with very similar names yet slightly different meanings, and approved P1754R1's proposed renaming.

LWG 3149(i) was moved to "Ready" but not approved by WG21 until Belfast — after the application of P1754R1 to the working draft — so this renaming has not happened, but the rationale remains valid.

[2019-11-30 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4835.

1. Change the stable name "[concept.defaultconstructible]" to "[concept.default.init]" and retitle "Concept `default_constructible`" to "Concept `default_initializable`". Replace all references to the name `default_constructible` with `default_initializable` (There are 20 occurrences).

### 3340(i). Formatting functions should throw on argument/format string mismatch in §[format.functions]

Section: 28.5.5 [format.functions] Status: C++20 Submitter: Great Britain Opened: 2019-11-17 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [format.functions].

View all issues with C++20 status.

Discussion:

Addresses GB 229

Formatting functions don't allow throwing on incorrect arguments. `std::format` is only allowed to throw if `fmt` is not a format string, but the intention is it also throws for errors during formatting, e.g. there are fewer arguments than required by the format string.

Proposed change:

Allow exceptions even when the format string is valid. Possibly state the Effects: more precisely.

Victor Zverovich:

LEWG approved resolution of this NB comment as an LWG issue. Previous resolution [SUPERSEDED]:

> This wording is relative to N4835.

> > [Drafting Note: Depending on whether LWG 3336(i)'s wording has been accepted when this issue's wording has been accepted, two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

> Option A (LWG 3336(i) has been accepted)

> 1. Change 28.5.2.1 [format.string.general] as follows:

>    > -1- A format string for arguments `args` is a (possibly empty) sequence of replacement fields, escape sequences, and characters other than `{` and `}`. […]

>    > -2- The arg-id field specifies the index of the argument in `args` whose value is to be formatted and inserted into the output instead of the replacement field. If there is no argument with the index arg-id in `args`, the string is not a format string. The optional format-specifier field explicitly specifies a format for the replacement value.

>    > […]

>    > -5- The format-spec field contains format specifications that define how the value should be presented. Each type can define its own interpretation of the format-spec field. If format-spec doesn't conform to the format specifications for the argument in `args` referred to by arg-id, the string is not a format string. […]

> 2. Before 28.5.5 [format.functions] insert a new sub-clause as indicated:

>    > 20.20.? Error reporting [format.err.report]

>    > -?- Formatting functions throw exceptions to report formatting and other errors. They throw `format_error` if an argument `fmt` is passed that is not a format string for arguments `args` and propagate exceptions thrown by `formatter` specializations and iterator operations. Failure to allocate storage is reported by throwing an exception as described in 16.4.6.14 [res.on.exception.handling].

> 3. Modify 28.5.5 [format.functions] as indicated:

>    >     string vformat(string_view fmt, format_args args);
>    >     wstring vformat(wstring_view fmt, wformat_args args);
>    >     string vformat(const locale& loc, string_view fmt, format_args args);
>    >     wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);

>    > > -6- […]

>    > > -7- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

>    > […]

>    >     template<class Out>
>    >       Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, const locale& loc, string_view fmt,
>    >                      format_args_t<Out, char> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, const locale& loc, wstring_view fmt,
>    >                      format_args_t<Out, wchar_t> args);

>    > > […]

>    > > -15- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

>    > […]

>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           string_view fmt, const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           wstring_view fmt, const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           const locale& loc, string_view fmt,
>    >                                           const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           const locale& loc, wstring_view fmt,
>    >                                           const Args&... args);

>    > > […]

>    > > -21- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

>    > […]

>    >     template<class... Args>
>    >       size_t formatted_size(string_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(wstring_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(const locale& loc, string_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);

>    > > […]

>    > > -25- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

> Option B (LWG 3336(i) has not been accepted)

> 1. Change 28.5.2.1 [format.string.general] as follows:

>    > -1- A format string for arguments `args` is a (possibly empty) sequence of replacement fields, escape sequences, and characters other than `{` and `}`. […]

>    > -2- The arg-id field specifies the index of the argument in `args` whose value is to be formatted and inserted into the output instead of the replacement field. If there is no argument with the index arg-id in `args`, the string is not a format string. The optional format-specifier field explicitly specifies a format for the replacement value.

>    > […]

>    > -5- The format-spec field contains format specifications that define how the value should be presented. Each type can define its own interpretation of the format-spec field. If format-spec doesn't conform to the format specifications for the argument in `args` referred to by arg-id, the string is not a format string. […]

> 2. Modify 28.5.5 [format.functions] as indicated:

>    >     string vformat(string_view fmt, format_args args);
>    >     wstring vformat(wstring_view fmt, wformat_args args);
>    >     string vformat(const locale& loc, string_view fmt, format_args args);
>    >     wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);

>    > > -6- […]

>    > > -7- Throws: `format_error` if `fmt` is not a format string for `args`.

>    > […]

>    >     template<class Out>
>    >       Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, const locale& loc, string_view fmt,
>    >                      format_args_t<Out, char> args);
>    >     template<class Out>
>    >       Out vformat_to(Out out, const locale& loc, wstring_view fmt,
>    >                      format_args_t<Out, wchar_t> args);

>    > > […]

>    > > -15- Throws: `format_error` if `fmt` is not a format string for `args`.

>    > […]

>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           string_view fmt, const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           wstring_view fmt, const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           const locale& loc, string_view fmt,
>    >                                           const Args&... args);
>    >     template<class Out, class... Args>
>    >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
>    >                                           const locale& loc, wstring_view fmt,
>    >                                           const Args&... args);

>    > > […]

>    > > -21- Throws: `format_error` if `fmt` is not a format string for `args`.

>    > […]

>    >     template<class... Args>
>    >       size_t formatted_size(string_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(wstring_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(const locale& loc, string_view fmt, const Args&... args);
>    >     template<class... Args>
>    >       size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);

>    > > […]

>    > > -25- Throws: `format_error` if `fmt` is not a format string for `args`.

[2020-02-12, Prague; LWG discussion]

Option A is the only one we look at to resolve LWG 3336(i) as well. During the discussions some wording refinements have been suggested that are integrated below.

Proposed resolution:

This wording is relative to N4849.

1. Change 28.5.2.1 [format.string.general] as follows:

   > -1- A format string for arguments `args` is a (possibly empty) sequence of replacement fields, escape sequences, and characters other than `{` and `}`. […]

   > -2- The arg-id field specifies the index of the argument in `args` whose value is to be formatted and inserted into the output instead of the replacement field. If there is no argument with the index arg-id in `args`, the string is not a format string for `args`. The optional format-specifier field explicitly specifies a format for the replacement value.

   > […]

   > -5- The format-spec field contains format specifications that define how the value should be presented. Each type can define its own interpretation of the format-spec field. If format-spec does not conform to the format specifications for the argument type referred to by arg-id, the string is not a format string for `args`. […]

2. Before 28.5.5 [format.functions] insert a new sub-clause as indicated:

   > 20.20.? Error reporting [format.err.report]

   > -?- Formatting functions throw `format_error` if an argument `fmt` is passed that is not a format string for `args`. They propagate exceptions thrown by operations of `formatter` specializations and iterators. Failure to allocate storage is reported by throwing an exception as described in 16.4.6.14 [res.on.exception.handling].

3. Modify 28.5.5 [format.functions] as indicated:

   >     string vformat(string_view fmt, format_args args);
   >     wstring vformat(wstring_view fmt, wformat_args args);
   >     string vformat(const locale& loc, string_view fmt, format_args args);
   >     wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);

   > > -6- […]

   > > -7- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

   > […]

   >     template<class Out>
   >       Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args);
   >     template<class Out>
   >       Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args);
   >     template<class Out>
   >       Out vformat_to(Out out, const locale& loc, string_view fmt,
   >                      format_args_t<Out, char> args);
   >     template<class Out>
   >       Out vformat_to(Out out, const locale& loc, wstring_view fmt,
   >                      format_args_t<Out, wchar_t> args);

   > > […]

   > > -15- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

   > […]

   >     template<class Out, class... Args>
   >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
   >                                           string_view fmt, const Args&... args);
   >     template<class Out, class... Args>
   >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
   >                                           wstring_view fmt, const Args&... args);
   >     template<class Out, class... Args>
   >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
   >                                           const locale& loc, string_view fmt,
   >                                           const Args&... args);
   >     template<class Out, class... Args>
   >       format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
   >                                           const locale& loc, wstring_view fmt,
   >                                           const Args&... args);

   > > […]

   > > -21- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

   > […]

   >     template<class... Args>
   >       size_t formatted_size(string_view fmt, const Args&... args);
   >     template<class... Args>
   >       size_t formatted_size(wstring_view fmt, const Args&... args);
   >     template<class... Args>
   >       size_t formatted_size(const locale& loc, string_view fmt, const Args&... args);
   >     template<class... Args>
   >       size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);

   > > […]

   > > -25- Throws: `format_error` if `fmt` is not a format stringAs specified in 28.5.3 [format.err.report].

### 3343(i). Ordering of calls to `unlock()` and `notify_all()` in Effects element of `notify_all_at_thread_exit()` should be reversed

Section: 32.7.3 [thread.condition.nonmember] Status: C++26 Submitter: Lewis Baker Opened: 2019-11-21 Last modified: 2026-06-09

Priority: 3

View all issues with C++26 status.

Discussion:

32.7.3 [thread.condition.nonmember] p2 states:

> Effects: Transfers ownership of the lock associated with `lk` into internal storage and schedules `cond` to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification shall be as if:

> >     lk.unlock();
> >     cond.notify_all();

One common use-cases for the `notify_all_at_thread_exit()` is in conjunction with `thread::detach()` to allow detached threads to signal when they complete and to allow another thread to wait for them to complete using the `condition_variable/mutex` pair.

However, the current wording for `notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk)` makes it impossible to know when it is safe to destroy the `condition_variable` in the presence of spurious wake-ups and detached threads.

For example: Consider the following code-snippet:

>     #include <condition_variable>
>     #include <mutex>
>     #include <thread>

>     int main() {
>       std::condition_variable cv;
>       std::mutex mut;
>       bool complete = false;

>       std::thread{[&] {
>         // do work here

>         // Signal thread completion
>         std::unique_lock lk{mut};
>         complete = true;
>         std::notify_all_at_thread_exit(cv, std::move(lk));
>       }}.detach();

>       // Wait until thread completes
>       std::unique_lock lk{mut};
>       cv.wait(lk, [&] { return complete; });

>       // condition_variable destroyed on scope exit
>       return 0;
>     }

This seems to an intended usage of `thread::detach()` and `std::notify_all_at_thread_exit()` and yet this code contains a race involving the call to `cv.notify_all()` on the created thread, and the destructor of the `condition_variable`.

To highlight the issue, consider the following case:

Let `T0` be the thread that executes `main()` and `T1` be the thread created by the `std::thread` construction.

> `T0`: creates thread `T1`
> `T0`: context-switched out by OS
> `T1`: starts running

> `T1`: acquires mutex lock
> `T1`: sets `complete = true`

> `T1`: calls `notify_all_at_thread_exit()`
> `T1`: returns from thread-main function and runs all thread-local destructors
> `T1`: calls `lk.unlock()`
> `T1`: context-switched out by OS
> `T0`: resumes execution
> `T0`: acquires mutex lock
> `T0`: calls `cv.wait()` which returns immediately as `complete` is `true`

> `T0`: returns from `main()`, destroying `condition_variable`
> `T1`: resumes execution

> `T1`: calls `cv.notify_all()` on a dangling `cv` reference (undefined behaviour)

Other sequencings are possible involving spurious wake-ups of the `cv.wait()` call.

A proof-of-concept showing this issue can be found here.

The current wording requires releasing the mutex lock before calling `cv.notify_all()`. In the presence of spurious wake-ups of a `condition_variable::wait()`, there is no way to know whether or not a detached thread that called `std::notify_all_at_thread_exit()` has finished calling `cv.notify_all()`. This means there is no portable way to know when it will be safe for the waiting thread to destroy that `condition_variable`.

However, if we were to reverse the order of the calls to `lk.unlock()` and `cond.notify_all()` then the thread waiting for the detached thread to exit would not be able to observe the completion of the thread (in the above case, this would be observing the assignment of `true` to the `complete` variable) until the mutex lock was released by that thread and subsequently acquired by the waiting thread which would only happen after the completion of the call to `cv.notify_all()`. This would allow the above code example to eliminate the race between a subsequent destruction of the condition-variable and the call to `cv.notify_all()`.

[2019-12-08 Issue Prioritization]

Priority to 3 after reflector discussion.

[2019-12-15; Daniel synchronizes wording with N4842]

[2020-02, Prague]

Response from SG1: "We discussed it in Prague. We agree it’s an error and SG1 agreed with the PR."

Previous resolution [SUPERSEDED]

This wording is relative to N4842.

1. Change 32.7.3 [thread.condition.nonmember] as indicated:

   >     void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);

   > > […]

   > > -2- Effects: Transfers ownership of the lock associated with `lk` into internal storage and schedules `cond` to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification is equivalent to:

   > > >     lk.unlock();
   > > >     cond.notify_all();
   > > >     lk.unlock();

[2023-06-13, Varna; Tim provides improved wording]

Addressed mailing list comments. Ask SG1 to check.

[Kona 2025-11-05; SG1 unanimously approved the proposed resolution.]

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Change 32.7.3 [thread.condition.nonmember] as indicated:

   >     void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);

   > > […]

   > > -2- Effects: Transfers ownership of the lock associated with `lk` into internal storage and schedules `cond` to be notified when the current thread exits,. This notification is sequenced after all objects of thread storage duration associated with the current thread have been destroyed. This notification and is equivalent to:

   > > >     lk.unlock();
   > > >     cond.notify_all();
   > > >     lk.unlock();

### 3345(i). Incorrect usages of "models" versus "satisfies"

Section: 18.4.9 [concept.swappable], 24.3.3.2 [iterator.cust.swap], 24.4.4.2 [range.iter.op.advance], 24.4.4.3 [range.iter.op.distance], 24.5.1.2 [reverse.iterator], 24.5.4.2 [move.iterator], 24.5.4.7 [move.iter.nav], 24.5.5.2 [common.iter.types], 24.5.5.5 [common.iter.nav], 25.3 [range.access], 25.6.4.3 [range.iota.iterator], 25.7 [range.adaptors], 26 [algorithms] Status: Resolved Submitter: Daniel Krügler Opened: 2019-11-23 Last modified: 2020-05-01

Priority: 2

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Discussion:

The current working draft uses at several places within normative wording the term "models" instead of "satisfies" even though it is clear from the context that these are conditions to be tested by the implementation. Since "models" includes both syntactic requirements as well as semantic requirements, such wording would require (at least in general) heroic efforts for an implementation. Just to name a few examples for such misusage:

- The specification of the customization objects in 18.4.9 [concept.swappable], 24.3.3.2 [iterator.cust.swap], 25.3 [range.access]

- The algorithmic decision logic for the effects of the functions specified in 24.4.4.2 [range.iter.op.advance], 24.4.4.3 [range.iter.op.distance]

The correct way to fix these presumably unintended extra requirements is to use the term "satisfies" at the places where it is clear that an implementation has to test them.

Note: There exists similar misusage in regard to wording for types that "meet Cpp17XX requirements, but those are not meant to be covered as part of this issue. This additional wording problem should be handled by a separate issue.

[2019-12-08 Issue Prioritization]

Priority to 2 after reflector discussion.

[2019-12-15; Daniel synchronizes wording with N4842]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4842.

> > [Drafting note: The proposed wording does intentionally not touch the definition of `enable_view`, whose definition is radically changed by LWG 3326(i) in a manner that does no longer need similar adjustments.]

> 1. Modify 18.4.9 [concept.swappable] as indicated:

>    > -2- The name `ranges::swap` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::swap(E1, E2)` for some subexpressions `E1` and `E2` is expression-equivalent to an expression `S` determined as follows:

>    > 1. (2.1) — […]

>    > 2. (2.2) — […]

>    > 3. (2.3) — Otherwise, if `E1` and `E2` are lvalues of the same type `T` that modelssatisfies `move_constructible<T>` and `assignable_from<T&, T>`, `S` is an expression that exchanges the denoted values. `S` is a constant expression if […]

>    > 4. (2.4) — […]

> 2. Modify 24.3.3.2 [iterator.cust.swap] as indicated:

>    > -4- The expression `ranges::iter_swap(E1, E2)` for some subexpressions `E1` and `E2` is expression-equivalent to:

>    > 1. (4.1) — […]

>    > 2. (4.2) — Otherwise, if the types of `E1` and `E2` each modelsatisfy `indirectly_readable`, and if the reference types of `E1` and `E2` modelsatisfy `swappable_with` (18.4.9 [concept.swappable]), then `ranges::swap(*E1, *E2)`.

>    > 3. (4.3) — Otherwise, if the types `T1` and `T2` of `E1` and `E2` modelsatisfy `indirectly_movable_storable<T1, T2>` and `indirectly_movable_storable<T2, T1>`, then `(void)(*E1 = iter-exchange-move(E2, E1))`, except that `E1` is evaluated only once.

>    > 4. (4.4) — […]

> 3. Modify 24.4.4 [range.iter.ops] as indicated:

>    > -1- The library includes the function templates `ranges::advance`, `ranges::distance`, `ranges::next`, and `ranges::prev` to manipulate iterators. These operations adapt to the set of operators provided by each iterator category to provide the most efficient implementation possible for a concrete iterator type. [Example: `ranges::advance` uses the `+` operator to move a `random_access_iterator` forward `n` steps in constant time. For an iterator type that does not modelsatisfy `random_access_iterator`, `ranges::advance` instead performs `n` individual increments with the `++` operator. — end example]

> 4. Modify 24.4.4.2 [range.iter.op.advance] as indicated:

>    >     template<input_or_output_iterator I>
>    >       constexpr void ranges::advance(I& i, iter_difference_t<I> n);

>    > > -1- Preconditions: […]

>    > > -2- Effects:

>    > > 1. (2.1) — If `I` modelssatisfies `random_access_iterator`, equivalent to `i += n`.

>    > > 2. (2.2) — […]

>    > > 3. (2.3) — […]

>    >     template<input_or_output_iterator I, sentinel_for<I> S>
>    >       constexpr void ranges::advance(I& i, S bound);

>    > > -3- Preconditions: […]

>    > > -4- Effects:

>    > > 1. (4.1) — If `I` and `S` modelsatisfy `assignable_from<I&, S>`, equivalent to `i = std::move(bound)`.

>    > > 2. (4.2) — Otherwise, if `S` and `I` modelsatisfy `sized_sentinel_for<S, I>`, equivalent to `ranges::advance(i, bound - i)`.

>    > > 3. (4.3) — […]

>    >     template<input_or_output_iterator I, sentinel_for<I> S>
>    >       constexpr iter_difference_t<I> ranges::advance(I& i, iter_difference_t<I> n, S bound);

>    > > -5- Preconditions: […]

>    > > -6- Effects:

>    > > 1. (6.1) — If `S` and `I` modelsatisfy `sized_sentinel_for<S, I>`: […]

>    > > 2. (6.2) — […]

> 5. Modify 24.4.4.3 [range.iter.op.distance] as indicated:

>    >     template<input_or_output_iterator I, sentinel_for<I> S>
>    >       constexpr iter_difference_t<I> ranges::distance(I first, S last);

>    > > -1- Preconditions: […]

>    > > -2- Effects: If `S` and `I` modelsatisfy `sized_sentinel_for<S, I>`, returns `(last - first)`; otherwise, returns the number of increments needed to get from `first` to `last`.

>    >     template<range R>
>    >       range_difference_t<R> ranges::distance(R&& r);

>    > > -3- Effects: If `R` modelssatisfies `sized_range`, equivalent to:

>    > > >     return static_cast<range_difference_t<R>>(ranges::size(r)); // 25.3.10 [range.prim.size]

>    > > Otherwise, equivalent to:

>    > > >     return ranges::distance(ranges::begin(r), ranges::end(r)); // 25.3 [range.access]

> 6. Modify 24.5.1.2 [reverse.iterator] as indicated:

>    > -1- The member typedef-name `iterator_concept` denotes

>    > 1. (1.1) — `random_access_iterator_tag` if `Iterator` modelssatisfies `random_access_iterator`, and

>    > 2. (1.2) — `bidirectional_iterator_tag` otherwise.

>    > -2- The member typedef-name `iterator_category` denotes

>    > 1. (2.1) — `random_access_iterator_tag` if the type `iterator_traits<Iterator>::iterator_category` modelssatisfies `derived_from<random_access_iterator_tag>`, and

>    > 2. (2.2) — `iterator_traits<Iterator>::iterator_category` otherwise.

> 7. Modify 24.5.4.2 [move.iterator] as indicated:

>    > -1- The member typedef-name `iterator_category` denotes

>    > 1. (1.1) — `random_access_iterator_tag` if the type `iterator_traits<Iterator>::iterator_category` modelssatisfies `derived_from<random_access_iterator_tag>`, and

>    > 2. (1.2) — `iterator_traits<Iterator>::iterator_category` otherwise.

> 8. Modify 24.5.4.7 [move.iter.nav] as indicated:

>    >     constexpr auto operator++(int);

>    > > -3- Effects: `Iterator` modelssatisfies `forward_iterator`, equivalent to:

>    > > >     move_iterator tmp = *this;
>    > > >     ++current;
>    > > >     return tmp;

>    > > Otherwise, equivalent to `++current`.

> 9. Modify 24.5.5.2 [common.iter.types] as indicated:

>    > -1- The nested typedef-names of the specialization of `iterator_traits` for `common_iterator<I, S>` are defined as follows.

>    > 1. (1.1) — `iterator_concept` denotes `forward_iterator_tag` if `I` modelssatisfies `forward_iterator`; otherwise it denotes `input_iterator_tag`.

>    > 2. (1.2) — `iterator_category` denotes `forward_iterator_tag` if `iterator_traits<I>::iterator_category` modelssatisfies `derived_from<forward_iterator_tag>`; otherwise it denotes `input_iterator_tag`.

>    > 3. (1.3) — […]

> 10. Modify 24.5.5.5 [common.iter.nav] as indicated:

>     >     decltype(auto) operator++(int);

>     > > -4- Preconditions: […]

>     > > -5- Effects: If `I` modelssatisfies `forward_iterator`, equivalent to:

>     > > >     common_iterator tmp = *this;
>     > > >     ++*this;
>     > > >     return tmp;

>     > > Otherwise, equivalent to `return get<I>(v_)++;`

> 11. Modify 25.3.2 [range.access.begin] as indicated:

>     > -1- The name `ranges::begin` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a subexpression `E` and an lvalue `t` that denotes the same object as `E`, if `E` is an rvalue and `enable_safe_range<remove_cvref_t<decltype((E))>>` is `false`, `ranges::begin(E)` is ill-formed. Otherwise, `ranges::begin(E)` is expression-equivalent to:

>     > 1. (1.1) — […]

>     > 2. (1.2) — Otherwise, `decay-copy(t.begin())` if it is a valid expression and its type `I` modelssatisfies `input_or_output_iterator`.

>     > 3. (1.3) — Otherwise, `decay-copy(begin(t))` if it is a valid expression and its type `I` modelssatisfies `input_or_output_iterator` with overload resolution performed in a context that includes the declarations:

>     >    >     template<class T> void begin(T&&) = delete;
>     >    >     template<class T> void begin(initializer_list<T>&&) = delete;

>     >    and does not include a declaration of `ranges::begin`.

>     > 4. (1.4) — […]

> 12. Modify 25.3.3 [range.access.end] as indicated:

>     > -1- The name `ranges::end` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a subexpression `E` and an lvalue `t` that denotes the same object as `E`, if `E` is an rvalue and `enable_safe_range<remove_cvref_t<decltype((E))>>` is `false`, `ranges::end(E)` is ill-formed. Otherwise, `ranges::end(E)` is expression-equivalent to:

>     > 1. (1.1) — […]

>     > 2. (1.2) — Otherwise, `decay-copy(t.end())` if it is a valid expression and its type `S` modelssatisfies `sentinel_for<decltype(ranges::begin(E))>`.

>     > 3. (1.3) — Otherwise, `decay-copy(end(t))` if it is a valid expression and its type `S` modelssatisfies `sentinel_for<decltype(ranges::begin(E))>` with overload resolution performed in a context that includes the declarations:

>     >    >     template<class T> void end(T&&) = delete;
>     >    >     template<class T> void end(initializer_list<T>&&) = delete;

>     >    and does not include a declaration of `ranges::end`.

>     > 4. (1.4) — […]

> 13. Modify 25.3.6 [range.access.rbegin] as indicated:

>     > -1- The name `ranges::rbegin` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a subexpression `E` and an lvalue `t` that denotes the same object as `E`, if `E` is an rvalue and `enable_safe_range<remove_cvref_t<decltype((E))>>` is `false`, `ranges::rbegin(E)` is ill-formed. Otherwise, `ranges::rbegin(E)` is expression-equivalent to:

>     > 1. (1.1) — `decay-copy(t.rbegin())` if it is a valid expression and its type `I` modelssatisfies `input_or_output_iterator`.

>     > 2. (1.2) — Otherwise, `decay-copy(rbegin(t))` if it is a valid expression and its type `I` modelssatisfies `input_or_output_iterator` with overload resolution performed in a context that includes the declaration:

>     >    >     template<class T> void rbegin(T&&) = delete;

>     >    and does not include a declaration of `ranges::rbegin`.

>     > 3. (1.3) — Otherwise, `make_reverse_iterator(ranges::end(t))` if both `ranges::begin(t)` and `ranges::end(t)` are valid expressions of the same type `I` which modelssatisfies `bidirectional_iterator` (24.3.4.12 [iterator.concept.bidir]).

>     > 4. (1.4) — […]

> 14. Modify 25.3.7 [range.access.rend] as indicated:

>     > -1- The name `ranges::rend` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a subexpression `E` and an lvalue `t` that denotes the same object as `E`, if `E` is an rvalue and `enable_safe_range<remove_cvref_t<decltype((E))>>` is `false`, `ranges::rend(E)` is ill-formed. Otherwise, `ranges::rend(E)` is expression-equivalent to:

>     > 1. (1.1) — `decay-copy(t.rend())` if it is a valid expression and its type `S` modelssatisfies `sentinel_for<decltype(ranges::rbegin(E))>`.

>     > 2. (1.2) — Otherwise, `decay-copy(rend(t))` if it is a valid expression and its type `S` modelssatisfies `sentinel_for<decltype(ranges::rbegin(E))>` with overload resolution performed in a context that includes the declaration:

>     >    >     template<class T> void rend(T&&) = delete;

>     >    and does not include a declaration of `ranges::rend`.

>     > 3. (1.3) — Otherwise, `make_reverse_iterator(ranges::begin(t))` if both `ranges::begin(t)` and `ranges::end(t)` are valid expressions of the same type `I` which modelssatisfies `bidirectional_iterator` (24.3.4.12 [iterator.concept.bidir]).

>     > 4. (1.4) — […]

> 15. Modify 25.3.10 [range.prim.size] as indicated:

>     > [Drafting note: The term "is integer-like" is very specifically defined to be related to semantic requirements as well, and the term "satisfy integer-like" seems to be undefined. Fortunately, 24.3.4.4 [iterator.concept.winc] also introduces the exposition-only variable template `is-integer-like` which we use as predicate below to describe the syntactic constraints of "integer-like" alone. This wording change regarding "is integer-like" is strictly speaking not required because of the "if and only if" definition provided in 24.3.4.4 [iterator.concept.winc] p12, but it has been made nonetheless to improve the consistency between discriminating compile-time tests from potentially semantic requirements]

>     > -1- The name `size` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::size(E)` for some subexpression `E` with type `T` is expression-equivalent to:

>     > 1. (1.1) — […]

>     > 2. (1.2) — Otherwise, if `disable_sized_range<remove_cv_t<T>>` (25.4.4 [range.sized]) is `false`:

>     >    1. (1.2.1) — `decay-copy(E.size())` if it is a valid expression and its type `I` is integer-like `is-integer-like<I>` is `true` (24.3.4.4 [iterator.concept.winc]).

>     >    2. (1.2.2) — Otherwise, `decay-copy(size(E))` if it is a valid expression and its type `I` is integer-like `is-integer-like<I>` is `true` with overload resolution performed in a context that includes the declaration:

>     >       >     template<class T> void size(T&&) = delete;

>     >       and does not include a declaration of `ranges::size`.

>     > 3. (1.3) — Otherwise, `make-unsigned-like(ranges::end(E) - ranges::begin(E))` (25.5.4 [range.subrange]) if it is a valid expression and the types `I` and `S` of `ranges::begin(E)` and `ranges::end(E)` (respectively) modelsatisfy both `sized_sentinel_for<S, I>` (24.3.4.8 [iterator.concept.sizedsentinel]) and `forward_iterator<I>`. However, `E` is evaluated only once.

>     > 4. (1.4) — […]

> 16. Modify 25.3.13 [range.prim.empty] as indicated:

>     > -1- The name `empty` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::empty(E)` for some subexpression `E` is expression-equivalent to:

>     > 1. (1.1) — […]

>     > 2. (1.2) — […]

>     > 3. (1.3) — Otherwise, `EQ`, where `EQ` is `bool(ranges::begin(E) == ranges::end(E))` except that `E` is only evaluated once, if `EQ` is a valid expression and the type of `ranges::begin(E)` modelssatisfies `forward_iterator`.

>     > 4. (1.4) — […]

> 17. Modify 25.3.14 [range.prim.data] as indicated:

>     > -1- The name `data` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::data(E)` for some subexpression `E` is expression-equivalent to:

>     > 1. (1.1) — […]

>     > 2. (1.2) — Otherwise, if `ranges::begin(E)` is a valid expression whose type modelssatisfies `contiguous_iterator`, `to_address(ranges::begin(E))`.

>     > 3. (1.3) — […]

> 18. Modify 25.5.5 [range.dangling] as indicated:

>     > -1- The tag type `dangling` is used together with the template aliases `safe_iterator_t` and `safe_subrange_t` to indicate that an algorithm that typically returns an iterator into or subrange of a range argument does not return an iterator or subrange which could potentially reference a range whose lifetime has ended for a particular rvalue range argument which does not modelsatisfy `forwarding-range` (25.4.2 [range.range]).

>     > […]

>     > -2- [Example:

>     > >     vector<int> f();
>     > >     auto result1 = ranges::find(f(), 42); // #1
>     > >     static_assert(same_as<decltype(result1), ranges::dangling>);
>     > >     auto vec = f();
>     > >     auto result2 = ranges::find(vec, 42); // #2
>     > >     static_assert(same_as<decltype(result2), vector<int>::iterator>);
>     > >     auto result3 = ranges::find(subrange{vec}, 42); // #3
>     > >     static_assert(same_as<decltype(result3), vector<int>::iterator>);

>     > The call to `ranges::find` at #1 returns `ranges::dangling` since `f()` is an rvalue `vector`; the `vector` could potentially be destroyed before a returned iterator is dereferenced. However, the calls at #2 and #3 both return iterators since the lvalue `vec` and specializations of subrange modelsatisfy `forwarding-range`. — end example]

> 19. Modify 25.6.4.3 [range.iota.iterator] as indicated:

>     > -1- `iterator::iterator_category` is defined as follows:

>     > 1. (1.1) — If `W` modelssatisfies `advanceable`, then `iterator_category` is `random_access_iterator_tag`.

>     > 2. (1.2) — Otherwise, if `W` modelssatisfies `decrementable`, then `iterator_category` is `bidirectional_iterator_tag`.

>     > 3. (1.3) — Otherwise, if `W` modelssatisfies `incrementable`, then `iterator_category` is `forward_iterator_tag`.

>     > 4. (1.4) — Otherwise, `iterator_category` is `input_iterator_tag`.

> 20. Modify [range.semi.wrap] as indicated:

>     > -1- Many types in this subclause are specified in terms of an exposition-only class template `semiregular-box`. `semiregular-box<T>` behaves exactly like `optional<T>` with the following differences:

>     > 1. (1.1) — […]

>     > 2. (1.2) — If `T` modelssatisfies `default_constructible`, the default constructor of `semiregular-box<T>` is equivalent to: […]

>     > 3. (1.3) — If `assignable_from<T&, const T&>` is not modeledsatisfied, the copy assignment operator is equivalent to: […]

>     > 4. (1.4) — If `assignable_from<T&, T>` is not modeledsatisfied, the move assignment operator is equivalent to: […]

> 21. Modify 25.7.6 [range.all] as indicated:

>     > -2- The name `views::all` denotes a range adaptor object (25.7.2 [range.adaptor.object]). For some subexpression `E`, the expression `views::all(E)` is expression-equivalent to:

>     > 1. (2.1) — `decay-copy(E)` if the decayed type of `E` modelssatisfies `view`.

>     > 2. (2.2) — […]

>     > 3. (2.3) — […]

> 22. Modify 25.7.8.3 [range.filter.iterator] as indicated:

>     > -2- `iterator::iterator_concept` is defined as follows:

>     > 1. (2.1) — If `V` modelssatisfies `bidirectional_range`, then `iterator_concept` denotes `bidirectional_iterator_tag`.

>     > 2. (2.2) — Otherwise, if `V` modelssatisfies `forward_range`, then `iterator_concept` denotes `forward_iterator_tag`.

>     > 3. (2.3) — […]

>     > -3- `iterator::iterator_category` is defined as follows:

>     > 1. (3.1) — Let `C` denote the type `iterator_traits<iterator_t<V>>::iterator_category`.

>     > 2. (3.2) — If `C` modelssatisfies `derived_from<bidirectional_iterator_tag>`, then `iterator_category` denotes `bidirectional_iterator_tag`.

>     > 3. (3.3) — Otherwise, if `C` modelssatisfies `derived_from<forward_iterator_tag>`, then `iterator_category` denotes `forward_iterator_tag`.

>     > 4. (3.4) — […]

> 23. Modify 25.7.9.3 [range.transform.iterator] as indicated:

>     > -1- `iterator::iterator_concept` is defined as follows:

>     > 1. (1.1) — If `V` modelssatisfies `random_access_range`, then `iterator_concept` denotes `random_access_iterator_tag`.

>     > 2. (1.2) — Otherwise, if `V` modelssatisfies `bidirectional_range`, then `iterator_concept` denotes `bidirectional_iterator_tag`.

>     > 3. (1.3) — Otherwise, if `V` modelssatisfies `forward_range`, then `iterator_concept` denotes `forward_iterator_tag`.

>     > 4. (1.4) — […]

>     > -2- Let `C` denote the type `iterator_traits<iterator_t<Base>>::iterator_category`. If `C` modelssatisfies `derived_from<contiguous_iterator_tag>`, then `iterator_category` denotes `random_access_iterator_tag`; otherwise, `iterator_category` denotes `C`.

> 24. Modify 25.7.14.3 [range.join.iterator] as indicated:

>     > -1- `iterator::iterator_concept` is defined as follows:

>     > 1. (1.1) — If `ref_is_glvalue` is `true` and `Base` and `range_reference_t<Base>` each modelsatisfy `bidirectional_range`, then `iterator_concept` denotes `bidirectional_iterator_tag`.

>     > 2. (1.2) — Otherwise, if `ref_is_glvalue` is `true` and `Base` and `range_reference_t<Base>` each modelsatisfy `forward_range`, then `iterator_concept` denotes `forward_iterator_tag`.

>     > 3. (1.3) — […]

>     > -2- `iterator::iterator_category` is defined as follows:

>     > 1. (2.1) — Let […]

>     > 2. (2.2) — If `ref_is_glvalue` is `true` and `OUTERC` and `INNERC` each modelsatisfy `derived_from<bidirectional_iterator_tag>`, `iterator_category` denotes `bidirectional_iterator_tag`.

>     > 3. (2.3) — Otherwise, if `ref_is_glvalue` is `true` and `OUTERC` and `INNERC` each modelsatisfy `derived_from<forward_iterator_tag>`, `iterator_category` denotes `forward_iterator_tag`.

>     > 4. (2.4) — Otherwise, if `OUTERC` and `INNERC` each modelsatisfy `derived_from<input_iterator_tag>`, `iterator_category` denotes `input_iterator_tag`.

>     > 5. (2.5) — […]

> 25. Modify [range.split.outer] as indicated:

>     >     […]
>     >     Parent* parent_ = nullptr; // exposition only
>     >     iterator_t<Base> current_ = // exposition only, present only if V modelssatisfies forward_range
>     >       iterator_t<Base>();
>     >     […]

>     > -1- Many of the following specifications refer to the notional member `current` of `outer_iterator`. `current` is equivalent to `current_` if `V` modelssatisfies `forward_range`, and `parent_->current_` otherwise.

> 26. Modify [range.split.inner] as indicated:

>     > -1- The typedef-name `iterator_category` denotes

>     > 1. (1.1) — `forward_iterator_tag` if `iterator_traits<iterator_t<Base>>::iterator_category` modelssatisfies `derived_from<forward_iterator_tag>`;

>     > 2. (1.2) — otherwise, `iterator_traits<iterator_t<Base>>::iterator_category`.

> 27. Modify 25.7.19 [range.counted] as indicated:

>     > -2- The name `views::counted` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Let `E` and `F` be expressions, and let `T` be `decay_t<decltype((E))>`. Then the expression `views::counted(E, F)` is expression-equivalent to:

>     > 1. (2.1) — If `T` modelssatisfies `input_or_output_iterator` and `decltype((F))` modelssatisfies `convertible_to<iter_difference_t<T>>`,

>     >    1. (2.1.1) — `subrange{E, E + static_cast<iter_difference_t<T>>(F)}` if `T` modelssatisfies `random_access_iterator`.

>     >    2. (2.1.2) — […]

>     > 2. (2.2) — […]

> 28. Modify [range.common.adaptor] as indicated:

>     > -1- The name `views::common denotes` a range adaptor object (25.7.2 [range.adaptor.object]). For some subexpression `E`, the expression `views::common(E)` is expression-equivalent to:

>     > 1. (1.1) — `views::all(E)`, if `decltype((E))` modelssatisfies `common_range` and `views::all(E)` is a well-formed expression.

>     > 2. (1.2) — […]

> 29. Modify 26.6.13 [alg.equal] as indicated:

>     >     template<class InputIterator1, class InputIterator2>
>     >       constexpr bool equal(InputIterator1 first1, InputIterator1 last1,
>     >                            InputIterator2 first2);
>     >     […]
>     >     template<input_range R1, input_range R2, class Pred = ranges::equal_to,
>     >               class Proj1 = identity, class Proj2 = identity>
>     >       requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
>     >       constexpr bool ranges::equal(R1&& r1, R2&& r2, Pred pred = {},
>     >                                    Proj1 proj1 = {}, Proj2 proj2 = {});

>     > > […]

>     > > -3- Complexity: If the types of `first1`, `last1`, `first2`, and `last2`:

>     > > 1. (3.1) — meet the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]) for the overloads in namespace `std`, or

>     > > 2. (3.2) — pairwise modelsatisfy `sized_sentinel_for` (24.3.4.8 [iterator.concept.sizedsentinel]) for the overloads in namespace `ranges`, and `last1 - first1 != last2 - first2`, then no applications of the corresponding predicate and each projection; otherwise,

>     > > 3. (3.3) — […]

>     > > 4. (3.4) — […]

> 30. Modify 26.6.14 [alg.is.permutation] as indicated:

>     >     template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2,
>     >              sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = identity,
>     >              class Proj2 = identity>
>     >       requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
>     >       constexpr bool ranges::is_permutation(I1 first1, S1 last1, I2 first2, S2 last2,
>     >                                             Pred pred = {},
>     >                                             Proj1 proj1 = {}, Proj2 proj2 = {});
>     >     template<forward_range R1, forward_range R2, class Pred = ranges::equal_to,
>     >              class Proj1 = identity, class Proj2 = identity>
>     >       requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
>     >       constexpr bool ranges::is_permutation(R1&& r1, R2&& r2, Pred pred = {},
>     >                                             Proj1 proj1 = {}, Proj2 proj2 = {});

>     > > […]

>     > > -7- Complexity: No applications of the corresponding predicate and projections if:

>     > > 1. (7.1) — `S1` and `I1` modelsatisfy `sized_sentinel_for<S1, I1>`,

>     > > 2. (7.2) — `S2` and `I2` modelsatisfy `sized_sentinel_for<S2, I2>`, and

>     > > 3. (7.3) — […]

> 31. Modify 26.8.5 [alg.partitions] as indicated:

>     >     template<class ForwardIterator, class Predicate>
>     >       constexpr ForwardIterator
>     >         partition(ForwardIterator first, ForwardIterator last, Predicate pred);
>     >     […]
>     >     template<forward_range R, class Proj = identity,
>     >              indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
>     >       requires permutable<iterator_t<R>>
>     >       constexpr safe_subrange_t<R>
>     >         ranges::partition(R&& r, Pred pred, Proj proj = {});

>     > > […]

>     > > -8- Complexity: Let `N = last - first`:

>     > > 1. (8.1) — For the overload with no `ExecutionPolicy`, exactly `N` applications of the predicate and projection. At most `N/2` swaps if the type of `first` meets the Cpp17BidirectionalIterator requirements for the overloads in namespace `std` or modelssatisfies `bidirectional_iterator` for the overloads in namespace `ranges`, and at most `N` swaps otherwise.

>     > > 2. (8.2) […]

[2020-02-10, Prague; Daniel comments]

I expect that P2101R0 (See D2101R0) is going to resolve this issue.

This proposal should also resolve the corresponding NB comments US-298 and US-300.

[2020-05-01; reflector discussions]

Resolved by P2101R0 voted in in Prague.

Proposed resolution:

Resolved by P2101R0.

### 3346(i). `pair` and `tuple` copy and move constructor have backwards specification

Section: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr] Status: C++20 Submitter: Richard Smith Opened: 2019-11-26 Last modified: 2021-02-25

Priority: 0

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Discussion:

22.3.2 [pairs.pair] p2 and 22.4.4.2 [tuple.cnstr] p3 say:

> The defaulted move and copy constructor, respectively, of {`pair`,`tuple`} is a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.

Note that we specify the copy constructor in terms of element move operations and the move constructor in terms of element copy operations. Is that really the intent? This appears to be how this was originally specified when the wording was added by N3471.

[2019-12-01; Daniel comments and provides wording]

These inverted wording effects are an unintended oversight caused by N3471.

[2019-12-08 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after ten positive votes on the reflector.

Proposed resolution:

This wording is relative to N4835.

1. Modify 22.3.2 [pairs.pair] as indicated:

   > -2- The defaulted move and copy constructor, respectively, of `pair` shall be a constexpr function if and only if all required element-wise initializations for copymove and movecopy, respectively, would satisfy the requirements for a constexpr function.

2. Modify 22.4.4.2 [tuple.cnstr] as indicated:

   > -3- The defaulted move and copy constructor, respectively, of `tuple` shall be a constexpr function if and only if all required element-wise initializations for copymove and movecopy, respectively, would satisfy the requirements for a constexpr function. The defaulted move and copy constructor of `tuple<>` shall be constexpr functions.

### 3347(i). `std::pair<T, U>` now requires `T` and `U` to be less-than-comparable

Section: 22.3.3 [pairs.spec], 23.3.3.1 [array.overview] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-12-03 Last modified: 2021-02-25

Priority: 1

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Discussion:

P1614R2 added `operator<=>` as a hidden friend to `std::pair`:

>     friend constexpr common_comparison_category_t<synth-three-way-result<T1>,
>                                                   synth-three-way-result<T2>>
>       operator<=>(const pair& x, const pair& y) { see below }

That is not a function template, so is not a SFINAE context. If one or both of `synth-three-way-result<T1>` or `synth-three-way-result<T2>` is an invalid type then the declaration of `operator<=>` is ill-formed, and so the specialization `std::pair<T1, T2>` is ill-formed.

A similar problem exists for `std::array`.

There are at least two ways to fix this:

1. Constrain the function and delay the use of synth-three-way-result until we know it's valid.

2. Replace the hidden friend with a namespace-scope function template, so invalid `synth-three-way-result` types cause substitution failure.

The first option is somewhat hard to specify, because current policy is to avoid the use of requires-clauses in most of the library clauses. Even with a requires-clause, the potentially-invalid `synth-three-way-result` types cannot be used in the function declarator. Furthermore, the `operator<=>` for `std::array` is currently specified in Table [tab:container.opt] and so there's nowhere to add a Constraints: element.

The second option would partially revert the P1614R2 changes for `std::pair` and `std::array` and bring them closer to what was in C++17. The main motivation for making `operator==` a hidden friend was to allow it to be defaulted, so that `std::pair` and `std::array` would be usable as non-type template parameters. Following the acceptance of P1907 in Belfast it isn't necessary to default it, so we can go back to what was in C++17.

[2019-12-12 Issue Prioritization]

Priority to 1 after reflector discussion.

[2020-02-10 Move to Immediate Monday afternoon in Prague]

Proposed resolution:

This wording is relative to N4842.

1. Modify 22.2.1 [utility.syn] as indicated:

   > [Drafting note: This restores the pre-P1614R2 `operator==` and uses `operator<=>` as replacement for `operator<`, `operator<=`, `operator>`, `operator>=`.]

   >     […]

   >     // 22.3 [pairs], class template pair
   >     template<class T1, class T2>
   >     struct pair;

   >     // 22.3.3 [pairs.spec], pair specialized algorithms
   >     template<class T1, class T2>
   >       constexpr bool operator==(const pair<T1, T2>&, const pair<T1, T2>&);
   >     template<class T1, class T2>
   >       constexpr common_comparison_category_t<synth-three-way-result<T1>,
   >                                              synth-three-way-result<T2>>
   >       operator<=>(const pair<T1, T2>&, const pair<T1, T2>&);

   >     template<class T1, class T2>
   >     constexpr void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));
   >     […]

2. Modify 22.3.2 [pairs.pair] as indicated:

   >       […]
   >       constexpr void swap(pair& p) noexcept(see below);

   >       // 22.3.3 [pairs.spec], pair specialized algorithms
   >       friend constexpr bool operator==(const pair&, const pair&) = default;
   >       friend constexpr bool operator==(const pair& x, const pair& y)
   >         requires (is_reference_v<T1> || is_reference_v<T2>)
   >         { return x.first == y.first && x.second == y.second; }
   >       friend constexpr common_comparison_category_t<synth-three-way-result<T1>,
   >                                                     synth-three-way-result<T2>>
   >         operator<=>(const pair& x, const pair& y) { see below }
   >     };

   >     template<class T1, class T2>
   >     pair(T1, T2) -> pair<T1, T2>;
   >     […]

3. Modify 22.3.3 [pairs.spec] as indicated:

   > 20.4.3 Specialized algorithms [pairs.spec]

   >     template<class T1, class T2>
   >       constexpr bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y);

   > > -?- Returns: `x.first == y.first && x.second == y.second`.

   >     template<class T1, class T2>
   >     friend constexpr
   >       common_comparison_category_t<synth-three-way-result<T1>, synth-three-way-result<T2>>
   >         operator<=>(const pair<T1, T2>& x, const pair<T1, T2>& y);

   > > -1- Effects: Equivalent to:

   > > >     if (auto c = synth-three-way(x.first, y.first); c != 0) return c;
   > > >     return synth-three-way(x.second, y.second);

4. Modify 23.3.2 [array.syn] as indicated:

   > [Drafting note: This restores the pre-P1614R2 `operator==` and uses `operator<=>` as replacement for `operator<`, `operator<=`, `operator>`, `operator>=`.]

   >     namespace std {

   >     // 23.3.3 [array], class template array
   >     template<class T, size_t N> struct array;

   >     template<class T, size_t N>
   >       constexpr bool operator==(const array<T, N>& x, const array<T, N>& y);
   >     template<class T, size_t N>
   >       constexpr synth-three-way-result<T>
   >         operator<=>(const array<T, N>& x, const array<T, N>& y);

   >     template<class T, size_t N>
   >     constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));
   >     […]

5. Modify 23.3.3.1 [array.overview] as indicated:

   > [Drafting note: there is no need to add definitions of `operator==` and `operator<=>` to [array.spec] because they are defined by Table 71: Container requirements [tab:container.req] and Table 73: Optional container operations [tab:container.opt] respectively.]

   >       […]
   >       constexpr T * data() noexcept;
   >       constexpr const T * data() const noexcept;

   >       friend constexpr bool operator==(const array&, const array&) = default;
   >       friend constexpr synth-three-way-result<value_type>
   >         operator<=>(const array&, const array&);
   >     };

   >     template<class T, class... U>
   >     array(T, U...) -> array<T, 1 + sizeof...(U)>;
   >     […]

### 3348(i). `__cpp_lib_unwrap_ref` in wrong header

Section: 17.3.2 [version.syn], 99 [refwrap.unwrapref] Status: C++20 Submitter: Barry Revzin Opened: 2019-12-03 Last modified: 2021-02-25

Priority: 2

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Discussion:

cpplearner points out in this github comment that:

> Since `unwrap_reference` and `unwrap_ref_decay` are defined in `<functional>` ([functional.syn]), their feature test macro should also be defined there.

P1902R1 adds this feature test macro in `<type_traits>` instead. The feature test macro and the type traits should go into the same header: either both in `<functional>` or both in `<type_traits>`.

The smallest diff is just to move the macro into `<functional>`.

[2019-12-12 Issue Prioritization]

Priority to 2 after reflector discussion.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4842.

> 1. Modify 17.3.2 [version.syn] p2 as indicated:

>    >     […]
>    >     #define __cpp_lib_unordered_map_try_emplace 201411L // also in <unordered_map>
>    >     #define __cpp_lib_unwrap_ref                201811L // also in <type_traitsfunctional>
>    >     #define __cpp_lib_variant                   201606L // also in <variant>
>    >     […]

[2020-02-13, Prague]

During LWG discussions it had been suggested that they considered it is an improvement to move the definitions of `unwrap_reference` and `unwrap_ref_decay` from `<functional>` to `<type_traits>`. This is what the alternative wording tries to accomplish.

[Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 22.10.2 [functional.syn], header `<functional>` synopsis, as indicated:

   >     namespace std {
   >     […]
   >     template<class T> struct unwrap_reference;
   >     template<class T> using unwrap_reference_t = typename unwrap_reference<T>::type;
   >     template<class T> struct unwrap_ref_decay;
   >     template<class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;
   >     […]
   >     }

2. Delete sub-clause 99 [refwrap.unwrapref] completely, as indicated:

   > 20.14.5.6 Transformation type trait `unwrap_reference` [refwrap.unwrapref]

   >     template<class T>
   >       struct unwrap_reference;

   > > -1- If `T` is a specialization `reference_wrapper<X>` for some type `X`, the member typedef `type` of `unwrap_reference<T>` is `X&`, otherwise it is `T`.

   >     template<class T>
   >       struct unwrap_ref_decay;

   > > -2- The member typedef `type` of `unwrap_ref_decay<T>` denotes the type `unwrap_reference_t<decay_t<T>>`.

3. Modify 21.3.3 [meta.type.synop], header `<type_traits>` synopsis, as indicated:

   >     namespace std {
   >     […]
   >     // 21.3.8.7 [meta.trans.other], other transformations
   >     […]
   >     template<class T> struct underlying_type;
   >     template<class Fn, class... ArgTypes> struct invoke_result;
   >     template<class T> struct unwrap_reference;
   >     template<class T> struct unwrap_ref_decay;

   >     template<class T>
   >       using type_identity_t = typename type_identity<T>::type;
   >     […]
   >     template<class Fn, class... ArgTypes>
   >       using invoke_result_t = typename invoke_result<Fn, ArgTypes...>::type;
   >     template<class T>
   >       using unwrap_reference_t = typename unwrap_reference<T>::type;
   >     template<class T>
   >       using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;
   >     template<class...>
   >       using void_t = void;
   >     […]
   >     }

4. Modify 21.3.8.7 [meta.trans.other], Table 55 — "Sign modifications" in [tab:meta.trans.sign] as indicated:

   Table 52 — Other transformations [tab:meta.trans.other]
   Template | Comments
   --- | ---
   `[…]`
   `template <class T> struct unwrap_reference;` | If `T` is a specialization `reference_wrapper<X>` for some type `X`, the member typedef `type` of `unwrap_reference<T>` is `X&`, otherwise it is `T`.
   `template <class T> struct unwrap_ref_decay;` | The member typedef `type` of `unwrap_ref_decay<T>` denotes the type `unwrap_reference_t<decay_t<T>>`.

5. Insert between 21.3.8.7 [meta.trans.other] p1 and p2 as indicated:

   > In addition to being available via inclusion of the `<type_traits>` header, the templates `unwrap_reference`, `unwrap_ref_decay`, `unwrap_reference_t`, and `unwrap_ref_decay_t` are available when the header `<functional>` (22.10.2 [functional.syn]) is included.

### 3349(i). Missing `__cpp_lib_constexpr_complex` for P0415R1

Section: 17.3.2 [version.syn] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2019-12-04 Last modified: 2021-02-25

Priority: 0

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Discussion:

P1902R1 "Missing feature-test macros 2017-2019", accepted in Belfast, said:

> [P0415R1] (Constexpr for `std::complex`): this paper proposes to introduce the macro `__cpp_lib_constexpr_complex`. That is, introducing a new macro for this header.

However, `__cpp_lib_constexpr_complex` wasn't mentioned in the Wording section, and doesn't appear in the latest WP N4842.

P0415R1 was accepted in Albuquerque (November 2017), and the paper itself said "For the purposes of SG10, we recommend the feature-testing macro name `__cpp_lib_constexpr_complex`.", so this has been accepted and then overlooked by WG21 twice.

[2019-12-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after twelve positive votes on the reflector.

Proposed resolution:

This wording is relative to N4842.

1. Modify 17.3.2 [version.syn] p2 as indicated:

   >     […]
   >     #define __cpp_lib_constexpr_algorithms    201806L // also in <algorithm>
   >     #define __cpp_lib_constexpr_complex       201711L // also in <complex>
   >     #define __cpp_lib_constexpr_dynamic_alloc 201907L // also in <memory>
   >     […]

### 3350(i). Simplify return type of `lexicographical_compare_three_way`

Section: 26.8.12 [alg.three.way] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-12-04 Last modified: 2021-02-25

Priority: 0

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Discussion:

The current return type is:

>     common_comparison_category_t<decltype(comp(*b1, *b2)), strong_ordering>

Finding the common category with `strong_ordering` doesn't do anything. The common category of `X` and `strong_ordering` is always `X`, so we can simplify it to:

>     common_comparison_category_t<decltype(comp(*b1, *b2))>

This can further be simplified, because the common category of any comparison category type is just that type. If it's not a comparison category then the result would be `void`, but the function would be ill-formed in that case anyway, as we have:

> Mandates: `decltype(comp(*b1, *b2))` is a comparison category type.

So the only effect of the complicated return type seems to be to cause the return type to be deduced as `void` for specializations of the function template that are ill-formed if called. That doesn't seem useful.

[2019-12-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.

Proposed resolution:

This wording is relative to N4842.

1. Modify 26.4 [algorithm.syn] as indicated:

   >     […]
   >     // 26.8.12 [alg.three.way], three-way comparison algorithms
   >     template<class InputIterator1, class InputIterator2, class Cmp>
   >       constexpr auto
   >         lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
   >                                           InputIterator2 b2, InputIterator2 e2,
   >                                           Cmp comp)
   >           -> common_comparison_category_t<decltype(comp(*b1, *b2)), strong_ordering>;
   >     template<class InputIterator1, class InputIterator2>
   >       constexpr auto
   >         lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
   >                                           InputIterator2 b2, InputIterator2 e2);
   >     […]

2. Modify 26.8.12 [alg.three.way] as indicated:

   >     template<class InputIterator1, class InputIterator2, class Cmp>
   >       constexpr auto
   >         lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
   >                                           InputIterator2 b2, InputIterator2 e2,
   >                                           Cmp comp)
   >           -> common_comparison_category_t<decltype(comp(*b1, *b2)), strong_ordering>;

   > > -1- Mandates: `decltype(comp(*b1, *b2))` is a comparison category type.

   > > […]

### 3351(i). `ranges::enable_safe_range` should not be constrained

Section: 25.2 [ranges.syn] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-12-05 Last modified: 2021-02-25

Priority: 0

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Discussion:

Currently `ranges::enable_safe_range` is constrained with `ranges::range`, which not only forces the compiler to do unnecessary satisfaction checking when it's used, but also creates a tricky dependency cycle (`ranges::range` depends on `ranges::begin` which depends on `ranges::enable_safe_range` which depends on `ranges::range`).

The only place the variable template is expected to be used is in the `ranges::safe_range` concept, which already checks `range<T>` before using `enable_safe_range<T>` anyway.

The constraint serves no purpose and should be removed.

[2019-12-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4842.

1. Modify 25.2 [ranges.syn] as indicated:

   > [Drafting note: The definition in 25.4.2 [range.range] p7 is already unconstrained, which contradicts the synopsis.]

   >     […]
   >     // 25.4.2 [range.range], ranges
   >     template<class T>
   >     concept range = see below;

   >     template<rangeclass T>
   >     inline constexpr bool enable_safe_range = false;

   >     template<class T>
   >     concept safe_range = see below;
   >     […]

### 3352(i). `strong_equality` isn't a thing

Section: 23.2.2 [container.requirements.general] Status: C++20 Submitter: Casey Carter Opened: 2019-12-06 Last modified: 2021-02-25

Priority: 1

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Discussion:

[tab:container.req] includes the row:

> Expression: `i <=> j`

> Return Type: `strong_ordering` if `X::iterator` meets the random access iterator requirements, otherwise `strong_equality`.

> Complexity: constant

"`strong_equality`" is (now) a meaningless term that appears nowhere else in the working draft. Presumably we want to make the Return Type unconditionally `strong_ordering`, and require this expression to be valid only when `i` and `j` are random access iterators. It's not clear to me if the best way to do so would be to add a "Constraints" to the "Assertion/note/pre-/post-condition" column of this table row, or if we should pull the row out into yet another "conditionally-supported iterator requirements" table.

[2019-12-21 Issue Prioritization]

Priority to 1 after reflector discussion.

[2020-02-10, Prague; David Olsen provides new wording based upon Tim Songs suggestion]

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

> Table 71 — Container requirements [tab:container.req]
> Expression | Return type | Operational semantics | Assertion/note pre/post-condition | Complexity
> --- | --- | --- | --- | ---
> `[…]`
> `i <=> j` | `strong_ordering` if `X::iterator` meets the random access iterator requirements, otherwise `strong_equality` |  | Constraints: `X::iterator` meets the random access iterator requirements. | constant
> `[…]`

### 3354(i). `has_strong_structural_equality` has a meaningless definition

Section: 21.3.5.4 [meta.unary.prop], 17.3.2 [version.syn] Status: C++20 Submitter: Daniel Krügler Opened: 2019-12-08 Last modified: 2021-02-25

Priority: 1

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Discussion:

After the Belfast 2019 meeting with the acceptance of P1907R1 the core language term "strong structural equality" has been removed and instead a more general definition of a "structural type" has been introduced that is suitable to be used as non-type template parameter. These changes have caused the current definition of the `has_strong_structural_equality` type trait (which had originally been introduced by P1614R2 during the Cologne 2019 meeting) to become meaningless since it is currently defined as follows:

> The type `T` has strong structural equality (11.10.1 [class.compare.default]).

Besides the now undefined term "strong structural equality", the reference to 11.10.1 [class.compare.default] doesn't make sense anymore, assuming that the trait definition is supposed to refer now to a type that can be used as non-type template parameter.

During library reflector discussions several informal naming suggestions has been mentioned, such as `is_structural[_type]`, `can_be_nttp`, `is_nontype_template_parameter_type`.

Albeit `is_structural_type` would come very near to the current core terminology, core experts have argued that the term "structural type" should be considered as a "core-internal" term that may easily be replaced post-C++20.

In addition to that definition and naming question of that type trait it should be discussed whether there should exist a specific feature macro for just this type trait, similar to the reason why we introduced the `__cpp_lib_has_unique_object_representations` test macro for the `has_unique_object_representations` type trait while voting in P0258R2. The submitter of this issue believes that such a feature macro should be added and given that its final definition should be related to the date of the acceptance of this issue (and should not be related to any historically accepted papers such as P1614R2).

[2020-01 Priority set to 1 and questions to LEWG after review on the reflector.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4842.

> 1. Modify 17.3.2 [version.syn], header `<version>` synopsis, as indicated (The symbolic `??????L` represents the date to be defined by the project editor):

>    >     […]
>    >     #define __cpp_lib_is_swappable               201603L // also in <type_traits>
>    >     #define __cpp_lib_is_template_parameter_type ??????L // also in <type_traits>
>    >     #define __cpp_lib_jthread                    201911L // also in <stop_token>, <thread>
>    >     […]

> 2. Modify 21.3.3 [meta.type.synop], header `<type_traits>` synopsis, as indicated:

>    >     […]
>    >     template<class T> struct has_unique_object_representations;

>    >     template<class T> struct has_strong_structural_equalityis_template_parameter_type;

>    >     // 21.3.6 [meta.unary.prop.query], type property queries
>    >     […]
>    >     template<class T>
>    >       inline constexpr bool has_unique_object_representations_v
>    >         = has_unique_object_representations<T>::value;

>    >     template<class T>
>    >       inline constexpr bool has_strong_structural_equality_vis_template_parameter_type_v
>    >         = has_strong_structural_equalityis_template_parameter_type<T>::value;

>    >     // 21.3.6 [meta.unary.prop.query], type property queries
>    >     […]

> 3. Modify 21.3.3 [meta.type.synop], Table 47 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:

>    > Table 47: Type property predicates [tab:meta.unary.prop]
>    > Template | Condition | Preconditions
>    > --- | --- | ---
>    > `…`
>    > `template<class T> struct has_strong_structural_equalityis_template_parameter_type;` | The type `T` has strong structural equality (11.10.1 [class.compare.default]) can be used as non-type template-parameter (13.2 [temp.param]). | `T` shall be a complete type, cv `void`, or an array of unknown bound. If `T` is a class type, `T` shall be a complete type.
>    > `…`

[2020-02, Prague]

LEWG looked at this and suggested to remove the existing trait `has_strong_structural_equality` for now until someone demonstrates which usecases exist that would justify its existence.

[2020-02-13, Prague]

Set to Immediate.

Proposed resolution:

This wording is relative to N4849.

1. Modify 21.3.3 [meta.type.synop], header `<type_traits>` synopsis, as indicated:

   >     […]
   >     template<class T> struct has_unique_object_representations;

   >     template<class T> struct has_strong_structural_equality;

   >     // 21.3.6 [meta.unary.prop.query], type property queries
   >     […]
   >     template<class T>
   >       inline constexpr bool has_unique_object_representations_v
   >         = has_unique_object_representations<T>::value;

   >     template<class T>
   >       inline constexpr bool has_strong_structural_equality_v
   >         = has_strong_structural_equality<T>::value;

   >     // 21.3.6 [meta.unary.prop.query], type property queries
   >     […]

2. Modify 21.3.3 [meta.type.synop], Table 47 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:

   > Table 47: Type property predicates [tab:meta.unary.prop]
   > Template | Condition | Preconditions
   > --- | --- | ---
   > `…`
   > `template<class T> struct has_strong_structural_equality;` | The type `T` has strong structural equality (11.10.1 [class.compare.default]). | `T` shall be a complete type, cv `void`, or an array of unknown bound.

### 3355(i). The memory algorithms should support move-only input iterators introduced by P1207

Section: 26.11.5 [uninitialized.copy], 26.11.6 [uninitialized.move] Status: C++20 Submitter: Corentin Jabot Opened: 2019-11-12 Last modified: 2021-02-25

Priority: 2

View all other issues in [uninitialized.copy].

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Discussion:

P1207 introduced move-only input iterators but did not modify the specialized memory algorithms to support them.

[2020-01 Priority set to 2 after review on the reflector.]

[2020-02 Status to Immediate on Friday morning in Prague.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 26.11.5 [uninitialized.copy] as indicated:

   >     namespace ranges {
   >       template<input_iterator I, sentinel_for<I> S1,
   >                no-throw-forward-iterator O, no-throw-sentinel<O> S2>
   >           requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
   >         uninitialized_copy_result<I, O>
   >           uninitialized_copy(I ifirst, S1 ilast, O ofirst, S2 olast);
   >       template<input_range IR, no-throw-forward-range OR>
   >           requires constructible_from<range_value_t<OR>, range_reference_t<IR>>
   >         uninitialized_copy_result<safe_iterator_t<IR>, safe_iterator_t<OR>>
   >           uninitialized_copy(IR&& in_range, OR&& out_range);
   >     }

   > > -4- Preconditions: `[ofirst, olast)` shall not overlap with `[ifirst, ilast)`.

   > > -5- Effects: Equivalent to:

   > > >     for (; ifirst != ilast && ofirst != olast; ++ofirst, (void)++ifirst) {
   > > >       ::new (voidify(*ofirst)) remove_reference_t<iter_reference_t<O>>(*ifirst);
   > > >     }
   > > >     return {std::move(ifirst), ofirst};

   > […]

   >     namespace ranges {
   >       template<input_iterator I, no-throw-forward-iterator O, no-throw-sentinel<O> S>
   >           requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
   >         uninitialized_copy_n_result<I, O>
   >           uninitialized_copy_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
   >     }

   > > -9- Preconditions: `[ofirst, olast)` shall not overlap with `[ifirst, n)`.

   > > -10- Effects: Equivalent to:

   > > >     auto t = uninitialized_copy(counted_iterator(ifirst, n),
   > > >                                 default_sentinel, ofirst, olast);
   > > >     return {std::move(t.in).base(), t.out};

2. Modify 26.11.6 [uninitialized.move] as indicated:

   >     namespace ranges {
   >       template<input_iterator I, sentinel_for<I> S1,
   >                no-throw-forward-iterator O, no-throw-sentinel<O> S2>
   >           requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
   >         uninitialized_move_result<I, O>
   >           uninitialized_move(I ifirst, S1 ilast, O ofirst, S2 olast);
   >       template<input_range IR, no-throw-forward-range OR>
   >           requires constructible_from<range_value_t<OR>, range_rvalue_reference_t<IR>>
   >         uninitialized_move_result<safe_iterator_t<IR>, safe_iterator_t<OR>>
   >           uninitialized_move(IR&& in_range, OR&& out_range);
   >     }

   > > -3- Preconditions: `[ofirst, olast)` shall not overlap with `[ifirst, ilast)`.

   > > -4- Effects: Equivalent to:

   > > >     for (; ifirst != ilast && ofirst != olast; ++ofirst, (void)++ifirst) {
   > > >       ::new (voidify(*ofirst))
   > > >         remove_reference_t<iter_reference_t<O>>(ranges::iter_move(*ifirst));
   > > >     }
   > > >     return {std::move(ifirst), ofirst};

   > […]

   >     namespace ranges {
   >       template<input_iterator I, no-throw-forward-iterator O, no-throw-sentinel<O> S>
   >           requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
   >         uninitialized_move_n_result<I, O>
   >           uninitialized_move_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
   >     }

   > > -8- Preconditions: `[ofirst, olast)` shall not overlap with `[ifirst, n)`.

   > > -9- Effects: Equivalent to:

   > > >     auto t = uninitialized_move(counted_iterator(ifirst, n),
   > > >                                 default_sentinel, ofirst, olast);
   > > >     return {std::move(t.in).base(), t.out};

### 3356(i). `__cpp_lib_nothrow_convertible` should be `__cpp_lib_is_nothrow_convertible`

Section: 17.3.2 [version.syn] Status: C++20 Submitter: Barry Revzin Opened: 2019-12-09 Last modified: 2021-02-25

Priority: 0

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++20 status.

Discussion:

P1902R1 introduced the feature test macro `__cpp_lib_nothrow_convertible`, but every other example in SD-6 of a feature test macro testing for the presence of a single type trait FOO is named `__cpp_lib_FOO`. This macro should be renamed `__cpp_lib_is_nothrow_convertible`.

This naming convention should probably be documented in SD-6 as policy.

[2019-12-21 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector. A convincing argument was that currently no vendor had published a release with the previous feature macro.

Proposed resolution:

This wording is relative to N4842.

1. Modify 17.3.2 [version.syn], header `<version>` synopsis, as indicated:

   >     […]
   >     #define __cpp_lib_not_fn                 201603L  // also in <functional>
   >     #define __cpp_lib_is_nothrow_convertible 201806L  // also in <type_traits>
   >     #define __cpp_lib_null_iterators         201304L  // also in <iterator>
   >     […]

### 3358(i). §[span.cons] is mistaken that `to_address` can throw

Section: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Casey Carter Opened: 2019-12-10 Last modified: 2021-02-25

Priority: 0

View all other issues in [span.cons].

View all issues with C++20 status.

Discussion:

[span.cons] paragraphs 6 and 9:

> Throws: When and what `to_address(first)` throws.

could equivalently be "Throws: Nothing." since all overloads of `std::to_address` are `noexcept`. However, paragraph 9 fails to account for the fact that paragraph 8:

> Effects: Initializes `data_` with `to_address(first)` and `size_` with `last - first`.

must evaluate `last - first`.

[2020-01-14 Status set to Tentatively Ready after ten positive votes on the reflector.]

[2020-01-14; Daniel comments]

The fixed wording in 23.7.2.2.2 [span.cons] p9 depends on the no-throw-guarantee of integer-class conversions to integral types. This guarantee is specified by LWG 3367(i).

Proposed resolution:

This wording is relative to N4842.

1. Modify 23.7.2.2.2 [span.cons] paragraphs 6 and 9 as indicated:

   > [Drafting note:

   > 1. The missing paragraph number of the Preconditions element at p7/p8 has already been reported as editorial issue

   > 2. The effective change to "Throws: Nothing." in p6 has already been applied editorially.

   > ]

   >     template<class It>
   >       constexpr span(It first, size_type count);

   > > […]

   > > -4- Preconditions: […]

   > > -5- Effects: Initializes `data_` with `to_address(first)` and `size_` with `count`.

   > > -6- Throws: When and what `to_address(first)` throwsNothing.

   >     template<class It, class End>
   >       constexpr span(It first, End last);

   > > […]

   > > -?- Preconditions: […]

   > > -8- Effects: Initializes `data_` with `to_address(first)` and `size_` with `last - first`.

   > > -9- Throws: When and what `to_address(first)last - first` throws.

### 3359(i). `<chrono>` leap second support should allow for negative leap seconds

Section: 30.11.8 [time.zone.leap] Status: C++20 Submitter: Asher Dunn Opened: 2019-12-16 Last modified: 2021-02-25

Priority: 3

View all other issues in [time.zone.leap].

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Discussion:

`class leap` (which is expected to be renamed by P1981R0 to `leap_second`) defined in 30.11.8 [time.zone.leap] should include support for both positive leap seconds (`23:59:60` added to UTC at a specified time) and negative leap seconds (`23:59:59` removed from UTC at a specified time). While only positive leap seconds have been inserted to date, the definition of UTC allows for both.

Update 30.11.8 [time.zone.leap] to specify the value of leap seconds in addition to their insertion date, and update wording and examples in 30.7 [time.clock] and 30.12 [time.format] that involve leap seconds to account for both types of leap second.

[2020-01 Priority set to 3 after review on the reflector.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4842.

> 1. Modify 30.7.3.2 [time.clock.utc.members] as indicated:

>    >     template<class Duration>
>    >       static sys_time<common_type_t<Duration, seconds>>
>    >         to_sys(const utc_time<Duration>& u);

>    > > -2- Returns: A `sys_time t`, such that `from_sys(t) == u` if such a mapping exists. Otherwise `u` represents a `time_point` during a positive leap second insertion, the conversion counts that leap second as not inserted, and the last representable value of `sys_time` prior to the insertion of the leap second is returned.

>    >     template<class Duration>
>    >       static utc_time<common_type_t<Duration, seconds>>
>    >         from_sys(const sys_time<Duration>& t);

>    > > -3- Returns: A `utc_time u`, such that `u.time_since_epoch() - t.time_since_epoch()` is equal to the numbersum of leap seconds that were inserted between `t` and 1970-01-01. If `t` is exactly the date of leap second insertion, then the conversion counts that leap second as inserted.

>    > > […]

> 2. Modify 30.7.3.3 [time.clock.utc.nonmembers] as indicated:

>    >     template<class Duration>
>    >       leap_second_info get_leap_second_info(const utc_time<Duration>& ut);

>    > > -6- Returns: A `leap_second_info` where `is_leap_second` is `true` if `ut` is during a positive leap second insertion, and otherwise `false`. `elapsed` is the numbersum of leap seconds between 1970-01-01 and `ut`. If `is_leap_second` is `true`, the leap second referred to by `ut` is included in the count.

> 3. Modify 30.7.4.1 [time.clock.tai.overview] as indicated:

>    > -1- The clock `tai_clock` measures seconds since 1958-01-01 00:00:00 and is offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into TAI. Therefore every time a leap second is inserted into UTC, UTC falls another second behindshifts another second with respect to TAI. For example by 2000-01-01 there had been 22 positive and 0 negative leap seconds inserted so 2000-01-01 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the initial 10s offset).

> 4. Modify 30.7.5.1 [time.clock.gps.overview] as indicated:

>    > -1- The clock `gps_clock` measures seconds since the first Sunday of January, 1980 00:00:00 UTC. Leap seconds are not inserted into GPS. Therefore every time a leap second is inserted into UTC, UTC falls another second behindshifts another second with respect to GPS. Aside from the offset from `1958y/January/1` to `1980y/January/Sunday[1]`, GPS is behind TAI by 19s due to the 10s offset between 1958 and 1970 and the additional 9 leap seconds inserted between 1970 and 1980.

> 5. Modify 30.11.8.1 [time.zone.leap.overview] as indicated:

>    >     namespace std::chrono {
>    >       class leap {
>    >       public:
>    >         leap(const leap&) = default;
>    >         leap& operator=(const leap&) = default;

>    >         // unspecified additional constructors

>    >         constexpr sys_seconds date() const noexcept;
>    >         constexpr seconds value() const noexcept;
>    >       };
>    >     }

>    > -1- Objects of type `leap` representing the date and value of the leap second insertions are constructed and stored in the time zone database when initialized.

>    > -2- [Example:

>    > >     for (auto& l : get_tzdb().leaps)
>    > >       if (l <= 2018y/March/17d)
>    > >         cout << l.date() << ": " << l.value() << '\n';

>    > Produces the output:

>    > >     1972-07-01 00:00:00: 1s
>    > >     1973-01-01 00:00:00: 1s
>    > >     1974-01-01 00:00:00: 1s
>    > >     1975-01-01 00:00:00: 1s
>    > >     1976-01-01 00:00:00: 1s
>    > >     1977-01-01 00:00:00: 1s
>    > >     1978-01-01 00:00:00: 1s
>    > >     1979-01-01 00:00:00: 1s
>    > >     1980-01-01 00:00:00: 1s
>    > >     1981-07-01 00:00:00: 1s
>    > >     1982-07-01 00:00:00: 1s
>    > >     1983-07-01 00:00:00: 1s
>    > >     1985-07-01 00:00:00: 1s
>    > >     1988-01-01 00:00:00: 1s
>    > >     1990-01-01 00:00:00: 1s
>    > >     1991-01-01 00:00:00: 1s
>    > >     1992-07-01 00:00:00: 1s
>    > >     1993-07-01 00:00:00: 1s
>    > >     1994-07-01 00:00:00: 1s
>    > >     1996-01-01 00:00:00: 1s
>    > >     1997-07-01 00:00:00: 1s
>    > >     1999-01-01 00:00:00: 1s
>    > >     2006-01-01 00:00:00: 1s
>    > >     2009-01-01 00:00:00: 1s
>    > >     2012-07-01 00:00:00: 1s
>    > >     2015-07-01 00:00:00: 1s
>    > >     2017-01-01 00:00:00: 1s

>    > — end example]

> 6. Modify 30.11.8.2 [time.zone.leap.members] as indicated:

>    >     constexpr sys_seconds date() const noexcept;

>    > > -1- Returns: The date and time at which the leap second was inserted.

>    >     constexpr seconds value() const noexcept;

>    > > -?- Returns: The value of the leap second. Always `+1s` to indicate a positive leap second or `-1s` to indicate a negative leap second. All leap seconds inserted up through 2017 were positive leap seconds.

> 7. Modify 30.12 [time.format] as indicated:

>    >     template<class Duration, class charT>
>    >       struct formatter<chrono::utc_time<Duration>, charT>;

>    > > -7- Remarks: If `%Z` is used, it is replaced with `STATICALLY-WIDEN<charT>("UTC")`. If `%z` (or a modified variant of `%z`) is used, an offset of `0min` is formatted. If the argument represents a time during a positive leap second insertion, and if a seconds field is formatted, the integral portion of that format is `STATICALLY-WIDEN<charT>("60")`.

[2020-02-14; Prague]

LWG Review. Some wording improvements have been made and lead to revised wording.

Proposed resolution:

This wording is relative to N4849.

1. Modify 30.7.3.2 [time.clock.utc.members] as indicated:

   >     template<class Duration>
   >       static sys_time<common_type_t<Duration, seconds>>
   >         to_sys(const utc_time<Duration>& u);

   > > -2- Returns: A `sys_time t`, such that `from_sys(t) == u` if such a mapping exists. Otherwise `u` represents a `time_point` during a positive leap second insertion, the conversion counts that leap second as not inserted, and the last representable value of `sys_time` prior to the insertion of the leap second is returned.

   >     template<class Duration>
   >       static utc_time<common_type_t<Duration, seconds>>
   >         from_sys(const sys_time<Duration>& t);

   > > -3- Returns: A `utc_time u`, such that `u.time_since_epoch() - t.time_since_epoch()` is equal to the numbersum of leap seconds that were inserted between `t` and 1970-01-01. If `t` is exactly the date of leap second insertion, then the conversion counts that leap second as inserted.

   > > […]

2. Modify 30.7.3.3 [time.clock.utc.nonmembers] as indicated:

   >     template<class Duration>
   >       leap_second_info get_leap_second_info(const utc_time<Duration>& ut);

   > > -6- Returns: A `leap_second_info`, `lsi`, where `lsi.is_leap_second` is `true` if `ut` is during a positive leap second insertion, and otherwise `false`. `lsi.elapsed` is the numbersum of leap seconds between 1970-01-01 and `ut`. If `lsi.is_leap_second` is `true`, the leap second referred to by `ut` is included in the countsum.

3. Modify 30.7.4.1 [time.clock.tai.overview] as indicated:

   > -1- The clock `tai_clock` measures seconds since 1958-01-01 00:00:00 and is offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into TAI. Therefore every time a leap second is inserted into UTC, UTC falls another second behindshifts another second with respect to TAI. For example by 2000-01-01 there had been 22 positive and 0 negative leap seconds inserted so 2000-01-01 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the initial 10s offset).

4. Modify 30.7.5.1 [time.clock.gps.overview] as indicated:

   > -1- The clock `gps_clock` measures seconds since the first Sunday of January, 1980 00:00:00 UTC. Leap seconds are not inserted into GPS. Therefore every time a leap second is inserted into UTC, UTC falls another second behindshifts another second with respect to GPS. Aside from the offset from `1958y/January/1` to `1980y/January/Sunday[1]`, GPS is behind TAI by 19s due to the 10s offset between 1958 and 1970 and the additional 9 leap seconds inserted between 1970 and 1980.

5. Modify 30.11.8.1 [time.zone.leap.overview] as indicated:

   >     namespace std::chrono {
   >       class leap {
   >       public:
   >         leap(const leap&) = default;
   >         leap& operator=(const leap&) = default;

   >         // unspecified additional constructors

   >         constexpr sys_seconds date() const noexcept;
   >         constexpr seconds value() const noexcept;
   >       };
   >     }

   > -1- Objects of type `leap` representing the date and value of the leap second insertions are constructed and stored in the time zone database when initialized.

   > -2- [Example:

   > >     for (auto& l : get_tzdb().leaps)
   > >       if (l <= 2018y/March/17d)
   > >         cout << l.date() << ": " << l.value() << '\n';

   > Produces the output:

   > >     1972-07-01 00:00:00: 1s
   > >     1973-01-01 00:00:00: 1s
   > >     1974-01-01 00:00:00: 1s
   > >     1975-01-01 00:00:00: 1s
   > >     1976-01-01 00:00:00: 1s
   > >     1977-01-01 00:00:00: 1s
   > >     1978-01-01 00:00:00: 1s
   > >     1979-01-01 00:00:00: 1s
   > >     1980-01-01 00:00:00: 1s
   > >     1981-07-01 00:00:00: 1s
   > >     1982-07-01 00:00:00: 1s
   > >     1983-07-01 00:00:00: 1s
   > >     1985-07-01 00:00:00: 1s
   > >     1988-01-01 00:00:00: 1s
   > >     1990-01-01 00:00:00: 1s
   > >     1991-01-01 00:00:00: 1s
   > >     1992-07-01 00:00:00: 1s
   > >     1993-07-01 00:00:00: 1s
   > >     1994-07-01 00:00:00: 1s
   > >     1996-01-01 00:00:00: 1s
   > >     1997-07-01 00:00:00: 1s
   > >     1999-01-01 00:00:00: 1s
   > >     2006-01-01 00:00:00: 1s
   > >     2009-01-01 00:00:00: 1s
   > >     2012-07-01 00:00:00: 1s
   > >     2015-07-01 00:00:00: 1s
   > >     2017-01-01 00:00:00: 1s

   > — end example]

6. Modify 30.11.8.2 [time.zone.leap.members] as indicated:

   >     constexpr sys_seconds date() const noexcept;

   > > -1- Returns: The date and time at which the leap second was inserted.

   >     constexpr seconds value() const noexcept;

   > > -?- Returns: `+1s` to indicate a positive leap second or `-1s` to indicate a negative leap second. [Note: All leap seconds inserted up through 2019 were positive leap seconds. — end note]

7. Modify 30.12 [time.format] as indicated:

   >     template<class Duration, class charT>
   >       struct formatter<chrono::utc_time<Duration>, charT>;

   > > -7- Remarks: If `%Z` is used, it is replaced with `STATICALLY-WIDEN<charT>("UTC")`. If `%z` (or a modified variant of `%z`) is used, an offset of `0min` is formatted. If the argument represents a time during a positive leap second insertion, and if a seconds field is formatted, the integral portion of that format is `STATICALLY-WIDEN<charT>("60")`.

### 3360(i). `three_way_comparable_with` is inconsistent with similar concepts

Section: 17.12.4 [cmp.concept] Status: C++20 Submitter: Casey Carter Opened: 2019-12-18 Last modified: 2021-02-25

Priority: 0

View all other issues in [cmp.concept].

View all issues with C++20 status.

Discussion:

The concept `three_way_comparable_with` is defined in 17.12.4 [cmp.concept] as:

>     template<class T, class U, class Cat = partial_ordering>
>       concept three_way_comparable_with =
>         weakly-equality-comparable-with<T, U> &&
>         partially-ordered-with<T, U> &&
>         three_way_comparable<T, Cat> &&
>         three_way_comparable<U, Cat> &&
>         common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
>         three_way_comparable<
>           common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> &&
>         requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) {
>           { t <=> u } -> compares-as<Cat>;
>           { u <=> t } -> compares-as<Cat>;
>         };

Which notably doesn't follow the requirement ordering:

1. same-type requirements on `T`

2. same-type requirements on `U`

3. `common_reference_with` requirement

4. same-type requirements on `common_reference_t<T, U>`

5. cross-type requirements on `T` and `U`

that the other cross-type comparison concepts (18.5.4 [concept.equalitycomparable], 18.5.5 [concept.totallyordered]) use. There were some motivating reasons for that ordering:

1. The existence of a common reference type is effectively an opt-in to cross-type concepts. Avoiding checking cross-type expressions when no common reference type exists can enable the concepts to work even in the presence of poorly-constrained "accidental" cross-type operator templates which could otherwise produce compile errors instead of dissatisfied concepts.

2. Putting the simpler same-type requirements first can help produce simpler error messages when applying the wrong concept to a pair of types, or the right concept to the wrong pair of types. "`Frobnozzle <=> Frobnozzle` is not a valid expression" is more easily deciphered than is "`std::common_reference<int, FrobNozzle>` has no member named type".

`three_way_comparable_with` should be made consistent with `equality_comparable_with` and `totally_ordered_with` for the above reasons and to make it easier to reason about comparison concepts in general.

[2020-01 Status set to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 17.12.4 [cmp.concept] as indicated:

   >     template<class T, class U, class Cat = partial_ordering>
   >       concept three_way_comparable_with =
   >         weakly-equality-comparable-with<T, U> &&
   >         partially-ordered-with<T, U> &&
   >         three_way_comparable<T, Cat> &&
   >         three_way_comparable<U, Cat> &&
   >         common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
   >         three_way_comparable<
   >           common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> &&
   >         weakly-equality-comparable-with<T, U> &&
   >         partially-ordered-with<T, U> &&
   >         requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) {
   >           { t <=> u } -> compares-as<Cat>;
   >           { u <=> t } -> compares-as<Cat>;
   >         };

### 3361(i). `safe_range<SomeRange&>` case

Section: 25.4.2 [range.range] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2019-12-19 Last modified: 2023-11-22

Priority: 3

View all other issues in [range.range].

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Discussion:

25.5.5 [range.dangling] p2 hints at how `safe_range` should allow lvalue ranges to model it. However, its wording doesn't take into account that case.

[2020-01 Priority set to 3 after review on the reflector.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4842.

> 1. Modify 25.4.2 [range.range] as indicated:

>    >     template<class T>
>    >       concept safe_range =
>    >         range<T> &&
>    >           (is_lvalue_reference_v<T> || enable_safe_range<remove_cvref_t<T>>);

>    > > -5- Given an expression `E` such that `decltype((E))` is `T`,A type `T` models `safe_range` only if:

>    > > 1. (5.1) — `is_lvalue_reference_v<T>` is `true`, or

>    > > 2. (5.2) — given an expression `E` such that `decltype((E))` is `T`, if the validity of iterators obtained from the object denoted by `E` is not tied to the lifetime of that object.

>    > > -6- [Note: Since the validity of iterators is not tied to the lifetime of an object whose type models `safe_range`, aA function can accept arguments of such a type that models `safe_range` by value and return iterators obtained from it without danger of dangling. — end note]

[2021-05-19 Tim updates wording]

The new wording below attempts to keep the "borrowed" property generally applicable to all models of `borrowed_range`, instead of bluntly carving out lvalue reference types.

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll in June.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 25.4.2 [range.range] as indicated:

   >     template<class T>
   >       concept borrowed_range =
   >         range<T> &&
   >           (is_lvalue_reference_v<T> || enable_borrowed_range<remove_cvref_t<T>>);

   > > -5- Let `U` be `remove_reference_t<T>` if `T` is an rvalue reference type, and `T` otherwise. Given an expression `E` such that `decltype((E))` is `T` a variable `u` of type `U`, `T` models `borrowed_range` only if the validity of iterators obtained from the object denoted by `E` u is not tied to the lifetime of that object variable.

   > > -6- [Note: Since the validity of iterators is not tied to the lifetime of an objecta variable whose type models `borrowed_range`, a function can accept arguments of with a parameter of such a type by value and can return iterators obtained from it without danger of dangling. — end note]

### 3362(i). Strike `stop_source`'s `operator!=`

Section: 32.3.5 [stopsource] Status: C++20 Submitter: Tim Song Opened: 2020-01-03 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

Just like `stop_token` (see LWG 3254(i)), `stop_source` in 32.3.5 [stopsource] declares an `operator!=` friend that is unnecessary in light of the new core language rules and should be struck.

[2020-01-14 Status set to Tentatively Ready after six positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 32.3.5 [stopsource], class `stop_source` synopsis, as indicated:

   >     namespace std {
   >       […]

   >       class stop_source {
   >       public:
   >         […]

   >         [[nodiscard]] friend bool
   >         operator==(const stop_source& lhs, const stop_source& rhs) noexcept;
   >         [[nodiscard]] friend bool
   >         operator!=(const stop_source& lhs, const stop_source& rhs) noexcept;
   >         friend void swap(stop_source& lhs, stop_source& rhs) noexcept;
   >       };
   >     }

2. Modify [stopsource.cmp] and [stopsource.special] as indicated:

   > 32.3.4.3 Non-member functionsComparisons [stopsource.nonmemberscmp]

   >     [[nodiscard]] bool operator==(const stop_source& lhs, const stop_source& rhs) noexcept;

   > > -1- Returns: `true` if `lhs` and `rhs` have ownership of the same stop state or if both `lhs` and `rhs` do not have ownership of a stop state; otherwise `false`.

   >     [[nodiscard]] bool operator!=(const stop_source& lhs, const stop_source& rhs) noexcept;

   > > -2- Returns: `!(lhs==rhs)`.

   > 32.3.4.4 Specialized algorithms [stopsource.special]

   >     friend void swap(stop_source& x, stop_source& y) noexcept;

   > > -1- Effects: Equivalent to: `x.swap(y)`.

### 3363(i). `drop_while_view` should opt-out of `sized_range`

Section: 25.7.13.2 [range.drop.while.view] Status: C++20 Submitter: Johel Ernesto Guerrero Peña Opened: 2020-01-07 Last modified: 2021-02-25

Priority: 1

View all other issues in [range.drop.while.view].

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Discussion:

If `drop_while_view`'s `iterator_t` and `sentinel_t` model `forward_iterator` and `sized_sentinel_for`, it will incorrectly satisfy `sized_range` thanks to the size member function inherited from `view_interface`.

Because it has to compute its `begin()`, it can never model `sized_range` due to not meeting its non-amortized O(1) requirement.

[2020-01-16 Priority set to 1 after discussion on the reflector.]

[2020-02-10; Prague 2020; Casey comments and provides alternative wording]

The fundamental issue here is that both `ranges::size` and `view_interface::size` (it should be unsurprising that many of the "default" implementations of member `meow` in `view_interface` look just like fallback cases of the `ranges::meow` CPO) have a case that returns the difference of `ranges::end` and `ranges::begin`. If `begin` and `end` are amortized* O(1) but not "true" O(1), then the resulting `size` operation is amortized O(1) and not "true" O(1) as required by the `sized_range` concept. I don't believe we can or should fix this on a case by case basis, but we should instead relax the complexity requirement for `sized_range` to be handwavy-amortized O(1).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Add the following specialization to 25.2 [ranges.syn]:

>    >     // [drop.while.view], drop while view
>    >       template<view V, class Pred>
>    >         requires input_range<V> && is_object_v<Pred> &&
>    >           indirect_unary_predicate<const Pred, iterator_t<V>>
>    >         class drop_while_view;

>    >       template<view V, class Pred>
>    >         inline constexpr bool disable_sized_range<drop_while_view<V, Pred>> = true;

>    >       namespace views { inline constexpr unspecified drop_while = unspecified; }

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.4.4 [range.sized] as indicated:

   > -2- Given an lvalue `t` of type `remove_reference_t<T>`, `T` models `sized_range` only if

   > 1. (2.1) — `ranges::size(t)` is amortized 𝒪(`1`), does not modify `t`, and is equal to `ranges::distance(t)`, and

   > 2. […]

   > -3- [Note: The complexity requirement for the evaluation of `ranges::size` is non-amortized, unlike the case for the complexity of the evaluations of `ranges::begin` and `ranges::end` in the `range` concept. — end note]

### 3364(i). Initialize data members of ranges and their iterators

Section: 25.7.11.2 [range.take.while.view], 25.7.12.2 [range.drop.view], 25.7.13.2 [range.drop.while.view], 25.7.23.3 [range.elements.iterator] Status: C++20 Submitter: Johel Ernesto Guerrero Peña Opened: 2020-01-07 Last modified: 2021-02-25

Priority: 0

View all other issues in [range.take.while.view].

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Discussion:

Before P1035 was accepted, no data member in [ranges] whose type could potentially be an aggregate or fundamental type was left without initializer. P1035 left some such data members without initializer, so it is possible to have them have indeterminate values. We propose restoring consistency.

[2020-01-14 Status set to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 25.7.11.2 [range.take.while.view] as follows:

   >     class take_while_view : public view_interface<take_while_view<V, Pred>> {
   >       template<bool> class sentinel;                      // exposition only

   >       V base_ = V();                                      // exposition only
   >       semiregular-box<Pred> pred_;                        // exposition only

   >     public:

2. Modify 25.7.12.2 [range.drop.view] as follows:

   >     private:
   >       V base_ = V();                                     // exposition only
   >       range_difference_t<V> count_ = 0;                  // exposition only
   >     };

3. Modify 25.7.13.2 [range.drop.while.view] as follows:

   >     private:
   >       V base_ = V();                                     // exposition only
   >       semiregular-box<Pred> pred_;                       // exposition only
   >     };

4. Modify 25.7.23.3 [range.elements.iterator] as follows:

   >     class elements_view<V, N>::iterator {                // exposition only
   >       using base-t = conditional_t<Const, const V, V>;
   >       friend iterator<!Const>;

   >       iterator_t<base-t> current_ = iterator_t<base-t>();
   >     public:

### 3366(i). Narrowing conversions between integer and integer-class types

Section: 24.3.4.4 [iterator.concept.winc] Status: Resolved Submitter: Casey Carter Opened: 2020-01-07 Last modified: 2021-10-23

Priority: 3

View other active issues in [iterator.concept.winc].

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Discussion:

The working draft ignores the possibility that:

1. the value of an expression of integer-class type might not be representable by the target integer type of a conversion, and

2. the value of an expression of integer type might not be representable by the target integer-class type of a conversion.

Presumably the behavior of these cases is undefined by omission; is this actually the intent?

Notably (2) could be specified away by mandating that all integer-class types are capable of representing the value range of all integer types of the same signedness.

[2020-01-25 Issue Prioritization]

Priority to 3 after reflector discussion.

[2021-10-23 Resolved by the adoption of P2393R1 at the October 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

### 3367(i). Integer-class conversions should not throw

Section: 24.3.4.4 [iterator.concept.winc] Status: C++20 Submitter: Casey Carter Opened: 2020-01-07 Last modified: 2021-02-25

Priority: 0

View other active issues in [iterator.concept.winc].

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Discussion:

It's widely established that neither conversions of integral types to `bool` nor conversions between different integral types throw exceptions. These properties are crucial to supporting exception guarantees in algorithms, containers, and other uses of iterators and their difference types. Integer-class types must provide the same guarantees to support the same use cases as do integer types.

[2020-01-14; Daniel comments]

1. We probably need to think about providing the stronger guarantee that all integer-class operations are also `noexcept` in addition to the guarantee that they do not throw any exceptions.

2. The fixed wording in LWG 3358(i), 23.7.2.2.2 [span.cons] p9 depends on the no-throw-guarantee of integer-class conversions to integral types.

[2020-01-25 Status set to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 24.3.4.4 [iterator.concept.winc] as indicated:

   > [Drafting note: There's a bit of drive-by editing here to change occurrences of the meaningless "type is convertible to type" to "expression is convertible to type". Paragraph 7 only has drive-by edits. ]

   > -6- AllExpressions of integer-class types are explicitly convertible to allany integral types and. Expressions of integral type are both implicitly and explicitly convertible from all integral typesto any integer-class type. Conversions between integral and integer-class types do not exit via an exception.

   > -7- AllExpressions `E` of integer-class types `I` are contextually convertible to `bool` as if by `bool(aE != I(0))`, where `a` is an instance of the integral-class type `I`.

### 3368(i). Exactly when does `size` return `end - begin`?

Section: 25.3.10 [range.prim.size] Status: Resolved Submitter: Casey Carter Opened: 2020-01-07 Last modified: 2021-02-25

Priority: 0

View all issues with Resolved status.

Discussion:

The specification of `ranges::size` in 25.3.10 [range.prim.size] suggests that bullet 1.3 ("Otherwise, `make-unsigned-like(ranges::end(E) - ranges::begin(E)) ...")` only applies when `disable_sized_range<remove_cv_t<T>>` is `true`. This is not the design intent, but the result of an erroneous attempt to factor out the common "`disable_sized_range` is `false`" requirement from the member and non-member `size` cases in bullets 1.2.1 and 1.2.2 that occurred between P0896R3 and P0896R4. The intended design has always been that a range with member or non-member `size` with the same syntax but different semantics may opt-out of being sized by specializing `disable_sized_range`. It has never been intended that arrays or ranges whose iterator and sentinel model `sized_sentinel_for` be able to opt out of being sized via `disable_sized_range`. `disable_sized_sentinel_for` can/must be used to opt out in the latter case so that library functions oblivious to the range type that operate on the iterator and sentinel of such a range will avoid subtraction.

[2020-01-25 Status set to Tentatively Ready after six positive votes on the reflector.]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

[2020-11-18 This was actually resolved by P2091R0 in Prague. Status changed: WP → Resolved.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 25.3.10 [range.prim.size] as indicated:

   > [Drafting note: There are drive-by changes here to (1) avoid introducing unused type placeholders, (2) avoid reusing "`T`" as both the type of the subexpression and the template parameter of the poison pill, and (3) fix the cross-reference for `make-unsigned-like` which is defined in [ranges.syn]/1, not in [range.subrange].]

   > -1- The name `size` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::size(E)` for some subexpression `E` with type `T` is expression-equivalent to:

   > 1. (1.1) — `decay-copy(extent_v<T>)` if `T` is an array type (6.9.4 [basic.compound]).

   > 2. (1.2) — Otherwise, if `disable_sized_range<remove_cv_t<T>>` (25.4.4 [range.sized]) is `false`:

   > 3. (1.?2.1) — Otherwise, if `disable_sized_range<remove_cv_t<T>>` (25.4.4 [range.sized]) is `false` and `decay-copy(E.size())` if it is a valid expression and its type `I` isof integer-like type (24.3.4.4 [iterator.concept.winc]), `decay-copy(E.size())`.

   > 4. (1.?2.2) — Otherwise, if `disable_sized_range<remove_cv_t<T>>` is `false` and `decay-copy(size(E))` if it is a valid expression and its type `I` isof integer-like type with overload resolution performed in a context that includes the declaration:

   >    >     template<class T> void size(Tauto&&) = delete;

   >    and does not include a declaration of `ranges::size`, `decay-copy(size(E))`.

2. (1.3) — Otherwise, `make-unsigned-like(ranges::end(E) - ranges::begin(E))` (25.5.4 [range.subrange]25.2 [ranges.syn]) if it is a valid expression and the types `I` and `S` of `ranges::begin(E)` and `ranges::end(E)` (respectively) model both `sized_sentinel_for<S, I>` (24.3.4.8 [iterator.concept.sizedsentinel]) and `forward_iterator<I>`. However, `E` is evaluated only once.

3. (1.4) — […]

### 3369(i). `span`'s deduction-guide for built-in arrays doesn't work

Section: 23.7.2.2.1 [span.overview] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2020-01-08 Last modified: 2021-02-25

Priority: 0

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Discussion:

N4842 22.7.3.1 [span.overview] depicts:

>     template<class T, size_t N>
>     span(T (&)[N]) -> span<T, N>;

This isn't constrained by 22.7.3.3 [span.deduct]. Then, 22.7.3.2 [span.cons]/10 specifies:

>     template<size_t N> constexpr span(element_type (&arr)[N]) noexcept;
>     template<size_t N> constexpr span(array<value_type, N>& arr) noexcept;
>     template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;

> > Constraints:

> > - `extent == dynamic_extent || N == extent` is `true`, and

> > - `remove_pointer_t<decltype(data(arr))>(*)[]` is convertible to `ElementType(*)[]`.

Together, these cause CTAD to behave unexpectedly. Here's a minimal test case, reduced from libcxx's test suite:

>     C:\Temp>type span_ctad.cpp
>     #include <stddef.h>
>     #include <type_traits>

>     inline constexpr size_t dynamic_extent = static_cast<size_t>(-1);

>     template <typename T, size_t Extent = dynamic_extent>
>     struct span {
>       template <size_t Size>
>       requires (Extent == dynamic_extent || Extent == Size)
>     #ifdef WORKAROUND_WITH_TYPE_IDENTITY_T
>       span(std::type_identity_t<T> (&)[Size]) {}
>     #else
>       span(T (&)[Size]) {}
>     #endif
>     };

>     template <typename T, size_t Extent>
>     #ifdef WORKAROUND_WITH_REQUIRES_TRUE
>     requires (true)
>     #endif
>     span(T (&)[Extent]) -> span<T, Extent>;

>     int main() {
>       int arr[] = {1,2,3};
>       span s{arr};
>       static_assert(std::is_same_v<decltype(s), span<int, 3>>,
>         "CTAD should deduce span<int, 3>.");
>     }

>     C:\Temp>cl /EHsc /nologo /W4 /std:c++latest span_ctad.cpp
>     span_ctad.cpp
>     span_ctad.cpp(26): error C2338: CTAD should deduce span<int, 3>.

>     C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /DWORKAROUND_WITH_TYPE_IDENTITY_T span_ctad.cpp
>     span_ctad.cpp

>     C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /DWORKAROUND_WITH_REQUIRES_TRUE span_ctad.cpp
>     span_ctad.cpp

>     C:\Temp>

(MSVC and GCC 10 demonstrate this behavior. Clang is currently affected by LLVM#44484.)

Usually, when there's an explicit deduction-guide, we can ignore any corresponding constructor, because the overload resolution tiebreaker 12.4.3 [over.match.best]/2.10 prefers deduction-guides. However, this is a mental shortcut only, and it's possible for guides generated from constructors to out-compete deduction-guides during CTAD. That's what's happening here.

Specifically, the constructor is constrained, while the deduction-guide is not constrained. This activates the "more specialized" tiebreaker first (12.4.3 [over.match.best]/2.5 is considered before /2.10 for deduction-guides). That goes through 13.7.6.2 [temp.func.order]/2 and 13.5.4 [temp.constr.order] to prefer the more constrained overload.

(In the test case, this results in `span<int, dynamic_extent>` being deduced. That's because the constructor allows `T` to be deduced to be `int`. The constructor's `Size` template parameter is deduced to be 3, but that's unrelated to the class's `Extent` parameter. Because `Extent` has a default argument of `dynamic_extent`, CTAD succeeds and deduces `span<int, dynamic_extent>`.)

There are at least two possible workarounds: we could alter the constructor to prevent it from participating in CTAD, or we could constrain the deduction-guide, as depicted in the test case. Either way, we should probably include a Note, following the precedent of 21.3.2.2 [string.cons]/12.

Note that there are also deduction-guides for span from `std::array`. However, the constructors take `array<value_type, N>` with `using value_type = remove_cv_t<ElementType>;` so that prevents the constructors from interfering with CTAD.

I'm currently proposing to alter the constructor from built-in arrays. An alternative resolution to constrain the deduction-guide would look like: "Constraints: `true`. [Note: This affects class template argument deduction. — end note]"

[2020-01-25 Status set to Tentatively Ready after seven positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 23.7.2.2.1 [span.overview], class template `span` synopsis, as indicated:

   >     namespace std {
   >     template<class ElementType, size_t Extent = dynamic_extent>
   >     class span {
   >     public:
   >       […]
   >       // 23.7.2.2.2 [span.cons], constructors, copy, and assignment
   >       constexpr span() noexcept;
   >       […]
   >       template<size_t N>
   >       constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
   >       […]
   >     };
   >     […]

2. Modify 23.7.2.2.2 [span.cons] as indicated:

   >     template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
   >     template<size_t N> constexpr span(array<value_type, N>& arr) noexcept;
   >     template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;

   > > -10- Constraints:

   > > 1. (10.1) — `extent == dynamic_extent || N == extent` is `true`, and

   > > 2. (10.2) — `remove_pointer_t<decltype(data(arr))>(*)[]` is convertible to `ElementType(*)[]`.

   > > -11- Effects: Constructs a `span` that is a view over the supplied array. [Note: `type_identity_t` affects class template argument deduction. — end note]

   > > -12- Postconditions: `size() == N && data() == data(arr)`.

### 3371(i). `visit_format_arg` and `make_format_args` are not hidden friends

Section: 28.5.8.1 [format.arg] Status: C++20 Submitter: Tim Song Opened: 2020-01-16 Last modified: 2021-02-25

Priority: 0

View all other issues in [format.arg].

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Discussion:

After P1965R0, friend function and function template declarations always introduce hidden friends under the new blanket wording in 16.4.6.6 [hidden.friends]. However, 28.5.8.1 [format.arg] contains "exposition only" friend declarations of `visit_format_arg` and `make_format_args`, and those are not intended to be hidden. The only reason to have these declarations in the first place is because these function templates are specified using the exposition-only private data members of `basic_format_arg`, but that's unnecessary — for example, `shared_ptr`'s constructors are not exposition-only friends of `enable_shared_from_this`, even though the former are shown as assigning to the latter's exposition-only `weak_this` private data member (see 20.3.2.2.2 [util.smartptr.shared.const]p1).

[2020-02-01 Status set to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Edit 28.5.8.1 [format.arg], class template `basic_format_arg` synopsis, as indicated:

   >     namespace std {
   >       template<class Context>
   >       class basic_format_arg {

   >         […]

   >         template<class Visitor, class Ctx>
   >           friend auto visit_format_arg(Visitor&& vis,
   >                                        basic_format_arg<Ctx> arg);                  // exposition only

   >         template<class Ctx, class... Args>
   >           friend format-arg-store<Ctx, Args...>
   >             make_format_args(const Args&... args);                                  // exposition only

   >         […]
   >       };
   >     }

### 3372(i). `vformat_to` should not try to deduce `Out` twice

Section: 28.5.5 [format.functions] Status: C++20 Submitter: Tim Song Opened: 2020-01-16 Last modified: 2021-02-25

Priority: 0

View all other issues in [format.functions].

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Discussion:

`vformat_to` currently deduces `Out` from its first and last arguments. This requires its last argument's type to be a specialization of `basic_format_args`, which notably prevents the use of `format-arg-store` arguments directly. This is unnecessary: we should only deduce from the first argument.

[2020-02-01 Status set to Tentatively Ready after six positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Edit 28.5.1 [format.syn], header `<format>` synopsis, as indicated:

   >     namespace std {

   >       […]

   >       template<class Out>
   >         Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args);
   >       template<class Out>
   >         Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args);
   >       template<class Out>
   >         Out vformat_to(Out out, const locale& loc, string_view fmt,
   >                        format_args_t<type_identity_t<Out>, char> args);
   >       template<class Out>
   >         Out vformat_to(Out out, const locale& loc, wstring_view fmt,
   >                        format_args_t<type_identity_t<Out>, wchar_t> args);

   >       […]
   >     }

2. Edit 28.5.5 [format.functions] p8 through p10 as indicated:

   >     template<class Out, class... Args>
   >       Out format_to(Out out, string_view fmt, const Args&... args);
   >     template<class Out, class... Args>
   >       Out format_to(Out out, wstring_view fmt, const Args&... args);

   > > -8- Effects: Equivalent to:

   > > >     using context = basic_format_context<Out, decltype(fmt)::value_type>;
   > > >     return vformat_to(out, fmt, {make_format_args<context>(args...)});

   >     template<class Out, class... Args>
   >       Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args);
   >     template<class Out, class... Args>
   >       Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);

   > > -9- Effects: Equivalent to:

   > > >     using context = basic_format_context<Out, decltype(fmt)::value_type>;
   > > >     return vformat_to(out, loc, fmt, {make_format_args<context>(args...)});

   >       template<class Out>
   >         Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args);
   >       template<class Out>
   >         Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args);
   >       template<class Out>
   >         Out vformat_to(Out out, const locale& loc, string_view fmt,
   >                        format_args_t<type_identity_t<Out>, char> args);
   >       template<class Out>
   >         Out vformat_to(Out out, const locale& loc, wstring_view fmt,
   >                        format_args_t<type_identity_t<Out>, wchar_t> args);

   > > -10- Let `charT` be `decltype(fmt)::value_type`.

   > > […]

### 3373(i). `{to,from}_chars_result` and `format_to_n_result` need the "we really mean what we say" wording

Section: 28.2.1 [charconv.syn], 28.5.1 [format.syn] Status: C++20 Submitter: Tim Song Opened: 2020-01-16 Last modified: 2021-02-25

Priority: 0

View all other issues in [charconv.syn].

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Discussion:

To ensure that `to_chars_result`, `from_chars_result`, and `format_to_n_result` can be used in structured bindings, they need the special wording we use to negate the general library permission to add private data members and bases.

[2020-02-01 Status set to Tentatively Ready after six positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Add a paragraph at the end of 28.2.1 [charconv.syn] as follows:

   > -?- The types `to_chars_result` and `from_chars_result` have the data members and special members specified above. They have no base classes or members other than those specified.

2. Add a paragraph at the end of 28.5.1 [format.syn] as follows:

   > -1- The class template `format_to_n_result` has the template parameters, data members, and special members specified above. It has no base classes or members other than those specified.

### 3374(i). P0653 + P1006 should have made the other `std::to_address` overload `constexpr`

Section: 20.2.4 [pointer.conversion] Status: C++20 Submitter: Billy O'Neal III Opened: 2020-01-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [pointer.conversion].

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Discussion:

While reviewing some interactions with P0653 + P1006, Billy discovered that one of the overloads was missing the `constexpr` tag. This might be a typo or a missed merge interaction between P0653 (which adds `to_address` with the pointer overload being `constexpr`) and P1006 (which makes `pointer_traits::pointer_to constexpr`). Mail was sent the LWG reflector, and Glen Fernandes, the author of P0653, indicates that this might have been an oversight.

[2020-02-01 Status set to Tentatively Ready after seven positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     […]
   >     // 20.2.4 [pointer.conversion], pointer conversion
   >     template<class T>
   >       constexpr T* to_address(T* p) noexcept;
   >     template<class Ptr>
   >       constexpr auto to_address(const Ptr& p) noexcept;
   >     […]

2. Modify 20.2.4 [pointer.conversion] as indicated:

   >     template<class Ptr> constexpr auto to_address(const Ptr& p) noexcept;

   > > -3- Returns: `pointer_traits<Ptr>::to_address(p)` if that expression is well-formed (see 20.2.3.4 [pointer.traits.optmem]), otherwise `to_address(p.operator->())`.

### 3375(i). `decay` in `viewable_range` should be `remove_cvref`

Section: 25.4.6 [range.refinements] Status: C++20 Submitter: Casey Carter Opened: 2020-01-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [range.refinements].

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Discussion:

The viewable_range concept is defined in 25.4.6 [range.refinements] as:

>     template<class T>
>       concept viewable_range =
>         range<T> && (safe_range<T> || view<decay_t<T>>);

Since neither pointer types, array types, nor function types model `view`, `view<decay_t<T>>` here could simplified to `view<remove_cvref_t<T>>`. The use of `decay_t` is an artifact of the Ranges TS being based on C++14 which didn't have `remove_cvref_t`. [Note that the proposed change is not purely editorial since the difference is observable to subsumption.]

[2020-02-01 Status set to Tentatively Ready after five positive votes on the reflector.]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4842.

> 1. Modify 25.4.6 [range.refinements] as indicated:

>    > -4- The `viewable_range` concept specifies the requirements of a `range` type that can be converted to a `view` safely.

>    >     template<class T>
>    >       concept viewable_range =
>    >         range<T> && (safe_range<T> || view<decay_tremove_cvref<T>>);

[2020-02-06 Casey provides a corrected P/R]

... in response to Jonathan's observation that `remove_cvref<T>` is both the wrong type and not what the discussion argues for.

[2020-02 Status to Immediate on Thursday morning in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.4.6 [range.refinements] as indicated:

   > -4- The `viewable_range` concept specifies the requirements of a `range` type that can be converted to a `view` safely.

   >     template<class T>
   >       concept viewable_range =
   >         range<T> && (safe_range<T> || view<decay_tremove_cvref_t<T>>);

### 3376(i). "integer-like class type" is too restrictive

Section: 24.3.4.4 [iterator.concept.winc] Status: Resolved Submitter: Jonathan Wakely Opened: 2020-01-16 Last modified: 2021-10-23

Priority: 3

View other active issues in [iterator.concept.winc].

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Discussion:

24.3.4.4 [iterator.concept.winc] says:

> A type `I` is an integer-class type if it is in a set of implementation-defined class types that behave as integer types do, as defined in below.

and

> A type `I` is integer-like if it models `integral<I>` or if it is an integer-class type.

Some implementations support built-in integer types that do not necessarily model `std::integral`, e.g. with libstdc++ whether `std::is_integral_v<__int128>` is `true` depends whether "strict" or "extensions" mode is in use. Because `__int128` is not a class type, it can't be used as an integer-like type in strict mode (which effectively means it can't be used at all, to avoid unwanted ABI differences between modes).

The requirement should be relaxed to permit non-class types which are integer-like but not one of the standard integer types (nor extended integer types). If we do that, the name "integer-like class type" should probably change.

[2020-02-08 Issue Prioritization]

Priority to 3 after reflector discussion.

[2021-10-23 Resolved by the adoption of P2393R1 at the October 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

### 3377(i). `elements_view::iterator` befriends a specialization of itself

Section: 25.7.23.3 [range.elements.iterator] Status: C++20 Submitter: Casey Carter Opened: 2020-01-18 Last modified: 2021-02-25

Priority: 0

View other active issues in [range.elements.iterator].

View all other issues in [range.elements.iterator].

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Discussion:

The synopsis of the exposition-only class template `elements_view::iterator` in 25.7.23.3 [range.elements.iterator] includes the declaration "`friend iterator<!Const>;`". We typically don't depict such friend relationships in the Library specification, leaving the choice of how to implement access to private data from external sources to implementer magic. For consistency, we should strike this occurrence from `elements_view::iterator`.

[2020-02-08 Status set to Tentatively Ready after ten positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Modify 25.7.23.3 [range.elements.iterator], class template `elements_view::iterator` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<class V, size_t N>
   >       template<bool Const>
   >       class elements_view<V, N>::iterator { // exposition only
   >         using base_t = conditional_t<Const, const V, V>;
   >         friend iterator<!Const>;

   >         iterator_t<base_t> current_;
   >       public:
   >         […]
   >       };
   >       […]
   >     }

### 3379(i). "`safe`" in several library names is misleading

Section: 25.2 [ranges.syn] Status: C++20 Submitter: Casey Carter Opened: 2020-01-21 Last modified: 2021-02-25

Priority: 1

View other active issues in [ranges.syn].

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Discussion:

Various WG21 members have commented that the use of "safe" in the names `safe_range`, `enable_safe_range`, `safe_iterator_t`, and `safe_subrange_t` to mean "fairly unlikely to produce dangling iterators" is inappropriate. The term "safe" has very strong connotations in some application domains, and these names don't intend such connotations. We could avoid confusion by changing these names to avoid the use of "safe". The Ranges Illuminati has deemed "borrowed" to be more appropriate: it reflects the fact that such ranges often "borrow" iterators from an adapted range which allows them to not be lifetime-bound to the adapting range.

[2020-02-08 Issue Prioritization]

Priority to 1 after reflector discussion. This issue needs to be looked at by LEWG.

[2020-02-13, Prague]

Set to Immediate.

Proposed resolution:

This wording is relative to N4849.

1. Replace all occurrences of `safe_range`, `enable_safe_range`, `safe_iterator_t`, and `safe_subrange_t` in the working draft with `borrowed_range`, `enable_borrowed_range`, `borrowed_iterator_t`, and `borrowed_subrange_t`.

### 3380(i). `common_type` and comparison categories

Section: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Casey Carter Opened: 2020-01-23 Last modified: 2021-02-25

Priority: 0

View all other issues in [meta.trans.other].

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Discussion:

There are two paragraphs in the the definition of `common_type`:

- Otherwise, if both `D1` and `D2` denote comparison category types (17.12.2.1 [cmp.categories.pre]), let `C` denote the common comparison type (11.10.3 [class.spaceship]) of `D1` and `D2`.

- Otherwise, if `decay_t<decltype(false ? declval<D1>() : declval<D2>())>` denotes a valid type, let `C` denote that type.

P1614R2 added the first bullet so that `common_type_t<strong_equality, T>` would be the same type as `common_comparison_category_t<strong_equality, T>`; other cases are correctly handled by the second (pre-existing) bullet. After application of P1959R0 in Belfast, `std::strong_equality` is no more. We can now strike the first bullet without changing the behavior of `common_type`.

[2020-02-08 Status set to Tentatively Ready after seven positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 21.3.8.7 [meta.trans.other] as indicated:

   > -3- Note A: For the `common_type` trait applied to a template parameter pack `T` of types, the member `type` shall be either defined or not present as follows:

   > 1. (3.1) — […]

   > 2. […]

   > 3. (3.3) — If `sizeof...(T)` is two, let the first and second types constituting `T` be denoted by `T1` and `T2`, respectively, and let `D1` and `D2` denote the same types as `decay_t<T1>` and `decay_t<T2>`, respectively.

   >    1. (3.3.1) — If `is_same_v<T1, D1>` is `false` or `is_same_v<T2, D2>` is `false`, let `C` denote the same type, if any, as `common_type_t<D1, D2>`.

   >    2. (3.3.2) — [Note: None of the following will apply if there is a specialization `common_type<D1, D2>`. — end note]

   >    3. (3.3.3) — Otherwise, if both `D1` and `D2` denote comparison category types (17.12.2.1 [cmp.categories.pre]), let `C` denote the common comparison type (11.10.3 [class.spaceship]) of `D1` and `D2`.

   >    4. (3.3.4) — Otherwise, if

   >       >     decay_t<decltype(false ? declval<D1>() : declval<D2>())>

   >       denotes a valid type, let `C` denote that type.

   >    5. (3.3.5) — […]

   > 4. […]

### 3381(i). `begin` and `data` must agree for `contiguous_range`

Section: 25.4.6 [range.refinements] Status: C++20 Submitter: Casey Carter Opened: 2020-01-25 Last modified: 2021-02-25

Priority: 0

View all other issues in [range.refinements].

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Discussion:

The definition of the `contiguous_range` concept in 25.4.6 [range.refinements]/2 requires that `ranges::data(r)` be valid for a `contiguous_range r`, but fails to impose the obvious semantic requirement that `to_address(ranges::begin(r)) == ranges::data(r)`. In other words, `data` and `begin` must agree so that `[begin(r), end(r))` and the counted range `data(r) + [0, size(r))` (this is the new "counted range" specification syntax per working draft issue 2932) denote the same sequence of elements.

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.4.6 [range.refinements] as indicated:

   > -2- `contiguous_range` additionally requires that the `ranges::data` customization point (25.3.14 [range.prim.data]) is usable with the range.

   >     template<class T>
   >       concept contiguous_range =
   >         random_access_range<T> && contiguous_iterator<iterator_t<T>> &&
   >         requires(T& t) {
   >           { ranges::data(t) } -> same_as<add_pointer_t<range_reference_t<T>>>;
   >         };

   > -?- Given an expression `t` such that `decltype((t))` is `T&`, `T` models `contiguous_range` only if `(to_address(ranges::begin(t)) == ranges::data(t))`.

   > -3- The `common_range` concept […]

### 3382(i). NTTP for `pair` and `array`

Section: 22.3.2 [pairs.pair], 23.3.3 [array] Status: C++20 Submitter: Barry Revzin Opened: 2020-01-27 Last modified: 2021-02-25

Priority: 2

View other active issues in [pairs.pair].

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Discussion:

We had this NB ballot issue, to ensure that `std::array` could be a NTTP. But after P1907, we still need some kind of wording to ensure that `std::array` (and also `std::pair`) have no extra private members or base classes.

This is similar to LWG 3373(i) — maybe we just need to add:

> The class template `pair`/`array` has the data members specified above. It has no base classes or data members other than those specified.

[2020-02 Prioritized as P2 Monday morning in Prague]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 22.3.2 [pairs.pair] as indicated:

>    > -1- Constructors and member functions of `pair` do not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.

>    > -2- The defaulted move and copy constructor, respectively, of `pair` is a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.

>    > -3- If `(is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>)` is `true`, then the destructor of `pair` is trivial.

>    > -?- The class template `pair` has the data members specified above. It has no base classes or data members other than those specified.

> 2. Modify 23.3.3.1 [array.overview] as indicated:

>    > -1- The header `<array>` defines a class template for storing fixed-size sequences of objects. An `array` is a contiguous container (23.2.2 [container.requirements.general]). An instance of `array<T, N>` stores `N` elements of type `T`, so that `size() == N` is an invariant.

>    > -2- An `array` is an aggregate (9.5.2 [dcl.init.aggr]) that can be list-initialized with up to `N` elements whose types are convertible to `T`.

>    > -3- An `array` meets all of the requirements of a container and of a reversible container (23.2 [container.requirements]), except that a default constructed `array` object is not empty and that `swap` does not have constant complexity. An `array` meets some of the requirements of a sequence container (23.2.4 [sequence.reqmts]). Descriptions are provided here only for operations on `array` that are not described in one of these tables and for operations where there is additional semantic information.

>    > -?- The class template `array` has the data members specified in subclauses 23.3.3.1 [array.overview] and 23.3.3.5 [array.zero]. It has no base classes or data members other than those specified.

>    > -4- […]

[2020-02-13, Prague]

Tim Song and Tomasz were trying to come up with general wording that could be reused for both `pair` and `array` (and other types). They suggest that if it should be in scope for C++20, it would be better to provide non-general wording for `pair` and `array` (that is easier to get right).

For completeness (and future wording) the generalized wording is included. The definition of structurally compatible with:

> The type `T` is structurally compatible with `subs`, if for the values `t1` and `t2` of type `T`:

> - `T` is a structural type (13.2 [temp.param]) if the types of subobjects of `t1` designated by `subs` are all structural types.

> - `t1` is template-argument-equivalent (13.6 [temp.type]) to `t2`, if and only if, for each subject designed by `subs`, the value of subobject of `t1` is template-argument-equivalent to the value of the correponding subobject of `t2`.

Then changes for `array`/`pair` would then look like:

> `pair<T, U>` is structurally compatible (<some-reference>) with `first` and `second`.

> `array<T, N>` is structurally compatible with its elements (if any).

[2020-02 Status to Immediate on Friday morning in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 22.3.2 [pairs.pair] as indicated:

   > -1- Constructors and member functions of `pair` do not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.

   > -2- The defaulted move and copy constructor, respectively, of `pair` is a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.

   > -3- If `(is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>)` is `true`, then the destructor of `pair` is trivial.

   > -?- `pair<T, U>` is a structural type (13.2 [temp.param]) if `T` and `U` are both structural types. Two values `p1` and `p2` of type `pair<T, U>` are template-argument-equivalent (13.6 [temp.type]) if and only if `p1.first` and `p2.first` are template-argument-equivalent and `p1.second` and `p2.second` are template-argument-equivalent.

2. Modify 23.3.3.1 [array.overview] as indicated:

   > -1- The header `<array>` defines a class template for storing fixed-size sequences of objects. An `array` is a contiguous container (23.2.2 [container.requirements.general]). An instance of `array<T, N>` stores `N` elements of type `T`, so that `size() == N` is an invariant.

   > -2- An `array` is an aggregate (9.5.2 [dcl.init.aggr]) that can be list-initialized with up to `N` elements whose types are convertible to `T`.

   > -3- An `array` meets all of the requirements of a container and of a reversible container (23.2 [container.requirements]), except that a default constructed `array` object is not empty and that `swap` does not have constant complexity. An `array` meets some of the requirements of a sequence container (23.2.4 [sequence.reqmts]). Descriptions are provided here only for operations on `array` that are not described in one of these tables and for operations where there is additional semantic information.

   > -?- `array<T, N>` is a structural type (13.2 [temp.param]) if `T` is a structural type. Two values `a1` and `a2` of type `array<T, N>` are template-argument-equivalent (13.6 [temp.type]) if and only if each pair of corresponding elements in `a1` and `a2` are template-argument-equivalent.

   > -4- […]

### 3383(i). §[time.zone.leap.nonmembers] `sys_seconds` should be replaced with `seconds`

Section: 30.11.8.3 [time.zone.leap.nonmembers] Status: C++20 Submitter: Jiang An Opened: 2020-01-30 Last modified: 2021-02-25

Priority: 1

View all issues with C++20 status.

Discussion:

In N4849 30.11.8.3 [time.zone.leap.nonmembers]/12, the type template parameter `Duration` is constrained by `three_way_comparable_with<sys_seconds>`. However, since `std::chrono::sys_seconds` is a time point type and `Duration` must be a duration type, they can never be compared directly via `operator<=>`.

I guess that the actual intent is comparing `Duration` with the duration type of `std::chrono::sys_seconds`, i.e. `std::chrono::seconds`. And thus `sys_seconds` should be replaced with `seconds` here.

[2020-02 Prioritized as P1 Monday morning in Prague]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 30.2 [time.syn], header `<chrono>` synopsis, as indicated:

>    >     namespace std {
>    >       […]
>    >       namespace chrono {
>    >         […]
>    >         template<three_way_comparable_with<sys_seconds> Duration>
>    >           auto operator<=>(const leap& x, const sys_time<Duration>& y);
>    >         […]
>    >       }
>    >       […]
>    >     }

> 2. Modify 30.11.8.3 [time.zone.leap.nonmembers] as indicated:

>    >     template<three_way_comparable_with<sys_seconds> Duration>
>    >       constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;

>    > > -12- Returns: `x.date() <=> y`.

[2020-02-10, Prague; Howard suggests alternative wording]

The below shown alternative wording does more directly describe the constrained code (by comparing `time_point`s), even though in the very end the code specifying the effects finally goes down to actually return `x.date().time_since_epoch() <=> y.time_since_epoch()` (thus comparing `duration`s).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 30.2 [time.syn], header `<chrono>` synopsis, as indicated:

>    > The synopsis does provide an additional drive-by fix to eliminate the mismatch of the `constexpr` and `noexcept` in declaration and prototype specification.

>    >     namespace std {
>    >       […]
>    >       namespace chrono {
>    >         […]
>    >         template<three_way_comparable_with<sys_seconds>class Duration>
>    >           requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
>    >             constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;
>    >         […]
>    >       }
>    >       […]
>    >     }

> 2. Modify 30.11.8.3 [time.zone.leap.nonmembers] as indicated:

>    >     template<three_way_comparable_with<sys_seconds>class Duration>
>    >       requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
>    >         constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;

>    > > -12- Returns: `x.date() <=> y`.

[2020-02-11, Prague; Daniel suggests alternative wording]

During today's LWG discussion of this issue the observation was made that there also exists a mismatch regarding the `noexcept` specifier for both declarations, but for this second deviation a corresponding change does not seem to be a good drive-by fix candidate, because we have a function template here that allows supporting user-defined types, whose comparison may throw (Note that the corresponding `operator<=>` or other comparison function declarations of the `duration` and `time_point` templates are not specified as `noexcept` function templates). The revised wording below does therefore intentionally not change the currently existing `noexcept`-specifier mismatch, but a separate issue should instead be opened for the general `noexcept`-specifier mismatches for all comparison function templates of `std::chrono::leap`. Daniel has volunteered to take care for this issue.

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 30.2 [time.syn], header `<chrono>` synopsis, as indicated:

   > [Drafting note: The synopsis does provide an additional drive-by fix to eliminate the mismatch of the `constexpr` in declaration and prototype specification, but does not so for a similar mismatch of the exception-specifications of both declarations.]

   >     namespace std {
   >       […]
   >       namespace chrono {
   >         […]
   >         template<three_way_comparable_with<sys_seconds>class Duration>
   >           requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
   >             constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y);
   >         […]
   >       }
   >       […]
   >     }

2. Modify 30.11.8.3 [time.zone.leap.nonmembers] as indicated:

   >     template<three_way_comparable_with<sys_seconds>class Duration>
   >       requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
   >         constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;

   > > -12- Returns: `x.date() <=> y`.

### 3384(i). `transform_view::sentinel` has an incorrect `operator-`

Section: 25.7.9.4 [range.transform.sentinel] Status: C++20 Submitter: Ville Voutilainen Opened: 2020-01-31 Last modified: 2021-02-25

Priority: 0

View all other issues in [range.transform.sentinel].

View all issues with C++20 status.

Discussion:

1. `transform_view::iterator` has an exposition-only member `current_` (25.7.9.3 [range.transform.iterator])

2. `transform_view::sentinel` has an exposition-only member `end_` (25.7.9.4 [range.transform.sentinel])

3. at 25.7.9.4 [range.transform.sentinel]/6 we have:

    friend constexpr range_difference_t<Base>
      operator-(const sentinel& y, const iterator<Const>& x)
        requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

> Effects: Equivalent to: `return x.end_ - y.current_;`

`x` is an `iterator`, so it has `current_`, not `end_`. `y` is a `sentinel`, so it has `end_`, not `current_`.

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.7.9.4 [range.transform.sentinel] as indicated:

   >     friend constexpr range_difference_t<Base>
   >       operator-(const sentinel& y, const iterator<Const>& x)
   >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

   > > -6- Effects: Equivalent to: `return xy.end_ - yx.current_;`

### 3385(i). `common_iterator` is not sufficiently constrained for non-copyable iterators

Section: 24.5.5.1 [common.iterator] Status: C++20 Submitter: Corentin Jabot Opened: 2020-01-31 Last modified: 2021-02-25

Priority: 0

View other active issues in [common.iterator].

View all other issues in [common.iterator].

View all issues with C++20 status.

Discussion:

We don't actually specify anywhere that non-copyable iterators do not have a `common_iterator` (and it would make little sense for them to as algorithms dealing with C++17 iterators are not expecting non-copyable things) As it stands `common_iterator` can be created from move only iterator but are then non-copyable themselves. P1862 already constrains `common_view` in a similar fashion

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 24.5.5 [iterators.common], common iterators
   >       template<input_or_output_iterator I, sentinel_for<I> S>
   >         requires (!same_as<I, S> && copyable<I>)
   >           class common_iterator;
   >       […]
   >     }

2. Modify 24.5.5.1 [common.iterator], class template `common_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<input_or_output_iterator I, sentinel_for<I> S>
   >         requires (!same_as<I, S> && copyable<I>)
   >       class common_iterator {
   >       public:
   >         […]
   >       };
   >       […]
   >     }

### 3386(i). `elements_view` needs its own `sentinel` type

Section: 25.7.23 [range.elements] Status: C++20 Submitter: Tim Song Opened: 2020-02-07 Last modified: 2021-02-25

Priority: 1

View all issues with C++20 status.

Discussion:

`elements_view` is effectively a specialized version of `transform_view`. The latter has a custom `sentinel` type, and so should `elements_view`.

In particular, it should not use the underlying range's sentinel directly, for that sentinel could encode a generic predicate that is equally meaningful for the adapted range. Consider a range `[i, s)` whose `value_type` is `pair<array<int, 2>, long>`, where `s` is a generic sentinel that checks if the second element (for this range in particular, the `long`) is zero:

>     struct S {
>       friend bool operator==(input_iterator auto const& i, S) /* additional constraints */
>       { return get<1>(*i) == 0; }
>     };

If we adapt `[i, s)` with `views::keys`, then the resulting adapted range would have surprising behavior when used with `S{}`: even though it's nominally a range of `array<int, 2>`, when its iterator is used with the sentinel `S{}` it doesn't actually check the second element of the array, but the `long` that's not even part of the `value_type`:

>     void algo(input_range auto&& r) /* constraints */{
>       // We want to stop at the first element of the range r whose second element is zero.
>       for (auto&& x : subrange{ranges::begin(r), S{}})
>       {
>         std::cout << get<0>(x);
>       }
>     }

>     using P = pair<array<int, 2>, long>;
>     vector<P> vec =  {
>         { {0, 1}, 1L },
>         { {1, 0}, 1L },
>         { {2, 2}, 0L }
>     };

>     subrange r{vec.begin(), S{}};              // range with two elements: {0, 1}, {1, 0}

>     algo(r | views::keys);                     // checks the long, prints '01'
>     algo(r | views::transform(&P::first));     // checks the second element of the array, prints '0'

This is an API break since it changes the return type of `end()`, so it should be fixed before we ship C++20.

[2020-02 Prioritized as P1 Monday morning in Prague]

[2020-06-11 Voted into the WP in Prague. Status changed: New → WP.]

Proposed resolution:

The proposed wording is contained in P1994R0.

### 3387(i). §[range.reverse.view] `reverse_view<V>` unintentionally requires `range<const V>`

Section: 25.7.21.2 [range.reverse.view] Status: C++20 Submitter: Patrick Palka Opened: 2020-02-04 Last modified: 2021-02-25

Priority: 0

View other active issues in [range.reverse.view].

View all other issues in [range.reverse.view].

View all issues with C++20 status.

Discussion:

`reverse_view<V>` requires `bidirectional_range<V>`, but not `range<const V>`, which means that `iterator_t<const V>` might be an invalid type. The return types of the `begin() const` and `end() const` overloads make use of `iterator_t<const V>` in a non-SFINAE context, which means that instantiating `reverse_view<X>` is ill-formed unless `range<const X>` is satisfied.

Code like `x | views::filter(p) | views::reverse` fails to compile because `const filter_view<…>` does not model `range`, so `iterator_t<const filter_view<…>>` is invalid.

Either `range<const V>` needs to be in the class' `requires`-clause, or the return types of the `const`-qualified `begin()` and `end()` need to delay use of `iterator_t<const V>` until `range<const V>` is known to be satisfied.

Giving these overloads an `auto` return type means the type is determined when the member is called. The constraint `common_range<const V>` appropriately restricts the selection of these overloads, so they can only be called when the type is valid. This is what `cmcstl2` does. `range-v3` makes the `begin() const` and `end() const` members into function templates, so that they are SFINAE contexts. This is related to 3347(i).

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.7.21.2 [range.reverse.view] as indicated:

   > >     namespace std::ranges {
   > >       template<view V>
   > >         requires bidirectional_range<V>
   > >       class reverse_view : public view_interface<reverse_view<V>> {
   > >         […]
   > >         constexpr reverse_iterator<iterator_t<V>> begin();
   > >         constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
   > >         constexpr reverse_iterator<iterator_t<const V>>auto begin() const
   > >           requires common_range<const V>;

   > >         constexpr reverse_iterator<iterator_t<V>> end();
   > >         constexpr reverse_iterator<iterator_t<const V>>auto end() const
   > >           requires common_range<const V>;
   > >         […]
   > >       };
   > >       […]
   > >     }

   > > […]

   > >     constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
   > >     constexpr reverse_iterator<iterator_t<const V>>auto begin() const
   > >       requires common_range<const V>;

   > > > -5- Effects: Equivalent to: `return make_reverse_iterator(ranges::end(base_));`

   > >     constexpr reverse_iterator<iterator_t<V>> end();
   > >     constexpr reverse_iterator<iterator_t<const V>>auto end() const
   > >       requires common_range<const V>;

   > > > -6- Effects: Equivalent to: `return make_reverse_iterator(ranges::begin(base_));`

### 3388(i). `view` iterator types have ill-formed `<=>` operators

Section: 25.6.4.3 [range.iota.iterator], 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++20 Submitter: Jonathan Wakely Opened: 2020-02-07 Last modified: 2021-02-25

Priority: 0

View other active issues in [range.iota.iterator].

View all other issues in [range.iota.iterator].

View all issues with C++20 status.

Discussion:

25.6.4.3 [range.iota.iterator] and 25.7.9.3 [range.transform.iterator] and 25.7.23.3 [range.elements.iterator] all declare `operator<=>` similar to this:

>     friend constexpr compare_three_way_result_t<W> operator<=>(
>         const iterator& x, const iterator& y)
>       requires totally_ordered<W> && three_way_comparable<W>;

Similar to issue 3347(i) and issue 3387(i), this is ill-formed if `three_way_comparable<W>` is not satisfied, because `compare_three_way_result_t<W>` is invalid. This declaration is instantiated when the enclosing iterator type is instantiated, making any use of `iota_view<W, B>::iterator` ill-formed when `three_way_comparable<W>` is not satisfied.

We can either add an exposition-only `safe-compare-three-way-result-t` alias that denotes `void` or `std::nonesuch` for spaceship-less types, so the declaration is valid (and then disabled by the constraints), or simply make them return `auto`.

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.6.4.3 [range.iota.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<class W, class Bound>
   > >       struct iota_view<W, Bound>::iterator {
   > >         […]
   > >         friend constexpr compare_three_way_result_t<W>auto operator<=>(
   > >             const iterator& x, const iterator& y)
   > >           requires totally_ordered<W> && three_way_comparable<W>;
   > >         […]
   > >       };
   > >       […]
   > >     }

   > > […]

   > >     friend constexpr compare_three_way_result_t<W>auto
   > >       operator<=>(const iterator& x, const iterator& y)
   > >         requires totally_ordered<W> && three_way_comparable<W>;

   > > > -19- Effects: Equivalent to: `return x.value_ <=> y.value_;`

2. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<class V, class F>
   > >       template<bool Const>
   > >       class transform_view<V, F>::iterator {
   > >         […]
   > >         friend constexpr compare_three_way_result_t<iterator_t<Base>>auto
   > >           operator<=>(const iterator& x, const iterator& y)
   > >             requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;
   > >         […]
   > >       };
   > >       […]
   > >     }

   > > […]

   > >     friend constexpr compare_three_way_result_t<iterator_t<Base>>auto
   > >       operator<=>(const iterator& x, const iterator& y)
   > >         requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;

   > > > -19- Effects: Equivalent to: `return x.current_ <=> y.current_;`

3. Modify 25.7.23.3 [range.elements.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<class V, size_t N>
   > >       template<bool Const>
   > >       class elements_view<V, N>::iterator {
   > >         […]
   > >         friend constexpr compare_three_way_result_t<iterator_t<base-t>>auto
   > >           operator<=>(const iterator& x, const iterator& y)
   > >             requires random_access_range<base-t> && three_way_comparable<iterator_t<base-t>>;
   > >         […]
   > >       };
   > >       […]
   > >     }

   > > […]

   > >     friend constexpr compare_three_way_result_t<iterator_t<base-t>>auto
   > >       operator<=>(const iterator& x, const iterator& y)
   > >         requires random_access_range<base-t> && three_way_comparable<iterator_t<base-t>>;

   > > > -18- Effects: Equivalent to: `return x.current_ <=> y.current_;`

### 3389(i). A move-only iterator still does not have a `counted_iterator`

Section: 24.5.7.2 [counted.iter.const] Status: C++20 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2021-02-25

Priority: 0

View all issues with C++20 status.

Discussion:

P1207R4 ("Movability of single-pass iterators") introduces the notion of a move-only non-forward iterator and makes some changes to the iterator adaptor `counted_iterator` in order to support move-only iterators.

The problem is that the constructor of `counted_iterator` (24.5.7.2 [counted.iter.const] p2) accepting such an iterator is specified as "Initializes `current` with `i`" which would attempt copy-constructing `current` from `i` instead of move-constructing it.

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 24.5.7.2 [counted.iter.const] as indicated:

   >     constexpr counted_iterator(I i, iter_difference_t<I> n);

   > > -1- Preconditions: `n >= 0`.

   > > -2- Effects: Initializes `current` with `std::move(i)` and `length` with `n`.

### 3390(i). `make_move_iterator()` cannot be used to construct a `move_iterator` for a move-only iterator

Section: 24.5.4.9 [move.iter.nonmember] Status: C++20 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2021-02-25

Priority: 0

View all other issues in [move.iter.nonmember].

View all issues with C++20 status.

Discussion:

P1207R4 ("Movability of single-pass iterators") introduces the notion of a move-only non-forward iterator and makes some changes to the existing specification to realize that support.

The problem is that the specification of `make_move_iterator()` provided in 24.5.4.9 [move.iter.nonmember] p6 does attempt to construct a `move_iterator<Iterator>` with an lvalue of `i` instead of an rvalue, having the effect of copying it instead of moving it, thus preventing to accept move-only iterators.

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 24.5.4.9 [move.iter.nonmember] as indicated:

   >     template<class Iterator>
   >     constexpr move_iterator<Iterator> make_move_iterator(Iterator i);

   > > -6- Returns: `move_iterator<Iterator>(std::move(i))`.

### 3391(i). Problems with `counted_iterator/move_iterator::base() const &`

Section: 24.5.4 [move.iterators], 24.5.7 [iterators.counted] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2023-11-22

Priority: 2

View all other issues in [move.iterators].

View all issues with C++23 status.

Discussion:

It is not possible to use the `const &` overloads of `counted_iterator::base()` or `move_iterator::base()` to get at an underlying move-only iterator in order to compare it against a sentinel.

More concretely, assuming issue LWG 3389(i) is fixed, this means that

>     auto v = r | views::take(5);
>     ranges::begin(v) == ranges::end(v);

is invalid when `r` is a `view` whose `begin()` is a move-only input iterator. The code is invalid because `ranges::take_view::sentinel::operator==()` must call `counted_iterator::base()` to compare the underlying iterator against its sentinel, and therefore this `operator==()` requires that the underlying iterator is `copy_constructible`.

Suggested resolution:

Make these `const & base()` overloads return the underlying iterator by `const` reference. Remove the `copy_constructible` constraint on these overloads. Perhaps the `base()` overloads for the iterator wrappers in 25.7 [range.adaptors] could use the same treatment?

[2020-02 Prioritized as P2 Monday morning in Prague]

[2021-01-28; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 24.5.4.2 [move.iterator], class template `move_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<class Iterator>
   >       class move_iterator {
   >       public:
   >         using iterator_type = Iterator;
   >         […]
   >         constexpr const iterator_type& base() const &;
   >         constexpr iterator_type base() &&;
   >         […]
   >       };
   >     }

2. Modify 24.5.4.5 [move.iter.op.conv] as indicated:

   >     constexpr const Iterator& base() const &;

   > > -1- Constraints: `Iterator` satisfies `copy_constructible`.

   > > -2- Preconditions: `Iterator` models `copy_constructible`.

   > > -3- Returns: `current`.

3. Modify 24.5.7.1 [counted.iterator], class template `counted_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<input_or_output_iterator I>
   >       class counted_iterator {
   >       public:
   >         using iterator_type = I;
   >         […]
   >         constexpr const I& base() const & requires copy_constructible<I>;
   >         constexpr I base() &&;
   >         […]
   >       };
   >     }

4. Modify 24.5.7.3 [counted.iter.access] as indicated:

   >     constexpr const I& base() const & requires copy_constructible<I>;

   > > -1- Effects: Equivalent to: `return current;`

### 3392(i). `ranges::distance()` cannot be used on a move-only iterator with a sized sentinel

Section: 24.4.4.3 [range.iter.op.distance] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2023-11-22

Priority: 3

View all other issues in [range.iter.op.distance].

View all issues with C++23 status.

Discussion:

One cannot use `ranges::distance(I, S)` to compute the distance between a move-only `counted_iterator` and the `default_sentinel`. In other words, the following is invalid

>     // iter is a counted_iterator with an move-only underlying iterator
>     ranges::distance(iter, default_sentinel);

and yet

>     (default_sentinel - iter);

is valid. The first example is invalid because `ranges::distance()` takes its first argument by value so when invoking it with an iterator lvalue argument the iterator must be copyable, which a move-only iterator is not. The second example is valid because `counted_iterator::operator-()` takes its iterator argument by `const` reference, so it doesn't require copyability of the `counted_iterator`.

This incongruency poses an inconvenience in generic code which uses `ranges::distance()` to efficiently compute the distance between two iterators or between an iterator-sentinel pair. Although it's a bit of an edge case, it would be good if `ranges::distance()` does the right thing when the iterator is a move-only lvalue with a sized sentinel.

If this is worth fixing, one solution might be to define a separate overload of `ranges::distance(I, S)` that takes its arguments by `const` reference, as follows.

[2020-02 Prioritized as P3 and LEWG Monday morning in Prague]

[2020-05-28; LEWG issue reviewing]

LEWG issue processing voted to accept the direction of 3392. Status change to Open.

>     Accept the direction of LWG3392

>     SF F N A SA
>     14 6 0 0 0

Previous resolution [SUPERSEDED]:

> 1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

>    >     #include <concepts>

>    >     namespace std {
>    >       […]
>    >       // 24.4.4 [range.iter.ops], range iterator operations
>    >       namespace ranges {
>    >         […]
>    >         // 24.4.4.3 [range.iter.op.distance], ranges::distance
>    >         template<input_or_output_iterator I, sentinel_for<I> S>
>    >           requires (!sized_sentinel_for<S, I>)
>    >             constexpr iter_difference_t<I> distance(I first, S last);
>    >         template<input_or_output_iterator I, sized_sentinel_for<I> S>
>    >           constexpr iter_difference_t<I> distance(const I& first, const S& last);
>    >         template<range R>
>    >           constexpr range_difference_t<R> distance(R&& r);
>    >         […]
>    >       }
>    >       […]
>    >     }

> 2. Modify 24.4.4.3 [range.iter.op.distance] as indicated:

>    >     template<input_or_output_iterator I, sentinel_for<I> S>
>    >       requires (!sized_sentinel_for<S, I>)
>    >         constexpr iter_difference_t<I> ranges::distance(I first, S last);

>    > > -1- Preconditions: `[first, last)` denotes a range, or `[last, first)` denotes a range and `S` and `I` model `same_as<S, I> && sized_sentinel_for<S, I>`.

>    > > -2- Effects: If `S` and `I` model `sized_sentinel_for<S, I>`, returns `(last - first)`; otherwise, rReturns the number of increments needed to get from `first` to `last`.

>    >     template<input_or_output_iterator I, sized_sentinel_for<I> S>
>    >       constexpr iter_difference_t<I> ranges::distance(const I& first, const S& last);

>    > > -?- Preconditions: `S` and `I` model `sized_sentinel_for<S, I>` and either:

>    > > 1. (?.1) — `[first, last)` denotes a range, or

>    > > 2. (?.2) — `[last, first)` denotes a range and `S` and `I` model `same_as<S, I>`.

>    > > -? Effects: Returns `(last - first)`;

[2021-05-19 Tim updates wording]

The wording below removes the explicit precondition on the `sized_sentinel_for` overload of `distance`, relying instead on the semantic requirements of that concept and the "Effects: Equivalent to:" word of power. This also removes the potentially surprising inconsistency that given a non-empty `std::vector<int> v`, `ranges::distance(v.begin(), v.cend())` is well-defined but `ranges::distance(v.cend(), v.begin())` is currently undefined.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     […]

   >     namespace std {
   >       […]
   >       // 24.4.4 [range.iter.ops], range iterator operations
   >       namespace ranges {
   >         […]
   >         // 24.4.4.3 [range.iter.op.distance], ranges::distance
   >         template<input_or_output_iterator I, sentinel_for<I> S>
   >           requires (!sized_sentinel_for<S, I>)
   >             constexpr iter_difference_t<I> distance(I first, S last);
   >         template<input_or_output_iterator I, sized_sentinel_for<I> S>
   >           constexpr iter_difference_t<I> distance(const I& first, const S& last);
   >         template<range R>
   >           constexpr range_difference_t<R> distance(R&& r);
   >         […]
   >       }
   >       […]
   >     }

2. Modify 24.4.4.3 [range.iter.op.distance] as indicated:

   >     template<input_or_output_iterator I, sentinel_for<I> S>
   >       requires (!sized_sentinel_for<S, I>)
   >         constexpr iter_difference_t<I> ranges::distance(I first, S last);

   > > -1- Preconditions: `[first, last)` denotes a range, or `[last, first)` denotes a range and `S` and `I` model `same_as<S, I> && sized_sentinel_for<S, I>`.

   > > -2- Effects: If `S` and `I` model `sized_sentinel_for<S, I>`, returns `(last - first)`; otherwise, returns the Returns: The number of increments needed to get from `first` to `last`.

   >     template<input_or_output_iterator I, sized_sentinel_for<I> S>
   >       constexpr iter_difference_t<I> ranges::distance(const I& first, const S& last);

   > > -?- Effects: Equivalent to `return last - first;`

### 3393(i). Missing/incorrect feature test macro for coroutines

Section: 17.3.2 [version.syn] Status: C++20 Submitter: Barry Revzin Opened: 2020-01-25 Last modified: 2021-02-25

Priority: 0

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++20 status.

Discussion:

We have a policy, established in P1353 (and needing to be added to SD-6):

In some cases a feature requires two macros, one for the language and one for the library. For example, the library does not want to define its three-way comparison operations unless the compiler supports the feature.

For end-users, it is suggested that they test only the library macro, as that will only be true if the language macro is also true. As a result, the language macros contain "impl" to distinguish them from the more general version that is expected to be set by the library. That paper added two papers of macros: one for `<=>` and one for destroying `delete`. We have a third such example in coroutines: there is library machinery that needs to be provided only when the compiler has language support for it, and the end user should just check the library macro.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 15.12 [cpp.predefined], Table [tab:cpp.predefined.ft], as indicated:

>    > Table 18: Feature-test macros [tab:cpp.predefined.ft]
>    > Macro name | Value
>    > --- | ---
>    > `[…]`
>    > `__cpp_impl_coroutines` | `201902L`
>    > `[…]`

> 2. Modify 17.3.2 [version.syn], header `<version>` synopsis, as indicated:

>    >     […]
>    >     #define __cpp_lib_constexpr_vector    201907L // also in <vector>
>    >     #define __cpp_lib_coroutines          201902L // also in <coroutine>
>    >     #define __cpp_lib_destroying_delete   201806L // also in <new>
>    >     […]

[2020-02-11, Prague]

LWG observed that the naming suggestion didn't follow naming conventions of SG-10 because of the plural form `corountines`. The submitter agreed with that complaint, so the revised wording uses now the singular form.

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 15.12 [cpp.predefined], Table [tab:cpp.predefined.ft], as indicated:

   > Table 18: Feature-test macros [tab:cpp.predefined.ft]
   > Macro name | Value
   > --- | ---
   > `[…]`
   > `__cpp_impl_coroutines` | `201902L`
   > `[…]`

2. Modify 17.3.2 [version.syn], header `<version>` synopsis, as indicated:

   >     […]
   >     #define __cpp_lib_constexpr_vector    201907L // also in <vector>
   >     #define __cpp_lib_coroutine           201902L // also in <coroutine>
   >     #define __cpp_lib_destroying_delete   201806L // also in <new>
   >     […]

### 3395(i). Definition for three-way comparison needs to be updated (US 152)

Section: 99 [defns.comparison] Status: C++20 Submitter: Jeff Garland Opened: 2020-02-10 Last modified: 2021-02-25

Priority: 1

View all issues with C++20 status.

Discussion:

Addresses US 152

This definition in 99 [defns.comparison] should be updated to accommodate the new 3-way comparison operator (7.6.8 [expr.spaceship]) as well.

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 99 [defns.comparison] as indicated:

   > comparison function

   > operator function (12.4 [over.oper]) for any of the equality (7.6.10 [expr.eq]), or relational (7.6.9 [expr.rel]), or three-way comparison (7.6.8 [expr.spaceship]) operators

### 3396(i). Clarify point of reference for `source_location::current()` (DE 169)

Section: 17.8.2.2 [support.srcloc.cons] Status: C++20 Submitter: Jens Maurer Opened: 2020-02-13 Last modified: 2021-02-25

Priority: 2

View all other issues in [support.srcloc.cons].

View all issues with C++20 status.

Discussion:

Addresses DE 169

The expectation of the note that a default argument expression involving current() causes a source_location to be constructed that refers to the site of a function call where that default argument is needed has no basis in normative text. In particular, 9.2.3.6 paragraph 5 seems to imply that the name "current" and its semantics are bound where it appears lexically in the function declaration.

Proposed change:

Add normative text to express the desired semantics.

[2020-02 Moved to Immediate on Thursday afternoon in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 17.8.2.2 [support.srcloc.cons] as indicated:

   >     static consteval source_location current() noexcept;

   > > -1- […]

   > > -2- Remarks: When a default member initializer is used to initialize a non-static data member, any calls to `current` Any call to `current` that appears as a default member initializer (11.4 [class.mem]), or as a subexpression thereof, should correspond to the location of the constructor definition or aggregate initialization that initializes the memberuses the default member initializer. Any call to `current` that appears as a default argument (9.3.4.7 [dcl.fct.default]), or as a subexpression thereof, should correspond to the location of the invocation of the function that uses the default argument (7.6.1.3 [expr.call]).

   > > -3- [Note: When used as a default argument (9.3.4.7 [dcl.fct.default]), the value of the `source_location` will be the location of the call to `current` at the call site. — end note]

### 3397(i). `ranges::basic_istream_view::iterator` should not provide `iterator_category`

Section: 25.6.6.3 [range.istream.iterator] Status: C++20 Submitter: Tomasz Kamiński Opened: 2020-02-13 Last modified: 2021-02-25

Priority: 1

View all other issues in [range.istream.iterator].

View all issues with C++20 status.

Discussion:

The `ranges::basic_istream_view::iterator` is a move-only type, and as such it does not meets the Cpp17 iterator requirements, yet it does provides `iterator_category` (intended to be used for Cpp17 iterators). We should provide `iterator_concept` instead.

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.6.6.3 [range.istream.iterator] as indicated:

   >     namespace std::ranges {
   >       template<class Val, class CharT, class Traits>
   >       class basic_istream_view<Val, CharT, Traits>::iterator { // exposition only
   >       public:
   >         using iterator_categoryiterator_concept = input_iterator_tag;
   >         using difference_type = ptrdiff_t;
   >         using value_type = Val;

   >         iterator() = default;
   >         […]
   >       };
   >     }

### 3398(i). `tuple_element_t` is also wrong for `const subrange`

Section: 25.2 [ranges.syn] Status: C++20 Submitter: Casey Carter Opened: 2019-02-13 Last modified: 2021-02-25

Priority: 0

View other active issues in [ranges.syn].

View all other issues in [ranges.syn].

View all issues with C++20 status.

Discussion:

As currently specified, it uses the cv-qualified partial specialization, which incorrectly adds cv-qualification to the element type.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

>    >     […]
>    >     namespace std {
>    >       namespace views = ranges::views;

>    >       template<class I, class S, ranges::subrange_kind K>
>    >       struct tuple_size<ranges::subrange<I, S, K>>
>    >         : integral_constant<size_t, 2> {};
>    >       template<class I, class S, ranges::subrange_kind K>
>    >       struct tuple_element<0, ranges::subrange<I, S, K>> {
>    >         using type = I;
>    >       };
>    >       template<class I, class S, ranges::subrange_kind K>
>    >       struct tuple_element<1, ranges::subrange<I, S, K>> {
>    >         using type = S;
>    >       };
>    >       template<class I, class S, ranges::subrange_kind K>
>    >       struct tuple_element<0, const ranges::subrange<I, S, K>> {
>    >         using type = I;
>    >       };
>    >       template<class I, class S, ranges::subrange_kind K>
>    >       struct tuple_element<1, const ranges::subrange<I, S, K>> {
>    >         using type = S;
>    >       };
>    >     }

> 2. Add the following wording to Annex D:

>    > D.? Deprecated `subrange` tuple interface [depr.ranges.syn]

>    > 1 The header `<ranges>` (25.2 [ranges.syn]) has the following additions:

>    > >     namespace std {
>    > >       template<size_t X, class I, class S, ranges::subrange_kind K>
>    > >       struct tuple_element<X, volatile ranges::subrange<I, S, K>> {};
>    > >       template<size_t X, class I, class S, ranges::subrange_kind K>
>    > >       struct tuple_element<X, const volatile ranges::subrange<I, S, K>> {};
>    > >     }

[2020-02-14, Prague]

LWG decided to remove the volatile support, we shouldn't give the impression to support an idiom that wouldn't work.

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     […]
   >     namespace std {
   >       namespace views = ranges::views;

   >       template<class I, class S, ranges::subrange_kind K>
   >       struct tuple_size<ranges::subrange<I, S, K>>
   >         : integral_constant<size_t, 2> {};
   >       template<class I, class S, ranges::subrange_kind K>
   >       struct tuple_element<0, ranges::subrange<I, S, K>> {
   >         using type = I;
   >       };
   >       template<class I, class S, ranges::subrange_kind K>
   >       struct tuple_element<1, ranges::subrange<I, S, K>> {
   >         using type = S;
   >       };
   >       template<class I, class S, ranges::subrange_kind K>
   >       struct tuple_element<0, const ranges::subrange<I, S, K>> {
   >         using type = I;
   >       };
   >       template<class I, class S, ranges::subrange_kind K>
   >       struct tuple_element<1, const ranges::subrange<I, S, K>> {
   >         using type = S;
   >       };
   >     }

### 3403(i). Domain of `ranges::ssize(E)` doesn't match `ranges::size(E)`

Section: 25.3.11 [range.prim.ssize] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-02-19 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

`ranges::size(E)` works with a non-range for which `E.size()` or `size(E)` is valid. But `ranges::ssize(E)` requires the type `range_difference_t` which requires `ranges::begin(E)` to be valid. This means there are types for which `ranges::size(E)` is valid but `ranges::ssize(E)` is not.

Casey's reaction to this is:

I believe we want `ranges::ssize` to work with any argument that `ranges::size` accepts. That suggest to me that we're going to need `make-signed-like-t<T>` after all, so we can "Let `E` be an expression, and let `D` be the wider of `ptrdiff_t` or `decltype(ranges::size(E))`. Then `ranges::ssize(E)` is expression-equivalent to `static_cast<make-signed-like-t<D>>(ranges::size(E))`." Although this wording is still slightly icky since `D` isn't a valid type when `ranges::size(E)` isn't a valid expression, I think it's an improvement?

[2020-03-11 Issue Prioritization]

Priority to 2 after reflector discussion.

[2020-07-22 Casey provides wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Add a new paragraph after paragraph 1 in 25.2 [ranges.syn]:

>    > -?- Also within this clause, `make-signed-like-t<X>` for an integer-like type `X` denotes `make_signed_t<X>` if `X` is an integer type; otherwise, it denotes a corresponding unspecified signed-integer-like type of the same width as `X`.

> 2. Modify 25.3.11 [range.prim.ssize] as indicated:

>    > -1- The name `ranges::ssize` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::ssize(E)` for a subexpression `E` of type `T` is expression-equivalent to:

>    > (1.1) — If `range_difference_t<T>` has width less than `ptrdiff_t`, `static_cast<ptrdiff_t>(ranges::size(E))`.

>    > (1.2) — Otherwise, `static_cast<range_difference_t<T>>(ranges::size(E))`.

>    > -?- Given a subexpression `E` with type `T`, let `t` be an lvalue that denotes the reified object for `E`. If `ranges::size(t)` is ill-formed, `ranges::ssize(E)` is ill-formed. Otherwise, let `D` be the wider of `ptrdiff_t` or `decltype(ranges::size(t))`; `ranges::ssize(E)` is expression-equivalent to `static_cast<make-signed-like-t<D>>(ranges::size(t))`.

[2020-07-31 Casey provides updated wording]

Per discussion on the reflector.

[2020-08-21; Issue processing telecon: Tentatively Ready]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Add a new paragraph after paragraph 1 in 25.2 [ranges.syn]:

   > [Drafting note: The following does not define an analog `to-signed-like` for `to-unsigned-like` since we don't need it at this time.]

   > -?- Also within this Clause, `make-signed-like-t<X>` for an integer-like type `X` denotes `make_signed_t<X>` if `X` is an integer type; otherwise, it denotes a corresponding unspecified signed-integer-like type of the same width as `X`.

2. Modify 25.3.11 [range.prim.ssize] as indicated:

   > -1- The name `ranges::ssize` denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression `ranges::ssize(E)` for a subexpression `E` of type `T` is expression-equivalent to:

   > (1.1) — If `range_difference_t<T>` has width less than `ptrdiff_t`, `static_cast<ptrdiff_t>(ranges::size(E))`.

   > (1.2) — Otherwise, `static_cast<range_difference_t<T>>(ranges::size(E))`.

   > -?- Given a subexpression `E` with type `T`, let `t` be an lvalue that denotes the reified object for `E`. If `ranges::size(t)` is ill-formed, `ranges::ssize(E)` is ill-formed. Otherwise let `D` be `make-signed-like-t<decltype(ranges::size(t))>`, or `ptrdiff_t` if it is wider than that type; `ranges::ssize(E)` is expression-equivalent to `static_cast<D>(ranges::size(t))`.

### 3404(i). Finish removing `subrange`'s conversions from `pair-like`

Section: 25.5.4 [range.subrange] Status: C++23 Submitter: Casey Carter Opened: 2020-02-20 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.subrange].

View all issues with C++23 status.

Discussion:

Both LWG 3281(i) "Conversion from `pair-like` types to `subrange` is a silent semantic promotion" and LWG 3282(i) "`subrange` converting constructor should disallow derived to base conversions" removed `subrange`'s hated implicit conversions from `pair-like` types. Notably, neither issue removed the two "`iterator-sentinel-pair`" deduction guides which target the removed constructors nor the exposition-only `iterator-sentinel-pair` concept itself, all of which are now useless.

[2020-03-11 Issue Prioritization]

Status set to Tentatively Ready after seven positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.5.4 [range.subrange] as indicated:

   >     […]
   >     template<class T, class U, class V>
   >       concept pair-like-convertible-from = // exposition only
   >         !range<T> && pair-like<T> && constructible_from<T, U, V>;

   >     template<class T>
   >       concept iterator-sentinel-pair = // exposition only
   >         !range<T> && pair-like<T> &&
   >         sentinel_for<tuple_element_t<1, T>, tuple_element_t<0, T>>;

   >     […]

   >     template<iterator-sentinel-pair P>
   >       subrange(P) -> subrange<tuple_element_t<0, P>, tuple_element_t<1, P>>;

   >     template<iterator-sentinel-pair P>
   >       subrange(P, make-unsigned-like-t(iter_difference_t<tuple_element_t<0, P>>)) ->
   >         subrange<uple_element_t<0, P>, tuple_element_t<1, P>, subrange_kind::sized>;
   >     […]

### 3405(i). `common_view`'s converting constructor is bad, too

Section: 25.7.20.2 [range.common.view] Status: C++23 Submitter: Casey Carter Opened: 2020-02-20 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.common.view].

View all issues with C++23 status.

Discussion:

LWG 3280(i) struck the problematic/extraneous converting constructor templates from the `meow_view` range adaptor types in the standard library with the exception of `common_view`. The omission of `common_view` seems to have been simply an oversight: its converting constructor template is no less problematic or extraneous. We should remove `common_view`'s converting constructor template as well to finish the task. Both `cmcstl2` and `range-v3` removed the converting constructor template from `common_view` when removing the other converting constructor templates, so we have implementation experience that this change is good as well as consistent with the general thrust of LWG 3280(i).

[2020-03-11 Issue Prioritization]

Status set to Tentatively Ready after seven positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.7.20.2 [range.common.view], class template `common_view` synopsis, as indicated:

   >       […]
   >       constexpr explicit common_view(V r);

   >       template<viewable_range R>
   >         requires (!common_range<R> && constructible_from<V, all_view<R>>)
   >       constexpr explicit common_view(R&& r);

   >       constexpr V base() const& requires copy_constructible<V> { return base_; }
   >       […]

   > […]

   >     template<viewable_range R>
   >       requires (!common_range<R> && constructible_from<V, all_view<R>>)
   >     constexpr explicit common_view(R&& r);

   > > -2- Effects: Initializes `base_` with `views::all(std::forward<R>(r))`.

### 3406(i). `elements_view::begin()` and `elements_view::end()` have incompatible constraints

Section: 25.7.23.2 [range.elements.view] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-21 Last modified: 2023-11-22

Priority: 1

View all other issues in [range.elements.view].

View all issues with C++23 status.

Discussion:

P1994R1 (`elements_view` needs its own `sentinel`) introduces a distinct `sentinel` type for `elements_view`. In doing so, it replaces the two existing overloads of `elements_view::end()` with four new ones:

>     -    constexpr auto end() requires (!simple-view<V>)
>     -    { return ranges::end(base_); }
>     -
>     -    constexpr auto end() const requires simple-view<V>
>     -    { return ranges::end(base_); }

>     +    constexpr auto end()
>     +    { return sentinel<false>{ranges::end(base_)}; }
>     +
>     +    constexpr auto end() requires common_range<V>
>     +    { return iterator<false>{ranges::end(base_)}; }
>     +
>     +    constexpr auto end() const
>     +      requires range<const V>
>     +    { return sentinel<true>{ranges::end(base_)}; }
>     +
>     +    constexpr auto end() const
>     +      requires common_range<const V>
>     +    { return iterator<true>{ranges::end(base_)}; }

But now these new overloads of `elements_view::end()` have constraints that are no longer consistent with the constraints of `elements_view::begin()`:

>          constexpr auto begin() requires (!simple-view<V>)
>          { return iterator<false>(ranges::begin(base_)); }

>          constexpr auto begin() const requires simple-view<V>
>          { return iterator<true>(ranges::begin(base_)); }

This inconsistency means that we can easily come up with a view `V` for which `elements_view<V>::begin()` returns an `iterator<true>` and `elements_view<V>::end()` returns an `sentinel<false>`, i.e. incomparable things of opposite constness. For example:

>     tuple<int, int> x[] = {{0,0}};
>     ranges::subrange r = {counted_iterator(x, 1), default_sentinel};
>     auto v = r | views::elements<0>;
>     v.begin() == v.end(); // ill-formed

Here, overload resolution for `begin()` selects the `const` overload because the subrange `r` models `simple-view`. But overload resolution for `end()` selects the non-`const` non-`common_range` overload. Hence the last line of this snippet is ill-formed because it is comparing an iterator and sentinel of opposite constness, for which we have no matching `operator==` overload. So in this example `v` does not even model range because its `begin()` and `end()` are incomparable.

This issue can be resolved by making sure the constraints on `elements_view::begin()` and on `elements_view::end()` are consistent and compatible. The following proposed resolution seems to be one way to achieve that and takes inspiration from the design of `transform_view`.

[2020-04-04 Issue Prioritization]

Priority to 1 after reflector discussion.

[2020-07-17; telecon]

Should be considered together with 3448(i) and 3449(i).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849 after application of P1994R1.

> 1. Modify 25.7.23.2 [range.elements.view], class template `elements_view` synopsis, as indicated:

>    >     namespace std::ranges {
>    >       […]
>    >       template<input_range V, size_t N>
>    >         requires view<V> && has-tuple-element<range_value_t<V>, N> &&
>    >           has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
>    >       class elements_view : public view_interface<elements_view<V, N>> {
>    >       public:
>    >         […]
>    >         constexpr V base() && { return std::move(base_); }

>    >         constexpr auto begin() requires (!simple-view<V>)
>    >         { return iterator<false>(ranges::begin(base_)); }
>    >         constexpr auto begin() const requires simple-view<V>range<const V>
>    >         { return iterator<true>(ranges::begin(base_)); }
>    >         […]
>    >       };
>    >     }

[2020-06-05 Tim updates P/R in light of reflector discussions and LWG 3448(i) and comments]

The fact that, as currently specified, `sentinel<false>` is not comparable with `iterator<true>` is a problem with the specification of this comparison, as noted in LWG 3448(i). The P/R below repairs this problem along the lines suggested in that issue. The constraint mismatch does make this problem easier to observe for `elements_view`, but the mismatch is not independently a problem: since `begin` can only add constness on `simple-view`s for which constness is immaterial, whether `end` also adds constness or not ought not to matter.

However, there is a problem with the `begin` overload set: if `const V` is a range, but `V` is not a `simple-view`, then a `const elements_view<V, N>` has no viable `begin` at all (the simplest example of such non-simple-views is probably `single_view`). That's simply broken; the fix is to constrain the `const` overload of `begin` with just `range<const V>` instead of `simple-view<V>`. Notably, this is how many other uses of `simple-view` work already (see, e.g., `take_view` in 25.7.10.2 [range.take.view]).

The previous `simple-view` constraint on `end` served a useful purpose (when done correctly): it reduces template instantiations if the underlying view is const-agnostic. This was lost in P1994 because that paper modeled the overload set on `transform_view`; however, discussion with Eric Niebler confirmed that the reason `transform_view` doesn't have the `simple-view` optimization is because it would add constness to the callable object as well, which can make a material difference in the result. Such concerns are not present in `elements_view` where the "callable object" is effectively hard-coded into the type and unaffected by const-qualification. The revised P/R below therefore restores the `simple-view` optimization.

I have implemented this P/R together with P1994 on top of libstdc++ trunk and can confirm that no existing test cases were broken by these changes, and that it fixes the issue reported here as well as in LWG 3448 (as it relates to `elements_view`).

[2020-10-02; Status to Tentatively Ready after five positive votes on the reflector]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861. It assumes the `maybe-const` helper introduced by the P/R of LWG 3448(i).

1. Modify 25.7.23.2 [range.elements.view], class template `elements_view` synopsis, as indicated:

   >     namespace std::ranges {
   >       […]
   >       template<input_range V, size_t N>
   >         requires view<V> && has-tuple-element<range_value_t<V>, N> &&
   >           has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
   >       class elements_view : public view_interface<elements_view<V, N>> {
   >       public:
   >         […]
   >         constexpr V base() && { return std::move(base_); }

   >         constexpr auto begin() requires (!simple-view<V>)
   >         { return iterator<false>(ranges::begin(base_)); }

   >         constexpr auto begin() const requires simple-view<V>range<const V>
   >         { return iterator<true>(ranges::begin(base_)); }

   >         constexpr auto end() requires (!simple-view<V> && !common_range<V>)
   >         { return sentinel<false>{ranges::end(base_)}; }

   >         constexpr auto end() requires (!simple-view<V> && common_range<V>)
   >         { return iterator<false>{ranges::end(base_)}; }

   >         constexpr auto end() const requires range<const V>
   >         { return sentinel<true>{ranges::end(base_)}; }

   >         constexpr auto end() const requires common_range<const V>
   >         { return iterator<true>{ranges::end(base_)}; }
   >         […]
   >       };
   >     }

2. Modify 25.7.23.4 [range.elements.sentinel] as indicated:

   > [Drafting note: Unlike the current P/R of LWG 3448(i), this P/R also changes the return type of `operator-` to depend on the constness of iterator rather than that of the sentinel. This is consistent with `sized_sentinel_for<S, I>` (24.3.4.8 [iterator.concept.sizedsentinel]), which requires `decltype(i - s)` to be `iter_difference_t<I>`.]

   > >     namespace std::ranges {
   > >       template<input_range V, size_t N>
   > >         requires view<V> && has-tuple-element<range_value_t<V>, N> &&
   > >           has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
   > >       template<bool Const>
   > >       class elements_view<V, F>::sentinel {
   > >         […]
   > >         constexpr sentinel_t<Base> base() const;

   > >         template<bool OtherConst>
   > >           requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >         friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

   > >         template<bool OtherConst>
   > >           requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >         friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
   > >           operator-(const iterator<OtherConst>& x, const sentinel& y)
   > >             requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

   > >         template<bool OtherConst>
   > >           requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >         friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
   > >           operator-(const sentinel& x, const iterator<OtherConst>& y)
   > >             requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
   > >       };
   > >     }

   > […]

   >     template<bool OtherConst>
   >       requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

   > > -4- Effects: Equivalent to: `return x.current_ == y.end_;`

   >     template<bool OtherConst>
   >       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
   >       operator-(const iterator<OtherConst>& x, const sentinel& y)
   >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

   > > -5- Effects: Equivalent to: `return x.current_ - y.end_;`

   >     template<bool OtherConst>
   >       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
   >       operator-(const sentinel& x, const iterator<OtherConst>& y)
   >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

   > > -6- Effects: Equivalent to: `return x.end_ - y.current_;`

### 3407(i). Some problems with the wording changes of P1739R4

Section: 25.7.10.1 [range.take.overview], 25.6.4 [range.iota] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-21 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.take.overview].

View all issues with C++23 status.

Discussion:

Section 6.1 of P1739R4 changes the specification of `views::take` as follows:

> -2- The name `views::take` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given subexpressions `E` and `F`, the expression `views::take(E, F)` is expression-equivalent to `take_view{E, F}`. Let `E` and `F` be expressions, let `T` be `remove_cvref_t<decltype((E))>`, and let `D` be `range_difference_t<decltype((E))>`. If `decltype((F))` does not model `convertible_to<D>`, `views::take(E, F)` is ill-formed. Otherwise, the expression `views::take(E, F)` is expression-equivalent to:

> 1. — if `T` is a specialization of `ranges::empty_view` (25.6.2.2 [range.empty.view]), then `((void) F, decay-copy(E))`;

> 2. — otherwise, if `T` models `random_access_range` and `sized_range` and is

>    1. — a specialization of `span` (23.7.2.2 [views.span]) where `T::extent == dynamic_extent`,

>    2. — a specialization of `basic_string_view` (27.3 [string.view]),

>    3. — a specialization of `ranges::iota_view` (25.6.4.2 [range.iota.view]), or

>    4. — a specialization of `ranges::subrange` (25.5.4 [range.subrange]),

>    then `T{ranges::begin(E), ranges::begin(E) + min<D>(ranges::size(E), F)}`, except that `E` is evaluated only once;

> 3. — otherwise, `ranges::take_view{E, F}`.

Consider the case when `T = subrange<counted_iterator<int>, default_sentinel_t>`. Then according to the above wording, `views::take(E, F)` is expression-equivalent to

>     T{ranges::begin(E), ranges:begin(E) + min<D>(ranges::size(E), F)};   (*)

But this expression is ill-formed for the `T` we chose because `subrange<counted_iterator<int>, default_sentinel_t>` has no matching constructor that takes an `iterator-iterator` pair.

More generally the above issue applies anytime `T` is a specialization of `subrange` that does not model `common_range`. But a similar issue also exists when `T` is a specialization of `iota_view` whose value type differs from its bound type. In this case yet another issue arises: In order for the expression `(*)` to be well-formed when `T` is a specialization of `iota_view`, we need to be able to construct an `iota_view` out of an `iterator-iterator` pair, and for that it seems we need to add another constructor to `iota_view`.

[2020-02-24, Casey comments]

Furthermore, the pertinent subrange constructor is only available when `subrange::StoreSize` is `false` (i.e., when either the `subrange` specialization's third template argument is not `subrange_kind::sized` or its iterator and sentinel types `I` and `S` model `sized_sentinel_for<S, I>`).

[2020-03-16, Tomasz comments]

A similar problem occurs for the `views::drop` for the `subrange<I, S, subrange_kind::sized>`, that explicitly stores size (i.e. `sized_sentinel_for<I, S>` is `false`). In such case, the `(iterator, sentinel)` constructor that `views::drop` will be expression-equivalent is not available.

[2020-03-29 Issue Prioritization]

Priority to 2 after reflector discussion.

[2021-05-18 Tim adds wording]

The proposed resolution below is based on the MSVC implementation, with one caveat: the MSVC implementation uses a SCARY iterator type for `iota_view` and therefore its `iota_view` case for `take` is able to directly construct the new view from the iterator type of the original. This is not required to work, so the wording below constructs the `iota_view` from the result of dereferencing the iterators instead in this case.

[2021-06-18 Tim syncs wording to the current working draft]

The wording below also corrects the size calculation in the presence of integer-class types.

[2021-08-20; LWG telecon]

Set status to Tentatively Ready after telecon review.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Edit 25.7.10.1 [range.take.overview] as indicated:

   > -2- The name `views::take` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Let `E` and `F` be expressions, let `T` be `remove_cvref_t<decltype((E))>`, and let `D` be `range_difference_t<decltype((E))>`. If `decltype((F))` does not model `convertible_to<D>`, `views::take(E, F)` is ill-formed. Otherwise, the expression `views::take(E, F)` is expression-equivalent to:

   > 1. (2.1) — if `T` is a specialization of `ranges::empty_view` (25.6.2.2 [range.empty.view]), then `((void) F, decay-copy(E))`, except that the evaluations of `E` and `F` are indeterminately sequenced;

   > 2. (2.2) — otherwise, if `T` models `random_access_range` and `sized_range`, and is a specialization of `span` (23.7.2.2 [views.span]), `basic_string_view` (27.3 [string.view]), or `ranges::subrange` (25.5.4 [range.subrange]), then `U(ranges::begin(E), ranges::begin(E) + std::min<D>(ranges::distance(E), F))`, except that `E` is evaluated only once, where `U` is a type determined as follows:

   >    1. (2.2.1) — if `T` is a specialization of `span` (23.7.2.2 [views.span]) where `T::extent == dynamic_extent`, then `U` is `span<typename T::element_type>`;

   >    2. (2.2.2) — otherwise, if `T` is a specialization of `basic_string_view` (27.3 [string.view]), then `U` is `T`;

   >    3. (2.2.3) — a specialization of `ranges::iota_view` (25.6.4.2 [range.iota.view]), or

   >    4. (2.2.4) — otherwise, `T` is a specialization of `ranges::subrange` (25.5.4 [range.subrange]), and `U` is `ranges::subrange<iterator_t<T>>`;

   >    then `T{ranges::begin(E), ranges::begin(E) + min<D>(ranges::size(E), F)}`, except that `E` is evaluated only once;

   > 3. (2.?) — otherwise, if `T` is a specialization of `ranges::iota_view` (25.6.4.2 [range.iota.view]) that models `random_access_range` and `sized_range`, then `ranges::iota_view(*ranges::begin(E), *(ranges::begin(E) + std::min<D>(ranges::distance(E), F)))`, except that `E` and `F` are each evaluated only once;

   > 4. (2.3) — otherwise, `ranges::take_view(E, F)`.

2. Edit 25.7.12.1 [range.drop.overview] as indicated:

   > -2- The name `views::drop` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Let `E` and `F` be expressions, let `T` be `remove_cvref_t<decltype((E))>`, and let `D` be `range_difference_t<decltype((E))>`. If `decltype((F))` does not model `convertible_to<D>`, `views::drop(E, F)` is ill-formed. Otherwise, the expression `views::drop(E, F)` is expression-equivalent to:

   > 1. (2.1) — if `T` is a specialization of `ranges::empty_view` (25.6.2.2 [range.empty.view]), then `((void) F, decay-copy(E))`, except that the evaluations of `E` and `F` are indeterminately sequenced;

   > 2. (2.2) — otherwise, if `T` models `random_access_range` and `sized_range`, and is

   >    1. (2.2.1) —a specialization of `span` (23.7.2.2 [views.span]) where `T::extent == dynamic_extent`,

   >    2. (2.2.2) — a specialization of `basic_string_view` (27.3 [string.view]),

   >    3. (2.2.3) — a specialization of `ranges::iota_view` (25.6.4.2 [range.iota.view]), or

   >    4. (2.2.4) — a specialization of `ranges::subrange` (25.5.4 [range.subrange]) where `T::StoreSize` is `false`,

   >    then `TU(ranges::begin(E) + std::min<D>(ranges::sizedistance(E), F), ranges::end(E))`, except that `E` is evaluated only once, where `U` is `span<typename T::element_type>` if `T` is a specialization of `span` and `T` otherwise;

   > 3. (2.?) — otherwise, if `T` is a specialization of `ranges::subrange` (25.5.4 [range.subrange]) that models `random_access_range` and `sized_range`, then `T(ranges::begin(E) + std::min<D>(ranges::distance(E), F), ranges::end(E), to-unsigned-like(ranges::distance(E) - std::min<D>(ranges::distance(E), F)))`, except that `E` and `F` are each evaluated only once;

   > 4. (2.3) — otherwise, `ranges::drop_view(E, F)`.

### 3408(i). LWG 3291 reveals deficiencies in `counted_iterator`

Section: 24.5.7.1 [counted.iterator] Status: Resolved Submitter: Patrick Palka Opened: 2020-02-25 Last modified: 2021-05-18

Priority: 2

View all other issues in [counted.iterator].

View all issues with Resolved status.

Discussion:

LWG 3291(i) makes changes to `iota_view` so that it no longer falsely claims it's a C++17 forward iterator. A side effect of this change is that the `counted_iterator` of an `iota_view` can no longer model `forward_iterator`, no matter what the properties of the underlying `weakly_incrementable` type are. For example, the following snippet is ill-formed after LWG 3291:

>     auto v = views::iota(0);
>     auto i = counted_iterator{v.begin(), 5};
>     static_assert(random_access_iterator<decltype(i)>); // fails after LWG 3291

The problem seems to be that `counted_iterator` populates its `iterator_traits` but it does not specify its `iterator_concept`, so we fall back to looking at its `iterator_category` to determine its C++20 iterator-ness. It seems `counted_iterator` should specify its `iterator_concept` appropriately based on the `iterator_concept` of the iterator it wraps.

[2020-03-29 Issue Prioritization]

Priority to 2 after reflector discussion.

[2021-05-18 Resolved by the adoption of P2259R1 at the February 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

### 3410(i). `lexicographical_compare_three_way` is overspecified

Section: 26.8.12 [alg.three.way] Status: C++23 Submitter: Casey Carter Opened: 2020-02-27 Last modified: 2023-11-22

Priority: 3

View all other issues in [alg.three.way].

View all issues with C++23 status.

Discussion:

26.8.12 [alg.three.way]/2 specifies the effects of the `lexicographical_compare_three_way` algorithm via a large "equivalent to" codeblock. This codeblock specifies one more iterator comparison than necessary when the first input sequence is greater than the second, and two more than necessary in other cases. Requiring unnecessary work is the antithesis of C++.

[2020-03-29 Issue Prioritization]

Priority to 3 after reflector discussion.

[2021-05-19 Tim adds wording]

The wording below simply respecifies the algorithm in words. It seems pointless to try to optimize the code when the reading in the discussion above would entirely rule out things like `memcmp` optimizations for arbitrary contiguous iterators of bytes.

[2021-05-26; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 26.8.12 [alg.three.way] as indicated:

   >     template<class InputIterator1, class InputIterator2, class Cmp>
   >       constexpr auto
   >         lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
   >                                           InputIterator2 b2, InputIterator2 e2,
   >                                           Cmp comp)
   >             -> decltype(comp(*b1, *b2));

   > > -?- Let N be min(`e1 - b1, e2 - b2`). Let E(n) be `comp(*(b1 + n), *(b2 + n))`.

   > > -1- Mandates: `decltype(comp(*b1, *b2))` is a comparison category type.

   > > -?- Returns: E(i), where i is the smallest integer in [0, N) such that E(i) `!= 0` is `true`, or `(e1 - b1) <=> (e2 - b2)` if no such integer exists.

   > > -?- Complexity: At most N applications of `comp`.

   > > -2- Effects: Lexicographically compares two ranges and produces a result of the strongest applicable comparison category type. Equivalent to:

   > > >     for ( ; b1 != e1 && b2 != e2; void(++b1), void(++b2) )
   > > >       if (auto cmp = comp(*b1,*b2); cmp != 0)
   > > >           return cmp;
   > > >     return b1 != e1 ? strong_ordering::greater :
   > > >            b2 != e2 ? strong_ordering::less :
   > > >                       strong_ordering::equal;

### 3411(i). [fund.ts.v3] Contradictory namespace rules in the Library Fundamentals TS

Section: 8.3 [fund.ts.v3::memory.resource.syn] Status: WP Submitter: Thomas Köppe Opened: 2020-02-28 Last modified: 2026-02-21

Priority: 3

View all issues with WP status.

Discussion:

Addresses: fund.ts.v3

The Library Fundamentals TS, N4840, contains a rule about the use of namespaces (paragraph 1), with the consequence:

> "This TS does not define `std::experimental::fundamentals_v3::pmr`"

However, the TS then goes on to define exactly that namespace.

At the time when the subclause memory.resource.syn was added, the IS didn't use to contain a namespace `pmr`. When the IS adopted that namespace and the TS was rebased, the namespace rule started conflicting with the material in the TS.

I do not have a workable proposed resolution at this point.

[2020-04-07 Issue Prioritization]

Priority to 3 after reflector discussion.

[2021-11-17; Jiang An comments and provides wording]

Given namespaces `std::chrono::experimental::fundamentals_v2` and `std::experimental::fundamentals_v2::pmr` are used in LFTS v2, I think that the intent is that

- the prefix `experimental::fundamentals_vN` is only added to a namespace specified by the standard, and

- every name specified by the standard in the outer namespace of that of the related component in the TS.

If we follow the convention, perhaps we should relocate `resource_adaptor` from `std::experimental::fundamentals_v3::pmr` to `std::pmr::experimental::fundamentals_v3` in LFTS v3. If it's decided that `resource_adaptor` shouldn't be relocated, I suppose that LWG 3411 can be by striking the wrong wording in 4.1 [fund.ts.v3::general.namespaces] and using qualified `std::pmr::memory_resource` when needed.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4853.

> > [Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

> Option A:

> 1. Modify 4.1 [fund.ts.v3::general.namespaces] as indicated:

>    > -2- Each header described in this technical specification shall import the contents of `outer-namespacestd::experimental::fundamentals_v3` into `outer-namespacestd::experimental` as if by

>    > >     namespace outer-namespacestd::experimental::inline fundamentals_v3 {}

>    > where `outer-namespace` is a namespace defined in the C++ Standard Library.

> 2. Modify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:

>    > Table 5 — Computed `memory_resource` for type-erased allocator
>    > If the type of `alloc` is | then the value of `rptr` is
>    > --- | ---
>    > `[…]`
>    > any other type meeting the requirements (C++20 ¶16.5.3.5) | a pointer to a value of type `pmr::experimental::resource_adaptor<A>` where `A` is the type of `alloc`. `rptr` remains valid only for the lifetime of `X`.
>    > `[…]`

> 3. Modify 8.3 [fund.ts.v3::memory.resource.syn], header `<experimental/memory_resource>` synopsis, as indicated:

>    >     namespace std::pmr::experimental::inline fundamentals_v3::pmr {
>    >     […]
>    >     } // namespace std::pmr::experimental::inline fundamentals_v3::pmr

> Option B:

> 1. Modify 4.1 [fund.ts.v3::general.namespaces] as indicated:

>    > -1- Since the extensions described in this technical specification are experimental and not part of the C++ standard library, they should not be declared directly within namespace `std`. Unless otherwise specified, all components described in this technical specification either:

>    > 1. — modify an existing interface in the C++ Standard Library in-place,

>    > 2. — are declared in a namespace whose name appends `::experimental::fundamentals_v3` to a namespace defined in the C++ Standard Library, such as `std` or `std::chrono`, or

>    > 3. — are declared in a subnamespace of a namespace described in the previous bullet, whose name is not the same as an existing subnamespace of namespace `std`.

>    > [Example: This TS does not define `std::experimental::fundamentals_v3::pmr` because the C++ Standard Library defines `std::pmr`. — end example]

> 2. Modify 7.2 [fund.ts.v3::func.wrap.func], class template `function` synopsis, as indicated:

>    >     namespace std {
>    >       namespace experimental::inline fundamentals_v3 {

>    >         template<class> class function; // undefined

>    >         template<class R, class... ArgTypes>
>    >         class function<R(ArgTypes...)> {
>    >         public:
>    >           […]
>    >           std::pmr::memory_resource* get_memory_resource() const noexcept;
>    >         };
>    >     […]
>    >     } // namespace experimental::inline fundamentals_v3
>    >     […]
>    >     } // namespace std

> 3. Modify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:

>    > Table 5 — Computed `memory_resource` for type-erased allocator
>    > If the type of `alloc` is | then the value of `rptr` is
>    > --- | ---
>    > `[…]`
>    > any other type meeting the requirements (C++20 ¶16.5.3.5) | a pointer to a value of type `experimental::pmr::resource_adaptor<A>` where `A` is the type of `alloc`. `rptr` remains valid only for the lifetime of `X`.
>    > `[…]`

> 4. Modify 8.4.1 [fund.ts.v3::memory.resource.adaptor.overview] as indicated:

>    > -1- An instance of `resource_adaptor<Allocator>` is an adaptor that wraps a `std::pmr::memory_resource` interface around `Allocator`. […]

>    > >     // The name resource_adaptor_imp is for exposition only.
>    > >     template<class Allocator>
>    > >     class resource_adaptor_imp : public std::pmr::memory_resource {
>    > >     public:
>    > >       […]

>    > >       virtual bool do_is_equal(const std::pmr::memory_resource& other) const noexcept;
>    > >     };

> 5. Modify 8.4.3 [fund.ts.v3::memory.resource.adaptor.mem] as indicated:

>    >     -6- bool do_is_equal(const std::pmr::memory_resource& other) const noexcept;

>    > > […]

> 6. Modify 32.10.6 [futures.promise], class template `promise` synopsis, as indicated:

>    >     namespace std {
>    >       namespace experimental::inline fundamentals_v3 {

>    >         template<class R>
>    >         class promise {
>    >         public:
>    >           […]
>    >           std::pmr::memory_resource* get_memory_resource() const noexcept;
>    >         };
>    >     […]
>    >     } // namespace experimental::inline fundamentals_v3
>    >     […]
>    >     } // namespace std

> 7. Modify 32.10.10 [futures.task], class template `packaged_task` synopsis, as indicated:

>    >     namespace std {
>    >       namespace experimental::inline fundamentals_v3 {

>    >         template<class R, class... ArgTypes>
>    >         class packaged_task<R(ArgTypes...)> {
>    >         public:
>    >           […]
>    >           std::pmr::memory_resource* get_memory_resource() const noexcept;
>    >         };
>    >     […]
>    >     } // namespace experimental::inline fundamentals_v3
>    >     […]
>    >     } // namespace std

[2022-10-12; Jonathan provides updated wording]

The LWG telecon decided on a simpler form of Option A. The changes to 4.1 [fund.ts.v3::general.namespaces] generated some questions and disagreement, but it was decided that they are not needed anyway. The normative synopses already depict the use of inline namespaces with the stated effects. That paragraph seems more informative than normative, and there were suggestions to strike it entirely. It was decided to keep it but without making the edits. As such, it remains correct for the contents of `std::experimental::fundamentals_v3`. It doesn't apply to `pmr::resource_adaptor`, but is not incorrect for that either. The rest of the proposed resolution fully specifies the `pmr` parts.

[2022-10-19; Reflector poll]

Set status to "Tentatively Ready" after eight votes in favour in reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4920.

1. Modify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:

   > Table 5 — Computed `memory_resource` for type-erased allocator
   > If the type of `alloc` is | then the value of `rptr` is
   > --- | ---
   > `[…]`
   > any other type meeting the requirements (C++20 ¶16.5.3.5) | a pointer to a value of type `pmr::experimental::resource_adaptor<A>` where `A` is the type of `alloc`. `rptr` remains valid only for the lifetime of `X`.
   > `[…]`

2. Modify 8.3 [fund.ts.v3::memory.resource.syn], header `<experimental/memory_resource>` synopsis, as indicated:

   >     namespace std::pmr::experimental::inline fundamentals_v3::pmr {
   >     […]
   >     } // namespace std::pmr::experimental::inline fundamentals_v3::pmr

### 3412(i). §[format.string.std] references to "Unicode encoding" unclear

Section: 28.5.2.2 [format.string.std] Status: Resolved Submitter: Hubert Tong Opened: 2020-02-29 Last modified: 2023-03-23

Priority: 3

View other active issues in [format.string.std].

View all other issues in [format.string.std].

View all issues with Resolved status.

Discussion:

In 28.5.2.2 [format.string.std], the meaning of "Unicode encoding" in the text added by P1868R2 (the "Unicorn width" paper) is unclear.

One interpretation of what is meant by "Unicode encoding" is "UCS encoding scheme" (as defined by ISO/IEC 10646). Another interpretation is an encoding scheme capable of encoding all UCS code points that have been assigned to characters. Yet another interpretation is an encoding scheme capable of encoding all UCS scalar values.

SG16 reflector discussion (with the LWG reflector on CC) indicates that the third option above is the closest to the intent of the study group. The situation of the current wording, which readers can easily read as indicating the first option above, is undesirable.

[2020-07-17; Priority set to 3 in telecon]

[2023-03-22 Resolved by the adoption of P2736R2 in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

### 3413(i). [fund.ts.v3] `propagate_const`'s `swap`'s `noexcept` specification needs to be constrained and use a trait

Section: 6.1.2.8 [fund.ts.v3::propagate_const.modifiers], 6.1.2.10 [fund.ts.v3::propagate_const.algorithms] Status: WP Submitter: Thomas Köppe Opened: 2020-02-29 Last modified: 2026-02-21

Priority: 0

View all issues with WP status.

Discussion:

Addresses: fund.ts.v3

In the Fundamentals TS, the `noexcept` specifications of both the member and non-member `swap` functions for `propagate_const` are using the old, ill-formed pattern of attempting to use "`noexcept(swap(...))`" as the boolean predicate. According to LWG 2456(i), this is ill-formed, and a resolution such as in P0185R1 is required.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4840.

> 1. Modify 6.1.1 [fund.ts.v3::propagate_const.syn], header `<experimental/propagate_const>` synopsis, as indicated:

>    >     // 6.1.2.10 [fund.ts.v3::propagate_const.algorithms], propagate_const specialized algorithms
>    >     template <class T>
>    >       constexpr void swap(propagate_const<T>& pt, propagate_const<T>& pt2) noexcept(see belowis_nothrow_swappable_v<T>);

> 2. Modify 6.1.2.1 [fund.ts.v3::propagate_const.overview], class template `propagate_const` synopsis, as indicated:

>    >     // 6.1.2.8 [fund.ts.v3::propagate_const.modifiers], propagate_const modifiers
>    >     constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);

> 3. Modify 6.1.2.8 [fund.ts.v3::propagate_const.modifiers] as indicated:

>    >     constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);

>    > > -2- The constant-expression in the exception-specification is `noexcept(swap(t_, pt.t_))`.

>    > > -3- Effects: `swap(t_, pt.t_)`.

> 4. Modify 6.1.2.10 [fund.ts.v3::propagate_const.algorithms] as indicated:

>    >     template <class T>
>    >       constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see belowis_nothrow_swappable_v<T>);

>    > > -2- The constant-expression in the exception-specification is `noexcept(pt1.swap(pt2))`.

>    > > -3- Effects: `pt1.swap(pt2)`.

[2020-03-30; Reflector discussion]

This issue has very much overlap with LWG 2561(i), especially now that the library fundamentals has been rebased to C++20 the there reported problem for the corresponding `swap` problem for `optional` is now moot. During the reflector discussion of this issue here it was also observed that the free `swap` template for `propagate_const` needs to be constrained. This has been done below in the revised wording which also attempts to use a similar style as the IS.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4840.

> 1. Modify 6.1.2.1 [fund.ts.v3::propagate_const.overview], class template `propagate_const` synopsis, as indicated:

>    >     // 6.1.2.8 [fund.ts.v3::propagate_const.modifiers], propagate_const modifiers
>    >     constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);

> 2. Modify 6.1.2.8 [fund.ts.v3::propagate_const.modifiers] as indicated:

>    >     constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);

>    > > -2- The constant-expression in the exception-specification is `noexcept(swap(t_, pt.t_))`.

>    > > -3- Effects: `swap(t_, pt.t_)`.

> 3. Modify 6.1.2.10 [fund.ts.v3::propagate_const.algorithms] as indicated:

>    >     template <class T>
>    >       constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see below);

>    > > -?- Constraints: `is_swappable_v<T>` is `true`.

>    > > -2- The constant-expression in the exception-specification is `noexcept(pt1.swap(pt2))`.

>    > > -3- Effects: `pt1.swap(pt2)`.

>    > > -?- Remarks: The expression inside `noexcept` is equivalent to:

>    > > >     noexcept(pt1.swap(pt2))

[2020-04-06; Wording update upon reflector discussions]

[2020-05-03 Issue Prioritization]

Status set to Tentatively Ready after five positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4840.

1. Modify 6.1.2.1 [fund.ts.v3::propagate_const.overview], class template `propagate_const` synopsis, as indicated:

   >     // 6.1.2.8 [fund.ts.v3::propagate_const.modifiers], propagate_const modifiers
   >     constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);

2. Modify 6.1.2.8 [fund.ts.v3::propagate_const.modifiers] as indicated:

   >     constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);

   > > -?- Preconditions: Lvalues of type `T` are swappable (C++17 §20.5.3.2).

   > > -2- The constant-expression in the exception-specification is `noexcept(swap(t_, pt.t_))`.

   > > -3- Effects: `swap(t_, pt.t_)`.

3. Modify 6.1.2.10 [fund.ts.v3::propagate_const.algorithms] as indicated:

   >     template <class T>
   >       constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see below);

   > > -?- Constraints: `is_swappable_v<T>` is `true`.

   > > -2- The constant-expression in the exception-specification is `noexcept(pt1.swap(pt2))`.

   > > -3- Effects: Equivalent to: `pt1.swap(pt2)`.

   > > -?- Remarks: The expression inside `noexcept` is equivalent to:

   > > >     noexcept(pt1.swap(pt2))

### 3414(i). [networking.ts] `service_already_exists` has no usable constructors

Section: 13.7 [networking.ts::async.exec.ctx] Status: WP Submitter: Jonathan Wakely Opened: 2020-03-17 Last modified: 2026-02-21

Priority: 0

View all issues with WP status.

Discussion:

Addresses: networking.ts

In the Networking TS, the `service_already_exists` exception type has no constructors declared. The `logic_error` base class is not default constructible, so `service_already_exists`'s implicit default constructor is defined as deleted.

Implementations can add one or more private constructors that can be used by `make_service`, but there seems to be little benefit to that. The Boost.Asio type of the same name has a public default constructor.

[2020-04-18 Issue Prioritization]

Status set to Tentatively Ready after six positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4734.

1. Modify 13.7 [networking.ts::async.exec.ctx] p1, as indicated:

   >     // service access
   >     template<class Service> typename Service::key_type&
   >     use_service(execution_context& ctx);
   >     template<class Service, class... Args> Service&
   >     make_service(execution_context& ctx, Args&&... args);
   >     template<class Service> bool has_service(const execution_context& ctx) noexcept;
   >     class service_already_exists : public logic_error { };
   >     {
   >     public:
   >       service_already_exists();
   >     };

2. Add a new subclause after [async.exec.ctx.globals]:

   > 13.7.6 Class `service_already_exists` [async.exec.ctx.except]

   > -1- The class `service_already_exists` defines the type of objects thrown as exceptions to report an attempt to add an existing service to an `execution_context`.

   >     service_already_exists();

   > > -2- Postconditions: `what()` returns an implementation-defined NTBS.

### 3417(i). Missing `volatile` atomic deprecations

Section: 32.5.8.2 [atomics.types.operations] Status: WP Submitter: Alisdair Meredith Opened: 2020-03-19 Last modified: 2026-06-15

Priority: 3

View all other issues in [atomics.types.operations].

View all issues with WP status.

Discussion:

Paper P1831R1 aimed to deprecate all the atomic overloads for `volatile` qualified member function of `std::atomic` unless `is_always_lock_free` is `true`. There are a few omissions in the wording.

First, `operator++` and `operator--` are correctly constrained, but the deprecated overloads are not restored in Annex D, unlike the other member functions. I confirmed with the paper author this is an accidental oversight, and not an intended change of behavior for C++20.

Secondly, the wait/notify APIs were added after the initial wording For this paper was drafted, and the paper did not catch up. Again, I confirmed with the paper author that these functions should be similarly constrained and deprecated.

[2020-04-04 Issue Prioritization]

Priority to 3 after reflector discussion. The suggested wording was generally accepted, but there were considerable doubts that it is necessary to add deprecated functions of the new `wait/notify` functions instead of declaring only the non-volatile overloads. The wish was expressed that both SG1 and LEWG should express their opinion here.

[Kona 2025-11-05; SG1 had strong consensus for the proposed resolution. Status changed: SG1 → Open.]

[2026-04-14; Approved by LWG in Kona, but not added to the plenary polls. Status changed: Open → Ready.]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 32.5.8.2 [atomics.types.operations] as indicated:

   >     void wait(T old, memory_order order = memory_order::seq_cst) const volatile noexcept;
   >     void wait(T old, memory_order order = memory_order::seq_cst) const noexcept;

   > > Constraints: For the `volatile` overload of this function, `is_always_lock_free` is `true`.

   > > -29- Preconditions: […]

   > > […]

   >     void notify_one() volatile noexcept;
   >     void notify_one() noexcept;

   > > Constraints: For the `volatile` overload of this function, `is_always_lock_free` is `true`.

   > > -32- Effects: […]

   > > […]

   >     void notify_all() volatile noexcept;
   >     void notify_all() noexcept;

   > > Constraints: For the `volatile` overload of this function, `is_always_lock_free` is `true`.

   > > -34- Effects: […]

   > > […]

2. Modify D.24.2 [depr.atomics.volatile], annex D, as indicated:

   > If an atomic specialization has one of the following overloads, then that overload participates in overload resolution even if `atomic<T>::is_always_lock_free` is `false`:

   > >     void store(T desired, memory_order order = memory_order::seq_cst) volatile noexcept;
   > >     […]
   > >     T* fetch_key(ptrdiff_t operand, memory_order order = memory_order::seq_cst) volatile noexcept;
   > >     value_type operator++(int) volatile noexcept;
   > >     value_type operator--(int) volatile noexcept;
   > >     value_type operator++() volatile noexcept;
   > >     value_type operator--() volatile noexcept;
   > >     void wait(T old, memory_order order = memory_order::seq_cst) const volatile noexcept;
   > >     void notify_one() volatile noexcept;
   > >     void notify_all() volatile noexcept;

### 3419(i). §[algorithms.requirements]/15 doesn't reserve as many rights as it intends to

Section: 26.2 [algorithms.requirements] Status: C++23 Submitter: Richard Smith Opened: 2020-03-24 Last modified: 2023-11-22

Priority: 0

View other active issues in [algorithms.requirements].

View all other issues in [algorithms.requirements].

View all issues with C++23 status.

Discussion:

26.2 [algorithms.requirements]/15 says:

> "The number and order of deducible template parameters for algorithm declarations are unspecified, except where explicitly stated otherwise. [Note: Consequently, the algorithms may not be called with explicitly-specified template argument lists. — end note]"

But the note doesn't follow from the normative rule. For example, we felt the need to explicitly allow deduction for `min`'s template parameter:

>     template<typename T> const T& min(const T&, const T&);

… but if only the order and number of deducible template parameters is permitted to vary, then because of the required deduction behavior of this function template, there are only three possible valid declarations:

>     template<typename T> ??? min(const T&, const T&);
>     template<typename T, typename U> ??? min(const T&, const U&);
>     template<typename T, typename U> ??? min(const U&, const T&);

(up to minor differences in the parameter type). This doesn't prohibit calls with an explicitly-specified template argument list, contrary to the claim in the note. (Indeed, because a call such as `min(1, {})` is valid, either the first of the above three overloads must be present or there must be a default template argument `typename U = T`, which further adds to the fact that there may be valid calls with an explicitly-specified template argument list.)

Also, the "explicitly stated otherwise" cases use phrasing such as: "An invocation may explicitly specify an argument for the template parameter `T` of the overloads in namespace `std`." which doesn't "specify otherwise" the normative rule, but does "specify otherwise" the claim in the note.

All this leads me to believe that [algorithms.requirements]/15 is backwards: the normative rule should be a note and the note should be the normative rule.

[2020-04-04 Issue Prioritization]

Status set to Tentatively Ready after six positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 26.2 [algorithms.requirements] as indicated:

   > -15- The well-formedness and behavior of a call to an algorithm with an explicitly-specified template argument list isnumber and order of deducible template parameters for algorithm declarations are unspecified, except where explicitly stated otherwise. [Note: Consequently, an implementation can declare an algorithm with different template parameters than those presentedthe algorithms may not be called with explicitly-specified template argument lists. — end note]

### 3420(i). `cpp17-iterator` should check that the type looks like an iterator first

Section: 24.3.2.3 [iterator.traits] Status: C++23 Submitter: Tim Song Opened: 2020-02-29 Last modified: 2023-11-22

Priority: 0

View all other issues in [iterator.traits].

View all issues with C++23 status.

Discussion:

It is common in pre-C++20 code to rely on SFINAE-friendly `iterator_traits` to rule out non-iterators in template constraints (`std::filesystem::path` is one example in the standard library).

C++20 changed `iterator_traits` to automatically detect its members in some cases, and this detection can cause constraint recursion. LWG 3244(i) tries to fix this for `path` by short-circuiting the check when the source type is `path` itself, but this isn't sufficient:

>     struct Foo
>     {
>       Foo(const std::filesystem::path&);
>     };

>     static_assert(std::copyable<Foo>);

Here the copyability determination will ask whether a `path` can be constructed from a `Foo`, which asks whether `Foo` is an iterator, which checks whether `Foo` is `copyable` […].

To reduce the risk of constraint recursion, we should change `cpp17-iterator` so that it does not ask about copyability until the type is known to resemble an iterator.

[2020-04-04 Issue Prioritization]

Status set to Tentatively Ready after six positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 24.3.2.3 [iterator.traits] as indicated:

   > -2- The definitions in this subclause make use of the following exposition-only concepts:

   > >     template<class I>
   > >     concept cpp17-iterator =
   > >       copyable<I> && requires(I i) {
   > >         { *i } -> can-reference;
   > >         { ++i } -> same_as<I&>;
   > >         { *i++ } -> can-reference;
   > >       } && copyable<I>;

   > >     […]

### 3421(i). Imperfect ADL emulation for `boolean-testable`

Section: 18.5.2 [concept.booleantestable] Status: C++23 Submitter: Davis Herring Opened: 2020-03-24 Last modified: 2023-11-22

Priority: 0

View all issues with C++23 status.

Discussion:

18.5.2 [concept.booleantestable]/4 checks for "a specialization of a class template that is a member of the same namespace as `D`", which ignores the possibility of inline namespaces.

[2020-04-18 Issue Prioritization]

Status set to Tentatively Ready after six positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 18.5.2 [concept.booleantestable] as indicated:

   > -4- A key parameter of a function template `D` is a function parameter of type cv `X` or reference thereto, where `X` names a specialization of a class template that has the same innermost enclosing non-inlineis a member of the same namespace as `D`, and `X` contains at least one template parameter that participates in template argument deduction. […]

### 3422(i). Issues of `seed_seq`'s constructors

Section: 29.5.8.1 [rand.util.seedseq] Status: C++23 Submitter: Jiang An Opened: 2020-03-25 Last modified: 2023-11-22

Priority: 3

View all other issues in [rand.util.seedseq].

View all issues with C++23 status.

Discussion:

29.5.8.1 [rand.util.seedseq] says that `std::seed_seq` has following 3 constructors:

1. #1: `seed_seq()`

2. #2: `template<class T> seed_seq(initializer_list<T> il)`

3. #3: `template<class InputIterator> seed_seq(InputIterator begin, InputIterator end)`

The default constructor (#1) has no precondition and does not throw, and `vector<result_type>`'s default constructor is already `noexcept` since C++17, so #1 should also be `noexcept`.

Despite that the `vector<result_type>` member is exposition-only, current implementations (at least libc++, libstdc++ and MSVC STL) all hold it as the only data member of `seed_seq`, even with different names. And #1 is already `noexcept` in libc++ and libstdc++.

These constructors are not constrained, so #3 would never be matched in list-initialization. Consider following code:

>     #include <random>
>     #include <vector>

>     int main()
>     {
>       std::vector<int> v(32);
>       std::seed_seq{std::begin(v), std::end(v)}; // error: #2 matched and T is not an integer type
>       std::seed_seq(std::begin(v), std::end(v)); // OK
>     }

#3 should be made available in list-initialization by changing Mandates in 29.5.8.1 [rand.util.seedseq]/3 to Constraints IMO.

[2020-04-18 Issue Prioritization]

Priority to 3 after reflector discussion.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 29.5.8.1 [rand.util.seedseq] as indicated:

   >     class seed_seq {
   >     public:
   >       // types
   >       using result_type = uint_least32_t;

   >       // constructors
   >       seed_seq() noexcept;
   >       […]
   >     };

   >     seed_seq() noexcept;

   > > -1- Postconditions: `v.empty()` is `true`.

   > > -2- Throws: Nothing.

   >     template<class T>
   >       seed_seq(initializer_list<T> il);

   > > -3- ConstraintsMandates: `T` is an integer type.

   > > -4- Effects: Same as `seed_seq(il.begin(), il.end())`.

### 3425(i). `condition_variable_any` fails to constrain its `Lock` parameters

Section: 32.7.5 [thread.condition.condvarany] Status: C++23 Submitter: Casey Carter Opened: 2020-04-04 Last modified: 2023-11-22

Priority: 0

View all other issues in [thread.condition.condvarany].

View all issues with C++23 status.

Discussion:

32.7.5 [thread.condition.condvarany]/1 says "A `Lock` type shall meet the Cpp17BasicLockable requirements (32.2.5.2 [thread.req.lockable.basic]).", which is fine, but it notably doesn't require anything to be a `Lock` type or meet the requirements of a `Lock` type. Given that every member template of `condition_variable_any` has a template parameter named `Lock`, the intent is clearly to impose a requirement on the template arguments supplied for those parameters but the use of code font for "`Lock`" in the definition of "`Lock` type" is a bit subtle to establish that connection. We should specify this more clearly.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Modify 32.7.5 [thread.condition.condvarany] as indicated:

>    > -1- A `Lock` typeTemplate arguments for template parameters of member templates of `conditional_variable_any` named `Lock` shall meet the Cpp17BasicLockable requirements (32.2.5.2 [thread.req.lockable.basic]). [Note: All of the standard mutex types meet this requirement. If a `Lock` type other than one of the standard mutex types or a `unique_lock` wrapper for a standard mutex type is used with `condition_variable_any`, the user should ensure that any necessary synchronization is in place with respect to the predicate associated with the `condition_variable_any` instance. — end note]

[2020-04-06; Tim improves wording]

[2020-04-11 Issue Prioritization]

Status set to Tentatively Ready after seven positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 32.7.5 [thread.condition.condvarany] as indicated:

   > -1- A `Lock` typeIn this subclause 32.7.5 [thread.condition.condvarany], template arguments for template parameters named `Lock` shall meet the Cpp17BasicLockable requirements (32.2.5.2 [thread.req.lockable.basic]). [Note: All of the standard mutex types meet this requirement. If a `Lock` type other than one of the standard mutex types or a `unique_lock` wrapper for a standard mutex type is used with `condition_variable_any`, the user should ensure that any necessary synchronization is in place with respect to the predicate associated with the `condition_variable_any` instance. — end note]

### 3426(i). `operator<=>(const unique_ptr<T, D>&, nullptr_t)` can't get no satisfaction

Section: 20.3.1.6 [unique.ptr.special] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-04-09 Last modified: 2023-11-22

Priority: 0

View all other issues in [unique.ptr.special].

View all issues with C++23 status.

Discussion:

The constraint for `operator<=>(const unique_ptr<T, D>&, nullptr_t)` cannot be satisfied, because `std::three_way_comparable<nullptr_t>` is `false`, because `nullptr <=> nullptr` is ill-formed.

We can make it work as intended by comparing `x.get()` to `pointer(nullptr)` instead of to `nullptr` directly.

[2020-04-14; Replacing the functional cast by a `static_cast` as result of reflector discussion]

[2020-04-18 Issue Prioritization]

Status set to Tentatively Ready after seven positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 20.2.2 [memory.syn] as indicated:

   >     […]
   >     template<class T, class D>
   >       bool operator>=(nullptr_t, const unique_ptr<T, D>& y);
   >     template<class T, class D>
   >       requires three_way_comparable_with<typename unique_ptr<T, D>::pointer, nullptr_t>
   >       compare_three_way_result_t<typename unique_ptr<T, D>::pointer, nullptr_t>
   >         operator<=>(const unique_ptr<T, D>& x, nullptr_t);
   >     […]

2. Modify 20.3.1.6 [unique.ptr.special] as indicated:

   >     template<class T, class D>
   >       requires three_way_comparable_with<typename unique_ptr<T, D>::pointer, nullptr_t>
   >       compare_three_way_result_t<typename unique_ptr<T, D>::pointer, nullptr_t>
   >         operator<=>(const unique_ptr<T, D>& x, nullptr_t);

   > > -18- Returns: `compare_three_way()(x.get(), static_cast<typename unique_ptr<T, D>::pointer>(nullptr))`.

### 3427(i). `operator<=>(const shared_ptr<T>&, nullptr_t)` definition ill-formed

Section: 20.3.2.2.8 [util.smartptr.shared.cmp] Status: C++23 Submitter: Daniel Krügler Opened: 2020-04-11 Last modified: 2023-11-22

Priority: 0

View all other issues in [util.smartptr.shared.cmp].

View all issues with C++23 status.

Discussion:

This is similar to LWG 3426(i): This time the definition of `operator<=>(const shared_ptr<T>&, nullptr_t)` is ill-formed, whose effects are essentially specified as calling:

>     compare_three_way()(a.get(), nullptr)

This call will be ill-formed by constraint-violation, because both `nullptr <=> nullptr` as well as `((T*) 0) <=> nullptr` are ill-formed.

As a short-term solution we can make it work as intended — equivalent to LWG 3426(i) — by comparing `a.get()` to `(element_type*) nullptr` instead of to `nullptr` directly.

As a long-term solution we should at least consider to deprecate the mixed relational operators as well as the mixed three-way comparison operator of all our smart-pointers with `std::nullptr_t` since the core language has eliminated relational comparisons of pointers with `std::nullptr_t` with N3624 four years after they had been originally accepted by CWG 879. Consequently, for C++20, the mixed three-way comparison between pointers and `nullptr` is not supported either. For this long-term solution I'm suggesting to handle this via a proposal.

[2020-05-16 Reflector discussions]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 20.3.2.2.8 [util.smartptr.shared.cmp] as indicated:

   >     template<class T>
   >       strong_ordering operator<=>(const shared_ptr<T>& a, nullptr_t) noexcept;

   > > -5- Returns: `compare_three_way()(a.get(), static_cast<typename shared_ptr<T>::element_type*>(nullptr))`.

### 3428(i). `single_view`'s in place constructor should be explicit

Section: 25.6.3.2 [range.single.view] Status: C++23 Submitter: Tim Song Opened: 2020-04-07 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.single.view].

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Discussion:

The `in_place_t` constructor template of `single_view` is not explicit:

>     template<class... Args>
>       requires constructible_from<T, Args...>
>     constexpr single_view(in_place_t, Args&&... args);

so it defines an implicit conversion from `std::in_place_t` to `single_view<T>` whenever `constructible_from<T>` is modeled, which seems unlikely to be the intent.

[2020-04-18 Issue Prioritization]

Status set to Tentatively Ready after six positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.6.3.2 [range.single.view] as indicated:

   > >     namespace std::ranges {
   > >       template<copy_constructible T>
   > >         requires is_object_v<T>
   > >       class single_view : public view_interface<single_view<T>> {
   > >         […]
   > >       public:
   > >         […]
   > >         template<class... Args>
   > >           requires constructible_from<T, Args...>
   > >         constexpr explicit single_view(in_place_t, Args&&... args);

   > >         […]
   > >       };
   > >     }

   > […]

   >     template<class... Args>
   >     constexpr explicit single_view(in_place_t, Args&&... args);

   > > -3- Effects: Initializes `value_` as if by `value_{in_place, std::forward<Args>(args)...}`.

### 3430(i). `std::fstream` & co. should be constructible from `string_view`

Section: 31.10.1 [fstream.syn] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-04-15 Last modified: 2023-11-22

Priority: 3

View all issues with C++23 status.

Discussion:

We have:

>     basic_fstream(const char*, openmode);
>     basic_fstream(const filesystem::path::value_type*, openmode); // wide systems only
>     basic_fstream(const string&, openmode);
>     basic_fstream(const filesystem::path&, openmode);

I think the omission of a `string_view` overload was intentional, because the underlying OS call (such as `fopen`) needs a NTBS. We wanted the allocation required to turn a `string_view` into an NTBS to be explicitly requested by the user. But then we added the `path` overload, which is callable with a `string_view`. Converting to a `path` is more expensive than converting to `std::string`, because a path has to at least construct a `basic_string`, and potentially also does an encoding conversion, parses the path, and potentially allocates a sequence of `path` objects for the path components.

This means the simpler, more obvious code is slower and uses more memory:

>     string_view sv = "foo.txt";
>     fstream f1(sv); // bad
>     fstream f2(string(sv)); // good

We should just allow passing a `string_view` directly, since it already compiles but doesn't do what anybody expects or wants.

Even with a `string_view` overload, passing types like `const char16_t*` or `u32string_view` will still implicitly convert to `filesystem::path`, but that seems reasonable. In those cases the encoding conversion is necessary. For Windows we support construction from `const wchar_t*` but not from `wstring` or `wstring_view`, which means those types will convert to `filesystem::path`. That seems suboptimal, so we might also want to add `wstring` and `wstring_view` overloads for "wide systems only", as per 31.10.1 [fstream.syn] p3.

Daniel:

LWG 2883(i) has a more general view on that but does not consider potential cost differences in the presence of `path` overloads (Which didn't exist at this point yet).

[2020-05-09; Reflector prioritization]

Set priority to 3 after reflector discussions.

[2020-08-10; Jonathan comments]

An alternative fix would be to retain the original design and not allow construction from a `string_view`. The `path` parameters could be changed to template parameters which are constrained to be exactly `path`, and not things like `string_view` which can convert to `path`.

[2020-08-21; Issue processing telecon: send to LEWG]

Just adding support for `string_view` doesn't prevent expensive conversions from other types that convert to `path`. Preference for avoiding all expensive implicit conversions to `path`, maybe via abbreviated function templates:

    basic_fstream(same_as<filesystem::path> auto const&, openmode);

It's possible `path_view` will provide a better option at some point.

It was noted that 2676(i) did intentionally allow conversions from "strings of character types `wchar_t`, `char16_t`, and `char32_t`". Those conversions don't need to be implicit for that to be supported.

[2020-09-11; Tomasz comments and provides wording]

During the LEWG 2020-08-24 telecon the LEWG provided following guidance on the issue:

> We took one poll (exact wording in the notes) to constrain the constructor which takes `filesystem::path` to only take `filesystem::path` and not things convertible to it, but only 9 out of 26 people present actually voted. Our interpretation: LWG should go ahead with making this change. There is still plenty of time for someone who hasn't yet commented on this to bring it up even if it is in a tentatively ready state. It would be nice to see a paper to address the problem of the templated path constructor, but no one has yet volunteered to do so. Note: the issue description is now a misnomer, as adding a `string_view` constructor is no longer being considered at this time.

The proposed P/R follows original LWG proposal and makes the `path` constructor of the `basic_*fstreams` "`explicit`". To adhere to current policy, we refrain from use of requires clauses and abbreviated function syntax, and introduce a Constraints element.

[2021-05-21; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 31.10.4 [ifstream], class template `basic_ifstream` synopsis, as indicated:

   >     […]
   >     explicit basic_ifstream(const string& s,
   >                             ios_base::openmode mode = ios_base::in);
   >     template<class T>
   >     explicit basic_ifstream(const filesystem::pathT& s,
   >                             ios_base::openmode mode = ios_base::in);
   >     […]

2. Modify 31.10.4.2 [ifstream.cons] as indicated:

   >     explicit basic_ifstream(const string& s,
   >                             ios_base::openmode mode = ios_base::in);

   > > -?- Effects: Equivalent to: `basic_ifstream(s.c_str(), mode)`.

   >     template<class T>
   >     explicit basic_ifstream(const filesystem::pathT& s,
   >                             ios_base::openmode mode = ios_base::in);

   > > -?- Constraints: `is_same_v<T, filesystem::path>` is `true`.

   > > -3- Effects: Equivalent to: `basic_ifstream(s.c_str(), mode)`.

3. Modify 31.10.5 [ofstream], class template `basic_ofstream` synopsis, as indicated:

   >     […]
   >     explicit basic_ofstream(const string& s,
   >                             ios_base::openmode mode = ios_base::out);
   >     template<class T>
   >     explicit basic_ofstream(const filesystem::pathT& s,
   >                             ios_base::openmode mode = ios_base::out);
   >     […]

4. Modify 31.10.5.2 [ofstream.cons] as indicated:

   >     explicit basic_ofstream(const string& s,
   >                             ios_base::openmode mode = ios_base::out);

   > > -?- Effects: Equivalent to: `basic_ofstream(s.c_str(), mode)`.

   >     template<class T>
   >     explicit basic_ofstream(const filesystem::pathT& s,
   >                             ios_base::openmode mode = ios_base::out);

   > > -?- Constraints: `is_same_v<T, filesystem::path>` is `true`.

   > > -3- Effects: Equivalent to: `basic_ofstream(s.c_str(), mode)`.

5. Modify 31.10.6 [fstream], class template `basic_fstream` synopsis, as indicated:

   >     […]
   >     explicit basic_fstream(
   >       const string& s,
   >       ios_base::openmode mode = ios_base::in | ios_base::out);
   >     template<class T>
   >     explicit basic_fstream(
   >       const filesystem::pathT& s,
   >       ios_base::openmode mode = ios_base::in | ios_base::out);
   >     […]

6. Modify 31.10.6.2 [fstream.cons] as indicated:

   >     explicit basic_fstream(
   >       const string& s,
   >       ios_base::openmode mode = ios_base::in | ios_base::out);

   > > -?- Effects: Equivalent to: `basic_fstream(s.c_str(), mode)`.

   >     template<class T>
   >     explicit basic_fstream(
   >       const filesystem::pathT& s,
   >       ios_base::openmode mode = ios_base::in | ios_base::out);

   > > -?- Constraints: `is_same_v<T, filesystem::path>` is `true`.

   > > -3- Effects: Equivalent to: `basic_fstream(s.c_str(), mode)`.

### 3431(i). `<=>` for containers should require `three_way_comparable<T>` instead of `<=>`

Section: 23.2.2.4 [container.opt.reqmts] Status: C++26 Submitter: Jonathan Wakely Opened: 2020-04-17 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

The precondition for `<=>` on containers is:

"Either `<=>` is defined for values of type (possibly `const`) `T`, or `<` is defined for values of type (possibly `const`) `T` and `<` is a total ordering relationship."

I don't think `<=>` is sufficient, because `synth-three-way` won't use `<=>` unless `three_way_comparable<T>` is satisfied, which requires `weakly-equality-comparable-with<T, T>` as well as `<=>`.

So to use `<=>` I think the type also requires `==`, or more precisely, it must satisfy `three_way_comparable`.

The problem becomes clearer with the following example:

>     #include <compare>
>     #include <vector>

>     struct X
>     {
>       friend std::strong_ordering operator<=>(X, X) { return std::strong_ordering::equal; }
>     };

>     std::vector<X> v(1);
>     std::strong_ordering c = v <=> v;

This doesn't compile, because despite `X` meeting the preconditions for `<=>` in [tab:container.opt], `synth-three-way` will return `std::weak_ordering`.

Here is another example:

>     #include <compare>
>     #include <vector>

>     struct X
>     {
>       friend bool operator<(X, X) { return true; } // The return value is intentional, see below
>       friend std::strong_ordering operator<=>(X, X) { return std::strong_ordering::equal; }
>     };

>     std::vector<X> v(1);
>     std::weak_ordering c = v <=> v;

This meets the precondition because it defines `<=>`, but the result of `<=>` on `vector<X>` will be nonsense, because `synth-three-way` will use `operator<` not `operator<=>` and that defines a broken ordering.

So we're stating a precondition which implies "if you do this, you don't get garbage results" and then we give garbage results anyway.

The proposed resolution is one way to fix that, by tightening the precondition so that it matches what `synth-three-way` actually does.

[2020-04-25 Issue Prioritization]

Priority to 2 after reflector discussion.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Modify 23.2.2 [container.requirements.general], Table [tab:container.opt], as indicated:

>    > Table 75: Optional container operations [tab:container.opt]
>    > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
>    > --- | --- | --- | --- | ---
>    > `…`
>    > `a <=> b` | `synth-three-way-result<value_type>` | `lexicographical_compare_three_way( a.begin(), a.end(), b.begin(), b.end(), synth-three-way)` | Preconditions: Either `<=>` is defined for values of type (possibly `const`) `T` satisfies `three_way_comparable`, or `<` is defined for values of type (possibly `const`) `T` and `<` is a total ordering relationship. | linear
>    > `…`

[2022-04-24; Daniel rebases wording on N4910]

Previous resolution [SUPERSEDED]

This wording is relative to N4910.

1. Modify 23.2.2.4 [container.opt.reqmts] as indicated:

   >     a <=> b

   > > -2- Result: `synth-three-way-result<X::value_type>`.

   > > -3- Preconditions: Either `<=>` is defined for values of type (possibly `const`) `T` satisfies `three_way_comparable`, or `<` is defined for values of type (possibly `const`) `T` and `<` is a total ordering relationship.

   > > -4- Returns: `lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(), synth-three-way)`

   > > [Note 1: The algorithm `lexicographical_compare_three_way` is defined in Clause 27. — end note]

   > > -5- Complexity: Linear.

[2023-06-13; Varna]

The group liked the previously suggested wording but would prefer to say "models" instead of "satisfies" in preconditions.

[2023-06-14 Varna; Move to Ready]

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 23.2.2.4 [container.opt.reqmts] as indicated:

   >     a <=> b

   > > -2- Result: `synth-three-way-result<X::value_type>`.

   > > -3- Preconditions: Either `<=>` is defined for values of type (possibly `const`) `T` models `three_way_comparable`, or `<` is defined for values of type (possibly `const`) `T` and `<` is a total ordering relationship.

   > > -4- Returns: `lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(), synth-three-way)`

   > > [Note 1: The algorithm `lexicographical_compare_three_way` is defined in Clause 27. — end note]

   > > -5- Complexity: Linear.

### 3432(i). Missing requirement for `comparison_category`

Section: 27.3.4 [string.view.comparison] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-04-19 Last modified: 2023-11-22

Priority: 0

View all issues with C++23 status.

Discussion:

It's not clear what happens if a program-defined character traits type defines `comparison_category` as a synonym for `void`, or some other bogus type.

Discussion on the LWG reflector settled on making it ill-formed at the point of use.

[2020-07-17; Moved to Ready in telecon]

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 27.3.4 [string.view.comparison] by adding a new paragraph after p3:

   > As result of reflector discussion we decided to make a drive-by fix in p3 below.

   >     template<class charT, class traits>
   >       constexpr see below operator<=>(basic_string_view<charT, traits> lhs,
   >                                       basic_string_view<charT, traits> rhs) noexcept;

   > > -3- Let `R` denote the type `traits::comparison_category` if that qualified-id is valid and denotes a type (13.10.3 [temp.deduct])it exists, otherwise `R` is `weak_ordering`.

   > > -?- Mandates: `R` denotes a comparison category type (17.12.2 [cmp.categories]).

   > > -4- Returns: `static_cast<R>(lhs.compare(rhs) <=> 0)`.

### 3433(i). `subrange::advance(n)` has UB when `n < 0`

Section: 25.5.4.3 [range.subrange.access] Status: C++23 Submitter: Casey Carter Opened: 2020-04-21 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.subrange.access].

View all issues with C++23 status.

Discussion:

`subrange::advance(n)` is specified to call `ranges::advance(begin_, n, end_)` in both `StoreSize` and `!StoreSize` cases (25.5.4.3 [range.subrange.access]/9). Unfortunately, `ranges::advance(begin_, n, end_)` has undefined behavior when `n < 0` unless `[end_, begin_)` denotes a valid range (24.4.4.2 [range.iter.op.advance]/5). This would all be perfectly fine — the UB is exposed to the caller via effects-equivalent-to — were it not the design intent that `subrange::advance(-n)` be usable to reverse the effects of `subrange::advance(n)` when the `subrange` has a bidirectional iterator type. That is, however, clearly the design intent: `subrange::prev()`, for example, is equivalent to `subrange::advance(-1)`.

[2020-05-11; Reflector prioritization]

Set priority to 2 after reflector discussions.

[2021-01-15; Issue processing telecon]

Set status to Tentatively Ready after discussion and poll.

F | A | N
--- | --- | ---
7 | 0 | 3

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.5.4.3 [range.subrange.access] as indicated:

   >     constexpr subrange& advance(iter_difference_t<I> n);

   > > -9- Effects: Equivalent to:

   > > 1. (9.1) — If `StoreSize` is `true`,

   > >    >     auto d = n - ranges::advance(begin_, n, end_);
   > >    >     if (d >= 0)
   > >    >       size_ -= to-unsigned-like(d);
   > >    >     else
   > >    >       size_ += to-unsigned-like(-d);
   > >    >     return *this;

   > > 2. (9.2) — Otherwise,

   > >    >     ranges::advance(begin_, n, end_);
   > >    >     return *this;

   > > >     if constexpr (bidirectional_iterator<I>) {
   > > >       if (n < 0) {
   > > >         ranges::advance(begin_, n);
   > > >         if constexpr (StoreSize)
   > > >           size_ += to-unsigned-like(-n);
   > > >         return *this;
   > > >       }
   > > >     }

   > > >     auto d = n - ranges::advance(begin_, n, end_);
   > > >     if constexpr (StoreSize)
   > > >       size_ -= to-unsigned-like(d);
   > > >     return *this;

### 3434(i). `ios_base` never reclaims memory for `iarray` and `parray`

Section: 31.5.2.8 [ios.base.cons] Status: C++23 Submitter: Alisdair Meredith Opened: 2020-04-26 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [ios.base.cons].

View all issues with C++23 status.

Discussion:

According to 31.5.2.6 [ios.base.storage] the class `ios_base` allocates memory, represented by two exposition-only pointers, `iarray` and `parray` in response to calls to `iword` and `pword`. However, the specification for the destructor in 31.5.2.8 [ios.base.cons] says nothing about reclaiming any allocated memory.

[2020-07-17; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 31.5.2.8 [ios.base.cons] as indicated:

   > [Drafting note: Wording modeled on container requirements]

   >     ~ios_base();

   > > -2- Effects: Calls each registered callback pair `(fn, idx)` (31.5.2.7 [ios.base.callback]) as `(*fn)(erase_event, *this, idx)` at such time that any `ios_base` member function called from within `fn` has well-defined results. Then, any memory obtained is deallocated.

### 3435(i). `three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>`

Section: 24.5.4.4 [move.iter.cons], 24.5.1.4 [reverse.iter.cons] Status: C++23 Submitter: Casey Carter Opened: 2020-04-26 Last modified: 2023-11-22

Priority: 2

View all other issues in [move.iter.cons].

View all issues with C++23 status.

Discussion:

Despite that `reverse_iterator<int*>` and `reverse_iterator<const int*>` are comparable with `<=>`, `three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>` is `false`. This unfortunate state of affairs results from the absence of constraints on `reverse_iterator`'s converting constructor: both `convertible_to<reverse_iterator<int*>, reverse_iterator<const int*>>` and `convertible_to<reverse_iterator<const int*>, reverse_iterator<int*>>` evaluate to `true`, despite that `reverse_iterator<int*>`'s converting constructor template is ill-formed when instantiated for `reverse_iterator<const int*>`. This apparent bi-convertibility results in ambiguity when trying to determine `common_reference_t<const reverse_iterator<int*>&, const reverse_iterator<const int*>&>`, causing the `common_reference` requirement in `three_way_comparable_with` to fail.

I think we should correct this by constraining `reverse_iterator`'s conversion constructor (and converting assignment operator, while we're here) correctly so we can use the concept to determine when it's ok to compare specializations of `reverse_iterator` with `<=>`.

`move_iterator` has similar issues due to its similarly unconstrained conversions. We should fix both similarly.

[2020-07-17; Priority set to 2 in telecon]

[2020-07-26; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 24.5.4.4 [move.iter.cons] as indicated:

   > [Drafting note: this incorporates and supercedes the P/R of LWG 3265(i).]

   >     constexpr move_iterator();

   > > -1- Effects: Constructs a `move_iterator`, vValue-initializesing `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value-initialized iterator of type `Iterator`.

   >     constexpr explicit move_iterator(Iterator i);

   > > -2- Effects: Constructs a `move_iterator`, iInitializesing `current` with `std::move(i)`.

   >     template<class U> constexpr move_iterator(const move_iterator<U>& u);

   > > -3- Mandates: `U` is convertible to `Iterator` Constraints: `is_same_v<U, Iterator>` is `false` and `const U&` models `convertible_to<Iterator>`.

   > > -4- Effects: Constructs a `move_iterator`, iInitializesing `current` with `u.currentbase()`.

   >     template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);

   > > -5- Mandates: `U` is convertible to `Iterator` Constraints: `is_same_v<U, Iterator>` is `false`, `const U&` models `convertible_to<Iterator>`, and `assignable_from<Iterator&, const U&>` is modeled.

   > > -6- Effects: Assigns `u.currentbase()` to `current`.

2. Modify 24.5.1.4 [reverse.iter.cons] as indicated:

   >     template<class U> constexpr reverse_iterator(const reverse_iterator<U>& u);

   > > -?- Constraints: `is_same_v<U, Iterator>` is `false` and `const U&` models `convertible_to<Iterator>`.

   > > -3- Effects: Initializes `current` with `u.current`.

   >     template<class U>
   >       constexpr reverse_iterator&
   >         operator=(const reverse_iterator<U>& u);

   > > -?- Constraints: `is_same_v<U, Iterator>` is `false`, `const U&` models `convertible_to<Iterator>`, and `assignable_from<Iterator&, const U&>` is modeled.

   > > -4- Effects: Assigns `u.currentbase()` to `current`.

   > > -5- Returns: `*this`.

### 3436(i). `std::construct_at` should support arrays

Section: 26.11.8 [specialized.construct] Status: C++26 Submitter: Jonathan Wakely Opened: 2020-04-29 Last modified: 2026-06-09

Priority: 2

View all other issues in [specialized.construct].

View all issues with C++26 status.

Discussion:

`std::construct_at` is ill-formed for array types, because the type of the `new`-expression is `T` not `T*` so it cannot be converted to the return type.

In C++17 `allocator_traits::construct` did work for arrays, because it returns `void` so there is no ill-formed conversion. On the other hand, in C++17 `allocator_traits::destroy` didn't work for arrays, because `p->~T()` isn't valid.

In C++20 `allocator_traits::destroy` does work, because `std::destroy_at` treats arrays specially, but `allocator_traits::construct` no longer works because it uses `std::construct_at`.

It seems unnecessary and/or confusing to remove support for arrays in `construct` when we're adding it in `destroy`.

I suggest that `std::construct_at` should also handle arrays. It might be reasonable to restrict that support to the case where `sizeof...(Args) == 0`, if supporting parenthesized aggregate-initialization is not desirable in `std::construct_at`.

[2020-05-09; Reflector prioritization]

Set priority to 2 after reflector discussions.

[2021-01-16; Zhihao Yuan provides wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4878.

> 1. Modify 26.11.8 [specialized.construct] as indicated:

>    >     template<class T, class... Args>
>    >       constexpr T* construct_at(T* location, Args&&... args);

>    >     namespace ranges {
>    >       template<class T, class... Args>
>    >         constexpr T* construct_at(T* location, Args&&... args);
>    >     }

>    > > -1- Constraints: The expression `::new (declval<void*>()) T(declval<Args>()...)` is well-formed when treated as an unevaluated operand.

>    > > -2- Effects: Equivalent to:

>    > > >     returnauto ptr = ::new (voidify(*location)) T(std::forward<Args>(args)...);
>    > > >     if constexpr (is_array_v<T>)
>    > > >       return launder(location);
>    > > >     else
>    > > >       return ptr;

[2021-12-07; Zhihao Yuan comments and provides improved wording]

The previous PR allows constructing arbitrary number of elements when `T` is an array of unknown bound:

>     extern int a[];
>     std::construct_at(&a, 0, 1, 2);

and leads to a UB.

Previous resolution [SUPERSEDED]

This wording is relative to N4901.

1. Modify 26.11.8 [specialized.construct] as indicated:

   >     template<class T, class... Args>
   >       constexpr T* construct_at(T* location, Args&&... args);

   >     namespace ranges {
   >       template<class T, class... Args>
   >         constexpr T* construct_at(T* location, Args&&... args);
   >     }

   > > -1- Constraints: The expression `::new (declval<void*>()) T(declval<Args>()...)` is well-formed when treated as an unevaluated operand (7.2.3 [expr.context]) and `is_unbounded_array_v<T>` is `false`.

   > > -2- Effects: Equivalent to:

   > > >     returnauto ptr = ::new (voidify(*location)) T(std::forward<Args>(args)...);
   > > >     if constexpr (is_array_v<T>)
   > > >       return launder(location);
   > > >     else
   > > >       return ptr;

[2024-03-18; Jonathan provides new wording]

During Core review in Varna, Hubert suggested creating `T[1]` for the array case.

Previous resolution [SUPERSEDED]

This wording is relative to N4971.

1. Modify 26.11.8 [specialized.construct] as indicated:

   >     template<class T, class... Args>
   >       constexpr T* construct_at(T* location, Args&&... args);

   >     namespace ranges {
   >       template<class T, class... Args>
   >         constexpr T* construct_at(T* location, Args&&... args);
   >     }

   > > -1- Constraints: `is_unbounded_array_v<T>` is `false`. The expression `::new (declval<void*>()) T(declval<Args>()...)` is well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).

   > > -2- Effects: Equivalent to:

   > > >     if constexpr (is_array_v<T>)
   > > >       return ::new (voidify(*location)) T[1]{{std::forward<Args>(args)...}};
   > > >     else
   > > >       return ::new (voidify(*location)) T(std::forward<Args>(args)...);

[St. Louis 2024-06-24; Jonathan provides improved wording]

Why not support unbounded arrays, deducing the bound from `sizeof...(Args)`?
JW: There's no motivation to support that here in `construct_at`. It isn't possible to create unbounded arrays via allocators, nor via any of the `uninitialized_xxx` algorithms. Extending `construct_at` that way seems like a design change, not restoring support for something that used to work with allocators and then got broken in C++20.

Tim observed that the proposed resolution is ill-formed if `T` has an explicit default constructor. Value-initialization would work for that case, and there seems to be little motivation for supplying arguments to initialize the array. In C++17 the `allocator_traits::construct` case only supported value-initialization.

[St. Louis 2024-06-24; move to Ready.]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 26.11.8 [specialized.construct] as indicated:

   >     template<class T, class... Args>
   >       constexpr T* construct_at(T* location, Args&&... args);

   >     namespace ranges {
   >       template<class T, class... Args>
   >         constexpr T* construct_at(T* location, Args&&... args);
   >     }

   > > -1- Constraints: `is_unbounded_array_v<T>` is `false`. The expression `::new (declval<void*>()) T(declval<Args>()...)` is well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).

   > > -?- Mandates: If `is_array_v<T>` is `true`, `sizeof...(Args)` is zero.

   > > -2- Effects: Equivalent to:

   > > >     if constexpr (is_array_v<T>)
   > > >       return ::new (voidify(*location)) T[1]();
   > > >     else
   > > >       return ::new (voidify(*location)) T(std::forward<Args>(args)...);

### 3437(i). `__cpp_lib_polymorphic_allocator` is in the wrong header

Section: 17.3.2 [version.syn] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-04-29 Last modified: 2023-11-22

Priority: 0

View other active issues in [version.syn].

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Discussion:

17.3.2 [version.syn] says that `__cpp_lib_polymorphic_allocator` is also defined in `<memory>`, but `std::polymorphic_allocator` is defined in `<memory_resource>`. This seems like an error in P1902R1. The macro should be in the same header as the feature it relates to.

[2020-05-09; Reflector prioritization]

Set status to Tentatively Ready after six votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 17.3.2 [version.syn] as indicated:

   >     […]
   >     #define __cpp_lib_parallel_algorithm    201603L // also in <algorithm>, <numeric>
   >     #define __cpp_lib_polymorphic_allocator 201902L // also in <memorymemory_resource>
   >     #define __cpp_lib_quoted_string_io      201304L // also in <iomanip>
   >     […]

### 3441(i). Misleading note about calls to customization points

Section: 16.4.5.2.1 [namespace.std] Status: C++23 Submitter: Michael Park Opened: 2020-05-08 Last modified: 2023-11-22

Priority: 1

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Discussion:

P0551 (Thou Shalt Not Specialize `std` Function Templates!) added a clause in [namespace.std]/7:

> Other than in namespace `std` or in a namespace within namespace `std`, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point. (footnote 174) [Note: This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments. — end note]

Given that `std::swap` is a designated customization point, the note seems to suggest the following:

>     namespace N {
>       struct X {};
>       void swap(X&, X&) {}
>     }

>     N::X a, b;
>     std::swap(a, b); // qualified call to customization point finds N::swap?

This is not what happens, as the call to `std::swap` does not find `N::swap`.

[2020-07-17; Priority set to 1 in telecon]

Related to 3442(i).

[2020-09-11; discussed during telecon]

The note is simply bogus, not backed up by anything normative. The normative part of the paragraph is an unacceptable landgrab on those identifiers. We have no right telling users they can't use the names `data` and `size` unless they do exactly what we say `std::data` and `std::size` do. The library only ever uses `swap` unqualified, so the effect of declaring the others as designated customization points is unclear.

The rule only needs to apply to such overloads when actually found by overload resolution in a context that expects the semantics of the customization point.

Frank: do we need to designate `operator<<` as a customization point? Users overload that in their own namespaces all the time.

Walter: This clearly needs a paper.

[2020-10-02; status to Open]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Modify 16.4.5.2.1 [namespace.std] as indicated:

>    > -7- Other than in namespace `std` or in a namespace within namespace `std`, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point. (footnote 173) [Note: This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments. — end note]

[Issaquah 2023-02-09; Jonathan provides improved wording]

The normative part of 16.4.5.2.1 [namespace.std] paragraph 7 (i.e. not the note and the footnote) does not actually do anything except tell users they're not allowed to use the names `begin`, `size` etc. unless they conform to the provisions of paragraph 7. This means a program that contains the following declaration might have undefined behaviour:

    namespace user { int data(int, int); }

This is not OK! It's not even clear what "the requirements for the customization point" are, if users wanted to meet them. It's not even clear what "provide an overload" means.

In particular, paragraph 7 does not give permission for the designated customization points to actually use such overloads. Just by forbidding users from using `data` for their own functions isn't sufficient to allow the library to use ADL to call `data`, and it's unclear why we'd want that anyway (what problem with `std::data` are we trying to solve that way?). As shown in LWG 3442(i), if `std::data` became a customization point that would be a backwards-incompatible change, and create a portability problem if some implementations did that and others didn't.

So the non-normative note and footnote make claims that do not follow from the normative wording, and should be removed.

The only one of the designated customization points that is actually looked up using ADL is `std::swap`, but how that works is fully specified by 16.4.2.2 [contents] and 16.4.4.3 [swappable.requirements]. The additional specification that it's a designated customization point serves no benefit. In particular, the permission that the note and footnote claim exists is not needed. We specify precisely how `swap` calls are performed. We don't even need to say that user overloads of `swap` in their own namespaces must meet the library requirements, because the "swappable with" and Cpp17Swappable requirements state the required semantics, and functions that use `swap` have preconditions that the types are swappable. So we correctly impose preconditions in the places that actually call swap, and don't need to globally make it undefined for any function called `swap` to not meet the requirements, even if that function is never found by ADL by the library (e.g. because it's in a namespace that is never an associated namespace of any types used with library components that require swappable types).

Paragraph 7 and its accompanying notes should go.

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 16.4.5.2.1 [namespace.std] as indicated:

   > [Drafting note: This should also remove the only index entry for "customization point". ]

   > -7- Other than in namespace `std` or in a namespace within namespace `std`, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point. 163

   > [Note 3: This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments. — end note]

   > 163) Any library customization point must be prepared to work adequately with any user-defined overload that meets the minimum requirements of this document. Therefore an implementation can elect, under the as-if rule (6.10.1 [intro.execution]), to provide any customization point in the form of an instantiated function object (22.10 [function.objects]) even though the customization point's specification is in the form of a function template. The template parameters of each such function object and the function parameters and return type of the object's `operator()` must match those of the corresponding customization point's specification.

2. Modify 22.2.2 [utility.swap] as indicated:

   >     template<class T>
   >       constexpr void swap(T& a, T& b) noexcept(see below);

   > -1- Constraints: `is_move_constructible_v<T>` is `true` and `is_move_assignable_v<T>` is `true`.

   > -2- Preconditions: Type `T` meets the Cpp17MoveConstructible (Table 32) and Cpp17MoveAssignable (Table 34) requirements.

   > -3- Effects: Exchanges values stored in two locations.

   > -4- Remarks: This function is a designated customization point (16.4.5.2.1 [namespace.std]). The exception specification is equivalent to:

   >     is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>

3. Modify 24.7 [iterator.range] as indicated:

   > -1- In addition to being available via inclusion of the <iterator> header, the function templates in 24.7 [iterator.range] are available when any of the following headers are included: <array> (23.3.2 [array.syn]), <deque> (23.3.4 [deque.syn]), <forward_list> (23.3.6 [forward.list.syn]), <list> (23.3.10 [list.syn]), <map> (23.4.2 [associative.map.syn]), <regex> (28.6.3 [re.syn]), <set> (23.4.5 [associative.set.syn]), <span> (23.7.2.1 [span.syn]), <string> (27.4.2 [string.syn]), <string_view> ( [string.view.syn]), <unordered_map> (23.5.2 [unord.map.syn]), <unordered_set> (23.5.5 [unord.set.syn]), and <vector> (23.3.12 [vector.syn]). Each of these templates is a designated customization point (16.4.5.2.1 [namespace.std]).

### 3442(i). Unsatisfiable suggested implementation of customization points

Section: 16.4.5.2.1 [namespace.std] Status: Resolved Submitter: Michael Park Opened: 2020-05-08 Last modified: 2023-03-22

Priority: 1

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Discussion:

Footnote 173 under [namespace.std]/7 reads (emphasis mine):

> Any library customization point must be prepared to work adequately with any user-defined overload that meets the minimum requirements of this document. Therefore an implementation may elect, under the as-if rule (6.10.1 [intro.execution]), to provide any customization point in the form of an instantiated function object (22.10 [function.objects]) even though the customization point's specification is in the form of a function template. The template parameters of each such function object and the function parameters and return type of the object's `operator()` must match those of the corresponding customization point's specification.

This implementation suggestion doesn't seem to be satisfiable with the as-if rule.

1. In order to maintain as-if rule for qualified calls to `std::swap`, `std::swap` cannot perform ADL look-up like `std::ranges::swap` does.

2. But then we cannot maintain as-if rule for two-step calls to `std::swap`, since ADL is turned off when function objects are involved.

   >     #include <iostream>

   >     namespace S {
   >     #ifdef CPO
   >       static constexpr auto swap = [](auto&, auto&) { std::cout << "std"; };
   >     #else
   >       template<typename T> swap(T&, T&) { std::cout << "std"; }
   >     #endif
   >     }

   >     namespace N {
   >       struct X {};
   >       void swap(X&, X&) { std::cout << "mine"; }
   >     }

   >     int main() {
   >       N::X a, b;
   >       S::swap(a, b);  // (1) prints std in both cases
   >       using S::swap;
   >       swap(a, b);     // (2) prints std with -DCPO, and mine without.
   >     }

3. We can try to satisfy the as-if rule for (2) by having `std::swap` perform ADL like `std::ranges::swap`, but then that would break (1) since qualified calls to `std::swap` would also ADL when it did not do so before.

[2020-07-17; Priority set to 1 in telecon]

Related to 3441(i).

[2020-10-02; status to Open]

[Issaquah 2023-02-09; Jonathan adds note]

This would be resolved by the new proposed resolution of LWG 3441(i).

[2023-03-22 LWG 3441 was approved in Issaquah. Status changed: Open → Resolved.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 16.4.5.2.1 [namespace.std], footnote 173, as indicated:

   > footnote 173) Any library customization point must be prepared to work adequately with any user-defined overload that meets the minimum requirements of this document. Therefore an implementation may elect, under the as-if rule (6.10.1 [intro.execution]), to provide any customization point in the form of an instantiated function object (22.10 [function.objects]) even though the customization point's specification is in the form of a function template. The template parameters of each such function object and the function parameters and return type of the object's `operator()` must match those of the corresponding customization point's specification.

### 3443(i). [networking.ts] `net::basic_socket_iostream` should use `addressof`

Section: 19.2.1 [networking.ts::socket.iostream.cons] Status: WP Submitter: Jonathan Wakely Opened: 2020-05-14 Last modified: 2026-02-21

Priority: 0

View all other issues in [networking.ts::socket.iostream.cons].

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Discussion:

Addresses: networking.ts

19.2.1 [networking.ts::socket.iostream.cons] uses `&sb_` which could find something bad via ADL.

[2020-07-17; Moved to Ready in telecon]

Jens suggested we should have blanket wording saying that when the library uses the `&` operator, it means `std::addressof`.

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4771.

1. Modify 19.2.1 [networking.ts::socket.iostream.cons] as indicated:

   >     basic_socket_iostream();

   > > -1- Effects: Initializes the base class as `basic_iostream<char>(&addressof(sb_))`, value-initializes `sb_`, and performs `setf(std::ios_base::unitbuf)`.

   >     explicit basic_socket_iostream(basic_stream_socket<protocol_type> s);

   > > -2- Effects: Initializes the base class as `basic_iostream<char>(&addressof(sb_))`, initializes `sb_` with `std::move(s)`, and performs `setf(std::ios_base::unitbuf)`.

   >     basic_socket_iostream(basic_socket_iostream&& rhs);

   > > -3- Effects: Move constructs from the rvalue `rhs`. This is accomplished by move constructing the base class, and the contained `basic_socket_streambuf`. Next `basic_iostream<char>::set_rdbuf(&addressof(sb_))` is called to install the contained `basic_socket_streambuf`.

   >     template<class... Args>
   >       explicit basic_socket_iostream(Args&&... args);

   > > -4- Effects: Initializes the base class as `basic_iostream<char>(&addressof(sb_))`, value-initializes `sb_`, and performs `setf(std::ios_base::unitbuf)`. Then calls `rdbuf()->connect(forward<Args>(args)...)`. If that function returns a null pointer, calls `setstate(failbit)`.

### 3446(i). `indirectly_readable_traits` ambiguity for types with both `value_type` and `element_type`

Section: 24.3.2.2 [readable.traits] Status: C++23 Submitter: Casey Carter Opened: 2020-05-15 Last modified: 2023-11-22

Priority: 2

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Discussion:

Per 24.3.2.2 [readable.traits], `indirectly_readable_traits<T>::value_type` is the same type as `remove_cv_t<T::value_type>` if it denotes an object type, or `remove_cv_t<T::element_type>` if it denotes an object type. If both `T::value_type` and `T::element_type` denote types, `indirectly_readable_traits<T>::value_type` is ill-formed. This was perhaps not the best design, given that there are iterators in the wild (Boost's unordered containers) that define both nested types. `indirectly_readable_traits` should tolerate iterators that define both nested types consistently.

[2020-07-17; Priority set to 2 in telecon]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Modify 24.3.2.2 [readable.traits] as indicated:

>    >     […]
>    >     template<class> struct cond-value-type { }; // exposition only

>    >     template<class T>
>    >       requires is_object_v<T>
>    >     struct cond-value-type<T> {
>    >       using value_type = remove_cv_t<T>;
>    >     };

>    >     template<class> struct indirectly_readable_traits { };

>    >     […]

>    >     template<class T>
>    >       requires requires { typename T::value_type; }
>    >     struct indirectly_readable_traits<T>
>    >       : cond-value-type<typename T::value_type> { };

>    >     template<class T>
>    >       requires requires { typename T::element_type; }
>    >     struct indirectly_readable_traits<T>
>    >       : cond-value-type<typename T::element_type> { };

>    >     template<class T>
>    >       requires requires {
>    >         typename T::element_type;
>    >         typename T::value_type;
>    >         requires same_as<
>    >           remove_cv_t<typename T::element_type>,
>    >           remove_cv_t<typename T::value_type>>;
>    >       }
>    >     struct indirectly_readable_traits<T>
>    >       : cond-value-type<typename T::value_type> { };

>    >     […]

[2020-07-23; Casey improves wording per reflector discussion]

[2020-08-21; moved to Tentatively Ready after five votes in favour in reflector poll]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 24.3.2.2 [readable.traits] as indicated:

   >     […]
   >     template<class> struct cond-value-type { }; // exposition only

   >     template<class T>
   >       requires is_object_v<T>
   >     struct cond-value-type<T> {
   >       using value_type = remove_cv_t<T>;
   >     };

   >     template<class T>
   >     concept has-member-value-type = requires { typename T::value_type; }; // exposition only

   >     template<class T>
   >     concept has-member-element-type = requires { typename T::element_type; }; // exposition only

   >     template<class> struct indirectly_readable_traits { };

   >     […]

   >     template<classhas-member-value-type T>
   >       requires requires { typename T::value_type; }
   >     struct indirectly_readable_traits<T>
   >       : cond-value-type<typename T::value_type> { };

   >     template<classhas-member-element-type T>
   >       requires requires { typename T::element_type; }
   >     struct indirectly_readable_traits<T>
   >       : cond-value-type<typename T::element_type> { };

   >     template<classhas-member-value-type T>
   >       requires has-member-element-type<T> &&
   >                same_as<remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>>
   >     struct indirectly_readable_traits<T>
   >       : cond-value-type<typename T::value_type> { };

   >     […]

### 3447(i). Deduction guides for `take_view` and `drop_view` have different constraints

Section: 25.7.10.2 [range.take.view] Status: C++23 Submitter: Jens Maurer Opened: 2020-05-15 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.take.view].

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Discussion:

From this editorial issue request:

(Note "`range R`" vs "`class R`".)

In 25.7.10.2 [range.take.view], the deduction guide for `take_view` is declared as:

>     template<range R>
>       take_view(R&&, range_difference_t<R>)
>         -> take_view<views::all_t<R>>;

In 25.7.12.2 [range.drop.view], the deduction guide for `drop_view` is declared as:

>     template<class R>
>       drop_view(R&&, range_difference_t<R>) -> drop_view<views::all_t<R>>;

Note the difference between their template parameter lists.

Suggested resolution:

Change the deduction guide of `take_view` from

>     template<range R>

to

>     template<class R>

[2020-07-17; Moved to Ready in telecon]

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.7.10.2 [range.take.view], class template `take_view` synopsis, as indicated:

   >     […]
   >     template<rangeclass R>
   >       take_view(R&&, range_difference_t<R>)
   >         -> take_view<views::all_t<R>>;
   >     […]

### 3448(i). `transform_view`'s `sentinel<false>` not comparable with `iterator<true>`

Section: 25.7.9.4 [range.transform.sentinel], 25.7.14.4 [range.join.sentinel] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-05-26 Last modified: 2023-11-22

Priority: 1

View all other issues in [range.transform.sentinel].

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Discussion:

A user reported that this doesn't compile:

>     #include <list>
>     #include <ranges>

>     std::list v{1, 2}; // works if std::vector
>     auto view1 = v | std::views::take(2);
>     auto view2 = view1 | std::views::transform([] (int i) { return i; });
>     bool b = std::ranges::cbegin(view2) == std::ranges::end(view2);

The comparison is supposed to use `operator==(iterator<Const>, sentinel<Const>)` after converting `sentinel<false>` to `sentinel<true>`. However, the `operator==` is a hidden friend so is not a candidate when comparing `iterator<true>` with `sentinel<false>`. The required conversion would only happen if we'd found the operator, but we can't find the operator until after the conversion happens.

As Patrick noted, the `join_view` sentinel has a similar problem.

The proposed wording shown below has been suggested by Casey and has been implemented and tested in GCC's libstdc++.

[2020-07-17; Priority set to 1 in telecon]

Should be considered together with 3406(i) and 3449(i).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

>    > [Drafting note: The project editor is kindly asked to consider replacing editorially all of the

>    > "`using Base = conditional_t<Const, const V, V>;`"

>    > occurrences by

>    > "`using Base = maybe-const<Const, V>;`"

>    > ]

>    >     […]
>    >     namespace std::ranges {
>    >     […]
>    >       namespace views { inline constexpr unspecified filter = unspecified; }

>    >       template<bool Const, class T>
>    >         using maybe-const = conditional_t<Const, const T, T>; // exposition-only

>    >       // 25.7.9 [range.transform], transform view
>    >       template<input_range V, copy_constructible F>
>    >         requires view<V> && is_object_v<F> &&
>    >                  regular_invocable<F&, range_reference_t<V>>
>    >       class transform_view;
>    >     […]
>    >     }

> 2. Modify 25.7.9.4 [range.transform.sentinel], class template `transform_view::sentinel` synopsis, as indicated:

>    > >     namespace std::ranges {
>    > >       template<input_range V, copy_constructible F>
>    > >         requires view<V> && is_object_v<F> &&
>    > >                  regular_invocable<F&, range_reference_t<V>> &&
>    > >                  can-reference<invoke_result_t<F&, range_reference_t<V>>>
>    > >       template<bool Const>
>    > >       class transform_view<V, F>::sentinel {
>    > >         […]
>    > >         constexpr sentinel_t<Base> base() const;

>    > >         template<bool OtherConst>
>    > >           requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    > >         friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

>    > >         template<bool OtherConst>
>    > >           requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    > >         friend constexpr range_difference_t<Base>
>    > >           operator-(const iterator<OtherConst>& x, const sentinel& y)
>    > >             requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

>    > >         template<bool OtherConst>
>    > >           requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    > >         friend constexpr range_difference_t<Base>
>    > >           operator-(const sentinel& y, const iterator<OtherConst>& x)
>    > >             requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
>    > >       };
>    > >     }

>    > […]

>    >     template<bool OtherConst>
>    >       requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    >     friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

>    > > -4- Effects: Equivalent to: `return x.current_ == y.end_;`

>    >     template<bool OtherConst>
>    >       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    >     friend constexpr range_difference_t<Base>
>    >       operator-(const iterator<OtherConst>& x, const sentinel& y)
>    >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

>    > > -5- Effects: Equivalent to: `return x.current_ - y.end_;`

>    >     template<bool OtherConst>
>    >       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    >     friend constexpr range_difference_t<Base>
>    >       operator-(const sentinel& y, const iterator<OtherConst>& x)
>    >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

>    > > -6- Effects: Equivalent to: `return y.end_ - x.current_;`

> 3. Modify 25.7.14.4 [range.join.sentinel], class template `join_view::sentinel` synopsis, as indicated:

>    > >     namespace std::ranges {
>    > >       template<input_range V>
>    > >         requires view<V> && input_range<range_reference_t<V>> &&
>    > >                  (is_reference_v<range_reference_t<V>> ||
>    > >                  view<range_value_t<V>>)
>    > >       template<bool Const>
>    > >       class join_view<V>::sentinel {
>    > >         […]

>    > >         template<bool OtherConst>
>    > >           requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    > >         friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
>    > >     };
>    > >     }

>    > […]

>    >     template<bool OtherConst>
>    >       requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    >     friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

>    > > -3- Effects: Equivalent to: `return x.outer_ == y.end_;`

[2020-08-21 Tim updates PR per telecon discussion]

As noted in the PR of LWG 3406(i), the return type of `operator-` should be based on the constness of the iterator rather than that of the sentinel, as `sized_sentinel_for<S, I>` (24.3.4.8 [iterator.concept.sizedsentinel]) requires `decltype(i - s)` to be `iter_difference_t<I>`.

[2020-10-02; Status to Tentatively Ready after five positive votes on the reflector]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   > [Drafting note: The project editor is kindly asked to consider replacing editorially all of the

   > "`using Base = conditional_t<Const, const V, V>;`"

   > occurrences by

   > "`using Base = maybe-const<Const, V>;`"

   > ]

   >     […]
   >     namespace std::ranges {
   >     […]
   >       namespace views { inline constexpr unspecified filter = unspecified; }

   >       template<bool Const, class T>
   >         using maybe-const = conditional_t<Const, const T, T>; // exposition-only

   >       // 25.7.9 [range.transform], transform view
   >       template<input_range V, copy_constructible F>
   >         requires view<V> && is_object_v<F> &&
   >                  regular_invocable<F&, range_reference_t<V>>
   >       class transform_view;
   >     […]
   >     }

2. Modify 25.7.9.4 [range.transform.sentinel], class template `transform_view::sentinel` synopsis, as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, copy_constructible F>
   > >         requires view<V> && is_object_v<F> &&
   > >                  regular_invocable<F&, range_reference_t<V>> &&
   > >                  can-reference<invoke_result_t<F&, range_reference_t<V>>>
   > >       template<bool Const>
   > >       class transform_view<V, F>::sentinel {
   > >         […]
   > >         constexpr sentinel_t<Base> base() const;

   > >         template<bool OtherConst>
   > >           requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >         friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

   > >         template<bool OtherConst>
   > >           requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >         friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
   > >           operator-(const iterator<OtherConst>& x, const sentinel& y)
   > >             requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

   > >         template<bool OtherConst>
   > >           requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >         friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
   > >           operator-(const sentinel& y, const iterator<OtherConst>& x)
   > >             requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
   > >       };
   > >     }

   > […]

   >     template<bool OtherConst>
   >       requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

   > > -4- Effects: Equivalent to: `return x.current_ == y.end_;`

   >     template<bool OtherConst>
   >       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
   >       operator-(const iterator<OtherConst>& x, const sentinel& y)
   >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

   > > -5- Effects: Equivalent to: `return x.current_ - y.end_;`

   >     template<bool OtherConst>
   >       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
   >       operator-(const sentinel& y, const iterator<OtherConst>& x)
   >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

   > > -6- Effects: Equivalent to: `return y.end_ - x.current_;`

3. Modify 25.7.14.4 [range.join.sentinel], class template `join_view::sentinel` synopsis, as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V>
   > >         requires view<V> && input_range<range_reference_t<V>> &&
   > >                  (is_reference_v<range_reference_t<V>> ||
   > >                  view<range_value_t<V>>)
   > >       template<bool Const>
   > >       class join_view<V>::sentinel {
   > >         […]

   > >         template<bool OtherConst>
   > >           requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >         friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
   > >     };
   > >     }

   > […]

   >     template<bool OtherConst>
   >       requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

   > > -3- Effects: Equivalent to: `return x.outer_ == y.end_;`

### 3449(i). `take_view` and `take_while_view`'s `sentinel<false>` not comparable with their const iterator

Section: 25.7.10.3 [range.take.sentinel], 25.7.11.3 [range.take.while.sentinel] Status: C++23 Submitter: Tim Song Opened: 2020-06-06 Last modified: 2023-11-22

Priority: 1

View all other issues in [range.take.sentinel].

View all issues with C++23 status.

Discussion:

During reflector discussion it was noted that the issue observed in LWG 3448(i) applies to `take_view::sentinel` and `take_while_view::sentinel` as well.

[2020-06-15 Tim corrects a typo in the PR]

[2020-10-02; Priority set to 1 in telecon]

Should be considered together with 3406(i) and 3448(i).

[2020-10-02; Status to Tentatively Ready after five positive votes on the reflector]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861. It assumes the `maybe-const` helper introduced by the P/R of LWG 3448(i).

1. Modify 25.7.10.3 [range.take.sentinel] as indicated:

   > [Drafting note: Unlike LWG 3448(i), these operators can't be easily templatized, so the approach taken here is to add an overload for the opposite constness instead. However, because `const V` may not even be a range (and therefore `iterator_t<const V>` may not be well-formed), we need to defer instantiating the signature of the new overload until the point of use. ]

   > >     namespace std::ranges {
   > >       template<view V>
   > >       template<bool Const>
   > >       class take_view<V>::sentinel {
   > >       private:
   > >         using Base = conditional_t<Const, const V, V>;  // exposition only
   > >         template<bool OtherConst>
   > >         using CI = counted_iterator<iterator_t<Basemaybe-const<OtherConst, V>>>;  // exposition only

   > >         […]

   > >         constexpr sentinel_t<Base> base() const;

   > >         friend constexpr bool operator==(const CI<Const>& y, const sentinel& x);

   > >         template<bool OtherConst = !Const>
   > >             requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >         friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);

   > >       };
   > >     }

   > […]

   >     friend constexpr bool operator==(const CI<Const>& y, const sentinel& x);

   >     template<bool OtherConst = !Const>
   >         requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);

   > > -4- Effects: Equivalent to: `return y.count() == 0 || y.base() == x.end_;`

2. Modify 25.7.11.3 [range.take.while.sentinel] as indicated:

   > >     namespace std::ranges {
   > >         template<view V, class Pred>
   > >         requires input_range<V> && is_object_v<Pred> &&
   > >                     indirect_unary_predicate<const Pred, iterator_t<V>>
   > >         template<bool Const>
   > >         class take_while_view<V>::sentinel {
   > >           […]

   > >           constexpr sentinel_t<Base> base() const { return end_; }

   > >           friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y);

   > >           template<bool OtherConst = !Const>
   > >               requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   > >           friend constexpr bool operator==(const iterator_t<maybe-const<OtherConst, V>>& x, const sentinel& y);

   > >         };
   > >     }

   > […]

   >     friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y);

   >     template<bool OtherConst = !Const>
   >         requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   >     friend constexpr bool operator==(const iterator_t<maybe-const<OtherConst, V>>& x, const sentinel& y);

   > > -3- Effects: Equivalent to: `return y.end_ == x || !invoke(*y.pred_, *x);`

### 3450(i). The `const` overloads of `take_while_view::begin/end` are underconstrained

Section: 25.7.11.2 [range.take.while.view] Status: C++23 Submitter: Tim Song Opened: 2020-06-06 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.take.while.view].

View all issues with C++23 status.

Discussion:

The `const` overloads of `take_while_view::begin` and `take_while_view::end` are underconstrained: they should require the predicate to model `indirect_unary_predicate<iterator_t<const V>>`. A simple example is

>         #include <ranges>

>         auto v = std::views::single(1) | std::views::take_while([](int& x) { return true;});
>         static_assert(std::ranges::range<decltype(v)>);
>         static_assert(std::ranges::range<decltype(v) const>);
>         bool b = std::ranges::cbegin(v) == std::ranges::cend(v);

Here, the `static_assert`s pass but the comparison fails to compile. The other views using `indirect_unary_predicate` (`filter_view` and `drop_while_view`) do not have this problem because they do not support const-iteration.

[2020-07-17; Moved to Ready in telecon]

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.7.11.2 [range.take.while.view], class template `take_while_view` synopsis, as indicated:

   > >     namespace std::ranges {
   > >         template<view V, class Pred>
   > >           requires input_range<V> && is_object_v<Pred> &&
   > >                    indirect_unary_predicate<const Pred, iterator_t<V>>
   > >         class take_while_view : public view_interface<take_while_view<V, Pred>> {
   > >         […]

   > >         constexpr auto begin() requires (!simple-view<V>)
   > >         { return ranges::begin(base_); }

   > >         constexpr auto begin() const requires range<const V> && indirect_unary_predicate<const Pred, iterator_t<const V>>
   > >         { return ranges::begin(base_); }

   > >         constexpr auto end() requires (!simple-view<V>)
   > >         { return sentinel<false>(ranges::end(base_), addressof(*pred_)); }

   > >         constexpr auto end() const requires range<const V> && indirect_unary_predicate<const Pred, iterator_t<const V>>
   > >         { return sentinel<true>(ranges::end(base_), addressof(*pred_)); }
   > >         };
   > >     }

### 3452(i). Are views really supposed to have strict `𝒪(1)` destruction?

Section: 25.4.5 [range.view] Status: Resolved Submitter: Mathias Stearn Opened: 2020-06-16 Last modified: 2021-10-23

Priority: 2

View all other issues in [range.view].

View all issues with Resolved status.

Discussion:

The second bullet of 25.4.5 [range.view] paragraph 3 says "Examples of views are:[…] A range type that holds its elements by `shared_ptr` and shares ownership with all its copies". That clearly does not have `𝒪(1)` destruction in all cases. However, that does seem like a useful type of view, and is related to the discussions around the proposed `std::generator`.

What is the purpose of requiring `𝒪(1)` destruction? Would it still be achieved by weakening it slightly to something like "If N copies and/or moves are made from a view that yields M values, destroying all of them takes time proportional to at worst `𝒪(N+M)`"? This in particular prevents the `𝒪(N*M)` case that I think the rules are trying to prevent, while still allowing some more interesting types of views.

If instead we actually really do want strict `𝒪(1)` destruction, then the example needs to be fixed.

[2020-06-26; Reflector prioritization]

Set priority to 2 after reflector discussions.

[2021-01-15; Telecon discussion]

Set status to LEWG for guidance on the issue of what it means to model view.

[2021-02-16; Library Evolution Telecon Minutes]

Poll: The `shared_ptr` part of the example mentioned in LWG3452 should be removed.

    SF F N A SA
    4  6 3 1 0

Attendance: 25

Outcome: Remove the `shared_ptr` part.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4878.

> 1. Modify 25.4.5 [range.view] as indicated:

>    >     template<class T>
>    >       concept view =
>    >         range<T> && movable<T> && default_initializable<T> && enable_view<T>;

>    > […]

>    > > -3- [Example 1: Examples of `view`s are:

>    > > 1. (3.1) — A `range` type that wraps a pair of iterators.

>    > > 2. (3.2) — A `range` type that holds its elements by shared_ptr and shares ownership with all its copies.

>    > > 3. (3.3) — A `range` type that generates its elements on demand.

>    > > Most containers (Clause 22) are not views since destruction of the container destroys the elements, which cannot be done in constant time. — end example]

[2021-10-23 Resolved by the adoption of P2415R2 at the October 2021 plenary. Status changed: LEWG → Resolved.]

Proposed resolution:

### 3453(i). Generic code cannot call `ranges::advance(i, s)`

Section: 24.4.4.2 [range.iter.op.advance], 24.3.4.7 [iterator.concept.sentinel] Status: C++23 Submitter: Casey Carter Opened: 2020-06-18 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.iter.op.advance].

View all issues with C++23 status.

Discussion:

The specification of the iterator & sentinel overload of `ranges::advance` in 24.4.4.2 [range.iter.op.advance] reads:

>     template<input_or_output_iterator I, sentinel_for<I> S>
>       constexpr void ranges::advance(I& i, S bound);

> -3- Preconditions: `[i, bound)` denotes a range.

> -4- Effects:

> 1. (4.1) — If `I` and `S` model `assignable_from<I&, S>`, equivalent to `i = std::move(bound)`.

> 2. (4.2) — […]

The assignment optimization in bullet 4.1 is just fine for callers with concrete types who can decide whether or not to call `advance` depending on the semantics of the assignment performed. However, since this assignment operation isn't part of the `input_or_output_iterator` or `sentinel_for` requirements its semantics are unknown for arbitrary types. Effectively, generic code is forbidden to call this overload of `advance` when `assignable_from<I&, S>` is satisfied and non-generic code must tread lightly. This seems to make the library dangerously unusable. We can correct this problem by either:

1. Making the assignment operation in question an optional part of the `sentinel_for` concept with well-defined semantics. This concept change should be relatively safe given that `assignable_from<I&, S>` requires `common_reference_with<const I&, const S&>`, which is very rarely satisfied inadvertently.

2. Requiring instead `same_as<I, S>` to trigger the assignment optimization in bullet 4.1 above. `S` is `semiregular`, so `i = std::move(s)` is certainly well-formed (and has well-defined semantics thanks to `semiregular`) when `I` and `S` are the same type. The optimization will not apply in as many cases, but we don't need to make a scary concept change, either.

[2020-06-26; Reflector prioritization]

Set priority to 2 after reflector discussions.

[2020-08-21; Issue processing telecon: Option A is Tentatively Ready]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> Wording for both Option A and Option B are provided.

> Option A:

> 1. Modify 24.3.4.7 [iterator.concept.sentinel] as indicated:

>    >     template<class S, class I>
>    >       concept sentinel_for =
>    >         semiregular<S> &&
>    >         input_or_output_iterator<I> &&
>    >         weakly-equality-comparable-with<S, I>; // See 18.5.4 [concept.equalitycomparable]

>    > > -2- Let `s` and `i` be values of type `S` and `I` such that `[i, s)` denotes a range. Types `S` and `I` model `sentinel_for<S, I>` only if

>    > > 1. (2.1) — `i == s` is well-defined.

>    > > 2. (2.2) — If `bool(i != s)` then `i` is dereferenceable and `[++i, s)` denotes a range.

>    > > 3. (2.?) — `assignable_from<I&, S>` is either modeled or not satisfied.

> Option B:

> 1. Modify 24.4.4.2 [range.iter.op.advance] as indicated:

>    >     template<input_or_output_iterator I, sentinel_for<I> S>
>    >       constexpr void ranges::advance(I& i, S bound);

>    > > -3- Preconditions: `[i, bound)` denotes a range.

>    > > -4- Effects:

>    > > 1. (4.1) — If `I` and `S` model `assignable_from<I&, S>same_as<I, S>`, equivalent to `i = std::move(bound)`.

>    > > 2. (4.2) — […]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 24.3.4.7 [iterator.concept.sentinel] as indicated:

   >     template<class S, class I>
   >       concept sentinel_for =
   >         semiregular<S> &&
   >         input_or_output_iterator<I> &&
   >         weakly-equality-comparable-with<S, I>; // See 18.5.4 [concept.equalitycomparable]

   > > -2- Let `s` and `i` be values of type `S` and `I` such that `[i, s)` denotes a range. Types `S` and `I` model `sentinel_for<S, I>` only if

   > > 1. (2.1) — `i == s` is well-defined.

   > > 2. (2.2) — If `bool(i != s)` then `i` is dereferenceable and `[++i, s)` denotes a range.

   > > 3. (2.?) — `assignable_from<I&, S>` is either modeled or not satisfied.

### 3454(i). `pointer_traits::pointer_to` should be `constexpr`

Section: 20.2.3 [pointer.traits] Status: C++26 Submitter: Alisdair Meredith Opened: 2020-06-21 Last modified: 2026-06-09

Priority: 4

View all other issues in [pointer.traits].

View all issues with C++26 status.

Discussion:

Trying to implement a `constexpr std::list` (inspired by Tim Song's note on using variant members in the node) as part of evaluating the `constexpr` container and adapters proposals, I hit problems I could not code around in `pointer_traits`, as only the specialization for native pointers has a `constexpr pointer_to` function.

This means that containers of my custom allocator, that delegates all allocation behavior to `std::allocator<T>` but adds extra telemetry and uses a fancy pointer, does not work with the approach I tried for implementing `list` (common link type, shared between nodes, and stored as `end` sentinel directly in the `list` object).

[2020-07-17; Forwarded to LEWG after review in telecon]

[2022-07-19; Casey Carter comments]

This is no longer simply a theoretical problem that impedes implementing `constexpr std::list`, but an actual defect affecting current implementations of `constexpr std::string`. More specifically, it makes it impossible to support so-called "fancy pointers" in a `constexpr basic_string` that performs the small string optimization (SSO). (`pointer_traits::pointer_to` is critically necessary to get a pointer that designates the SSO buffer.) As things currently stand, `constexpr basic_string` can support fancy pointers or SSO, but not both.

[Wrocław 2024-11-18; LEWG approves the direction]

Should there be an Annex C entry noting that program-defined specializations need to add `constexpr` to be conforming?

[2025-10-20; Set priority to 4 based on age of issue and lack of recent interest.]

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 20.2.3 [pointer.traits] as indicated:

   > -1- The class template `pointer_traits` supplies a uniform interface to certain attributes of pointer-like types.

   > >     namespace std {
   > >       template<class Ptr> struct pointer_traits {
   > >         using pointer         = Ptr;
   > >         using element_type    = see below;
   > >         using difference_type = see below;

   > >         template<class U> using rebind = see below;

   > >         static constexpr pointer pointer_to(see below r);
   > >       };
   > >       […]
   > >     }

2. Modify 20.2.3.3 [pointer.traits.functions] as indicated:

   >     static constexpr pointer pointer_traits::pointer_to(see below r);
   >     static constexpr pointer pointer_traits<T*>::pointer_to(see below r) noexcept;

   > > -1- Mandates: For the first member function, `Ptr::pointer_to(r)` is well-formed.

   > > -2- Preconditions: For the first member function, `Ptr::pointer_to(r)` returns a pointer to `r` through which indirection is valid.

   > > -3- Returns: The first member function returns `Ptr::pointer_to(r)`. The second member function returns `addressof(r)`.

   > > -4- Remarks: If `element_type` is cv `void`, the type of `r` is unspecified; otherwise, it is `element_type&`.

### 3455(i). Incorrect Postconditions on `unique_ptr` move assignment

Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++23 Submitter: Howard Hinnant Opened: 2020-06-22 Last modified: 2023-11-22

Priority: 0

View all other issues in [unique.ptr.single.asgn].

View all issues with C++23 status.

Discussion:

20.3.1.3.4 [unique.ptr.single.asgn]/p5 says this about the `unique_ptr` move assignment operator:

> Postconditions: `u.get() == nullptr`.

But this is only true if `this != &u`. For example:

>     #include <iostream>
>     #include <memory>

>     int main()
>     {
>       auto x = std::unique_ptr<int>(new int{3});
>       x = std::move(x);
>       if (x)
>         std::cout << *x << '\n';
>       else
>         std::cout << "nullptr\n";
>     }

Output:

>     3

An alternative resolution to that proposed below is to just delete the Postcondition altogether as the Effects element completely specifies everything. If we do that, then we should also remove p10, the Postconditions element for the converting move assignment operator. I have a slight preference for the proposed change below as it is more informative, at the expense of being a little more repetitive.

[2020-06-26; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after seven votes in favor during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 20.3.1.3.4 [unique.ptr.single.asgn] as indicated:

   >     unique_ptr& operator=(unique_ptr&& u) noexcept;

   > > -1- Constraints: `is_move_assignable_v<D>` is `true`.

   > > -2- Preconditions: If `D` is not a reference type, `D` meets the Cpp17MoveAssignable requirements (Table [tab:cpp17.moveassignable]) and assignment of the deleter from an rvalue of type `D` does not throw an exception. Otherwise, `D` is a reference type; `remove_reference_t<D>` meets the Cpp17CopyAssignable requirements and assignment of the deleter from an lvalue of type `D` does not throw an exception.

   > > -3- Effects: Calls `reset(u.release())` followed by `get_deleter() = std::forward<D>(u.get_deleter())`.

   > > -4- Returns: `*this`.

   > > -5- Postconditions: If `this != addressof(u)`, `u.get() == nullptr`, otherwise `u.get()` is unchanged.

### 3458(i). Is `shared_future` intended to work with arrays or function types?

Section: 32.10.7 [futures.unique.future], 32.10.8 [futures.shared.future] Status: Resolved Submitter: Tomasz Kamiński Opened: 2020-06-28 Last modified: 2020-11-09

Priority: 0

View all other issues in [futures.unique.future].

View all issues with Resolved status.

Discussion:

Currently there is no prohibition of creating `shared_future<T>` where `T` is either an array type or a function type. However such objects cannot be constructed, as the corresponding `future<T>`, is ill-formed due the signature `get()` method being ill-formed — it will be declared as function returning an array or function type, respectively. [Note: For `shared_future` `get` always returns an reference, so it is well-formed for array or function types.]

[2020-07-17; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> > Ideally the wording below would use a Mandates: element, but due to the still open issue LWG 3193(i) the wording below uses instead the more general "ill-formed" vocabulary.

> 1. Modify 32.10.7 [futures.unique.future] as indicated:

>    > >     namespace std {
>    > >       template<class R>
>    > >       class future {
>    > >         […]
>    > >       };
>    > >     }

>    > -?- If `is_array_v<R>` is `true` or `is_function_v<R>` is `true`, the program is ill-formed.

>    > -4- The implementation provides the template `future` and two specializations, `future<R&>` and `future<void>`. These differ only in the return type and return value of the member function `get`, as set out in its description, below.

> 2. Modify 32.10.8 [futures.shared.future] as indicated:

>    > >     namespace std {
>    > >       template<class R>
>    > >       class shared_future {
>    > >         […]
>    > >       };
>    > >     }

>    > -?- If `is_array_v<R>` is `true` or `is_function_v<R>` is `true`, the program is ill-formed.

>    > -4- The implementation provides the template `shared_future` and two specializations, `shared_future<R&>` and `shared_future<void>`. These differ only in the return type and return value of the member function `get`, as set out in its description, below.

[2020-08-02; Daniel comments]

I'm request to reopen the issue and to follow the recent wording update of LWG 3466(i) instead.

[2020-11-09 Resolved by acceptance of 3466(i). Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

Resolved by 3466(i)

### 3460(i). Unimplementable `noop_coroutine_handle` guarantees

Section: 17.13.5.2.4 [coroutine.handle.noop.resumption] Status: C++23 Submitter: Casey Carter Opened: 2020-07-01 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

17.13.5.2.4 [coroutine.handle.noop.resumption]/2 states "Remarks: If `noop_coroutine_handle` is converted to `coroutine_handle<>`, calls to `operator()`, `resume` and `destroy` on that handle will also have no observable effects." This suggests that e.g. in this function:

>     void f(coroutine_handle<> meow)
>     {
>       auto woof = noop_coroutine();
>       static_cast<coroutine_handle<>&>(woof) = meow;
>       static_cast<coroutine_handle<>&>(woof).resume();
>     }

the final call to `coroutine_handle<>::resume` must have no effect regardless of what coroutine (if any) `meow` refers to, contradicting the specification of `coroutine_handle<>::resume`. Even absent this contradiction, implementing the specification requires `coroutine_handle<>::resume` to determine if `*this` is a base subobject of a `noop_coroutine_handle`, which seems pointlessly expensive to implement.

17.13.5.2.6 [coroutine.handle.noop.address]/2 states "Remarks: A `noop_coroutine_handle`'s `ptr` is always a non-null pointer value." Similar to the above case, a slicing assignment of a default-initialized `coroutine_handle<>` to a `noop_coroutine_handle` must result in `ptr` having a null pointer value — another contradiction between the requirements of `noop_coroutine_handle` and `coroutine_handle<>`.

[2020-07-12; Reflector prioritization]

Set priority to 2 after reflector discussions.

[2020-07-29 Tim adds PR and comments]

The root cause for this issue as well as issue 3469(i) is the unnecessary public derivation from `coroutine_handle<void>`. The proposed resolution below replaces the derivation with a conversion function and adds explicit declarations for members that were previously inherited. It also modifies the preconditions on `from_address` with goal of making it impossible to obtain a `coroutine_handle<P>` to a coroutine whose promise type is not `P` in well-defined code.

[2020-08-21; Issue processing telecon: moved to Tentatively Ready]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861 and also resolves LWG issue 3469(i).

1. Edit 17.13.4 [coroutine.handle] as indicated:

   >     namespace std {

   >       […]

   >       template<class Promise>
   >       struct coroutine_handle : coroutine_handle<>
   >       {
   >         // [coroutine.handle.con], construct/reset
   >         using coroutine_handle<>::coroutine_handle;
   >         constexpr coroutine_handle() noexcept;
   >         constexpr coroutine_handle(nullptr_t) noexcept;
   >         static coroutine_handle from_promise(Promise&);
   >         coroutine_handle& operator=(nullptr_t) noexcept;

   >         // [coroutine.handle.export.import], export/import
   >         constexpr void* address() const noexcept;
   >         static constexpr coroutine_handle from_address(void* addr);

   >         // [coroutine.handle.conv], conversion
   >         constexpr operator coroutine_handle<>() const noexcept;

   >         // [coroutine.handle.observers], observers
   >         constexpr explicit operator bool() const noexcept;
   >         bool done() const;

   >         // [coroutine.handle.resumption], resumption
   >         void operator()() const;
   >         void resume() const;
   >         void destroy() const;

   >         // [coroutine.handle.promise], promise access
   >         Promise& promise() const;

   >       private:
   >         void* ptr;  // exposition only
   >       };
   >     }

   > -1- An object of type `coroutine_handle<T>` is called a coroutine handle and can be used to refer to a suspended or executing coroutine. A default-constructed `coroutine_handle` object whose member `address()` returns a null pointer value does not refer to any coroutine. Two `coroutine_handle` objects refer to the same coroutine if and only if their member `address()` returns the same value.

2. Add the following subclause under 17.13.4 [coroutine.handle], immediately after 17.13.4.2 [coroutine.handle.con]:

   > ?.?.?.? Conversion [coroutine.handle.conv]

   >         constexpr operator coroutine_handle<>() const noexcept;

   > > -1- Effects: Equivalent to: `return coroutine_handle<>::from_address(address());`.

3. Edit 17.13.4.4 [coroutine.handle.export.import] as indicated, splitting the two versions:

   >     static constexpr coroutine_handle<> coroutine_handle<>::from_address(void* addr);

   > > -?- Preconditions: `addr` was obtained via a prior call to `address` on an object whose type is a specialization of `coroutine_handle`.

   > > -?- Postconditions: `from_address(address()) == *this`.

   >     static constexpr coroutine_handle<Promise> coroutine_handle<Promise>::from_address(void* addr);

   > > -2- Preconditions: `addr` was obtained via a prior call to `address` on an object of type cv `coroutine_handle<Promise>`.

   > > -3- Postconditions: `from_address(address()) == *this`.

4. Edit 17.13.5.2 [coroutine.handle.noop] as indicated:

   >     namespace std {
   >       template<>
   >       struct coroutine_handle<noop_coroutine_promise> : coroutine_handle<>
   >       {
   >         // [coroutine.handle.noop.conv], conversion
   >         constexpr operator coroutine_handle<>() const noexcept;

   >         // [coroutine.handle.noop.observers], observers
   >         constexpr explicit operator bool() const noexcept;
   >         constexpr bool done() const noexcept;

   >         // [coroutine.handle.noop.resumption], resumption
   >         constexpr void operator()() const noexcept;
   >         constexpr void resume() const noexcept;
   >         constexpr void destroy() const noexcept;

   >         // [coroutine.handle.noop.promise], promise access
   >         noop_coroutine_promise& promise() const noexcept;

   >         // [coroutine.handle.noop.address], address
   >         constexpr void* address() const noexcept;

   >       private:
   >         coroutine_handle(unspecified);
   >         void* ptr; // exposition only
   >       };
   >     }

5. Add the following subclause under 17.13.5.2 [coroutine.handle.noop], immediately before 17.13.5.2.3 [coroutine.handle.noop.observers]:

   > ?.?.?.?.? Conversion [coroutine.handle.noop.conv]

   >         constexpr operator coroutine_handle<>() const noexcept;

   > > -1- Effects: Equivalent to: `return coroutine_handle<>::from_address(address());`.

### 3461(i). `convertible_to`'s description mishandles cv-qualified `void`

Section: 18.4.4 [concept.convertible] Status: C++23 Submitter: Tim Song Opened: 2020-07-03 Last modified: 2023-11-22

Priority: 0

View other active issues in [concept.convertible].

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Discussion:

There are no expressions of type cv-qualified `void` because any such expression must be prvalues and 7.2.2 [expr.type]/2 states:

> If a prvalue initially has the type "cv `T`", where `T` is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to `T` prior to any further analysis.

However, 18.4.4 [concept.convertible] p1 states:

> Given types `From` and `To` and an expression `E` such that `decltype((E))` is `add_rvalue_reference_t<From>`, `convertible_to<From, To>` requires `E` to be both implicitly and explicitly convertible to type `To`.

When `From` is cv-qualified `void`, `E` does not exist, yet we do want `convertible_to<const void, void>` to be modeled.

[2020-07-12; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 18.4.4 [concept.convertible] as indicated:

   > -1- Given types `From` and `To` and an expression `E` such that `decltype((E))` is `add_rvalue_reference_t<From>`whose type and value category are the same as those of `declval<From>()`, `convertible_to<From, To>` requires `E` to be both implicitly and explicitly convertible to type `To`. The implicit and explicit conversions are required to produce equal results.

### 3462(i). §[formatter.requirements]: Formatter requirements forbid use of `fc.arg()`

Section: 28.5.6.1 [formatter.requirements] Status: C++23 Submitter: Alberto Barbati Opened: 2020-06-30 Last modified: 2023-11-22

Priority: 3

View all other issues in [formatter.requirements].

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Discussion:

The requirements on the expression `f.format(t, fc)` in [tab:formatter] say

> Formats `t` according to the specifiers stored in `*this`, writes the output to `fc.out()` and returns an iterator past the end of the output range. The output shall only depend on `t`, `fc.locale()`, and the range `[pc.begin(), pc.end())` from the last call to `f.parse(pc)`.

Strictly speaking, this wording effectively forbids `f.format(t, fc)` from calling `fc.arg(n)`, whose motivation is precisely to allow a formatter to rely on arguments different from `t`. According to this interpretation, there's no conforming way to implement the `"{ arg-id }"` form of the width and precision fields of standard format specifiers. Moreover, the formatter described in the example if paragraph 28.5.6.7 [format.context]/8 would also be non-conforming.

[2020-07-12; Reflector prioritization]

Set priority to 3 after reflector discussions.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Modify 28.5.6.1 [formatter.requirements], Table [tab:formatter], as indicated:

>    > Table 67: Formatter requirements [tab:formatter]
>    > Expression | Return type | Requirement
>    > --- | --- | ---
>    > `…`
>    > `f.format(t, fc)` | `FC::iterator` | Formats `t` according to the specifiers stored in `*this`, writes the output to `fc.out()` and returns an iterator past the end of the output range. The output shall only depend on `t`, `fc.locale()`, and the range `[pc.begin(), pc.end())` from the last call to `f.parse(pc)`, and `fc.arg(n)`, where `n` is a `size_t` index value that has been validated with a call to `pc.check_arg_id(n)` in the last call to `f.parse(pc)`.
>    > `…`

[2021-05-20 Tim comments and updates wording]

During reflector discussion Victor said that the formatter requirements should allow dependency on any of the format arguments in the context. The wording below reflects that.

[2021-05-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 28.5.6.1 [formatter.requirements], Table [tab:formatter], as indicated:

   > Table 67: Formatter requirements [tab:formatter]
   > Expression | Return type | Requirement
   > --- | --- | ---
   > `…`
   > `f.format(t, fc)` | `FC::iterator` | Formats `t` according to the specifiers stored in `*this`, writes the output to `fc.out()` and returns an iterator past the end of the output range. The output shall only depend on `t`, `fc.locale()`, `fc.arg(n)` for any value `n` of type `size_t`, and the range `[pc.begin(), pc.end())` from the last call to `f.parse(pc)`.
   > `…`

### 3464(i). `istream::gcount()` can overflow

Section: 31.7.5.4 [istream.unformatted] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-07-10 Last modified: 2023-11-22

Priority: 0

View all other issues in [istream.unformatted].

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Discussion:

The standard doesn't say what `gcount()` should return if the last unformatted input operation extracted more than `numeric_limits<streamsize>::max()` characters. This is possible when using `istream::ignore(numeric_limits<streamsize>::max(), delim)`, which will keep extracting characters until the delimiter is found. On a 32-bit platform files larger than 2GB can overflow the counter, so can a streambuf reading from a network socket, or producing random characters.

Libstdc++ saturates the counter in `istream::ignore`, so that `gcount()` returns `numeric_limits<streamsize>::max()`. Libc++ results in an integer overflow.

As far as I'm aware, only `istream::ignore` can extract more than `numeric_limits<streamsize>::max()` characters at once. We could either fix it in the specification of `ignore`, or in `gcount`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> Option A:

> 1. Modify 31.7.5.4 [istream.unformatted] as indicated:

>    >     streamsize gcount() const;

>    > > -2- Effects: None. This member function does not behave as an unformatted input function (as described above).

>    > > -3- Returns: The number of characters extracted by the last unformatted input member function called for the object. If the number cannot be represented, returns `numeric_limits<streamsize>::max()`.

> Option B:

> 1. Modify 31.7.5.4 [istream.unformatted] as indicated:

>    >     basic_istream<charT, traits>& ignore(streamsize n = 1, int_type delim = traits::eof());

>    > > -25- Effects: Behaves as an unformatted input function (as described above). After constructing a sentry object, extracts characters and discards them. Characters are extracted until any of the following occurs:

>    > > 1. (25.1) — `n != numeric_limits<streamsize>::max()` (17.3.5 [numeric.limits]) and `n` characters have been extracted so far

>    > > 2. (25.2) — end-of-file occurs on the input sequence (in which case the function calls `setstate(eofbit)`, which may throw `ios_base::failure` (31.5.4.4 [iostate.flags]));

>    > > 3. (25.3) — `traits::eq_int_type(traits::to_int_type(c), delim)` for the next available input character `c` (in which case `c` is extracted).

>    > > -?- If the number of characters extracted is greater than `numeric_limits<streamsize>::max()` then for the purposes of `gcount()` the number is treated as `numeric_limits<streamsize>::max()`.

>    > > -26- Remarks: The last condition will never occur if `traits::eq_int_type(delim, traits::eof())`.

>    > > -27- Returns: `*this`.

[2020-07-17; Moved to Ready in telecon]

On the reflector Davis pointed out that there are other members which can cause `gcount()` to overflow. There was unanimous agreement on the reflector and the telecon that Option A is better.

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 31.7.5.4 [istream.unformatted] as indicated:

   >     streamsize gcount() const;

   > > -2- Effects: None. This member function does not behave as an unformatted input function (as described above).

   > > -3- Returns: The number of characters extracted by the last unformatted input member function called for the object. If the number cannot be represented, returns `numeric_limits<streamsize>::max()`.

### 3465(i). `compare_partial_order_fallback` requires `F < E`

Section: 17.12.6 [cmp.alg] Status: C++23 Submitter: Stephan T. Lavavej Opened: 2020-07-18 Last modified: 2023-11-22

Priority: 0

View other active issues in [cmp.alg].

View all other issues in [cmp.alg].

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Discussion:

`compare_partial_order_fallback` uses three expressions, but requires only two. The decayed types of `E` and `F` are required to be identical, but variations in constness might make a difference.

[2020-07-26; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after seven votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 17.12.6 [cmp.alg] as indicated:

   > -6- The name `compare_partial_order_fallback` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given subexpressions `E` and `F`, the expression `compare_partial_order_fallback(E, F)` is expression-equivalent ( [defns.expression-equivalent]) to:

   > 1. (6.1) — If the decayed types of `E` and `F` differ, `compare_partial_order_fallback(E, F)` is ill-formed.

   > 2. (6.2) — Otherwise, `partial_order(E, F)` if it is a well-formed expression.

   > 3. (6.3) — Otherwise, if the expressions `E == F`, and `E < F`, and `F < E` are allboth well-formed and convertible to `bool`,

   >    >     E == F ? partial_ordering::equivalent :
   >    >     E < F  ? partial_ordering::less :
   >    >     F < E  ? partial_ordering::greater :
   >    >              partial_ordering::unordered

   >    except that `E` and `F` are evaluated only once.

   > 4. (6.4) — Otherwise, `compare_partial_order_fallback(E, F)` is ill-formed.

### 3466(i). Specify the requirements for `promise`/`future`/`shared_future` consistently

Section: 32.10.6 [futures.promise] Status: C++23 Submitter: Tomasz Kamiński Opened: 2020-07-18 Last modified: 2023-11-22

Priority: 3

View all other issues in [futures.promise].

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Discussion:

The resolution of the LWG 3458(i) clearly specified the requirement that `future`/`shared_future` are ill-formed in situations when `T` is native array or function type. This requirement was not strictly necessary for `future<T>` as it was already ill-formed due the signature of the `get` function (that would be ill-formed in such case), however it was still added for consistency of specification. Similar, requirement should be introduced for the `promise<T>`, for which any call to `get_future()` would be ill-formed, if `T` is of array or function type.

[Note: `promise<int[10]>` is ill-formed for libstdc++ and libc++, see this code]

[2020-07-26; Reflector prioritization]

Set priority to 3 after reflector discussions. Tim Song made the suggestion to replace the P/R wording by the following alternative wording:

> For the primary template, `R` shall be an object type that meets the Cpp17Destructible requirements.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> > Ideally the wording below would use a Mandates: element, but due to the still open issue LWG 3193(i) the wording below uses instead the more general "ill-formed" vocabulary.

> 1. Modify 32.10.6 [futures.promise] as indicated:

>    > >     namespace std {
>    > >       template<class R>
>    > >       class promise {
>    > >         […]
>    > >       };
>    > >       […]
>    > >     }

>    > -?- If `is_array_v<R>` is `true` or `is_function_v<R>` is `true`, the program is ill-formed.

[2020-08-02; Daniel comments and provides alternative wording]

Following the suggestion of Tim Song a revised wording is provided which is intended to replace the currently agreed on wording for LWG 3458(i).

[2020-08-21; Issue processing telecon: Tentatively Ready]

Discussed a note clarifying that Cpp17Destructible disallows arrays (as well as types without accessible destructors). Can be added editorially.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 32.10.6 [futures.promise] as indicated:

   > >     namespace std {
   > >       template<class R>
   > >       class promise {
   > >         […]
   > >       };
   > >       […]
   > >     }

   > -?- For the primary template, `R` shall be an object type that meets the Cpp17Destructible requirements.

   > -1- The implementation provides the template `promise` and two specializations, `promise<R&>` and `promise<void>`. These differ only in the argument type of the member functions `set_value` and `set_value_at_thread_exit`, as set out in their descriptions, below.

2. Modify 32.10.7 [futures.unique.future] as indicated:

   > >     namespace std {
   > >       template<class R>
   > >       class future {
   > >         […]
   > >       };
   > >     }

   > -?- For the primary template, `R` shall be an object type that meets the Cpp17Destructible requirements.

   > -4- The implementation provides the template `future` and two specializations, `future<R&>` and `future<void>`. These differ only in the return type and return value of the member function `get`, as set out in its description, below.

3. Modify 32.10.8 [futures.shared.future] as indicated:

   > >     namespace std {
   > >       template<class R>
   > >       class shared_future {
   > >         […]
   > >       };
   > >     }

   > -?- For the primary template, `R` shall be an object type that meets the Cpp17Destructible requirements.

   > -4- The implementation provides the template `shared_future` and two specializations, `shared_future<R&>` and `shared_future<void>`. These differ only in the return type and return value of the member function `get`, as set out in its description, below.

### 3467(i). `bool` can't be an integer-like type

Section: 24.3.4.4 [iterator.concept.winc] Status: C++23 Submitter: Casey Carter Opened: 2020-07-23 Last modified: 2023-11-22

Priority: 0

View other active issues in [iterator.concept.winc].

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Discussion:

Per 25.2 [ranges.syn]/1, the Standard Library believes it can convert an integer-like type `X` to an unsigned integer-like type with the exposition-only type alias `make-unsigned-like-t`. `make-unsigned-like-t<X>` is specified as being equivalent to `make_unsigned_t<X>` when `X` is an integral type. However, despite being an integral type, `bool` is not a valid template type argument for `make_unsigned_t` per [tab:meta.trans.sign].

This problem with `bool` was an oversight when we added support for integer-like types: it was certainly not the design intent to allow `ranges::size(r)` to return `false`! While we could devise some more-complicated metaprogramming to allow use of `bool`, it seems easier — and consistent with the design intent — to simply exclude `bool` from the set of integer-like types.

[2020-08-02; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 24.3.4.4 [iterator.concept.winc] as indicated:

   > -11- A type `I` other than `cv bool` is integer-like if it models `integral<I>` or if it is an integer-class type. An integer-like type `I` is signed-integer-like if it models `signed_integral<I>` or if it is a signed-integer-class type. An integer-like type `I` is unsigned-integer-like if it models `unsigned_integral<I>` or if it is an unsigned-integer-class type.

### 3469(i). Precondition of `coroutine_handle::promise` may be insufficient

Section: 17.13.4.7 [coroutine.handle.promise] Status: Resolved Submitter: Jiang An Opened: 2020-07-25 Last modified: 2020-11-09

Priority: 2

View all issues with Resolved status.

Discussion:

The issue is related to LWG 3460(i).

Because the `coroutine_handle<>` base subobject of a `coroutine_handle<P1>` can be assigned from the one of a `coroutine_handle<P2>`, a `coroutine_handle<P1>` may refer to a coroutine whose promise type is `P2`. If a `coroutine_handle<P>` refers to a coroutine with difference, a call to `promise()` should result in undefined behavior IMO.

I think that 17.13.4.7 [coroutine.handle.promise]/1 should be changed to: "Preconditions: `*this` refers to a coroutine whose promise type is `Promise`.", and the same precondition should be added to 17.13.5.2.5 [coroutine.handle.noop.promise], and hence `noexcept` should be removed from `coroutine_handle<noop_coroutine_promise>::promise`.

[2020-08-21; Reflector prioritization]

Set priority to 2 after reflector discussions.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Modify 17.13.4.7 [coroutine.handle.promise] as indicated:

>    >     Promise& promise() const;

>    > > -1- Preconditions: `*this` refers to a coroutine whose promise type is `Promise`.

>    > > -2- Returns: A reference to the promise of the coroutine.

> 2. Modify 17.13.5.2 [coroutine.handle.noop], class `coroutine_handle<noop_coroutine_promise>` synopsis, as indicated:

>    >     […]
>    >     // 17.13.5.2.5 [coroutine.handle.noop.promise], promise access
>    >     noop_coroutine_promise& promise() const noexcept;
>    >     […]

> 3. Modify 17.13.5.2.5 [coroutine.handle.noop.promise] as indicated:

>    >     noop_coroutine_promise& promise() const noexcept;

>    > > -?- Preconditions: `*this` refers to a coroutine whose promise type is `noop_coroutine_promise`.

>    > > -1- Returns: A reference to the promise object associated with this coroutine handle.

[2020-11-09 Resolved by acceptance of 3460(i). Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

This issue is resolved by the resolution of issue 3460(i).

### 3470(i). `convertible-to-non-slicing` seems to reject valid case

Section: 25.5.4 [range.subrange] Status: C++23 Submitter: S. B. Tam Opened: 2020-07-26 Last modified: 2023-11-22

Priority: 3

View all other issues in [range.subrange].

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Discussion:

Consider

>     #include <ranges>

>     int main()
>     {
>       int a[3] = { 1, 2, 3 };
>       int* b[3] = { &a[2], &a[0], &a[1] };
>       auto c = std::ranges::subrange<const int*const*>(b);
>     }

The construction of `c` is ill-formed because `convertible-to-non-slicing<int**, const int*const*>` is `false`, although the conversion does not involve object slicing.

I think `subrange` should allow such qualification conversion, just like `unique_ptr<T[]>` already does.

(Given that this constraint is useful in more than one context, maybe it deserves a named type trait?)

[2020-08-21; Reflector prioritization]

Set priority to 3 after reflector discussions.

[2021-05-19 Tim adds wording]

The wording below, which has been implemented and tested on top of libstdc++, uses the same technique we use for `unique_ptr`, `shared_ptr`, and `span`. It seems especially appropriate to have feature parity between `subrange` and `span` in this respect.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 25.5.4 [range.subrange] as indicated:

   >     namespace std::ranges {
   >       template<class From, class To>
   >         concept uses-nonqualification-pointer-conversion = // exposition only
   >           is_pointer_v<From> && is_pointer_v<To> &&
   >           !convertible_to<remove_pointer_t<From>(*)[], remove_pointer_t<To>(*)[]>;

   >       template<class From, class To>
   >         concept convertible-to-non-slicing = // exposition only
   >           convertible_to<From, To> &&
   >           !uses-nonqualification-pointer-conversion<decay_t<From>, decay_t<To>>;
   >           !(is_pointer_v<decay_t<From>> &&
   >           is_pointer_v<decay_t<To>> &&
   >           not-same-as<remove_pointer_t<decay_t<From>>, remove_pointer_t<decay_t<To>>>
   >           );
   >       […]
   >     }
   >     […]

### 3471(i). `polymorphic_allocator::allocate` does not satisfy Cpp17Allocator requirements

Section: 20.5 [mem.res] Status: C++23 Submitter: Alisdair Meredith Opened: 2020-07-27 Last modified: 2023-11-22

Priority: 3

View all other issues in [mem.res].

View all issues with C++23 status.

Discussion:

With the adoption of P0593R6 in Prague, `std::ptr::polymorphic_allocator` no longer satisfies the of Cpp17Allocator requirements. Specifically, all calls to `allocate(n)` need to create an object for an array of `n T`s (but not initialize any of those elements). `std::pmr::polymorphic_allocator` calls its underlying memory resource to allocate sufficient bytes of storage, but it does not create (and start the lifetime of) the array object within that storage.

[2020-08-03; Billy comments]

It's worth noting that the resolution of CWG 2382 has impact on implementors for this issue.

[2020-08-21; Reflector prioritization]

Set priority to 3 after reflector discussions.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> > [Drafting note: The proposed wording is inspired by the example given in the "Assertion/note" column of the expression `a.allocate(n)` in table [tab:cpp17.allocator].]

> 1. Modify 20.5.2.2 [mem.res.public] as indicated:

>    >     [[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);

>    > > -2- Effects: Equivalent to: `return do_allocate(bytes, alignment);`

>    > > >     void* p = do_allocate(bytes, alignment);
>    > > >     return launder(new (p) byte[bytes]);

[2021-05-20 Tim comments and updates wording]

`memory_resource::allocate` is the PMR equivalent of `malloc` and `operator new`. It therefore needs the "suitable created object" wording (see 6.8.2 [intro.object], 20.2.12 [c.malloc]). This ensures that it creates the requisite array object automatically for all the allocation functions of `polymorphic_allocator`, and returns the correct pointer value in all cases.

[2022-01-31; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 20.5.2.2 [mem.res.public] as indicated:

   >     [[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);

   > > -2- Effects: Equivalent to: `return` Allocates storage by calling `do_allocate(bytes, alignment)``;` and implicitly creates objects within the allocated region of storage.

   > > -?- Returns: A pointer to a suitable created object (6.8.2 [intro.object]) in the allocated region of storage.

   > > -?- Throws: What and when the call to `do_allocate` throws.

### 3472(i). `counted_iterator` is missing preconditions

Section: 24.5.7 [iterators.counted] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-07-29 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

C++20 introduces a new iterator `counted_iterator` that keeps track of the end of its range via an additional exposition only member `length`.

Consequently, there are several preconditions for many member functions of `counted_iterator`, but it seems some are missing:

1. `operator*`

   Here we have no precondition regarding `length`. However, given that `length` denotes the distance to the end of the range it should be invalid to dereference a `counted_iterator` with `length 0`.

   Moreover, `operator[]` has a precondition of "`n < length`". Consider the following code snippet:

   >     int some_ints[] = {0,1,2};
   >     counted_iterator<int*> i{some_ints, 0};

   Here "`i[0]`" would be invalid due to the precondition "`n < length`". However, "`*i`" would be a valid expression. This violates the definition of `operator[]` which states according to 7.6.1.2 [expr.sub] p1:

   > […] The expression `E1[E2]` is identical (by definition) to `*((E1)+(E2))` […]

   Substituting `E2->0` we get

   > […] The expression `E1[0]` is identical (by definition) to `*(E1)` […]

   With the current wording `counted_iterator` violates that definition and we should add to `operator*`:

   > Preconditions: `length > 0`.

2. `iter_move`

   This is a similar case. We have only the Effects element:

   > Effects: Equivalent to: `return ranges::iter_move(i.current);`

   However, looking at the requirements of `ranges::iter_move` we have in 24.3.3.1 [iterator.cust.move] p2:

   > If `ranges::iter_move(E)` is not equal to `*E`, the program is ill-formed, no diagnostic required.

   This clearly requires that for `counted_iterator::iter_move` to be well-formed, we need `counted_iterator::operator*` to be well formed. Consequently we should also add the same precondition to `counted_iterator::iter_move`:

   > Preconditions: `length > 0`.

3. `iter_swap`

   This is essentially the same arguing as for `counted_iterator::iter_move`. The essential observation is that `ranges::iter_swap` is defined in terms of `ranges::iter_move` (see 24.3.3.2 [iterator.cust.swap]) so it must have the same preconditions and we should add:

   > Preconditions: `length > 0`.

[2020-08-21 Issue processing telecon: moved to Tentatively Ready.]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 24.5.7.4 [counted.iter.elem] as indicated:

   >     constexpr decltype(auto) operator*();
   >     constexpr decltype(auto) operator*() const
   >       requires dereferenceable<const I>;

   > > -?- Preconditions: `length > 0`.

   > > -1- Effects: Equivalent to: `return *current;`

2. Modify 24.5.7.7 [counted.iter.cust] as indicated:

   >     friend constexpr iter_rvalue_reference_t<I>
   >       iter_move(const counted_iterator& i)
   >         noexcept(noexcept(ranges::iter_move(i.current)))
   >         requires input_iterator<I>;

   > > -?- Preconditions: `i.length > 0`.

   > > -1- Effects: Equivalent to: `return ranges::iter_move(i.current);`

   >     template<indirectly_swappable<I> I2>
   >       friend constexpr void
   >         iter_swap(const counted_iterator& x, const counted_iterator<I2>& y)
   >           noexcept(noexcept(ranges::iter_swap(x.current, y.current)));

   > > -?- Preconditions: `x.length > 0` and `y.length > 0`.

   > > -1- Effects: Equivalent to: `return ranges::iter_swap(x.current, y.current);`

### 3473(i). Normative encouragement in non-normative note

Section: 28.5.8.3 [format.args] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-07-31 Last modified: 2023-11-22

Priority: 0

View all other issues in [format.args].

View all issues with C++23 status.

Discussion:

The note in the final paragraph of 28.5.8.3 [format.args] gives encouragement to implementations, which is not allowed in a note.

It needs to be normative text, possibly using "should", or if left as a note could be phrased as "Implementations can optimize the representation […]".

[2020-08-09; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 28.5.8.3 [format.args] as indicated:

   > -1- An instance of `basic_format_args` provides access to formatting arguments. Implementations should optimize the representation of `basic_format_args` for a small number of formatting arguments. [Note: For example, by storing indices of type alternatives separately from values and packing the former. — end note]

   >     basic_format_args() noexcept;

   > > -2- Effects: Initializes `size_` with `0`.

   > […]

   >     basic_format_arg<Context> get(size_t i) const noexcept;

   > > -4- Returns: `i < size_ ? data_[i] : basic_format_arg<Context>()`.

   > [Note: Implementations are encouraged to optimize the representation of `basic_format_args` for small number of formatting arguments by storing indices of type alternatives separately from values and packing the former. — end note]

### 3474(i). Nesting `join_view`s is broken because of CTAD

Section: 25.7.14 [range.join] Status: C++23 Submitter: Barry Revzin Opened: 2020-08-04 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.join].

View all issues with C++23 status.

Discussion:

Let's say I had a range of range of ranges and I wanted to recursively flatten it. That would involve repeated invocations of `join`. But this doesn't work:

>     std::vector<std::vector<std::vector<int>>> nested_vectors = {
>       {{1, 2, 3}, {4, 5}, {6}},
>       {{7},       {8, 9}, {10, 11, 12}},
>       {{13}}
>     };
>     auto joined = nested_vectors | std::views::join | std::views::join;

The expectation here is that the `value_type` of joined is `int`, but it's actually `vector<int>` — because the 2nd invocation of `join` ends up just copying the first. This is because `join` is specified to do:

> The name `views::join` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpression `E`, the expression `views::join(E)` is expression-equivalent to `join_view{E}`.

And `join_view{E}` for an `E` that's already a specialization of a `join_view` just gives you the same `join_view` back. Yay CTAD. We need to do the same thing with `join` that we did with `reverse` in P1252. We can do that either in exposition (Option A) my modifying 25.7.14.1 [range.join.overview] p2

> The name `views::join` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpression `E`, the expression `views::join(E)` is expression-equivalent to `join_view<views::all_t<decltype((E))>>{E}`.

Or in code (Option B) add a deduction guide to 25.7.14.2 [range.join.view]:

>       template<class R>
>         explicit join_view(R&&) -> join_view<views::all_t<R>>;

>       template<class V>
>         explicit join_view(join_view<V>) -> join_view<join_view<V>>;

[2020-08-21; Issue processing telecon: Option A is Tentatively Ready]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> > [Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

> Option A:

> 1. Modify 25.7.14.1 [range.join.overview] as indicated:

>    > -2- `The name views::join` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpression `E`, the expression `views::join(E)` is expression-equivalent to `join_view<views::all_t<decltype((E))>>{E}`.

> Option B:

> 1. Modify 25.7.14.2 [range.join.view] as indicated:

>    >     namespace std::ranges {
>    >       […]

>    >       template<class R>
>    >         explicit join_view(R&&) -> join_view<views::all_t<R>>;

>    >       template<class V>
>    >         explicit join_view(join_view<V>) -> join_view<join_view<V>>;
>    >     }

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.7.14.1 [range.join.overview] as indicated:

   > -2- `The name views::join` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpression `E`, the expression `views::join(E)` is expression-equivalent to `join_view<views::all_t<decltype((E))>>{E}`.

### 3476(i). `thread` and `jthread` constructors require that the parameters be move-constructible but never move construct the parameters

Section: 32.4.3.3 [thread.thread.constr], 32.4.4.2 [thread.jthread.cons], 32.10.9 [futures.async] Status: C++23 Submitter: Billy O'Neal III Opened: 2020-08-18 Last modified: 2023-11-22

Priority: 0

View other active issues in [thread.thread.constr].

View all other issues in [thread.thread.constr].

View all issues with C++23 status.

Discussion:

I think this was upgraded to Mandates because C++17 and earlier had "`F` and each `Ti` in `Args` shall satisfy the Cpp17MoveConstructible requirements." And for those, I think the requirement was attempting to make the subsequent `decay-copy` valid. However, the 'Mandating the standard library' papers added `is_constructible` requirements which already serve that purpose; `std::(j)thread` has no reason to move the elements after they have been `decay-copy`'d to transfer to the launched thread.

[2020-08-26; Reflector discussion]

Jonathan noticed that the wording for `std::async` is affected by exactly the same unnecessary move-constructible requirements. The proposed wording has been updated to cope for that as well.

[2020-09-02; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 32.4.3.3 [thread.thread.constr] as indicated:

   >     template<class F, class... Args> explicit thread(F&& f, Args&&... args);

   > > -3- Constraints: `remove_cvref_t<F>` is not the same type as `thread`.

   > > -4- Mandates: The following are all `true`:

   > > 1. (4.1) — `is_constructible_v<decay_t<F>, F>`,

   > > 2. (4.2) — `(is_constructible_v<decay_t<Args>, Args> && ...)`, and

   > > 3. (4.3) — `is_move_constructible_v<decay_t<F>>`,

   > > 4. (4.4) — `(is_move_constructible_v<decay_t<Args>> && ...)`, and

   > > 5. (4.5) — `is_invocable_v<decay_t<F>, decay_t<Args>...>`.

   > > -5- Preconditions: `decay_t<F>` and each type in `decay_t<Args>` meet the Cpp17MoveConstructible requirements.

2. Modify 32.4.4.2 [thread.jthread.cons] as indicated:

   >     template<class F, class... Args> explicit jthread(F&& f, Args&&... args);

   > > -3- Constraints: `remove_cvref_t<F>` is not the same type as `jthread`.

   > > -4- Mandates: The following are all `true`:

   > > 1. (4.1) — `is_constructible_v<decay_t<F>, F>`,

   > > 2. (4.2) — `(is_constructible_v<decay_t<Args>, Args> && ...)`, and

   > > 3. (4.3) — `is_move_constructible_v<decay_t<F>>`,

   > > 4. (4.4) — `(is_move_constructible_v<decay_t<Args>> && ...)`, and

   > > 5. (4.5) — `is_invocable_v<decay_t<F>, decay_t<Args>...> || is_invocable_v<decay_t<F>, stop_token, decay_t<Args>...>`.

   > > -5- Preconditions: `decay_t<F>` and each type in `decay_t<Args>` meet the Cpp17MoveConstructible requirements.

3. Modify 32.10.9 [futures.async] as indicated:

   >     template<class F, class... Args>
   >       [[nodiscard]] future<invoke_result_t<decay_t<F>, decay_t<Args>...>>
   >         async(F&& f, Args&&... args);
   >     template<class F, class... Args>
   >       [[nodiscard]] future<invoke_result_t<decay_t<F>, decay_t<Args>...>>
   >         async(launch policy, F&& f, Args&&... args);

   > > -2- Mandates: The following are all `true`:

   > > 1. (2.1) — `is_constructible_v<decay_t<F>, F>`,

   > > 2. (2.2) — `(is_constructible_v<decay_t<Args>, Args> && ...)`, and

   > > 3. (2.3) — `is_move_constructible_v<decay_t<F>>`,

   > > 4. (2.4) — `(is_move_constructible_v<decay_t<Args>> && ...)`, and

   > > 5. (2.5) — `is_invocable_v<decay_t<F>, decay_t<Args>...>`.

   > > -3- Preconditions: `decay_t<F>` and each type in `decay_t<Args>` meet the Cpp17MoveConstructible requirements.

### 3477(i). Simplify constraints for `semiregular-box`

Section: 25.7.3 [range.move.wrap] Status: C++23 Submitter: Casey Carter Opened: 2020-08-19 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.move.wrap].

View all issues with C++23 status.

Discussion:

The exposition-only `semiregular-box` class template specified in [range.semi.wrap] implements a default constructor, copy assignment operator, and move assignment operator atop the facilities provided by `std::optional` when the wrapped type is not default constructible, copy assignable, or move assignable (respectively). The constraints on the copy and move assignment operator implementations go out of their way to be unnecessarily minimal. The meaning of the constraint on the copy assignment operator — `!assignable<T, const T&>` — has even changed since this wording was written as a result of LWG reformulating the implicit expression variations wording in 18.2 [concepts.equality].

It would be much simpler for implementors and users if we recall that minimality is not the primary goal of constraints and instead constrain these assignment operators more simply with `!movable<T>` and `!copyable<T>`.

[2020-09-03; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify [range.semi.wrap] as indicated:

   > -1- Many types in this subclause are specified in terms of an exposition-only class template `semiregular-box`. `semiregular-box<T>` behaves exactly like `optional<T>` with the following differences:

   > 1. (1.1) — […]

   > 2. (1.2) — […]

   > 3. (1.3) — If `assignable_from<T&, const T&>copyable<T>` is not modeled, the copy assignment operator is equivalent to:

   >        semiregular-box& operator=(const semiregular-box& that)
   >          noexcept(is_nothrow_copy_constructible_v<T>)
   >        {
   >          if (that) emplace(*that);
   >          else reset();
   >          return *this;
   >        }

   > 4. (1.4) — If `assignable_from<T&, T>movable<T>` is not modeled, the move assignment operator is equivalent to:

   >        semiregular-box& operator=(semiregular-box&& that)
   >          noexcept(is_nothrow_move_constructible_v<T>)
   >        {
   >          if (that) emplace(std::move(*that));
   >          else reset();
   >          return *this;
   >        }

### 3478(i). `views::split` drops trailing empty range

Section: 25.7.17 [range.split] Status: Resolved Submitter: Barry Revzin Opened: 2020-08-20 Last modified: 2021-06-14

Priority: 2

View all issues with Resolved status.

Discussion:

From StackOverflow, the program:

>     #include <iostream>
>     #include <string>
>     #include <ranges>

>     int main()
>     {
>       std::string s = " text ";
>       auto sv = std::ranges::views::split(s, ' ');
>       std::cout << std::ranges::distance(sv.begin(), sv.end());
>     }

prints 2 (as specified), but it really should print 3. If a range has `N` delimiters in it, splitting should produce `N+1` pieces. If the `N`th delimiter is the last element in the input range, `views::split` produces only `N` pieces — it doesn't emit a trailing empty range.

Going through a bunch of languages gets a sense of what they all do here. There are basically two groups (and Haskell goes in both because it has several different split functions)

1. Rust, Python, Javascript, Go, Kotlin, Haskell's `"splitOn"` all provide `N+1` parts if there were `N` delimiters.

2. APL, D, Elixir, Haskell's `"words"`, Ruby, and Clojure all compress all empty words. Splitting `" x "` on `" "` would give `["x"]` here, whereas the languages in the above group would give `["", "x", ""]`

Java is distinct from both groups in that it is mostly a first category language, except that by default it removes all trailing empty strings (but it keeps all leading and intermediate empty strings, unlike the second category languages) — although it has a parameter that lets you keep the trailing ones too.

C++20's behavior is closest to Java's default, except that it only removes one trailing empty string instead of every trailing empty string — and this behavior is not parameterizeable. But I think the intent is to be squarely in the first category, so I think the current behavior is just a specification error.

Many of these languages also provide an additional extra parameter to limit how many splits happen (e.g. Java, Kotlin, Python, Rust, JavaScript), but that's a separate design question.

[2020-09-02; Reflector prioritization]

Set priority to 2 as result of reflector discussions.

[2021-06-13 Resolved by the adoption of P2210R2 at the June 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

### 3479(i). `semiregular-box` mishandles self-assignment

Section: 25.7.3 [range.move.wrap] Status: Resolved Submitter: Casey Carter Opened: 2020-08-25 Last modified: 2023-02-07

Priority: 3

View all other issues in [range.move.wrap].

View all issues with Resolved status.

Discussion:

The exposition-only wrapper type `semiregular-box` — specified in [range.semi.wrap] — layers behaviors onto `std::optional` so `semiregular-box<T>` is semiregular even when `T` is only copy constructible. It provides copy and move assignment operators when `optional<T>`'s are deleted:

> 1. (1.1) — […]

> 2. (1.2) — […]

> 3. (1.3) — If `assignable_from<T&, const T&>` is not modeled, the copy assignment operator is equivalent to:

>        semiregular-box& operator=(const semiregular-box& that)
>          noexcept(is_nothrow_copy_constructible_v<T>)
>        {
>          if (that) emplace(*that);
>          else reset();
>          return *this;
>        }

> 4. (1.4) — If `assignable_from<T&, T>` is not modeled, the move assignment operator is equivalent to:

>        semiregular-box& operator=(semiregular-box&& that)
>          noexcept(is_nothrow_move_constructible_v<T>)
>        {
>          if (that) emplace(std::move(*that));
>          else reset();
>          return *this;
>        }

How do these assignment operators handle self-assignment? When `*this` is empty, `that` will test as `false` and `reset()` has no effect, so the result state of the object is the same. No problems so far. When `*this` isn't empty, `that` will test as `true`, and we evaluate `optional::emplace(**this)` (resp. `optional::emplace(std::move(**this)))`. This outcome is not as pretty: `emplace` is specified in 22.5.3.4 [optional.assign]/30: "Effects: Calls `*this = nullopt`. Then initializes the contained value as if direct-non-list-initializing an object of type `T` with the arguments `std::forward<Args>(args)...`." When the sole argument is an lvalue (resp. xvalue) of type `T` that denotes the `optional`'s stored value, `emplace` will destroy that stored value and then try to copy/move construct a new object at the same address from the dead object that used to live there resulting in undefined behavior. Mandatory undefined behavior does not meet the semantic requirements for the `copyable` or `movable` concepts, we should do better.

[2020-09-13; Reflector prioritization]

Set priority to 3 during reflector discussions.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> 1. Modify [range.semi.wrap] as indicated:

>    > -1- Many types in this subclause are specified in terms of an exposition-only class template `semiregular-box`. `semiregular-box<T>` behaves exactly like `optional<T>` with the following differences:

>    > 1. (1.1) — […]

>    > 2. (1.2) — […]

>    > 3. (1.3) — If `assignable_from<T&, const T&>` is not modeled, the copy assignment operator is equivalent to:

>    >        semiregular-box& operator=(const semiregular-box& that)
>    >          noexcept(is_nothrow_copy_constructible_v<T>)
>    >        {
>    >          if (this != addressof(that)) {
>    >            if (that) emplace(*that);
>    >            else reset();
>    >          }
>    >          return *this;
>    >        }

>    > 4. (1.4) — If `assignable_from<T&, T>` is not modeled, the move assignment operator is equivalent to:

>    >        semiregular-box& operator=(semiregular-box&& that)
>    >          noexcept(is_nothrow_move_constructible_v<T>)
>    >        {
>    >          reset();
>    >          if (that) emplace(std::move(*that));
>    >          else reset();
>    >          return *this;
>    >        }

[2021-06-13 Resolved by the adoption of P2325R3 at the June 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

### 3480(i). `directory_iterator` and `recursive_directory_iterator` are not C++20 ranges

Section: 31.12.11 [fs.class.directory.iterator], 31.12.12 [fs.class.rec.dir.itr] Status: C++23 Submitter: Barry Revzin Opened: 2020-08-27 Last modified: 2023-11-22

Priority: 3

View all other issues in [fs.class.directory.iterator].

View all issues with C++23 status.

Discussion:

`std::filesystem::directory_iterator` and `std::filesystem::recursive_directory_iterator` are intended to be ranges, but both fail to satisfy the concept `std::ranges::range`.

They both opt in to being a range the same way, via non-member functions:

>     directory_iterator begin(directory_iterator iter) noexcept;
>     directory_iterator end(const directory_iterator&) noexcept;

>     recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept;
>     recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;

This is good enough for a range-based for statement, but for the `range` concept, non-member `end` is looked up in a context that includes (25.3.3 [range.access.end]/2.6) the declarations:

>     void end(auto&) = delete;
>     void end(const auto&) = delete;

Which means that non-`const` `directory_iterator` and non-`const` `recursive_directory_iterator`, the `void end(auto&)` overload ends up being a better match and thus the CPO `ranges::end` doesn't find a candidate. Which means that `{recursive_,}directory_iterator` is not a range, even though `const {recursive_,}directory_iterator` is a range.

This could be fixed by having the non-member `end` for both of these types just take by value (as libstdc++ currently does anyway) or by adding member functions `begin() const` and `end() const`.

A broader direction would be to consider removing the poison pill overloads. Their motivation from P0970 was to support what are now called borrowed ranges — but that design now is based on specializing a variable template instead of providing a non-member `begin` that takes an rvalue, so the initial motivation simply no longer exists. And, in this particular case, causes harm.

[2020-09-06; Reflector prioritization]

Set priority to 3 during reflector discussions.

[2021-02-22, Barry Revzin comments]

When we do make whichever of the alternative adjustments necessary such that `range<directory_iterator>` is `true`, we should also remember to specialize `enable_borrowed_range` for both types to be `true` (since the iterator is the range, this is kind of trivially true).

[2021-05-17, Tim provides wording]

Both MSVC and libstdc++'s `end` already take its argument by value, so the wording below just does that. Any discussion about changing or removing the poison pills is probably better suited for a paper.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Edit 31.12.4 [fs.filesystem.syn], header `<filesystem>` synopsis, as indicated:

   >     […]
   >     namespace std::filesystem {
   >       […]

   >       // 31.12.11.3 [fs.dir.itr.nonmembers], range access for directory iterators
   >       directory_iterator begin(directory_iterator iter) noexcept;
   >       directory_iterator end(const directory_iterator&) noexcept;

   >       […]

   >       // 31.12.12.3 [fs.rec.dir.itr.nonmembers], range access for recursive directory iterators
   >       recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept;
   >       recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;

   >       […]
   >     }

   >     namespace std::ranges {
   >       template<>
   >       inline constexpr bool enable_borrowed_range<filesystem::directory_iterator> = true;
   >       template<>
   >       inline constexpr bool enable_borrowed_range<filesystem::recursive_directory_iterator> = true;

   >       template<>
   >       inline constexpr bool enable_view<filesystem::directory_iterator> = true;
   >       template<>
   >       inline constexpr bool enable_view<filesystem::recursive_directory_iterator> = true;
   >     }

2. Edit 31.12.11.3 [fs.dir.itr.nonmembers] as indicated:

   > -1- These functions enable range access for `directory_iterator`.

   >     directory_iterator begin(directory_iterator iter) noexcept;

   > > -2- Returns: `iter`.

   >     directory_iterator end(const directory_iterator&) noexcept;

   > > -3- Returns: `directory_iterator()`.

3. Edit 31.12.12.3 [fs.rec.dir.itr.nonmembers] as indicated:

   > -1- These functions enable use of `recursive_directory_iterator` with range-based for statements.

   >     recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept;

   > > -2- Returns: `iter`.

   >     recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;

   > > -3- Returns: `recursive_directory_iterator()`.

### 3481(i). `viewable_range` mishandles lvalue move-only views

Section: 25.4.6 [range.refinements] Status: C++23 Submitter: Casey Carter Opened: 2020-08-29 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.refinements].

View all issues with C++23 status.

Discussion:

The `viewable_range concept` (25.4.6 [range.refinements]) and the `views:all` range adaptor (25.7.6 [range.all]) are duals: `viewable_range` is intended to admit exactly types `T` for which `views::all(declval<T>())` is well-formed. (Recall that `views::all(meow)` is a prvalue whose type models `view` when it is well-formed.) Before the addition of move-only view types to the design, this relationship was in place (modulo an incredibly pathological case: a `volatile` value of a view type with `volatile`-qualified `begin` and `end` models `viewable_range` but is rejected by `views::all` unless it also has a `volatile`-qualified copy constructor and copy assignment operator). Adding move-only views to the design punches a bigger hole, however: `viewable_range` admits lvalues of move-only view types for which `views::all` is ill-formed because these lvalues cannot be decay-copied.

It behooves us to restore the correspondence between `viewable_range` and `views::all` so that instantiations of components constrained with `viewable_range` (which often appears indirectly as `views::all_t<R>` in deduction guides) continue to be well-formed when the constraints are satisfied.

[2020-09-06; Reflector prioritization]

Set priority to 2 during reflector discussions.

[2021-05-24; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.4.6 [range.refinements] as indicated:

   > -5- The `viewable_range` concept specifies the requirements of a `range` type that can be converted to a `view` safely.

   >     template<class T>
   >       concept viewable_range =
   >         range<T> && (borrowed_range<T> || view<remove_cvref_t<T>>);
   >         ((view<remove_cvref_t<T>> && constructible_from<remove_cvref_t<T>, T>) ||
   >         (!view<remove_cvref_t<T>> && borrowed_range<T>));

### 3482(i). `drop_view`'s `const begin` should additionally require `sized_range`

Section: 25.7.12.2 [range.drop.view] Status: C++23 Submitter: Casey Carter Opened: 2020-08-31 Last modified: 2023-11-22

Priority: 0

View other active issues in [range.drop.view].

View all other issues in [range.drop.view].

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Discussion:

When the underlying range models both `random_access_range` and `sized_range`, a `drop_view` can easily calculate its first iterator in `𝒪`(1) as by the underlying range's first iterator plus the minimum of the number of elements to drop and the size of the underlying range. In this case `drop_view::begin` need not "cache the result within the `drop_view` for use on subsequent calls" "in order to provide the amortized constant-time complexity required by the `range` concept" (25.7.12.2 [range.drop.view]/4). However, `drop_view::begin() const` does not require `sized_range`, it requires only `random_access_range`. There's no way to implementing what amounts to a requirement that calls to `begin` after the first must be `𝒪`(1) without memoization.

Performing memoization in a `const` member function in a manner consistent with 16.4.6.10 [res.on.data.races] is impossible without some kind of thread synchronization. It is not the intended design for anything in current Range library to require such implementation heroics, we typically fall back to mutable-only iteration to avoid thread synchronization concerns. (Note that both `range-v3` and `cmcstl2` handle `drop_view::begin() const` incorrectly by performing `𝒪`(`N`) lookup of the first iterator on each call to `begin`, which is consistent with 16.4.6.10 [res.on.data.races] but fails to meet the complexity requirements imposed by the `range` concept.) We should fall back to mutable-only iteration here as well when the underlying range is not a `sized_range`.

For `drop_view`, changing the constraints on the `const` overload of `begin` also requires changing the constraints on the non-`const` overload. The non-`const begin` tries to constrain itself out of overload resolution when the `const` overload would be valid if the underlying range models the exposition-only `simple-view` concept. (Recall that `T` models simple-view iff `T` models `view`, `const T` models `range`, and `T` and `const T` have the same iterator and sentinel types.) Effectively this means the constraints on the non-`const` overload must require either that the underlying range fails to model `simple-view` or that the constraints on the `const` overload would not be satisfied. So when we add a new `sized_range` requirement to the `const` overload, we must also add its negation to the mutable overload. (The current form of the constraint on the mutable `begin` overload is `!(simple-view<V> && random_access_range<V>)` instead of `!(simple-view<V> && random_access_range<const V>)` because of an unstated premise that `V` and `const V` should both have the same category when both are ranges. Avoiding this unstated premise would make it easier for future readers to grasp what's happening here; we should formulate our new constraints in terms of `const V` instead of `V`.)

[2020-09-29; Reflector discussions]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.7.12.2 [range.drop.view] as indicated:

   > >     namespace std::ranges {
   > >       template<view V>
   > >       class drop_view : public view_interface<drop_view<V>> {
   > >       public:
   > >         […]
   > >         constexpr auto begin()
   > >           requires (!(simple-view<V> &&
   > >             random_access_range<const V> && sized_range<const V>));
   > >         constexpr auto begin() const
   > >           requires random_access_range<const V> && sized_range<const V>;
   > >         […]
   > >       };
   > >     }

   > […]

   >     constexpr auto begin()
   >       requires (!(simple-view<V> &&
   >         random_access_range<const V> && sized_range<const V>));
   >     constexpr auto begin() const
   >       requires random_access_range<const V> && sized_range<const V>;

   > > -3- Returns: `ranges::next(ranges::begin(base_), count_, ranges::end(base_))`.

   > > -4- Remarks: In order to provide the amortized constant-time complexity required by the `range` concept when `drop_view` models `forward_range`, the first overload caches the result within the `drop_view` for use on subsequent calls. [Note: Without this, applying a `reverse_view` over a `drop_view` would have quadratic iteration complexity. — end note]

### 3483(i). `transform_view::iterator`'s difference is overconstrained

Section: 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Casey Carter Opened: 2020-09-04 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.transform.iterator].

View all issues with C++23 status.

Discussion:

The difference operation for `transform_view::iterator` is specified in 25.7.9.3 [range.transform.iterator] as:

>     friend constexpr difference_type operator-(const iterator& x, const iterator& y)
>       requires random_access_range<Base>;

> > -22- Effects: Equivalent to: `return x.current_ - y.current_;`

The member `current_` is an iterator of type `iterator_t<Base>`, where `Base` is `V` for `transform_view<V, F>::iterator<false>` and `const V` for `transform_view<V, F>::iterator<true>`. The difference of iterators that appears in the above Effects: element is notably well-defined if their type models `sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>` which `random_access_range<Base>` refines. This overstrong requirement seems to be simply the result of an oversight; it has been present since P0789R0, without — to my recollection — ever having been discussed. We should relax this requirement to provide difference capability for `transform_view`'s iterators whenever the underlying iterators do.

[2020-09-08; Reflector discussion]

During reflector discussions it was observed that `elements_view::iterator` has the same issue and the proposed wording has been extended to cover this template as well.

[2020-09-13; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after seven votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, copy_constructible F>
   > >         requires view<V> && is_object_v<F> &&
   > >                  regular_invocable<F&, range_reference_t<V>> &&
   > >                  can-reference<invoke_result_t<F&, range_reference_t<V>>>
   > >       template<bool Const>
   > >       class transform_view<V, F>::iterator {
   > >       public:
   > >         […]
   > >         friend constexpr iterator operator-(iterator i, difference_type n)
   > >           requires random_access_range<Base>;
   > >         friend constexpr difference_type operator-(const iterator& x, const iterator& y)
   > >           requires random_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
   > >         […]
   > >       };
   > >     }

   > […]

   >     friend constexpr difference_type operator-(const iterator& x, const iterator& y)
   >       requires random_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;

   > > -22- Effects: `return x.current_ - y.current_;`

2. Modify 25.7.23.3 [range.elements.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, size_t N>
   > >         requires view<V> && has-tuple-element<range_value_t<V>, N> &&
   > >                  has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
   > >       template<bool Const>
   > >       class elements_view<V, N>::iterator {  // exposition only
   > >         […]
   > >         friend constexpr iterator operator-(iterator x, difference_type y)
   > >           requires random_access_range<Base>;
   > >         friend constexpr difference_type operator-(const iterator& x, const iterator& y)
   > >           requires random_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
   > >       };
   > >     }

   > […]

   >     constexpr difference_type operator-(const iterator& x, const iterator& y)
   >       requires random_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;

   > > -21- Effects: `return x.current_ - y.current_;`

### 3490(i). `ranges::drop_while_view::begin()` is missing a precondition

Section: 25.7.13.2 [range.drop.while.view] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-10-13 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.drop.while.view].

View all issues with C++23 status.

Discussion:

Similar to `ranges::filter_view` 25.7.8.2 [range.filter.view] p3, `ranges::drop_while_view` should have a precondition on its `begin()` method that the predicate is set.

I propose to add as 25.7.13.2 [range.drop.while.view] p3:

>     Preconditions: pred_.has_value().

[2020-11-07; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4868.

1. Modify 25.7.13.2 [range.drop.while.view] as indicated:

   > Since we usually don't rely on implicit `bool` conversion in Preconditions: elements an explicit "is `true`" has been added. Editorial fixes of the referenced paragraph 25.7.13.2 [range.drop.while.view] p3 and similar places have been requested separately.

   >     constexpr auto begin();

   > > -?- Preconditions: `pred_.has_value()` is `true`.

   > > -3- Returns: `ranges::find_if_not(base_, cref(*pred_))`.

   > > -4- […]

### 3492(i). Minimal improvements to `elements_view::iterator`

Section: 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-10-28 Last modified: 2023-11-22

Priority: 0

View other active issues in [range.elements.iterator].

View all other issues in [range.elements.iterator].

View all issues with C++23 status.

Discussion:

During code review of `elements_view` for MSVC-STL we found two issues that should be easily addressed:

1. `elements_view::iterator` constraints both `operator++(int)` member functions

   >     constexpr void operator++(int) requires (!forward_range<Base>);
   >     constexpr iterator operator++(int) requires forward_range<Base>;

   However, given that a constrained method would be preferred we only need to constrain one of those. The proposal would be to remove the constraint from the `void` returning overload and change the declaration to

   >     constexpr void operator++(int);
   >     constexpr iterator operator++(int) requires forward_range<Base>;

2. `elements_view::iterator operator-` is constrained as follows:

   >     friend constexpr difference_type operator-(const iterator& x, const iterator& y)
   >       requires random_access_range<Base>;

   However, that requires its base to have `operator-` defined. We should change the constraint to `sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>`:

   >     friend constexpr difference_type operator-(const iterator& x, const iterator& y)
   >       requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;

[2020-11-01; Daniel comments]

Bullet (2) of the discussion has already been resolved by LWG 3483(i), it has therefore been omitted from the proposed wording below.

[2020-11-15; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4868.

> This wording intentionally only touches `operator++(int)` and not `operator-`, see the 2020-11-01 comment for the reason why.

1. Modify 25.7.23.3 [range.elements.iterator], class template `elements_view::iterator` synopsis, as indicated:

   > >     […]
   > >     constexpr iterator& operator++();
   > >     constexpr void operator++(int) requires (!forward_range<Base>);
   > >     constexpr iterator operator++(int) requires forward_range<Base>;
   > >     […]

   > […]

   >     constexpr void operator++(int) requires (!forward_range<Base>);

   > > -6- Effects: Equivalent to: `++current_`.

   >     constexpr iterator operator++(int) requires forward_range<Base>;

   > […]

### 3494(i). Allow ranges to be conditionally borrowed

Section: 25.7.21 [range.reverse], 25.7.10 [range.take], 25.7.12 [range.drop], 25.7.13 [range.drop.while], 25.7.20 [range.common], 25.7.13 [range.drop.while], 25.7.23 [range.elements] Status: C++23 Submitter: Barry Revzin Opened: 2020-11-01 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.reverse].

View all issues with C++23 status.

Discussion:

Consider the following approach to trimming a `std::string`:

>     auto trim(std::string const& s) {
>       auto isalpha = [](unsigned char c){ return std::isalpha(c); };
>       auto b = ranges::find_if(s, isalpha);
>       auto e = ranges::find_if(s | views::reverse, isalpha).base();
>       return subrange(b, e);
>     }

This is a fairly nice and, importantly, safe way to implement `trim`. The iterators `b` and `e` returned from `find_if` will not dangle, since they point into the `string s` whose lifetime outlives the function. But the status quo in C++20 is that `s | views::reverse` is not a borrowed range (because `reverse_view<V>` is never a borrowed range for any `V`). As a result, `find_if(s | views::reverse, isalpha)` returns dangling rather than a real iterator.

Instead, you have to write it this way, introducing a new named variable for the reversed view:

>     auto trim(std::string const& s) {
>       auto isalpha = [](unsigned char c){ return std::isalpha(c); };
>       auto b = ranges::find_if(s, isalpha);
>       auto reversed = s | views::reverse;
>       auto e = ranges::find_if(reversed, isalpha).base();
>       return subrange(b, e);
>     }

But borrowed range can be a transitive property. `s` itself is a borrowed range (as all lvalue references are) so `s | views::reverse` could be made to be too, which would allow the first example above to work with really no downside. We know such an iterator would not dangle, we just need to teach the library this.

P2017R1 resolves this by making `reverse_view<V>` a borrowed range when `V` is a borrowed range (and likewise several other range adapters).

[2021-01-15; Telecon prioritization]

Set status to Tentatively Ready after five P0 votes in reflector discussion.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Rationale:

Resolved by P2017R1.

Proposed resolution:

### 3495(i). `constexpr launder` makes pointers to inactive members of unions usable

Section: 17.6.5 [ptr.launder] Status: C++23 Submitter: Hubert Tong Opened: 2020-11-10 Last modified: 2023-11-22

Priority: 3

View all other issues in [ptr.launder].

View all issues with C++23 status.

Discussion:

The wording in 17.6.5 [ptr.launder] paragraph 4:

> An invocation of this function may be used in a core constant expression whenever the value of its argument may be used in a core constant expression.

can be taken to mean that the invocation may be used only when the value of its argument can be used in place of the invocation itself.

That interpretation is not particularly obvious, but based on comments on the CWG reflector (see here), that is the interpretation that matches the design intent.

Consider:

>     #include <new>

>     struct A { int x; int y; };
>     struct B { float x; int y; };

>     union U {
>       A a;
>       B b;
>     };

>     constexpr A foo() {
>       U u;
>       int* byp = &u.b.y;
>       static_assert(&u.b.y == static_cast<void*>(&u.a.y));
>       u.a.y = 42;
>       *std::launder(byp) = 13;
>       return u.a;
>     }

>     extern constexpr A globA = foo();

If the `static_assert` succeeds, then a possible interpretation is that the source file above compiles because the call to `std::launder` produces a pointer to `u.a.y`. That interpretation is apparently not desirable.

[2020-11-21; Reflector prioritization]

Set priority to 3 during reflector discussions.

[2020-12-07; Davis Herring comments]

This issue is related to CWG 2464.

[2021-02-08; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4868.

1. Modify 17.6.5 [ptr.launder] as indicated:

   >     template<class T> [[nodiscard]] constexpr T* launder(T* p) noexcept;

   > > […]

   > > -4- Remarks: An invocation of this function may be used in a core constant expression whenever theif and only if the (converted) value of its argument may be used in a core constant expressionplace of the function invocation. A byte of storage `b` is reachable through a pointer value that points to an object `Y` if there is an object `Z`, pointer-interconvertible with `Y`, such that `b` is within the storage occupied by `Z`, or the immediately-enclosing array object if `Z` is an array element.

   > > […]

### 3498(i). Inconsistent `noexcept`-specifiers for `basic_syncbuf`

Section: 31.11.2.1 [syncstream.syncbuf.overview], 31.11.2.3 [syncstream.syncbuf.assign] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-11-10 Last modified: 2023-11-22

Priority: 3

View all other issues in [syncstream.syncbuf.overview].

View all issues with C++23 status.

Discussion:

The synopsis in 31.11.2.1 [syncstream.syncbuf.overview] shows the move assignment operator and `swap` member as potentially throwing. The detailed descriptions in 31.11.2.3 [syncstream.syncbuf.assign] are `noexcept`.

Daniel:

This mismatch is already present in the originally accepted paper P0053R7, so this is nothing that could be resolved editorially.

[2020-11-21; Reflector prioritization]

Set priority to 3 during reflector discussions.

[2021-05-22 Tim adds PR]

The move assignment is specified to call `emit()` which can throw, and there's nothing in the wording providing for catching/ignoring the exception, so it can't be `noexcept`. The `swap` needs to call `basic_streambuf::swap`, which isn't `noexcept`, so it shouldn't be `noexcept` either.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 31.11.2.3 [syncstream.syncbuf.assign] as indicated:

   >     basic_syncbuf& operator=(basic_syncbuf&& rhs) noexcept;

   > > -1- Effects: […]

   > > -2- Postconditions: […]

   > > -3- Returns: […]

   > > -4- Remarks: […]

   >     void swap(basic_syncbuf& other) noexcept;

   > > -5- Preconditions: […]

   > > -6- Effects: […]

### 3500(i). `join_view::iterator::operator->()` is bogus

Section: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-11-15 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.join.iterator].

View all issues with C++23 status.

Discussion:

There seems to be a copy & paste error in the `join_view::iterator::operator->()` specification. According to 25.7.14.3 [range.join.iterator] p8 it is specified as:

>     constexpr iterator_t<Base> operator->() const
>       requires has-arrow<iterator_t<Base>> && copyable<iterator_t<Base>>;

> > -8- Effects: Equivalent to `return inner_;`

Now `inner_` is of type `iterator_t<range_reference_t<Base>>`. So it is unclear how that should be converted to `iterator_t<Base>`, or why the constraints concern the outer iterator type `iterator_t<Base>`. On the other hand returning `outer_` would not make any sense here.

As far as I can tell we should replace `iterator_t<Base>` with `iterator_t<range_reference_t<Base>>` so that the new specification would read

>     constexpr iterator_t<range_reference_t<Base>> operator->() const
>      requires has-arrow<iterator_t<range_reference_t<Base>>>
>        && copyable<iterator_t<range_reference_t<Base>>>;

> > -8- Effects: Equivalent to `return inner_;`

Generally it would help readability of the specification a lot if we would introduce some exposition only aliases:

>     using OuterIter = iterator_t<Base>; //exposition-only
>     using InnerIter = iterator_t<range_reference_t<Base>> //exposition-only

and use them throughout `join_view::iterator`.

[2020-11-21; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4868.

1. Modify 25.7.14.3 [range.join.iterator], class template `join_view::iterator` synopsis, as indicated:

   > >     template<input_range V>
   > >       requires view<V> && input_range<range_reference_t<V>> &&
   > >                (is_reference_v<range_reference_t<V>> ||
   > >                 view<range_value_t<V>>)
   > >     struct join_view<V>::iterator {
   > >     private:
   > >       using Parent = // exposition only
   > >         conditional_t<Const, const join_view, join_view>;
   > >       using Base = conditional_t<Const, const V, V>; // exposition only
   > >       using OuterIter = iterator_t<Base>; //exposition-only
   > >       using InnerIter = iterator_t<range_reference_t<Base>> //exposition-only
   > >       static constexpr bool ref-is-glvalue = // exposition only
   > >         is_reference_v<range_reference_t<Base>>;
   > >       OuterIteriterator_t<Base> outer_ = OuterIteriterator_t<Base>(); // exposition only
   > >       InnerIteriterator_t<range_reference_t<Base>> inner_ = // exposition only
   > >         InnerIteriterator_t<range_reference_t<Base>>();
   > >       Parent* parent_ = nullptr; // exposition only
   > >       constexpr void satisfy(); // exposition only
   > >     public:
   > >       […]
   > >       iterator() = default;
   > >       constexpr iterator(Parent& parent, OuterIteriterator_t<Base> outer);
   > >       constexpr iterator(iterator<!Const> i)
   > >         requires Const &&
   > >                  convertible_to<iterator_t<V>, OuterIteriterator_t<Base>> &&
   > >                  convertible_to<iterator_t<InnerRng>,
   > >                                 InnerIteriterator_t<range_reference_t<Base>>>;

   > >       constexpr decltype(auto) operator*() const { return *inner_; }

   > >       constexpr InnerIteriterator_t<Base> operator->() const
   > >         requires has-arrow<InnerIteriterator_t<Base>>
   > >           && copyable<InnerIteriterator_t<Base>>;

   > >       constexpr iterator& operator++();
   > >       […]

   > […]

   >     constexpr void satisfy(); // exposition only

   > > -5- Effects: Equivalent to:

   > > >     […]
   > > >     if constexpr (ref-is-glvalue)
   > > >       inner_ = InnerIteriterator_t<range_reference_t<Base>>();

   >     constexpr iterator(Parent& parent, OuterIteriterator_t<Base> outer);

   > > […]

   >     constexpr iterator(iterator<!Const> i)
   >       requires Const &&
   >                convertible_to<iterator_t<V>, OuterIteriterator_t<Base>> &&
   >                convertible_to<iterator_t<InnerRng>,
   >                               InnerIteriterator_t<range_reference_t<Base>>>;

   > > […]

   >     constexpr InnerIteriterator_t<Base> operator->() const
   >       requires has-arrow<InnerIteriterator_t<Base>>
   >         && copyable<InnerIteriterator_t<Base>>;

   > > -8- Effects: Equivalent to `return inner_;`

### 3502(i). `elements_view` should not be allowed to return dangling references

Section: 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Tim Song Opened: 2020-11-18 Last modified: 2023-11-22

Priority: 2

View other active issues in [range.elements.iterator].

View all other issues in [range.elements.iterator].

View all issues with C++23 status.

Discussion:

This compiles but the resulting view is full of dangling references:

>     std::vector<int> vec = {42};
>     auto r = vec | std::views::transform([](auto c) { return std::make_tuple(c, c); })
>                  | std::views::keys;

This is because `elements_view::iterator::operator*` is specified as

>     constexpr decltype(auto) operator*() const { return get<N>(*current_); }

Here `*current_` is a prvalue, and so the `get<N>` produces a reference into the materialized temporary that becomes dangling as soon as `operator*` returns.

We should either ban this case altogether, or make `operator*` (and `operator[]`) return by value when `*current_` is a prvalue and the corresponding tuple element is not a reference (since this `get` is `std::get`, we need not worry about weird user-defined overloads.)

[2020-11-29; Reflector prioritization]

Set priority to 2 during reflector discussions.

[2021-01-31 Tim adds PR]

[2021-02-08; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 25.7.23.2 [range.elements.view], as indicated:

   >     namespace std::ranges {
   >       template<class T, size_t N>
   >         concept has-tuple-element =                   // exposition only
   >           requires(T t) {
   >             typename tuple_size<T>::type;
   >             requires N < tuple_size_v<T>;
   >             typename tuple_element_t<N, T>;
   >             { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
   >           };

   >       template<class T, size_t N>
   >       concept returnable-element = is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;

   >       template<input_range V, size_t N>
   >         requires view<V> && has-tuple-element<range_value_t<V>, N> &&
   >                  has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
   >                  returnable-element<range_reference_t<V>, N>
   >       class elements_view : public view_interface<elements_view<V, N>> {
   >         […]
   >       };
   >     }

2. Modify 25.7.23.3 [range.elements.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, size_t N>
   > >         requires view<V> && has-tuple-element<range_value_t<V>, N> &&
   > >                  has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
   > >                  returnable-element<range_reference_t<V>, N>
   > >       template<bool Const>
   > >       class elements_view<V, N>::iterator {                 // exposition only
   > >         using Base = maybe-const<Const, V>;                 // exposition only

   > >         iterator_t<Base> current_ = iterator_t<Base>();     // exposition only

   > >         static constexpr decltype(auto) get-element(const iterator_t<Base>& i);    // exposition only
   > >       public:
   > >         […]
   > >         constexpr decltype(auto) operator*() const
   > >         { return get<N>get-element(*current_); }

   > >         […]
   > >         constexpr decltype(auto) operator[](difference_type n) const
   > >         requires random_access_range<Base>
   > >         { return get<N>get-element(*(current_ + n)); }
   > >       };
   > >     }

   >     static constexpr decltype(auto) get-element(const iterator_t<Base>& i);    // exposition only

   > > -?- Effects: Equivalent to:

   > > >     if constexpr (is_reference_v<range_reference_t<Base>>) {
   > > >       return get<N>(*i);
   > > >     }
   > > >     else {
   > > >       using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>;
   > > >       return static_cast<E>(get<N>(*i));
   > > >     }

3. Modify 25.7.23.4 [range.elements.sentinel] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, size_t N>
   > >         requires view<V> && has-tuple-element<range_value_t<V>, N> &&
   > >                  has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
   > >                  returnable-element<range_reference_t<V>, N>
   > >       template<bool Const>
   > >       class elements_view<V, N>::sentinel {                 // exposition only
   > >         […]
   > >       };
   > >     }

### 3504(i). `condition_variable::wait_for` is overspecified

Section: 32.7.4 [thread.condition.condvar] Status: C++26 Submitter: Jonathan Wakely Opened: 2020-11-18 Last modified: 2026-06-09

Priority: 3

View all other issues in [thread.condition.condvar].

View all issues with C++26 status.

Discussion:

32.7.4 [thread.condition.condvar] p24 says:

> Effects: Equivalent to: `return wait_until(lock, chrono::steady_clock::now() + rel_time);`

This is overspecification, removing implementer freedom to make `cv.wait_for(duration<float>(1))` work accurately.

The type of `steady_clock::now() + duration<float>(n)` is `time_point<steady_clock, duration<float, steady_clock::period>>`. If the steady clock's period is `std::nano` and its epoch is the time the system booted, then in under a second a 32-bit `float` becomes unable to exactly represent those `time_points`! Every second after boot makes `duration<float, nano>` less precise.

This means that adding a `duration<float>` to a `time_point` (or `duration`) measured in nanoseconds is unlikely to produce an accurate value. Either it will round down to a value less than `now()`, or round up to one greater than `now() + 1s`. Either way, the `wait_for(rel_time)` doesn't wait for the specified time, and users think the implementation is faulty.

A possible solution is to use `steady_clock::now() + ceil<steady_clock::duration>(rel_time)` instead. This converts the relative time to a suitably large integer, and then the addition is not affected by floating-point rounding errors due to the limited precision of 32-bit `float`. Libstdc++ has been doing this for nearly three years. Alternatively, the standard could just say that the relative timeout is converted to an absolute timeout measured against the steady clock, and leave the details to the implementation. Some implementations might not be affected by the problem (e.g. if the steady clock measures milliseconds, or processes only run for a few seconds and use the process start as the steady clock's epoch).

This also affects the other overload of `condition_variable::wait_for`, and both overloads of `condition_variable_any::wait_for`.

[2020-11-29; Reflector prioritization]

Set priority to 3 during reflector discussions.

[2024-06-18; Jonathan adds wording]

Previous resolution [SUPERSEDED]

This wording is relative to N4981.

1. Modify 32.7.1 [thread.condition.general] as indicated, adding a new paragraph to the end:

   > -6- The definitions in 32.7 [thread.condition] make use of the following exposition-only function:

   > >     template<class Dur>
   > >       chrono::steady_clock::time_point rel-to-abs(const Dur& rel_time)
   > >       { return chrono::steady_clock::now() + chrono::ceil<chrono::steady_clock::duration>(rel_time); }

2. Modify 32.7.4 [thread.condition.condvar] as indicated:

   >     template<class Rep, class Period>
   >       cv_status wait_for(unique_lock<mutex>& lock,
   >                          const chrono::duration<Rep, Period>& rel_time);

   > -23- Preconditions: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either [...]

   > -24- Effects: Equivalent to:

   > > `return wait_until(lock, chrono::steady_clock::now() + rel_time rel-to-abs(rel_time));`

   > [...]

   >     template<class Rep, class Period, class Predicate>
   >       cv_status wait_for(unique_lock<mutex>& lock,
   >                          const chrono::duration<Rep, Period>& rel_time,
   >                          Predicate pred);

   > -35- Preconditions: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either [...]

   > -36- Effects: Equivalent to:

   > > `return wait_until(lock, chrono::steady_clock::now() + rel_time rel-to-abs(rel_time), std::move(pred));`

   > [Note 8: There is no blocking if `pred()` is initially `true`, even if the timeout has already expired. — end note]

3. Modify 32.7.5.2 [thread.condvarany.wait] as indicated:

   >     template<class Lock, class Rep, class Period>
   >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

   > -11- Effects: Equivalent to:

   > > `return wait_until(lock, chrono::steady_clock::now() + rel_time rel-to-abs(rel_time));`

   > [...]

   >     template<class Lock, class Rep, class Period, class Predicate>
   >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

   > -19- Effects: Equivalent to:

   > > `return wait_until(lock, chrono::steady_clock::now() + rel_time rel-to-abs(rel_time), std::move(pred));`

[2026-02-20; Jonathan provides new wording]

Need to add another change for the interruptible `wait_for`.

[2026-02-20; LWG telecon; Status changed: New → Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 32.7.1 [thread.condition.general] as indicated, adding a new paragraph to the end:

   > -6- The definitions in 32.7 [thread.condition] make use of the following exposition-only function:

   > >     template<class Dur>
   > >       chrono::steady_clock::time_point rel-to-abs(const Dur& rel_time)
   > >       { return chrono::steady_clock::now() + chrono::ceil<chrono::steady_clock::duration>(rel_time); }

2. Modify 32.7.4 [thread.condition.condvar] as indicated:

   >     template<class Rep, class Period>
   >       cv_status wait_for(unique_lock<mutex>& lock,
   >                          const chrono::duration<Rep, Period>& rel_time);

   > -23- Preconditions: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either [...]

   > -24- Effects: Equivalent to:

   > > `return wait_until(lock, chrono::steady_clock::now() + rel_time rel-to-abs(rel_time));`

   > [...]

   >     template<class Rep, class Period, class Predicate>
   >       cv_status wait_for(unique_lock<mutex>& lock,
   >                          const chrono::duration<Rep, Period>& rel_time,
   >                          Predicate pred);

   > -35- Preconditions: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either [...]

   > -36- Effects: Equivalent to:

   > > `return wait_until(lock, chrono::steady_clock::now() + rel_time rel-to-abs(rel_time), std::move(pred));`

   > [Note 8: There is no blocking if `pred()` is initially `true`, even if the timeout has already expired. — end note]

3. Modify 32.7.5.2 [thread.condvarany.wait] as indicated:

   >     template<class Lock, class Rep, class Period>
   >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

   > -11- Effects: Equivalent to:

   > > `return wait_until(lock, chrono::steady_clock::now() + rel_time rel-to-abs(rel_time));`

   > [...]

   >     template<class Lock, class Rep, class Period, class Predicate>
   >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

   > -19- Effects: Equivalent to:

   > > `return wait_until(lock, chrono::steady_clock::now() + rel_time rel-to-abs(rel_time), std::move(pred));`

4. Modify 32.7.5.3 [thread.condvarany.intwait] as indicated:

   >     template<class Lock, class Rep, class Period, class Predicate>
   >       bool wait_for(Lock& lock, stop_token stoken,
   >                     const chrono::duration<Rep, Period>& rel_time, Predicate pred);

   > -13- Effects: Equivalent to:

   > > `return wait_until(lock, std::move(stoken), chrono::steady_clock::now() + rel_time rel-to-abs(rel_time), std::move(pred));`

### 3505(i). `split_view::outer-iterator::operator++` misspecified

Section: 25.7.16.3 [range.lazy.split.outer] Status: C++23 Submitter: Tim Song Opened: 2020-11-20 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.lazy.split.outer].

View all issues with C++23 status.

Discussion:

Prior to the application of P1862R1, the part of `split_view::outer-iterator::operator++` that searches for the pattern is specified as:

>     do {
>       const auto [b, p] = ranges::mismatch(current, end, pbegin, pend);
>       if (p == pend) {
>         current = b; // The pattern matched; skip it
>         break;
>       }
>     } while (++current != end);

P1862R1, trying to accommodate move-only iterators, respecified this as

>     do {
>       auto [b, p] = ranges::mismatch(std::move(current), end, pbegin, pend);
>       current = std::move(b);
>       if (p == pend) {
>         break; // The pattern matched; skip it
>       }
>     } while (++current != end);

but this is not correct, because if the pattern didn't match, it advances `current` to the point of first mismatch, skipping elements before that point. This is totally wrong if the pattern contains more than one element.

Consider `std::views::split("xxyx"sv, "xy"sv)`:

- at the beginning, `current` points to the first `'x'`

- `ranges::mismatch` produces `[`iterator to second `'x'`, iterator to `'y'` in pattern`]`

- `current` now points to second `'x'`

- we increment `current` in the condition, so it now points to `'y'`

At this point there's no way we can find the `"xy"` in the middle. In fact, in this particular example, we'll increment past the end of the source range at the end of the third iteration.

[2020-11-29; Reflector prioritization]

Set priority to 2 during reflector discussions.

[2021-02-08; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4868.

1. Modify [range.split.outer] as indicated:

   >     constexpr outer-iterator& operator++();

   > > -6- Effects: Equivalent to:

   > > >     const auto end = ranges::end(parent_->base_);
   > > >     if (current == end) return *this;
   > > >     const auto [pbegin, pend] = subrange{parent_->pattern_};
   > > >     if (pbegin == pend) ++current;
   > > >     else if constexpr (tiny-range<Pattern>) {
   > > >       current = ranges::find(std::move(current), end, *pbegin);
   > > >       if (current != end) {
   > > >         ++current;
   > > >       }
   > > >     }
   > > >     else {
   > > >       do {
   > > >         auto [b, p] = ranges::mismatch(std::move(current), end, pbegin, pend);
   > > >         current = std::move(b);
   > > >         if (p == pend) {
   > > >           current = b;
   > > >           break; // The pattern matched; skip it
   > > >         }
   > > >       } while (++current != end);
   > > >     }
   > > >     return *this;

### 3506(i). Missing allocator-extended constructors for `priority_queue`

Section: 23.6.4 [priority.queue] Status: C++23 Submitter: Tim Song Opened: 2020-11-21 Last modified: 2023-11-22

Priority: 3

View all other issues in [priority.queue].

View all issues with C++23 status.

Discussion:

`priority_queue` has two constructor templates taking a pair of input iterators in addition to a comparator and a container, but it does not have allocator-extended constructors corresponding to these constructor templates:

>     template<class InputIterator>
>       priority_queue(InputIterator first, InputIterator last, const Compare& x,
>                      const Container&);
>     template<class InputIterator>
>       priority_queue(InputIterator first, InputIterator last,
>                      const Compare& x = Compare(), Container&& = Container());

[2020-11-29; Reflector prioritization]

Set priority to 3 during reflector discussions. It has been pointed out that this issue is related to LWG 1199(i), LWG 2210(i), and LWG 2713(i).

[2021-02-17 Tim adds PR]

[2021-02-26; LWG telecon]

Set status to Tentatively Ready after discussion and poll.

F | A | N
--- | --- | ---
11 | 0 | 0

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Add the following paragraph at the end of 23.6.1 [container.adaptors.general]:

   > -6- The exposition-only alias template `iter-value-type` defined in 23.3.1 [sequences.general] may appear in deduction guides for container adaptors.

2. Modify 23.6.4 [priority.queue], class template `priority_queue` synopsis, as indicated:

   >     namespace std {
   >       template<class T, class Container = vector<T>,
   >                 class Compare = less<typename Container::value_type>>
   >       class priority_queue {

   >       // […]

   >       public:
   >         priority_queue() : priority_queue(Compare()) {}
   >         explicit priority_queue(const Compare& x) : priority_queue(x, Container()) {}
   >         priority_queue(const Compare& x, const Container&);
   >         priority_queue(const Compare& x, Container&&);
   >         template<class InputIterator>
   >           priority_queue(InputIterator first, InputIterator last, const Compare& x,
   >                           const Container&);
   >         template<class InputIterator>
   >           priority_queue(InputIterator first, InputIterator last,
   >                           const Compare& x = Compare(), Container&& = Container());
   >         template<class Alloc> explicit priority_queue(const Alloc&);
   >         template<class Alloc> priority_queue(const Compare&, const Alloc&);
   >         template<class Alloc> priority_queue(const Compare&, const Container&, const Alloc&);
   >         template<class Alloc> priority_queue(const Compare&, Container&&, const Alloc&);
   >         template<class Alloc> priority_queue(const priority_queue&, const Alloc&);
   >         template<class Alloc> priority_queue(priority_queue&&, const Alloc&);
   >         template<class InputIterator, class Alloc>
   >           priority_queue(InputIterator, InputIterator, const Alloc&);
   >         template<class InputIterator, class Alloc>
   >           priority_queue(InputIterator, InputIterator, const Compare&, const Alloc&);
   >         template<class InputIterator, class Alloc>
   >           priority_queue(InputIterator, InputIterator, const Compare&, const Container&, const Alloc&);
   >         template<class InputIterator, class Alloc>
   >           priority_queue(InputIterator, InputIterator, const Compare&, Container&&, const Alloc&);

   >       // […]

   >       };

   >       template<class Compare, class Container>
   >         priority_queue(Compare, Container)
   >           -> priority_queue<typename Container::value_type, Container, Compare>;

   >       template<class InputIterator,
   >                 class Compare = less<typename iterator_traitsiter-value-type<InputIterator>::value_type>,
   >                 class Container = vector<typename iterator_traitsiter-value-type<InputIterator>::value_type>>
   >         priority_queue(InputIterator, InputIterator, Compare = Compare(), Container = Container())
   >           -> priority_queue<typename iterator_traitsiter-value-type<InputIterator>::value_type, Container, Compare>;

   >       template<class Compare, class Container, class Allocator>
   >         priority_queue(Compare, Container, Allocator)
   >           -> priority_queue<typename Container::value_type, Container, Compare>;

   >       template<class InputIterator, class Allocator>
   >         priority_queue(InputIterator, InputIterator, Allocator)
   >           -> priority_queue<iter-value-type<InputIterator>,
   >                             vector<iter-value-type<InputIterator>, Allocator>,
   >                             less<iter-value-type<InputIterator>>>;

   >       template<class InputIterator, class Compare, class Allocator>
   >         priority_queue(InputIterator, InputIterator, Compare, Allocator)
   >           -> priority_queue<iter-value-type<InputIterator>,
   >                             vector<iter-value-type<InputIterator>, Allocator>, Compare>;

   >       template<class InputIterator, class Compare, class Container, class Allocator>
   >         priority_queue(InputIterator, InputIterator, Compare, Container, Allocator)
   >           -> priority_queue<typename Container::value_type, Container, Compare>;

   >       // […]
   >     }

3. Add the following paragraphs to 23.6.4.3 [priqueue.cons.alloc]:

   >     template<class InputIterator, class Alloc>
   >       priority_queue(InputIterator first, InputIterator last, const Alloc& a);

   > > -?- Effects: Initializes `c` with `first` as the first argument, `last` as the second argument, and `a` as the third argument, and value-initializes `comp`; calls `make_heap(c.begin(), c.end(), comp)`.

   >     template<class InputIterator, class Alloc>
   >       priority_queue(InputIterator first, InputIterator last, const Compare& compare, const Alloc& a);

   > > -?- Effects: Initializes `c` with `first` as the first argument, `last` as the second argument, and `a` as the third argument, and initializes `comp` with `compare`; calls `make_heap(c.begin(), c.end(), comp)`.

   >     template<class InputIterator, class Alloc>
   >       priority_queue(InputIterator first, InputIterator last, const Compare& compare, const Container& cont, const Alloc& a);

   > > -?- Effects: Initializes `c` with `cont` as the first argument and `a` as the second argument, and initializes `comp` with `compare`; calls `c.insert(c.end(), first, last)`; and finally calls `make_heap(c.begin(), c.end(), comp)`.

   >     template<class InputIterator, class Alloc>
   >       priority_queue(InputIterator first, InputIterator last, const Compare& compare, Container&& cont, const Alloc& a);

   > > -?- Effects: Initializes `c` with `std::move(cont)` as the first argument and `a` as the second argument, and initializes `comp` with `compare`; calls `c.insert(c.end(), first, last)`; and finally calls `make_heap(c.begin(), c.end(), comp)`.

### 3508(i). `atomic_ref<cv T>` is not well-specified

Section: 32.5.7.1 [atomics.ref.generic.general] Status: Resolved Submitter: Casey Carter Opened: 2020-12-02 Last modified: 2025-10-14

Priority: 2

View all other issues in [atomics.ref.generic.general].

View all issues with Resolved status.

Discussion:

`atomic_ref<T>` requires only that its template parameter `T` is trivially copyable, which is not sufficient to implement many of the class template's member functions. Consider, for example:

>     int main() {
>       static const int x = 42;
>       std::atomic_ref<const int> meow{x};
>       meow.store(0);
>       return meow.load();
>     }

Since `const int` is trivially copyable, this is a well-formed C++20 program that returns `0`. That said, the `store` call that mutates the value of a constant object is (a) not implementable with library technology, and (b) pure madness that violates the language semantics. `atomic_ref::store` should presumably require `is_copy_assignable_v<T>`, and other operations need to have their requirements audited as well. (Related: LWG 3012(i) made similar changes to `atomic`.)

As a related issue, `volatile atomic<T>` recently had its member functions deprecated when they are not lock-free. Presumably `atomic_ref<volatile T>` should require that atomic operations on `T` be lock-free for consistency.

Jonathan:

The last point is related to LWG 3417(i) (another case where we screwed up the `volatile` deprecations).

[2020-12-19; Reflector prioritization]

Set priority to 2 during reflector discussions.

[2024-06; Related to issue 4069(i).]

[St. Louis 2024-06-28; SG1 feedback]

SG1 forwarded P3323R0 to LEWG to resolve LWG issues 3508(i) and 4069(i).

[2025-02-07 Status changed: Open → Resolved.]

Resolved by P3323R1 in Wrocław.

Proposed resolution:

### 3509(i). Range adaptor objects are underspecified

Section: 25.7.2 [range.adaptor.object] Status: Resolved Submitter: Tim Song Opened: 2020-12-15 Last modified: 2021-06-14

Priority: 2

View all other issues in [range.adaptor.object].

View all issues with Resolved status.

Discussion:

There is implementation divergence in the handling of this example:

>     template<class F>
>     auto filter(F f) {
>       return std::views::filter(f);
>     }
>     std::vector<int> v = {1, 2, 3, 4};
>     auto f = filter([i = std::vector{4}] (auto x) { return x == i[0]; });
>     auto x = v | f;

libstdc++'s `views::filter` stores a reference to the argument if it is passed as an lvalue, so `f` contains a dangling reference in the above example. MSVC's `views::filter` always stores a copy, and forwards that as either an lvalue or an rvalue depending on the range adaptor closure object's value category.

MSVC's behavior here seems desirable and is consistent with range-v3's behavior as well: the ability to form range adapter pipelines like this is an important feature, and there's nothing in the above example that even hints at a dangling reference problem.

However, the wording in 25.7.2 [range.adaptor.object] is unclear at best about exactly how the arguments are stored in the `adaptor(args...)` case; arguably, the requirement that `adaptor(range, args...)` be equivalent to `adaptor(args...)(range)` may even rule out the ability to make an extra copy. Certainly nothing in the wording guarantees that it is safe to reuse lvalue range pipelines (that is, `v | f` doesn't move from `f`).

[2021-01-15; Telecon prioritization]

Set priority to 2 following reflector and telecon discussions.

[2021-06-13 Resolved by the adoption of P2281R1 at the June 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

### 3510(i). Customization point objects should be invocable as non-`const` too

Section: 16.3.3.3.5 [customization.point.object] Status: Resolved Submitter: Tim Song Opened: 2020-12-15 Last modified: 2021-06-14

Priority: 3

View all other issues in [customization.point.object].

View all issues with Resolved status.

Discussion:

16.3.3.3.5 [customization.point.object] promises that customization point objects are semiregular and that all copies are equal, which permits copies to be freely made. In C++, making copies tends to drop cv-qualification, but there does not appear to be anything that guarantees that you can invoke these objects as non-`const` (or as rvalues, for that matter).

It is possible that the use of `invocable<const F&, Args...>` here was meant to bring in implicit expression variations, but P2102R0 has since clarified that this formulation doesn't do that.

[2021-01-15; Telecon prioritization]

Set priority to 3 following reflector and telecon discussions.

[2021-06-13 Resolved by the adoption of P2281R1 at the June 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

### 3514(i). `stacktrace` should add type alias `pmr::stacktrace`

Section: 19.6.2 [stacktrace.syn] Status: Resolved Submitter: Hiroaki Ando Opened: 2021-01-11 Last modified: 2021-10-23

Priority: 3

View all other issues in [stacktrace.syn].

View all issues with Resolved status.

Discussion:

`std::stacktrace` is almost `std::vector<stacktrace_entry>`. This makes it an obvious candidate to define an alias for `std::polymorphic_allocator`.

Daniel:

It should be pointed out that a theoretically possible (additional) template alias

>     namespace pmr {
>       template<class T>
>         using basic_stacktrace = std::basic_stacktrace<polymorphic_allocator<T>>;
>     }

— albeit it would seem a natural choice when comparing with existing `pmr` typedef additions — would not provide any advantage for the user, because template parameter `T` would essentially need to be constrained to be equal to `std::stacktrace_entry`.

[2021-03-12; Reflector poll]

Set priority to 3 and status to LEWG following reflector poll.

P2301 would resolve this.

[2021-10-23 Resolved by the adoption of P2301R1 at the October 2021 plenary. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 19.6.2 [stacktrace.syn], header `<stacktrace>` synopsis, as indicated:

   >     namespace std {
   >       // 19.6.3 [stacktrace.entry], class stacktrace_entry
   >       class stacktrace_entry;

   >       // 19.6.4 [stacktrace.basic], class template basic_stacktrace
   >       template<class Allocator>
   >       class basic_stacktrace;

   >       // basic_stacktrace typedef names
   >       using stacktrace = basic_stacktrace<allocator<stacktrace_entry>>;

   >       […]

   >       // 19.6.6 [stacktrace.basic.hash], hash support
   >       template<class T> struct hash;
   >       template<> struct hash<stacktrace_entry>;
   >       template<class Allocator> struct hash<basic_stacktrace<Allocator>>;

   >       namespace pmr {
   >         using stacktrace = std::basic_stacktrace<polymorphic_allocator<stacktrace_entry>>;
   >       }
   >     }

### 3515(i). §[stacktrace.basic.nonmem]: `operator<<` should be less templatized

Section: 19.6.2 [stacktrace.syn], 19.6.4.6 [stacktrace.basic.nonmem] Status: C++23 Submitter: Jiang An Opened: 2021-01-25 Last modified: 2023-11-22

Priority: 2

View all other issues in [stacktrace.syn].

View all issues with C++23 status.

Discussion:

According to 27.4.4.4 [string.io], the `operator<<` overloads in 19.6.4.6 [stacktrace.basic.nonmem] are well-formed only if the template parameters `charT` and `traits` are `char` and `std::char_traits<char>` (that of `std::string`) respectively, because it is required in Effects: that these overloads behave as-if insert a `std::string`.

I think the declarations of these overloads should be changed to:

>     ostream& operator<<(ostream& os, const stacktrace_entry& f);

>     template<class Allocator>
>     ostream& operator<<(ostream& os, const basic_stacktrace<Allocator>& st);

[2021-03-12; Reflector poll]

Set priority to 2 and status to LEWG following reflector poll.

[2022-11-07; Kona]

Move to Immediate.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 19.6.2 [stacktrace.syn], header `<stacktrace>` synopsis, as indicated:

   >     namespace std {
   >       // 19.6.3 [stacktrace.entry], class stacktrace_entry
   >       class stacktrace_entry;

   >       // 19.6.4 [stacktrace.basic], class template basic_stacktrace
   >       template<class Allocator>
   >       class basic_stacktrace;

   >       […]

   >       // 19.6.4.6 [stacktrace.basic.nonmem], non-member functions
   >       […]

   >       string to_string(const stacktrace_entry& f);

   >       template<class Allocator>
   >         string to_string(const basic_stacktrace<Allocator>& st);

   >       template<class charT, class traits>
   >         basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const stacktrace_entry& f);

   >       template<class charT, class traits, class Allocator>
   >         basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st);

   >       […]
   >     }

2. Modify 19.6.4.6 [stacktrace.basic.nonmem] as indicated:

   >     template<class charT, class traits>
   >     basic_ostream<charT, traits>&
   >       operator<<(basic_ostream<charT, traits>& os, const stacktrace_entry& f);

   > > -4- Effects: Equivalent to: `return os << to_string(f);`

   >     template<class charT, class traits, class Allocator>
   >     basic_ostream<charT, traits>&
   >       operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st);

   > > -5- Effects: Equivalent to: `return os << to_string(st);`

### 3517(i). `join_view::iterator`'s `iter_swap` is underconstrained

Section: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Casey Carter Opened: 2021-01-28 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.join.iterator].

View all issues with C++23 status.

Discussion:

`std::ranges::join_view::iterator`'s hidden friend `iter_swap` is specified in 25.7.14.3 [range.join.iterator]/16 as:

>     friend constexpr void iter_swap(const iterator& x, const iterator& y)
>       noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)));

> > -16- Effects: Equivalent to: `return ranges::iter_swap(x.inner_, y.inner_);`

Notably, the expression `ranges::iter_swap(meow, woof)` is not valid for all iterators `meow` and `woof`, or even all input iterators of the same type as is the case here. This `iter_swap` overload should be constrained to require the type of `iterator::inner_` (`iterator_t<range_reference_t<maybe-const<Const, V>>>)` to satisfy `indirectly_swappable`. Notably this is already the case for `iter_swap` friends of every other iterator adaptor in the Standard Library (`reverse_iterator`, `move_iterator`, `common_iterator`, `counted_iterator`, `filter_view::iterator`, `transform_view::iterator`, and `split_view::inner-iterator`). The omission for `join_view::iterator` seems to have simply been an oversight.

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 25.7.14.3 [range.join.iterator] as indicated:

   > [Drafting note: If 3500(i) is accepted before this issue, it is kindly suggested to the Project Editor to apply the equivalent replacement of "`iterator_t<range_reference_t<Base>>`" by "`InnerIter`" to the newly inserted `requires`.

   >     namespace std::ranges {
   >       template<input_range V>
   >         requires view<V> && input_range<range_reference_t<V>> &&
   >                  (is_reference_v<range_reference_t<V>> ||
   >                  view<range_value_t<V>>)
   >       template<bool Const>
   >       struct join_view<V>::iterator {
   >         […]

   >         friend constexpr void iter_swap(const iterator& x, const iterator& y)
   >           noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)))
   >           requires indirectly_swappable<iterator_t<range_reference_t<Base>>>;
   >       };
   >     }

   […]

   >     friend constexpr void iter_swap(const iterator& x, const iterator& y)
   >       noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)))
   >       requires indirectly_swappable<iterator_t<range_reference_t<Base>>>;

   > > -16- Effects: Equivalent to: `return ranges::iter_swap(x.inner_, y.inner_);`

### 3518(i). Exception requirements on char trait operations unclear

Section: 27.2.2 [char.traits.require] Status: C++23 Submitter: Zoe Carver Opened: 2021-02-01 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [char.traits.require].

View all other issues in [char.traits.require].

View all issues with C++23 status.

Discussion:

27.2.2 [char.traits.require] p1 says:

> `X` denotes a traits class defining types and functions for the character container type `C` […] Operations on `X` shall not throw exceptions.

It should be clarified what "operations on `X`" means. For example, in this patch, there was some confusion around the exact meaning of "operations on `X`". If it refers to the expressions specified in [tab:char.traits.req] or if it refers to all member functions of `X`, this should be worded in some clearer way.

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 27.2.2 [char.traits.require] as indicated:

   > -1- In Table [tab:char.traits.req], `X` denotes a traits class defining types and functions for the character container type `C`; […] Operations on `X` shall not throw exceptionsNo expression which is part of the character traits requirements specified in this subclause 27.2.2 [char.traits.require] shall exit via an exception.

### 3519(i). Incomplete synopses for `<random>` classes

Section: 29.5.4 [rand.eng], 29.5.5 [rand.adapt], 29.5.9 [rand.dist] Status: C++23 Submitter: Jens Maurer Opened: 2021-02-02 Last modified: 2023-11-22

Priority: 3

View all other issues in [rand.eng].

View all issues with C++23 status.

Discussion:

The synopses for the engine and distribution classes in `<random>` do not show declarations `operator==`, `operator!=`, `operator<<`, and `operator>>`, although they are part of the engine and distribution requirements.

Suggested resolution:

Add these operators as hidden friends to the respective class synopses.

[2021-02-07: Daniel provides concrete wording]

The proposed wording attempts to use a conservative approach in regard to exception specifications. Albeit 29.5.4.1 [rand.eng.general] p3 says that "no function described in 29.5.4 [rand.eng] throws an exception" (unless specified otherwise), not all implementations have marked the equality operators as `noexcept` (But some do for their engines, such as VS 2019). [No implementations marks the IO stream operators as `noexcept` of-course, because these cannot be prevented to throw exceptions in general by design].

The wording also uses hidden friends of the IO streaming operators ("inserters and extractors") as suggested by the issue discussion, but it should be noted that at least some existing implementations use out-of-class free function templates for their distributions.

Note that we intentionally don't declare any `operator!=`, because the auto-generated form already has the right semantics. The wording also covers the engine adaptor class templates because similar requirements apply to them.

[2021-03-12; Reflector poll]

Set priority to 3 following reflector poll.

[2021-03-12; LWG telecon]

Set status to Tentatively Ready after discussion and poll.

F | A | N
--- | --- | ---
10 | 0 | 0

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 29.5.4.2 [rand.eng.lcong] as indicated:

   >     template<class UIntType, UIntType a, UIntType c, UIntType m>
   >     class linear_congruential_engine {
   >       […]
   >       // constructors and seeding functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const linear_congruential_engine& x, const linear_congruential_engine& y);

   >       // generating functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const linear_congruential_engine& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, linear_congruential_engine& x);
   >     };

2. Modify 29.5.4.3 [rand.eng.mers] as indicated:

   >     template<class UIntType, size_t w, size_t n, size_t m, size_t r,
   >                 UIntType a, size_t u, UIntType d, size_t s,
   >                 UIntType b, size_t t,
   >                 UIntType c, size_t l, UIntType f>
   >     class mersenne_twister_engine {
   >       […]
   >       // constructors and seeding functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const mersenne_twister_engine& x, const mersenne_twister_engine& y);

   >       // generating functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const mersenne_twister_engine& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, mersenne_twister_engine& x);
   >     };

3. Modify 29.5.4.4 [rand.eng.sub] as indicated:

   >     template<class UIntType, size_t w, size_t s, size_t r>
   >     class subtract_with_carry_engine {
   >       […]
   >       // constructors and seeding functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const subtract_with_carry_engine& x, const subtract_with_carry_engine& y);

   >       // generating functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const subtract_with_carry_engine& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, subtract_with_carry_engine& x);
   >     };

4. Modify 29.5.5.2 [rand.adapt.disc] as indicated:

   >     template<class Engine, size_t p, size_t r>
   >     class discard_block_engine {
   >       […]
   >       // constructors and seeding functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const discard_block_engine& x, const discard_block_engine& y);

   >       // generating functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const discard_block_engine& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, discard_block_engine& x);
   >     };

5. Modify 29.5.5.3 [rand.adapt.ibits] as indicated:

   >     template<class Engine, size_t w, class UIntType>
   >     class independent_bits_engine {
   >       […]
   >       // constructors and seeding functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const independent_bits_engine& x, const independent_bits_engine& y);

   >       // generating functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const independent_bits_engine& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, independent_bits_engine& x);
   >     };

6. Modify 29.5.5.4 [rand.adapt.shuf] as indicated:

   >     template<class Engine, size_t k>
   >     class shuffle_order_engine {
   >       […]
   >       // constructors and seeding functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const shuffle_order_engine& x, const shuffle_order_engine& y);

   >       // generating functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const shuffle_order_engine& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, shuffle_order_engine& x);
   >     };

7. Modify 29.5.9.2.1 [rand.dist.uni.int] as indicated:

   >     template<class IntType = int>
   >     class uniform_int_distribution {
   >       […]
   >       // constructors and reset functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const uniform_int_distribution& x, const uniform_int_distribution& y);

   >       // generating functions
   >       […]

   >       // property functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const uniform_int_distribution& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, uniform_int_distribution& x);
   >     };

8. Modify 29.5.9.2.2 [rand.dist.uni.real] as indicated:

   >     template<class RealType = double>
   >     class uniform_real_distribution {
   >       […]
   >       // constructors and reset functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const uniform_real_distribution& x, const uniform_real_distribution& y);

   >       // generating functions
   >       […]

   >       // property functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const uniform_real_distribution& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, uniform_real_distribution& x);
   >     };

9. Modify 29.5.9.3.1 [rand.dist.bern.bernoulli] as indicated:

   >     class bernoulli_distribution {
   >       […]
   >       // constructors and reset functions
   >       […]

   >       // equality operators
   >       friend bool operator==(const bernoulli_distribution& x, const bernoulli_distribution& y);

   >       // generating functions
   >       […]

   >       // property functions
   >       […]

   >       // inserters and extractors
   >       template<class charT, class traits>
   >         friend basic_ostream<charT, traits>&
   >           operator<<(basic_ostream<charT, traits>& os, const bernoulli_distribution& x);
   >       template<class charT, class traits>
   >         friend basic_istream<charT, traits>&
   >           operator>>(basic_istream<charT, traits>& is, bernoulli_distribution& x);
   >     };

10. Modify 29.5.9.3.2 [rand.dist.bern.bin] as indicated:

    >     template<class IntType = int>
    >     class binomial_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const binomial_distribution& x, const binomial_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const binomial_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, binomial_distribution& x);
    >     };

11. Modify 29.5.9.3.3 [rand.dist.bern.geo] as indicated:

    >     template<class IntType = int>
    >     class geometric_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const geometric_distribution& x, const geometric_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const geometric_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, geometric_distribution& x);
    >     };

12. Modify 29.5.9.3.4 [rand.dist.bern.negbin] as indicated:

    >     template<class IntType = int>
    >     class negative_binomial_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const negative_binomial_distribution& x, const negative_binomial_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const negative_binomial_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, negative_binomial_distribution& x);
    >     };

13. Modify 29.5.9.4.1 [rand.dist.pois.poisson] as indicated:

    >     template<class IntType = int>
    >     class poisson_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const poisson_distribution& x, const poisson_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const poisson_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, poisson_distribution& x);
    >     };

14. Modify 29.5.9.4.2 [rand.dist.pois.exp] as indicated:

    >     template<class RealType = double>
    >     class exponential_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const exponential_distribution& x, const exponential_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const exponential_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, exponential_distribution& x);
    >     };

15. Modify 29.5.9.4.3 [rand.dist.pois.gamma] as indicated:

    >     template<class RealType = double>
    >     class gamma_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const gamma_distribution& x, const gamma_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const gamma_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, gamma_distribution& x);
    >     };

16. Modify 29.5.9.4.4 [rand.dist.pois.weibull] as indicated:

    >     template<class RealType = double>
    >     class weibull_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const weibull_distribution& x, const weibull_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const weibull_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, weibull_distribution& x);
    >     };

17. Modify 29.5.9.4.5 [rand.dist.pois.extreme] as indicated:

    >     template<class RealType = double>
    >     class extreme_value_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const extreme_value_distribution& x, const extreme_value_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const extreme_value_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, extreme_value_distribution& x);
    >     };

18. Modify 29.5.9.5.1 [rand.dist.norm.normal] as indicated:

    >     template<class RealType = double>
    >     class normal_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const normal_distribution& x, const normal_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const normal_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, normal_distribution& x);
    >     };

19. Modify 29.5.9.5.2 [rand.dist.norm.lognormal] as indicated:

    >     template<class RealType = double>
    >     class lognormal_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const lognormal_distribution& x, const lognormal_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const lognormal_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, lognormal_distribution& x);
    >     };

20. Modify 29.5.9.5.3 [rand.dist.norm.chisq] as indicated:

    >     template<class RealType = double>
    >     class chi_squared_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const chi_squared_distribution& x, const chi_squared_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const chi_squared_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, chi_squared_distribution& x);
    >     };

21. Modify 29.5.9.5.4 [rand.dist.norm.cauchy] as indicated:

    >     template<class RealType = double>
    >     class cauchy_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const cauchy_distribution& x, const cauchy_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const cauchy_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, cauchy_distribution& x);
    >     };

22. Modify 29.5.9.5.5 [rand.dist.norm.f] as indicated:

    >     template<class RealType = double>
    >     class fisher_f_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const fisher_f_distribution& x, const fisher_f_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const fisher_f_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, fisher_f_distribution& x);
    >     };

23. Modify 29.5.9.5.6 [rand.dist.norm.t] as indicated:

    >     template<class RealType = double>
    >     class student_t_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const student_t_distribution& x, const student_t_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const student_t_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, student_t_distribution& x);
    >     };

24. Modify 29.5.9.6.1 [rand.dist.samp.discrete] as indicated:

    >     template<class IntType = int>
    >     class discrete_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const discrete_distribution& x, const discrete_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const discrete_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, discrete_distribution& x);
    >     };

25. Modify 29.5.9.6.2 [rand.dist.samp.pconst] as indicated:

    >     template<class RealType = double>
    >     class piecewise_constant_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const piecewise_constant_distribution& x, const piecewise_constant_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const piecewise_constant_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, piecewise_constant_distribution& x);
    >     };

26. Modify 29.5.9.6.3 [rand.dist.samp.plinear] as indicated:

    >     template<class RealType = double>
    >     class piecewise_linear_distribution {
    >       […]
    >       // constructors and reset functions
    >       […]

    >       // equality operators
    >       friend bool operator==(const piecewise_linear_distribution& x, const piecewise_linear_distribution& y);

    >       // generating functions
    >       […]

    >       // property functions
    >       […]

    >       // inserters and extractors
    >       template<class charT, class traits>
    >         friend basic_ostream<charT, traits>&
    >           operator<<(basic_ostream<charT, traits>& os, const piecewise_linear_distribution& x);
    >       template<class charT, class traits>
    >         friend basic_istream<charT, traits>&
    >           operator>>(basic_istream<charT, traits>& is, piecewise_linear_distribution& x);
    >     };

### 3520(i). `iter_move` and `iter_swap` are inconsistent for `transform_view::iterator`

Section: 25.7.9.3 [range.transform.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-02-03 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.transform.iterator].

View all issues with C++23 status.

Discussion:

For `transform_view::iterator`, `iter_move` is specified to operate on the transformed value but `iter_swap` is specified to operate on the underlying iterator.

Consider the following test case:

>     struct X { int x; int y; };
>     std::vector<X> v = {...};
>     auto t = v | views::transform(&X::x);
>     ranges::sort(t);

`iter_swap` on `t`'s iterators would swap the whole `X`, including the `y` part, but `iter_move` will only move the `x` data member and leave the `y` part intact. Meanwhile, `ranges::sort` can use both `iter_move` and `iter_swap`, and does so in at least one implementation. The mixed behavior means that we get neither "sort `X`s by their `x` data member" (as `ranges::sort(v, {}, &X::x)` would do), nor "sort the `x` data member of these `X`s and leave the rest unchanged", as one might expect, but instead some arbitrary permutation of `y`. This seems like a questionable state of affairs.

[2021-03-12; Reflector poll]

Set priority to 2 following reflector poll.

[2021-03-12; LWG telecon]

Set status to Tentatively Ready after discussion and poll.

F | A | N
--- | --- | ---
9 | 0 | 0

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, copy_constructible F>
   > >         requires view<V> && is_object_v<F> &&
   > >                  regular_invocable<F&, range_reference_t<V>> &&
   > >                  can-reference<invoke_result_t<F&, range_reference_t<V>>>
   > >       template<bool Const>
   > >       class transform_view<V, F>::iterator {
   > >         […]
   > >         friend constexpr void iter_swap(const iterator& x, const iterator& y)
   > >           noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)))
   > >           requires indirectly_swappable<iterator_t<Base>>;
   > >       };
   > >     }

   > […]

   >     friend constexpr void iter_swap(const iterator& x, const iterator& y)
   >       noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)))
   >       requires indirectly_swappable<iterator_t<Base>>;

   > > -23- Effects: Equivalent to `ranges::iter_swap(x.current_, y.current_)`.

### 3521(i). Overly strict requirements on `qsort` and `bsearch`

Section: 26.13 [alg.c.library] Status: C++23 Submitter: Richard Smith Opened: 2021-02-02 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [alg.c.library].

View all issues with C++23 status.

Discussion:

Per 26.13 [alg.c.library]/2, for `qsort` and `bsearch`, we have:

> Preconditions: The objects in the array pointed to by `base` are of trivial type.

This seems like an unnecessarily strict requirement. `qsort` only needs the objects to be of a trivially-copyable type (because it will use `memcpy` or equivalent to relocate them), and `bsearch` doesn't need any particular properties of the array element type. Presumably it would be in improvement to specify the more-precise requirements instead.

We should also reconsider the other uses of the notion of a trivial type. It's really not a useful or meaningful type property by itself, because it doesn't actually require that any operations on the type are valid (due to the possibility of them being ambiguous or only some of them being available) and the places that consider it very likely actually mean `is_trivially_copyable` plus `is_trivially_default_constructible` instead, or perhaps `is_trivially_copy_constructible` and `is_trivially_move_constructible` and so on.

Other than `qsort` and `bsearch`, the only uses of this type property in the standard are to constrain `max_align_t`, `aligned_storage`, `aligned_union`, and the element type of `basic_string` (and in the definition of the deprecated `is_pod` trait), all of which (other than `is_pod`) I think really mean "is trivially default constructible", not "has at least one eligible default constructor and all eligible default constructors are trivial". And in fact I think the alignment types are underspecified — we don't want to require merely that they be trivially-copyable, since that doesn't require any particular operation on them to actually be valid — we also want to require that they actually model `semiregular`.

[2021-02-23; Casey Carter provides concrete wording]

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 26.13 [alg.c.library] as indicated:

   >     void* bsearch(const void* key, const void* base, size_t nmemb, size_t size,
   >                  c-compare-pred* compar);
   >     void* bsearch(const void* key, const void* base, size_t nmemb, size_t size,
   >                   compare-pred* compar);
   >     void qsort(void* base, size_t nmemb, size_t size, c-compare-pred* compar);
   >     void qsort(void* base, size_t nmemb, size_t size, compare-pred* compar);

   > > -2- Preconditions: For `qsort`, tThe objects in the array pointed to by base are of trivialtrivially copyable type.

### 3522(i). Missing requirement on `InputIterator` template parameter for `priority_queue` constructors

Section: 23.6.4 [priority.queue] Status: C++23 Submitter: Tim Song Opened: 2021-02-17 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [priority.queue].

View all issues with C++23 status.

Discussion:

There is nothing in 23.6.4 [priority.queue] or more generally 23.6 [container.adaptors] saying that `InputIterator` in the following constructor templates has to be an input iterator.

>     template<class InputIterator>
>         priority_queue(InputIterator first, InputIterator last, const Compare& x,
>                         const Container&);
>     template<class InputIterator>
>         priority_queue(InputIterator first, InputIterator last,
>                         const Compare& x = Compare(), Container&& = Container());

The second constructor template above therefore accepts

>     std::priority_queue<int> x = {1, 2};

to produce a `priority_queue` that contains a single element `2`. This behavior seems extremely questionable.

[2021-02-26; LWG telecon]

Set status to Tentatively Ready after discussion and poll.

F | A | N
--- | --- | ---
11 | 0 | 0

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

> [Drafting note: Because an upcoming paper provides iterator-pair constructors for other container adaptors, the wording below adds the restriction to 23.6.1 [container.adaptors.general] so that it also covers the constructors that will be added by that paper. — end drafting note]

This wording is relative to N4878.

1. Add the following paragraph to 23.6.1 [container.adaptors.general] after p3:

   > -?- A constructor template of a container adaptor shall not participate in overload resolution if it has an `InputIterator` template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

   > -4- A deduction guide for a container adaptor shall not participate in overload resolution if any of the following are true:

   > 1. (4.1) — It has an `InputIterator` template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

   > 2. […]

### 3523(i). `iota_view::sentinel` is not always `iota_view`'s sentinel

Section: 25.6.4.2 [range.iota.view] Status: C++23 Submitter: Tim Song Opened: 2021-02-17 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.iota.view].

View all issues with C++23 status.

Discussion:

P1739R4 added the following constructor to `iota_view`:

>     constexpr iota_view(iterator first, sentinel last) : iota_view(*first, last.bound_) {}

However, while `iota_view`'s iterator type is always `iota_view::iterator`, its sentinel type is not always `iota_view::sentinel`. First, if `Bound` is `unreachable_sentinel_t`, then the sentinel type is `unreachable_sentinel_t` too - we don't add an unnecessary level of wrapping on top. Second, when `W` and `Bound` are the same type, `iota_view` models `common_range`, and the sentinel type is the same as the iterator type - that is, `iterator`, not `sentinel`.

Presumably the intent is to use the view's actual sentinel type, rather than always use the `sentinel` type.

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Edit 25.6.4.2 [range.iota.view], as indicated:

   >     namespace std::ranges {
   >       // [...]

   >       template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t>
   >         requires weakly-equality-comparable-with<W, Bound> && semiregular<W>
   >       class iota_view : public view_interface<iota_view<W, Bound>> {
   >       private:
   >         // [range.iota.iterator], class iota_view​::​iterator
   >         struct iterator;            // exposition only
   >         // [range.iota.sentinel], class iota_view​::​sentinel
   >         struct sentinel;            // exposition only
   >         W value_ = W();             // exposition only
   >         Bound bound_ = Bound();     // exposition only
   >       public:
   >         iota_view() = default;
   >         constexpr explicit iota_view(W value);
   >         constexpr iota_view(type_identity_t<W> value,
   >                             type_identity_t<Bound> bound);
   >         constexpr iota_view(iterator first, sentinelsee below last);: iota_view(*first, last.bound_) {}

   >         constexpr iterator begin() const;
   >         constexpr auto end() const;
   >         constexpr iterator end() const requires same_as<W, Bound>;

   >         constexpr auto size() const requires see below;
   >       };

   >       template<class W, class Bound>
   >           requires (!is-integer-like<W> || !is-integer-like<Bound> ||
   >                   (is-signed-integer-like<W> == is-signed-integer-like<Bound>))
   >           iota_view(W, Bound) -> iota_view<W, Bound>;
   >     }

   > [...]

   >     constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);

   > > -8- Preconditions: `Bound` denotes `unreachable_sentinel_t` or `bound` is reachable from `value`. When `W` and `Bound` model `totally_ordered_with`, then `bool(value <= bound)` is true.

   > > -9- Effects: Initializes `value_` with `value` and `bound_` with `bound`.

   >     constexpr iota_view(iterator first, see below last);

   > > -?- Effects: Equivalent to:

   > > 1. (?.1) — If `same_as<W, Bound>` is `true`, `iota_view(first.value_, last.value_)`.

   > > 2. (?.2) — Otherwise, if `Bound` denotes `unreachable_sentinel_t`, `iota_view(first.value_, last)`.

   > > 3. (?.3) — Otherwise, `iota_view(first.value_, last.bound_)`.

   > > -?- Remarks: The type of `last` is:

   > > 1. (?.1) — If `same_as<W, Bound>` is `true`, `iterator`.

   > > 2. (?.2) — Otherwise, if `Bound` denotes `unreachable_sentinel_t`, `Bound`.

   > > 3. (?.3) — Otherwise, `sentinel`.

### 3524(i). Unimplementable narrowing and evaluation order requirements for range adaptors

Section: 25.7 [range.adaptors] Status: Resolved Submitter: Tim Song Opened: 2021-02-19 Last modified: 2021-06-14

Priority: 3

View all issues with Resolved status.

Discussion:

The specification of various range factory and adaptor objects generally says that some function call expression on them is expression-equivalent to an expression that performs list-initialization or in some cases a comma expression. This imposes evaluation order requirements that are unlikely to be intended and sometimes outright contradictory. For example, 25.7.12.1 [range.drop.overview] says that `views::drop(E, F)` is expression-equivalent to "`((void) F, decay-copy(E))`" in one case, and `drop_view{E, F}` in another. The first expression requires `F` to be sequenced before `E`, while the second expression requires `E` to be sequenced before `F`. They can't both hold in the absence of high levels of compiler magic.

Additionally, because the core language narrowing check in list-initialization considers the value of constant expressions, "expression-equivalent" also requires the constantness to be propagated. For example, given a range `E` whose `difference_type` is `int32_t`, `views::drop(E, int64_t())` is required to work, but `int64_t l = 0; views::drop(E, l)` is required to be ill-formed. This seems unlikely to be the intent either.

[2021-03-12; Reflector poll]

Set priority to 3 following reflector poll.

[2021-06-13 Resolved by the adoption of P2367R0 at the June 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

### 3525(i). `uses_allocator_construction_args` fails to handle types convertible to `pair`

Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Tim Song Opened: 2021-02-23 Last modified: 2023-11-22

Priority: 3

View all other issues in [allocator.uses.construction].

View all issues with C++23 status.

Discussion:

As currently specified, the following program is ill-formed (and appears to have been since LWG 2975(i)):

>     struct S {
>       operator std::pair<const int, int>() const {
>         return {};
>       }
>     };

>     void f() {
>       std::pmr::map<int, int> s;
>       s.emplace(S{});
>     }

There's no matching overload for `uses_allocator_construction_args<pair<const int, int>>(alloc, S&&)`, since `S` is not a `pair` and every overload for constructing `pair`s that takes one non-allocator argument expects a `pair` from which template arguments can be deduced.

[2021-02-27 Tim adds PR and comments]

The superseded resolution below attempts to solve this issue by adding two additional overloads of `uses_allocator_construction_args` to handle this case. However, the new overloads forces implicit conversions at the call to `uses_allocator_construction_args`, which requires the result to be consumed within the same full-expression before any temporary created from the conversion is destroyed. This is not the case for the `piecewise_construct` overload of `uses_allocator_construction_args`, which recursively calls `uses_allocator_construction_args` for the two elements of the pair, which might themselves be `pair`s.

The approach taken in the revised PR is to produce an exposition-only `pair-constructor` object instead. The object holds the allocator and the argument by reference, implicitly converts to the specified specialization of `pair`, and when so converted return a `pair` that is constructed by uses-allocator construction with the converted value of the original argument. This maintains the existing design that `pair` itself doesn't know anything about allocator construction.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4878.

> 1. Edit 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

>    >     namespace std {
>    >       […]
>    >       // 20.2.8.2 [allocator.uses.construction], uses-allocator construction
>    >       […]

>    >       template<class T, class Alloc>
>    >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                         const remove_cv_t<T>& pr) noexcept;
>    >       template<class T, class Alloc, class U, class V>
>    >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                         const pair<U, V>& pr) noexcept -> see below;

>    >       template<class T, class Alloc>
>    >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                         remove_cv_t<T>&& pr) noexcept;
>    >       template<class T, class Alloc, class U, class V>
>    >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                         pair<U, V>&& pr) noexcept -> see below;
>    >       […]
>    >     }

> 2. Edit 20.2.8.2 [allocator.uses.construction] as indicated:

>    >     template<class T, class Alloc>
>    >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                       const remove_cv_t<T>& pr) noexcept;
>    >     template<class T, class Alloc, class U, class V>
>    >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                       const pair<U, V>& pr) noexcept -> see below;

>    > > -12- Constraints: `T` is a specialization of `pair`. For the second overload, `is_same_v<pair<U, V>, remove_cv_t<T>>` is `false`.

>    > > -13- Effects: Equivalent to:

>    > > >     return uses_allocator_construction_args<T>(alloc, piecewise_construct,
>    > > >                                                 forward_as_tuple(pr.first),
>    > > >                                                 forward_as_tuple(pr.second));

>    >     template<class T, class Alloc>
>    >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                       remove_cv_t<T>&& pr) noexcept;
>    >     template<class T, class Alloc, class U, class V>
>    >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                       pair<U, V>&& pr) noexcept -> see below;

>    > > -14- Constraints: `T` is a specialization of `pair`. For the second overload, `is_same_v<pair<U, V>, remove_cv_t<T>>` is `false`.

>    > > -15- Effects: Equivalent to:

>    > > >     return uses_allocator_construction_args<T>(alloc, piecewise_construct,
>    > > >                                                 forward_as_tuple(std::move(pr).first),
>    > > >                                                 forward_as_tuple(std::move(pr).second));

[2021-03-12; Reflector poll]

Set priority to 3 following reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4878.

> 1. Edit 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

>    >     namespace std {
>    >       […]
>    >       // 20.2.8.2 [allocator.uses.construction], uses-allocator construction
>    >       […]

>    >       template<class T, class Alloc, class U, class V>
>    >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                         const pair<U, V>& pr) noexcept -> see below;

>    >       template<class T, class Alloc, class U, class V>
>    >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>    >                                                         pair<U, V>&& pr) noexcept -> see below;

>    >       template<class T, class Alloc, class U>
>    >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;
>    >       […]
>    >     }

> 2. Add the following to 20.2.8.2 [allocator.uses.construction]:

>    >       template<class T, class Alloc, class U>
>    >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;

>    > > -?- Let `FUN` be the function template:

>    > > >       template<class A, class B>
>    > > >       void FUN(const pair<A, B>&);

>    > > -?- Constraints: `T` is a specialization of `pair`, and the expression `FUN(u)` is not well-formed when considered as an unevaluated operand.

>    > > -?- Effects: Equivalent to:

>    > > >     return make_tuple(pair-constructor{alloc, u});

>    > > where `pair-constructor` is an exposition-only class defined as follows:

>    > > >     struct pair-constructor {
>    > > >       using pair-type = remove_cv_t<T>;            // exposition only

>    > > >       constexpr operator pair-type() const {
>    > > >         return do-construct(std::forward<U>(u));
>    > > >       }

>    > > >       constexpr auto do-construct(const pair-type& p) const {  // exposition only
>    > > >         return make_obj_using_allocator<pair-type>(alloc, p);
>    > > >       }
>    > > >       constexpr auto do-construct(pair-type&& p) const {  // exposition only
>    > > >         return make_obj_using_allocator<pair-type>(alloc, std::move(p));
>    > > >       }

>    > > >       const Alloc& alloc;  // exposition only
>    > > >       U& u;                // exposition only
>    > > >     };

[2021-12-02 Tim updates PR to avoid public exposition-only members]

[2022-01-31; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Edit 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 20.2.8.2 [allocator.uses.construction], uses-allocator construction
   >       […]
   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         pair<U, V>& pr) noexcept;

   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         const pair<U, V>& pr) noexcept;

   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         pair<U, V>&& pr) noexcept;

   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         const pair<U, V>&& pr) noexcept;

   >       template<class T, class Alloc, class U>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;
   >       […]
   >     }

2. Add the following to 20.2.8.2 [allocator.uses.construction]:

   >       template<class T, class Alloc, class U>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;

   > > -?- Let `FUN` be the function template:

   > > >       template<class A, class B>
   > > >       void FUN(const pair<A, B>&);

   > > -?- Constraints: `T` is a specialization of `pair`, and the expression `FUN(u)` is not well-formed when considered as an unevaluated operand.

   > > -?- Let `pair-constructor` be an exposition-only class defined as follows:

   > > >     class pair-constructor {
   > > >       using pair-type = remove_cv_t<T>;            // exposition only

   > > >       constexpr auto do-construct(const pair-type& p) const {  // exposition only
   > > >         return make_obj_using_allocator<pair-type>(alloc_, p);
   > > >       }
   > > >       constexpr auto do-construct(pair-type&& p) const {  // exposition only
   > > >         return make_obj_using_allocator<pair-type>(alloc_, std::move(p));
   > > >       }

   > > >       const Alloc& alloc_;  // exposition only
   > > >       U& u_;                // exposition only

   > > >     public:
   > > >       constexpr operator pair-type() const {
   > > >         return do-construct(std::forward<U>(u_));
   > > >       }
   > > >     };

   > > -?- Returns: `make_tuple(pc)`, where `pc` is a `pair-constructor` object whose `alloc_` member is initialized with `alloc` and whose `u_` member is initialized with `u`.

### 3526(i). Return types of `uses_allocator_construction_args` unspecified

Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Casey Carter Opened: 2021-02-25 Last modified: 2023-11-22

Priority: 3

View all other issues in [allocator.uses.construction].

View all issues with C++23 status.

Discussion:

The synopsis of `<memory>` in 20.2.2 [memory.syn] declares six overloads of `uses_allocator_construction_args` with return types "see below":

>     template<class T, class Alloc, class... Args>
>       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>                                                       Args&&... args) noexcept -> see below;
>     template<class T, class Alloc, class Tuple1, class Tuple2>>
>       constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
>                                                       Tuple1&& x, Tuple2&& y)
>                                                       noexcept -> see below;
>     template<class T, class Alloc>
>       constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below;
>     template<class T, class Alloc, class U, class V>
>       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>                                                       U&& u, V&& v) noexcept -> see below;
>     template<class T, class Alloc, class U, class V>
>       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>                                                       const pair<U, V>& pr) noexcept -> see below;
>     template<class T, class Alloc, class U, class V>
>       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
>                                                       pair<U, V>&& pr) noexcept -> see below;

The "see belows" also appear in the detailed specification of these overloaded function templates in 20.2.8.2 [allocator.uses.construction] para 4 through 15. Despite that the values these function templates return are specified therein, the return types are not. Presumably LWG wanted to specify deduced return types, but couldn't figure out how to do so, and just gave up?

[2021-02-27; Daniel comments and provides wording]

My interpretation is that the appearance of the trailing-return-type was actually unintended and that these functions where supposed to use the return type placeholder to signal the intention that the actual return type is deduced by the consistent sum of all return statements as they appear in the prototype specifications. Given that at least one implementation has indeed realized this form, I suggest to simply adjust the specification to remove the trailing-return-type. Specification-wise we have already existing practice for this approach (See e.g. `to_address`).

[2021-03-12; Reflector poll]

Set priority to 3 following reflector poll.

[2021-05-26; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Edit 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 20.2.8.2 [allocator.uses.construction], uses-allocator construction
   >       template<class T, class Alloc, class... Args>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         Args&&... args) noexcept -> see below;
   >       template<class T, class Alloc, class Tuple1, class Tuple2>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
   >                                                         Tuple1&& x, Tuple2&& y)
   >                                                         noexcept -> see below;
   >       template<class T, class Alloc>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below;
   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         U&& u, V&& v) noexcept -> see below;
   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         const pair<U, V>& pr) noexcept -> see below;
   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         pair<U, V>&& pr) noexcept -> see below;
   >       […]
   >     }

2. Edit 20.2.8.2 [allocator.uses.construction] as indicated:

   >     template<class T, class Alloc, class... Args>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                       Args&&... args) noexcept -> see below;

   > > […]

   > > -5- Returns: A `tuple` value determined as follows:

   > > 1. (5.1) — if […], return `forward_as_tuple(std::forward<Args>(args)...)`.

   > > 2. (5.2) — Otherwise, if […], return

   > >    >     tuple<allocator_arg_t, const Alloc&, Args&&...>(
   > >    >       allocator_arg, alloc, std::forward<Args>(args)...)

   > > 3. (5.3) — Otherwise, if […], return `forward_as_tuple(std::forward<Args>(args)..., alloc)`.

   > > 4. (5.4) — Otherwise, the program is ill-formed.

   >     template<class T, class Alloc, class Tuple1, class Tuple2>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
   >                                                       Tuple1&& x, Tuple2&& y)
   >                                                       noexcept -> see below;

   > > […]

   > > -7- Effects: For `T` specified as `pair<T1, T2>`, equivalent to:

   > > >     return make_tuple(
   > > >       piecewise_construct,
   > > >       apply([&alloc](auto&&... args1) {
   > > >               return uses_allocator_construction_args<T1>(
   > > >                 alloc, std::forward<decltype(args1)>(args1)...);
   > > >             }, std::forward<Tuple1>(x)),
   > > >       apply([&alloc](auto&&... args2) {
   > > >               return uses_allocator_construction_args<T2>(
   > > >                 alloc, std::forward<decltype(args2)>(args2)...);
   > > >             }, std::forward<Tuple2>(y)));

   >     template<class T, class Alloc>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below;

   > > […]

   > > -9- Effects: Equivalent to:

   > > >     return uses_allocator_construction_args<T>(alloc, piecewise_construct,
   > > >                                                tuple<>{}, tuple<>{});

   >     template<class T, class Alloc, class U, class V>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                       U&& u, V&& v) noexcept -> see below;

   > > […]

   > > -11- Effects: Equivalent to:

   > > >     return uses_allocator_construction_args<T>(alloc, piecewise_construct,
   > > >                                                forward_as_tuple(std::forward<U>(u)),
   > > >                                                forward_as_tuple(std::forward<V>(v));

   >     template<class T, class Alloc, class U, class V>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                       const pair<U, V>& pr) noexcept -> see below;

   > > […]

   > > -13- Effects: Equivalent to:

   > > >     return uses_allocator_construction_args<T>(alloc, piecewise_construct,
   > > >                                                forward_as_tuple(pr.first),
   > > >                                                forward_as_tuple(pr.second));

   >     template<class T, class Alloc, class U, class V>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                       pair<U, V>&& pr) noexcept -> see below;

   > > […]

   > > -15- Effects: Equivalent to:

   > > >     return uses_allocator_construction_args<T>(alloc, piecewise_construct,
   > > >                                                forward_as_tuple(std::move(pr).first),
   > > >                                                forward_as_tuple(std::move(pr).second));

### 3527(i). `uses_allocator_construction_args` handles rvalue pairs of rvalue references incorrectly

Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Tim Song Opened: 2021-02-27 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [allocator.uses.construction].

View all issues with C++23 status.

Discussion:

For an rvalue pair `pr`, `uses_allocator_construction_args` is specified to forward `std::move(pr).first` and `std::move(pr).second`. This is correct for non-references and lvalue references, but wrong for rvalue refrences because the class member access produces an lvalue (see 7.6.1.5 [expr.ref]/6). `get` produces an xvalue, which is what is desired here.

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Edit 20.2.8.2 [allocator.uses.construction] as indicated:

   >     template<class T, class Alloc, class U, class V>
   >       constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                       pair<U, V>&& pr) noexcept -> see below;

   > > […]

   > > -15- Effects: Equivalent to:

   > > >     return uses_allocator_construction_args<T>(alloc, piecewise_construct,
   > > >                                                 forward_as_tuple(std::move(pr).firstget<0>(std::move(pr))),
   > > >                                                 forward_as_tuple(std::move(pr).secondget<1>(std::move(pr))));

### 3528(i). `make_from_tuple` can perform (the equivalent of) a C-style cast

Section: 22.4.6 [tuple.apply] Status: C++23 Submitter: Tim Song Opened: 2021-02-28 Last modified: 2023-11-22

Priority: 3

View all issues with C++23 status.

Discussion:

`make_from_tuple` is specified to return `T(get<I>(std::forward<Tuple>(t))...)`. When there is only a single tuple element, this is equivalent to a C-style cast that may be a `reinterpret_cast`, a `const_cast`, or an access-bypassing `static_cast`.

[2021-03-12; Reflector poll]

Set priority to 3 following reflector poll. Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Edit 22.4.6 [tuple.apply] as indicated:

   >     template<class T, class Tuple>
   >       constexpr T make_from_tuple(Tuple&& t);

   > > […]

   > > -2- Effects: Given the exposition-only function:

   > > >     template<class T, class Tuple, size_t... I>
   > > >       requires is_constructible_v<T, decltype(get<I>(declval<Tuple>()))...>
   > > >     constexpr T make-from-tuple-impl(Tuple&& t, index_sequence<I...>) {     // exposition only
   > > >       return T(get<I>(std::forward<Tuple>(t))...);
   > > >     }

   > > Equivalent to:

   > > >     return make-from-tuple-impl<T>(
   > > >       std::forward<Tuple>(t),
   > > >       make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});

   > > [Note 1: The type of T must be supplied as an explicit template parameter, as it cannot be deduced from the argument list. — end note]

### 3529(i). `priority_queue(first, last)` should construct `c` with `(first, last)`

Section: 23.6.4 [priority.queue] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2021-03-01 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [priority.queue].

View all issues with C++23 status.

Discussion:

Tim's new constructors for `priority_queue` (LWG 3506(i)) are specified so that when you construct

>     auto pq = PQ(first, last, a);

it calls this new-in-LWG3506 constructor:

>     template<class InputIterator, class Alloc>
>       priority_queue(InputIterator first, InputIterator last, const Alloc& a);

> Effects: Initializes `c` with `first` as the first argument, `last` as the second argument, and `a` as the third argument, and value-initializes `comp`; calls `make_heap(c.begin(), c.end(), comp)`.

But the pre-existing constructors are specified so that when you construct

>     auto pq = PQ(first, last);

it calls this pre-existing constructor:

>     template<class InputIterator>
>       priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& y = Container());

> Preconditions: `x` defines a strict weak ordering ([alg.sorting]).

> Effects: Initializes `comp` with `x` and `c` with `y` (copy constructing or move constructing as appropriate); calls `c.insert(c.end(), first, last)`; and finally calls `make_heap(c.begin(), c.end(), comp)`.

In other words,

>     auto pq = PQ(first, last);

will default-construct a `Container`, then move-construct `c` from that object, then `c.insert(first, last)`, and finally `make_heap`.

But our new

>     auto pq = PQ(first, last, a);

will simply construct `c` with `(first, last)`, then `make_heap`.

The latter is obviously better.

Also, Corentin's P1425R3 specifies the new iterator-pair constructors for `stack` and `queue` to construct `c` from `(first, last)`. Good.

LWG should refactor the existing constructor overload set so that the existing non-allocator-taking constructors simply construct `c` from `(first, last)`. This will improve consistency with the resolutions of LWG3506 and P1425, and reduce the surprise factor for users.

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Edit 23.6.4.1 [priqueue.overview] as indicated:

   >     template<class InputIterator>
   >         priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare());
   >     template<class InputIterator>
   >         priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container& y);
   >     template<class InputIterator>
   >         priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& y = Container());

2. Edit 23.6.4.2 [priqueue.cons] as indicated:

   >     template<class InputIterator>
   >         priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare());

   > Preconditions: `x` defines a strict weak ordering ([alg.sorting]).

   > Effects: Initializes `c` with `first` as the first argument and `last` as the second argument, and initializes `comp` with `x`; then calls `make_heap(c.begin(), c.end(), comp)`.

   >     template<class InputIterator>
   >         priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container& y);
   >     template<class InputIterator>
   >         priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& y = Container());

   > Preconditions: `x` defines a strict weak ordering ([alg.sorting]).

   > Effects: Initializes `comp` with `x` and `c` with `y` (copy constructing or move constructing as appropriate); calls `c.insert(c.end(), first, last)`; and finally calls `make_heap(c.begin(), c.end(), comp)`.

### 3530(i). `BUILTIN-PTR-MEOW` should not opt the type out of syntactic checks

Section: 22.10.8.8 [comparisons.three.way], 22.10.9 [range.cmp] Status: C++23 Submitter: Tim Song Opened: 2021-03-04 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

The use of `BUILTIN-PTR-MEOW` for the constrained comparison function objects was needed to disable the semantic requirements on the associated concepts when the comparison resolves to a built-in operator comparing pointers: the comparison object is adding special handling for this case to produce a total order despite the core language saying otherwise, so requiring the built-in operator to then produce a total order as part of the semantic requirements doesn't make sense.

However, because it is specified as a disjunction on the constraint, it means that the comparison function objects are now required to accept types that don't even meet the syntactic requirements of the associated concept. For example, `ranges::less` requires all six comparison operators (because of `totally_ordered_with`) to be present … except when `operator<` on the arguments resolves to a built-in operator comparing pointers, in which case it just requires `operator<` and `operator==` (except that the latter isn't even required to be checked — it comes from the use of `ranges::equal_to` in the precondition of `ranges::less`). This seems entirely arbitrary.

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Edit 22.10.8.8 [comparisons.three.way] as indicated:

   > -1- In this subclause, `BUILTIN-PTR-THREE-WAY(T, U)` for types `T` and `U` is a boolean constant expression. `BUILTIN-PTR-THREE-WAY(T, U)` is true if and only if `<=>` in the expression

   > >     declval<T>() <=> declval<U>()

   > resolves to a built-in operator comparing pointers.

   >     struct compare_three_way {
   >       template<class T, class U>
   >         requires three_way_comparable_with<T, U> || BUILTIN-PTR-THREE-WAY(T, U)
   >       constexpr auto operator()(T&& t, U&& u) const;

   >       using is_transparent = unspecified;
   >     };

   >     template<class T, class U>
   >       requires three_way_comparable_with<T, U> || BUILTIN-PTR-THREE-WAY(T, U)
   >     constexpr auto operator()(T&& t, U&& u) const;

   > > -?- Constraints: `T` and `U` satisfy `three_way_comparable_with`.

   > > -2- Preconditions: If the expression `std​::​forward<T>(t) <=> std​::​forward<U>(u)` results in a call to a built-in operator `<=>` comparing pointers of type `P`, the conversion sequences from both `T` and `U` to `P` are equality-preserving (18.2 [concepts.equality]); otherwise, `T` and `U` model `three_way_comparable_with`.

   > > -3- Effects:

   > > 1. (3.1) — If the expression `std​::​forward<T>(t) <=> std​::​forward<U>(u)` results in a call to a built-in operator `<=>` comparing pointers of type `P`, returns `strong_ordering​::​less` if (the converted value of) `t` precedes `u` in the implementation-defined strict total order over pointers (3.28 [defns.order.ptr]), `strong_ordering​::​greater` if `u` precedes `t`, and otherwise `strong_ordering​::​equal`.

   > > 2. (3.2) — Otherwise, equivalent to: `return std​::​forward<T>(t) <=> std​::​forward<U>(u);`

2. Edit 22.10.9 [range.cmp] as indicated:

   > -1- In this subclause, `BUILTIN-PTR-CMP(T,` op`, U)` for types `T` and `U` and where op is an equality (7.6.10 [expr.eq]) or relational operator (7.6.9 [expr.rel]) is a boolean constant expression. `BUILTIN-PTR-CMP(T,` op`, U)` is true if and only if op in the expression `declval<T>()` op `declval<U>()` resolves to a built-in operator comparing pointers.

   >     struct ranges::equal_to {
   >       template<class T, class U>
   >         requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)
   >       constexpr bool operator()(T&& t, U&& u) const;

   >       using is_transparent = unspecified;
   >     };

   >     template<class T, class U>
   >       requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)
   >     constexpr bool operator()(T&& t, U&& u) const;

   > > -?- Constraints: `T` and `U` satisfy `equality_comparable_with`.

   > > -2- Preconditions: If the expression `std​::​forward<T>(t) == std​::​forward<U>(u)` results in a call to a built-in operator `==` comparing pointers of type `P`, the conversion sequences from both `T` and `U` to `P` are equality-preserving (18.2 [concepts.equality]); otherwise, `T` and `U` model `equality_comparable_with`.

   > > -3- Effects:

   > > 1. (3.1) — If the expression `std​::​forward<T>(t) == std​::​forward<U>(u)` results in a call to a built-in operator `==` comparing pointers of type `P`, returns `false` if either (the converted value of) `t` precedes `u` or `u` precedes `t` in the implementation-defined strict total order over pointers (3.28 [defns.order.ptr]) and otherwise `true`.

   > > 2. (3.2) — Otherwise, equivalent to: `return std​::​forward<T>(t) == std​::​forward<U>(u);`

   >     struct ranges::not_equal_to {
   >         template<class T, class U>
   >             requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)
   >         constexpr bool operator()(T&& t, U&& u) const;

   >         using is_transparent = unspecified;
   >     };

   >     template<class T, class U>
   >     constexpr bool operator()(T&& t, U&& u) const;

   > > -?- Constraints: `T` and `U` satisfy `equality_comparable_with`.

   > > -4- `operator()` has effects eEffects: Equivalent to:

   > > >     return !ranges::equal_to{}(std::forward<T>(t), std::forward<U>(u));

   >     struct ranges::greater {
   >       template<class T, class U>
   >         requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
   >       constexpr bool operator()(T&& t, U&& u) const;

   >       using is_transparent = unspecified;
   >     };

   >     template<class T, class U>
   >     constexpr bool operator()(T&& t, U&& u) const;

   > > -?- Constraints: `T` and `U` satisfy `totally_ordered_with`.

   > > -5- `operator()` has effects eEffects: Equivalent to:

   > > >     return ranges::less{}(std::forward<U>(u), std::forward<T>(t));

   >     struct ranges::less {
   >       template<class T, class U>
   >         requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
   >       constexpr bool operator()(T&& t, U&& u) const;

   >       using is_transparent = unspecified;
   >     };

   >     template<class T, class U>
   >       requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
   >     constexpr bool operator()(T&& t, U&& u) const;

   > > -?- Constraints: `T` and `U` satisfy `totally_ordered_with`.

   > > -6- Preconditions: If the expression `std​::​forward<T>(t) < std​::​forward<U>(u)` results in a call to a built-in operator `<` comparing pointers of type `P`, the conversion sequences from both `T` and `U` to `P` are equality-preserving (18.2 [concepts.equality]); otherwise, `T` and `U` model `totally_ordered_with`. For any expressions `ET` and `EU` such that `decltype((ET))` is `T` and `decltype((EU))` is `U`, exactly one of `ranges::less{}(ET, EU)`, `ranges::less{}(EU, ET)`, or `ranges::equal_to{}(ET, EU)` is `true`.

   > > -7- Effects:

   > > 1. (7.1) — If the expression `std​::​forward<T>(t) < std​::​forward<U>(u)` results in a call to a built-in operator `<` comparing pointers of type `P`, returns `true` if (the converted value of) `t` precedes `u` in the implementation-defined strict total order over pointers (3.28 [defns.order.ptr]) and otherwise `false`.

   > > 2. (7.2) — Otherwise, equivalent to: `return std​::​forward<T>(t) < std​::​forward<U>(u);`

   >     struct ranges::greater_equal {
   >       template<class T, class U>
   >         requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
   >       constexpr bool operator()(T&& t, U&& u) const;

   >       using is_transparent = unspecified;
   >     };

   >     template<class T, class U>
   >     constexpr bool operator()(T&& t, U&& u) const;

   > > -?- Constraints: `T` and `U` satisfy `totally_ordered_with`.

   > > -8- `operator()` has effects eEffects: Equivalent to:

   > > >     return !ranges::less{}(std::forward<T>(t), std::forward<U>(u));

   >     struct ranges::less_equal {
   >       template<class T, class U>
   >         requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
   >       constexpr bool operator()(T&& t, U&& u) const;

   >       using is_transparent = unspecified;
   >     };

   >     template<class T, class U>
   >     constexpr bool operator()(T&& t, U&& u) const;

   > > -?- Constraints: `T` and `U` satisfy `totally_ordered_with`.

   > > -9- `operator()` has effects eEffects: Equivalent to:

   > > >     return !ranges::less{}(std::forward<U>(u), std::forward<T>(t));

### 3532(i). `split_view<V, P>::inner-iterator<true>::operator++(int)` should depend on `Base`

Section: 25.7.16.5 [range.lazy.split.inner] Status: C++23 Submitter: Casey Carter Opened: 2021-03-11 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.lazy.split.inner].

View all issues with C++23 status.

Discussion:

`split_view<V, P>::inner-iterator<Const>::operator++(int)` is specified directly in the synopsis in [range.split.inner] as:

>     constexpr decltype(auto) operator++(int) {
>       if constexpr (forward_range<V>) {
>         auto tmp = *this;
>         ++*this;
>         return tmp;
>       } else
>         ++*this;
>     }

The dependency on the properties of `V` here is odd given that we are wrapping an iterator obtained from a `maybe-const<Const, V>` (aka `Base`). It seems like this function should instead be concerned with `forward_range<Base>`.

[2021-04-20; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify [range.split.inner] as indicated:

   >     constexpr decltype(auto) operator++(int) {
   >       if constexpr (forward_range<VBase>) {
   >         auto tmp = *this;
   >         ++*this;
   >         return tmp;
   >       } else
   >         ++*this;
   >     }

### 3533(i). Make `base() const &` consistent across iterator wrappers that supports `input_iterator`s

Section: 25.7.8.3 [range.filter.iterator], 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Tomasz Kamiński Opened: 2021-03-14 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

The resolution of LWG issue 3391(i) changed the `base()` function for the `counted_iterator`/`move_iterator` to return `const &`, because the previous specification prevented non-mutating uses of the base iterator (comparing against sentinel) and made `take_view` unimplementable. However, this change was not applied for all other iterators wrappers, that may wrap move-only input iterators. As consequence, we end-up with inconsistency where a user can perform the following operations on some adapters, but not on others (e. g. `take_view` uses `counted_iterator` so it supports them).

1. read the original value of the base iterator, by calling `operator*`

2. find position of an element in the underlying iterator, when sentinel is sized by calling `operator-` (e.g. all input iterators wrapped into `counted_iterator`).

To fix above, the proposed wording below proposes to modify the signature of `iterator::base() const &` member function for all iterators adapters that support input iterator. These include:

- `filter_view::iterator` (uses case B)

- `transform_view::iterator` (uses case A)

- `elements_view::iterator` (uses case B)

- `lazy_split_view<V, Pattern>::inner-iterator` (uses case B) if P2210 is accepted

Note: `common_iterator` does not expose the `base()` function (because it can either point to iterator or sentinel), so changes to above are not proposed. However, both (A) and (B) use cases are supported.

[2021-04-20; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

> If P2210 would become accepted, the corresponding subclause [range.lazy.split.inner] (?) for `lazy_split_view::inner-iterator` would require a similar change.

1. Modify 25.7.8.3 [range.filter.iterator] as indicated:

   > >     […]
   > >     constexpr const iterator_t<V>& base() const &
   > >       requires copyable<iterator_t<V>>;
   > >     […]

   […]

   >     constexpr const iterator_t<V>& base() const &
   >       requires copyable<iterator_t<V>>;

   > > -5- Effects: Equivalent to: `return current_;`

2. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > >     […]
   > >     constexpr const iterator_t<Base>& base() const &
   > >       requires copyable<iterator_t<Base>>;
   > >     […]

   […]

   >     constexpr const iterator_t<Base>& base() const &
   >       requires copyable<iterator_t<Base>>;

   > > -5- Effects: Equivalent to: `return current_;`

3. Modify 25.7.23.3 [range.elements.iterator] as indicated:

   > >     […]
   > >     constexpr const iterator_t<Base>& base() const &
   > >       requires copyable<iterator_t<Base>>;
   > >     […]

   […]

   >     constexpr const iterator_t<Base>& base() const &
   >       requires copyable<iterator_t<Base>>;

   > > -3- Effects: Equivalent to: `return current_;`

### 3535(i). `join_view::iterator::iterator_category` and `::iterator_concept` lie

Section: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Casey Carter Opened: 2021-03-16 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.join.iterator].

View all issues with C++23 status.

Discussion:

Per 25.7.14.3 [range.join.iterator]/1, `join_view::iterator::iterator_concept` denotes `bidirectional_iterator_tag` if `ref-is-glvalue` is `true` and `Base` and `range_reference_t<Base>` each model `bidirectional_range`. Similarly, paragraph 2 says that `join_view::iterator::iterator_category` is present if `ref-is-glvalue` is `true`, and denotes `bidirectional_iterator_tag` if the categories of the iterators of both `Base` and `range_reference_t<Base>` derive from `bidirectional_iterator_tag`.

However, the constraints on the declarations of `operator--` and `operator--(int)` in the synopsis that immediately precedes paragraph 1 disagree. Certainly they also consistently require `ref-is-glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>>`, but they additionally require `common_range<range_reference_t<Base>>`. So as currently specified, this iterator sometimes declares itself to be bidirectional despite not implementing `--`. This is not incorrect for iterator_concept — recall that `iterator_concept` is effectively an upper bound since the concepts require substantial syntax — but slightly misleading. It is, however, very much incorrect for `iterator_category` which must not denote a type derived from a tag that corresponds to a stronger category than that iterator implements.

It's worth pointing out, that LWG 3313(i) fixed the constraints on `operator--()` and `operator--(int)` by adding the `common_range` requirements, but failed to make a consistent change to the definitions of `iterator_concept` and `iterator_category`.

[2021-04-04; Daniel comments]

The below proposed wording can be compared as being based on N4885.

[2021-04-20; Reflector poll]

Priority set to 2.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to the post-2021-February-virtual-meeting working draft.

1. Modify 25.7.14.3 [range.join.iterator] as indicated:

   > -1- `iterator::iterator_concept` is defined as follows:

   > 1. (1.1) — If `ref-is-glvalue` is `true`, and `Base` and `range_reference_t<Base>` each models `bidirectional_range`, and `range_reference_t<Base>` models both `bidirectional_range` and `common_range`, then `iterator_concept` denotes `bidirectional_iterator_tag`.

   > 2. […]

   > […]

   > -2- The member typedef-name `iterator_category` is defined if and only if `ref-is-glvalue` is `true`, `Base` models `forward_range`, and `range_reference_t<Base>` models `forward_range`. In that case, `iterator::iterator_category` is defined as follows:

   > 1. (2.1) — Let `OUTERC` denote `iterator_traits<iterator_t<Base>>::iterator_category`, and let `INNERC` denote `iterator_traits<iterator_t<range_reference_t<Base>>>::iterator_category`.

   > 2. (2.2) — If `OUTERC` and `INNERC` each model `derived_from<bidirectional_iterator_tag>`, and `range_reference_t<Base>` models `common_range`, `iterator_category` denotes `bidirectional_iterator_tag`.

   > 3. […]

### 3536(i). Should `chrono::from_stream()` assign zero to `duration` for failure?

Section: 30.5.11 [time.duration.io] Status: C++23 Submitter: Matt Stephanson Opened: 2021-03-19 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [time.duration.io].

View all issues with C++23 status.

Discussion:

The `duration` specialization of `from_stream` says in N4878 30.5.11 [time.duration.io]/3:

> If the parse parses everything specified by the parsing format flags without error, and yet none of the flags impacts a duration, `d` will be assigned a zero value.

This is in contrast to the other specializations that say, for example, 30.8.3.3 [time.cal.day.nonmembers]/8:

> If the parse fails to decode a valid day, `is.setstate(ios_base::failbit)` is called and `d` is not modified.

The wording ("none of the flags impacts a duration" vs. "parse fails to decode a valid [meow]") and semantics ("assigned a zero value" vs. "not modified") are different, and it's not clear why that should be so. It also leaves unspecified what should be done in case of a parse failure, for example parsing `"%j"` from a stream containing `"meow"`. 30.13 [time.parse]/12 says that `failbit` should be set, but neither it nor 30.5.11 [time.duration.io]/3 mention the `duration` result if parsing fails.

This has been discussed at the Microsoft STL project, where Howard Hinnant, coauthor of P0355R7 that added these functions, commented:

> This looks like a bug in the standard to me, due to two errors on my part.

> I believe that the standard should clearly say that if `failbit` is set, the parsed variable (`duration`, `time_point`, whatever) is not modified. I mistakenly believed that the definition of unformatted input function covered this behavior. But after review, I don't believe it does. Instead each extraction operator seems to say this separately.

> I also at first did not have my example implementation coded to leave the duration unchanged. So that's how the wording got in in the first place. Here's the commit where I fixed my implementation: HowardHinnant/date@d53db7a. And I failed to propagate that fix into the proposal/standard.

It would be clearer and simpler for users if the `from_stream` specializations were consistent in wording and behavior.

Thanks to Stephan T. Lavavej, Miya Natsuhara, and Howard Hinnant for valuable investigation and discussion of this issue.

[2021-04-20; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 30.5.11 [time.duration.io] as indicated:

   >     template<class charT, class traits, class Rep, class Period, class Alloc = allocator<charT>>
   >       basic_istream<charT, traits>&
   >         from_stream(basic_istream<charT, traits>& is, const charT* fmt,
   >                     duration<Rep, Period>& d,
   >                     basic_string<charT, traits, Alloc>* abbrev = nullptr,
   >                     minutes* offset = nullptr);

   > > -3- Effects: Attempts to parse the input stream `is` into the duration `d` using the format flags given in the NTCTS `fmt` as specified in 30.13 [time.parse]. If the parse parses everything specified by the parsing format flags without error, and yet none of the flags impacts a duration, `d` will be assigned a zero valueIf the parse fails to decode a valid duration, `is.setstate(ios_base::failbit)` is called and `d` is not modified. If `%Z` is used and successfully parsed, that value will be assigned to `*abbrev` if `abbrev` is non-null. If `%z` (or a modified variant) is used and successfully parsed, that value will be assigned to `*offset` if `offset` is non-null.

### 3537(i). §[iterator.range] Missing `noexcept` for `std::rbegin`/`rend` for arrays and `initializer_list`

Section: 24.7 [iterator.range] Status: Resolved Submitter: Jiang An Opened: 2021-03-21 Last modified: 2025-11-11

Priority: 3

View all other issues in [iterator.range].

View all issues with Resolved status.

Discussion:

Overloads for `std::rbegin`/`rend` for built-in arrays and `std::initializer_list`'s has no preconditions and never throw exceptions, thus should be `noexcept`. LWG 2280(i) addressed a similar issue for `std::begin/end`.

Suggestion: change these declarations in 24.7 [iterator.range] (p10, 11, 12, 13) as indicated:

>     template<class T, size_t N> constexpr reverse_iterator<T*> rbegin(T (&array)[N]) noexcept;

>     template<class T, size_t N> constexpr reverse_iterator<T*> rend(T (&array)[N]) noexcept;

>     template<class E> constexpr reverse_iterator<const E*> rbegin(initializer_list<E> il) noexcept;

>     template<class E> constexpr reverse_iterator<const E*> rend(initializer_list<E> il) noexcept;

If this change is accepted, we may also specify conditional `noexcept` for `std::crbegin/crend` (in 24.7 [iterator.range] p14, 15), by adding `noexcept(noexcept(std::rbegin/crend(c)))`, like in LWG 2280(i).

Conditional noexcept for other functions in 24.7 [iterator.range] (p2, 3, 7, 8, 16, 18, 20 22) may also be added for consistency.

[2021-03-21; Daniel comments]

There is intentionally no P/R provided at this point, but I'm volunteering to provide it if we got feedback whether adding conditional `noexcept` specifiers similar to those provided by LWG 2280(i) would be preferred or not.

[2021-04-20; Reflector poll]

Priority set to 3.

Jonathan: This would create a strange situation where `std::rbegin` and `std::crbegin` on an `initializer_list` are noexcept but `std::begin` and `std::cbegin` aren't guaranteed to be (because an `initializer_list` uses the generic `std::begin` and `std::cbegin` overloads, which have no conditional noexcept).

Casey: I don't think we should mark these rbegin/rend overloads noexcept without making the pertinent `reverse_iterator` constructors conditionally noexcept.

[2025-07-01; P3623R0 would resolve this]

[2025-11-03; P3016R6 would resolve this (replacing P3623R0)]

[2025-11-11; Resolved by P3016R6, approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

### 3539(i). `format_to` must not copy models of `output_iterator<const charT&>`

Section: 28.5.5 [format.functions] Status: C++23 Submitter: Casey Carter Opened: 2021-03-31 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [format.functions].

View all issues with C++23 status.

Discussion:

28.5.5 [format.functions] specifies the overloads of `format_to` as:

>     template<class Out, class... Args>
>       Out format_to(Out out, string_view fmt, const Args&... args);
>     template<class Out, class... Args>
>       Out format_to(Out out, wstring_view fmt, const Args&... args);

> > -8- Effects: Equivalent to:

> > >     using context = basic_format_context<Out, decltype(fmt)::value_type>;
> > >     return vformat_to(out, fmt, make_format_args<context>(args...));

>     template<class Out, class... Args>
>       Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args);
>     template<class Out, class... Args>
>       Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);
>       Out format_to(Out out, wstring_view fmt, const Args&... args);

> > -9- Effects: Equivalent to:

> > >     using context = basic_format_context<Out, decltype(fmt)::value_type>;
> > >     return vformat_to(out, loc, fmt, make_format_args<context>(args...));

but the overloads of `vformat_to` take their first argument by value (from the same subclause):

>     template<class Out>
>       Out vformat_to(Out out, string_view fmt,
>                      format_args_t<type_identity_t<Out>, char> args);
>     template<class Out>
>       Out vformat_to(Out out, wstring_view fmt,
>                      format_args_t<type_identity_t<Out>, wchar_t> args);
>     template<class Out>
>       Out vformat_to(Out out, const locale& loc, string_view fmt,
>                      format_args_t<type_identity_t<Out>, char> args);
>     template<class Out>
>       Out vformat_to(Out out, const locale& loc, wstring_view fmt,
>                      format_args_t<type_identity_t<Out>, wchar_t> args);

> > -10- Let `charT` be `decltype(fmt)::value_type`.

> > -11- Constraints: `Out` satisfies `output_iterator<const charT&>`.

> > -12- Preconditions: `Out` models `output_iterator<const charT&>`.

and require its type to model `output_iterator<const charT&>`. `output_iterator<T, U>` refines `input_or_output_iterator<T>` which refines `movable<T>`, but it notably does not refine `copyable<T>`. Consequently, the "Equivalent to" code for the `format_to` overloads is copying an iterator that could be move-only. I suspect it is not the intent that calls to `format_to` with move-only iterators be ill-formed, but that it was simply an oversight that this wording needs updating to be consistent with the change to allow move-only single-pass iterators in C++20.

[2021-04-20; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 28.5.5 [format.functions] as indicated:

   >     template<class Out, class... Args>
   >       Out format_to(Out out, string_view fmt, const Args&... args);
   >     template<class Out, class... Args>
   >       Out format_to(Out out, wstring_view fmt, const Args&... args);

   > > -8- Effects: Equivalent to:

   > > >     using context = basic_format_context<Out, decltype(fmt)::value_type>;
   > > >     return vformat_to(std::move(out), fmt, make_format_args<context>(args...));

   >     template<class Out, class... Args>
   >       Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args);
   >     template<class Out, class... Args>
   >       Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);

   > > -9- Effects: Equivalent to:

   > > >     using context = basic_format_context<Out, decltype(fmt)::value_type>;
   > > >     return vformat_to(std::move(out), loc, fmt, make_format_args<context>(args...));

### 3540(i). §[format.arg] There should be no `const` in `basic_format_arg(const T* p)`

Section: 28.5.8.1 [format.arg] Status: C++23 Submitter: S. B. Tam Opened: 2021-04-07 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [format.arg].

View all issues with C++23 status.

Discussion:

When P0645R10 "Text formatting" was merged into the draft standard, there was a typo: an exposition-only constructor of `basic_format_arg` is declared as accepting `const T*` in the class synopsis, but is later declared to accept `T*`. This was editorial issue 3461 and was resolved by adding `const` to the redeclaration.

As it is, constructing `basic_format_arg` from `void*` will select `template<class T> explicit basic_format_arg(const T& v)` and store a `basic_format_arg::handle` instead of select `template<class T> basic_format_arg(const T*)` and store a `const void*`, because `void*` → `void*const&` is identity conversion, while `void*` → `const void*` is qualification conversion.

While this technically works, it seems that storing a `const void*` would be more intuitive.

Hence, I think `const` should be removed from both declarations of `basic_format_arg(const T*)`, so that construction from `void*` will select this constructor, resulting in more intuitive behavior.

[2021-04-20; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 28.5.8.1 [format.arg] as indicated:

   > >     namespace std {
   > >       template<class Context>
   > >       class basic_format_arg {
   > >       public:
   > >         class handle;

   > >       private:
   > >         using char_type = typename Context::char_type;                     // exposition only

   > >         variant<monostate, bool, char_type,
   > >           int, unsigned int, long long int, unsigned long long int,
   > >           float, double, long double,
   > >           const char_type*, basic_string_view<char_type>,
   > >           const void*, handle> value;                                      // exposition only

   > >         template<class T> explicit basic_format_arg(const T& v) noexcept;  // exposition only
   > >         explicit basic_format_arg(float n) noexcept;                       // exposition only
   > >         explicit basic_format_arg(double n) noexcept;                      // exposition only
   > >         explicit basic_format_arg(long double n) noexcept;                 // exposition only
   > >         explicit basic_format_arg(const char_type* s);                     // exposition only

   > >         template<class traits>
   > >           explicit basic_format_arg(
   > >             basic_string_view<char_type, traits> s) noexcept;              // exposition only

   > >         template<class traits, class Allocator>
   > >           explicit basic_format_arg(
   > >             const basic_string<char_type, traits, Allocator>& s) noexcept; // exposition only

   > >         explicit basic_format_arg(nullptr_t) noexcept;                     // exposition only

   > >         template<class T>
   > >           explicit basic_format_arg(const T* p) noexcept;                  // exposition only
   > >       public:
   > >         basic_format_arg() noexcept;

   > >         explicit operator bool() const noexcept;
   > >       };
   > >     }

   > […]

   >     template<class T> explicit basic_format_arg(const T* p) noexcept;

   > > -12- Constraints: `is_void_v<T>` is `true`.

   > > -13- Effects: Initializes `value` with `p`.

   > > -14- [Note 1: Constructing `basic_format_arg` from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type. — end note]

### 3541(i). `indirectly_readable_traits` should be SFINAE-friendly for all types

Section: 24.3.2.2 [readable.traits] Status: C++23 Submitter: Christopher Di Bella Opened: 2021-04-08 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [readable.traits].

View all issues with C++23 status.

Discussion:

Following the outcome of LWG 3446(i), a strict implementation of `std::indirectly_readable_traits` isn't SFINAE-friendly for types that have different `value_type` and `element_type` members due to the ambiguity between the `has-member-value-type` and `has-member-element-type` specialisations. Other traits types are empty when requirements aren't met; we should follow suit here.

[2021-04-20; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 24.3.2.2 [readable.traits] p1 as indicated:

   >     […]
   >     template<class T>
   >       concept has-member-value-type = requires { typename T::value_type; }; // exposition only

   >     template<class T>
   >       concept has-member-element-type = requires { typename T::element_type; }; // exposition only

   >     template<class> struct indirectly_readable_traits { };

   >     […]

   >     template<has-member-value-type T>
   >     struct indirectly_readable_traits<T>
   >       : cond-value-type<typename T::value_type> { };

   >     template<has-member-element-type T>
   >     struct indirectly_readable_traits<T>
   >       : cond-value-type<typename T::element_type> { };

   >     template<has-member-value-type T>
   >       requires has-member-element-type<T>
   >     struct indirectly_readable_traits<T> { };

   >     template<has-member-value-type T>
   >       requires has-member-element-type<T> &&
   >                same_as<remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>>
   >     struct indirectly_readable_traits<T>
   >       : cond-value-type<typename T::value_type> { };
   >     […]

### 3542(i). `basic_format_arg` mis-handles `basic_string_view` with custom traits

Section: 28.5.8.1 [format.arg] Status: C++23 Submitter: Casey Carter Opened: 2021-04-20 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [format.arg].

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Discussion:

`basic_format_arg` has a constructor that accepts a `basic_string_view` of an appropriate character type, with any `traits` type. The constructor is specified in 28.5.8.1 [format.arg] as:

>     template<class traits>
>     explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;

> > -9- Effects: Initializes `value` with `s`.

Recall that `value` is a `variant<monostate, bool, char_type, int, unsigned int, long long int, unsigned long long int, float, double, long double, const char_type*, basic_string_view<char_type>, const void*, handle>` as specified earlier in the subclause. Since `basic_string_view<meow, woof>` cannot be initialized with an lvalue `basic_string_view<meow, quack>` — and certainly none of the other alternative types can be initialized by such an lvalue — the effects of this constructor are ill-formed when `traits` is not `std::char_traits<char_type>`.

The `basic_string` constructor deals with this same issue by ignoring the deduced traits type when initializing `value`'s `basic_string_view` member:

>     template<class traits, class Allocator>
>       explicit basic_format_arg(
>         const basic_string<char_type, traits, Allocator>& s) noexcept;

> > -10- Effects: Initializes `value` with `basic_string_view<char_type>(s.data(), s.size())`.

which immediately begs the question of "why doesn't the `basic_string_view` constructor do the same?"

[2021-05-10; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 28.5.8.1 [format.arg] as indicated:

   >     template<class traits>
   >       explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;

   > > -9- Effects: Initializes `value` with `sbasic_string_view<char_type>(s.data(), s.size())`.

### 3543(i). Definition of when `counted_iterators` refer to the same sequence isn't quite right

Section: 24.5.7.1 [counted.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-04-21 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [counted.iterator].

View all issues with C++23 status.

Discussion:

24.5.7.1 [counted.iterator]/3 says:

> Two values `i1` and `i2` of types `counted_iterator<I1>` and `counted_iterator<I2>` refer to elements of the same sequence if and only if `next(i1.base(), i1.count())` and `next(i2.base(), i2.count())` refer to the same (possibly past-the-end) element.

However, some users of `counted_iterator` (such as `take_view`) don't guarantee that there are `count()` elements past `base()`. It seems that we need to rephrase this definition to account for such uses.

[2021-05-10; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 24.5.7.1 [counted.iterator] as indicated:

   > -3- Two values `i1` and `i2` of types `counted_iterator<I1>` and `counted_iterator<I2>` refer to elements of the same sequence if and only if there exists some integer `n` such that `next(i1.base(), i1.count() + n)` and `next(i2.base(), i2.count() + n)` refer to the same (possibly past-the-end) element.

### 3544(i). `format-arg-store::args` is unintentionally not exposition-only

Section: 28.5.8.2 [format.arg.store] Status: C++23 Submitter: Casey Carter Opened: 2021-04-22 Last modified: 2023-11-22

Priority: 3

View all issues with C++23 status.

Discussion:

Despite the statement in 28.5.8.3 [format.args]/1:

> An instance of `basic_format_args` provides access to formatting arguments. Implementations should optimize the representation of `basic_format_args` for a small number of formatting arguments. [Note 1: For example, by storing indices of type alternatives separately from values and packing the former. — end note]

`make_format_args` and `make_wformat_args` are specified to return an object whose type is a specialization of the exposition-only class template `format-arg-store` which has a public non-static data member that is an `array` of `basic_format_arg`. In order to actually "optimize the representation of `basic_format_args`" an implementation must internally avoid using `make_format_args` (and `make_wformat_args`) and instead use a different mechanism to type-erase arguments. `basic_format_args` must still be convertible from `format-arg-store` as specified, however, so internally `basic_format_args` must support both the bad/slow standard mechanism and a good/fast internal-only mechanism for argument storage.

While this complicated state of affairs is technically implementable, it greatly complicates the implementation of `<format>` with no commensurate benefit. Indeed, naive users may make the mistake of thinking that e.g. `vformat(fmt, make_format_args(args...))` is as efficient as `format(fmt, args...)` — that's what the "Effects: Equivalent to" in 28.5.5 [format.functions]/2 implies — and inadvertently introduce performance regressions. It would be better for both implementers and users if `format-arg-store` had no public data members and its member `args` were made exposition-only.

[2021-05-10; Reflector poll]

Priority set to 3. Tim: "The current specification of `make_format_args` depends on `format-arg-store` being an aggregate, which is no longer true with this PR."

Previous resolution [SUPERSEDED]:

> This wording is relative to N4885.

> 1. Modify 28.5.1 [format.syn], header `<format>` synopsis, as indicated:

>    >     […]
>    >     // 28.5.8.2 [format.arg.store], class template format-arg-store
>    >     template<class Context, class... Args> structclass format-arg-store; // exposition only
>    >     […]

> 2. Modify 28.5.8.2 [format.arg.store] as indicated:

>    >     namespace std {
>    >       template<class Context, class... Args>
>    >       structclass format-arg-store { // exposition only
>    >         array<basic_format_arg<Context>, sizeof...(Args)> args; // exposition only
>    >       };
>    >     }

[2021-05-18; Tim updates wording.]

[2021-05-20; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 28.5.1 [format.syn], header `<format>` synopsis, as indicated:

   >     […]
   >     // 28.5.8.2 [format.arg.store], class template format-arg-store
   >     template<class Context, class... Args> structclass format-arg-store; // exposition only

   >     template<class Context = format_context, class... Args>
   >       format-arg-store<Context, Args...>
   >         make_format_args(const Args&... fmt_args);
   >     […]

2. Modify 28.5.8.2 [format.arg.store] as indicated:

   >     namespace std {
   >       template<class Context, class... Args>
   >       structclass format-arg-store { // exposition only
   >         array<basic_format_arg<Context>, sizeof...(Args)> args; // exposition only
   >       };
   >     }

   > -1- An instance of `format-arg-store` stores formatting arguments.

   >     template<class Context = format_context, class... Args>
   >       format-arg-store<Context, Args...> make_format_args(const Args&... fmt_args);

   > > -2- Preconditions: The type `typename Context::template formatter_type<Ti>` meets the Formatter requirements (28.5.6.1 [formatter.requirements]) for each `Ti` in `Args`.

   > > -3- Returns: An object of type `format-arg-store<Context, Args...>` whose `args` data member is initialized with `{basic_format_arg<Context>(fmt_args)...}`.

### 3545(i). `std::pointer_traits` should be SFINAE-friendly

Section: 20.2.3 [pointer.traits] Status: C++23 Submitter: Glen Joseph Fernandes Opened: 2021-04-20 Last modified: 2023-11-22

Priority: 2

View all other issues in [pointer.traits].

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Discussion:

P1474R1 chose to use `std::to_address` (a mechanism of converting pointer-like types to raw pointers) for contiguous iterators. `std::to_address` provides an optional customization point via an optional member in `std::pointer_traits`. However all iterators are not pointers, and the primary template of `std::pointer_traits<Ptr>` requires that either `Ptr::element_type` is valid or `Ptr` is of the form `template<T, Args...>` or the `pointer_traits` specialization is ill-formed. This requires specializing `pointer_traits` for those contiguous iterator types which is inconvenient for users. P1474 should have also made `pointer_traits` SFINAE friendly.

[2021-05-10; Reflector poll]

Priority set to 2. Send to LEWG. Daniel: "there is no similar treatment for the `rebind` member template and I think it should be clarified whether `pointer_to`'s signature should exist and in which form in the offending case."

[2022-01-29; Daniel comments]

This issue has some overlap with LWG 3665(i) in regard to the question how we should handle the `rebind_alloc` member template of the `allocator_traits` template as specified by 20.2.9.2 [allocator.traits.types]/11. It would seem preferable to decide for the same approach in both cases.

[2022-02-22 LEWG telecon; Status changed: LEWG → Open]

No objection to unanimous consent for Jonathan's suggestion to make `pointer_traits` an empty class when there is no `element_type`. Jonathan to provide a paper.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4885.

> 1. Modify 20.2.3.2 [pointer.traits.types] as indicated:

>    > As additional drive-by fix the improper usage of the term "instantiation" has been corrected.

>    >     using element_type = see below;

>    > > -1- Type: `Ptr::element_type` if the qualified-id `Ptr::element_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `T` if `Ptr` is a class template instantiationspecialization of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the specialization is ill-formed `pointer_traits` has no member `element_type`.

[2022-09-27; Jonathan provides new wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 20.2.3.1 [pointer.traits.general] as indicated:

>    > -1- The class template `pointer_traits` supplies a uniform interface to certain attributes of pointer-like types.

>    > >     namespace std {
>    > >       template<class Ptr> struct pointer_traits {
>    > >         using pointer         = Ptr;
>    > >         using element_type    = see below;
>    > >         using difference_type = see below;

>    > >         template<class U> using rebind = see below;
>    > >         static pointer pointer_to(see below r);

>    > >         see below;
>    > >       };

>    > >       template<class T> struct pointer_traits<T*> {
>    > >         using pointer         = T*;
>    > >         using element_type    = T;
>    > >         using difference_type = ptrdiff_t;

>    > >         template<class U> using rebind = U*;
>    > >         static constexpr pointer pointer_to(see below r) noexcept;
>    > >       };
>    > >     }

> 2. Modify 20.2.3.2 [pointer.traits.types] as indicated:

>    > -?- The definitions in this subclause make use of the following exposition-only class template and concept:

>    > >     template<class T>
>    > >     struct ptr-traits-elem // exposition only
>    > >     { };

>    > >     template<class T> requires requires { typename T::element_type; }
>    > >     struct ptr-traits-elem<T>
>    > >     { using type = typename T::element_type; };

>    > >     template<template<class...> class SomePointer, class T, class... Args>
>    > >     requires (!requires { typename SomePointer<T, Args...>::element_type; })
>    > >     struct ptr-traits-elem<SomePointer<T, Args...>>
>    > >     { using type = T; };

>    > >     template<class Ptr>
>    > >       concept has-elem-type = // exposition only
>    > >         requires { typename ptr-traits-elem<Ptr>::type; }

>    > -?- If `Ptr` satisfies `has-elem-type`, a specialization `pointer_traits<Ptr>` generated from the `pointer_traits` primary template has the members described in 20.2.3.2 [pointer.traits.types] and 20.2.3.3 [pointer.traits.functions]; otherwise, such a specialization has no members by any of the names described in those subclauses or in 20.2.3.4 [pointer.traits.optmem].

>    >     using pointer = Ptr;

>    >     using element_type = see below typename ptr-traits-elem<Ptr>::type;

>    > > -1- Type: `Ptr::element_type` if the qualified-id `Ptr::element_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `T` if `Ptr` is a class template instantiation of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the specialization is ill-formed.

>    >     using difference_type = see below;

>    > > -2- Type: `Ptr::difference_type` if the qualified-id `Ptr::difference_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `ptrdiff_t`.

>    >     template<class U> using rebind = see below;

>    > > -3- Alias template: `Ptr::rebind<U>` if the qualified-id `Ptr::rebind<U>` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `SomePointer<U, Args>` if `Ptr` is a class template instantiation of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the instantiation of `rebind` is ill-formed.

[2022-10-11; Jonathan provides improved wording]

[2022-10-19; Reflector poll]

Set status to "Tentatively Ready" after six votes in favour in reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 20.2.3.1 [pointer.traits.general] as indicated:

   > -1- The class template `pointer_traits` supplies a uniform interface to certain attributes of pointer-like types.

   > >     namespace std {
   > >       template<class Ptr> struct pointer_traits {
   > >         using pointer         = Ptr;
   > >         using element_type    = see below;
   > >         using difference_type = see below;

   > >         template<class U> using rebind = see below;
   > >         static pointer pointer_to(see below r);

   > >         see below;
   > >       };

   > >       template<class T> struct pointer_traits<T*> {
   > >         using pointer         = T*;
   > >         using element_type    = T;
   > >         using difference_type = ptrdiff_t;

   > >         template<class U> using rebind = U*;
   > >         static constexpr pointer pointer_to(see below r) noexcept;
   > >       };
   > >     }

2. Modify 20.2.3.2 [pointer.traits.types] as indicated:

   > -?- The definitions in this subclause make use of the following exposition-only class template and concept:

   > >     template<class T>
   > >     struct ptr-traits-elem // exposition only
   > >     { };

   > >     template<class T> requires requires { typename T::element_type; }
   > >     struct ptr-traits-elem<T>
   > >     { using type = typename T::element_type; };

   > >     template<template<class...> class SomePointer, class T, class... Args>
   > >     requires (!requires { typename SomePointer<T, Args...>::element_type; })
   > >     struct ptr-traits-elem<SomePointer<T, Args...>>
   > >     { using type = T; };

   > >     template<class Ptr>
   > >       concept has-elem-type = // exposition only
   > >         requires { typename ptr-traits-elem<Ptr>::type; }

   > -?- If `Ptr` satisfies `has-elem-type`, a specialization `pointer_traits<Ptr>` generated from the `pointer_traits` primary template has the following members as well as those described in 20.2.3.3 [pointer.traits.functions]; otherwise, such a specialization has no members by any of those names.

   >     using pointer = see below;

   > > -?- Type: `Ptr`.

   >     using element_type = see below;

   > > -1- Type: `typename ptr-traits-elem<Ptr>::type`. `Ptr::element_type` if the qualified-id `Ptr::element_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `T` if `Ptr` is a class template instantiation of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the specialization is ill-formed.

   >     using difference_type = see below;

   > > -2- Type: `Ptr::difference_type` if the qualified-id `Ptr::difference_type` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `ptrdiff_t`.

   >     template<class U> using rebind = see below;

   > > -3- Alias template: `Ptr::rebind<U>` if the qualified-id `Ptr::rebind<U>` is valid and denotes a type (13.10.3 [temp.deduct]); otherwise, `SomePointer<U, Args>` if `Ptr` is a class template instantiation of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the instantiation of `rebind` is ill-formed.

3. Modify 20.2.3.4 [pointer.traits.optmem] as indicated:

   > -1- Specializations of `pointer_traits` may define the member declared in this subclause to customize the behavior of the standard library. A specialization generated from the `pointer_traits` primary template has no member by this name.

   >     static element_type* to_address(pointer p) noexcept;

   > > -1- Returns: A pointer of type `element_type*` that references the same location as the argument `p`.

### 3546(i). `common_iterator`'s `postfix-proxy` is not quite right

Section: 24.5.5.5 [common.iter.nav] Status: C++23 Submitter: Tim Song Opened: 2021-04-23 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [common.iter.nav].

View all issues with C++23 status.

Discussion:

P2259R1 modeled `common_iterator::operator++(int)`'s `postfix-proxy` class on the existing proxy class used by `common_iterator::operator->`, but in doing so it overlooked two differences:

- `operator->`'s proxy is only used when `iter_reference_t<I>` is not a reference type; this is not the case for `postfix-proxy`;

- `operator->` returns a prvalue proxy, while `operator++`'s `postfix-proxy` is returned by (elidable) move, so the latter needs to require `iter_value_t<I>` to be `move_constructible`.

The proposed wording has been implemented and tested.

[2021-05-10; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-05-17; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 24.5.5.5 [common.iter.nav] as indicated:

   >     decltype(auto) operator++(int);

   > > -4- Preconditions: `holds_alternative<I>(v_)` is `true`.

   > > -5- Effects: If `I` models `forward_iterator`, equivalent to:

   > > >     common_iterator tmp = *this;
   > > >     ++*this;
   > > >     return tmp;

   > > Otherwise, if `requires (I& i) { { *i++ } -> can-reference; }` is `true` or `constructible_from<iter_value_t<I>, iter_reference_t<I>> && move_constructible<iter_value_t<I>>` is `false`, equivalent to:

   > > >     return get<I>(v_)++;

   > > Otherwise, equivalent to:

   > > >     postfix-proxy p(**this);
   > > >     ++*this;
   > > >     return p;

   > > where `postfix-proxy` is the exposition-only class:

   > > >     class postfix-proxy {
   > > >       iter_value_t<I> keep_;
   > > >       postfix-proxy(iter_reference_t<I>&& x)
   > > >         : keep_(std::moveforward<iter_reference_t<I>>(x)) {}
   > > >     public:
   > > >       const iter_value_t<I>& operator*() const {
   > > >         return keep_;
   > > >       }
   > > >     };

### 3547(i). Time formatters should not be locale sensitive by default

Section: 30.12 [time.format] Status: Resolved Submitter: Corentin Jabot Opened: 2021-04-27 Last modified: 2021-10-23

Priority: 2

View other active issues in [time.format].

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Discussion:

In 30.12 [time.format] it is specified:

> Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise.

This is not consistent with the format design after the adoption of P1892. In 28.5.2.2 [format.string.std] we say:

> When the `L` option is used, the form used for the conversion is called the locale-specific form. The `L` option is only valid for arithmetic types, and its effect depends upon the type.

This has two issues: First, it is inconsistent.

>     format("{}, 0.0"); // locale independent
>     format("{:L}", 0.0); // use locale
>     format("{:%r}, some_time); // use globale locale
>     format("{:%rL}, some_time); // error

And second it perpetuates the issues P1892 intended to solve. It is likely that this inconsistency resulted from both papers being in flight around the same time.

The `L` option should be used and consistent with floating point. We suggest using the C locale which is the non-locale locale, see also here.

[2021-05-17; Reflector poll]

Priority set to 2. This will be resolved by P2372. Tim noted: "P1892 didn't change format's ignore-locale-by-default design, so that paper being in flight at the same time as P1361 cannot explain why the latter is locale-sensitive-by-default."

[2021-10-23 Resolved by the adoption of P2372R3 at the October 2021 plenary. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 30.12 [time.format] as indicated:

   > >     chrono-format-spec:
   > >              fill-and-alignopt widthopt precisionopt Lopt chrono-specsopt
   > >     […]

   > -1- […]

   > -2- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table [tab:time.format.spec]; the formats specified in ISO 8601:2004 shall be used where so described. Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwisea locale. If the `L` option is used, that locale is the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise. If the `L` option is not used, that locale is the `"C"` locale. If the formatted object does not contain the information the conversion specifier refers to, an exception of type `format_error` is thrown.

### 3548(i). `shared_ptr` construction from `unique_ptr` should move (not copy) the deleter

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++23 Submitter: Thomas Köppe Opened: 2021-05-04 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [util.smartptr.shared.const].

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Discussion:

The construction of a `shared_ptr` from a suitable `unique_ptr` rvalue `r` (which was last modified by LWG 2415(i), but not in ways relevant to this issue) calls for `shared_ptr(r.release(), r.get_deleter())` in the case where the deleter is not a reference.

This specification requires the deleter to be copyable, which seems like an unnecessary restriction. Note that the constructor `unique_ptr(unique_ptr&& u)` only requires `u`'s deleter to be Cpp17MoveConstructible. By analogy, the constructor `shared_ptr(unique_ptr<Y, D>&&)` should also require `D` to be only move-, not copy-constructible.

[2021-05-17; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   >     template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > > -28- Constraints: `Y*` is compatible with `T*` and `unique_ptr<Y, D>::pointer` is convertible to `element_type*`.

   > > -29- Effects: If `r.get() == nullptr`, equivalent to `shared_ptr()`. Otherwise, if `D` is not a reference type, equivalent to `shared_ptr(r.release(), std::move(r.get_deleter()))`. Otherwise, equivalent to `shared_ptr(r.release(), ref(r.get_deleter()))`. If an exception is thrown, the constructor has no effect.

### 3549(i). `view_interface` is overspecified to derive from `view_base`

Section: 25.5.3 [view.interface], 25.4.5 [range.view] Status: C++23 Submitter: Tim Song Opened: 2021-05-09 Last modified: 2023-11-22

Priority: 2

View other active issues in [view.interface].

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Discussion:

`view_interface<D>` is currently specified to publicly derive from `view_base`. This derivation requires unnecessary padding in range adaptors like `reverse_view<V>`: the `view_base` subobject of the `reverse_view` is required to reside at a different address than the `view_base` subobject of `V` (assuming that `V` similarly opted into being a view through derivation from `view_interface` or `view_base`).

This appears to be a case of overspecification: we want public and unambiguous derivation from `view_interface` to make the class a `view`; the exact mechanism of how that opt-in is accomplished should have been left as an implementation detail.

The proposed resolution below has been implemented on top of libstdc++ master and passes its ranges testsuite, with the exception of a test that checks for the size of various range adaptors (and therefore asserts the existence of `view_base`-induced padding).

[2021-05-17; Reflector poll]

Priority set to 2.

[2021-05-26; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 25.4.5 [range.view] as indicated:

   >     template<class T>
   >       inline constexpr bool is-derived-from-view-interface = see below;   // exposition only
   >     template<class T>
   >       inline constexpr bool enable_view = derived_from<T, view_base> || is-derived-from-view-interface<T>;

   > > -?- For a type `T`, `is-derived-from-view-interface<T>` is `true` if and only if `T` has exactly one public base class `view_interface<U>` for some type `U` and `T` has no base classes of type `view_interface<V>` for any other type `V`.

   > > -5- Remarks: Pursuant to 16.4.5.2.1 [namespace.std], users may specialize `enable_view` to `true` for cv-unqualified program-defined types which model `view`, and `false` for types which do not. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have type `const bool`.

2. Modify 25.5.3.1 [view.interface.general] as indicated:

   >     namespace std::ranges {
   >       template<class D>
   >         requires is_class_v<D> && same_as<D, remove_cv_t<D>>
   >       class view_interface : public view_base {

   >       […]

   >       };
   >     }

### 3551(i). `borrowed_{iterator,subrange}_t` are overspecified

Section: 25.5.5 [range.dangling] Status: C++23 Submitter: Tim Song Opened: 2021-05-12 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

As discussed in P1715R0, there are ways to implement something equivalent to `std::conditional_t` that are better for compile times. However, `borrowed_{iterator,subrange}_t` are currently specified to use `conditional_t`, and this appears to be user-observable due to the transparency of alias templates. We should simply specify the desired result and leave the actual definition to the implementation.

[2021-05-20; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >       […]
   >     namespace std::ranges {
   >       […]
   >       // 25.5.5 [range.dangling], dangling iterator handling
   >       struct dangling;

   >       template<range R>
   >         using borrowed_iterator_t = see belowconditional_t<borrowed_range<R>, iterator_t<R>, dangling>;

   >       template<range R>
   >         using borrowed_subrange_t = see below
   >           conditional_t<borrowed_range<R>, subrange<iterator_t<R>>, dangling>;

   >       […]
   >     }

2. Modify 25.5.5 [range.dangling] as indicated:

   > -1- The tag type `dangling` is used together with the template aliases `borrowed_iterator_t` and `borrowed_subrange_t`. When an algorithm that typically returns an iterator into, or a subrange of, a range argument is called with an rvalue range argument that does not model `borrowed_range` (25.4.2 [range.range]), the return value possibly refers to a range whose lifetime has ended. In such cases, the tag type `dangling` is returned instead of an iterator or subrange.

   > >     namespace std::ranges {
   > >       struct dangling {
   > >         […]
   > >       };
   > >     }

   > -2- [Example 1: […] — end example]

   > -?- For a type `R` that models `range`:

   > 1. (?.1) — if `R` models `borrowed_range`, then `borrowed_iterator_t<R>` denotes `iterator_t<R>`, and `borrowed_subrange_t<R>` denotes `subrange<iterator_t<R>>`;

   > 2. (?.2) — otherwise, both `borrowed_iterator_t<R>` and `borrowed_subrange_t<R>` denote `dangling`.

### 3552(i). Parallel specialized memory algorithms should require forward iterators

Section: 20.2.2 [memory.syn] Status: C++23 Submitter: Tim Song Opened: 2021-05-16 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

The parallel versions of `uninitialized_{copy,move}{,_n}` are currently depicted as accepting input iterators for their source range. Similar to the non-uninitialized versions, they should require the source range to be at least forward.

[2021-05-20; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >       […]
   >     namespace std {
   >       […]

   >       template<class InputIterator, class NoThrowForwardIterator>
   >         NoThrowForwardIterator uninitialized_copy(InputIterator first, InputIterator last,
   >                                                   NoThrowForwardIterator result);
   >       template<class ExecutionPolicy, class InputForwardIterator, class NoThrowForwardIterator>
   >         NoThrowForwardIterator uninitialized_copy(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
   >                                                   InputForwardIterator first, InputForwardIterator last,
   >                                                   NoThrowForwardIterator result);
   >       template<class InputIterator, class Size, class NoThrowForwardIterator>
   >         NoThrowForwardIterator uninitialized_copy_n(InputIterator first, Size n,
   >                                                     NoThrowForwardIterator result);
   >       template<class ExecutionPolicy, class InputForwardIterator, class Size, class NoThrowForwardIterator>
   >         NoThrowForwardIterator uninitialized_copy_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
   >                                                     InputForwardIterator first, Size n,
   >                                                     NoThrowForwardIterator result);
   >       […]

   >       template<class InputIterator, class NoThrowForwardIterator>
   >         NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last,
   >                                                   NoThrowForwardIterator result);
   >       template<class ExecutionPolicy, class InputForwardIterator, class NoThrowForwardIterator>
   >         NoThrowForwardIterator uninitialized_move(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
   >                                                   InputForwardIterator first, InputForwardIterator last,
   >                                                   NoThrowForwardIterator result);
   >       template<class InputIterator, class Size, class NoThrowForwardIterator>
   >         pair<InputIterator, NoThrowForwardIterator>
   >           uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);
   >       template<class ExecutionPolicy, class InputForwardIterator, class Size, class NoThrowForwardIterator>
   >         pair<InputForwardIterator, NoThrowForwardIterator>
   >           uninitialized_move_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]
   >                                InputForwardIterator first, Size n, NoThrowForwardIterator result);

   >       […]
   >     }

### 3553(i). Useless constraint in `split_view::outer-iterator::value_type::begin()`

Section: 25.7.16.4 [range.lazy.split.outer.value] Status: C++23 Submitter: Hewill Kang Opened: 2021-05-18 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.lazy.split.outer.value].

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Discussion:

The `copyable` constraint in `split_view::outer-iterator::value_type::begin()` is useless because `outer-iterator` always satisfies `copyable`.

When `V` does not model `forward_range`, the `outer-iterator` only contains a pointer, so it is obviously copyable; When `V` is a `forward_range`, the `iterator_t<Base>` is a `forward_iterator`, so it is copyable, which makes `outer-iterator` also copyable.

Suggested resolution: Remove the no-`const begin()` and useless `copyable` constraint in [range.split.outer.value].

[2021-05-26; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify [range.split.outer.value], class `split_view::outer-iterator::value_type` synopsis, as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, forward_range Pattern>
   > >         requires view<V> && view<Pattern> &&
   > >                  indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
   > >                  (forward_range<V> || tiny-range<Pattern>)
   > >       template<bool Const>
   > >       struct split_view<V, Pattern>::outer-iterator<Const>::value_type
   > >         : view_interface<value_type> {
   > >       private:
   > >         outer-iterator i_ = outer-iterator(); // exposition only
   > >       public:
   > >         value_type() = default;
   > >         constexpr explicit value_type(outer-iterator i);
   > >         constexpr inner-iterator<Const> begin() const requires copyable<outer-iterator>;
   > >         constexpr inner-iterator<Const> begin() requires (!copyable<outer-iterator>);
   > >         constexpr default_sentinel_t end() const;
   > >       };
   > >     }

   > […]

   >     constexpr inner-iterator<Const> begin() const requires copyable<outer-iterator>;

   > > -2- Effects: Equivalent to: `return inner-iterator<Const>{i_};`

   >     constexpr inner-iterator<Const> begin() requires (!copyable<outer-iterator>);

   > > -3- Effects: Equivalent to: `return inner-iterator<Const>{std::move(i_)};`

   > […]

### 3554(i). `chrono::parse` needs `const charT*` and `basic_string_view<charT>` overloads

Section: 30.13 [time.parse] Status: C++23 Submitter: Howard Hinnant Opened: 2021-05-22 Last modified: 2023-11-22

Priority: 2

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Discussion:

The `chrono::parse` functions take `const basic_string<charT, traits, Alloc>&` parameters to specify the format strings for the parse. Due to an oversight on my part in the proposal, overloads taking `const charT*` and `basic_string_view<charT, traits>` were omitted. These are necessary when the supplied arguments is a string literal or `string_view` respectively:

>     in >> parse("%F %T", tp);

These overloads have been implemented in the example implementation.

[2021-05-26; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4885.

> 1. Modify 30.2 [time.syn], header `<chrono>` synopsis, as indicated:

>    >     […]
>    >     namespace chrono {
>    >       // 30.13 [time.parse], parsing
>    >       template<class charT, class Parsable>
>    >         unspecified
>    >           parse(const charT* fmt, Parsable& tp);

>    >       template<class charT, class traits, class Parsable>
>    >         unspecified
>    >           parse(basic_string_view<charT, traits> fmt, Parsable& tp);

>    >       template<class charT, class traits, class Alloc, class Parsable>
>    >         unspecified
>    >           parse(const charT* fmt, Parsable& tp,
>    >                 basic_string<charT, traits, Alloc>& abbrev);

>    >       template<class charT, class traits, class Alloc, class Parsable>
>    >         unspecified
>    >           parse(basic_string_view<charT, traits> fmt, Parsable& tp,
>    >                 basic_string<charT, traits, Alloc>& abbrev);

>    >       template<class charT, class Parsable>
>    >         unspecified
>    >           parse(const charT* fmt, Parsable& tp, minutes& offset);

>    >       template<class charT, class traits, class Parsable>
>    >         unspecified
>    >           parse(basic_string_view<charT, traits> fmt, Parsable& tp, minutes& offset);

>    >       template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(const charT* fmt, Parsable& tp,
>    >               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

>    >       template<class charT, class traits, class Alloc, class Parsable>
>    >         unspecified
>    >           parse(basic_string_view<charT, traits> fmt, Parsable& tp,
>    >                 basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

>    >       template<class charT, class traits, class Alloc, class Parsable>
>    >         unspecified
>    >           parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp);

>    >       template<class charT, class traits, class Alloc, class Parsable>
>    >         unspecified
>    >           parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
>    >                 basic_string<charT, traits, Alloc>& abbrev);

>    >       template<class charT, class traits, class Alloc, class Parsable>
>    >         unspecified
>    >           parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
>    >                 minutes& offset);

>    >       template<class charT, class traits, class Alloc, class Parsable>
>    >         unspecified
>    >           parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
>    >                 basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

>    >       […]
>    >     }
>    >     […]

> 2. Modify 30.13 [time.parse] as indicated:

>    >     template<class charT, class Parsable>
>    >       unspecified
>    >         parse(const charT* fmt, Parsable& tp);

>    > > -?- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT>&>(), fmt, tp)

>    > > is well-formed when treated as an unevaluated operand.

>    > > -?- Effects: Equivalent to `return parse(basic_string<charT>{fmt}, tp);`

>    >     template<class charT, class traits, class Parsable>
>    >       unspecified
>    >         parse(basic_string_view<charT, traits> fmt, Parsable& tp);

>    > > -?- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp)

>    > > is well-formed when treated as an unevaluated operand.

>    > > -?- Effects: Equivalent to `return parse(basic_string<charT, traits>{fmt}, tp);`

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(const charT* fmt, Parsable& tp,
>    >               basic_string<charT, traits, Alloc>& abbrev);

>    > > -?- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt, tp, addressof(abbrev))

>    > > is well-formed when treated as an unevaluated operand.

>    > > -?- Effects: Equivalent to `return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);`

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(basic_string_view<charT, traits> fmt, Parsable& tp,
>    >               basic_string<charT, traits, Alloc>& abbrev);

>    > > -?- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, addressof(abbrev))

>    > > is well-formed when treated as an unevaluated operand.

>    > > -?- Effects: Equivalent to `return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);`

>    >     template<class charT, class Parsable>
>    >       unspecified
>    >         parse(const charT* fmt, Parsable& tp, minutes& offset);

>    > > -?- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT>&>(), fmt, tp,
>    > > >                 declval<basic_string<charT>*>(),
>    > > >                 &offset)

>    > > is well-formed when treated as an unevaluated operand.

>    > > -?- Effects: Equivalent to `return parse(basic_string<charT>{fmt}, tp, offset);`

>    >     template<class charT, class traits, class Parsable>
>    >       unspecified
>    >         parse(basic_string_view<charT, traits> fmt, Parsable& tp, minutes& offset);

>    > > -?- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp,
>    > > >                 declval<basic_string<charT, traits>*>(),
>    > > >                 &offset)

>    > > is well-formed when treated as an unevaluated operand.

>    > > -?- Effects: Equivalent to `return parse(basic_string<charT, traits>{fmt}, tp, offset);`

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >     unspecified
>    >       parse(const charT* fmt, Parsable& tp,
>    >             basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

>    > > -?- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt, tp, addressof(abbrev), &offset)

>    > > is well-formed when treated as an unevaluated operand.

>    > > -?- Effects: Equivalent to `return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev, offset);`

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(basic_string_view<charT, traits> fmt, Parsable& tp,
>    >               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

>    > > -?- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, addressof(abbrev), &offset)

>    > > is well-formed when treated as an unevaluated operand.

>    > > -?- Effects: Equivalent to `return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev, offset);`

>    >     template<class charT, class traits, class Alloc, class Parsable>
>    >       unspecified
>    >         parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp);

>    > > -2- Constraints: The expression

>    > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp)

>    > > is well-formed when treated as an unevaluated operand.

>    > > -3- Returns: A manipulator such that the expression `is >> parse(fmt, tp)` has type `I`, has value `is`, and calls `from_stream(is, fmt.c_str(), tp)`.

[2021-06-02 Tim comments and provides updated wording]

The current specification suggests that `parse` takes a reference to the format string (stream extraction on the resulting manipulator is specified to call `from_stream` on `fmt.c_str()`, not some copy), so we can't call it with a temporary string.

Additionally, since the underlying API (`from_stream`) requires a null-terminated string, the usual practice is to avoid providing a `string_view` overload (see, e.g., LWG 3430(i)). The wording below therefore only provides `const charT*` overloads. It does not use "Equivalent to" to avoid requiring that the `basic_string` overload and the `const charT*` overload return the same type. As the manipulator is intended to be immediately consumed, the wording also adds normative encouragement to make misuse more difficult.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 30.2 [time.syn], header `<chrono>` synopsis, as indicated:

   >     […]
   >     namespace chrono {
   >       // 30.13 [time.parse], parsing
   >       template<class charT, class Parsable>
   >         unspecified
   >           parse(const charT* fmt, Parsable& tp);

   >       template<class charT, class traits, class Alloc, class Parsable>
   >         unspecified
   >           parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp);

   >       template<class charT, class traits, class Alloc, class Parsable>
   >         unspecified
   >           parse(const charT* fmt, Parsable& tp,
   >                 basic_string<charT, traits, Alloc>& abbrev);

   >       template<class charT, class traits, class Alloc, class Parsable>
   >         unspecified
   >           parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
   >                 basic_string<charT, traits, Alloc>& abbrev);

   >       template<class charT, class Parsable>
   >         unspecified
   >           parse(const charT* fmt, Parsable& tp, minutes& offset);

   >       template<class charT, class traits, class Alloc, class Parsable>
   >         unspecified
   >           parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
   >                 minutes& offset);

   >       template<class charT, class traits, class Alloc, class Parsable>
   >         unspecified
   >           parse(const charT* fmt, Parsable& tp,
   >                 basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

   >       template<class charT, class traits, class Alloc, class Parsable>
   >         unspecified
   >           parse(const basic_string<charT, traits, Alloc>& formatfmt, Parsable& tp,
   >                 basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

   >       […]
   >     }
   >     […]

2. Modify 30.13 [time.parse] as indicated:

   -1- Each `parse` overload specified in this subclause calls `from_stream` unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, let `is` denote an object of type `basic_istream<charT, traits>` and let `I` be `basic_istream<charT, traits>&`, where `charT` and `traits` are template parameters in that context.

   -?- Recommended practice: Implementations should make it difficult to accidentally store or use a manipulator that may contain a dangling reference to a format string, for example by making the manipulators produced by `parse` immovable and preventing stream extraction into an lvalue of such a manipulator type.

   >     template<class charT, class Parsable>
   >       unspecified
   >         parse(const charT* fmt, Parsable& tp);
   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);

   > > -?- Let F be `fmt` for the first overload and `fmt.c_str()` for the second overload. Let `traits` be `char_traits<charT>` for the first overload.

   > > -2- Constraints: The expression

   > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str()F, tp)

   > > is well-formed when treated as an unevaluated operand.

   > > -3- Returns: A manipulator such that the expression `is >> parse(fmt, tp)` has type `I`, has value `is`, and calls `from_stream(is, fmt.c_str()F, tp)`.

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const charT* fmt, Parsable& tp,
   >               basic_string<charT, traits, Alloc>& abbrev);
   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   >               basic_string<charT, traits, Alloc>& abbrev);

   > > -?- Let F be `fmt` for the first overload and `fmt.c_str()` for the second overload.

   > > -4- Constraints: The expression

   > > >     from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str()F, tp, addressof(abbrev))

   > > is well-formed when treated as an unevaluated operand.

   > > -5- Returns: A manipulator such that the expression `is >> parse(fmt, tp, abbrev)` has type `I`, has value `is`, and calls `from_stream(is, fmt.c_str()F, tp, addressof(abbrev))`.

   >     template<class charT, class Parsable>
   >       unspecified
   >         parse(const charT* fmt, Parsable& tp, minutes& offset);

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   >               minutes& offset);

   > > -?- Let F be `fmt` for the first overload and `fmt.c_str()` for the second overload. Let `traits` be `char_traits<charT>` and `Alloc` be `allocator<charT>` for the first overload.

   > > -6- Constraints: The expression

   > > >     from_stream(declval<basic_istream<charT, traits>&>(),
   > > >                 fmt.c_str()F, tp,
   > > >                 declval<basic_string<charT, traits, Alloc>*>(),
   > > >                 &offset)

   > > is well-formed when treated as an unevaluated operand.

   > > -7- Returns: A manipulator such that the expression `is >> parse(fmt, tp, offset)` has type `I`, has value `is`, and calls:

   > > >     from_stream(is,
   > > >                 fmt.c_str()F, tp,
   > > >                 static_cast<basic_string<charT, traits, Alloc>*>(nullptr),
   > > >                 &offset)

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const charT* fmt, Parsable& tp,
   >               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

   >     template<class charT, class traits, class Alloc, class Parsable>
   >       unspecified
   >         parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
   >               basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

   > > -?- Let F be `fmt` for the first overload and `fmt.c_str()` for the second overload.

   > > -8- Constraints: The expression

   > > >     from_stream(declval<basic_istream<charT, traits>&>(),
   > > >                 fmt.c_str()F, tp, addressof(abbrev), &offset)

   > > is well-formed when treated as an unevaluated operand.

   > > -9- Returns: A manipulator such that the expression `is >> parse(fmt, tp, abbrev, offset)` has type `I`, has value `is`, and calls `from_stream(is, fmt.c_str()F, tp, addressof(abbrev), &offset)`.

### 3555(i). `{transform,elements}_view::iterator::iterator_concept` should consider const-qualification of the underlying range

Section: 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-05-23 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.transform.iterator].

View all issues with C++23 status.

Discussion:

`transform_view::iterator<true>::iterator_concept` and `elements_view::iterator<true>::iterator_concept` (i.e., the const versions) are currently specified as looking at the properties of `V` (i.e., the underlying view without const), while the actual iterator operations are all correctly specified as using `Base` (which includes the const). `iterator_concept` should do so too.

The proposed resolution has been implemented and tested on top of libstdc++.

[2021-05-26; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > -1- `iterator::iterator_concept` is defined as follows:

   > 1. (1.1) — If `VBase` models `random_access_range`, then `iterator_concept` denotes `random_access_iterator_tag`.

   > 2. (1.2) — Otherwise, if `VBase` models `bidirectional_range`, then `iterator_concept` denotes `bidirectional_iterator_tag`.

   > 3. (1.3) — Otherwise, if `VBase` models `forward_range`, then `iterator_concept` denotes `forward_iterator_tag`.

   > 4. (1.4) — Otherwise, `iterator_concept` denotes `input_iterator_tag`.

2. Modify 25.7.23.3 [range.elements.iterator] as indicated:

   > -1- The member typedef-name `iterator_concept` is defined as follows:

   > 1. (1.1) — If `VBase` models `random_access_range`, then `iterator_concept` denotes `random_access_iterator_tag`.

   > 2. (1.2) — Otherwise, if `VBase` models `bidirectional_range`, then `iterator_concept` denotes `bidirectional_iterator_tag`.

   > 3. (1.3) — Otherwise, if `VBase` models `forward_range`, then `iterator_concept` denotes `forward_iterator_tag`.

   > 4. (1.4) — Otherwise, `iterator_concept` denotes `input_iterator_tag`.

### 3557(i). The `static_cast` expression in `convertible_to` has the wrong operand

Section: 18.4.4 [concept.convertible] Status: C++23 Submitter: Tim Song Opened: 2021-05-26 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [concept.convertible].

View all other issues in [concept.convertible].

View all issues with C++23 status.

Discussion:

The specification of `convertible_to` implicitly requires `static_cast<To>(f())` to be equality-preserving. Under 18.2 [concepts.equality] p1, the operand of this expression is `f`, but what we really want is for `f()` to be treated as the operand. We should just use `declval` (which is treated specially by the definition of "operand" for this purpose) instead of reinventing the wheel.

[2021-06-07; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 18.4.4 [concept.convertible] as indicated:

   >     template<class From, class To>
   >       concept convertible_to =
   >         is_convertible_v<From, To> &&
   >         requires(add_rvalue_reference_t<From> (&f)()) {
   >           static_cast<To>(fdeclval<From>());
   >         };

### 3559(i). Semantic requirements of `sized_range` is circular

Section: 25.4.4 [range.sized] Status: C++23 Submitter: Tim Song Opened: 2021-06-03 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.sized].

View all issues with C++23 status.

Discussion:

25.4.4 [range.sized] p2.1 requires that for a `sized_range t`, `ranges::size(t)` is equal to `ranges::distance(t)`. But for a `sized_range`, `ranges::distance(t)` is simply `ranges::size(t)` cast to the difference type.

Presumably what we meant is that `distance(begin, end)` — the actual distance you get from traversing the range — is equal to what `ranges::size` produces, and not merely that casting from the size type to difference type is value-preserving. Otherwise, `sized_range` would be more or less meaningless.

[2021-06-14; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 25.4.4 [range.sized] as indicated:

   >     template<class T>
   >       concept sized_range =
   >         range<T> &&
   >         requires(T& t) { ranges::size(t); };

   > > -2- Given an lvalue `t` of type `remove_reference_t<T>`, `T` models `sized_range` only if

   > > 1. (2.1) — `ranges::size(t)` is amortized 𝒪(1), does not modify `t`, and is equal to `ranges::distance(ranges::begin(t), ranges::end(t)t)`, and

   > > 2. (2.2) — if `iterator_t<T>` models `forward_iterator`, `ranges::size(t)` is well-defined regardless of the evaluation of `ranges::begin(t)`.

### 3560(i). `ranges::equal` and `ranges::is_permutation` should short-circuit for `sized_ranges`

Section: 26.6.13 [alg.equal], 26.6.14 [alg.is.permutation] Status: C++23 Submitter: Tim Song Opened: 2021-06-03 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [alg.equal].

View all issues with C++23 status.

Discussion:

`ranges::equal` and `ranges::is_permutation` are currently only required to short-circuit on different sizes if the iterator and sentinel of the two ranges pairwise model `sized_sentinel_for`. They should also short-circuit if the ranges are sized.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 26.6.13 [alg.equal] as indicated:

   >     template<class InputIterator1, class InputIterator2>
   >       constexpr bool equal(InputIterator1 first1, InputIterator1 last1,
   >                            InputIterator2 first2);
   >     […]
   >     template<class InputIterator1, class InputIterator2,
   >              class BinaryPredicate>
   >       constexpr bool equal(InputIterator1 first1, InputIterator1 last1,
   >                            InputIterator2 first2, InputIterator2 last2,
   >                            BinaryPredicate pred);
   >     […]
   >     template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
   >              class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
   >       requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
   >       constexpr bool ranges::equal(I1 first1, S1 last1, I2 first2, S2 last2,
   >                                    Pred pred = {},
   >                                    Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<input_range R1, input_range R2, class Pred = ranges::equal_to,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
   >       constexpr bool ranges::equal(R1&& r1, R2&& r2, Pred pred = {},
   >                                    Proj1 proj1 = {}, Proj2 proj2 = {});

   > > […]

   > > -3- Complexity: If the types of `first1`, `last1`, `first2`, and `last2`:

   > > 1. (3.1) — the types of `first1`, `last1`, `first2`, and `last2` meet the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]) and `last1 - first1 != last2 - first2` for the overloads in namespace `std`;

   > > 2. (3.2) — the types of `first1`, `last1`, `first2`, and `last2` pairwise model `sized_sentinel_for` (24.3.4.8 [iterator.concept.sizedsentinel]) and `last1 - first1 != last2 - first2` for the first overloads in namespace `ranges`,

   > > 3. (3.3) — `R1` and `R2` each model `sized_range` and `ranges::distance(r1) != ranges::distance(r2)` for the second overload in namespace `ranges`,

   > > and `last1 - first1 != last2 - first2`, then no applications of the corresponding predicate and each projection; otherwise, […]

2. Modify 26.6.14 [alg.is.permutation] as indicated:

   >     template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2,
   >              sentinel_for<I2> S2, class Proj1 = identity, class Proj2 = identity,
   >              indirect_equivalence_relation<projected<I1, Proj1>,
   >                                            projected<I2, Proj2>> Pred = ranges::equal_to>
   >       constexpr bool ranges::is_permutation(I1 first1, S1 last1, I2 first2, S2 last2,
   >                                             Pred pred = {},
   >                                             Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<forward_range R1, forward_range R2,
   >              class Proj1 = identity, class Proj2 = identity,
   >              indirect_equivalence_relation<projected<iterator_t<R1>, Proj1>,
   >                                            projected<iterator_t<R2>, Proj2>> Pred = ranges::equal_to>
   >       constexpr bool ranges::is_permutation(R1&& r1, R2&& r2, Pred pred = {},
   >                                             Proj1 proj1 = {}, Proj2 proj2 = {});

   > > […]

   > > -7- Complexity: No applications of the corresponding predicate and projections if:

   > > 1. (7.1) — for the first overload,

   > >    1. (7.1.1) — `S1` and `I1` model `sized_sentinel_for<S1, I1>`,

   > >    2. (7.1.2) — `S2` and `I2` model `sized_sentinel_for<S2, I2>`, and

   > >    3. (7.1.3) — `last1 - first1 != last2 - first2`.;

   > > 2. (7.2) — for the second overload, `R1` and `R2` each model `sized_range`, and `ranges::distance(r1) != ranges::distance(r2)`.

   > > Otherwise, exactly `last1 - first1` applications of the corresponding predicate and projections if `ranges::equal(first1, last1, first2, last2, pred, proj1, proj2)` would return `true`; otherwise, at worst 𝒪(`N2`), where `N` has the value `last1 - first1`.

### 3561(i). Issue with internal counter in `discard_block_engine`

Section: 29.5.5.2 [rand.adapt.disc] Status: C++23 Submitter: Ilya Burylov Opened: 2021-06-03 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [rand.adapt.disc].

View all issues with C++23 status.

Discussion:

A `discard_block_engine` engine adaptor is described in 29.5.5.2 [rand.adapt.disc]. It produces random numbers from the underlying engine discarding `(p - r)` last values of `p - size` block, where `r` and `p` are template parameters of `std::size_t` type:

>     template<class Engine, size_t p, size_t r>
>     class discard_block_engine;

The transition algorithm for `discard_block_engine` is described as follows (paragraph 2):

> The transition algorithm discards all but `r > 0` values from each block of `p = r` values delivered by `ℯ`. The state transition is performed as follows: If `n = r`, advance the state of `e` from `ei` to `ei+p-r` and set `n` to `0`. In any case, then increment `n` and advance `e`'s then-current state `ej` to `ej+1`.

Where `n` is of integer type. In the API of discard block engine, `n` is represented in the following way:

>     […]
>     int n; // exposition only

In cases where `int` is equal to `int32_t`, overflow is possible for `n` that leads to undefined behavior. Such situation can happen when the `p` and `r` template parameters exceed `INT_MAX`.

This misleading exposition block leads to differences in implementations:

- GNU Libstdc++ uses `size_t` for `n` — in this case no overflow happened even if template parameters exceed `INT_MAX`

- LLVM Libcxx uses `int` for `n` together with a `static_assert` that is checking that `p` and `r` values are `<= INT_MAX`

Such difference in implementation makes code not portable and may potentially breaks random number sequence consistency between different implementors of C++ std lib.

The problematic use case is the following one:

>     #include <iostream>
>     #include <random>
>     #include <limits>

>     int main() {
>       std::minstd_rand0 generator;

>       constexpr std::size_t skipped_size = static_cast<std::size_t>(std::numeric_limits<int>::max());
>       constexpr std::size_t block_size = skipped_size + 5;
>       constexpr std::size_t used_block = skipped_size;

>       std::cout << "block_size = " << block_size << std::endl;
>       std::cout << "used_block = " << used_block << "\n" << std::endl;

>       std::discard_block_engine<std::minstd_rand0, block_size, used_block> discard_generator;

>       // Call discard procedures
>       discard_generator.discard(used_block);
>       generator.discard(block_size);

>       // Call generation. Results should be equal
>       for (std::int32_t i = 0; i < 10; ++i)
>       {
>         std::cout << discard_generator() << " should be equal " << generator() << std::endl;
>       }
>     }

We see no solid reason for `n` to be an `int`, given that the relevant template parameters are `std::size_t`. It seems like a perfectly fine use case to generate random numbers in amounts larger than `INT_MAX`.

The proposal is to change exposition text to `std::size_t`:

>     size_t n; // exposition only

It will not mandate the existing libraries to break ABI, but at least guide for better implementation.

[2021-06-14; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 29.5.5.2 [rand.adapt.disc], class template `discard_block_engine` synopsis, as indicated:

   >       […]
   >       // generating functions
   >       result_type operator()();
   >       void discard(unsigned long long z);

   >       // property functions
   >       const Engine& base() const noexcept { return e; };

   >     private:
   >       Engine e; // exposition only
   >       intsize_t n; // exposition only
   >     };

### 3563(i). `keys_view` example is broken

Section: 25.7.23.1 [range.elements.overview] Status: C++23 Submitter: Barry Revzin Opened: 2021-05-29 Last modified: 2023-11-22

Priority: 3

View all issues with C++23 status.

Discussion:

In 25.7.23.1 [range.elements.overview] we have:

> `keys_view` is an alias for `elements_view<views::all_t<R>, 0>`, and is useful for extracting keys from associative containers.

> [Example 2:

> >     auto names = keys_view{historical_figures};
> >     for (auto&& name : names) {
> >       cout << name << ' ';  // prints Babbage Hamilton Lovelace Turing
> >     }

> — end example]

Where `keys_view` is defined as:

>     template<class R>
>       using keys_view = elements_view<views::all_t<R>, 0>;

There's a similar example for `values_view`.

But class template argument deduction cannot work here, `keys_view(r)` cannot deduce `R` — it's a non-deduced context. At the very least, the example is wrong and should be rewritten as `views::keys(historical_figures)`. Or, I guess, as `keys_view<decltype(historical_figures)>(historical_figures)>` (but really, not).

[2021-05-29 Tim comments and provides wording]

I have some ideas about making CTAD work for `keys_view` and `values_view`, but current implementations of alias template CTAD are not yet in a shape to permit those ideas to be tested. What is clear, though, is that the current definitions of `keys_view` and `values_view` can't possibly ever work for CTAD, because `R` is only used in a non-deduced context and so they can never meet the "deducible" constraint in 12.2.2.9 [over.match.class.deduct] p2.3.

The proposed resolution below removes the `views::all_t` transformation in those alias templates. This makes these aliases transparent enough to support CTAD out of the box when a view is used, and any questions about deduction guides on `elements_view` can be addressed later once the implementations become more mature. This also makes them consistent with all the other views: you can't write `elements_view<map<int, int>&, 0>` or `reverse_view<map<int, int>&>`, so why do we allow `keys_view<map<int, int>&>`? It is, however, a breaking change, so it is important that we get this in sooner rather than later.

The proposed resolution has been implemented and tested.

[2021-06-14; Reflector poll]

Set priority to 3 after reflector poll in August. Partially addressed by an editorial change.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4885.

> 1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

>    >     […]
>    >     namespace std::ranges {
>    >     […]

>    >       // 25.7.23 [range.elements], elements view
>    >       template<input_range V, size_t N>
>    >         requires see below
>    >       class elements_view;

>    >       template<class T, size_t N>
>    >         inline constexpr bool enable_borrowed_range<elements_view<T, N>> = enable_borrowed_range<T>;

>    >       template<class R>
>    >         using keys_view = elements_view<views::all_t<R>, 0>;
>    >       template<class R>
>    >         using values_view = elements_view<views::all_t<R>, 1>;

>    >     […]
>    >     }

> 2. Modify 25.7.23.1 [range.elements.overview] as indicated:

>    > -3- `keys_view` is an alias for `elements_view<views::all_t<R>, 0>`, and is useful for extracting keys from associative containers.

>    > [Example 2:

>    > >     auto names = keys_view{views::all(historical_figures)};
>    > >     for (auto&& name : names) {
>    > >       cout << name << ' '; // prints Babbage Hamilton Lovelace Turing
>    > >     }

>    > — end example]

>    > -4- `values_view` is an alias for `elements_view<views::all_t<R>, 1>`, and is useful for extracting values from associative containers.

>    > [Example 3:

>    > >     auto is_even = [](const auto x) { return x % 2 == 0; };
>    > >     cout << ranges::count_if(values_view{views::all(historical_figures)}, is_even); // prints 2

>    > — end example]

[2021-06-18 Tim syncs wording to latest working draft]

The examples have been editorially corrected, so the new wording simply changes the definition of `keys_view` and `values_view`.

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     […]
   >     namespace std::ranges {
   >     […]

   >       // 25.7.23 [range.elements], elements view
   >       template<input_range V, size_t N>
   >         requires see below
   >       class elements_view;

   >       template<class T, size_t N>
   >         inline constexpr bool enable_borrowed_range<elements_view<T, N>> = enable_borrowed_range<T>;

   >       template<class R>
   >         using keys_view = elements_view<views::all_t<R>, 0>;
   >       template<class R>
   >         using values_view = elements_view<views::all_t<R>, 1>;

   >     […]
   >     }

2. Modify 25.7.23.1 [range.elements.overview] as indicated:

   > -3- `keys_view` is an alias for `elements_view<views::all_t<R>, 0>`, and is useful for extracting keys from associative containers.

   > [Example 2: … — end example]

   > -4- `values_view` is an alias for `elements_view<views::all_t<R>, 1>`, and is useful for extracting values from associative containers.

   > [Example 3: … — end example]

### 3564(i). `transform_view::iterator<true>::value_type` and `iterator_category` should use `const F&`

Section: 25.7.9.3 [range.transform.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-06-06 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.transform.iterator].

View all issues with C++23 status.

Discussion:

Iterators obtained from a `const transform_view` invoke the transformation function as `const`, but the `value_type` and `iterator_category` determination uses plain `F&`, i.e., non-`const`.

[2021-06-14; Reflector poll]

Set priority to 2 after reflector poll, send to SG9 for design clarification. Should `r` and `as_const(r)` guarantee same elements?

[2022-07-08; Reflector poll]

SG9 has decided to proceed with this PR in its 2021-09-13 telecon and not block the issue for a paper on the more general const/non-const problem.

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, copy_constructible F>
   > >         requires view<V> && is_object_v<F> &&
   > >                  regular_invocable<F&, range_reference_t<V>> &&
   > >                  can-reference<invoke_result_t<F&, range_reference_t<V>>>
   > >       template<bool Const>
   > >       class transform_view<V, F>::iterator {
   > >       private:
   > >         […]
   > >       public:
   > >         using iterator_concept = see below;
   > >         using iterator_category = see below; // not always present
   > >         using value_type =
   > >           remove_cvref_t<invoke_result_t<maybe-const<Const, F>&,
   > >           range_reference_t<Base>>>;
   > >         using difference_type = range_difference_t<Base>;
   > >         […]
   > >       };
   > >     }

   > -1- […]

   > -2- The member typedef-name `iterator_category` is defined if and only if `Base` models `forward_range`. In that case, `iterator::iterator_category` is defined as follows: Let `C` denote the type `iterator_traits<iterator_t<Base>>::iterator_category`.

   > 1. (2.1) — If `is_lvalue_reference_v<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>>` is `true`, then

   >    1. (2.1.1) — […]

   >    2. (2.1.2) — […]

   > 2. (2.2) — Otherwise, `iterator_category` denotes `input_iterator_tag`.

### 3565(i). Handling of encodings in localized formatting of `chrono` types is underspecified

Section: 30.12 [time.format] Status: Resolved Submitter: Victor Zverovich Opened: 2021-05-31 Last modified: 2023-03-23

Priority: 2

View other active issues in [time.format].

View all other issues in [time.format].

View all issues with Resolved status.

Discussion:

When formatting chrono types using a locale the result is underspecified, possibly a mix of the literal and locale encodings. For example:

>     std::locale::global(std::locale("Russian.1251"));
>     auto s = std::format("День недели: {}", std::chrono::Monday);

(Note that "`{}`" should be replaced with "`{:L}`" if P2372 is adopted but that's non-essential.)

If the literal encoding is UTF-8 and the `"Russian.1251"` locale exists we have a mismatch between encodings. As far as I can see the standard doesn't specify what happens in this case.

One possible and undesirable result is

>     "День недели: \xcf\xed"

where `"\xcf\xed"` is `"Пн"` (Mon in Russian) in CP1251 and is not valid UTF-8.

Another possible and desirable result is

>     "День недели: Пн"

where everything is in one encoding (UTF-8).

This issue is not resolved by LWG 3547(i) / P2372 but the resolution proposed here is compatible with P2372 and can be rebased onto its wording if the paper is adopted.

[2021-06-14; Reflector poll]

Set priority to 2 after reflector poll. Send to SG16.

Previous resolution [SUPERSEDED]

This wording is relative to N4885.

1. Modify 30.12 [time.format] as indicated:

   > -2- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table [tab:time.format.spec]; the formats specified in ISO 8601:2004 shall be used where so described. Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise. If the string literal encoding is UTF-8 the replacement of a conversion specifier that depends on the locale is transcoded to UTF-8 for narrow strings, otherwise the replacement is taken as is. If the formatted object does not contain the information the conversion specifier refers to, an exception of type `format_error` is thrown.

[2023-03-22 Resolved by the adoption of P2419R2 in the July 2022 virtual plenary. Status changed: SG16 → Resolved.]

Proposed resolution:

### 3566(i). Constraint recursion for `operator<=>(optional<T>, U)`

Section: 22.5.9 [optional.comp.with.t] Status: C++23 Submitter: Casey Carter Opened: 2021-06-07 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [optional.comp.with.t].

View all issues with C++23 status.

Discussion:

The three-way-comparison operator for optionals and non-optionals is defined in [optional.comp.with.t] paragraph 25:

>     template<class T, three_way_comparable_with<T> U>
>       constexpr compare_three_way_result_t<T, U>
>         operator<=>(const optional<T>& x, const U& v);

> > -25- Effects: Equivalent to: `return bool(x) ? *x <=> v : strong_ordering::less;`

Checking `three_way_comparable_with` in particular requires checking that `x < v` is well-formed for an lvalue `const optional<T>` and lvalue `const U`. `x < v` can be rewritten as `(v <=> x) > 0`. If `U` is a specialization of `optional`, this overload could be used for `v <=> x`, but first we must check the constraints…

The straightforward fix for this recursion seems to be to refuse to check `three_way_comparable_with<T>` when `U` is a specialization of `optional`. MSVC has been shipping such a fix; our initial tests for this new operator triggered the recursion.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4885.

> 1. Modify 22.5.2 [optional.syn], header `<optional>` synopsis, as indicated:

>    >     […]
>    >     // 22.5.9 [optional.comp.with.t], comparison with T
>    >     […]
>    >     template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
>    >     template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
>    >     template<class T, three_way_comparable_with<T>class U>
>    >       requires see below
>    >         constexpr compare_three_way_result_t<T, U>
>    >           operator<=>(const optional<T>&, const U&);

>    >     […]

> 2. Modify 22.5.9 [optional.comp.with.t] as indicated:

>    >     template<class T, three_way_comparable_with<T>class U>
>    >       requires (!is-specialization-of<U, optional>) && three_way_comparable_with<T, U>
>    >         constexpr compare_three_way_result_t<T, U>
>    >           operator<=>(const optional<T>&, const U&);

>    > > -?- The exposition-only trait template `is-specialization-of<A, B>` is a constant expression with value `true` when `A` denotes a specialization of the class template `B`, and `false` otherwise.

>    > > -25- Effects: Equivalent to: `return bool(x) ? *x <=> v : strong_ordering::less;`

[2021-06-14; Improved proposed wording based on reflector discussion]

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 22.5.2 [optional.syn], header `<optional>` synopsis, as indicated:

   >     […]
   >     // 22.5.3 [optional.optional], class template optional
   >     template<class T>
   >     class optional;

   >     template<class T>
   >       constexpr bool is-optional = false;               // exposition only
   >     template<class T>
   >       constexpr bool is-optional<optional<T>> = true;   // exposition only

   >     […]
   >     // 22.5.9 [optional.comp.with.t], comparison with T
   >     […]
   >     template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
   >     template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
   >     template<class T, three_way_comparable_with<T>class U>
   >       requires (!is-optional<U>) && three_way_comparable_with<T, U>
   >         constexpr compare_three_way_result_t<T, U>
   >           operator<=>(const optional<T>&, const U&);

   >     […]

2. Modify 22.5.9 [optional.comp.with.t] as indicated:

   >     template<class T, three_way_comparable_with<T>class U>
   >       requires (!is-optional<U>) && three_way_comparable_with<T, U>
   >         constexpr compare_three_way_result_t<T, U>
   >           operator<=>(const optional<T>& x, const U& v);

   > > -25- Effects: Equivalent to: `return bool(x) ? *x <=> v : strong_ordering::less;`

### 3567(i). Formatting move-only iterators take two

Section: 28.5.6.7 [format.context] Status: C++23 Submitter: Casey Carter Opened: 2021-06-08 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [format.context].

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Discussion:

LWG 3539(i) fixed copies of potentially-move-only iterators in the `format_to` and `vformat_to` overloads, but missed the fact that member functions of `basic_format_context` are specified to copy iterators as well. In particular, 28.5.6.7 [format.context] states:

>     iterator out();

> > -7- Returns: `out_`.

>     void advance_to(iterator it);

> > -8- Effects: Equivalent to: `out_ = it;`

both of which appear to require copyability.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 28.5.6.7 [format.context] as indicated:

   >     iterator out();

   > > -7- Returns: `out_`.Effects: Equivalent to: `return std::move(out_);`

   >     void advance_to(iterator it);

   > > -8- Effects: Equivalent to: `out_ = std::move(it);`

### 3568(i). `basic_istream_view` needs to initialize `value_`

Section: 25.6.6.2 [range.istream.view] Status: C++23 Submitter: Casey Carter Opened: 2021-06-15 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

P2325R3 removes the default member initializers for `basic_istream_view`'s exposition-only `stream_` and `value_` members. Consequently, copying a `basic_istream_view` before the first call to `begin` may produce undefined behavior since doing so necessarily copies the uninitialized `value_`. We should restore `value_`'s default member initializer and remove this particular footgun.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

Wording relative to the post-202106 working draft (as near as possible). This PR is currently being implemented in MSVC.

1. Modify 25.6.6.2 [range.istream.view] as indicated:

   >     namespace std::ranges {
   >       […]
   >       template<movable Val, class CharT, class Traits>
   >         requires default_initializable<Val> &&
   >                  stream-extractable<Val, CharT, Traits>
   >       class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> {
   >         […]
   >         Val value_ = Val(); // exposition only
   >       };
   >     }

### 3569(i). `join_view` fails to support ranges of ranges with non-`default_initializable` iterators

Section: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Casey Carter Opened: 2021-06-16 Last modified: 2023-11-22

Priority: 3

View all other issues in [range.join.iterator].

View all issues with C++23 status.

Discussion:

`join_view::iterator` has exposition-only members `outer_` — which holds an iterator into the adapted range — and `inner_` — which holds an iterator into the range denoted by `outer_`. After application of P2325R3 "Views should not be required to be default constructible" to the working draft, single-pass iterators can be `non-default_initializable`. P2325R3 constrains `join_view::iterator`'s default constructor to require that the types of both `outer_` and `inner_` are `default_initializable`, indicating an intent to support such iterator types. However, the effect of the non-default constructor specified in 25.7.14.3 [range.join.iterator] paragraph 6 is to default-initialize `inner_`, which is ill-formed if its type is not `default_initializable`.

[2021-06-23; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]:

> Wording relative to the post 2021-06 virtual plenary working draft. This PR is currently being implemented in MSVC.

> 1. Modify 25.7.14.3 [range.join.iterator] as indicated:

>    > >     namespace std::ranges {
>    > >       template<input_range V>
>    > >         requires view<V> && input_range<range_reference_t<V>> &&
>    > >                  (is_reference_v<range_reference_t<V>> ||
>    > >                   view<range_value_t<V>>)
>    > >       template<bool Const>
>    > >       struct join_view<V>::iterator {
>    > >         […]
>    > >         optional<InnerIter> inner_ = InnerIter();
>    > >         […]
>    > >         constexpr decltype(auto) operator*() const { return **inner_; }
>    > >         […]
>    > >         friend constexpr decltype(auto) iter_move(const iterator& i)
>    > >         noexcept(noexcept(ranges::iter_move(*i.inner_))) {
>    > >           return ranges::iter_move(*i.inner_);
>    > >         }

>    > >         friend constexpr void iter_swap(const iterator& x, const iterator& y)
>    > >           noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_)))
>    > >           requires indirectly_swappable<InnerIter>;
>    > >       };
>    > >     }

>    > […]

>    >     constexpr void satisfy();       // exposition only

>    > > -5- Effects: Equivalent to:

>    > > >     auto update_inner = [this](const iterator_t<Base>& x) -> auto&& {
>    > > >     […]
>    > > >     };
>    > > >     for (; outer_ != ranges::end(parent_->base_); ++outer_) {
>    > > >       auto&& inner = update_inner(*outer_);
>    > > >       inner_ = ranges::begin(inner);
>    > > >       if (*inner_ != ranges::end(inner))
>    > > >         return;
>    > > >     }
>    > > >     if constexpr (ref-is-glvalue)
>    > > >       inner_.reset() = InnerIter();

>    > […]

>    >     constexpr InnerIter operator->() const
>    >       requires has-arrow<InnerIter> && copyable<InnerIter>;

>    > > -8- Effects: Equivalent to: `return *inner_;`

>    >     constexpr iterator& operator++();

>    > > -9- Let inner-range be:

>    > > […]

>    > > -10- Effects: Equivalent to:

>    > > >     auto&& inner_rng = inner-range;
>    > > >     if (++*inner_ == ranges::end(inner_rng)) {
>    > > >       ++outer_;
>    > > >       satisfy();
>    > > >     }
>    > > >     return *this;

>    > […]

>    >     constexpr iterator& operator--()
>    >       requires ref-is-glvalue && bidirectional_range<Base> &&
>    >                bidirectional_range<range_reference_t<Base>> &&
>    >                common_range<range_reference_t<Base>>;

>    > > -13- Effects: Equivalent to:

>    > > >     if (outer_ == ranges::end(parent_->base_))
>    > > >       inner_ = ranges::end(*--outer_);
>    > > >     while (*inner_ == ranges::begin(*outer_))
>    > > >       *inner_ = ranges::end(*--outer_);
>    > > >     --*inner_;
>    > > >     return *this;

>    > […]

>    >     friend constexpr void iter_swap(const iterator& x, const iterator& y)
>    >       noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_)))
>    >       requires indirectly_swappable<InnerIter>;

>    > > -16- Effects: Equivalent to: return `ranges::iter_swap(*x.inner_, *y.inner_);`

[2021-08-23; Louis Dionne comments and provides improved wording]

I believe the currently proposed resolution is missing the removal of the `default_initializable<InnerIter>` constraint on `join_view::iterator`'s default constructor in 25.7.14.3 [range.join.iterator]. Indeed, after the currently-proposed resolution, `join_view::iterator` reads like:

>     template<input_range V>
>       requires […]
>     struct join_view<V>::iterator {
>     private:
>       optional<InnerIter> inner_; // exposition only
>       […]
>     public:
>       iterator() requires default_initializable<OuterIter> &&
>                           default_initializable<InnerIter> = default;
>         […]
>     };

I believe we should drop the `default_initializable<InnerIter>` constraint from the default constructor (that seems like an oversight unless I missed something):

>     template<input_range V>
>       requires […]
>     struct join_view<V>::iterator {
>     private:
>       optional<InnerIter> inner_; // exposition only
>       […]
>     public:
>       iterator() requires default_initializable<OuterIter> = default;
>       […]
>     };

[Kona 2022-11-08; Accepted at joint LWG/SG9 session. Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.7.14.3 [range.join.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V>
   > >         requires view<V> && input_range<range_reference_t<V>> &&
   > >                  (is_reference_v<range_reference_t<V>> ||
   > >                   view<range_value_t<V>>)
   > >       template<bool Const>
   > >       struct join_view<V>::iterator {
   > >         […]
   > >         optional<InnerIter> inner_ = InnerIter();
   > >         […]
   > >         iterator() requires default_initializable<OuterIter> &&
   > >                             default_initializable<InnerIter> = default;
   > >         […]
   > >         constexpr decltype(auto) operator*() const { return **inner_; }
   > >         […]
   > >         friend constexpr decltype(auto) iter_move(const iterator& i)
   > >         noexcept(noexcept(ranges::iter_move(*i.inner_))) {
   > >           return ranges::iter_move(*i.inner_);
   > >         }

   > >         friend constexpr void iter_swap(const iterator& x, const iterator& y)
   > >           noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_)))
   > >           requires indirectly_swappable<InnerIter>;
   > >       };
   > >     }

   > […]

   >     constexpr void satisfy();       // exposition only

   > > -5- Effects: Equivalent to:

   > > >     auto update_inner = [this](const iterator_t<Base>& x) -> auto&& {
   > > >     […]
   > > >     };
   > > >     for (; outer_ != ranges::end(parent_->base_); ++outer_) {
   > > >       auto&& inner = update_inner(*outer_);
   > > >       inner_ = ranges::begin(inner);
   > > >       if (*inner_ != ranges::end(inner))
   > > >         return;
   > > >     }
   > > >     if constexpr (ref-is-glvalue)
   > > >       inner_.reset() = InnerIter();

   > […]

   >     constexpr InnerIter operator->() const
   >       requires has-arrow<InnerIter> && copyable<InnerIter>;

   > > -8- Effects: Equivalent to: `return *inner_;`

   >     constexpr iterator& operator++();

   > > -9- Let inner-range be:

   > > […]

   > > -10- Effects: Equivalent to:

   > > >     auto&& inner_rng = inner-range;
   > > >     if (++*inner_ == ranges::end(inner_rng)) {
   > > >       ++outer_;
   > > >       satisfy();
   > > >     }
   > > >     return *this;

   > […]

   >     constexpr iterator& operator--()
   >       requires ref-is-glvalue && bidirectional_range<Base> &&
   >                bidirectional_range<range_reference_t<Base>> &&
   >                common_range<range_reference_t<Base>>;

   > > -13- Effects: Equivalent to:

   > > >     if (outer_ == ranges::end(parent_->base_))
   > > >       inner_ = ranges::end(*--outer_);
   > > >     while (*inner_ == ranges::begin(*outer_))
   > > >       *inner_ = ranges::end(*--outer_);
   > > >     --*inner_;
   > > >     return *this;

   > […]

   >     friend constexpr void iter_swap(const iterator& x, const iterator& y)
   >       noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_)))
   >       requires indirectly_swappable<InnerIter>;

   > > -16- Effects: Equivalent to: return `ranges::iter_swap(*x.inner_, *y.inner_);`

### 3570(i). `basic_osyncstream::emit` should be an unformatted output function

Section: 31.11.3.3 [syncstream.osyncstream.members] Status: C++23 Submitter: Tim Song Opened: 2021-06-19 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

`basic_osyncstream::emit` seems rather similar to `basic_ostream::flush` — both are "flushing" operations that forward to member functions of the `streambuf` and set `badbit` if those functions fail. But the former isn't currently specified as an unformatted output function, so it a) doesn't construct or check a `sentry` and b) doesn't set `badbit` if `emit` exits via an exception. At least the latter appears to be clearly desirable — a `streambuf` operation that throws ordinarily results in `badbit` being set.

The reference implementation in P0053R7 constructs a `sentry` and only calls `emit` on the `streambuf` if the `sentry` converts to `true`, so this seems to be an accidental omission in the wording.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 31.11.3.3 [syncstream.osyncstream.members] as indicated:

   >     void emit();

   > > -1- Effects: Behaves as an unformatted output function (31.7.6.4 [ostream.unformatted]). After constructing a `sentry` object, cCalls `sb.emit()`. If that call returns `false`, calls `setstate(ios_base::badbit)`.

### 3571(i). `flush_emit` should set `badbit` if the `emit` call fails

Section: 31.7.6.5 [ostream.manip] Status: C++23 Submitter: Tim Song Opened: 2021-06-19 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [ostream.manip].

View all issues with C++23 status.

Discussion:

`basic_osyncstream::emit` is specified to set `badbit` if it fails (31.11.3.3 [syncstream.osyncstream.members] p1), but the `emit` part of the `flush_emit` manipulator isn't, even though the `flush` part does set `badbit` if it fails.

More generally, given an `osyncstream s`, `s << flush_emit;` should probably have the same behavior as `s.flush(); s.emit()`.

The reference implementation linked in P0753R2 does set `badbit` on failure, so at least this part appears to be an oversight. As discussed in LWG 3570(i), `basic_osyncstream::emit` should probably be an unformatted output function, so the `emit` part of `flush_emit` should do so too.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 31.7.6.5 [ostream.manip] as indicated:

   >     template<class charT, class traits>
   >       basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os);

   > > -12- Effects: Calls `os.flush()`. Then, if `os.rdbuf()` is a `basic_syncbuf<charT, traits, Allocator>*`, called `buf` for the purpose of exposition, behaves as an unformatted output function (31.7.6.4 [ostream.unformatted]) of `os`. After constructing a `sentry` object, calls `buf->emit()`. If that call returns `false`, calls `os.setstate(ios_base::badbit)`.

### 3572(i). `copyable-box` should be fully `constexpr`

Section: 25.7.3 [range.move.wrap] Status: C++23 Submitter: Tim Song Opened: 2021-06-19 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.move.wrap].

View all issues with C++23 status.

Discussion:

P2231R1 made `optional` fully `constexpr`, but missed its cousin `semiregular-box` (which was renamed to `copyable-box` by P2325R3). Most operations of `copyable-box` are already `constexpr` simply because `copyable-box` is specified in terms of `optional`; the only missing ones are the assignment operators layered on top when the wrapped type isn't assignable itself.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify [range.copy.wrap] as indicated:

   > -1- Many types in this subclause are specified in terms of an exposition-only class template `copyable-box`. `copyable-box<T>` behaves exactly like `optional<T>` with the following differences:

   > 1. (1.1) — […]

   > 2. (1.2) — […]

   > 3. (1.3) — If `copyable<T>` is not modeled, the copy assignment operator is equivalent to:

   >    >     constexpr copyable-box& operator=(const copyable-box& that)
   >    >       noexcept(is_nothrow_copy_constructible_v<T>) {
   >    >       if (this != addressof(that)) {
   >    >         if (that) emplace(*that);
   >    >         else reset();
   >    >       }
   >    >       return *this;
   >    >     }

   > 4. (1.4) — If `movable<T>` is not modeled, the move assignment operator is equivalent to:

   >    >     constexpr copyable-box& operator=(copyable-box&& that)
   >    >       noexcept(is_nothrow_move_constructible_v<T>) {
   >    >       if (this != addressof(that)) {
   >    >         if (that) emplace(std::move(*that));
   >    >         else reset();
   >    >       }
   >    >       return *this;
   >    >     }

### 3573(i). Missing Throws element for `basic_string_view(It begin, End end)`

Section: 27.3.3.2 [string.view.cons] Status: C++23 Submitter: Hewill Kang Opened: 2021-07-13 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [string.view.cons].

View all other issues in [string.view.cons].

View all issues with C++23 status.

Discussion:

The standard does not specify the exceptions of this constructor, but since `std::to_address` is a `noexcept` function, this constructor throws if and only when `end - begin` throws, we should add a Throws element for it.

[2021-08-20; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 27.3.3.2 [string.view.cons] as indicated:

   >     template<class It, class End>
   >       constexpr basic_string_view(It begin, End end);

   > > -7- Constraints:

   > > […]

   > > -8- Preconditions:

   > > […]

   > > -9- Effects: Initializes `data_` with `to_address(begin)` and initializes `size_` with `end - begin`.

   > > -?- Throws: When and what `end - begin` throws.

### 3574(i). `common_iterator` should be completely `constexpr`-able

Section: 24.5.5.1 [common.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2021-07-21 Last modified: 2023-11-22

Priority: 3

View other active issues in [common.iterator].

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Discussion:

After P2231R1, `variant` becomes completely `constexpr`-able, which means that `common_iterator` can also be completely `constexpr`-able.

[2021-08-20; Reflector poll]

Set priority to 3 after reflector poll.

[2021-08-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 24.5.5.1 [common.iterator], class template `common_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<input_or_output_iterator I, sentinel_for<I> S>
   >         requires (!same_as<I, S> && copyable<I>)
   >       class common_iterator {
   >       public:
   >         constexpr common_iterator() requires default_initializable<I> = default;
   >         constexpr common_iterator(I i);
   >         constexpr common_iterator(S s);
   >         template<class I2, class S2>
   >           requires convertible_to<const I2&, I> && convertible_to<const S2&, S>
   >             constexpr common_iterator(const common_iterator<I2, S2>& x);

   >         template<class I2, class S2>
   >           requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
   >                    assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
   >           constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);

   >         constexpr decltype(auto) operator*();
   >         constexpr decltype(auto) operator*() const
   >           requires dereferenceable<const I>;
   >         constexpr decltype(auto) operator->() const
   >           requires see below;

   >         constexpr common_iterator& operator++();
   >         constexpr decltype(auto) operator++(int);

   >         template<class I2, sentinel_for<I> S2>
   >           requires sentinel_for<S, I2>
   >         friend constexpr bool operator==(
   >           const common_iterator& x, const common_iterator<I2, S2>& y);
   >         template<class I2, sentinel_for<I> S2>
   >           requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
   >         friend constexpr bool operator==(
   >           const common_iterator& x, const common_iterator<I2, S2>& y);

   >         template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
   >           requires sized_sentinel_for<S, I2>
   >         friend constexpr iter_difference_t<I2> operator-(
   >           const common_iterator& x, const common_iterator<I2, S2>& y);

   >         friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i)
   >           noexcept(noexcept(ranges::iter_move(declval<const I&>())))
   >             requires input_iterator<I>;
   >         template<indirectly_swappable<I> I2, class S2>
   >           friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
   >             noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));

   >       private:
   >         variant<I, S> v_; // exposition only
   >       };
   >       […]
   >     }

2. Modify 24.5.5.3 [common.iter.const] as indicated:

   >     template<class I2, class S2>
   >       requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
   >                assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
   >         constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);

   > > -5- Preconditions: `x.v_.valueless_by_exception()` is `false`.

   > > […]

3. Modify 24.5.5.4 [common.iter.access] as indicated:

   >     constexpr decltype(auto) operator*();
   >     constexpr decltype(auto) operator*() const
   >       requires dereferenceable<const I>;

   > > -1- Preconditions: `holds_alternative<I>(v_)` is `true`.

   > > […]

   >     constexpr decltype(auto) operator->() const
   >       requires see below;

   > > -3- The expression in the requires-clause is equivalent to:

   > > […]

4. Modify 24.5.5.5 [common.iter.nav] as indicated:

   >     constexpr common_iterator& operator++();

   > > -1- Preconditions: `holds_alternative<I>(v_)` is `true`.

   > > […]

   >     constexpr decltype(auto) operator++(int);

   > > -4- Preconditions: `holds_alternative<I>(v_)` is `true`.

   > > […]

5. Modify 24.5.5.6 [common.iter.cmp] as indicated:

   >     template<class I2, sentinel_for<I> S2>
   >       requires sentinel_for<S, I2>
   >     friend constexpr bool operator==(
   >       const common_iterator& x, const common_iterator<I2, S2>& y);

   > > -1- Preconditions: […]

   >     template<class I2, sentinel_for<I> S2>
   >       requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
   >     friend constexpr bool operator==(
   >       const common_iterator& x, const common_iterator<I2, S2>& y);

   > > -3- Preconditions: […]

   >     template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
   >       requires sized_sentinel_for<S, I2>
   >     friend constexpr iter_difference_t<I2> operator-(
   >       const common_iterator& x, const common_iterator<I2, S2>& y);

   > > -5- Preconditions: […]

6. Modify 24.5.5.7 [common.iter.cust] as indicated:

   >     friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i)
   >       noexcept(noexcept(ranges::iter_move(declval<const I&>())))
   >         requires input_iterator<I>;

   > > -1- Preconditions: […]

   >     template<indirectly_swappable<I> I2, class S2>
   >       friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
   >         noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));

   > > -3- Preconditions: […]

### 3575(i). `<=>` for integer-class types isn't consistently specified

Section: 24.3.4.4 [iterator.concept.winc] Status: Resolved Submitter: Jiang An Opened: 2021-07-25 Last modified: 2021-10-23

Priority: 3

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Discussion:

It seems that the return type of `<=>` for integer-class types is not specified consistently with other comparison operators. Even P2393R0 has ignored it.

IMO `strong_ordering` should be added to 24.3.4.4 [iterator.concept.winc]/(5.3), and `three_way_comparable<strong_ordering>` should be added to 24.3.4.4 [iterator.concept.winc]/8.

[2021-07-31, Daniel comments]

The upcoming revision P2393R1 will provide additional wording to solve this issue.

[2021-08-20; Reflector poll]

Set priority to 3 after reflector poll. Tentatively Resolved by P2393R1 which has been approved by LWG.

[2021-10-23 Resolved by the adoption of P2393R1 at the October 2021 plenary. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

### 3576(i). Clarifying fill character in `std::format`

Section: 28.5.2.2 [format.string.std] Status: Resolved Submitter: Mark de Wever Opened: 2021-08-01 Last modified: 2023-03-23

Priority: 2

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Discussion:

The paper P1868 "width: clarifying units of width and precision in `std::format`" added optional Unicode support to the `format` header. This paper didn't update the definition of the fill character, which is defined as

> "The fill character can be any character other than `{` or `}`."

This wording means the fill is a character and not a Unicode grapheme cluster. Based on the current wording the range of available fill characters depends on the `char_type` of the format string. After P1868 the determination of the required padding size is Unicode aware, but it's not possible to use a Unicode grapheme clusters as padding. This looks odd from a user's perspective and already lead to implementation divergence between libc++ and MSVC STL:

- The WIP libc++ implementation stores one `char_type`, strictly adhering to the wording of the Standard.

- MSVC STL stores one code point, regardless of the `char_type` used. This is already better from a user's perspective; all 1 code point grapheme clusters are properly handled.

For the width calculation the width of a Unicode grapheme cluster is estimated to be 1 or 2. Since padding with a 2 column width can't properly pad an odd number of columns the grapheme cluster used should always have a column width of 1.

The responsibility for precondition can be either be validated in the library or by the user. It would be possible to do the validation compile time and make the code ill-formed when the precondition is violated. For the following reason I think it's better to not validate the width:

- P1868 14. Implementation

  > "More importantly, our approach permits refining the definition in the future if there is interest in doing so. It will mostly require researching the status of Unicode support on terminals and minimal or no changes to the implementation."

  When an estimated width of 1 is required it means that improving the Standard may make previously valid code ill-formed after the improvement.

- P1868 13. Examples

  The example of the family grapheme cluster is only rendered properly on the MacOS terminal. So even when the library does a proper validation it's not certain the output will be rendered properly.

Changing the fill type changes the size of the `std::formatter` and thus will be an ABI break.

The proposed resolution probably needs some additional changes since the Unicode and output width are specified later in the standard, specifically 28.5.2.2 [format.string.std]/9 - 12.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4892.

> 1. Modify 28.5.2.2 [format.string.std] as indicated:

>    > -2- [Note 2: The fill character can be any character other than `{` or `}`. For a string in a Unicode encoding, the fill character can be any Unicode grapheme cluster other than `{` or `}`. For a string in a non-Unicode encoding, the fill character can be any character other than `{` or `}`. The output width of the fill character is always assumed to be one column. The presence of a fill character is signaled by the character following it, which must be one of the alignment options. If the second character of std-format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent. — end note]

[2021-08-09; Mark de Wever provides improved wording]

[2021-08-20; Reflector poll]

Set priority to 2 and status to "SG16" after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4892.

> 1. Modify 28.5.2.2 [format.string.std] as indicated:

>    > -1- […] The syntax of format specifications is as follows:

>    > >     […]
>    > >     fill:
>    > >                  any Unicode grapheme cluster or character other than { or }
>    > >     […]

>    > -2- [Note 2: The fill character can be any character other than `{` or `}`. For a string in a Unicode encoding, the fill character can be any Unicode grapheme cluster other than `{` or `}`. For a string in a non-Unicode encoding, the fill character can be any character other than `{` or `}`. The output width of the fill character is always assumed to be one column.

>    > [Note 2: The presence of a fill character is signaled by the character following it, which must be one of the alignment options. If the second character of std-format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent. — end note]

[2021-08-26; SG16 reviewed and provides alternative wording]

[2023-01-11; LWG telecon]

P2572 would resolve this issue and LWG 3639(i).

Previous resolution [SUPERSEDED]

This wording is relative to N4892.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > -1- […] The syntax of format specifications is as follows:

   > >     […]
   > >     fill:
   > >                     any charactercodepoint of the literal encoding other than { or }
   > >     […]

   > -2- [Note 2: The fill character can be any charactercodepoint other than `{` or `}`. The presence of a fill character is signaled by the character following it, which must be one of the alignment options. If the second character of std-format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent. — end note]

[2023-03-22 Resolved by the adoption of P2572R1 in Issaquah. Status changed: SG16 → Resolved.]

Proposed resolution:

### 3578(i). Iterator SCARYness in the context of associative container merging

Section: 23.2.7.1 [associative.reqmts.general] Status: C++26 Submitter: Joaquín M López Muñoz Opened: 2021-08-04 Last modified: 2026-06-09

Priority: 3

View other active issues in [associative.reqmts.general].

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Discussion:

For the expression `a.merge(a2)`, postconditions say that "iterators referring to the transferred elements […] now behave as iterators into `a` […]". When `a` and `a2` are of different types, this seems to imply, under the widest interpretation for "behaving as", that `a`-iterators and `a2`-iterators are actually of the same type, that is, that associative containers have SCARY iterators, which is, to the best of my knowledge, not currently mandated by the standard.

There are (at least) three possible resolutions to this ambiguity, ordered by intrusiveness:

- Indicate that "behaving as" only applies to the case where `a` and `a2` are of the same type.

- Clarify what "behaving as" means. A non-SCARY interpretation is that an `a2`-iterator to a transferred element can still be dereferenced, incremented (if not past the last element of `a`) and decremented (if not pointing to the first element of `a`), while comparison with `a`-iterators and use in the interface of `a` is not guaranteed.

- Mandate SCARY iterators by, for instance, requiring that associative containers with compatible nodes (23.2.5.1 [container.node.overview]/1) have the same iterator types.

Note that this issue does not extend to unordered associative containers, as there (23.2.8.1 [unord.req.general]) iterators to transferred elements are invalidated, which makes the point of SCARYness irrelevant. That said, if SCARY iterators are finally required for associative containers, it makes much sense to extend the requirement to unordered associative containers as well.

[2021-08-20; Reflector poll]

Set priority to 3 after reflector poll.

[2024-12-04; Jonathan provides wording]

If we want to require SCARY iterators then that should be a proposal that goes through LEWG design review. I propose an almost minimal change to make the spec consistent without imposing any new requirements on implementations.

The minimal change would be to say that iterators remain valid if `a` and `a2` have the same type, which is the minimum portable guarantee that always holds. However what matters in practice is whether the iterator types are the same. That would not be a portable guarantee, because it depends on whether the implementation uses SCARY iterators for its maps and sets, so users could write code that works on one implementation and fails to compile when moved to a different implementation. But that portability trap would be present even if we only say iterators remain valid when `a` and `a2` are the same type. If the code compiles and works on an implementation with SCARY iterators, then users will rely on that, even if unintentionally. Leaving that case unspecified or undefined in the standard doesn't prevent users from relying on it. It doesn't seem to serve any benefit to pretend it doesn't work when it actually does on some implementations.

N.B. Libstdc++ associative container iterators are SCARY by default, but non-SCARY when `-D_GLIBCXX_DEBUG` is used to enable Debug Mode (see Bug 62169). I believe libc++ iterators are SCARY even when `-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG` is used, and MSVC STL iterators are SCARY even when `/D_ITERATOR_DEBUG_LEVEL` is used.

[2024-12-09; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4993.

1. Modify 23.2.7.1 [associative.reqmts.general] as indicated:

   >     a.merge(a2)

   > -112- Result: `void`

   > -113- Preconditions: `a.get_allocator() == a2.get_allocator()` is `true`.

   > -114- Effects: Attempts to extract each element in `a2` and insert it into `a` using the comparison object of `a`. In containers with unique keys, if there is an element in `a` with key equivalent to the key of an element from `a2`, then that element is not extracted from `a2`.

   > -115- Postconditions: Pointers and references to the transferred elements of `a2` refer to those same elements but as members of `a`. If `a.begin()` and `a2.begin()` have the same type, iterators Iterators referring to the transferred elements will continue to refer to their elements, but they now behave as iterators into `a`, not into `a2`.

   > -116- Throws: Nothing unless the comparison objects throws.

   > -117- Complexity: N log(`a.size()`+N), where N has the value `a2.size()`.

### 3580(i). `iota_view's iterator`'s binary `operator+` should be improved

Section: 25.6.4.3 [range.iota.iterator] Status: C++23 Submitter: Zoe Carver Opened: 2021-08-14 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

`iota_view`'s iterator's `operator+` could avoid a copy construct by doing "`i += n; return i`" rather than "`return i += n;`" as seen in 25.6.4.3 [range.iota.iterator].

This is what libc++ has implemented and shipped, even though it may not technically be conforming (if a program asserted the number of copies, for example).

It might be good to update this operator and the minus operator accordingly.

[2021-08-20; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.6.4.3 [range.iota.iterator] as indicated:

   >     friend constexpr iterator operator+(iterator i, difference_type n)
   >       requires advanceable<W>;

   > > -20- Effects: Equivalent to:

   > > >     return i += n;
   > > >     return i;

   >     friend constexpr iterator operator+(difference_type n, iterator i)
   >       requires advanceable<W>;

   > > -21- Effects: Equivalent to: `return i + n;`

   >     friend constexpr iterator operator-(iterator i, difference_type n)
   >       requires advanceable<W>;

   > > -22- Effects: Equivalent to:

   > > >     return i -= n;
   > > >     return i;

### 3581(i). The range constructor makes `basic_string_view` not trivially move constructible

Section: 27.3.3.2 [string.view.cons] Status: C++23 Submitter: Jiang An, Casey Carter Opened: 2021-08-16 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

Jiang An:

The issue is found in this Microsoft STL pull request.

I think an additional constraint should be added to 27.3.3.2 [string.view.cons]/11 (a similar constraint is already present for `reference_wrapper`):

> (11.?) — `is_same_v<remove_cvref_t<R>, basic_string_view>` is `false`.

Casey Carter:

P1989R2 "Range constructor for `std::string_view` 2: Constrain Harder" added a converting constructor to `basic_string_view` that accepts (by forwarding reference) any sized contiguous range with a compatible element type. This constructor unfortunately intercepts attempts at move construction which previously would have resulted in a call to the copy constructor. (I suspect the authors of P1989 were under the mistaken impression that `basic_string_view` had a defaulted move constructor, which would sidestep this issue by being chosen by overload resolution via the "non-template vs. template" tiebreaker.)

The resulting inefficiency could be corrected by adding a defaulted move constructor and move assignment operator to `basic_string_view`, but it would be awkward to add text to specify that these moves always leave the source intact. Presumably this is why the move operations were omitted in the first place. We therefore recommend constraining the conversion constructor template to reject arguments whose decayed type is `basic_string_view` (which we should probably do for all conversion constructor templates in the Standard Library).

Implementation experience: MSVC STL is in the process of implementing this fix. Per Jonathan Wakely, libstdc++ has a similar constraint.

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 27.3.3.2 [string.view.cons] as indicated:

   >     template<class R>
   >       constexpr basic_string_view(R&& r);

   > > -10- Let `d` be an lvalue of type `remove_cvref_t<R>`.

   > > -11- Constraints:

   > > 1. (11.?) — `remove_cvref_t<R>` is not the same type as `basic_string_view`,

   > > 2. (11.1) — `R` models `ranges::contiguous_range` and `ranges::sized_range`,

   > > 3. (11.2) — `is_same_v<ranges::range_value_t<R>, charT>` is `true`,

   > > 4. […]

### 3585(i). Variant converting assignment with immovable alternative

Section: 22.6.3.4 [variant.assign] Status: C++23 Submitter: Barry Revzin Opened: 2021-09-01 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

Example originally from StackOverflow but with a more reasonable example from Tim Song:

>     #include <variant>
>     #include <string>

>     struct A {
>       A() = default;
>       A(A&&) = delete;
>     };

>     int main() {
>       std::variant<A, std::string> v;
>       v = "hello";
>     }

There is implementation divergence here: libstdc++ rejects, libc++ and msvc accept.

22.6.3.4 [variant.assign] bullet (13.3) says that if we're changing the alternative in assignment and it is not the case that the converting construction won't throw (as in the above), then "Otherwise, equivalent to `operator=(variant(std::forward<T>(t)))`." That is, we defer to move assignment.

`variant<A, string>` isn't move-assignable (because `A` isn't move constructible). Per the wording in the standard, we have to reject this. libstdc++ follows the wording of the standard.

But we don't actually have to do a full move assignment here, since we know the situation we're in is changing the alternative, so the fact that `A` isn't move-assignable shouldn't matter. libc++ and msvc instead do something more direct, allowing the above program.

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 22.6.3.4 [variant.assign] as indicated:

   > [Drafting note: This should cover the case that we want to cover: that if construction of `Tj` from `T` throws, we haven't yet changed any state in the variant.]

   >     template<class T> constexpr variant& operator=(T&& t) noexcept(see below);

   > > -11- Let `Tj` be a type that is determined as follows: build an imaginary function `FUN(Ti)` for each alternative type `Ti` for which `Ti x[] = {std::forward<T>(t)};` is well-formed for some invented variable `x`. The overload `FUN(Tj)` selected by overload resolution for the expression `FUN(std::forward<T>(t))` defines the alternative `Tj` which is the type of the contained value after assignment.

   > > -12- Constraints: […]

   > > -13- Effects:

   > > 1. (13.1) — If `*this` holds a `Tj`, assigns `std::forward<T>(t)` to the value contained in `*this`.

   > > 2. (13.2) — Otherwise, if `is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj>` is `true`, equivalent to `emplace<j>(std::forward<T>(t))`.

   > > 3. (13.3) — Otherwise, equivalent to `operator=(variant(std::forward<T>(t)))emplace<j>(Tj(std::forward<T>(t)))`.

### 3589(i). The `const` lvalue reference overload of `get` for `subrange` does not constrain `I` to be `copyable` when `N == 0`

Section: 25.5.4 [range.subrange] Status: C++23 Submitter: Hewill Kang Opened: 2021-09-08 Last modified: 2023-11-22

Priority: 3

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Discussion:

The `const` lvalue reference overload of `get` used for `subrange` only constraint that `N < 2`, this will cause the "discards qualifiers" error inside the function body when applying `get` to the lvalue `subrange` that stores a non-`copyable` iterator since its `begin()` is non-`const` qualified, we probably need to add a constraint for it.

[2021-09-20; Reflector poll]

Set priority to 3 after reflector poll.

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.5.4 [range.subrange] as indicated:

   >     namespace std::ranges {
   >       […]
   >       template<size_t N, class I, class S, subrange_kind K>
   >         requires ((N < 2== 0 && copyable<I>) || N == 1)
   >         constexpr auto get(const subrange<I, S, K>& r);

   >       template<size_t N, class I, class S, subrange_kind K>
   >         requires (N < 2)
   >         constexpr auto get(subrange<I, S, K>&& r);
   >     }

2. Modify 25.5.4.3 [range.subrange.access] as indicated:

   >       […]
   >       template<size_t N, class I, class S, subrange_kind K>
   >         requires ((N < 2== 0 && copyable<I>) || N == 1)
   >         constexpr auto get(const subrange<I, S, K>& r);
   >       template<size_t N, class I, class S, subrange_kind K>
   >         requires (N < 2)
   >         constexpr auto get(subrange<I, S, K>&& r);

   > > -10- Effects: Equivalent to:

   > >     if constexpr (N == 0)
   > >       return r.begin();
   > >     else
   > >       return r.end();

### 3590(i). `split_view::base() const &` is overconstrained

Section: 25.7.17.2 [range.split.view] Status: C++23 Submitter: Tim Song Opened: 2021-09-13 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

Unlike every other range adaptor, `split_view::base() const &` is constrained on `copyable<V>` instead of `copy_constructible<V>`.

Since this function just performs a copy construction, there is no reason to require all of `copyable` — `copy_constructible` is sufficient.

[2021-09-24; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.7.17.2 [range.split.view], class template `split_view` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<forward_range V, forward_range Pattern>
   >         requires view<V> && view<Pattern> &&
   >                  indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
   >       class split_view : public view_interface<split_view<V, Pattern>> {
   >       private:
   >         […]
   >       public:
   >         […]

   >         constexpr V base() const& requires copyablecopy_constructible<V> { return base_; }
   >         constexpr V base() && { return std::move(base_); }

   >         […]
   >       };

### 3591(i). `lazy_split_view<input_view>::inner-iterator::base() &&` invalidates outer iterators

Section: 25.7.16.5 [range.lazy.split.inner] Status: C++23 Submitter: Tim Song Opened: 2021-09-13 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.lazy.split.inner].

View all issues with C++23 status.

Discussion:

The `base() &&` members of iterator adaptors (and iterators of range adaptors) invalidate the adaptor itself by moving from the contained iterator. This is generally unobjectionable — since you are calling `base()` on an rvalue, it is expected that you won't be using it afterwards.

But `lazy_split_view<input_view>::inner-iterator::base() &&` is special: the iterator being moved from is stored in the `lazy_split_view` itself and shared between the inner and outer iterators, so the operation invalidates not just the `inner-iterator` on which it is called, but also the `outer-iterator` from which the `inner-iterator` was obtained. This spooky-action-at-a-distance behavior can be surprising, and the value category of the inner iterator seems to be too subtle to base it upon.

The PR below constrains this overload to forward ranges. Forward iterators are copyable anyway, but moving could potentially be more efficient.

[2021-09-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.7.16.5 [range.lazy.split.inner] as indicated:

   > [Drafting note: The constraint uses `forward_range<V>` since that's the condition for caching in `lazy_split_view`.]

   > >     namespace std::ranges {
   > >       template<input_range V, forward_range Pattern>
   > >         requires view<V> && view<Pattern> &&
   > >                  indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
   > >                  (forward_range<V> || tiny-range<Pattern>)
   > >       template<bool Const>
   > >       struct lazy_split_view<V, Pattern>::inner-iterator {
   > >       private:
   > >         […]
   > >       public:
   > >         […]

   > >         constexpr const iterator_t<Base>& base() const &;
   > >         constexpr iterator_t<Base> base() && requires forward_range<V>;

   > >         […]
   > >       };

   > […]

   >     constexpr iterator_t<Base> base() && requires forward_range<V>;

   > > -4- Effects: Equivalent to: `return std::move(i_.current);`

### 3592(i). `lazy_split_view` needs to check the simpleness of `Pattern`

Section: 25.7.16.2 [range.lazy.split.view] Status: C++23 Submitter: Tim Song Opened: 2021-09-15 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [range.lazy.split.view].

View all other issues in [range.lazy.split.view].

View all issues with C++23 status.

Discussion:

For forward ranges, the non-`const` versions of `lazy_split_view::begin()` and `lazy_split_view::end()` returns an `outer-iterator<simple-view<V>>`, promoting to `const` if `V` models `simple-view`. However, this failed to take `Pattern` into account, even though that will also be accessed as `const` if `const`-promotion takes place. There is no requirement that `const Pattern` is a range, however.

[2021-09-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.7.16.2 [range.lazy.split.view], class template `lazy_split_view` synopsis, as indicated:

   > >     namespace std::ranges {

   > >       […]

   > >       template<input_range V, forward_range Pattern>
   > >         requires view<V> && view<Pattern> &&
   > >                  indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
   > >                 (forward_range<V> || tiny-range<Pattern>)
   > >       class lazy_split_view : public view_interface<lazy_split_view<V, Pattern>> {
   > >       private:
   > >         […]
   > >       public:
   > >         […]

   > >         constexpr auto begin() {
   > >           if constexpr (forward_range<V>)
   > >             return outer-iterator<simple-view<V> &&
   > >               simple-view<Pattern>>{*this, ranges::begin(base_)};
   > >           else {
   > >             current_ = ranges::begin(base_);
   > >             return outer-iterator<false>{*this};
   > >           }
   > >         }

   > >         […]

   > >         constexpr auto end() requires forward_range<V> && common_range<V> {
   > >           return outer-iterator<simple-view<V> &&
   > >             simple-view<Pattern>>{*this, ranges::end(base_)};
   > >         }

   > >         […]
   > >       };

   > >       […]

   > >     }

### 3593(i). Several iterators' `base() const &` and `lazy_split_view::outer-iterator::value_type::end()` missing `noexcept`

Section: 24.5.4 [move.iterators], 24.5.7 [iterators.counted], 25.7.8.3 [range.filter.iterator], 25.7.9.3 [range.transform.iterator], 25.7.16.4 [range.lazy.split.outer.value], 25.7.16.5 [range.lazy.split.inner], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2021-09-14 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [move.iterators].

View all issues with C++23 status.

Discussion:

LWG 3391(i) and 3533(i) changed some iterators' `base() const &` from returning value to returning `const` reference, which also prevents them from throwing exceptions, we should add `noexcept` for them. Also, `lazy_split_view::outer-iterator::value_type::end()` can be `noexcept` since it only returns `default_sentinel`.

[2021-09-24; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 24.5.4 [move.iterators], class template `move_iterator` synopsis, as indicated:

   >     […]
   >     constexpr const iterator_type& base() const & noexcept;
   >     constexpr iterator_type base() &&;
   >     […]

2. Modify 24.5.4.5 [move.iter.op.conv] as indicated:

   >     constexpr const Iterator& base() const & noexcept;

   > > -1- Returns: `current`.

3. Modify 24.5.7.1 [counted.iterator], class template `counted_iterator` synopsis, as indicated:

   >     […]
   >     constexpr const I& base() const & noexcept;
   >     constexpr I base() &&;
   >     […]

4. Modify 24.5.7.3 [counted.iter.access] as indicated:

   >     constexpr const I& base() const & noexcept;

   > > -1- Effects: Equivalent to: `return current;`

5. Modify 25.7.8.3 [range.filter.iterator] as indicated:

   > >     […]
   > >     constexpr const iterator_t<V>& base() const & noexcept;
   > >     constexpr iterator_t<V> base() &&;
   > >     […]

   > […]

   >     constexpr const iterator_t<V>& base() const & noexcept;

   > > -5- Effects: Equivalent to: `return current_;`

6. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > >     […]
   > >     constexpr const iterator_t<Base>& base() const & noexcept;
   > >     constexpr iterator_t<Base> base() &&;
   > >     […]

   > […]

   >     constexpr const iterator_t<Base>& base() const & noexcept;

   > > -5- Effects: Equivalent to: `return current_;`

7. Modify 25.7.16.4 [range.lazy.split.outer.value] as indicated:

   > >     […]
   > >     constexpr inner-iterator<Const> begin() const;
   > >     constexpr default_sentinel_t end() const noexcept;
   > >     […]

   > […]

   >     constexpr default_sentinel_t end() const noexcept;

   > > -3- Effects: Equivalent to: `return default_sentinel;`

8. Modify 25.7.16.5 [range.lazy.split.inner] as indicated:

   > >     […]
   > >     constexpr const iterator_t<Base>& base() const & noexcept;
   > >     constexpr iterator_t<Base> base() &&;
   > >     […]

   > […]

   >     constexpr const iterator_t<Base>& base() const & noexcept;

   > > -3- Effects: Equivalent to: `return i_.current;`

9. Modify 25.7.23.3 [range.elements.iterator] as indicated:

   > >     […]
   > >     constexpr const iterator_t<Base>& base() const& noexcept;
   > >     constexpr iterator_t<Base> base() &&;
   > >     […]

   > […]

   >     constexpr const iterator_t<Base>& base() const& noexcept;

   > > -6- Effects: Equivalent to: `return current_;`

### 3594(i). `inout_ptr` — inconsistent `release()` in destructor

Section: 20.3.4.3 [inout.ptr.t] Status: C++23 Submitter: JeanHeyd Meneide Opened: 2021-09-16 Last modified: 2023-11-22

Priority: 1

View all other issues in [inout.ptr.t].

View all issues with C++23 status.

Discussion:

More succinctly, the model for `std::out_ptr_t` and `std::inout_ptr_t` both have a conditional release in their destructors as part of their specification (20.3.4.1 [out.ptr.t] p8) (Option #2 below). But, if the wording is followed, they have an unconditional release in their constructor (Option #1 below). This is not exactly correct and can cause issues with double-free in `inout_ptr` in particular.

Consider a function `MyFunc` that sets `rawPtr` to `nullptr` when freeing an old value and deciding not to produce a new value, as shown below:

>     // Option #1:
>     auto uptr = std::make_unique<BYTE[]>(25);
>     auto rawPtr = uptr.get();
>     uptr.release(); // UNCONDITIONAL
>     MyFunc(&rawPtr);
>     If (rawPtr)
>     {
>       uptr.reset(rawPtr);
>     }

>     // Option #2:
>     auto uptr = std::make_unique<BYTE[]>(25);
>     auto rawPtr = uptr.get();
>     MyFunc(&rawPtr);
>     If (rawPtr)
>     {
>       uptr.release(); // CONDITIONAL
>       uptr.reset(rawPtr);
>     }

This is no problem if the implementation selected Option #1 (release in the constructor), but results in double-free if the implementation selected option #2 (release in the destructor).

As the paper author and after conferring with others, the intent was that the behavior was identical and whether a choice between the constructor or destructor is made. The reset should be unconditional, at least for `inout_ptr_t`. Suggested change for the `~inout_ptr_t` destructor text is to remove the "`if (p) { ... }`" wrapper from around the code in 20.3.4.3 [inout.ptr.t] p11.

[2021-09-24; Reflector poll]

Set priority to 1 after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4892.

> 1. Modify 20.3.4.3 [inout.ptr.t] as indicated:

>    >     ~inout_ptr_t();

>    > > -9- Let `SP` be `POINTER_OF_OR(Smart, Pointer)` (20.2.1 [memory.general]).

>    > > -10- Let `release-statement` be `s.release()`; if an implementation does not call `s.release()` in the constructor. Otherwise, it is empty.

>    > > -11- Effects: Equivalent to:

>    > > 1. (11.1) —

>    > >    >     if (p) {
>    > >    >       apply([&](auto&&... args) {
>    > >    >       s = Smart( static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
>    > >    >     }

>    > >    if `is_pointer_v<Smart>` is `true`;

>    > > 2. (11.2) — otherwise,

>    > >    >     if (p) {
>    > >    >       apply([&](auto&&... args) {
>    > >    >       release-statement;
>    > >    >       s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
>    > >    >     }

>    > >    if the expression `s.reset(static_cast<SP>(p), std::forward<Args>(args)...)` is well-formed;

>    > > 3. (11.3) — otherwise,

>    > >    >     if (p) {
>    > >    >       apply([&](auto&&... args) {
>    > >    >       release-statement;
>    > >    >       s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
>    > >    >     }

>    > >    if `is_constructible_v<Smart, SP, Args...>` is `true`;

>    > > 4. (11.4) — otherwise, the program is ill-formed.

[2021-10-28; JeanHeyd Meneide provides improved wording]

[2022-08-24 Approved unanimously in LWG telecon.]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 20.3.4.3 [inout.ptr.t] as indicated:

   >     ~inout_ptr_t();

   > > -9- Let `SP` be `POINTER_OF_OR(Smart, Pointer)` (20.2.1 [memory.general]).

   > > -10- Let `release-statement` be `s.release()`; if an implementation does not call `s.release()` in the constructor. Otherwise, it is empty.

   > > -11- Effects: Equivalent to:

   > > 1. (11.1) —

   > >    >     if (p) {
   > >    >       apply([&](auto&&... args) {
   > >    >       s = Smart( static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
   > >    >     }

   > >    if `is_pointer_v<Smart>` is `true`;

   > > 2. (11.2) — otherwise,

   > >    >     release-statement;
   > >    >     if (p) {
   > >    >       apply([&](auto&&... args) {
   > >    >       release-statement;
   > >    >       s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
   > >    >     }

   > >    if the expression `s.reset(static_cast<SP>(p), std::forward<Args>(args)...)` is well-formed;

   > > 3. (11.3) — otherwise,

   > >    >     release-statement;
   > >    >     if (p) {
   > >    >       apply([&](auto&&... args) {
   > >    >       release-statement;
   > >    >       s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
   > >    >     }

   > >    if `is_constructible_v<Smart, SP, Args...>` is `true`;

   > > 4. (11.4) — otherwise, the program is ill-formed.

### 3595(i). Exposition-only classes `proxy` and `postfix-proxy` for `common_iterator` should be fully `constexpr`

Section: 24.5.5.4 [common.iter.access], 24.5.5.5 [common.iter.nav] Status: C++23 Submitter: Hewill Kang Opened: 2021-09-18 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [common.iter.access].

View all issues with C++23 status.

Discussion:

LWG 3574(i) added `constexpr` to all member functions and friends of `common_iterator` to make it fully `constexpr`, but accidentally omitted the exposition-only classes `proxy` and `postfix-proxy` defined for some member functions. We should make these two classes fully `constexpr`, too.

[2021-09-24; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 24.5.5.4 [common.iter.access] as indicated:

   >     decltype(auto) operator->() const
   >       requires see below;

   > > -3- […]

   > > -4- […]

   > > -5- Effects:

   > > 1. (5.1) — […]

   > > 2. (5.2) — […]

   > > 3. (5.3) — Otherwise, equivalent to: `return proxy(*get<I>(v_));` where `proxy` is the exposition-only class:

   > >    >     class proxy {
   > >    >       iter_value_t<I> keep_;
   > >    >       constexpr proxy(iter_reference_t<I>&& x)
   > >    >         : keep_(std::move(x)) {}
   > >    >     public:
   > >    >       constexpr const iter_value_t<I>* operator->() const noexcept {
   > >    >         return addressof(keep_);
   > >    >       }
   > >    >     };

2. Modify 24.5.5.5 [common.iter.nav] as indicated:

   >     decltype(auto) operator++(int);

   > > -4- […]

   > > -5- Effects: If I models forward_iterator, equivalent to:

   > > […]

   > > Otherwise, if […]

   > > […]

   > > Otherwise, equivalent to:

   > > >     postfix-proxy p(**this);
   > > >     ++*this;
   > > >     return p;

   > > where `postfix-proxy` is the exposition-only class:

   > > >     class postfix-proxy {
   > > >       iter_value_t<I> keep_;
   > > >       constexpr postfix-proxy(iter_reference_t<I>&& x)
   > > >         : keep_(std::forward<iter_reference_t<I>>(x)) {}
   > > >     public:
   > > >       constexpr const iter_value_t<I>& operator*() const noexcept {
   > > >         return keep_;
   > > >       }
   > > >     };

### 3597(i). Unsigned integer types don't model `advanceable`

Section: 25.6.4.2 [range.iota.view] Status: C++23 Submitter: Jiang An Opened: 2021-09-23 Last modified: 2023-11-22

Priority: 3

View all other issues in [range.iota.view].

View all issues with C++23 status.

Discussion:

Unsigned integer types satisfy `advanceable`, but don't model it, since

> every two values of an unsigned integer type are reachable from each other, and modular arithmetic is performed on unsigned integer types,

which makes the last three bullets of the semantic requirements of `advanceable` (25.6.4.2 [range.iota.view]/5) can't be satisfied, and some (if not all) uses of `iota_view`s of unsigned integer types ill-formed, no diagnostic required.

Some operations that are likely to expect the semantic requirements of `advanceable` behave incorrectly for unsigned integer types. E.g. according to 25.6.4.2 [range.iota.view]/6 and 25.6.4.2 [range.iota.view]/16, `std::ranges::iota_view<std::uint8_t, std::uint8_t>(std::uint8_t(1)).size()` is well-defined IMO, because

> `Bound()` is `std::uint8_t(0)`, which is reachable from `std::uint8_t(1)`, and not modeling `advanceable` shouldn't affect the validity, as both `W` and `Bound` are integer types.

However, it returns `unsigned(-1)` on common implementations (where `sizeof(int) > sizeof(std::uint8_t))`, which is wrong.

Perhaps the semantic requirements of `advanceable` should be adjusted, and a refined definition of reachability in 25.6.4 [range.iota] is needed to avoid reaching `a` from `b` when `a > b` (the iterator type is also affected).

[2021-10-14; Reflector poll]

Set priority to 3 after reflector poll.

[Tim Song commented:]

The advanceable part of the issue is NAD. This is no different from NaN and `totally_ordered`, see 16.3.2.3 [structure.requirements]/8.

The part about `iota_view<uint8_t, uint8_t>(1)` is simply this: when we added "When `W` and `Bound` model ..." to 25.6.4.2 [range.iota.view]/8, we forgot to add its equivalent to the single-argument constructor. We should do that.

[2022-10-12; Jonathan provides wording]

[2022-10-19; Reflector poll]

Set status to "Tentatively Ready" after seven votes in favour in reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.6.4.2 [range.iota.view] as indicated:

   >     constexpr explicit iota_view(W value);

   > > -6- Preconditions: `Bound` denotes `unreachable_sentinel_t` or `Bound()` is reachable from `value`. When `W` and `Bound` model `totally_ordered_with`, then `bool(value <= Bound())` is `true`.

   > > -7- Effects: Initializes `value_` with `value`.

   >     constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);

   > > -8- Preconditions: `Bound` denotes `unreachable_sentinel_t` or `bound` is reachable from `value`. When `W` and `Bound` model `totally_ordered_with`, then `bool(value <= bound)` is `true`.

   > > -9- Effects: Initializes `value_` with `value` and `bound_` with `bound`.

### 3598(i). `system_category().default_error_condition(0)` is underspecified

Section: 19.5.3.5 [syserr.errcat.objects] Status: C++23 Submitter: Jonathan Wakely Opened: 2021-09-23 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [syserr.errcat.objects].

View all issues with C++23 status.

Discussion:

19.5.3.5 [syserr.errcat.objects] says:

> If the argument `ev` corresponds to a POSIX `errno` value `posv`, the function shall return `error_condition(posv, generic_category())`. Otherwise, the function shall return `error_condition(ev, system_category())`. What constitutes correspondence for any given operating system is unspecified.

What is "a POSIX `errno` value"? Does it mean a value equal to one of the `<cerrno>` `Exxx` macros? Because in that case, the value `0` is not "a POSIX `errno` value". So arguably `system_category().default_error_condition(0)` is required to return an `error_condition` using the "system" category, which means that `error_code{} == error_condition{}` is required to be `false`. This seems wrong.

For POSIX-based systems the value `0` should definitely correspond to the generic category. Arguably that needs to be `true` for all systems, because the `std::error_code` API strongly encourages a model where zero means "no error".

The proposed resolution has been implemented in libstdc++. Libc++ has always treated system error code `0` as corresponding to generic error code `0`.

[2021-10-14; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 19.5.3.5 [syserr.errcat.objects] as indicated:

   >     const error_category& system_category() noexcept;

   > > -3- Returns: […]

   > > -4- Remarks: The object's `equivalent` virtual functions shall behave as specified for class `error_category`. The object's `name` virtual function shall return a pointer to the string `"system"`. The object's `default_error_condition` virtual function shall behave as follows:

   > > If the argument `ev` is equal to `0`, the function returns `error_condition(0, generic_category())`. Otherwise, if `ev` corresponds to a POSIX `errno` value `posv`, the function shall returnreturns `error_condition(posv, generic_category())`. Otherwise, the function shall returnreturns `error_condition(ev, system_category())`. What constitutes correspondence for any given operating system is unspecified.

   > > [Note 1: The number of potential system error codes is large and unbounded, and some might not correspond to any POSIX `errno` value. Thus implementations are given latitude in determining correspondence. — end note]

### 3599(i). The `const` overload of `lazy_split_view::begin` should be constrained by `const Pattern`

Section: 25.7.16.2 [range.lazy.split.view] Status: C++26 Submitter: Hewill Kang Opened: 2021-09-23 Last modified: 2026-06-09

Priority: 3

View other active issues in [range.lazy.split.view].

View all other issues in [range.lazy.split.view].

View all issues with C++26 status.

Discussion:

Consider the following code snippet:

>     #include <ranges>

>     int main() {
>       auto p = std::views::iota(0)
>              | std::views::take(1)
>              | std::views::reverse;
>       auto r = std::views::single(42)
>              | std::views::lazy_split(p);
>       auto f = r.front();
>     }

`r.front()` is ill-formed even if `r` is a `forward_range`.

This is because the `const` overload of `lazy_split_view::begin` is not constrained by the `const Pattern`, which makes it still well-formed in such cases. When the `const` overload of `view_interface<lazy_split_view<V, Pattern>>::front` is instantiated, the `subrange{parent_->pattern_}` inside `lazy_split_view::outer-iterator<true>::operator++()` will cause a hard error since `const Pattern` is not a `range`.

[2021-09-24; Daniel comments]

This issue is related to LWG 3592(i).

[2021-10-14; Reflector poll]

Set priority to 3 after reflector poll.

[2026-01-16; LWG telecon. Move to Ready]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.7.16.2 [range.lazy.split.view], class template `lazy_split_view` synopsis, as indicated:

   > >     namespace std::ranges {

   > >       […]

   > >       template<input_range V, forward_range Pattern>
   > >         requires view<V> && view<Pattern> &&
   > >                  indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
   > >                 (forward_range<V> || tiny-range<Pattern>)
   > >       class lazy_split_view : public view_interface<lazy_split_view<V, Pattern>> {
   > >       private:
   > >         […]
   > >       public:
   > >         […]

   > >         constexpr auto begin() {
   > >           […]
   > >         }

   > >         constexpr auto begin() const requires forward_range<V> && forward_range<const V> &&
   > >                                               forward_range<const Pattern>{
   > >           return outer-iterator<true>{*this, ranges::begin(base_)};
   > >         }

   > >         constexpr auto end() requires forward_range<V> && common_range<V> {
   > >           […]
   > >         }

   > >         constexpr auto end() const {
   > >           if constexpr (forward_range<V> && forward_range<const V> && common_range<const V> &&
   > >                         forward_range<const Pattern>)
   > >             return outer-iterator<true>{*this, ranges::end(base_)};
   > >           else
   > >             return default_sentinel;
   > >         }
   > >       };

   > >       […]

   > >     }

### 3600(i). Making `istream_iterator` copy constructor trivial is an ABI break

Section: 24.6.2.2 [istream.iterator.cons] Status: C++23 Submitter: Jonathan Wakely Opened: 2021-09-23 Last modified: 2023-11-22

Priority: 3

View all other issues in [istream.iterator.cons].

View all issues with C++23 status.

Discussion:

Libstdc++ never implemented this change made between C++03 and C++11 (by N2994):

24.6.1.1 [istream.iterator.cons] p3:

>     istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;

> > -3- Effects: Constructs a copy of `x`. If `T` is a literal type, then this constructor shall be a trivial copy constructor.

This breaks our ABI, as it changes the argument passing convention for the type, meaning this function segfaults if compiled with today's libstdc++ and called from one that makes the triviality change:

>     #include <iterator>
>     #include <istream>

>     int f(std::istream_iterator<int> i)
>     {
>       return *i++;
>     }

As a result, it's likely that libstdc++ will never implement the change.

There is no reason to require this constructor to be trivial. It was required for C++0x at one point, so the type could be literal, but that is not true in the current language. We should strike the requirement, to reflect reality. MSVC and libc++ are free to continue to define it as defaulted (and so trivial when appropriate) but we should not require it from libstdc++. The cost of an ABI break is not worth the negligible benefit from making it trivial.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4892.

> 1. Modify 24.6.2.2 [istream.iterator.cons] as indicated:

>    >     istream_iterator(const istream_iterator& x) = default;

>    > > -5- Postconditions: `in_stream == x.in_stream` is `true`.

>    > > -6- Remarks: If `is_trivially_copy_constructible_v<T>` is `true`, then this constructor is trivial.

[2021-09-30; Jonathan revises wording after reflector discussion]

A benefit of triviality is that it is constexpr, want to preserve that.

[2021-10-14; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4892.

> 1. Modify the class synopsis in 24.6.2.1 [istream.iterator.general] as indicated:

>    >     constexpr istream_iterator();
>    >     constexpr istream_iterator(default_sentinel_t);
>    >     istream_iterator(istream_type& s);
>    >     constexpr istream_iterator(const istream_iterator& x) = default;
>    >     ~istream_iterator() = default;
>    >     istream_iterator& operator=(const istream_iterator&) = default;

> 2. Modify 24.6.2.2 [istream.iterator.cons] as indicated:

>    >       constexpr istream_iterator(const istream_iterator& x) = default;

>    > > -5- Postconditions: `in_stream == x.in_stream` is `true`.

>    > > -6- Remarks: If `is_trivially_copy_constructible_v<T>` is `true`, then this constructor is trivial. If the initializer `T(x.value)` in the declaration `auto val = T(x.value);` is a constant initializer ([expr.const]), then this constructor is a constexpr constructor.

[2022-10-12; Jonathan provides improved wording]

Discussed on the reflector September 2021.

[2022-10-13; Jonathan revises wording to add a noexcept-specifier]

[2022-11-07; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify the class synopsis in 24.6.2.1 [istream.iterator.general] as indicated:

   >     constexpr istream_iterator();
   >     constexpr istream_iterator(default_sentinel_t);
   >     istream_iterator(istream_type& s);
   >     constexpr istream_iterator(const istream_iterator& x) noexcept(see below) = default;
   >     ~istream_iterator() = default;
   >     istream_iterator& operator=(const istream_iterator&) = default;

2. Modify 24.6.2.2 [istream.iterator.cons] as indicated:

   >       constexpr istream_iterator(const istream_iterator& x) noexcept(see below) = default;

   > > -5- Postconditions: `in_stream == x.in_stream` is `true`.

   > > -?- Effects: Initializes `in_stream` with `x.in_stream` and initializes `value` with `x.value`.

   > > -6- Remarks: If `is_trivially_copy_constructible_v<T>` is `true`, then this constructor is trivial. An invocation of this constructor may be used in a core constant expression if and only if the initialization of `value` from `x.value` is a constant subexpression ([defns.const.subexpr]). The exception specification is equivalent to `is_nothrow_copy_constructible_v<T>`.

### 3601(i). `common_iterator`'s `postfix-proxy` needs `indirectly_readable`

Section: 24.5.5.5 [common.iter.nav] Status: C++23 Submitter: Casey Carter Opened: 2021-09-24 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [common.iter.nav].

View all issues with C++23 status.

Discussion:

It would appear that when P2259R1 added `postfix-proxy` to `common_iterator::operator++(int)` LWG missed a crucial difference between `operator++(int)` and `operator->` which uses a similar proxy: `operator->` requires the wrapped type to be `indirectly_readable`, but `operator++(int)` does not. Consequently, operations that read from the wrapped type for the `postfix-proxy` case in `operator++(int)` are not properly constrained to be valid.

[2021-10-14; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 24.5.5.5 [common.iter.nav] as indicated:

   >     decltype(auto) operator++(int);

   > > -4- Preconditions: `holds_alternative<I>(v_)` is `true`.

   > > -5- Effects: If `I` models `forward_iterator`, equivalent to:

   > > >     common_iterator tmp = *this;
   > > >     ++*this;
   > > >     return tmp;

   > > Otherwise, if `requires (I& i) { { *i++ } -> can-reference; }` is `true` or `indirectly_readable<I> && constructible_from<iter_value_t<I>, iter_reference_t<I>> && move_constructible<iter_value_t<I>>` is `false`, equivalent to:

   > > >     return get<I>(v_)++;

   > > Otherwise, equivalent to: […]

### 3607(i). `contiguous_iterator` should not be allowed to have custom `iter_move` and `iter_swap` behavior

Section: 24.3.4.14 [iterator.concept.contiguous] Status: C++23 Submitter: Tim Song Opened: 2021-09-28 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [iterator.concept.contiguous].

View all issues with C++23 status.

Discussion:

The `iter_move` and `iter_swap` customization points were introduced primarily for proxy iterators. Whatever their application to non-proxy iterators in general, they should not be allowed to have custom behavior for contiguous iterators — this new iterator category was introduced in large part to permit better optimizations, and allowing custom `iter_move/iter_swap` prevents such optimizations for a wide variety of algorithms that are specified to call them.

[2021-10-14; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 24.3.4.14 [iterator.concept.contiguous] as indicated:

   >     template<class I>
   >       concept contiguous_iterator =
   >         random_access_iterator<I> &&
   >         derived_from<ITER_CONCEPT(I), contiguous_iterator_tag> &&
   >         is_lvalue_reference_v<iter_reference_t<I>> &&
   >         same_as<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> &&
   >         requires(const I& i) {
   >           { to_address(i) } -> same_as<add_pointer_t<iter_reference_t<I>>>;
   >         };

   > > -2- Let `a` and `b` be dereferenceable iterators and `c` be a non-dereferenceable iterator of type `I` such that `b` is reachable from `a` and `c` is reachable from `b`, and let `D` be `iter_difference_t<I>`. The type `I` models `contiguous_iterator` only if

   > > 1. (2.1) — `to_address(a) == addressof(*a)`,

   > > 2. (2.2) — `to_address(b) == to_address(a) + D(b - a)`, and

   > > 3. (2.3) — `to_address(c) == to_address(a) + D(c - a)`.,

   > > 4. (2.?) — `ranges::iter_move(a)` has the same type, value category, and effects as `std::move(*a)`, and

   > > 5. (2.?) — if `ranges::iter_swap(a, b)` is well-formed, it has effects equivalent to `ranges::swap(*a, *b)`.

### 3610(i). `iota_view::size` sometimes rejects integer-class types

Section: 25.6.4.2 [range.iota.view] Status: C++23 Submitter: Jiang An Opened: 2021-09-29 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.iota.view].

View all issues with C++23 status.

Discussion:

It seems that the `iota_view` tends to accept integer-class types as its value types, by using `is-integer-like` or `is-signed-integer-like` through the specification, although it's unspecified whether any of them satisfies `weakly_incrementable`. However, the requires-clause of `iota_view::size` (25.6.4.2 [range.iota.view] p16) uses `(integral<W> && integral<Bound>)`, which sometimes rejects integer-class types.

Should we relax the restrictions by changing this part to `(is-integer-like<W> && is-integer-like<Bound>)`?

[2021-10-14; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.6.4.2 [range.iota.view] as indicated:

   >     constexpr auto size() const requires see below;

   > > -15- Effects: Equivalent to:

   > > >     if constexpr (is-integer-like<W> && is-integer-like<Bound>)
   > > >       return (value_ < 0)
   > > >         ? ((bound_ < 0)
   > > >           ? to-unsigned-like(-value_) - to-unsigned-like(-bound_)
   > > >           : to-unsigned-like(bound_) + to-unsigned-like(-value_))
   > > >         : to-unsigned-like(bound_) - to-unsigned-like(value_);
   > > >     else
   > > >       return to-unsigned-like(bound_ - value_);

   > > -16- Remarks: The expression in the requires-clause is equivalent to:

   > > >     (same_as<W, Bound> && advanceable<W>) || (integralis-integer-like<W> && integralis-integer-like<Bound>) ||
   > > >       sized_sentinel_for<Bound, W>

### 3612(i). Inconsistent pointer alignment in `std::format`

Section: 28.5.2.2 [format.string.std] Status: C++23 Submitter: Victor Zverovich Opened: 2021-10-02 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [format.string.std].

View all other issues in [format.string.std].

View all issues with C++23 status.

Discussion:

According to [tab:format.type.ptr] pointers are formatted as hexadecimal integers (at least in the common case when `uintptr_t` is available). However, it appears that they have left alignment by default according to [tab:format.align]:

> Forces the field to be aligned to the start of the available space. This is the default for non-arithmetic types, `charT`, and `bool`, unless an integer presentation type is specified.

because pointers are not arithmetic types.

For example:

>     void* p = …
>     std::format("{:#16x}", std::bit_cast<uintptr_t>(p));
>     std::format("{:16}", p);

may produce `" 0x7fff88716c84"` and `"0x7fff88716c84 "` (the actual output depends on the value of `p`).

This is inconsistent and clearly a bug in specification that should have included pointers together with arithmetic types in [tab:format.align].

[2021-10-14; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 28.5.2.2 [format.string.std], Table [tab:format.align], as indicated:

   > Table 59 — Meaning of align options [tab:format.align]
   > Option | Meaning
   > --- | ---
   > `<` | Forces the field to be aligned to the start of the available space. This is the default for non-arithmetic non-pointer types, `charT`, and `bool`, unless an integer presentation type is specified.
   > `>` | Forces the field to be aligned to the end of the available space. This is the default for arithmetic types other than `charT` and `bool`, pointer types or when an integer presentation type is specified.
   > `[…]`

   > [Drafting note: The wording above touches a similar area as LWG 3586(i). To help solving the merge conflict the following shows the delta of this proposed wording on top of the LWG 3586(i) merge result]

   > Table 59 — Meaning of align options [tab:format.align]
   > Option | Meaning
   > --- | ---
   > `<` | Forces the field to be aligned to the start of the available space. This is the default when the presentation type is a non-arithmetic non-pointer type.
   > `>` | Forces the field to be aligned to the end of the available space. This is the default when the presentation type is an arithmetic or pointer type.
   > `[…]`

### 3613(i). Specify that `nullopt_t` is copyable

Section: 22.5.5 [optional.nullopt] Status: Resolved Submitter: Frank Birbacher Opened: 2021-10-01 Last modified: 2026-06-15

Priority: 3

View all other issues in [optional.nullopt].

View all issues with Resolved status.

Discussion:

The Standard defines a number of types that are used to create overload disambiguators for constructors, like `nullopt_t` and `allocator_tag_t`. Values of such types are passed by value to such constructors to give it particular meaning. For pass-by-value these types need to be copy-constructible and for consistency should also be copy-assignable. Of those types the specification of `nullopt_t` doesn't clearly state that the type is copyable, 22.5.5 [optional.nullopt].

The reason that `nullopt_t` is defined differently from other such types is to avoid ambiguity in expressions that assign an empty brace initializer to an `optional`. The meaning of such assignment should be to engage the `optional` instead of taking the braces to create a temporary `nullopt_t` for assignment and thus reset the `optional`. The RHS of such assignment should be a temporary empty `optional` instead of a temporary `nullopt_t`.

Types that aren't affected: `nullptr_t` (fundamental type), `allocator_tag_t`, `piecewise_construct_t`, `in_place_t`, `in_place_type_t`, `in_place_index_t` (all basically defined as a `class T { explicit T() = default; }` which works fine for pass-by-value)

[2021-10-14; Reflector poll]

Set priority to 3 after reflector poll.

[Daniel commented:]

I would prefer to see the wording use `is_trivially_copy_constructible_v` and `trivially_copy_assignable_v`, which is consistent with similar usage of trivial-requirements in `std::optional`.

[Tim commented:]

We need to say that it models copyable and is trivially copyable (not sure if we need the latter but might as well - does anyone do it differently?). "has a copy constructor" isn't enough - `T(T&)` is a copy constructor.

[Brno 2026-06-12; Status changed: New → Resolved.]

LWG does not think it necessary to guarantee triviality in the standard. The remainder of the issue is covered by the resolution of LWG 4497(i).

Proposed resolution:

This wording is relative to N4892.

1. Modify 22.5.5 [optional.nullopt] as indicated:

   >     struct nullopt_t{see below};
   >     inline constexpr nullopt_t nullopt(unspecified);

   > > -1- The struct `nullopt_t` is an empty class type used as a unique type to indicate the state of not containing a value for `optional` objects. In particular, `optional<T>` has a constructor with `nullopt_t` as a single argument; this indicates that an optional object not containing a value shall be constructed.

   > > -2- Type `nullopt_t` shall not have a default constructor or an initializer-list constructor, and shall not be an aggregate, and shall have a copy constructor and a copy assignment operator, both shall be public and trivial.

### 3616(i). LWG 3498 seems to miss the non-member `swap` for `basic_syncbuf`

Section: 31.11.2.6 [syncstream.syncbuf.special] Status: C++23 Submitter: S. B. Tam Opened: 2021-10-07 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

LWG 3498(i) fixes the inconsistent `noexcept`-specifiers for member functions of `basic_syncbuf`, but the proposed resolution in LWG 3498(i) seems to miss the non-member `swap`, which also has inconsistent `noexcept`-specifier: 31.11.2.6 [syncstream.syncbuf.special] says it's `noexcept`, while 31.11.1 [syncstream.syn] says it's not.

Since the non-member `swap` and the member `swap` have equivalent effect, and LWG 3498 removes `noexcept` from the latter, I think it's pretty clear that the former should not be `noexcept`.

[2021-10-14; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

[2022-02-10; Jens comments]

The desired fix was applied editorially while applying

P2450 C++ Standard Library Issues to be moved in Virtual Plenary, Oct. 2021

Proposed resolution:

This wording is relative to N4892.

1. Modify 31.11.2.6 [syncstream.syncbuf.special] as indicated:

   >     template<class charT, class traits, class Allocator>
   >       void swap(basic_syncbuf<charT, traits, Allocator>& a,
   >                 basic_syncbuf<charT, traits, Allocator>& b) noexcept;

   > > -1- Effects: Equivalent to `a.swap(b)`.

### 3617(i). `function`/`packaged_task` deduction guides and deducing `this`

Section: 22.10.17.3.2 [func.wrap.func.con], 32.10.10.2 [futures.task.members] Status: C++23 Submitter: Barry Revzin Opened: 2021-10-09 Last modified: 2023-11-22

Priority: 2

View all other issues in [func.wrap.func.con].

View all issues with C++23 status.

Discussion:

With the adoption of deducing `this` (P0847), we can now create types whose call operator is an explicit object member function, which means that `decltype(&F::operator())` could have pointer-to-function type rather than pointer-to-member-function type. This means that the deduction guides for `std::function` (22.10.17.3.2 [func.wrap.func.con]) and `std::packaged_task` (32.10.10.2 [futures.task.members]) will simply fail:

>     struct F {
>       int operator()(this const F&) { return 42; }
>     };

>     std::function g = F{}; // error: decltype(&F::operator()) is not of the form R(G::*)(A...) cv &opt noexceptopt

We should update the deduction guides to keep them in line with the core language.

[2021-10-14; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4892.

> 1. Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:

>    >     template<class F> function(F) -> function<see below>;

>    > > -16- Constraints: `&F::operator()` is well-formed when treated as an unevaluated operand and `decltype(&F::operator())` is either of the form `R(G::*)(A...) cv &opt noexceptopt` or of the form `R(*)(G cv &opt, A...) noexceptopt` for a class type `G`.

>    > > -17- Remarks: The deduced type is `function<R(A...)>`.

> 2. Modify 32.10.10.2 [futures.task.members] as indicated:

>    >     template<class F> packaged_task(F) -> packaged_task<see below>;

>    > > -7- Constraints: `&F::operator()` is well-formed when treated as an unevaluated operand and `decltype(&F::operator())` is either of the form `R(G::*)(A...) cv &opt noexceptopt` or of the form `R(*)(G cv &opt, A...) noexceptopt` for a class type `G`.

>    > > -8- Remarks: The deduced type is `packaged_task<R(A...)>`.

[2021-10-17; Improved wording based on Tim Song's suggestion]

[2022-07-01; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:

   >     template<class F> function(F) -> function<see below>;

   > > -16- Constraints: `&F::operator()` is well-formed when treated as an unevaluated operand and `decltype(&F::operator())` is either of the form `R(G::*)(A...) cv &opt noexceptopt` or of the form `R(*)(G, A...) noexceptopt` for a class type `G`.

   > > -17- Remarks: The deduced type is `function<R(A...)>`.

2. Modify 32.10.10.2 [futures.task.members] as indicated:

   >     template<class F> packaged_task(F) -> packaged_task<see below>;

   > > -7- Constraints: `&F::operator()` is well-formed when treated as an unevaluated operand and `decltype(&F::operator())` is either of the form `R(G::*)(A...) cv &opt noexceptopt` or of the form `R(*)(G, A...) noexceptopt` for a class type `G`.

   > > -8- Remarks: The deduced type is `packaged_task<R(A...)>`.

### 3618(i). Unnecessary `iter_move` for `transform_view::iterator`

Section: 25.7.9.3 [range.transform.iterator] Status: C++23 Submitter: Barry Revzin Opened: 2021-10-12 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.transform.iterator].

View all issues with C++23 status.

Discussion:

`transform_view`'s iterator currently specifies a customization point for `iter_move`. This customization point does the same thing that the default implementation would do — `std::move(*E)` if `*E` is an lvalue, else `*E` — except that it is only there to provide the correct conditional `noexcept` specification. But for `iota_view`, we instead provide a `noexcept`-specifier on the iterator's `operator*`. We should do the same thing for both, and the simplest thing would be:

Change 25.7.9.3 [range.transform.iterator] to put the whole body into `noexcept`:

>     constexpr decltype(auto) operator*() const noexcept(noexcept(invoke(*parent_->fun_, *current_))) {
>       return invoke(*parent_->fun_, *current_);
>     }

And to remove this:

>     friend constexpr decltype(auto) iter_move(const iterator& i)
>       noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_))) {
>       if constexpr (is_lvalue_reference_v<decltype(*i)>)
>         return std::move(*i);
>       else
>         return *i;
>     }

[2022-01-29; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 25.7.9.3 [range.transform.iterator], class template `transform_view::iterator`, as indicated:

   >     […]
   >     constexpr decltype(auto) operator*() const noexcept(noexcept(invoke(*parent_->fun_, *current_))) {
   >       return invoke(*parent_->fun_, *current_);
   >     }
   >     […]
   >     friend constexpr decltype(auto) iter_move(const iterator& i)
   >       noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_))) {
   >       if constexpr (is_lvalue_reference_v<decltype(*i)>)
   >         return std::move(*i);
   >       else
   >         return *i;
   >     }
   >     […]

### 3619(i). Specification of `vformat_to` contains ill-formed `formatted_size` calls

Section: 28.5.5 [format.functions] Status: C++23 Submitter: Tim Song Opened: 2021-10-17 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [format.functions].

View all issues with C++23 status.

Discussion:

The specification of `vformat_to` says that it formats "into the range `[out, out + N)`, where `N` is `formatted_size(fmt, args...)` for the functions without a `loc` parameter and `formatted_size(loc, fmt, args...)` for the functions with a `loc` parameter".

This is wrong in at least two ways:

- First, `args` is a `(w)format_args`, not a pack, so it doesn't make sense to use `...` to expand it.

- Second, `fmt` is a `(w)string_view` parameter, and it is never a constant expression, and so the call is ill-formed after P2216 added compile-time format string checking.

[2022-01-29; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 28.5.5 [format.functions] as indicated:

   >     template<class Out>
   >       Out vformat_to(Out out, string_view fmt, format_args args);
   >     template<class Out>
   >       Out vformat_to(Out out, wstring_view fmt, wformat_args args);
   >     template<class Out>
   >       Out vformat_to(Out out, const locale& loc, string_view fmt, format_args args);
   >     template<class Out>
   >       Out vformat_to(Out out, const locale& loc, wstring_view fmt, wformat_args args);

   > > -12- Let `charT` be `decltype(fmt)::value_type`.

   > > -13- Constraints: `Out` satisfies `output_iterator<const charT&>`.

   > > -14- Preconditions: `Out` models `output_iterator<const charT&>`.

   > > -15- Effects: Places the character representation of formatting the arguments provided by `args`, formatted according to the specifications given in `fmt`, into the range `[out, out + N)`, where `N` is `formatted_size(fmt, args...)` for the functions without a `loc` parameter and `formatted_size(loc, fmt, args...)` for the functions with a `loc` parameterthe number of characters in that character representation. If present, `loc` is used for locale-specific formatting.

   > > -16- Returns: `out + N`.

   > > -17- […]

### 3621(i). Remove feature-test macro `__cpp_lib_monadic_optional`

Section: 17.3.2 [version.syn] Status: C++23 Submitter: Jens Maurer Opened: 2021-10-18 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++23 status.

Discussion:

P0798R8 "Monadic operations for `std::optional`" created a new feature-test macro `__cpp_lib_monadic_optional` for a relatively minor enhancement.

We should instead increment the value of the existing feature-test macro `__cpp_lib_optional`.

[2022-01-29; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 17.3.2 [version.syn] as indicated:

   >     […]
   >     #define __cpp_lib_monadic_optional 202110L // also in <optional>
   >     […]
   >     #define __cpp_lib_optional 202106L202110L // also in <optional>
   >     […]

### 3622(i). Misspecified transitivity of equivalence in §[unord.req.general]

Section: 23.2.8.1 [unord.req.general] Status: C++23 Submitter: Thomas Köppe Opened: 2021-10-20 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

The paper P2077R3 ("Heterogeneous erasure overloads for associative containers") adds a new variable `kx` with specific meaning for use in the Table of Unordered Associative Container Requirements, 23.2.8.1 [unord.req.general] p11, which is meant to stand for an equivalence class of heterogeneous values that can be compared with container keys.

One property required of `kx` is transitivity of equality/equivalence, but this is currently specified as:

> "`kx` is a value such that […] `(eq(r1, kx) && eq(r1, r2)) == eq(r2, kx)` […], where `r1` and `r2` are [any] keys".

But this doesn't seem right. Transitivity means that `eq(r1, kx) && eq(r1, r2)` being true implies `eq(r2, kx)` being true, but it does not imply that both sides are equal in general. In particular, `eq(r2, kx)` can be true even when `eq(r1, kx) && eq(r1, r2)` is false.

More abstractly, equality is transitive, but inequality is not.

The new wording appears to have been copied from the pre-existing wording for the variable "`ke`", which suffers from the same problem, and so we propose to fix both cases.

[2022-01-29; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N4901.

1. Modify 23.2.8.1 [unord.req.general] as indicated:

   > 1. […]

   > 2. (11.19) — `ke` is a value such that

   >    1. (11.19.1) — `eq(r1, ke) == eq(ke, r1)`,

   >    2. (11.19.2) — `hf(r1) == hf(ke)` if `eq(r1, ke)` is `true`, and

   >    3. (11.19.3) — `(eq(r1, ke) && eq(r1, r2)) == eq(r2, ke)``eq(ke, r2)` is `true` if `eq(ke, r1) && eq(r1, r2)` is `true`,

   >    where `r1` and `r2` are keys of elements in `a_tran`,

   > 3. (11.20) — `kx` is a value such that

   >    1. (11.20.1) — `eq(r1, kx) == eq(kx, r1)`,

   >    2. (11.20.2) — `hf(r1) == hf(kx)` if `eq(r1, kx)` is `true`,

   >    3. (11.20.3) — `(eq(r1, kx) && eq(r1, r2)) == eq(r2, kx)``eq(kx, r2)` is `true` if `eq(kx, r1) && eq(r1, r2)` is `true`, and

   >    4. (11.20.4) — `kx` is not convertible to either `iterator` or `const_iterator`,

   >    where `r1` and `r2` are keys of elements in `a_tran`,

   > 4. […]

[2022-02-07 Tim comments and provides updated wording]

For heterogeneous lookup on unordered containers to work properly, we need all keys comparing equal to the transparent key to be grouped together. Since the only keys guaranteed to be so grouped are the ones that are equal according to `eq`, we cannot allow `eq(r1, r2) == false` but `eq(r1, ke) == true && eq(r2, ke) == true`. The one-way transitivity of equality is insufficient.

We need both of the following:

- if `eq(r1, ke)` is `true` and `eq(r1, r2)` is `true` then `eq(r2, ke)` is `true`.
- if `eq(r1, ke)` is `true` and `eq(r2, ke)` is `true` then `eq(r1, r2)` is `true`

In a table:

eq(r1, ke) | eq(r1, r2) | eq(r2, ke) | OK?
--- | --- | --- | ---
T | T | T | Y
T | T | F | N
T | F | T | N
T | F | F | Y
F | T | T | N
F | T | F | Y
F | F | T | Y
F | F | F | Y

[Issaquah 2023-02-08; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 23.2.8.1 [unord.req.general] as indicated:

   > 1. […]

   > 2. (10.20) — `ke` is a value such that

   >    1. (10.20.1) — `eq(r1, ke) == eq(ke, r1)`,

   >    2. (10.20.2) — `hf(r1) == hf(ke)` if `eq(r1, ke)` is `true`, and

   >    3. (10.20.3) — `(eq(r1, ke) && eq(r1, r2)) == eq(r2, ke)`if any two of `eq(r1, ke)`, `eq(r2, ke)` and `eq(r1, r2)` are `true`, then all three are `true`,

   >    where `r1` and `r2` are keys of elements in `a_tran`,

   > 3. (10.21) — `kx` is a value such that

   >    1. (10.21.1) — `eq(r1, kx) == eq(kx, r1)`,

   >    2. (10.21.2) — `hf(r1) == hf(kx)` if `eq(r1, kx)` is `true`,

   >    3. (10.21.3) — `(eq(r1, kx) && eq(r1, r2)) == eq(r2, kx)`if any two of `eq(r1, kx)`, `eq(r2, kx)` and `eq(r1, r2)` are `true`, then all three are `true`, and

   >    4. (10.21.4) — `kx` is not convertible to either `iterator` or `const_iterator`,

   >    where `r1` and `r2` are keys of elements in `a_tran`,

   > 4. […]

### 3624(i). Inconsistency of `<typeinfo>`, `<initializer_list>`, and `<compare>` in the standard library

Section: 17.7 [support.rtti], 17.11 [support.initlist], 17.12 [cmp] Status: Resolved Submitter: Jiang An Opened: 2021-10-23 Last modified: 2025-11-11

Priority: 3

View all issues with Resolved status.

Discussion:

Standard library headers `<typeinfo>`, `<initializer_list>`, and `<compare>` are required for some core language features, as specified in 7.6.1.8 [expr.typeid]/7, 9.5.5 [dcl.init.list]/2, and 7.6.8 [expr.spaceship]/8. In C++11 (via N2930), every header that has dependency on `std::initializer_list` is required to include `<initializer_list>`. The similar requirements are added for `operator<=>` and `<compare>` in C++20 (via LWG 3330(i)).

As N2930 and LWG3330 have been adpoted, IMO there are some inconsistencies in the standard library now:

- No operation is done for `<typeinfo>`, although `<typeindex>` (`std::type_index`), `<functional>` (`std::function`, since C++11), and `<any>` (`std::any`) have dependency on `std::type_info`;

- `<iterator>` has dependency on `std::initializer_list` since C++14/LWG 2128(i) (the `std::rbegin` overload and its friends), but it is not required to include `<initializer_list>`;

- `<stacktrace>` is not required to include `<compare>` while it provides operator `<=>` overloads.

The situation may be quite serious for `std::type_index`. Perhaps no expected operation on `std::type_index` is guaranteed to work when only `<typeindex>` but not `<typeinfo>` is included.

libc++, libstdc++, and MSVC STL include `<typeinfo>` and `<initializer_list>` when the required standard interface depends on them. I think we should standardize the existing practice (except that `<stackstrace>` has not been implemented now) to reduce uncertainty for users.

[2021-10-24; Daniel comments]

This issue is related to and depending on LWG 3625(i).

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.

[2025-10-22; Expected to be resolved by P3016R6.]

[2025-11-11; Resolved by P3016R6, approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4901.

> [Drafting note: The proposed wording below contains one conditional change, it is therefore depending upon LWG 3625(i).]

1. Add `#include <typeinfo>` to 22.7.2 [any.synop], 22.10.2 [functional.syn], and 17.7.6 [type.index.synopsis].

2. Add `#include <initializer_list>` to 24.2 [iterator.synopsis].

3. Add `#include <compare>` to 19.6.2 [stacktrace.syn].

4. If we decide to add range access function templates (24.7 [iterator.range]) to `<stacktrace>`, we should also add `#include <initializer_list>` to 19.6.2 [stacktrace.syn].

### 3625(i). Should `<stacktrace>` provide range access function templates?

Section: 19.6.2 [stacktrace.syn], 24.7 [iterator.range] Status: Resolved Submitter: Jiang An Opened: 2021-10-23 Last modified: 2025-11-11

Priority: 3

View all other issues in [stacktrace.syn].

View all issues with Resolved status.

Discussion:

Range access function templates (24.7 [iterator.range]) are available in every standard header for containers. As `std::basic_stacktrace` provides some container-like interfaces (member functions `begin`, `end`, `size`, etc.), should we add these free function templates to `<stacktrace>` for consistency?

[2021-10-24; Daniel comments]

This issue is related to LWG 3624(i).

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.

[2025-10-07; Status updated New → Open]

This will be resolved by P3016R6. The `<initializer_list>` part is covered by 3624(i).

[2025-11-11; Resolved by P3016R6, approved in Kona. Status changed: Open → Resolved.]

Proposed resolution:

This wording is relative to N4901.

> [Drafting note: The proposed wording below contains one conditional change, it is therefore depending upon a decision]

1. Modify 24.7 [iterator.range] as indicated:

   > -1- In addition to being available via inclusion of the `<iterator>` header, the function templates in 24.7 [iterator.range] are available when any of the following headers are included: `<array>` (23.3.2 [array.syn]), `<deque>` (23.3.4 [deque.syn]), `<forward_list>` (23.3.6 [forward.list.syn]), `<list>` (23.3.10 [list.syn]), `<map>` (23.4.2 [associative.map.syn]), `<regex>` (28.6.3 [re.syn]), `<set>` (23.4.5 [associative.set.syn]), `<span>` (23.7.2.1 [span.syn]), `<stacktrace>` (19.6.2 [stacktrace.syn]), `<string>` (27.4.2 [string.syn]), `<string_view>` (27.3.2 [string.view.synop]), `<unordered_map>` (23.5.2 [unord.map.syn]), `<unordered_set>` (23.5.5 [unord.set.syn]), and `<vector>` (23.3.12 [vector.syn]). […]

2. If we decide that `<initializer_list>` should be included if the header has dependency on `std::initializer_list` (it may be introduce by `std::rbegin`, `std::data`, etc.), `#include <initializer_list>` should also be added to 19.6.2 [stacktrace.syn].

### 3627(i). Inconsistent specifications for `std::make_optional` overloads

Section: 22.5.10 [optional.specalg] Status: C++26 Submitter: Jiang An Opened: 2021-10-23 Last modified: 2026-06-09

Priority: 3

View all other issues in [optional.specalg].

View all issues with C++26 status.

Discussion:

Three `std::make_optional` overloads are specified in 22.5.10 [optional.specalg]. The first one is specified by "Returns:" and the other two are specified by "Effects: Equivalent to:". According to 16.3.2.4 [structure.specifications]/4, such uses of "Effects: Equivalent to:" propagate the Constraints specified for constructors. As the selected constructor for the first overload has "Constraints:" (22.5.3.2 [optional.ctor]/22), it seems that inconsistency is introduced here.

Existing implementations are inconsistent: libstdc++ constrains all three overloads, while libc++ and MSVC STL do not constrain any of them.

IMO all three overloads should be constrained.

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.

[2025-10-16; Status changed: New → Tentatively Ready.]

Reflector poll in 2024-07 with eight supporting votes.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 22.5.10 [optional.specalg] as indicated:

   >     template<class T> constexpr optional<decay_t<T>> make_optional(T&& v);

   > > -3- ReturnsEffects: Equivalent to: `return optional<decay_t<T>>(std::forward<T>(v));`.

### 3629(i). `make_error_code` and `make_error_condition` are customization points

Section: 19.5 [syserr] Status: C++23 Submitter: Jonathan Wakely Opened: 2021-10-31 Last modified: 2023-11-22

Priority: 2

View all other issues in [syserr].

View all issues with C++23 status.

Discussion:

The rule in 16.4.2.2 [contents] means that the calls to `make_error_code` in 19.5.4.2 [syserr.errcode.constructors] and 19.5.4.3 [syserr.errcode.modifiers] are required to call `std::make_error_code`, which means program-defined error codes do not work. The same applies to the `make_error_condition` calls in 19.5.5.2 [syserr.errcondition.constructors] and 19.5.5.3 [syserr.errcondition.modifiers].

They need to use ADL. This is what all known implementations (including Boost.System) do.

[2022-01-29; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4901.

> 1. Modify 19.5.2 [system.error.syn] as indicated:

>    > -1- The value of each `enum errc` constant shall be the same as the value of the `<cerrno>` macro shown in the above synopsis. Whether or not the `<system_error>` implementation exposes the `<cerrno>` macros is unspecified.

>    > -?- Invocations of `make_error_code` and `make_error_condition` shown in subclause 19.5 [syserr] select a function to call via overload resolution (12.2 [over.match]) on a candidate set that includes the lookup set found by argument dependent lookup (6.5.4 [basic.lookup.argdep]).

>    > -2- The `is_error_code_enum` and `is_error_condition_enum` templates may be specialized for program-defined types to indicate that such types are eligible for `class error_code` and `class error_condition` implicit conversions, respectively.

>    > [Note 1: Conversions from such types are done by program-defined overloads of `make_error_code` and `make_error_condition`, found by ADL. —end note]

[2022-08-25; Jonathan Wakely provides improved wording]

Discussed in LWG telecon and decided on new direction:

- Add `make_error_code` and `make_error_condition` to 16.4.2.2 [contents] as done for `swap`. Describe form of lookup used for them.
- Respecify `error_code` and `error_condition` constructors in terms of "Effects: Equivalent to" so that the requirements on program-defined overloads found by ADL are implied by those effects.

[2022-09-07; Jonathan Wakely revises wording]

Discussed in LWG telecon. Decided to change "established as-if by performing unqualified name lookup and argument-dependent lookup" to simply "established as-if by performing argument-dependent lookup".

This resolves the question of whether `std::make_error_code(errc)`, `std::make_error_code(io_errc)`, etc. should be visible to the unqualified name lookup. This affects whether a program-defined type that specializes `is_error_code_enum` but doesn't provide an overload of `make_error_code` should find the overloads in namespace `std` and consider them for overload resolution, via implicit conversion to `std::errc`, `std::io_errc`, etc.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

- Modify 16.4.2.2 [contents] as indicated:

  > -3- Whenever an unqualified name other than `swap`, `make_error_code`, or `make_error_condition` is used in the specification of a declaration `D` in 17 [support] through 32 [thread] or D [depr], its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of `D`.

  > [Note 1: Argument-dependent lookup is not performed. — end note]

  > Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of `D`.

  > [Example 1: The reference to `is_array_v` in the specification of `std::to_array` (23.3.3.6 [array.creation]) refers to `::std::is_array_v`. — end example]

  > [Note 2: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]

  > The meaning of the unqualified name `swap` is established in an overload resolution context for swappable values (16.4.4.3 [swappable.requirements]). The meanings of the unqualified names `make_error_code` and `make_error_condition` are established as-if by performing argument-dependent lookup (6.5.4 [basic.lookup.argdep]).

- Modify 19.5.4.2 [syserr.errcode.constructors] as indicated:

  >     error_code() noexcept;

  > -1- Postconditions: `val_ == 0` and `cat_ == &system_category()`.
  > -1- Effects: Initializes `val_` with `0` and `cat_` with `&system_category()`.

  >     error_code(int val, const error_category& cat) noexcept;

  > -2- Postconditions: `val_ == val` and `cat_ == &cat`.
  > -2- Effects: Initializes `val_` with `val` and `cat_` with `&cat`.

  >     template<class ErrorCodeEnum>
  >       error_code(ErrorCodeEnum e) noexcept;

  > -3- Constraints: `is_error_code_enum_v<ErrorCodeEnum>` is `true`.

  > -4- Postconditions: `*this == make_error_code(e)`.
  > -4- Effects: Equivalent to:

  > >     error_code ec = make_error_code(e);
  > >     assign(ec.value(), ec.category());

- Modify 19.5.4.3 [syserr.errcode.modifiers] as indicated:

  >     template<class ErrorCodeEnum>
  >       error_code& operator=(ErrorCodeEnum e) noexcept;

  > -2- Constraints: `is_error_code_enum_v<ErrorCodeEnum>` is `true`.

  > -3- Postconditions: `*this == make_error_code(e)`.
  > -3- Effects: Equivalent to:

  > >     error_code ec = make_error_code(e);
  > >     assign(ec.value(), ec.category());

  > -4- Returns: `*this`.

- Modify 19.5.5.2 [syserr.errcondition.constructors] as indicated:

  >     error_condition() noexcept;

  > -1- Postconditions: `val_ == 0` and `cat_ == &generic_category()`.
  > -1- Effects: Initializes `val_` with `0` and `cat_` with `&generic_category()`.

  >     error_condition(int val, const error_category& cat) noexcept;

  > -2- Postconditions: `val_ == val` and `cat_ == &cat`.
  > -2- Effects: Initializes `val_` with `val` and `cat_` with `&cat`.

  >     template<class ErrorConditionEnum>
  >       error_condition(ErrorConditionEnum e) noexcept;

  > -3- Constraints: `is_error_condition_enum_v<ErrorConditionEnum>` is `true`.

  > -4- Postconditions: `*this == make_error_condition(e)`.
  > -4- Effects: Equivalent to:

  > >     error_condition ec = make_error_condition(e);
  > >     assign(ec.value(), ec.category());

- Modify 19.5.5.3 [syserr.errcondition.modifiers] as indicated:

  >     template<class ErrorConditionEnum>
  >       error_condition& operator=(ErrorConditionEnum e) noexcept;

  > -2- Constraints: `is_error_condition_enum_v<ErrorConditionEnum>` is `true`.

  > -3- Postconditions: `*this == make_error_condition(e)`.
  > -3- Effects: Equivalent to:

  > >     error_condition ec = make_error_condition(e);
  > >     assign(ec.value(), ec.category());

  > -4- Returns: `*this`.

### 3631(i). `basic_format_arg(T&&)` should use `remove_cvref_t<T>` throughout

Section: 28.5.8.1 [format.arg] Status: C++23 Submitter: Tim Song Opened: 2021-11-03 Last modified: 2023-11-22

Priority: 3

View all other issues in [format.arg].

View all issues with C++23 status.

Discussion:

P2418R2 changed `basic_format_arg`'s constructor to take a forwarding reference but didn't change 28.5.8.1 [format.arg]/5 which inspects various properties of `T`. Now that the deduced type can be cvref-qualified, they need to be removed before the checks.

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.

Two suggestions to just change it to be `T&` because we don't need forwarding references here, and only accepting lvalues prevents forming dangling references.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4901.

> 1. Modify 28.5.8.1 [format.arg] as indicated:

>    >     template<class T> explicit basic_format_arg(T&& v) noexcept;

>    > > -?- Let `TD` be `remove_cvref_t<T>`.

>    > > -4- Constraints: The template specialization

>    > > >     typename Context::template formatter_type<remove_cvref_t<T>TD>

>    > > meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression

>    > > >     typename Context::template formatter_type<remove_cvref_t<T>TD>()
>    > > >       .format(declval<T&>(), declval<Context&>())

>    > > shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).

>    > > -5- Effects:

>    > > 1. (5.1) — if `TD` is `bool` or `char_type`, initializes `value` with `v`;

>    > > 2. (5.2) — otherwise, if `TD` is `char` and `char_type` is `wchar_t`, initializes `value` with `static_cast<wchar_t>(v)`;

>    > > 3. (5.3) — otherwise, if `TD` is a signed integer type (6.9.2 [basic.fundamental]) and `sizeof(TD) <= sizeof(int)`, initializes `value` with `static_cast<int>(v)`;

>    > > 4. (5.4) — otherwise, if `TD` is an unsigned integer type and `sizeof(TD) <= sizeof(unsigned int)`, initializes `value` with `static_cast<unsigned int>(v)`;

>    > > 5. (5.5) — otherwise, if `TD` is a signed integer type and `sizeof(TD) <= sizeof(long long int)`, initializes `value` with `static_cast<long long int>(v)`;

>    > > 6. (5.6) — otherwise, if `TD` is an unsigned integer type and `sizeof(TD) <= sizeof(unsigned long long int)`, initializes `value` with `static_cast<unsigned long long int>(v)`;

>    > > 7. (5.7) — otherwise, initializes `value` with `handle(v)`.

[2022-10-19; Jonathan provides improved wording]

During reflector prioritization it was pointed out that forwarding references are unnecessary (arguments are always lvalues), and so using `T&` would be simpler.

In order to resolve the overload ambiguities discussed in 3718(i) replace all unary constructor overloads with a single constructor that works for any formattable type.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 28.5.8.1 [format.arg] as indicated:

>    >     template<class T> explicit basic_format_arg(T&& v) noexcept;

>    > > -4- Constraints: `T` satisfies `formattable<char_type>`. The template specialization

>    > > >     typename Context::template formatter_type<remove_cvref_t<T>>

>    > > meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression

>    > > >     typename Context::template formatter_type<remove_cvref_t<T>>()
>    > > >       .format(declval<T&>(), declval<Context&>())

>    > > shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).

>    > > -?- Preconditions: If `decay_t<T>` is `char_type*` or `const char_type*`, `static_cast<const char_type*>(v)` points to a NTCTS (3.36 [defns.ntcts]).

>    > > -5- Effects: Let `TD` be `remove_const_t<T>`.

>    > > 1. (5.1) — if `TD` is `bool` or `char_type`, initializes `value` with `v`;

>    > > 2. (5.2) — otherwise, if `TD` is `char` and `char_type` is `wchar_t`, initializes `value` with `static_cast<wchar_t>(v)`;

>    > > 3. (5.3) — otherwise, if `TD` is a signed integer type (6.9.2 [basic.fundamental]) and `sizeof(TD) <= sizeof(int)`, initializes `value` with `static_cast<int>(v)`;

>    > > 4. (5.4) — otherwise, if `TD` is an unsigned integer type and `sizeof(TD) <= sizeof(unsigned int)`, initializes `value` with `static_cast<unsigned int>(v)`;

>    > > 5. (5.5) — otherwise, if `TD` is a signed integer type and `sizeof(TD) <= sizeof(long long int)`, initializes `value` with `static_cast<long long int>(v)`;

>    > > 6. (5.6) — otherwise, if `TD` is an unsigned integer type and `sizeof(TD) <= sizeof(unsigned long long int)`, initializes `value` with `static_cast<unsigned long long int>(v)`;

>    > > 7. (5.?) — otherwise, if `TD` is a standard floating-point type, initializes `value` with `v`;

>    > > 8. (5.?) — otherwise, if `TD` is a specialization of `basic_string_view` or `basic_string` and `TD::value_type` is `char_type`, initializes `value` with `basic_string_view<char_type>(v.data(), v.size())`;

>    > > 9. (5.?) — otherwise, if `decay_t<TD>` is `char_type*` or `const char_type*`, initializes `value` with `static_cast<const char_type*>(v)`;

>    > > 10. (5.?) — otherwise, if `is_void_v<remove_pointer_t<TD>>` is `true` or `is_null_pointer_v<TD>` is `true`, initializes `value` with `static_cast<const void*>(v)`;

>    > > 11. (5.7) — otherwise, initializes `value` with `handle(v)`.

>    > > -?- [Note: Constructing `basic_format_arg` from a pointer to a member is ill-formed unless the user provides an enabled specialization of `formatter` for that pointer to member type. — end note]

>    >     explicit basic_format_arg(float n) noexcept;
>    >     explicit basic_format_arg(double n) noexcept;
>    >     explicit basic_format_arg(long double n) noexcept;

>    > > -6- Effects: Initializes `value` with `n`.

>    >     explicit basic_format_arg(const char_type* s) noexcept;

>    > > -7- Preconditions: `s` points to a NTCTS (3.36 [defns.ntcts]).

>    > > -8- Effects: Initializes `value` with `s`.

>    >     template<class traits>>
>    >       explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;

>    > > -9- Effects: Initializes `value` with `basic_string_view<char_type>(s.data(), s.size())`.

>    >     template<class traits, class Allocator>>
>    >       explicit basic_format_arg(
>    >         const basic_string<char_type, traits, Allocator>& s) noexcept;

>    > > -10- Effects: Initializes `value` with `basic_string_view<char_type>(s.data(), s.size())`.

>    >     explicit basic_format_arg(nullptr_t) noexcept;

>    > > -11- Effects: Initializes `value` with `static_cast<const void*>(nullptr)`.

>    >     template<class T> explicit basic_format_arg(T* p) noexcept;

>    > > -12- Constraints: `is_void_v<T>` is `true`.

>    > > -13- Effects: Initializes `value` with `p`.

>    > > -14- [Note: Constructing `basic_format_arg` from a pointer to a member is ill-formed unless the user provides an enabled specialization of `formatter` for that pointer to member type. — end note]

> 2. Modify 28.5.8.1 [format.arg] p17 and p18 as indicated:

>    >     template<class T> explicit handle(T&& v) noexcept;

>    > > -17- Let

>    > > 1. — (17.1) `TD` be `remove_cvrefconst_t<T>`,

>    > > 2. — (17.2) `const-formattable` be `true` if `typename Context::template formatter_type<TD>().format(declval<const TD&>(), declval<Context&>())` is well-formed, otherwise `false`,

>    > > 3. — (17.3) `TQ` be `const TD` if `const-formattable` is `true` `const TD` satisfies `formattable<char_type>` and `TD` otherwise.

>    > > -18- Mandates: `const-formattable `formattable<const TD, char_type>` || !is_const_v<remove_reference_t<T>` is `true`.

>    > > -19- Effects: Initializes `ptr_` with `addressof(val)` and `format_` with

>    > > >     [](basic_format_parse_context<char_type>& parse_ctx,
>    > > >        Context& format_ctx, const void* ptr) {
>    > > >       typename Context::template formatter_type<TD> f;
>    > > >       parse_ctx.advance_to(f.parse(parse_ctx));
>    > > >       format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)),
>    > > >                                      format_ctx));
>    > > >     }

[2022-11-10; Jonathan provides improved wording]

[2022-11-29; Casey expands the issue to also cover `make_format_args`]

[2023-01-11; Jonathan adds the missing edits to the class synopsis]

[Issaquah 2023-02-07; LWG]

Edited proposed resolution to remove extra `=` in concept definition and capitialize start of (5.1). Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 28.5.6.3 [format.formattable] as indicated:

   -1- Let `fmt-iter-for<charT>` be an unspecified type that models `output_iterator<const charT&>` (24.3.4.10 [iterator.concept.output]).

   >       template<class T, class Context,
   >                class Formatter = typename Context::template formatter_type<remove_const_t<T>>>
   >         concept formattable-with =         // exposition only
   >           semiregular<Formatter> &&
   >             requires (Formatter& f, const Formatter& cf, T&& t, Context fc,
   >                      basic_format_parse_context<typename Context::char_type> pc)
   >            {
   >              { f.parse(pc) } -> same_as<typename decltype(pc)::iterator>;
   >              { cf.format(t, fc) } -> same_as<typename Context::iterator>;
   >            };

   >     template<class T, class charT>
   >       concept formattable =
   >         semiregular<formatter<remove_cvref_t<T>, charT>> &&
   >         requires(formatter<remove_cvref_t<T>, charT> f,
   >                  const formatter<remove_cvref_t<T>, charT> cf,
   >                  T t,
   >                  basic_format_context<fmt-iter-for<charT>, charT> fc,
   >                  basic_format_parse_context<charT> pc) {
   >             { f.parse(pc) } -> same_as<basic_format_parse_context<charT>::iterator>;
   >             { cf.format(t, fc) } -> same_as<fmt-iter-for<charT>>;
   >         };
   >         formattable-with<remove_reference_t<T>, basic_format_context<fmt-iter-for<charT>>>;

2. Modify 28.5.8.1 [format.arg] as indicated:

       namespace std {
         template<class Context>
         class basic_format_arg {
         public:
           class handle;

         private:
           using char_type = typename Context::char_type;                              // exposition only

           variant<monostate, bool, char_type,
                   int, unsigned int, long long int, unsigned long long int,
                   float, double, long double,
                   const char_type*, basic_string_view<char_type>,
                   const void*, handle> value;                                         // exposition only

           template<class T> explicit basic_format_arg(T&& v) noexcept;                // exposition only
           explicit basic_format_arg(float n) noexcept;                                // exposition only
           explicit basic_format_arg(double n) noexcept;                               // exposition only
           explicit basic_format_arg(long double n) noexcept;                          // exposition only
           explicit basic_format_arg(const char_type* s);                              // exposition only

           template<class traits>
             explicit basic_format_arg(
               basic_string_view<char_type, traits> s) noexcept;                       // exposition only

           template<class traits, class Allocator>
             explicit basic_format_arg(
               const basic_string<char_type, traits, Allocator>& s) noexcept;          // exposition only

           explicit basic_format_arg(nullptr_t) noexcept;                              // exposition only

           template<class T>
             explicit basic_format_arg(T* p) noexcept;                                 // exposition only

   …

   >     template<class T> explicit basic_format_arg(T&& v) noexcept;

   > > -4- Constraints: `T` satisfies `formattable-with<Context>`. The template specialization

   > > >     typename Context::template formatter_type<remove_cvref_t<T>>

   > > meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression

   > > >     typename Context::template formatter_type<remove_cvref_t<T>>()
   > > >       .format(declval<T&>(), declval<Context&>())

   > > shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).

   > > -?- Preconditions: If `decay_t<T>` is `char_type*` or `const char_type*`, `static_cast<const char_type*>(v)` points to a NTCTS (3.36 [defns.ntcts]).

   > > -5- Effects: Let `TD` be `remove_const_t<T>`.

   > > 1. (5.1) — If if `TD` is `bool` or `char_type`, initializes `value` with `v`;

   > > 2. (5.2) — otherwise, if `TD` is `char` and `char_type` is `wchar_t`, initializes `value` with `static_cast<wchar_t>(v)`;

   > > 3. (5.3) — otherwise, if `TD` is a signed integer type (6.9.2 [basic.fundamental]) and `sizeof(TD) <= sizeof(int)`, initializes `value` with `static_cast<int>(v)`;

   > > 4. (5.4) — otherwise, if `TD` is an unsigned integer type and `sizeof(TD) <= sizeof(unsigned int)`, initializes `value` with `static_cast<unsigned int>(v)`;

   > > 5. (5.5) — otherwise, if `TD` is a signed integer type and `sizeof(TD) <= sizeof(long long int)`, initializes `value` with `static_cast<long long int>(v)`;

   > > 6. (5.6) — otherwise, if `TD` is an unsigned integer type and `sizeof(TD) <= sizeof(unsigned long long int)`, initializes `value` with `static_cast<unsigned long long int>(v)`;

   > > 7. (5.?) — otherwise, if `TD` is a standard floating-point type, initializes `value` with `v`;

   > > 8. (5.?) — otherwise, if `TD` is a specialization of `basic_string_view` or `basic_string` and `TD::value_type` is `char_type`, initializes `value` with `basic_string_view<char_type>(v.data(), v.size())`;

   > > 9. (5.?) — otherwise, if `decay_t<TD>` is `char_type*` or `const char_type*`, initializes `value` with `static_cast<const char_type*>(v)`;

   > > 10. (5.?) — otherwise, if `is_void_v<remove_pointer_t<TD>>` is `true` or `is_null_pointer_v<TD>` is `true`, initializes `value` with `static_cast<const void*>(v)`;

   > > 11. (5.7) — otherwise, initializes `value` with `handle(v)`.

   > > -?- [Note: Constructing `basic_format_arg` from a pointer to a member is ill-formed unless the user provides an enabled specialization of `formatter` for that pointer to member type. — end note]

   >     explicit basic_format_arg(float n) noexcept;
   >     explicit basic_format_arg(double n) noexcept;
   >     explicit basic_format_arg(long double n) noexcept;

   > > -6- Effects: Initializes `value` with `n`.

   >     explicit basic_format_arg(const char_type* s) noexcept;

   > > -7- Preconditions: `s` points to a NTCTS (3.36 [defns.ntcts]).

   > > -8- Effects: Initializes `value` with `s`.

   >     template<class traits>>
   >       explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;

   > > -9- Effects: Initializes `value` with `basic_string_view<char_type>(s.data(), s.size())`.

   >     template<class traits, class Allocator>>
   >       explicit basic_format_arg(
   >         const basic_string<char_type, traits, Allocator>& s) noexcept;

   > > -10- Effects: Initializes `value` with `basic_string_view<char_type>(s.data(), s.size())`.

   >     explicit basic_format_arg(nullptr_t) noexcept;

   > > -11- Effects: Initializes `value` with `static_cast<const void*>(nullptr)`.

   >     template<class T> explicit basic_format_arg(T* p) noexcept;

   > > -12- Constraints: `is_void_v<T>` is `true`.

   > > -13- Effects: Initializes `value` with `p`.

   > > -14- [Note: Constructing `basic_format_arg` from a pointer to a member is ill-formed unless the user provides an enabled specialization of `formatter` for that pointer to member type. — end note]

3. Modify 28.5.8.1 [format.arg] p17 and p18 as indicated:

   >     template<class T> explicit handle(T&& v) noexcept;

   > > -17- Let

   > > 1. — (17.1) `TD` be `remove_cvrefconst_t<T>`,

   > > 2. — (17.2) `const-formattable` be `true` if `typename Context::template formatter_type<TD>().format(declval<const TD&>(), declval<Context&>())` is well-formed, otherwise `false`,

   > > 3. — (17.3) `TQ` be `const TD` if `const-formattable` is `true` `const TD` satisfies `formattable-with<Context>` and `TD` otherwise.

   > > -18- Mandates: `const-formattable || !is_const_v<remove_reference_t<T>>` is `true`. `TQ` satisfies `formattable-with<Context>`.

   > > -19- Effects: Initializes `ptr_` with `addressof(val)` and `format_` with

   > > >     [](basic_format_parse_context<char_type>& parse_ctx,
   > > >        Context& format_ctx, const void* ptr) {
   > > >       typename Context::template formatter_type<TD> f;
   > > >       parse_ctx.advance_to(f.parse(parse_ctx));
   > > >       format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)),
   > > >                                      format_ctx));
   > > >     }

4. Modify 28.5.8.2 [format.arg.store] p2 as indicated:

   >     template<class Context = format_context, class... Args>
   >       format-arg-store<Context, Args...> make_format_args(Args&&... fmt_args);

   > > -2- Preconditions: The type `typename Context::template formatter_type<remove_cvref_t<T`i`>>` meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]) for each `T`i in `Args`.

### 3632(i). `unique_ptr` "Mandates: This constructor is not selected by class template argument deduction"

Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2021-11-03 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [unique.ptr.single.ctor].

View all issues with C++23 status.

Discussion:

P1460R1 changed the wording for `unique_ptr`'s constructor `unique_ptr(pointer)`. In C++17, it said in [unique.ptr.single.ctor] p5:

>     explicit unique_ptr(pointer p) noexcept;

> > Preconditions: […]

> > Effects: […]

> > Postconditions: […]

> > Remarks: If `is_pointer_v<deleter_type>` is `true` or `is_default_constructible_v<deleter_type>` is `false`, this constructor shall not participate in overload resolution. If class template argument deduction would select the function template corresponding to this constructor, then the program is ill-formed.

In C++20, it says in [unique.ptr.single.ctor] p5:

>     explicit unique_ptr(pointer p) noexcept;

> > Constraints: `is_pointer_v<deleter_type>` is `false` and `is_default_constructible_v<deleter_type>` is `true`.

> > Mandates: This constructor is not selected by class template argument deduction.

> > Preconditions: […]

> > Effects: […]

> > Postconditions: […]

Normally, we use "Mandates:" for `static_assert`-like stuff, not just to indicate that some constructor doesn't contribute to CTAD. Both libstdc++ and Microsoft (and soon libc++, see LLVM issue) seem to agree about the intent of this wording: It's basically asking for the constructor to be implemented with a CTAD firewall, as

>     explicit unique_ptr(type_identity_t<pointer> p) noexcept;

and there is no actual `static_assert` corresponding to this "Mandates:" element. In particular, the following program is well-formed on all vendors:

>     // godbolt link
>     template<class T> auto f(T p) -> decltype(std::unique_ptr(p));
>     template<class T> constexpr bool f(T p) { return true; }
>     static_assert(f((int*)nullptr));

I claim that this is a confusing and/or wrong use of "Mandates:". My proposed resolution is simply to respecify the constructor as

>     explicit unique_ptr(type_identity_t<pointer> p) noexcept;

> > Constraints: `is_pointer_v<deleter_type>` is `false` and `is_default_constructible_v<deleter_type>` is `true`.

> > Preconditions: […]

> > Effects: […]

> > Postconditions: […]

with no Mandates: or Remarks: elements at all.

[2022-01-29; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 20.3.1.3.1 [unique.ptr.single.general], class template `unique_ptr` synopsis, as indicated:

   >     […]
   >     // 20.3.1.3.2 [unique.ptr.single.ctor], constructors
   >     constexpr unique_ptr() noexcept;
   >     explicit unique_ptr(type_identity_t<pointer> p) noexcept;
   >     unique_ptr(type_identity_t<pointer> p, see below d1) noexcept;
   >     unique_ptr(type_identity_t<pointer> p, see below d2) noexcept;
   >     […]

2. Modify 20.3.1.3.2 [unique.ptr.single.ctor] as indicated:

   >     explicit unique_ptr(type_identity_t<pointer> p) noexcept;

   > > -5- Constraints: `is_pointer_v<deleter_type>` is `false` and `is_default_constructible_v<deleter_type>` is `true`.

   > > -6- Mandates: This constructor is not selected by class template argument deduction (12.2.2.9 [over.match.class.deduct]).

   > > -7- Preconditions: […]

   > > -8- Effects: […]

   > > -9- Postconditions: […]

   >     unique_ptr(type_identity_t<pointer> p, const D& d) noexcept;
   >     unique_ptr(type_identity_t<pointer> p, remove_reference_t<D>&& d) noexcept;

   > > -10- Constraints: `is_constructible_v<D, decltype(d)>` is `true`.

   > > -11- Mandates: These constructors are not selected by class template argument deduction (12.2.2.9 [over.match.class.deduct]).

   > > -12- Preconditions: […]

   > > -13- Effects: […]

   > > -14- Postconditions: […]

   > > -15- Remarks: If `D` is a reference type, the second constructor is defined as deleted.

   > > -16- [Example 1: […] — end example]

### 3636(i). `formatter<T>::format` should be `const`-qualified

Section: 28.5.6.1 [formatter.requirements] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2021-11-11 Last modified: 2023-11-22

Priority: 1

View all other issues in [formatter.requirements].

View all issues with C++23 status.

Discussion:

In libc++ review, we've noticed that we don't understand the implications of 28.5.6.1 [formatter.requirements] bullet 3.1 and Table [tab:formatter.basic]: (emphasize mine):

> (3.1) — `f` is a value of type `F`,

> […]

> Table 70: BasicFormatter requirements [tab:formatter.basic]

> […]

> `f.parse(pc)` [must compile] […]

> `f.format(u, fc)` [must compile] […]

According to Victor Zverovich, his intent was that `f.parse(pc)` should modify the state of `f`, but `f.format(u, fc)` should merely read `f`'s state to support format string compilation where formatter objects are immutable and therefore the `format` function must be `const`-qualified.

That is, a typical formatter should look something like this (modulo errors introduced by me):

>     struct WidgetFormatter {
>       auto parse(std::format_parse_context&) -> std::format_parse_context::iterator;
>       auto format(const Widget&, std::format_context&) const -> std::format_context::iterator;
>     };

However, this is not reflected in the wording, which treats `parse` and `format` symmetrically. Also, there is at least one example that shows a non-const `format` method:

>     template<> struct std::formatter<color> : std::formatter<const char*> {
>       auto format(color c, format_context& ctx) {
>         return formatter<const char*>::format(color_names[c], ctx);
>       }
>     };

Victor writes:

> Maybe we should […] open an LWG issue clarifying that all standard formatters have a `const` format function.

I'd like to be even more assertive: Let's open an LWG issue clarifying that all formatters must have a `const` format function!

[2022-01-30; Reflector poll]

Set priority to 1 after reflector poll.

[2022-08-24 Approved unanimously in LWG telecon.]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 28.5.6.1 [formatter.requirements] as indicated:

   > [Drafting note: It might also be reasonable to do a drive-by clarification that when the Table 70 says "Stores the parsed format specifiers in `*this`," what it actually means is "Stores the parsed format specifiers in `g`." (But I don't think anyone's seriously confused by that wording.)

   > -3- Given character type `charT`, output iterator type `Out`, and formatting argument type `T`, in Table 70 and Table 71:

   > 1. (3.1) — `f` is a value of type (possibly `const`) `F`,

   > 2. (3.?) — `g` is an lvalue of type `F`,

   > 3. (3.2) — `u` is an lvalue of type `T`,

   > 4. (3.3) — `t` is a value of a type convertible to (possibly `const`) `T`,

   > 5. […]

   > […]

   > > Table 70: Formatter requirements [tab:formatter]
   > > Expression | Return type | Requirement
   > > --- | --- | ---
   > > `fg.parse(pc)` | `PC::iterator` | […] Stores the parsed format specifiers in `*this` and returns an iterator past the end of the parsed range.
   > > `…`

2. Modify 28.5.6.4 [format.formatter.spec] as indicated:

   > -6- An enabled specialization `formatter<T, charT>` meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]).

   > [Example 1:

   > >     #include <format>

   > >     enum color { red, green, blue };
   > >     const char* color_names[] = { "red", "green", "blue" };

   > >     template<> struct std::formatter<color> : std::formatter<const char*> {
   > >       auto format(color c, format_context& ctx) const {
   > >         return formatter<const char*>::format(color_names[c], ctx);
   > >       }
   > >     };

   > >     […]

   > — end example]

3. Modify 28.5.6.7 [format.context] as indicated:

   >     void advance_to(iterator it);

   > > -8- Effects: Equivalent to: `out_ = std::move(it);`

   > > [Example 1:

   > > >     struct S { int value; };

   > > >     template<> struct std::formatter<S> {
   > > >       size_t width_arg_id = 0;

   > > >       // Parses a width argument id in the format { digit }.
   > > >       constexpr auto parse(format_parse_context& ctx) {
   > > >         […]
   > > >       }

   > > >       // Formats an S with width given by the argument width_arg_id.
   > > >       auto format(S s, format_context& ctx) const {
   > > >         int width = visit_format_arg([](auto value) -> int {
   > > >           if constexpr (!is_integral_v<decltype(value)>)
   > > >             throw format_error("width is not integral");
   > > >           else if (value < 0 || value > numeric_limits<int>::max())
   > > >             throw format_error("invalid width");
   > > >           else
   > > >             return value;
   > > >           }, ctx.arg(width_arg_id));
   > > >         return format_to(ctx.out(), "{0:x<{1}}", s.value, width);
   > > >       }
   > > >     };

   > > >     […]

   > > — end example]

4. Modify 30.12 [time.format] as indicated:

   >     template<class Duration, class charT>
   >     struct formatter<chrono::local-time-format-t<Duration>, charT>;

   > > -15- Let `f` be […]

   > > -16- Remarks: […]

   > >     template<class Duration, class TimeZonePtr, class charT>
   > >     struct formatter<chrono::zoned_time<Duration, TimeZonePtr>, charT>
   > >       : formatter<chrono::local-time-format-t<Duration>, charT> {
   > >       template<class FormatContext>
   > >         typename FormatContext::iterator
   > >           format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const;
   > >     };

   >     template<class FormatContext>
   >       typename FormatContext::iterator
   >         format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const;

   > > -17- Effects: Equivalent to:

   > > >     sys_info info = tp.get_info();
   > > >     return formatter<chrono::local-time-format-t<Duration>, charT>::
   > > >              format({tp.get_local_time(), &info.abbrev, &info.offset}, ctx);

### 3638(i). `vector<bool>::swap(reference, reference)` is useless

Section: 23.3.14 [vector.bool] Status: Resolved Submitter: Jonathan Wakely Opened: 2021-11-12 Last modified: 2025-11-11

Priority: 3

View all other issues in [vector.bool].

View all issues with Resolved status.

Discussion:

`vector<bool>` provides a static member function that can be used to swap rvalues of type `vector<bool>::reference` like so:

>     vector<bool> v{true, false};
>     vector<bool>::swap(v[0], v[1]);

This is not useful. Nobody calls `swap` like that. This fails to make `v[0]` swappable with `v[1]` as per 16.4.4.3 [swappable.requirements]. The similar SGI STL `bit_vector` class that `vector<bool>` is partially inspired by has a "global function" with the same signature, described as:

> "Swaps the bits referred to by `x` and `y`. This is a global function, not a member function. It is necessary because the ordinary version of `swap` takes arguments of type `T&`, and `bit_vector::reference` is a class, not a built-in C++ reference."

For some reason this became a static member function of `vector<bool>` in the C++ standard.

We should restore the intended functionality, and deprecate the useless function.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4901.

> 1. Create a new subclause [vector.bool.general] below 23.3.14 [vector.bool] and move paragraphs p1-p3 (including the class template `vector<bool, Allocator>` partial specialization synopsis) into that subclause.

> 2. Add to the synopsis in [vector.bool.general] p1 (née 23.3.14 [vector.bool] p1):

>    >     […]
>    >     // bit reference
>    >     class reference {
>    >       friend class vector;
>    >       constexpr reference() noexcept;
>    >     public:
>    >       constexpr reference(const reference&) = default;
>    >       constexpr ~reference();
>    >       constexpr operator bool() const noexcept;
>    >       constexpr reference& operator=(bool x) noexcept;
>    >       constexpr reference& operator=(const reference& x) noexcept;
>    >       constexpr const reference& operator=(bool x) const noexcept;
>    >       constexpr void flip() noexcept; // flips the bit

>    >       friend constexpr void swap(reference x, reference y) noexcept;
>    >       friend constexpr void swap(reference x, bool& y) noexcept;
>    >       friend constexpr void swap(bool& x, reference y) noexcept;
>    >     };
>    >     […]

> 3. Remove the static `swap` function from the class template `vector<bool, Allocator>` partial specialization synopsis:

>    >     […]
>    >     constexpr void swap(vector&);
>    >     constexpr static void swap(reference x, reference y) noexcept;
>    >     constexpr void flip() noexcept; // flips all bits
>    >     […]

> 4. Create a new subclause [vector.bool.ref] after p3, with p4 as its first paragraph, and add after it:

>    > 22.3.12.? Class `vector<bool, Allocator>::reference` [vector.bool.ref]

>    > -1- `reference` is a class that simulates the behavior of references of a single bit in `vector<bool>`. The conversion function returns `true` when the bit is set, and `false` otherwise. The assignment operators set the bit when the argument is (convertible to) `true` and clear it otherwise. `flip` reverses the state of the bit.

>    >     constexpr void flip() noexcept;

>    > > -?- Effects: `*this = !*this;`

>    >      friend constexpr void swap(reference x, reference y) noexcept;
>    >      friend constexpr void swap(reference x, bool& y) noexcept;
>    >      friend constexpr void swap(bool& x, reference y) noexcept;

>    > > -?- Effects: Exchanges the contents of `x` and `y` as if by:

>    > > >     bool b = x;
>    > > >     x = y;
>    > > >     y = b;

> 5. Create a new subclause [vector.bool.mem] after that, containing the paragraphs describing `flip()` and the `hash` specialization:

>    > 22.3.12.? Class `vector<bool, Allocator>` members [vector.bool.mem]

>    >     constexpr void flip() noexcept;

>    > > -1- Effects: Replaces each element in the container with its complement.

>    >      constexpr static void swap(reference x, reference y) noexcept;

>    > > -6- Effects: Exchanges the contents of `x` and `y` as if by:

>    > > >     bool b = x;
>    > > >     x = y;
>    > > >     y = b;

>    >     template<class Allocator> struct hash<vector<bool, Allocator>>;

>    > > -7- The specialization is enabled (22.10.19 [unord.hash]).

> 6. Create a new subclause [depr.vector.bool.swap] after [depr.string.capacity]

>    > D.? Deprecated `vector<bool, Allocator>` swap [depr.vector.bool.swap]

>    > -?- The following member is declared in addition to those members specified in 23.3.14 [vector.bool]:

>    >     namespace std {
>    >       template<class Allocator> class vector<bool, Allocator> {
>    >       public:
>    >         constexpr static void swap(reference x, reference y) noexcept;
>    >       };
>    >     }

>    >     constexpr static void swap(reference x, reference y) noexcept;

>    > > -?- Effects: `swap(x, y)`.

[2022-01-22; Jonathan replaces `swap(x, y)` in the Annex D wording, following reflector discussion about lookup for `swap` finding itself in that context. ]

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N4901.

1. Create a new subclause [vector.bool.general] below 23.3.14 [vector.bool] and move paragraphs p1-p3 (including the class template `vector<bool, Allocator>` partial specialization synopsis) into that subclause.

2. Add to the synopsis in [vector.bool.general] p1 (née 23.3.14 [vector.bool] p1):

   >     […]
   >     // bit reference
   >     class reference {
   >       friend class vector;
   >       constexpr reference() noexcept;
   >     public:
   >       constexpr reference(const reference&) = default;
   >       constexpr ~reference();
   >       constexpr operator bool() const noexcept;
   >       constexpr reference& operator=(bool x) noexcept;
   >       constexpr reference& operator=(const reference& x) noexcept;
   >       constexpr const reference& operator=(bool x) const noexcept;
   >       constexpr void flip() noexcept; // flips the bit

   >       friend constexpr void swap(reference x, reference y) noexcept;
   >       friend constexpr void swap(reference x, bool& y) noexcept;
   >       friend constexpr void swap(bool& x, reference y) noexcept;
   >     };
   >     […]

3. Remove the static `swap` function from the class template `vector<bool, Allocator>` partial specialization synopsis:

   >     […]
   >     constexpr void swap(vector&);
   >     constexpr static void swap(reference x, reference y) noexcept;
   >     constexpr void flip() noexcept; // flips all bits
   >     […]

4. Create a new subclause [vector.bool.ref] after p3, with p4 as its first paragraph, and add after it:

   > 22.3.12.? Class `vector<bool, Allocator>::reference` [vector.bool.ref]

   > -1- `reference` is a class that simulates the behavior of references of a single bit in `vector<bool>`. The conversion function returns `true` when the bit is set, and `false` otherwise. The assignment operators set the bit when the argument is (convertible to) `true` and clear it otherwise. `flip` reverses the state of the bit.

   >     constexpr void flip() noexcept;

   > > -?- Effects: `*this = !*this;`

   >      friend constexpr void swap(reference x, reference y) noexcept;
   >      friend constexpr void swap(reference x, bool& y) noexcept;
   >      friend constexpr void swap(bool& x, reference y) noexcept;

   > > -?- Effects: Exchanges the contents of `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

5. Create a new subclause [vector.bool.mem] after that, containing the paragraphs describing `flip()` and the `hash` specialization:

   > 22.3.12.? Class `vector<bool, Allocator>` members [vector.bool.mem]

   >     constexpr void flip() noexcept;

   > > -1- Effects: Replaces each element in the container with its complement.

   >      constexpr static void swap(reference x, reference y) noexcept;

   > > -6- Effects: Exchanges the contents of `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

   >     template<class Allocator> struct hash<vector<bool, Allocator>>;

   > > -7- The specialization is enabled (22.10.19 [unord.hash]).

6. Create a new subclause [depr.vector.bool.swap] after [depr.string.capacity]

   > D.? Deprecated `vector<bool, Allocator>` swap [depr.vector.bool.swap]

   > -?- The following member is declared in addition to those members specified in 23.3.14 [vector.bool]:

   >     namespace std {
   >       template<class Allocator> class vector<bool, Allocator> {
   >       public:
   >         constexpr static void swap(reference x, reference y) noexcept;
   >       };
   >     }

   >     constexpr static void swap(reference x, reference y) noexcept;

   > > -?- Effects: Exchanges the contents of `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

[2024-08-21; Jonathan provides improved wording]

Rebase on the current draft, change "exchanges the contents" to "exchanges the denoted values", and don't split the subclause into new subclauses.

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

1. Add to the synopsis in 23.3.14.1 [vector.bool.pspc] p1:

   >     […]
   >     // bit reference
   >     class reference {
   >       friend class vector;
   >       constexpr reference() noexcept;

   >     public:
   >       constexpr reference(const reference&) = default;
   >       constexpr ~reference();
   >       constexpr operator bool() const noexcept;
   >       constexpr reference& operator=(bool x) noexcept;
   >       constexpr reference& operator=(const reference& x) noexcept;
   >       constexpr const reference& operator=(bool x) const noexcept;
   >       constexpr void flip() noexcept;   // flips the bit

   >       friend constexpr void swap(reference x, reference y) noexcept;
   >       friend constexpr void swap(reference x, bool& y) noexcept;
   >       friend constexpr void swap(bool& x, reference y) noexcept;
   >     };
   >     […]

2. Remove the static `swap` function from the same synopsis:

   >     […]
   >     constexpr void swap(vector&)
   >       noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);
   >     static constexpr void swap(reference x, reference y) noexcept;
   >     constexpr void flip() noexcept;    // flips all bits
   >     constexpr void clear() noexcept;
   >     […]

3. Modify the paragraphs below the synopsis as shown:

   > -4- `reference` is a class that simulates the behavior of references of a single bit in `vector<bool>`. The conversion function returns `true` when the bit is set, and `false` otherwise. The assignment operators set the bit when the argument is (convertible to) converts to `true` and clear it otherwise. `flip` reverses the state of the bit.

   >     constexpr void reference::flip() noexcept;

   > > -?- Effects: `*this = !*this;`

   >      constexpr void swap(reference x, reference y) noexcept;
   >      constexpr void swap(reference x, bool& y) noexcept;
   >      constexpr void swap(bool& x, reference y) noexcept;

   > > -?- Effects: Exchanges the values denoted by `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

   >     constexpr void flip() noexcept;

   > > -1- Effects: Replaces each element in the container with its complement.

   >      constexpr static void swap(reference x, reference y) noexcept;

   > > -6- Effects: Exchanges the contents of `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

   >     template<class Allocator> struct hash<vector<bool, Allocator>>;

   > > -7- The specialization is enabled (22.10.19 [unord.hash]).

4. Create a new subclause [depr.vector.bool.swap] after D.21 [depr.format]

   > D.? Deprecated `vector<bool, Allocator>` swap [depr.vector.bool.swap]

   > -?- The following member is declared in addition to those members specified in 23.3.14 [vector.bool]:

   >     namespace std {
   >       template<class Allocator> class vector<bool, Allocator> {
   >       public:
   >         static constexpr void swap(reference x, reference y) noexcept;
   >       };
   >     }

   >     static constexpr void swap(reference x, reference y) noexcept;

   > > -?- Effects: Exchanges the values denoted by `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

[2025-02-07; Jonathan provides improved wording]

Add `swap` for `bitset::reference`, as proposed in LWG 4187(i).

[2025-11-11; Resolved by P3612R1, approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 22.9.2.1 [template.bitset.general] as indicated:

   >     namespace std {
   >       template<size_t N> class bitset {
   >       public:
   >         // bit reference
   >         class reference {
   >         public:
   >           constexpr reference(const reference&) = default;
   >           constexpr ~reference();
   >           constexpr reference& operator=(bool x) noexcept;           // for b[i] = x;
   >           constexpr reference& operator=(const reference&) noexcept; // for b[i] = b[j];
   >           constexpr bool operator~() const noexcept;                 // flips the bit
   >           constexpr operator bool() const noexcept;                  // for x = b[i];
   >           constexpr reference& flip() noexcept;                      // for b[i].flip();
   >           friend constexpr void swap(reference x, reference y) noexcept;
   >           friend constexpr void swap(reference x, bool& y) noexcept;
   >           friend constexpr void swap(bool& x, reference y) noexcept;
   >         };
   >         […]
   >       };
   >       […]
   >     }

2. Add to the synopsis in 23.3.14.1 [vector.bool.pspc] p1:

   >     […]
   >     // bit reference
   >     class reference {
   >       friend class vector;
   >       constexpr reference() noexcept;

   >     public:
   >       constexpr reference(const reference&) = default;
   >       constexpr ~reference();
   >       constexpr operator bool() const noexcept;
   >       constexpr reference& operator=(bool x) noexcept;
   >       constexpr reference& operator=(const reference& x) noexcept;
   >       constexpr const reference& operator=(bool x) const noexcept;
   >       constexpr void flip() noexcept;   // flips the bit

   >       friend constexpr void swap(reference x, reference y) noexcept;
   >       friend constexpr void swap(reference x, bool& y) noexcept;
   >       friend constexpr void swap(bool& x, reference y) noexcept;
   >     };
   >     […]

3. Remove the static `swap` function from the same synopsis:

   >     […]
   >     constexpr void swap(vector&)
   >       noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);
   >     static constexpr void swap(reference x, reference y) noexcept;
   >     constexpr void flip() noexcept;    // flips all bits
   >     constexpr void clear() noexcept;
   >     […]

4. Modify the paragraphs below the synopsis as shown:

   > -4- `reference` is a class that simulates the behavior of references of a single bit in `vector<bool>`. The conversion function returns `true` when the bit is set, and `false` otherwise. The assignment operators set the bit when the argument is (convertible to) converts to `true` and clear it otherwise. `flip` reverses the state of the bit.

   >     constexpr void reference::flip() noexcept;

   > > -?- Effects: `*this = !*this;`

   >      constexpr void swap(reference x, reference y) noexcept;
   >      constexpr void swap(reference x, bool& y) noexcept;
   >      constexpr void swap(bool& x, reference y) noexcept;

   > > -?- Effects: Exchanges the values denoted by `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

   >     constexpr void flip() noexcept;

   > > -1- Effects: Replaces each element in the container with its complement.

   >      constexpr static void swap(reference x, reference y) noexcept;

   > > -6- Effects: Exchanges the contents of `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

   >     template<class Allocator> struct hash<vector<bool, Allocator>>;

   > > -7- The specialization is enabled (22.10.19 [unord.hash]).

5. Create a new subclause [depr.vector.bool.swap] after D.21 [depr.format]

   > D.? Deprecated `vector<bool, Allocator>` swap [depr.vector.bool.swap]

   > -?- The following member is declared in addition to those members specified in 23.3.14 [vector.bool]:

   >     namespace std {
   >       template<class Allocator> class vector<bool, Allocator> {
   >       public:
   >         static constexpr void swap(reference x, reference y) noexcept;
   >       };
   >     }

   >     static constexpr void swap(reference x, reference y) noexcept;

   > > -?- Effects: Exchanges the values denoted by `x` and `y` as if by:

   > > >     bool b = x;
   > > >     x = y;
   > > >     y = b;

### 3639(i). Handling of fill character width is underspecified in `std::format`

Section: 28.5.2.2 [format.string.std] Status: Resolved Submitter: Victor Zverovich Opened: 2021-11-13 Last modified: 2023-03-23

Priority: 3

View other active issues in [format.string.std].

View all other issues in [format.string.std].

View all issues with Resolved status.

Discussion:

28.5.2.2 [format.string.std] doesn't specify if implementations should consider the estimated width of the fill character when substituting it into the formatted results.

For example:

>     auto s = std::format("{:🤡>10}", 42);

`"🤡"` (U+1F921) is a single code point but its estimated display width is two.

There are at least three possible resolutions:

1. `s == "🤡🤡🤡🤡42"`: use the estimated display width, correctly displayed on compatible terminals.

2. `s == "🤡🤡🤡🤡🤡🤡🤡🤡42"`: assume the display width of 1, incorrectly displayed.

3. Require the fill character to have the estimated width of 1.

[2021-11-14; Daniel comments]

Resolving this issue should be harmonized with resolving LWG 3576(i).

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll. Sent to SG16.

[2023-01-11; LWG telecon]

P2572 would resolve this issue and LWG 3576(i).

[2023-03-22 Resolved by the adoption of P2572R1 in Issaquah. Status changed: SG16 → Resolved.]

Proposed resolution:

### 3643(i). Missing `constexpr` in `std::counted_iterator`

Section: 24.5.7.5 [counted.iter.nav] Status: C++23 Submitter: Jiang An Opened: 2021-11-21 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

One overload of `std::counted_operator::operator++` is not `constexpr` currently, which is seemly because of that a try-block (specified in 24.5.7.5 [counted.iter.nav]/4) is not allowed in a constexpr function until C++20. Given a try-block is allowed in a constexpr function in C++20, IMO this overload should also be `constexpr`.

MSVC STL has already added `constexpr` at first. The situation of this overload is originally found by Casey Carter, but no LWG issue has been submitted.

[2022-01-30; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 24.5.7.1 [counted.iterator], class template `counted_iterator` synopsis, as indicated:

   >     […]
   >     constexpr counted_iterator& operator++();
   >     constexpr decltype(auto) operator++(int);
   >     constexpr counted_iterator operator++(int)
   >       requires forward_iterator<I>;
   >     constexpr counted_iterator& operator--()
   >       requires bidirectional_iterator<I>;
   >     constexpr counted_iterator operator--(int)
   >       requires bidirectional_iterator<I>;
   >     […]

2. Modify 24.5.7.5 [counted.iter.nav] as indicated:

   >     constexpr decltype(auto) operator++(int);

   > > -3- Preconditions: `length > 0`.

   > > -4- Effects: Equivalent to:

   > > >     --length;
   > > >     try { return current++; }
   > > >     catch(...) { ++length; throw; }

### 3645(i). `resize_and_overwrite` is overspecified to call its callback with lvalues

Section: 27.4.3.5 [string.capacity] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2021-11-28 Last modified: 2023-11-22

Priority: 2

View all other issues in [string.capacity].

View all issues with C++23 status.

Discussion:

27.4.3.5 [string.capacity] p7 says:

- [Let] `OP` be the expression `std::move(op)(p, n)`.

- [Precondition:] `OP` does not throw an exception or modify `p` or `n`.

Notice that `p` and `n` above are lvalue expressions.

Discussed with Mark Zeren, Casey Carter, Jonathan Wakely. We observe that:

A. This wording requires vendors to reject

>     s.resize_and_overwrite(100, [](char*&&, size_t&&){ return 0; });

which is surprising.

B. This wording requires vendors to accept

>     s.resize_and_overwrite(100, [](char*&, size_t&){ return 0; });

which is even more surprising, and also threatens to allow the user to corrupt the internal state (which is why we need to specify the Precondition above).

C. A user who writes

>     s.resize_and_overwrite(100, [](auto&&, auto&&){ return 0; });

can detect that they're being passed lvalues instead of rvalues. If we change the wording to permit implementations to pass either lvalues or rvalues (their choice), then this will be detectable by the user, so we don't want that if we can help it.

1. X. We want to enable implementations to say `move(op)(__p, __n)` and then use `__p` and `__n`.

2. Y. We have one implementation which wants to say `move(op)(data(), __n)`, which is not currently allowed, but arguably should be.

3. Z. We have to do or say something about disallowing writes to any internal state to which `Op` might get a reference.

Given all of this, Mark and Arthur think that the simplest way out is to say that the arguments are prvalues. It prevents X, but fixes the surprises in A, B, Y, Z. We could do this in the Let bullets. Either like so:

- [Let] `p` be a prvalue of type `charT*` …

- `m` be a prvalue of type `size_type` equal to `n`,

- `OP` be the expression `std::move(op)(p, m)`.

or (Arthur's preference) by specifying prvalues in the expression `OP` itself:

- [Let] `OP` be the expression `std::move(op)(auto(p), auto(n))`.

No matter which specification approach we adopt, we can also simplify the Preconditions bullet to:

- [Precondition:] `OP` does not throw an exception.

because once the user is receiving prvalue copies, it will no longer be physically possible for the user to modify the library's original variables `p` and `n`.

[2021-11-29; Arthur O'Dwyer provides wording]

[2022-01-30; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4901.

> 1. Modify 27.4.3.5 [string.capacity] as indicated:

>    >     template<class Operation> constexpr void resize_and_overwrite(size_type n, Operation op);

>    > > -7- Let

>    > > 1. (7.1) — `o = size()` before the call to `resize_and_overwrite`.

>    > > 2. (7.2) — `k` be `min(o, n)`.

>    > > 3. (7.3) — `p` be a `charT*`, such that the range `[p, p + n]` is valid and `this->compare(0, k, p, k) == 0` is `true` before the call. The values in the range `[p + k, p + n]` may be indeterminate (6.8.5 [basic.indet]).

>    > > 4. (7.4) — `OP` be the expression `std::move(op)(auto(p), auto(n))`.

>    > > 5. (7.5) — `r = OP`.

>    > > -8- Mandates: `OP` has an integer-like type (24.3.4.4 [iterator.concept.winc]).

>    > > -9- Preconditions:

>    > > 1. (9.1) — `OP` does not throw an exception or modify `p` or `n`.

>    > > 2. (9.2) — `r ≥ 0`.

>    > > 3. (9.3) — `r ≤ n`.

>    > > 4. (9.4) — After evaluating `OP` there are no indeterminate values in the range `[p, p + r)`.

>    > > -10- Effects: Evaluates `OP`, replaces the contents of `*this` with `[p, p + r)`, and invalidates all pointers and references to the range `[p, p + n]`.

>    > > -11- Recommended practice: Implementations should avoid unnecessary copies and allocations by, for example, making `p` a pointer into internal storage and by restoring `*(p + r)` to `charT()` after evaluating `OP`.

[2023-01-11; Jonathan Wakely provides new wording requested by LWG]

[Issaquah 2023-02-07; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 27.4.3.5 [string.capacity] as indicated:

   >     template<class Operation> constexpr void resize_and_overwrite(size_type n, Operation op);

   > > -7- Let

   > > 1. (7.1) — `o = size()` before the call to `resize_and_overwrite`.

   > > 2. (7.2) — `k` be `min(o, n)`.

   > > 3. (7.3) — `p` be a value of type `charT*` or `charT* const`, such that the range `[p, p + n]` is valid and `this->compare(0, k, p, k) == 0` is `true` before the call. The values in the range `[p + k, p + n]` may be indeterminate (6.8.5 [basic.indet]).

   > > 4. (7.?) — `m` be a value of type `size_type` or `const size_type` equal to `n`.

   > > 5. (7.4) — `OP` be the expression `std::move(op)(p, nm)`.

   > > 6. (7.5) — `r = OP`.

   > > -8- Mandates: `OP` has an integer-like type (24.3.4.4 [iterator.concept.winc]).

   > > -9- Preconditions:

   > > 1. (9.1) — `OP` does not throw an exception or modify `p` or `nm`.

   > > 2. (9.2) — `r ≥ 0`.

   > > 3. (9.3) — `r ≤ nm`.

   > > 4. (9.4) — After evaluating `OP` there are no indeterminate values in the range `[p, p + r)`.

   > > -10- Effects: Evaluates `OP`, replaces the contents of `*this` with `[p, p + r)`, and invalidates all pointers and references to the range `[p, p + n]`.

   > > -11- Recommended practice: Implementations should avoid unnecessary copies and allocations by, for example, making `p` a pointer into internal storage and by restoring `*(p + r)` to `charT()` after evaluating `OP`.

### 3646(i). `std::ranges::view_interface::size` returns a signed type

Section: 25.5.3.1 [view.interface.general] Status: C++23 Submitter: Jiang An Opened: 2021-11-29 Last modified: 2023-11-22

Priority: 3

View all other issues in [view.interface.general].

View all issues with C++23 status.

Discussion:

According to 25.5.3.1 [view.interface.general], `view_interface::size` returns the difference between the sentinel and the beginning iterator, which always has a signed-integer-like type. However, IIUC the decision that a `size` member function should return an unsigned type by default was made when adopting P1227R2, and the relative changes of the ranges library were done in P1523R1. I don't know why `view_interface::size` was unchanged, while `ranges::size` returns an unsigned type in similar situations (25.3.10 [range.prim.size] (2.5)).

If we want to change `views_interface::size` to return an unsigned type, the both overloads should be changed as below:

>     constexpr auto size() requires forward_range<D> &&
>       sized_sentinel_for<sentinel_t<D>, iterator_t<D>> {
>         return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived()));
>       }
>     constexpr auto size() const requires forward_range<const D> &&
>       sized_sentinel_for<sentinel_t<const D>, iterator_t<const D>> {
>         return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived()));
>       }

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll.

[2022-06-22; Reflector poll]

LEWG poll approved the proposed resolution

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll in July 2022.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 25.5.3.1 [view.interface.general], class template `view_interface` synopsis, as indicated:

   >     […]
   >     constexpr auto size() requires forward_range<D> &&
   >       sized_sentinel_for<sentinel_t<D>, iterator_t<D>> {
   >         return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived()));
   >       }
   >     constexpr auto size() const requires forward_range<const D> &&
   >       sized_sentinel_for<sentinel_t<const D>, iterator_t<const D>> {
   >         return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived()));
   >       }
   >     […]

### 3648(i). `format` should not print `bool` with `'c'`

Section: 28.5.2.2 [format.string.std] Status: C++23 Submitter: Zhihao Yuan Opened: 2021-11-30 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [format.string.std].

View all other issues in [format.string.std].

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Discussion:

P1652R1 prints integral inputs as characters with `'c'` and preserves the wording to treat `bool` as a one-byte unsigned integer; this accidentally asks the implementation to cast `bool` into a 1-bit character if a user asks for the `'c'` presentation type.

Recent wording improvements made this implied behavior obvious.

[2021-12-04; Daniel comments]

This issue relates to LWG 3644(i).

[2022-01-30; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 28.5.2.2 [format.string.std], Table 67 [tab:format.type.bool], as indicated:

   > Table 67 — Meaning of type options for `bool` [tab:format.type.bool]
   > Type | Meaning
   > --- | ---
   > none, `s` | Copies textual representation, either `true` or `false`, to the output.
   > `b`, `B`, `c`, `d`, `o`, `x`, `X` | As specified in Table 65 [tab:format.type.int] for the value `static_cast<unsigned char>(value)`.

### 3649(i). [fund.ts.v3] Reinstate and bump `__cpp_lib_experimental_memory_resource` feature test macro

Section: 4.2 [fund.ts.v3::general.feature.test] Status: TS Submitter: Thomas Köppe Opened: 2021-12-03 Last modified: 2022-07-11

Priority: Not Prioritized

View all issues with TS status.

Discussion:

Addresses: fund.ts.v3

The rebase on C++17 in P0996 had the effect of deleting the feature test macro `__cpp_lib_experimental_memory_resource`: the macro was ostensibly tied to the `memory_resource` facility that had become part of C++17, but we overlooked that there was a residual piece that has not been adopted in the IS, namely the `resource_adaptor` class template.

It is still useful to be able to detect the presence of `resource_adaptor`, so we should reinstate the feature test macro and bump its value.

[2022-01-30; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-11 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → TS.]

Proposed resolution:

This wording is relative to N4853.

1. Modify 4.2 [fund.ts.v3::general.feature.test], Table 2, as indicated:

   > Table 2 — Significant features in this technical specification
   > Doc. No. | Title | Primary Section | Macro Name Suffix | Value | Header
   > --- | --- | --- | --- | --- | ---
   > `…`
   > N3916 | Type-erased allocator for std::function | 4.2 | `function_erased_allocator` | `201406` | `<experimental/functional>`
   > N3916 | Polymorphic Memory Resources | 8.3 [fund.ts.v3::memory.resource.syn] | `memory_resources` | [new value] | `<experimental/memory_resource>`
   > N4282 | The World's Dumbest Smart Pointer | 8.12 | `observer_ptr` | `201411` | `<experimental/memory>`
   > `…`

### 3650(i). Are `std::basic_string`'s `iterator` and `const_iterator` constexpr iterators?

Section: 27.4.3.1 [basic.string.general] Status: C++23 Submitter: Jiang An Opened: 2021-12-04 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [basic.string.general].

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Discussion:

`std::vector`'s `iterator` and `const_iterator` are required to meet constexpr iterator requirements in C++20 per P1004R2, but it seems that the similar wording is missing for `std::basic_string` in both P0980R1 and the current working draft.

I think we should add a bullet "The types `iterator` and `const_iterator` meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general])." to 27.4.3.1 [basic.string.general].

[2022-01-30; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 27.4.3.1 [basic.string.general], as indicated:

   > -3- In all cases, `[data(), data() + size()]` is a valid range, `data() + size()` points at an object with value `charT()` (a "null terminator"), and `size() <= capacity()` is `true`.

   > -4- A `size_type` parameter type in a `basic_string` deduction guide refers to the `size_type` member type of the type deduced by the deduction guide.

   > -?- The types `iterator` and `const_iterator` meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

### 3653(i). `<coroutine>` is freestanding, but uses `std::hash` which is not

Section: 17.13.2 [coroutine.syn] Status: Resolved Submitter: Jonathan Wakely Opened: 2021-12-17 Last modified: 2026-06-15

Priority: 3

View all other issues in [coroutine.syn].

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Discussion:

The `<coroutine>` header is required for freestanding implementations, but it defines a specialization of `std::hash`, which is not required for freestanding implementations.

Presumably we should make the `std::hash` specialization conditionally present.

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll. Would be resolved by P1642 for C++23, but it's still a defect in C++20.

[2025-11-11; Jonathan adds wording for C++20.]

This wording would resolve the defect for C++20, but the issue should be closed as Resolved in C++23.

[Brno 2026-06-12; Status changed: New → Resolved.]

Resolved by P1642 for C++23 and later. LWG has no objection to the proposed wording for C++20.

Proposed resolution:

This wording is relative to ISO/IEC 14882:2020(E).

1. Modify 17.13.4.9 [coroutine.handle.hash] as indicated:

   >     template<class P> struct hash<coroutine_handle<P>>;

   > -1- The specialization is enabled (22.10.19 [unord.hash]) in a hosted implementation.

### 3654(i). `basic_format_context::arg(size_t)` should be `noexcept`

Section: 28.5.6.7 [format.context] Status: C++23 Submitter: Hewill Kang Opened: 2021-12-26 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [format.context].

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Discussion:

`basic_format_context::arg(size_t)` simply returns `args_.get(id)` to get the elements of `args_`, where the type of `args_` is `basic_format_args<basic_format_context>`. Since `basic_format_args`'s `get(size_t)` is `noexcept`, this function can also be `noexcept`.

[2022-01-30; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 28.5.6.7 [format.context] as indicated:

   > >     namespace std {
   > >       template<class Out, class charT>
   > >       class basic_format_context {
   > >         basic_format_args<basic_format_context> args_; // exposition only
   > >         Out out_; // exposition only
   > >       public:
   > >         using iterator = Out;
   > >         using char_type = charT;
   > >         template<class T> using formatter_type = formatter<T, charT>;

   > >         basic_format_arg<basic_format_context> arg(size_t id) const noexcept;
   > >         std::locale locale();

   > >         iterator out();
   > >         void advance_to(iterator it);
   > >       };
   > >     }

   > […]

   >     basic_format_arg<basic_format_context> arg(size_t id) const noexcept;

   > > -5- Returns: `args_.get(id)`.

### 3655(i). The `INVOKE` operation and `union` types

Section: 22.10.4 [func.require] Status: C++23 Submitter: Jiang An Opened: 2021-12-29 Last modified: 2023-11-22

Priority: 3

View other active issues in [func.require].

View all other issues in [func.require].

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Discussion:

There are two cases of the `INVOKE` operation specified with `std::is_base_of_v` (22.10.4 [func.require] (1.1), (1,4)), which means the following code snippet is ill-formed, as `std::is_base_of_v<B, D>` is `false` when either `B` or `D` is a `union` type.

>     union Foo { int x; };
>     static_assert(std::is_invocable_v<int Foo::*, Foo&>);

Currently libstdc++ accepts this code, because it uses slightly different conditions that handle `union` types. libc++ and MSVC STL reject this code as specified in 22.10.4 [func.require].

Should we change the conditions in 22.10.4 [func.require] (1.1) and (1.4) to match libstdc++ and correctly handle `union` types?

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll.

[2023-02-07; Jonathan adds wording]

This is a regression introduced by LWG 2219(i). In C++14 `std::result_of<int Foo::*(Foo&)>::type` was valid, because the `INVOKE` wording used to say "`f` is a pointer to member data of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`". Since LWG 2219 we use `is_base_of` which is always false for union types. I don't think LWG 2219 intended to break this case, so we should fix it.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4928.

> 1. Modify 22.10.4 [func.require] as indicated:

>    > -1- Define `INVOKE(f, t1, t2, …, tN)` as follows:

>    > - (1.1) — `(t1.*f)(t2, …, tN)` when `f` is a pointer to a member function of a class `T` and `is_same_v<T, remove_cvref_t<decltype(t1)>> || is_base_of_v<T, remove_reference_t<decltype(t1)>>` is `true`;
>    > - (1.2) — `(t1.get().*f)(t2, …, tN)` when `f` is a pointer to a member function of a class `T` and `remove_cvref_t<decltype(t1)>` is a specialization of `reference_wrapper`;
>    > - (1.3) — `((*t1).*f)(t2, …, tN)` when `f` is a pointer to a member function of a class `T` and `t1` does not satisfy the previous two items;
>    > - (1.4) — `t1.*f` when `N == 1` and `f` is a pointer to data member of a class `T` and `is_same_v<T, remove_cvref_t<decltype(t1)>> || is_base_of_v<T, remove_reference_t<decltype(t1)>>` is `true`;
>    > - (1.5) — `t1.get().*f` when `N == 1` and `f` is a pointer to data member of a class `T` and `remove_cvref_t<decltype(t1)>` is a specialization of `reference_wrapper`;
>    > - (1.6) — `(*t1).*f` when `N == 1` and `f` is a pointer to data member of a class `T` and `t1` does not satisfy the previous two items;
>    > - (1.7) — `f(t1, t2, …, tN)` in all other cases.

[2023-02-07; Jonathan provides wording change requested by LWG]

Change `remove_reference_t` to `remove_cvref_t`. `is_base_of` ignores cv-qualifiers, so this isn't necessary, but just using the same transformation in both cases seems simpler to grok.

[Issaquah 2023-02-07; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 22.10.4 [func.require] as indicated:

   > -1- Define `INVOKE(f, t1, t2, …, tN)` as follows:

   > - (1.1) — `(t1.*f)(t2, …, tN)` when `f` is a pointer to a member function of a class `T` and `is_same_v<T, remove_cvref_t<decltype(t1)>> || is_base_of_v<T, remove_referencecvref_t<decltype(t1)>>` is `true`;
   > - (1.2) — `(t1.get().*f)(t2, …, tN)` when `f` is a pointer to a member function of a class `T` and `remove_cvref_t<decltype(t1)>` is a specialization of `reference_wrapper`;
   > - (1.3) — `((*t1).*f)(t2, …, tN)` when `f` is a pointer to a member function of a class `T` and `t1` does not satisfy the previous two items;
   > - (1.4) — `t1.*f` when `N == 1` and `f` is a pointer to data member of a class `T` and `is_same_v<T, remove_cvref_t<decltype(t1)>> || is_base_of_v<T, remove_referencecvref_t<decltype(t1)>>` is `true`;
   > - (1.5) — `t1.get().*f` when `N == 1` and `f` is a pointer to data member of a class `T` and `remove_cvref_t<decltype(t1)>` is a specialization of `reference_wrapper`;
   > - (1.6) — `(*t1).*f` when `N == 1` and `f` is a pointer to data member of a class `T` and `t1` does not satisfy the previous two items;
   > - (1.7) — `f(t1, t2, …, tN)` in all other cases.

### 3656(i). Inconsistent bit operations returning a count

Section: 22.11.5 [bit.pow.two] Status: C++23 Submitter: Nicolai Josuttis Opened: 2021-12-30 Last modified: 2023-11-22

Priority: 3

View all issues with C++23 status.

Discussion:

Among the bit operations returning a count, `bit_width()` is the only one not returning an `int`.

This has the following consequences:

>     std::uint64_t b64 = 1;
>     b64 = std::rotr(b64, 1);
>     int count1 = std::popcount(b64);     // OK
>     int count2 = std::countl_zero(b64);  // OK
>     int count3 = std::bit_width(b64);    // OOPS

The last line may result in a warning such as:

> Warning: conversion from `long long unsigned` to `int` may change value

You have to use a `static_cast` to avoid the warning.

Note that P0553R4 explicitly states the following design considerations, which I would also assume to apply to the later added functions from P0556R3:

> The counting operations return "`int`" quantities, consistent with the rule "use an `int` unless you need something else". This choice does not reflect, in the type, the fact that counts are always non-negative.

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll. Eight votes for P0, but request LEWG confirmation before setting it to Tentatively Ready.

[2022-02-22 LEWG telecon; Status changed: LEWG → Open]

No objection to unanimous consent to send the proposed resolution for LWG3656 to LWG for C++23. The changes in P1956 changed the functions to be more counting than mathematical.

[2022-07-08; Reflector poll]

Set status to Tentatively Ready after ten votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 22.11.2 [bit.syn], header `<bit>` synopsis, as indicated:

   >     […]
   >     template<class T>
   >       constexpr Tint bit_width(T x) noexcept;
   >     […]

2. Modify 22.11.5 [bit.pow.two], as indicated:

   >     template<class T>
   >       constexpr Tint bit_width(T x) noexcept;

   > > -11- Constraints: `T` is an unsigned integer type (6.9.2 [basic.fundamental]).

   > > -12- Returns: If `x == 0`, `0`; otherwise one plus the base-2 logarithm of `x`, with any fractional part discarded.

### 3657(i). `std::hash<std::filesystem::path>` is not enabled

Section: 31.12.6 [fs.class.path] Status: C++23 Submitter: Jiang An Opened: 2022-01-02 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [fs.class.path].

View all issues with C++23 status.

Discussion:

The hash support of `std::filesystem::path` is provided by `std::filesystem::hash_value`, but the specialization `std::hash<std::filesystem::path>` is currently disabled. IMO the specialization should be enabled, and its `operator()` should return the same value as `hash_value`.

[2022-01-15; Daniel provides wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4901.

> 1. Modify 31.12.4 [fs.filesystem.syn], header `<filesystem>` synopsis, as indicated:

>    >     #include <compare> // see 17.12.1 [compare.syn]

>    >     namespace std::filesystem {
>    >       // 31.12.6 [fs.class.path], paths
>    >       class path;

>    >       // 31.12.6.8 [fs.path.nonmember], path non-member functions
>    >       void swap(path& lhs, path& rhs) noexcept;
>    >       size_t hash_value(const path& p) noexcept;

>    >       […]
>    >     }

>    >     // [fs.path.hash], hash support
>    >     namespace std {
>    >       template<class T> struct hash;
>    >       template<> struct hash<filesystem::path>;
>    >     }

> 2. Following subclause 31.12.6.8 [fs.path.nonmember], introduce a new subclause [fs.path.hash], as indicated:

>    > 29.12.6.? Hash support [fs.path.hash]

>    >     template<> struct hash<filesystem::path>;

>    > > -?- For an object `p` of type `filesystem::path`, `hash<filesystem::path>()(p)` shall evaluate to the same result as `hash_value(p)`.

[2022-01-18; Daniel improves wording based on reflector discussion feedback]

[2022-01-30; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 31.12.4 [fs.filesystem.syn], header `<filesystem>` synopsis, as indicated:

   >     #include <compare> // see 17.12.1 [compare.syn]

   >     namespace std::filesystem {
   >       // 31.12.6 [fs.class.path], paths
   >       class path;

   >       // 31.12.6.8 [fs.path.nonmember], path non-member functions
   >       void swap(path& lhs, path& rhs) noexcept;
   >       size_t hash_value(const path& p) noexcept;

   >       […]
   >     }

   >     // [fs.path.hash], hash support
   >     namespace std {
   >       template<class T> struct hash;
   >       template<> struct hash<filesystem::path>;
   >     }

2. Following subclause 31.12.6.8 [fs.path.nonmember], introduce a new subclause [fs.path.hash], as indicated:

   > 29.12.6.? Hash support [fs.path.hash]

   >     template<> struct hash<filesystem::path>;

   > > -?- For an object `p` of type `filesystem::path`, `hash<filesystem::path>()(p)` evaluates to the same result as `filesystem::hash_value(p)`.

### 3659(i). Consider `ATOMIC_FLAG_INIT` undeprecation

Section: 32.5.10 [atomics.flag] Status: C++23 Submitter: Aaron Ballman Opened: 2022-01-18 Last modified: 2023-11-22

Priority: 3

View all other issues in [atomics.flag].

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Discussion:

P0883R2 deprecated `ATOMTIC_VAR_INIT` and `ATOMIC_FLAG_INIT` largely based on rationale from WG14. However, WG14 only deprecated `ATOMIC_VAR_INIT` because we were told by implementers that `ATOMIC_FLAG_INIT` is still necessary for some platforms (platforms for which "clear" is actually not all zero bits, which I guess exist).

I'd like to explore whether WG21 should undeprecate `ATOMIC_FLAG_INIT` as there was no motivation that I could find in the paper on the topic or in the discussion at P0883R0 [Jacksonville 2018].

One possible approach would be to undeprecate it from `<stdatomic.h>` only (C++ can still use the constructors from `<atomic>` and shared code can use the macros from `<stdatomic.h>`).

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll. Send to SG1.

[2022-07-06; SG1 confirm the direction, Jonathan adds wording]

"In response to LWG 3659, add ATOMIC_FLAG_INIT to <stdatomic.h> as undeprecated."

SF | F | N | A | SA
3 | 0 | 0 | 0 | 0

"In response to LWG 3659, undeprecate ATOMIC_FLAG_INIT in <atomic>."

SF | F | N | A | SA
2 | 3 | 1 | 0 | 0

[2022-07-11; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 32.5.2 [atomics.syn], Header `<atomic>` synopsis, as indicated:

   >     void atomic_flag_notify_all(volatile atomic_flag*) noexcept;
   >     void atomic_flag_notify_all(atomic_flag*) noexcept;
   >     #define ATOMIC_FLAG_INIT see below

   >     // [atomics.fences], fences
   >     extern "C" void atomic_thread_fence(memory_order) noexcept;
   >     extern "C" void atomic_signal_fence(memory_order) noexcept;

2. Move the content of [depr.atomics.flag] from Annex D to the end of 32.5.10 [atomics.flag].

   >     #define ATOMIC_FLAG_INIT see below

   > > Remarks: The macro `ATOMIC_FLAG_INIT` is defined in such a way that it can be used to initialize an object of type `atomic_flag` to the clear state. The macro can be used in the form:

   > >       atomic_flag guard = ATOMIC_FLAG_INIT;

   > > It is unspecified whether the macro can be used in other initialization contexts. For a complete static-duration object, that initialization shall be static.

3. Modify 32.5.12 [stdatomic.h.syn] C compatibility, as indicated:

   >     using std::atomic_flag_clear;            // see below
   >     using std::atomic_flag_clear_explicit;   // see below
   >     #define ATOMIC_FLAG_INIT see below

   >     using std::atomic_thread_fence;          // see below
   >     using std::atomic_signal_fence;          // see below

4. Modify D.24.1 [depr.atomics.general] in Annex D as indicated:

   >     #define ATOMIC_VAR_INIT(value) see below

   >     #define ATOMIC_FLAG_INIT see below
   >     }

### 3660(i). `iterator_traits<common_iterator>::pointer` should conform to §[iterator.traits]

Section: 24.5.5.2 [common.iter.types] Status: C++23 Submitter: Casey Carter Opened: 2022-01-20 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

24.3.2.3 [iterator.traits]/1 says:

> […] In addition, the types

> >     iterator_traits<I>::pointer
> >     iterator_traits<I>::reference

> shall be defined as the iterator's pointer and reference types; that is, for an iterator object `a` of class type, the same type as `decltype(a.operator->())` and `decltype(*a)`, respectively. The type `iterator_traits<I>::pointer` shall be `void` for an iterator of class type `I` that does not support `operator->`. […]

24.5.5.2 [common.iter.types]/1 slightly contradicts this:

> The nested typedef-names of the specialization of `iterator_traits` for `common_iterator<I, S>` are defined as follows.

> 1. […]

> 2. (1.3) — If the expression `a.operator->()` is well-formed, where `a` is an lvalue of type `const common_iterator<I, S>`, then `pointer` denotes the type of that expression. Otherwise, `pointer` denotes `void`.

"The type of `a.operator->()`" is not necessarily the same as `decltype(a.operator->())`: when the expression is an lvalue or xvalue of type `T`, "the type of `a.operator->()`" is `T` but `decltype(a.operator->())` is either `T&` or `T&&`. An implementation therefore cannot conform to the requirements of both cited paragraphs for some specializations of `common_iterator`.

The most likely explanation for this contradiction is that the writer of the phrase "type of `a.operator->()`" was not cognizant of the difference in meaning and intended to actually write `decltype(a.operator->())`.

[2022-01-30; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

> [Drafting Note: The wording change below includes an additional drive-by fix that ensures that the last sentence "Otherwise, `pointer` denotes `void`" cannot be misinterpreted (due to the leading "If") to apply also for a situation when the outcome of the expression `a.operator->()` for a non-`const` `common_iterator<I, S>` is reflected upon.]

1. Modify 24.5.5.2 [common.iter.types] as indicated:

   > -1- The nested typedef-names of the specialization of `iterator_traits` for `common_iterator<I, S>` are defined as follows.

   > 1. […]

   > 2. (1.3) — Let `a` denote an lvalue of type `const common_iterator<I, S>`. If the expression `a.operator->()` is well-formed, where `a` is an lvalue of type `const common_iterator<I, S>`, then `pointer` denotes `decltype(a.operator->())`the type of that expression. Otherwise, `pointer` denotes `void`.

### 3661(i). `constinit atomic<shared_ptr<T>> a(nullptr);` should work

Section: 32.5.8.7.2 [util.smartptr.atomic.shared] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-01-21 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [util.smartptr.atomic.shared].

View all issues with C++23 status.

Discussion:

All the following are valid except for the last line:

>     constinit int i1{};
>     constinit std::atomic<int> a1{};
>     constinit int i2{0};
>     constinit std::atomic<int> a2{0};
>     constinit std::shared_ptr<int> i3{};
>     constinit std::atomic<std::shared_ptr<int>> a3{};
>     constinit std::shared_ptr<int> i4{nullptr};
>     constinit std::atomic<std::shared_ptr<int>> a4{nullptr}; // error

The initializer for `a4` will create a `shared_ptr<int>` temporary (using the same constructor as `i4`) but then try to use `atomic(shared_ptr<int>)` which is not `constexpr`.

This is an unnecessary inconsistency in the API for `atomic<shared_ptr<T>>` that can easily be fixed. The proposed resolution has been implemented in libstdc++.

There is no need to also change `atomic<weak_ptr<T>>` because `weak_ptr` doesn't have a constructor taking `nullptr`.

[2022-01-30; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 32.5.8.7.2 [util.smartptr.atomic.shared], class template partial specialization `atomic<shared_ptr<T>>` synopsis, as indicated:

   >     […]
   >     constexpr atomic() noexcept;
   >     constexpr atomic(nullptr_t) noexcept : atomic() { }
   >     atomic(shared_ptr<T> desired) noexcept;
   >     atomic(const atomic&) = delete;
   >     void operator=(const atomic&) = delete;
   >     […]

### 3662(i). `basic_string::append/assign(NTBS, pos, n)` suboptimal

Section: 27.4.3.7.2 [string.append], 27.4.3.7.3 [string.assign] Status: C++26 Submitter: Jonathan Wakely Opened: 2022-01-21 Last modified: 2026-06-09

Priority: 3

View all other issues in [string.append].

View all issues with C++26 status.

Discussion:

Addresses RU-268

This will allocate temporary string, then copy one byte from it:

>     std::string s;
>     s.append("a very very long string that doesn't fit in SSO buffer", 13, 1);

The problem is that there is no overload accepting an NTBS there, so it converts to a temporary string using:

>     append(const basic_string&, size_t, size_t = npos);

C++17 added a new overload for a `string_view`:

>     append(const T&, size_t, size_t = npos);

But that overload is constrained to not allow `const char*` arguments, so we still create a temporary string.

If we're going to accept those arguments, we should do so efficiently, without forcing an unnecessary allocation.

We could do better if we split the `append(const T&, ...)` overload into:

>     append(const T&, size_t);
>     append(const T&, size_t, size_t);

and then remove the `!is_convertible_v<const T&, const charT*>` constraint from the second overload (Option A in the proposed resolution).

Alternatively, we can leave the `append(const T&, size_t, size_t)` overload alone and just add one taking exactly the arguments used above. That seems simpler to me, and I prefer that (Option B in the Proposed Resolution).

The same problem applies to `assign("...", pos, n)`.

[2022-01-30; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N4901.

> [Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A:

1. Modify 27.4.3.1 [basic.string.general], class template `basic_string` synopsis, as indicated:

   >     […]
   >     // 27.4.3.7 [string.modifiers], modifiers
   >     […]
   >     template<class T>
   >       constexpr basic_string& append(const T& t);
   >     template<class T>
   >       constexpr basic_string& append(const T& t, size_type pos);
   >     template<class T>
   >       constexpr basic_string& append(const T& t, size_type pos, size_type n = npos);
   >     constexpr basic_string& append(const charT* s, size_type n);
   >     […]
   >     template<class T>
   >       constexpr basic_string& assign(const T& t);
   >     template<class T>
   >       constexpr basic_string& assign(const T& t, size_type pos);
   >     template<class T>
   >       constexpr basic_string& assign(const T& t, size_type pos, size_type n = npos);
   >     constexpr basic_string& assign(const charT* s, size_type n);
   >     […]

2. Modify 27.4.3.7.2 [string.append] as indicated:

   >     template<class T>
   >       constexpr basic_string& append(const T& t);

   > > -3- Constraints:

   > > 1. (3.1) — `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and

   > > 2. (3.2) — `is_convertible_v<const T&, const charT*>` is `false`.

   > > -4- Effects: Equivalent to:

   > > >     basic_string_view<charT, traits> sv = t;
   > > >     return append(sv.data(), sv.size());

   >     template<class T>
   >       constexpr basic_string& append(const T& t, size_type pos);

   > > -?- Constraints:

   > > 1. (?.1) — `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and

   > > 2. (?.2) — `is_convertible_v<const T&, const charT*>` is `false`.

   > > -?- Effects: Equivalent to:

   > > >     basic_string_view<charT, traits> sv = t;
   > > >     return append(sv.substr(pos));

   >     template<class T>
   >       constexpr basic_string& append(const T& t, size_type pos, size_type n = npos);

   > > -5- Constraints:

   > > 1. (5.1) — `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and

   > > 2. (5.2) — `is_convertible_v<const T&, const charT*>` is `false`.

   > > -6- Effects: Equivalent to:

   > > >     basic_string_view<charT, traits> sv = t;
   > > >     return append(sv.substr(pos, n));

3. Modify 27.4.3.7.3 [string.assign] as indicated:

   >     template<class T>
   >       constexpr basic_string& assign(const T& t);

   > > -4- Constraints:

   > > 1. (4.1) — `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and

   > > 2. (4.2) — `is_convertible_v<const T&, const charT*>` is `false`.

   > > -5- Effects: Equivalent to:

   > > >     basic_string_view<charT, traits> sv = t;
   > > >     return assign(sv.data(), sv.size());

   >     template<class T>
   >       constexpr basic_string& assign(const T& t, size_type pos);

   > > -?- Constraints:

   > > 1. (?.1) — `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and

   > > 2. (?.2) — `is_convertible_v<const T&, const charT*>` is `false`.

   > > -?- Effects: Equivalent to:

   > > >     basic_string_view<charT, traits> sv = t;
   > > >     return assign(sv.substr(pos));

   >     template<class T>
   >       constexpr basic_string& assign(const T& t, size_type pos, size_type n = npos);

   > > -6- Constraints:

   > > 1. (6.1) — `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true` and

   > > 2. (6.2) — `is_convertible_v<const T&, const charT*>` is `false`.

   > > -7- Effects: Equivalent to:

   > > >     basic_string_view<charT, traits> sv = t;
   > > >     return assign(sv.substr(pos, n));

Option B:

1. Modify 27.4.3.1 [basic.string.general], class template `basic_string` synopsis, as indicated:

   >     […]
   >     // 27.4.3.7 [string.modifiers], modifiers
   >     […]
   >     constexpr basic_string& append(const charT* s, size_type n);
   >     constexpr basic_string& append(const charT* s);
   >     constexpr basic_string& append(const charT* s, size_type pos, size_type n);
   >     constexpr basic_string& append(size_type n, charT c);
   >     […]
   >     constexpr basic_string& assign(const charT* s, size_type n);
   >     constexpr basic_string& assign(const charT* s);
   >     constexpr basic_string& assign(const charT* s, size_type pos, size_type n);
   >     constexpr basic_string& assign(size_type n, charT c);
   >     […]

2. Modify 27.4.3.7.2 [string.append] as indicated:

   >     constexpr basic_string& append(const charT* s, size_type n);

   > > -7- Preconditions: `[s, s + n)` is a valid range.

   > > -8- Effects: Appends a copy of the range `[s, s + n)` to the string.

   > > -9- Returns: `*this`.

   >     constexpr basic_string& append(const charT* s);

   > > -10- Effects: Equivalent to: `return append(s, traits::length(s));`

   >     constexpr basic_string& append(const charT* s, size_type pos, size_type n);

   > > -?- Effects: Equivalent to: `return append(basic_string_view<charT, traits>(s).substr(pos, n));`

3. Modify 27.4.3.7.3 [string.assign] as indicated:

   >     constexpr basic_string& assign(const charT* s, size_type n);

   > > -8- Preconditions: `[s, s + n)` is a valid range.

   > > -9- Effects: Replaces the string controlled by `*this` with a copy of the range `[s, s + n)`.

   > > -10- Returns: `*this`.

   >     constexpr basic_string& assign(const charT* s);

   > > -11- Effects: Equivalent to: `return assign(s, traits::length(s));`

   >     constexpr basic_string& assign(const charT* s, size_type pos, size_type n);

   > > -?- Effects: Equivalent to: `return assign(basic_string_view<charT, traits>(s).substr(pos, n));`

[2026-03-23 Croydon; accept Option B and move to Immediate]

We would like to make this even more efficient, by not doing `char_traits::length(s)` for the whole thing. That would mean that `s` does not have to be null-terminated. But would need something in `char_traits` like `strnlen`. That's a pre-existing problem though, Option B improves on the status quo even if it's not perfect.

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 27.4.3.1 [basic.string.general], class template `basic_string` synopsis, as indicated:

   >     […]
   >     // 27.4.3.7 [string.modifiers], modifiers
   >     […]
   >     constexpr basic_string& append(const charT* s, size_type n);
   >     constexpr basic_string& append(const charT* s);
   >     constexpr basic_string& append(const charT* s, size_type pos, size_type n);
   >     constexpr basic_string& append(size_type n, charT c);
   >     […]
   >     constexpr basic_string& assign(const charT* s, size_type n);
   >     constexpr basic_string& assign(const charT* s);
   >     constexpr basic_string& assign(const charT* s, size_type pos, size_type n);
   >     constexpr basic_string& assign(size_type n, charT c);
   >     […]

2. Modify 27.4.3.7.2 [string.append] as indicated:

   >     constexpr basic_string& append(const charT* s, size_type n);

   > > -7- Preconditions: `[s, s + n)` is a valid range.

   > > -8- Effects: Appends a copy of the range `[s, s + n)` to the string.

   > > -9- Returns: `*this`.

   >     constexpr basic_string& append(const charT* s);

   > > -10- Effects: Equivalent to: `return append(s, traits::length(s));`

   >     constexpr basic_string& append(const charT* s, size_type pos, size_type n);

   > > -?- Effects: Equivalent to: `return append(basic_string_view<charT, traits>(s).substr(pos, n));`

3. Modify 27.4.3.7.3 [string.assign] as indicated:

   >     constexpr basic_string& assign(const charT* s, size_type n);

   > > -8- Preconditions: `[s, s + n)` is a valid range.

   > > -9- Effects: Replaces the string controlled by `*this` with a copy of the range `[s, s + n)`.

   > > -10- Returns: `*this`.

   >     constexpr basic_string& assign(const charT* s);

   > > -11- Effects: Equivalent to: `return assign(s, traits::length(s));`

   >     constexpr basic_string& assign(const charT* s, size_type pos, size_type n);

   > > -?- Effects: Equivalent to: `return assign(basic_string_view<charT, traits>(s).substr(pos, n));`

### 3664(i). LWG 3392 broke `std::ranges::distance(a, a+3)`

Section: 24.4.4.3 [range.iter.op.distance] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2022-01-23 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.iter.op.distance].

View all issues with C++23 status.

Discussion:

Consider the use of `std::ranges::distance(first, last)` on a simple C array. This works fine with `std::distance`, but currently does not work with `std::ranges::distance`.

>     // godbolt link
>     #include <ranges>
>     #include <cassert>

>     int main() {
>       int a[] = {1, 2, 3};
>       assert(std::ranges::distance(a, a+3) == 3);
>       assert(std::ranges::distance(a, a) == 0);
>       assert(std::ranges::distance(a+3, a) == -3);
>     }

Before LWG 3392(i), we had a single iterator-pair overload:

>     template<input_or_output_iterator I, sentinel_for<I> S>
>       constexpr iter_difference_t<I> distance(I first, S last);

which works fine for C pointers. After LWG 3392(i), we have two iterator-pair overloads:

>     template<input_or_output_iterator I, sentinel_for<I> S>
>       requires (!sized_sentinel_for<S, I>)
>         constexpr iter_difference_t<I> distance(I first, S last);

>     template<input_or_output_iterator I, sized_sentinel_for<I> S>
>       constexpr iter_difference_t<I> distance(const I& first, const S& last);

and unfortunately the one we want — `distance(I first, S last)` — is no longer viable because [with `I=int*`, `S=int*`], we have `sized_sentinel_for<S, I>` and so its constraints aren't satisfied. So we look at the other overload [with `I=int[3]`, `S=int[3]`], but unfortunately its constraints aren't satisfied either, because `int[3]` is not an `input_or_output_iterator`.

[2022-01-30; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4901.

> > [Drafting Note: Thanks to Casey Carter. Notice that `sentinel_for<S, I>` already implies and subsumes `input_or_output_iterator<I>`, so that constraint wasn't doing anything; personally I'd prefer to remove it for symmetry (and to save the environment). Otherwise you'll have people asking why one of the `I`'s is constrained and the other isn't.]

> 1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

>    >     […]
>    >     // 24.4.4.3 [range.iter.op.distance], ranges::distance
>    >     template<classinput_or_output_iterator I, sentinel_for<I> S>
>    >       requires (!sized_sentinel_for<S, I>)
>    >       constexpr iter_difference_t<I> distance(I first, S last);
>    >     template<classinput_or_output_iterator I, sized_sentinel_for<decay_t<I>> S>
>    >       constexpr iter_difference_t<I> distance(const I& first, const S& last);
>    >     […]

> 2. Modify 24.4.4.3 [range.iter.op.distance] as indicated:

>    >     template<classinput_or_output_iterator I, sentinel_for<I> S>
>    >       requires (!sized_sentinel_for<S, I>)
>    >       constexpr iter_difference_t<I> ranges::distance(I first, S last);

>    > > -1- Preconditions: `[first, last)` denotes a range.

>    > > -2- Effects: Increments `first` until `last` is reached and returns the number of increments.

>    >     template<classinput_or_output_iterator I, sized_sentinel_for<decay_t<I>> S>
>    >       constexpr iter_difference_t<I> ranges::distance(const I& first, const S& last);

>    > > -3- Effects: Equivalent to: `return last - first;`

[2022-02-16; Arthur and Casey provide improved wording]

[Kona 2022-11-08; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4901.

> [Drafting Note: Arthur thinks it's a bit "cute" of the Effects: element to `static_cast` from `T(&)[N]` to `T* const&` in the array case, but it does seem to do the right thing in all cases, and it saves us from having to use an `if constexpr (is_array_v...)` or something like that.]

1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     […]
   >     // 24.4.4.3 [range.iter.op.distance], ranges::distance
   >     template<classinput_or_output_iterator I, sentinel_for<I> S>
   >       requires (!sized_sentinel_for<S, I>)
   >       constexpr iter_difference_t<I> distance(I first, S last);
   >     template<classinput_or_output_iterator I, sized_sentinel_for<decay_t<I>> S>
   >       constexpr iter_difference_t<decay_t<I>> distance(const I&& first, const S& last);
   >     […]

2. Modify 24.4.4.3 [range.iter.op.distance] as indicated:

   >     template<classinput_or_output_iterator I, sentinel_for<I> S>
   >       requires (!sized_sentinel_for<S, I>)
   >       constexpr iter_difference_t<I> ranges::distance(I first, S last);

   > > -1- Preconditions: `[first, last)` denotes a range.

   > > -2- Effects: Increments `first` until `last` is reached and returns the number of increments.

   >     template<classinput_or_output_iterator I, sized_sentinel_for<decay_t<I>> S>
   >       constexpr iter_difference_t<decay_t<I>> ranges::distance(const I&& first, const S& last);

   > > -3- Effects: Equivalent to: `return last - static_cast<const decay_t<I>&>(first);`

### 3670(i). Cpp17InputIterators don't have integer-class difference types

Section: 25.6.4.3 [range.iota.iterator] Status: C++23 Submitter: Casey Carter Opened: 2022-02-04 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [range.iota.iterator].

View all other issues in [range.iota.iterator].

View all issues with C++23 status.

Discussion:

25.6.4.3 [range.iota.iterator] defines

>     using iterator_category = input_iterator_tag; // present only if W models incrementable

but when `difference_type` is an integer-class type the iterator does not meet the Cpp17InputIterator requirements.

[2022-02-07; Daniel comments]

As requested by LWG 3376(i) and wording implemented by P2393R1, integer-class types are no longer required to have class type.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4901.

> 1. Modify 25.6.4.3 [range.iota.iterator], class `iota_view::iterator` synopsis, as indicated:

>    >     namespace std::ranges {
>    >       template<weakly_incrementable W, semiregular Bound>
>    >         requires weakly-equality-comparable-with<W, Bound> && copyable<W>
>    >       struct iota_view<W, Bound>::iterator {
>    >         […]
>    >         using iterator_category = input_iterator_tag; // present only if W models incrementable and
>    >                                                       // IOTA-DIFF-T(W) is not a class type
>    >         using value_type = W;
>    >         using difference_type = IOTA-DIFF-T(W);
>    >         […]
>    >       };
>    >     }

[2022-02-07; Casey Carter provides improved wording]

[2022-03-04; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 25.6.4.3 [range.iota.iterator], class `iota_view::iterator` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<weakly_incrementable W, semiregular Bound>
   >         requires weakly-equality-comparable-with<W, Bound> && copyable<W>
   >       struct iota_view<W, Bound>::iterator {
   >         […]
   >         using iterator_category = input_iterator_tag; // present only if W models incrementable and
   >                                                       // IOTA-DIFF-T(W) is an integral type
   >         using value_type = W;
   >         using difference_type = IOTA-DIFF-T(W);
   >         […]
   >       };
   >     }

### 3671(i). `atomic_fetch_xor` missing from `stdatomic.h`

Section: 32.5.12 [stdatomic.h.syn] Status: C++23 Submitter: Hubert Tong Opened: 2022-02-09 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [stdatomic.h.syn].

View all issues with C++23 status.

Discussion:

C17 subclause 7.17.7.5 provides `atomic_fetch_xor` and `atomic_fetch_xor_explicit`. `stdatomic.h` in the working draft (N4901) does not.

[2022-02-09; Jonathan comments and provides wording]

C++20 32.5.2 [atomics.syn] has both of them, too, so it should definitely be in the common subset.

[2022-03-04; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 32.5.12 [stdatomic.h.syn], header `<stdatomic.h>` synopsis, as indicated:

   > [Drafting Note: The editor is kindly requested to reorder these "`atomic_fetch_KEY`" declarations to match the other synopses in Clause 32.5 [atomics]: `add`, `sub`, `and`, `or`, `xor`.]

   >     […]
   >     using std::atomic_fetch_or;                    // see below
   >     using std::atomic_fetch_or_explicit;           // see below
   >     using std::atomic_fetch_xor;                   // see below
   >     using std::atomic_fetch_xor_explicit;          // see below
   >     using std::atomic_fetch_and;                   // see below
   >     using std::atomic_fetch_and_explicit;          // see below
   >     […]

### 3672(i). `common_iterator::operator->()` should return by value

Section: 24.5.5.4 [common.iter.access] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-02-12 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [common.iter.access].

View all issues with C++23 status.

Discussion:

24.5.5.4 [common.iter.access] p5 (5.1) says that `common_iterator<T*, S>::operator->()` returns `std::get<T*>(v_)` which has type `T* const&`. That means that `iterator_traits::pointer` is `T* const&` as well (this was recently clarified by LWG 3660(i)). We have an actual pointer here, why are we returning it by reference?

For the three bullet points in 24.5.5.4 [common.iter.access] p5, the second and third return by value anyway, so `decltype(auto)` is equivalent to `auto`. For the first bullet, it would make a lot more sense for raw pointers to be returned by value. That leaves the case where the iterator has an `operator->()` member, which could potentially benefit from returning by reference. But it must return something that is iterator-like or pointer-like, which we usually just pass by value. Casey suggested we should just change `common_iterator<I, S>::operator->()` to return by value in all cases.

Libstdc++ has always returned by value, as an unintended consequence of using a union instead of `std::variant<I, S>`, so that it doesn't use `std::get<I>` to return the member.

[2022-03-04; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 24.5.5.1 [common.iterator], class template `common_iterator` synopsis, as indicated:

   >     […]
   >     constexpr decltype(auto) operator*();
   >     constexpr decltype(auto) operator*() const
   >       requires dereferenceable<const I>;
   >     constexpr decltype(auto) operator->() const
   >       requires see below;
   >     […]

2. Modify 24.5.5.4 [common.iter.access] as indicated:

   >     constexpr decltype(auto) operator->() const
   >       requires see below;

   > > -3- The expression in the requires-clause is equivalent to: […]

   > > -4- Preconditions: `holds_alternative<I>(v_)` is `true`.

   > > -5- Effects:

   > > 1. (5.1) — If `I` is a pointer type or if the expression `get<I>(v_).operator->()` is well-formed, equivalent to: `return get<I>(v_);`

   > > 2. (5.2) — Otherwise, if `iter_reference_t<I>` is a reference type, equivalent to:

   > >    >     auto&& tmp = *get<I>  (v_);
   > >    >     return addressof(tmp);

   > > 3. (5.3) — Otherwise, equivalent to: `return proxy(*get<I>(v_));` where `proxy` is the exposition-only class:

   > >    >     class proxy {
   > >    >       iter_value_t<I> keep_;
   > >    >       constexpr proxy(iter_reference_t<I>&& x)
   > >    >         : keep_(std::move(x)) {}
   > >    >     public:
   > >    >       constexpr const iter_value_t<I>* operator->() const noexcept {
   > >    >         return addressof(keep_);
   > >    >       }
   > >    >     };

### 3673(i). §[locale.cons] Ambiguous argument in Throws for locale+name+category constructor

Section: 28.3.3.1.3 [locale.cons] Status: Resolved Submitter: Hubert Tong Opened: 2022-02-12 Last modified: 2023-03-22

Priority: 3

View all other issues in [locale.cons].

View all issues with Resolved status.

Discussion:

>     locale(const locale& other, const char* std_name, category);

has

> Throws: `runtime_error` if the argument is not valid, or is null.

There being three arguments, the statement is rather problematic. It looks like a copy/paste from

>     explicit locale(const char* std_name);

The conclusion, assuming that "the argument" is also `std_name` in the problem case, seems to be that the statement should be changed to read:

> Throws: `runtime_error` if `std_name` is not valid, or is null.

However there is implementation divergence over whether or not values for the category argument not explicitly described as valid by 28.3.3.1.2.1 [locale.category] result in `runtime_error`.

libc++ does not throw. libstdc++ does.

Code:

>     #include <locale>
>     #include <stdio.h>
>     #include <exception>
>     int main(void) {
>       std::locale Generic("C");
>       try {
>         std::locale Abomination(Generic, Generic, 0x7fff'ffff);
>       } catch (std::runtime_error&) {
>         fprintf(stderr, "Threw\n");
>       }
>     }

Compiler Explorer link.

[2022-03-04; Reflector poll]

Set priority to 3 after reflector poll.

[2022-11-01; Jonathan comments]

The proposed resolution of 2295(i) would resolve this too.

The implementation divergence is not a problem. 28.3.3.1.2.1 [locale.category] p2 makes invalid `category` values undefined, so silently ignoring them or throwing exceptions are both valid.

[2023-03-22 LWG 2295 was approved in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

> Preconditions: The `category` argument is a valid value 28.3.3.1.2.1 [locale.category].

> Throws: `runtime_error` if the argument `std_name` is not valid, or is null.

### 3676(i). Name of locale composed using `std::locale::none`

Section: 28.3.3.1.3 [locale.cons] Status: Resolved Submitter: Hubert Tong Opened: 2022-02-14 Last modified: 2023-03-22

Priority: 3

View all other issues in [locale.cons].

View all issues with Resolved status.

Discussion:

For

>     locale(const locale& other, const locale& one, category cats);

the Remarks say:

> The resulting locale has a name if and only if the first two arguments have names.

The case where `cats` is `locale::none` seems very similar to the issue that LWG 2295(i) reports with the case where the provided facet pointer is `nullptr`. That is, if no composition is actually occurring, then using the name of `other` seems to be reasonable.

It would help consistency if both cases are treated the same way.

Note: The `locale::all` case should not imply using the name of `one`. `locale::all` does not necessarily cover all of the C locale categories on a platform.

[2022-03-04; Reflector poll]

Set priority to 3 after reflector poll.

[2022-11-01; Jonathan comments]

The proposed resolution of 2295(i) would resolve this too.

[2023-03-22 LWG 2295 was approved in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

### 3677(i). Is a cv-qualified `pair` specially handled in uses-allocator construction?

Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Jiang An Opened: 2022-02-16 Last modified: 2023-11-22

Priority: 2

View all other issues in [allocator.uses.construction].

View all issues with C++23 status.

Discussion:

It seems unclear whether cv-qualified `pair` specializations are considered as specializations of `pair` in 20.2.8.2 [allocator.uses.construction].

Currently MSVC STL only considered cv-unqualified pair types as such specializations, while libstdc++ accept both cv-unqualified and `const`-qualified pair types as such specializations. The resolution of LWG 3525(i) uses `remove_cv_t`, which possibly imply that the specialization of pair may be cv-qualified.

The difference can be observed via the following program:

>     #include <utility>
>     #include <memory>
>     #include <vector>
>     #include <cassert>

>     template<class T>
>     class payload_ator {

>       int payload{};

>     public:
>       payload_ator() = default;

>       constexpr explicit payload_ator(int n) noexcept : payload{n} {}

>       template<class U>
>       constexpr explicit payload_ator(payload_ator<U> a) noexcept : payload{a.payload} {}

>       friend bool operator==(payload_ator, payload_ator) = default;

>       template<class U>
>       friend constexpr bool operator==(payload_ator x, payload_ator<U> y) noexcept
>       {
>         return x.payload == y.payload;
>       }

>       using value_type = T;

>       constexpr T* allocate(std::size_t n) { return std::allocator<T>{}.allocate(n); }

>       constexpr void deallocate(T* p, std::size_t n) { return std::allocator<T>{}.deallocate(p, n); }

>       constexpr int get_payload() const noexcept { return payload; }
>     };

>     bool test()
>     {
>       constexpr int in_v = 42;
>       using my_pair_t = std::pair<int, std::vector<int, payload_ator<int>>>;
>       auto out_v = std::make_obj_using_allocator<const my_pair_t>(payload_ator<int>{in_v}).second.get_allocator().get_payload();
>       return in_v == out_v;
>     }

>     int main()
>     {
>       assert(test()); // passes only if a const-qualified pair specialization is considered as a pair specialization
>     }

[2022-03-04; Reflector poll]

Set priority to 2 after reflector poll.

[2022-08-24; LWG telecon]

Change every `T` to `remove_cv_t<T>`.

[2022-08-25; Jonathan Wakely provides wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> - Modify 20.2.8.2 [allocator.uses.construction] as indicated, using `remove_cv_t` in every Constraints: element:

>   > Constraints: `remove_cv_t<T>` [is|is not] a specialization of `pair`

[2022-09-23; Jonathan provides improved wording]

[2022-09-30; moved to Tentatively Ready after seven votes in reflector poll]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 20.2.8.2 [allocator.uses.construction] as indicated, using `remove_cv_t` in every Constraints: element (paragraphs 4, 6, 8, 10, 12, 14, 17):

   > Constraints: `remove_cv_t<T>` [is|is not] a specialization of `pair`

2. Add `remove_cv_t` in paragraph 5:

   > -5- Returns: A `tuple` value determined as follows:

   > (5.1) — If `uses_allocator_v<remove_cv_t<T>, Alloc>` is `false` and `is_constructible_v<T, Args...>` is `true`, return `forward_as_tuple(std::forward<Args>(args)...)`.

   > (5.2) — Otherwise, if `uses_allocator_v<remove_cv_t<T>, Alloc>` is `true` and `is_constructible_v<T, allocator_arg_t, const Alloc&, Args...>` is `true`, return

   >     tuple<allocator_arg_t, const Alloc&, Args&&...>(
   >       allocator_arg, alloc, std::forward<Args>(args)...)

   > (5.3) — Otherwise, if `uses_allocator_v<remove_cv_t<T>, Alloc>` is `true` and `is_constructible_v<T, Args..., const Alloc&>` is `true`, return `forward_as_tuple(std::forward<Args>(args)..., alloc)`.

   > (5.4) — Otherwise, the program is ill-formed.

3. Rephrase paragraph 7 in terms of the pair member types:

   > -?- Let `T1` be `T::first_type`. Let `T2` be `T::second_type`.

   > -6- Constraints: `remove_cv_t<T>` is a specialization of `pair`

   > -7- Effects:: For `T` specified as `pair<T1, T2>`, equivalent Equivalent to:

### 3683(i). `operator==` for `polymorphic_allocator` cannot deduce template argument in common cases

Section: 20.5.3 [mem.poly.allocator.class] Status: C++23 Submitter: Pablo Halpern Opened: 2022-03-18 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [mem.poly.allocator.class].

View all issues with C++23 status.

Discussion:

In `<memory_resource>`, the equality comparison operator for `pmr::polymorphic_allocator` is declared in namespace scope as:

>     template<class T1, class T2>
>       bool operator==(const polymorphic_allocator<T1>& a,
>                       const polymorphic_allocator<T2>& b) noexcept;

Since `polymorphic_allocator` is implicitly convertible from `memory_resource*`, one would naively expect — and the author of `polymorphic_allocator` intended — the following code to work:

>     std::pmr::unsynchronized_pool_resource pool_rsrc;
>     std::pmr::vector<int> vec(&pool_rsrc); // Converts to std::pmr::polymorphic_allocator<int>
>     […]
>     assert(vec.get_allocator() == &pool_rsrc);  // (1) Compare polymorphic_allocator to memory_resource*

Unfortunately, the line labeled (1) is ill-formed because the type `T2` in `operator==` cannot be deduced.

Possible resolution 1 (PR1) is to supply a second `operator==`, overloaded for comparison to `memory_resource*`:

>     template<class T1, class T2>
>       bool operator==(const polymorphic_allocator<T1>& a,
>                       const polymorphic_allocator<T2>& b) noexcept;
>     template<class T>
>       bool operator==(const polymorphic_allocator<T>& a,
>                       memory_resource* b) noexcept;

The rules for implicitly defined spaceship and comparison operators obviates defining `operator!=` or `operator==(b, a)`. This PR would allow `polymorphic_allocator` to be compared for equality with `memory_resource*`, but not with any other type that is convertible to `polymorphic_allocator`.

Possible resolution 2 (PR2) is to replace `operator==` with a homogeneous version where type deduction occurs only for one template parameter:

>     template<class T1, class T2>
>       bool operator==(const polymorphic_allocator<T1>& a,
>                       const polymorphic_allocator<T2>& b) noexcept;
>     template<class T>
>       bool operator==(const polymorphic_allocator<T>& a,
>                       const type_identity_t<polymorphic_allocator<T>>& b) noexcept;

This version will work with any type that is convertible to `polymorphic_allocator`.

Possible resolution 3 (PR3), the proposed resolution, below, is to add a homogeneous equality operator as a "hidden friend", such that it is found by ADL only if one argument is a `polymorphic_allocator` and the other argument is convertible to `polymorphic_allocator`. As with PR2, this PR will work with any type that is convertible to `polymorphic_allocator`.

> Note to reader: Proof of concept for the three possible resolutions can be seen at this godbolt link. Uncomment one of PR1, PR2, or PR3 macros to see the effects of each PR.

[2022-05-17; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 20.5.3 [mem.poly.allocator.class], class template `polymorphic_allocator` synopsis, as indicated:

   >     namespace std::pmr {
   >       template<class Tp = byte> class polymorphic_allocator {
   >         memory_resource* memory_rsrc; // exposition only

   >       public:
   >         using value_type = Tp;

   >         […]
   >         memory_resource* resource() const;

   >         // friends
   >         friend bool operator==(const polymorphic_allocator& a,
   >                                const polymorphic_allocator& b) noexcept {
   >           return *a.resource() == *b.resource();
   >         }
   >       };
   >     }

### 3687(i). `expected<cv void, E>` move constructor should move

Section: 22.8.7.4 [expected.void.assign] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-03-23 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [expected.void.assign].

View all issues with C++23 status.

Discussion:

For `expected<cv void>::operator=(expected&&)` we have this in the last bullet of the Effects element:

> Otherwise, equivalent to `unex = rhs.error()`.

That should be a move assignment, not a copy assignment.

[2022-05-17; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 22.8.7.4 [expected.void.assign] as indicated:

   >     constexpr expected& operator=(expected&& rhs) noexcept(see below);

   > > -4- Effects:

   > > 1. (4.1) — If `this->has_value() && rhs.has_value()` is `true`, no effects.

   > > 2. (4.2) — Otherwise, if `this->has_value()` is `true`, equivalent to:

   > >    >     construct_at(addressof(unex), std::move(rhs.unex));
   > >    >     has_val = false;

   > > 3. (4.3) — Otherwise, if `rhs.has_value()` is `true`, destroys `unex` and sets `has_val` to `true`.

   > > 4. (4.4) — Otherwise, equivalent to `unex = std::move(rhs.error())`.

### 3692(i). `zip_view::iterator`'s `operator<=>` is overconstrained

Section: 25.7.25.3 [range.zip.iterator] Status: C++23 Submitter: S. B. Tam Opened: 2022-04-21 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

`zip_view::iterator`'s `operator<=>` is constrained with `(three_way_comparable<iterator_t<maybe-const<Const, Views>>> && ...)`. This is unnecessary, because the comparison is performed on the stored `tuple-or-pair`, and both `std::tuple` and `std::pair` provide `operator<=>` regardless of whether the elements are three-way comparable.

Note that, because neither `std::tuple` nor `std::pair` provides `operator<` since C++20, comparing two `zip::iterator`s with `operator<` (which is specified to return `x.current_ < y.current_`, where `current_` is a `tuple-or-pair`) eventually uses `tuple` or `pair`'s `operator<=>` anyway.

Thus, I think it's possible to make `operator<=>` not require `three_way_comparable`. This also makes it possible to remove the operator functions for `<`, `>`, `<=`, `>=` and rely on the operators synthesized from `operator<=>`.

[2022-04-24; Daniel comments and provides wording]

It should be pointed out that by still constraining `operator<=>` with `all-random-access<Const, Views...>` we also constrain by `random_access_iterator<iterator_t<maybe-const<Const, Views>>>` which again means that we constrain by `totally_ordered<iterator_t<maybe-const<Const, Views>>>`, so this operator will only be satisfied with iterators `I` that satisfy `partially-ordered-with<I, I>`. Based on this argument the delegation to `tuple-or-pair`'s `operator<=>` that solely depends on `synth-three-way` should be appropriate.

[2022-05-17; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.25.3 [range.zip.iterator] as indicated:

   > >     namespace std::ranges {
   > >       […]
   > >       template<input_range... Views>
   > >         requires (view<Views> && ...) && (sizeof...(Views) > 0)
   > >       template<bool Const>
   > >       class zip_view<Views...>::iterator {
   > >         tuple-or-pair<iterator_t<maybe-const<Const, Views>>...> current_; // exposition only
   > >         constexpr explicit iterator(tuple-or-pair<iterator_t<maybe-const<Const, Views>>...>); // exposition only
   > >       public:
   > >         […]
   > >         friend constexpr bool operator==(const iterator& x, const iterator& y)
   > >           requires (equality_comparable<iterator_t<maybe-const<Const, Views>>> && ...);

   > >         friend constexpr bool operator<(const iterator& x, const iterator& y)
   > >           requires all-random-access<Const, Views...>;
   > >         friend constexpr bool operator>(const iterator& x, const iterator& y)
   > >           requires all-random-access<Const, Views...>;
   > >         friend constexpr bool operator<=(const iterator& x, const iterator& y)
   > >           requires all-random-access<Const, Views...>;
   > >         friend constexpr bool operator>=(const iterator& x, const iterator& y)
   > >           requires all-random-access<Const, Views...>;
   > >         friend constexpr auto operator<=>(const iterator& x, const iterator& y)
   > >           requires all-random-access<Const, Views...> &&
   > >                    three_way_comparable<iterator_t<maybe-const<Const, Views>>> && ...);
   > >         […]
   > >       };
   > >       […]
   > >     }

   > […]

   >     friend constexpr bool operator<(const iterator& x, const iterator& y)
   >       requires all-random-access<Const, Views...>;

   > > -16- Returns: `x.current_ < y.current_`.

   >     friend constexpr bool operator>(const iterator& x, const iterator& y)
   >       requires all-random-access<Const, Views...>;

   > > -17- Effects: Equivalent to: `return y < x;`

   >     friend constexpr bool operator<=(const iterator& x, const iterator& y)
   >       requires all-random-access<Const, Views...>;

   > > -18- Effects: Equivalent to: `return !(y < x);`

   >     friend constexpr bool operator>=(const iterator& x, const iterator& y)
   >       requires all-random-access<Const, Views...>;

   > > -19- Effects: Equivalent to: `return !(x < y);`

   >     friend constexpr auto operator<=>(const iterator& x, const iterator& y)
   >       requires all-random-access<Const, Views...> &&
   >               (three_way_comparable<iterator_t<maybe-const<Const, Views>>> && ...);

   > > -20- Returns: `x.current_ <=> y.current_`.

### 3698(i). `regex_iterator` and `join_view` don't work together very well

Section: 28.6.11 [re.iter], 25.7.14 [range.join] Status: Resolved Submitter: Barry Revzin Opened: 2022-05-12 Last modified: 2023-03-23

Priority: 2

View all other issues in [re.iter].

View all issues with Resolved status.

Discussion:

Consider this example (from StackOverflow):

>     #include <ranges>
>     #include <regex>
>     #include <iostream>

>     int main() {
>       char const text[] = "Hello";
>       std::regex regex{"[a-z]"};

>       auto lower = std::ranges::subrange(
>             std::cregex_iterator(
>                 std::ranges::begin(text),
>                 std::ranges::end(text),
>                 regex),
>             std::cregex_iterator{}
>         )
>         | std::views::join
>         | std::views::transform([](auto const& sm) {
>             return std::string_view(sm.first, sm.second);
>         });

>       for (auto const& sv : lower) {
>         std::cout << sv << '\n';
>       }
>     }

This example seems sound, having `lower` be a range of `string_view` that should refer back into `text`, which is in scope for all this time. The `std::regex` object is also in scope for all this time.

Yet, if run this through address sanitizer, this blows up in the first call to the dereference operator of the underlying `transform_view`'s iterator with heap-use-after-free.

The problem here is ultimately that `regex_iterator` is a stashing iterator (it has a member `match_results`) yet advertises itself as a `forward_iterator` (despite violating 24.3.5.5 [forward.iterators] p6 and 24.3.4.11 [iterator.concept.forward] p3.

Then, `join_view`'s iterator stores an outer iterator (the `regex_iterator`) and an `inner_iterator` (an iterator into the container that the `regex_iterator` stashes). Copying that iterator effectively invalidates it — since the new iterator's inner iterator will refer to the old iterator's outer iterator's container. These aren't (and can't be) independent copies. In this particular example, `join_view`'s `begin` iterator is copied into the `transform_view`'s iterator, and then the original is destroyed (which owns the container that the new inner iterator still points to), which causes us to have a dangling iterator.

Note that the example is well-formed in libc++ because libc++ moves instead of copying an iterator, which happens to work. But I can produce other non-transform-view related examples that fail.

This is actually two different problems:

1. `regex_iterator` is really an input iterator, not a forward iterator. It does not meet either the C++17 or the C++20 forward iterator requirements.

2. `join_view` can't handle stashing iterators, and would need to additionally store the outer iterator in a non-propagating-cache for input ranges (similar to how it already potentially stores the inner iterator in a non-propagating-cache).

(So potentially this could be two different LWG issues, but it seems nicer to think of them together.)

[2022-05-17; Reflector poll]

Set priority to 2 after reflector poll.

[Kona 2022-11-08; Move to Open]

Tim to write a paper

[2023-01-16; Tim comments]

The paper P2770R0 is provided with proposed wording.

[2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: Open → Resolved.]

Proposed resolution:

### 3700(i). The `const begin` of the `join_view` family does not require `InnerRng` to be a range

Section: 25.7.14.2 [range.join.view], 25.7.15.2 [range.join.with.view] Status: Resolved Submitter: Hewill Kang Opened: 2022-05-17 Last modified: 2023-03-23

Priority: 3

View all other issues in [range.join.view].

View all issues with Resolved status.

Discussion:

This is a follow-up of LWG 3599(i).

Currently, the `const` version of `join_view::begin` has the following constraints

>     constexpr auto begin() const
>       requires input_range<const V> &&
>                is_reference_v<range_reference_t<const V>>
>     { return iterator<true>{*this, ranges::begin(base_)}; }

which only requires `InnerRng` to be a reference type, but in some cases, `InnerRng` may not be a `range`. Consider

>     #include <ranges>

>     int main() {
>       auto r = std::views::iota(0, 5)
>              | std::views::split(1);
>       auto s = std::views::single(r);
>       auto j = s | std::views::join;
>       auto f = j.front();
>     }

The reference type of `single_view` is `const split_view&`, which only has the non-`const` version of `begin`, which will cause `view_interface`'s `const front` to be incorrectly instantiated, making `r.front()` unnecessarily ill-formed.

We should add this check for `join_view`'s `const begin`, as well as `join_with_view`.

[2022-06-21; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N4910.

1. Modify 25.7.14.2 [range.join.view], class template `join_view` synopsis, as indicated:

   >     namespace std::ranges {

   >       […]

   >       template<input_range V>
   >         requires view<V> && input_range<range_reference_t<V>>
   >       class join_view : public view_interface<join_view<V>> {
   >       private:
   >         […]
   >       public:
   >         […]

   >         constexpr auto begin() {
   >           […]
   >         }

   >         constexpr auto begin() const
   >           requires input_range<const V> &&
   >                     input_range<range_reference_t<const V>> &&
   >                     is_reference_v<range_reference_t<const V>>
   >         { return iterator<true>{*this, ranges::begin(base_)}; }

   >         constexpr auto end() {
   >           […]
   >         }

   >         constexpr auto end() const
   >           requires input_range<const V> &&
   >                     input_range<range_reference_t<const V>> &&
   >                     is_reference_v<range_reference_t<const V>> {
   >           if constexpr (forward_range<const V> &&
   >                         forward_range<range_reference_t<const V>> &&
   >                         common_range<const V> &&
   >                         common_range<range_reference_t<const V>>)
   >             return iterator<true>{*this, ranges::end(base_)};
   >           else
   >             return sentinel<true>{*this};
   >         }
   >       };

   >       […]

   >     }

2. Modify 25.7.15.2 [range.join.with.view], class template `join_with_view` synopsis, as indicated:

   >     namespace std::ranges {

   >       […]

   >       template<input_range V, forward_range Pattern>
   >       requires view<V> && input_range<range_reference_t<V>>
   >             && view<Pattern>
   >             && compatible-joinable-ranges<range_reference_t<V>, Pattern>
   >       class join_with_view : public view_interface<join_with_view<V, Pattern>> {
   >       private:
   >         […]
   >       public:
   >         […]

   >         constexpr auto begin() {
   >           […]
   >         }
   >         constexpr auto begin() const
   >           requires input_range<const V> &&
   >                     forward_range<const Pattern> &&
   >                     input_range<range_reference_t<const V>> &&
   >                     is_reference_v<range_reference_t<const V>> {
   >           return iterator<true>{*this, ranges::begin(base_)};
   >         }

   >         constexpr auto end() {
   >           […]
   >         }
   >         constexpr auto end() const
   >           requires input_range<const V> && forward_range<const Pattern> &&
   >                     input_range<range_reference_t<const V>> &&
   >                     is_reference_v<range_reference_t<const V>> {
   >           using InnerConstRng = range_reference_t<const V>;
   >           if constexpr (forward_range<const V> && forward_range<InnerConstRng> &&
   >                         common_range<const V> && common_range<InnerConstRng>)
   >             return iterator<true>{*this, ranges::end(base_)};
   >           else
   >             return sentinel<true>{*this};
   >         }
   >       };

   >       […]

   >     }

[2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

### 3701(i). Make `formatter<remove_cvref_t<const charT[N]>, charT>` requirement explicit

Section: 28.5.6.4 [format.formatter.spec] Status: C++23 Submitter: Mark de Wever Opened: 2022-05-17 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [format.formatter.spec].

View all other issues in [format.formatter.spec].

View all issues with C++23 status.

Discussion:

The wording in 28.5.5 [format.functions]/20 and 28.5.5 [format.functions]/25 both contain

> `formatter<remove_cvref_t<Ti>, charT>` meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]) for each `Ti` in `Args`.

The issue is that `remove_cvref_t<const charT[N]>` becomes `charT[N]`. 28.5.6.4 [format.formatter.spec]/2.2 requires a specialization for

>     template<size_t N> struct formatter<const charT[N], charT>;

but there's no requirement to provide

>      template<size_t N> struct formatter<charT[N], charT>;

There's no wording preventing library vendors from providing additional specializations. So it's possible to implement the current specification but the indirect requirement is odd. I noticed this while implementing a `formattable` concept. The concept is based on the `formattable` concept of P2286 "Formatting Ranges" (This paper is targeting C++23.)

It could be argued that the specialization

>     template<size_t N> struct formatter<const charT[N], charT>

is not needed and should be removed from the Standard. This will be an API break. Vendors can decide to keep the no longer required specialization as an extension; which would lead to implementation divergence. Microsoft is already shipping this specialization as stable and Victor doesn't like the removal too.

Therefore I only propose to add the required `formatter` specialization.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 28.5.6.4 [format.formatter.spec] as indicated:

   > -2- Let `charT` be either `char` or `wchar_t`. Each specialization of `formatter` is either enabled or disabled, as described below. Each header that declares the template `formatter` provides the following enabled specializations:

   > 1. (2.1) — The specializations […]

   > 2. (2.2) — For each `charT`, the string type specializations

   >    >     template<> struct formatter<charT*, charT>;
   >    >     template<> struct formatter<const charT*, charT>;
   >    >     template<size_t N> struct formatter<charT[N], charT>;
   >    >     template<size_t N> struct formatter<const charT[N], charT>;
   >    >     template<class traits, class Allocator>
   >    >       struct formatter<basic_string<charT, traits, Allocator>, charT>;
   >    >     template<class traits>
   >    >       struct formatter<basic_string_view<charT, traits>, charT>;

   > 3. (2.3) — […]

   > 4. (2.4) — […]

### 3702(i). Should `zip_transform_view::iterator` remove `operator<`?

Section: 25.7.26.3 [range.zip.transform.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-05-21 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

After LWG 3692(i), `zip_view::iterator` only provides `operator<=>`. Since the comparison of `zip_transform_view::iterator` uses `zip_view::iterator`'s `operator<=>`, it is possible to remove `zip_transform_view::iterator`'s `operator<`, `>`, `<=`, `>=` and just detect if `ziperator`'s `operator<=>` is available.

Since the `ziperator`'s `operator<=>` is valid only when `zip_view` is a `random_access_range`, we don't need to additionally constrain the `ziperator` to be `three_way_comparable`.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.26.3 [range.zip.transform.iterator] as indicated:

   > >     namespace std::ranges {
   > >     […]
   > >     template<copy_constructible F, input_range... Views>
   > >     requires (view<Views> && ...) && (sizeof...(Views) > 0) && is_object_v<F> &&
   > >              regular_invocable<F&, range_reference_t<Views>...> &&
   > >              can-reference<invoke_result_t<F&, range_reference_t<Views>...>>
   > >     template<bool Const>
   > >     class zip_transform_view<F, Views...>::iterator {
   > >       using Parent = maybe-const<Const, zip_transform_view>;        // exposition only
   > >       using Base = maybe-const<Const, InnerView>;                   // exposition only
   > >       Parent* parent_ = nullptr;                                    // exposition only
   > >       ziperator<Const> inner_;                                      // exposition only

   > >       constexpr iterator(Parent& parent, ziperator<Const> inner);   // exposition only
   > >     public:
   > >       […]
   > >       friend constexpr bool operator==(const iterator& x, const iterator& y)
   > >         requires equality_comparable<ziperator<Const>>;

   > >       friend constexpr bool operator<(const iterator& x, const iterator& y)
   > >         requires random_access_range<Base>;
   > >       friend constexpr bool operator>(const iterator& x, const iterator& y)
   > >         requires random_access_range<Base>;
   > >       friend constexpr bool operator<=(const iterator& x, const iterator& y)
   > >         requires random_access_range<Base>;
   > >       friend constexpr bool operator>=(const iterator& x, const iterator& y)
   > >         requires random_access_range<Base>;
   > >       friend constexpr auto operator<=>(const iterator& x, const iterator& y)
   > >         requires random_access_range<Base>&& three_way_comparable<ziperator<Const>>;
   > >       […]
   > >     };
   > >     […]
   > >     }

   > […]

   >     friend constexpr bool operator==(const iterator& x, const iterator& y)
   >       requires equality_comparable<ziperator<Const>>;
   >     friend constexpr bool operator<(const iterator& x, const iterator& y)
   >       requires random_access_range<Base>;
   >     friend constexpr bool operator>(const iterator& x, const iterator& y)
   >       requires random_access_range<Base>;
   >     friend constexpr bool operator<=(const iterator& x, const iterator& y)
   >       requires random_access_range<Base>;
   >     friend constexpr bool operator>=(const iterator& x, const iterator& y)
   >       requires random_access_range<Base>;
   >     friend constexpr auto operator<=>(const iterator& x, const iterator& y)
   >       requires random_access_range<Base>&& three_way_comparable<ziperator<Const>>;

   > > -14- Let `op` be the operator.

   > > -15- Effects: Equivalent to: `return x.inner_ op y.inner_;`

### 3703(i). Missing requirements for `expected<T, E> requires is_void<T>`

Section: 22.8.7.1 [expected.void.general] Status: C++23 Submitter: Casey Carter Opened: 2022-05-24 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

The partial specialization `expected<T, E> requires is_void<T>` specified in 22.8.7.1 [expected.void.general] is missing some template parameter requirements that should have been copied from 22.8.6.1 [expected.object.general]. We should copy the pertinent requirements from the first two paragraphs of the latter subclause into new paragraphs in the first subclause (the pertinent requirement from the third paragraph is already present in 22.8.7.1 [expected.void.general]).

[2022-06-21; Jonathan adds "Member" before "`has_val`"]

[2022-06-21; Reflector poll]

Set priority to 2 after reflector poll.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

> [Drafting note: There is some drive-by cleanup that I couldn't resist while touching this wording: (1) strike the redundant "suitably aligned" guarantee, (2) Don't repeat in prose that the exposition-only members are exposition-only.]

1. Modify 22.8.6.1 [expected.object.general] as indicated:

   > -1- Any object of type `expected<T, E>` either contains a value of type `T` or a value of type `E` within its own storage. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of type `T` or the object of type `E`. These objects are allocated in a region of the `expected<T, E>` storage suitably aligned for the types `T` and `E`. Members `has_val`, `val`, and `unex` are provided for exposition only. Member `has_val` indicates whether the `expected<T, E>` object contains an object of type `T`.

2. Modify 22.8.7.1 [expected.void.general] as indicated:

   > -?- Any object of type `expected<T, E>` either represents a value of type `T`, or contains a value of type `E` within its own storage. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of type `E`. Member `has_val` indicates whether the `expected<T, E>` object represents a value of type `T`.

   > -?- A program that instantiates the definition of the template `expected<T, E>` with a type for the `E` parameter that is not a valid template argument for `unexpected` is ill-formed.

   > -1- `E` shall meet the requirements of Cpp17Destructible (Table [tab:cpp17.destructible]).

### 3704(i). LWG 2059 added overloads that might be ill-formed for sets

Section: 23.4.6.1 [set.overview], 23.4.7.1 [multiset.overview], 23.5.6.1 [unord.set.overview], 23.5.7.1 [unord.multiset.overview] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-05-25 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

The restored `erase(iterator)` overloads introduced by LWG 2059(i) would be duplicates of the `erase(const_iterator)` ones if `iterator` and `const_iterator` are the same type, which is allowed for sets.

We should constrain them (or add prose) so that the `erase(iterator)` overloads are only present when the iterator types are distinct.

This applies to `set`, `multiset`, `unordered_set`, `unordered_multiset` (and `flat_set` and `flat_multiset`).

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 23.4.6.1 [set.overview], class template `set` synopsis, as indicated:

   >     iterator erase(iterator position) requires (!same_as<iterator, const_iterator>);
   >     iterator erase(const_iterator position);

2. Modify 23.4.7.1 [multiset.overview], class template `multiset` synopsis, as indicated:

   >     iterator erase(iterator position) requires (!same_as<iterator, const_iterator>);
   >     iterator erase(const_iterator position);

3. Modify 23.5.6.1 [unord.set.overview], class template `unordered_set` synopsis, as indicated:

   >     iterator erase(iterator position) requires (!same_as<iterator, const_iterator>);
   >     iterator erase(const_iterator position);

4. Modify 23.5.7.1 [unord.multiset.overview], class template `unordered_multiset` synopsis, as indicated:

   >     iterator erase(iterator position) requires (!same_as<iterator, const_iterator>);
   >     iterator erase(const_iterator position);

### 3705(i). Hashability shouldn't depend on `basic_string`'s allocator

Section: 27.4.6 [basic.string.hash] Status: C++23 Submitter: Casey Carter Opened: 2022-05-26 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [basic.string.hash].

View all issues with C++23 status.

Discussion:

27.4.6 [basic.string.hash] says:

>     template<> struct hash<string>;
>     template<> struct hash<u8string>;
>     template<> struct hash<u16string>;
>     template<> struct hash<u32string>;
>     template<> struct hash<wstring>;
>     template<> struct hash<pmr::string>;
>     template<> struct hash<pmr::u8string>;
>     template<> struct hash<pmr::u16string>;
>     template<> struct hash<pmr::u32string>;
>     template<> struct hash<pmr::wstring>;

> > -1- If `S` is one of these string types, `SV` is the corresponding string view type, and `s` is an object of type `S`, then `hash<S>()(s) == hash<SV>()(SV(s))`

Despite that the hash value of a `basic_string` object is equivalent to the hash value of a corresponding `basic_string_view` object, which has no allocator, the capability to hash a `basic_string` depends on its allocator. All of the enabled specializations have specific allocators, which fact becomes more clear if we expand the type aliases:

>     template<> struct hash<basic_string<char, char_traits<char>, allocator<char>>;
>     template<> struct hash<basic_string<char8_t, char_traits<char8_t>, allocator<char8_t>>;

>     template<> struct hash<basic_string<char16_t, char_traits<char16_t>, allocator<char16_t>>;

>     template<> struct hash<basic_string<char32_t, char_traits<char32_t>, allocator<char32_t>>;

>     template<> struct hash<basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t>>;

>     template<> struct hash<basic_string<char, char_traits<char>, pmr::polymorphic_allocator<char>>;
>     template<> struct hash<basic_string<char8_t, char_traits<char8_t>, pmr::polymorphic_allocator<char8_t>>;

>     template<> struct hash<basic_string<char16_t, char_traits<char16_t>, pmr::polymorphic_allocator<char16_t>>;

>     template<> struct hash<basic_string<char32_t, char_traits<char32_t>, pmr::polymorphic_allocator<char32_t>>;

>     template<> struct hash<basic_string<wchar_t, char_traits<wchar_t>, pmr::polymorphic_allocator<wchar_t>>;

If the hash value doesn't depend on the allocator type, why should we care about the allocator type? I posit that we should not, and that these ten explicit specializations should be replaced by 5 partial specializations that enable hashing `basic_string` specializations using these combinations of character type and traits type with any allocator type.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 27.4.2 [string.syn], header `<string>` synopsis, as indicated:

   >     […]

   >     // 27.4.6 [basic.string.hash], hash support
   >     template<class T> struct hash;
   >     template<> struct hash<string>;
   >     template<> struct hash<u8string>;
   >     template<> struct hash<u16string>;
   >     template<> struct hash<u32string>;
   >     template<> struct hash<wstring>;
   >     template<> struct hash<pmr::string>;
   >     template<> struct hash<pmr::u8string>;
   >     template<> struct hash<pmr::u16string>;
   >     template<> struct hash<pmr::u32string>;
   >     template<> struct hash<pmr::wstring>;
   >     template<class A> struct hash<basic_string<char, char_traits<char>, A>>;
   >     template<class A> struct hash<basic_string<char8_t, char_traits<char8_t>, A>>;
   >     template<class A> struct hash<basic_string<char16_t, char_traits<char16_t>, A>>;
   >     template<class A> struct hash<basic_string<char32_t, char_traits<char32_t>, A>>;
   >     template<class A> struct hash<basic_string<wchar_t, char_traits<wchar_t>, A>>;

   >     […]

2. Modify 27.4.6 [basic.string.hash] as indicated:

   >     template<> struct hash<string>;
   >     template<> struct hash<u8string>;
   >     template<> struct hash<u16string>;
   >     template<> struct hash<u32string>;
   >     template<> struct hash<wstring>;
   >     template<> struct hash<pmr::string>;
   >     template<> struct hash<pmr::u8string>;
   >     template<> struct hash<pmr::u16string>;
   >     template<> struct hash<pmr::u32string>;
   >     template<> struct hash<pmr::wstring>;
   >     template<class A> struct hash<basic_string<char, char_traits<char>, A>>;
   >     template<class A> struct hash<basic_string<char8_t, char_traits<char8_t>, A>>;
   >     template<class A> struct hash<basic_string<char16_t, char_traits<char16_t>, A>>;
   >     template<class A> struct hash<basic_string<char32_t, char_traits<char32_t>, A>>;
   >     template<class A> struct hash<basic_string<wchar_t, char_traits<wchar_t>, A>>;

   > > -1- If `S` is one of these string types, `SV` is the corresponding string view type, and `s` is an object of type `S`, then `hash<S>()(s) == hash<SV>()(SV(s))`

### 3707(i). `chunk_view::outer-iterator::value_type::size` should return unsigned type

Section: 25.7.29.4 [range.chunk.outer.value] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-01 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.chunk.outer.value].

View all issues with C++23 status.

Discussion:

Currently, the `size` function of `chunk_view::outer-iterator::value_type` returns the result of `ranges::min`, since the operands are of type `range_difference_t<V>`, this will return a signed type, which is inconsistent with the return type of `size` of the `forward`-version of `chunk_view::iterator::value_type` (25.7.29.7 [range.chunk.fwd.iter]), which always returns an unsigned type.

I think it's more reasonable to return an unsigned type, since this is intentional behavior and doesn't fall back to using the default `view_interface::size`. And if LWG 3646(i) is eventually adopted, there's no reason why it shouldn't be made unsigned.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.29.4 [range.chunk.outer.value] as indicated:

   >     constexpr auto size() const
   >       requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

   > > -4- Effects: Equivalent to:

   > > >     return to-unsigned-like(ranges::min(parent_->remainder_, ranges::end(parent_->base_) - *parent_->current_));

### 3708(i). `take_while_view::sentinel`'s conversion constructor should move

Section: 25.7.11.3 [range.take.while.sentinel] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-03 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

The conversion constructor of `take_while_view::sentinel` requires `sentinel_t<V>` must satisfy `convertible_to<sentinel_t<Base>>`, which indicates that the rvalue reference of `sentinel_t<V>` can be converted to `sentinel_t<Base>`, but in the Effects element, we assign the lvalue `s.end_` to `end_`.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.11.3 [range.take.while.sentinel] as indicated:

   >     constexpr sentinel(sentinel<!Const> s)
   >       requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

   > > -2- Effects: Initializes `end_` with `std::move(s.end_)` and `pred_` with `s.pred_`.

### 3709(i). LWG-3703 was underly ambitious

Section: 22.5.3.1 [optional.optional.general], 22.6.3.1 [variant.variant.general] Status: C++23 Submitter: Casey Carter Opened: 2022-06-08 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [optional.optional.general].

View all issues with C++23 status.

Discussion:

LWG 3703(i) struck redundant language from 22.8.7.1 [expected.void.general] specifying that (1) space allocated for an object is suitably aligned, and (2) a member annotated `// exposition only` in a class synopsis "is provided for exposition only."

Let's also strike similar occurrences from the wording for `optional` and `variant`.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 22.5.3.1 [optional.optional.general] as indicated:

   > -1- Any instance of `optional<T>` at any given time either contains a value or does not contain a value. When an instance of `optional<T>` contains a value, it means that an object of type `T`, referred to as the optional object's contained value, is allocated within the storage of the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value. The contained value shall be allocated in a region of the `optional<T>` storage suitably aligned for the type `T`. When an object of type `optional<T>` is contextually converted to `bool`, the conversion returns `true` if the object contains a value; otherwise the conversion returns `false`.

   > -2- Member `val` is provided for exposition only. When an `optional<T>` object contains a value, member `val` points to the contained value.

2. Modify 22.6.3.1 [variant.variant.general] as indicated:

   > -1- Any instance of `variant` at any given time either holds a value of one of its alternative types or holds no value. When an instance of `variant` holds a value of alternative type `T`, it means that a value of type `T`, referred to as the `variant` object's contained value, is allocated within the storage of the `variant` object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value. The contained value shall be allocated in a region of the variant storage suitably aligned for all types in `Types`.

### 3710(i). The `end` of `chunk_view` for input ranges can be `const`

Section: 25.7.29.2 [range.chunk.view.input] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-09 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.chunk.view.input].

View all issues with C++23 status.

Discussion:

The input range version of `chunk_view`'s `end` is a very simple function that only returns `default_sentinel`, and simple `end`s like this also appear in other range adaptors, such as `basic_istream_view`, `lazy_split_view::outer-iterator::value_type`, and `chunk_view::outer-iterator::value_type`.

However, unlike `chunk_view`, their `end`s all are `const`-qualified, which allows us to freely get `default_sentinel` through the `end` of these `const` objects even though they may not themselves be `range`s.

I think we should add `const` to this `chunk_view`'s `end` as I don't see any harm in doing this, and in some cases, it may have a certain value. Also, this makes it consistent with `basic_istream_view` and the upcoming `std::generator`, which, like it, only has a non-`const begin`.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.29.2 [range.chunk.view.input] as indicated:

   > >     namespace std::ranges {
   > >       […]

   > >       template<view V>
   > >         requires input_range<V>
   > >       class chunk_view : public view_interface<chunk_view<V>> {
   > >         V base_ = V();                                        // exposition only
   > >         […]
   > >       public:
   > >         […]

   > >         constexpr outer-iterator begin();
   > >         constexpr default_sentinel_t end() const noexcept;

   > >         constexpr auto size() requires sized_range<V>;
   > >         constexpr auto size() const requires sized_range<const V>;
   > >       };
   > >       […]
   > >     }

   > […]

   >     constexpr default_sentinel_t end() const noexcept;

   > > -4- Returns: `default_sentinel`.

### 3711(i). Missing preconditions for `slide_view` constructor

Section: 25.7.30.2 [range.slide.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-10 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.slide.view].

View all issues with C++23 status.

Discussion:

It doesn't make sense for `slide_view` when `n` is not positive, we should therefore add a precondition for its constructor just like we did for `chunk_view`.

Daniel:

Indeed the accepted paper describes the intention in P2442R1 Section 4.2 by saying that "It is a precondition that `N` is positive" in the design wording but omitted to add a normative precondition in the proposed wording.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.30.2 [range.slide.view] as indicated:

   >     constexpr explicit slide_view(V base, range_difference_t<V> n);

   > > -?- Preconditions: `n > 0` is `true`.

   > > -1- Effects: Initializes `base_` with `std::move(base)` and `n_` with `n`.

### 3712(i). `chunk_view` and `slide_view` should not be `default_initializable`

Section: 25.7.29.2 [range.chunk.view.input], 25.7.29.6 [range.chunk.view.fwd], 25.7.30.2 [range.slide.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-10 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.chunk.view.input].

View all issues with C++23 status.

Discussion:

Both `chunk_view` and `slide_view` have a precondition that `N` must be positive, but they are still `default_initializable` when the underlying range is `default_initializable`, which makes the member variable `n_` initialized with an invalid value `0` when they are default-constructed, which produces the following unexpected result:

>     #include <ranges>

>     using V = std::ranges::iota_view<int, int>;
>     static_assert(std::ranges::slide_view<V>().empty()); // fails
>     static_assert(std::ranges::chunk_view<V>().empty()); // division by zero is not a constant expression

Although we could provide a default positive value for `n_`, I think a more appropriate solution would be to not provide the default constructor, since default-constructed values for integer types will never be valid.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.29.2 [range.chunk.view.input] as indicated:

   >     namespace std::ranges {
   >       […]

   >       template<view V>
   >         requires input_range<V>
   >       class chunk_view : public view_interface<chunk_view<V>> {
   >         V base_ = V();                                        // exposition only
   >         range_difference_t<V> n_ = 0;                         // exposition only
   >         range_difference_t<V> remainder_ = 0;                 // exposition only
   >         […]
   >       public:
   >         chunk_view() requires default_initializable<V> = default;
   >         constexpr explicit chunk_view(V base, range_difference_t<V> n);
   >         […]
   >       };
   >       […]
   >     }

2. Modify 25.7.29.6 [range.chunk.view.fwd] as indicated:

   >     namespace std::ranges {
   >       template<view V>
   >         requires forward_range<V>
   >       class chunk_view<V> : public view_interface<chunk_view<V>> {
   >         V base_ = V();                   // exposition only
   >         range_difference_t<V> n_ = 0;    // exposition only
   >         […]
   >       public:
   >         chunk_view() requires default_initializable<V> = default;
   >         constexpr explicit chunk_view(V base, range_difference_t<V> n);

   >         […]
   >       };
   >     }

3. Modify 25.7.30.2 [range.slide.view] as indicated:

   >     namespace std::ranges {
   >       […]

   >       template<forward_range V>
   >         requires view<V>
   >       class slide_view : public view_interface<slide_view<V>> {
   >         V base_ = V();                      // exposition only
   >         range_difference_t<V> n_ = 0;       // exposition only
   >         […]
   >       public:
   >         slide_view() requires default_initializable<V> = default;
   >         constexpr explicit slide_view(V base, range_difference_t<V> n);

   >         […]
   >       };
   >       […]
   >     }

### 3713(i). Sorted with respect to comparator (only)

Section: 26.8.1 [alg.sorting.general] Status: C++23 Submitter: Casey Carter Opened: 2022-06-10 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

P0896R4 changed the term of art "sorted with respect to comparator" defined in 26.8.1 [alg.sorting.general] paragraph 5 to "sorted with respect to comparator and projection." That proposal updated the algorithm specifications consistently. However, there were uses of the old term outside of 26 [algorithms] that are now without meaning. We should bring back the term "sorted with respect to comparator" to fix that lack.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 26.8.1 [alg.sorting.general] as indicated:

   > -5- A sequence is sorted with respect to a `comp` and `proj` for a comparator and projection `comp` and `proj` if for every iterator `i` pointing to the sequence and every non-negative integer `n` such that `i + n` is a valid iterator pointing to an element of the sequence,

   > >     bool(invoke(comp, invoke(proj, *(i + n)), invoke(proj, *i)))

   > is `false`.

   > -?- A sequence is sorted with respect to a comparator `comp` for a comparator `comp` if it is sorted with respect to `comp` and `identity{}` (the identity projection).

### 3715(i). `view_interface::empty` is overconstrained

Section: 25.5.3.1 [view.interface.general] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-12 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [view.interface.general].

View all issues with C++23 status.

Discussion:

Currently, `view_interface::empty` has the following constraints

>     constexpr bool empty() requires forward_range<D> {
>       return ranges::begin(derived()) == ranges::end(derived());
>     }

which seems reasonable, since we need to guarantee the equality preservation of the expression `ranges::begin(r)`.

However, this prevents a more efficient way in some cases, i.e., when `D` models `sized_range`, we only need to determine whether the value of `ranges::size` is `0`. Since `sized_range` and `forward_range` are orthogonal to each other, this also prevents any range that models `sized_range` but not `forward_range`.

Consider:

>     #include <iostream>
>     #include <ranges>

>     int main() {
>       auto f = std::views::iota(0, 5)
>              | std::views::filter([](int) { return true; });
>       auto r = std::views::counted(f.begin(), 4)
>              | std::views::slide(2);
>       std::cout << (r.size() == 0) << "\n"; // #1
>       std::cout << r.empty() << "\n";       // #2, calls r.begin() == r.end()
>     }

Since `r` models `sized_range`, `#1` will invoke `slide_view::size`, which mainly invokes `ranges::distance`; However, `#2` invokes `view_interface::empty` and evaluates `r.begin() == r.end()`, which constructs the iterator, invokes `ranges::next`, and caches the result, which is unnecessary.

Also consider:

>     #include <iostream>
>     #include <ranges>

>     int main() {
>       auto i = std::views::istream<int>(std::cin);
>       auto r = std::views::counted(i.begin(), 4)
>              | std::views::chunk(2);
>       std::cout << (r.size() == 0) << "\n"; // #1
>       std::cout << !r << "\n";              // #2, equivalent to r.size() == 0
>       std::cout << r.empty() << "\n";       // #3, ill-formed
>     }

Since `r` is still `sized_range`, `#1` will invoke `chunk_view::size`. `#2` is also well-formed since `view_interface::operator bool` only requires the expression `ranges::empty(r)` to be well-formed, which first determines the validity of `r.empty()`, and ends up evaluating `#1`; However, `#3` is ill-formed since `r` is not a `forward_range`.

Although we can still use `ranges::empty` to determine whether `r` is empty, this inconsistency of the validity of `!r` and `r.empty()` is quite unsatisfactory.

I see no reason to prevent `view_interface::empty` when `D` is `sized_range`, since checking whether `ranges::size(r) == 0` is an intuitive way to check for empty, as `ranges::empty` does.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.5.3.1 [view.interface.general] as indicated:

   >     namespace std::ranges {
   >       template<class D>
   >         requires is_class_v<D> && same_as<D, remove_cv_t<D>>
   >       class view_interface {
   >       private:
   >         constexpr D& derived() noexcept {                  // exposition only
   >           return static_cast<D&>(*this);
   >         }
   >         constexpr const D& derived() const noexcept {      // exposition only
   >           return static_cast<const D&>(*this);
   >         }

   >       public:
   >         constexpr bool empty() requires sized_range<D> || forward_range<D> {
   >           if constexpr (sized_range<D>)
   >             return ranges::size(derived()) == 0;
   >           else
   >             return ranges::begin(derived()) == ranges::end(derived());
   >         }
   >         constexpr bool empty() const requires sized_range<const D> || forward_range<const D> {
   >           if constexpr (sized_range<const D>)
   >             return ranges::size(derived()) == 0;
   >           else
   >             return ranges::begin(derived()) == ranges::end(derived());
   >         }
   >         […]
   >       };
   >     }

### 3717(i). `common_view::end` should improve `random_access_range` case

Section: 25.7.20.2 [range.common.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-15 Last modified: 2023-11-22

Priority: 3

View all other issues in [range.common.view].

View all issues with C++23 status.

Discussion:

This issue is part of NB comment US 47-109 26 [ranges] Resolve open issues

In view of the fact that random access iterators are only required to work with its difference type, P2393R1 improves the wording of `take_view`, which first convert the integer type to difference type and then operate with the iterator. However, the paper omits the handling of random access iterators in `common_view::end`, which directly operates on the return types of `ranges::begin` and `ranges::size`. We should improve this, too.

[2022-07-06; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> 1. Modify 25.7.20.2 [range.common.view] as indicated:

>    >     namespace std::ranges {
>    >       template<view V>
>    >         requires (!common_range<V> && copyable<iterator_t<V>>)
>    >       class common_view : public view_interface<common_view<V>> {
>    >       private:
>    >         V base_ = V();  // exposition only
>    >       public:
>    >         […]
>    >         constexpr auto begin() {
>    >           if constexpr (random_access_range<V> && sized_range<V>)
>    >             return ranges::begin(base_);
>    >           else
>    >             return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_));
>    >         }

>    >         constexpr auto begin() const requires range<const V> {
>    >           if constexpr (random_access_range<const V> && sized_range<const V>)
>    >             return ranges::begin(base_);
>    >           else
>    >             return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_));
>    >         }

>    >         constexpr auto end() {
>    >           if constexpr (random_access_range<V> && sized_range<V>)
>    >             return ranges::begin(base_) + range_difference_t<V>(size())ranges::size(base_);
>    >           else
>    >             return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_));
>    >         }

>    >         constexpr auto end() const requires range<const V> {
>    >           if constexpr (random_access_range<const V> && sized_range<const V>)
>    >             return ranges::begin(base_) + range_difference_t<const V>(size())ranges::size(base_);
>    >           else
>    >             return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_));
>    >         }

>    >         constexpr auto size() requires sized_range<V> {
>    >           return ranges::size(base_);
>    >         }
>    >         constexpr auto size() const requires sized_range<const V> {
>    >           return ranges::size(base_);
>    >         }
>    >       };
>    >       […]
>    >     }

[Kona 2022-11-08; Discussed at joint LWG/SG9 session. Move to Open]

[2022-11-09 Tim updates wording per LWG discussion]

[Kona 2022-11-10; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.20.2 [range.common.view] as indicated:

   >     namespace std::ranges {
   >       template<view V>
   >         requires (!common_range<V> && copyable<iterator_t<V>>)
   >       class common_view : public view_interface<common_view<V>> {
   >       private:
   >         V base_ = V();  // exposition only
   >       public:
   >         […]
   >         constexpr auto begin() {
   >           if constexpr (random_access_range<V> && sized_range<V>)
   >             return ranges::begin(base_);
   >           else
   >             return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_));
   >         }

   >         constexpr auto begin() const requires range<const V> {
   >           if constexpr (random_access_range<const V> && sized_range<const V>)
   >             return ranges::begin(base_);
   >           else
   >             return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_));
   >         }

   >         constexpr auto end() {
   >           if constexpr (random_access_range<V> && sized_range<V>)
   >             return ranges::begin(base_) + ranges::sizedistance(base_);
   >           else
   >             return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_));
   >         }

   >         constexpr auto end() const requires range<const V> {
   >           if constexpr (random_access_range<const V> && sized_range<const V>)
   >             return ranges::begin(base_) + ranges::sizedistance(base_);
   >           else
   >             return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_));
   >         }

   >         constexpr auto size() requires sized_range<V> {
   >           return ranges::size(base_);
   >         }
   >         constexpr auto size() const requires sized_range<const V> {
   >           return ranges::size(base_);
   >         }
   >       };
   >       […]
   >     }

### 3718(i). P2418R2 broke the overload resolution for `std::basic_format_arg`

Section: 28.5.8.1 [format.arg] Status: Resolved Submitter: Jiang An Opened: 2022-06-17 Last modified: 2023-03-23

Priority: 2

View all other issues in [format.arg].

View all issues with Resolved status.

Discussion:

While correcting some bugs in MSVC STL, it is found that P2418R2 broke the overload resolution involving non-const lvalue: constructing `basic_format_arg` from a non-`const basic_string` lvalue incorrectly selects the `T&&` overload and uses `handle`, while the separated `basic_string` overload should be selected (i.e. the old behavior before P2418R2 did the right thing). Currently MSVC STL is using a workaround that treats `basic_string` to be as if passed by value during overload resolution.

I think the `T&&` overload should not interfere the old result of overload resolution, which means that when a type is `const`-formattable, the newly added non-`const` mechanism shouldn't be considered.

[2022-07-06; Reflector poll]

Set priority to 2 after reflector poll.

[2022-10-19; Would be resolved by 3631(i)]

Previous resolution [SUPERSEDED]

This wording is relative to N4910.

> [Drafting note: The below presented wording adds back the `const T&` constructor and its specification that were present before P2418R2. Furthermore it adds an additional constraint to the `T&&` constructor and simplifies the Effects of this constructors to the `handle` construction case.]

1. Modify 28.5.8.1 [format.arg] as indicated:

   > >     namespace std {
   > >       template<class Context>
   > >       class basic_format_arg {
   > >       public:
   > >         class handle;

   > >       private:
   > >         […]
   > >         template<class T> explicit basic_format_arg(T&& v) noexcept; // exposition only
   > >         template<class T> explicit basic_format_arg(const T& v) noexcept; // exposition only
   > >         […]
   > >       };
   > >     }

   > […]

   >     template<class T> explicit basic_format_arg(T&& v) noexcept;

   > > -4- Constraints: The template specialization

   > > >     typename Context::template formatter_type<remove_cvref_t<T>>

   > > meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression

   > > >     typename Context::template formatter_type<remove_cvref_t<T>>()
   > > >       .format(declval<T&>(), declval<Context&>())

   > > shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]), and if this overload were not declared, the overload resolution would find no usable candidate or be ambiguous. [Note ?: This overload has no effect if the overload resolution among other overloads succeeds. — end note].

   > > -5- Effects:

   > > 1. (5.1) — if `T` is `bool` or `char_type`, initializes `value` with `v`;

   > > 2. (5.2) — otherwise, if `T` is `char` and `char_type` is `wchar_t`, initializes `value` with `static_cast<wchar_t>(v)`;

   > > 3. (5.3) — otherwise, if `T` is a signed integer type (6.9.2 [basic.fundamental]) and `sizeof(T) <= sizeof(int)`, initializes `value` with `static_cast<int>(v)`;

   > > 4. (5.4) — otherwise, if `T` is an unsigned integer type and `sizeof(T) <= sizeof(unsigned int)`, initializes `value` with `static_cast<unsigned int>(v)`;

   > > 5. (5.5) — otherwise, if `T` is a signed integer type and `sizeof(T) <= sizeof(long long int)`, initializes `value` with `static_cast<long long int>(v)`;

   > > 6. (5.6) — otherwise, if `T` is an unsigned integer type and `sizeof(T) <= sizeof(unsigned long long int)`, initializes `value` with `static_cast<unsigned long long int>(v)`;

   > > 7. (5.7) — otherwise, iInitializes `value` with `handle(v)`.

   >     template<class T> explicit basic_format_arg(const T& v) noexcept;

   > > -?- Constraints: The template specialization

   > > >     typename Context::template formatter_type<T>

   > > meets the Formatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the Formatter requirements is unspecified, except that as a minimum the expression

   > > >     typename Context::template formatter_type<T>()
   > > >       .format(declval<const T&>(), declval<Context&>())

   > > shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).

   > > -?- Effects:

   > > 1. (?.1) — if `T` is `bool` or `char_type`, initializes `value` with `v`;

   > > 2. (?.2) — otherwise, if `T` is `char` and `char_type` is `wchar_t`, initializes `value` with `static_cast<wchar_t>(v)`;

   > > 3. (?.3) — otherwise, if `T` is a signed integer type (6.9.2 [basic.fundamental]) and `sizeof(T) <= sizeof(int)`, initializes `value` with `static_cast<int>(v)`;

   > > 4. (?.4) — otherwise, if `T` is an unsigned integer type and `sizeof(T) <= sizeof(unsigned int)`, initializes `value` with `static_cast<unsigned int>(v)`;

   > > 5. (?.5) — otherwise, if `T` is a signed integer type and `sizeof(T) <= sizeof(long long int)`, initializes `value` with `static_cast<long long int>(v)`;

   > > 6. (?.6) — otherwise, if `T` is an unsigned integer type and `sizeof(T) <= sizeof(unsigned long long int)`, initializes `value` with `static_cast<unsigned long long int>(v)`;

   > > 7. (?.7) — otherwise, initializes `value` with `handle(v)`.

[2023-03-22 Resolved by the adoption of 3631(i) in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

### 3719(i). Directory iterators should be usable with default sentinel

Section: 31.12.11.1 [fs.class.directory.iterator.general], 31.12.12.1 [fs.class.rec.dir.itr.general] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-06-17 Last modified: 2024-01-29

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

We added comparisons with `default_sentinel_t` to the stream and streambuf iterators, because their past-the-end iterator is just a default-constructed iterator. We didn't do the same for filesystem directory iterators, but they also use a default-constructed value as the sentinel.

The proposed resolution addresses this oversight.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> 1. Modify 31.12.11.1 [fs.class.directory.iterator.general], class `directory_iterator` synopsis, as indicated:

>    >     namespace std::filesystem {
>    >       class directory_iterator {
>    >         […]

>    >         const directory_entry& operator*() const;
>    >         const directory_entry* operator->() const;
>    >         directory_iterator& operator++();
>    >         directory_iterator& increment(error_code& ec);

>    >         friend bool operator==(const directory_iterator& lhs, default_sentinel_t) noexcept
>    >         { return lhs == end(lhs); }

>    >         // other members as required by 24.3.5.3 [input.iterators], input iterators
>    >       };
>    >     }

> 2. Modify 31.12.12.1 [fs.class.rec.dir.itr.general], class `recursive_directory_iterator` synopsis, as indicated:

>    >     namespace std::filesystem {
>    >       class recursive_directory_iterator {
>    >         […]

>    >         void pop();
>    >         void pop(error_code& ec);
>    >         void disable_recursion_pending();

>    >         friend bool operator==(const recursive_directory_iterator& lhs, default_sentinel_t) noexcept
>    >         { return lhs == end(lhs); }

>    >         // other members as required by 24.3.5.3 [input.iterators], input iterators
>    >       };
>    >     }

[2022-07-06; Jonathan Wakely revises proposed resolution and adds regex iterators as suggested on the reflector.]

[2022-07-11; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 31.12.11.1 [fs.class.directory.iterator.general], class `directory_iterator` synopsis, as indicated:

   >     namespace std::filesystem {
   >       class directory_iterator {
   >         […]

   >         const directory_entry& operator*() const;
   >         const directory_entry* operator->() const;
   >         directory_iterator& operator++();
   >         directory_iterator& increment(error_code& ec);

   >         bool operator==(default_sentinel_t) const noexcept
   >         { return *this == directory_iterator(); }

   >         // other members as required by 24.3.5.3 [input.iterators], input iterators
   >       };
   >     }

2. Modify 31.12.12.1 [fs.class.rec.dir.itr.general], class `recursive_directory_iterator` synopsis, as indicated:

   >     namespace std::filesystem {
   >       class recursive_directory_iterator {
   >         […]

   >         void pop();
   >         void pop(error_code& ec);
   >         void disable_recursion_pending();

   >         bool operator==(default_sentinel_t) const noexcept
   >         { return *this == recursive_directory_iterator(); }

   >         // other members as required by 24.3.5.3 [input.iterators], input iterators
   >       };
   >     }

3. Modify 28.6.11.1.1 [re.regiter.general], `regex_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<class BidirectionalIterator,
   >                 class charT = typename iterator_traits<BidirectionalIterator>::value_type,
   >                 class traits = regex_traits<charT>>
   >         class regex_iterator {
   >           […]
   >           regex_iterator& operator=(const regex_iterator&);
   >           bool operator==(const regex_iterator&) const;
   >           bool operator==(default_sentinel_t) const { return *this == regex_iterator(); }
   >           const value_type& operator*() const;
   >           const value_type* operator->() const;

4. Modify 28.6.11.2.1 [re.tokiter.general], `regex_token_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<class BidirectionalIterator,
   >                 class charT = typename iterator_traits<BidirectionalIterator>::value_type,
   >                 class traits = regex_traits<charT>>
   >         class regex_token_iterator {
   >           […]
   >           regex_iterator& operator=(const regex_token_iterator&);
   >           bool operator==(const regex_token_iterator&) const;
   >           bool operator==(default_sentinel_t) const { return *this == regex_token_iterator(); }
   >           const value_type& operator*() const;
   >           const value_type* operator->() const;

### 3720(i). Restrict the valid types of arg-id for width and precision in std-format-spec

Section: 28.5.2.2 [format.string.std] Status: C++23 Submitter: Mark de Wever Opened: 2022-06-19 Last modified: 2023-11-22

Priority: 2

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Discussion:

Per 28.5.2.2 [format.string.std]/7

> If `{ arg-idopt }` is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integral type, or its value is negative for precision or non-positive for width, an exception of type `format_error` is thrown.

The issue is the integral type requirement. The following code is currently valid:

>     std::cout << std::format("{:*^{}}\n", 'a', '0');
>     std::cout << std::format("{:*^{}}\n", 'a', true);

The output of the first example depends on the value of `'0'` in the implementation. When a `char` has `signed char` as underlying type negative values are invalid, while the same value would be valid when the underlying type is `unsigned char`. For the second example the range of a boolean is very small, so this seems not really useful.

Currently libc++ rejects these two examples and MSVC STL accepts them. The members of the MSVC STL team, I spoke, agree these two cases should be rejected.

The following integral types are rejected by both libc++ and MSVC STL:

>     std::cout << std::format("{:*^{}}\n", 'a', L'0');
>     std::cout << std::format("{:*^{}}\n", 'a', u'0');
>     std::cout << std::format("{:*^{}}\n", 'a', U'0');
>     std::cout << std::format("{:*^{}}\n", 'a', u8'0');

In order to accept these character types they need to meet the basic formatter requirements per 28.5.5 [format.functions]/20 and 28.5.5 [format.functions]/25

> `formatter<remove_cvref_t<Ti>, charT>` meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]) for each `Ti` in `Args`.

which requires adding the following enabled formatter specializations to 28.5.6.4 [format.formatter.spec].

>     template<> struct formatter<wchar_t, char>;

>     template<> struct formatter<char8_t, charT>;
>     template<> struct formatter<char16_t, charT>;
>     template<> struct formatter<char32_t, charT>;

Note, that the specialization `template<> struct formatter<char, wchar_t>` is already required by the Standard.

Not only do they need to be added, but it also needs to be specified how they behave when their value is not in the range of representable values for `charT`.

Instead of requiring these specializations, I propose to go the other direction and limit the allowed types to signed and unsigned integers.

[2022-07-08; Reflector poll]

Set priority to 2 after reflector poll. Tim Song commented:

> "This is technically a breaking change, so we should do it sooner rather than later.

> "I don't agree with the second part of the argument though - I don't see how this wording requires adding those transcoding specializations. Nothing in this wording requires integral types that cannot be packed into basic_format_arg to be accepted.

> "I also think we need to restrict this to signed or unsigned integer types with size no greater than sizeof(long long). Larger types get type-erased into a handle and the value isn't really recoverable without heroics."

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> 1. Modify 28.5.2.2 [format.string.std] as indicated:

>    > -7- If `{ arg-idopt }` is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integralsigned or unsigned integer type, or its value is negative for precision or non-positive for width, an exception of type `format_error` is thrown.

> 2. Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:

>    > Affected subclause: 28.5 [format]

>    > Change: Requirement changes of arg-id of the width and precision fields of std-format-spec. arg-id now requires a signed or unsigned integer type instead of an integral type.

>    > Rationale: Avoid types that are not useful and the need to specify enabled formatter specializations for all character types.

>    > Effect on original feature: Valid C++ 2020 code that passes a boolean or character type as arg-id becomes invalid. For example:

>    > >     std::format("{:*^{}}", "", true); // ill-formed, previously returned "*"

[2022-11-01; Jonathan provides improved wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 28.5.2.2 [format.string.std] as indicated:

>    > -8- If `{ arg-idopt }` is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integralstandard signed or unsigned integer type, or its value is negative, an exception of type `format_error` is thrown.

> 2. Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:

>    > Affected subclause: 28.5.2.2 [format.string.std]

>    > Change: Restrict types of formatting arguments used as width or precision in a std-format-spec.

>    > Rationale: Avoid types that are not useful or do not have portable semantics.

>    > Effect on original feature: Valid C++ 2020 code that passes a boolean or character type as arg-id becomes invalid. For example:

>    > >     std::format("{:*^{}}", "", true); // ill-formed, previously returned "*"
>    > >     std::format("{:*^{}}", "", '1'); // ill-formed, previously returned an implementation-defined number of '*' characters

[2022-11-10; Jonathan revises wording]

Improve Annex C entry.

[Kona 2022-11-10; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > -8- If `{ arg-idopt }` is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integralstandard signed or unsigned integer type, or its value is negative, an exception of type `format_error` is thrown.

2. Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:

   > Affected subclause: 28.5.2.2 [format.string.std]

   > Change: Restrict types of formatting arguments used as width or precision in a std-format-spec.

   > Rationale: Disallow types that do not have useful or portable semantics as a formatting width or precision.

   > Effect on original feature: Valid C++ 2020 code that passes a boolean or character type as arg-id becomes invalid. For example:

   > >     std::format("{:*^{}}", "", true); // ill-formed, previously returned "*"
   > >     std::format("{:*^{}}", "", '1'); // ill-formed, previously returned an implementation-defined number of '*' characters

### 3721(i). Allow an arg-id with a value of zero for width in std-format-spec

Section: 28.5.2.2 [format.string.std] Status: C++23 Submitter: Mark de Wever Opened: 2022-06-19 Last modified: 2023-11-22

Priority: 3

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Discussion:

Per 28.5.2.2 [format.string.std]/7

> If `{ arg-idopt }` is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integral type, or its value is negative for precision or non-positive for width, an exception of type `format_error` is thrown.

During a libc++ code review Victor mentioned it would be nice to allow zero as a valid value for the arg-id when used for the width. This would simplify the code by having the same requirements for the arg-id for the width and precision fields. A width of zero has no effect on the output.

In the std-format-spec the width is restricted to a positive-integer to avoid parsing ambiguity with the zero-padding option. This ambiguity doesn't happen using an arg-id with the value zero. Therefore I only propose to change the arg-id's requirement.

Note the Standard doesn't specify the width field's effect on the output. Specifically [tab:format.align] doesn't refer to the width field. This is one of the items addressed by P2572. The proposed resolution works in combination with the wording of that paper.

[2022-07-08; Reflector poll]

Set priority to 3 after reflector poll.

[2022-07-11; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > -7- If `{ arg-idopt }` is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integral type, or its value is negative for precision or non-positive for width, an exception of type `format_error` is thrown.

### 3723(i). `priority_queue::push_range` needs to `append_range`

Section: 23.6.4.4 [priqueue.members] Status: C++23 Submitter: Casey Carter Opened: 2022-06-20 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

The `push_range` members of the `queue` (23.6.3.4 [queue.mod]) and `stack` (23.6.6.5 [stack.mod]) container adaptors are both specified as "Effects: Equivalent to `c.append_range(std::forward<R>(rg))` if that is a valid expression, otherwise `ranges::copy(rg, back_inserter(c))`.". For `priority_queue`, however, we have instead (23.6.4.4 [priqueue.members]):

> -3- Effects: Insert all elements of `rg` in `c`.

> -4- Postconditions: `is_heap(c.begin(), c.end(), comp)` is `true`.

Since `append_range` isn't one of the operations required of the underlying container, "Insert all elements of `rg`" must be implemented via potentially less efficient means. It would be nice if this `push_back` could take advantage of `append_range` when it's available just as do the other two overloads.

[2022-07-08; Reflector poll]

Set priority to 2 after reflector poll.

[Issaquah 2023-02-08; LWG]

Unanimous consent to move to Immediate.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 23.6.4.4 [priqueue.members] as indicated:

   >     template<container-compatible-range<T> R>
   >       void push_range(R&& rg);

   > > -3- Effects: Inserts all elements of `rg` in `c` via `c.append_range(std::forward<R>(rg))` if that is a valid expression, or `ranges::copy(rg, back_inserter(c))` otherwise. Then restores the heap property as if by `make_heap(c.begin(), c.end(), comp)`.

   > > -4- Postconditions: `is_heap(c.begin(), c.end(), comp)` is `true`.

### 3724(i). `decay-copy` should be constrained

Section: 16.3.3.2 [expos.only.entity] Status: C++23 Submitter: Hui Xie Opened: 2022-06-23 Last modified: 2023-11-22

Priority: 3

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Discussion:

The spec of `views::all` 25.7.6.1 [range.all.general] p2 says:

> Given a subexpression `E`, the expression `views::all(E)` is expression-equivalent to:

> 1. (2.1) — `decay-copy(E)` if the decayed type of E models `view`.

> 2. […]

If `E` is an lvalue move-only `view`, according to the spec, `views::all(E)` would be expression-equivalent to `decay-copy(E)`.

However, [expos.only.func] p2 defines `decay-copy` as follows

>     template<class T> constexpr decay_t<T> decay-copy(T&& v)
>         noexcept(is_nothrow_convertible_v<T, decay_t<T>>)         // exposition only
>       { return std::forward<T>(v); }

It is unconstrained. As a result, for the above example, `views::all(E)` is a well-formed expression and it would only error on the template instantiation of the function body of `decay-copy`, because `E` is an lvalue of move-only type and not copyable.

I think this behaviour is wrong, instead, we should make `decay-copy(E)` ill-formed if it is not copyable, so that `views::all(E)` can be SFINAE friendly.

[2022-07-08; Reflector poll]

Set priority to 3 after reflector poll.

[2022-07-11; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify [expos.only.func] as indicated:

   > -2- The following are defined for exposition only to aid in the specification of the library:

   > >     namespace std {
   > >       template<class T>
   > >         requires convertible_to<T, decay_t<T>>
   > >           constexpr decay_t<T> decay-copy(T&& v)
   > >             noexcept(is_nothrow_convertible_v<T, decay_t<T>>) // exposition only
   > >           { return std::forward<T>(v); }
   > >       […]
   > >     }

### 3732(i). `prepend_range` and `append_range` can't be amortized constant time

Section: 23.2.4 [sequence.reqmts] Status: C++23 Submitter: Tim Song Opened: 2022-07-06 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [sequence.reqmts].

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Discussion:

23.2.4 [sequence.reqmts]/69 says "An implementation shall implement them so as to take amortized constant time." followed by a list of operations that includes the newly added `append_range` and `prepend_range`. Obviously these operations cannot be implemented in amortized constant time.

Because the actual complexity of these operations are already specified in the concrete container specification, we can just exclude them here.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   > -69- The following operations are provided for some types of sequence containers but not others. An implementation shall implement themOperations other than `prepend_range` and `append_range` are implemented so as to take amortized constant time.

### 3733(i). `ranges::to` misuses `cpp17-input-iterator`

Section: 25.5.7.2 [range.utility.conv.to] Status: C++23 Submitter: S. B. Tam Opened: 2022-07-10 Last modified: 2024-01-29

Priority: 2

View other active issues in [range.utility.conv.to].

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Discussion:

`ranges::to` uses `cpp17-input-iterator<iterator_t<R>>` to check whether an iterator is a Cpp17InputIterator, which misbehaves if there is a `std::iterator_traits` specialization for that iterator (e.g. if the iterator is a `std::common_iterator`).

>     struct MyContainer {
>         template<class Iter>
>         MyContainer(Iter, Iter);

>         char* begin();
>         char* end();
>     };

>     auto nul_terminated = std::views::take_while([](char ch) { return ch != '\0'; });
>     auto c = nul_terminated("") | std::views::common | std::ranges::to<MyContainer>();  // error

I believe that `ranges::to` should instead use `derived_from<typename iterator_traits<iterator_t<R>>::iterator_category, input_iterator_tag>`, which correctly detects the iterator category of a `std::common_iterator`.

[2022-08-23; Reflector poll]

Set priority to 2 after reflector poll. Set status to Tentatively Ready after five votes in favour during reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> 1. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

>    >     template<class C, input_range R, class... Args> requires (!view<C>)
>    >       constexpr C to(R&& r, Args&&... args);

>    > > -1- Returns: An object of type `C` constructed from the elements of `r` in the following manner:

>    > > 1. (1.1) — If `convertible_to<range_reference_t<R>, range_value_t<C>>` is `true`:

>    > >    1. (1.1.1) — If `constructible_from<C, R, Args...>` is `true`:

>    > >       > `C(std::forward<R>(r), std::forward<Args>(args)...)`

>    > >    2. (1.1.2) — Otherwise, if `constructible_from<C, from_range_t, R, Args...>` is `true`:

>    > >       > `C(from_range, std::forward<R>(r), std::forward<Args>(args)...)`

>    > >    3. (1.1.3) — Otherwise, if

>    > >       1. (1.1.3.1) — `common_range<R>` is `true`,

>    > >       2. (1.1.3.2) — `cpp17-input-iteratorderived_from<typename iterator_traits<iterator_t<R>>::iterator_category, input_iterator_tag>` is `true`, and

>    > >       3. (1.1.3.3) — `constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...>` is `true`:

>    > >          > `C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)`

>    > >    4. (1.1.4) — Otherwise, if

>    > >       1. (1.1.4.1) — `constructible_from<C, Args...>` is `true`, and

>    > >       2. (1.1.4.2) — `container-insertable<C, range_reference_t<R>>` is `true`:

>    > >          >     […]

>    > > 2. (1.2) — Otherwise, if `input_range<range_reference_t<R>>` is `true`:

>    > >    >     to<C>(r | views::transform([](auto&& elem) {
>    > >    >       return to<range_value_t<C>>(std::forward<decltype(elem)>(elem));
>    > >    >     }), std::forward<Args>(args)...);

>    > > 3. (1.3) — Otherwise, the program is ill-formed.

[2022-08-27; Hewill Kang reopens and suggests a different resolution]

This issue points out that the standard misuses `cpp17-input-iterator` to check whether the iterator meets the requirements of Cpp17InputIterator, and proposes to use `iterator_traits<I>::iterator_category` to check the iterator's category directly, which may lead to the following potential problems:

First, for the range types that model both `common_range` and `input_range`, the expression `iterator_traits<I>::iterator_category` may not be valid, consider

>     #include <ranges>

>     struct I {
>       using difference_type = int;
>       using value_type = int;
>       int operator*() const;
>       I& operator++();
>       void operator++(int);
>       bool operator==(const I&) const;
>       bool operator==(std::default_sentinel_t) const;
>     };

>     int main() {
>       auto r = std::ranges::subrange(I{}, I{});
>       auto v = r | std::ranges::to<std::vector<int>>(0);
>     }

Although `I` can serve as its own sentinel, it does not model `cpp17-input-iterator` since postfix `operator++` returns `void`, which causes `iterator_traits<R>` to be an empty class, making the expression `derived_from<iterator_traits<I>::iterator_category, input_iterator_tag>` ill-formed.

Second, for `common_iterator`, `iterator_traits<I>::iterator_category` does not guarantee a strictly correct iterator category in the current standard.

For example, when the above `I::operator*` returns a non-`copyable` object that can be converted to `int`, this makes `common_iterator<I, default_sentinel_t>` unable to synthesize a C++17-conforming postfix `operator++`, however, `iterator_traits<common_iterator<I, S>>::iterator_category` will still give `input_iterator_tag` even though it's not even a C++17 iterator.

Another example is that for `input_iterator`s with difference type of integer-class type, the difference type of the `common_iterator` wrapped on it is still of the integer-class type, but the `iterator_category` obtained by the `iterator_traits` is `input_iterator_tag`.

The proposed resolution only addresses the first issue since I believe that the problem with `common_iterator` requires a paper.

[2023-01-11; LWG telecon]

Set status to Tentatively Ready (poll results F6/A0/N1)

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   >     template<class C, input_range R, class... Args> requires (!view<C>)
   >       constexpr C to(R&& r, Args&&... args);

   > > -1- Returns: An object of type `C` constructed from the elements of `r` in the following manner:

   > > 1. (1.1) — If `convertible_to<range_reference_t<R>, range_value_t<C>>` is `true`:

   > >    1. (1.1.1) — If `constructible_from<C, R, Args...>` is `true`:

   > >       > `C(std::forward<R>(r), std::forward<Args>(args)...)`

   > >    2. (1.1.2) — Otherwise, if `constructible_from<C, from_range_t, R, Args...>` is `true`:

   > >       > `C(from_range, std::forward<R>(r), std::forward<Args>(args)...)`

   > >    3. (1.1.3) — Otherwise, if

   > >       1. (1.1.3.1) — `common_range<R>` is `true`,

   > >       2. (1.1.3.2) — `cpp17-input-iterator` if the qualified-id `iterator_traits``<iterator_t<R>>``::iterator_category` is `true` valid and denotes a type that models `derived_from<input_iterator_tag>`, and

   > >       3. (1.1.3.3) — `constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...>`:

   > >          > `C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)`

   > >    4. (1.1.4) — Otherwise, if

   > >       1. (1.1.4.1) — `constructible_from<C, Args...>` is `true`, and

   > >       2. (1.1.4.2) — `container-insertable<C, range_reference_t<R>>` is `true`:

   > >          >     […]

   > > 2. (1.2) — Otherwise, if `input_range<range_reference_t<R>>` is `true`:

   > >    >     to<C>(r | views::transform([](auto&& elem) {
   > >    >       return to<range_value_t<C>>(std::forward<decltype(elem)>(elem));
   > >    >     }), std::forward<Args>(args)...);

   > > 3. (1.3) — Otherwise, the program is ill-formed.

### 3734(i). Inconsistency in `inout_ptr` and `out_ptr` for empty case

Section: 20.3.4.1 [out.ptr.t] Status: C++23 Submitter: Doug Cook Opened: 2022-07-11 Last modified: 2023-11-22

Priority: 2

View all issues with C++23 status.

Discussion:

`out_ptr` and `inout_ptr` are inconsistent when a pointer-style function returns `nullptr`.

- `out_ptr` leaves the stale value in `smart` (not the value returned by the pointer-style function).

- `inout_ptr` (as resolved by LWG 3594(i)) leaves `nullptr` in `smart` (the value returned by the pointer-style function).

Assume we have the following pointer-style functions that return `nullptr` in case of failure:

>     void ReplaceSomething(/*INOUT*/ int** pp) {
>       delete *pp;
>       *pp = nullptr;
>       return; // Failure!
>     }

>     void GetSomething(/*OUT*/ int** pp) {
>       *pp = nullptr;
>       return; // Failure!
>     }

In the scenario that led to the creation of issue LWG 3594(i):

>     // Before the call, inout contains a stale value.
>     auto inout = std::make_unique<int>(1);
>     ReplaceSomething(std::inout_ptr(inout));
>     // (1) If ReplaceSomething failed (returned nullptr), what does inout contain?

Assuming LWG 3594(i) is resolved as suggested, `inout` will be empty. (The original N4901 text allows `inout` to be either empty or to hold a pointer to already-deleted memory.) Using the resolution suggested by LWG 3594(i), it expands to something like the following (simplified to ignore exceptions and opting to perform the `release()` before the `ReplaceSomething()` operation):

>     // Before the call, inout contains a stale value.
>     auto inout = std::make_unique<int>(1);
>     int* p = inout.release();
>     ReplaceSomething(&p);
>     if (p) {
>       inout.reset(p);
>     }
>     // (1) If ReplaceSomething failed (returned nullptr), inout contains nullptr.

This behavior seems reasonable.

Now consider the corresponding scenario with `out_ptr`:

>     // Before the call, out contains a stale value.
>     auto out = std::make_unique<int>(2);
>     GetSomething(std::out_ptr(out));
>     // (2) If GetSomething failed (returned nullptr), what does out contain?

Based on N4901, `out` contains the stale value (from `make_unique`), not the `nullptr` value returned by `GetSomething()`. The N4901 model (simplified to ignore exceptions) expands to the following:

>     // Before the call, out contains a stale value.
>     auto out = std::make_unique<int>(2);
>     int* p{};
>     GetSomething(&p);
>     if (p) {
>       out.reset(p);
>     }
>     // (2) If GetSomething failed (returned nullptr), out contains a pointer to "2".

This behavior seems incorrect to me. It is inconsistent with the behavior of `inout_ptr` and it is inconsistent with my expectation that `out` should contain the value returned by `GetSomething()`, even if that value is `nullptr`. Intuitively, I expect it to behave as if the `out.reset(p)` were unconditional.

The `reset(p)` is conditional as an optimization for cases where `reset` is non-trivial. For example, `shared_ptr`'s `reset(p)` requires the allocation of a control block even if `p` is `nullptr`. As such, simply making the `reset` unconditional may be sub-optimal.

I see two primary options for making `out_ptr`'s behavior consistent with `inout_ptr`:

- Perform an unconditional `out.reset()` or `out = Smart()` in the `out_ptr_t` constructor.

- Add an else clause to the if statement, containing `out.reset()` or `out = Smart()`.

I note that these solutions do not make use of the additional `args...`, leaving the `out` pointer in an empty state. This is analogous to the corresponding state in the similar `inout` scenario where the `inout` pointer is left empty as a result of the call to `smart.release()`.

I favor the first resolution, freeing any existing value in the `out_ptr_t` constructor.

[2022-08-23; Reflector poll]

Set priority to 2 after reflector poll. "A bit like design."

[Issaquah 2023-02-07; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 20.3.4.1 [out.ptr.t] as indicated:

   >     explicit out_ptr_t(Smart& smart, Args... args);

   > > -6- Effects: Initializes `s` with `smart`, `a` with `std::forward<Args>(args)...`, and value-initializes `p`. Then, equivalent to:

   > > - (6.1) —

   > >   >     s.reset();

   > >   if the expression `s.reset()` is well-formed;

   > > - (6.2) — otherwise,

   > >   >     s = Smart();

   > >   if `is_constructible_v<Smart>` is `true`;

   > > - (6.3) — otherwise, the program is ill-formed.

   > > -7- [Note 2: The constructor is not `noexcept` to allow for a variety of non-terminating and safe implementation strategies. For example, an implementation can allocate a `shared_ptr`'s internal node in the constructor and let implementation-defined exceptions escape safely. The destructor can then move the allocated control block in directly and avoid any other exceptions. — end note]

### 3736(i). `move_iterator` missing `disable_sized_sentinel_for` specialization

Section: 24.2 [iterator.synopsis] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-14 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [iterator.synopsis].

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Discussion:

Since `reverse_iterator::operator-` is not constrained, the standard adds a `disable_sized_sentinel_for` specialization for it to avoid situations where the underlying iterator can be subtracted making `reverse_iterator` accidentally model `sized_sentinel_for`.

However, given that `move_iterator::operator-` is also unconstrained and the standard does not have the `disable_sized_sentinel_for` specialization for it, this makes `subrange<move_iterator<I>, move_iterator<I>>` unexpectedly satisfy `sized_range` and incorrectly use `I::operator-` to get size when `I` can be subtracted but not modeled `sized_sentinel_for<I>`.

In addition, since P2520 makes `move_iterator` no longer just `input_iterator`, `ranges::size` can get the size of the range by subtracting two `move_iterator` pairs, this also makes `r | views::as_rvalue` may satisfy `sized_range` when neither `r` nor `r | views::reverse` is `sized_range`.

We should add a `move_iterator` version of the `disable_sized_sentinel_for` specialization to the standard to avoid the above situation.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

"but I don't think the issue text is quite right - both `move_iterator` and `reverse_iterator`'s `operator-` are constrained on `x.base() - y.base()` being valid."

Does anyone remember why we did this for `reverse_iterator` and not `move_iterator`?

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     namespace std::ranges {
   >       […]
   >       template<class Iterator>
   >         constexpr move_iterator<Iterator> make_move_iterator(Iterator i);

   >       template<class Iterator1, class Iterator2>
   >           requires (!sized_sentinel_for<Iterator1, Iterator2>)
   >         inline constexpr bool disable_sized_sentinel_for<move_iterator<Iterator1>,
   >                                                          move_iterator<Iterator2>> = true;

   >       […]
   >     }

### 3737(i). `take_view::sentinel` should provide `operator-`

Section: 25.7.10.3 [range.take.sentinel] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-15 Last modified: 2023-11-22

Priority: 3

View all other issues in [range.take.sentinel].

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Discussion:

This issue is part of NB comment US 47-109 26 [ranges] Resolve open issues

When the underlying range is not a `sized_range`, the `begin` and `end` functions of `take_view` return `counted_iterator` and `take_view::sentinel` respectively. However, the sentinel type of the underlying range may still model `sized_sentinel_for` for its iterator type, and since `take_view::sentinel` can only be compared to `counted_iterator`, this makes `take_view` no longer able to compute the distance between its iterator and sentinel.

We are needlessly losing functionality here. Since calculating the distance, in this case, is still simple, i.e. we just need to compute the minimum of `counted_iterator::count` and the difference between the underlying iterator and sentinel, I think providing `operator-` for `take_view::sentinel` does bring some value.

[2022-08-23; Reflector poll]

Set priority to 3 after reflector poll.

Some P0 votes, but with objections: "This seems like a) a feature not a bug - of fairly limited utility?, and b) I’d like to see an implementation (maybe it’s in MSVC?) to be sure there isn’t a negative interaction we’re not thinking of."

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> 1. Modify 25.7.10.3 [range.take.sentinel], class template `take_view::sentinel` synopsis, as indicated:

>    > >     namespace std::ranges {
>    > >       template<view V>
>    > >       template<bool Const>
>    > >       class take_view<V>::sentinel {
>    > >       private:
>    > >         using Base = maybe-const<Const, V>;                                     // exposition only
>    > >         template<bool OtherConst>
>    > >           using CI = counted_iterator<iterator_t<maybe-const<OtherConst, V>>>;  // exposition only
>    > >         sentinel_t<Base> end_ = sentinel_t<Base>();                             // exposition only
>    > >       public:
>    > >         […]
>    > >         friend constexpr bool operator==(const CI<Const>& y, const sentinel& x);

>    > >         template<bool OtherConst = !Const>
>    > >           requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    > >         friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);

>    > >         friend constexpr range_difference_t<Base>
>    > >           operator-(const sentinel& x, const CI<Const>& y)
>    > >             requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

>    > >         template<bool OtherConst = !Const>
>    > >           requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    > >         friend constexpr range_difference_t<maybe-const<OtherConst, V>>
>    > >           operator-(const sentinel& x, const CI<OtherConst>& y);

>    > >         friend constexpr range_difference_t<Base>
>    > >           operator-(const CI<Const>& x, const sentinel& y)
>    > >             requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

>    > >         template<bool OtherConst = !Const>
>    > >           requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    > >         friend constexpr range_difference_t<maybe-const<OtherConst, V>>
>    > >           operator-(const CI<OtherConst>& x, const sentinel& y);
>    > >       };
>    > >     }

>    > […]

>    >     friend constexpr bool operator==(const CI<Const>& y, const sentinel& x);

>    >     template<bool OtherConst = !Const>
>    >       requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    >     friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);

>    > > -4- Effects: Equivalent to: `return y.count() == 0 || y.base() == x.end_;`

>    >     friend constexpr range_difference_t<Base>
>    >       operator-(const sentinel& x, const CI<Const>& y)
>    >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

>    >     template<bool OtherConst = !Const>
>    >       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    >     friend constexpr range_difference_t<maybe-const<OtherConst, V>>
>    >       operator-(const sentinel& x, const CI<OtherConst>& y);

>    > > -?- Effects: Equivalent to: `return ranges::min(y.count(), x.end_ - y.base());`

>    >     friend constexpr range_difference_t<Base>
>    >       operator-(const CI<Const>& x, const sentinel& y)
>    >         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

>    >     template<bool OtherConst = !Const>
>    >       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
>    >     friend constexpr range_difference_t<maybe-const<OtherConst, V>>
>    >       operator-(const CI<OtherConst>& x, const sentinel& y);

>    > > -?- Effects: Equivalent to: `return -(y - x);`

[Kona 2022-11-08; Discussed at joint LWG/SG9 session. Move to Open]

[2022-11-09 Tim updates wording following LWG discussion]

This case is only possible if the source view is not a `sized_range`, yet its iterator/sentinel types model `sized_sentinel_for` (typically when source is an input range). In such a case we should just have `begin` compute the correct size so that `end` can just return `default_sentinel`.

[Kona 2022-11-10; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.10.2 [range.take.view], class template `take_view` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<view V>
   >       class take_view : public view_interface<take_view<V>> {
   >       private:
   >         […]
   >       public:
   >         […]
   >         constexpr auto begin() requires (!simple-view<V>) {
   >           if constexpr (sized_range<V>) {
   >             if constexpr (random_access_range<V>) {
   >               return ranges::begin(base_);
   >             } else {
   >               auto sz = range_difference_t<V>(size());
   >               return counted_iterator(ranges::begin(base_), sz);
   >             }
   >           } else if constexpr (sized_sentinel_for<sentinel_t<V>, iterator_t<V>>) {
   >             auto it = ranges::begin(base_);
   >             auto sz = std::min(count_, ranges::end(base_) - it);
   >             return counted_iterator(std::move(it), sz);
   >           } else {
   >             return counted_iterator(ranges::begin(base_), count_);
   >           }
   >         }

   >         constexpr auto begin() const requires range<const V> {
   >           if constexpr (sized_range<const V>) {
   >             if constexpr (random_access_range<const V>) {
   >               return ranges::begin(base_);
   >             } else {
   >               auto sz = range_difference_t<const V>(size());
   >               return counted_iterator(ranges::begin(base_), sz);
   >             }
   >           } else if constexpr (sized_sentinel_for<sentinel_t<const V>, iterator_t<const V>>) {
   >             auto it = ranges::begin(base_);
   >             auto sz = std::min(count_, ranges::end(base_) - it);
   >             return counted_iterator(std::move(it), sz);
   >           } else {
   >             return counted_iterator(ranges::begin(base_), count_);
   >           }
   >         }

   >         constexpr auto end() requires (!simple-view<V>) {
   >           if constexpr (sized_range<V>) {
   >             if constexpr (random_access_range<V>)
   >               return ranges::begin(base_) + range_difference_t<V>(size());
   >             else
   >               return default_sentinel;
   >           } else if constexpr (sized_sentinel_for<sentinel_t<V>, iterator_t<V>>) {
   >             return default_sentinel;
   >           } else {
   >             return sentinel<false>{ranges::end(base_)};
   >           }
   >         }

   >         constexpr auto end() const requires range<const V> {
   >           if constexpr (sized_range<const V>) {
   >             if constexpr (random_access_range<const V>)
   >               return ranges::begin(base_) + range_difference_t<const V>(size());
   >             else
   >               return default_sentinel;
   >           } else if constexpr (sized_sentinel_for<sentinel_t<const V>, iterator_t<const V>>) {
   >             return default_sentinel;
   >           } else {
   >             return sentinel<true>{ranges::end(base_)};
   >           }
   >         }

   >         […]
   >       };
   >       […]
   >     }

### 3738(i). Missing preconditions for `take_view` constructor

Section: 25.7.10.2 [range.take.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-15 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.take.view].

View all issues with C++23 status.

Discussion:

When `V` does not model `sized_range`, `take_view::begin` returns `counted_iterator(ranges::begin(base_), count_)`. Since the `counted_iterator` constructor (24.5.7.2 [counted.iter.const]) already has a precondition that `n >= 0`, we should add this to `take_view` as well, which is consistent with `drop_view`.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.10.2 [range.take.view] as indicated:

   >     constexpr take_view(V base, range_difference_t<V> count);

   > > -?- Preconditions: `count >= 0` is `true`.

   > > -1- Effects: Initializes `base_` with `std::move(base)` and `count_` with `count`.

### 3742(i). `deque::prepend_range` needs to permute

Section: 23.2.4 [sequence.reqmts] Status: C++23 Submitter: Casey Carter Opened: 2022-07-16 Last modified: 2023-11-22

Priority: 2

View other active issues in [sequence.reqmts].

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Discussion:

When the range to be inserted is neither bidirectional nor sized, it's simpler to prepend elements one at a time, and then reverse the prepended elements. When the range to be inserted is neither forward nor sized, I believe this approach is necessary to implement `prepend_range` at all — there is no way to determine the length of the range modulo the block size of the `deque` ahead of time so as to insert the new elements in the proper position.

The container requirements do not allow `prepend_range` to permute elements in a `deque`. I believe we must allow permutation when the range is neither forward nor sized, and we should allow permutation when the range is not bidirectional to allow implementations the freedom to make a single pass through the range.

[2022-07-17; Daniel comments]

The below suggested wording follows the existing style used in the specification of `insert` and `insert_range`, for example. Unfortunately, this existing practice violates the usual wording style that a Cpp17XXX requirement shall be met and that we should better say that "lvalues of type `T` are swappable (16.4.4.3 [swappable.requirements])" to be clearer about the specific swappable context. A separate editorial issue will be reported to take care of this problem.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4910.

> 1. Modify 23.2.4 [sequence.reqmts] as indicated:

>    >     a.prepend_range(rg)

>    > > -94- Result: `void`

>    > > -95- Preconditions: `T` is Cpp17EmplaceConstructible into `X` from `*ranges::begin(rg)`. For `deque`, `T` is also `Cpp17MoveInsertable` into `X`, Cpp17MoveConstructible, Cpp17MoveAssignable, and swappable (16.4.4.3 [swappable.requirements]).

>    > > -96- Effects: Inserts copies of elements in `rg` before `begin()`. Each iterator in the range `rg` is dereferenced exactly once.

>    > > [Note 3: The order of elements in `rg` is not reversed. — end note]

>    > > -97- Remarks: Required for `deque`, `forward_list`, and `list`.

[2022-11-07; Daniel reopens and comments]

The proposed wording has two problems:

1. It still uses "swappable" instead of "swappable lvalues", which with the (informal) acceptance of P2696R0 should now become Cpp17Swappable.

2. It uses an atypical form to say "`T` (is) Cpp17MoveConstructible, Cpp17MoveAssignable" instead of the more correct form "`T` meets the Cpp17MoveConstructible and Cpp17MoveAssignable requirements". This form was also corrected by P2696R0.

The revised wording uses the P2696R0 wording approach to fix both problems.

[Kona 2022-11-12; Set priority to 2]

[2022-11-30; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917 assuming that P2696R0 has been accepted.

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   >     a.prepend_range(rg)

   > > -94- Result: `void`

   > > -95- Preconditions: `T` is Cpp17EmplaceConstructible into `X` from `*ranges::begin(rg)`. For `deque`, `T` is also `Cpp17MoveInsertable` into `X`, and `T` meets the Cpp17MoveConstructible, Cpp17MoveAssignable, and Cpp17Swappable (16.4.4.3 [swappable.requirements]) requirements.

   > > -96- Effects: Inserts copies of elements in `rg` before `begin()`. Each iterator in the range `rg` is dereferenced exactly once.

   > > [Note 3: The order of elements in `rg` is not reversed. — end note]

   > > -97- Remarks: Required for `deque`, `forward_list`, and `list`.

### 3743(i). `ranges::to`'s `reserve` may be ill-formed

Section: 25.5.7.2 [range.utility.conv.to] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-21 Last modified: 2024-01-29

Priority: Not Prioritized

View other active issues in [range.utility.conv.to].

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Discussion:

When the "reserve" branch is satisfied, `ranges::to` directly passes the result of `ranges::size(r)` into the `reserve` call. However, given that the standard only guarantees that integer-class type can be explicitly converted to any integer-like type (24.3.4.4 [iterator.concept.winc] p6), this makes the call potentially ill-formed, since `ranges::size(r)` may return an integer-class type:

>     #include <ranges>
>     #include <vector>

>     int main() {
>       auto r = std::ranges::subrange(std::views::iota(0ULL) | std::views::take(5), 5);
>       auto v = r | std::ranges::to<std::vector<std::size_t>>(0); // cannot implicitly convert _Unsigned128 to size_t in MSVC-STL
>     }

We should do an explicit cast before calling `reserve`.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Are we all happy that the result of conversion to the container's size type may be less than the length of the source range, so the reservation is too small but we don't realize until pushing the max_size() + 1st element fails? I think it's acceptable that converting pathologically large ranges to containers fails kind of messily, but I could imagine throwing if the range length is greater than container's max_size().

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   >     template<class C, input_range R, class... Args> requires (!view<C>)
   >       constexpr C to(R&& r, Args&&... args);

   > > -1- Returns: An object of type `C` constructed from the elements of `r` in the following manner:

   > > 1. (1.1) — If `convertible_to<range_reference_t<R>, range_value_t<C>>` is `true`:

   > >    1. (1.1.1) — If `constructible_from<C, R, Args...>` is `true`:

   > >       > `C(std::forward<R>(r), std::forward<Args>(args)...)`

   > >    2. (1.1.2) — Otherwise, if `constructible_from<C, from_range_t, R, Args...>` is `true`:

   > >       > `C(from_range, std::forward<R>(r), std::forward<Args>(args)...)`

   > >    3. (1.1.3) — Otherwise, if

   > >       1. (1.1.3.1) — `common_range<R>` is `true`,

   > >       2. (1.1.3.2) — `cpp17-input-iterator<iterator_t<R>>` is `true`, and

   > >       3. (1.1.3.3) — `constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...>` is `true`:

   > >          > `C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)`

   > >    4. (1.1.4) — Otherwise, if

   > >       1. (1.1.4.1) — `constructible_from<C, Args...>` is `true`, and

   > >       2. (1.1.4.2) — `container-insertable<C, range_reference_t<R>>` is `true`:

   > >          >     C c(std::forward<Args>(args)...);
   > >          >     if constexpr (sized_range<R> && reservable-container<C>)
   > >          >       c.reserve(static_cast<range_size_t<C>>(ranges::size(r)));
   > >          >     ranges::copy(r, container-inserter<range_reference_t<R>>(c));

   > > 2. (1.2) — Otherwise, if `input_range<range_reference_t<R>>` is `true`:

   > >    >     to<C>(r | views::transform([](auto&& elem) {
   > >    >       return to<range_value_t<C>>(std::forward<decltype(elem)>(elem));
   > >    >     }), std::forward<Args>(args)...);

   > > 3. (1.3) — Otherwise, the program is ill-formed.

### 3745(i). `std::atomic_wait` and its friends lack `noexcept`

Section: 32.5.2 [atomics.syn] Status: C++23 Submitter: Jiang An Opened: 2022-07-25 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [atomics.syn].

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Discussion:

Currently function templates `std::atomic_wait`, `std::atomic_wait_explicit`, `std::atomic_notify_one`, and `std::atomic_notify_all` are not `noexcept` in the Working Draft, but the equivalent member functions are all `noexcept`. I think these function templates should be specified as `noexcept`, in order to be consistent with the `std::atomic_flag_*` free functions, the corresponding member functions, and other `std::atomic_*` function templates.

Mainstream implementations (libc++, libstdc++, and MSVC STL) have already added `noexcept` to them.

[2022-07-30; Daniel provides wording]

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

"Technically there's a difference between these and the member functions - the pointer can be null - but we don't seem to have let that stop us from adding noexcept to the rest of these functions."

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 32.5.2 [atomics.syn], header `<atomic>` synopsis, as indicated:

   >     […]
   >     template<class T>
   >       void atomic_wait(const volatile atomic<T>*, typename atomic<T>::value_type) noexcept;
   >     template<class T>
   >       void atomic_wait(const atomic<T>*, typename atomic<T>::value_type) noexcept;
   >     template<class T>
   >       void atomic_wait_explicit(const volatile atomic<T>*, typename atomic<T>::value_type,
   >                                 memory_order) noexcept;
   >     template<class T>
   >       void atomic_wait_explicit(const atomic<T>*, typename atomic<T>::value_type,
   >                                 memory_order) noexcept;
   >     template<class T>
   >       void atomic_notify_one(volatile atomic<T>*) noexcept;
   >     template<class T>
   >       void atomic_notify_one(atomic<T>*) noexcept;
   >     template<class T>
   >       void atomic_notify_all(volatile atomic<T>*) noexcept;
   >     template<class T>
   >       void atomic_notify_all(atomic<T>*) noexcept;
   >     […]

### 3746(i). `optional`'s spaceship with `U` with a type derived from `optional` causes infinite constraint meta-recursion

Section: 22.5.9 [optional.comp.with.t] Status: C++23 Submitter: Ville Voutilainen Opened: 2022-07-25 Last modified: 2023-11-22

Priority: Not Prioritized

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View all issues with C++23 status.

Discussion:

What ends up happening is that the constraints of `operator<=>(const optional<T>&, const U&)` end up in `three_way_comparable_with`, and then in `partially-ordered-with`, and the expressions there end up performing a conversion from `U` to an `optional`, and we end up instantiating the same `operator<=>` again, evaluating its constraints again, until the compiler bails out.

See an online example here.

All implementations end up with infinite meta-recursion.

The solution to the problem is to stop the meta-recursion by constraining the spaceship with `U` so that `U` is not publicly and unambiguously derived from a specialization of optional, SFINAEing that candidate out, and letting 22.5.7 [optional.relops]/20 perform the comparison instead.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

> [Drafting note: This proposed wording removes the only use of `is-optional`. ]

1. Modify 22.5.2 [optional.syn], header `<optional>` synopsis, as indicated:

   >     […]
   >     namespace std {
   >       // 22.5.3 [optional.optional], class template optional
   >       template<class T>
   >         class optional;

   >       template<class T>
   >         constexpr bool is-optional = false; // exposition only
   >       template<class T>
   >         constexpr bool is-optional<optional<T>> = true; // exposition only
   >       template<class T>
   >         concept is-derived-from-optional = requires(const T& t) { // exposition only
   >           []<class U>(const optional<U>&){ }(t);
   >         };
   >       […]
   >       // 22.5.9 [optional.comp.with.t], comparison with T
   >       […]
   >       template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
   >       template<class T, class U> requires (!is-derived-from-optional<U>) && three_way_comparable_with<T, U>
   >         constexpr compare_three_way_result_t<T, U>
   >           operator<=>(const optional<T>&, const U&);
   >       […]
   >     }

2. Modify 22.5.9 [optional.comp.with.t] as indicated:

   >     template<class T, class U> requires (!is-derived-from-optional<U>) && three_way_comparable_with<T, U>
   >       constexpr compare_three_way_result_t<T, U>
   >         operator<=>(const optional<T>&, const U&);

   > > -25- Effects: Equivalent to: `return x.has_value() ? *x <=> v : strong_ordering::less;`

### 3747(i). `ranges::uninitialized_copy_n`, `ranges::uninitialized_move_n`, and `ranges::destroy_n` should use `std::move`

Section: 26.11.5 [uninitialized.copy], 26.11.6 [uninitialized.move], 26.11.9 [specialized.destroy] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-28 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [uninitialized.copy].

View all issues with C++23 status.

Discussion:

Currently, `ranges::uninitialized_copy_n` has the following equivalent Effects:

>     auto t = uninitialized_copy(counted_iterator(ifirst, n),
>                                 default_sentinel, ofirst, olast);
>     return {std::move(t.in).base(), t.out};

Given that `ifirst` is just an `input_iterator` which is not guaranteed to be `copyable`, we should move it into `counted_iterator`. The same goes for `ranges::uninitialized_move_n` and `ranges::destroy_n`.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 26.11.5 [uninitialized.copy] as indicated:

   >     namespace ranges {
   >       template<input_iterator I, nothrow-forward-iterator O, nothrow-sentinel-for<O> S>
   >         requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
   >         uninitialized_copy_n_result<I, O>
   >           uninitialized_copy_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
   >     }

   > > -9- Preconditions: [`ofirst, olast`) does not overlap with `ifirst +` [`0, n`) .

   > > -10- Effects: Equivalent to:

   > > >     auto t = uninitialized_copy(counted_iterator(std::move(ifirst), n),
   > > >                                 default_sentinel, ofirst, olast);
   > > >     return {std::move(t.in).base(), t.out};

2. Modify 26.11.6 [uninitialized.move] as indicated:

   >     namespace ranges {
   >       template<input_iterator I, nothrow-forward-iterator O, nothrow-sentinel-for<O> S>
   >         requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
   >         uninitialized_move_n_result<I, O>
   >           uninitialized_move_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
   >     }

   > > -8- Preconditions: [`ofirst, olast`) does not overlap with `ifirst +` [`0, n`) .

   > > -9- Effects: Equivalent to:

   > > >     auto t = uninitialized_move(counted_iterator(std::move(ifirst), n),
   > > >                                 default_sentinel, ofirst, olast);
   > > >     return {std::move(t.in).base(), t.out};

3. Modify 26.11.9 [specialized.destroy] as indicated:

   >     namespace ranges {
   >       template<nothrow-input-iterator I>
   >         requires destructible<iter_value_t<I>>
   >         constexpr I destroy_n(I first, iter_difference_t<I> n) noexcept;
   >     }

   > > -5- Effects: Equivalent to:

   > > >     return destroy(counted_iterator(std::move(first), n), default_sentinel).base();

### 3749(i). `common_iterator` should handle integer-class difference types

Section: 24.5.5 [iterators.common] Status: C++26 Submitter: Hewill Kang Opened: 2022-08-01 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

The partial specialization of `iterator_traits` for `common_iterator` is defined in 24.5.5.1 [common.iterator] as

    template<input_iterator I, class S>
    struct iterator_traits<common_iterator<I, S>> {
      using iterator_concept = see below;
      using iterator_category = see below;
      using value_type = iter_value_t<I>;
      using difference_type = iter_difference_t<I>;
      using pointer = see below;
      using reference = iter_reference_t<I>;
    };

where `difference_type` is defined as `iter_difference_t<I>` and `iterator_category` is defined as at least `input_iterator_tag`. However, when `difference_type` is an integer-class type, `common_iterator` does not satisfy Cpp17InputIterator, which makes `iterator_category` incorrectly defined as `input_iterator_tag`.

Since the main purpose of `common_iterator` is to be compatible with the legacy iterator system, which is reflected in its efforts to try to provide the operations required by C++17 iterators even if the underlying iterator does not support it. We should handle this case of difference type incompatibility as well.

The proposed solution is to provide a C++17 conforming difference type by clamping the integer-class type to `ptrdiff_t`.

Daniel:

> The second part of this issue provides an alternative resolution for the first part of LWG 3748(i) and solves the casting problem mentioned in LWG 3748(i) as well.

[2022-08-23; Reflector poll]

Set priority to 2 after reflector poll.

"I think `common_iterator` should reject iterators with integer-class difference types since it can't possibly achieve the design intent of adapting them to Cpp17Iterators."

"I'm not yet convinced that we need to outright reject such uses, but I'm pretty sure that we shouldn't mess with the difference type and that the PR is in the wrong direction."

Previous resolution [SUPERSEDED]

This wording is relative to N4910.

1. Modify 24.5.5.1 [common.iterator] as indicated:

   > [Drafting note: `common_iterator` requires iterator type `I` must model `input_or_output_iterator` which ensures that `iter_difference_t<I>` is a signed-integer-like type. The modification of `common_iterator::operator-` is to ensure that the pair of `common_iterator<I, S>` models `sized_sentinel_for` when `sized_sentinel_for<I, S>` is modeled for iterator type `I` with an integer-class difference type and its sentinel type `S`.]

   >     namespace std {
   >       template<class D>
   >         requires is-signed-integer-like<D>
   >       using make-cpp17-diff-t = conditional_t<signed_integral<D>, D, ptrdiff_t>;  // exposition only

   >       template<input_or_output_iterator I, sentinel_for<I> S>
   >         requires (!same_as<I, S> && copyable<I>)
   >       class common_iterator {
   >       public:
   >         […]
   >         template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
   >           requires sized_sentinel_for<S, I2>
   >         friend constexpr make-cpp17-diff-t<iter_difference_t<I2>> operator-(
   >           const common_iterator& x, const common_iterator<I2, S2>& y);
   >         […]
   >       };

   >       template<class I, class S>
   >       struct incrementable_traits<common_iterator<I, S>> {
   >         using difference_type = make-cpp17-diff-t<iter_difference_t<I>>;
   >       };

   >       template<input_iterator I, class S>
   >       struct iterator_traits<common_iterator<I, S>> {
   >         using iterator_concept = see below;
   >         using iterator_category = see below;
   >         using value_type = iter_value_t<I>;
   >         using difference_type = make-cpp17-diff-t<iter_difference_t<I>>;
   >         using pointer = see below;
   >         using reference = iter_reference_t<I>;
   >       };
   >     }

2. Modify 24.5.5.6 [common.iter.cmp] as indicated:

   > [Drafting note: If this issue is voted in at the same time as LWG 3748(i), the editor is kindly informed that the changes indicated below supersede those of the before mentioned issues first part.]

   >     template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
   >       requires sized_sentinel_for<S, I2>
   >     friend constexpr make-cpp17-diff-t<iter_difference_t<I2>> operator-(
   >       const common_iterator& x, const common_iterator<I2, S2>& y);

   > > -5- Preconditions: `x.v_.valueless_by_exception()` and `y.v_.valueless_by_exception()` are each `false`.

   > > -6- Returns: `0` if `i` and `j` are each `1`, and otherwise `static_cast<make-cpp17-diff-t<iter_difference_t<I2>>>(``get<i>(x.v_) - get<j>(y.v_)``)`, where `i` is `x.v_.index()` and `j` is `y.v_.index()`.

[2023-06-13; Varna; Tomasz provides wording]

[2023-06-14 Varna; Move to Ready]

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 24.5.5.1 [common.iterator] as indicated:

   >     namespace std {
   >       […]

   >       template<input_iterator I, class S>
   >       struct iterator_traits<common_iterator<I, S>> {
   >         using iterator_concept = see below;
   >         using iterator_category = see below; // not always present
   >         using value_type = iter_value_t<I>;
   >         using difference_type = iter_difference_t<I>;
   >         using pointer = see below;
   >         using reference = iter_reference_t<I>;
   >       };
   >     }

2. Modify 24.5.5.2 [common.iter.types] as indicated:

   > -?- The nested typedef-name `iterator_category` of the specialization of `iterator_traits` for `common_iterator<I, S>` is defined if and only if `iter_difference_t<I>` is an integral type. In that case, `iterator_category` denotes `forward_iterator_tag` if the qualified-id `iterator_traits<I>::iterator_category` is valid and denotes a type that models `derived_from<forward_iterator_tag>`; otherwise it denotes `input_iterator_tag`.

   > -1- The remaining nested typedef-names of the specialization of `iterator_traits` for `common_iterator<I, S>` are defined as follows.:

   > 1. (1.1) — `iterator_concept` denotes `forward_iterator_tag` if `I` models `forward_iterator`; otherwise it denotes `input_iterator_tag`.

   > 2. (1.2) — `iterator_category` denotes `forward_iterator_tag` if the qualified-id `iterator_traits<I>::iterator_category` is valid and denotes a type that models `derived_from<forward_iterator_tag>`; otherwise it denotes `input_iterator_tag`.

   > 3. (1.3) — Let `a` denote an lvalue of type `const common_iterator<I, S>`. If the expression `a.operator->()` is well-formed, then `pointer` denotes `decltype(a.operator->())`. Otherwise, `pointer` denotes `void`.

### 3750(i). Too many papers bump `__cpp_lib_format`

Section: 17.3.2 [version.syn] Status: C++23 Submitter: Barry Revzin Opened: 2022-08-04 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++23 status.

Discussion:

As pointed out by Casey Carter, four papers approved at the recent July 2022 plenary:

- P2419R2 "Clarify handling of encodings in localized formatting of chrono types"

- P2508R1 "Expose `std::basic-format-string<charT, Args...>`"

- P2286R8 "Formatting Ranges"

- P2585R1 "Improve container default formatting"

all bump the value of `__cpp_lib_format`. We never accounted for all of these papers being moved at the same time, and these papers have fairly different implementation complexities.

Victor Zverovich suggests that we instead add `__cpp_lib_format_ranges` (with value `202207L`) for the two formatting ranges papers (P2286 and P2585, which should probably be implemented concurrently anyway, since the latter modifies the former) and `bump __cpp_lib_format` for the other two, which are both minor changes.

We should do that.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after 12 votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

1. Modify 17.3.2 [version.syn] as indicated:

   >     #define __cpp_lib_format          202207L // also in <format>
   >     #define __cpp_lib_format_ranges   202207L // also in <format>

### 3751(i). Missing feature macro for `flat_set`

Section: 17.3.2 [version.syn] Status: C++23 Submitter: Barry Revzin Opened: 2022-08-04 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++23 status.

Discussion:

As pointed out by Casey Carter, while there is a feature macro for `flat_map` in P0429, there is no corresponding macro for flat_set in P1222. We should add one.

[2022-08-23; Reflector poll]

Set status to Tentatively Ready after 10 votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

1. Modify 17.3.2 [version.syn] as indicated:

   >     #define __cpp_lib_flat_map  202207L // also in <flat_map>
   >     #define __cpp_lib_flat_set  202207L // also in <flat_set>

### 3753(i). Clarify entity vs. freestanding entity

Section: 16.3.3.7 [freestanding.item] Status: C++23 Submitter: Ben Craig Opened: 2022-08-23 Last modified: 2023-11-22

Priority: 2

View all other issues in [freestanding.item].

View all issues with C++23 status.

Discussion:

This addresses NB comment GB-075 ( [freeestanding.entity] "Freestanding entities" are not entities)

[freestanding.entity] p1 defines a freestanding entity as a declaration or macro definition.

[freestanding.entity] p3 then says "entities followed with a comment […] are freestanding entities".

This is inconsistent, and breaks with macros, because macros are not entities, but they can be freestanding entities.

[2022-09-23; Reflector poll]

Set priority to 2 after reflector poll.

It's confusing for "freestanding entities" to be two things, neither of which are entities. Declarations may declare entities, they are not entities themselves. Given this definition, p6/7/8 makes no sense. A namespace can't be a freestanding entity since it's neither a declaration nor a macro definition.

"freestanding entities" is not best name, given the collision with core entity, but I think that this is separable issue.

[2022-09-28; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Previous resolution [SUPERSEDED]:

> This wording is relative to the forthcoming C++23 CD.

> - Modify [freestanding.entity] as indicated:

>   > -3- In a header synopsis, entities declarations and macro definitions followed with a comment that includes freestanding are freestanding entities.

[2022-11-06; Ben Craig provides new wording]

[2022-11-07; Kona - move to open]

New proposed resolution to be added.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify [freestanding.entity] as indicated:

>    > [Drafting note: Replace the section name [freestanding.entity] by [freestanding.item] throughout the working draft]

>    > 16.3.3.6 Freestanding entitiesitems [freestanding.itementity]

>    > -1- A freestanding entityitem is a declarationan entity or macro definition that is present in a freestanding implementation and a hosted implementation.

>    > -2- Unless otherwise specified, the requirements on freestanding entitiesitems on a freestanding implementation are the same as the corresponding requirements in a hosted implementation.

>    > -3- In a header synopsis, entities introduced by declarations followed with a comment that includes freestanding are freestanding entitiesitems.

>    > -?- In a header synopsis, macro definitions followed with a comment that includes freestanding are freestanding items.

>    > […]

>    > -4- If a header synopsis begins with a comment that includes all freestanding, then all of the entities introduced by declarations and macro definitions in the header synopsis are freestanding entitiesitems.

>    > -?- If a header synopsis begins with a comment that includes all freestanding, then all of the macro definitions in the header synopsis are freestanding items.

>    > […]

>    > -5- Deduction guides for freestanding entityitem class templates are freestanding entitiesitems.

>    > -6- Enclosing namespaces of freestanding entitiesitems are freestanding entitiesitems.

>    > -7- Friends of freestanding entitiesitems are freestanding entitiesitems.

>    > -8- Entities denoted by freestanding entityitem typedef-names and freestanding entityitem alias templates are freestanding entitiesitems.

> 2. Modify 16.4.2.5 [compliance] as indicated:

>    > -3- For each of the headers listed in Table 28, a freestanding implementation provides at least the freestanding entitiesitems ( [freestanding.entity]) declared in the header.

> 3. Modify 22.10.15.5 [func.bind.place] as indicated:

>    > -3- Placeholders are freestanding entitiesitems ( [freestanding.entity]).

[2022-11-08; Ben Craig provides improved wording]

This combined resolution addresses both 3753 and LWG 3815(i), and has already been reviewed by LWG.

This resolves ballot comment GB-75. It also partially addresses GB-130 (along with LWG 3814(i)).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 16.3.3.6 [freestanding.entity] as indicated:

>    > [Drafting note: Replace the section name [freestanding.entity] by [freestanding.item] throughout the working draft]

>    > 16.3.3.6 Freestanding entitiesitems [freestanding.itementity]

>    > -1- A freestanding entityitem is a declarationan entity or macro definition that is present in a freestanding implementation and a hosted implementation.

>    > -2- Unless otherwise specified, the requirements on non-namespace freestanding entitiesitems on a freestanding implementation are the same as the corresponding requirements in a hosted implementation.

>    > [Note: Enumerators impose requirements on their enumerations. Freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations. Members and deduction guides impose requirements on their class types. Class types have the same deduction guides and members on freestanding implementations and hosted implementations. — end note]

>    > -3- In a header synopsis, entities each entity introduced by a declaration followed withby a comment that includes freestanding areis a freestanding entitiesitem.

>    > -?- In a header synopsis, each macro definition followed by a comment that includes freestanding is a freestanding item.

>    > […]

>    > -4- If a header synopsis begins with a comment that includes all freestanding, then all of the declarations and macro definitionseach entity introduced by a declaration in the header synopsis areis a freestanding entitiesitem.

>    > -?- If a header synopsis begins with a comment that includes all freestanding, then each macro definition in the header synopsis is a freestanding item.

>    > […]

>    > -5- Deduction guides for freestanding entity class templates are freestanding entities.

>    > -6- Enclosing namespaces of freestanding entities are freestanding entities. Each enclosing namespace of each freestanding item is a freestanding item.

>    > -7- Friends of freestanding entities are freestanding entities. Each friend of each freestanding item is a freestanding item.

>    > -8- Entities denoted by freestanding entity typedef-names and freestanding entity alias templates are freestanding entities. Each entity denoted by each freestanding item typedef-name and each freestanding item alias template is a freestanding item.

> 2. Modify 16.4.2.5 [compliance] as indicated:

>    > -3- For each of the headers listed in Table 28, a freestanding implementation provides at least the freestanding entitiesitems (16.3.3.6 [freestanding.entityitem]) declared in the header.

> 3. Modify 22.10.15.5 [func.bind.place] as indicated:

>    > -3- Placeholders are freestanding entitiesitems (16.3.3.6 [freestanding.entityitem]).

[2022-11-09; Ben Craig and Tomasz provide improved wording]

This new resolution merges definition of freestanding item for entity in macro into bullet lists. It still addresses both 3753 and LWG 3815(i).

This resolves ballot comment GB-75. It also partially addresses GB-130 (along with LWG 3814(i)).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify [freestanding.entity] as indicated:

>    > [Drafting note: Replace the section name [freestanding.entity] by [freestanding.item] throughout the working draft]

>    > 16.3.3.6 Freestanding entitiesitems [freestanding.itementity]

>    > -1- A freestanding entityitem is a declarationan entity or macro definition that is present in a freestanding implementation and a hosted implementation.

>    > -2- Unless otherwise specified, the requirements on freestanding entitiesitems, except namespaces, onfor a freestanding implementation are the same as the corresponding requirements infor a hosted implementation.

>    > [Note: This implies that freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations. Furthermore, class types have the same deduction guides and members on freestanding implementations and hosted implementations. — end note]

>    > -3- An entity is a freestanding item, if it is:

>    > 1. (3.1) — introduced by a declaration in the header synopsis, and

>    >    1. (3.1.1) — the declaration is followed by a comment that includes freestanding, or

>    >    2. (3.1.2) — the header synopsis begins with a comment that includes all freestanding;

>    > 2. (3.2) — an enclosing namespace of a freestanding item,

>    > 3. (3.3) — a friend of a freestanding item,

>    > 4. (3.4) — denoted by a typedef-name, that is a freestanding item, or

>    > 5. (3.5) — denoted by a template alias, that is a freestanding item.

>    > -4- A macro definition is a freestanding item, if it is defined in the header synopsis and

>    > 1. (4.1) — the definition is followed by a comment that includes freestanding in the header synopsis, or

>    > 2. (4.2) — the header synopsis begins with a comment that includes all freestanding.

>    > -3- In a header synopsis, entities followed with a comment that includes freestanding are freestanding entities.

>    > [Example 1: … — end example]

>    > -4- If a header synopsis begins with a comment that includes all freestanding, then all of the declarations and macro definitions in the header synopsis are freestanding entities..

>    > [Example 2: … — end example]

>    > -5- Deduction guides for freestanding entity class templates are freestanding entities.

>    > -6- Enclosing namespaces of freestanding entities are freestanding entities.

>    > -7- Friends of freestanding entities are freestanding entities.

>    > -8- Entities denoted by freestanding entity typedef-names and freestanding entity alias templates are freestanding entities.

> 2. Modify 16.4.2.5 [compliance] as indicated:

>    > -3- For each of the headers listed in Table 28, a freestanding implementation provides at least the freestanding entitiesitems (16.3.3.6 [freestanding.entityitem]) declared in the header.

> 3. Modify 22.10.15.5 [func.bind.place] as indicated:

>    > -3- Placeholders are freestanding entitiesitems (16.3.3.6 [freestanding.entityitem]).

[2022-11-10; Tomasz provide improved wording]

Updated wording to support freestanding typedef-names and using declaration that are not entities. It still addresses both 3753 and LWG 3815(i).

This resolves ballot comment GB-75. It also partially addresses GB-130 (along with LWG 3814(i)).

[Kona 2022-11-10; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify [freestanding.entity] as indicated:

   > [Drafting note: Replace the section name [freestanding.entity] by [freestanding.item] throughout the working draft]

   > 16.3.3.6 Freestanding entitiesitems [freestanding.itementity]

   > -1- A freestanding entityitem is a declaration, entity, typedef-name, or macro definition that is required to be present in a freestanding implementation and a hosted implementation.

   > -2- Unless otherwise specified, the requirements on freestanding entitiesitems onfor a freestanding implementation are the same as the corresponding requirements infor a hosted implementation, except that not all of the members of the namespaces are required to be present.

   > [Note: This implies that freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations. Furthermore, class types have the same members and class templates have the same deduction guides on freestanding implementations and hosted implementations. — end note]

   > -3- A declaration in a header synopsis is a freestanding item if

   > 1. (3.1) — it is followed by a comment that includes freestanding, or

   > 2. (3.1) — the header synopsis begins with a comment that includes all freestanding.

   > -4- An entity or typedef-name is a freestanding item if it is:

   > 1. (4.1) — introduced by a declaration that is a freestanding item,

   > 2. (4.2) — an enclosing namespace of a freestanding item,

   > 3. (4.3) — a friend of a freestanding item,

   > 4. (4.4) — denoted by a typedef-name that is a freestanding item, or

   > 5. (4.5) — denoted by an alias template that is a freestanding item.

   > -5- A macro is a freestanding item if it is defined in a header synopsis and

   > 1. (5.1) — the definition is followed by a comment that includes freestanding, or

   > 2. (5.2) — the header synopsis begins with a comment that includes all freestanding.

   > -3- In a header synopsis, entities followed with a comment that includes freestanding are freestanding entities.

   > [Example 1: … — end example]

   > -4- If a header synopsis begins with a comment that includes all freestanding, then all of the declarations and macro definitions in the header synopsis are freestanding entities..

   > [Example 2: … — end example]

   > -5- Deduction guides for freestanding entity class templates are freestanding entities.

   > -6- Enclosing namespaces of freestanding entities are freestanding entities.

   > -7- Friends of freestanding entities are freestanding entities.

   > -8- Entities denoted by freestanding entity typedef-names and freestanding entity alias templates are freestanding entities.

2. Modify 16.4.2.5 [compliance] as indicated:

   > -3- For each of the headers listed in Table 28, a freestanding implementation provides at least the freestanding entitiesitems (16.3.3.6 [freestanding.entityitem]) declared in the header.

3. Modify 22.10.15.5 [func.bind.place] as indicated:

   > -3- Placeholders are freestanding entitiesitems (16.3.3.6 [freestanding.entityitem]).

### 3754(i). Class template `expected` synopsis contains declarations that do not match the detailed description

Section: 22.8.6.1 [expected.object.general] Status: C++23 Submitter: S. B. Tam Opened: 2022-08-23 Last modified: 2024-01-29

Priority: Not Prioritized

View all other issues in [expected.object.general].

View all issues with C++23 status.

Discussion:

22.8.6.1 [expected.object.general] declares the following constructors:

>     template<class G>
>       constexpr expected(const unexpected<G>&);
>     template<class G>
>       constexpr expected(unexpected<G>&&);

But in [expected.object.ctor], these constructors are declared as:

>     template<class G>
>       constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
>     template<class G>
>       constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);

Note that they have no explicit-specifiers in 22.8.6.1 [expected.object.general], but are conditionally explicit in [expected.object.ctor].

I presume that 22.8.6.1 [expected.object.general] is missing a few `explicit(see below)`.

The same inconsistency exists in 22.8.7 [expected.void].

[2022-09-05; Jonathan Wakely provides wording]

In N4910 the `expected` synopses had `explicit(see below)` on the copy and move constructors. That was fixed editorially, but this other inconsistency was not noticed.

[2022-09-07; Moved to "Ready" at LWG telecon]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Change 22.8.6.1 [expected.object.general] as indicated:

   >     // 22.8.6.2, constructors
   >     constexpr expected();
   >     constexpr expected(const expected&);
   >     constexpr expected(expected&&) noexcept(see below);
   >     template<class U, class G>
   >       constexpr explicit(see below) expected(const expected<U, G>&);
   >     template<class U, class G>
   >       constexpr explicit(see below) expected(expected<U, G>&&);

   >     template<class U = T>
   >       constexpr explicit(see below) expected(U&& v);

   >     template<class G>
   >       constexpr explicit(see below) expected(const unexpected<G>&);
   >     template<class G>
   >       constexpr explicit(see below) expected(unexpected<G>&&);

   >     template<class... Args>
   >       constexpr explicit expected(in_place_t, Args&&...);

2. Change 22.8.7.1 [expected.void.general] as indicated:

   >     // 22.8.7.2, constructors
   >     constexpr expected() noexcept;
   >     constexpr expected(const expected&);
   >     constexpr expected(expected&&) noexcept(see below);
   >     template<class U, class G>
   >       constexpr explicit(see below) expected(const expected<U, G>&&);

   >     template<class G>
   >       constexpr explicit(see below) expected(const unexpected<G>&);
   >     template<class G>
   >       constexpr explicit(see below) expected(unexpected<G>&&);

   >     constexpr explicit expected(in_place_t) noexcept;

### 3755(i). `tuple-for-each` can call user-defined `operator,`

Section: 25.7.25.2 [range.zip.view] Status: C++23 Submitter: Nicole Mazzuca Opened: 2022-08-26 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [range.zip.view].

View all other issues in [range.zip.view].

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Discussion:

The specification for `tuple-for-each` is:

>     template<class F, class Tuple>
>     constexpr auto tuple-for-each(F&& f, Tuple&& t) { // exposition only
>       apply([&]<class... Ts>(Ts&&... elements) {
>         (invoke(f, std::forward<Ts>(elements)), ...);
>       }, std::forward<Tuple>(t));
>     }

Given

>     struct Evil {
>       void operator,(Evil) {
>             abort();
>         }
>     };

and `tuple<int, int> t`, then `tuple-for-each([](int) { return Evil{}; }, t)`, the program will (unintentionally) abort.

It seems likely that our `Evil`'s `operator,` should not be called.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

Feedback from reviewers:

> "NAD. This exposition-only facility is only used with things that return `void`. As far as I know, users can't define `operator,` for `void`." "If I see the `void` cast, I don't need to audit the uses or be concerned that we'll add a broken use in the future."

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to the forthcoming C++23 CD.

- Modify [range.adaptor.tuple] as indicated:

  >     template<class F, class Tuple>
  >     constexpr auto tuple-for-each(F&& f, Tuple&& t) { // exposition only
  >       apply([&]<class... Ts>(Ts&&... elements) {
  >         (static_cast<void>(invoke(f, std::forward<Ts>(elements))), ...);
  >       }, std::forward<Tuple>(t));
  >     }

### 3756(i). Is the `std::atomic_flag` class signal-safe?

Section: 17.14.5 [support.signal], 32.5.10 [atomics.flag] Status: C++23 Submitter: Ruslan Baratov Opened: 2022-08-18 Last modified: 2023-11-22

Priority: 3

View all other issues in [support.signal].

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Discussion:

Following document number N4910 about signal-safe instructions 17.14.5 [support.signal] Signal handlers, and it's unclear whether `std::atomic_flag` is signal-safe.

Formally it doesn't fit any of the mentioned conditions:

- `f` is a non-static member function invoked on an object `A`, such that `A.is_lock_free()` yields `true`, or

  > (there is no `is_lock_free` method in `std::atomic_flag` class)

- `f` is a non-member function, and for every pointer-to-atomic argument `A` passed to `f`, `atomic_is_lock_free(A)` yields `true`

  > (`std::atomic_flag` object can't be passed to `atomic_is_lock_free` as argument)

However, `std::atomic_flag` seem to fit well here, it's atomic, and it's always lock-free.

The suggestion is as follows: If `std::atomic_flag` is signal-safe, then it should be explicitly mentioned in 17.14.5 [support.signal], e.g., the following lines should be added:

> - `f` is a non-static member function invoked on an `atomic_flag` object, or

> - `f` is a non-member function, and every pointer-to-atomic argument passed to `f` is `atomic_flag`, or

If the `std::atomic_flag` is not signal-safe, the following note could be added:

> [Note: Even though `atomic_flag` is atomic and lock-free, it's not signal-safe. — end note]

[2022-09-23; Reflector poll]

Set priority to 3 after reflector poll. Send to SG1.

Another way to fix this is to add `is_always_lock_free (=true)` and `is_lock_free() { return true; }` to `atomic_flag`.

[Kona 2022-11-10; SG1 yields a recommendation]

Poll: Adopt the proposed resolution for LWG3756
"f is a non-static member function invoked on an atomic_flag object, or"
"f is a non-member function, and every pointer-to- atomic argument passed to f is atomic_flag, or"

    SF F N A SA
    11 3 0 0 0

Unanimous consent

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 17.14.5 [support.signal] as indicated:

>    > -1- A call to the function `signal` synchronizes with any resulting invocation of the signal handler so installed.

>    > -2- A plain lock-free atomic operation is an invocation of a function `f` from 32.5 [atomics], such that:

>    > 1. (2.1) — `f` is the function `atomic_is_lock_free()`, or
>    > 2. (2.2) — `f` is the member function `is_lock_free()`, or
>    > 3. (2.?) — `f` is a non-static member function invoked on an `atomic_flag` object, or
>    > 4. (2.?) — `f` is a non-member function, and every pointer-to-atomic argument passed to `f` is `atomic_flag`, or
>    > 5. (2.3) — `f` is a non-static member function invoked on an object `A`, such that `A.is_lock_free()` yields `true`, or
>    > 6. (2.4) — `f` is a non-member function, and for every pointer-to-atomic argument `A` passed to `f`, `atomic_is_lock_free(A)` yields `true`.

>    > -3- An evaluation is signal-safe unless it includes one of the following:

>    > 1. (3.1) — a call to any standard library function, except for plain lock-free atomic operations and functions explicitly identified as signal-safe;

>    >    [Note 1: This implicitly excludes the use of `new` and `delete` expressions that rely on a library-provided memory allocator. — end note]

>    > 2. (3.2) — an access to an object with thread storage duration;
>    > 3. (3.3) — a `dynamic_cast` expression;
>    > 4. (3.4) — throwing of an exception;
>    > 5. (3.5) — control entering a try-block or function-try-block;
>    > 6. (3.6) — initialization of a variable with static storage duration requiring dynamic initialization (6.10.3.3 [basic.start.dynamic], 8.10 [stmt.dcl])206; or
>    > 7. (3.7) — waiting for the completion of the initialization of a variable with static storage duration (8.10 [stmt.dcl]).

>    > A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe.

[2022-11-11; Jonathan provides improved wording]

[Kona 2022-11-11; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 17.14.5 [support.signal] as indicated:

   > -1- A call to the function `signal` synchronizes with any resulting invocation of the signal handler so installed.

   > -2- A plain lock-free atomic operation is an invocation of a function `f` from 32.5 [atomics], such that:

   > 1. (2.1) — `f` is the function `atomic_is_lock_free()`, or
   > 2. (2.2) — `f` is the member function `is_lock_free()`, or
   > 3. (2.?) — `f` is a non-static member function of class `atomic_flag`, or
   > 4. (2.?) — `f` is a non-member function, and the first parameter of `f` has type cv `atomic_flag*`, or
   > 5. (2.3) — `f` is a non-static member function invoked on an object `A`, such that `A.is_lock_free()` yields `true`, or
   > 6. (2.4) — `f` is a non-member function, and for every pointer-to-atomic argument `A` passed to `f`, `atomic_is_lock_free(A)` yields `true`.

   > -3- An evaluation is signal-safe unless it includes one of the following:

   > 1. (3.1) — a call to any standard library function, except for plain lock-free atomic operations and functions explicitly identified as signal-safe;

   >    [Note 1: This implicitly excludes the use of `new` and `delete` expressions that rely on a library-provided memory allocator. — end note]

   > 2. (3.2) — an access to an object with thread storage duration;
   > 3. (3.3) — a `dynamic_cast` expression;
   > 4. (3.4) — throwing of an exception;
   > 5. (3.5) — control entering a try-block or function-try-block;
   > 6. (3.6) — initialization of a variable with static storage duration requiring dynamic initialization (6.10.3.3 [basic.start.dynamic], 8.10 [stmt.dcl])206; or
   > 7. (3.7) — waiting for the completion of the initialization of a variable with static storage duration (8.10 [stmt.dcl]).

   > A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe.

### 3757(i). What's the effect of `std::forward_like<void>(x)`?

Section: 22.2.4 [forward] Status: C++23 Submitter: Jiang An Opened: 2022-08-24 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [forward].

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Discussion:

Currently the return type of `std::forward_like` is specified by the following bullet:

> — Let `V` be

> >     OVERRIDE_REF(T&&, COPY_CONST(remove_reference_t<T>, remove_reference_t<U>))

where `T&&` is not always valid, e.g. it's invalid when `T` is `void`.

A strait forward reading may suggest that there is a hard error when `T` is not referenceable (which is currently implemented in MSVC STL), but this seems not clarified. It is unclear to me whether the intent is that hard error, substitution failure, or no error is caused when `T&&` is invalid.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 22.2.4 [forward] as indicated:

   >     template<class T, class U>
   >       [[nodiscard]] constexpr auto forward_like(U&& x) noexcept -> see below;

   > > Mandates: `T` is a referenceable type (3.46 [defns.referenceable]).

   > > […]

### 3759(i). `ranges::rotate_copy` should use `std::move`

Section: 26.7.11 [alg.rotate] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-25 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [alg.rotate].

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Discussion:

The range version of `ranges::rotate_copy` directly passes the `result` to the iterator-pair version. Since the type of `result` only models `weakly_incrementable` and may not be copied, we should use `std::move` here.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 26.7.11 [alg.rotate] as indicated:

   >     template<forward_range R, weakly_incrementable O>
   >       requires indirectly_copyable<iterator_t<R>, O>
   >       constexpr ranges::rotate_copy_result<borrowed_iterator_t<R>, O>
   >         ranges::rotate_copy(R&& r, iterator_t<R> middle, O result);

   > > -11- Effects: Equivalent to:

   > > >     return ranges::rotate_copy(ranges::begin(r), middle, ranges::end(r), std::move(result));

### 3760(i). `cartesian_product_view::iterator`'s `parent_` is never valid

Section: 25.7.33 [range.cartesian] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

`cartesian_product_view::iterator` has a pointer member `parent_` that points to `cartesian_product_view`, but its constructor only accepts a `tuple` of iterators, which makes `parent_` always default-initialized to `nullptr`.

The proposed resolution is to add an aliased `Parent` parameter to the constructor and initialize `parent_` with `addressof`, as we usually do.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.33.2 [range.cartesian.view] as indicated:

   >     constexpr iterator<false> begin()
   >       requires (!simple-view<First> || ... || !simple-view<Vs>);

   > > -2- Effects: Equivalent to: `return iterator<false>(*this, tuple-transform(ranges::begin, bases_));`

   >     constexpr iterator<true> begin() const
   >       requires (range<const First> && ... && range<const Vs>);

   > > -3- Effects: Equivalent to: `return iterator<true>(*this, tuple-transform(ranges::begin, bases_));`

   >     constexpr iterator<false> end()
   >       requires ((!simple-view<First> || ... || !simple-view<Vs>)
   >         && cartesian-product-is-common<First, Vs...>);
   >     constexpr iterator<true> end() const
   >       requires cartesian-product-is-common<const First, const Vs...>;

   > > -4- Let:

   > > 1. (4.1) — `is-const` be `true` for the const-qualified overload, and `false` otherwise;

   > > 2. (4.2) — `is-empty` be `true` if the expression `ranges::empty(rng)` is `true` for any `rng` among the underlying ranges except the first one and `false` otherwise; and

   > > 3. (4.3) — `begin-or-first-end(rng)` be expression-equivalent to `is-empty ? ranges::begin(rng) : cartesian-common-arg-end(rng)` if `rng` is the first underlying range and `ranges::begin(rng)` otherwise.

   > > -5- Effects: Equivalent to:

   > > >     iterator<is-const> it(*this, tuple-transform(
   > > >       [](auto& rng){ return begin-or-first-end(rng); }, bases_));
   > > >     return it;

2. Modify [ranges.cartesian.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range First, forward_range... Vs>
   > >         requires (view<First> && ... && view<Vs>)
   > >       template<bool Const>
   > >       class cartesian_product_view<First, Vs...>::iterator {
   > >       public:
   > >         […]

   > >       private:
   > >         using Parent = maybe-const<Const, cartesian_product_view>;           // exposition only
   > >         Parentmaybe-const<Const, cartesian_product_view>* parent_ = nullptr; // exposition only
   > >         tuple<iterator_t<maybe-const<Const, First>>,
   > >           iterator_t<maybe-const<Const, Vs>>...> current_;                   // exposition only

   > >         template<size_t N = sizeof...(Vs)>
   > >           constexpr void next();                                             // exposition only

   > >         template<size_t N = sizeof...(Vs)>
   > >           constexpr void prev();                                             // exposition only

   > >         template<class Tuple>
   > >           constexpr difference_type distance-from(Tuple t);                  // exposition only

   > >         constexpr explicit iterator(Parent& parent, tuple<iterator_t<maybe-const<Const, First>>,
   > >           iterator_t<maybe-const<Const, Vs>>...> current);                   // exposition only
   > >       };
   > >     }

   > […]

   >     constexpr explicit iterator(Parent& parent, tuple<iterator_t<maybe-const<Const, First>>,
   >       iterator_t<maybe-const<Const, Vs>>...> current);

   > > -10- Effects: Initializes `parent_` with `addressof(parent)` and `current_` with `std::move(current)`.

   >     constexpr iterator(iterator<!Const> i) requires Const &&
   >       (convertible_to<iterator_t<First>, iterator_t<const First>> &&
   >         ... && convertible_to<iterator_t<Vs>, iterator_t<const Vs>>);

   > > -11- Effects: Initializes `parent_` with `i.parent_` and `current_` with `std::move(i.current_)`.

### 3761(i). `cartesian_product_view::iterator::operator-` should pass by reference

Section: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.cartesian.iterator].

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Discussion:

There are two problems with `cartesian_product_view::iterator::operator-`.

First, `distance-from` is not `const`-qualified, which would cause the common version of `operator-` to produce a hard error inside the function body since it invokes `distance-from` on a const `iterator` object.

Second, the non-common version of `operator-` passes `iterator` by value, which unnecessarily prohibits subtracting `default_sentinel` from an lvalue `iterator` whose first underlying iterator is non-`copyable`. Even if we `std::move` it into `operator-`, the other overload will still be ill-formed since it returns `-(i - s)` which will calls `i`'s deleted copy constructor.

The proposed resolution is to add `const` qualification to `distance-from` and make the non-common version of `iterator::operator-` pass by const reference. Although the `Tuple` parameter of `distance-from` is guaranteed to be `copyable`, I think it would be more appropriate to pass it by reference.

[2022-09-08]

As suggested by Tim Song, the originally submitted proposed wording has been refined to use `const Tuple&` instead of `Tuple&`.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify [ranges.cartesian.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range First, forward_range... Vs>
   > >         requires (view<First> && ... && view<Vs>)
   > >       template<bool Const>
   > >       class cartesian_product_view<First, Vs...>::iterator {
   > >       public:
   > >         […]

   > >         friend constexpr difference_type operator-(const iterator& i, default_sentinel_t)
   > >           requires cartesian-is-sized-sentinel<Const, sentinel_t, First, Vs...>;
   > >         friend constexpr difference_type operator-(default_sentinel_t, const iterator& i)
   > >           requires cartesian-is-sized-sentinel<Const, sentinel_t, First, Vs...>;

   > >         […]
   > >       private:
   > >         […]

   > >         template<class Tuple>
   > >           constexpr difference_type distance-from(const Tuple& t) const;            // exposition only

   > >         […]
   > >       };
   > >     }

   > […]

   >     template<class Tuple>
   >       constexpr difference_type distance-from(const Tuple& t) const;

   > > -7- Let:

   > > 1. (7.1) — `scaled-size(N)` be the product of `static_cast<difference_type>(ranges::size(std::get<N>(parent_->bases_)))` and `scaled-size(N + 1)` if `N < sizeof...(Vs)`, otherwise `static_cast<difference_type>(1)`;

   > > 2. (7.2) — `scaled-distance(N)` be the product of `static_cast<difference_type>(std::get<N>(current_) - std::get<N>(t))` and `scaled-size(N + 1)`; and

   > > 3. (7.3) — `scaled-sum` be the sum of `scaled-distance(N)` for every integer `0 ≤ N ≤ sizeof...(Vs)`.

   > > -8- Preconditions: `scaled-sum` can be represented by `difference_type`.

   > > -9- Returns: `scaled-sum`.

   > […]

   >     friend constexpr difference_type operator-(const iterator& i, default_sentinel_t)
   >       requires cartesian-is-sized-sentinel<Const, sentinel_t, First, Vs...>;

   > > -32- Let `end-tuple` be an object of a type that is a specialization of `tuple`, such that:

   > > 1. (32.1) — `std::get<0>(end-tuple)` has the same value as `ranges::end(std::get<0>(i.parent_->bases_))`;

   > > 2. (32.2) — `std::get<N>(end-tuple)` has the same value as `ranges::begin(std::get<N>(i.parent_->bases_))` for every integer `1 ≤ N ≤ sizeof...(Vs)`.

   > > -33- Effects: Equivalent to: `return i.distance-from(end-tuple);`

   >     friend constexpr difference_type operator-(default_sentinel_t, const iterator& i)
   >       requires cartesian-is-sized-sentinel<Const, sentinel_t, First, Vs...>;

   > > -34- Effects: Equivalent to: `return -(i - s);`

### 3762(i). `generator::iterator::operator==` should pass by reference

Section: 25.8.6 [coro.generator.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [coro.generator.iterator].

View all other issues in [coro.generator.iterator].

View all issues with C++23 status.

Discussion:

Currently, `generator::iterator::operator==` passes `iterator` by value. Given that `iterator` is a move-only type, this makes it impossible for `generator` to model `range`, since `default_sentinel` cannot be compared to the `iterator` of `const` lvalue and is not eligible to be its sentinel.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.8.6 [coro.generator.iterator] as indicated:

   > >     namespace std {
   > >       template<class Ref, class V, class Allocator>
   > >       class generator<Ref, V, Allocator>::iterator {
   > >       public:
   > >         using value_type = value;
   > >         using difference_type = ptrdiff_t;

   > >         iterator(iterator&& other) noexcept;
   > >         iterator& operator=(iterator&& other) noexcept;

   > >         reference operator*() const noexcept(is_nothrow_copy_constructible_v<reference>);
   > >         iterator& operator++();
   > >         void operator++(int);

   > >         friend bool operator==(const iterator& i, default_sentinel_t);

   > >       private:
   > >         coroutine_handle<promise_type> coroutine_; // exposition only
   > >       };
   > >     }

   > […]

   >     friend bool operator==(const iterator& i, default_sentinel_t);

   > > -10- Effects: Equivalent to: `return i.coroutine_.done();`

### 3764(i). `reference_wrapper::operator()` should propagate `noexcept`

Section: 22.10.6.5 [refwrap.invoke] Status: C++23 Submitter: Zhihao Yuan Opened: 2022-08-31 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [refwrap.invoke].

View all issues with C++23 status.

Discussion:

The following assertion does not hold, making the type less capable than the function pointers.

>     void f() noexcept;

>     std::reference_wrapper fn = f;
>     static_assert(std::is_nothrow_invocable_v<decltype(fn)>);

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Already implemented in MSVC and libstdc++.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 22.10.6.1 [refwrap.general], class template `reference_wrapper` synopsis, as indicated:

   >     // 22.10.6.5 [refwrap.invoke], invocation
   >     template<class... ArgTypes>
   >       constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) const
   >         noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);

2. Modify 22.10.6.5 [refwrap.invoke] as indicated:

   >     template<class... ArgTypes>
   >       constexpr invoke_result_t<T&, ArgTypes...>
   >         operator()(ArgTypes&&... args) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);

   > > -1- Mandates: `T` is a complete type.

   > > -2- Returns: `INVOKE(get(), std::forward<ArgTypes>(args)...)`. (22.10.4 [func.require])

### 3765(i). `const_sentinel` should be constrained

Section: 24.2 [iterator.synopsis], 24.5.3.2 [const.iterators.alias] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-03 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [iterator.synopsis].

View all issues with C++23 status.

Discussion:

The current standard does not have any constraints on `const_sentinel` template parameters, which makes it possible to pass almost any type of object to `make_const_sentinel`. It would be more appropriate to constrain the type to meet the minimum requirements of the sentinel type such as `semiregular` as `move_sentinel` does.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <concepts>             // see 18.3 [concepts.syn]

   >     namespace std {
   >       […]

   >       // 24.5.3 [const.iterators], constant iterators and sentinels
   >       // 24.5.3.2 [const.iterators.alias], alias templates
   >       […]
   >       template<input_iterator I>
   >         using const_iterator = see below;                                               // freestanding
   >       template<semiregularclass S>
   >         using const_sentinel = see below;                                               // freestanding

   >       […]
   >       template<input_iterator I>
   >         constexpr const_iterator<I> make_const_iterator(I it) { return it; }            // freestanding

   >       template<semiregularclass S>
   >         constexpr const_sentinel<S> make_const_sentinel(S s) { return s; }              // freestanding
   >       […]
   >     }

2. Modify 24.5.3.2 [const.iterators.alias] as indicated:

   >     template<semiregularclass S>
   >       using const_sentinel = see below;

   > > -2- Result: If `S` models `input_iterator`, `const_iterator<S>`. Otherwise, `S`.

### 3766(i). `view_interface::cbegin` is underconstrained

Section: 25.5.3.1 [view.interface.general] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-04 Last modified: 2023-11-22

Priority: 2

View all other issues in [view.interface.general].

View all issues with C++23 status.

Discussion:

Currently, `view_interface::cbegin` simply returns `ranges::cbegin(derived())`, which returns the type alias `const_iterator` for its iterator, which requires that the template parameter `I` must model the `input_iterator`.

Given that `view_interface::cbegin` does not have any constraints, when `D` models only `output_range`, calling its `cbegin()` will result in a hard error inside the function body:

>     #include <ranges>
>     #include <vector>

>     int main() {
>       std::vector<int> v;
>       auto r = std::views::counted(std::back_inserter(v), 3);
>       auto b = r.cbegin(); // hard error
>     }

We should add a constraint for `view_interface::cbegin` that `D` must model `input_range`.

[2022-09-23; Reflector poll]

Set priority to 2 after reflector poll.

This should be done for `cend` too.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 25.5.3.1 [view.interface.general] as indicated:

>    >     namespace std::ranges {
>    >       template<class D>
>    >         requires is_class_v<D> && same_as<D, remove_cv_t<D>>
>    >       class view_interface {
>    >         […]
>    >       public:
>    >         […]
>    >         constexpr auto cbegin() requires input_range<D> {
>    >           return ranges::cbegin(derived());
>    >         }
>    >         constexpr auto cbegin() const requires input_range<const D> {
>    >           return ranges::cbegin(derived());
>    >         }
>    >         […]
>    >       };
>    >     }

[2022-09-25; Hewill provides improved wording]

[Kona 2022-11-08; Accepted at joint LWG/SG9 session. Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.5.3.1 [view.interface.general] as indicated:

   >     namespace std::ranges {
   >       template<class D>
   >         requires is_class_v<D> && same_as<D, remove_cv_t<D>>
   >       class view_interface {
   >         […]
   >       public:
   >         […]
   >         constexpr auto cbegin() requires input_range<D> {
   >           return ranges::cbegin(derived());
   >         }
   >         constexpr auto cbegin() const requires input_range<const D> {
   >           return ranges::cbegin(derived());
   >         }
   >         constexpr auto cend() requires input_range<D> {
   >           return ranges::cend(derived());
   >         }
   >         constexpr auto cend() const requires input_range<const D> {
   >           return ranges::cend(derived());
   >         }
   >         […]
   >       };
   >     }

### 3767(i). `codecvt<charN_t, char8_t, mbstate_t>` incorrectly added to locale

Section: 28.3.3.1.2.1 [locale.category], 28.3.4.2.5.1 [locale.codecvt.general] Status: C++26 Submitter: Victor Zverovich Opened: 2022-09-05 Last modified: 2026-06-09

Priority: 3

View all other issues in [locale.category].

View all issues with C++26 status.

Discussion:

Table [tab:locale.category.facets] includes the following two facets:

- `codecvt<char16_t, char8_t, mbstate_t>`

- `codecvt<char32_t, char8_t, mbstate_t>`

However, neither of those actually has anything to do with a locale and therefore it doesn't make sense to dynamically register them with `std::locale`. Instead they provide conversions between fixed encodings (UTF-8, UTF-16, UTF-32) that are unrelated to locale encodings other than they may happen to coincide with encodings of some locales by accident.

The issue was introduced when adding `codecvt<char[16|32]_t, char, mbstate_t>` in N2035 which gave no design rationale for using `codecvt` in the first place. Likely it was trying to do a minimal amount of changes and copied the wording for `codecvt<wchar_t, char, mbstate_t>` but unfortunately didn't consider encoding implications.

P0482 changed `char` to `char8_t` in these facets which made the issue more glaring but unfortunately, despite the breaking change, it failed to address it.

Apart from an obvious design mistake this also adds a small overhead for every locale construction because the implementation has to copy these pseudo-facets for no good reason violating "don't pay for what you don't use" principle.

A simple fix is to remove the two facets from table [tab:locale.category.facets] and make them directly constructible.

[2022-09-23; Reflector poll]

Set priority to 3 after reflector poll. Send to SG16 (then maybe LEWG).

[2022-09-28; SG16 responds]

SG16 agrees that the codecvt facets mentioned in LWG3767 "`codecvt<charN_t, char8_t, mbstate_t>` incorrectly added to locale" are intended to be invariant with respect to locale. Unanimously in favor.

[Issaquah 2023-02-10; LWG issue processing]

Removing these breaks most code using them today, because the most obvious way to use them is via `use_facet` on a locale, which would throw if they're removed (and because they were guaranteed to be present, code using them might have not bothered to check for them using `has_facet`). Instead of removing them, deprecate the guarantee that they're always present (so move them to D.20 [depr.locale.category]). Don't bother changing the destructor. Victor to update wording.

Previous resolution [SUPERSEDED]

This wording is relative to N4917.

1. Modify 28.3.3.1.2.1 [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — as indicated:

   > Table 105: Locale category facets [tab:locale.category.facets]
   > Category | Includes facets
   > --- | ---
   > `…`
   > ctype | `ctype<char>, ctype<wchar_t> codecvt<char, char, mbstate_t> codecvt<char16_t, char8_t, mbstate_t> codecvt<char32_t, char8_t, mbstate_t> codecvt<wchar_t, char, mbstate_t>`
   > `…`

2. Modify 28.3.4.2.5.1 [locale.codecvt.general] as indicated:

   > >     namespace std {
   > >       […]
   > >       template<class internT, class externT, class stateT>
   > >         class codecvt : public locale::facet, public codecvt_base {
   > >         public:
   > >           using intern_type = internT;
   > >           using extern_type = externT;
   > >           using state_type = stateT;

   > >           explicit codecvt(size_t refs = 0);
   > >           ~codecvt();

   > >           […]
   > >         protected:
   > >           ~codecvt();
   > >           […]
   > >         };
   > >     }

   > […]

   > -3- The specializations required in Table 105 [tab:locale.category.facets]106 [tab:locale.spec] (28.3.3.1.2.1 [locale.category]) convert the implementation-defined native character set. `codecvt<char, char, mbstate_t>` implements a degenerate conversion; it does not convert at all. The specialization `codecvt<char16_t, char8_t, mbstate_t>` converts between the UTF-16 and UTF-8 encoding forms, and the specialization `codecvt<char32_t, char8_t, mbstate_t>` converts between the UTF-32 and UTF-8 encoding forms. `codecvt<wchar_t, char, mbstate_t>` converts between the native character sets for ordinary and wide characters. Specializations on `mbstate_t` perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-defined `stateT` type. Objects of type `stateT` can contain any state that is useful to communicate to or from the specialized `do_in` or `do_out` members.

[2023-02-10; Victor Zverovich comments and provides improved wording]

Per today's LWG discussion the following changes have been implemented in revised wording:

- Deprecated the facets instead of removing them (also `_byname` variants which were previously missed).

- Removed the changes to facet dtor since with deprecation it's no longer critical to provide other ways to access them.

[Kona 2023-11-07; move to Ready]

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 28.3.3.1.2.1 [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — and Table 106 ([tab:locale.spec]) "Required specializations" as indicated:

   > Table 105: Locale category facets [tab:locale.category.facets]
   > Category | Includes facets
   > --- | ---
   > `…`
   > ctype | `ctype<char>, ctype<wchar_t> codecvt<char, char, mbstate_t> codecvt<char16_t, char8_t, mbstate_t> codecvt<char32_t, char8_t, mbstate_t> codecvt<wchar_t, char, mbstate_t>`
   > `…`

   > […]

   > Table 106: Required specializations [tab:locale.spec]
   > Category | Includes facets
   > --- | ---
   > `…`
   > ctype | `ctype_byname<char>, ctype_byname<wchar_t> codecvt_byname<char, char, mbstate_t> codecvt_byname<char16_t, char8_t, mbstate_t> codecvt_byname<char32_t, char8_t, mbstate_t> codecvt_byname<wchar_t, char, mbstate_t>`
   > `…`

2. Modify 28.3.4.2.5.1 [locale.codecvt.general] as indicated:

   > […]

   > -3- The specializations required in Table 105 (28.3.3.1.2.1 [locale.category]) convert the implementation-defined native character set. `codecvt<char, char, mbstate_t>` implements a degenerate conversion; it does not convert at all. The specialization `codecvt<char16_t, char8_t, mbstate_t>` converts between the UTF-16 and UTF-8 encoding forms, and the specialization `codecvt<char32_t, char8_t, mbstate_t>` converts between the UTF-32 and UTF-8 encoding forms. `codecvt<wchar_t, char, mbstate_t>` converts between the native character sets for ordinary and wide characters. Specializations on `mbstate_t` perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-defined `stateT` type. Objects of type `stateT` can contain any state that is useful to communicate to or from the specialized `do_in` or `do_out` members.

3. Modify D.20 [depr.locale.category] (Deprecated locale category facets) in Annex D as indicated:

   > -1- The `ctype` locale category includes the following facets as if they were specified in table Table 105 [tab:locale.category.facets] of 28.3.4.2.5.1 [locale.codecvt.general].

   > >     codecvt<char16_t, char, mbstate_t>
   > >     codecvt<char32_t, char, mbstate_t>
   > >     codecvt<char16_t, char8_t, mbstate_t>
   > >     codecvt<char32_t, char8_t, mbstate_t>

   > -1- The `ctype` locale category includes the following facets as if they were specified in table Table 106 [tab:locale.spec] of 28.3.4.2.5.1 [locale.codecvt.general].

   > >     codecvt_byname<char16_t, char, mbstate_t>
   > >     codecvt_byname<char32_t, char, mbstate_t>
   > >     codecvt_byname<char16_t, char8_t, mbstate_t>
   > >     codecvt_byname<char32_t, char8_t, mbstate_t>

   > -3- The following class template specializations are required in addition to those specified in 28.3.4.2.5 [locale.codecvt]. The specializations `codecvt<char16_t, char, mbstate_t>` and `codecvt<char16_t, char8_t, mbstate_t>` converts between the UTF-16 and UTF-8 encoding forms, and the specializations `codecvt<char32_t, char, mbstate_t>` and `codecvt<char32_t, char8_t, mbstate_t>` converts between the UTF-32 and UTF-8 encoding forms.

### 3769(i). `basic_const_iterator::operator==` causes infinite constraint recursion

Section: 24.5.3 [const.iterators] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-05 Last modified: 2023-11-22

Priority: 1

View other active issues in [const.iterators].

View all other issues in [const.iterators].

View all issues with C++23 status.

Discussion:

Currently, `basic_const_iterator::operator==` is defined as a `friend` function:

>     template<sentinel_for<Iterator> S>
>       friend constexpr bool operator==(const basic_const_iterator& x, const S& s);

which only requires `S` to model `sentinel_for<Iterator>`, and since `basic_const_iterator` has a conversion constructor that accepts `I`, this will result in infinite constraint checks when comparing `basic_const_iterator<int*>` with `int*` (online example):

>     #include <iterator>

>     template<std::input_iterator I>
>     struct basic_const_iterator {
>       basic_const_iterator() = default;
>       basic_const_iterator(I);
>       template<std::sentinel_for<I> S>
>       friend bool operator==(const basic_const_iterator&, const S&);
>     };

>     static_assert(std::sentinel_for<basic_const_iterator<int*>, int*>); // infinite meta-recursion

That is, `sentinel_for` ends with `weakly-equality-comparable-with` and instantiates `operator==`, which in turn rechecks `sentinel_for` and instantiates the same `operator==`, making the circle closed.

The proposed resolution is to change `operator==` to be a member function so that `S` is no longer accidentally instantiated as `basic_const_iterator`. The same goes for `basic_const_iterator::operator-`.

[2022-09-23; Reflector poll]

Set priority to 1 after reflector poll.

"Although I am not a big fan of member ==, the proposed solution seems to be simple." "prefer if we would keep `operator==` as non-member for consistency."

Previous resolution from Hewill [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 24.5.3.3 [const.iterators.iterator], class template `basic_const_iterator` synopsis, as indicated:

>    >     namespace std {
>    >       template<class I>
>    >         concept not-a-const-iterator = see below;

>    >       template<input_iterator Iterator>
>    >       class basic_const_iterator {
>    >         Iterator current_ = Iterator();
>    >         using reference = iter_const_reference_t<Iterator>;         // exposition only

>    >       public:
>    >         […]
>    >         template<sentinel_for<Iterator> S>
>    >           friend constexpr bool operator==(const basic_const_iterator& x, const S& s) const;
>    >         […]
>    >         template<sized_sentinel_for<Iterator> S>
>    >           friend constexpr difference_type operator-(const basic_const_iterator& x, const S& y) const;
>    >         template<not-a-const-iteratorsized_sentinel_for<Iterator> S>
>    >           requires sized_sentinel_fordifferent-from<S, Iteratorbasic_const_iterator>
>    >           friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);
>    >       };
>    >     }

> 2. Modify 24.5.3.5 [const.iterators.ops] as indicated:

>    > […]

>    >       template<sentinel_for<Iterator> S>
>    >         friend constexpr bool operator==(const basic_const_iterator& x, const S& s) const;

>    > > -16- Effects: Equivalent to: `return x.current_ == s;`.

>    > […]

>    >       template<sized_sentinel_for<Iterator> S>
>    >         friend constexpr difference_type operator-(const basic_const_iterator& x, const S& y) const;

>    > > -24- Effects: Equivalent to: `return x.current_ - y;`.

>    >       template<not-a-const-iteratorsized_sentinel_for<Iterator> S>
>    >         requires sized_sentinel_fordifferent-from<S, Iteratorbasic_const_iterator>
>    >         friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);

>    > > -25- Effects: Equivalent to: `return x - y.current_;`.

[2022-11-04; Tomasz comments and improves proposed wording]

Initially, LWG requested an investigation of alternative resolutions that would avoid using member functions for the affected operators. Later, it was found that in addition to `==`/`-`, all comparison operators (`<`, `>`, `<=`, `>=`, `<=>`) are affected by same problem for the calls with `basic_const_iterator<basic_const_iterator<int*>>` and `int*` as arguments, i.e. `totally_ordered_with<basic_const_iterator<basic_const_iterator<int*>>, int*>` causes infinite recursion in constraint checking.

The new resolution, change all of the friends overloads for operators `==`, `<`, `>`, `<=`, `>=`, `<=>` and `-` that accept `basic_const_iterator` as lhs, to `const` member functions. This change is applied to homogeneous `(basic_const_iterator, basic_const_iterator)` for consistency. For the overload of `<`, `>`, `<=`, `>=` and `-` that accepts `(I, basic_const_iterator)` we declared them as friends and consistently constrain them with `not-const-iterator`. Finally, its put (now member) `operator`<=>`(I)` in the block with other heterogeneous overloads in the synopsis.

The use of member functions addresses issues, because:

- it disallows conversion to `basic_const_iterator` in the left-hand side of op, i.e. eliminates issues for `(sized_)sentinel_for<basic_const_iterator<int*>, int*>` and `totally_ordered<basic_const_iterator<int*>, int*>`
- member functions (in contrast to friends) are not found by ADL, so we do not get multiple candidates for `basic_const_iterator<basic_const_iterator<S>>`, so we address recursion for nested iterators

[Kona 2022-11-08; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 24.5.3.3 [const.iterators.iterator], class template `basic_const_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<class I>
   >         concept not-a-const-iterator = see below;

   >       template<input_iterator Iterator>
   >       class basic_const_iterator {
   >         Iterator current_ = Iterator();
   >         using reference = iter_const_reference_t<Iterator>;         // exposition only

   >       public:
   >         […]
   >         template<sentinel_for<Iterator> S>
   >           friend constexpr bool operator==(const basic_const_iterator& x, const S& s) const;

   >         friend constexpr bool operator<(const basic_const_iterator& x, const basic_const_iterator& y) const
   >           requires random_access_iterator<Iterator>;
   >         friend constexpr bool operator>(const basic_const_iterator& x, const basic_const_iterator& y) const
   >           requires random_access_iterator<Iterator>;
   >         friend constexpr bool operator<=(const basic_const_iterator& x, const basic_const_iterator& y) const
   >           requires random_access_iterator<Iterator>;
   >         friend constexpr bool operator>=(const basic_const_iterator& x, const basic_const_iterator& y) const
   >           requires random_access_iterator<Iterator>;
   >         friend constexpr auto operator<=>(const basic_const_iterator& x, const basic_const_iterator& y) const
   >           requires random_access_iterator<Iterator> && three_way_comparable<Iterator>;

   >         template<different-from<basic_const_iterator> I>
   >         friend constexpr bool operator<(const basic_const_iterator& x, const I& y) const
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<different-from<basic_const_iterator> I>
   >         friend constexpr bool operator>(const basic_const_iterator& x, const I& y) const
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<different-from<basic_const_iterator> I>
   >         friend constexpr bool operator<=(const basic_const_iterator& x, const I& y) const
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<different-from<basic_const_iterator> I>
   >         friend constexpr bool operator>=(const basic_const_iterator& x, const I& y) const
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<different-from<basic_const_iterator> I>
   >         constexpr auto operator<=>(const I& y) const
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I> &&
   >        	       three_way_comparable_with<Iterator, I>;
   >         template<not-a-const-iterator I>
   >         friend constexpr bool operator<(const I& y, const basic_const_iterator& x)
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<not-a-const-iterator I>
   >         friend constexpr bool operator>(const I& y, const basic_const_iterator& x)
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<not-a-const-iterator I>
   >         friend constexpr bool operator<=(const I& y, const basic_const_iterator& x)
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<not-a-const-iterator I>
   >         friend constexpr bool operator>=(const I& y, const basic_const_iterator& x)
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<different-from<basic_const_iterator> I>
   >         friend constexpr auto operator<=>(const basic_const_iterator& x, const I& y)
   >           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I> &&
   >           	       three_way_comparable_with<Iterator, I>;

   >         […]
   >         template<sized_sentinel_for<Iterator> S>
   >           friend constexpr difference_type operator-(const basic_const_iterator& x, const S& y) const;
   >         template<not-a-const-iteratorsized_sentinel_for<Iterator> S>
   >           requires sized_sentinel_fordifferent-from<S, Iteratorbasic_const_iterator>
   >           friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);
   >       };
   >     }

2. Modify 24.5.3.5 [const.iterators.ops] as indicated:

   > […]

   >     template<sentinel_for<Iterator> S>
   >       friend constexpr bool operator==(const basic_const_iterator& x, const S& s) const;

   > > -16- Effects: Equivalent to: `return x.current_ == s;`

   >     friend constexpr bool operator<(const basic_const_iterator& x, const basic_const_iterator& y) const
   >       requires random_access_iterator<Iterator>;
   >     friend constexpr bool operator>(const basic_const_iterator& x, const basic_const_iterator& y) const
   >       requires random_access_iterator<Iterator>;
   >     friend constexpr bool operator<=(const basic_const_iterator& x, const basic_const_iterator& y) const
   >       requires random_access_iterator<Iterator>;
   >     friend constexpr bool operator>=(const basic_const_iterator& x, const basic_const_iterator& y) const
   >       requires random_access_iterator<Iterator>;
   >     friend constexpr auto operator<=>(const basic_const_iterator& x, const basic_const_iterator& y) const
   >       requires random_access_iterator<Iterator> && three_way_comparable<Iterator>;

   > > -17- Let op be the operator.

   > > -18- Effects: Equivalent to: `return x.current_ op y.current_;`

   >     template<different-from<basic_const_iterator> I>
   >     friend constexpr bool operator<(const basic_const_iterator& x, const I& y) const
   >       requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >     template<different-from<basic_const_iterator> I>
   >     friend constexpr bool operator>(const basic_const_iterator& x, const I& y) const
   >       requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >     template<different-from<basic_const_iterator> I>
   >     friend constexpr bool operator<=(const basic_const_iterator& x, const I& y) const
   >       requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >     template<different-from<basic_const_iterator> I>
   >     friend constexpr bool operator>=(const basic_const_iterator& x, const I& y) const
   >       requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >     template<different-from<basic_const_iterator> I>
   >     friend constexpr auto operator<=>(const basic_const_iterator& x, const I& y) const
   >       requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I> &&
   >       	       three_way_comparable_with<Iterator, I>;

   > > -19- Let `op` be the operator.

   > > -20- ReturnsEffects: Equivalent to: `return x.current_ op y;`

   > […]

   >     template<sized_sentinel_for<Iterator> S>
   >       friend constexpr difference_type operator-(const basic_const_iterator& x, const S& y) const;

   > > -24- Effects: Equivalent to: `return x.current_ - y;`

   >     template<not-a-const-iteratorsized_sentinel_for<Iterator> S>
   >       requires sized_sentinel_fordifferent-from<S, Iteratorbasic_const_iterator>
   >       friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);

   > > -25- Effects: Equivalent to: `return x - y.current_;`

### 3770(i). `const_sentinel_t` is missing

Section: 25.2 [ranges.syn] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-05 Last modified: 2023-11-22

Priority: 3

View other active issues in [ranges.syn].

View all other issues in [ranges.syn].

View all issues with C++23 status.

Discussion:

The type alias pair `const_iterator` and `const_sentinel` and the factory function pair `make_const_iterator` and `make_const_sentinel` are defined in `<iterator>`, however, only `const_iterator_t` exists in `<ranges>`, which lacks `const_sentinel_t`, we should add it to ensure consistency.

The proposed resolution also lifts the type constraint of `const_iterator_t` to `input_range` to make it consistent with `const_iterator`, which already requires that its template parameter must model `input_iterator`.

[2022-09-23; Reflector poll]

Set priority to 3 after reflector poll.

Comment from a reviewer:

> "I don't see why we should redundantly constrain `const_iterator_t` with `input_range`; it's not as if we can make it more ill-formed or more SFINAE-friendly than it is now."

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

>    >     #include <compare>              // see 17.12.1 [compare.syn]
>    >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
>    >     #include <iterator>             // see 24.2 [iterator.synopsis]

>    >     namespace std::ranges {
>    >       […]
>    >       template<class T>
>    >         using iterator_t = decltype(ranges::begin(declval<T&>()));                      // freestanding
>    >       template<range R>
>    >         using sentinel_t = decltype(ranges::end(declval<R&>()));                        // freestanding
>    >       template<input_range R>
>    >         using const_iterator_t = const_iterator<iterator_t<R>>;                         // freestanding
>    >       template<range R>
>    >         using const_sentinel_t = const_sentinel<sentinel_t<R>>;                         // freestanding
>    >       […]
>    >     }

[2022-09-25; Daniel and Hewill provide alternative wording]

The alternative solution solely adds the suggested `const_sentinel_t` alias template and doesn't touch the `const_iterator_t` constraints.

[2022-10-12; Reflector poll]

Set status to "Tentatively Ready" after seven votes in favour.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   >     #include <iterator>             // see 24.2 [iterator.synopsis]

   >     namespace std::ranges {
   >       […]
   >       template<class T>
   >         using iterator_t = decltype(ranges::begin(declval<T&>()));                      // freestanding
   >       template<range R>
   >         using sentinel_t = decltype(ranges::end(declval<R&>()));                        // freestanding
   >       template<range R>
   >         using const_iterator_t = const_iterator<iterator_t<R>>;                         // freestanding
   >       template<range R>
   >         using const_sentinel_t = const_sentinel<sentinel_t<R>>;                         // freestanding
   >       […]
   >     }

### 3771(i). [fund.ts.v3] remove binders typedefs from `function`

Section: 7.2.1 [fund.ts.v3::func.wrap.func.overview] Status: WP Submitter: Alisdair Meredith Opened: 2022-09-12 Last modified: 2026-02-21

Priority: Not Prioritized

View all issues with WP status.

Discussion:

Addresses: fund.ts.v3

The LFTSv3 bases its specification for `experimental::function` on `std::function` in the C++20 standard. However, the wording was largely copied over from LFTSv2 which based its wording on the C++14 standard.

For C++17, we removed the conditionally defined typedefs for the legacy binders API, but this removal was not reflected in the TS. We are now left with a specification referring to unknown types, `T1` and `T2`.

These typedefs should be excised to match the referenced standard.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after ten votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4920.

1. Modify the synopsis in 7.2.1 [fund.ts.v3::func.wrap.func.overview] as indicated:

   >     namespace std {
   >       namespace experimental::inline fundamentals_v3 {

   >         template<class> class function; // undefined

   >         template<class R, class... ArgTypes>
   >         class function<R(ArgTypes...)> {
   >         public:
   >           using result_type = R;
   >           using argument_type = T1;
   >           using first_argument_type T1;
   >           using second_argument_type = T2;

   >           using allocator_type = erased_type;

   >           // ...
   >       }
   >     }

### 3772(i). `repeat_view`'s piecewise constructor is missing Postconditions

Section: 25.6.5.2 [range.repeat.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-12 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.repeat.view].

View all issues with C++23 status.

Discussion:

The first two value-bound pair constructors of `repeat_view` have the Preconditions that the integer-like object `bound` must be non-negative. However, the piecewise constructor does not mention the valid values for `bound_args`. It would be nice to add a Postconditions that the initialized `bound_` must be greater than or equal to `0` here.

[2022-09-23; Reflector poll]

Set priority to 2 after reflector poll.

This is trying to state a requirement on users, but that's not what Postconditions: means. Should be something more like:
Precondition: If `Bound` is not `unreachable_sentinel_t`, the `bound_` ≥ 0 after its initialization from `bound_args`.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 25.6.5.2 [range.repeat.view] as shown:

>    >     template<class... WArgs, class... BoundArgs>
>    >       requires constructible_from<W, WArgs...> &&
>    >                constructible_from<Bound, BoundArgs...>
>    >     constexpr explicit repeat_view(piecewise_construct_t,
>    >       tuple<Wargs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{});

>    > > -5- Effects: Initializes `value_` with arguments of types `WArgs...` obtained by forwarding the elements of `value_args` and initializes `bound_` with arguments of types `BoundArgs...` obtained by forwarding the elements of `bound_args`. (Here, forwarding an element `x` of type `U` within a tuple object means calling `std::forward<U>(x)`.)

>    > > -?- Postconditions: If `Bound` is not `unreachable_sentinel_t`, `bound_` ≥ 0.

[2022-09-23; Jonathan provides improved wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 25.6.5.2 [range.repeat.view] as shown:

>    >     template<class... WArgs, class... BoundArgs>
>    >       requires constructible_from<W, WArgs...> &&
>    >                constructible_from<Bound, BoundArgs...>
>    >     constexpr explicit repeat_view(piecewise_construct_t,
>    >       tuple<Wargs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{});

>    > > -?- Preconditions: `Bound` is `unreachable_sentinel_t`, or the initialization of `bound_` yields a non-negative value.

>    > > -5- Effects: Initializes `value_` with arguments of types `WArgs...` obtained by forwarding the elements of `value_args` and initializes `bound_` with arguments of types `BoundArgs...` obtained by forwarding the elements of `bound_args`. (Here, forwarding an element `x` of type `U` within a tuple object means calling `std::forward<U>(x)`.)

[2023-01-11; Jonathan provides new wording requested by LWG]

[Issaquah 2023-02-07; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.6.5.2 [range.repeat.view] as shown:

   >     template<class... TArgs, class... BoundArgs>
   >       requires constructible_from<T, TArgs...> &&
   >                constructible_from<Bound, BoundArgs...>
   >     constexpr explicit repeat_view(piecewise_construct_t,
   >       tuple<Targs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{});

   > > -5- Effects: Initializes `value_` with arguments of types `TArgs...` obtained by forwarding the elements of `value_args` `make_from_tuple<T>(std::move(value_args))` and initializes `bound_` with arguments of types `BoundArgs...` obtained by forwarding the elements of `bound_args`. (Here, forwarding an element `x` of type `U` within a tuple object means calling `std::forward<U>(x)`.) `make_from_tuple<Bound>(std::move(bound_args))`. The behavior is undefined if `Bound` is not `unreachable_sentinel_t` and `bound_` is negative.

### 3773(i). `views::zip_transform` still requires `F` to be `copy_constructible` when empty pack

Section: 25.7.26.1 [range.zip.transform.overview] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-12 Last modified: 2024-01-29

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

After P2494R2, range adaptors only require callable to be `move_constructible`, however, for `views::zip_transform`, when empty pack, it still requires callable to be `copy_constructible`. We should relax this requirement, too.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.26.1 [range.zip.transform.overview] as indicated:

   > -2- The name `views::zip_transform` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Let `F` be a subexpression, and let `Es...` be a pack of subexpressions.

   > 1. (2.1) — If `Es` is an empty pack, let `FD` be `decay_t<decltype((F))>`.

   >    1. (2.1.1) — If `move_constructiblecopy_constructible<FD> && regular_invocable<FD&>` is `false`, or if `decay_t<invoke_result_t<FD&>>` is not an object type, `views::zip_transform(F, Es...)` is ill-formed.

   >    2. (2.1.2) — Otherwise, the expression `views::zip_transform(F, Es...)` is expression-equivalent to

   >                 ((void)F, auto(views::empty<decay_t<invoke_result_t<FD&>>>))

   > 2. (2.2) — Otherwise, the expression `views::zip_transform(F, Es...)` is expression-equivalent to `zip_transform_view(F, Es...)`.

### 3774(i). `<flat_set>` should include `<compare>`

Section: 23.6.10 [flat.set.syn] Status: C++23 Submitter: Jiang An Opened: 2022-09-12 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

This was originally editorial PR #5789 which is considered not-editorial.

`std::flat_set` and `std::flat_multiset` have `operator<=>` so `<compare>` should be included in the corresponding header, in the spirit of LWG 3330(i). `#include <compare>` is also missing in the adopted paper P1222R4.

[2022-09-23; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify the synopsis in 23.6.10 [flat.set.syn] as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]

### 3775(i). Broken dependencies in the Cpp17Allocator requirements

Section: 16.4.4.6.1 [allocator.requirements.general] Status: C++23 Submitter: Alisdair Meredith Opened: 2022-09-22 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [allocator.requirements.general].

View all other issues in [allocator.requirements.general].

View all issues with C++23 status.

Discussion:

This issue is extracted from P0177R2 as that paper stalled on the author's ability to update in time for C++17. While the issue was recorded and going to be resolved in the paper, we did not file an issue for the list when work on that paper stopped.

Many of the types and expressions in the Cpp17Allocator requirements are optional, and as such a default is provided that is exposed through `std::allocator_traits`. However, some types and operations are specified directly in terms of the allocator member, when really they should be specified allowing for reliance on the default, obtained through `std::allocator_traits`. For example, `X::pointer` is an optional type and not required to exist; `XX::pointer` is either `X::pointer` when it is present, or the default formula otherwise, and so is guaranteed to always exist, and the intended interface for user code. Observe that bullet list in p2, which acts as the key to the names in the Cpp17Allocator requirements, gets this right, unlike most of the text that follows.

This change corresponds to the known implementations, which meet the intended contract rather than that currently specified. For example, `std::allocator` does not provide any of the `pointer` related typedef members, so many of the default semantics indicated today would be ill-formed if implementations were not already implementing the fix.

An alternative resolution might be to add wording around p1-3 to state that if a name lookup fails then the default formula is used. However, it is simply clearer to write the constraints as intended, in the form of code that users can write, rather than hide behind a layer of indirect semantics that may be interpreted as requiring another layer of SFINAE metaprogramming.

[2022-10-12; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 16.4.4.6.1 [allocator.requirements.general] as indicated:

   >     typename X::pointer

   > > -4- Remarks: Default: `T*`

   >     typename X::const_pointer

   > > -5- Mandates: `XX::pointer` is convertible to `XX::const_pointer`.

   > > -6- Remarks: Default: `pointer_traits<XX::pointer>::rebind<const T>`

   >     typename X::void_pointer
   >     typename Y::void_pointer

   > > -7- Mandates: `XX::pointer` is convertible to `XX::void_pointer`. `XX::void_pointer` and `YY::void_pointer` are the same type.

   > > -8- Remarks: Default: `pointer_traits<XX::pointer>::rebind<void>`

   >     typename X::const_void_pointer
   >     typename Y::const_void_pointer

   > > -9- Mandates: `XX::pointer`, `XX::const_pointer`, and `XX::void_pointer` are convertible to `XX::const_void_pointer`. `XX::const_void_pointer` and `YY::const_void_pointer` are the same type.

   > > -10- Remarks: Default: `pointer_traits<XX::pointer>::rebind<const void>`

   >     typename X::value_type

   > > -11- Result: Identical to `T`.

   >     typename X::size_type

   > > -12- Result: An unsigned integer type that can represent the size of the largest object in the allocation model.

   > > -13- Remarks: Default: `make_unsigned_t<XX::difference_type>`

   >     typename X::difference_type

   > > -14- Result: A signed integer type that can represent the difference between any two pointers in the allocation model.

   > > -15- Remarks: Default: `pointer_traits<XX::pointer>::difference_type`

   >     typename X::template rebind<U>::other

   > > -16- Result: `Y`

   > > -17- Postconditions: For all `U` (including `T`), `YY::template rebindrebind_alloc<T>::other` is `X`.

   > > -18- Remarks: If `Allocator` is a class template instantiation of the form `SomeAllocator<T, Args>`, where `Args` is zero or more type arguments, and `Allocator` does not supply a `rebind` member template, the standard `allocator_traits` template uses `SomeAllocator<U, Args>` in place of `Allocator::rebind<U>::other` by default. For allocator types that are not template instantiations of the above form, no default is provided.

   > > -19- [Note 1: The member class template `rebind` of `X` is effectively a typedef template. In general, if the name `Allocator` is bound to `SomeAllocator<T>`, then `Allocator::rebind<U>::other` is the same type as `SomeAllocator<U>`, where `SomeAllocator<T>::value_type` is `T` and `SomeAllocator<U>::value_type` is `U`. — end note]

   > […]

   >     static_cast<XX::pointer>(w)

   > > -29- Result: `XX::pointer`

   > > -30- Postconditions: `static_cast<XX::pointer>(w) == p`.

   >     static_cast<XX::const_pointer>(x)

   > > -31- Result: `XX::const_pointer`

   > > -32- Postconditions: `static_cast<XX::const_pointer>(x) == q`.

   >     pointer_traits<XX::pointer>::pointer_to(r)

   > > -33- Result: `XX::pointer`

   > > -34- Postconditions: Same as `p`.

   >     a.allocate(n)

   > > -35- Result: `XX::pointer`

   > > […]

   >     a.allocate(n, y)

   > > -40- Result: `XX::pointer`

   > > […]

   >     a.allocate_at_least(n)

   > > -43- Result: `allocation_result<XX::pointer>`

   > > -44- Returns: `allocation_result<XX::pointer>{ptr, count}` where `ptr` is memory allocated for an array of count `T` and such an object is created but array elements are not constructed, such that `count = n`. If `n == 0`, the return value is unspecified.

   > > […]

   > […]

   >     a.max_size()

   > > -50- Result: `XX::size_type`

   > > -51- Returns: The largest value `n` that can meaningfully be passed to `X::a.allocate(n)`.

   > > -52- Remarks: Default: `numeric_limits<size_type>::max() / sizeof(value_type)`

   > […]

   >     a == b

   > > -59- Result: `bool`

   > > -60- Returns: `a == YY::rebind_alloc<T>::other(b)`.

   > […]

   > -92- An allocator type `X` shall meet the Cpp17CopyConstructible requirements (Table 33). The `XX::pointer`, `XX::const_pointer`, `XX::void_pointer`, and `XX::const_void_pointer` types shall meet the Cpp17NullablePointer requirements (Table 37). No constructor, comparison operator function, copy operation, move operation, or swap operation on these pointer types shall exit via an exception. `XX::pointer` and `XX::const_pointer` shall also meet the requirements for a Cpp17RandomAccessIterator (25.3.5.7) and the additional requirement that, when `ap` and `(ap + n)` are dereferenceable pointer values for some integral value `n`, `addressof(*(ap + n)) == addressof(*ap) + n` is `true`.

   > -93- Let `x1` and `x2` denote objects of (possibly different) types `XX::void_pointer`, `XX::const_void_pointer`, `XX::pointer`, or `XX::const_pointer`. Then, `x1` and `x2` are equivalently-valued pointer values, if and only if both `x1` and `x2` can be explicitly converted to the two corresponding objects `px1` and `px2` of type `XX::const_pointer`, using a sequence of `static_cast`s using only these four types, and the expression `px1 == px2` evaluates to `true`.

   > -94- Let `w1` and `w2` denote objects of type `XX::void_pointer`. Then for the expressions

   > >     w1 == w2
   > >     w1 != w2

   > either or both objects may be replaced by an equivalently-valued object of type `XX::const_void_pointer` with no change in semantics.

   > -95- Let `p1` and `p2` denote objects of type `XX::pointer`. Then for the expressions

   > >     p1 == p2
   > >     p1 != p2
   > >     p1 < p2
   > >     p1 <= p2
   > >     p1 >= p2
   > >     p1 > p2
   > >     p1 - p2

   > either or both objects may be replaced by an equivalently-valued object of type `XX::const_pointer` with no change in semantics.

### 3777(i). Common `cartesian_product_view` produces an invalid range if the first range is input and one of the ranges is empty

Section: 25.7.33.2 [range.cartesian.view] Status: C++26 Submitter: Tomasz Kamiński Opened: 2022-09-12 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

In case when `cartesian_product_view` is common and one of the inner ranges is empty, it needs to produce equal iterators from `begin`/`end`. We currently create a sequence of `begin` iterators as both `begin` and `end` iterators. This assumes that `begin` iterator is copyable, which may not be the case with the input range, even in the case if that range is common — in such case, we require that only sentinel is semantically copy-constructible, not begin even if they are the same type.

To illustrate, C++98 input iterators (like `directory_iterator`) are syntactically copy-constructible, but only default constructed object, that corresponds to sentinels are semantically copyable — the copy produces an equivalent result. As a consequence for `directory_iterator d`, and empty `std::string_view sv`, the `view::cartesian_product(d, sv)` produces an invalid range.

To fix the problem, we need to move the logic of adjusting the first range iterator to return `[end, begin, ..., begin]` for `begin`. This is safe, as we require the end to be always semantically copy-constructible. This again can be done only if computing the end can be done in `𝒪(1)` i.e. the first range is common.

[2022-09-28; Reflector poll]

Set priority to 2 after reflector poll.

[2022-09-28; LWG telecon]

Discussed issue. Tim suggested to add a new semantic requirement to `sentinel_for` that when `S` and `I` are the same type then `i == i` is true for any non-singular `i` of type `I`.

Previous resolution [SUPERSEDED]

This wording is relative to N4910.

1. Modify 25.7.33.2 [range.cartesian.view] as indicated:

   > [Drafting note: We can optimize the comparison with `default_sentinel_t` to compare only the iterator to the first range if the range is common. This is observable, as we call comparison of user-provided iterators.]

   >     constexpr iterator<false> begin()
   >       requires (!simple-view<First> || ... || !simple-view<Vs>);

   > > -2- Effects: Equivalent to: `return iterator<false>(tuple-transform(ranges::begin, bases_));`

   >     constexpr iterator<true> begin() const
   >       requires (range<const First> && ... && range<const Vs>);

   > > -3- Effects: Equivalent to: `return iterator<true>(tuple-transform(ranges::begin, bases_));`

   >     constexpr iterator<false> end()
   >       requires ((!simple-view<First> || ... || !simple-view<Vs>)
   >         && cartesian-product-is-common<First, Vs...>);
   >     constexpr iterator<true> end() const
   >       requires cartesian-product-is-common<const First, const Vs...>;

   > > -4- Let:

   > > 1. (4.1) — `is-const` be `true` for the const-qualified overloads, and `false` otherwise;

   > > 2. (4.?) — `is-end` be `true` for the `end` overloads, and `false` otherwise;

   > > 3. (4.2) — `is-empty` be `true` if the expression `ranges::empty(rng)` is `true` for any `rng` among the underlying ranges except the first one and `false` otherwise; and

   > > 4. (4.3) — `begin-or-first-end(rng)` be expression-equivalent to `is-end || is-empty ? cartesian-common-arg-end(rng) : ranges::begin(rng)is-empty ? ranges::begin(rng) : cartesian-common-arg-end(rng)` if `cartesian-product-common-arg<maybe-const<is-const, First>>` is `true` and `rng` is the first underlying range, and `ranges::begin(rng)` otherwise.

   > > -5- Effects: Equivalent to:

   > > >     iterator<is-const> it(tuple-transform(
   > > >       [](auto& rng){ return begin-or-first-end(rng); }, bases_));
   > > >     return it;

2. Modify 25.7.33.3 [range.cartesian.iterator] as indicated:

   >     friend constexpr bool operator==(const iterator& x, default_sentinel_t);

   > > -26- Returns:

   > > 1. (?.1) — If `cartesian-product-common-arg<maybe-const<Const, First>>` is `true`, returns `std::get<0>(x.current_) == ranges::end(std::get<0>(x.parent_->bases_))`.

   > > 2. (?.2) — Otherwise, `true` if `std::get<i>(x.current_) == ranges::end(std::get<i>(x.parent_->bases_))` is `true` for any integer `0 ≤ i ≤ sizeof...(Vs)`,; otherwise, `false` returns `true`.

   > > 3. (?.3) — Otherwise, returns `false`.

[2026-03-27; Tim provides new wording]

[Croydon 2026-03-27; Status changed: Open → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Edit 24.3.4.7 [iterator.concept.sentinel], as indicated:

   >     template<class S, class I>
   >       concept sentinel_for =
   >         semiregular<S> &&
   >         input_or_output_iterator<I> &&
   >         weakly-equality-comparable-with<S, I>;      // see [concept.equalitycomparable]

   > > -2- Let `s` and `i` be values of type `S` and `I` such that `[i, s)` denotes a range. Types `S` and `I` model `sentinel_for<S, I>` only if:

   > > 1. (2.1) — `i == s` is well-defined.

   > > 2. (2.2) — If `bool(i != s)` then `i` is dereferenceable and `[++i, s)` denotes a range.

   > > 3. (2.3) — `assignable_from<I&, S>` is either modeled or not satisfied.

   > > 4. (2.4) — If `I` and `S` are the same type, then `i == i` is `true`.

### 3778(i). `vector<bool>` missing exception specifications

Section: 23.3.14.1 [vector.bool.pspc] Status: C++23 Submitter: Alisdair Meredith Opened: 2022-09-13 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [vector.bool.pspc].

View all issues with C++23 status.

Discussion:

As noted back in P0177R2, the primary template for `vector` has picked up some exception specification, but the partial specialization for `vector<bool>` has not been so updated.

Several other changes have been made to `vector` in the intervening years, but these particular exception specifications have still not been updated. Note that the free-function `swap` in the header synopsis will automatically pick up this update once applied.

[2022-09-28; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 23.3.14.1 [vector.bool.pspc], partial class template `vector<bool, Allocator>` synopsis, as indicated:

   >     namespace std {
   >       template<class Allocator>
   >       class vector<bool, Allocator> {
   >       public:
   >         […]
   >         // construct/copy/destroy
   >         constexpr vector() noexcept(noexcept(Allocator())) : vector(Allocator()) { }
   >         constexpr explicit vector(const Allocator&) noexcept;
   >         constexpr explicit vector(size_type n, const Allocator& = Allocator());
   >         constexpr vector(size_type n, const bool& value, const Allocator& = Allocator());
   >         template<class InputIterator>
   >           constexpr vector(InputIterator first, InputIterator last, const Allocator& = Allocator());
   >         template<container-compatible-range <bool> R>
   >           constexpr vector(from_range_t, R&& rg, const Allocator& = Allocator());
   >         constexpr vector(const vector& x);
   >         constexpr vector(vector&& x) noexcept;
   >         constexpr vector(const vector&, const type_identity_t<Allocator>&);
   >         constexpr vector(vector&&, const type_identity_t<Allocator>&);
   >         constexpr vector(initializer_list<bool>, const Allocator& = Allocator());
   >         constexpr ~vector();
   >         constexpr vector& operator=(const vector& x);
   >         constexpr vector& operator=(vector&& x)
   >           noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                    allocator_traits<Allocator>::is_always_equal::value);
   >         constexpr vector& operator=(initializer_list<bool>);
   >         template<class InputIterator>
   >           constexpr void assign(InputIterator first, InputIterator last);
   >         template<container-compatible-range <bool> R>
   >           constexpr void assign_range(R&& rg);
   >         constexpr void assign(size_type n, const bool& t);
   >         constexpr void assign(initializer_list<bool>);
   >         constexpr allocator_type get_allocator() const noexcept;

   >         […]
   >         constexpr iterator erase(const_iterator position);
   >         constexpr iterator erase(const_iterator first, const_iterator last);
   >         constexpr void swap(vector&)
   >           noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
   >                    allocator_traits<Allocator>::is_always_equal::value);
   >         constexpr static void swap(reference x, reference y) noexcept;
   >         constexpr void flip() noexcept; // flips all bits
   >         constexpr void clear() noexcept;
   >       };
   >     }

### 3780(i). `format`'s width estimation is too approximate and not forward compatible

Section: 28.5.2.2 [format.string.std] Status: Resolved Submitter: Corentin Jabot Opened: 2022-09-15 Last modified: 2023-03-23

Priority: 3

View other active issues in [format.string.std].

View all other issues in [format.string.std].

View all issues with Resolved status.

Discussion:

For the purpose of width estimation, `format` considers ranges of codepoints initially derived from an implementation of `wcwidth` with modifications (see P1868R1).

This however present a number of challenges:

> - From a reading of the spec, it is not clear how these ranges were selected.

> - Poor forward compatibility with future Unicode versions. The list will become less and less meaningful overtime or require active maintenance at each Unicode release (which we have not done for Unicode 14 already).

> - Some of these codepoints are unassigned or otherwise reserved, which is another forward compatibility concern.

Instead, we propose to

> - Rely on UAX-11 for most of the codepoints)

> - Grand-father specific and fully assigned, blocks of codepoints to support additional pictograms per the original intent of the paper and existing practices. We add the name of these blocks in the wording for clarity.

Note that per UAX-11

> - Most emojis are considered `East_Asian_Width="W"`

> - By design, `East_Asian_Width="W"` includes specific unassigned ranges, which should always be treated as Wide.

This change:

> - Considers 8477 extra codepoints as having a width 2 (as of Unicode 15) (mostly Tangut Ideographs)

> - Change the width of 85 unassigned code points from 2 to 1

> - Change the width of 8 codepoints (in the range `U+3248 CIRCLED NUMBER TEN ON BLACK SQUARE` … `U+324F CIRCLED NUMBER EIGHTY ON BLACK SQUARE`) from 2 to 1, because it seems questionable to make an exception for those without input from Unicode

For the following code points, the estimated width used to be 1, and is 2 after the suggested change:

- U+231A WATCH .. U+231B HOURGLASS

- U+23E9 BLACK RIGHT-POINTING DOUBLE TRIANGLE .. U+23EC BLACK DOWN-POINTING DOUBLE TRIANGLE

- U+23F0 ALARM CLOCK

- U+23F3 HOURGLASS WITH FLOWING SAND

- U+25FD WHITE MEDIUM SMALL SQUARE .. U+25FE BLACK MEDIUM SMALL SQUARE

- U+2614 UMBRELLA WITH RAIN DROPS .. U+2615 HOT BEVERAGE

- U+2648 ARIES .. U+2653 PISCES

- U+267F WHEELCHAIR SYMBOL

- U+2693 ANCHOR

- U+26A1 HIGH VOLTAGE SIGN

- U+26AA MEDIUM WHITE CIRCLE .. U+26AB MEDIUM BLACK CIRCLE

- U+26BD SOCCER BALL .. U+26BE BASEBALL

- U+26C4 SNOWMAN WITHOUT SNOW .. U+26C5 SUN BEHIND CLOUD

- U+26CE OPHIUCHUS

- U+26D4 NO ENTRY

- U+26EA CHURCH

- U+26F2 FOUNTAIN .. U+26F3 FLAG IN HOLE

- U+26F5 SAILBOAT

- U+26FA TENT

- U+26FD FUEL PUMP

- U+2705 WHITE HEAVY CHECK MARK

- U+270A RAISED FIST .. U+270B RAISED HAND

- U+2728 SPARKLES

- U+274C CROSS MARK

- U+274E NEGATIVE SQUARED CROSS MARK

- U+2753 BLACK QUESTION MARK ORNAMENT .. U+2755 WHITE EXCLAMATION MARK ORNAMENT

- U+2757 HEAVY EXCLAMATION MARK SYMBOL

- U+2795 HEAVY PLUS SIGN .. U+2797 HEAVY DIVISION SIGN

- U+27B0 CURLY LOOP

- U+27BF DOUBLE CURLY LOOP

- U+2B1B BLACK LARGE SQUARE .. U+2B1C WHITE LARGE SQUARE

- U+2B50 WHITE MEDIUM STAR

- U+2B55 HEAVY LARGE CIRCLE

- U+A960 HANGUL CHOSEONG TIKEUT-MIEUM .. U+A97C HANGUL CHOSEONG SSANGYEORINHIEUH

- U+16FE0 TANGUT ITERATION MARK .. U+16FE4 KHITAN SMALL SCRIPT FILLER

- U+16FF0 VIETNAMESE ALTERNATE READING MARK CA .. U+16FF1 VIETNAMESE ALTERNATE READING MARK NHAY

- U+17000 TANGUT IDEOGRAPH-# .. U+187F7 TANGUT IDEOGRAPH-#

- U+18800 TANGUT COMPONENT-001 .. U+18CD5 KHITAN SMALL SCRIPT CHARACTER-#

- U+18D00 TANGUT IDEOGRAPH-# .. U+18D08 TANGUT IDEOGRAPH-#

- U+1AFF0 KATAKANA LETTER MINNAN TONE-2 .. U+1AFF3 KATAKANA LETTER MINNAN TONE-5

- U+1AFF5 KATAKANA LETTER MINNAN TONE-7 .. U+1AFFB KATAKANA LETTER MINNAN NASALIZED TONE-5

- U+1AFFD KATAKANA LETTER MINNAN NASALIZED TONE-7 .. U+1AFFE KATAKANA LETTER MINNAN NASALIZED TONE-8

- U+1B000 KATAKANA LETTER ARCHAIC E .. U+1B122 KATAKANA LETTER ARCHAIC WU

- U+1B132 HIRAGANA LETTER SMALL KO

- U+1B150 HIRAGANA LETTER SMALL WI .. U+1B152 HIRAGANA LETTER SMALL WO

- U+1B155 KATAKANA LETTER SMALL KO

- U+1B164 KATAKANA LETTER SMALL WI .. U+1B167 KATAKANA LETTER SMALL N

- U+1B170 NUSHU CHARACTER-# .. U+1B2FB NUSHU CHARACTER-#

- U+1F004 MAHJONG TILE RED DRAGON

- U+1F0CF PLAYING CARD BLACK JOKER

- U+1F18E NEGATIVE SQUARED AB

- U+1F191 SQUARED CL .. U+1F19A SQUARED VS

- U+1F200 SQUARE HIRAGANA HOKA .. U+1F202 SQUARED KATAKANA SA

- U+1F210 SQUARED CJK UNIFIED IDEOGRAPH-624B .. U+1F23B SQUARED CJK UNIFIED IDEOGRAPH-914D

- U+1F240 TORTOISE SHELL BRACKETED CJK UNIFIED IDEOGRAPH-672C .. U+1F248 TORTOISE SHELL BRACKETED CJK UNIFIED IDEOGRAPH-6557

- U+1F250 CIRCLED IDEOGRAPH ADVANTAGE .. U+1F251 CIRCLED IDEOGRAPH ACCEPT

- U+1F260 ROUNDED SYMBOL FOR FU .. U+1F265 ROUNDED SYMBOL FOR CAI

- U+1F680 ROCKET .. U+1F6C5 LEFT LUGGAGE

- U+1F6CC SLEEPING ACCOMMODATION

- U+1F6D0 PLACE OF WORSHIP .. U+1F6D2 SHOPPING TROLLEY

- U+1F6D5 HINDU TEMPLE .. U+1F6D7 ELEVATOR

- U+1F6DC WIRELESS .. U+1F6DF RING BUOY

- U+1F6EB AIRPLANE DEPARTURE .. U+1F6EC AIRPLANE ARRIVING

- U+1F6F4 SCOOTER .. U+1F6FC ROLLER SKATE

- U+1F7E0 LARGE ORANGE CIRCLE .. U+1F7EB LARGE BROWN SQUARE

- U+1F7F0 HEAVY EQUALS SIGN

- U+1FA70 BALLET SHOES .. U+1FA7C CRUTCH

- U+1FA80 YO-YO .. U+1FA88 FLUTE

- U+1FA90 RINGED PLANET .. U+1FABD WING

- U+1FABF GOOSE .. U+1FAC5 PERSON WITH CROWN

- U+1FACE MOOSE .. U+1FADB PEA POD

- U+1FAE0 MELTING FACE .. U+1FAE8 SHAKING FACE

- U+1FAF0 HAND WITH INDEX FINGER AND THUMB CROSSED .. U+1FAF8 RIGHTWARDS PUSHING HAND

For the following code points, the estimated width used to be 2, and is 1 after the suggested change:

- U+2E9A

- U+2EF4 .. U+2EFF

- U+2FD6 .. U+2FEF

- U+2FFC .. U+2FFF

- U+3040

- U+3097 .. U+3098

- U+3100 .. U+3104

- U+3130

- U+318F

- U+31E4 .. U+31EF

- U+321F

- U+A48D .. U+A48F

- U+A4C7 .. U+A4CF

- U+FE53

- U+FE67

- U+FE6C .. U+FE6F

- U+FF00

- U+3248 CIRCLED NUMBER TEN ON BLACK SQUARE .. U+324F CIRCLED NUMBER EIGHTY ON BLACK SQUARE

[2022-10-12; Reflector poll]

Set priority to 3 after reflector poll. Send to SG16.

Previous resolution [SUPERSEDED]

This wording is relative to N4917.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > -12- For a string in a Unicode encoding, implementations should estimate the width of a string as the sum of estimated widths of the first code points in its extended grapheme clusters. The extended grapheme clusters of a string are defined by UAX #29. The estimated width of the following code points is 2:

   > 1. (12.1) — U+1100 – U+115F

   > 2. (12.2) — U+2329 – U+232A

   > 3. (12.3) — U+2E80 – U+303E

   > 4. (12.4) — U+3040 – U+A4CF

   > 5. (12.5) — U+AC00 – U+D7A3

   > 6. (12.6) — U+F900 – U+FAFF

   > 7. (12.7) — U+FE10 – U+FE19

   > 8. (12.8) — U+FE30 – U+FE6F

   > 9. (12.9) — U+FF00 – U+FF60

   > 10. (12.10) — U+FFE0 – U+FFE6

   > 11. (12.11) — U+1F300 – U+1F64F

   > 12. (12.12) — U+1F900 – U+1F9FF

   > 13. (12.13) — U+20000 – U+2FFFD

   > 14. (12.14) — U+30000 – U+3FFFD

   > 15. (?.1) — Any code point with the `East_Asian_Width="W"` or `East_Asian_Width="F"` Derived Extracted Property as described by UAX #44

   > 16. (?.2) — U+4DC0 – U+4DFF (Yijing Hexagram Symbols)

   > 17. (?.3) — U+1F300 – U+1F5FF (Miscellaneous Symbols and Pictographs)

   > 18. (?.4) — U+1F900 – U+1F9FF (Supplemental Symbols and Pictographs)

   > The estimated width of other code points is 1.

[2023-03-22 Resolved by the adoption of P2675R1 in Issaquah. Status changed: SG16 → Resolved.]

Proposed resolution:

### 3781(i). The exposition-only alias templates `cont-key-type` and `cont-mapped-type` should be removed

Section: 23.6.1 [container.adaptors.general] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-16 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

P0429R9 removed the `flat_map`'s single-range argument constructor and uses C++23-compatible `from_range_t` version constructor with alias templates `range-key-type` and `range-mapped-type` to simplify the deduction guide.

This makes `cont-key-type` and `cont-mapped-type` no longer useful, we should remove them.

[2022-10-12; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 23.6.1 [container.adaptors.general] as indicated:

   > -7- The exposition-only alias template `iter-value-type` defined in 23.3.1 [sequences.general] and the exposition-only alias templates `iter-key-type` and `iter-mapped-type` defined in 23.4.1 [associative.general] may appear in deduction guides for container adaptors.

   > -8- The following exposition-only alias templates may appear in deduction guides for container adaptors:

   > >     template<class Container>
   > >       using cont-key-type =                                // exposition only
   > >         remove_const_t<typename Container::value_type::first_type>;
   > >     template<class Container>
   > >       using cont-mapped-type =                             // exposition only
   > >         typename Container::value_type::second_type;

### 3782(i). Should `<math.h>` declare `::lerp`?

Section: 17.15.7 [support.c.headers.other] Status: C++23 Submitter: Jiang An Opened: 2022-09-17 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [support.c.headers.other].

View all other issues in [support.c.headers.other].

View all issues with C++23 status.

Discussion:

According to 17.15.7 [support.c.headers.other]/1, `<math.h>` is required to provide `::lerp`, despite that it is a C++-only component. In P0811R3, neither `<math.h>` nor C compatibility is mentioned, so it seems unintended not to list `lerp` as an exception in 17.15.7 [support.c.headers.other].

Currently there is implementation divergence: libstdc++ provide `::lerp` in `<math.h>` but not in `<cmath>`, while MSVC STL and libc++ don't provide `::lerp` in `<math.h>` or `<cmath>` at all.

I'm not sure whether this should be considered together with LWG 3484(i). Since `nullptr_t` has become a part of the C standard library as of C23 (see WG14-N3042 and WG14-N3048), I believe we should keep providing `::nullptr_t` in `<stddef.h>`.

[2022-10-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 17.15.7 [support.c.headers.other] as indicated:

   > -1- Every C header other than `<complex.h>` (17.15.2 [complex.h.syn]), `<iso646.h>` (17.15.3 [iso646.h.syn]), `<stdalign.h>` (17.15.4 [stdalign.h.syn]), `<stdatomic.h>` (32.5.12 [stdatomic.h.syn]), `<stdbool.h>` (17.15.5 [stdbool.h.syn]), and `<tgmath.h>` (17.15.6 [tgmath.h.syn]), each of which has a name of the form `<name.h>`, behaves as if each name placed in the standard library namespace by the corresponding `<cname>` header is placed within the global namespace scope, except for the functions described in 29.7.6 [sf.cmath], the `std::lerp` function overloads (29.7.4 [c.math.lerp]), the declaration of `std::byte` (17.2.1 [cstddef.syn]), and the functions and function templates described in 17.2.5 [support.types.byteops]. […]

### 3784(i). `std.compat` should not provide `::byte` and its friends

Section: 16.4.2.4 [std.modules] Status: C++23 Submitter: Jiang An Opened: 2022-09-19 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

Currently 16.4.2.4 [std.modules]/3 seemly requires that the `std.compat` module has to provide `byte` (via `<cstddef>`), `beta` (via `<cmath>`) etc. in the global namespace, which is defective to me as these components are C++-only, and doing so would increase the risk of conflict.

I think we should only let `std.compat` provide the same set of global declarations as `<xxx.h>` headers.

[2022-10-12; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 16.4.2.4 [std.modules] as indicated:

   > -3- The named module `std.compat` exports the same declarations as the named module `std`, and additionally exports declarations in the global namespace corresponding to the declarations in namespace `std` that are provided by the C++ headers for C library facilities (Table 26), except the explicitly excluded declarations described in 17.15.7 [support.c.headers.other].

### 3785(i). `ranges::to` is over-constrained on the destination type being a range

Section: 25.5.7.2 [range.utility.conv.to] Status: C++23 Submitter: Barry Revzin Opened: 2022-09-19 Last modified: 2024-01-29

Priority: Not Prioritized

View other active issues in [range.utility.conv.to].

View all other issues in [range.utility.conv.to].

View all issues with C++23 status.

Discussion:

The current wording in 25.5.7.2 [range.utility.conv.to] starts:

> If `convertible_to<range_reference_t<R>, range_value_t<C>>` is `true`

and then tries to do `C(r, args...)` and then `C(from_range, r, args...)`. The problem is that `C` might not be a range — indeed we explicitly removed that requirement from an earlier revision of the paper — which makes this check ill-formed. One example use-case is using `ranges::to` to convert a range of `expected<T, E>` into a `expected<vector<T>, E>` — `expected` isn't any kind of range, but it could support this operation, which is quite useful. Unfortunately, the wording explicitly rejects that. This change happened between R6 and R7 of the paper and seems to have unintentionally rejected this use-case.

[2022-09-28; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll. During telecon review we agreed that supporting non-ranges was an intended part of the original design, but was inadvertently broken when adding `range_value_t` for other reasons.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   > [Drafting note: We need to be careful that this short-circuits, since if `C` does not satisfy `input_range`, then `range_value_t<C>` will be ill-formed.]

   >     template<class C, input_range R, class... Args> requires (!view<C>)
   >       constexpr C to(R&& r, Args&&... args);

   > > -1- Returns: An object of type `C` constructed from the elements of `r` in the following manner:

   > > 1. (1.1) — If `C` does not satisfy `input_range` or `convertible_to<range_reference_t<R>, range_value_t<C>>` is `true`:

   > >    1. (1.1.1) — If `constructible_from<C, R, Args...>` is `true`:

   > >       > `C(std::forward<R>(r), std::forward<Args>(args)...)`

   > >    2. (1.1.2) — Otherwise, if `constructible_from<C, from_range_t, R, Args...>` is `true`:

   > >       > `C(from_range, std::forward<R>(r), std::forward<Args>(args)...)`

   > >    3. (1.1.3) — Otherwise, if

   > >       1. (1.1.3.1) — `common_range<R>` is `true`,

   > >       2. (1.1.3.2) — `cpp17-input-iterator<iterator_t<R>>` is `true`, and

   > >       3. (1.1.3.3) — `constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...>` is `true`:

   > >          > `C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)`

   > >    4. (1.1.4) — Otherwise, if

   > >       1. (1.1.4.1) — `constructible_from<C, Args...>` is `true`, and

   > >       2. (1.1.4.2) — `container-insertable<C, range_reference_t<R>>` is `true`:

   > >          >     C c(std::forward<Args>(args)...);
   > >          >     if constexpr (sized_range<R> && reservable-container<C>)
   > >          >       c.reserve(ranges::size(r));
   > >          >     ranges::copy(r, container-inserter<range_reference_t<R>>(c));

   > > 2. (1.2) — Otherwise, if `input_range<range_reference_t<R>>` is `true`:

   > >    >     to<C>(r | views::transform([](auto&& elem) {
   > >    >       return to<range_value_t<C>>(std::forward<decltype(elem)>(elem));
   > >    >     }), std::forward<Args>(args)...);

   > > 3. (1.3) — Otherwise, the program is ill-formed.

### 3786(i). Flat maps' deduction guide needs to default `Allocator` to be useful

Section: 23.6.8.2 [flat.map.defn], 23.6.9.2 [flat.multimap.defn] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2022-09-25 Last modified: 2023-11-22

Priority: 2

View all other issues in [flat.map.defn].

View all issues with C++23 status.

Discussion:

This originated from the editorial issue #5800.

P0429R9 added some deduction guides with a non-defaulted `Allocator` template parameter and a corresponding function parameter that is defaulted. Since the template parameter `Allocator` is not defaulted, these deduction guides are never used.

[2022-09-28; LWG telecon]

We should not just ignore the allocator, it should be rebound and used for the two container types.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 23.6.8.2 [flat.map.defn], class template `flat_map` synopsis, as indicated:

>    >     […]
>    >     template<ranges::input_range R, class Compare = less<range-key-type<R>>,
>    >              class Allocator = allocator<void>>
>    >       flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
>    >         -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare>;
>    >     […]

> 2. Modify 23.6.9.2 [flat.multimap.defn], class template `flat_multimap` synopsis, as indicated:

>    >     […]
>    >     template<ranges::input_range R, class Compare = less<range-key-type<R>>,
>    >              class Allocator = allocator<void>>
>    >       flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
>    >         -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare>;
>    >     […]

[2022-10-19; Jonathan provides improved wording]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 23.6.1 [container.adaptors.general] as indicated:

>    > -8- The following exposition-only alias templates may appear in deduction guides for container adaptors:

>    > >     template<class Container>
>    > >       using cont-key-type =                                // exposition only
>    > >         remove_const_t<typename Container::value_type::first_type>;
>    > >     template<class Container>
>    > >       using cont-mapped-type =                             // exposition only
>    > >         typename Container::value_type::second_type;
>    > >     template<class Allocator, class T>
>    > >       using alloc-rebind =                             // exposition only
>    > >         typename allocator_traits<Allocator>::template rebind_alloc<T>;

> 2. Modify 23.6.8.2 [flat.map.defn], class template `flat_map` synopsis, as indicated:

>    >     […]
>    >     template<ranges::input_range R, class Compare = less<range-key-type<R>>,
>    >              class Allocator = allocator<void>>
>    >       flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
>    >         -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare,
>    >              vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>,
>    >              vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>;

>    >     template<ranges::input_range R, class Allocator>
>    >       flat_map(from_range_t, R&&, Allocator)
>    >         -> flat_map<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>,
>    >              vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>,
>    >              vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>;
>    >     […]

> 3. Modify 23.6.9.2 [flat.multimap.defn], class template `flat_multimap` synopsis, as indicated:

>    >     […]
>    >     template<ranges::input_range R, class Compare = less<range-key-type<R>>,
>    >              class Allocator = allocator<void>>
>    >       flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
>    >         -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare,
>    >              vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>,
>    >              vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>;

>    >     template<ranges::input_range R, class Allocator>
>    >       flat_multimap(from_range_t, R&&, Allocator)
>    >         -> flat_multimap<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>,
>    >              vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>,
>    >              vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>;
>    >     […]

> 4. Modify 23.6.11.2 [flat.set.defn], class template `flat_set` synopsis, as indicated:

>    >     […]
>    >     template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>,
>    >              class Allocator = allocator<ranges::range_value_t<R>>>
>    >       flat_set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
>    >         -> flat_set<ranges::range_value_t<R>, Compare,
>    >              vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;

>    >     template<ranges::input_range R, class Allocator>
>    >       flat_set(from_range_t, R&&, Allocator)
>    >         -> flat_set<ranges::range_value_t<R>, less<ranges::range_value_t<R>>,
>    >              vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;
>    >     […]

> 5. Modify 23.6.12.2 [flat.multiset.defn], class template `flat_multiset` synopsis, as indicated:

>    >     […]
>    >     template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>,
>    >              class Allocator = allocator<ranges::range_value_t<R>>>
>    >       flat_multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
>    >         -> flat_multiset<ranges::range_value_t<R>, Compare,
>    >              vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;

>    >     template<ranges::input_range R, class Allocator>
>    >       flat_multiset(from_range_t, R&&, Allocator)
>    >         -> flat_multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>,
>    >              vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;
>    >     […]

[2023-01-11; Jonathan Wakely provides improved wording]

During LWG telecon Tim pointed out that because `allocator<void>` does not meet the Cpp17Allocator requirements, it might not "qualify as an allocator" and so would cause the deduction guides to not participate in overload resolution, as per 23.6.1 [container.adaptors.general] p6 (6.4). Use `allocator<byte>` instead.

[Issaquah 2023-02-07; LWG]

Edited proposed resolution to fix missing `>` in guides for maps. Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 23.6.1 [container.adaptors.general] as indicated:

   > -8- The following exposition-only alias template may appear in deduction guides for container adaptors:

   > >     template<class Allocator, class T>
   > >       using alloc-rebind =                                 // exposition only
   > >         typename allocator_traits<Allocator>::template rebind_alloc<T>;

2. Modify 23.6.8.2 [flat.map.defn], class template `flat_map` synopsis, as indicated:

   >     […]
   >     template<ranges::input_range R, class Compare = less<range-key-type<R>>,
   >              class Allocator = allocator<byte>>
   >       flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
   >         -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare,
   >              vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>,
   >              vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>;

   >     template<ranges::input_range R, class Allocator>
   >       flat_map(from_range_t, R&&, Allocator)
   >         -> flat_map<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>,
   >              vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>,
   >              vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>;
   >     […]

3. Modify 23.6.9.2 [flat.multimap.defn], class template `flat_multimap` synopsis, as indicated:

   >     […]
   >     template<ranges::input_range R, class Compare = less<range-key-type<R>>,
   >              class Allocator = allocator<byte>>
   >       flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
   >         -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare,
   >              vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>,
   >              vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>;

   >     template<ranges::input_range R, class Allocator>
   >       flat_multimap(from_range_t, R&&, Allocator)
   >         -> flat_multimap<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>,
   >              vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>,
   >              vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>;
   >     […]

4. Modify 23.6.11.2 [flat.set.defn], class template `flat_set` synopsis, as indicated:

   >     […]
   >     template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>,
   >              class Allocator = allocator<ranges::range_value_t<R>>>
   >       flat_set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
   >         -> flat_set<ranges::range_value_t<R>, Compare,
   >              vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;

   >     template<ranges::input_range R, class Allocator>
   >       flat_set(from_range_t, R&&, Allocator)
   >         -> flat_set<ranges::range_value_t<R>, less<ranges::range_value_t<R>>,
   >              vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;
   >     […]

5. Modify 23.6.12.2 [flat.multiset.defn], class template `flat_multiset` synopsis, as indicated:

   >     […]
   >     template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>,
   >              class Allocator = allocator<ranges::range_value_t<R>>>
   >       flat_multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
   >         -> flat_multiset<ranges::range_value_t<R>, Compare,
   >              vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;

   >     template<ranges::input_range R, class Allocator>
   >       flat_multiset(from_range_t, R&&, Allocator)
   >         -> flat_multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>,
   >              vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;
   >     […]

### 3787(i). `ranges::to`'s template parameter `C` should not be a reference type

Section: 25.5.7.2 [range.utility.conv.to], 25.5.7.3 [range.utility.conv.adaptors] Status: Resolved Submitter: Hewill Kang Opened: 2022-09-25 Last modified: 2024-01-29

Priority: 3

View other active issues in [range.utility.conv.to].

View all other issues in [range.utility.conv.to].

View all issues with Resolved status.

Discussion:

It doesn't make sense for the template parameter `C` of `ranges::to` be a reference, which allows us to write something like `ranges::to<vector<int>&&>(std::move(v))` or `ranges::to<const vector<int>&&>(v)`, we should forbid this.

The proposed resolution constrains the template parameter `C` to be object type to conform to its description: "The range conversion functions construct an object (usually a container) from a range".

[2022-09-28; Reflector poll]

Set priority to 3 after reflector poll. Some suggestions that it should be Mandates: not Constraints:, but no consensus.

[Issaquah 2023-02-08; LWG]

This would be resolved by LWG 3847(i).

[2023-03-22 LWG 3847 was approved in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   >     #include <iterator>             // see 24.2 [iterator.synopsis]

   >     namespace std::ranges {
   >       […]
   >       // 25.5.7 [range.utility.conv], range conversions
   >       template<class C, input_range R, class... Args> requires is_object_v<C> && (!view<C>)
   >         constexpr C to(R&& r, Args&&... args);                                          // freestanding
   >       template<template<class...> class C, input_range R, class... Args>
   >         constexpr auto to(R&& r, Args&&... args);                                       // freestanding
   >       template<class C, class... Args> requires is_object_v<C> && (!view<C>)
   >         constexpr auto to(Args&&... args);                                              // freestanding
   >       template<template<class...> class C, class... Args>
   >         constexpr auto to(Args&&... args);                                              // freestanding
   >       […]
   >     }

2. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   >     template<class C, input_range R, class... Args> requires is_object_v<C> && (!view<C>)
   >     constexpr C to(R&& r, Args&&... args);

   > > -1- Returns: An object of type `C` constructed from the elements of `r` in the following manner:

   > > […]

3. Modify 25.5.7.3 [range.utility.conv.adaptors] as indicated:

   >     template<class C, class... Args> requires is_object_v<C> && (!view<C>)
   >       constexpr auto to(Args&&... args);
   >     template<template<class...> class C, class... Args>
   >       constexpr auto to(Args&&... args);

   > > -1- Returns: A range adaptor closure object (25.7.2 [range.adaptor.object]) `f` that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:

   > > […]

### 3788(i). `jthread::operator=(jthread&&)` postconditions are unimplementable under self-assignment

Section: 32.4.4.2 [thread.jthread.cons] Status: C++23 Submitter: Nicole Mazzuca Opened: 2022-09-22 Last modified: 2023-11-22

Priority: 3

View all issues with C++23 status.

Discussion:

In the Postconditions element of `jthread& jthread::operator=(jthread&&)` (32.4.4.2 [thread.jthread.cons] p13), we have the following:

> Postconditions: `x.get_id() == id()`, and `get_id()` returns the value of `x.get_id()` prior to the assignment. `ssource` has the value of `x.ssource` prior to the assignment and `x.ssource.stop_possible()` is `false`.

Assume `j` is a joinable `jthread`. Then, `j = std::move(j);` results in the following post-conditions:

- Let `old_id = j.get_id()` (and since `j` is joinable, `old_id != id()`)

- Since `x.get_id() == id()`, `j.get_id() == id()`

- Since `get_id() == old_id`, `j.get_id() == old_id`

- Thus, `id() == j.get_id() == old_id`, and `old_id != id()`, which is a contradiction.

One can see that these postconditions are therefore unimplementable.

Two standard libraries – the MSVC STL and libstdc++ – currently implement `jthread`.

The MSVC STL chooses to follow the letter of the Effects element, which results in unfortunate behavior. It first `request_stop()`s, then `join()`s; then, it assigns the values over. This results in `j.get_id() == id()` – this means that `std::swap(j, j)` stops and joins `j`.

libstdc++ chooses instead to implement this move assignment operator via the move-swap idiom. This results in `j.get_id() == old_id`, and `std::swap(j, j)` is a no-op.

It is the opinion of the issue writer that libstdc++'s choice is the correct one, and should be taken into the standard.

[2022-09-28; Reflector poll]

Set priority to 3 after reflector poll.

[2022-09-28; Jonathan provides wording]

[2022-09-29; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 32.4.4.2 [thread.jthread.cons] as indicated:

   >     jthread& operator=(jthread&& x) noexcept;

   > > -12- Effects: If `&x == this` is `true`, there are no effects. Otherwise, if `joinable()` is `true`, calls `request_stop()` and then `join()`. Assigns, then assigns the state of `x` to `*this` and sets `x` to a default constructed state.

   > > -13- Postconditions: `x.get_id() == id()` and `get_id()` returns the value of `x.get_id()` prior to the assignment. `ssource` has the value of `x.ssource` prior to the assignment and `x.ssource.stop_possible()` is `false`.

   > > -14- Returns: `*this`.

### 3790(i). P1467 accidentally changed `nexttoward`'s signature

Section: 29.7.1 [cmath.syn] Status: C++23 Submitter: David Olsen Opened: 2022-09-30 Last modified: 2023-11-22

Priority: 1

View other active issues in [cmath.syn].

View all other issues in [cmath.syn].

View all issues with C++23 status.

Discussion:

P1467 (Extended floating-point types), which was adopted for C++23 at the July plenary, has a typo (which is entirely my fault) that no one noticed during wording review. The changes to the `<cmath>` synopsis in the paper included changing this:

>     constexpr float nexttoward(float x, long double y);       // see [library.c]
>     constexpr double nexttoward(double x, long double y);
>     constexpr long double nexttoward(long double x, long double y);   // see [library.c]

to this:

>     constexpr floating-point-type nexttoward(floating-point-type x, floating-point-type y);

That changed the second parameter of `nexttoward` from always being `long double` to being `floating-point-type`, which matches the type of the first parameter.

The change is obviously incorrect. The purpose of the changes to `<cmath>` was to add overloads of the functions for extended floating-point types, not to change any existing signatures.

[2022-10-10; Reflector poll]

Set priority to 1 after reflector poll. Discussion during prioritization revolved around whether to delete `nexttoward` for new FP types or just restore the C++20 signatures, which might accept the new types via implicit conversions (and so return a different type, albeit with the same representation and same set of values).

"When the first argument to `nexttoward` is an extended floating-point type that doesn't have the same representation as a standard floating-point type, such as `std::float16_t`, `std::bfloat16_t`, or `std::float128_t` (on some systems), the call to `nexttoward` is ambiguous and ill-formed, so the unexpected return type is not an issue. Going through the extra effort of specifying '= delete' for `nexttoward` overloads that have extended floating-point arguments is a solution for a problem that doesn't really exist."

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 29.7.1 [cmath.syn], header `<cmath>` synopsis, as indicated:

>    >     […]
>    >     constexpr floating-point-type nexttoward(floating-point-type x, floating-point-typelong double y);
>    >     constexpr float nexttowardf(float x, long double y);
>    >     constexpr long double nexttowardl(long double x, long double y);
>    >     […]

[2022-10-04; David Olsen comments and provides improved wording]

C23 specifies variants of most of the functions in `<math.h>` for the `_FloatN` types (which are C23's equivalent of C++23's `std::floatN_t` types). But it does not specify those variants for `nexttoward`.

Based on what C23 is doing, I think it would be reasonable to leave `nexttoward`'s signature unchanged from C++20. There would be no requirement to provide overloads for extended floating-point types, only for the standard floating-point types. Instead of explicitly deleting the overloads with extended floating-point types, we can just never declare them in the first place.
Previous resolution [SUPERSEDED]

This wording is relative to N4917.

1. Modify 29.7.1 [cmath.syn], header `<cmath>` synopsis, as indicated:

   >     […]
   >     constexpr float nexttoward(float x, long double y);
   >     constexpr double nexttoward(double x, long double y);
   >     constexpr long double nexttoward(long double x, long double y);
   >     constexpr floating-point-type nexttoward(floating-point-type x, floating-point-type y);
   >     constexpr float nexttowardf(float x, long double y);
   >     constexpr long double nexttowardl(long double x, long double y);
   >     […]

[2022-11-12; Tomasz comments and provides improved wording]

During 2022-10-26 LWG telecon we decided that we want to make the calls of the `nexttoward` to be ill-formed (equivalent of Mandates) when the first argument is extended floating-point type.

[2023-01-11; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 29.7.1 [cmath.syn], header `<cmath>` synopsis, as indicated:

   >     […]
   >     constexpr floating-point-type nexttoward(floating-point-type x, floating-point-typelong double y);
   >     constexpr float nexttowardf(float x, long double y);
   >     constexpr long double nexttowardl(long double x, long double y);
   >     […]

2. Add following paragraph at the end of 29.7.1 [cmath.syn], header `<cmath>` synopsis:

   > -?- An invocation of `nexttoward` is ill-formed if the argument corresponding to the floating-point-type parameter has extended floating-point type.

### 3791(i). `join_view::iterator::operator--` may be ill-formed

Section: 25.7.14.3 [range.join.iterator] Status: Resolved Submitter: Hewill Kang Opened: 2022-09-30 Last modified: 2023-03-23

Priority: 3

View all other issues in [range.join.iterator].

View all issues with Resolved status.

Discussion:

Currently, `join_view::iterator::operator--` has the following Effects:

>     if (outer_ == ranges::end(parent_->base_))
>       inner_ = ranges::end(*--outer_);
>     while (inner_ == ranges::begin(*outer_))
>       inner_ = ranges::end(*--outer_);
>     --inner_;
>     return *this;

which uses `ranges::end(*--outer_)` to get the sentinel of the inner range. However, `*--outer_` may return an rvalue reference to a non-borrowed range, in which case calling `ranges::end` will be ill-formed, for example:

>     #include <ranges>
>     #include <vector>

>     int main() {
>       std::vector<std::vector<int>> v;
>       auto r = v | std::views::transform([](auto& x) -> auto&& { return std::move(x); })
>                  | std::views::join;
>       auto e = --r.end(); // hard error
>     }

The proposed resolution uses a temporary reference to bind `*--outer_`, so that `ranges::end` is always invoked on an lvalue range, which is consistent with the behavior of `join_with_view::iterator::operator--`.

[2022-10-12; Reflector poll]

Set priority to 3 after reflector poll.

"Could introduce an `as_lvalue` lambda (like `auto as_lvalue = []<class T>(T&& x) -> T& { return (T&)x; };`) and use it throughout."

Previous resolution [SUPERSEDED]

This wording is relative to N4917.

1. Modify 25.7.14.3 [range.join.iterator] as indicated:

   >     constexpr iterator& operator--()
   >       requires ref-is-glvalue && bidirectional_range<Base> &&
   >                 bidirectional_range<range_reference_t<Base>> &&
   >                 common_range<range_reference_t<Base>>;

   -13- Effects: Equivalent to:

   >     if (outer_ == ranges::end(parent_->base_)) {
   >       auto&& inner = *--outer_;
   >       inner_ = ranges::end(inner*--outer_);
   >     }
   >     while (trueinner_ == ranges::begin(*outer_)) {
   >       if (auto&& tmp = *outer_; inner_ == ranges::begin(tmp)) {
   >         auto&& inner = *--outer_;
   >         inner_ = ranges::end(inner*--outer_);
   >       } else {
   >         break;
   >       }
   >     }
   >     --inner_;
   >     return *this;

[2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

### 3792(i). `__cpp_lib_constexpr_algorithms` should also be defined in `<utility>`

Section: 17.3.2 [version.syn] Status: C++23 Submitter: Marc Mutz Opened: 2022-10-05 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++23 status.

Discussion:

17.3.2 [version.syn] says that `__cpp_lib_constexpr_algorithms` is only defined in `<version>` and `<algorithm>`, but the macro applies to `std::exchange()` in `<utility>` as well (via P0202R3), so it should also be available from `<utility>`.

[2022-10-12; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 17.3.2 [version.syn] as indicated (It is suggested to retroactively apply to C++20):

   >     […]
   >     #define __cpp_lib_concepts             202207L // also in <concepts>, <compare>
   >     #define __cpp_lib_constexpr_algorithms 201806L // also in <algorithm>, <utility>
   >     #define __cpp_lib_constexpr_bitset     202202L // also in <bitset>
   >     […]

### 3795(i). Self-move-assignment of `std::future` and `std::shared_future` have unimplementable postconditions

Section: 32.10.7 [futures.unique.future], 32.10.8 [futures.shared.future] Status: C++23 Submitter: Jiang An Opened: 2022-10-19 Last modified: 2023-11-22

Priority: 3

View all other issues in [futures.unique.future].

View all issues with C++23 status.

Discussion:

The move assignment operators of `std::future` and `std::shared_future` have their postconditions specified as below:

> Postconditions:

> - `valid()` returns the same value as `rhs.valid()` returned prior to the assignment.

> - `rhs.valid() == false`.

It can be found that when `*this` and `rhs` is the same object and `this->valid()` is `true` before the assignment, the postconditions can't be achieved.

Mainstream implementations (libc++, libstdc++, msvc stl) currently implement such self-move-assignment as no-op, which doesn't meet the requirements in the Effects: element. As discussed in LWG 3788(i), I think we should say self-move-assignment has no effects for these types.

[2022-11-01; Reflector poll]

Set priority to 3 after reflector poll.

[2022-11-01; Jonathan provides wording]

[2022-11-07; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 32.10.7 [futures.unique.future] as indicated:

   > >     future& operator=(future&& rhs) noexcept;

   > > -11- Effects: If `addressof(rhs) == this` is `true`, there are no effects. Otherwise:

   > > 1. (11.1) — Releases any shared state (32.10.5 [futures.state]).
   > > 2. (11.2) — move assigns the contents of `rhs` to `*this`.

   > > -12- Postconditions:

   > > 1. (12.1) — `valid()` returns the same value as `rhs.valid()` prior to the assignment.
   > > 2. (12.2) — If `addressof(rhs) == this` is `false`, `rhs.valid() == false`.

2. Modify 32.10.8 [futures.shared.future] as indicated:

   > >     shared_future& operator=(shared_future&& rhs) noexcept;

   > > -13- Effects: If `addressof(rhs) == this` is `true`, there are no effects. Otherwise:

   > > 1. (13.1) — Releases any shared state (32.10.5 [futures.state]).
   > > 2. (13.2) — move assigns the contents of `rhs` to `*this`.

   > > -14- Postconditions:

   > > 1. (14.1) — `valid()` returns the same value as `rhs.valid()` returned prior to the assignment.
   > > 2. (14.2) — If `addressof(rhs) == this` is `false`, `rhs.valid() == false`.

   > >     shared_future& operator=(const shared_future& rhs) noexcept;

   > > -15- Effects: If `addressof(rhs) == this` is `true`, there are no effects. Otherwise:

   > > 1. (15.1) — Releases any shared state (32.10.5 [futures.state]).
   > > 2. (15.2) — assigns the contents of `rhs` to `*this`.

   > >    [Note 3: As a result, `*this` refers to the same shared state as `rhs` (if any). — end note]

   > > -16- Postconditions: `valid() == rhs.valid()`.

### 3796(i). `movable-box` as member should use default-initialization instead of copy-initialization

Section: 25.6.5.2 [range.repeat.view], 25.7.31.2 [range.chunk.by.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-10-20 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.repeat.view].

View all issues with C++23 status.

Discussion:

Currently, the member variable `value_` of `repeat_view` is initialized with the following expression:

>     movable-box<W> value_ = W();

which will result in the following unexpected error in libstdc++:

>     #include <ranges>

>     int main() {
>       std::ranges::repeat_view<int> r; // error: could not convert '0' from 'int' to 'std::ranges::__detail::__box<int>'
>     }

This is because the single-argument constructors of the simple version of `movable-box` in libstdc++ are declared as `explicit`, which is different from the conditional `explicit` declared by the `optional`'s constructor, resulting in no visible conversion between those two types.

For MSVC-STL, the simple version of `movable-box` does not provide a single-argument constructor, so this form of initialization will also produce a hard error.

This may be a bug of existing implementations, but we don't need such "copy-initialization", the default-constructed `movable-box` already value-initializes the underlying type.

Simply using default initialization, as most other range adaptors do, guarantees consistency, which also eliminates extra move construction.

[2022-11-01; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.6.5.2 [range.repeat.view] as indicated:

   >     namespace std::ranges {
   >       template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
   >         requires (is_object_v<W> && same_as<W, remove_cv_t<W>> &&
   >                   (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>))
   >        class repeat_view : public view_interface<repeat_view<W, Bound>> {
   >        private:
   >          // 25.6.5.3 [range.repeat.iterator], class repeat_view::iterator
   >          struct iterator;                            // exposition only

   >          movable-box<W> value_ = W();                // exposition only, see 25.7.3 [range.move.wrap]
   >          Bound bound_ = Bound();                     // exposition only
   >          […]
   >        };
   >        […]
   >     }

2. Modify 25.7.31.2 [range.chunk.by.view] as indicated:

   >     namespace std::ranges {
   >       template<forward_range V, indirect_binary_predicate<iterator_t<V>, iterator_t<V>> Pred>
   >         requires view<V> && is_object_v<Pred>
   >       class chunk_by_view : public view_interface<chunk_by_view<V, Pred>> {
   >         V base_ = V();                                          // exposition only
   >         movable-box<Pred> pred_ = Pred();                       // exposition only

   >         // 25.7.31.3 [range.chunk.by.iter], class chunk_by_view::iterator
   >         class iterator;                                         // exposition only
   >         […]
   >       };
   >       […]
   >     }

### 3797(i). `elements_view` insufficiently constrained

Section: 25.7.23.2 [range.elements.view] Status: C++26 Submitter: Hui Xie Opened: 2022-10-21 Last modified: 2026-06-09

Priority: 2

View all other issues in [range.elements.view].

View all issues with C++26 status.

Discussion:

This issue came up when I tried to integrate the C++23 changes to tuple-like into `ranges::elements_view` in libc++. Given the following test:

>     Using SubRange = ranges::subrange<MoveOnlyIter, Sent>;
>     std::vector<SubRange> srs = ...;  // a vector of subranges
>     for(auto&& iter : srs | views::elements<0>){
>     }

The above code results in a hard error in deciding the `iterator_category` (The base is a random access range so it should exist). The immediate hard error complains that the following expression is invalid.

>     std::get<N>(*current_);

Note that even if `iterator_category` does not complain, it will complain later when we dereference the iterator.

Here are the declarations of the "get" overloads for `subrange`:

>     template<size_t N, class I, class S, subrange_kind K>
>       requires ((N == 0 && copyable<I>) || N == 1)
>       constexpr auto get(const subrange<I, S, K>& r);

>     template<size_t N, class I, class S, subrange_kind K>
>       requires (N < 2)
>       constexpr auto get(subrange<I, S, K>&& r);

Note that the first overload requires `copyable<I>` which is `false` and the second overload requires an rvalue, which is also not the case. So we don't have a valid "get" in this case.

But why does `elements_view` allow the instantiation in the first place? Let's look at its requirements:

>     template<class T, size_t N>
>       concept returnable-element =                  // exposition only
>         is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;

>     template<input_range V, size_t N>
>         requires view<V> && has-tuple-element<range_value_t<V>, N> &&
>                  has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
>                  returnable-element<range_reference_t<V>, N>
>       class elements_view;

It passed the "`is_reference_v<range_reference_t<V>>`" requirement, because it is "`subrange&`". Here the logic has an assumption: if the tuple-like is a reference, then we can always "`get`" and return a reference. This is not the case for `subrange`. `subrange`'s `get` always return by value.

[2022-11-01; Reflector poll]

Set priority to 2 after reflector poll.

"The actual issue is that P2165 broke `has-tuple-element` for this case. We should unbreak it."

Previous resolution [SUPERSEDED]

This wording is relative to N4917.

> [Drafting Note: Three mutually exclusive options are prepared, depicted below by Option A, Option B, and Option C, respectively.]

Option A: Properly disallow this case (preferred solution)

1. Modify 25.7.23.2 [range.elements.view] as indicated:

   >     namespace std::ranges {
   >       […]
   >       template<class T, size_t N>
   >       concept returnable-element =              // exposition only
   >         requires { std::get<N>(declval<T>()); } &&
   >         is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;
   >       […]
   >     }

Option B: Relax `subrange`'s `get` to have more overloads. Since `subrange`'s non-`const begin` unconditionally moves the iterator (even for lvalue-reference),

>     [[nodiscard]] constexpr I begin() requires (!copyable<I>);
>     Effects: Equivalent to: return std::move(begin_);

if we add more `get` overloads, it would work. The non-const lvalue-ref overload would work (and it also moves because non-`const` lvalue begin moves). This solution would make another way to let `subrange`'s iterator in moved-from state, which is not good.

1. Modify 25.2 [ranges.syn] as indicated:

   >     […]
   >     namespace std::ranges {
   >       […]

   >       template<size_t N, class I, class S, subrange_kind K>
   >         requires ((N == 0 && copyable<I>) || N == 1)
   >         constexpr auto get(const subrange<I, S, K>& r);

   >       template<size_t N, class I, class S, subrange_kind K>
   >         requires (N < 2)
   >         constexpr auto get(subrange<I, S, K>&& r);

   >       template<size_t N, class I, class S, subrange_kind K>
   >         requires ((N == 0 && constructible_from<I, const I&&>) || N == 1)
   >         constexpr auto get(const subrange<I, S, K>&& r);

   >       template<size_t N, class I, class S, subrange_kind K>
   >         requires (N < 2)
   >         constexpr auto get(subrange<I, S, K>& r);
   >     }
   >     […]

Option C: Make `subrange`'s get to return by reference. This seems to significantly change the `subrange`'s tuple protocol, which is not ideal.

[2026-03-27; Tim provides new wording]

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 25.7.23.2 [range.elements.view] as indicated:

   >     namespace std::ranges {
   >       template<class T, size_t N>
   >       concept has-tuple-element =                   // exposition only
   >         tuple-like<T> && N < tuple_size_v<T> &&
   >         requires(T t) {
   >           { std::get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
   >         };

   >       […]
   >     }

### 3798(i). Rvalue reference and `iterator_category`

Section: 24.3.2.3 [iterator.traits] Status: C++23 Submitter: Jiang An Opened: 2022-10-22 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [iterator.traits].

View all issues with C++23 status.

Discussion:

Since C++11, a forward iterator may have an rvalue reference as its `reference` type. I think this is an intentional change made by N3066. However, several components added (or changed) in C++20/23 recognize an iterator as a Cpp17ForwardIterator (by using `forward_iterator_tag` or a stronger iterator category tag type as the `iterator_category` type) only if the `reference` type would be an lvalue reference type, which seems a regression.

[2022-11-01; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 24.3.2.3 [iterator.traits] as indicated:

   > -2- The definitions in this subclause make use of the following exposition-only concepts:

   > >     […]
   > >     template<class I>
   > >     concept cpp17-forward-iterator =
   > >       cpp17-input-iterator<I> && constructible_from<I> &&
   > >       is_lvalue_reference_v<iter_reference_t<I>> &&
   > >       same_as<remove_cvref_t<iter_reference_t<I>>,
   > >               typename indirectly_readable_traits<I>::value_type> &&
   > >       requires(I i) {
   > >         {  i++ } -> convertible_to<const I&>;
   > >         { *i++ } -> same_as<iter_reference_t<I>>;
   > >       };
   > >     […]

2. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   > -2- The member typedef-name `iterator_category` is defined if and only if Base models `forward_range`. In that case, `iterator::iterator_category` is defined as follows: Let `C` denote the type `iterator_traits<iterator_t<Base>>::iterator_category`.

   > 1. (2.1) — If `is_lvalue_reference_v<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>>` is `true`, then

   >    1. (2.1.1) — if `C` models `derived_from<contiguous_iterator_tag>`, `iterator_category denotes random_access_iterator_tag`;

   >    2. (2.1.2) — otherwise, `iterator_category` denotes `C`.

   > 2. (2.2) — Otherwise, `iterator_category` denotes `input_iterator_tag`.

3. Modify 25.7.15.3 [range.join.with.iterator] as indicated:

   > -2- The member typedef-name `iterator_category` is defined if and only if `ref-is-glvalue` is `true`, and `Base` and `InnerBase` each model `forward_range`. In that case, `iterator::iterator_category` is defined as follows:

   > 1. (2.1) — […]

   > 2. (2.2) — If

   >    >     is_lvalue_reference_v<common_reference_t<iter_reference_t<InnerIter>,
   >    >                           iter_reference_t<PatternIter>>>

   >    is `false`, `iterator_category` denotes `input_iterator_tag`.

   > 3. (2.3) — […]

4. Modify 25.7.26.3 [range.zip.transform.iterator] as indicated:

   > -1- The member typedef-name `iterator::iterator_category` is defined if and only if `Base` models `forward_range`. In that case, `iterator::iterator_category` is defined as follows:

   > 1. (1.1) — If

   >    >     invoke_result_t<maybe-const<Const, F>&, range_reference_t<maybe-const<Const, Views>>...>

   >    is not an lvalue reference, `iterator_category` denotes `input_iterator_tag`.

   > 2. (1.2) — […]

5. Modify 25.7.28.3 [range.adjacent.transform.iterator] as indicated:

   > -1- The member typedef-name `iterator::iterator_category` is defined as follows:

   > 1. (1.1) — If `invoke_result_t<maybe-const<Const, F>&, REPEAT(range_reference_t<Base>, N)...>` is not an lvalue reference, `iterator_category` denotes `input_iterator_tag`.

   > 2. (1.2) — […]

### 3801(i). `cartesian_product_view::iterator::distance-from` ignores the size of last underlying range

Section: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Patrick Palka Opened: 2022-10-25 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.cartesian.iterator].

View all issues with C++23 status.

Discussion:

The helper `scaled-size(N)` from the wording for P2374R4's `cartesian_product_view::iterator::distance-from` is recursively specified as:

> Let `scaled-size(N)` be the product of `static_cast<difference_type>(ranges::size(std::get<N>(parent_->bases_)))` and `scaled-size(N + 1)` if `N < sizeof...(Vs)`, otherwise `static_cast<difference_type>(1)`;

Intuitively, `scaled-size(N)` is the size of the cartesian product of all but the first `N` underlying ranges. Thus `scaled-size(sizeof...(Vs))` ought to just yield the size of the last underlying range (since there are `1 + sizeof...(Vs)` underlying ranges), but according to this definition it yields `1`. Similarly at the other extreme, `scaled-size(0)` should yield the product of the sizes of all the underlying ranges, but it instead yields that of all but the last underlying range.

For `cartesian_product_view`s of two or more underlying ranges, this causes the relevant `operator-` overloads to compute wrong distances, e.g.

>     int x[] = {1, 2, 3};
>     auto v = views::cartesian_product(x, x);
>     auto i = v.begin() + 5;  // *i == {2, 3}
>     assert(*i == tuple{2, 3});
>     assert(i - v.begin() == 5); // fails, expects 3, because:
>     // scaled-sum = scaled-distance(0) + scaled-distance(1)
>     //            = ((1 - 0) * scaled-size(1)) + ((2 - 0) * scaled-size(2))
>     //            = 1 + 2 instead of 3 + 2

The recursive condition should probably use `<=` instead of `<`.

[2022-11-04; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify [ranges.cartesian.iterator] as indicated:

   >     template<class Tuple>
   >       constexpr difference_type distance-from(Tuple t);

   > > -7- Let:

   > > 1. (7.1) — `scaled-size(N)` be the product of `static_cast<difference_type>(ranges::size(std::get<N>(parent_->bases_)))` and `scaled-size(N + 1)` if `N ≤< sizeof...(Vs)`, otherwise `static_cast<difference_type>(1)`;

   > > 2. (7.2) — `scaled-distance(N)` be the product of `static_cast<difference_type>(std::get<N>(current_) - std::get<N>(t))` and `scaled-size(N + 1)`; and

   > > 3. (7.3) — `scaled-sum` be the sum of `scaled-distance(N)` for every integer `0 ≤ N ≤ sizeof...(Vs)`.

### 3803(i). `flat_foo` constructors taking `KeyContainer` lack `KeyCompare` parameter

Section: 23.6.8 [flat.map], 23.6.9 [flat.multimap], 23.6.11 [flat.set], 23.6.12 [flat.multiset] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2022-10-25 Last modified: 2023-11-22

Priority: 1

View other active issues in [flat.map].

View all other issues in [flat.map].

View all issues with C++23 status.

Discussion:

`flat_set`'s current constructor overload set has these two overloads:

>     explicit flat_set(container_type cont);
>     template<class Allocator>
>       flat_set(const container_type& cont, const Allocator& a);

I believe it should have these two in addition:

>     flat_set(const key_compare& comp, container_type cont);
>     template<class Allocator>
>       flat_set(const key_compare& comp, const container_type& cont, const Allocator& a);

with corresponding deduction guides. Similar wording changes would have to be made to all the `flat_foo` containers.

Tony Table:

>     struct LessWhenDividedBy {
>       int divisor_;
>       bool operator()(int x, int y) const { return x/divisor_ < y/divisor_; }
>     };
>     std::flat_set<int, LessWhenDividedBy> s(data.begin(), data.end(), LessWhenDividedBy(10));
>     // BEFORE AND AFTER: okay, but cumbersome
>     std::flat_set<int, LessWhenDividedBy> s(data);
>     // BEFORE AND AFTER: oops, this default-constructs the comparator

>     std::flat_set<int, LessWhenDividedBy> s(LessWhenDividedBy(10), data);
>     // BEFORE: fails to compile
>     // AFTER: compiles successfully

[2022-11-04; Reflector poll]

Set priority to 1 after reflector poll.

[2023-02-09 Tim adds wording]

For each construction that takes containers, this wording allow a comparator to be specified as well. Differing from the suggestion in the issue, the comparator goes last (but before the allocator), for consistency with every other constructor of `flat_meow` taking a comparator. (This is inconsistent with `priority_queue`, but consistency among the type's own constructors seems more important.)

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Add a new bullet to 23.6.1 [container.adaptors.general] p6 as indicated:

   > -6- A deduction guide for a container adaptor shall not participate in overload resolution if any of the following are true:

   > 1. — (6.?) It has both `KeyContainer` and `Compare` template parameters, and `is_invocable_v<const Compare&, const typename KeyContainer::value_type&, const typename KeyContainer::value_type&>` is not a valid expression or is `false`.

2. Modify 23.6.8.2 [flat.map.defn] as indicated:

   >     namespace std {
   >       template<class Key, class T, class Compare = less<Key>,
   >                class KeyContainer = vector<Key>, class MappedContainer = vector<T>>
   >       class flat_map {
   >       public:
   >         […]
   >         // 23.6.8.3 [flat.map.cons], construct/copy/destroy
   >         flat_map() : flat_map(key_compare()) { }

   >         flat_map(key_container_type key_cont, mapped_container_type mapped_cont,
   >                  const key_compare& comp = key_compare());
   >         template<class Allocator>
   >           flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
   >                    const Allocator& a);
   >         template<class Allocator>
   >           flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
   >                    const key_compare& comp, const Allocator& a);

   >         flat_map(sorted_unique_t, key_container_type key_cont, mapped_container_type mapped_cont,
   >                  const key_compare& comp = key_compare());
   >         template<class Allocator>
   >           flat_map(sorted_unique_t, const key_container_type& key_cont,
   >                    const mapped_container_type& mapped_cont, const Allocator& a);
   >         template<class Allocator>
   >           flat_map(sorted_unique_t, const key_container_type& key_cont,
   >                    const mapped_container_type& mapped_cont,
   >                    const key_compare& comp, const Allocator& a);
   >         […]
   >       };

   >       template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>>
   >         flat_map(KeyContainer, MappedContainer, Compare = Compare())
   >           -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       Compareless<typename KeyContainer::value_type>, KeyContainer, MappedContainer>;

   >       template<class KeyContainer, class MappedContainer, class Allocator>
   >         flat_map(KeyContainer, MappedContainer, Allocator)
   >           -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>;
   >       template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
   >         flat_map(KeyContainer, MappedContainer, Compare, Allocator)
   >           -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       Compare, KeyContainer, MappedContainer>;

   >       template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>>
   >         flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare = Compare())
   >           -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       Compareless<typename KeyContainer::value_type>, KeyContainer, MappedContainer>;

   >       template<class KeyContainer, class MappedContainer, class Allocator>
   >         flat_map(sorted_unique_t, KeyContainer, MappedContainer, Allocator)
   >           -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>;
   >       template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
   >         flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare, Allocator)
   >           -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       Compare, KeyContainer, MappedContainer>;
   >       […]
   >     }

3. Modify 23.6.8.3 [flat.map.cons] as indicated:

   >     flat_map(key_container_type key_cont, mapped_container_type mapped_cont,
   >              const key_compare& comp = key_compare());

   > > -1- Effects: Initializes `c.keys` with `std::move(key_cont)`, and `c.values` with `std::move(mapped_cont)`, and `compare` with `comp` ; value-initializes `compare`; sorts the range `[begin(), end())` with respect to `value_comp()`; and finally erases the duplicate elements as if by:

   > > >     auto zv = ranges::zip_view(c.keys, c.values);
   > > >     auto it = ranges::unique(zv, key_equiv(compare)).begin();
   > > >     auto dist = distance(zv.begin(), it);
   > > >     c.keys.erase(c.keys.begin() + dist, c.keys.end());
   > > >     c.values.erase(c.values.begin() + dist, c.values.end());

   > > -2- Complexity: Linear in N if the container arguments are already sorted with respect to `value_comp()` and otherwise N log N, where N is the value of `key_cont.size()` before this call.

   >     template<class Allocator>
   >       flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
   >                const Allocator& a);
   >     template<class Allocator>
   >       flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
   >                const key_compare& comp, const Allocator& a);

   > > -3- Constraints: `uses_allocator_v<key_container_type, Allocator>` is `true` and `uses_allocator_v<mapped_container_type, Allocator>` is `true`.

   > > -4- Effects: Equivalent to `flat_map(key_cont, mapped_cont)` and `flat_map(key_cont, mapped_cont, comp)`, respectively, except that `c.keys` and `c.values` are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -5- Complexity: Same as `flat_map(key_cont, mapped_cont)` and `flat_map(key_cont, mapped_cont, comp)`, respectively.

   >     flat_map(sorted_unique_t, key_container_type key_cont, mapped_container_type mapped_cont,
   >              const key_compare& comp = key_compare());

   > > -6- Effects: Initializes `c.keys` with `std::move(key_cont)`, and `c.values` with `std::move(mapped_cont)`, and `compare` with `comp` ; value-initializes `compare`.

   > > -7- Complexity: Constant.

   >     template<class Allocator>
   >       flat_map(sorted_unique_t s, const key_container_type& key_cont,
   >                const mapped_container_type& mapped_cont, const Allocator& a);
   >     template<class Allocator>
   >       flat_map(sorted_unique_t s, const key_container_type& key_cont,
   >                const mapped_container_type& mapped_cont, const key_compare& comp,
   >                const Allocator& a);

   > > -8- Constraints: `uses_allocator_v<key_container_type, Allocator>` is `true` and `uses_allocator_v<mapped_container_type, Allocator>` is `true`.

   > > -9- Effects: Equivalent to `flat_map(s, key_cont, mapped_cont)` and `flat_map(s, key_cont, mapped_cont, comp)`, respectively, except that `c.keys` and `c.values` are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -10- Complexity: Linear.

4. Modify 23.6.9.2 [flat.multimap.defn] as indicated:

   >     namespace std {
   >       template<class Key, class T, class Compare = less<Key>,
   >                class KeyContainer = vector<Key>, class MappedContainer = vector<T>>
   >       class flat_multimap {
   >       public:
   >         […]
   >         // 23.6.9.3 [flat.multimap.cons], construct/copy/destroy
   >         flat_multimap() : flat_multimap(key_compare()) { }

   >         flat_multimap(key_container_type key_cont, mapped_container_type mapped_cont,
   >                       const key_compare& comp = key_compare());
   >         template<class Allocator>
   >           flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
   >                         const Allocator& a);
   >         template<class Allocator>
   >           flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
   >                         const key_compare& comp, const Allocator& a);

   >         flat_multimap(sorted_equivalent_t,
   >                       key_container_type key_cont, mapped_container_type mapped_cont,
   >                       const key_compare& comp = key_compare());
   >         template<class Allocator>
   >           flat_multimap(sorted_equivalent_t, const key_container_type& key_cont,
   >                         const mapped_container_type& mapped_cont, const Allocator& a);
   >         template<class Allocator>
   >           flat_multimap(sorted_equivalent_t, const key_container_type& key_cont,
   >                         const mapped_container_type& mapped_cont,
   >                         const key_compare& comp, const Allocator& a);
   >         […]
   >       };

   >       template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>>
   >         flat_multimap(KeyContainer, MappedContainer, Compare = Compare())
   >           -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       Compareless<typename KeyContainer::value_type>, KeyContainer, MappedContainer>;

   >       template<class KeyContainer, class MappedContainer, class Allocator>
   >         flat_multimap(KeyContainer, MappedContainer, Allocator)
   >           -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>;
   >       template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
   >         flat_multimap(KeyContainer, MappedContainer, Compare, Allocator)
   >           -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       Compare, KeyContainer, MappedContainer>;

   >       template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>>
   >         flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare = Compare())
   >           -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       Compareless<typename KeyContainer::value_type>, KeyContainer, MappedContainer>;

   >       template<class KeyContainer, class MappedContainer, class Allocator>
   >         flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Allocator)
   >           -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>;
   >       template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
   >         flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare, Allocator)
   >           -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type,
   >                       Compare, KeyContainer, MappedContainer>;
   >       […]
   >     }

5. Modify 23.6.9.3 [flat.multimap.cons] as indicated:

   >     flat_multimap(key_container_type key_cont, mapped_container_type mapped_cont,
   >                   const key_compare& comp = key_compare());

   > > -1- Effects: Initializes `c.keys` with `std::move(key_cont)`, and `c.values` with `std::move(mapped_cont)`, and `compare` with `comp` ; value-initializes `compare`; and sorts the range `[begin(), end())` with respect to `value_comp()`.

   > > -2- Complexity: Linear in N if the container arguments are already sorted with respect to `value_comp()` and otherwise N log N, where N is the value of `key_cont.size()` before this call.

   >     template<class Allocator>
   >       flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
   >                     const Allocator& a);
   >     template<class Allocator>
   >       flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
   >                     const key_compare& comp, const Allocator& a);

   > > -3- Constraints: `uses_allocator_v<key_container_type, Allocator>` is `true` and `uses_allocator_v<mapped_container_type, Allocator>` is `true`.

   > > -4- Effects: Equivalent to `flat_multimap(key_cont, mapped_cont)` and `flat_multimap(key_cont, mapped_cont, comp)`, respectively, except that `c.keys` and `c.values` are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -5- Complexity: Same as `flat_multimap(key_cont, mapped_cont)` and `flat_multimap(key_cont, mapped_cont, comp)`, respectively.

   >     flat_multimap(sorted_equivalent_t, key_container_type key_cont, mapped_container_type mapped_cont,
   >                   const key_compare& comp = key_compare());

   > > -6- Effects: Initializes `c.keys` with `std::move(key_cont)`, and `c.values` with `std::move(mapped_cont)`, and `compare` with `comp` ; value-initializes `compare`.

   > > -7- Complexity: Constant.

   >     template<class Allocator>
   >       flat_multimap(sorted_equivalent_t s, const key_container_type& key_cont,
   >                    const mapped_container_type& mapped_cont, const Allocator& a);
   >     template<class Allocator>
   >       flat_multimap(sorted_equivalent_t s, const key_container_type& key_cont,
   >                     const mapped_container_type& mapped_cont, const key_compare& comp,
   >                     const Allocator& a);

   > > -8- Constraints: `uses_allocator_v<key_container_type, Allocator>` is `true` and `uses_allocator_v<mapped_container_type, Allocator>` is `true`.

   > > -9- Effects: Equivalent to `flat_multimap(s, key_cont, mapped_cont)` and `flat_multimap(s, key_cont, mapped_cont, comp)`, respectively, except that `c.keys` and `c.values` are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -10- Complexity: Linear.

6. Modify 23.6.11.2 [flat.set.defn] as indicated:

   >     namespace std {
   >       template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
   >       class flat_set {
   >       public:
   >         […]

   >         // [flat.set.cons], constructors
   >         flat_set() : flat_set(key_compare()) { }

   >         explicit flat_set(container_type cont, const key_compare& comp = key_compare());
   >         template<class Allocator>
   >           flat_set(const container_type& cont, const Allocator& a);
   >         template<class Allocator>
   >           flat_set(const container_type& cont, const key_compare& comp, const Allocator& a);

   >         flat_set(sorted_unique_t, container_type cont, const key_compare& comp = key_compare())
   >           : c(std::move(cont)), compare(compkey_compare()) { }
   >         template<class Allocator>
   >           flat_set(sorted_unique_t, const container_type& cont, const Allocator& a);
   >         template<class Allocator>
   >           flat_set(sorted_unique_t, const container_type& cont,
   >                    const key_compare& comp, const Allocator& a);

   >         […]
   >       };

   >       template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>>
   >         flat_set(KeyContainer, Compare = Compare())
   >           -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>;
   >       template<class KeyContainer, class Allocator>
   >         flat_set(KeyContainer, Allocator)
   >           -> flat_set<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>;
   >       template<class KeyContainer, class Compare, class Allocator>
   >         flat_set(KeyContainer, Compare, Allocator)
   >           -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>;

   >       template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>>
   >         flat_set(sorted_unique_t, KeyContainer, Compare = Compare())
   >           -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>;
   >       template<class KeyContainer, class Allocator>
   >         flat_set(sorted_unique_t, KeyContainer, Allocator)
   >           -> flat_set<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>;
   >       template<class KeyContainer, class Compare, class Allocator>
   >         flat_set(sorted_unique_t, KeyContainer, Compare, Allocator)
   >           -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>;

   >       […]
   >     }

7. Modify 23.6.11.3 [flat.set.cons] as indicated:

   >     flat_set(container_type cont, const key_compare& comp = key_compare());

   > > -1- Effects: Initializes `c` with `std::move(cont)` and `compare` with `comp` , value-initializes `compare`, sorts the range `[begin(), end())` with respect to `compare`, and finally erases all but the first element from each group of consecutive equivalent elements.

   > > -2- Complexity: Linear in N if `cont` is already sorted with respect to `compare` and otherwise N log N, where N is the value of `cont.size()` before this call.

   >     template<class Allocator>
   >       flat_set(const container_type& cont, const Allocator& a);
   >     template<class Allocator>
   >       flat_set(const container_type& cont, const key_compare& comp, const Allocator& a);

   > > -3- Constraints: `uses_allocator_v<container_type, Allocator>` is `true`.

   > > -4- Effects: Equivalent to `flat_set(cont)` and `flat_set(cont, comp)`, respectively, except that `c` is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -5- Complexity: Same as `flat_set(cont)` and `flat_set(cont, comp)`, respectively.

   >     template<class Allocator>
   >       flat_set(sorted_unique_t s, const container_type& cont, const Allocator& a);
   >     template<class Allocator>
   >       flat_set(sorted_unique_t s, const container_type& cont, const key_compare& comp, const Allocator& a);

   > > -6- Constraints: `uses_allocator_v<container_type, Allocator>` is `true`.

   > > -7- Effects: Equivalent to `flat_set(s, cont)` and `flat_set(s, cont, comp)`, respectively, except that `c` is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -8- Complexity: Linear.

8. Modify 23.6.12.2 [flat.multiset.defn] as indicated:

   >     namespace std {
   >       template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
   >       class flat_multiset {
   >       public:
   >         […]

   >         // [flat.multiset.cons], constructors
   >         flat_multiset() : flat_multiset(key_compare()) { }

   >         explicit flat_multiset(container_type cont, const key_compare& comp = key_compare());
   >         template<class Allocator>
   >           flat_multiset(const container_type& cont, const Allocator& a);
   >         template<class Allocator>
   >           flat_multiset(const container_type& cont, const key_compare& comp, const Allocator& a);

   >         flat_multiset(sorted_equivalent_t, container_type cont, const key_compare& comp = key_compare())
   >           : c(std::move(cont)), compare(compkey_compare()) { }
   >         template<class Allocator>
   >           flat_multiset(sorted_equivalent_t, const container_type& cont, const Allocator& a);
   >         template<class Allocator>
   >           flat_multiset(sorted_equivalent_t, const container_type& cont,
   >                         const key_compare& comp, const Allocator& a);

   >         […]
   >       };

   >       template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>>
   >         flat_multiset(KeyContainer, Compare = Compare())
   >           -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>;
   >       template<class KeyContainer, class Allocator>
   >         flat_multiset(KeyContainer, Allocator)
   >           -> flat_multiset<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>;
   >       template<class KeyContainer, class Compare, class Allocator>
   >         flat_multiset(KeyContainer, Compare, Allocator)
   >           -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>;

   >       template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>>
   >         flat_multiset(sorted_equivalent_t, KeyContainer, Compare = Compare())
   >           -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>;
   >       template<class KeyContainer, class Allocator>
   >         flat_multiset(sorted_equivalent_t, KeyContainer, Allocator)
   >           -> flat_multiset<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>;
   >       template<class KeyContainer, class Compare, class Allocator>
   >         flat_multiset(sorted_equivalent_t, KeyContainer, Compare, Allocator)
   >           -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>;

   >       […]
   >     }

9. Modify 23.6.12.3 [flat.multiset.cons] as indicated:

   >     flat_multiset(container_type cont, const key_compare& comp = key_compare());

   > > -1- Effects: Initializes `c` with `std::move(cont)` and `compare` with `comp` , value-initializes `compare`, and sorts the range `[begin(), end())` with respect to `compare`.

   > > -2- Complexity: Linear in N if `cont` is already sorted with respect to `compare` and otherwise N log N, where N is the value of `cont.size()` before this call.

   >     template<class Allocator>
   >       flat_multiset(const container_type& cont, const Allocator& a);
   >     template<class Allocator>
   >       flat_multiset(const container_type& cont, const key_compare& comp, const Allocator& a);

   > > -3- Constraints: `uses_allocator_v<container_type, Allocator>` is `true`.

   > > -4- Effects: Equivalent to `flat_multiset(cont)` and `flat_multiset(cont, comp)`, respectively, except that `c` is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -5- Complexity: Same as `flat_multiset(cont)` and `flat_multiset(cont, comp)`, respectively.

   >     template<class Allocator>
   >       flat_multiset(sorted_equivalent_t s, const container_type& cont, const Allocator& a);
   >     template<class Allocator>
   >       flat_multiset(sorted_equivalent_t s, const container_type& cont, const key_compare& comp, const Allocator& a);

   > > -6- Constraints: `uses_allocator_v<container_type, Allocator>` is `true`.

   > > -7- Effects: Equivalent to `flat_multiset(s, cont)` and `flat_multiset(s, cont, comp)`, respectively, except that `c` is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -8- Complexity: Linear.

### 3807(i). The feature test macro for `ranges::find_last` should be renamed

Section: 17.3.2 [version.syn] Status: C++23 Submitter: Daniel Marshall Opened: 2022-11-02 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++23 status.

Discussion:

The current feature test macro is `__cpp_lib_find_last` which is inconsistent with almost all other ranges algorithms which are in the form `__cpp_lib_ranges_xxx`.

Proposed resolution is to rename the macro to `__cpp_lib_ranges_find_last`.

[Kona 2022-11-12; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 17.3.2 [version.syn], header `<version>` synopsis, as indicated:

   >     […]
   >     #define __cpp_lib_filesystem              201703L // also in <filesystem>
   >     #define __cpp_lib_ranges_find_last        202207L // also in <algorithm>
   >     #define __cpp_lib_flat_map                202207L // also in <flat_map>
   >     […]

### 3809(i). Is `std::subtract_with_carry_engine<uint16_t>` supposed to work?

Section: 29.5.4.4 [rand.eng.sub] Status: C++26 Submitter: Jonathan Wakely Opened: 2022-11-02 Last modified: 2026-06-09

Priority: 3

View all other issues in [rand.eng.sub].

View all issues with C++26 status.

Discussion:

The standard requires `subtract_with_carry_engine<T>` to use:

>     linear_congruential_engine<T, 40014u, 0u, 2147483563u>

where each of those values is converted to `T`.

This appears to mean `subtract_with_carry_engine` cannot be used with `uint16_t`, because `2147483563u` cannot be converted to `uint16_t` without narrowing.

What is the intention here? Should it be ill-formed? Should the seed engine be `linear_congruential_engine<uint_least32_t, …>` instead? The values from the `linear_congruential_engine` are used modulo 2^32 so getting 64-bit values from it is pointless, and getting 16-bit values from it doesn't compile.

[Kona 2022-11-12; Set priority to 3]

[Kona 2022-11-25; Jonathan provides wording]

[2023-05; reflector poll]

Status to Tentatively Ready after six votes in favour.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

[This resolution had to be fixed by 4014(i)]

Proposed resolution:

This wording is relative to N4917.

1. Modify the class synopsis in 29.5.4.4 [rand.eng.sub] as indicated:

   >     namespace std {
   >       template<class UIntType, size_t w, size_t s, size_t r>
   >       class subtract_with_carry_engine {
   >       public:
   >         // types
   >         using result_type = UIntType;
   >         // engine characteristics
   >         static constexpr size_t word_size = w;
   >         static constexpr size_t short_lag = s;
   >         static constexpr size_t long_lag = r;
   >         static constexpr result_type min() { return 0; }
   >         static constexpr result_type max() { return m − 1; }
   >         static constexpr result_typeuint_least32_t default_seed = 19780503u;
   >         // constructors and seeding functions
   >         subtract_with_carry_engine() : subtract_with_carry_engine(default_seed0u) {}
   >         explicit subtract_with_carry_engine(result_type value);
   >         template<class Sseq> explicit subtract_with_carry_engine(Sseq& q);
   >         void seed(result_type value = default_seed0u);
   >         template<class Sseq> void seed(Sseq& q);

2. Modify 29.5.4.4 [rand.eng.sub] p7 as indicated:

   >     explicit subtract_with_carry_engine(result_type value);

   > -7- Effects: Sets the values of X − r , … , X − 1 , in that order, as specified below. If X − 1 is then 0 , sets c to 1 ; otherwise sets c to 0 .

   > To set the values X k , first construct `e`, a `linear_congruential_engine` object, as if by the following definition:

   > >     linear_congruential_engine<result_typeuint_least32_t,
   > >                                40014u,0u,2147483563u> e(value == 0u ? default_seed : value);

   > Then, to set each X k , obtain new values z 0 , … , z n − 1 from n = ⌈ w ⁄ 32 ⌉ successive invocations of `e`. Set X k to ( ∑ j = 0 n − 1 z j ∙ 2 32 j ) mod m .

### 3810(i). CTAD for `std::basic_format_args`

Section: 28.5.8.3 [format.args] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-11-03 Last modified: 2023-11-22

Priority: 3

View all other issues in [format.args].

View all issues with C++23 status.

Discussion:

It seems desirable for this should work:

>     auto args_store = std::make_format_args<C>(1,2,3);
>     // …
>     std::basic_format_args args = args_store;

i.e. CTAD should deduce the `Context` argument from the `fmt-store-args<C, int, int, int>` object returned by `make_format_args`.

Another example (from Tomasz Kamiński):

Given:

>     template<typename Context>
>     void foo(basic_format_args<Context> c);

>     foo(make_format_args<SomeContext>(…)); // won't work
>     foo(basic_format_args(make_format_args<SomeContext>(…))); // should work

Since `fmt-store-args` is exposition-only, it's not entirely clear that it must have exactly the form shown in 28.5.8.2 [format.arg.store]. E.g. maybe it can have different template arguments, or could be a nested type defined inside `basic_format_args`. I don't know how much of the exposition-only spec is actually required for conformance. If CTAD is already intended to be required, it's a bit subtle.

If we want the CTAD to work (and I think it's nice if it does) we could make that explicit by adding a deduction guide.

[Kona 2022-11-12; Set priority to 3, status to LEWG]

[2023-01-10; LEWG telecon]

Unanimous consensus in favor.

[Issaquah 2023-02-06; LWG]

Unanimous consent (9/0/0) to move to Immediate for C++23.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 28.5.8.3 [format.args] as indicated:

   >     namespace std {
   >       template<class Context>
   >       class basic_format_args {
   >         size_t size_;                           // exposition only
   >         const basic_format_arg<Context>* data_; // exposition only

   >       public:
   >         basic_format_args() noexcept;

   >         template<class... Args>
   >           basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;

   >         basic_format_arg<Context> get(size_t i) const noexcept;
   >       };

   >       template<class Context, class... Args>
   >         basic_format_args(format-arg-store<Context, Args...>) -> basic_format_args<Context>;
   >     }

### 3811(i). `views::as_const` on `ref_view<T>` should return `ref_view<const T>`

Section: 25.7.22.1 [range.as.const.overview] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-03 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.as.const.overview].

View all issues with C++23 status.

Discussion:

For `v` being a non-const lvalue of type `std::vector<int>`, `views::as_const(v)` produces `ref_view<std::vector<int> const>`. However, when `v` is converted to `ref_view` by using `views::all`, `views::as_const(views::all(v))` produces `as_const_view<ref_view<std::vector<int>>>`.

Invoking `views::as_const` on `ref_view<T>` should produce `ref_view<const T>` when `const T` models a constant range. This will reduce the number of instantiations, and make a behavior of `views::as_const` consistent on references and `ref_view` to containers.

[Kona 2022-11-08; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.22.1 [range.as.const.overview] as indicated:

   > [Drafting note: If we have `ref_view<V>`, when `V` is constant propagating view (`single_view`, `owning_view`), we still can (and should) produce `ref_view<V const>`. This wording achieves that.]

   > -2- The name `views::as_const` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Let `E` be an expression, let `T` be `decltype((E))`, and let `U` be `remove_cvref_t<T>`. The expression `views::as_const(E)` is expression-equivalent to:

   > 1. (2.1) — If `views::all_t<T>` models `constant_range`, then `views::all(E)`.

   > 2. (2.2) — Otherwise, if `U` denotes `span<X, Extent>` for some type `X` and some extent `Extent`, then `span<const X, Extent>(E)`.

   > 3. (2.?) — Otherwise, if `U` denotes `ref_view<X>` for some type `X` and `const X` models `constant_range`, then `ref_view(static_cast<const X&>(E.base()))`.

   > 4. (2.3) — Otherwise, if `E` is an lvalue, `const U` models `constant_range`, and `U` does not model `view`, then `ref_view(static_cast<const U&>(E))`.

   > 5. (2.4) — Otherwise, `as_const_view(E)`.

### 3814(i). Add freestanding items requested by NB comments

Section: 20.2.2 [memory.syn], 25.2 [ranges.syn], 32.5.2 [atomics.syn] Status: C++23 Submitter: Ben Craig Opened: 2022-11-06 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [memory.syn].

View all issues with C++23 status.

Discussion:

This addresses the following NB comments:

- GB-085 (20.2.2 [memory.syn] `start_lifetime_as` and `start_lifetime_as_array` should be freestanding)

- GB-110 (25.2 [ranges.syn] New views should be freestanding (`repeat`, `stride`, `cartesian_product`))

- (partial) GB-130 (32.5.2 [atomics.syn] `memory_order_acquire` etc should be freestanding)

The explicit lifetime management functions requested by GB-085 have not been reviewed by LEWG in the context of freestanding, but they seem non-controversial in that context. None of the requested lifetime management functions run any code. I believe these were missed in post-merge conflict searches because the papers weren't targeted to LEWG or LWG at the time of those searches.

The ranges facilities requested by GB-110 have been reviewed on the LEWG mailing list in the context of freestanding. P1642R11 mentions the `repeat`, `stride`, and `cartesian_product` papers in "Potential Post-LEWG merge conflicts". All were discussed in an April 2022 reflector discussion and received six votes in favor of allowing these papers into freestanding, with no opposition.

The atomics facilities requested by GB-130 are essentially new names for existing facilities. Marking these as freestanding isn't concerning. There are concerns in GB-130 dealing with the specification details of freestanding enums, but those concerns won't be addressed in this issue.

[Kona 2022-11-07; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     […]
   >     // 20.2.6 [obj.lifetime], explicit lifetime management
   >     template<class T>
   >       T* start_lifetime_as(void* p) noexcept; // freestanding
   >     template<class T>
   >       const T* start_lifetime_as(const void* p) noexcept; // freestanding
   >     template<class T>
   >       volatile T* start_lifetime_as(volatile void* p) noexcept; // freestanding
   >     template<class T>
   >       const volatile T* start_lifetime_as(const volatile void* p) noexcept; // freestanding
   >     template<class T>
   >       T* start_lifetime_as_array(void* p, size_t n) noexcept; // freestanding
   >     template<class T>
   >       const T* start_lifetime_as_array(const void* p, size_t n) noexcept; // freestanding
   >     template<class T>
   >       volatile T* start_lifetime_as_array(volatile void* p, size_t n) noexcept; // freestanding
   >     template<class T>
   >       const volatile T* start_lifetime_as_array(const volatile void* p, size_t n) noexcept; // freestanding
   >     […]

2. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     […]
   >     // 25.6.5 [range.repeat], repeat view
   >     template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
   >       requires (is_object_v<W> && same_as<W, remove_cv_t<W>>
   >         && (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>))
   >     class repeat_view; // freestanding

   >     namespace views { inline constexpr unspecified repeat = unspecified; } // freestanding
   >     […]
   >     // 25.7.32 [range.stride], stride view
   >     template<input_range V>
   >       requires view<V>
   >     class stride_view; // freestanding

   >     template<class V>
   >       inline constexpr bool enable_borrowed_range<stride_view<V>> = enable_borrowed_range<V>; // freestanding

   >     namespace views { inline constexpr unspecified stride = unspecified; } // freestanding

   >     // 25.7.33 [range.cartesian], cartesian product view
   >     template<input_range First, forward_range... Vs>
   >       requires (view<First> && ... && view<Vs>)
   >     class cartesian_product_view; // freestanding

   >     namespace views { inline constexpr unspecified cartesian_product = unspecified; } // freestanding
   >     […]

3. Modify 32.5.2 [atomics.syn], header `<atomic>` synopsis, as indicated:

   >     namespace std {
   >       // 32.5.4 [atomics.order], order and consistency
   >       enum class memory_order : unspecified; // freestanding
   >       inline constexpr memory_order memory_order_relaxed = memory_order::relaxed; // freestanding
   >       inline constexpr memory_order memory_order_consume = memory_order::consume; // freestanding
   >       inline constexpr memory_order memory_order_acquire = memory_order::acquire; // freestanding
   >       inline constexpr memory_order memory_order_release = memory_order::release; // freestanding
   >       inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel; // freestanding
   >       inline constexpr memory_order memory_order_seq_cst = memory_order::seq_cst; // freestanding
   >       […]
   >     }

### 3815(i). Freestanding enumerators specification is lacking

Section: 16.3.3.7 [freestanding.item], 32.5.2 [atomics.syn] Status: Resolved Submitter: Ben Craig Opened: 2022-11-06 Last modified: 2023-02-07

Priority: Not Prioritized

View all other issues in [freestanding.item].

View all issues with Resolved status.

Discussion:

This is a partial resolution of GB-130 (32.5.2 [atomics.syn] `memory_order_acquire` etc should be freestanding).

It's not entirely clear whether the enumerators for the `std::memory_order` enum type are also freestanding, as those enumerators aren't shown in the synopsis, only in 32.5.4 [atomics.order].

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917 assuming that LWG 3753(i) has been accepted (freestanding entity -> freestanding item).

> > [Drafting Note: Four mutually exclusive options are prepared, depicted below by Option A, Option B, Option C, and Option D, respectively.]

> Option A: Not a defect; no change required. Enumerators of freestanding enums are already part of freestanding with the current wording. [freestanding.entity]#2 says the requirements of freestanding entities are the same as for hosted entities. The existence of the right enumerators (with the appropriate name scoping) are part of the requirements of the enum type.

> Option B: Not a defect; provide a clarifying note. Same rationale as above, but we make this stance clear in [freestanding.entity]#2.

> 1. Modify [freestanding.entity] (Which has been renamed to [freestanding.item]) as indicated:

>    > 16.3.3.6 Freestanding items [freestanding.item]

>    > […]

>    > -2- Unless otherwise specified, the requirements on freestanding items on a freestanding implementation are the same as the corresponding requirements in a hosted implementation.

>    > [Note: An enumerator places requirements on its enumeration. Freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations. — end note]

>    > […]

> Option C: Add additional normative wording that makes enumerators freestanding if their enumeration types are freestanding.

> 1. Modify [freestanding.entity] (Which has been renamed to [freestanding.item]) as indicated:

>    > 16.3.3.6 Freestanding items [freestanding.item]

>    > -1- A freestanding item is an entity or macro definition that is present in a freestanding implementation and a hosted implementation.

>    > -?- An enumerator of a freestanding item enumeration is a freestanding item.

>    > […]

> Option D: This is Option C, plus it handles class types as well. If enumerators aren't automatically included with the existing wording, then arguably, neither are class type members.

> 1. Modify [freestanding.entity] (Which has been renamed to [freestanding.item]) as indicated:

>    > 16.3.3.6 Freestanding items [freestanding.item]

>    > -1- A freestanding item is an entity or macro definition that is present in a freestanding implementation and a hosted implementation.

>    > -?- An enumerator of a freestanding item enumeration is a freestanding item.

>    > -?- Members of freestanding item class types are freestanding items.

>    > […]

[2022-11-08; Kona - move to open]

This will be resolved by the combined resolution in 3753(i).

[2022-11-22 Resolved by 3753(i) accepted in Kona. Status changed: Open → Resolved.]

Proposed resolution:

### 3816(i). `flat_map` and `flat_multimap` should impose sequence container requirements

Section: 23.6.8.1 [flat.map.overview], 23.6.9.1 [flat.multimap.overview] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-08 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [flat.map.overview].

View all issues with C++23 status.

Discussion:

This is resolution of US 42-103 (23.6.8.1 [flat.map.overview] p6 23.6.9.1 [flat.multimap.overview] p6 Clearing when restoring invariants).

Currently both 23.6.8.1 [flat.map.overview] p7 and 23.6.9.1 [flat.multimap.overview] p7 claims that `flat_(multi)map` supports "Any sequence container (23.2.4 [sequence.reqmts]) `C` supporting Cpp17RandomAccessIterator", which arguably includes `std::array` (see LWG 617(i)). This is incorrect as `std::array` does not provide operations required to restored these adaptors invariant, including `clear`. We should require that `C` meets sequence container requirements, and we state that fact explicitly in 23.3.3.1 [array.overview] p3: "An `array` meets some of the requirements of a sequence container (23.2.4 [sequence.reqmts])".

[Kona 2022-11-08; Move to Immediate status]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 23.6.8.1 [flat.map.overview] as indicated:

   > -7- Any sequence container (23.2.4 [sequence.reqmts])type `C` supporting Cpp17RandomAccessIteratorthat meets sequence container requirements (23.2.4 [sequence.reqmts]) can be used to instantiate `flat_map`, as long as `C::iterator` meets the Cpp17RandomAccessIterator requirements and invocations of member functions `C::size` and `C::max_size` do not exit via an exception. In particular, `vector` (23.3.13 [vector]) and `deque` (23.3.5 [deque]) can be used.

2. Modify 23.6.9.1 [flat.multimap.overview] as indicated:

   > -7- Any sequence container (23.2.4 [sequence.reqmts])type `C` supporting Cpp17RandomAccessIteratorthat meets sequence container requirements (23.2.4 [sequence.reqmts]) can be used to instantiate `flat_multimap`, as long as `C::iterator` meets the Cpp17RandomAccessIterator requirements and invocations of member functions `C::size` and `C::max_size` do not exit via an exception. In particular, `vector` (23.3.13 [vector]) and `deque` (23.3.5 [deque]) can be used.

### 3817(i). Missing preconditions on `forward_list` modifiers

Section: 23.3.7.5 [forward.list.modifiers] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-08 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [forward.list.modifiers].

View all issues with C++23 status.

Discussion:

This is resolution of GB-101 (23.3.7.5 [forward.list.modifiers] p12,15,20,21 Missing preconditions on forward_list modifiers).

Some of the modifiers to `forward_list` are special to that container and accordingly are not described in 23.2 [container.requirements]. Specifically, `insert_after` (iterator overload), `insert_range_after` and `emplace_after` do not verify that the `value_type` is Cpp17EmplaceConstructible from the appropriate argument(s). Furthermore `insert_after` (value overloads) are missing Cpp17CopyInsertable/Cpp17MoveInsertable requirements.

[Kona 2022-11-08; Move to Ready]

[Kona 2022-11-12; Correct status to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 23.3.7.5 [forward.list.modifiers] as indicated:

   > [Drafting note: `emplace_front`, `push_front`, and `prepend_range` are already covered by 23.2 [container.requirements]. ]

       iterator insert_after(const_iterator position, const T& x);

   > -?- Preconditions: `T` is Cpp17CopyInsertable into `forward_list`. `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`, `end()`).

   > -?- Effects: Inserts a copy of `x` after `position`.

   > -?- Returns: An iterator pointing to the copy of `x`.

       iterator insert_after(const_iterator position, T&& x);

   > -6- Preconditions: `T` is Cpp17MoveInsertable into `forward_list`. `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`, `end()`).

   > -7- Effects: Inserts a copy of `x` after `position`.

   > -8- Returns: An iterator pointing to the copy of `x`.

       iterator insert_after(const_iterator position, size_type n, const T& x);

   > -9- Preconditions: `T` is Cpp17CopyInsertable into `forward_list`. `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`, `end()`).

   > -10- Effects: Inserts `n` copies of `x` after `position`.

   > -11- Returns: An iterator pointing to the last inserted copy of `x`, or `position` if `n == 0` is `true`.

       template<class InputIterator>
         iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

   > -12- Preconditions: `T` is Cpp17EmplaceConstructible into `forward_list` from `*first`. `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`, `end()`). Neither `first` nor `last` are iterators in `*this`.

   > -13- Effects: Inserts copies of elements in [`first`, `last`) after `position`.

   > -14- Returns: An iterator pointing to the last inserted element, or `position` if `first == last` is `true`.

       template<container-compatible-range<T> R>
         iterator insert_after(const_iterator position, R&& rg);

   > -15- Preconditions: `T` is Cpp17EmplaceConstructible into `forward_list` from `*ranges::begin(rg)`. `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`, `end()`). `rg` and `*this` do not overlap.

   > -16- Effects: Inserts copies of elements in range `rg` after `position`.

   > -17- Returns: An iterator pointing to the last inserted element, or `position` if `rg` is empty.

       iterator insert_after(const_iterator position, initializer_list<T> il);

   > -18- Effects: Equivalent to: `return` `insert_after(position, il.begin(), il.end())`.;

   > -19- Returns: An iterator pointing to the last inserted element or `position` if `il` is empty.

       template<class... Args>
         iterator emplace_after(const_iterator position, Args&&... args);

   > -20- Preconditions: `T` is Cpp17EmplaceConstructible into `forward_list` from `std::forward<Args>(args)...`. `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`, `end()`).

   > -21- Effects: Inserts an object of type `value_type` constructeddirect-non-list-initialized with `value_type(std::forward<Args>(args)...)` after `position`.

   > -22- Returns: An iterator pointing to the new object.

### 3818(i). Exposition-only concepts are not described in library intro

Section: 16.3.3 [conventions] Status: C++23 Submitter: Tim Song Opened: 2022-11-08 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

This is the resolution for GB-074.

The comment is:

> [expos.only.func] introduces exposition-only function templates. [expos.only.types] introduces exposition-only types. 16.3.3.6 [objects.within.classes] introduces exposition-only private members.

> There is nothing about exposition-only concepts, despite them being used extensively in the library clauses.

> The same problem exists for exposition-only variable templates.

[Kona 2022-11-08; Move to Immediate status]

Previous resolution [SUPERSEDED]:

> 1. Modify [expos.only.func] as indicated, changing the stable name:

>    > 16.3.3.2 Exposition-only functionsentities [expos.only.funcentity]

>    > -1- Several function templatesentities defined in 17 [support] through 32 [thread] and D [depr] are only defined for the purpose of exposition. The declaration of such a functionan entity is followed by a comment ending in exposition only.

> 2. Strike [expos.only.types] as redundant:

>    > 16.3.3.3.2 Exposition-only types [expos.only.types]

>    > -1- Several types defined in 17 [support] through 32 [thread] and D [depr] are defined for the purpose of exposition. The declaration of such a type is followed by a comment ending in exposition only.

>    > [Example 1:

>    > >     namespace std {
>    > >       extern "C" using some-handler = int(int, void*, double);  // exposition only
>    > >     }

>    > The type placeholder `some-handler` can now be used to specify a function that takes a callback parameter with C language linkage. — end example]

[2022-11-09 Tim reopens]

During LWG review of 3753(i), it was pointed out that typedef-names are not necessarily entities.

[Kona 2022-11-11; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify [expos.only.func] as indicated, changing the stable name:

   > 16.3.3.2 Exposition-only functionsentities, etc. [expos.only.funcentity]

   > -1- Several function templatesentities and typedef-names defined in 17 [support] through 32 [thread] and D [depr] are only defined for the purpose of exposition. The declaration of such a functionan entity or typedef-name is followed by a comment ending in exposition only.

2. Strike [expos.only.types] as redundant:

   > 16.3.3.3.2 Exposition-only types [expos.only.types]

   > -1- Several types defined in 17 [support] through 32 [thread] and D [depr] are defined for the purpose of exposition. The declaration of such a type is followed by a comment ending in exposition only.

   > [Example 1:

   > >     namespace std {
   > >       extern "C" using some-handler = int(int, void*, double);  // exposition only
   > >     }

   > The type placeholder `some-handler` can now be used to specify a function that takes a callback parameter with C language linkage. — end example]

### 3819(i). `reference_meows_from_temporary` should not use `is_meowible`

Section: 21.3.5.4 [meta.unary.prop] Status: C++23 Submitter: Tim Song Opened: 2022-11-08 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

View all other issues in [meta.unary.prop].

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Discussion:

The intent of P2255R2 is for the `reference_meows_from_temporary` traits to fully support cases where a prvalue is used as the source. Unfortunately the wording fails to do so because it tries to use the `is_meowible` traits to say "the initialization is well-formed", but those traits only consider initialization from xvalues, not prvalues. For example, given:

>     struct U {
>       U();
>       U(U&&) = delete;
>     };

>     struct T {
>       T(U);
>     };

`reference_constructs_from_temporary_v<const T&, U>` should be true, but is currently defined as `false`. We need to spell out the "is well-formed" condition directly.

[Kona 2022-11-08; Move to Tentatively Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

> [Drafting note: The note is already repeated every time we talk about "immediate context".]

1. Modify 21.3.3 [meta.type.synop], Table 46 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:

   > Table 46: Type property predicates [tab:meta.unary.prop]
   > Template | Condition | Preconditions
   > --- | --- | ---
   > `…`
   > `template<class T, class U> struct reference_constructs_from_temporary;` | `conjunction_v<is_reference<T>, is_constructible<T, U>>` is `true` `T` is a reference type, and the initialization `T t(VAL<U>);` is well-formed and binds `t` to a temporary object whose lifetime is extended (6.8.7 [class.temporary]). Access checking is performed as if in a context unrelated to `T` and `U`. Only the validity of the immediate context of the variable initialization is considered. [Note ?: The initialization can result in effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such effects are not in the "immediate context" and can result in the program being ill-formed. — end note] | `T` and `U` shall be complete types, cv `void`, or arrays of unknown bound.
   > `template<class T, class U> struct reference_converts_from_temporary;` | `conjunction_v<is_reference<T>, is_convertible<U, T>>` is `true` `T` is a reference type, and the initialization `T t = VAL<U>;` is well-formed and binds `t` to a temporary object whose lifetime is extended (6.8.7 [class.temporary]). Access checking is performed as if in a context unrelated to `T` and `U`. Only the validity of the immediate context of the variable initialization is considered. [Note ?: The initialization can result in effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such effects are not in the "immediate context" and can result in the program being ill-formed. — end note] | `T` and `U` shall be complete types, cv `void`, or arrays of unknown bound.

### 3820(i). `cartesian_product_view::iterator::prev` is not quite right

Section: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-11-08 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.cartesian.iterator].

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Discussion:

Currently, `cartesian_product_view::iterator::prev` has the following Effects:

>     auto& it = std::get<N>(current_);
>     if (it == ranges::begin(std::get<N>(parent_->bases_))) {
>       it = cartesian-common-arg-end(std::get<N>(parent_->bases_));
>       if constexpr (N > 0) {
>         prev<N - 1>();
>       }
>     }
>     --it;

which decrements the underlying iterator one by one using recursion. However, when `N == 0`, it still detects if the first iterator has reached the beginning and assigns it to the end, which is not only unnecessary, but also causes `cartesian-common-arg-end` to be applied to the first range, making it ill-formed in some cases, for example:

>     #include <ranges>

>     int main() {
>       auto r = std::views::cartesian_product(std::views::iota(0));
>       r.begin() += 3; // hard error
>     }

This is because, for the first range, `cartesian_product_view::iterator::operator+=` only requires it to model `random_access_range`. However, when `x` is negative, this function will call `prev` and indirectly calls `cartesian-common-arg-end`, since the latter constrains its argument to satisfy `cartesian-product-common-arg`, that is, `common_range<R> || (sized_range<R> && random_access_range<R>)`, which is not the case for the unbounded `iota_view`, resulting in a hard error in `prev`'s function body.

The proposed resolution changes the position of the `if constexpr` so that we just decrement the first iterator and nothing else.

[Kona 2022-11-08; Move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify [ranges.cartesian.iterator] as indicated:

   >     template<size_t N = sizeof...(Vs)>
   >       constexpr void prev();

   > > -6- Effects: Equivalent to:

   > >     auto& it = std::get<N>(current_);
   > >     if constexpr (N > 0) {
   > >       if (it == ranges::begin(std::get<N>(parent_->bases_))) {
   > >         it = cartesian-common-arg-end(std::get<N>(parent_->bases_));
   > >         if constexpr (N > 0) {
   > >           prev<N - 1>();
   > >         }
   > >       }
   > >     }
   > >     --it;

### 3821(i). `uses_allocator_construction_args` should have overload for `pair-like`

Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Tim Song Opened: 2022-11-08 Last modified: 2023-11-22

Priority: 2

View all other issues in [allocator.uses.construction].

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Discussion:

P2165R4 added a `pair-like` constructor to `std::pair` but didn't add a corresponding `uses_allocator_construction_args` overload. It was in P2165R3 but incorrectly removed during the small group review.

Without LWG 3525(i), not having the overload would have caused emplacing a `pair-like` into a `pmr::vector<pair>` to be outright ill-formed.

With that issue's resolution, in cases where the constructor is not explicit we would create a temporary pair and then do uses-allocator construction using its pieces, and it still won't work when the constructor is explicit.

We should just do this properly.

[2022-11-09 Tim updates wording following LWG review]

During review of this issue LWG noticed that neither the constructor nor the new overload should accept `subrange`.

The `remove_cv_t` in the new paragraph is added for consistency with LWG 3677(i).

[Kona 2022-11-12; Set priority to 2]

[2023-01-11; LWG telecon]

Replace `P` with `U` in p17 and set status to Tentatively Ready (poll result: 8/0/0).

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917 after the application of LWG 3677(i).

1. Edit 22.3.2 [pairs.pair] as indicated:

   >     template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>& p);
   >     template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>& p);
   >     template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>&& p);
   >     template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>&& p);
   >     template<pair-like P> constexpr explicit(see below) pair(P&& p);

   > > -14- Let `FWD(u)` be `static_cast<decltype(u)>(u)`.

   > > -15- Constraints:

   > > 1. (15.?) — For the last overload, `remove_cvref_t<P>` is not a specialization of `ranges::subrange`,

   > > 2. (15.1) — `is_constructible_v<T1, decltype(get<0>(FWD(p)))>` is `true` and

   > > 3. (15.2) — `is_constructible_v<T2, decltype(get<1>(FWD(p)))>` is `true`.

   > > -16- Effects: Initializes `first` with `get<0>(FWD(p))` and `second` with `get<1>(FWD(p))`.

2. Edit 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 20.2.8.2 [allocator.uses.construction], uses-allocator construction
   >       […]
   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         pair<U, V>& pr) noexcept;

   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         const pair<U, V>& pr) noexcept;

   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         pair<U, V>&& pr) noexcept;

   >       template<class T, class Alloc, class U, class V>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc,
   >                                                         const pair<U, V>&& pr) noexcept;
   >       template<class T, class Alloc, pair-like P>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, P&& p) noexcept;

   >       template<class T, class Alloc, class U>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;
   >       […]
   >     }

3. Add the following to 20.2.8.2 [allocator.uses.construction]:

   >       template<class T, class Alloc, pair-like P>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, P&& p) noexcept;

   > > -?- Constraints: `remove_cv_t<T>` is a specialization of `pair` and `remove_cvref_t<P>` is not a specialization of `ranges::subrange`.

   > > -?- Effects: Equivalent to:

   > > >     return uses_allocator_construction_args<T>(alloc, piecewise_construct,
   > > >                                                 forward_as_tuple(get<0>(std::forward<P>(p))),
   > > >                                                 forward_as_tuple(get<1>(std::forward<P>(p))));

4. Edit 20.2.8.2 [allocator.uses.construction] p17:

   >       template<class T, class Alloc, class U>
   >         constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;

   > > -16- Let `FUN` be the function template:

   > > >       template<class A, class B>
   > > >       void FUN(const pair<A, B>&);

   > > -17- Constraints: `remove_cv_t<T>` is a specialization of `pair`, and either:

   > > 1. (17.1) — `remove_cvref_t<U>` is a specialization of `ranges::subrange`, or

   > > 2. (17.2) — `U` does not satisfy `pair-like` and the expression `FUN(u)` is not well-formed when considered as an unevaluated operand..

### 3822(i). Avoiding normalization in `filesystem::weakly_canonical`

Section: 31.12.13.40 [fs.op.weakly.canonical] Status: C++23 Submitter: US Opened: 2022-11-08 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

This addresses NB comment US-60-125 (31.12.13.40 [fs.op.weakly.canonical] Avoiding normalization)

NB comment: "Implementations cannot avoid normalization because arbitrary file system changes may have occurred since any previous call. Proposed change: Remove the paragraph."

[Kona 2022-11-07; LWG review]

Discussion revolved around two different interpretations of the Remarks:

- It's normative recommendation for `path` objects to cache some kind of flag that indicates they are in a normalized form, which would be checked in `weakly_canonical` to avoid normalizing again. This is the interpretation assumed by the NB comment, which correctly notes that such caching would be unreliable.
- It's an optimization hint to implementors that they should not normalize the result if each component has been canonicalized one-by-one. That is, when canonicalizing `"/a/b/c"` by incrementally building up canonicalized components, if the entire path exists then the result will already have been normalized and should not be normalized again explicitly.

For the first interpretation, the recommendation is a bad recommendation and should be removed as suggested by the comment. For the second interpretation, we don't need to give hints to implementors about not doing unnecessary work; they already know they shouldn't do that. Either way, it should go.

[Kona 2022-11-09; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 31.12.13.40 [fs.op.weakly.canonical] as indicated:

   >     path filesystem::weakly_canonical(const path& p);
   >     path filesystem::weakly_canonical(const path& p, error_code& ec);

   > > -1- Effects: Using `status(p)` or `status(p, ec)`, respectively, to determine existence, return a path composed by `operator/=` from the result of calling `canonical()` with a path argument composed of the leading elements of `p` that exist, if any, followed by the elements of `p` that do not exist, if any. For the first form, `canonical()` is called without an `error_code` argument. For the second form, `canonical()` is called with `ec` as an `error_code` argument, and `path()` is returned at the first error occurrence, if any.

   > > -2- Postconditions: The returned path is in normal form (31.12.6.2 [fs.path.generic]).

   > > -3- Returns: `p` with symlinks resolved and the result normalized (31.12.6.2 [fs.path.generic]).

   > > -4- Throws: As specified in 31.12.5 [fs.err.report].

   > > -5- Remarks: Implementations should avoid unnecessary normalization such as when `canonical` has already been called on the entirety of `p`.

### 3823(i). Unnecessary precondition for `is_aggregate`

Section: 21.3.5.4 [meta.unary.prop] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-09 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

This is resolution of GB-090 (21.3.5.4 [meta.unary.prop] Unnecessary precondition for `is_aggregate`).

The precondition for `is_aggregate` is "`remove_all_extents_t<T>` shall be a complete type or cv `void`." This means that `is_aggregate_v<Incomplete[2]>` is undefined, but an array is always an aggregate, we don't need a complete element type to know that.

Historically the `is_aggregate` was introduced by LWG 2911(i) as part of the resolution of the NB comments. The comment proposed to introduce this trait with requirement "`remove_all_extents_t<T>` shall be a complete type, an array type, or (possibly cv-qualified) `void`.", that is close to resolution proposed in this issue. According to notes this was simplified during review, after realizing that `remove_all_extents_t<T>` is never an array type.

[Kona 2022-11-09; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 21.3.3 [meta.type.synop], Table 46 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:

   > Table 48: Type property predicates [tab:meta.unary.prop]
   > Template | Condition | Preconditions
   > --- | --- | ---
   > `…`
   > `template<class T, class U> struct is_aggregate;` | `T` is an aggregate type (9.5.2 [dcl.init.aggr]) | `remove_all_extents_t<T>` shall be an array type, a complete type, or cv `void`.

### 3824(i). Number of `bind` placeholders is underspecified

Section: 22.10.15.5 [func.bind.place] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-09 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

This is resolution of GB-95 (22.10.15.5 [func.bind.place] Number of `bind` placeholders is underspecified).

22.10.2 [functional.syn] and 22.10.15.5 [func.bind.place] both contain a comment that says "M is the implementation-defined number of placeholders". There is no wording anywhere to say that there is any such number, only that comment.

[Kona 2022-11-09; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 22.10.15.5 [func.bind.place] as indicated:

   >     namespace std::placeholders {
   >       // M is the implementation-defined number of placeholders
   >       see below _1;
   >       see below _2;
   >                   .
   >                   .
   >                   .
   >       see below _M;
   >     }

   > > -?- The number `M` of placeholders is implementation-defined.

   > > -1- All placeholder types meet the Cpp17DefaultConstructible and Cpp17CopyConstructible requirements, and their default constructors and copy/move constructors are constexpr functions that do not throw exceptions. It is implementation-defined whether placeholder types meet the Cpp17CopyAssignable requirements, but if so, their copy assignment operators are constexpr functions that do not throw exceptions.

### 3825(i). Missing compile-time argument `id` check in `basic_format_parse_context::next_arg_id`

Section: 28.5.6.6 [format.parse.ctx] Status: C++23 Submitter: Victor Zverovich Opened: 2022-11-09 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

The definition of `check_arg_id` in 28.5.6.6 [format.parse.ctx] includes a (compile-time) argument `id` check in the Remarks element:

>     constexpr void check_arg_id(size_t id);

> > […]

> > Remarks: Call expressions where `id >= num_args_` are not core constant expressions (7.7 [expr.const]).

However, a similar check is missing from `next_arg_id` which means that there is no argument id validation in user-defined format specification parsing code that invokes this function (e.g. when parsing automatically indexed dynamic width).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 28.5.6.6 [format.parse.ctx] as indicated:

>    >     constexpr size_t next_arg_id();

>    > > -7- Effects: If `indexing_ != manual`, equivalent to:

>    > > >     if (indexing_ == unknown)
>    > > >       indexing_ = automatic;
>    > > >     return next_arg_id_++;

>    > > -8- Throws: `format_error` if `indexing_ == manual` which indicates mixing of automatic and manual argument indexing.

>    > > -?- Remarks: Call expressions where `next_arg_id_ >= num_args_` are not core constant expressions (7.7 [expr.const]).

[2022-11-11; Tomasz provide improved wording; Move to Open]

Clarify that the value of `next_arg_id_` is used, and add missing "is `true`."

[Kona 2022-11-11; move to Ready]

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 28.5.6.6 [format.parse.ctx] as indicated:

   >     constexpr size_t next_arg_id();

   > > -7- Effects: If `indexing_ != manual` is `true`, equivalent to:

   > > >     if (indexing_ == unknown)
   > > >       indexing_ = automatic;
   > > >     return next_arg_id_++;

   > > -8- Throws: `format_error` if `indexing_ == manual` is `true` which indicates mixing of automatic and manual argument indexing.

   > > -?- Remarks: Let cur-arg-id be the value of `next_arg_id_` prior to this call. Call expressions where `cur-arg-id >= num_args_` is `true` are not core constant expressions (7.7 [expr.const]).

   >     constexpr size_t check_arg_id(size_t id);

   > > -9- Effects: If `indexing_ != automatic` is `true`, equivalent to:

   > > >     if (indexing_ == unknown)
   > > >       indexing_ = manual;

   > > -10- Throws: `format_error` if `indexing_ == automatic` is `true` which indicates mixing of automatic and manual argument indexing.

   > > -11- Remarks: Call expressions where `id >= num_args_` is `true` are not core constant expressions (7.7 [expr.const]).

### 3826(i). Redundant specification [for overload of `yield_value`]

Section: 25.8.5 [coro.generator.promise] Status: C++23 Submitter: US Opened: 2022-11-10 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

This is in resolution of US 56-118 (25.8.5 [coro.generator.promise] Redundant specification).

[Paragraph 14] is redundant given [paragraphs] 13 and 12. Remove it.

Paragraphs 12 and 14 are identical: "Remarks: A yield-expression that calls this function has type `void` (7.6.17 [expr.yield])." Paragraph 13 states that the overload of `yield_value` that accepts `ranges::elements_of` for arbitrary ranges has "Effects: Equivalent to:" calling the overload of `yield_value` that accepts specializations of `generator`, which paragraph 12 specifies. Per 16.3.2.4 [structure.specifications] paragraph 4, the former overload "inherits" the Remarks of paragraph 12 making paragraph 14 redundant.

LWG is concerned that the redundancy is not immediately obvious — it depends on an understanding of how await expressions function — so we'd like to preserve comment despite that we agree that it is normatively redundant.

[2022-11-10 Casey provides wording]

[Kona 2022-11-11; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.8.5 [coro.generator.promise] as indicated:

   >     template<ranges::input_range R, class Alloc>
   >       requires convertible_to<ranges::range_reference_t<R>, yielded>
   >         auto yield_value(ranges::elements_of<R, Alloc> r) noexcept;

   > > -13- Effects: Equivalent to: […]

   > > -14- Remarks: [Note 1: A yield-expression that calls this function has type `void` (7.6.17 [expr.yield]). — end note]

### 3827(i). Deprecate `<stdalign.h>` and `<stdbool.h>` macros

Section: 17.15.4 [stdalign.h.syn], 17.15.5 [stdbool.h.syn] Status: C++23 Submitter: GB Opened: 2022-11-10 Last modified: 2024-01-29

Priority: Not Prioritized

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Discussion:

This is the resolution for NB comments:

- GB-081 17.14.4 [stdalign.h.syn] Deprecate contents of <stdalign.h>.
- GB-082 17.14.5 [stdbool.h.syn] Deprecate contents of <stdbool.h>.

GB-081:

C2x defines `alignas` as a keyword, so `<stdalign.h>` is empty in C2x. C++23 should deprecate the `__alignas_is_defined` macro now, rather than wait until a future C++ standard is based on C2x. That gives users longer to prepare for the removal of the macro.

Recommended change: Deprecate `__alignas_is_defined` and move it to Annex D. Maybe keep a note in 17.15.4 [stdalign.h.syn] that the macro is present but deprecated.

GB-082:

C2x supports `bool` as a built-in type, and `true` and `false` as keywords. Consequently, C2x marks the `__bool_true_false_are_defined` as obsolescent. C++23 should deprecate that attribute now, rather than wait until a future C++ standard is based on C2x. That gives users longer to prepare for the removal of the macro.

Recommended change: Deprecate `__bool_true_false_are_defined` and move it to Annex D. Maybe keep a note in 17.15.5 [stdbool.h.syn] that the macro is present but deprecated.

[Kona 2022-11-10; Jonathan provides wording]

[Kona 2022-11-10; Waiting for LEWG electronic polling]

[2022-11-08; Kona LEWG]

Strong consensus to accept GB-81. Strong consensus to accept GB-82.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 17.15.4 [stdalign.h.syn] as indicated:

>    > 17.14.4 Header `<stdalign.h>` synopsis [stdalign.h.syn]

>    > >     #define __alignas_is_defined 1

>    > -1- The contents of the C++ header `<stdalign.h>` are the same as the C standard library header `<stdalign.h>`, with the following changes: The header `<stdalign.h>` does not define a macro named `alignas`. The macro `__alignas_is_defined` (D.11 [depr.c.macros]) is deprecated.

>    > See also: ISO C 7.15

> 2. Modify 17.15.5 [stdbool.h.syn] as indicated:

>    > 17.14.5 Header `<stdbool.h>` synopsis [stdbool.h.syn]

>    > >     #define __bool_true_false_are_defined 1

>    > -1- The contents of the C++ header `<stdbool.h>` are the same as the C standard library header `<stdbool.h>`, with the following changes: The header `<stdbool.h>` does not define macros named `bool`, `true`, or `false`. The macro `__bool_true_false_are_defined` (D.11 [depr.c.macros]) is deprecated.

>    > See also: ISO C 7.18

> 3. Add a new subclause to Annex D [depr] between [depr.res.on.required] and D.14 [depr.relops], with this content:

>    > D?? Deprecated C macros [depr.c.macros]

>    > -1- The header `<stdalign.h>` has the following macro:

>    > >     #define __alignas_is_defined 1

>    > -2- The header `<stdbool.h>` has the following macro:

>    > >     #define __bool_true_false_are_defined 1

[Issaquah 2023-02-06; LWG]

Green text additions in Clause 17 was supposed to be adding notes. Drop them instead. Unanimous consent (14/0/0) to move to Immediate for C++23.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 17.15.4 [stdalign.h.syn] as indicated:

   > 17.14.4 Header `<stdalign.h>` synopsis [stdalign.h.syn]

   > >     #define __alignas_is_defined 1

   > -1- The contents of the C++ header `<stdalign.h>` are the same as the C standard library header `<stdalign.h>`, with the following changes: The header `<stdalign.h>` does not define a macro named `alignas`.

   > See also: ISO C 7.15

2. Modify 17.15.5 [stdbool.h.syn] as indicated:

   > 17.14.5 Header `<stdbool.h>` synopsis [stdbool.h.syn]

   > >     #define __bool_true_false_are_defined 1

   > -1- The contents of the C++ header `<stdbool.h>` are the same as the C standard library header `<stdbool.h>`, with the following changes: The header `<stdbool.h>` does not define macros named `bool`, `true`, or `false`.

   > See also: ISO C 7.18

3. Add a new subclause to Annex D [depr] between [depr.res.on.required] and D.14 [depr.relops], with this content:

   > D?? Deprecated C macros [depr.c.macros]

   > -1- The header `<stdalign.h>` has the following macro:

   > >     #define __alignas_is_defined 1

   > -2- The header `<stdbool.h>` has the following macro:

   > >     #define __bool_true_false_are_defined 1

### 3828(i). Sync `intmax_t` and `uintmax_t` with C2x

Section: 17.4.1 [cstdint.syn] Status: C++23 Submitter: GB Opened: 2022-11-10 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

This is the resolution for NB comment GB-080 17.4.1 [cstdint.syn] Sync `intmax_t` and `uintmax_t` with C2x.

With the approval of WG14 N2888 the next C standard will resolve the long-standing issue that implementations cannot support 128-bit integer types properly without ABI breaks. C++ should adopt the same fix now, rather than waiting until a future C++ standard is rebased on C2x.

31.13.2 [cinttypes.syn] also mentions those types, but doesn't need a change. The proposed change allows `intmax_t` to be an extended integer type of the same width as `long long`, in which case we'd still want those `abs` overloads.

Recommended change: Add to 31.13.2 [cinttypes.syn] p2 "except that `intmax_t` is not required to be able to represent all values of extended integer types wider than `long long`, and `uintmax_t` is not required to be able to represent all values of extended integer types wider than `unsigned long long`."

[Kona 2022-11-10; Waiting for LEWG electronic polling]

[2022-11; LEWG electronic polling]

Unanimous consensus in favor.

[Issaquah 2023-02-06; LWG]

Unanimous consent (13/0/0) to move to Immediate for C++23.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 31.13.2 [cinttypes.syn] as indicated:

   > -1- The contents and meaning of the header `<cinttypes>` are the same as the C standard library header `<inttypes.h>`, with the following changes:

   > 1. (1.1) — The header `<cinttypes>` includes the header `<cstdint>` instead of `<stdint.h>`, and

   > 2. (1.?) — `intmax_t` and `uintmax_t` are not required to be able to represent all values of extended integer types wider than `long long` and `unsigned long long` respectively, and

   > 3. (1.2) — if and only if the type `intmax_t` designates an extended integer type, the following function signatures are added:

   >    >     intmax_t abs(intmax_t);
   >    >     imaxdiv_t div(intmax_t, intmax_t);

   >    which shall have the same semantics as the function signatures `intmax_t imaxabs(intmax_t)` and `imaxdiv_t imaxdiv(intmax_t, intmax_t)`, respectively.

   > See also: ISO C 7.8

### 3831(i). Two-digit formatting of negative `year` is ambiguous

Section: 30.12 [time.format], 30.13 [time.parse] Status: C++26 Submitter: Matt Stephanson Opened: 2022-11-18 Last modified: 2026-06-09

Priority: 3

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Discussion:

An issue has been identified regarding the two-digit formatting of negative years according to Table [tab:time.format.spec] (30.12 [time.format]):

>     cout << format("{:%y} ", 1976y)  // "76"
>          << format("{:%y}", -1976y); // also "76"?

The relevant wording is

> The last two decimal digits of the year. If the result is a single digit it is prefixed by `0`. The modified command `%Oy` produces the locale's alternative representation. The modified command `%Ey` produces the locale's alternative representation of offset from `%EC` (year only).

MSVC STL treats the regular modified form symmetrically. Just as `%Ey` is the offset from `%EC`, so `%y` is the offset from `%C`, which is itself "[t]he year divided by 100 using floored division." (emphasis added). Because -1976 is the 24th year of the -20th century, the above code will print "76 24" using MSVC STL. However, many users expect, and libc++ gives, a result based on the literal wording, "76 76".

IEEE 1003.1-2008 strftime expects the century to be nonnegative, but the glibc implementation prints 24 for -1976. My own opinion is that this is the better result, because it consistently interprets `%C` and `%y` as the quotient and remainder of floored division by 100.

Howard Hinnant, coauthor of the original 30.12 [time.format] wording in P0355 adds:

> On the motivation for this design it is important to remember a few things:

> - POSIX `strftime`/`strptime` doesn't handle negative years in this department, so this is an opportunity for an extension in functionality.

> - This is a formatting/parsing issue, as opposed to a computational issue. This means that human readability of the string syntax is the most important aspect. Computational simplicity takes a back seat (within reason).

> - `%C` can't be truncated division, otherwise the years [-99, -1] would map to the same century as the years [0, 99]. So floored division is a pretty easy and obvious solution.

> - `%y` is obvious for non-negative years: The last two decimal digits, or `y % 100`.

> This leaves how to represent negative years with `%y`. I can think of 3 options:

> 1. Use the last two digits without negating: -1976 → 76.

> 2. Use the last two digits and negate it: -1976 → -76.

> 3. Use floored modulus arithmetic: -1976 → 24.

> The algorithm to convert `%C` and `%y` into a year is not important to the client because these are both strings, not integers. The client will do it with `parse`, not `100*C + y`.

> I discounted solution 3 as not sufficiently obvious. If the output for -1976 was 23, the human reader wouldn't immediately know that this is off by 1. The reader is expecting the POSIX spec:

> > the last two digits of the year as a decimal number [00,99].

> 24 just doesn't cut it.

> That leaves solution 1 or 2. I discounted solution 2 because having the negative in 2 places (the `%C` and `%y`) seemed overly complicated and more error prone. The negative sign need only be in one place, and it has to be in `%C` to prevent ambiguity.

> That leaves solution 1. I believe this is the solution for an extension of the POSIX spec to negative years with the property of least surprise to the client. The only surprise is in `%C`, not `%y`, and the surprise in `%C` seems unavoidable.

[2022-11-30; Reflector poll]

Set priority to 3 after reflector poll.

A few votes for priority 2. Might need to go to LEWG.

Previous resolution [SUPERSEDED]

This wording is relative to N4917.

> [Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A: This is Howard Hinnant's choice (3)

1. Modify 30.12 [time.format], Table [tab:time.format.spec] as indicated:

   > Table 102 — Meaning of conversion specifiers [tab:time.format.spec]
   > Specifier | Replacement
   > --- | ---
   > `[…]`
   > `%y` | The last two decimal digits of the yearremainder after dividing the year by 100 using floored division. If the result is a single digit it is prefixed by `0`. The modified command `%Oy` produces the locale's alternative representation. The modified command `%Ey` produces the locale's alternative representation of offset from `%EC` (year only).
   > `[…]`

2. Modify 30.13 [time.parse], Table [tab:time.parse.spec] as indicated:

   > Table 103 — Meaning of `parse` flags [tab:time.parse.spec]
   > Flag | Parsed value
   > --- | ---
   > `[…]`
   > `%y` | The last two decimal digits of the yearremainder after dividing the year by 100 using floored division. If the century is not otherwise specified (e.g. with `%C`), values in the range [`69`, `99`] are presumed to refer to the years 1969 to 1999, and values in the range [`00`, `68`] are presumed to refer to the years 2000 to 2068. The modified command `%N y` specifies the maximum number of characters to read. If N is not specified, the default is 2. Leading zeroes are permitted but not required. The modified commands `%Ey` and `%Oy` interpret the locale's alternative representation.
   > `[…]`

Option B: This is Howard Hinnant's choice (1)

1. Modify 30.12 [time.format], Table [tab:time.format.spec] as indicated:

   > Table 102 — Meaning of conversion specifiers [tab:time.format.spec]
   > Specifier | Replacement
   > --- | ---
   > `[…]`
   > `%y` | The last two decimal digits of the year, regardless of the sign of the year. If the result is a single digit it is prefixed by `0`. The modified command `%Oy` produces the locale's alternative representation. The modified command `%Ey` produces the locale's alternative representation of offset from `%EC` (year only). [Example ?: `cout << format("{:%C %y}", -1976y);` prints `-20 76`. — end example]
   > `[…]`

2. Modify 30.13 [time.parse], Table [tab:time.parse.spec] as indicated:

   > Table 103 — Meaning of `parse` flags [tab:time.parse.spec]
   > Flag | Parsed value
   > --- | ---
   > `[…]`
   > `%y` | The last two decimal digits of the year, regardless of the sign of the year. If the century is not otherwise specified (e.g. with `%C`), values in the range [`69`, `99`] are presumed to refer to the years 1969 to 1999, and values in the range [`00`, `68`] are presumed to refer to the years 2000 to 2068. The modified command `%N y` specifies the maximum number of characters to read. If N is not specified, the default is 2. Leading zeroes are permitted but not required. The modified commands `%Ey` and `%Oy` interpret the locale's alternative representation. [Example ?: `year y; istringstream{"-20 76"} >> parse("%3C %y", y);` results in `y == -1976y`. — end example]
   > `[…]`

[2025-10-17; Jonathan provides updated wording using Option B]

[2025-12-04; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 30.12 [time.format], Table [tab:time.format.spec] as indicated:

   > Table 133 — Meaning of conversion specifiers [tab:time.format.spec]
   > Specifier | Replacement
   > --- | ---
   > `[…]`
   > `%y` | The last two decimal digits of the year, regardless of the sign of the year. If the result is a single digit it is prefixed by `0`. The modified command `%Oy` produces the locale's alternative representation. The modified command `%Ey` produces the locale's alternative representation of offset from `%EC` (year only). [Example ?: `cout << format("{:%C %y}", -1976y);` prints `-20 76`. — end example]
   > `[…]`

2. Modify 30.13 [time.parse], Table [tab:time.parse.spec] as indicated:

   > Table 103 — Meaning of `parse` flags [tab:time.parse.spec]
   > Flag | Parsed value
   > --- | ---
   > `[…]`
   > `%y` | The last two decimal digits of the year, regardless of the sign of the year. If the century is not otherwise specified (e.g. with `%C`), values in the range [`69`, `99`] are presumed to refer to the years 1969 to 1999, and values in the range [`00`, `68`] are presumed to refer to the years 2000 to 2068. The modified command `%N y` specifies the maximum number of characters to read. If N is not specified, the default is 2. Leading zeroes are permitted but not required. The modified commands `%Ey` and `%Oy` interpret the locale's alternative representation. [Example ?: `year y; istringstream{"-20 76"} >> parse("%3C %y", y);` results in `y == -1976y`. — end example]
   > `[…]`

### 3833(i). Remove specialization `template<size_t N> struct formatter<const charT[N], charT>`

Section: 28.5.6.4 [format.formatter.spec] Status: C++23 Submitter: Mark de Wever Opened: 2022-11-27 Last modified: 2023-11-22

Priority: 2

View other active issues in [format.formatter.spec].

View all other issues in [format.formatter.spec].

View all issues with C++23 status.

Discussion:

In the past I discussed with Victor and Charlie to remove

>     template<size_t N> struct formatter<const charT[N], charT>;

Charlie disliked that since MSVC STL already shipped it and Victor mentioned it was useful. Instead of proposing to remove the specialization in LWG 3701(i) ("Make `formatter<remove_cvref_t<const charT[N]>, charT>` requirement explicit") I proposed to keep it. It's unused but it doesn't hurt.

That was until P2585R0 "Improve default container formatting". This paper makes it no longer possible to instantiate this specialization. See here for an example and a possible work-around.

The relevant wording is 28.5.1 [format.syn]:

>     template<class R>
>       constexpr unspecified format_kind = unspecified;

>     template<ranges::input_range R>
>         requires same_as<R, remove_cvref_t<R>>
>       constexpr range_format format_kind<R> = see below;

combined with 28.5.7.1 [format.range.fmtkind] p1:

> A program that instantiates the primary template of `format_kind` is ill-formed.

The issue is that `const charT[N]` does not satisfy the requirement `same_as<R, remove_cvref_t<R>>`. So it tries to instantiate the primary template, which is ill-formed.

I see two possible solutions:

1. Removing the specialization

   >     template<size_t N> struct formatter<const charT[N], charT>;

2. Adding a specialization

   >     template<class charT, size_t N>
   >       constexpr range_format format_kind<const charT[N]> =
   >         range_format::disabled;

I discussed this issue privately and got no objection for solution 1, therefore I propose to take that route. Implementations can still implement solution 2 as an extension until they are ready to ship an API/ABI break.

[2022-11-30; Reflector poll]

Set priority to 2 after reflector poll.

"Rationale is not really convincing why the first option is the right one."

"The point is not that we would need to add a format_kind specialization, it is that the specialization is inconsistent with the design of `formatter`, which is supposed to be instantiated only for cv-unqualified non-reference types."

Previous resolution [SUPERSEDED]

This wording is relative to N4917.

1. Modify 28.5.6.4 [format.formatter.spec] as indicated:

   > -2- Let `charT` be either `char` or `wchar_t`. Each specialization of `formatter` is either enabled or disabled, as described below. A debug-enabled specialization of `formatter` additionally provides a public, constexpr, non-static member function `set_debug_format()` which modifies the state of the `formatter` to be as if the type of the std-format-spec parsed by the last call to `parse` were `?`. Each header that declares the template `formatter` provides the following enabled specializations:

   > 1. (2.1) — The debug-enabled specializations […]

   > 2. (2.2) — For each `charT`, the debug-enabled string type specializations

   >    >     template<> struct formatter<charT*, charT>;
   >    >     template<> struct formatter<const charT*, charT>;
   >    >     template<size_t N> struct formatter<charT[N], charT>;
   >    >     template<size_t N> struct formatter<const charT[N], charT>;
   >    >     template<class traits, class Allocator>
   >    >       struct formatter<basic_string<charT, traits, Allocator>, charT>;
   >    >     template<class traits>
   >    >       struct formatter<basic_string_view<charT, traits>, charT>;

   > 3. (2.3) — […]

   > 4. (2.4) — […]

2. Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:

   > Affected subclause: 28.5.6.4 [format.formatter.spec]

   > Change: Remove the `formatter` specialization `template<size_t N> struct formatter<const charT[N], charT>`.

   > Rationale: The `formatter` specialization was not used in the Standard library. Keeping the specialization well-formed required an additional `format_kind` specialization.

   > Effect on original feature: Valid C++ 2020 code that instantiated the removed specialization is now ill-formed.

[2023-02-07 Tim provides updated wording]

[Issaquah 2023-02-08; LWG]

Unanimous consent to move to Immediate.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 28.5.6.4 [format.formatter.spec] as indicated:

   > -2- Let `charT` be either `char` or `wchar_t`. Each specialization of `formatter` is either enabled or disabled, as described below. A debug-enabled specialization of `formatter` additionally provides a public, constexpr, non-static member function `set_debug_format()` which modifies the state of the `formatter` to be as if the type of the std-format-spec parsed by the last call to `parse` were `?`. Each header that declares the template `formatter` provides the following enabled specializations:

   > 1. (2.1) — The debug-enabled specializations […]

   > 2. (2.2) — For each `charT`, the debug-enabled string type specializations

   >    >     template<> struct formatter<charT*, charT>;
   >    >     template<> struct formatter<const charT*, charT>;
   >    >     template<size_t N> struct formatter<charT[N], charT>;
   >    >     template<size_t N> struct formatter<const charT[N], charT>;
   >    >     template<class traits, class Allocator>
   >    >       struct formatter<basic_string<charT, traits, Allocator>, charT>;
   >    >     template<class traits>
   >    >       struct formatter<basic_string_view<charT, traits>, charT>;

   > 3. (2.3) — […]

   > 4. (2.4) — […]

2. Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:

   > Affected subclause: 28.5.6.4 [format.formatter.spec]

   > Change: Removed the `formatter` specialization `template<size_t N> struct formatter<const charT[N], charT>`.

   > Rationale: The specialization is inconsistent with the design of `formatter`, which is intended to be instantiated only with cv-unqualified object types.

   > Effect on original feature: Valid C++ 2020 code that instantiated the removed specialization can become ill-formed.

### 3834(i). Missing `constexpr` for `std::intmax_t` math functions in `<cinttypes>`

Section: 31.13.2 [cinttypes.syn] Status: C++23 Submitter: George Tokmaji Opened: 2022-11-27 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

P0533R9 adds `constexpr` to math functions in `<cmath>` and `<cstdlib>`, which includes `std::abs` and `std::div`. This misses the overloads for `std::intmax_t` in `<cinttypes>`, as well as `std::imaxabs` and `std::imaxdiv`, which seems like an oversight.

[2023-01-06; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 31.13.2 [cinttypes.syn], header `<cinttypes>` synopsis, as indicated:

   > >     […]
   > >     namespace std {
   > >       using imaxdiv_t = see below;

   > >       constexpr intmax_t imaxabs(intmax_t j);
   > >       constexpr imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom);
   > >       intmax_t strtoimax(const char* nptr, char** endptr, int base);
   > >       uintmax_t strtoumax(const char* nptr, char** endptr, int base);
   > >       intmax_t wcstoimax(const wchar_t* nptr, wchar_t** endptr, int base);
   > >       uintmax_t wcstoumax(const wchar_t* nptr, wchar_t** endptr, int base);

   > >       constexpr intmax_t abs(intmax_t);            // optional, see below
   > >       constexpr imaxdiv_t div(intmax_t, intmax_t); // optional, see below
   > >       […]
   > >     }
   > >     […]

   > -1- The contents and meaning of the header `<cinttypes>` are the same as the C standard library header `<inttypes.h>`, with the following changes:

   > 1. (1.1) — The header `<cinttypes>` includes the header `<cstdint>` (17.4.1 [cstdint.syn]) instead of `<stdint.h>`, and

   > 2. (1.2) — if and only if the type `intmax_t` designates an extended integer type (6.9.2 [basic.fundamental]), the following function signatures are added:

   >    >     constexpr intmax_t abs(intmax_t);
   >    >     constexpr imaxdiv_t div(intmax_t, intmax_t);

   >    which shall have the same semantics as the function signatures `constexpr intmax_t imaxabs(intmax_t)` and `constexpr imaxdiv_t imaxdiv(intmax_t, intmax_t)`, respectively.

### 3836(i). `std::expected<bool, E1>` conversion constructor `expected(const expected<U, G>&)` should take precedence over `expected(U&&)` with operator `bool`

Section: 22.8.6.2 [expected.object.cons] Status: C++23 Submitter: Hui Xie Opened: 2022-11-30 Last modified: 2023-11-22

Priority: 1

View all other issues in [expected.object.cons].

View all issues with C++23 status.

Discussion:

The issue came up when implementing `std::expected` in libc++. Given the following example:

>     struct BaseError{};
>     struct DerivedError : BaseError{};

>     std::expected<int, DerivedError> e1(5);
>     std::expected<int, BaseError> e2(e1);  // e2 holds 5

In the above example, `e2` is constructed with the conversion constructor

>     expected::expected(const expected<U, G>&)

and the value `5` is correctly copied into `e2` as expected.

However, if we change the type from `int` to `bool`, the behaviour is very surprising.

>     std::expected<bool, DerivedError> e1(false);
>     std::expected<bool, BaseError> e2(e1);  // e2 holds true

In this example `e2` is constructed with

>     expected::expected(U&&)

together with

>     expected::operator bool() const

Instead of copying `e1`'s "`false`" into `e2`, it uses `operator bool`, which returns `true` in this case and `e2` would hold "`true`" instead.

This is surprising behaviour given how inconsistent between `int` and `bool`.

The reason why the second example uses a different overload is that the constructor `expected(const expected<U, G>& rhs);` has the following constraint (22.8.6.2 [expected.object.cons] p17):

> 1. (17.3) — `is_constructible_v<T, expected<U, G>&>` is `false`; and

> 2. (17.4) — `is_constructible_v<T, expected<U, G>>` is `false`; and

> 3. (17.5) — `is_constructible_v<T, const expected<U, G>&>` is `false`; and

> 4. (17.6) — `is_constructible_v<T, const expected<U, G>>` is `false`; and

> 5. (17.7) — `is_convertible_v<expected<U, G>&, T>` is `false`; and

> 6. (17.8) — `is_convertible_v<expected<U, G>&&, T>` is `false`; and

> 7. (17.9) — `is_convertible_v<const expected<U, G>&, T>` is `false`; and

> 8. (17.10) — `is_convertible_v<const expected<U, G>&&, T>` is `false`; and

Since `T` is `bool` in the second example, and `bool` can be constructed `from` `std::expected`, this overload will be removed. and the overload that takes `U&&` will be selected.

I would suggest to special case `bool`, i.e.

- (The above 8 constraints); or

- `is_same_v<remove_cv_t<T>, bool>` is `true`

And we need to make sure this overload and the overload that takes `expected(U&&)` be mutually exclusive.

[2023-01-06; Reflector poll]

Set priority to 1 after reflector poll.

There was a mix of votes for P1 and P2 but also one for NAD ("The design of forward/repack construction for expected matches optional, when if the stored value can be directly constructed, we use that."). `std::optional<bool>` is similarly affected. Any change should consider the effects on `expected<expected<>>` use cases.

[Issaquah 2023-02-08; Jonathan provides wording]

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 22.5.3.2 [optional.ctor] as indicated:

   >     template<class U = T> constexpr explicit(see below) optional(U&& v);

   > -23- Constraints:

   > > [Drafting note: Change this paragraph to a bulleted list.]

   > - (23.1) — `is_constructible_v<T, U>` is `true`,
   > - (23.2) — `is_same_v<remove_cvref_t<U>, in_place_t>` is `false`, and
   > - (23.3) — `is_same_v<remove_cvref_t<U>, optional>` is `false`, and
   > - (23.4) — if `T` is cv `bool`, `remove_cvref_t<U>` is not a specialization of `optional`.

   > -24- Effects: Direct-non-list-initializes the contained value with `std::forward>U>(v)`.

   > -25- Postconditions: `*this` has a value.

   > -26- Throws: Any exception thrown by the selection constructor of `T`.

   > -27- Remarks: If `T`'s selected constructor is a constexpr constructor, this constructor is a constexpr constructor. The expression inside `explicit` is equivalent to:

   >       !is_convertible_v<U, T>

   >     template<class U> constexpr explicit(see below) optional(const optional<U>& rhs);

   > -28- Constraints:

   > - (28.1) — `is_constructible_v<T, const U&>` is `true`, and
   > - (28.1) — if `T` is not cv `bool`, `converts-from-any-cvref<T, optional<U>>` is `false`.

   > -29- Effects: If `rhs` contains a value, direct-non-list-initializes the contained value with `*rhs`.

   > -30- Postconditions: `rhs.has_value() == this->has_value()`.

   > -31- Throws: Any exception thrown by the selection constructor of `T`.

   > -32- Remarks: The expression inside `explicit` is equivalent to:

   >       !is_convertible_v<const U&, T>

   >     template<class U> constexpr explicit(see below) optional(optional<U>&& rhs);

   > -33- Constraints:

   > - (33.1) — `is_constructible_v<T, U>` is `true`, and
   > - (33.1) — if `T` is not cv `bool`, `converts-from-any-cvref<T, optional<U>>` is `false`.

   > -34- Effects: If `rhs` contains a value, direct-non-list-initializes the contained value with `std::move(*rhs)`. `rhs.has_value()` is unchanged.

   > -35- Postconditions: `rhs.has_value() == this->has_value()`.

   > -36- Throws: Any exception thrown by the selection constructor of `T`.

   > -37- Remarks: The expression inside `explicit` is equivalent to:

   >       !is_convertible_v<U, T>

2. Modify 22.8.6.2 [expected.object.cons] as indicated:

   >     template<class U, class G>
   >       constexpr explicit(see below) expected(const expected<U, G>& rhs);
   >     template<class U, class G>
   >       constexpr explicit(see below) expected(expected<U, G>&& rhs);

   > -17- Let:

   > - (17.1) — `UF` be `const U&` for the first overload and `U` for the second overload.
   > - (17.2) — `GF` be `const G&` for the first overload and `G` for the second overload.

   > -18- Constraints:

   > - (18.1) — `is_constructible_v<T, UF>` is `true`; and
   > - (18.2) — `is_constructible_v<E, GF>` is `true`; and
   > - (18.3) — if `T` is not cv `bool`, `converts-from-any-cvref<T, expected<U, G>>` is `false`; and
   > - (18.4) — `is_constructible_v<unexpected<E>, expected<U, G>&>` is `false`; and
   > - (18.5) — `is_constructible_v<unexpected<E>, expected<U, G>>` is `false`; and
   > - (18.6) — `is_constructible_v<unexpected<E>, const expected<U, G>&>` is `false`; and
   > - (18.7) — `is_constructible_v<unexpected<E>, const expected<U, G>>` is `false`.

   > -19- Effects: If `rhs.has_value()`, direct-non-list-initializes `val` with `std::forward>UF>(*rhs)`. Otherwise, direct-non-list-initializes `unex` with `std::forward>GF>(rhs.error())`.

   > -20- Postconditions: `rhs.has_value()` is unchanged; `rhs.has_value() == this->has_value()` is `true`.

   > -21- Throws: Any exception thrown by the initialization of `val` or `unex`.

   > -22- Remarks: The expression inside `explicit` is equivalent to `!is_convertible_v<UF, T> || !is_convertible_v<GF, E>`.

   >     template<class U = T>
   >       constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);

   > -23- Constraints:

   > - (23.1) — `is_same_v<remove_cvref_t<U>, in_place_t>` is `false`; and
   > - (23.2) — `is_same_v<expected, remove_cvref_t<U>>` is `false`; and
   > - (23.3) — `is_constructible_v<T, U>` is `true`; and
   > - (23.4) — `remove_cvref_t<U>` is not a specialization of `unexpected`; and
   > - (23.5) — if `T` is cv `bool`, `remove_cvref_t<U>` is not a specialization of `expected`.

   > -24- Effects: Direct-non-list-initializes `val` with `std::forward>U>(v)`.

   > -25- Postconditions: `has_value()` is `true`.

   > -26- Throws: Any exception thrown by the initialization of `val`.

### 3839(i). `range_formatter`'s `set_separator`, `set_brackets`, and `underlying` functions should be `noexcept`

Section: 28.5.7.2 [format.range.formatter], 28.5.7.3 [format.range.fmtdef], 28.5.9 [format.tuple] Status: C++23 Submitter: Hewill Kang Opened: 2022-12-11 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [format.range.formatter].

View all issues with C++23 status.

Discussion:

The `set_separator` and `set_brackets` of `range_formatter` only invoke `basic_string_view`'s assignment operator, which is `noexcept`, we should add `noexcept` specifications for them.

In addition, its `underlying` function returns a reference to the underlying `formatter`, which never throws, they should also be `noexcept`.

Similar rules apply to `range-default-formatter` and `formatter`'s `tuple` specialization.

[2023-01-06; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 28.5.7.2 [format.range.formatter] as indicated:

   > >     namespace std {
   > >       template<class T, class charT = char>
   > >         requires same_as<remove_cvref_t<T>, T> && formattable<T, charT>
   > >       class range_formatter {
   > >         formatter<T, charT> underlying_;                                          // exposition only
   > >         basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", ");      // exposition only
   > >         basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("["); // exposition only
   > >         basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>("]"); // exposition only

   > >       public:
   > >         constexpr void set_separator(basic_string_view<charT> sep) noexcept;
   > >         constexpr void set_brackets(basic_string_view<charT> opening,
   > >                                     basic_string_view<charT> closing) noexcept;
   > >         constexpr formatter<T, charT>& underlying() noexcept { return underlying_; }
   > >         constexpr const formatter<T, charT>& underlying() const noexcept { return underlying_; }

   > >         […]
   > >       };
   > >     }

   > […]

   >     constexpr void set_separator(basic_string_view<charT> sep) noexcept;

   > > -7- Effects: Equivalent to: `separator_ = sep;`

   >     constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;

   > > -8- Effects: Equivalent to:

   > >     opening-bracket_ = opening;
   > >     closing-bracket_ = closing;

2. Modify 28.5.7.3 [format.range.fmtdef] as indicated:

   > >     namespace std {
   > >       template<ranges::input_range R, class charT>
   > >       struct range-default-formatter<range_format::sequence, R, charT> {    // exposition only
   > >       private:
   > >         using maybe-const-r = fmt-maybe-const<R, charT>;                    // exposition only
   > >         range_formatter<remove_cvref_t<ranges::range_reference_t<maybe-const-r>>,
   > >                         charT> underlying_;                                 // exposition only

   > >       public:
   > >         constexpr void set_separator(basic_string_view<charT> sep) noexcept;
   > >         constexpr void set_brackets(basic_string_view<charT> opening,
   > >                                     basic_string_view<charT> closing) noexcept;

   > >         […]
   > >       };
   > >     }

   >     constexpr void set_separator(basic_string_view<charT> sep) noexcept;

   > > -1- Effects: Equivalent to: `underlying_.set_separator(sep);`.

   >     constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;

   > > -2- Effects: Equivalent to: `underlying_.set_brackets(opening, closing);`.

3. Modify 28.5.9 [format.tuple] as indicated:

   > >     namespace std {
   > >       template<class charT, formattable<charT>... Ts>
   > >       struct formatter<pair-or-tuple<Ts...>, charT> {
   > >       private:
   > >         tuple<formatter<remove_cvref_t<Ts>, charT>...> underlying_;               // exposition only
   > >         basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", ");      // exposition only
   > >         basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("("); // exposition only
   > >         basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>(")"); // exposition only

   > >       public:
   > >         constexpr void set_separator(basic_string_view<charT> sep) noexcept;
   > >         constexpr void set_brackets(basic_string_view<charT> opening,
   > >                                     basic_string_view<charT> closing) noexcept;

   > >         […]
   > >       };
   > >     }

   > […]

   >     constexpr void set_separator(basic_string_view<charT> sep) noexcept;

   > > -5- Effects: Equivalent to: `separator_ = sep;`

   >     constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;

   > > -6- Effects: Equivalent to:

   > >     opening-bracket_ = opening;
   > >     closing-bracket_ = closing;

### 3841(i). `<version>` should not be "all freestanding"

Section: 17.3.2 [version.syn] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-12-14 Last modified: 2024-01-29

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++23 status.

Discussion:

It's reasonable for the `<version>` header to be required for freestanding, so that users can include it and see the "implementation-dependent information … (e.g. version number and release date)", and also to ask which features are present (which is the real intended purpose of `<version>`). It seems less reasonable to require every macro to be present on freestanding implementations, even the ones that correspond to non-freestanding features.

P2198R7 will fix this situation for C++26, but we should also do something for C++23 before publishing it. It seems sensible not to require any of the macros to be present, and then allow implementations to define them for the features that they support.

[2023-01-06; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 17.3.2 [version.syn], header `<version>` synopsis, as indicated:

   > -2- Each of the macros defined in `<version>` is also defined after inclusion of any member of the set of library headers indicated in the corresponding comment in this synopsis.

   > [Note 1: Future revisions of C++ might replace the values of these macros with greater values. — end note]

   > >     // all freestanding
   > >     #define __cpp_lib_addressof_constexpr          201603L // also in <memory>
   > >     […]

### 3842(i). Unclear wording for precision in chrono-format-spec

Section: 30.12 [time.format] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-12-14 Last modified: 2024-01-29

Priority: Not Prioritized

View other active issues in [time.format].

View all other issues in [time.format].

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Discussion:

30.12 [time.format] says:

> […] Giving a precision specification in the chrono-format-spec is valid only for `std::chrono::duration` types where the representation type `Rep` is a floating-point type. For all other `Rep` types, an exception of type `format_error` is thrown if the chrono-format-spec contains a precision specification. […]

It's unclear whether the restriction in the first sentence applies to all types, or only duration types. The second sentence seems to restrict the exceptional case to only types with a non-floating-point `Rep`, but what about types with no `Rep` type at all?

Can you use a precision with `sys_time<duration<float>>`? That is not a `duration` type at all, so does the restriction apply? What about `hh_mm_ss<duration<int>>`? That's not a `duration` type, but it uses one, and its `Rep` is not a floating-point type. What about `sys_info`? That's not a `duration` and doesn't have any associated `duration`, or `Rep` type.

What is the intention here?

Less importantly, I don't like the use of `Rep` here. That's the template parameter of the `duration` class template, but that name isn't in scope here. Why aren't we talking about the `duration` type's `rep` type, which is the public name for it? Or about a concrete specialization `duration<Rep, Period>`, instead of the class template?

The suggested change below would preserve the intended meaning, but with more … precision.

[2023-01-06; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 30.12 [time.format] as indicated:

   > -1- […]

   > The productions fill-and-align, width, and precision are described in 28.5.2 [format.string]. Giving a precision specification in the chrono-format-spec is valid only for types that are specializations of `std::chrono::duration` types where the representation type `Rep` isfor which the nested typedef-name `rep` denotes a floating-point type. For all other `Rep` types, an exception of type `format_error` is thrown if the chrono-format-spec contains a precision specification. […]

### 3843(i). `std::expected<T,E>::value() &` assumes `E` is copy constructible

Section: 22.8.6.6 [expected.object.obs] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-12-20 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

22.8.6.6 [expected.object.obs] p9 says:

> Throws: `bad_expected_access(error())` if `has_value()` is `false`.

But if `error()` returns a reference to a move-only type then it can't be copied and the function body is ill-formed. Should it be constrained with `is_copy_constructible_v<E>`? Or just mandate it?

Similarly, the `value()&&` and `value() const&&` overloads require `is_move_constructible_v<E>` to be true for `bad_expected_access(std::move(error()))` to be valid. Casey Carter pointed out they also require it to be copyable so that the exception can be thrown, as per 14.2 [except.throw] p5.

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

1. Modify 22.8.6.6 [expected.object.obs] as indicated:

   > >     constexpr const T& value() const &;
   > >     constexpr T& value() &;

   > -?- Mandates: `is_copy_constructible_v<E>` is `true`.

   > -8- Returns: `val`, if `has_value()` is `true`.

   > -9- Throws: `bad_expected_access(as_const(error()))` if `has_value()` is `false`.

   > >     constexpr T&& value() &&;
   > >     constexpr const T&& value() const &&;

   > -?- Mandates: `is_copy_constructible_v<E>` is `true` and `is_constructible_v<E, decltype(std::move(error()))>` is `true`.

   > -10- Returns: `std::move(val)`, if `has_value()` is `true`.

   > -11- Throws: `bad_expected_access(std::move(error()))` if `has_value()` is `false`.

### 3847(i). `ranges::to` can still return views

Section: 25.5.7.2 [range.utility.conv.to], 25.5.7.3 [range.utility.conv.adaptors] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2024-01-29

Priority: 2

View other active issues in [range.utility.conv.to].

View all other issues in [range.utility.conv.to].

View all issues with C++23 status.

Discussion:

The intention of `ranges::to` is to construct a non-view object, which is reflected in its constraint that object type `C` should not model a `view`.

The specification allows `C` to be a `const`-qualified type which does not satisfy `view` such as `const string_view`, making `ranges::to` return a dangling view. We should ban such cases.

[2023-02-01; Reflector poll]

Set priority to 2 after reflector poll. Just require C to be a cv-unqualified object type.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4917.

> 1. Modify 25.2 [ranges.syn] as indicated:

>    >     #include <compare>              // see 17.12.1 [compare.syn]
>    >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
>    >     #include <iterator>             // see 24.2 [iterator.synopsis]

>    >     namespace std::ranges {
>    >       […]
>    >       // 25.5.7 [range.utility.conv], range conversions
>    >       template<class C, input_range R, class... Args> requires (!view<remove_cv_t<C>>)
>    >         constexpr C to(R&& r, Args&&... args);                                          // freestanding
>    >       template<template<class...> class C, input_range R, class... Args>
>    >         constexpr auto to(R&& r, Args&&... args);                                       // freestanding
>    >       template<class C, class... Args> requires (!view<remove_cv_t<C>>)
>    >         constexpr auto to(Args&&... args);                                              // freestanding
>    >       template<template<class...> class C, class... Args>
>    >         constexpr auto to(Args&&... args);                                              // freestanding
>    >       […]
>    >     }

> 2. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

>    >     template<class C, input_range R, class... Args> requires (!view<remove_cv_t<C>>)
>    >     constexpr C to(R&& r, Args&&... args);

>    > > -1- Returns: An object of type `C` constructed from the elements of `r` in the following manner:

>    > > […]

> 3. Modify 25.5.7.3 [range.utility.conv.adaptors] as indicated:

>    >     template<class C, class... Args> requires (!view<remove_cv_t<C>>)
>    >       constexpr auto to(Args&&... args);
>    >     template<template<class...> class C, class... Args>
>    >       constexpr auto to(Args&&... args);

>    > > -1- Returns: A range adaptor closure object (25.7.2 [range.adaptor.object]) `f` that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:

>    > > […]

[2023-02-02; Jonathan provides improved wording]

[Issaquah 2023-02-08; LWG]

Unanimous consent to move to Immediate. This also resolves LWG 3787(i).

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   >     template<class C, input_range R, class... Args> requires (!view<C>)
   >     constexpr C to(R&& r, Args&&... args);

   > > -?- Mandates: `C` is a cv-unqualified class type.

   > > -1- Returns: An object of type `C` constructed from the elements of `r` in the following manner:

   > > […]

2. Modify 25.5.7.3 [range.utility.conv.adaptors] as indicated:

   >     template<class C, class... Args> requires (!view<C>)
   >       constexpr auto to(Args&&... args);
   >     template<template<class...> class C, class... Args>
   >       constexpr auto to(Args&&... args);

   > > -?- Mandates: For the first overload, `C` is a cv-unqualified class type.

   > > -1- Returns: A range adaptor closure object (25.7.2 [range.adaptor.object]) `f` that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:

   > > […]

### 3848(i). `adjacent_view`, `adjacent_transform_view` and `slide_view` missing `base` accessor

Section: 25.7.27.2 [range.adjacent.view], 25.7.28.2 [range.adjacent.transform.view], 25.7.30.2 [range.slide.view] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.adjacent.view].

View all issues with C++23 status.

Discussion:

Like most range adaptors, these three views accept a single range and store it as a member variable. However, they do not provide a `base()` member function for accessing the underlying range, which seems like an oversight.

[2023-02-01; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.27.2 [range.adjacent.view] as indicated:

   > >     namespace std::ranges {
   > >       template<forward_range V, size_t N>
   > >         requires view<V> && (N > 0)
   > >       class adjacent_view : public view_interface<adjacent_view<V, N>> {
   > >         V base_ = V();                      // exposition only
   > >         […]
   > >       public:
   > >         adjacent_view() requires default_initializable<V> = default;
   > >         constexpr explicit adjacent_view(V base);

   > >         constexpr V base() const & requires copy_constructible<V> { return base_; }
   > >         constexpr V base() && { return std::move(base_); }
   > >         […]
   > >       };
   > >     }

2. Modify 25.7.28.2 [range.adjacent.transform.view] as indicated:

   > >     namespace std::ranges {
   > >       template<forward_range V, move_constructible F, size_t N>
   > >         requires view<V> && (N > 0) && is_object_v<F> &&
   > >                  regular_invocable<F&, REPEAT(range_reference_t<V>, N)...> &&
   > >                  can-reference<invoke_result_t<F&, REPEAT(range_reference_t<V>, N)...>>
   > >       class adjacent_transform_view : public view_interface<adjacent_transform_view<V, F, N>> {
   > >         movable-box<F> fun_;                        // exposition only
   > >         adjacent_view<V, N> inner_;                 // exposition only

   > >         using InnerView = adjacent_view<V, N>;      // exposition only
   > >         […]
   > >       public:
   > >         adjacent_transform_view() = default;
   > >         constexpr explicit adjacent_transform_view(V base, F fun);

   > >         constexpr V base() const & requires copy_constructible<InnerView> { return inner_.base(); }
   > >         constexpr V base() && { return std::move(inner_).base(); }
   > >         […]
   > >       };
   > >     }

3. Modify 25.7.30.2 [range.slide.view] as indicated:

   > >     namespace std::ranges {
   > >       […]
   > >       template<forward_range V>
   > >         requires view<V>
   > >       class slide_view : public view_interface<slide_view<V>> {
   > >         V base_;                            // exposition only
   > >         range_difference_t<V> n_;           // exposition only
   > >         […]
   > >       public:
   > >         constexpr explicit slide_view(V base, range_difference_t<V> n);

   > >         constexpr V base() const & requires copy_constructible<V> { return base_; }
   > >         constexpr V base() && { return std::move(base_); }
   > >         […]
   > >       };
   > >       […]
   > >     }

### 3849(i). `cartesian_product_view::iterator`'s default constructor is overconstrained

Section: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.cartesian.iterator].

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Discussion:

Currently, `cartesian_product_view::iterator` only provides the default constructor when the first range models `forward_range`, which seems too restrictive since several input iterators like `istream_iterator` are still default-constructible.

It would be more appropriate to constrain the default constructor only by whether the underlying iterator satisfies `default_initializable`, as most other range adaptors do. Since `cartesian_product_view::iterator` contains a `tuple` member that already has a constrained default constructor, the proposed resolution simply removes the constraint.

[2023-02-01; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify [ranges.cartesian.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range First, forward_range... Vs>
   > >         requires (view<First> && ... && view<Vs>)
   > >       template<bool Const>
   > >       class cartesian_product_view<First, Vs...>::iterator {
   > >       public:
   > >         […]
   > >         iterator() requires forward_range<maybe-const<Const, First>> = default;
   > >         […]
   > >       private:
   > >         using Parent = maybe-const<Const, cartesian_product_view>;          // exposition only
   > >         Parent* parent_ = nullptr;                                          // exposition only
   > >         tuple<iterator_t<maybe-const<Const, First>>,
   > >           iterator_t<maybe-const<Const, Vs>>...> current_;                  // exposition only
   > >         […]
   > >       };
   > >     }

### 3850(i). `views::as_const` on `empty_view<T>` should return `empty_view<const T>`

Section: 25.7.22.1 [range.as.const.overview] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.as.const.overview].

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Discussion:

Currently, applying `views::as_const` to an `empty_view<int>` will result in an `as_const_view<empty_view<int>>`, and its iterator type will be `basic_const_iterator<int*>`.

This amount of instantiation is not desirable for such a simple view, in which case simply returning `empty_view<const int>` should be enough.

[2023-02-01; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.22.1 [range.as.const.overview] as indicated:

   > -2- The name `views::as_const` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Let `E` be an expression, let `T` be `decltype((E))`, and let `U` be `remove_cvref_t<T>`. The expression `views::as_const(E)` is expression-equivalent to:

   > 1. (2.1) — If `views::all_t<T>` models `constant_range`, then `views::all(E)`.

   > 2. (2.?) — Otherwise, if `U` denotes `empty_view<X>` for some type `X`, then `auto(views::empty<const X>)`.

   > 3. (2.2) — Otherwise, if `U` denotes `span<X, Extent>` for some type `X` and some extent `Extent`, then `span<const X, Extent>(E)`.

   > 4. (2.3) — Otherwise, if `E` is an lvalue, `const U` models `constant_range`, and `U` does not model `view`, then `ref_view(static_cast<const U&>(E))`.

   > 5. (2.4) — Otherwise, `as_const_view(E)`.

### 3851(i). `chunk_view::inner-iterator` missing custom `iter_move` and `iter_swap`

Section: 25.7.29.5 [range.chunk.inner.iter] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

For the input version of `chunk_view`, its `inner-iterator::operator*()` simply dereferences the underlying input iterator. However, there are no customized `iter_move` and `iter_swap` overloads for the `inner-iterator`, which prevents customized behavior when applying `views::chunk` to a range whose iterator type is a proxy iterator, for example:

        #include <algorithm>
        #include <cassert>
        #include <ranges>
        #include <sstream>
        #include <vector>

        int main() {
          auto ints = std::istringstream{"0 1 2 3 4"};
          std::vector<std::string> vs{"the", "quick", "brown", "fox"};
          auto r = std::views::zip(vs, std::views::istream<int>(ints))
                  | std::views::chunk(2)
                  | std::views::join;
          std::vector<std::tuple<std::string, int>> res;
          std::ranges::copy(std::move_iterator(r.begin()), std::move_sentinel(r.end()),
                            std::back_inserter(res));
          assert(vs.front().empty()); // assertion failed
        }

`zip` iterator has a customized `iter_move` behavior, but since there is no `iter_move` specialization for `inner-iterator`, when we try to move elements in `chunk_view`, `move_iterator` will fallback to use the default implementation of `iter_move`, making `string`s not moved as expected from the original `vector` but copied instead.

[2023-02-01; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.29.5 [range.chunk.inner.iter] as indicated:

   > >     namespace std::ranges {
   > >       template<view V>
   > >         requires input_range<V>
   > >       class chunk_view<V>::inner-iterator {
   > >         chunk_view* parent_;                                                // exposition only

   > >         constexpr explicit inner-iterator(chunk_view& parent) noexcept;     // exposition only

   > >       public:
   > >         […]
   > >         friend constexpr difference_type operator-(default_sentinel_t y, const inner-iterator& x)
   > >           requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
   > >         friend constexpr difference_type operator-(const inner-iterator& x, default_sentinel_t y)
   > >           requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

   > >         friend constexpr range_rvalue_reference_t<V> iter_move(const inner-iterator& i)
   > >           noexcept(noexcept(ranges::iter_move(*i.parent_->current_)));

   > >         friend constexpr void iter_swap(const inner-iterator& x, const inner-iterator& y)
   > >           noexcept(noexcept(ranges::iter_swap(*x.parent_->current_, *y.parent_->current_)))
   > >           requires indirectly_swappable<iterator_t<V>>;
   > >       };
   > >     }

   > […]

   >     friend constexpr range_rvalue_reference_t<V> iter_move(const inner-iterator& i)
   >       noexcept(noexcept(ranges::iter_move(*i.parent_->current_)));

   > > -?- Effects: Equivalent to: `return ranges::iter_move(*i.parent_->current_);`

   >     friend constexpr void iter_swap(const inner-iterator& x, const inner-iterator& y)
   >       noexcept(noexcept(ranges::iter_swap(*x.parent_->current_, *y.parent_->current_)))
   >       requires indirectly_swappable<iterator_t<V>>;

   > > -?- Effects: Equivalent to: `ranges::iter_swap(*x.parent_->current_, *y.parent_->current_)`.

### 3853(i). `basic_const_iterator<volatile int*>::operator->` is ill-formed

Section: 24.5.3 [const.iterators] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [const.iterators].

View all other issues in [const.iterators].

View all issues with C++23 status.

Discussion:

Currently, `basic_const_iterator::operator->` constrains the value type of the underlying iterator to be only the cv-unqualified type of its reference type, which is true for raw pointers.

However, since it also explicitly specifies returning a pointer to a `const` value type, this will cause a hard error when the value type is actually `volatile`-qualified:

>     std::basic_const_iterator<volatile int*> it;
>     auto* p = it.operator->(); // invalid conversion from 'volatile int*' to 'const int*'

The proposed resolution changes the return type from `const value_type*` to `const auto*`, which makes it deduce the correct type in the above example, i.e. `const volatile int*`.

[2023-02-01; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 24.5.3 [const.iterators] as indicated:

   >     namespace std {
   >       template<class I>
   >         concept not-a-const-iterator = see below;

   >       template<input_iterator Iterator>
   >       class basic_const_iterator {
   >         Iterator current_ = Iterator();                             // exposition only
   >         using reference = iter_const_reference_t<Iterator>;         // exposition only

   >       public:
   >         […]
   >         constexpr reference operator*() const;
   >         constexpr const autovalue_type* operator->() const
   >            requires is_lvalue_reference_v<iter_reference_t<Iterator>> &&
   >                     same_as<remove_cvref_t<iter_reference_t<Iterator>>, value_type>;
   >         […]
   >       };
   >     }

   > […]

   >     constexpr const autovalue_type* operator->() const
   >       requires is_lvalue_reference_v<iter_reference_t<Iterator>> &&
   >                same_as<remove_cvref_t<iter_reference_t<Iterator>>, value_type>;

   > -7- Returns: If `Iterator` models `contiguous_iterator`, `to_address(current_)`; otherwise, `addressof(*current_)`.

### 3857(i). `basic_string_view` should allow explicit conversion when only traits vary

Section: 27.3.3.2 [string.view.cons] Status: C++23 Submitter: Casey Carter Opened: 2023-01-10 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [string.view.cons].

View all other issues in [string.view.cons].

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Discussion:

`basic_string_view` has a constructor that converts appropriate contiguous ranges to `basic_string_view`. This constructor accepts an argument by forwarding reference (`R&&`), and has several constraints including that specified in 27.3.3.2 [string.view.cons]/12.6:

> if the qualified-id `remove_reference_t<R>::traits_type` is valid and denotes a type, `is_same_v<remove_reference_t<R>::traits_type, traits>` is `true`.

This constraint prevents conversions from `basic_string_view<C, T1>` and `basic_string<C, T1, A>` to `basic_string_view<C, T2>`. Preventing such seemingly semantic-affecting conversions from happening implicitly was a good idea, but since the constructor was changed to be explicit it no longer seems necessary to forbid these conversions. If a user wants to convert a `basic_string_view<C, T2>` to `basic_string_view<C, T1>` with `static_cast<basic_string_view<C, T1>>(meow)` instead of by writing out `basic_string_view<C, T1>{meow.data(), meow.size()}` that seems fine to me. Indeed, if we think conversions like this are so terribly dangerous we probably shouldn't be performing them ourselves in 28.5.8.1 [format.arg]/9 and 28.5.8.1 [format.arg]/10.

[2023-02-01; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 27.3.3.2 [string.view.cons] as indicated:

   >     template<class R>
   >       constexpr explicit basic_string_view(R&& r);

   > > -11- Let `d` be an lvalue of type `remove_cvref_t<R>`.

   > > -12- Constraints:

   > > 1. (12.1) — `remove_cvref_t<R>` is not the same type as `basic_string_view`,

   > > 2. (12.2) — `R` models `ranges::contiguous_range` and `ranges::sized_range`,

   > > 3. (12.3) — `is_same_v<ranges::range_value_t<R>, charT>` is `true`,

   > > 4. (12.4) — `is_convertible_v<R, const charT*>` is `false`, and

   > > 5. (12.5) — `d.operator ::std::basic_string_view<charT, traits>()` is not a valid expression, and.

   > > 6. (12.6) — if the qualified-id `remove_reference_t<R>::traits_type` is valid and denotes a type, `is_same_v<remove_reference_t<R>::traits_type, traits>` is `true`.

### 3859(i). `std::projected` cannot handle proxy iterator

Section: 24.3.6.4 [projected] Status: Resolved Submitter: Hewill Kang Opened: 2023-01-24 Last modified: 2023-03-23

Priority: 3

View all other issues in [projected].

View all issues with Resolved status.

Discussion:

Currently, `std::projected` is heavily used in `<algorithm>` to transform the original iterator into a new readable type for concept checking, which has the following definition:

>     template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
>     struct projected {
>       using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>;
>       indirect_result_t<Proj&, I> operator*() const;              // not defined
>     };

It provides the member type `value_type`, which is defined as the cvref-unqualified of a projection function applied to the reference of the iterator, this seems reasonable since this is how iterators are usually defined for the value type.

However, this does not apply to C++20 proxy iterators such as `zip_view::iterator`, we cannot obtain the `tuple` of value by simply removing the cvref-qualifier of the `tuple` of reference.
This incorrect definition allows us to unethically bypass the constraint checking of the constraint algorithm, for example:

>     #include <algorithm>
>     #include <ranges>
>     #include <vector>

>     struct Cmp {
>       bool operator()(std::tuple<int&>, std::tuple<int&>) const;
>       bool operator()(auto, auto) const = delete;
>     };

>     int main() {
>       std::vector<int> v;
>       std::ranges::sort(std::views::zip(v), Cmp{}); // hard error
>     }

In the above example, the value type and reference of the original iterator `I` are `tuple<int>` and `tuple<int&>` respectively, however, the value type and reference of `projected<I, identity>` will be `tuple<int&>` and `tuple<int&>&&`, which makes the constraint only require that the comparator can compare two `tuple<int&>`s, resulting in a hard error in the implementation.

[2023-02-06; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N4928.

> [Drafting note: The proposed resolution is to alias `projected` as `I` when the projection function is exactly `identity`. This form of type aliasing has similarities to the proposed wording of P2538R1, except for the nested `struct type`. — end drafting note]

1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 24.3.6.4 [projected], projected
   >       template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
   >         usingstruct projected = see below;                         // freestanding

   >       template<weakly_incrementable I, class Proj>
   >         struct incrementable_traits<projected<I, Proj>>;           // freestanding
   >       […]
   >     }

2. Modify 24.3.6.4 [projected] as indicated:

   > -1- Class template `projected` is used to constrain algorithms that accept callable objects and projections (3.44 [defns.projection]). It combines a `indirectly_readable` type `I` and a callable object type `Proj` into a new `indirectly_readable` type whose `reference` type is the result of applying `Proj` to the `iter_reference_t` of `I`.

   > >     namespace std {
   > >       template<classindirectly_readable I, classindirectly_regular_unary_invocable<I> Proj>
   > >       struct projected-implprojected { // exposition only
   > >         using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>;
   > >         using difference_type = iter_difference_t<I>;               // present only if I models weakly_incrementable
   > >         indirect_result_t<Proj&, I> operator*() const;              // not defined
   > >       };

   > >       template<weakly_incrementable I, class Proj>
   > >       struct incrementable_traits<projected<I, Proj>> {
   > >         using difference_type = iter_difference_t<I>;
   > >       };

   > >       template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
   > >         using projected = conditional_t<same_as<Proj, identity>, I, projected-impl<I, Proj>>;
   > >     }

[2023-03-22 Resolved by the adoption of P2609R3 in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

### 3860(i). `range_common_reference_t` is missing

Section: 25.2 [ranges.syn] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-24 Last modified: 2023-11-22

Priority: Not Prioritized

View other active issues in [ranges.syn].

View all other issues in [ranges.syn].

View all issues with C++23 status.

Discussion:

For the alias template `iter_meow_t` in `<iterator>`, there are almost all corresponding `range_meow_t` in `<ranges>`, except for `iter_common_reference_t`, which is used to calculate the common reference type shared by `reference` and `value_type` of the iterator.

Given that it has a highly similar formula form to `iter_const_reference_t`, and the latter has a corresponding sibling, I think we should add a `range_common_reference_t` for `<ranges>`.

This increases the consistency of the two libraries and simplifies the text of getting common reference from a range. Since C++23 brings proxy iterators and `tuple` enhancements, I believe such introduction can bring some value.

[2023-02-06; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   >     #include <iterator>             // see 24.2 [iterator.synopsis]

   >     namespace std::ranges {
   >       […]
   >       template<range R>
   >         using range_reference_t = iter_reference_t<iterator_t<R>>;                      // freestanding
   >       template<range R>
   >         using range_const_reference_t = iter_const_reference_t<iterator_t<R>>;          // freestanding
   >       template<range R>
   >         using range_rvalue_reference_t = iter_rvalue_reference_t<iterator_t<R>>;        // freestanding
   >       template<range R>
   >         using range_common_reference_t = iter_common_reference_t<iterator_t<R>>;        // freestanding
   >       […]
   >     }

### 3861(i). `mdspan` `layout_stride::mapping` default constructor problem

Section: 23.7.3.4.7 [mdspan.layout.stride] Status: Resolved Submitter: Christian Robert Trott Opened: 2023-01-25 Last modified: 2023-06-13

Priority: 1

View all other issues in [mdspan.layout.stride].

View all issues with Resolved status.

Discussion:

During work on some follow on proposals for C++26 (like the padded layouts and `submdspan`) we stumbled over an issue in the C++23 `mdspan` definition.

If you have a `layout_stride::mapping` with just static extents, its default constructor will produce an inconsistent mapping. All its strides will be `0` but its extents are not. Thus the `is_unique` property is violated. We wrote a paper to fix this: P2763R0. We describe two options to fix it and state why we prefer one over the other, with proposed wording changes.

[2023-02-06; Reflector poll]

Set priority to 1 after reflector poll.

[2023-06-12 Status changed: New → Resolved.]

Resolved in Issaquah by P2763R0.

Proposed resolution:

### 3862(i). `basic_const_iterator`'s `common_type` specialization is underconstrained

Section: 24.2 [iterator.synopsis] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-25 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [iterator.synopsis].

View all issues with C++23 status.

Discussion:

To make `basic_const_iterator` compatible with its unwrapped iterators, the standard defines the following `common_type` specialization:

>     template<class T, common_with<T> U>
>     struct common_type<basic_const_iterator<T>, U> {
>       using type = basic_const_iterator<common_type_t<T, U>>;
>     };

For type `U`, when it shares a common type with the unwrapped type `T` of `basic_const_iterator`, the common type of both is `basic_const_iterator` of the common type of `T` and `U`.

However, since this specialization only constrains `U` and `T` to have a common type, this allows `U` to be any type that satisfies such requirement, such as `optional<T>`, in which case computing the common type of both would produce a hard error inside the specialization, because `basic_const_iterator` requires the template parameter to be `input_iterator`, while `optional` clearly isn't.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4928.

> 1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

>    >     #include <compare>              // see 17.12.1 [compare.syn]
>    >     #include <concepts>             // see 18.3 [concepts.syn]

>    >     namespace std {
>    >       […]
>    >       template<class T, common_with<T> U>
>    >         requires input_iterator<U>
>    >       struct common_type<basic_const_iterator<T>, U> {                                  // freestanding
>    >         using type = basic_const_iterator<common_type_t<T, U>>;
>    >       };
>    >       template<class T, common_with<T> U>
>    >         requires input_iterator<U>
>    >       struct common_type<U, basic_const_iterator<T>> {                                  // freestanding
>    >         using type = basic_const_iterator<common_type_t<T, U>>;
>    >       };
>    >       […]
>    >     }

[2023-02-06; Jonathan provides improved wording based on Casey's suggestion during the prioritization poll.]

[Issaquah 2023-02-07; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <concepts>             // see 18.3 [concepts.syn]

   >     namespace std {
   >       […]
   >       template<class T, common_with<T> U>
   >         requires input_iterator<common_type_t<T, U>>
   >       struct common_type<basic_const_iterator<T>, U> {                                  // freestanding
   >         using type = basic_const_iterator<common_type_t<T, U>>;
   >       };
   >       template<class T, common_with<T> U>
   >         requires input_iterator<common_type_t<T, U>>
   >       struct common_type<U, basic_const_iterator<T>> {                                  // freestanding
   >         using type = basic_const_iterator<common_type_t<T, U>>;
   >       };
   >       template<class T, common_with<T> U>
   >         requires input_iterator<common_type_t<T, U>>
   >       struct common_type<basic_const_iterator<T>, basic_const_iterator<U>> {            // freestanding
   >         using type = basic_const_iterator<common_type_t<T, U>>;
   >       };
   >       […]
   >     }

### 3865(i). Sorting a range of `pair`s

Section: 22.3.3 [pairs.spec] Status: C++23 Submitter: Barry Revzin Opened: 2023-01-28 Last modified: 2023-11-22

Priority: 2

View all other issues in [pairs.spec].

View all issues with C++23 status.

Discussion:

Consider this example:

>     #include <algorithm>
>     #include <ranges>

>     int main() {
>       int a[3] = {1, 2, -1};
>       int b[3] = {1, 4, 1};
>       std::ranges::sort(std::views::zip(a, b));
>     }

This is currently valid C++23 code, but wasn't before P2165 (Compatibility between `tuple`, `pair` and tuple-like objects). Before P2165, `zip(a, b)` returned a range whose reference was `std::pair<int&, int&>` and whose `value_type` was `std::pair<int, int>` and `std::pair`, unlike `std::tuple`, does not have any heterogeneous comparisons — which is required to satisfy the `sortable` concept.

While the zip family of range adapters no longer has this problem, nothing prevents users from themselves creating a range whose reference type is `pair<T&, U&>` and whose `value_type` is `pair<T, U>` (which is now a valid range after the zip paper) and then discovering that this range isn't sortable, even though the equivalent using `tuple` is.

Suggested resolution:

Change `pair`'s comparison operators from comparing two arguments of type `const pair<T1, T2>&` to instead comparing arguments of types `const pair<T1, T2>&` and `const pair<U1, U2>&`.

[2023-02-05; Barry provides wording]

[2023-02-06; Reflector poll]

Set priority to 2 after reflector poll.

[Issaquah 2023-02-07; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 22.2.1 [utility.syn], header `<utility>` synopsis, as indicated:

   >     […]
   >     // 22.3.3 [pairs.spec], pair specialized algorithms
   >     template<class T1, class T2, class U1, class U2>
   >       constexpr bool operator==(const pair<T1, T2>&, const pair<TU1, TU2>&);
   >     template<class T1, class T2, class U1, class U2>
   >       constexpr common_comparison_category_t<synth-three-way-result<T1, U1>,
   >                                              synth-three-way-result<T2, U2>>
   >         operator<=>(const pair<T1, T2>&, const pair<TU1, TU2>&);
   >     […]

2. Modify 22.3.3 [pairs.spec] as indicated:

   >     template<class T1, class T2, class U1, class U2>
   >       constexpr bool operator==(const pair<T1, T2>& x, const pair<TU1, TU2>& y);

   > > -1- […]

   > > -2- […]

   >     template<class T1, class T2, class U1, class U2>
   >       constexpr common_comparison_category_t<synth-three-way-result<T1, U1>,
   >                                              synth-three-way-result<T2, U2>>
   >         operator<=>(const pair<T1, T2>& x, const pair<TU1, TU2>& y);

   > > -3- […]

### 3866(i). Bad Mandates for `expected::transform_error` overloads

Section: 22.8.6.7 [expected.object.monadic], 22.8.7.7 [expected.void.monadic] Status: C++23 Submitter: Casey Carter Opened: 2023-01-29 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [expected.object.monadic].

View all issues with C++23 status.

Discussion:

The overloads of `expected::transform_error` mandate that "[type] `G` is a valid value type for `expected`" (22.8.6.7 [expected.object.monadic]/27 and 31 as well as 22.8.7.7 [expected.void.monadic]/24 and 27)).

All of these overloads then instantiate `expected<T, G>` (for some type `T`) which doesn't require `G` to be a valid value type for `expected` (22.8.6.1 [expected.object.general]/2) but instead requires that `G` is "a valid template argument for `unexpected`" (22.8.6.1 [expected.object.general]/2). Comparing 22.8.6.1 [expected.object.general]/2 with 22.8.3.1 [expected.un.general]/2 it's clear that there are types — `const int`, for example — which are valid value types for `expected` but not valid template arguments for `unexpected`. Presumably this unimplementable requirement is a typo, and the subject paragraphs intended to require that `G` be a valid template argument for `unexpected`.

[2023-02-06; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 22.8.6.7 [expected.object.monadic] as indicated:

   >     template<class F> constexpr auto transform_error(F&& f) &;
   >     template<class F> constexpr auto transform_error(F&& f) const &;

   > > […]

   > > -27- Mandates: `G` is a valid value type for `expected` template argument for `unexpected` (22.8.3.1 [expected.un.general]) and the declaration

   > > >     G g(invoke(std::forward<F>(f), error()));

   > > is well-formed.

   > > […]

   > […]

   >     template<class F> constexpr auto transform_error(F&& f) &&;
   >     template<class F> constexpr auto transform_error(F&& f) const &&;

   > > […]

   > > -31- Mandates: `G` is a valid value type for `expected` template argument for `unexpected` (22.8.3.1 [expected.un.general]) and the declaration

   > > >     G g(invoke(std::forward<F>(f), std::move(error())));

   > > is well-formed.

   > > […]

2. Modify 22.8.7.7 [expected.void.monadic] as indicated:

   >     template<class F> constexpr auto transform_error(F&& f) &;
   >     template<class F> constexpr auto transform_error(F&& f) const &;

   > > […]

   > > -24- Mandates: `G` is a valid value type for `expected` template argument for `unexpected` (22.8.3.1 [expected.un.general]) and the declaration

   > > >     G g(invoke(std::forward<F>(f), error()));

   > > is well-formed.

   > > […]

   > […]

   >     template<class F> constexpr auto transform_error(F&& f) &&;
   >     template<class F> constexpr auto transform_error(F&& f) const &&;

   > > […]

   > > -27- Mandates: `G` is a valid value type for `expected` template argument for `unexpected` (22.8.3.1 [expected.un.general]) and the declaration

   > > >     G g(invoke(std::forward<F>(f), std::move(error())));

   > > is well-formed.

   > > […]

### 3867(i). Should `std::basic_osyncstream`'s move assignment operator be `noexcept`?

Section: 31.11.3.1 [syncstream.osyncstream.overview] Status: C++23 Submitter: Jiang An Opened: 2023-01-29 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [syncstream.osyncstream.overview].

View all issues with C++23 status.

Discussion:

The synopsis of `std::basic_osyncstream` (31.11.3.1 [syncstream.osyncstream.overview]) indicates that it's member functions behave as if it hold a `std::basic_syncbuf` as its subobject, and according to 16.3.3.5 [functions.within.classes], `std::basic_osyncstream`'s move assignment operator should call `std::basic_syncbuf`'s move assignment operator.

However, currently `std::basic_osyncstream`'s move assignment operator is `noexcept`, while `std::basic_syncbuf`'s is not. So when an exception is thrown from move assignment between `std::basic_syncbuf` objects, `std::terminate` should be called.

It's clarified in LWG 3498(i) that an exception can escape from `std::basic_syncbuf`'s move assignment operator. Is there any reason that an exception shouldn't escape from `std::basic_osyncstream`'s move assignment operator?

[2023-02-06; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 31.11.3.1 [syncstream.osyncstream.overview] as indicated:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
   >       class basic_osyncstream : public basic_ostream<charT, traits> {
   >       public:
   >         […]
   >         using syncbuf_type = basic_syncbuf<charT, traits, Allocator>;
   >         […]

   >         // assignment
   >         basic_osyncstream& operator=(basic_osyncstream&&) noexcept;

   >         […]

   >       private:
   >         syncbuf_type sb; // exposition only
   >       };
   >     }

### 3869(i). Deprecate `std::errc` constants related to UNIX STREAMS

Section: 19.5.2 [system.error.syn] Status: C++23 Submitter: Jonathan Wakely Opened: 2023-01-30 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [system.error.syn].

View all issues with C++23 status.

Discussion:

This is the resolution for NB comment GB-084

The error numbers `ENODATA`, `ENOSR`, `ENOSTR` and `ETIME` are all marked "obsolecent" in POSIX 2017 (the current normative reference for C++) and they are absent in the current POSIX 202x draft. They related to the obsolete STREAMS API, which was optional and not required for conformance to the previous POSIX standard (because popular unix-like systems refused to implement it). C++11 added those error numbers to `<errno.h>` and also defined corresponding `errc` enumerators: `errc::no_message_available`, `errc::no_stream_resources`, `errc::not_a_stream` and `errc::stream_timeout`.

Given the obsolescent status of those constants in the current normative reference and their absence from the next POSIX standard, WG21 should consider deprecating them now. A deprecation period will allow removing them when C++ is eventually rebased to a new POSIX standard. Otherwise C++ will be left with dangling references to `ENODATA`, `ENOSR`, `ENOSTR` and `ETIME` that are not defined in the POSIX reference.

After a period of deprecation they can be removed from Annex D, and the names added to 16.4.5.3.2 [zombie.names] so that implementations can continue to define them if they need to.

[Issaquah 2023-02-06; LWG]

Unanimous consent (9/0/0) to move to Immediate for C++23.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 19.4.2 [cerrno.syn], header `<cerrno>` synopsis, as indicated:

   >       #define ENETUNREACH see below
   >       #define ENFILE see below
   >       #define ENOBUFS see below
   >       #define ENODATA see below
   >       #define ENODEV see below
   >       #define ENOENT see below
   >       #define ENOEXEC see below
   >       #define ENOLCK see below
   >       #define ENOLINK see below
   >       #define ENOMEM see below
   >       #define ENOMSG see below
   >       #define ENOPROTOOPT see below
   >       #define ENOSPC see below
   >       #define ENOSR see below
   >       #define ENOSTR see below
   >       #define ENOSYS see below
   >       #define ENOTCONN see below
   >       #define ENOTDIR see below
   >       #define ENOTEMPTY see below
   >       ...
   >       #define EROFS see below
   >       #define ESPIPE see below
   >       #define ESRCH see below
   >       #define ETIME see below
   >       #define ETIMEDOUT see below
   >       #define ETXTBSY see below
   >       #define EWOULDBLOCK see below
   >       #define EXDEV see below

   > -1- The meaning of the macros in this header is defined by the POSIX standard.

2. Modify 19.5.2 [system.error.syn], header `<system_error>` synopsis, as indicated:

   >         no_child_process,                   // ECHILD
   >         no_link,                            // ENOLINK
   >         no_lock_available,                  // ENOLCK
   >         no_message_available,               // ENODATA
   >         no_message,                         // ENOMSG
   >         no_protocol_option,                 // ENOPROTOOPT
   >         no_space_on_device,                 // ENOSPC
   >         no_stream_resources,                // ENOSR
   >         no_such_device_or_address,          // ENXIO
   >         no_such_device,                     // ENODEV
   >         no_such_file_or_directory,          // ENOENT
   >         no_such_process,                    // ESRCH
   >         not_a_directory,                    // ENOTDIR
   >         not_a_socket,                       // ENOTSOCK
   >         not_a_stream,                       // ENOSTR
   >         not_connected,                      // ENOTCONN
   >         not_enough_memory,                  // ENOMEM
   >         ...
   >         result_out_of_range,                // ERANGE
   >         state_not_recoverable,              // ENOTRECOVERABLE
   >         stream_timeout,                     // ETIME
   >         text_file_busy,                     // ETXTBSY
   >         timed_out,                          // ETIMEDOUT

3. Modify D [depr], Annex D, Compatibility Features, by adding a new subclause before [depr.default.allocator]: :

   > D.?? Deprecated error numbers [depr.cerrno]

   > -1- The following macros are defined in addition to those specified in 19.4.2 [cerrno.syn]:

   >       #define ENODATA see below
   >       #define ENOSR see below
   >       #define ENOSTR see below
   >       #define ETIME see below

   > -2- The meaning of these macros is defined by the POSIX standard.

   > -4- The following `enum errc` enumerators are defined in addition to those specified in 19.5.2 [system.error.syn]:

   >       no_message_available,               // ENODATA
   >       no_stream_resources,                // ENOSR
   >       not_a_stream,                       // ENOSTR
   >       stream_timeout,                     // ETIME

   > -4- The value of each `enum errc` enumerator above is the same as the value of the `<cerrno>` macro shown in the above synopsis.

### 3870(i). Remove `voidify`

Section: 26.11.1 [specialized.algorithms.general] Status: C++23 Submitter: Jonathan Wakely Opened: 2023-01-30 Last modified: 2024-03-18

Priority: Not Prioritized

View all other issues in [specialized.algorithms.general].

View all issues with C++23 status.

Discussion:

This is the resolution for NB comment GB-121

The `voidify` helper breaks const-correctness, for no tangible benefit. C++20 ballot comment US 215 also suggested removing it, but failed to achieve consensus. That should be reconsidered.

The only claimed benefits are:

- Allowing the `uninitialized_xxx` algorithms to create objects in const storage (including overwriting objects declared as const which is usually UB). The caller should be responsible for using `const_cast` if that's really desirable. Implicitly removing 'const' is unsafe and unnecessary.
- Supporting creation of const objects with `std::construct_at`. This seems reasonable, but should be supported by adding a dedicated function that doesn't conflate the type of the storage to write to and the object to create, e.g. `construct_at<const T>(ptr)`.

[Issaquah 2023-02-06; LWG]

Casey noted:

> The claimed benefit is allowing the `uninitialized_xxx` algorithms to create objects of const and/or volatile type, which they cannot otherwise do since they deduce the type of object to be created from the reference type of the pertinent iterator. Creating const objects has some (marginal?) benefits over using const pointers to mutable objects. For example, their non-mutable members cannot be modified via casting away const without undefined behavior. A unit test might take advantage of this behavior to force a compiler to diagnose such undefined behavior in a constant expression.

The issue submitter was aware of this, but an open Core issue, CWG 2514, would invalidate that benefit. If accepted, objects with dynamic storage duration (such as those created by `std::construct_as` and the `std::uninitialized_xxx` algorithms) would never be const objects, so casting away the const would not be undefined. So implicitly removing const in `voidify` would still allow modifying "truly const" objects (resulting in undefined behaviour), without being able to create "truly const" objects in locations where that actually is safe. If CWG 2514 is accepted, the `voidify` behaviour would be all downside.

LWG requested removing the remaining casts from the proposed resolution, relying on an implicit conversion to `void*` instead. Move to Immediate for C++23.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4928.

> 1. Modify 26.11.1 [specialized.algorithms.general], General, as indicated:

>    > -4- Some algorithms specified in 26.11 [specialized.algorithms] make use of the exposition-only function `voidify`:

>    > >     template<class T>
>    > >       constexpr void* voidify(T& obj) noexcept {
>    > >         return const_cast<void*>(static_cast<const volatile void*>(addressof(obj)));
>    > >       }

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 26.11.1 [specialized.algorithms.general], General, as indicated:

   > -4- Some algorithms specified in 26.11 [specialized.algorithms] make use of the exposition-only function `voidify`:

   > >     template<class T>
   > >       constexpr void* voidify(T& obj) noexcept {
   > >         return const_cast<void*>(static_cast<const volatile void*>(addressof(obj)));
   > >       }

### 3871(i). Adjust note about `terminate`

Section: 16.4.2.5 [compliance] Status: C++23 Submitter: CA Opened: 2023-02-01 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [compliance].

View all issues with C++23 status.

Discussion:

This is the resolution for NB comment CA-076

16.4.2.5 [compliance] p4 has this note:

> [Note 1: Throwing a standard library provided exception is not observably different from `terminate()` if the implementation does not unwind the stack during exception handling (14.4 [except.handle]) and the user's program contains no catch blocks. — end note]

Even under the conditions described by the note, a call to `terminate()` is observably different from throwing an exception if the current `terminate_handler` function observes what would have been the currently handled exception in the case where the exception was thrown.

The set of conditions should be extended to include something along the lines of "and the current `terminate_handler` function simply calls `abort()`".

Previous resolution [SUPERSEDED]:

> This wording is relative to N4928.

> 1. Modify 16.4.2.5 [compliance], "Freestanding implementations", as indicated:

>    > -4- [Note 1: Throwing a standard library provided exception is not observably different from `terminate()` if the implementation does not unwind the stack during exception handling (14.4 [except.handle]) and the user's program contains no catch blocks and the current `terminate_handler` function simply calls `abort()`. — end note]

[Issaquah 2023-02-06; LWG]

If the note isn't true then remove it.
Poll: keep note and change as proposed? 3/1/10.
Poll: drop the note entirely? 10/0/5.
Drop the note and move to Immediate for C++20: 9/0/2.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 16.4.2.5 [compliance], "Freestanding implementations", as indicated:

   > -4- [Note 1: Throwing a standard library provided exception is not observably different from `terminate()` if the implementation does not unwind the stack during exception handling (14.4 [except.handle]) and the user's program contains no catch blocks. — end note]

### 3872(i). `basic_const_iterator` should have custom `iter_move`

Section: 24.5.3 [const.iterators] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-31 Last modified: 2023-11-22

Priority: 3

View other active issues in [const.iterators].

View all other issues in [const.iterators].

View all issues with C++23 status.

Discussion:

The standard does not currently customize `iter_move` for `basic_const_iterator`, which means that applying `iter_move` to `basic_const_iterator` will invoke the default behavior. Although the intent of such an operation is unpredictable, it does introduce some inconsistencies:

>     int x[] = {1, 2, 3};
>     using R1 = decltype(           x  | views::as_rvalue | views::as_const);
>     using R2 = decltype(           x  | views::as_const  | views::as_rvalue);
>     using Z1 = decltype(views::zip(x) | views::as_rvalue | views::as_const);
>     using Z2 = decltype(views::zip(x) | views::as_const  | views::as_rvalue);

>     static_assert(same_as<ranges::range_reference_t<R1>,       const int&&>);
>     static_assert(same_as<ranges::range_reference_t<R2>,       const int&&>);
>     static_assert(same_as<ranges::range_reference_t<Z1>, tuple<const int&&>>);
>     static_assert(same_as<ranges::range_reference_t<Z2>, tuple<const int&&>>); // failed

In the above example, `views::zip(x) | views::as_const` will produce a range whose iterator type is `basic_const_iterator` with reference of `tuple<const int&>`. Since `iter_move` adopts the default behavior, its rvalue reference will also be `tuple<const int&>`, so applying `views::as_rvalue` to it won't have any effect.

Such an inconsistency seems undesirable.

The proposed resolution adds an `iter_move` specialization for `basic_const_iterator` and specifies the return type as `common_reference_t<const iter_value_t<It>&&, iter_rvalue_reference_t<It>>`, which is the type that `input_iterator` is guaranteed to be valid. This is also in sync with the behavior of range-v3.

[Issaquah 2023-02-10; LWG issue processing]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 24.5.3 [const.iterators] as indicated:

   >     namespace std {
   >       template<class I>
   >         concept not-a-const-iterator = see below;

   >       template<indirectly_readable I>
   >         using iter-const-rvalue-reference-t =  // exposition only
   >           common_reference_t<const iter_value_t<I>&&, iter_rvalue_reference_t<I>>;

   >       template<input_iterator Iterator>
   >       class basic_const_iterator {
   >         Iterator current_ = Iterator();                             // exposition only
   >         using reference = iter_const_reference_t<Iterator>;         // exposition only
   >         using rvalue-reference = iter-const-rvalue-reference-t<Iterator>;  // exposition only

   >       public:
   >         […]
   >         template<sized_sentinel_for<Iterator> S>
   >           requires different-from<S, basic_const_iterator>
   >           friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);
   >           friend constexpr rvalue-reference iter_move(const basic_const_iterator& i)
   >             noexcept(noexcept(static_cast<rvalue-reference>(ranges::iter_move(i.current_))))
   >           {
   >             return static_cast<rvalue-reference>(ranges::iter_move(i.current_));
   >           }
   >       };
   >     }

### 3875(i). `std::ranges::repeat_view<T, IntegerClass>::iterator` may be ill-formed

Section: 25.6.5 [range.repeat] Status: C++23 Submitter: Jiang An Opened: 2023-02-05 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

25.6.5.3 [range.repeat.iterator] specifies `difference_type` as

>     using difference_type = conditional_t<is-signed-integer-like<index-type>,
>       index-type,
>       IOTA-DIFF-T(index-type)>;

which always instantiates `IOTA-DIFF-T(index-type)`, and thus possibly makes the program ill-formed when `index-type` is an integer-class type (`index-type` is same as `Bound` in this case), because `IOTA-DIFF-T(index-type)` is specified to be `iter_difference_t<index-type>` which may be ill-formed (25.6.4.2 [range.iota.view]/1.1).

I think the intent is using `index-type` as-is without instantiating `IOTA-DIFF-T` when `is-signed-integer-like<index-type>` is `true`.

However, when `Bound` is an unsigned integer-class type, it's unclear which type should the difference type be, or whether `repeat_view` should be well-formed when the possibly intended `IOTA-DIFF-T(Bound)` is ill-formed.

[2023-02-09 Tim adds wording]

We should reject types for which there is no usable difference type.

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     // […]
   >     namespace std::ranges {
   >       template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
   >         requires see below(is_object_v<W> && same_as<W, remove_cv_t<W>> &&
   >                   (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>))
   >       class repeat_view;

2. Modify 25.6.5.2 [range.repeat.view] as indicated:

   >     namespace std::ranges {

   >       template<class T>
   >       concept integer-like-with-usable-difference-type =   //exposition only
   >         is-signed-integer-like<T> || (is-integer-like<T> && weakly_incrementable<T>);

   >       template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
   >         requires (is_object_v<W> && same_as<W, remove_cv_t<W>> &&
   >                   (is-integer-likeinteger-like-with-usable-difference-type<Bound> || same_as<Bound, unreachable_sentinel_t>))
   >       class repeat_view {
   >         […]
   >       };
   >     }

3. Modify 25.6.5.3 [range.repeat.iterator] as indicated:

   >     namespace std::ranges {
   >       template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
   >         requires (is_object_v<W> && same_as<W, remove_cv_t<W>> &&
   >                   (is-integer-likeinteger-like-with-usable-difference-type<Bound> || same_as<Bound, unreachable_sentinel_t>))
   >       class repeat_view<W, Bound>::iterator {
   >       private:
   >         using index-type =                  // exposition only
   >           conditional_t<same_as<Bound, unreachable_sentinel_t>, ptrdiff_t, Bound>;
   >         […]
   >       public:
   >         […]
   >         using difference_type = see belowconditional_t<is-signed-integer-like<index-type>,
   >             index-type,
   >             IOTA-DIFF-T(index-type)>;
   >         […]
   >       };
   >     }

   -?- If `is-signed-integer-like<index-type>` is `true`, the member typedef-name `difference_type` denotes `index-type`. Otherwise, it denotes `IOTA-DIFF-T(index-type)` (25.6.4.2 [range.iota.view]).

### 3876(i). Default constructor of `std::layout_XX::mapping` misses precondition

Section: 23.7.3.4 [mdspan.layout] Status: C++23 Submitter: Christian Trott Opened: 2023-02-09 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [mdspan.layout].

View all issues with C++23 status.

Discussion:

As shortly discussed during the LWG review of a `layout_stride` defect, there is currently no protection against creating layout mappings with all static extents where the product of those extents exceeds the representable value range of the `index_type`.

For example, the following statement does not violate any preconditions or mandates:

>     layout_left::mapping<extents<int, 100000, 100000>> a{};

But `a.required_span_size()` would overflow since the implied span size is 10B and thus exceeds what `int` can represent.

This is only a problem for all static extents, since with any dynamic extent in the mix the implied span size is `0`. Hence we can check for this via a mandates check on the class.

The paper P2798R0 has been provided with the proposed wording as shown below.

[Issaquah 2023-02-10; LWG issue processing]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 23.7.3.4.5.1 [mdspan.layout.left.overview] as indicated:

   > -2- If `Extents` is not a specialization of `extents`, then the program is ill-formed.

   > -3- `layout_left::mapping<E>` is a trivially copyable type that models `regular` for each `E`.

   > -?- Mandates: If `Extents::rank_dynamic() == 0` is `true`, then the size of the multidimensional index space `Extents()` is representable as a value of type `typename Extents::index_type`.

2. Modify 23.7.3.4.6.1 [mdspan.layout.right.overview] as indicated:

   > -2- If `Extents` is not a specialization of `extents`, then the program is ill-formed.

   > -3- `layout_right::mapping<E>` is a trivially copyable type that models `regular` for each `E`.

   > -?- Mandates: If `Extents::rank_dynamic() == 0` is `true`, then the size of the multidimensional index space `Extents()` is representable as a value of type `typename Extents::index_type`.

3. Modify 23.7.3.4.7.1 [mdspan.layout.stride.overview] as indicated:

   > -2- If `Extents` is not a specialization of `extents`, then the program is ill-formed.

   > -3- `layout_stride::mapping<E>` is a trivially copyable type that models `regular` for each `E`.

   > -?- Mandates: If `Extents::rank_dynamic() == 0` is `true`, then the size of the multidimensional index space `Extents()` is representable as a value of type `typename Extents::index_type`.

### 3877(i). Incorrect constraints on `const`-qualified monadic overloads for `std::expected`

Section: 22.8.6.7 [expected.object.monadic], 22.8.7.7 [expected.void.monadic] Status: C++23 Submitter: Sy Brand Opened: 2023-02-09 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [expected.object.monadic].

View all issues with C++23 status.

Discussion:

The constraints for `and_then`, `transform`, `transform_error`, and `or_else` for `std::expected` seem incorrect for `const` overloads. E.g., from 22.8.6.7 [expected.object.monadic]

>     template<class F> constexpr auto transform(F&& f) &&;
>     template<class F> constexpr auto transform(F&& f) const &&;
>     […]

> > Constraints: `is_move_constructible_v<E>` is `true`.

That constraint should likely be `is_move_constructible_v<const E>` for the `const`-qualified version. Same for the lvalue overloads, and for the three other functions, including in the `void` partial specialization. For example, currently this code would result in a hard compiler error inside the body of `transform` rather than failing the constraint:

>     const std::expected<int, std::unique_ptr<int>> e;
>     std::move(e).transform([](auto) { return 42; });

Previous resolution [SUPERSEDED]:

> This wording is relative to N4928.

> 1. Modify 22.8.6.7 [expected.object.monadic] as indicated:

>    >     template<class F> constexpr auto and_then(F&& f) &;
>    >     template<class F> constexpr auto and_then(F&& f) const &;

>    > > […]

>    > > -2- Constraints: `is_copy_constructible_v<Edecltype((error()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto and_then(F&& f) &&;
>    >     template<class F> constexpr auto and_then(F&& f) const &&;

>    > > […]

>    > > -6- Constraints: `is_move_constructible_v<Edecltype((error()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto or_else(F&& f) &;
>    >     template<class F> constexpr auto or_else(F&& f) const &;

>    > > […]

>    > > -10- Constraints: `is_copy_constructible_v<Tdecltype((value()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto or_else(F&& f) &&;
>    >     template<class F> constexpr auto or_else(F&& f) const &&;

>    > > […]

>    > > -14- Constraints: `is_move_constructible_v<Tdecltype((value()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto transform(F&& f) &;
>    >     template<class F> constexpr auto transform(F&& f) const &;

>    > > […]

>    > > -18- Constraints: `is_copy_constructible_v<Edecltype((error()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto transform(F&& f) &&;
>    >     template<class F> constexpr auto transform(F&& f) const &&;

>    > > […]

>    > > -22- Constraints: `is_move_constructible_v<Edecltype((error()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto transform_error(F&& f) &;
>    >     template<class F> constexpr auto transform_error(F&& f) const &;

>    > > […]

>    > > -26- Constraints: `is_copy_constructible_v<Tdecltype((value()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto transform_error(F&& f) &&;
>    >     template<class F> constexpr auto transform_error(F&& f) const &&;

>    > > […]

>    > > -30- Constraints: `is_move_constructible_v<Tdecltype((value()))>` is `true`.

> 2. Modify 22.8.7.7 [expected.void.monadic] as indicated:

>    >     template<class F> constexpr auto and_then(F&& f) &;
>    >     template<class F> constexpr auto and_then(F&& f) const &;

>    > > […]

>    > > -2- Constraints: `is_copy_constructible_v<Edecltype((error()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto and_then(F&& f) &&;
>    >     template<class F> constexpr auto and_then(F&& f) const &&;

>    > > […]

>    > > -6- Constraints: `is_move_constructible_v<Edecltype((error()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto transform(F&& f) &;
>    >     template<class F> constexpr auto transform(F&& f) const &;

>    > > […]

>    > > -16- Constraints: `is_copy_constructible_v<Edecltype((error()))>` is `true`.

>    > > […]

>    >     template<class F> constexpr auto transform(F&& f) &&;
>    >     template<class F> constexpr auto transform(F&& f) const &&;

>    > > […]

>    > > -20- Constraints: `is_move_constructible_v<Edecltype((error()))>` is `true`.

>    > > […]

[Issaquah 2023-02-09; Jonathan provides improved wording]

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 22.8.6.7 [expected.object.monadic] as indicated:

   >     template<class F> constexpr auto and_then(F&& f) &;
   >     template<class F> constexpr auto and_then(F&& f) const &;

   > > […]

   > > -2- Constraints: `is_copy_constructible_v<E, decltype(error())>` is `true`.

   > > […]

   >     template<class F> constexpr auto and_then(F&& f) &&;
   >     template<class F> constexpr auto and_then(F&& f) const &&;

   > > […]

   > > -6- Constraints: `is_copy_constructible_v<E, decltype(std::move(error()))>` is `true`.

   > > […]

   >     template<class F> constexpr auto or_else(F&& f) &;
   >     template<class F> constexpr auto or_else(F&& f) const &;

   > > […]

   > > -10- Constraints: `is_copy_constructible_v<T, decltype(value())>` is `true`.

   > > […]

   >     template<class F> constexpr auto or_else(F&& f) &&;
   >     template<class F> constexpr auto or_else(F&& f) const &&;

   > > […]

   > > -14- Constraints: `is_copy_constructible_v<T, decltype(std::move(value()))>` is `true`.

   > > […]

   >     template<class F> constexpr auto transform(F&& f) &;
   >     template<class F> constexpr auto transform(F&& f) const &;

   > > […]

   > > -18- Constraints: `is_copy_constructible_v<E, decltype(error())>` is `true`.

   > > […]

   >     template<class F> constexpr auto transform(F&& f) &&;
   >     template<class F> constexpr auto transform(F&& f) const &&;

   > > […]

   > > -22- Constraints: `is_copy_constructible_v<E, decltype(std::move(error()))>` is `true`.

   > > […]

   >     template<class F> constexpr auto transform_error(F&& f) &;
   >     template<class F> constexpr auto transform_error(F&& f) const &;

   > > […]

   > > -26- Constraints: `is_copy_constructible_v<T, decltype(value())>` is `true`.

   > > […]

   >     template<class F> constexpr auto transform_error(F&& f) &&;
   >     template<class F> constexpr auto transform_error(F&& f) const &&;

   > > […]

   > > -30- Constraints: `is_copy_constructible_v<T, decltype(std::move(value()))>` is `true`.

2. Modify 22.8.7.7 [expected.void.monadic] as indicated:

   >     template<class F> constexpr auto and_then(F&& f) &;
   >     template<class F> constexpr auto and_then(F&& f) const &;

   > > […]

   > > -2- Constraints: `is_copy_constructible_v<E, decltype(error())>` is `true`.

   > > […]

   >     template<class F> constexpr auto and_then(F&& f) &&;
   >     template<class F> constexpr auto and_then(F&& f) const &&;

   > > […]

   > > -6- Constraints: `is_copy_constructible_v<E, decltype(std::move(error()))>` is `true`.

   > > […]

   >     template<class F> constexpr auto transform(F&& f) &;
   >     template<class F> constexpr auto transform(F&& f) const &;

   > > […]

   > > -16- Constraints: `is_copy_constructible_v<E, decltype(error())>` is `true`.

   > > […]

   >     template<class F> constexpr auto transform(F&& f) &&;
   >     template<class F> constexpr auto transform(F&& f) const &&;

   > > […]

   > > -20- Constraints: `is_copy_constructible_v<E, decltype(std::move(error()))>` is `true`.

   > > […]

### 3878(i). `import std;` should guarantee initialization of standard iostreams objects

Section: 31.4.2 [iostream.objects.overview] Status: C++23 Submitter: Tim Song Opened: 2023-02-09 Last modified: 2024-01-29

Priority: Not Prioritized

View all other issues in [iostream.objects.overview].

View all issues with C++23 status.

Discussion:

In the old world, `#include <iostream>` behaves as if it defined a static-storage-duration `ios_base::Init` object, which causes the standard iostreams objects to be initialized (if necessary) on startup and flushed on shutdown.

But we don't include headers with `import std;`, so we need separate wording to provide this guarantee. The proposed resolution below was adapted from a suggestion by Mathias Stearn on the reflector.

[2023-02-09 Tim updates wording following LWG discussion]

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 31.4.2 [iostream.objects.overview]p5 as indicated:

   -5- The results of including `<iostream>` in a translation unit shall be as if `<iostream>` defined an instance of `ios_base::Init` with static storage duration. Each C++ library module (16.4.2.4 [std.modules]) in a hosted implementation shall behave as if it contains an interface unit that defines an unexported `ios_base::Init` variable with ordered initialization (6.10.3.3 [basic.start.dynamic]).

   [Note ?: As a result, the definition of that variable is appearance-ordered before any declaration following the point of importation of a C++ library module. Whether such a definition exists is unobservable by a program that does not reference any of the standard iostream objects. — end note]

### 3879(i). `erase_if` for `flat_{,multi}set` is incorrectly specified

Section: 23.6.11.6 [flat.set.erasure], 23.6.12.6 [flat.multiset.erasure] Status: C++23 Submitter: Tim Song Opened: 2023-02-09 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

The current specification of `erase_if` for `flat_{,multi}set` calls `ranges::remove_if` on the set, which is obviously incorrect — the set only present constant views of its elements.

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 23.6.11.6 [flat.set.erasure] as indicated:

   >     template<class Key, class Compare, class KeyContainer, class Predicate>
   >       typename flat_set<Key, Compare, KeyContainer>::size_type
   >         erase_if(flat_set<Key, Compare, KeyContainer>& c, Predicate pred);

   > > -1- Effects: Equivalent to:

   > > >     auto [erase_first, erase_last] = ranges::remove_if(c, pred);
   > > >     auto n = erase_last - erase_first;
   > > >     c.erase(erase_first, erase_last);
   > > >     return n;

   > > -1- Preconditions: `Key` meets the Cpp17MoveAssignable requirements.

   > > -2- Effects: Let E be `bool(pred(as_const(e)))`. Erases all elements `e` in `c` for which E holds.

   > > -3- Returns: The number of elements erased.

   > > -4- Complexity: Exactly `c.size()` applications of the predicate.

   > > -5- Remarks: Stable (16.4.6.8 [algorithm.stable]). If an invocation of `erase_if` exits via an exception, `c` is in a valid but unspecified state (3.68 [defns.valid]).

   > > [Note 1: `c` still meets its invariants, but can be empty. — end note]

2. Modify 23.6.12.6 [flat.multiset.erasure] as indicated:

   >     template<class Key, class Compare, class KeyContainer, class Predicate>
   >       typename flat_multiset<Key, Compare, KeyContainer>::size_type
   >         erase_if(flat_multiset<Key, Compare, KeyContainer>& c, Predicate pred);

   > > -1- Effects: Equivalent to:

   > > >     auto [erase_first, erase_last] = ranges::remove_if(c, pred);
   > > >     auto n = erase_last - erase_first;
   > > >     c.erase(erase_first, erase_last);
   > > >     return n;

   > > -1- Preconditions: `Key` meets the Cpp17MoveAssignable requirements.

   > > -2- Effects: Let E be `bool(pred(as_const(e)))`. Erases all elements `e` in `c` for which E holds.

   > > -3- Returns: The number of elements erased.

   > > -4- Complexity: Exactly `c.size()` applications of the predicate.

   > > -5- Remarks: Stable (16.4.6.8 [algorithm.stable]). If an invocation of `erase_if` exits via an exception, `c` is in a valid but unspecified state (3.68 [defns.valid]).

   > > [Note 1: `c` still meets its invariants, but can be empty. — end note]

### 3880(i). Clarify `operator+=` complexity for `{chunk,stride}_view::iterator`

Section: 25.7.29.7 [range.chunk.fwd.iter], 25.7.32.3 [range.stride.iterator] Status: C++23 Submitter: Tim Song Opened: 2023-02-09 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

The intent was that the precondition allows the call to `ranges::advance`, which otherwise would have time linear in the argument of `operator+=`, to actually be implemented using `operator+=` or equivalent for all but the last step. This is at best very non-obvious and should be clarified.

[Issaquah 2023-02-10; LWG issue processing]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.7.29.7 [range.chunk.fwd.iter] p13 as indicated:

   >     constexpr iterator& operator+=(difference_type x)
   >       requires random_access_range<Base>;

   > > -12- Preconditions: If `x` is positive, `ranges::distance(current_, end_) > n_ * (x - 1)` is `true`.

   > > [Note 1: If `x` is negative, the Effects paragraph implies a precondition. — end note]

   > > -13- Effects: Equivalent to:

   > > >     if (x > 0) {
   > > >       ranges::advance(current_, n_ * (x - 1));
   > > >       missing_ = ranges::advance(current_, n_ * x, end_);
   > > >     } else if (x < 0) {
   > > >       ranges::advance(current_, n_ * x + missing_);
   > > >       missing_ = 0;
   > > >     }
   > > >     return *this;

2. Modify 25.7.32.3 [range.stride.iterator] p14 as indicated:

   >     constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>;

   > > -13- Preconditions: If `n` is positive, `ranges::distance(current_, end_) > stride_ * (n - 1)` is `true`.

   > > [Note 1: If `n` is negative, the Effects paragraph implies a precondition. — end note]

   > > -14- Effects: Equivalent to:

   > > >     if (n > 0) {
   > > >       ranges::advance(current_, stride_ * (n - 1));
   > > >       missing_ = ranges::advance(current_, stride_ * n, end_);
   > > >     } else if (n < 0) {
   > > >       ranges::advance(current_, stride_ * n + missing_);
   > > >       missing_ = 0;
   > > >     }
   > > >     return *this;

### 3881(i). Incorrect formatting of container adapters backed by `std::string`

Section: 23.6.13 [container.adaptors.format] Status: C++23 Submitter: Victor Zverovich Opened: 2023-02-10 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

According to 23.6.13 [container.adaptors.format] container adapters such as `std::stack` are formatted by forwarding to the underlying container:

>     template<class FormatContext>
>       typename FormatContext::iterator
>         format(maybe-const-adaptor& r, FormatContext& ctx) const;

> > Effects: Equivalent to: `return underlying_.format(r.c, ctx);`

This gives expected results for `std::stack<T>` and most types of underlying container:

>     auto s = std::format("{}", std::stack(std::deque{'a', 'b', 'c'}));
>     // s == "['a', 'b', 'c']"

However, when the underlying container is `std::string` the output is:

>     auto s = std::format("{}", std::stack{std::string{"abc"}});
>     // s == "abc"

This is clearly incorrect because `std::stack` itself is not a string (it is only backed by a string) and inconsistent with formatting of ranges where non-string range types are formatted as comma-separated values delimited by `'['` and `']'`. The correct output in this case would be `['a', 'b', 'c']`.

Here is an illustration of this issue on godbolt using {fmt} and an implementation of the formatter for container adapters based on the one from the standard: https://godbolt.org/z/P1nrM1986.

A simple fix is to wrap the underlying container in `std::views::all(_t)` (https://godbolt.org/z/8MT1be838).
Previous resolution [SUPERSEDED]

This wording is relative to N4928.

1. Modify 23.6.13 [container.adaptors.format] as indicated:

   > -1- For each of `queue`, `priority_queue`, and `stack`, the library provides the following formatter specialization where `adaptor-type` is the name of the template:

   > >     namespace std {
   > >       template<class charT, class T, formattable<charT> Container, class... U>
   > >       struct formatter<adaptor-type<T, Container, U...>, charT> {
   > >       private:
   > >         using maybe-const-adaptor =                       // exposition only
   > >           fmt-maybe-const<adaptor-type<T, Container, U...>, charT>;
   > >         formatter<views::all_t<const Container&>, charT> underlying_;          // exposition only

   > >       public:
   > >         template<class ParseContext>
   > >           constexpr typename ParseContext::iterator
   > >             parse(ParseContext& ctx);

   > >         template<class FormatContext>
   > >           typename FormatContext::iterator
   > >             format(maybe-const-adaptor& r, FormatContext& ctx) const;
   > >       };
   > >     }

   > […]

   >     template<class FormatContext>
   >       typename FormatContext::iterator
   >         format(maybe-const-adaptor& r, FormatContext& ctx) const;

   > > -3- Effects: Equivalent to: `return underlying_.format(views::all(r.c), ctx);`

[2023-02-10 Tim provides updated wording]

The container elements may not be const-formattable so we cannot use the `const` formatter unconditionally. Also the current wording is broken because an adaptor is not range and we cannot use `fmt-maybe-const` on the adaptor — only the underlying container.

[Issaquah 2023-02-10; LWG issue processing]

Move to Immediate for C++23

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 23.6.13 [container.adaptors.format] as indicated:

   > -1- For each of `queue`, `priority_queue`, and `stack`, the library provides the following formatter specialization where `adaptor-type` is the name of the template:

   > >     namespace std {
   > >       template<class charT, class T, formattable<charT> Container, class... U>
   > >       struct formatter<adaptor-type<T, Container, U...>, charT> {
   > >       private:
   > >         using maybe-const-container =                     // exposition only
   > >           fmt-maybe-const<Container, charT>;
   > >         using maybe-const-adaptor =                       // exposition only
   > >           fmt-maybe-const<is_const_v<maybe-const-container>, adaptor-type<T, Container, U...>, charT>; // see 25.2 [ranges.syn]

   > >         formatter<ranges::ref_view<maybe-const-container>Container, charT> underlying_;          // exposition only

   > >       public:
   > >         template<class ParseContext>
   > >           constexpr typename ParseContext::iterator
   > >             parse(ParseContext& ctx);

   > >         template<class FormatContext>
   > >           typename FormatContext::iterator
   > >             format(maybe-const-adaptor& r, FormatContext& ctx) const;
   > >       };
   > >     }

   > […]

   >     template<class FormatContext>
   >       typename FormatContext::iterator
   >         format(maybe-const-adaptor& r, FormatContext& ctx) const;

   > > -3- Effects: Equivalent to: `return underlying_.format(r.c, ctx);`

### 3884(i). `flat_foo` is missing allocator-extended copy/move constructors

Section: 23.6.8 [flat.map], 23.6.9 [flat.multimap], 23.6.11 [flat.set], 23.6.12 [flat.multiset] Status: C++26 Submitter: Arthur O'Dwyer Opened: 2023-02-06 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [flat.map].

View all other issues in [flat.map].

View all issues with C++26 status.

Discussion:

This issue is part of the "`flat_foo`" sequence, LWG 3786(i), 3802(i), 3803(i), 3804(i). `flat_set`, `flat_multiset`, `flat_map`, and `flat_multimap` all have implicitly defaulted copy and move constructors, but they lack allocator-extended versions of those constructors. This means that the following code is broken:

>     // https://godbolt.org/z/qezv5rTrW
>     #include <cassert>
>     #include <flat_set>
>     #include <memory_resource>
>     #include <utility>
>     #include <vector>

>     template<class T, class Comp = std::less<T>>
>     using pmr_flat_set = std::flat_set<T, Comp, std::pmr::vector<T>>;

>     int main() {
>       std::pmr::vector<pmr_flat_set<int>> vs = {
>         {1,2,3},
>         {4,5,6},
>       };
>       std::pmr::vector<int> v = std::move(vs[0]).extract();
>       assert(v.get_allocator().resource() == std::pmr::get_default_resource());
>     }

`pmr_flat_set<int>` advertises that it "`uses_allocator`" `std::pmr::polymorphic_allocator<int>`, but in fact it lacks the allocator-extended constructor overload set necessary to interoperate with `std::pmr::vector`.

[2023-03-22; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 23.6.8.2 [flat.map.defn] as indicated:

   >     namespace std {
   >       template<class Key, class T, class Compare = less<Key>,
   >       class KeyContainer = vector<Key>, class MappedContainer = vector<T>>
   >       class flat_map {
   >       public:
   >         […]
   >         // 23.6.8.3 [flat.map.cons], construct/copy/destroy
   >         flat_map() : flat_map(key_compare()) { }

   >         template<class Allocator>
   >           flat_map(const flat_map&, const Allocator& a);
   >         template<class Allocator>
   >           flat_map(flat_map&&, const Allocator& a);

   >         […]
   >       };
   >       […]
   >     }

2. Modify 23.6.8.3 [flat.map.cons] as indicated:

   > [Drafting note: As a drive-by fix, a missing closing `>` is inserted in p11.]

   >     template<class Allocator>
   >       flat_map(const flat_map&, const Allocator& a);
   >     template<class Allocator>
   >       flat_map(flat_map&&, const Allocator& a);
   >     template<class Allocator>
   >       flat_map(const key_compare& comp, const Allocator& a);
   >     template<class Allocator>
   >       explicit flat_map(const Allocator& a);
   >     template<class InputIterator, class Allocator>
   >       flat_map(InputIterator first, InputIterator last, const key_compare& comp, const Allocator& a);
   >     template<class InputIterator, class Allocator>
   >       flat_map(InputIterator first, InputIterator last, const Allocator& a);
   >     […]

   > > -11- Constraints: `uses_allocator_v<key_container_type, Allocator>` is `true` and `uses_allocator_v<mapped_container_type, Allocator>` is `true`.

   > > -12- Effects: Equivalent to the corresponding non-allocator constructors except that `c.keys` and `c.values` are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

3. Modify 23.6.9.2 [flat.multimap.defn] as indicated:

   >     namespace std {
   >       template<class Key, class T, class Compare = less<Key>,
   >       class KeyContainer = vector<Key>, class MappedContainer = vector<T>>
   >       class flat_multimap {
   >       public:
   >         […]
   >         // 23.6.9.3 [flat.multimap.cons], construct/copy/destroy
   >         flat_multimap() : flat_multimap(key_compare()) { }

   >         template<class Allocator>
   >           flat_multimap(const flat_multimap&, const Allocator& a);
   >         template<class Allocator>
   >           flat_multimap(flat_multimap&&, const Allocator& a);

   >         […]
   >       };
   >       […]
   >     }

4. Modify 23.6.9.3 [flat.multimap.cons] as indicated:

   >     template<class Allocator>
   >       flat_multimap(const flat_multimap&, const Allocator& a);
   >     template<class Allocator>
   >       flat_multimap(flat_multimap&&, const Allocator& a);
   >     template<class Allocator>
   >       flat_multimap(const key_compare& comp, const Allocator& a);
   >     […]

   > > -11- Constraints: `uses_allocator_v<key_container_type, Allocator>` is `true` and `uses_allocator_v<mapped_container_type, Allocator>` is `true`.

   > > -12- Effects: Equivalent to the corresponding non-allocator constructors except that `c.keys` and `c.values` are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

5. Modify 23.6.11.2 [flat.set.defn] as indicated:

   >     namespace std {
   >       template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
   >       class flat_set {
   >       public:
   >         […]
   >         // 23.6.11.3 [flat.set.cons], constructors
   >         flat_set() : flat_set(key_compare()) { }

   >         template<class Allocator>
   >           flat_set(const flat_set&, const Allocator& a);
   >         template<class Allocator>
   >           flat_set(flat_set&&, const Allocator& a);

   >         […]
   >       };
   >       […]
   >     }

6. Modify 23.6.11.3 [flat.set.cons] as indicated:

   >     template<class Allocator>
   >       flat_set(const flat_set&, const Allocator& a);
   >     template<class Allocator>
   >       flat_set(flat_set&&, const Allocator& a);
   >     template<class Allocator>
   >       flat_set(const key_compare& comp, const Allocator& a);
   >     […]

   > > -9- Constraints: `uses_allocator_v<container_type, Allocator>` is `true`.

   > > -10- Effects: Equivalent to the corresponding non-allocator constructors except that `c` is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

7. Modify 23.6.12.2 [flat.multiset.defn] as indicated:

   >     namespace std {
   >       template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
   >       class flat_multiset {
   >       public:
   >         […]
   >         // 23.6.12.3 [flat.multiset.cons], constructors
   >         flat_multiset() : flat_multiset(key_compare()) { }

   >         template<class Allocator>
   >           flat_multiset(const flat_multiset&, const Allocator& a);
   >         template<class Allocator>
   >           flat_multiset(flat_multiset&&, const Allocator& a);

   >         […]
   >       };
   >       […]
   >     }

8. Modify 23.6.12.3 [flat.multiset.cons] as indicated:

   >     template<class Allocator>
   >       flat_multiset(const flat_multiset&, const Allocator& a);
   >     template<class Allocator>
   >       flat_multiset(flat_multiset&&, const Allocator& a);
   >     template<class Allocator>
   >       flat_multiset(const key_compare& comp, const Allocator& a);
   >     […]

   > > -9- Constraints: `uses_allocator_v<container_type, Allocator>` is `true`.

   > > -10- Effects: Equivalent to the corresponding non-allocator constructors except that `c` is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

### 3885(i). '`op`' should be in [zombie.names]

Section: 16.4.5.3.2 [zombie.names] Status: C++26 Submitter: Jonathan Wakely Opened: 2023-02-11 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [zombie.names].

View all issues with C++26 status.

Discussion:

C++98 and C++11 defined a protected member `std::bind1st::op` so '`op`' should still be a reserved name, just like '`bind1st`' itself.

[2023-03-22; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 16.4.5.3.2 [zombie.names] as indicated:

   > -2- The following names are reserved as members for previous standardization, and may not be used as a name for object-like macros in portable code:

   > 1. (2.1) — `argument_type`,

   > 2. […]

   > 3. (2.3) — `io_state`,

   > 4. (2.?) — `op`,

   > 5. (2.4) — `open_mode`,

   > 6. […]

### 3886(i). Monad mo' problems

Section: 22.5.3.1 [optional.optional.general], 22.8.6.1 [expected.object.general] Status: C++26 Submitter: Casey Carter Opened: 2023-02-13 Last modified: 2026-06-09

Priority: 3

View all other issues in [optional.optional.general].

View all issues with C++26 status.

Discussion:

While implementing P2505R5 "Monadic Functions for `std::expected`" we found it odd that the template type parameter for the assignment operator that accepts an argument by forwarding reference is defaulted, but the template type parameter for `value_or` is not. For consistency, it would seem that `meow.value_or(woof)` should accept the same arguments `woof` as does `meow = woof`, even when those arguments are braced-initializers.

That said, it would be peculiar to default the template type parameter of `value_or` to `T` instead of `remove_cv_t<T>`. For `expected<const vector<int>, int> meow{unexpect, 42};`, for example, `meow.value_or({1, 2, 3})` would create a temporary `const vector<int>` for the argument and return a copy of that argument. Were the default template argument instead `remove_cv_t<T>`, `meow.value_or({1, 2, 3})` could move construct its return value from the argument `vector<int>`. For the same reason, the constructor that accepts a forwarding reference with a default template argument of `T` should default that argument to `remove_cv_t<T>`.

For consistency, it would be best to default the template argument of the perfect-forwarding construct, perfect-forwarding assignment operator, and `value_or` to `remove_cv_t<T>`. Since all of the arguments presented apply equally to `optional`, we believe `optional` should be changed consistently with `expected`. MSVCSTL has prototyped these changes successfully.

[2023-03-22; Reflector poll]

Set priority to 3 after reflector poll.

[2024-09-18; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 22.5.3.1 [optional.optional.general] as indicated:

   >     namespace std {
   >       template<class T>
   >       class optional {
   >       public:
   >         […]
   >         template<class U = remove_cv_t<T>>
   >           constexpr explicit(see below) optional(U&&);
   >         […]
   >         template<class U = remove_cv_t<T>> constexpr optional& operator=(U&&);
   >         […]
   >         template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const &;
   >         template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&;
   >         […]
   >       };
   >       […]
   >     }

2. Modify 22.5.3.2 [optional.ctor] as indicated:

   >     template<class U = remove_cv_t<T>> constexpr explicit(see below) optional(U&& v);

   > > -23- Constraints: […]

3. Modify 22.5.3.4 [optional.assign] as indicated:

   >     template<class U = remove_cv_t<T>> constexpr optional& operator=(U&& v);

   > > -12- Constraints: […]

4. Modify 22.5.3.7 [optional.observe] as indicated:

   >     template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;

   > > -15- Mandates: […]

   > > […]

   >     template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;

   > > -17- Mandates: […]

5. Modify 22.8.6.1 [expected.object.general] as indicated:

   >     namespace std {
   >       template<class T, class E>
   >       class expected {
   >       public:
   >         […]
   >         template<class U = remove_cv_t<T>>
   >           constexpr explicit(see below) expected(U&& v);
   >         […]
   >         template<class U = remove_cv_t<T>> constexpr expected& operator=(U&&);
   >         […]
   >         template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const &;
   >         template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&;
   >         […]
   >       };
   >       […]
   >     }

6. Modify 22.8.6.2 [expected.object.cons] as indicated:

   >     template<class U = remove_cv_t<T>>
   >       constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);

   > > -23- Constraints: […]

7. Modify 22.8.6.4 [expected.object.assign] as indicated:

   >     template<class U = remove_cv_t<T>>
   >       constexpr expected& operator=(U&& v);

   > > -9- Constraints: […]

8. Modify 22.8.6.6 [expected.object.obs] as indicated:

   >     template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;

   > > -16- Mandates: […]

   > > […]

   >     template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;

   > > -18- Mandates: […]

### 3887(i). Version macro for `allocate_at_least`

Section: 17.3.2 [version.syn] Status: C++26 Submitter: Alisdair Meredith Opened: 2023-02-14 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++26 status.

Discussion:

In Issaquah, we just adopted P2652R0, forbidding user specialization of `allocator_traits`.

It looks like we did not deem that worthy of a feature macro.

However, it also changed the interface of the C++23 addition, `allocate_at_least`, which is covered by the macro `__cpp_lib_allocate_at_least`.

I believe we should have updated that macro for this significant change in how that function is accessed (from free function to a member of `allocator_traits`). Unfortunately, there are no more meetings to process a comment to that effect, and I suspect this rises beyond the purview of an editorial change, although I live in hope (as the original paper left the value of the macro to the editor, although we approved existing working papers where that macro does have a value, i.e., status quo).

[2023-03-22; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll. But "if we don’t get the editors to do it for C++23 there doesn’t seem to be any point doing it."

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 17.3.2 [version.syn], header `<version>` synopsis, as indicated and replace the placeholder `YYYYMM` by the year and month of adoption of P2652R0:

   >     […]
   >     #define __cpp_lib_algorithm_iterator_requirements  202207L
   >       // also in <algorithm>, <numeric>, <memory>
   >     #define __cpp_lib_allocate_at_least                202106YYYYMML // also in <memory>
   >     #define __cpp_lib_allocator_traits_is_always_equal 201411L
   >       // also in […]
   >     […]

### 3891(i). LWG 3870 breaks `std::expected<cv T, E>`

Section: 22.8.6.1 [expected.object.general] Status: C++26 Submitter: Jiang An Opened: 2023-02-19 Last modified: 2026-06-09

Priority: 2

View all other issues in [expected.object.general].

View all issues with C++26 status.

Discussion:

Currently the `value_type` of `std::expected` can be a cv-qualified type, which is possibly intended. However, LWG 3870(i) disallows `std::construct_at` to construct objects via `cv T*`, which breaks `std::expected<cv T, E>` because some operations are specified with `std::construct_at` (22.8.6.4 [expected.object.assign], 22.8.6.5 [expected.object.swap]).

I think when `T` is cv-qualified, it would be better to store `std::remove_cv_t<T>` subobject while sometimes (other than construction/destruction) access it via a cv-qualified glvalue, which can also avoid UB associated with `const`/`volatile` objects.

[2023-03-22; Reflector poll]

Set priority to 2 after reflector poll. "Not clear if all these wording changes are needed or desired." "Unconvinced that the mixed-value-error swap should use `value()`, source is destroyed immediately anyway. The else branch should use `remove_cv_t` too."

Previous resolution [SUPERSEDED]

This wording is relative to N4928.

> [Drafting note: When assignment and `swap` need to backup the old value by move construction, the source should be considered cv-unqualified, as the backup mechanism is only used internally.]

1. Modify 22.8.6.1 [expected.object.general] as indicated:

   >     […]
   >     bool has_val;      // exposition only
   >     union {
   >       remove_cv_t<T> val;       // exposition only
   >       E unex;          // exposition only
   >     };
   >     […]

2. Modify 22.8.6.4 [expected.object.assign] as indicated:

   >     constexpr expected& operator=(const expected& rhs);

   > > -2- Effects:

   > > 1. (2.1) — If `this->has_value() && rhs.has_value()` is `true`, equivalent to `value()val = *rhs`.

   > > 2. […]

   > […]

   >     constexpr expected& operator=(expected&& rhs) noexcept(see below);

   > > […]

   > > -6- Effects:

   > > 1. (6.1) — If `this->has_value() && rhs.has_value()` is `true`, equivalent to `value()val = std::move(*rhs)`.

   > > 2. […]

   > […]

   >     template<class U = T>
   >       constexpr expected& operator=(U&& v);

   > > […]

   > > -10- Effects:

   > > 1. (10.1) — If `has_value()` is `true`, equivalent to `value()val = std::forward<U>(v)`.

   > > 2. […]

3. Modify Table 64: `swap(expected&)` effects [tab:expected.object.swap] as indicated:

   > Table 64 — `swap(expected&)` effects [tab:expected.object.swap]
   > | `this->has_value()` | `!this->has_value()`
   > --- | --- | ---
   > `rhs.has_value()` | equivalent to: `using std::swap;` `swap(value()val, rhs.value()val);` | calls `rhs.swap(*this)`
   > `[…]`

4. Modify 22.8.6.5 [expected.object.swap] as indicated:

   >     constexpr void swap(expected& rhs) noexcept(see below);

   > > -1- Constraints: […]

   > > -2- Effects: See Table 64 [tab:expected.object.swap].

   > > For the case where `rhs.value()` is `false` and `this->has_value()` is `true`, equivalent to:

   > > >     if constexpr (is_nothrow_move_constructible_v<E>) {
   > > >       E tmp(std::move(rhs.unex));
   > > >       destroy_at(addressof(rhs.unex));
   > > >       try {
   > > >         construct_at(addressof(rhs.val), std::move(value()val));
   > > >         destroy_at(addressof(val));
   > > >         construct_at(addressof(unex), std::move(tmp));
   > > >       } catch(...) {
   > > >         construct_at(addressof(rhs.unex), std::move(tmp));
   > > >         throw;
   > > >       }
   > > >     } else {
   > > >       T tmp(std::move(val));
   > > >       destroy_at(addressof(val));
   > > >       try {
   > > >         construct_at(addressof(unex), std::move(rhs.unex));
   > > >         destroy_at(addressof(rhs.unex));
   > > >         construct_at(addressof(rhs.val), std::move(tmp));
   > > >       } catch (...) {
   > > >         construct_at(addressof(val), std::move(tmp));
   > > >         throw;
   > > >       }
   > > >     }
   > > >     has_val = false;
   > > >     rhs.has_val = true;

[2024-10-02; Jonathan provides improved wording]

Removed the use of `value()` in the [expected.object.swap] p2 Effects: and added `remove_cv_t` to the local `T` in the `else`-branch.

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

> [Drafting note: When assignment and `swap` need to backup the old value by move construction, the source should be considered cv-unqualified, as the backup mechanism is only used internally.]

1. Modify 22.8.6.1 [expected.object.general] as indicated:

   >     […]
   >     bool has_val;      // exposition only
   >     union {
   >       remove_cv_t<T> val;       // exposition only
   >       E unex;          // exposition only
   >     };
   >     […]

2. Modify 22.8.6.4 [expected.object.assign] as indicated:

   >     constexpr expected& operator=(const expected& rhs);

   > > -2- Effects:

   > > 1. (2.1) — If `this->has_value() && rhs.has_value()` is `true`, equivalent to `value()val = *rhs`.

   > > 2. […]

   > […]

   >     constexpr expected& operator=(expected&& rhs) noexcept(see below);

   > > […]

   > > -6- Effects:

   > > 1. (6.1) — If `this->has_value() && rhs.has_value()` is `true`, equivalent to `value()val = std::move(*rhs)`.

   > > 2. […]

   > […]

   >     template<class U = T>
   >       constexpr expected& operator=(U&& v);

   > > […]

   > > -10- Effects:

   > > 1. (10.1) — If `has_value()` is `true`, equivalent to `value()val = std::forward<U>(v)`.

   > > 2. […]

3. Modify Table 64: `swap(expected&)` effects [tab:expected.object.swap] as indicated:

   > Table 64 — `swap(expected&)` effects [tab:expected.object.swap]
   > | `this->has_value()` | `!this->has_value()`
   > --- | --- | ---
   > `rhs.has_value()` | equivalent to: `using std::swap;` `swap(value()val, rhs.value()val);` | calls `rhs.swap(*this)`
   > `[…]`

4. Modify 22.8.6.5 [expected.object.swap] as indicated:

   >     constexpr void swap(expected& rhs) noexcept(see below);

   > > -1- Constraints: […]

   > > -2- Effects: See Table 64 [tab:expected.object.swap].

   > > For the case where `rhs.value()` is `false` and `this->has_value()` is `true`, equivalent to:

   > > >     if constexpr (is_nothrow_move_constructible_v<E>) {
   > > >       E tmp(std::move(rhs.unex));
   > > >       destroy_at(addressof(rhs.unex));
   > > >       try {
   > > >         construct_at(addressof(rhs.val), std::move(val));
   > > >         destroy_at(addressof(val));
   > > >         construct_at(addressof(unex), std::move(tmp));
   > > >       } catch(...) {
   > > >         construct_at(addressof(rhs.unex), std::move(tmp));
   > > >         throw;
   > > >       }
   > > >     } else {
   > > >       remove_cv_t<T> tmp(std::move(val));
   > > >       destroy_at(addressof(val));
   > > >       try {
   > > >         construct_at(addressof(unex), std::move(rhs.unex));
   > > >         destroy_at(addressof(rhs.unex));
   > > >         construct_at(addressof(rhs.val), std::move(tmp));
   > > >       } catch (...) {
   > > >         construct_at(addressof(val), std::move(tmp));
   > > >         throw;
   > > >       }
   > > >     }
   > > >     has_val = false;
   > > >     rhs.has_val = true;

[Croydon 2026-03-27; Jonathan provides new wording]

LWG review pointed out that using `value()` relies on a freestanding-deleted function. The equivalent functions in `optional` just use `val = rhs.val` and we can do the same here. We don't need to disallow assignment to `const T` here in the general case, because the assignments and swaps are already constrained to only work if the type is assignable/swappable. If your type satisfies both `assignable_from<const T&, const T&>` and `assignable_from<T&, const T&>` but with different semantics, this function might have surprising effects. That seems fine.

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

> [Drafting note: When assignment and `swap` need to backup the old value by move construction, the source should be considered cv-unqualified, as the backup mechanism is only used internally.]

1. Modify 22.8.6.1 [expected.object.general] as indicated:

   >     […]
   >     bool has_val;      // exposition only
   >     union {
   >       remove_cv_t<T> val;       // exposition only
   >       E unex;          // exposition only
   >     };
   >     […]

2. Modify 22.8.6.5 [expected.object.swap] as indicated:

   >     constexpr void swap(expected& rhs) noexcept(see below);

   > > -1- Constraints: […]

   > > -2- Effects: See Table 64 [tab:expected.object.swap].

   > > For the case where `rhs.value()` is `false` and `this->has_value()` is `true`, equivalent to:

   > > >     if constexpr (is_nothrow_move_constructible_v<E>) {
   > > >       E tmp(std::move(rhs.unex));
   > > >       destroy_at(addressof(rhs.unex));
   > > >       try {
   > > >         construct_at(addressof(rhs.val), std::move(val));
   > > >         destroy_at(addressof(val));
   > > >         construct_at(addressof(unex), std::move(tmp));
   > > >       } catch(...) {
   > > >         construct_at(addressof(rhs.unex), std::move(tmp));
   > > >         throw;
   > > >       }
   > > >     } else {
   > > >       remove_cv_t<T> tmp(std::move(val));
   > > >       destroy_at(addressof(val));
   > > >       try {
   > > >         construct_at(addressof(unex), std::move(rhs.unex));
   > > >         destroy_at(addressof(rhs.unex));
   > > >         construct_at(addressof(rhs.val), std::move(tmp));
   > > >       } catch (...) {
   > > >         construct_at(addressof(val), std::move(tmp));
   > > >         throw;
   > > >       }
   > > >     }
   > > >     has_val = false;
   > > >     rhs.has_val = true;

### 3892(i). Incorrect formatting of nested ranges and tuples

Section: 28.5.7.2 [format.range.formatter], 28.5.9 [format.tuple] Status: C++26 Submitter: Victor Zverovich Opened: 2023-02-20 Last modified: 2026-06-09

Priority: 2

View all other issues in [format.range.formatter].

View all issues with C++26 status.

Discussion:

`formatter` specializations for ranges and tuples set debug format for underlying element formatters in their `parse` functions e.g. 28.5.7.2 [format.range.formatter] p9:

>     template<class ParseContext>
>       constexpr typename ParseContext::iterator
>         parse(ParseContext& ctx);

> > Effects: Parses the format specifier as a range-format-spec and stores the parsed specifiers in `*this`. The values of `opening-bracket_`, `closing-bracket_`, and `separator_` are modified if and only if required by the range-type or the `n` option, if present. If:

> > 1. — the range-type is neither `s` nor `?s`,

> > 2. — `underlying_.set_debug_format()` is a valid expression, and

> > 3. — there is no range-underlying-spec,

> > then calls `underlying_.set_debug_format()`.

However, they don't say anything about calling `parse` functions of those formatters. As as result, formatting of nested ranges can be incorrect, e.g.

>     std::string s = std::format("{}", std::vector<std::vector<std::string>>{{"a, b", "c"}});

With the current specification `s` is `[[a, b, c]]` instead of `[["a, b", "c"]]`, i.e. strings in the output are not correctly escaped. The same is true for nested tuples and combinations of tuples and ranges.

The fix approved by LEWG as part of P2733 (which was trying to address a different issue) is to always call `parse` for underlying formatter. Additionally the standard should clarify that format-spec cannot start with `'}'` because that's the implicit assumption in range formatting and what happens when format-spec is not present.

[2023-03-22; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N4928.

1. Modify 28.5.2.1 [format.string.general] p1 as indicated:

   > […]
   > format-specifier:
   >   : format-spec
   > format-spec:
   >   as specified by the `formatter` specialization for the argument type; cannot start with `}`

2. Modify 28.5.6.1 [formatter.requirements] as indicated:

   > -3- Given character type `charT`, output iterator type `Out`, and formatting argument type `T`, in Table 74 [tab:formatter.basic] and Table 75 [tab:formatter]:

   > […]

   > `pc.begin()` points to the beginning of the format-spec (28.5.2 [format.string]) of the replacement field being formatted in the format string. If format-spec is not present or empty then either `pc.begin() == pc.end()` or `*pc.begin() == '}'`.

3. Modify 28.5.7.2 [format.range.formatter] as indicated:

   >     template<class ParseContext>
   >       constexpr typename ParseContext::iterator
   >         parse(ParseContext& ctx);

   > > -9- Effects: Parses the format specifiers as a range-format-spec and stores the parsed specifiers in `*this`. Calls `underlying_.parse(ctx)` to parse format-spec in range-format-spec or, if the latter is not present, empty format-spec. The values of `opening-bracket_`, `closing-bracket_`, and `separator_` are modified if and only if required by the range-type or the `n` option, if present. If:

   > > 1. (9.1) — the range-type is neither `s` nor `?s`,

   > > 2. (9.2) — `underlying_.set_debug_format()` is a valid expression, and

   > > 3. (9.3) — there is no range-underlying-spec,

   > > then calls `underlying_.set_debug_format()`.

4. Modify 28.5.9 [format.tuple] as indicated:

   >     template<class ParseContext>
   >       constexpr typename ParseContext::iterator
   >         parse(ParseContext& ctx);

   > > -7- Effects: Parses the format specifiers as a tuple-format-spec and stores the parsed specifiers in `*this`. The values of `opening-bracket_`, `closing-bracket_`, and `separator_` are modified if and only if required by the tuple-type, if present. For each element `e` in `underlying_`, calls `e.parse(ctx)` to parse empty format-spec and, if `e.set_debug_format()` is a valid expression, calls `e.set_debug_format()`.

[Varna 2023-06-16; Jonathan provides tweaked wording]

Add "an" in two places.

[Varna 2023-06-16; Move to Ready]

This would allow resolving LWG 3776(i) as NAD.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 28.5.2.1 [format.string.general] p1 as indicated:

   > […]
   > format-specifier:
   >   : format-spec
   > format-spec:
   >   as specified by the `formatter` specialization for the argument type; cannot start with `}`

2. Modify 28.5.6.1 [formatter.requirements] as indicated:

   > -3- Given character type `charT`, output iterator type `Out`, and formatting argument type `T`, in Table 74 [tab:formatter.basic] and Table 75 [tab:formatter]:

   > […]

   > `pc.begin()` points to the beginning of the format-spec (28.5.2 [format.string]) of the replacement field being formatted in the format string. If format-spec is not present or empty then either `pc.begin() == pc.end()` or `*pc.begin() == '}'`.

3. Modify 28.5.7.2 [format.range.formatter] as indicated:

   >     template<class ParseContext>
   >       constexpr typename ParseContext::iterator
   >         parse(ParseContext& ctx);

   > > -9- Effects: Parses the format specifiers as a range-format-spec and stores the parsed specifiers in `*this`. Calls `underlying_.parse(ctx)` to parse format-spec in range-format-spec or, if the latter is not present, an empty format-spec. The values of `opening-bracket_`, `closing-bracket_`, and `separator_` are modified if and only if required by the range-type or the `n` option, if present. If:

   > > 1. (9.1) — the range-type is neither `s` nor `?s`,

   > > 2. (9.2) — `underlying_.set_debug_format()` is a valid expression, and

   > > 3. (9.3) — there is no range-underlying-spec,

   > > then calls `underlying_.set_debug_format()`.

4. Modify 28.5.9 [format.tuple] as indicated:

   >     template<class ParseContext>
   >       constexpr typename ParseContext::iterator
   >         parse(ParseContext& ctx);

   > > -7- Effects: Parses the format specifiers as a tuple-format-spec and stores the parsed specifiers in `*this`. The values of `opening-bracket_`, `closing-bracket_`, and `separator_` are modified if and only if required by the tuple-type, if present. For each element `e` in `underlying_`, calls `e.parse(ctx)` to parse an empty format-spec and, if `e.set_debug_format()` is a valid expression, calls `e.set_debug_format()`.

### 3893(i). LWG 3661 broke `atomic<shared_ptr<T>> a; a = nullptr;`

Section: 32.5.8.7.2 [util.smartptr.atomic.shared] Status: C++26 Submitter: Zachary Wassall Opened: 2023-02-22 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [util.smartptr.atomic.shared].

View all issues with C++26 status.

Discussion:

LWG 3661(i), "`constinit atomic<shared_ptr<T>> a(nullptr);` should work" added the following to `atomic<shared_ptr<T>>`:

>     constexpr atomic(nullptr_t) noexcept : atomic() { }

I believe that doing so broke the following example:

>     atomic<shared_ptr<T>> a;
>     a = nullptr;

For reference, `atomic<shared_ptr<T>>` provides two assignment operator overloads:

>     void operator=(const atomic&) = delete;          // #1
>     void operator=(shared_ptr<T> desired) noexcept;  // #2

Prior to LWG 3661(i), the assignment in the example unambiguously matches #2. #1 is not viable because `nullptr_t` is not convertible to `atomic<shared_ptr<T>>`. After LWG 3611, #1 is viable and the assignment is ambiguous between #1 and #2.

I believe this could be remedied easily enough by adding an assignment operator to match the added `nullptr_t` constructor:

>     void operator=(nullptr_t) noexcept;

[2023-02-25; Daniel comments and provides wording]

The suggested delegation below to `store(nullptr)` is not ambiguous and calls the constructor `shared_ptr(nullptr_t)`, which is guaranteed to be no-throwing.

[2023-03-22; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 32.5.8.7.2 [util.smartptr.atomic.shared] as indicated:

   > >     namespace std {
   > >       template<class T> struct atomic<shared_ptr<T>> {
   > >         […]
   > >         constexpr atomic() noexcept;
   > >         constexpr atomic(nullptr_t) noexcept : atomic() { }
   > >         atomic(shared_ptr<T> desired) noexcept;
   > >         atomic(const atomic&) = delete;
   > >         void operator=(const atomic&) = delete;
   > >         […]
   > >         void operator=(shared_ptr<T> desired) noexcept;
   > >         void operator=(nullptr_t) noexcept;
   > >         […]
   > >       private:
   > >         shared_ptr<T> p; // exposition only
   > >       };
   > >     }

   > […]

   >     void operator=(shared_ptr<T> desired) noexcept;

   > > -5- Effects: Equivalent to `store(desired)`.

   >     void operator=(nullptr_t) noexcept;

   > > -?- Effects: Equivalent to `store(nullptr)`.

### 3894(i). `generator::promise_type::yield_value(ranges::elements_of<Rng, Alloc>)` should not be `noexcept`

Section: 25.8.5 [coro.generator.promise] Status: C++26 Submitter: Tim Song Opened: 2023-02-25 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [coro.generator.promise].

View all issues with C++26 status.

Discussion:

The overload of `yield_value` for yielding elements of arbitrary ranges does so by creating a nested generator, but to do so it needs to:

- call `ranges::begin`/`ranges::end` on the range

- allocate a new coroutine frame (unless the allocation is elided by the compiler, which isn't guaranteed)

- copy/move the iterator and sentinel into the coroutine frame

All of these are allowed to throw, so this overload should not be `noexcept`.

[2023-03-22; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.8.5 [coro.generator.promise] as indicated:

   > >     namespace std {
   > >       template<class Ref, class V, class Allocator>
   > >       class generator<Ref, V, Allocator>::promise_type {
   > >       public:
   > >         […]
   > >         auto yield_value(const remove_reference_t<yielded>& lval)
   > >           requires is_rvalue_reference_v<yielded> &&
   > >             constructible_from<remove_cvref_t<yielded>, const remove_reference_t<yielded>&>;

   > >         template<class R2, class V2, class Alloc2, class Unused>
   > >           requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded>
   > >             auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept;

   > >         template<ranges::input_range R, class Alloc>
   > >           requires convertible_to<ranges::range_reference_t<R>, yielded>
   > >             auto yield_value(ranges::elements_of<R, Alloc> r) noexcept;
   > >         […]
   > >        };
   > >     }

   > […]

   >     template<ranges::input_range R, class Alloc>
   >       requires convertible_to<ranges::range_reference_t<R>, yielded>
   >       auto yield_value(ranges::elements_of<R, Alloc> r) noexcept;

   > > -13- Effects: Equivalent to:

   > > >     auto nested = [](allocator_arg_t, Alloc, ranges::iterator_t<R> i, ranges::sentinel_t<R> s)
   > > >       -> generator<yielded, ranges::range_value_t<R>, Alloc> {
   > > >         for (; i != s; ++i) {
   > > >           co_yield static_cast<yielded>(*i);
   > > >         }
   > > >       };
   > > >     return yield_value(ranges::elements_of(nested(
   > > >       allocator_arg, r.allocator, ranges::begin(r.range), ranges::end(r.range))));

   > > […]

### 3897(i). `inout_ptr` will not update raw pointer to 0

Section: 20.3.4.3 [inout.ptr.t] Status: C++26 Submitter: Doug Cook Opened: 2023-02-27 Last modified: 2026-06-09

Priority: 2

View all other issues in [inout.ptr.t].

View all issues with C++26 status.

Discussion:

`inout_ptr` seems useful for two purposes:

1. Using smart pointers with C-style APIs.

2. Annotating raw pointers for use with C-style APIs.

Unfortunately, as presently specified, it is not safe for developers to use `inout_ptr` for the second purpose. It is not safe to change code from

>     void* raw_ptr1;
>     InitSomething(&raw_ptr1);
>     UpdateSomething(&raw_ptr1); // In some cases may set raw_ptr1 = nullptr.
>     CleanupSomething(raw_ptr1);

to

>     void* raw_ptr2;
>     InitSomething(std::out_ptr(raw_ptr2));
>     UpdateSomething(std::inout_ptr(raw_ptr2)); // May leave dangling pointer
>     CleanupSomething(raw_ptr2);                // Possible double-delete

In the case where `UpdateSomething` would set `raw_ptr1 = nullptr`, the currently-specified `inout_ptr` implementation will leave `raw_ptr2` at its old value. This would likely lead to a double-delete in `CleanupSomething`.

The issue occurs because `inout_ptr` is specified as follows:

1. Constructor: If the user's pointer is a smart pointer, perform a "`release`" operation.

2. (C-style API executes)

3. If the C-style API returns a non-`NULL` pointer, propagate the returned value to the user's pointer.

If the user's pointer is not a smart pointer, no "`release`" operation occurs, and if the C-style API returns a `NULL` pointer, no propagation of the `NULL` occurs. We're left with a dangling raw pointer which is different from the original behavior using `&`.

I see two potential solutions:

1. Make the "`release`" operation unconditional (i.e. it applies to both smart and raw pointers). For raw pointers, define the "`release`" operation as setting the raw pointer to `nullptr`.

2. Make the return value propagation unconditional for raw pointers.

Solution #2 seems likely to lead to more optimal code as it avoids an unnecessary branch.

[2023-03-22; Reflector poll]

Set priority to 2 after reflector poll.

[Varna 2023-06-16; Move to Ready]

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 20.3.4.3 [inout.ptr.t] as indicated:

   >     ~inout_ptr_t();

   > > -9- Let `SP` be `POINTER_OF_OR(Smart, Pointer)` (20.2.1 [memory.general]).

   > > -10- Let release-statement be `s.release();` if an implementation does not call `s.release()` in the constructor. Otherwise, it is empty.

   > > -11- Effects: Equivalent to:

   > > 1. (11.1) —

   > >    >     if (p) {
   > >    >       apply([&](auto&&... args) {
   > >    >         s = Smart( static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
   > >    >     }

   > >    if `is_pointer_v<Smart>` is `true`;

   > > 2. (11.2) — otherwise,

   > >    >     release-statement;
   > >    >     if (p) {
   > >    >       apply([&](auto&&... args) {
   > >    >         s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
   > >    >     }

   > >    if the expression `s.reset(static_cast<SP>(p), std::forward<Args>(args)...)` is well-formed;

   > > 3. (11.3) — otherwise,

   > >    >     release-statement;
   > >    >     if (p) {
   > >    >       apply([&](auto&&... args) {
   > >    >         s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));
   > >    >     }

   > >    if `is_constructible_v<Smart, SP, Args...>` is `true`;

   > > 4. (11.4) — otherwise, the program is ill-formed.

### 3899(i). `co_yield`ing elements of an lvalue `generator` is unnecessarily inefficient

Section: 25.8.5 [coro.generator.promise] Status: C++26 Submitter: Tim Song Opened: 2023-03-04 Last modified: 2026-06-09

Priority: 3

View all other issues in [coro.generator.promise].

View all issues with C++26 status.

Discussion:

Consider:

>     std::generator<int> f();
>     std::generator<int> g() {
>         auto gen = f();
>         auto gen2 = f();
>         co_yield std::ranges::elements_of(std::move(gen));   // #1
>         co_yield std::ranges::elements_of(gen2);             // #2
>         // other stuff
>     }

Both #1 and #2 compile. The differences are:

- #2 is significantly less efficient (it uses the general overload of `yield_value`, so it creates a new coroutine frame and doesn't do symmetric transfer into `gen2`'s coroutine)

- the coroutine frame of `gen` and `gen2` are destroyed at different times: `gen`'s frame is destroyed at the end of #1, but `gen2`'s is not destroyed until the closing brace.

But as far as the user is concerned, neither `gen` nor `gen2` is usable after the `co_yield`. In both cases the only things you can do with the objects are:

- destroying them;

- assigning to them;

- call `end()` on them to get a copy of `default_sentinel`.

We could make #2 ill-formed, but that seems unnecessary: there is no meaningful difference between `generator` and any other single-pass input range (or a `generator` with a different yielded type that has to go through the general overload) in this regard. We should just make #2 do the efficient thing too.

[2023-03-22; Reflector poll]

Set priority to 3 after reflector poll.

[St. Louis 2024-06-28; move to Ready]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.8.5 [coro.generator.promise] as indicated:

   > >     namespace std {
   > >       template<class Ref, class V, class Allocator>
   > >       class generator<Ref, V, Allocator>::promise_type {
   > >       public:
   > >         […]
   > >         auto yield_value(const remove_reference_t<yielded>& lval)
   > >           requires is_rvalue_reference_v<yielded> &&
   > >             constructible_from<remove_cvref_t<yielded>, const remove_reference_t<yielded>&>;

   > >         template<class R2, class V2, class Alloc2, class Unused>
   > >           requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded>
   > >             auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept;
   > >         template<class R2, class V2, class Alloc2, class Unused>
   > >           requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded>
   > >             auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&, Unused> g) noexcept;

   > >         template<ranges::input_range R, class Alloc>
   > >           requires convertible_to<ranges::range_reference_t<R>, yielded>
   > >             auto yield_value(ranges::elements_of<R, Alloc> r) noexcept;
   > >         […]
   > >        };
   > >     }

   > […]

   >     template<class R2, class V2, class Alloc2, class Unused>
   >       requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded>
   >       auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept;
   >     template<class R2, class V2, class Alloc2, class Unused>
   >       requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded>
   >       auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&, Unused> g) noexcept;

   > > -10- Preconditions: A handle referring to the coroutine whose promise object is `*this` is at the top of `*active_` of some generator object `x`. The coroutine referred to by `g.range.coroutine_` is suspended at its initial suspend point.

   > > -11- Returns: An awaitable object of an unspecified type (7.6.2.4 [expr.await]) into which `g.range` is moved, whose member `await_ready` returns `false`, whose member `await_suspend` pushes `g.range.coroutine_` into `*x.active_` and resumes execution of the coroutine referred to by `g.range.coroutine_`, and whose member `await_resume` evaluates `rethrow_exception(except_)` if `bool(except_)` is `true`. If `bool(except_)` is `false`, the `await_resume` member has no effects.

   > > -12- Remarks: A yield-expression that calls this functionone of these functions has type `void` (7.6.17 [expr.yield]).

### 3900(i). The `allocator_arg_t` overloads of `generator::promise_type::operator new` should not be constrained

Section: 25.8.5 [coro.generator.promise] Status: C++26 Submitter: Tim Song Opened: 2023-03-04 Last modified: 2026-06-09

Priority: 3

View all other issues in [coro.generator.promise].

View all issues with C++26 status.

Discussion:

When the allocator is not type-erased, the `allocator_arg_t` overloads of `generator::promise_type::operator new` are constrained on `convertible_to<const Alloc&, Allocator>`. As a result, if the the allocator is default-constructible (like `polymorphic_allocator` is) but the user accidentally provided a wrong type (say, `memory_resource&` instead of `memory_resource*`), their code will silently fall back to using a default-constructed allocator. It would seem better to take the tag as definitive evidence of the user's intent to supply an allocator for the coroutine, and error out if the supplied allocator cannot be used.

This change does mean that the user cannot deliberately pass an incompatible allocator (preceded by an `std::allocator_arg_t` tag) for their own use inside the coroutine, but that sort of API seems fragile and confusing at best, since the usual case is that allocators so passed will be used by `generator`.

[2023-03-22; Reflector poll]

Set priority to 3 after reflector poll.

[St. Louis 2024-06-28; move to Ready]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.8.5 [coro.generator.promise] as indicated:

   > >     namespace std {
   > >       template<class Ref, class V, class Allocator>
   > >       class generator<Ref, V, Allocator>::promise_type {
   > >       public:
   > >         […]
   > >         void* operator new(size_t size)
   > >           requires same_as<Allocator, void> || default_initializable<Allocator>;

   > >         template<class Alloc, class... Args>
   > >           requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>
   > >             void* operator new(size_t size, allocator_arg_t, const Alloc& alloc, const Args&...);

   > >         template<class This, class Alloc, class... Args>
   > >           requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>
   > >             void* operator new(size_t size, const This&, allocator_arg_t, const Alloc& alloc,
   > >                                const Args&...);
   > >         […]
   > >        };
   > >     }

   > […]

   >     void* operator new(size_t size)
   >       requires same_as<Allocator, void> || default_initializable<Allocator>;

   >     template<class Alloc, class... Args>
   >       requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>
   >       void* operator new(size_t size, allocator_arg_t, const Alloc& alloc, const Args&...);

   >     template<class This, class Alloc, class... Args>
   >       requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>
   >       void* operator new(size_t size, const This&, allocator_arg_t, const Alloc& alloc,
   >                          const Args&...);

   > > -17- Let `A` be

   > > 1. (17.1) — `Allocator`, if it is not `void`,

   > > 2. (17.2) — `Alloc` for the overloads with a template parameter `Alloc`, or

   > > 3. (17.3) — `allocator<void>` otherwise.

   > > Let `B` be `allocator_traits<A>::template rebind_alloc<U>` where `U` is an unspecified type whose size and alignment are both `__STDCPP_DEFAULT_NEW_ALIGNMENT__`.

   > > -18- Mandates: `allocator_traits<B>::pointer` is a pointer type. For the overloads with a template parameter `Alloc`, `same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>` is modeled.

   > > -19- Effects: Initializes an allocator `b` of type `B` with `A(alloc)`, for the overloads with a function parameter `alloc`, and with `A()` otherwise. Uses `b` to allocate storage for the smallest array of `U` sufficient to provide storage for a coroutine state of size `size`, and unspecified additional state necessary to ensure that `operator delete` can later deallocate this memory block with an allocator equal to `b`.

   > > -20- Returns: A pointer to the allocated storage.

### 3903(i). `span` destructor is redundantly `noexcept`

Section: 23.7.2.2.1 [span.overview] Status: C++26 Submitter: Ben Craig Opened: 2023-03-11 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [span.overview].

View all issues with C++26 status.

Discussion:

The `span` class template synopsis in 23.7.2.2.1 [span.overview] has this declaration:

>     ~span() noexcept = default;

The `noexcept` is redundant, as `~span` is `noexcept` automatically. I think the entire declaration is unnecessary as well. There is no additional specification for `~span()` at all, much less some that warrants inclusion in the class template synopsis.

Recommended fix:

>     ~span() noexcept = default;

Alternative fix:

>     ~span() noexcept = default;

[2023-03-22; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 23.7.2.2.1 [span.overview], class template `span` synopsis, as indicated:

   >     […]

   >     ~span() noexcept = default;

   >     […]

### 3904(i). `lazy_split_view::outer-iterator`'s `const`-converting constructor isn't setting `trailing_empty_`

Section: 25.7.16.3 [range.lazy.split.outer] Status: C++26 Submitter: Patrick Palka Opened: 2023-03-13 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.lazy.split.outer].

View all issues with C++26 status.

Discussion:

It seems the `const`-converting constructor for `lazy_split_view`'s iterator fails to propagate `trailing_empty_` from the argument, which effectively results in the trailing empty range getting skipped over:

>     auto r = views::single(0) | views::lazy_split(0); // r is { {}, {} }
>     auto i = r.begin();
>     ++i; // i.trailing_empty_ is correctly true
>     decltype(std::as_const(r).begin()) j = i; // j.trailing_empty_ is incorrectly false
>     auto k = r.end(); // k.trailing_empty_ is correctly false, and we wrongly have j == k

[2023-05-24; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.7.16.3 [range.lazy.split.outer] as indicated:

   >     constexpr outer-iterator(outer-iterator<!Const> i)
   >       requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;

   > > -4- Effects: Initializes `parent_` with `i.parent_`, and `current_` with `std::move(i.current_)`, and `trailing_empty_` with `i.trailing_empty_`.

### 3905(i). Type of `std::fexcept_t`

Section: 29.3.1 [cfenv.syn] Status: C++26 Submitter: Sam Elliott Opened: 2023-03-13 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

`<cfenv>`, as specified in 29.3.1 [cfenv.syn], requires `fexcept_t` to be an integer type:

>     using fexcept_t = integer type;

`<cfenv>` was initially added to the (first) Technical Report on C++ Library Extensions via N1568 and then integrated into the C++ Working Draft N2009 in Berlin (April, 2006).

However, C99 does not actually require that `fexcept_t` is an integer type, it only requires:

> The type `fexcept_t` represents the floating-point status flags collectively, including any status the implementation associates with the flags.

Relaxing this requirement should not cause conforming C++ implementations to no longer be conforming. In fact, this should enable conforming C implementations to become conforming C++ implementations without an ABI break. The only incompatibility I foresee is where a user's program is initializing a `std::fexcept_t` with an integer value, which would become invalid on some C++ implementations (but not those that were previously conforming).

[2023-05-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 29.3.1 [cfenv.syn], header `<cfenv>` synopsis, as indicated:

   >     […]
   >     namespace std {
   >       // types
   >       using fenv_t = object type;
   >       using fexcept_t = integerobject type;
   >       […]
   >     }

### 3912(i). `enumerate_view::iterator::operator-` should be `noexcept`

Section: 25.7.24.3 [range.enumerate.iterator] Status: C++26 Submitter: Hewill Kang Opened: 2023-03-27 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.enumerate.iterator].

View all issues with C++26 status.

Discussion:

The distance between two `enumerate_view::iterator` is calculated by subtracting two integers, which never throws.

[2023-05-24; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4944.

1. Modify 25.7.24.3 [range.enumerate.iterator] as indicated:

   >     namespace std::ranges {
   >       template<view V>
   >         requires range-with-movable-references<V>
   >       template<bool Const>
   >       class enumerate_view<V>::iterator {
   >         […]
   >         public:
   >         […]
   >         friend constexpr difference_type operator-(const iterator& x, const iterator& y) noexcept;
   >         […]
   >       }
   >       […]
   >     }

   > […]

   >     friend constexpr difference_type operator-(const iterator& x, const iterator& y) noexcept;

   > -19- Effects: Equivalent to: `return x.pos_ - y.pos_;`

### 3914(i). Inconsistent template-head of `ranges::enumerate_view`

Section: 25.2 [ranges.syn], 25.7.24 [range.enumerate] Status: C++26 Submitter: Johel Ernesto Guerrero Peña Opened: 2023-03-30 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [ranges.syn].

View all other issues in [ranges.syn].

View all issues with C++26 status.

Discussion:

Originally editorial issue Editorial issue #6151.

The template-head of `ranges::enumerate_view` in the header synopsis is different from those in the class synopses of itself and its iterator/sentinel pair.

[2023-05-24; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4944.

1. Modify 25.2 [ranges.syn] as indicated:

   >     […]
   >     // 25.7.24 [range.enumerate], enumerate view
   >     template<view Vinput_range View>
   >       requires see belowview<View>
   >     class enumerate_view;
   >     […]

### 3915(i). Redundant paragraph about expression variations

Section: 25.4.2 [range.range] Status: C++26 Submitter: Johel Ernesto Guerrero Peña Opened: 2023-04-01 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.range].

View all issues with C++26 status.

Discussion:

Originally editorial issue Editorial issue #4431.

Expression variations kick in for "an expression that is non-modifying for some constant lvalue operand", but `std::ranges::range`'s is an non-constant lvalue, so 25.4.2 [range.range] p2 is redundant.

I suppose that the change that clarified the template parameters' cv-qualification for purposes of equality-preservation and requiring additional variations happened concurrently with the change of `std::ranges::range`'s operand from a forwarding reference to a non-constant lvalue reference.

[2023-05-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4944.

1. Modify 25.2 [ranges.syn] as indicated:

   > -1- The `range` concept defines the requirements of a type that allows iteration over its elements by providing an iterator and sentinel that denote the elements of the range.

   >     template<class T>
   >       concept range =
   >         requires(T& t) {
   >           ranges::begin(t); // sometimes equality-preserving (see below)
   >           ranges::end(t);
   >         };

   > > -2- The required expressions `ranges::begin(t)` and `ranges::end(t)` of the `range` concept do not require implicit expression variations (18.2 [concepts.equality]).

   > > -3- […]

### 3918(i). `std::uninitialized_move/_n` and guaranteed copy elision

Section: 26.11.6 [uninitialized.move] Status: C++26 Submitter: Jiang An Opened: 2023-04-04 Last modified: 2026-06-09

Priority: 3

View all issues with C++26 status.

Discussion:

Currently `std::move` is unconditionally used in `std::uninitialized_move` and `std::uninitialized_move_n`, which may involve unnecessary move construction if dereferencing the input iterator yields a prvalue.

The status quo was mentioned in paper issue #975, but no further process is done since then.

[2023-06-01; Reflector poll]

Set priority to 3 after reflector poll. Send to LEWG.

"P2283 wants to remove guaranteed elision here." "Poorly motivated, not clear anybody is using these algos with proxy iterators." "Consider using `iter_move` in the move algos."

Previous resolution [SUPERSEDED]

This wording is relative to N4944.

1. Modify 26.11.1 [specialized.algorithms.general] as indicated:

   > -3- Some algorithms specified in 26.11 [specialized.algorithms] make use of the following exposition-only functions `voidify`:

   > >     template<class T>
   > >       constexpr void* voidify(T& obj) noexcept {
   > >         return addressof(obj);
   > >       }

   > >     template<class I>
   > >       decltype(auto) deref-move(const I& it) {
   > >         if constexpr (is_lvalue_reference_v<decltype(*it)>)
   > >           return std::move(*it);
   > >         else
   > >           return *it;
   > >       }

2. Modify 26.11.6 [uninitialized.move] as indicated:

   >     template<class InputIterator, class NoThrowForwardIterator>
   >       NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last,
   >                                                 NoThrowForwardIterator result);

   > > -1- Preconditions: `result + [0, (last - first))` does not overlap with `[first, last)`.

   > > -2- Effects: Equivalent to:

   > > >     for (; first != last; (void)++result, ++first)
   > > >       ::new (voidify(*result))
   > > >         typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first));
   > > >     return result;

   > […]

   >     template<class InputIterator, class Size, class NoThrowForwardIterator>
   >       pair<InputIterator, NoThrowForwardIterator>
   >         uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);

   > > -6- Preconditions: `result + [0, n)` does not overlap with `first + [0, n)`.

   > > -7- Effects: Equivalent to:

   > > >     for (; n > 0; ++result,(void) ++first, --n)
   > > >       ::new (voidify(*result))
   > > >         typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first));
   > > >     return {first, result};

[2024-03-22; Tokyo: Jonathan updates wording after LEWG review]

LEWG agrees it would be good to do this. Using `iter_move` was discussed, but it was noted that the versions of these algos in the `ranges` namespace already use it and introducing `ranges::iter_move` into the non-ranges versions wasn't desirable. It was observed that the proposed deref-move has a `const I&` parameter which would be ill-formed for any iterator with a non-const `operator*` member. Suggested removing the const and recommended LWG to accept the proposed resolution.

Previous resolution [SUPERSEDED]

This wording is relative to N4971.

1. Modify 26.11.1 [specialized.algorithms.general] as indicated:

   > -3- Some algorithms specified in 26.11 [specialized.algorithms] make use of the following exposition-only functions `voidify`:

   > >     template<class T>
   > >       constexpr void* voidify(T& obj) noexcept {
   > >         return addressof(obj);
   > >       }

   > >     template<class I>
   > >       decltype(auto) deref-move(I& it) {
   > >         if constexpr (is_lvalue_reference_v<decltype(*it)>)
   > >           return std::move(*it);
   > >         else
   > >           return *it;
   > >       }

2. Modify 26.11.6 [uninitialized.move] as indicated:

   >     template<class InputIterator, class NoThrowForwardIterator>
   >       NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last,
   >                                                 NoThrowForwardIterator result);

   > > -1- Preconditions: `result + [0, (last - first))` does not overlap with `[first, last)`.

   > > -2- Effects: Equivalent to:

   > > >     for (; first != last; (void)++result, ++first)
   > > >       ::new (voidify(*result))
   > > >         typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first));
   > > >     return result;

   > […]

   >     template<class InputIterator, class Size, class NoThrowForwardIterator>
   >       pair<InputIterator, NoThrowForwardIterator>
   >         uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);

   > > -6- Preconditions: `result + [0, n)` does not overlap with `first + [0, n)`.

   > > -7- Effects: Equivalent to:

   > > >     for (; n > 0; ++result,(void) ++first, --n)
   > > >       ::new (voidify(*result))
   > > >         typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first));
   > > >     return {first, result};

[St. Louis 2024-06-24; revert P/R and move to Ready]

Tim observed that the iterator requirements require all iterators to be const-dereferenceable, so there was no reason to remove the const. Restore the original resolution and move to Ready.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 26.11.1 [specialized.algorithms.general] as indicated:

   > -3- Some algorithms specified in 26.11 [specialized.algorithms] make use of the following exposition-only functions `voidify`:

   > >     template<class T>
   > >       constexpr void* voidify(T& obj) noexcept {
   > >         return addressof(obj);
   > >       }

   > >     template<class I>
   > >       decltype(auto) deref-move(I& it) {
   > >         if constexpr (is_lvalue_reference_v<decltype(*it)>)
   > >           return std::move(*it);
   > >         else
   > >           return *it;
   > >       }

2. Modify 26.11.6 [uninitialized.move] as indicated:

   >     template<class InputIterator, class NoThrowForwardIterator>
   >       NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last,
   >                                                 NoThrowForwardIterator result);

   > > -1- Preconditions: `result + [0, (last - first))` does not overlap with `[first, last)`.

   > > -2- Effects: Equivalent to:

   > > >     for (; first != last; (void)++result, ++first)
   > > >       ::new (voidify(*result))
   > > >         typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first));
   > > >     return result;

   > […]

   >     template<class InputIterator, class Size, class NoThrowForwardIterator>
   >       pair<InputIterator, NoThrowForwardIterator>
   >         uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);

   > > -6- Preconditions: `result + [0, n)` does not overlap with `first + [0, n)`.

   > > -7- Effects: Equivalent to:

   > > >     for (; n > 0; ++result,(void) ++first, --n)
   > > >       ::new (voidify(*result))
   > > >         typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first));
   > > >     return {first, result};

### 3919(i). `enumerate_view` may invoke UB for sized common non-forward underlying ranges

Section: 25.7.24 [range.enumerate] Status: C++26 Submitter: Patrick Palka Opened: 2023-04-07 Last modified: 2026-06-09

Priority: 3

View all issues with C++26 status.

Discussion:

For a sized common range, `enumerate_view::end()` is specified to call `ranges::distance`. But `ranges::distance` is not necessarily well-defined for a sized non-forward range after calling `ranges::begin` (according to 25.4.4 [range.sized]).

So for a sized common non-forward underlying range, it seems calling `enumerate_view::begin()` followed by `enumerate_view::end()` may invoke UB and thus make `enumerate_view` potentially unusable for such ranges.

I suppose we might need to instead call and cache the result of `ranges::distance` from `enumerate_view::begin()` for such ranges.

[2022-04-12; Patrick Palka provides wording]

The proposed wording follows the suggestion provided by Tim Song, to simply make `enumerate` non-common for this case.

[2023-05-24; Reflector poll]

Set priority to 3 after reflector poll.

[Kona 2023-11-10; move to Ready]

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4944.

1. Modify 25.7.24.2 [range.enumerate.view], class template class `enumerate_view` synopsis, as indicated:

   > >     […]
   > >     constexpr auto end() requires (!simple-view<V>) {
   > >       if constexpr (forward_range<V> && common_range<V> && sized_range<V>)
   > >         return iterator<false>(ranges::end(base_), ranges::distance(base_));
   > >       else
   > >         return sentinel<false>(ranges::end(base_));
   > >     }
   > >     constexpr auto end() const requires range-with-movable-references<const V> {
   > >       if constexpr (forward_range<const V> && common_range<const V> && sized_range<const V>)
   > >         return iterator<true>(ranges::end(base_), ranges::distance(base_));
   > >       else
   > >         return sentinel<true>(ranges::end(base_));
   > >     }
   > >     […]

### 3925(i). Concept `formattable`'s definition is incorrect

Section: 28.5.6.3 [format.formattable] Status: C++26 Submitter: Mark de Wever Opened: 2023-04-16 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [format.formattable].

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Discussion:

LWG 3631(i) modified the `formattable` concept. The new wording contains a small issue: `basic_format_context` requires two template arguments, but only one is provided.

[2023-05-24; Reflector poll]

Set status to Tentatively Ready after ten votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4944.

1. Modify 28.5.6.3 [format.formattable] as indicated:

   > -1- Let `fmt-iter-for<charT>` be an unspecified type that models `output_iterator<const charT&>` (24.3.4.10 [iterator.concept.output]).

   > >     […]
   > >     template<class T, class charT>
   > >       concept formattable =
   > >         formattable-with<remove_reference_t<T>, basic_format_context<fmt-iter-for<charT>, charT>>;

### 3927(i). Unclear preconditions for `operator[]` for sequence containers

Section: 23.2.4 [sequence.reqmts] Status: C++26 Submitter: Ville Voutilainen Opened: 2023-04-24 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [sequence.reqmts].

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Discussion:

In 23.2.4 [sequence.reqmts]/118, we specify what `a[n]` means for a sequence container `a`, but we don't state that it actually has preconditions, other than implied ones.

When we want to use implied preconditions, we can actually get them by using the "Effects: Equivalent to..." wording.

[2023-05-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4944.

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   >     a[n]

   > > -118- Result: `reference`; `const_reference` for constant `a`

   > > -119- ReturnsEffects: Equivalent to: `return *(a.begin() + n);`

   > > -120- Remarks: Required for `basic_string`, `array`, `deque`, and `vector`.

### 3935(i). `template<class X> constexpr complex& operator=(const complex<X>&)` has no specification

Section: 29.4.3 [complex] Status: C++26 Submitter: Daniel Krügler Opened: 2023-05-21 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [complex].

View all other issues in [complex].

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Discussion:

The class template `complex` synopsis in 29.4.3 [complex] shows the following member function template:

>     template<class X> constexpr complex& operator= (const complex<X>&);

but does not specify its semantics. This affects a code example such as the following one:

>     #include <complex>

>     int main()
>     {
>       std::complex<double> zd(1, 1);
>       std::complex<float> zf(2, 0);
>       zd = zf;
>     }

This problem exists since the 1998 version of the standard (at that time this was declared in subclause [lib.complex]), and even though this looks like a "copy-assignment-like" operation, its effects aren't implied by our general 16.3.3.5 [functions.within.classes] wording (a function template is never considered as copy assignment operator, see 11.4.6 [class.copy.assign]).

It should be point out that the refactoring done by P1467R9 had caused some other members to become finally specified that were not specified before, but in addition to LWG 3934(i)'s observation about the missing specification of the assignment operator taking a value type as parameter, this is now an additional currently unspecified member, but unaffected by any decision on LWG 3933(i).

[2023-06-01; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Add a new prototype specification between the current paragraphs 8 and 9 of 29.4.5 [complex.member.ops] as indicated:

   > [Drafting note: Similar to the proposed wording for LWG 3934(i), the wording form used below intentionally deviates from the rest of the [complex.member.ops] wording forms, because it seems much simpler and clearer to follow the wording forms used that specify the effects of `imag` and `real` functions plus borrowing relevant parts from 29.4.4 [complex.members] p2.]

   >     constexpr complex& operator/=(const T& rhs);

   > > […]

   >     template<class X> constexpr complex& operator=(const complex<X>& rhs);

   > > -?- Effects: Assigns the value `rhs.real()` to the real part and the value `rhs.imag()` to the imaginary part of the complex value `*this`.

   > > -?- Returns: `*this`.

   >     template<class X> constexpr complex& operator+=(const complex<X>& rhs);

   > > […]

### 3938(i). Cannot use `std::expected` monadic ops with move-only `error_type`

Section: 22.8.6.7 [expected.object.monadic] Status: C++26 Submitter: Jonathan Wakely Opened: 2023-05-25 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [expected.object.monadic].

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Discussion:

The monadic ops for `std::expected` are specified in terms of calls to `value()` and `error()`, but LWG 3843(i) ("`std::expected<T,E>::value()&` assumes `E` is copy constructible") added additional Mandates requirements to `value()`. This means that you can never call `value()` for a move-only `error_type`, even the overloads of `value()` with rvalue ref-qualifiers.

The changes to `value()` are because it needs to be able to throw a `bad_expected_access<E>` which requires a copyable `E`. But in the monadic ops we know it can't throw, because we always check. All the monadic ops are of the form:

>     if (has_value())
>       do something with value();
>     else
>       do something with error();

We know that `value()` won't throw here, but because we use "Effects: Equivalent to ..." the requirement for `E` to be copyable is inherited from `value()`.

Should we have changed the monadic ops to use `operator*()` instead of `value()`? For example, for the first `and_then` overloads the change would be:

> -4- Effects: Equivalent to:

> >     if (has_value())
> >       return invoke(std::forward<F>(f), value()**this);
> >     else
> >       return U(unexpect, error());

[2023-06-01; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. For each Effects: element in 22.8.6.7 [expected.object.monadic], replace `value()` with `**this` as indicated:

   >     template<class F> constexpr auto and_then(F&& f) &;
   >     template<class F> constexpr auto and_then(F&& f) const &;

   > -1- Let `U` be `remove_cvref_t<invoke_result_t<F, decltype(value()**this)>>`.

   > -2- Constraints: `is_constructible_v<E, decltype(error())>` is `true`.

   > -3- Mandates: `U` is a specialization of `expected` and `is_same_v<U::error_type, E>` is `true`.

   > -4- Effects: Equivalent to:

   > >     if (has_value())
   > >       return invoke(std::forward<F>(f), value()**this);
   > >     else
   > >       return U(unexpect, error());

   >     template<class F> constexpr auto and_then(F&& f) &&;
   >     template<class F> constexpr auto and_then(F&& f) const &&;

   > -5- Let `U` be `remove_cvref_t<invoke_result_t<F, decltype(std::move(value()**this))>>`.

   > -6- Constraints: `is_constructible_v<E, decltype(std::move(error()))>` is `true`.

   > -7- Mandates: `U` is a specialization of `expected` and `is_same_v<U::error_type, E>` is `true`.

   > -8- Effects: Equivalent to:

   > >     if (has_value())
   > >       return invoke(std::forward<F>(f), std::move(value()**this));
   > >     else
   > >       return U(unexpect, std::move(error()));

   >     template<class F> constexpr auto or_else(F&& f) &;
   >     template<class F> constexpr auto or_else(F&& f) const &;

   > -9- Let `G` be `remove_cvref_t<invoke_result_t<F, decltype(error())>>`.

   > -10- Constraints: `is_constructible_v<T, decltype(value()**this)>` is `true`.

   > -11- Mandates: `G` is a specialization of `expected` and `is_same_v<G::value_type, T>` is `true`.

   > -12- Effects: Equivalent to:

   > >     if (has_value())
   > >       return G(in_place, value()**this);
   > >     else
   > >       return invoke(std::forward<F>(f), error());

   >     template<class F> constexpr auto or_else(F&& f) &&;
   >     template<class F> constexpr auto or_else(F&& f) const &&;

   > -13- Let `G` be `remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>`.

   > -14- Constraints: `is_constructible_v<T, decltype(std::move(value()**this))>` is `true`.

   > -15- Mandates: `G` is a specialization of `expected` and `is_same_v<G::value_type, T>` is `true`.

   > -16- Effects: Equivalent to:

   > >     if (has_value())
   > >       return G(in_place, std::move(value()**this));
   > >     else
   > >       return invoke(std::forward<F>(f), std::move(error()));

   >     template<class F> constexpr auto transform(F&& f) &;
   >     template<class F> constexpr auto transform(F&& f) const &;

   > -17- Let `U` be `remove_cv_t<invoke_result_t<F, decltype(value()**this)>>`.

   > -18- Constraints: `is_constructible_v<E, decltype(error())>` is `true`.

   > -19- Mandates: `U` is a valid value type for `expected`. If `is_void_v<U>` is `false`, the declaration

   >       U u(invoke(std::forward<F>(f), value()**this));

   > is well-formed.

   > -20- Effects:

   > 1. (20.1) — If `has_value()` is `false`, returns `expected<U, E>(unexpect, error())`.
   > 2. (20.2) — Otherwise, if `is_void_v<U>` is `false`, returns an `expected<U, E>` object whose `has_val` member is `true` and `val` member is direct-non-list-initialized with `invoke(std::forward<F>(f), value()**this)`.
   > 3. (20.3) — Otherwise, evaluates `invoke(std::forward<F>(f), value()**this)` and then returns `expected<U, E>()`.

   >     template<class F> constexpr auto transform(F&& f) &&;
   >     template<class F> constexpr auto transform(F&& f) const &&;

   > -21- Let `U` be `remove_cv_t<invoke_result_t<F, decltype(std::move(value()**this))>>`.

   > -22- Constraints: `is_constructible_v<E, decltype(std::move(error()))>` is `true`.

   > -23- Mandates: `U` is a valid value type for `expected`. If `is_void_v<U>` is `false`, the declaration

   >       U u(invoke(std::forward<F>(f), std::move(value()**this)));

   > is well-formed for some invented variable `u`.

   > > [Drafting Note: The removal of "for some invented variable u" in paragraph 23 is a drive-by fix for consistency with paragraphs 19, 27 and 31.]

   > -24- Effects:

   > 1. (24.1) — If `has_value()` is `false`, returns `expected<U, E>(unexpect, error())`.
   > 2. (24.2) — Otherwise, if `is_void_v<U>` is `false`, returns an `expected<U, E>` object whose `has_val` member is `true` and `val` member is direct-non-list-initialized with `invoke(std::forward<F>(f), std::move(value()**this))`.
   > 3. (24.3) — Otherwise, evaluates `invoke(std::forward<F>(f), std::move(value()**this))` and then returns `expected<U, E>()`.

   >     template<class F> constexpr auto transform_error(F&& f) &;
   >     template<class F> constexpr auto transform_error(F&& f) const &;

   > -25- Let `G` be `remove_cv_t<invoke_result_t<F, decltype(error())>>`.

   > -26- Constraints: `is_constructible_v<T, decltype(value()**this)>` is `true`.

   > -27- Mandates: `G` is a valie template argument for `unexpected` ( [unexpected.un.general]) and the declaration

   >       G g(invoke(std::forward<F>(f), error()));

   > is well-formed.

   > -28- Returns: If `has_value()` is `true`, `expected<T, G>(in_place, value()**this);`; otherwise, an `expected<T, G>` object whose `has_val` member is `false` and `unex` member is direct-non-list-initialized with `invoke(std::forward<F>(f), error())`.

   >     template<class F> constexpr auto transform_error(F&& f) &&;
   >     template<class F> constexpr auto transform_error(F&& f) const &&;

   > -29- Let `G` be `remove_cv_t<invoke_result_t<F, decltype(std::move(error()))>>`.

   > -30- Constraints: `is_constructible_v<T, decltype(std::move(value()**this))>` is `true`.

   > -31- Mandates: `G` is a valie template argument for `unexpected` ( [unexpected.un.general]) and the declaration

   >       G g(invoke(std::forward<F>(f), std::move(error())));

   > is well-formed.

   > -32- Returns: If `has_value()` is `true`, `expected<T, G>(in_place, std::move(value()**this));`; otherwise, an `expected<T, G>` object whose `has_val` member is `false` and `unex` member is direct-non-list-initialized with `invoke(std::forward<F>(f), std::move(error()))`.

### 3940(i). `std::expected<void, E>::value()` also needs `E` to be copy constructible

Section: 22.8.7.6 [expected.void.obs] Status: C++26 Submitter: Jiang An Opened: 2023-05-26 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

LWG 3843(i) added Mandates: to `std::expected::value`, but the similar handling is missing for `expected<cv void, E>`.

[2023-06-01; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 22.8.7.6 [expected.void.obs] as indicated:

   >     constexpr void value() const &;

   > > -?- Mandates: `is_copy_constructible_v<E>` is `true`.

   > > -3- Throws: `bad_expected_access(error())` if `has_value()` is `false`.

   >     constexpr void value() &&;

   > > -?- Mandates: `is_copy_constructible_v<E>` is `true` and `is_move_constructible_v<E>` is `true`.

   > > -4- Throws: `bad_expected_access(std::move(error()))` if `has_value()` is `false`.

### 3944(i). Formatters converting sequences of `char` to sequences of `wchar_t`

Section: 28.5.6.4 [format.formatter.spec] Status: C++26 Submitter: Mark de Wever Opened: 2023-06-01 Last modified: 2026-06-09

Priority: 3

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Discussion:

I noticed some interesting features introduced by the range based formatters in C++23

>     // Ill-formed in C++20 and C++23
>     const char* cstr = "hello";
>     char* str = const_cast<char*>(cstr);
>     std::format(L"{}", str);
>     std::format(L"{}",cstr);

>     // Ill-formed in C++20
>     // In C++23 they give L"['h', 'e', 'l', 'l', 'o']"
>     std::format(L"{}", "hello"); // A libc++ bug prevents this from working.
>     std::format(L"{}", std::string_view("hello"));
>     std::format(L"{}", std::string("hello"));
>     std::format(L"{}", std::vector{'h', 'e', 'l', 'l', 'o'});

An example is shown here. This only shows libc++ since libstdc++ and MSVC STL have not implemented the formatting ranges papers (P2286R8 and P2585R0) yet.

The difference between C++20 and C++23 is the existence of range formatters. These formatters use the formatter specialization `formatter<char, wchar_t>` which converts the sequence of `char`s to a sequence of `wchar_t`s.

In this conversion `same_as<char, charT>` is `false`, thus the requirements of the range-type `s` and `?s` ([tab:formatter.range.type]) aren't met. So the following is ill-formed:

>     std::format(L"{:s}", std::string("hello")); // Not L"hello"

It is surprising that some string types can be formatted as a sequence of wide-characters, but others not. A sequence of characters can be a sequence UTF-8 code units. This is explicitly supported in the width estimation of string types. The conversion of `char` to `wchar_t` will convert the individual code units, which will give incorrect results for multi-byte code points. It will not transcode UTF-8 to UTF-16/32. The current behavior is not in line with the note in 28.5.6.4 [format.formatter.spec]/2

> [Note 1: Specializations such as `formatter<wchar_t, char>` and `formatter<const char*, wchar_t>` that would require implicit multibyte / wide string or character conversion are disabled. — end note]

Disabling this could be done by explicitly disabling the `char` to `wchar_t` sequence formatter. Something along the lines of

>     template<ranges::input_range R>
>       requires(format_kind<R> == range_format::sequence &&
>                same_as<remove_cvref_t<ranges::range_reference_t<R>>, char>)
>     struct formatter<R, wchar_t> : __disabled_formatter {};

where `__disabled_formatter` satisfies 28.5.6.4 [format.formatter.spec]/5, would do the trick. This disables the conversion for all sequences not only the string types. So `vector`, `array`, `span`, etc. would be disabled.

This does not disable the conversion in the `range_formatter`. This allows users to explicitly opt in to this formatter for their own specializations.

An alternative would be to only disable this conversion for string type specializations (28.5.6.4 [format.formatter.spec]/2.2) where `char` to `wchar_t` is used:

>     template<size_t N> struct formatter<charT[N], charT>;
>     template<class traits, class Allocator>
>       struct formatter<basic_string<charT, traits, Allocator>, charT>;
>     template<class traits>
>       struct formatter<basic_string_view<charT, traits>, charT>;

Disabling following the following two is not strictly required:

>     template<> struct formatter<char*, wchar_t>;
>     template<> struct formatter<const char*, wchar_t>;

However, if (`const`) `char*` becomes an `input_range` in a future version C++, these formatters would become enabled. Disabling all five instead of the three required specializations seems like a future proof solution.

Since there is no enabled narrowing formatter specialization

>     template<> struct formatter<wchar_t, char>;

there are no issues for `wchar_t` to `char` conversions.

Before proceeding with a proposed resolution the following design questions need to be addressed:

- Do we want to allow string types of `char`s to be formatted as sequences of `wchar_t`s?

- Do we want to allow non string type sequences of `char`s to be formatted as sequences of `wchar_t`s?

- Should we disable `char` to `wchar_t` conversion in the `range_formatter`?

SG16 has indicated they would like to discuss this issue during a telecon.

[2023-06-08; Reflector poll]

Set status to SG16 and priority to 3 after reflector poll.

[2023-07-26; Mark de Wever provides wording confirmed by SG16]

[2024-03-18; Tokyo: move to Ready]

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 28.5.6.4 [format.formatter.spec] as indicated:

   > [Drafting note: The unwanted conversion happens due to the `formatter` base class specialization (28.5.7.3 [format.range.fmtdef])

   >     struct range-default-formatter<range_format::sequence, R, charT>

   > which is defined the header `<format>`. Therefore the disabling is only needed in this header) — end drafting note]

   > -2- […]

   > The `parse` member functions of these formatters interpret the format specification as a std-format-spec as described in 28.5.2.2 [format.string.std].

   > [Note 1: Specializations such as `formatter<wchar_t, char>` and `formatter<const char*, wchar_t>` that would require implicit multibyte / wide string or character conversion are disabled. — end note]

   > -?- The header `<format>` provides the following disabled specializations:

   > 1. (?.1) — The string type specializations

   >    >     template<> struct formatter<char*, wchar_t>;
   >    >     template<> struct formatter<const char*, wchar_t>;
   >    >     template<size_t N> struct formatter<char[N], wchar_t>;
   >    >     template<class traits, class Allocator>
   >    >       struct formatter<basic_string<char, traits, Allocator>, wchar_t>;
   >    >     template<class traits>
   >    >       struct formatter<basic_string_view<char, traits>, wchar_t>;

   > -3- For any types `T` and `charT` for which neither the library nor the user provides an explicit or partial specialization of the class template `formatter`, `formatter<T, charT>` is disabled.

### 3946(i). The definition of `const_iterator_t` should be reworked

Section: 25.2 [ranges.syn] Status: C++26 Submitter: Christopher Di Bella Opened: 2023-06-13 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [ranges.syn].

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Discussion:

During the reflector discussion of P2836, consensus was reached that `const_iterator_t<R>` doesn't necessarily provide the same type as `decltype(ranges::cbegin(r))`, and that it should be changed to the proposed resolution below so that they're consistent.

[Varna 2023-06-14; Move to Ready]

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     […]
   >     template<range R>
   >       using const_iterator_t = decltype(ranges::cbegin(declval<R&>()))const_iterator<iterator_t<R>>; // freestanding
   >     template<range R>
   >       using const_sentinel_t = decltype(ranges::cend(declval<R&>()))const_sentinel<sentinel_t<R>>;   // freestanding
   >     […]

### 3947(i). Unexpected constraints on `adjacent_transform_view::base()`

Section: 25.7.28.2 [range.adjacent.transform.view] Status: C++26 Submitter: Bo Persson Opened: 2023-06-17 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

In section 25.7.28.2 [range.adjacent.transform.view] the class `ranges::adjacent_transform_view` got two new `base()` members from 3848(i).

The first one looks like

>     constexpr V base() const & requires copy_constructible<InnerView>
>     { return inner_.base(); }

Here the requirement is that `InnerView` is copy constructible, when it in fact returns an object of type `V`. That seems odd.

I would expect the constraint to instead be `copy_constructible<V>`.

[2023-10-27; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 25.7.28.2 [range.adjacent.transform.view], class template `adjacent_transform_view` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<forward_range V, move_constructible F, size_t N>
   >         requires view<V> && (N > 0) && is_object_v<F> &&
   >           regular_invocable<F&, REPEAT(range_reference_t<V>, N)...> &&
   >           can-reference<invoke_result_t<F&, REPEAT(range_reference_t<V>, N)...>>
   >       class adjacent_transform_view : public view_interface<adjacent_transform_view<V, F, N>> {
   >         […]
   >         adjacent_view<V, N> inner_; // exposition only

   >         using InnerView = adjacent_view<V, N>; // exposition only
   >         […]
   >       public:
   >         […]
   >         constexpr V base() const & requires copy_constructible<VInnerView> { return inner_.base(); }
   >         constexpr V base() && { return std::move(inner_).base(); }
   >         […]
   >       };
   >     }

### 3948(i). `possibly-const-range` and `as-const-pointer` should be `noexcept`

Section: 25.2 [ranges.syn], 25.3.15 [range.prim.cdata] Status: C++26 Submitter: Jiang An Opened: 2023-06-20 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [ranges.syn].

View all other issues in [ranges.syn].

View all issues with C++26 status.

Discussion:

As of P2278R4, several range access CPOs are specified with `possibly-const-range` and `as-const-pointer`. These helper functions never throw exceptions, but are not marked with `noexcept`. As a result, implementations are currently allowed to make a call to `ranges::ccpo` potentially throwing while the underlying `ranges::cpo` call is non-throwing, which doesn't seem to be intended.

[2023-10-27; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     […]
   >     // 25.7.22 [range.as.const], as const view
   >     template<input_range R>
   >       constexpr auto& possibly-const-range(R& r) noexcept {      // exposition only
   >         if constexpr (constant_range<const R> && !constant_range<R>) {
   >           return const_cast<const R&>(r);
   >         } else {
   >           return r;
   >         }
   >       }
   >     […]

2. Modify 25.3.15 [range.prim.cdata] before p1 as indicated:

   >     template<class T>
   >     constexpr auto as-const-pointer(const T* p) noexcept { return p; }    // exposition only

### 3949(i). `std::atomic<bool>`'s trivial destructor dropped in C++17 spec wording

Section: 32.5.8.1 [atomics.types.generic.general] Status: C++26 Submitter: Jeremy Hurwitz Opened: 2023-06-20 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

`std::atomic<bool>` was originally required to have a trivial default constructor and a trivial destructor [C++11 N3337: Section [atomics.types.generic], Paragraph 5], the same as the integral [C++11 N3337: Section [atomics.types.generic], Paragraph 5] and pointer specializations [C++11 N3337: Section [atomics.types.generic], Paragraph 6]. P0558 rearranged the text, accidentally (as far as we can tell) removing the constructor and destructor requirements from `std::atomic<bool>`, which has the surprising effect that `std::atomic<bool>` has no longer the same constructor/destructor guarantees as `std::atomic<int>`.

C++20 removed the "trivial default constructor" requirement from all specializations. A specialization for floating-point types was added with a trivial destructor [N4861: Section [atomics.types.float], Paragraph 2)].

[2023-10-27; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 32.5.8.1 [atomics.types.generic.general] as indicated:

   > -1- […]

   > -2- The specialization `atomic<bool>` is a standard-layout struct. It has a trivial destructor.

### 3950(i). `std::basic_string_view` comparison operators are overspecified

Section: 27.3.2 [string.view.synop] Status: C++26 Submitter: Giuseppe D'Angelo Opened: 2023-06-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The `<string_view>` synopsis in 27.3.2 [string.view.synop] has these signatures for `operator==` and `operator<=>`:

>     // 27.3.4 [string.view.comparison], non-member comparison functions
>     template<class charT, class traits>
>       constexpr bool operator==(basic_string_view<charT, traits> x,
>                                 basic_string_view<charT, traits> y) noexcept;
>     template<class charT, class traits>
>       constexpr see below operator<=>(basic_string_view<charT, traits> x,
>                                       basic_string_view<charT, traits> y) noexcept;

>     // see 27.3.4 [string.view.comparison], sufficient additional overloads of comparison functions

In 27.3.4 [string.view.comparison], paragraph 1 states that "Implementations shall provide sufficient additional overloads" so that all comparisons between a `basic_string_view<C, T>` object and an object of a type convertible to `basic_string_view<C, T>` work (with the reasonable semantics).

The associated Example 1 proposes this implementation strategy for `operator==`:

>     template<class charT, class traits>
>       constexpr bool operator==(basic_string_view<charT, traits> lhs,
>                                 basic_string_view<charT, traits> rhs) noexcept {
>         return lhs.compare(rhs) == 0;
>       }
>     template<class charT, class traits>
>       constexpr bool operator==(basic_string_view<charT, traits> lhs,
>                                 type_identity_t<basic_string_view<charT, traits>> rhs) noexcept {
>         return lhs.compare(rhs) == 0;
>       }

With the current semantics of rewritten candidates for the comparison operators, it is however superfluous to actually specify both overloads (the same applies for `operator<=>`).

The second overload (using `type_identity_t`) is indeed necessary to implement the "sufficient additional overloads" part of 27.3.4 [string.view.comparison], but it is also sufficient, as all the following cases

- `sv == sv`

- `sv == convertible_to_sv`

- `convertible_to_sv == sv`

can in fact use it (directly, or after being rewritten e.g. with the arguments swapped).

The reason why we still do have both operators seems to be historical; there is an explanation offered here by Barry Revzin.

Basically, there were three overloads before a bunch of papers regarding `operator<=>` and `operator==` were merged:

1. `operator==(bsv, bsv)` to deal with `sv == sv`;

2. `operator==(bsv, type_identity_t<bsv>)` and

3. `operator==(type_identity_t<bsv>, bsv)` to deal with `sv == convertible_to_sv` and vice versa.

Overload (1) was necessary because with only (2) and (3) a call like `sv == sv` would otherwise be ambiguous. With the adoption of the rewriting rules, overload (3) has been dropped, without realizing that overload (1) would then become redundant.

The specification of these overloads can be greatly simplified by adjusting the signatures to explicitly use `type_identity_t`.

[Kona 2023-11-10; move to Ready]

Editorial issue 6324 provides the changes as a pull request to the draft.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 27.3.2 [string.view.synop], header `<string_view>` synopsis, as indicated:

   >     […]
   >     // 27.3.4 [string.view.comparison], non-member comparison functions
   >     template<class charT, class traits>
   >       constexpr bool operator==(basic_string_view<charT, traits> x,
   >                                 type_identity_t<basic_string_view<charT, traits>> y) noexcept;

   >     template<class charT, class traits>
   >       constexpr see below operator<=>(basic_string_view<charT, traits> x,
   >                                       type_identity_t<basic_string_view<charT, traits>> y) noexcept;

   >     // see 27.3.4 [string.view.comparison], sufficient additional overloads of comparison functions
   >     […]

2. Modify 27.3.4 [string.view.comparison] as indicated:

   > -1- Let `S` be `basic_string_view<charT, traits>`, and `sv` be an instance of `S`. Implementations shall provide sufficient additional overloads marked `constexpr` and `noexcept` so that an object `t` with an implicit conversion to `S` can be compared according to Table 81 [tab:string.view.comparison.overloads].

   > Table 81: Additional `basic_string_view` comparison overloads [tab:string.view.comparison.overloads]
   > Expression | Equivalent to
   > --- | ---
   > `t == sv` | `S(t) == sv`
   > `sv == t` | `sv == S(t)`
   > `t != sv` | `S(t) != sv`
   > `sv != t` | `sv != S(t)`
   > `t < sv` | `S(t) < sv`
   > `sv < t` | `sv < S(t)`
   > `t > sv` | `S(t) > sv`
   > `sv > t` | `sv > S(t)`
   > `t <= sv` | `S(t) <= sv`
   > `sv <= t` | `sv <= S(t)`
   > `t >= sv` | `S(t) >= sv`
   > `sv >= t` | `sv >= S(t)`
   > `t <=> sv` | `S(t) <=> sv`
   > `sv <=> t` | `sv <=> S(t)`

   > [Example 1: A sample conforming implementation for `operator==` would be:

   > >     template<class charT, class traits>
   > >       constexpr bool operator==(basic_string_view<charT, traits> lhs,
   > >                                 basic_string_view<charT, traits> rhs) noexcept {
   > >         return lhs.compare(rhs) == 0;
   > >       }
   > >     template<class charT, class traits>
   > >       constexpr bool operator==(basic_string_view<charT, traits> lhs,
   > >                                 type_identity_t<basic_string_view<charT, traits>> rhs) noexcept {
   > >         return lhs.compare(rhs) == 0;
   > >       }

   > — end example]

   >     template<class charT, class traits>
   >       constexpr bool operator==(basic_string_view<charT, traits> lhs,
   >                                 type_identity_t<basic_string_view<charT, traits>> rhs) noexcept;

   > > -2- Returns: `lhs.compare(rhs) == 0`.

   >     template<class charT, class traits>
   >       constexpr see below operator<=>(basic_string_view<charT, traits> lhs,
   >                                       type_identity_t<basic_string_view<charT, traits>> rhs) noexcept;

   > > -3- Let `R` denote the type `traits::comparison_category` if that qualified-id is valid and denotes a type (13.10.3 [temp.deduct]), otherwise `R` is `weak_ordering`.

   > > -4- Mandates: `R` denotes a comparison category type (17.12.2 [cmp.categories]).

   > > -5- Returns: `static_cast<R>(lhs.compare(rhs) <=> 0)`.

   > > [Note: The usage of `type_identity_t` as parameter ensures that an object of type `basic_string_view<charT, traits>` can always be compared with an object of a type `T` with an implicit conversion to `basic_string_view<charT, traits>`, and vice versa, as per 12.2.2.3 [over.match.oper]. — end note]

### 3951(i). §[expected.object.swap]: Using `value()` instead of `has_value()`

Section: 22.8.6.5 [expected.object.swap], 22.8.7.5 [expected.void.swap] Status: C++26 Submitter: Ben Craig Opened: 2023-06-25 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

22.8.6.5 [expected.object.swap] p2 has the following text in it:

> For the case where `rhs.value()` is `false` and `this->has_value()` is `true`, equivalent to: […]

The table preceding that text is a table of `this->has_value()` vs. `rhs.has_value()`. The `rhs.value()` in the text is almost certainly a typo, as a `.value()` call here doesn't make any sense, especially if this is an `expected<non-bool, E>`.

The same issue is there for 22.8.7.5 [expected.void.swap] p2.

[2023-10-27; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 22.8.6.5 [expected.object.swap] as indicated:

   >     constexpr void swap(expected& rhs) noexcept(see below);

   > > -1- […]

   > > -2- Effects: See Table 63 [tab:expected.object.swap].

   > > For the case where `rhs.has_value()` is `false` and `this->has_value()` is `true`, equivalent to: […]

2. Modify 22.8.7.5 [expected.void.swap] as indicated:

   >     constexpr void swap(expected& rhs) noexcept(see below);

   > > -1- […]

   > > -2- Effects: See Table 64 [tab:expected.void.swap].

   > > For the case where `rhs.has_value()` is `false` and `this->has_value()` is `true`, equivalent to: […]

### 3953(i). `iter_move` for `common_iterator` and `counted_iterator` should return `decltype(auto)`

Section: 24.5.5.7 [common.iter.cust], 24.5.7.7 [counted.iter.cust] Status: C++26 Submitter: Hewill Kang Opened: 2023-06-30 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Although the customized `iter_move` of both requires the underlying iterator `I` to be `input_iterator`, they still explicitly specify the return type as `iter_rvalue_reference_t<I>`, which makes it always instantiated.

From the point of view that its validity is only specified in the `input_iterator` concept, it would be better to remove such unnecessary type instantiation, which does not make much sense for an `output_iterator` even if it is still valid.

[2023-10-27; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 24.5.5.1 [common.iterator], class template `common_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<input_or_output_iterator I, sentinel_for<I> S>
   >         requires (!same_as<I, S> && copyable<I>)
   >       class common_iterator {
   >       public:
   >         […]
   >         friend constexpr iter_rvalue_reference_t<I>decltype(auto) iter_move(const common_iterator& i)
   >           noexcept(noexcept(ranges::iter_move(declval<const I&>())))
   >             requires input_iterator<I>;
   >         […]
   >       };
   >       […]
   >     }

2. Modify 24.5.5.7 [common.iter.cust] as indicated:

   >     friend constexpr iter_rvalue_reference_t<I>decltype(auto) iter_move(const common_iterator& i)
   >       noexcept(noexcept(ranges::iter_move(declval<const I&>())))
   >         requires input_iterator<I>;

   > > -1- Preconditions: […]

   > > -2- Effects: Equivalent to: `return ranges::iter_move(get<I>(i.v_));`

3. Modify 24.5.7.1 [counted.iterator], class template `counted_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<input_or_output_iterator I>
   >       class counted_iterator {
   >       public:
   >         […]
   >         friend constexpr iter_rvalue_reference_t<I>decltype(auto) iter_move(const counted_iterator& i)
   >           noexcept(noexcept(ranges::iter_move(i.current)))
   >             requires input_iterator<I>;
   >         […]
   >       };
   >       […]
   >     }

4. Modify 24.5.7.7 [counted.iter.cust] as indicated:

   >     friend constexpr iter_rvalue_reference_t<I>decltype(auto)
   >       iter_move(const counted_iterator& i)
   >         noexcept(noexcept(ranges::iter_move(i.current)))
   >         requires input_iterator<I>;

   > > -1- Preconditions: […]

   > > -2- Effects: Equivalent to: `return ranges::iter_move(i.current);`

### 3956(i). `chrono::parse` uses `from_stream` as a customization point

Section: 30.13 [time.parse] Status: C++26 Submitter: Jonathan Wakely Opened: 2023-07-15 Last modified: 2026-06-09

Priority: 3

View other active issues in [time.parse].

View all other issues in [time.parse].

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Discussion:

30.13 [time.parse] says: "Each `parse` overload specified in this subclause calls `from_stream` unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep])." That name should be added to 16.4.2.2 [contents] along with `swap`, `make_error_code`, and `make_error_condition`.

We should decide whether calls to `from_stream` should use normal lookup (i.e. unqualified lookup plus ADL) or just ADL, as was done for `make_error_code` and `make_error_condition` (see LWG 3629(i)).

[2023-10-30; Reflector poll]

Set priority to 3 after reflector poll.

[2024-12-02; Jonathan provides wording]

I suggest that `from_stream` should only be found via ADL, not unqualified lookup. This is consistent with what we did for `make_error_code` and `make_error_condition`, and more recently for `submdspan_mapping`. I see no reason to treat `from_stream` differently. This implies that implementations might need a poison poll in `std::chrono` so that unqualified lookup stops as soon as those are found.

[2024-12-09; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4993.

1. Modify 16.4.2.2 [contents] as indicated:

   > -3- Whenever an unqualified name other than `swap`, `make_error_code`, `make_error_condition`, `from_stream`, or `submdspan_mapping` is used in the specification of a declaration `D` in Clause 17 through Clause 33 or Annex D, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of `D`.

   > [Note 1: Argument-dependent lookup is not performed. — end note]

   > Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of `D`.

   > [Example 1: The reference to `is_array_v` in the specification of `std::to_array` (23.3.3.6 [array.creation]) refers to `::std::is_array_v`. — end example]

   > [Note 2: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]

   > The meaning of the unqualified name `swap` is established in an overload resolution context for swappable values (16.4.4.3 [swappable.requirements]). The meanings of the unqualified names `make_error_code`, `make_error_condition`, `from_stream`, and `submdspan_mapping` are established as-if by performing argument-dependent lookup (6.5.4 [basic.lookup.argdep]).

### 3957(i). §[container.alloc.reqmts] The value category of `v` should be claimed

Section: 23.2.2.5 [container.alloc.reqmts] Status: C++26 Submitter: jim x Opened: 2023-07-10 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [container.alloc.reqmts].

View all issues with C++26 status.

Discussion:

23.2.2.5 [container.alloc.reqmts] p2 says:

> […] an expression `v` of type `T` or `const T`, […]

Then 23.2.2.5 [container.alloc.reqmts] bullet (2.4) says:

> `T` is Cpp17CopyInsertable into `X` means that, in addition to `T` being Cpp17MoveInsertable into `X`, the following expression is well-formed:

> >     allocator_traits<A>::construct(m, p, v)

So, what is the value category of the expression `v`? We didn't explicitly phrase the wording. The intent may be that the value category of `v` is any defined value category in 7.2.1 [basic.lval], however, the intent is not clear in the current wording. Maybe, we can say:

> […] the following expression is well-formed:

> >     allocator_traits<A>::construct(m, p, v)

> for `v` of any value category.

which can make the intent meaning clearer.

[2023-10-27; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

1. Modify 23.2.2.5 [container.alloc.reqmts] as indicated:

   > -2- Given an allocator type `A` and given a container type `X` having a `value_type` identical to `T` and an `allocator_type` identical to `allocator_traits<A>::rebind_alloc<T>` and given an lvalue `m` of type `A`, a pointer `p` of type `T*`, an expression `v` that denotes an lvalue of type `T` or `const T` or an rvalue of type `const T`, and an rvalue `rv` of type `T`, the following terms are defined. […]

   > 1. […]

   > 2. (2.3) — `T` is Cpp17MoveInsertable into `X` means that the following expression is well-formed:

   >    >     allocator_traits<A>::construct(m, p, rv)

   >    and its evaluation causes the following postcondition to hold: The value of `*p` is equivalent to the value of `rv` before the evaluation.

   >    [Note 1: `rv` remains a valid object. Its state is unspecified — end note]

   > 3. (2.4) — `T` is Cpp17CopyInsertable into `X` means that, in addition to `T` being Cpp17MoveInsertable into `X`, the following expression is well-formed:

   >    >     allocator_traits<A>::construct(m, p, v)

   >    and its evaluation causes the following postcondition to hold: The value of `v` is unchanged and is equivalent to `*p`.

   > 4. […]

### 3965(i). Incorrect example in [format.string.escaped] p3 for formatting of combining characters

Section: 28.5.6.5 [format.string.escaped] Status: C++26 Submitter: Tom Honermann Opened: 2023-07-31 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [format.string.escaped].

View all issues with C++26 status.

Discussion:

The C++23 DIS contains the following example in 28.5.6.5 [format.string.escaped] p3. (This example does not appear in the most recent N4950 WP or on https://eel.is/c++draft because the project editor has not yet merged changes needed to support rendering of some of the characters involved).

>     string s6 = format("[{:?}]", "🤷‍♂️"); // s6 has value: ["🤷\u{200d}♂\u{fe0f}"]

The character to be formatted (🤷‍♂️) consists of the following sequence of code points in the order presented:

- U+1F937 (SHRUG)

- U+200D (ZERO WIDTH JOINER)

- U+2642 (MALE SIGN)

- U+FE0F (VARIATION SELECTOR-16)

28.5.6.5 [format.string.escaped] bullet 2.2.1 specifies which code points are to be formatted as a `\u{hex-digit-sequence}` escape sequence:

1. (2.2.1) — If X encodes a single character C, then:

   1. (2.2.1.1) — If C is one of the characters in Table 75 [tab:format.escape.sequences], then the two characters shown as the corresponding escape sequence are appended to E.

   2. (2.2.1.2) — Otherwise, if C is not U+0020 SPACE and

      1. (2.2.1.2.1) — CE is UTF-8, UTF-16, or UTF-32 and C corresponds to a Unicode scalar value whose Unicode property `General_Category` has a value in the groups `Separator` (`Z`) or `Other` (`C`), as described by UAX #44 of the Unicode Standard, or

      2. (2.2.1.2.2) — CE is UTF-8, UTF-16, or UTF-32 and C corresponds to a Unicode scalar value with the Unicode property `Grapheme_Extend=Yes` as described by UAX #44 of the Unicode Standard and C is not immediately preceded in S by a character P appended to E without translation to an escape sequence, or

      3. (2.2.1.2.3) — CE is neither UTF-8, UTF-16, nor UTF-32 and C is one of an implementation-defined set of separator or non-printable characters

      then the sequence `\u{hex-digit-sequence}` is appended to E, where `hex-digit-sequence` is the shortest hexadecimal representation of C using lower-case hexadecimal digits.

   3. (2.2.1.3) — Otherwise, C is appended to E.

The example is not consistent with the above specification for the final code point. U+FE0F is a single character, is not one of the characters in Table 75, is not U+0020, has a `General_Category` of `Nonspacing Mark (Mn)` which is neither `Z` nor `C`, has `Grapheme_Extend=Yes` but the prior character (U+2642) is not formatted as an escape sequence, and is not one of an implementation-defined set of separator or non-printable characters (for the purposes of this example; the example assumes a UTF-8 encoding). Thus, formatting for this character falls to the last bullet point and the character should be appended as is (without translation to an escape sequence). Since this character is a combining character, it should combine with the previous character and thus alter the appearance of U+2642 (thus producing `"♂️"` instead of `"♂\u{fe0f}"`).

[2023-10-27; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950 plus missing editorial pieces from P2286R8.

1. Modify the example following 28.5.6.5 [format.string.escaped] p3 as indicated:

   > [Drafting note: The presented example was voted in as part of P2286R8 during the July 2022 Virtual Meeting but is not yet accessible in the most recent working draft N4950.

   > Note that the final character (♂️) is composed from the two code points U+2642 and U+FE0F. ]

   > >     string s6 = format("[{:?}]", "🤷‍♂️"); // s6 has value: ["🤷\u{200d}♂\u{fe0f}"]["🤷\u{200d}♂️"]

### 3970(i). §[mdspan.syn] Missing definition of `full_extent_t` and `full_extent`

Section: 23.7.3.2 [mdspan.syn] Status: C++26 Submitter: S. B. Tam Opened: 2023-08-16 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [mdspan.syn].

View all issues with C++26 status.

Discussion:

`submdspan` uses a type called `full_extent_t`, but there isn't a definition for that type.

It appears that `full_extent_t` (along with `full_extent`) was proposed in P0009 before `submdspan` was moved into its own paper, and its definition failed to be included in P2630 Submdspan.

[2023-10-27; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4958.

1. Modify 23.7.3.2 [mdspan.syn] as indicated:

   >     […]
   >     //  [mdspan.submdspan], submdspan creation
   >     template<class OffsetType, class LengthType, class StrideType>
   >       struct strided_slice;

   >     template<class LayoutMapping>
   >       struct submdspan_mapping_result;

   >     struct full_extent_t { explicit full_extent_t() = default; };
   >     inline constexpr full_extent_t full_extent{};
   >     […]

### 3973(i). Monadic operations should be ADL-proof

Section: 22.8.6.7 [expected.object.monadic], 22.5.3.8 [optional.monadic] Status: C++26 Submitter: Jiang An Opened: 2023-08-10 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [expected.object.monadic].

View all issues with C++26 status.

Discussion:

LWG 3938(i) switched to use `**this` to access the value stored in `std::expected`. However, as shown in LWG 3969(i), `**this` can trigger ADL and find an unwanted overload, and thus may caused unintended behavior.

Current implementations behave correctly (Godbolt link): they don't direct use `**this`, but use the name of the union member instead.

Moreover, P2407R5 will change the monadic operations of `std::optional` to use `**this`, which is also problematic.

[2023-09-19; Wording update]

Several people preferred to replace `operator*()` by the corresponding union members, so this part of the proposed wording has been adjusted, which is a rather mechanical replacement.

[2023-10-30; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4958.

1. Modify 22.8.6.7 [expected.object.monadic] as indicated:

   > [Drafting note: Effectively replace all occurrences of `**this` by `val`, except for `decltype`.]

   >     template<class F> constexpr auto and_then(F&& f) &;
   >     template<class F> constexpr auto and_then(F&& f) const &;

   > > -1- Let `U` be `remove_cvref_t<invoke_result_t<F, decltype(**this(val))>>`.

   > > -2- […]

   > > -3- […]

   > > -4- Effects: Equivalent to:

   > > >     if (has_value())
   > > >       return invoke(std::forward<F>(f), **thisval);
   > > >     else
   > > >       return U(unexpect, error());

   >     template<class F> constexpr auto and_then(F&& f) &&;
   >     template<class F> constexpr auto and_then(F&& f) const &&;

   > > -5- Let `U` be `remove_cvref_t<invoke_result_t<F, decltype((std::move(**thisval))>>`.

   > > -6- […]

   > > -7- […]

   > > -8- Effects: Equivalent to:

   > > >     if (has_value())
   > > >       return invoke(std::forward<F>(f), std::move(**thisval));
   > > >     else
   > > >       return U(unexpect, std::move(error()));

   >     template<class F> constexpr auto or_else(F&& f) &;
   >     template<class F> constexpr auto or_else(F&& f) const &;

   > > -9- Let `G` be `remove_cvref_t<invoke_result_t<F, decltype(error())>>`.

   > > -10- Constraints: `is_constructible_v<T, decltype(**this(val))>` is `true`.

   > > -11- […]

   > > -12- Effects: Equivalent to:

   > > >     if (has_value())
   > > >       return G(in_place, **thisval);
   > > >     else
   > > >       return invoke(std::forward<F>(f), error());

   >     template<class F> constexpr auto or_else(F&& f) &&;
   >     template<class F> constexpr auto or_else(F&& f) const &&;

   > > -13- Let `G` be `remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>`.

   > > -14- Constraints: `is_constructible_v<T, decltype(std::move(**thisval))>` is `true`.

   > > -15- […]

   > > -16- Effects: Equivalent to:

   > > >     if (has_value())
   > > >       return G(in_place, std::move(**thisval));
   > > >     else
   > > >       return invoke(std::forward<F>(f), std::move(error()));

   >     template<class F> constexpr auto transform(F&& f) &;
   >     template<class F> constexpr auto transform(F&& f) const &;

   > > -17- Let `U` be `remove_cvref_t<invoke_result_t<F, decltype(**this(val))>>`.

   > > -18- […]

   > > -19- Mandates: `U` is a valid value type for `expected`. If `is_void_v<U>` is `false`, the declaration

   > > >     U u(invoke(std::forward<F>(f), **thisval));

   > > is well-formed.

   > > -20- Effects:

   > > 1. (20.1) — […]

   > > 2. (20.2) — Otherwise, if `is_void_v<U>` is `false`, returns an `expected<U, E>` object whose `has_val` member is `true` and `val` member is direct-non-list-initialized with `invoke(std::forward<F>(f), **thisval)`.

   > > 3. (20.3) — Otherwise, evaluates `invoke(std::forward<F>(f), **thisval)` and then returns `expected<U, E>()`.

   >     template<class F> constexpr auto transform(F&& f) &&;
   >     template<class F> constexpr auto transform(F&& f) const &&;

   > > -21- Let `U` be `remove_cvref_t<invoke_result_t<F, decltype(std::move(**thisval))>>`.

   > > -22- […]

   > > -23- Mandates: `U` is a valid value type for `expected`. If `is_void_v<U>` is `false`, the declaration

   > > >     U u(invoke(std::forward<F>(f), std::move(**thisval)));

   > > is well-formed.

   > > -24- Effects:

   > > 1. (24.1) — […]

   > > 2. (24.2) — Otherwise, if `is_void_v<U>` is `false`, returns an `expected<U, E>` object whose `has_val` member is `true` and `val` member is direct-non-list-initialized with `invoke(std::forward<F>(f), std::move(**thisval))`.

   > > 3. (24.3) — Otherwise, evaluates `invoke(std::forward<F>(f), std::move(**thisval))` and then returns `expected<U, E>()`.

   >     template<class F> constexpr auto transform_error(F&& f) &;
   >     template<class F> constexpr auto transform_error(F&& f) const &;

   > > -25- Let `G` be `remove_cvref_t<invoke_result_t<F, decltype(error())>>`.

   > > -26- Constraints: `is_constructible_v<T, decltype(**this(val))>` is `true`.

   > > -27- Mandates: […]

   > > -28- Returns: If `has_value()` is `true`, `expected<T, G>(in_place, **thisval)`; otherwise, an `expected<T, G>` object whose `has_val` member is `false` and `unex` member is direct-non-list-initialized with `invoke(std::forward<F>(f), error())`.

   >     template<class F> constexpr auto transform_error(F&& f) &&;
   >     template<class F> constexpr auto transform_error(F&& f) const &&;

   > > -29- Let `G` be `remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>`.

   > > -30- Constraints: `is_constructible_v<T, decltype(std::move(**thisval))>` is `true`.

   > > -31- Mandates: […]

   > > -32- Returns: If `has_value()` is `true`, `expected<T, G>(in_place, std::move(**thisval))`; otherwise, an `expected<T, G>` object whose `has_val` member is `false` and `unex` member is direct-non-list-initialized with `invoke(std::forward<F>(f), std::move(error()))`.

2. Modify 22.5.3.8 [optional.monadic] as indicated:

   > [Drafting note: Effectively replace all occurrences of `value()` by `*val`.]

   >     template<class F> constexpr auto and_then(F&& f) &;
   >     template<class F> constexpr auto and_then(F&& f) const &;

   > > -1- Let `U` be `invoke_result_t<F, decltype(value()*val)>`.

   > > -2- […]

   > > -3- Effects: Equivalent to:

   > > >     if (*this) {
   > > >       return invoke(std::forward<F>(f), value()*val);
   > > >     } else {
   > > >       return remove_cvref_t<U>();
   > > >     }

   >     template<class F> constexpr auto and_then(F&& f) &&;
   >     template<class F> constexpr auto and_then(F&& f) const &&;

   > > -4- Let `U` be `invoke_result_t<F, decltype(std::move(value()*val))>`.

   > > -5- […]

   > > -6- Effects: Equivalent to:

   > > >     if (*this) {
   > > >       return invoke(std::forward<F>(f), std::move(value()*val));
   > > >     } else {
   > > >       return remove_cvref_t<U>();
   > > >     }

   >     template<class F> constexpr auto transform(F&& f) &;
   >     template<class F> constexpr auto transform(F&& f) const &;

   > > -7- Let `U` be `remove_cv_t<invoke_result_t<F, decltype(value()*val)>>`.

   > > -8- Mandates: `U` is a non-array object type other than `in_place_t` or `nullopt_t`. The declaration

   > > >     U u(invoke(std::forward<F>(f), value()*val));

   > > is well-formed for some invented variable `u`.

   > > […]

   > > -9- Returns: If `*this` contains a value, an `optional<U>` object whose contained value is direct-non-list-initialized with `invoke(std::forward<F>(f), value()*val)`; otherwise, `optional<U>()`.

   >     template<class F> constexpr auto transform(F&& f) &&;
   >     template<class F> constexpr auto transform(F&& f) const &&;

   > > -10- Let `U` be `remove_cv_t<invoke_result_t<F, decltype(std::move(value()*val))>>`.

   > > -11- Mandates: `U` is a non-array object type other than `in_place_t` or `nullopt_t`. The declaration

   > > >     U u(invoke(std::forward<F>(f), std::move(value()*val)));

   > > is well-formed for some invented variable `u`.

   > > […]

   > > -12- Returns: If `*this` contains a value, an `optional<U>` object whose contained value is direct-non-list-initialized with `invoke(std::forward<F>(f), std::move(value()*val))`; otherwise, `optional<U>()`.

### 3974(i). `mdspan::operator[]` should not copy `OtherIndexTypes`

Section: 23.7.3.6.3 [mdspan.mdspan.members] Status: C++26 Submitter: Casey Carter Opened: 2023-08-12 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The wording for `mdspan`'s `operator[]` overloads that accept `span` and `array` is in 23.7.3.6.3 [mdspan.mdspan.members] paragraphs 5 and 6:

>     template<class OtherIndexType>
>       constexpr reference operator[](span<OtherIndexType, rank()> indices) const;
>     template<class OtherIndexType>
>       constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;

> -5- Constraints:

> 1. (5.1) — `is_convertible_v<const OtherIndexType&, index_type>` is `true`, and

> 2. (5.2) — `is_nothrow_constructible_v<index_type, const OtherIndexType&>` is `true`.

> -6- Effects: Let `P` be a parameter pack such that

> >     is_same_v<make_index_sequence<rank()>, index_sequence<P...>>

> is `true`. Equivalent to:

> >     return operator[](as_const(indices[P])...);

The equivalent code calls the other `operator[]` overload:

>     template<class... OtherIndexTypes>
>       constexpr reference operator[](OtherIndexTypes... indices) const;

with a pack of `const OtherIndexType` lvalues, but we notably haven't required `OtherIndexTypes` to be copyable — we only require that we can convert them to `index_type`. While one could argue that the use in "Effects: equivalent to" implies a requirement of copyability, it's odd that this implicit requirement would be the only requirement for copyable `OtherIndexTypes` in the spec. We could fix this by changing the `operator[]` overload accepting `OtherIndexTypes` to take them by `const&`, but that would be inconsistent with virtually every other place in the spec where types convertible to `index_type` are taken by-value. I think the best localized fix is to perform the conversion to `index_type` in the "Effects: equivalent to" code so the actual arguments have type `index_type` which we know is copyable.

[2023-11-02; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4958.

1. Modify 23.7.3.6.3 [mdspan.mdspan.members] as indicated:

   >     template<class OtherIndexType>
   >       constexpr reference operator[](span<OtherIndexType, rank()> indices) const;
   >     template<class OtherIndexType>
   >       constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;

   > > -5- Constraints:

   > > 1. (5.1) — `is_convertible_v<const OtherIndexType&, index_type>` is `true`, and

   > > 2. (5.2) — `is_nothrow_constructible_v<index_type, const OtherIndexType&>` is `true`.

   > > -6- Effects: Let `P` be a parameter pack such that

   > > >     is_same_v<make_index_sequence<rank()>, index_sequence<P...>>

   > > is `true`. Equivalent to:

   > > >     return operator[](extents_type::index-cast(as_const(indices[P]))...);

### 3975(i). Specializations of `basic_format_context` should not be permitted

Section: 28.5.6.7 [format.context] Status: C++26 Submitter: Brian Bi Opened: 2023-08-13 Last modified: 2026-06-09

Priority: 3

View all other issues in [format.context].

View all issues with C++26 status.

Discussion:

The current wording allows users to specialize `std::basic_format_context`. However, an implementation is not likely to accept a program that uses the library in a way that would instantiate such a specialization, because 28.5.6.7 [format.context] does not provide a complete description of the interface that such a specialization would need to have (e.g., it does not provide a means to initialize the exposition-only `args_` member). Since the library was not designed to be able to work with user specializations of `std::basic_format_context`, declaring such specializations should be explicitly disallowed.

Previous resolution [SUPERSEDED]

This wording is relative to N4958.

1. Modify the 28.5.6.7 [format.context] as indicated:

   > -1- An instance of `basic_format_context` holds formatting state consisting of the formatting arguments and the output iterator.

   > -?- The behavior of a program that adds specializations of `basic_format_context` is undefined.

   > -2- `Out` shall model `output_iterator<const charT&>`.

[2023-09-23; Daniel comments and provides improved wording]

During the reflector discussion, Dietmar pointed out that the constraint can in principle be checked statically (e.g. when the Library creates or refers to an instantiation of `basic_format_context`), so we can reduce the rather draconian consequence of "undefined behaviour" to "ill-formed, no diagnostics required". Furthermore, the new wording also adds the same constraint to `basic_format_parse_context` as suggested by Tim. This is needed, since only one public constructor is specified, but that specification does not allow to construct an object a non-zero `num_args_` or with the type information necessary for the `check_dynamic_spec*` functions, so the library has an unspecified way to realize this.

[2023-10-30; Reflector poll]

Set priority to 3 after reflector poll.

[Kona 2023-11-07; move to Ready]

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4958.

> [Drafting note: The suggested wording form is borrowed from exactly the same wording form used for `allocator_traits`.]

1. Modify 28.5.6.7 [format.context] as indicated:

   > -1- An instance of `basic_format_context` holds formatting state consisting of the formatting arguments and the output iterator.

   > -?- If a program declares an explicit or partial specialization of `basic_format_context`, the program is ill-formed, no diagnostic required.

   > -2- `Out` shall model `output_iterator<const charT&>`.

2. Modify 28.5.6.6 [format.parse.ctx] as indicated:

   > -1- An instance of `basic_format_parse_context` holds the format string parsing state consisting of the format string range being parsed and the argument counter for automatic indexing.

   > -?- If a program declares an explicit or partial specialization of `basic_format_parse_context`, the program is ill-formed, no diagnostic required.

### 3978(i). The "no effect" requirement for `std::ignore` is unimplementable for `volatile` bit-fields

Section: 22.4.5 [tuple.creation] Status: Resolved Submitter: Jiang An Opened: 2023-08-19 Last modified: 2024-07-25

Priority: 4

View all other issues in [tuple.creation].

View all issues with Resolved status.

Discussion:

22.4.5 [tuple.creation]/5 currently says:

> […] When an argument in `t` is `ignore`, assigning any value to the corresponding tuple element has no effect.

which is unimplementable for `volatile`-qualified bit-field glvalues.

In order to passing such a value to the `operator=` function, a distinct object needs to be created and thus the read from the `volatile` glvalue, which is a side effect (6.10.1 [intro.execution]/7), is unavoidable.

P2968R0 addresses the impossibility of assignment from `void` values, but doesn't talk about `volatile` bit-fields. Perhaps we should explicitly say that the program is ill-formed if a `volatile` bit-field value is assigned to `std::ignore` (which is implemented in libstdc++ and MSVC STL, but not in libc++).

[2023-11-03; Reflector poll]

Set priority to 4 after reflector poll. "Specify it as code." "P2968 should fix this."

[2024-07-25 Status changed: New → Resolved.]

Resolved by P2968R2, approved in St. Louis.

Proposed resolution:

### 3984(i). `ranges::to`'s recursion branch may be ill-formed

Section: 25.5.7.2 [range.utility.conv.to] Status: C++26 Submitter: Hewill Kang Opened: 2023-08-23 Last modified: 2026-06-09

Priority: 3

View other active issues in [range.utility.conv.to].

View all other issues in [range.utility.conv.to].

View all issues with C++26 status.

Discussion:

When `r` is a nested range, `ranges::to` constructs the object recursively through `r | views::transform(...)`.

However, the expression is ill-formed when the type of lvalue `r` does not model `viewable_range` (demo):

>     #include <ranges>
>     #include <vector>
>     #include <list>

>     int main() {
>       std::vector<std::vector<int>> v;
>       auto r = std::views::all(std::move(v));
>       auto l = std::ranges::to<std::list<std::list<int>>>(r); // hard error in MSVC-STL and libc++
>     }

[2023-11-03; Reflector poll]

Set priority to 3 after reflector poll. "Should be `std::forward<R>(r)` instead?"

[Kona 2023-11-07; move to Ready]

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4958.

1. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   >     template<class C, input_range R, class... Args> requires (!view<C>)
   >       constexpr C to(R&& r, Args&&... args);

   > > -1- Mandates: `C` is a cv-unqualified class type.

   > > -2- Returns: An object of type `C` constructed from the elements of `r` in the following manner:

   > > 1. (2.1) — If `C` does not satisfy `input_range` or `convertible_to<range_reference_t<R>, range_value_t<C>>` is `true`:

   > >    1. […]

   > > 2. (2.2) — Otherwise, if `input_range<range_reference_t<R>>` is `true`:

   > >    >     to<C>(ref_view(r) | views::transform([](auto&& elem) {
   > >    >       return to<range_value_t<C>>(std::forward<decltype(elem)>(elem));
   > >    >     }), std::forward<Args>(args)...);

   > > 3. (2.3) — Otherwise, the program is ill-formed.

### 3987(i). Including `<flat_foo>` doesn't provide `std::begin/end`

Section: 24.7 [iterator.range] Status: C++26 Submitter: Hewill Kang Opened: 2023-08-27 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [iterator.range].

View all issues with C++26 status.

Discussion:

It seems that 24.7 [iterator.range] should also add `<flat_foo>` to the list as the latter provides a series of range access member functions such as `begin/end`.

[2023-11-02; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4958.

1. Modify 24.7 [iterator.range] as indicated:

   > -1- In addition to being available via inclusion of the `<iterator>` header, the function templates in [iterator.range] are available when any of the following headers are included: `<array>`, `<deque>`, `<flat_map>`, `<flat_set>`, `<forward_list>`, `<list>`, `<map>`, `<regex>`, `<set>`, `<span>`, `<string>`, `<string_view>`, `<unordered_map>`, `<unordered_set>`, and `<vector>`.

### 3990(i). Program-defined specializations of `std::tuple` and `std::variant` can't be properly supported

Section: 22.4.4 [tuple.tuple], 22.6.3.1 [variant.variant.general] Status: C++26 Submitter: Jiang An Opened: 2023-08-29 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [tuple.tuple].

View all issues with C++26 status.

Discussion:

Currently, program-defined specializations of `std::tuple` and `std::variant` are not explicitly disallowed. However, they can't be properly supported by standard library implementations, because the corresponding `std::get` function templates have to inspect the implementation details of these types, and users have no way to make `std::get` behave correctly for a program-defined specializations.

Perhaps we should explicitly disallow specializing `std::tuple` and `std::variant`.

[2023-11-02; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4958.

1. Modify 22.4.4 [tuple.tuple] as indicated:

   >     namespace std {
   >       template<class... Types>
   >       class tuple {
   >         […]
   >       };

   >       […]
   >     }

   > -?- If a program declares an explicit or partial specialization of `tuple`, the program is ill-formed, no diagnostic required.

2. Modify 22.6.3.1 [variant.variant.general] as indicated:

   > […]

   > -3- A program that instantiates the definition of `variant` with no template arguments is ill-formed.

   > -?- If a program declares an explicit or partial specialization of `variant`, the program is ill-formed, no diagnostic required.

### 4000(i). `flat_map::insert_range`'s Effects is not quite right

Section: 23.6.8.7 [flat.map.modifiers] Status: Resolved Submitter: Hewill Kang Opened: 2023-10-23 Last modified: 2025-11-11

Priority: 3

View all other issues in [flat.map.modifiers].

View all issues with Resolved status.

Discussion:

`flat_map::insert_range` adds elements to the `containers` member via:

>     for (const auto& e : rg) {
>       c.keys.insert(c.keys.end(), e.first);
>       c.values.insert(c.values.end(), e.second);
>     }

which is incorrect because `rg`'s value type may not be a `pair` (`tuple`, for instance), which means that `.first` and `.second` are not valid in such cases.

[2024-02-22; Reflector poll]

Set priority to 3 after reflector poll in October 2023.

"This is P2767 section 6 which LWG looked at in Varna."

[2025-09-05; LWG telecon]

This will be resolved by P3567.

[2025-11-11; Resolved by P3567R2, approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 23.6.8.7 [flat.map.modifiers] as indicated:

   >     template<container-compatible-range<value_type> R>
   >       void insert_range(R&& rg);

   > > -12- Effects: Adds elements to `c` as if by:

   > > >     for (value_typeconst auto& e : rg) {
   > > >       c.keys.insert(c.keys.end(), std::move(e.first));
   > > >       c.values.insert(c.values.end(), std::move(e.second));
   > > >     }

   > > […]

### 4001(i). `iota_view` should provide `empty`

Section: 25.6.4.2 [range.iota.view] Status: C++26 Submitter: Hewill Kang Opened: 2023-10-27 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.iota.view].

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Discussion:

When `iota_view`'s template parameter is not an integer type and does not model `advanceable`, its `size` member will not be provided as constraints are not satisfied.

If the type further fails to model the `incrementable`, this results in its `view_interface` base being unable to synthesize a valid `empty` member as `iota_view` will just be an `input_range` (demo):

>     #include <ranges>
>     #include <vector>
>     #include <iostream>

>     int main() {
>       std::vector<int> v;
>       auto it = std::back_inserter(v);
>       auto s = std::ranges::subrange(it, std::unreachable_sentinel);
>       auto r = std::views::iota(it);
>       std::cout << s.empty() << "\n"; // 0
>       std::cout << r.empty() << "\n"; // ill-formed
>     }

This seems to be an oversight. I don't see a reason why `iota_view` doesn't provide `empty` as it does store the start and end like `subrange`, in which case it's easy to tell if it's empty just by comparing the two.

[2023-11-02; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 25.6.4.2 [range.iota.view], class template `iota_view` synopsis, as indicated:

   > >     namespace std::ranges {
   > >       […]
   > >       template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t>
   > >         requires weakly-equality-comparable-with<W, Bound> && copyable<W>
   > >       class iota_view : public view_interface<iota_view<W, Bound>> {
   > >       private:
   > >         […]
   > >         W value_ = W();                     // exposition only
   > >         Bound bound_ = Bound();             // exposition only
   > >       public:
   > >         […]
   > >         constexpr iterator begin() const;
   > >         constexpr auto end() const;
   > >         constexpr iterator end() const requires same_as<W, Bound>;

   > >         constexpr bool empty() const;
   > >         constexpr auto size() const requires see below;
   > >       };
   > >       […]
   > >     }

   > […]

   >     constexpr iterator end() const requires same_as<W, Bound>;

   > > -14- Effects: Equivalent to: `return iterator{bound_};`

   >     constexpr bool empty() const;

   > > -?- Effects: Equivalent to: `return value_ == bound_;`

   > > […]

### 4011(i). "Effects: Equivalent to return" in [span.elem]

Section: 23.7.2.2.6 [span.elem] Status: C++26 Submitter: Arthur O'Dwyer Opened: 2023-11-09 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

In reviewing the wording for P2821 `span.at()`, it had been noticed that 23.7.2.2.6 [span.elem] uses a lot of "Effects: Equivalent to return […];" which could be simply "Returns: […]".

For comparison, 27.3.3.6 [string.view.access] uses "Returns: ..." instead, so I suggest that 23.7.2.2.6 [span.elem] should be consistent with that.

[2024-03-11; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 23.7.2.2.6 [span.elem] as indicated:

       constexpr reference operator[](size_type idx) const;

   > -1- Preconditions: `idx < size()` is `true`.

   > -2- EffectsReturns: Equivalent to: `return *(data() + idx);`.

   > -?- Throws: Nothing.

       constexpr reference front() const;

   > -3- Preconditions: `empty()` is `false`.

   > -4- EffectsReturns: Equivalent to: `return *data();`.

   > -?- Throws: Nothing.

       constexpr reference back() const;

   > -5- Preconditions: `empty()` is `false`.

   > -6- EffectsReturns: Equivalent to: `return *(data() + (size() - 1));`.

   > -?- Throws: Nothing.

       constexpr pointer data() const noexcept;

   > -7- EffectsReturns: Equivalent to: `return data_;`.

### 4012(i). `common_view::begin/end` are missing the `simple-view` check

Section: 25.7.20.2 [range.common.view] Status: C++26 Submitter: Hewill Kang Opened: 2023-11-11 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.common.view].

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Discussion:

`common_view::begin/end` have exactly the same implementation as their corresponding `const` versions, which implies that when the underlying `V` satisfies `simple-view`, it is sufficient to just provide `const`-qualified members.

[2024-03-11; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 25.7.20.2 [range.common.view] as indicated:

   >     namespace std::ranges {
   >       template<view V>
   >         requires (!common_range<V> && copyable<iterator_t<V>>)
   >       class common_view : public view_interface<common_view<V>> {
   >       private:
   >         V base_ = V();  // exposition only

   >       public:
   >         […]
   >         constexpr auto begin() requires (!simple-view<V>) {
   >           if constexpr (random_access_range<V> && sized_range<V>)
   >             return ranges::begin(base_);
   >           else
   >             return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_));
   >         }

   >         constexpr auto begin() const requires range<const V> {
   >           if constexpr (random_access_range<const V> && sized_range<const V>)
   >             return ranges::begin(base_);
   >           else
   >             return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_));
   >         }

   >         constexpr auto end() requires (!simple-view<V>) {
   >           if constexpr (random_access_range<V> && sized_range<V>)
   >             return ranges::begin(base_) + ranges::distance(base_);
   >           else
   >             return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_));
   >         }

   >         constexpr auto end() const requires range<const V> {
   >           if constexpr (random_access_range<const V> && sized_range<const V>)
   >             return ranges::begin(base_) + ranges::distance(base_);
   >           else
   >             return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_));
   >         }

   >         […]
   >       };
   >     […]
   >     }

### 4013(i). `lazy_split_view::outer-iterator::value_type` should not provide default constructor

Section: 25.7.16.4 [range.lazy.split.outer.value] Status: C++26 Submitter: Hewill Kang Opened: 2023-11-11 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.lazy.split.outer.value].

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Discussion:

After P2325, there is no reason for `lazy_split_view::outer-iterator::value_type` to provide a default constructor, which only leads to unexpected behavior:

>     #include <ranges>

>     constexpr int arr[] = {42};
>     constexpr auto split = arr | std::views::lazy_split(0);
>     static_assert(!std::ranges::range_value_t<decltype(split)>{});  // UB, dereferencing a null pointer

Also, the other constructor should be private because it makes no sense for the user to construct it arbitrarily, which is not the intention.

[2024-03-11; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 25.7.16.4 [range.lazy.split.outer.value], class `split_view::outer-iterator::value_type` synopsis, as indicated:

   >     namespace std::ranges {
   >       template<input_range V, forward_range Pattern>
   >         requires view<V> && view<Pattern> &&
   >                  indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
   >                  (forward_range<V> || tiny-range<Pattern>)
   >       template<bool Const>
   >       struct lazy_split_view<V, Pattern>::outer-iterator<Const>::value_type
   >         : view_interface<value_type> {
   >       private:
   >         outer-iterator i_ = outer-iterator();               // exposition only

   >         constexpr explicit value_type(outer-iterator i);    // exposition only

   >       public:
   >         value_type() = default;
   >         constexpr explicit value_type(outer-iterator i);

   >         constexpr inner-iterator<Const> begin() const;
   >         constexpr default_sentinel_t end() const noexcept;
   >       };
   >     }

### 4014(i). LWG 3809 changes behavior of some existing `std::subtract_with_carry_engine` code

Section: 29.5.4.4 [rand.eng.sub] Status: C++26 Submitter: Matt Stephanson Opened: 2023-11-15 Last modified: 2026-06-09

Priority: 2

View all other issues in [rand.eng.sub].

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Discussion:

Issue 3809(i) pointed out that `subtract_with_carry_engine<T>` can be seeded with values from a `linear_congruential_engine<T, 40014u, 0u, 2147483563u>` object, which results in narrowing when `T` is less than 32 bits. Part of the resolution was to modify the LCG seed sequence as follows:

>     explicit subtract_with_carry_engine(result_type value);

> -7- Effects: Sets the values of X - r , … , X - 1 , in that order, as specified below. If X - 1 is then 0 , sets c to 1 ; otherwise sets c to 0 .

> To set the values X k , first construct `e`, a `linear_congruential_engine` object, as if by the following definition:

> >     linear_congruential_engine<result_typeuint_least32_t,
> >                                40014u,0u,2147483563u> e(value == 0u ? default_seed : value);

> Then, to set each X k , obtain new values z 0 , … , z n - 1 from n = ⌈ w / 32 ⌉ successive invocations of `e`. Set X k to ( ∑ j = 0 n - 1 z j ∙ 2 32 j ) mod m .

Inside `linear_congruential_engine`, the seed is reduced modulo 2147483563, so `uint_least32_t` is fine from that point on. This resolution, however, forces `value`, the user-provided seed, to be truncated from `result_type` to `uint_least32_t` before the reduction, which generally will change the result. It also breaks the existing behavior that two seeds are equivalent if they're in the same congruence class modulo the divisor.

[2024-01-11; Reflector poll]

Set priority to 2 after reflector poll.

[2024-01-11; Jonathan comments]

More precisely, the resolution forces `value` to be converted to `uint_least32_t`, which doesn't necessarily truncate, and if it does truncate, it doesn't necessarily change the value. But it will truncate whenever `value_type` is wider than `uint_least32_t`, e.g. for 32-bit `uint_least32_t` you get a different result for `std::ranlux48_base(UINT_MAX + 1LL)()`. The new proposed resolution below restores the old behaviour for that type.

[2024-10-09; LWG telecon: Move to Ready]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964 after the wording changes applied by LWG 3809(i), which had been accepted into the working paper during the Kona 2023-11 meeting.

1. Modify 29.5.4.4 [rand.eng.sub] as indicated:

   >     explicit subtract_with_carry_engine(result_type value);

   > -7- Effects: Sets the values of X - r , … , X - 1 , in that order, as specified below. If X - 1 is then 0 , sets c to 1 ; otherwise sets c to 0 .

   > To set the values X k , first construct `e`, a `linear_congruential_engine` object, as if by the following definition:

   > >     linear_congruential_engine<uint_least32_t,
   > >                                40014u,0u,2147483563u> e(value == 0u ? default_seed :
   > >                                static_cast<uint_least32_t>(value % 2147483563u));

   > Then, to set each X k , obtain new values z 0 , … , z n - 1 from n = ⌈ w / 32 ⌉ successive invocations of `e`. Set X k to ( ∑ j = 0 n - 1 z j ∙ 2 32 j ) mod m .

### 4015(i). LWG 3973 broke `const` overloads of `std::optional` monadic operations

Section: 22.5.3.8 [optional.monadic] Status: C++26 Submitter: Jonathan Wakely Opened: 2023-11-24 Last modified: 2026-06-09

Priority: 1

View all other issues in [optional.monadic].

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Discussion:

The resolution of LWG 3973(i) (adopted in Kona) changed all occurrences of `value()` to `*val`. The intention was not to change the meaning, just avoid the non-freestanding `value()` function, and avoid ADL that would be caused by using `**this`. However, in the `const` overloads such as `and_then(F&&) const` the type of `value()` was `const T&`, but the type of `*val` is always `T&`. This implies that the const overloads invoke the callable with a non-const argument, which is incorrect (and would be undefined behaviour for a `const std::optional<T>`).

On the LWG reflector it was suggested that we should rewrite the specification of `std::optional` to stop using an exposition-only data member of type `T*`. No such member ever exists in real implemetations, so it is misleading and leads to specification bugs of this sort.

Change the class definition in 22.5.3.1 [optional.optional.general] to use a union, and update every use of `val` accordingly throughout 22.5.3 [optional.optional]. For consistency with 22.8.6.1 [expected.object.general] we might also want to introduce a `bool has_val` member and refer to that in the specification.

>       private:
>         T *val;         // exposition only
>         bool has_val;   // exposition only
>         union {
>           T val;        // exposition only
>         };
>       };

For example, in 22.5.3.9 [optional.mod]:

> -1- Effects: If `*this` contains a value, calls `val->.T::~T()` to destroy the contained value and sets `has_val` to `false`; otherwise no effect.

[2023-11-26; Daniel provides wording]

The proposed wording is considerably influenced by that of the specification of `expected`, but attempts to reduce the amount of changes to not perfectly mimic it. Although "the contained value" is a magic word of power it seemed feasible and simpler to use the new exposition-only member `val` directly in some (but not all) places, usually involved with initializations.

Furthermore, I have only added "and sets `has_val` to `true/false`" where either the Effects wording says "otherwise no effect" or in other cases if the postconditions did not already say that indirectly. I also added extra mentioning of `has_val` changes in tables where different cells had very different effects on that member (unless these cells specify postconditions), to prevent misunderstanding.

[2024-03-11; Reflector poll]

Set priority to 1 after reflector poll in November 2023. Six votes for 'Tentatively Ready' but enough uncertainty to deserve discussion at a meeting.

Previous resolution [SUPERSEDED]

This wording is relative to N4964 after application of the wording of LWG 3973(i).

1. Modify 22.5.3.1 [optional.optional.general], class template `optional` synopsis, as indicated:

   >     namespace std {
   >       template<class T>
   >       class optional {
   >       public:
   >         using value_type = T;
   >         […]
   >       private:
   >         bool has_val; // exposition only
   >         union {
   >           T val*val; // exposition only
   >         };
   >       };

   >       […]
   >     }

2. Modify 22.5.3.1 [optional.optional.general] as indicated:

   > -2- Member `has_val` indicates whether an `optional<T>` object contains a valueWhen an `optional<T>` object contains a value, member `val` points to the contained value.

3. Modify 22.5.3.2 [optional.ctor] as indicated:

   > [Drafting note: Normatively, this subclause doesn't require any changes, but I'm suggesting to replace phrases of the form "[…]initializes the contained value with"] by "[…]initializes `val` with" as we do in 22.8.6.2 [expected.object.cons]. I intentionally did not add extra "and sets `has_val` to `true/false`" since those effects are already guaranteed by the postconditions]

   >     constexpr optional(const optional& rhs);

   > > -4- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `*rhs.val`.

   > > -5- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     constexpr optional(optional&& rhs) noexcept(see below);

   > > -8- Constraints: […]

   > > -9- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)`. `rhs.has_value()` is unchanged.

   > > -10- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);

   > > -13- Constraints: […]

   > > -14- Effects: Direct-non-list-initializes `val` the contained value with `std::forward<Args>(args)...`.

   > > -15- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U, class... Args>
   >       constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

   > > -18- Constraints: […]

   > > -19- Effects: Direct-non-list-initializes `val` the contained value with `il, std::forward<Args>(args)...`.

   > > -20- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U = T> constexpr explicit(see below) optional(U&& v);

   > > -23- Constraints: […]

   > > -24- Effects: Direct-non-list-initializes `val` the contained value with `std::forward<U>(v)`.

   > > -25- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U> constexpr explicit(see below) optional(const optional<U>& rhs);

   > > -28- Constraints: […]

   > > -29- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `*rhs.val`.

   > > -30- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     template<class U> constexpr explicit(see below) optional(optional<U>&& rhs);

   > > -33- Constraints: […]

   > > -34- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)`. `rhs.has_value()` is unchanged.

   > > -35- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

4. Modify 22.5.3.3 [optional.dtor] as indicated:

   >     constexpr ~optional();

   > > -1- Effects: If `is_trivially_destructible_v<T> != true` and `*this` contains a value, calls `val->val.T::~T()`.

5. Modify 22.5.3.4 [optional.assign] as indicated:

   >     constexpr optional<T>& operator=(nullopt_t) noexcept;

   > > -1- Effects: If `*this` contains a value, calls `val->val.T::~T()` to destroy the contained value and sets `has_val` to `false`; otherwise no effect.

   > > -2- Postconditions: `*this` does not contain a value.

   >     constexpr optional<T>& operator=(const optional& rhs);

   > > -4- Effects: See Table 58.

   > > Table 58 — `optional::operator=(const optional&)` effects [tab:optional.assign.copy]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*rhs.val` to `val` the contained value | direct-non-list-initializes `val` the contained value with `*rhs.val` and sets `has_val` to `true`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` and sets `has_val` to `false` | no effect

   > > -5- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     constexpr optional<T>& operator=(optional&& rhs) noexcept(see below);

   > > -8- Constraints: […]

   > > -9- Effects: See Table 59. The result of the expression `rhs.has_value()` remains unchanged.

   > > -10- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -11- Returns: `*this`.

   > > Table 59 — `optional::operator=(optional&&)` effects [tab:optional.assign.move]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `std::move(*rhs.val)` to `val` the contained value | direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)` and sets `has_val` to `true`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()`and sets `has_val` to `false` | no effect

   > > -12- Remarks: […]

   > > -13- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s move constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s move constructor. If an exception is thrown during the call to `T`'s move assignment, the state of `*valval` and `*rhs.valval` is determined by the exception safety guarantee of `T`'s move assignment.

   >     template<class U = T> constexpr optional<T>& operator=(U&& v);

   > > -14- Constraints: […]

   > > -15- Effects: If `*this` contains a value, assigns `std::forward<U>(v)` to `val` the contained value; otherwise direct-non-list-initializes `val` the contained value with `std::forward<U>(v)`.

   > > -16- Postconditions: `*this` contains a value.

   > > -17- Returns: `*this`.

   > > -18- Remarks: If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `val*val` and `v` is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class U> constexpr optional<T>& operator=(const optional<U>& rhs);

   > > -19- Constraints: […]

   > > -20- Effects: See Table 60.

   > > Table 60 — `optional::operator=(const optional<U>&)` effects [tab:optional.assign.copy.templ]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*rhs.val` to `val` the contained value | direct-non-list-initializes `val` the contained value with `*rhs.val` and sets `has_val` to `true`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` and sets `has_val` to `false` | no effect

   > > -21- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -22- Returns: `*this`.

   > > -23- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `val*val` and `*rhs.valval` is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class U> constexpr optional<T>& operator=(optional<U>&& rhs);

   > > -24- Constraints: […]

   > > -25- Effects: See Table 61. The result of the expression `rhs.has_value()` remains unchanged.

   > > Table 61 — `optional::operator=(optional<U>&&)` effects [tab:optional.assign.move.templ]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `std::move(*rhs.val)` to `val` the contained value | direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)` and sets `has_val` to `true`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` and sets `has_val` to `false` | no effect

   > > -26- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -27- Returns: `*this`.

   > > -28- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the state of `val*val` and `*rhs.valval` is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class... Args> constexpr T& emplace(Args&&... args);

   > > -29- Mandates: […]

   > > -30- Effects: Calls `*this = nullopt`. Then direct-non-list-initializes `val` the contained value with `std::forward<Args>(args)...`.

   > > -31- Postconditions: `*this` contains a value.

   > > -32- Returns: `val` A reference to the new contained value.

   > > […]

   > > -34- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and the previous `val*val` (if any) has been destroyed.

   >     template<class U, class... Args> constexpr T& emplace(initializer_list<U> il, Args&&... args);

   > > -35- Constraints: […]

   > > -36- Effects: Calls `*this = nullopt`. Then direct-non-list-initializes `val` the contained value with `il, std::forward<Args>(args)...`.

   > > -37- Postconditions: `*this` contains a value.

   > > -38- Returns: `val` A reference to the new contained value.

   > > […]

   > > -40- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and the previous `val*val` (if any) has been destroyed.

6. Modify 22.5.3.5 [optional.swap] as indicated:

   >     constexpr void swap(optional& rhs) noexcept(see below);

   > > -1- Mandates: […]

   > > -2- Preconditions: […]

   > > -3- Effects: See Table 62.

   > > Table 62 — `optional::swap(optional&)` effects [tab:optional.swap]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | calls `swap(val*(*this), *rhs.val)` | direct-non-list-initializes `val` the contained value of `*this` with `std::move(*rhs.val)`, followed by `rhs.val.val->T::~T()`; postcondition is that `*this` contains a value and `rhs` does not contain a value
   > > `rhs` does not contain a value | direct-non-list-initializes the contained value of `rhs.val` with `std::move(val*(*this))`, followed by `val.val->T::~T()`; postcondition is that `*this` does not contain a value and `rhs` contains a value | no effect

   > > -4- Throws: […]

   > > -5- Remarks: […]

   > > -6- If any exception is thrown, the results of the expressions `this->has_value()` and `rhs.has_value()` remain unchanged. If an exception is thrown during the call to function `swap`, the state of `val*val` and `*rhs.valval` is determined by the exception safety guarantee of `swap` for lvalues of `T`. If an exception is thrown during the call to `T`'s move constructor, the state of `val*val` and `*rhs.valval` is determined by the exception safety guarantee of `T`'s move constructor.

7. Modify 22.5.3.7 [optional.observe] as indicated:

   >     constexpr const T* operator->() const noexcept;
   >     constexpr T* operator->() noexcept;

   > > -1- Preconditions: `*this` contains a value.

   > > -2- Returns: `addressof(val)val`.

   > > -3- […]

   >     constexpr const T& operator*() const & noexcept;
   >     constexpr T& operator*() & noexcept;

   > > -4- Preconditions: `*this` contains a value.

   > > -5- Returns: `val*val`.

   > > -6- […]

   >     constexpr T&& operator*() && noexcept;
   >     constexpr const T&& operator*() const && noexcept;

   > > -7- Preconditions: `*this` contains a value.

   > > -8- Effects: Equivalent to: `return std::move(val*val);`

   >     constexpr explicit operator bool() const noexcept;

   > > -9- Returns: `true` if and only if `*this` contains a value.

   > > -10- Remarks: This function is a constexpr function.

   >     constexpr bool has_value() const noexcept;

   > > -11- Returns: `has_val` `true` if and only if `*this` contains a value.

   > > -12- Remarks: These functions areThis function is a constexpr functions.

   >     constexpr const T& value() const &;
   >     constexpr T& value() &;

   > > -13- Effects: Equivalent to:

   > > >     return has_value() ? val*val : throw bad_optional_access();

   >     constexpr T&& value() &&;
   >     constexpr const T&& value() const &&;

   > > -14- Effects: Equivalent to:

   > > >     return has_value() ? std::move(val*val) : throw bad_optional_access();

   >     template<class U> constexpr T value_or(U&& v) const &;

   > > -15- Mandates: […]

   > > -16- Effects: Equivalent to:

   > > >     return has_value() ? val**this : static_cast<T>(std::forward<U>(v));

   >     template<class U> constexpr T value_or(U&& v) &&;

   > > -17- Mandates: […]

   > > -18- Effects: Equivalent to:

   > > >     return has_value() ? std::move(val**this) : static_cast<T>(std::forward<U>(v));

8. Modify 22.5.3.8 [optional.monadic] as indicated:

   >     template<class F> constexpr auto and_then(F&& f) &;
   >     template<class F> constexpr auto and_then(F&& f) const &;

   > > -1- Let `U` be `invoke_result_t<F, decltype((val)*val)>`.

   > > -2- Mandates: […]

   > > -3- Effects: Equivalent to:

   > > >     if (*this) {
   > > >       return invoke(std::forward<F>(f), val*val);
   > > >     } else {
   > > >       return remove_cvref_t<U>();
   > > >     }

   >     template<class F> constexpr auto and_then(F&& f) &&;
   >     template<class F> constexpr auto and_then(F&& f) const &&;

   > > -4- Let `U` be `invoke_result_t<F, decltype(std::move(val*val))>`.

   > > -5- Mandates: […]

   > > -6- Effects: Equivalent to:

   > > >     if (*this) {
   > > >       return invoke(std::forward<F>(f), std::move(val*val));
   > > >     } else {
   > > >       return remove_cvref_t<U>();
   > > >     }

   >     template<class F> constexpr auto transform(F&& f) &;
   >     template<class F> constexpr auto transform(F&& f) const &;

   > > -7- Let `U` be `remove_cv_t<invoke_result_t<F, decltype((val)*val)>>`.

   > > -8- Mandates: `U` is a non-array object type other than `in_place_t` or `nullopt_t`. The declaration

   > > >     U u(invoke(std::forward<F>(f), val*val));

   > > is well-formed for some invented variable `u`.

   > > […]

   > > -9- Returns: If `*this` contains a value, an `optional<U>` object whose contained value is direct-non-list-initialized with `invoke(std::forward<F>(f), val*val)`; otherwise, `optional<U>()`.

   >     template<class F> constexpr auto transform(F&& f) &&;
   >     template<class F> constexpr auto transform(F&& f) const &&;

   > > -10- Let `U` be `remove_cv_t<invoke_result_t<F, decltype(std::move(val*val))>>`.

   > > -11- Mandates: `U` is a non-array object type other than `in_place_t` or `nullopt_t`. The declaration

   > > >     U u(invoke(std::forward<F>(f), std::move(val*val)));

   > > is well-formed for some invented variable `u`.

   > > […]

   > > -12- Returns: If `*this` contains a value, an `optional<U>` object whose contained value is direct-non-list-initialized with `invoke(std::forward<F>(f), std::move(val*val))`; otherwise, `optional<U>()`.

9. Modify 22.5.3.9 [optional.mod] as indicated:

   >     constexpr void reset() noexcept;

   > > -1- Effects: If `*this` contains a value, calls `val->val.T::~T()` to destroy the contained value and sets `has_val` to `false`; otherwise no effect.

   > > -2- Postconditions: `*this` does not contain a value.

[St. Louis 2024-06-24; Jonathan provides improved wording]

[2024-08-21; LWG telecon]

During telecon review it was suggested to replace 22.5.3.1 [optional.optional.general] p1 and p2. On the reflector Daniel requested to keep the "additional storage" prohibition, so that will be addressed by issue 4141(i) instead.

[2024-10-02; Jonathan tweaks proposed resolution]

On the reflector we decided that the union member should use `remove_cv_t`, as proposed for `expected` by issue 3891(i). The rest of the proposed resolution is unchanged, so that edit was made in-place below, instead of as a new resolution that supersedes the old one.

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

1. Modify 22.5.3.1 [optional.optional.general], class template `optional` synopsis, as indicated:

   >     namespace std {
   >       template<class T>
   >       class optional {
   >       public:
   >         using value_type = T;
   >         […]
   >       private:
   >         *val // exposition only;
   >         union {
   >           remove_cv_t<T> val; // exposition only
   >         };
   >       };

   >       […]
   >     }

2. Modify 22.5.3.1 [optional.optional.general] as indicated:

   > -1- When its member `val` is active (11.5.1 [class.union.general]), an instance of `optional<T>` is said to contain a value, and `val` is referred to as its contained value. Any instance of `optional<T>` at any given time either contains a value or does not contain a value. When an instance of `optional<T>` contains a value, it means that an object of type `T`, referred to as the An optional object's contained value contained value, is allocated within the storage of the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value. When an object of type `optional<T>` is contextually converted to `bool`, the conversion returns `true` if the object contains a value; otherwise the conversion returns `false`.

   > -2- When an `optional<T>` object contains a value, member `val` points to the contained value.

3. Modify 22.5.3.2 [optional.ctor] as indicated:

   >     constexpr optional(const optional& rhs);

   > > -4- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `*rhs.val`.

   > > -5- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     constexpr optional(optional&& rhs) noexcept(see below);

   > > -8- Constraints: […]

   > > -9- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)`. `rhs.has_value()` is unchanged.

   > > -10- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);

   > > -13- Constraints: […]

   > > -14- Effects: Direct-non-list-initializes `val` the contained value with `std::forward<Args>(args)...`.

   > > -15- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U, class... Args>
   >       constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

   > > -18- Constraints: […]

   > > -19- Effects: Direct-non-list-initializes `val` the contained value with `il, std::forward<Args>(args)...`.

   > > -20- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U = T> constexpr explicit(see below) optional(U&& v);

   > > -23- Constraints: […]

   > > -24- Effects: Direct-non-list-initializes `val` the contained value with `std::forward<U>(v)`.

   > > -25- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U> constexpr explicit(see below) optional(const optional<U>& rhs);

   > > -28- Constraints: […]

   > > -29- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `*rhs.val`.

   > > -30- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     template<class U> constexpr explicit(see below) optional(optional<U>&& rhs);

   > > -33- Constraints: […]

   > > -34- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)`. `rhs.has_value()` is unchanged.

   > > -35- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

4. Modify 22.5.3.3 [optional.dtor] as indicated:

   >     constexpr ~optional();

   > > -1- Effects: If `is_trivially_destructible_v<T> != true` and `*this` contains a value, calls `val->val.T::~T()`.

5. Modify 22.5.3.4 [optional.assign] as indicated:

   >     constexpr optional<T>& operator=(nullopt_t) noexcept;

   > > -1- Effects: If `*this` contains a value, calls `val->val.T::~T()` to destroy the contained value; otherwise no effect.

   > > -2- Postconditions: `*this` does not contain a value.

   >     constexpr optional<T>& operator=(const optional& rhs);

   > > -4- Effects: See Table 58.

   > > Table 58 — `optional::operator=(const optional&)` effects [tab:optional.assign.copy]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*rhs.val` to `val` the contained value | direct-non-list-initializes `val` the contained value with `*rhs.val`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` | no effect

   > > -5- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     constexpr optional<T>& operator=(optional&& rhs) noexcept(see below);

   > > -8- Constraints: […]

   > > -9- Effects: See Table 59. The result of the expression `rhs.has_value()` remains unchanged.

   > > -10- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -11- Returns: `*this`.

   > > Table 59 — `optional::operator=(optional&&)` effects [tab:optional.assign.move]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `std::move(*rhs.val)` to `val` the contained value | direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` | no effect

   > > -12- Remarks: […]

   > > -13- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s move constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s move constructor. If an exception is thrown during the call to `T`'s move assignment, the states state of `*valval` and `*rhs.valval` are is determined by the exception safety guarantee of `T`'s move assignment.

   >     template<class U = T> constexpr optional<T>& operator=(U&& v);

   > > -14- Constraints: […]

   > > -15- Effects: If `*this` contains a value, assigns `std::forward<U>(v)` to `val` the contained value; otherwise direct-non-list-initializes `val` the contained value with `std::forward<U>(v)`.

   > > -16- Postconditions: `*this` contains a value.

   > > -17- Returns: `*this`.

   > > -18- Remarks: If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the states state of `val*val` and `v` are is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class U> constexpr optional<T>& operator=(const optional<U>& rhs);

   > > -19- Constraints: […]

   > > -20- Effects: See Table 60.

   > > Table 60 — `optional::operator=(const optional<U>&)` effects [tab:optional.assign.copy.templ]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*rhs.val` to `val` the contained value | direct-non-list-initializes `val` the contained value with `*rhs.val`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` | no effect

   > > -21- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -22- Returns: `*this`.

   > > -23- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the states state of `val*val` and `*rhs.valval` are is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class U> constexpr optional<T>& operator=(optional<U>&& rhs);

   > > -24- Constraints: […]

   > > -25- Effects: See Table 61. The result of the expression `rhs.has_value()` remains unchanged.

   > > Table 61 — `optional::operator=(optional<U>&&)` effects [tab:optional.assign.move.templ]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `std::move(*rhs.val)` to `val` the contained value | direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` | no effect

   > > -26- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -27- Returns: `*this`.

   > > -28- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the states state of `val*val` and `*rhs.valval` are is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class... Args> constexpr T& emplace(Args&&... args);

   > > -29- Mandates: […]

   > > -30- Effects: Calls `*this = nullopt`. Then direct-non-list-initializes `val` the contained value with `std::forward<Args>(args)...`.

   > > -31- Postconditions: `*this` contains a value.

   > > -32- Returns: `val` A reference to the new contained value.

   > > […]

   > > -34- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and the previous `val*val` (if any) has been destroyed.

   >     template<class U, class... Args> constexpr T& emplace(initializer_list<U> il, Args&&... args);

   > > -35- Constraints: […]

   > > -36- Effects: Calls `*this = nullopt`. Then direct-non-list-initializes `val` the contained value with `il, std::forward<Args>(args)...`.

   > > -37- Postconditions: `*this` contains a value.

   > > -38- Returns: `val` A reference to the new contained value.

   > > […]

   > > -40- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and the previous `val*val` (if any) has been destroyed.

6. Modify 22.5.3.5 [optional.swap] as indicated:

   >     constexpr void swap(optional& rhs) noexcept(see below);

   > > -1- Mandates: […]

   > > -2- Preconditions: […]

   > > -3- Effects: See Table 62.

   > > Table 62 — `optional::swap(optional&)` effects [tab:optional.swap]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | calls `swap(val*(*this), *rhs.val)` | direct-non-list-initializes `val` the contained value of `*this` with `std::move(*rhs.val)`, followed by `rhs.val.val->T::~T()`; postcondition is that `*this` contains a value and `rhs` does not contain a value
   > > `rhs` does not contain a value | direct-non-list-initializes the contained value of `rhs.val` with `std::move(val*(*this))`, followed by `val.val->T::~T()`; postcondition is that `*this` does not contain a value and `rhs` contains a value | no effect

   > > -4- Throws: […]

   > > -5- Remarks: […]

   > > -6- If any exception is thrown, the results of the expressions `this->has_value()` and `rhs.has_value()` remain unchanged. If an exception is thrown during the call to function `swap`, the state of `val*val` and `*rhs.valval` is determined by the exception safety guarantee of `swap` for lvalues of `T`. If an exception is thrown during the call to `T`'s move constructor, the states state of `val*val` and `*rhs.valval` are is determined by the exception safety guarantee of `T`'s move constructor.

7. Modify 22.5.3.7 [optional.observe] as indicated:

   >     constexpr const T* operator->() const noexcept;
   >     constexpr T* operator->() noexcept;

   > > -1- Preconditions: `*this` contains a value.

   > > -2- Returns: `addressof(val)val`.

   > > -3- […]

   >     constexpr const T& operator*() const & noexcept;
   >     constexpr T& operator*() & noexcept;

   > > -4- Preconditions: `*this` contains a value.

   > > -5- Returns: `val*val`.

   > > -6- […]

   >     constexpr T&& operator*() && noexcept;
   >     constexpr const T&& operator*() const && noexcept;

   > > -7- Preconditions: `*this` contains a value.

   > > -8- Effects: Equivalent to: `return std::move(val*val);`

   >     constexpr explicit operator bool() const noexcept;

   > > -9- Returns: `true` if and only if `*this` contains a value.

   > > -10- Remarks: This function is a constexpr function.

   >     constexpr bool has_value() const noexcept;

   > > -11- Returns: `true` if and only if `*this` contains a value.

   > > -12- Remarks: This function is a constexpr function.

   >     constexpr const T& value() const &;
   >     constexpr T& value() &;

   > > -13- Effects: Equivalent to:

   > > >     return has_value() ? val*val : throw bad_optional_access();

   >     constexpr T&& value() &&;
   >     constexpr const T&& value() const &&;

   > > -14- Effects: Equivalent to:

   > > >     return has_value() ? std::move(val*val) : throw bad_optional_access();

   >     template<class U> constexpr T value_or(U&& v) const &;

   > > -15- Mandates: […]

   > > -16- Effects: Equivalent to:

   > > >     return has_value() ? val**this : static_cast<T>(std::forward<U>(v));

   >     template<class U> constexpr T value_or(U&& v) &&;

   > > -17- Mandates: […]

   > > -18- Effects: Equivalent to:

   > > >     return has_value() ? std::move(val**this) : static_cast<T>(std::forward<U>(v));

8. Modify 22.5.3.8 [optional.monadic] as indicated:

   >     template<class F> constexpr auto and_then(F&& f) &;
   >     template<class F> constexpr auto and_then(F&& f) const &;

   > > -1- Let `U` be `invoke_result_t<F, decltype((val)*val)>`.

   > > -2- Mandates: […]

   > > -3- Effects: Equivalent to:

   > > >     if (*this) {
   > > >       return invoke(std::forward<F>(f), val*val);
   > > >     } else {
   > > >       return remove_cvref_t<U>();
   > > >     }

   >     template<class F> constexpr auto and_then(F&& f) &&;
   >     template<class F> constexpr auto and_then(F&& f) const &&;

   > > -4- Let `U` be `invoke_result_t<F, decltype(std::move(val*val))>`.

   > > -5- Mandates: […]

   > > -6- Effects: Equivalent to:

   > > >     if (*this) {
   > > >       return invoke(std::forward<F>(f), std::move(val*val));
   > > >     } else {
   > > >       return remove_cvref_t<U>();
   > > >     }

   >     template<class F> constexpr auto transform(F&& f) &;
   >     template<class F> constexpr auto transform(F&& f) const &;

   > > -7- Let `U` be `remove_cv_t<invoke_result_t<F, decltype((val)*val)>>`.

   > > -8- Mandates: `U` is a non-array object type other than `in_place_t` or `nullopt_t`. The declaration

   > > >     U u(invoke(std::forward<F>(f), val*val));

   > > is well-formed for some invented variable `u`.

   > > […]

   > > -9- Returns: If `*this` contains a value, an `optional<U>` object whose contained value is direct-non-list-initialized with `invoke(std::forward<F>(f), val*val)`; otherwise, `optional<U>()`.

   >     template<class F> constexpr auto transform(F&& f) &&;
   >     template<class F> constexpr auto transform(F&& f) const &&;

   > > -10- Let `U` be `remove_cv_t<invoke_result_t<F, decltype(std::move(val*val))>>`.

   > > -11- Mandates: `U` is a non-array object type other than `in_place_t` or `nullopt_t`. The declaration

   > > >     U u(invoke(std::forward<F>(f), std::move(val*val)));

   > > is well-formed for some invented variable `u`.

   > > […]

   > > -12- Returns: If `*this` contains a value, an `optional<U>` object whose contained value is direct-non-list-initialized with `invoke(std::forward<F>(f), std::move(val*val))`; otherwise, `optional<U>()`.

9. Modify 22.5.3.9 [optional.mod] as indicated:

   >     constexpr void reset() noexcept;

   > > -1- Effects: If `*this` contains a value, calls `val->val.T::~T()` to destroy the contained value; otherwise no effect.

   > > -2- Postconditions: `*this` does not contain a value.

[2025-11-03; Tomasz tweaks proposed resolution]

Updated converting constructor and assignments to use `operator*()` directly, required to correctly support `optional<T&>`. Also update corresponding constructor in specialization.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.3.1 [optional.optional.general], class template `optional` synopsis, as indicated:

   >     namespace std {
   >       template<class T>
   >       class optional {
   >       public:
   >         using value_type = T;
   >         […]
   >       private:
   >         T* val; // exposition only
   >         union {
   >           remove_cv_t<T> val; // exposition only
   >         };
   >       };

   >       […]
   >     }

2. Modify 22.5.3.1 [optional.optional.general] as indicated:

   > -1- An instance of `optional<T>` is said to contain a value when and only when its member `val` is active (11.5.1 [class.union.general]); `val` is referred to as its contained value. An object of type `optional<T>` at any given time either contains a value or does not contain a value. When an object of type `optional<T>` contains a value, it means that an object of type `T`, referred to as the An optional object's contained value contained value, is nested within (6.8.2 [intro.object]) the optional object. When an object of type `optional<T>` is contextually converted to `bool`, the conversion returns `true` if the object contains a value; otherwise the conversion returns `false`.

   > -2- When an `optional<T>` object contains a value, member `val` points to the contained value.

3. Modify 22.5.3.2 [optional.ctor] as indicated:

   >     constexpr optional(const optional& rhs);

   > > -4- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `*rhs.val`.

   > > -5- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     constexpr optional(optional&& rhs) noexcept(see below);

   > > -8- Constraints: […]

   > > -9- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)`. `rhs.has_value()` is unchanged.

   > > -10- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);

   > > -13- Constraints: […]

   > > -14- Effects: Direct-non-list-initializes `val` the contained value with `std::forward<Args>(args)...`.

   > > -15- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U, class... Args>
   >       constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

   > > -18- Constraints: […]

   > > -19- Effects: Direct-non-list-initializes `val` the contained value with `il, std::forward<Args>(args)...`.

   > > -20- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U = T> constexpr explicit(see below) optional(U&& v);

   > > -23- Constraints: […]

   > > -24- Effects: Direct-non-list-initializes `val` the contained value with `std::forward<U>(v)`.

   > > -25- Postconditions: `*this` contains a value.

   > > […]

   >     template<class U> constexpr explicit(see below) optional(const optional<U>& rhs);

   > > -28- Constraints: […]

   > > -29- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `*rhs.operator*()`.

   > > -30- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     template<class U> constexpr explicit(see below) optional(optional<U>&& rhs);

   > > -33- Constraints: […]

   > > -34- Effects: If `rhs` contains a value, direct-non-list-initializes `val` the contained value with `*std::move(rhs).operator*()`. `rhs.has_value()` is unchanged.

   > > -35- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

4. Modify 22.5.3.3 [optional.dtor] as indicated:

   >     constexpr ~optional();

   > > -1- Effects: If `is_trivially_destructible_v<T> != true` and `*this` contains a value, calls `val->val.T::~T()`.

5. Modify 22.5.3.4 [optional.assign] as indicated:

   >     constexpr optional<T>& operator=(nullopt_t) noexcept;

   > > -1- Effects: If `*this` contains a value, calls `val->val.T::~T()` to destroy the contained value; otherwise no effect.

   > > -2- Postconditions: `*this` does not contain a value.

   >     constexpr optional<T>& operator=(const optional& rhs);

   > > -4- Effects: See Table 58.

   > > Table 58 — `optional::operator=(const optional&)` effects [tab:optional.assign.copy]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*rhs.val` to `val` the contained value | direct-non-list-initializes `val` the contained value with `*rhs.val`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` | no effect

   > > -5- Postconditions: `rhs.has_value() == this->has_value()`.

   > > […]

   >     constexpr optional<T>& operator=(optional&& rhs) noexcept(see below);

   > > -8- Constraints: […]

   > > -9- Effects: See Table 59. The result of the expression `rhs.has_value()` remains unchanged.

   > > -10- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -11- Returns: `*this`.

   > > Table 59 — `optional::operator=(optional&&)` effects [tab:optional.assign.move]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `std::move(*rhs.val)` to `val` the contained value | direct-non-list-initializes `val` the contained value with `std::move(*rhs.val)`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` | no effect

   > > -12- Remarks: […]

   > > -13- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s move constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s move constructor. If an exception is thrown during the call to `T`'s move assignment, the states state of `*valval` and `*rhs.valval` are is determined by the exception safety guarantee of `T`'s move assignment.

   >     template<class U = T> constexpr optional<T>& operator=(U&& v);

   > > -14- Constraints: […]

   > > -15- Effects: If `*this` contains a value, assigns `std::forward<U>(v)` to `val` the contained value; otherwise direct-non-list-initializes `val` the contained value with `std::forward<U>(v)`.

   > > -16- Postconditions: `*this` contains a value.

   > > -17- Returns: `*this`.

   > > -18- Remarks: If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `v` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the states state of `val*val` and `v` are is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class U> constexpr optional<T>& operator=(const optional<U>& rhs);

   > > -19- Constraints: […]

   > > -20- Effects: See Table 60.

   > > Table 60 — `optional::operator=(const optional<U>&)` effects [tab:optional.assign.copy.templ]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*rhs.operator*()` to `val` the contained value | direct-non-list-initializes `val` the contained value with `*rhs.operator*()`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` | no effect

   > > -21- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -22- Returns: `*this`.

   > > -23- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the states state of `val*val` and `*rhs.valval` are is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class U> constexpr optional<T>& operator=(optional<U>&& rhs);

   > > -24- Constraints: […]

   > > -25- Effects: See Table 61. The result of the expression `rhs.has_value()` remains unchanged.

   > > Table 61 — `optional::operator=(optional<U>&&)` effects [tab:optional.assign.move.templ]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | assigns `*std::move(rhs).operator*()` to `val` the contained value | direct-non-list-initializes `val` the contained value with `*std::move(rhs).operator*()`
   > > `rhs` does not contain a value | destroys the contained value by calling `val->val.T::~T()` | no effect

   > > -26- Postconditions: `rhs.has_value() == this->has_value()`.

   > > -27- Returns: `*this`.

   > > -28- If any exception is thrown, the result of the expression `this->has_value()` remains unchanged. If an exception is thrown during the call to `T`'s constructor, the state of `*rhs.valval` is determined by the exception safety guarantee of `T`'s constructor. If an exception is thrown during the call to `T`'s assignment, the states state of `val*val` and `*rhs.valval` are is determined by the exception safety guarantee of `T`'s assignment.

   >     template<class... Args> constexpr T& emplace(Args&&... args);

   > > -29- Mandates: […]

   > > -30- Effects: Calls `*this = nullopt`. Then direct-non-list-initializes `val` the contained value with `std::forward<Args>(args)...`.

   > > -31- Postconditions: `*this` contains a value.

   > > -32- Returns: `val` A reference to the new contained value.

   > > […]

   > > -34- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and the previous `val*val` (if any) has been destroyed.

   >     template<class U, class... Args> constexpr T& emplace(initializer_list<U> il, Args&&... args);

   > > -35- Constraints: […]

   > > -36- Effects: Calls `*this = nullopt`. Then direct-non-list-initializes `val` the contained value with `il, std::forward<Args>(args)...`.

   > > -37- Postconditions: `*this` contains a value.

   > > -38- Returns: `val` A reference to the new contained value.

   > > […]

   > > -40- Remarks: If an exception is thrown during the call to `T`'s constructor, `*this` does not contain a value, and the previous `val*val` (if any) has been destroyed.

6. Modify 22.5.3.5 [optional.swap] as indicated:

   >     constexpr void swap(optional& rhs) noexcept(see below);

   > > -1- Mandates: […]

   > > -2- Preconditions: […]

   > > -3- Effects: See Table 62.

   > > Table 62 — `optional::swap(optional&)` effects [tab:optional.swap]
   > > | `*this` contains a value | `*this` does not contain a value
   > > --- | --- | ---
   > > `rhs` contains a value | calls `swap(val*(*this), *rhs.val)` | direct-non-list-initializes `val` the contained value of `*this` with `std::move(*rhs.val)`, followed by `rhs.val.val->T::~T()`; postcondition is that `*this` contains a value and `rhs` does not contain a value
   > > `rhs` does not contain a value | direct-non-list-initializes the contained value of `rhs.val` with `std::move(val*(*this))`, followed by `val.val->T::~T()`; postcondition is that `*this` does not contain a value and `rhs` contains a value | no effect

   > > -4- Throws: […]

   > > -5- Remarks: […]

   > > -6- If any exception is thrown, the results of the expressions `this->has_value()` and `rhs.has_value()` remain unchanged. If an exception is thrown during the call to function `swap`, the state of `val*val` and `*rhs.valval` is determined by the exception safety guarantee of `swap` for lvalues of `T`. If an exception is thrown during the call to `T`'s move constructor, the states state of `val*val` and `*rhs.valval` are is determined by the exception safety guarantee of `T`'s move constructor.

7. Modify 22.5.3.7 [optional.observe] as indicated:

   >     constexpr const T* operator->() const noexcept;
   >     constexpr T* operator->() noexcept;

   > > -1- Preconditions: `*this` contains a value.

   > > -2- Returns: `addressof(val)val`.

   > > -3- […]

   >     constexpr const T& operator*() const & noexcept;
   >     constexpr T& operator*() & noexcept;

   > > -4- Preconditions: `*this` contains a value.

   > > -5- Returns: `val*val`.

   > > -6- […]

   >     constexpr T&& operator*() && noexcept;
   >     constexpr const T&& operator*() const && noexcept;

   > > -7- Preconditions: `*this` contains a value.

   > > -8- Effects: Equivalent to: `return std::move(val*val);`

   >     constexpr explicit operator bool() const noexcept;

   > > -9- Returns: `true` if and only if `*this` contains a value.

   > > -10- Remarks: This function is a constexpr function.

   >     constexpr bool has_value() const noexcept;

   > > -11- Returns: `true` if and only if `*this` contains a value.

   > > -12- Remarks: This function is a constexpr function.

   >     constexpr const T& value() const &;
   >     constexpr T& value() &;

   > > -13- Effects: Equivalent to:

   > > >     return has_value() ? val*val : throw bad_optional_access();

   >     constexpr T&& value() &&;
   >     constexpr const T&& value() const &&;

   > > -14- Effects: Equivalent to:

   > > >     return has_value() ? std::move(val*val) : throw bad_optional_access();

   >     template<class U> constexpr T value_or(U&& v) const &;

   > > -15- Mandates: […]

   > > -16- Effects: Equivalent to:

   > > >     return has_value() ? val**this : static_cast<T>(std::forward<U>(v));

   >     template<class U> constexpr T value_or(U&& v) &&;

   > > -17- Mandates: […]

   > > -18- Effects: Equivalent to:

   > > >     return has_value() ? std::move(val**this) : static_cast<T>(std::forward<U>(v));

8. Modify 22.5.3.8 [optional.monadic] as indicated:

   >     template<class F> constexpr auto and_then(F&& f) &;
   >     template<class F> constexpr auto and_then(F&& f) const &;

   > > -1- Let `U` be `invoke_result_t<F, decltype((val)*val)>`.

   > > -2- Mandates: […]

   > > -3- Effects: Equivalent to:

   > > >     if (*this) {
   > > >       return invoke(std::forward<F>(f), val*val);
   > > >     } else {
   > > >       return remove_cvref_t<U>();
   > > >     }

   >     template<class F> constexpr auto and_then(F&& f) &&;
   >     template<class F> constexpr auto and_then(F&& f) const &&;

   > > -4- Let `U` be `invoke_result_t<F, decltype(std::move(val*val))>`.

   > > -5- Mandates: […]

   > > -6- Effects: Equivalent to:

   > > >     if (*this) {
   > > >       return invoke(std::forward<F>(f), std::move(val*val));
   > > >     } else {
   > > >       return remove_cvref_t<U>();
   > > >     }

   >     template<class F> constexpr auto transform(F&& f) &;
   >     template<class F> constexpr auto transform(F&& f) const &;

   > > -7- Let `U` be `remove_cv_t<invoke_result_t<F, decltype((val)*val)>>`.

   > > -8- Mandates: `U` is a non-array object type other than `in_place_t` or `nullopt_t`. The declaration

   > > >     U u(invoke(std::forward<F>(f), val*val));

   > > is well-formed for some invented variable `u`.

   > > […]

   > > -9- Returns: If `*this` contains a value, an `optional<U>` object whose contained value is direct-non-list-initialized with `invoke(std::forward<F>(f), val*val)`; otherwise, `optional<U>()`.

   >     template<class F> constexpr auto transform(F&& f) &&;
   >     template<class F> constexpr auto transform(F&& f) const &&;

   > > -10- Let `U` be `remove_cv_t<invoke_result_t<F, decltype(std::move(val*val))>>`.

   > > -11- Mandates: `U` is a non-array object type other than `in_place_t` or `nullopt_t`. The declaration

   > > >     U u(invoke(std::forward<F>(f), std::move(val*val)));

   > > is well-formed for some invented variable `u`.

   > > […]

   > > -12- Returns: If `*this` contains a value, an `optional<U>` object whose contained value is direct-non-list-initialized with `invoke(std::forward<F>(f), std::move(val*val))`; otherwise, `optional<U>()`.

9. Modify 22.5.3.9 [optional.mod] as indicated:

   >     constexpr void reset() noexcept;

   > > -1- Effects: If `*this` contains a value, calls `val->val.T::~T()` to destroy the contained value; otherwise no effect.

   > > -2- Postconditions: `*this` does not contain a value.

10. Modify 22.5.4.2 [optional.ref.ctor] as indicated:

        template<class U>
          constexpr explicit(!is_convertible_v<U&, T&>)
          optional(optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, U&>);

    > -8- Constraints: […]

    > -9- Effects: Equivalent to:

    >       if (rhs.has_value()) convert-ref-init-val(*rhs.operator*());

    > -10- Remarks: […]

        template<class U>
          constexpr explicit(!is_convertible_v<const U&, T&>)
          optional(const optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, const U&>);

    > -11- Constraints: […]

    > -12- Effects: Equivalent to:

    >       if (rhs.has_value()) convert-ref-init-val(*rhs.operator*());

    > -13- Remarks: […]

        template<class U>
          constexpr explicit(!is_convertible_v<U, T&>)
          optional(optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, U>);

    > -14- Constraints: […]

    > -15- Effects: Equivalent to:

    >       if (rhs.has_value()) convert-ref-init-val(*std::move(rhs).operator*());

    > -16- Remarks: […]

        template<class U>
          constexpr explicit(!is_convertible_v<const U, T&>)
          optional(const optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, const U>);

    > -17- Constraints: […]

    > -18- Effects: Equivalent to:

    >       if (rhs.has_value()) convert-ref-init-val(*std::move(rhs).operator*());

    > -19- Remarks: […]

### 4016(i). `container-insertable` checks do not match what `container-inserter` does

Section: 25.5.7 [range.utility.conv] Status: C++26 Submitter: Jonathan Wakely Opened: 2023-11-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The exposition-only helper `container-inserter` uses either `std::back_inserter` or `std::inserter`. Both `std::back_insert_iterator` and `std::insert_iterator` require `C::value_type` to be a valid type, and we do not check for that in `container-insertable`. The insert iterators can also incur a conversion to construct a `C::value_type` which then gets moved into the container. Using emplace instead of insert would avoid that temporary object. It's also possible (although arguably not worth caring about) that `range_value_t<C>` is not the same type as `C::value_type`, and that conversion to `C::value_type` could be ill-formed (we only check that conversion from `range_reference_t<R>` to `range_value_t<C>` is well-formed).

It seems preferable to remove the use of insert iterators, so that we don't need to check their requirements at all.

[2023-1-26; Rename exposition-only concept and function after reflector discussion.]

[2024-03-11; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 25.5.7.1 [range.utility.conv.general] as indicated:

   > -4- Let `container-insertableappendable` be defined as follows:

   > >     template<class Container, class Ref>
   > >     constexpr bool container-insertableappendable =         // exposition only
   > >       requires(Container& c, Ref&& ref) {
   > >                requires (requires { c.emplace_back(std::forward<Ref>(ref)); } ||
   > >                          requires { c.push_back(std::forward<Ref>(ref)); } ||
   > >                          requires { c.emplace(c.end(), std::forward<Ref>(ref)); } ||
   > >                          requires { c.insert(c.end(), std::forward<Ref>(ref)); });
   > >     };

   > -5- Let `container-inserterappend` be defined as follows:

   > >     template<class Container, class Ref>
   > >     constexpr auto container-inserterappend(Container& c) {     // exposition only
   > >       if constexpr (requires { c.push_back(declval<Ref>()); })
   > >         return back_inserter(c);
   > >       else
   > >         return inserter(c, c.end());
   > >       return [&c]<class Ref>(Ref&& ref) {
   > >         if constexpr (requires { c.emplace_back(declval<Ref>()); })
   > >           c.emplace_back(std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.push_back(declval<Ref>()); })
   > >           c.push_back(std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.emplace(c.end(), declval<Ref>()); })
   > >           c.emplace(c.end(), std::forward<Ref>(ref));
   > >         else
   > >           c.insert(c.end(), std::forward<Ref>(ref));
   > >       };
   > >     };

2. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   > (2.1.4) Otherwise, if

   > - — `constructible_from<C, Args...>` is `true`, and
   > - — `container-insertableappendable<C, range_reference_t<R>>` is `true`:

   > >     C c(std::forward<Args>(args)...);
   > >     if constexpr (sized_range<R> && reservable-container<C>)
   > >     c.reserve(static_cast<range_size_t<C>>(ranges::size(r)));
   > >     ranges::copyfor_each(r, container-inserterappend<range_reference_t<R>>(c));

### 4020(i). `extents::index-cast` weirdness

Section: 23.7.3.3.2 [mdspan.extents.expo] Status: C++26 Submitter: Casey Carter Opened: 2023-11-29 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The exposition-only static member `index-cast` of `extents` is specified as (23.7.3.3.2 [mdspan.extents.expo]/9):

>     template<class OtherIndexType>
>     static constexpr auto index-cast(OtherIndexType&& i) noexcept;

> > -9- Effects:

> > 1. (9.1) — If `OtherIndexType` is an integral type other than `bool`, then equivalent to `return i;`,

> > 2. (9.2) — otherwise, equivalent to `return static_cast<index_type>(i);`.

> > [Note 1: This function will always return an integral type other than `bool`. Since this function's call sites are constrained on convertibility of `OtherIndexType` to `index_type`, integer-class types can use the `static_cast` branch without loss of precision. — end note]

This function returns `T` when passed an rvalue of cv-unqualified integral type `T`, but `index_type` when passed a cv-qualified and/or lvalue argument of any integral type. It would seem more consistent and easier to reason about if 9.1 was instead conditional on `remove_cvref_t<OtherIndexType>`.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

"Doesn't matter in this case, but logically `decay_t` seems like a better fit."

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 23.7.3.3.2 [mdspan.extents.expo] as indicated:

   >     template<class OtherIndexType>
   >       static constexpr auto index-cast(OtherIndexType&& i) noexcept;

   > > -9- Effects:

   > > 1. (9.1) — If `remove_cvref_t<OtherIndexType>` is an integral type other than `bool`, then equivalent to `return i;`,

   > > 2. (9.2) — otherwise, equivalent to `return static_cast<index_type>(i);`.

   > > [Note 1: This function will always return an integral type other than `bool`. Since this function's call sites are constrained on convertibility of `OtherIndexType` to `index_type`, integer-class types can use the `static_cast` branch without loss of precision. — end note]

### 4023(i). Preconditions of `std::basic_streambuf::setg/setp`

Section: 31.6.3.4 [streambuf.protected] Status: C++26 Submitter: Jiang An Opened: 2023-12-08 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

It seems that operations of `std::basic_streambuf` expect that

- `[eback(), egptr())` is a valid range and `gptr()` points into that range, and

- `[pbase(), pptr())` is a valid range and `epptr()` points into that range.

However, it is currently not specified for `setg/setp` that such invariants need to be established.

[2024-03-11; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 31.6.3.4.2 [streambuf.get.area] as indicated:

   >     void setg(char_type* gbeg, char_type* gnext, char_type* gend);

   > > -?- Preconditions: `[gbeg, gnext)`, `[gbeg, gend)`, and `[gnext, gend)` are all valid ranges.

   > > -5- Postconditions: `gbeg == eback()`, `gnext == gptr()`, and `gend == egptr()` are all `true`.

2. Modify 31.6.3.4.3 [streambuf.put.area] as indicated:

   >     void setp(char_type* pbeg, char_type* pend);

   > > -?- Preconditions: `[pbeg, pend)` is a valid range.

   > > -5- Postconditions: `pbeg == pbase()`, `pbeg == pptr()`, and `pend == epptr()` are all `true`.

### 4024(i). Underspecified destruction of objects created in `std::make_shared_for_overwrite/std::allocate_shared_for_overwrite`

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++26 Submitter: Jiang An Opened: 2023-12-16 Last modified: 2026-06-09

Priority: 2

View all other issues in [util.smartptr.shared.create].

View all issues with C++26 status.

Discussion:

Currently, only destructions of non-array (sub)objects created in `std::make_shared` and `std::allocate_shared` are specified in 20.3.2.2.7 [util.smartptr.shared.create]. Presumably, objects created in `std::make_shared_for_overwrite` and `std::allocate_shared_for_overwrite` should be destroyed by plain destructor calls.

[2024-03-11; Reflector poll]

Set priority to 2 after reflector poll in December 2023.

This was the P1020R1 author's intent (see LWG reflector mail in November 2018) but it was never clarified in the wording. This fixes that.

[2024-08-21; Move to Ready at LWG telecon]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4964.

1. Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:

   >     template<class T, ...>
   >       shared_ptr<T> make_shared(args);
   >     template<class T, class A, ...>
   >       shared_ptr<T> allocate_shared(const A& a, args);
   >     template<class T, ...>
   >       shared_ptr<T> make_shared_for_overwrite(args);
   >     template<class T, class A, ...>
   >       shared_ptr<T> allocate_shared_for_overwrite(const A& a, args);

   > > […]

   > > -7- Remarks:

   > > 1. […]

   > > 2. (7.11) — When a (sub)object of non-array type `U` that was initialized by `make_shared`, `make_shared_for_overwrite`, or `allocate_shared_for_overwrite` is to be destroyed, it is destroyed via the expression `pv->~U()` where `pv` points to that object of type `U`.

   > > 3. […]

### 4025(i). Move assignment operator of `std::expected<cv void, E>` should not be conditionally deleted

Section: 22.8.7.4 [expected.void.assign] Status: C++26 Submitter: Jiang An Opened: 2023-12-16 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [expected.void.assign].

View all issues with C++26 status.

Discussion:

It seems intended that copy functions of `std::optional`, `std::variant`, and `std::expected` are conditionally deleted, while move functions are constrained. However, the move assignment operator of `std::expected<cv void, E>` is currently conditionally deleted, which is inconsistent.

[2024-03-11; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 22.8.7.4 [expected.void.assign] as indicated:

   >     constexpr expected& operator=(expected&& rhs) noexcept(see below);

   > > -?- Constraints: `is_move_constructible_v<E>` is `true` and `is_move_assignable_v<E>` is `true`.

   > > […]

   > > -6- Remarks: The exception specification is equivalent to `is_nothrow_move_constructible_v<E> && is_nothrow_move_assignable_v<E>`.

   > > -7- This operator is defined as deleted unless `is_move_constructible_v<E>` is `true` and `is_move_assignable_v<E>` is `true`.

### 4026(i). Assignment operators of `std::expected` should propagate triviality

Section: 22.8.6.4 [expected.object.assign], 22.8.7.4 [expected.void.assign] Status: C++26 Submitter: Jiang An Opened: 2023-12-16 Last modified: 2026-06-09

Priority: 2

View all other issues in [expected.object.assign].

View all issues with C++26 status.

Duplicate of: 4195

Discussion:

Addresses US 135-216 and US 136-217

Currently, only copy and move constructors of `std::expected` are required to propagate triviality, while copy and move assignment operators are not. Given that the assignment operators of `std::optional` and `std::variant` are already required to propagate triviality, it seems to me that we should also apply such requirements for `std::expected`.

Such changes are being made in libc++ (llvm/llvm-project#74768). And it may be desired to make the triviality improvement portable.

[2024-03-11; Reflector poll]

Set priority to 2 after reflector poll in January 2024. A few votes for Tentatively Ready, others thought it needed more consideration.

[2026-01-29; added links to C++26 NB comments.]

[Croydon 2026-03-25; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 22.8.6.4 [expected.object.assign] as indicated:

   >     constexpr expected& operator=(const expected& rhs);

   > > -2- Effects: […]

   > > […]

   > > -4- Remarks: This operator is defined as deleted unless:

   > > 1. […]

   > > -?- This operator is trivial if:

   > > 1. (?.1) — `is_trivially_copy_constructible_v<T>` is `true`, and

   > > 2. (?.2) — `is_trivially_copy_assignable_v<T>` is `true`, and

   > > 3. (?.3) — `is_trivially_destructible_v<T>` is `true`, and

   > > 4. (?.4) — `is_trivially_copy_constructible_v<E>` is `true`, and

   > > 5. (?.5) — `is_trivially_copy_assignable_v<E>` is `true`, and

   > > 6. (?.6) — `is_trivially_destructible_v<E>` is `true`.

   >     constexpr expected& operator=(expected&& rhs) noexcept(see below);

   > > -5- Constraints: […]

   > > […]

   > > -8- Remarks: The exception specification is equivalent to:

   > > […]

   > > -?- This operator is trivial if:

   > > 1. (?.1) — `is_trivially_move_constructible_v<T>` is `true`, and

   > > 2. (?.2) — `is_trivially_move_assignable_v<T>` is `true`, and

   > > 3. (?.3) — `is_trivially_destructible_v<T>` is `true`, and

   > > 4. (?.4) — `is_trivially_move_constructible_v<E>` is `true`, and

   > > 5. (?.5) — `is_trivially_move_assignable_v<E>` is `true`, and

   > > 6. (?.6) — `is_trivially_destructible_v<E>` is `true`.

2. Modify 22.8.7.4 [expected.void.assign] as indicated:

   >     constexpr expected& operator=(const expected& rhs);

   > > -1- Effects: […]

   > > […]

   > > -3- Remarks: This operator is defined as deleted unless `is_copy_assignable_v<E>` is `true` and `is_copy_constructible_v<E>` is `true`.

   > > -?- This operator is trivial if `is_trivially_copy_constructible_v<E>`, `is_trivially_copy_assignable_v<E>`, and `is_trivially_destructible_v<E>` are all `true`.

   >     constexpr expected& operator=(expected&& rhs) noexcept(see below);

   > > -4- Effects: […]

   > > […]

   > > -6- Remarks: The exception specification is equivalent to `is_nothrow_move_constructible_v<E> && is_nothrow_move_assignable_v<E>`.

   > > -7- This operator is defined as deleted unless `is_move_constructible_v<E>` is `true` and `is_move_assignable_v<E>` is `true`.

   > > -?- This operator is trivial if `is_trivially_move_constructible_v<E>`, `is_trivially_move_assignable_v<E>`, and `is_trivially_destructible_v<E>` are all `true`.

### 4027(i). `possibly-const-range` should prefer returning `const R&`

Section: 25.2 [ranges.syn] Status: C++26 Submitter: Hewill Kang Opened: 2023-12-17 Last modified: 2026-06-09

Priority: 2

View other active issues in [ranges.syn].

View all other issues in [ranges.syn].

View all issues with C++26 status.

Discussion:

`possibly-const-range` currently only returns `const R&` when `R` does not satisfy `constant_range` and `const R` satisfies `constant_range`.

Although it's not clear why we need the former condition, this does diverge from the legacy `std::cbegin` (demo):

>     #include <ranges>

>     int main() {
>       auto r = std::views::single(0)
>             | std::views::transform([](int) { return 0; });
>       using C1 = decltype(std::ranges::cbegin(r));
>       using C2 = decltype(std::cbegin(r));
>       static_assert(std::same_as<C1, C2>); // failed
>     }

Since `R` itself is `constant_range`, so `possibly-const-range`, above just returns `R&` and `C1` is `transform_view::iterator<false>`; `std::cbegin` specifies to return `as_const(r).begin()`, which makes that `C2` is `transform_view::iterator<true>` which is different from `C1`.

I believe `const R&` should always be returned if it's a range, regardless of whether `const R` or `R` is a `constant_range`, just as `fmt-maybe-const` in format ranges always prefers `const R` over `R`.

Although it is theoretically possible for `R` to satisfy `constant_range` and that `const R` is a mutable range, such nonsense range type should not be of interest.

This relaxation of constraints allows for maximum consistency with `std::cbegin`, and in some cases can preserve constness to the greatest extent (demo):

>     #include <ranges>

>     int main() {
>       auto r = std::views::single(0) | std::views::lazy_split(0);
>       (*std::ranges::cbegin(r)).front() = 42; // ok
>       (*std::cbegin(r)).front() = 42; // not ok
>     }

Above, `*std::ranges::cbegin` returns a range of type `const lazy_split_view::outer-iterator<false>::value_type`, which does not satisfy `constant_range` because its reference type is `int&`.

However, `*std::cbegin(r)` returns `lazy_split_view::outer-iterator<true>::value_type` whose reference type is `const int&` and satisfies `constant_range`.

[2024-03-11; Reflector poll]

Set priority to 2 after reflector poll. Send to SG9.

[St. Louis 2024-06-28; LWG and SG9 joint session: move to Ready]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   >     #include <iterator>             // see 24.2 [iterator.synopsis]

   >     namespace std::ranges {
   >       […]

   >       // 25.7.22 [range.as.const], as const view
   >       template<input_range R>
   >         constexpr auto& possibly-const-range(R& r) noexcept { // exposition only
   >           if constexpr (inputconstant_range<const R> && !constant_range<R>) {
   >             return const_cast<const R&>(r);
   >           } else {
   >             return r;
   >           }
   >         }

   >       […]
   >     }

### 4030(i). Clarify whether arithmetic expressions in [numeric.sat.func] are mathematical or C++

Section: 26.10.17.1 [numeric.sat.func] Status: C++26 Submitter: Thomas Köppe Opened: 2023-12-18 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

During the application of P0543R0, "Saturation arithmetic", it was pointed out that it might not be entirely clear what we want something like "`x` + `y`" to mean. The paper does not suggest any formatting for those symbols, and a non-normative note explains that the intention is for the expression to be considered mathematically.

I wonder if this is clear enough. Notes are not normative, and the document "shall be usable without notes". I have formatted this as `$\tcode{x} + \tcode{y}$` throughout, i.e. the variables are in code font, but the symbol is maths, not code. This is quite subtle. (See also GitHub discussion.)

I think it would be an improvement if we simply made the note not be a note. It seems to contain entirely reasonable, mandatory content.

[2024-03-11; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 26.10.17.1 [numeric.sat.func] as indicated:

   > -1- [Note 1: In the following descriptions, an arithmetic operation is performed as a mathematical operation with infinite range and then it is determined whether the mathematical result fits into the result type. — end note]

### 4031(i). `bad_expected_access<void>` member functions should be `noexcept`

Section: 22.8.5 [expected.bad.void] Status: C++26 Submitter: Cassio Neri Opened: 2023-12-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

According to 17.9.3 [exception]/2:

> Each standard library class `T` that derives from class `exception` has the following publicly accessible member functions, each of them having a non-throwing exception specification (14.5):

> 1. (2.1) — default constructor (unless the class synopsis shows other constructors)

> 2. (2.2) — copy constructor

> 3. (2.3) — copy assignment operator

For good reasons, `bad_expected_access<void>` overrules from this general rule by protecting its special member functions. However, there's no reason these functions should not be `noexcept`.

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 22.8.5 [expected.bad.void] as indicated:

   >     namespace std {
   >       template<>
   >       class bad_expected_access<void> : public exception {
   >       protected:
   >         bad_expected_access() noexcept;
   >         bad_expected_access(const bad_expected_access&) noexcept;
   >         bad_expected_access(bad_expected_access&&) noexcept;
   >         bad_expected_access& operator=(const bad_expected_access&) noexcept;
   >         bad_expected_access& operator=(bad_expected_access&&) noexcept;
   >         ~bad_expected_access();
   >       public:
   >         const char* what() const noexcept override;
   >       };
   >     }

### 4035(i). `single_view` should provide `empty`

Section: 25.6.3.2 [range.single.view] Status: C++26 Submitter: Hewill Kang Opened: 2023-12-31 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.single.view].

View all issues with C++26 status.

Discussion:

Although `single_view::empty` can be synthesized through `view_interface`, it seems more worthwhile to provide a static `empty` for it which eliminates the need to pass in an object parameter, guarantees `noexcept`-ness, and is consistent with the design of `empty_view` (demo):

>     #include <ranges>
>     auto empty = std::views::empty<int>;
>     static_assert(noexcept(empty.empty()));
>     static_assert(noexcept(empty.size()));
>     auto single = std::views::single(0);
>     static_assert(noexcept(single.empty())); // fire
>     static_assert(noexcept(single.size()));

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 25.6.3.2 [range.single.view] as indicated:

   > >     namespace std::ranges {
   > >       template<move_constructible T>
   > >         requires is_object_v<T>
   > >       class single_view : public view_interface<single_view<T>> {
   > >         […]
   > >       public:
   > >         […]
   > >         constexpr T* begin() noexcept;
   > >         constexpr const T* begin() const noexcept;
   > >         constexpr T* end() noexcept;
   > >         constexpr const T* end() const noexcept;
   > >         static constexpr bool empty() noexcept;
   > >         static constexpr size_t size() noexcept;
   > >         constexpr T* data() noexcept;
   > >         constexpr const T* data() const noexcept;
   > >       };
   > >       […]
   > >     }

   > […]

   >     constexpr T* end() noexcept;
   >     constexpr const T* end() const noexcept;

   > > -5- Effects: Equivalent to: `return data() + 1;`

   >     static constexpr bool empty() noexcept;

   > > -?- Effects: Equivalent to: `return false;`

### 4036(i). `__alignof_is_defined` is only implicitly specified in C++ and not yet deprecated

Section: D.11 [depr.c.macros] Status: C++26 Submitter: Jiang An Opened: 2024-01-12 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Currently 17.15.4 [stdalign.h.syn] states

> The contents of the C++ header `<stdalign.h>` are the same as the C standard library header `<stdalign.h>`, with the following changes: The header `<stdalign.h>` does not define a macro named `alignas`.

> See also: ISO/IEC 9899:2018, 7.15

which implicitly specifies that `__alignof_is_defined` is also provided in C++, because C17 specified that the macro is provided in `<stdaligh.h>`.

However, there's no explicit mentioning of `__alignof_is_defined` in the C++ standard wording. And D.11 [depr.c.macros]/1 (added by LWG 3827(i)) seemingly contradicts with 17.15.4 [stdalign.h.syn] and only makes `__alignas_is_defined` deprecated.

It seems that we should explicitly mention `__alignof_is_defined` in D.11 [depr.c.macros] at this moment.

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

[2024-10-22; Note: this should have been handled by LWG 2241(i) but that issue was closed as Resolved without properly resolving it.]

Proposed resolution:

This wording is relative to N4971.

1. Modify D.11 [depr.c.macros] as indicated:

   > -1- The header `<stdalign.h>` has the following macros:

   > >     #define __alignas_is_defined 1
   > >     #define __alignof_is_defined 1

### 4037(i). Static data members of `ctype_base` are not yet required to be usable in constant expressions

Section: 28.3.4.2.1 [category.ctype.general] Status: C++26 Submitter: Jiang An Opened: 2024-01-12 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

It may be desired that static data members `ctype_base` are "real constants", i.e. usable in constant expressions. However, this is not strictly required because `mask` is only required to be a bitmask type that can be a class type, which makes the plain `const` potentially insufficient.

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 28.3.4.2.1 [category.ctype.general] as indicated:

   > >     namespace std {
   > >       class ctype_base {
   > >       public:
   > >         using mask = see below;

   > >         // numeric values are for exposition only.
   > >         static constexpr mask space  = 1 << 0;
   > >         static constexpr mask print  = 1 << 1;
   > >         static constexpr mask cntrl  = 1 << 2;
   > >         static constexpr mask upper  = 1 << 3;
   > >         static constexpr mask lower  = 1 << 4;
   > >         static constexpr mask alpha  = 1 << 5;
   > >         static constexpr mask digit  = 1 << 6;
   > >         static constexpr mask punct  = 1 << 7;
   > >         static constexpr mask xdigit = 1 << 8;
   > >         static constexpr mask blank  = 1 << 9;
   > >         static constexpr mask alnum  = alpha | digit;
   > >         static constexpr mask graph  = alnum | punct;
   > >       };
   > >     }

   > -1- The type `mask` is a bitmask type (16.3.3.3.3 [bitmask.types]).

### 4038(i). `std::text_encoding::aliases_view` should have constexpr iterators

Section: 28.4.2.5 [text.encoding.aliases] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-01-16 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

`aliases_view::begin()` and `aliases_view::end()` are constexpr functions, but there is no requirement that you can use the returned iterator and sentinel in constant expressions.

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 28.4.2.5 [text.encoding.aliases] as indicated:

   > >     struct text_encoding::aliases_view : ranges::view_interface<text_encoding::aliases_view> {
   > >       constexpr implementation-defined begin() const;
   > >       constexpr implementation-defined end() const;
   > >     };

   > -1- `text_encoding::aliases_view` models `copyable`, `ranges::view`, `ranges::random_access_range`, and `ranges::borrowed_range`.

   > -2- Both `ranges::range_value_t<text_encoding::aliases_view>` and `ranges::range_reference_t<text_encoding::aliases_view>` denote `const char*`.

   > -?- `ranges::iterator_t<text_encoding::aliases_view>` is a constexpr iterator (24.3.1 [iterator.requirements.general]).

### 4042(i). `std::print` should permit an efficient implementation

Section: 31.7.10 [print.fun] Status: Resolved Submitter: Victor Zverovich Opened: 2024-01-20 Last modified: 2025-03-10

Priority: 3

View all other issues in [print.fun].

View all issues with Resolved status.

Discussion:

`std::print`/`std::vprint*` is currently defined in terms of formatting into a temporary string, e.g. 31.7.10 [print.fun]:

>     void vprint_nonunicode(FILE* stream, string_view fmt, format_args args);

> > Preconditions: `stream` is a valid pointer to an output C stream.

> > Effects: Writes the result of `vformat(fmt, args)` to `stream`.

> > Throws: Any exception thrown by the call to `vformat` (28.5.3 [format.err.report]). `system_error` if writing to `stream` fails. May throw `bad_alloc`.

This is done to make it clear that noninterleaved output is desired while keeping specification simple and portable.

Unfortunately, the current specification seems to prohibit a more efficient implementation that performs formatting directly into a stream buffer under a lock (`flockfile`/`funlockfile` in POSIX) like `printf` does. The difference can be observable in case of an I/O error that occurs before a custom formatter is called. In the (double buffered) implementation that directly follows the spec all formatters will be called, while in a more efficient (locking) implementation subsequent formatters may not be called.

The easiest fix, given in the current proposed resolution, is to say that some arguments may not be formatted in case of a write error. It might be a bit weird considering that the spec says that we construct a string first so an alternative resolution is to replace `vformat` with `vformat_to` info some unspecified buffer iterator and state noninterleaving requirement separately.

[2024-02-19; Feb 2024 mailing]

This would be resolved by P3107.

[2024-03-12; Reflector poll]

Set priority to 3 and status to LEWG after reflector poll in January 2024.

"This loses the guarantee that if the formatting throws then there's no output."

[2025-03-10 Status changed: LEWG → Resolved.]

Resolved by P3107R5, approved in Tokyo, March 2024.

Proposed resolution:

This wording is relative to N4971.

1. Modify 31.7.10 [print.fun] as indicated:

   >     void vprint_unicode(FILE* stream, string_view fmt, format_args args);

   > > -6- Preconditions: `stream` is a valid pointer to an output C stream.

   > > -7- Effects: The function initializes an automatic variable via

   > > >     string out = vformat(fmt, args);

   > > If `stream` refers to a terminal capable of displaying Unicode, writes out to the terminal using the native Unicode API; if out contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. Otherwise writes `out` to `stream` unchanged. If the native Unicode API is used, the function flushes `stream` before writing `out`.

   > > If writing to the terminal or `stream` fails, some arguments in `args` may not be formatted.

   > > […]

   > […]

   >     void vprint_nonunicode(FILE* stream, string_view fmt, format_args args);

   > > -11- Preconditions: `stream` is a valid pointer to an output C stream.

   > > -12- Effects: Writes the result of `vformat(fmt, args)` to `stream`. If writing to `stream` fails, some arguments in `args` may not be formatted.

   > > -13- Throws: […].

### 4043(i). `"ASCII"` is not a registered character encoding

Section: 28.4.2.2 [text.encoding.general] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-01-23 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The IANA Charater Sets registry does not contain "ASCII" as an alias of the "US-ASCII" encoding. This is apparently for historical reasons, because there used to be some ambiguity about exactly what "ASCII" meant. I don't think those historical reasons are relevant to C++26, but the absence of "ASCII" in the IANA registry means that it's not a registered character encoding as defined by 28.4.2.2 [text.encoding.general].

This means that the encoding referred to by notes in the C++ standard (31.12.6.2 [fs.path.generic], 28.3.4.4.1.3 [facet.numpunct.virtuals]) and by an example in the `std::text_encoding` proposal (P1885) isn't actually usable in portable code. So `std::text_encoding("ASCII")` creates an object with `mib() == std::text_encoding::other`, which is not the same encoding as `std::text_encoding("US-ASCII")`. This seems surprising.

[2024-03-12; Reflector poll]

SG16 approved the proposed resolution. Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 28.4.2.2 [text.encoding.general] as indicated:

   > -1- A registered character encoding is a character encoding scheme in the IANA Character Sets registry.

   > [Note 1: The IANA Character Sets registry uses the term “character sets” to refer to character encodings. — end note]

   > The primary name of a registered character encoding is the name of that encoding specified in the IANA Character Sets registry.

   > -2- The set of known registered character encodings contains every registered character encoding specified in the IANA Character Sets registry except for the following:

   > 1. (2.1) – NATS-DANO (33)
   > 2. (2.2) – NATS-DANO-ADD (34)

   > -3- Each known registered character encoding is identified by an enumerator in `text_encoding::id`, and has a set of zero or more aliases.

   > -4- The set of aliases of a known registered character encoding is an implementation-defined superset of the aliases specified in the IANA Character Sets registry. The set of aliases for US-ASCII includes `"ASCII"`. No two aliases or primary names of distinct registered character encodings are equivalent when compared by `text_encoding::comp-name`.

### 4044(i). Confusing requirements for `std::print` on POSIX platforms

Section: 31.7.10 [print.fun] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-01-24 Last modified: 2026-06-09

Priority: 3

View all other issues in [print.fun].

View all issues with C++26 status.

Discussion:

The effects for `vprintf_unicode` say:

> If `stream` refers to a terminal capable of displaying Unicode, writes `out` to the terminal using the native Unicode API; if `out` contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. Otherwise writes `out` to `stream` unchanged. If the native Unicode API is used, the function flushes `stream` before writing `out`.

> [Note 1: On POSIX and Windows, `stream` referring to a terminal means that, respectively, `isatty(fileno(stream))` and `GetConsoleMode(_get_osfhandle(_fileno(stream)), ...)` return nonzero. — end note]

> [Note 2: On Windows, the native Unicode API is `WriteConsoleW`. — end note]

> -8- Throws: [...]

> -9- Recommended practice: If invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.

The very explicit mention of `isatty` for POSIX platforms has confused at least two implementers into thinking that we're supposed to use `isatty`, and supposed to do something differently based on what it returns. That seems consistent with the nearly identical wording in 28.5.2.2 [format.string.std] paragraph 12, which says "Implementations should use either UTF-8, UTF-16, or UTF-32, on platforms capable of displaying Unicode text in a terminal" and then has a note explicitly saying this is the case for Windows-based and many POSIX-based operating systems. So it seems clear that POSIX platforms are supposed to be considered to have "a terminal capable of displaying Unicode text", and so `std::print` should use `isatty` and then use a native Unicode API, and diagnose invalid code units.

This is a problem however, because `isatty` needs to make a system call on Linux, adding 500ns to every `std::print` call. This results in a 10x slowdown on Linux, where `std::print` can take just 60ns without the `isatty` check.

From discussions with Tom Honermann I learned that the "native Unicode API" wording is only relevant on Windows. This makes sense, because for POSIX platforms, writing to a terminal is done using the usual stdio functions, so there's no need to treat a terminal differently to any other file stream. And substitution of invalid code units with u+fffd is recommended for Windows because that's what typical modern terminals do on POSIX platforms, so requiring the implementation to do that on Windows gives consistent behaviour. But the implementation doesn't need to do anything to make that happen with a POSIX terminal, it happens anyway. So the `isatty` check is unnecessary for POSIX platforms, and the note mentioning it just causes confusion and has no benefit.

Secondly, there initially seems to be a contradiction between the "implementations are encouraged to diagnose it" wording and the later Recommended practice. In fact, there's no contradiction because the native Unicode API might accept UTF-8 and therefore require no transcoding, and so the Recommended practice wouldn't apply. The intention is that diagnosing invalid UTF-8 is still desirable in this case, but how should it be diagnosed? By writing an error to the terminal alongside the formatted string? Or by substituting u+fffd maybe? If the latter is the intention, why is one suggestion in the middle of the Effects, and one given as Recommended practice?

The proposed resolution attempts to clarify that a "native Unicode API" is only needed if that's how you display Unicode on the terminal. It also moves the flushing requirement to be adjacent to the other requirements for systems using a native Unicode API instead of on its own later in the paragraph. And the suggestion to diagnose invalid code units is moved into the Recommended practice and clarified that it's only relevant if using a native Unicode API. I'm still not entirely happy with encouragement to diagnose invalid code units without giving any clue as to how that should be done. What does it mean to diagnose something at runtime? That's novel for the C++ standard. The way it's currently phrased seems to imply something other than u+fffd substitution should be done, although that seems the most obvious implementation to me.

[2024-03-12; Reflector poll]

Set priority to 3 after reflector poll and send to SG16.

Previous resolution [SUPERSEDED]

This wording is relative to N4971.

1. Modify 31.7.6.3.5 [ostream.formatted.print] as indicated:

   >     void vprint_unicode(ostream& os, string_view fmt, format_args args);
   >     void vprint_nonunicode(ostream& os, string_view fmt, format_args args);

   > -3- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of `os`, except that:

   > 1. (3.1) – failure to generate output is reported as specified below, and
   > 2. (3.2) – any exception thrown by the call to `vformat` is propagated without regard to the value of `os.exceptions()` and without turning on `ios_base::badbit` in the error state of `os`.

   > After constructing a `sentry` object, the function initializes an automatic variable via

   >       string out = vformat(os.getloc(), fmt, args);

   > If the function is `vprint_unicode` and `os` is a stream that refers to a terminal capable of displaying Unicode via a native Unicode API, which is determined in an implementation-defined manner, flushes `os` and then writes `out` to the terminal using the native Unicode API; if `out` contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. If the native Unicode API is used, the function flushes `os` before writing `out`. Otherwise, (if `os` is not such a stream or the function is `vprint_nonunicode`), inserts the character sequence [`out.begin()`,`out.end()`) into `os`. If writing to the terminal or inserting into `os` fails, calls `os.setstate(ios_base::badbit)` (which may throw `ios_base::failure`).

   > -4- Recommended practice: For `vprint_unicode`, if invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion. If invoking the native Unicode API does not require transcoding, implementations are encouraged to diagnose invalid code units.

2. Modify 31.7.10 [print.fun] as indicated:

   >     void vprint_unicode(FILE* stream, string_view fmt, format_args args);

   > -6- Preconditions: `stream` is a valid pointer to an output C stream.

   > -7- Effects: The function initializes an automatic variable via

   >       string out = vformat(fmt, args);

   > If `stream` refers to a terminal capable of displaying Unicode via a native Unicode API, flushes `stream` and then writes `out` to the terminal using the native Unicode API; if `out` contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. Otherwise writes `out` to `stream` unchanged. If the native Unicode API is used, the function flushes `stream` before writing `out`.

   > [Note 1: On POSIX and Windows, the native Unicode API is `WriteConsoleW` and `stream` referring to a terminal means that, respectively, `isatty(fileno(stream))` and `GetConsoleMode(_get_osfhandle(_fileno(stream)), ...)` return nonzero. — end note]

   > [Note 2: On Windows, the native Unicode API is `WriteConsoleW`. — end note]

   > -8- Throws: [...]

   > -9- Recommended practice: If invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion. If invoking the native Unicode API does not require transcoding, implementations are encouraged to diagnose invalid code units.

[2024-03-12; Jonathan updates wording based on SG16 feedback]

SG16 reviewed the issue and approved the proposed resolution with the wording about diagnosing invalid code units removed.

SG16 favors removing the following text (both occurrences) from the proposed wording. This is motivated by a lack of understanding regarding what it means to diagnose such invalid code unit sequences given that the input is likely provided at run-time.

> If invoking the native Unicode API does not require transcoding, implementations are encouraged to diagnose invalid code units.

Some concern was expressed regarding how the current wording is structured. At present, the wording leads with a Windows centric perspective; if the stream refers to a terminal ... use the native Unicode API ... otherwise write code units to the stream. It might be an improvement to structure the wording such that use of the native Unicode API is presented as a fallback for implementations that require its use when writing directly to the stream is not sufficient to produce desired results. In other words, the wording should permit direct writing to the stream even when the stream is directed to a terminal and a native Unicode API is available when the implementation has reason to believe that doing so will produce the correct results. For example, Microsoft's HoloLens has a Windows based operating system, but it only supports use of UTF-8 as the system code page and therefore would not require the native Unicode API bypass; implementations for it could avoid the overhead of checking to see if the stream is directed to a console.

Previous resolution [SUPERSEDED]

This wording is relative to N4971.

1. Modify 31.7.6.3.5 [ostream.formatted.print] as indicated:

   >     void vprint_unicode(ostream& os, string_view fmt, format_args args);
   >     void vprint_nonunicode(ostream& os, string_view fmt, format_args args);

   > -3- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of `os`, except that:

   > 1. (3.1) – failure to generate output is reported as specified below, and
   > 2. (3.2) – any exception thrown by the call to `vformat` is propagated without regard to the value of `os.exceptions()` and without turning on `ios_base::badbit` in the error state of `os`.

   > After constructing a `sentry` object, the function initializes an automatic variable via

   >       string out = vformat(os.getloc(), fmt, args);

   > If the function is `vprint_unicode` and `os` is a stream that refers to a terminal that is only capable of displaying Unicode via a native Unicode API, which is determined in an implementation-defined manner, flushes `os` and then writes `out` to the terminal using the native Unicode API; if `out` contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. If the native Unicode API is used, the function flushes `os` before writing `out`. Otherwise, (if `os` is not such a stream or the function is `vprint_nonunicode`), inserts the character sequence [`out.begin()`,`out.end()`) into `os`. If writing to the terminal or inserting into `os` fails, calls `os.setstate(ios_base::badbit)` (which may throw `ios_base::failure`).

   > -4- Recommended practice: For `vprint_unicode`, if invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.

2. Modify 31.7.10 [print.fun] as indicated:

   >     void vprint_unicode(FILE* stream, string_view fmt, format_args args);

   > -6- Preconditions: `stream` is a valid pointer to an output C stream.

   > -7- Effects: The function initializes an automatic variable via

   >       string out = vformat(fmt, args);

   > If `stream` refers to a terminal that is only capable of displaying Unicode via a native Unicode API, flushes `stream` and then writes `out` to the terminal using the native Unicode API; if `out` contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. Otherwise writes `out` to `stream` unchanged. If the native Unicode API is used, the function flushes `stream` before writing `out`.

   > [Note 1: On POSIX and Windows, the native Unicode API is `WriteConsoleW` and `stream` referring to a terminal means that, respectively, `isatty(fileno(stream))` and `GetConsoleMode(_get_osfhandle(_fileno(stream)), ...)` return nonzero. — end note]

   > [Note 2: On Windows, the native Unicode API is `WriteConsoleW`. — end note]

   > -8- Throws: [...]

   > -9- Recommended practice: If invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.

[2024-03-19; Tokyo: Jonathan updates wording after LWG review]

Split the Effects: into separate bullets for the "native Unicode API" and "otherwise" cases. Remove the now-redundant "if `os` is not such a stream" parenthesis.

[St. Louis 2024-06-24; move to Ready.]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 31.7.6.3.5 [ostream.formatted.print] as indicated:

   >     void vprint_unicode(ostream& os, string_view fmt, format_args args);
   >     void vprint_nonunicode(ostream& os, string_view fmt, format_args args);

   > -3- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of `os`, except that:

   > 1. (3.1) – failure to generate output is reported as specified below, and
   > 2. (3.2) – any exception thrown by the call to `vformat` is propagated without regard to the value of `os.exceptions()` and without turning on `ios_base::badbit` in the error state of `os`.

   > -?- After constructing a `sentry` object, the function initializes an automatic variable via

   >       string out = vformat(os.getloc(), fmt, args);

   > 1. (?.1) – If the function is `vprint_unicode` and `os` is a stream that refers to a terminal that is only capable of displaying Unicode via a native Unicode API, which is determined in an implementation-defined manner, flushes `os` and then writes `out` to the terminal using the native Unicode API; if `out` contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. If the native Unicode API is used, the function flushes `os` before writing `out`.
   > 2. (?.2) – Otherwise, (if `os` is not such a stream or the function is `vprint_nonunicode`), inserts the character sequence [`out.begin()`,`out.end()`) into `os`.

   > -?- If writing to the terminal or inserting into `os` fails, calls `os.setstate(ios_base::badbit)` (which may throw `ios_base::failure`).

   > -4- Recommended practice: For `vprint_unicode`, if invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.

2. Modify 31.7.10 [print.fun] as indicated:

   >     void vprint_unicode(FILE* stream, string_view fmt, format_args args);

   > -6- Preconditions: `stream` is a valid pointer to an output C stream.

   > -7- Effects: The function initializes an automatic variable via

   >       string out = vformat(fmt, args);

   > 1. (7.1) – If `stream` refers to a terminal that is only capable of displaying Unicode via a native Unicode API, flushes `stream` and then writes `out` to the terminal using the native Unicode API; if `out` contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it.
   > 2. (7.2) – Otherwise writes `out` to `stream` unchanged.

   > If the native Unicode API is used, the function flushes `stream` before writing `out`.

   > [Note 1: On POSIX and Windows, the native Unicode API is `WriteConsoleW` and `stream` referring to a terminal means that, respectively, `isatty(fileno(stream))` and `GetConsoleMode(_get_osfhandle(_fileno(stream)), ...)` returns nonzero. — end note]

   > [Note 2: On Windows, the native Unicode API is `WriteConsoleW`. — end note]

   > -8- Throws: [...]

   > -9- Recommended practice: If invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.

### 4045(i). `tuple` can create dangling references from `tuple-like`

Section: 22.4.4.2 [tuple.cnstr] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-01-24 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [tuple.cnstr].

View all other issues in [tuple.cnstr].

View all issues with C++26 status.

Discussion:

P2165R4 (Compatibility between tuple, pair and tuple-like objects) added two new constructors to `std::tuple`:

      template<tuple-likeUTuple>
        constexpr explicit(see below ) tuple(UTuple&& u);

and the allocator-extended equivalent. Unlike the existing constructors taking a single parameter of tuple type, these new constructors are not defined as deleted if they would create a dangling reference to a temporary. The existing constructors gained that restriction from P2255R2 (A type trait to detect reference binding to temporary) which was approved one meeting before P2165R4 so LWG seem to have missed the inconsistency.

The proposal also added a new constructor for `std::pair`:

      template<pair-like P> constexpr explicit(see below) pair(P&& p);

This is deleted if it would create a dangling reference, although that seems to be an almost accidental consequence of adding the new signature after existing ones which already have the Remarks: about being deleted.

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after eleven votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 22.4.4.2 [tuple.cnstr] as indicated:

   >     template<tuple-like UTuple>
   >       constexpr explicit(see below) tuple(UTuple&& u);

   > -28- Let `I` be the pack `0, 1, ..., (sizeof...(Types) - 1)`.

   > -29- Constraints:

   > 1. (29.1) – `different-from<UTuple, tuple>` (25.5.2 [range.utility.helpers]) is `true`,
   > 2. (29.2) – `remove_cvref_t<UTuple>` is not a specialization of `ranges::subrange`,
   > 3. (29.3) – `sizeof...(Types)` equals `tuple_size_v<remove_cvref_t<UTuple>>`,
   > 4. (29.4) – `(is_constructible_v<Types, decltype(get<I>(std::forward<UTuple>(u)))> && ...)` is `true`, and
   > 5. (29.5) – either `sizeof...(Types)` is not 1, or (when `Types...` expands to `T`) `is_convertible_v<UTuple, T>` and `is_constructible_v<T, UTuple>` are both `false`.

   > -30- Effects: For all i, initializes the ith element of `*this` with `get<i>(std::forward<UTuple>(u))`.

   > -31- Remarks: The expression inside `explicit` is equivalent to:

   >       !(is_convertible_v<decltype(get<I>(std::forward<UTuple>(u))), Types> && ...)

   > The constructor is defined as deleted if

   >       (reference_constructs_from_temporary_v<Types, decltype(get<I>(std::forward<UTuple>(u)))> || ...)

   > is `true`.

### 4053(i). Unary call to `std::views::repeat` does not decay the argument

Section: 25.6.5.2 [range.repeat.view] Status: C++26 Submitter: Jiang An Opened: 2024-02-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.repeat.view].

View all issues with C++26 status.

Discussion:

Currently, a binary call to `std::views::repeat` decay the arguments due to the deduction guide, but a unary call doesn't, which is inconsistent.

For example:

>     #include <concepts>
>     #include <ranges>

>     using RPV = std::ranges::repeat_view<const char*>;

>     static_assert(std::same_as<decltype(std::views::repeat("foo", std::unreachable_sentinel)), RPV>); // OK
>     static_assert(std::same_as<decltype(std::views::repeat(+"foo", std::unreachable_sentinel)), RPV>); // OK
>     static_assert(std::same_as<decltype(std::views::repeat("foo")), RPV>); // ill-formed
>     static_assert(std::same_as<decltype(std::views::repeat(+"foo")), RPV>); // OK

Presumably we should extend the deduction guide of `std::ranges::repeat_view` to cover the unary cases.

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 25.6.5.2 [range.repeat.view], class template `repeat_view` synopsis, as indicated:

   > [Drafting note: The proposed wording has been suggested by Casey Carter, see microsoft/STL#3576]

   >     namespace std::ranges {
   >       […]

   >       template<class T, class Bound = unreachable_sentinel_t>
   >         repeat_view(T, Bound = Bound()) -> repeat_view<T, Bound>;
   >     }

### 4054(i). Repeating a `repeat_view` should repeat the view

Section: 25.6.5.1 [range.repeat.overview] Status: C++26 Submitter: Tim Song Opened: 2024-02-12 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

`views::repeat(views::repeat(5))` should be a view of `repeat_view`s, but it's currently a view of `ints` due to the use of CTAD in the specification of `views::repeat`.

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 25.6.5.1 [range.repeat.overview] as indicated:

   > -1- `repeat_view` generates a sequence of elements by repeatedly producing the same value.

   > -2- The name `views::repeat` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given subexpressions `E` and `F`, the expressions `views::repeat(E)` and `views::repeat(E, F)` are expression-equivalent to `repeat_view<decay_t<decltype((E))>>(E)` and `repeat_view(E, F)`, respectively.

### 4060(i). `submdspan` preconditions do not forbid creating invalid pointer

Section: 23.7.3.7.8 [mdspan.sub.sub] Status: C++26 Submitter: Mark Hoemmen Opened: 2024-03-26 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Oliver Lee and Ryan Wooster pointed out to us that creating a `submdspan` with zero-length `tuple-like` or `strided_slice` slice specifiers at the upper extent can cause the Standard `submdspan_mapping` overloads to access the input `mdspan`'s mapping out of bounds. This happens in the following line of specification (23.7.3.7.7 [mdspan.sub.map] p8 in N4971 moved to [mdspan.sub.map.common] p8 after the merge of P2642R6).

> Let `offset` be a value of type `size_t` equal to `(*this)(first_<index_type, P>(slices...)...)`.

If `data_handle_type` is a pointer to an array, then the resulting offset can be larger than `required_span_size()`, thus making the pointer invalid (not just one past the end). In a constexpr context, the result is ill-formed. With the reference `mdspan` implementation, Clang can actually report a build error (e.g., for out-of-bounds access to a `std::array`). The contributed example illustrates this.

Oliver and Ryan offer the following example and analysis:

> Example 1:

> >     auto x = std::array<int, 3>{};
> >     auto A = mdspan{x.data(), extents{3}};
> >     auto B = submdspan(A, pair{3, 3});

> B is an `mdspan` with zero elements.

> Example 2:

> >     auto y = std::array<int, 9>{};
> >     auto C = mdspan{y.data(), extents{3, 3}};
> >     auto D = submdspan(C, pair{3, 3}, pair{3, 3});

> A precondition for each slice specifier is (23.7.3.7.6 [mdspan.sub.extents]):

> >     0 ≤ first_<index_type, k>(slices...) ≤ last_<k>(src.extents(), slices...) ≤ src.extent(k).

> Our understanding is that precondition is satisfied. In the second example, `first_<0>` is 3 and `first_<1>` is also 3.

> However, the submapping offset is defined as `(*this)(first_<index_type, P>(slices...)...)`, which then can result in an invalid data handle of the `submdspan`, even if the data handle is never accessed/dereferenced.

> godbolt demo

We expect this situation to come up in practice.

Suppose we have an `N x N` mdspan representing a matrix `A`, and we want to partition it into a `2 x 2` "matrix of matrices" (also called a "block matrix"). This partitioning is a common operation in linear algebra algorithms such as matrix factorizations. Examples of this `2 x 2` partitioning appear in P2642 and P1673.

>     mdspan A{A_ptr, N, N};

>     size_t p = partition_point(N); // integer in 0, 1, …, N (inclusive)
>     auto A_00 = submdspan(A, tuple{0, p}, tuple{0, p});
>     auto A_10 = submdspan(A, tuple{p, N}, tuple{0, 0});
>     auto A_01 = submdspan(A, tuple{0, p}, tuple{p, N});
>     auto A_11 = submdspan(A, tuple{p, N}, tuple{p, N});

Table illustrating the resulting `2 x 2` block matrix follows:

`A_00` | `A_01`
`A_10` | `A_11`

It's valid for `p` to be `0`. That makes every block but `A_11` have zero size. Thus, it should also be valid for `p` to be `N`. That makes every block but `A_00` have zero size. However, that leads to the aforementioned UB.

It doesn't make sense to change `first_` or `last_`. The definitions of `first_` and `last_` are meant to turn the slice specifier into a pair of bounds. Since `submdspan(A, tuple{p, N}, tuple{p, N})` is valid even if `p` equals `N`, then that strongly suggests that `first_<0>` and `first_<1>` should always be `p`, even if `p` equals `N`.

As a result, we find ourselves needing to change `submdspan_mapping`. This will affect both the Standard `submdspan_mapping` overloads, and any custom (user-defined) overloads.

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971 after the merge of P2642R6.

1. Modify the new 23.7.3.7.7.2 [mdspan.sub.map.common] as indicated:

   > -8- If `first_<index_type, k>(slices...)` equals `extents().extent(k)` for any rank index `k` of `extents()`, then lLet `offset` be a value of type `size_t` equal to `(*this).required_span_size()`. Otherwise, let `offset` be a value of type `size_t` equal to `(*this)(first_<index_type, P>(slices...)...)`.

2. Modify 23.7.3.7.8 [mdspan.sub.sub] as indicated:

   > As a drive-by readability fix, we also propose changing a variable name in paragraph 6 as indicated below.

   >     template<class ElementType, class Extents, class LayoutPolicy,
   >              class AccessorPolicy, class... SliceSpecifiers>
   >       constexpr auto submdspan(
   >         const mdspan<ElementType, Extents, LayoutPolicy, AccessorPolicy>& src,
   >         SliceSpecifiers... slices) -> see below;

   > > -1- Let `index_type` be `typename Extents::index_type`.

   > > -2- Let `sub_map_offset` be the result of `submdspan_mapping(src.mapping(), slices...)`.

   > > […]

   > > -3- Constraints: […]

   > > -4- Mandates: […]

   > > -5-Preconditions: […]

   > > -6- Effects: Equivalent to:

   > > >     auto sub_map_resultoffset = submdspan_mapping(src.mapping(), slices...);
   > > >     return mdspan(src.accessor().offset(src.data(), sub_map_resultoffset.offset),
   > > >                   sub_map_resultoffset.mapping,
   > > >                   AccessorPolicy::offset_policy(src.accessor()));

### 4061(i). Should `std::basic_format_context` be default-constructible/copyable/movable?

Section: 28.5.6.7 [format.context] Status: C++26 Submitter: Jiang An Opened: 2024-03-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [format.context].

View all issues with C++26 status.

Discussion:

Per 28.5.6.7 [format.context], it seems that `std::basic_format_context` has a default constructor that is effectively defaulted, which means that it is default constructible if and only if `OutIt` is default constructible. Currently only libstdc++ makes it conditionally default constructible, while libc++ and MSVC STL (together with fmtlib) make it never default constructible.

It seems that `basic_format_context` objects are supposed to be created by the implementation in some internal way, and user codes are only supposed to modify existing `basic_format_context` objects during formatting.

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 28.5.6.7 [format.context] as indicated:

   >     namespace std {
   >       template<class Out, class charT>
   >       class basic_format_context {
   >         basic_format_args<basic_format_context> args_; // exposition only
   >         Out out_;                                      // exposition only

   >         basic_format_context(const basic_format_context&) = delete;
   >         basic_format_context& operator=(const basic_format_context&) = delete;
   >       public:
   >         using iterator = Out;
   >         using char_type = charT;
   >         template<class T> using formatter_type = formatter<T, charT>;

   >         basic_format_arg<basic_format_context> arg(size_t id) const noexcept;
   >         std::locale locale();

   >         iterator out();
   >         void advance_to(iterator it);
   >       };
   >     }

### 4064(i). Clarify that `std::launder` is not needed when using the result of `std::memcpy`

Section: 27.5.1 [cstring.syn] Status: C++26 Submitter: Jan Schultke Opened: 2024-04-05 Last modified: 2026-06-09

Priority: 3

View all issues with C++26 status.

Discussion:

>     int x = 0;
>     alignas(int) std::byte y[sizeof(int)];
>     int z = *static_cast<int*>(std::memcpy(y, &x, sizeof(int)));

This example should be well-defined, even without the use of `std::launder`. `std::memcpy` implicitly creates an `int` inside `y`, and https://www.iso-9899.info/n3047.html#7.26.2.1p3 states that

> The `memcpy` function returns the value of [the destination operand].

In conjunction with 27.5.1 [cstring.syn] p3, this presumably means that `std::memcpy` returns a pointer to the (first) implicitly-created object, and no use of `std::launder` is necessary.

The wording should be clarified to clearly support this interpretation or reject it.

[2024-06-24; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N4971.

1. Modify 27.5.1 [cstring.syn] as indicated:

   > -3- The functions `memcpy` and `memmove` are signal-safe (17.14.5 [support.signal]). Both functions implicitly create objects (6.8.2 [intro.object]) in the destination region of storage immediately prior to copying the sequence of characters to the destination. Both functions return a pointer to a suitable created object.

[St. Louis 2024-06-26; CWG suggested improved wording]

[St. Louis 2024-06-28; LWG: move to Ready]

[Wrocław 2024-11-18; approved by Core (again)]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 27.5.1 [cstring.syn] as indicated:

   > -3- The functions `memcpy` and `memmove` are signal-safe (17.14.5 [support.signal]). Both Each of these functions implicitly create creates objects (6.8.2 [intro.object]) in the destination region of storage immediately prior to copying the sequence of characters to the destination. Each of these functions returns a pointer to a suitable created object, if any, otherwise the value of the first parameter.

### 4069(i). `std::atomic<volatile T>` should be ill-formed

Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Jonathan Wakely Opened: 2024-04-19 Last modified: 2025-02-07

Priority: 2

View all other issues in [atomics.types.generic].

View all issues with Resolved status.

Discussion:

As a result of Core DR 2094 (concerning triviality of volatile-qualified subobjects), `is_trivially_copyable_v<volatile int>` is now true, which means that `volatile int` is a valid type for `std::atomic`.

Libstdc++ and libc++ can't actually compile that type though, and that seems very sensible to me.

Even worse is that `std::atomic<volatile int>` will not select the specialization for `int`, because that is clearly specified by 32.5.8.3 [atomics.types.int] to only be for cv-unqualified types. 32.5.8.4 [atomics.types.float] also says it's only for cv-unqualified floating-point types. And 32.5.8.5 [atomics.types.pointer] will only match cv-unqualified pointer types. This means that even when `std::atomic<volatile int>` compiles (as with MSVC) you can't use members like `fetch_add` because that only exists on the specialization, not the primary template.

Should we add something to `std::atomic` to make it not valid again, as was the case (and presumably the original intent) before CWG DR 2094?

A similar question exists for `std::atomic_ref<volatile int>` although there are apparently valid uses for that type. However, the `atomic_ref` specializations for integers, floats, and pointers are only for cv-unqualified types, so it doesn't work usefully. For `atomic_ref` we might want to allow `volatile`-qualified types and make the specializations match them.

[2024-04-29; Reflector poll]

Set priority to 2 after reflector poll and send to SG1.

[2024-06; Related to issue 3508(i).]

[St. Louis 2024-06-28; SG1 feedback]

SG1 forwarded P3323R0 to LEWG to resolve LWG issues 3508(i) and 4069(i).

[2025-02-07 Status changed: Open → Resolved.]

Resolved by P3323R0 in Wrocław.

Proposed resolution:

### 4071(i). `reference_wrapper` comparisons are not SFINAE-friendly

Section: 22.10.6.6 [refwrap.comparisons] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-04-19 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

P2944R3 added these hidden friends to `reference_wrapper`:

       friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, reference_wrapper);
       friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, const T&);
       friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, reference_wrapper<const T>);

These functions are not templates, and so their declarations are ill-formed for any type that does have any comparison operators, e.g.

        struct A { } a;
        std::reference_wrapper<A> r(a);

Instantiating `reference_wrapper<A>` will instantiate the declarations of the hidden friends, which will attempt to determine the return types of the `operator<=>` functions. That fails because `synth-three-way` is constrained and can't be called with arguments of type `A`.

This can be solved by changing those functions into templates, so they aren't instantiated eagerly, e.g.,

        template<class U = T>
        friend constexpr synth-three-way-result<TU> operator<=>(reference_wrapper, reference_wrapper);

or by giving them a deduced return type (so that it isn't instantiated eagerly) and constraining them to only be callable when valid:

        friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper y)
        requires requires (const T t) { synth-three-way(t, t); }

The second alternative is used in the proposed resolution.

In practice the requires-clause can be implemented more simply (and efficiently) by checking the constraints of synth-three-way directly:

        requires (const T t) { { t < t } -> boolean-testable; }

but when specified in prose in a Constraints: element it seems clearer to just use `synth-three-way(x.get(), y.get())`.

The proposed resolution has been committed to libstdc++'s master branch.

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 22.10.6.1 [refwrap.general] as indicated:

   >        // [refwrap.comparisons], comparisons
   >        friend constexpr bool operator==(reference_wrapper, reference_wrapper);
   >        friend constexpr bool operator==(reference_wrapper, const T&);
   >        friend constexpr bool operator==(reference_wrapper, reference_wrapper<const T>);

   >        friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper, reference_wrapper);
   >        friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper, const T&);
   >        friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper, reference_wrapper<const T>);

2. Modify 22.10.6.6 [refwrap.comparisons] as indicated:

   >     friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper y);

   > -?- Constraints: The expression `synth-three-way(x.get(), y.get())` is well-formed.

   > -7- Returns: `synth-three-way(x.get(), y.get())`.

   >     friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, const T& y);

   > -?- Constraints: The expression `synth-three-way(x.get(), y)` is well-formed.

   > -8- Returns: `synth-three-way(x.get(), y)`.

   >     friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper<const T> y);

   > -9- Constraints: `is_const_v<T>` is `false`. The expression `synth-three-way(x.get(), y.get())` is well-formed.

   > -10- Returns: `synth-three-way(x.get(), y.get())`.

### 4072(i). `std::optional` comparisons: constrain harder

Section: 22.5.9 [optional.comp.with.t] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-04-19 Last modified: 2026-06-09

Priority: 1

View all other issues in [optional.comp.with.t].

View all issues with C++26 status.

Discussion:

P2944R3 added constraints to `std::optional`'s comparisons, e.g.

>     template<class T, class U> constexpr bool operator==(const optional<T>& x, const optional<U>& y);

> -1- MandatesConstraints: The expression `*x == *y` is well-formed and its result is convertible to `bool`.

> …

>     template<class T, class U> constexpr bool operator==(const optional<T>& x, const U& v);

> -1- MandatesConstraints: The expression `*x == v` is well-formed and its result is convertible to `bool`.

But I don't think the constraint on the second one (the "compare with value") is correct. If we try to compare two optionals that can't be compared, such as `optional<void*>` and `optional<int>`, then the first overload is not valid due to the new constraints, and so does not participate in overload resolution. But that means we now consider the second overload, but that's ambiguous. We could either use `operator==<void*, optional<int>>` or we could use `operator==<optional<void*>, int>` with the arguments reversed (using the C++20 default comparison rules). We never even get as far as checking the new constraints on those overloads, because they're simply ambiguous.

Before P2944R3 overload resolution always would have selected the first overload, for comparing two optionals. But because that is now constrained away, we consider an overload that should never be used for comparing two optionals. The solution is to add an additional constraint to the "compare with value" overloads so that they won't be used when the "value" is really another optional.

A similar change was made to `optional`'s `operator<=>` by LWG 3566(i), and modified by LWG 3746(i). I haven't analyzed whether we need the modification here too.

The proposed resolution (without `is-derived-from-optional`) has been implemented and tested in libstdc++.

[2024-06-24; Reflector poll]

Set priority to 1 after reflector poll. LWG 3746(i) changes might be needed here too, i.e, consider types derived from `optional`, not only `optional` itself.

[2024-08-21; Move to Ready at LWG telecon]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 22.5.9 [optional.comp.with.t] as indicated:

   >     template<class T, class U> constexpr bool operator==(const optional<T>& x, const U& v);

   > -1- Constraints: `U` is not a specialization of `optional`. The expression `*x == v` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator==(const T& v, const optional<U>& x);

   > -3- Constraints: `T` is not a specialization of `optional`. The expression `v == *x` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator!=(const optional<T>& x, const U& v);

   > -5- Constraints: `U` is not a specialization of `optional`. The expression `*x != v` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator!=(const T& v, const optional<U>& x);

   > -7- Constraints: `T` is not a specialization of `optional`. The expression `v != *x` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator<(const optional<T>& x, const U& v);

   > -9- Constraints: `U` is not a specialization of `optional`. The expression `*x < v` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator<(const T& v, const optional<U>& x);

   > -11- Constraints: `T` is not a specialization of `optional`. The expression `v < *x` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator>(const optional<T>& x, const U& v);

   > -13- Constraints: `U` is not a specialization of `optional`. The expression `*x > v` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator>(const T& v, const optional<U>& x);

   > -15- Constraints: `T` is not a specialization of `optional`. The expression `v > *x` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator<=(const optional<T>& x, const U& v);

   > -17- Constraints: `U` is not a specialization of `optional`. The expression `*x <= v` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator<=(const T& v, const optional<U>& x);

   > -19- Constraints: `T` is not a specialization of `optional`. The expression `v <= *x` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator>=(const optional<T>& x, const U& v);

   > -21- Constraints: `U` is not a specialization of `optional`. The expression `*x &gt;= v` is well-formed and its result is convertible to `bool`.

   >     template<class T, class U> constexpr bool operator>=(const T& v, const optional<U>& x);

   > -23- Constraints: `T` is not a specialization of `optional`. The expression `v >= *x` is well-formed and its result is convertible to `bool`.

### 4074(i). `compatible-joinable-ranges` is underconstrained

Section: 25.7.15.2 [range.join.with.view] Status: C++26 Submitter: Hewill Kang Opened: 2024-04-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.join.with.view].

View all issues with C++26 status.

Discussion:

`join_with_view` requires the value type, reference and rvalue reference of the inner range and pattern range to share common (reference) types through `compatible-joinable-ranges`.

However, unlike what `concat_view` and `generator` do, this concept only requires that these three types be valid and does not further check the relationship between them to be compatible with the `indirectly_readable` requirement for `input_iterator`. This results in a validly-constructed `join_with_view` that may not model `input_range`, which seems unintended.

The proposed resolution aliases `compatible-joinable-ranges` to `concatable` i.e. specialization for two ranges to fully constrain, and I believe this could also be a better fit for LWG 3971(i).
Previous resolution [SUPERSEDED]

This wording is relative to N4981.

1. Modify 25.7.15.2 [range.join.with.view] as indicated:

   >     namespace std::ranges {
   >       template<class R, class P>
   >         concept compatible-joinable-ranges = concatable<R, P>;  // exposition only
   >             common_with<range_value_t<R>, range_value_t<P>> &&
   >             common_reference_with<range_reference_t<R>, range_reference_t<P>> &&
   >             common_reference_with<range_rvalue_reference_t<R>, range_rvalue_reference_t<P>>;

   >       […]
   >     }

[2024-04-24; Hewill Kang provides improved wording]

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 25.2 [ranges.syn] as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   >     #include <iterator>             // see 24.2 [iterator.synopsis]

   >     namespace std::ranges {
   >       […]
   >       // 25.7.15 [range.join.with], join with view
   >       template<class R, class P>
   >         concept compatible-joinable-ranges = see below; // exposition only

   >       template<input_range V, forward_range Pattern>
   >         requires view<V> && input_range<range_reference_t<V>>
   >               && view<Pattern>
   >               && compatible-joinable-ranges<range_reference_t<V>, Pattern>
   >               see below
   >       class join_with_view;                                                             // freestanding
   >       […]
   >     }

2. Modify 25.7.15.2 [range.join.with.view] as indicated:

   >     namespace std::ranges {
   >       template<class R, class P>
   >         concept compatible-joinable-ranges =            // exposition only
   >             common_with<range_value_t<R>, range_value_t<P>> &&
   >             common_reference_with<range_reference_t<R>, range_reference_t<P>> &&
   >             common_reference_with<range_rvalue_reference_t<R>, range_rvalue_reference_t<P>>;

   >       […]

   >       template<input_range V, forward_range Pattern>
   >         requires view<V> && input_range<range_reference_t<V>>
   >               && view<Pattern>
   >               && compatible-joinable-rangesconcatable<range_reference_t<V>, Pattern>
   >       class join_with_view : public view_interface<join_with_view<V, Pattern>> {
   >         […]
   >         constexpr auto begin() const
   >           requires forward_range<const V> &&
   >                    forward_range<const Pattern> &&
   >                    is_reference_v<range_reference_t<const V>> &&
   >                    input_range<range_reference_t<const V>> &&
   >                    concatable<range_reference_t<const V>, const Pattern> {
   >           return iterator<true>{*this, ranges::begin(base_)};
   >         }
   >         […]
   >         constexpr auto end() const
   >           requires forward_range<const V> && forward_range<const Pattern> &&
   >                    is_reference_v<range_reference_t<const V>> &&
   >                    input_range<range_reference_t<const V>> &&
   >                    concatable<range_reference_t<const V>, const Pattern> {
   >           […]
   >         }
   >       };
   >     }

3. Modify 25.7.15.3 [range.join.with.iterator] as indicated:

   >     namespace std::ranges {
   >       template<input_range V, forward_range Pattern>
   >         requires view<V> && input_range<range_reference_t<V>>
   >               && view<Pattern> && compatible-joinable-rangesconcatable<range_reference_t<V>, Pattern>
   >       template<bool Const>
   >       class join_with_view<V, Pattern>::iterator {
   >         […]
   >       };
   >     }

4. Modify 25.7.15.4 [range.join.with.sentinel] as indicated:

   >     namespace std::ranges {
   >       template<input_range V, forward_range Pattern>
   >         requires view<V> && input_range<range_reference_t<V>>
   >               && view<Pattern> && compatible-joinable-rangesconcatable<range_reference_t<V>, Pattern>
   >       template<bool Const>
   >       class join_with_view<V, Pattern>::sentinel {
   >         […]
   >       };
   >     }

### 4076(i). `concat_view` should be freestanding

Section: 17.3.2 [version.syn] Status: C++26 Submitter: Hewill Kang Opened: 2024-04-21 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++26 status.

Discussion:

`concat_view` can be freestanding, but this never seems to come up in the discussion, which seems to be an oversight.

[2024-04-21; Daniel comments]

The specification of some member functions of `concat_view` seem to depend on freestanding-deleted `get` overloads for `variant`, but so does `join_with_view`, which is marked as freestanding, so it does not seem to be a good reason to accept `join_with_view` but not `concat_view` as freestanding.

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 17.3.2 [version.syn] as indicated:

   >     #define __cpp_lib_ranges_concat 202403L // freestanding, also in <ranges>

2. Modify 25.2 [ranges.syn] as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   >     #include <iterator>             // see 24.2 [iterator.synopsis]

   >     namespace std::ranges {
   >       […]
   >       // 25.7.18 [range.concat], concat view
   >       template<input_range... Views>
   >         requires see below
   >       class concat_view;                                                                // freestanding

   >       namespace views { inline constexpr unspecified concat = unspecified; }            // freestanding
   >       […]
   >     }

### 4079(i). Missing Preconditions in `concat_view::iterator`'s conversion constructor

Section: 25.7.18.3 [range.concat.iterator] Status: C++26 Submitter: Hewill Kang Opened: 2024-04-26 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [range.concat.iterator].

View all other issues in [range.concat.iterator].

View all issues with C++26 status.

Discussion:

This conversion constructor obtains the alternative iterator of the argument through `std::get`, which will throw when the `variant` is valueless. We seem to be missing a Preconditions element here.

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 25.7.18.3 [range.concat.iterator] as indicated:

   >     constexpr iterator(iterator<!Const> it)
   >       requires Const &&
   >                (convertible_to<iterator_t<Views>, iterator_t<const Views>> && ...);

   > > -?- Preconditions: `it.it_.valueless_by_exception()` is `false`.

   > > -8- Effects: Initializes `parent_` with `it.parent_`, and let `i` be `it.it_.index()`, initializes `it_` with `base-iter(in_place_index<i>, std::get<i>(std::move(it.it_)))`.

### 4082(i). `views::concat(r)` is well-formed when `r` is an `output_range`

Section: 25.7.18.1 [range.concat.overview] Status: C++26 Submitter: Hewill Kang Opened: 2024-04-27 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Currently, `views::concat` will return `views::all(r)` when it takes only one argument, which only requires that the type of `r` models `viewable_range` which includes `output_range`:

>     std::vector<int> v;
>     auto r = std::views::counted(std::back_inserter(v), 3);
>     auto c = std::views::concat(r); // well-formed

Since `concat_view` requires all ranges to be `input_range`, this seems inconsistent. We should reject the above just like `views::zip_transform` still requires `F` to be `move_constructible` in the case of an empty pack.

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 25.7.18.1 [range.concat.overview] as indicated:

   > -2- The name `views::concat` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a pack of subexpressions `Es...`, the expression `views::concat(Es...)` is expression-equivalent to

   > 1. (2.1) — `views::all(Es...)` if `Es` is a pack with only one element whose type models `input_range`,

   > 2. (2.2) — otherwise, `concat_view(Es...)`.

### 4083(i). `views::as_rvalue` should reject non-input ranges

Section: 25.7.7.1 [range.as.rvalue.overview] Status: C++26 Submitter: Hewill Kang Opened: 2024-04-27 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.as.rvalue.overview].

View all issues with C++26 status.

Discussion:

`views::as_rvalue(r)` equivalent to `views::all(r)` when `r`'s reference and rvalue reference are of the same type, which means that in this case we only need to check whether the type of `r` models `viewable_range`.

However, libstdc++'s implementation always requires `as_rvalue_view{r}` to be valid, which leads to divergence when `r` is not an `input_range` (demo):

>     #include <ranges>

>     struct I {
>       int operator*();
>       using difference_type = int;
>       I& operator++();
>       void operator++(int);
>     };

>     std::ranges::range auto r = std::ranges::subrange{I{}, std::unreachable_sentinel}
>                               | std::views::as_rvalue; // // well-formed in libc++/MSVC-STL, ill-formed in libstdc++

Although this is precisely a bug in libstdc++ that does not conform to the current wording, it is reasonable to require `r` to be an `input_range` to be consistent with the constraints of `as_rvalue_view`.

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 25.7.7.1 [range.as.rvalue.overview] as indicated:

   > -2- The name `views::as_rvalue` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Let `E` be an expression and let `T` be `decltype((E))`. The expression `views::as_rvalue(E)` is expression-equivalent to:

   > 1. (2.1) — `views::all(E)` if `T` models `input_range` and `same_as<range_rvalue_reference_t<T>, range_reference_t<T>>` is `true`.

   > 2. (2.2) — Otherwise, `as_rvalue_view(E)`.

### 4084(i). `std::fixed` ignores `std::uppercase`

Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-04-30 Last modified: 2026-06-09

Priority: 3

View other active issues in [facet.num.put.virtuals].

View all other issues in [facet.num.put.virtuals].

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Discussion:

In Table 114 – Floating-point conversions [tab:facet.num.put.fp] we specify that a floating-point value should be printed as if by `%f` when `(flags & floatfield) == fixed`. This ignores whether `uppercase` is also set in `flags`, meaning there is no way to use the conversion specifier `%F` that was added to `printf` in C99.

That's fine for finite values, because 1.23 in fixed format has no exponent character and no hex digits that would need to use uppercase. But `%f` and `%F` are not equivalent for non-finite values, because `%F` prints `"NAN"` and `"INF"` (or `"INFINITY"`). It seems there is no way to print `"NAN"` or `"INF"` using `std::num_put`.

Libstdc++ and MSVC print `"inf"` for the following code, but libc++ prints `"INF"` which I think is non-conforming:

        std::cout << std::uppercase << std::fixed << std::numeric_limits<double>::infinity();

The libc++ behaviour seems more useful and less surprising.

[2024-05-08; Reflector poll]

Set priority to 3 after reflector poll. Send to LEWG.

[2024-09-17; LEWG mailing list vote]

Set status to Open after LEWG approved the proposed change.

[2024-09-19; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 28.3.4.3.3.3 [facet.num.put.virtuals] as indicated:

   > Table 114 – Floating-point conversions [tab:facet.num.put.fp]
   > State | `stdio` equivalent
   > --- | ---
   > `floatfield == ios_base::fixed` `&& !uppercase` | `%f`
   > `floatfield == ios_base::fixed` | `%F`
   > `floatfield == ios_base::scientific && !uppercase` | `%e`
   > `floatfield == ios_base::scientific` | `%E`
   > `floatfield == (ios_base::fixed | ios_base::scientific)` && !uppercase` | `%a`
   > `floatfield == (ios_base::fixed | ios_base::scientific)` | `%A`
   > `!uppercase` | `%g`
   > otherwise | `%G`

### 4085(i). `ranges::generate_random`'s helper lambda should specify the return type

Section: 26.12.2 [alg.rand.generate] Status: C++26 Submitter: Hewill Kang Opened: 2024-04-28 Last modified: 2026-06-09

Priority: 2

View all other issues in [alg.rand.generate].

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Discussion:

The non-specialized case of `generate_random(r, g, d)` would call `ranges::generate(r, [&d, &g] { return invoke(d, g); })`. However, the lambda does not explicitly specify the return type, which leads to a hard error when `invoke` returns a reference to the object that is not copyable or `R` is not the `output_range` for `decay_t<invoke_result_t<D&, G&>>`.

Previous resolution [SUPERSEDED]

This wording is relative to N4981.

1. Modify 26.12.2 [alg.rand.generate] as indicated:

   >     template<class R, class G, class D>
   >       requires output_range<R, invoke_result_t<D&, G&>> && invocable<D&, G&> &&
   >                uniform_random_bit_generator<remove_cvref_t<G>>
   >     constexpr borrowed_iterator_t<R> ranges::generate_random(R&& r, G&& g, D&& d);

   > -5- Effects:

   > 1. (5.1) — […]

   > 2. (5.2) — […]

   > 3. (5.3) — Otherwise, calls:

   >    >     ranges::generate(std::forward<R>(r), [&d, &g] -> decltype(auto) { return invoke(d, g); });

   > -6- Returns: `ranges::end(r)`

   > -7- Remarks: The effects of `generate_random(r, g, d)` shall be equivalent to

   > >     ranges::generate(std::forward<R>(r), [&d, &g] -> decltype(auto) { return invoke(d, g); })

[2024-05-12, Hewill Kang provides improved wording]

[2024-08-02; Reflector poll]

Set priority to 2 after reflector poll.

[2024-10-09; LWG telecon: Move to Ready]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 29.5.2 [rand.synopsis], header `<random>` synopsis, as indicated:

   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]

   >     namespace std {
   >       […]
   >       namespace ranges {
   >         […]
   >         template<class R, class G, class D>
   >           requires output_range<R, invoke_result_t<D&, G&>> && invocable<D&, G&> &&
   >                    uniform_random_bit_generator<remove_cvref_t<G>> &&
   >                    is_arithmetic_v<invoke_result_t<D&, G&>>
   >         constexpr borrowed_iterator_t<R> generate_random(R&& r, G&& g, D&& d);

   >         template<class G, class D, output_iterator<invoke_result_t<D&, G&>> O, sentinel_for<O> S>
   >           requires invocable<D&, G&> && uniform_random_bit_generator<remove_cvref_t<G>> &&
   >                    is_arithmetic_v<invoke_result_t<D&, G&>>
   >         constexpr O generate_random(O first, S last, G&& g, D&& d);
   >       }
   >       […]
   >     }

2. Modify 26.12.2 [alg.rand.generate] as indicated:

   >     template<class R, class G, class D>
   >       requires output_range<R, invoke_result_t<D&, G&>> && invocable<D&, G&> &&
   >                uniform_random_bit_generator<remove_cvref_t<G>> &&
   >                is_arithmetic_v<invoke_result_t<D&, G&>>
   >     constexpr borrowed_iterator_t<R> ranges::generate_random(R&& r, G&& g, D&& d);

   > > -5- Effects:

   > > […]

   >     template<class G, class D, output_iterator<invoke_result_t<D&, G&>> O, sentinel_for<O> S>
   >       requires invocable<D&, G&> && uniform_random_bit_generator<remove_cvref_t<G>> &&
   >                is_arithmetic_v<invoke_result_t<D&, G&>>
   >     constexpr O ranges::generate_random(O first, S last, G&& g, D&& d);

   > > -8- Effects: Equivalent to:

   > > […]

### 4088(i). `println` ignores the locale imbued in `std::ostream`

Section: 31.7.6.3.5 [ostream.formatted.print] Status: C++26 Submitter: Jens Maurer Opened: 2024-04-30 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [ostream.formatted.print].

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Discussion:

31.7.6.3.5 [ostream.formatted.print] specifies that `std::print` uses the locale imbued in the `std::ostream&` argument for formatting, by using this equivalence:

>     vformat(os.getloc(), fmt, args);

(in the `vformat_(non)unicode` delegation).

However, `std::println` ignores the `std::ostream`'s locale for its locale-dependent formatting:

>     print(os, "{}\n", format(fmt, std::forward<Args>(args)...));

This is inconsistent.

[2024-10-03; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 31.7.6.3.5 [ostream.formatted.print] as indicated:

   >     template<class... Args>
   >       void println(ostream& os, format_string<Args...> fmt, Args&&... args);

   > > -2- Effects: Equivalent to:

   > > >     print(os, "{}\n", format(os.getloc(), fmt, std::forward<Args>(args)...));

### 4090(i). Underspecified use of locale facets for locale-dependent `std::format`

Section: 28.5.2.2 [format.string.std] Status: C++26 Submitter: Jens Maurer Opened: 2024-04-30 Last modified: 2026-06-09

Priority: 3

View other active issues in [format.string.std].

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Discussion:

There are `std::format` variants that take an explicit `std::locale` parameter. There is the `"L"` format specifier that uses that locale (or some environment locale) for formatting, according to 28.5.2.2 [format.string.std] p17:

> "For integral types, the locale-specific form causes the context's locale to be used to insert the appropriate digit group separator characters."

It is unclear which specific facets are used to make this happen. This is important, because users can install their own facets into a given locale. Specific questions include:

- Is `num_put<>` being used? Or just `numpunct<>`?

- Are any of the `_byname` facets being used?

Assuming the encoding for `char` is UTF-8, the use of a user-provided `num_put<>` facet (as opposed to `std::format` creating the output based on `numpunct<>`) would allow digit separators that are not expressibly as a single UTF-8 code unit.

[2024-05-08; Reflector poll]

Set priority to 3 after reflector poll.

[2024-06-12; SG16 meeting]

The three major implementations all use `numpunct` but not `num_put`, clarify that this is the intended behaviour.

[2025-06-12; Jonathan provides wording]

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > -17- When the `L` option is used, the form used for the conversion is called the locale-specific form. The `L` option is only valid for arithmetic types, and its effect depends upon the type.
   > 1. (17.1) — For integral types, the locale-specific form causes the context’s locale to be used to insert the appropriate digit group separator characters as if obtained with `numpunct<charT>::grouping` and `numpunct<charT>::thousands_sep` .
   > 2. (17.2) — For floating-point types, the locale-specific form causes the context’s locale to be used to insert the appropriate digit group and radix separator characters as if obtained with `numpunct<charT>::grouping`, `numpunct<charT>::thousands_sep`, and `numpunct<charT>::decimal_point` .
   > 3. (17.3) — For the textual representation of `bool`, the locale-specific form causes the context’s locale to be used to insert the appropriate string as if obtained with `numpunct<charT>::truename` or `numpunct<charT>::falsename`.

[2025-08-27; SG16 meeting]

SG16 unanimously approved new wording from Victor. The new wording incorporates similar wording as added by P2419R2 to address 3565(i). Status updated SG16 → Open.

[2026-01-16; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > -17- When the `L` option is used, the form used for the conversion is called the locale-specific form. The `L` option is only valid for arithmetic types, and its effect depends upon the type.
   > 1. (17.1) — For integral types, the locale-specific form causes the context’s locale to be used to insert the appropriate digit group separator characters as if obtained with `numpunct<charT>::grouping` and `numpunct<charT>::thousands_sep` .
   > 2. (17.2) — For floating-point types, the locale-specific form causes the context’s locale to be used to insert the appropriate digit group and radix separator characters as if obtained with `numpunct<charT>::grouping`, `numpunct<charT>::thousands_sep`, and `numpunct<charT>::decimal_point` .
   > 3. (17.3) — For the textual representation of `bool`, the locale-specific form causes the context’s locale to be used to insert the appropriate string as if obtained with `numpunct<charT>::truename` or `numpunct<charT>::falsename`.
   > If the string literal encoding is a Unicode encoding form and the locale is among an implementation-defined set of locales, each replacement that depends on the locale is performed as if the replacement character sequence is converted to the string literal encoding.

### 4096(i). `views::iota(views::iota(0))` should be rejected

Section: 25.6.4.1 [range.iota.overview] Status: C++26 Submitter: Hewill Kang Opened: 2024-05-08 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.iota.overview].

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Discussion:

`views::iota(E)` literally means incrementing element E endlessly, but `views::iota(views::iota(0))` is currently well-formed due to CTAD, rejecting such unreasonable spelling seems therefore reasonable.

[2024-06-24; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 25.6.4.1 [range.iota.overview] as indicated:

   > -1- `iota_view` generates a sequence of elements by repeatedly incrementing an initial value.

   > -2- The name `views::iota` denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given subexpressions `E` and `F`, the expressions `views::iota(E)` and `views::iota(E, F)` are expression-equivalent to `iota_view<decay_t<decltype((E))>>(E)` and `iota_view(E, F)`, respectively.

### 4098(i). `views::adjacent<0>` should reject non-forward ranges

Section: 25.7.27.1 [range.adjacent.overview], 25.7.28.1 [range.adjacent.transform.overview] Status: C++26 Submitter: Hewill Kang Opened: 2024-05-10 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.adjacent.overview].

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Discussion:

Following-up LWG 4082(i) and LWG 4083(i), the current wording makes `views::adjacent<0>(r)` and `views::adjacent_transform<0>(r, [] { return 0; })` well-formed even when `r` is just an input range or an output range, which seems to be an oversight.

[2024-06-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 25.7.27.1 [range.adjacent.overview] as indicated:

   > -2- The name `views::adjacent<N>` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpression `E` and a constant expression `N`, the expression `views::adjacent<N>(E)` is expression-equivalent to

   > 1. (2.1) — `((void)E, auto(views::empty<tuple<>>))` if `N` is equal to `0` and `decltype((E))` models `forward_range`,

   > 2. (2.2) — otherwise, `adjacent_view<views::all_t<decltype((E))>, N>(E)`.

2. Modify 25.7.28.1 [range.adjacent.transform.overview] as indicated:

   > -2- The name `views::adjacent_transform<N>` denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given subexpressions `E` and `F` and a constant expression `N`:

   > 1. (2.1) — If `N` is equal to `0` and `decltype((E))` models `forward_range`, `views::adjacent_transform<N>(E, F)` is expression-equivalent to `((void)E, views::zip_transform(F))`, except that the evaluations of `E` and `F` are indeterminately sequenced.

   > 2. (2.2) — Otherwise, the expression `views::adjacent_transform<N>(E, F)` is expression-equivalent to `adjacent_transform_view<views::all_t<decltype((E))>, decay_t<decltype((F))>, N>(E, F)`.

### 4105(i). `ranges::ends_with`'s Returns misses difference casting

Section: 26.6.17 [alg.ends.with] Status: C++26 Submitter: Hewill Kang Opened: 2024-05-17 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The Returns of the ranges version of `ranges::ends_with` are specified as `ranges::equal(ranges::drop_view(ranges::ref_view(r1), N1 - N2), r2, ...)` which is not quite right when `N2` is an integer-class type and `N1` is an integer type, because in this case `N1 - N2` will be an integer-class type which cannot be implicitly converted to the `difference_type` of `r1` leading to the construction of `drop_view` being ill-formed.

[2024-06-24; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 26.6.17 [alg.ends.with] as indicated:

   >     template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity,
   >              class Proj2 = identity>
   >       requires (forward_range<R1> || sized_range<R1>) &&
   >                (forward_range<R2> || sized_range<R2>) &&
   >                indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
   >       constexpr bool ranges::ends_with(R1&& r1, R2&& r2, Pred pred = {},
   >                                        Proj1 proj1 = {}, Proj2 proj2 = {});

   > > -3- Let `N1` be `ranges::distance(r1)` and `N2` be `ranges::distance(r2)`.

   > > -4- Returns: `false` if `N1 < N2`, otherwise

   > > >     ranges::equal(ranges::drop_view(ranges::ref_view(r1), N1 - static_cast<decltype(N1)>(N2)), r2, pred, proj1, proj2)

### 4106(i). `basic_format_args` should not be default-constructible

Section: 28.5.8.3 [format.args] Status: C++26 Submitter: Hewill Kang Opened: 2024-05-17 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [format.args].

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Discussion:

It's unclear why `basic_format_args` originally provided a default constructor and its actual use cases, all three major libraries declare them as `default`, which allows value initialization of `basic_format_args` at compile time.

This does not fully conform to the current wording as its signature does not have the `constexpr` specifier.

Additionally, the current wording only initializes the `size_` member in the default constructor, which may lead to undefined behavior when copying `basic_format_args` as uninitialized `data_` member is copied. There is also an implementation divergence (demo):

>     #include <format>
>     constexpr std::format_args fmt_args; // only well-formed in MSVC-STL

One option is to add default member initializers for all members and `default` the default constructor to best match the status quo, which guarantees that `basic_format_args` is `constexpr`-able.

However, given that `basic_format_args` has different implementation details in the three libraries, its actual members may not be `size_` and `data_`. It is unnecessary to ensure that all the internal members are initialized when default-constructed `basic_format_args`, indicating that not providing one is reasonable.

The proposed solution is to prefer the more aggressive one.

[2024-05-19; Daniel comments]

The here suggested proposal to remove the default constructor implicitly depends on the decision of LWG 4061(i) to remove `basic_format_context`'s default constructor, since its usage would require that the exposition-only member `args_` of type `basic_format_args<basic_format_context>` can be default-constructed as well.

[2024-06-24; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 28.5.8.3 [format.args] as indicated:

   > >     namespace std {
   > >       template<class Context>
   > >       class basic_format_args {
   > >         size_t size_;                               // exposition only
   > >         const basic_format_arg<Context>* data_;     // exposition only

   > >       public:
   > >         basic_format_args() noexcept;

   > >         template<class... Args>
   > >           basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;

   > >         basic_format_arg<Context> get(size_t i) const noexcept;
   > >       };
   > >       […]
   > >     }

   > […]

   >     basic_format_args() noexcept;

   > > -2- Effects Initializes `size_` with `0`.

### 4109(i). Instantiating templates in §[rand] with `int8_t`/`uint8_t` is undefined behavior

Section: 29.5.3.1 [rand.req.genl] Status: Resolved Submitter: Peter Dimov Opened: 2024-05-25 Last modified: 2026-03-31

Priority: 3

View all other issues in [rand.req.genl].

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Discussion:

Addresses RU-272

As pointed out in LWG issue 2326(i) (closed as NAD) and on reddit, instantiating e.g. `uniform_int_distribution<uint8_t>` is undefined behavior because of the requirement 29.5.3.1 [rand.req.genl] bullet (1.5):

> -1- Throughout this subclause 29.5 [rand], the effect of instantiating a template:

> 1. […]

> 2. (1.4) — that has a template type parameter named `RealType` is undefined unless the corresponding template argument is cv-unqualified and is one of `float`, `double`, or `long double`.

> 3. (1.5) — that has a template type parameter named `IntType` is undefined unless the corresponding template argument is cv-unqualified and is one of `short`, `int`, `long`, `long long`, `unsigned short`, `unsigned int`, `unsigned long`, or `unsigned long long`.

> 4. (1.6) — that has a template type parameter named `UIntType` is undefined unless the corresponding template argument is cv-unqualified and is one of `unsigned short`, `unsigned int`, `unsigned long`, or `unsigned long long`.

This is, in my opinion, a defect; such uses should either be rejected at compile time (made ill-formed), or permitted (as 2326 proposes.)

UB here has undesirable safety implications, because it's possible to write code that produces a random, or a seemingly random, sequence of `uint8_t` numbers on platform A, but an arbitrarily non-random sequence on platform B (e.g. all zeroes.)

If that sequence is then used in e.g. a cryptographic algorithm, bad things will happen on platform B, and the tests on platform A won't catch the issue.

[2024-05-26; Daniel comments]

I think that all violations of the bullets 29.5.3.1 [rand.req.genl] (1.4), (1.5), and (1.6) are missed opportunities of Mandates (That is: Make the program ill-formed), because they can be all checked (easily) at compile-time, regardless whether we agree on the question to support `int8_t`/`uint8_t` (Violations of (1.1), (1.2), and (1.3) still have to be remain undefined because of additional runtime requirements imposed).

Given that I also think that we should consider to either normatively extend all of (1.4), (1.5), and (1.6) to corresponding extended floating point types and extended (unsigned) integer types, or to the minimum make these extended types conditionally-supported with implementation-defined semantics (A word of power that is used at several places).

[2025-10-21; Reflector poll.]

Set priority to 3 after reflector poll.

"The only benefit of making it UB is allowing it as an extension."

"Does anybody actually want to support a BigInt as an extension? If not, non-integral types should be ill-formed and integral types not in the list should be conditionally-supported."

"I'd like extended arithmetic types to work in general, it's useful for writing generic math libraries to be able to generate random numbers of any arithmetic type that the implementation provides, including short integers and all floating-point types."

"Can have overflows here, differences between whether `char` is signed or unsigned lead to subtle bugs. Most users are better off picking `int` or `unsigned int` (which is what they get from integral promotions anyway) and just using values that are in range for their intended destination type."

[2026-03-31 P4037R1 was approved in Croydon. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4981.

> [Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A: As suggested in issue LWG 2326(i)

1. Modify 29.5.3.1 [rand.req.genl] as indicated:

   > -1- Throughout this subclause 29.5 [rand], the effect of instantiating a template:

   > 1. […]

   > 2. (1.4) — that has a template type parameter named `RealType` is undefined unless the corresponding template argument is cv-unqualified and is one of `float`, `double`, or `long double`.

   > 3. (1.5) — that has a template type parameter named `IntType` is undefined unless the corresponding template argument is cv-unqualified and is a standard integer type (6.9.2 [basic.fundamental])one of `short`, `int`, `long`, `long long`, `unsigned short`, `unsigned int`, `unsigned long`, or `unsigned long long`.

   > 4. (1.6) — that has a template type parameter named `UIntType` is undefined unless the corresponding template argument is cv-unqualified and is a standard unsigned integer type (6.9.2 [basic.fundamental])one of `unsigned short`, `unsigned int`, `unsigned long`, or `unsigned long long`.

Option B: Make ill-formed.

1. Modify 29.5.3.1 [rand.req.genl] as indicated:

   > -1- Throughout this subclause 29.5 [rand], the effect of instantiating a template:

   > 1. […]

   > 2. (1.4) — that has a template type parameter named `RealType` is undefined unless the corresponding template argument is cv-unqualified and is one of `float`, `double`, or `long double`.

   > 3. (1.5) — that has a template type parameter named `IntType` renders the program ill-formedis undefined unless the corresponding template argument is cv-unqualified and is one of `short`, `int`, `long`, `long long`, `unsigned short`, `unsigned int`, `unsigned long`, or `unsigned long long`.

   > 4. (1.6) — that has a template type parameter named `UIntType` renders the program ill-formedis undefined unless the corresponding template argument is cv-unqualified and is one of `unsigned short`, `unsigned int`, `unsigned long`, or `unsigned long long`.

### 4112(i). `has-arrow` should required `operator->()` to be `const`-qualified

Section: 25.5.2 [range.utility.helpers] Status: C++26 Submitter: Hewill Kang Opened: 2024-06-22 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The helper concept `has-arrow` in 25.5.2 [range.utility.helpers] does not require that `I::operator->()` be `const`-qualified, which is inconsistent with the constraints on `reverse_iterator` and `common_iterator`'s `operator->()` as the latter two both require the underlying iterator has `const operator->()` member.

We should enhance the semantics of `has-arrow` so that implicit expression variations (18.2 [concepts.equality]) prohibit silly games.

[St. Louis 2024-06-24; move to Ready]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 25.5.2 [range.utility.helpers] as indicated:

   >     […]

   >     template<class I>
   >       concept has-arrow =                                       // exposition only
   >         input_iterator<I> && (is_pointer_v<I> || requires(const I i) { i.operator->(); });

   >     […]

### 4113(i). Disallow `has_unique_object_representations<Incomplete[]>`

Section: 21.3.5.4 [meta.unary.prop] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-06-25 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

View all other issues in [meta.unary.prop].

View all issues with C++26 status.

Discussion:

The type completeness requirements for `has_unique_object_representations` say:

> `T` shall be a complete type, cv `void`, or an array of unknown bound.

This implies that the trait works for all arrays of unknown bound, whether the element type is complete or not. That seems to be incorrect, because `has_unique_object_representations_v<Incomplete[]>` is required to have the same result as `has_unique_object_representations_v<Incomplete>` which is ill-formed if `Incomplete` is an incomplete class type.

I think we need the element type to be complete to be able to give an answer. Alternatively, if the intended result for an array of unknown bound is false (maybe because there can be no objects of type `T[]`, or because we can't know that two objects declared as `extern T a[];` and `extern T b[];` have the same number of elements?) then the condition for the trait needs to be special-cased as `false` for arrays of unknown bound. The current spec is inconsistent, we can't allow arrays of unknown bound and apply the current rules to determine the trait's result.

[2024-08-02; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 21.3.5.4 [meta.unary.prop] as indicated:

   > Template | Condition | Preconditions
   > --- | --- | ---
   > … | … | …
   >     template<class T>
   >     struct has_unique_object_representations;
   > | For an array type `T`, the same result as `has_unique_object_representations_v<remove_all_extents_t<T>>`, otherwise see below. | `remove_all_extents_t<T>` `T` shall be a complete type, or cv `void`, or an array of unknown bound.

   > [Drafting note: We could use `remove_extent_t<T>` to remove just the first array dimension, because only that first one can have an unknown bound. The proposed resolution uses `remove_all_extents_t<T>` for consistency with the Condition column.]

### 4119(i). `generator::promise_type::yield_value(ranges::elements_of<R, Alloc>)`'s nested `generator` may be ill-formed

Section: 25.8.5 [coro.generator.promise] Status: C++26 Submitter: Hewill Kang Opened: 2024-07-11 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [coro.generator.promise].

View all issues with C++26 status.

Discussion:

The nested coroutine is specified to return `generator<yielded, ranges::range_value_t<R>, Alloc>` which can be problematic as the value type of `R` is really irrelevant to `yielded`, unnecessarily violating the `generator`'s Mandates (demo):

>     #include <generator>
>     #include <vector>

>     std::generator<std::span<int>> f() {
>       std::vector<int> v;
>       co_yield v; // ok
>     }

>     std::generator<std::span<int>> g() {
>       std::vector<std::vector<int>> v;
>       co_yield std::ranges::elements_of(v); // hard error
>     }

This proposed resolution is to change the second template parameter from `range_value_t<R>` to `void` since that type doesn't matter to us.

[2024-08-02; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4986.

1. Modify 25.8.5 [coro.generator.promise] as indicated:

   >     template<ranges::input_range R, class Alloc>
   >       requires convertible_to<ranges::range_reference_t<R>, yielded>
   >       auto yield_value(ranges::elements_of<R, Alloc> r);

   > > -13- Effects: Equivalent to:

   > > >     auto nested = [](allocator_arg_t, Alloc, ranges::iterator_t<R> i, ranges::sentinel_t<R> s)
   > > >       -> generator<yielded, ranges::range_value_t<R>void, Alloc> {
   > > >         for (; i != s; ++i) {
   > > >           co_yield static_cast<yielded>(*i);
   > > >         }
   > > >       };
   > > >     return yield_value(ranges::elements_of(nested(
   > > >       allocator_arg, r.allocator, ranges::begin(r.range), ranges::end(r.range))));

   > > […]

### 4121(i). `ranges::to` constructs associative containers via `c.emplace(c.end(), *it)`

Section: 25.5.7.1 [range.utility.conv.general] Status: WP Submitter: Hewill Kang Opened: 2024-07-16 Last modified: 2026-06-15

Priority: 2

View all issues with WP status.

Discussion:

When `ranges::to` constructs an associative container, if there is no range version constructor for `C` and the input range is not common range, `ranges::to` will dispatch to the bullet 25.5.7.2 [range.utility.conv.to] (2.1.4), which first default-constructs the container and emplaces the element through `c.emplace(c.end(), *it)`.

However, this is not the correct way to call `emplace()` on an associative container as it does not expect an iterator as the first argument, and since `map::emplace()`, for instance, is not constrained, which turns out a hard error because we are trying to make a `pair` with `{it, pair}`.

Given that libstdc++ currently does not implement the range constructor for associative containers, the following illustrates the issue:

>     #include <ranges>
>     #include <set>

>     auto s = std::views::iota(0)
>            | std::views::take(5)
>            | std::ranges::to<std::set>(); // hard error

The proposed resolution simply removes the `emplace()` branch. Although this means that we always use `insert()` to fill associative containers, such an impact seems negligible.

[2024-07-23; This was caused by LWG 4016(i).]

[2024-08-02; Reflector poll]

Set priority to 2 after reflector poll. "Would like to preserve the ability to use `emplace`. Tim suggested trying `emplace_hint` first, then `emplace`." "I tried it, it gets very verbose, because we might also want to try `insert(*it)` instead of `insert(c.end(), *it)` if `emplace(*it)` is not valid for associative containers, because `c.end()` might not be a good hint." "It might be suboptimal, but it still works."

Previous resolution [SUPERSEDED]

This wording is relative to N4986.

1. Modify 25.5.7.1 [range.utility.conv.general] as indicated:

   > -4- Let `container-appendable` be defined as follows:

   > >     template<class Container, class Ref>
   > >     constexpr bool container-appendable =         // exposition only
   > >       requires(Container& c, Ref&& ref) {
   > >                requires (requires { c.emplace_back(std::forward<Ref>(ref)); } ||
   > >                          requires { c.push_back(std::forward<Ref>(ref)); } ||
   > >                          requires { c.emplace(c.end(), std::forward<Ref>(ref)); } ||
   > >                          requires { c.insert(c.end(), std::forward<Ref>(ref)); });
   > >     };

   > -5- Let `container-append` be defined as follows:

   > >     template<class Container>
   > >     constexpr auto container-append(Container& c) {     // exposition only
   > >       return [&c]<class Ref>(Ref&& ref) {
   > >         if constexpr (requires { c.emplace_back(declval<Ref>()); })
   > >           c.emplace_back(std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.push_back(declval<Ref>()); })
   > >           c.push_back(std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.emplace(c.end(), declval<Ref>()); })
   > >           c.emplace(c.end(), std::forward<Ref>(ref));
   > >         else
   > >           c.insert(c.end(), std::forward<Ref>(ref));
   > >       };
   > >     };

[2025-03-13; Jonathan provides improved wording for Tim's suggestion]

It's true that for some cases it might be optimal to use `c.emplace(ref)` instead of `c.emplace_hint(c.end(), ref)` but I don't think I care. A bad hint is not expensive, it's just an extra comparison then the hint is ignored. And this code path isn't going to be used for `std::set` or `std::map`, only for user-defined associative containers that don't have a `from_range_t` constructor or a `C(Iter,Sent)` constructor. I think just fixing the original issue is all we need, rather than trying to handle every possible way to insert elements.

This is a simpler, portable reproducer that doesn't depend on the current implementation status of `std::set` in libstdc++:

>     #include <ranges>
>     #include <set>
>     struct Set : std::set<int> {
>       Set() { }; // No other constructors
>     };
>     int main() {
>       int a[1];
>       auto okay = std::ranges::to<std::set<int>>(a);
>       auto ohno = std::ranges::to<Set>(a);
>     }

Previous resolution [SUPERSEDED]

This wording is relative to N5001.

1. Modify 25.5.7.1 [range.utility.conv.general] as indicated:

   > -4- Let `container-appendable` be defined as follows:

   > >     template<class Container, class Ref>
   > >     constexpr bool container-appendable =         // exposition only
   > >       requires(Container& c, Ref&& ref) {
   > >                requires (requires { c.emplace_back(std::forward<Ref>(ref)); } ||
   > >                          requires { c.push_back(std::forward<Ref>(ref)); } ||
   > >                          requires { c.emplace_hint(c.end(), std::forward<Ref>(ref)); } ||
   > >                          requires { c.emplace(c.end(), std::forward<Ref>(ref)); } ||
   > >                          requires { c.insert(c.end(), std::forward<Ref>(ref)); });
   > >     };

   > -5- Let `container-append` be defined as follows:

   > >     template<class Container>
   > >     constexpr auto container-append(Container& c) {     // exposition only
   > >       return [&c]<class Ref>(Ref&& ref) {
   > >         if constexpr (requires { c.emplace_back(declval<Ref>()); })
   > >           c.emplace_back(std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.push_back(declval<Ref>()); })
   > >           c.push_back(std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.emplace_hint(c.end(), declval<Ref>()); })
   > >           c.emplace_hint(c.end(), std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.emplace(c.end(), declval<Ref>()); })
   > >           c.emplace(c.end(), std::forward<Ref>(ref));
   > >         else
   > >           c.insert(c.end(), std::forward<Ref>(ref));
   > >       };
   > >     };

[2026-05-22; Jonathan provides new wording]

Hewill noted that all the standard sequence containers that could use `c.emplace(c.end(), ref)` would already have used the `emplace_back` branch, so there's no point using `c.emplace`. Just change that branch to use `emplace_hint` for associative containers.

[2026-05-29; Reflector poll.]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5046.

1. Modify 25.5.7.1 [range.utility.conv.general] as indicated:

   > -4- Let `container-appendable` be defined as follows:

   > >     template<class Container, class Ref>
   > >     constexpr bool container-appendable =         // exposition only
   > >       requires(Container& c, Ref&& ref) {
   > >                requires (requires { c.emplace_back(std::forward<Ref>(ref)); } ||
   > >                          requires { c.push_back(std::forward<Ref>(ref)); } ||
   > >                          requires { c.emplace_hint(c.end(), std::forward<Ref>(ref)); } ||
   > >                          requires { c.insert(c.end(), std::forward<Ref>(ref)); });
   > >     };

   > -5- Let `container-append` be defined as follows:

   > >     template<class Container>
   > >     constexpr auto container-append(Container& c) {     // exposition only
   > >       return [&c]<class Ref>(Ref&& ref) {
   > >         if constexpr (requires { c.emplace_back(declval<Ref>()); })
   > >           c.emplace_back(std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.push_back(declval<Ref>()); })
   > >           c.push_back(std::forward<Ref>(ref));
   > >         else if constexpr (requires { c.emplace_hint(c.end(), declval<Ref>()); })
   > >           c.emplace_hint(c.end(), std::forward<Ref>(ref));
   > >         else
   > >           c.insert(c.end(), std::forward<Ref>(ref));
   > >       };
   > >     };

### 4122(i). Ill-formed `operator<=>` can cause hard error when instantiating `std::inplace_vector`

Section: 23.3.16.1 [inplace.vector.overview] Status: C++26 Submitter: Jiang An Opened: 2024-07-20 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

This is almost the same problem as LWG 4071(i) except that it happens to `std::inplace_vector`. As the `operator<=>` overload for `std::inplace_vector` is a non-template function whose return type (`synth-three-way-result<T>`) is not deduced, when the return type is ill-formed, hard error occurs in the instantiation of the enclosing `std::inplace_vector<T, N>`.

[2024-08-02; Reflector poll]

Set priority to 2 after reflector poll.

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4986.

1. Modify 23.3.16.1 [inplace.vector.overview], class template `std::inplace_vector` synopsis, as indicated:

   >     namespace std {
   >       template<class T, size_t N>
   >       class inplace_vector {
   >       public:
   >         […]
   >         constexpr friend bool operator==(const inplace_vector& x,
   >                                          const inplace_vector& y);
   >         constexpr friend synth-three-way-result<T>auto
   >           operator<=>(const inplace_vector& x, const inplace_vector& y);
   >             requires requires (const T t) { synth-three-way(t, t); }
   >           {
   >             return lexicographical_compare_three_way(x.begin(), x.end(), y.begin(), y.end(),
   >                                                      synth-three-way);
   >           }
   >         constexpr friend void swap(inplace_vector& x, inplace_vector& y)
   >           noexcept(N == 0 || (is_nothrow_swappable_v<T> &&
   >                               is_nothrow_move_constructible_v<T>))
   >         { x.swap(y); }
   >       };
   >     };

### 4124(i). Cannot format `zoned_time` with resolution coarser than seconds

Section: 30.12 [time.format] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-07-26 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [time.format].

View all other issues in [time.format].

View all issues with C++26 status.

Discussion:

The `std::formatter<std::chrono::zoned_time<Duration, TimeZonePtr>>` specialization calls `tp.get_local_time()` for the object it passes to its base class' `format` function. But `get_local_time()` does not return a `local_time<Duration>`, it returns `local_time<common_type_t<Duration, seconds>>`. The base class' `format` function is only defined for `local_time<Duration>`. That means this is ill-formed, even though the static assert passes:

    using namespace std::chrono;
      static_assert( std::formattable<zoned_time<minutes>, char> );
      zoned_time<minutes> zt;
      (void) std::format("{}", zt); // error: cannot convert local_time<seconds> to local_time<minutes>

Additionally, it's not specified what output you should get for:

    std::format("{}", local_time_format(zt.get_local_time()));

30.12 [time.format] p7 says it's formatted as if by streaming to an `ostringstream`, but there is no `operator<<` for `local-time-format-t`. Presumably it should give the same result as `operator<<` for a `zoned_time`, i.e. `"{:L%F %T %Z}"` with padding adjustments etc.

The proposed resolution below has been implemented in libstdc++.

[2024-08-02; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4986.

1. Modify 30.12 [time.format] as indicated:

   >     template<classDuration, class charT>
   >       struct formatter<chrono::local-time-format-t<Duration>, charT>;

   > -17- Let `f` be a `locale-time-format-t<Duration>` object passed to `formatter::format`.

   > -18- Remarks: If the chrono-specs is omitted, the result is equivalent to using `%F %T %Z` as the chrono-specs. If `%Z` is used, it is replaced with `*f.abbrev` if `f.abbrev` is not a null pointer value. If `%Z` is used and `f.abbrev` is a null pointer value, an exception of type `format_error` is thrown. If `%z` (or a modified variant of `%z`) is used, it is formatted with the value of `*f.offset_sec` if `f.offset_sec` is not a null pointer value. If `%z` (or a modified variant of `%z`) is used and `f.offset_sec` is a null pointer value, then an exception of type `format_error` is thrown.

   >       template<class Duration, class TimeZonePtr, class charT>
   >       struct formatter<chrono::zoned_time<Duration, TimeZonePtr>, charT>
   >           : formatter<chrono::local-time-format-t<common_type_t<Duration, seconds>>, charT> {
   >         template<class FormatContext>
   >           typename FormatContext::iterator
   >           format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const;
   >       };

   >     template<class FormatContext>
   >       typename FormatContext::iterator
   >         format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const;

   > -19- Effects: Equivalent to:

   > >     sys_info info = tp.get_info();
   > >     return formatter<chrono::local-time-format-t<common_type_t<Duration, seconds>>, charT>::
   > >              format({tp.get_local_time(), &info.abbrev, &info.offset}, ctx);

### 4125(i). `move_iterator`'s default constructor should be constrained

Section: 24.5.4.2 [move.iterator], 24.5.4.4 [move.iter.cons] Status: WP Submitter: Hewill Kang Opened: 2024-07-22 Last modified: 2026-06-15

Priority: 3

View all other issues in [move.iterator].

View all issues with WP status.

Discussion:

Although it is unclear why P2325 did not apply to `move_iterator`, there is implementation divergence among the current libraries (demo):

>     #include <istream>
>     #include <iterator>
>     #include <ranges>

>     using R = std::ranges::istream_view<int>;
>     using I = std::ranges::iterator_t<R>;
>     using MI = std::move_iterator<I>;
>     static_assert(std::default_initializable<MI>); // libstdc++ passes, libc++ fails

As libc++ additionally requires that its default constructors satisfy `is_constructible_v<Iterator>`.

Although this is not current standard-conforming behavior, such constraint does make sense since `move_iterator` only requires the underlying iterator to be `input_iterator` which may not be `default_initializable`.

[2024-08-02; Reflector poll]

Set priority to 3 after reflector poll. Six P0 (tentatively ready) votes but one for NAD: "design change, needs paper. Not constraining this seems to be intended by P2325R3. Even if we want to constrain it, why `default_initializable` rather than just `is_constructible_v`?" Author of P2325R3 had no such intent and voted P0. Issue submitter used `default_initializable` to align with `forward_iterator` requirements, but would not object to implementers using `is_constructible_v` for backported DRs.

[2026-05-29 LWG telecon; status: New → Ready]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4986.

1. Modify 24.5.4.2 [move.iterator] as indicated:

   >     namespace std {
   >       template<class Iterator>
   >       class move_iterator {
   >       public:
   >         […]
   >         constexpr move_iterator() requires default_initializable<Iterator> = default;
   >         […]
   >       private:
   >         Iterator current = Iterator();   // exposition only
   >       };
   >     }

2. Modify 24.5.4.4 [move.iter.cons] as indicated:

   >     constexpr move_iterator();

   > > -1- Effects: Value-initializes `current`.

### 4126(i). Some feature-test macros for fully freestanding features are not yet marked freestanding

Section: 17.3.2 [version.syn] Status: C++26 Submitter: Jiang An Opened: 2024-07-24 Last modified: 2026-06-09

Priority: 2

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++26 status.

Discussion:

Currently (N4986), it's a bit weird in 17.3.2 [version.syn] that some feature-test macros are not marked freestanding, despite the indicated features being fully freestanding. The freestanding status seems sometimes implicitly covered by "also in" headers that are mostly or all freestanding, but sometimes not.

I think it's more consistent to ensure feature-test macros for fully freestanding features are also freestanding.

[2024-08-02; Reflector poll]

Set priority to 2 and set status to Tentatively Ready after seven votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4986.

1. Modify 17.3.2 [version.syn] as indicated:

   > [Drafting note: `<charconv>` is not fully freestanding, but all functions made `constexpr` by P2291R3 are furtherly made freestanding by P2338R4. ]

   >     […]
   >     #define __cpp_lib_common_reference                  202302L // freestanding, also in <type_traits>
   >     #define __cpp_lib_common_reference_wrapper          202302L // freestanding, also in <functional>
   >     […]
   >     #define __cpp_lib_constexpr_charconv                202207L // freestanding, also in <charconv>
   >     […]
   >     #define __cpp_lib_coroutine                         201902L // freestanding, also in <coroutine>
   >     […]
   >     #define __cpp_lib_is_implicit_lifetime              202302L // freestanding, also in <type_traits>
   >     […]
   >     #define __cpp_lib_is_virtual_base_of                202406L // freestanding, also in <type_traits>
   >     […]
   >     #define __cpp_lib_is_within_lifetime                202306L // freestanding, also in <type_traits>
   >     […]
   >     #define __cpp_lib_mdspan                            202406L // freestanding, also in <mdspan>
   >     […]
   >     #define __cpp_lib_ratio                             202306L // freestanding, also in <ratio>
   >     […]
   >     #define __cpp_lib_span_initializer_list             202311L // freestanding, also in <span>
   >     […]
   >     #define __cpp_lib_submdspan                         202403L // freestanding, also in <mdspan>
   >     […]
   >     #define __cpp_lib_to_array                          201907L // freestanding, also in <array>
   >     […]

### 4130(i). Preconditions for `std::launder` might be overly strict

Section: 17.6.5 [ptr.launder] Status: C++26 Submitter: Jiang An Opened: 2024-07-30 Last modified: 2026-06-09

Priority: 3

View all other issues in [ptr.launder].

View all issues with C++26 status.

Discussion:

From issue cplusplus/draft#4553 which is considered non-editorial.

Currently, `std::launder` has a precondition that the pointed to object must be within its lifetime. However, per the example added by P0593R6, it's probably intended that the result of `std::launder` should be allowed to point to an array element subobject whose lifetime has not started yet.

[2024-10-02; Reflector poll]

Set priority to 3 after reflector poll. Needs review by Core.

[Wrocław 2024-11-18; approved by Core]

[2026-02-13; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4986.

1. Modify 17.6.5 [ptr.launder] as indicated:

   >     template<class T> [[nodiscard]] constexpr T* launder(T* p) noexcept;

   > > -1- Mandates: `!is_function_v<T> && !is_void_v<T>` is `true`.

   > > -2- Preconditions: `p` represents the address A of a byte in memory. An object X that is within its lifetime (6.8.4 [basic.life]) and whose type is similar (7.3.6 [conv.qual]) to `T` is located at the address A, and is either within its lifetime (6.8.4 [basic.life]) or is an array element subobject whose containing array object is within its lifetime. All bytes of storage that would be reachable through (6.9.4 [basic.compound]) the result are reachable through `p`.

   > > -3- Returns: A value of type `T*` that points to X.

   > > […]

### 4131(i). Including `<optional>` doesn't provide `std::begin/end`

Section: 24.7 [iterator.range] Status: Resolved Submitter: Hewill Kang Opened: 2024-08-02 Last modified: 2025-11-11

Priority: 3

View all other issues in [iterator.range].

View all issues with Resolved status.

Discussion:

Since `optional` now provides `begin/end` members, it is reasonable to ensure the validity of `std::begin/end` after including `<optional>`.

[2024-08-21; Reflector poll]

Set priority to 3 after reflector poll.

"I would like to hear opinion of P3168 authors, or have the change discussed as a part of P3016.

[2025-11-11; Resolved by P3016R6, approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4986.

1. Modify 24.7 [iterator.range] as indicated:

   > -1- In addition to being available via inclusion of the `<iterator>` header, the function templates in 24.7 [iterator.range] are available when any of the following headers are included: `<array>` (23.3.2 [array.syn]), `<deque>` (23.3.4 [deque.syn]), `<flat_map>` (23.6.7 [flat.map.syn]), `<flat_set>` (23.6.10 [flat.set.syn]), `<forward_list>` (23.3.6 [forward.list.syn]), `<inplace_vector>` (23.3.15 [inplace.vector.syn]), `<list>` (23.3.10 [list.syn]), `<map>` (23.4.2 [associative.map.syn]), `<optional>` (22.5.2 [optional.syn]), `<regex>` (28.6.3 [re.syn]), `<set>` (23.4.5 [associative.set.syn]), `<span>` (23.7.2.1 [span.syn]), `<string>` (27.4.2 [string.syn]), `<string_view>` (27.3.2 [string.view.synop]), `<unordered_map>` (23.5.2 [unord.map.syn]), `<unordered_set>` (23.5.5 [unord.set.syn]), and `<vector>` (23.3.12 [vector.syn]).

### 4133(i). `awaitable-receiver`'s members are potentially throwing

Section: 33.13.1 [exec.as.awaitable] Status: C++26 Submitter: Eric Niebler Opened: 2024-07-30 Last modified: 2026-06-09

Priority: 1

View all other issues in [exec.as.awaitable].

View all issues with C++26 status.

Discussion:

The specification of `awaitable-receiver` in 33.13.1 [exec.as.awaitable]/p4 as of N4988 is not taking into consideration the fact that the `resume()` and `promise()` member functions on `coroutine_handle<P>` are not marked `noexcept`. `awaitable-receiver`'s member functions must all be `noexcept`, but they are specified as being "equivalent to" statement that call `resume()` and `promise()` outside of `try`/`catch` blocks.

[2024-08-21; Reflector poll]

Set priority to 1 after reflector poll.

`promise()` can probably be Throws: Nothing (along with a bunch of other coroutine_handle members), but `resume()` certainly can throw. Also AS-EXCEPT-PTR can throw for the `error_code` case (that might be worth a separate issue though).

[2026-03-27; Tim adds wording following LWG discussion]

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 33.13.1 [exec.as.awaitable] as indicated:

   > -4- Let `rcvr` be an rvalue expression of type `awaitable-receiver`, let `crcvr` be a const lvalue that refers to `rcvr`, let `vs` be a pack of subexpressions, and let `err` be an expression of type `Err`. Let `MAKE-NOEXCEPT(expr)` for some subexpression `expr` be expression-equivalent to `[&] noexcept -> decltype(auto) { return (expr); }()`. Then:

   > 1. (4.1) — If `constructible_from<result-type, decltype((vs))...>` is satisfied, the expression `set_value(rcvr, vs...)` is equivalent to:

   >    >     try {
   >    >       rcvr.result-ptr->template emplace<1>(vs...);
   >    >     } catch(...) {
   >    >       rcvr.result-ptr->template emplace<2>(current_exception());
   >    >     }
   >    >     MAKE-NOEXCEPT(rcvr.continuation.resume());

   >    Otherwise, `set_value(rcvr, vs...)` is ill-formed.

   > 2. (4.2) — The expression `set_error(rcvr, err)` is equivalent to:

   >    >     try {
   >    >       rcvr.result-ptr->template emplace<2>(AS-EXCEPT-PTR(err));
   >    >     } catch(...) {
   >    >       rcvr.result-ptr->template emplace<2>(current_exception());
   >    >     }
   >    >     MAKE-NOEXCEPT(rcvr.continuation.resume());

   > 3. (4.3) — The expression `set_stopped(rcvr)` is equivalent to:

   >    >     MAKE-NOEXCEPT(static_cast<coroutine_handle<>>(rcvr.continuation.promise().unhandled_stopped()).resume());

   > 4. (4.4) — For any expression `tag` whose type satisfies `forwarding-query` and for any pack of subexpressions `as`, `get_env(crcvr).query(tag, as...)` is expression-equivalent to:

   >    >     tag(get_env(as_const(MAKE-NOEXCEPT(crcvr.continuation.promise()))), as...)

### 4134(i). Issue with Philox algorithm specification

Section: 29.5.4.5 [rand.eng.philox] Status: C++26 Submitter: Ilya A. Burylov Opened: 2024-08-06 Last modified: 2026-06-09

Priority: 1

View all other issues in [rand.eng.philox].

View all issues with C++26 status.

Discussion:

The P2075R6 proposal introduced the Philox engine and described the algorithm closely following the original paper (further Random123sc11).

Matt Stephanson implemented P2075R6 and the 10000'th number did not match. Further investigation revealed several places in Random123sc11 algorithm description, which either deviate from the reference implementation written by Random123sc11 authors or loosely defined, which opens the way for different interpretations.

All major implementations of the Philox algorithm (NumPy, Intel MKL, Nvidia cuRAND, etc.) match the reference implementation written by Random123sc11 authors and we propose to align wording with that.

The rationale of proposed changes:

1. Random123sc11 refers to the permutation step as "the inputs are permuted using the Threefish N-word P-box", thus the P2075R6 permutation table ([tab:rand.eng.philox.f]) is taken from Threefish algorithm. But the permutation for N=4 in this table does not match the reference implementations. It's worth noting that while Random123sc11 described the Philox algorithm for N=8 and N=16, there are no known reference implementations of it either provided by authors or implemented by other libraries. We proposed to drop N=8 and N=16 for now and update the permutation indices for N=4 to match the reference implementation. Note: the proposed resolution allows extending N > 4 cases in the future.

2. The original paper describes the S-box update for X values in terms of `L'` and `R'` but does not clarify their ordering as part of the counter. In order to match Random123sc11 reference implementation the ordering should be swapped.

3. Philox alias templates should be updated, because the current ordering of constants matches the specific optimization in the reference Random123sc11 implementation and not the generic algorithm description.

All proposed modifications below are confirmed by:

- Philox algorithm coauthor Mark Moraes who is planning to publish errata for the original Random123sc11 Philox paper.

- Matt Stephanson, who originally found the mismatch in P2075R6

- The updated reference implementation.

[2024-08-21; Reflector poll]

Set priority to 1 after reflector poll.

[2024-08-21; Move to Ready at LWG telecon]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 29.5.4.5 [rand.eng.philox], [tab:rand.eng.philox.f] as indicated (This effectively reduces 16 data columns to 4 data columns and 4 data rows to 2 data rows):

   > Table 101 — Values for the word permutation fn(j) [tab:rand.eng.philox.f]
   > fn(j) | `j`
   > --- | ---
   > 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15
   > n
   > 2 | 0 | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |
   > 4 | 20 | 13 | 02 | 31 |  |  |  |  |  |  |  |  |  |  |  |
   > 8 | 2 | 1 | 4 | 7 | 6 | 5 | 0 | 3 |  |  |  |  |  |  |  |
   > 16 | 0 | 9 | 2 | 13 | 6 | 11 | 4 | 15 | 10 | 7 | 12 | 3 | 14 | 5 | 8 | 1

2. Modify 29.5.4.5 [rand.eng.philox] as indicated:

   > -4- […]

   > 1. (4.1) — […]

   > 2. (4.2) — The following computations are applied to the elements of the V sequence:

   >    > X 2 k+0 = mulhimullo(V 2 k+1,M k,w) xor key k q xor V 2 k+1

   >    > X 2 k+1 = mullomulhi(V 2 k+1,M k,w) xor key k q xor V 2 k

   > -5- […]

   > -6- Mandates:

   > 1. (6.1) — […]

   > 2. (6.2) — `n == 2 || n == 4 || n == 8 || n == 16` is `true`, and

   > 3. (6.3) — […]

   > 4. (6.4) — […]

3. Modify 29.5.6 [rand.predef] as indicated:

   >     using philox4x32 =
   >           philox_engine<uint_fast32_t, 32, 4, 10,
   >            0xCD9E8D570xD2511F53, 0x9E3779B9, 0xD2511F530xCD9E8D57, 0xBB67AE85>;

   > > -11- Required behavior: The 10000th consecutive invocation a default-constructed object of type `philox4x32` produces the value `1955073260`.

   >     using philox4x64 =
   >           philox_engine<uint_fast64_t, 64, 4, 10,
   >            0xCA5A8263951211570xD2E7470EE14C6C93, 0x9E3779B97F4A7C15, 0xD2E7470EE14C6C930xCA5A826395121157, 0xBB67AE8584CAA73B>;

   > > -12- Required behavior: The 10000th consecutive invocation a default-constructed object of type `philox4x64` produces the value 3409172418970261260.

### 4135(i). The helper lambda of `std::erase` for `list` should specify return type as `bool`

Section: 23.3.7.7 [forward.list.erasure], 23.3.11.6 [list.erasure] Status: C++26 Submitter: Hewill Kang Opened: 2024-08-07 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

`std::erase` for `list` is specified to return `erase_if(c, [&](auto& elem) { return elem == value; })`. However, the template parameter `Predicate` of `erase_if` only requires that the type of `decltype(pred(...))` satisfies `boolean-testable`, i.e., the return type of `elem == value` is not necessarily `bool`.

This means it's worth explicitly specifying the lambda's return type as `bool` to avoid some pedantic cases (demo):

>     #include <list>

>     struct Bool {
>       Bool(const Bool&) = delete;
>       operator bool() const;
>     };

>     struct Int {
>       Bool& operator==(Int) const;
>     };

>     int main() {
>       std::list<Int> l;
>       std::erase(l, Int{}); // unnecessary hard error
>     }

[2024-08-21; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 23.3.7.7 [forward.list.erasure] as indicated:

   >     template<class T, class Allocator, class U = T>
   >       typename forward_list<T, Allocator>::size_type
   >         erase(forward_list<T, Allocator>& c, const U& value);

   > > -1- Effects: Equivalent to: `return erase_if(c, [&](const auto& elem) -> bool { return elem == value; });`

2. Modify 23.3.11.6 [list.erasure] as indicated:

   >     template<class T, class Allocator, class U = T>
   >       typename list<T, Allocator>::size_type
   >         erase(list<T, Allocator>& c, const U& value);

   > > -1- Effects: Equivalent to: `return erase_if(c, [&](const auto& elem) -> bool { return elem == value; });`

### 4136(i). Specify behavior of [linalg] Hermitian algorithms on diagonal with nonzero imaginary part

Section: 29.9.3 [linalg.general] Status: C++26 Submitter: Mark Hoemmen Opened: 2024-08-09 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 170-278

Mathematically, the diagonal elements of a Hermitian matrix must all have zero imaginary part (if they are complex numbers). 29.9.3 [linalg.general] paragraphs 3 and 4 govern the behavior of [linalg] functions that operate on "symmetric," "Hermitian," or "triangular" matrices. All such functions only access the specified triangle (lower or upper) of the matrix. Whatever is in the other triangle of the matrix doesn't matter; it's not even accessed. That gives well-defined behavior for "symmetric" and "triangular" matrices. However, both triangles include the diagonal. What should the "Hermitian" functions do if they encounter a diagonal element with nonzero imaginary part?

The current wording says that both the operation performed and the matrix itself are Hermitian, but does not clarify what happens if the latter is not true. For example, 29.9.14.3 [linalg.algs.blas2.hemv] says that `hermitian_matrix_vector_product` performs a

> Hermitian matrix-vector product, taking into account the `Triangle` parameter that applies to the Hermitian matrix `A` (29.9.3 [linalg.general]).

Language like this appears in the specifications of all the functions whose names start with `hermitian`. The implication is that if the diagonal has an element with nonzero imaginary part, then the matrix is not Hermitian and therefore a precondition of the function has been violated. The result is undefined behavior.

We can get rid of this undefined behavior by defining what happens in this case. It turns out that Chapter 2 of the BLAS Standard already does this: It says that the Hermitian algorithms do not access the imaginary parts of diagonal elements. The reference Fortran BLAS implementations of `CHEMV` (single-precision Complex HErmitian Matrix-Vector product) and `ZHEMV` (double-precision complex HErmitian Matrix-Vector product) follow the BLAS Standard. `CHEMV` uses `real(A(j,j))` and `ZHEMV` uses `dble(A(j,j))`, which means that the BLAS routines only access the real part of each diagonal element.

The clear design intent of P1673R13 was to imitate the behavior of the BLAS Standard and reference BLAS unless otherwise specified. Thus, we propose to specify this behavior in the wording; we do not think this requires LEWG re-review.

In my view, it's fine to retain the existing wording like that in 29.9.14.3 [linalg.algs.blas2.hemv], that refers to a "Hermitian matrix". The matrix defined by the revised [linalg.general] instructions is unambiguously a Hermitian matrix. Just like the "other triangle" of a symmetric or triangular matrix does not affect the behavior of [linalg] symmetric resp. triangular algorithms, the wording fix here will ensure that any imaginary parts of diagonal elements will not affect the behavior of [linalg] Hermitian algorithms.

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 29.9.3 [linalg.general] as indicated:

   > -4- For any function `F` that takes a parameter named `t`, `t` applies to accesses done through the parameter preceding `t` in the parameter list of `F`. Let `m` be such an access-modified function parameter. `F` will only access the triangle of `m` specified by `t`. For accesses of diagonal elements `m[i, i]`, `F` will use the value `real-if-needed(m[i, i])` if the name of `F` starts with `hermitian`. For accesses `m[i, j]` outside the triangle specified by `t`, `F` will use the value […]

### 4137(i). Fix Mandates, Preconditions, and Complexity elements of [linalg] algorithms

Section: 29.9.14 [linalg.algs.blas2], 29.9.15 [linalg.algs.blas3] Status: C++26 Submitter: Mark Hoemmen Opened: 2024-08-08 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 172-275

As pointed out by Raffaele Solcà (CSCS Swiss National Supercomputing Centre), some of the Mandates, Preconditions, and Complexity elements of some BLAS 2 and BLAS 3 algorithms in [linalg] are incorrect.

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

1. Modify 29.9.14.1 [linalg.algs.blas2.gemv] as indicated:

   > [Drafting note: This change is needed because the matrix `A` does not need to be square. `x.extents(0)` must equal `A.extents(1)`, while `y.extents(0)` must equal `A.extents(0)`.]

   > -3- Mandates:

   > 1. (3.1) — `possibly-multipliable<decltype(A), decltype(x), decltype(y)>()` is `true`, and

   > 2. (3.2) — `possibly-addable<decltype(yx), decltype(y), decltype(z)>()` is `true` for those overloads that take a `z` parameter.

   > -4- Preconditions:

   > 1. (4.1) — `multipliable(A, x, y)` is `true`, and

   > 2. (4.2) — `addable(yx, y, z)` is `true` for those overloads that take a `z` parameter.

   > -5- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

2. Modify 29.9.14.2 [linalg.algs.blas2.symv] as indicated:

   > -3- Mandates:

   > 1. (3.1) — […]

   > 2. (3.2) — […]

   > 3. (3.3) — `possibly-multipliable<decltype(A), decltype(x), decltype(y)>()` is `true`, and

   > 4. (3.4) — `possibly-addable<decltype(yx), decltype(y), decltype(z)>()` is `true` for those overloads that take a `z` parameter.

   > -4- Preconditions:

   > 1. (4.1) — `A.extent(0)` equals `A.extent(1)`,

   > 2. (4.2) — `multipliable(A, x, y)` is `true`, and

   > 3. (4.3) — `addable(yx, y, z)` is `true` for those overloads that take a `z` parameter.

   > -5- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

3. Modify 29.9.14.3 [linalg.algs.blas2.hemv] as indicated:

   > -3- Mandates:

   > 1. (3.1) — […]

   > 2. (3.2) — […]

   > 3. (3.3) — `possibly-multipliable<decltype(A), decltype(x), decltype(y)>()` is `true`, and

   > 4. (3.4) — `possibly-addable<decltype(yx), decltype(y), decltype(z)>()` is `true` for those overloads that take a `z` parameter.

   > -4- Preconditions:

   > 1. (4.1) — `A.extent(0)` equals `A.extent(1)`,

   > 2. (4.2) — `multipliable(A, x, y)` is `true`, and

   > 3. (4.3) — `addable(yx, y, z)` is `true` for those overloads that take a `z` parameter.

   > -5- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

4. Modify 29.9.14.4 [linalg.algs.blas2.trmv] as indicated:

   > [Drafting note: The extents compatibility conditions are expressed differently than in the above matrix-vector multiply sections, perhaps more for consistency with the TRSV section below. They look correct here. The original Complexity elements adjusted below are technically correct, since A is square, but changing this would improve consistency with 29.9.14.1 [linalg.algs.blas2.gemv]]

   >     template<in-matrix InMat, class Triangle, class DiagonalStorage, in-vector InVec,
   >              out-vector OutVec>
   >       void triangular_matrix_vector_product(InMat A, Triangle t, DiagonalStorage d, InVec x, OutVec y);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat, class Triangle, class DiagonalStorage, in-vector InVec,
   >              out-vector OutVec>
   >       void triangular_matrix_vector_product(ExecutionPolicy&& exec,
   >                                             InMat A, Triangle t, DiagonalStorage d, InVec x, OutVec y);

   > > -5- […]

   > > -6- Effects: Computes y = A x.

   > > -5- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

   >     template<in-matrix InMat, class Triangle, class DiagonalStorage, inout-vector InOutVec>
   >       void triangular_matrix_vector_product(InMat A, Triangle t, DiagonalStorage d, InOutVec y);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat, class Triangle, class DiagonalStorage, inout-vector InOutVec>
   >       void triangular_matrix_vector_product(ExecutionPolicy&& exec,
   >                                             InMat A, Triangle t, DiagonalStorage d, InOutVec y);

   > > -8- […]

   > > -9- Effects: […]

   > > -10- Complexity: 𝒪(`Ay.extent(0)` × `yA.extent(01)`).

   >     template<in-matrix InMat, class Triangle, class DiagonalStorage,
   >              in-vector InVec1, in-vector InVec2, out-vector OutVec>
   >       void triangular_matrix_vector_product(InMat A, Triangle t, DiagonalStorage d,
   >                                             InVec1 x, InVec2 y, OutVec z);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat, class Triangle, class DiagonalStorage,
   >              in-vector InVec1, in-vector InVec2, out-vector OutVec>
   >       void triangular_matrix_vector_product(ExecutionPolicy&& exec,
   >                                             InMat A, Triangle t, DiagonalStorage d,
   >                                             InVec1 x, InVec2 y, OutVec z);

   > > -11- […]

   > > -12- Effects: Computes z = y + A x.

   > > -13- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

5. Modify 29.9.15.4 [linalg.algs.blas3.rankk] as indicated:

   > [Drafting note: P3371R0, to be submitted in the August 15 mailing for LEWG review, contains the same wording changes to 29.9.15.4 [linalg.algs.blas3.rankk] and 29.9.15.5 [linalg.algs.blas3.rank2k] as proposed here, with additional changes corresponding to that proposal. Please apply this LWG issue's changes first, before P3371 merges]

   > -3- Mandates:

   > 1. (3.1) — If `InOutMat` has `layout_blas_packed` layout, then the layout's `Triangle` template argument has the same type as the function's `Triangle` template argument; and

   > 2. (3.2) — `possibly-multipliable<decltype(A), decltype(transposed(A)), decltype(C)> compatible-static-extents<decltype(A), decltype(A)>(0, 1)` is `true`.;

   > 3. (3.3) — `compatible-static-extents<decltype(C), decltype(C)>(0, 1)` is `true`; and

   > 4. (3.4) — `compatible-static-extents<decltype(A), decltype(C)>(0, 0)` is `true`.

   > -4- Preconditions: `multipliable(A, transposed(A), C)` is `true`.

   > 1. (4.1) — `A.extent(0)` equals `A.extent(1)`,

   > 2. (4.2) — `C.extent(0)` equals `C.extent(1)`, and

   > 3. (4.3) — `A.extent(0)` equals `C.extent(0)`.

   > -5- Complexity: 𝒪(`A.extent(0)` × `A.extent(1)` × `AC.extent(0)`).

6. Modify 29.9.15.5 [linalg.algs.blas3.rank2k] as indicated:

   > -3- Mandates:

   > 1. (3.1) — If `InOutMat` has `layout_blas_packed` layout, then the layout's `Triangle` template argument has the same type as the function's `Triangle` template argument;

   > 2. (3.2) — `possibly-multipliable<decltype(A), decltype(transposed(B)), decltype(C)>() possibly-addable<decltype(A), decltype(B), decltype(C)>()` is `true`; and

   > 3. (3.3) — `possibly-multipliable<decltype(B), decltype(transposed(A)), decltype(C)>(0, 1) compatible-static-extents<decltype(A), decltype(A)>(0, 1)` is `true`.

   > -4- Preconditions:

   > 1. (4.1) — `multipliable(A, transposed(B), C) addable(A, B, C)` is `true`, and

   > 2. (4.2) — `multipliable(B, transposed(A), C)` is `true` `A.extent(0)` equals `A.extent(1)`.

   > -5- Complexity: 𝒪(`A.extent(0)` × `A.extent(1)` × `BC.extent(0)`).

7. Modify 29.9.15.6 [linalg.algs.blas3.trsm] as indicated:

   > [Drafting note: Nothing is wrong here, but it's nice to make the complexity clauses depend only on input if possible]

   >     template<in-matrix InMat1, class Triangle, class DiagonalStorage,
   >              in-matrix InMat2, out-matrix OutMat, class BinaryDivideOp>
   >       void triangular_matrix_matrix_left_solve(InMat1 A, Triangle t, DiagonalStorage d,
   >                                                InMat2 B, OutMat X, BinaryDivideOp divide);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat1, class Triangle, class DiagonalStorage,
   >              in-matrix InMat2, out-matrix OutMat, class BinaryDivideOp>
   >       void triangular_matrix_matrix_left_solve(ExecutionPolicy&& exec,
   >                                                InMat1 A, Triangle t, DiagonalStorage d,
   >                                                InMat2 B, OutMat X, BinaryDivideOp divide);

   > > […]

   > > -6- Complexity: 𝒪(`A.extent(0)` × `BX.extent(1)` × `BX.extent(1)`).

8. Modify 29.9.15.7 [linalg.algs.blas3.inplacetrsm] as indicated:

   > [Drafting note: Nothing is wrong here, but it's nice to make the complexity clauses depend only on input if possible]

   >     template<in-matrix InMat, class Triangle, class DiagonalStorage,
   >              inout-matrix InOutMat, class BinaryDivideOp>
   >       void triangular_matrix_matrix_right_solve(InMat A, Triangle t, DiagonalStorage d,
   >                                                InOutMat B, BinaryDivideOp divide);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat, class Triangle, class DiagonalStorage,
   >              inout-matrix InOutMat, class BinaryDivideOp>
   >       void triangular_matrix_matrix_right_solve(ExecutionPolicy&& exec,
   >                                                InMat A, Triangle t, DiagonalStorage d,
   >                                                InOutMat B, BinaryDivideOp divide);

   > > […]

   > > -13- Complexity: 𝒪(`BA.extent(0)` × `A.extent(01)` × `AB.extent(1)`).

[LWG telecon 2025-10-10; Fix proposed resolution after review]

Add missing `()` in one place and change `(0, 1)` to `()` in another.

[LWG telecon 2025-10-10; Status updated New → Ready]

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.9.14.1 [linalg.algs.blas2.gemv] as indicated:

   > [Drafting note: This change is needed because the matrix `A` does not need to be square. `x.extents(0)` must equal `A.extents(1)`, while `y.extents(0)` must equal `A.extents(0)`.]

   > -3- Mandates:

   > 1. (3.1) — `possibly-multipliable<decltype(A), decltype(x), decltype(y)>()` is `true`, and

   > 2. (3.2) — `possibly-addable<decltype(yx), decltype(y), decltype(z)>()` is `true` for those overloads that take a `z` parameter.

   > -4- Preconditions:

   > 1. (4.1) — `multipliable(A, x, y)` is `true`, and

   > 2. (4.2) — `addable(yx, y, z)` is `true` for those overloads that take a `z` parameter.

   > -5- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

2. Modify 29.9.14.2 [linalg.algs.blas2.symv] as indicated:

   > -3- Mandates:

   > 1. (3.1) — […]

   > 2. (3.2) — […]

   > 3. (3.3) — `possibly-multipliable<decltype(A), decltype(x), decltype(y)>()` is `true`, and

   > 4. (3.4) — `possibly-addable<decltype(yx), decltype(y), decltype(z)>()` is `true` for those overloads that take a `z` parameter.

   > -4- Preconditions:

   > 1. (4.1) — `A.extent(0)` equals `A.extent(1)`,

   > 2. (4.2) — `multipliable(A, x, y)` is `true`, and

   > 3. (4.3) — `addable(yx, y, z)` is `true` for those overloads that take a `z` parameter.

   > -5- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

3. Modify 29.9.14.3 [linalg.algs.blas2.hemv] as indicated:

   > -3- Mandates:

   > 1. (3.1) — […]

   > 2. (3.2) — […]

   > 3. (3.3) — `possibly-multipliable<decltype(A), decltype(x), decltype(y)>()` is `true`, and

   > 4. (3.4) — `possibly-addable<decltype(yx), decltype(y), decltype(z)>()` is `true` for those overloads that take a `z` parameter.

   > -4- Preconditions:

   > 1. (4.1) — `A.extent(0)` equals `A.extent(1)`,

   > 2. (4.2) — `multipliable(A, x, y)` is `true`, and

   > 3. (4.3) — `addable(yx, y, z)` is `true` for those overloads that take a `z` parameter.

   > -5- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

4. Modify 29.9.14.4 [linalg.algs.blas2.trmv] as indicated:

   > [Drafting note: The extents compatibility conditions are expressed differently than in the above matrix-vector multiply sections, perhaps more for consistency with the TRSV section below. They look correct here. The original Complexity elements adjusted below are technically correct, since A is square, but changing this would improve consistency with 29.9.14.1 [linalg.algs.blas2.gemv]]

   >     template<in-matrix InMat, class Triangle, class DiagonalStorage, in-vector InVec,
   >              out-vector OutVec>
   >       void triangular_matrix_vector_product(InMat A, Triangle t, DiagonalStorage d, InVec x, OutVec y);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat, class Triangle, class DiagonalStorage, in-vector InVec,
   >              out-vector OutVec>
   >       void triangular_matrix_vector_product(ExecutionPolicy&& exec,
   >                                             InMat A, Triangle t, DiagonalStorage d, InVec x, OutVec y);

   > > -5- […]

   > > -6- Effects: Computes y = A x.

   > > -5- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

   >     template<in-matrix InMat, class Triangle, class DiagonalStorage, inout-vector InOutVec>
   >       void triangular_matrix_vector_product(InMat A, Triangle t, DiagonalStorage d, InOutVec y);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat, class Triangle, class DiagonalStorage, inout-vector InOutVec>
   >       void triangular_matrix_vector_product(ExecutionPolicy&& exec,
   >                                             InMat A, Triangle t, DiagonalStorage d, InOutVec y);

   > > -8- […]

   > > -9- Effects: […]

   > > -10- Complexity: 𝒪(`Ay.extent(0)` × `yA.extent(01)`).

   >     template<in-matrix InMat, class Triangle, class DiagonalStorage,
   >              in-vector InVec1, in-vector InVec2, out-vector OutVec>
   >       void triangular_matrix_vector_product(InMat A, Triangle t, DiagonalStorage d,
   >                                             InVec1 x, InVec2 y, OutVec z);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat, class Triangle, class DiagonalStorage,
   >              in-vector InVec1, in-vector InVec2, out-vector OutVec>
   >       void triangular_matrix_vector_product(ExecutionPolicy&& exec,
   >                                             InMat A, Triangle t, DiagonalStorage d,
   >                                             InVec1 x, InVec2 y, OutVec z);

   > > -11- […]

   > > -12- Effects: Computes z = y + A x.

   > > -13- Complexity: 𝒪(`Ax.extent(0)` × `xA.extent(01)`).

5. Modify 29.9.15.4 [linalg.algs.blas3.rankk] as indicated:

   > [Drafting note: P3371R0, to be submitted in the August 15 mailing for LEWG review, contains the same wording changes to 29.9.15.4 [linalg.algs.blas3.rankk] and 29.9.15.5 [linalg.algs.blas3.rank2k] as proposed here, with additional changes corresponding to that proposal. Please apply this LWG issue's changes first, before P3371 merges]

   > -3- Mandates:

   > 1. (3.1) — If `InOutMat` has `layout_blas_packed` layout, then the layout's `Triangle` template argument has the same type as the function's `Triangle` template argument; and

   > 2. (3.2) — `possibly-multipliable<decltype(A), decltype(transposed(A)), decltype(C)>() compatible-static-extents<decltype(A), decltype(A)>(0, 1)` is `true`.;

   > 3. (3.3) — `compatible-static-extents<decltype(C), decltype(C)>(0, 1)` is `true`; and

   > 4. (3.4) — `compatible-static-extents<decltype(A), decltype(C)>(0, 0)` is `true`.

   > -4- Preconditions: `multipliable(A, transposed(A), C)` is `true`.

   > 1. (4.1) — `A.extent(0)` equals `A.extent(1)`,

   > 2. (4.2) — `C.extent(0)` equals `C.extent(1)`, and

   > 3. (4.3) — `A.extent(0)` equals `C.extent(0)`.

   > -5- Complexity: 𝒪(`A.extent(0)` × `A.extent(1)` × `AC.extent(0)`).

6. Modify 29.9.15.5 [linalg.algs.blas3.rank2k] as indicated:

   > -3- Mandates:

   > 1. (3.1) — If `InOutMat` has `layout_blas_packed` layout, then the layout's `Triangle` template argument has the same type as the function's `Triangle` template argument;

   > 2. (3.2) — `possibly-multipliable<decltype(A), decltype(transposed(B)), decltype(C)>() possibly-addable<decltype(A), decltype(B), decltype(C)>()` is `true`; and

   > 3. (3.3) — `possibly-multipliable<decltype(B), decltype(transposed(A)), decltype(C)>() compatible-static-extents<decltype(A), decltype(A)>(0, 1)` is `true`.

   > -4- Preconditions:

   > 1. (4.1) — `multipliable(A, transposed(B), C) addable(A, B, C)` is `true`, and

   > 2. (4.2) — `multipliable(B, transposed(A), C)` is `true` `A.extent(0)` equals `A.extent(1)`.

   > -5- Complexity: 𝒪(`A.extent(0)` × `A.extent(1)` × `BC.extent(0)`).

7. Modify 29.9.15.6 [linalg.algs.blas3.trsm] as indicated:

   > [Drafting note: Nothing is wrong here, but it's nice to make the complexity clauses depend only on input if possible]

   >     template<in-matrix InMat1, class Triangle, class DiagonalStorage,
   >              in-matrix InMat2, out-matrix OutMat, class BinaryDivideOp>
   >       void triangular_matrix_matrix_left_solve(InMat1 A, Triangle t, DiagonalStorage d,
   >                                                InMat2 B, OutMat X, BinaryDivideOp divide);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat1, class Triangle, class DiagonalStorage,
   >              in-matrix InMat2, out-matrix OutMat, class BinaryDivideOp>
   >       void triangular_matrix_matrix_left_solve(ExecutionPolicy&& exec,
   >                                                InMat1 A, Triangle t, DiagonalStorage d,
   >                                                InMat2 B, OutMat X, BinaryDivideOp divide);

   > > […]

   > > -6- Complexity: 𝒪(`A.extent(0)` × `BX.extent(1)` × `BX.extent(1)`).

8. Modify 29.9.15.7 [linalg.algs.blas3.inplacetrsm] as indicated:

   > [Drafting note: Nothing is wrong here, but it's nice to make the complexity clauses depend only on input if possible]

   >     template<in-matrix InMat, class Triangle, class DiagonalStorage,
   >              inout-matrix InOutMat, class BinaryDivideOp>
   >       void triangular_matrix_matrix_right_solve(InMat A, Triangle t, DiagonalStorage d,
   >                                                InOutMat B, BinaryDivideOp divide);
   >     template<class ExecutionPolicy,
   >              in-matrix InMat, class Triangle, class DiagonalStorage,
   >              inout-matrix InOutMat, class BinaryDivideOp>
   >       void triangular_matrix_matrix_right_solve(ExecutionPolicy&& exec,
   >                                                InMat A, Triangle t, DiagonalStorage d,
   >                                                InOutMat B, BinaryDivideOp divide);

   > > […]

   > > -13- Complexity: 𝒪(`BA.extent(0)` × `A.extent(01)` × `AB.extent(1)`).

### 4140(i). Useless default constructors for bit reference types

Section: 22.9.2.1 [template.bitset.general], 23.3.14.1 [vector.bool.pspc] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-08-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [template.bitset.general].

View all issues with C++26 status.

Discussion:

The standard shows a private default constructor for `bitset<N>::reference` but does not define its semantics, and nothing in the spec refers to it. It was present in C++98, then in C++11 it got `noexcept` added to it, and in C++23 it was made `constexpr` by P2417R2. That's quite a lot of churn for an unusuable member function with no definition.

In libstdc++ it's declared as private, but never defined. In libc++ it doesn't exist at all. In MSVC it is private and defined (and presumably used somewhere). There's no reason for the standard to declare it. Implementers can define it as private if they want to, or not. The spec doesn't need to say anything for that to be true. We can also remove the friend declaration, because implementers know how to do that too.

I suspect it was added as private originally so that it didn't look like `reference` should have an implicitly-defined default constructor, which would have been the case in previous standards with no other constructors declared. However, C++20 added `reference(const reference&) = default;` which suppresses the implicit default constructor, so declaring the default constructor as private is now unnecessary.

Jiang An pointed out in an editorial pull request that `vector<bool, Alloc>::reference` has exactly the same issue.

[2024-09-18; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 22.9.2.1 [template.bitset.general] as indicated:

   >     namespace std {
   >       template<size_t N> class bitset {
   >       public:
   >         // bit reference
   >         class reference {
   >           friend class bitset;
   >           constexpr reference() noexcept;

   >         public:
   >           constexpr reference(const reference&) = default;
   >           constexpr ~reference();
   >           constexpr reference& operator=(bool x) noexcept;            // for b[i] = x;
   >           constexpr reference& operator=(const reference&) noexcept;  // for b[i] = b[j];
   >           constexpr bool operator~() const noexcept;                  // flips the bit
   >           constexpr operator bool() const noexcept;                   // for x = b[i];
   >           constexpr reference& flip() noexcept;                       // for b[i].flip();
   >         };

2. Modify 23.3.14.1 [vector.bool.pspc], `vector<bool, Allocator>` synopsis, as indicated:

   >     namespace std {
   >       template<class Allocator>
   >       class vector<bool, Allocator> {
   >       public:
   >         // types
   >         […]
   >         // bit reference
   >         class reference {
   >           friend class vector;
   >           constexpr reference() noexcept;

   >         public:
   >           constexpr reference(const reference&) = default;
   >           constexpr ~reference();
   >           constexpr operator bool() const noexcept;
   >           constexpr reference& operator=(bool x) noexcept;
   >           constexpr reference& operator=(const reference& x) noexcept;
   >           constexpr const reference& operator=(bool x) const noexcept;
   >           constexpr void flip() noexcept;   // flips the bit
   >         };

### 4141(i). Improve prohibitions on "additional storage"

Section: 22.5.3.1 [optional.optional.general], 22.6.3.1 [variant.variant.general], 22.8.6.1 [expected.object.general], 22.8.7.1 [expected.void.general] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-08-22 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [optional.optional.general].

View all issues with C++26 status.

Discussion:

This issue was split out from issue 4015(i).

`optional`, `variant` and `expected` all use similar wording to require their contained value to be a subobject, rather than dynamically allocated and referred to by a pointer, e.g.

> When an instance of `optional<T>` contains a value, it means that an object of type `T`, referred to as the optional object’s contained value, is allocated within the storage of the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value.

During the LWG reviews of P2300 in St. Louis, concerns were raised about the form of this wording and whether it's normatively meaningful. Except for the special case of standard-layout class types, the standard has very few requirements on where or how storage for subobjects is allocated. The library should not be trying to dictate more than the language guarantees. It would be better to refer to wording from 6.8.2 [intro.object] such as subobject, provides storage, or nested within. Any of these terms would provide the desired properties, without using different (and possibly inconsistent) terminology.

Using an array of bytes to provide storage for the contained value would make it tricky to meet the constexpr requirements of types like `optional`. This means in practice, the most restrictive of these terms, subobject, is probably accurate and the only plausible implementation strategy. However, I don't see any reason to outlaw other implementation strategies that might be possible in future (say, with a constexpr type cast, or non-standard compiler-specific instrinics). For this reason, the proposed resolution below uses nested within, which provides the desired guarantee without imposing additional restrictions on implementations.

[2024-09-18; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 22.5.3.1 [optional.optional.general] as indicated:

   > [Drafting note: This edit modifies the same paragraph as issue 4015(i), but that other issue intentionally doesn't touch the affected sentence here (except for removing the italics on "contained value"). The intention is that the merge conflict can be resolved in the obvious way: "An optional object's contained value is nested within (6.8.2 [intro.object]) the optional object."]

   > -1- Any instance of `optional<T>` at any given time either contains a value or does not contain a value. When an instance of `optional<T>` contains a value, it means that an object of type `T`, referred to as the optional object's contained value, is allocated within the storage of nested within (6.8.2 [intro.object]) the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value. When an object of type `optional<T>` is contextually converted to `bool`, the conversion returns `true` if the object contains a value; otherwise the conversion returns `false`.

2. Modify 22.6.3.1 [variant.variant.general] as indicated:

   > -1- Any instance of `variant` at any given time either holds a value of one of its alternative types or holds no value. When an instance of `variant` holds a value of alternative type `T`, it means that a value of type `T`, referred to as the `variant` object's contained value, is allocated within the storage of nested within (6.8.2 [intro.object]) the `variant` object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value.

3. Modify 22.8.6.1 [expected.object.general] as indicated:

   > -1- Any object of type `expected<T, E>` either contains a value of type `T` or a value of type `E` within its own storage nested within (6.8.2 [intro.object]) it. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of type `T` or the object of type `E`. Member `has_val` indicates whether the `expected<T, E>` object contains an object of type `T`.

4. Modify 22.8.7.1 [expected.void.general] as indicated:

   > -1- Any object of type `expected<T, E>` either represents a value of type `T`, or contains a value of type `E` within its own storage nested within (6.8.2 [intro.object]) it. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of type `E`. Member `has_val` indicates whether the `expected<T, E>` represents a value of type `T`.

### 4142(i). `format_parse_context::check_dynamic_spec` should require at least one type

Section: 28.5.6.6 [format.parse.ctx] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-08-28 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [format.parse.ctx].

View all issues with C++26 status.

Discussion:

The Mandates: conditions for `format_parse_context::check_dynamic_spec` are:

> -14- Mandates: The types in `Ts...` are unique. Each type in `Ts...` is one of `bool`, `char_type`, `int`, `unsigned int`, `long long int`, `unsigned long long int`, `float`, `double`, `long double`, `const char_type*`, `basic_string_view<char_type>`, or `const void*`.

There seems to be no reason to allow `Ts` to be an empty pack, that's not useful. There is no valid arg-id value that can be passed to it if the list of types is empty, since `arg(n)` will never be one of the types in an empty pack. So it's never a constant expression if the pack is empty.

[2024-09-18; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 28.5.6.6 [format.parse.ctx] as indicated:

       template<class... Ts>
         constexpr void check_dynamic_spec(size_t id) noexcept;

   > -14- Mandates: `sizeof...(Ts)` ≥ 1. The types in `Ts...` are unique. Each type in `Ts...` is one of `bool`, `char_type`, `int`, `unsigned int`, `long long int`, `unsigned long long int`, `float`, `double`, `long double`, `const char_type*`, `basic_string_view<char_type>`, or `const void*`.

   > -15- Remarks: A call to this function is a core constant expression only if:

   > 1. (15.1) — `id < num_args_` is `true` and
   > 2. (15.2) — the type of the corresponding format argument (after conversion to `basic_format_arg<Context>`) is one of the types in `Ts...`.

### 4143(i). `execution::set_value/set_error/set_stopped/start` should always return `void`

Section: 33.7.2 [exec.set.value], 33.7.3 [exec.set.error], 33.7.4 [exec.set.stopped], 33.8.2 [exec.opstate.start] Status: C++26 Submitter: Jiang An Opened: 2024-08-20 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

In editorial issue #7222, it was observed that currently `execution::start` may have a non-`void` return value, which possibly interacts with overloaded `operator,`. But the return value of `execution::start` doesn't seem used anywhere.

In addition to `execution::start`, the return values of `execution::set_value`, `execution::set_error`, and `execution::set_stopped` also seem never used, and the return type of these CPOs are always `void` in stdexec. Perhaps it would be better to specified in the standard that these CPOs always return `void`.

[2024-09-18; Reflector poll]

Set priority to 2 after reflector poll. Should require the expressions to have type `void`, rather than just discarding anything that is returned.

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

1. Modify 33.7.2 [exec.set.value] as indicated:

   > -1- `set_value` is a value completion function (33.3 [exec.async.ops]). Its associated completion tag is `set_value_t`. The expression `set_value(rcvr, vs...)` for a subexpression `rcvr` and pack of subexpressions `vs` is ill-formed if `rcvr` is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to `MANDATE-NOTHROW(void(rcvr.set_value(vs...)))`.

2. Modify 33.7.3 [exec.set.error] as indicated:

   > -1- `set_error` is an error completion function (33.3 [exec.async.ops]). Its associated completion tag is `set_error_t`. The expression `set_error(rcvr, err)` for some subexpressions `rcvr` and `err` is ill-formed if `rcvr` is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to `MANDATE-NOTHROW(void(rcvr.set_error(err)))`.

3. Modify 33.7.4 [exec.set.stopped] as indicated:

   > -1- `set_stopped` is a stopped completion function (33.3 [exec.async.ops]). Its associated completion tag is `set_stopped_t`. The expression `set_stopped(rcvr)` for a subexpression `rcvr` is ill-formed if `rcvr` is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to `MANDATE-NOTHROW(void(rcvr.set_stopped()))`.

4. Modify 33.8.2 [exec.opstate.start] as indicated:

   > -1- The name `start` denotes a customization point object that starts (33.3 [exec.async.ops]) the asynchronous operation associated with the operation state object. For a subexpression `op`, the expression `start(op)` is ill-formed if `op` is an rvalue. Otherwise, it is expression-equivalent to `MANDATE-NOTHROW(void(op.start()))`.

[2026-03-27; Tim provides new wording]

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 33.7.2 [exec.set.value] as indicated:

   > -1- `set_value` is a value completion function (33.3 [exec.async.ops]). Its associated completion tag is `set_value_t`. The expression `set_value(rcvr, vs...)` for a subexpression `rcvr` and pack of subexpressions `vs` is ill-formed if `rcvr` is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to `MANDATE-NOTHROW(rcvr.set_value(vs...))`.

   > Mandates: If the expression above is well-formed, its type is `void`.

2. Modify 33.7.3 [exec.set.error] as indicated:

   > -1- `set_error` is an error completion function (33.3 [exec.async.ops]). Its associated completion tag is `set_error_t`. The expression `set_error(rcvr, err)` for some subexpressions `rcvr` and `err` is ill-formed if `rcvr` is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to `MANDATE-NOTHROW(rcvr.set_error(err))`.

   > Mandates: If the expression above is well-formed, its type is `void`.

3. Modify 33.7.4 [exec.set.stopped] as indicated:

   > -1- `set_stopped` is a stopped completion function (33.3 [exec.async.ops]). Its associated completion tag is `set_stopped_t`. The expression `set_stopped(rcvr)` for a subexpression `rcvr` is ill-formed if `rcvr` is an lvalue or an rvalue of const type. Otherwise, it is expression-equivalent to `MANDATE-NOTHROW(rcvr.set_stopped())`.

   > Mandates: If the expression above is well-formed, its type is `void`.

4. Modify 33.8.2 [exec.opstate.start] as indicated:

   > -1- The name `start` denotes a customization point object that starts (33.3 [exec.async.ops]) the asynchronous operation associated with the operation state object. For a subexpression `op`, the expression `start(op)` is ill-formed if `op` is an rvalue. Otherwise, it is expression-equivalent to `MANDATE-NOTHROW(op.start())`.

   > Mandates: If the expression above is well-formed, its type is `void`.

### 4144(i). Disallow `unique_ptr<T&, D>`

Section: 20.3.1.3.1 [unique.ptr.single.general] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-08-30 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [unique.ptr.single.general].

View all issues with C++26 status.

Discussion:

It seems that we currently allow nonsensical specializations of `unique_ptr` such as `unique_ptr<int&, D>` and `unique_ptr<void()const, D>` (a custom deleter that defines `D::pointer` is needed, because otherwise the `pointer` type would default to invalid types like `int&*` or `void(*)()const`). There seems to be no reason to support these "unique pointer to reference" and "unique pointer to abominable function type" specializations, or any specialization for a type that you couldn't form a raw pointer to.

Prior to C++17, the major library implementations rejected such specializations as a side effect of the constraints for the `unique_ptr(auto_ptr<U>&&)` constructor being defined in terms of `is_convertible<U*, T*>`. This meant that overload resolution for any constructor of `unique_ptr` would attempt to form the type `T*` and fail if that was invalid. With the removal of `auto_ptr` in C++17, that constructor was removed and now `unique_ptr<int&, D>` can be instantiated (assuming any zombie definition of `auto_ptr` is not enabled by the library). This wasn't intentional, but just an accident caused by not explicitly forbidding such types.

Discussion on the LWG reflector led to near-unanimous support for explicitly disallowing these specializations for non-pointable types.

[2024-11-13; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 20.3.1.3.1 [unique.ptr.single.general] as indicated:

   > -?- A program that instantiates the definition of `unique_ptr<T, D>` is ill-formed if `T*` is an invalid type.
   > [Note: This prevents the intantiation of specializations such as `unique_ptr<T&, D>` and `unique_ptr<int() const, D>`. — end note]

   > -1- The default type for the template parameter `D` is `default_delete`. A client-supplied template argument `D` shall be a function object type (22.10 [function.objects]), lvalue reference to function, or lvalue reference to function object type for which, given a value `d` of type `D` and a value `ptr` of type `unique_ptr<T, D>::pointer`, the expression `d(ptr)` is valid and has the effect of disposing of the pointer as appropriate for that deleter.

   > -2- If the deleter’s type `D` is not a reference type, `D` shall meet the Cpp17Destructible requirements (Table 35).

   > -3- If the qualified-id `remove_reference_t<D>::pointer` is valid and denotes a type (13.10.3 [temp.deduct]), then `unique_ptr<T, D>::pointer` shall be a synonym for `remove_reference_t<D>::pointer`. Otherwise `unique_ptr<T, D>::pointer` shall be a synonym for `element_type*`. The type `unique_ptr<T, D>::pointer` shall meet the Cpp17NullablePointer requirements (Table 36).

   > -4- [Example 1:  Given an allocator type `X` (16.4.4.6.1 [allocator.requirements.general]) and letting `A` be a synonym for `allocator_traits<X>`, the types `A::pointer`, `A::const_pointer`, `A::void_pointer`, and `A::const_void_pointer` may be used as `unique_ptr<T, D>::pointer`. — end example]

### 4146(i). §[format.formatter.spec]/3 unconditionally enables nonlocking for container adaptors

Section: 28.5.6.4 [format.formatter.spec], 23.6.13 [container.adaptors.format] Status: Resolved Submitter: Casey Carter Opened: 2024-08-31 Last modified: 2026-02-27

Priority: 2

View other active issues in [format.formatter.spec].

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Discussion:

28.5.6.4 [format.formatter.spec]/3 says that the library provides a specialization of `enable_nonlocking_formatter_optimization` with value `true` corresponding to each library-provided specialization of `formatter`, unless otherwise specified. Although it actually states "for each type `T`", the intent is that partial specializations are also provided corresponding to library-provided partial specializations of formatter.

23.6.13 [container.adaptors.format]/1 says the library provides a partial specialization of `formatter` for each of the container adaptor templates `priority_queue`, `queue`, and `stack`. Together with 28.5.6.4 [format.formatter.spec]/3, that means that e.g. `enable_nonlocking_formatter_optimization<stack<T>> == true`. Formatting a stack of that type will enable the nonlocking optimization even if `enable_nonlocking_formatter_optimization<T> == false`. To avoid this, the author of `T` must partially specialize `enable_nonlocking_formatter_optimization` to `false` for all container adaptors when they adapt a container of `T`.

It is clearly not the design intent that programmers must explicitly opt out of the nonlocking optimization, so this is a defect that LWG should correct. Since P3235R3 was applied as a Defect Report to C++23, the resolution of this issue should be so applied as well.

Suggested Resolution:

LEWG was reticent to apply the optimization to general ranges — ostensibly due to the possibility of deadlock in program-defined iterator operations — but apparently unconcerned about the iterators of program-defined containers nor the fancy pointers of program-defined allocators. It seems consistent with that design to ignore the "container" part of "container adaptors" and only pay attention to the elements that are going to be formatted. (I have prototyped this resolution on MSVCSTL, albeit slightly modified since neither MSVC nor Clang like this partial specialization form.)

[2025-10-14; Reflector poll]

Set priority to 2 after reflector poll.

This is a duplicate of LWG 4398(i), with a different proposed resolution.

[2026-02-27; LWG 4398(i) was approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 23.6.13 [container.adaptors.format] as indicated:

   > -1- For each of `queue`, `priority_queue`, and `stack`, the library provides the following formatter specializations where `adaptor-type` is the name of the template:

   > >     namespace std {
   > >       template<class charT, class T, formattable<charT> Container, class... U>
   > >       struct formatter<adaptor-type<T, Container, U...>, charT> {
   > >         […]
   > >       };

   > >       template<class T, class Container, class... U>
   > >       constexpr bool enable_nonlocking_formatter_optimization<adaptor-type<T, Container, U...>> =
   > >         enable_nonlocking_formatter_optimization<T>;
   > >     }

### 4147(i). Precondition on `inplace_vector::emplace`

Section: 23.2.4 [sequence.reqmts] Status: C++26 Submitter: Arthur O'Dwyer Opened: 2024-08-26 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [sequence.reqmts].

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Discussion:

Inserting into the middle of an `inplace_vector`, just like inserting into the middle of a `vector` or `deque`, requires that we construct the new element out-of-line, shift down the trailing elements (Cpp17MoveAssignable), and then move-construct the new element into place (Cpp17MoveInsertable). P0843R14 failed to make this change, but it should have.

[2024-09-18; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   >     a.emplace(p, args)

   > > -19- Result: `iterator`.

   > > -20- Preconditions: `T` is Cpp17EmplaceConstructible into `X` from `args`. For `vector`, inplace_vector, and `deque`, `T` is also Cpp17MoveInsertable into `X` and Cpp17MoveAssignable.

   > > -21- Effects: Inserts an object of type `T` constructed with `std::forward<Args>(args)...` before `p`.

   > > [Note 1: `args` can directly or indirectly refer to a value in `a`. — end note]

   > > -22- Returns: An iterator that points to the new element constructed from `args` into `a`.

### 4148(i). `unique_ptr::operator*` should not allow dangling references

Section: 20.3.1.3.5 [unique.ptr.single.observers] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-09-02 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [unique.ptr.single.observers].

View all issues with C++26 status.

Discussion:

If `unique_ptr<T,D>::element_type*` and `D::pointer` are not the same type, it's possible for `operator*()` to return a dangling reference that has undefined behaviour.

      struct deleter {
        using pointer = long*;
        void operator()(pointer) const {}
      };
      long l = 0;
      std::unique_ptr<const int, deleter> p(&l);
      int i = *p; // undefined

We should make this case ill-formed.

[2024-09-18; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 20.3.1.3.5 [unique.ptr.single.observers] as indicated:

   >     constexpr add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));

   > > -?- Mandates: `reference_converts_from_temporary_v<add_lvalue_reference_t<T>, decltype(*declval<pointer>())>` is `false`.

   > > -1- Preconditions: `get() != nullptr` is `true`.

   > > -2- Returns: `*get()`.

### 4149(i). User defined macros without standard headers (294 redux)

Section: 16.4.5.3.3 [macro.names] Status: Resolved Submitter: Jonathan Wakely Opened: 2024-09-05 Last modified: 2025-10-14

Priority: Not Prioritized

View all other issues in [macro.names].

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Discussion:

Issue 294(i) changed 16.4.5.3.3 [macro.names] from:

> A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.

to:

> A translation unit that includes a standard library header shall not `#define` or `#undef` names declared in any standard library header.

> A translation unit shall not `#define` or `#undef` names lexically identical to keywords.

Note that the second sentence of the original says "such a translation unit" when prohibiting things like `#define while`. This means the prohibition only applies to "a translation unit that includes a header". The replacement has the prohibition in a separate paragraph and does not clearly say that it only applies when a header is included.

The issue discussion seems clear that the concern is about C++ headers including other unspecified headers, which is allowed in C++ (though not in C). There is no justification for broadening the second sentence to apply unconditionally. Such a rule would belong in 15 [cpp] anyway, not in library wording. That overreach doesn't appear to have been intended, and we should clarify what the library is prohibiting.

It was pointed out on the reflector that it's not enough to only prohibit defining such macros after including headers, because in some cases that could still break the library header. For example, the library macro `assert` typically uses `void` and so `#define void ...` would break `assert` even if it happens after including `<cassert>`.

[2025-10-14; Resolved by P2843R3. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4986.

1. Modify 16.4.5.3.3 [macro.names] as indicated:

   > -1- A translation unit that includes a standard library header shall not `#define` or `#undef` names declared in any standard library header.

   > -2- A translation unit that includes a standard library header shall not `#define` or `#undef` names lexically identical to keywords, to the identifiers listed in Table 4, or to the attribute-tokens described in 9.13 [dcl.attr], except that the names `likely` and `unlikely` may be defined as function-like macros (15.7 [cpp.replace]).

### 4150(i). The preconditions on `run_loop::run()` are too strict

Section: 33.12.1.4 [exec.run.loop.members] Status: Resolved Submitter: Eric Niebler Opened: 2024-09-05 Last modified: 2025-02-07

Priority: Not Prioritized

View all issues with Resolved status.

Discussion:

From sender-receiver/issues #280:

The precondition on `run_loop::run()` is that the `state` is starting. Given that `run_loop::finish()` puts the loop in the finishing state, the implication is that one cannot call `finish()` before calling `run()`.

However, in most cases `sync_wait` is calling `finish()` on its `run_loop` before calling `run()`, violating this precondition. `sync_wait` does the following:

>     run_loop loop;
>     auto op = connect(sndr, sync_wait_receiver{&loop});
>     start(op);

>     loop.run();

If `sndr` completes synchronously, the `sync_wait_receiver` will call `finish()` on the loop before `loop.run()` is called, violating `run`'s precondition.

[2025-02-07 Status changed: New → Resolved.]

Resolved by P3396R1.

Proposed resolution:

This wording is relative to N4988.

1. Modify 33.12.1.4 [exec.run.loop.members] as indicated:

   >     void run();

   > > -5- Preconditions: `state` is starting or finishing.

   > > -6- Effects: Sets the `state` to running. Then, equivalent to:

   > > >     while (auto* op = pop-front()) {
   > > >       op->execute();
   > > >     }

   > > -7- Remarks: When `state` changes, it does so without introducing data races.

### 4151(i). Precondition of `inplace_vector::swap`

Section: 23.3.16.5 [inplace.vector.modifiers] Status: C++26 Submitter: Arthur O'Dwyer Opened: 2024-09-07 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

Right now `inplace_vector::swap` has only a declaration in the class synopsis; it doesn't specify what the behavior of `swap` for `inplace_vector`s actually is. So I think the behavior ends up being governed by 23.2.2.2 [container.reqmts], which are written from the point of view of a container that manages an external heap allocation so that swapping containers doesn't touch the elements at all, just changes their ownership.

`inplace_vector::swap` actually works more like `array::swap`, which has its own specification in 23.3.3.3 [array.members], where it is defined in terms of `std::swap_ranges`. The `std::swap_ranges` algorithm (26.7.3 [alg.swap]) has a precondition! This precondition is missing from `inplace_vector::swap`.

That is, I think we currently have no wording that explains why

>     std::pmr::monotonic_buffer_resource mr1;
>     std::inplace_vector<std::pmr::vector<int>, 2> v1, v2;
>     v1.emplace_back(1, &mr1);
>     v2.emplace_back(1);
>     v1.swap(v2);

is undefined behavior. The current spec seems to say this Just Works, even though it physically cannot work because `v1[0]` and `v2[0]` don't dynamically meet the semantic requirements of being "swappable with" each other, and `v1.swap(v2)` is necessarily going to try to swap them.

[2024-09-18; Reflector poll]

Set priority to 2 after reflector poll. The Preconditions: (but not the Effects:) would be changed again if P3160R2 is approved.

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

1. Modify 23.3.16.5 [inplace.vector.modifiers] as indicated:

   > [Drafting note: It is suggested to add the new wording to the end of the existing subclause]

   >     constexpr void swap(inplace_vector& x) noexcept(N == 0 ||
   >       (is_nothrow_swappable_v<T> && is_nothrow_move_constructible_v<T>));

   > > -?- Preconditions: Let `M` be `min(size(), x.size())`. For each non-negative integer `n < M`, `(*this)[n]` is swappable with (16.4.4.3 [swappable.requirements]) `x[n]`.

   > > -?- Effects: Exchanges the contents of `*this` and `x`.

[Croydon 2026-03-27; Jonathan provides new wording]

Also require Cpp17MoveConstructible.

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 23.3.16.5 [inplace.vector.modifiers] as indicated:

   > [Drafting note: It is suggested to add the new wording to the end of the existing subclause]

   >     constexpr void swap(inplace_vector& x) noexcept(N == 0 ||
   >       (is_nothrow_swappable_v<T> && is_nothrow_move_constructible_v<T>));

   > > -?- Preconditions: `T` meets the Cpp17MoveConstructible requirements. Let `M` be `min(size(), x.size())`. For each non-negative integer `n < M`, `(*this)[n]` is swappable with `x[n]` (16.4.4.3 [swappable.requirements]).

   > > -?- Effects: Exchanges the contents of `*this` and `x`.

### 4153(i). Fix extra "-1" for `philox_engine::max()`

Section: 29.5.4.5 [rand.eng.philox] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2024-09-18 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [rand.eng.philox].

View all issues with C++26 status.

Discussion:

There is a typo in `philox_engine` wording that makes "-1" two times instead of one for `max()` method. The reason for that typo is that the wording was originally inspired by `mersenne_twister_engine` but after getting feedback that what is written in the `philox_engine` synopsis is not C++ code, the authors introduced the m variable (as in `subtract_with_carry_engine`) but forgot to remove "-1" in the m definition.

Note: after the proposed resolution below is applied the m variable could be reused in other places: basically in all places where the `mod 2^w` pattern appears (like `subtract_with_carry_engine` does). The authors don’t think it’s worth changing the rest of the wording to reuse the m variable. If somebody thinks otherwise, please provide such feedback.

[2024-10-02; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 29.5.4.5 [rand.eng.philox] as indicated:

   > -1- A `philox_engine` random number engine produces unsigned integer random numbers in the closed interval [0, m]), where m = 2w − 1 and the template parameter w defines the range of the produced numbers.

### 4154(i). The Mandates for `std::packaged_task`'s constructor from a callable entity should consider decaying

Section: 32.10.10.2 [futures.task.members] Status: C++26 Submitter: Jiang An Opened: 2024-09-18 Last modified: 2026-06-09

Priority: 3

View all other issues in [futures.task.members].

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Discussion:

Currently, 32.10.10.2 [futures.task.members]/3 states:

> Mandates: `is_invocable_r_v<R, F&, ArgTypes...>` is `true`.

where `F&` can be a reference to a cv-qualified function object type.

However, in mainstream implementations (libc++, libstdc++, and MSVC STL), the stored task object always has a cv-unqualified type, and thus the cv-qualification is unrecognizable in `operator()`.

Since 22.10.17.3.2 [func.wrap.func.con] uses a decayed type, perhaps we should also so specify for `std::packaged_task`.

[2024-10-02; Reflector poll]

Set priority to 3 after reflector poll.

"Fix preconditions, `f` doesn't need to be invocable, we only invoke the copy."

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

1. Modify 32.10.10.2 [futures.task.members] as indicated:

   > -3- Mandates: `is_invocable_r_v<R, Fdecay_t<F>&, ArgTypes...>` is `true`.

   > [...]

   > -5- Effects: Constructs a new `packaged_task` object with a shared state and initializes the object's stored task of type `decay_t<F>` with `std::forward<F>(f)`.

[2024-10-02; Jonathan provides improved wording]

Drop preconditions as suggested on reflector.

[2024-10-02; LWG telecon]

Clarify that "of type `decay_t<F>`" is supposed to be specifying the type of the stored task.

[2024-10-09; LWG telecon: Move to Ready]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 32.10.10.2 [futures.task.members] as indicated:

   >     template<class F>
   >       explicit packaged_task(F&& f);

   > -2- Constraints: `remove_cvref_t<F>` is not the same type as `packaged_task<R(ArgTypes...)>`.

   > -3- Mandates: `is_invocable_r_v<R, Fdecay_t<F>&, ArgTypes...>` is `true`.

   > -4- Preconditions: Invoking a copy of `f` behaves the same as invoking `f`.

   > -5- Effects: Constructs a new `packaged_task` object with a stored task of type `decay_t<F>` and a shared state . Initializes and initializes the object's stored task with `std::forward<F>(f)`.

### 4157(i). The resolution of LWG3465 was damaged by P2167R3

Section: 17.12.6 [cmp.alg] Status: C++26 Submitter: Jiang An Opened: 2024-09-18 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [cmp.alg].

View all other issues in [cmp.alg].

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Discussion:

In the resolution of LWG 3465(i), `F < E` was required to be well-formed and implicitly convertible to `bool`. However, P2167R3 replaced the convertibility requirements with just "each of `decltype(E == F)` and `decltype(E < F)` models `boolean-testable`", which rendered the type of `F < E` underconstrained.

[2024-10-02; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 17.12.6 [cmp.alg] as indicated:

   > (6.3) — Otherwise, if the expressions `E == F`, `E < F`, and `F < E` are all well-formed and each of `decltype(E == F)` and, `decltype(E < F)` , and `decltype(F < E)` models `boolean-testable`,

   >       E == F ? partial_ordering::equivalent :
   >       E < F  ? partial_ordering::less :
   >       F < E  ? partial_ordering::greater :
   >                partial_ordering::unordered

   > except that `E` and `F` are evaluated only once.

### 4158(i). `packaged_task::operator=` should abandon its shared state

Section: 32.10.10.2 [futures.task.members] Status: WP Submitter: Jonathan Wakely Opened: 2024-09-19 Last modified: 2026-06-15

Priority: 3

View all other issues in [futures.task.members].

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Discussion:

The `packaged_task` move assignment operator is specified to release the previous shared state. This means it releases ownership, but does not make the shared state ready. Any future that shares ownership of the shared state will never receive a result, because the provider is gone. This means that any thread that waits on the future will block forever.

There is a note on `packaged_task::reset()` which claims that assignment abandons the state, which is not supported by any normative wording:

>     void reset();

> -26- Effects: As if `*this = packaged_task(std::move(f))`, where `f` is the task stored in `*this`.

> [Note 2: This constructs a new shared state for `*this`. The old state is abandoned (32.10.5 [futures.state]). — end note]

Presumably, the intended behaviour of assignment was to abandon the shared state, i.e. make it ready with a `broken_promise` error, and then release it. That is what the `std::promise` move assignment does (see 32.10.6 [futures.promise] p9). Both libstdc++ and libc++ abandon the state, despite what the standard says.

[2024-10-02; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

1. Modify 32.10.10.2 [futures.task.members] as indicated:

   >     packaged_task& operator=(packaged_task&& rhs) noexcept;

   > -11- Effects:

   > 1. (11.1) — Releases Abandons any shared state (32.10.5 [futures.state]);
   > 2. (11.2) — calls `packaged_task(std::move(rhs)).swap(*this)`.

   > -?- Returns: `*this`.

[2024-10-02; Jonathan provides improved wording]

Following reflector discussion, remove the "Releases any shared state" text completely. The remaining effects imply that the state will be abandoned anyway. This makes it safe against self-assignment.

[2024-10-02; LWG telecon]

Agreed to change `promise` the same way, which is safe for self-assignment and matches what all three of libstdc++, libc++ and MSVC do today. Ask SG1 to review.

[2026-05-19; LWG telecon. Status changed: New → Ready.]

SG1 agreed with the resolution. LWG moved to Ready.

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Modify 32.10.6 [futures.promise] as indicated:

   >     promise& operator=(promise&& rhs) noexcept;

   > -9- Effects: Abandons any shared state (32.10.5 [futures.state]) and then as if Equivalent to `promise(std::move(rhs)).swap(*this)`.

   > -10- Returns: `*this`.

2. Modify 32.10.10.2 [futures.task.members] as indicated:

   >     packaged_task& operator=(packaged_task&& rhs) noexcept;

   > -11- Effects:

   > 1. (11.1) — Releases any shared state (32.10.5 [futures.state]);
   > 2. (11.2) — calls Equivalent to `packaged_task(std::move(rhs)).swap(*this)`.

   > -?- Returns: `*this`.

### 4164(i). Missing guarantees for `forward_list` modifiers

Section: 23.3.7.5 [forward.list.modifiers] Status: C++26 Submitter: Jonathan Wakely Opened: 2024-10-05 Last modified: 2026-06-09

Priority: 3

View all other issues in [forward.list.modifiers].

View all issues with C++26 status.

Discussion:

The new `std::list` members added by P1206R7, `insert_range(const_iterator, R&&)`, `prepend_range(R&&)`, and `append_range(R&&)`, have the same exception safety guarantee as `std::list::insert(const_iterator, InputIterator, InputIterator)`, which is:

> Remarks: Does not affect the validity of iterators and references. If an exception is thrown, there are no effects.

This guarantee was achieved for the new `list` functions simply by placing them in the same set of declarations as the existing `insert` overloads, at the start of 23.3.11.4 [list.modifiers].

However, the new `std::forward_list` members, `insert_range_after(const_iterator, R&&)` and `prepend_range(R&&)`, do not have the same guarantee as `forward_list::insert_after`. This looks like an omission caused by the fact that `insert_after`'s exception safety guarantee is given in a separate paragraph at the start of 23.3.7.5 [forward.list.modifiers]:

> None of the overloads of `insert_after` shall affect the validity of iterators and references, and `erase_after` shall invalidate only iterators and references to the erased elements. If an exception is thrown during `insert_after` there shall be no effect.

I think we should give similar guarantees for `insert_range_after` and `prepend_range`. The change might also be appropriate for `emplace_after` as well. A "no effects" guarantee is already given for `push_front` and `emplace_front` in 23.2.2.2 [container.reqmts] p66, although that doesn't say anything about iterator invalidation so we might want to add that to 23.3.7.5 [forward.list.modifiers] too. For the functions that insert a single element, it's trivial to not modify the list if allocating a new node of constructing the element throws. The strong exception safety guarantee for the multi-element insertion functions is easily achieved by inserting into a temporary `forward_list` first, then using `splice_after` which is non-throwing.

[2024-10-09; Reflector poll]

Set priority to 3 after reflector poll.

It was suggested to change "If an exception is thrown by any of these member functions that insert elements there is no effect on the forward_list" to simply "If an exception is thrown by any of these member functions there is no effect on the forward_list"

Previous resolution [SUPERSEDED]

This wording is relative to N4988.

1. Change 23.3.7.5 [forward.list.modifiers] as indicated:

   > None of the overloads of `insert_after` shall member functions in this subclause that insert elements affect the validity of iterators and references, and `erase_after` shall invalidate invalidates only iterators and references to the erased elements. If an exception is thrown during `insert_after` by any of these member functions there shall be is no effect on the `forward_list`.

[2024-10-09; LWG suggested improved wording]

The proposed resolution potentially mandates a change to `resize` when increasing the size, requiring implementations to "roll back" earlier insertions if a later one throws, so that the size is left unchanged. It appears that libstdc++ and MSVC already do this, libc++ does not.

[2024-10-09; LWG telecon: Move to Ready]

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4988.

1. Change 23.3.7.5 [forward.list.modifiers] as indicated:

   > None of the overloads of `insert_after` shall affect the validity of iterators and references, and `erase_after` shall invalidate only iterators and references to the erased elements. The member functions in this subclause do not affect the validity of iterators and references when inserting elements, and when erasing elements invalidate iterators and references to the erased elements only. If an exception is thrown during `insert_after` by any of these member functions there shall be is no effect on the container.

### 4166(i). `concat_view::end()` should be more constrained in order to support noncopyable iterators

Section: 25.7.18.2 [range.concat.view] Status: C++26 Submitter: Yaito Kakeyama & Nana Sakisaka Opened: 2024-10-13 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [range.concat.view].

View all other issues in [range.concat.view].

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Discussion:

There is a case that `concat(a, b)` compiles but `concat(b, a)` does not.

>     auto range_copyable_it = std::vector<int>{1, 2, 3};

>     std::stringstream ss{"4 5 6"};
>     auto range_noncopyable_it = std::views::istream<int>(ss);

>     auto view1 = std::views::concat(range_copyable_it, range_noncopyable_it);
>     static_assert(std::ranges::range<decltype(view1)>);               // ok
>     assert(std::ranges::equal(view1, std::vector{1, 2, 3, 4, 5, 6})); // ok

>     auto view2 = std::views::concat(range_noncopyable_it, range_copyable_it);
>     // static_assert(std::ranges::range<decltype(view2)>);               // error
>     // assert(std::ranges::equal(view2, std::vector{4, 5, 6, 1, 2, 3})); // error

The reason behind this is as follows:

Firstly, if all `Views...` satisfy the `std::ranges::range` concept, then `concat_view` should also satisfy it. However, if any of the `Views...` have a noncopyable iterator and the last view is `common_range`, the current `concat_view` fails to model a range.

For `concat_view` to model a range, its sentinel must satisfy `std::semiregular`, but `concat_view::end()` returns a `concat_view::iterator`, which is noncopyable if the underlying iterator is noncopyable. This issue arises from the proposed implementation where the iterator uses `std::variant`. Although this specification is exposition-only, even if an alternative type-erasure mechanism is used, copying is still required if the user attempts to copy an iterator.

To resolve the issue, `concat_view::end()` can and should fallback to returning `std::default_sentinel` in such cases.

Unfortunately, as a side effect, this fix would prevent `concat_view` from being a `common_range` in certain situations. According to P2542R8:

> `concat_view` can be `common_range` if the last underlying range models `common_range`

However, this is no longer true after applying our fix. That said, these two issues cannot be resolved simultaneously due to implementability. Therefore, we suggest applying our fix regardless and accepting that `concat_view` will not always inherit `common_range`. Note that the current draft (N4988) does not explicitly specify when `concat_view` can model `common_range`, so no addition is required for mentioning this point.

A similar issue had been reported as 3385(i), which was eventually adopted as a C++20 DR. This DR indicates that LWG approved the decision to require `copyable` in order to model a `common_iterator`.
Previous resolution [SUPERSEDED]

This wording is relative to N4993.

1. Modify 25.7.18.2 [range.concat.view] as indicated:

   >     constexpr auto end() const
   >       requires (range<const Views> && ...) && concatable<const Views...>;

   > > -7- Effects: Let `is-const` be `true` for the const-qualified overload, and `false` otherwise. Equivalent to:

   > > >     constexpr auto N = sizeof...(Views);
   > > >     if constexpr ((semiregular<iterator_t<maybe-const<is-const, Views>>> && ...) &&
   > > >                   common_range<maybe-const<is-const, Views...[N - 1]>>) {
   > > >       return iterator<is-const>(this, in_place_index<N - 1>,
   > > >                                 ranges::end(std::get<N - 1>(views_)));
   > > >     } else {
   > > >       return default_sentinel;
   > > >     }

[2025-03-05; Hewill Kang provides improved wording]

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 25.7.18.2 [range.concat.view] as indicated:

   >     constexpr auto end() const
   >       requires (range<const Views> && ...) && concatable<const Views...>;

   > > -7- Effects: Let `is-const` be `true` for the const-qualified overload, and `false` otherwise. Equivalent to:

   > > >     constexpr auto N = sizeof...(Views);
   > > >     if constexpr (all-forward<is-const, Views...> &&
   > > >                   common_range<maybe-const<is-const, Views...[N - 1]>>) {
   > > >       return iterator<is-const>(this, in_place_index<N - 1>,
   > > >                                 ranges::end(std::get<N - 1>(views_)));
   > > >     } else {
   > > >       return default_sentinel;
   > > >     }

### 4169(i). `std::atomic<T>`'s default constructor should be constrained

Section: 32.5.8.2 [atomics.types.operations] Status: C++26 Submitter: Giuseppe D'Angelo Opened: 2024-10-15 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [atomics.types.operations].

View all issues with C++26 status.

Discussion:

The current wording for `std::atomic`'s default constructor in 32.5.8.2 [atomics.types.operations] specifies:

>     constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>);

> > Mandates: `is_default_constructible_v<T>` is `true`.

This wording has been added by P0883R2 for C++20, which changed `std::atomic`'s default constructor to always value-initialize. Before, the behavior of this constructor was not well specified (this was LWG issue 2334(i)).

The usage of a Mandates element in the specification has as a consequence that `std::atomic<T>` is always default constructible, even when `T` is not. For instance:

>     // not default constructible:
>     struct NDC { NDC(int) {} };

>     static_assert(std::is_default_constructible<std::atomic<NDC>>); // OK

The above check is OK as per language rules, but this is user-hostile: actually using `std::atomic<NDC>`'s default constructor results in an error, despite the detection saying otherwise.

Given that `std::atomic<T>` already requires `T` to be complete anyhow (32.5.8.1 [atomics.types.generic.general] checks for various type properties which require completeness) it would be more appropriate to use a constraint instead, so that `std::atomic<T>` is default constructible if and only if `T` also is.

[2024-11-13; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4993.

1. Modify 32.5.8.2 [atomics.types.operations] as indicated:

   > [Drafting note: There is implementation divergence at the moment; libstdc++ already implements the proposed resolution and has done so for a while.]

   >     constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>);

   > > -1- ConstraintsMandates: `is_default_constructible_v<T>` is `true`.

   > > -2- Effects: […]

### 4170(i). `contiguous_iterator` should require `to_address(I{})`

Section: 24.3.4.14 [iterator.concept.contiguous] Status: C++26 Submitter: Casey Carter Opened: 2024-11-01 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [iterator.concept.contiguous].

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Discussion:

The design intent of the `contiguous_iterator` concept is that iterators can be converted to pointers denoting the same sequence of elements. This enables a common range `[i, j)` or counted range `i + [0, n)` to be processed with extremely efficient low-level C or assembly code that operates on `[to_address(i), to_address(j))` (respectively `to_address(i) + [0, n)`).

A value-initialized iterator `I{}` can be used to denote the empty ranges `[I{}, I{})` and `I{} + [0, 0)`. While the existing semantic requirements of `contiguous_iterator` enable us to convert both dereferenceable and past-the-end iterators with `to_address`, converting ranges involving value-initialized iterators to pointer ranges additionally needs `to_address(I{})` to be well-defined. Note that `to_address` is already implicitly equality-preserving for `contiguous_iterator` arguments. Given this additional requirement `to_address(I{}) == to_address(I{})` and `to_address(I{}) == to_address(I{)) + 0` both hold, so the two types of empty ranges involving value-initialized iterators convert to empty pointer ranges as desired.

[2024-11-13; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4993.

1. Modify 24.3.4.14 [iterator.concept.contiguous] as indicated:

   > -1- The `contiguous_iterator` concept provides a guarantee that the denoted elements are stored contiguously in memory.

   > >     template<class I>
   > >       concept contiguous_iterator =
   > >         random_access_iterator<I> &&
   > >         derived_from<ITER_CONCEPT(I), contiguous_iterator_tag> &&
   > >         is_lvalue_reference_v<iter_reference_t<I>> &&
   > >         same_as<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> &&
   > >         requires(const I& i) {
   > >           { to_address(i) } -> same_as<add_pointer_t<iter_reference_t<I>>>;
   > >         };

   > -2- Let `a` and `b` be dereferenceable iterators and `c` be a non-dereferenceable iterator of type `I` such that `b` is reachable from `a` and `c` is reachable from `b`, and let `D` be `iter_difference_t<I>`. The type `I` models `contiguous_iterator` only if

   > 1. (2.1) — `to_address(a) == addressof(*a)`,

   > 2. (2.2) — `to_address(b) == to_address(a) + D(b - a)`,

   > 3. (2.3) — `to_address(c) == to_address(a) + D(c - a)`,

   > 4. (2.?) — `to_address(I{})` is well-defined,

   > 5. (2.4) — `ranges::iter_move(a)` has the same type, value category, and effects as `std::move(*a)`, and

   > 6. (2.5) — if `ranges::iter_swap(a, b)` is well-formed, it has effects equivalent to `ranges::swap(*a, *b)`.

### 4172(i). unique_lock self-move-assignment is broken

Section: 32.6.5.4.2 [thread.lock.unique.cons], 32.6.5.5.2 [thread.lock.shared.cons] Status: C++26 Submitter: Casey Carter Opened: 2024-11-13 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [thread.lock.unique.cons].

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Discussion:

The postconditions in 32.6.5.4.2 [thread.lock.unique.cons] paragraph 19:

> Postconditions: `pm == u_p.pm` and `owns == u_p.owns` (where `u_p` is the state of `u` just prior to this construction), `u.pm == 0` and `u.owns == false`.

contradict themselves if `*this` and the parameter `u` refer to the same object. (Presumably "this construction" means the assignment, and it is copy-pasta from the move constructor postconditions.) Apparently `unique_lock` didn't get the memo that we require well-defined behavior from self-move-assignment as of LWG 2839(i).

Also, the move assignment operator doesn't specify what it returns.

[2024-11-18; Casey expands the PR to cover `shared_lock`]

`shared_lock` has the same problems, and can be fixed in the same way.

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

"Should use parentheses not braces for the initializations." Jonathan volunteers to do that editorially after this gets approved.

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4993.

> Drafting Note: I've chosen to use the move-into-temporary-and-swap idiom here to keep things short and sweet. Since move construction, swap, and destruction are all `noexcept`, I've promoted move assignment from "Throws: Nothing" to `noexcept` as well. This is consistent with eliminating the implicit narrow contract condition that `*this` and `u` refer to distinct objects.

1. In the class synopsis in 32.6.5.4.1 [thread.lock.unique.general], annotate the move assignment operator as `noexcept`:

   >       namespace std {
   >         template<class Mutex>
   >         class unique_lock {
   >           [...]
   >           unique_lock& operator=(unique_lock&& u) noexcept;
   >           [...]
   >         };
   >       }

2. Modify 32.6.5.4.2 [thread.lock.unique.cons] as follows:

   >     unique_lock& operator=(unique_lock&& u) noexcept;

   > -18- Effects: If `owns` calls `pm->unlock()`. Equivalent to: `unique_lock{std::move(u)}.swap(*this)`.

   > -?- Returns: `*this`.

   > -19- Postconditions: `pm == u_p.pm` and `owns == u_p.owns` (where `u_p` is the state of `u` just prior to this construction), `u.pm == 0` and `u.owns == false`.

   > -20- [Note 1: With a recursive mutex it is possible for both `*this` and u to own the same mutex before the assignment. In this case, *this will own the mutex after the assignment and u will not. — end note]

   > -21- Throws: Nothing.

3. Modify 32.6.5.5.2 [thread.lock.shared.cons] as follows:

   >     shared_lock& operator=(shared_lock&& sl) noexcept;

   > -17- Effects: If `owns` calls `pm->unlock_shared()`. Equivalent to: `shared_lock{std::move(sl)}.swap(*this)`.

   > -?- Returns: `*this`.

   > -18- Postconditions: `pm == sl_p.pm` and `owns == sl_p.owns` (where `sl_p` is the state of `sl` just prior to this assignment), `sl.pm == nullptr` and `sl.owns == false`.

### 4175(i). `get_env()` specified in terms of `as_const()` but this doesn't work with rvalue senders

Section: 33.5.3 [exec.get.allocator], 33.5.4 [exec.get.stop.token], 33.5.5 [exec.get.env], 33.5.6 [exec.get.domain], 33.5.7 [exec.get.scheduler], 33.5.9 [exec.get.delegation.scheduler], 33.5.10 [exec.get.fwd.progress], 33.5.11 [exec.get.compl.sched] Status: C++26 Submitter: Lewis Baker Opened: 2024-11-10 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The current specification of `std::execution::get_env()` defines `get_env(o)` as `as_const(o).get_env()`. However, the `as_const()` function has a deleted rvalue-taking overload, meaning that you cannot pass temporaries to it.

This means that several uses of `get_env()` which pass expressions which are either potentially rvalues (e.g. in definition of `connect(sndr, rcvr)` it uses the expression `get_env(rcvr)`, but `rcvr` could be, and usually is, a prvalue) or always rvalues (e.g. `scheduler` concept has the expression `get_env(schedule(std::forward<Sch>(sch)))`).

The intent here was that `get_env()` is a function that takes as an argument a `const T&` and thus allows prvalues to bind to it. We basically just want to require that `get_env()` finds a const-qualified member-function. The use of `as_const()` does not seem to mirror the semantics of a function with a `const T&` parameter, so I suggest we change it to something else that expresses the intent.

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

This could use the "reified object" idea from 25.3 [range.access].

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4993.

1. Add to the end of 33.1 [exec.general] as indicated:

   > -?- For a subexpression `expr`, let `AS-CONST(expr)` be expression-equivalent to

   > >     [](const auto& x) noexcept -> const auto& { return x; }(expr)

2. Modify 33.5.3 [exec.get.allocator] as indicated:

   > -1- `get_allocator` asks a queryable object for its associated allocator.

   > -2- The name `get_allocator` denotes a query object. For a subexpression `env`, `get_allocator(env)` is expression-equivalent to `MANDATE-NOTHROW(as_constAS-CONST(env).query(get_allocator))`.

3. Modify 33.5.4 [exec.get.stop.token] as indicated:

   > -2- The name `get_stop_token` denotes a query object. For a subexpression `env`, `get_stop_token(env)` is expression-equivalent to:

   > 1. (2.1) — `MANDATE-NOTHROW(as_constAS-CONST(env).query(get_stop_token))` if that expression is well-formed.

4. Modify 33.5.5 [exec.get.env] as indicated:

   > -1- `execution::get_env` is a customization point object. For a subexpression `o`, `execution::get_env(o)` is expression-equivalent to:

   > 1. (1.1) — `MANDATE-NOTHROW(as_constAS-CONST(o).get_env())` if that expression is well-formed.

5. Modify 33.5.6 [exec.get.domain] as indicated:

   > -2- The name `get_domain` denotes a query object. For a subexpression `env`, `get_domain(env)` is expression-equivalent to `MANDATE-NOTHROW(as_constAS-CONST(env).query(get_domain))`.

6. Modify 33.5.7 [exec.get.scheduler] as indicated:

   > -2- The name `get_scheduler` denotes a query object. For a subexpression `env`, `get_scheduler(env)` is expression-equivalent to `MANDATE-NOTHROW(as_constAS-CONST(env).query(get_scheduler))`.

7. Modify 33.5.9 [exec.get.delegation.scheduler] as indicated:

   > -2- The name `get_delegation_scheduler` denotes a query object. For a subexpression `env`, `get_delegation_scheduler(env)` is expression-equivalent to `MANDATE-NOTHROW(as_constAS-CONST(env).query(get_delegation_scheduler))`.

8. Modify 33.5.10 [exec.get.fwd.progress] as indicated:

   > -2- The name `get_forward_progress_guarantee` denotes a query object. For a subexpression `sch`, let `Sch` be `decltype((sch))`. If `Sch` does not satisfy `scheduler`, `get_forward_progress_guarantee` is ill-formed. Otherwise, `get_forward_progress_guarantee(sch)` is expression-equivalent to:

   > 1. (2.1) — `MANDATE-NOTHROW(as_constAS-CONST(sch).query(get_forward_progress_guarantee))` if that expression is well-formed.

9. Modify 33.5.11 [exec.get.compl.sched] as indicated:

   > -2- The name `get_completion_scheduler` denotes a query object template. For a subexpression `q`, the expression `get_completion_scheduler<completion-tag>(q)` is ill-formed if `completion-tag` is not one of `set_value_t`, `set_error_t`, or `set_stopped_t`. Otherwise, `get_completion_scheduler<completion-tag>(q)` is expression-equivalent to

   > >     MANDATE-NOTHROW(as_constAS-CONST(q).query(get_completion_scheduler<completion-tag>))

### 4179(i). Wrong range in [alg.search]

Section: 26.6.15 [alg.search] Status: C++26 Submitter: Oscar Slotosch Opened: 2024-12-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [alg.search].

View all issues with C++26 status.

Discussion:

Originally reported as editorial request #7474:

During the qualification of the C++ STL Validas has pointed me to the following issue:

The specification of 26.6.15 [alg.search] has a wrong range. Currently the range is "`[first1, last1 - (last2 - first2))`" (exclusive) but should be "`[first1, last1 - (last2 - first2)]`" (inclusive). So please correct the closing ")" to "]". Otherwise the last occurrence will not be found. We observed the issue in C++14 and C++17 and cppreference.com.

The implementations do the right thing and work correctly and find even the last occurrence.

For example in the list `{1, 2, 3, 4, 5}` the pattern `{4, 5}` should be found (obviously). In the case the last element is not included it will not be found.

Demonstration on godbolt shows that the implementation is working and finds the pattern.

[2024-12-07; Daniel comments and provides wording]

The mentioned wording issue is present in all previous standard versions, including the SGI STL. It needs to be fixed in all search variants specified in 26.6.15 [alg.search] (except for the form using a `Searcher`).

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after ten votes in favour during reflector poll.

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4993.

1. Modify 26.6.15 [alg.search] as indicated:

   >     template<class ForwardIterator1, class ForwardIterator2>
   >       constexpr ForwardIterator1
   >         search(ForwardIterator1 first1, ForwardIterator1 last1,
   >                ForwardIterator2 first2, ForwardIterator2 last2);
   >     […]
   >     template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2,
   >              class BinaryPredicate>
   >     ForwardIterator1
   >       search(ExecutionPolicy&& exec,
   >              ForwardIterator1 first1, ForwardIterator1 last1,
   >              ForwardIterator2 first2, ForwardIterator2 last2,
   >              BinaryPredicate pred);

   > > -1- Returns: The first iterator `i` in the range `[first1, last1 - (last2 - first2)])` such that for every nonnegative integer `n` less than `last2 - first2` the following corresponding conditions hold: `*(i + n) == *(first2 + n), pred(*(i + n)`, `*(first2 + n)) != false`. Returns `first1` if `[first2, last2)` is empty, otherwise returns `last1` if no such iterator is found.

   > > -2- […]

   >     template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2,
   >              sentinel_for<I2> S2, class Pred = ranges::equal_to,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
   >       constexpr subrange<I1>
   >         ranges::search(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
   >                        Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<forward_range R1, forward_range R2, class Pred = ranges::equal_to,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
   >       constexpr borrowed_subrange_t<R1>
   >         ranges::search(R1&& r1, R2&& r2, Pred pred = {},
   >                        Proj1 proj1 = {}, Proj2 proj2 = {});

   > > -3- Returns:

   > > 1. (3.1) — `{i, i + (last2 - first2)}`, where `i` is the first iterator in the range `[first1, last1 - (last2 - first2)])` such that for every non-negative integer `n` less than `last2 - first2` the condition

   > >    >     bool(invoke(pred, invoke(proj1, *(i + n)), invoke(proj2, *(first2 + n))))

   > >    is `true`.

   > > 2. (3.2) — Returns `{last1, last1}` if no such iterator exists.

   > > -4- […]

   >     template<class ForwardIterator, class Size, class T = iterator_traits<ForwardIterator>::value_type>
   >       constexpr ForwardIterator
   >         search_n(ForwardIterator first, ForwardIterator last,
   >                  Size count, const T& value);
   >     […]
   >     template<class ExecutionPolicy, class ForwardIterator, class Size,
   >              class T = iterator_traits<ForwardIterator>::value_type,
   >              class BinaryPredicate>
   >       ForwardIterator
   >         search_n(ExecutionPolicy&& exec,
   >                  ForwardIterator first, ForwardIterator last,
   >                  Size count, const T& value,
   >                  BinaryPredicate pred);

   > > -5- […]

   > > -6- […]

   > > -7- Returns: The first iterator `i` in the range `[first, last-count])` such that for every non-negative integer `n` less than `count` the condition `E` is `true`. Returns `last` if no such iterator is found.

   > > -8- […]

   >     template<forward_iterator I, sentinel_for<I> S,
   >              class Pred = ranges::equal_to, class Proj = identity,
   >              class T = projected_value_t<I, Proj>>
   >       requires indirectly_comparable<I, const T*, Pred, Proj>
   >       constexpr subrange<I>
   >         ranges::search_n(I first, S last, iter_difference_t<I> count,
   >                          const T& value, Pred pred = {}, Proj proj = {});
   >     template<forward_range R, class Pred = ranges::equal_to,
   >              class Proj = identity, class T = projected_value_t<iterator_t<R>, Proj>>
   >       requires indirectly_comparable<iterator_t<R>, const T*, Pred, Proj>
   >       constexpr borrowed_subrange_t<R>
   >         ranges::search_n(R&& r, range_difference_t<R> count,
   >                          const T& value, Pred pred = {}, Proj proj = {});

   > > -9- Returns: `{i, i + count}` where `i` is the first iterator in the range `[first, last - count])` such that for every non-negative integer `n` less than `count`, the following condition holds: `invoke(pred, invoke(proj, *(i + n)), value)`. Returns `{last, last}` if no such iterator is found.

   > > -10- […]

### 4182(i). Definition of `NULL` is too broad

Section: 17.2.3 [support.types.nullptr] Status: WP Submitter: Janet Cobb Opened: 2024-12-09 Last modified: 2026-06-15

Priority: 3

View all issues with WP status.

Discussion:

7.3.12 [conv.ptr]/1 reads in part: "A null pointer constant is an integer literal (5.13.2 [lex.icon]) with value zero or a prvalue of type `std::nullptr_t`.".

17.2.3 [support.types.nullptr]/2 reads: "The macro `NULL` is an implementation-defined null pointer constant.".

Together, these imply that `#define NULL (::std::unreachable(), nullptr)` is a conforming definition. The expression is a prvalue of type `std::nullptr_t`, so it is a null pointer constant. This makes it implementation-defined whether any program that evaluates `NULL` has undefined behavior.

[2025-02-07; Reflector poll]

Set priority to 3 after reflector poll.

"I'd very much like to see `nullptr` added to the footnote."

Previous resolution [SUPERSEDED]

This wording is relative to N5001.

1. Modify 17.2.3 [support.types.nullptr] as indicated:

   > -2- The macro `NULL` is an implementation-defined null pointer constant that is a literal (5.13.2 [lex.icon], 5.13.8 [lex.nullptr]).footnote 161

   > 161) Possible definitions include `0` and `0L`, but not `(void*)0`.

[2025-02-07; Jonathan provides improved wording]

[2026-05-29 LWG telecon; status: New → Ready]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 17.2.3 [support.types.nullptr] as indicated:

   > -2- The macro `NULL` is an implementation-defined null pointer constant that is a literal (5.13.2 [lex.icon], 5.13.8 [lex.nullptr]).footnote 161

   > 161) Possible definitions include `nullptr`, `0` and `0L`, but not `(void*)0`.

### 4186(i). `regex_traits::transform_primary` mistakenly detects `typeid` of a function

Section: 28.6.6 [re.traits] Status: C++26 Submitter: Jiang An Opened: 2024-12-18 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [re.traits].

View all issues with C++26 status.

Discussion:

28.6.6 [re.traits]/7 currently says `typeid(use_facet<collate<charT>>) == typeid(collate_byname<charT>)`, which is always `false` because `use_facet<collate<charT>>` is a function template specialization while `collate_byname<charT>` is a class template specialization. This looks like misuse, and has been present in TR1 (N1836).

Presumably the intended operand is `use_facet<collate<charT>>(getloc())`.

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 28.6.6 [re.traits] as indicated:

   >     template<class ForwardIterator>
   >       string_type transform_primary(ForwardIterator first, ForwardIterator last) const;

   > > -7- Effects: If

   > > >     typeid(use_facet<collate<charT>>(getloc())) == typeid(collate_byname<charT>)

   > > and the form of the sort key returned by `collate_byname<charT>::transform(first, last)` is known and can be converted into a primary sort key then returns that key, otherwise returns an empty string.

### 4187(i). `bitset::reference` should be const-assignable

Section: 22.9.2 [template.bitset] Status: Resolved Submitter: Arthur O'Dwyer Opened: 2024-12-21 Last modified: 2025-11-11

Priority: 3

View all other issues in [template.bitset].

View all issues with Resolved status.

Discussion:

LWG 3638(i), which proposes changes to `vector<bool>::reference`, is related. Should `vector<bool>::reference` and `bitset<N>::reference` behave differently in any respect? I think there's no good reason for them to behave differently, and good technical incentives to permit them to behave the same. We already have implementation divergence: libc++ makes `bitset::reference` const-assignable, whereas libstdc++ and MS STL do not. This means that libc++'s `bitset::reference` successfully avoids false positives from Arthur's proposed -Wassign-to-class-rvalue diagnostic, while libstdc++'s does not (See Godbolt).

The proposed resolution applies P2321's approach. We can't just insert `const` into the existing spec, because ABI. But also, since our goal is consistency with the post-P2321 `vector<bool>::reference`, we should do the same thing here as P2321, not invent anything novel.

Open questions related to the current P/R:

1. LWG 3638 proposes to add these three `swap` overloads to `vector<bool>::reference`. Should we also, consistently, add them to `bitset::reference`? I think we should.

   >     friend constexpr void swap(reference x, reference y) noexcept;
   >     friend constexpr void swap(reference x, bool& y) noexcept;
   >     friend constexpr void swap(bool& x, reference y) noexcept;

2. Both `vector<bool>::reference` and `bitset::reference` right now are specified with

   >     constexpr reference(const reference&) = default;

   which is meaningless because we don't know the data members of `reference`. So this isn't actually specifying that the constructor is trivial, let alone that it's `noexcept`. I think we should re-specify both types' copy constructors as simply `constexpr` and `noexcept`; and then if we want them to be trivial, we should say so in English prose.

[2025-02-07; Reflector poll]

Set priority to 3 after reflector poll.

"Just const-quality the existing assignment operators." "Would need to change the return type (breaking) or use `const_cast` (weird)." "And it would be needlessly inconsistent with `vector<bool>::reference`."

"The `swap` part belongs in LWG 3638(i)."

Previous resolution [SUPERSEDED]

This wording is relative to N5001.

1. Modify 22.9.2.1 [template.bitset.general] as indicated:

   >     namespace std {
   >       template<size_t N> class bitset {
   >       public:
   >         // bit reference
   >         class reference {
   >         public:
   >           constexpr reference(const reference&) = default;
   >           constexpr ~reference();
   >           constexpr reference& operator=(bool x) noexcept;           // for b[i] = x;
   >           constexpr reference& operator=(const reference&) noexcept; // for b[i] = b[j];
   >           constexpr const reference& operator=(bool x) const noexcept;
   >           constexpr bool operator~() const noexcept;                 // flips the bit
   >           constexpr operator bool() const noexcept;                  // for x = b[i];
   >           constexpr reference& flip() noexcept;                      // for b[i].flip();
   >           friend constexpr void swap(reference x, reference y) noexcept;
   >           friend constexpr void swap(reference x, bool& y) noexcept;
   >           friend constexpr void swap(bool& x, reference y) noexcept;
   >         };
   >         […]
   >       };
   >       […]
   >     }

[2025-02-07; Jonathan provides improved wording]

Moved `swap` changes to LWG 3638(i).

[2025-11-11; Resolved by P3612R1, approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 22.9.2.1 [template.bitset.general] as indicated:

   >     namespace std {
   >       template<size_t N> class bitset {
   >       public:
   >         // bit reference
   >         class reference {
   >         public:
   >           constexpr reference(const reference&) = default;
   >           constexpr ~reference();
   >           constexpr reference& operator=(bool x) noexcept;           // for b[i] = x;
   >           constexpr reference& operator=(const reference&) noexcept; // for b[i] = b[j];
   >           constexpr const reference& operator=(bool x) const noexcept;
   >           constexpr bool operator~() const noexcept;                 // flips the bit
   >           constexpr operator bool() const noexcept;                  // for x = b[i];
   >           constexpr reference& flip() noexcept;                      // for b[i].flip();
   >         };
   >         […]
   >       };
   >       […]
   >     }

### 4188(i). `ostream::sentry` destructor should handle exceptions

Section: 31.7.6.2.4 [ostream.sentry] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-01-14 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [ostream.sentry].

View all issues with C++26 status.

Discussion:

LWG 397(i) suggested changing 31.7.6.2.4 [ostream.sentry] to say that the `ostream::sentry` destructor doesn't throw any exceptions. That issue was closed as resolved by LWG 835(i) which included the "Throws: Nothing" change to the `sentry` destructor. However, that part of the resolution never seems to have been applied to the working draft. N3091 mentions applying LWG 835 for N3090 but the destructor change is missing, maybe because the paragraph for the sentry destructor had been renumbered from p17 to p4 and LWG 835 didn't show sufficient context to indicate the intended location.

The problem described in LWG 397(i) is still present: the streambuf operations can fail, and the sentry needs to handle that. The changes for LWG 835(i) ensure no exception is thrown if `rdbuf()->pubsync()` returns -1 on failure, but do nothing for the case where it throws an exception (the original topic of LWG 397!). Because C++11 made `~sentry` implicitly `noexcept`, an exception from `rdbuf()->pubsync()` will terminate the process. That needs to be fixed.

Libstdc++ does terminate if `pubsync()` throws when called by `~sentry`. Both MSVC and Libc++ silently swallow exceptions. It seems preferable to handle the exception and report an error, just as we do when `pubsync()` returns -1.

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 31.7.6.2.4 [ostream.sentry] as indicated:

   >     ~sentry();

   > > -4- If `(os.flags() & ios_base::unitbuf) && !uncaught_exceptions() && os.good()` is `true`, calls `os.rdbuf()->pubsync()`. If that function returns −1 or exits via an exception, sets `badbit` in `os.rdstate()` without propagating an exception.

### 4189(i). `cache_latest_view` should be freestanding

Section: 17.3.2 [version.syn], 25.2 [ranges.syn] Status: C++26 Submitter: Hewill Kang Opened: 2024-12-23 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with C++26 status.

Discussion:

`cache_latest_view` can be freestanding, but this never comes up in the discussion, which seems to be an oversight.

Previous resolution [SUPERSEDED]

This wording is relative to N5001.

1. Modify 17.3.2 [version.syn] as indicated:

   >     #define __cpp_lib_ranges_cache_latest 202411L // freestanding, also in <ranges>

2. Modify 25.2 [ranges.syn] as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   >     #include <iterator>             // see 24.2 [iterator.synopsis]

   >     namespace std::ranges {
   >       […]
   >       // 25.7.34 [range.cache.latest], cache latest view
   >       template<input_range V>
   >         requires view<V>
   >       class cache_latest_view;                                                                // freestanding

   >       namespace views { inline constexpr unspecified cache_latest = unspecified; }            // freestanding
   >       […]
   >     }

[2025-01-04; Tim Song suggests alternative wording]

While we are here, we can use the new convention from P2407 to dramatically simplify `<ranges>`. Most future additions to this header should have no problem being freestanding, so that is the right default.

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 17.3.2 [version.syn] as indicated:

   >     #define __cpp_lib_ranges_cache_latest 202411L // freestanding, also in <ranges>

2. Delete all "// freestanding" comments in 25.2 [ranges.syn], header `<ranges>` synopsis, and then modify as indicated:

   >     // mostly freestanding
   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   >     #include <iterator>             // see 24.2 [iterator.synopsis]

   >     […]
   >     namespace std::ranges {
   >       // 25.5.6 [range.elementsof], class template elements_of
   >       template<range R, class Allocator = allocator<byte>>
   >         struct elements_of;  // hosted
   >       […]

   >       // 25.6.6 [range.istream], istream view
   >       template<movable Val, class CharT, class Traits = char_traits<CharT>>
   >         requires see below
   >       class basic_istream_view;  // hosted
   >       template<class Val>
   >         using istream_view = basic_istream_view<Val, char>;   // hosted
   >       template<class Val>
   >         using wistream_view = basic_istream_view<Val, wchar_t>;   // hosted

   >       namespace views {
   >         template<class T> constexpr unspecified istream = unspecified; // hosted
   >       }
   >       […]
   >     }
   >     […]

### 4190(i). Specification of completion-signatures-for in [exec.snd.expos]/p39 is recursive

Section: 33.9.2 [exec.snd.expos] Status: Resolved Submitter: Eric Niebler Opened: 2025-01-02 Last modified: 2026-04-01

Priority: 2

View all other issues in [exec.snd.expos].

View all issues with Resolved status.

Discussion:

33.9.2 [exec.snd.expos]/p39 reads:

> For a subexpression `sndr` let `Sndr` be `decltype((sndr))`. Let `rcvr` be a receiver with an associated environment of type `Env` such that `sender_in<Sndr, Env>` is `true`. `completion-signatures-for<Sndr, Env>` denotes a specialization of `completion_signatures`, the set of whose template arguments correspond to the set of completion operations that are potentially evaluated as a result of starting (33.3 [exec.async.ops]) the operation state that results from connecting `sndr` and `rcvr`. When `sender_in<Sndr, Env>` is `false`, the type denoted by `completion-signatures-for<Sndr, Env>`, if any, is not a specialization of `completion_signatures`.

This paragraph is trying to specify the return type of `basic-sender::get_completion_signatures`, but it immediately goes off the rails when it tests for the satisfaction of `sender_in<Sndr, Env>`. The `sender_in<Sndr, Env>` concept requires that `get_completion_signatures(sndr, env)` is well-formed and that its type is a specialization of `completion_signatures`. But the return type of `get_completion_signatures(sndr, env)` is exactly the thing this para is trying to define!

[2025-02-07; Reflector poll]

Set priority to 2 after reflector poll.

First sentence of p39 should be either "Let the type `Sndr` be ..." or "Let `Sndr` be ...", but not "Let type `Sndr` be ...".

[2026-04-01 Resolved by the adoption of P3557R3 at Sofia, 2025. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.9.2 [exec.snd.expos] as indicated:

   > -39- Let type `Sndr` be a (possibly `const`-qualified) specialization of `basic-sender` or an lvalue reference of such, and let `Rcvr` be the type of a receiver with an associated environment of type `Env`. If the type `basic-operation<Sndr, Rcvr>` is well-formed, let `op` be an lvalue subexpression of that type.For a subexpression `sndr` let `Sndr` be `decltype((sndr))`. Let `rcvr` be a receiver with an associated environment of type `Env` such that `sender_in<Sndr, Env>` is `true`. `completion-signatures-for<Sndr, Env>` denotes a specialization of `completion_signatures`, the set of whose template arguments correspond to the set of completion operations that are potentially evaluated (6.3 [basic.def.odr]) as a result of evaluating `op.start()`.starting (33.3 [exec.async.ops]) the operation state that results from connecting `sndr` and `rcvr`. When `sender_in<Sndr, Env>` is `false` Otherwise, the type denoted by `completion-signatures-for<Sndr, Env>`, if any, is not a specialization of `completion_signatures`.

   > Recommended practice: When the type `basic-operation<Sndr, Rcvr>` is ill-formed `sender_in<Sndr, Env>` is `false`, implementations are encouraged to use the type denoted by `completion-signatures-for<Sndr, Env>` to communicate to users why.

### 4191(i). P1467 changed the return type of `pow(complex<float>, int)`

Section: 29.4.10 [cmplx.over] Status: C++26 Submitter: Tim Song Opened: 2025-01-10 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [cmplx.over].

View all issues with C++26 status.

Discussion:

Before C++20, 29.4.10 [cmplx.over] says that this produces a `complex<double>`. This was confirmed by LWG 844(i) and consistent with C99.

P1467 changed the return type to `complex<common_type_t<float, int>>`, which is `complex<float>`. This is a breaking change that does not appear to have been intentional.

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 29.4.10 [cmplx.over] as indicated:

   > -3- Function template `pow` has additional constexpr overloads sufficient to ensure, for a call with one argument of type `complex<T1>` and the other argument of type `T2` or `complex<T2>`, both arguments are effectively cast to `complex<common_type_t<T1, T3T2>>`, where `T3` is `double` if `T2` is an integer type and `T2` otherwise. If `common_type_t<T1, T3T2>` is not well-formed, then the program is ill-formed.

### 4196(i). Complexity of `inplace_merge()` is incorrect

Section: 26.8.6 [alg.merge] Status: C++26 Submitter: Stephen Howe Opened: 2025-01-22 Last modified: 2026-06-09

Priority: 4

View all other issues in [alg.merge].

View all issues with C++26 status.

Discussion:

For N5001, section 26.8.6 [alg.merge] p5, it says for `merge()` Complexity (emphasis mine):

> For the overloads with no `ExecutionPolicy`, at most `N - 1` comparisons and applications of each projection

For N5001, section 26.8.6 [alg.merge] p11, it says from `inplace_merge()` Complexity (emphasis mine):

> Complexity: Let `N = last - first`:

> 1. (11.1) — For the overloads with no `ExecutionPolicy`, and if enough additional memory is available, exactly `N - 1` comparisons.

> 2. (11.2) — Otherwise, `𝒪(N log N)` comparisons.

This wording should be (emphasis mine)

> Complexity: Let `N = last - first`:

> 1. (11.1) — For the overloads with no `ExecutionPolicy`, and if enough additional memory is available, at most `N - 1` comparisons.

> 2. (11.2) — Otherwise, `𝒪(N log N)` comparisons.

Consider the 2 sequences in a `std::vector` of `int`s and assume that `inplace_merge` has enough memory:

>     { 1 }, { 2, 3, 4, 5, 6, 7 )

`N` is `7`, 7 elements. So `N - 1` is `6`

If you `inplace_merge()` the two sequences, the `1` is compared with `2` and `1` is output. But now the 1st sequence is over, so the 2nd sequence is copied. Only 1 comparison is done, not 6.

[2025-01-25; Daniel comments]

The SGI STL archive correctly says "at most" as well.

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll. There were nine votes for P0 (Tentatively Ready), but also several for NAD Editorial, because 16.3.2.4 [structure.specifications]/7 explicitly says that all complexity requirements are upper bounds and it's conforming to do less work. The consensus was that this is still a clarifying improvement.

[Hagenberg 2025-02-16; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 26.8.6 [alg.merge] as indicated:

   >     template<class BidirectionalIterator>
   >       constexpr void inplace_merge(BidirectionalIterator first,
   >                                    BidirectionalIterator middle,
   >                                    BidirectionalIterator last);
   >     template<class ExecutionPolicy, class BidirectionalIterator>
   >       void inplace_merge(ExecutionPolicy&& exec,
   >                          BidirectionalIterator first,
   >                          BidirectionalIterator middle,
   >                          BidirectionalIterator last);
   >     template<class BidirectionalIterator, class Compare>
   >       constexpr void inplace_merge(BidirectionalIterator first,
   >                                    BidirectionalIterator middle,
   >                                    BidirectionalIterator last, Compare comp);
   >     template<class ExecutionPolicy, class BidirectionalIterator, class Compare>
   >       void inplace_merge(ExecutionPolicy&& exec,
   >                          BidirectionalIterator first,
   >                          BidirectionalIterator middle,
   >                          BidirectionalIterator last, Compare comp);
   >     template<bidirectional_iterator I, sentinel_for<I> S, class Comp = ranges::less,
   >              class Proj = identity>
   >       requires sortable<I, Comp, Proj>
   >       constexpr I ranges::inplace_merge(I first, I middle, S last, Comp comp = {}, Proj proj = {});

   > > -7- […]

   > > -8- Preconditions: […]

   > > -9- Effects: Merges two sorted consecutive ranges `[first, middle)` and `[middle, last)`, putting the result of the merge into the range `[first, last)`. The resulting range is sorted with respect to `comp` and `proj`.

   > > -10- Returns: `last` for the overload in namespace `ranges`.

   > > -11- Complexity: Let `N = last - first`:

   > > 1. (11.1) — For the overloads with no `ExecutionPolicy`, and if enough additional memory is available, exactlyat most `N - 1` comparisons.

   > > 2. (11.2) — Otherwise, `𝒪(N log N)` comparisons.

   > > In either case, twice as many projections as comparisons.

### 4198(i). `schedule_from` isn't starting the schedule sender if decay-copying results throws

Section: 33.9.12.7 [exec.schedule.from] Status: C++26 Submitter: Eric Niebler Opened: 2025-02-03 Last modified: 2026-06-09

Priority: 1

View all other issues in [exec.schedule.from].

View all issues with C++26 status.

Discussion:

Imported from cplusplus/sender-receiver #304.

33.9.12.7 [exec.schedule.from]/p11 specifies `schedule_from`'s completion operation as follows:

    []<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept
        -> void {
      using result_t = decayed-tuple<Tag, Args...>;
      constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>;

      try {
        state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
      } catch (...) {
        if constexpr (!nothrow) {
          set_error(std::move(rcvr), current_exception());
          return;
        }
      }
      start(state.op-state);
    };

so if emplacing the result tuple throws, `set_error` is immediately called on the downstream receiver. no attempt is made to post the completion to the specified scheduler. this is probably not the right behavior.

Suggested resolution

The right thing, i think, is to catch the exception, emplace the `exception_ptr` into the `async-result` variant, and then start the schedule operation, as shown below:

    []<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept
        -> void {
      using result_t = decayed-tuple<Tag, Args...>;
      constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>;

      try {
        state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
      } catch(...) {
        if constexpr (!nothrow)
          state.async-result.template emplace<tuple<set_error_t, exception_ptr>>(set_error, current_exception());
      }

      start(state.op-state);
    }

we also need to change how we specify the variant type of `state.async-result`:

> Let `Sigs` be a pack of the arguments to the `completion_signatures` specialization named by `completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>`. Let `as-tuple` be an alias template that transforms a completion signature `Tag(Args...)` into the tuple specialization `decayed-tuple<Tag, Args...>`. such that `as-tuple<Tag(Args...)>` denotes the tuple specialization `decayed-tuple<Tag, Args...>`, and let `is-nothrow-decay-copy-sig` be a variable template such that `is-nothrow-decay-copy-sig<Tag(Args...)>` is a core constant expression of type `bool const` and whose value is `true` if the types `Args...` are all nothrow decay-copyable, and `false` otherwise. Let `error-completion` be a pack consisting of the type `set_error_t(exception_ptr)` if `(is-nothrow-decay-copy-sig<Sigs> &&...)` is `false`, and an empty pack otherwise. Then `variant_t` denotes the type `variant<monostate, as-tuple<Sigs>..., error-completion...>`, except with duplicate types removed.

[This touches the same text as LWG 4203(i).]

[2025-02-07; Reflector poll]

Set priority to 1 after reflector poll.

[Hagenberg 2025-02-11; LWG]

Direction seems right. Decay-copyable is not a defined term.

[2025-02-12 Tim adds PR]

This also corrects the issue that `nothrow` is currently relying on the unspecified exception specification of `tuple`'s constructor.

[Hagenberg 2025-02-12; move to Ready]

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.9.12.7 [exec.schedule.from] p8 as indicated:

   > -8- Let `Sigs` be a pack of the arguments to the `completion_signatures` specialization named by `completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>`. Let `as-tuple` be an alias template that transforms a completion signature `Tag(Args...)` into the tuple specialization `decayed-tuple<Tag, Args...>`. such that `as-tuple<Tag(Args...)>` denotes the type `decayed-tuple<Tag, Args...>`, and let `is-nothrow-decay-copy-sig` be a variable template such that `auto(is-nothrow-decay-copy-sig<Tag(Args...)>)` is a constant expression of type `bool` and equal to `(is_nothrow_constructible_v<decay_t<Args>, Args> && ...)`. Let `error-completion` be a pack consisting of the type `set_error_t(exception_ptr)` if `(is-nothrow-decay-copy-sig<Sigs> &&...)` is `false`, and an empty pack otherwise. Then `variant_t` denotes the type `variant<monostate, as-tuple<Sigs>..., error-completion...>`, except with duplicate types removed.

2. Modify 33.9.12.7 [exec.schedule.from] p11 as indicated:

   > -11- The member `impls-for<schedule_from_t>::complete` is initialized with a callable object equivalent to the following lambda:

   > >     []<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept
   > >         -> void {
   > >       using result_t = decayed-tuple<Tag, Args...>;
   > >       constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>
   > >                                (is_nothrow_constructible_v<decay_t<Args>, Args> && ...);

   > >       try {
   > >         state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
   > >       } catch (...) {
   > >         if constexpr (!nothrow) {
   > >           state.async-result.template emplace<tuple<set_error_t, exception_ptr>>(set_error, current_exception());
   > >           set_error(std::move(rcvr), current_exception());
   > >           return;
   > >         }
   > >       }
   > >       start(state.op-state);
   > >     };

### 4200(i). The `operation_state` concept can be simplified

Section: 33.8.1 [exec.opstate.general] Status: C++26 Submitter: Eric Niebler Opened: 2025-02-03 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Imported from cplusplus/sender-receiver #312.

The current defn of the `operation_state` concept is:

    template<class O>
      concept operation_state =
        derived_from<typename O::operation_state_concept, operation_state_t> &&
        is_object_v<O> &&
        requires (O& o) {
          { start(o) } noexcept;
        };

I think the `is_object_v<O>` constraint is not needed because the `derived_from` constraint has already established that `O` is a class type.

And `start(o)` is always `noexcept` now that `start` mandates the `noexcept`-ness of `op.start()`.

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.8.1 [exec.opstate.general] as indicated:

   >     template<class O>
   >       concept operation_state =
   >         derived_from<typename O::operation_state_concept, operation_state_t> &&
   >         is_object_v<O> &&
   >         requires (O& o) {
   >           { start(o) } noexcept;
   >         };

### 4201(i). `with-await-transform::await_transform` should not use a deduced return type

Section: 33.9.4 [exec.awaitable] Status: C++26 Submitter: Brian Bi Opened: 2025-02-03 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Imported from cplusplus/sender-receiver #309.

33.9.4 [exec.awaitable]/p5

The use of the deduced return type causes the definition of the sender's `as_awaitable` method to be instantiated too early, e.g., when the sender is passed to `get_completion_signatures`.

[Eric provides wording]

[2025-02-07; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.9.4 [exec.awaitable] as indicated:

   > -5- Let `with-await-transform` be the exposition-only class template:

   >     namespace std::execution {
   >       template<class T, class Promise>
   >         concept has-as-awaitable =                                  // exposition only
   >           requires (T&& t, Promise& p) {
   >             { std::forward<T>(t).as_awaitable(p) } -> is-awaitable<Promise&>;
   >           };

   >       template<class Derived>
   >         struct with-await-transform {                               // exposition only
   >           template<class T>
   >             T&& await_transform(T&& value) noexcept {
   >               return std::forward<T>(value);
   >             }

   >           template<has-as-awaitable<Derived> T>
   >             decltype(auto)auto await_transform(T&& value)
   >               noexcept(noexcept(std::forward<T>(value).as_awaitable(declval<Derived&>())))
   >             -> decltype(std::forward<T>(value).as_awaitable(declval<Derived&>())) {
   >               return std::forward<T>(value).as_awaitable(static_cast<Derived&>(*this));
   >             }
   >         };
   >     }

### 4202(i). `enable-sender` should be a variable template

Section: 33.9.3 [exec.snd.concepts] Status: C++26 Submitter: Eric Niebler Opened: 2025-02-03 Last modified: 2026-06-09

Priority: 1

View all issues with C++26 status.

Discussion:

Imported from cplusplus/sender-receiver #305 and cplusplus/sender-receiver #306.

We require an opt-in to satisfy the `sender` concept. Making your type awaitable with an empty environment is one way to opt in. If your awaitable requires an environment, you have two options:

- give your awaitable type a nested `sender_concept` typedef, or
- specialize `enable_sender` for your awaitable type.

The problem is that somewhere along the way, the `enable_sender` variable template was turned into an exposition-only `enable-sender` concept. We should roll back that change.

[2025-02-07; Reflector poll]

Set priority to 1 after reflector poll.

[Hagenberg 2025-02-11; move to Ready]

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Change 33.9.3 [exec.snd.concepts] as indicated:

   >       template<class Sndr>
   >         concept is-sender =                                         // exposition only
   >           derived_from<typename Sndr::sender_concept, sender_t>;

   >       template<class Sndr>
   >         concept enable-sender =                                     // exposition only
   >           is-sender<Sndr> ||
   >           is-awaitable<Sndr, env-promise<empty_env>>;               // [exec.awaitable]

   >       template<class Sndr>
   >         inline constexpr bool enable_sender = enable-sender<Sndr>;

   >       template<class Sndr>
   >         concept sender =
   >           bool(enable-senderenable_sender<remove_cvref_t<Sndr>>) &&
   >           requires (const remove_cvref_t<Sndr>& sndr) {
   >             { get_env(sndr) } -> queryable;
   >           } &&
   >           move_constructible<remove_cvref_t<Sndr>> &&
   >           constructible_from<remove_cvref_t<Sndr>, Sndr>;

   > …

   > -2- Given a subexpression `sndr`, […]

   > -?- Remarks: Pursuant to 16.4.5.2.1 [namespace.std], users may specialize `enable_sender` to `true` for cv-unqualified program-defined types that model `sender`, and `false` for types that do not. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have type `const bool`.

### 4203(i). Constraints on `get-state` functions are incorrect

Section: 33.9.12.7 [exec.schedule.from] Status: C++26 Submitter: Eric Niebler Opened: 2025-02-03 Last modified: 2026-06-09

Priority: 1

View all other issues in [exec.schedule.from].

View all issues with C++26 status.

Discussion:

Imported from: cplusplus/sender-receiver #313 and cplusplus/sender-receiver #314.

33.9.12.7 [exec.schedule.from] p6 reads:

> The member `impls-for<schedule_from_t>::get-state` is initialized with a callable object equivalent to the following lambda:

>       []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below)
>          requires sender_in<child-type<Sndr>, env_of_t<Rcvr>> {

The constraint should account for the fact that the child sender will be connected with `FWD-ENV(get_env(rcvr))`.

[ The resolution touches some of the same text as LWG 4198(i), but without conflicting. ]

Imported from: cplusplus/sender-receiver #315.

33.9.12.12 [exec.when.all] p6 reads:

> The member `impls-for<when_all_t>::get-env` is initialized with a callable object equivalent to the following lambda expression:

>       []<class State, class Rcvr>(auto&&, State& state, const Receiver& rcvr) noexcept {
>         return see below;
>       }

> Returns an object `e` such that
> 1. (6.1) — `decltype(e)` models `queryable`, and
> 2. (6.2) — `e.query(get_stop_token)` is expression-equivalent to `state.stop-src.get_token()`, and
> 3. (6.3) — given a query object `q` with type other than cv `stop_token_t`, `e.query(q)` is expression-equivalent to `get_env(rcvr).query(q)`.

The problem is in (6.3). It should be forwarding on `forwarding-query`'s to `get_env(rcvr)` but is is instead forwarding all queries.

Imported from: cplusplus/sender-receiver #316.

The child senders should only see the parent's queries if they are forwarding queries.

Imported from: cplusplus/sender-receiver #311.

33.9.12.14 [exec.stopped.opt]/para 3 reads:

> Let `sndr` and `env` be subexpressions such that `Sndr` is `decltype((sndr))` and `Env` is `decltype((env))`. If `sender-for<Sndr, stopped_as_optional_t>` is `false`, or if the type `single-sender-value-type<Sndr, Env>` is ill-formed or `void`, then the expression `stopped_as_optional.transform_sender(sndr, env)` is ill-formed; otherwise, it is equivalent to:

the test for `single-sender-value-type<Sndr, Env>` is incorrect. It should be testing its child for single-sender-ness.

In addition, it should be applying `FWD-ENV-T` to `Env` so that only forwarding queries are forwarded.

[2025-02-07; Reflector poll]

Set priority to 1 after reflector poll.

[Hagenberg 2025-02-11; move to Ready]

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Change 33.9.2 [exec.snd.expos] as indicated:

   > -2- For a queryable object `env`, `FWD-ENV(env)` is an expression whose type satisfies `queryable` such that for a query object `q` and a pack of subexpressions `as`, the expression `FWD-ENV(env).query(q, as...)` is ill-formed if `forwarding_query(q)` is `false`; otherwise, it is expression-equivalent to `env.query(q, as...)`. The type `FWD-ENV-T(Env)` is `decltype(FWD-ENV(declval<Env>()))`.

2. Change 33.9.12.7 [exec.schedule.from] as indicated:

   > -6- The member `impls-for<schedule_from_t>::get-state` is initialized with a callable object equivalent to the following lambda:

   >       []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below)
   >        requires sender_in<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)> {

   > …

   > -8- Let `Sigs` be a pack of the arguments to the `completion_signatures` specialization named by `completion_signatures_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>`. Let `as-tuple` be an alias template that transforms a completion signature `Tag(Args...)` into the tuple specialization `decayed-tuple<Tag, Args...>`. Then `variant_t` denotes the type `variant<monostate, as-tuple<Sigs>...>`, except with duplicate types removed.

3. Change 33.9.12.10 [exec.let] as indicated:

   > -6- Let `receiver2` denote the following exposition-only class template:

   >       namespace std::execution {
   >         …
   >       }

   > Invocation of the function `receiver2::get_env` returns an object `e` such that
   > 1. (6.1) — `decltype(e)` models `queryable` and
   > 2. (6.2) — given a query object `q`, the expression `e.query(q)` is expression-equivalent to `env.query(q)` if that expression is valid,; otherwise, if the type of `q` satisfies `forwarding-query`, `e.query(q)` is expression-equivalent to `get_env(rcvr).query(q)` ; otherwise, `e.query(q)` is ill-formed.

   > -7- `impls-for<decayed-typeof<let-cpo>>::get-state` is initialized with a callable object […]

   > -8- Let `Sigs` be a pack of the arguments to the `completion_signatures` specialization named by `completion_signatures_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>`. Let `LetSigs` be a pack of those types in `Sigs` with a return type of `decayed-typeof<set-cpo>`. Let `as-tuple` be an alias template such that `as-tuple<Tag(Args...)>` denotes the type `decayed-tuple<Args...>`. Then `args_variant_t` denotes the type `variant<monostate, as-tuple<LetSigs>...>` except with duplicate types removed.

4. Change 33.9.12.12 [exec.when.all] as indicated:

   > -6- The member `impls-for<when_all_t>::get-env` is initialized with a callable object equivalent to the following lambda expression:

   >       []<class State, class Rcvr>(auto&&, State& state, const Receiver& rcvr) noexcept {
   >         return see below;
   >       }

   > Returns an object `e` such that
   > 1. (6.1) — `decltype(e)` models `queryable`, and
   > 2. (6.2) — `e.query(get_stop_token)` is expression-equivalent to `state.stop-src.get_token()`, and
   > 3. (6.3) — given a query object `q` with type other than cv `stop_token_t` and whose type satisfies `forwarding-query`, `e.query(q)` is expression-equivalent to `get_env(rcvr).query(q)`.

   > -7- The member `impls-for<when_all_t>::get-state` is initialized with a callable object equivalent to the following lambda expression:

   >       []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(e) -> decltype(e) {
   >         return e;
   >       }

   > where e is the expression

   >       std::forward<Sndr>(sndr).apply(make-state<Rcvr>())

   > and `make-state` is the following exposition-only class template:

   >       template<class Sndr, class Env>
   >       concept max-1-sender-in = sender_in<Sndr, Env> && // exposition only
   >         (tuple_size_v<value_types_of_t<Sndr, Env, tuple, tuple>> <= 1);

   >       enum class disposition { started, error, stopped }; // exposition only

   >       template<class Rcvr>
   >       struct make-state {
   >        template<max-1-sender-in<FWD-ENV-T(env_of_t<Rcvr>)>... Sndrs>

   > …

   > -8- Let `copy_fail` be `exception_ptr` if […]

   > -9- The alias `values_tuple` denotes the type

   >       tuple<value_types_of_t<Sndrs, FWD-ENV-T(env_of_t<Rcvr>), decayed-tuple, optional>...>

   > if that type is well-formed; otherwise, `tuple<>`.

5. Change 33.9.12.13 [exec.into.variant] as indicated:

   > -5- The member `impls-for<into_variant_t>::get-state` is initialized with a callable object equivalent to the following lambda:

   >       []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept
   >         -> type_identity<value_types_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>> {
   >         return {};
   >       }

6. Change 33.9.12.14 [exec.stopped.opt] as indicated:

   > -3- Let `sndr` and `env` be subexpressions such that `Sndr` is `decltype((sndr))` and `Env` is `decltype((env))`. If `sender-for<Sndr, stopped_as_optional_t>` is `false`, or if the type `single-sender-value-type<child-type<Sndr>, FWD-ENV-T(Env)>` is ill-formed or `void`, then the expression `stopped_as_optional.transform_sender(sndr, env)` is ill-formed; otherwise, it is equivalent to:

   >       auto&& [_, _, child] = sndr;
   >       using V = single-sender-value-type<child-type<Sndr>, FWD-ENV-T(Env)>;

   > …

### 4204(i). specification of `as-sndr2(Sig)` in [exec.let] is incomplete

Section: 33.9.12.10 [exec.let] Status: C++26 Submitter: Eric Niebler Opened: 2025-02-03 Last modified: 2026-06-09

Priority: 1

View all other issues in [exec.let].

View all issues with C++26 status.

Discussion:

33.9.12.10 [exec.let]/p9 reads:

> Given a type `Tag` and a pack `Args`, let `as-sndr2` be an alias template such that `as-sndr2<Tag(Args...)>` denotes the type `call-result-t<Fn, decay_t<Args>&...>`. Then `ops2_variant_t` denotes the type

>      variant<monostate, connect_result_t<as-sndr2<LetSigs>, receiver2<Rcvr, Env>>...>

> except with duplicate types removed.

The type `Env` is not specified. It should be `env_t` from paragraph 7.

Paragraphs 8, 9, and 10 only make sense in relation to the lambda in paragraph 7, but that is not at all clear from the current wording. I suggest making paragraphs 8, 9, and 10 sub-bullets of paragraph 7.

[2025-02-07; Reflector poll]

Set priority to 1 after reflector poll.

[Hagenberg 2025-02-11; move to Ready]

Dropped the suggestion to nest p8-10 under p7.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Change 33.9.12.10 [exec.let] as indicated:

   > -7- `impls-for<decayed-typeof<let-cpo>>::get-state` is initialized with a callable object equivalent to the following:

   >       []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) requires see below {
   >         auto& [_, fn, child] = sndr;
   >         using fn_t = decay_t<decltype(fn)>;
   >         using env_t = decltype(let-env(child));
   >         using args_variant_t = see below;
   >         using ops2_variant_t = see below;

   >         struct state-type {
   >           fn_t fn;                    // exposition only
   >           env_t env;                  // exposition only
   >           args_variant_t args;        // exposition only
   >           ops2_variant_t ops2;        // exposition only
   >         };
   >         return state-type{std::forward_like<Sndr>(fn), let-env(child), {}, {}};
   >       }

   > -8- Let `Sigs` be a pack of the arguments to the `completion_signatures` specialization named by `completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>`. Let `LetSigs` be a pack of those types in `Sigs` with a return type of `decayed-typeof<set-cpo>`. Let `as-tuple` be an alias template such that `as-tuple<Tag(Args...)>` denotes the type `decayed-tuple<Args...>`. Then `args_variant_t` denotes the type `variant<monostate, as-tuple<LetSigs>...>` except with duplicate types removed.

   > -9- Given a type `Tag` and a pack `Args`, let `as-sndr2` be an alias template such that `as-sndr2<Tag(Args...)>` denotes the type `call-result-t<Fn, decay_t<Args>&...>`. Then `ops2_variant_t` denotes the type

   >      variant<monostate, connect_result_t<as-sndr2<LetSigs>, receiver2<Rcvr, Envenv_t>>...>

   > except with duplicate types removed.

   > -10- The requires-clause constraining the above lambda is satisfied if and only if the types `args_variant_t` and `ops2_variant_t` are well-formed.

### 4205(i). `let_[*].transform_env` is specified in terms of the `let_*` sender itself instead of its child

Section: 33.9.12.10 [exec.let] Status: C++26 Submitter: Eric Niebler Opened: 2025-02-04 Last modified: 2026-06-09

Priority: 1

View all other issues in [exec.let].

View all issues with C++26 status.

Discussion:

Imported from cplusplus/sender-receiver #319.

33.9.12.10 [exec.let] para 13 reads:

> 13. Let `sndr` and `env` be subexpressions, and let `Sndr` be `decltype((sndr))`. If `sender-for<Sndr, decayed-typeof<let-cpo>>` is `false`, then the expression `let-cpo.transform_env(sndr, env)` is ill-formed. Otherwise, it is equal to `JOIN-ENV(let-env(sndr), FWD-ENV(env))`.

The sender passed to `let-env` here should be the child of `sndr`.

[2025-02-07; Reflector poll]

Set priority to 1 after reflector poll.

"We seem to be missing a guarantee that `auto [_,_,child] = sndr;` works. We guarantee that it can be used in a structured binding, but not that it must work with a size of three."

[Hagenberg 2025-02-11; move to Ready]

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.9.12.10 [exec.let] as indicated:

   > -13- Let `sndr` and `env` be subexpressions, and let `Sndr` be `decltype((sndr))`. If `sender-for<Sndr, decayed-typeof<let-cpo>>` is `false`, then the expression `let-cpo.transform_env(sndr, env)` is ill-formed. Otherwise, it is equal to `JOIN-ENV(let-env(sndr), FWD-ENV(env))`.

   >       auto& [_, _, child] = sndr;
   >       return JOIN-ENV(let-env(child), FWD-ENV(env));

### 4208(i). Wording needs to ensure that in `connect(sndr, rcvr)` that `rcvr` expression is only evaluated once

Section: 33.9.10 [exec.connect] Status: C++26 Submitter: Eric Niebler Opened: 2025-02-07 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [exec.connect].

View all other issues in [exec.connect].

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Discussion:

Imported from cplusplus/sender-receiver #325.

The current wording of `connect(sndr, rcvr)` defines the `new_sndr` expression as `transform_sender(decltype(get-domain-late(sndr, get_env(rcvr))){}, sndr, get_env(rcvr))`.

It then defines `connect(sndr, rcvr)` as expression equivalent to `new_sndr.connect(rcvr)`.

As currently worded, this requires evaluating the `rcvr` expression twice. Note that the first usage in the `new_sndr` expression is unevaluated, but the second usage in `get_env(rcvr)` is evaluated.

I think we need to add an extra sentence at the end of this section saying "Where the expression `rcvr` is only evaluated once." or similar.

[Hagenberg 2025-02-11; move to Ready]

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.9.10 [exec.connect] as indicated:

   > -6- The expression `connect(sndr, rcvr)` is expression-equivalent to:

   > 1. (6.1) — `new_sndr.connect(rcvr)` if that expression is well-formed.

   >    Mandates: The type of the expression above satisfies `operation_state`.

   > 2. (6.2) — Otherwise, `connect-awaitable(new_sndr, rcvr)`.

   > except that `rcvr` is evaluated only once.

   > Mandates: `sender<Sndr> && receiver<Rcvr>` is `true`.

### 4209(i). `default_domain::transform_env` should be returning `FWD-ENV(env)`

Section: 33.9.6 [exec.domain.default] Status: C++26 Submitter: Eric Niebler Opened: 2025-02-07 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [exec.domain.default].

View all issues with C++26 status.

Discussion:

Imported from cplusplus/sender-receiver #168.

When writing a generic recursive sender transform, you need to ability to unpack an unknown sender `S` and recursively transform the children.

For that, it can be useful to know the type of the environment that `S` will use when connecting its child senders, which is why `transform_env` exists.

For an environment `E` and a sender `S` with tag `T` child `C`, the expression `default_domain().transform_env(S, E)` should return an environment `E2` that is identical to the environment of the receiver that `S` uses to connect `C`.

`default_domain().transform_env(S, E)` will first check whether `T().transform_env(S, E)` is well-formed. If so, it will return that (e.g. `when_all_t` has a `transform_env` that adds a stop token to the environment).

If `T().transform_env(S, E)` is not well-formed, what should `default_domain::transform_env` do? At present, it returns `E` unmodified.

But 33.9.12.1 [exec.adapt.general] has this:

> [unless otherwise specified, when] a parent sender is connected to a receiver `rcvr`, any receiver used to connect a child sender has an associated environment equal to `FWD-ENV(get_env(rcvr))`.

So the correct thing for `default_domain::transform_env` to do is to return `FWD-ENV(get_env(rcvr))`.

[Hagenberg 2025-02-11; move to Ready]

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.9.6 [exec.domain.default] as indicated:

   >     template<sender Sndr, queryable Env>
   >       constexpr queryable decltype(auto) transform_env(Sndr&& sndr, Env&& env) noexcept;

   > > -5- Let `e` be the expression

   > > >     tag_of_t<Sndr>().transform_env(std::forward<Sndr>(sndr), std::forward<Env>(env))

   > > if that expression is well-formed; otherwise, `static_cast<Env>FWD-ENV(std::forward<Env>(env))`.

   > > -6- Mandates: `noexcept(e)` is `true`.

   > > -7- Returns: `e`.

### 4213(i). Sender spec depends on unspecified order of evaluation

Section: 33.9 [exec.snd] Status: Resolved Submitter: Frank Birbacher Opened: 2025-02-14 Last modified: 2026-04-01

Priority: 2

View all issues with Resolved status.

Discussion:

In certain clauses for defining senders the unspecified order of evaluation of function arguments can lead to retrieving values from a move-from state of a sender. An example is 33.9.12.6 [exec.continues.on] where paragraph 3 states:

>     transform_sender(get-domain-early(sndr), make-sender(continues_on, sch, sndr))

In this expression the evaluation of `get-domain-early(sndr)` can happen before or after the `make-sender`. The latter can steal the value from `sndr` by moving from it. So `get-domain-early` may see the moved-from state of `sndr` and fail to obtain anything.

Repetitions are at least in:

1. 33.9.12.9 [exec.then] p3

2. 33.9.12.10 [exec.let] p4

3. 33.9.12.11 [exec.bulk] p2

4. [exec.split] p4

5. 33.9.12.12 [exec.when.all] p3

6. 33.9.12.13 [exec.into.variant] p3

7. 33.9.12.14 [exec.stopped.opt] p2

8. 33.9.12.15 [exec.stopped.err] p2

9. 33.9.13.2 [exec.sync.wait.var] p1

[2025-10-23; Reflector poll.]

Set priority to 2 after reflector poll.

"Don't want to have to consider whether you can call `get_env` on a moved-from sender. If the domain is a stateless tag type, then get-domain-early's return value can't depend on the actual value of the sender, just its type. If that's correct, we can just spell this `decltype(get-domain-early(sndr)){}`. We already do this for get-domain-late."

"The point of all these exposition-only things was to make the wording more compact and readable. Adding `decltype` and parens everywhere wouldn't help that. Maybe change get-domain-early and get-domain-late from function templates to all-caps wording macros, e.g. `GET-DOMAIN-EARLY``(sndr)`?"

"NAD, I think the issue is covered by “except sndr is evaluated just once.”"

[2026-04-01 Resolved by the adoption of P3826R5 at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

### 4215(i). `run_loop::finish` should be `noexcept`

Section: 33.12.1 [exec.run.loop] Status: Resolved Submitter: Eric Niebler Opened: 2025-02-13 Last modified: 2026-04-01

Priority: 2

View all issues with Resolved status.

Discussion:

Imported from cplusplus/sender-receiver #329.

`run_loop::finish` puts the `run_loop` into the `finishing` state so that the next time the work queue is empty, `run_loop::run` will return instead of waiting for more work.

Calling `.finish()` on a `run_loop` instance can potentially throw (`finish()` is not marked `noexcept`), that is because one valid implementation involves acquiring a lock on a `std::mutex` — a potentially throwing operation.

But failing to put the `run_loop` into the `finishing` state is problematic in the same way that a failing destructor is problematic: shutdown and clean-up code depends on it succeeding.

Consider `sync_wait`'s use of `run_loop`:

>     sync-wait-state<Sndr> state;
>     auto op = connect(sndr, sync-wait-receiver<Sndr>{&state});
>     start(op);

>     state.loop.run();
>     if (state.error) {
>       rethrow_exception(std::move(state.error));
>     }
>     return std::move(state.result);

It is the job of `sync-wait-receiver` to put the `run_loop` into the `finishing` state so that the invocation of `state.loop.run()` will return. It does that in its completion functions, like so:

>     void set_stopped() && noexcept;

> > Effects: Equivalent to `state->loop.finish()`.

Here we are not handling the fact that `state->loop.finish()` is potentially throwing. Given that this function is `noexcept`, this will lead to the application getting terminated. Not good.

But even if we handle the exception and save it into `state.result` to be rethrown later, we still have a problem. Since `run_loop::finish()` threw, the `run_loop` has not been placed into the `finishing` state. That means that `state.loop.run()` will never return, and `sync_wait` will hang forever.

Simply put, `run_loop::finish()` has to be `noexcept`. The implementation must find a way to put the `run_loop` into the `finishing` state. If it cannot, it should terminate. Throwing an exception and foisting the problem on the caller — who has no recourse — is simply wrong.

[2025-06-13; Reflector poll]

Set priority to 2 after reflector poll.

"If this can call `terminate()`, we should explicitly say so (c.f. 32.7.4 [thread.condition.condvar] p11)"

[2026-04-01 Resolved by the adoption of issue 4476(i) at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.12.1.1 [exec.run.loop.general] as indicated:

   >     namespace std::execution {
   >       class run_loop {
   >         // 33.12.1.2 [exec.run.loop.types], associated types
   >         class run-loop-scheduler; // exposition only
   >         class run-loop-sender;    // exposition only
   >         struct run-loop-opstate-base { // exposition only
   >           virtual void execute() = 0;  // exposition only
   >           run_loop* loop;              // exposition only
   >           run-loop-opstate-base* next; // exposition only
   >         };
   >         template<class Rcvr>
   >           using run-loop-opstate = unspecified; // exposition only

   >         // 33.12.1.4 [exec.run.loop.members], member functions
   >         run-loop-opstate-base* pop-front(); // exposition only
   >         void push-back(run-loop-opstate-base*); // exposition only

   >       public:
   >         // 33.12.1.3 [exec.run.loop.ctor], constructor and destructor
   >         run_loop() noexcept;
   >         run_loop(run_loop&&) = delete;
   >         ~run_loop();

   >         // 33.12.1.4 [exec.run.loop.members], member functions
   >         run-loop-scheduler get_scheduler();
   >         void run();
   >         void finish() noexcept;
   >       };
   >     }

2. Modify 33.12.1.4 [exec.run.loop.members] as indicated:

   >     void finish() noexcept;

   > > -8- Preconditions: `state` is either `starting` or `running`.

   > > -9- Effects: Changes `state` to `finishing`.

   > > -10- Synchronization: `finish` synchronizes with the `pop-front` operation that returns `nullptr`.

### 4217(i). Clarify `mdspan` layout mapping requirements for `rank == 0`

Section: 23.7.3.4.2 [mdspan.layout.reqmts] Status: C++26 Submitter: Mark Hoemmen Opened: 2025-03-03 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

23.7.3.4.2 [mdspan.layout.reqmts] p19-21 says that a layout mapping needs to provide `m.stride(r)`. However, 23.7.3.4.5.3 [mdspan.layout.left.obs] p5 constrains `layout_left::mapping<Extents>::stride(r)` on `Extents::rank() > 0` being `true`. The same is true of `layout_right::mapping` (23.7.3.4.6.3 [mdspan.layout.right.obs] p5). (The other Standard mappings in 23.7.3 [views.multidim] and 29.9 [linalg] do not have this constraint.) This suggests that a rank-zero `layout_{left,right}::mapping` does not conform with the layout mapping requirements.

On the other hand, other parts of 23.7.3.4.2 [mdspan.layout.reqmts] imply that `r` must be in the range [0, `rank()`) for the layout mapping's extents type. If such an `r` does not exist, which is the case for a rank-zero layout mapping, then the `m.stride(r)` requirement is vacuous. This implies that a rank-zero `layout_{left,right}::mapping` fully conforms with the layout mapping requirements.

It is definitely the design intent for rank-zero `mdspan` to work, and for it to represent a view of a single element. Users can create rank-zero `mdspan` by invoking its constructor, or by using `submdspan` where all the slice arguments are convertible to `index_type`. Even though the normative wording permits this, adding a Note would clarify the design intent without making the wording redundant. This was the preferred change per LWG reflector discussion.

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 23.7.3.4.2 [mdspan.layout.reqmts] as indicated:

   >     m.stride(r)

   > > -19- Preconditions: `m.is_strided()` is `true`.

   > > -20- Result: `typename M::index_type`

   > > -21- Returns: `sr` as defined in `m.is_strided()` above.

   > > [Note ?: It is not required for `m.stride(r)` to be well-formed if `m.extents().rank()` is zero, even if `m.is_always_strided()` is `true`. — end note]

### 4218(i). Constraint recursion in `basic_const_iterator`'s relational operators due to ADL + CWG 2369

Section: 24.5.3.5 [const.iterators.ops] Status: WP Submitter: Patrick Palka Opened: 2025-03-03 Last modified: 2026-06-15

Priority: 2

View all issues with WP status.

Discussion:

Consider the example (devised by Hewill Kang)

>     using RCI = reverse_iterator<basic_const_iterator<vector<int>::iterator>>;
>     static_assert(std::totally_ordered<RCI>);

Checking `RCI` is `totally_ordered` entails checking

>     requires (RCI x) { x RELOP x; } for each RELOP in {<, >, <=, >=}

which we expect to be straightforwardly satisfied by `reverse_iterator`'s namespace-scope operators (24.5.1.8 [reverse.iter.cmp]):

>     template<class Iterator1, class Iterator2>
>       constexpr bool operator<(
>         const reverse_iterator<Iterator1>& x,
>         const reverse_iterator<Iterator2>& y);
>     // etc

But due to ADL we find ourselves also considering the `basic_const_iterator` relop friends (24.5.3.5 [const.iterators.ops]/24).

>     template<input_iterator Iterator>
>     class basic_const_iterator {
>       template<not-a-const-iterator I>
>         friend constexpr bool operator<(const I& x, const basic_const_iterator& y)
>           requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>
>       // etc
>     };

Before CWG 2369 these candidates would quickly get discarded since for the second operand RCI clearly isn't convertible to `basic_const_iterator`. But after CWG 2369 implementations must first check these operators' constraints (with `Iterator = vector<int>::iterator` and `I = RCI`), which entails checking `totally_ordered<RCI>` recursively, causing the example to be ill-formed.

The constraint recursion is diagnosed by GCC (See godbolt demo). Other compilers accept the example because they don't implement CWG 2369, as far as I know.

GCC trunk works around this issue by giving these friend relational operators a dependent second operand of the form `basic_const_iterator<J>` where `J` is constrained to match `Iterator`:

>     template<not-a-const-iterator I, same_as<Iterator> J>
>       friend constexpr bool operator<(const I& x, const basic_const_iterator<J>& y)
>         requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>
>     // etc

So that deduction fails earlier, before constraints get checked, for a second operand that isn't a specialization of `basic_const_iterator` (or derived from one).

LWG 3769(i) is an earlier issue about constraint recursion in `basic_const_iterator`'s operators, but there the recursion was independent of CWG 2369.

[2025-06-13; Reflector poll]

Set priority to 2 after reflector poll.

[2026-05-19; LWG telecon. Status changed: New → Ready.]

Something seems to be wrong or missing with CWG 2369 because it's silly to have to keep doing this in the library. We're changing a non-deduced parameter into a deduced one with a constraint that requries it to be the same type as it was anyway. In this particular case, P2600 might have helped.

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 24.5.3.3 [const.iterators.iterator], class template `basic_const_iterator` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       template<input_iterator Iterator>
   >       class basic_const_iterator {
   >         […]
   >         template<not-a-const-iterator I, same_as<Iterator> J>
   >           friend constexpr bool operator<(const I& x, const basic_const_iterator<J>& y)
   >             requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<not-a-const-iterator I, same_as<Iterator> J>
   >           friend constexpr bool operator>(const I& x, const basic_const_iterator<J>& y)
   >             requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<not-a-const-iterator I, same_as<Iterator> J>
   >           friend constexpr bool operator<=(const I& x, const basic_const_iterator<J>& y)
   >             requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         template<not-a-const-iterator I, same_as<Iterator> J>
   >           friend constexpr bool operator>=(const I& x, const basic_const_iterator<J>& y)
   >             requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >         […]
   >       };
   >     }

2. Modify 24.5.3.5 [const.iterators.ops] as indicated:

   >     template<not-a-const-iterator I, same_as<Iterator> J>
   >       friend constexpr bool operator<(const I& x, const basic_const_iterator<J>& y)
   >         requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >     template<not-a-const-iterator I, same_as<Iterator> J>
   >       friend constexpr bool operator>(const I& x, const basic_const_iterator<J>& y)
   >         requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >     template<not-a-const-iterator I, same_as<Iterator> J>
   >       friend constexpr bool operator<=(const I& x, const basic_const_iterator<J>& y)
   >         requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;
   >     template<not-a-const-iterator I, same_as<Iterator> J>
   >       friend constexpr bool operator>=(const I& x, const basic_const_iterator<J>& y)
   >         requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;

   > > -23- Let `op` be the operator.

   > > -24- Effects: Equivalent to: `return x op y.current_;`

### 4222(i). `expected` constructor from a single value missing a constraint

Section: 22.8.6.2 [expected.object.cons] Status: C++26 Submitter: Bronek Kozicki Opened: 2025-03-12 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [expected.object.cons].

View all issues with C++26 status.

Discussion:

When an `expected` object is initialized with a constructor taking first parameter of type `unexpect_t` , the expectation is that the object will be always initialized in disengaged state (i.e. the user expected postcondition is that `has_value()` will be `false`), as in the example:

>     struct T { explicit T(auto) {} };
>     struct E { E() {} };

>     int main() {
>       expected<T, E> a(unexpect);
>       assert(!a.has_value());
>     }

This does not hold when both value type `T` and error type `E` have certain properties. Observe:

>     struct T { explicit T(auto) {} };
>     struct E { E(int) {} }; // Only this line changed from the above example

>     int main() {
>       expected<T, E> a(unexpect);
>       assert(!a.has_value()); // This assert will now fail
>     }

In the example above the overload resolution of `a` finds the universal single parameter constructor for initializing `expected` in engaged state (22.8.6.2 [expected.object.cons] p23):

>     template<class U = remove_cv_t<T>>
>       constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);

This constructor has a list of constraints which does not mention `unexpect_t` (but it mentions e.g. `unexpected` and `in_place_t`). Email exchange with the author of `expected` confirmed that it is an omission.

The proposed resolution is to add the following additional constraint to this constructor:

> `is_same_v<remove_cvref_t<U>, unexpect_t>` is `false`

This will result in the above, most likely buggy, program to become ill-formed. If the user intent was for the object to be constructed in an engaged state, passing `unexpect_t` to the `T` constructor, they can fix the compilation error like so:

>     expected<T, E> a(in_place, unexpect);

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 22.8.6.2 [expected.object.cons] as indicated:

   >     template<class U = remove_cv_t<T>>
   >     constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);

   > > -23- Constraints:

   > > 1. (23.1) — `is_same_v<remove_cvref_t<U>, in_place_t>` is `false`; and

   > > 2. (23.2) — `is_same_v<expected, remove_cvref_t<U>>` is `false`; and

   > > 3. (23.?) — `is_same_v<remove_cvref_t<U>, unexpect_t>` is `false`; and

   > > 4. (23.3) — `remove_cvref_t<U>` is not a specialization of `unexpected`; and

   > > 5. (23.4) — `is_constructible_v<T, U>` is `true`; and

   > > 6. (23.5) — if `T` is cv `bool`, `remove_cvref_t<U>` is not a specialization of `expected`.

   > > -24- Effects: Direct-non-list-initializes `val` with `std::forward<U>(v)`.

   > > -25- Postconditions: `has_value()` is `true`.

   > > -26- Throws: Any exception thrown by the initialization of `val`.

### 4223(i). Deduction guides for maps are mishandling tuples and references

Section: 23.4.1 [associative.general] Status: C++26 Submitter: Tomasz Kaminski Opened: 2025-03-14 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

The `from_range` deduction guide for maps currently do not handle ranges of tuple of two elements:

>     std::vector<int> v;
>     auto zv = std::views::zip(v, v);
>     std::map m4(std::from_range, zv); // Ill-formed, no-deduction guide

This seems to be result of merge conflict between P2165 (Compatibility between tuple, pair and tuple-like objects) and P1206R4 (Conversions from ranges to containers): The helper `range-key-type` and `range-mapped-type` aliases introduced by the later use the old formulation of `::first_type`, `::second::type` instead of `tuple_element`.

Furthermore, both iterator and range deduction guides do not correctly handle iterators with a pair of references as value types, and deduce key or value type as reference, which is ill-formed:

>     std::flat_map<int, float> fm; // iterator value_type is pair<int, float>
>     std::map m1(fm.begin(), fm.end()); // OK, deduces std::map<int, float>

>     auto tv = fm | std::views::transform(std::identity{}); // iterator value value_type is pair<int const&, float const&>
>     std::map m3(tv.begin(), tv.end()); // Ill-formed, deduces std::map<int const&, float&>

[2025-08-29; Reflector poll]

Set priority to 2 after reflector poll.

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5001.

> [Drafting note: The proposed change also strips `const` from the value type of the `map`, changing the behavior of previously working code:

> >     std::pair<int const, float const> tp[2];
> >     std::map m(std::begin(tp), std::end(tp)); // Was std::map<int, float const>, now std::map<int, float>

> ]

1. Modify 23.4.1 [associative.general] as indicated:

   >     template<class InputIterator>
   >       using iter-value-type =
   >         typename iterator_traits<InputIterator>::value_type; // exposition only

   >     template<class InputIterator>
   >       using iter-key-type = remove_const_tremove_cvref_t<
   >         tuple_element_t<0, iter-value-type<InputIterator>>>; // exposition only

   >     template<class InputIterator>
   >       using iter-mapped-type = remove_cvref_t<
   >         tuple_element_t<1, iter-value-type<InputIterator>>>; // exposition only

   >     template<class InputIterator>
   >       using iter-to-alloc-type = pair<
   >         add_const_t<
   >           tuple_element_t<0, iter-value-type<InputIterator>>
   >           iter-key-type<InputIterator>
   >         >,
   >         tuple_element_t<1, iter-value-type<InputIterator>>
   >         iter-mapped-type<InputIterator>
   >         >; // exposition only

   >     template<ranges::input_range Range>
   >       using range-key-type =
   >         remove_const_t<typename ranges::range_value_t<Range>::first_type>
   >         remove_cvref_t<tuple_element_t<0, ranges::range_value_t<Range>>>; // exposition only

   >     template<ranges::input_range Range>
   >       using range-mapped-type =
   >         typename ranges::range_value_t<Range>::second_type
   >         remove_cvref_t<tuple_element_t<1, ranges::range_value_t<Range>>>; // exposition only

   >     template<ranges::input_range Range>
   >       using range-to-alloc-type =
   >         pair<add_const_t<
   >           typename ranges::range_value_t<Range>::first_type
   >           range-key-type<Range>
   >         >,
   >         typename ranges::range_value_t<Range>::second_type
   >         range-mapped-type<Range>
   >         >; // exposition only

### 4224(i). Philox engines should be freestanding

Section: 29.5.4.5 [rand.eng.philox] Status: C++26 Submitter: Jiang An Opened: 2025-03-15 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [rand.eng.philox].

View all issues with C++26 status.

Discussion:

Philox engines don't seem to require floating-point operations or support from the operating system, so they are probably suitable for freestanding. However, as P2976R1 was finished before the adoption of P2075R6, these engines are not made freestanding yet.

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after ten votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 29.5.2 [rand.synopsis], header `<random>` synopsis, as indicated:

   >     […]
   >     // 29.5.4.5 [rand.eng.philox], class template philox_engine
   >     template<class UIntType, size_t w, size_t n, size_t r, UIntType... consts>
   >       class philox_engine;               // partially freestanding

   >     […]
   >     using philox4x32 = see below; // freestanding
   >     using philox4x64 = see below; // freestanding
   >     […]

2. Modify 29.5.4.5 [rand.eng.philox], class template `philox_engine` synopsis, as indicated:

   >     namespace std {
   >       template<class UIntType, size_t w, size_t n, size_t r, UIntType... consts>
   >       class philox_engine {
   >         […]
   >         // inserters and extractors
   >         template<class charT, class traits>
   >           friend basic_ostream<charT, traits>&
   >             operator<<(basic_ostream<charT, traits>& os, const philox_engine& x); // hosted
   >         template<class charT, class traits>
   >           friend basic_istream<charT, traits>&
   >             operator>>(basic_istream<charT, traits>& is, philox_engine& x);       // hosted
   >       };
   >     }

### 4227(i). Missing `noexcept` operator in [exec.when.all]

Section: 33.9.12.12 [exec.when.all] Status: C++26 Submitter: Ian Petersen Opened: 2025-03-17 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [exec.when.all].

View all issues with C++26 status.

Discussion:

In 33.9.12.12 [exec.when.all] p7, the `impls-for<when_all_t>::get-state` member is defined to be equivalent to the following lambda:

>     []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(e) -> decltype(e) {
>       return e;
>     }

and `e` is later defined to be:

>     std::forward<Sndr>(sndr).apply(make-state<Rcvr>())

Together, the two definitions imply that the `noexcept` clause on the provided lambda is:

>     noexcept(std::forward<Sndr>(sndr).apply(make-state<Rcvr>()))

which is invalid.

Presumably, the lambda should be defined like so (with an extra `noexcept` operator in the `noexcept` clause):

>     []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(noexcept(e)) -> decltype(e) {
>       return e;
>     }

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 33.9.12.12 [exec.when.all] as indicated:

   > -7- The member `impls-for<when_all_t>::get-state` is initialized with a callable object equivalent to the following lambda expression:

   > >     []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(noexcept(e)) -> decltype(e) {
   > >       return e;
   > >     }

### 4230(i). `simd<complex>::real/imag` is overconstrained

Section: 29.10.7.4 [simd.complex.access] Status: C++26 Submitter: Matthias Kretz Opened: 2025-03-18 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

29.10.7.4 [simd.complex.access] overconstrains the arguments to `real` and `imag`. `complex<T>::real/imag` allows conversions to `T` whereas `simd<complex<T>>` requires basically an exact match (`same_as<simd<T>>` modulo ABI tag differences).

>     complex<double> c = {};
>     c.real(1.f); // OK

>     simd<complex<double>> sc = {};
>     sc.real(simd<float>(1.f)); // ill-formed, should be allowed

The design intent was to match the `std::complex<T>` interface. In which case the current wording doesn't match that intent. `complex` doesn't say `real(same_as<T> auto)` but 'real(T)', which allows conversions.

This issue is also present in the `basic_simd(real, imag)` constructor. It deduces the type for the real/imag arguments instead of using a dependent type derived from `value_type` and ABI tag.

>     // OK:
>     complex<double> c{1., 1.f};

>     // Ill-formed, should be allowed:
>     simd<complex<double>> sc0(1., 1.);
>     simd<complex<double>, 4> sc1(simd<double, 4>(1.), simd<float, 4>(1.f));

[2025-06-13; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 29.10.7.1 [simd.overview], class template `basic_simd` synopsis, as indicated:

   >     namespace std::datapar {
   >       template<class T, class Abi> class basic_simd {
   >       public:
   >         using value_type = T;
   >         using mask_type = basic_simd_mask<sizeof(T), Abi>;
   >         using abi_type = Abi;

   >         using real-type = rebind_t<typename T::value_type, basic_simd> // exposition-only

   >         // 29.10.7.2 [simd.ctor], basic_simd constructors
   >         […]
   >         template<simd-floating-point V>
   >           constexpr explicit(see below) basic_simd(const real-typeV& reals, const real-typeV& imags = {}) noexcept;
   >         […]
   >         // 29.10.7.4 [simd.complex.access], basic_simd complex-value accessors
   >         constexpr real-typeauto real() const noexcept;
   >         constexpr real-typeauto imag() const noexcept;
   >         template<simd-floating-point V>
   >           constexpr void real(const real-typeV& v) noexcept;
   >         template<simd-floating-point V>
   >           constexpr void imag(const real-typeV& v) noexcept;
   >         […]
   >       };
   >       […]
   >     }

2. Modify 29.10.7.2 [simd.ctor] as indicated:

   >     template<simd-floating-point V>
   >       constexpr explicit(see below) basic_simd(const real-typeV& reals, const real-typeV& imags = {}) noexcept;

   > > -19- Constraints:

   > > 1. (19.1) — `simd-complex<basic_simd>` is modeled., and

   > > 2. (19.2) — `V::size() == size()` is `true`.

   > > […]

   > > -21- Remarks: The expression inside `explicit` evaluates to `false` if and only if the floating-point conversion rank of `T::value_type` is greater than or equal to the floating-point conversion rank of `real-typeV::value_type`.

3. Modify 29.10.7.4 [simd.complex.access] as indicated:

   >     constexpr real-typeauto real() const noexcept;
   >     constexpr real-typeauto imag() const noexcept;

   > > -1- Constraints: `simd-complex<basic_simd>` is modeled.

   > > -2- Returns: An object of type `real-typerebind_t<typename T::value_type, basic_simd>` where the `i`th element is initialized to the result of `cmplx-func(operator[](i))` for all `i` in the range `[0, size())`, where `cmplx-func` is the corresponding function from `<complex>`.

   >     template<simd-floating-point V>
   >       constexpr void real(const real-typeV& v) noexcept;
   >     template<simd-floating-point V>
   >       constexpr void imag(const real-typeV& v) noexcept;

   > > -3- Constraints:

   > > 1. (3.1) — `simd-complex<basic_simd>` is modeled.,

   > > 2. (3.2) — `same_as<typename V::value_type, typename T::value_type>` is modeled, and

   > > 3. (3.3) — `V::size() == size()` is `true`.

   > > […]

[2025-07-21; Matthias Kretz comments]

The currently shown P/R says:

> Remarks: The expression inside `explicit` evaluates to `false` if and only if the floating-point conversion rank of `T::value_type` is greater than or equal to the floating-point conversion rank of `real-type::value_type`.

But, by construction, `real-type::value_type` is the same as `T::value_type`. So we get an elaborately worded `explicit(false)` here (which is correct). Consequently, the proposed resolution needs to strike `explicit(<i>see below</i>)` from 29.10.7.1 [simd.overview] and 29.10.7.2 [simd.ctor] and drop the Remarks paragraph (21).

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.1 [simd.overview], class template `basic_vec` synopsis, as indicated:

   >     namespace std::simd {
   >       template<class T, class Abi> class basic_vec {
   >         using real-type = see below; // exposition-only
   >       public:
   >         using value_type = T;
   >         using mask_type = basic_mask<sizeof(T), Abi>;
   >         using abi_type = Abi;
   >         using iterator = simd-iterator<basic_vec>;
   >         using const_iterator = simd-iterator<const basic_vec>;

   >         // 29.10.7.2 [simd.ctor], basic_vec constructors
   >         […]
   >         template<simd-floating-point V>
   >           constexpr explicit(see below) basic_vec(const real-typeV& reals, const real-typeV& imags = {}) noexcept;
   >         […]
   >         // 29.10.7.4 [simd.complex.access], basic_vec complex-value accessors
   >         constexpr real-typeauto real() const noexcept;
   >         constexpr real-typeauto imag() const noexcept;
   >         template<simd-floating-point V>
   >           constexpr void real(const real-typeV& v) noexcept;
   >         template<simd-floating-point V>
   >           constexpr void imag(const real-typeV& v) noexcept;
   >         […]
   >       };
   >       […]
   >     }

   > -2- Recommended practice: […]

   > > [Note 1: …]

   > -?- If `T` is a specialization of `complex`, `real-type` denotes the same type as `rebind_t<typename T::value_type, basic_vec<T, Abi>>`, otherwise an unspecified non-array object type.

2. Modify 29.10.7.2 [simd.ctor] as indicated:

   >     template<simd-floating-point V>
   >       constexpr explicit(see below)
   >         basic_vec(const real-typeV& reals, const real-typeV& imags = {}) noexcept;

   > > -19- Constraints:

   > > 1. (19.1) — `simd-complex<basic_vec>` is modeled., and

   > > 2. (19.2) — `V::size() == size()` is `true`.

   > > […]

   > > -21- Remarks: The expression inside `explicit` evaluates to `false` if and only if the floating-point conversion rank of `T::value_type` is greater than or equal to the floating-point conversion rank of `V::value_type`.

3. Modify 29.10.7.4 [simd.complex.access] as indicated:

   >     constexpr real-typeauto real() const noexcept;
   >     constexpr real-typeauto imag() const noexcept;

   > > -1- Constraints: `simd-complex<basic_vec>` is modeled.

   > > -2- Returns: An object of type `real-typerebind_t<typename T::value_type, basic_vec>` where the `i`th element is initialized to the result of `cmplx-func(operator[](i))` for all `i` in the range `[0, size())`, where `cmplx-func` is the corresponding function from `<complex>`.

   >     template<simd-floating-point V>
   >       constexpr void real(const real-typeV& v) noexcept;
   >     template<simd-floating-point V>
   >       constexpr void imag(const real-typeV& v) noexcept;

   > > -3- Constraints:

   > > 1. (3.1) — `simd-complex<basic_vec>` is modeled.,

   > > 2. (3.2) — `same_as<typename V::value_type, typename T::value_type>` is modeled, and

   > > 3. (3.3) — `V::size() == size()` is `true`.

   > > […]

### 4231(i). `datapar::chunk<N>` should use `simd-size-type` instead of `size_t`

Section: 29.10.3 [simd.syn], 29.10.8.11 [simd.creation] Status: C++26 Submitter: Matthias Kretz Opened: 2025-03-22 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [simd.syn].

View all issues with C++26 status.

Discussion:

All index values and simd widths in subclause "Data-parallel types" use the type `simd-size-type`. Specifically, the NTTP of `std::datapar::resize` uses `simd-size-type` and `std::datapar::chunk` is "implemented" using `std::datapar::resize`.

For `chunk<N>`, `N` is of type `size_t` and needs to be converted to `simd-size-type` in the effects clause where it is passed to resize. The NTTP of chunk should use `simd-size-type` instead of `size_t`.

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 29.10.3 [simd.syn], header `<simd>` synopsis, as indicated:

   >     namespace std::datapar {
   >       […]
   >       template<simd-size-typesize_t N, class T, class Abi>
   >         constexpr auto chunk(const basic_simd<T, Abi>& x) noexcept;
   >       template<simd-size-typesize_t N, size_t Bytes, class Abi>
   >         constexpr auto chunk(const basic_simd_mask<Bytes, Abi>& x) noexcept;
   >       […]
   >     }

2. Modify 29.10.8.11 [simd.creation] as indicated:

   >     template<simd-size-typesize_t N, class T, class Abi>
   >       constexpr auto chunk(const basic_simd<T, Abi>& x) noexcept;

   > > -4- Effects: Equivalent to: `return chunk<resize_t<N, basic_simd<T, Abi>>>(x);`

   >     template<simd-size-typesize_t N, size_t Bytes, class Abi>
   >       constexpr auto chunk(const basic_simd_mask<Bytes, Abi>& x) noexcept;

   > > -5- Effects: Equivalent to: `return chunk<resize_t<N, basic_simd_mask<Bytes, Abi>>>(x);`

### 4232(i). `datapar::resize` does not resize

Section: 29.10.4 [simd.traits] Status: C++26 Submitter: Tim Song Opened: 2025-03-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [simd.traits].

View all issues with C++26 status.

Discussion:

The wording actually requires the size to be left unchanged.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 29.10.4 [simd.traits] as indicated:

   >     template<simd-size-type N, class V> struct resize { using type = see below; };

   > > […]

   > > -9- If `V` is a specialization of `basic_simd`, let `Abi1` denote an ABI tag such that `basic_simd<T, Abi1>::size()` equals `V::size()N`. If `V` is a specialization of `basic_simd_mask`, let `Abi1` denote an ABI tag such that `basic_simd_mask<sizeof(T), Abi1>::size()` equals `V::size()N`.

### 4233(i). The helper lambda of `std::erase` for `hive` should specify return type as `bool`

Section: 23.3.9.6 [hive.erasure] Status: C++26 Submitter: Hewill Kang Opened: 2025-03-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [hive.erasure].

View all issues with C++26 status.

Discussion:

This is a follow up to LWG 4135(i), which incidentally adds a default template parameter for `U` to be consistent with other `erase` functions, which is editorial since the declaration already has it.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.3.9.6 [hive.erasure] as indicated:

   >     template<class T, class Allocator, class U = T>
   >       typename hive<T, Allocator>::size_type
   >         erase(hive<T, Allocator>& c, const U& value);

   > > -1- Effects: Equivalent to:

   > > >     return erase_if(c, [&](const auto& elem) -> bool { return elem == value; });

### 4234(i). Including `<hive>` doesn't provide `std::begin/end`

Section: 24.7 [iterator.range] Status: C++26 Submitter: Hewill Kang Opened: 2025-03-25 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [iterator.range].

View all issues with C++26 status.

Discussion:

24.7 [iterator.range] should add `<hive>` to the list as the latter provides a series of range access member functions such as `begin/end`.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.7 [iterator.range] as indicated:

   > -1- In addition to being available via inclusion of the `<iterator>` header, the function templates in 24.7 [iterator.range] are available when any of the following headers are included: `<array>`, `<deque>`, `<flat_map>`, `<flat_set>`, `<forward_list>`, `<hive>`, `<inplace_vector>`, `<list>`, `<map>`, `<regex>`, `<set>`, `<span>`, `<string>`, `<string_view>`, `<unordered_map>`, `<unordered_set>`, and `<vector>`.

### 4235(i). `cache_latest_view` and `to_input_view` miss `reserve_hint`

Section: 25.7.34.2 [range.cache.latest.view], 99 [range.to.input.view] Status: C++26 Submitter: Hewill Kang Opened: 2025-03-25 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

Intuitively, both view classes should also have `reserve_hint` members.

[2025-06-12; Reflector poll]

Set priority to 2 after reflector poll.

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 25.7.34.2 [range.cache.latest.view] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V>
   > >         requires view<V>
   > >       class cache_latest_view : public view_interface<cache_latest_view<V>> {
   > >         […]
   > >         constexpr auto size() requires sized_range<V>;
   > >         constexpr auto size() const requires sized_range<const V>;

   > >         constexpr auto reserve_hint() requires approximately_sized_range<V>;
   > >         constexpr auto reserve_hint() const requires approximately_sized_range<const V>;
   > >       };
   > >       […]
   > >     }

   > […]

   >     constexpr auto size() requires sized_range<V>;
   >     constexpr auto size() const requires sized_range<const V>;

   > > -4- Effects: Equivalent to: `return ranges::size(base_);`

   >     constexpr auto reserve_hint() requires approximately_sized_range<V>;
   >     constexpr auto reserve_hint() const requires approximately_sized_range<const V>;

   > > -?- Effects: Equivalent to: `return ranges::reserve_hint(base_);`

2. Modify 99 [range.to.input.view] as indicated:

   > >     template<input_range V>
   > >       requires view<V>
   > >     class to_input_view : public view_interface<to_input_view<V>> {
   > >       […]
   > >       constexpr auto size() requires sized_range<V>;
   > >       constexpr auto size() const requires sized_range<const V>;

   > >       constexpr auto reserve_hint() requires approximately_sized_range<V>;
   > >       constexpr auto reserve_hint() const requires approximately_sized_range<const V>;
   > >     };
   > >     […]

   > […]

   >     constexpr auto size() requires sized_range<V>;
   >     constexpr auto size() const requires sized_range<const V>;

   > > -5- Effects: Equivalent to: `return ranges::size(base_);`

   >     constexpr auto reserve_hint() requires approximately_sized_range<V>;
   >     constexpr auto reserve_hint() const requires approximately_sized_range<const V>;

   > > -?- Effects: Equivalent to: `return ranges::reserve_hint(base_);`

### 4236(i). `chunk_view::outer-iterator::value_type` should provide `reserve_hint`

Section: 25.7.29.4 [range.chunk.outer.value] Status: C++26 Submitter: Hewill Kang Opened: 2025-03-26 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.chunk.outer.value].

View all issues with C++26 status.

Discussion:

Consider:

>     views::istream<int>(is) | views::chunk(N) | ranges::to<std::list<std::vector<int>>>();

When the stream is large enough, each chunk will be of size `N` in most cases, except it is the last chunk.

In this case, there is no reason not to provide a `reserve_hint` as this can be easily done by just return `remainder_`. Otherwise, when `N` is large, each vector will be reallocated multiple times unnecessarily.

This is also consistent with the `forward_range` version, since its value type is `views::take(subrange(current_, end_), n_)`, which always has a `reserve_hint` as `take_view` unconditionally provides it.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 25.7.29.4 [range.chunk.outer.value] as indicated:

   > >     namespace std::ranges {
   > >       template<view V>
   > >         requires input_range<V>
   > >       struct chunk_view<V>::outer-iterator::value_type : view_interface<value_type> {
   > >         […]
   > >         constexpr auto size() const
   > >           requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

   > >         constexpr auto reserve_hint() const noexcept;
   > >       };
   > >     }

   > […]

   >     constexpr auto size() const
   >       requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;

   > > -4- Effects: Equivalent to:

   > > >     return to-unsigned-like(ranges::min(parent_->remainder_,
   > > >                                         ranges::end(parent_->base_) - *parent_->current_));

   >     constexpr auto reserve_hint() const noexcept;

   > > -?- Effects: Equivalent to:

   > > >     return to-unsigned-like(parent_->remainder_);

### 4238(i). `simd_mask<complex<double>>::operator+/-/~` return a disabled `simd` specialization

Section: 29.10.9.4 [simd.mask.unary] Status: Resolved Submitter: Matthias Kretz Opened: 2025-03-27 Last modified: 2026-01-26

Priority: 1

View all other issues in [simd.mask.unary].

View all issues with Resolved status.

Discussion:

Addresses DE 297

Consider:

>     simd<complex<double>> c = {};
>     simd_mask<complex<double>> k = (c == c); // sizeof(complex<double>) == 16
>     auto i = -k; // simd<__int128> !

`basic_simd_mask` unary `+`, `-`, and `~` return `basic_simd<integer-from<Bytes, Abi>>`. 29.10.2.1 [simd.expos.defn]/2 says:

> `integer-from<Bytes>` is an alias for a signed integer type `T` such that `sizeof(T)` equals `Bytes`.

But `__int128` isn't a vectorizable type. Consequently, `simd<__int128>` currently is a disabled specialization. So it seems `simd<complex<double>>` wants `__int128` to be added to the list of vectorizable types.

Options:

1. "Specialize" `basic_simd_mask<16, Abi>` to return `rebind_t<integer-from<8>, basic_simd_mask<integer-from<16>, Abi>>` (reduces a 128-bit value of a vector-mask to a 64-bit value in the simd)

2. "Specialize" `basic_simd_mask<16, Abi>` to return `resize_t<size() * 2, rebind_t<integer-from<8>, basic_simd_mask<integer-from<16>, Abi>>>` (duplicates a 128-bit value of a vector-mask to two 64-bit values in the simd)

3. delete unary `+`, `-`, and `~` for `basic_simd_mask<16, Abi>` (closest to the status quo)

[2025-06-12; Reflector poll]

Set priority to 1 after reflector poll.

[2025-10-17; related to 4376(i).]

[2026-01-26; Resolved by LWG 4376(i). Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 29.10.9.4 [simd.mask.unary] as indicated:

   >     constexpr basic_simd_mask operator!() const noexcept;
   >     constexpr basic_simd<integer-from<Bytes>, Abi> operator+() const noexcept;
   >     constexpr basic_simd<integer-from<Bytes>, Abi> operator-() const noexcept;
   >     constexpr basic_simd<integer-from<Bytes>, Abi> operator~() const noexcept;

   > > -1- Let `op` be the operator.

   > > -2- Returns: A data-parallel object where the `i`th element is initialized to the results of applying `op` to `operator[](i)` for all `i` in the range of `[0, size())`.

   > > -?- Remarks: If `Bytes` is greater than `8`, `operator+()`, `operator-()`, and `operator~()` are deleted.

### 4239(i). `flat_map`'s transparent comparator no longer works for string literals

Section: 23.6.8.7 [flat.map.modifiers] Status: C++26 Submitter: Hui Xie Opened: 2025-03-29 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [flat.map.modifiers].

View all issues with C++26 status.

Discussion:

According to the spec, the following code should hard error

>     std::flat_map<std::string, int, std::less<>> m;
>     m.try_emplace("abc", 5);  // hard error

The reason is that we specify in 23.6.8.7 [flat.map.modifiers] p21 the effect to be as if `ranges::upper_bound` is called.

`ranges::upper_bound` requires `indirect_strict_weak_order`, which requires the comparator to be invocable for all combinations. In this case, it requires

>     const char (&)[4] < const char (&)[4]

to be well-formed, which is no longer the case in C++26 after P2865R6.

We should just use `std::upper_bound` instead. libstdc++ already uses `std::upper_bound`. libc++ uses `ranges::upper_bound` but clang has not yet implemented P2865 properly.

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

We should make `ranges::upper_bound` work for this case, but literals never met its `indirect_strict_weak_order` semantic requirements anyway. Making that work would be a new design to be seen be LEWG. The proposed resolution solves the immediate problem here.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.6.8.7 [flat.map.modifiers] as indicated:

   >     template<class K, class... Args>
   >       constexpr pair<iterator, bool> try_emplace(K&& k, Args&&... args);
   >     template<class K, class... Args>
   >       constexpr iterator try_emplace(const_iterator hint, K&& k, Args&&... args);

   > > -19- Constraints: […]

   > > -20- Preconditions: […]

   > > -21- Effects: If the map already contains an element whose key is equivalent to `k`, `*this` and `args...` are unchanged. Otherwise equivalent to:

   > > >     auto key_it = upper_bound(c.keys.begin(), c.keys.end(), k, compare)ranges::upper_bound(c.keys, k, compare);
   > > >     auto value_it = c.values.begin() + distance(c.keys.begin(), key_it);
   > > >     c.keys.emplace(key_it, std::forward<K>(k));
   > > >     c.values.emplace(value_it, std::forward<Args>(args)...);

### 4242(i). `ranges::distance` does not work with volatile iterators

Section: 24.4.4.3 [range.iter.op.distance] Status: C++26 Submitter: Hewill Kang Opened: 2025-04-12 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [range.iter.op.distance].

View all issues with C++26 status.

Discussion:

After LWG 3664(i), `ranges::distance` computes the distance between `last` and `first` by returning `last - static_cast<const decay_t<I>&>(first)` when the two are subtractable. However, this will cause a hard error if `first` is volatile-qualified (demo):

>     #include <iterator>

>     int main() {
>       int arr[] = {1, 2, 3};
>       int* volatile ptr = arr;
>       // return std::distance(ptr, arr + 3);      // this is ok
>       return std::ranges::distance(ptr, arr + 3); // hard error
>     }

The resolution changes the Effects of LWG 3664(i) from "cute" to "noncute".

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

"Would prefer `auto(first)` but the current wording is correct."

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.4.4.3 [range.iter.op.distance] as indicated:

   >     template<class I, sized_sentinel_for<decay_t<I>> S>
   >       constexpr iter_difference_t<decay_t<I>> ranges::distance(I&& first, S last);

   > > -3- Effects: Equivalent to:

   > >     if constexpr (!is_array_v<remove_reference_t<I>>)
   > >       return last - first;
   > >     else
   > >       return last - static_cast<const decay_t<I>&>(first);

### 4243(i). `as_bytes`/`as_writable_bytes` is broken with `span<volatile T>`

Section: 23.7.2.3 [span.objectrep] Status: C++26 Submitter: Hewill Kang Opened: 2025-04-12 Last modified: 2026-06-09

Priority: 4

View all issues with C++26 status.

Discussion:

They both use `reinterpret_cast` to cast the underlying pointer type of `span` to `const byte*` and `byte*` respectively, which leads to a hard error when the element type is `volatile`-qualified (demo):

>     #include <span>

>     int main() {
>       std::span<volatile int> span;
>       auto bytes = as_bytes(span);                   // hard error
>       auto writable_bytes = as_writable_bytes(span); // hard error
>     }

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 23.7.2.1 [span.syn], header `<span>` synopsis, as indicated:

   >     […]
   >     namespace std {
   >       […]
   >       // 23.7.2.3 [span.objectrep], views of object representation
   >       template<class ElementType, size_t Extent>
   >         span<const conditional_t<is_volatile_v<ElementType>, volatile byte, byte>,
   >              Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
   >           as_bytes(span<ElementType, Extent> s) noexcept;

   >       template<class ElementType, size_t Extent>
   >         span<conditional_t<is_volatile_v<ElementType>, volatile byte, byte>,
   >              Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
   >           as_writable_bytes(span<ElementType, Extent> s) noexcept;
   >     }

2. Modify 23.7.2.3 [span.objectrep] as indicated:

   >     template<class ElementType, size_t Extent>
   >       span<const conditional_t<is_volatile_v<ElementType>, volatile byte, byte>,
   >            Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
   >         as_bytes(span<ElementType, Extent> s) noexcept;

   > > -1- Effects: Equivalent to: `return R{reinterpret_cast<R::pointerconst byte*>(s.data()), s.size_bytes()};`
   > > where `R` is the return type.

   >     template<class ElementType, size_t Extent>
   >       span<conditional_t<is_volatile_v<ElementType>, volatile byte, byte>,
   >            Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
   >         as_writable_bytes(span<ElementType, Extent> s) noexcept;

   > > -2- Constraints: `is_const_v<ElementType>` is `false`.

   > > -3- Effects: Equivalent to: `return R{reinterpret_cast<R::pointerbyte*>(s.data()), s.size_bytes()};`
   > > where `R` is the return type.

[2025-04-16; Hewill Kang provides alternative wording]

Based on reflector feedback, the revised wording just improves the current state of not supporting support for `volatile`.

[2025-06-12; Reflector poll]

Set priority to 4 after reflector poll.

[Sofia 2025-06-17; Move to Ready]

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.2.3 [span.objectrep] as indicated:

   >     template<class ElementType, size_t Extent>
   >       span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
   >         as_bytes(span<ElementType, Extent> s) noexcept;

   > > -?- Constraints: `is_volatile_v<ElementType>` is `false`.

   > > -1- Effects: Equivalent to: `return R{reinterpret_cast<const byte*>(s.data()), s.size_bytes()};`
   > > where `R` is the return type.

   >     template<class ElementType, size_t Extent>
   >       span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
   >         as_writable_bytes(span<ElementType, Extent> s) noexcept;

   > > -2- Constraints: `is_const_v<ElementType>` is `false` and `is_volatile_v<ElementType>` is `false`.

   > > -3- Effects: Equivalent to: `return R{reinterpret_cast<byte*>(s.data()), s.size_bytes()};`
   > > where `R` is the return type.

### 4245(i). Operators that interact with `counted_iterator` and `default_sentinel_t` should be `noexcept`

Section: 24.5.7.1 [counted.iterator], 24.5.7.5 [counted.iter.nav], 24.5.7.6 [counted.iter.cmp] Status: C++26 Submitter: Hewill Kang Opened: 2025-04-18 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [counted.iterator].

View all issues with C++26 status.

Discussion:

`counted_iterator` can be compared or subtracted from `default_sentinel_t`, which only involves simple integer arithmetic and does not have any Preconditions.

In this case, it is reasonable to declare them as `noexcept`.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.5.7.1 [counted.iterator] as indicated:

   >     namespace std {
   >       template<input_or_output_iterator I>
   >       class counted_iterator {
   >       public:
   >         […]
   >         friend constexpr iter_difference_t<I> operator-(
   >           const counted_iterator& x, default_sentinel_t) noexcept;
   >         friend constexpr iter_difference_t<I> operator-(
   >           default_sentinel_t, const counted_iterator& y) noexcept;
   >         […]
   >         friend constexpr bool operator==(
   >           const counted_iterator& x, default_sentinel_t) noexcept;
   >         […]
   >       };
   >       […]
   >     }

2. Modify 24.5.7.5 [counted.iter.nav] as indicated:

   >     friend constexpr iter_difference_t<I> operator-(
   >       const counted_iterator& x, default_sentinel_t) noexcept;

   > > -15- Effects: Equivalent to: `return -x.length;`

   >     friend constexpr iter_difference_t<I> operator-(
   >       default_sentinel_t, const counted_iterator& y) noexcept;

   > > -16- Effects: Equivalent to: `return y.length;`

3. Modify 24.5.7.6 [counted.iter.cmp] as indicated:

   >     friend constexpr bool operator==(
   >       const counted_iterator& x, default_sentinel_t) noexcept;

   > > -3- Effects: Equivalent to: `return x.length == 0;`

### 4247(i). Header `<stdbit.h>` is not yet freestanding

Section: 22.12 [stdbit.h.syn] Status: C++26 Submitter: Jiang An Opened: 2025-04-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Per C23/WG14 N3220 4 p7, `<stdbit.h>` is freestanding in C23, but C++ hasn't required it for a freestanding implementation. LWG 4049(i) is related but doesn't cover this, because there's no `<cstdbit>` standard header.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify Table 27 [tab:headers.cpp.fs] as indicated:

   > Table 27: C++ headers for freestanding implementations [tab:headers.cpp.fs]
   > | Subclause | Header
   > --- | --- | ---
   > `[…]`
   > 22.11 [bit] | Bit manipulation | `<bit>`
   > 22.12 [stdbit.h.syn] | C-compatible bit manipulation | `<stdbit.h>`
   > 23.3.3 [array] | Class template `array` | `<array>`
   > `[…]`

2. Modify 22.12 [stdbit.h.syn], header `<stdbit.h>` synopsis, as indicated:

   >     // all freestanding
   >     #define __STDC_VERSION_STDBIT_H__ 202311L

   >     #define __STDC_ENDIAN_BIG__    see below
   >     #define __STDC_ENDIAN_LITTLE__ see below
   >     #define __STDC_ENDIAN_NATIVE__ see below

   >     […]

### 4248(i). Late domain-based dispatching of `schedule_from` and `continues_on` are flipped

Section: 33.9.2 [exec.snd.expos] Status: Resolved Submitter: Eric Niebler Opened: 2025-04-26 Last modified: 2026-03-31

Priority: 1

View all other issues in [exec.snd.expos].

View all issues with Resolved status.

Discussion:

The reason for having two different customization points for transitioning between two execution contexts is described in 33.9.2 [exec.snd.expos] bullet (14.1) Note 1, to wit:

> [Note 1: The `continues_on` algorithm works in tandem with `schedule_from` (33.9.12.7 [exec.schedule.from]) to give scheduler authors a way to customize both how to transition onto (`continues_on`) and off of (`schedule_from`) a given execution context. Thus, `continues_on` ignores the domain of the predecessor and uses the domain of the destination scheduler to select a customization, a property that is unique to `continues_on`. That is why it is given special treatment here. — end note]

The exposition-only `get-domain-late` function treats `continues_on` senders specially to make sure the correct domain (that of the destination scheduler) is used to find customizations at connect time.

However, customizations are also searched for early when the sender is first constructed. And here the dispatching of `continues_on` and `schedule_from` are reversed.

`continues_on(sndr, sch)` is defined as (33.9.12.6 [exec.continues.on]):

>     transform_sender(get-domain-early(sndr), make-sender(continues_on, sch, sndr))

which is using the domain of the predecessor rather than ignoring it as 33.9.2 [exec.snd.expos] p14.1 says it does. And `schedule_from(sch, sndr)` is currently defined as (33.9.12.7 [exec.schedule.from]):

>     transform_sender(
>       query-or-default(get_domain, sch, default_domain()),
>       make-sender(schedule_from, sch, sndr))

which is using the domain of the destination scheduler to find customizations. The logic for determining the domain to use for early customization of these two algorithms are opposite what they are for late customization. This is a bug. They should be consistent.

"Lazy" customization (at connect time) was added to P2300 later in the process, and this inconsistency was a mistake on my part. The correct thing to do is to change `get-domain-late` to treat `schedule_from` as special, not `continues_on`.

[2025-06-12; Reflector poll]

Set priority to 1 after reflector poll.

"Names are a bit misleading but the change looks right."

[2026-03-31 P3826R5 was approved in Croydon. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 33.9.2 [exec.snd.expos] as indicated:

   >     template<class Sndr, class Env>
   >       constexpr auto get-domain-late(const Sndr& sndr, const Env& env) noexcept;

   > > -14- Effects: Equivalent to:

   > > 1. (14.1) — If `sender-for<Sndr, continues_on_tschedule_from_t>` is `true`, then

   > >    >     return Domain();

   > >    where `Domain` is the type of the following expression:

   > >    >     [] {
   > >    >       auto [_, sch, _] = sndr;
   > >    >       return query-or-default(get_domain, sch, default_domain());
   > >    >     }();

   > >    [Note 1: The `continues_onschedule_from` algorithm works in tandem with `schedule_fromcontinues_on` (33.9.12.7 [exec.schedule.from]33.9.12.6 [exec.continues.on]) to give scheduler authors a way to customize both how to transition onto (`continues_onschedule_from`) and off of (`schedule_fromcontinues_on`) a given execution context. Thus, `continues_onschedule_from` ignores the domain of the predecessor and uses the domain of the destination scheduler to select a customization, a property that is unique to `continues_onschedule_from`. That is why it is given special treatment here. — end note]

   > > 2. […]

### 4249(i). The past end issue for `lazy_split_view`

Section: 25.7.16.3 [range.lazy.split.outer] Status: WP Submitter: Hewill Kang Opened: 2025-04-26 Last modified: 2026-06-15

Priority: 2

View all other issues in [range.lazy.split.outer].

View all issues with WP status.

Discussion:

Consider (demo):

>     #include <print>
>     #include <ranges>
>     #include <sstream>

>     int main() {
>       std::istringstream is{"1 0 2 0 3"};
>       auto r = std::views::istream<int>(is)
>              | std::views::lazy_split(0)
>              | std::views::stride(2);
>       std::println("{}", r); // should print [[1], [3]]
>     }

The above leads to a hardened precondition failure in libstdc++, the reason is that we are iterating over the nested range as:

>     for (auto&& inner : r) {
>       for (auto&& elem : inner) {
>         // […]
>       }
>     }

which is disassembled as:

>     auto outer_it = r.begin();
>     std::default_sentinel_t out_end = r.end();
>     for(; outer_it != out_end; ++outer_it) {
>       auto&& inner_r = *outer_it;
>       auto inner_it = inner_r.begin();
>       std::default_sentinel_t inner_end = inner_r.end();
>       for(; inner_it != inner_end; ++inner_it) {
>         auto&& elem = *inner_it;
>         // […]
>       }
>     }

Since `inner_it` and `output_it` actually update the same iterator, when we back to the outer loop, `lazy_split_view::outer-iterator` is now equal to `default_sentinel`, which makes `output_it` reach the end, so `++outer_it` will increment the iterator past end, triggering the assertion.

Note that this also happens in MSVC-STL when `_ITERATOR_DEBUG_LEVEL` is turned on.

It seems that extra flags are needed to fix this issue because `output_it` should not be considered to reach the end when we back to the outer loop.

[2025-10-21; Reflector poll.]

Set priority to 2 after reflector poll.

"This is unfortunate. `lazy_split` is probably not very commonly used, but handling input ranges was half the reason why we kept it around. Can we reuse `trailing_empty_` for this instead of adding a new flag? Both flags have the same meaning, and the two cases where they are true are disjoint: we need `has_next_` when iterating through the inner range exhausted the source range; we need `trailing_empty_` when we find a delimiter at the end of the source range when incrementing the outer iterator, which by definition means that iterating through the inner range didn't exhaust it."

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 25.7.16.3 [range.lazy.split.outer] as indicated:

   > >     namespace std::ranges {
   > >       template<input_range V, forward_range Pattern>
   > >         requires view<V> && view<Pattern> &&
   > >                  indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
   > >                  (forward_range<V> || tiny-range<Pattern>)
   > >       template<bool Const>
   > >       struct lazy_split_view<V, Pattern>::outer-iterator {
   > >       private:
   > >         using Parent = maybe-const<Const, lazy_split_view>;     // exposition only
   > >         using Base = maybe-const<Const, V>;                     // exposition only
   > >         Parent* parent_ = nullptr;                              // exposition only

   > >         iterator_t<Base> current_ = iterator_t<Base>();         // exposition only, present only
   > >                                                                 // if V models forward_range

   > >         bool trailing_empty_ = false;                           // exposition only
   > >         bool has_next_ = false;                                 // exposition only, present only
   > >                                                                 // if forward_range<V> is false
   > >       public:
   > >         […]
   > >       };
   > >     }

   > […]

   >     constexpr explicit outer-iterator(Parent& parent)
   >       requires (!forward_range<Base>);

   > > -2- Effects: Initializes `parent_` with `addressof(parent)` and `has_next_` with `current != ranges::end(parent_->base_)`.

   > […]

   >     constexpr outer-iterator& operator++();

   > > -6- Effects: Equivalent to:

   > > >     const auto end = ranges::end(parent_->base_);
   > > >     if (current == end) {
   > > >       trailing_empty_ = false;
   > > >       if constexpr (!forward_range<V>)
   > > >         has_next_ = false;
   > > >       return *this;
   > > >     }
   > > >     const auto [pbegin, pend] = subrange{parent_->pattern_};
   > > >     if (pbegin == pend) ++current;
   > > >     else if constexpr (tiny-range<Pattern>) {
   > > >       current = ranges::find(std::move(current), end, *pbegin);
   > > >       if (current != end) {
   > > >         ++current;
   > > >         if (current == end)
   > > >           trailing_empty_ = true;
   > > >       }
   > > >     }
   > > >     else {
   > > >       do {
   > > >         auto [b, p] = ranges::mismatch(current, end, pbegin, pend);
   > > >         if (p == pend) {
   > > >           current = b;
   > > >           if (current == end)
   > > >             trailing_empty_ = true;
   > > >           break;            // The pattern matched; skip it
   > > >         }
   > > >       } while (++current != end);
   > > >     }
   > > >     if constexpr (!forward_range<V>)
   > > >       if (current == end)
   > > >         has_next_ = false;
   > > >     return *this;

   > […]

   >     friend constexpr bool operator==(const outer-iterator& x, default_sentinel_t);

   > > -8- Effects: Equivalent to:

   > > >     if constexpr (!forward_range<V>)
   > > >       return !x.has_next_ && !x.trailing_empty_;
   > > >     else
   > > >       return x.current == ranges::end(x.parent_->base_) && !x.trailing_empty_;

[2025-10-21; Hewill Kang provides simpler wording]

[2026-05-22 LWG telecon; Status changed: New → Ready]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 25.7.16.3 [range.lazy.split.outer] as indicated:

   >     constexpr outer-iterator& operator++();

   > > -6- Effects: Equivalent to:

   > > >     const auto end = ranges::end(parent_->base_);
   > > >     if (current == end) {
   > > >       trailing_empty_ = false;
   > > >       return *this;
   > > >     }
   > > >     const auto [pbegin, pend] = subrange{parent_->pattern_};
   > > >     if (pbegin == pend) ++current;
   > > >     else if constexpr (tiny-range<Pattern>) {
   > > >       current = ranges::find(std::move(current), end, *pbegin);
   > > >       if (current != end) {
   > > >         ++current;
   > > >         if (current == end)
   > > >           trailing_empty_ = true;
   > > >         else if constexpr (!forward_range<V>)
   > > >           trailing_empty_ = true;
   > > >       }
   > > >     }
   > > >     else {
   > > >       do {
   > > >         auto [b, p] = ranges::mismatch(current, end, pbegin, pend);
   > > >         if (p == pend) {
   > > >           current = b;
   > > >           if (current == end)
   > > >             trailing_empty_ = true;
   > > >           break;            // The pattern matched; skip it
   > > >         }
   > > >       } while (++current != end);
   > > >     }
   > > >     return *this;

### 4251(i). Move assignment for `indirect` unnecessarily requires copy construction

Section: 20.4.1.5 [indirect.assign], 20.4.2.5 [polymorphic.assign] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-05-01 Last modified: 2026-06-09

Priority: 1

View all issues with C++26 status.

Discussion:

The move assignment operator for `indirect` says:

> Mandates: `is_copy_constructible_t<T>` is `true`.

However, the only way it ever construct an object is:

> constructs a new owned object with the owned object of `other` as the argument as an rvalue

and that only ever happens when `alloc == other.alloc` is false.

It seems like we should require `is_move_constructible_v` instead, and only if the allocator traits mean we need to construct an object. (Technically move-constructible might not be correct, because the allocator's `construct` member might use a different constructor).

Additionally, the noexcept-specifier for the move assignment doesn't match the effects. The noexcept-specifier says it can't throw if POCMA is true, but nothing in the effects says that ownership can be transferred in that case; we only do a non-throwing transfer when the allocators are equal. I think we should transfer ownership when POCMA is true, which would make the noexcept-specifier correct.

[2025-06-12; Reflector poll]

Set priority to 1 after reflector poll.

Similar change needed for `std::polymorphic`.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 20.4.1.5 [indirect.assign] as indicated:

   >     constexpr indirect& operator=(indirect&& other)
   >       noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);

   > > -5- Mandates: If `allocator_traits<Allocator>::propagate_on_container_move_assignment::value` is `false` and `allocator_traits<Allocator>::is_always_equal::value` is `false`, `is_copymove_constructible_t<T>` is `true`.

   > > -6- Effects: If `addressof(other) == this` is `true`, there are no effects. Otherwise:

   > > 1. (6.1) — The allocator needs updating if `allocator_traits<Allocator>::propagate_on_container_move_assignment::value` is `true`.
   > > 2. (6.2) — If `other` is valueless, `*this` becomes valueless and the owned object in `*this`, if any, is destroyed using `allocator_traits<Allocator>::destroy` and then the storage is deallocated.
   > > 3. (6.3) — Otherwise, if the allocator needs updating or if `alloc == other.alloc` is `true`, swaps the owned objects in `*this` and `other`; the owned object in `other`, if any, is then destroyed using `allocator_traits<Allocator>::destroy` and then the storage is deallocated `*this` takes ownership of the owned object of `other`.
   > > 4. (6.4) — Otherwise, constructs a new owned object with the owned object of `other` as the argument as an rvalue, using either the allocator in `*this` or the allocator in `other` if the allocator needs updating.
   > > 5. (6.5) — The previously owned object in `*this`, if any, is destroyed using `allocator_traits<Allocator>::destroy` and then the storage is deallocated.
   > > 6. (6.6) — If the allocator needs updating, the allocator in `*this` is replaced with a copy of the allocator in `other`.

   > > -7- Postcondition: `other` is valueless.

[2025-11-03; Tomasz provides wording.]

[Kona 2025-11-03; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 20.4.1.5 [indirect.assign] as indicated:

   >     constexpr indirect& operator=(indirect&& other)
   >       noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);

   > > -5- Mandates: If `allocator_traits<Allocator>::propagate_on_container_move_assignment::value` is `false` and `allocator_traits<Allocator>::is_always_equal::value` is `false`, `is_copymove_constructible_t<T>` is `true`.

   > > -6- Effects: If `addressof(other) == this` is `true`, there are no effects. Otherwise:

   > > 1. (6.1) — The allocator needs updating if `allocator_traits<Allocator>::propagate_on_container_move_assignment::value` is `true`.
   > > 2. (6.2) — If `other` is valueless, `*this` becomes valueless and the owned object in `*this`, if any, is destroyed using `allocator_traits<Allocator>::destroy` and then the storage is deallocated.
   > > 3. (6.3) — Otherwise, if the allocator needs updating or if `alloc == other.alloc` is `true`, swaps the owned objects in `*this` and `other`; the owned object in `other`, if any, is then destroyed using `allocator_traits<Allocator>::destroy` and then the storage is deallocated `*this` takes ownership of the owned object of `other`.
   > > 4. (6.4) — Otherwise, constructs a new owned object with the owned object of `other` as the argument as an rvalue, using either the allocator in `*this` or the allocator in `other` if the allocator needs updating.
   > > 5. (6.5) — The previously owned object in `*this`, if any, is destroyed using `allocator_traits<Allocator>::destroy` and then the storage is deallocated.
   > > 6. (6.6) — If the allocator needs updating, the allocator in `*this` is replaced with a copy of the allocator in `other`.

   > > -7- Postcondition: `other` is valueless.

2. Modify 20.4.2.5 [polymorphic.assign] as indicated:

   >     constexpr polymorphic& operator=(polymorphic&& other)
   >       noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);

   > > -5- Mandates: If `allocator_traits<Allocator>::propagate_on_container_move_assignment::value` is `false` and `allocator_traits<Allocator>::is_always_equal::value` is `false`, `T` is complete type.

   > > -6- Effects: If `addressof(other) == this` is `true`, there are no effects. Otherwise:

   > > 1. (6.1) — The allocator needs updating if `allocator_traits<Allocator>::propagate_on_container_move_assignment::value` is `true`.
   > > 2. (6.?) — If `other` is valueless, `*this` becomes valueless.
   > > 3. (6.2) — Otherwise, if the allocator needs updating orIf `alloc == other.alloc` is `true`, swaps the owned objects in `*this` and `other`; the owned object in `other`, if any, is then destroyed using `allocator_traits<Allocator>::destroy` and then the storage is deallocated `*this` takes ownership of the owned object of `other`.
   > > 4. (6.3) — Otherwise, if `alloc != other.alloc` is `true`; if `other` is not valueless, a new owned object is constructed in `*this` using `allocator_traits::construct` with the owned object from Otherwise, constructs a new owned object with the owned object of `other` as the argument as an rvalue, using either the allocator in `*this` or the allocator in `other` if the allocator needs updating.
   > > 5. (6.4) — The previously owned object in `*this`, if any, is destroyed using `allocator_traits<Allocator>::destroy` and then the storage is deallocated.
   > > 6. (6.5) — If the allocator needs updating, the allocator in `*this` is replaced with a copy of the allocator in `other`.
   > > […]

### 4253(i). `basic_const_iterator` should provide `iterator_type`

Section: 24.5.3.3 [const.iterators.iterator] Status: C++26 Submitter: Hewill Kang Opened: 2025-04-29 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [const.iterators.iterator].

View all issues with C++26 status.

Discussion:

Currently, iterator adaptors in `<iterator>` that wrap a single iterator such as `reverse_iterator`, `move_iterator`, and `counted_iterator` all provide a public `iterator_type` member for users to access the underlying iterator type, except for `basic_const_iterator` (demo):

>     #include <iterator>

>     using I  = int*;
>     using RI = std::reverse_iterator<I>;
>     using MI = std::move_iterator<I>;
>     using CI = std::counted_iterator<I>;
>     using BI = std::basic_const_iterator<I>;

>     static_assert(std::same_as<RI::iterator_type, I>);
>     static_assert(std::same_as<MI::iterator_type, I>);
>     static_assert(std::same_as<CI::iterator_type, I>);
>     static_assert(std::same_as<BI::iterator_type, I>); // error

It seems reasonable to add one for `basic_const_iterator` for consistency.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.5.3.3 [const.iterators.iterator] as indicated:

   >     namespace std {
   >       […]
   >       template<input_iterator Iterator>
   >       class basic_const_iterator {
   >         Iterator current_ = Iterator();                             // exposition only
   >         using reference = iter_const_reference_t<Iterator>;         // exposition only
   >         using rvalue-reference =                                    // exposition only
   >           iter-const-rvalue-reference-t<Iterator>;

   >         public:
   >           using iterator_type = Iterator;
   >           using iterator_concept = see below;
   >           using iterator_category = see below;  // not always present
   >           using value_type = iter_value_t<Iterator>;
   >           using difference_type = iter_difference_t<Iterator>;
   >           […]
   >       };
   >     }

### 4255(i). `move_only_function` constructor should recognize empty `copyable_function`s

Section: 22.10.17.4.3 [func.wrap.move.ctor] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-05-12 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [func.wrap.move.ctor].

View all other issues in [func.wrap.move.ctor].

View all issues with C++26 status.

Discussion:

The standard currently requires that constructing `move_only_function` from empty `copyable_function`, creates an non-empty `move_only_function`, that contains an empty `copyable_function` as the target. For example:

>     std::copyable_function<int(int)> ce;
>     std::move_only_function<int(int)> me(ce);

We require that invoking `me(1)` is undefined behavior (as it leads to call to the `ce(1)`), however it cannot be detected in the user code, as `me != nullptr` is true.

We should require the `move_only_function(F&& f)` constructor to create an empty object, if `f` is an instantiation of `copyable_function` and `f == nullptr` is true, i.e. `f` does not contain target object.

This simplifies implementing avoidance of double wrapping per 22.10.17.1 [func.wrap.general] p2, as transferring the target produces an empty functor.

The `copyable_function` cannot be constructed from `move_only_function`, as it requires functor to be copyable. Invoking an empty `std::function` has well defined behavior (throws `bad_function_call`), and wrapping such object into other functors should reproduce that behavior.

[2025-10-22; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 22.10.17.4.3 [func.wrap.move.ctor] as indicated:

   >     template<class F> move_only_function(F&& f);

   > > […]

   > > -8- Postconditions:: `*this` has no target object if any of the following hold:

   > > 1. (8.1) — `f` is a null function pointer value, or

   > > 2. (8.2) — `f` is a null member pointer value, or

   > > 3. (8.2) — `remove_cvref_t<F>` is a specialization of the `move_only_function` or `copyable_function` class template, and `f` has no target object.

### 4256(i). Incorrect constrains for `function_ref` constructors from `nontype_t`

Section: 22.10.17.6.3 [func.wrap.ref.ctor] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-05-14 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

For the following class:

    struct M
    {
       void operator()();
    };

The constructor of `function_ref<void()>` from `nontype_t` is considered to be valid candidate (`is_constructible_v<function_ref<void()>, nontype_t<M{}>>` is `true`), despite the fact that the corresponding invocation of template argument object, that is const lvalue, is ill-formed. As consequence we produce a hard error from inside of this constructor.

This is caused by the fact that for constructors with non-type `auto f` parameter, we are checking if `is-invocable-using<F>` is `true`, where `F` is `decltype(f)` i.e. `M` for the example. We should use `const F&` or `decltype((f))`.

[2025-10-21; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 22.10.17.6.3 [func.wrap.ref.ctor] as indicated:

   >     template<auto f> constexpr function_ref(nontype_t<f>) noexcept;

   > > -8- Let `F` be `decltype(f)`.

   > > -9- Constraints: `is-invocable-using<const F&>` is `true`.

   > > […]

   >     template<auto f, class U> constexpr function_ref(nontype_t<f>, U&& obj) noexcept;

   > > -12- Let `T` be `remove_reference_t<U>` and `F` be `decltype(f)`.

   > > -13- Constraints::

   > > 1. (13.1) — `is_rvalue_reference_v<U&&>` is false, and

   > > 2. (13.2) — `is-invocable-using<const F&, T cv&>` is `true`.

   > > […]

   >     template<auto f, class T> constexpr function_ref(nontype_t<f>, T cv* obj) noexcept;

   > > -17- Let `F` be `decltype(f)`.

   > > -16- Constraints: `is-invocable-using<const F&, T cv*>` is `true`.

   > > […]

### 4257(i). Stream insertion for `chrono::local_time` should be constrained

Section: 30.7.9 [time.clock.local] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-05-16 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Stream insertion for `chrono::local_time` is defined in terms of conversion to `chrono::sys_time`, but not all `chrono::sys_time` specializations can be inserted into an ostream, because one of the overloads is constrained and the other requires convertibility to `chrono::sys_days` (see 30.7.2.3 [time.clock.system.nonmembers]).

This means the following code fails to compile:

    #include <iostream>
    #include <chrono>

    template<typename T>
    concept ostream_insertable = requires (std::ostream& o, const T& t) { o << t; };

    using D = std::chrono::duration<double>;

    int main() {
      if constexpr (ostream_insertable<std::chrono::sys_time<D>>)
        std::cout << std::chrono::sys_time<D>{};
      if constexpr (ostream_insertable<std::chrono::local_time<D>>)
        std::cout << std::chrono::local_time<D>{}; // FAIL
    }

The first condition is false, because there's no overload that's suitable. The second is true, because the `operator<<` overload for `chrono::local_time` isn't constrained and so insertion appears to be valid. But actually trying to use it is ill-formed, because it tries to convert the `local_time<D>` to a `sys_time<D>` and then insert that, which isn't valid.

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 30.7.9 [time.clock.local] as indicated:

   >     template<class charT, class traits, class Duration>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os, const local_time<Duration>& lt);

   > -?- Constraints: `os << sys_time<Duration>{lt.time_since_epoch()}` is a valid expression.

   > -2- Effects:

   >      os << sys_time<Duration>{lt.time_since_epoch()};

   > -3- Returns: `os`.

### 4259(i). P1148R0 changed the return values of searching functions of `std::basic_string` on some platforms

Section: 27.4.3.8.2 [string.find] Status: C++26 Submitter: Jiang An Opened: 2025-05-05 Last modified: 2026-06-09

Priority: 3

View all other issues in [string.find].

View all issues with C++26 status.

Discussion:

P1148R0 respecified the searching functions of `std::basic_string` to return corresponding string view type's `npos` member constant (equal to `std::size_t(-1)`), converted to the string type `S`'s member `S::size_type`, when the search fails. Before the change, `S::npos` (equal to `S::size_type(-1)`) was returned on failure.

On platforms where `std::size_t` isn't the widest unsigned integer type (e.g. on usual 32-bit platforms), the return value can change. Because there can be an allocator with a wider size_type, and when the basic_string type `S` uses such an allocator, `S::size_type` is specified to be that type, which in turn makes `S::size_type(std::size_t(-1))` not equal to `S::size_type(-1)`.

Do we want to restore the old return values?
Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 27.4.3.8.2 [string.find] as indicated:

   >     template<class T>
   >       constexpr size_type find(const T& t, size_type pos = 0) const noexcept(see below);
   >     […]
   >     template<class T>
   >       constexpr size_type find_last_not_of(const T& t, size_type pos = npos) const noexcept(see below);

   > > -2- Constraints: […]

   > > -3- Effects: Let `G` be the name of the function. Equivalent to:

   > > >     basic_string_view<charT, traits> s = *this, sv = t;
   > > >     return s.G(sv, pos);
   > > >     if (auto result = s.G(sv, pos); result == size_t(-1))
   > > >       return npos;
   > > >     else
   > > >       return result;

[2025-06-10, reflector discussion]

During reflector discussion of this issue there was a preference to adjust the proposed wording to use `s.npos` instead of `size_t(-1)`.

[2025-10-21; Reflector poll.]

Set priority to 3 after reflector poll.

[2025-12-04; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 27.4.3.8.2 [string.find] as indicated:

   >     template<class T>
   >       constexpr size_type find(const T& t, size_type pos = 0) const noexcept(see below);
   >     […]
   >     template<class T>
   >       constexpr size_type find_last_not_of(const T& t, size_type pos = npos) const noexcept(see below);

   > > -2- Constraints: […]

   > > -3- Effects: Let `G` be the name of the function. Equivalent to:

   > > >     basic_string_view<charT, traits> s = *this, sv = t;
   > > >     return s.G(sv, pos);
   > > >     if (auto result = s.G(sv, pos); result == s.npos)
   > > >       return npos;
   > > >     else
   > > >       return result;

### 4260(i). Query objects must be default constructible

Section: 33.2.1 [exec.queryable.general] Status: C++26 Submitter: Eric Niebler Opened: 2025-05-07 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

Imported from cplusplus/sender-receiver #333.

We require the types of query objects such as `get_scheduler` to be customization point objects. 16.3.3.3.5 [customization.point.object] requires them to be `semiregular` but that concept does not require default constructability. Much of `std::execution` assumes query object types to be default constructible.

I propose adding a (nothrow) default-constructibility requirement.

[2025-10-23; Reflector poll.]

Set priority to 2 after reflector poll.

"The discussion is wrong, `semiregular` requires `default_initializable`. If we want to mandate nothrow construction (a.k.a the implementation isn't out to get you), I'd rather we do it for all CPOs."

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 33.2.1 [exec.queryable.general] as indicated:

   > -1- A queryable object is a read-only collection of key/value pair where each key is a customization point object known as a query object. The type of a query object satisfies `default_initializable`, and its default constructor is not potentially throwing. A query is an invocation of a query object with a queryable object as its first argument and a (possibly empty) set of additional arguments. A query imposes syntactic and semantic requirements on its invocations.

[2025-11-05; Tim provides improved wording]

LWG decided to guarantee some additional properties for CPOs.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 16.3.3.3.5 [customization.point.object] as indicated:

   > -1- A customization point object is a function object (22.10 [function.objects]) with a literal class type that interacts with program-defined types while enforcing semantic requirements on that interaction.

   > -2- The type of a customization point object, ignoring cv-qualifiers, shall model `semiregular` (18.6 [concepts.object]) and shall be a structural type (13.2 [temp.param]) and a trivially copyable type (11.2 [class.prop]). Every constructor of this type shall have a non-throwing exception specification (14.5 [except.spec]).

### 4264(i). Skipping indirection is not allowed for `function_ref`

Section: 22.10.17.1 [func.wrap.general] Status: Resolved Submitter: Tomasz Kamiński Opened: 2025-05-15 Last modified: 2026-04-01

Priority: 2

View all issues with Resolved status.

Discussion:

Addresses RU-220

Currently the wording in 22.10.17.1 [func.wrap.general] allows implementation to avoid double indirection when constructing owning functions wrappers from another one:

> -2- Let `t` be an object of a type that is a specialization of `function`, `copyable_function`, or `move_only_function`, such that the target object `x` of `t` has a type that is a specialization of `function`, `copyable_function`, or `move_only_function`. Each argument of the invocation of `x` evaluated as part of the invocation of `t` may alias an argument in the same position in the invocation of `t` that has the same type, even if the corresponding parameter is not of reference type.

However, the wording does not cover a `function_ref`, disallowing implementation to perform this optimization when signatures are compatible, for example:

>     std::function_ref<void() noexcept> f1(ptr);
>     std::function_ref<void()> f1(f2);

We should include `function_ref` in the list. Note that this allows, but does not require, an implementation to perform such an optimization. As a consequence, it is acceptable to specify the allowance for all combinations of polymorphic wrappers, even for creating an owning wrapper from a non-owning one, where implementing such an optimization may not be possible.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 22.10.17.1 [func.wrap.general] as indicated:

   > -2- Let `t` be an object of a type that is a specialization of `function`, `copyable_function`, or `move_only_function`, or `function_ref`, such that the target object `x` of `t` has a type that is a specialization of `function`, `copyable_function`, or `move_only_function`, or `function_ref`. Each argument of the invocation of `x` evaluated as part of the invocation of `t` may alias an argument in the same position in the invocation of `t` that has the same type, even if the corresponding parameter is not of reference type.

[2024-05-21; Tomasz's comment and upates proposed resolution]

After implementing double indirection avoidance in the libstdc++, I have realized that above wording change is insufficient to cover all user observable effects of the change. Revelant quote from the Avoid double indirection in function_ref from libstdc++ mailing lists:

> To avoidance of double indirection requires that constructed `function_ref`, refers directly to the target function of the source, instead of source, and this is visible after the assigment:

>     void foo() noexcept;
>     void bar() noexcept;

>     std::function_ref<void() noexcept> sr(&foo);
>     std::function_ref<void()> dr(sr);
>     dr(); // calls foo regardless of implementation

>     sr = &bar;
>     sr(); // calls bar
>     dr(); // still calls foo if we avoid indirection,
>           // calls bar if we do not

> Similary for `move_only_function`/`copyable_function` source:

>     std::move_only_function<void()> sm;
>     std::function_ref<void()> dm(sm);

>     dm(); // UB because sm is empty

>     sm = &foo;

>     dm(); // remains UB if we avoid indirection,
>           // calls bar if we do not.

> While we may want to allow skipping indirection for function_ref, as this produces same behavior as in case for copy constructor (matching signatures):

>     void foo() noexcept;
>     void bar() noexcept;

>     std::function_ref<void() noexcept> sr(&foo);
>     std::function_ref<void() noexcept> dr(sr); // copy-cosntructor
>     dr(); // calls foo regardless of implementation

>     sr = &bar;
>     sr(); // calls bar
>     dr(); // still calls foo if we avoid indirection

> I do not think this is acceptable for `move_only_function`. …

> Note that for the same reason, implementations are not free to avoid dangling when constructing `function_ref` from `reference_wrapper`:

>     auto srw = std::ref(&foo);
>     std::function_ref<void()> drw(srw);
>     drw(); // calls foo

>     srw = std::ref(&bar);
>     drw(); // calls foo if we unwrap referenc wrapper,
>            // calls bar otherwise.

> Note that this is limited to `function_ref` due reference nature of this wrapper.

The updated resolution allows indirection but making it unspecified if `function_ref` constructed from other `function_ref` specialization, will refer to source object or its target.

[2025-10-21; Reflector poll.]

Set priority to 2 after reflector poll.

Strong oposition to making it unspecified whether `function_ref` constructed from other `function_ref` will reference source object or its target directly.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 22.10.17.1 [func.wrap.general] as indicated:

   > -2- Let `t` be an object of a type that is a specialization of `function`, `copyable_function`, or `move_only_function`, or `function_ref`, such that the target object `x` of `t` has a type that is a specialization of `function`, `copyable_function`, or `move_only_function`, or `function_ref`. Each argument of the invocation of `x` evaluated as part of the invocation of `t` may alias an argument in the same position in the invocation of `t` that has the same type, even if the corresponding parameter is not of reference type.

2. Modify 22.10.17.6.3 [func.wrap.ref.ctor] as indicated:

   >     template<class F> constexpr function_ref(F&&) noexcept;

   > > […]

   > > -7- Effects: Initializes `bound-entity` with `addressof(f)` and `thunk-ptr` with the address of a function `thunk` such that `thunk(bound-entity, call-args...)` is expression-equivalent (3.22 [defns.expression.equivalent]) to `invoke_r<R>(static_cast<cv T&>(f), call-args...)`.

   > > -?- Remarks: If `remove_cvref_t<F>` is a specialization of `function_ref` an implementation may initialize `bound-entity` with `bound-entity` of `f`. [Example::

   > >     void f1() noexcept;
   > >     void f2() noexcept;

   > >     function_ref<void() noexcept> r1(&r1);
   > >     function_ref<void()> r2(r1);
   > >     r1 = &f2;
   > >     f2(); // it is unspecified if f1 or f2 is invoked

   > > — end example]

[2025-11-06, Tomasz provides updated wording correcting example]

[2026-04-01 Resolved by the adoption of P3961R1 at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.10.17.1 [func.wrap.general] as indicated:

   > -2- Let `t` be an object of a type that is a specialization of `function`, `copyable_function`, or `move_only_function`, or `function_ref`, such that the target object `x` of `t` has a type that is a specialization of `function`, `copyable_function`, or `move_only_function`, or `function_ref`. Each argument of the invocation of `x` evaluated as part of the invocation of `t` may alias an argument in the same position in the invocation of `t` that has the same type, even if the corresponding parameter is not of reference type.

2. Modify 22.10.17.6.3 [func.wrap.ref.ctor] as indicated:

   >     template<class F> constexpr function_ref(F&&) noexcept;

   > > […]

   > > -7- Effects: Initializes `bound-entity` with `addressof(f)` and `thunk-ptr` with the address of a function `thunk` such that `thunk(bound-entity, call-args...)` is expression-equivalent (3.22 [defns.expression.equivalent]) to `invoke_r<R>(static_cast<cv T&>(f), call-args...)`.

   > > -?- Remarks: If `remove_cvref_t<F>` is a specialization of `function_ref` an implementation may initialize `bound-entity` with `bound-entity` of `f`. [Example:

   > >     void f1() noexcept;
   > >     void f2() noexcept;

   > >     function_ref<void() noexcept> r1(&f1);
   > >     function_ref<void()> r2(r1);
   > >     r2(); // f1 is invoked
   > >     r1 = &f2;
   > >     r2(); // it is unspecified if f1 or f2 is invoked

   > > — end example]

### 4265(i). `std::midpoint` should not accept `const bool`

Section: 26.10.16 [numeric.ops.midpoint] Status: C++26 Submitter: Jan Schultke Opened: 2025-05-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [numeric.ops.midpoint].

View all issues with C++26 status.

Discussion:

The constraints of the first overload of `std::midpoint` are as follows:

>     template<class T>
>       constexpr T midpoint(T a, T b) noexcept;

> > -1- Constraints: `T` is an arithmetic type other than `bool`.

It does not appear intentional that `const bool` is supported considering that 26.10.14 [numeric.ops.gcd] excludes cv `bool`.

More generally, it is questionable that cv-qualified arithmetic types aren't excluded in general, considering that 26.10.17 [numeric.sat] excludes them with the "signed or unsigned integer type" constraint. However, tightening the constraints for these other functions seems evolutionary, not like fixing an obvious oversight, and so it likely requires a separate issue.

[2025-10-21; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

This requires template argument to be explicty specified, i.e. `midpoint<const bool>`. I would prefer to address all cv-qualified types at once, e.g. Constraints: `remove_cv_t<T>` is an arithmetic type other than `bool`."

"This is locally consistent with `gcd` and `lcm` which only exclude cv `bool`. [algorithms.requirement] p15 makes it unspecified to use an explicit template argument list here, so `midpoint<const bool>` and `midpoint<const int>` are already unspecified, this issue just ensures that `const bool` is explicitly rejected, like `bool`."

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 26.10.16 [numeric.ops.midpoint] as indicated:

   >     template<class T>
   >       constexpr T midpoint(T a, T b) noexcept;

   > > -1- Constraints: `T` is an arithmetic type other than cv `bool`.

### 4266(i). `layout_stride::mapping` should treat empty mappings as exhaustive

Section: 23.7.3.4.7 [mdspan.layout.stride] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-05-22 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [mdspan.layout.stride].

View all issues with C++26 status.

Discussion:

Mapping over an empty multidimensional index space is always exhaustive according to the corresponding definitions from 23.7.3.4.2 [mdspan.layout.reqmts] p16.

However, the current specification of `layout_stride::mapping` does not consider whether some of the empty multidimensional index spaces are unique or exhaustive. For illustration, the mapping with the following configuration is not considered exhaustive according to the current specification of 23.7.3.4.7.4 [mdspan.layout.stride.obs] bullet 5.2:

>     extents: 2, 2, 0
>     strides: 2, 6, 20

This prevents the implementation from implementing `sm.is_exhaustive()` as `sm.fwd-prod-of-extents(sm::extents_type::rank()) == sm.required_span_size()`. For all mappings with size greater than zero, such an expression provides an answer consistent with the standard. However, it always returns `true` for an empty mapping, such as shown in the example.

We should make such implementation conforming, and require `is_exhaustive()` to return `true` for empty mappings.

For consistency, we could update `is_always_exhaustive()` to recognize mapping with `rank() == 0`, and one for which at least one of the static extents is equal to zero (i.e., they always represent a multidimensional index space).

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.3.4.7.1 [mdspan.layout.stride.overview] as indicated:

   >     namespace std {
   >       template<class Extents>
   >       class layout_stride::mapping {
   >         […]
   >         static constexpr bool is_always_unique() noexcept { return true; }
   >         static constexpr bool is_always_exhaustive() noexcept; { return false; }
   >         static constexpr bool is_always_strided() noexcept { return true; }
   >         […]
   >       };
   >     }

2. Modify 23.7.3.4.7.4 [mdspan.layout.stride.obs] as indicated:

   > […]

   >     static constexpr bool is_always_exhaustive() noexcept;

   > > -?- Returns: `true` if `rank_` is `0` or if there is a rank index `r` of `extents()` such that `extents_type::static_extent(r)` is `0`, otherwise `false`.

   >     constexpr bool is_exhaustive() const noexcept;

   > > -5- Returns:

   > > 1. (5.1) — `true` if `rank_` or the size of the multidimensional index space `m.extents()` is `0`.

   > > 2. (5.2) — […]

   > > 3. (5.3) — […]

### 4267(i). Uses-allocator construction is meaningless for tuple of references

Section: 22.4.4.2 [tuple.cnstr] Status: WP Submitter: Jiang An Opened: 2025-05-24 Last modified: 2026-06-15

Priority: 3

View other active issues in [tuple.cnstr].

View all other issues in [tuple.cnstr].

View all issues with WP status.

Discussion:

Per 20.2.8.2 [allocator.uses.construction]/1, uses-allocator construction is only defined for objects. And presumably, an attempt to construct `std::tuple` of reference from an `allocator_arg_t` constructor causes a hard error.

Since C++23/P2255R2, it seems that these `allocator_arg_t` constructors are conditionally deleted according to 22.4.4.2 [tuple.cnstr]/33. However, it's confusing that these constructors are sometimes non-deleted when the tuple contains a reference, while there are hard errors in an instantiation instead.

[2025-10-14; Reflector poll]

Set priority to 3 after reflector poll.

"There is a defect, but the right fix is to just define uses-allocator construction of references as equivalent to normal construction."

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 22.4.4.2 [tuple.cnstr] as indicated:

   >     template<class Alloc>
   >       constexpr explicit(see below)
   >         tuple(allocator_arg_t, const Alloc& a);
   >     […]
   >     template<class Alloc, tuple-like UTuple>
   >       constexpr explicit(see below)
   >         tuple(allocator_arg_t, const Alloc& a, UTuple&&);

   > > -32- Preconditions: `Alloc` meets the Cpp17Allocator requirements (16.4.4.6.1 [allocator.requirements.general]).

   > > -33- Effects: Equivalent to the preceding constructors except that each element is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

   > > -?- Remarks: These constructors are defined as deleted if `is_reference_v<Ti>` is `true` for at least one `Ti`.

[2025-10-15; Jonathan provides new wording]

I don't think we care about uses-allocator construction of references in general. You can't construct a reference using `allocator_traits::construct` nor put references in containers. For a `std::pair` containing a reference, I think `make_obj_using_allocator` and `uses_allocator_construction_args` work fine. The problem only exists in the `std::tuple` wording where we say that the elements are constructed using uses-allocator construction. So let's just fix that.

[2026-05-22 LWG telecon; Status changed: New → Ready]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.4.4.2 [tuple.cnstr] as indicated:

   >     template<class Alloc>
   >       constexpr explicit(see below)
   >         tuple(allocator_arg_t, const Alloc& a);
   >     […]
   >     template<class Alloc, tuple-like UTuple>
   >       constexpr explicit(see below)
   >         tuple(allocator_arg_t, const Alloc& a, UTuple&&);

   > > -32- Preconditions: `Alloc` meets the Cpp17Allocator requirements (16.4.4.6.1 [allocator.requirements.general]).

   > > -33- Effects: Equivalent to the preceding constructors except that each element of non-reference type is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).

### 4269(i). `unique_copy` passes arguments to its predicate backwards

Section: 26.7.9 [alg.unique] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-05-29 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [alg.unique].

View all issues with C++26 status.

Discussion:

For the `unique` algorithms, 26.7.9 [alg.unique] p1 says:

> 1. Let `pred` be `equal_to{}` for the overloads with no parameter `pred`, and let E be
> 1. (1.1) — `bool(pred(*(i - 1), *i))` for the overloads in namespace `std`;
> 2. (1.2) — `bool(invoke(comp, invoke(proj, *(i - 1)), invoke(proj, *i)))` for the overloads in namespace `ranges`.

However for the `unique_copy` algorithms, 26.7.9 [alg.unique] p6 says that the arguments `*i` and `*(i-1)` should be reversed:

> 6. Let `pred` be `equal_to{}` for the overloads with no parameter `pred`, and let E be
> 1. (6.1) — `bool(pred(*i, *(i - 1)))` for the overloads in namespace `std`;
> 2. (6.2) — `bool(invoke(comp, invoke(proj, *i), invoke(proj, *(i - 1))))` for the overloads in namespace `ranges`.

This reversed order is consistent with the documentation for SGI STL `unique_copy`, although the docs for SGI STL `unique` show reversed arguments too, and the C++ standard doesn't match that.

A survey of known implementations shows that all three of libstdc++, libc++, and MSVC STL use the `pred(*(i - 1), *i)` order for all of `std::unique`, `std::unique_copy`, `ranges::unique`, and `ranges::unique_copy`. The range-v3 library did the same, and even the SGI STL did too (despite what its docs said). Only two implementations were found which match the spec and use a different argument order for `unique` and `unique_copy`, Casey Carter's (cmcstl2) and Fraser Gordon's.

In the absence of any known rationale for `unique` and `unique_copy` to differ, it seems sensible to make `unique_copy` more consistent with `unique` (and with the majority of implementations stretching back three decades).

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Fixed misplaced `)` in the (6.1) change as pointed out on reflector, and rebased on N5014.

"I remain inconvinced that this actually matters given the equivalence relation requirement."

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.7.9 [alg.unique] as indicated:

   > 6. Let `pred` be `equal_to{}` for the overloads with no parameter `pred`, and let E(`i`) be
   > 1. (6.1) — `bool(pred(*i, *(i - 1), *i))` for the overloads in namespace `std`;
   > 2. (6.2) — `bool(invoke(comp, invoke(proj, *i), invoke(proj, *(i - 1)), invoke(proj, *i)))` for the overloads in namespace `ranges`.

### 4272(i). For `rank == 0`, `layout_stride` is atypically convertible

Section: 23.7.3.4 [mdspan.layout] Status: C++26 Submitter: Luc Grosheintz Opened: 2025-06-02 Last modified: 2026-06-09

Priority: 2

View all other issues in [mdspan.layout].

View all issues with C++26 status.

Discussion:

Commonly, two layouts are considered convertible, if the underlying `extent_types` are convertible.

However, for the two ctors `layout_left::mapping(layout_stride::mapping)` and `layout_right::mapping(layout_stride::mapping)`, the condition is `rank > 0`. Therefore,

>     using E1 = std::extents<int>;
>     using E2 = std::extents<unsigned int>;

>     static_assert(std::is_convertible_v<
>         std::layout_stride::mapping<E2>,
>         std::layout_right::mapping<E1>
>       >);

even though:

>     static_assert(!std::is_convertible_v<E2, E1>);

Moreover, for rank 0 `layout_stride` can be converted to any specialization of `layout_left` or `layout_right`; but not to every specialization of `layout_stride`.

[2025-06-12; Reflector poll]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

> [Drafting note: As drive-by fixes the edits for `layout_left_padded<>::mapping` and `layout_right_padded<>::mapping` also correct an editorial asymmetry between class header synopsis declaration form and prototype specification form of the corresponding constructors and adjust to the correct formatting of the exposition-only data member `rank_`.]

1. Modify 23.7.3.4.5.1 [mdspan.layout.left.overview] as indicated:

   >     namespace std {
   >       template<class Extents>
   >       class layout_left::mapping {
   >         […]
   >         // 23.7.3.4.5.2 [mdspan.layout.left.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);

   >         constexpr mapping& operator=(const mapping&) noexcept = default;
   >         […]
   >       };
   >     }

2. Modify 23.7.3.4.5.2 [mdspan.layout.left.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(extents_type::rank() > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -13- Constraints: […]

   > > -14- Preconditions: […]

   > > -15- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

3. Modify 23.7.3.4.6.1 [mdspan.layout.right.overview] as indicated:

   >     namespace std {
   >       template<class Extents>
   >       class layout_right::mapping {
   >         […]
   >         // 23.7.3.4.6.2 [mdspan.layout.right.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);

   >         constexpr mapping& operator=(const mapping&) noexcept = default;
   >         […]
   >       };
   >     }

4. Modify 23.7.3.4.6.2 [mdspan.layout.right.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(extents_type::rank() > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -13- Constraints: […]

   > > -14- Preconditions: […]

   > > -15- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

5. Modify 23.7.3.4.8.1 [mdspan.layout.leftpad.overview] as indicated:

   >     namespace std {
   >       template<size_t PaddingValue>
   >       template<class Extents>
   >       class layout_left_padded<PaddingValue>::mapping {
   >         […]
   >         // 23.7.3.4.8.3 [mdspan.layout.leftpad.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);
   >         […]
   >       };
   >     }

6. Modify 23.7.3.4.8.3 [mdspan.layout.leftpad.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(rank_ > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -10- Constraints: […]

   > > -11- Preconditions: […]

   > > -12- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

7. Modify 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] as indicated:

   >     namespace std {
   >       template<size_t PaddingValue>
   >       template<class Extents>
   >       class layout_right_padded<PaddingValue>::mapping {
   >         […]
   >         // 23.7.3.4.9.3 [mdspan.layout.rightpad.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(rank_ > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);
   >         […]
   >       };
   >     }

8. Modify 23.7.3.4.9.3 [mdspan.layout.rightpad.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(rank_ > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -10- Constraints: […]

   > > -11- Preconditions: […]

   > > -12- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

[2025-06-20, Luc Grosheintz provides further wording improvements]

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

> [Drafting note: As drive-by fixes the edits for `layout_left_padded<>::mapping` and `layout_right_padded<>::mapping` also correct an editorial asymmetry between class header synopsis declaration form and prototype specification form of the corresponding constructors and adjust to the correct formatting of the exposition-only data member `rank_`.]

1. Modify 23.7.3.4.5.1 [mdspan.layout.left.overview] as indicated:

   >     namespace std {
   >       template<class Extents>
   >       class layout_left::mapping {
   >         […]
   >         // 23.7.3.4.5.2 [mdspan.layout.left.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);

   >         constexpr mapping& operator=(const mapping&) noexcept = default;
   >         […]
   >       };
   >     }

2. Modify 23.7.3.4.5.2 [mdspan.layout.left.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(extents_type::rank() > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -13- Constraints: […]

   > > -14- Preconditions: […]

   > > -15- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

3. Modify 23.7.3.4.6.1 [mdspan.layout.right.overview] as indicated:

   >     namespace std {
   >       template<class Extents>
   >       class layout_right::mapping {
   >         […]
   >         // 23.7.3.4.6.2 [mdspan.layout.right.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);

   >         constexpr mapping& operator=(const mapping&) noexcept = default;
   >         […]
   >       };
   >     }

4. Modify 23.7.3.4.6.2 [mdspan.layout.right.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(extents_type::rank() > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -13- Constraints: […]

   > > -14- Preconditions: […]

   > > -15- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

5. Modify 23.7.3.4.8.1 [mdspan.layout.leftpad.overview] as indicated:

   >     namespace std {
   >       template<size_t PaddingValue>
   >       template<class Extents>
   >       class layout_left_padded<PaddingValue>::mapping {
   >         […]
   >         // 23.7.3.4.8.3 [mdspan.layout.leftpad.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);
   >         […]
   >       };
   >     }

6. Modify 23.7.3.4.8.3 [mdspan.layout.leftpad.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(rank_ > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -10- Constraints: […]

   > > -11- Preconditions: […]

   > > -12- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

   >     template<class LayoutLeftPaddedMapping>
   >       constexpr explicit(see below)
   >         mapping(const LayoutLeftPaddedMapping& other);

   > > -13- Constraints: […]

   > > […]

   > > -16- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type> &&
   > > >     rank_> 1 &&
   > > >     (padding_value != dynamic_extent ||
   > > >      LayoutLeftPaddedMapping::padding_value == dynamic_extent)

7. Modify 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] as indicated:

   >     namespace std {
   >       template<size_t PaddingValue>
   >       template<class Extents>
   >       class layout_right_padded<PaddingValue>::mapping {
   >         […]
   >         // 23.7.3.4.9.3 [mdspan.layout.rightpad.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(rank_ > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);
   >         […]
   >       };
   >     }

8. Modify 23.7.3.4.9.3 [mdspan.layout.rightpad.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(rank_ > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -10- Constraints: […]

   > > -11- Preconditions: […]

   > > -12- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

   >     template<class LayoutRightPaddedMapping>
   >       constexpr explicit(see below)
   >         mapping(const LayoutRightPaddedMapping& other);

   > > -13- Constraints: […]

   > > […]

   > > -17- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type> &&
   > > >     rank_ > 1 &&
   > > >     (padding_value != dynamic_extent ||
   > > >     LayoutRightPaddedMapping::padding_value == dynamic_extent)

[2025-09-27, Tomasz Kamiński fixes constraints in constructors from padded layouts]

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5008.

> [Drafting note: As drive-by fixes the edits for `layout_left_padded<>::mapping` and `layout_right_padded<>::mapping` also correct an editorial asymmetry between class header synopsis declaration form and prototype specification form of the corresponding constructors and adjust to the correct formatting of the exposition-only data member `rank_`.]

1. Modify 23.7.3.4.5.1 [mdspan.layout.left.overview] as indicated:

   >     namespace std {
   >       template<class Extents>
   >       class layout_left::mapping {
   >         […]
   >         // 23.7.3.4.5.2 [mdspan.layout.left.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);

   >         constexpr mapping& operator=(const mapping&) noexcept = default;
   >         […]
   >       };
   >     }

2. Modify 23.7.3.4.5.2 [mdspan.layout.left.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(extents_type::rank() > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -13- Constraints: […]

   > > -14- Preconditions: […]

   > > -15- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

3. Modify 23.7.3.4.6.1 [mdspan.layout.right.overview] as indicated:

   >     namespace std {
   >       template<class Extents>
   >       class layout_right::mapping {
   >         […]
   >         // 23.7.3.4.6.2 [mdspan.layout.right.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);

   >         constexpr mapping& operator=(const mapping&) noexcept = default;
   >         […]
   >       };
   >     }

4. Modify 23.7.3.4.6.2 [mdspan.layout.right.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(extents_type::rank() > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -13- Constraints: […]

   > > -14- Preconditions: […]

   > > -15- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

5. Modify 23.7.3.4.8.1 [mdspan.layout.leftpad.overview] as indicated:

   >     namespace std {
   >       template<size_t PaddingValue>
   >       template<class Extents>
   >       class layout_left_padded<PaddingValue>::mapping {
   >         […]
   >         // 23.7.3.4.8.3 [mdspan.layout.leftpad.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(extents_type::rank() > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);
   >         […]
   >       };
   >     }

6. Modify 23.7.3.4.8.3 [mdspan.layout.leftpad.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(rank_ > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -10- Constraints: […]

   > > -11- Preconditions: […]

   > > -12- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

   >     template<class LayoutLeftPaddedMapping>
   >       constexpr explicit(see below)
   >         mapping(const LayoutLeftPaddedMapping& other);

   > > -13- Constraints: […]

   > > […]

   > > -16- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type> ||
   > > >     rank_> 1 &&
   > > >     (padding_value != dynamic_extent ||
   > > >      LayoutLeftPaddedMapping::padding_value == dynamic_extent)

7. Modify 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] as indicated:

   >     namespace std {
   >       template<size_t PaddingValue>
   >       template<class Extents>
   >       class layout_right_padded<PaddingValue>::mapping {
   >         […]
   >         // 23.7.3.4.9.3 [mdspan.layout.rightpad.cons], constructors
   >         […]
   >         template<class OtherExtents>
   >           constexpr explicit(rank_ > 0see below)
   >             mapping(const layout_stride::mapping<OtherExtents>&);
   >         […]
   >       };
   >     }

8. Modify 23.7.3.4.9.3 [mdspan.layout.rightpad.cons] as indicated:

   >     template<class OtherExtents>
   >       constexpr explicit(rank_ > 0see below)
   >         mapping(const layout_stride::mapping<OtherExtents>& other);

   > > -10- Constraints: […]

   > > -11- Preconditions: […]

   > > -12- Effects: […]

   > > -?- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

   >     template<class LayoutRightPaddedMapping>
   >       constexpr explicit(see below)
   >         mapping(const LayoutRightPaddedMapping& other);

   > > -13- Constraints: […]

   > > […]

   > > -17- Remarks: The expression inside `explicit` is equivalent to:

   > > >     !is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type> ||
   > > >     rank_ > 1 &&
   > > >     (padding_value != dynamic_extent ||
   > > >     LayoutRightPaddedMapping::padding_value == dynamic_extent)

### 4274(i). The `chrono::hh_mm_ss` constructor is ill-formed for unsigned durations

Section: 30.9.2 [time.hms.members] Status: C++26 Submitter: Michael Welsh Duggan Opened: 2025-06-04 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

In 30.9.2 [time.hms.members], paragraph 3, the current wording for the constructor of `hh_mm_ss` expresses some of its requirements in terms of `abs(d)`, which is assumed to be `chrono::abs(chrono::duration)`. `chrono::abs` is not defined, however, for durations with an unsigned representation. I believe that not being able to create `hh_mm_ss` objects from unsigned durations is unintentional.

Moreover, `is_constructible_v<hh_mm_ss<ud>, ud>` is required to be true by the standard for any duration, so making it actually work makes a lot of sense.

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 30.9.2 [time.hms.members] as indicated:

   >     constexpr explicit hh_mm_ss(Duration d);

   > > -3- Effects: Constructs an object of type `hh_mm_ss` which represents the `Duration d` with precision `precision`.

   > > 1. (3.1) — Initializes `is_neg` with `d < Duration::zero()`. Let `ABS_D` represent `-d` if `is_neg` is `true` and `d` otherwise.

   > > 2. (3.2) — Initializes `h` with `duration_cast<chrono::hours>(abs(d)ABS_D)`.

   > > 3. (3.3) — Initializes `m` with `duration_cast<chrono::minutes>(abs(d)ABS_D - hours())`.

   > > 4. (3.4) — Initializes `s` with `duration_cast<chrono::seconds>(abs(d)ABS_D - hours() - minutes())`.

   > > 5. (3.5) — If `treat_as_floating_point_v<precision::rep>` is `true`, initializes `ss` with `abs(d)ABS_D - hours() - minutes() - seconds()`. Otherwise, initializes `ss` with `duration_cast<precision>(abs(d)ABS_D - hours() - minutes() - seconds())`.

### 4275(i). `std::dynamic_extent` should also be defined in `<mdspan>`

Section: 23.7.3.2 [mdspan.syn] Status: C++26 Submitter: Aiden Grossman Opened: 2025-06-06 Last modified: 2026-06-09

Priority: 3

View all other issues in [mdspan.syn].

View all issues with C++26 status.

Discussion:

`std::dynamic_extent` can be used in certain circumstances in `std::mdspan`, such as with padded layouts. However, `std::dynamic_extent` is currently only defined in `<span>` which necessitates including `<span>` solely for the `std::dynamic_extent` definition.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 23.7.3.2 [mdspan.syn], header `<span>` synopsis, as indicated:

   >     // all freestanding
   >     namespace std {
   >       // constants
   >       inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();

   >       // 23.7.3.3 [mdspan.extents], class template extents
   >       template<class IndexType, size_t... Extents>
   >       class extents;

   >       […]
   >     }

[2025-06-10; Jonathan provides improved wording]

[2025-10-15; Reflector poll]

Set priority to 3 after reflector poll.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.1 [views.general] as indicated:

   > The header `<span>` (23.7.2.1 [span.syn]) defines the view `span`. The header `<mdspan>` (23.7.3.2 [mdspan.syn]) defines the class template `mdspan` and other facilities for interacting with these multidimensional views.

   > -?- In addition to being available via inclusion of the `<span>` header, `dynamic_extent` is available when the header `<mdspan>` is included.

### 4276(i). `front()` and `back()` are not hardened for zero-length `std::array`s

Section: 23.3.3.5 [array.zero] Status: C++26 Submitter: Jan Schultke Opened: 2025-06-08 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [array.zero].

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Discussion:

The intent of P3471 "Standard library hardening" is clearly to provide hardened preconditions for members of sequence containers, including `std::array`. However, a zero-length `std::array` dodges this hardening by having undefined behavior for `front()` and `back()` explicitly specified in 23.3.3.5 [array.zero] paragraph 3.

Without this paragraph, `front()` and `back()` would be hardened as well, as specified in 23.2.4 [sequence.reqmts].

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.3.3.5 [array.zero] as indicated:

   > -3- The effect of calling `front()` or `back()` for a zero-sized array is undefined.

### 4278(i). Consider changing how `get-domain-early(sndr)` works

Section: 33.9.2 [exec.snd.expos] Status: Resolved Submitter: Jonathan Wakely Opened: 2025-06-16 Last modified: 2026-04-01

Priority: 2

View all other issues in [exec.snd.expos].

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Discussion:

During LWG review of P3481R3 it was noted that we need to say "except that `sndr` is only evaluated once" because it's used in `get-domain-early(sndr)`, but that exposition-only-function doesn't actually use its argument, it only cares about the type, and it default constructs a `Domain` of a type determined by using the type of `sndr`.

If we had something like:

>     template<class Sender>
>     using get-domain-early-t = decltype(get-domain-early(declval<const Sender&>()));

then we wouldn't need to use `sndr` twice in expressions that currently use `get-domain-early(sndr)`.

[2026-01-16; Reflector poll.]

Set priority to 2 after reflector poll.

A re-design, but we're already doing it for get-domain-late.

[2026-04-01 Resolved by the adoption of P3826R5 at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

### 4280(i). `simd::partial_load` uses undefined identifier `T`

Section: 29.10.8.6 [simd.loadstore] Status: C++26 Submitter: Tim Song Opened: 2025-06-21 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [simd.loadstore].

View all other issues in [simd.loadstore].

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Discussion:

The Effects: element of `std::simd::partial_load` (after the latest rename) uses `T` but that is not defined anywhere. It should be `V::value_type`.

Also, this paragraph should be a Returns: element.

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 29.10.8.6 [simd.loadstore] as indicated:

   >     template<class V = see below, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R>
   >       constexpr V partial_load(R&& r, flags<Flags...> f = {});
   >     […]
   >     template<class V = see below, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       constexpr V partial_load(I first, S last, const typename V::mask_type& mask,
   >                                flags<Flags...> f = {});

   > > -6- […]

   > > -7- Mandates: […]

   > > -8- Preconditions: […]

   > > -9- Effects: Initializes theReturns: A `basic_simd` object whose `i`th element is initialized with `mask[i] && i < ranges::size(r) ? static_cast<T>(ranges::data(r)[i]) : T()` for all `i` in the range of `[0, V::size())`, where `T` is `V::value_type`.

   > > -10- Remarks: The default argument for template parameter `V` is `basic_simd<ranges::range_value_t<R>>`.

### 4282(i). Imprecise Throws: clause in `std::relocate`

Section: 20.2.6 [obj.lifetime] Status: Resolved Submitter: Giuseppe D'Angelo Opened: 2025-06-23 Last modified: 2026-03-27

Priority: 2

View other active issues in [obj.lifetime].

View all other issues in [obj.lifetime].

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Discussion:

The current specification of `std::relocate` in 20.2.6 [obj.lifetime] says

> Throws: Nothing.

This is imprecise. A trivially relocatable type may feature a throwing move constructor (or a throwing destructor); for instance, a linked-list implementation with an externally allocated sentinel node may have a throwing move constructor, but still be trivially relocatable. Such a type is nothrow relocatable (as per `is_nothrow_relocatable_v`, cf. 21.3.5.4 [meta.unary.prop]) and therefore one can call `std::relocate` on objects of that type.

During constant evaluation, a call to `std::relocate` is specified to relocate objects via move construction and destruction. (`std::trivially_relocate` is unavailable during constant evaluation.)

Since P3068 we are allowed to throw during constant evaluation, and therefore it's unclear whether `std::relocate` should propagate such an exception to the caller or should instead make the evaluation not a constant expression (and therefore make the program ill-formed, since by construction we end up in this possibility during constant evaluation).

Given the rationale brought forward for `std::relocate` in P2786 is to be always a nofail operation, I'm proposing a resolution that goes in that direction.

[2025-10-22; Reflector poll.]

Set priority to 2 after reflector poll.

"PR is incomplete, needs to be 'and either `is_move_constructible_v<T>` is false or an exception is thrown...'"

"I think we should use Constant when:."

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 20.2.6 [obj.lifetime] as indicated:

   >     template<class T>
   >       constexpr T* relocate(T* first, T* last, T* result);

   > > -16- Mandates: […]

   > > -17- Preconditions: […]

   > > -18- Effects: […]

   > > -19- Returns: `result + (last - first)`.

   > > -20- Throws: Nothing.

   > > -?- Remarks: If `relocate` is called during constant evaluation, and an exception is thrown by a constructor or destructor of `T`, the call to `relocate` is not a core constant expression (7.7 [expr.const]).

   > > [Note 3: Overlapping ranges are supported. — end note]

[2026-03-26; Resolved by adoption of P3920R0 in Kona, 2025]

Proposed resolution:

### 4283(i). `std::trivially_relocate` needs stronger preconditions on "nested" objects with dynamic lifetime

Section: 20.2.6 [obj.lifetime] Status: Resolved Submitter: Giuseppe D'Angelo Opened: 2025-06-23 Last modified: 2026-03-27

Priority: 2

View other active issues in [obj.lifetime].

View all other issues in [obj.lifetime].

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Discussion:

In 20.2.6 [obj.lifetime] the `std::trivially_relocate` function is missing a precondition, that is, that any object alive in the range being relocated is itself trivially relocatable.

We know the objects in the range are trivially relocatable, because there is a Mandates: element for this. The current draft has precise rules to determine whether a type is trivially relocatable or not; in general, subobjects are considered there (cf. 11.2 [class.prop], "eligible for trivial relocation", which discusses base classes and non-static data members).

However these rules do not take into account objects with dynamic lifetime whose storage is being provided by (sub)objects in the range.

For instance, given a `wrapper` type like:

>     // wraps a T
>     template<typename T>
>     struct wrapper {
>       alignas(T) std::byte data[sizeof(T)];
>     };

then one can build a non-trivially relocatable object into `wrapper` objects:

>     struct NTR { ~NTR() {} };
>     static_assert(not std::is_trivially_relocatable_v<NTR>);

>     using WS = wrapper<NTR>;
>     static_assert(std::is_trivially_relocatable_v<WS>); // OK

And now one can do this:

>     WS* ws = /* … */;     // create a wrapper
>     new (&ws->data) NTR();  // create a NTR object into it

>     std::trivially_relocate(ws, ws+1, dest); // should be UB

Attempting to trivially relocate `*ws` should result in undefined behavior because `NTR` isn't trivially relocatable. I don't believe that this fact is correctly captured by the preconditions of `std::trivially_relocate`.

A similar issue is present for polymorphic types. In P2786's design polymorphic types can be trivially relocatable (assuming all the other conditions hold). Given a trivially relocatable polymorphic type `P`, then this code:

>     struct P { virtual void f(); };
>     static_assert(std::is_trivially_relocatable_v<P>);

>     using WP = wrapper<P>;
>     WP* wp = /* … */;   // create a wrapper
>     new (&wp->data) P();  // create a P object into it

>     std::trivially_relocate(wp, wp+1, dest); // implementation defined

is well-defined or UB, depending on the implementation. This is because on some implementations trivially relocating a polymorphic type requires patching its virtual table pointer; cf. the discussion in chapter 15.1 of P2786R13. However the "type erasure" done by `wrapper<P>` in the example (ultimately, it is just an array of bytes) does not allow implementations to do such patching, and the code is going to fail at runtime. Therefore this case also needs to be discussed by `std::trivially_relocate`'s specification.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 20.2.6 [obj.lifetime] as indicated:

   > [Drafting note: For the general part of the issue (all objects in the range must be of trivially relocatable type), we append another point at the end of the existing Preconditions: element of `trivially_relocate`.
   > For the specifics of polymorphic types, we amend at the end of the description the existing Remarks: element]

   >     template<class T>
   >       T* trivially_relocate(T* first, T* last, T* result);

   > > -9- Mandates: […]

   > > -10- Preconditions:

   > > 1. (10.1) — `[first, last)` is a valid range.

   > > 2. (10.2) — `[result, result + (last - first))` denotes a region of storage that is a subset of the region reachable through `result` (6.9.4 [basic.compound]) and suitably aligned for the type `T`.

   > > 3. (10.3) — No element in the range `[first, last)` is a potentially-overlapping subobject.

   > > 4. (10.?) — All objects whose storage is being provided for (6.8.2 [intro.object]) by objects in the `[first, last)` range are of trivially relocatable type.

   > > -11- Postconditions: […]

   > > -12- Returns: `result + (last - first)`.

   > > -13- Throws: Nothing.

   > > -14- Complexity: Linear in the length of the source range.

   > > -15- Remarks: The destination region of storage is considered reused (6.8.4 [basic.life]). No constructors or destructors are invoked. If any polymorphic object (11.7.3 [class.virtual]) exists in storage provided for (6.8.2 [intro.object]) by objects in the `[first, last)` range, it is implementation-defined whether the behavior is undefined.

   > > [Note 2: Overlapping ranges are supported. — end note]

[2025-07-01; Pablo and Giuseppe improve wording]

[2025-10-23; Reflector poll.]

Set priority to 2 after reflector poll.

It is unclear if the wording covers objects that are nested within (but not subobjects) of relocated objects. For trivially-copyable objectes nested within char array members, they are recreated by-implicit object creation.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 20.2.6 [obj.lifetime] as indicated:

   > [Drafting note: For the general part of the issue (all objects in the range must be of trivially relocatable type), we append another point at the end of the existing Preconditions: element of `trivially_relocate`.
   > For the specifics of polymorphic types, we amend at the end of the description the existing Remarks: element]

   >     template<class T>
   >       T* trivially_relocate(T* first, T* last, T* result);

   > > -9- Mandates: […]

   > > -10- Preconditions:

   > > 1. (10.1) — `[first, last)` is a valid range.

   > > 2. (10.2) — `[result, result + (last - first))` denotes a region of storage that is a subset of the region reachable through `result` (6.9.4 [basic.compound]) and suitably aligned for the type `T`.

   > > 3. (10.3) — No element in the range `[first, last)` is a potentially-overlapping subobject.

   > > 4. (10.?) — All objects whose storage is being provided for (6.8.2 [intro.object]) by objects in the `[first, last)` range are of a type such that a union containing a non-static member of that type would be eligible for trivial relocation.

   > > -11- Postconditions: […]

   > > -12- Returns: `result + (last - first)`.

   > > -13- Throws: Nothing.

   > > -14- Complexity: Linear in the length of the source range.

   > > -15- Remarks: The destination region of storage is considered reused (6.8.4 [basic.life]). No constructors or destructors are invoked.

   > > [Note 2: Overlapping ranges are supported. — end note]

[2026-03-26; Resolved by adoption of P3920R0 in Kona, 2025]

Proposed resolution:

### 4286(i). Some more feature-test macros for fully freestanding features are not marked freestanding

Section: 17.3.2 [version.syn], 20.2.2 [memory.syn] Status: C++26 Submitter: Yihe Li Opened: 2025-06-17 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

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Discussion:

P1642R11 (accepted in C++23) plus LWG 4189(i) (accepted in Hagenberg) added nearly the entire `<ranges>` header to freestanding. However, the only feature-test macro being added to freestanding is `__cpp_lib_ranges_cache_latest`, which seems weird, since `views::enumerate` is also added to freestanding following the blanket comment strategy, but its feature-test macro remains not in freestanding. In retrospective, since all range algorithms are in freestanding via P2976, all `__cpp_lib_ranges_*` FTMs (except `__cpp_lib_ranges_generate_random` since `ranges::generate_random` is not in freestanding) should probably be marked as freestanding.

Furthermore, LWG 4126(i) left out some other FTMs for fully freestanding features. They are also added in the following wording.

A note about `is_sufficiently_aligned`: P2897R7 does indicate in 5.7.6.1 that the function should be freestanding, but somehow the wording didn't say so. The following wording includes the function and its FTM anyway since hopefully this is just an omission when wording the paper.

[2025-10-14; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 17.3.2 [version.syn], header `<version>` synopsis, as indicated:

   >     […]
   >     #define __cpp_lib_aligned_accessor             202411L // freestanding, also in <mdspan>
   >     […]
   >     #define __cpp_lib_array_constexpr              201811L // freestanding, also in <iterator>, <array>
   >     […]
   >     #define __cpp_lib_clamp                        201603L // freestanding, also in <algorithm>
   >     […]
   >     #define __cpp_lib_constexpr_numeric            201911L // freestanding, also in <numeric>
   >     […]
   >     #define __cpp_lib_function_ref                 202306L // freestanding, also in <functional>
   >     #define __cpp_lib_gcd_lcm                      201606L // freestanding, also in <numeric>
   >     […]
   >     #define __cpp_lib_integer_comparison_functions 202002L // freestanding, also in <utility>
   >     […]
   >     #define __cpp_lib_is_sufficiently_aligned      202411L // freestanding, also in <memory>
   >     […]
   >     #define __cpp_lib_ranges_contains              202207L // freestanding, also in <algorithm>
   >     #define __cpp_lib_ranges_enumerate             202302L // freestanding, also in <ranges>
   >     #define __cpp_lib_ranges_find_last             202207L // freestanding, also in <algorithm>
   >     #define __cpp_lib_ranges_fold                  202207L // freestanding, also in <algorithm>
   >     […]
   >     #define __cpp_lib_ranges_iota                  202202L // freestanding, also in <numeric>
   >     […]
   >     #define __cpp_lib_ranges_starts_ends_with      202106L // freestanding, also in <algorithm>
   >     […]
   >     #define __cpp_lib_robust_nonmodifying_seq_ops  201304L // freestanding, also in <algorithm>
   >     #define __cpp_lib_sample                       201603L // freestanding, also in <algorithm>
   >     #define __cpp_lib_saturation_arithmetic        202311L // freestanding, also in <numeric>
   >     […]

2. Modify 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   >     […]
   >     template<size_t Alignment, class T>
   >       bool is_sufficiently_aligned(T* ptr);      // freestanding
   >     […]

### 4290(i). Missing Mandates clauses on `is_sufficiently_aligned`

Section: 20.2.5 [ptr.align] Status: C++26 Submitter: Damien Lebrun-Grandie Opened: 2025-07-03 Last modified: 2026-06-09

Priority: 2

View all other issues in [ptr.align].

View all issues with C++26 status.

Discussion:

`is_sufficiently_aligned` should mandate that the alignment template argument is a power of two and that it is greater equal to the byte alignment of its type template argument.

In 20.2.5 [ptr.align] `is_sufficiently_aligned` has no Mandates element. It is an oversight that we realized when implementing P2897R7 into libc++ (in https://github.com/llvm/llvm-project/pull/122603). The function template was originally proposed as a static member function of the `aligned_accessor` class template which has these two Mandates clauses and therefore applied (see 23.7.3.5.4.1 [mdspan.accessor.aligned.overview] p1). It revision P2897R4, `is_sufficiently_aligned` was moved out the class template definition to become the free function memory helper that was voted into C++26 but the Mandates were lost in the process.

We propose to correct that oversight and reintroduce the following Mandates clauses right above 20.2.5 [ptr.align] p10.

[2025-10-23; Reflector poll.]

Set priority to 2 after reflector poll.

The Mandates of `Alignment >= alignof(T)` was put into question, as being too restrictive for general purpose free function.

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

1. Modify 20.2.5 [ptr.align] as indicated:

   >     template<size_t Alignment, class T>
   >       bool is_sufficiently_aligned(T* ptr);

   > > -?- Mandates:

   > > 1. (?.1) — `Alignment` is a power of two, and

   > > 2. (?.2) — `Alignment >= alignof(T)` is `true`.

   > > -10- Preconditions: `p` points to an object `X` of a type similar (7.3.6 [conv.qual]) to `T`.

   > > -11- Returns: `true` if `X` has alignment at least `Alignment`, otherwise `false`.

   > > -12- Throws: Nothing.

[2026-01-30; Jonathan provides new wording]

[2026-01-30; LWG telecon. Status → Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 20.2.5 [ptr.align] as indicated:

   >     template<size_t Alignment, class T>
   >       bool is_sufficiently_aligned(T* ptr);

   > > -?- Mandates: `Alignment` is a power of two.

   > > -10- Preconditions: `p` points to an object `X` of a type similar (7.3.6 [conv.qual]) to `T`.

   > > -11- Returns: `true` if `X` has alignment at least `Alignment`, otherwise `false`.

   > > -12- Throws: Nothing.

### 4291(i). `explicit map(const Allocator&)` should be `constexpr`

Section: 23.4.3.1 [map.overview] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-07-10 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [map.overview].

View all issues with C++26 status.

Discussion:

The intent of P3372R3 was for all container constructors to be constexpr, but during application of the paper to the working draft it was observed that one `map` constructor was missed.

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.4.3.1 [map.overview] as indicated:

   >     // 23.4.3.2, construct/copy/destroy
   >     constexpr map() : map(Compare()) { }
   >     constexpr explicit map(const Compare& comp, const Allocator& = Allocator());
   >     template<class InputIterator>
   >       constexpr map(InputIterator first, InputIterator last,
   >                     const Compare& comp = Compare(), const Allocator& = Allocator());
   >     template<container-compatible-range <value_type> R>
   >       constexpr map(from_range_t, R&& rg, const Compare& comp = Compare(),
   >                     const Allocator& = Allocator());
   >     constexpr map(const map& x);
   >     constexpr map(map&& x);
   >     constexpr explicit map(const Allocator&);
   >     constexpr map(const map&, const type_identity_t<Allocator>&);
   >     constexpr map(map&&, const type_identity_t<Allocator>&);
   >     constexpr map(initializer_list<value_type>, const Compare& = Compare(),
   >                   const Allocator& = Allocator());

### 4292(i). Unordered container local iterators should be constexpr iterators

Section: 23.5.3.1 [unord.map.overview], 23.5.4.1 [unord.multimap.overview], 23.5.6.1 [unord.set.overview], 23.5.7.1 [unord.multiset.overview] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-07-10 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The intent of P3372R3 was for all container iterators to be constexpr iterators, but during application of the paper to the working draft it was observed that unordered containers don't say it for their local iterators.

[2025-08-29; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.5.3.1 [unord.map.overview] as indicated:

   > -4- The types `iterator`, and `const_iterator`, `local_iterator`, and `const_local_iterator` meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

2. Modify 23.5.4.1 [unord.multimap.overview] as indicated:

   > -4- The types `iterator`, and `const_iterator`, `local_iterator`, and `const_local_iterator` meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

3. Modify 23.5.6.1 [unord.set.overview] as indicated:

   > -4- The types `iterator`, and `const_iterator`, `local_iterator`, and `const_local_iterator` meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

4. Modify 23.5.7.1 [unord.multiset.overview] as indicated:

   > -4- The types `iterator`, and `const_iterator`, `local_iterator`, and `const_local_iterator` meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

### 4293(i). span::subspan/first/last chooses wrong constructor when T is const-qualified bool

Section: 23.7.2.2.4 [span.sub] Status: C++26 Submitter: Yuhan Liu Opened: 2025-07-11 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [span.sub].

View all issues with C++26 status.

Discussion:

In section 23.7.2.2.4 [span.sub], paragraphs p12, p14, and p16 erroneously use the initializer list constructor for span instead of the intended iterator/count constructor.

Specifically, in these paragraphs, the standard states:

> Effects: Equivalent to: `return {data(), count};`

or some variant of `return {pointer, size}`. As reported in GCC bug 120997 this results in a span that points to invalid stack memory. This can be reproduced on GCC 15.1 for subspan, first, and last: https://godbolt.org/z/r9nrdWscq.

A proposed fix (thanks to Jonathan Wakely) could look like this following:

> `return span<element_type>(data(), count);`

for the affected paragraphs, which would explicitly specify the constructor used.

[2025-07-11; Jonathan adds proposed resolution]

The meaning of those Effects: paragraphs was changed for C++26 by P2447R6 which added the `span(initializer_list)` constructor. A simpler demo is:

>     bool a[5]{};
>     std::span<const bool> s(a);
>     std::span<const bool> s2 = s.first(5);
>     assert(s2.size() == 5); // OK in C++23, fails in C++26
>     assert(s2.data() == a); // OK in C++23, fails in C++26

The proposed resolution is to use `R(data(), count)` instead of `{data(), count}`. The former always (uniformly) means the same thing, but for the latter the meaning of list-initialization depends on the types. The list-initialization form will choose the initializer-list constructor when `data()` and `count` are both convertible to the element type.

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.2.2.4 [span.sub] as indicated:

   >     template<size_t Count> constexpr span<element_type, Count> first() const;

   > > -1- Mandates: `Count <= Extent` is `true`.

   > > -2- Hardened preconditions: `Count <= size()` is `true`.

   > > -3- Effects: Equivalent to: `return R({data(), Count});` where `R` is the return type.

   >     template<size_t Count> constexpr span<element_type, Count> last() const;

   > > -4- Mandates: `Count <= Extent` is `true`.

   > > -5- Hardened preconditions: `Count <= size()` is `true`.

   > > -6- Effects: Equivalent to: `return R({data() + (size() - Count), Count});` where `R` is the return type.

   >     template<size_t Offset, size_t Count = dynamic_extent>
   >       constexpr span<element_type, see below> subspan() const;

   > > -7- Mandates:

   > > >     Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset)

   > > is `true`.

   > > -8- Hardened preconditions:

   > > >     Offset <= size() && (Count == dynamic_extent || Count <= size() - Offset)

   > > is `true`.

   > > -9- Effects: Equivalent to:

   > > >     return span<ElementType, see below>(
   > > >         data() + Offset, Count != dynamic_extent ? Count : size() - Offset);

   > > -10- Remarks: The second template argument of the returned `span` type is:

   > > >     Count != dynamic_extent ? Count
   > > >                             : (Extent != dynamic_extent ? Extent - Offset
   > > >                                                         : dynamic_extent)

   >     constexpr span<element_type, dynamic_extent> first(size_type count) const;

   > > -11- Hardened preconditions: `count <= size()` is `true`.

   > > -12- Effects: Equivalent to: `return R({data(), count});` where `R` is the return type.

   >     constexpr span<element_type, dynamic_extent> last(size_type count) const;

   > > -13- Hardened preconditions: `count <= size()` is `true`.

   > > -14- Effects: Equivalent to: `return R({data() + (size() - count), count});` where `R` is the return type.

   >     constexpr span<element_type, dynamic_extent> subspan(
   >       size_type offset, size_type count = dynamic_extent) const;

   > > -15- Hardened preconditions:

   > > >     offset <= size() && (count == dynamic_extent || count <= size() - offset

   > > is `true`.

   > > -16- Effects: Equivalent to:

   > > >     return R({data() + offset, count == dynamic_extent ? size() - offset :  count});

   > > where `R` is the return type.

### 4294(i). `bitset(const CharT*)` constructor needs to be constrained

Section: 22.9.2.2 [bitset.cons] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-07-12 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [bitset.cons].

View all issues with C++26 status.

Discussion:

This code might be ill-formed, with an error outside the immediate context that users cannot prevent:

>     #include <bitset>
>     struct NonTrivial { ~NonTrivial() { } };
>     static_assert( ! std::is_constructible_v<std::bitset<1>, NonTrivial*> );

The problem is that the `bitset(const CharT*)` constructor tries to instantiate `basic_string_view<NonTrivial>` to find its `size_type`, and that instantiation might ill-formed, e.g. if `std::basic_string_view` or `std::char_traits` has a static assert enforcing the requirement for their character type to be sufficiently char-like. 27.1 [strings.general] defines a char-like type as "any non-array trivially copyable standard-layout (6.9.1 [basic.types.general]) type `T` where `is_trivially_default_constructible_v<T>` is `true`."

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 22.9.2.2 [bitset.cons] as indicated:

   >     template<class charT>
   >       constexpr explicit bitset(
   >         const charT* str,
   >         typename basic_string_view<charT>::size_type n = basic_string_view<charT>::npos,
   >         charT zero = charT(’0’),
   >         charT one = charT(’1’));

   > > -?- Constraints:

   > > - `is_array_v<charT>` is `false`,
   > > - `is_trivially_copyable_v<charT>` is `true`,
   > > - `is_standard_layout_v<charT>` is `true`, and
   > > - `is_trivially_default_constructible_v<charT>` is `true`.

   > > -8- Effects: As if by:

   > > >     bitset(n == basic_string_view<charT>::npos
   > > >               ? basic_string_view<charT>(str)
   > > >               : basic_string_view<charT>(str, n),
   > > >            0, n, zero, one)

### 4297(i). Missing `permutable` constraint for iterator overloads in Parallel Range Algorithms

Section: 26.4 [algorithm.syn], 26.7.8 [alg.remove], 26.8.5 [alg.partitions] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2025-06-27 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [algorithm.syn].

View all issues with C++26 status.

Discussion:

The P3179R9: Parallel Range Algorithms paper was accepted to C++ working draft for C++ 26. Unfortunately, there is an oversight for three algorithms — `remove`, `remove_if` and `partition` — where the `permutable` constraint is missing. This applies to "Iterator and Sentinel" overloads only. The issue exists in 26.4 [algorithm.syn] as well as in per-algorithm sections: 26.8.5 [alg.partitions] and 26.7.8 [alg.remove].

[2025-10-21; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.4 [algorithm.syn], header `<algorithm>`, as indicated:

   >     […]
   >     template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >              class Proj = identity, class T = projected_value_t<I, Proj>>
   >       requires permutable<I> &&
   >                indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>
   >       subrange<I> remove(Ep& exec, I first, S last, const T& value,
   >                          Proj proj = {}); // freestanding-deleted
   >     template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >              class T = projected_value_t<iterator_t<R>, Proj>>
   >       requires permutable<iterator_t<R>> &&
   >                indirect_binary_predicate<ranges::equal_to,
   >                                          projected<iterator_t<R>, Proj>, const T*>
   >       borrowed_subrange_t<R>
   >         remove(Ep&& exec, R&& r, const T& value, Proj proj = {}); // freestanding-deleted
   >     […]
   >     template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >              class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred>
   >       requires permutable<I>
   >       subrange<I>
   >         remove_if(Ep& exec, I first, S last, Pred pred, Proj proj = {}); // freestanding-deleted
   >     template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >              indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires permutable<iterator_t<R>>
   >       borrowed_subrange_t<R>
   >         remove_if(Ep& exec, R& r, Pred pred, Proj proj = {}); // freestanding-deleted
   >     […]
   >     template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >              class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred>
   >       requires permutable<I>
   >       subrange<I>
   >         partition(Ep&& exec, I first, S last, Pred pred, Proj proj = {}); // freestanding-deleted
   >     template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >              indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires permutable<iterator_t<R>>
   >       borrowed_subrange_t<R>
   >         partition(Ep&& exec, R&& r, Pred pred, Proj proj = {}); // freestanding-deleted
   >     […]

2. Modify 26.7.8 [alg.remove] as indicated:

   >     […]
   >     template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >              class Proj = identity, class T = projected_value_t<I, Proj>>
   >       requires permutable<I> &&
   >                indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>
   >       subrange<I>
   >         ranges::remove(Ep& exec, I first, S last, const T& value, Proj proj = {});
   >     template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >              class T = projected_value_t<iterator_t<R>, Proj>>
   >       requires permutable<iterator_t<R>> &&
   >                indirect_binary_predicate<ranges::equal_to,
   >                                          projected<iterator_t<R>, Proj>, const T*>
   >       borrowed_subrange_t<R>
   >         ranges::remove(Ep&& exec, R&& r, const T& value, Proj proj = {});
   >     […]
   >     template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >              class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred>
   >       requires permutable<I>
   >       subrange<I>
   >         ranges::remove_if(Ep& exec, I first, S last, Pred pred, Proj proj = {});
   >     template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >              indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires permutable<iterator_t<R>>
   >       borrowed_subrange_t<R>
   >         ranges::remove_if(Ep& exec, R& r, Pred pred, Proj proj = {});

   > > -1- Let `E` be […]

3. Modify 26.8.5 [alg.partitions] as indicated:

   >     […]
   >     template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >              class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred>
   >       requires permutable<I>
   >       subrange<I>
   >         ranges::partition(Ep&& exec, I first, S last, Pred pred, Proj proj = {});
   >     template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >              indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires permutable<iterator_t<R>>
   >       borrowed_subrange_t<R>
   >         ranges::partition(Ep&& exec, R&& r, Pred pred, Proj proj = {});

   > > -1- Let `proj` be `identity{}` for the overloads with no parameter named `proj`.

### 4299(i). Missing Mandates: part in `optional<T&>::transform`

Section: 22.5.4.7 [optional.ref.monadic] Status: C++26 Submitter: Giuseppe D'Angelo Opened: 2025-07-15 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [optional.ref.monadic].

View all issues with C++26 status.

Discussion:

In 22.5.4.7 [optional.ref.monadic] the specification of `optional<T&>::transform` is missing an additional part of the Mandates: element compared to the primary template's `transform` (in 22.5.3.8 [optional.monadic] p8); that is, is missing to enforce that the `U` type is a valid contained type for `optional`.

The definition of "valid contained type" comes from P2988R12. The paper amended the Mandates: element of the primary template's `transform` to use this definition. The fact that the same wording has not been applied to `optional<T&>::transform` as well looks like an oversight. I would suggest to apply it.

[2025-10-16; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

As `optional<remove_cv_t<invoke_result_t<F, T&>>>` is part of the signature (return type), we never enter the body to trigger the Mandates, so it's already implicitly ill-formed if the result of `f` is not a valid contained type. It's worth clarifying that though."

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.7 [optional.ref.monadic] as indicated:

   >     template<class F>
   >       constexpr optional<remove_cv_t<invoke_result_t<F, T&>>> transform(F&& f) const;

   > > -4- Let `U` be `remove_cv_t<invoke_result_t<F, T&>>`.

   > > -5- Mandates: `U` is a valid contained type for `optional`. The declaration

   > > >     U u(invoke(std::forward<F>(f), *val ));

   > > is well-formed for some invented variable `u`.

### 4300(i). Missing Returns: element in `optional<T&>::emplace`

Section: 22.5.4.3 [optional.ref.assign] Status: C++26 Submitter: Giuseppe D'Angelo Opened: 2025-07-15 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The specification for `optional<T&>::emplace` in 22.5.4.3 [optional.ref.assign] is not specifying the returned value via a Returns: element; however the function does indeed return something (a `T&`). Such a Returns: element is there for the primary template's `emplace` (cf. 22.5.3.4 [optional.assign]).

[2025-08-27; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.3 [optional.ref.assign] as indicated:

   >     template<class U>
   >       constexpr T& emplace(U&& u) noexcept(is_nothrow_constructible_v<T&, U>);

   > > -4- Constraints: […]

   > > -5- Effects: Equivalent to: `convert-ref-init-val(std::forward<U>(u))`.

   > > -?- Returns: `*val`.

### 4301(i). `condition_variable{_any}::wait_{for, until}` should take timeout by value

Section: 32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany] Status: C++26 Submitter: Hui Xie Opened: 2025-07-19 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [thread.condition.condvar].

View all issues with C++26 status.

Discussion:

At the moment, both `condition_variable` and `condition_variable_any`'s `wait_for` and `wait_until` take the timeout `time_point`/`duration` by `const` reference. This can cause surprising behaviour. Given the following example (thanks to Tim Song):

>     struct Task {
>       system_clock::time_point deadline;
>       // stuff
>     };

>     std::mutex mtx;
>     std::condition_variable cv;
>     std::priority_queue<Task, vector<Task>, CompareDeadlines> queue;

>     // thread 1
>     std::unique_lock lck(mtx);
>     if (queue.empty()) { cv.wait(lck); }
>     else { cv.wait_until(lck, queue.top().deadline); }

>     // thread 2
>     std::lock_guard lck(mtx);
>     queue.push(/* some task */);
>     cv.notify_one();

From the user's point of view, it is sufficiently locked on both threads. However, due to the fact that the `time_point` is taken by reference, and that both libc++ and libstdc++'s implementation will read the value again after waking up, this will read a dangling reference of the `time_point`.

Another example related to this issue:

We (libc++) recently received a bug report on `condition_variable{_any}::wait_{for, until}`.

Basically the user claims that these functions take `time_point`/`duration` by `const` reference, if the user modifies the `time_point`/`duration` on another thread with the same mutex, they can get unexpected return value for `condition_variable`, and data race for `conditional_variable_any`.

Bug report here.

Reproducer (libstdc++ has the same behaviour as ours) on godbolt.

>     std::mutex mutex;
>     std::condition_variable cv;
>     auto timeout = std::chrono::steady_clock::time_point::max();

>     // Thread 1:
>     std::unique_lock lock(mutex);
>     const auto status = cv.wait_until(lock, timeout);

>     // Thread 2:
>     std::unique_lock lock(mutex);
>     cv.notify_one();
>     timeout = std::chrono::steady_clock::time_point::min();

So basically the problem was that when we return whether there is `no_timeout` or `timeout` at the end of the function, we read the `const` reference again, which can be changed since the beginning of the function. For `condition_variable`, it is "unexpected results" according to the user. And in `conditional_variable_any`, we actually unlock the user lock and acquire our internal lock, then read the input again, so this is actually a data race.

For `wait_for`, the spec has

> Effects: Equivalent to: `return wait_until(lock, chrono::steady_clock::now() + rel_time);`

So the user can claim our implementation is not conforming because the spec says there needs to be a temporary `time_point` (`now + duration`) created and since it should operate on this temporary `time_point`. There shouldn't be any unexpected behaviour or data race .

For `wait_until` it is unclear whether the spec has implications that implementations are allowed to read `abs_time` while the user's lock is unlocked.

it is also unclear if an implementation is allowed to return `timeout` if `cv` indeed does not wait longer than the original value of `timeout`. If it is not allowed, implementations will have to make a local copy of the input `rel_time` or `abs_time`, which defeats the purpose of taking arguments by `const` reference.

For both of the examples, Ville has a great comment in the reflector:

> It seems like a whole bag of problems goes away if these functions just take the timeout by value?

libc++ implementers have strong preference just changing the API to take these arguments by value, and it is not an ABI break for us as the function signature has changed.

[2025-08-29; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 32.7.4 [thread.condition.condvar] as indicated:

   > >     namespace std {
   > >       class condition_variable {
   > >       public:
   > >         […]
   > >         template<class Predicate>
   > >           void wait(unique_lock<mutex>& lock, Predicate pred);
   > >         template<class Clock, class Duration>
   > >           cv_status wait_until(unique_lock<mutex>& lock,
   > >                                const chrono::time_point<Clock, Duration>& abs_time);
   > >         template<class Clock, class Duration, class Predicate>
   > >           bool wait_until(unique_lock<mutex>& lock,
   > >                           const chrono::time_point<Clock, Duration>& abs_time,
   > >                           Predicate pred);
   > >         template<class Rep, class Period>
   > >           cv_status wait_for(unique_lock<mutex>& lock,
   > >                              const chrono::duration<Rep, Period>& rel_time);
   > >         template<class Rep, class Period, class Predicate>
   > >           bool wait_for(unique_lock<mutex>& lock,
   > >                         const chrono::duration<Rep, Period>& rel_time,
   > >                         Predicate pred);
   > >         […]
   > >       };
   > >     }

   > […]

   >     template<class Clock, class Duration>
   >       cv_status wait_until(unique_lock<mutex>& lock,
   >                            const chrono::time_point<Clock, Duration>& abs_time);

   > > -17- Preconditions: […]

   > > […]

   >     template<class Rep, class Period>
   >       cv_status wait_for(unique_lock<mutex>& lock,
   >                          const chrono::duration<Rep, Period>& rel_time);

   > > -23- Preconditions: […]

   > > […]

   >     template<class Clock, class Duration, class Predicate>
   >       bool wait_until(unique_lock<mutex>& lock,
   >                       const chrono::time_point<Clock, Duration>& abs_time,
   >                       Predicate pred);

   > > -29- Preconditions: […]

   > > […]

   >     template<class Rep, class Period, class Predicate>
   >       bool wait_for(unique_lock<mutex>& lock,
   >                     const chrono::duration<Rep, Period>& rel_time,
   >                     Predicate pred);

   > > -35- Preconditions: […]

   > > […]

2. Modify 32.7.5.1 [thread.condition.condvarany.general] as indicated:

   >     namespace std {
   >       class condition_variable_any {
   >       public:
   >         […]
   >         // 32.7.5.2 [thread.condvarany.wait], noninterruptible waits
   >         template<class Lock>
   >           void wait(Lock& lock);
   >         template<class Lock, class Predicate>
   >           void wait(Lock& lock, Predicate pred);

   >         template<class Lock, class Clock, class Duration>
   >           cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);
   >         template<class Lock, class Clock, class Duration, class Predicate>
   >           bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time,
   >                           Predicate pred);
   >         template<class Lock, class Rep, class Period>
   >           cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);
   >         template<class Lock, class Rep, class Period, class Predicate>
   >           bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

   >         // 32.7.5.3 [thread.condvarany.intwait], interruptible waits
   >         template<class Lock, class Predicate>
   >           bool wait(Lock& lock, stop_token stoken, Predicate pred);
   >         template<class Lock, class Clock, class Duration, class Predicate>
   >           bool wait_until(Lock& lock, stop_token stoken,
   >                           const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);
   >         template<class Lock, class Rep, class Period, class Predicate>
   >           bool wait_for(Lock& lock, stop_token stoken,
   >                         const chrono::duration<Rep, Period>& rel_time, Predicate pred);
   >       };
   >     }

3. Modify 32.7.5.2 [thread.condvarany.wait] as indicated:

   > […]

   >     template<class Lock, class Clock, class Duration>
   >       cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);

   > > -6- Effects: […]

   > > […]

   >     template<class Lock, class Rep, class Period>
   >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

   > > -11- Effects: […]

   > > […]

   >     template<class Lock, class Clock, class Duration, class Predicate>
   >       bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time,
   >                       Predicate pred);

   > > -16- Effects: […]

   > > […]

   >     template<class Lock, class Rep, class Period, class Predicate>
   >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

   > > -19- Effects: […]

4. Modify 32.7.5.3 [thread.condvarany.intwait] as indicated:

   > […]

   >     template<class Lock, class Clock, class Duration, class Predicate>
   >       bool wait_until(Lock& lock, stop_token stoken,
   >                       const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);

   > > -7- Effects: […]

   > > […]

   >     template<class Lock, class Rep, class Period, class Predicate>
   >       bool wait_for(Lock& lock, stop_token stoken,
   >                     const chrono::duration<Rep, Period>& rel_time, Predicate pred);

   > > -13- Effects: […]

### 4302(i). Problematic `vector_sum_of_squares` wording

Section: 99 [linalg.algs.blas1.ssq] Status: C++26 Submitter: Mark Hoemmen Opened: 2025-07-23 Last modified: 2026-06-09

Priority: 1

View all issues with C++26 status.

Discussion:

Addresses US 169-276

The current wording for `vector_sum_of_squares` 99 [linalg.algs.blas1.ssq] has three problems with its specification of the value of `result.scaling_factor`.

1. The function permits `InVec::value_type` and `Scalar` to be any linear algebra value types. However, computing `result.scaling_factor` that satisfies both (3.1) and (3.2) requires more operations, such as division. Even if those operations are defined, they might not make `result.scaling_factor` satisfy the required properties. For example, integers have division, but integer division won't help here.

2. LAPACK's xLASSQ (the algorithm to which Note 1 in 99 [linalg.algs.blas1.ssq] refers) changed its algorithm recently (see Reference-LAPACK/lapack/pull/#494) so that the scaling factor is no longer necessarily the maximum of the input scaling factor and the absolute value of all the input elements. It's a better algorithm and we would like to be able to use it.

3. Both members of `sum_of_squares_result<Scalar>` have the same type, `Scalar`. If the input `mdspan`'s `value_type` represents a quantity with units, this would not be correct. For example, if `value_type` has units of distance (say `[m]`), the sum of squares should have units of area (`[m2]`), while the scaling factor should have units of distance (`[m]`).

Problem (1) means that the current wording is broken. I suggest two different ways to fix this.

1. Remove `vector_sum_of_squares` entirely (both overloads from 29.9.2 [linalg.syn], and the entire 99 [linalg.algs.blas1.ssq]). That way, we won't be baking an old, less good algorithm into the Standard. Remove Note 3 from 29.9.13.8 [linalg.algs.blas1.nrm2], which is the only other reference to `vector_sum_of_squares` in the Standard.

2. Fix 99 [linalg.algs.blas1.ssq] by adding to the Mandates element (para 2) that `InVec::value_type` and `Scalar` are both floating-point types (so that we could fix this later if we want), and remove 99 [linalg.algs.blas1.ssq] 3.1. Optionally add Recommended Practice, though Note 1 already suggests the intent.

I prefer just removing `vector_sum_of_squares`. Implementers who care about QoI of `vector_two_norm` should already know what to do. If somebody cares sufficiently, they can propose it back for C++29 and think about how to make it work for generic number types.

[2025-10-17; Reflector poll. Status changed: New → LEWG.]

Set priority to 1 after reflector poll. Send to LEWG.

This is the subject of NB comment 169-276. LWG took a poll in the 2025-10-10 telecon and recommends that LEWG confirms this resolution.

[Kona 2025-11-05; approved by LEWG to resolve US 169-276.]

[Kona 2025-11-05; approved by LWG. Status changed: LEWG → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

> [Drafting note: The wording below implements option 1 of the issue discussion]

1. Modify 29.9.2 [linalg.syn], header `<linalg>` synopsis, as indicated:

   >     namespace std::linalg {
   >       […]
   >       // 99 [linalg.algs.blas1.ssq], scaled sum of squares of a vector's elements
   >       template<class Scalar>
   >         struct sum_of_squares_result {
   >           Scalar scaling_factor;
   >         };
   >       template<in-vector InVec, class Scalar>
   >         sum_of_squares_result<Scalar>
   >           vector_sum_of_squares(InVec v, sum_of_squares_result<Scalar> init);
   >       template<class ExecutionPolicy, in-vector InVec, class Scalar>
   >         sum_of_squares_result<Scalar>
   >           vector_sum_of_squares(ExecutionPolicy&& exec,
   >                                 InVec v, sum_of_squares_result<Scalar> init);
   >       […]
   >     }

2. Delete the entire 99 [linalg.algs.blas1.ssq] as indicated:

   > 29.9.13.8 Scaled sum of squares of a vector's elements [linalg.algs.blas1.ssq]

   >     template<in-vector InVec, class Scalar>
   >       sum_of_squares_result<Scalar> vector_sum_of_squares(InVec v, sum_of_squares_result<Scalar> init);
   >     template<class ExecutionPolicy, in-vector InVec, class Scalar>
   >       sum_of_squares_result<Scalar> vector_sum_of_squares(ExecutionPolicy&& exec,
   >                                                                 InVec v, sum_of_squares_result<Scalar> init);

   > > -1- [Note 1: These functions correspond to the LAPACK function xLASSQ[20]. — end note]

   > > -2- Mandates: `decltype(abs-if-needed(declval<typename InVec::value_type>()))` is convertible to `Scalar`.

   > > -3- Effects: Returns a value `result` such that

   > > 1. (3.1) — `result.scaling_factor` is the maximum of `init.scaling_factor` and `abs-if-needed(x[i])` for all `i` in the domain of `v`; and

   > > 2. (3.2) — let `s2init` be

   > >    >     init.scaling_factor * init.scaling_factor * init.scaled_sum_of_squares

   > >    then `result.scaling_factor * result.scaling_factor * result.scaled_sum_of_squares` equals the sum of `s2init` and the squares of `abs-if-needed(x[i])` for all `i` in the domain of `v`.

   > > -4- Remarks: If `InVec::value_type`, and `Scalar` are all floating-point types or specializations of `complex`, and if `Scalar` has higher precision than `InVec::value_type`, then intermediate terms in the sum use `Scalar`'s precision or greater.

3. Modify 29.9.13.8 [linalg.algs.blas1.nrm2] as indicated:

   >     template<in-vector InVec, class Scalar>
   >       Scalar vector_two_norm(InVec v, Scalar init);
   >     template<class ExecutionPolicy, in-vector InVec, class Scalar>
   >       Scalar vector_two_norm(ExecutionPolicy&& exec, InVec v, Scalar init);

   > > -1- [Note 1: […] ]

   > > -2- Mandates: […]

   > > -3- Returns: […]

   > > [Note 2: […] ]

   > > -4- Remarks: […]

   > > [Note 3: An implementation of this function for floating-point types `T` can use the `scaled_sum_of_squares` result from `vector_sum_of_squares(x, {.scaling_factor=1.0, .scaled_sum_of_squares=init})`. — end note]

### 4304(i). `std::optional<NonReturnable&>` is ill-formed due to `value_or`

Section: 22.5.4.6 [optional.ref.observe] Status: C++26 Submitter: Jiang An Opened: 2025-07-25 Last modified: 2026-06-09

Priority: 1

View all issues with C++26 status.

Discussion:

Currently, if `T` is an array type or a function type, instantiation of `std::optional<T&>` is still ill-formed, because the return type of its `value_or` member function is specified as `remove_cv_t<T>`, which is invalid as a return type.

However, we don't exclude such `T&` from valid contained types. Given only `value_or` is problematic here, perhaps we can avoid providing it if `T` is not returnable.

[2025-10-16; Reflector poll]

Set priority to 1 after reflector poll.

Why not just add Constraints: and use `decay_t<T>` for the return type, instead of "not always present" which is currently only used for member types, not member functions.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 22.5.4.1 [optional.optional.ref.general], header `<iterator>` synopsis, as indicated:

   >     namespace std {
   >       template<class T>
   >       class optional<T&> {
   >         […]
   >         constexpr T& value() const; // freestanding-deleted
   >         template<class U = remove_cv_t<T>>
   >           constexpr remove_cv_t<T> value_or(U&& u) const; // not always present
   >         […]
   >       };
   >     }

2. Modify 22.5.4.6 [optional.ref.observe] as indicated:

   >     template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& u) const;

   > > -8- Let `X` be `remove_cv_t<T>`.

   > > -9- Mandates: `is_constructible_v<X, T&> && is_convertible_v<U, X>` is `true`.

   > > -10- Effects: Equivalent to:

   > > >     return has_value() ? *val : static_cast<X>(std::forward<U>(u));

   > > -?- Remarks: This function template is present if and only if `T` is a non-array object type.

[2025-10-16; Jonathan provides new wording]

[Kona 2025-11-03; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.6 [optional.ref.observe] as indicated:

   >     template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& u) const;

   > > -?- Constraints: `T` is a non-array object type.

   > > -8- Let `X` be `remove_cv_t<T>`.

   > > -9- Mandates: `is_constructible_v<X, T&> && is_convertible_v<U, X>` is `true`.

   > > -10- Effects: Equivalent to:

   > > >     return has_value() ? *val : static_cast<X>(std::forward<U>(u));

   > > -?- Remarks: The return type is unspecified if `T` is an array type or a non-object type. [Note ?: This is to avoid the declaration being ill-formed. — end note]

### 4305(i). Missing user requirements on `type_order` template

Section: 17.12.7 [compare.type] Status: C++26 Submitter: Daniel Krügler Opened: 2025-07-27 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The recently approved paper P2830R10 proposes to add the new `type_order` type traits to 17.12 [cmp] (and thus outside of 21.3 [type.traits]), which has the subtle and most likely unintended effect, that it doesn't fall under the general requirement expressed in 21.3.2 [meta.rqmts] p4,

> Unless otherwise specified, the behavior of a program that adds specializations for any of the templates specified in 21.3.2 [meta.rqmts] is undefined.

and so in principle the explicit allowance specified in 16.4.5.2.1 [namespace.std] p2,

> Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace `std` […]

holds. So we need to add extra wording to the `type_order` specification in 17.12.7 [compare.type] to prohibit such program specializations.

This was reported shortly before the Sofia meeting during reflector discussion but seems to be forgotten before the final paper appeared on plenary.

During the reflector discussion two possible ways to solve this issue were pointed out:

1. The most simple one would mimic the wording in 21.3.2 [meta.rqmts] p4 quoted above.

2. Instead of introducing just another undefined opportunity to run into undefined behaviour it has been pointed out that we could follow the approach taken by `std::initializer_list` and make the program ill-formed in this case, as specified in 17.11.2 [initializer.list.syn] p2:

   > If an explicit specialization or partial specialization of `initializer_list` is declared, the program is ill-formed.

Jonathan Wakely responded to the reflector discussion:

> I think ill-formed would be better. It shouldn't be difficult for implementations to have special cases that are disallowed.

Given the already existing experience with `std::initializer_list` the proposed wording below therefore follows the ill-formed program approach.

[2025-10-14; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 17.12.7 [compare.type] as indicated:

   >     template<class T, class U>
   >       struct type_order;

   > > -2- The name `type_order` denotes a Cpp17BinaryTypeTrait (21.3.2 [meta.rqmts]) with a base characteristic of `integral_constant<strong_ordering, TYPE-ORDER(T, U)>`.

   > > -?- If an explicit specialization or partial specialization of `type_order` is declared, the program is ill-formed.

   > > -3- Recommended practice: The order should be lexicographical on parameter-type-lists and template argument lists.

### 4308(i). `std::optional<T&>::iterator` can't be a contiguous iterator for some `T`

Section: 22.5.4.5 [optional.ref.iterators] Status: C++26 Submitter: Jiang An Opened: 2025-08-05 Last modified: 2026-06-09

Priority: 1

View all issues with C++26 status.

Discussion:

This is related to LWG 4304(i). When `T` is function type or an incomplete array type, it is impossible to implement all requirements in 22.5.4.5 [optional.ref.iterators]/1.

When `T` is an incomplete object type, we may want to support `std::optional<T&>` as it's sometimes a replacement of `T*`. Perhaps we can require that the iterator type is always a random access iterator, and additional models `contiguous_iterator` when `T` is complete.

When `T` is a function type, the possibly intended iterator would be not even an actual iterator. But it seems that range-for loop over such an `std::optional<T&>` can work.

[2025-08-29; Reflector poll]

Set priority to 1 after reflector poll.

"How can `end()` work for a pointer to incomplete type? `begin`/`end` should be constrained on object types, and Mandate complete object types. The aliases shouldn't be defined for non-object types, but probably harmless."

[Kona 2025-11-05; Should only be a range for an object type.]

`optional<T&>` doesn't currently allow incomplete types anyway.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 22.5.4.5 [optional.ref.iterators] as indicated:

   >     using iterator = implementation-defined;

   > > -1- TIf `T` is an object type, this type models `contiguous_iterator` (24.3.4.14 [iterator.concept.contiguous])`random_access_iterator` (24.3.4.13 [iterator.concept.random.access]), meets the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]), and meets the requirements for constexpr iterators (24.3.1 [iterator.requirements.general]), with value type `remove_cv_t<T>`. The reference type is `T&` for `iterator`. When `T` is a complete object type, iterator additionally models `contiguous_iterator` (24.3.4.14 [iterator.concept.contiguous]).
   > > -2- All requirements on container iterators (23.2.2.2 [container.reqmts]) apply to `optional::iterator`.

   > > -?- If `T` is a function type, `iterator` supports all operators required by the `random_access_iterator` concept (24.3.4.13 [iterator.concept.random.access]) along with the `<=>` operator as specified for container iterators (23.2.2.2 [container.reqmts]). `iterator` dereferences to a `T` lvalue. These operators behave as if `iterator` were an actual iterator iterating over a range of `T`, and result in constant subexpressions whenever the behavior is well-defined. [Note ?: Such an `optional::iterator` does not need to declare any member type because it is not an actual iterator type. — end note]

[Kona 2025-11-06, Tomasz provides updated wording]

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

1. Modify 22.5.4.1 [optional.optional.ref.general] as indicated:

   >     namespace std {
   >       template<class T>
   >       class optional<T&> {
   >       public:
   >         using value_type     = T;
   >         using iterator       = implementation-defined;  // present only if T is an object type other than an array of unknown bound; see [optional.ref.iterators]
   >       public:
   >         […]

   >         // [optional.ref.iterators], iterator support
   >         constexpr iteratorauto begin() const noexcept;
   >         constexpr iteratorauto end() const noexcept;

   >         […]
   >       };
   >     }

2. Modify 22.5.4.5 [optional.ref.iterators] as indicated:

   >     using iterator = implementation-defined; // present only if T is an object type other than an array of unknown bound

   > > -1- This type models `contiguous_iterator` (24.3.4.14 [iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]), and meets the requirements for constexpr iterators (24.3.1 [iterator.requirements.general]), with value type `remove_cv_t<T>`. The reference type is `T&` for `iterator`.

   > > -2- All requirements on container iterators (23.2.2.2 [container.reqmts]) apply to `optional::iterator`.

   >     constexpr iteratorauto begin() const noexcept;

   > > -?- Constraints: T is an object type other than an array of unknown bound.

   > > -3- Returns: An object `i` of type `iterator`, such that If `has_value()` is `true`, `i` is an iterator referring to `*val` if `has_value()` is `true`, and Otherwise, a past-the-end iterator value otherwise.

   >     constexpr iteratorauto end() const noexcept;

   > > -?- Constraints: T is an object type other than an array of unknown bound.

   > > -4- Returns: `begin() + has_value()`.

### 4312(i). Const and value category mismatch for `allocator_arg_t`/`allocator_arg` in the description of uses-allocator construction

Section: 20.2.8.2 [allocator.uses.construction] Status: C++26 Submitter: Jiang An Opened: 2025-08-06 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [allocator.uses.construction].

View all issues with C++26 status.

Discussion:

Currently, 20.2.8.2 [allocator.uses.construction] bullet 2.2 states:

> Otherwise, if `T` has a constructor invocable as `T(allocator_arg, alloc, args...)` (leading-allocator convention), […]

However, when forming construction arguments in the utility functions, we're actually using cv-unqualified rvalue of `allocator_arg_t`, which can be inferred from using plain `allocator_arg_t` but not `const allocator_arg_t&` in 20.2.8.2 [allocator.uses.construction] bullet 5.2.

It seems that such mismatch was present even since C++11 (per N3337 [allocator.uses.construction]/1.2). If the use of plain `allocator_arg_t` is considered correct, I think we should fix the description.

[2025-10-14; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Unless the `std::allocator_arg` tag object is not supposed to be used, wouldn't it make more sense to preserve the "if `T` has a constructor invocable as `T(allocator_arg, alloc, args...)`" wording and change every `allocator_arg_t` into `const allocator_arg_t&`, so that we check for construction from the const tag object, and then actually use a const value in the constructor arguments. Strongly don't care though.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 20.2.8.2 [allocator.uses.construction] as indicated:

   > -2- The following utility functions support three conventions for passing `alloc` to a constructor:

   > 1. (2.1) — […]

   > 2. (2.2) — Otherwise, if `T` has a constructor invocable as `T(allocator_argallocator_arg_t{}, alloc, args...)` (leading-allocator convention), then uses-allocator construction chooses this constructor form.

   > 3. (2.3) — […]

### 4314(i). Missing move in `mdspan` layout `mapping::operator()`

Section: 23.7.3.4 [mdspan.layout] Status: C++26 Submitter: Luc Grosheintz Opened: 2025-08-13 Last modified: 2026-06-09

Priority: 2

View all other issues in [mdspan.layout].

View all issues with C++26 status.

Discussion:

Numerous template classes in `<mdspan>` have template parameter `IndexType`. While this template parameter is restricted to be a signed or unsigned integer, these classes often accept user-defined classes that convert to `IndexType`.

They're either passed as an array/span of `OtherIndexType`; or as a template parameter pack. When passed as a template parameter pack, the common pattern is

>     template<class... OtherIndexTypes>
>       requires std::is_convertible_v<OtherIndexTypes, IndexType> && ...
>       void dummy(OtherIndexTypes... indices)
>       {
>         something(static_cast<IndexType>(std::move(indices))...);
>       }

This pattern allows passing in objects that convert to IndexType only as an rvalue reference, e.g.

>     class RValueInt
>     {
>       constexpr
>       operator int() && noexcept
>       { return m_int; }

>     private:
>       int m_int;
>     };

This pattern can be found in:

- a ctor of `extents`,

- a ctor of `mdspan`,

- `mdspan::operator[]`.

The five standardized layout mappings use a different pattern in their operator(), namely,

>     static_cast<IndexType>(indices)...

This prevents the passing in objects of type `RValueInt`, because the conversion isn't happening from an rvalue reference. This is addressed by Items 1 - 5 in the Proposed Resolution.

A different pattern can be found a ctor for `layout_{left,right}_padded`. Namely, directly passing an object of type `OtherIndexType` to `LEAST-MULTIPLE-AT-LEAST`. This is addressed in Items 6 and 7 in the Proposed Resolution.

This inconsistency was noticed while fixing PR121061 and these changes have been applied to all layout mappings implemented in libstdc++.

[2025-10-21; Reflector poll.]

Set priority to 2 after reflector poll.

The resolution bypasses `extents_type::index-cast` that would validate if input value is representable.

We should require convertibility without regard to const and value category.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 23.7.3.4.5.3 [mdspan.layout.left.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... i) const noexcept;

   > > -2- Constraints: […]

   > > -3- Preconditions: […]

   > > -4- Effects: Let `P` be a parameter pack such that

   > > >     is_same_v<index_sequence_for<Indices...>, index_sequence<P...>>

   > > is `true`. Equivalent to:

   > > >     return ((static_cast<index_type>(std::move(i)) * stride(P)) + ... + 0);

2. Modify 23.7.3.4.6.3 [mdspan.layout.right.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... i) const noexcept;

   > > -2- Constraints: […]

   > > -3- Preconditions: […]

   > > -4- Effects: Let `P` be a parameter pack such that

   > > >     is_same_v<index_sequence_for<Indices...>, index_sequence<P...>>

   > > is `true`. Equivalent to:

   > > >     return ((static_cast<index_type>(std::move(i)) * stride(P)) + ... + 0);

3. Modify 23.7.3.4.7.4 [mdspan.layout.stride.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... i) const noexcept;

   > > -2- Constraints: […]

   > > -3- Preconditions: […]

   > > -4- Effects: Let `P` be a parameter pack such that

   > > >     is_same_v<index_sequence_for<Indices...>, index_sequence<P...>>

   > > is `true`. Equivalent to:

   > > >     return ((static_cast<index_type>(std::move(i)) * stride(P)) + ... + 0);

4. Modify 23.7.3.4.8.4 [mdspan.layout.leftpad.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... idxs) const noexcept;

   > > -3- Constraints: […]

   > > -4- Preconditions: […]

   > > -5- Returns: `((static_cast<index_type>(std::move(idxs)) * stride(P_rank)) + ... + 0)`.

5. Modify 23.7.3.4.9.4 [mdspan.layout.rightpad.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... idxs) const noexcept;

   > > -3- Constraints: […]

   > > -4- Preconditions: […]

   > > -5- Returns: `((static_cast<index_type>(std::move(idxs)) * stride(P_rank)) + ... + 0)`.

6. Modify 23.7.3.4.8.3 [mdspan.layout.leftpad.cons] as indicated:

   >     template<class OtherIndexType>
   >     constexpr mapping(const extents_type& ext, OtherIndexType padding);

   > > Let `pad = static_cast<index_type>(std::move(padding))`.

   > > -3- Constraints: […]

   > > -4- Preconditions:

   > > 1. (4.1) — `padding` is representable as a value of type `index_type`.

   > > 2. (4.2) — `extents_type::index-cast(pad)`pad is greater than zero.

   > > 3. (4.3) — If `rank_` is greater than one, then `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(0))` is representable as a value of type `index_type`.

   > > 4. (4.4) — If `rank_` is greater than one, then the product of `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(0))` and all values `ext.extent(k)` with `k` in the range of `[1, rank_)` is representable as a value of type `index_type`.

   > > 5. (4.5) — If `padding_value` is not equal to `dynamic_extent`, `padding_value` equals `extents_type::index-cast(pad)pad`.

   > > -5- Effects: […]

7. Modify 23.7.3.4.9.3 [mdspan.layout.rightpad.cons] as indicated:

   >     template<class OtherIndexType>
   >     constexpr mapping(const extents_type& ext, OtherIndexType padding);

   > > Let `pad = static_cast<index_type>(std::move(padding))`.

   > > -3- Constraints: […]

   > > -4- Preconditions:

   > > 1. (4.1) — `padding` is representable as a value of type `index_type`.

   > > 2. (4.2) — `extents_type::index-cast(pad)`pad is greater than zero.

   > > 3. (4.3) — If `rank_` is greater than one, then `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1))` is representable as a value of type `index_type`.

   > > 4. (4.4) — If `rank_` is greater than one, then the product of `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1))` and all values `ext.extent(k)` with `k` in the range of `[1, rank_ - 1)` is representable as a value of type `index_type`.

   > > 5. (4.5) — If `padding_value` is not equal to `dynamic_extent`, `padding_value` equals `extents_type::index-cast(pad)pad`.

   > > -5- Effects: […]

[2025-10-05; Tomasz provides upated wording after LWG review]

[Croydon 2026-03-25; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.7.3.3.2 [mdspan.extents.expo] as indicated:

   >     template<class OtherIndexType>
   >       static constexpr auto index-cast(OtherIndexType&& i) noexcept;

   > > -9- Effects:

   > > - -9.1- If `OtherIndexType` is an integral type other than `bool`, then equivalent to `return i;`,
   > > - -9.2- otherwise, equivalent to `return static_cast<index_type>(std::forward<OtherIndexType>(i));`.

2. Modify 23.7.3.4.5.3 [mdspan.layout.left.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... i) const noexcept;

   > > -2- Constraints: […]

   > > -3- Preconditions: […]

   > > -4- Effects: Let `P` be a parameter pack such that

   > > >     is_same_v<index_sequence_for<Indices...>, index_sequence<P...>>

   > > is `true`. Equivalent to:

   > > >     return ((static_cast<index_type>(std::move(i)) * stride(P)) + ... + 0);

3. Modify 23.7.3.4.6.3 [mdspan.layout.right.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... i) const noexcept;

   > > -2- Constraints: […]

   > > -3- Preconditions: […]

   > > -4- Effects: Let `P` be a parameter pack such that

   > > >     is_same_v<index_sequence_for<Indices...>, index_sequence<P...>>

   > > is `true`. Equivalent to:

   > > >     return ((static_cast<index_type>(std::move(i)) * stride(P)) + ... + 0);

4. Modify 23.7.3.4.7.4 [mdspan.layout.stride.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... i) const noexcept;

   > > -2- Constraints: […]

   > > -3- Preconditions: […]

   > > -4- Effects: Let `P` be a parameter pack such that

   > > >     is_same_v<index_sequence_for<Indices...>, index_sequence<P...>>

   > > is `true`. Equivalent to:

   > > >     return ((static_cast<index_type>(std::move(i)) * stride(P)) + ... + 0);

5. Modify 23.7.3.4.8.4 [mdspan.layout.leftpad.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... idxs) const noexcept;

   > > -3- Constraints: […]

   > > -4- Preconditions: […]

   > > -5- Returns: `((static_cast<index_type>(std::move(idxs)) * stride(P_rank)) + ... + 0)`.

6. Modify 23.7.3.4.9.4 [mdspan.layout.rightpad.obs] as indicated:

   >     template<class... Indices>
   >       constexpr index_type operator()(Indices... idxs) const noexcept;

   > > -3- Constraints: […]

   > > -4- Preconditions: […]

   > > -5- Returns: `((static_cast<index_type>(std::move(idxs)) * stride(P_rank)) + ... + 0)`.

7. Modify 23.7.3.4.8.3 [mdspan.layout.leftpad.cons] as indicated:

   >     template<class OtherIndexType>
   >     constexpr mapping(const extents_type& ext, OtherIndexType padding);

   > > Let `pad` be `extents_type::index-cast(std::move(padding))`.

   > > -3- Constraints: […]

   > > -4- Preconditions:

   > > 1. (4.1) — `pad` is representable as a value of type `index_type`.

   > > 2. (4.2) — `extents_type::index-cast(pad)``pad` is greater than zero.

   > > 3. (4.3) — If `rank_` is greater than one, then `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(0))` is representable as a value of type `index_type`.

   > > 4. (4.4) — If `rank_` is greater than one, then the product of `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(0))` and all values `ext.extent(k)` with `k` in the range of `[1, rank_)` is representable as a value of type `index_type`.

   > > 5. (4.5) — If `padding_value` is not equal to `dynamic_extent`, `padding_value` equals `extents_type::index-cast(pad)pad`.

   > > -5- Effects: Direct-non-list-initializes `extents_` with `ext`, and if `rank_` is greater than one, direct-non-list-initializes `stride-rm2` with `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1))`.

8. Modify 23.7.3.4.9.3 [mdspan.layout.rightpad.cons] as indicated:

   >     template<class OtherIndexType>
   >     constexpr mapping(const extents_type& ext, OtherIndexType padding);

   > > Let `pad` be `extents_type::index-cast(std::move(padding))`.

   > > -3- Constraints: […]

   > > -4- Preconditions:

   > > 1. (4.1) — `pad` is representable as a value of type `index_type`.

   > > 2. (4.2) — `extents_type::index-cast(pad)``pad` is greater than zero.

   > > 3. (4.3) — If `rank_` is greater than one, then `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1))` is representable as a value of type `index_type`.

   > > 4. (4.4) — If `rank_` is greater than one, then the product of `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1))` and all values `ext.extent(k)` with `k` in the range of `[1, rank_ - 1)` is representable as a value of type `index_type`.

   > > 5. (4.5) — If `padding_value` is not equal to `dynamic_extent`, `padding_value` equals `extents_type::index-cast(pad)pad`.

   > > -5- Effects: Direct-non-list-initializes `extents_` with `ext`, and if `rank_` is greater than one, direct-non-list-initializes `stride-rm2` with `LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1))`.

### 4315(i). Insufficient specification of `vector_two_norm` and `matrix_frob_norm`

Section: 29.9.13.8 [linalg.algs.blas1.nrm2], 29.9.13.11 [linalg.algs.blas1.matfrobnorm] Status: C++26 Submitter: Mark Hoemmen Opened: 2025-08-14 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [linalg.algs.blas1.nrm2].

View all issues with C++26 status.

Discussion:

Addresses US 171-274

The Returns clauses of `vector_two_norm` 29.9.13.8 [linalg.algs.blas1.nrm2] and `matrix_frob_norm` 29.9.13.11 [linalg.algs.blas1.matfrobnorm] say that the functions return the "square root" of the sum of squares of the initial value and the absolute values of the elements of the input `mdspan`. However, nowhere in 29.9 [linalg] explains how to compute a square root.

1. The input `mdspan`'s `value_type` and the initial value type are not constrained in a way that would ensure that calling `std::sqrt` on this expression would be well-formed.

2. There is no provision to find `sqrt` via argument-dependent lookup, even though 29.9 [linalg] has provisions to find `abs`, `conj`, `real`, and `imag` via argument-dependent lookup. There is no "`sqrt-if-needed`" analog to `abs-if-needed`, `conj-if-needed`, `real-if-needed`, and `imag-if-needed`.

The easiest fix for both issues is just to Constrain both `Scalar` and the input `mdspan`'s `value_type` to be floating-point numbers or specializations of `std::complex` for these two functions. This presumes that relaxing this Constraint and fixing the above two issues later would be a non-breaking change. If that is not the case, then I would suggest removing the two functions entirely.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

> [Drafting note: As a drive-by fix the proposed wording adds a missing closing parentheses in 29.9.13.8 [linalg.algs.blas1.nrm2] p2.]

1. Modify 29.9.13.8 [linalg.algs.blas1.nrm2] as indicated:

   >     template<in-vector InVec, class Scalar>
   >       Scalar vector_two_norm(InVec v, Scalar init);
   >     template<class ExecutionPolicy, in-vector InVec, class Scalar>
   >       Scalar vector_two_norm(ExecutionPolicy&& exec, InVec v, Scalar init);

   > > -1- [Note 1: […] — end note]

   > > -?- Constraints: `InVec::value_type` and `Scalar` are either a floating-point type, or a specialization of `complex`.

   > > -2- Mandates: Let `a` be `abs-if-needed(declval<typename InVec::value_type>())`. Then, `decltype(init + a * a)` is convertible to `Scalar`.

   > > -3- Returns: The square root of the sum of the square of `init` and the squares of the absolute values of the elements of `v`.

   > > [Note 2: For `init` equal to zero, this is the Euclidean norm (also called 2-norm) of the vector `v`. — end note]

   > > -4- Remarks: If `InVec::value_type`, and `Scalar` are all floating-point types or specializations of `complex`, and if `Scalar` has higher precision than `InVec::value_type`, then intermediate terms in the sum use `Scalar`'s precision or greater.

   > > [Note 3: An implementation of this function for floating-point types `T` can use the `scaled_sum_of_squares` result `from vector_sum_of_squares(x, {.scaling_factor=1.0, .scaled_sum_of_squares=init})`. — end note]

2. Modify 29.9.13.11 [linalg.algs.blas1.matfrobnorm] as indicated:

   >     template<in-matrix InMat, class Scalar>
   >       Scalar matrix_frob_norm(InMat A, Scalar init);
   >     template<class ExecutionPolicy, in-matrix InMat, class Scalar>
   >       Scalar matrix_frob_norm(ExecutionPolicy&& exec, InMat A, Scalar init);

   > > -?- Constraints: `InVec::value_type` and `Scalar` are either a floating-point type, or a specialization of `complex`.

   > > -2- Mandates: Let `a` be `abs-if-needed(declval<typename InMat::value_type>())`. Then, `decltype(init + a * a)` is convertible to `Scalar`.

   > > -3- Returns: The square root of the sum of squares of `init` and the absolute values of the elements of `A`.

   > > [Note 2: For `init` equal to zero, this is the Frobenius norm of the matrix `A`. — end note]

   > > -4- Remarks: If `InMat::value_type` and `Scalar` are all floating-point types or specializations of `complex`, and if `Scalar` has higher precision than `InMat::value_type`, then intermediate terms in the sum use `Scalar`'s precision or greater.

[LWG telecon 2025-10-10; Update proposed resolution after review]

Use Mandates: for the new requirements, because we plan to change this later so want to make it ill-formed, not something that is statically checkable as part of the API.

[LWG telecon 2025-10-10; Status updated New → Ready]

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

> [Drafting note: As a drive-by fix the proposed wording adds a missing closing parentheses in 29.9.13.8 [linalg.algs.blas1.nrm2] p2.]

1. Modify 29.9.13.8 [linalg.algs.blas1.nrm2] as indicated:

   >     template<in-vector InVec, class Scalar>
   >       Scalar vector_two_norm(InVec v, Scalar init);
   >     template<class ExecutionPolicy, in-vector InVec, class Scalar>
   >       Scalar vector_two_norm(ExecutionPolicy&& exec, InVec v, Scalar init);

   > > -1- [Note 1: […] — end note]

   > > -2- Mandates: `InVec::value_type` and `Scalar` are either a floating-point type, or a specialization of `complex`. Let `a` be `abs-if-needed(declval<typename InVec::value_type>())`. Then, `decltype(init + a * a)` is convertible to `Scalar`.

   > > -3- Returns: The square root of the sum of the square of `init` and the squares of the absolute values of the elements of `v`.

   > > [Note 2: For `init` equal to zero, this is the Euclidean norm (also called 2-norm) of the vector `v`. — end note]

   > > -4- Remarks: If `InVec::value_type`, and `Scalar` are all floating-point types or specializations of `complex`, and if `Scalar` has higher precision than `InVec::value_type`, then intermediate terms in the sum use `Scalar`'s precision or greater.

   > > [Note 3: An implementation of this function for floating-point types `T` can use the `scaled_sum_of_squares` result `from vector_sum_of_squares(x, {.scaling_factor=1.0, .scaled_sum_of_squares=init})`. — end note]

2. Modify 29.9.13.11 [linalg.algs.blas1.matfrobnorm] as indicated:

   >     template<in-matrix InMat, class Scalar>
   >       Scalar matrix_frob_norm(InMat A, Scalar init);
   >     template<class ExecutionPolicy, in-matrix InMat, class Scalar>
   >       Scalar matrix_frob_norm(ExecutionPolicy&& exec, InMat A, Scalar init);

   > > -2- Mandates: `InVec::value_type` and `Scalar` are either a floating-point type, or a specialization of `complex`. Let `a` be `abs-if-needed(declval<typename InMat::value_type>())`. Then, `decltype(init + a * a)` is convertible to `Scalar`.

   > > -3- Returns: The square root of the sum of squares of `init` and the absolute values of the elements of `A`.

   > > [Note 2: For `init` equal to zero, this is the Frobenius norm of the matrix `A`. — end note]

   > > -4- Remarks: If `InMat::value_type` and `Scalar` are all floating-point types or specializations of `complex`, and if `Scalar` has higher precision than `InMat::value_type`, then intermediate terms in the sum use `Scalar`'s precision or greater.

### 4316(i). `{can_}substitute` specification is ill-formed

Section: 21.4.15 [meta.reflection.substitute] Status: C++26 Submitter: Matthias Wippich Opened: 2025-08-15 Last modified: 2026-06-09

Priority: 1

View all other issues in [meta.reflection.substitute].

View all issues with C++26 status.

Discussion:

Addresses US 114-175

`can_substitute` and `substitute` are currently specified in terms of splices in a template argument list:

21.4.15 [meta.reflection.substitute] p3:

> Returns: `true` if `Z<[:Args:]...>` is a valid template-id (13.3 [temp.names]) that does not name a function whose type contains an undeduced placeholder type. Otherwise, `false`.

21.4.15 [meta.reflection.substitute] p7:

> Returns: `^^Z<[:Args:]...>`.

This wording was introduced in P2996R11. However, merging in changes from P3687 "Final Adjustments to C++26 Reflection" in P2996R13 changed the rules for splices in this context. This makes `can_substitute` and `substitute` as specified currently ill-formed. We cannot use the given syntax to splice an arbitrary choice of values, types and templates anymore.

While the intent seems clear, this should be rephrased to be more technically correct.

[2025-10-22; Reflector poll.]

Set priority to 1 after reflector poll.

[2025-10-27; Tomasz provides wording.]

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 21.4.15 [meta.reflection.substitute] as indicated:

   > > -1- For value `x` of type `info`, and prvalue constant expression `X` that computes the reflection held by `x`, let `TARG-SPLICE(x)` be:

   > > - -1.1- `template [: X :]` if `is_template(x)` is `true`, otherwise
   > > - -1.2- `typename [: X :]` if `is_type(x)` is `true`, otherwise
   > > - -1.3- `([: X :])`

   >     template<reflection_range R = initializer_list<info>>
   >       consteval bool can_substitute(info templ, R&& arguments);

   > > -1- Let `Z` be the template represented by `templ` and let `Args...` be a sequence of prvalue constant expressions that compute the reflections held by the elements of `arguments`, in order. Let n be the number of elements in `arguments`, and ei be the ith element of `arguments`.

   > > -2- Returns: `true` if `Z<[:Args:]...TARG-SPLICE(e0), ..., TARG-SPLICE(en-1)>` is a valid template-id (13.3 [temp.names]) that does not name a function whose type contains an undeduced placeholder type. Otherwise, `false`.

   > > -3- Throws: `meta::exception` unless `templ` represents a template, and every reflection in `arguments` represents a construct usable as a template argument (13.4 [temp.arg]).

   > > -4- [Note: If forming `Z<[:Args:]...TARG-SPLICE(e0), ..., TARG-SPLICE(en-1)>` leads to a failure outside of the immediate context, the program is ill-formed. — end note]

   >     template<reflection_range R = initializer_list<info>>
   >       consteval info substitute(info templ, R&& arguments);

   > > -5- Let `Z` be the template represented by `templ` and let `Args...` be a sequence of prvalue constant expressions that compute the reflections held by the elements of `arguments`, in order. Let n be the number of elements in `arguments`, and ei be the ith element of `arguments`.

   > > -6- Returns: `^^Z<[:Args:]...TARG-SPLICE(e0) ..., TARG-SPLICE(en-1)>`.

   > > -7- Throws: `meta::exception` unless `can_substitute(templ, arguments)` is `true`.

   > > -8- [Note: If forming `Z<[:Args:]...TARG-SPLICE(e0), ..., TARG-SPLICE(en-1)>` leads to a failure outside of the immediate context, the program is ill-formed. — end note]

[2025-10-27; Reflector comments.]

We lost definition of `Z`. Use `TARG-SPLICE([:Args:])...`.

[2025-11-03; Tomasz provides wording.]

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.15 [meta.reflection.substitute] as indicated:

   > > -1- Let `TARG-SPLICE(x)` be:

   > > - -1.1- `template [: x :]` if `is_template(x)` is `true`, otherwise
   > > - -1.2- `typename [: x :]` if `is_type(x)` is `true`, otherwise
   > > - -1.3- `([: x :])`

   >     template<reflection_range R = initializer_list<info>>
   >       consteval bool can_substitute(info templ, R&& arguments);

   > > -1- Let `Z` be the template represented by `templ` and let `Args...` be a sequence of prvalue constant expressions that compute the reflections held by the elements of `arguments`, in order.

   > > -2- Returns: `true` if `Z<TARG-SPLICE([:Args:])...>` is a valid template-id (13.3 [temp.names]) that does not name a function whose type contains an undeduced placeholder type. Otherwise, `false`.

   > > -3- Throws: `meta::exception` unless `templ` represents a template, and every reflection in `arguments` represents a construct usable as a template argument (13.4 [temp.arg]).

   > > -4- [Note: If forming `Z<TARG-SPLICE([:Args:])...>` leads to a failure outside of the immediate context, the program is ill-formed. — end note]

   >     template<reflection_range R = initializer_list<info>>
   >       consteval info substitute(info templ, R&& arguments);

   > > -5- Let `Z` be the template represented by `templ` and let `Args...` be a sequence of prvalue constant expressions that compute the reflections held by the elements of `arguments`, in order.

   > > -6- Returns: `Z<TARG-SPLICE([:Args:])...>`.

   > > -7- Throws: `meta::exception` unless `can_substitute(templ, arguments)` is `true`.

   > > -8- [Note: If forming `Z<TARG-SPLICE([:Args:])...>` leads to a failure outside of the immediate context, the program is ill-formed. — end note]

### 4317(i). The meaning of "resource" in the Cpp17Destructible requirements is undefined

Section: 16.4.4.2 [utility.arg.requirements] Status: C++26 Submitter: Jiang An Opened: 2025-08-15 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [utility.arg.requirements].

View all issues with C++26 status.

Discussion:

The meaning of "resource" in the Cpp17Destructible requirements cannot be inferred from the standard wording and it seems unlikely that the standard will determine its meaning in the future. What are considered as resources generally depends on users' intent, so the standard shouldn't determine the well-definedness of a program execution due to it. Moreover, the wording doesn't seem to consider shared ownership, which can be represented by `shared_ptr`.

[2025-10-14; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 16.4.4.2 [utility.arg.requirements], Table 35 [tab:cpp17.destructible] as indicated:

   > Table 35 — `Cpp17Destructible` requirements [tab:cpp17.destructible]
   > Expression | Post-condition
   > --- | ---
   > `u.~T()` | All resources owned by `u` are reclaimed, nNo exception is propagated.
   > [Note 3: Array types and non-object types are not Cpp17Destructible. — end note]

### 4318(i). Have `hive::erase_if` reevaluate `end()` to avoid UB

Section: 23.3.9.6 [hive.erasure] Status: C++26 Submitter: Frank Birbacher Opened: 2025-08-16 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [hive.erasure].

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Discussion:

Background: https://github.com/cplusplus/draft/pull/8162

For 23.3.9.6 [hive.erasure] p2, the defining code must not cache the end-iterator. In case the last element of the sequence is removed, the past-the-end iterator will be invalidated. This will trigger UB in the loop condition. Instead, re-evaluate `end()` each time.

[2025-08-29; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

> [Drafting note: There are other ways to fix this code while keeping the caching behaviour, but I don't see any particular reason to do so for the definition of the effects.]

1. Modify 23.3.9.6 [hive.erasure] as indicated:

   >     template<class T, class Allocator, class Predicate>
   >       typename hive<T, Allocator>::size_type
   >         erase_if(hive<T, Allocator>& c, Predicate pred);

   > > -2- Effects: Equivalent to:

   > > >     auto original_size = c.size();
   > > >     for (auto i = c.begin(), last = c.end(); i != lastc.end(); ) {
   > > >       if (pred(*i)) {
   > > >         i = c.erase(i);
   > > >       } else {
   > > >         ++i;
   > > >       }
   > > >     }
   > > >     return original_size - c.size();

### 4324(i). `unique_ptr<void>::operator*` is not SFINAE-friendly

Section: 20.3.1.3.5 [unique.ptr.single.observers] Status: C++26 Submitter: Hewill Kang Opened: 2025-08-24 Last modified: 2026-06-09

Priority: 3

View all other issues in [unique.ptr.single.observers].

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Discussion:

LWG 2762(i) added a conditional `noexcept` specification to `unique_ptr::operator*` to make it consistent with `shared_ptr::operator*`:

    constexpr add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));

This unexpectedly makes `unique_ptr<void>::operator*` no longer SFINAE-friendly, for example:

>     #include <memory>

>     template<class T> concept dereferenceable = requires(T& t) { *t; };

>     static_assert( dereferenceable<int *>);
>     static_assert(!dereferenceable<void*>);

>     static_assert( dereferenceable<std::shared_ptr<int >>);
>     static_assert(!dereferenceable<std::shared_ptr<void>>);

>     static_assert( dereferenceable<std::unique_ptr<int >>);
>     static_assert( dereferenceable<std::unique_ptr<void>>); // hard error

Given that the standard intends for `operator*` of `shared_ptr` and `unique_ptr` to be SFINAE-friendly based on 20.3.2.2.6 [util.smartptr.shared.obs], regardless of the value of `static_assert`, it is reasonable to assume that there should be no hard error here.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 20.3.1.3.5 [unique.ptr.single.observers] as indicated:

   >     constexpr add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>())see below);

   > > -1- Mandates: `reference_converts_from_temporary_v<add_lvalue_reference_t<T>, decltype(*declval<pointer>())>` is `false`.

   > > -2- Preconditions: `get() != nullptr` is `true`.

   > > -3- Returns: `*get()`.

   > > -?- Remarks:: The exception specification is equivalent to:

   > > >     !requires { *declval<pointer>(); } || requires { { *declval<pointer>() } noexcept; }

[2025-08-26; Reflector discussion]

During reflector triaging it had been pointed out that a better solution would be to constrain the `operator*` directly. The proposed wording has been updated to that effect.

[2025-10-22; Reflector poll.]

Set priority to 3 after reflector poll.

[2025-12-04; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 20.3.1.3.5 [unique.ptr.single.observers] as indicated:

   >     constexpr add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));

   > > -?- Constraints: `*declval<pointer>()` is a well-formed expression.

   > > -1- Mandates: `reference_converts_from_temporary_v<add_lvalue_reference_t<T>, decltype(*declval<pointer>())>` is `false`.

   > > -2- Preconditions: `get() != nullptr` is `true`.

   > > -3- Returns: `*get()`.

### 4325(i). `std::indirect`'s `operator==` still does not support incomplete types

Section: 20.4.1.8 [indirect.relops], 20.4.1.9 [indirect.comp.with.t] Status: C++26 Submitter: Hewill Kang Opened: 2025-08-24 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

`std::indirect`'s `operator== intentionally uses Mandates instead of Constraints to support incomplete types. However, its function signature has the following `noexcept` specification:

>     template<class U, class AA>
>       constexpr bool operator==(const indirect& lhs, const indirect<U, AA>& rhs)
>         noexcept(noexcept(*lhs == *rhs));

That is, we check whether the expression `*lhs == *rhs` throws, which unfortunately leads to the following hard error:

>     struct Incomplete;
>     static_assert(std::equality_comparable<std::indirect<Incomplete>>);
>     // hard error, no match for 'operator==' (operand types are 'const Incomplete' and 'const Incomplete')

This makes `operator==` not SFINAE-friendly for incomplete types, which defeats the purpose.

Also, checking `noexcept(*lhs == *rhs)` seems insufficient because the result of `*lhs == *rhs` might still throw during conversion to `bool`.

We could add a note similar to 22.10.6.1 [refwrap.general].

[2025-10-14; Reflector poll]

Set priority to 2 after reflector poll.

Even if we don't want to support incomplete types, we still need to fix the noexcept-specifier to consider whether the conversion to `bool` throws, e.g. `noexcept(noexcept(bool(*lhs == *rhs)))`.

"The proposed resolution may result in the noexcept operator giving different results when evaluated in different translation units where the type `T` of `indirect` was incomplete or not. Ill-formed seems safer than inconsistent."

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

> [Drafting note:: We introduce the exposition-only function `FUN` below to mimic the implicit conversion to `bool`. As a drive-by effect this helps us simplifying (and clarifying, see LWG 484(i)) the existing Mandates element.

> Please note that the seemingly unresolved `T` in the `requires` expression below names the first template parameter of the `indirect` class template. ]

1. Modify 20.4.1.8 [indirect.relops] as indicated:

   >     template<class U, class AA>
   >       constexpr bool operator==(const indirect& lhs, const indirect<U, AA>& rhs)
   >         noexcept(noexcept(*lhs == *rhs)see below);

   > > -?- Let `FUN` denote the exposition-only function

   > > >     bool FUN(bool) noexcept;

   > > -1- Mandates: The expression `FUN(*lhs == *rhs)` is well-formed and its result is convertible to `bool`.

   > > -2- Returns: If `lhs` is valueless or `rhs` is valueless, `lhs.valueless_after_move() == rhs.valueless_after_move()`; otherwise `*lhs == *rhs`.

   > > -?- Remarks: The exception specification is equivalent to:

   > > >     requires (const T& lhs, const U& rhs) { { FUN(lhs == rhs) } noexcept; }

2. Modify 20.4.1.9 [indirect.comp.with.t] as indicated:

   >     template<class U>
   >       constexpr bool operator==(const indirect& lhs, const U& rhs) noexcept(noexcept(*lhs == rhs)see below);

   > > -?- Let `FUN` denote the exposition-only function

   > > >     bool FUN(bool) noexcept;

   > > -1- Mandates: The expression `FUN(*lhs == rhs)` is well-formed and its result is convertible to `bool`.

   > > -2- Returns: If `lhs` is valueless, `false`; otherwise `*lhs == rhs`.

   > > -?- Remarks: The exception specification is equivalent to:

   > > >     requires (const T& lhs, const U& rhs) { { FUN(lhs == rhs) } noexcept; }

[2026-03-26; Tim provides new wording]

This only fixes the exception specification to include conversion to `bool`.

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 20.4.1.8 [indirect.relops] as indicated:

   >     template<class U, class AA>
   >       constexpr bool operator==(const indirect& lhs, const indirect<U, AA>& rhs)
   >         noexcept(noexcept(bool(*lhs == *rhs)));

   > > -1- […]

2. Modify 20.4.1.9 [indirect.comp.with.t] as indicated:

   >     template<class U>
   >       constexpr bool operator==(const indirect& lhs, const U& rhs) noexcept(noexcept(bool(*lhs == rhs)));

   > > -1- […]

### 4328(i). Remove note in §[exec.sched] regarding waiting for completion of scheduled operations

Section: 33.6 [exec.sched] Status: C++26 Submitter: Lewis Baker Opened: 2025-08-25 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [exec.sched].

View all issues with C++26 status.

Discussion:

The note at the end of 33.6 [exec.sched] says:

> [Note: The ability to wait for completion of submitted function objects can be provided by the associated execution resource of the scheduler — end note]

The suggestion that the execution resource should be used to join/wait on scheduled work is problematic in situations that may involve more than one execution context, as an execution resource having an empty queue of scheduled work does not necessarily imply that tasks currently running on another execution context will not later schedule additional work on this execution resource.

With the introduction of `counting_scope` with P3149 we now have a better recommended way of waiting for tasks that use a resource (including execution resources) to complete.

The note as it stands represents bad guidance and should either be removed or updated to refer to `counting_scope` and `simple_counting_scope` (33.14.2 [exec.counting.scopes]).

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

"What is this telling me as a user? That a custom execution resource can add a non-standard 'wait for completion' facility?"

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.6 [exec.sched] as indicated:

   > -7- A scheduler type's destructor shall not block pending completion of any receivers connected to the sender objects returned from schedule.

   > [Note 1: The ability to wait for completion of submitted function objects can be provided by the associated execution resource of the scheduler. — end note]

### 4329(i). Customising affine_on for other algorithms

Section: 99 [exec.affine.on] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-06-10

Priority: 3

View all other issues in [exec.affine.on].

View all issues with Resolved status.

Discussion:

Addresses US 232-366

There are no customisations of `affine_on` for other algorithms specified. For example, `affine_on(just(), sched)` can be equivalent to `just()` because `just()` completes inline and, thus, on the correct execution context.

[2025-10-23; Reflector poll. Status changed: New → LEWG]

Set priority to 3 after reflector poll. Send to LEWG.

"Becomes moot if we remove customization entirely."

[2026-06-07 Resolved by the adoption of P3941R4 at Croydon, 2026. Status changed: LEWG → Resolved.]

Proposed resolution:

### 4330(i). `affine_on` semantics

Section: 99 [exec.affine.on] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-04-01

Priority: 2

View all other issues in [exec.affine.on].

View all issues with Resolved status.

Discussion:

Addresses US 233-365

The specification of `affine_on` uses "current execution resource" and it is unclear what that means exactly. Additionally, it is unclear what the difference between `affine_on` and `continues_on` is. The intended difference for `affine_on` is to avoid unnecessary scheduling which `continues_on` is already allowed to do in some cases, too.

The intended semantics is that `affine_on` will either complete inline on whatever execution agent it was started on or it will complete asynchronously on the specified execution context. With this formulation `affine_on` may complete on one of two different execution context if it is started on an execution context that is different from the one specified by the scheduler.

[2025-10-23; Reflector poll. Status changed: New → LEWG]

Set priority to 2 after reflector poll. Send to LEWG.

[2026-04-01 Resolved by the adoption of P3941R4 at Croydon, 2026. Status changed: LEWG → Resolved.]

Proposed resolution:

### 4331(i). `affine_on` shape may be wrong

Section: 99 [exec.affine.on] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-04-01

Priority: 2

View all other issues in [exec.affine.on].

View all issues with Resolved status.

Discussion:

Addresses US 234-364

`affine_on` is specified to take a sender and a scheduler as arguments. The scheduler is meant to match the scheduler obtained from the `get_scheduler` query on the receiver's environment. Thus, the scheduler argument is redundant and the semantics become weird if these two schedulers don't match. The `affine_on` algorithm should only take the sender as argument.

[2025-10-17; Reflector poll. Status changed: New → LEWG.]

Set priority to 2 after reflector poll. Send to LEWG.

[2026-04-01 Resolved by the adoption of P3941R4 at Croydon, 2026. Status changed: LEWG → Resolved.]

Proposed resolution:

### 4332(i). `affine_on` shouldn't forward the stop token to the scheduling operation

Section: 99 [exec.affine.on] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-04-01

Priority: 2

View all other issues in [exec.affine.on].

View all issues with Resolved status.

Discussion:

Addresses US 235-363

The main purpose of `affine_on` is to make sure work resumes on a specific execution context. The scheduling operation may take some time and work may be cancelled in the meantime. If this cancellation causes the scheduling to be cancelled work cleaning up after the cancellation would be executed on the wrong execution context. Thus, the stop token from the receiver's environment should only be forwarded when `connect`ing the sender but not to the scheduling operation.

[2025-10-23; Reflector poll. Status changed: New → LEWG]

Set priority to 2 after reflector poll. Send to LEWG.

"This affects the cancelation behaviour so should also be seen by SG1."

[2026-04-01 Resolved by the adoption of P3941R4 at Croydon, 2026. Status changed: LEWG → Resolved.]

Proposed resolution:

### 4333(i). `task` uses unusual allocator customisation

Section: 33.13.6.5 [task.promise] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-04-01

Priority: 2

View all other issues in [task.promise].

View all issues with Resolved status.

Discussion:

Unlike `generator` the allocator customisation of `task` constraints the allocator type used for the coroutine to be convertible to the configured `allocator_type`. This prevents easy use of an allocator especially when no allocator is configured and the default (`std::allocator<std::byte>`) is used. The reason for this constraint is that the `get_allocator` is forwarded to `co_await`ed senders and is intended to be the same as the allocator used for the coroutine frame.

It may be reasonable to allow use of an arbitrary allocator when there is no explicit configuration of the `allocator_type`. In this case it may also be resonable to not support the `get_allocator` query when `co_await`ing senders.

[2025-10-17; Reflector poll. Status changed: New → LEWG.]

Set priority to 2 after reflector poll. Send to LEWG.

[2026-04-01 Resolved by the adoption of P3980R1 at Croydon, 2026. Status changed: LEWG → Resolved.]

Proposed resolution:

### 4334(i). `task<...>::promise_type` supports arbitrary `allocator_arg` position

Section: 33.13.6.5 [task.promise] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-04-01

Priority: 2

View all other issues in [task.promise].

View all issues with Resolved status.

Discussion:

Normally the `allocator_arg` argument has to be the first argument when present. For `task<...>::promise_type` the `allocator_arg` can appear at an arbitrary position (except the last because it always needs to be followed by the allocator). This permission is inconsistent and the position of the `allocator_arg` argument and the allocator should be limited to come first.

For containers the optional support for allocators is implemented once for every container. For coroutines the optional support for allocators is implemented once for every coroutine definition. To support an optional allocator the coroutine definition needs to use an allocator and either gets duplicated not using an allocator or a forwarding function is added which adds the default allocator. With the flexible allocator position optional allocator support can be provided using a trailing argument list, i.e., adding `, auto&&...`. Instead of constraining `task` it may be more reasonable to add the flexibility to `generator`.

[2025-10-17; Reflector poll. Status changed: New → LEWG.]

Set priority to 2 after reflector poll.

[2026-04-01 Resolved by the adoption of P3980R1 at Croydon, 2026. Status changed: LEWG → Resolved.]

Proposed resolution:

### 4335(i). `task` shadows the environment's allocator

Section: 33.13.6.5 [task.promise] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-04-01

Priority: 2

View all other issues in [task.promise].

View all issues with Resolved status.

Discussion:

Addresses US 255-384

Normally, the `get_allocator` query forwards the allocator from the receiver's environment. For `task` the `get_allocator` query used for `co_await`ed senders uses the allocator passed when creating the coroutine or the default if there was none. It should use the receiver's environment, at least, if the receiver's environment supports a `get_allocator` query.

Supporting the receiver's allocator isn't always possible: the used allocator type needs to be known when the coroutine is created. At that time the receiver isn't known, yet. As a result the receiver's environment may provide an allocator which is incompatible with the allocator type used by the coroutine. It may be possible to use the receiver's allcoator if it is convertible to the allocator type used by the coroutine and to produce a compile-time error otherwise.

[2025-10-17; Reflector poll. Status changed: New → LEWG.]

Set priority to 2 after reflector poll. Send to LEWG.

[2026-04-01 Resolved by the adoption of P3980R1 at Croydon, 2026. Status changed: LEWG → Resolved.]

Proposed resolution:

### 4336(i). `bulk` vs. `task_scheduler`

Section: 33.13.5 [exec.task.scheduler] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-04-01

Priority: 2

View all other issues in [exec.task.scheduler].

View all issues with Resolved status.

Discussion:

Normally, the scheduler type used by an operation can be deduced when a sender is `connect`ed to a receiver from the receiver's environment. The body of a coroutine cannot know about the receiver the `task` sender gets `connect`ed to. The implication is that the type of the scheduler used by the coroutine needs to be known when the `task` is created. To still allow custom schedulers used when connecting, the type-erased scheduler `task_scheduler` is used. However, that leads to surprises when algorithms are customised for a scheduler as is, e.g., the case for `bulk` when used with a `parallel_scheduler`: if `bulk` is `co_awaited` within a coroutine using `task_scheduler` it will use the default implementation of `bulk` which sequentially executes the work, even if the `task_scheduler` was initialised with a `parallel_scheduler` (the exact invocation may actually be slightly different or need to use `bulk_chunked` or `bulk_unchunked` but that isn't the point being made):

    struct env {
        auto query(ex::get_scheduler_t) const noexcept { return ex::parallel_scheduler(); }
    };
    struct work {
        auto operator()(std::size_t s){ /*...*/ };
    };

    ex::sync_wait(
        ex::write_env(ex::bulk(ex::just(), 16u, work{}),
        env{}
    ));
    ex::sync_wait(ex::write_env(
        []()->ex::task<void, ex::env<>>>{ co_await ex::bulk(ex::just(), 16u, work{}); }(),
        env{}
    ));

The two invocations should probably both execute the work in parallel but the coroutine version doesnt: it uses the `task_scheduler` which doesnt have a specialised version of `bulk` to potentially delegate in a type-erased form to the underlying scheduler. It is straight forward to move the `write_env` wrapper inside the coroutine which fixes the problem in this case but this need introduces the potential for a subtle performance bug. The problem is sadly not limited to a particular scheduler or a particular algorithm: any scheduler/algorithm combination which may get specialised can suffer from the specialised algorithm not being picked up.

There are a few ways this problem can be addressed (this list of options is almost certainly incomplete):

- Accept the situation as is and advise users to be careful about customised algorithms like bulk when using `task_scheduler`.
- Extend the interface of `task_scheduler` to deal with a set of algorithms for which it provides a type-erased interface. The interface would likely be more constrained and it would use virtual dispatch at run-time. However, the set of covered algorithms would necessarily be limited in some form.
- To avoid the trap, make the use of known algorithms incompatible with the use of `task_scheduler`, i.e., customise these algorithms for `task_scheduler` such that a compile-time error is produced.

A user who knows that the main purpose of a coroutine is to executed an algorithm customised for a certain scheduler can use `task<T, E>` with an environment `E` specifying exactly that scheduler type. However, this use may be nested within some sender being `co_awaited` and users need to be aware that the customisation wouldnt be picked up. Any approach I'm currently aware of will have the problem that customised versions of an algorithm are not used for algorithms we are currently unaware of.

[2025-10-23; Reflector poll. Status changed: New → LEWG]

Set priority to 2 after reflector poll. Send to LEWG.

"It seems like there are several kinds of problems with sender algorithm customization that have related causes and might have a single solution."

[2026-04-01 Resolved by the adoption of P3927R2 at Croydon, 2026. Status changed: LEWG → Resolved.]

Proposed resolution:

### 4339(i). `task`'s coroutine frame may be released late

Section: 33.13.6 [exec.task] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

Addresses US 242-372

The specification of `task` doesn't spell out when the coroutine frame is destroyed (i.e., when `handle.destroy()` is called). As a result the lifetime of arguments passed to the coroutine and/or of local variables in the coroutine body may be longer than expected.

The intention is that the coroutine frame is destroyed before any of the completion functions is called. One implication of this requirement is that the result and error objects can't be stored in the `promise_type` when the completion function is called although the exposition-only members `result` and `errors` imply exactly that. Instead the data needs to be stored in the operation state object or it needs to be moved to a different place before calling `destroy()`.

The proposed resolution is to add a paragraph to the specification of `promise_type` in 33.13.6.5 [task.promise] that spells out that the coroutine frame is destroyed before any of the completion functions is called. To actually do that the exposition-only members `result` and `errors` can't remain as members of the `promise_type`. While writing the relevant change it turned out that `errors` is a `variant` which only ever stores an `exception_ptr` (the other potential errors are immediately reported via the awaiter return from `yield_value`). Thus, the `variant` can be replaced with an `exception_ptr`.

[2025-10-17; Reflector poll.]

Set priority to 2 after reflector poll.

"P/R is incomplete - also needs to update at least `promise_type::uncaught_exception()`."

[Kona 2025-11-06]

SG1 think resolution looks good, but noted pre-existing issue that 7.3 doesn't use `std::move` on `*st.result`. (Would need to be `*std::move(st.result)` to handle `optional<T&>` correctly.)

LWG 4415(i) renames `uncaught_exception` to `unhandled_exception`.

Previous resolution [SUPERSEDED]

In 33.13.6.4 [task.state], add exposition-only data members `result` and `error` to the exposition-only class `state`:

>     namespace std::execution {
>       template<class T, class Environment>
>       template<receiver Rcvr>
>       class task<T, Environment>::state {           // exposition only
>       ...
>       Environment               environment;   // exposition only
>       optional<T>               result;        // exposition only; present only if is_void_v<T> is false
>       exception_ptr             error;         // exposition-only
>     };
>     }

Remove the exposition-only data members `result` and `errors` from the class `promise_type` in 33.13.6.5 [task.promise]:

>     namespace std::execution {
>       template<class T, class Environment>
>       class task<T, Environment>::promise_type {
>       ...
>       stop_token_type   token;  // exposition only
>       optional<T>       result; // exposition only; present only if is_void_v<T> is false
>       error-variant     errors; // exposition only
>     };
>     }

The definition of `error-variant` isn't needed, i.e., remove 33.13.6.5 [task.promise] paragraph 2:

> -2- `error-variant` is a `variant<monostate, remove_cvref_t<E>...>`, with duplicate types removed, where `E...` are template arguments of the specialization of `execution::completion_signatures` denoted by `error_types`.

In 33.13.6.5 [task.promise] change paragraph 7 to use the members added to `state`:

>     auto final_suspend() noexcept

> -7- Returns: An awaitable object of unspecified type (7.6.2.4 [expr.await]) whose member functions arrange for the completion of the asynchronous operation associated with `STATE(*this)` by invoking. Let `st` be a reference to `STATE(*this)`. The asynchronous completion first destroys the coroutine frame using `st.handle.destroy()` and then invokes:

> - -7.1- `set_error(std::move(RCVR(*this)st.rcvr), std::move(est.error))` if `errors.index()` is greater than zero and `e` is the value held by `errors` `bool(st.error)` is `true`, otherwise
> - -7.2- `set_value(std::move(RCVR(*this)st.rcvr))` if `is_void<T>` is `true`, and otherwise
> - -7.3- `set_value(std::move(RCVR(*this)st.rcvr), *st.result)`.

Change the specification of `yield_value` to destroy the coroutine frame before invoking the `set_error` completion, i.e., change 33.13.6.5 [task.promise] paragraph 9:

> -9- Returns: An awaitable object of unspecified type ([expr.await]) whose member functions arrange for the calling coroutine to be suspended and then completes the asynchronous operation associated with `STATE(*this)` by. Let `st` be a reference to `STATE(*this)`. Then the asynchronous operation completes by first destroying the coroutine frame using `st.handle.destroy()` and then invoking `set_error(std::move(RCVR(*this)st.rcvr), Cerr(std::move(err.error)))`.

Change the specification of `unhandled_stopped` to destroy the coroutine frame before invoking the `set_stopped` completion, i.e., change 33.13.6.5 [task.promise] paragraph 13:

>     coroutine_handle<> unhandled_stopped();

> -13- Effects: Completes the asynchronous operation associated with `STATE(*this)` by. Let `st` be a reference to `STATE(*this)`. The asynchronous operation is completed by first destroying the coroutine frame using `st.handle.destroy()` and then invoking `set_stopped(std::move(RCVR(*this)st.rcvr))`.

[2025-11-07; Jonathan provides improved wording]

Incorporate change from LWG 4466(i) and add `_v` to `is_void`.

LWG moved this to Ready but then reopened it because it still needs an update w.r.t `unhandled_exception`.

[2026-01-16; LWG telecon. Dietmar will add a paragraph changing how `unhandled_exception` reports errors.]

Previous resolution [SUPERSEDED]

In 33.13.6.4 [task.state], add exposition-only data members `result` and `error` to the exposition-only class `state`:

>     namespace std::execution {
>       template<class T, class Environment>
>       template<receiver Rcvr>
>       class task<T, Environment>::state {           // exposition only
>       ...
>       Environment               environment;   // exposition only
>       optional<T>               result;        // exposition only; present only if is_void_v<T> is false
>       exception_ptr             error;         // exposition-only
>     };
>     }

Remove the exposition-only data members `result` and `errors` from the class `promise_type` in 33.13.6.5 [task.promise]:

>     namespace std::execution {
>       template<class T, class Environment>
>       class task<T, Environment>::promise_type {
>       ...
>       stop_token_type   token;  // exposition only
>       optional<T>       result; // exposition only; present only if is_void_v<T> is false
>       error-variant     errors; // exposition only
>     };
>     }

The definition of `error-variant` isn't needed, i.e., remove 33.13.6.5 [task.promise] paragraph 2:

> -2- `error-variant` is a `variant<monostate, remove_cvref_t<E>...>`, with duplicate types removed, where `E...` are template arguments of the specialization of `execution::completion_signatures` denoted by `error_types`.

In 33.13.6.5 [task.promise] change paragraph 7 to use the members added to `state`:

>     auto final_suspend() noexcept

> -7- Returns: An awaitable object of unspecified type (7.6.2.4 [expr.await]) whose member functions arrange for the completion of the asynchronous operation associated with `STATE(*this)` by invoking. Let `st` be a reference to `STATE(*this)`. The asynchronous completion first destroys the coroutine frame using `st.handle.destroy()` and then invokes:

> - -7.1- `set_error(std::move(RCVR(*this)st.rcvr), std::move(est.error))` if `errors.index()` is greater than zero and `e` is the value held by `errors` `bool(st.error)` is `true`, otherwise
> - -7.2- `set_value(std::move(RCVR(*this)st.rcvr))` if `is_void_v<T>` is `true`, and otherwise
> - -7.3- `set_value(std::move(RCVR(*this)st.rcvr), *std::move(st.result))`.

Change the specification of `yield_value` to destroy the coroutine frame before invoking the `set_error` completion, i.e., change 33.13.6.5 [task.promise] paragraph 9:

> -9- Returns: An awaitable object of unspecified type ([expr.await]) whose member functions arrange for the calling coroutine to be suspended and then completes the asynchronous operation associated with `STATE(*this)` by. Let `st` be a reference to `STATE(*this)`. Then the asynchronous operation completes by first destroying the coroutine frame using `st.handle.destroy()` and then invoking `set_error(std::move(RCVR(*this)st.rcvr), Cerr(std::move(err.error)))`.

Change the specification of `unhandled_stopped` to destroy the coroutine frame before invoking the `set_stopped` completion, i.e., change 33.13.6.5 [task.promise] paragraph 13:

>     coroutine_handle<> unhandled_stopped();

> -13- Effects: Completes the asynchronous operation associated with `STATE(*this)` by. Let `st` be a reference to `STATE(*this)`. The asynchronous operation is completed by first destroying the coroutine frame using `st.handle.destroy()` and then invoking `set_stopped(std::move(RCVR(*this)st.rcvr))`.

[2026-01-22; Dietmar Kühl provides improved wording]

[Croydon 2026-03-27; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. In 33.13.6.4 [task.state], add exposition-only data members `result` and `error` to the exposition-only class `state`:

   >     namespace std::execution {
   >       template<class T, class Environment>
   >       template<receiver Rcvr>
   >       class task<T, Environment>::state {           // exposition only
   >       ...
   >       Environment               environment;   // exposition only
   >       optional<T>               result;        // exposition only; present only if is_void_v<T> is false
   >       exception_ptr             error;         // exposition-only
   >     };
   >     }

2. Remove the exposition-only data members `result` and `errors` from the class `promise_type` in 33.13.6.5 [task.promise]:

   >     namespace std::execution {
   >       template<class T, class Environment>
   >       class task<T, Environment>::promise_type {
   >       ...
   >       stop_token_type   token;  // exposition only
   >       optional<T>       result; // exposition only; present only if is_void_v<T> is false
   >       error-variant     errors; // exposition only
   >     };
   >     }

3. The definition of `error-variant` isn't needed, i.e., remove 33.13.6.5 [task.promise] paragraph 2:

   > -2- `error-variant` is a `variant<monostate, remove_cvref_t<E>...>`, with duplicate types removed, where `E...` are template arguments of the specialization of `execution::completion_signatures` denoted by `error_types`.

4. In 33.13.6.5 [task.promise] change paragraph 6 to use the members added to `state`:

   >     auto final_suspend() noexcept

   > > -6- Returns: An awaitable object of unspecified type (7.6.2.4 [expr.await]) whose member functions arrange for the completion of the asynchronous operation associated with `STATE(*this)` by invoking. Let `st` be a reference to `STATE(*this)`. The asynchronous completion first destroys the coroutine frame using `st.handle.destroy()` and then invokes:

   > > - (6.1) — `set_error(std::move(RCVR(*this)st.rcvr), std::move(est.error))` if `errors.index()` is greater than zero and `e` is the value held by `errors` `bool(st.error)` is `true`, otherwise
   > > - (6.2) — `set_value(std::move(RCVR(*this)st.rcvr))` if `is_void_v<T>` is `true`, and otherwise
   > > - (6.3) — `set_value(std::move(RCVR(*this)st.rcvr), *std::move(st.result))`.

5. In 33.13.6.5 [task.promise] change the specification of `yield_value` to destroy the coroutine frame before invoking the `set_error` completion:

   > -8- Returns: An awaitable object of unspecified type (7.6.2.4 [expr.await]) whose member functions arrange for the calling coroutine to be suspended and then completes the asynchronous operation associated with `STATE(*this)` by. Let `st` be a reference to `STATE(*this)`. Then the asynchronous operation completes by first destroying the coroutine frame using `st.handle.destroy()` and then invoking `set_error(std::move(RCVR(*this)st.rcvr), Cerr(std::move(err.error)))`.

6. In 33.13.6.5 [task.promise] change the specification of `unhandled_exception` to use the `error` member of `STATE(*this)`:

   >     void unhandled_exception();

   > > -11- Effects: If the signature `set_error_t(exception_ptr)` is not an element of `error_types`, calls `terminate()` (14.6.2 [except.terminate]). Otherwise, stores `current_exception()` into `STATE(*this).errorerrors`.

7. In 33.13.6.5 [task.promise] change the specification of `unhandled_stopped` to destroy the coroutine frame before invoking the `set_stopped` completion:

   >     coroutine_handle<> unhandled_stopped();

   > > -12- Effects: Completes the asynchronous operation associated with `STATE(*this)` by. Let `st` be a reference to `STATE(*this)`. The asynchronous operation is completed by first destroying the coroutine frame using `st.handle.destroy()` and then invoking `set_stopped(std::move(RCVR(*this)st.rcvr))`.

### 4340(i). `task::promise_type::unhandled_stopped()` should be `noexcept`

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 252-387

The function `task::promise_type::unhandled_stopped()` is called from `set_stopped()` of a receiver and calls `set_stopped` itself. These functions are required to be `noexcept`. Thus, `unhandled_stopped()` can't throw an exception and should be marked `noexcept`. All other declarations of `unhandled_stopped()` are already marked `noexcept` but `task::promise_type::unhandled_stopped()` isn't.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

In the synopsis in 33.13.6.5 [task.promise] add `noexcept` to the declaration of `task::promise_type::unhandled_stopped()`:

>     namespace std::execution {
>       template<class T, class Environment>
>       class task<T, Environment>::promise_type {
>          ...
>         coroutine_handle<> unhandled_stopped() noexcept;
>         ...
>       };
>     }

In the specification in 33.13.6.5 [task.promise] paragraph 13 add `noexcept`:

>     coroutine_handle<> unhandled_stopped() noexcept;

> -13- Effects: Completes the asynchronous operation associated with `STATE(*this)` by invoking `set_stopped(std::move(RCVR(*this)))`.

### 4341(i). Missing rvalue reference qualification for `task::connect()`

Section: 33.13.6.2 [task.class] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 244-375

Coroutines can't be copied. Thus, a `task` can be `connect()` just once. To represent that `task::connect()` should be rvalue reference qualified but currently it isn't.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

"It's nice to rvalue qualify such a function, but it is not strictly necessary."

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

In the synopsis in 33.13.6.2 [task.class] add rvalue reference qualification to `task::connect()`:

>     namespace std::execution {
>       template<class T, class Environment>
>       class task {
>         ...
>         template<receiver Rcvr>
>             state<Rcvr> connect(Rcvr&& rcvr) &&;
>         ...
>       }
>     }

In the specification in 33.13.6.3 [task.members] paragraph 3 add rvalue reference qualification to `task::connect()`:

>     template<receiver Rcvr>
>         state<Rcvr> connect(Rcvr&& rcvr) &&;

> -3- Precondition: `bool(handle)` is `true`.

> -4- Effects: Equivalent to:

>         return state<Rcvr>(exchange(handle, {}), std::forward<Rcvr>(recv));

### 4342(i). Missing rvalue reference qualification for `task_scheduler::ts-sender::connect()`

Section: 33.13.5 [exec.task.scheduler] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 237-369

The result of `schedule(sched)` for a scheduler `sched` is only required to be movable. An object of this type may need to be forwarded to an operation state constructor by `task_scheduler::ts-sender::connect`. Thus, this function should be qualified with an rvalue reference.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

Add an rvalue qualifier to the declaration of `connect` in 33.13.5 [exec.task.scheduler] paragraph 8:

>     namespace std::execution {
>       class task_scheduler::ts-sender {     // exposition only
>       public:
>         using sender_concept = sender_t;

>         template<receiver Rcvr>
>           state<Rcvr> connect(Rcvr&& rcvr) &&;
>       };
>     }

In the specification in 33.13.5 [exec.task.scheduler] paragraph 10 add an rvalue qualifier to `connect`:

>     template<receiver Rcvr>
>       state<Rcvr> connect(Rcvr&& rcvr) &&;

> -10- Effects: Let r be an object of a type that models receiver and whose completion handlers result in invoking the corresponding completion handlers of `rcvr` or copy thereof. Returns an object of type `state<Rcvr>` containing an operation state object initialized with `connect(SENDER(*this), std::move(r))`.

### 4343(i). Missing default template arguments for `task`

Section: 33.13.6.2 [task.class] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 243-376

The design discussion of `task` describes defaults for the two template parameters `T` and `Environment` of `task` but these defaults are not reflected in the synopsis of 33.13.6.2 [task.class]. This is an oversight and should be fixed. The default for `T` should be `void` and the default for `Environment` should be `env<>` (the design paper used `empty_env` but this `struct` was replaced by the class template `env` by P3325R5).

There could be a counter argument to defining a default for the `Environment` template parameter: this type is used to determine various customizations of `task`, e.g., the `allocator_type`, the `scheduler_type`, and the `stop_source_type`. Leaving the type a required argument means that a future standard could choose a possibly better default than the types determined when the `Environment` doesn't define them. On the other hand, a future standard could provide a suitable alias with modified types under a different name and/or a different namespace. Based on the LEWG discussion on 2025-08-26 the direction is to add the default arguments.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

Add default template arguments for `task` for `T = void` and `Environment = env<>` in the synopsis of 33.13.6.2 [task.class]:

>     namespace std::execution {
>       template<class T = void, class Environment = env<>>
>       class task {
>          ...
>       };
>     }

### 4344(i). `affine_on` has no specification for the default

Section: 99 [exec.affine.on] Status: Resolved Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2026-04-01

Priority: 2

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Discussion:

Addresses US 236-362

The wording of `affine_on` doesnt have a specification for the default implementation. For other algorithms the default implementation is specified.

The intention for `affine_on` was to all optimisation/customisation in a way reducing the necessary scheduling: if the implementation can determine if a sender completed already on the correct execution agent it should be allowed to avoid scheduling. A specification should provide enough lattitude to allow that.

[2025-10-17; Reflector poll.]

Set priority to 2 after reflector poll.

[2026-04-01 Resolved by the adoption of P3941R4 at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

Add a new paragraph to the specification of `affine_on` in 99 [exec.affine.on] providing a specification for the default implementation:

> -6- Let `sndr` and `env` be subexpressions such that `Sndr` is `decltype((sndr))`. If `sender-for<Sndr, affine_on_t>` is `false`, then the expression `affine_on.transform_sender(sndr, env)` is ill-formed; otherwise, it is equivalent to:

>     auto [_, sch, child] = sndr;
>     return transform_sender(
>       query-with-default(get_domain, sch, default_domain()),
>       continues_on(std::move(child), std::move(sch)));

> except that `sch` is only evaluated once.

### 4345(i). `task::promise_type::return_value` default template parameter

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 251-388

The template parameter `V` of `task::promise_type::return_value` doesn't have a default template argument specified. Specifying a default template argument of `T` would enable use of `co_return { ... }` which would be consistent with normal `return` statements. This feature was not discussed in the design paper but based on the LEWG discussion on 2025-08-26 it is considered to be more a bug fix than a new feature.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

Add a default template argument of `T` to the template parameter `V` of `task::promise_type::return_value` in the synopsis of 33.13.6.5 [task.promise]:

>     namespace std::execution {
>       template<class T, class Environment>
>       class task<T, Environment>::promise_type {
>          ...
>         template<typename V = T>
>         void return_value(V&& value);
>         ...
>       };

>     }

### 4346(i). `task::promise_type::return_void`/`value` lack a specification

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 250-389

The synopsis for `std::execution::task<T, E>::promise_type` declares `return_void()` or `return_value(V&&)`. However, there is no specification of what these functions actually do. `return_void()` doesn’t need to do anything at all. `return_value(V&& v)` needs to store `v` into the `result`.

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

Insert the following paragraphs in 33.13.6.5 [task.promise] after the specification of `unhandled_stopped`:

>     coroutine_handle<> unhandled_stopped();

> -13- Effects: Completes the asynchronous operation associated with `STATE(*this)` by invoking `set_stopped(std::move(RCVR(*this)))`.

> -14- Returns: `noop_coroutine()`.

>     void return_void();

> -?- Effects: does nothing.

>     template<class V>
>       void return_value(V&& v);

> -?- Effects: Equivalent to `result.emplace(std::forward<V>(v))`.

>     unspecified get_env() const noexcept;

> -15- Returns: An object `env` such that queries are forwarded as follows:

### 4347(i). `task`'s stop source is always created

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2026-06-09

Priority: 2

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Discussion:

Addresses US 257-382

The type `task<...>::promise_type` has exposition-only members `source` and `token`. These can be interpreted as always existing which would be a performance issue for former and an unnecessary constraints for the latter (because stop tokens aren't required to be default constructible).

The intent is that the stop token obtained from the `get_stop_token` query of the receiver's environment is used. Only if this type is different from the `task`'s `stop_token_type` a stop source of type `stop_source_type` needs to be created when the `get_stop_token` query is used on the promise type's environment. The stop token doesn't need to be stored at all: it can either be obtained from the receiver's environment or from the stop source. The fix is to show the stop source as an optionally present member of of the operation state and it should be of type `std::optional<stop_source_type>` to imply that it is only created when accessed.

[2025-10-17; Reflector poll.]

Set priority to 2 after reflector poll.

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Change the synopsis in 33.13.6.4 [task.state] to include an `optional<stop_source_type>` member and an exposition-only function to get a stop token:

   >     namespace std::execution {
   >       template<class T, class Environment>
   >       template<receiver Rcvr>
   >       class task<T, Environment>::state {           // exposition only
   >       public:
   >         using operation_state_concept = operation_state_t;

   >         template<class R>
   >           state(coroutine_handle<promise_type> h, R&& rr);

   >         ~state();

   >         void start() & noexcept;

   >         stop_token_type get-stop-token(); // exposition only

   >       private:
   >         using own-env-t = see below;                // exposition only
   >         coroutine_handle<promise_type> handle;      // exposition only
   >         remove_cvref_t<Rcvr>           rcvr;        // exposition only
   >         optional<stop_source_type>     source;      // exposition only
   >         own-env-t                      own-env;     // exposition only
   >         Environment                    environment; // exposition only
   >       };
   >     }

2. In 33.13.6.4 [task.state] change paragraph 4 into two paragraphs: one for `start()` and one for `get-stop-token()` such that the initialization of `source` is in the second paragraph:

   >     void start() & noexcept;

   > -4- Effects: Let `prom` be the object `handle.promise()`. Associates `STATE(prom)`, `RCVR(prom)`, and `SCHED(prom)` with *this as follows:

   > -4.1- -- `STATE(prom)` is `*this`.

   > -4.2- -- `RCVR(prom)` is `rcvr`.

   > -4.3- -- `SCHED(prom)` is the object initialized with `scheduler_type(get_scheduler(get_env(rcvr)))` if that expression is valid and `scheduler_type()` otherwise. If neither of these expressions is valid, the program is ill-formed.

   > Let `st` be `get_stop_token(get_env(rcvr))`. Initializes `prom.token` and `prom.source` such that

   > -4.4- -- `prom.token.stop_requested()` returns `st.stop_requested()`;

   > -4.5- -- `prom.token.stop_possible()` returns `st.stop_possible()`; and

   > -4.6- -- for types `Fn` and `Init` such that both `invocable<Fn>` and `constructible_from<Fn, Init> are modeled,` stop_token_type::callback_type<Fn> models `stoppable-callback-for<Fn, stop_token_type, Init>`.

   > -5- After that invokes `handle.resume()`.

   >     stop_token_type get-stop-token();

   > -6- Effects: If `same_as<decltype(declval<stop_source_type>().get_token()), decltype(get_stop_token(get_env(rcvr)))>` is `true` returns `get_stop_token(get_env(rcvr))`. Otherwise, if `source.has_value()` is `false`, initializes the contained value of `source` such that

   > - -6.1- -- `source->stop_requested()` returns `get_stop_token(get_env(rcvr))->stop_requested()`; and
   > - -6.2- -- `source->stop_possible()` returns `get_stop_token(get_env(rcvr))->stop_possible()`

   > Finally, returns `source->get_token()`.

3. Remove `token` and `source` from the synopsis of `promise_type` in 33.13.6.5 [task.promise]:

   >     namespace std::execution {
   >       template<class T, class Environment>
   >       class task<T, Environment>::promise_type {
   >       public:
   >         ...

   >       private:
   >         using error-variant = see below;    // exposition only

   >         allocator_type    alloc;            // exposition only
   >         stop_source_type  source;           // exposition only
   >         stop_token_type   token;            // exposition only
   >         optional<T>       result;           // exposition only; present only if is_void_v<T> is false
   >         error-variant     errors;           // exposition only
   >       };
   >     }

4. Change the specification of `get_env` in 33.13.6.5 [task.promise] paragraph 16.3 to use `STATE(*this).get-stop-token()`:

   >     unspecified get_env() const noexcept;

   > -16- Returns: An object `env` such that queries are forwarded as follows:

   > -16.1- -- `env.query(get_scheduler)` returns `scheduler_type(SCHED(*this))`.

   > -16.2- -- `env.query(get_allocator)` returns `alloc`.

   > -16.3- -- `env.query(get_stop_token)` returns `token` `STATE(*this).get-stop-token()`.

   > -16.4- -- For any other query `q` and arguments `a...` a call to `env.query(q, a...)` returns `STATE(*this).environmentenvironment.query(q, a...)` if this expression is well-formed and `forwarding_query(q)` is well-formed and is `true`. Otherwise `env.query(q, a...)` is ill-formed.

### 4349(i). `task` is not actually started lazily

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 258-381

The wording for `task<...>::promise_type::initial_suspend` in 33.13.6.5 [task.promise] paragraph 6 (second bullet) may imply that a task is eagerly started, i.e., that the awaiter return from `initial_suspend()` immediately starts the scheduling operation and cause the `task` to be resumed. At the very least the second bullet of the wording should be clarified such that the scheduling operation is only started when the coroutine gets resumed.

An alternative resolution it have `initial_suspend()` return `std::suspend_always` implicitly requiring that the `task` gets `start()`ed from the correct execution context. This approach has the advantage of avoiding unnecessary scheduling operations for the likely common case when `task`s are started from the correct context.

[2025-10-18; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

Change the declaration of `initial_suspend()` in the synopsis of 33.13.6.5 [task.promise] to use `suspend_always`, directly provide a definition, and add various qualifiers:

>     namespace std::execution {
>       template<class T, class Environment>
>       class task<T, Environment>::promise_type {
>         ...
>         autostatic constexpr suspend_always initial_suspend() noexcept;{ return {}; }
>         ...
>       };

>     }

Remove 33.13.6.5 [task.promise] paragraph 6 entirely:

>     auto initial_suspend() noexcept;

> -6- Returns: An awaitable object of unspecified type ([expr.await]) whose member functions arrange for

> -6.1- - the calling coroutine to be suspended,

> -6.2- - the coroutine to be resumed on an execution agent of the execution resource associated with `SCHED(*this)`.

### 4351(i). `integral-constant-like` needs more `remove_cvref_t`

Section: 23.7.2.1 [span.syn] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-09-05 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

P2781R9 tweaked the definition of `integral-constant-like` to work with `constant_wrapper`, like so:

>     template<class T>
>         concept integral-constant-like =                    // exposition only
>           is_integral_v<remove_cvref_t<decltype(T::value)>> &&
>           !is_same_v<bool, remove_const_t<decltype(T::value)>> &&
>           convertible_to<T, decltype(T::value)> &&
>           equality_comparable_with<T, decltype(T::value)> &&
>           bool_constant<T() == T::value>::value &&
>           bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;

This was done so that `cw<5>` models the concept, but it needs an additional tweak so that `cw<true>` does not model it.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.7.2.1 [span.syn] as indicated:

   >     template<class T>
   >         concept integral-constant-like =                    // exposition only
   >           is_integral_v<remove_cvref_t<decltype(T::value)>> &&
   >           !is_same_v<bool, remove_cvrefconst_t<decltype(T::value)>> &&
   >           convertible_to<T, decltype(T::value)> &&
   >           equality_comparable_with<T, decltype(T::value)> &&
   >           bool_constant<T() == T::value>::value &&
   >           bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;

### 4354(i). Reconsider `weakly_parallel` as the default `forward_progress_guarantee`

Section: 33.5.10 [exec.get.fwd.progress] Status: C++26 Submitter: Lewis Baker Opened: 2025-08-25 Last modified: 2026-06-09

Priority: 1

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Discussion:

The `get_forward_progress_guarantee` CPO is intended to allow querying a scheduler as for what sort of forward-progress guarantee it provides. Algorithms may use this to determine whether it is safe to execute certain operations on a given scheduler. If a scheduler does not customize this query, the query will fall back to returning a forward-progress guarantee of weakly_parallel.

I think we should reconsider what this default should be returning and suggest it should instead return `parallel` by default, as this will be by far the most common kind of scheduler, i.e. a scheduler that executes on `std::thread`-like execution agents and that maintains a queue of scheduled tasks.

I expect it to be common that authors of schedulers may forget to customize the `get_forward_progress_guarantee_t` query and just leave it at the default. This will likely leave their scheduler reporting a weaker guarantee than it actually provides and thus not being usable within generic algorithms that require at least parallel forward progress.

For example, the `run_loop` execution context defined in 33.12.1 [exec.run.loop] does not define its scheduler to customize the `get_forward_progress_guarantee_t`. This means it will report the default value of `weakly_parallel`.

However, the scheduled operations will run on the thread that calls `run_loop::run()` and thus will inherit its forward-progress guarantees. As this function might block and is therefore unsafe to invoke it from a thread/agent with `weakly_parallel` forward progress guarantees (which should probably be explicitly specified as having undefined-behaviour) we can safely assume that `run_loop`'s scheduler can provide parallel forward-progress guarantee.

It's not clear whether the current `run_loop` specification defaulting to its scheduler having `weakly_parallel` forward progress guarantee is intentional or unintentional here. However, forgetting to define the `get_forward_progress_guarantee` query on a scheduler is something I expect to be fairly common.

Schedulers that provide `weakly_parallel` (or in future, `concurrent`) forward progress guarantees require implementations to be much more aware of the fact that these are the guarantees they are providing and thus could be more expected to customize the `get_forward_progress_guarantee` query to return the respective values.

[2025-10-20; Reflector poll. Status changed: New → SG1]

Set priority to 1 after reflector poll. Send to SG1 and LEWG.

"If there is a default, it should be the weakest possible one. If that is an unfortunate choice I’d rather prefer no default and mandate that the query gets implemented. Providing a default which is stronger than the weakest possible creates logic errors. Accidentally claiming weaker than the actual value is only a performance error."

"This is tension between the default being promising the least and the default being the most likely thing a user wants to do. Assuming the least powerful guarantees unless the user has opted in is safer. Changing this choice requires going back to LEWG or SG1."

"Plenty of reasonable schedulers are weakly parallel at best. It's the right default. If your scheduler offers better than that, you would naturally remember to customize it."

"It seems that the authors of `run_loop::scheduler` did not naturally remember to customize it. It's possible the intent was that `run_loop::scheduler` should not offer better than `weakly_parallel` forward progress, but it was not discussed in P2300. The absence of an explicit implementation of the query could either be intentional or an accidental omission. Perhaps this is an indication that there should not be a default forward-progress guarantee for schedulers?"

[Kona 2025-11-06; SG1 feedback.]

SG1 recommends to remove the default `get_forward_progress_guarantee` implementation instead of adopting LWG4354 current proposed resolution and encourages a paper that explores that direction.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 33.5.10 [exec.get.fwd.progress] as indicated:

   > -2- The name `get_forward_progress_guarantee` denotes a query object. For a subexpression `sch`, let `Sch` be `decltype((sch))`. If `Sch` does not satisfy `scheduler`, `get_forward_progress_guarantee` is ill-formed. Otherwise, `get_forward_progress_guarantee(sch)` is expression-equivalent to:

   > 1. (2.1) — `MANDATE-NOTHROW(AS-CONST(sch).query(get_forward_progress_guarantee))` if that expression is well-formed.

   >    Mandates: The type of the expression above is `forward_progress_guarantee`.

   > 2. (2.2) — Otherwise, `forward_progress_guarantee::weakly_parallel`.

[2026-03-26; Jonathan provides new wording]

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Modify 33.5.10 [exec.get.fwd.progress] as indicated:

   > -2- The name `get_forward_progress_guarantee` denotes a query object. For a subexpression `sch`, let `Sch` be `decltype((sch))`. If `Sch` does not satisfy `scheduler`, `get_forward_progress_guarantee` is ill-formed. Otherwise, `get_forward_progress_guarantee(sch)` is expression-equivalent to:

   > 1. (2.1) — `MANDATE-NOTHROW(AS-CONST(sch).query(get_forward_progress_guarantee))` if that expression is well-formed.

   >    Mandates: The type of the expression above is `forward_progress_guarantee`.

   > 2. (2.2) — Otherwise, `forward_progress_guarantee::weakly_parallel`.

[2026-03-26; Ruslan provides new wording]

[Croydon 2026-03-27; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 33.5.10 [exec.get.fwd.progress] as indicated:

   > -2- The name `get_forward_progress_guarantee` denotes a query object. For a subexpression `sch`, let `Sch` be `decltype((sch))`. If `Sch` does not satisfy `scheduler`, `get_forward_progress_guarantee` is ill-formed. Otherwise, `get_forward_progress_guarantee(sch)` is expression-equivalent to:

   > 1. (2.1) — `MANDATE-NOTHROW(AS-CONST(sch).query(get_forward_progress_guarantee))` if that expression is well-formed.

   >    Mandates: The type of the expression above is `forward_progress_guarantee`.

   > 2. (2.2) — Otherwise, `forward_progress_guarantee::weakly_parallel`.

2. Modify 33.6 [exec.sched] as indicated:

   >       template<class Sch>
   >         concept scheduler =
   >           derived_from<typename remove_cvref_t<Sch>::scheduler_concept, scheduler_t> &&
   >           queryable<Sch> &&
   >           requires(Sch&& sch) {
   >             { schedule(std::forward<Sch>(sch)) } -> sender;
   >             { get_forward_progress_guarantee(sch) } -> same_as<forward_progress_guarantee>;
   >             { auto(get_completion_scheduler<set_value_t>(
   >                 get_env(schedule(std::forward<Sch>(sch))))) }
   >                   -> same_as<remove_cvref_t<Sch>>;
   >           } &&
   >           equality_comparable<remove_cvref_t<Sch>> &&
   >           copyable<remove_cvref_t<Sch>>;

### 4358(i). §[exec.as.awaitable] is using "Preconditions:" when it should probably be described in the constraint

Section: 33.13.1 [exec.as.awaitable] Status: C++26 Submitter: Lewis Baker Opened: 2025-08-27 Last modified: 2026-06-09

Priority: 2

View all other issues in [exec.as.awaitable].

View all issues with C++26 status.

Discussion:

In 33.13.1 [exec.as.awaitable] bullet 7.2 it states:

> 1. (7.2) — Otherwise, `(void(p), expr)` if `is-awaitable<Expr, U>` is `true`, where `U` is an unspecified class type that is not `Promise` and that lacks a member named `await_transform`.

>    Preconditions: `is-awaitable<Expr, Promise>` is `true` and the expression `co_await expr` in a coroutine with promise type `U` is expression-equivalent to the same expression in a coroutine with promise type `Promise`.

The "Preconditions:" sentence there refers to static properties of the program and so seems like a better fit for a Mandates: element or for folding into the constraint.

Also, in the part of the precondition above which says "… and the expression `co_await expr` in a coroutine with promise type `U` is expression-equivalent to the same expression in a coroutine with promise type `Promise`" it is unclear how this can be satisfied, as the types involved are different and therefore the expression cannot be expression-equivalent.

I think perhaps what is intended here is something along the lines of the first expression having "effects equivalent to" the second expression, instead of "expression-equivalent to"?

However, I think there is a more direct way to express the intent here, by instead just requiring that `decltype(GET-AWAITER(expr))` satisfies `is-awaiter<Promise>`. This checks whether `expr` would be a valid type to return from a `Promise::await_transform()` function.

[2025-10-23; Reflector poll.]

Set priority to 2 after reflector poll.

"Intent of the original wording seems to be that GET-AWAITER(expr) should be the same as GET-AWAITER(expr, p) and this rewording loses that. Don't understand the rationale for the new wording either." (More details in the reflector thread in Sept. 2025)

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.13.1 [exec.as.awaitable] as indicated:

   > -7- `as_awaitable` is a customization point object. For subexpressions `expr` and `p` where `p` is an lvalue, `Expr` names the type `decltype((expr))` and `Promise` names the type `decay_t<decltype((p))>`, `as_awaitable(expr, p)` is expression-equivalent to, except that the evaluations of `expr` and `p` are indeterminately sequenced:

   > 1. (7.1) — `expr.as_awaitable(p)` if that expression is well-formed.

   >    Mandates: `is-awaitable<A, Promise>` is `true`, where `A` is the type of the expression above.

   > 2. (7.2) — Otherwise, `(void(p), expr)` if `decltype(GET-AWAITER(expr))` satisfies `is-awaiter<Promise>`.`is-awaitable<Expr, U>` is `true`, where `U` is an unspecified class type that is not `Promise` and that lacks a member named `await_transform`.

   >    Preconditions: `is-awaitable<Expr, Promise>` is `true` and the expression `co_await expr` in a coroutine with promise type `U` is expression-equivalent to the same expression in a coroutine with promise type `Promise`.

   > 3. (7.3) — […]

   > 4. (7.4) — […]

   > 5. (7.5) — […]

### 4359(i). `as_awaitable(expr, p)` does not define semantics of call if `p` is not an lvalue

Section: 33.13.1 [exec.as.awaitable] Status: WP Submitter: Lewis Baker Opened: 2025-08-27 Last modified: 2026-06-15

Priority: 2

View all other issues in [exec.as.awaitable].

View all issues with WP status.

Discussion:

The wording in 33.13.1 [exec.as.awaitable] p7 defines the semantics of a call to `as_awaitable(expr, p)` where `p` is an lvalue. However, it does not specify what the behaviour is if `p` is not an lvalue.

We should probably say that `as_awaitable(expr, p)` is ill-formed if `p` is not an lvalue.

[2026-01-16; Reflector poll.]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 33.13.1 [exec.as.awaitable] as indicated:

   > -7- `as_awaitable` is a customization point object. For subexpressions `expr` and `p` where `p` is an lvalue, `Expr` names the type `decltype((expr))` and `Promise` names the type `decay_t<decltype((p))>`, if `p` is not an lvalue, `as_awaitable(expr, p)` is ill-formed, otherwise `as_awaitable(expr, p)` is expression-equivalent to, except that the evaluations of `expr` and `p` are indeterminately sequenced:

[2026-01-16; Jonathan provides updated wording]

[2026-05-19; LWG telecon. Status changed: New → Ready.]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 33.13.1 [exec.as.awaitable] as indicated:

   > -7- `as_awaitable` is a customization point object. For subexpressions `expr` and `p` where `p` is an lvalue, `Expr` names the type `decltype((expr))` and `Promise` names the type `decay_t<decltype((p))>`, `as_awaitable(expr, p)` is ill-formed if `p` is not an lvalue, otherwise `as_awaitable(expr, p)` is expression-equivalent to the expression specified below, except that the evaluations of `expr` and `p` are indeterminately sequenced:

### 4360(i). `awaitable-sender` concept should qualify use of `awaitable-receiver` type

Section: 33.13.1 [exec.as.awaitable] Status: C++26 Submitter: Lewis Baker Opened: 2025-08-27 Last modified: 2026-06-09

Priority: 2

View all other issues in [exec.as.awaitable].

View all issues with C++26 status.

Discussion:

In 33.13.1 [exec.as.awaitable] p1 there is an exposition-only helper concept `awaitable-sender` defined as follows:

>     namespace std::execution {
>       template<class Sndr, class Promise>
>         concept awaitable-sender =
>           single-sender<Sndr, env_of_t<Promise>> &&
>           sender_to<Sndr, awaitable-receiver> && // see below
>           requires (Promise& p) {
>             { p.unhandled_stopped() } -> convertible_to<coroutine_handle<>>;
>           };
>     }

The mention of the type `awaitable-receiver` here does not refer to any exposition-only type defined at namespace-scope. It seems to, instead, be referring to the nested member-type `sender-awaitable<Sndr, Promise>::awaitable-receiver` and so should be qualified as such.

[2025-10-23; Reflector poll.]

Set priority to 2 after reflector poll.

"We should move the declaration of `sender-awaitable` before the concept."

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.13.1 [exec.as.awaitable] as indicated:

   > -1- `as_awaitable` transforms an object into one that is awaitable within a particular coroutine. Subclause 33.13 [exec.coro.util] makes use of the following exposition-only entities:

   > >     namespace std::execution {
   > >       template<class Sndr, class Promise>
   > >         concept awaitable-sender =
   > >           single-sender<Sndr, env_of_t<Promise>> &&
   > >           sender_to<Sndr, typename sender-awaitable<Sndr, Promise>::awaitable-receiver> && // see below
   > >           requires (Promise& p) {
   > >             { p.unhandled_stopped() } -> convertible_to<coroutine_handle<>>;
   > >           };
   > >       […]
   > >     }

### 4361(i). `awaitable-receiver::set_value` should use Mandates instead of constraints

Section: 33.13.1 [exec.as.awaitable] Status: C++26 Submitter: Lewis Baker Opened: 2025-08-28 Last modified: 2026-06-09

Priority: 1

View all other issues in [exec.as.awaitable].

View all issues with C++26 status.

Discussion:

In 33.13.1 [exec.as.awaitable] bullet 4.1 the `awaitable-receiver::set_value` member function is defined as having a constraint that the `result-type` is constructible from the values.

> If `constructible_from<result-type, decltype((vs))...>` is satisfied, the expression `set_value(rcvr, vs...)` is equivalent to:

> >     try {
> >       rcvr.result-ptr->template emplace<1>(vs...);
> >     } catch(...) {
> >       rcvr.result-ptr->template emplace<2>(current_exception());
> >     }
> >     rcvr.continuation.resume();

> Otherwise, `set_value(rcvr, vs...)` is ill-formed.

Should we be using mandates here instead of constraints (or alternatively just drop the constraint altogether)? There shouldn't be any need to change behaviour based on whether or not the receiver's completion methods are well-formed or not.

It is worth noting that there is inconsistent use of constraints on `set_value` methods in other receiver implementations throughout 33 [exec].

For example: The following `set_value` member function applies constraints:

- In 33.9.2 [exec.snd.expos] `basic-receiver::set_value` constrains that check that it can accept those specific value arguments

While the following `set_value` member functions do not apply constraints:

- In 33.9.12.10 [exec.let] `receiver2::set_value`

- In 33.9.12.18 [exec.spawn.future] `spawn-future-receiver::set_value`

- in 33.9.13.1 [exec.sync.wait] `sync-wait-receiver::set_value`

We should probably try to be consistent on whether or not `set_value` implementations should use constraints or mandates. Given that it is not allowed to form calls to the receiver unless that overload is present in the `completion_signatures`, it may be worth just making them all mandates. This would tend to make uses of the `receiver_of` concept less useful as satisfying `receiver_of<R, Sig>` would not necessarily guarantee that actually trying to call each of `R`'s corresponding completion functions will result in a well-formed program. It is arguable that this is already the status-quo, however.

[2025-10-23; Reflector poll. Status changed: New → LEWG]

Set priority to 1 after reflector poll.

"Send to LEWG if we want to go this way, which makes `receiver_of` mostly meaningless and calls into question why we even have the concept."

[Croydon 2026-03-27; LEWG confirmed the direction, P4159R0 removes the concept.]

[Croydon 2026-03-27; Status changed: LEWG → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.13.1 [exec.as.awaitable] as indicated:

   > -4- Let `rcvr` be an rvalue expression of type `awaitable-receiver`, let `crcvr` be a const lvalue that refers to `rcvr`, let `vs` be a pack of subexpressions, and let `err` be an expression of type `Err`. Then:

   > 1. (4.1) — If `constructible_from<result-type, decltype((vs))...>` is satisfied, tThe expression `set_value(rcvr, vs...)` is equivalent to:

   >    >     try {
   >    >       rcvr.result-ptr->template emplace<1>(vs...);
   >    >     } catch(...) {
   >    >       rcvr.result-ptr->template emplace<2>(current_exception());
   >    >     }
   >    >     rcvr.continuation.resume();

   >    Otherwise, `set_value(rcvr, vs...)` is ill-formedMandates: `constructible_from<result-type, decltype((vs))...>` is satisfied.

   > 2. (4.2) — […]

   > 3. (4.3) — […]

   > 4. (4.4) — […]

2. Modify 33.9.2 [exec.snd.expos] after p25 as indicated:

   >     […]
   >     template<class Sndr, class Rcvr, class Index>
   >       requires valid-specialization<env-type, Index, Sndr, Rcvr>
   >     struct basic-receiver { // exposition only
   >       using receiver_concept = receiver_t;

   >       using tag-t = tag_of_t<Sndr>; // exposition only
   >       using state-t = state-type<Sndr, Rcvr>; // exposition only
   >       static constexpr const auto& complete = impls-for<tag-t>::complete; // exposition only

   >       template<class... Args>
   >         requires callable<decltype(complete), Index, state-t&, Rcvr&, set_value_t, Args...>
   >       void set_value(Args&&... args) && noexcept {
   >         complete(Index(), op->state, op->rcvr, set_value_t(), std::forward<Args>(args)...);
   >       }

   >       template<class Error>
   >         requires callable<decltype(complete), Index, state-t&, Rcvr&, set_error_t, Error>
   >       void set_error(Error&& err) && noexcept {
   >         complete(Index(), op->state, op->rcvr, set_error_t(), std::forward<Error>(err));
   >       }

   >       void set_stopped() && noexcept
   >         requires callable<decltype(complete), Index, state-t&, Rcvr&, set_stopped_t> {
   >         complete(Index(), op->state, op->rcvr, set_stopped_t());
   >       }

   >       auto get_env() const noexcept -> env-type<Index, Sndr, Rcvr> {
   >         return impls-for<tag-t>::get-env(Index(), op->state, op->rcvr);
   >       }

   >       basic-state<Sndr, Rcvr>* op; // exposition only
   >     };
   >     […]

### 4366(i). Heterogeneous comparison of `expected` may be ill-formed

Section: 22.8.6.8 [expected.object.eq], 22.8.7.8 [expected.void.eq] Status: C++26 Submitter: Hewill Kang Opened: 2025-09-06 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

These comparison functions all explicitly `static_cast` the result of the underlying comparison to `bool`. However, the Constraints only require the implicit conversion, not the explicit one (i.e., "convertible to `bool`" rather than "models `boolean-testable`").

This means that in some pathological cases it will lead to hard errors (demo):

>     #include <expected>

>     struct E1 {};
>     struct E2 {};

>     struct Bool {
>       operator bool() const;
>       explicit operator bool() = delete;
>     };
>     Bool operator==(E1, E2);

>     int main() {
>       std::unexpected e1{E1{}};
>       std::unexpected e2{E2{}};
>       return std::expected<int, E1>{e1} == e2; // fire
>     }

It is reasonable to specify return consistency with actual Constraints.

[2025-10-16; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Related to LWG 4366(i), but the wording styles are inconsistent. `optional` uses "Effects: Equivalent to ..." and `expected` just uses Returns:.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.8.6.8 [expected.object.eq] as indicated:

   >     template<class T2> friend constexpr bool operator==(const expected& x, const T2& v);

   > > -3- Constraints: `T2` is not a specialization of `expected`. The expression `*x == v` is well-formed and its result is convertible to `bool`.

   > > [Note 1: `T` need not be Cpp17EqualityComparable. — end note]

   > > -4- Returns: If `x.has_value()` is `true`, `&& static_cast<bool>(*x == v)`; otherwise `false`.

   >     template<class E2> friend constexpr bool operator==(const expected& x, const unexpected<E2>& e);

   > > -5- Constraints: The expression `x.error() == e.error()` is well-formed and its result is convertible to `bool`.

   > > -6- Returns: If `!x.has_value()` is `true`, `&& static_cast<bool>(x.error() == e.error())`; otherwise `false`.

2. Modify 22.8.7.8 [expected.void.eq] as indicated:

   >     template<class T2, class E2> requires is_void_v<T2>
   >       friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);

   > > -1- Constraints: The expression `x.error() == y.error()` is well-formed and its result is convertible to `bool`.

   > > -2- Returns: If `x.has_value()` does not equal `y.has_value()`, `false`; otherwise if `x.has_value()` is `true`, `true`; otherwise `|| static_cast<bool>(x.error() == y.error())`.

   >     template<class E2>
   >       friend constexpr bool operator==(const expected& x, const unexpected<E2>& e);

   > > -3- Constraints: The expression `x.error() == e.error()` is well-formed and its result is convertible to `bool`.

   > > -4- Returns: If `!x.has_value()` is `true`, `&& static_cast<bool>(x.error() == e.error())` ; otherwise `false`.

### 4368(i). Potential dangling reference returned from `transform_sender`

Section: 33.9.6 [exec.domain.default] Status: Resolved Submitter: Eric Niebler Opened: 2025-08-31 Last modified: 2026-03-31

Priority: 1

View all other issues in [exec.domain.default].

View all issues with Resolved status.

Discussion:

The following has been reported by Trevor Gray to me:

> There is a potential stack-use-after-scope in `execution::transform_sender` with `execution::default_domain::transform_sender`.

> I'll give an example of the problem using `starts_on` with the `default_domain`.

> `starts_on` defines a `transform_sender` so `execution::transform_sender` will expand to:

> >     return transform_sender(
> >         dom,
> >         dom.transform_sender(std::forward<Sndr>(sndr), env...),
> >         env...);

> where `dom` is the `default_domain` and `sndr` is `starts_on`.

> Execution flow:

> - `dom.transform_sender(std::forward<Sndr>(sndr), env...)` uses `default_domain` to invoke `start_on`'s `transform_sender`. The return type is `T` (where `T` is a `let_value` sender)

> - `transform_sender(dom, declval<T>(), env...)` is then run which uses `default_domain` to just return `std::forward<T>(t)`.

> This means the value returned from the entire expression is `T&&` which a reference to a temporary variable in the frame of `transform_sender` which is no longer valid after the return.

In the reference implementation, this scenario does not create a dangling reference because its implementation of `default_domain::transform_sender` does not conform to the spec. By default, it returns an rvalue sender as a prvalue instead of an xvalue as the spec requires.

The fix is for the spec to follow suit and return prvalues when an xvalue would otherwise be returned.

[2025-10-23; Reflector poll.]

Set priority to 1 after reflector poll.

"There's an NB comment to remove `transform_sender` entirely." "Nothing seems to prevent `Sndr` from being a reference type, so I would expect `decay_t` or `remove_cvref_t` (or `decayed-typeof`) instead of the `static_cast`."

"I think that's intentional. The dangling happens if `transform_sender` is invoked with an xvalue (where `Sndr` is deduced as a type). When invoked with and lvalue (where `Sndr` is deduced as an lvalue reference) we don't want to make a copy. So the `static_cast` makes a copy of an xvalue and not of an lvalue."

[2026-03-31 P3826R5 was approved in Croydon. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.6 [exec.domain.default] as indicated:

   >     template<sender Sndr, queryable... Env>
   >       requires (sizeof...(Env) <= 1)
   >     constexpr sender decltype(auto) transform_sender(Sndr&& sndr, const Env&... env)
   >       noexcept(see below);

   > > -2- Let `e` be the expression

   > > >     tag_of_t<Sndr>().transform_sender(std::forward<Sndr>(sndr), env...)

   > > if that expression is well-formed; otherwise, `static_cast<Sndr>(std::forward<Sndr>(sndr))`.

   > > -3- Returns: `e`.

   > > -4- Remarks: The exception specification is equivalent to `noexcept(e)`.

### 4369(i). `check-types` function for `upon_error` and `upon_stopped` is wrong

Section: 33.9.12.9 [exec.then] Status: C++26 Submitter: Eric Niebler Opened: 2025-08-31 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

Addresses US 219-350

The following has been reported by Trevor Gray:

In 33.9.12.9 [exec.then] p5, the `impls-for<decayed-typeof<then-cpo>>::check-types` unction is specified as follows:

>     template<class Sndr, class... Env>
>       static consteval void check-types();

> > Effects: Equivalent to:

> > >     auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>();
> > >     auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {
> > >       if constexpr (!invocable<remove_cvref_t<data-type<Sndr>>, Ts...>)
> > >         throw unspecified-exception();
> > >     };
> > >     cs.for-each(overload-set{fn, [](auto){}});

> > where `unspecified-exception` is a type derived from `exception`.

The line `auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {` is correct when `then-cpo` is `then` but not when it is `upon_error` or `upon_stopped`.

For `upon_error` it should be:

>     auto fn = []<class... Ts>(set_error_t(*)(Ts...)) {

and for `upon_stopped` it should be:

>     auto fn = []<class... Ts>(set_stopped_t(*)(Ts...)) {

We can achieve that by replacing `set_value_t` in the problematic line with `decayed-typeof<set-cpo>`.

[2025-10-23; Reflector poll.]

Set priority to 2 after reflector poll.

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.12.9 [exec.then] as indicated:

   >     template<class Sndr, class... Env>
   >       static consteval void check-types();

   > > -5- Effects: Equivalent to:

   > > >     auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>();
   > > >     auto fn = []<class... Ts>(set_value_tdecayed-typeof<set-cpo>(*)(Ts...)) {
   > > >       if constexpr (!invocable<remove_cvref_t<data-type<Sndr>>, Ts...>)
   > > >         throw unspecified-exception();
   > > >     };
   > > >     cs.for-each(overload-set{fn, [](auto){}});

   > > where `unspecified-exception` is a type derived from `exception`.

### 4370(i). Comparison of `optional<T>` to `T` may be ill-formed

Section: 22.5.9 [optional.comp.with.t] Status: C++26 Submitter: Hewill Kang Opened: 2025-09-06 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [optional.comp.with.t].

View all issues with C++26 status.

Discussion:

When comparing an `optional` with its value type, the current wording specifies that the result is the ternary expression of `x.has_value() ? *x == v : false`, where `*x == v` returns a result that can be implicitly converted to `bool`.

However, when the result can also be constructed using `bool` (which is common), the ternary operation will be ill-formed due to ambiguity (demo):

>     #include <optional>

>     struct Bool {
>       Bool(bool);
>       operator bool() const;
>     };

>     struct S {
>       Bool operator==(S) const;
>     };

>     int main() {
>       return std::optional<S>{} == S{}; // fire
>     }

[2025-10-16; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

"Alternatively could keep the conditional operator but cast one side to `bool`, but that would do an explicit conversion, which might not be what we want."

"Should just require boolean-testable."

Related to LWG 4366(i).

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.9 [optional.comp.with.t] as indicated:

   >     template<class T, class U> constexpr bool operator==(const optional<T>& x, const U& v);

   > > -1- Constraints: `U` is not a specialization of `optional`. The expression `*x == v` is well-formed and its result is convertible to `bool`.

   > > [Note 1: `T` need not be Cpp17EqualityComparable. — end note]

   > > -2- Effects: Equivalent to: `return x.has_value() ? *x == v : false;`

   > > >     if (x.has_value())
   > > >       return *x == v;
   > > >     return false;

   >     template<class T, class U> constexpr bool operator==(const T& v, const optional<U>& x);

   > > -3- Constraints: `T` is not a specialization of `optional`. The expression `v == *x` is well-formed and its result is convertible to `bool`.

   > > -4- Effects: Equivalent to: `return x.has_value() ? v == *x : false;`

   > > >     if (x.has_value())
   > > >       return v == *x;
   > > >     return false;

   >     template<class T, class U> constexpr bool operator!=(const optional<T>& x, const U& v);

   > > -5- Constraints: `U` is not a specialization of `optional`. The expression `*x != v` is well-formed and its result is convertible to `bool`.

   > > -6- Effects: Equivalent to: `return x.has_value() ? *x != v : true;`

   > > >     if (x.has_value())
   > > >       return *x != v;
   > > >     return true;

   >     template<class T, class U> constexpr bool operator!=(const T& v, const optional<U>& x);

   > > -7- Constraints: `T` is not a specialization of `optional`. The expression `v != *x` is well-formed and its result is convertible to `bool`.

   > > -8- Effects: Equivalent to: `return x.has_value() ? v != *x : true;`

   > > >     if (x.has_value())
   > > >       return v != *x;
   > > >     return true;

   >     template<class T, class U> constexpr bool operator<(const optional<T>& x, const U& v);

   > > -9- Constraints: `U` is not a specialization of `optional`. The expression `*x < v` is well-formed and its result is convertible to `bool`.

   > > -10- Effects: Equivalent to: `return x.has_value() ? *x < v : true;`

   > > >     if (x.has_value())
   > > >       return *x < v;
   > > >     return true;

   >     template<class T, class U> constexpr bool operator<(const T& v, const optional<U>& x);

   > > -11- Constraints: `T` is not a specialization of `optional`. The expression `v < *x` is well-formed and its result is convertible to `bool`.

   > > -12- Effects: Equivalent to: `return x.has_value() ? v < *x : false;`

   > > >     if (x.has_value())
   > > >       return v < *x;
   > > >     return false;

   >     template<class T, class U> constexpr bool operator>(const optional<T>& x, const U& v);

   > > -13- Constraints: `U` is not a specialization of `optional`. The expression `*x > v` is well-formed and its result is convertible to `bool`.

   > > -14- Effects: Equivalent to: `return x.has_value() ? *x > v : false;`

   > > >     if (x.has_value())
   > > >       return *x > v;
   > > >     return false;

   >     template<class T, class U> constexpr bool operator>(const T& v, const optional<U>& x);

   > > -15- Constraints: `T` is not a specialization of `optional`. The expression `v > *x` is well-formed and its result is convertible to `bool`.

   > > -16- Effects: Equivalent to: `return x.has_value() ? v > *x : true;`

   > > >     if (x.has_value())
   > > >       return v > *x;
   > > >     return true;

   >     template<class T, class U> constexpr bool operator<=(const optional<T>& x, const U& v);

   > > -17- Constraints: `U` is not a specialization of `optional`. The expression `*x <= v` is well-formed and its result is convertible to `bool`.

   > > -18- Effects: Equivalent to: `return x.has_value() ? *x <= v : true;`

   > > >     if (x.has_value())
   > > >       return *x <= v;
   > > >     return true;

   >     template<class T, class U> constexpr bool operator<=(const T& v, const optional<U>& x);

   > > -19- Constraints: `T` is not a specialization of `optional`. The expression `v <= *x` is well-formed and its result is convertible to `bool`.

   > > -20- Effects: Equivalent to: `return x.has_value() ? v <= *x : false;`

   > > >     if (x.has_value())
   > > >       return v <= *x;
   > > >     return false;

   >     template<class T, class U> constexpr bool operator>=(const optional<T>& x, const U& v);

   > > -21- Constraints: `U` is not a specialization of `optional`. The expression `*x >= v` is well-formed and its result is convertible to `bool`.

   > > -22- Effects: Equivalent to: `return x.has_value() ? *x >= v : false;`

   > > >     if (x.has_value())
   > > >       return *x >= v;
   > > >     return false;

   >     template<class T, class U> constexpr bool operator>=(const T& v, const optional<U>& x);

   > > -23- Constraints: `T` is not a specialization of `optional`. The expression `v >= *x` is well-formed and its result is convertible to `bool`.

   > > -24- Effects: Equivalent to: `return x.has_value() ? v >= *x : true;`

   > > >     if (x.has_value())
   > > >       return v >= *x;
   > > >     return true;

### 4372(i). Weaken Mandates: for dynamic padding values in padded layouts

Section: 23.7.3.4.8.1 [mdspan.layout.leftpad.overview], 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] Status: C++26 Submitter: Luc Grosheintz Opened: 2025-09-09 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Two new layouts were added to `<mdspan>` in C++26. Both have a template parameter `size_t PaddingValue`. This value is allowed to be `std::dynamic_extent` to signal that the padding value isn't known at compile time.

A class Mandates: element (in 23.7.3.4.8.1 [mdspan.layout.leftpad.overview] (5.2) and 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] (5.2), respectively) requires (unconditionally) that

- `PaddingValue` is representable as a value of `index_type`.

Since `std::dynamic_extent` is defined as `size_t(-1)` (in 23.7.2.1 [span.syn]) this immediately prohibits all dynamically padded layout mappings for any `index_type` for which:

>     numeric_limit<index_type>::max() < numeric_limit<size_t>::max()

One example is `int` on a 64-bit system.

The proposed resolution states that the modified representability Mandates: element holds for `rank <= 1`, even though in that case the `PaddingValue` has no other effect. Hence, the Mandates: element could be weakened further.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

"This matches the wording in 23.7.3.3.1 [mdspan.extents.overview] 1.2"

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.7.3.4.8.1 [mdspan.layout.leftpad.overview] as indicated:

   > -5- Mandates:

   > 1. (5.1) — […]

   > 2. (5.2) — if `padding_value` is not equal to `dynamic_extent`, then `padding_value` is representable as a value of type `index_type`.

   > 3. (5.3) — […]

   > 4. (5.4) — […]

2. Modify 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] as indicated:

   > -5- Mandates:

   > 1. (5.1) — […]

   > 2. (5.2) — if `padding_value` is not equal to `dynamic_extent`, then `padding_value` is representable as a value of type `index_type`.

   > 3. (5.3) — […]

   > 4. (5.4) — […]

### 4375(i). `std::simd::bit_ceil` should not be `noexcept`

Section: 29.10.8.14 [simd.bit] Status: C++26 Submitter: Matthias Kretz Opened: 2025-08-29 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

`std::simd::bit_ceil` is declared 'noexcept' in 29.10.3 [simd.syn] and 29.10.8.14 [simd.bit]. But

1. `std::bit_ceil` is not 'noexcept' (22.11.2 [bit.syn] and 22.11.5 [bit.pow.two]) and

2. `std::simd::bit_ceil` has a precondition.

[2025-10-22; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.3 [simd.syn], header `<simd>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 29.10.8.14 [simd.bit], bit manipulation
   >       template<simd-vec-type V> constexpr V byteswap(const V& v) noexcept;
   >       template<simd-vec-type V> constexpr V bit_ceil(const V& v) noexcept;
   >       template<simd-vec-type V> constexpr V bit_floor(const V& v) noexcept;
   >       […]
   >     }

2. Modify 29.10.8.14 [simd.bit] as indicated:

   >     template<simd-vec-type V> constexpr V bit_ceil(const V& v) noexcept;

   > > -3- Constraints: The type `V::value_type` is an unsigned integer type (6.9.2 [basic.fundamental]).

   > > -4- Preconditions: […]

   > > […]

### 4376(i). ABI tag in return type of [simd.mask.unary] is overconstrained

Section: 29.10.9.4 [simd.mask.unary] Status: C++26 Submitter: Matthias Kretz Opened: 2025-09-15 Last modified: 2026-06-09

Priority: 1

View all other issues in [simd.mask.unary].

View all issues with C++26 status.

Discussion:

Addresses DE 298

29.10.9.4 [simd.mask.unary] spells out the return type with the ABI tag of the `basic_mask` specialization. That's problematic / overconstrained.

1. Consider Intel SandyBridge/IvyBridge-like targets:

   >     vec<float>::size() -> 8
   >     vec<int>::size() -> 4
   >     mask<float>::size() -> 8

   The ABI tag in this case encodes for `vec<float>` that one object holds 8 elements and is passed via one register. `vec<int>` uses a different ABI tag that says 4 elements passed via one register. `vec<int, 8>`'s ABI tag says 8 elements passed via two registers.

   Now what should `+mask<float>()` return? The working draft says it must return a `basic_vec<int, mask<float>::abi_type>`. And `mask<float>::abi_type` is constrained to be the same as `vec<float>::abi_type`. The working draft thus makes it impossible to implement ABI tags that encode number of elements + number of registers (+ bit-masks vs. vector-masks, but that's irrelevant for this issue). Instead, an ABI tag would have to encode the native SIMD width of all vectorizable types. And that's unnecessarily making compatible types incompatible. Also we make it harder to add to the set of vectorizable types in the future.

2. The issue is even worse for an implementation that implements `vec<complex<T>>` using different ABI tags. Encoding whether the value-type is complex into the ABI is useful because it impacts how the mask is stored (`mask<complex<float>, 8>` is internally stored as a 16-element bit-mask (for interleaved `complex`), while `mask<double, 8>` is stored as an 8-element bit-mask). The ABI tag can also be used to implement interleaved vs. contiguous storage, which is useful for different architectures. If we require `+mask<complex<float>>()` to be of a different type than any `vec<long long>` would ever be, that's just brittle and unnecessary template bloat.

[2025-10-17; Reflector poll.]

Set priority to 1 after reflector poll.

"Should be addressed together with 4238(i)."

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

> [Drafting note: LWG 4238(i) is closely related.]

1. Modify 29.10.2 [simd.expos] as indicated:

   >     using simd-size-type = see below;                      // exposition only
   >     template<size_t Bytes> using integer-from = see below; // exposition only

   >     template<class T, class Abi>
   >       constexpr simd-size-type simd-size-v = see below;               // exposition only
   >     template<class T> constexpr size_t mask-element-size = see below; // exposition only

   >     template <size_t Bytes, class Abi>
   >       using simd-vec-from-mask-t = see below;                         // exposition only
   >     […]

2. Modify 29.10.2.1 [simd.expos.defn] as indicated:

   >     template<class T> constexpr size_t mask-element-size = see below; // exposition only

   > > -4- `mask-element-size<basic_mask<Bytes, Abi>>` has the value `Bytes`.

   >     template <size_t Bytes, class Abi>
   >       using simd-vec-from-mask-t = see below;

   > > -?- `simd-vec-from-mask-t<Bytes, Abi>` is an alias for an enabled specialization of `basic_vec` if and only if `basic_mask<Bytes, Abi>` is a data-parallel type and `integer-from<Bytes>` is valid and a vectorizable type.

   > > -?- `simd-vec-from-mask-t<Bytes, Abi>::size() == basic_mask<Bytes, Abi>::size()` is `true`.

   > > -?- `typename simd-vec-from-mask-t<Bytes, Abi>::value_type` is `integer-from<Bytes>`

3. Modify 29.10.9.1 [simd.mask.overview], class template `basic_mask overview` synopsis, as indicated:

   >     namespace std::simd {
   >       template<size_t Bytes, class Abi> class basic_mask {
   >       public:
   >         […]
   >         // 29.10.9.4 [simd.mask.unary], basic_mask unary operators
   >         constexpr basic_mask operator!() const noexcept;
   >         constexpr basic_vec<integer-from<Bytes>, Abi>simd-vec-from-mask-t<Bytes, Abi> operator+() const noexcept;
   >         constexpr basic_vec<integer-from<Bytes>, Abi>simd-vec-from-mask-t<Bytes, Abi> operator-() const noexcept;
   >         constexpr basic_vec<integer-from<Bytes>, Abi>simd-vec-from-mask-t<Bytes, Abi> operator~() const noexcept;
   >         […]
   >     }

4. Modify 29.10.9.4 [simd.mask.unary] as indicated:

   >     constexpr basic_mask operator!() const noexcept;
   >     constexpr basic_vec<integer-from<Bytes>, Abi>simd-vec-from-mask-t<Bytes, Abi> operator+() const noexcept;
   >     constexpr basic_vec<integer-from<Bytes>, Abi>simd-vec-from-mask-t<Bytes, Abi> operator-() const noexcept;
   >     constexpr basic_vec<integer-from<Bytes>, Abi>simd-vec-from-mask-t<Bytes, Abi> operator~() const noexcept;

   > > -1- Let `op` be the operator.

   > > -2- Returns: […]

[2025-11-04; Matthias Kretz provides new wording]

This also resolves 4238(i) and addresses DE 297.

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.9.1 [simd.mask.overview], class template `basic_mask overview` synopsis, as indicated:

   >     namespace std::simd {
   >       template<size_t Bytes, class Abi> class basic_mask {
   >       public:
   >         […]
   >         // 29.10.9.4 [simd.mask.unary], basic_mask unary operators
   >         constexpr basic_mask operator!() const noexcept;
   >         constexpr basic_vec<integer-from<Bytes>, Abi>see below operator+() const noexcept;
   >         constexpr basic_vec<integer-from<Bytes>, Abi>see below operator-() const noexcept;
   >         constexpr basic_vec<integer-from<Bytes>, Abi>see below operator~() const noexcept;
   >         […]
   >     }

2. Modify 29.10.9.4 [simd.mask.unary] as indicated:

   >     constexpr basic_mask operator!() const noexcept;
   >     constexpr basic_vec<integer-from<Bytes>, Abi>see below operator+() const noexcept;
   >     constexpr basic_vec<integer-from<Bytes>, Abi>see below operator-() const noexcept;
   >     constexpr basic_vec<integer-from<Bytes>, Abi>see below operator~() const noexcept;

   > > -1- Let `op` be the operator.

   > > -2- Returns: A data-parallel object where the i-th element is initialized to the results of applying `op` to `operator[](i)` for all i in the range of [0, `size()`).

   > > -?- Remarks: If there exists a vectorizable signed integer type `I` such that `sizeof(I) == Bytes` is `true`, `operator+`, `operator-`, and `operator~` return an enabled specialization `R` of `basic_vec` such that `R::value_type` denotes `integer-from<Bytes>` and `R::size() == size()` is `true`. Otherwise, these operators are defined as deleted and their return types are unspecified.

### 4377(i). Misleading note about lock-free property of `std::atomic_ref`

Section: 32.5.7.2 [atomics.ref.ops] Status: C++26 Submitter: Brian Bi Opened: 2025-09-15 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [atomics.ref.ops].

View all issues with C++26 status.

Discussion:

Note 1 to 32.5.7.2 [atomics.ref.ops] states:

> Hardware could require an object referenced by an `atomic_ref` to have stricter alignment (6.8.3 [basic.align]) than other objects of type `T`. Further, whether operations on an `atomic_ref` are lock-free could depend on the alignment of the referenced object. For example, lock-free operations on `std::complex<double>` could be supported only if aligned to `2*alignof(double)`.

By using the word "Further", the note misleadingly implies that `required_alignment` may need to be greater than `alignof(T)` even before considering lock freedom, i.e., that `std::atomic_ref<T>` may be completely unimplementable on given hardware if the stricter alignment requirement is not met. However, that can never be true because falling back to a lock-based implementation is always possible.

The note could also be misinterpreted to imply that even though an object may be aligned to `required_alignment` and thus referenceable by an `atomic_ref`, operations could still fail to be lock-free because there is a stricter alignment requirement that the object does not meet. Such an interpretation is, however, at odds with p4.

The example given by the note is also confusing in that it does not necessarily demonstrate a situation in which `std::atomic_ref<T>::required_alignment` is greater than `alignof(T)`.

In conclusion, this note appears to be a convoluted way of saying that, in order to ensure that operations on `atomic_ref<T>` are lock-free, the implementation may define `required_alignment` to a value greater than `alignof(T)`. The note should be modified to say this much more clearly.

[2025-10-20; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll. Also ask SG1 to take a look.

[Kona 2025-11-06; SG1 had weak consensus for approving the resolution.]

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 32.5.7.2 [atomics.ref.ops] as indicated:

   >     static constexpr size_t required_alignment;

   > > -1- The alignment required for an object to be referenced by an atomic reference, which is at least `alignof(T)`.

   > > -2- [Note 1: Hardware could require an object referenced by an `atomic_ref` to have stricter alignment (6.8.3 [basic.align]) than other objects of type `T`. Further, whether operations on an `atomic_ref` are lock-free could depend on the alignment of the referenced object. For example, lock-free operations on `std::complex<double>` could be supported only if aligned to `2*alignof(double)`An implementation can choose to define `atomic_ref<T>::required_alignment` to a value greater than `alignof(T)` in order to ensure that operations on all objects of type `atomic_ref<T>` are lock-free. — end note]

### 4378(i). Inconsistency between `std::basic_string`'s `data()` and `operator[]` specification

Section: 27.4.3.6 [string.access] Status: C++26 Submitter: Peter Bindels Opened: 2025-09-16 Last modified: 2026-06-09

Priority: 4

View all other issues in [string.access].

View all issues with C++26 status.

Discussion:

From the working draft N5014, the specification for `operator[]` in 27.4.3.6 [string.access] p2 says:

> Returns: `*(begin() + pos)` if `pos < size()`. Otherwise, returns a reference to an object of type `charT` with value `charT()`, where modifying the object to any value other than `charT()` leads to undefined behavior.

The specification for data() in 27.4.3.8.1 [string.accessors] p1 (and p4) says, however:

> Returns: A pointer `p` such that `p + i == addressof(operator[](i))` for each `i` in `[0, size()]`.

The former implies that `str[str.size()]` is allowed to be the address of any null terminator, while the latter restricts it to only being the null terminator belonging to the string.

Suggested fix: Change wording around `operator[]` to

> Returns: `*(begin() + pos)` if `pos <= size()`. The program shall not modify the value stored at `size()` to any value other than `charT()`; otherwise, the behavior is undefined.

This moves it inline with the `data()` specification. Given the hardened precondition that `pos <= size()` this does not change behavior for any in-contract access, and we do not define what the feature does when called with broken preconditions. I have been looking at the latter but that will be an EWG paper instead.

[2025-10-21; Reflector poll.]

Set priority to 4 after reflector poll.

"NAD. `begin() + size()` is not dereferenceable and should remain that way."

"Saying "if `pos <= size()` is redundant given the precondition above."

"The resolution removes any guarantee that the value at `str[str.size()]` is `charT()`. Furthermore, the premise of the issue is incorrect, returning the address of a different null terminator not belonging to the string would make traversing it with other string operations UB, so it has to return a reference to a terminator that's within the same array."

"`*(begin() = size())` is UB, but could use `*(data() + size())` instead. Personally I'd like `*end()` to be valid, but that's certainly LEWG business requiring a paper."

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 27.4.3.6 [string.access] as indicated:

   >     constexpr const_reference operator[](size_type pos) const;
   >     constexpr       reference operator[](size_type pos);

   > > -1- Hardened preconditions: `pos <= size()` is `true`.

   > > -2- Returns: `*(begin() + pos)` if `pos <= size()`. Otherwise, returns a reference to an object of type `charT` with value `charT()`, where modifying the object to any value other than `charT()` leads to undefined behavior.

   > > -3- Throws: Nothing.

   > > -4- Complexity: Constant time.

   > > -?- Remarks The program shall not modify the value stored at `size()` to any value other than `charT()`; otherwise, the behavior is undefined

[2025-11-11; Jonathan provides new wording]

We say that `basic_string` is a contiguous container, which makes the `addressof` wording in `c_str()` and `data()` redundant. The front matter says that there's a null terminator present, so we can move the rule about not modifying the terminator there instead of repeating it in `operator[]` and `c_str()`.

We can also permit modifying the string contents through `const_cast<char*>(str.c_str())[0]`. There's no reason for that to be undefined when `const_cast<string&>(str)[0]` and `const_cast<string&>(str).data()[0]` are both allowed. The only restriction should be on changing the null terminator. Changing any other characters through `c_str() const` or `data() const` is no different to changing them through the non-const `data()`, and does not need to cause undefined behaviour.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 27.4.3.1 [basic.string.general] as indicated:

   > -3- In all cases, [`data()`, `data() + size()`] is a valid range, `data() + size()` points at an object with value `charT()` (a "null terminator"), and `size() <= capacity()` is `true`. Non-const access to the null terminator is possible, e.g. using `*(data()+size())`, but the program has undefined behavior if the null terminator is modified to any value other than `charT()`.

2. Modify 27.4.3.6 [string.access] as indicated:

   >     constexpr const_reference operator[](size_type pos) const;
   >     constexpr reference       operator[](size_type pos);

   > > -1- Hardened Preconditions: `pos <= size()` is `true`.

   > > -2- Returns: `*(data() + pos)`. `*(begin() + pos)` if `pos < size()`. Otherwise, returns a reference to an object of type `charT` with value `charT()`, where modifying the object to any value other than `charT()` leads to undefined behavior.

   > > -3- Throws: Nothing.

   > > -4- Complexity: Constant time.

3. Modify 27.4.3.8.1 [string.accessors] as indicated:

   >     constexpr const charT* c_str() const noexcept;
   >     constexpr const charT* data() const noexcept;
   >     constexpr charT* data() noexcept;

   > > -1- Returns: `to_address(begin())`. A pointer `p` such that `p + i == addressof(operator[](i))` for each `i` in [`0`, `size()`].

   > > -2- Complexity: Constant time.

   > > -3- Remarks: The program shall not modify any of the values stored in the character array; otherwise, the behavior is undefined.

   >     constexpr charT* data() noexcept;

   > > -4- Returns: A pointer `p` such that `p + i == addressof(operator[](i))` for each `i` in [`0`, `size()`].

   > > -5- Complexity: Constant time.

   > > -6- Remarks: The program shall not modify the value stored at `p + size()` to any value other than `charT()`; otherwise, the behavior is undefined.

[2026-02-27; Jonathan provides new wording]

[2026-03-06; Reflector poll.]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 27.4.3.1 [basic.string.general] as indicated:

   > -3- In all cases, [`data()`, `data() + size()`] is a valid range, `data() + size()` points at an object with value `charT()` (a "null terminator"), and `size() <= capacity()` is `true`. The program has undefined behavior if the null terminator is modified to any value other than `charT()`.

2. Modify 27.4.3.6 [string.access] as indicated:

   >     constexpr const_reference operator[](size_type pos) const;
   >     constexpr reference       operator[](size_type pos);

   > > -1- Hardened Preconditions: `pos <= size()` is `true`.

   > > -2- Returns: `data()[pos]`. `*(begin() + pos)` if `pos < size()`. Otherwise, returns a reference to an object of type `charT` with value `charT()`, where modifying the object to any value other than `charT()` leads to undefined behavior.

   > > -3- Throws: Nothing.

   > > -4- Complexity: Constant time.

3. Modify 27.4.3.8.1 [string.accessors] as indicated:

   >     constexpr const charT* c_str() const noexcept;
   >     constexpr const charT* data() const noexcept;
   >     constexpr charT* data() noexcept;

   > > -1- Returns: `to_address(begin())`. A pointer `p` such that `p + i == addressof(operator[](i))` for each `i` in [`0`, `size()`].

   > > -2- Complexity: Constant time.

   > > -3- Remarks: The program shall not modify any of the values stored in the character array; otherwise, the behavior is undefined.

   >     constexpr charT* data() noexcept;

   > > -4- Returns: A pointer `p` such that `p + i == addressof(operator[](i))` for each `i` in [`0`, `size()`].

   > > -5- Complexity: Constant time.

   > > -6- Remarks: The program shall not modify the value stored at `p + size()` to any value other than `charT()`; otherwise, the behavior is undefined.

### 4379(i). `hive::reserve()` needs Throws: element adjusted to match block min/max considerations

Section: 23.3.9.3 [hive.capacity] Status: C++26 Submitter: Matt Bentley Opened: 2025-09-17 Last modified: 2026-06-09

Priority: 3

View all other issues in [hive.capacity].

View all issues with C++26 status.

Discussion:

Addresses US 142-236.

This issue comes from Bloomberg as part of their C++26 comments via Incits. To summarize their case, in a call to `reserve(n)`,

>     if (n > capacity() && capacity() + current-limits.min > max_size()),

`reserve` should throw, e.g when `max_size=100`, `capacity=80`, `current-limits.min` and `current-limits.max` are `40` and `n=90`.

In addition, in the above scenario if we increase `max_size()` to 140 and `n` to 130, we can see that although we could add one block with a capacity of `current-limits.min`, adding another would be impossible; we still cannot make `capacity >= n` without also being `> max_size`.

This is currently not stated in the Throws: element. I've implemented the requested additional throws and they are easily achievable.

[2025-10-22; Reflector poll.]

Set priority to 3 after reflector poll.

There was discussion if `reserve()` is allowed to deallocate unused blocks, that materialized into LWG 4380(i).

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 23.3.9.3 [hive.capacity] as indicated:

   >     void reserve(size_type n);

   > > -3- Effects: If `n <= capacity()` is `true`, there are no effects. Otherwise increases `capacity()` by allocating reserved blocks.

   > > -4- Postconditions: `capacity() >= n` is `true`.

   > > -5- Throws: `length_error` if `n` `capacity()` cannot be made `>= n` without being `> max_size()`, as well as any exceptions thrown by the allocator.

   > > […]

[2026-03-24; Matt provides new wording]

[Croydon 2026-03-25; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 23.3.9.3 [hive.capacity] as indicated:

   >     void reserve(size_type n);

   > > -3- Effects: If `n <= capacity()` is `true`, there are no effects. Otherwise increases `capacity()` by allocating reserved blocks.

   > > -4- Postconditions: `capacity() >= n` is `true`.

   > > -5- Throws: `length_error` if `n >` satisfying the postcondition would cause `capacity()` to exceed `max_size()`, as well as any exceptions thrown by the allocator.

   > > […]

### 4381(i). `std::ranges::to specification` using CTAD not supported by core language

Section: 25.5.7.2 [range.utility.conv.to] Status: Resolved Submitter: Jens Maurer Opened: 2025-09-23 Last modified: 2026-04-01

Priority: 2

View other active issues in [range.utility.conv.to].

View all other issues in [range.utility.conv.to].

View all issues with Resolved status.

Discussion:

25.5.7.2 [range.utility.conv.to] p4 defines a `DEDUCE_EXPR` that attempts CTAD (class template argument deduction) on a template template parameter `C`.

This is not supported by the core language; CWG 3003 will clarify the core wording accordingly.

Suggested resolution: Remove 25.5.7.2 [range.utility.conv.to] p3, p4, p5 and the respective entry in the synopsis as unimplementable.

[2025-10-20; Reflector poll.]

Set priority to 2 after reflector poll.

The core language needs to be fixed so this works.

[2026-04-01 Resolved by the adoption of P3865R3 at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 25.2 [ranges.syn], header `<ranges>` synopsis, as indicated:

   >     […]
   >     namespace std::ranges {
   >       […]
   >       // 25.5.7 [range.utility.conv], range conversions
   >       template<class C, input_range R, class... Args> requires (!view<C>)
   >         constexpr C to(R&& r, Args&&... args);
   >       template<template<class...> class C, input_range R, class... Args>
   >         constexpr auto to(R&& r, Args&&... args);
   >       template<class C, class... Args> requires (!view<C>)
   >         constexpr auto to(Args&&... args);
   >       template<template<class...> class C, class... Args>
   >         constexpr auto to(Args&&... args);
   >       […]
   >     }

2. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   >     template<template<class...> class C, input_range R, class... Args>
   >       constexpr auto to(R&& r, Args&&... args);

   > > -3- Let `input-iterator` be an exposition-only type:

   > > >     struct input-iterator { // exposition only
   > > >       using iterator_category = input_iterator_tag;
   > > >       using value_type = range_value_t<R>;
   > > >       using difference_type = ptrdiff_t;
   > > >       using pointer = add_pointer_t<range_reference_t<R>>;
   > > >       using reference = range_reference_t<R>;
   > > >       reference operator*() const;
   > > >       pointer operator->() const;
   > > >       input-iterator& operator++();
   > > >       input-iterator operator++(int);
   > > >       bool operator==(const input-iterator&) const;
   > > >     };

   > > [Note 1: `input-iterator` meets the syntactic requirements of Cpp17InputIterator. — end note]

   > > -4- Let `DEDUCE_EXPR` be defined as follows:

   > > 1. (4.1) — `C(declval<R>(), declval<Args>()...)` if that is a valid expression,

   > > 2. (4.2) — otherwise, `C(from_range, declval<R>(), declval<Args>()...)` if that is a valid expression,

   > > 3. (4.3) — otherwise,

   > >    >     C(declval<input-iterator>(), declval<input-iterator>(), declval<Args>()...)

   > >    if that is a valid expression,

   > > 4. (4.4) — otherwise, the program is ill-formed.

   > > -5- Returns: `to<decltype(DEDUCE_EXPR)>(std::forward<R>(r), std::forward<Args>(args)...)`.

### 4382(i). The `simd::basic_mask(bool)` overload needs to be more constrained

Section: 29.10.9.2 [simd.mask.ctor] Status: C++26 Submitter: Matthias Kretz Opened: 2025-09-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

29.10.9.2 [simd.mask.ctor] defines the overloads `basic_mask(bool)` and `basic_mask(unsigned_integral auto)`. This leads to the following pitfall:

>     auto g0() {
>       unsigned short k = 0xf;
>       return simd::mask<float, 8>(k); // mov eax, 15
>     }

>     auto g1() {
>       unsigned short k = 0xf;
>       return simd::mask<float, 8>(k >> 1); // mov eax, -1 ⚠️
>     }

>     auto g2() {
>       unsigned int k = 0xf;
>       return simd::mask<float, 8>(k >> 1); // mov eax, 7
>     }

In `g1`, `k` is promoted to `int`, shifted and then passed to the mask constructor. Instead of failing, `int(0x7)` is converted to `bool` and the mask thus initialized to all `true`.

Also consider:

1. `simd::mask<float>(true_type());`

2. `unsigned_integral<bool>` is `true` => `same_as<bool> auto` instead of 'bool' makes the overload set ambiguous

3. `float` is convertible to `bool`, thus `simd::mask<float>(1.f)` continues to compile

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 29.10.9.1 [simd.mask.overview], `class template basic_mask` synopsis, as indicated:

   >     namespace std::simd {
   >       template<size_t Bytes, class Abi> class basic_mask {
   >       public:
   >         […]

   >         constexpr basic_mask() noexcept = default;

   >         // 29.10.9.2 [simd.mask.ctor], basic_mask constructors
   >         constexpr explicit basic_mask(value_type) noexcept;
   >         template<size_t UBytes, class UAbi>
   >           constexpr explicit basic_mask(const basic_mask<UBytes, UAbi>&) noexcept;
   >         template<class G>
   >           constexpr explicit basic_mask(G&& gen) noexcept;
   >         constexpr basic_mask(const bitset<size()>& b) noexcept;
   >         constexpr explicit basic_mask(unsigned_integral auto val) noexcept;
   >         basic_mask(signed_integral auto) = delete;

   >         […]
   >       };
   >     }

[2025-10-06; Matthias Kretz improves wording after reflector discussion]

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.9.1 [simd.mask.overview], `class template basic_mask` synopsis, as indicated:

   >     namespace std::simd {
   >       template<size_t Bytes, class Abi> class basic_mask {
   >       public:
   >         […]

   >         constexpr basic_mask() noexcept = default;

   >         // 29.10.9.2 [simd.mask.ctor], basic_mask constructors
   >         constexpr explicit basic_mask(same_as<value_type> auto) noexcept;
   >         template<size_t UBytes, class UAbi>
   >           constexpr explicit basic_mask(const basic_mask<UBytes, UAbi>&) noexcept;
   >         template<class G>
   >           constexpr explicit basic_mask(G&& gen) noexcept;
   >         template<same_as<bitset<size()>> T>
   >           constexpr basic_mask(const Tbitset<size()>& b) noexcept;
   >         template<unsigned_integral T> requires (!same_as<T, value_type>)
   >           constexpr explicit basic_mask(Tunsigned_integral auto val) noexcept;

   >         […]
   >       };
   >     }

2. Modify 29.10.9.2 [simd.mask.ctor] as indicated:

   >     constexpr explicit basic_mask(same_as<value_type> auto x) noexcept;

   > > -1- Effects: Initializes each element with `x`.

   > […]

   >     template<same_as<bitset<size()>> T>
   >       constexpr basic_mask(const Tbitset<size()>& b) noexcept;

   > > -7- Effects: Initializes the `i`th element with `b[i]` for all `i` in the range `[0, size())`.

   >     template<unsigned_integral T> requires (!same_as<T, value_type>)
   >       constexpr explicit basic_mask(Tunsigned_integral auto val) noexcept;

   > > -8- Effects: Initializes the first `M` elements to the corresponding bit values in `val`, […]

### 4383(i). `constant_wrapper`'s pseudo-mutators are underconstrained

Section: 22.2.10 [const.wrap.class] Status: C++26 Submitter: Hewill Kang Opened: 2025-09-24 Last modified: 2026-06-09

Priority: 1

View all other issues in [const.wrap.class].

View all issues with C++26 status.

Discussion:

Unlike other operators, `constant_wrapper`'s pseudo-mutators only require that the wrapped type has corresponding mutators, but do not require them to be `constexpr` or to return a sensible value. This inconsistency loses the SFINAE friendliness (demo):

>     #include <type_traits>

>     void test(auto t) {
>       if constexpr (requires { +t; })  // ok
>         +t;
>       if constexpr (requires { -t; })  // ok
>         -t;
>       if constexpr (requires { ++t; }) // hard error
>         ++t;
>       if constexpr (requires { --t; }) // hard error
>         --t;
>     }

>     struct S {
>       /* constexpr */ int operator+() const { return 0; }
>       /* constexpr */ int operator++() { return 0; }
>       constexpr void operator-() const { }
>       constexpr void operator--() { }
>     };

>     int main() {
>       test(std::cw<S{}>);
>     }

Since these pseudo-mutators have constraints, it is reasonable to further require constant expressions.

[2025-10-17; Reflector poll.]

Set priority to 1 after reflector poll.

`operator+=` changed between P2781R4 and P2781R5, intent is unclear.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 22.2.10 [const.wrap.class], class template `constant_wrapper` synopsis, as indicated:

   > [Drafting note: The requires clause follows the form of `constant_wrapper`'s function call operator.]

   >     struct cw-operators {                                                           // exposition only
   >       […]
   >       // pseudo-mutators
   >       template<constexpr-param T>
   >         constexpr auto operator++(this T) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<++x>(); }
   >             { return constant_wrapper<[] { auto c = T::value; return ++c; }()>{}; }
   >       template<constexpr-param T>
   >         constexpr auto operator++(this T, int) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x++>(); }
   >             { return constant_wrapper<[] { auto c = T::value; return c++; }()>{}; }

   >       template<constexpr-param T>
   >         constexpr auto operator--(this T) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<--x>(); }
   >             { return constant_wrapper<[] { auto c = T::value; return --c; }()>{}; }
   >       template<constexpr-param T>
   >         constexpr auto operator--(this T, int) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x-->(); }
   >             { return constant_wrapper<[] { auto c = T::value; return c--; }()>{}; }

   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator+=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x += R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v += R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator-=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x -= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v -= R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator*=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x *= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v *= R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator/=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x /= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v /= R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator%=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x %= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v %= R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator&=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x &= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v &= R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator|=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x |= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v |= R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator^=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x ^= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v ^= R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator<<=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x <<= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v <<= R::value; }()>{}; }
   >       template<constexpr-param T, constexpr-param R>
   >         constexpr auto operator>>=(this T, R) noexcept
   >           requires requires(T::value_type x) { constant_wrapper<x >>= R::value>(); }
   >             { return constant_wrapper<[] { auto v = T::value; return v >>= R::value; }()>{}; }
   >     };

[2025-11-05; Zach provides improved wording]

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.2.10 [const.wrap.class], class template `constant_wrapper` synopsis, as indicated:

   > [Drafting note: The requires clause follows the form of `constant_wrapper`'s function call operator.]

   >       struct cw-operators {                                                           // exposition only
   >         […]
   >         // pseudo-mutators
   >         template<constexpr-param T>
   >           constexpr auto operator++(this T) noexcept requires requires(T::value_type x) { ++x; }
   >             { return constant_wrapper<[] { auto c = T::value; return ++c; }()>{}; }
   >         template<constexpr-param T>
   >           constexpr auto operator++(this T, int) noexcept requires requires(T::value_type x) { x++; }
   >             { return constant_wrapper<[] { auto c = T::value; return c++; }()>{}; }

   >         template<constexpr-param T>
   >           constexpr auto operator--(this T) noexcept requires requires(T::value_type x) { --x; }
   >             { return constant_wrapper<[] { auto c = T::value; return --c; }()>{}; }
   >         template<constexpr-param T>
   >           constexpr auto operator--(this T, int) noexcept requires requires(T::value_type x) { x--; }
   >             { return constant_wrapper<[] { auto c = T::value; return c--; }()>{}; }

   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator+=(this T, R) noexcept requires requires(T::value_type x) { x += R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v += R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator-=(this T, R) noexcept requires requires(T::value_type x) { x -= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v -= R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator*=(this T, R) noexcept requires requires(T::value_type x) { x *= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v *= R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator/=(this T, R) noexcept requires requires(T::value_type x) { x /= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v /= R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator%=(this T, R) noexcept requires requires(T::value_type x) { x %= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v %= R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator&=(this T, R) noexcept requires requires(T::value_type x) { x &= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v &= R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator|=(this T, R) noexcept requires requires(T::value_type x) { x |= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v |= R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator^=(this T, R) noexcept requires requires(T::value_type x) { x ^= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v ^= R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator<<=(this T, R) noexcept requires requires(T::value_type x) { x <<= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v <<= R::value; }()>{}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator>>=(this T, R) noexcept requires requires(T::value_type x) { x >>= R::value; }
   >             { return constant_wrapper<[] { auto v = T::value; return v >>= R::value; }()>{}; }
   >         template<constexpr-param T>
   >           constexpr auto operator++(this T) noexcept -> constant_wrapper<++Y> { return {}; }
   >         template<constexpr-param T>
   >           constexpr auto operator++(this T, int) noexcept -> constant_wrapper<Y++> { return {}; }
   >         template<constexpr-param T>
   >           constexpr auto operator--(this T) noexcept -> constant_wrapper<--Y> { return {}; }
   >         template<constexpr-param T>
   >           constexpr auto operator--(this T, int) noexcept -> constant_wrapper<Y--> { return {}; }

   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator+=(this T, R) noexcept -> constant_wrapper<(T::value += R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator-=(this T, R) noexcept -> constant_wrapper<(T::value -= R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator*=(this T, R) noexcept -> constant_wrapper<(T::value *= R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator/=(this T, R) noexcept -> constant_wrapper<(T::value /= R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator%=(this T, R) noexcept -> constant_wrapper<(T::value %= R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator&=(this T, R) noexcept -> constant_wrapper<(T::value &= R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator|=(this T, R) noexcept -> constant_wrapper<(T::value |= R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator^=(this T, R) noexcept -> constant_wrapper<(T::value ^= R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator<<=(this T, R) noexcept -> constant_wrapper<(T::value <<= R::value)> { return {}; }
   >         template<constexpr-param T, constexpr-param R>
   >           constexpr auto operator>>=(this T, R) noexcept -> constant_wrapper<(T::value >>= R::value)> { return {}; }
   >       };
   >     }

   >     template<cw-fixed-value X, typename>
   >     struct constant_wrapper: cw-operators {
   >       static constexpr const auto & value = X.data;
   >       using type = constant_wrapper;
   >       using value_type = typename decltype(X)::type;

   >       template<constexpr-param R>
   >         constexpr auto operator=(R) const noexcept requires requires(value_type x) { x = R::value; }
   >           { return constant_wrapper<[] { auto v = value; return v = R::value; }()>{}; }
   >       template<constexpr-param R>
   >         constexpr auto operator=(R) const noexcept -> constant_wrapper<X = R::value> { return {}; }

   >       constexpr operator decltype(auto)() const noexcept { return value; }
   >     };

### 4384(i). `flat_set::erase(iterator)` is underconstrained

Section: 23.6.11.2 [flat.set.defn], 23.6.12.2 [flat.multiset.defn] Status: C++26 Submitter: Hewill Kang Opened: 2025-09-25 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

This is a follow-up of LWG 3704(i) since we now have `flat_set` and `flat_multiset`.

[2025-10-21; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.6.11.2 [flat.set.defn] as indicated:

   >     iterator erase(iterator position) requires (!same_as<iterator, const_iterator>);
   >     iterator erase(const_iterator position);

2. Modify 23.6.12.2 [flat.multiset.defn] as indicated:

   >     iterator erase(iterator position) requires (!same_as<iterator, const_iterator>);
   >     iterator erase(const_iterator position);

### 4385(i). Including `<simd>` doesn't provide `std::begin/end`

Section: 24.7 [iterator.range] Status: WP Submitter: Hewill Kang Opened: 2025-09-26 Last modified: 2026-06-15

Priority: 3

View all other issues in [iterator.range].

View all issues with WP status.

Discussion:

`std::simd::basic_vec` and `std::simd::basic_mask` are ranges since P3480R6, it is reasonable to enable range access utilities when introducing `<simd>`.

[2025-10-17; Reflector poll.]

Set priority to 3 after reflector poll.

"This will be the first time we add the free-function begin/end interface to support a type where `end` returns a sentinel. I would not want to set precedent here without at least taking the time to think about it, and acknowledge a deliberate intent."

"The intent has been deliberate since P0184R0."

[2026-05-22 LWG telecon; Status changed: New → Ready]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 24.7 [iterator.range] as indicated:

   > -1- In addition to being available via inclusion of the `<iterator>` header, the function templates in 24.7 [iterator.range] are available when any of the following headers are included: `<array>`, `<deque>`, `<flat_map>`, `<flat_set>`, `<forward_list>`, `<hive>`, `<inplace_vector>`, `<list>`, `<map>`, `<regex>`, `<set>`, `<simd>`, `<span>`, `<string>`, `<string_view>`, `<unordered_map>`, `<unordered_set>`, and `<vector>`.

### 4387(i). Including `<stacktrace>` doesn't provide `std::begin/end`

Section: 24.7 [iterator.range] Status: Resolved Submitter: Hewill Kang Opened: 2025-09-27 Last modified: 2025-11-11

Priority: 3

View all other issues in [iterator.range].

View all issues with Resolved status.

Discussion:

`basic_stacktrace` is explicitly specified as a reversible container, an allocator-aware container, and a const-qualified sequence container, with members `begin`/`end`, `rbegin`/`rend`, `cbegin`/`cend`, `crbegin`/`crend`, `empty`, `size`, etc.

Thus, it's worth making the corresponding free functions available when introducing `<stacktrace>`, just like other containers.

[2025-10-07; Reflector poll]

Approved as Tentatively Ready, but this is a duplicate of 3625(i) which will be resolved by P3016R6. So move Status New → Open.

[2025-10-20; Set to same priority as 3625(i) (i.e. P3).]

[2025-11-11; Resolved by P3016R6, approved in Kona. Status changed: Open → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 24.7 [iterator.range] as indicated:

   > -1- In addition to being available via inclusion of the `<iterator>` header, the function templates in 24.7 [iterator.range] are available when any of the following headers are included: `<array>`, `<deque>`, `<flat_map>`, `<flat_set>`, `<forward_list>`, `<hive>`, `<inplace_vector>`, `<list>`, `<map>`, `<regex>`, `<set>`, `<span>`, `<stacktrace>`, `<string>`, `<string_view>`, `<unordered_map>`, `<unordered_set>`, and `<vector>`.

### 4388(i). Align new definition of `va_start` with C23

Section: 17.14.2 [cstdarg.syn] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-10-01 Last modified: 2026-06-09

Priority: 1

View other active issues in [cstdarg.syn].

View all other issues in [cstdarg.syn].

View all issues with C++26 status.

Discussion:

P3348R4 changed the `va_start` macro to match C23, but the following wording from C is not present in C++:

> If any additional arguments expand to include unbalanced parentheses, or a preprocessing token that does not convert to a token, the behavior is undefined.

The importance of that wording was not realized during review of P3348R4. The wording is intended to ensure that any discarded arguments to `va_start` are actually lexable by the compiler, rather than containing unbalanced parentheses or brackets. It also makes the following undefined:

    #define BAD ); format_disk(
    va_start(ap, BAD);

[2025-10-14; Reflector poll]

Set priority to 1 after reflector poll.

[Kona 2025-11-05; LWG had questions about requiring some cases to be ill-formed.]

The submitter clarified that it would constrain implementations (effectively requiring `va_start` to be implemented as a magic keyword in the preprocessor, in order to be able to diagnose misuses when preprocessing separately from compilation).

Core approved the new wording.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 17.14.2 [cstdarg.syn] as indicated:

   > (1.2) — If more than one argument is present for `va_start` and any of the second or subsequent arguments expands to include unbalanced parentheses, or a preprocessing token that does not convert to a token, the program is ill-formed, no diagnostic required. The preprocessing tokens comprising the second and subsequent arguments to `va_start` (if any) are discarded. [Note 1: `va_start` accepts a second argument for compatibility with prior revisions of C++. — end note]

### 4390(i). `simd::basic_vec(U&&)` default template parameter

Section: 29.10.7.2 [simd.ctor] Status: Resolved Submitter: Hewill Kang Opened: 2025-09-29 Last modified: 2026-04-14

Priority: 4

View all other issues in [simd.ctor].

View all issues with Resolved status.

Discussion:

`simd::basic_vec` supports `complex` after P2663R7, defaulting the template parameter of its broadcast constructor seems to be reasonable, as this allows the intuitive spelling:

    simd::vec<complex<double>> sc1   ({-1.0, 0.5}); // current ill-formed
    simd::vec<complex<double>> sc2 = {{-1.0, 0.5}}; // current ill-formed

[2025-10-22; Reflector poll. Status changed: New → LEWG with priority 4.]

This narrows design space wrt using braces for initialization of `basic_vec`, especially if we allow user-defined types.

LWG 4230(i) would address this by allowing `vec<complex<double>>(1., 1.);`

[2026-04-14; LWG 4230(i) was approved in Kona. Status changed: LEWG → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.1 [simd.overview] as indicated:

   >     namespace std::simd {
   >       template<class T, class Abi> class basic_vec {
   >       public:
   >         […]
   >         template<class U = T>
   >           constexpr explicit(see below) basic_vec(U&& value) noexcept;
   >         […]
   >       };
   >     }

2. Modify 29.10.7.2 [simd.ctor] as indicated:

   >     template<class U = T> constexpr explicit(see below) basic_vec(U&& value) noexcept;

   > > -1- […]

### 4393(i). `simd::unchecked_scatter_to` is underconstrained

Section: 29.10.8.10 [simd.permute.memory] Status: Resolved Submitter: Hewill Kang Opened: 2025-09-29 Last modified: 2025-11-28

Priority: 2

View all issues with Resolved status.

Discussion:

Both `simd::unchecked_scatter_to` and `simd::partial_scatter_to` are used to write a `simd::vec` into a range `R`.

However, they only require `R` to be `contiguous_range` and `sized_range`. Requiring `R` to be `output_range` is also necessary; otherwise, the `constant_range` cannot be written.

[2025-10-22; Reflector poll.]

Set priority to 2 after reflector poll.

This issue is related to LWG 4420(i). The Constrains needs to be updated to handle conversions between `float` and `float16_t` and similar cases.

[2025-11-28; Resolved by LWG 4420(i), approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.3 [simd.syn] as indicated:

   >     namespace std::simd {
   >       […]
   >       template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >         requires ranges::sized_range<R> && ranges::output_range<R, typename V::value_type>
   >         constexpr void
   >           unchecked_scatter_to(const V& v, R&& out,
   >                                const I& indices, flags<Flags...> f = {});
   >       template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >         requires ranges::sized_range<R> && ranges::output_range<R, typename V::value_type>
   >         constexpr void
   >           unchecked_scatter_to(const V& v, R&& out, const typename I::mask_type& mask,
   >                                const I& indices, flags<Flags...> f = {});

   >       template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >         requires ranges::sized_range<R> && ranges::output_range<R, typename V::value_type>
   >         constexpr void
   >           partial_scatter_to(const V& v, R&& out,
   >                              const I& indices, flags<Flags...> f = {});
   >       template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >         requires ranges::sized_range<R> && ranges::output_range<R, typename V::value_type>
   >         constexpr void
   >           partial_scatter_to(const V& v, R&& out, const typename I::mask_type& mask,
   >                              const I& indices, flags<Flags...> f = {});
   >       […]
   >     }

2. Modify 29.10.8.10 [simd.permute.memory] as indicated:

   >     template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >       requires ranges::sized_range<R> && ranges::output_range<R, typename V::value_type>
   >       constexpr void unchecked_scatter_to(const V& v, R&& out, const I& indices,
   >                                           flags<Flags...> f = {});
   >     template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >       requires ranges::sized_range<R> && ranges::output_range<R, typename V::value_type>
   >       constexpr void unchecked_scatter_to(const V& v, R&& out, const typename I::mask_type& mask,
   >                                           const I& indices, flags<Flags...> f = {});

   > > -10- Let `mask` be `typename I::mask_type(true)` for the overload with no `mask` parameter.

   > > […]

   >     template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >       requires ranges::sized_range<R> && ranges::output_range<R, typename V::value_type>
   >       constexpr void
   >       partial_scatter_to(const V& v, R&& out, const I& indices, flags<Flags...> f = {});
   >     template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >       requires ranges::sized_range<R> && ranges::output_range<R, typename V::value_type>
   >       constexpr void partial_scatter_to(const V& v, R&& out, const typename I::mask_type& mask,
   >                                         const I& indices, flags<Flags...> f = {});

   > > -13- Let `mask` be `typename I::mask_type(true)` for the overload with no `mask` parameter.

   > > […]

### 4396(i). Improve `inplace_vector(from_range_t, R&& rg)`

Section: 23.2.4 [sequence.reqmts], 23.3.16.2 [inplace.vector.cons] Status: C++26 Submitter: Hewill Kang Opened: 2025-10-01 Last modified: 2026-06-09

Priority: 3

View other active issues in [sequence.reqmts].

View all other issues in [sequence.reqmts].

View all issues with C++26 status.

Discussion:

Consider:

>     std::array<int, 42> a;
>     std::inplace_vector<int, 5> v(std::from_range, a);

The above throws `std::bad_alloc` at runtime because the size of `array` is larger than capacity of `inplace_vector`. However, we should reject it at compile time since the `array` size is a constant expression.

Given that we do a lot of compile-time size checking in `<simd>`, it's worth applying that here as well. Compile-time errors are better than runtime ones.

[2025-10-22; Reflector poll. Status changed: New → LEWG and P3.]

General support for change, after LEWG approval.

Suggestion was made that this could be extended to all containers, but is unlikely to be triggred in real word, as it requires ranges with static size greater than `size_t(-1)`.

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   >     a.assign_range(rg)

   > > -60- Result: `void`

   > > -61- Mandates: `assignable_from<T&, ranges::range_reference_t<R>>` is modeled. For `inplace_vector`, if `ranges::size(rg)` is a constant expression then `ranges::size(rg)` ≤ `a.max_size()`.

2. Modify 23.3.16.2 [inplace.vector.cons] as indicated:

   >     template<container-compatible-range<T> R>
   >       constexpr inplace_vector(from_range_t, R&& rg);

   > > -?- Mandates: If `ranges::size(rg)` is a constant expression then `ranges::size(rg)` ≤ `N`.

   > > -9- Effects: Constructs an `inplace_vector` with the elements of the range `rg`.

   > > -10- Complexity: Linear in `ranges::distance(rg)`.

### 4398(i). `enable_nonlocking_formatter_optimization` should be disabled for container adaptors

Section: 23.6.2 [queue.syn], 23.6.5 [stack.syn] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-10-02 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

As the standard currently defines formatters for `queue`, `prioriy_queue`, and `stack` `enable_nonlocking_formatter_optimization` is specialized to `true` for these adaptors per 28.5.6.4 [format.formatter.spec] p3:

> Unless specified otherwise, for each type `T` for which a formatter specialization is provided by the library, each of the headers provides the following specialization:

> >     template<> inline constexpr bool enable_nonlocking_formatter_optimization<T> = true;

However, formatting an adaptor requires formatting of the underlying range in terms of `ranges::ref_view`, and we disable the nonlocking_optimizations for all ranges, including `ranges::ref_view`.

This problem does not occur for the `flat_set`, `flat_map` adaptors, which are also ranges, but unlike `stack` etc. they do not have a specialized formatter. They use the `formatter` specialization for ranges and we already disable the optimization for that formatter.

The proposed wording has recently been implemented in gcc's libstdc++.

[2025-10-14; Reflector poll]

Set priority to 2 after reflector poll.

This is a duplicate of LWG 4146(i), with a different proposed resolution.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.6.2 [queue.syn], header `<queue>` synopsis, as indicated:

   >     […]
   >     // 23.6.13 [container.adaptors.format], formatter specialization for queue
   >     template<class charT, class T, formattable<charT> Container>
   >       struct formatter<queue<T, Container>, charT>;

   >     template<class T, class Container>
   >       constexpr bool enable_nonlocking_formatter_optimization<queue<T, Container>> = false;

   >     // 23.6.4 [priority.queue], class template priority_queue
   >     template<class T, class Container = vector<T>,
   >              class Compare = less<typename Container::value_type>>
   >       class priority_queue;
   >     […]
   >     // 23.6.13 [container.adaptors.format], formatter specialization for priority_queue
   >     template<class charT, class T, formattable<charT> Container, class Compare>
   >       struct formatter<priority_queue<T, Container, Compare>, charT>;

   >     template<class T, class Container, class Compare>
   >       constexpr bool enable_nonlocking_formatter_optimization<priority_queue<T, Container, Compare>> = false;
   >     […]

2. Modify 23.6.5 [stack.syn], header `<stack>` synopsis, as indicated:

   >     […]

   >     // 23.6.13 [container.adaptors.format], formatter specialization for stack
   >     template<class charT, class T, formattable<charT> Container>
   >       struct formatter<stack<T, Container>, charT>;

   >     template<class T, class Container>
   >       constexpr bool enable_nonlocking_formatter_optimization<stack<T, Container>> = false;

   >     […]

### 4399(i). `enable_nonlocking_formatter_optimization` for `pair` and `tuple` needs `remove_cvref_t`

Section: 28.5.9 [format.tuple] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-10-02 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

The `enable_nonlocking_formatter_optimization` variable template is specialized only for cv-unqualified types. However, the specialization for `pair` and `tuple` does not remove the references and cv-qualifiers from the elements:

>     template<class... Ts>
>       constexpr bool enable_nonlocking_formatter_optimization<pair-or-tuple<Ts...>> =
>         (enable_nonlocking_formatter_optimization<Ts> && ...);

As consequence `pair<const std::string, int>` or `pair<const std::string&, int&>` (`map` and `flat_map` reference types) will not use unbuffered prints.

The proposed wording has recently been implemented in gcc's libstdc++.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 28.5.9 [format.tuple] as indicated:

   > -1- For each of `pair` and `tuple`, the library provides the following formatter specialization where `pair-or-tuple` is the name of the template:

   > >     namespace std {
   > >       […]

   > >       template<class... Ts>
   > >         constexpr bool enable_nonlocking_formatter_optimization<pair-or-tuple<Ts...>> =
   > >           (enable_nonlocking_formatter_optimization<remove_cvref_t<Ts>> && ...);
   > >     }

### 4403(i). `simd::basic_vec` CTAD misses difference type casting

Section: 29.10.7.2 [simd.ctor] Status: C++26 Submitter: Hewill Kang Opened: 2025-10-04 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [simd.ctor].

View all issues with C++26 status.

Discussion:

Currently, `basic_vec` can take an object `r` of range type `R` whose size is a constant expression and deduced to `vec<ranges::range_value_t<R>, ranges::size(r)>`.

However, such a deduced type is ill-formed when `R` has a an integer-class type size which cannot be implicitly converted to `simd-size-type`, which is a signed integer type.

It is necessary to perform difference type casting here, and the narrowing conversion will still correctly be rejected due to the constructor's constraints.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.2 [simd.ctor] as indicated:

   >     template<class R, class... Ts>
   >       basic_vec(R&& r, Ts...) -> see below;

   > > -17- Constraints:

   > > 1. (17.1) — `R` models `ranges::contiguous_range` and `ranges::sized_range`, and

   > > 2. (17.2) — `ranges::size(r)` is a constant expression.

   > > -18- Remarks: The deduced type is equivalent to `vec<ranges::range_value_t<R>, static_cast<simd-size-type>(ranges::size(r))>`

### 4407(i). `constexpr-wrapper-like` needs `remove_cvref_t` in `simd::basic_vec` constructor

Section: 29.10.7.2 [simd.ctor] Status: C++26 Submitter: Hewill Kang Opened: 2025-10-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [simd.ctor].

View all issues with C++26 status.

Discussion:

`decltype(From::value)` would be `const int&` if `From` is a type of `std::cw<42>`, so the reference also needs to be removed for checking the arithmetic type.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.2 [simd.ctor] as indicated:

   >     template<class U> constexpr explicit(see below) basic_vec(U&& value) noexcept;

   > > -1- Let `From` denote the type `remove_cvref_t<U>`.

   > > […]

   > > -4- Remarks: The expression inside `explicit` evaluates to `false` if and only if `U` satisfies `convertible_to<value_type>`, and either

   > > 1. (4.1) — `From` is not an arithmetic type and does not satisfy `constexpr-wrapper-like`,

   > > 2. (4.2) — `From` is an arithmetic type and the conversion from `From` to `value_type` is value-preserving (29.10.1 [simd.general]), or

   > > 3. (4.3) — `From` satisfies `constexpr-wrapper-like`, `remove_cvref_tremove_const_t<decltype(From::value)>` is an arithmetic type, and `From::value` is representable by `value_type`.

### 4412(i). Fix declaration of `zero_element` and `uninit_element`

Section: 29.10.3 [simd.syn] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-10-17 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [simd.syn].

View all issues with C++26 status.

Discussion:

Addresses US-174-282

`zero_element` and `uninit_element` should be inline and not static

[2025-10-22; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.3 [simd.syn] as indicated:

   >     // 29.10.8.7, permute by static index generator
   >     inline static constexpr simd-size-type zero_element = implementation-defined;
   >     inline static constexpr simd-size-type uninit_element = implementation-defined;

### 4413(i). Unused/left-over `simd::alignment` specialization for `basic_mask`

Section: 29.10.4 [simd.traits] Status: C++26 Submitter: Matthias Kretz Opened: 2025-10-15 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [simd.traits].

View all issues with C++26 status.

Discussion:

29.10.4 [simd.traits] describes a `value` member for `simd::alignment<basic_mask<…>, bool>`. This was used for loads and stores for masks from/to arrays of `bool`. However, these load/store functions were removed after `bitset`/`unsigned_integral` conversions were introduced. This left-over TS wording should be removed.

[2025-10-22; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.3 [simd.syn] as indicated:

   >     template<class T, class U = typename T::value_type> struct alignment { see below };

   > > -1- `alignment<T, U>` has a member `value` if and only if

   > > 1. (1.1) — `T` is a specialization of `basic_mask` and `U` is `bool`, or

   > > 2. (1.2) — `T` is a specialization of `basic_vec` and `U` is a vectorizable type.

### 4414(i). §[simd.expos.abi] `deduce-abi-t` is underspecified and incorrectly referenced from `rebind` and `resize`

Section: 29.10.2.2 [simd.expos.abi], 29.10.4 [simd.traits] Status: Resolved Submitter: Matthias Kretz Opened: 2025-10-15 Last modified: 2026-03-31

Priority: 1

View all issues with Resolved status.

Discussion:

In 29.10.2.2 [simd.expos.abi], `deduce-abi-t` is specified to be defined for some arguments. For all remaining arguments, nothing is specified. This could be interpreted to make such specializations ill-formed. But that does not match the intent of making

>     simd::vec<std::string>

and

>     simd::vec<int, INT_MAX>

disabled specializations of `basic_vec`. (If `INT_MAX` is not supported by the implementation.)

The wording needs to clarify what happens in those cases.

In 29.10.4 [simd.traits], `rebind` and `resize` say "`deduce-abi-t<T, V::size()>` has a member type `type`". But that's not how `deduce-abi-t` is specified.
Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 29.10.4 [simd.traits] as indicated:

   >     template<class T, class V> struct rebind { using type = see below; };

   > > -4- The member `type` is present if and only if

   > > 1. (4.1) — `V` is a data-parallel type,

   > > 2. (4.2) — `T` is a vectorizable type, and

   > > 3. (4.3) — `deduce-abi-t<T, V::size()>` has a member type `type` is defined.

   > > […]

   >     template<simd-size-type N, class V> struct resize { using type = see below; };

   > > -7- Let `T` denote […]

   > > -8- The member `type` is present if and only if

   > > 1. (8.1) — `V` is a data-parallel type, and

   > > 2. (8.2) — `deduce-abi-t<T, N>` has a member type `type` is defined.

[2025-10-21; Matthias Kretz improves discussion and proposed wording]

[2026-01-16; Reflector poll.]

Set priority to 1 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 29.10.2.2 [simd.expos.abi] as indicated:

   >     template<class T> using native-abi = see below;
   >     template<class T, simd-size-type N> using deduce-abi-t = see below;

   > > -1- An ABI tag is a type that indicates a choice of size and binary representation for objects of data-parallel type.

   > > […]

   > > -3- An implementation defines ABI tag types as necessary for the following aliases.

   > > -4- `deduce-abi-t<T, N>` is definednames an ABI tag type if and only if

   > > 1. (4.1) — `T` is a vectorizable type,

   > > 2. (4.2) — `N` is greater than zero, and

   > > 3. (4.3) — `N` is not larger than an implementation-defined maximum.

   > > Otherwise, `deduce-abi-t<T, N>` names an unspecified type.

   > > The implementation-defined maximum for `N` is not smaller than 64 and can differ depending on `T`. […]

   > > -5- Where present,If `deduce-abi-t<T, N>` names an ABI tag type such that, the following is `true`:

   > > 1. (5.1) — `simd-size-v<T, deduce-abi-t<T, N>>` equals `N`,

   > > 2. (5.2) — `basic_vec<T, deduce-abi-t<T, N>>` is enabled (29.10.7.1 [simd.overview]), and

   > > 3. (5.3) — `basic_mask<sizeof(T), deduce-abi-t<integer-from<sizeof(T)>, N>>` is enabled.

2. Modify 29.10.4 [simd.traits] as indicated:

   >     template<class T, class V> struct rebind { using type = see below; };

   > > -4- The member `type` is present if and only if

   > > 1. (4.1) — `V` is a data-parallel type,

   > > 2. (4.2) — `T` is a vectorizable type, and

   > > 3. (4.3) — `deduce-abi-t<T, V::size()>` has a member type `type` names an ABI tag type.

   > > […]

   >     template<simd-size-type N, class V> struct resize { using type = see below; };

   > > -7- Let `T` denote […]

   > > -8- The member `type` is present if and only if

   > > 1. (8.1) — `V` is a data-parallel type, and

   > > 2. (8.2) — `deduce-abi-t<T, N>` has a member type `type` names an ABI tag type.

[2026-02-13; LWG telecon.]

P3932R0 resolving the issue was approved.

[2026-03-31 P3932R0 was approved in Croydon. Status changed: New → Resolved.]

Proposed resolution:

### 4415(i). `task::promise_type::uncaught_exception` seems to be misnamed

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Jiang An Opened: 2025-10-17 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [task.promise].

View all issues with C++26 status.

Discussion:

According to 9.6.4 [dcl.fct.def.coroutine], a function of name `unhandled_exception` may be called by the language mechanisms. However, `std::execution::task<T, Environment>::promise_type` has an `uncaught_exception function` instead, which won't be implicitly called.

In P3552R3, `unhandled_exception` was discussed, but the wording specified `uncaught_exception`, which looks like a mistake. Moreover, the paper didn't talk about the status of `uncaught_exception` in the zombie name list ([tab:zombie.names.std]). Perhaps `unhandled_exception` is the correct function name.

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.13.6.5 [task.promise] as indicated:

   > >     namespace std::execution {
   > >       template<class T, class Environment>
   > >       class task<T, Environment>::promise_type {
   > >       public:
   > >         […]
   > >         void uncaughtunhandled_exception();
   > >         coroutine_handle<> unhandled_stopped();
   > >         […]
   > >       };
   > >     }

   > […]

   >     void uncaughtunhandled_exception();

   > > -12- Effects: If the signature `set_error_t(exception_ptr)` is not an element of `error_types`, calls `terminate()` (14.6.2 [except.terminate]). Otherwise, stores `current_exception()` into `errors`.

### 4416(i). `<meta>` should include `<compare>`

Section: 21.4.1 [meta.syn] Status: C++26 Submitter: Jiang An Opened: 2025-10-15 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [meta.syn].

View all issues with C++26 status.

Discussion:

Some inclusions from `<meta>` were removed between P2996R7 and P2996R8. However, given `std::meta::member_offsets` has `operator<=>`, perhaps we should still include `<compare>` in `<meta>`. This is also noticed in P3429R1.

[2025-10-20; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.1 [meta.syn], header `<meta>` synopsis, as indicated:

   >     #include <compare> // see 17.12.1 [compare.syn]
   >     #include <initializer_list> // see 17.11.2 [initializer.list.syn]

   >     namespace std {
   >       […]
   >     }

### 4419(i). `let_value/error/stopped` should specify the attributes of their senders

Section: 33.9.12.10 [exec.let] Status: Resolved Submitter: Eric Niebler Opened: 2025-10-16 Last modified: 2026-03-31

Priority: 1

View all other issues in [exec.let].

View all issues with Resolved status.

Discussion:

Imported from https://github.com/cplusplus/sender-receiver/issues/346.

`let_value(sndr, fn)` returns `make-sender(let-tag, fn, sndr)`, which means that the `let_value` sender has one child. 33.9.12.1 [exec.adapt.general] bullet (3.2) says:

> A parent sender with a single child sender `sndr` has an associated attribute object equal to `FWD-ENV(get_env(sndr))`.

However, `let_value(sndr, fn)` does not know where it will be started or where it will complete in the general case. The function `fn` could return `schedule(some-scheduler)`, making the `let_value` operation complete on `some-scheduler`. It would be wrong for the `let_value(sndr, fn)` sender to have `sndr`'s completion schedulers or domains.

`let_value/error/stopped` must define `impls-for<let_xxx_t>::get-attrs` to return `FWD-ENV(get_env(sndr))` except that it handles the `get_completion_scheduler_t<Tag>` (for any completion tag `Tag`) and `get_domain_t` queries specially.

[2025-10-23; Reflector poll.]

Set priority to 1 after reflector poll.

"Looks like a special case of the problem that P3826 addresses."

[2026-03-31 P3826R5 was approved in Croydon. Status changed: New → Resolved.]

Proposed resolution:

### 4420(i). §[simd] conversions (constructor, load, stores, gather, and scatter) are incorrectly constrained for `<stdfloat>` types

Section: 29.10 [simd] Status: C++26 Submitter: Matthias Kretz Opened: 2025-10-15 Last modified: 2026-06-09

Priority: 1

View all other issues in [simd].

View all issues with C++26 status.

Discussion:

Addresses DE-288 and DE-285

29.10.8.6 [simd.loadstore] `unchecked_store` and `partial_store` are constrained with `indirectly_writable` in such a way that `basic_vec`'s `value_type` must satisfy `convertible_to<range value-type>`. But that is not the case, e.g. for `float → float16_t` or `double → float32_t`. However, if `simd::flag_convert` is passed, these conversions were intended to work. The implementation thus must `static_cast` the `basic_vec` values to the range's value-type before storing to the range.

`unchecked_store(vec<float>, span<complex<float16_t>>, flag_convert)` does not work for a different reason. The `complex(const float16_t&, const float16_t&)` constructor simply does not allow construction from `float`, irrespective of using implicit or explicit conversion. The only valid conversion from `float → complex<float16_t>` is via an extra step through `complex<float16_t>::value_type`. This issue considers it a defect of `complex` that an explicit conversion from `float → complex<float16_t>` is ill-formed and therefore no workaround/special case is introduced.

Conversely, the conversion constructor in 29.10.7.2 [simd.ctor] does not reject conversion from `vec<complex<float>, 4>` to `vec<float, 4>`. I.e. `convertible_to<vec<complex<float>, 4>, vec<float, 4>>` is `true`, which is a lie. This is NB comment DE-288. However, the NB comment's proposed resolution is too strict, in that it would disallow conversion from `float` to `float16_t`.

The conversion/load from static-sized range constructor in 29.10.7.2 [simd.ctor] has a similar problem:

> `convertible_to<array<std::string, 4>, vec<int, 4>>` is `true`

but when fixing this

>     vec<float16_t, 4>(array<float, 4>, flag_convert)

must continue to be valid.

`unchecked_load` and `partial_load` in 29.10.8.6 [simd.loadstore] currently Mandate the range's value-type to be vectorizable, but converting loads from `complex<float>` to `float` are not covered. It is unclear what a conversion from `complex<float>` to `float` should do, so it needs to be added (again without breaking `float → float16_t`).

29.10.8.10 [simd.permute.memory] is analogous to 29.10.8.6 [simd.loadstore] and needs equivalent constraints.

29.10.7.2 [simd.ctor] p2 requires `constructible_from<U>`, which makes explicit construction of `vec<float16_t>` from `float` ill-formed. For consistency this should also be constrained with `explicitly-convertible-to`.

[2025-10-22; Reflector poll.]

Set priority to 1 after reflector poll.

We also need to update Effects. There are more places in 29.10 [simd] where `float` to `float16_t` and similar conversion are not supported.

It was pointed out that similar issues happen for `complex<float16_t>`. There seem to be mismatch between language initialization rules and the intended usage based on library API.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. In 29.10.3 [simd.syn] and 29.10.8.6 [simd.loadstore] replace all occurrences of

   >     indirectly_writable<ranges::iterator_t<R>, T>

   with

   >     indirectly_writable<ranges::iterator_t<R>, Tranges::range_value_t<R>>

   and all occurrences of

   >     indirectly_writable<I, T>

   with

   >     indirectly_writable<I, Titer_value_t<I>>

2. Modify 29.10.8.6 [simd.loadstore] as indicated:

   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, R&& r, flags<Flags...> f = {});
   >     […]
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first, S last,
   >         const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});

   > > -11- Let […]

   > > -?- Constraints: The expression `static_cast<ranges::range_value_t<R>>(x)` where `x` is an object of type `T` is well-formed.

   > > -12- Mandates: If `ranges::size(r)` is a constant expression then `ranges::size(r) ≥ simd-size-v<T, Abi>`.

   > > […]

   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, R&& r, flags<Flags...> f = {});
   >     […]
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, I first, S last,
   >         const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});

   > > -15- Let […]

   > > -?- Constraints: The expression `static_cast<iter_value_t<I>>(x)` where `x` is an object of type `T` is well-formed.

   > > -16- Mandates: […]

[2025-10-22; Matthias Kretz improves discussion and provides new wording]

[Kona 2025-11-04; Also resolves LWG 4393(i).]

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.2 [simd.expos] as indicated:

   >     […]

   >     template<class T>
   >       concept constexpr-wrapper-like =                   // exposition only
   >         […]
   >         bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;

   >     template<class From, class To>
   >       concept explicitly-convertible-to =                // exposition-only
   >         requires {
   >           static_cast<To>(declval<From>());
   >         };

   >     template<class T> using deduced-vec-t = see below; // exposition only
   >     […]

2. Modify 29.10.3 [simd.syn] as indicated:

   >     […]
   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, R&& r,
   >                                      flags<Flags...> f = {});
   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, R&& r,
   >                                      const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first,
   >                                      iter_difference_t<I> n, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first,
   >                                      iter_difference_t<I> n, const typename basic_vec<T, Abi>::mask_type& mask,
   >                                      flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first, S last,
   >                                      flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first, S last,
   >                                      const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});

   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, R&& r,
   >                                    flags<Flags...> f = {});
   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, R&& r,
   >         const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(
   >         const basic_vec<T, Abi>& v, I first, iter_difference_t<I> n, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(
   >         const basic_vec<T, Abi>& v, I first, iter_difference_t<I> n,
   >         const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, I first, S last,
   >                                    flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, I first, S last,
   >         const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});
   >     […]

3. Modify 29.10.7.2 [simd.ctor] as indicated:

   >     template<class U> constexpr explicit(see below) basic_vec(U&& value) noexcept;

   > > -1- Let `From` denote the type `remove_cvref_t<U>`.

   > > -2- Constraints: `value_typeU` satisfies `constructible_from<U>explicitly-convertible-to<value_type>`. […]

   >     template<class U, class UAbi>
   >       constexpr explicit(see below) basic_vec(const basic_vec<U, UAbi>& x) noexcept;

   > > -5- Constraints:

   > > 1. (5.1) — `simd-size-v<U, UAbi> == size()` is `true`, and

   > > 2. (5.2) — `U` satisfies `explicitly-convertible-to<T>`.

   > > […]

   >     template<class R, class... Flags>
   >       constexpr basic_vec(R&& r, flags<Flags...> = {});
   >     template<class R, class... Flags>
   >       constexpr basic_vec(R&& r, const mask_type& mask, flags<Flags...> = {});

   > > -12- Let `mask` be `mask_type(true)` for the overload with no `mask` parameter.

   > > -13- Constraints:

   > > 1. (13.1) — `R` models `ranges::contiguous_range` and `ranges::sized_range`,

   > > 2. (13.2) — `ranges::size(r)` is a constant expression, and

   > > 3. (13.3) — `ranges::size(r)` is equal to `size()`, and

   > > 4. (13.?) — `ranges::range_value_t<R>` is a vectorizable type and satisfies `explicitly-convertible-to<T>`.

   > > -14- Mandates:

   > > 1. (14.1) — `ranges::range_value_t<R>` is a vectorizable type, and

   > > 2. (14.2) — ifIf the template parameter pack `Flags` does not contain `convert-flag`, then the conversion from `ranges::range_value_t<R>` to `value_type` is value-preserving.

4. Modify 29.10.8.6 [simd.loadstore] as indicated:

   >     template<class V = see below , ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R>
   >       constexpr V partial_load(R&& r, flags<Flags...> f = {});
   >     template<class V = see below , ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R>
   >       constexpr V partial_load(R&& r, const typename V::mask_type& mask, flags<Flags...> f = {});
   >     template<class V = see below , contiguous_iterator I, class... Flags>
   >       constexpr V partial_load(I first, iter_difference_t<I> n, flags<Flags...> f = {});
   >     template<class V = see below , contiguous_iterator I, class... Flags>
   >       constexpr V partial_load(I first, iter_difference_t<I> n, const typename V::mask_type& mask,
   >                                flags<Flags...> f = {});
   >     template<class V = see below , contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       constexpr V partial_load(I first, S last, flags<Flags...> f = {});
   >     template<class V = see below , contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       constexpr V partial_load(I first, S last, const typename V::mask_type& mask,
   >                                flags<Flags...> f = {});

   > > -6- Let

   > > 1. (6.1) — `mask` be `V::mask_type(true)` for the overloads with no `mask` parameter;

   > > 2. (6.2) — `R` be `span<const iter_value_t<I>>` for the overloads with no template parameter `R`;

   > > 3. (6.3) — `r` be `R(first, n)` for the overloads with an `n` parameter and `R(first, last)` for the overloads with a `last` parameter.;

   > > 4. (6.?) — `T` be `typename V::value_type`.

   > > -7- Mandates:

   > > 1. (7.1) — `ranges::range_value_t<R>` is a vectorizable type and satisfies `explicitly-convertible-to<T>`,

   > > 2. (7.2) — `same_as<remove_cvref_t<V>, V>` is `true`,

   > > 3. (7.3) — `V` is an enabled specialization of `basic_vec`, and

   > > 4. (7.4) — if the template parameter pack `Flags` does not contain `convert-flag`, then the conversion from `ranges::range_value_t<R>` to `V::value_type` is value-preserving.

   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, R&& r,
   >                                      flags<Flags...> f = {});
   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, R&& r,
   >                                      const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first,
   >                                      iter_difference_t<I> n, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first,
   >                                      iter_difference_t<I> n, const typename basic_vec<T, Abi>::mask_type& mask,
   >                                      flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first, S last,
   >                                      flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void unchecked_store(const basic_vec<T, Abi>& v, I first, S last,
   >                                      const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});

   > > -11- Let […]

   > > […]

   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, R&& r,
   >                                    flags<Flags...> f = {});
   >     template<class T, class Abi, ranges::contiguous_range R, class... Flags>
   >       requires ranges::sized_range<R> && indirectly_writable<ranges::iterator_t<R>, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, R&& r,
   >         const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(
   >         const basic_vec<T, Abi>& v, I first, iter_difference_t<I> n, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(
   >         const basic_vec<T, Abi>& v, I first, iter_difference_t<I> n,
   >         const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, I first, S last,
   >                                    flags<Flags...> f = {});
   >     template<class T, class Abi, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
   >       requires indirectly_writable<I, T>
   >       constexpr void partial_store(const basic_vec<T, Abi>& v, I first, S last,
   >         const typename basic_vec<T, Abi>::mask_type& mask, flags<Flags...> f = {});

   > > -15- Let […]

   > > -?- Constraints:

   > > 1. (?.1) — `ranges::iterator_t<R>` models `indirectly_writable<ranges::range_value_t<R>>`, and

   > > 2. (?.2) — `T` satisfies `explicitly-convertible-to<ranges::range_value_t<R>>`

   > > -16- Mandates: […]

   > > -17- Preconditions: […]

   > > -18- Effects: For all `i` in the range of `[0, basic_vec<T, Abi>::size())`, if `mask[i] && i < ranges::size(r)` is `true`, evaluates `ranges::data(r)[i] = static_cast<ranges::range_value_t<R>>(v[i])`.

5. Modify 29.10.8.10 [simd.permute.memory] as indicated:

   >     template<class V = see below, ranges::contiguous_range R, simd-integral I, class... Flags>
   >       requires ranges::sized_range<R>
   >       constexpr V partial_gather_from(R&& in, const I& indices, flags<Flags...> f = {});
   >     template<class V = see below, ranges::contiguous_range R, simd-integral I, class... Flags>
   >       requires ranges::sized_range<R>
   >       constexpr V partial_gather_from(R&& in, const typename I::mask_type& mask,
   >                                       const I& indices, flags<Flags...> f = {});

   > > -5- Let: […]

   > > -?- Constraints: `ranges::range_value_t<R>` is a vectorizable type and satisfies `explicitly-convertible-to<T>`.

   > > -6- Mandates: […]

   > > […]

   >     template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >       requires ranges::sized_range<R>
   >       constexpr void
   >       partial_scatter_to(const V& v, R&& out, const I& indices, flags<Flags...> f = {});
   >     template<simd-vec-type V, ranges::contiguous_range R, simd-integral I, class... Flags>
   >       requires ranges::sized_range<R>
   >       constexpr void partial_scatter_to(const V& v, R&& out, const typename I::mask_type& mask,
   >                                         const I& indices, flags<Flags...> f = {});

   > > -13- Let `mask` be `I::mask_type(true)` for the overload with no `mask` parameter.

   > > -14- Constraints:

   > > 1. (14.1) — `V::size() == I::size()` is `true`,

   > > 2. (14.2) — `ranges::iterator_t<R>` models `indirectly_writable<ranges::range_value_t<R>>`, and

   > > 3. (14.3) — `typename V::value_type` satisfies `explicitly-convertible-to<ranges::range_value_t<R>>`.

   > > […]

   > > -17- Effects: For all i in the range [`0`, `I::size()`), if `mask[i] && (indices[i] < ranges::size(out))` is `true`, evaluates `ranges::data(out)[indices[i]] = static_cast<ranges::range_value_t<R>>(v[i])`.

### 4422(i). `meta::access_context` should be a consteval-only type

Section: 21.4.9 [meta.reflection.access.context] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-10-20 Last modified: 2026-06-09

Priority: 2

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Discussion:

The `meta::access_context` type is expected to contain some `meta::info` objects, which would make it a consteval-only type. But we don't actually specify any members, so nothing in the current specification says you can't persist one until runtime.

[2025-10-23; Reflector poll. Adjust proposed wording.]

Set priority to 2 after reflector poll.

Reflector discussion requested that 'non-aggregate' and 'consteval-only' both be put in paragraph 3, adjacent to 'structural'. Also added a drive-by editorial change to paragraph 1.

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.9 [meta.reflection.access.context] as indicated:

   > -1- The class `access_context` class is a non-aggregate type that represents a namespace, class, or function from which queries pertaining to access rules may be performed, as well as the designating class (11.8.3 [class.access.base]), if any.

   > -2- An `access_context` has an associated scope and designating class.

   > >     namespace std::meta {
   > >       struct access_context {
   > >         access_context() = delete;

   > >         consteval info scope() const;
   > >         consteval info designating_class() const;

   > >         static consteval access_context current() noexcept;
   > >         static consteval access_context unprivileged() noexcept;
   > >         static consteval access_context unchecked() noexcept;
   > >         consteval access_context via(info cls) const;
   > >       };
   > >     }

   > -3- `access_context` is a structural, consteval-only, non-aggregate type. Two values `ac1` and `ac2` of type `access_context` are template-argument-equivalent (13.6 [temp.type]) if `ac1.scope()` and `ac2.scope()` are template-argument-equivalent and `ac1.designating_class()` and `ac2.designating_class()` are template-argument-equivalent.

### 4423(i). `meta::data_member_spec` allows negative bit-field widths

Section: 21.4.17 [meta.reflection.define.aggregate] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-10-20 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [meta.reflection.define.aggregate].

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Discussion:

The `meta::data_member_spec` function doesn't restrict `options.bit_width` to be non-negative.

[2025-10-24; LWG telecon; Status changed: New → Ready.]

Poll to move issue to Ready: 10/0/0

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.17 [meta.reflection.define.aggregate] as indicated:

   > - (5.3) — if `options.name` does not contain a value, then `options.bit_width` contains a value;
   > - (5.4) — if `options.bit_width` contains a value V, then
   >   - (5.4.1) — `is_integral_type(type) || is_enum_type(type)` is `true`,
   >   - (5.4.2) — `options.alignment` does not contain a value,
   >   - (5.4.3) — `options.no_unique_address` is `false`, and
   >   - (5.4.?) — V is not negative, and
   >   - (5.4.4) — if V equals `0`, then `options.name` does not contain a value; and
   > - (5.5) — if `options.alignment` contains a value, it is an alignment value (6.8.3 [basic.align]) not less than `alignment_of(type)`.

### 4424(i). `meta::define_aggregate` should require a class type

Section: 21.4.17 [meta.reflection.define.aggregate] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-10-20 Last modified: 2026-06-09

Priority: 1

View all other issues in [meta.reflection.define.aggregate].

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Discussion:

Addresses US 125-188

The `meta::define_aggregate` function doesn't say what happens if C does not represent a class type.

It's also unclear whether it should work with aliases to class types, e.g.

    struct S; using A = S; ... meta::define_aggregate(^^A, {});

And what happens if you try to define a cv-qualified type:

    struct S; meta::define_aggregate(^^const S, {});

Should this be an error, or inject a definition of the unqualified type?

[2025-10-23; Reflector poll.]

Set priority to 1 after reflector poll.

[Kona 2025-11-03; approved by LWG. Status changed: New → Immediate.]

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 21.4.17 [meta.reflection.define.aggregate] as indicated:

   > -7- Let C be the class represented by `dealias(class_type)` and rK be the Kth reflection value in `mdescrs`. For every rK in `mdescrs`, let (TK, NK, AK, WK, NUAK) be the corresponding data member description represented by rK.

   > -8- Constant when:

   > - (8.?) — `dealias(class_type)` represents a class type;
   > - (8.1) — C is incomplete from every point in the evaluation context;

[2025-10-24; LWG telecon. Jonathan updates wording]

Make a minimal change for now, can add support for aliases later.

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.17 [meta.reflection.define.aggregate] as indicated:

   > -7- Let C be the class type represented by `class_type` and rK be the Kth reflection value in `mdescrs`. For every rK in `mdescrs`, let (TK, NK, AK, WK, NUAK) be the corresponding data member description represented by rK.

   > -8- Constant when:

   > - (8.?) — `class_type` represents a cv-unqualified class type;
   > - (8.1) — C is incomplete from every point in the evaluation context;

### 4425(i). CTAD `function_ref` of data member pointer should produce `noexcept` signature

Section: 22.10.17.6.5 [func.wrap.ref.deduct] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-10-20 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses PL-005.

For data member pointer `M C::* dmp`, template argument deduction in form `std::function_ref fr(std::nontype<dmp>, x)` results in `std::function_ref<M&()>`, that returns a reference to designated member. As accessing a data member can never throw exception and `function_ref` support `noexcept` qualifed function types, the deduction guide should be updated to produce `noexcept`-qualified signature.

[2025-10-21; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.10.17.6.5 [func.wrap.ref.deduct] as indicated:

   >     template<class F>
   >     function_ref(nontype_t<f>, T&&) -> function_ref<see below>;

   > > Let `F` be `decltype(f)`.

   > > -6- Constraint:

   > > 1. (6.1) — `F` is of the form `R(G::*)(A...) cv &opt noexcept(E)` for type `G`, or
   > > 2. (6.2) — `F` is of the form `M G::*` for type `G` and object type `M`, in which case let `R` be `invoke_result_t<F, T&>`, `A...` be an empty pack, and `E` be `false` `true`, or
   > > 3. (6.3) — `F` is of the form `R(*)(G, A...) noexcept(E)` for type `G`.

   > > -7- Remarks: The deduced type is `function_ref<R(A...) noexcept(E)>`.

### 4426(i). Clarify what `meta::reflect_constant_string` considers a string literal

Section: 99 [meta.reflection.array] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-10-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

`meta::reflect_constant_string` says:

> Let V be the pack of values of type `CharT` whose elements are the corresponding elements of `r`, except that if `r` refers to a string literal object, then V does not include the trailing null terminator of `r`.

It's unclear how the implementation should decide whether `r` refers to a string literal. If `R` models `contiguous_iterator`, should it use `meta::is_string_literal(ranges::data(r))`? Should it omit the `'\0'` from `string_view("abc", 3)`?

Also, "null terminator" is only defined in 27.4.3.1 [basic.string.general] and not used for string literal objects (5.13.5 [lex.string]).

[2025-10-23; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Kona 2025-11-08; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 99 [meta.reflection.array] as indicated:

   >     template<ranges::input_range R>
   >       consteval info reflect_constant_string(R&& r);

   > > -2- Let `CharT` be `ranges::range_value_t<R>`.

   > > -3- Mandates: `CharT` is one of `char`, `wchar_t`, `char8_t`, `char16_t`, `char32_t`.

   > > Let V be the pack of values of type `CharT` whose elements are the corresponding elements of `r`, except that if `r` refers is a reference to a string literal object, then V does not include the trailing null terminator terminating u+0000 null character of `r`.

### 4427(i). `meta::dealias` needs to work with things that aren't entities

Section: 21.4.7 [meta.reflection.queries] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-10-24 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [meta.reflection.queries].

View all other issues in [meta.reflection.queries].

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Discussion:

Addresses US 99-205

Several uses of `dealias` assume that it can be used with reflections that represent direct base class relationships, which are not entities. The spec for `dealias` says that such uses should fail with an exception.

In the 2025-10-24 LWG telecon it was agreed that `dealias` should just be the identity function for non-entities.

[Kona 2025-11-03; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.7 [meta.reflection.queries] as indicated:

   >     consteval info dealias(info r);

   > > -49- Returns: If `r` represents an entity, then a A reflection representing the underlying entity of what `r` represents. Otherwise, `r`.

   > > [Example 5:
   > > ...
   > > — end example]

   > > -50- Throws: `meta::exception` unless `r` represents an entity.

### 4428(i). Metafunctions should not be defined in terms of constant subexpressions

Section: 21.4.10 [meta.reflection.access.queries], 21.4.13 [meta.reflection.annotation], 99 [meta.reflection.array] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-10-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [meta.reflection.access.queries].

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Discussion:

Addresses US 102-209

"is a constant (sub)expression" is incorrect now that errors are reported via exceptions.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 21.4.10 [meta.reflection.access.queries] as indicated:

   >     consteval bool has_inaccessible_nonstatic_data_members(info r, access_context ctx);

   > > -5- Returns: `true` if `is_accessible(`R`, ctx)` is `false` for any R in `nonstatic_data_members_of(r, access_context::unchecked())`. Otherwise, `false`.

   > > -6- Throws: `meta::exception` unless

   > > - (6.1) — the evaluation of `nonstatic_data_members_of(r, access_context::unchecked())` is a constant subexpression would not exit via an exception and
   > > - (6.2) — `r` does not represent a closure type.

   >     consteval bool has_inaccessible_bases(info r, access_context ctx);

   > > -5- Returns: `true` if `is_accessible(`R`, ctx)` is `false` for any R in `bases_of(r, access_context::unchecked())`. Otherwise, `false`.

   > > -6- Throws: `meta::exception` unless the evaluation of `bases_of(r, access_context::unchecked())` is a constant subexpression would not exit via an exception.

2. Modify 21.4.13 [meta.reflection.annotation] as indicated:

   >     consteval vector<info> annotations_of_with_type(info item, info type);

   > > -4- Returns: A `vector` containing each element `e` of `annotations_of(item)` where `remove_const(type_of(e)) == remove_const(type)` is `true`, preserving their order.

   > > -5- Throws: `meta::exception` unless

   > > - (5.1) — the evaluation of `annotations_of(item)` is a constant subexpression would not exit via an exception and
   > > - (5.2) — `dealias(type)` represents a type that is complete from some point in the evaluation context.

3. Modify 99 [meta.reflection.array] as indicated:

   >     template<ranges::input_range R>
   >       consteval info reflect_constant_array(R&& r);

   > > -8- Let `T` be `ranges::range_value_t<R>`.

   > > -9- Mandates: `T` is a structural type (13.2 [temp.param]), `is_constructible_v<T, ranges::range_reference_t<R>>` is `true`, and `is_copy_constructible_v<T>` is `true`.

   > > -10- Let V be the pack of values of type `info` of the same size as `r`, where the ith element is `reflect_constant(ei)`, where `ei` is the ith element of `r`.

   > > -11- Let P be

   > > - (11.1) — If `sizeof...(`V`) > 0` is `true`, then the template parameter object (13.2 [temp.param]) of type `const T[sizeof...(`V`)]` initialized with `{[:V:]...}`.
   > > - (11.2) — Otherwise, the template parameter object of type `array<T, 0>` initialized with `{}`.

   > > -12- Returns: `^^`P.

   > > -13- Throws: `meta::exception` unless the evaluation of `reflect_constant(e)` is a constant subexpression would not exit via an exception for every element `e` of `r`.

[Kona 2025-11-06; Jonathan removes change to 99 [meta.reflection.array] that was handled by LWG 4432(i).]

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.10 [meta.reflection.access.queries] as indicated:

   >     consteval bool has_inaccessible_nonstatic_data_members(info r, access_context ctx);

   > > -5- Returns: `true` if `is_accessible(`R`, ctx)` is `false` for any R in `nonstatic_data_members_of(r, access_context::unchecked())`. Otherwise, `false`.

   > > -6- Throws: `meta::exception` unless if

   > > - (6.1) — the evaluation of `nonstatic_data_members_of(r, access_context::unchecked())` is a constant subexpression would exit via an exception and or
   > > - (6.2) — `r` does not represent represents a closure type.

   >     consteval bool has_inaccessible_bases(info r, access_context ctx);

   > > -7- Returns: `true` if `is_accessible(`R`, ctx)` is `false` for any R in `bases_of(r, access_context::unchecked())`. Otherwise, `false`.

   > > -8- Throws: `meta::exception` unless if the evaluation of `bases_of(r, access_context::unchecked())` is a constant subexpression would exit via an exception.

2. Modify 21.4.13 [meta.reflection.annotation] as indicated:

   >     consteval vector<info> annotations_of_with_type(info item, info type);

   > > -4- Returns: A `vector` containing each element `e` of `annotations_of(item)` where `remove_const(type_of(e)) == remove_const(type)` is `true`, preserving their order.

   > > -5- Throws: `meta::exception` unless

   > > - (5.1) — the evaluation of `annotations_of(item)` is a constant subexpression would not exit via an exception and
   > > - (5.2) — `dealias(type)` represents a type that is complete from some point in the evaluation context.

### 4429(i). `meta::alignment_of` should exclude data member description of bit-field

Section: 21.4.12 [meta.reflection.layout] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-10-24 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 109-170

21.4.12 [meta.reflection.layout] p#8 This should similarly disallow data member descriptions of bit-fields.

[Kona 2025-11-03; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.12 [meta.reflection.layout] as indicated:

       consteval size_t alignment_of(info r);

   > -7- Returns: […]

   > -8- Throws: `meta::exception` unless all of the following conditions are met:

   > - (8.1) — `dealias(r)` is a reflection of a type, object, variable of non-reference type, non-static data member that is not a bit-field, direct base class relationship, or data member description (T,N,A,W,NUA) (11.4.1 [class.mem.general]) where W is ⊥..
   > - (8.2) — If `dealias(r)` represents a type, then `is_complete_type(r)` is true.

### 4430(i). `from_chars` should not parse `"0b"` base prefixes

Section: 28.2.3 [charconv.from.chars] Status: C++26 Submitter: Jan Schultke Opened: 2025-10-20 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [charconv.from.chars].

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Discussion:

C23 added support for the `"0b"` and `"0B"` base prefix to `strtol`, and since the wording of `from_chars` for integers is based on `strol`, this inadvertently added support for parsing `"0b"` prefixes to `from_chars`.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 28.2.3 [charconv.from.chars] as indicated:

   >     constexpr from_chars_result from_chars(const char* first, const char* last,
   >                                            integer-type& value, int base = 10);

   > > -2- Preconditions: `base` has a value between 2 and 36 (inclusive).

   > > -3- Effects: The pattern is the expected form of the subject sequence in the "C" locale for the given nonzero base, as described for `strtol`, except that no `"0b"` or `"0B"` prefix shall appear if the value of `base` is 2, no `"0x"` or `"0X"` prefix shall appear if the value of `base` is 16, and except that `'-'` is the only sign that may appear, and only if `value` has a signed type.

   > > -4- Throws: Nothing.

### 4431(i). Parallel `std::ranges::destroy` should allow exceptions

Section: 20.2.2 [memory.syn] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2025-10-24 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

The serial `std::ranges::destroy` algorithm is marked as `noexcept`. However, the parallel counterpart should not be marked `noexcept`.

While we generally don't expect any exceptions from the `destroy` algorithm when called with the standard execution policies (`seq`, `unseq`, `par`, `par_unseq`), the implementation-defined policies for parallel algorithms are allowed by the C++ standard, and it is up to the particular execution policy to decide which exceptions can be thrown from parallel algorithms.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 20.2.2 [memory.syn], header `<memory>` synopsis, as indicated:

   > [Drafting note: There are no further prototype definitions for the affected `execution-policy` overloads in 26.11.9 [specialized.destroy].]

   >     […]
   >     // 26.11.9, 26.11.9 [specialized.destroy]
   >     template<class T>
   >       constexpr void destroy_at(T* location);                                  // freestanding
   >     template<class NoThrowForwardIterator>
   >       constexpr void destroy(NoThrowForwardIterator first,                     // freestanding
   >                              NoThrowForwardIterator last);
   >     template<class ExecutionPolicy, class NoThrowForwardIterator>
   >       void destroy(ExecutionPolicy&& exec,                                     // freestanding-deleted,
   >                    NoThrowForwardIterator first,                               // see 26.3.5 [algorithms.parallel.overloads]
   >                    NoThrowForwardIterator last);
   >     template<class NoThrowForwardIterator, class Size>
   >       constexpr NoThrowForwardIterator destroy_n(NoThrowForwardIterator first, // freestanding
   >                                                  Size n);
   >     template<class ExecutionPolicy, class NoThrowForwardIterator, class Size>
   >       NoThrowForwardIterator destroy_n(ExecutionPolicy&& exec,                 // freestanding-deleted,
   >                                        NoThrowForwardIterator first, Size n);  // see 26.3.5 [algorithms.parallel.overloads]
   >     namespace ranges {
   >       template<destructible T>
   >         constexpr void destroy_at(T* location) noexcept;                       // freestanding

   >       template<nothrow-input-iterator I, nothrow-sentinel-for <I> S>
   >         requires destructible<iter_value_t<I>>
   >           constexpr I destroy(I first, S last) noexcept;                       // freestanding
   >       template<nothrow-input-range R>
   >         requires destructible<range_value_t<R>>
   >           constexpr borrowed_iterator_t<R> destroy(R&& r) noexcept;            // freestanding

   >       template<nothrow-input-iterator I>
   >         requires destructible<iter_value_t<I>>
   >         constexpr I destroy_n(I first, iter_difference_t<I> n) noexcept;       // freestanding

   >       template<execution-policy Ep, nothrow-random-access-iterator I,
   >                nothrow-sized-sentinel-for <I> S>
   >         requires destructible<iter_value_t<I>>
   >           I destroy(Ep&& exec, I first, S last) noexcept;                       // freestanding-deleted,
   >                                                                                 // see 26.3.5 [algorithms.parallel.overloads]
   >       template<execution-policy Ep, nothrow-sized-random-access-range R>
   >         requires destructible<range_value_t<R>>
   >           borrowed_iterator_t<R> destroy(Ep&& exec, R&& r) noexcept;            // freestanding-deleted,
   >                                                                                 // see 26.3.5 [algorithms.parallel.overloads]
   >       template<execution-policy Ep, nothrow-random-access-iterator I>
   >         requires destructible<iter_value_t<I>>
   >           I destroy_n(Ep&& exec, I first, iter_difference_t<I> n) noexcept;     // freestanding-deleted,
   >                                                                                 // see 26.3.5 [algorithms.parallel.overloads]
   >     }
   >     […]

### 4432(i). Clarify element initialization for `meta::reflect_constant_array`

Section: 21.4.3 [meta.define.static] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-10-27 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 120-181 and US 121-182

99 [meta.reflection.array] p10 Clarify ei type. It is not clear what ei is when proxy references are involved.

99 [meta.reflection.array] Clarify copy-initialization vs. direct-initialization use The initialization of `P` uses copy-initialization but the Mandates clause uses direct-initialization.

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 21.4.3 [meta.define.static] as indicated:

       template<ranges::input_range R>
         consteval info reflect_constant_array(R&& r);

   > -8- Let `T` be `ranges::range_value_t<R>`.

   > -9- Mandates: `T` is a structural type (13.2 [temp.param]), `is_constructible_v<T, ranges::range_reference_t<R>>` is `true`, and `is_copy_constructible_v<T>` is `true` `T` satisfies `copy_constructible`.

   > -10- Let `V` be the pack of values of type info of the same size as `r`, where the ith element is `reflect_constant(eistatic_cast<T>(*iti))`, where eiiti is an iterator to the ith element of `r`.

   > […]

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.3 [meta.define.static] as indicated:

       template<ranges::input_range R>
         consteval info reflect_constant_array(R&& r);

   > -8- Let `T` be `ranges::range_value_t<R>` and ei be `static_cast<T>(*iti)`, where iti is an iterator to the ith element of `r`.

   > -9- Mandates: `T` is a structural type (13.2 [temp.param]), `is_constructible_v<T, ranges::range_reference_t<R>>` is `true`, and `is_copy_constructible_v<T>` is `true` `T` satisfies `copy_constructible`.

   > -10- Let `V` be the pack of values of type `info` of the same size as `r`, where the ith element is `reflect_constant(ei)`, where ei is an iterator to the ith element of `r`.

   > […]

   > -13- Throws: Any exception thrown by the evaluation of any ei, or `meta::exception` unlessif evaluation of any `reflect_constant(eei)` would exit via an exception is a constant subexpression for every element `e` of `r`.

### 4433(i). Incorrect query for C language linkage

Section: 21.4.7 [meta.reflection.queries] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-10-27 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [meta.reflection.queries].

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Discussion:

Addresses US 97-203

21.4.7 [meta.reflection.queries] Language linkage is a property of functions, variables, and function types (6.7 [basic.link]), not of names.

[ The wording below contains a drive-by fix for a misapplication of P2996R13 ]

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.7 [meta.reflection.queries] as indicated:

       consteval bool has_internal_linkage(info r);
       consteval bool has_module_linkage(info r);
       consteval bool has_external_linkage(info r);
       consteval bool has_c_language_linkage(info r);
       consteval bool has_linkage(info r);

   > -25- Returns: `true` if `r` represents a variable, function, type, template, or namespace whose name has internal linkage, module linkage, C languageexternal linkage, or any linkage, respectively (6.7 [basic.link]). Otherwise, `false`.

       consteval bool has_c_language_linkage(info r);

   > -??- Returns: `true` if `r` represents a variable, function, or function type with C language linkage. Otherwise, `false`.

### 4434(i). `meta::is_accessible` does not need to consider incomplete D

Section: 21.4.10 [meta.reflection.access.queries] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-10-27 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [meta.reflection.access.queries].

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Discussion:

21.4.10 [meta.reflection.access.queries] says that `is_accessible(r, ctx)` throws if:

> `r` represents a direct base class relationship (D,B) for which D is incomplete.

However, the only way to get access to a direct base relationship is through `bases_of`/`subobjects_of` and those throw if the class is incomplete, so I don't see how an `is_base` reflection could have ever incomplete D.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.10 [meta.reflection.access.queries] as indicated:

   > -4- Throws: `meta::exception` if:

   > - (4.1) — `r` represents a class member for which `PARENT-CLS(r)` is an incomplete class or.
   > - (4.2) — `r` represents a direct base class relationship (D,B) for which D is incomplete.

### 4435(i). `meta::has_identifier` doesn't handle all types

Section: 21.4.6 [meta.reflection.names] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-10-27 Last modified: 2026-06-09

Priority: 2

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Discussion:

The wording for `meta::has_identifier` doesn't specify what it returns for `^^int` or `^^void` or `^^Enum`.

[2025-10-29; Reflector poll.]

Set priority to 2 after reflector poll.

Move bullet point for aliases before bullet for types. Add "cv-unqualified" to class type and enumeration type. Simplify "`!has_template_arguments()` is `true`" to "`has_template_arguments()` is `false`".

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.6 [meta.reflection.names] as indicated:

   >     consteval bool has_identifier(info r);

   > > -1- Returns:

   > > - (1.1) — If `r` represents an entity that has a typedef name for linkage purposes (9.2.4 [dcl.typedef]), then `true`.
   > > - (1.2) — Otherwise, if `r` represents an unnamed entity, then `false`.
   > > - (1.?) — Otherwise, if `r` represents a type alias, then `!has_template_arguments(r)`.
   > > - (1.3) — Otherwise, if `r` represents a type, then `true` if class type, then `!has_template_arguments(r)`.
   > >   - (1.3.1) — `r` represents a cv-unqualified class type and `has_template_arguments(r)` is `false`, or
   > >   - (1.3.2) — `r` represents a cv-unqualified enumeration type.
   > >   Otherwise, `false`.
   > > - (1.4) — Otherwise, if `r` represents a function, then `true` if `has_template_arguments(r)` is `false` and the function is not a constructor, destructor, operator function, or conversion function. Otherwise, `false`.
   > > - (1.5) — Otherwise, if `r` represents a template, then `true` if `r` does not represent a constructor template, operator function template, or conversion function template. Otherwise, `false`.
   > > - (1.6) — Otherwise, if `r` represents the ith parameter of a function F that is an (implicit or explicit) specialization of a templated function T and the ith parameter of the instantiated declaration of T whose template arguments are those of F would be instantiated from a pack, then `false`.
   > > - (1.7) — Otherwise, if `r` represents the parameter P of a function F, then let S be the set of declarations, ignoring any explicit instantiations, that precede some point in the evaluation context and that declare either F or a templated function of which F is a specialization; `true` if
   > >   - (1.7.1) — there is a declaration D in S that introduces a name N for either P or the parameter corresponding to P in the templated function that D declares and
   > >   - (1.7.2) — no declaration in S does so using any name other than N.
   > >   Otherwise, `false`.
   > > - (1.8) — Otherwise, if `r` represents a variable, then `false` if the declaration of that variable was instantiated from a function parameter pack. Otherwise, `!has_template_arguments(r)`.
   > > - (1.9) — Otherwise, if `r` represents a structured binding, then `false` if the declaration of that structured binding was instantiated from a structured binding pack. Otherwise, `true`.
   > > - (1.10) — Otherwise, if `r` represents a type alias, then `!has_template_arguments(r)`.
   > > - (1.11) — Otherwise, if `r` represents an enumerator, non-static-data member, namespace, or namespace alias, then `true`.
   > > - (1.12) — Otherwise, if `r` represents a direct base class relationship, then `has_identifier(type_of(r))`.
   > > - (1.13) — Otherwise, `r` represents a data member description (T,N,A,W,NUA) (11.4.1 [class.mem.general]); `true` if N is not ⊥. Otherwise, `false`.

### 4436(i). `simd` broadcast is overconstrained — `std::cw<0.f>` is not convertible to `simd::vec<float16_t>`

Section: 29.10.7.2 [simd.ctor] Status: Resolved Submitter: Matthias Kretz Opened: 2025-10-28 Last modified: 2026-03-31

Priority: 1

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Discussion:

29.10.7.2 [simd.ctor] p4 requires all implicit conversions via broadcast constructor to satisfy `convertible_to<value_type>`. However, `std::cw<0.f>` is not convertible to `float16_t` because `float` does not satisfy `convertible_to<float16_t>`. Consequently, with the current wording we don't arrive at bullet (4.3) which allows not-value-preserving conversions if the value is known to not change on conversion. The `convertible_to` constraint is implicit in bullet (4.2), not needed/wrong for bullet (4.3) and thus needs to be moved into bullet (4.1).

[2026-01-16; Reflector poll.]

Set priority to 1 after reflector poll.

"Resolution should be rebased on LWG 4420(i), constraint was already changed to not use `constructible_from`. Might as well drive-by fix 4.3 to use `remove_cvref_t` so it works for `constant_wrapper`.

[Croydon 2026-03-25; Resolved by P4012R1.]

[2026-03-31 P4012R1 was approved in Croydon. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.2 [simd.ctor] as indicated:

   >     template<class U> constexpr explicit(see below) basic_vec(U&& value) noexcept;

   > > -1- Let `From` denote the type `remove_cvref_t<U>`.

   > > -2- Constraints: `value_type` satisfies `constructible_from<U>`.

   > > -3- Effects: Initializes each element to the value of the argument after conversion to `value_type`.

   > > -4- Remarks: The expression inside explicit evaluates to `false` if and only if `U` satisfies `convertible_to<value_type>`, and either
   > > - (4.1) — `U` satisfies `convertible_to<value_type>` and `From` is not an arithmetic type and does not satisfy `constexpr-wrapper-like`,
   > > - (4.2) — `From` is an arithmetic type and the conversion from `From` to `value_type` is value-preserving (29.10.1 [simd.general]), or
   > > - (4.3) — `From` satisfies `constexpr-wrapper-like`, `remove_const_t<decltype(From::value)>` is an arithmetic type, and `From::value` is representable by `value_type`.

### 4437(i). `constant_of(^^v)` for variable `v` of array type produces reflection of pointer constant

Section: 21.4.7 [meta.reflection.queries] Status: Resolved Submitter: Tomasz Kamiński Opened: 2025-10-29 Last modified: 2025-11-11

Priority: Not Prioritized

View other active issues in [meta.reflection.queries].

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Discussion:

The unintended consequence of the late change of `reflect_constant` to accept its parameter by value is that `constant_of(a)` returns a reflection of the value `&a[0]` for a variable of array type.

For reference, `constant_of` is specified as:

>     if constexpr (is_annotation(R))  {
>       return C;
>     } else {
>       return reflect_constant([: R :]);
>     }

In case when `[: R :]` is a reference to array, it will decay to a pointer to the first element when accepted by value. I believe this is unintended and we should return an reflection of an array object instead.

[Kona 2025-11-07; Will be resolved by CWG 3111.]

[2025-11-11; Resolved by CWG 3111, approved in Kona. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.7 [meta.reflection.queries] as indicated:

   >     consteval info constant_of(info r);

   > > -6 Let `R` be a constant expression of type `info` such that `R == r` is `true`. If `r` represents an annotation, then let `C` be its underlying constant.

   > > -7- Effects: Equivalent to:

   > > >     if constexpr (is_annotation(R)) {
   > > >       return C;
   > > >     } else if constexpr (is_array_type(type_of(R))) {
   > > >       return reflect_constant_array([: R :]);
   > > >     } else {
   > > >       return reflect_constant([: R :]);
   > > >     }

### 4438(i). Bad expression in [exec.when.all]

Section: 33.9.12.12 [exec.when.all] Status: C++26 Submitter: Eric Niebler Opened: 2025-10-30 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 220-344 and US 224-342

33.9.12.12 [exec.when.all] p5 reads as follows:

> -5- Let `make-when-all-env` be the following exposition-only function template:

> >     template<class Env>
> >       constexpr auto make-when-all-env(inplace_stop_source& stop_src,        // exposition only
> >                                        Env&& env) noexcept {
> >       return see below;
> >     }

> Returns an object `e` such that

> - (5.1) — `decltype(e)` models `queryable`, and
> - (5.2) — `e.query(get_stop_token)` is expression-equivalent to `state.stop-src.get_token()`, and
> - (5.3) — given a query object `q` with type other than cv `stop_token_t` and whose type satisfies `forwarding-query`, `e.query(q)` is expression-equivalent to `get_env(rcvr).query(q)`.

The problem is with "`state.stop-src.get_token()`" in bullet (5.2). There is no `state` object here. This expression should be `stop_src.get_token()`.

[Kona 2025-11-04; add edits to address NB comments.]

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.12.12 [exec.when.all] as indicated:

   > -5- Let `make-when-all-env` be the following exposition-only function template:

   > >     template<class Env>
   > >       constexpr auto make-when-all-env(inplace_stop_source& stop_src,        // exposition only
   > >                                        Env&& env) noexcept; {
   > >       return see below;
   > >     }

   > -?- Returns: An Returns and object `e` such that

   > - (5.1) — `decltype(e)` models `queryable`, and
   > - (5.2) — `e.query(get_stop_token)` is expression-equivalent to `state.stop-srcstop_src.get_token()`, and
   > - (5.3) — given a query object `q` with type other than cv `get_stop_token_t` and whose type satisfies `forwarding-query`, `e.query(q)` is expression-equivalent to `get_env(rcvr)``env``.query(q)` if the type of `q` satisfies `forwarding-query`, and ill-formed otherwise.

### 4439(i). `std::optional<T&>::swap` possibly selects ADL-found `swap`

Section: 22.5.4.4 [optional.ref.swap] Status: C++26 Submitter: Jiang An Opened: 2025-10-31 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Currently, 22.5.4.4 [optional.ref.swap] p1 specifies an "unqualified" `swap` call, which possibly selects an ADL-found swap function due to 16.4.2.2 [contents] and 16.4.4.3 [swappable.requirements].

It's unlike to be intentional to call ADL-found `swap` on pointers (given `ranges::swap` doesn't), and the unconditional `noexcept` also suggests that user-provided `swap` functions shouldn't interfere with `optional<T&>::swap`.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.4 [optional.ref.swap] as indicated:

   >     constexpr void swap(optional& rhs) noexcept;

   > > -1- Effects: Equivalent to: `std::swap(val, rhs.val)`.

### 4440(i). Forward declarations of entities need also in entries

Section: 17.3.2 [version.syn] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-03 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 65-116

There are forward declarations of entities from `<spanstream>` and `<syncstream>` in `<iosfwd>` so their feature macros should be added to that header too. Proposed change: Add `<iosfwd>` to the "also in" entries for `__cpp_lib_char8_t`, `__cpp_lib_spanstream`, and `__cpp_lib_syncbuf`.

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 17.3.2 [version.syn] as indicated:

   >     #define __cpp_lib_char8_t                           201907L
   >       // freestanding, also in <atomic>, <filesystem>, <iosfwd>, <istream>, <limits>, <locale>, <ostream>, <string>,
   >       // <string_view>
   >     […]
   >     #define __cpp_lib_spanstream                        202106L // also in <iosfwd>, <spanstream>
   >     […]
   >     #define __cpp_lib_syncbuf                           201803L // also in <iosfwd>, <syncstream>

### 4441(i). `ranges::rotate` do not handle sized-but-not-sized-sentinel ranges correctly

Section: 26.7.11 [alg.rotate], 26.8.2.3 [partial.sort], 26.8.3 [alg.nth.element], 26.8.6 [alg.merge] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-03 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [alg.rotate].

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Discussion:

Addresses US 161-258

These do not handle sized-but-not-sized-sentinel ranges correctly.

[Kona 2025-11-03; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.7.11 [alg.rotate] as indicated:

       template<execution-policy Ep, sized-random-access-range R>
         requires permutable<iterator_t<R>>
         borrowed_subrange_t<R> ranges::rotate(Ep&& exec, R&& r, iterator_t<R> middle);

   > -6- Effects Equivalent to: `return ranges::rotate(std::forward<Ep>(exec), ranges::begin(r), middle, ranges::end(r)ranges::begin(r) + ranges::distance(r));`

   […]

       template<execution-policy Ep, sized-random-access-range R, sized-random-access-range OutR>
         requires indirectly_copyable<iterator_t<R>, iterator_t<OutR>>
         ranges::rotate_copy_truncated_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
       	  ranges::rotate_copy(Ep&& exec, R&& r, iterator_t<R> middle, OutR&& result_r);

   > -18- Effects Equivalent to: `return ranges::rotate(std::forward<Ep>(exec), ranges::begin(r), middle, ranges::end(r)ranges::begin(r) + ranges::distance(r), ranges::begin(result_r), ranges::end(result_r)ranges::begin(result_r) + ranges::distance(result_r));`

2. Modify 26.8.2.3 [partial.sort] as indicated:

       template<execution-policy Ep, sized-random-access-range R,
                class Comp = ranges::less, class Proj = identity>
         requires sortable<iterator_t<R>, Comp, Proj>
         borrowed_iterator_t<R>
           ranges::partial_sort(Ep&& exec, R&& r, iterator_t<R> middle, Comp comp = {},
                                Proj proj = {});

   > -7- Effects Equivalent to: `return ranges::partial_sort(std::forward<Ep>(exec), ranges::begin(r), middle, ranges::end(r)ranges::begin(r) + ranges::distance(r), comp, proj);`

3. Modify 26.8.3 [alg.nth.element] as indicated:

       template<execution-policy Ep, sized-random-access-range R, class Comp = ranges::less,
                class Proj = identity>
         requires sortable<iterator_t<R>, Comp, Proj>
         borrowed_iterator_t<R>
           ranges::nth_element(Ep&& exec, R&& r, iterator_t<R> nth, Comp comp = {}, Proj proj = {});

   > -7- Effects Equivalent to: `return ranges::nth_element(std::forward<Ep>(exec), ranges::begin(r), nth, ranges::end(r)ranges::begin(r) + ranges::distance(r), comp, proj);`

4. Modify 26.8.6 [alg.merge] as indicated:

       template<execution-policy Ep, sized-random-access-range R, class Comp = ranges::less,
                class Proj = identity>
         requires sortable<iterator_t<R>, Comp, Proj>
         borrowed_iterator_t<R>
           ranges::inplace_merge(Ep&& exec, R&& r, iterator_t<R> middle, Comp comp = {},
                                 Proj proj = {});

   > -14- Effects Equivalent to: `return ranges::inplace_merge(std::forward<Ep>(exec), ranges::begin(r), middle, ranges::end(r)ranges::begin(r) + ranges::distance(r), comp, proj);`

### 4442(i). Clarify `expr` and `fn` for `meta::reflect_object` and `meta::reflect_function`

Section: 21.4.16 [meta.reflection.result] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-04 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 118-179

This should talk about the object/function designated by `expr`/`fn`, rather than `expr`/`fn`.

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.16 [meta.reflection.result] as indicated:

       template<class T>
         consteval info reflect_object(T& expr);

   > -7- Mandates: `T` is an object type.

   > -8- Returns: A reflection of the object designated by `expr`.

   > -9- Throws: `meta::exception` unless `expr` is if `E` is not suitable for use as a constant template argument for a constant template parameter of type `T&` (13.4.3 [temp.arg.nontype]) , where `E` is an lvalue constant expression that computes the object that `expr` refers to.

       template<class T>
         consteval info reflect_function(T& fn);

   > -10- Mandates: `T` is an function type.

   > -11- Returns: A reflection of the function designated by `fn`.

   > -12- Throws: `meta::exception` unless `fn` is if `F` is not suitable for use as a constant template argument for a constant template parameter of type `T&` (13.4.3 [temp.arg.nontype]) , where `F` is an lvalue constant expression that computes the function that `fn` refers to.

### 4443(i). Clean up identifier comparisons in `meta::define_aggregate`

Section: 21.4.17 [meta.reflection.define.aggregate] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-04 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [meta.reflection.define.aggregate].

View all issues with C++26 status.

Discussion:

Addresses US 127-190

NK is defined as an identifier (see 11.4.1) and should not be compared with code or with string literals in bullet 8.4. Similarly, 9.5.1 should not talk about “character sequence encoded by NK”

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.17 [meta.reflection.define.aggregate] as indicated:

       template<reflection_range R = initializer_list<info>>
         consteval info define_aggregate(info class_type, R&& mdescrs);

   > -7- […]

   > -8- Constant When:

   > - -8.1- […]
   > - -8.2- […]
   > - -8.3- […]
   > - -8.4- for every pair (rK, rL) where K<L, if NK is not ⊥ and NL is not ⊥, then either:
   >   - -8.4.1- NK`!=` NL is `true` NK is not the same identifier as NL or
   >   - -8.4.2- NK`== u8"_"` is `true` NK is the identifier `_` (u+005f low line).

   > -9- Effects: Produces an injected declaration D (7.7 [expr.const]) that defines C and has properties as follows:

   > - -9.1- […]
   > - -9.2- […]
   > - -9.3- […]
   > - -9.4- […]
   > - -9.5- for every rK, there is corresponding entity MK belonging to the class scope of D with the following properties: K<L, if NK is not ⊥ and NL is not ⊥, then either:
   >   - -9.5.1- if NK is ⊥, MK is an unnamed bit-field. Otherwise, MK is a non-static data member whose name is the identifier determined by the character sequence encoded by NK in UTF-8.
   >   - -9.5.2- […]
   >   - -9.5.3- […]
   >   - -9.5.4- […]
   >   - -9.5.5- […]
   > - -9.6- […]

### 4444(i). Fix default template arguments for `ranges::replace` and `ranges::replace_if`

Section: 26.7.5 [alg.replace], 26.4 [algorithm.syn] Status: C++26 Submitter: Tim Song Opened: 2025-11-04 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [alg.replace].

View all issues with C++26 status.

Discussion:

Addresses US 159-259

The default template argument for the type of the new value in `ranges::replace` and `ranges::replace_if` should not have projections applied.

[Kona 2025-11-04; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.4 [algorithm.syn], header `<algorithm>` synopsis, as indicated:

   >     […]
   >       namespace ranges {
   >         template<input_iterator I, sentinel_for<I> S, class Proj = identity,
   >                  class T1 = projected_value_t<I, Proj>, class T2 = T1iter_value_t<I>>
   >           requires indirectly_writable<I, const T2&> &&
   >                    indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*>
   >           constexpr I
   >             replace(I first, S last, const T1& old_value, const T2& new_value, Proj proj = {});
   >         template<input_range R, class Proj = identity,
   >                  class T1 = projected_value_t<iterator_t<R>, Proj>, class T2 = T1range_value_t<R>>
   >           requires indirectly_writable<iterator_t<R>, const T2&> &&
   >                    indirect_binary_predicate<ranges::equal_to,
   >                                              projected<iterator_t<R>, Proj>, const T1*>
   >           constexpr borrowed_iterator_t<R>
   >             replace(R&& r, const T1& old_value, const T2& new_value, Proj proj = {});

   >         template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >                  class Proj = identity, class T1 = projected_value_t<I, Proj>, class T2 = T1iter_value_t<I>>
   >           requires indirectly_writable<I, const T2&> &&
   >                    indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*>
   >           I replace(Ep&& exec, I first, S last,
   >                     const T1& old_value, const T2& new_value, Proj proj = {});  // freestanding-deleted
   >         template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >                  class T1 = projected_value_t<iterator_t<R>, Proj>, class T2 = T1range_value_t<R>>
   >           requires indirectly_writable<iterator_t<R>, const T2&> &&
   >                    indirect_binary_predicate<ranges::equal_to,
   >                                              projected<iterator_t<R>, Proj>, const T1*>
   >           borrowed_iterator_t<R>
   >             replace(Ep&& exec, R&& r, const T1& old_value, const T2& new_value,
   >                     Proj proj = {});                                            // freestanding-deleted

   >         template<input_iterator I, sentinel_for<I> S, class Proj = identity,
   >                  class T = projectediter_value_t<I, Proj>,
   >                  indirect_unary_predicate<projected<I, Proj>> Pred>
   >           requires indirectly_writable<I, const T&>
   >           constexpr I replace_if(I first, S last, Pred pred, const T& new_value, Proj proj = {});
   >         template<input_range R, class Proj = identity, class T = projected_value_t<Irange_value_t<R, Proj>,
   >                  indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >           requires indirectly_writable<iterator_t<R>, const T&>
   >           constexpr borrowed_iterator_t<R>
   >             replace_if(R&& r, Pred pred, const T& new_value, Proj proj = {});

   >         template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >                  class Proj = identity, class T = projectediter_value_t<I, Proj>,
   >                  indirect_unary_predicate<projected<I, Proj>> Pred>
   >           requires indirectly_writable<I, const T&>
   >           I replace_if(Ep&& exec, I first, S last, Pred pred,
   >                        const T& new_value, Proj proj = {});         // freestanding-deleted
   >         template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >                  class T = projected_value_t<iterator_t<R>range_value_t<R, Proj>,
   >                  indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >           requires indirectly_writable<iterator_t<R>, const T&>
   >           borrowed_iterator_t<R>
   >             replace_if(Ep&& exec, R&& r, Pred pred, const T& new_value,
   >                        Proj proj = {});                             // freestanding-deleted
   >       }
   >     […]

2. Modify 26.7.5 [alg.replace] as indicated:

   >     […]
   >     template<input_iterator I, sentinel_for<I> S, class Proj = identity,
   >              class T1 = projected_value_t<I, Proj>, class T2 = T1iter_value_t<I>>
   >       requires indirectly_writable<I, const T2&> &&
   >                indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*>
   >       constexpr I
   >         ranges::replace(I first, S last, const T1& old_value, const T2& new_value, Proj proj = {});
   >     template<input_range R, class Proj = identity,
   >              class T1 = projected_value_t<iterator_t<R>, Proj>, class T2 = T1range_value_t<R>>
   >       requires indirectly_writable<iterator_t<R>, const T2&> &&
   >                indirect_binary_predicate<ranges::equal_to,
   >                                          projected<iterator_t<R>, Proj>, const T1*>
   >       constexpr borrowed_iterator_t<R>
   >         ranges::replace(R&& r, const T1& old_value, const T2& new_value, Proj proj = {});

   >     template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >              class Proj = identity, class T1 = projected_value_t<I, Proj>, class T2 = T1iter_value_t<I>>
   >       requires indirectly_writable<I, const T2&> &&
   >                indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*>
   >       I ranges::replace(Ep&& exec, I first, S last,
   >                         const T1& old_value, const T2& new_value, Proj proj = {});
   >     template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >              class T1 = projected_value_t<iterator_t<R>, Proj>, class T2 = T1range_value_t<R>>
   >       requires indirectly_writable<iterator_t<R>, const T2&> &&
   >                indirect_binary_predicate<ranges::equal_to,
   >                                          projected<iterator_t<R>, Proj>, const T1*>
   >       borrowed_iterator_t<R>
   >         ranges::replace(Ep&& exec, R&& r, const T1& old_value, const T2& new_value,
   >                         Proj proj = {});

   >     template<input_iterator I, sentinel_for<I> S, class Proj = identity,
   >              class T = projectediter_value_t<I, Proj>,
   >              indirect_unary_predicate<projected<I, Proj>> Pred>
   >       requires indirectly_writable<I, const T&>
   >       constexpr I ranges::replace_if(I first, S last, Pred pred, const T& new_value, Proj proj = {});
   >     template<input_range R, class Proj = identity, class T = projected_value_t<Irange_value_t<R, Proj>,
   >              indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires indirectly_writable<iterator_t<R>, const T&>
   >       constexpr borrowed_iterator_t<R>
   >         ranges::replace_if(R&& r, Pred pred, const T& new_value, Proj proj = {});

   >     template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >              class Proj = identity, class T = projectediter_value_t<I, Proj>,
   >              indirect_unary_predicate<projected<I, Proj>> Pred>
   >       requires indirectly_writable<I, const T&>
   >       I ranges::replace_if(Ep&& exec, I first, S last, Pred pred,
   >                            const T& new_value, Proj proj = {});
   >     template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >              class T = projected_value_t<iterator_t<R>range_value_t<R, Proj>,
   >              indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires indirectly_writable<iterator_t<R>, const T&>
   >       borrowed_iterator_t<R>
   >         ranges::replace_if(Ep&& exec, R&& r, Pred pred, const T& new_value,
   >                            Proj proj = {});

   > > -1- […]

### 4445(i). `sch_` must not be in moved-from state

Section: 33.13.5 [exec.task.scheduler] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [exec.task.scheduler].

View all issues with C++26 status.

Discussion:

Addresses US 239-367

As specified, there is an implicit precondition that `sch_` is not moved from on all the member functions. If that is intended, the precondition should be made explicit.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.13.5 [exec.task.scheduler] as indicated:

       namespace std::execution {
         class task_scheduler {
           class ts-sender;                    // exposition only

           template<receiver R>
             class state;                      // exposition only

         public:
           using scheduler_concept = scheduler_t;

           template<class Sch, class Allocator = allocator<void>>
             requires (!same_as<task_scheduler, remove_cvref_t<Sch>>)
               && scheduler<Sch>
           explicit task_scheduler(Sch&& sch, Allocator alloc = {});

           task_scheduler(const task_scheduler&) = default;
           task_scheduler& operator=(const task_scheduler&) = default;

           ts-sender schedule();

           friend bool operator==(const task_scheduler& lhs, const task_scheduler& rhs)
               noexcept;
           template<class Sch>
             requires (!same_as<task_scheduler, Sch>)
             && scheduler<Sch>
           friend bool operator==(const task_scheduler& lhs, const Sch& rhs) noexcept;

         private:
           shared_ptr<void> sch_; // exposition only
         };
       }

### 4446(i). Bad phrasing for `SCHED(s)`

Section: 33.13.5 [exec.task.scheduler] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [exec.task.scheduler].

View all issues with C++26 status.

Discussion:

Addresses US 240-370

`shared_ptr` owns a pointer (or `nullptr_t`), not the pointee, but `SCHED` wants the pointee.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.13.5 [exec.task.scheduler] as indicated:

   > -1- `task_scheduler` is a class that models `scheduler` (33.6 [exec.sched]). Given an object `s` of type `task_scheduler`, let `SCHED` be the object pointed to by the pointer owned by `s.sch_`.

### 4447(i). Remove unnecessary `sizeof…(Env) > 1` condition

Section: 33.4 [execution.syn] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [execution.syn].

View all issues with C++26 status.

Discussion:

Addresses US 206-325

The “`sizeof…(Env) &gt; 1` is `true`” part seems unreachable because `CS` is ill-formed in that case.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.4 [execution.syn] as indicated:

   > -2- For type `Sndr` and pack of types `Env`, let `CS` be `completion_signatures_of_t&lt;Sndr, Env...&gt;`. Then `single-sender-value-type&lt;Sndr, Env...&gt;` is ill-formed if `CS` is ill-formed or if `sizeof...(Env) > 1` is `true`; otherwise, it is an alias for: […]

### 4448(i). Do not forward `fn` in `completion_signatures`

Section: 33.10 [exec.cmplsig] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 230-360

`fn` can be called multiple times and therefore should not be forwarded.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.10 [exec.cmplsig] as indicated:

   -8-

       namespace std::execution {
         template<completion-signature... Fns>
           struct completion_signatures {
             template<class Tag>
               static constexpr size_t count-of(Tag) { return see below; }

             template<class Fn>
               static constexpr void for-each(Fn&& fn) {               // exposition only
                 (std::forward<Fn>(fn)fn(static_cast<Fns*>(nullptr)), ...);
               }
           };

         […]
       }

### 4449(i). `define_aggregate` members must be public

Section: 21.4.17 [meta.reflection.define.aggregate] Status: C++26 Submitter: Daniel Katz Opened: 2025-11-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [meta.reflection.define.aggregate].

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Discussion:

The access of members of classes defined by injected declarations produced by evaluations of `std::meta::define_aggregate` is unspecified.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 21.4.17 [meta.reflection.define.aggregate] as indicated:

   >     template<reflection_range R = initializer_list<info>>
   >       consteval info define_aggregate(info class_type, R&& mdescrs);

   > > -7- Let `C` be the class represented by `class_type` and `rK` be the `K`th reflection value in `mdescrs`. […]

   > > -8- Constant When: […]

   > > -9- Effects: Produces an injected declaration `D` (7.7 [expr.const]) that defines `C` and has properties as follows:
   > > - (9.1) — […]
   > > - (9.2) — […]
   > > - (9.3) — […]
   > > - (9.4) — […]
   > > - (9.5) — For each `rK`, there is a corresponding entity `MK` with public access belonging to the class scope of `D` with the following properties: […]
   > > - (9.6) — […]

### 4450(i). `std::atomic_ref<T>::store_key` should be disabled for const `T`

Section: 32.5.7.3 [atomics.ref.int] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 193-311

The new `store_key` functions modify the object, so it can't be const.

[Kona 2025-11-05; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 32.5.7.3 [atomics.ref.int], as indicated:

   >     constexpr void store_key(value_type operand,
   >                              memory_order order = memory_order::seq_cst) const noexcept;

   > > -?- Constraints: `is_const_v<integral-type>` is `false`.

   > > -10- Preconditions: `order` is `memory_order::relaxed`, `memory_order::release`, or `memory_order::seq_cst`.

   > > -11- Effects: Atomically replaces the value referenced by `*ptr` with the result of the computation applied to the value referenced by `*ptr` and the given `operand`. Memory is affected according to the value of `order`. These operations are atomic modify-write operations (32.5.4 [atomics.order]).

2. Modify 32.5.7.4 [atomics.ref.float], as indicated:

   >     constexpr void store_key(value_type operand,
   >                              memory_order order = memory_order::seq_cst) const noexcept;

   > > -?- Constraints: `is_const_v<floating-point-type>` is `false`.

   > > -10- Preconditions: `order` is `memory_order::relaxed`, `memory_order::release`, or `memory_order::seq_cst`.

   > > -11- Effects: Atomically replaces the value referenced by `*ptr` with the result of the computation applied to the value referenced by `*ptr` and the given `operand`. Memory is affected according to the value of `order`. These operations are atomic modify-write operations (32.5.4 [atomics.order]).

3. Modify 32.5.7.5 [atomics.ref.pointer], as indicated:

   >     constexpr void store_key(see above operand,
   >                              memory_order order = memory_order::seq_cst) const noexcept;

   > > -?- Constraints: `is_const_v<pointer-type>` is `false`.

   > > -11- Mandates: `T` is a complete object type.

   > > -12- Preconditions: `order` is `memory_order::relaxed`, `memory_order::release`, or `memory_order::seq_cst`.

   > > -13- Effects: Atomically replaces the value referenced by `*ptr` with the result of the computation applied to the value referenced by `*ptr` and the given `operand`. Memory is affected according to the value of `order`. These operations are atomic modify-write operations (32.5.4 [atomics.order]).

### 4451(i). `make_shared` should not refer to a type `U[N]` for runtime N

Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [util.smartptr.shared.create].

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Discussion:

Addresses US 76-139

The overloads of `make_shared` and `allocate_shared` for creating `shared_ptr<T[]>` refer to an object a type `U[N]` where `N` is a function parameter not a constant expression. Since `N` is allowed to be zero, this also allows `U[0]` which is an invalid type and so totally ill-formed.

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 20.3.2.2.7 [util.smartptr.shared.create], as indicated:

   >     template<class T>
   >       constexpr shared_ptr<T> make_shared(size_t N); // T is U[]
   >     template<class T, class A>
   >       constexpr shared_ptr<T> allocate_shared(const A& a, size_t N); // T is U[]

   > > -12- Constraints: `T` is of the form `U[]` an array of unknown bound.

   > > -13- Returns: A `shared_ptr` to an object of type `U[N]` array of `N` elements of type `remove_extent_t<T>` with a default initial value, where `U` is `remove_extent_t<T>`.

   > > -14- [Example 2: ...]

   >     template<class T>
   >       constexpr shared_ptr<T> make_shared(); // T is U[N]
   >     template<class T, class A>
   >       constexpr shared_ptr<T> allocate_shared(const A& a); // T is U[N]

   > > -15- Constraints: `T` is of the form `U[N]` an array of known bound.

   > > -16- Returns: A `shared_ptr` to an object of type `T` with a default initial value.

   > > -17- [Example 3: ...]

   >     template<class T>
   >       constexpr shared_ptr<T> make_shared(size_t N,
   >                                              const remove_extent_t<T>& u); // T is U[]
   >     template<class T, class A>
   >       constexpr shared_ptr<T> allocate_shared(const A& a, size_t N,
   >                                                  const remove_extent_t<T>& u); // T is U[]

   > > -18- Constraints: `T` is of the form `U[]` an array of unknown bound.

   > > -19- Returns: A `shared_ptr` to an object of type `U[N]` array of `N` elements of type `remove_extent_t<T>` where `U` is `remove_extent_t<T>` and each array element has an initial value of `u`.

   > > -20- [Example 4: ...]

   >     template<class T>
   >       constexpr shared_ptr<T> make_shared(const remove_extent_t<T>& u); // T is U[N]
   >     template<class T, class A>
   >       constexpr shared_ptr<T> allocate_shared(const A& a,
   >                                           const remove_extent_t<T>& u); // T is U[N]

   > > -21- Constraints: `T` is of the form `U[N]` an array of known bound.

   > > -22- Returns: A `shared_ptr` to an object of type `T`, where each array element of type `remove_extent_t<T>` has an initial value of `u`.

   > > -23- [Example 5: ...]

   >     template<class T>
   >       constexpr shared_ptr<T> make_shared_for_overwrite(); // T is U[N]
   >     template<class T, class A>
   >       constexpr shared_ptr<T> allocate_shared_for_overwrite(const A& a); // T is U[N]

   > > -24- Constraints: `T` is not an array of unknown bound.

   > > -25- Returns: A `shared_ptr` to an object of type `T`.

   > > -26- [Example 6: ...]

   >     template<class T>
   >       constexpr shared_ptr<T> make_shared_for_overwrite(size_t N); // T is U[]
   >     template<class T, class A>
   >       constexpr shared_ptr<T> allocate_shared_for_overwrite(const A& a, size_t N); // T is U[]

   > > -27- Constraints: `T` is an array of unknown bound.

   > > -28- Returns: A `shared_ptr` to an object of type `U[N]` array of `N` elements of type `remove_extent_t<T>` , where `U` is `remove_extent_t<T>`.

   > > -29- [Example 7: ...]

### 4452(i). Make deref-move constexpr

Section: 26.11.1 [specialized.algorithms.general] Status: C++26 Submitter: S.B. Tam Opened: 2025-11-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [specialized.algorithms.general].

View all issues with C++26 status.

Discussion:

`std::uninitialized_move` and `std::uninitialized_move_n` are constexpr and invoke `deref-move`, but `deref-move` is not constexpr. This looks like an obvious mistake.

Jiang An pointed out that P3508R0 and LWG 3918(i), both touching `std::uninitialized_move(_n)`, were adopted at the same meeting, and unfortunately none of them was aware of the other.

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.11.1 [specialized.algorithms.general], as indicated:

   >     template<class I>
   >       constexpr decltype(auto) deref-move(I& it) {
   >         if constexpr (is_lvalue_reference_v<decltype(*it)>)
   >           return std::move(*it);
   >         else
   >           return *it;
   >       }

### 4453(i). `atomic_ref<cv T>::required_alignment` should be the same as for `T`

Section: 32.5.7.2 [atomics.ref.ops] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-06 Last modified: 2026-06-09

Priority: 2

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Discussion:

When P3323R1 fixed support for `atomic_ref<cv T>` we didn't consider that an implementation could define `required_alignment` to be different from `atomic_ref<T>::required_alignment`. For example, a processor could support atomic loads on any `int` object, but could require a greater alignment for atomic stores (and read-modify-write and modify-write operations). So the `required_alignment` could be `alignof(int)` for `const int` but `2 * alignof(int)` for `int`, because the latter is needed for stores.

A malicious implementation could even define `required_alignment` to be greater for the `const T` specialization, which would make it undefined to use the new converting constructor added by P3860R1.

We should constrain implementations to use the same `required_alignment` for cv `T`, so that users can use `atomic_ref<const T>` on any object that is referenced by a `atomic_ref<T>`.

[2026-01-16; Reflector poll.]

Set priority to 2 after reflector poll.

"Should be broaded to include all similar types, not just top-level cv."

"Should we have the same guarantee for `is_always_lock_free`? Would allow checking if `atomic<volatile T>` is usable by checking `atomic<T>`."

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify [atomics.refs.ops], as indicated:

   >     static constexpr size_t required_alignment;

   > -1- The alignment required for an object to be referenced by an atomic reference, which is at least `alignof(T)`. The value of `required_alignment` is the same as `atomic_ref<remove_cv_t<T>>::required_alignment`

   > -2- [Note 1: Hardware could require an object referenced by an `atomic_ref` to have stricter alignment (6.8.3 [basic.align]) than other objects of type `T`. Further, whether operations on an `atomic_ref` are lock-free could depend on the alignment of the referenced object. For example, lock-free operations on `std::complex<double>` could be supported only if aligned to `2*alignof(double)`. — end note]

[2026-02-16; Tomasz provides updated resolution.]

Guarantee that `required_alignment` and `is_always_lock_free` values are the same for any similar types.

We define equality to `atomic_ref<remove_cv_t<U>`, as `volatile` atomics may be ill-formed.

[2026-02-27 telecon; Move to Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify [atomics.refs.ops], as indicated:

   >     static constexpr size_t required_alignment;

   > -1- The alignment required for an object to be referenced by an atomic reference, which is at least `alignof(T)`. For any type `U` similar to `T`, the value of `required_alignment` is the same as `atomic_ref<remove_cv_t<U>>::required_alignment`.

   > -2- [Note 1: Hardware could require an object referenced by an `atomic_ref` to have stricter alignment (6.8.3 [basic.align]) than other objects of type `T`. Further, whether operations on an `atomic_ref` are lock-free could depend on the alignment of the referenced object. For example, lock-free operations on `std::complex<double>` could be supported only if aligned to `2*alignof(double)`. — end note]

   >     static constexpr bool is_always_lock_free;

   > -3- The static data member `is_always_lock_free` is `true` if the `atomic_ref` type's operations are always lock-free, and false otherwise. For any type `U` similar to `T` the value of `is_always_lock_free` is the same as `atomic_ref<remove_cv_t<U>>::is_always_lock_free`.

### 4454(i). `assert` should forbid `co_await` and `co_yield`

Section: 19.3.3 [assertions.assert] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-06 Last modified: 2026-06-09

Priority: 1

View all issues with C++26 status.

Discussion:

Addresses GB 05-129

The new implementation of `assert` introduced by P2264R7 might require something like `sizeof(bool(__VA_ARGS__))`. Using something like `assert((co_yield 1, true))` would be ill-formed if the implementation of `assert` uses `sizeof`, because `co_yield` cannot appear in an unevaluated context.

The specification for `assert` should forbid using any constructs which are not valid in unevaluated contexts, unless we are certain that the new `assert` requirements can be implemented without such tricks using unevaluated contexts. "Ill-formed; no diagnostic required" seems appropriate.

[2026-03-06 Tim provides wording]

[2026-03-06 LWG telecon; move to Ready]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

- Modify 19.3.3 [assertions.assert] as indicated:

  > -3- If `__VA_ARGS__` does not expand to ana well-formed assignment-expression, the program is ill-formed. If such an assignment-expression is ill-formed when treated as an unevaluated operand (7.6.2.4 [expr.await], 7.6.17 [expr.yield]), the program is ill-formed, no diagnostic required.

### 4455(i). Add missing constraint to `basic-sender::get_completion_signatures` definition

Section: 33.9.2 [exec.snd.expos] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-06 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 215-356

The definition of `basic-sender::get_completion_signatures` is missing the `decays-to<basic-sender>` type constraint.

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.2 [exec.snd.expos], as indicated (just above paragraph 43):

   >     template<class Tag, class Data, class... Child>
   >       template<class decays-to<basic-sender> Sndr, class... Env>
   >         constexpr auto basic-sender<Tag, Data, Child...>::get_completion_signatures();

### 4456(i). Decay `Data` and `Child` in `make-sender`

Section: 33.9.2 [exec.snd.expos] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-06 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 211-351

The Mandates: for `make-sender` defines `Sndr` as a type that is different from what the Returns: element uses.

[Kona 2025-11-06; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.2 [exec.snd.expos], as indicated:

   >     template<class Tag, class Data = see below, class... Child>
   >       constexpr auto make-sender(Tag tag, Data&& data, Child&&... child);

   > > -24- Mandates: The following expressions are `true`:

   > > 1. (24.1) — `semiregular<Tag>`
   > > 2. (24.2) — `movable-value<Data>`
   > > 3. (24.3) — `(sender<Child> && ...)`
   > > 4. (24.4) — `dependent_sender<Sndr> || sender_in<Sndr>`, where `Sndr` is `basic-sender<Tag, Data, Child decay_t<Data>, decay_t<Child>...>` as defined below.

   > >    > Recommended practice: If evaluation of `sender_in<Sndr>` results in an uncaught exception from the evaluation of `get_completion_signatures<Sndr>()`, the implementation should include information about that exception in the resulting diagnostic.

   > > -25- Returns: A prvalue of type `basic-sender<Tag, decay_t<Data>, decay_t<Child>...>` that has been direct-list-initialized with the forwarded arguments, where `basic-sender` is the following exposition-only class template except as noted below.

### 4457(i). freestanding for `stable_sort`, `stable_partition` and `inplace_merge`

Section: 26.4 [algorithm.syn] Status: C++26 Submitter: Braden Ganetsky Opened: 2025-11-06 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 157-255

This applies the resolution for US 157-255.

[2026-01-16; Reflector poll.]

Set status to Tentatively Ready after ten votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.4 [algorithm.syn], as indicated:

   >     namespace ranges {
   >       template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
   >                class Proj = identity>
   >         requires sortable<I, Comp, Proj>
   >         constexpr I stable_sort(I first, S last, Comp comp = {}, Proj proj = {});         // hosted
   >       template<random_access_range R, class Comp = ranges::less, class Proj = identity>
   >         requires sortable<iterator_t<R>, Comp, Proj>
   >         constexpr borrowed_iterator_t<R>
   >           stable_sort(R&& r, Comp comp = {}, Proj proj = {});                             // hosted

   >       template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >                class Comp = ranges::less, class Proj = identity>
   >         requires sortable<I, Comp, Proj>
   >         I stable_sort(Ep&& exec, I first, S last, Comp comp = {},
   >                       Proj proj = {});                                        // freestanding-deletedhosted
   >       template<execution-policy Ep, sized-random-access-range R, class Comp = ranges::less,
   >                class Proj = identity>
   >         requires sortable<iterator_t<R>, Comp, Proj>
   >         borrowed_iterator_t<R>
   >           stable_sort(Ep&& exec, R&& r, Comp comp = {}, Proj proj = {});      // freestanding-deletedhosted
   >     }

2. Modify 26.4 [algorithm.syn], as indicated:

   >     namespace ranges {
   >       template<bidirectional_iterator I, sentinel_for<I> S, class Proj = identity,
   >                indirect_unary_predicate<projected<I, Proj>> Pred>
   >         requires permutable<I>
   >         constexpr subrange<I> stable_partition(I first, S last, Pred pred,            // hosted
   >                                                Proj proj = {});
   >       template<bidirectional_range R, class Proj = identity,
   >                indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >         requires permutable<iterator_t<R>>
   >         constexpr borrowed_subrange_t<R> stable_partition(R&& r, Pred pred,           // hosted
   >                                                           Proj proj = {});

   >       template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >                class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred>
   >         requires permutable<I>
   >         subrange<I>
   >           stable_partition(Ep&& exec, I first, S last, Pred pred,
   >                            Proj proj = {});                                   // freestanding-deletedhosted
   >       template<execution-policy Ep, sized-random-access-range R, class Proj = identity,
   >                indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >         requires permutable<iterator_t<R>>
   >         borrowed_subrange_t<R>
   >           stable_partition(Ep&& exec, R&& r, Pred pred, Proj proj = {});      // freestanding-deletedhosted
   >     }

3. Modify 26.4 [algorithm.syn], as indicated:

   >     namespace ranges {
   >       template<bidirectional_iterator I, sentinel_for<I> S, class Comp = ranges::less,
   >                class Proj = identity>
   >         requires sortable<I, Comp, Proj>
   >         constexpr I
   >           inplace_merge(I first, I middle, S last, Comp comp = {}, Proj proj = {});       // hosted
   >       template<bidirectional_range R, class Comp = ranges::less, class Proj = identity>
   >         requires sortable<iterator_t<R>, Comp, Proj>
   >         constexpr borrowed_iterator_t<R>
   >           inplace_merge(R&& r, iterator_t<R> middle, Comp comp = {}, Proj proj = {});     // hosted

   >       template<execution-policy Ep, random_access_iterator I, sized_sentinel_for<I> S,
   >                class Comp = ranges::less, class Proj = identity>
   >         requires sortable<I, Comp, Proj>
   >         I inplace_merge(Ep&& exec, I first, I middle, S last, Comp comp = {},
   >                         Proj proj = {});                          // freestanding-deletedhosted
   >       template<execution-policy Ep, sized-random-access-range R, class Comp = ranges::less,
   >                class Proj = identity>
   >         requires sortable<iterator_t<R>, Comp, Proj>
   >         borrowed_iterator_t<R>
   >           inplace_merge(Ep&& exec, R&& r, iterator_t<R> middle, Comp comp = {},
   >                         Proj proj = {});                          // freestanding-deletedhosted
   >     }

### 4459(i). Protect `get_completion_signatures` fold expression from overloaded commas

Section: 33.9.2 [exec.snd.expos] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-07 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 214-355

The fold expression can pick up overloaded comma operators.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.2 [exec.snd.expos], as indicated:

   >     template<class Sndr, class... Env>
   >       static consteval void default-impls::check-types();

   > > -40- Let `Is` be the pack of integral template arguments of the `integer_sequence` specialization denoted by `indices-for<Sndr>`.

   > > -41- Effects: Equivalent to: `((void)get_completion_signatures<child-type<Sndr, Is>, FWD-ENV-T(Env)...>(), ...);`

### 4460(i). Missing Throws: for last `variant` constructor

Section: 22.6.3.2 [variant.ctor] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-07 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

All `variant` constructors except the last one have a Throws: element saying what they're allowed to throw.

This originates from an editorial pull request, where the submitter said:

> "It looks like this defect is an artifact of a change between P0088R0 and P0088R1. Note how in R0 neither one of the `emplaced_type_t`/`emplaced_index_t` (as they were then called) + `initializer_list` constructors have a throws clause. In R1 only one of them gained it."

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 22.6.3.2 [variant.ctor], as indicated:

   >     template<size_t I, class U, class... Args>
   >       constexpr explicit variant(in_place_index_t<I>, initializer_list<U> il, Args&&... args);

   > > -35- Constraints:

   > > 1. (35.1) — `I` is less than `sizeof...(Types)` and
   > > 2. (35.2) — `is_constructible_v<TI, initializer_list<U>&, Args...>` is `true`.

   > > -36- Effects: Direct-non-list-initializes the contained value of type `TI` with `il, std::forward<Args>(args)...`.

   > > -37- Postconditions: `index()` is `I`.

   > > -?- Throws: Any exception thrown by calling the selected constructor of `TI`.

   > > -38- Remarks: If `TI`’s selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

[2025-11-11; Jonathan provides improved wording]

[2025-12-04; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.6.3.2 [variant.ctor], as indicated:

   >     constexpr variant() noexcept(see below);

   > > -2- Constraints: `is_default_constructible_v<T0>` is `true`.

   > > -3- Effects: Constructs a `variant` holding a value-initialized value of type `T0`.

   > > -4- Postconditions: `valueless_by_exception()` is `false` and `index()` is `0`.

   > > -5- Throws: Any exception thrown by the value-initialization of `T0`.

   > > -6- Remarks: […]

   >     constexpr variant(const variant&);

   > > -7- Effects: If `w` holds a value, initializes the `variant` to hold the same alternative as `w` and direct-initializes the contained value with `GET<j>(w)`, where `j` is `w.index()`. Otherwise, initializes the `variant` to not hold a value.

   > > -8- Throws: Any exception thrown by direct-initializating any `Ti` for all i the initialization of the contained value.

   > > -9- Remarks: […]

   >     constexpr variant(variant&&) noexcept(see below);

   > > -10- Constraints: `is_move_constructible_v<Ti>` is `true` for all i.

   > > -11- Effects: If `w` holds a value, initializes the `variant` to hold the same alternative as `w` and direct-initializes the contained value with `GET<j>(std::move(w))`, where `j` is `w.index()`. Otherwise, initializes the `variant` to not hold a value.

   > > -12- Throws: Any exception thrown by move-constructing any `Ti` for all i the initialization of the contained value.

   > > -13- Remarks: […]

   >     template<class T> constexpr variant(T&&) noexcept(see below);

   > > -14- Let `Tj` be a type that is determined as follows: build an imaginary function `FUN(Ti)` for each alternative type `Ti` for which `Ti x[] = {std::forward<T>(t)};` is well-formed for some invented variable `x`. The overload `FUN(Tj)` selected by overload resolution for the expression `FUN(std::forward<T>(t))` defines the alternative `Tj` which is the type of the contained value after construction.

   > > -15- Constraints: […]

   > > -16- Effects: Initializes `*this` to hold the alternative type `Tj` and direct-non-list-initializes the contained value with `std::forward<T>(t)`.

   > > -17- Postconditions: […]

   > > -18- Throws: Any exception thrown by the initialization of the selected alternative `Tj` contained value.

   > > -19- Remarks: […]

   >     template<class T, class... Args> constexpr variant(in_place_type_t<T>, Args&&... args);

   > > -20- Constraints: […]

   > > -21- Effects: Direct-non-list-initializes the contained value of type `T` with `std::forward<Args>(args)...`.

   > > -22- Postconditions: […]

   > > -23- Throws: Any exception thrown by the selected constructor of `T` the initialization of the contained value.

   > > -24- Remarks: […]

   >     template<class T, class U, class... Args>
   >       constexpr variant(in_place_type_t<T>, initializer_list<U> li, Args&&... args);

   > > -25- Constraints: […]

   > > -26- Effects: Direct-non-list-initializes the contained value of type `T` with `il, std::forward<Args>(args)...`.

   > > -27- Postconditions: […]

   > > -28- Throws: Any exception thrown by the selected constructor of `T` the initialization of the contained value.

   > > -29- Remarks: […]

   >     template<size_t I, class... Args>
   >       constexpr explicit variant(in_place_index_t<I>, Args&&... args);

   > > -30- Constraints:

   > > 1. (30.1) — `I` is less than `sizeof...(Types)` and
   > > 2. (30.2) — `is_constructible_v<TI, Args...>` is `true`.

   > > -31- Effects: Direct-non-list-initializes the contained value of type `TI` with `std::forward<Args>(args)...`.

   > > -32- Postconditions: `index()` is `I`.

   > > -33- Throws: Any exception thrown by the selected constructor of `Ti` the initialization of the contained value.

   > > -34- Remarks: If `TI`’s selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

   >     template<size_t I, class U, class... Args>
   >       constexpr explicit variant(in_place_index_t<I>, initializer_list<U> il, Args&&... args);

   > > -35- Constraints:

   > > 1. (35.1) — `I` is less than `sizeof...(Types)` and
   > > 2. (35.2) — `is_constructible_v<TI, initializer_list<U>&, Args...>` is `true`.

   > > -36- Effects: Direct-non-list-initializes the contained value of type `TI` with `il, std::forward<Args>(args)...`.

   > > -37- Postconditions: `index()` is `I`.

   > > -?- Throws: Any exception thrown by the initialization of the contained value.

   > > -38- Remarks: If `TI`’s selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

### 4461(i). `stop-when` needs to evaluate unstoppable tokens

Section: 33.9.12.17 [exec.stop.when] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-06 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 226-345

For the case where `token` models `unstoppable_token` the expression that `stop-when` is expression-equivalent to needs to include the fact that `token` is evaluated (even if not used).

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.12.17 [exec.stop.when], as indicated:

   > -2- The name `stop-when` denotes an exposition-only sender adaptor. For subexpressions `sndr` and `token`:

   > 1. (2.1) — If `decltype((sndr))` does not satisfy `sender`, or `remove_cvref_t<decltype((token))>` does not satisfy `stoppable_token`, then `stop-when(sndr, token)` is ill-formed.
   > 2. (2.2) — Otherwise, if `remove_cvref_t<decltype((token))>` models `unstoppable_token` then `stop-when(sndr, token)` is expression-equivalent to `(void)token, sndr`, except that `token` and `sndr` are indeterminately sequenced.

### 4462(i). Algorithm requirements don't describe semantics of `s - i` well

Section: 26.2 [algorithms.requirements] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-07 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 154-252

“the semantics of `s - i` has” is not grammatically correct. Additionally, “type, value, and value category” are properties of expressions, not “semantics”.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.2 [algorithms.requirements], as indicated:

   > -11- In the description of the algorithms, operator `+` is used for some of the iterator categories for which it does not have to be defined. In these cases the semantics of `a + n` are the same as those of

   >         auto tmp = a;
   >         for (; n < 0; ++n) --tmp;
   >         for (; n > 0; --n) ++tmp;
   >         return tmp;

   > Similarly, operator `-` is used for some combinations of iterators and sentinel types for which it does not have to be defined. If [`a`, `b`) denotes a range, the semantics of `b - a` in these cases are the same as those of

   >         iter_difference_t<decltype(a)> n = 0;
   >         for (auto tmp = a; tmp != b; ++tmp) ++n;
   >         return n;

   > and if [`b`, `a`) denotes a range, the same as those of

   >         iter_difference_t<decltype(b)> n = 0;
   >         for (auto tmp = b; tmp != a; ++tmp) --n;
   >         return n;

   > For each iterator `i` and sentinel `s` produced from a range `r`, the semantics of `s - i` are the same as those of an expression that has the same type, value, and value category as `ranges::distance(i, s)`.

   > [Note 3: The implementation can use `ranges::distance(r)` when that produces the same value as `ranges::distance(i, s)`. This can be more efficient for sized ranges. — end note]

### 4463(i). Change wording to 'model' from 'subsumes' in [algorithms.parallel.user]

Section: 26.3.2 [algorithms.parallel.user] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2025-11-07 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 155-253

“Subsumes” word does not work here because `regular_invocable` and `invocable` subsume each other.

Proposed change: Say that the type is required to model `regular_invocable`.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.3.2 [algorithms.parallel.user], as indicated:

   > -1- Unless otherwise specified, invocable objects passed into parallel algorithms as objects of a type denoted by a template parameter named `Predicate`, `BinaryPredicate`, `Compare`, `UnaryOperation`, `BinaryOperation`, `BinaryOperation1`, `BinaryOperation2`, `BinaryDivideOp`, or constrained by a concept whose semantic requirements include that subsumes the type models `regular_invocable` and the operators used by the analogous overloads to these parallel algorithms that are formed by an invocation with the specified default predicate or operation (where applicable) shall not directly or indirectly modify objects via their arguments, nor shall they rely on the identity of the provided objects.

### 4464(i). §[alg.merge] Wording tweaks

Section: 26.8.6 [alg.merge] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2025-11-07 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 163-262

The original text of the “US 163-262” issue says: “Bullets 1.3 and 1.4 and paragraph 3 should say `E(e1, e2)` instead of `E(e1, e1)`” in [alg.merge]. The problem, though, was introduced when merging P3179R9 “Parallel Range Algorithms” proposal. The original wording of P3179 does not have parentheses after `E`. Those extra parameters in `E` do not bring clarity to `merge` algorithm. The proposed resolution is to strike them through.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.8.6 [alg.merge], as indicated:

   >     template<class InputIterator1, class InputIterator2,
   >              class OutputIterator>
   >       constexpr OutputIterator
   >         merge(InputIterator1 first1, InputIterator1 last1,
   >               InputIterator2 first2, InputIterator2 last2,
   >               OutputIterator result);
   >     […]
   >     template<execution-policy Ep, sized-random-access-range R1, sized-random-access-range R2,
   >              sized-random-access-range OutR, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, iterator_t<OutR>, Comp, Proj1, Proj2>
   >       ranges::merge_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, borrowed_iterator_t<OutR>>
   >         ranges::merge(Ep&& exec, R1&& r1, R2&& r2, OutR&& result_r,
   >                       Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});

   > > -1- Let:

   > > - (1.1) — `N` be: […]
   > > - (1.2) — `comp` be `less{}`, `proj1` be `identity{}`, and `proj2` be `identity{}`, for the overloads with no parameters by those names;
   > > - (1.3) — `E(e1, e1)` be `bool(invoke(comp, invoke(proj2, e2), invoke(proj1, e1)))`;
   > > - (1.4) — `K` be the smallest integer in `[0, last1 - first1)` such that for the element `e1` in the position `first1 + K` there are at least `N − K` elements `e2` in `[first2, last2)` for which `E(e1, e1)` holds, and be equal to `last1 - first1` if no such integer exists.

   > > -2- Preconditions: The ranges `[first1, last1)` and `[first2, last2)` are sorted with respect to `comp` and `proj1` or `proj2`, respectively. The resulting range does not overlap with either of the original ranges.

   > > -3- Effects: Copies the first `K` elements of the range `[first1, last1)` and the first `N − K` elements of the range `[first2, last2)` into the range `[result, result + N)`. If an element `a` precedes `b` in an input range, `a` is copied into the output range before `b`. If `e1` is an element of `[first1, last1)` and `e2` of `[first2, last2)`, `e2` is copied into the output range before `e1` if and only if `E(e1, e1)` is `true`.

### 4465(i). §[alg.partitions] Clarify Returns: element

Section: 26.8.5 [alg.partitions] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2025-11-07 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses US 162-261

In 26.8.5 [alg.partitions] p21 the wording is unclear what happens if there is no such element. The proposed resolution tries to clarify that without complicating the wording. If the proposed resolution (or something along those lines) fails, the recommendation is to reject US 162-261.

[Kona 2025-11-07; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.8.5 [alg.partitions], as indicated:

   >     template<class InputIterator, class OutputIterator1, class OutputIterator2, class Predicate>
   >       constexpr pair<OutputIterator1, OutputIterator2>
   >         partition_copy(InputIterator first, InputIterator last,
   >                        OutputIterator1 out_true, OutputIterator2 out_false, Predicate pred);
   >     […]
   >     template<execution-policy Ep, sized-random-access-range R,
   >              sized-random-access-range OutR1, sized-random-access-range OutR2,
   >              class Proj = identity,
   >              indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
   >       requires indirectly_copyable<iterator_t<R>, iterator_t<OutR1>> &&
   >                 indirectly_copyable<iterator_t<R>, iterator_t<OutR2>>
   >       ranges::partition_copy_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR1>,
   >                                     borrowed_iterator_t<OutR2>>
   >         ranges::partition_copy(Ep&& exec, R&& r, OutR1&& out_true_r, OutR2&& out_false_r,
   >                                Pred pred, Proj proj = {});

   > > -14- Let `proj` be `identity{}` for the overloads with no parameter named `proj` and let `E(x)` be `bool(invoke(pred, invoke(proj, x)))`.

   > > […]

   > > -19- Preconditions: The input range and output ranges do not overlap. […]

   > > -20- Effects: For each iterator `i` in `[first, first + N)`, copies `*i` to the output range `[out_true, last_true)` if `E(*i)` is `true`, or to the output range `[out_false, last_false)` otherwise.

   > > -21- Returns: Let `o1` `Q` be the iterator past the last number of elements copied element into the output range `[out_true, last_true)`, and `o2` `V` be the iterator past the last number of elements copied element into the output range `[out_false, last_false)`. Returns:
   > > - (21.1) — `{o1out_true + Q, o2out_false + V}` for the overloads in namespace `std`.
   > > - (21.2) — `{first + N, o1out_true + Q, o2out_false + V}` for the overloads in namespace `ranges`.

   > > -22- Complexity: At `most last - first` applications of `pred` and `proj`.

### 4466(i). `task` should move its result

Section: 33.13.6.5 [task.promise] Status: Resolved Submitter: Robert A.H. Leahy Opened: 2025-11-05 Last modified: 2026-04-01

Priority: 2

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Discussion:

In 33.13.6.5 [task.promise] there's the following bullet (7.3):

>     set_value(std::move(RCVR(*this)), *result).

This has the effect of sending the result as an lvalue whereas an rvalue is almost certainly what's intended, see the bullet (7.1) above:

>     set_error(std::move(RCVR(*this)), std::move(e)) […]

Resolution would be to change the bullet (7.3) to:

>     set_value(std::move(RCVR(*this)), *std::move(result)).

[2025-11-07; This would be resolved by LWG 4339(i).]

[2025-12-04; Reflector poll.]

Set priority to 2 after reflector poll.

[2026-04-01 Resolved by the adoption of issue 4339(i) at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.13.6.5 [task.promise], as indicated:

   >     auto final_suspend() noexcept;

   > > -7- Returns: An awaitable object of unspecified type (7.6.2.4 [expr.await]) whose member functions arrange for the completion of the asynchronous operation associated with `STATE(*this)` by invoking:

   > > - (7.1) — `set_error(std::move(RCVR(*this)), std::move(e))` if `errors.index()` is greater than zero and `e` is the value held by `errors`, otherwise
   > > - (7.2) — `set_value(std::move(RCVR(*this)))` if `is_void<T>` is `true`, and otherwise
   > > - (7.3) — `set_value(std::move(RCVR(*this)), *std::move(result))`.

### 4467(i). `hive::splice` can throw `bad_alloc`

Section: 23.3.9.5 [hive.operations] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-07 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Moving blocks from the source `hive` to the destination `hive` might require reallocating the array of pointers to blocks, so the Throws: element should allow this.

[2025-12-04; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.3.9.5 [hive.operations], as indicated:

   >     void splice(hive& x);
   >     void splice(hive&& x);

   > > -2- Preconditions: `get_allocator() == x.get_allocator()` is `true`.

   > > -3- Effects: If `addressof(x) == this` is `true`, the behavior is erroneous and there are no effects. Otherwise, inserts the contents of `x` into `*this` and `x` becomes empty. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

   > > -4- Throws: `length_error` if any of `x`'s active blocks are not within the bounds of `current-limits` , as well as any exceptions thrown by the allocator.

   > > -5- Complexity: Linear in the sum of all element blocks in `x` plus all element blocks in `*this`.

   > > -6- Remarks: Reserved blocks in `x` are not transferred into `*this`. If `addressof(x) == this` is `false`, invalidates the past-the-end iterator for both `x` and `*this`.

### 4468(i). §[const.wrap.class] "`operator decltype(auto)`" is ill-formed

Section: 22.2.10 [const.wrap.class] Status: C++26 Submitter: Jiang An Opened: 2025-11-07 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Following the approval of CWG 1670 in Kona 2025, the following declaration in class template `constant_wrapper`, 22.2.10 [const.wrap.class], is ill-formed:

>     constexpr operator decltype(auto)() const noexcept { return value; }

[2025-12-04; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.2.10 [const.wrap.class], class template `constant_wrapper` synopsis, as indicated:

   >     […]
   >     template<cw-fixed-value X, class>
   >     struct constant_wrapper : cw-operators {
   >       static constexpr const auto & value = X.data;

   >       using type = constant_wrapper;
   >       using value_type = typename decltype(X)::type;

   >       […]
   >       constexpr operator decltype(autovalue)() const noexcept { return value; }
   >     };

### 4469(i). Names of parameters of addressable function shall remain unspecified

Section: 16.4.5.2.1 [namespace.std] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-14 Last modified: 2026-06-09

Priority: 2

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Discussion:

The wording in 16.4.5.2.1 [namespace.std] p7 guarantees that reflection of the addressable function can be reliably produced. With the addition of the function parameter name, it is possible to access the parameter name of such functions:

    constexpr auto p = parameters_of(^^std::endl)[0];
    static_assert( identifier_of(p) == "os" ); // guaranteed?

We should clarify that parameter names used by standard library implementation remain unspecified, by making result of `has_identifier` and `identifier_of` unspecified on reflection of such parameter.

[2026-01-16; Reflector poll.]

Set priority to 2 after reflector poll.

[2026-01-16; LWG telecon. Move to Ready]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 16.4.5.2.1 [namespace.std], as indicated:

   > Let F denote a standard library function or function template. Unless F is designated an addressable function, it is unspecified if or how a reflection value designating the associated entity can be formed. For any value `p` of type `meta::info` that represents a reflection of a parameter of F, it is unspecified if `has_identifier(p)` returns `true` or `false`, and if `meta::has_identifier(p)` is `true`, then the `NTMBS` produced by `meta::identifier_of(p)` and `meta::u8identifier_of(p)` is unspecified.

### 4470(i). The use of `integer-from<Bytes>` all over [simd] is incorrect for `Bytes=sizeof(complex<double>)`

Section: 29.10 [simd] Status: Resolved Submitter: Matthias Kretz Opened: 2025-11-04 Last modified: 2026-03-31

Priority: 1

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Discussion:

After the introduction of `complex<double>` to the set of vectorizable types, the `integer-from<Bytes>` trait does not work as intended anymore. All uses of `integer-from` need to be reviewed and the wording needs to be adjusted to work for `Bytes=16`.

[2025-12-04; Reflector poll.]

Set priority to 1 after reflector poll.

[2026-02-13; LWG telecon.]

P3932R0 resolving the issue was approved.

[2026-03-31 P3932R0 was approved in Croydon. Status changed: New → Resolved.]

Proposed resolution:

### 4471(i). Remove test for `get_env noexcept`-ness from `inline_scheduler`

Section: 33.13.4 [exec.inline.scheduler] Status: WP Submitter: Eric Niebler Opened: 2025-11-05 Last modified: 2026-06-15

Priority: 4

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Discussion:

33.13.4 [exec.inline.scheduler] bullet (3.2) reads:

> - (3.2) — the expression `get_completion_scheduler<set_value_t>(get_env(sndr))` has type `inline_scheduler` and is potentially-throwing if and only if `get_env(sndr)` is potentially-throwing.

`get_env(sndr)` is never potentially-throwing because it mandates that `sndr.get_env()` cannot throw, see 33.5.5 [exec.get.env].

also, `get_completion_scheduler<set_*_t>(attrs)` mandates that `attrs.query(get_completion_scheduler<set_*_t>)` cannot throw.

[2026-01-16; Reflector poll.]

Set priority to 4 after reflector poll.

"33.12.1.2 [exec.run.loop.types] (7.2) has the same redundancy."

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 33.13.4 [exec.inline.scheduler] as indicated:

   > -3- Let `sndr` be an expression of type `inline-sender`, let `rcvr` be an expression such that `receiver_of<decltype((rcvr)), CS>` is `true` where `CS` is `completion_signatures<set_value_t()>`, then:

   > - (3.1) — the expression `connect(sndr, rcvr)` has type `inline-state<remove_cvref_t<decltype((rcvr))>>` and is potentially-throwing if and only if `((void)sndr, auto(rcvr))` is potentially-throwing, and
   > - (3.2) — the expression `get_completion_scheduler<set_value_t>(get_env(sndr))` has type `inline_scheduler` and is potentially-throwing if and only if `get_env(sndr)` is potentially-throwing.

[2026-01-16; Jonathan provides updated wording]

LWG believes the original issue discussion is wrong to say that `get_env(sndr)` can never throw, because the expression `sndr` itself could throw. However, we already state that `get_env` and `get_completion_scheduler` do not throw, so it's redundant to say that `get_completion_scheduler(get_env(sndr))` only throws if `sndr` throws. If nothing except `sndr` in that expression is allowed to throw, then it's just stating the obvious.

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Modify 33.13.4 [exec.inline.scheduler] as indicated:

   > -3- Let `sndr` be an expression of type `inline-sender`, let `rcvr` be an expression such that `receiver_of<decltype((rcvr)), CS>` is `true` where `CS` is `completion_signatures<set_value_t()>`, then:

   > - (3.1) — the expression `connect(sndr, rcvr)` has type `inline-state<remove_cvref_t<decltype((rcvr))>>` and is potentially-throwing if and only if `((void)sndr, auto(rcvr))` is potentially-throwing, and
   > - (3.2) — the expression `get_completion_scheduler<set_value_t>(get_env(sndr))` has type `inline_scheduler` and is potentially-throwing if and only if `get_env(sndr)` is potentially-throwing.

2. Modify 33.12.1.2 [exec.run.loop.types] as indicated:

   > -7- Let `sndr` be an expression of type `run-loop-sender`, let `rcvr` be an expression such that `receiver_of<decltype((rcvr)), CS>` is `true` where `CS` is the `completion_signatures` specialization above. Let `C` be either `set_value_t` or `set_stopped_t`. Then:

   > - (7.1) — The expression `connect(sndr, rcvr)` has type `run-loop-opstate<decay_t<decltype((rcvr))>>` and is potentially-throwing if and only if `(void(sndr), auto(rcvr))` is potentially-throwing.
   > - (7.2) — The expression `get_completion_scheduler<C>(get_env(sndr))` is potentially-throwing if and only if `sndr` is potentially-throwing, has type `run-loop-scheduler`, and compares equal to the `run-loop-scheduler` instance from which `sndr` was obtained.

[2026-05-15; Tim rebases wording to new working draft]

The `inline_scheduler` change is no longer needed.

[2026-05-19; LWG telecon. Status changed: New → Ready.]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5046.

1. Modify 33.12.1.2 [exec.run.loop.types] as indicated:

   > -8- Let `sndr` be an expression of type `run-loop-sender`, let `rcvr` be an expression such that `receiver-of<decltype((rcvr)), CS>` is `true` where `CS` is the `completion_signatures` specialization above. Let `C` be either `set_value_t` or `set_stopped_t`. Then:

   > - (8.1) — The expression `connect(sndr, rcvr)` has type `run-loop-opstate<decay_t<decltype((rcvr))>>` and is potentially-throwing if and only if `(void(sndr), auto(rcvr))` is potentially-throwing.
   > - (8.2) — The expression `get_completion_scheduler<C>(get_env(sndr))` is potentially-throwing if and only if `sndr` is potentially-throwing, has type `run-loop-scheduler`, and compares equal to the `run-loop-scheduler` instance from which `sndr` was obtained.

### 4472(i). `std::atomic_ref<const T>` can be constructed from temporaries

Section: 32.5.7 [atomics.ref.generic] Status: C++26 Submitter: Jiang An Opened: 2025-11-11 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

`std::atomic_ref<T>` has a constructor taking `T&`, so when `T` is a `const` but not `volatile` object type, the constructor parameter can be bound to a temporary expression, which doesn't seem to make sense. Even after P3860R1, explicitly constructing `std::atomic_ref<const T>` from `std::atomic_ref<volatile T>` can be well-formed with the undesired semantics when it is well-formed to instantiate `std::atomic_ref<volatile T>` and its operator `T` conversion function, because the construction calls the conversion function and creates a temporary object. Probably it's better to disallow such reference binding.

[2025-12-05; LWG telecon. Moved to Ready]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014 after application of P3860R1.

> [Drafting note: The deleted overloads were mirrored from the design of `reference_wrapper` before LWG 2993(i). As these overloads don't participate in implicit conversion, I don't think there will be any similar issue introduced.]

1. Modify 32.5.7.1 [atomics.ref.generic.general], primary class template `atomic_ref` synopsis, as indicated:

   >     namespace std {
   >       template<class T> struct atomic_ref {
   >       private:
   >         T* ptr;               // exposition only
   >       public:
   >         […]
   >         constexpr explicit atomic_ref(T&);
   >         explicit atomic_ref(T&&) = delete;
   >         constexpr atomic_ref(const atomic_ref&) noexcept;
   >         template<class U>
   >           constexpr atomic_ref(const atomic_ref<U>&) noexcept;
   >         […]
   >       };
   >     }

2. Modify 32.5.7.3 [atomics.ref.int], class template `atomic_ref` `integral-type` specialization synopsis, as indicated:

   >     namespace std {
   >       template<> struct atomic_ref<integral-type> {
   >       private:
   >         integral-type* ptr;               // exposition only
   >       public:
   >         […]
   >         constexpr explicit atomic_ref(integral-type&);
   >         explicit atomic_ref(integral-type&&) = delete;
   >         constexpr atomic_ref(const atomic_ref&) noexcept;
   >         template<class U>
   >           constexpr atomic_ref(const atomic_ref<U>&) noexcept;
   >         […]
   >       };
   >     }

3. Modify 32.5.7.4 [atomics.ref.float], class template `atomic_ref` `floating-point-type` specialization synopsis, as indicated:

   >     namespace std {
   >       template<> struct atomic_ref<floating-point-type> {
   >       private:
   >         floating-point-type* ptr;               // exposition only
   >       public:
   >         […]
   >         constexpr explicit atomic_ref(floating-point-type&);
   >         explicit atomic_ref(floating-point-type&&) = delete;
   >         constexpr atomic_ref(const atomic_ref&) noexcept;
   >         template<class U>
   >           constexpr atomic_ref(const atomic_ref<U>&) noexcept;
   >         […]
   >       };
   >     }

4. Modify 32.5.7.5 [atomics.ref.pointer], class template `atomic_ref` `pointer-type` specialization synopsis, as indicated:

   >     namespace std {
   >       template<> struct atomic_ref<pointer-type> {
   >       private:
   >         pointer-type* ptr;               // exposition only
   >       public:
   >         […]
   >         constexpr explicit atomic_ref(pointer-type&);
   >         explicit atomic_ref(pointer-type&&) = delete;
   >         constexpr atomic_ref(const atomic_ref&) noexcept;
   >         template<class U>
   >           constexpr atomic_ref(const atomic_ref<U>&) noexcept;
   >         […]
   >       };
   >     }

### 4474(i). "`round_to_nearest`" rounding mode is unclear

Section: 17.3.4 [round.style] Status: C++26 Submitter: Jan Schultke Opened: 2025-11-13 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Consider the specification of `round_to_nearest` in 17.3.4 [round.style]:

> `round_to_nearest` if the rounding style is to the nearest representable value

It is unclear how exact ties are rounded. For example, with this rounding, would a value that is equidistant to zero and `numeric_limits<float>::min()` be rounded towards zero or away from zero?

In 17.3.5.2 [numeric.limits.members], there exists a footnote for `numeric_limits<T>::round_style`:

> 185) Equivalent to `FLT_ROUNDS`. Required by ISO/IEC 10967-1:2012.

In C23 5.2.4.2.2 [Characteristics of floating types `<float.h>`], it is specified that a value of `1` for `FLT_ROUNDS` (which equals `round_to_nearest`) means

> to nearest, ties to even

This is also the default ISO/IEC 60559 rounding mode, and chosen by standard libraries such as libstdc++ for `numeric_limits` under that assumption. Do note that C23 no longer references ISO/IEC 10967 in any normative wording, so presumably, matching `FLT_ROUNDS` values means to match the value that exists in C23, including its meaning.

[2026-01-16; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

"IEEE 754-2019 talks about what to do in case of a tie for single digit precision types, such as FP4 (E2M1). Will raise this with WG14." [E2M1 is 1 sign bit, 2 exponent bits, 1 explicit mantissa bit]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 17.3.4 [round.style] as indicated:

   > -1- The rounding mode for floating-point arithmetic is characterized by the values:

   > - (1.1) — `round_indeterminate` if the rounding style is indeterminable
   > - (1.2) — `round_toward_zero` if the rounding style is toward zero
   > - (1.3) — `round_to_nearest` if the rounding style is to the nearest representable value; if there are two equally near such values, the one with an even least significant digit is chosen
   > - (1.4) — `round_toward_infinity` if the rounding style is toward infinity
   > - (1.5) — `round_toward_neg_infinity` if the rounding style is toward negative infinity

### 4476(i). `run_loop` should not have a `set_error` completion

Section: 33.12.1.1 [exec.run.loop.general] Status: C++26 Submitter: Eric Niebler Opened: 2025-11-16 Last modified: 2026-06-09

Priority: 2

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Discussion:

When `run_loop` was proposed, the only implementation we had synchronized with a mutex and a condition variable. Operations on those can theoretically throw exceptions, so `run_loop` got a `set_error_t(exception_ptr)` completion signature.

Since then, a lock-free implementation of `run_loop` has been found. Atomic operations cannot fail with an exception, so an atomic `run_loop` can never complete with an error.

[2026-01-16; Reflector poll.]

Set priority to 2 after reflector poll.

"Very helpful to know if something completes without exceptions, it affects whether completions on some types of scheduler are correct."

"Want confirmation from LEWG as `run()` and `finish()` have narrow contracts."

[2026-03-26; Status New → LEWG.]

[Croydon 2026-03-27; LEWG confirmed they want `noexcept`. Status changes: LEWG → Open.]

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

> [Drafting note: It should be pointed out that member function `finish` should be `noexcept` for other reasons as well, see LWG 4215(i).]

1. Modify 33.12.1.1 [exec.run.loop.general], class `run_loop` synopsis, as indicated:

   >     namespace std::execution {
   >       class run_loop {
   >         // 33.12.1.2 [exec.run.loop.types], associated types
   >         class run-loop-scheduler;                        // exposition only
   >         class run-loop-sender;                           // exposition only
   >         struct run-loop-opstate-base {                   // exposition only
   >           virtual void execute() noexcept = 0;           // exposition only
   >           run_loop* loop;                                // exposition only
   >           run-loop-opstate-base* next;                   // exposition only
   >         };
   >         template<class Rcvr>
   >           using run-loop-opstate = unspecified;          // exposition only

   >         // 33.12.1.4 [exec.run.loop.members], member functions
   >         run-loop-opstate-base* pop-front() noexcept;     // exposition only
   >         void push-back(run-loop-opstate-base*) noexcept; // exposition only

   >       public:
   >         // 33.12.1.3 [exec.run.loop.ctor], constructor and destructor
   >         run_loop() noexcept;
   >         run_loop(run_loop&&) = delete;
   >         ~run_loop();

   >         // 33.12.1.4 [exec.run.loop.members] member functions
   >         run-loop-scheduler get_scheduler() noexcept;
   >         void run() noexcept;
   >         void finish() noexcept;
   >       };
   >     }

2. Modify 33.12.1.2 [exec.run.loop.types] as indicated:

   >     class run-loop-sender;

   > -5- `run-loop-sender` is an exposition-only type that satisfies sender. `completion_signatures_of_t<runloop-sender>` is

   > >     completion_signatures<set_value_t(), set_error_t(exception_ptr), set_stopped_t()>

   > […]

   > -9- Let `o` be a non-const lvalue of type `run-loop-opstate<Rcvr>`, and let `REC(o)` be a non-const lvalue reference to an instance of type `Rcvr` that was initialized with the expression `rcvr` passed to the invocation of connect that returned `o`. Then:
   > - (9.1) — […]
   > - (9.2) — […]
   > - (9.3) — The expression `start(o)` is equivalent to:

   >   >     try {
   >   >       o.loop->push-back(addressof(o));
   >   >     } catch(...) {
   >   >       set_error(std::move(REC(o)), current_exception());
   >   >     }

3. Modify 33.12.1.4 [exec.run.loop.members] as indicated:

   >     run-loop-opstate-base* pop-front() noexcept;

   > > -1- Effects: Blocks (3.6 [defns.block]) until one of the following conditions is `true`: […]

   >     void push-back(run-loop-opstate-base* item) noexcept;

   > > -2- Effects: Adds `item` to the back of the queue and increments `count` by `1`.

   > > -3- Synchronization: This operation synchronizes with the `pop-front` operation that obtains `item`.

   >     run-loop-scheduler get_scheduler() noexcept;

   > > -4- Returns: An instance of `run-loop-scheduler` that can be used to schedule work onto this `run_loop` instance.

   >     void run() noexcept;

   > > -5- Preconditions: is either `starting` or `finishing`.

   > > -6- Effects: If `state` is `starting`, sets the `state` to `running`, otherwise leaves `state` unchanged. Then, equivalent to:

   > > >     while (auto* op = pop-front()) {
   > > >       op->execute();
   > > >     }

   > > -7- Remarks: When `state` changes, it does so without introducing data races.

   >     void finish() noexcept;

   > > -8- Preconditions: `state` is either `starting` or `running`.

   > > -9- Effects: Changes `state` to `finishing`.

   > > -10- Synchronization: `finish` synchronizes with the `pop-front` operation that returns `nullptr`.

[2026-03-27; Jonathan provides new wording]

Change to [exec.run.loop.types]/5 superseded by P3941.

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

> [Drafting note: It should be pointed out that member function `finish` should be `noexcept` for other reasons as well, see LWG 4215(i).]

1. Modify 33.12.1.1 [exec.run.loop.general], class `run_loop` synopsis, as indicated:

   >     namespace std::execution {
   >       class run_loop {
   >         // 33.12.1.2 [exec.run.loop.types], associated types
   >         class run-loop-scheduler;                        // exposition only
   >         class run-loop-sender;                           // exposition only
   >         struct run-loop-opstate-base {                   // exposition only
   >           virtual void execute() noexcept = 0;           // exposition only
   >           run_loop* loop;                                // exposition only
   >           run-loop-opstate-base* next;                   // exposition only
   >         };
   >         template<class Rcvr>
   >           using run-loop-opstate = unspecified;          // exposition only

   >         // 33.12.1.4 [exec.run.loop.members], member functions
   >         run-loop-opstate-base* pop-front() noexcept;     // exposition only
   >         void push-back(run-loop-opstate-base*) noexcept; // exposition only

   >       public:
   >         // 33.12.1.3 [exec.run.loop.ctor], constructor and destructor
   >         run_loop() noexcept;
   >         run_loop(run_loop&&) = delete;
   >         ~run_loop();

   >         // 33.12.1.4 [exec.run.loop.members] member functions
   >         run-loop-scheduler get_scheduler() noexcept;
   >         void run() noexcept;
   >         void finish() noexcept;
   >       };
   >     }

2. Modify 33.12.1.2 [exec.run.loop.types] as indicated:

   > -9- Let `o` be a non-const lvalue of type `run-loop-opstate<Rcvr>`, and let `REC(o)` be a non-const lvalue reference to an instance of type `Rcvr` that was initialized with the expression `rcvr` passed to the invocation of connect that returned `o`. Then:

   > - (9.1) — […]
   > - (9.2) — […]
   > - (9.3) — The expression `start(o)` is equivalent to:

   >   >     try {
   >   >       o.loop->push-back(addressof(o));
   >   >     } catch(...) {
   >   >       set_error(std::move(REC(o)), current_exception());
   >   >     }

3. Modify 33.12.1.4 [exec.run.loop.members] as indicated:

   >     run-loop-opstate-base* pop-front() noexcept;

   > > -1- Effects: Blocks (3.6 [defns.block]) until one of the following conditions is `true`: […]

   >     void push-back(run-loop-opstate-base* item) noexcept;

   > > -2- Effects: Adds `item` to the back of the queue and increments `count` by `1`.

   > > -3- Synchronization: This operation synchronizes with the `pop-front` operation that obtains `item`.

   >     run-loop-scheduler get_scheduler() noexcept;

   > > -4- Returns: An instance of `run-loop-scheduler` that can be used to schedule work onto this `run_loop` instance.

   >     void run() noexcept;

   > > -5- Preconditions: is either `starting` or `finishing`.

   > > -6- Effects: If `state` is `starting`, sets the `state` to `running`, otherwise leaves `state` unchanged. Then, equivalent to:

   > > >     while (auto* op = pop-front()) {
   > > >       op->execute();
   > > >     }

   > > -7- Remarks: When `state` changes, it does so without introducing data races.

   >     void finish() noexcept;

   > > -8- Preconditions: `state` is either `starting` or `running`.

   > > -9- Effects: Changes `state` to `finishing`.

   > > -10- Synchronization: `finish` synchronizes with the `pop-front` operation that returns `nullptr`.

### 4477(i). Placement `operator delete` should be constexpr

Section: 17.6.3.4 [new.delete.placement] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-11-18 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [new.delete.placement].

View all issues with C++26 status.

Discussion:

The P2747R2 paper made placement operator new constexpr. At that time constexpr exceptions weren't in C++26, so that was all that was needed. But later on when P3068R5 was voted in, the P2747R2 changes look insufficient. The problem is that when you throw from a constructor during operator new, it invokes placement operator delete. And P2747R2 didn't touch that.

This makes it impossible to handle an exception thrown by a placement new-expression during constant evaluation.

[2025-11-26; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 17.6.2 [new.syn] as indicated:

       constexpr void* operator new  (std::size_t size, void* ptr) noexcept;
       constexpr void* operator new[](std::size_t size, void* ptr) noexcept;
       constexpr void operator delete  (void* ptr, void*) noexcept;
       constexpr void operator delete[](void* ptr, void*) noexcept;

2. Modify 17.6.3.4 [new.delete.placement] as indicated:

   >     constexpr void* operator new(std::size_t size, void* ptr) noexcept;

   > > […]

   >     constexpr void* operator new[](std::size_t size, void* ptr) noexcept;

   > > […]

   >     constexpr void operator delete(void* ptr, void*) noexcept;

   > > -7- Effects: Intentionally performs no action.

   > > -8- Remarks: Default function called when any part of the initialization in a placement new-expression that invokes the library's non-array placement operator new terminates by throwing an exception (7.6.2.8 [expr.new]).

   >     constexpr void operator delete[](void* ptr, void*) noexcept;

   > > -9- Effects: Intentionally performs no action.

   > > -10- Remarks: Default function called when any part of the initialization in a placement new-expression that invokes the library's array placement operator new terminates by throwing an exception (7.6.2.8 [expr.new]).

### 4478(i). `meta::has_identifier` is not specified for annotations

Section: 21.4.6 [meta.reflection.names] Status: C++26 Submitter: Jakub Jelinek Opened: 2025-11-18 Last modified: 2026-06-09

Priority: 1

View all other issues in [meta.reflection.names].

View all issues with C++26 status.

Discussion:

The `std::meta::has_identifier` metafunction doesn't say what it returns for an annotation. An annotation is not an entity, so it's not an unnamed entity, and it's also not a type, type alias, function, or any of the other things in the list.

We should add an item to the list for annotations, or just add a catch-all "Otherwise false" to the end of the list.

[2025-11-26; Reflector poll.]

Set priority to 1 after reflector poll.

'value' and 'object' are missing too.

[Croydon 2026-03-25; LWG adds wording]

[Croydon 2026-03-25; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.6 [meta.reflection.names] as indicated:

   >     consteval bool has_identifier(info r);

   > > -1- Returns:
   > > 1. — [...]
   > > 2. — Otherwise, if `r` represents a data member description (T,N,A,W,NUA) (11.4.1 [class.mem.general]); `true` if N is not ⊥. Otherwise, `false`.
   > > 3. — Otherwise, `false`.

### 4480(i). `<stdatomic.h>` should provide `ATOMIC_CHAR8_T_LOCK_FREE`

Section: 32.5.12 [stdatomic.h.syn] Status: C++26 Submitter: Jiang An Opened: 2025-11-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [stdatomic.h.syn].

View all issues with C++26 status.

Discussion:

Currently, `<stdatomic.h>` is specified to provide `atomic_char8_t` but not its corresponding `ATOMIC_CHAR8_T_LOCK_FREE` macro, which is self-inconsistent. Also, given WG14 N2653 added `ATOMIC_CHAR8_T_LOCK_FREE` to C's `<stdatomic.h>` in C23, perhaps C++ should do the same thing in the spirit of P3348R4.

[2025-12-04; Reflector poll.]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 32.5.12 [stdatomic.h.syn], header `<stdatomic.h>` synopsis, as indicated:

   >     template<class T>
   >       using std-atomic = std::atomic<T>; // exposition only

   >     #define _Atomic(T) std-atomic<T>

   >     #define ATOMIC_BOOL_LOCK_FREE see below
   >     #define ATOMIC_CHAR_LOCK_FREE see below
   >     #define ATOMIC_CHAR8_T_LOCK_FREE see below
   >     #define ATOMIC_CHAR16_T_LOCK_FREE see below
   >     #define ATOMIC_CHAR32_T_LOCK_FREE see below
   >     #define ATOMIC_WCHAR_T_LOCK_FREE see below
   >     #define ATOMIC_SHORT_LOCK_FREE see below
   >     #define ATOMIC_INT_LOCK_FREE see below
   >     #define ATOMIC_LONG_LOCK_FREE see below
   >     #define ATOMIC_LLONG_LOCK_FREE see below
   >     #define ATOMIC_POINTER_LOCK_FREE see below

   >     […]

### 4481(i). Disallow `chrono::duration<const T, P>`

Section: 30.5.1 [time.duration.general] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-11-26 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Using a const type as the `rep` for a `chrono::duration` causes various problems but there seems to be no rule preventing it.

The non-const member operators that modify a `duration` don't work if the `rep` type is const, e.g. `duration<const int>::operator++()` is typically ill-formed (unless the implementation chooses to store `remove_cv_t<rep>` as the data member). `hash<duration<const int>>` uses `hash<const int>` which is not enabled, so you can't hash a `duration` with a const rep. Generic code that wants to perform arithmetic with the `rep` type would need to remember to consistently use `remove_cv_t<rep>` to work correctly with const types.

We should just disallow const `rep` types. If you want a non-modifiable duration, use `const duration<R,P>` not `duration<const R, P>`

[2026-01-16; Reflector poll.]

Set status to Tentatively Ready after ten votes in favour during reflector poll.

This loses support for `duration<volatile T, P>` but that seems OK.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 30.5.1 [time.duration.general] as indicated:

   > -2- `Rep` shall be an arithmetic type or a class emulating an arithmetic type. If a specialization of `duration` is instantiated with a cv-qualified type or a specialization of `duration` type as the argument for the template parameter `Rep`, the program is ill-formed.

   > -3- If `Period` is not a specialization of `ratio`, the program is ill-formed. If `Period::num` is not positive, the program is ill-formed.

   > -4- Members of `duration` do not throw exceptions other than those thrown by the indicated operations on their representations.

### 4483(i). Multidimensional arrays are not supported by `meta::reflect_constant_array` and related functions

Section: 21.4.3 [meta.define.static] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-11-27 Last modified: 2026-06-09

Priority: 2

View all other issues in [meta.define.static].

View all issues with C++26 status.

Discussion:

As any array type (even of structural types) is not considered an structural type, per 13.2 [temp.param] p12, any invocation of `reflect_constant_array`/`define_static_array` with a multidimensional array or `span` of arrays is ill-formed due to the Mandates in 21.4.3 [meta.define.static] p8 that requires range value type to be structural.

As a consequence, `constant_of` currently supports only single-dimensional arrays (`reflect_constant_array` strips outermost extents), while multi-dimensional arrays are rejected.

Furthermore, `define_static_object` currently uses `define_static_array(span(addressof(t), 1)).data()`, for array types. Since for `T[N]` input this creates an multidimensional `T[1][N]` constant parameter object, this function does not support arrays at all. Creating a distinct template parameter object leads to emission of (otherwise unnecessary) additional symbols, and breaks the invariant that for all supported object types `&constant_of(o) == define_static_object(o)`. We should use `reflect_constant_array` for arrays directly.

The Throws clause of `reflect_constant_array` was updated to include any exception thrown by iteration over range.

[2025-12-05; LWG telecon. Wording tweaks]

[2025-12-12; LWG telecon. Further wording tweaks]

[2026-01-16; Reflector poll.]

Set priority to 2 after reflector poll.

"The new Throws: paragraph implies that `meta::exception` is thrown if a copy constructor of an array element throws, but we want to propagate the thrown exception not catch it an throw a `meta::exception`. Maybe something like:"

> If evaluation of `reflect_constant` (including evaluation resulting from `reflect_constant_array` invocations) would result in an exception `E` being thrown, then:
> - `E` if an exception was thrown by evaluation of an argument (if any), and
> - `meta::exception` otherwise.

"I suggest:"

> Throws: Any of
> - an exception thrown by any operation on `r` or on iterators and sentinels referring to `r`,
> - an exception thrown by the evaluation of any argument of `reflect_constant` or by any evaluation of `reflect_constant_array`, or
> - `meta::exception` if any invocation of `reflect_constant` would exit via an exception.

Previous resolution [SUPERSEDED]

This wording is relative to N5014 amended with changes from LWG 4432(i).

1. Modify 21.4.3 [meta.define.static] as indicated:

       template<ranges::input_range R>
         consteval info reflect_constant_array(R&& r);

   > -8- Let `TU` be `ranges::range_value_t<R>` and `T` be `remove_all_extents_t<U>` ei be `static_cast<T>(*iti)`, where iti is an iterator to the ith element of `r`.

   > -9- Mandates:

   > - (9.1) — `T` is a structural type (13.2 [temp.param]), `is_constructible_v<T, ranges::range_reference_t<R>>` is `true`, and
   > - (9.2) — `T` satisfies `copy_constructible`, and
   > - (9.3) — if `U` is not an array type, then `is_constructible_v<T, ranges::range_reference_t<R>>` is `true`.

   > -10- Let `V` be the pack of values of type `info` of the same size as `r`, where the ith element is

   > - (10.1) — `reflect_constant_array(*iti)` if `U` is an array type,
   > - (10.2) — `reflect_constant(static_cast<T>(*iti)ei)` otherwise,
   > and iti is an iterator to the ith element of `r`.

   > -11- Let `P` be

   > - (11.1) — If `sizeof...(V) > 0` is `true`, then the template parameter object (13.2 [temp.param]) of type const `T[sizeof...(V)]` initialized with `{[:V:]...}`, such that `P[I]` is template-argument-equivalent (13.6 [temp.type]) to the object represented by `V...[I]` for all `I` in the range [`0`, `sizeof...(V)`).
   > - (11.2) — Otherwise, the template parameter object of type `const array<T, 0>` initialized with `{}`.

   > -12- Returns: `^^P`.

   > -13- Throws: Any exception thrown by operations on `r` or on iterators and sentinels referring to `r`. Any exception thrown by the evaluation of any argument of `reflect_constant`.ei, or `meta::exception` if evaluation of any `reflect_constant(ei)` any invocation of `reflect_constant` or `reflect_constant_array` would exit via an exception.

   […]

       template<class T>
         consteval const remove_cvref_t<T>* define_static_object(T&& t);

   > -15- Effects:Equivalent to:

   >     using U = remove_cvref_t<T>;
   >     if constexpr (meta::is_class_type(^^U)) {
   >       return addressof(meta::extract<const U&>(meta::reflect_constant(std::forward<T>(t))));
   >     } else if constexpr (meta::is_array_type(^^U)) {
   >       return addressof(meta::extract<const U&>(meta::reflect_constant_array(std::forward<T>(t))));
   >     } else {
   >       return define_static_array(span(addressof(t), 1)).data();
   >     }

[2026-02-27; LWG telecon. Further wording tweaks]

Rebased on top of N5032

Used alternative suggestion for Throws clause.

`define_static_object` now uses `reflect_constant` also for `union` types.

[2026-03-06; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.3 [meta.define.static] as indicated:

       template<ranges::input_range R>
         consteval info reflect_constant_array(R&& r);

   > -8- Let `TU` be `ranges::range_value_t<R>` and `T` be `remove_all_extents_t<U>` ei be `static_cast<T>(*iti)`, where iti is an iterator to the ith element of `r`.

   > -9- Mandates:

   > - (9.1) — `T` is a structural type (13.2 [temp.param]), `is_constructible_v<T, ranges::range_reference_t<R>>` is `true`, and
   > - (9.2) — `T` satisfies `copy_constructible`, and
   > - (9.3) — if `U` is not an array type, then `is_constructible_v<T, ranges::range_reference_t<R>>` is `true`.

   > -10- Let `V` be the pack of values of type `info` of the same size as `r`, where the ith element is

   > - (10.1) — `reflect_constant_array(*iti)` if `U` is an array type,
   > - (10.2) — `reflect_constant(static_cast<T>(*iti)ei)` otherwise,
   > and iti is an iterator to the ith element of `r`.

   > -11- Let `P` be

   > - (11.1) — If `sizeof...(V) > 0` is `true`, then the template parameter object (13.2 [temp.param]) of type const `T[sizeof...(V)]` initialized with `{[:V:]...}`, such that `P[I]` is template-argument-equivalent (13.6 [temp.type]) to the object represented by `V...[I]` for all `I` in the range [`0`, `sizeof...(V)`).
   > - (11.2) — Otherwise, the template parameter object of type `const array<T, 0>` initialized with `{}`.

   > -12- Returns: `^^P`.

   > -13- Throws: Any exception thrown by the evaluation of any ei, or `meta::exception` if evaluation of any `reflect_constant(ei)` would exit via an exception. Any of

   > - -13.1- an exception thrown by any operation on `r` or on iterators and sentinels referring to `r`,
   > - -13.2- an exception thrown by the evaluation of any argument of `reflect_constant` or by any evaluation of `reflect_constant_array`, or
   > - -13.3- `meta::exception` if any invocation of `reflect_constant` would exit via an exception.

   […]

       template<class T>
         consteval const remove_cvref_t<T>* define_static_object(T&& t);

   > -15- Effects:Equivalent to:

   >     using U = remove_cvref_t<T>;
   >     if constexpr (meta::is_class_type(^^U) || meta::is_union_type(^^U)) {
   >       return addressof(meta::extract<const U&>(meta::reflect_constant(std::forward<T>(t))));
   >     } else if constexpr (meta::is_array_type(^^U)) {
   >       return addressof(meta::extract<const U&>(meta::reflect_constant_array(std::forward<T>(t))));
   >     } else {
   >       return define_static_array(span(addressof(t), 1)).data();
   >     }

### 4485(i). Move specification for `task::stop_token_type`

Section: 33.13.6.4 [task.state] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-12-08 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 249-379

It is not clear what bullet 33.13.6.4 [task.state] p4.6 is – it reads like a requirement on `stop_token_type`, but if so, this paragraph is a poor place for it.

[2025-12-05 Tomasz comments]

The paragraph expresses requirements on user supplied `Environment::stop_source_type`, that needs to model `stoppable-source`, which includes `stoppable-callback-for` on associated token. We should also require that `Environment::scheduler_type` shall satisfy `scheduler`.

We also need to clarify that `stop_possible` and `stop_requested` on `prom.token` returns same value as `st` during lifetime of asynchronous operation.

[2026-01-16; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Croydon 2026-03-27; reopen due to conflict with LWG 4347(i).]

Previous resolution [SUPERSEDED]

This wording is relative to N5014.

1. Modify 33.13.6 [exec.task] as indicated:

   > -5- `allocator_type` shall meet the Cpp17Allocator requirements, `scheduler_type` shall model `scheduler`, and `stop_source_type` shall model `stoppable-source`.

2. Modify 33.13.6.4 [task.state] as indicated:

       void start() & noexcept;

   > -4- Effects: Effects: Let `prom` be the object `handle.promise()`. Associates `STATE(prom)`, `RCVR(prom)`, and `SCHED(prom)` with `*this` as follows:

   > - -4.1- […]
   > - -4.2- […]
   > - -4.3- […]
   > Let `st` be `get_stop_token(get_env(rcvr))`. Initializes `prom.token` and `prom.source` such that during the lifetime of the asynchronous operation (33.3 [exec.async.ops]) associated with `*this`
   > - -4.4- `prom.token.stop_requested()` returns `st.stop_requested()`;
   > - -4.5- `prom.token.stop_possible()` returns `st.stop_possible()`;.
   > - -4.6- for types `Fn` and `Init` such that both `invocable<Fn>` and `constructible_from<Fn, Init>` are modeled, `stop_token_type::callback_type<Fn>` models `stoppable-callback-for<Fn, stop_token_type, Init>`.

[Croydon 2026-03-27; Jonathan provides new wording]

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 33.13.6.2 [task.class] as indicated:

   > -5- `allocator_type` shall meet the Cpp17Allocator requirements, `scheduler_type` shall model `scheduler`, and `stop_source_type` shall model `stoppable-source`.

### 4486(i). `integral-constant-like` and `constexpr-wrapper-like` exposition-only concept duplication

Section: 23.7.2.1 [span.syn], 29.10.2 [simd.expos] Status: C++26 Submitter: Jonathan Wakely Opened: 2025-12-12 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [span.syn].

View all issues with C++26 status.

Discussion:

Addresses US 151-242

NB comment:

> We have two exposition-only concepts for similar things (`integral-constant-like` and `constant-wrapper-like`). The former can be expressed in terms of the latter.
> Proposed change: Move `constant-wrapper-like` to the library introduction and update `integral-constant-like` to use it.

[2025-12-12; LWG telecon; Status changed: New → Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 16.3.3.2 [expos.only.entity] by moving the concept (unchanged) from 29.10.2 [simd.expos]:

   >       template<class T, class U = T>
   >       using synth-three-way-result =                        // exposition only
   >         decltype(synth-three-way(declval<T&>(), declval<U&>()));

   >       template<class T>
   >         concept constexpr-wrapper-like =                  // exposition only
   >           convertible_to<T, decltype(T::value)> &&
   >           equality_comparable_with<T, decltype(T::value)> &&
   >           bool_constant<T() == T::value>::value &&
   >           bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;
   >     }

2. Modify 23.7.2.1 [span.syn] as indicated:

   >     template<class T>
   >       concept integral-constant-like =                    // exposition only
   >         is_integral_v<remove_cvref_t<decltype(T::value)>> &&
   >         !is_same_v<bool, remove_const_t<decltype(T::value)>> &&
   >         constexpr-wrapper-like<T>;
   >         convertible_to<T, decltype(T::value)> &&
   >         equality_comparable_with<T, decltype(T::value)> &&
   >         bool_constant<T() == T::value>::value &&
   >         bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;

3. Modify 29.10.2 [simd.expos] as indicated:

   >     template<class T>
   >       concept constexpr-wrapper-like =                    // exposition only
   >         convertible_to<T, decltype(T::value)> &&
   >         equality_comparable_with<T, decltype(T::value)> &&
   >         bool_constant<T() == T::value>::value &&
   >         bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;

### 4487(i). Is member `is_steady` of a Cpp17Clock type required to be usable in constant expressions?

Section: 30.3 [time.clock.req] Status: WP Submitter: Jiang An Opened: 2025-11-26 Last modified: 2026-06-15

Priority: 3

View all other issues in [time.clock.req].

View all issues with WP status.

Discussion:

In 30.3 [time.clock.req]/[tab:time.clock], the static data member `C1::is_steady` seemingly indicates the property of the clock statically, as the requirements say "always".

However, it is not clear whether `C1::steady` should be usable in a constant expression. A hostile reading may indicate that it is allowed to make `C1::steady` only defined in some separated translated unit and/or initialized from some non-constant expression.

Also, it is not very clear that the "always" means the conditions always hold in all executions, or only always hold in the current execution. The latter reading allows `C1::steady` to vary between executions.

[2026-01-16; Reflector poll.]

Set priority to 3 after reflector poll.

Unanimous preference for Option A.

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

> [Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A: Requiring usability in constant expressions.

1. Modify Table [tab:time.clock] — Cpp17Clock requirements as indicated:

   > Table 131 — Cpp17Clock requirements [tab:time.clock]
   > Expression | Return type | Operational semantics
   > --- | --- | ---
   > `[…]`
   > `C1::is_steady` | `const bool` | `true` if `t1 <= t2` is always `true` and the time between clock ticks is constant, otherwise `false`. `C1::is_steady` shall be usable in constant expressions (7.7 [expr.const]).
   > `[…]`

Option B: Allowing varying between executions.

1. Modify Table [tab:time.clock] — Cpp17Clock requirements as indicated:

   > Table 131 — Cpp17Clock requirements [tab:time.clock]
   > Expression | Return type | Operational semantics
   > --- | --- | ---
   > `[…]`
   > `C1::is_steady` | `const bool` | `true` if `t1 <= t2` is always `true` and the time between clock ticks is constant for the duration of the current program execution, otherwise `false`. [Note ?: `C1::is_steady` is not required to be usable in constant expressions (7.7 [expr.const]) and can vary between different executions. — end note]
   > `[…]`

[2026-05-22; Tim updates wording to use Option A based on reflector discussion]

[2026-05-22 LWG telecon; Status changed: New → Ready]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify Table [tab:time.clock] — Cpp17Clock requirements as indicated:

   > Table 131 — Cpp17Clock requirements [tab:time.clock]
   > Expression | Return type | Operational semantics
   > --- | --- | ---
   > `[…]`
   > `C1::is_steady` | `const bool` | `true` if `t1 <= t2` is always `true` and the time between clock ticks is constant, otherwise `false`. `C1::is_steady` shall be usable in constant expressions (7.7 [expr.const]).
   > `[…]`

### 4490(i). Allow calling `std::ranges::size` in ranges algorithms

Section: 26.2 [algorithms.requirements] Status: WP Submitter: Ruslan Arutyunyan Opened: 2025-12-12 Last modified: 2026-06-15

Priority: 3

View other active issues in [algorithms.requirements].

View all other issues in [algorithms.requirements].

View all issues with WP status.

Discussion:

26.2 [algorithms.requirements] paragraph 14 says that the algorithms in `ranges` namespace are implemented as if they are dispatched to the corresponding overload with iterator and sentinel. However, there are two problems with the current wording:

1. We only allow to call either `std::ranges::end(r)` or `std::ranges::next(std::ranges::begin(r)`, `std::ranges::end())` to calculate a corresponding sentinel. However, this is a pessimization for some ranges because we can have sized ranges without sized_sentinel_for. Consider the following example:

   >     const char str[] = "something";
   >     ranges::subrange<const char*,
   >                     null_sentinel_t,
   >                     subrange_kind::sized> sr(ranges::begin(str),
   >                                               null_sentinel,
   >                                               ranges::size(str) - 1);
   >     my::ranges::next(sr.begin(), sr.end()); // this line serially calculates iterator that is equal to sr.end()

   Despite the fact that we know the size of the range and that the range is the random access one, `std::ranges::next` calculates the iterator serially, because it only has constant time complexity optimization for `sized_sentinel_for`. We could clearly achieve better complexity with a different API or with the different overload of the same API.

2. It is unclear when an algorithm calls `std::ranges::end(r)` and when it calls `std::ranges::next(std::ranges::begin(r), std::ranges::end())` to calculate the corresponding sentinel because there is no established priority between them. Maybe, it's smaller problem but still it's worth clarifying in my opinion.

[2026-01-16; Reflector poll.]

Set priority to 3 after reflector poll.

"Should be 'the type of `r` models `sized_range`"

"It's a lot of words, could be shorter. Would prefer not to establish priority, but let implementation decide if/when to use `ranges::next`."

"Would `counted_iterator(ranges::begin(r), ranges::distance(r))` and `default_sentinel` be better?"

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Modify 26.2 [algorithms.requirements] as indicated:

   > […]

   > -14- Overloads of algorithms that take `range` arguments (25.4.2 [range.range]) behave as if they are implemented by dispatching to the overload in namespace `ranges` that takes separate iterator and sentinel arguments, where for each range argument `r`

   > - (14.1) — a corresponding iterator argument is initialized with `ranges::begin(r)` and

   > - (14.2) — a corresponding sentinel argument is initialized with `ranges::end(r)`, or `ranges::next(ranges::begin(r), ranges::end(r))` if the type of `r` models `forward_range` and computing `ranges::next` meets the specified complexity requirements.one of the following:

   >   - (14.2.1) — if the type of `r` models `forward_range` and computing `ranges::next` meets the specified complexity requirements then

   >     - (14.2.1.1) — `ranges::next(ranges::begin(r), ranges::size(r))` if `r` models `sized_range`, otherwise

   >     - (14.2.1.2) — `ranges::next(ranges::begin(r), ranges::end(r))`.

   >   - (14.2.2) — Otherwise, `std::ranges::end(s)`.

[2026-01-30; Jonathan provides new wording]

We can just say that the value is either the end iterator, or begin+N, without saying when or how the implementation obtains begin+N. It might use `ranges::next(ranges::begin(r), std::ranges::end(r))` or `ranges::next(ranges::begin(r), ranges::distance(r))` or something else, only the value matters not how it's obtained. Paragraph 11 in the same subclause defines what begin+N means if the iterator is not a random access iterator.

[2026-01-30; LWG telecon. Status → Open.]

[2026-05-19; LWG telecon. Status changed: Open → Ready.]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 26.2 [algorithms.requirements] as indicated:

   > […]

   > -14- Overloads of algorithms that take `range` arguments (25.4.2 [range.range]) behave as if they are implemented by dispatching to the overload in namespace `ranges` that takes separate iterator and sentinel arguments, where for each range argument `r`

   > - (14.1) — a corresponding iterator argument is initialized with `ranges::begin(r)` and

   > - (14.2) — a corresponding sentinel argument is initialized with `ranges::end(r)`, or `ranges::next(ranges::begin(r), ranges::end(r))` if the type of `r` models `forward_range` and computing `ranges::next` meets the specified complexity requirements. `ranges::begin(r) + N` where `N` is equal to `ranges::distance(r)`.

### 4491(i). Rename `submdspan_extents` and `submdspan_canonicalize_slices`

Section: 23.7.3.7 [mdspan.sub] Status: C++26 Submitter: Tomasz Kamiński Opened: 2025-12-16 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 152-243 and PL-008.

Rename `submdspan_extents` to `subextents` and `submdspan_canonicalize_slices` to `canonical_slices`.

[2026-01-16; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 23.7.3.2 [mdspan.syn] as follows:

   >     // 23.7.3.7 [mdspan.sub], submdspan creation
   >     template<class OffsetType, class LengthType, class StrideType>
   >       struct strided_slice;

   >     template<class LayoutMapping>
   >       struct submdspan_mapping_result;

   >     struct full_extent_t { explicit full_extent_t() = default; };
   >     inline constexpr full_extent_t full_extent{};

   >     template<class IndexType, size_t... Extents, class... SliceSpecifiers>
   >       constexpr auto submdspan_extents(const extents<IndexType, Extents...>&, SliceSpecifiers...);

   >     // 23.7.3.7.5 [mdspan.sub.canonical], submdspan slice canonicalization
   >     template<class IndexType, size_t... Extents, class... Slices>
   >       constexpr auto submdspan_canonicalizecanonical_slices(const extents<IndexType, Extents...>& src,
   >                                                    Slices... slices);

2. Modify 23.7.3.7.5 [mdspan.sub.canonical] as follows:

   > #### `submdspan` slice canonicalization

   >     template<class IndexType, size_t... Extents, class... Slices>
   >       constexpr auto submdspan_canonicalizecanonical_slices(const extents<IndexType, Extents...>& src,
   >                                                    Slices... slices);

3. Modify 23.7.3.7.6 [mdspan.sub.extents] as follows:

   > #### 23.7.3.7.6 [mdspan.sub.extents] `submdspan_extents` function

   >     template<class IndexType, size_t... Extents, class... SliceSpecifiers>
   >       constexpr auto submdspan_extents(const extents<IndexType, Extents...>& src,
   >                                        SliceSpecifiers... raw_slices);

   > > -1- Let `slices` be the pack introduced by the following declaration:

   > >       auto [...slices] = submdspan_canonicalizecanonical_slices(src, raw_slices...)

4. Modify 23.7.3.7.7.1 [mdspan.sub.map.sliceable] as follows:

   > > -5- Returns: An object `smr` of type `SMR` such that

   > > - -5.1- `smr.mapping.extents() == submdspan_extents(m.extents(), valid_slices...)` is `true`; and
   > > - -5.2- […]

5. Modify 23.7.3.7.7.2 [mdspan.sub.map.common] as follows:

   > > -5- Let `sub_ext` be the result of `submdspan_extents(extents(), slices...)` and let `SubExtents` be `decltype(sub_ext)`.

6. Modify 23.7.3.7.8 [mdspan.sub.sub] as follows:

   > > -2- Let `slices` be the pack introduced by the following declaration:

   > >       auto [...slices] = submdspan_canonicalizecanonical_slices(src, raw_slices...)

### 4492(i). `std::generate` and `std::ranges::generate` wording is unclear for parallel algorithms

Section: 26.7.7 [alg.generate] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2025-12-12 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

`std::generate` and `std::ranges::generate` are confusing for parallel algorithms. For both of those Effects says "Assigns the result of successive evaluations of `gen()` through each iterator in the range `[first, first + N)`." The word "successive" is confusing when we talk about parallelism. This wording was the preexisting one; P3179 "Parallel Range Algorithms" didn't modify that, so the problem existed even before.

Another problem is that there is nothing about what is allowed to do with the `Generator` template parameter type for C++17 parallel `generate`. Intel, NVIDIA, and GNU libstdc++ implementations do multiple copies of `Generator` object to maximize parallelism, while it's not technically allowed if I am reading the standard correctly. But without `Generator` being copyable we have point of contention and extra synchronization (or we put synchronization part as users' responsibility, which is also not obvious).

There is no clear solution right away. We could try to fix the wording for both ranges and C++17 parallel generate but it seems like it requires extra effort because we need to at least verify the new behavior with LLVM libc++ implementation if we want to make `Generator` copyable; libc++ currently does not create per-thread copy of `gen` object. Perhaps, the best strategy for now is to remove `ranges::generate` and `ranges::generate_n` and return them back in C++29 time frame when we figure out the proper fix.

[2026-01-16; Reflector poll.]

Set priority to 2 after reflector poll.

[2026-01-30; LWG telecon. Status → Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 26.4 [algorithm.syn], header `<algorithm>` synopsis, as indicated:

   >     […]
   >     // 26.7.7 [alg.generate], generate
   >     template<class ForwardIterator, class Generator>
   >       constexpr void generate(ForwardIterator first, ForwardIterator last,
   >                               Generator gen);
   >     template<class ExecutionPolicy, class ForwardIterator, class Generator>
   >       void generate(ExecutionPolicy&& exec,   // freestanding-deleted, see 26.3.5 [algorithms.parallel.overloads]
   >                     ForwardIterator first, ForwardIterator last,
   >                     Generator gen);
   >     template<class OutputIterator, class Size, class Generator>
   >       constexpr OutputIterator generate_n(OutputIterator first, Size n, Generator gen);
   >     template<class ExecutionPolicy, class ForwardIterator, class Size, class Generator>
   >       ForwardIterator generate_n(ExecutionPolicy&& exec,   // freestanding-deleted, see 26.3.5 [algorithms.parallel.overloads]
   >                                  ForwardIterator first, Size n, Generator gen);

   >     namespace ranges {
   >       template<input_or_output_iterator O, sentinel_for<O> S, copy_constructible F>
   >         requires invocable<F&> & indirectly_writable<O, invoke_result_t<F&>>
   >         constexpr O generate(O first, S last, F gen);
   >       template<class R, copy_constructible F>
   >         requires invocable<F&> & output_range<R, invoke_result_t<F&>>
   >         constexpr borrowed_iterator_t<R> generate(R&& r, F gen);
   >       template<input_or_output_iterator O, copy_constructible F>
   >         requires invocable<F&> & indirectly_writable<O, invoke_result_t<F&>>
   >         constexpr O generate_n(O first, iter_difference_t<O> n, F gen);
   >       template<execution-policy Ep, random_access_iterator O, sized_sentinel_for<O> S,
   >                copy_constructible F>
   >         requires invocable<F&> & indirectly_writable<O, invoke_result_t<F&>>
   >         O generate(Ep&& exec, O first, S last, F gen); // freestanding-deleted
   >       template<execution-policy Ep, sized-random-access-range R, copy_constructible F>
   >         requires invocable<F&> & indirectly_writable<iterator_t<R>, invoke_result_t<F&>>
   >         borrowed_iterator_t<R> generate(Ep&& exec, R&& r, F gen); // freestanding-deleted
   >       template<execution-policy Ep, random_access_iterator O, copy_constructible F>
   >         requires invocable<F&> & indirectly_writable<O, invoke_result_t<F&>>
   >         O generate_n(Ep&& exec, O first, iter_difference_t<O> n, F gen); // freestanding-deleted
   >     }
   >     […]

2. Modify 26.7.7 [alg.generate] as indicated:

   >     template<class ForwardIterator, class Generator>
   >       constexpr void generate(ForwardIterator first, ForwardIterator last,
   >                               Generator gen);
   >     template<class ExecutionPolicy, class ForwardIterator, class Generator>
   >       void generate(ExecutionPolicy&& exec,
   >                     ForwardIterator first, ForwardIterator last,
   >                     Generator gen);

   >     template<class OutputIterator, class Size, class Generator>
   >       constexpr OutputIterator generate_n(OutputIterator first, Size n, Generator gen);
   >     template<class ExecutionPolicy, class ForwardIterator, class Size, class Generator>
   >       ForwardIterator generate_n(ExecutionPolicy&& exec,
   >                                  ForwardIterator first, Size n, Generator gen);

   >     template<input_or_output_iterator O, sentinel_for<O> S, copy_constructible F>
   >       requires invocable<F&> & indirectly_writable<O, invoke_result_t<F&>>
   >       constexpr O ranges::generate(O first, S last, F gen);
   >     template<class R, copy_constructible F>
   >       requires invocable<F&> & output_range<R, invoke_result_t<F&>>
   >       constexpr borrowed_iterator_t<R> ranges::generate(R&& r, F gen);
   >     template<input_or_output_iterator O, copy_constructible F>
   >       requires invocable<F&> & indirectly_writable<O, invoke_result_t<F&>>
   >       constexpr O ranges::generate_n(O first, iter_difference_t<O> n, F gen);
   >     template<execution-policy Ep, random_access_iterator O, sized_sentinel_for<O> S,
   >              copy_constructible F>
   >       requires invocable<F&> & indirectly_writable<O, invoke_result_t<F&>>
   >       O ranges::generate(Ep&& exec, O first, S last, F gen);
   >     template<execution-policy Ep, sized-random-access-range R, copy_constructible F>
   >       requires invocable<F&> & indirectly_writable<iterator_t<R>, invoke_result_t<F&>>
   >       borrowed_iterator_t<R> ranges::generate(Ep&& exec, R&& r, F gen);
   >     template<execution-policy Ep, random_access_iterator O, copy_constructible F>
   >       requires invocable<F&> & indirectly_writable<O, invoke_result_t<F&>>
   >       O ranges::generate_n(Ep&& exec, O first, iter_difference_t<O> n, F gen);

   > > -1- Let `N` be `max(0, n)` for the `generate_n` algorithms, and `last - first` for the `generate` algorithms.

   > > […]

### 4493(i). Specification for some functions of bit reference types seems missing

Section: 22.9.2.1 [template.bitset.general], 23.3.14.1 [vector.bool.pspc] Status: C++26 Submitter: Jiang An Opened: 2025-12-16 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [template.bitset.general].

View all issues with C++26 status.

Discussion:

We haven't explicitly specified the return values of `bitset<N>::reference::operator bool()` and `vector<bool, A>::reference::operator bool()`, although the intended return values can be inferred from "a class that simulates a reference to a single bit in the sequence". Moreover, specification for `bitset<N>::reference::operator~` seems completely missing, and the comment "flip the bit" seems misleading. Implementations consistently make the `operator~` return `!operator bool()`.

[2026-02-18; Reflector poll.]

Changed resolution to say "the bit referred to by `*this`" instead of "the referred to bit".

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 22.9.2.1 [template.bitset.general] as indicated:

   > >     […]
   > >     // bit reference
   > >     class reference {
   > >     public:
   > >       constexpr reference(const reference& x) noexcept;
   > >       constexpr ~reference();
   > >       constexpr reference& operator=(bool x) noexcept; // for b[i] = x;
   > >       constexpr reference& operator=(const reference& x) noexcept; // for b[i] = b[j];
   > >       constexpr const reference& operator=(bool x) const noexcept;
   > >       constexpr operator bool() const noexcept; // for x = b[i];
   > >       constexpr bool operator~() const noexcept; // flips the bit
   > >       constexpr reference& flip() noexcept; // for b[i].flip();

   > >       friend constexpr void swap(reference x, reference y) noexcept;
   > >       friend constexpr void swap(reference x, bool& y) noexcept;
   > >       friend constexpr void swap(bool& x, reference y) noexcept;
   > >     };
   > >     […]

   > […]

   >     constexpr reference& reference::operator=(bool x) noexcept;
   >     constexpr reference& reference::operator=(const reference& x) noexcept;
   >     constexpr const reference& reference::operator=(bool x) const noexcept;

   > > -6- Effects: Sets the bit referred to by `*this` if `bool(x)` is `true`, and clears it otherwise.

   > > -7- Returns: `*this`.

   >     constexpr reference::operator bool() const noexcept;

   > > -?- Returns: `true` if the value of bit referred to by `*this` is one, `false` otherwise.

   >     constexpr bool reference::operator~() const noexcept;

   > > -?- Returns: `!bool(*this)`.

2. Modify 23.3.14.1 [vector.bool.pspc] as indicated:

   > […]

   >     constexpr reference& reference::operator=(bool x) noexcept;
   >     constexpr reference& reference::operator=(const reference& x) noexcept;
   >     constexpr const reference& reference::operator=(bool x) const noexcept;

   > > -7- Effects: Sets the bit referred to by `*this` when `bool(x)` is `true`, and clears it otherwise.

   > > -8- Returns: `*this`.

   >     constexpr reference::operator bool() const noexcept;

   > > -?- Returns: `true` if the value of the bit referred to by `*this` is one, `false` otherwise.

### 4496(i). Precedes vs Reachable in [meta.reflection]

Section: 21.4 [meta.reflection] Status: C++26 Submitter: Daniel Katz Opened: 2025-12-11 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

Discussion on the Core mailing list surfaced a handful of places in 21.4 [meta.reflection] that use the "precedes" relation (defined in 6.5 [basic.lookup] and primarily used for name lookup) when the "reachable" relation (defined in 10.7 [module.reach]) is really what we want.

[2026-02-18; Reflector poll.]

Set priority to 2 after reflector poll.

[2026-02-20; LWG telecon; Status changed: New → Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.6 [meta.reflection.names] as indicated:

   >     consteval bool has_identifier(info r);

   > > -1- Returns:

   > > - (1.1) — […]

   > > - […]

   > > - (1.8) — Otherwise, if `r` represents the parameter `P` of a function `F`, then let `S` be the set of declarations, ignoring any explicit instantiations, that precede someare reachable from a point in the evaluation context and that declare either `F` or a templated function of which `F` is a specialization; […]

   > > - […]

   >     consteval string_view identifier_of(info r);
   >     consteval u8string_view u8identifier_of(info r);

   > > -2- Let `E` be UTF-8 for `u8identifier_of`, and otherwise the ordinary literal encoding.

   > > -3- Returns: An ntmbs, encoded with `E`, determined as follows:

   > > - (3.1) — […]

   > > - (3.2) — […]

   > > - (3.3) — Otherwise, if `r` represents the parameter `P` of a function `F`, then let `S` be the set of declarations, ignoring any explicit instantiations, that precede someare reachable from a point in the evaluation context and that declare either `F` or a templated function of which `F` is a specialization; the name that was introduced by a declaration in `S` for the parameter corresponding to `P`.

   > > - […]

2. Modify 21.4.7 [meta.reflection.queries] as indicated:

   >     consteval info type_of(info r);

   > > -2- Returns:

   > > - (2.1) — […]

   > > - […]

   > > - (2.4) — Otherwise, if `r` represents an enumerator `N` of an enumeration `E`, then:

   > >   - (2.4.1) — If `E` is defined by a declaration `D` that precedesis reachable from a point `P` in the evaluation context and `P` does not occur within an enum-specifier of `D`, then a reflection of `E`.

   > >   - (2.4.2) — Otherwise, a reflection of the type of `N` prior to the closing brace of the enum-specifier as specified in 9.8.1 [dcl.enum].

   > > - […]

   > […]

   >     consteval bool has_default_argument(info r);

   > > -41- Returns: If `r` represents a parameter `P` of a function `F`, then:

   > > - (41.1) — If `F` is a specialization of a templated function `T`, then `true` if there exists a declaration `D` of `T` that precedes someis reachable from a point in the evaluation context and `D` specifies a default argument for the parameter of `T` corresponding to `P`. Otherwise, `false`.

   > > - (41.2) — Otherwise, if there exists a declaration `D` of `F` that precedes someis reachable from a point in the evaluation context and `D` specifies a default argument for `P`, then `true`.

3. Modify 21.4.13 [meta.reflection.annotation] as indicated:

   >     consteval vector<info> annotations_of(info item);

   > > -1- Let `E` be […]

   > > -2- Returns: A `vector` containing all of the reflections `R` representing each annotation applying to each declaration of `E` that precedesis reachable from either somea point in the evaluation context (7.7 [expr.const]) or a point immediately following the class-specifier of the outermost class for which such a point is in a complete-class context. […]

### 4497(i). `std::nullopt_t` should be comparable

Section: 22.5.5 [optional.nullopt] Status: C++26 Submitter: Barry Revzin Opened: 2025-12-21 Last modified: 2026-06-09

Priority: 2

View all other issues in [optional.nullopt].

View all issues with C++26 status.

Discussion:

`std::nullopt_t` currently has no comparison operators. This prevents perfectly reasonable code from working, like `ranges::find(v, nullopt)` where `v` is a `vector<optional<T>>`, for no good reason.

Additionally, `optional<T>` has the full set of comparisons. But `optional<T>` is conceptually a `variant<nullopt_t, T>`, which wouldn't be comparable... because of `nullopt_t`. Other empty types like `tuple<>` and `monostate` are also comparable.

Proposed resolution: Add a defaulted member `operator<=>` to `nullopt_t`.

[2026-02-18; Reflector poll.]

Set priority to 2 after reflector poll.

Discussion whether this needs LEWG approval. Paper P2405, for which the `nullopt` part received LEWG support, is relevant here.

[2026-03-26; Tim provides wording]

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Edit 22.5.5 [optional.nullopt], as indicated:

   >     struct nullopt_t{see below};
   >     inline constexpr nullopt_t nullopt(unspecified);

   -1- […]

   -2- Type `nullopt_t` shalldoes not have a default constructor or an initializer-list constructor, and shall not beis not an aggregate. `nullopt_t` models `copyable` and `three_way_comparable<strong_ordering>`.

### 4499(i). `flat_set::insert_range` specification may be problematic

Section: 23.6.11.5 [flat.set.modifiers], 23.6.12.5 [flat.multiset.modifiers] Status: C++26 Submitter: Hewill Kang Opened: 2025-12-22 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

The function adds elements via:

>     ranges::for_each(rg, [&](auto&& e) {
>       c.insert(c.end(), std::forward<decltype(e)>(e));
>     });

Here, `e` is an element of the input range.

However, this can lead to ambiguity when `e` can also be converted to `initializer_list`, as `vector::insert` has an overload of `insert(const_iterator, initializer_list<T>)`.

[2026-02-18; Reflector poll.]

Set priority to 2 after reflector poll.

Pessimizing a very common case (matching type), for uncommon one.

[2026-02-20; LWG telecon; Status changed: New → Ready.]

Implementations can optimize away the extra move for the case where the range's value type is already the same as the flat set's value type.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 23.6.11.5 [flat.set.modifiers] as indicated:

   >     template<container-compatible-range<value_type> R>
   >       constexpr void insert_range(R&& rg);

   > > -10- Effects: Adds elements to `c` as if by:

   > > >     ranges::for_each(rg, [&](value_typeauto&& e) {
   > > >       c.insert(c.end(), std::movestd::forward<decltype(e)>(e));
   > > >     });

2. Modify 23.6.12.5 [flat.multiset.modifiers] as indicated:

   >     template<container-compatible-range<value_type> R>
   >       void insert_range(R&& rg);

   > > -9- Effects: Adds elements to `c` as if by:

   > > >     ranges::for_each(rg, [&](value_typeauto&& e) {
   > > >       c.insert(c.end(), std::movestd::forward<decltype(e)>(e));
   > > >     });

### 4500(i). `constant_wrapper` wording problems

Section: 22.2.10 [const.wrap.class] Status: C++26 Submitter: Matthias Wippich Opened: 2026-01-07 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [const.wrap.class].

View all issues with C++26 status.

Discussion:

During resolution of LWG 4383(i) prefix and postfix increment and decrement operators were changed to

>     template<constexpr-param T>
>       constexpr auto operator++(this T) noexcept -> constant_wrapper<++Y>
>         { return {}; }
>     template<constexpr-param T>
>       constexpr auto operator++(this T, int) noexcept ->
>         constant_wrapper<Y++> { return {}; }
>     template<constexpr-param T>
>       constexpr auto operator--(this T) noexcept -> constant_wrapper<--Y>
>         { return {}; }
>     template<constexpr-param T>
>       constexpr auto operator--(this T, int) noexcept ->
>         constant_wrapper<Y--> { return {}; }

However, we do not actually specify what `Y` is. Additionally, the assignment operator has been changed to

>     template<constexpr-param R>
>       constexpr auto operator=(R) const noexcept
>         -> constant_wrapper<X = R::value> { return {}; }

This is grammatically not valid C++. The assignment must be parenthesized.

[2026-01-16; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 22.2.10 [const.wrap.class], class template `constant_wrapper` synopsis, as indicated:

   >     struct cw-operators { // exposition only
   >       …
   >       // pseudo-mutators
   >       template<constexpr-param T>
   >         constexpr auto operator++(this T) noexcept
   >           -> constant_wrapper<++Y (++T::value)> { return {}; }
   >       template<constexpr-param T>
   >         constexpr auto operator++(this T, int) noexcept
   >           -> constant_wrapper<Y++ (T::value++)> { return {}; }
   >       template<constexpr-param T>
   >         constexpr auto operator--(this T) noexcept
   >           -> constant_wrapper<--Y (--T::value)> { return {}; }
   >       template<constexpr-param T>
   >         constexpr auto operator--(this T, int) noexcept
   >           -> constant_wrapper<Y-- (T::value--)> { return {}; }
   >       …
   >     };

   >     template<cw-fixed-value X, class>
   >     struct constant_wrapper : cw-operators {
   >       static constexpr const auto & value = X.data;
   >       using type = constant_wrapper;
   >       using value_type = decltype(X)::type;

   >       template<constexpr-param R>
   >         constexpr auto operator=(R) const noexcept
   >           -> constant_wrapper<(X = R::value)> { return {}; }
   >       constexpr operator decltype(auto)() const noexcept { return value; }
   >     };

### 4501(i). `std::execution::let_`* Unconditionally Potentially Evaluates `std::execution::set_error`

Section: 33.9.12.10 [exec.let] Status: Resolved Submitter: Robert Leahy Opened: 2026-01-11 Last modified: 2026-04-01

Priority: 2

View all other issues in [exec.let].

View all issues with Resolved status.

Discussion:

The adoption of P3388 as a resolution to CA-334 allowed asynchronous operations to determine, without a concrete receiver, whether connecting some sub-operation thereof can throw an exception.

The above-described capability is critical when an asynchronous operation connects another asynchronous operation as part of its asynchronous execution, which is exactly what `std::execution::set_value`, `::set_error`, and `::set_stopped` do. Enabling the aforementioned algorithms to correctly advertise a `std::execution::set_error_t(std::exception_ptr)` completion signature was the concrete motivation for P3388

Unfortunately the current library wording for the above-enumerated algorithms prohibits them from taking advantage of P3388.

33.9.2 [exec.snd.expos] paragraph 43.1 describes the process by which the completion signatures of an algorithm are determined:

> [...] `CS` is the specialization of `completion_signatures` the set of whose template arguments correspond to the set of completion operations that are potentially evaluated as a result of evaluating `op.start()`.

With this in mind we look to the specification of `std::execution::let_value` et al. in 33.9.12.10 [exec.let]. Connection of the sub-operation is performed via `let-bind` whose signature is (paragraph 5):

>     template<class State, class Rcvr, class... Args>
>     void let-bind(State& state, Rcvr& rcvr, Args&& args);

Note there is no `noexcept`, conditionally or otherwise.

The effects of `let-bind` are equivalent to (paragraph 12):

>     using args_t = decayed-tuple<Args...>;
>     auto mkop2 = [&] {
>       return connect(
>         apply(std::move(state.fn),
>               state.args.template emplace<args_t>(std::forward<Args>(args)...)),
>         receiver2{rcvr, std::move(state.env)});
>     };
>     start(state.ops2.template emplace<decltype(mkop2())>(emplace-from{mkop2}));

Note that `mkop2` is not `noexcept`, conditionally or otherwise.

The point of use of `let-bind` is (paragraph 13):

>     TRY-EVAL(rcvr, let-bind(state, rcvr, std::forward<Args>(args)...));

Where `TRY-EVAL` is described in 33.9.2 [exec.snd.expos] paragraph 7:

> `TRY-EVAL(rcvr, expr)` is equivalent to:

>     try {
>       expr;
>     } catch(...) {
>       set_error(std::move(rcvr), current_exception());
>     }

> if `expr` is potentially-throwing; otherwise, `expr`.

The meaning of "potentially-throwing" is drawn from 14.5 [except.spec] paragraph 5:

> An expression E is potentially-throwing if

> - E is a function call whose postfix-expression has a function type, or a pointer-to-function type, with a potentially-throwing exception specification, or
> - [...]

As such even if connecting the sub-operation cannot throw `std::execution::let_value` et al. all potentially evaluate a potentially throwing expression and therefore must advertise `std::execution::set_error_t(std::exception_ptr)` as a possible completion signature.

Note that the wording proposed by P3373 obsoletes this issue.

[2026-02-18; Reflector poll.]

Set priority to 2 after reflector poll.

The wording needs to account for `emplace` not having a `noexcept` specification.

[2026-04-01 Resolved by the adoption of P3373R4 at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

Update 33.9.12.10 [exec.let] as follows:

> [...]

>     namespace std::execution {
>       template<class State, class Rcvr, class... Args>
>       void let-bind(State& state, Rcvr& rcvr, Args&&... args) noexcept(see below); // exposition only

>       template<>
>       struct impls-for<decayed-typeof<let-cpo>> : default-impls {
>         static constexpr auto get-state = see below;
>         static constexpr auto complete = see below;

>         template<class Sndr, class... Env>
>           static consteval void check-types();
>       };
>     }

> [...]

> The exposition-only function template `let-bind` has effects equivalent to:

>     using args_t = decayed-tuple<Args...>;
>     auto mkop2 = [&] noexcept(see below) {
>       return connect(
>         apply(std::move(state.fn),
>               state.args.template emplace<args_t>(std::forward<Args>(args)...)),
>         receiver2{rcvr, std::move(state.env)});
>     };
>     start(state.ops2.template emplace<decltype(mkop2())>(emplace-from{mkop2}));

> Where the exception specification of `mkop2` and `let-bind` evaluates to `true` if each expression contained thereby is not potentially-throwing, `false` otherwise.

> [...]

### 4504(i). Wording problem in `{simple_}counting_scope`

Section: 33.14.2.2.3 [exec.simple.counting.mem] Status: C++26 Submitter: Ian Petersen Opened: 2025-12-24 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

I discovered while implementing P3149 + P3815 in NVIDIA's stdexec that there's a bug in 33.14.2.2.3 [exec.simple.counting.mem], paragraph 9. The status quo says:

>     template<class State>
>       bool start-join-sender(State& st) noexcept;

> > -9- Effects: If `state` is

> > - (9.1) — `unused`, `unused-and-closed`, or `joined`, then changes `state` to `joined` and returns `true`;

> > - (9.2) — `open` or `open-and-joining`, then changes `state` to `open-and-joining`, registers `st` with `*this` and returns `false`;

> > - (9.3) — `closed` or `closed-and-joining`, then changes `state` to `closed-and-joining`, registers `st` with `*this` and returns `false`.

The problem is that if either bullet 9.2 or 9.3 applies when `count` happens to be zero then there's nothing to guarantee that `st.complete()` is ever invoked, leading to deadlock unless another sender is associated and disassociated with the scope (because the subsequent disassociation will trigger the join-sender's completion).

One fix is to change bullet 9.1 of the Effects element such that `start-join-sender` changes `state` to `joined` and returns `true` if `count` is zero, regardless of the value of `state`; bullets 9.2 and 9.3 can be left alone. I might add a note that points out that, when `state` is `unused`, `unused-and-closed`, or `joined`, `count` must be zero.

Another fix would be to delete bullet 9.1, to modify bullets 9.2 and 9.3 by adding "and `count` is greater than zero", and to add a trailing bullet that says "otherwise, changes `state` to `joined` and returns `true`."

[2026-02-18; Reflector poll.]

Set priority to 2 after reflector poll.

[2026-03-25; Tomasz provides updated wording.]

[Croydon 2026-03-26; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 33.14.2.2.3 [exec.simple.counting.mem] as indicated:

   >     template<class State>
   >       bool start-join-sender(State& st) noexcept;

   > > -9- Effects: If `count` is zero, then changes `state` to `joined` and returns `true`. Otherwise, if `state` is:

   > > - (9.1) — `unused`, `unused-and-closed`, or `joined`, then changes `state` to `joined` and returns `true`;

   > > - (9.2) — `open` or `open-and-joining`, then changes `state` to `open-and-joining`, registers `st` with `*this` and returns `false`;

   > > - (9.3) — `closed` or `closed-and-joining`, then changes `state` to `closed-and-joining`, registers `st` with `*this` and returns `false`.

### 4506(i). `source_location` is explicitly unspecified if is constexpr or not

Section: 17.8.2 [support.srcloc.class] Status: C++26 Submitter: Hana Dusíková Opened: 2025-12-27 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

Basically in 17.8.2.1 [support.srcloc.class.general] following p1 there is a note from Jens from six years ago which says (emphasis mine):

> [Note 1: The intent of `source_location` is to have a small size and efficient copying. It is unspecified whether the copy/move constructors and the copy/move assignment operators are trivial and/or constexpr. — end note]

But also reflection's `std::meta::source_location_of` returns it by value and is `consteval`. This means `source_location` needs to be specified to constexpr. And good news is ... all three major implementations have it constexpr implicitly.

Options are (from my perspective) to remove that part of the note "and/or constexpr" and just call it a day, or add

>     constexpr source_location(const source_location&) = default;
>     constexpr source_location(source_location&&) noexcept = default;
>     constexpr source_location& operator=(const source_location&) = default;
>     constexpr source_location& operator=(source_location&&) noexcept = default;

AFAIK this is how to explicitly say it must be constexpr but it can still be trivial, see demo.

[2026-02-18; Reflector poll.]

Set priority to 2 after reflector poll.

[2026-03-26; Tim adds wording]

[Croydon 2026-03-27; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

> [Drafting note: 16.3.3.5 [functions.within.classes] can be read to say that the functions are constexpr already, but given the note that contradicts this interpretion, explicit declarations can't hurt. — end drafting note]

1. Modify 17.8.2.1 [support.srcloc.class.general] as indicated:

   >     namespace std {
   >       struct source_location {
   >         // source location construction
   >         static consteval source_location current() noexcept;
   >         constexpr source_location() noexcept;

   >         constexpr source_location(const source_location&) noexcept = default;
   >         constexpr source_location& operator=(const source_location&) noexcept = default;

   >         // source location field access
   >         constexpr uint_least32_t line() const noexcept;
   >         constexpr uint_least32_t column() const noexcept;
   >         constexpr const char* file_name() const noexcept;
   >         constexpr const char* function_name() const noexcept;

   >       private:
   >         uint_least32_t line_;               // exposition only
   >         uint_least32_t column_;             // exposition only
   >         const char* file_name_;             // exposition only
   >         const char* function_name_;         // exposition only
   >       };
   >     }

   > - 1- The type `source_location` meets the Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, Cpp17Swappable, and Cpp17Destructible requirements (16.4.4.2 [utility.arg.requirements], 16.4.4.3 [swappable.requirements]) models `semiregular`. `is_nothrow_swappable_v<source_location>` is `true`. All of the following conditions are `true`:

   > 1. (1.1) — `is_nothrow_move_constructible_v<source_location>`

   > 2. (1.2) — `is_nothrow_move_assignable_v<source_location>`

   > 3. (1.3) — `is_nothrow_swappable_v<source_location>`

   > [Note 1: The intent of `source_location` is to have a small size and efficient copying. It is unspecified whether the copy/move constructors and the copy/move assignment operators are trivial and/or constexpr. — end note]

### 4510(i). Ambiguity of `std::ranges::advance` and `std::ranges::next` when the difference type is also a sentinel type

Section: 24.4.4.2 [range.iter.op.advance], 24.4.4.4 [range.iter.op.next] Status: C++26 Submitter: Jiang An Opened: 2026-01-09 Last modified: 2026-06-09

Priority: 3

View all other issues in [range.iter.op.advance].

View all issues with C++26 status.

Discussion:

Currently, `ranges::advance` and `ranges::next` have `operator()` overloads that accept `(iterator, sentinel)` and `(iterator, difference)`. However, when the difference type of the iterator is also a sentinel type, there may be ambiguity in both overloads.

E.g. the following example is rejected when compiling with libc++ or libstdc++ (demo):

>     #include <cstddef>
>     #include <iterator>
>     #include <type_traits>

>     template<class T>
>     struct FwdIter { // triggers ADL for T
>       FwdIter();

>       using value_type      = std::remove_cv_t<T>;
>       using difference_type = int;

>       T& operator*() const;

>       FwdIter& operator++();
>       FwdIter operator++(int);

>       friend bool operator==(const FwdIter&, const FwdIter&);
>     };

>     static_assert(std::forward_iterator<FwdIter<int>>);

>     struct OmniConv {
>       OmniConv(const auto&);
>       friend bool operator==(OmniConv, OmniConv); // found by ADL via things related to OmniConv
>     };

>     int main() {
>       FwdIter<OmniConv> it{};
>       std::ranges::advance(it, 0); // ambiguous
>       std::ranges::next(it, 0); // ambiguous
>     }

Perhaps it would be better to ensure that calling `ranges::advance` or `ranges::next` with an iterator value and a value of the difference type is unambiguous. Note that wrapping iterators that trigger ADL for the value type like the `FwdIter` in this example are common in standard library implementations, so ambiguity can be easily raised from containers with such `OmniConv` being their element type.

[2026-02-20; Reflector poll.]

Set priority to 3 after reflector poll.

The type in the example is basically a `std::any` that additionally type-erases equality. That causes a problem for templated iterators that have their value type's namespace as an associated namespace for ADL.

Instead of allowing integers to be sentinels in general but just restricting them from being used with these overloads, we could just change the concept so that integers do not model `sentinel_for`. That's similar to what we did for `span`'s constructor which has a similar problem (though that one's more aggressive due to compatibility constraints).

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Modify 24.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     […]
   >     namespace std {
   >       template<class T> using with-reference = T&; // exposition only
   >       template<class T> concept can-reference      // exposition only
   >         = requires { typename with-reference<T>; };
   >       template<class T> concept dereferenceable    // exposition only
   >         = requires(T& t) {
   >           { *t } -> can-reference; // not required to be equality-preserving
   >         };

   >       template<class T>
   >         constexpr bool is-integer-like = see below;           // exposition only

   >       […]
   >       // 24.4.4 [range.iter.ops], range iterator operations
   >       namespace ranges {
   >         // 24.4.4.2 [range.iter.op.advance], ranges::advance
   >         template<input_or_output_iterator I>
   >           constexpr void advance(I& i, iter_difference_t<I> n);                // freestanding
   >         template<input_or_output_iterator I, sentinel_for<I>class S>
   >           requires (!is-integer-like<S>) && sentinel_for<S, I>
   >           constexpr void advance(I& i, S bound);                               // freestanding
   >         template<input_or_output_iterator I, sentinel_for<I> S>
   >           constexpr iter_difference_t<I> advance(I& i, iter_difference_t<I> n, // freestanding
   >                                                  S bound);
   >        […]
   >         // 24.4.4.4 [range.iter.op.next], ranges::next
   >         template<input_or_output_iterator I>
   >           constexpr I next(I x);                                  // freestanding
   >         template<input_or_output_iterator I>
   >           constexpr I next(I x, iter_difference_t<I> n);          // freestanding
   >         template<input_or_output_iterator I, sentinel_for<I>class S>
   >           requires (!is-integer-like<S>) && sentinel_for<S, I>
   >           constexpr I next(I x, S bound);                         // freestanding
   >         template<input_or_output_iterator I, sentinel_for<I> S>
   >           constexpr I next(I x, iter_difference_t<I> n, S bound); // freestanding
   >       }
   >       […]
   >     }

2. Modify 24.4.4.2 [range.iter.op.advance] as indicated:

   >     template<input_or_output_iterator I, sentinel_for<I>class S>
   >       requires (!is-integer-like<S>) && sentinel_for<S, I>
   >       constexpr void advance(I& i, S bound);

   > > -3- Preconditions: […]

3. Modify 24.4.4.4 [range.iter.op.next] as indicated:

   >     template<input_or_output_iterator I, sentinel_for<I>class S>
   >       requires (!is-integer-like<S>) && sentinel_for<S, I>
   >       constexpr I next(I x, S bound);

   > > -3- Effects: […]

[2026-02-20; Jonathan provides new wording]

[2026-02-27; LWG telecon: move to Ready]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 24.3.4.7 [iterator.concept.sentinel] as indicated:

   >     template<class S, class I>
   >       concept sentinel_for =
   >         semiregular<S> &&
   >         !is-integer-like<S> &&
   >         input_or_output_iterator<I> &&
   >         weakly-equality-comparable-with<S, I>; // see 18.5.4 [concept.equalitycomparable]

### 4511(i). Inconsistency between the deduction guide of `std::mdspan` taking `(data_handle_type, mapping_type, accessor_type)` and the corresponding constructor

Section: 23.7.3.6.1 [mdspan.mdspan.overview] Status: C++26 Submitter: Jiang An Opened: 2026-01-09 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [mdspan.mdspan.overview].

View all other issues in [mdspan.mdspan.overview].

View all issues with C++26 status.

Discussion:

Currently, the following deduction guide of std::mdspan takes the data handle by reference:

>     template<class MappingType, class AccessorType>
>       mdspan(const typename AccessorType::data_handle_type&, const MappingType&,
>              const AccessorType&)
>         -> mdspan<typename AccessorType::element_type, typename MappingType::extents_type,
>                   typename MappingType::layout_type, AccessorType>;

But the corresponding constructor takes the data handle by value:

>     constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);

The distinction is observable with `volatile` glvalues. E.g., in the following example, CTAD fails but explicitly specifying template arguments works (demo):

>     #include <cstddef>
>     #include <mdspan>

>     int main() {
>       int a[1]{};
>       int * volatile p = a;
>       std::mdspan(
>         p,
>         std::layout_right::mapping<std::extents<std::size_t, 1>>{},
>         std::default_accessor<int>{}); // error (but accidentally accepted by libc++)
>       std::mdspan<int, std::extents<std::size_t, 1>>(
>         p,
>         std::layout_left::mapping<std::extents<std::size_t, 1>>{},
>         std::default_accessor<int>{}); // OK
>     }

Given we're generally passing data handle by value, it seems better to remove `const &` from `const typename AccessorType::data_handle_type&` in the deduction guide, which is more consistent with the constructor and accept more seemingly valid uses.

Note that libc++ is accidentally doing this now by forgetting adding the `&`, see llvm/llvm-project#175024.

[2026-02-20; LWG telecon; Status changed: New → Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 23.7.3.6.1 [mdspan.mdspan.overview], class template `mdspan` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       template<class MappingType, class AccessorType>
   >       mdspan(const typename AccessorType::data_handle_type&, const MappingType&,
   >              const AccessorType&)
   >         -> mdspan<typename AccessorType::element_type, typename MappingType::extents_type,
   >                   typename MappingType::layout_type, AccessorType>;
   >     }

### 4512(i). The `system_encoded_string()` and `generic_system_encoded_string()` member functions of `std::filesystem::path` are misnamed

Section: 31.12.6.1 [fs.class.path.general], 31.12.6.3.2 [fs.path.type.cvt], 31.12.6.5.6 [fs.path.native.obs], 31.12.6.5.7 [fs.path.generic.obs], D.23.2 [depr.fs.path.obs] Status: C++26 Submitter: Tom Honermann Opened: 2026-01-11 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 189-304

The `system_encoded_string()` and `generic_system_encoded_string()` member functions of `std::filesystem::path` are misnamed. These functions were added as renamed versions of `string()` and `generic_string()` respectively by P2319R5 ("Prevent path presentation problems"). The original function names are now deprecated.

The C++ standard does not define "system encoding" and casual use of such a term is at best ambiguous. In common language, one might expect the "system encoding" to correspond to the environment encoding (for which `std::text_encoding::environment()` provides a definition) or the encoding of the current locale. However, neither of these encodings corresponds to the encoding these functions are expected to use. 31.12.6.3.2 [fs.path.type.cvt] defines "native encoding" and, for `char`-based filesystem paths, states (in a note) that "For Windows-based operating systems, the native ordinary encoding is determined by calling a Windows API function." It is not specified which Windows API function is consulted, but existing implementations call `AreFileApisANSI()` to choose between the Active Code Page (`CP_ACP`) and the OEM Code Page (`CP_OEMCP`). Microsoft's implementation also checks for a thread-local locale and, if the locale encoding is UTF-8, prefers that (`CP_UTF8`) over either of the other two.

[2026-02-20; LWG telecon; Status changed: New → Ready.]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 31.12.6.1 [fs.class.path.general], class `path` synopsis, as indicated:

   >     namespace std::filesystem {
   >       class path {
   >       public:
   >         […]
   >         // 31.12.6.5.6 [fs.path.native.obs], native format observers
   >         […]
   >         std::string display_string() const;
   >         std::string systemnative_encoded_string() const;
   >         std::wstring wstring() const;
   >         […]

   >         // 31.12.6.5.7 [fs.path.generic.obs], generic format observers
   >         […]
   >         std::string generic_display_string() const;
   >         std::string generic_systemnative_encoded_string() const;
   >         std::wstring generic_wstring() const;
   >         […]
   >       };
   >     }

2. Modify 31.12.6.3.2 [fs.path.type.cvt] as indicated:

   > -2- For member function arguments that take character sequences representing paths and for member functions returning strings, value type and encoding conversion is performed if the value type of the argument or return value differs from `path::value_type`. For the argument or return value, the method of conversion and the encoding to be converted to is determined by its value type:

   > - (2.1) — `char`: The encoding is the native ordinary encoding. The method of conversion, if any, is operating system dependent.

   >   [Note 1: For POSIX-based operating systems `path::value_type` is `char` so no conversion from `char` value type arguments or to `char` value type return values is performed. For Windows-based operating systems, the native ordinary encoding is determined by calling a Windows API functionthe current locale encoding and the Windows `AreFileApisANSI` function. — end note]

   >   […]

   > - (2.2) — […]

   > - […]

3. Modify 31.12.6.5.6 [fs.path.native.obs] as indicated:

   >     std::string systemnative_encoded_string() const;
   >     std::wstring wstring() const;
   >     std::u8string u8string() const;
   >     std::u16string u16string() const;
   >     std::u32string u32string() const;

   > > -7- Returns: `native()`.

   > > -8- Remarks: Conversion, if any, is performed as specified by 31.12.6.3 [fs.path.cvt].

4. Modify 31.12.6.5.7 [fs.path.generic.obs] as indicated:

   >     std::string generic_systemnative_encoded_string() const;
   >     std::wstring generic_wstring() const;
   >     std::u8string generic_u8string() const;
   >     std::u16string generic_u16string() const;
   >     std::u32string generic_u32string() const;

   > > -5- Returns: The pathname in the generic format.

   > > -6- Remarks: Conversion, if any, is specified by 31.12.6.3 [fs.path.cvt].

5. Modify D.23.2 [depr.fs.path.obs] as indicated:

   >     std::string string() const;

   > > -2- Returns: `systemnative_encoded_string()`.

   >     std::string generic_string() const;

   > > -3- Returns: `generic_systemnative_encoded_string()`.

### 4513(i). `meta::exception::what()` should be `consteval`

Section: 21.4.4 [meta.reflection.exception] Status: Resolved Submitter: Jonathan Wakely Opened: 2026-01-28 Last modified: 2026-02-13

Priority: 2

View all other issues in [meta.reflection.exception].

View all issues with Resolved status.

Discussion:

As mentioned in LWG 4494(i), `meta::exception::what()` can be (and should be) marked `consteval` now that CWG 3117 has been approved.

[2026-02-13; Reflector poll.]

Set priority to 2 after reflector poll.

Discussion about whether this is really necessary, or redundant.

[2026-06-15 Status changed: New → Resolved.]

Resolved by P4101R1, approved in Brno.

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.4 [meta.reflection.exception] as indicated:

   >     namespace std::meta {
   >       class exception : public std::exception {
   >       private:
   >         optional<string> what_;  // exposition only
   >         u8string u8what_;        // exposition only
   >         info from_;              // exposition only
   >         source_location where_;  // exposition only
   >       public:
   >         consteval exception(u8string_view what, info from,
   >                             source_location where = source_location::current()) noexcept;

   >         consteval exception(string_view what, info from,
   >                             source_location where = source_location::current()) noexcept;

   >         exception(const exception&) = default;
   >         exception(exception&&) = default;

   >         exception& operator=(const exception&) = default;
   >         exception& operator=(exception&&) = default;

   >         constexpr consteval const char* what() const noexcept override;
   >         consteval u8string_view u8what() const noexcept;
   >         consteval info from() const noexcept;
   >         consteval source_location where() const noexcept;
   >       };
   >     }

   > …

   >     constexpr consteval const char* what() const noexcept override;

   > > -5- Constant when: `what_.has_value()` is `true`.

### 4514(i). Missing absolute value of `init` in `vector_two_norm` and `matrix_frob_norm`

Section: 29.9.13.8 [linalg.algs.blas1.nrm2], 29.9.13.11 [linalg.algs.blas1.matfrobnorm] Status: C++26 Submitter: Mark Hoemmen Opened: 2026-01-28 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [linalg.algs.blas1.nrm2].

View all issues with C++26 status.

Discussion:

This pull request discussion points out two issues with the current wording of `vector_two_norm`:

1. If `Scalar` is the complex number `1 + i`, then the current wording would make `vector_two_norm` return `sqrt(2i + nonnegative_real_number)`. This would have nonzero imaginary part.

2. If `Scalar` is the real number `-1.0`, then the current wording would make `vector_two_norm` return `sqrt(-1.0 + nonnegative_real_number)`. If `nonnegative_real_number` is less than `1`, then the result would be imaginary.

The analogous issues would also apply to `matrix_frob_norm`.

It's acceptable for the return type `Scalar` to be `complex`. However, the point of letting `init` be nonzero is to support computing the 2-norm of a vector (or the Frobenius norm of a matrix) in multiple steps. The value of `init` should thus always represent a valid 2-norm (or Frobenius norm) result.

There are two ways to fix this:

1. Impose a precondition on `init`, that it have nonnegative real part and zero imaginary part.

2. Change the Returns element to take the absolute value of `init`. (The 29.9 [linalg] clause generally uses "absolute value" to mean the magnitude of a complex number, or the absolute value of a non-complex number.)

We offer (2) as the Proposed Fix, as it is consistent with the approach we took for fixing LWG 4136(i).

[2026-02-13; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 29.9.13.8 [linalg.algs.blas1.nrm2] as indicated:

   > [Drafting note: As a drive-by fix the missing closing parenthesis of the `decltype` form has been added.]

   >     template<in-vector InVec, scalar Scalar>
   >       Scalar vector_two_norm(InVec v, Scalar init);
   >     template<class ExecutionPolicy, in-vector InVec, scalar Scalar>
   >       Scalar vector_two_norm(ExecutionPolicy&& exec, InVec v, Scalar init);

   > > -1- [Note 1: These functions correspond to the BLAS function `xNRM2`[17]. — end note]

   > > -2- Mandates: `InVec::value_type` and `Scalar` are either a floating-point type, or a specialization of `complex`. Let `a` be `abs-if-needed(declval<typename InVec::value_type>())` and let `init_abs` be `abs-if-needed(init)`. Then, `decltype(init_abs * init_abs + a * a)` is convertible to `Scalar`.

   > > -3- Returns: The square root of the sum of the square of `init` and the squares of the absolute values of the elements of `v`.whose terms are the following:

   > > - (3.1) — the square of the absolute value of `init`, and

   > > - (3.2) — the squares of the absolute values of the elements of `v`.

   > > [Note 2: For `init` equal to zero, this is the Euclidean norm (also called 2-norm) of the vector `v`. — end note]

   > > -4- Remarks: If `Scalar` has higher precision than `InVec::value_type`, then intermediate terms in the sum use `Scalar`'s precision or greater.

2. Modify 29.9.13.11 [linalg.algs.blas1.matfrobnorm] as indicated:

   > [Drafting note: As a drive-by fix one spurious `InVec` has been changed to `InMat`]

   >     template<in-matrix InMat, scalar Scalar>
   >       Scalar matrix_frob_norm(InMat A, Scalar init);
   >     template<class ExecutionPolicy, in-matrix InMat, scalar Scalar>
   >       Scalar matrix_frob_norm(ExecutionPolicy&& exec, InMat A, Scalar init);

   > > -2- Mandates: `InVecInMat::value_type` and `Scalar` are either a floating-point type, or a specialization of `complex`. Let `a` be `abs-if-needed(declval<typename InMat::value_type>())` and let `init_abs` be `abs-if-needed(init)`. Then, `decltype(init_abs * init_abs + a * a)` is convertible to `Scalar`.

   > > -3- Returns: The square root of the sum of the squares of `init` and the absolute values of the elements of `A`.whose terms are the following:

   > > - (3.1) — the square of the absolute value of `init`, and

   > > - (3.2) — the squares of the absolute values of the elements of `A`.

   > > [Note 2: For `init` equal to zero, this is the Frobenius norm of the matrix `A`. — end note]

   > > -4- Remarks: If `Scalar` has higher precision than `InMat::value_type`, then intermediate terms in the sum use `Scalar`'s precision or greater.

### 4517(i). `data_member_spec` should throw for cv-qualified unnamed bit-fields

Section: 21.4.17 [meta.reflection.define.aggregate] Status: C++26 Submitter: Marek Polacek Opened: 2026-02-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [meta.reflection.define.aggregate].

View all issues with C++26 status.

Discussion:

DR 2229 changed 11.4.10 [class.bit] p2 to say:

> An unnamed bit-field shall not be declared with a cv-qualified type.

It follows that `define_aggregate` should have the same limitation, and thus in:

>     #include <meta>

>     constexpr auto d1 = std::meta::data_member_spec(^^const int, { .bit_width = 1 });
>     constexpr auto d2 = std::meta::data_member_spec(^^volatile int, { .bit_width = 1 });
>     constexpr auto d3 = std::meta::data_member_spec(^^const volatile int, { .bit_width = 1 });

all three `data_member_spec` should throw.

[2026-02-27; Reflector poll.]

Set status to Tentatively Ready after 8 votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.17 [meta.reflection.define.aggregate] as indicated:

   >     consteval info data_member_spec(info type, data_member_options options);

   > > -4- Returns: […]

   > > -5- Throws: `meta::exception` unless the following conditions are met:

   > > - (5.1) — `dealias(type)` represents either an object type or a reference type;

   > > - (5.2) — if `options.name` contains a value, then: […]

   > > - (5.3) — if `options.name` does not contain a value, then `options.bit_width` contains a value;

   > > - (5.4) — if `options.bit_width` contains a value `V`, then

   > >   - (5.4.1) — `is_integral_type(type) || is_enum_type(type)` is `true`,

   > >   - (5.4.2) — `options.alignment` does not contain a value,

   > >   - (5.4.3) — `options.no_unique_address` is `false`,

   > >   - (5.4.4) — `V` is not negative, and

   > >   - (5.4.5) — if `V` equals `0`, then `options.name` does not contain a value; and

   > >   - (5.4.?) — if `options.name` does not contain a value, then `is_const(type) || is_volatile(type)` is `false`; and

   > > - (5.5) — if `options.alignment` contains a value, it is an alignment value (6.8.3 [basic.align]) not less than `alignment_of(type)`.

### 4518(i). `simd::cat` return type requires inefficient ABI tag change/conversion

Section: 29.10.3 [simd.syn], 29.10.8.11 [simd.creation] Status: Resolved Submitter: Matthias Kretz Opened: 2025-11-25 Last modified: 2026-04-01

Priority: 2

View all other issues in [simd.syn].

View all issues with Resolved status.

Discussion:

The return type of `simd::cat` is defined using `deduce-abi-t` rather than `resize_t`. This can lead to:

>     basic_vec<T, Abi> x = […]
>     auto [...vs] = simd::chunk<2>(x);
>     auto y = simd::cat(vs...);
>     static_assert(is_same_v<decltype(x), decltype(y)>); // can fail

[2026-02-13; LWG telecon.]

Setting priority to P2, after P3932R0 (Fix LWG4470: Fix integer-from in [simd]), resolving the issue was accepted on telecon.

[2026-04-01 Resolved by the adoption of P3932R0 at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

### 4520(i). Breaking change in `std::span` new `initializer_list` constructor

Section: 23.7.2.2.2 [span.cons] Status: Resolved Submitter: Michael Schellenberger Costa Opened: 2026-02-10 Last modified: 2026-03-31

Priority: 1

View all other issues in [span.cons].

View all issues with Resolved status.

Discussion:

NVIDIA C++ standard library libcu++ received a bug report against our `span` implementation.

However, looking into this a bit it seems that this is a source-breaking change from the new C++26 `initializer_list` constructor of `span` in conjunction with a bad conversion sequence.

The code in question is:

>     #include <span>

>     int main() {
>       bool data[4] = {true, false, true, false};
>       bool* ptr = data;
>       size_t size = 4;

>       ::std::span<const bool> bad_span{ptr, size};

>       return 0;
>     }

The issue is that the compiler tries a conversion sequence from `bool*` to `bool` and `size_t` to `bool`.

We probably should disallow conversions to `bool` by that constructor.

[2026-02-27; Reflector poll.]

Set priority to 1 after reflector poll.

Votes being split between LEWG/P1 and considering this new information from deployment experience, and NAD as the breakage in the paper was already covered in [diff.cpp23.containers]. It was pointed out, that in contrast to the silent change of meaning already discussed with paper, this snippet becomes ill-formed.

[Croydon 2026-03-26; will be resolved by P4144R0.]

[2026-03-31 P4144R1 was approved in Croydon. Status changed: New → Resolved.]

Proposed resolution:

### 4521(i). Improve [atomics.order] p10 to have a consistent way with [intro.races]

Section: 32.5.4 [atomics.order] Status: WP Submitter: jim x Opened: 2026-02-10 Last modified: 2026-06-15

Priority: 4

View other active issues in [atomics.order].

View all other issues in [atomics.order].

View all issues with WP status.

Discussion:

32.5.4 [atomics.order] p10 says:

> Atomic read-modify-write operations shall always read the last value (in the modification order) written before the write associated with the read-modify-write operation.

We do not write side effects to the modification order, instead we write side effects to an atomic object. These modifications occur in a single total order, called the modification order. That is, the modification order describes how these operations are ordered.

Moreover, we called the compared element in the modification order "modification" or "side effect", as shown in 6.10.2.2 [intro.races]. In addition, we don't use "a side effect `X` written before `Y` in the modification order"; we use "`X` precedes/follows `Y` in the modification order" to describe the relationship of order between two modifications. Suggested Resolution:

To make the wording more formal and have a consistent way with 6.10.2.2 [intro.races], the proposed wording is:

> An atomic read-modify-write operation shall always read the value of a side effect `X`, where `X` is the last side effect that precedes the side effect of the read-modify-write operation in the modification order.

Another option is:

> An atomic read-modify-write operation shall always read the value of a side effect that immediately precedes the side effect of the read-modify-write operation in the modification order.

[2026-02-27; Reflector poll.]

Set priority to 4 after reflector poll.

The existing wording was considered to be correct. Support for option B as wording improvement was expressed, as making the wording more formal and "de-shalling".

[2026-03-23: Croydon, SG1]

Poll: Approve option B to resolve LWG 4521 and send it back to LWG

Outcome: No objection to unanimous consent
Previous resolution [SUPERSEDED]

This wording is relative to N5032.

> [Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.

> As a drive-by fix, in both options we also replace the "shall" wording by an indicative voice, because 32.5.4 [atomics.order] p10 already imposes a requirement on the implementation/language. ]

Option A:

1. Modify 32.5.4 [atomics.order] as indicated:

   > -10- An atomic read-modify-write operations shall always reads the last value from the side effect `X`, where `X` is the last side effect that precedes the side effect of the read-modify-write operation (in the modification order) written before the write associated with the read-modify-write operation.

Option B:

1. Modify 32.5.4 [atomics.order] as indicated:

   > -10- An atomic read-modify-write operations shall always reads the last value from the side effect that immediately precedes the side effect of the read-modify-write operation (in the modification order) written before the write associated with the read-modify-write operation.

[2026-05-19; LWG telecon. Status changed: New → Ready.]

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

> [Drafting Note: As a drive-by fix, we also replace the "shall" wording by an indicative voice, because 32.5.4 [atomics.order] p10 already imposes a requirement on the implementation/language. ]

1. Modify 32.5.4 [atomics.order] as indicated:

   > -10- An atomic read-modify-write operations shall always reads the last value from the side effect that immediately precedes the side effect of the read-modify-write operation (in the modification order) written before the write associated with the read-modify-write operation.

### 4522(i). Clarify that `std::format` transcodes for `std::wformat_strings`

Section: 28.5.2.2 [format.string.std] Status: C++26 Submitter: Jan Schultke Opened: 2026-02-12 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [format.string.std].

View all other issues in [format.string.std].

View all issues with C++26 status.

Discussion:

The current wording in 28.5.2.2 [format.string.std] paragraph 20 suggests that the expression `format(L"{}", 0)` could produce a garbage or malformed string:

> […] Formatting is done as if by calling `to_chars` as specified and copying the output through the output iterator of the format context.

It is not stated that transcoding takes place, so a plausible interpretation is that the output of `to_chars` is copied to the output iterator directly. If the ordinary literal encoding is, say, EBCDIC and the wide literal encoding is, say, UTF-32, simply converting `char` to `wchar_t` is not meaningful.

During the 2026-02-11 SG16 Telecon, when reviewing P3876R0, SG16 examined this issue and concluded that transcoding is intended here, affirmed by the author of P0645R10 Text Formatting. An LWG issue was requested.

[2026-02-27; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

P3876 would address that, but we need to clarify the wording for existing standards.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 28.5.2.2 [format.string.std] as indicated:

   > [Drafting note: 28.5.6.5 [format.string.escaped] paragraph 2 similarly references the wide literal encoding (as the associated character encoding for `charT = wchar_t`. ]

   > -20- The meaning of some non-string presentation types is defined in terms of a call to `to_chars`. In such cases, let `[first, last)` be a range large enough to hold the `to_chars` output and value be the formatting argument value. Formatting is done as if by calling `to_chars` as specified , transcoding the `to_chars` output to the wide literal encoding if `charT` is `wchar_t`, and copying the output through the output iterator of the format context.

   > [Note 7: Additional padding and adjustments are performed prior to copying the output through the output iterator as specified by the format specifiers. — end note]

### 4523(i). `constant_wrapper` should assign to `value`

Section: 22.2.10 [const.wrap.class] Status: C++26 Submitter: Tomasz Kamiński Opened: 2026-02-17 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [const.wrap.class].

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Discussion:

During resolution of LWG 4383(i) assignment operator of `constant_wrapper`, was changed to:

>     template<constexpr-param R>
>       constexpr auto operator=(R) const noexcept -> constant_wrapper<X = R::value> { return {}; }

This is incorrect (and inconsistent with other operators), as we assign to `X` that is object of specialization of exposition only `cw-fixed-value`, instead of `value`.

[2026-02-27; Reflector poll.]

Set status to Tentatively Ready after 9 votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032 with changes from 4500(i).

1. Modify 22.2.10 [const.wrap.class], class template `constant_wrapper` synopsis, as indicated:

   >     template<cw-fixed-value X, class>
   >     struct constant_wrapper : cw-operators {
   >       static constexpr const auto & value = X.data;
   >       using type = constant_wrapper;
   >       using value_type = decltype(X)::type;

   >       template<constexpr-param R>
   >         constexpr auto operator=(R) const noexcept
   >       -> constant_wrapper<(valueX = R::value)> { return {}; }

   >       constexpr operator decltype(auto)() const noexcept { return value; }
   >     };

### 4525(i). `task`'s `final_suspend` should `move` the result

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Dietmar Kühl Opened: 2026-02-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [task.promise].

View all issues with C++26 status.

Discussion:

In 33.13.6.5 [task.promise] p6.3 the `*result` is passed to `set_value` without `std::move`ing: `set_value(std::move(RCVR(*this)), *result)`. Once `set_value` is called the operation state object where `result` is stored just gets destroyed. The second argument to `set_value` should be `std::move(*result)`.

[2026-02-27; Reflector poll.]

Set status to Tentatively Ready after 6 votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Change 33.13.6.5 [task.promise] p6.3 to `std::move` the `*result`:

   >     auto final_suspend() noexcept;

   > -6- Returns: An awaitable object of unspecified type (7.6.2.4 [expr.await]) whose member functions arrange for the completion of the asynchronous operation associated with `STATE(*this)` by invoking:

   > -6.1- -- `set_error(std::move(RCVR(*this)), std::move(e))` if `errors.index()` is greater than zero and `e` is the value held by `errors`, otherwise

   > -6.2- -- `set_value(std::move(RCVR(*this)))` if `is_void<T>` is `true`, and otherwise

   > -6.3- -- `set_value(std::move(RCVR(*this)), std::move(*result))`.

### 4526(i). `std::monostate` could be a valid error for `task`

Section: 33.13.6.5 [task.promise] Status: Resolved Submitter: Dietmar Kühl Opened: 2026-02-21 Last modified: 2026-04-01

Priority: Not Prioritized

View all other issues in [task.promise].

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Discussion:

The current specification of the `error-variant` in 33.13.6.5 [task.promise] p2 uses `std::monostate` as a placeholder for the absence of an error: `variant<monostate, remove_cvref_t<E>...>`. However, `std::monostate` can be an actual error type. It should be replaced by an implementation-defined, default constructible type.

[2026-02-27; Reflector discussion.]

This issue seems to be resolved by 4339(i).

[2026-04-01 Resolved by the adoption of issue 4339(i) at Croydon, 2026. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N5032.

1. In 33.13.6.5 [task.promise] p2 replace `monostate` with an implementation-defined, default constructible type:

   > -2- `error-variant` is a `variant<monostateempty, remove_cvref_t<E>...>`, with duplicate types removed, where `empty` is an implementation-defined, default constructible type and `E...` are the parameter types of the template arguments of the specialization of `execution::completion_signatures` denoted by `error_types`.

### 4527(i). `await_transform` needs to use `as_awaitable`

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Dietmar Kühl Opened: 2026-02-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [task.promise].

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Discussion:

In the `await_transform` overload for `change_coroutine_scheduler` the `return` statement repeats the function name instead of using `as_awaitable`. The `return` statement needs to be fixed. The change may be editorial.

Note that P3941 recommends removing `change_coroutine_scheduler` and the corresponding overload of `await_transform` entirely.

[2026-02-27; Reflector poll.]

Set status to Tentatively Ready after 7 votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Change 33.13.6.5 [task.promise] p10 to use `as_awaitable` in the Effects clause:

   >     template<class Sch>
   >       auto await_transform(change_coroutine_scheduler<Sch> sch) noexcept;

   > -10- Effects: Equivalent to:

   >     return await_transformas_awaitable(just(exchange(SCHED(*this), scheduler_type(sch.scheduler))), *this);

### 4528(i). `task` needs `get_completion_signatures()`

Section: 33.13.6.2 [task.class] Status: C++26 Submitter: Dietmar Kühl Opened: 2026-02-21 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [task.class].

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Discussion:

The changes made by P3164 mean that it isn't sufficient to have a `completion_signatures` type alias in the `task` class. Instead, it needs to have a `static consteval get_completion_signatures()` member function that returns the completion signatures. Instead of defining a type alias, the function can return the same type.

[2026-02-27; Reflector poll.]

Set status to Tentatively Ready after 6 votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Change the `task` synopsis in 33.13.6.2 [task.class] to not have a `completion_signatures` type alias, but instead have a `static consteval get_completion_signatures()` member function that returns the completion signatures:

   >     namespace std::execution {
   >       template<class T = void, class Environment = env<>>
   >       class task {
   >         // [task.state]
   >         template<receiver Rcvr>
   >           class state;                              // exposition only

   >       public:
   >         using sender_concept = sender_t;
   >         using completion_signatures = see below;
   >         using allocator_type = see below;
   >         using scheduler_type = see below;
   >         using stop_source_type = see below;
   >         using stop_token_type = decltype(declval<stop_source_type>().get_token());
   >         using error_types = see below;

   >         // [task.promise]
   >         class promise_type;

   >         task(task&&) noexcept;
   >         ~task();

   >         template<class Self, class... Env>
   >           static consteval auto get_completion_signatures();

   >         template<receiver Rcvr>
   >           state<Rcvr> connect(Rcvr&& rcvr) &&;

   >       private:
   >         coroutine_handle<promise_type> handle;      // exposition only
   >       };
   >     }

2. Remove 33.13.6.2 [task.class] paragraph 4 (the specification moves to 33.13.6.3 [task.members]):

   > -4- The type alias `completion_signatures` is a specialization of `execution::completion_signatures` with the template arguments (in unspecified order):

   > -4.1- -- `set_value_t()` if `T` is `void`, and `set_value_t(T)` otherwise;

   > -4.2- -- template arguments of the specialization of `execution::completion_signatures` denoted by `error_types`; and

   > -4.3- -- `set_stopped_t()`.

3. Add a new paragraph to 33.13.6.3 [task.members] before the specification of the `connect` member function, with the specification of the new `get_completion_signatures` member function:

   >     template<class Self, class... Env>
   >       static consteval auto get_completion_signatures();

   > -?- Let the type `C` be a specialization of `execution::completion_signatures` with the template arguments (in unspecified order):

   > -?.1- -- `set_value_t()` if `T` is `void`, and `set_value_t(T)` otherwise;

   > -?.2- -- template arguments of the specialization of `execution::completion_signatures` denoted by `error_types`; and

   > -?.3- -- `set_stopped_t()`.

   > -?- Returns: `C()`.

### 4529(i). `task::promise_type::await_transform` declaration and definition mismatch

Section: 33.13.6.5 [task.promise] Status: C++26 Submitter: Dietmar Kühl Opened: 2026-02-21 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [task.promise].

View all issues with C++26 status.

Discussion:

The declarations of `await_transform` mismatch between the synopsis in 33.13.6.5 [task.promise] and the definition in 33.13.6.5 [task.promise] paragraph 9:

        template<class A>
          auto await_transform(A&& a);
        template<class Sch>
          auto await_transform(change_coroutine_scheduler<Sch> sch);

vs.

        template<sender Sender>
          auto await_transform(Sender&& sndr) noexcept;
        template<class Sch>
          auto await_transform(change_coroutine_scheduler<Sch> sch) noexcept;

The declaration should be fixed to match the definition.

[2026-02-27; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Change the declarations of `await_transform` in the synopsis of `promise_type` in 33.13.6.5 [task.promise] to match the definition in 33.13.6.5 [task.promise] p9 except for omitting the `noexcept` specifier:

   >     namespace std::execution {
   >       template<class T, class Environment>
   >       class task<T, Environment>::promise_type {
   >       public:
   >         ...
   >         template<class E>
   >           unspecified yield_value(with_error<E> error);

   >         template<class Asender Sender>
   >           auto await_transform(A&& aSender&& sndr);
   >         template<class Sch>
   >           auto await_transform(change_coroutine_scheduler<Sch> sch);

   >         unspecified get_env() const noexcept;
   >         ...
   >       };
   >     }

2. Remove the `noexcept` specifier from the declarations of `await_transform` in the definition of `promise_type` in 33.13.6.5 [task.promise] p9:

   >     template<sender Sender>
   >       auto await_transform(Sender&& sndr) noexcept;

   > -9- Returns: If `same_as<inline_scheduler, scheduler_type>` is `true` returns `as_awaitable(std::forward<Sender>(sndr), *this);` otherwise returns `as_awaitable(affine_on(std::forward<Sender>(sndr), SCHED(*this)), *this)`.

   >     template<class Sch>
   >       auto await_transform(change_coroutine_scheduler<Sch> sch) noexcept;

   > -10- Effects: Equivalent to:

   >          return await_transform(just(exchange(SCHED(*this), scheduler_type(sch.scheduler))), *this);

### 4531(i). Should there be a feature-test macro update for constexpr `std::to_(w)string`?

Section: 17.3.2 [version.syn] Status: WP Submitter: Jiang An Opened: 2026-02-25 Last modified: 2026-06-15

Priority: 2

View other active issues in [version.syn].

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Discussion:

P3391R2 added `constexpr` to integral overloads of `std::to_string` and `std::to_wstring`. However, the paper only added one feature-test macro `__cpp_lib_constexpr_format`, which doesn't seem clearly related to `std::to_(w)string`.

Given that P3391R2 contained the constexpr additions in P3438R0 which proposed to bump `__cpp_lib_to_string`, perhaps we should bump that FTM. It might be also reasonable to bump `__cpp_lib_constexpr_string` for pure constexpr additions.

[2026-04-13; Reflector poll.]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

> [Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A:

1. Modify 17.3.2 [version.syn] as indicated:

   >     […]
   >     #define __cpp_lib_to_string                         202306L202511L // also in <string>
   >     […]

Option B:

1. Modify 17.3.2 [version.syn] as indicated:

   >     […]
   >     #define __cpp_lib_constexpr_string                  201907L202511L // also in <string>
   >     […]

[2026-05-11; Select single resolution.]

Update wording to option B, that had slight preference in reflector discussion.

[2026-05-22; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 17.3.2 [version.syn] as indicated:

   >     […]
   >     #define __cpp_lib_constexpr_string                  201907L202511L // also in <string>
   >     […]

### 4532(i). Imprecise `std::polymorphic` wording seems to imply slicing

Section: 20.4.2.3 [polymorphic.ctor], 20.4.2.5 [polymorphic.assign], 20.4.2.4 [polymorphic.dtor] Status: C++26 Submitter: Fraser Gordon Opened: 2026-02-27 Last modified: 2026-06-09

Priority: 2

View all issues with C++26 status.

Discussion:

The wording for some member functions of `std::polymorphic` is imprecise, leading to plausible interpretations that undermine the purpose of the type (i.e causing slicing on copy/move).

The wording on most copy/assign methods includes a clause "of type `U`, where `U` is the type of the owned object in `other`" but the copy/move assignment methods are missing this, which allows a reading where `allocator_traits<Allocator>::construct` is called to construct an instance of `value_type` instead of `U`.

A similar reading of the destructor effects would cause `~value_type()` to be used instead of `~U()`, so that should also be cleaned up.

Finally, the last sentence of the effects of one of the constructors (20.4.2.3 [polymorphic.ctor] p11) doesn't make sense in context ("constructs an object of type `polymorphic`, considering the owned object in `other` as an rvalue"). As written, this is creating a new polymorphic object and doing nothing with it when the intended effect would be to construct a new owned object.

[2026-03-25; Reflector poll.]

Set priority to 2 after reflector poll.

[Croydon 2026-03-25; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 20.4.2.3 [polymorphic.ctor] as indicated:

   >     constexpr polymorphic(allocator_arg_t, const Allocator& a, polymorphic&& other)
   >       noexcept(allocator_traits<Allocator>::is_always_equal::value);

   > > -11- Effects: `alloc` is direct-non-list-initialized with `a`. If `other` is valueless, `*this is valueless`. Otherwise, if `alloc == other.alloc` is `true`, either constructs an object of type `polymorphic` that owns the owned object of `other`, making `other` valueless; or, owns an object of the same type constructed from the owned object of other considering that owned object as an rvalue. Otherwise, if `alloc != other.alloc` is `true`, constructs an object of type `polymorphic`, consideringowned object of type `U`, where `U` is the type of the owned object in `other`, with the owned object in `other` as an rvalue, using the allocator `alloc`.

2. Modify 20.4.2.5 [polymorphic.assign] as indicated:

   >     constexpr polymorphic& operator=(const polymorphic& other);

   > > -1- Mandates: `T` is a complete type.

   > > -2- Effects: If `addressof(other) == this` is `true`, there are no effects. Otherwise:

   > > - (2.1) — […]

   > > - (2.2) — If `other` is not valueless, a new owned object of type `U`, where `U` is the type of the owned object in other, is constructed in `*this` using `allocator_traits<Allocator>::construct` with the owned object from `other` as the argument, using either the allocator in `*this` or the allocator in `other` if the allocator needs updating.

   > > - (2.3) — […]

   > > - (2.4) — […]

   >     constexpr polymorphic& operator=(polymorphic&& other)
   >       noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);

   > > -5- Mandates: If `allocator_traits<Allocator>::propagate_on_container_move_assignment::value` is `false` and `allocator_traits<Allocator>::is_always_equal::value` is `false`, `T` is a complete type.

   > > -6- Effects: If `addressof(other) == this` is `true`, there are no effects. Otherwise:

   > > - (6.1) — […]

   > > - (6.2) — […]

   > > - (6.3) — […]

   > > - (6.4) — Otherwise, constructs a new owned object of type `U`, where `U` is the type of the owned object in `other`, with the owned object of `other` as the argument as an rvalue, using the allocator in `*this`.

   > > - (6.5) — […]

   > > - (6.6) — […]

3. Modify 20.4.2.4 [polymorphic.dtor] as indicated:

   >     constexpr ~polymorphic();

   > > -1- Mandates: `T` is a complete type.

   > > -2- Effects: If `*this` is not valueless, destroys the owned object using `allocator_traits<Allocator>::destroy and` calls `allocator_traits<Allocator>::destroy(p)`, where `p` is a pointer of type `U*` to the owned object and `U` is the type of the owned object; then the storage is deallocated.

### 4533(i). `not_fn<f>` is unimplementable

Section: 22.10.13 [func.not.fn], 22.10.14 [func.bind.partial] Status: C++26 Submitter: Tim Song Opened: 2026-03-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [func.not.fn].

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Discussion:

The current specification of `not_fn<f>` says that the perfect forwarding call wrapper it returns does not have state entities. 22.10.4 [func.require]p8 says that such wrappers "returned by a given standard library function template have the same type if the types of their corresponding state entities are the same." That condition is trivially true.

It follows that `not_fn<f>()` must return the same type regardless of the value or type of `f`. This is, as far as I know, impossible to implement, and certainly not the intent.

`bind_front<f>` and `bind_back<f>` suffer from the same problem.

[Croydon 2026-03-24; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

[2026-03-24 Tim replaces `constant_arg` with `cw` in light of P3948.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 22.10.3 [func.def] as indicated:

   > -5- A call wrapper type is a type that holds a target object, which is either a callable object or an object representing a callable object, and supports a call operation that forwards to that callable object.

   > -6- […]

   > -7- A target object is the callable object held by a call wrapper.

   > -8- A call wrapper type may additionally hold a sequence of objects and references that may be passed as arguments to the targetcallable object. These entities are collectively referred to as bound argument entities.

2. Modify 22.10.13 [func.not.fn] as indicated:

   >     template<auto f> constexpr unspecified not_fn() noexcept;

   > > -6- […]

   > > -7- […]

   > > -8- Returns: A perfect forwarding call wrapper (22.10.4 [func.require]) `g` that does not have state entities whose target object is a copy of `cw<f>`, and has thewhose call pattern is `!invoke(f, call_args...)`.

3. Modify 22.10.14 [func.bind.partial] as indicated:

   >     template<auto f, class... Args>
   >       constexpr unspecified bind_front(Args&&... args);
   >     template<auto f, class... Args>
   >       constexpr unspecified bind_back(Args&&... args);

   > > -6- […]

   > > -7- […]

   > > -8- […]

   > > -9- Returns: A perfect forwarding call wrapper (22.10.4 [func.require]) `g` that does not have a target object whose target object is a copy of `cw<f>`, and has thewhose call pattern is:

   > > - (2.1) — `invoke(f, bound_args..., call_args...)` for a `bind_front` invocation, or

   > > - (2.2) — `invoke(f, call_args..., bound_args...)` for a `bind_back` invocation.

### 4534(i). `is_consteval_only` preconditions are too weak

Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: Tim Song Opened: 2026-03-05 Last modified: 2026-04-01

Priority: Not Prioritized

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Discussion:

Addresses US 81-149.

The precondition for `is_consteval_only` is insufficient. A definitive false answer requires every type that `T` is compounded from to be complete or cv void.

[2026-03-06 LWG telecon; move to Ready]

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Combine 21.3.5.4 [meta.unary.prop] p4-5 into one paragraph with a bullet list, then add a new bullet, as indicated:

   > -4- For the purpose of defining the templates in this subclause,:

   > - (4.1) — a function call expression `declval<T>()` for any type `T` is considered to be a trivial (D.13 [depr.meta.types], 11.4.4 [special]) function call that is not an odr-use (6.3 [basic.def.odr]) of `declval` in the context of the corresponding definition notwithstanding the restrictions of 22.2.6 [declval].

   > - (4.2) — -5- For the purpose of defining the templates in this subclause, let `VAL<T>` for some type `T` be an expression defined as follows:

   >   - (5.14.2.1) — […]

   >   - (5.24.2.2) — […]

   > - (4.3) — let `C(T)` be the set of types defined as follows:

   >   - (4.3.1) — if `remove_cv_t<T>` is the type "array of `U`", "pointer to `U`", or "reference to `U`", then `C(T)` is `C(U)`;

   >   - (4.3.2) — otherwise, if `remove_cv_t<T>` is the type "pointer to member of class `X` of type `U`", then `C(T)` is the union of `C(X)` and `C(U)`;

   >   - (4.3.3) — otherwise, if `T` is the type "`noexcept` opt function of parameter-type-list cv-qualifier-seqopt ref-qualifieropt returning `U`", then `C(T)` is the union of `C(U)` and `C(X)` for each type `X` in the parameter-type-list;

   >   - (4.3.4) — otherwise, if `remove_cv_t<T>` is a complete class type, then `C(T)` is the union of the sets `C(U)`, for each `U` that is the type of a (possibly indirect) non-static data member of `remove_cv_t<T>`;

   >   - (4.3.5) — otherwise, `C(T)` is the set containing the single element `remove_cv_t<T>`.

2. Modify [tab:meta.unary.prop], Table 54 — Type property predicates as indicated:

   > Template | Condition | Preconditions
   > --- | --- | ---
   > … | … | …
   >     template<class T>
   >     struct is_consteval_only;
   > | T is consteval-only (6.9.1 [basic.types.general]) | `remove_all_extents_t<T>` shall be a complete type or cv `void`. If `C(T)` does not contain `std::meta::info`, then it shall not contain an incomplete class type.
   > … | … | …

[2026-03-24; Reopened due to issue 4555(i).]

[2026-04-01 Resolved by the adoption of issue 4555(i) at Croydon, 2026. Status changed: Open → Resolved.]

Proposed resolution:

This issue is resolved by the resolution of issue 4555(i).

### 4535(i). Disallow user specialization of `<simd>` templates

Section: 29.10.1 [simd.general] Status: C++26 Submitter: Tim Song Opened: 2026-03-05 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 176-280.

It does not appear that any of the class templates in `<simd>` can be profitably specialized by the user – or that the non-member functions can be used with such user specializations.

[2026-03-06 LWG telecon; move to Ready]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Add the following paragraph to the end of 29.10.1 [simd.general]:

   > -?- If a program declares an explicit or partial specialization of any of the templates specified in subclause 29.10 [simd], the program is ill-formed, no diagnostic required.

2. Strike 29.10.4 [simd.traits] p3 as redundant:

   > -3- The behavior of a program that adds specializations for `alignment` is undefined.

### 4536(i). Type traits have inconsistent interactions with immediate functions

Section: 21.3.5.4 [meta.unary.prop], 21.3.7 [meta.rel] Status: C++26 Submitter: Tim Song Opened: 2026-03-05 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

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Discussion:

Per 7.7 [expr.const]p23, calls to immediate functions are not immediate invocations, and therefore are not required to result in constant expressions, if the call occurs in an unevaluated operand. Currently, some type traits (e.g. `is_assignable`, `is_invocable`) specify that the construct at issue is treated as an unevaluated operand, while others (`is_constructible`, `is_convertible`) do not. This seems like a bad state of affairs.

The wording below makes everything unevaluated (and hence not check for constant-ness) to match the behavior of `requires` expressions (and implementations). This arguably makes the traits less useful in some contexts where false negatives are tolerable but false positives are not.

[2026-03-06 LWG telecon; move to Ready]

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify [tab:meta.unary.prop], Table 54 — Type property predicates as indicated:

   > Template | Condition | Preconditions
   > --- | --- | ---
   > … | … | …
   >     template<class T, class U>
   >     struct reference_constructs_from_temporary;
   > | `T` is a reference type, and the initialization `T t(VAL<U>);` is well-formed and binds `t` to a temporary object whose lifetime is extended (6.8.7 [class.temporary]). The full-expression of the variable initialization is treated as an unevaluated operand (7.2.3 [expr.context]). Access checking is performed as if in a context unrelated to `T` and `U`. Only the validity of the immediate context of the variable initialization is considered. [Note 5: … — end note] | […]
   >     template<class T, class U>
   >     struct reference_converts_from_temporary;
   > | `T` is a reference type, and the initialization `T t = VAL<U>;` is well-formed and binds `t` to a temporary object whose lifetime is extended (6.8.7 [class.temporary]). The full-expression of the variable initialization is treated as an unevaluated operand (7.2.3 [expr.context]). Access checking is performed as if in a context unrelated to `T` and `U`. Only the validity of the immediate context of the variable initialization is considered. [Note 6: … — end note] | […]
   > … | … | …

2. Modify 21.3.5.4 [meta.unary.prop] p9 as indicated:

   > -9- The predicate condition for a template specialization `is_constructible<T, Args...>` shall be satisfied if and only if the following variable definition would be well-formed for some invented variable `t`:

   > >     T t(declval<Args>()...);

   > [Note 7: … — end note]

   > The full-expression of the variable initialization is treated as an unevaluated operand (7.2.3 [expr.context]). Access checking is performed as if in a context unrelated to `T` and any of the `Args`. Only the validity of the immediate context of the variable initialization is considered.

   > [Note 8: … — end note]

3. Modify 21.3.7 [meta.rel] p6 as indicated:

   > -9- The predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied if and only if the return expressionstatement (8.8.4 [stmt.return]) in the following code would be well-formed, including any implicit conversions to the return type of the function:

   > >     To test() {
   > >       return declval<From>();
   > >     }

   > [Note 4: … — end note]

   > Access checking is performed in a context unrelated to `To` and `From`. The operand of the `return` statement (including initialization of the returned object or reference, if any) is treated as an unevaluated operand (7.2.3 [expr.context]), and only the validity of its immediate context is considered. Only the validity of the immediate context of the expression of the `return` statement (8.8.4 [stmt.return]) (including initialization of the returned object or reference) is considered.

   > [Note 5: … — end note]

### 4537(i). Improve `define_static_array`

Section: 21.4.3 [meta.define.static] Status: C++26 Submitter: Hewill Kang Opened: 2026-03-09 Last modified: 2026-06-09

Priority: 2

View all other issues in [meta.define.static].

View all issues with C++26 status.

Discussion:

`define_static_array` can transform any input range, even a non-contiguous one, into a `span`.

However, the returned `span` always has a dynamic extent. If the size of the input range is known at compile time, returning a static `span` seems reasonable and may offer slight runtime efficiency.

[2026-03-25; Reflector poll.]

Set priority to 2 after reflector poll.

[Croydon 2026-03-25; move to Immediate.]

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Modify 21.4.1 [meta.syn], header `<meta>` synopsis, as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]

   >     namespace std {
   >       […]
   >       // 21.4.3 [meta.define.static], promoting to static storage
   >       […]
   >       template<ranges::input_range R>
   >         consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);
   >       […]
   >     }

2. Modify 21.4.3 [meta.define.static] as indicated:

   >     template<ranges::input_range R>
   >       consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);

   > > -16- Effects: Equivalent to:

   > > >     using T = ranges::range_value_t<R>;
   > > >     meta::info array = meta::reflect_constant_array(r);
   > > >     if (meta::is_array_type(meta::type_of(array))) {
   > > >       return span<const T, see below>(meta::extract<const T*>(array), meta::extent(meta::type_of(array)));
   > > >     } else {
   > > >       return span<const T, see below>();
   > > >     }

   > > -?- Remarks: The second template argument of the returned `span` type is `static_cast<size_t>(ranges::size(r))` if `ranges::size(r)` is a constant expression, and `dynamic_extent` otherwise.

[2026-03-26; Tim reopens and provides revised wording]

The `else` branch is ill-formed when the returned has non-zero static extent.

[Croydon 2026-03-27; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.1 [meta.syn], header `<meta>` synopsis, as indicated:

   >     #include <compare>              // see 17.12.1 [compare.syn]
   >     #include <initializer_list>     // see 17.11.2 [initializer.list.syn]

   >     namespace std {
   >       […]
   >       // 21.4.3 [meta.define.static], promoting to static storage
   >       […]
   >       template<ranges::input_range R>
   >         consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);
   >       […]
   >     }

2. Modify 21.4.3 [meta.define.static] as indicated:

   >     template<ranges::input_range R>
   >       consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);

   > > -16- Effects: Equivalent to:

   > > >     using T = ranges::range_value_t<R>;
   > > >     meta::info array = meta::reflect_constant_array(r);
   > > >     if (meta::is_array_type(meta::type_of(array))) {
   > > >       return span<const T, see below>(meta::extract<const T*>(array), meta::extent(meta::type_of(array)));
   > > >     } else {
   > > >       return span<const T, see below>(static_cast<const T*>(nullptr), 0);
   > > >     }

   > > -?- Remarks: The second template argument of the returned `span` type is `static_cast<size_t>(ranges::size(r))` if `ranges::size(r)` is a constant expression, and `dynamic_extent` otherwise.

### 4540(i). `future-sender`s returned from `spawn_future` do not forward stop requests to spawned work

Section: 33.9.12.18 [exec.spawn.future] Status: C++26 Submitter: Ian Petersen Opened: 2026-03-10 Last modified: 2026-06-09

Priority: 1

View all other issues in [exec.spawn.future].

View all issues with C++26 status.

Discussion:

The wording that describes `spawn_future` (specifically 33.9.12.18 [exec.spawn.future] paragraph 13 and paragraph 14) does not capture a critical element of the design intent originally expressed in P3149R11, section 5.5.

P3149R11, section 5.5 reads in part,

> When `fsop` is started, if `fsop` receives a stop request from its receiver before the eagerly-started work has completed then an attempt is made to abandon the eagerly-started work. Note that it's possible for the eagerly-started work to complete while `fsop` is requesting stop; once the stop request has been delivered, either `fsop` completes with the result of the eagerly-started work if it's ready, or it completes with `set_stopped()` without waiting for the eagerly-started work to complete.

In the foregoing, `fsop` is the name of an operation state constructed by connecting a `future-sender` (i.e. a sender returned from `spawn_future`) to a receiver.

Paragraphs 13 and 14 of 33.9.12.18 [exec.spawn.future] describe the behaviour of the `future-sender` returned from `spawn_future` in terms of the `basic-sender` machinery like so:

> -13- The exposition-only class template `impls-for` (33.9.2 [exec.snd.expos]) is specialized for `spawn_future_t` as follows:

> >     namespace std::execution {
> >       template<>
> >       struct impls-for<spawn_future_t> : default-impls {
> >         static constexpr auto start = see below;                    // exposition only
> >       };
> >     }

> -14- The member `impls-for<spawn_future_t>::start` is initialized with a callable object equivalent to the following lambda:

> >     [](auto& state, auto& rcvr) noexcept -> void {
> >       state->consume(rcvr);
> >     }

Since there's no specification for the behaviour of `std::execution::impls-for<spawn_future_t>::get-state`, the behaviour is the default provided by `std::execution::default-impls::get-state`, which just returns the "data" member of the original result of `make-sender`. In this case, that is the object named `u` defined in 33.9.12.18 [exec.spawn.future] bullet 16.2, which is an instance of a specialization of `std::unique_ptr`. There is therefore no wording to require that a `future-sender` that has been connected and started take any action in response to stop requests received through the receiver to which it was connected, contrary to the LEWG-approved design intent.

An implementation that addresses this issue is included in stdexec PR 1713, specifically in commit 5209ffdcaf9a3badf0079746b5578c12a1d0da4f, which is just the difference between the current wording and the intended design.

[2026-03-25; Reflector poll.]

Set priority to 1 after reflector poll.

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Modify 33.9.12.18 [exec.spawn.future] as indicated:

   > -2- The name `spawn_future` denotes a customization point object. For subexpressions `sndr`, `token`, and `env`,

   > […]

   > If any of `sender<Sndr>`, `scope_token<Token>`, or `queryable<Env>` are not satisfied, the expression `spawn_future(sndr, token, env)` is ill-formed.

   > -?- Let `try-cancelable` be the exposition-only class:

   > >     namespace std::execution {
   > >       struct try-cancelable {                   // exposition only
   > >         virtual void try-cancel() noexcept = 0; // exposition only
   > >       };
   > >     }

   > -3- Let `spawn-future-state-base` be the exposition-only class template: […]

   > >     namespace std::execution {
   > >       template<class Completions>
   > >       struct spawn-future-state-base;           // exposition only

   > >       template<class... Sigs>
   > >       struct spawn-future-state-base<completion_signatures<Sigs...>> { // exposition only
   > >         : try-cancelable {
   > >         using variant-t = see below;            // exposition only
   > >         variant-t result;                       // exposition only
   > >         virtual void complete() noexcept = 0;   // exposition only
   > >       };
   > >     }

   > […]

   > -7- Let `spawn-future-state` be the exposition-only class template:

   > >     namespace std::execution {
   > >       template<class Alloc, scope_token Token, sender Sender, class Env>
   > >       struct spawn-future-state                     // exposition only
   > >         : spawn-future-state-base<completion_signatures_of_t<future-spawned-sender<Sender, Env>>> {
   > >         […]
   > >         void complete() noexcept override;          // exposition only
   > >         void consume(receiver auto& rcvr) noexcept; // exposition only
   > >         void abandon() noexcept;                    // exposition only
   > >         void try-cancel() noexcept override {       // exposition only
   > >           ssource.request_stop();
   > >           try-set-stopped();
   > >         }
   > >         void try-set-stopped() noexcept; // exposition only
   > >         […]
   > >       };
   > >       […]
   > >     }

   > -8- For purposes of determining the existence of a data race, `complete`, `consume`, `try-set-stopped`, and `abandon` behave as atomic operations (6.10.2 [intro.multithread]). These operations on a single object of a type that is a specialization of `spawn-future-state` appear to occur in a single total order.

   >     void complete() noexcept;

   > > -9- Effects:

   > > - (9.1) — No effects if this invocation of `complete` happens before an invocation of `consume`, `try-set-stopped`, or `abandon` on `*this`;

   > > - (9.2) — otherwise, if an invocation of `consume` on `*this` happens before this invocation of `complete` and no invocation of `try-set-stopped` on `*this` happened before this invocation of `complete` then there is a receiver, `rcvr`, registered and that receiver is deregistered and completed as if by `consume(rcvr)`;

   > > - (9.3) — otherwise, `destroy` is invoked.

   >     void consume(receiver auto& rcvr) noexcept;

   > > -10- Effects:

   > > - (10.1) — If this invocation of `consume` happens before an invocation of `complete` on `*this` and no invocation of `try-cancel` on `*this` happened before this invocation of `consume` then `rcvr` is registered to be completed when `complete` is subsequently invoked on `*this`;

   > > - (10.?) — otherwise, if this invocation of `consume` happens after an invocation of `try-cancel` on `*this` and no invocation of `complete` on `*this` happened before this invocation of `consume` then `rcvr` is completed as if by `set_stopped(std::move(rcvr))`;

   > > - (10.2) — otherwise, `rcvr` is completed as if by: […]

   >     void try-cancel() noexcept;

   > > -?- Effects:

   > > - (?.1) — No effects if this invocation of `try-cancel` happens after an invocation of `complete` on `*this`;

   > > - (?.2) — otherwise, if this invocation of `try-cancel` happens before an invocation of `consume` on `*this` then invokes `ssource.request_stop()`;

   > > - (?.3) — otherwise,

   > >   - (?.3.1) — invokes `ssource.request_stop()`, and

   > >   - (?.3.2) — if there is a receiver, `rcvr`, still registered then that receiver is deregistered and completed as if by `set_stopped(std::move(rcvr))`.

   > >     [Note: an invocation of `complete` on `*this` may have happened after the just-described invocation of `ssource.request_stop()` and happened before the check to see if there is a receiver still registered; if so, it would have deregistered and completed the previously-registered receiver. Only one of `try-cancel` or `complete` completes the registered receiver and no data races are introduced between the two invocations. — end note]

   >     void abandon() noexcept;

   > > […]

   >     void destroy() noexcept;

   > > -12- Effects: Equivalent to:

   > > >     auto associated = std::move(this->associated);
   > > >     {
   > > >       using traits = allocator_traits<Alloc>::template rebind_traits<spawn-future-state>;
   > > >       typename traits::allocator_type alloc(std::move(this->alloc));
   > > >       traits::destroy(alloc, this);
   > > >       traits::deallocate(alloc, this, 1);
   > > >     }

   > > -?- Let `future-operation` be the exposition-only class template:

   > > >     namespace std::execution {
   > > >       template<class StatePtr, class Rcvr>
   > > >       struct future-operation {                              // exposition only
   > > >         struct callback {                                    // exposition only
   > > >           try-cancelable* state;                             // exposition only

   > > >           void operator()() noexcept {
   > > >             state->try-cancel();
   > > >           };
   > > >         };

   > > >         using stop-token-t =                                 // exposition only
   > > >           stop_token_of_t<env_of_t<Rcvr>>;

   > > >         using stop-callback-t =                              // exposition only
   > > >           stop_callback_for_t<stop-token-t, callback>;

   > > >         struct receiver {                                    // exposition only
   > > >           using receiver_concept = receiver_t;
   > > >           future-operation* op;                              // exposition only

   > > >           template<class... T>
   > > >           void set_value(T&&... ts) && noexcept {
   > > >             op->set-complete<set_value_t>(std::forward<T>(ts)...);
   > > >           }

   > > >           template<class E>
   > > >           void set_error(E&& e) && noexcept {
   > > >             op->set-complete<set_error_t>(std::forward<E>(e));
   > > >           }

   > > >           void set_stopped() && noexcept {
   > > >             op->set-complete<set_stopped_t>();
   > > >           }

   > > >           env_of_t<Rcvr> get_env() const noexcept {
   > > >             return op->rcvr.get_env();
   > > >           }
   > > >         };

   > > >         Rcvr rcvr;                                           // exposition only

   > > >         union {
   > > >           StatePtr state;                                    // exposition only
   > > >           receiver inner;                                    // exposition only
   > > >         };

   > > >         union {
   > > >           stop-callback-t stopCallback;                      // exposition only
   > > >         };

   > > >         future-operation(StatePtr state, Rcvr rcvr) noexcept // exposition only
   > > >           : rcvr(std::move(rcvr))
   > > >         {
   > > >           construct_at(addressof(state), std::move(state));
   > > >         }

   > > >         future-operation(future-operation&&) = delete;

   > > >         ~future-operation() {
   > > >           destroy_at(addressof(state));
   > > >         }

   > > >         void run() & noexcept {                              // exposition only
   > > >           constexpr bool nothrow =
   > > >               is_nothrow_constructible_v<stop-callback-t, stop-token-t, callback>;
   > > >           try {
   > > >             construct_at(addressof(stopCallback), get_stop_token(rcvr), callback(state.get()));
   > > >           }
   > > >           catch (...) {
   > > >             if constexpr (!nothrow) {
   > > >               set_error(std::move(rcvr), current_exception());
   > > >               return;
   > > >             }
   > > >           }

   > > >           auto* state = state.release();
   > > >           destroy_at(addressof(state));
   > > >           construct_at(addressof(inner), this);
   > > >           state->consume(inner);
   > > >         }

   > > >         template<class CPO, class... T>
   > > >         void set-complete(T&&... ts) noexcept {              // exposition only
   > > >           destroy_at(addressof(stopCallback));
   > > >           destroy_at(addressof(inner));
   > > >           construct_at(addressof(state), nullptr);
   > > >           CPO{}(std::move(rcvr), std::forward<T>(ts)...);
   > > >         }
   > > >       };
   > > >     }

   > > -13- The exposition-only class template `impls-for` (33.9.2 [exec.snd.expos]) is specialized for `spawn_future_t` as follows:

   > > >     namespace std::execution {
   > > >       template<>
   > > >       struct impls-for<spawn_future_t> : default-impls {
   > > >         static constexpr auto start = see below;       // exposition only
   > > >         static constexpr auto get-state = see below;   // exposition only
   > > >       };
   > > >     }

   > > -14- The member `impls-for<spawn_future_t>::start` is initialized with a callable object equivalent to the following lambda:

   > > >     [](auto& state, auto& rcvr) noexcept -> void {
   > > >       state.run()->consume(rcvr);
   > > >     }

   > > -?- The member `impls-for<spawn_future_t>::get-state` is initialized with a callable object equivalent to the following lambda:

   > > >     []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept {
   > > >       auto& [_, data] = sndr;
   > > >       using state_ptr = remove_cvref_t<decltype(data)>;
   > > >       return future-operation<state_ptr, Rcvr>(std::move(data), std::move(rcvr));
   > > >     }

[2026-03-25; Tomasz provides updated wording.]

The updated wording removes the union inside `future-operation`.

[Croydon 2026-03-27; Status changed: New → Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 33.9.12.18 [exec.spawn.future] as indicated:

   > -2- The name `spawn_future` denotes a customization point object. For subexpressions `sndr`, `token`, and `env`,

   > […]

   > If any of `sender<Sndr>`, `scope_token<Token>`, or `queryable<Env>` are not satisfied, the expression `spawn_future(sndr, token, env)` is ill-formed.

   > -?- Let `try-cancelable` be the exposition-only class:

   > >     namespace std::execution {
   > >       struct try-cancelable {                   // exposition only
   > >         virtual void try-cancel() noexcept = 0; // exposition only
   > >       };
   > >     }

   > -3- Let `spawn-future-state-base` be the exposition-only class template: […]

   > >     namespace std::execution {
   > >       template<class Completions>
   > >       struct spawn-future-state-base;           // exposition only

   > >       template<class... Sigs>
   > >       struct spawn-future-state-base<completion_signatures<Sigs...>> { // exposition only
   > >         : try-cancelable {
   > >         using variant-t = see below;            // exposition only
   > >         variant-t result;                       // exposition only
   > >         virtual void complete() noexcept = 0;   // exposition only
   > >       };
   > >     }

   > […]

   > -7- Let `spawn-future-state` be the exposition-only class template:

   > >     namespace std::execution {
   > >       template<class Alloc, scope_token Token, sender Sender, class Env>
   > >       struct spawn-future-state                     // exposition only
   > >         : spawn-future-state-base<completion_signatures_of_t<future-spawned-sender<Sender, Env>>> {
   > >         […]
   > >         void complete() noexcept override;          // exposition only
   > >         void consume(receiver auto& rcvr) noexcept; // exposition only
   > >         void abandon() noexcept;                    // exposition only
   > >         void try-cancel() noexcept override {       // exposition only
   > >           ssource.request_stop();
   > >           try-set-stopped();
   > >         }
   > >         void try-set-stopped() noexcept; // exposition only
   > >         […]
   > >       };
   > >       […]
   > >     }

   > -8- For purposes of determining the existence of a data race, `complete`, `consume`, `try-set-stopped`, and `abandon` behave as atomic operations (6.10.2 [intro.multithread]). These operations on a single object of a type that is a specialization of `spawn-future-state` appear to occur in a single total order.

   >     void complete() noexcept;

   > > -9- Effects:

   > > - (9.1) — No effects if this invocation of `complete` happens before an invocation of `consume`, `try-set-stopped`, or `abandon` on `*this`;

   > > - (9.2) — otherwise, if an invocation of `consume` on `*this` happens before this invocation of `complete` and no invocation of `try-set-stopped` on `*this` happened before this invocation of `complete` then there is a receiver, `rcvr`, registered and that receiver is deregistered and completed as if by `consume(rcvr)`;

   > > - (9.3) — otherwise, `destroy` is invoked.

   >     void consume(receiver auto& rcvr) noexcept;

   > > -10- Effects:

   > > - (10.1) — If this invocation of `consume` happens before an invocation of `complete` on `*this` and no invocation of `try-set-stopped` on `*this` happened before this invocation of `consume` then `rcvr` is registered to be completed when `complete` is subsequently invoked on `*this`;

   > > - (10.?) — otherwise, if this invocation of `consume` happens after an invocation of `try-set-stopped` on `*this` and no invocation of `complete` on `*this` happened before this invocation of `consume` then `rcvr` is completed as if by `set_stopped(std::move(rcvr))`;

   > > - (10.2) — otherwise, `rcvr` is completed as if by:

   > >       std::move(this->result).visit(
   > >         [&rcvr](auto&& tuple) noexcept {
   > >           if constexpr (!same_as<remove_reference_t<decltype(tuple)>, monostate>) {
   > >             apply([&rcvr](auto cpo, auto&&... vals) {
   > >               cpo(std::move(rcvr), std::move(vals)...);
   > >             }, std::move(tuple));
   > >           }
   > >         });
   > >       destroy();

   >     void try-set-stopped() noexcept;

   > > -?- Effects:

   > > - (?.1) — If an invocation of `consume` on `*this` happens before this invocation of `try-set-stopped` and no invocation of `complete` on `*this` happened before this invocation of `try-set-stopped` then there is a receiver, `rcvr`, registered and that receiver is deregistered and completed as if by `set_stopped(std::move(rcvr)), destroy()`;

   > > - (?.2) — otherwise, no effects.

   >     void abandon() noexcept;

   > > […]

   >     void destroy() noexcept;

   > > -12- Effects: Equivalent to:

   > > >     auto associated = std::move(this->associated);
   > > >     {
   > > >       using traits = allocator_traits<Alloc>::template rebind_traits<spawn-future-state>;
   > > >       typename traits::allocator_type alloc(std::move(this->alloc));
   > > >       traits::destroy(alloc, this);
   > > >       traits::deallocate(alloc, this, 1);
   > > >     }

   > > -?- Let `future-operation` be the exposition-only class template:

   > > >     namespace std::execution {
   > > >       template<class StatePtr, class Rcvr>
   > > >       struct future-operation {                              // exposition only
   > > >         struct callback {                                    // exposition only
   > > >           try-cancelable* state;                             // exposition only

   > > >           void operator()() noexcept {
   > > >             state->try-cancel();
   > > >           };
   > > >         };

   > > >         using stop-token-t =                                 // exposition only
   > > >           stop_token_of_t<env_of_t<Rcvr>>;

   > > >         using stop-callback-t =                              // exposition only
   > > >           stop_callback_for_t<stop-token-t, callback>;

   > > >         struct receiver {                                    // exposition only
   > > >           using receiver_concept = receiver_t;
   > > >           future-operation* op;                              // exposition only

   > > >           template<class... T>
   > > >           void set_value(T&&... ts) && noexcept {
   > > >             op->set-complete<set_value_t>(std::forward<T>(ts)...);
   > > >           }

   > > >           template<class E>
   > > >           void set_error(E&& e) && noexcept {
   > > >             op->set-complete<set_error_t>(std::forward<E>(e));
   > > >           }

   > > >           void set_stopped() && noexcept {
   > > >             op->set-complete<set_stopped_t>();
   > > >           }

   > > >           env_of_t<Rcvr> get_env() const noexcept {
   > > >             return execution::get_env(op->rcvr);
   > > >           }
   > > >         };

   > > >         Rcvr rcvr;                                       // exposition only
   > > >         StatePtr state;                                  // exposition only
   > > >         receiver inner;                                  // exposition only
   > > >         optional<stop-callback-t> stopCallback;          // exposition only

   > > >         future-operation(StatePtr state, Rcvr rcvr) noexcept // exposition only
   > > >           : rcvr(std::move(rcvr)), state(std::move(state)), inner(this)
   > > >         {}

   > > >         future-operation(future-operation&&) = delete;

   > > >         void run() & noexcept {                              // exposition only
   > > >           constexpr bool nothrow =
   > > >               is_nothrow_constructible_v<stop-callback-t, stop-token-t, callback>;
   > > >           try {
   > > >     	  stopCallback.emplace(get_stop_token(rcvr), callback(state.get()));
   > > >           }
   > > >           catch (...) {
   > > >             if constexpr (!nothrow) {
   > > >               set_error(std::move(rcvr), current_exception());
   > > >               return;
   > > >             }
   > > >           }

   > > >           state.release()->consume(inner);
   > > >         }

   > > >         template<class CPO, class... T>
   > > >         void set-complete(T&&... ts) noexcept {              // exposition only
   > > >           stopCallback.reset();
   > > >           CPO{}(std::move(rcvr), std::forward<T>(ts)...);
   > > >         }
   > > >       };
   > > >     }

   > > -13- The exposition-only class template `impls-for` (33.9.2 [exec.snd.expos]) is specialized for `spawn_future_t` as follows:

   > > >     namespace std::execution {
   > > >       template<>
   > > >       struct impls-for<spawn_future_t> : default-impls {
   > > >         static constexpr auto start = see below;       // exposition only
   > > >         static constexpr auto get-state = see below;   // exposition only
   > > >       };
   > > >     }

   > > -14- The member `impls-for<spawn_future_t>::start` is initialized with a callable object equivalent to the following lambda:

   > > >     [](auto& state, auto& rcvr) noexcept -> void {
   > > >       state.run()->consume(rcvr);
   > > >     }

   > > -?- The member `impls-for<spawn_future_t>::get-state` is initialized with a callable object equivalent to the following lambda:

   > > >     []<class Sndr, class Rcvr>(Sndr sndr, Rcvr& rcvr) noexcept {
   > > >       auto& [_, data] = sndr;
   > > >       using state_ptr = remove_cvref_t<decltype(data)>;
   > > >       return future-operation<state_ptr, Rcvr>(std::move(data), std::move(rcvr));
   > > >     }

### 4544(i). Parallel overload of `ranges::set_difference` should return `in_in_out_result`

Section: 26.8.7.5 [set.difference] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2026-03-18 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Serial `ranges::set_difference` returns `in_out_result`, even though it takes two input ranges. This is probably fine because it assumes that the output size is always sufficient for the algorithms to write all necessary elements. We only need to go to the end of the first input and we usually do not care where the second input stops even if we did not fully traverse it. However, for parallel range algorithms the output size can be insufficient to have every single element from input that fits. When the output has smaller size than the number of elements it is supposed to take (based on the input), it should be useful for users to understand where stop points are for both first and second inputs. Thus, we need to return `in_in_out_result`. The algorithm should return the stop point of the first input, which is the first element that has to be written to the output but doesn't fit and it should also return the stop point of the second input, which is the element that was compared with the `input1` element that does not fit. Otherwise, the algorithm returns an iterator that compares equal with `last2`, for the second input, meaning that it is already exhausted.

This design for parallel range `set_difference` will also be beneficial if and when we are going to add serial range algorithms with the range-as-the-output-design aspect. For the behavior illustration purposes please look at the serial implementation of such algorithm:

>     template <class In1, class In2, class Out>
>     using set_difference_truncated_result = std::ranges::in_in_out_result<In1, In2, Out>;

>     struct set_difference_fn
>     {
>         template<std::input_iterator I1, std::sentinel_for<I1> S1,
>                  std::input_iterator I2, std::sentinel_for<I2> S2,
>                  std::input_iterator O, std::sentinel_for<O> OutS,
>                  class Comp = std::ranges::less,
>                  class Proj1 = std::identity, class Proj2 = std::identity>
>           requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
>         constexpr set_difference_truncated_result<I1, I2, O>
>         operator()(I1 first1, S1 last1, I2 first2, S2 last2,
>                    O result, OutS result_last, Comp comp = {},
>                    Proj1 proj1 = {}, Proj2 proj2 = {}) const
>        {
>          while (!(first1 == last1 || first2 == last2))
>          {
>            if (std::invoke(comp, std::invoke(proj1, *first1), std::invoke(proj2, *first2)))
>            {
>              if (result == result_last)
>                break;

>              *result = *first1;
>              ++first1;
>              ++result;
>            }
>            else if (std::invoke(comp, std::invoke(proj2, *first2),
>                     std::invoke(proj1, *first1)))
>              ++first2;
>            else
>            {
>              ++first1;
>              ++first2;
>            }
>          }

>          while (first1 != last1 && result != result_last)
>          {
>            *result = *first1;
>            ++first1;
>            ++result;
>          }

>          return {first1, first2, result};
>        }

>        template<std::ranges::input_range R1, std::ranges::input_range R2,
>                 std::ranges::input_range OR, class Comp = std::ranges::less,
>                 class Proj1 = std::identity, class Proj2 = std::identity>
>            requires std::mergeable<std::ranges::iterator_t<R1>, std::ranges::iterator_t<R2>,
>        std::ranges::iterator_t<OR>, Comp, Proj1, Proj2>
>        constexpr set_difference_truncated_result<std::ranges::borrowed_iterator_t<R1>,
>                                                  std::ranges::borrowed_iterator_t<R2>,formatPainter
>                                                  std::ranges::borrowed_iterator_t<OR>>
>        operator()(R1&& r1, R2&& r2, OR&& result, Comp comp = {},
>                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
>        {
>          return (*this)(std::ranges::begin(r1), std::ranges::end(r1),
>                         std::ranges::begin(r2), std::ranges::end(r2),
>                         std::ranges::begin(result), std::ranges::end(result),
>                         std::move(comp), std::move(proj1), std::move(proj2));
>        }
>     };

>     inline constexpr set_difference_fn set_difference{};

For instance, if the first input is `std::vector v1{1, 2, 3, 4, 5, 6, 7}`, the second input is `std::vector v2{3, 4, 7}`, and the output is `std::vector<int> output(3)` then for the call

>     auto [in1, in2, out] = std::ranges::set_difference(v1, v2, output);

- `output` is `{1, 2, 5}`

- `in1` points to `6` in `v1`

- `in2` points to `7` in `v2`

- `out` points to `output.end()`

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Modify 26.4 [algorithm.syn], header `<algorithm>` synopsis, as indicated:

   >     […]
   >     namespace ranges {
   >       template<class I, class O>
   >         using set_difference_result = in_out_result<I, O>;
   >       template<class I1, class, I2, class O>
   >         using set_difference_truncated_result = in_in_out_result<I1, I2, O>;

   >       template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
   >                weakly_incrementable O, class Comp = ranges::less,
   >                class Proj1 = identity, class Proj2 = identity>
   >         requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >         constexpr set_difference_result<I1, O>
   >           set_difference(I1 first1, S1 last1, I2 first2, S2 last2, O result,
   >                          Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >       template<input_range R1, input_range R2, weakly_incrementable O,
   >                class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
   >         requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
   >         constexpr set_difference_result<borrowed_iterator_t<R1>, O>
   >           set_difference(R1&& r1, R2&& r2, O result,
   >                          Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});

   >       template<execution-policy Ep, random_access_iterator I1, sized_sentinel_for<I1> S1,
   >                random_access_iterator I2, sized_sentinel_for<I2> S2,
   >                random_access_iterator O, sized_sentinel_for<O> OutS, class Comp = ranges::less,
   >                class Proj1 = identity, class Proj2 = identity>
   >         requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >         set_difference_truncated_result<I1, I2, O>
   >           set_difference(Ep&& exec, I1 first1, S1 last1, I2 first2, S2 last2,
   >                          O result, OutS result_last, Comp comp = {}, Proj1 proj1 = {},
   >                          Proj2 proj2 = {}); // freestanding-deleted
   >       template<execution-policy Ep, sized-random-access-range R1, sized-random-access-range R2,
   >                sized-random-access-range OutR, class Comp = ranges::less,
   >                class Proj1 = identity, class Proj2 = identity>
   >         requires mergeable<iterator_t<R1>, iterator_t<R2>, iterator_t<OutR>, Comp, Proj1, Proj2>
   >         set_difference_truncated_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, borrowed_iterator_t<OutR>>
   >           set_difference(Ep&& exec, R1&& r1, R2&& r2, OutR&& result_r, Comp comp = {},
   >                          Proj1 proj1 = {}, Proj2 proj2 = {}); // freestanding-deleted
   >     }
   >     […]

2. Modify 26.8.7.5 [set.difference] as indicated:

   >     […]
   >     template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
   >              weakly_incrementable O, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >       constexpr ranges::set_difference_result<I1, O>
   >         ranges::set_difference(I1 first1, S1 last1, I2 first2, S2 last2, O result,
   >                                Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<input_range R1, input_range R2, weakly_incrementable O,
   >              class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
   >       constexpr ranges::set_difference_result<borrowed_iterator_t<R1>, O>
   >         ranges::set_difference(R1&& r1, R2&& r2, O result,
   >                                Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<execution-policy Ep, random_access_iterator I1, sized_sentinel_for<I1> S1,
   >              random_access_iterator I2, sized_sentinel_for<I2> S2,
   >              random_access_iterator O, sized_sentinel_for<O> OutS, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >       ranges::set_difference_truncated_result<I1, I2, O>
   >         ranges::set_difference(Ep&& exec, I1 first1, S1 last1,
   >                                I2 first2, S2 last2, O result, OutS result_last,
   >                                Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<execution-policy Ep, sized-random-access-range R1, sized-random-access-range R2,
   >              sized-random-access-range OutR, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, iterator_t<OutR>, Comp, Proj1, Proj2>
   >       ranges::set_difference_truncated_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, borrowed_iterator_t<OutR>>
   >         ranges::set_difference(Ep&& exec, R1&& r1, R2&& r2, OutR&& result_r, Comp comp = {},
   >                                Proj1 proj1 = {}, Proj2 proj2 = {});

   > > -1- Let: […]

   > > -2- Preconditions: The ranges `[first1, last1)` and `[first2, last2)` are sorted with respect to `comp` and `proj1` or `proj2`, respectively. The resulting range does not overlap with either of the original ranges.

   > > -3- Effects: Constructs a sorted difference between the elements from the two ranges; that is, the set of elements that are present in the range `[first1, last1)` but not `[first2, last2)`. If `[first1, last1)` contains `m` elements that are equivalent to each other and `[first2, last2)` contains `n` elements that are equivalent to them, the last max`(m − n, 0)` elements from `[first1, last1)` are included in the sorted difference, in order. Of those equivalent elements, the first min`(m, n)` elements in both ranges are considered skipped. An element from the second range is also considered skipped if it compares less than the min`(N + 1, M)`th element of the sorted difference. Copies the first `N` elements of the sorted difference to the range `[result, result + N)`.

   > > -4- Returns:

   > > - (4.1) — `result_last` for the overloads in namespace `std`.

   > > - (4.2) — `{last1, result + N}` for the overloads in namespace `ranges`, if `N` is equal to `M`.For non-parallel algorithm overloads in namespace `ranges`:

   > >   - (4.2.1) — `{last1, result + N}`, if `N` is equal to `M`.

   > >   - (4.2.2) — Otherwise, `{j1, result_last}`, where the iterator `j1` points to the position of the element in `[first1, last1)` corresponding to the `(N + 1)`th element of the sorted difference.

   > > - (4.3) — Otherwise, `{j1, result_last}` for the overloads in namespace `ranges`, where the iterator `j1` points to the position of the element in `[first1, last1)` corresponding to the `(N + 1)`th element of the sorted difference.For parallel algorithm overloads in namespace `ranges`:

   > >   - (4.3.1) — `{last1, first2 + B, result + N}`, if `N` is equal to `M`, where `B` is the number of skipped elements in `[first2, last2)`.

   > >   - (4.3.2) — Otherwise, `{ first1 + A, first2 + B, result_last}`, where `A` and `B` are the numbers of copied or skipped elements in `[first1, last1)` and `[first2, last2)`, respectively.

[Croydon 2026-03-24; update wording after LWG review]

[Croydon 2026-03-24; move to Immediate]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 26.4 [algorithm.syn], header `<algorithm>` synopsis, as indicated:

   >     […]
   >     namespace ranges {
   >       template<class I, class O>
   >         using set_difference_result = in_out_result<I, O>;
   >       template<class I1, class, I2, class O>
   >         using set_difference_truncated_result = in_in_out_result<I1, I2, O>;

   >       template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
   >                weakly_incrementable O, class Comp = ranges::less,
   >                class Proj1 = identity, class Proj2 = identity>
   >         requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >         constexpr set_difference_result<I1, O>
   >           set_difference(I1 first1, S1 last1, I2 first2, S2 last2, O result,
   >                          Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >       template<input_range R1, input_range R2, weakly_incrementable O,
   >                class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
   >         requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
   >         constexpr set_difference_result<borrowed_iterator_t<R1>, O>
   >           set_difference(R1&& r1, R2&& r2, O result,
   >                          Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});

   >       template<execution-policy Ep, random_access_iterator I1, sized_sentinel_for<I1> S1,
   >                random_access_iterator I2, sized_sentinel_for<I2> S2,
   >                random_access_iterator O, sized_sentinel_for<O> OutS, class Comp = ranges::less,
   >                class Proj1 = identity, class Proj2 = identity>
   >         requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >         set_difference_truncated_result<I1, I2, O>
   >           set_difference(Ep&& exec, I1 first1, S1 last1, I2 first2, S2 last2,
   >                          O result, OutS result_last, Comp comp = {}, Proj1 proj1 = {},
   >                          Proj2 proj2 = {}); // freestanding-deleted
   >       template<execution-policy Ep, sized-random-access-range R1, sized-random-access-range R2,
   >                sized-random-access-range OutR, class Comp = ranges::less,
   >                class Proj1 = identity, class Proj2 = identity>
   >         requires mergeable<iterator_t<R1>, iterator_t<R2>, iterator_t<OutR>, Comp, Proj1, Proj2>
   >         set_difference_truncated_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, borrowed_iterator_t<OutR>>
   >           set_difference(Ep&& exec, R1&& r1, R2&& r2, OutR&& result_r, Comp comp = {},
   >                          Proj1 proj1 = {}, Proj2 proj2 = {}); // freestanding-deleted
   >     }
   >     […]

2. Modify 26.8.7.5 [set.difference] as indicated:

   >     […]
   >     template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
   >              weakly_incrementable O, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >       constexpr ranges::set_difference_result<I1, O>
   >         ranges::set_difference(I1 first1, S1 last1, I2 first2, S2 last2, O result,
   >                                Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<input_range R1, input_range R2, weakly_incrementable O,
   >              class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
   >       constexpr ranges::set_difference_result<borrowed_iterator_t<R1>, O>
   >         ranges::set_difference(R1&& r1, R2&& r2, O result,
   >                                Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<execution-policy Ep, random_access_iterator I1, sized_sentinel_for<I1> S1,
   >              random_access_iterator I2, sized_sentinel_for<I2> S2,
   >              random_access_iterator O, sized_sentinel_for<O> OutS, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >       ranges::set_difference_truncated_result<I1, I2, O>
   >         ranges::set_difference(Ep&& exec, I1 first1, S1 last1,
   >                                I2 first2, S2 last2, O result, OutS result_last,
   >                                Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<execution-policy Ep, sized-random-access-range R1, sized-random-access-range R2,
   >              sized-random-access-range OutR, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, iterator_t<OutR>, Comp, Proj1, Proj2>
   >       ranges::set_difference_truncated_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, borrowed_iterator_t<OutR>>
   >         ranges::set_difference(Ep&& exec, R1&& r1, R2&& r2, OutR&& result_r, Comp comp = {},
   >                                Proj1 proj1 = {}, Proj2 proj2 = {});

   > > -1- Let: […]

   > > -2- Preconditions: The ranges `[first1, last1)` and `[first2, last2)` are sorted with respect to `comp` and `proj1` or `proj2`, respectively. The resulting range does not overlap with either of the original ranges.

   > > -3- Effects: Constructs a sorted difference between the elements from the two ranges; that is, the set of elements that are present in the range `[first1, last1)` but not `[first2, last2)`. If `[first1, last1)` contains `m` elements that are equivalent to each other and `[first2, last2)` contains `n` elements that are equivalent to them, the last max`(m − n, 0)` elements from `[first1, last1)` are included in the sorted difference, in order. Of those equivalent elements, the first min`(m, n)` elements in both ranges are considered skipped. An element from the second range is also considered skipped if it compares less than the min`(N + 1, M)`th element of the sorted difference. Copies the first `N` elements of the sorted difference to the range `[result, result + N)`.

   > > -4- Returns:

   > > - (4.1) — `result_last` for the overloads in namespace `std`.

   > > - (4.2) — `{last1, result + N}` for the non-parallel overloads in namespace `ranges`, if `N` is equal to `M`.

   > > - (4.3) — Otherwise, `{j1, result_last}` for the overloads in namespace `ranges`, where the iterator `j1` points to the position of the element in `[first1, last1)` corresponding to the `(N + 1)`th element of the sorted difference. For the parallel algorithm overloads in namespace `ranges`:

   > >   - (4.3.1) — `{last1, first2 + B, result + N}`, if `N` is equal to `M`, where `B` is the number of skipped elements in `[first2, last2)`.

   > >   - (4.3.2) — Otherwise, `{ first1 + A, first2 + B, result_last}`, where `A` and `B` are the numbers of copied or skipped elements in `[first1, last1)` and `[first2, last2)`, respectively.

### 4548(i). Parallel `ranges::set_intersection` should not do unnecessary work

Section: 26.8.7.4 [set.intersection] Status: C++26 Submitter: Ruslan Arutyunyan Opened: 2026-03-20 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [set.intersection].

View all issues with C++26 status.

Discussion:

Serial `ranges::set_intersection` returns `in_in_out_result` and it always goes to the end of both input ranges. However, when either of the input ranges reaches the end, there cannot be intersection anymore, so the algorithm could immediately stop but does not do that.

For example, if the first input is `{1}` and second input is `{1, 2, 3, 4, …, 1000000}` it's clear that the output will be `{1}` after the first iteration of `ranges::set_intersection`. The first input is already exhausted, so the algorithm could stop and return:

- `it1` that compared equal with first `last1`,

- `it2` that points to the element equal to 2 in the second input (in other words, stop point for the second input), and

- `result_it` that points to past the last written element

Unfortunately, the algorithm does 999999 "unnecessary" operations just to calculate the iterator that compared equal to `last2`, not to compute an intersection itself.

Of course, this case is problematic only for non-random access, non-sized ranges. Still, it looks like an oversight because:

- Serial overload can work with input iterators,

- We almost certainly do not want to pessimize the algorithm execution time to just calculate `last1` or `last2`, when the algorithm could potentially stop earlier, and

- `std::set_intersection` (not the `ranges` one) stops when it hits the end of either input.

For parallel `ranges::set_intersection` we just copied the current behavior of serial `ranges::set_intersection` because parallel overload requires `sized-random-access-range`, so it's not a problem to calculate both `last1` and `last2` iterators for a constant time in this case. Then we realized that this problem exists for the serial overload.

Fixing C++20 `ranges::set_intersection` is a breaking change, unfortunately. However, we can still change the parallel overload to mimic the serial behavior that we want. This is especially beneficial if and when we add serial range algorithms with the range-as-the-output-design aspect.

The proposed resolution:

- When the output size is not sufficient to copy all necessary elements from both inputs — no changes to the specified parallel `ranges::set_intersection` behavior.

- When the output size is sufficient, change the behavior of parallel `ranges::set_intersection` to return `last1` or `last2` — whichever input range is exhausted first — and a stop point in non-exhausted input range, which is the element that is:

  - Past the last element of non-exhausted range, if it compared equal with the last written element to the output, or otherwise

  - The last element that does not compare equal to `exhausted-range-last - 1` element.

The trade-off of the proposed behavior is that the overload with execution policy would return something different at runtime compared to the serial overload. However, if we don't apply the proposed resolution, we will lock ourselves into suboptimal `ranges::set_intersection` implementation by design, even for the future algorithms extension.

For the behavior illustration purposes please look at the serial implementation of serial `ranges::set_intersection` with range-as-the-output:

>     struct set_intersection_fn
>     {
>         template<std::input_iterator I1, std::sentinel_for<I1> S1,
>                  std::input_iterator I2, std::sentinel_for<I2> S2,
>                  std::input_iterator O, std::sentinel_for<O> OutS,
>                  class Comp = std::ranges::less,
>                  class Proj1 = std::identity, class Proj2 = std::identity>
>         requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
>         constexpr std::ranges::set_intersection_result<I1, I2, O>
>           operator()(I1 first1, S1 last1, I2 first2, S2 last2,
>                      O result, OutS result_last, Comp comp = {},
>                      Proj1 proj1 = {}, Proj2 proj2 = {}) const
>           {
>             while (first1 != last1 && first2 != last2)
>             {
>               if (std::invoke(comp, std::invoke(proj1, *first1),
>                                     std::invoke(proj2, *first2)))
>                 ++first1;
>               else if (std::invoke(comp, std::invoke(proj2, *first2),
>                                          std::invoke(proj1, *first1)))
>                 ++first2;
>               else
>               {
>                 if (result == result_last)
>                   break;

>                 *result = *first1;
>                 ++first1;
>                 ++first2;
>                 ++result;
>               }
>             }
>             return {std::move(first1), std::move(first2), std::move(result)};
>           }

>         template<std::ranges::input_range R1, std::ranges::input_range R2,
>                  std::ranges::input_range OutR, class Comp = std::ranges::less,
>                  class Proj1 = std::identity, class Proj2 = std::identity>
>         requires std::mergeable<std::ranges::iterator_t<R1>, std::ranges::iterator_t<R2>,
>                                 std::ranges::iterator_t<OutR>, Comp, Proj1, Proj2>
>         constexpr std::ranges::set_intersection_result<std::ranges::borrowed_iterator_t<R1>,
>                                                        std::ranges::borrowed_iterator_t<R2>,
>                                                        std::ranges::borrowed_iterator_t<OutR>>
>           operator()(R1&& r1, R2&& r2, OutR&& result, Comp comp = {},
>                      Proj1 proj1 = {}, Proj2 proj2 = {}) const
>           {
>             return (*this)(std::ranges::begin(r1), std::ranges::end(r1),
>                            std::ranges::begin(r2), std::ranges::end(r2),
>                            std::ranges::begin(result), std::ranges::end(result),
>                            std::move(comp), std::move(proj1), std::move(proj2));
>           }
>     };

>     inline constexpr set_intersection_fn set_intersection {};

For instance, if the first input is `std::vector v1{2, 4, 5}` the second input is `std::vector v2{1, 2, 3, 4, 5, 6, 7}`, and the output is `std::vector<int> output(3)` then for the call

>     auto [in1, in2, out] = std::ranges::set_difference(v1, v2, output);

- `output` is `{2, 4, 5}`

- `in1` points to `v1.end()`

- `in2` points to `6` in `v2`

- `out` points to `output.end()`

Another example: if the first input is `std::vector v1{1, 2, 3, 4}` the second input is `std::vector v2{0, 1, 2, 3, 5, 7}`, and the output is `std::vector<int> output(3)` then for the call

>     auto [in1, in2, out] = std::ranges::set_difference(v1, v2, output);

- `output` is `{1, 2, 3}`

- `in1` points to `v1.end()`

- `in2` points to `5` in `v2`

- `out` points to `output.end()`

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

1. Modify 26.8.7.4 [set.intersection] as indicated:

   >     […]
   >     template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
   >              weakly_incrementable O, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >       constexpr ranges::set_intersection_result<I1, I2, O>
   >         ranges::set_intersection(I1 first1, S1 last1, I2 first2, S2 last2, O result,
   >                                  Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<input_range R1, input_range R2, weakly_incrementable O,
   >              class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
   >       constexpr ranges::set_intersection_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O>
   >         ranges::set_intersection(R1&& r1, R2&& r2, O result,
   >                                  Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});

   >     template<execution-policy Ep, random_access_iterator I1, sized_sentinel_for<I1> S1,
   >              random_access_iterator I2, sized_sentinel_for<I2> S2,
   >              random_access_iterator O, sized_sentinel_for<O> OutS, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >       ranges::set_intersection_result<I1, I2, O>
   >         ranges::set_intersection(Ep&& exec, I1 first1, S1 last1,
   >                                  I2 first2, S2 last2, O result, OutS result_last,
   >                                  Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<execution-policy Ep, sized-random-access-range R1, sized-random-access-range R2,
   >              sized-random-access-range OutR, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, iterator_t<OutR>, Comp, Proj1, Proj2>
   >       ranges::set_intersection_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>,
   >                                       borrowed_iterator_t<OutR>>
   >         ranges::set_intersection(Ep&& exec, R1&& r1, R2&& r2, OutR&& result_r, Comp comp = {},
   >                                  Proj1 proj1 = {}, Proj2 proj2 = {});

   > > -1- Let: […]

   > > -2- Preconditions: The ranges `[first1, last1)` and `[first2, last2)` are sorted with respect to `comp` and `proj1` or `proj2`, respectively. The resulting range does not overlap with either of the original ranges.

   > > -3- Effects: Constructs a sorted intersection of the elements from the two ranges; that is, the set of elements that are present in both of the ranges. If `[first1, last1)` contains `m` elements that are equivalent to each other and `[first2, last2)` contains `n` elements that are equivalent to them, the first min`(m, n)` elements from the first range are included in the sorted intersection. If, of those elements, `k` elements from the first range are copied to the output range, then the first `k` elements from the second range are considered skipped. If `N < M`, a non-copied element is also considered skipped if it compares less than the `(N + 1)`th element of the sorted intersection. Copies the first `N` elements of the sorted intersection to the range `[result, result + N)`.

   > > -4- Returns:

   > > - (4.1) — `result_last` for the overloads in namespace `std`.

   > > - (4.2) — `{last1, last2, result + N}` for the non-parallel algorithm overloads in namespace `ranges`, if `N` is equal to `M`.

   > > - (4.?) — `{first1 + A, first2 + B, result + N}` for the parallel algorithm overloads in namespace `ranges`, if `N` is equal to `M` and where `A` and `B` are the numbers of copied or skipped elements in `[first1, last1)` and `[first2, last2)`, respectively.

   > > - (4.3) — Otherwise, `{first1 + A, first2 + B, result_last}` for the overloads in namespace `ranges`, where `A` and `B` are the numbers of copied or skipped elements in `[first1, last1)` and `[first2, last2)`, respectively.

[Croydon 2026-03-24; update wording after LWG review]

[Croydon 2026-03-24; move to Immediate]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 26.8.7.4 [set.intersection] as indicated:

   >     […]
   >     template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
   >              weakly_incrementable O, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >       constexpr ranges::set_intersection_result<I1, I2, O>
   >         ranges::set_intersection(I1 first1, S1 last1, I2 first2, S2 last2, O result,
   >                                  Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<input_range R1, input_range R2, weakly_incrementable O,
   >              class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
   >       constexpr ranges::set_intersection_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O>
   >         ranges::set_intersection(R1&& r1, R2&& r2, O result,
   >                                  Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});

   >     template<execution-policy Ep, random_access_iterator I1, sized_sentinel_for<I1> S1,
   >              random_access_iterator I2, sized_sentinel_for<I2> S2,
   >              random_access_iterator O, sized_sentinel_for<O> OutS, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
   >       ranges::set_intersection_result<I1, I2, O>
   >         ranges::set_intersection(Ep&& exec, I1 first1, S1 last1,
   >                                  I2 first2, S2 last2, O result, OutS result_last,
   >                                  Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});
   >     template<execution-policy Ep, sized-random-access-range R1, sized-random-access-range R2,
   >              sized-random-access-range OutR, class Comp = ranges::less,
   >              class Proj1 = identity, class Proj2 = identity>
   >       requires mergeable<iterator_t<R1>, iterator_t<R2>, iterator_t<OutR>, Comp, Proj1, Proj2>
   >       ranges::set_intersection_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>,
   >                                       borrowed_iterator_t<OutR>>
   >         ranges::set_intersection(Ep&& exec, R1&& r1, R2&& r2, OutR&& result_r, Comp comp = {},
   >                                  Proj1 proj1 = {}, Proj2 proj2 = {});

   > > -1- Let: […]

   > > -2- Preconditions: The ranges `[first1, last1)` and `[first2, last2)` are sorted with respect to `comp` and `proj1` or `proj2`, respectively. The resulting range does not overlap with either of the original ranges.

   > > -3- Effects: Constructs a sorted intersection of the elements from the two ranges; that is, the set of elements that are present in both of the ranges. If `[first1, last1)` contains `m` elements that are equivalent to each other and `[first2, last2)` contains `n` elements that are equivalent to them, the first min`(m, n)` elements from the first range are included in the sorted intersection. If, of those elements, `k` elements from the first range are copied to the output range, then the first `k` elements from the second range are considered skipped. If `N < M`, a A non-copied element is also considered skipped if it compares less than the min(`M`, `N` + 1)th `(N + 1)`th element of the sorted intersection. Copies the first `N` elements of the sorted intersection to the range `[result, result + N)`.

   > > -4- Returns:

   > > - (4.1) — `result_last` for the overloads in namespace `std`.

   > > - (4.2) — `{last1, last2, result + N}` for the non-parallel algorithm overloads in namespace `ranges`, if `N` is equal to `M`.

   > > - (4.3) — Otherwise, `{first1 + A, first2 + B,result_last result + N}` for the parallel algorithm overloads in namespace `ranges`, where `A` and `B` are the numbers of copied or skipped elements in `[first1, last1)` and `[first2, last2)`, respectively.

### 4549(i). `vprint_nonunicode_buffered` ignores its `stream` parameter

Section: 31.7.10 [print.fun] Status: C++26 Submitter: Abhinav Agarwal Opened: 2026-03-21 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

The Effects element of `vprint_nonunicode_buffered(FILE* stream, string_view fmt, format_args args)` in 31.7.10 [print.fun] p14 is specified as:

>     string out = vformat(fmt, args);
>     vprint_nonunicode("{}", make_format_args(out));

That call selects the two-argument overload `vprint_nonunicode(string_view, format_args)` (31.7.10 [print.fun] p13), whose Effects element delegates to `vprint_nonunicode(stdout, fmt, args)`. The caller-supplied stream is discarded and the output destination becomes `stdout`.

The parallel function `vprint_unicode_buffered` (31.7.10 [print.fun] p8) correctly passes `stream`:

>     string out = vformat(fmt, args);
>     vprint_unicode(stream, "{}", make_format_args(out));

This is user-visible: `print(FILE* stream, ...)` (31.7.10 [print.fun] p2) calls `vprint_nonunicode_buffered(stream, ...)` on the non-UTF-8 buffered path, so the current wording routes output to the wrong stream.

[Croydon 2026-03-24; move to Immediate]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 31.7.10 [print.fun] as indicated:

   >     void vprint_nonunicode_buffered(FILE* stream, string_view fmt, format_args args);

   > > -14- Effects: Equivalent to:

   > > >     string out = vformat(fmt, args);
   > > >     vprint_nonunicode(stream, "{}", make_format_args(out));

### 4550(i). Need new feature test macros for `<stdckdint.h>` and `<stdbit.h>`

Section: 17.3.2 [version.syn] Status: C++26 Submitter: Jonathan Wakely Opened: 2026-03-23 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses GB 07 and GB 11.

The `__STDC_VERSION_STDCKDINT_H__` macro defined by C is not sufficient to check if a C++26 implementation provides a usable `<stdckdint.h>` header. If a libc header of that name is found somewhere in the C++ compiler's include paths (e.g. in `/usr/include`) then it might be included and would define the macro. However, the libc version of the header might use C-specific features such as `_Bool` or `_Generic` and cause errors in a C++ program. A C++-specific macro would only be present in a C++-aware header, and so a C++ program could check if that C++-specific macro is defined in `<version>` to be sure that the header is usable in C++ code.

The same argument applies to `<stdbit.h>`.

[2026-03-23 Croydon; move to Immediate]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 17.3.2 [version.syn] as indicated:

   >     #define __cpp_lib_starts_ends_with             201711L // also in <string>, <string_view>
   >     #define __cpp_lib_stdatomic_h                  202011L // also in <stdatomic.h>
   >     #define __cpp_lib_stdbit_h                     20YYMML // also in <stdbit.h>
   >     #define __cpp_lib_stdckdint_h                  20YYMML // also in <stdckdint.h>
   >     #define __cpp_lib_string_contains              202011L // also in <string>, <string_view>
   >     #define __cpp_lib_string_resize_and_overwrite  202110L // also in <string>

### 4552(i). `compare_exchange_weak` writes a value on spurious failure, not memory contents

Section: 32.5.7.2 [atomics.ref.ops], 32.5.8.2 [atomics.types.operations] Status: C++26 Submitter: Jonathan Wakely Opened: 2026-03-24 Last modified: 2026-06-09

Priority: 3

View all other issues in [atomics.ref.ops].

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Discussion:

[atomic.ref.ops] says:

> -27- Remarks: A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by `expected` and `ptr` are equal, it may return `false` and store back to `expected` the same memory contents that were originally there.

I think this is missing an update from P0528R3 which should have changed "the same memory contents" to something like "the same value". When the comparison succeeds but the weak CAS fails anyway, the bits of `exchange` are replaced by the bits copied from `*ptr`. Those bits might differ from the original bits of `expected` in their padding.

[Croydon 2026-03-24; Set priority to 2 and send to SG1.]

[Croydon 2026-03-27; SG1 approved the change.]

[Croydon 2026-03-27; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 32.5.7.2 [atomics.ref.ops] as indicated:

   >     constexpr bool compare_exchange_weak(value_type& expected, value_type desired,
   >                                          memory_order order = memory_order::seq_cst) const noexcept;

   >     constexpr bool compare_exchange_strong(value_type& expected, value_type desired,
   >                                            memory_order order = memory_order::seq_cst) const noexcept;

   > > -23- Constraints: `is_const_v<T>` is `false`.

   > > -24- Preconditions: `failure` is `memory_order::relaxed`, `memory_order::acquire`, or `memory_order::seq_cst`.

   > > -25- Effects: Retrieves the value in `expected`. It then atomically compares the value representation of the value referenced by `*ptr` for equality with that previously retrieved from `expected`, and if `true`, replaces the value referenced by `*ptr` with that in `desired`. If and only if the comparison is `true`, memory is affected according to the value of `success`, and if the comparison is `false`, memory is affected according to the value of `failure`. When only one `memory_order` argument is supplied, the value of `success` is `order`, and the value of `failure` is `order` except that a value of `memory_order::acq_rel` shall be replaced by the value `memory_order::acquire` and a value of `memory_order::release` shall be replaced by the value `memory_order::relaxed`. If and only if the comparison is `false` then, after the atomic operation, the value in `expected` is replaced by the value read from the value referenced by `*ptr` during the atomic comparison. If the operation returns `true`, these operations are atomic read-modify-write operations (6.10.2.2 [intro.races]) on the value referenced by `*ptr`. Otherwise, these operations are atomic load operations on that memory.

   > > -26- Returns: The result of the comparison.

   > > -27- Remarks: A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory value representations referred to by `expected` and `ptr` are compare equal, it may return `false` and store back to `expected` the same memory contents that were value representation that was originally there.

   > > [Note 2: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop. When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable. — end note]

2. Modify 32.5.8.2 [atomics.types.operations] as indicated:

   >     bool compare_exchange_weak(T& expected, T desired,
   >                                memory_order success, memory_order failure) volatile noexcept;
   >     constexpr bool compare_exchange_weak(T& expected, T desired,
   >                                memory_order success, memory_order failure) noexcept;
   >     bool compare_exchange_strong(T& expected, T desired,
   >                                  memory_order success, memory_order failure) volatile noexcept;
   >     constexpr bool compare_exchange_strong(T& expected, T desired,
   >                                  memory_order success, memory_order failure) noexcept;
   >     bool compare_exchange_weak(T& expected, T desired,
   >                                memory_order order = memory_order::seq_cst) volatile noexcept;
   >     constexpr bool compare_exchange_weak(T& expected, T desired,
   >                                memory_order order = memory_order::seq_cst) noexcept;
   >     bool compare_exchange_strong(T& expected, T desired,
   >                                  memory_order order = memory_order::seq_cst) volatile noexcept;
   >     constexpr bool compare_exchange_strong(T& expected, T desired,
   >                                  memory_order order = memory_order::seq_cst) noexcept;

   > > -21- Constraints: For the `volatile` overload of this function, `is_always_lock_free` is `true`.

   > > -22- Preconditions: `failure` is `memory_order::relaxed`, `memory_order::acquire`, or `memory_order::seq_cst`.

   > > -25- Effects: Retrieves the value in `expected`. It then atomically compares the value representation of the value pointed to by `this` for equality with that previously retrieved from `expected`, and if `true`, replaces the value pointed to by `this` with that in `desired`. If and only if the comparison is `true`, memory is affected according to the value of `success`, and if the comparison is `false`, memory is affected according to the value of `failure`. When only one `memory_order` argument is supplied, the value of `success` is `order`, and the value of `failure` is `order` except that a value of `memory_order::acq_rel` shall be replaced by the value `memory_order::acquire` and a value of `memory_order::release` shall be replaced by the value `memory_order::relaxed`. If and only if the comparison is `false` then, after the atomic operation, the value in `expected` is replaced by the value read from the value pointed to by `this` during the atomic comparison. If the operation returns `true`, these operations are atomic read-modify-write operations (6.10.2 [intro.multithread] 6.10.2.2 [intro.races]) on the value pointed to by `this`. Otherwise, these operations are atomic load operations on that memory.

   > > -24- Returns: The result of the comparison.

   > > -25- [Note 4: For example, the effect of `compare_exchange_strong` on objects without padding bits (6.9.1 [basic.types.general]) is

   > > >     if (memcmp(this, &expected, sizeof(*this)) == 0)
   > > >       memcpy(this, &desired, sizeof(*this));
   > > >     else
   > > >       memcpy(&expected, this, sizeof(*this));

   > > — end note]

   > > [Example 1: The expected use of the compare-and-exchange operations is as follows. The compare-and-exchange operations will update expected when another iteration of the loop is needed.

   > > >     expected = current.load();
   > > >     do {
   > > >       desired = function(expected);
   > > >     } while (!current.compare_exchange_weak(expected, desired));

   > > — end example]

   > > [Example 2: Because the expected value is updated only on failure, code releasing the memory containing the expected value on success will work. For example, list head insertion will act atomically and would not introduce a data race in the following code:

   > > >     do {
   > > >       p->next = head;                                   // make new list node point to the current head
   > > >     } while (!head.compare_exchange_weak(p->next, p));  // try to insert

   > > — end example]

   > > -26- Recommended practice: Implementations should ensure that weak compare-and-exchange operations do not consistently return false unless either the atomic object has value different from `expected` or there are concurrent modifications to the atomic object.

   > > -27- Remarks: A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory value representations referred to by `expected` and `this` are compare equal, it may return `false` and store back to `expected` the same memory contents that were value representation that was originally there.

   > > [Note 2: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop. When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable. — end note]

### 4553(i). Wording for FR-025-246 25.7.18.2 Add a `reserve_hint` function to `concat_view`

Section: 25.7.18.2 [range.concat.view] Status: C++26 Submitter: Hui Xie Opened: 2026-03-24 Last modified: 2026-06-09

Priority: Not Prioritized

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Discussion:

Addresses FR-025-246

LEWG approved the NB comment "FR-025-246 25.7.18.2 Add a `reserve_hint` function to `concat_view`" without wording. This issue proposes the wording.

[Croydon 2026-03-24; move to Immediate]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 25.7.18.2 [range.concat.view] as indicated:

   >     namespace std::ranges {
   >       […]
   >       template<input_range... Views>
   >         requires (view<Views> && ...) && (sizeof...(Views) > 0) &&
   >                   concatable<Views...>
   >       class concat_view : public view_interface<concat_view<Views...>> {
   >         […]
   >         constexpr auto size() requires (sized_range<Views> && ...);
   >         constexpr auto size() const requires (sized_range<const Views> && ...);
   >         constexpr auto reserve_hint() requires (approximately_sized_range<Views> && ...);
   >         constexpr auto reserve_hint() const requires (approximately_sized_range<const Views> && ...);
   >       };
   >     }
   >     […]

   >     constexpr auto size() requires (sized_range<Views> && ...);
   >     constexpr auto size() const requires (sized_range<const Views> && ...);

   >     -8- Effects: Equivalent to:

   >     return apply(
   >       [](auto... sizes) {
   >         using CT = make-unsigned-like-t<common_type_t<decltype(sizes)...>>;
   >         return (CT(sizes) + ...);
   >       },
   >       tuple-transform(ranges::size, views_));

   >     constexpr auto reserve_hint() requires (approximately_sized_range<Views> && ...);
   >     constexpr auto reserve_hint() const requires (approximately_sized_range<const Views> && ...);

   >     -9- Effects: Equivalent to:

   >     return apply(
   >       [](auto... sizes) {
   >         using CT = make-unsigned-like-t<common_type_t<decltype(sizes)...>>;
   >         return (CT(sizes) + ...);
   >       },
   >       tuple-transform(ranges::reserve_hint, views_));

### 4554(i). Remove undefined behaviour from `hive` for invalid limits

Section: 23.3.9.1 [hive.overview] Status: C++26 Submitter: Jonathan Wakely Opened: 2026-03-24 Last modified: 2026-06-09

Priority: Not Prioritized

View all issues with C++26 status.

Discussion:

Addresses US 139-232

23.3.9.1 [hive.overview] says:

> -5- Limits can be placed on both the minimum and maximum element capacities of element blocks, both by users and implementations.
> - ...
> - (5.4) — If user-specified limits are not within hard limits, or if the specified minimum limit is greater than the specified maximum limit, the behavior is undefined.

The NB comment says:

> Paragraph 5.4 allows for UB when the user doesn't manage to keep the block limits within bounds and the min below the max, yet no tools are offered to check these conditions easily. In terms of providing safe facilities this UB is an easy trap to fall into. The methods that allow to set block limits on hive should rather check the conditions and throw in case of violation or at least some function should be provided to let the user check the conditions easily.

> Proposed change: Adjust all constructors and methods on hive to throw in case specified block limits violate the conditions set out here. Or at least provide a function to the user to correctly check the condition separately.

[2026-03-24; Jonathan adds wording]

Could throw when the precondition is violated, or make it a Hardened Precondition, or ... something else? Three options are presented, which are not entirely mutually exclusive.

Previous resolution [SUPERSEDED]

This wording is relative to N5032.

Option A - make `hive` check for invalid limits and throw

1. Modify 23.3.9.1 [hive.overview] as indicated:

   > -5- Limits can be placed on both the minimum and maximum element capacities of element blocks, both by users and implementations.
   > - (5.1) — The minimum limit shall be no larger than the maximum limit.
   > - (5.2) — When limits are not specified by a user during construction, the implementation's default limits are used.
   > - (5.3) — The default limits of an implementation are not guaranteed to be the same as the minimum and maximum possible capacities for an implementation's element blocks.

   >   [Note 2:  To allow latitude for both implementation-specific and user-directed optimization. — end note]

   >   The latter are defined as hard limits. The maximum hard limit shall be no larger than `std::allocator_traits<Allocator>::max_size()`.
   > - (5.4) — If user-specified limits are not within hard limits, or if the specified minimum limit is greater than the specified maximum limit, the behavior is undefined.
   > - (5.5) — An element block is said to be within the bounds of a pair of minimum/maximum limits when its capacity is greater-or-equal-to the minimum limit and less-than-or-equal-to the maximum limit.

2. Modify 23.3.9.2 [hive.cons] as indicated:

   >     constexpr hive(hive_limits block_limits, const Allocator&);

   > > -3- Effects: Constructs an empty `hive`, using the specified allocator. Initializes `current-limits` with `block_limits`.

   > > -4- Complexity: Constant.

   > > -?- Throws: `invalid_argument` if `block_limits.min` or `block_limits.max` is outside the hard limits, or if `block_limits.min` is greater than `block_limits.max`.

   > > Similar changes to other constructors and `reshape`

Option B - provide a helper to make limits valid

1. Modify 23.3.8 [hive.syn] as indicated:

   >     namespace std {
   >       struct hive_limits {
   >         size_t min;
   >         size_t max;
   >         constexpr hive_limits(size_t minimum, size_t maximum) noexcept
   >           : min(minimum), max(maximum) {}

   >         constexpr hive_limits clamp_to(const hive_limits& lim) const noexcept
   >         { return {clamp(min, lim.min, lim.max), clamp(max, lim.min, lim.max)}; }
   >       };

Option C - add a helper to validate limits against hard limits and define Preconditions using it.

1. Modify 23.3.8 [hive.syn] as indicated:

   >     namespace std {
   >       struct hive_limits {
   >         size_t min;
   >         size_t max;
   >         constexpr hive_limits(size_t minimum, size_t maximum) noexcept
   >           : min(minimum), max(maximum) {}

   >         constexpr bool valid_for(const hive_limits& lim) const noexcept
   >         { return min <= max && lim.min <= min && max <= lim.max; }
   >       };

2. Modify 23.3.9.2 [hive.cons] as indicated:

   >     constexpr hive(hive_limits block_limits, const Allocator&);

   > > -?- Preconditions: `block_limits.valid_for(block_capacity_hard_limits())` is `true`.

   > > -3- Effects: Constructs an empty `hive`, using the specified allocator. Initializes `current-limits` with `block_limits`.

   > > -4- Complexity: Constant.

   > > Similar changes to other constructors and `reshape`

[2026-03-26; Jonathan and Frank provide new wording]

Provide a new static member function on hive which compares a `hive_limits` to its hard limits. Change behaviour from undefined to erroneous with implementation-defined effects.

[Croydon 2026-03-26; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 23.3.9.1 [hive.overview] as indicated:

   > -5- Limits can be placed on both the minimum and maximum element capacities of element blocks, both by users and implementations.
   > - (5.1) — The minimum limit shall be no larger than the maximum limit.
   > - (5.2) — When limits are not specified by a user during construction, the implementation's default limits are used.
   > - (5.3) — The default limits of an implementation are not guaranteed to be the same as the minimum and maximum possible capacities for an implementation's element blocks.
   >   [Note 2:  To allow latitude for both implementation-specific and user-directed optimization. — end note]
   >   The latter are defined as hard limits. The maximum hard limit shall be no larger than `std::allocator_traits<Allocator>::max_size()`.
   > - (5.4) — If user-specified limits passed to a `hive` constructor or `reshape` are not within hard limits, or if the specified minimum limit is greater than the specified maximum limit, the behavior is undefined erroneous and the effects are implementation-defined.
   >   [Note ?: This condition can be checked using `is_within_hard_limits`. — end note]
   > - (5.5) — An element block is said to be within the bounds of a pair of minimum/maximum limits when its capacity is greater-or-equal-to the minimum limit and less-than-or-equal-to the maximum limit.

2. Modify 23.3.9.1 [hive.overview] as indicated:

   >       constexpr hive_limits block_capacity_limits() const noexcept;
   >       static constexpr hive_limits block_capacity_default_limits() noexcept;
   >       static constexpr hive_limits block_capacity_hard_limits() noexcept;
   >       static constexpr bool is_within_hard_limits(hive_limits) noexcept;
   >       void reshape(hive_limits block_limits);

3. Modify 23.3.9.3 [hive.capacity] as indicated:

   >     static constexpr hive_limits block_capacity_hard_limits() noexcept;

   > > -19- Returns: A `hive_limits` struct with the `min` and `max` members set to the implementation’s hard limits.

   >     static constexpr bool is_within_hard_limits(hive_limits lim) noexcept;

   > > -?- Let `hl` be `block_capacity_hard_limits()`.

   > > -?- Returns: `hl.min <= lim.min && lim.min <= lim.max && lim.max <= hl.max`.

### 4555(i). Remove `is_consteval_only`

Section: 21.3.5.4 [meta.unary.prop], 21.4.18 [meta.reflection.traits] Status: C++26 Submitter: Tim Song Opened: 2026-03-24 Last modified: 2026-06-09

Priority: Not Prioritized

View other active issues in [meta.unary.prop].

View all other issues in [meta.unary.prop].

View all issues with C++26 status.

Discussion:

Addresses US 81-149.

In light of recent EWG polls related to consteval-only types, it would be improvident to ship this trait in C++26. This also resolves the NB comment requesting respecification of its precondition.

[Croydon 2026-03-26; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.3.3 [meta.type.synop], header `<type_traits>` synopsis, as indicated:

   >     […]
   >       // 21.3.5.4 [meta.unary.prop], type properties
   >     […]
   >       template<class T> struct is_consteval_only;
   >     […]
   >       // 21.3.5.4 [meta.unary.prop], type properties
   >     […]
   >       template<class T>
   >         constexpr bool is_consteval_only_v = is_consteval_only<T>::value;
   >     […]

2. Modify [tab:meta.unary.prop], Table 54 — Type property predicates as indicated:

   > Template | Condition | Preconditions
   > --- | --- | ---
   > … | … | …
   >     template<class T>
   >     struct is_consteval_only;
   > | T is consteval-only (6.9.1 [basic.types.general]) | `remove_all_extents_t<T>` shall be a complete type or cv `void`.
   > … | … | …

3. Modify 21.4.1 [meta.syn], header `<meta>` synopsis, as indicated:

   >     […]
   >       // associated with 21.3.5.4 [meta.unary.prop], type properties
   >     […]
   >       consteval bool is_consteval_only_type(info type);
   >     […]

4. Modify 21.4.18 [meta.reflection.traits] as indicated:

   >     […]
   >       // associated with 21.3.5.4 [meta.unary.prop], type properties
   >     […]
   >       consteval bool is_consteval_only_type(info type);
   >     […]

   > > -3- […]

### 4556(i). Unclear properties of reflection strings

Section: 21.4.1 [meta.syn] Status: C++26 Submitter: Tim Song Opened: 2026-03-25 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [meta.syn].

View all issues with C++26 status.

Discussion:

We are missing a statement about the lifetime and other properties of the strings returned from the reflection functions.

[Croydon 2026-03-25; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.1 [meta.syn] as indicated:

   > -3- Any function in namespace `std::meta` whose return type is `string_view` or `u8string_view` returns an object `V` such that `V.data()[V.size()]` equals `'\0'`. Each element of the range `V.data()` + [0, `V.size()`] is a potentially non-unique object with static storage duration that is usable in constant expressions (6.8.2 [intro.object], 7.7 [expr.const]); a pointer to such an element is not suitable for use as a template argument for a constant template parameter of pointer type (13.4.3 [temp.arg.nontype]).

### 4557(i). Remove `constexpr` from `owner_less` and `owner_before`

Section: 20.3.2.2.6 [util.smartptr.shared.obs], 20.3.2.3.6 [util.smartptr.weak.obs], 20.3.2.4 [util.smartptr.ownerless] Status: C++26 Submitter: Tim Song Opened: 2026-03-25 Last modified: 2026-06-09

Priority: Not Prioritized

View all other issues in [util.smartptr.shared.obs].

View all issues with C++26 status.

Discussion:

Addresses US 75-138

LEWG decided to remove `constexpr` from `owner_before` and `owner_less`.

[Croydon 2026-03-25; move to Immediate.]

[Croydon 2026-03-28; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 20.3.2.2.1 [util.smartptr.shared.general], class template `shared_ptr` synopsis, as indicated:

   >     namespace std {
   >       template<class T> class shared_ptr {
   >       public:
   >         […]
   >         template<class U>
   >           constexpr bool owner_before(const shared_ptr<U>& b) const noexcept;
   >         template<class U>
   >           constexpr bool owner_before(const weak_ptr<U>& b) const noexcept;
   >         size_t owner_hash() const noexcept;
   >         template<class U>
   >           constexpr bool owner_equal(const shared_ptr<U>& b) const noexcept;
   >         template<class U>
   >           constexpr bool owner_equal(const weak_ptr<U>& b) const noexcept;
   >       };

   >       […]
   >     }

2. Modify 20.3.2.2.6 [util.smartptr.shared.obs] as indicated:

   >     template<class U> constexpr bool owner_before(const shared_ptr<U>& b) const noexcept;
   >     template<class U> constexpr bool owner_before(const weak_ptr<U>& b) const noexcept;

   > > -19- […]

3. Modify 20.3.2.3.1 [util.smartptr.weak.general], class template `weak_ptr` synopsis, as indicated:

   >     namespace std {
   >       template<class T> class weak_ptr {
   >       public:
   >         […]
   >         template<class U>
   >           constexpr bool owner_before(const shared_ptr<U>& b) const noexcept;
   >         template<class U>
   >           constexpr bool owner_before(const weak_ptr<U>& b) const noexcept;
   >         size_t owner_hash() const noexcept;
   >         template<class U>
   >           constexpr bool owner_equal(const shared_ptr<U>& b) const noexcept;
   >         template<class U>
   >           constexpr bool owner_equal(const weak_ptr<U>& b) const noexcept;
   >       };

   >       […]
   >     }

4. Modify 20.3.2.3.6 [util.smartptr.weak.obs] as indicated:

   >     template<class U> constexpr bool owner_before(const shared_ptr<U>& b) const noexcept;
   >     template<class U> constexpr bool owner_before(const weak_ptr<U>& b) const noexcept;

   > > -4- […]

5. Modify 20.3.2.4 [util.smartptr.ownerless], class template `owner_less` synopsis, as indicated:

   >     namespace std {
   >       template<class T = void> struct owner_less;

   >       template<class T> struct owner_less<shared_ptr<T>> {
   >         constexpr bool operator()(const shared_ptr<T>&, const shared_ptr<T>&) const noexcept;
   >         constexpr bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
   >         constexpr bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
   >       };

   >       template<class T> struct owner_less<weak_ptr<T>> {
   >         constexpr bool operator()(const weak_ptr<T>&, const weak_ptr<T>&) const noexcept;
   >         constexpr bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
   >         constexpr bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
   >       };

   >       template<> struct owner_less<void> {
   >         template<class T, class U>
   >           constexpr bool operator()(const shared_ptr<T>&, const shared_ptr<U>&) const noexcept;
   >         template<class T, class U>
   >           constexpr bool operator()(const shared_ptr<T>&, const weak_ptr<U>&) const noexcept;
   >         template<class T, class U>
   >           constexpr bool operator()(const weak_ptr<T>&, const shared_ptr<U>&) const noexcept;
   >         template<class T, class U>
   >           constexpr bool operator()(const weak_ptr<T>&, const weak_ptr<U>&) const noexcept;

   >         using is_transparent = unspecified;
   >       };
   >     }

### 4567(i). Feature test macro value for `apply_result`, `is_applicable`

Section: 17.3.2 [version.syn] Status: WP Submitter: Tomasz Kamiński Opened: 2026-04-08 Last modified: 2026-06-15

Priority: Not Prioritized

View other active issues in [version.syn].

View all other issues in [version.syn].

View all issues with WP status.

Discussion:

The paper P1317R2 "Remove return type deduction in `std::apply`", which changed the definition of `std::apply` and introduced the new traits `is_applicable`, `is_nothrow_applicable`, and `apply_result`, was accepted in Sofia.

In Croydon we moved the P3795R2 "Miscellaneous Reflection Cleanup", that adds the reflection equivalents of above, under `__cpp_lib_reflection`.

This leads to situation, when an implementation that does not yet implement P1317R2, and thus does not define the corresponding meta functions, should not define `__cpp_lib_reflection`.

I propose to assign the following `__cpp_lib_apply` macro values to account to above papers:

- `202506L` for P1317R2 also in `<tuple>`, `<type_traits>`

- `202603L` for the corresponding parts of P3795R2 also in `<meta>`

The proposed resolution for the standard working paper represents only the effective end state.

[2026-05-29; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 17.3.2 [version.syn] as indicated:

   >     #define __cpp_lib_apply 202506L202603L // freestanding, also in <tuple>, <type_traits>, <meta>

### 4568(i). `std::execution::spawn_future` is mishandling dependent senders

Section: 33.9.12.18 [exec.spawn.future] Status: WP Submitter: Eric Niebler Opened: 2026-04-11 Last modified: 2026-06-15

Priority: Not Prioritized

View all other issues in [exec.spawn.future].

View all issues with WP status.

Discussion:

In 33.9.12.18 [exec.spawn.future] p7, we see this:

> -7- Let `spawn-future-state` be the exposition-only class template:

> >     namespace std::execution {
> >       template<class Alloc, scope_token Token, sender Sender, class Env>
> >       struct spawn-future-state                   // exposition only
> >         : spawn-future-state-base<completion_signatures_of_t<future-spawned-sender<Sender, Env>>> {
> >         using sigs-t =                            // exposition only
> >           completion_signatures_of_t<future-spawned-sender<Sender, Env>>;
> >         using receiver-t =                        // exposition only
> >           spawn-future-receiver<sigs-t>;
> >         using op-t =                              // exposition only
> >           connect_result_t<future-spawned-sender<Sender, Env>, receiver-t>;
> >       […]
> >       };
> >     […]
> >     }

where `spawn-future-sender<Sender, Env>` is the type of `write_env(stop-when(declval<Sender>(), declval<stoken-t>()), declval<Env>())`.

The problem happens in the definition of `sigs-t`. There is a difference between asking for a sender's completion signatures with an empty environment, and asking with no environment.

In `sigs-t`, we are asking for the completion signatures with no environment. That is asking the sender for its non-dependent completions. But if `Sender` is a dependent sender, then so is `future-spawned-sender<Sender, Env>`, and so this line will not compile. A dependent sender cannot provide non-dependent completion signatures.

The `spawn_future` algorithm connects the `future-spawned-sender` with a receiver that has an empty environment. Therefore, we should be using the empty environment when computing the `future-spawned-sender`'s completion signatures.

[2026-05-29; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5032.

1. Modify 33.9.12.18 [exec.spawn.future] as indicated:

   > -7- Let `spawn-future-state` be the exposition-only class template:

   > >     namespace std::execution {
   > >       template<class Alloc, scope_token Token, sender Sender, class Env>
   > >       struct spawn-future-state                   // exposition only
   > >         : spawn-future-state-base<completion_signatures_of_t<future-spawned-sender<Sender, Env>, env<>>> {
   > >         using sigs-t =                            // exposition only
   > >           completion_signatures_of_t<future-spawned-sender<Sender, Env>, env<>>;
   > >         using receiver-t =                        // exposition only
   > >           spawn-future-receiver<sigs-t>;
   > >         using op-t =                              // exposition only
   > >           connect_result_t<future-spawned-sender<Sender, Env>, receiver-t>;
   > >       […]
   > >       };
   > >     […]
   > >     }

### 4579(i). `make-state<Rcvr>::state-type::complete` uses `Env` which is not in scope

Section: 33.9.12.12 [exec.when.all] Status: WP Submitter: Abhinav Agarwal Opened: 2026-05-12 Last modified: 2026-06-15

Priority: Not Prioritized

View all other issues in [exec.when.all].

View all issues with WP status.

Discussion:

In 33.9.12.12 [exec.when.all], the specification of `make-state::state-type::complete` (paragraph 15, bullet (15.3)) defines `sends-stopped` as true if and only if there exists an element `S` of `Sndrs` such that

>     completion_signatures_of_t<S, when-all-env<Env>>

contains `set_stopped_t()`.

However, no declaration named `Env` is in scope at this point. `state-type` is specified in the context of `make-state<Rcvr>::operator()<Sndrs...>`; the relevant named template parameters there are `Rcvr` and `Sndrs...`. `Env` is a template parameter of `check-types<Sndr, Env...>`, which is a separate member function whose scope does not extend into `make-state`.

Other environment-dependent wording within the `make-state` scope uses `env_of_t<Rcvr>`:

- The `stop_callback` alias (paragraph 12 within the `make-state` class template synopsis) uses `stop_callback_for_t<stop_token_of_t<env_of_t<Rcvr>>, on-stop-request>`

- The `values_tuple` alias (paragraph 13) uses `value_types_of_t<Sndrs, FWD-ENV-T(env_of_t<Rcvr>), […]>`

The `sends-stopped` definition in bullet (15.3) is the only use of bare `Env` in this scope.

This issue was introduced by P3887R1 ("Make when_all a Ronseal Algorithm"), whose wording contains the same text.

[2026-05-29; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Brno 2026-06-13; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5046.

1. Modify 33.9.12.12 [exec.when.all] as indicated:

   > -15- The member `void state-type::complete(Rcvr& rcvr) noexcept` behaves as follows:

   > 1. (15.1) — If `disp` is equal to `disposition::started`, evaluates: […]

   > 2. (15.2) — Otherwise, if `disp` is equal to `disposition::error`, evaluates: […]

   > 3. (15.3) — Otherwise, evaluates:

   >    >     if constexpr (sends-stopped) {
   >    >       on_stop.reset();
   >    >       set_stopped(std::move(rcvr));
   >    >     }

   >    where `sends-stopped` equals true if and only if there exists an element `S` of `Sndrs` such that `completion_signatures_of_t<S, when-all-env<Envenv_of_t<Rcvr>>>` contains `set_stopped_t()`.